Temporal dispersion of activation of phospholipase C-beta1 and -gamma isoforms by angiotensin II in vascular smooth muscle cells. Role of alphaq/11, alpha12, and beta gamma G protein subunits.

Activation of phospholipase C (PLC) is one of the earliest events in angiotensin II (Ang II) type 1 (AT1) receptor (R)-mediated signal transduction in vascular smooth muscle cells (VSMCs). The coupling mechanisms of AT1 Rs to PLC, however, are controversial, because both tyrosine phosphorylation of PLC-gamma and G protein-dependent PLC-beta activation pathways have been reported. The expression of PLC-beta1, furthermore, has not been consistently demonstrated in VSMCs. Here we identified the PLC subtypes and subunits of heterotrimeric G proteins involved in AT1 R-PLC coupling using cultured rat VSMCs. Western analysis revealed the expression of PLC-beta1, -gamma1, and -delta1 in VSMCs. Ang II-stimulated inositol trisphosphate (IP3) formation measured at 15 s, which corresponds to the peak response, was significantly inhibited by electroporation of antibodies against PLC-beta1, but not by anti-PLC-gamma and -delta antibodies. Electroporation of anti-Galphaq/11 and -Galpha12 antibodies also showed significant inhibition of the Ang II-induced IP3 generation at 15 s, while anti-Galphai and Galpha13 antibodies were ineffective. Furthermore, in VSMCs electroporated with anti-Gbeta antibody and cells stably transfected with the plasmid encoding the Gbetagamma-binding region of the carboxyl terminus of beta-adrenergic receptor kinase1, the peak Ang II-stimulated PLC activity (at 15 s) was significantly inhibited. The tyrosine kinase inhibitor, genistein, had no effect on the peak response to Ang II stimulation, but significantly inhibited IP3 production after 30 s, a time period which temporally correlated with PLC-gamma tyrosine phosphorylation in response to Ang II. Moreover, electropor-ation of anti-PLC-gamma antibody markedly inhibited the IP3 production measured at 30 s, indicating that tyrosine phosphorylation of PLC-gamma contributes mainly to the later phase of PLC activation. Thus, these results suggest that: 1) AT1 receptors sequentially couple to PLC-beta1 via a heterotrimeric G protein and to PLC-gamma via a downstream tyrosine kinase; 2) the initial AT1 receptor-PLC-beta1 coupling is mediated by Galphaq/11beta gamma and Galpha12 beta gamma; 3) Gbeta gamma acts as a signal transducer for activation of PLC in VSMCs. The sequential coupling of AT1 receptors to PLC-beta1 and PLC-gamma, as well as dual coupling of AT1 receptors to distinct Galpha proteins, suggests a novel mechanism for a temporally controlled, highly organized and convergent Ang II-signaling network in VSMCs.