Data from 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):498–510)