While it has been known for decades that androgen hormones influence normal breast development and breast carcinogenesis, the underlying mechanisms have only been recently elucidated. To date, most studies have focused on androgen action in breast cancer cell lines, yet these studies represent artificial systems that often do not faithfully replicate/recapitulate the cellular, molecular and hormonal environments of breast tumours in vivo. It is critical to have a better understanding of how androgens act in the normal mammary gland as well as in in vivo systems that maintain a relevant tumour microenvironment to gain insights into the role of androgens in the modulation of breast cancer development. This in turn will facilitate application of androgen-modulation therapy in breast cancer. This is particularly relevant as current clinical trials focus on inhibiting androgen action as breast cancer therapy but, depending on the steroid receptor profile of the tumour, certain individuals may be better served by selectively stimulating androgen action. Androgen receptor (AR) protein is primarily expressed by the hormone-sensing compartment of normal breast epithelium, commonly referred to as oestrogen receptor alpha (ERa (ESR1))-positive breast epithelial cells, which also express progesterone receptors (PRs) and prolactin receptors and exert powerful developmental influences on adjacent breast epithelial cells. Recent lineage-tracing studies, particularly those focussed on NOTCH signalling, and genetic analysis of cancer risk in the normal breast highlight how signalling via the hormone-sensing compartment can influence normal breast development and breast cancer susceptibility. This provides an impetus to focus on the relationship between androgens, AR and NOTCH signalling and the crosstalk between ERa and PR signalling in the hormone-sensing component of breast epithelium in order to unravel the mechanisms behind the ability of androgens to modulate breast cancer initiation and growth.
Gerard A Tarulli, Lisa M Butler, Wayne D Tilley and Theresa E Hickey
W R Miller
Both mammary adipose tissue and breast cancers have the ability to aromatize androgens into oestrogens. Such potential may maintain the growth of hormone-dependent tumours. It has therefore been important to determine the effects of new aromatase inhibitors such as formestane, exemestane, anastrozole and letrozole on oestrogen biosynthesis and concentrations of endogenous hormones within the breast. Studies based on in vitro incubations of breast cancer and cultures of mammary adipose tissue fibroblasts demonstrate that these drugs are highly effective inhibitors, with IC50 values ranging between 1 and 50 nM (although the relative efficacy varies between tissues and test systems). Despite this potential, in vitro incubations of breast tissues from patients treated with type II inhibitors such as aminoglutethimide and letrozole can display paradoxically high aromatase activity; this appears to be caused by the reversible nature of the inhibition, coupled with induction/stabilization of the aromatase enzyme. To assess in situ effects within the breast, postmenopausal women with large primary breast cancers have been treated neoadjuvantly with aromatase inhibitors using a protocol that included (i) breast biopsy before treatment, (ii) definitive surgery after 3 months of treatment and (iii) infusion of [3H]androstenedione and [14C]oestrone in the 18 h immediately before biopsy and surgery. With this study design, it has been shown that drugs such as letrozole profoundly inhibit in situ aromatase activity and reduce endogenous oestrogens within the breast.
Breast cancer is the most frequently diagnosed and the second cause of cancer death in women, thus making breast cancer a most feared disease. Since breast cancer metastasizes early and it is unlikely that improvements in the treatment of metastatic disease could permit a cure in most cases in the foreseeable future, it is clear that prevention is essential in order practically to eliminate deaths from breast cancer. Tamoxifen is the only selective estrogen receptor modulator (SERM) currently registered for use in breast cancer prevention; the tamoxifen versus raloxifene study should indicate the efficacy of this compound compared with raloxifene. The recent benefits of aromatase inhibitors over tamoxifen indicate the advantages of a blockade of estrogens more complete than the one achieved with tamoxifen, a SERM having some estrogenic activity in the mammary gland and an even higher estrogenic action in the uterus. However, it is unlikely that the general estrogen ablation achieved with aromatase inhibitors will be acceptable for the long-term use required for prevention. It is thus important to develop SERMs with highly potent and pure antagonistic activity in the mammary gland and uterus while possessing estrogen-like activity in tissues of particular importance for women’s health, namely the bones and the cardiovascular system. However, it is expected that a SERM alone will not meet all the requirements of women’s health at the postmenopause when ovarian estrogen secretion has ceased and peripheral formation of androgens and estrogens from DHEA by intracrine mechanisms is decreased by 60% or more. One possibility is to combine a SERM with DHEA, a precursor of sex steroids that permits, somewhat like SERMs, tissue-specific formation of androgens and/or estrogens according to the level of expression of the steroidogenic and steroid-inactivating enzymes. DHEA could thus compensate for the important loss of androgens that accompanies aging and could also permit sex steroid formation and action in the brain while breast cancer prevention would be achieved by the SERM.
Giorgio Secreto, Alessandro Girombelli and Vittorio Krogh
The aim of this review is to highlight the pivotal role of androgen excess in the development of breast cancer. Available evidence suggests that testosterone controls breast epithelial growth through a balanced interaction between its two active metabolites: cell proliferation is promoted by estradiol while it is inhibited by dihydrotestosterone. A chronic overproduction of testosterone (e.g. ovarian stromal hyperplasia) results in an increased estrogen production and cell proliferation that are no longer counterbalanced by dihydrotestosterone. This shift in the androgen/estrogen balance partakes in the genesis of ER-positive tumors. The mammary gland is a modified apocrine gland, a fact rarely considered in breast carcinogenesis. When stimulated by androgens, apocrine cells synthesize epidermal growth factor (EGF) that triggers the ErbB family receptors. These include the EGF receptor and the human epithelial growth factor 2, both well known for stimulating cellular proliferation. As a result, an excessive production of androgens is capable of directly stimulating growth in apocrine and apocrine-like tumors, a subset of ER-negative/AR-positive tumors. The key role of androgen excess in the genesis of different subtypes of breast cancer has significant clinical implications for both treatment and prevention. Our belief stems from a thorough analysis of the literature, where an abundance of evidence is present to justify a clinical trial that would investigate the effectiveness of treating the underlying excessive androgen production.
R Kaaks, S Rinaldi, T J Key, F Berrino, P H M Peeters, C Biessy, L Dossus, A Lukanova, S Bingham, K-T Khaw, N E Allen, H B Bueno-de-Mesquita, C H van Gils, D Grobbee, H Boeing, P H Lahmann, G Nagel, J Chang-Claude, F Clavel-Chapelon, A Fournier, A Thiébaut, C A González, J R Quirós, M-J Tormo, E Ardanaz, P Amiano, V Krogh, D Palli, S Panico, R Tumino, P Vineis, A Trichopoulou, V Kalapothaki, D Trichopoulos, P Ferrari, T Norat, R Saracci and E Riboli
Considerable experimental and epidemiological evidence suggests that elevated endogenous sex steroids — notably androgens and oestrogens — promote breast tumour development. In spite of this evidence, postmenopausal androgen replacement therapy with dehydroepiandrosterone (DHEA) or testosterone has been advocated for the prevention of osteoporosis and improved sexual well-being. We have conducted a case–control study nested within the European Prospective Investigation into Cancer and Nutrition. Levels of DHEA sulphate (DHEAS), (Δ4-androstenedione), testosterone, oestrone, oestradiol and sex-hormone binding globulin (SHBG) were measured in prediagnostic serum samples of 677 postmenopausal women who subsequently developed breast cancer and 1309 matched control subjects. Levels of free testosterone and free oestradiol were calculated from absolute concentrations of testosterone, oestradiol and SHBG. Logistic regression models were used to estimate relative risks of breast cancer by quintiles of hormone concentrations. For all sex steroids –the androgens as well as the oestrogens – elevated serum levels were positively associated with breast cancer risk, while SHBG levels were inversely related to risk. For the androgens, relative risk estimates (95% confidence intervals) between the top and bottom quintiles of the exposure distribution were: DHEAS 1.69 (1.23–2.33), androstenedione 1.94 (1.40–2.69), testosterone 1.85 (1.33–2.57) and free testosterone 2.50 (1.76–3.55). For the oestrogens, relative risk estimates were: oestrone 2.07 (1.42–3.02), oestradiol 2.28 (1.61–3.23) and free oestradiol (odds ratios 2.13 (1.52–2.98)). Adjustments for body mass index or other potential confounding factors did not substantially alter any of these relative risk estimates. Our results have shown that, among postmenopausal women, not only elevated serum oestrogens but also serum androgens are associated with increased breast cancer risk. Since DHEAS and androstenedione are largely of adrenal origin in postmenopausal women, our results indicated that elevated adrenal androgen synthesis is a risk factor for breast cancer. The results from this study caution against the use of DHEA(S), or other androgens, for postmenopausal androgen replacement therapy.
D K Wyld, J D Chester and T J Perren
Endocrine therapy has now been used as an effective treatment for breast cancer for 100 years. It was the first successful systemic treatment for cancer, initially becoming accepted following George Beatson's (1896) observations that, in a proportion of pre-menopausal women with advanced breast cancer, bilateral oophorectomy resulted in disease regression. Over the subsequent 75 years, several other endocrine therapies for breast cancer were developed, including other surgical approaches to hormone ablation therapy such as adrenalectomy (Huggins & Dao 1953) and hypophysectomy (Luft & Olivecrona 1953), and hormonal additive therapies, such as the use of pharmacological doses of androgens, oestrogens, progestogens and glucocorticoids. However, as none of these approaches led to a significant improvement in the rates of tumour regression, clinicians' enthusiasm in the 1960s and early 1970s became focused on the use of newly developing cytotoxic chemotherapy regimens. Then, in the 1970s, tamoxifen, and subsequently a range of other new endocrine agents, became available which were of low toxicity and generally well tolerated - high-dose oestrogens were quickly replaced by tamoxifen, and adrenalectomy by aromatase inhibitors. Measurement of hormone receptor levels also became available, allowing better selection of patients whose tumours might be hormonally responsive. In addition, some of the limitations of cytotoxic drugs in breast cancer were starting to become apparent.
Ian S Fentiman
Male breast cancer (MBC) is a rare disease but, as a result of epidemiological collaborations, there is now greater clarity concerning endocrine risk factors. The significant rise in global age-standardised mean BMI in men is likely to lead to increases in incidence of maturity-onset diabetes and MBC. The metabolic changes accompanying obesity decrease androgens and sex hormone-binding globulin (SHBG), thereby increasing available oestrogens. The higher rates of MBC in North and Equatorial Africa are largely due to liver damage from endemic bilharziasis and hepatitis B causing elevated oestradiol (E2) levels from hepatic conversion of androgen. Klinefelter’s syndrome (XXY) is associated with a 50-fold increase in incidence of MBC compared with XY males, and this is the most pronounced evidence for testicular malfunction amplifying risk. Delay in presentation means that up to 40% of cases have stage III or stage IV disease at diagnosis. No randomised controlled trials have been reported on endocrine treatment of advanced disease or adjuvant/neoadjuvant therapy following or preceding surgery. Tamoxifen is the most effective endocrine therapy, but side effects can lead to non-compliance in a substantial number of men. Aromatase inhibitors are less effective because they do not inhibit testicular oestrogen production. There is an urgent need for collaborative trials to provide an evidence base for the most effective endocrine and least toxic therapies for men with breast cancer.
Suman Rice and Saffron A Whitehead
The majority of breast cancers are oestrogen dependent and in postmenopausal women the supply of oestrogens in breast tissue is derived from the peripheral conversion of circulating androgens. There is, however, a paradox concerning the epidemiology of breast cancer and the dietary intake of phytoestrogens that bind weakly to oestrogen receptors and initiate oestrogen-dependent transcription. In Eastern countries, such as Japan, the incidence of breast cancer is approximately one-third that of Western countries whilst their high dietary intake of phytoestrogens, mainly in the form of soy products, can produce circulating levels of phytoestrogens that are known experimentally to have oestrogenic effects. Indeed, their weak oestrogenicity has been used to advantage by herbalist medicine to promote soy products as a natural alternative to conventional hormone replacement therapy (HRT). Such usage could increase in light of recent evidence that long-term HRT usage may be associated with an increased risk of breast cancer with a consequent reduction in prescription rates. So, are phytoestrogens safe as a natural alternative to HRT and could they be promoters or protectors of breast cancer? If they are promoters, then we must assume that it is due to their oestrogenic effect. If they are protectors, then other actions of phytoestrogens, including their ability to inhibit enzymes that are responsible for converting androgens and weak oestrogens into oestradiol, must be considered. This paper addresses these questions by reviewing the actions of phytoestrogens on oestrogen receptors and key enzymes that convert androgens to oestrogens in relation to the growth of breast cancer cells. In addition, it compares the experimental and epidemiological evidence pertinent to the potential beneficial or harmful effects of phytoestrogens in relation to the incidence/progression of breast cancer and their efficacy as natural alternatives to conventional HRT.
E M Rosen, S Fan and C Isaacs
The breast and ovarian cancer susceptibility gene-1 (BRCA1) located on chromosome 17q21 encodes a tumor suppressor gene that functions, in part, as a caretaker gene in preserving chromosomal stability. The observation that most BRCA1 mutant breast cancers are hormone receptor negative has led some to question whether hormonal factors contribute to the etiology of BRCA1-mutant breast cancers. Nevertheless, the caretaker function of BRCA1 is a generic one and does not explain why BRCA1 mutations confer a specific risk for tumor types that are hormone-responsive or that hormonal factors contribute to the etiology, including those of the breast, uterus, cervix, and prostate. An accumulating body of research indicates that in addition to its well-established roles in regulation of the DNA damage response, the BRCA1 protein interacts with steroid hormone receptors (estrogen receptor (ER-α) and androgen receptor (AR)) and regulates their activity, inhibiting ER-α activity and stimulating AR activity. The ability of BRCA1 to regulate steroid hormone action is consistent with clinical-epidemiological research suggesting that: (i) hormonal factors contribute to breast cancer risk in BRCA1 mutation carriers; and (ii) the spectrum of risk-modifying effects of hormonal factors in BRCA1 carriers is not identical to that observed in the general population. These data suggest a model for BRCA1 carcinogenesis in which genomic instability leads to the initiation of cancerous cell clones, while loss of normal restraint on hormonal stimulation of mammary epithelial cell proliferation allows amplification of these pre-existing clones. Further research will be required to substantiate this hypothesis.
Glenn T G Chang, Mila Jhamai, Wytske M van Weerden, Guido Jenster and Albert O Brinkmann
TRPS1 mRNA is more highly expressed in androgen-dependent lymph node carcinoma of prostate-fast growing colony (LNCaP-FGC) compared with androgen-independent lymph node carcinoma of prostate-lymph node original (LNCaP-LNO) prostate cancer cell lines. Furthermore, TRPS1 mRNA expression is down-regulated by androgens in LNCaP-FGC cells, a process mediated by the androgen receptor (AR). Here, we present TRPS1 protein expression in human prostate cancer material derived from a panel of six androgen-dependent and eight androgen-independent human prostate cancer xenografts. TRPS1 protein is expressed in all androgen-dependent xenografts, which also express AR and prostate-specific antigen (PSA). Androgen withdrawal by castration resulted in an increase in TRPS1 protein in two androgen-dependent xenografts, indicating relieved repression by action of AR. TRPS1 protein is expressed in four androgen-independent xenografts and is low or absent in the other four androgen-independent xenografts. Androgen withdrawal by castration demonstrates that TRPS1 protein levels remain the same in 1 androgen-independent xenograft, most likely due to the lack of AR expression. These data show that TRPS1 protein expression is regulated by androgens via the AR in human prostate cancer xenografts.
Analysis of TRPS1 mRNA expression in normal and tumour tissue of the prostate and 18 other human tissues, showed that TRPS1 had the highest mRNA expression levels in normal and tumour tissues of breast. In addition, high TRPS1 mRNA and protein expression levels were observed in four out of five human breast cancer cell lines.
In conclusion, TRPS1 protein expression is down-regulated by androgens in human prostate cancer, and analysis of TRPS1 mRNA expression levels in several human tissues showed that the highest levels were observed in normal and tumour breast tissue.