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Angiotensin II Signaling Mechanisms Involved in the Elevation of Arginase Activity/Expression and Vascular DysfunctionVascular endothelial dysfunction is a major cause of morbidity and mortality in patients with cardiovascular diseases such as hypertension, atherosclerosis and diabetes. Nitric oxide (NO) produced by endothelial nitric oxide synthase (NOS) is needed for normal vascular function. During hypertension, diabetes or atherosclerosis, elevated levels of arginase can compete with NOS for available L-arginine thus reducing vascular NO production. Elevated angiotensin II (Ang II) is a key participant of endothelial dysfunction in many cardiovascular diseases and has been linked to elevated arginase activity. In this study we explored the signaling pathway leading to increased arginase expression/activity in responses to Ang II in bovine aortic endothelial cells (BAEC). Treatment of BAEC with Ang II (10-7 M, 24 hrs) caused a 40±6% increase in arginase activity. This was accompanied by 30±8% decrease in NO production. Our studies indicate involvement of the RhoA/ROCK-p38 mitogen activated protein kinase (MAPK) in Ang II-induced arginase upregulation and reduced NO production, as inhibitors of ROCK or p38 MAPK prevented the Ang II-induced increase in arginase activity. Our studies in mice also show involvement of p38 MAPK in Ang II-induced vascular dysfunction associated with elevated arginase activity and expression. Ang II (42 μg/kg/h) caused impaired EC-dependent vasorelaxation in mouse aorta (55±7% vs. 75±8% for control). This impairment was prevented by treatment with p38 inhibitor SB203580 (5 μg/kg/day). Ang II also caused a 6.2 fold increase in vascular arginase activity/expression that was completely prevented by p38 MAPK inhibition. Additionally, treatment of BAEC with Ang II causes phosphorylation of activating transcription factor-2 (ATF-2) and enhancement of the binding of ATF-2 to arginase promotor through an AP-1 site as evident from electrophoretic mobility shift assay experiments. Transfection of BAEC with ATF-2 siRNA prevents Ang II-induced increases in arginase activity/expression and maintains NO production. These results indicate that ATF-2 is necessary for enhanced expression of arginase by Ang II. Collectively, our results indicate that Ang II increases endothelial arginase activity/expression through a RhoA/ROCK-p38 MAPK-ATF-2 pathway leading to reduced NO production and endothelial dysfunction. Targeting these signaling steps might be therapeutic points for preventing vascular endothelial dysfunction associated with elevated arginase activity/expression.