Direct Involvement of Breast Tumor Fibroblasts in the Modulation of Tamoxifen Sensitivity

Using contact-dependent three-dimensional coculture systems and serum-free conditions, we compared the ability of estrogen receptor (ER)-α+ tamoxifen-sensitive premalignant (EIII8) or tumorigenic (MCF-7), ER-α+ tamoxifen-resistant (EIII8-TAMR) or ER-α− MDA-MB-231 breast cancer cells to interact and undergo epithelial morphogenesis on association with breast tumor-derived fibroblasts. Although all breast cancer cell lines interacted with tumor fibroblasts, EIII8 and its intrinsically tamoxifen-resistant counterpart EIII8-TAMR cells were most receptive and responded with dramatic, albeit, aberrant epithelial morphogenesis. EIII8 cells underwent epithelial morphogenesis when cocultured with fibroblasts from ER-α−/PgR− or ER-α+/PgR+ breast tumors; however, EIII8 cells cocultured with ER-α−/PgR− tumor-derived fibroblasts exhibited decreased tamoxifen sensitivity compared with cells cocultured with ER-α+/PgR+ tumor fibroblasts. Fibroblast-induced tamoxifen resistance was accompanied by mitogen-activated protein kinase and Akt hyperactivation, reduced sensitivity to U0126 or LY294002, and ER-α hyperphosphorylation in the activation function-1 domain. The intrinsic tamoxifen resistance of EIII8-TamR cells correlated with constitutive ER-α hyperphosphorylation that was unaffected by the tumor fibroblasts. Our results suggest that tumor fibroblast-induced tamoxifen resistance of EIII8 cells is not mediated by epidermal growth factor receptor or insulin-like growth factor (IGF)-1R axes because no correlation was found between expression levels of IGF-1, IGF-2, phosphorylated IGF-1R, or epidermal growth factor receptor, and tamoxifen sensitivity of EIII8 fibroblast cultures.