Src and Cas mediate JNK activation but not ERK1/2 and p38 kinases by reactive oxygen species

c-Jun NH₂-terminal kinase (JNK) is activated by a number of cellular stimuli such as inflammatory cytokines and environmental stresses. Reactive oxygen species also cause activation of JNK; however, the signaling cascade that leads to JNK activation remains to be elucidated. Because recent reports showed that expression of Cas, a putative Src substrate, stimulates JNK activation, we hypothesized that the Src kinase family and Cas would be involved in JNK activation by reactive oxygen species. An essential role for both Src and Cas was demonstrated. First, the specific Src family tyrosine kinase inhibitor, PP2, inhibited JNK activation by H₂O₂ in a concentration- dependent manner but had no effect on extracellular signal-regulated kinases 1 and 2 and p38 activation. Second, JNK activation in response to H₂O₂ was completely inhibited in cells derived from transgenic mice deficient in Src but not Fyn. Third, expression of a dominant negative mutant of Cas prevented H₂O₂-mediated JNK activation but had no effect on extracellular signal- regulated kinases 1 and 2 and p38 activation. Finally, the importance of Src was further supported by the inhibition of both H₂O₂-mediated Cas tyrosine phosphorylation and Cas-Crk complex formation in Src-/- but not Fyn-/- cells. These results demonstrate an essential role for Src and Cas in H₂O₂- mediated activation of JNK and suggest a new redox-sensitive pathway for JNK activation mediated by Src.