Fatty Acid Amide Hydrolase but not Monoacyl Glycerol Lipase Is a Crucial Determinant of Endocannabinoid-induced Cell Death in Hepatic Cell Populations

Abstract

Background: We have shown that the endcannabinoids anandamide (AEA) and 2-arachidonoyl glycerol (2-AG) selectively induce cell death in activated hepatic stellate cells (HSCs), but not in hepatocytes. In contrast to HSCs, hepatocytes highly express the AEA-degrading enzyme fatty acid amide hydrolase (FAAH) that protects them from AEA-induced injury. However, the hepatic expression pattern and function of the 2-AG-degrading enzyme monoacyl glycerol lipase (MGL) has not been investigated. Methods: MGL expression was analyzed by western blot and real time PCR in primary rat hepatocytes and primary rat HSCs. MGL or FAAH activity was determined colorimetrically. Cell death was analyzed by LDH release, propidium iodide uptake and western blot for caspase 3- and PARP cleavage. MGL or FAAH overexpression was achieved by adenoviral infection. Results: In contrast to FAAH, MGL mRNA, protein and activity did not significantly differ in hepatocytes and HSCs. Hepatocytes which had been pretreated with the MGL inhibitor URB754 were not sensitized towards 2-AG-mediated death, indicating a minor role for MGL in the resistance against 2-AG. Moreover, MGL overexpression did not render HSCs resistant towards 2-AG. Conversely, FAAH overexpression prevented 2-AG-induced death in HSCs. Accordingly, 2-AG induced necrotic cell death in hepatocytes which had been pretreated with the FAAH inhibitor URB597 or depleted of GSH and 2-AG induced an increase in ROS production in hepatocytes which had been pretreated with the FAAH inhibitor URB597. Conclusion: The main 2-AG degrading enzyme MGL is not differentially expressed in hepatocytes or HSCs and does not appear to play a crucial role for the resistance of hepatocytes against 2-AG-induced cell death. In contrast, the high expression of the primary AEA-metabolizing enzyme FAAH in hepatocytes and its absence in HSCs accounts for the differential susceptibility of these hepatic cell populations towards cell death mediated by endocannabinoids.