The role of protein kinase C in activation and termination of mitogen-activated protein kinase activity in angiotensin II-stimulated rat aortic smooth-muscle cells

Mitogen-activated protein (MAP) kinases are a family of serine/threonine kinases activated by both tyrosine kinase and G-protein-linked receptor agonists. In rat aorta vascular smooth-muscle cells (VSMC), vasoconstrictors, angiotension II (AII), and α-thrombin (α-thr), as well as platelet-derived growth factor ββ (PDGF) stimulated the tyrosine phosphorylation and activation of MAP kinase in a time- and concentration-dependent manner. Pre-treatment of cells with the protein kinase C (PKC) inhibitor Ro-318220, inhibited the initial increase in tyrosine phosphorylation of MAP kinase in response to vasoconstrictors, suggesting the involvement of PKC. Four isoforms of PKC were identified in VSMC by western blotting: α, β, ϵ, and ζ. Downregulation of PKCα and PKCε isoforms following chronic phorbol myristate 12, 13-acetate (PMA) pre-treatment resulted in the abolition of AII-stimulated MAP kinase activation. Selective downregulation of PKCα following pre-treatment with bryostatin 1 did not affect AII-stimulated MAP kinase. Preincubation of cells with Ro-318220 enhanced the activation of MAP kinase at later time points. In addition, Ro-318220 pre-treatment inhibited the induction by AII of a novel transcriptionally regulated phosphatase, MAP kinase phosphatase-1 (MKP-1). However, AII-mediated activation of MAP kinase was not prolonged by cycloheximide pre-treatment and was not maintained indefinitely by Ro-318220. These results demonstrate a specific role for the Ca 2+-independent PKC isoform, PKCε, in the activation of MAP kinase in response to vasoconstrictors, and suggest that PKC-mediated induction of MKP-1 plays no role in the termination of transiently activated MAP kinase.