Angiotensin II plays a critical role in hypertrophy of vascular smooth muscle cells, however, the molecular underpinnings remain unclear. The present study indicated that AT(1)/PKC/PKD pathway was able to regulate downstream ERK5, affecting pro-hypertrophic responses to Ang II. Ang II-stimulated phosphorylation of ERK5 in a time- and dose-dependent manner in human aortic smooth muscle cells (HASMCs). The pharmacological inhibitors for AT(1) and PKCs significantly inhibited Ang II-induced ERK5 activation, suggesting the involvement of the AT(1)/PKC pathway. In particular, PKD was critical for Ang II-induced ERK5 activation since silencing PKD by siRNA markedly inhibited Ang II-induced ERK5 activation. Consequently, we found that Losartan, Go 6983 and PKD siRNA significantly attenuated ERK5 activated translocation and hypertrophy of HASMCs by Ang II. Taken together, we demonstrated for the first time that Ang II activates ERK5 via the AT(1)/PKC/PKD pathway and revealed a critical role of ERK5 in Ang II-induced HASMCs hypertrophy. (C) 2009 Elsevier Inc. All rights reserved.