Apoptotic action of 17beta-estradiol in raloxifene-resistant MCF-7 cells in vitro and in vivo.
BACKGROUND: Resistance to tamoxifen, a selective estrogen receptor modulator (SERM), involves changes that prevent apoptosis and enhance cell proliferation and survival. Paradoxically, estrogen treatment inhibits the growth of long-term tamoxifen-treated breast tumors. Because of the increasing use of raloxifene, another SERM, to prevent osteoporosis and potentially reduce breast cancer risk, some women will develop raloxifene-resistant breast cancer. We developed a raloxifene-resistant MCF-7 cell model (MCF-7/ Ral) and investigated the nature of raloxifene-resistant breast cancer and its response to estradiol. METHODS: Raloxifene resistance and hormone responsiveness were assessed by proliferation assays and cell cycle analysis in parental MCF-7 and MCF-7/ Ral cells. Nuclear factor kappaB (NF-kappaB) activity was investigated with a transient transfection assay. Apoptosis was investigated by annexin V staining, mRNA was measured by real-time polymerase chain reaction, and protein was measured by western blotting. Tumorigenesis was studied by injecting MCF-7 or MCF-7/ Ral cells into ovariectomized athymic mice (10 per group) and monitoring tumor size weekly. All statistical tests were two-sided. RESULTS: Basal NF-kappaB activity was higher in MCF-7/ Ral cells (1.6 U, 95% confidence interval [CI] = 1.2 to 2.0 U) than in MCF-7 cells (0.8 U, 95% CI = 0.4 to 1.1 U; P =.004). When cultured with 1 microM raloxifene, MCF-7/ Ral cells grew statistically significantly (P<.001) faster than MCF-7 cells. Estradiol treatment of MCF-7/ Ral cells arrested cells in G(2)/M phase of the cell cycle, decreased NF-kappaB activity (0.2 U, 95% CI = 0.2 to 0.3 U; P<.001), increased expression of Fas protein and mRNA (4.5-fold, 95% CI = 2.8- to 6.3-fold versus 0.5-fold, 95% CI = 0.3- to 0.8-fold for control treatment; P<.001), and induced apoptosis. Treatment with either raloxifene or tamoxifen stimulated MCF-7/ Ral tumor growth, suggesting that such tumors were resistant to both drugs. When a 9-week raloxifene or tamoxifen treatment was followed by a 5-week estradiol treatment, estradiol statistically significantly reduced the size of tumors stimulated by raloxifene or tamoxifen (at week 14, P =.004 for raloxifene and P<.001 for tamoxifen). CONCLUSIONS: Growth of raloxifene-resistant MCF-7/ Ral cells in vitro and in vivo is repressed by estradiol treatment by a mechanism involving G2/M-phase arrest, decreased NF-kappaB activity, and increased Fas expression to induce apoptosis.[1]References
- Apoptotic action of 17beta-estradiol in raloxifene-resistant MCF-7 cells in vitro and in vivo. Liu, H., Lee, E.S., Gajdos, C., Pearce, S.T., Chen, B., Osipo, C., Loweth, J., McKian, K., De Los Reyes, A., Wing, L., Jordan, V.C. J. Natl. Cancer Inst. (2003) [Pubmed]
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