Oxidized low-density lipoprotein (OxLDL) impairs NO signaling and endothelial function, and contributes to the pathogenesis of atherosclerosis. Arginase reciprocally regulates NO levels in endothelial cells by competing with NO synthase for the substrate l-arginine. In human aortic endothelial cells, OxLDL stimulation increased arginase enzyme activity in a time- and dose-dependent manner. Arginase activity reached its maximum as early as 5 minutes, was maintained for a period of more than 48 hours, and was associated with a reciprocal decrease in NO metabolite (NOx [nitrite and nitrate]) production. Furthermore, OxLDL induced arginase II mRNA expression after 4 hours. Small interfering RNA targeted to arginase II decreased both the quantity and the activity of arginase from baseline, prevented OxLDL-dependent increases in arginase activity, and induced an increase in NOx production. Immunofluorescence analysis revealed an association of arginase II with the microtubule cytoskeleton. Microtubule disruption with nocodazole caused a dramatic redistribution of arginase II to a diffuse cytosolic pattern, increased arginase activity, and decreased NOx production, which was restored in the presence of the specific arginase inhibitor (S)-(2-boronoethyl)-l-cysteine (BEC). On the other hand, epothilone B prevented microtubule disruption and inhibited OxLDL-dependent increases in arginase activity and attenuated OxLDL-dependent decreases in NOx. Preincubation of rat aortic rings with OxLDL resulted in an increase in arginase activity and a decrease in NOx production. This was reversed by arginase inhibition with the BEC. Thus, OxLDLs increase arginase activity by a sequence of regulatory events that involve early activation through decreased association with microtubules and a later increase in transcription. Furthermore, increased arginase activity contributes to OxLDL-dependent impairment of NOx production. Arginase, therefore, represents a novel target for therapy in atherosclerosis. © 2006 American Heart Association, Inc.