Paradoxical action of fulvestrant in estradiol-induced regression of tamoxifen-stimulated breast cancer

Background: Long-term tamoxifen treatment of breast cancer can result in tamoxifen-stimulated breast cancer, in which estrogen inhibits tumor growth after tamoxifen withdrawal. We investigated the molecular mechanism(s) of estradiol-induced tumor regression by using an in vivo model of tamoxifen-stimulated human breast cancer. Methods: Growth of parental estradiol-stimulated MCF-7E(2) and long-term tamoxifen-stimulated MCF-7TAMLT xenografts in athymic mice was measured during treatment with vehicle, estradiol, estradiol plus tamoxifen, tamoxifen alone, estradiol plus fulvestrant, or fulvestrant alone. Apoptosis was detected by the terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) assay. Protein expression was assessed by western blot analysis. mRNA expression was assessed by real-time reverse transcription-polymerase chain reaction. All statistical tests were two-sided. Results: MCF-7E2 tumor growth was stimulated by estradiol (cross-sectional area at week 13 = 1.06 cm(2), 95% confidence interval [CI] = 0.82 to 1.30 cm(2); p<.001) compared with control (0.06 cm(2), 95%C1 = -0.02 to 0.14 cm(2)), but tumor growth was inhibited by tamoxifen or fulvestrant. MCF-7TAMLT tumor growth was stimulated by tamoxifen) cross-sectional area at week 10 = 0.60 cm(2), 95% CI = 0.50 to 0.70 cm(2); P<.001) compared with control (0.02 cm(2), 95% CI = 0.00 to 0.04 cm(2)). For MCF-7TAMLT tumors that were initially 0.35 cm(2), estradiol-induced regression to 0.18 cm(2) (95% CI = 0.15 to 0.21 cm(2); P<.001), and tamoxifen or estradiol plus fulvestrant enhanced tumor growth to 1.00 cm(2) (95% CI = 0.88 to 1.22 cm(2)). Estradiol increased the number of apoptotic cells in tumors by 23% (95% CI = 20% to 26%; P<.001) compared with all other treatments, decreased estrogen receptor alpha(ERalpha) protein expression, increased the expression of Fas mRNA and protein, decreased the expression of HER2/neu mRNA and protein and nuclear factor kappaB (NF-kappaB) protein but did not affect Fas ligand protein expression compared with control. Paradoxically, fulvestrant reversed this effect and stimulated MCF-7TAMLT tumor growth apparently through ERalpha-mediated regulation of Fas, HER2/neu, and NF-kappaB. Conclusion: Physiologic levels of estradiol induced regression of tamoxifen-stimulated breast cancer tumors, apparently by inducing the death receptor Fas and suppressing the antiapoptotic/ prosurvival factors NF-kappaB and HER2/neu.