Dietary Vitamin A Modulates the Properties of Retinoic Acid and Glucocorticoid Receptors in Rat Liver

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Abstract
Properties of retinoic acid receptors and glucocorticoid receptors of rat liver were influenced by retinol status in a nonsimilar manner. The binding of the retinoic acid receptors which was lowered in vitamin A-deficient animals relative to controls was restored by a single dose (100 µg) of retinoic acid; in vitamin A-overloaded animals (40-fold the control intake) the binding was greater than in controls. The binding of the glucocorticoid receptor was higher in vitamin A-deficient rats than in controls and restored by retinoic acid supplementation, but did not differ from controls in the vitamin A-overloaded rats. The cellular actions of glucocorticoid hormone and retinoic acid were investigated by assaying the activity of some related enzymes. The activity of tyrosine aminotransferase reflected glucocorticoid receptor binding in vitamin A-deficient and vitamin A-restored rats. The decreased tyrosine amino transferase activity observed in vitamin A-overloaded rats could be related to the inhibition of expression of tyrosine amino transferase gene by retinoic acid. Alcohol dehydrogenase activity was unaffected or only slightly affected by vitamin A status. The known existence of glucocorticoid hormone- and retinoic acid-sensitive elements in the alcohol dehydrogenase gene could explain such observations. Furthermore, the changes in the binding of retinoic acid receptors and glucocorticoid receptors were often in opposite directions. These results provide new evidence for the mechanisms by which the amount of dietary vitamin A modulates hormonal status.