RhoBTB1 is a Novel Gene Protecting Against Hypertension

Peroxisome proliferator‐activated receptor gamma (PPARγ) is a ligand activated transcription factor regulating metabolic and vascular function. We previously reported that mice (S‐DN) expressing dominant‐negative PPARγ in smooth muscle cells (SMC) are hypertensive, exhibit impaired vasodilation, augmented vasoconstriction and reduced expression of a novel PPARγ target gene, RhoBTB1. RhoBTB1 belongs to class of proteins which act as substrate adaptors for the Cullin‐3 E3 ubiquitin ligase. Delivery of substrates to the Cullin‐3 CRL3 complex leads to ubiquitination of the substrate protein targeting it for degradation by the proteasome. We hypothesized that loss of RhoBTB1 expression in S‐DN mice may be responsible for impaired vasodilation. In this study, we tested the hypothesis that RhoBTB1 may play a protective role by preventing hypertension in response to stressors which cause hypertension, such as Angiotensin‐II (Ang‐II). To test this, we generated double transgenic mice with tamoxifen‐inducible, Cre‐dependent overexpression of RhoBTB1 specifically in SMC (S‐RhoBTB1). S‐RhoBTB1 and non‐transgenic (NT) mice were treated with tamoxifen (Tx; 75 mg/kg, ip, 5 days) or vehicle (corn oil) and then Ang‐II was infused (490 ng/kg/min, 2 weeks). Overexpression of RhoBTB1 did not alter baseline blood pressure (BP) in the absence of Ang‐II. However, increased BP caused by Ang‐II infusion was significantly attenuated by RhoBTB1 overexpression in S‐RhoBTB1 with Tx compared to Ang‐II‐infused control mice (either NT with Tx, NT with corn oil, or S‐RhoBTB1 with corn oil) in which RhoBTB1 was not overexpressed (p<0.05, n=4–5). We also observed increased heart weight in Ang II‐infused control mice, which was reversed in S‐RhoBTB1 with Tx (n=4). Thoracic aorta, carotid and basilar artery from Ang‐II‐infused control mice showed impaired acetylcholine (ACh)‐induced endothelial‐dependent relaxation (p<0.05, n=4–5), which was reversed by overexpression of RhoBTB1 in SMC (p<0.05, n=4–5). Aorta and carotid artery from Ang‐II‐infused control mice also displayed decreased sodium nitroprusside (SNP)‐induced endothelial‐independent relaxation with a right‐shifted dose‐response (p<0.05, n=4–5), which was reversed in Tx‐treated S‐RhoBTB1 mice (p<0.05, n=5). Despite the marked improvement in vasodilation, augmented vasoconstriction to serotonin (5‐HT) was preserved in aorta and carotid artery from Ang‐II‐infused mice and was not reversed in Ang‐II‐infused S‐RhoBTB1 with Tx (n=4–5). We conclude that the novel PPARγ target gene, RhoBTB1, functions in SMC to specifically facilitate vasodilation and through this mediates a protective anti‐hypertensive effect. We are currently assessing which proteins may be substrates for Cullin‐3 through RhoBTB1. Support or Funding Information Grants from the NIH and AHA; Support from the Roy J. Carver Trust