RETINOIC ACID DIFFERENTIALLY MODULATES TRIIODOTHYRONINE AND RETINOIC ACID RECEPTORS IN RAT-LIVER ACCORDING TO THYROID STATUS

Triiodothyronine (T-3) receptors (TRs) and retinoic acid (RA) receptor s (RARs) exert their effects on growth, differentiation and cellular h omeostasis by acting as transcription factors. The binding characteris tics of these receptors have been studied in liver of hypothyroid and hyperthyroid rats, with or without treatment with T-3, RA or T-3 + RA together. The changes in binding induced by RA treatment depended on t he hormonal status of the rat. In hypothyroid rats the Tg binding capa city was unaltered by administration of Tg or RA alone but increased b y 48% after treatment with Tg and RA together. In these rats administr ation of RA, T-3 or T-3 + RA increased the RAR binding capacity by 45, 79 and 112%, respectively. In hyperthyroid rats the administration of RA reduced the TR and RAR binding capacities by 22 and 37%, respectiv ely. We found also that the affinity constants of TRs and RARs were re duced in hypothyroid rats after treatment with T-3 Or T-3 + RA. It is suggested that this change of the properties of receptors is related t o a ligand-dependent conformational change in these receptors.