Reactive oxygen species activate p90 ribosomal S6 kinase via Fyn and Ras

Reactive oxygen species and growth factors stimulate similar intracellular signal transduction events including activation of Src kinase family members and extracellular signal-regulated kinases (ERK1/2). A potentially important downstream effector of Src and ERK1/2 is p90 ribosomal S6 kinase (p90RSK), which plays an important role in cell growth by activating several transcription factors as well as the Na⁺/H⁺ exchanger. In the present study, we determined whether H₂O₂ activates p90RSK to gain insight into signal transduction mechanisms activated by reactive oxygen species. H₂O₂ (200 μM) stimulated ERK1/2 and p90RSK activity in lymphocytes, endothelial cells, and fibroblasts. The MEK-1 inhibitor, PD98059 (30 μM), inhibited H₂O₂-mediated activation of ERK1/2 but not of p90RSK. An essential role for Fyn and Ras in p90RSK activation was suggested by five findings. 1) The tyrosine kinase inhibitor, herbimycin A, and the specific Src kinase family inhibitor, PP1, blocked p90RSK activation by H₂O₂ in a concentration-dependent manner. 2) p90RSK activation by H₂O₂ was significantly reduced in fibroblasts derived from transgenic mice deficient in Fyn, but not c-Src. 3) H₂O₂ rapidly activated Ras (peak at 2-5 min), which preceded p90RSK activation (peak at 20 min). 4) Dominant negative Ras completely blocked H₂O₂-induced activation of p90RSK. 5) In Fyn-/- fibroblasts, activation of Ras by H₂O₂ was significantly attenuated. These results show essential roles for Fyn and Ras in H₂O₂-mediated activation of p90RSK and establish redox-sensitive regulation of Ras and p90RSK as a new function for Fyn.