Cannabinoid Receptor 1 Mediates Palmitic Acid-Induced Apoptosis Via Endoplasmic Reticulum Stress in Human Renal Proximal Tubular Cells

The endocannabinoid system (ECS) is activated at the onset of obesity and diverse metabolic diseases. Endocannabinoids mediate their physiological and behavioral effects by activating specific cannabinoid receptors, mainly cannabinoid receptor 1 (CB1R). Diabetic nephropathy (DN) is induced by hyperlipidemia, and renal proximal tubule cells are an important site for the onset of DN. However, the pathophysiology of CB1R, especially in the hyperlipidemia of DN, has not been elucidated. Therefore, we examined the effect of palmitic acid (PA) on CB1R expression and its related signal pathways in human renal proximal tubular cells (HK-2 cells). PA significantly increased CB1R mRNA and protein levels and induced CB1R internalization. PA-induced activation of CB1R is prevented by the treatment of AACOCF(3) (a cPLA(2) inhibitor), indomethacin and NS398 (a COX 2 inhibitors). Indeed, PA increased cPLA(2), and COX-2 but not COX-1. We also investigated whether the PA-induced activation of CB1R is linked to apoptosis. As a result, AM251 (a CB1R antagonist) attenuated PA-mediated apoptosis in a concentration-dependent manner. Furthermore, PA decreased GRP78 expression and induced increases in the endoplasmic reticulum (ER) stress signaling pathways p-PERK, p-elF2 alpha, p-ATF4, and CHOP, which were blocked by AM251 treatment. Moreover, PA increased the Bax/Bcl-2 ratio, cleaved PARP, and caspase-3 levels. The PA-induced apoptotic effects were decreased with CB1R-specific antagonist (AM251) treatment and CB1 si-RNA transfection. In conclusion, PA induced apoptosis through ER stress via CB1R expression in human proximal tubule cells. Our results provide evidence that CB1R blockade may be a potential anti-diabetic therapy for the treatment of DN. J. Cell. Physiol. 225: 654-663, 2010. (C) 2010 Wiley-Liss, Inc.