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) receptors (RARs) exert their effects on growth, differentiation and cellular homeostasis by acting as transcription factors. The binding characteristics 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 the hormonal status of the rat. In hypothyroid rats the Tg binding capacity was unaltered by administration of Tg or RA alone but increased by 48% after treatment with Tg and RA together. In these rats administration 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%, respectively. We found also that the affinity constants of TRs and RARs were reduced 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 to a ligand-dependent conformational change in these receptors.