Requirement for Ras/Rac1-mediated p38 and c-Jun N-terminal kinase signaling in Stat3 transcriptional activity induced by the Src oncoprotein

Signal transducers and activators of transcription (STATs) are transcriptio n factors that mediate normal biologic responses to cytokines and growth fa ctors. However, abnormal activation of certain STAT family members, includi ng Stat3, is increasingly associated with oncogenesis. In fibroblasts expre ssing the Src oncoprotein, activation of Stat3 induces specific gene expres sion and is required for cell transformation. Although the Src tyrosine kin ase induces constitutive Stat3 phosphorylation on tyrosine, activation of S tat3-mediated gene regulation requires both tyrosine and serine phosphoryla tion of Stat3. We investigated the signaling pathways underlying the consti tutive Stat3 activation in Src oncogenesis. Expression of Ras or Rac1 domin ant negative protein blocks Stat3-mediated gene regulation induced by Src i n a manner consistent with dependence on p38 and c-Jun N-terminal kinase (J NK). Both of these serine/threonine kinases and Stat3 serine phosphorylatio n are constitutively induced in Src-transformed fibroblasts. Furthermore, i nhibition of p38 and JNK activities suppresses constitutive Stat3 serine ph osphorylation and Stat3-mediated gene regulation. In vitro kinase assays wi th purified full-length Stat3 as the substrate show that both JNK and p38 c an phosphorylate Stat3 on serine. Moreover, inhibition of p38 activity and thus of Stat3 serine phosphorylation results in suppression of transformati on by v-Src but not v-Ras, consistent with a requirement for Stat3 serine p hosphorylation in Src transformation. Our results demonstrate that Ras- and Rac1-mediated p38 and JNK signals are required for Stat3 transcriptional a ctivity induced by the Src oncoprotein. These findings delineate a network of tyrosine and serine/threonine kinase signaling pathways that converge on Stat3 in the context of oncogenesis.