Down-regulation of Bcl-2 enhances estrogen apoptotic action in long-term estradiol-depleted ER+ breast cancer cells

Postmenopausal women with estrogen receptor positive (ER+) breast cancer frequently respond paradoxically to estrogen administration with tumor regression. Using both LTED and E8CASS cells derived from MCF-7 breast cancer cells by long-term estrogen-deprivation, we previously reported that 17 beta-estradiol (estradiol) is a powerful, pro-apoptotic hormone which kills the cancer cells through activation of the Fas/FasL death receptor pathway. We postulated that the mitochondrial interactive protein BcI-2 might play a role in the regulation of estradiol-induced apoptosis in both LTED and E8CASS cells. In this study, we assessed estradiol effects on cell growth, proliferation and apoptosis. Additionally we investigated the effect of estradiol on caspase activation, NF-KB and BcI-2 expression. The functional role of BcI-2 in estradiol-induced apoptosis was further studied by knockdown or decrease of BcI-2 with siRNA. Our results show that estradiol significantly inhibited cell growth primarily through a pro-apoptotic action involving caspase-7 and 9 activations (p < 0.01). Basal BcI-2 and NF-KB levels were greatly elevated and estradiol decreased NF-KB, but not BcI-2 expression. Knockdown of BcI-2 expression with siRNA decreased the levels of this protein by 9 fold (p < 0.01). This reduction markedly sensitized both LTED and E8CASS cells to the pro-apoptotic action of estradiol, leading to a synergistic induction of apoptosis and a concomitant reduction in cell number (p < 0.01). Therefore, downregulation of BcI-2 synergistically enhanced estradiol-induced apoptosis in ER+ postmenopausal breast cancer cells.