Phospholipase A₂-mediated activation of mitogen-activated protein kinase by angiotensin II

In renal proximal tubule epithelial cells, a membrane-associated phospholipase A₂ (PLA₂) is a major signaling pathway linked to angiotensin II (Ang II) type 2 receptor (AT₂). The current studies were designed to test the hypothesis that membrane-associated PLA₂-induced release of arachidonic acid (AA) and/or its metabolites may serve as an upstream mediator of Ang II- induced mitogen-activated protein kinase (MAPK) activation. Ang II stimulated transient dose-dependent phosphorylation of MAPK with a maximum at 1 μM (10 min). Inhibition of PLA₂ by mepacrine diminished both AA release and MAPK phosphorylation, induced by Ang II. Furthermore, AA itself induced time- and dose-dependent phosphorylation of MAPK, supporting the importance of PLA₂ as a mediator of Ang II signaling. The effects of both Ang II and AA on MAPK phosphorylation were protein kinasc C independent and abolished by the inhibitor of cytochrome P450 isoenzyme, ketoconazole. Moreover, 5,6- epoxyeicosatrienoic acid and 14,15-epoxyeicosatrienoic acid, the cytochrome P450-dependent metabolites of AA, significantly stimulated MAPK activity in renal proximal tubule epithelial cells. These observations document a mechanism of Ang II-induced MAPK phosphorylation, mediated by PLA₂- dependent release of AA and cytochrome P450-dependent production of epoxy derivatives of AA.