There is abundant evidence that the endothelium plays a crucial role in the maintenance of vascular tone and structure. One of the major endothelium-derived vasoactive mediators is nitric oxide (NO), formed in healthy vascular endothelium from the amino acid precursor L-arginine. Endothelial dysfunction is increased by various cardiovascular risk factors, metabolic diseases, and systemic or local inflammation. One mechanism that has been implicated in the development of endothelial dysfunction is the presence of elevated levels of asymmetric dimethylarginine (ADMA). Free ADMA, which is formed during proteolysis, is actively degraded by the intracellular enzyme dimethylarginine dimethylaminohydrolase (DDAH) which catalyzes the conversion of ADMA to citrulline and dimethylamine. It has been estimated that more than 70% of ADMA is metabolized by DDAH 1 . Decreased DDAH expression/activity is evident in disease states associated with endothelial dysfunction and is believed to be the mechanism responsible for increased methylarginines and subsequent ADMA mediated eNOS impairment. However, recent studies suggest that DDAH may regulate eNOS activity and endothelial function through both ADMAdependent and independent mechanisms. In this regard, elevated plasma ADMA may serve as a marker of impaired methylarginine metabolism and the pathology previously attributed to elevated ADMA may be manifested, at least in part, through altered activity of the enzymes involved in ADMA regulation, specifically DDAH and PRMT.

Karuppiah K, Druhan LJ, Chen CA, Smith T, Zweier JL, Sessa WC, Cardounel AJ. Suppression of eNOS-derived superoxide by caveolin-1: a biopterin-dependent mechanism. Am J Physiol Heart Circ Physiol 301: H903-H911, 2011. First published July 1, 2011; doi:10.1152/ajpheart.00936.2010.-In the vasculature, nitric oxide (NO) is generated by endothelial NO synthase (eNOS) in a calcium/ calmodulin-dependent reaction. In the absence of the requisite eNOS cofactor tetrahydrobiopterin (BH4), NADPH oxidation is uncoupled from NO generation, leading to the production of superoxide. Although this phenomenon is apparent with purified enzyme, cellular studies suggest that formation of the BH4 oxidation product, dihydrobiopterin, is the molecular trigger for eNOS uncoupling rather than BH4 depletion alone. In the current study, we investigated the effects of both BH4 depletion and oxidation on eNOS-derived superoxide production in endothelial cells in an attempt to elucidate the molecular mechanisms regulating eNOS oxidase activity. Results demonstrated that pharmacological depletion of endothelial BH4 does not result in eNOS oxidase activity, whereas BH4 oxidation gave rise to significant eNOS-oxidase activity. These findings suggest that the endothelium possesses regulatory mechanisms, which prevent eNOS oxidase activity from pterin-free eNOS. Using a combination of gene silencing and pharmacological approaches, we demonstrate that eNOS-caveolin-1 association is increased under conditions of reduced pterin bioavailability and that this sequestration serves to suppress eNOS uncoupling. Using small interfering RNA approaches, we demonstrate that caveolin-1 gene silencing increases eNOS oxidase activity to 85% of that observed under conditions of BH 4 oxidation. Moreover, when caveolin-1 silencing was combined with a pharmacological inhibitor of AKT, BH 4 depletion increased eNOS-derived superoxide to 165% of that observed with BH 4 oxidation. This study identifies a critical role of caveolin-1 in the regulation of eNOS uncoupling and provides new insight into the mechanisms through which disease-associated changes in caveolin-1 expression may contribute to endothelial dysfunction.endothelial nitric oxide synthase; endothelium; oxidative stress; tetrahydrobiopterin ENDOTHELIUM-DERIVED NITRIC oxide (NO) is a potent vasodilator that plays a critical role in maintaining vascular homeostasis through its antiatherogenic and antiproliferative effects on the vascular wall. Reduced NO bioavailability is a key determinant of endothelial dysfunction and has been demonstrated to play a central role in the increased cardiovascular risks associated with metabolic syndrome (3, 26). Endothelial dysfunction has been observed in pre-diabetic stages of insulin resistance and subsequently contributes to smooth muscle cell proliferation and platelet and leukocyte adhesion as well as atherogenesis (4,10,22). Impaired endothelial function is apparent in experimental diabetes and in diabetic patients despite the fact that endothelial NO synthase (eNOS) e...

No abstract

No abstract