| Breast cancer growth can be stimulated by 17beta-estradiol (E2) or growth factors like epidermal growth factor (EGF). Early stage breast tumors are E2-dependent and overexpress the Estrogen Receptor (ER), whereas more advanced breast tumors are often ER-negative and overexpress human epidermal growth factor receptor (HER) family members such as EGFR. One potential common mediator of E2 and EGF signaling in breast cancer is the transcription factor STAT5b. We examined functional crosstalk between ERalpha and STAT5b signaling in ER-negative and ER-positive (ER+) breast cancer models. Introduction of ERalpha into ER-negative breast cancer cells overexpressing HER family members, along with E2 treatment, suppressed basal and EGF-induced STAT5-mediated transcription and EGF-induced STAT5b tyrosine phosphorylation and DNA-synthesis, suggesting ER interfered with signaling through EGFR in this context. We then concentrated on ERalpha and STAT5b crosstalk in the more physiologically relevant ER+ breast cancer model. We used STAT5b siRNA to show STAT5b is required for E2-stimulated breast cancer proliferation in ER+ MCF-7 and T47D breast cancer cells, as well as the E2-induced transcription of the endogenous proliferative genes cyclin D1 and c-Myc. E2 acted through ER, c-Src, and EGFR to stimulate STAT5b Y699 phosphorylation and STAT5-mediated transcription. EGFR and c-Src kinase activities were also required for E2-induced cyclin D1 and c-Myc mRNA, demonstrating positive crosstalk between ER, c-Src, EGFR, and STAT5b in ER+ breast cancer growth. In addition, constitutively active STAT5b conferred tamoxifen resistance. Chromatin immunoprecipitation studies revealed that E2 induced cyclical recruitment of STAT5b and ERalpha to the cyclin D1 and c-Myc promoters, which correlated with enhanced association of phosphorylated RNA Polymerase II. STAT5b is required for the E2-induced recruitment of ERalpha and phosphorylated RNA Polymerase II to both promoters. Overall, our research demonstrates that ERalpha-E2 has functional interactions with EGFR and c-Src to stimulate STAT5b phosphorylation and transcriptional activity in E2-dependent ER+ breast cancer cells, and this is required for E2-stimulated proliferation. Thus, ER and STAT5b pathways intersect in the cytoplasm and nucleus. Because STAT5b is also a target for growth factors, it may be an effective therapeutic target for many types of breast tumors, including those that are resistant to tamoxifen. |
