Purpose: 3-iodothyronamine (3-T1AM), a decarboxylated and deiodinated thyroid hormone metabolite, leads at pharmacological doses to hypoinsulinemia, hyperglucagonemia and hyperglycemia in vivo. As the pancreatic Langerhans islets express thyroid hormone transmembrane transporters (THTT), we tested the hypothesis that not only plasma membrane-mediated 3-T1AM binding to and activation of G-protein coupled receptors, but also 3-T1AM metabolite(s) generated by 3-T1AM uptake and metabolism might decrease glucose-stimulated insulin secretion (GSIS).
Methods: Murine pancreatic β-cells MIN6 were characterized for gene expression of THTT, deiodinases and monoamine oxidases. 3-T1AM uptake and intracellular metabolism to the corresponding 3-iodothyroacetic acid were analysed by liquid-chromatography tandem mass spectrometry (LC-MS/MS) at different time points in cells as well as the conditioned medium. Mitochondrial activity, especially ATP-production, was monitored real-time after 3-T1AM application using Seahorse Bioanalyzer technique. Effect of 3-T1AM on GSIS into the culture medium was assayed by ELISA.
Results: MIN6 cells express classical THTT, proposed to transport 3-T1AM, as well as 3-T1AM metabolizing enzymes comparable to murine primary pancreatic islets. 3-T1AM accumulates in MIN6 cells and is metabolized by intracellular MaoB to 3-iodothyroacetic, which in turn is rapidly exported. 3-T1AM decreases mitochondrial ATP-production concentration dependently. GSIS is diminished by 3-T1AM treatment. Using LC-MS/MS, no further 3-T1AM metabolites except 3-iodothyroacetic were detectable.
Conclusions: This data provides a first link between cellular 3-T1AM uptake and regulation of mitochondrial energy metabolism in ß-cells, resulting in reduced insulin secretion. We conclude that MIN6 is an appropriate cell model to study 3-T1AM-dependent (intra-)cellular biochemical mechanisms affecting insulin production in vitro.
Keywords: 3-Iodothyroacetic acid; Beta(β)-cell; Diabetes mellitus type I; MIN6 cells; Pancreas; Thyroid hormone metabolites.
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