G_q- and G_12/13-coupled activation of Rho mediates inhibitory regulation of cellular Rac and chemotaxis in VSMCs

Abstract

The lysophospholipid mediator sphingosine-1-phosphate (S1P) exerts pleiotropic actions on diverse cell types through Edg family G protein-coupled receptors. Among them, of particular interest is an inhibitory action of S1P on cell migration. We have previously shown that S1P induces activation of Rho via Edg-5, which in turn mediates inhibition of PDGF-induced Rac activation and chemotaxis in VSMCs. In non-muscle cells, it was previously demonstrated that G_αq as well as G_α12 and G_α13 mediate Rho activation. However, it is not fully understood in VSMCs which heterotrimeric G protein mediates Rho activation and how the G protein regulates the activity of Rho. The C-terminal peptides of G_α subunits (G_α-CT) act as specific inhibitors for respective G proteins. Adenovirus-mediated expression of either G_αq-CT, G_α12-CT, or G_α13-CT, but not G_αs-CT, abolished S1P stimulation of RhoA and S1P inhibition of PDGF-induced Rac activation and chemotaxis. Interestingly, the PLC inhibitor U-73122 and the Ca²⁺-chelator BAPTA-AM did not affect S1P-induced RhoA activation. These results suggest in VSMCs that both G_αq and G_α12/13 mediate Rho activation, which in turn mediates inhibition of cellular Rac and chemotaxis, and that the G_αq-mediated Rho activation is independent of calcium mobilization. [Jpn J Physiol 54 Suppl:S77 (2004)]