Activation of the MAPK pathway enhances sensitivity of MCF-7 breast cancer cells to the mitogenic effect of estradiol

Caveolin-1, a Determinant of the Fate of MCF-7 Breast Cancer Cells

Background

The scaffolding protein, caveolin-1 (Cav-1), participates in multiple cellular functions including promotion of sodium excretion from the kidney. Loss of expression of Cav-1 is associated with tumorigenesis of various types of cancer. We have shown the potential link between hypertension and breast cancer via abnormal function of the G protein-coupled receptor kinase type 4 (GRK4).

Objective

The current studies tested the hypothesis that Cav-1 acts as a tumor-suppressive factor in breast cancer cells and enhances the sensitivity to the inhibitory effect of the type 1 dopaminergic receptor (D1R).

Methods

Michigan Cancer Foundation (MCF) MCF-7 cells stably expressing a Cav-1/mCherry fusion protein or mCherry alone were used as models to examine the effect of Cav-1 on cell growth, apoptosis, and senescence. Cell proliferation was determined by cell counting, cell cycle analysis (flow cytometry), and BrdU incorporation. Apoptosis was determined using the Cell Death Detection ELISA kit from Roche Diagnosis. Senescence was determined using the senescence associated beta galactosidase (SA-β-gal) assay. Reactive oxygen species (ROS) was measured using 2',7'-dichlorodihydrofluorescein diacetate. Western blot analysis was used to measure activation of signaling pathway molecules. All statistical analyses were conducted with Microsoft Excel.

Results

Overexpression of Cav-1 in MCF-7 cells reduced cellular growth rate. Both inhibition of proliferation and induction of cell death are contributing factors. Multiple signaling pathways were activated in Cav-1-expressing MCF-7 cells. Activation of Akt was prominent. In MCF-7-expressing Cav-1 (MCF-7 Cav-1) cells, the levels of phosphorylated Akt at S473 and T308 were increased 28- and 8.7-fold, respectively. Instead of protecting cells from apoptosis, extremely high levels of activated Akt resulted in increased levels of ROS which led to apoptosis and senescence. The tumor-suppressive effect plus downregulation of GRK4 makes Cav-1-expressing MCF-7 cells significantly more sensitive to the inhibitory effect of the D1R agonist, SKF38393.

Conclusion

Caveolin-1 acts as a tumor-suppressing factor via extreme activation of Akt and down regulation of survival factors such as GRK4, survivin, and cyclin D1.

mTOR pathway candidate genes and obesity interaction on breast cancer risk in black women from the Women's Circle of Health Study

BACKGROUND

Obesity is known to stimulate the mammalian target of rapamycin (mTOR) signaling pathway and both obesity and the mTOR signaling pathway are implicated in breast carcinogenesis. We investigated potential gene-environment interactions between mTOR pathway genes and obesity in relation to breast cancer risk among Black women.

METHODS

The study included 1,655 Black women (821 incident breast cancer cases and 834 controls) from the Women's Circle of Health Study (WCHS). Obesity measures including body mass index (BMI); central obesity i.e., waist circumference (WC) and waist/hip ratio (WHR); and body fat distribution (fat mass, fat mass index and percent body fat) were obtained by trained research staff. We examined the associations of 43 candidate single-nucleotide polymorphisms (SNPs) in 20 mTOR pathway genes with breast cancer risk using multivariable logistic regression. We next examined interactions between these SNPs and measures of obesity using Wald test with 2-way interaction term.

RESULTS

The variant allele of BRAF (rs114729114 C > T) was associated with an increase in overall breast cancer risk [odds ratio (OR) = 1.81, 95% confidence interval (CI) 1.10-2.99, for each copy of the T allele] and the risk of estrogen receptor (ER)-defined subtypes (ER+ tumors: OR = 1.83, 95% CI 1.04,3.29, for each copy of the T allele; ER- tumors OR = 2.14, 95% CI 1.03,4.45, for each copy of the T allele). Genetic variants in AKT, AKT1, PGF, PRKAG2, RAPTOR, TSC2 showed suggestive associations with overall breast cancer risk and the risk of, ER+ and ER- tumors (range of p-values = 0.040-0.097). We also found interactions of several of the SNPs with BMI, WHR, WC, fat mass, fat mass index and percent body fat in relation to breast cancer risk. These associations and interactions, however, became nonsignificant after correction for multiple testing (FDR-adjusted p-value > 0.05).

CONCLUSION

We found associations between mTOR genetic variants and breast cancer risk as well as gene and body fatness interactions in relation to breast cancer risk. However, these associations and interactions became nonsignificant after correction for multiple testing. Future studies with larger sample sizes are required to confirm and validate these findings.

Systems and in vitro pharmacology profiling of diosgenin against breast cancer

Aim: The purpose of this study was to establish a mode of action for diosgenin against breast cancer employing a range of system biology tools and to corroborate its results with experimental facts. Methodology: The diosgenin-regulated domains implicated in breast cancer were enriched in the Kyoto Encyclopedia of Genes and Genomes database to establish diosgenin-protein(s)-pathway(s) associations. Later, molecular docking and the lead complexes were considered for molecular dynamics simulations, MMPBSA, principal component, and dynamics cross-correlation matrix analysis using GROMACS v2021. Furthermore, survival analysis was carried out for the diosgenin-regulated proteins that were anticipated to be involved in breast cancer. For gene expression analyses, the top three targets with the highest binding affinity for diosgenin and tumor expression were examined. Furthermore, the effect of diosgenin on cell proliferation, cytotoxicity, and the partial Warburg effect was tested to validate the computational findings using functional outputs of the lead targets. Results: The protein-protein interaction had 57 edges, an average node degree of 5.43, and a p-value of 3.83e-14. Furthermore, enrichment analysis showed 36 KEGG pathways, 12 cellular components, 27 molecular functions, and 307 biological processes. In network analysis, three hub proteins were notably modulated: IGF1R, MDM2, and SRC, diosgenin with the highest binding affinity with IGF1R (binding energy -8.6 kcal/mol). Furthermore, during the 150 ns molecular dynamics (MD) projection run, diosgenin exhibited robust intermolecular interactions and had the least free binding energy with IGF1R (-35.143 kcal/mol) compared to MDM2 (-34.619 kcal/mol), and SRC (-17.944 kcal/mol). Diosgenin exhibited the highest cytotoxicity against MCF7 cell lines (IC₅₀ 12.05 ± 1.33) µg/ml. Furthermore, in H₂O₂-induced oxidative stress, the inhibitory constant (IC₅₀ 7.68 ± 0.51) µg/ml of diosgenin was lowest in MCF7 cell lines. However, the reversal of the Warburg effect by diosgenin seemed to be maximum in non-cancer Vero cell lines (EC₅₀ 15.27 ± 0.95) µg/ml compared to the rest. Furthermore, diosgenin inhibited cell proliferation in SKBR3 cell lines more though. Conclusion: The current study demonstrated that diosgenin impacts a series of signaling pathways, involved in the advancement of breast cancer, including FoxO, PI3K-Akt, p53, Ras, and MAPK signaling. Additionally, diosgenin established a persistent diosgenin-protein complex and had a significant binding affinity towards IGF1R, MDM2, and SRC. It is possible that this slowed down cell growth, countered the Warburg phenomenon, and showed the cytotoxicity towards breast cancer cells.

A Potential Anticancer Mechanism of Finger Root (Boesenbergia rotunda) Extracts against a Breast Cancer Cell Line

Breast cancer is the most common type of cancer women suffer from worldwide in 2020 and the 4th leading cause of cancer death. Boesenbergia rotunda is an herb with high potential as an anticancer agent. This study explores the potential bioactive compounds in B. rotunda as anti-breast cancer agents using in silico and in vitro approaches. The in silico study was used for active compound analysis, selection of anticancer compound candidates, prediction of target protein, functional annotation, molecular docking, and molecular dynamics simulation, respectively. The in vitro study was conducted by measurement toxicity, rhodamine 123, and apoptosis assays on T47D cells. Based on the KNApSAcK database, B. rotunda contained 20 metabolites, which are dominated by chalcone and flavonoid groups. Seven of them were predicted to have anticancer activity, namely, sakuranetin, cardamonin, alpinetin, 2S-pinocembrin, 7.4'-dihydroxy-5-methoxyflavanone, 5.6-dehydrokawain, and pinostrobin chalcone. These compounds targeted proteins related to cancer progression pathways such as the PI3K/Akt, FOXO, JAK/STAT, and estrogen signaling pathways. Therefore, these compounds are predicted to inhibit growth and induce apoptosis of cancer cells through their interactions with MMP12, MMP13, CDK4, JAK3, VEGFR1, VEGFR2, and KCNA3. Anticancer activity of B. rotunda through in vitro study confirmed that B. rotunda extract is strong cytotoxic and induces apoptosis of breast cancer cell lines. This study concludes that Boesenbergia rotunda has potency as an anticancer candidate.

Mechanistic Effects of Estrogens on Breast Cancer

PURPOSE

Current concepts regarding estrogen and its mechanistic effects on breast cancer in women are evolving. This article reviews studies that address estrogen-mediated breast cancer development, the prevalence of occult tumors at autopsy, and the natural history of breast cancer as predicted by a newly developed tumor kinetic model.

METHODS

This article reviews previously published studies from the authors and articles pertinent to the data presented.

RESULTS

We discuss the concepts of adaptive hypersensitivity that develops in response to long-term deprivation of estrogen and results in both increased cell proliferation and apoptosis. The effects of menopausal hormonal therapy on breast cancer in postmenopausal women are interpreted based on the tumor kinetic model. Studies of the administration of a tissue selective estrogen complex in vitro, in vivo, and in patients are described. We review the various clinical studies of breast cancer prevention with selective estrogen receptor modulators and aromatase inhibitors. Finally, the effects of the underlying risk of breast cancer on the effects of menopausal hormone therapy are outlined.

DISCUSSION

The overall intent of this review is to present data supporting recent concepts, discuss pertinent literature, and critically examine areas of controversy.

Mechanism-Centric Approaches for Biomarker Detection and Precision Therapeutics in Cancer

Biomarker discovery is at the heart of personalized treatment planning and cancer precision therapeutics, encompassing disease classification and prognosis, prediction of treatment response, and therapeutic targeting. However, many biomarkers represent passenger rather than driver alterations, limiting their utilization as functional units for therapeutic targeting. We suggest that identification of driver biomarkers through mechanism-centric approaches, which take into account upstream and downstream regulatory mechanisms, is fundamental to the discovery of functionally meaningful markers. Here, we examine computational approaches that identify mechanism-centric biomarkers elucidated from gene co-expression networks, regulatory networks (e.g., transcriptional regulation), protein-protein interaction (PPI) networks, and molecular pathways. We discuss their objectives, advantages over gene-centric approaches, and known limitations. Future directions highlight the importance of input and model interpretability, method and data integration, and the role of recently introduced technological advantages, such as single-cell sequencing, which are central for effective biomarker discovery and time-cautious precision therapeutics.

β-Estradiol Enhanced Secretion of Lipoprotein Lipase from Mouse Mammary Tumor FM3A Cells

The role of β-estradiol (E₂) in lipoprotein metabolism in mammary tumors is unclear, therefore, we investigated the effect of E₂ on the secretion of lipoprotein lipase (LPL) from mouse mammary tumor FM3A cells. E₂-treated cells increased the secretion of active LPL from FM3A cells in a time- and dose-dependent manner. Activity of mitogen-activated protein kinase (MAPK) was increased in the tumor cells treated with E₂, and enhanced secretion of LPL was suppressed by MAPK kinase 1/2 inhibitor, PD98059, extracellular signal-regulated kinase (ERK) 1/2 inhibitor, FR180204, p38 MAPK inhibitor, SB202190, and phosphatidyl inositol 3-kinase (PI3K) inhibitor, LY294002. In addition, the effect of E₂ on LPL secretion was markedly suppressed by an inhibitor of mammalian target of rapamycin complex (mTORC) 1 and 2, KU0063794, but were not by a mTORC1 inhibitor, rapamycin. Furthermore, a small interfering RNA (siRNA)-mediated decrease in the expression of rapamycin-insensitive companion of mTOR (Rictor), a pivotal component of mTORC2, suppressed secretion of LPL by E₂. These results suggest that the stimulatory secretion of LPL by E₂ from the tumor cells is closely associated with an activation of mTORC2 rather than mTORC1 possibly via the MAPK cascade.

The Hypertension Related Gene G-Protein Coupled Receptor Kinase 4 Contributes to Breast Cancer Proliferation

PURPOSE

Clinical studies have shown that breast cancer risk is increased in hypertensive women. The underlying molecular mechanism remains undetermined. The current study tests our hypothesis that G protein coupled receptor kinase 4 (GRK4) is a molecule that links hypertension and breast cancer. GRK4 plays an important role in regulation of renal sodium excretion. Sustained activation of GRK4 as in the circumstances of single nucleotide polymorphism (SNPs) causes hypertension. Expression of GRK4 in the kidney is regulated by cMyc, an oncogene that is amplified in breast cancer.

METHODS

Western analysis was used to evaluate GRK4 protein expression in seven breast cancer cell lines. GRK4 gene single nucleotide polymorphism in breast cancer cell lines and in breast cancer cDNA arrays were determined using TaqMan Genotyping qPRC. The function of GRK4 was evaluated in MCF-7 cells with cMyc knock-down and GRK4 re-expression and in MDA-MB-468 cells expressing inducible GRK4 shRNA lentivirus constructs. Nuclei counting and 5-Bromo-2'-deoxy-uridine (BrdU) labeling were used to evaluate cell growth and proliferation.

RESULTS

Genotyping of GRK4 SNPs in breast cancer cDNA arrays (n = 94) revealed that the frequency of carrying two hypertension related SNPs A142 V or R65 L is threefold higher in breast cancer patients than in healthy people (P = 7.53E-11). GRK4 protein is expressed in seven breast cancer cell lines but not the benign mammary epithelial cell line, MCF-10A. Three hypertension related SNPs in the GRK4 gene were identified in the breast cancer cell lines. Except for BT20, all other breast cancer lines have 1-3 GRK4 SNPs of which A142 V occurs in all 6 lines. MDA-MB-468 cells contain homozygous A142 V and R65 L SNPs. Knocking down cMyc in MCF-7 cells significantly reduced the growth rate, which was rescued by re-expression of GRK4. We then generated three stable GRK4 knock-down MDA-MB-468 lines using inducible lentiviral shRNA vectors. Doxycycline (Dox) induced GRK4 silencing significantly reduced GRK4 mRNA and protein levels, growth rates, and proliferation. As a marker of cell proliferation, the percentage of BrdU-labeled cells decreased from 45 ± 3% in the cells without Dox to 32 ± 5% in the cells treated with 0.1 µg/mL Dox.

CONCLUSIONS

GRK4 acts as an independent proliferation promotor in breast cancer. Our results suggest that targeted inhibition of GRK4 could be a new therapy for both hypertension and breast cancer.

Trefoil factor-1 upregulation in estrogen-receptor positive breast cancer correlates with an increased risk of bone metastasis

Bone is one of the most preferred sites of metastatic spread from different cancer types, including breast cancer. However, different breast cancer subtypes exhibit distinct metastatic behavior in terms of kinetics and anatomic sites of relapse. Despite advances in the diagnosis, the identification of patients at high-risk of bone recurrence is still an unmet clinical need. We conducted a retrospective analysis, by gene expression and immunohistochemical assays, on 90 surgically resected breast cancer samples collected from patients who experienced no evidence of distant metastasis, bone or visceral metastasis in order to identify a primary tumor-derived marker of bone recurrence. We identified trefoil factor-1 (pS2 or TFF1) as strictly correlated to bone metastasis from ER+ breast cancer. In silico analysis was carried out to confirm this observation, linking gene expression data with clinical characteristics available from public clinical datasets. Then, we investigated TFF1 function in ER+ breast cancer tumorigenesis and bone metastasis through xenograft in vivo models of MCF 7 breast cancer with gain and loss of function of TFF1. As a response to microenvironmental features in primary tumors, TFF1 expression could modulate ER+ breast cancer growth, leading to a less proliferative phenotype. Our results showed it may not play a role in late stages of bone metastasis, however further studies are warranted to understand whether it could contribute in the early-stages of the metastatic cascade. In conclusion, TFF1 upregulation in primary ER+ breast cancer could be useful to identify patients at high-risk of bone metastasis. This could help clinicians in the identification of patients who likely can develop bone metastasis and who could benefit from personalized treatments and follow-up strategies to prevent metastatic disease.

Combined treatment with auranofin and trametinib induces synergistic apoptosis in breast cancer cells

Auranofin is a gold complex used as an anti-rheumatic agent and may act as a potent anticancer drug against breast tumors. Trametinib is a specific mitogen-activated protein kinase inhibitor, approved for the treatment of metastatic melanoma. The aim of this study was to examine the synergistic effects of auranofin and trametinib on apoptosis in MCF-7 human breast cancer cells. The combination treatment inhibited cancer cell proliferation and induced cell cycle arrest at the sub-G1 phase and apoptosis via poly (ADP-ribose) polymerase cleavage and caspase-3/7 activation. It is noteworthy that this treatment significantly increased p38 mitogen-activated protein kinase (MAPK) phosphorylation to induce mitochondrial stress, subsequently promoting cancer cell apoptosis through release of apoptosis-inducing factor. Further data demonstrated that combined treatment significantly induced increase in nuclear translocation of AIF. These results indicated that activation of the p38 MAPK signaling pathway and mitochondrial apoptosis may contribute to the synergistic consequences in MCF-7 cells. Collectively, our data demonstrated that combined treatment with auranofin and trametinib exhibited synergistic breast cancer cell death and this combination might be utilized as a novel therapeutic strategy for breast cancer.

Maslinic acid differentially exploits the MAPK pathway in estrogen-positive and triple-negative breast cancer to induce mitochondrion-mediated, caspase-independent apoptosis

Breast cancer accounts for 1.4 million new cases every year. Triple-negative breast cancer (TNBC) is one the leading cause of mortality in developing countries and is associated with early age onset (under 40 years old). Chemotherapy has a poor success rate in patients with TNBC as compared to other types of breast cancers. It is due to the lack of expression of three validated molecular markers for breast cancer, the estrogen and progesterone receptors, and the amplification of HER-2/Neu. Therefore, a clear need exists for a greater understanding of TNBC at all levels and for the development of better therapies. We have studied the anti-tumor effects of a potential drug, maslinic acid, which can be extracted from olive oil industry waste. This natural product showed inhibitory effect at concentrations ranging from 30 to 50 µM within 24 h. It exhibited divergent effects in cell cycle progression for the MCF7 (estrogen positive) cell line when compared with TNBCs like MDA-MB-231 and MDA-MB-468. Also, maslinic acid treatment altered the mitochondrial membrane electrochemical potential and the reactive oxygen species (ROS) levels to cause a caspase-independent programmed cell death. In silico approaches and immunoblotting suggested the involvement of the MAPK pathway explaining the variability in cell cycle progression along with the apoptotic cell death caused by maslinic acid.

HS2ST1-dependent signaling pathways determine breast cancer cell viability, matrix interactions, and invasive behavior

Heparan sulfate proteoglycans (HSPGs) act as signaling co‐receptors by interaction of their sulfated glycosaminoglycan chains with numerous signaling molecules. In breast cancer, the function of heparan sulfate 2‐O‐sulfotransferase (HS2ST1), the enzyme mediating 2‐O‐sulfation of HS, is largely unknown. Hence, a comparative study on the functional consequences of HS2ST1 overexpression and siRNA knockdown was performed in the breast cancer cell lines MCF‐7 and MDA‐MB‐231. HS2ST1 overexpression inhibited Matrigel invasion, while its knockdown reversed the phenotype. Likewise, cell motility and adhesion to fibronectin and laminin were affected by altered HS2ST1 expression. Phosphokinase array screening revealed a general decrease in signaling via multiple pathways. Fluorescent ligand binding studies revealed altered binding of fibroblast growth factor 2 (FGF‐2) to HS2ST1‐expressing cells compared with control cells. HS2ST1‐overexpressing cells showed reduced MAPK signaling responses to FGF‐2, and altered expression of epidermal growth factor receptor (EGFR), E‐cadherin, Wnt‐7a, and Tcf4. The increased viability of HS2ST1‐depleted cells was reduced to control levels by pharmacological MAPK pathway inhibition. Moreover, MAPK inhibitors generated a phenocopy of the HS2ST1‐dependent delay in scratch wound repair. In conclusion, HS2ST1 modulation of breast cancer cell invasiveness is a compound effect of altered E‐cadherin and EGFR expression, leading to altered signaling via MAPK and additional pathways.

G1T48, an oral selective estrogen receptor degrader, and the CDK4/6 inhibitor lerociclib inhibit tumor growth in animal models of endocrine-resistant breast cancer

Purpose

The combination of targeting the CDK4/6 and estrogen receptor (ER) signaling pathways with palbociclib and fulvestrant is a proven therapeutic strategy for the treatment of ER-positive breast cancer. However, the poor physicochemical properties of fulvestrant require monthly intramuscular injections to patients, which limit the pharmacokinetic and pharmacodynamic activity of the compound. Therefore, an orally available compound that more rapidly reaches steady state may lead to a better clinical response in patients. Here, we report the identification of G1T48, a novel orally bioavailable, non-steroidal small molecule antagonist of ER.

Methods

The pharmacological effects and the antineoplastic mechanism of action of G1T48 on tumors was evaluated using human breast cancer cells (in vitro) and xenograft efficacy models (in vivo).

Results

G1T48 is a potent and efficacious inhibitor of estrogen-mediated transcription and proliferation in ER-positive breast cancer cells, similar to the pure antiestrogen fulvestrant. In addition, G1T48 can effectively suppress ER activity in multiple models of endocrine therapy resistance including those harboring ER mutations and growth factor activation. In vivo, G1T48 has robust antitumor activity in a model of estrogen-dependent breast cancer (MCF7) and significantly inhibited the growth of tamoxifen-resistant (TamR), long-term estrogen-deprived (LTED) and patient-derived xenograft tumors with an increased response being observed with the combination of G1T48 and the CDK4/6 inhibitor lerociclib.

Conclusions

These data show that G1T48 has the potential to be an efficacious oral antineoplastic agent in ER-positive breast cancer.

Electronic supplementary material

The online version of this article (10.1007/s10549-020-05575-9) contains supplementary material, which is available to authorized users.

PRPF4 is a novel therapeutic target for the treatment of breast cancer by influencing growth, migration, invasion, and apoptosis of breast cancer cells via p38 MAPK signaling pathway

Pre-mRNA processing factor 4 (PRPF4), a core protein in U4/U6 snRNP, maintains snRNP structures by interacting with PRPF3 and cyclophilin H. Expression of the PRPF4 gene affects cell survival as well as apoptosis and is responsible for retinitis pigmentosa (RP). Proteomics analysis shows that PRPF4 may be a therapeutic target in human cancers. Nevertheless, the exact function and role of the PRPF4 gene are unclear. In this study, we assessed the expression of PRPF4 gene in human breast cancer cells. First, we confirmed that the PRPF4 gene was overexpressed in various breast cancer cell lines. Next, using breast cancer cell lines MCF7 and MDA-MB-468, we established stable cell lines with PRPF4 gene knockdown. We also performed microarray analysis to investigate molecular mechanisms underlying PRPF4 activity. All cell lines with PRPF4 gene knockdown exhibited reduced cell proliferation, remarkable reduction in anchorage-independent colony formation capacity, and reduction of PCNA protein, which is a marker cell of proliferation. Reduced expression of the PRPF4 gene induced apoptosis and changes in the expression of associated apoptotic markers in breast cancer cell lines. Knockdown of the PRPF4 gene reduced cellular capacity for migration and invasion (the key hallmarks of human cancers) and decreased the expression of genes involved in epithelial-mesenchymal transition (EMT). Microarray results showed that the expression of PPIP5K1, PPIPK2, and YWHAE genes was reduced at the transcriptional level, leading to reduced phosphorylation of p38 MAPK. These findings suggest that knockdown of PRPF4 gene slows down breast cancer progression via suppression of p38 MAPK phosphorylation. In conclusion, the PRPF4 gene plays an important role in the growth of breast cancer cells and is therefore a potential therapeutic target.

Pro-Apoptotic Effects of Estetrol on Long-Term Estrogen-Deprived Breast Cancer Cells and at Low Doses on Hormone-Sensitive Cells

Purpose:

Postmenopausal women with estrogen receptor-positive breast cancers often respond initially to tamoxifen or aromatase inhibitor therapy. Resistance to these treatments usually develops within 12 to 18 months. Clinical studies have demonstrated that high-dose estrogen can induce regression of these endocrine-resistant tumors. However, side-effects of high-dose estradiol (E₂) or diethylstilbestrol (DES) limit their usage. Estetrol (E₄) is the most abundant estrogen during pregnancy and has a long half-life and a low potential for side-effects. Estetrol might then provide benefits similar to DES on tumor regression but with lesser toxicity.

Methods:

In this study, we systematically evaluated the effects of E₄ on cell proliferation and apoptosis in wild-type MCF-7 and long-term estrogen-deprived (LTED) MCF-7 cells and compared its effects with E₂ and estriol (E₃).

Results:

Estetrol induced apoptosis in LTED cells but stimulated growth of MCF-7 cells at concentrations from 10⁻¹¹ to 10⁻⁸ M. These effects of E₄ are similar to those of E₂ but require much higher doses. Differing from E₂, E₄ at 10⁻¹² M induced apoptosis in MCF-7 cells and another pregnancy estrogen, E₃, acted similarly. No antagonistic effect of E₄ or E₃ against E₂ occurred when they were combined.

Conclusions:

The pro-apoptotic effects of E₄ and E₃ on LTED cells and at low doses on MCF-7 cells indicate that these steroids could be used as therapeutic agents for endocrine-resistant or sensitive breast cancer.

Estrogen Receptors in Cell Membranes: Regulation and Signaling

Estrogens can stimulate the development, proliferation, migration, and survival of target cells. These biological effects are mediated through their action upon the plasma membrane estrogen receptors (ERs). ERs regulate transcriptional processes by nuclear translocation and binding to specific response elements, which leads to the regulation of gene expression. This effect is termed genomic or nuclear. However, estrogens may exert their biological activity also without direct binding to DNA and independently of gene transcription or protein synthesis. This action is called non-genomic or non-nuclear. Through non-genomic mechanisms, estrogens can modify regulatory cascades such as MAPK, P13K, and tyrosine cascade as well as membrane-associated molecules such as ion channels and G-protein-coupled receptors. The recent studies on the mechanisms of estrogen action provide an evidence that non-genomic and genomic effects converge. An example of such convergence is the potential possibility to modulate gene expression through these two independent pathways. The understanding of the plasma membrane estrogen receptors is crucial for the development of novel drugs and therapeutic protocols targeting specific receptor actions.

Mechanism of action of the breast cancer-promoter hormone, 5α-dihydroprogesterone (5αP), involves plasma membrane-associated receptors and MAPK activation

Previous studies have shown that breast tissues and breast cell lines can convert progesterone to 5α-pregnane-3,20-dione (5aP), and that 5αP stimulates breast cell proliferation and detachment in vitro, and tumor formation in vivo, regardless of presence or absence of receptors for progesterone (PR) or estrogen (ER). Recently it was demonstrated, both in vitro and in vivo, that pro-cancer actions attributed to administered progesterone are due to the in situ produced 5αP. Because of the significant role of 5αP in breast cancers, it is important to understand its molecular mechanisms of action. The aims of the current studies were to identify 5αP binding sites and to determine if the mechanisms of action of 5αP involve the mitogen-activated protein kinase (MAPK), extracellular signal-regulated protein kinases (ERK1/2) pathway. Binding studies, using tritium-labeled 5αP ([(3)H]5αP), carried out on membrane, cytosol and nuclear fractions from human breast cells (MCF-7, PR/ER-positive; MDA-MB-231, PR/ER-negative) and on highly enriched membrane fractions, identified the plasma membrane as the site of ligand specific 5αP receptors. Localization of 5αP receptors to the cell membrane was confirmed visually with fluorescently labeled conjugate (5αP-BSA-FITC). Treatment of cells with either 5αP or membrane-impermeable 5αP-BSA resulted in significant increases in cell proliferation and detachment. 5αP and 5αP-BSA equally activated the MAPK/ERK1/2 pathway as evidenced by phosphorylation of ERK1/2. Inhibitors (PD98059, mevastatin and genistein) of specific sites along the Ras/Raf/MEK/ERK signaling pathway, blocked the phosphorylation and concomitantly inhibited 5αP-induced stimulation of cell proliferation and detachment. The study has identified high affinity, stereo-specific binding sites for 5αP that have the characteristics of a functional membrane 5αP receptor, and has shown that the cancer-promoter actions of 5αP are mediated from the liganded receptor via the MAPK/ERK1/2 signaling cascade. The findings enhance our understanding of the role of the progesterone metabolite 5αP in breast cancer and should promote new approaches to the development of breast cancer diagnostics and therapeutics.

A Role for Histone H2B Variants in Endocrine-Resistant Breast Cancer

Acquired resistance to aromatase inhibitors (AIs) remains a major clinical problem in the treatment of estrogen receptor-positive (ER+) breast cancer. We and others have previously reported widespread changes in DNA methylation using breast cancer cell line models of endocrine resistance. Here, we show that the histone variant HIST1H2BE is hypomethylated in estrogen deprivation-resistant C4-12 and long-term estrogen-deprived (LTED) cells compared with parental MCF-7 cells. As expected, this hypomethylation associates with increased expression of HIST1H2BE in C4-12 and LTED cells. Both overexpression and downregulation of HIST1H2BE caused decreased proliferation in breast cancer cell lines suggesting the need for tightly controlled expression of this histone variant. Gene expression analysis showed varied expression of HIST1H2BE in a large panel of breast cancer cell lines, without restriction to specific molecular subtypes. Analysis of HIST1H2BE messenger RNA (mRNA) expression in ER+ AI-treated breast tumors showed significantly higher expression in resistant (n = 19) compared with sensitive (n = 37) tumors (p = 0.01). Using nanostring analysis, we measured expression of all 61 histone variants in endocrine-resistant and endocrine-sensitive tumors. We found significant overexpression of 22 variant histone genes in tumors resistant to AI therapy. In silico The Cancer Genome Atlas (TCGA) analysis showed frequent amplification of the HIST1 locus. In summary, our studies show, for the first time, that overexpression of histone variants might be important in endocrine response in ER+ breast cancer, and that overexpression is at least in part mediated via epigenetic mechanisms and amplifications. Future studies addressing endocrine response should include a potential role of these currently understudied histone variants.

Crosstalk with insulin and dependence on PI3K/Akt/mTOR rather than MAPK pathways in upregulation of basal growth following long-term oestrogen deprivation in three human breast cancer cell lines

BACKGROUND

MCF-7, T-47-D, ZR-75-1 human breast cancer cell lines are dependent on oestrogen for growth but can adapt to grow during long-term oestrogen deprivation. This serves as a model for identification of therapeutic targets in endocrine-resistant breast cancer.

METHODS

An overlooked complication of this model is that it involves more than non-addition of oestrogen, and inadequate attention has been given to separating molecular events associated with each of the culture manipulations.

RESULTS

Insulin and oestradiol were shown to protect MCF-7 cells against upregulation of basal growth, demonstrating a crosstalk in the growth adaptation process. Increased phosphorylation of p44/42MAPK and c-Raf reflected removal of insulin from the medium and proliferation of all three cell lines was inhibited to a lesser extent by PD98059 and U0126 following long-term oestrogen/insulin withdrawal, demonstrating a reduced dependence on the MAPK pathway. By contrast, long-term oestrogen/insulin deprivation did not alter levels of phosphorylated Akt and did not alter the dose-response of growth inhibition with LY294002 in any of the three cell lines. The IGF1R inhibitor picropodophyllin inhibited growth of all MCF-7 cells but only in the long-term oestrogen/insulin-deprived cells was this paralleled by reduction in phosphorylated p70S6K, a downstream target of mTOR. Long-term oestrogen/insulin-deprived MCF-7 cells had higher levels of phosphorylated p70S6K and developed increased sensitivity to growth inhibition by rapamycin.

CONCLUSIONS

The greater sensitivity to growth inhibition by rapamycin in all three cell lines following long-term oestrogen/insulin deprivation suggests rapamycin-based therapies might be more effective in breast cancers with acquired oestrogen resistance.

Epigenetic reprogramming of HOXC10 in endocrine-resistant breast cancer

Resistance to aromatase inhibitors (AIs) is a major clinical problem in the treatment of estrogen receptor (ER)-positive breast cancer. In two breast cancer cell line models of AI resistance, we identified widespread DNA hyper- and hypomethylation, with enrichment for promoter hypermethylation of developmental genes. For the homeobox gene HOXC10, methylation occurred in a CpG shore, which overlapped with a functional ER binding site, causing repression of HOXC10 expression. Although short-term blockade of ER signaling caused relief of HOXC10 repression in both cell lines and breast tumors, it also resulted in concurrent recruitment of EZH2 and increased H3K27me3, ultimately transitioning to increased DNA methylation and silencing of HOXC10. Reduced HOXC10 in vitro and in xenografts resulted in decreased apoptosis and caused antiestrogen resistance. Supporting this, we used paired primary and metastatic breast cancer specimens to show that HOXC10 was reduced in tumors that recurred during AI treatment. We propose a model in which estrogen represses apoptotic and growth-inhibitory genes such as HOXC10, contributing to tumor survival, whereas AIs induce these genes to cause apoptosis and therapeutic benefit, but long-term AI treatment results in permanent repression of these genes via methylation and confers resistance. Therapies aimed at inhibiting AI-induced histone and DNA methylation may be beneficial in blocking or delaying AI resistance.

Acquired resistance to selective estrogen receptor modulators (SERMs) in clinical practice (tamoxifen & raloxifene) by selection pressure in breast cancer cell populations

Tamoxifen, a pioneering selective estrogen receptor modulator (SERM), has long been a therapeutic choice for all stages of estrogen receptor (ER)-positive breast cancer. The clinical application of long-term adjuvant antihormone therapy for the breast cancer has significantly improved breast cancer survival. However, acquired resistance to SERM remains a significant challenge in breast cancer treatment. The evolution of acquired resistance to SERMs treatment was primarily discovered using MCF-7 tumors transplanted in athymic mice to mimic years of adjuvant treatment in patients. Acquired resistance to tamoxifen is unique because the growth of resistant tumors is dependent on SERMs. It appears that acquired resistance to SERM is initially able to utilize either E2 or a SERM as the growth stimulus in the SERM-resistant breast tumors. Mechanistic studies reveal that SERMs continuously suppress nuclear ER-target genes even during resistance, whereas they function as agonists to activate multiple membrane-associated molecules to promote cell growth. Laboratory observations in vivo further show that three phases of acquired SERM-resistance exists, depending on the length of SERMs exposure. Tumors with Phase I resistance are stimulated by both SERMs and estrogen. Tumors with Phase II resistance are stimulated by SERMs, but are inhibited by estrogen due to apoptosis. The laboratory models suggest a new treatment strategy, in which limited-duration, low-dose estrogen can be used to purge Phase II-resistant breast cancer cells. This discovery provides an invaluable insight into the evolution of drug resistance to SERMs, and this knowledge is now being used to justify clinical trials of estrogen therapy following long-term antihormone therapy. All of these results suggest that cell populations that have acquired resistance are in constant evolution depending upon selection pressure. The limited availability of growth stimuli in any new environment enhances population plasticity in the trial and error search for survival.

Genomic and non-genomic actions of progestogens in the breast

Evidence is growing that progestogens may enhance breast cancer risk under hormone therapy in the postmenopause or hormonal contraception. However, differences may exist within the progestogen class and certain progestogens may have a higher potency in terms of breast cancer risk. The mechanism(s) by which these progestogens might influence breast cancer risk appear to be mediated via genomic and/or non-genomic effects triggered by activated progestogen receptors. In general, regulation of gene expression by progestogen receptors seems to be a multifactorial process involving both actions which often converge. In the present review, we describe the known genomic and non-genomic effects in the breast, especially focusing on the progestins. This article is part of a Special Issue entitled 'Menopause'.

AroER tri-screen is a biologically relevant assay for endocrine disrupting chemicals modulating the activity of aromatase and/or the estrogen receptor

Endocrine disrupting chemicals (EDCs) interfere with the biosynthesis, metabolism, and functions of steroid hormones, including estrogens and androgens. Aromatase enzyme converts androgen to estrogen. Thus, EDCs against aromatase significantly impact estrogen- and/or androgen-dependent functions, including the development of breast cancer. The current study aimed to develop a biologically relevant cell-based high-throughput screening assay to identify EDCs that act as aromatase inhibitors (AIs), estrogen receptor (ER) agonists, and/or ER antagonists. The AroER tri-screen assay was developed by stable transfection of ER-positive, aromatase-expressing MCF-7 breast cancer cells with an estrogen responsive element (ERE) driven luciferase reporter plasmid. The AroER tri-screen can identify: estrogenic EDCs, which increase luciferase signal without 17β-estradiol (E2); anti-estrogenic EDCs, which inhibit the E2-induced luciferase signal; and AI-like EDCs, which suppress a testosterone-induced luciferase signal. The assay was first optimized in a 96-well plate format and then miniaturized into a 1536-well plate format. The AroER tri-screen was demonstrated to be suitable for high-throughput screening in the 1536-well plate format, with a 6.9-fold signal-to-background ratio, a 5.4% coefficient of variation, and a screening window coefficient (Z-factor) of 0.78. The assay suggested that bisphenol A (BPA) functions mainly as an ER agonist. Results from screening the 446 drugs in the National Institutes of Health Clinical Collection revealed 106 compounds that modulated ER and/or aromatase activities. Among these, two AIs (bifonazole and oxiconazole) and one ER agonist (paroxetine) were confirmed through alternative aromatase and ER activity assays. These findings indicate that AroER tri-screen is a useful high-throughput screening system for identifying ER ligands and aromatase-inhibiting chemicals.

Novel cancer-targeting SPECT/NIRF dual-modality imaging probe (99m)Tc-PC-1007: synthesis and biological evaluation

Synthesis, characterization, in vitro and in vivo biological evaluation of a heptamethine cyanine based dual-mode single-photon emission computed tomography (SPECT)/near infrared fluorescence (NIRF) imaging probe (99m)Tc-PC-1007 is described. (99m)Tc-PC-1007 exhibited preferential accumulation in human breast cancer MCF-7 cells. Cancer-specific SPECT/CT and NIRF imaging of (99m)Tc-PC-1007 was performed in a breast cancer xenograft model. The probe uptake ratio of tumor to control (spinal cord) was calculated to be 4.02±0.56 at 6 h post injection (pi) and 8.50±1.41 at 20 h pi (P<0.0001). Pharmacokinetic parameters such as blood clearance and organ distribution were assessed.

Clinical instability of breast cancer markers is reflected in long-term in vitro estrogen deprivation studies

BACKGROUND

Long-term estrogen deprivation models are widely employed in an in vitro setting to recapitulate the hormonal milieu of breast cancer patients treated with endocrine therapy. Despite the wealth information we have garnered from these models thus far, a comprehensive time-course analysis of the estrogen (ER), progesterone (PR), and human epidermal growth factor 2 (HER-2/neu) receptors on the gene and protein level, coupled with expression array data is currently lacking. We aimed to address this knowledge gap in order to enhance our understanding of endocrine therapy resistance in breast cancer patients.

METHODS

ER positive MCF7 and BT474 breast cancer cells were grown in estrogen depleted medium for 10 months with the ER negative MDA-MB-231 cell line employed as control. ER, PR and HER-2/neu expression were analysed at defined short and long-term time points by immunocytochemistry (ICC), and quantitative real-time RT-PCR (qRT-PCR). Microarray analysis was performed on representative samples.

RESULTS

MCF7 cells cultured in estrogen depleted medium displayed decreasing expression of ER up to 8 weeks, which was then re-expressed at 10 months. PR was also down-regulated at early time points and remained so for the duration of the study. BT474 cells generally displayed no changes in ER during the first 8 weeks of deprivation, however its expression was significantly decreased at 10 months. PR expression was also down-regulated early in BT474 samples and was absent at later time points. Finally, microarray data revealed that genes and cell processes down-regulated in both cell lines at 6 weeks overlapped with those down-regulated in aromatase inhibitor treated breast cancer patients.

CONCLUSIONS

Our data demonstrate that expression of ER, PR, and cell metabolic/proliferative processes are unstable in response to long-term estrogen deprivation in breast cancer cell lines. These results mirror recent clinical findings and again emphasize the utility of LTED models in translational research.

Investigating new therapeutic strategies targeting hyperinsulinemia's mitogenic effects in a female mouse breast cancer model

Epidemiological and experimental studies have identified hyperinsulinemia as an important risk factor for breast cancer induction and for the poor prognosis in breast cancer patients with obesity and type 2 diabetes. Recently it was demonstrated that both the insulin receptor (IR) and the IGF-IR mediate hyperinsulinemia's mitogenic effect in several breast cancer models. Although IGF-IR has been intensively investigated, and anti-IGF-IR therapies are now in advanced clinical trials, the role of the IR in mediating hyperinsulinemia's mitogenic effect remains to be clarified. Here we aimed to explore the potential of IR inhibition compared to dual IR/IGF-IR blockade on breast tumor growth. To initiate breast tumors, we inoculated the mammary carcinoma Mvt-1 cell line into the inguinal mammary fat pad of the hyperinsulinemic MKR female mice, and to study the role of IR, we treated the mice bearing tumors with the recently reported high-affinity IR antagonist-S961, in addition to the well-documented IGF-IR inhibitor picropodophyllin (PPP). Although reducing IR activation, with resultant severe hyperglycemia and hyperinsulinemia, S961-treated mice had significantly larger tumors compared to the vehicle-treated group. This effect maybe secondary to the severe hyperinsulinemia mediated via the IGF-1 receptor. In contrast, PPP by partially inhibiting both IR and IGF-IR activity reduced tumor growth rate with only mild metabolic consequences. We conclude that targeting (even partially) both IR and IGF-IRs impairs hyperinsulinemia's effects in breast tumor development while simultaneously sparing the metabolic abnormalities observed when targeting IR alone with virtual complete inhibition.

Adaptive changes results in activation of alternate signaling pathways and resistance to aromatase inhibitor resistance

Hormone therapy is an effective approach for the treatment of breast cancer. The antiestrogen tamoxifen has had a major impact on the disease. Recently, aromatase inhibitors which reduce estrogen synthesis have proved to be more effective and these agents are now used as first line therapy for postmenopausal breast cancer. Nevertheless, resistance to treatment eventually may occur. We have investigated mechanisms involved in resistance to AIs and devised strategies to overcome the resistance. Using a xenograft model, we have identified adaptive changes that results in activation of alternate signaling pathways in tumors resistant to aromatase inhibitors. Expression of ERα and aromatase was decreased in the tumors after long term treatment with AIs. In contrast increased expression was observed of tyrosine kinase receptors such as HER-2 and IGFR as well as of downstream signaling proteins, including MAPK. We have demonstrated functional activation of the MAPKinase pathway and shown dependency on growth factor receptor signaling in letrozole resistant cells. Furthermore, our studies indicate that HER-2 is a negative regulator of ER. Thus, when HER-2 was blocked with antibody (herceptin, trastuzumab) ER expression was increased rendering the cells and tumors responsive to aromatase inhibitors and resulting in tumor regression.

The molecular mechanisms underlying the pharmacological actions of ER modulators: implications for new drug discovery in breast cancer

Our understanding of the molecular mechanisms underlying the pharmacological actions of estrogen receptor (ER) ligands has evolved considerably in recent years. Much of this knowledge has come from a detailed dissection of the mechanism(s) of action of the Selective Estrogen Receptor Modulators (SERMs) tamoxifen and raloxifene, so called for their ability to function as ER agonists or antagonists depending on the tissue in which they operate. These mechanistic insights have had a significant impact on the discovery of second generation SERMs, some of which are in late stage clinical development for the treatment/prevention of breast cancer as well as other estrogenopathies. In addition to the SERMs, however, have emerged the Selective Estrogen Degraders (SERDs), which as their name suggests, interact with and facilitate ER turnover in cells. One drug of this class, fulvestrant, has been approved as a third line treatment for ER-positive metastatic breast cancer. Whereas the first generation SERMs/SERDs were discovered in a serendipitous manner, this review will highlight how our understanding of the molecular pharmacology of ER ligands has been utilized in the development of the next generation of SERMs/SERDs, some of which are likely to have a major impact on the pharmacotherapy of breast cancer.

The turnover of estrogen receptor α by the selective estrogen receptor degrader (SERD) fulvestrant is a saturable process that is not required for antagonist efficacy

It has become apparent of late that even in tamoxifen and/or aromatase resistant breast cancers, ERα remains a bona fide therapeutic target. Not surprisingly, therefore, there has been considerable interest in developing Selective ER Degraders (SERDs), compounds that target the receptor for degradation. Currently, ICI 182,780 (ICI, fulvestrant) is the only SERD approved for the treatment of breast cancer. However, the poor pharmaceutical properties of this injectable drug and its lack of superiority over second line aromatase inhibitors in late stage breast cancer have negatively impacted its clinical use. These findings have provided the impetus to develop second generation, orally bioavailable SERDs with which quantitative turnover of ERα in tumors can be achieved. Interestingly however, the contribution of SERD activity to fulvestrant efficacy is unclear, making it difficult to define the characteristics desired of the next generation of ER antagonists. It is of significance therefore, that we have determined that the antagonist activity of ICI and its ability to induce ERα degradation are not coupled processes. Specifically, our results indicate that it is the ability of ICI to interact with ERα and to (a) competitively displace estradiol and (b) induce a conformational change in ER incompatible with transcriptional activation that are likely to be the most important pharmacological characteristics of this drug. Collectively, these data argue for a renewed emphasis on the development of high affinity, orally bioavailable pure antagonists and suggest that SERD activity though proven effective may not be required for ERα antagonism in breast cancer.

An "omics" approach to determine the mechanisms of acquired aromatase inhibitor resistance

Aromatase inhibitors (AIs) are the major types of drugs to treat hormone-dependent breast cancer. Although these drugs work effectively, cancer still recurs in many patients after treatment as a result of acquired resistance to the AIs. To characterize the resistant mechanisms, a set of MCF-7aro cell lines that acquired resistance to the AIs was generated. Through an "Omics" approach, we found that the resistance mechanisms of the three AIs (anastrozole, letrozole, and exemestane) differ and activation of estrogen receptor alpha (ERα) is critical for acquired AI resistance. Our results reveal that growth factor/signal transduction pathways are upregulated after ERα-dependent pathways are suppressed by AIs, and ERα can then be activated through different crosstalk mechanisms.

Targeting the receptor tyrosine kinase RET sensitizes breast cancer cells to tamoxifen treatment and reveals a role for RET in endocrine resistance

Endocrine therapy is the main therapeutic option for patients with estrogen receptor (ERalpha)-positive breast cancer. Resistance to this treatment is often associated with estrogen-independent activation of ERalpha. In this study, we show that in ERalpha-positive breast cancer cells, activation of the receptor tyrosine kinase RET (REarranged during Transfection) by its ligand GDNF results in increased ERalpha phosphorylation on Ser118 and Ser167 and estrogen-independent activation of ERalpha transcriptional activity. Further, we identify mTOR as a key component in this downstream signaling pathway. In tamoxifen response experiments, RET downregulation resulted in 6.2-fold increase in sensitivity of MCF7 cells to antiproliferative effects of tamoxifen, whereas GDNF stimulation had a protective effect against the drug. In tamoxifen-resistant (TAM(R)-1) MCF7 cells, targeting RET restored tamoxifen sensitivity. Finally, examination of two independent tissue microarrays of primary human breast cancers revealed that expression of RET protein was significantly associated with ERalpha-positive tumors and that in primary tumors from patients who subsequently developed invasive recurrence after adjuvant tamoxifen treatment, there was a twofold increase in the number of RET-positive tumors. Together these findings identify RET as a potentially important therapeutic target in ERalpha-positive breast cancers and in particular in tamoxifen-resistant tumors.

Mechanisms of resistance to structurally diverse antiestrogens differ under premenopausal and postmenopausal conditions: evidence from in vitro breast cancer cell models

This study questioned whether the mechanisms of resistance to antiestrogens differ when acquired under premenopausal (Pre-M) vs. postmenopausal (PM) conditions and whether structurally diverse antiestrogens induce adaptation of differing signaling pathways. To address this issue, we conducted systematic studies under Pre-M vs. PM culture conditions with long-term exposure to different antiestrogens and examined the resultant "specific biologic signatures" of the various resistant cells. Estradiol stimulated growth and inhibited apoptosis of "pre-menopausal" antiestrogen-resistant cells but exerted opposite effects on their "post-menopausal" counterparts. Under Pre-M conditions, tamoxifen (TAM)-resistant cells exhibited a marked translocation of estrogen receptor alpha from the nucleus into the cytoplasm, whereas this occurred to a lesser extent under PM conditions. MCF-7 cells exposed to PM but not Pre-M conditions exhibited up-regulation of basal epidermal growth factor (EGF) receptor (EGFR) levels, an effect exaggerated in cells exposed to 4-hydroxytamoxifen. Differing effects occurred in response to structurally divergent antiestrogens. Long-term treatment with both 4-hydroxytamoxifen and ICI182,780 increased EGFR levels, but this was not seen in response to TAM. Surprisingly, EGF administration slightly increased cell number in TAM-resistant cells, whereas only increasing cell weight and decreasing cell number in EGFR overexpressing-resistant cells. To assess potential differences among various parental cell lines, we induced resistance in cell lines obtained from other laboratories and confirmed the results from our own parental cells with minor differences. Together, these data demonstrate that culture of breast cancer cells under Pre-M and PM conditions and structurally diverse antiestrogens results in adaptive responses with differing biological signatures.

Aromatase inhibitors and breast cancer

We have developed a breast cancer intratumoral aromatase model to simulate the postmenopausal breast cancer patient in order to compare the antitumor efficacy of aromatase inhibitors (AIs) and antiestrogens (AEs). The AI letrozole sustained growth inhibition longer than the AE tamoxifen. Nevertheless, eventually tumors began to grow despite continued treatment. Estrogen receptor-alpha (ER-alpha) levels decreased below control levels concomitant with increased phosphorylation of ER-alpha and unaltered progesterone receptor (PgR) levels. Expression of Her-2, p-Shc, Grb-2, p-Raf, p-Mekl/2, and p-MAPK was increased in the letrozole-resistant tumors. When cells isolated from letrozole-resistant tumors (LTLTCa cells) were treated with inhibitors of the Her-2 signaling pathway, such as trastuzumab (herceptin), ER-alpha was restored. Furthermore, sensitivity of LTLTCa cells to AIs and AEs was regained. These findings suggest cross-talk between ER and Her-2 signaling. To prevent activation of the Her-2 pathway and resistance to AIs, mice were treated with a combination of an AI anastrozole and the ER downregulator fulvestrant. This resulted in no increase in Her-2 and p-MAPK levels, and tumor growth was significantly inhibited. Thus, blocking both ER and Her-2 signaling delayed development of resistance to AIs. This hypothesis was supported by the finding that growth of letrozole-resistant tumors was reduced when xenografts were treated with trastuzumab combined with letrozole. In addition, resistance to letrozole could be reversed by discontinuing letrozole. Our findings indicate that after letrozole treatment is stopped, the antitumor effect of letrozole can be restored when the AI treatment is resumed.

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.

Modulation of Delta9-tetrahydrocannabinol-induced MCF-7 breast cancer cell growth by cyclooxygenase and aromatase

Delta(9)-Tetrahydrocannabinol (Delta(9)-THC), a major constituent of marijuana, has been shown to stimulate the growth of MCF-7 breast cancer cells through cannabinoid receptor-independent signaling [Takeda, S., Yamaori, S., Motoya, E., Matsunaga, T., Kimura, T., Yamamoto, I., Watanabe, K., 2008. Delta(9)-Tetrahydrocannabinol enhances MCF-7 cell proliferation via cannabinoid receptor-independent signaling. Toxicology 245, 141-146]. Although the growth of MCF-7 cells is known to be stimulated by 17beta-estradiol (E(2)), the interaction of Delta(9)-THC and E(2) in MCF-7 cell growth is not fully clarified so far. In the present study, by using E(2)-sensitive MCF-7 cells that have expressed cyclooxygenase-2 (COX-2) and cytochrome P450 19 (aromatase), we studied whether or not COX-2 and aromatase are involved in Delta(9)-THC-mediated MCF-7 cell proliferation. It was shown that Delta(9)-THC-induced MCF-7 cell growth was inhibited by COX-2 inhibitors and was stimulated by arachidonic acid (a COX substrate). However, the growth of MCF-7 cells induced by Delta(9)-THC was not stimulated by PGE(2), and the expression of aromatase was not affected by COX-2 inhibitors, arachidonic acid, and PGE(2), suggesting that there is a disconnection between COX-2 (PGE(2)) and aromatase in Delta(9)-THC-mediated MCF-7 cell proliferation. On the other hand, Delta(9)-THC-induced MCF-7 cell growth was elevated by two kinds of aromatase inhibitors. Taken together with the evidence that Delta(9)-THC-induced MCF-7 cell proliferation was interfered with testosterone (an aromatase substrate) and exogenously provided E(2), it is suggested that (1) the growth stimulatory effects of Delta(9)-THC are mediated by the product(s) of COX-2 except for PGE(2), (2) the action of Delta(9)-THC is modulated by E(2), and (3) COX-2 and aromatase are individually engaged in the proliferation of MCF-7 cells induced by Delta(9)-THC.

Combination of anastrozole with fulvestrant in the intratumoral aromatase xenograft model

Although the aromatase inhibitor anastrozole has been shown to be very effective in the treatment of hormone-dependent postmenopausal breast cancer, some patients with advanced disease will develop resistance to treatment. To investigate therapeutic strategies to overcome resistance to anastrozole treatment, we have used an intratumoral aromatase model that simulates postmenopausal breast cancer patients with estrogen-dependent tumors. Growth of the tumors in the mice was inhibited by both anastrozole and fulvestrant compared with the control tumors. Nevertheless, tumors had doubled in size at 5 weeks of treatment. We therefore investigated whether switching the original treatments to anastrozole or fulvestrant alone or the combination of anastrozole plus fulvestrant would reduce tumor growth. The results showed that the best strategy to reverse the insensitivity to anastrozole or fulvestrant is to combine the two agents. Additionally, the tumors treated with anastrozole plus fulvestrant from the beginning had only just doubled their size after 14 weeks of treatment, whereas the anastrozole and fulvestrant treatments alone resulted in 9- and 12-fold increases in tumor size, respectively, in the same time period. Anastrozole plus fulvestrant from the beginning or in sequence was associated with down-regulation of signaling proteins involved in the development of hormonal resistance such as insulin-like growth factor type I receptor beta, mitogen-activated protein kinase (MAPK), p-MAPK, AKT, mammalian target of rapamycin (mTOR), p-mTOR, and estrogen receptor alpha compared with tumors treated with anastrozole or fulvestrant alone. These results suggest that blocking the estrogen receptor and aromatase may delay or reverse the development of resistance to aromatase inhibitors in advanced breast cancer patients.

Preclinical modeling of endocrine response and resistance: focus on aromatase inhibitors

The authors developed a breast cancer intratumoral aromatase model system to compare the antitumor efficacy of several aromatase inhibitors (AIs) and antiestrogens (AEs). Although the AI letrozole caused sustained growth inhibition, tumors eventually began to grow, even when treatment was maintained. For the current study, the mechanisms of resistance to letrozole during the course of treatment were investigated. Estrogen receptor alpha (ER-alpha) levels decreased below control levels in letrozole-resistant tumors. The decrease was simultaneous to an increase in phosphorylation of ER-alpha and an unaltered expression of progesterone receptor (PgR). Expression levels of HER-2, activated (phosphorylated) SHC-adaptor protein (p-Shc), growth factor receptor-bound protein 2 (Grb-2), p-Raf, phosphorylated mitogen-activated protein kinase kinase 1/2 (p-Mekl/2), and phosphorylated mitogen-activated protein kinase (p-MAPK) were increased. When cells isolated from letrozole-resistant tumors (LTLTCa cells) were treated with inhibitors of the HER-2 signaling pathway, ER-alpha expression and estradiol-stimulated transactivation was restored. The HER-2 blocker trastuzumab also restored the sensitivity of LTLTCa cells to AIs and AEs. These findings suggested that there is crosstalk between ER and HER-2 signaling. To prevent activation of the HER-2 pathway and resistance to AIs, mice were treated with a combination of AIs and the ER down-regulator fulvestrant. There was no increase in HER-2 or p-MAPK expression, and tumor growth was inhibited significantly. When trastuzumab was added to unresponsive tumors under letrozole treatment, it significantly inhibited tumors growth compared with switching to trastuzumab alone. However, the trastuzumab plus letrozole combination was more effective than letrozole alone only in refractory breast tumors. These results suggested that blocking both ER and HER-2 signaling may delay the development of resistance to AIs in patients with recurrent breast cancer.

17beta-estradiol promotes breast cancer cell proliferation-inducing stromal cell-derived factor-1-mediated epidermal growth factor receptor transactivation: reversal by gefitinib pretreatment

The coordinated activity of estrogens and epidermal growth factor receptor (EGFR) family agonists represents the main determinant of breast cancer cell proliferation. Stromal cell-derived factor-1 (SDF-1) enhances extracellular signal-regulated kinases 1 and 2 (ERK1/2) activity via the transactivation of EGFR and 17beta-estradiol (E2) induces SDF-1 production to exert autocrine proliferative effects. On this basis, we evaluated whether the inhibition of the tyrosine kinase (TK) activity of EGFR may control different mitogenic stimuli in breast tumors using the EGFR-TK inhibitor gefitinib to antagonize the proliferation induced by E2 in T47D human breast cancer cells. EGF, E2, and SDF-1 induced a dose-dependent T47D cell proliferation, that being nonadditive suggested the activation of common intracellular pathways. Gefitinib treatment inhibited not only the EGF-dependent proliferation and ERK1/2 activation but also the effects of SDF-1 and E2, suggesting that these activities were mediated by EGFR transactivation. Indeed, both SDF-1 and E2 caused EGFR tyrosine phosphorylation. The molecular link between E2 and SDF-1 proliferative effects was identified because 1,1'-(1,4-phenylenebis(methylene))-bis-1,4,8,11-tetraazacyclotetradecane octahydrochloride (AMD3100), a CXCR4 antagonist, inhibited SDF-1- and E2-dependent proliferation and EGFR and ERK1/2 phosphorylation. EGFR transactivation was dependent on c-Src activation. E2 treatment caused a powerful SDF-1 release from T47D cells. Finally, in SKBR3, E2-resistant cells, EGFR was constitutively activated, and AMD3100 reduced EGFR phosphorylation and cell proliferation, whereas HER2-neu was transactivated by SDF-1 in SKBR3 but not in T47D cells. In conclusion, we show that activation of CXCR4 transduces proliferative signals from the E2 receptor to EGFR, whose inhibition is able to revert breast cancer cell proliferation induced by multiple receptor activation.

Femara and the future: tailoring treatment and combination therapies with Femara

Long-term estrogen deprivation treatment for breast cancer can, in some patients, lead to the activation of alternate cellular pathways, resulting in the re-emergence of the disease. This is a distressing scenario for oncologists and patients, but recent intensive molecular and biochemical studies are beginning to unravel these pathways, revealing opportunities for new targeted treatments. Far from making present therapies redundant, these new discoveries open the door to novel combination therapies that promise to provide enhanced efficacy or overcome treatment resistance. Letrozole, one of the most potent aromatase inhibitors, is the ideal candidate for combination therapy; indeed, it is one of the most intensively studied aromatase inhibitors in the evolving combinatorial setting. Complementary to the use of combination therapy is the development of molecular tools to identify patients who will benefit the most from these new treatments. Microarray gene profiling studies, designed to detect letrozole-responsive targets, are currently under way to understand how the use of the drug can be tailored more efficiently to specific patient needs.

Letrozole in the extended adjuvant setting: MA.17

Relapse after completing adjuvant tamoxifen therapy is a persistent threat for women with hormone-responsive breast cancer. Third-generation aromatase inhibitors, such as letrozole, provide a new option for extended adjuvant hormonal therapy after 5 years of tamoxifen. MA.17 was conducted to determine whether letrozole improves outcome after discontinuation of tamoxifen. Postmenopausal women with hormone receptor-positive breast cancer (N = 5,187) were randomized to letrozole 2.5 mg or placebo once daily for 5 years. At a median follow-up of 30 months, letrozole significantly improved disease-free survival (DFS; P < 0.001), the primary end point, compared with placebo (hazard ratio [HR] for recurrence or contralateral breast cancer 0.58; 95% confidence interval [CI] 0.45, 0.76] P < 0.001). Furthermore, letrozole significantly improved distant DFS (HR = 0.60; 95% CI 0.43, 0.84; P = 0.002) and, in women with node-positive tumors, overall survival (HR = 0.61; 95% CI 0.38, 0.98; P = 0.04). Clinical benefits, including an overall survival advantage, were also seen in women who crossed over from placebo to letrozole after unblinding, indicating that tumors remain sensitive to hormone therapy despite a prolonged period since discontinuation of tamoxifen. The efficacy and safety of letrozole therapy beyond 5 years is being assessed in a re-randomization study, following the emergence of new data suggesting that clinical benefit correlates with the duration of letrozole. MA.17 showed that letrozole is extremely well-tolerated relative to placebo. Letrozole should be considered for all women completing tamoxifen; new results from the post-unblinding analysis suggest that letrozole treatment should also be considered for all disease-free women for periods up to 5 years following completion of adjuvant tamoxifen.

Mechanisms of acquired resistance to endocrine therapy in hormone-dependent breast cancer cells

Acquired resistance is a major problem limiting the clinical benefit of endocrine therapy. To investigate the mechanisms involved, two in vitro models were developed from MCF-7 cells. Long-term culture of MCF-7 cells in estrogen deprived medium (LTED) mimics aromatase inhibition in patients. Continued exposure of MCF-7 to tamoxifen represents a model of acquired resistance to antiestrogens (TAM-R). Long-term estrogen deprivation results in sustained activation of the ERK MAP kinase and the PI3 kinase/mTOR pathways. Using a novel Ras inhibitor, farnesylthiosalicylic acid (FTS), to achieve dual inhibition of the pathways, we found that the mTOR pathway plays the primary role in mediation of proliferation of LTED cells. In contrast to the LTED model, there is no sustained activation of ERK MAPK but enhanced responsiveness to rapid stimulation induced by E(2) and TAM in TAM-R cells. An increased amount of ERalpha formed complexes with EGFR and c-Src in TAM-R cells, which apparently resulted from extra-nuclear redistribution of ERalpha. Blockade of c-Src activity drove ERalpha back to the nucleus and reduced ERalpha-EGFR interaction. Prolonged blockade of c-Src activity restored sensitivity of TAM-R cells to tamoxifen. Our results suggest that different mechanisms are involved in acquired endocrine resistance and the necessity for individualized treatment of recurrent diseases.

Xenograft models for aromatase inhibitor studies

As several aromatase inhibitors are now available for treating breast cancer, we developed a model system to compare their antitumor efficacy and to explore strategies for their optimal use. Tumors are grown in ovariectomized, immunodeficient mice from MCF-7 human breast cancer cells transfected with the aromatase gene (MCF-7Ca) and can therefore synthesize as well as respond to estrogen. Results from this model have been predictive of clinical outcome. Thus, inhibiting estrogen action and estrogen synthesis by treating mice with the aromatase inhibitor letrozole and the antiestrogen tamoxifen in combination did not result in synergy. Moreover, when tamoxifen treatment was no longer effective, tumor growth was significantly reduced in response to sequential letrozole treatment. However, our findings indicate that letrozole alone was better than all other treatments. Although letrozole resulted in long sustained growth inhibition, tumors eventually grew despite continued treatment. Mechanisms of resistance to letrozole were investigated during the course of treatment. ER was initially upregulated in responding tumors, but subsequently decreased below control levels in tumors no longer responsive to letrozole. Her-2 as well as adapter proteins (p-Shc and Grb-2) and signaling proteins in the MAPK cascade (p-Raf, p-Mekl/2, and p-MAPK), were all increased in letrozole resistant tumors. In LTLT cells, isolated from the letrozole resistant tumors and treated with inhibitors of the MAPKinase pathway, MAPK activity was decreased and ER expression restored to control levels. Inhibitors of EGFR/Her-2 also restored the sensitivity of LTLT cells to letrozole. These results suggest that crosstalk occurs between ER and tyrosine kinase receptor signaling. Therefore, to investigate whether down-regulating ER would prevent activation of MAPK and resistance to letrozole, xenografts were treated with letrozole and faslodex in combination. Her-2 and MAPK were not increased and tumor growth was inhibited throughout 29 weeks of treatment. These results suggest that blocking both ER and growth factor mediated transcription may delay development of resistance to letrozole and maintain its growth inhibition of breast cancer.

Inhibition of the phosphatidylinositol 3-kinase/Akt pathway improves response of long-term estrogen-deprived breast cancer xenografts to antiestrogens

PURPOSE

Aromatase inhibitors that block the synthesis of estrogen are proving to be superior to antiestrogens and may replace tamoxifen as first-line treatment for postmenopausal estrogen receptor (ER)-positive breast cancer patients. However, acquisition of resistance to all forms of treatments is inevitable and a major clinical concern. In this study, we have investigated the effects of long-term estrogen deprivation in the breast cancer xenograft model and whether sensitivity to antiestrogens can be restored in vivo. We also compared whether combining wortmannin with tamoxifen or fulvestrant inhibited tumor growth better than either drug alone.

EXPERIMENTAL DESIGN

Long-term estrogen-deprived aromatase-transfected human ER-positive breast cancer cells (UMB-1Ca) were grown as tumors in ovariectomized athymic nude mice. Twelve weeks after inoculation, when tumors reached 300 mm(3), animals were grouped and injected with vehicle, Delta(4)A, letrozole, tamoxifen, fulvestrant, wortmannin, tamoxifen plus wortmannin, and wortmannin plus fulvestrant. Tumor volumes were measured weekly.

RESULTS

Tumors of UMB-1Ca cells grew equally well with and without androstenedione, indicating the ability of the cells to proliferate in the absence of estrogen. The combination of wortmannin with tamoxifen or fulvestrant inhibited tumor growth better than either drug alone. The combination of wortmannin plus fulvestrant was the most effective treatment that maintained tumor regression for a prolonged time.

CONCLUSION

These results suggest that blocking both ER and growth factor receptor pathways could provide effective control over tumor growth of long-term estrogen-deprived human breast cancers.

Demethylation of promoter C region of estrogen receptor alpha gene is correlated with its enhanced expression in estrogen-ablation resistant MCF-7 cells

Long-term estrogen deprivation (LTED) MCF-7 cells showing estrogen-independent growth, express estrogen receptor (ER) alpha at a much higher level than wild-type MCF-7 cells. Enhanced expression of ERalpha associated with partial localization of ERalpha to the plasma membranes in LTED cells is thought to be an important step for acquisition of estrogen-ablation resistance. In this study, we compared the regulation of ERalpha gene expression between wild type and LTED cells, examining the usage of the promoters A and C as well as their methylation status. We found that transcription from the promoter C was drastically enhanced in LTED cells, compared with that in wild-type cells. Furthermore, the promoter C region was highly unmethylated in LTED cells, but partially methylated in wild-type cells. Our findings imply that demethylation of promoter C region in the ERalpha gene is in part responsible for the enhanced expression of ERalpha gene in LTED cells.

A20/TNFAIP3, a new estrogen-regulated gene that confers tamoxifen resistance in breast cancer cells

The zinc-finger protein A20/TNFAIP3, an inhibitor of nuclear factor-kappaB (NF-kappaB) activation, has been shown to protect MCF-7 breast carcinoma cells from TNFalpha-induced apoptosis. As estrogen receptor (ER) status is an important parameter in the development and progression of breast cancer, we analysed the effect of 17beta-estradiol (E2) treatment on the expression of A20. We found that A20 is a new E2-regulated gene, whose expression correlates with ER expression in both cell lines and tumor samples. With the aim of investigating the impact of A20 expression on MCF-7 cells in response to ER ligands, we established stably transfected-MCF-7 cells overexpressing A20 (MCF-7-A20). These cells exhibited a phenotype of resistance to the 4-hydroxy-tamoxifen cytostatic and pro-apoptotic actions and of hyper-response to E2. Dysregulations in bax, bcl2, bak, phospho-bad, cyclin D1, cyclin E2, cyclin D2 and cyclin A2 proteins expression were shown to be related to the resistant phenotype developed by the MCF-7-A20 cells. Interestingly, we found that A20 was also overexpressed in MVLN and VP tamoxifen-resistant cell lines. Furthermore, high A20 expression levels were observed in more aggressive breast tumors (ER-negative, progesterone receptor-negative and high histological grade). These overall findings strongly suggest that A20 is a key protein involved in tamoxifen resistance, and thus represents both a new breast cancer marker and a promising target for developing new strategies to prevent the emergence of acquired mechanisms of drug resistance in breast cancer.

Farnesylthiosalicylic acid: inhibition of proliferation and enhancement of apoptosis of hormone-dependent breast cancer cells

Farnesyltransferase inhibitors (FTIs) are being developed to block Ras-mediated actions, but current data suggest that the FTIs act through other non-Ras pathways. A new agent, farnesylthiosalicylic acid (FTS), blocks the binding of Ras to membrane acceptor sites and causes a marked reduction in Ras levels. Accordingly, FTS could be a useful new agent for the treatment of hormone-dependent breast cancer. We examined the dose-response effects of FTS on the growth of MCF-7 breast cancer cells in vitro and in vivo. Further, we dissected out its specific effects on cell proliferation and apoptosis by measuring BrdU incorporation into DNA and by using an ELISA assay to quantitate the magnitude of apoptosis. FTS and its solubilized conjoiner FTS-cyclodextrin markedly inhibited cell growth in MCF-7 breast cancer cells in culture and in xenografts. This agent exerted dual effects to reduce cell proliferation as assessed by BrdU incorporation and to enhance apoptosis as quantitated by ELISA assay. These data suggest that FTS is a promising agent to be developed for treatment of hormone-dependent breast cancer.

Estrogenic effect of procymidone through activation of MAPK in MCF-7 breast carcinoma cell line

Procymidone modifies sexual differentiation in vitro and induces estrogenic activity in primary cultured rainbow trout hepatocytes, as shown by an increase in the contents of vitellogenin and heat shock proteins. Since this dicarboximide fungicide is found in human tissues, it was considered of interest to investigate its ability to induce endocrine damage in the MCF-7 human cell line. The mechanism of this estrogenic action was also evaluated. Procymidone 100 microM stimulated cell growth from day 3 up to day 12 and raised the level of pS2 on day 3. Although procymidone does not bind the estrogen receptor (ER), the antiestrogen ICI 182780 inhibited its effect on cell growth and pS2 content, suggesting that the ER is involved indirectly in these effects. In exploring the mechanism of ER indirect activation we found that the antibody against c-Neu receptor (9G6) did not modify procymidone's effects on cell growth and pS2 expression. Thus, procymidone does not bind the c-Neu membrane receptor, excluding this indirect ER activation pathway. We also found that procymidone induced mitogen-activated protein kinase (MAPK) at 15 and 30 min, and that PD 98059, a MAPK (Erk1/2) inhibitor, prevented procymidone's effects on cell growth and pS2, indicating that MAPK activation is responsible for procymidone ER activation. The production of reactive oxygen species (ROS) with these times and elimination of the phenomenon by alpha-tocopherol (alpha-T), a ROS scavenger, is proof that oxygen free-radical production is at the basis of the MAPK activation by procymidone.

Adaptation of estrogen-regulated genes in long-term estradiol deprived MCF-7 breast cancer cells

First line treatment of hormone dependent breast cancer initially causes tumor regression but later results in adaptive changes and tumor re-growth. Responses to second line treatments occur but tumors again begin to progress after a period of 12???18??months. In depth understanding of the adaptive process would allow the identification of targets to abrogate the development of hormonal resistance and prolong the efficacy of endocrine therapy. We have developed a model system to examine adaptive changes in human MCF-7 breast cancer cells. Upon deprivation of estradiol for a prolonged period of time, a maneuver analogous to surgical oophorectomy in pre-menopausal women and use of aromatase inhibitors in post-menopausal patients, tumor cells adapt and become hypersensitive to estradiol. We reasoned that the expression pattern of multiple genes would change in response to estradiol deprivation and that cDNA microarrays would provide an efficient means of assessing these changes. Accordingly, we examined the transcriptional responses to estradiol in long-term estradiol deprived (LTED) MCF-7 cells with a cDNA microarray containing 1901 known genes and ESTs. To assess the changes induced by long-term estradiol deprivation, we compared the effects of estradiol administration in LTED cells with those in MCF-7 cells, which we had previously reported, and confirmed with real time PCR using the parental and LTED cells. Seven genes and one EST were induced by estradiol in LTED but not in wild type MCF-7 cells, whereas ten genes were down-regulated by estradiol only in LTED cells. The expression of seven genes increased concurrently and five decreased in response to estradiol in both cell types. From these observations, we generated testable hypotheses regarding several genes including DKFZP, RAP-1, ribosomal protein S6, and TM4SF1. Based upon the known functions of these genes and the patterns of observed changes, we postulate that divergent regulation of these genes may contribute to the different biologic responses to estrogen in these cell lines. These results provide targets for further mechanistic studies in our experimental system. Our findings indicate that long-term estradiol deprivation causes expression changes in multiple genes and emphasizes the complexity of the process of cellular adaptation.