Abstract
NADPH oxidase (Nox)-derived oxyradicals contribute to atherosclerosis by oxidizing low-density lipoproteins (LDL), leading to their phagocytosis by vascular macrophages. Endocannabinoids, such as 2-arachidonoylglycerol (2-AG), might be an important link between oxidative stress and atherosclerosis. We hypothesized that 2-AG biosynthesis in macrophages is enhanced following ligation of oxidized LDL by scavenger receptors via a signal transduction pathway involving Nox-derived ROS that activates diacylglycerol lipase-β (DAGL-β), the 2-AG biosynthetic enzyme. To test this idea, we challenged macrophage cell lines and murine primary macrophages with a xanthine oxidase system or with nonphysiological and physiological Nox stimulants [phorbol 12-myristate 13-acetate (PMA) and arachidonic acid (AA)]. Each stressor increased cellular superoxide levels and enhanced 2-AG biosynthetic activity in a Nox-dependent manner. Levels of cytosolic phospholipase A2-dependent AA metabolites (eicosanoids) in primary macrophages were also dependent on Nox-mediated ROS. In addition, 2-AG levels in DAGL-β-overexpressing COS7 cells were attenuated by inhibitors of Nox and DAGL-β. Furthermore, ROS induced by menadione (a redox cycling agent) or PMA could be partially attenuated by the cannabinoid 1/2 receptor agonist (WIN 55,212-2). Finally, cells that overexpress Nox2 components (Phox-COS7) synthesized larger amounts of 2-AG compared with the parental COS7 cells. Together, the results suggest a positive correlation between heightened oxygen radical flux and 2-AG biosynthesis in macrophage cell lines and primary macrophages. Because of the antioxidant and anti-inflammatory effects associated with 2-AG, the increased levels of this bioactive lipid might be an adaptive response to oxidative stress. Thus oxyradical stress may be counteracted by the enhanced endocannabinoid tone.