ErbB receptor-induced activation of Stat transcription factors is mediatedby Src tyrosine kinases

Epidermal growth factor (EGF) binding to its receptor, ErbB1, triggers vari ous signal transduction pathways, one of which leads to the activation of s ignal transducer and activator of transcription (Stat) factors. The mechani sm underlying ErbB1-induced Stat activation and whether Stats are downstrea m targets of other ErbB receptors have not been explored. In this report we show that ErbB2, ErbB3, and ErbB4 do not potentiate Stat5 phosphorylation by EGF, However, neu differentiation factor-induced heterodimers of ErbB2 a nd ErbB4 activated Stat5. In A431 cells, Stat1, Stat3, and Stat5, were cons titutively complexed with ErbB1 and rapidly phosphorylated on tyrosine in r esponse to EGF. Neither mutation of the conserved tyrosine residue (Tyr(694 )) nor inactivation of the Stat5a SH2 domain disrupted this association. Ho wever, an intact SH2 domain was necessary for EGF-induced Stat5a phosphoryl ation, In contrast to prolactin, which induced only Tyr(694) phosphorylatio n of Stat5a, EGF promoted phosphorylation on Tyr(694) and additional tyrosi ne residue(s). Janus kinases (Jaks) were also constitutively associated wit h ErbB receptors and were phosphorylated in response to EGF-related ligands , However, we provide evidence that EGF- and neu differentiation factor-ind uced Stat activation are dependent on Src but not Jak kinases. Upon EGF sti mulation, c-Src was rapidly recruited to Stat/ ErbB receptor complexes. Pha rmacological Src kinase inhibitors and a dominant negative c-Src ablated bo th Stat and Jak tyrosine phosphorylation, However, dominant negative Jaks d id not affect EGF-induced Stat phosphorylation, Taken together, the experim ents establish two independent roles for Src kinases: (i) key molecules in ErbB receptor-mediated Stat signaling and (ii) potential upstream regulator s of Jak kinases.