title: STAT5b, a Mediator of Synergism between c-Src and the Epidermal Growth Factor Receptor.
abstract: Overexpression of the epidermal growth factor receptor (EGFR) and its association with the tyrosine kinase, c-Src, is correlated with increased cellular proliferation and tumorigenesis. Previous studies have shown that EGFR and c-Src co-overexpression and association leads to the c-Src-mediated phosphorylation of tyrosine 845 of the EGFR and that mutation of Tyr(845) ablates epidermal growth factor (EGF)-induced DNA synthesis. Here, we investigate the contribution of the signal transducers and activators of transcription (STAT5b) in the signaling pathways regulated by EGFR and c-Src overexpression in human breast tumor cell lines as well as in a mouse fibroblast model (C3H10T1/2). We demonstrate that 1) activation of STAT5b by EGF requires overexpression of the EGFR, 2) co-overexpression of c-Src alone does not result in EGF-induced activation of STAT5b but enhances that seen in EGFR-overexpressing cells, and 3) EGF-induced tyrosine phosphorylation of STAT5b requires Tyr(845) of the EGFR. Furthermore, the stable overexpression of a kinase-defective c-Src in the context of EGFR overexpression results in a decrease in the tyrosine phosphorylation of STAT5b in response to EGF and a more dramatic decrease in EGF-induced transcriptional activation of STAT5b, suggesting an integral role for c-Src in the physiological actions of STAT5b. Using a dominant negative STAT5b, we provide evidence that one such physiological action is to mediate EGF-induced DNA-synthesis. Finally, the use of site-specific tyrosine mutants demonstrates that EGF-induced phosphorylation of STAT5b involves not only tyrosine 699 of STAT5b, which is required for its transcriptional activation, but also three previously identified tyrosines in the C terminus of STAT5b (Tyr(725)/Tyr(740)/Tyr(743)).
authors.affiliation: Departments of Internal Medicine and Microbiology and the Cancer Center, University of Virginia Health System, Charlottesville, Virginia 22908.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Kloth
authors.forename: Kristen K
authors.initials: KK
authors.lastname: Laughlin
authors.forename: Jacqueline S
authors.initials: JS
authors.lastname: Biscardi
authors.forename: Julie L
authors.initials: JL
authors.lastname: Boerner
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Parsons
authors.forename: Corinne M
authors.initials: CM
authors.lastname: Silva
mesh_terms: D000818:Animals; D004261:DNA Replication / Q000502:physiology; D004268:DNA-Binding Proteins / Q000502:physiology*; D066246:ErbB Receptors / Q000737:chemistry / Q000502:physiology*; D051379:Mice; D008809:Mice, Inbred C3H; D008894:Milk Proteins; D010766:Phosphorylation; D016392:Proto-Oncogene Proteins pp60(c-src) / Q000502:physiology*; D050799:STAT5 Transcription Factor; D015534:Trans-Activators / Q000502:physiology*; D014407:Tumor Cells, Cultured; D014443:Tyrosine / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; D008894:Milk Proteins; D050799:STAT5 Transcription Factor; C494096:Stat5b protein, mouse; D015534:Trans-Activators; D014443:Tyrosine; D066246:ErbB Receptors; D016392:Proto-Oncogene Proteins pp60(c-src)