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

✍ Scribed by Xiang-Dong Wang; Chun Liu; Jayong Chung; Felix Stickel; Helmut K. Seitz; Robert M. Russell

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 192 KB
• Volume: 28
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.510280321

✦ Synopsis

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 (␣, ␤, ␥) 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 (6.5and 2.6-fold reduction, respectively). Ethanol did not modulate the expression of RAR␣, -␤, and -␥ 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. (HEPATOLOGY 1998;28:744-750.) Chronic ethanol consumption is associated with an increased incidence of a variety of illnesses, including cirrhosis and cancer. 1 Impaired nutritional status of vitamin A has been reported in alcoholics, i.e., reduced levels of vitamin A have been found in serum of alcoholics with and without liver disease and in liver biopsies from alcoholics. 2 These changes may be caused by the increased metabolism and mobilization of vitamin A from the liver, 3-5 the competition of alcohol dehydrogenase (ADH)-catalyzed retinol oxidation, 6 or the inhibition of retinoic acid biosynthesis. 7 Therefore, tissue retinoic acid deficiency may be one molecular mechanism for alcohol-induced liver injury, e.g., proliferative activation of cells or hepatic fibrosis.

Retinoic acid exerts profound effects on embryonic development, cellular growth, and differentiation. 8,9 Two families of nuclear receptors (RAR␣, -␤, -␥ and RXR␣, -␤, -␥) have been cloned and have been shown to be active in receptormediated regulation of gene transcription. Retinoic acid is currently used in the treatment of several types of cancers, including acute promyelocytic leukemia, 10 squamous cell carcinoma of the head and neck, 11 and skin cancer. 12 The chemoprotective effects of retinoic acid are thought to be mediated through proliferative control. 9 One of the proposed mechanisms for an antiproliferative effect of RARs is through a protein-protein interaction, such as activator protein-1 (AP-1 [c-Jun/c-Fos]), in which AP-1 activity and AP-1induced gene transcription can be inhibited by RAR and RXR in a ligand-dependent manner. [13][14][15][16] AP-1 mediates the signal from growth factors, inflammatory peptides, oncogenes, and tumor promoters, usually resulting in cell proliferation. 17 A recent study has been reported that diminished retinoic acid signaling in hepatic stellate cells in cholestatic liver fibrosis appears to be the result of retinoic acid deficiency and suppressed expression of RARs and RXR␣. 18 Furthermore, ethanol or its oxidative metabolite, acetaldehyde, may induce AP-1 transcription through a protein kinase C (PKC) pathway in vitro. 19 Because the stimulation of the PKC second messenger pathway can modulate tumor differentiation and transcriptional activation of a retinoid receptor associated with retinoic acid response, 20 and retinoic acid inhibits both the PKC stimulator, 12-O-tetradecanoylphorbol-13-acetateinduced mouse skin tumor formation, 21 and squamous differentiation of human tracheobronchial epithelial cells, 22 there may be a correlation between diminished retinoid signaling and increased AP-1 activity after ethanol exposure. However, the effects of chronic alcohol feeding on both retinoic acid concentration and RARs and AP-1 expression in in vivo animals has not previously been explored.

In the present study, we have examined the effect of chronic ethanol feeding (36% of total calorie intake) on retinoid concentrations, retinoic acid receptor expression, and AP-1 (c-Jun and c-Fos