Intrinsic mechanism of estradiol-induced apoptosis in breast cancer cells resistant to estrogen deprivation

Estrogen regulation of apoptosis: how can one hormone stimulate and inhibit?

The link between estrogen and the development and proliferation of breast cancer is well documented. Estrogen stimulates growth and inhibits apoptosis through estrogen receptor-mediated mechanisms in many cell types. Interestingly, there is strong evidence that estrogen induces apoptosis in breast cancer and other cell types. Forty years ago, before the development of tamoxifen, high-dose estrogen was used to induce tumor regression of hormone-dependent breast cancer in post-menopausal women. While the mechanisms by which estrogen induces apoptosis were not completely known, recent evidence from our laboratory and others demonstrates the involvement of the extrinsic (Fas/FasL) and the intrinsic (mitochondria) pathways in this process. We discuss the different apoptotic signaling pathways involved in E2 (17beta-estradiol)-induced apoptosis, including the intrinsic and extrinsic apoptosis pathways, the NF-kappaB (nuclear factor-kappa-B)-mediated survival pathway as well as the PI3K (phosphoinositide 3-kinase)/Akt signaling pathway. Breast cancer cells can also be sensitized to estrogen-induced apoptosis through suppression of glutathione by BSO (L-buthionine sulfoximine). This finding has implications for the control of breast cancer with low-dose estrogen and other targeted therapeutic drugs.

The Paradox of Oestradiol-Induced Breast Cancer Cell Growth and Apoptosis

High dose oestrogen therapy was used as a treatment for postmenopausal patients with breast cancer from the 1950s until the introduction of the safer antioestrogen, tamoxifen in the 1970s. The anti-tumour mechanism of high dose oestrogen therapy remained unknown. There was no enthusiasm to study these signal transduction pathways as oestrogen therapy has almost completely been eliminated from the treatment paradigm. Current use of tamoxifen and the aromatase inhibitors seek to create oestrogen deprivation that prevents the growth of oestrogen stimulated oestrogen receptor (ER) positive breast cancer cells. However, acquired resistance to antihormonal therapy does occur, but it is through investigation of laboratory models that a vulnerability of the cancer cell has been discovered and is being investigated to provide new opportunities in therapy with the potential for discovering new cancer-specific apoptotic drugs. Laboratory models of resistance to raloxifene and tamoxifen, the selective oestrogen receptor modulators (SERMs) and aromatase inhibitors demonstrate an evolution of drug resistance so that after many years of oestrogen deprivation, the ER positive cancer cell reconfigures the survival signal transduction pathways so oestrogen now becomes an apoptotic trigger rather than a survival signal. Current efforts are evaluating the mechanisms of oestrogen-induced apoptosis and how this new biology of oestrogen action can be amplified and enhanced, thereby increasing the value of this therapeutic opportunity for the treatment of breast cancer. Several synergistic approaches to therapeutic enhancement are being advanced which involve drug combinations to impair survival signaling with the use of specific agents and to impair bcl-2 that protects the cancer cell from apoptosis. We highlight the historical understanding of oestrogen's role in cell survival and death and specifically illustrate the progress that has been made in the last five years to understand the mechanisms of oestrogen-induced apoptosis. There are opportunities to harness knowledge from this new signal transduction pathway to discover the precise mechanism of this oestrogen-induced apoptotic trigger. Indeed, the new biology of oestrogen action also has significance for understanding the physiology of bone remodeling. Thus, the pathway has a broad appeal in both physiology and cancer research.

Effective but cost-prohibitive drugs in breast cancer treatment: a clinician's perspective

New pharmacologic treatments for early-stage breast cancer have been proven effective, but many of them are cost prohibitive in low economic settings. Differences in breast cancer mortality rates between developed and developing countries may be because of differences in screening and treatment options, some of which may be unavailable or limited by cost constraints in countries with limited resources. It is well recognized that treatment choices have to be made within budgetary constraints, and treatment guidelines that address the need to stratify treatment options by available resources have been published by the Breast Health Global Initiative. Practical treatment choices need to be made based on the best available cost-effective information. This article reviews new and emerging medical strategies that may improve the cost-effectiveness equation.

Use of global gene expression patterns in mechanistic studies of oestrogen action in MCF7 human breast cancer cells

Over the years, the MCF7 human breast cancer cell line has provided a model system for the study of cellular and molecular mechanisms in oestrogen regulation of cell proliferation and in progression to oestrogen and antioestrogen independent growth. Global gene expression profiling has shown that oestrogen action in MCF7 cells involves the coordinated regulation of hundreds of genes across a wide range of functional groupings and that more genes are downregulated than upregulated. Adaptation to long-term oestrogen deprivation, which results in loss of oestrogen-responsive growth, involves alterations to gene patterns not only at early time points (0-4 weeks) but continuing through to later times (20-55 weeks), and even involves alterations to patterns of oestrogen-regulated gene expression. Only 48% of the genes which were regulated > or =2-fold by oestradiol in oestrogen-responsive cells retained this responsiveness after long-term oestrogen deprivation but other genes developed de novo oestrogen regulation. Long-term exposure to fulvestrant, which resulted in loss of growth inhibition by the antioestrogen, resulted in some very large fold changes in gene expression up to 10,000-fold. Comparison of gene profiles produced by environmental chemicals with oestrogenic properties showed that each ligand gave its own unique expression profile which suggests that environmental oestrogens entering the human breast may give rise to a more complex web of interference in cell function than simply mimicking oestrogen action at inappropriate times.

Potential of l-buthionine sulfoximine to enhance the apoptotic action of estradiol to reverse acquired antihormonal resistance in metastatic breast cancer

L-Buthionine sulfoximine (BSO) is a potent inhibitor of glutathione biosynthesis and studies have shown that it is capable of enhancing the apoptotic effects of several chemotherapeutic agents. Previous studies have shown that long-term antihormonal therapy leads to acquired drug resistance and that estrogen, which is normally a survival signal, is a potent apoptotic agent in these resistant cells. Interestingly, we have developed an antihormone-resistant breast cancer cell line, MCF-7:2A, which is resistant to estrogen-induced apoptosis but has elevated levels of glutathione. In the present study, we examined whether BSO is capable of sensitizing antihormone-resistant MCF-7:2A cells to estrogen-induced apoptosis. Our results showed that treatment of MCF-7:2A cells with 1nM E2 plus 100muM BSO combination for 1 week reduced the growth of these cells by almost 80-90% whereas the individual treatments had no significant effect on growth. TUNEL and 4',6-diamidino-2-phenylindole (DAPI) staining showed that the inhibitory effect of the combination treatment was due to apoptosis. Our data indicates that glutathione participates in retarding apoptosis in antihormone-resistant human breast cancer cells and that depletion of this molecule by BSO may be critical in predisposing resistant cells to estrogen-induced apoptosis.

Application of microfluidic gradient chip in the analysis of lung cancer chemotherapy resistance

The major challenge of chemotherapy is the disease resistance for many lung cancer patients. Integrated microfluidic systems offer many desirable characteristics and can be used in cellular biological analysis. This work aimed to study the correlation between the expression of Glucose Regulated Protein-78 (GRP78) and the resistance to anticancer drug VP-16 in human lung squamous carcinoma cell line SK-MES-1 using an integrated microfluidic gradient chip device. We used A23187, a GRP78 inducer, with a gradient concentration in the upstream network of the device to induce the expression of GRP78 in the cells cultured in the downstream before the addition of VP-16. The expression of GRP78 was detected by immunofluorescence, the apoptosis for the cells treated by VP-16 was assessed morphologically by 4',6-diamidino-2-phenylindole (DAPI) staining. The results indicated that the expressions of GRP78 increased greatly for the cells under the induction of A23187 with a dose-depended manner, while the percentage of apoptotic cells decreased significantly after being treated by VP-16. Our results from this study confirmed the role of GRP78 played in the chemotherapy resistance to VP-16 in SK-MES-1 cell line, suggesting that the integrated microfluidic systems may be an unique approach for characterizing the cellular responses.

Buthionine sulfoximine sensitizes antihormone-resistant human breast cancer cells to estrogen-induced apoptosis

INTRODUCTION

Estrogen deprivation using aromatase inhibitors is one of the standard treatments for postmenopausal women with estrogen receptor (ER)-positive breast cancer. However, one of the consequences of prolonged estrogen suppression is acquired drug resistance. Our group is interested in studying antihormone resistance and has previously reported the development of an estrogen deprived human breast cancer cell line, MCF-7:5C, which undergoes apoptosis in the presence of estradiol. In contrast, another estrogen deprived cell line, MCF-7:2A, appears to have elevated levels of glutathione (GSH) and is resistant to estradiol-induced apoptosis. In the present study, we evaluated whether buthionine sulfoximine (BSO), a potent inhibitor of glutathione (GSH) synthesis, is capable of sensitizing antihormone resistant MCF-7:2A cells to estradiol-induced apoptosis.

METHODS

Estrogen deprived MCF-7:2A cells were treated with 1 nM 17beta-estradiol (E2), 100 microM BSO, or 1 nM E2 + 100 microM BSO combination in vitro, and the effects of these agents on cell growth and apoptosis were evaluated by DNA quantitation assay and annexin V and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) staining. The in vitro results of the MCF-7:2A cell line were further confirmed in vivo in a mouse xenograft model.

RESULTS

Exposure of MCF-7:2A cells to 1 nM E2 plus 100 microM BSO combination for 48 to 96 h produced a sevenfold increase in apoptosis whereas the individual treatments had no significant effect on growth. Induction of apoptosis by the combination treatment of E2 plus BSO was evidenced by changes in Bcl-2 and Bax expression. The combination treatment also markedly increased phosphorylated c-Jun N-terminal kinase (JNK) levels in MCF-7:2A cells and blockade of the JNK pathway attenuated the apoptotic effect of E2 plus BSO. Our in vitro findings corroborated in vivo data from a mouse xenograft model in which daily administration of BSO either as a single agent or in combination with E2 significantly reduced tumor growth of MCF-7:2A cells.

CONCLUSIONS

Our data indicates that GSH participates in retarding apoptosis in antihormone-resistant human breast cancer cells and that depletion of this molecule by BSO may be critical in predisposing resistant cells to E2-induced apoptotic cell death. We suggest that these data may form the basis of improving therapeutic strategies for the treatment of antihormone resistant ER-positive breast cancer.

Sex dimorphism in antitumor response of chemotherapeutic drug cisplatin in a murine host-bearing a T-cell lymphoma

Previously we have demonstrated that in-vivo growth of a murine T-cell lymphoma of spontaneous origin designated as Dalton's lymphoma (DL) shows sex dimorphism (J Rep Immunol 2005; 65:17-32). It remained unclear, however, if DL growth in female and male tumor-bearing hosts also shows a sex-dependent differential susceptibility to the antitumor action of cancer chemotherapeutic drugs. In this study we have demonstrated that in-vivo administration of anticancer drugs: cisplatin or doxorubicin to the DL-bearing host results in a sex-dependent different antitumor activity of the drugs, causing a sex dimorphism in the antitumor response of the drugs with respect to tumor growth inhibition. The antitumor effect of both drugs was found to be better in male tumor-bearing hosts compared with female tumor-bearing hosts. The study also shows that DL cells obtained from male and female tumor-bearing hosts display a differential growth response to following treatment with cisplatin in vitro. Cell growth regulatory proteins: interleukin-2, interferon-gamma, tumor growth factor-beta, p53, caspase-activated DNase, vascular endothelial growth factor, and interleukin-2 receptor were found to be involved in the observed sex-specific response of DL cells to the antitumor action of cisplatin. Moreover, gonadal hormones: androgen, estrogen, and their specific antagonists flutamide and tamoxifen were found to directly modulate the cytotoxicity of cisplatin against DL cells in vitro. This study, therefore, suggests for the first time that the efficacy of cancer chemotherapeutic may vary in a sex-specific manner in a host-bearing a T-cell lymphoma.

Low-dose estrogen therapy to reverse acquired antihormonal resistance in the treatment of breast cancer

Estrogen is a potent stimulus for growth in its target organs: the uterus, vagina, and some estrogen receptor-positive breast cancers. However, estrogen is also able to control menopausal symptoms and maintain bone density in postmenopausal women. Until recently, there was also believed to be a link between estrogen and the prevention of cardiovascular disease. For these reasons, hormone replacement therapy (HRT) with an orally active estrogen and progesterone has been used routinely for more than 50 years to maintain physiologic homeostasis after menopause. Not surprisingly, HRT increases the risk of developing breast cancer. The link between estrogen and breast cancer growth served as the incentive to develop long-term tamoxifen therapy and, subsequently, the aromatase inhibitors (AIs) as successful "anti-estrogenic" treatments. Unfortunately, the consequence of exhaustive therapy is drug resistance. Laboratory studies have defined the evolution of tumor drug resistance to tamoxifen, raloxifene (used for breast and osteoporosis chemoprevention), and the AIs. Remarkably, the long-term exposure of breast cancers to antihormonal therapy also exposes a vulnerability that is being exploited in the clinic. Years of antihormonal therapy alters the cellular response mechanism to estrogen. Normally, estrogen is classified as a survival signal in breast cancer, but in sensitive antihormone-resistant cells, estrogen induces apoptosis. When resistant cells are killed, antihormonal therapy is once again effective. This new targeted approach to the treatment of metastatic breast cancer could open the door to novel approaches to treatment with drug combinations.

Nuclear and extranuclear pathway inputs in the regulation of global gene expression by estrogen receptors

Whereas estrogens exert their effects by binding to nuclear estrogen receptors (ERs) and directly altering target gene transcription, they can also initiate extranuclear signaling through activation of kinase cascades. We have investigated the impact of estrogen-mediated extranuclear-initiated pathways on global gene expression by using estrogen-dendrimer conjugates (EDCs), which because of their charge and size remain outside the nucleus and can only initiate extranuclear signaling. Genome-wide cDNA microarray analysis of MCF-7 breast cancer cells identified a subset of 17beta-estradiol (E2)-regulated genes ( approximately 25%) as EDC responsive. The EDC and E2-elicited increases in gene expression were due to increases in gene transcription, as observed in nuclear run-on assays and RNA polymerase II recruitment and phosphorylation. Treatment with antiestrogen or ERalpha knockdown using small interfering RNA abolished EDC-mediated gene stimulation, whereas GPR30 knockdown or treatment with a GPR30-selective ligand was without effect, indicating ER as the mediator of these gene regulations. Inhibitors of MAPK kinase and c-Src suppressed both E2 and EDC stimulated gene expression. Of note, in chromatin immunoprecipitation assays, EDC was unable to recruit ERalpha to estrogen-responsive regions of regulated genes, whereas ERalpha recruitment by E2 was very effective. These findings suggest that other transcription factors or kinases that are downstream effectors of EDC-initiated extranuclear signaling cascades are recruited to regulatory regions of EDC-responsive genes in order to elicit gene stimulation. This study thus highlights the importance of inputs from both nuclear and extranuclear ER signaling pathways in regulating patterns of gene expression in breast cancer cells.

Overexpression of CEACAM6 promotes migration and invasion of oestrogen-deprived breast cancer cells

Carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) is an intercellular adhesion molecule that is overexpressed in a wide variety of human cancers, including colon, breast and lung and is associated with tumourigenesis, tumour cell adhesion, invasion and metastasis. In this study, we showed that CEACAM6 was overexpressed in a panel of oestrogen receptor (ERalpha)-positive human breast cancer cell lines (MCF-7:5C and MCF-7:2A) that have acquired resistance to oestrogen deprivation, and this overexpression was associated with a more aggressive invasive phenotype in vitro. Expression array analysis revealed that MCF-7:5C and MCF-7:2A cells overexpressed CEACAM6 mRNA by 27-fold and 12-fold, respectively, and were 6-15-times more invasive compared to non-invasive wild-type MCF-7 cells which expressed low levels of CEACAM6. Suppression of CEACAM6 expression using small interfering RNA (siRNA) completely reversed migration and invasion of MCF-7:5C and MCF-7:2A cells and it significantly reduced phosphorylated Akt and c-Src expression in these cells. In conclusion, our findings establish CEACAM6 as a unique mediator of migration and invasion of drug resistant oestrogen-deprived breast cancer cells and suggest that this protein could be an important biomarker of metastasis.

The 38th David A. Karnofsky lecture: the paradoxical actions of estrogen in breast cancer--survival or death?

During the first David A. Karnofsky Award lecture entitled "Thoughts on Chemical Therapy" in 1970, Sir Alexander Haddow commented about the dramatic regressions observed with estrogen in some breast cancers in postmenopausal women, but regrettably the mechanism was unknown. He was concerned that a cancer-specific target would remain elusive, without tests to predict response to therapy. At that time, I was conducting research for my PhD on an obscure group of estrogen derivatives called nonsteroidal antiestrogens. Antiestrogens had failed to fulfill their promise as postcoital contraceptives and were unlikely to be developed further by the pharmaceutical industry. In 1972, that perspective started to change and ICI 46,474 was subsequently reinvented as the first targeted therapy for breast cancer. The scientific strategy of targeting the estrogen receptor (ER) in the tumor, treating patients with long-term adjuvant therapy, examining active metabolites, and considering chemoprevention all translated through clinical trials to clinical practice during the next 35 years. Hundreds of thousands of women now have enhanced survivorship after their diagnosis of ER-positive breast cancer. However, it was the recognition of selective ER modulation (SERM) that created a new dimension in therapeutics. Nonsteroidal antiestrogens selectively turn on or turn off estrogen target tissues throughout the body. Patient care was immediately affected by the recognition in the laboratory that tamoxifen would potentially increase the growth of endometrial cancer during long-term adjuvant therapy. At that time, a failed breast cancer drug, keoxifene, was found to maintain bone density of rats (estrogenic action) while simultaneously preventing mammary carcinogenesis (antiestrogenic action). Perhaps a SERM used to prevent osteoporosis could simultaneously prevent breast cancer? Keoxifene was renamed raloxifene and became the first SERM for the treatment and prevention of osteoporosis as well as the prevention of breast cancer, but without an increase in endometrial cancer. There the story might have ended had the study of antihormone resistance not revealed a vulnerability of cancer cells that could be exploited in the clinic. The evolution of antihormone resistance over years of therapy reconfigures the survival mechanism of the breast cancer cell, so estrogen no longer is a survival signal but a death signal. Remarkably, remaining tumor tissue is again responsive to continuing antihormone therapy. This new discovery is currently being evaluated in clinical trials but it also solves the mystery mechanism of chemical therapy with estrogen noted by Haddow in the first Karnofsky lecture.

Combined estrogen and ghrelin administration decreases expression of p27(kip1) and proportion of isomyosin type I in the striated urethral and anal sphincters and levator ani of old ovariectomized rats

We compared estrogen and/or ghrelin effects on pelvic floor muscles in old versus young adult ovariectomized rats. Ovariectomized Fisher 344 rats (18 and 3 months old, n = 24 x 2) received 42 daily intraperitoneal 17-beta estradiol (10 microg kg(-1)), ghrelin (2 microg kg(-1)), both, or vehicle (n = 6 x 4/group). Cytoplasmic p27(kip1) expression and isomyosin I proportion in striated urethral and anal sphincters and levator ani were measured, respectively, by Western blot analysis and gel electrophoresis with immunohistochemistry of muscle ghrelin receptors and radioimmunoassay of circulating growth hormone. In young adult rats, estrogen significantly decreased cytoplasmic p27(kip1) and isomyosin I signal intensities. In old rats, ghrelin and estrogen/ghrelin significantly decreased both intensities with greater estrogen/ghrelin effect. Ghrelin receptors were not immunostained in any muscle. Estrogen and/or ghrelin significantly increased or decreased, respectively, circulating growth hormone in old and young adult rats. Estrogen/ghrelin administration reversed pelvic floor muscle ageing changes in old ovariectomized rats through growth hormone production.

Selective Estrogen-Receptor Modulators and Antihormonal Resistance in Breast Cancer

Selective estrogen-receptor (ER) modulators (SERMs) are synthetic nonsteroidal compounds that switch on and switch off target sites throughout the body. Tamoxifen, the pioneering SERM, blocks estrogen action by binding to the ER in breast cancers. Tamoxifen has been used ubiquitously in clinical practice during the last 30 years for the treatment of breast cancer and is currently available to reduce the risk of breast cancer in high-risk women. Raloxifene maintains bone density (estrogen-like effect) in postmenopausal osteoporotic women, but at the same time reduces the incidence of breast cancer in both high- and low-risk (osteoporotic) postmenopausal women. Unlike tamoxifen, raloxifene does not increase the incidence of endometrial cancer. Clearly, the simple ER model of estrogen action can no longer be used to explain SERM action at different sites around the body. Instead, a new model has evolved on the basis of the discovery of protein partners that modulate estrogen action at distinct target sites. Coactivators are the principal players that assemble a complex of functional proteins around the ligand ER complex to initiate transcription of a target gene at its promoter site. A promiscuous SERM ER complex creates a stimulatory signal in growth factor receptor–rich breast or endometrial cancer cells. These events cause drug-resistant, SERM-stimulated growth. The sometimes surprising pharmacology of SERMs has resulted in a growing interest in the development of new selective medicines for other members of the nuclear receptor superfamily. This will allow the precise treatment of diseases that was previously considered impossible.

The what, why and how of aromatase inhibitors: hormonal agents for treatment and prevention of breast cancer

The third-generation aromatase inhibitors (AIs) anastrozole, exemestane and letrozole have largely replaced tamoxifen as the preferred treatment for hormone receptor - positive breast cancer in postmenopausal women. Approximately 185,000 new cases of invasive breast cancer are diagnosed yearly, and at least half of these women are both postmenopausal and eligible for adjuvant therapy with AIs. In addition, AIs are currently being tested as primary prevention therapy in large randomised trials involving tens of thousands of women at increased risk for breast cancer. Given the volume of use, internists will increasingly see postmenopausal women who are taking or considering treatment with AIs. Physicians need to be able to: (i) briefly discuss the pros and cons of using a selective estrogen receptor modulator such as tamoxifen or raloxifene vs. an AI for risk reduction and (ii) recognise and manage AI-associated adverse events. The primary purpose of this review is to help internists with these two tasks.

Mechanism of estrogen-induced apoptosis in breast cancer cells: role of the NF-kappaB signaling pathway

The ability of sex steroid hormones to up-regulate the apoptotic signaling proteins is well documented; however, the apoptotic potential of sex hormones is not remarkable and fully compensated by their growth stimulatory action to target cells. In the present study using the long-term cultivation of estrogen-dependent MCF-7 breast cancer cells in steroid-free medium, we have established a cell subline, designed as MCF-7/LS, which was characterized by the resistance to growth stimulatory estradiol action and hypersensitivity to estrogen-induced apoptosis. We have demonstrated that estrogen treatment of the cells does not influence on the level of TNF-R1 or Fas, but dramatically decreases the transcriptional activity of NF-kappaB. Importantly, the MCF-7/LS cells, which are insensitive to growth stimulatory estrogen action, retain the ability to decrease in the NF-kappaB activity in response to estrogen stimulus. Furthermore, the significant increase in the basal (in the absence of ligand) estrogen receptor (ER)-dependent transcriptional activity in the MCF-7/LS cells was revealed and reciprocal transcriptional antagonism between ER and NF-kappaB was demonstrated. Finally, we proved the possible involvement of phosphatidylinositol-3 kinase (PI3K) in the ligand-independent ER activation. In general, the results presented suggest that long-term growth of MCF-7 breast cancer cells in steroid-free medium is accompanied with the increase in the basal ER-dependent transcriptional activity as well as the maintenance of the negative regulatory loop ER-NF-kappaB. The latter may be considered as one of the factors resulting in a disbalance between pro- and anti-apoptotic pathways and enhancement in estrogen apoptotic action in the cells.

Estrogen-regulated gene networks in human breast cancer cells: involvement of E2F1 in the regulation of cell proliferation

Estrogens generally stimulate the proliferation of estrogen receptor (ER)-containing breast cancer cells, but they also suppress proliferation of some ER-positive breast tumors. Using a genome-wide analysis of gene expression in two ER-positive human breast cancer cell lines that differ in their proliferative response to estrogen, we sought to identify genes involved in estrogen-regulated cell proliferation. To this end, we compared the transcriptional profiles of MCF-7 and MDA-MB-231ER+ cells, which have directionally opposite 17beta-estradiol (E2)-dependent proliferation patterns, MCF-7 cells being stimulated and 231ER+ cells suppressed by E2. We identified a set of approximately 70 genes regulated by E2 in both cells, with most being regulated by hormone in an opposite fashion. Using a variety of bioinformatics approaches, we found the E2F binding site to be overrepresented in the potential regulatory regions of many cell cycle-related genes stimulated by estrogen in MCF-7 but inhibited by estrogen in 231ER+ cells. Biochemical analyses confirmed that E2F1 and E2F downstream target genes were increased in MCF-7 and decreased in 231ER+ cells upon estrogen treatment. Furthermore, RNA interference-mediated knockdown of E2F1 blocked estrogen regulation of E2F1 target genes and resulted in loss of estrogen regulation of proliferation. These results demonstrate that regulation by estrogen of E2F1, and subsequently its downstream target genes, is critical for hormone regulation of the proliferative program of these breast cancer cells, and that gene expression profiling combined with bioinformatic analyses of transcription factor binding site enrichment in regulated genes can identify key components associated with nuclear receptor hormonal regulation of important cellular functions.

Development and evolution of therapies targeted to the estrogen receptor for the treatment and prevention of breast cancer

This article describes the origins and evolution of "antiestrogenic" medicines for the treatment and prevention of breast cancer. Developing drugs that target the estrogen receptor (ER) either directly (tamoxifen) or indirectly (aromatase inhibitors) has improved the prognosis of breast cancer and significantly advanced healthcare. The development of the principles for treatment and the success of the concept, in practice, has become a model for molecular medicine and presaged the current testing of numerous targeted therapies for all forms of cancer. The translational research with tamoxifen to target the ER with the appropriate duration (5 years) of adjuvant therapy has contributed to the falling national death rates from breast cancer. Additionally, exploration of the endocrine pharmacology of tamoxifen and related nonsteroidal antiestrogen (e.g. keoxifene now known as raloxifene) resulted in the laboratory recognition of selective ER modulation and the translation of the concept to use raloxifene for the prevention of osteoporosis and breast cancer. However, the extensive evaluation of tamoxifen treatment revealed small but significant side effects such as endometrial cancer, blood clots and the development of acquired resistance. The solution was to develop drugs that targeted the aromatase enzyme specifically to prevent the conversion of androstenedione to estrone and subsequently estradiol. The successful translational research with the suicide inhibitor 4-hydroxyandrostenedione (known as formestane) pioneered the development of a range of oral aromatase inhibitors that are either suicide inhibitors (exemestane) or competitive inhibitors (letrozole and anastrozole) of the aromatase enzyme. Treatment with aromatase inhibitors is proving effective and is associated with reduction in the incidence of endometrial cancer and blood clots when compared with tamoxifen and there is also limited cross resistance so treatment can be sequential. Current clinical trials are addressing the value of aromatase inhibitors as chemopreventive agents for postmenopausal women.

JNK pathway regulates estradiol-induced apoptosis in hormone-dependent human breast cancer cells

Estrogen is known to stimulate breast cancer development in humans. Ironically, high doses of estrogen can induce regression of hormone-dependent breast cancer in postmenopausal women. The mechanism by which estrogen induces tumour regression in breast cancer is still unknown. We found that under low growth-stimulated conditions, high concentrations of 17-beta-estradiol (estradiol) induces apoptosis and concomitantly increases phosphorylation of c-jun in estrogen receptor (ER)-positive breast cancer cell line, MCF-7, but not in ER-negative breast cancer cell line MDA-MB 231 suggesting an ER-mediated event. Interestingly, when the c-jun NH2-terminal kinase (JNK) signalling pathway was disrupted by the JNK inhibitor SP600125, the ability of estradiol to inhibit the growth of MCF-7 cells and to induce apoptosis was completely blocked. These data suggest that JNK plays a central role in mediating the anticancer effect of high concentrations of estradiol in MCF-7 cells. Our data showing the apoptotic effect of estradiol in low growth-stimulated conditions suggest potential implications for the pharmacological control of breast cancer with high dose estrogen in postmenopausal women. Furthermore, our results indicate that augmenting JNK activity could be an efficient novel approach for treating breast cancer.

Emerging principles for the development of resistance to antihormonal therapy: implications for the clinical utility of fulvestrant

We seek to evaluate the clinical consequences of resistance to antihormonal therapy by studying analogous animal xenograft models. Two approaches were taken: (1) MCF-7 tumors were serially transplanted into selective estrogen receptor modulator (SERM)-treated immunocompromised mice to mimic 5 years of SERM treatment. The studies in vivo were designed to replicate the development of acquired resistance to SERMs over years of clinical exposure. (2) MCF-7 cells were cultured long-term under SERM-treated or estrogen withdrawn conditions (to mimic aromatase inhibitors), and then injected into mice to generate endocrine-resistant xenografts. These tumor models have allowed us to define Phase I and Phase II antihormonal resistance according to their responses to E(2) and fulvestrant. Phase I SERM-resistant tumors were growth stimulated in response to estradiol (E(2)), but paradoxically, Phase II SERM and estrogen withdrawn-resistant tumors were growth inhibited by E(2). Fulvestrant did not support growth of Phases I and II SERM-resistant tumors, but did allow growth of Phase II estrogen withdrawn-resistant tumors. Importantly, fulvestrant plus E(2) in Phase II antihormone-resistant tumors reversed the E(2)-induced inhibition and instead resulted in growth stimulation. These data have important clinical implications. Based on these and prior laboratory findings, we propose a clinical strategy for optimal third-line therapy: patients who have responded to and then failed at least two antihormonal treatments may respond favorably to short-term low-dose estrogen due to E(2)-induced apoptosis, followed by treatment with fulvestrant plus an aromatase inhibitor to maintain low tumor burden and avoid a negative interaction between physiologic E(2) and fulvestrant.

Biologic basis of sequential and combination therapies for hormone-responsive breast cancer

Although pharmacologic therapies that reduce or block estrogen signaling are effective treatments of estrogen receptor (ER)-positive breast cancer, acquired resistance to individual drugs can develop. Furthermore, this approach is ineffective as initial therapy for a subgroup of receptor-positive patients. The mechanisms of drug resistance are not completely understood, but the presence of alternative signaling pathways for activating ER response appears to play a significant role. Cross-talk between signaling pathways can activate ERs when conventional ER pathways are blocked or inactivated. For example, signaling via epidermal growth factor or HER-2 receptors, mitogen-activated protein kinases, phosphatidylinositol 3' kinase/protein kinase B, and vascular endothelial growth factor receptor can lead to estrogen-independent stimulation of ERs and tumor growth. The discovery that alternative pathways are involved in estrogen signaling has prompted development of newer endocrine therapies, such as aromatase inhibitors and pure estrogen antagonists, with distinct mechanisms for interrupting signal transduction. The existence of multiple pathways may explain the effectiveness of follow-up therapy with a different class of endocrine agents after failure of prior endocrine treatment. Because they do not have the partial agonist activity of tamoxifen that is enhanced by the adaptive hypersensitivity process, these alternative endocrine agents may play an increasingly important role in the treatment of ER-positive breast cancer. Although optimal sequencing of these agents has not been determined and is continuing to evolve, current evidence allows rational recommendations to be made. The multiple pathways involved in activating ERs also provide a rationale for combining endocrine and non-endocrine therapies that block different signaling pathways, which may have synergistic and overlapping interactions.

Estrogen Receptor Pathway: Resistance to Endocrine Therapy and New Therapeutic Approaches

Endocrine therapy is widely accepted as the most important treatment for all patients with hormone receptor-positive breast cancer. However, despite the positive effect of endocrine therapy on clinical outcome, resistance to these drugs inevitably develops. This article reviews the problem of resistance to hormonal therapy and addresses potential approaches to overcome intrinsic or acquired mechanisms of resistance.

Hormone-replacement therapy influences gene expression profiles and is associated with breast-cancer prognosis: a cohort study

BACKGROUND

Postmenopausal hormone-replacement therapy (HRT) increases breast-cancer risk. The influence of HRT on the biology of the primary tumor, however, is not well understood.

METHODS

We obtained breast-cancer gene expression profiles using Affymetrix human genome U133A arrays. We examined the relationship between HRT-regulated gene profiles, tumor characteristics, and recurrence-free survival in 72 postmenopausal women.

RESULTS

HRT use in patients with estrogen receptor (ER) protein positive tumors (n = 72) was associated with an altered regulation of 276 genes. Expression profiles based on these genes clustered ER-positive tumors into two molecular subclasses, one of which was associated with HRT use and had significantly better recurrence free survival despite lower ER levels. A comparison with external data suggested that gene regulation in tumors associated with HRT was negatively correlated with gene regulation induced by short-term estrogen exposure, but positively correlated with the effect of tamoxifen.

CONCLUSION

Our findings suggest that post-menopausal HRT use is associated with a distinct gene expression profile related to better recurrence-free survival and lower ER protein levels. Tentatively, HRT-associated gene expression in tumors resembles the effect of tamoxifen exposure on MCF-7 cells.

Validating cancer drug targets

A cancer drug target is only truly validated by demonstrating that a given therapeutic agent is clinically effective and acts through the target against which it was designed. Nevertheless, it is desirable to declare an early-stage drug target as 'validated' before investing in a full-scale drug discovery programme dedicated to it. Although the outcome of validation studies can guide cancer research programmes, strictly defined universal validation criteria have not been established.