YAP and the RhoC regulator ARHGAP18, are required to mediate flow-dependent endothelial cell alignment.

BACKGROUND: Vascular endothelial cell alignment in the direction of flow is an adaptive response that protects against aortic diseases such as atherosclerosis. The RhoGTPases are known to regulate this alignment. We have shown previously that ARHGAP18 in endothelial cells is a negative regulator of RhoC and its expression is essential in flow-mediated alignment. Depletion of ARHGAP18 inhibits alignment and results in the induction of a pro-inflammatory phenotype. In embryogenesis, ARHGAP18 was identified as a downstream effector of the Yes-associated protein, YAP, which regulates cell shape and size. METHODS: We have used siRNA technology to deplete either ARHGAP18 or YAP in human endothelial cells. The in vitro studies were performed under athero-protective, laminar flow conditions. The analysis of YAP activity was also investigated, using high performance confocal imaging, in our ARHGAP18 knockout mutant mice. RESULTS: We show here that loss of ARHGAP18, although decreasing the expression of YAP results in its nuclear localisation consistent with activation. We further show that depletion of YAP itself results in its activation as defined by an in increase in its nuclear localisation and an increase in the YAP target gene, CyR61. Depletion of YAP, similar to that observed for ARHGAP18 depletion, results in loss of endothelial cell alignment under high shear stress mediated flow and also in the activation of NFkB, as determined by p65 nuclear localisation. In contrast, ARHGAP18 overexpression results in upregulation of YAP, its phosphorylation, and a decrease in the YAP target gene Cyr61, consistent with YAP inactivation. Finally, in ARHGAP18 deleted mice, in regions where there is a loss of endothelial cell alignment, a situation associated with a priming of the cells to a pro-inflammatory phenotype, YAP shows nuclear localisation. CONCLUSION: Our results show that YAP is downstream of ARHGAP18 in mature endothelial cells and that this pathway is involved in the athero-protective alignment of endothelial cells under laminar shear stress. ARHGAP18 depletion leads to a disruption of the junctions as seen by loss of VE-Cadherin localisation to these regions and a concomitant localisation of YAP to the nucleus.

ARHGAP18 Protects Against Thoracic Aortic Aneurysm Formation by Mitigating the Synthetic and Proinflammatory Smooth Muscle Cell Phenotype.

RATIONALE: Thoracic aortic aneurysm (TAA) is a potentially lethal condition, which can affect individuals of all ages. TAA may be complicated by the sudden onset of life-threatening dissection or rupture. The underlying mechanisms leading to TAA formation, particularly in the nonsyndromal idiopathic group of patients, are not well understood. Thus, identification of new genes and targets that are involved in TAA pathogenesis are required to help prevent and reverse the disease phenotype. OBJECTIVE: Here we explore the role of ARHGAP18, a novel Rho GAP expressed by smooth muscle cells (SMCs), in the pathogenesis of TAA. METHODS AND RESULTS: Using human and mouse aortic samples, we report that ARHGAP18 levels were significantly reduced in the SMC layer of aortic aneurysms. Arhgap18 global knockout (Arhgap18-/-) mice exhibited a highly synthetic, proteolytic, and proinflammatory smooth muscle phenotype under basal conditions and when challenged with angiotensin II, developed TAA with increased frequency and severity compared with littermate controls. Chromatin immunoprecipitation studies revealed this phenotype is partly associated with strong enrichment of H3K4me3 and depletion of H3K27me3 at the MMP2 and TNF-α promoters in Arhgap18-deficient SMC. We further show that TAA formation in the Arhgap18-/- mice is associated with loss of Akt activation. The abnormal SMC phenotype observed in the Arhgap18-/- mice can be partially rescued by pharmacological treatment with the mTORC1 inhibitor rapamycin, which reduces the synthetic and proinflammatory phenotype of Arhgap18-deficient SMC. CONCLUSION: We have identified ARHGAP18 as a novel protective gene against TAA formation and define an additional target for the future development of treatments to limit TAA pathogenesis.

Long-term effects of addition of mineralocorticoid receptor antagonist to angiotensin II receptor blocker in patients with diabetic nephropathy: a randomized clinical trial.

BACKGROUND: Addition of spironolactone (SPR) to angiotensin-converting enzyme (ACE) inhibitors or angiotensin II receptor blockers (ARBs) might provide antiproteinuric effects beyond what is gained by either medication alone. This study was designed to assess the long-term efficacy of SPR/ARB combination in comparison with the standard ACE/ARB regimen in diabetic nephropathy. METHODS: In an open-label, parallel-group, single-center, randomized clinical trial (NCT01667614), 136 patients with diabetes and proteinuria, already treated with enalapril and losartan, were included. In 74 patients, ACE inhibitors were discontinued. After a wash-out period of 2 weeks, 25 mg SPR daily was initiated. The remainder of the patients (n = 62) received ACE inhibitors and ARBs as before. Patients were followed every 3 months for 18 months. During each visit, systolic and diastolic blood pressure (BP), urinary albumin excretion (UAE), serum creatinine, estimated glomerular filtration rate (eGFR) and serum potassium concentrations were determined. RESULTS: After 18 months, three patients in the SPR/ARB group developed asymptomatic hyperkalemia. SPR/ARB significantly reduced both systolic and diastolic BP (P < 0.001 and 0.001, respectively). SPR/ARB decreased UAE by 46, 72 and 59% after 3, 12 and 18 months, respectively. Compared with the continuation regimen, SPR/ARB was superior in UAE reduction (P = 0.017 after 18 months), independent of BP change. In both groups, eGFR declined significantly over the trial course and the decline rate did not differ significantly between the two groups. CONCLUSIONS: Addition of SPR to ARB provides added benefits with respect to BP control and proteinuria diminution. These antiproteinuric effects are not accompanied by prevention of eGFR loss compared with conventional therapy with ACE/ARB.