Analysis of the functional state of T3 nuclear receptors expressed in thyroid cells

T3 nuclear receptors (TR) are present in thyroid cells. We have analyzed the ability of thyroid TR to function as transcriptional regulators. Studies were performed on pig thyrocytes in primary culture. Messenger RNA corresponding to TR alpha 1, alpha 2 and beta were detected in pig thyrocytes by RT-PCR and Northern blot; the alpha 2 mRNA was more abundant than the alpha 1 mRNA. Thyrocytes were transiently transfected with different plasmids containing the CAT (chloramphenicol acetyl transferase) gene placed under the control of different promoters (delta MTV, TK or delta SV40) and bearing a thyroid hormone response element, TREp or TRE DR + 4. It was found that TSH induced a concentration-dependent increase of the transfection efficiency, an effect reproduced by (Bu)2cAMP and Forskolin. Cells transfected with either delta MTV-, TK- or delta SV40-TREp-CAT expressed similar basal CAT activities. Addition of T3 produced a 3-fold increase of CAT activity expressed from each of these vectors. In contrast, CAT activity expressed from a vector containing the TRE DR + 4 was decreased by about 50% by T3. Thus, TREp and TRE DR + 4 gave distinct responses. These data demonstrate that TR physiologically expressed in thyroid cells can act as transcriptional regulators in a T3-dependent manner. This finding directly substantiates the concept of autocrine regulatory actions of thyroid hormones.