Chronic alcohol intake reduces retinoic acid concentration and enhances AP-1 (c-Jun and c-Fos) expression in rat liver

Chronic ethanol intake may interfere with retinoid signal transduction by inhibiting retinoic acid synthesis and by enhancing activator protein-1 (AP-1) (c-Jun and c-Fos) expression, thereby contributing to malignant transformation. To determine the effect of ethanol on hepatic retinoid levels, retinoic acid receptors (RARs) and AP-1 (c-Jun and c-Fos) gene expression, chronic ethanol (36% of total calorie intake) pair-feeding was conducted on rats for a 1-month period. Retinoic acid, retinol, and retinyl ester concentrations in both liver and plasma were examined by using high-performance liquid chromatography (HPLC). Both retinoic acid receptor (alpha, beta, gamma) and AP-1 (c-Jun and c-Fos) expression in the rat liver were examined by using Western blot analysis. Treatment with high-dose ethanol led to a significant reduction of retinoic acid concentration in both the liver and the plasma (11- and 8.5-fold reduction, respectively), as compared with animals pair-fed an isocaloric control diet containing the same amount of vitamin A. Similar to the retinoic acid reductions, both retinol and retinyl palmitate levels in the livers of the alcohol-fed group decreased significantly, but in smaller fold reduction (6.5- and 2.6-fold reduction, respectively). Ethanol did not modulate the expression of RAR alpha, -beta, and -gamma genes in the liver. However, chronic alcohol feeding enhanced AP-1 (c-Jun and c-Fos) expression by 7- to 8-fold, as compared with the control group. These data suggest that functional downregulation of RARs by inhibiting biosynthesis of retinoic acid and up-regulation of AP-1 gene expression may be important mechanisms for causing malignant transformation by ethanol.