Glucagon-like peptide-1 stimulates type 3 iodothyronine deiodinase expression in a mouse insulinoma cell line

The pathophysiological roles of thyroid hormones in glucose metabolism remain uncertain. Type 3 iodothyronine deiodinase (D3) converts thyroxine (T4) and 3,5,3'-triiodothyronine (T3) to 3,3',5'-triiodothyronine (rT3) and 3,3'-diiodothyronine (T2), respectively, inactivating thyroid hormones in a cell-specific fashion. In the present study, we identified D3 expression in MIN6 cells derived from a mouse insulinoma cell line and examined the mechanisms regulating D3 expression in these cells.

We characterized D3 activity using HPLC analysis, and examined the effect of GLP-1 or exendin-4 on D3 expression and cAMP accumulation in MIN6 cells. We also measured insulin secretion from MIN6 cells exposed to GLP-1 and T3.

We identified enzyme activity that catalyzes the conversion of T3 to T2 in MIN6 cells, which showed characteristics compatible with those for D3. D3 mRNA was identified in these cells using RT-PCR analysis. Forskolin rapidly stimulated D3 mRNA and D3 activity. Glucagon-like peptide-1 (GLP-1) increased D3 expression in a dose-dependent manner, and this effect was inhibited by the protein kinase A (PKA) inhibitor H-89. Exendin-4, a GLP-1 receptor agonist, also stimulated D3 expression in MIN6 cells. These results suggest that a cAMP-PKA-mediated pathway participates in GLP-1-stimulated D3 expression in MIN6 cells. Furthermore, GLP-1 stimulated insulin secretion was suppressed by the addition of T3 in MIN6 cells.

Our findings indicate that GLP-1 regulates intracellular T3 concentration in pancreatic β cells via a cAMP-PKA-D3-mediated pathway that may also regulate β-cell function.