The idea of breast cancer prevention by hormonal means stemmed from the results of treatment trials, many of them carried out by the National Surgical Adjuvant Breast and Bowel Project (NSABP). Over the years, a number of NSABP treatment studies demonstrated that breast cancer recurrence was reduced in women with the disease who were given tamoxifen, a selective estrogen receptor (ER) modulator (SERM). Five subsequent tamoxifen prevention trials with this agent have shown a 48% reduction in ER-positive cancers, but no effect for ER-negative cancers, and an increase in endometrial cancer and thromboembolic events. The drug raloxifene, another SERM, originally examined as an osteoporosis agent, has also shown promise for the prevention of breast cancer, although, as with tamoxifen, the drug carries a risk for thromboembolic events. There is recent evidence in a large treatment trial that the aromastase inhibitor anastrazole, a 'pure anti-estrogen', holds promise as a breast cancer preventive agent. Longer follow-up and the testing of additional agents is required before these drugs can be used widely for prevention. In addition, future research should focus on the identification of at-risk women who can perhaps be targeted for specific prevention agents.
R R Tekmal, N Kirma, K Gill, and K Fowler
To test directly the role of breast-tissue estrogen in initiation of breast cancer, we have developed the aromatase-transgenic mouse model and demonstrated for the first time that increased mammary estrogens resulting from the overexpression of aromatase in mammary glands lead to the induction of various preneoplastic and neoplastic changes that are similar to early breast cancer. Continued overexpression of aromatase that leads to increased breast-tissue estrogen contributes to a number of epigenetic changes in mammary tissue such as alteration in the regulation of genes involved in apoptosis, activation of genes involved in cell cycle and cell proliferation, and activation of a number of growth factors. Our current studies show aromatase overexpression is sufficient to induce and maintain early preneoplastic and neoplastic changes in female mice without circulating ovarian estrogen. Preneoplastic and neoplastic changes induced in mammary glands as a result of aromatase overexpression can be completely abrogated with the administration of the aromatase inhibitor, letrozole. Consistent with complete reduction in hyperplasia, we have also seen downregulation of estrogen receptor and a decrease in cell proliferation markers, suggesting aromatase-induced hyperplasia can be treated with aromatase inhibitors. Our studies demonstrate that aromatase overexpression alone, without circulating estrogen, is responsible for the induction of breast hyperplasia and these changes can be abrogated using aromatase inhibitors.
Daniela Gallo, Elisabetta Mantuano, Manuela Fabrizi, Cristiano Ferlini, Simona Mozzetti, Ilaria De Stefano, and Giovanni Scambia
The study reported here was designed to determine whether a phytoestrogen-containing soy extract (SSE) could negate/overwhelm the inhibitory effects of ICI 182 780 on the growth of estrogen-sustained human breast cancer xenografts (MCF-7), in ovariectomized athymic mice. As expected, estradiol-supplemented tumors did not grow over the study period in ICI 182 780-treated females; concomitant administration of 50 mg/kg per day SSE slightly potentiated the inhibitory activity of the drug, while at 100 mg/kg per day, SSE partially negated ICI 182 780 activity. In keeping with these in vivo outcomes, we observed that the level of cyclin D1 (and progesterone receptor) in MCF-7 xenografts was considerably reduced by ICI 182 780, an effect enhanced by concomitant treatment with 50 SSE, but reduced by the higher dosage (i.e. 100 mg/kg per day). Thrombospondin-1 (TSP-1) and kallikrein 6 (KLK6) levels were also reduced following ICI 182 780, although to a lesser degree; again, combined anti-estrogen and SSE produced a dose-dependent regulation in TSP-1 and KLK6 tumor level, with a further reduction in the mRNA gene expression at 50 SSE (compared with ICI 182 780) and a partial reversion of the drug-induced down-regulation at 100 mg/kg per day. No modulation was detected in the serum concentration of IGF-1 (a potent mitogen for estrogen receptor-positive breast cancer cell lines) either upon treatment with ICI 182 780 or concomitant administration of the anti-estrogen with SSE. In conclusion, results from this study raise concerns about the consumption of isoflavone supplements in conjunction with ICI 182 780 therapy, in postmenopausal women with estrogen-dependent breast cancer.
Zane Hammoud, Bailin Tan, Sunil Badve, and Robert M Bigsby
Numerous epidemiological observations point to sex differences in lung cancer etiology and progression. The present study was aimed at understanding the bases of these sex differences. To test the effect of estradiol on tumor progression, we used a mouse model based on conditional Kras expression and concurrent deletion of Tp53 following inhalation of an adenoviral vector expressing Cre recombinase (AdeCre). Ovariectomized females and males were treated with estradiol via a continuous-release capsule. Tumor multiplicity, tumor volume, and histological grade were determined at 10 weeks after AdeCre administration. Cell proliferation was monitored by Ki67 immunohistochemistry at 4 and 10 weeks after AdeCre administration. At 10 weeks, female mice had more than twice the number of tumors evident on the surface of the lungs than male mice; ovariectomy eliminated this sex difference. The estrogen treatment significantly increased tumor number and volume in ovariectomized females and in males. Histological character of the tumors ranged from adenoma to adenocarcinoma. Ovary-intact females exhibited higher grade tumors than ovariectomized females or males. Progression to higher histological grade was stimulated by estrogen in male mice but not in ovariectomized females. At 10 weeks after AdeCre administration, tumor cell Ki67-labeling varied widely, precluding assessment of an estrogen effect; however, at 4 weeks, Ki67 labeling of lung parenchymal cells was increased 3.5-fold by estrogen. In conclusion, estrogen acts as a promoter for lung adenocarcinoma in a genetically defined lung cancer model; estrogen-induced cell proliferation in the oncogene-initiated cells is likely to play a role in this tumor promoter activity.
Oliver Zierau, Jacintha O’Sullivan, Colm Morrissey, Dana McDonald, Winfried Wünsche, Martin R Schneider, Martin P Tenniswood, and Günter Vollmer
Tamoxifen is the most widely prescribed anti-neoplastic drug for the treatment of both localized and metastatic breast cancer. It is also the prototype for a class of drugs that are referred to as selective estrogen receptor modifiers (SERMs), most of which have both estrogenic and anti-estrogenic activity in estrogen target tissues including the breast and endometrium. The underlying mechanisms of action of SERMs in the breast and endometrium that lead to profound differences in the tissue-specific effects of tamoxifen have not yet been elucidated.
We have compared the effects of tamoxifen and the pure anti-estrogen ICI 182,780 (Faslodex) in the RUCA-I hormone-responsive rat endometrial cell line in vitro and in vivo. In cell culture, RUCA-I cells responded to both estrogens and anti-estrogens, and the expression of clusterin and complement C3 mRNAs required the presence of estradiol and was repressed in the absence of estradiol or in the presence of the pure anti-estrogen ICI 182,780. Tamoxifen, on the other hand, induced both complement C3 and clusterin mRNA in the absence of estradiol and failed to repress their expression in the presence of estradiol. When grown as subcutaneous xenografts in syngeneic Da/Han rats for 5 weeks, the RUCA-I cells retained their sensitivity to estradiol, as demonstrated by significantly enhanced tumor growth in intact female rats compared with the growth in ovariectomized rats. But neither ICI 182,780 nor tamoxifen had a significant impact on tumor growth in cycling or ovariectomized animals. On the other hand, tamoxifen was potently estrogenic in metastatic lymph nodes, increasing the size of the lymph node tumors almost 6-fold over that seen in the intact cycling animals. In primary tumors, the expression of complement C3 mirrored that seen in vitro, although tamoxifen showed some agonist activity in ovariectomized animals. Tamoxifen also displayed marked agonist activity with respect to clusterin expression and enhanced clusterin mRNA levels and protein in both the primary tumors and lymph metastases in intact and ovariectomized animals.
Given the recent demonstration that over-expression of clusterin increases the metastatic potential of breast cancer cells, these data may provide a mechanistic explanation for the increased incidence of endometrial cancer in postmenopausal patients treated with tamoxifen.
Evangelia-Ourania Fourkala, Alexey Zaikin, Matthew Burnell, Aleksandra Gentry-Maharaj, Jeremy Ford, Richard Gunu, Christina Soromani, Guido Hasenbrink, Ian Jacobs, Anne Dawnay, Martin Widschwendter, Hella Lichtenberg-Fraté, and Usha Menon
Postmenopausal women with elevated serum sex steroids have an increased risk of breast cancer. Most of this risk is believed to be exerted through binding of the sex steroids to their receptors. For the first time, we investigate the association of estrogen receptor (ER) and androgen receptor (AR) serum bioactivity (SB) in addition to hormone levels in samples from women with breast cancer collected before diagnosis. Two hundred postmenopausal women participating in the UK Collaborative Trial of Ovarian Cancer Screening who developed ER-positive breast cancer 0.6–5 years after sample donation were identified and matched to 400 controls. ER and AR bioassays were used to measure ERα, ERβ, and AR SB. Androgen and estrogen levels were measured with immunoassays. Subjects were classified according to quintiles of the respective marker among controls and the associations between SB and hormones with breast cancer risk were determined by logistic regression analysis. ERα and ERβ SB were significantly higher before diagnosis compared with controls, while estrogens showed no difference. Women had a twofold increased breast cancer risk if ERα SB (odds ratio (OR), 2.114; 95% confidence interval (CI), 1.050–4.425; P=0.040) was in the top quintile >2 years before diagnosis or estrone (OR, 2.205; 95% CI, 1.104–4.586; P=0.029) was in the top quintile <2 years before diagnosis. AR showed no significant association with breast cancer while androstenedione (OR, 3.187; 95% CI, 1.738–6.044; P=0.0003) and testosterone (OR, 2.145; 95% CI, 1.256–3.712; P=0.006) were significantly higher compared with controls and showed a strong association with an almost threefold increased breast cancer risk independent of time to diagnosis. This study provides further evidence on the association of androgens and estrogens with breast cancer. In addition, it reports that high ER but not AR SB is associated with increased breast risk >2 years before diagnosis.
E Enmark and J Å Gustafsson
The recent discovery of a second estrogen receptor, ER&B;, shows that the mechanisms behind the effects of estrogen are far more complicated than previously assumed, and gives unique opportunities to gain a better understanding of these phenomena. ER&B; is expressed in many important target tissues for estrogen, and a better insight into the respective mechanisms of action of ERalpha and ER&B; might give clues concerning the etiology and pathogenesis of for example prostate or ovarian cancer. Development of ERalpha and ER&B; specific ligands may furthermore open up interesting new possibilities for treatment of e.g. postmenopausal symptoms and breast cancer. In this review, we will try to summarize what is known sofar about estrogen receptor &B;, with some emphasis on the human receptor and its expression. We will furthermore try so summarize what is known about different isoforms of the receptor, in view of what is known about isoforms and variants of other receptors, in particular estrogen receptor alpha (ERalpha) and the progesterone receptor (PR).
This study was supported by a grant from the Swedish Cancer Society.
Tim Byers and Rebecca L Sedjo
Carrying excess body fat is a leading cause of cancer. Epidemiologic evidence gives strong clues about the mechanisms that link excess adiposity to risk for several cancer sites. For postmenopausal breast cancer and endometrial cancer, the hyper-estrogenic state that is induced by excess body fatness is the likely cause. For esophageal cancer and gallbladder cancer, chronic local inflammation induced by acid reflux and gallstones is the likely cause, and for liver cancer, local inflammation induced by hepatic fatty infiltration is the likely cause. However, for several other cancers known to be associated with excess adiposity, including cancers of the colon, pancreas, ovary, kidney, and prostate, specific causes are not known. Possible candidates include elevated systemic or local tissue inflammation induced by adiposity and effects of the elevated levels of leptin, insulin, IGFs, and depressed immune function that are seen with excess adiposity. There is growing evidence that intentional weight loss not only reduces circulating levels of cancer-associated factors but that it also reduces cancer incidence and recurrence. Better research is needed to understand the mechanisms that link excess body fat to cancer risk as well as to understand the amount of weight loss needed for substantial cancer risk reduction. Finally, as we develop better understanding of the mediators of the effects of excess body fatness on cancer risk, we should identify pharmacologic interventions that target those mediators so that they can be used to complement weight loss in order to reduce cancer risk.
W Yue, R J Santen, J P Wang, C J Hamilton, and L M Demers
In situ aromatization and enhanced uptake of estradiol from plasma are two potential mechanisms for maintenance of high concentrations of estradiol found in breast tumors of postmenopausal patients. To test the relative importance of these two mechanisms, a nude mouse model was established by inoculating aromatase (A+) and/or sham (A-) transfected MCF-7 cells into ovariectomized mice. Postmenopausal hormonal status was simulated by providing estradiol Silastic implants which clamped plasma estradiol levels at 5-20 pg/ml. We demonstrated that in situ aromatization rather than the uptake mechanism is the key determinant of tumor estradiol levels and tumor growth rate under conditions reflecting the postmenopausal state. The importance of intratumoral aromatase was also suggested by the findings that long-term estrogen deprivation increases sensitivity to estradiol and enhances aromatase activity in MCF-7 cells. The results of our in vivo and in vitro studies suggest that complete blockade of in situ aromatization in the breast would provide added benefit to postmenopausal breast cancer patients, especially those who relapse from antiestrogen therapy.
N Harada, S I Honda, and O Hatano
The effects of two steroidal (4-hydroxyandrostenedione and atamestane) and three non-steroidal (fadrozole, vorozole, and pentrozole) aromatase inhibitors on the levels of aromatase mRNA and protein were examined in vitro and in vivo. Immunocytochemistry revealed increased quantities of immunoreactive aromatase in human choriocarcinoma-derived JEG-3 cells in response to pretreatment with the non-steroidal inhibitors. To elucidate this effect in detail, aromatase protein in JEG-3 cells after treatment with various inhibitors was quantified using an enzyme-linked immunosorbent assay (ELISA). A time-dependent increase in aromatase protein in the cells was observed with all the aromatase inhibitors except 4-hydroxyandrostenedione, whereas aromatase mRNA levels in the cells remained unchanged during the inhibitor treatment. The three non-steroidal agents caused an approximately fourfold increase in aromatase protein in the cells 24 h after the treatment, as compared with untreated controls. The increase in aromatase protein in the cells was not blocked by treatment with cycloheximide, an inhibitor of protein synthesis. The inhibitors also appeared to block the rapid degradation observed in JEG-3 cells after induction by forskolin. In vivo, daily injection of the inhibitors into adult female mice caused increases in levels of both aromatase mRNA and protein in the ovary. The increase in aromatase mRNA in this in vivo study could be explained by an increase in gonadotropin concentrations in response to decreased plasma concentrations of estrogens. In conclusion, we suggest that aromatase inhibitors increase aromatase protein through stabilization and reduced protein turnover.