Novel mechanisms of vascular-specific peroxisome proliferator-activated receptor gamma in hypertension and atherosclerosis

The nuclear hormone receptor peroxisome proliferator-activated receptor Γ (PPARΓ) is a ligand-dependent transcription factor of increasing importance in cardiovascular physiology. Treatment of type II diabetes patients with thiazolidinediones (TZD), synthetic ligands of PPARΓ, improves insulin sensitivity and also lowers blood pressure despite increased water and salt retention by the kidneys. In 1999, Stephen O'Rahilly's group reported that patients carrying mutations in PPARΓ exhibit severe type II diabetes and early-onset hypertension. The missense mutations in PPARΓ (e.g. V290M, P467L) affect the ligand-binding domain and render the transcription factor dominant negative (DN). These findings suggested that the PPARΓ activation plays a vital role in cardiovascular regulation but they did not differentiate whether the cardiovascular protective effects of TZDs result from systemic metabolic changes or direct actions of PPARΓ in the vasculature. To answer this, our group generated transgenic mice that express DN PPARΓ specifically in vascular endothelial or vascular smooth muscle cell types. Herein we are reporting the molecular and physiological mechanism linking mutation of PPARΓ to impaired vascular function leading to hypertension. Smooth muscle-specific expression of DN PPARΓ in transgenic mice causes increased arterial pressure and enhanced agonist-mediated contraction and blunting of nitric oxide-mediated relaxation in aorta via a RhoA/Rho-kinase-dependent mechanism. Our results demonstrate that interference with PPARΓ in smooth muscle impairs Cullin-3 RING E3 ubiquitin ligase-mediated regulation of RhoA/Rho-kinase signaling and identify Cullin-3 as a novel regulator of vascular function. Hypertension, insulin resistance and atherosclerosis are major targets for therapeutic intervention against morbidity and mortality caused by coronary artery disease. In addition to the beneficial blood-pressure lowering and insulin-sensitizing effects of treatment with TZD PPARΓ agonists, they also have potent inhibitory effects on atherosclerosis progression. Given the concerns over TZD use including weight gain and edema, it is essential to understand the fundamental mechanisms by which vascular PPARΓ affects atherosclerosis lesion development. We tested this by crossing our transgenic mice onto the apolipoprotein E-deficient (ApoE^-/-) mouse model of hypercholesterolemia. Either endothelial- or smooth muscle-specific expression of DN PPARΓ on the ApoE^-/- background led to enhanced atherosclerotic lesion formation in aorta without altering levels of plasma cholesterol and triglycerides. Furthermore, endothelial- or smooth muscle-specific DN PPARΓ induced distinct alterations in the signature of genes related to atherogenesis when comparing aortic tissue from either model with its respective non-transgenic control. Our results obtained using PPARΓ-interfering mutations provide novel mechanistic insight into pathways critical to the pathogenesis of cardiovascular diseases.