Estradiol-Induced Regression in T47D:A18/PKCα Tumors Requires the Estrogen Receptor and Interaction with the Extracellular Matrix

Several breast cancer tumor models respond to estradiol (E2) by undergoing apoptosis, a phenomenon known to occur in clinical breast cancer. Before the application of tamoxifen as an endocrine therapy, high-dose E2 or diethystilbesterol treatment was successfully used, albeit with unfavorable side effects. It is now recognized that such an approach may be a potential endocrine therapy option. We have explored the mechanism of E2-induced tumor regression in our T47D:A18/PKCα tumor model that exhibits autonomous growth, tamoxifen resistance, and E2-induced tumor regression. Fulvestrant, a selective estrogen receptor (ER) down-regulator, prevents T47D:A18/PKCα E2-induced tumor growth inhibition and regression when given before or after tumor establishment, respectively. Interestingly, E2-induced growth inhibition is only observed in vivo or when cells are grown in Matrigel but not in two-dimensional tissue culture, suggesting the requirement of the extracellular matrix. Tumor regression is accompanied by increased expression of the proapoptotic FasL/FasL ligand proteins and down-regulation of the prosurvival Akt pathway. Inhibition of colony formation in Matrigel by E2 is accompanied by increased expression of FasL and short hairpin RNA knockdown partially reverses colony formation inhibition. Classic estrogen-responsive element-regulated transcription of pS2, PR, transforming growth factor-α, C3, and cathepsin D is independent of the inhibitory effects of E2. A membrane-impermeable E2-BSA conjugate is capable of mediating growth inhibition, suggesting the involvement of a plasma membrane ER. We conclude that E2-induced T47D:A18/PKCα tumor regression requires participation of ER-α, the extracellular matrix, FasL/FasL ligand, and Akt pathways, allowing the opportunity to explore new predictive markers and therapeutic targets. (Mol Cancer Res 2009;7(4)