Restoration of retinoic acid concentration supresses ethanol-enhanced c-Jun expression and hepatocyte proliferation in rat liver

Chronic and excessive ethanol intake decreases hepatic retinoic acid (RA) c oncentrations, which may play a critical role in ethanol-induced hyperproli feration in hepatocytes. The present study was conducted to determine wheth er RA supplementation in chronic ethanol-fed rats could restore hepatic RA concentrations to normal levels and modulate hepatocyte hyperproliferation. Male Sprague-Dawley rats were divided into four groups: control, ethanol-f ed, ethanol-fed + 50 mug all-trans-RA/kg body wt and ethanol-fed + 100 mug all-trans-RA/kg body wt. Ethanol was given to rats at 6.2% (v/v) in a liqui d diet to provide 36% of total caloric intake. Control animals received the same amount of liquid diet with isocaloric maltodextrin in place of ethano l. Results show that the ethanol treatment in rats for a month significantl y increased the mean number of proliferating cell nuclear antigen (PCNA)-po sitive hepatocytes [4.96 +/- 1.36% (ethanol-fed) versus 0.29 +/- 0.08% (con trol), P < 0.05]. This increase was associated with the induction of hepati c c-Jun protein (6.5-fold increase) and cyclin DI protein (3-fold increase) in ethanol-fed animals as compared with controls. Furthermore, activator p rotein 1 (AP-1) DNA-binding activity was significantly higher in hepatic nu clear extracts from ethanol-fed rats than those from controls. In contrast, RA supplementation in ethanol-fed rats raised hepatic RA concentration to normal levels and almost completely abolished the ethanol-enhanced c-Jun, c yclin D and AP-1 DNA-binding activities. Moreover, RA supplementation at bo th doses markedly suppressed the ethanol-induced PCNA-positive hepatocytes by similar to 80%. These results demonstrate that the restoration of hepati c RA concentrations by dietary RA supplementation suppresses ethanol-induce d hepatocyte proliferation via inhibiting c-Jun overexpression, and suggest that RA may play a role in preventing or reversing certain types of ethano l-induced liver injury.