Expression of thyroid hormone receptor isoform alpha1 in pancreatic islets

Thyroid hormone receptors (TR) mediate the action of thyroid hormones. Genetic studies revealed that the individual TR isoforms possess different functions. In the present paper we studied the expression of the isoforms TRalpha1 and TRbeta1 in the murine pancreatic islet. TRalpha1 and TRbeta1 mRNA transcripts and proteins were detected in islets using reverse transcription-polymerase chain reaction and Western blotting analyses, respectively. In immunohistochemical studies individual cells in the periphery of islets were labelled using an anti-TRalpha1 antibody. No labelled cells were detected in the exocrine pancreas. A similar staining pattern was obtained with an anti-glucagon antibody, but not with an anti-insulin antibody, which suggests that TRalpha1 is mainly expressed in alpha-cells. In order to address a potential function of TRalpha1 in this cell type, the regulation of glucagon gene expression by triiodothyronine was studied in a glucagon-producing cell line by Northern blot analysis and transient transfection assays using glucagon promoter luciferase fusion gene constructs. In these assays, triiodothyronine did not regulate the glucagon mRNA level or the glucagon promoter activity. The predominant localization of TRalpha1 in pancreatic alpha-cells suggests that this receptor isoform mediates a specific, yet unknown, function of thyroid hormones in this cell type.