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Abstract:The thyrotropin-releasing hormone (TRH) receptor (TRHR) is widely distributed throughout the central and peripheral nervous systems. In addition to its role in controlling the synthesis and secretion of thyroid-stimulating hormone and prolactin from the anterior pituitary, TRH is believed to act as a neurotransmitter as well as a neuromodulator. We have isolated genomic and P1-derived artificial chromosome clones encoding the human TRHR. The gene was found to be 35 kb with three exons and two introns. A 541-bp intron 1 (Ϫ629 to Ϫ89 relative to the translation start site) is conserved between human and mouse. A large intron 2 of 31 kb disrupts the open reading frame (starting in position ϩ790) in the sequence encoding the supposed junction between the third intracellular loop and the putative sixth transmembrane domain. A similar intron was found in chimpanzee and sheep but not in rat and mouse. Promoter analysis of upstream regions demonstrated cell type-specific reporter activation, and sequencing of 2.5 kb of the promoter revealed putative cis-acting regulatory elements for several transcription factors that may contribute to the regulation of the TRHR gene expression. Functional analysis of potential response elements for the anterior pituitary-specific transcription factor Pit-1 revealed cell typespecific binding that was competed out with a Pit-1 response element from the GH gene promoter.

Thyrotropin-releasing hormone (TRH) receptor (TRHR) is a G-protein-coupled receptor playing a crucial role in the anterior pituitary where it controls the synthesis and secretion of thyroidstimulating hormone and prolactin. Its widespread presence not only in the central nervous system, but also in peripheral tissues, including thymus, indicates other important, but unknown, functions. One hypothesis is that the neuropeptide TRH could play a role in the immune system. We report here that the human TRHR promoter contains 11 putative response elements for the haematopoietic transcription factor c-Myb and is highly Mybresponsive in transfection assays. Analysis of Myb binding to putative response elements revealed one preferred binding site in intron 1 of the receptor gene. Transfection studies of promoter deletions confirmed that this high-affinity element is necessary for efficient Myb-dependent transactivation of reporter plasmids in CV-1 cells. The Myb-dependent activation of the TRHR promoter was strongly suppressed by expression of a dominant negative Myb-Engrailed fusion. In line with these observations, reverse transcriptase PCR analysis of rat tissues showed that the TRHR gene is expressed both in thymocytes and bone marrow. Furthermore, specific, high-affinity TRH agonist binding to cell-surface receptors was demonstrated in thymocytes and a haematopoietic cell line. Our findings imply a novel functional link between the neuroendocrine and the immune systems at the level of promoter regulation.