Effect of long-term estrogen deprivation on apoptotic responses of breast cancer cells to 17 beta-estradiol

Background: High doses of estrogen can promote tumor regression in postmeno pausal women with hormone-dependent breast cancer, but the mechanism is unk nown. We investigated the molecular basis of this process by using LTED cel ls, which were derived by growing MCF-7 breast cancer cells under long-term (6-24 months) estrogen-deprived conditions. Methods: We treated LTED and M CF-7 cells with various concentrations of 17 beta -estradiol (estradiol) an d assayed their growth by counting the cells and measured apoptosis by anne xin V staining and DNA fragmentation. Using western blot analysis, we also examined the expression of the apoptosis-inducing system of the Fas death r eceptor protein and its ligand, FasL, in these cells. To assess the involve ment of Fas and FasL in the induction of apoptosis in LTED cells, we used a ctivating anti-Fas antibodies and the universal caspase inhibitor Z-VAD. Fi nally, we examined the expression of Fas protein in E8CASS and BSK3 cells, two other cell lines derived by depriving MCF-7 cells of estrogen long term , and the responses of these cells to high-dose estradiol: All statistical tests were two-sided. Results: High concentrations of estradiol (greater th an or equal to0.1 nM) resulted in a statistically significant, 60% reductio n in the growth of LTED cells (P < .001) and in a sevenfold increase in apo ptosis (P < .001) as compared with levels in vehicle-treated cells. Both LT ED and MCF-7 cells expressed FasL, but only LTED cells expressed Fas. Treat ment of LTED cells with 0.1 nM estradiol increased the expression of FasL. Activating anti-Fas antibodies increased apoptosis of LTED cells, which was further stimulated by estradiol. Z-VAD blocked estradiol-induced apoptosis . E8CASS cells, which express Fas protein, but not BSK3 cells, which do not , also responded to 0.1 nM estradiol by increasing apoptosis. Conclusion: T umor regression induced by high-dose estrogen therapy in postmenopausal wom an may result from estrogen activation of Fas-mediated apoptosis.