97 The similar mechanisms of diet-induced and alcohol-induced steatosis, together with ethanol’s ability to increase endocannabinoid levels, at least in the brain,98 suggest ECS involvement
in the alcoholic fatty liver. Indeed, the exposure of male mice click here to a low-fat, liquid ethanol diet for 4 weeks increased hepatic CB1 expression and 2-AG levels but not AEA levels. 2-AG was increased in HSCs but not in hepatocytes. The expression of diacylglycerol lipase β was also increased in HSCs,23 and this suggests increased biosynthesis of 2-AG. Rimonabant treatment attenuated ethanol-induced steatosis without affecting alcohol intake and blood ethanol levels, and this suggests CB1 involvement. This was further supported by the resistance of both CB1−/− and LCB1−/− ACP-196 solubility dmso mice to ethanol-induced steatosis.23 The hepatic nuclear expression of SREBP1c and its target FAS was increased, whereas CPT1 expression and activity decreased in ethanol-fed mice, in agreement with earlier findings.96 In both CB1−/− and LCB1−/−
mice, the effects of ethanol on SREBP1c, FAS, and CPT1 were blunted or absent. Furthermore, CPT1 activity was increased and resistant to suppression by ethanol in both CB1 knockout strains.23 This supports the notion that in alcoholic fatty liver disease (AFLD), hepatic lipogenesis is increased and fatty acid oxidation is decreased via CB1 activation. CB1−/− hepatocytes are resistant to ethanol-induced steatosis, whereas ethanol increases 2-AG exclusively in HSCs. This suggests a paracrine mechanism by which HSC-derived 2-AG activates CB1 receptors on adjacent hepatocytes to stimulate lipogenesis and inhibit fatty acid oxidation in the latter. Indeed, coculturing HSCs from alcohol-fed mice with hepatocytes from control mice resulted in increased lipogenic gene expression in the latter. The paracrine effect of ethanol-primed HSCs was blunted when the hepatocytes in the coculture were from LCB1−/− mice, and this confirmed the role of CB1 receptors.23 This paracrine interaction, together with high levels of retinoic acid in HSCs and its well-known role
in the control of gene expression, prompted a study of the possible role of retinoic acid and its receptors in regulating hepatic CB1 expression. CB1 expression in isolated mouse or human hepatocytes was up-regulated by retinoid Selleck Gefitinib A receptor γ (RARγ) or pan-RAR agonists, and the effect could be attenuated by small interfering RNA knockdown of RARγ but not other RAR subtypes.25 Both CB1 and RARγ were up-regulated in hepatocytes from mice fed either a high-fat diet or a liquid alcohol diet. Furthermore, 2-AG up-regulated CB1 in normal hepatocytes but not in retinaldehyde dehydrogenase 1−/− hepatocytes, which are deficient in retinoic acid. Thus, CB1 autoinduction may also involve retinoic acid.25 Interestingly, autoinduction of hepatic CB1 receptors is also suggested by the finding that chronic rimonabant treatment of DIO mice reversed the diet-induced up-regulation of hepatic CB1.