Role of smooth muscle Rac1 in the vascular remodeling associated with pulmonary hypertension

Abstract

Pulmonary arterial hypertension (PAH) is a chronic disease characterized by an increase in pulmonary artery (PA) resistance leading to right ventricle (RV) failure. Endothelial dysfunction and alteration of NO/cGMP signaling play a major role in PAH. We previously described the major role of smooth muscle cell (SMC) Rac1 in NO-induced dilation of systemic arteries. Here, our aim was to analyze the role of SMC Rac1 in pulmonary circulation and in PAH. Rac1 activity was assessed by immunofluorescence in lung biopsies. PAH has been induced by exposure of wild-type (WT) and SMC Rac1-deficient mice (SM-Rac1-KO mice) to chronic hypoxia (10% O2, 4 weeks). In vitro, the proliferation of PASMC in hypoxic condition was measured by EdU staining and contractility of PA rings was analyzed by myography. Oxidative stress was assessed using the DHE probe and by electron paramagnetic resonance (EPR). A 3-fold increase of Rac-GTP in PA was observed in PAH patients compared to control ( n = 4-6). To determine whether Rac1 activation plays a causal role in PAH, WT and SM-Rac1-KO mice were exposed to hypoxia. As expected, exposure to chronic hypoxia induced an increase in systolic pressure in RV and RV hypertrophy which were reduced by 32.5% and 60% respectively, in SM-Rac1-KO mice compared to WT ( n = 5–7). PA remodeling was also significantly reduced in hypoxic SM-Rac1-KO compared to WT mice, while PA contractility was not affected by SMC Rac1 deletion. In vitro, exposure of primary PASMC to hypoxia stimulated proliferation and ROS production that were decreased by 50% and 66%, respectively, by the Rac1 inhibitor, EHT1864. Our results show that SMC Rac1 activity is increased in PA of PAH patients and plays a causal role in hypoxia-induced PAH in mice by stimulating ROS-dependent PA remodeling. These data suggest that pharmacological Rac1 inhibition might have a beneficial effect in the treatment of PAH.