Fyn and JAK2 mediate Ras activation by reactive oxygen species

Reactive oxygen species (ROS) activate Ras and the extracellular signal- regulated kinase (ERK) cascade. Because JAK2 is a critical mediator for Ras/Raf/ERK activation by several hormones, we examined the role of JAK2 in ROS signal 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 activation and its downstream events, we used fibroblasts derived from transgenic mice deficient in Src (Src-/-) or Fyn (Fyn-/-). H₂O₂-stimulated JAK2 activity was completely inhibited in Fyn-/- cells. Shc tyrosine phosphorylation and Ras activation by H₂O₂ were also significantly reduced in Fyn-/- cells, 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 and Ras activation. H₂O₂-mediated ERK1/2 activation in Fyn- /- cells and AG-490 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 redox-sensitive pathway for Ras activation and rapid ERK1/2 activation, which is mediated by Fyn and JAK2.