Elevated endocannabinoid levels in liver fibrosis are driven by diminished hepatic expression and activity of endocannabinoid degrading enzymes

Aims: We and others have previously shown, that the hepatic endocannabinoid (EC) system becomes activated during fibrogenensis. In particular, the expression of cannabinoid receptors and levels of ECs (anandamide, AEA; 2-arachydonoyl glycerol, 2-AG) are upregulated. However, the mechanisms that lead to the increased levels of ECs during liver fibrogenensis are unknown. AIM: We sought to analyse the hepatic expression and activity of the major AEA-degrading enzyme fatty acid amide hydrolase (FAAH) and the 2-AG-degrading enzyme monoacyl glycerol lipase (MGL) in normal and fibrotic mouse livers at different timepoints after bile duct ligation (BDL) in correlation to intrahepatic EC levels. METHODS: AEA and 2-AG were measured by liquid chromatography/mass spectrometry in liver tissue from male BALB/C mice 2, 5, 7 and 14 days after BDL or sham operation (n=4 each). Alpha-SMA expression was analysed by Western blot. FAAH and MGL mRNA or protein expression were determined by Real Time PCR and Western blot. Enzymatic FAAH and MGL activities in liver tissue were measured colorimetrically. RESULTS: As a sign of hepatic fibrogenesis, alpha-SMA expression significantly increased 5, 7 and 14 days after BDL. Hepatic AEA and 2-AG levels were increasingly elevated up to 2 to 2.5-fold 14 days (p<0.05) after BDL. The mRNA and protein expression of FAAH and MGL decreased significantly during the BDL timecourse. Enzymatic EC degradation activity of FAAH and MGL also decreased significantly during fibrogenesis. Linear regression analysis revealed a significant correlation between the increase in EC levels and the decreased expression and activity of FAAH or MGL, respectively. CONCLUSION: Elevated EC levels in fibrotic liver are due to decreased expression and activity of EC-degrading enzymes FAAH and MGL. Modification of expression or activity of EC-degrading enzymes may represent a tool to modify the regulation of the endocannabinoid tone in the injured or fibrotic liver.

Liver fibrosis - endocannabinoids - fatty acid amide hydrolase - monoacyl glycerol lipase