Date Published: January 27, 2017
Publisher: Public Library of Science
Author(s): Luis Rodrigo Cataldo, María L. Mizgier, Roberto Bravo Sagua, Fabián Jaña, César Cárdenas, Paola Llanos, Dolores Busso, Pablo Olmos, José E. Galgani, José L. Santos, Víctor A. Cortés, Bridget Wagner.
Pancreatic β-cells synthesize and release serotonin (5 hydroxytryptamine, 5HT); however, the role of 5HT receptors on glucose stimulated insulin secretion (GSIS) and the mechanisms mediating this function is not fully understood. The aims of this study were to determine the expression profile of 5HT receptors in murine MIN6 β-cells and to examine the effects of pharmacological activation of 5HT receptor Htr2b on GSIS and mitochondrial function.
mRNA levels of 5HT receptors in MIN6 cells were quantified by RT qPCR. GSIS was assessed in MIN6 cells in response to global serotonergic activation with 5HT and pharmacological Htr2b activation or inhibition with BW723C86 or SB204741, respectively. In response to Htr2b activation also was evaluated the mRNA and protein levels of PGC1α and PPARy by RT-qPCR and western blotting and mitochondrial function by oxygen consumption rate (OCR) and ATP cellular content.
We found that mRNA levels of most 5HT receptors were either very low or undetectable in MIN6 cells. By contrast, Htr2b mRNA was present at moderate levels in these cells. Preincubation (6 h) of MIN6 cells with 5HT or BW723C86 reduced GSIS and the effect of 5HT was prevented by SB204741. Preincubation with BW723C86 increased PGC1α and PPARy mRNA and protein levels and decreased mitochondrial respiration and ATP content in MIN6 cells.
Our results indicate that prolonged Htr2b activation in murine β-cells decreases glucose-stimulated insulin secretion and mitochondrial activity by mechanisms likely dependent on enhanced PGC1α/PPARy expression.
Although most studies on serotonin (5-hydroxytryptamine, 5HT) focus on its role as a central neurotransmitter, the bulk of 5HT (>90%) is synthetized by enteroendocrine cells, secreted to systemic circulation and stored in platelets . Additional peripheral tissues are also able to synthesize and release small amounts of 5HT [1, 2]. These microserotoninergic systems have been involved in auto and paracrine regulatory circuits. Nowadays, pancreatic β-cells are recognized as a bona fide microserotonergic system, able to synthesize, store and release 5HT in response to glucose stimulation [3–6]. Whether 5HT has physiological or pathophysiological implications on glucose-stimulated insulin secretion (GSIS) is still controversial.
We previously found that MIN6 cells as well as islets from WT and db/db mice express several serotonergic genes, including those encoding for 5HT biosynthetic enzymes Tph1/2 and Ddc, vesicle transporters Vmat1/2 and catabolic enzymes Maoa/b [5, 6]. Others have described equivalent findings in mouse islets [3, 4]. Additionally, in this and previous work , we have shown that 5HT precursor 5HTP and Mao inhibitor Pargyline, increases 5HT content and extracellular 5HT levels in MIN6 cells, indicating these cells synthesize, degrade, store and secrete 5HT, constituting a functional microserotonergic system.