Suzuki Atsushi

The intricate link between glucose metabolism, ATP production, and glucose-stimulated insulin secretion (GIIS) in pancreatic β-cells has been well established. However, the effects of other digestible monosaccharides on this mechanism remain unclear. This study examined the interaction between intracellular fructose metabolism and GIIS using MIN6-K8 β-cell lines and mouse pancreatic islets. Fructose at millimolar concentrations potentiated insulin secretion in the presence of stimulatory levels (8.8 mM) of glucose. This potentiation was dependent on sweet taste receptor-activated phospholipase Cβ2 (PLCβ2) signaling. Concurrently, metabolic tracing using 13C-labeled fructose and glucose in conjunction with biochemical analyses demonstrated that fructose blunted the glucose-induced increase in the ATP/ADP ratio. Mechanistically, fructose is substantially converted to fructose 1-phosphate (F1P) at the expense of ATP. F1P directly inhibited PKM2 (pyruvate kinase M2), thereby reducing the later glycolytic flux used for ATP production. Remarkably, F1P-mediated PKM2 inhibition was counteracted by TEPP-46, a small-molecule PKM2 activator. TEPP-46 restored glycolytic flux and the ATP/ADP ratio, leading to the enhancement of fructose-potentiated GIIS in MIN6-K8 cells, normal mouse islets, and fructose-unresponsive diabetic mouse islets. These findings reveal an antagonistic interplay between glucose and fructose metabolism in β-cells, highlighting PKM2 as a crucial regulator and broadening our understanding of the relationship between β-cell fuel metabolism and insulin secretion.

β-adrenergic blockers (β-blockers) are extensively used to inhibit β-adrenoceptor activation and subsequent cAMP production in many cell types. In this study, we characterized the effects of β-blockers on mouse pancreatic β-cells. Unexpectedly, high concentrations (100 μM) of β-blockers (propranolol and bisoprolol) paradoxically increased cAMP levels 5-10 fold, enhanced Ca2+ influx, and stimulated a 2-4 fold increase in glucose- and glimepiride-induced insulin secretion in MIN6-K8 clonal β-cells and isolated mouse pancreatic islets. These effects were observed despite minimal expression of β-adrenoceptors in these cells. Mechanistically, the cAMP increase led to ryanodine receptor 2 (RYR2) phosphorylation via protein kinase A (PKA), triggering Ca2+-induced Ca2+ release (CICR). CICR then activates transient receptor potential cation channel subfamily M member 5 (TRPM5), resulting in increased Ca2+ influx via voltage-dependent Ca2+ channels. These effects contradict the conventional understanding of the pharmacology of β-blockers, highlighting the variability in β-blocker actions depending on the experimental context.