Abstract
Hormones are key drivers of cancer development. To date, interest has largely been focussed on the classical model of hormonal gene regulation, but there is increasing evidence for a role of hormone signalling pathways in post-translational regulation of gene expression. In particular, a complex and dynamic network of bi-directional interactions with microRNAs (miRs) at all stages of biogenesis and during target gene repression is emerging. miRs, which act mainly by negatively regulating gene expression through association with 3′-UTRs of mRNA species, are increasingly understood to be important in development, normal physiology and pathogenesis. Given recent demonstrations of altered miR profiles in a diverse range of cancers, their ability to function as oncogenes or tumour suppressors, and hormonal regulation of miRs, understanding mechanisms by which miRs are generated and regulated is vitally important. miRs are transcribed by RNA polymerase II and then processed in the nucleus by the Drosha-containing Microprocessor complex and in the cytoplasm by Dicer, before mature miRs are incorporated into the RNA-induced silencing complex. It is increasingly evident that multiple cellular signalling pathways converge upon the miR biogenesis cascade, adding further layers of regulatory complexity to modulate miR maturation. This review summarises recent advances in identification of novel components and regulators of the Microprocessor and Dicer complexes, with particular emphasis on the role of hormone signalling pathways in regulating their activity. Understanding hormone regulation of miR production and how this is perturbed in cancer are critical for the development of miR-based therapeutics and biomarkers.
An overview of microRNA biogenesis
MicroRNAs (miRs) were first discovered in 1993, with two independent studies describing the Caenorhabditis elegans lin-4 gene product as a small non-coding RNA (ncRNA; Lee et al. 1993, Wightman et al. 1993). Later it was shown that the C. elegans let7 gene product, also an ncRNA, could associate with the 3′-UTR of target genes to initiate a cascade of regulatory processes acting on heterochronic genes (Reinhart et al. 2000). The functionality of miRs in other cell systems was first demonstrated by three groups in 2001 (Lagos-Quintana et al. 2001, Lau et al. 2001, Lee & Ambros 2001). It is now understood that miRs act as gene regulators in almost all plant and animal species.
Mammalian miRs are transcribed by RNA polymerase II, generating a primary miR (pri-miR) transcript containing one or more hairpin structures, comprising a stem and terminal loop. Following transcription, pri-miRs are 5′ capped and adenylated. The majority are polycistronic and generate several functional mature miRs. In the nucleus, the pri-miR is cleaved into one or more ∼70 nt precursor miRs (pre-miRs), which have a hairpin structure; this is performed by the Microprocessor complex, which contains the RNase III enzyme, Drosha, and the double-stranded RNA-binding domain protein DiGeorge Critical Region 8 (DGCR8)/Partner of Drosha (Pasha). Purified Drosha can cleave pri-miRs in vitro (Gregory et al. 2004), but requires cofactor proteins for increased efficiency and specificity; therefore the Drosha Microprocessor is very large multi-protein complex (>650 kDa in human cells (Gregory et al. 2004)) containing at least 20 different polypeptides, thought to promote fidelity, specificity and/or Drosha cleavage activity and to act as scaffolding proteins. Some general cofactors modulate biogenesis of the majority of miRs, e.g. DGCR8, whilst others promote or inhibit biogenesis of a defined miR subgroup, e.g. KH-type splicing regulatory protein 1 (KHSRP1), heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1), and the DEAD box RNA helicases, p68 (DDX5) and p72 (DDX17) (Fukuda et al. 2007). The primary function of DGCR8 is to bind to the junction between single-stranded and double-stranded regions of the pri-miR stem via its two RNA-binding domains, leading Drosha to cleave ∼11 bp from the junction. DGCR8 ensures correct orientation of Drosha on the miR hairpin (Landthaler et al. 2004). Only about 25–100 nucleotides surrounding both ends of the stem–loop are required for Drosha cleavage and pri-miR processing (Zeng et al. 2002, Lee et al. 2003).
Drosha cleavage results in a product with a 2 nt 3′ overhang, which is vital both for recognition by Exportin 5, which facilitates Ran-GTP-dependent export of the pre-miR to the cytoplasm, and for Dicer-mediated pre-miR cleavage, because processing occurs at a defined distance from the free RNA end produced by Drosha. In the cytoplasm, pre-miRs are further processed by another RNase III enzyme, Dicer (enhanced by the accessory dsRBD protein TAR RNA-binding protein (TRBP)/PACT), which cleaves the stem loop to produce a ∼22 nt miR duplex. The two strands subsequently separate and one strand associates with Argonaute 2 (AGO2), a protein component of the RNA-induced silencing complex (RISC). Unwinding of the duplex to generate the guide strand and the passenger strand is achieved by a combination of helicases associated with the RISC complex (exactly which remains to be determined, although helicases associated with RISC formation and/or activity include p68, p72, RNA helicase A, TNRC6B and Gemin3/4 (Meister et al. 2005, Robb & Rana 2007, Salzman et al. 2007)). The mature miR guides RISC to complementary sequences within the 3′-UTR of target mRNAs, resulting in translational repression and/or transcript degradation. For a subset of intronic miRs (mirtrons), following transcription, pre-miR hairpins are released from host introns by splicing and debranching to bypass Drosha-mediated cleavage (Okamura et al. 2007, Ruby et al. 2007). These pre-miRs re-enter the conventional processing pathway in the cytoplasm, where they are further processed by Dicer. An overview of miR biogenesis is shown in Fig. 1.
Regulators of microRNA biogenesis. Schematic illustration of proteins that modulate miR maturation through multiple mechanisms. miRs regulated by a particular factor are displayed beside that protein. miR, microRNA; O-methyl, O-methylation; AI, adenosine to inosine; ADAR, deaminase acting on RNA; pri-miR, primary microRNA; pre-miR, precursor microRNA; ERα, estrogen receptor alpha; hnRNPA1, heterogeneous nuclear ribonucleoprotein A1; KHSRP1, KH-type splicing regulatory protein 1; ARS2, arsenite-resistance protein 2; AR, androgen receptor; BRCA1, breast cancer type 1 susceptibility protein; TDP43, TAR DNA-binding protein 43; DGCR8, DiGeorge critical region 8; TRIM32, tripartite motif-containing protein 32; NHL2, NDR1/HIN1-like protein 2; XRN2, 5′-3′ exoribonuclease 2; GLD2, defective in germ line development 2; AGO2, Argonaute 2; TUT4, terminal uridylyltransferase 4; TRBP, HIV 1 TAR RNA-binding protein; BCDIN3D, BCDIN3 domain-containing protein; MCPIP1, monocyte chemotactic protein-induced protein 1.
Citation: Endocrine-Related Cancer 21, 5; 10.1530/ERC-14-0208
Currently, little is known about the factors determining which strand of the miR duplex is incorporated into RISC. One important parameter may be thermodynamic asymmetry of the miR duplex, because the strand with the less stably base-paired 5′ end is more frequently incorporated into RISC (Kawamata et al. 2009, Pratt & MacRae 2009, Yang et al. 2011). The other strand (passenger strand or miR* strand) was thought to be degraded (Yang et al. 2011). However, recent deep-sequencing experiments in Drosophila have revealed that frequently miR*s are not degraded but associated with AGO1 or AGO2, hence both strands of the miR duplex are functional (Okamura et al. 2008, Mah et al. 2010, Yang et al. 2011), and 3′-UTR-repressive activities have been assigned to several human miR*s (Kim et al. 2011, Yang et al. 2011). Humans express four different AGO proteins demonstrating similar preferences for miR duplex structures, in that central mismatches promote RISC association (Murphy et al. 2008). However, it is likely that many other factors beyond the scope of this review could impinge upon strand selection.
Regulation of miR maturation
miR biogenesis is emerging as a stringently controlled and remarkably complex pathway; coordinated regulation likely serves to prevent mis-expression of miRs, both spatially and temporally. miR levels are thought to be particularly important in development and tumourigenesis. For example, in early development many pri-miRs are expressed but not efficiently converted into their mature forms, while reduced processing has been shown to contribute to widespread downregulation of many miRs in human cancers. It is widely accepted that miRs can function as oncogenes and tumour suppressors (Shenouda & Alahari 2009). Global miR downregulation has been identified as a general characteristic of a number of human cancers (Kumar et al. 2007, Ozen et al. 2008), suggesting abrogation of miR biogenesis may induce or maintain tumourigenesis. Indeed, downregulation of Drosha and Dicer and mutations in genes encoding TRBP and Exportin 5 have been reported in several cancers (Karube et al. 2005, Merritt et al. 2008, Melo et al. 2009), although overexpression of Dicer is found in precursor lung adenocarcinoma lesions and correlates with poor survival in colorectal cancer (Chiosea et al. 2007, Faber et al. 2011). Furthermore, disregulation of miRs as a consequence of activated oncogenic signalling pathways may represent a major driver of tumour development, because oncogenesis-related proteins including p53, SMAD, estrogen receptor alpha (ERα) and androgen receptor (AR) appear to modulate biogenesis of miR. Despite the relative infancy of this research area, it is clear that disregulation of miR biogenesis plays a vital role in a multitude of human pathologies including hormone-dependent malignancies. Important regulators of this process are summarised below and shown in Fig. 1.
miR-specific regulation by Microprocessor
The Microprocessor complex contains many proteins and there is evidence that certain of these may be important for processing distinct subsets of miRs, suggesting these proteins can act as specificity factors; for instance, depletion of Microprocessor cofactors p68 or p72 in knockout mice results in reduced levels of a subset of mature miRs, whilst others remain unchanged (Fukuda et al. 2007). In addition, the tumour-suppressor protein p53 and TGFβ/BMP4-stimulated SMAD are known to enhance the processing of distinct subsets of miRs through interaction with p68 and Drosha to increase pri-miR cleavage (shown in Fig. 1; Davis et al. 2008, Suzuki et al. 2009). One miR whose biogenesis is enhanced by SMAD is miR-21, which targets several tumour-suppressor genes including PTEN, therefore may act as an oncomiR (Kumarswamy et al. 2011). Also BRCA1, which is vital for maintenance of genomic stability, increased the levels of mature cancer-associated miRs including let7a-1, miR-16-1 and miR-34a via a mechanism involving direct association of BRCA1 with Drosha and pri-miRs through recognition of branched secondary structures (Kawai & Amano 2012). Processing of the BRCA1-regulated miR subset is likely to be lost in BRCA1-mutant cells, perhaps a mechanism by which BRCA1 mutation enhances tumourigenesis.
Self-regulation of the Microprocessor
Drosha and DGCR8 can mutually regulate each other's function post-transcriptionally: DGCR8 interacts with the central domain of Drosha to stabilise Drosha protein (Yeom et al. 2006), while the Drosha–DGCR8 complex cleaves hairpin structures in the 5′-UTR and coding region of the DGCR8 transcript, leading to the degradation of DGCR8 mRNA, generating a negative feedback loop to regulate DGCR8 levels if Microprocessor activity is sufficient (Han et al. 2009). These data imply collaborative and dynamic mutual regulation of pathways that regulate both mRNA stability and miR maturation.
Regulation of miR maturation by RNA processing modulators
mRNA splicing regulators have been shown to preferentially induce maturation and accumulation of one or more miRs from a polycistronic pri-miR transcript (Michlewski et al. 2008). hnRNPA1 is a multi-functional RNA-binding protein involved in many aspects of RNA processing, including alternative splicing. Its direct association with its consensus binding site, UAGGGA/U, in pri-miR stem–loops can either increase Drosha processing by inducing a stem–loop conformational shift, in the case of pri-miR-18a (Guil & Caceres 2007, Newman & Hammond 2010), or repress maturation by inhibiting Drosha association, as is the case for let7a (Michlewski & Caceres 2010). TAR DNA-binding protein 43 shares significant sequence homology with hnRNPs and promotes miR biogenesis as a component of both the Microprocessor, where it binds pri-miRs and Drosha, and the cytoplasmic Dicer complex, where it associates with the terminal loop regions of a subset of pre-miRs to enhance their maturation (Kawahara & Mieda-Sato 2012).
The RNA-binding protein KHSRP1 (also known as FBP2) also regulates miR biogenesis as a component of both the Microprocessor and Dicer complexes (Trabucchi et al. 2009). KHSRP1 (previously shown to interact with single-stranded, AU-rich mRNAs to mediate mRNA decay (Gherzi et al. 2004)) promotes maturation of a miR subset through association of its KH3 domain with a specific 5′ GGG triplet within terminal loops of target pre-miRs, such as pre-let7a-1 and pre-miR-155 (Ruggiero et al. 2009, Trabucchi et al. 2009). Furthermore, KHSRP is itself a target of AR signalling hence could mediate androgen regulation of this miR subset (Massie et al. 2007; C E Fletcher, D A Dart and C L Bevan, unpublished observations).
TRBP is a key component of the Dicer-containing complex (Melo et al. 2009). Truncating TRBP mutations have been identified in colorectal and endometrial carcinoma cell lines with microsatellite instability, leading to decreased TRBP protein, Dicer destabilisation and defective pre-miR processing. Reintroduction of TRBP into TRBP-deficient colon cancer cells restored processing of known tumour-suppressive miRs such as miR-26a (Sander et al. 2008) and miR-125a (Scott et al. 2007), and interestingly, repressed the cells' ability to form tumours in nude mice (Melo et al. 2009), suggesting that TRBP may function as a tumour suppressor by maintaining Dicer-mediated biogenesis of tumour-suppressive miRs.
Other factors regulating miR maturation
Enzymes that catalyse RNA/DNA modifications can strongly influence miR biogenesis. For instance, BCDIN3D, an RNA-methyltransferase, O-methylates 5′ monophosphates of target pre-miRs, resulting in loss of negative charge and consequently of Dicer binding and miR maturation (Xhemalce et al. 2012). Microprocessor-mediated pri-miR cleavage is regulated by RNA-editing adenosine deaminase acting on RNA (ADARs). ADAR conversion of adenosine (A) to inosine (I) inhibits effective pri-miR processing by Drosha and can alter sequence and structural properties, because base pairing of inosine is similar to that of guanosine (O'Connell & Keegan 2006, Yang et al. 2006). This editing can target the pri-miR for degradation, produce modified mature miRs that regulate a different set of targets, modify extent of target regulation or alter pre-miR processing by DGCR8 (O'Connell & Keegan 2006, Yang et al. 2006).
In addition, an immune regulator ribonuclease, monocyte chemoattractant protein 1-induced protein 1 (MCPIP1), suppresses biogenesis of a subset of miRs through pre-miR terminal loop cleavage, antagonising Dicer activity (Suzuki et al. 2011). Commensurate with a physiological role for this mode of regulation, an inverse correlation was identified between Dicer and MCPIP1 expression in lung cancer, and high MCPIP1 expression was associated with poor prognosis in lung adenocarcinoma patients (Karube et al. 2005, Shedden et al. 2008).
Together, these observations highlight the potential for signalling cascades to regulate the biogenesis of individual miRs or miR subsets, and suggest that interaction of Microprocessor cofactors with accessory proteins such as p53, hnRNPA1, KHSRP1 or SMAD can alter the specificity and/or activity of Drosha processing. It may be speculated that this is mediated through conformational changes in the Microprocessor and/or pri-miR, or through displacement of repressive proteins bound to RNA.
Hormonal regulation of miR biogenesis
There are clear indications in the literature to support direct and indirect hormonal regulation of miR biogenesis, with hormone signalling altering miR biogenesis at all stages of maturation, from transcription to control of mature miR stability. Such regulatory mechanisms provide numerous opportunities to impart subtle but stringent control over gene expression in order to maintain appropriate levels of key protein activity during normal cellular homeostasis. Aberrations in these subtle mechanisms of hormone-modulated miR biogenesis may promote disease pathogenesis, particularly in hormone-related cancers.
Estrogen modulation of miR biogenesis
Estrogens have been described as key regulators of miR biogenesis in breast cancer cells, endometrial stromal cells and myometrial smooth muscle cells; these effects are achieved through liganded ERα directly regulating miR transcription, ERα modulation of miR maturation, or via direct or indirect autoregulatory feedback loops, both positive and negative (Castellano et al. 2009, Klinge 2009).
High throughput profiling identifies estrogen-regulated miRs
As for protein-coding genes, hormone stimulation alters transcription of numerous miR-coding genes, and ERα has been demonstrated to directly associate with response elements in the promoters of such to initiate gene expression (Bhat-Nakshatri et al. 2009). miRNAome-wide profiling arrays, such as microarrays and qPCR arrays, have helped to define a signature of estrogen-regulated miRs. The first examples of such arrays are limited in their applicability to ERα signalling in humans, being performed in zebrafish, rat breast tissue and mouse spleen (Kovalchuk et al. 2007, Cohen et al. 2008, Dai et al. 2008), and to date only around 15 studies have examined estrogen regulation of miRs in human cells (Iorio et al. 2005, Bhat-Nakshatri et al. 2009, Castellano et al. 2009, Klinge 2009, Maillot et al. 2009, Wickramasinghe et al. 2009, Cicatiello et al. 2010, Cochrane et al. 2010, Di Leva et al. 2010, 2013, Hah et al. 2011, Ferraro et al. 2012, Masuda et al. 2012, Zhao et al. 2013). Despite the majority of these being performed in MCF7 breast cancer cells, inconsistencies are evident, presumably attributable to differences in ligand, treatment duration, array platform, statistical processing and threshold setting as well as lab-specific cell line differences. As such, very few estrogen-regulated miRs have been consistently identified and validated to date (Fig. 2). In contrast, considerable efforts have been focussed on identification of miRs that show differential expression in breast cancer, resulting in upwards of 50 publications. Such studies are vital in the identification of novel diagnostic/prognostic biomarkers and therapeutic targets to improve clinical management and treatment of a disease that is a very considerable cause of morbidity and mortality. However, the relationship between miRs that are disregulated in breast cancer and those that are estrogen-regulated is complex and the degree of overlap is currently unclear.
Mutual regulation by steroid hormone receptors and microRNAs (miRs). A schematic representation of the mechanisms by which steroid hormone receptors (SHRs) and miRs mutually regulate each other's activity through feedback loops, permitting a myriad of complex and subtle responses to stimuli. (1) ERα represses miR-206, miR-221 and miR-222, which directly target ERα, thereby reducing the levels of ERα inhibitory miRs. (2) ERα represses miR-26a, miR-26b, miR-181a, miR-181b, miR-23a and miR-23b, which target the ERα effector and growth-stimulatory protein, PR to inhibit cell growth and proliferation. (3) A negative-feedback loop whereby ERα upregulates c-MYC, a transcription factor required for the expression of the miR-17∼92 cluster, whose component miRs directly targets ERα. This estrogen-upregulated cluster also targets the ERα coactivator AIB1. (4) miR-18a, miR-18b, miR-193b and miR-302c represses ERα protein translation through direct association with its 3′-UTR. (5) ERα has been demonstrated to directly upregulate miR-20a, miR-19a/b, miR-92a-1, miR-21, let7a, let7g, miR-203, miR-200a, miR-30b and miR-182, amongst others. (6) ERα represses Drosha cleavage of a subset of pri-miRs through association with p68/p72 to promote Drosha dissociation from the Microprocessor. (7) miR-140-3p and miR-22 indirectly repress ERα transcriptional activity by targeting the ERα coactivators, RIP140 and SRC1 respectively. (8) miR-27a targets ZBTB10, an inhibitor of Sp transcription factors that function to enhance ERα transcriptional activity. By repressing an inhibitor of ERα, miR-27a indirectly increases ERα activity. (9) miR-27b targets CYP1B1, an enzyme responsible for estrogen synthesis, thereby preventing ERα activation. (10) PR inhibits miR-633, miR-29c, miR-29*, miR-193b and miR-29, which targets the PR effector protein, ATP1B1. (11) PR upregulates miR-625*, miR-21, miR-142-5p, miR-146b-5p and miR-513a-5p, which targets PR in a negative-feedback loop. (12) AR transcriptionally upregulates miR-141, miR-17-5p, miR-22, miR-27a miR-29a, miR-29b, miR-32, miR-101, miR-125b, miR-148a, miR-594 and others. (13) miR-331-3p indirectly reduces AR activity by targeting ERBB2, which can activate transcription of AR target genes in the absence of androgens. (14) AR indirectly maintains its own activity in positive feedback loop whereby AR induces transcription of miR-21, which targets the 3′-UTR of PTEN (known to inhibit AR nuclear translocation and promotes its degradation). Androgen-upregulated miR-21 also targets the tumour suppressor, TGFBR2, to stimulate growth. (15) AR directly represses transcription of miR-221 and miR-126*. (16) miR-205, miR-185, miR-34a/b, miR-135b, miR-297, miR-299-3p, miR-371-3p, miR-421, miR-449a/b, miR-634, miR-654-5p and miR-9 directly repress AR through binding to its 3′-UTR. (17) AR indirectly increases its own activity by enhancing transcription of miR-27a, which targets the AR corepressor, PHB. AR also inhibits PHB transcription. (18) Let7 indirectly inhibits AR by targeting the AR transcription factor, c-Myc. CYP1B1, cytochrome P450 family 1B1; E2, estradiol; Sp, specificity protein; ZBTB10, zinc finger and BTB domain containing 10; RIP-140, receptor interacting protein 140; SRC1, steroid receptor coactivator 1; ERα, estrogen receptor alpha; AIB1, amplified in breast cancer 1; PR, progesterone receptor; ATP1B1, ATPase Na+/K+ transporting beta 1; AR, androgen receptor; ERBB2, v-erb-b2 erythroblastic leukaemia viral oncogene homologue 2; TGFBR2, transforming growth factor beta receptor 2; PTEN, phosphatase and tensin homologue; PHB, prohibitin.
Citation: Endocrine-Related Cancer 21, 5; 10.1530/ERC-14-0208
A summary of the most commonly identified estrogen-regulated miRs is shown in Table 1 and Figure 2. In contrast to the identification of numerous estrogen-regulated miRs in MCF7 cells, no miRs demonstrated estrogen regulation following 24 h treatment of T47D breast cancer cells with 10 nM estradiol (E2; Katchy et al. 2012). In one of the most widely cited studies, Bhat-Nakshatri et al. (2009) demonstrated estrogen upregulation of 21 miRs (several of which target ERα coactivators in vitro) and downregulation of seven in MCF7 cells through microarray analysis. Several of the identified miRs were located within intragenic regions of estrogen-regulated genes. Of the identified estrogen-upregulated miRs, let7f, miR-98 and miR-21 were shown to reduce levels of c-MYC and E2F2 proteins (required for optimal ERα activity and secondary estrogen response respectively), providing evidence for a negative feedback loop to repress ERα activity. Another microarray analysis identified 17 miRs as significantly being upregulated by estrogen treatment in MCF7 cells. Interestingly, however, only miR-7 (which targets EGFR in ERα-positive but not ERα-negative breast cancer cells) was subsequently found to be suppressed by treatment with an anti-estrogen, Faslodex (Masuda et al. 2012). Reasons for the lack of converse effects of anti-estrogens on estrogen-regulated miRs in this case remain to be identified, but this approach is one means of validating ER-dependent regulation.
MicroRNAs regulated by estrogen receptor alpha in at least two studies
As a more direct, and arguably more physiologically relevant method for identification of transcriptionally estrogen-regulated miRs, Hah et al. (2011) used global nuclear run-on and sequencing (GRO-seq) to map the position and orientation of all RNA polymerases across the MCF7 genome following treatment with 100 nM E2 for 10, 60 or 160 min. 119 miR-containing transcripts that were regulated by E2 at one or more time points were identified. Interestingly, MCF7 cells were enriched for 2700 predicted targets of these E2-regulated miRs compared with other cells, indicating the presence of a complex regulatory network of miRs and their mRNA targets (Hah et al. 2011). Other studies have combined microarray expression profiling of miRs with global ERα-binding site mapping through ChIP-seq to provide mechanistic insights: Bhat-Nakshatri et al. examined functional ERα-binding sites within 20 kb regions 5′ of miR genes following estrogen stimulation that resulted in increased pri-miR transcription. One estrogen-responsive ERE was found in the miR-21 promoter, with five and four EREs identified in the miR-23b∼27b∼24-1 and miR-23a∼27a∼24-2 promoters respectively, although different cluster members showed differing extents of regulation (Bhat-Nakshatri et al. 2008, 2009). Ferraro et al. (2012) used this approach to compare responses in estrogen-treated MCF7 and ZR-75.1 cells. They demonstrated that miR-135a-2, miR-181c, miR-26b and the clustered miR-23a, miR-27a and miR-24-2 were estrogen-regulated in both cell lines and located within 10 kb of an ERα-binding site, while estrogen-regulated miR-25, miR-26a, miR-424, miR-618, miR-760 and miR-942 were located in the intragenic region of estrogen-regulated genes. Furthermore, Di Leva et al. (2013) demonstrated that binding of ERα to the promoter of the miR-191/-425-DALRD3 unit leads to increased levels of miR-191 and -425 and loss of DALRD3 levels, with subsequent loss of miR-191 and miR-425 targets that include the tumour suppressor EGR1. Both these studies provide evidence for mutual estrogen-regulation of protein-coding genes and their intragenic miRs.
In a further study, Maillot et al. (2009) identified 23 miRs as repressed by estrogen in MCF7 breast cancer cells, with at least some of the inhibitory effects attributable to ERα repression of miR cluster transcription. Most interesting was the observation that a number of estrogen-downregulated miRs reduce DNA synthesis and proliferation of breast cancer cells upon their transient transfection (Maillot et al. 2009). Analysis of predicted mRNA targets of these estrogen-downregulated, growth inhibitory miRs demonstrated enrichment for genes with roles in cell growth and proliferation. Of particular interest was the finding that six of seven miRs predicted to target the progesterone receptor (PR) 3′-UTR (PR is an ER-upregulated target gene and a vital mediator of ER signalling) are downregulated by estrogen (Maillot et al. 2009). These observations provide further evidence for a positive feedback loop, whereby ERα downregulates miRs that repress mediators of estrogen signalling in order to amplify its own activity. Hence, estrogen signalling has two ways to increase activity of a vital mediator of its own activity (PR) – direct and indirect feedback loops (Fig. 2).
In addition, hormone treatment can induce recruitment of other transcription factors to miR promoter regions. For example, estrogen-inducible c-MYC is recruited to the promoters of the paralogous miR-17-92 and miR-106a-363 clusters and increases mature levels of encoded miRs (Castellano et al. 2009). This generates a feedback loop whereby ERα self-regulates through increasing levels of ERα pathway-targeting miRs, since miRs 18a, 19b and 20b directly target and downregulate ERα, whilst miRs 20a, 20b, 17-5p and 106a inhibit translation of the ERα coactivator AIB1 (Castellano et al. 2009). These constitute a complex and highly sensitive network of interactions for subtle manipulation of ERα signalling in breast cancer cells, and highlight that negative as well as positive feedback loops contribute to this.
It is clear that greater research effort is required to isolate the effects of estrogen on pri-miR transcription from the post-transcriptional effects of liganded ERα to the latter stages of miR biogenesis. As an example, let7g demonstrated direct regulation by ERα, but not at the levels of transcription, because estrogen treatment reduced levels of mature let7g, but not pri-let7g in a MAPK-dependent manner in MCF7 cells (Qian et al. 2011). Although the precise mechanism of such regulation remains unclear, reduction in mature let7g may be attributable to p68/p72-mediated ERα inhibition of Drosha (discussed below).
ERα modulation of Microprocessor activity
A seminal study by Yamagata et al. (2009) provided the first in vivo evidence for ERα regulation of miR expression. It was also the first to demonstrate ERα modulation of Microprocessor function. Expression of a set of mature miRs was upregulated in female mouse embryos deficient in ERα, whilst estrogen treatment of ovariectomised female mice inhibited the production of a subset of mature miRs in the uterus, a target organ of estrogen signalling (Yamagata et al. 2009). Upon further examination, it was found that the levels of the relevant pri-miRs were unchanged following estrogen treatment, but levels of the cognate mature miRs and pre-miRs were downregulated, suggesting that estrogen-mediated modulation of miR levels occurs at the level of pri-miR to pre-miR processing. The authors hypothesised that estrogen signalling via ERα may modulate Drosha activity through physical association of ERα with the RNA helicases p68 (DDX5) and/or p72 (DDX17), which are important components of the Drosha Microprocessor complex and vital to miR maturation. Indeed, it was found that mouse ERα was associated with mouse Drosha, but only in the presence of E2 and that this requires p72 and p68, as confirmed by immunoprecipitation assays (Yamagata et al. 2009).
In another study, RNA-ChIP experiments showed dissociation of pri-miRs from Drosha and p72 in the presence of estrogen. Interestingly, the N-terminal of ERα associates with the C-terminal of Drosha, which contains the RNaseIII- and dsRNA-binding motifs (Macias et al. 2009). This could explain the observations of Castellano et al., who found that the extent of estrogen regulation of pri-miRs was greater than that of their encoded mature miRs (Castellano et al. 2009). It is possible that these miRs are also subject to ERα-mediated inhibition of pri-miR processing through repression of Drosha activity in an E2- and p68/p72-dependent manner, yielding the observed biphasic regulatory pattern.
These data indicate a mechanism whereby ERα inhibits Drosha-mediated processing of a subset of miRs through association with Drosha in a p68/p72- and E2-dependent manner, resulting in the dissociation of the Microprocessor complex from the pri-miR. However, it is likely that further auxillary factors dictate the specificity of estrogen-mediated miR regulation and that liganded ERα acts at other steps of miR biogenesis, for example, pre-miR processing or mature miR stability.
Regulation of miR biogenesis pathway proteins by estrogen
Further evidence for estrogen regulation of miR maturation comes from the findings of Nothnick et al. (2010), who demonstrated that estrogen and progesterone significantly upregulate the levels of Exportin 5 mRNA in mouse uterine tissue, an effect abrogated by the addition of the anti-estrogen ICI 182 780. They found that the levels of miR-451 were concurrently increased, but did not demonstrate that these effects are specifically attributable to the effects of these steroid hormones on Exportin 5 expression, failing to exclude the possibility of estrogen-enhanced transcription of pri-miR-451 independent of Exportin 5. The authors also showed that P4, but not E2, increased Dicer expression (Nothnick et al. 2010). However, evidence for estrogen regulation of Dicer in human cells comes from Lin et al. (2007) and Bhat-Nakshatri et al. (2009) who demonstrated that Dicer protein levels are upregulated by estrogen in MCF7 breast cancer cells, with ERα binding to the enhancer region of the Dicer promoter.
In a further study, Cheng et al. evaluated the expression of miR biogenesis-associated genes in ER-positive and ER-negative cell lines. They found that ER-positive cell lines had increased levels of TRBP and Dicer transcripts and decreased levels of AGO1 and AGO2 compared with ER-negative cell lines (Cheng et al. 2009). This is supported by Adams et al. (2009) who showed reduced AGO2 expression in ER-positive breast cancer cell lines. It has also been demonstrated that Dicer is regulated by MAPK, and that phosphorylation of TRBP by this kinase pathway increases the expression of the growth-promoting miRs 17 and 20 and inhibits maturation of the let7 tumour suppressor in HeLa cells (Paroo et al. 2009). As it is well established that ERα activates MAPK in several cell lines (Levin 2011), together these data highlight a putative indirect mechanism for estrogen regulation of Dicer activity through activation of hormone-stimulated MAPK pathway. Furthermore, the Dicer-targeting tumour-suppressive let7, which reduces Dicer protein levels in several cell lines (Tokumaru et al. 2008), is downregulated by estrogen treatment (Qian et al. 2011), constituting a mechanism for indirect upregulation of Dicer activity by estrogen modulation of miRs. Further, Dicer mRNA is targeted by estrogen-regulated miR-29a, miR-125, let7 and miR-21, and TRBP by let7 (Tokumaru et al. 2008, Cochrane et al. 2010, Shu et al. 2011). From these studies (summarised in Table 2), it is clear that a pattern of regulation is increasingly observed whereby feedback loops involving steroid signalling permit the maintenance of hormone-regulated miR biogenesis at appropriate levels.
Steroid hormone regulation of microRNA biogenesis pathway components
| miR biogenesis protein | Regulating SHR | Direction of regulation | References |
|---|---|---|---|
| Exportin-5 | ERα, PR | ↑ | Nothnick et al. (2010) |
| Dicer | ERα, PR | ↑ | Lin et al. (2007), Bhat-Nakshatri et al. (2009) and Cheng et al. (2009) |
| TRBP | ERα | ↑ | Cheng et al. (2009) |
| p68 | AR | ↑ | Clark et al. (2008) |
| AGO2 | ERα | ↓ | Adams et al. (2007) and Cheng et al. (2009) |
A potential novel role for estrogen in modulating miR:3′-UTR interaction
Finally, a novel mechanism has been proposed whereby, complementary to its role in the modulation of miR levels, ERα may modulate miR activity by altering the availability of miR-binding sites within target 3′-UTRs. One report has demonstrated that ERα signalling can induce alternative polyadenylation and 3′-UTR shortening of CDC6, which led to loss of miR-binding sites and increased CDC6 protein levels (Akman et al. 2012). This represents a possible additional mechanism for estrogen regulation of gene expression through modulating the potential for repressive miR–3′-UTR interactions; however, additional independent studies are required to corroborate these observations, and a defined mechanism for this phenomenon remains to be elucidated. If confirmed, such a mechanism would allow ERα not only to regulate its target genes by altering miR-mediated repression, but also to regulate cellular availability of some miRs in a transcription-independent manner.
Targeting of ER pathway by miRs: feedback circuits involving hormone-regulated miRs modulate ER expression and activity
miRs targeting ER
miRs known to target ER itself are summarised in Table 3. One of the first to be identified was miR-206. Adams et al. (2007) demonstrated that miR-206 targets ERα (ESR1) mRNA and decreases ERα protein levels to reduce the growth of breast cancer cells. The tumour-suppressive let7 family of miRs has also been demonstrated to repress ERα expression in MCF7 cells following their transient overexpression (Zhao et al. 2011). Leivonen et al. employed protein lysate microarray technology to identify ERα-targeting miRs following transfection of 319 pre-miRs into MCF7 and BT-474 cells. Twenty-one miRs were confirmed to downregulate Erα, as validated by western blotting and qPCR analyses, and miRs 18a, 18b, 193b, 206 and 302c were confirmed to target ERα 3′-UTR in reporter assays (Leivonen et al. 2009). Notably, clustering analysis of mRNA targets of these miRs demonstrated enrichment for estrogen-induced genes.
MicroRNAs targeting estrogen receptor alpha
| miR | ERα activity | References |
|---|---|---|
| 206 | ↓ | Iorio et al. (2005), Adams et al. (2007), Kondo et al. (2008) and Leivonen et al. (2009) |
| 221/222 | ↓ | Zhao et al. (2008) and Leivonen et al. (2009) |
| 22 | ↓ | Leivonen et al. (2009) and Pandey et al. (2009) |
| 17∼92 cluster | ↓ | Castellano et al. (2009) and Leivonen et al. (2009) |
| Let7 | ↓ | Zhao et al. (2010) |
| 27a | ↑ | Mertens-Talcott et al. (2007) and Li et al. (2010) |
| 219 | ↓ | Leivonen et al. (2009) |
| 9 | ↓ | Leivonen et al. (2009) |
| 517a/c | ↓ | Leivonen et al. (2009) |
| 373 | ↓ | Leivonen et al. (2009) |
| 181a/c/d | ↓ | Leivonen et al. (2009) |
| 302c | ↓ | Leivonen et al. (2009) |
| 301 | ↓ | Leivonen et al. (2009) |
| 372 | ↓ | Leivonen et al. (2009) |
| 193b | ↓ | Leivonen et al. (2009) |
| 130a/b | ↓ | Leivonen et al. (2009) |
| 93 | ↓ | Leivonen et al. (2009) |
| 520d | ↓ | Leivonen et al. (2009) |
Other miRs identified as directly targeting ERα are miR-221 and -222 (Leivonen et al. 2009). Indeed, knockdown of either miR in ERα-negative MDA-MB-468 cells restores ERα expression and initiates estrogen responsiveness and anti-estrogen sensitivity (Zhao et al. 2008). Again, the authors propose that miR-221/-222 also act by additionally targeting downstream components of the ERα signalling pathway. Interestingly, although miR-221/-222 decrease ERα protein levels, proliferation of the transfected cells was accelerated through decreased expression of tumour-suppressor proteins, thereby highlighting the importance of examining all miR targets and utilising pathway analysis tools to predict the effects of miR manipulation (Di Leva et al. 2010, Rao et al. 2011). In the context of miR-221/-222 targeting of ERα, it is interesting that several studies have demonstrated dramatic downregulation of these miRs upon estrogen treatment (Cochrane et al. 2010), and that ERα directly inhibits transcription of these miRs by recruiting the transcriptional corepressors NCoR and SMRT to the defined miR-221/-222 promoter (Di Leva et al. 2010). Together, these data provide further strong evidence for autoregulation of ERα activity through direct downregulation of miRs that directly target its 3′-UTR. Similarly, the ERα-targeting miRs 18a and 206 are estrogen-regulated, suggestive of a potential feedback loop to modulate ERα activity. Interestingly, whilst miR-18a is upregulated through estrogen stimulation of MCF7 cells (Castellano et al. 2009), miR-206, which also targets the ERα coactivators SRC1 and AIB1, is downregulated via inhibition of Drosha-mediated maturation (Adams et al. 2007, 2009). The opposite direction of regulation of these ERα-targeting miRs suggests a complex regulatory mechanism that permits subtle yet stringent control of ERα activity, allowing cells to adjust their transcriptional output to varied challenges.
Modulation of ERα activity regulators and synthesis of proteins by miRs
ERα-mediated gene transcription requires a plethora of accessory factors and comodulators in order to modify chromatin structure such that it is compatible with RNA polymerase binding, and there is increasing evidence that miRs can modulate ERα activity through targeting coregulators. For example, miR-17-5p reduced translation of AIB1 and reduced ER activity on transfected and endogenous target genes by up to 50% in CHO-K1 cells (Hossain et al. 2006). In addition, miR-206 was demonstrated to bind to the 3′-UTRs of both SRC1 and AIB1 and the transcription factor GATA3 in MCF7 cells (Adams et al. 2009). This led to a significant reduction in ERE luciferase reporter activity and inhibition of ERα target gene transcripts, for example, PR (Adams et al. 2009). Furthermore, levels of the ERα target gene and effector protein, cyclin D1, were reduced upon infection of MCF7 cells with a retroviral vector containing the miR-17∼20 cluster, although effects of cluster expression on ERα activity were not examined (Yu et al. 2008). Additional studies have confirmed miR-22 repression of SRC1 through 3′-UTR binding in Huh7 hepatocarcinoma cells, whilst miR-140-3p directly inhibits RIP-140 translation through association with 3′-UTR seed region (Takata et al. 2011).
Corepressors of ERα also display miR-mediated regulation. miRs 10a/b and 284 have been shown to directly suppress SMRT expression through 3′-UTR binding and translational inhibition in neuroblastoma and HeLa cells respectively (Foley et al. 2011, Wu et al. 2011), although the impact of these miRs on ERα activity was not assessed. Steroid-regulated miR-27a provides a further example of a miR targeting downstream regulators of steroid signalling: miR-27a represses both the steroid receptor corepressor Prohibitin and also ZBTB10, a zinc finger gene which functions to repress Sp transcription factors that have pro-proliferative and angiogenic effects in breast cancer cells (Mertens-Talcott et al. 2007, He et al. 2008, Fletcher et al. 2012). Thus miR-27a is an indirect oncomiR by repressing transcription factor repressors. miR27a also indirectly activates ERα, as ERα expression and E2 response are dependent on Sp1, which is derepressed by miR-27a (Li et al. 2010), Interestingly, and in keeping with these findings, a single-nucleotide polymorphism in the terminal loop of miR-27a that is predicted to inhibit pre-miR-27a processing is associated with reduced risk of familial premenopausal breast cancer (Yang et al. 2010). However, the effect of this mutation on the ability of miR-27a to indirectly increase ERα activity through repression of ZBTB10 remains to be evaluated.
It has also been postulated that miRs may modulate estrogen synthesis and release from cells, and there is some limited evidence to support this: transfection of a miR precursor library into primary human granulosa cells (the principal site of pre-menopausal estrogen production) identified 51 miRs that suppress the release of estrogen, including miR-15a, miR-24, miR-125b, and members of the let7 family. Interestingly, no miRs were found to increase estrogen release in this study (Sirotkin et al. 2009). Local production of estrogen in breast and other tissues is mediated by aromatase (encoded by CYP1B1) and 17β-hydroxysteroid dehydrogenase. It has been shown that the CYP1B1 3′-UTR contains putative miR-27b-binding sites, and that exogenous miR-27b reduced CYP1B1 levels and enzymatic activity, thus presumably decreasing production of local estrogen (Tsuchiya et al. 2006, Nakajima & Yokoi 2011). Of note, miR-27b is widely reported as estrogen regulated, although studies are conflicting in the directionality of such regulation (Mattie et al. 2006, Bhat-Nakshatri et al. 2009, Klinge 2009, Maillot et al. 2009, Cochrane et al. 2010). It is then possible that this represents a feedback mechanism whereby an ERα-regulated miR can feedback to estrogen synthesis enzymes to indirectly regulate ER activity. Another miR shown to modulate estrogen synthesis is miR-133b, which increases estrogen synthesis in the ovaries by targeting Foxl2, a negative regulator of the steroidogenic enzymes, STAR protein and CYP19A1. miR-133b was shown to associate with FOXL2 3′-UTR, reducing FOXL2 protein levels, and its binding to STAR and CYP19A1 promoter sequences in granulosa cells, thereby increasing estrogen synthesis and secretion (Dai et al. 2013). It is likely that further miRs will emerge as regulating hormone synthesis enzymes, and given the clinical importance of intratumoural steroid synthesis for both breast and prostate cancer it will be of major interest to determine whether these are disregulated in both tumour types and whether they are regulated by steroid signalling themselves.
Androgen regulation of miR biogenesis
Androgens have been demonstrated to alter miR biogenesis in a range of cell lines and in vivo models. ChIP-seq data have provided evidence for the association of AR with the promoters of a number of miR genes, and AR, like ER, demonstrates autoregulation through miRs (Fig. 2). A number of studies have described the roles of miRs both as mediators or effectors of AR action and regulators of AR activity, discussed below.
Androgen regulation of miR expression
Androgen-responsive tissues express distinct sets of miRs, as demonstrated by castration of Sprague–Dawley rats, which resulted in a dramatic reduction in a large subset of miRs in the prostate compared with intact animals, whilst treatment of castrated animals with androgen increased miR expression in a tissue-specific manner (Narayanan et al. 2010). The miR profile was not altered in the liver upon androgen treatment, but the levator ani muscle (which has an anabolic response to androgens) demonstrated an upregulated miR profile that differed significantly from that of the prostate, preferentially targeting components of the Raf/MEK/ERK signalling pathway (Narayanan et al. 2010). Interestingly, Dicer siRNA completely inhibited dihydrotestosterone (DHT) induction of the well-characterised androgen-responsive gene PSA in LNCaP cells, and Dicer−/− mice developed androgen-insensitivity syndrome, indicating that androgen-regulated miRs are vital mediators of AR action in vivo (Narayanan et al. 2010). It was also hypothesised that reduced AR function upon loss of androgen-stimulated miR synthesis may be a result of androgen-regulated miR targeting of AR corepressors. Consistent with this, 75% of predicted NCoR or SMRT-targeting miRs were upregulated in the prostate by DHT, and Dicer siRNA treatment increased transcription of both NCoR and SMRT (Narayanan et al. 2010). Taken together, these innovative data identify androgen-responsive tissue-specific miR profiles, and confirm androgen-upregulated miRs as vital mediators and enhancers of AR signalling through corepressor targeting (discussed also below).
Takayama et al. were amongst the first to describe the association of AR with miR promoters in prostate cancer cells through integration of 5′ cap analysis of gene expression (CAGE), to identify androgen-regulated transcription start sites, with ChIP on array (ChIP-chip) to identify AR-binding sites throughout the human genome. In addition to protein-coding genes, miR-222, miR-221, miR-21, miR-125b, miR-218-1, miR-218-2, let7c and miR-100 were found to be located adjacent to AR-binding sites (Takayama et al. 2011). In corroboration, Ribas et al. (2009) demonstrated androgen-induced AR binding to the miR-21 cluster promoter, which resulted in increased levels of oncomiR-21. This increased androgen-dependent growth and induced androgen-independent growth of LNCaP and LAPC-4 cells. It should be noted, however, that the CAGE-with-ChIP-chip approach favoured by Takayama et al. may not identify all miRs regulated by AR binding to ARE promoters, because the authors only assessed one time point of androgen treatment, thus possibly overlooking earlier or later androgen binding events.
In one of the first miR microarray studies to examine androgen regulation of miRs, Waltering et al. (2011) showed that 17 miRs were significantly modulated by treating VCaP cells with DHT, at least half of which were confirmed as androgen-regulated in subsequent studies (given in Table 4). Later, Mo et al. (2013) combined time course microarrays with bioinformatics to identify androgen-regulated miRs and their targets in androgen-stimulated LNCaP cells, focussing on those that showed an early and enduring response to androgen, a subgroup they term ‘primary targets’. Amongst 137 miRs primary targets, 22 miRs had existing well-documented roles in prostate cancer (Bonci et al. 2008, Ozen et al. 2008, Rokhlin et al. 2008, Tong et al. 2008). Of these, miR-101, miR-145, miR-34a, miR-182, miR-375, miR-181a, miR-92b and miR-125 were upregulated by androgen, whilst miR-16, miR-126*, miR-23b, miR-100, miR-222, miR-133a, miR-499 and miR-340 were downregulated by androgen, corroborating previous demonstrations of androgen regulation by miRs and altered expression in prostate cancer (Ambs et al. 2008, Bonci et al. 2008, Tong et al. 2008). Two additional primary targets, miR-19a and miR-27a, showed significant enrichment for roles in key cellular processes upon pathway analysis. Concordant with this, ChIP analysis confirmed AR binding to putative AREs within the promoter regions of these miRs (Mo et al. 2013). These data are consistent with our earlier findings, in which we also demonstrated AR binding to the miR-27a promoter after androgen stimulation (Fletcher et al. 2012). Similar extents of miR-27a upregulation were observed in the two studies, thereby supporting AR association with the miR-27a promoter as a bona fide physiological event. Pathway analysis of predicted target sets of these miRs showed enrichment for proteins with previously described roles in prostate cancer. For example ABCA1, targeted by miR-27a, inhibits proliferation of prostate cancer cells (Fukuchi et al. 2004, Yang et al. 2012) whilst RAB13, a GTPase that positively regulates epithelial cell tight junction formation (Kanda et al. 2007), is a target of miR-19a.
MicroRNAs identified as androgen receptor-regulated in at least two studies
Shi et al. (2007) have demonstrated that miR-125b is significantly transcriptionally upregulated by androgen treatment in LNCaP cells, and its transient transfection increases prostate cancer cell growth whilst reducing Bax1 expression and 3′-UTR activity. Given that a subset of estrogen-upregulated miRs with putative oncogenic function was demonstrated to target this tumour suppressor in estrogen-treated endometrial carcinoma cells to positively reinforce ERα activity (Zhang et al. 2012), it is very interesting that the same phenomenon and mechanism of action were observed for an androgen-regulated miR in prostate cancer (Shi et al. 2007).
Positive feedback regulation of AR through androgen-upregulated miRs
Several studies have now identified mechanisms whereby AR exploits miRs to maintain high levels of its own expression and activity. Mishra et al. (2013) demonstrated that AR inhibition reduced the levels of miR-21 in AR-positive prostate cancer cells, whilst overexpression of AR increases it. Interestingly, miR-21 mimic increased AR mRNA levels, restored AR protein to AR-negative PC3 cells and enhanced PSA promoter reporter activity. This therefore represents a positive feedback loop whereby androgen-regulated miR-21 increases AR levels to further increase miR-21, which targets several tumour suppressors. The mechanism whereby miR-21 increases AR activity was not addressed, although it was hypothesised this may involve miR-21 targeting PTEN, which inhibits AR nuclear translocation and promotes degradation of AR in several cancers (Lin et al. 2004, Mulholland et al. 2006). Our own studies demonstrated a similar positive feedback loop: androgen-upregulated miR-27a increases AR activity by targeting the AR corepressor and tumour suppressor, PHB, in prostate cancer cells (Fletcher et al. 2012). These data combined suggest that androgen upregulation of miRs that target tumour suppressors and corepressors of AR transcriptional activity to positively reinforce AR signalling (Fig. 2) may be commonplace in androgen-responsive tissues. Such mechanisms could potentially play a vital role in increasing AR activity in the absence of circulating androgens in the case of advanced, castrate-resistant prostate cancer (CRPC), particularly if such miRs are overexpressed in cancerous tissue, as has been demonstrated for miR-21 (Krichevsky & Gabriely 2009, Dart et al. 2011).
Androgen regulation of protein components of the miR biogenesis pathway
Regulation of one or more components of the miR processing complexes is a possible mechanism for androgen regulation of miR biogenesis. For example, Dicer and DGCR8 are upregulated by DHT treatment of LNCaP cells (Mo et al. 2013), consistent with the observed increase in Dicer protein levels in prostate cancer (Chiosea et al. 2006). In addition, AR was shown to accelerate miR biogenesis through increasing Drosha cleavage of pri-miR species, although the mechanism of androgen-increased Drosha activity remains to be described (Fletcher et al. 2012).
Given the general similarities between AR and ERα signalling and since ERα modulates miR biogenesis via Drosha, it may be logical that AR also regulates Drosha activity. Indeed, we demonstrated that androgens increase Drosha reporter activity towards pri-miRs (Fletcher et al. 2012). However this effect is opposite to the inhibition of Drosha cleavage by ERα. It may be that both ERα and AR can act to regulate Drosha and Dicer activities, exerting distinct and opposing effects in the nucleus and the cytoplasm to control hormone-stimulated miR biogenesis in different tissues and developmental stages. It may be hypothesised that these regulatory mechanisms are perturbed in cancer, because altered expression of Drosha or Dicer has been reported in a number of cancers (Chiosea et al. 2006, 2007, Merritt et al. 2008, Faber et al. 2011, Passon et al. 2012).
There is also circumstantial evidence for links between androgen signalling and miR processing pathways. The Microprocessor component p68 was identified as a novel AR-interacting protein in a yeast two-hybrid screen (Clark et al. 2008) and demonstrated to be recruited to AREs upon ligand treatment and to act as an AR coactivator in LNCaP cells. Interestingly, mutation of Y593 (a putative phosphorylation site) in p68 abolished AR coactivation, indicating that post-translational modifications of p68 may be vital for this role (Clark et al. 2008), although effects of this mutation on RNA helicase activity were not investigated. The above study also found increased P68 immunohistochemical staining in a cohort of prostate cancer patients compared with benign hyperplasia patients (Clark et al. 2008). These data potentially support a link between miR processing and AR-mediated transcription, and also suggest that P68-dependent miR processing could be altered in prostate cancer.
miR regulation of androgen signalling
miRs targeting the AR
It is well established that miRs can directly regulate AR signalling through targeting the AR 3′-UTR (Table 5). In one study, Östling et al. (2011) coupled protein lysate microarrays in LNCaP, LAPC-4, CWR-R1, 22Rv1 and MDA-PCa-2b cells with miR microarray expression analysis in 13 prostate cell lines to quantify miR-mediated changes in AR protein content in prostate cancer. Seventy-one miRs that altered AR protein levels in all five cell lines were selected for validation, with 52 decreasing and 19 increasing AR protein levels, and 21 AR-targeting miRs were shown to reduce both AR protein levels and androgen-stimulated growth upon transient transfection of the miR mimics into LNCaP and 22Rv-1 cells, and MDA-PCa-2b cells respectively. Thirteen of these were then further validated as significantly reducing AR 3′-UTR activity: miR-135b, miR-185, miR-297, miR-299-3p, miR-34a, miR-34c, miR-371-3p, miR-421, miR-449a, miR-449b, miR-634, miR-654-5p and miR-9 (Östling et al. 2011) (Fig. 2). Targeting of AR by miR-185 is consistent with the findings of Qu et al. (2013), who demonstrated that miR-185 significantly reduces the levels of AR protein, but not mRNA levels, in LNCaP cells, with concomitant decreases in LNCaP cell proliferation and xenograft tumourigenicity upon miR-185 overexpression. Furthermore, targeting of AR by miR-34a is in keeping with data from Kashat et al. (2013): in an observational study showing that miR-34a reduced protein and mRNA levels of AR in C4-2B and CWR22Rv-1 cells.
MicroRNAs targeting the androgen receptor
| miR | Direction of AR activity | References |
|---|---|---|
| 205 | ↓ | Hagman et al. (2013) |
| 185 | ↓/↓ | Östling et al. (2011) and Qu et al. (2013) |
| 34 | ↓/↓ | Östling et al. (2011) and Kashat et al. (2013) |
| 135b | ↓ | Östling et al. (2011) |
| 297 | ↓ | Östling et al. (2011) |
| 299-3p | ↓ | Östling et al. (2011) |
| 371-3p | ↓ | Östling et al. (2011) |
| 421 | ↓ | Östling et al. (2011) |
| 449a/b | ↓ | Östling et al. (2011) |
| 634 | ↓ | Östling et al. (2011) |
| 654-5p | ↓ | Östling et al. (2011) |
| 9 | ↓ | Östling et al. (2011) |
| 124 | ↓ | Shi et al. (2011) |
| 488* | ↓ | Sikand et al. (2011a,b) |
| 141 | ↓ (indirect via targeting Shp) | Xiao et al. (2012) |
| Let7c | ↓ (indirect via targeting Myc) | Nadiminty et al. (2012) |
| 17-5p | ↓ (indirect by targeting PCAF) | Gong et al. (2012) |
| 27a | ↑ (indirect via targeting AR corepressor PHB) | Fletcher et al. (2012) |
| 331-3p | ↓ (indirect via targeting ERBB2) | Epis et al. (2009) |
Other miRs are also reported to target AR directly. Hagman et al. (2013) demonstrated that ectopic miR-205 overexpression significantly decreased AR protein levels and activity of an AR 3′-UTR reporter in VCaP cells, although this was not recapitulated in LNCaP cells. However, as levels of miR-205 were low in all seven but one in prostate cancer cell lines, the physiological relevance of miR-205 targeting of AR remains to be ascertained. Similarly, AR levels were decreased both at the RNA and protein level following ectopic expression of miR-488* in androgen-dependent and -independent prostate cancer cells, and both AR transcriptional activity and AR 3′-UTR reporter activity were also reduced (Sikand et al. 2011a).
A potential novel role and mechanism for regulation of AR involving miRs involves shortening of the 3′-UTR. Recent studies have reported shortening of 3′-UTRs in cancer cells, leading to enhanced oncogene activation. Indeed, mRNAs with shorter 3′-UTRs produced tenfold more protein, in part due to loss of miR-mediated repression (Mayr & Bartel 2009). In this context, it is interesting that the AR 3′-UTR length in several PCa cell lines was found to be ∼6680 bp; far greater than the sequence length of 436 bp specified by the AR Ref Seq file (NM_0000444) and used by target prediction algorithms. This dramatically increases the potential for direct miR regulation of the AR transcript, although it should be noted that there is evidence to suggest that the extent of regulation of a transcript by a miR is generally greater closer to the 5′ end of the 3′-UTR (Grimson et al. 2007, Bartel 2009). It has been proposed that the AR 3′-UTR can function as dynamic entity, and active 3′-UTR shortening in response to environmental stimuli may represent a further mechanism of PCa progression (Sikand et al. 2011b), permitting AR to evade regulation by tumour-suppressive miRs, such as long AR 3′-UTR-binding miR-34a and miR-34c (Östling et al. 2011), which are proposed potent mediators of p53 tumour suppression. However, very limited information is currently available on miR targeting of the relatively poorly characterised longer AR 3′-UTR isoforms. In fact, there remains no definitive evidence for reduced miR binding and consequently higher AR protein levels following AR 3′-UTR shortening. Further studies are required to compare AR 3′-UTR length in malignant vs benign prostate tissue, and in hormone-naïve PCa vs CRPC, to assess its potential role in cancer progression.
Inhibition of AR activity by tumour-suppressive miRs
LET7 expression is frequently decreased, or the gene is deleted, in human cancers (Calin et al. 2004, Jiang et al. 2005, Ozen et al. 2008). Its targets include the oncogenes RAS and MYC (Johnson et al. 2005, Kumar et al. 2007) and it has thus been proposed as a tumour-suppressor miR. This tumour suppressor capacity may be achieved in prostate cancer through decreased AR activity, since let7c was demonstrated to target c-MYC, which is required for transcription of AR (Nadiminty et al. 2012). It was found that let7c reduced AR activity and decreased growth of C4-22B cells, attributable to let7c association with c-MYC 3′-UTR leading to a loss of c-MYC-mediated AR transcription (Nadiminty et al. 2012).
Epis et al. (2009) investigated the regulation of AR activity by miR-331-3p. It was found that miR-331-3p reduces AR activity in LNCaP cells by targeting the oncogene ERBB2, because AR 3′-UTR lacks miR-331-3p-binding sites and ERBB2 can activate transcription of AR-target genes in the absence of androgen (Craft et al. 1999, Mellinghoff et al. 2004). ERBB2 was demonstrated to be a direct target of miR-331-3p through 3′-UTR reporter assays and protein expression studies in multiple prostate cancer cell lines. The above theory of indirect miR targeting of AR via ERBB2 is supported by a decrease in PSA promoter activity upon miR-331-3p ectopic expression (Epis et al. 2009).
Such tumour-suppressive, AR-inhibitory miRs could be exploited as potential therapeutics for prostate cancer, either alone or in combination with anti-androgens.
Nuclear RISC components as coactivators of ERα and AR
Further evidence for cooperation and functional interaction between the miR biogenesis cascade and steroid signalling pathways has been reported from the recent study by Redfern et al. which demonstrated that components of the RISC complex, such as Dicer, PACT and TRBP, bind to an RNA transcript that functions as a coactivator of nuclear receptor (NR) activity, called steroid receptor RNA activator (SRA) (Lanz et al. 2002, Hube et al. 2006, Redfern et al. 2013). PACT, TRBP and Dicer increased SRA-enhanced ERα and AR reporter activities, and were recruited to AR-target gene promoters, whilst siRNA-mediated depletion of PACT and TRBP reduced the expression of AR and ERα-target genes (Redfern et al. 2013). Interestingly, RISC protein-bound pre-miRs were identified in the nucleus as well as in the cytoplasm, suggesting the existence of a nuclear RISC complex in which SRA functions as a scaffold for PACT, TRBP and Dicer (Redfern et al. 2013), raising the possibility of coupling pre-miR processing and NR coregulation of transcription within a single-nuclear complex. It is also possible that SRA could sequester RISC components away from their pre-miR substrates to regulate RISC activity. It is feasible that other proteins may fulfil similar roles to SRA in coupling NR transcriptional activity with Drosha Microprocessor-mediated pri-miR cleavage, since both FUS, a Microprocessor component that stimulates pri-miR processing (Morlando et al. 2012), and p68, known to promote cleavage of a subset of pri-miRs (Fukuda et al. 2007, Salzman et al. 2007, Kawai & Amano 2012, Samaan et al. 2013), are known AR cofactors (Clark et al. 2008, Brooke et al. 2011, Haile et al. 2011).
Conclusions and summary
The multiple steps of the miR biogenesis cascade provide a plethora of opportunities for hormone regulation of miR synthesis, adding further complexity to allow the most stringent regulation and fine-tuning of a pathway with vital roles in almost all physiological processes, and the dysregulation of which has undesirable consequences for the pathogenesis of human cancers. It is clear that distinct subsets of miRs are regulated by estrogen and androgen signalling in responsive tissues, but that these hormones share common mechanisms for regulating the activity of miR biogenesis pathway components, altering transcription of miRs and exploiting target miRs to reinforce their own activity. From in vivo studies, we evidenced that miRs are vital effectors of AR and ERα signalling pathways, and conversely, it is increasingly apparent that multiple miRs can directly target the 3′-UTRs of ERα and AR to regulate hormone signalling. Furthermore, NRs preferentially regulate miRs that target their own effectors and cofactors, resulting in feedback that may amplify or restrict their signalling. In addition, several fascinating emerging theories highlighted herein may yet have sizeable implications for the bi-directional regulation of miR biogenesis and hormone signalling. We should also remember that miRs themselves have been postulated to function similarly to hormones, influencing gene expression, microenvironment and disease pathogenesis at distant sites, and potentially functioning as readily assayable biomarkers. The interplay between hormone signalling and miR biogenesis, and further characterisation of its components, provides significant opportunity for increased understanding of steroid-related cancer progression and the potential for exploitation of such interactions for designing novel cancer therapeutics.
Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the review.
Funding
This review did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.
Acknowledgements
The authors are grateful for support from the Rosetrees Trust, the Wellcome Trust, Prostate Cancer UK and Cancer Research UK during the writing of this review.
References
Adams BD, Furneaux H & White BA 2007 The micro-ribonucleic acid (miRNA) miR-206 targets the human estrogen receptor-α (ERα) and represses ERα messenger RNA and protein expression in breast cancer cell lines. Molecular Endocrinology 21 1132–1147. (doi:10.1210/me.2007-0022).
Adams BD, Cowee DM & White BA 2009 The role of miR-206 in the epidermal growth factor (EGF) induced repression of estrogen receptor-α (ERα) signaling and a luminal phenotype in MCF-7 breast cancer cells. Molecular Endocrinology 23 1215–1230. (doi:10.1210/me.2009-0062).
Akman BH, Can T & Erson-Bensan AE 2012 Estrogen-induced upregulation and 3′-UTR shortening of CDC6. Nucleic Acids Research 40 10679–10688. (doi:10.1093/nar/gks855).
Ambs S, Prueitt RL, Yi M, Hudson RS, Howe TM, Petrocca F, Wallace TA, Liu CG, Volinia S & Calin GA et al. 2008 Genomic profiling of microRNA and messenger RNA reveals deregulated microRNA expression in prostate cancer. Cancer Research 68 6162–6170. (doi:10.1158/0008-5472.CAN-08-0144).
Bartel DP 2009 MicroRNAs: target recognition and regulatory functions. Cell 136 215–233. (doi:10.1016/j.cell.2009.01.002).
Bhat-Nakshatri P, Wang G, Appaiah H, Luktuke N, Carroll JS, Geistlinger TR, Brown M, Badve S, Liu Y & Nakshatri H 2008 AKT alters genome-wide estrogen receptor α binding and impacts estrogen signaling in breast cancer. Molecular and Cellular Biology 28 7487–7503. (doi:10.1128/MCB.00799-08).
Bhat-Nakshatri P, Wang G, Collins NR, Thomson MJ, Geistlinger TR, Carroll JS, Brown M, Hammond S, Srour EF & Liu Y et al. 2009 Estradiol-regulated microRNAs control estradiol response in breast cancer cells. Nucleic Acids Research 37 4850–4861. (doi:10.1093/nar/gkp500).
Blenkiron C, Goldstein L, Thorne N, Spiteri I, Chin S-F, Dunning M, Barbosa-Morais N, Teschendorff A, Green A & Ellis I et al. 2007 MicroRNA expression profiling of human breast cancer identifies new markers of tumor subtype. Genome Biology 8 R214. (doi:10.1186/gb-2007-8-10-r214).
Bonci D, Coppola V, Musumeci M, Addario A, Giuffrida R, Memeo L, D'Urso L, Pagliuca A, Biffoni M & Labbaye C et al. 2008 The miR-15a-miR-16-1 cluster controls prostate cancer by targeting multiple oncogenic activities. Nature Medicine 14 1271–1277. (doi:10.1038/nm.1880).
Brooke GN, Culley RL, Dart DA, Mann DJ, Gaughan L, McCracken SR, Robson CN, Spencer-Dene B, Gamble SC & Powell SM et al. 2011 FUS/TLS is a novel mediator of androgen-dependent cell-cycle progression and prostate cancer growth. Cancer Research 71 914–924. (doi:10.1158/0008-5472.CAN-10-0874).
Calin GA, Sevignani C, Dumitru CD, Hyslop T, Noch E, Yendamuri S, Shimizu M, Rattan S, Bullrich F & Negrini M et al. 2004 Human microRNA genes are frequently located at fragile sites and genomic regions involved in cancers. PNAS 101 2999–3004. (doi:10.1073/pnas.0307323101).
Cao P, Deng Z, Wan M, Huang W, Cramer S, Xu J, Lei M & Sui G 2010 MicroRNA-101 negatively regulates Ezh2 and its expression is modulated by androgen receptor and HIF-1alpha/HIF-1beta. Molecular Cancer 9 108.
Castellano L, Giamas G, Jacob J, Coombes RC, Lucchesi W, Thiruchelvam P, Barton G, Jiao LR, Wait R & Waxman J et al. 2009 The estrogen receptor-α-induced microRNA signature regulates itself and its transcriptional response. PNAS 106 15732–15737. (doi:10.1073/pnas.0906947106).
Cheng C, Fu X, Alves P & Gerstein M 2009 mRNA expression profiles show differential regulatory effects of microRNAs between estrogen receptor-positive and estrogen receptor-negative breast cancer. Genome Biology 10 R90. (doi:10.1186/gb-2009-10-9-r90).
Chiosea S, Jelezcova E, Chandran U, Acquafondata M, McHale T, Sobol RW & Dhir R 2006 Up-regulation of dicer, a component of the microRNA machinery, in prostate adenocarcinoma. American Journal of Pathology 169 1812–1820. (doi:10.2353/ajpath.2006.060480).
Chiosea S, Jelezcova E, Chandran U, Luo J, Mantha G, Sobol RW & Dacic S 2007 Overexpression of dicer in precursor lesions of lung adenocarcinoma. Cancer Research 67 2345–2350. (doi:10.1158/0008-5472.CAN-06-3533).
Cicatiello L, Mutarelli M, Grober OM, Paris O, Ferraro L, Ravo M, Tarallo R, Luo S, Schroth GP & Seifert M et al. 2010 Estrogen receptor α controls a gene network in luminal-like breast cancer cells comprising multiple transcription factors and microRNAs. American Journal of Pathology 176 2113–2130. (doi:10.2353/ajpath.2010.090837).
Clark EL, Coulson A, Dalgliesh C, Rajan P, Nicol SM, Fleming S, Heer R, Gaughan L, Leung HY & Elliott DJ et al. 2008 The RNA helicase p68 is a novel androgen receptor coactivator involved in splicing and is overexpressed in prostate cancer. Cancer Research 68 7938–7946. (doi:10.1158/0008-5472.CAN-08-0932).
Cochrane D, Cittelly D, Howe E, Spoelstra N, McKinsey E, LaPara K, Elias A, Yee D & Richer J 2010 MicroRNAs link estrogen receptor α status and Dicer levels in breast cancer. Hormones & Cancer 1 306–319. (doi:10.1007/s12672-010-0043-5).
Cohen A, Shmoish M, Levi L, Cheruti U, Levavi-Sivan B & Lubzens E 2008 Alterations in micro-ribonucleic acid expression profiles reveal a novel pathway for estrogen regulation. Endocrinology 149 1687–1696. (doi:10.1210/en.2007-0969).
Craft N, Shostak Y, Carey M & Sawyers CL 1999 A mechanism for hormone-independent prostate cancer through modulation of androgen receptor signaling by the HER-2/neu tyrosine kinase. Nature Medicine 5 280–285. (doi:10.1038/6495).
Dai R, Phillips RA, Zhang Y, Khan D, Crasta O & Ahmed SA 2008 Suppression of LPS-induced interferon-γ and nitric oxide in splenic lymphocytes by select estrogen-regulated microRNAs: a novel mechanism of immune modulation. Blood 112 4591–4597. (doi:10.1182/blood-2008-04-152488).
Dai A, Sun H, Fang T, Zhang Q, Wu S, Jiang Y, Ding L, Yan G & Hu Y 2013 MicroRNA-133b stimulates ovarian estradiol synthesis by targeting Foxl2. FEBS Letters 587 2474–2482. (doi:10.1016/j.febslet.2013.06.023).
Dart DA, Brooke GN, Sita-Lumsden A, Waxman J & Bevan CL 2011 Reducing prohibitin increases histone acetylation, and promotes androgen independence in prostate tumours by increasing androgen receptor activation by adrenal androgens. Oncogene 31 4588–4598. (doi:10.1038/onc.2011.591).
Davis BN, Hilyard AC, Lagna G & Hata A 2008 SMAD proteins control DROSHA-mediated microRNA maturation. Nature 454 56–61. (doi:10.1038/nature07086).
Di Leva G, Gasparini P, Piovan C, Ngankeu A, Garofalo M, Taccioli C, Iorio MV, Li M, Volinia S & Alder H et al. 2010 MicroRNA cluster 221–222 and estrogen receptor α interactions in breast cancer. Journal of the National Cancer Institute 102 706–721. (doi:10.1093/jnci/djq102).
Di Leva G, Piovan C, Gasparini P, Ngankeu A, Taccioli C, Briskin D, Cheung DG, Bolon B, Anderlucci L & Alder H et al. 2013 Estrogen mediated-activation of miR-191/425 cluster modulates tumorigenicity of breast cancer cells depending on estrogen receptor status. PLoS Genetics 9 e1003311. (doi:10.1371/journal.pgen.1003311).
Epis MR, Giles KM, Barker A, Kendrick TS & Leedman PJ 2009 miR-331-3p regulates ERBB-2 expression and androgen receptor signaling in prostate cancer. Journal of Biological Chemistry 284 24696–24704. (doi:10.1074/jbc.M109.030098).
Faber C, Horst D, Hlubek F & Kirchner T 2011 Overexpression of Dicer predicts poor survival in colorectal cancer. European Journal of Cancer 47 1414–1419. (doi:10.1016/j.ejca.2011.01.006).
Ferraro L, Ravo M, Nassa G, Tarallo R, Filippo M, Giurato G, Cirillo F, Stellato C, Silvestro S & Cantarella C et al. 2012 Effects of oestrogen on microRNA expression in hormone-responsive breast cancer cells. Hormones & Cancer 3 65–78. (doi:10.1007/s12672-012-0102-1).
Fletcher CE, Dart DA, Sita-Lumsden A, Cheng H, Rennie PS & Bevan CL 2012 Androgen-regulated processing of the oncomir MiR-27a, which targets Prohibitin in prostate cancer. Human Molecular Genetics 21 3112–3127. (doi:10.1093/hmg/dds139).
Foley N, Bray I, Watters K, Das S, Bryan K, Bernas T, Prehn J & Stallings R 2011 MicroRNAs 10a and 10b are potent inducers of neuroblastoma cell differentiation through targeting of nuclear receptor corepressor 2. Cell Death and Differentiation 18 1089–1098. (doi:10.1038/cdd.2010.172).
Fukuchi J, Hiipakka RA, Kokontis JM, Hsu S, Ko AL, Fitzgerald ML & Liao S 2004 Androgenic suppression of ATP-binding cassette transporter A1 expression in LNCaP human prostate cancer cells. Cancer Research 64 7682–7685. (doi:10.1158/0008-5472.CAN-04-2647).
Fukuda T, Yamagata K, Fujiyama S, Matsumoto T, Koshida I, Yoshimura K, Mihara M, Naitou M, Endoh H & Nakamura T et al. 2007 DEAD-box RNA helicase subunits of the Drosha complex are required for processing of rRNA and a subset of microRNAs. Nature Cell Biology 9 604–611. (doi:10.1038/ncb1577).
Gherzi R, Lee KY, Briata P, Wegmüller D, Moroni C, Karin M & Chen CY 2004 A KH domain RNA binding protein, KSRP, promotes ARE-directed mRNA turnover by recruiting the degradation machinery. Molecular Cell 14 571–583. (doi:10.1016/j.molcel.2004.05.002).
Gong A-Y, Eischeid A, Xiao J, Zhao J, Chen D, Wang Z-Y, Young C & Chen X-M 2012 miR-17-5p targets the p300/CBP-associated factor and modulates androgen receptor transcriptional activity in cultured prostate cancer cells. BMC Cancer 12 492.
Gregory RI, Yan KP, Amuthan G, Chendrimada T, Doratotaj B, Cooch N & Shiekhattar R 2004 The Microprocessor complex mediates the genesis of microRNAs. Nature 432 235–240. (doi:10.1038/nature03120).
Grimson A, Farh KK, Johnston WK, Garrett-Engele P, Lim LP & Bartel DP 2007 MicroRNA targeting specificity in mammals: determinants beyond seed pairing. Molecular Cell 27 91–105. (doi:10.1016/j.molcel.2007.06.017).
Guil S & Caceres JF 2007 The multifunctional RNA-binding protein hnRNP A1 is required for processing of miR-18a. Nature Structural & Molecular Biology 14 591–596. (doi:10.1038/nsmb1250).
Hagman Z, Haflidadottir BS, Ceder JA, Larne O, Bjartell A, Lilja H, Edsjo A & Ceder Y 2013 miR-205 negatively regulates the androgen receptor and is associated with adverse outcome of prostate cancer patients. British Journal of Cancer 108 1668–1676. (doi:10.1038/bjc.2013.131).
Hah N, Danko CG, Core L, Waterfall JJ, Siepel A, Lis JT & Kraus WL 2011 A rapid, extensive, and transient transcriptional response to estrogen signaling in breast cancer cells. Cell 145 622–634. (doi:10.1016/j.cell.2011.03.042).
Haile S, Lal A, Myung JK & Sadar MD 2011 FUS/TLS is a co-activator of androgen receptor in prostate cancer cells. PLoS ONE 6 e24197. (doi:10.1371/journal.pone.0024197).
Han J, Pedersen JS, Kwon SC, Belair CD, Kim YK, Yeom KH, Yang WY, Haussler D, Blelloch R & Kim VN 2009 Posttranscriptional crossregulation between drosha and DGCR8. Cell 136 75–84. (doi:10.1016/j.cell.2008.10.053).
He B, Feng Q, Mukherjee A, Lonard DM, DeMayo FJ, Katzenellenbogen BS, Lydon JP & O'Malley BW 2008 A repressive role for prohibitin in estrogen signaling. Molecular Endocrinology 22 344–360. (doi:10.1210/me.2007-0400).
Hossain A, Kuo MT & Saunders GF 2006 Mir-17-5p regulates breast cancer cell proliferation by inhibiting translation of AIB1 mRNA. Molecular and Cellular Biology 26 8191–8201. (doi:10.1128/MCB.00242-06).
Hu Z, Shen W-J, Cortez Y, Tang X, Liu L-F, Kraemer FB & Azhar S 2013 Hormonal regulation of microRNA expression in steroid producing cells of the ovary, testis and adrenal gland. PLoS ONE 8 e78040.
Hube F, Guo J, Chooniedass-Kothari S, Cooper C, Hamedani M, Dibrov A, Blanchard A, Wang X, Deng G & Myal Y et al. 2006 Alternative splicing of the first intron of the steroid receptor RNA activator (SRA) participates in the generation of coding and noncoding RNA isoforms in breast cancer cell lines. DNA and Cell Biology 25 418–428. (doi:10.1089/dna.2006.25.418).
Iorio MV, Ferracin M, Liu CG, Veronese A, Spizzo R, Sabbioni S, Magri E, Pedriali M, Fabbri M & Campiglio M et al. 2005 MicroRNA gene expression deregulation in human breast cancer. Cancer Research 65 7065–7070. (doi:10.1158/0008-5472.CAN-05-1783).
Jalava SE, Urbanucci A, Latonen L, Waltering KK, Sahu B, Janne OA, Seppala J, Lahdesmaki H, Tammela TL & Visakorpi T 2012 Androgen-regulated miR-32 targets BTG2 and is overexpressed in castration-resistant prostate cancer. Oncogene 31 4460–4471. (doi:10.1038/onc.2011.624).
Jiang J, Lee EJ, Gusev Y & Schmittgen TD 2005 Real-time expression profiling of microRNA precursors in human cancer cell lines. Nucleic Acids Research 33 5394–5403. (doi:10.1093/nar/gki863).
Johnson SM, Grosshans H, Shingara J, Byrom M, Jarvis R, Cheng A, Labourier E, Reinert KL, Brown D & Slack FJ 2005 RAS is regulated by the let-7 microRNA family. Cell 120 635–647. (doi:10.1016/j.cell.2005.01.014).
Kanda I, Nishimura N, Nakatsuji H, Yamamura R, Nakanishi H & Sasaki T 2007 Involvement of Rab13 and JRAB/MICAL-L2 in epithelial cell scattering. Oncogene 27 1687–1695. (doi:10.1038/sj.onc.1210812).
Karube Y, Tanaka H, Osada H, Tomida S, Tatematsu Y, Yanagisawa K, Yatabe Y, Takamizawa J, Miyoshi S & Mitsudomi T et al. 2005 Reduced expression of Dicer associated with poor prognosis in lung cancer patients. Cancer Science 96 111–115. (doi:10.1111/j.1349-7006.2005.00015.x).
Kashat M, Azzouz L, Sarkar S, Kong D, Li Y & Sarkar F 2013 Inactivation of AR and Notch-1 signaling by miR-34a attenuates prostate cancer aggressiveness. American Journal of Translational Research 4 432–442.
Katchy A, Edvardsson K, Aydogdu E & Williams C 2012 Estradiol-activated estrogen receptor α does not regulate mature microRNAs in T47D breast cancer cells. Journal of Steroid Biochemistry and Molecular Biology 128 145–153. (doi:10.1016/j.jsbmb.2011.10.008).
Kawahara Y & Mieda-Sato A 2012 TDP-43 promotes microRNA biogenesis as a component of the Drosha and Dicer complexes. PNAS 109 3347–3352. (doi:10.1073/pnas.1112427109).
Kawai S & Amano A 2012 BRCA1 regulates microRNA biogenesis via the DROSHA microprocessor complex. Journal of Cell Biology 197 201–208. (doi:10.1083/jcb.201110008).
Kawamata T, Seitz H & Tomari Y 2009 Structural determinants of miRNAs for RISC loading and slicer-independent unwinding. Nature Structural & Molecular Biology 16 953–960. (doi:10.1038/nsmb.1630).
Kim T-D, Lee SU, Yun S, Sun H-N, Lee SH, Kim JW, Kim HM, Park S-K, Lee CW & Yoon SR et al. 2011 Human microRNA-27a* targets Prf1 and GzmB expression to regulate NK cell cytotoxicity. Blood 118 5476–5486. (doi:10.1182/blood-2011-04-347526).
Klinge C 2009 Estrogen regulation of microRNA expression. Current Genomics 10 169–183. (doi:10.2174/138920209788185289).
Kondo N, Toyama T, Sugiura H, Fujii Y & Yamashita H 2008 miR-206 Expression is down-regulated in estrogen receptor alpha-positive human breast cancer. Cancer research 68 5004–5008. (doi:10.1158/0008-5472.CAN-08-0180).
Kovalchuk O, Tryndyak V, Montgomery B, Boyko A, Kutanzi K, Zemp F, Warbritton A, Latendresse J, Kovalchuk I & Beland F et al. 2007 Estrogen-induced rat breast carcinogenesis is characterized by alterations in DNA methylation, histone modifications and aberrant microRNA expression. Cell Cycle 6 2010–2018. (doi:10.4161/cc.6.16.4549).
Krichevsky AM & Gabriely G 2009 miR-21: a small multi-faceted RNA. Journal of Cellular and Molecular Medicine 13 39–53. (doi:10.1111/j.1582-4934.2008.00556.x).
Kumar MS, Lu J, Mercer KL, Golub TR & Jacks T 2007 Impaired microRNA processing enhances cellular transformation and tumorigenesis. Nature Genetics 39 673–677. (doi:10.1038/ng2003).
Kumarswamy R, Volkmann I & Thum T 2011 Regulation and function of miRNA-21 in health and disease. RNA Biology 8 706–713. (doi:10.4161/rna.8.5.16154).
Lagos-Quintana M, Rauhut R, Lendeckel W & Tuschl T 2001 Identification of novel genes coding for small expressed RNAs. Science 294 853–858. (doi:10.1126/science.1064921).
Landthaler M, Yalcin A & Tuschl T 2004 The human DiGeorge syndrome critical region gene 8 and Its D. melanogaster homolog are required for miRNA biogenesis. Current Biology 14 2162–2167. (doi:10.1016/j.cub.2004.11.001).
Lanz RB, Razani B, Goldberg AD & O'Malley BW 2002 Distinct RNA motifs are important for coactivation of steroid hormone receptors by steroid receptor RNA activator (SRA). PNAS 99 16081–16086. (doi:10.1073/pnas.192571399).
Lau NC, Lim LP, Weinstein EG & Bartel DP 2001 An abundant class of tiny RNAs with probable regulatory roles in Caenorhabditis elegans. Science 294 858–862. (doi:10.1126/science.1065062).
Lee RC & Ambros V 2001 An extensive class of small RNAs in Caenorhabditis elegans. Science 294 862–864. (doi:10.1126/science.1065329).
Lee RC, Feinbaum RL & Ambros V 1993 The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14. Cell 75 843–854. (doi:10.1016/0092-8674(93)90529-Y).
Lee Y, Ahn C, Han J, Choi H, Kim J, Yim J, Lee J, Provost P, Radmark O & Kim S et al. 2003 The nuclear RNase III Drosha initiates microRNA processing. Nature 425 415–419. (doi:10.1038/nature01957).
Leivonen SK, Makela R, Ostling P, Kohonen P, Haapa-Paananen S, Kleivi K, Enerly E, Aakula A, Hellstrom K & Sahlberg N et al. 2009 Protein lysate microarray analysis to identify microRNAs regulating estrogen receptor signaling in breast cancer cell lines. Oncogene 28 3926–3936. (doi:10.1038/onc.2009.241).
Levin ER 2011 Minireview: Extranuclear steroid receptors: roles in modulation of cell functions. Molecular Endocrinology 25 377–384. (doi:10.1210/me.2010-0284).
Li X, Mertens-Talcott SU, Zhang S, Kim K, Ball J & Safe S 2010 MicroRNA-27a indirectly regulates estrogen receptor α expression and hormone responsiveness in MCF-7 breast cancer cells. Endocrinology 151 2462–2473. (doi:10.1210/en.2009-1150).
Lin HK, Hu YC, Lee DK & Chang C 2004 Regulation of androgen receptor signaling by PTEN (phosphatase and tensin homolog deleted on chromosome 10) tumor suppressor through distinct mechanisms in prostate cancer cells. Molecular Endocrinology 18 2409–2423. (doi:10.1210/me.2004-0117).
Lin CY, Vega VB, Thomsen JS, Zhang T, Kong SL, Xie M, Chiu KP, Lipovich L, Barnett DH & Stossi F et al. 2007 Whole-genome cartography of estrogen receptor α binding sites. PLoS Genetics 3 e87. (doi:10.1371/journal.pgen.0030087).
Macias S, Michlewski G & Cáceres JF 2009 Hormonal regulation of microRNA biogenesis. Molecular Cell 36 172–173. (doi:10.1016/j.molcel.2009.10.006).
Mah S, Buske C, Humphries R & Kuchenbauer F 2010 miRNA*: a passenger stranded in RNA-induced silencing complex? Critical Reviews in Eukaryotic Gene Expression 20 141–148. (doi:10.1615/CritRevEukarGeneExpr.v20.i2.40).
Maillot G, Lacroix-Triki M, Pierredon S, Gratadou L, Schmidt S, Bénès V, Roché H, Dalenc F, Auboeuf D & Millevoi S et al. 2009 Widespread estrogen-dependent repression of microRNAs involved in breast tumor cell growth. Cancer Research 69 8332–8340. (doi:10.1158/0008-5472.CAN-09-2206).
Massie CE, Adryan B, Barbosa-Morais NL, Lynch AG, Tran MG, Neal DE & Mills IG 2007 New androgen receptor genomic targets show an interaction with the ETS1 transcription factor. EMBO Reports 8 871–878.
Masuda M, Miki Y, Hata S, Takagi K, Sakurai M, Ono K, Suzuki K, Yang Y, Abe E & Hirakawa H et al. 2012 An induction of microRNA, miR-7 through estrogen treatment in breast carcinoma. Journal of Translational Medicine 10 S2. (doi:10.1186/1479-5876-10-S1-S2).
Mattie M, Benz C, Bowers J, Sensinger K, Wong L, Scott G, Fedele V, Ginzinger D, Getts R & Haqq C 2006 Optimized high-throughput microRNA expression profiling provides novel biomarker assessment of clinical prostate and breast cancer biopsies. Molecular Cancer 5 24. (doi:10.1186/1476-4598-5-24).
Mayr C & Bartel DP 2009 Widespread shortening of 3′UTRs by alternative cleavage and polyadenylation activates oncogenes in cancer cells. Cell 138 673–684. (doi:10.1016/j.cell.2009.06.016).
Meister G, Landthaler M, Peters L, Chen PY, Urlaub H, Lührmann R & Tuschl T 2005 Identification of novel argonaute-associated proteins. Current Biology 15 2149–2155. (doi:10.1016/j.cub.2005.10.048).
Mellinghoff IK, Vivanco I, Kwon A, Tran C, Wongvipat J & Sawyers CL 2004 HER2/neu kinase-dependent modulation of androgen receptor function through effects on DNA binding and stability. Cancer Cell 6 517–527. (doi:10.1016/j.ccr.2004.09.031).
Melo SA, Ropero S, Moutinho C, Aaltonen LA, Yamamoto H, Calin GA, Rossi S, Fernandez AF, Carneiro F & Oliveira C et al. 2009 A TARBP2 mutation in human cancer impairs microRNA processing and DICER1 function. Nature Genetics 41 365–370. (doi:10.1038/ng.317).
Merritt WM, Lin YG, Han LY, Kamat AA, Spannuth WA, Schmandt R, Urbauer D, Pennacchio LA, Cheng JF & Nick AM et al. 2008 Dicer, Drosha, and outcomes in patients with ovarian cancer. New England Journal of Medicine 359 2641–2650. (doi:10.1056/NEJMoa0803785).
Mertens-Talcott SU, Chintharlapalli S, Li X & Safe S 2007 The oncogenic microRNA-27a targets genes that regulate specificity protein transcription factors and the G2-M checkpoint in MDA-MB-231 breast cancer cells. Cancer Research 67 11001–11011. (doi:10.1158/0008-5472.CAN-07-2416).
Michlewski G & Caceres JF 2010 Antagonistic role of hnRNP A1 and KSRP in the regulation of let-7a biogenesis. Nature Structural & Molecular Biology 17 1011–1018. (doi:10.1038/nsmb.1874).
Michlewski G, Guil S, Semple CA & Cáceres JF 2008 Posttranscriptional regulation of miRNAs harboring conserved terminal loops. Molecular Cell 32 383–393. (doi:10.1016/j.molcel.2008.10.013).
Mishra S, Deng JJ, Gowda PS, Rao MK, Lin CL, Chen CL, Huang T & Sun LZ 2013 Androgen receptor and microRNA-21 axis downregulates transforming growth factor beta receptor II (TGFBR2) expression in prostate cancer. Oncogene 33 4097–4106. (doi:10.1038/onc.2013.374).
Mo W, Zhang J, Li X, Meng D, Gao Y, Yang S, Wan X, Zhou C, Guo F & Huang Y et al. 2013 Identification of novel AR-targeted microRNAs mediating androgen signalling through critical pathways to regulate cell viability in prostate cancer. PLoS ONE 8 e56592. (doi:10.1371/journal.pone.0056592).
Morlando M, Dini Modigliani S, Torrelli G, Rosa A, Di Carlo V, Caffarelli E & Bozzoni I 2012 FUS stimulates microRNA biogenesis by facilitating co-transcriptional Drosha recruitment. EMBO Journal 31 4502–4510. (doi:10.1038/emboj.2012.319).
Mulholland DJ, Dedhar S, Wu H & Nelson CC 2006 PTEN and GSK3β: key regulators of progression to androgen-independent prostate cancer. Oncogene 25 329–337. (doi:10.1038/sj.onc.1209020).
Munagala R, Aqil F, Vadhanam MV & Gupta RC 2013 MicroRNA ‘signature’ during estrogen-mediated mammary carcinogenesis and its reversal by ellagic acid intervention. Cancer Letters 339 175–184. (doi:10.1016/j.canlet.2013.06.012).
Murata T, Takayama K, Katayama S, Urano T, Horie-Inoue K, Ikeda K, Takahashi S, Kawazu C, Hasegawa A & Ouchi Y et al. 2010 miR-148a is an androgen-responsive microRNA that promotes LNCaP prostate cell growth by repressing its target CAND1 expression. Prostate Cancer and Prostatic Disease 13 356–361.
Murphy D, Dancis B & Brown J 2008 The evolution of core proteins involved in microRNA biogenesis. BMC Evolutionary Biology 8 92. (doi:10.1186/1471-2148-8-92).
Nadiminty N, Tummala R, Lou W, Zhu Y, Zhang J, Chen X, deVere White RW, Kung HJ, Evans CP & Gao AC 2012 MicroRNA let-7c suppresses androgen receptor expression and activity via regulation of Myc expression in prostate cancer cells. Journal of Biological Chemistry 287 1527–1537. (doi:10.1074/jbc.M111.278705).
Nakajima M & Yokoi T 2011 MicroRNAs from biology to future pharmacotherapy: regulation of cytochrome P450s and nuclear receptors. Pharmacology & Therapeutics 131 330–337. (doi:10.1016/j.pharmthera.2011.04.009).
Narayanan R, Jiang J, Gusev Y, Jones A, Kearbey JD, Miller DD, Schmittgen TD & Dalton JT 2010 MicroRNAs are mediators of androgen action in prostate and muscle. PLoS ONE 5 e13637. (doi:10.1371/journal.pone.0013637).
Newman MA & Hammond SM 2010 Emerging paradigms of regulated microRNA processing. Genes and Development 24 1086–1092. (doi:10.1101/gad.1919710).
Nothnick W, Healy C & Hong X 2010 Steroidal regulation of uterine miRNAs is associated with modulation of the miRNA biogenesis components Exportin-5 and Dicer1. Endocrine 37 265–273. (doi:10.1007/s12020-009-9293-9).
O'Connell MA & Keegan LP 2006 Drosha versus ADAR: wrangling over pri-miRNA. Nature Structural & Molecular Biology 13 3–4. (doi:10.1038/nsmb0106-3).
Okamura K, Hagen JW, Duan H, Tyler DM & Lai EC 2007 The Mirtron pathway generates microRNA-class regulatory RNAs in Drosophila. Cell 130 89–100. (doi:10.1016/j.cell.2007.06.028).
Okamura K, Phillips MD, Tyler DM, Duan H, Chou YT & Lai EC 2008 The regulatory activity of microRNA species has substantial influence on microRNA and 3′ UTR evolution. Nature Structural & Molecular Biology 15 354–363. (doi:10.1038/nsmb.1409).
Östling P, Leivonen SK, Aakula A, Kohonen P, Mäkelä R, Hagman Z, Edsjö A, Kangaspeska S, Edgren H & Nicorici D et al. 2011 Systematic analysis of microRNAs targeting the androgen receptor in prostate cancer cells. Cancer Research 71 1956–1967. (doi:10.1158/0008-5472.CAN-10-2421).
Ozen M, Creighton CJ, Ozdemir M & Ittmann M 2008 Widespread deregulation of microRNA expression in human prostate cancer. Oncogene 27 1788–1793. (doi:10.1038/sj.onc.1210809).
Pandey DP & Picard D 2009 miR-22 inhibits estrogen signaling by directly targeting the estrogen receptor alpha mRNA. Molecular and Cellular Biology 29 3783–3790. (doi:10.1128/MCB.01875-08).
Paroo Z, Ye X, Chen S & Liu Q 2009 Phosphorylation of the human microRNA-generating complex mediates MAPK/Erk signaling. Cell 139 112–122. (doi:10.1016/j.cell.2009.06.044).
Passon N, Gerometta A, Puppin C, Lavarone E, Puglisi F, Tell G, Di Loreto C & Damante G 2012 Expression of Dicer and Drosha in triple-negative breast cancer. Journal of Clinical Pathology 65 320–326. (doi:10.1136/jclinpath-2011-200496).
Pratt AJ & MacRae IJ 2009 The RNA-induced silencing complex: a versatile gene-silencing machine. Journal of Biological Chemistry 284 17897–17901. (doi:10.1074/jbc.R900012200).
Qian P, Zuo Z, Wu Z, Meng X, Li G, Wu Z, Zhang W, Tan S, Pandey V & Yao Y et al. 2011 Pivotal role of reduced let-7g expression in breast cancer invasion and metastasis. Cancer Research 71 6463–6474. (doi:10.1158/0008-5472.CAN-11-1322).
Qu F, Cui X, Hong Y, Wang J, Li Y, Chen L, Liu Y, Gao Y, Xu D & Wang Q 2013 MicroRNA-185 suppresses proliferation, invasion, migration, and tumorigenicity of human prostate cancer cells through targeting androgen receptor. Molecular and Cellular Biochemistry 377 121–130. (doi:10.1007/s11010-013-1576-z).
Rao X, Di Leva G, Li M, Fang F, Devlin C, Hartman-Frey C, Burow ME, Ivan M, Croce CM & Nephew KP 2011 MicroRNA-221/222 confers breast cancer fulvestrant resistance by regulating multiple signaling pathways. Oncogene 30 1082–1097. (doi:10.1038/onc.2010.487).
Redfern AD, Colley SM, Beveridge DJ, Ikeda N, Epis MR, Li X, Foulds CE, Stuart LM, Barker A & Russell VJ et al. 2013 RNA-induced silencing complex (RISC) proteins PACT, TRBP, and Dicer are SRA binding nuclear receptor coregulators. PNAS 110 6536–6541. (doi:10.1073/pnas.1301620110).
Reinhart BJ, Slack FJ, Basson M, Pasquinelli AE, Bettinger JC, Rougvie AE, Horvitz HR & Ruvkun G 2000 The 21-nucleotide let-7 RNA regulates developmental timing in Caenorhabditis elegans. Nature 403 901–906.
Ribas J, Ni X, Haffner M, Wentzel EA, Salmasi AH, Chowdhury WH, Kudrolli TA, Yegnasubramanian S, Luo J & Rodriguez R et al. 2009 miR-21: an androgen receptor-regulated microRNA that promotes hormone-dependent and hormone-independent prostate cancer growth. Cancer Research 69 7165–7169. (doi:10.1158/0008-5472.CAN-09-1448).
Robb GB & Rana TM 2007 RNA helicase A interacts with RISC in human cells and functions in RISC loading. Molecular Cell 26 523–537. (doi:10.1016/j.molcel.2007.04.016).
Rokhlin OW, Scheinker V, Taghiyev AF, Bumcrot D, Glover RA & Cohen MB 2008 MicroRNA-34 mediates AR-dependent p53-induced apoptosis in prostate cancer. Cancer Biology & Therapy 7 1288–1296. (doi:10.4161/cbt.7.8.6284).
Ruby JG, Jan CH & Bartel DP 2007 Intronic microRNA precursors that bypass Drosha processing. Nature 448 83–86. (doi:10.1038/nature05983).
Ruggiero T, Trabucchi M, De Santa F, Zupo S, Harfe BD, McManus MT, Rosenfeld MG, Briata P & Gherzi R 2009 LPS induces KH-type splicing regulatory protein-dependent processing of microRNA-155 precursors in macrophages. FASEB Journal 23 2898–2908. (doi:10.1096/fj.09-131342).
Salzman DW, Shubert-Coleman J & Furneaux H 2007 P68 RNA helicase unwinds the human let-7 microRNA precursor duplex and is required for let-7-directed silencing of gene expression. Journal of Biological Chemistry 282 32773–32779. (doi:10.1074/jbc.M705054200).
Samaan S, Tranchevent LC, Dardenne E, Polay Espinoza M, Zonta E, Germann S, Gratadou L, Dutertre M & Auboeuf D 2013 The Ddx5 and Ddx17 RNA helicases are cornerstones in the complex regulatory array of steroid hormone-signaling pathways. Nucleic Acids Research 42 2197–2207. (doi:10.1093/nar/gkt1216).
Sander S, Bullinger L, Klapproth K, Fiedler K, Kestler HA, Barth TF, Möller P, Stilgenbauer S, Pollack JR & Wirth T 2008 MYC stimulates EZH2 expression by repression of its negative regulator miR-26a. Blood 112 4202–4212. (doi:10.1182/blood-2008-03-147645).
Scott GK, Goga A, Bhaumik D, Berger CE, Sullivan CS & Benz CC 2007 Coordinate suppression of ERBB2 and ERBB3 by enforced. Expression of micro-RNA miR-125a or miR-125b*. Journal of Biological Chemistry 282 1479–1486. (doi:10.1074/jbc.M609383200).
Shedden K, Taylor JM, Enkemann SA, Tsao MS, Yeatman TJ, Gerald WL, Eschrich S, Jurisica I, Giordano TJ & Misek DE et al. 2008 Gene expression-based survival prediction in lung adenocarcinoma: a multi-site, blinded validation study. Nature Medicine 14 822–827. (doi:10.1038/nm.1790).
Shenouda S & Alahari S 2009 MicroRNA function in cancer: oncogene or a tumor suppressor? Cancer Metastasis Reviews 28 369–378. (doi:10.1007/s10555-009-9188-5).
Shi XB, Xue L, Yang J, Ma AH, Zhao J, Xu M, Tepper CG, Evans CP, Kung HJ & deVere White RW 2007 An androgen-regulated miRNA suppresses Bak1 expression and induces androgen-independent growth of prostate cancer cells. PNAS 104 19983–19988. (doi:10.1073/pnas.0706641104).
Shi XB, Xue L, Ma AH, Tepper CG, Kung HJ & White RW 2011 miR-125b promotes growth of prostate cancer xenograft tumor through targeting pro-apoptotic genes. Prostate 71 538–549. (doi:10.1002/pros.21270).
Shu J, Kren BT, Xia Z, Wong PY, Li L, Hanse EA, Min MX, Li B, Albrecht JH & Zeng Y et al. 2011 Genomewide microRNA down-regulation as a negative feedback mechanism in the early phases of liver regeneration. Hepatology 54 609–619. (doi:10.1002/hep.24421).
Sikand K, Slaibi JE, Singh R, Slane SD & Shukla GC 2011a miR 488* inhibits androgen receptor expression in prostate carcinoma cells. International Journal of Cancer 129 810–819. (doi:10.1002/ijc.25753).
Sikand K, Barik S & Shukla G 2011b MicroRNAs and androgen receptor 3′ untranslated region: a missing link in castration-resistant prostate cancer? Molecular and Cellular Pharmacology 3 107–113.
Sirotkin AV, Ovcharenko D, Grossmann R, Lauková M & Mlynček M 2009 Identification of microRNAs controlling human ovarian cell steroidogenesis via a genome-scale screen. Journal of Cellular Physiology 219 415–420. (doi:10.1002/jcp.21689).
Sun T, Wang Q, Balk S, Brown M, Lee G-SM & Kantoff P 2009 The role of microRNA-221 and microRNA-222 in androgen-independent prostate cancer cell lines. Cancer research 69 3356–3363. (doi:10.1158/0008-5472.can-08-4112).
Suzuki HI, Yamagata K, Sugimoto K, Iwamoto T, Kato S & Miyazono K 2009 Modulation of microRNA processing by p53. Nature 460 529–533. (doi:10.1038/nature08199).
Suzuki HI, Arase M, Matsuyama H, Choi YL, Ueno T, Mano H, Sugimoto K & Miyazono K 2011 MCPIP1 ribonuclease antagonizes dicer and terminates microRNA biogenesis through precursor microRNA degradation. Molecular Cell 44 424–436. (doi:10.1016/j.molcel.2011.09.012).
Takata A, Otsuka M, Kojima K, Yoshikawa T, Kishikawa T, Yoshida H & Koike K 2011 MicroRNA-22 and microRNA-140 suppress NF-κB activity by regulating the expression of NF-κB coactivators. Biochemical and Biophysical Research Communications 411 826–831. (doi:10.1016/j.bbrc.2011.07.048).
Takayama K, Tsutsumi S, Katayama S, Okayama T, Horie-Inoue K, Ikeda K, Urano T, Kawazu C, Hasegawa A & Ikeo K et al. 2011 Integration of cap analysis of gene expression and chromatin immunoprecipitation analysis on array reveals genome-wide androgen receptor signaling in prostate cancer cells. Oncogene 30 619–630. (doi:10.1038/onc.2010.436).
Tokumaru S, Suzuki M, Yamada H, Nagino M & Takahashi T 2008 let-7 regulates Dicer expression and constitutes a negative feedback loop. Carcinogenesis 29 2073–2077. (doi:10.1093/carcin/bgn187).
Tong AW, Fulgham P, Jay C, Chen P, Khalil I, Liu S, Senzer N, Eklund AC, Han J & Nemunaitis J 2008 MicroRNA profile analysis of human prostate cancers. Cancer Gene Therapy 16 206–216.
Trabucchi M, Briata P, Garcia-Mayoral M, Haase AD, Filipowicz W, Ramos A, Gherzi R & Rosenfeld MG 2009 The RNA-binding protein KSRP promotes the biogenesis of a subset of microRNAs. Nature 459 1010–1014. (doi:10.1038/nature08025).
Tsuchiya Y, Nakajima M, Takagi S, Taniya T & Yokoi T 2006 MicroRNA regulates the expression of human cytochrome P450 1B1. Cancer Research 66 9090–9098. (doi:10.1158/0008-5472.CAN-06-1403).
Waltering KK, Porkka KP, Jalava SE, Urbanucci A, Kohonen PJ, Latonen LM, Kallioniemi OP, Jenster G & Visakorpi T 2011 Androgen regulation of micro-RNAs in prostate cancer. Prostate 71 604–614. (doi:10.1002/pros.21276).
Wang Q, Li W, Zhang Y, Yuan X, Xu K, Yu J, Chen Z, Beroukhim R, Wang H & Lupien M et al. 2009 Androgen receptor regulates a distinct transcription program in androgen-independent prostate cancer. Cell 138 245–256. (doi:10.1016/j.cell.2009.04.056).
Wickramasinghe NS, Manavalan TT, Dougherty SM, Riggs KA, Li Y & Klinge CM 2009 Estradiol downregulates miR-21 expression and increases miR-21 target gene expression in MCF-7 breast cancer cells. Nucleic Acids Research 37 2584–2595. (doi:10.1093/nar/gkp117).
Wightman B, Ha I & Ruvkun G 1993 Posttranscriptional regulation of the heterochronic gene lin-14 by lin-4 mediates temporal pattern formation in C. elegans. Cell 75 855–862. (doi:10.1016/0092-8674(93)90530-4).
Wu J, Bao J, Wang L, Hu Y & Xu C 2011 MicroRNA-184 downregulates nuclear receptor corepressor 2 in mouse spermatogenesis. BMC Developmental Biology 11 64. (doi:10.1186/1471-213X-11-64).
Xhemalce B, Robson SC & Kouzarides T 2012 Human RNA methyltransferase BCDIN3D regulates microRNA processing. Cell 151 278–288. (doi:10.1016/j.cell.2012.08.041).
Xiao J, Gong AY, Eischeid AN, Chen D, Deng C, Young CY & Chen XM 2012 miR-141 modulates androgen receptor transcriptional activity in human prostate cancer cells through targeting the small heterodimer partner protein. Prostate 72 1514–1522. (doi:10.1002/pros.22501).
Yamagata K, Fujiyama S, Ito S, Ueda T, Murata T, Naitou M, Takeyama KI, Minami Y, O'Malley BW & Kato S 2009 Maturation of microRNA is hormonally regulated by a nuclear receptor. Molecular Cell 36 340–347. (doi:10.1016/j.molcel.2009.08.017).
Yang W, Chendrimada TP, Wang Q, Higuchi M, Seeburg PH, Shiekhattar R & Nishikura K 2006 Modulation of microRNA processing and expression through RNA editing by ADAR deaminases. Nature Structural & Molecular Biology 13 13–21. (doi:10.1038/nsmb1041).
Yang R, Schlehe B, Hemminki K, Sutter C, Bugert P, Wappenschmidt B, Volkmann J, Varon R, Weber B & Niederacher D et al. 2010 A genetic variant in the pre-miR-27a oncogene is associated with a reduced familial breast cancer risk. Breast Cancer Research and Treatment 121 693–702. (doi:10.1007/s10549-009-0633-5).
Yang JS, Phillips MD, Betel D, Mu P, Ventura A, Siepel AC, Chen KC & Lai EC 2011 Widespread regulatory activity of vertebrate microRNA* species. RNA 17 312–326. (doi:10.1261/rna.2537911).
Yang CM, Lu IH, Chen HY & Hu ML 2012 Lycopene inhibits the proliferation of androgen-dependent human prostate tumor cells through activation of PPARγ–LXRα–ABCA1 pathway. Journal of Nutritional Biochemistry 23 8–17. (doi:10.1016/j.jnutbio.2010.10.006).
Yeom KH, Lee Y, Han J, Suh MR & Kim VN 2006 Characterization of DGCR8/Pasha, the essential cofactor for Drosha in primary miRNA processing. Nucleic Acids Research 34 4622–4629. (doi:10.1093/nar/gkl458).
Yu Z, Wang C, Wang M, Li Z, Casimiro MC, Liu M, Wu K, Whittle J, Ju X & Hyslop T et al. 2008 A cyclin D1/microRNA 17/20 regulatory feedback loop in control of breast cancer cell proliferation. Journal of Cell Biology 182 509–517. (doi:10.1083/jcb.200801079).
Zeng Y, Wagner EJ & Cullen BR 2002 Both natural and designed micro RNAs can inhibit the expression of cognate mRNAs when expressed in human cells. Molecular Cell 9 1327–1333. (doi:10.1016/S1097-2765(02)00541-5).
Zhang R, He Y, Zhang X, Xing B, Sheng Y, Lu H & Wei Z 2012 Estrogen receptor-regulated microRNAs contribute to the BCL2/BAX imbalance in endometrial adenocarcinoma and precancerous lesions. Cancer Letters 314 155–165. (doi:10.1016/j.canlet.2011.09.027).
Zhao JJ, Lin J, Yang H, Kong W, He L, Ma X, Coppola D & Cheng JQ 2008 MicroRNA-221/222 negatively regulates estrogen receptor α and is associated with tamoxifen resistance in breast cancer. Journal of Biological Chemistry 283 31079–31086. (doi:10.1074/jbc.M806041200).
Zhao C, Sun G, Li S, Lang MF, Yang S, Li W & Shi Y 2010 MicroRNA let-7b regulates neural stem cell proliferation and differentiation by targeting nuclear receptor TLX signaling. PNAS 107 1876–1881. (doi:10.1073/pnas.0908750107).
Zhao Y, Deng C, Wang J, Xiao J, Gatalica Z, Recker R & Xiao G 2011 Let-7 family miRNAs regulate estrogen receptor α signaling in estrogen receptor positive breast cancer. Breast Cancer Research and Treatment 127 69–80. (doi:10.1007/s10549-010-0972-2).
Zhao J, Imbrie GA, Baur WE, Iyer LK, Aronovitz MJ, Kershaw TB, Haselmann GM, Lu Q & Karas RH 2013 Estrogen receptor-mediated regulation of microRNA inhibits proliferation of vascular smooth muscle cells. Arteriosclerosis, Thrombosis, and Vascular Biology 33 257–265. (doi:10.1161/ATVBAHA.112.300200).