Search

Recently, a unimolecular tri-agonist with activity at glucagon-like peptide 1 receptor (GLP-1R), glucose dependent insulinotropic receptor, and the glucagon receptor was reported to improve glycemic control in mice. Here, we defined the underlying molecular mechanisms of enhanced insulin secretion in murine pancreatic islets for a specific tri-agonist. The tri-agonist induced an increase in insulin secretion from murine islets compared to the respective mono-agonists. GLP-1R mainly signals via activation of the Gα s pathway, but inhibition of protein kinase A (H89) and exchange protein activation by cAMP (EPAC) (ESI-09) could not completely block insulin release induced by tri-agonist. Electrophysiological observations identified a strong increase of intracellular Ca 2+ concentration and whole-cell currents induced by tri-agonist via transient receptor potential channels (TRPs). Although, EPAC activation mobilizes intracellular Ca 2+ via TRPs, the TRPs blockers (La 3+ and Ruthenium Red) had a larger inhibitory impact than ESI-09 on tri-agonist stimulatory effects. To test for other potential mechanisms, we blocked PLC activity (U73122) which reduced the superior effect of tri-agonist to induce insulin secretion, and partially inhibited the induced Ca 2+ influx. This result suggests that the relative effect of tri-agonist on insulin secretion caused by GLP-1R agonism is mediated mainly via Gα s signaling and partially by activation of PLC. Therefore, the large portion of the increased intracellular Ca 2+ concentration and the enhanced whole-cell currents induced by tri-agonist might be attributable to TRP channel activation resulting from signaling through multiple G-proteins. Here, we suggest that broadened intracellular signaling may account for the superior in vivo effects observed with tri-agonism., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Transient receptor potential channel vanilloid type 4 (TRPV4) is a Ca(2+)- and Mg(2+)-permeable cation channel that influences oxidative metabolism and insulin sensitivity. The role of TRPV4 in pancreatic beta cells is largely unknown. Here, we characterize the role of TRPV4 in controlling intracellular Ca(2+) and insulin secretion in INS-1E beta cells. Osmotic, thermal or pharmacological activation of TRPV4 caused a rapid rise of intracellular Ca(2+) and enhanced glucose-stimulated insulin secretion. In the presence of the TRPV channel blocker ruthenium red (RuR) or after suppression of TRPV4 protein production, TRPV4 activators failed to increase [Ca(2+)]i and insulin secretion in INS-1E cells., (Copyright © 2013 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

Transient receptor potential channel vanilloid type 6 (TRPV6) is a non-selective cation channel with high permeability for Ca 2 + ions. So far, the role of TRPV6 in pancreatic beta cells is unknown. In the present study, we characterized the role of TRPV6 in controlling calcium signaling, cell proliferation as well as insulin expression, and secretion in experimental INS-1E beta cell model. TRPV6 protein production was downregulated using siRNA by approx. 70%, as detected by Western blot. Intracellular free Ca 2 + ([Ca 2 + ] i ) was measured by fluorescence Ca 2 + imaging using fura-2. Calcineurin/NFAT signaling was analyzed using a NFAT reporter assay as well as a calcineurin activity assay. TRPV6 downregulation resulted in impaired cellular calcium influx. Its downregulation also reduced cell proliferation and decreased insulin mRNA expression. These changes were companied by the inhibition of the calcineurin/NFAT signaling. In contrast, insulin exocytosis was not affected by TRPV6 downregulation. In conclusion, this study demonstrates for the first time the expression of TRPV6 in INS-1E cells and rat pancreatic beta cells and describes its role in modulating calcium signaling, beta cell proliferation and insulin mRNA expression. In contrast, TRPV6 fails to influence insulin secretion. [ABSTRACT FROM AUTHOR]