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We have previously reported an unexpected relationship between retinoic acid-induced inhibition of cell growth and the ability of various cell lines to metabolize the retinoid. Here, we report that stable expression of the truncated retinoic acid receptor RAR alpha403, transduced in NIH-3T3 cells by a retroviral vector, rendered the cells resistant to retinoic acid for growth inhibition and reduced their ability to metabolize the retinoid at the same time as it blunted the induction of the target gene transglutaminase II. The data suggest that retinoic acid receptors mediate the growth-inhibitory action of retinoic acid as well as its metabolism and the induction of transglutaminase II.

Livers from aryl hydrocarbon receptor-null mice showed a 3-fold increase in retinoids and a 65% decrease in retinoic acid metabolism. Levels of expression of the retinoic acid 4-hydroxylase, P450RAI, did not change, whereas cytochrome P4501A2 levels were lower in the null mouse, as shown earlier; however, this enzyme was found not to be active toward retinoic acid. These data suggest that aryl hydrocarbon receptor controls retinoic acid catabolism, through modulation of an unidentified target gene. Aldehyde dehydrogenases 1 and 2 were down-regulated markedly in the aryl hydrocarbon receptor-deficient mouse liver. 2,3,7,8-Tetrachlorodibenzo-p-dioxin induced cytochrome P4501A2 but not the aldehyde dehydrogenases in wild-type mice, suggesting that aryl hydrocarbon receptor is not involved directly in the down-regulation of this gene. Transglutaminase II, a retinoic acid-responsive gene product, was increased 2-fold, consistent with the liver fibrosis phenotype observed in the null mice. These findings suggest a molecular connection between xenobiotic-activated receptor signaling and retinoid homeostasis.