Cyclooxygenase-2 contributes to the selective induction of cell death by the endocannabinoid 2-arachidonoyl glycerol in hepatic stellate cells

The endogenous cannabinoid 2-arachidonoyl glycerol (2-AG) is an anti-fibrotic lipid mediator that induces apoptosis in hepatic stellate cells (HSCs), but not in hepatocytes. However, the exact molecular mechanisms of this selective induction of HSC death are still unresolved. Interestingly, the inducible isoform of cyclooxygenase, COX-2, can metabolize 2-AG to pro-apoptotic prostaglandin glycerol esters (PG-GEs). We analyzed the roles of COX-2 and endocannabinoid-derived PG-GEs in the differential susceptibility of primary activated HSCs and hepatocytes toward 2-AG-induced cell death. HSCs displayed significant COX-2 expression in contrast to hepatocytes. Similar to 2-AG, treatment of HSCs with PGD₂-GE dose-dependently induced cell death independently from cannabinoid receptors that was accompanied by PARP- and caspase 3-cleavage. In contrast to 2-AG, PGD₂-GE failed to induce significant ROS formation in HSCs, and depletion of membrane cholesterol did not rescue HSCs from PGD₂-GE-induced apoptosis. These findings indicate differential engagement of initial intracellular signaling pathways by 2-AG and its COX-2-derived metabolite PGD₂-GE, but similar final cell death pathways. Other PG-GEs, such as PGE₂-or PGF_2α-GE did not induce apoptosis in HSCs. Primary rat hepatocytes were mainly resistant against 2-AG- and PGD₂-GE-induced apoptosis. HSCs, but not hepatocytes were able to metabolize 2-AG to PGD₂-GE. As a proof of principle, HSCs from COX-2^-/- mice lacked PDG₂-GE production after 2-AG treatment. Accordingly, COX-2^-/- HSCs were resistant against 2-AG-induced apoptosis. In conclusion, the divergent expression of COX-2 in HSCs and hepatocytes contributes to the different susceptibility of these cell types towards 2-AG-induced cell death due to the generation of pro-apoptotic PGD₂-GE by COX-2 in HSCs. Modulation of COX-2-driven metabolization of 2-AG may provide a novel physiological concept allowing the specific targeting of HSCs in liver fibrosis.