Expression of Thyrotropin-Releasing Hormone (TRH) receptor subtype 1 in mouse pancreatic islets and HIT-T15, an insulin-secreting clonal β cell line

Abstract

Thyrotropin-releasing hormone (TRH), originally isolated as a hypothalamic hormone, has been reported to be present and released from the pancreatic β cells, affecting pancreatic functions. However, it still remains unclear whether TRH receptor is expressed in the pancreas. In the present study, we characterized TRH receptors (TRHR) in mouse pancreatic islets and HIT-T15 cells, a hamster clonal β cell line. RT-PCR study showed significant expression of TRHR subtype 1 (TRHR1) mRNA in both mouse pancreatic islets and HIT-T15 (HIT) cells. In contrast, there was no expression of TRHR2 mRNA, a novel subtype of TRHR which is expressed predominantly in the central nervous system. Sequencing analysis demonstrated that TRHR1 of the islets was identical to that in the pituitary, and cloned hamster TRHR1 shared 93.3% homology with that of the mouse at the nucleic acid level. Northern blot analysis of TRHR 1 mRNA in HIT-T15 cells showed a single strong hybridization signal approximately 3.7 kb in length. Furthermore, Scatchard plot analysis in HIT-T15 cells revealed that the Kd value for MeTRH was 0.63 nM. Significant elevation of intracellular calcium concentration was observed in response to as little as 10 nM TRH, and this was not affected by removal of extracellular calcium. This is the first description indicating the presence of functional TRH receptor subtype 1 in the pancreatic β cells, and our observations suggested the regulation of pancreatic function by TRH through autocrine or paracrine mechanisms.