Reactive oxygen species (ROS) have been identified as central mediators in certain signalling events. In the heart, ROS have important functions in ischaemia/reperfusion-induced cardiac injury and in cytokine-stimulated hypertrophy. Extracellular signal-regulated kinase (ERK) is one of the ROS-responsive serine/threonine kinases. Previous studies showed that tyrosine kinases and small G proteins are involved in the activation of ERK by ROS; however, the initial target protein of ROS that leads to ERK activation remains unknown. Here we show that inhibition of the βγ-subunit of G protein (Gβγ) attenuates hydrogen peroxide (H2O2)-induced ERK activation in rat neonatal cardiomyocytes. The Gβγ-responsive ERK activation induced by H2O2 is independent of ligands binding to G(i)-coupled receptors, but requires phosphatidylinositol-3-kinase and Src activation. In in vitro studies, however, treatment with H2O2 increases [35S]GTPγS binding to cardiac membranes and directly activates purified heterotrimeric G(i) and G(o) but not G(s). Analysis using heterotrimeric G(o) and its individual subunits indicates that H2O2 modifies Gα(o) but not Gβγ, which leads to subunit dissociation. We conclude that Gα(i) and Gα(o) are critical targets of oxidative stress for activation of ERK.
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