William L. McGuire Memorial Lecture. Antiestrogens: mechanisms of action and resistance in breast cancer

Altered expression pattern of alternatively spliced estrogen receptor β transcripts in breast carcinoma

Dysregulation of total estrogen receptor beta (ERbeta) expression has been implicated in breast tumorigenesis. The ERbeta gene yields five exon 8 alternatively spliced transcripts (ERbeta1-5), which encode proteins with different C-terminal amino acids. Individual expression analysis of these transcripts may provide new insights into estrogen signaling in breast cancer. We measured mRNA levels of total ERbeta and its five isoforms in normal tissues, breast carcinomas from post-menopausal patients, and breast cancer cell lines by means of real-time reverse transcription-polymerase chain reaction and fluorescent fragment analysis. In various normal human tissues, ERbeta1-5 isoforms displayed different qualitative and quantitative expression patterns that were consistent with previous reports. Total ERbeta mRNA levels were significantly lower in breast tumors than in normal breast tissues (38-fold lower, P < 0.001), mainly due to lower expression of ERbeta1 and ERbeta2 (ERbeta5 expression was similar in the two tissue types). This altered expression pattern of ERbeta isoforms in breast cancer should be taken into account in future ERbeta-based clinical applications.

Reduction of tamoxifen resistance in human breast carcinomas by tamoxifen-containing liposomes in vivo

We investigated whether it is possible to reduce anti-estrogen resistance using liposomally encapsulated tamoxifen in vivo. Small liposomal vesicles containing up to 5.1 mg tamoxifen/ml liposomal suspension, together with an alkylphospholipid to enhance the cellular uptake, were prepared and characterized. Mice transplanted with different tumor models were treated with tamoxifen liposomes administered i.p. or orally as a bolus dose of 50 mg/kg once a week or as a daily dose of 10 mg/kg/day, both during a 4-week period. After orally administered tamoxifen liposomes, tumor growth was significantly reduced for the 3366/tamoxifen (acquired resistance) and for the MCF-7 (inherent resistance) models to 47 and 16%, respectively (treated to control value of relative tumor volume). Intraperitoneal treatment with tamoxifen liposomes revealed similar results. Investigation of biodistribution revealed especially an accumulation of liposomal tamoxifen in MCF-7 tumors and livers of the treated mice. These liposomes had uterotrophic properties comparable to the dissolved compound. This study demonstrates for the first time that a liposomal formulation of tamoxifen was able to induce pharmacological effects and to improve the therapeutic efficacy in several anti-estrogen-resistant xenografts.

From ligand structure to biological activity: modified estratrienes and their estrogenic and antiestrogenic effects in MCF-7 cells

A variety of compounds, including the selective estrogen receptor (ER) modulators tamoxifen and raloxifene, phytoestrogens such as genistein, and xenoestrogens such as bisphenol, bind to the estrogen receptor and elicit biological responses. Structural studies have linked the altered activity of compounds such as 4-hydroxytamoxifen, raloxifene, genistein, and tetrahydrochrysene, which have substantially different structures from estradiol (E2), to differences in the positioning of the critical "helix 12" within the ligand-binding domain (LBD) of the ER-ligand complex. However, subtle permutations of the E2 molecule would also be expected to modulate the pattern of responses within a cell. Forty-two ligands were constructed by the addition or relocation of double bonds, hydroxyl, keto, amino, and nitro substituents throughout the estra-l,3,5(10)-triene (estratriene) ring system. In this review, we summarize the effects of subtle changes in the estratriene molecule on the ability of the receptor complex to stimulate the growth of MCF-7 cells, or affect the expression of four estrogen-regulated genes (progesterone receptor, pS2 protein, cathepsin D, and tissue plasminogen activator), as well as undergo nuclear processing and downregulate ERalpha mRNA. The affinity of these ligands for, and mechanism of their binding with, the ERalpha have been measured, along with their effect on the conformation of the ER-ERE complex. In particular, two A-ring isomers of E2, 2- and 4-hydroxyestratriene-17beta-ol, display gene selective activity within MCF-7 cells which is dependent on complex endogenous promoters, an intact AF-2 and is sensitive to the level of SRC-1. Both of these A-ring isomers function as antiestrogens. Molecular modeling of these two A-ring isomers complexed with the ER ligand-binding domain supports the idea that the conformation of the LBD is affected by subtle changes in the estratriene structure.

A new MCF-7 breast cancer cell line resistant to the arzoxifene metabolite desmethylarzoxifene

The development of resistance in tamoxifen-treated breast cancer patients and the estrogenic side effects of tamoxifen have lead to the design of many new drugs. The new SERM arzoxifene and its active metabolite desmethylarzoxifene (ARZm) inhibits growth of breast cancer cells and has less estrogenic effects than tamoxifen on gene expression. A cell line with acquired resistance to ARZm (MCF-7/ARZm(R)-1) was established from MCF-7 cells. MCF-7/ARZm(R)-1 cells responded to treatment with tamoxifen and the pure antiestrogen ICI 182,7870. The estrogen receptor alpha (ERalpha) level in MCF-7/ARZm(R)-1 cells was lower than in MCF-7 cells due to a destabilization of the receptor by ARZm. A significant reduction in the mRNA and protein level of some estrogen-regulated genes was observed in MCF-7/ARZm(R)-1 compared to MCF-7. However, both the level of the ERalpha and several ER-regulated gene products increased towards parental MCF-7 level upon withdrawal from ARZm, concomitant with an increase in the sensitivity of MCF-7/ARZm(R)-1 cells to ARZm treatment. These data show that ARZm resistant cells remain sensitive to treatment with both tamoxifen and to ICI 182,780. Furthermore, the partial reversion to ARZm sensitivity upon withdrawal of the SERM suggests that patients may benefit from a rechallenge with ARZm.

In vivo models for endocrine-dependent breast carcinomas: special considerations of clinical relevance

Tumours in hormone-regulated organs such as the breast, prostate or ovaries are among the most frequent malignancies. Because of their endocrine-dependent development and growth, they offer a unique opportunity for antihormonal treatment either single or long-term or in combination with radio- or chemotherapy. A prominent example is breast carcinoma, for which the anti-oestrogen tamoxifen has been used successfully for several years. Unfortunately, a substantial number of tumours are intrinsically tamoxifen-resistant, despite oestrogen-receptor positivity, and, eventually, almost all breast carcinomas acquire resistance towards tamoxifen. The recently developed pure anti-oestrogen Faslodex and the third-generation aromatase inhibitors (Letrozol, anastrozole (Arimidex) offer the possibility of alternative therapies. Preclinical models are needed, as most of the mechanisms of hormonal tumour dependence and the causes of the appearance of antihormone resistance are not yet fully understood. This review focuses on the development and characterisation of breast cancer xenografts derived directly from surgical resections. With their help, a deeper insight into the mechanisms of hormone regulation and anti-oestrogen resistance can be gained. The xenograft models have already been used in differential gene-expression analysis on DNA microarrays and for the evaluation of approaches to overcoming tamoxifen resistance.

Relationship between intratumoral expression of genes coding for xenobiotic-metabolizing enzymes and benefit from adjuvant tamoxifen in estrogen receptor alpha-positive postmenopausal breast carcinoma

INTRODUCTION

Little is known of the function and clinical significance of intratumoral dysregulation of xenobiotic-metabolizing enzyme expression in breast cancer. One molecular mechanism proposed to explain tamoxifen resistance is altered tamoxifen metabolism and bioavailability.

METHODS

To test this hypothesis, we used real-time quantitative RT-PCR to quantify the mRNA expression of a large panel of genes coding for the major xenobiotic-metabolizing enzymes (12 phase I enzymes, 12 phase II enzymes and three members of the ABC transporter family) in a small series of normal breast (and liver) tissues, and in estrogen receptor alpha (ERalpha)-negative and ERalpha-positive breast tumors. Relevant genes were further investigated in a well-defined cohort of 97 ERalpha-positive postmenopausal breast cancer patients treated with primary surgery followed by adjuvant tamoxifen alone.

RESULTS

Seven of the 27 genes showed very weak or undetectable expression in both normal and tumoral breast tissues. Among the 20 remaining genes, seven genes (CYP2A6, CYP2B6, FMO5, NAT1, SULT2B1, GSTM3 and ABCC11) showed significantly higher mRNA levels in ERalpha-positive breast tumors than in normal breast tissue, or showed higher mRNA levels in ERalpha-positive breast tumors than in ERalpha-negative breast tumors. In the 97 ERalpha-positive breast tumor series, most alterations of these seven genes corresponded to upregulations as compared with normal breast tissue, with an incidence ranging from 25% (CYP2A6) to 79% (NAT1). Downregulation was rare. CYP2A6, CYP2B6, FMO5 and NAT1 emerged as new putative ERalpha-responsive genes in human breast cancer. Relapse-free survival was longer among patients with FMO5-overexpressing tumors or NAT1-overexpressing tumors (P = 0.0066 and P = 0.000052, respectively), but only NAT1 status retained prognostic significance in Cox multivariate regression analysis (P = 0.0013).

CONCLUSIONS

Taken together, these data point to a role of genes coding for xenobiotic-metabolizing enzymes in breast tumorigenesis, NAT1 being an attractive candidate molecular predictor of antiestrogen responsiveness.

Characterization of new estrogen receptor destabilizing compounds: effects on estrogen-sensitive and tamoxifen-resistant breast cancer

BACKGROUND

Antiestrogens of the selective estrogen receptor modulator (SERM) type, such as tamoxifen, have two major limitations: their mixed agonist and antagonist profile and the development of tumor resistance. We characterized two new pure antiestrogens-ZK-703 and ZK-253-that belong to the class of specific estrogen receptor destabilizers (SERDs), which includes fulvestrant, and compared their activity with that of fulvestrant and tamoxifen.

METHODS

Effects of antiestrogens on the growth of estrogen-dependent breast tumors in vivo were determined using several mouse xenograft models (including the tamoxifen-sensitive tumors MCF7, T47D, and MV3366 and the tamoxifen-resistant tumors ZR75-1 and MCF7/TAM) and chemically induced (nitrosomethyl urea [NMU] and dimethylbenzanthracene [DMBA]) rat breast cancer models (groups of 10 animals). We determined the initial response and effects on hormone receptor levels and the time to relapse after treatment (i.e., time to reach a predetermined tumor size threshold). Estrogen receptor (ER) levels were determined by immunoassay.

RESULTS

ZK-703 (administered subcutaneously) and ZK-253 (administered orally) were more effective than tamoxifen or fulvestrant at inhibiting the growth of ER-positive breast cancer in all xenograft models. For example, MCF7 tumors relapsed (i.e., reached the size threshold) in 10 weeks in mice treated with tamoxifen but in 30 weeks in mice treated with ZK-703. ZK-703 and ZK-253 also prevented further tumor progression in tamoxifen-resistant breast cancer models to a similar extent (more than 30 weeks in mice with ZR75-1 and MCF7/TAM tumors). In the chemically induced rat breast cancer models, orally administered ZK-703 and ZK-253 caused a nearly complete (>80%) inhibition of tumor growth. ER levels were dramatically reduced in MCF7 tumors after 5 weeks of ZK-703 treatment compared with ER levels in vehicle-treated tumors; by contrast, ER levels in tamoxifen-treated tumors were higher than those in control tumors.

CONCLUSION

ZK-703 and ZK-253 are potent, long-term inhibitors of growth in both tamoxifen-sensitive and tamoxifen-resistant breast cancer models.

Therapeutic targeting in the estrogen receptor hormonal pathway

Estrogens work along with genetic changes to promote the development and growth of breast cancers. Because estrogenic hormones act via the estrogen receptors (ERs), ER-alpha and ER-beta, and the ER is present in more than half of breast tumors, this receptor has been the most widely targeted protein in breast cancer therapy. The presence of the ER in breast tumors predicts improved disease-free survival and response to selective ER modulators (SERMs), such as tamoxifen, or other forms of endocrine therapy. Suppression of ER activity by SERMs has proven to be a great benefit in the treatment of breast cancers and also in the prevention of breast cancer in women at high risk for the disease. The Study of Tamoxifen and Raloxifene trial comparing tamoxifen versus raloxifene effectiveness in breast cancer prevention is currently under way. To understand the balance of beneficial and undesirable effects of SERMs and to optimize their effectiveness, current investigations seek to characterize the genes activated or suppressed by these agents. Elucidation of the gene networks and cell signaling pathways under estrogen and SERM regulation and a clearer definition of the respective roles of ER-alpha and ER-beta and their coregulators in the actions of selective ER ligands, should enable the identification of new gene targets for therapeutic intervention and the development of novel drugs for the optimal treatment and prevention of breast cancer.

Ligand-independent activation of estrogen receptor alpha by XBP-1

The estrogen receptor (ER) is a member of a large superfamily of nuclear receptors that regulates the transcription of estrogen-responsive genes. Several recent studies have demonstrated that XBP-1 mRNA expression is associated with ERalpha status in breast tumors. However, the role of XBP-1 in ERalpha signaling remains to be elucidated. More recently, two forms of XBP-1 were identified due to its unconventional splicing. We refer to the spliced and unspliced forms of XBP-1 as XBP-1S and XBP-1U, respectively. Here, we report that XBP-1S and XBP-1U enhanced ERalpha-dependent transcriptional activity in a ligand-independent manner. XBP-1S had stronger activity than XBP-1U. The maximal effects of XBP-1S and XBP-1U on ERalpha transactivation were observed when they were co-expressed with full-length ERalpha. SRC-1, the p160 steroid receptor coactivator family member, synergized with XBP-1S or XBP-1U to potentiate ERalpha activity. XBP-1S and XBP-1U bound to the ERalpha both in vitro and in vivo in a ligand-independent fashion. XBP-1S and XBP-1U interacted with the ERalpha region containing the DNA-binding domain. The ERalpha-interacting regions on XBP-1S and XBP-1U have been mapped to two regions, including the N-terminal basic region leucine zipper domain (bZIP) and the C-terminal activation domain. The bZIP-deleted mutants of XBP-1S and XBP-1U completely abolished ERalpha transactivation by XBP-1S and XBP-1U. These findings suggest that XBP-1S and XBP-1U may directly modulate ERalpha signaling in both the absence and presence of estrogen and, therefore, may play important roles in the proliferation of normal and malignant estrogen-regulated tissues.

Effect of estradiol on estrogen receptor-alpha gene expression and activity can be modulated by the ErbB2/PI 3-K/Akt pathway

Epidermal growth factor (EGF), insulin-like growth factor-I (IGF-I), and heregulin-beta1 (HRG-beta1), can modulate the expression and activity of the estrogen receptor-alpha (ER-alpha) via the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway in the ER-alpha-positive breast cancer cell line, MCF-7. Estradiol can also rapidly activate PI 3-K/Akt in these cells (nongenomic effect). The recent study examines whether Akt is involved in the ER-alpha regulation by estradiol (genomic effect). Stable transfection of parental MCF-7 cells with a dominant-negative Akt mutant, as well as the PI 3-K inhibitors wortmannin and LY 294,002, blocked the effect of estradiol on ER-alpha expression and activity by 70-80 and 55-63%, respectively. Stable transfection of MCF-7 cells with a constitutively active Akt mimicked the effect of estradiol. The changes in ER-alpha expression and activity were abrogated in response to estradiol by an arginine to cysteine mutation in the pleckstrin homology (PH) domain of Akt (R25C), suggesting the involvement of this amino acid in the interaction between Akt and ER-alpha. Experiments employing selective ErbB inhibitors demonstrate that the effect of estradiol on ER-alpha expression and activity is mediated by ErbB2 and not by EGFR. Moreover, anchorage-dependent and -independent growth assays, cell cycle and membrane ruffling analyses showed that Akt exerts estrogen-like activity on cell growth and membrane ruffling and that a selective ErbB2 inhibitor, but not anti-ErbB2 antibodies directed to the extracellular domain, can block these effects. In the presence of constitutively active Akt, tamoxifen only partially inhibits cell growth. In contrast, in cells stably transfected with either a dominant-negative Akt or with R25C-Akt, as well as in parental cells in the presence of a selective ErbB2 inhibitor, the effect of estradiol on anchorage-dependent and -independent cell growth was inhibited by 50-75 and 100%, respectively. Dominant-negative Akt inhibited membrane ruffling by 54%; however, R25C-Akt did not have any effect, suggesting that kinase activity plays an important role in this process. Scatchard analysis demonstrated a 67% reduction in estrogen-binding capacity in cells transfected with constitutively active Akt. No change in binding affinity of estradiol to the receptor was observed upon transfection with either Akt mutant. Taken together, our results suggest that estradiol treatment results in binding to membrane ER-alpha and interaction with a heterodimer containing ErbB2, leading to tyrosine phosphorylation. This results in the activation of PI 3-K and Akt. Akt, in turn, may interact with nuclear ER-alpha, altering its expression and activity.

Interactions between estrogen and growth factor receptors in human breast cancers and the tumor-associated vasculature

Estrogens and growth factors stimulate the proliferation of human breast cancer cells by primary binding and activation of specific receptors that regulate downstream signaling events. Receptors for estrogen are phosphoproteins, and the biologic function of these proteins can be modulated by changes in their phosphorylation state. Signal transduction by growth factor receptors, including HER-2/neu and epidermal growth factor (EGF) receptors, can alter the phosphorylation of estrogen receptor (ER) and the biologic activity of ER-dependent signaling networks both in the presence and in the absence of estrogenic ligands. In addition, both estrogen and growth factor signaling pathways regulate the secretion of vascular endothelial growth factors that stimulate tumor-associated angiogenesis. These molecular interactions significantly impact breast cancer cell growth and survival, and integration of selected signal transduction inhibitors with antiestrogen therapies show promise as a new antitumor treatment strategy that will soon be evaluated in the clinic. Sensitive and reliable assays of estrogen, HER-2/neu, and EGF receptors and tumor-associated angiogenesis will be important biologic factors to consider in the choice of optimal antitumor therapies for patients with breast cancer.

Oestrogen Receptor-Mediated Modulation of the EGFR/MAPK Pathway in Tamoxifen-Resistant MCF-7 Cells

Oestrogen receptor (ER) levels are usually maintained on acquisition of tamoxifen resistance in the clinic, however, tumour re-growth is associated with increased expression of epidermal growth factor receptor (EGFR) and activation of the mitogen activated protein kinase (MAPK) pathway. In the present study we have used the ER down-regulator fulvestrant ('Faslodex') to investigate the influence of the ER on growth of a tamoxifen-resistant (TAM-R) human breast cancer cell line. Expression levels of ER mRNA and protein were equivalent in parental wild-type MCF-7 (WT) and TAM-R cells. Fulvestrant eliminated ER protein expression and inhibited proliferation in both cell lines. The growth inhibitory effects of fulvestrant were associated with a decrease in basal EGFR, c-erbB2 and ERK1/2 activity in TAM-R but not WT cells. ER functionality as determined by oestrogen response element (ERE)-luciferase reporter activity and expression of PgR, pS2 and transforming growth factor alpha (TGFalpha) was significantly reduced in TAM-R compared to WT cells and was further decreased by fulvestrant treatment in both cell lines. Epidermal growth factor (EGF) and TGFalpha significantly increased EGFR/MAPK pathway activity in both cell lines. Ligand-induced EGFR/MAPK activation promoted TAM-R cell growth in both the absence and presence of fulvestrant, whereas no proliferative activity was observed under the same conditions in WT cells. These results suggest that the ER modulates EGFR/MAPK signalling efficiency in TAM-R cells possibly through the regulation of TGFalpha availability. This effect may be overcome by the action of exogenous EGFR ligands, which strengthen EGFR/MAPK signalling activity to generate endocrine-insensitive cell growth.

Modulation of estrogen receptor activity by selective coregulators

The estrogen receptor (ER), a member of the nuclear hormone receptor superfamily, is a hormone-regulated transcription factor that mediates the effects of estrogens and antiestrogens in breast cancer and other estrogen target cells. Because of the role of estrogens in promoting the growth and progression of breast cancer, there is great interest in exploring ways to functionally inactivate the ER, thereby suppressing ER-mediated gene expression and cell proliferation. These approaches have involved the use of antiestrogens such as tamoxifen, dominant negative ERs and, more recently, the use of corepressors. Through the use of two-hybrid screening, we have recently identified a selective repressor of estrogen receptor activity (REA). This protein is recruited to the hormone-occupied ER and selectively represses its transcriptional activity but not the other steroid and non-steroid nuclear receptors. REA also interacts with a protein, prothymosin-alpha (PTalpha), that selectively enhances ER transcriptional activity by recruiting the repressive REA protein away from ER. Analysis of the mechanisms underlying the activities of these two proteins highlights a new role for REA and PTalpha as activity-modulating proteins that confer receptor specificity.

Randomized trial of tamoxifen versus tamoxifen plus aminoglutethimide as adjuvant treatment in postmenopausal breast cancer patients with hormone receptor-positive disease: Austrian breast and colorectal cancer study group trial 6

PURPOSE

To determine whether the addition of aminoglutethimide to tamoxifen is able to improve the outcome in postmenopausal patients with hormone receptor-positive, early-stage breast cancer.

PATIENTS AND METHODS

A total of 2,021 postmenopausal women were randomly assigned to receive either tamoxifen for 5 years alone or tamoxifen in combination with aminoglutethimide (500 mg/d) for the first 2 years of treatment. Tamoxifen was administered at 40 mg/d for the first 2 years and at 20 mg/d for 3 years.

RESULTS

All randomized and eligible patients were included in the analysis according to the intention-to-treat principle. After a median follow-up of 5.3 years, the 5-year disease-free survival in the aminoglutethimide plus tamoxifen group was 83.6% versus 83.7% in the monotherapy group (P =.89). The corresponding data for overall survival at 5 years were 91.4% and 91.2%, respectively (P =.74). More patients failed to complete combination treatment (13.7%) because of side effects as compared to tamoxifen alone (5.2%; P =.0001).

CONCLUSION

Aminoglutethimide given for 2 years in addition to tamoxifen for 5 years does not improve the prognosis of postmenopausal patients with receptor-positive, lymph node-negative or lymph node-positive breast cancer.

Inhibiting Ras signaling in the therapy of breast cancer

Ras is a small guanosine triphosphate-binding protein that plays an important role in signal transduction pathways that influence cellular proliferation, apoptosis, cytoskeletal organization, and other important biological processes. Prenylation of Ras proteins by the enzyme farnesyltransferase renders the protein hydrophobic, causing localization to the inner surface of the cell membrane, where it exerts its biological effects. Ras mutations that result in constitutive activation of the Ras pathway are common in certain human cancers, and transfection of cell lines with mutant Ras renders them tumorigenic. Farnesyltransferase inhibitors (FTIs) were initially developed to inhibit growth of cancers harboring Ras mutations, but preclinical data suggests that they also have antiproliferative effects in cell lines with wild-type Ras. Preclinical data suggest that FTIs have antiproliferative and antitumor effects in breast cancer cell lines, but the precise target(s) remain to be defined. One phase II trial has demonstrated that one orally administered FTI has significant antitumor activity in metastatic breast cancer. In addition, preclinical evidence suggests that FTIs may augment the activity of cytotoxic agents and hormonal therapy. Clinical trials are currently underway evaluating whether these agents have a useful role in the management of advanced breast cancer.

Suppression of estrogen receptor-mediated transcription and cell growth by interaction with TR2 orphan receptor

The transcriptional activity of the estrogen receptor (ER) is known to be highly modulated by the character and amount of coregulator proteins present in the cells. TR2 orphan receptor (TR2), a member of the nuclear receptor superfamily without identified ligands, is found to be expressed in the breast cancer cell lines and to function as a repressor to suppress ER-mediated transcriptional activity. Utilizing an interaction blocker, ER-6 (amino acids 312-340), responsible for TR2 interaction, the suppression of ER by TR2 could be reversed, suggesting that this suppression is conferred by the direct protein-protein interaction. Administration of antisense TR2, resulting in an enhancement of ER transcriptional activity in MCF7 cells, indicates that endogenous TR2 normally suppresses ER-mediated signaling. To gain insights into the molecular mechanism by which TR2 suppresses ER, we found that TR2 could interrupt ER DNA binding via formation of an ER-TR2 heterodimer that disrupted the ER homodimerization. The suppression of ER transcription by TR2 consequently caused the inhibition of estrogen-induced cell growth and G(1)/S transition in estrogen-dependent breast cancer cells. Taken together in addition to the potential roles in spermatogenesis and neurogenesis, our data provide a novel biological function of TR2 that may exert an important repressor in regulating ER activity in mammary glands.

Chemoprevention of breast cancer: implications for postmenopausal women

Estrogen administration is associated with reduction in perimenopausal symptoms and the risk for several conditions affecting postmenopausal women. As estrogen administration also increases the risk for breast cancer, a common dilemma facing many women and their physicians is whether to use estrogen replacement therapy (ERT), a selective estrogen receptor modulator (SERM) that antagonises estrogenic effects in breast tissue but retains some estrogen agonist properties in other organs, or neither. For women with average to moderate risk of breast cancer and with perimenopausal symptoms, ERT may be the best short-term choice. For very high-risk women (>1% per year) with menopausal symptoms, alternatives to ERT might be offered and tried first. A diagnosis of ductal carcinoma in situ or invasive breast cancer within the last 2 to 5 years should be considered a relative contraindication for ERT unless the tumour was estrogen receptor negative. High-risk women without menopausal symptoms are the best candidates for the only currently approved drug for breast cancer risk reduction, tamoxifen. Although the drug is approved for women with a 5-year risk of breast cancer > or = 1.7% (0.34% per year), postmenopausal women most likely to experience a favourable benefit/risk ratio are those with a Gail estimated risk of >0.5% per year without a uterus or >1% per year if they retain their uterus. Tamoxifen should not be used in women with prior history of thromboembolic or precancerous uterine conditions. Tamoxifen is often used in Europe in conjunction with transdermal ERT in hysterectomised women without obvious loss of efficacy or increased risk of thromboembolism. Raloxifene is a second generation SERM with estrogen-like agonist effects on bone but with less uterine estrogen agonist activity than tamoxifen. Raloxifene may have less potent breast antiestrogenic effects than tamoxifen, particularly in a moderate- to high-estrogen environment. Raloxifene is approved for use in reducing risk of osteoporosis, but not breast cancer. Whether it is as effective as tamoxifen in reducing breast cancer risk in postmenopausal women is the subject of a current trial. All women regardless of breast cancer risk are advised to employ nonpharmacological risk reduction measures, including normalisation of bodyweight, exercise, adequate calcium and vitamin D intake, and avoidance of smoking and alcohol. The preventive options are best weighed during an individualised consultation where a woman's menopausal symptoms and risk for breast cancer and other diseases can be examined, and the options for improving postmenopausal health can be discussed.

Tamoxifen resistance in breast cancer: elucidating mechanisms

Tamoxifen has been used for the systemic treatment of patients with breast cancer for nearly three decades. Treatment success is primarily dependent on the presence of the estrogen receptor (ER) in the breast carcinoma. While about half of patients with advanced ER-positive disease immediately fail to respond to tamoxifen, in the responding patients the disease ultimately progresses to a resistant phenotype. The possible causes for intrinsic and acquired resistance have been attributed to the pharmacology of tamoxifen, alterations in the structure and function of the ER, the interactions with the tumour environment and genetic alterations in the tumour cells. So far no prominent mechanism leading to resistance has been identified. The recent results of a functional screen for breast cancer antiestrogen resis- tance (BCAR) genes responsible for development of tamoxifen resistance in human breast cancer cells are reviewed. Individual BCAR genes can transform estrogen-dependent breast cancer cells into estrogen-independent and tamoxifen-resistant cells in vitro. Furthermore, high levels of BCAR1/pl30Cas protein in ER-positive primary breast tumours are associated with intrinsic resistance to tamoxifen treatment. These results indicate a prominent role for alternative growth control pathways independent of ER signalling in intrinsic tamoxifen resistance of ER-positive breast carcinomas. Deciphering the differentiation characteristics of normal and malignant breast epithelial cells with respect to proliferation control and regulation of cell death (apoptosis) is essential for understanding therapy response and development of resistance of breast carcinoma.

A negative coregulator for the human ER

ERalpha is a ligand-activated transcription factor and a key regulator of the processes involved in cellular proliferation and differentiation. In addition, aberrant ERalpha activity is linked to several pathological conditions including breast cancer. A complex network of coregulatory proteins is largely believed to determine the transcriptional activity of ERalpha. We report here the isolation of a protein, denoted RTA for repressor of tamoxifen transcriptional activity, which contains an RNA recognition motif and interacts with the receptor N-terminal activation domain. RTA interacts with RNA in vitro, and its overexpression inhibits the partial agonist activity manifest by the antiestrogen tamoxifen while minimally affecting E2-activated transcription. Mutation of the RNA recognition motif alters RNA binding specificity and results in a dominant negative form of RTA that leads to derepression of ERalpha transcriptional activity, allowing all classes of antiestrogens to manifest partial agonist activity and enhancing agonist efficacy. These findings suggest a role for RNA binding proteins as coregulatory factors of the nuclear receptor family and reveal a novel mechanism by which antiestrogens can manifest agonist activities in some tissues.

Epidermal growth factor receptor (EGFR) transactivation by estrogen via the G-protein-coupled receptor, GPR30: a novel signaling pathway with potential significance for breast cancer

The biological and biochemical effects of estrogen have been ascribed to its known receptors, which function as ligand-inducible transcription factors. However, estrogen also triggers rapid activation of classical second messengers (cAMP, calcium, and inositol triphosphate) and stimulation of intracellular signaling cascades mitogen-activated protein kinase (MAP K), PI3K and eNOS. These latter events are commonly activated by membrane receptors that either possess intrinsic tyrosine kinase activity or couple to heterotrimeric G-proteins. We have shown that estrogen transactivates the epidermal growth factor receptor (EGFR) to MAP K signaling axis via the G-protein-coupled receptor (GPCR), GPR30, through the release of surface-bound proHB-EGF from estrogen receptor (ER)-negative human breast cancer cells [Molecular Endocrinology 14 (2000) 1649]. This finding is consistent with a growing body of evidence suggesting that transactivation of EGFRs by GPCRs is a recurrent theme in cell signaling. GPCR-mediated transactivation of EGFRs by estrogen provides a previously unappreciated mechanism of cross-talk between estrogen and serum growth factors, and explains prior data reporting the EGF-like effects of estrogen. This novel mechanism by which estrogen activates growth factor-dependent signaling and its implications for breast cancer biology are discussed further in this review.

Novel estrogen and tamoxifen induced genes identified by SAGE (Serial Analysis of Gene Expression)

The breast cancer promoting effects of estrogen and the chemopreventive effects of tamoxifen are thought to be mediated by the estrogen receptor, a ligand-dependent transcription factor. Therefore, comprehensive analysis of gene expression profiles following estrogen or tamoxifen treatment may help us better understand the role estrogen plays in tumorigenesis. We utilized SAGE (Serial Analysis of Gene Expression) technology to identify genes regulated by estrogen and tamoxifen in the ZR75-1 estrogen dependent breast cancer cell line. In this manner we have identified several genes that were regulated by estrogen or tamoxifen. Here we report the identification and initial characterization of EIT-6 (Estrogen Induced Tag-6), a novel nuclear protein and a new member of the evolutionarily conserved SM-20 family of growth regulatory immediate-early genes. EIT-6 appears to be a direct transcriptional target of the estrogen receptor and constitutive expression of EIT-6 promotes colony growth in human breast cancer cells. These data indicate that EIT-6 may play a role in estrogen induced cell growth.

DNA methylation of nuclear receptor genes--possible role in malignancy

The members of the nuclear receptor superfamily are known to mediate a wide array of basic biological processes, such as regulation of cell growth and differentiation, and induction of apoptosis. In several human malignancies, this central control function of nuclear receptors is disturbed, which seems to play an important role in tumor development and progression. Many nuclear receptor genes have been reported to be downregulated in malignancies; however, only a few mutations, gene arrangements, deletions or similar genetic changes have been shown to occur in these tumors. During the last decade, increasing attention has been directed towards epigenetic mechanisms of gene regulation such as DNA methylation. Many nuclear receptor genes can be silenced through aberrant methylation in tumors; epigenetic silencing, therefore, represents an additional mechanism that modifies expression of key genes during carcinogenesis. This review will give insights into the role of DNA methylation in the silencing of nuclear receptor genes and its involvement in human malignancies.

Natural modulators of estrogen biosynthesis and function as chemopreventive agents

There is clearly a need for novel breast cancer chemopreventive agents with enhanced potency and specificity with little or no side effects. To this end, several new chemical moieties have been synthesized or isolated from natural sources. In this review, we have described some agents currently in use or under development for treatment or prevention of breast cancer, as well as our own strategies for the discovery of natural product modulators of estrogen biosynthesis and function. In particular, bioassay-guided fractionation of active plant extracts is a unique method for identifying agents with novel mechanisms of action, some of which should be useful for prevention of human cancer. Further, with the advent of combinatorial chemistry and high throughput screening, even greater progress may now be expected with natural product leads.

A vision for the future?

Systemic adjuvant therapy is recommended immediately following surgical removal of the primary tumour in the majority of patients with early breast cancer, to prevent the recurrence of distant metastases. Significant progress has been made in the development and evaluation of endocrine therapies for systemic adjuvant therapy. In pre- and perimenopausal women, ovarian ablation has proven to be a valuable treatment option, though not always desirable for young patients. Thus, reversible medical ovarian suppression with a luteinizing hormone releasing hormone agonist, such as goserelin (Zoladex), may provide an attractive alternative for such patients. International trials have indicated that goserelin provides an important addition to the choice of adjuvant therapies now available to pre- and perimenopausal patients. For postmenopausal patients, it is hoped that the ATAC (Arimidex, tamoxifen, alone or in combination) trial will reveal whether or not the benefits of anastrozole (Arimidex) observed in advanced disease, where it has proven to be well tolerated and at least as effective as tamoxifen in recent trials, will translate to the early setting to provide further management options for these patients. On the horizon is yet another exciting endocrine agent, ICI 182,780 (Fulvestrant), which has also been shown to be as effective as anastrozole in advanced disease. In terms of the future, these agents are likely to provide additional valuable treatment choices for early breast cancer across the patient spectrum.

Evaluation of monoclonal antibody MIB-1 in the mammary epithelium adjacent to fibroadenomas in premenopausal women treated with tamoxifen

The purpose of this study was to study the monoclonal antibody MIB-1 in the normal breast epithelium adjacent to a fibroadenoma in women in the luteal phase of the menstrual cycle who were treated with tamoxifen at doses of 10 and 20 mg for 22 days. The proliferative activity of the mammary epithelium adjacent to the fibroadenoma was studied by immunohistochemistry on the basis of the monoclonal antibody MIB-1 (Immunotech, catalog No. 0505, lot 001). The study was randomized and double blind and was conducted on 44 women with fibroadenomas divided into three groups: A (n=16, placebo), B (n=15, tamoxifen, 10 mg), and C (n=13, tamoxifen, 20 mg). Tamoxifen was administered for 22 days starting on the 2nd day of the menstrual cycle, and a biopsy was taken on the 23rd day. Serum estradiol, progesterone, sex hormone binding globulin, follicle-stimulating hormone, luteinizing hormone, and prolactin were measured before treatment (21st and 24th day of the previous menstrual cycle) and on the day of the biopsy. The mean percentage of stained nuclei per 1,000 cells was 9.2 in group A, 4.5 in group B, and 3.2 in group C. The Fisher's test revealed that tamoxifen significantly reduced MIB-1 at doses of 10 and 20 mg compared with the placebo group (p < 0.0001), with no significant differences between doses in terms of proliferative activity (p=0.21). Groups B and C presented a significant increase in progesterone (p=0.038), estradiol (p < 0.001), and sex hormone binding globulin (p=0.001) levels. Elevation of serum follicle-stimulating hormone concentration (p=0.0045) and a fall in prolactin levels (p=0.0055) were observed. We conclude that tamoxifen significantly reduced the proliferative activity of the mammary epithelium at the doses of 10 and 20 mg/day.

Hypoxia reduces hormone responsiveness of human breast cancer cells

Resistance to hormonal therapy frequently occurs following successful treatment in breast cancer. The mechanism responsible for this acquired resistance is still unknown. It has been suggested that a hypoxic tumor microenvironment promotes malignant progression of cancer, i.e., hypoxia may promote estrogen-independent growth (a more malignant phenotype) of breast cancer. To clarify this hypothesis, the effects of hypoxia on the growth responses to hormonal agents and the expression levels of estrogen receptor (ER)-alpha and progesterone receptor (PgR) were investigated in two human breast cancer cell lines, ML-20 and KPL-1. The expression level of ER-alpha was significantly decreased by hypoxia (1% O(2)) in a time-dependent manner in both cell lines. Hypoxia also significantly reduced the growth-promoting effect of estradiol (E2) and the growth-inhibitory effects of an antiestrogen, ICI 182 780, and a progestin, medroxyprogesterone acetate, in both cell lines. In addition, hypoxia markedly suppressed the induction of PgR mRNA and protein by E2 in both cell lines. To clarify further the effect of hypoxia on ER-alpha expression, the expression levels of hypoxia-inducible factor-1 alpha (HIF-1 alpha), a marker of hypoxia and ER-alpha were immunohistochemically examined in 36 breast cancer specimens. ER-alpha expression (both its proportion and intensity) was significantly lower in nuclear HIF-1 alpha-positive tumors than in negative tumors. These findings indicate that hypoxia down-regulates ER-alpha expression as well as ER-alpha function in breast cancer cells. These processes may lead to an acquired resistance to hormonal therapy in breast cancer.

IGF status is altered by tamoxifen in patients with breast cancer

AIMS

An increased concentration of insulin-like growth factor 1 (IGF-1) is an independent risk factor for premenopausal breast cancer. Tamoxifen is thought initially to reduce concentrations of IGF-1 and increase concentrations of the IGF binding proteins. The aim of this study was to compare concentrations of IGF-1, IGF binding protein 1 (IGF-BP1), and IGF-BP3 in patients with breast cancer (n = 14) with those seen in control subjects (n = 23) and to assess the effect of tamoxifen on IGF status in these patients.

METHODS

Non-fasting blood samples were collected from patients with breast cancer before surgery and after nine, 18, and 27 months of tamoxifen treatment. The baseline concentrations were compared with those of age and sex matched healthy control subjects.

RESULTS

IGF-1, IGF-BP3, and IGF-BP1 concentrations were not significantly different in cases and controls. Tamoxifen treatment significantly increased IGF-BP1 after 18 and 27 months (baseline: mean, 21.6 ng/ml; SD, 16.6; 18 months: mean, 52.0 ng/ml; SD, 41.8; p = 0.019; 27 months: mean, 40.7 ng/ml; SD, 24.9; p = 0.043) and IGF-BP3 after nine, 18, and 27 months (baseline: mean, 3119 ng/ml; SD, 507; nine months: mean, 3673 ng/ml; SD, 476; p = 0.004; 18 months: mean, 3445 ng/ml; SD, 634; p = 0.034; 27 months: 3409 ng/ml; SD, 501; p = 0.043) when compared with baseline values. IGF-1 was not altered significantly from baseline at any time point. However, the IGF-1 to IGF-BP3 ratio was significantly decreased at both nine and 18 months (baseline: mean, 0.058; SD, 0.014; nine months: mean, 0.039; SD, 0.008; p = 0.033; 18 months: mean, 0.044; SD, 0.012; p = 0.01). This ratio was not significantly different from baseline at 27 months (mean, 0.054; SD, 0.01; p = 0.08).

CONCLUSIONS

Tamoxifen increases IGF-BP3 and IGF-BP1 concentrations. It also decreases the IGF-1 to IGF-BP3 ratio but this effect may be limited after long term use. Longer follow up, with larger numbers of patients, should determine when, and for how long, tamoxifen can reduce circulating IGF-1.

Chemoprevention for high-risk women: tamoxifen and beyond

The demonstration by the National Surgical Adjuvant Breast Project (NSABP) that 5 years of tamoxifen therapy is associated with an approximate 50% reduction in breast cancer incidence in high-risk women was a milestone in breast cancer prevention. Because tamoxifen is associated with increased risk of side-effects such as hot flashes, menstrual abnormalities, uterine cancer, and thromboembolic phenomena, its use will not be advisable or acceptable for all high-risk women. Women over 50 years of age appear to be at highest risk for serious adverse events, such as uterine cancer and thromboembolic phenomena. Individuals in whom tamoxifen-associated breast cancer risk reduction appears to outweigh risk of serious side-effects include women with prior in situ or estrogen receptor (ER)-positive invasive cancer, atypical hyperplasia, and/or women ages 35-49 with a calculated Gail 5-year risk of > or =1.7%, hysterectomized women aged 50 and older with a 5-year Gail risk of > or =2.5%, and nonhysterectomized women aged 50 and older with a 5-year Gail risk of >5.0%. It is not yet clear whether tamoxifen can reduce breast cancer incidence in women with BRCA1 and BRCA2 mutations, although preliminary evidence favors benefit for at least those with a BRCA2 mutation. Raloxifene is a selective ER modulator with less uterine estrogen agonist activity than tamoxifen, and it is hoped that it will result in fewer uterine cancers but will be equally efficacious in reducing the risk of breast cancer. The NSABP is currently conducting a randomized study of tamoxifen versus raloxifene in high-risk postmenopausal women. Approximately one third of invasive cancers are ER negative. Tamoxifen does not reduce the incidence of ER-negative cancers, nor does it appear to be effective in preventing the appearance of one third of ER-positive cancers. Priorities in prevention research are to develop (a) biomarkers to refine short-term risk assessments based on epidemiologic models, (b) biomarkers predictive of response to specific classes of preventive agents, (c) drugs with fewer side-effects and/or effective in ER-negative or ER-positive tamoxifen-resistant precancerous disease, and (d) efficient clinical trial models to assess new agent efficacy. Breast intraepithelial neoplasia (IEN) may be sampled by minimally invasive techniques and is an attractive short-term risk biomarker. Molecular abnormalities observed in IEN may be used to select potential agents for testing/therapy, and modulation of these abnormalities may be used in phase I trials to select appropriate doses and in phase II trials to assess response. Breast density volume and certain serum markers such as insulin-like growth factor-1 are also being studied as potential risk and response biomarkers. Reversal or prevention of advanced IEN as well as modulation of other risk biomarkers in randomized phase II and phase III trials is being evaluated as a means of more efficiently evaluating prevention drugs in the future. A number of agents are being developed that target molecular abnormalities in IEN, have fewer or different side effects than tamoxifen, and may be effective in ER-negative or tamoxifen-resistant disease.

Biomonitoring of urinary tamoxifen and its metabolites from breast cancer patients using nonaqueous capillary electrophoresis with electrospray mass spectrometry

Tamoxifen is an antiestrogen drug used to treat breast cancer. We have extracted tamoxifen and several of its metabolites from urine of patients with both metastatic (stage IV) and locally confined (stages I, II, and III) breast cancer. Analysis of these metabolites was performed by nonaqueous capillary electrophoresis with electrospray-mass spectrometry. Peak heights from extracted ion current electropherograms of the metabolites were used to establish a metabolic profile for each patient. We demonstrate substantial variation among patient profiles, statistically significant differences in the amount of urinary tamoxifen N-oxide found in stages I, II, and III compared to stage IV breast cancer patients, and statistically significant differences in the amount of 3,4-dihydroxytamoxifen found in progressors compared to nonprogressors with metastatic (stage IV) cancer.

Regulation of prothymosin alpha gene expression by estrogen in estrogen receptor-containing breast cancer cells via upstream half-palindromic estrogen response element motifs

Prothymosin alpha (PTalpha), a protein associated with cell proliferation and chromatin remodeling, and found to selectively enhance ER transcriptional activity by interacting with a repressor of ER activity, is shown to be a primary response gene to estrogen. Prothymosin alpha mRNA was rapidly increased by estrogen, followed by a 6-fold increase in prothymosin alpha protein content in ER-containing breast cancer cells. Analysis of the prothymosin alpha promoter and 5'-flanking region, and electrophoretic gel mobility shift studies showed the strong inducibility by the estradiol-ER complex to be mediated by two consensus half-palindromic estrogen response elements at -750 and -1051, which directly bind the ER. Estrogenic stimulation of prothymosin alpha required a DNA binding form of ER with a functional activation function-2 domain. The prothymosin alpha 5'-regulatory region also contains multiple Sp1 sites. Although addition of Sp1 did not further enhance estradiol-ER stimulated prothymosin alpha transcriptional activity in breast cancer cells, transfection and response element mutagenesis studies using Drosophila cells, which are deficient in Sp1, revealed that Sp1 and the estradiol occupied-ER can each activate the prothymosin alpha gene independently of the other and act in an additive manner. These observations, documenting robust prothymosin alpha up-regulation by the estradiol-ER complex via widely spaced half-palindromic estrogen response element motifs, are reminiscent of those shown previously for the ovalbumin gene and suggest that the use of multiple half response elements may be a more common mode for regulation of gene expression by the ER than previously appreciated. In addition, these observations suggest interrelationships between cell proliferation and gene transcriptional activities and indicate a positive mechanism by which PTalpha, which increases ER transcriptional effectiveness, is itself up-regulated by the estrogen-ER complex.

Identification of an estrogen-inducible phosphatase (PP5) that converts MCF-7 human breast carcinoma cells into an estrogen-independent phenotype when expressed constitutively

The proliferation of many estrogen receptor (ER)-positive breast cancer cells depends on estradiol, and tumors arising from these cells are often responsive initially to treatment with selective ER modulators, which produce an antiestrogen effect. However, tumors that are refractory to the antiestrogenic effects of selective ER modulators often reemerge, and the prognosis for these patients is poor because of the lack of additional effective therapy. Accordingly, deciphering the cellular events associated with estrogen-dependent growth and the subsequent outgrowth of tumors with an estrogen-independent phenotype is of considerable interest. Here we show that the expression of PP5, an evolutionarily conserved Ser/Thr phosphatase that functions as an inhibitor of glucocorticoid- and p53-induced signaling cascades leading to growth suppression, is responsive to 17beta-estradiol (E(2)) in ER-positive human breast carcinoma cells (MCF-7). Northern analysis revealed that E(2)-induced PP5 expression is blocked by treatment with tamoxifen, and a consensus ER recognition element was identified in the PP5 promoter. The PP5-ER recognition element associates with human ERs and confers E(2)-induced transcriptional activation to reporter plasmids. The specific inhibition of PP5 expression ablates E(2)-mediated proliferation in MCF-7 cells without having an apparent effect on E(2)-induced expression of c-myc or cyclin D1. Thus, although critical for cell growth, PP5 likely acts either downstream or independently of c-Myc and Cyclin D1. To further characterize the role of PP5 in E(2)-regulated growth control, we constructed stable MCF-7 cell lines in which the expression of PP5 was placed under the control of tetracycline-regulated transactivator and operator plasmids. Studies with these cells revealed that the constitutive overexpression of PP5 affords E(2)-dependent MCF-7 cells with the ability to proliferate in E(2)-depleted media. Together, these studies indicate that E(2)-induced PP5 expression functions to enhance E(2)-initiated signaling cascades leading to cell division and that aberrant PP5 expression may contribute to the development of MCF-7 cells with an estrogen-independent phenotype.

Human estrogen receptor beta-specific monoclonal antibodies: characterization and use in studies of estrogen receptor beta protein expression in reproductive tissues

Investigation of the role of the second, more recently described estrogen receptor, denoted ERbeta, will be critical in understanding the molecular mechanisms underlying tissue-specific gene regulation by estrogens. Expression of ERbeta in a variety of tissues has been examined predominantly at the mRNA level, and there is little information regarding the cellular localization and size of the endogenous ERbeta protein, due, in part, to the limited availability of human ERbeta-specific antibodies. Thus, our aim was to generate specific antibodies to human ERbeta and use them to determine the tissue-specific distribution and size(s) of the ERbeta protein. To this end, we have cloned three different hybridoma cell lines that produce monoclonal antibodies specific for the hormone-binding domain of human ERbeta. The antibodies, made in mice against human ERbeta amino acids 256-505 (hormone binding domain lacking the F domain), are designated CFK-E12 (E12), CMK-A9 (A9) and CWK-F12 (F12) and were determined to be the IgG gamma1 isotype for E12, and IgG gamma2b for A9 and F12. All three monoclonal antibodies could be used to detect in vitro translated, baculovirus expressed, and cell transfected and expressed ERbeta protein by Western blot analyses, and all failed to detect ERalpha. A9 and F12 were able to immunoprecipitate efficiently the native form of ERbeta protein in the presence and absence of estradiol. Epitope mapping studies indicate that the E12 and F12 antibodies recognize overlapping peptide sequences in the N-terminal region of the hormone-binding domain, a region that is highly conserved among species. Immunocytochemical studies with these antibodies reveal nuclear-specific localization of the ERbeta protein in granulosa cells of the rat ovary. Nuclear ERbeta is also specifically localized in epithelial and some stromal cells of mouse and rat epididymis. Western blot analysis with protein extracts from ovarian granulosa cells of human, rat, mouse, and pig showed a ca. 52 kDa and an additional ca. 62-64 kDa band in these species. These results indicate the presence of two predominant molecular size forms of the ERbeta protein in ovarian granulosa cells and demonstrate the utility of these antibodies for detection of ERbeta in the human and in several other mammalian species.

Enhanced epidermal growth factor receptor signaling in MCF7 breast cancer cells after long-term culture in the presence of the pure antiestrogen ICI 182,780 (Faslodex)

This paper describes the establishment of an antiestrogen-resistant MCF7 breast cancer cell subline (FASMCF) by continuous culture of the estrogen-responsive parental line in steroid-depleted, ICI 182,780 (Faslodex; 10(-7) M)-supplemented medium. After a 3-month period of growth suppression, cells began to proliferate in ICI 182,780 at rates similar to those of untreated wild-type cells. Immunocytochemistry showed these cells to have reduced estrogen receptor and an absence of progesterone receptor proteins. RT-PCR and transient transfection studies with estrogen response element-reporter constructs confirmed that ICI 182,780-suppressed estrogen response element-mediated signaling. FASMCF cells show increased dependence upon epidermal growth factor receptor (EgfR)/mitogen-activated protein kinase (MAPK)-mediated signaling. Thus, EgfR protein and messenger RNA, growth responses to transforming growth factor-alpha, and extracellular signal-regulated kinase 1/2 MAPK activation levels are all increased. Unlike wild-type cells, FASMCF cells are highly sensitive to growth inhibition by an EgfR-specific tyrosine-kinase inhibitor (TKI), ZD1839 (Iressa), and an inhibitor of the activation of MEK1 (MAPKK), PD098059. Short-term ( approximately 3 weeks) withdrawal of cells from antiestrogen had no effect on growth or phenotype, whereas longer withdrawal (>10 weeks) appeared to partially reverse the cellular phenotype with increasing estrogen receptor and decreasing EgfR levels. In subsequent studies FASMCF cells were maintained in TKI, where their growth was again suppressed and secondary TKI resistance failed to develop within the 3-month period in which initial ICI 182,780 resistance arose. Furthermore, wild-type cells similarly maintained in combination ICI 182,780 and TKI treatment conditions remained growth arrested (>6 months), with notable cell loss through both reduced rates of cellular proliferation and increased cell death.

Increase in the stability and helical content of estrogen receptor alpha in the presence of the estrogen response element: analysis by circular dichroism spectroscopy

Ligand-dependent stabilization of the estrogen receptor (ER) is often postulated, with limited support from experimental data. We studied the thermal unfolding of recombinant ERalpha by circular dichroism (CD) spectroscopy. The T(M) of unfolding of ERalpha was 38 +/- 2.4 degrees C, and the van't Hoff enthalpy of unfolding was 31.7 +/- 3.4 kcal/mol in the absence of ligands. Addition of estradiol (E(2)) increased the T(M) to 43.6 +/- 2.3 degrees C, while addition of E(2) and an oligonucleotide harboring the estrogen response element (ERE) increased the T(M) to 47.9 +/- 1.6 degrees C. Addition of the antiestrogen 4-hydroxytamoxifen (HT) alone did not increase the T(M); however, a combination of HT and the ERE increased the T(M) to 48.9 +/- 1.0 degrees C. The ERE alone increased the T(M) to 46.1 +/- 0.9 degrees C. Addition of E(2) alone had no effect on the apparent enthalpy of unfolding; however, the ERE alone increased the apparent enthalpy from 31.7 to 36.1 kcal/mol. ERalpha samples containing the ERE also exhibited an increase in the negative ellipticity at 208 and 222 nm, relative to that of ligand-free ERalpha, suggesting a stabilization of the alpha-helix. CD data analysis further showed that the presence of the ERE caused a large increase in alpha-helical content of ERalpha in both the presence and absence of the ligands. This increase in alpha-helical content of ERalpha was not observed in the presence of a nonspecific oligonucleotide. These results show that the ERE can increase the thermal stability of ERalpha, enhance its alpha-helical content, and facilitate the cooperativity of the folding transition.

ICI 182,780 acts as a partial agonist and antagonist of estradiol effects in specific cells of the sheep uterus

We assessed the ability of ICI 182,780 (ICI) to block the estradiol (E2) responses of genes within the sheep uterus. Ovariectomized ewes in the 'ICI+E2' treatment group received a uterine infusion with 10(-7) M ICI for 14 h, an injection of 50 microg E2 6 h after the infusion started, and were hysterectomized 18 h postinjection. Other groups received only ICI or E2, or neither treatment ('Con'). Both E2 and ICI increased the wet weight of dissected endometrium: averaging 10.0+/-1.2 g for ICI+E2, ICI, and E2 groups compared to 6.8+/-0.6 g for Con. Slot blot analyses of endometrial RNA showed that estrogen receptor-alpha (ER), progesterone receptor (PR), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), cyclophilin, actin and c-fos mRNAs responded to E2 treatment: the first five increased an average of 60% while the last decreased 38%. In situ hybridization identified more subtle ICI effects: agonistic up-regulation of GAPDH mRNA in superficial endometrial cells, and antagonistic down-regulation of ER and PR mRNAs in the inner layer of the myometrium. Thus, we conclude that the agonist versus antagonist effects of ICI relative to those of E2 are a function of the gene examined as well as the specific cell within the uterus.

Involvement of cyclic AMP response element binding protein (CREB) and estrogen receptor phosphorylation in the synergistic activation of the estrogen receptor by estradiol and protein kinase activators

Estrogen receptor (ER) and cAMP signaling pathways interact in a number of estrogen target tissues including mammary and uterine tissues. One aspect of this interaction is that estradiol and protein kinase A (PKA) activators can cooperate synergistically to activate ER-mediated transcription of both endogenous genes and reporter genes containing only estrogen response elements. The purpose of this study was to investigate the molecular mechanism of this interaction between signaling pathways. Site-directed mutagenesis of the potential PKA phosphorylation sites in the ER indicated that phosphorylation of these sites was not necessary for the observed transcriptional synergy. In transient transfection assays in two different cell lines using reporter constructs containing either cAMP response elements, estrogen response elements or both types of elements, with the addition or absence of cAMP response element binding protein (CREB) expression plasmid, we observed that only one of these cell lines exhibited estrogen/PKA transcriptional synergy. Experiments demonstrated that CREB itself was involved in the transcriptional synergy, and that transfection of CREB restored transcriptional synergy in the cell line in which it was lacking. A functional interaction between ER and CREB was also demonstrated using a mammalian cell protein interaction assay; a dominant negative mutant of CREB did not exhibit this interaction. Therefore, these data indicate that CREB protein is required for the transcriptional synergy between cAMP and estrogen signaling pathways. Furthermore, CREB cooperated with the ER on genes that did not contain cAMP response elements, but contained only estrogen response elements. We propose that activated CREB is recruited to estrogen responsive genes by an ER--coactivator complex containing proteins such as CREB binding protein (CBP) and that the interaction of CREB with ER may assist in stabilizing its interaction with CBP and in promoting estrogen-ER and PKA transcriptional synergy.

From adjuvant therapy to breast cancer prevention: BCPT and STAR

The continued widespread prevalence of breast cancer supports placing a high priority on research aimed at its primary prevention, particularly among women who are at increased risk for developing this disease. The suggestion of potential agents for the primary chemoprevention of breast cancer evolved out of the treatment setting. Extensive experience with tamoxifen, a first-generation selective estrogen receptor modulator (SERM) showing efficacy, first, in the treatment of advanced breast cancer and, subsequently, as adjuvant therapy for early stage disease established the safety of this agent. Cumulative data from multiple adjuvant studies documented the efficacy of tamoxifen in reducing second primary breast cancers in the contralateral breast, supporting its potential as a chemopreventive agent for breast cancer. The safety and second primary data on tamoxifen, together with extensive information on its pharmacokinetics, metabolism, and antitumor effects, as well as its potentially beneficial effects on lipid metabolism and osteoporosis, led the National Surgical Adjuvant Breast and Bowel Project (NSABP) to select tamoxifen for testing in the first prospective randomized phase III trial of the efficacy of a chemopreventive agent for preventing breast cancer in women at increased risk of the disease. Accordingly, in 1992 the NSABP started the Breast Cancer Prevention Trial (P-1) in which 13,388 women > or = 35 years of age who were at increased risk of breast cancer according to Gail model risk factors [family history, age, and personal history (i.e., age at first birth, age at menarche, previous breast biopsies)] were randomized to tamoxifen 20 mg/day or placebo for 5 years. Through 69 months of follow-up tamoxifen reduced the risk of invasive breast cancer, primarily estrogen receptor-positive tumors, by 49% (two-sided p < 0.00001). Tamoxifen reduced the risk of noninvasive breast cancer by 50% (two-sided p < 0.002). In addition, tamoxifen reduced fractures of the hip, radius, and spine, but it had no effect on the rate of ischemic heart disease. As previously shown, the rates of endometrial cancer and vascular events increased with tamoxifen. With the P-1 results establishing tamoxifen as the standard of care for the primary chemoprevention of breast cancer in high-risk women, concern over the side effects of tamoxifen has prompted a continuing search for an agent that displays a more desirable efficacy/toxicity profile. Raloxifene, a second-generation SERM approved for the prevention of osteoporosis in postmenopausal women, displays antiestrogenic properties in the breast and possibly the endometrium, and estrogenic effects in the bone and on the lipid profile, suggesting it as a candidate for comparison with the chemopreventive standard, tamoxifen. Raloxifene will be compared to tamoxifen in an equivalency trial, the Study of Tamoxifen and Raloxifene (STAR) NSABP P-2, which began in July 1999 at almost 500 centers in North America. The plan is to randomize 22,000 postmenopausal women > or = 35 years of age at increased risk of breast cancer by Gail criteria to tamoxifen 20 mg/day or raloxifene 60 mg/day for 5 years. Study endpoints include invasive and noninvasive breast cancer, cardiovascular disease, endometrial cancer, bone fractures, and vascular events.

Expression of p21Waf1, p27Kip1 and cyclin D1 proteins in breast ductal carcinoma in situ: Relation with clinicopathologic characteristics and with p53 expression and estrogen receptor status

p21Waf1 (p21), p27Kip1 (p27) and cyclin D1 have recently been reported as useful prognostic markers for patients with breast carcinoma. However, studies on these cell cycle regulators in ductal carcinoma in situ (DCIS) have been extremely limited. Therefore, we studied the immunohistochemical expression of p21, p27 and cyclin D1 proteins in 49 DCIS cases and compared the findings with the clinicopathologic parameters (age, tumor size, gross type, histologic type, histologic grade, necrosis and mitotic index), p53 and estrogen receptor (ER) status. A significant correlation was found between positive p21 immunoreactivity (67.3% of the cases) and well-differentiated histologic grade, non-comedo type, ER-positive and p53-negative (p53-) status. DCIS with p21+/p53- is likely to be the non-comedo type. The overexpression of cyclin D1 (59.2% of the cases) correlated positively with the ER expression (P = 0.001). The p27 protein expression (46.9% of the cases) correlated with the cyclin D1 immunopositivity (P = 0.0003) and ER expression (P = 0.005). No significant associations were seen in the p27 or cyclin D1 expression and other clinicopathologic parameters. Our results suggest that p21 might be more related to the useful biologic markers in DCIS than p27 or cyclin D1. The significant positive association between p21, p27 or cyclin D1 and ER status, and close association of p27 and cyclin D1 expression might be implicated in the tumor biology of DCIS.

Molecular mechanisms of estrogen action: selective ligands and receptor pharmacology

Estrogens exert profound effects on the physiology of diverse target cells and these effects appear to be mediated by two estrogen receptor (ER) subtypes, ERalpha and ERbeta. We have investigated how ER ligands, ranging from pure agonists to antagonists, interact with ERalpha and ERbeta, and regulate their transcriptional activity on different genes. Mutational mapping-structure activity studies indicate that different residues of the ER ligand binding domain are involved in the recognition of structurally distinct estrogens and antiestrogens. We have identified from ligands of diverse structure, several particularly interesting ones that are high potency selective agonists via ERalpha and others that are full agonists through ERalpha while being full antagonists through ERbeta. Antiestrogens such as hydroxytamoxifen, which are mixed agonist/antagonists through ERalpha, are pure antagonists through ERbeta at estrogen response element-containing gene sites. Studies with ERalpha/beta chimeric proteins reveal that tamoxifen agonism requires the activation function 1 region of ERalpha. Through two-hybrid assays, we have isolated an ER-specific coregulator that potentiates antiestrogen antagonist effectiveness and represses ER transcriptional activity. We have also focused on understanding the distinct pharmacologies of antiestrogen- and estrogen-regulated genes. Although antiestrogens are thought to largely act by antagonizing the actions of estrogens, we have found among several new ER-regulated genes, quinone reductase (QR), a detoxifying phase II antioxidant enzyme, that has its activity up-regulated by antiestrogens in an ER-dependent manner in breast cancer cells. This response is antagonized by estrogens, thus showing 'reversed pharmacology'. Increased QR activity by antiestrogens requires a functional ER (ERalpha or ERbeta) and is, interestingly, mediated via the electrophile response element in the QR gene 5' regulatory region. The up-regulation of QR may contribute to the beneficial effects of tamoxifen, raloxifene, and other antiestrogens in breast cancer prevention and treatment. Estrogens rapidly up-regulate expression of several genes associated with cell cytoarchitectural changes including NHE-RF, the sodium hydrogen exchanger regulatory factor, also known as EBP50. NHE-RF/EBP50 is enriched in microvilli, and may serve as a scaffold adaptor protein in regulating early changes in cell architecture and signal transduction events induced by estrogen. Analyses of the regulatory regions of these primary response genes, and the antioxidant and other signaling pathways involved, are providing considerable insight into the mechanisms by which ligands, that function as selective estrogen receptor modulators or SERMs, exert their marked effects on the activities and properties of target cells. The intriguing biology of estrogens in its diverse target cells is thus determined by the structure of the ligand, the ER subtype involved, the nature of the hormone-responsive gene promoter, and the character and balance of coactivators and corepressors that modulate the cellular response to the ER-ligand complex. The continuing development of ligands that function as selective estrogens or antiestrogens for ERalpha or ERbeta should allow optimized tissue selectivity of these agents for menopausal hormone replacement therapy and the treatment and prevention of breast cancer.

Platelet-derived growth factor receptor association with Na(+)/H(+) exchanger regulatory factor potentiates receptor activity

Platelet-derived growth factor (PDGF) is a potent mitogen for many cell types. The PDGF receptor (PDGFR) is a receptor tyrosine kinase that mediates the mitogenic effects of PDGF by binding to and/or phosphorylating a variety of intracellular signaling proteins upon PDGF-induced receptor dimerization. We show here that the Na(+)/H(+) exchanger regulatory factor (NHERF; also known as EBP50), a protein not previously known to interact with the PDGFR, binds to the PDGFR carboxyl terminus (PDGFR-CT) with high affinity via a PDZ (PSD-95/Dlg/Z0-1 homology) domain-mediated interaction and potentiates PDGFR autophosphorylation and extracellular signal-regulated kinase (ERK) activation in cells. A point-mutated version of the PDGFR, with the terminal leucine changed to alanine (L1106A), cannot bind NHERF in vitro and is markedly impaired relative to the wild-type receptor with regard to PDGF-induced autophosphorylation and activation of ERK in cells. NHERF potentiation of PDGFR signaling depends on the capacity of NHERF to oligomerize. NHERF oligomerizes in vitro when bound with PDGFR-CT, and a truncated version of the first NHERF PDZ domain that can bind PDGFR-CT but which does not oligomerize reduces PDGFR tyrosine kinase activity when transiently overexpressed in cells. PDGFR activity in cells can also be regulated in a NHERF-dependent fashion by stimulation of the beta(2)-adrenergic receptor, a known cellular binding partner for NHERF. These findings reveal that NHERF can directly bind to the PDGFR and potentiate PDGFR activity, thus elucidating both a novel mechanism by which PDGFR activity can be regulated and a new cellular role for the PDZ domain-containing adapter protein NHERF.

Prothymosin alpha selectively enhances estrogen receptor transcriptional activity by interacting with a repressor of estrogen receptor activity

We find that prothymosin alpha (PTalpha) selectively enhances transcriptional activation by the estrogen receptor (ER) but not transcriptional activity of other nuclear hormone receptors. This selectivity for ER is explained by PTalpha interaction not with ER, but with a 37-kDa protein denoted REA, for repressor of estrogen receptor activity, a protein that we have previously shown binds to ER, blocking coactivator binding to ER. We isolated PTalpha, known to be a chromatin-remodeling protein associated with cell proliferation, using REA as bait in a yeast two-hybrid screen with a cDNA library from MCF-7 human breast cancer cells. PTalpha increases the magnitude of ERalpha transcriptional activity three- to fourfold. It shows lesser enhancement of ERbeta transcriptional activity and has no influence on the transcriptional activity of other nuclear hormone receptors (progesterone receptor, glucocorticoid receptor, thyroid hormone receptor, or retinoic acid receptor) or on the basal activity of ERs. In contrast, the steroid receptor coactivator SRC-1 increases transcriptional activity of all of these receptors. Cotransfection of PTalpha or SRC-1 with increasing amounts of REA, as well as competitive glutathione S-transferase pulldown and mammalian two-hybrid studies, show that REA competes with PTalpha (or SRC-1) for regulation of ER transcriptional activity and suppresses the ER stimulation by PTalpha or SRC-1, indicating that REA can function as an anticoactivator in cells. Our data support a model in which PTalpha, which does not interact with ER, selectively enhances the transcriptional activity of the ER but not that of other nuclear receptors by recruiting the repressive REA protein away from ER, thereby allowing effective coactivation of ER with SRC-1 or other coregulators. The ability of PTalpha to directly interact in vitro and in vivo with REA, a selective coregulator of the ER, thereby enabling the interaction of ER with coactivators, appears to explain its ability to selectively enhance ER transcriptional activity. These findings highlight a new role for PTalpha as a coregulator activity-modulating protein that confers receptor specificity. Proteins such as PTalpha represent an additional regulatory component that defines a novel paradigm enabling receptor-selective enhancement of transcriptional activity by coactivators.

Altered bone marrow cell sensitivity in the lupus-prone NZB/W mouse: regulation of CFU-GM colony formation by estrogen, tamoxifen and thrombopoietin

Estrogen is thought to contribute to the onset of systemic lupus erythematosus (SLE) in women through mechanisms that are not completely understood. Although estrogen serves as a negative regulator in normal hematopoietic development, little research has been conducted examining alteration in hematopoietic development triggered by estrogen in lupus-susceptible individuals. We examined whether estrogen and other factors could influence colony formation of bone marrow cells obtained from normal and lupus-susceptible mice. Bone marrow cells isolated from New Zealand Black (NZB) and lupus-prone New Zealand Black and New Zealand White cross (NZB/W) mice were cultured in the presence of granulocyte-macrophage colony stimulating factor (GM-CSF) alone or in combination with estrogen, thrombopoietin (TPO), tamoxifen, estrogen and TPO. or estrogen and tamoxifen, and plated in methylcellulose culture medium. Plates were scored for the number of CFU-GM (colony forming unit granulocyte-macrophage) colonies after 6d in culture. For females of both mouse strains, estrogen significantly decreased (P < 0.05) the number of GM colonies. Co-treatment of NZB/W cells, but not NZB cells, with TPO or tamoxifen reversed the suppressive action of estrogen (P < 0.05). In contrast, while estrogen did suppress colony formation from cells of NZB/W males (P < 0.05), neither TPO nor tamoxifen reversed this effect. Our results indicate that the sensitivity of bone marrow cells isolated from both female and male NZB/W lupus-prone mice to hormones/growth factors is qualitatively different from cells of NZB mice, and suggest that hematopoietic alterations at the level of the bone marrow may be related to the pathogenesis of SLE.

Development and characterization of a tamoxifen-resistant breast carcinoma xenograft

A human tamoxifen-resistant mammary carcinoma, MaCa 3366/TAM, originating from a sensitive parental xenograft 3366 was successfully established by treatment of tumour-bearing nude mice with 1-50 mg kg(-1) tamoxifen for 3 years during routine passaging. Both tumours did not differ significantly in OR- and PR-positivity, however, when compared with the sensitive tumour line, the mean OR content of the TAM-resistant subline is slightly lower. An OR-upregulation following withdrawal of oestradiol treatment was observed in the parental tumours but not in the resistant xenografts. Following long-term treatment with tamoxifen, the histological pattern of the breast carcinoma changed. The more differentiated structures being apparent after treatment with 17beta-oestradiol in the original 3366 tumour were not induced in the resistant line. Tamoxifen failed to induce a tumour growth inhibition in comparison to the tamoxifen-sensitive line. The pure anti-oestrogen, ICI 182 780, revealed cross-resistance. Sequence analysis of the hormone-binding domain of the OR of both lines showed no differences, suggesting that either mutations in other regions of the OR are involved in the TAM-resistance phenotype or that mechanisms outside of this protein induced this phenotype. Oestrogen and anti-oestrogen regulate pS2 and cathepsin D expression in 3366 tumours as in the human breast cancer cell line MCF-7. The resistant 3366/TAM tumours have lost this regulation. The established breast cancer xenografts 3366 and 3366/TAM offer the possibility of investigating mechanisms of anti-oestrogen resistance in an in vivo situation. They can be used to test novel approaches to prevent, or to overcome, this resistance in a clinically related manner.

BCAR1, a human homologue of the adapter protein p130Cas, and antiestrogen resistance in breast cancer cells

BACKGROUND

Treatment of breast cancer with the antiestrogen tamoxifen is effective in approximately one half of the patients with estrogen receptor-positive disease, but tumors recur frequently because of the development of metastases that are resistant to tamoxifen. We have previously shown that mutagenesis of human estrogen-dependent ZR-75-1 breast cancer cells by insertion of a defective retrovirus genome caused the cells to become antiestrogen resistant. In this study, we isolated and characterized the crucial gene at the breast cancer antiestrogen resistance 1 (BCAR1) locus.

METHODS/RESULTS

Transfer of the BCAR1 locus from retrovirus-mutated, antiestrogen-resistant cells to estrogen-dependent ZR-75-1 cells by cell fusion conferred an antiestrogen-resistant phenotype on the recipient cells. The complete coding sequence of BCAR1 was isolated by use of exon-trapping and complementary DNA (cDNA) library screening. Sequence analysis of human BCAR1 cDNA predicted a protein of 870 amino acids that was strongly homologous to rat p130Cas-adapter protein. Genomic analysis revealed that BCAR1 consists of seven exons and is located at chromosome 16q23.1. BCAR1 transcripts were detected in multiple human tissues and were similar in size to transcripts produced by retrovirus-mutated ZR-75-1 cells. Transfection of BCAR1 cDNA into ZR-75-1 cells again resulted in sustained cell proliferation in the presence of antiestrogens, confirming that BCAR1 was the responsible gene in the locus.

CONCLUSIONS

Overexpression of the BCAR1 gene confers antiestrogen resistance on human ZR-75-1 breast cancer cells. Overexpression of BCAR1 in retrovirus-mutated cells appears to result from activation of the gene's promoter. The isolation and characterization of this gene open new avenues to elucidating mechanisms by which the growth of human breast cancer becomes independent of estrogen.

Estrogen receptor transcription and transactivation: Estrogen receptor alpha and estrogen receptor beta: regulation by selective estrogen receptor modulators and importance in breast cancer

Estrogens display intriguing tissue-selective action that is of great biomedical importance in the development of optimal therapeutics for the prevention and treatment of breast cancer, for menopausal hormone replacement, and for fertility regulation. Certain compounds that act through the estrogen receptor (ER), now referred to as selective estrogen receptor modulators (SERMs), can demonstrate remarkable differences in activity in the various estrogen target tissues, functioning as agonists in some tissues but as antagonists in others. Recent advances elucidating the tripartite nature of the biochemical and molecular actions of estrogens provide a good basis for understanding these tissue-selective actions. As discussed in this thematic review, the development of optimal SERMs should now be viewed in the context of two estrogen receptor subtypes, ERalpha and ERbeta, that have differing affinities and responsiveness to various SERMs, and differing tissue distribution and effectiveness at various gene regulatory sites. Cellular, biochemical, and structural approaches have also shown that the nature of the ligand affects the conformation assumed by the ER-ligand complex, thereby regulating its state of phosphorylation and the recruitment of different coregulator proteins. Growth factors and protein kinases that control the phosphorylation state of the complex also regulate the bioactivity of the ER. These interactions and changes determine the magnitude of the transcriptional response and the potency of different SERMs. As these critical components are becoming increasingly well defined, they provide a sound basis for the development of novel SERMs with optimal profiles of tissue selectivity as medical therapeutic agents.

Acquired antiestrogen resistance in MCF-7 human breast cancer sublines is not accomplished by altered expression of receptors in the ErbB-family

Development of acquired resistance against antiestrogen treatment is a serious problem in human breast cancer, and knowledge of alterations resulting in resistance is important for selection of further treatment. To mimic the clinical situation we have established a series of MCF-7 human breast cancer cell lines by long term treatment with the antiestrogens tamoxifen, ICI 164,384, and ICI 182,780. Common for these cell lines is a decreased expression of the estrogen receptor alpha (ER alpha). In human breast cancer, lack of response to endocrine therapy is often associated with decreased expression of the estrogen receptor and increased expression of epidermal growth factor receptor (EGFR) and/or HER-2/neu (ErbB-2). Our antiestrogen resistant cell lines did not express altered levels of EGFR, HER-2/neu, ErbB-3, or ErbB-4. Estrogen and antiestrogen regulation of HER-2/neu expression was essentially similar in parent and resistant MCF-7 cells. Treatment with antibodies to HER-2/neu (Herceptin) did not affect growth of MCF-7 cells or resistant cells, indicating that in this in vitro model system, acquired antiestrogen resistance does not emerge from activation of the HER-2/neu signaling pathway. In MCF-7 cells transfected with HER-2/neu and/or ErbB-3, overexpression alone did not result in resistance. However, addition of heregulinl-beta1 abolished the inhibitory activity of ICI 182,780 on both vector and HER-2/neu/ErbB-3 transfected MCF-7 cells, demonstrating that activation of the HER-2/neu receptor signaling pathway can override the growth inhibitory effect of ICI 182,780.