The IGF Pathway Regulates ERα through a S6K1-Dependent Mechanism in Breast Cancer Cells

• Published in: Molecular endocrinology (Baltimore, Md.) Md.), 2011-03, Vol.25 (3), p.516-528
• Main Authors: Becker, Marc A, Ibrahim, Yasir H, Cui, Xiaojiang, Lee, Adrian V, Yee, Douglas
• Format: Article
• Language: English
• Subjects: Blotting, Western; Breast Neoplasms - metabolism; Cell Line, Tumor; Cell Proliferation - drug effects; Chromatin Immunoprecipitation; Estrogen Receptor alpha - genetics; Estrogen Receptor alpha - metabolism; Humans; Immunoprecipitation; Insulin-Like Growth Factor I - pharmacology; Original Research; Phosphorylation - drug effects; Protein Binding - drug effects; Reverse Transcriptase Polymerase Chain Reaction; Ribosomal Protein S6 Kinases, 70-kDa - antagonists & inhibitors; Ribosomal Protein S6 Kinases, 70-kDa - genetics; Ribosomal Protein S6 Kinases, 70-kDa - metabolism
• ISSN: 0888-8809; 1944-9917
• DOI: 10.1210/me.2010-0373

In estrogen receptor (ER) positive breast cancer cells, insulin-like growth factor signaling activates S6K1 to initiate ER-mediated gene transcription and cell growth. The IGF pathway stimulates malignant behavior of breast cancer cells. Herein we identify the mammalian target of rapamycin (mTOR)/S6 kinase 1 (S6K1) axis as a critical component of IGF and estrogen receptor (ER)α cross talk. The insulin receptor substrate (IRS) adaptor molecules function downstream of IGF-I receptor and dictate a specific biological response, in which IRS-1 drives proliferation and IRS-2 is linked to motility. Although rapamycin-induced mTOR inhibition has been shown to block IGF-induced IRS degradation, we reveal differential effects on motility (up-regulation) and proliferation (down-regulation). Because a positive correlation between IRS-1 and ERα expression is thought to play a central role in the IGF growth response, we investigated the potential role of ERα as a downstream mTOR target. Small molecule inhibition and targeted knockdown of S6K1 blocked the IGF-induced ERαS167 phosphorylation and did not influence ligand-dependent ERαS118 phosphorylation. Inhibition of S6K1 kinase activity consequently ablated IGF-stimulated S6K1/ERα association, estrogen response element promoter binding and ERα target gene transcription. Moreover, site-specific ERαS167 mutation reduced ERα target gene transcription and blocked IGF-induced colony formation. These findings support a novel link between the IGF pathway and ERα, in which the translation factor S6K1 affects transcription of ERα-regulated genes.