STAT5b, c-Src, and EGFR Signaling Play Integral Roles in Estrogen-Stimulated Proliferation of ER-Positive Breast Cancer Cells

Authors:
Fox EM, Bernaciak TM, Wen J, Weaver AM, Shupnik MA, Silva CM
In:
Source: Mol Endocrinol
Publication Date: (2008)
Issue: 22(8): 1781-96
Research Area:
Cancer Research/Cell Biology
Cells used in publication:
MCF7
Species: human
Tissue Origin: breast
T-47D
Species: human
Tissue Origin: breast
Platform:
Nucleofectorâ„¢ I/II/2b
Abstract
17beta-estradiol (E2) acts through the estrogen receptor alpha (ERalpha) to stimulate breast cancer proliferation. Here, we investigated the functional relationship between ERalpha and STAT5b activity in ER+ MCF-7 and T47D human breast cancer cells after specific knockdown of STAT5b. STAT5b siRNA inhibited E2-induced BrdU incorporation in both cell lines, as well as the E2-induced increase in MCF-7 cell number, cyclin D1 and c-myc mRNA, and cyclin D1 protein expression, indicating that STAT5b is required for E2-stimulated breast cancer proliferation. E2 treatment stimulated STAT5b tyrosine phosphorylation at the activating tyrosine Y699, resulting in increased STAT5-mediated transcriptional activity, which was inhibited by a Y669F STAT5b mutant. E2-induced STAT5-mediated transcriptional activity was inhibited by overexpressing a kinase-defective EGFR, or the EGFR tyrosine kinase inhibitor tyrphostin AG1478, indicating a requirement for EGFR kinase activity. Both E2-induced STAT5b tyrosine phosphorylation and STAT5-mediated transcription were also inhibited by the ER antagonist ICI 182,780 and the c-Src inhibitor PP2, indicating additional requirements for the ER and c-Src kinase activity. EGFR and c-Src kinase activities were also required for E2-induced cyclin D1 and c-myc mRNA. Together, these studies demonstrate positive crosstalk between ER, c-Src, EGFR, and STAT5b in ER+ breast cancer cells. Increased EGFR and c-Src signaling is associated with tamoxifen resistance in ER+ breast cancer cells. Here we show that constitutively active STAT5b not only increased basal DNA-synthesis, but also conferred tamoxifen resistance. Since STAT5b plays an integral role in E2-stimulated proliferation and tamoxifen resistance, it may be an effective therapeutic target in ER+ breast tumors.