Expression of thyrotropin-releasing hormone receptor in immortalized β-cell lines and rat pancreas

Abstract

Thyrotropin-releasing hormone (TRH), a hypothalamic tripeptide, is expressed in pancreatic islets at peak levels during the late gestation and early neonate period. TRH increases insulin production in cultured β-cells, suggesting that it might play a role in regulating pancreatic β-cell function. However, there is limited information on TRH receptor expression in the pancreas. The aim of the present study was to explore the distribution of the TRH receptor in the pancreas and its function in pancreatic β-cells. TRH receptor type 1 (TRHR1) gene expression was detected by RT-PCR and verified by Northern blotting and immunoblotting in the β-cell lines, INS-1 and βTC-6, and the rat pancreatic organ. The absence of TRH receptor type 2 expression in the tissue and cells indicated the tissue specificity of TRH receptor expression in the pancreas. The TRHR1 signals (detected by in situ hybridization) were distributed not only in islets but also in the surrounding areas of the pancreatic ductal and vasal epithelia. The apparent dissociation constant value for the affinity of [3H]3-methyl-histidine TRH (MeTRH) is 4.19 in INS-1 and 3.09 nM in βTC-6. In addition, TRH induced epidermal growth factor (EGF) receptor phosphorylation with a half-maximum concentration of approximately 50 nM, whereas the high affinity analogue of TRH, MeTRH, was 1 nM. This suggested that the affinity of TRH ligands for the TRH receptor influences the activation of EGF receptor phosphorylation in βTC-6 cells. Our observations suggested that the biological role of TRH in pancreatic β-cells is via the activation of TRHR1. Further research is required to identify the role of TRHR1 in the pancreas aside from the islets. © 2004 Society for Endocrinology.