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Endocrinology |
1Departments of Obstetrics, Gynecology, and Reproductive Sciences, Cellular and Molecular Pharmacology and 2Anatomy, and 3Center for Reproductive Sciences, University of California, San Francisco, and 4Department of Molecular and Cellular Biology, University of California, Berkeley, California
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ABSTRACT |
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 Top ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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mediates breast cancer-promoting effects of estrogens. The role of ERß in breast cancer is unknown. Elucidating the role of ERß in the pathogenesis of breast cancer is important because many human breast tumors express both ER and ERß. We show that adenovirus-mediated expression of ERß changes the phenotype of ER-positive MCF-7 cells. Estradiol increases cell proliferation and causes tumor formation of MCF-7 cells expressing only ER. In contrast, introducing ERß into MCF-7 cells causes an inhibition of proliferation in vitro and prevents tumor formation in a mouse xenograft model in response to estradiol. ERß inhibits proliferation by repressing c-myc, cyclin D1, and cyclin A gene transcription, and increasing the expression of p21Cip1 and p27Kip1, which leads to a G2 cell cycle arrest. These results demonstrate that ER and ERß produce opposite effects in MCF-7 cells on cell proliferation and tumor formation. Natural or synthetic ERß-selective estrogens may lack breast cancer promoting properties exhibited by estrogens in hormone replacement regimens and may be useful for chemoprevention of breast cancer. |
INTRODUCTION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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and ERß (5, 6, 7, 8)
. Understanding the role of each ER in the pathogenesis of breast cancer is vital, because an urgent need exists to develop estrogens for long-term hormone replacement therapy that do not promote breast cancer.
ER knockout mice clearly indicate that ER or ERß have distinct roles in breast development. ER knockout mice have primitive mammary development (9)
, whereas ERß knockout mice develop normal mammary glands (10)
. These observations demonstrate that only ER is required for growth and differentiation of the mouse mammary gland. The precise roles of ER or ERß in breast cancer are unknown. Some studies indicate that ER mediates the tumor promoting effects of estrogens. Estradiol stimulates proliferation of MCF-7 breast cancer cells that express only ER (11)
. A MCF-7 cell line that lost ER does not proliferate with E2, but recovers its capacity to proliferate when ER is reintroduced (12)
. A mutation in ER has been identified in human premalignant breast hyperplasias that leads to enhanced proliferation of MCF-7 cells in response to E2 (13)
.
The role of ERß in breast cancer remains elusive (14
, 15) . Most studies correlating the presence of ERs in human breast tumors with clinical outcomes use antibodies that only detect ER. However, more recent studies demonstrate that ERß is also expressed in many human breast tumors (14)
. Approximately 70% of breast tumors express ERß, and most tumors coexpress both ER and ERß (16
, 17)
. Several studies indicate that ERß expression in human breast tumors is associated with a poorer prognosis, compared with tumors that only express ER (18)
. ERß expression is associated with elevated cell proliferation markers, Ki67 and cyclin A, in human breast tumors (19)
. ERß mRNA is also elevated significantly in the tamoxifen-resistant tumors compared with tamoxifen-sensitive tumors (20)
. These studies suggest that ERß may promote cell proliferation and breast tumor formation.
In contrast, other studies indicate that the presence of ERß in breast tumors confers a more favorable prognosis compared with tumors that contain only ER (21)
. The levels of ERß are highest in normal mammary tissue and it decreases as tumors progress from preinvasive to invasive tumors (22
, 23)
. ERß expression is associated with negative axillary node status, low-grade tumors, and low S phase fraction (24)
, and a greater disease-free survival rate (21)
. ERß expression also showed a strong association with the presence of progesterone receptors and well-differentiated breast tumors (25)
. The presence of ERß in >10% of cancer cells confers a better survival in women treated with tamoxifen (26)
. These studies indicate that ERß may function as a tumor suppressor and that the loss of ERß promotes breast carcinogenesis. Clearly, additional studies are needed to clarify the role of ERß in breast cancer. Because many breast tumors express both ER and ERß, we investigated the effects of ERß on proliferation and tumor formation of MCF-7 breast cancer cells that contain only ER.
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MATERIALS AND METHODS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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or ERß (530 amino acids) were prepared according to the manufacturer’s protocol (BD Biosciences Clontech, Palo Alto, CA). The control virus, Ad-lacZ, was purchased from BD Biosciences Clontech. MCF-7 breast cancer cells were cultured in phenol red-free DMEM/F-12 medium containing 5% fetal bovine serum, 2 mM glutamine, 50 units/ml penicillin, and 50 µg/ml streptomycin. Before the addition of 17ß-estradiol (E2), cells were grown in DMEM/F-12 medium containing 4% stripped fetal bovine serum for 1 week. Cells (5000) were plated in 24-well plates and treated with vehicle or 1 nM E2 for 10 days. [3H]thymidine incorporation was used to quantify DNA synthesis. All of the experiments were done at least three times, and the data were similar between experiments. The data points were done in triplicate, and SE was <%10.
Xenograft Studies in Nude Mice.
MCF-7 cells grown in DMEM/F-12 medium containing 4% stripped fetal bovine serum were infected with Ads for 24 h. The cells were collected, and 250,000 cells were aggregated in suspension and then resuspended in 200 µl of neutralized collagen (27)
. After an overnight incubation, the cells were then grafted under the kidney capsule of nude mouse as described and illustrated in detail elsewhere.5
One month after grafting, tumors were harvested, fixed in 10% phosphate-buffered formalin (Fisher Scientific, Fairlawn, NJ), embedded in paraffin, sectioned, and stained with H&E. Immunohistochemistry of paraffin sections of the tumors was done with Ki67 antibodies (Novocastra Laboratories Ltd., Newcastle, United Kingdom), and the proliferation index was determined as described (27)
. The animal studies were carried out with approval from the University of California, San Francisco committee on animal care.
Immunoblotting.
Cells infected with Ad-LacZ or Ad-ERß were grown in six-well tissue culture plates and treated with 10 nM E2 for times indicated in the figures. At the end of treatment, proteins were extracted in radioimmunoprecipitation assay buffer containing protease and phosphatase inhibitors. Immunoblotting of proteins was performed using standard procedures using antibodies to ER (DAKO Corporation, Carpinteria, CA), ERß (Genetex, San Antonio, TX), and c-myc, cyclin D1, cyclin A, and actin (Oncogene Research Products, Boston, MA). Proteins were visualized using ECL kits (Amersham Life Science, Arlington Heights, IL).
Flow Cytometry.
Cells infected with Ad-LacZ or Ad-ERß were treated with 1 nM E2 for 24, 48, or 96 h. The cells were lysed in 1 ml hypotonic DNA staining solution (0.5 mg/ml propidium iodide, 0.1% sodium citrate, and 0.05% Triton X-100). Nuclear emitted fluorescence with wavelength >585 nm was measured with a Coulter Elite instrument with laser output adjusted to deliver 15 mW at 488 nm. Nuclei (10,000) were analyzed from each sample at a rate of 300–500 cells/s. The percentages of cells in the G1, S, and G2-M phases of the cell cycle were determined with the multicycle computer program (Phoenix Flow Systems, San Diego, CA) as described (28)
.
Real-Time Quantitative Reverse Transcription-PCR Analysis.
Cells infected with Ad-LacZ or Ad-ERß were washed with PBS, and then 1 ml Trizol (Life Technologies, Inc., Grand Island, NY) was added to the cells. Total RNA was prepared according to the manufacturer’s protocol. Real-time quantitative PCR was performed using SYBR Green Supermix (BIO-RAD, Hercules, CA) with an iCycler thermal cycler (BIO-RAD). We used the following primers: p21 Forward 5'-GGCGGCAGACCAGCATGACAGATT-3' and Reverse 5'-GCAGGGGGCGGCCAGGGTAT-3'; p27 Forward 5'-GGGGCTCGTCTTTTCGGGGTGTTT-3' and Reverse 5'GAGCGGGAGGGCGGAGAGGAG-3'; c-myc Forward 5'-GCCCCTCAACGTTAGCTTCA-3' and Reverse 5' TTCCAGATATCCTCGCTGGG-3'; Cyclin D1 Forward 5'-AACTACCTGGACCGCTTCCT-3' and Reverse 5'-CCACTTGAG CTTGTTCACCA-3'; and GUS Forward 5'-CTCATTTGGAATTTTGCCGATT-3' and Reverse 5'-CCGAGTGAAGATCCCCTTTTTA-3'. The data were collected and analyzed using the comparative Ct (threshold cycle) method using GUS expression as the reference gene.
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RESULTS |
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(11)
, whereas ER-negative cells stably transfected with ER display anomalous behavior, because estrogens inhibit proliferation (29)
. Furthermore, MCF-7 cells are the best-characterized ER-positive cell line in terms of known genes regulated by estrogens that promote proliferation. The MCF-7 cells used for these studies expressed ER (Fig. 1A
, top panel, Lane 0) by immunoblotting, but not ERß (Fig. 1A
, bottom panel, Lane 0). The cells were infected for 24 h with 50 or 100 multiplicity of infection (MOI) Ad-ER, Ad-ERß, or Ad-LacZ to control for potential nonspecific effects of the virus. The infected cells were then grown for 10 days in the absence or presence of E2, after which DNA synthesis was measured by [3H]thymidine incorporation in vitro. The expression of ERß resulted in a 48% reduction in cell proliferation of MCF-7 cells in the absence of E2 compared with cells infected with 50 MOI of Ad-LacZ (Fig. 1B)
. E2 augmented the inhibition of cell proliferation to 71% in the Ad-ERß-infected cells. Similar results were observed using 100 MOI of Ad-ERß (Fig. 1B)
.
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). The addition of E2 produced an additional increase in G2-M cells to 19.5%. Similar results were observed after 24 and 48 h of treatment with E2.6
These results indicate that ERß inhibits proliferation of MCF-7 cells by causing a G2 cell cycle arrest independently of E2 and that E2 produces a modest G2 arrest enhancement.
ERß Down-Regulates c-Myc, Cyclin D1, and Cyclin A.
Estrogens regulate the production of multiple proteins involved in cell proliferation and cell cycle regulation (30)
. Cyclin D1 (31)
, cyclin A (32)
, and c-myc (33)
are known to be estrogen-inducible genes in MCF-7 cells. These genes are likely targets for ERß to cause inhibition of cell proliferation and cell cycle arrest, because they cause quiescent cells to progress through the cell cycle. Cyclin D1 interacts with cyclin-dependent kinase 4 and 6, which causes progression through G1, whereas cyclin A interacts with cyclin-dependent kinase 2 to promote the transition from the S phase to G2 (34)
. To investigate whether ERß regulates cyclin D1, cyclin A, or c-myc gene expression, we infected MCF-7 cells with Ad-LacZ or Ad-ERß and then measured mRNA levels by real-time quantitative PCR and protein levels by immunoblotting. E2 produced a time-dependent increase in c-myc, cyclin D1, and cyclin A mRNA (Fig. 2, A, B, and C
, respectively) and protein levels (Fig. 2D)
in cells infected with Ad-LacZ. The increase in gene expression and protein production is mediated by ER because only ER is expressed in these cells (11)
. ERß inhibited the induction of c-myc, cyclin D1, and cyclin A mRNA (Fig. 2, A, B, and C
, respectively) and protein by immunoblotting (Fig. 2D)
.
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and protein (Fig. 3B)
levels in MCF-7 cells. These results indicate that ERß reduces MCF-7 cell proliferation by inhibiting the induction of cell proliferation genes and activating the antiproliferation genes, p21 and p27.
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, or ERß were initially aggregated, then resuspended in polymerized collagen gel and grafted under the kidney capsule of female nude mice. The mice were also implanted with an estrogen pellet to stimulate tumor cell growth. One-month after the cells were grafted, tumors of comparable size developed (Fig. 4A)
from uninfected MCF-7 cells, and cells infected with Ad-LacZ. The size of the tumor derived from cells infected with Ad-ER were not larger than uninfected MCF-7 cells, suggesting the level of endogenous ER is sufficient to produce a maximal stimulation of growth. ERß produced a dose-dependent inhibition of tumor formation. A small tumor developed with 50 MOI Ad-ERß, whereas no significant tumor developed from MCF-7 cells infected with 100 MOI Ad-ERß (Fig. 4A)
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(Fig. 4B)
as measured by H&E staining. In contrast, no significant tumor was observed in MCF-7 cells infected with ERß by H&E staining. The Ki67 proliferation index found that 
70% of noninfected MCF-7 cells and cells infected with Ad-LacZ or Ad-ER stained for Ki67 compared with 5% of cells infected with Ad-ERß (Fig. 4C)
. Ki67-positive cells also stained with antibodies to keratin 8, a specific marker for MCF-7 cells, confirming that the proliferating cells were derived from the tumor.7
Our studies demonstrate that introducing Ad-ERß into MCF-7 cells but not Ad-ER prevents tumor formation in mouse xenografts. Similar levels of expression of ER and ERß from the Ads were detected in the infected cells by immunoblots (Fig. 1A)
making it unlikely that our results are due to overexpression and nonspecific squelching of cofactors or transcription factors by ERß. Furthermore, if squelching was the mechanism whereby ERß prevents tumor formation then similar results should have been observed with cells infected with Ad-ER. |
DISCUSSION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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and ERß have distinct roles in mammary gland development (9)
. However, the exact role of ER and ERß in the pathogenesis of breast cancer is unclear. Whereas many breast tumors express ERß, it is unclear if ERß participates in breast carcinogenesis, tumor progression, or resistance to antiestrogens. Furthermore, studies correlating the presence of ERß in human breast tumors with prognosis have been inconsistent. In this study, we used another approach to explore the role of ERß in breast cancer by introducing ERß into MCF-7 cells that express exclusively ER to mimic the majority of ER-positive breast tumors. Our studies provide additional evidence that ER and ERß have distinct roles in breast cancer cells. We found that ER promotes proliferation in MCF-7 cells, whereas ERß inhibits cell proliferation and tumor formation. The opposite effects exhibited ER and ERß on breast cancer cells are consistent with the proposal by Weihua et al. (37)
that ER and ERß have yin/yang relationship in some tissues. The growth inhibitory effects of ERß suggest that ERß functions as a tumor suppressor in breast cells. A tumor suppressor function for ERß is consistent with the observations that ERß knockout mice develop prostate hyperplasia (10)
and leukemia (38)
.
ERß can form a heterodimer with ER (39
, 40)
to inhibit ER-mediated transcriptional activation of a classical estrogen response element (41
, 42)
and the cyclin D1 promoter (43)
. These results indicate that ERß might inhibit cell proliferation and tumor formation of MCF-7 cells by functioning as a dominant negative of ER-mediated induction of growth promoting genes, such as cyclin D1 (43)
, cyclin A, and c-myc. However, we found that ERß did not inhibit some other genes induced by ER,8
demonstrating that ERß does not function as a dominant negative of all ER-inducible genes. ERß also inhibits proliferation of an ER-negative breast cancer cell (44)
. Furthermore, we found that most genes regulated by ER in response to E2 are distinct from those regulated by ERß in U2OS osteosarcoma cells.9
These observations suggest that in addition to antagonizing the effects of ER, ERß may inhibit cell proliferation by directly regulating distinct genes or by exerting other mechanisms independent of ER.
The inhibition of cell proliferation by ERß and activation of p21 and p27 was predominantly or totally ligand independent, respectively. It is possible that this observation results from residual E2 remaining in stripped serum or retained in cells infected with ERß. Alternatively, several studies indicate that steroid receptors can elicit ligand-independent effects. Ciana et al. (45) found that ERs can activate gene transcription in the absence of ligand in a transgenic mouse model engineered with an estrogen response element-luciferase construct. These studies found that the luciferase reporter was active in some nonreproductive tissues in ovariectomized adult mice and immature mice that do not produce estrogens. Progesterone receptor A receptor also regulates several genes in the absence of ligand (46) .
Our studies demonstrate that ERß changes the phenotype of MCF-7 cells in response to E2. In ER-expressing MCF-7 cells, E2 causes proliferation and tumor formation. In contrast, when ERß is expressed along with ER, MCF-7 cells are directed to antiproliferation and antitumor pathways even in the presence of estrogens. These results suggest that ERß can alter the response to estrogens and provide a possible explanation for the findings that ERß expression in breast tumors is associated with a more favorable prognosis (21, 22, 23, 24)
. A potentially important clinical application of our studies is that ERß-selective estrogens may be more potent at eliciting antiproliferative pathways compared with ER nonselective estrogens, such as E2, that also activate ER. High intake of dietary plant estrogens (phytoestrogens) is associated with a low incidence of breast cancer (47
, 48)
. Whereas E2 binds equally to ER and ERß, phytoestrogens selectively bind to ERß (49
, 50)
and recruit coregulators to ERß to trigger transcriptional activation and repression (11)
. Our results indicate that phytoestrogens and estrogens designed to selectively trigger ERß transcriptional pathways might not promote breast cancer, making them a safer alternative to estrogens used in current hormone replacement therapy formulations.
Selective ER modulators, such as tamoxifen and raloxifene, reduce the incidence of ER-positive breast cancer tumors (51 , 52) . These drugs block transcriptional activation of growth promoting genes by recruiting corepressor proteins to ERs bound to the promoter region (53 , 54) . However, selective ER modulators are not ideal drugs for breast cancer chemoprevention, because they can cause serious adverse effects, such as thromboembolisms (51 , 52) . Our results showing that ERß inhibits proliferation and tumor formation of breast cancer cells suggests that dietary or synthetic ERß-selective estrogens may be an alternative to selective ER modulators for chemoprevention of breast cancer.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Dale Leitman, HSE 1619, Center for Reproductive Sciences, 513 Parnassus Avenue, University of California San Francisco, San Francisco, CA 94143-0556. Phone: (415) 502-5261; Fax: (415) 753-3271; E-mail: leitmand{at}obgyn.ucsf.edu
5 Internet address: http://mammary.nih.gov/tools/mousework/Cunha001/index/html. 
6 V. Kerekatte, unpublished observations.
7 H. Parmar, unpublished observations.
8 S. Paruthiyil, unpublished observations.
9 M. K. Tee et al. Estradiol and selective estrogen receptor modulators differentially regulate target genes with estrogen receptors and ß. Mol. Biol. Cell., in press, 2004.
Received 8/ 6/03. Revised 10/10/03. Accepted 10/27/03.
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T Suzuki, S Hayashi, Y Miki, Y Nakamura, T Moriya, A Sugawara, T Ishida, N Ohuchi, and H Sasano Peroxisome proliferator-activated receptor {gamma} in human breast carcinoma: a modulator of estrogenic actions. Endocr. Relat. Cancer, March 1, 2006; 13(1): 233 - 250. [Abstract] [Full Text] [PDF]
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J E Sanchez-Criado, J M. de las Mulas, C Bellido, V M Navarro, R Aguilar, J C Garrido-Gracia, M M Malagon, M Tena-Sempere, and A Blanco Gonadotropin-secreting cells in ovariectomized rats treated with different oestrogen receptor ligands: a modulatory role for ER{beta} in the gonadotrope? J. Endocrinol., February 1, 2006; 188(2): 167 - 177. [Abstract] [Full Text] [PDF]
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A Rody, U Holtrich, C Solbach, K Kourtis, G von Minckwitz, K Engels, S Kissler, R Gatje, T Karn, and M Kaufmann Methylation of estrogen receptor {beta} promoter correlates with loss of ER-{beta} expression in mammary carcinoma and is an early indication marker in premalignant lesions Endocr. Relat. Cancer, December 1, 2005; 12(4): 903 - 916. [Abstract] [Full Text] [PDF]
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 P. Totta, F. Acconcia, F. Virgili, A. Cassidy, P. D. Weinberg, G. Rimbach, and M. Marino Daidzein-Sulfate Metabolites Affect Transcriptional and Antiproliferative Activities of Estrogen Receptor-{beta} in Cultured Human Cancer Cells J. Nutr., November 1, 2005; 135(11): 2687 - 2693. [Abstract] [Full Text] [PDF]
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 S K Nair, T J Thomas, N J Greenfield, A Chen, H He, and T Thomas Conformational dynamics of estrogen receptors {alpha} and {beta} as revealed by intrinsic tryptophan fluorescence and circular dichroism J. Mol. Endocrinol., October 1, 2005; 35(2): 211 - 223. [Abstract] [Full Text] [PDF]
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V. Martineti, L. Picariello, I. Tognarini, S. C. Sala, A. Gozzini, C. Azzari, C. Mavilia, A. Tanini, A. Falchetti, G. Fiorelli, et al. ER{beta} is a potent inhibitor of cell proliferation in the HCT8 human colon cancer cell line through regulation of cell cycle components Endocr. Relat. Cancer, June 1, 2005; 12(2): 455 - 469. [Abstract] [Full Text] [PDF]
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L C Murphy, B Peng, A Lewis, J R Davie, E Leygue, A Kemp, K Ung, M Vendetti, and R Shiu Inducible upregulation of oestrogen receptor-{beta}1 affects oestrogen and tamoxifen responsiveness in MCF7 human breast cancer cells J. Mol. Endocrinol., April 1, 2005; 34(2): 553 - 566. [Abstract] [Full Text] [PDF]
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M. F. McCarty Targeting Multiple Signaling Pathways as a Strategy for Managing Prostate Cancer: Multifocal Signal Modulation Therapy Integr Cancer Ther, December 1, 2004; 3(4): 349 - 380. [Abstract] [PDF]
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 J. A. Crowell, P. J. Korytko, R. L. Morrissey, T. D. Booth, and B. S. Levine Resveratrol-Associated Renal Toxicity Toxicol. Sci., December 1, 2004; 82(2): 614 - 619. [Abstract] [Full Text] [PDF]
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M. Razandi, A. Pedram, I. Merchenthaler, G. L. Greene, and E. R. Levin Plasma Membrane Estrogen Receptors Exist and Functions as Dimers Mol. Endocrinol., December 1, 2004; 18(12): 2854 - 2865. [Abstract] [Full Text] [PDF]
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T. A. Hopp, H. L. Weiss, I. S. Parra, Y. Cui, C. K. Osborne, and S. A. W. Fuqua Low Levels of Estrogen Receptor {beta} Protein Predict Resistance to Tamoxifen Therapy in Breast Cancer Clin. Cancer Res., November 15, 2004; 10(22): 7490 - 7499. [Abstract] [Full Text] [PDF]
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 D. M. Selva, O. M. Tirado, N. Toran, C. A. Suarez-Quian, J. Reventos, and F. Munell Estrogen Receptor {beta} Expression and Apoptosis of Spermatocytes of Mice Overexpressing a Rat Androgen-Binding Protein Transgene Biol Reprod, November 1, 2004; 71(5): 1461 - 1468. [Abstract] [Full Text] [PDF]
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 N. G. Forger, G. J. Rosen, E. M. Waters, D. Jacob, R. B. Simerly, and G. J. de Vries Deletion of Bax eliminates sex differences in the mouse forebrain PNAS, September 14, 2004; 101(37): 13666 - 13671. [Abstract] [Full Text] [PDF]
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A Bardin, N Boulle, G Lazennec, F Vignon, and P Pujol Loss of ER{beta} expression as a common step in estrogen-dependent tumor progression Endocr. Relat. Cancer, September 1, 2004; 11(3): 537 - 551. [Abstract] [Full Text] [PDF]
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A. Bardin, P. Hoffmann, N. Boulle, D. Katsaros, F. Vignon, P. Pujol, and G. Lazennec Involvement of Estrogen Receptor {beta} in Ovarian Carcinogenesis Cancer Res., August 15, 2004; 64(16): 5861 - 5869. [Abstract] [Full Text] [PDF]
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) or presence of 1 nM E2 (
). DNA synthesis was determined by [3H]thymidine incorporation in triplicate samples. The data are expressed as % growth inhibition of cells infected with ER
