Isolation and Characterization of Mouse Complementary DNAs Encoding α and β Thyroid Hormone Receptors from Thyrotrope Cells: The Mouse Pituitary-Specific β2 Isoform Differs at the Amino Terminus from the Corresponding Species fromRat Pituitary Tumor Cells

Abstract

Thyroid hormones (T3) and their receptors (TR) play a critical role in the function of the pituitary gland, particularly in thyrotropes, where they regulate expression of the alpha- and beta-subunits of TSH. Since the pituitary gland is composed of several cell types, we undertook a characterization of TR subtypes in a murine thyrotropic tumor (TtT-97), an excellent model in which to study thyroid hormone action in thyrotropes. We screened a thyrotrope cDNA library with rat TR alpha 1 and TR beta 1 cDNA probes and isolated cDNAs encoding the mouse TR alpha 1 and TR beta 1 isoforms as well as a partial clone corresponding to the non-T3 binding carboxy-terminal alpha 2 variant. The polymerase chain reaction was used to amplify additional cDNAs for the specific 5' domains of the mouse TR beta 1 and the pituitary-specific TR beta 2 amino-terminal variant. Using hybridization probes that discriminate between the alpha and beta isoforms and their variants, we demonstrated that thyrotropes contain TR alpha 1 and alpha 2 mRNAs as well as transcripts encoding Rev-erbA, which arise by transcription from the opposite strand of the TR alpha gene. In thyrotropes, the ratio of alpha 2 to TR alpha 1 mRNA levels more closely resembled the distribution in mouse brain than that in heart, where the mRNA levels of TR alpha 1 and alpha 2 are comparable. TR beta 1 and TR beta 2 mRNAs were detected in thyrotropes and were of similar size (approximately 6.4 kilobases). Despite the almost complete conservation between the rat and mouse TR beta 1 sequences at the protein level, the mouse and rat TR beta 2-specific N-terminal domains were less conserved, and the mouse protein was shorter by 39 amino acids at the N-terminus. Of the receptor species, only the mRNA encoding the TR beta 2 isoform, which was restricted to thyrotropes, was decreased by T3 treatment, although the mRNA for the alpha 2 variant was also reduced by T3 in thyrotropes and heart tissue. Levels of TR beta 1 mRNA were not changed in liver, but were increased in thyrotropic tumors and also somewhat in brain, an organ that is not responsive to T3 by classical criteria.