Activation of 44 and 42 kDa extracellular signal-regulated kinases (ERK)1/2 by angiotensin II (angII) plays an important role in vascular smooth muscle cell (VSMC) function. The dual specificity mitogen-actived protein (MAP) kinase/ERK kinase (MEK) activates ERK1/2 in response to angII, but the MEK activating kinases remain undefined. Raf is a candidate MEK kinase. However, a kinase other than Raf appears responsible for angII-mediated signal transduction because we showed previously that treatment with 1 microM phorbol 12, 13-dibutyrate (PDBU) for 24 h completely blocked Raf-Ras association in VSMC but did not inhibit activation of MEK and ERK1/2 by angII. We hypothesized that an atypical protein kinase C (PKC) isoform, which lacks a phorbol ester binding domain, mediated ERK1/2 activation by angII. Western blot analysis of rat aortic VSMC with PKC isoform-specific antibodies showed PKC-alpha, -beta1, -delta, -epsilon, and -zeta in relative abundance. All isoforms except PKC-zeta were down-regulated by 1 microM PDBU for 24 h suggesting that PKC-zeta was responsible for angII-mediated ERK1/2 activation. In response to angII, PKC-zeta associated with Ras as shown by co-precipitation of PKC-zeta with anti-H-Ras antibody. To characterize further the role of PKC-zeta, PKC-zeta protein was depleted specifically by transfection with antisense PKC-zeta oligonucleotides. Antisense PKC-zeta oligonucleotide treatment significantly decreased PKC-zeta protein expression (without effect on other PKC isoforms) and angII-mediated ERK1/2 activation in a concentration-dependent manner. In contrast, ERK1/2 activation by platelet-derived growth factor and phorbol ester was not significantly inhibited. These results demonstrate an important difference in signal transduction by angII compared with PDGF and phorbol ester in VSMC, and suggest a critical role for PKC-zeta and Ras in angII stimulation of ERK1/2.