0176 : Smooth muscle specific Rac1 deficiency induces hypertension by over activation of RhoA/Rho-kinase signaling pathway

Abstract

The small GTPase RhoA and its target, Rho kinase, play essential role in both blood pressure regulation and vascular smooth muscle contraction. Increasing evidence implicates over activation of this signaling pathway as a critical event in the pathogenesis of hypertension. Interestingly, it has been recently described that Rac1 could antagonize RhoA-dependent functions. However, the role of Rac1 in the regulation of vascular tone and blood pressure is not fully elucidated. Here we assessed the hypothesis that smooth muscle Rac1 participates in the control of blood pressure. To define the role of vascular smooth muscle cells (vSMC) Rac1 in vivo in the control of blood pressure, we generated inducible, smooth muscle (SM)-specific Rac1 knockout mice (SM-Rac1-KO) and analyzed cardiovascular parameters. Following induction of Rac1 deletion, SM-Rac1-KO mice rapidly develop high systolic blood pressure (+15±2 mmHg, n=19, p=0.02). Treatment with the Rho kinase inhibitor fasudil decreases systolic arterial pressure to a level similar in control and SM-Rac1-KO mice suggesting that RhoA/Rho kinase signaling is involved in the rise in systolic blood pressure induced by SM-Rac1 deletion. This is confirmed by immunoblot analysis in aorta that reveals an increase of activated RhoA (348± 19% of control, n=4, p<0.05) and Rho-kinase activity (390±25% of control, n=4, p<0.05) in SM-Rac1-KO compared to control mice. This overactivation of RhoA signaling in SM-Rac1-KO mice is associated with a 75%-reduction of the inhibitory phosphorylation of RhoA on Ser188 compared to control mice. At functional level, while NO production is not modified, NO-dependent vasodilation is decreased in SM-Rac1-KO arteries (≈56% of control). By in vitro experiments in vSMC, we demonstrate that the loss of Rac1 leads to a defective production of cGMP and a down-regulation of the cGMP-dependent kinase-mediated inhibition of RhoA/Rho kinase signaling. These results thus identified a blood pressurelowering effect of vSMC Rac1, that tonically antagonizes RhoA/Rho kinase activity to modulate arterial tone and blood pressure through cGMP/ cGMP-dependent kinase pathway.