Fyn and JAK2 mediate Ras activation by reactive oxygen species

Reactive oxygen species (ROS) activate Ras and the extracellular signal-reg ulated kinase (ERK) cascade. Because JAK2 is a critical mediator for Ras/Ra f/ERK activation by several hormones, we examined the role of JAK2 in ROS s ignal events. H2O2 stimulated JAK2 activity in fibroblasts with peak at 2-5 min. To determine the specific role of Src and Fyn as mediators of JAK2 ac tivation and its downstream events, we used fibroblasts derived from transg enic mice deficient in Src (Src-/-) or Fyn (Fyn-/-). H2O2-stimulated JAK2 a ctivity was completely inhibited in Fyn-/- cells. Shc tyrosine phosphorylat ion and Ras activation by H2O2 were also significantly reduced in Fyn-/- ce lls, but not altered in Src-/- cells. Activation of JAK2 was restored when Fyn-/- cells were transfected with B-Fyn but not with Src. Inhibiting JAK2 activity with the specific inhibitor AG-490 prevented H2O2 stimulated Shc a nd Ras activation. H2O2-mediated ERK1/2 activation in Fyn-/- cells and AG-4 90 treated cells was completely inhibited at an early time (5 min), but not at late times (20-40 min) after stimulation. These results define a new re dox-sensitive pathway for Ras activation and rapid ERK1/2 activation, which is mediated by Fyn and JAK2.