THYROTROPIN EXPRESSION IN HYPOPHYSEAL PARS TUBERALIS-SPECIFIC CELLS IS 3,5,3'-TRIIODOTHYRONINE, THYROTROPIN-RELEASING-HORMONE, AND PIT-1 INDEPENDENT

The expression of TSH subunit genes (TSH alpha and -beta) in pituitary thyrotropes is primarily regulated via circulating thyroid hormone le vels (T-3) and the hypothalamic TRH. Hypophyseal pars tuberalis (PT)-s pecific cells also express both hormonal subunits of TSH, but do not r esemble thyrotropes of the pars distalis (PD) with respect to their di stinct morphology, secretion, and direct modulation of TSH expression by photoperiodic inputs and melatonin. To investigate whether this dis tinct regulation of TSH is related to a different molecular structure or different signaling cascades, we analyzed PT-specific TSH and its t ranscriptional regulation in ovine PT-specific cells. After constructi on of PT- and PD-specific complementary DNA (cDNA) libraries, the clon ing and sequencing of several TSH alpha and -beta subunit clones revea led identical sizes and sequences for the translated and untranslated regions in both hypophyseal compartments. Transcription start site ana lysis also displayed three identical start sites for the transcription of TSH beta in PT and PD. After cloning of the ovine TRH receptor cDN A and a partial T-3 receptor cDNA, in situ hybridization, Northern blo t analysis, and PCR experiments showed that TRH and T-3 receptors are not expressed in specific cells of the PT. The transcription factor Pi t-1 that is involved in TSH expression of thyrotropes could only be de tected in the PD. In additional experiments rats were treated with T-4 or TRH, and subsequent in situ hybridization studies showed that TSH beta messenger RNA (mRNA) formation was not altered in the PT. In the PD, however, TSH beta mRNA was significantly reduced in the T-4-treate d group, but was enhanced in the TRH-treated group. We conclude that P T-specific cells of the pituitary are characterized by the transcripti on of TSH subunits that are identical to TSH expressed in thyrotropes of the PD. The absence of TRH, T-3 receptor mRNA, and Pit-1, respectiv ely, as well as the different reactions compared to PD thyrotropes in in vivo experiments lead to the conclusion that the expression of TSH in PT-specific cells of the pituitary is not regulated via the classic al thyrotrope receptors and their intracellular pathways, but through a novel, photoperiod-dependent mechanism.