Aging decreases retinoic acid and triiodothyronine nuclear expression in rat liver: exogenous retinol and retinoic acid differentially modulate this decreased expression.

The expression of nuclear receptors of retinoic acid (RAR) and triiodothyronine (TR) was analyzed in the liver of rats aged 2.5 (young), 6 (adult) and 24 (aged) months. In aged rats, decreased binding properties, binding capacity (Cmax) and affinity (Ka), of nuclear receptors were observed. This resulted, at least in part, from decreased transcription of receptor genes in that the amount of their mRNA also decreased. Moreover, the activity of malic enzyme (ME) and tissue transglutaminase (tTG), whose genes are TR and RAR responsive, respectively, was reduced in aged rats. These results are in agreement with the decreased binding capacity of these receptors. An inducer-related increase of RAR and TR expression was observed 24 h after a single dose of retinoic acid administration (5 mg/kg), while retinol administration (retinyl palmitate, 13 mg/kg) was without incidence on nuclear receptor expression in aged rats.