Role of cannabinoid receptors in alcoholic hepatic injury: steatosis and fibrogenesis are increased in CB2 receptor-deficient mice and decreased in CB1 receptor knockouts

Background Alcohol is a common cause of hepatic liver injury with steatosis and fibrosis. Cannabinoid receptors (CB) modulate steatosis, inflammation and fibrogenesis. To investigate the differences between CB1 and CB2 in the hepatic response to chronic alcohol intake, we examined CB knockout mice (CB1-/-, CB2-/-). Methods Eight- to 10-week-old CB1-/-, CB2-/- and wild-type mice received 16% ethanol for 35 weeks. Animals receiving water served as controls. We analysed triglyceride and hydroxyproline contents in liver homogenates. mRNA levels of CBs, pro-inflammatory cytokines [tumour necrosis factor (TNF)-alpha, monocyte chemotactic protein (MCP)-1, interleukin (IL)-1 beta] and profibrotic factors [alpha-smooth muscle actin (alpha-SMA), procollagen-Ia, platelet-derived growth factor beta receptor (PDGF beta-R)] were analysed by reverse transcription-polymerase chain reaction (RT-PCR). Histology (hemalaun and eosin, oil-red O, CD3, CD45R, CD45, F4/80, Sirius red) characterized hepatic steatosis, inflammation and fibrosis. Activation of lipogenic pathways, activation and proliferation of hepatic stellate cell (HSC) were assessed by western blot [fatty acid synthase (FAS), sterol regulatory element binding protein 1c (SREBP-1c), alpha-SMA, proliferating cell nuclear antigen (PCNA), cathepsin D]. Results Hepatic mRNA levels of the respective CBs were increased in wild-type animals and in CB1-/- mice after ethanol intake. Ethanol intake in CB2-/- mice induced much higher steatosis (SREBP-1c mediated) and inflammation (B-cell predominant infiltrates) compared with wild-type animals and CB1-/- mice. HSC activation and collagen production were increased in all groups after forced ethanol intake, being most pronounced in CB2-/- mice and least pronounced in CB1-/- mice. Discussion The fact that CB2 receptor knockout mice exhibited the most pronounced liver damage after ethanol challenge indicates a protective role of CB2 receptor expression in chronic ethanol intake. By contrast, in CB1 knockouts, the effect of ethanol was attenuated, suggesting aggravation of fibrogenesis and SREBP-1c-mediated steatosis via CB1 receptor expression after ethanol intake.