Reversal of Arterial Stiffness by Rho‐related BTB Domain‐containing Protein 1 (RhoBTB1)

Abstract

Rho-related BTB domain-containing protein (RhoBTB1) is a novel transcriptional target of peroxisome proliferator activated receptor Gamma (PPARg). The physiological and molecular function of RhoBTB1 is poorly understood. Human individuals carrying PPARg dominant negative mutation P467L develop early onset hypertension. Mice expressing smooth muscle-specific P467L also develop hypertension, vascular dysfunction, and arterial stiffness. Interestingly, P467L-mediated hypertension and its sequelae can be reversed by restoration of RhoBTB1 in smooth muscle, suggesting that RhoBTB1 mediates the anti-hypertension effect of PPARg in vascular smooth muscle cells. We recently found that RhoBTB1 preserves the nitric oxide-cGMP pathway by serving as a substrate adaptor for Cullin-3 and promoting phosphodiesterase 5 ubiquitination. Our current study explores the role of RhoBTB1 in angiotensin II (Ang-II) induced hypertension, a model that represents a broader hypertension population. We hypothesize that RhoBTB1 transgene expression in vascular smooth muscle will attenuate Ang-II-mediated hypertension and its sequelae. Mice expressing smooth muscle-specific, Tamoxifen inducible RhoBTB1 transgene (S-RhoBTB1) were used as experimental animals, whereas mice expressing smooth muscle-specific, Tamoxifen inducible Cre-recombinase (ISM-Cre) were used as control. Mice received 6-weeks of Ang-II infusion via osmotic mini pump at 490ng/kg/min. Transgene was induced by five daily Tamoxifen injections two weeks after Ang-II pumps implantation, allowing hypertension to be established before transgene activation. Blood pressure was monitored by radiotelemetry. Systolic, diastolic, and mean arterial blood pressure was significantly increased in response to Ang-II. RhoBTB1 transgene expression was not sufficient to reverse the high blood pressure. We also tested vascular reactivity in conduit and resistant arteries using wire myography. Ang-II infusion significantly impaired the vasodilation response to acetylcholine in carotid and mesenteric arteries, suggesting a defect in endothelium-mediated vasodilation. Like blood pressure, RhoBTB1 transgene expression was not sufficient to reverse the vasodilation impairment. Finally, we measured arterial stiffness using pulse-wave velocity (PWV). Both ISM-Cre and S-RhoBTB1 mice showed increased arterial stiffness two weeks after Ang-II infusion, indicated by a higher PWV. PWV in S-RhoBTB1 mice started to decline one-week post Tamoxifen and became significantly lowered (nearly reversed) two weeks after Tamoxifen. In comparison, PWV in ISM-Cre mice remained high till the end of the 6-weeks protocol. To explore the underlying mechanism, we evaluated the aortic compliance using pressure myograph in the absence of vascular tone and reactivity. Both pressure-diameter and stress-strain relationship indicate that RhoBTB1 transgene expression in smooth muscle improved the compliance of aorta in the presence of Ang-II, suggesting RhoBTB1 might attenuate arterial stiffness by acting on the extracellular matrix. Our study revealed a novel protective effect of RhoBTB1: attenuating established Ang-II-induced arterial stiffness.