Response to tamoxifen in estrogen receptor-positive cell line MCF-7 is independent of p53 expression

The study was concerned with identification of predictive value of p53 expression on sensitivity to tamoxifen in breast cancer management. Estrogen receptor-positive cell line MCF-7 was used to establish p53 expression influence on the rate of cell proliferation after tamoxifen. The investigation demonstrated the absence of that effect when p53 was silenced.     

Differences in protein kinase C and estrogen receptor alpha, beta expression and signaling correlate with apoptotic sensitivity of MCF-7 breast cancer cell variants

Widespread use of MCF-7 human breast cancer cells as a model system for breast cancer has lead to variations in these cells between different laboratories. Although several reports have addressed these differences in terms of proliferation and estrogenic response, differences in sensitivity to apoptosis have just begun to be described. Based on the possible differences in apoptotic sensitivity that may arise due to the existence of MCF-7 cell variants, we determined the relative sensitivity of MCF-7 cell variants from three established laboratories (designated M, L and N) to known inducers of apoptosis. Consistent with our previous studies we demonstrate that differences exist among these variants in regards to tumor necrosis factor alpha (TNF)-induced cell death and inhibition of proliferation in a dose-dependent manner. To establish if the difference in apoptotic susceptibility was specific to TNF, the three MCF-7 cell variants were tested for their response to other known inducers of apoptosis: okadaic acid, staurosporine and 4-hydroxy-tamoxifen. Viability and DNA fragmentation analysis revealed a similar pattern of resistance to apoptosis by all agents in the MCF-7 M variant. The MCF-7 L variant was resistant to okadaic acid and 4-hydroxy-tamoxifen but not staurosporine. In contrast, MCF-7 N cells were sensitive to induction of apoptosis by all agents. The role of both protein kinase C (PKC) and estrogen signaling in the regulation of cell survival prompted investigation of these pathways as a mechanism for differential sensitivity of MCF-7 cell variants to apoptosis. While both estrogen receptor alpha (ERalpha) and ERbeta were expressed in MCF-7 M and N cells, the absence of ERbeta in MCF-7 L cells correlated with decreased estrogen responsiveness of the L variant. Variations in estrogenic responsiveness and PKC isoform expression may account for the enhanced susceptibility of both the L and N variants to staurosporine.     

Bcl2-negative MCF7 cells overexpress p53: implications for the cell cycle and sensitivity to cytotoxic drugs

PURPOSE: Bcl2 is a mitochondrial protein endowed with cytostatic and antiapoptotic activities. In this work we studied the effects of the lack of Bcl2 in MCF7 cells. METHODS: The breast cancer cell line MCF7 (Bcl2-positive) and its derivative MCF7/50B (Bcl2-negative) were compared in terms of the level of p53 expression, doubling time and distribution of cells among the cycle phases. Sensitivities to the proapoptotic drugs cisplatinum and staurosporine were measured using a clonogenic assay and the contribution of apoptosis to cytotoxicity was determined with a mitochondrial membrane potential-sensitive dye. RESULTS: Relative to MCF7, MCF7/50B cells overexpressed p53 and slowly proliferated with a significant accumulation at G(0)/G(1) and depletion in S phase. The cytotoxicity of the DNA-damaging agent cisplatinum was decreased, while that of the protein kinase inhibitor staurosporine was increased. The induced cytotoxicity was essentially due to apoptosis and necrosis, respectively. CONCLUSIONS: These results suggest that the lack of Bcl2 accompanied by p53 overexpression affects the distribution of cells among the cell cycle phases and modifies the sensitivity to cytotoxic drugs and the type of cell death.     

Opposite effects of tamoxifen on in vitro protein kinase C activity and endogenous protein phosphorylation in intact MCF-7 cells

We investigated the effects of the antiestrogen tamoxifen on MCF-7 cell protein kinase C either by using the in vitro histone kinase assay or by studying the phosphorylation of its endogenous Mr 28,000 protein substrate in intact cells. In the in vitro assay, tamoxifen inhibited the enzyme competitively with respect to phospholipid, whereas estradiol and morpholinobenzyl phenoxy ethanamine, a specific ligand for antiestrogen binding sites, were considerably less efficient. In contrast, tamoxifen did not affect phosphorylation of the Mr 28,000 protein induced by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate in intact MCF-7 cells. Estradiol and morpholinobenzyl phenoxy ethanamine also had no effect. At high concentration (100 microM), tamoxifen itself stimulated specific phosphorylation of this Mr 28,000 protein. Estradiol and morpholinobenzyl phenoxy ethanamine neither mimicked nor interfered with this effect. Our data suggest that the effect of tamoxifen on protein kinase C activity depends on the phospholipid environment of the enzyme, and opposite effects may be observed in intact cells to those seen in disrupted cells. The action of tamoxifen on endogenous protein phosphorylation was thought to be due to direct interaction with the phospholipid binding domain of the enzyme rather than by interaction with the estrogen receptor or the antiestrogen binding site. Nevertheless, our results do not rule out a possible activation by tamoxifen of specific protein kinase(s) and phosphatase(s). In any case, the antiproliferative activity of tamoxifen on MCF-7 cells cannot be attributed to its effects on protein kinase C.     

Re-expression of estrogen receptor alpha in estrogen receptor alpha-negative MCF-7 cells restores both estrogen and insulin-like growth factor-mediated signaling and growth

Estrogen can increase insulin-like growth factor-I receptor (IGF-IR) and insulin receptor substrate-1 (IRS-1) expression, two key components of IGF-I-mediated signaling. The result is sensitization of breast cancer cells to IGF-I and synergistic growth in the presence of estrogen and IGF-I. We hypothesized that loss of estrogen receptor alpha (ERalpha) would result in reduced IGF-mediated signaling and growth. To test this hypothesis, we examined IGF-I effects in MCF-7 breast cancer cell sublines that have been selected for loss of ERalpha (C4 and C4-12 cells are ERalpha-negative) by long-term estrogen withdrawal. C4 and C4-12 cells had reduced IGF-IR and IRS-1 mRNA and protein expression (compared with MCF-7 cells) that was not inducible by estrogen. Furthermore, C4 and C4-12 cells showed reduced IGF-I signaling and failed to show any growth response to either estrogen or IGF-I. To prove that loss of IGF and estrogen-mediated signaling and growth was a consequence of loss of ERalpha, we re-expressed ERalpha in C4-12 cells by stable transfection with HA-tagged ERalpha. Three independent C4-12 ERalpha-HA clones expressed a functional ERalpha that (a) was down-regulated by estrogen, (b) conferred estrogen-induction of cyclin D1 expression, and (c) caused estrogen-mediated increase in the number of cells in S phase. All of the effects were completely blocked by antiestrogens. Interestingly, ERalpha-HA expression in C4-12 cells did not restore estrogen induction of progesterone receptor expression. However, ERalpha-positive C4-12 cells now exhibited estrogen-induction of IGF-IR and IRS-1 levels and responded mitogenically to both estrogen and IGF-I. These data show that ERalpha is a critical requirement for IGF signaling, and to our knowledge this is the first report of functional ERalpha expression that confers estrogen-mediated growth of an ER-negative breast cancer cell line.     

2-Methoxyestradiol inhibits proliferation and induces apoptosis independently of estrogen receptors alpha and beta

2-Methoxyestradiol (2ME(2)) is an endogenous metabolite of 17beta-estradiol (E(2)) that arises from the hydroxylation and subsequent methylation at the 2-position. In vitro 2ME(2) inhibits a large variety of tumor and nontumor cell lines from diverse origins, as well as several stages of the angiogenic cascade. In vivo it has been shown to be a very effective inhibitor of tumor growth and angiogenesis in numerous models. Although various molecular targets have been proposed for this compound, the mechanism of action is still uncertain. As this molecule emerges as a drug candidate it is important to assess the estrogen receptors (ERs) as molecular targets for 2ME(2). The purpose of this study was to investigate whether 2ME(2) is able to engage ERs as an agonist and whether its antiproliferative activities are mediated through ERs. We confirm that 2ME(2) has a lower binding affinity for ERalpha as compared with E(2) and other E(2) metabolites and antagonists, and we demonstrate that the affinity of 2ME(2) for ERbeta is even lower. When assessed in the presence of galangin, a cytochrome P450 enzyme inhibitor, at concentrations at which 2ME(2) interacts with ERalpha in an in vitro binding assay, it does not stimulate the proliferation of an estrogen-dependent breast carcinoma cell line. Similar IC(50) values for inhibition of proliferation and induction of apoptosis are obtained in estrogen-dependent and estrogen-independent human breast cancer cell lines, irrespective of the expression of ERalpha and ERbeta. Moreover, the estrogen antagonist ICI 182,780 does not inhibit the antiproliferative activity of 2ME(2). In E(2)-responsive cells such as MCF-7 and human umbilical vascular endothelial cells, high levels of E(2) inhibit the antiproliferative activity of ICI 182,780 but not of 2ME(2). Collectively, these results suggest that 2ME(2) is distinct among estradiol metabolites because of its inability to engage ERs as an agonist, and its unique antiproliferative and apoptotic activities are mediated independently of ERalpha and ERbeta.     

Time-related effects of estrogen withdrawal on proliferation- and cell death-related events in MCF-7 xenografts

Endocrine treatments for human breast cancer have been based largely upon the removal of estrogenic stimuli. The regression of tumors after estrogen deprivation has generally been characterized as being due to reduced proliferation but more recently has been recognized to also involve increased apoptosis. The aim of our experiments was to define the associated changes in certain proliferation- and cell death-related biological parameters after hormone withdrawal from estrogen-dependent MCF-7 xenografts in athymic nude mice using immunohistochemical techniques. The baseline estrogen receptor (ER) level of this MCF-7 xenograft was relatively low (average H score 23) but it was strongly Bcl-2-, PgR- and pS2-positive, indicating the functional integrity of estrogen signaling. Changes in proliferation (Ki-67), apoptosis, ER, progesterone receptor (PgR), cyclin D1, p27kip1, Bcl-2 and Bax expression were assessed during the 2 weeks after estrogen deprivation. ER levels rose markedly after estrogen ablation, whereas PgR levels fell to about 10% of baseline and pS2 levels halved. The proportion of Ki-67-positive cells was unchanged after 24 hr but by day 14 had reduced by about 80%. The normal levels of cyclin D1 also reduced after estrogen withdrawal in contrast to the rapid increase in levels of cyclin-dependent kinase inhibitor p27kip1. This latter increase appeared to occur in advance of the changes in Ki-67. The proportion of apoptotic cells increased from a mean 1.5% at baseline to 2.9% after 3 days and 4.7% after 14 days. There were reductions in both Bcl-2 and Bax staining but these appeared to be greater for Bcl-2, effectively decreasing the Bcl-2/Bax ratio. Our results provide a framework for the use of these parameters as intermediate markers in comparisons of hormonal agents for human breast cancer treatment.     

Prepubertal physical activity up-regulates estrogen receptor beta, BRCA1 and p53 mRNA expression in the rat mammary gland

Findings in BRCA1 mutation carriers suggest that physical activity, particularly during childhood, may be linked to a reduced risk of developing breast cancer. We investigated whether physical activity at puberty alters the expression of BRCA1 and two other tumor suppressor genes—p53 and estrogen receptor (ER)-β—in rats. In addition, the effects on ER-α expression, mammary proliferation and functional epithelial differentiation were investigated as markers of altered mammary cancer risk in rats exposed to regular physical activity at puberty. Female Sprague Dawley rat pups were randomized to voluntary exercise, sham-exercise control and non-manipulated control groups. Treadmill training (20–25 m/min, 15% grade, 30 min/day, 5 days/week) started on postnatal day 14 and continued through day 32. Third thoracic mammary glands (n = 5 per group and age) were obtained at days 32, 48 and 100 and assessed for changes in morphology through wholemounts, and at 100 days cell proliferation by using Ki67 staining, protein levels of ER-α and ER-β by immunohistochemistry, and mRNA expression levels of BRCA1, p53, ER-α and ER-β by real-time PCR. Mammary glands of rats exposed to exercise during puberty contained fewer terminal end buds (TEBs) and a higher number of differentiated alveolar buds and lobules than the sham controls. However, cell proliferation was not significantly altered among the groups. ER-α protein levels were significantly reduced, while ER-β levels were increased in the mammary ducts and lobular epithelial structures of 100-day old rays which were voluntarily exercised at puberty, compared to sham controls. ER-β, BRCA1 and p53 mRNA levels were significantly higher in the mammary glands of 100-day-old exercised versus sham control rats. Pubertal physical activity reduced mammary epithelial targets for neoplastic transformation through epithelial differentiation and it also up-regulated tumor suppressor genes BRCA1, p53 and ER-β, and reduced ER-α/ER-β ratio in the mammary gland. It remains to be determined whether the up-regulation of BRCA1, and perhaps p53, explains the protective effect of childhood physical activity against breast cancer in women who carry a germline mutation in one of the BRCA1 alleles.     

Pharmacological inhibition of fatty acid synthase activity produces both cytostatic and cytotoxic effects modulated by p53

Fatty acid synthetic metabolism is abnormally elevated in tumor cells, and pharmacological inhibitors of the anabolic enzyme fatty acid synthase (FAS), including the natural product cerulenin and the novel synthetic compound c75, are selective inhibitors of tumor cell growth. We have recently reported that these two FAS inhibitors both produce rapid, potent inhibition of DNA replication and S-phase progression in human cancer cells, as well as apoptotic death. Here we report an additional characterization of the cellular response to FAS inhibition. RKO colon carcinoma cells were selected for study because they undergo little apoptosis within the first 24 h after FAS inhibition. Instead, RKO cells exhibited a biphasic stress response with a transient accumulation in S and G2 at 4 and 8 h that corresponds to a marked reduction in cyclin A- and B1-associated kinase activities, and then by accumulation of p53 and p21 proteins at 16 and 24 h and growth arrest in G1 and G2. The response of RKO cells to FAS inhibition resembled a genotoxic stress response, but DNA damage did not appear to be an important downstream effect of FAS inhibition, because none was detected using the single cell gel electrophoresis assay (comet assay) to assess DNA damage. p53 function is probably important in protecting RKO cells from FAS inhibition because, similar to many other tumor lines, RKO cells expressing a dominant negative mutant p53 gene underwent extensive apoptosis within 24 h after FAS inhibition. Sensitization of cells to FAS inhibitors by the loss of p53 raises the possibility that these agents may be clinically useful against malignancies carrying p53 mutations. Whereas induction of apoptosis appeared related to accumulation of the substrate, malonyl-CoA, after FAS inhibition, the cytostatic effects were independent of malonyl-CoA accumulation and may have resulted from product depletion.     

Mechanisms of estrogen action on the proliferation of MCF-7 human breast cancer cells in an improved culture medium

The effects of 17 beta-estradiol and tamoxifen (TAM) on the proliferation of responsive MCF-7 and unresponsive HBC-4 human breast cancer cells were studied in a defined culture medium containing insulin (2 micrograms/ml), transferrin (2 micrograms/ml), ethanolamine (2 microM), and selenite (25 nM). MCF-7 cells grew at a population-doubling rate of 2.0 days in serum-free medium and at a rate of 1.7 days in the medium containing 1 mg/ml of the 55-70% ammonium sulfate fraction of bovine serum or 1% dextrancoated charcoal-treated fetal bovine serum. Increasing concentrations of the ammonium sulfate fraction and/or dextran-coated charcoal-treated fetal bovine serum increasingly inhibited the growth of MCF-7 cells but did not inhibit HBC-4 cell growth, indicating that such serum preparations contain some growth inhibitor specific for estradiol-responsive MCF-7 cells. A sufficiently high concentration of exogenous estradiol (100 pM) had the dual action of neutralizing the growth inhibition by the 55-70% ammonium sulfate fraction of bovine serum and dextran-treated charcoal-treated fetal bovine: serum and enhancing directly the MCF-7 cell growth maximally 2-fold. Bovine serum albumin fraction V containing globulin remnants also inhibited growth, but globulin-free bovine serum albumin did not. Eliminating growth inhibition by the use of globulin-free bovine serum albumin enabled us to develop an ideal medium for assaying the direct effects of estradiol and TAM on MCF-7 cells. With this medium, we clearly identified (a) a direct mitogenic effect of exogenous estradiol on MCF-7 cells which was initiated at 3 pM and maximized at 0.2 to 10 nM, (b) an acute lethal effect of 1 microM TAM and its prevention by 100 pM estradiol, and (c) a nearly 50-fold increase in the concentration of exogenous estradiol (10 nM) required for maximum growth enhancement in the presence of 1 microM TAM than without TAM (0.2-0.3 nM).     

乳腺癌耐药细胞ER表达状态影响细胞对屈洛昔芬和阿霉素的敏感性

背景与目的：雌激素受体（estrogen receptor,ER)阳性乳腺癌细胞株来源的耐药细胞株中,ER表达缺失或下降,且细胞生长速度减慢,本文的目的是研究MCF－7／Adr乳腺癌耐药细胞株中ER表达状态与细胞对出洛昔芬（droloxifene,Dro)和阿霉素（Adriamycin,Adr)敏感性之间的关系。方法：Western blot法检测MCF－7／Adr及其亲本MCF－7细胞中ER蛋白的表达,构建ER的真核细胞表达质粒（pCER),利用LipofectAMINE^TM将ER基因导入MCF－7／Adr细胞,经G418抗性筛选获得阳性克隆（MTER／Adr),PCR,Western blot法鉴定并检测ER基因的整合和蛋白表达,流式细胞仪检测细胞周期变化,MTT法检测Dro及Adr对细胞增殖的影响。结果：在MCF－7细胞中可检测到ER蛋白 的表达,而MCF－7／Adr细胞中使用Westernblot检测不到ER蛋白的表达,成功构建真核细胞表达质粒pCER并转染MCF－7／Adr细胞,阳性克隆MTER／Adr整合了ER基因并获得表达。经MTT分析,10μmol/LDro对MCF－7细胞的生长有明显的抑制作用。而到20μmol/L时对MCF－7／Adr细胞的生长有抑制作用。ER转染MCF－7／Adr细胞后,15μmol/L的Dro对其生长出现抑制作用,使细胞多分布于G0／G1期。同时细胞对Adro的敏感性下降,结论：MCF－7／MCF－7／Adr细胞部分恢复对Dro和Adr的敏感性。     

Comparative effects of histone deacetylase inhibitors on p53 target gene expression, cell cycle and apoptosis in MCF-7 breast cancer cells

Histone deacetylase inhibitors are currently being evaluated for their therapeutic potential and have shown considerable promise as adjuvant therapies for a number of cancers. This study compared the effects of 2 hydroxamic acid based inhibitors, CG-1521 and SAHA, on gene expression, cell cycle and cell death in MCF-7 human breast cancer cells. Both compounds show a dose- and time-dependent effect on cell number (evaluated using crystal violet), however CG-1521 exerts its effects significantly earlier than SAHA, and CG-1521 induces apoptosis (assessed by Apo-BrdU staining and flow cytometry) more rapidly than SAHA. qPCR of cell cycle regulatory and apoptotic genes shows that CG-1521 and SAHA modulate similar cohorts of p53-responsive genes, however, the levels of induction and the timing of the induction differs significantly between the 2 inhibitors. In particular SAHA downregulates cell cycle-associated genes that modulate the G1/S transition (including cyclin D1 and cdc25a) and the G2/M transition [cyclin B1, Plk1, Stk6 (serine-threonine kinase 6, Aurora kinase A) and Kntc2] more significantly than CG-1521. In contrast, CG-1521 significantly induces the expression of several p53 target genes associated with apoptosis including Bnip3/Bnip3L, p21/p21B and Gdf15. The differential levels of gene induction provide molecular evidence of both cell cycle arrest and apoptosis, and suggest a molecular mechanism that explains the difference in the biological effects of the 2 histone deacetylase inhibitors.     

Effect of growth on the estrogen receptor levels in MCF-7 cells

MCF-7 cells have been shown to contain estrogen receptor in several cell fractions following homogenization: nuclei, microsomes, and cytosol. The amount of 17 beta-estradiol-binding capacity found in each cellular compartment depended on the inclusion of detergent in homogenization buffers and on the use of 0.25 M sucrose in the nuclear washes. 17 beta-Estradiol receptor (E2R) associated with nuclei (whole nuclei exchange assay, 0.6 M KCl soluble, and that found on membranes sheared from crude nuclear pellets by centrifugation in 0.25 M sucrose buffer) displayed a dissociation constant (Kd) of 0.77 +/- 0.01 (S.D.) nM (n = 7). KdS of the cytoplasmic (microsomes and soluble) receptors were determined to be 0.33 +/- 0.10 nM (n = 9). Exchangeable ligand on partially purified nuclei assumed its highest level in MCF-7 cells during logarithmic growth in serum-containing media (0.8 pmol/micrograms DNA) but declined after the culture reached confluence (0.2 pmol/micrograms DNA). Seventy-five % of the nuclear E2R declined linearly after feeding MCF-7 cells in logarithmic growth phase an estrogen- and serum-free medium (t1/2 3.5 days). Another class of salt-extractable nuclear receptor (0.2 pmol/micrograms DNA) persisted in postconfluent cultures whether fed estrogen (serum-containing media) or not (serum-free media). This residual binding capacity remained in nuclei of MCF-7 cells for an extended period of time. MCF-7 cells demonstrated functionality of E2R throughout their growth phases as evidenced by the replenishment of cytosolic E2R and the induction of progesterone receptor when given 17 beta-estradiol.