Thyrotropin Expression in Hypophyseal Pars Tuberalis-Specific Cells is 3,5,3'-Triiodothyronine, Thyrotropin-Releasing Hormone, and Pit-1 Independent

Abstract

The expression of TSH subunit genes (TSHα and -β) in pituitary thyrotropes is primarily regulated via circulating thyroid hormone levels (T3) and the hypothalamic TRH. Hypophyseal pars tuberalis (PT)-specific cells also express both hormonal subunits of TSH, but do not resemble thyrotropes of the pars distalis (PD) with respect to their distinct morphology, secretion, and direct modulation of TSH expression by photoperiodic inputs and melatonin. To investigate whether this distinct regulation of TSH is related to a different molecular structure or different signaling cascades, we analyzed PT-specific TSH and its transcriptional regulation in ovine PT-specific cells. After construction of PT- and PD-specific complementary DNA (cDNA) libraries, the cloning and sequencing of several TSHα and -β subunit clones revealed identical sizes and sequences for the translated and untranslated regions in both hypophyseal compartments. Transcription start site analysis also displayed three identical start sites for the transcription of TSHβ in PT and PD. After cloning of the ovine TRH receptor cDNA and a partial T3 receptor cDNA, in situ hybridization, Northern blot analysis, and PCR experiments showed that TRH and T3 receptors are not expressed in specific cells of the PT. The transcription factor Pit-1 that is involved in TSH expression of thyrotropes could only be detected in the PD.