Analysis of the functional state of T 3 nuclear receptors expressed in thyroid cells

Abstract

T 3 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 α 1, α 2 and β were detected in pig thyrocytes by RT-PCR and Northern blot; the α 2 mRNA was more abundant than the α 1 mRNA. Thyrocytes were transiently transfected with different plasmids containing the CAT (chloramphenicol acetyl transferase) gene placed under the control of different promoters (ΔMTV, TK or ΔSV 40) and bearing a thyroid hormone response element, TREp or TRE DR + 4. It was found that TSH induced a concentration-dependant increase of the transfection efficiency, an effect reproduced by (Bu) 2cAMP and Forskolin. Cells transfected with either ΔMTV-, TK- or ΔSV 40-TREp-CAT expressed similar basal CAT activities. Addition of T 3 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 T 3. 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 T 3-dependent manner. This finding directly substantiates the concept of autocrine regulatory actions of thyroid hormones.