Abstract 695: Nox4 NAD(P)H Oxidase Mediates Angiotensin II-Induced Mesangial Cell Hypertrophy and Fibronectin Expression via Activation of a Src/PDK-1/Akt/PKB-Dependent Pathway
We have previously identified the serine-threonine kinase Akt/PKB as a signal transducer of the Nox4 NAD(P)H oxidase-dependent redox pathway engaged by angiotensin II (Ang II) to exert its hypertrophic and fibrotic effects in mesangial cells (MCs). Here, we demonstrate that in MCs, Ang II induces tyrosine phosphorylation of 3-phosphoinositide-dependent protein kinase-1 (PDK-1), the upstream activator of Akt/PKB. This increase in tyrosine phosphorylation is accompanied by an increase in phosphorylation of PDK-1 on Serine 241, another indicator of enzyme activation. Ang II also causes c-Src activation and phosphorylation on Tyrosine 416. Ang II-induced Tyrosine phosphorylation of PDK-1 is inhibited in MCs transfected with siRNA for Src (siSrc), but not scrambled siRNA, indicating that Src is positioned upstream of PDK-1. Moreover, Ang II activation and phosphorylation of Akt/PKB on Threonine 308 is abrogated by siSrc. Inhibition of Src activity by siSrc also blocks Ang II-induced fibronectin expression and protein synthesis. Similarly, depletion and inhibition of PDK-1 by transfection of MCs with siRNA for PDK-1 prevents the stimulatory effect of Ang II on fibronectin synthesis and MC hypertrophy. Hydrogen peroxide activates Src, PDK-1 and Akt/PKB, demonstrating that the kinases are redox sensitive. Downregulation of Nox4 protein by siNox4 abrogates Ang II-induced intracellular reactive oxygen species (ROS) generation. SiNox4 almost totally inhibits Ang II-induced phosphorylation of Src on Tyrosine 416, Serine 241 and tyrosine phosphorylation of PDK-1, and phosphorylation of Akt/PKB on Threonine 308, demonstrating that Nox4-derived ROS are critical for Ang II-induced Src, PDK-1 and Akt/PKB activation. In summary, we propose that Nox4-mediated generation of ROS is responsible for Ang II-induced activation. In turn, Src regulates Akt/PKB activity through stimulation and tyrosine phosphorylation of PDK-1. Importantly, this signaling pathway contributes to Ang II-induced MC hypertrophy and fibronectin expression. These data shed light on molecular processes underlying the oxidative signaling cascade engaged by Ang II in MCs and identify potential targets for the elaboration of intervention to prevent renal hypertrophy and fibrosis.