3-Iodothyronamine reduces insulin secretion in vitro via a mitochondrial mechanism

Purpose: 3-iodothyronamine (3-T(1)AM), 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-T(1)AM binding to and activation of G-protein coupled receptors, but also 3-T(1)AM metabolite(s) generated by 3-T(1)AM uptake and metabolism might decrease glucose-stimulated insulin secretion (GSIS). Methods: Murine pancreatic beta-cells MIN6 were characterized for gene expression of THTT, deiodinases and monoamine oxidases. 3-T(1)AM 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. Mitochondria( activity, especially ATP-production, was monitored real-time after 3-T(1)AM application using Seahorse Bioanalyzer technique. Effect of 3-T(1)AM on GSIS into the culture medium was assayed by ELISA. Results: MIN6 cells express classical THTT, proposed to transport 3-T(1)AM, as well as 3-T(1)AM metabolizing enzymes comparable to murine primary pancreatic islets. 3-T(1)AM accumulates in MIN6 cells and is metabolized by intracellular MaoB to 3-iodothyroacetic, which in turn is rapidly exported. 3-T(1)AM decreases mitochondrial ATP-production concentration dependently. GSIS is diminished by 3-T(1)AM treatment. Using LC-MS/MS, no further 3-T(1)AM metabolites except 3-iodothyroacetic were detectable. Conclusions: This data provides a first link between cellular 3-T(1)AM uptake and regulation of mitochondrial energy metabolism in beta-cells, resulting in reduced insulin secretion. We conclude that MIN6 is an appropriate cell model to study 3-T(1)AM-dependent (intra-)cellular biochemical mechanisms affecting insulin production in vitro. (C) 2017 Elsevier B.V. All rights reserved.