The purposes of this study were to determine whether expression of the gene encoding ornithine aminotransferase (OAT) in the rat liver and kidney is regulated by retinoic acid (RA) and to characterize further the role of thyroid hormone in regulating the expression of this gene. The level of OAT messenger RNA (mRNA) was reduced 70% in the liver of animals fed a vitamin A-deficient diet relative to that in animals fed a vitamin A-sufficient diet. RA, administered at a dose of 20 μgrams/rat to A-deficient rats for 1 or 3 days, restored OAT mRNA to near the level observed in animals fed the A-sufficient diet. Retinol was also effective in this regard. T3, when injected alone at a dose of 10 μgrams/100 g BW, had no effect on the level of OAT mRNA in the liver. However, when injected concurrently with RA, T3 blocked the ability of RA to induce OAT mRNA in the liver of rats fed the vitamin A-deficient diet. Animals made both vitamin A deficient and hypothyroid responded to RA in a manner similar to vitamin A-deficient animals. The vitamin A-deficient, hypothyroid rats responded somewhat differently to T3, however. T3 was unable to block the induction of OAT mRNA in the liver of vitamin A-deficient, hypothyroid rats when injected concurrently with RA for 1 day, but did block the induction of OAT mRNA by RA when these two hormones were injected concurrently for 3 days. These data indicate that RA and T3 exert opposing effects on the level of OAT mRNA in the liver. The effects of RA and T3 on OAT mRNA were markedly different in the kidney. Neither vitamin A deficiency nor RA had any apparent affect on the level of OAT mRNA in the kidney. T3, however, increased the level of OAT mRNA in the kidney of vitamin A-deficient rats. In the kidney of vitamin A-deficient, hypothyroid rats, T3 was unable to increase OAT mRNA when injected for 1 day, but did increase this mRNA when injected for 3 days. Together, these data indicate cell-type specific effects of both RA and T3 on the OAT gene.
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