Hypoxia promotes human pulmonary artery smooth muscle cell proliferation through induction of arginase

Abstract

Alterations in the metabolism of L-arginine via arginase and nitric oxide synthase play a critical role in the endothelial dysfunction seen in PAH. L-arginine metabolism by arginase produces L-ornithine and urea. L-ornithine is a precursor for polyamine and proline synthesis, ultimately leading to an increase in cellular proliferation. Given the integral role of the smooth muscle layer in the pathogenesis of hypoxia-induced PAH, we hypothesized that hypoxia would increase cellular proliferation via arginase induction in human pulmonary artery smooth muscle cells (hPASMC). We found that arginase II mRNA and protein expression were significantly increased in cultured hPASMC exposed to 1% O 2 for 24 and 48 h, which coincided with an increase in arginase activity at 48 h. There were no hypoxia-induced changes in levels of arginase I mRNA or protein in cultured hPASMC. Exposure to hypoxia resulted in more than one and a half times as many viable cells after 120 h than normoxic exposure. The addition of the arginase inhibitor, S-(2-boronoethyl)-L-cysteine, completely prevented both the hypoxia-induced increase in arginase activity and proliferation in hPASMC. Furthermore, transfection of small interfering RNA (siRNA) targeting arginase II in hPASMC resulted in knockdown of arginase II protein levels and complete prevention of the hypoxia-induced cellular proliferation. These data support our hypothesis that hypoxia increases proliferation of hPASMC through the induction of arginase II. pulmonary hypertension; L-arginine; vascular remodeling PULMONARY ARTERY HYPERTENSION (PAH) is a life-threatening complication of chronic hypoxic lung diseases characterized by vasoconstriction, thrombosis, and the pathogenic hallmark of vascular remodeling that involves all layers of the vessel wall (10, 15). The smooth muscle layer plays an integral role in the pathogenesis of PAH with the extension of smooth muscle into smaller, normally nonmuscular pulmonary arteries within the respiratory acinus, a feature common to all forms of PAH remodeling (10, 25). In addition, pulmonary artery smooth muscle cells (PASMC) markedly proliferate, resulting in decreased luminal diameters and ultimately the obstruction of resistance level pulmonary arteries (10, 32).The L-arginine metabolic pathway has been shown to be important in maintaining vascular tone. L-arginine is the substrate for nitric oxide synthase (NOS) that generates the signaling molecule nitric oxide (NO) with the coproduct L-citrulline (31). Endogenous NO maintains vascular integrity by maintaining vasodilator tone and modulating vascular smooth muscle cell proliferation (12,18,23). Additionally, L-arginine is the substrate for arginase, of which there are two described isoforms (16,24). Arginase I is a cytosolic enzyme that is highly expressed in the liver. Conversely, arginase II is a mitochondrial protein that is not expressed in the liver (8). Both arginase isoforms are expressed in the lung (27). The metabolism of L-arginine by arginase results in the production of L-ornithin...

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Silencing of Sodium–Hydrogen Exchanger 1 Attenuates the Proliferation, Hypertrophy, and Migration of Pulmonary Artery Smooth Muscle Cells via E2F1

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