Angiotensin II Stimulates p21-Activated Kinase in Vascular Smooth Muscle Cells

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Abstract
—Angiotensin II (Ang II) has been previously shown to stimulate the extracellular signal–regulated kinase (ERK)1/2 and c-Jun N-terminal kinase (JNK) mitogen-activated protein (MAP) kinase family members. Little is known regarding the upstream signaling molecules involved in Ang II–mediated JNK activation. Ang II has been shown to activate the Janus kinase/signal transducer(s) and activator(s) of transcription (JAK/STAT) pathway, suggesting similarities to cytokine signaling. In response to cytokines such as interleukin-1 and tumor necrosis factor-α, the p21-activated kinase (PAK) has been identified as an upstream component in JNK activation. Therefore, we hypothesized that PAK may be involved in JNK activation by Ang II in vascular smooth muscle cells (VSMCs). αPAK activity was measured by myelin basic protein phosphorylation in rat aortic VSMCs. In response to Ang II, αPAK was rapidly stimulated within 1 minute, with a peak (5-fold increase) at 30 minutes. αPAK stimulation preceded activation of JNK in VSMCs. Ang II–mediated activation of both αPAK and JNK was Ca2+ dependent and inhibited by downregulation of phorbol ester–sensitive protein kinase C isoforms (by pretreatment with phorbol 12,13-dibutyrate) but not by pretreatment with GF109203X. Activation of both PAK and JNK was partially inhibited by tyrosine kinase inhibitors but not by specific Src inhibitors, suggesting regulation by a tyrosine kinase other than c-Src. Finally, introduction of dominant negative PAK markedly reduced the JNK activation by Ang II in both Chinese hamster ovary and COS cells stably expressing the Ang II type 1 receptor (AT1R). Our data provide evidence for αPAK as an upstream mediator of JNK in Ang II signaling and extend the role of Ang II as a proinflammatory mediator for VSMCs.