Abstract 124: RhoBTB1, a Novel PPARγ Target Gene, Rescues Hypertension and Vascular Dysfunction Caused by PPARγ Dysfunction

Abstract

We reported that mice (S-DN) expressing dominant-negative peroxisome proliferator-activated receptor gamma (PPARγ) in smooth muscle cells (SMC) are hypertensive and exhibit impaired vascular relaxation due to increased RhoA/Rho kinase (ROCK) activity, and display reduced expression of a novel PPARγ target gene, RhoBTB1. We hypothesize that RhoBTB1 plays a protective role in vascular function and that the function of RhoBTB1 is disrupted in S-DN mice. To test this, we generated double transgenic mice (termed S-RhoBTB1) with tamoxifen-inducible, Cre-dependent expression of RhoBTB1 in SMC. S-RhoBTB1 mice were crossed with S-DN to produce mice (S-DN/S-RhoBTB1) in which tamoxifen-treatment (75 mg/kg, ip, 5 days) restored RhoBTB1 expression in aorta to normal. Thoracic aorta and basilar artery from S-DN showed severely impaired vasodilation to acetylcholine (ACh) and sodium nitroprusside, which was reversed by restoration of RhoBTB1 in SMC (Aorta, 46±5 vs 80±2% ACh-induced relaxation, p<0.01, n=6-9). Replacement of RhoBTB1 also reversed the hypertensive phenotype and aortic stiffness observed in S-DN mice within 1 week of treatment (Radiotelemetry SBP, 141±6 vs 124±3 mmHg, p<0.01, n=8-10; Aortic Pulse Wave Velocity, 3.8±0.2 vs 2.5±0.1 mm/ms, p<0.01, n=11-13). Increased phosphorylation of myosin phosphatase targeting protein was preserved in both S-DN and S-DN/S-RhoBTB1 aorta, suggesting that restoration of RhoBTB1 did not affect increased RhoA/ROCK activity (p<0.01, n=6). A phosphodiesterase (PDE) 5 inhibitor, Zaprinast improved vasodilation in S-DN (p<0.01, n=8). Consistent with this, a cGMP analog that is resistant to PDE hydrolysis, 8-pCPT-cGMP, induced equivalent relaxation in S-DN and non-transgenic mice (n=4), while S-DN exhibited impaired relaxation induced by PDE-sensitive 8-Bromo-cGMP (p<0.01, n=7). PDE activity was increased in S-DN aorta and was reduced to normal levels in S-DN/S-RhoBTB1 (p<0.01, n=6). We conclude: a) loss of RhoBTB1 function explains the vascular dysfunction and hypertension observed in response to interference with PPARγ in smooth muscle, b) DN PPARγ in SMC causes vascular dysfunction via promoting PDE activity, and c) restoration of RhoBTB1 in SMC facilitates vasodilatation by normalizing PDE activity.