Background & aims: Long-term and excessive ethanol intake results in decreased plasma and hepatic levels of retinoic acid (RA), the most active derivative of vitamin A. The decrease of RA by ethanol treatment has been proposed to be a cytochrome P450 enzyme (CYP)-dependent process. However, the role of the major ethanol-induced CYP, CYP2E1, in the metabolism of RA has not been defined.
Methods: In vitro incubations of RA with microsomal fractions of liver tissue containing CYPs from either ethanol-exposed or non-ethanol-exposed rats were carried out using chemical inhibitors and antibodies against various CYPs. In vivo, both ethanol-exposed and non-ethanol-exposed rats were treated with or without chlormethiazole, a specific CYP2E1 inhibitor, for 1 month. RA and its catabolic metabolites were analyzed by high-performance liquid chromatography and spectral analysis.
Results: Incubation of RA with the liver microsomal fraction from ethanol-exposed rats resulted in greater disappearance of RA and increased appearance of 18-hydroxy-RA and 4-oxo-RA compared with control rat liver microsomal fractions. The enhancement of RA catabolism by ethanol was inhibited by both CYP2E1 antibody and specific inhibitors (allyl sulfide and chlormethiazole) in a dose-dependent fashion, whereas the metabolism of RA into polar metabolites was abolished completely by nonspecific CYP inhibitors (disulfiram and liarozole). Furthermore, treatment with chlormethiazole in ethanol-fed rats in vivo restored both hepatic and plasma RA concentrations to normal levels.
Conclusions: Ethanol-induced CYP2E1 plays a major role in the degradation of RA, which may provide a possible biochemical mechanism for chronic and excessive ethanol intake as a risk for both hepatic and extrahepatic cell proliferation and carcinogenesis.