If Metformin Inhibited the Mitochondrial Glycerol Phosphate Dehydrogenase It Might Not Benefit Diabetes

The mitochondrial glycerol phosphate dehydrogenase is the rate-limiting enzyme of the glycerol phosphate shuttle. It was recently claimed that metformin, a first line drug used worldwide for the treatment of type 2 diabetes, works by inhibiting the mitochondrial glycerol phosphate dehydrogenase 30-50% thus suppressing hepatic gluconeogenesis. This enzyme is 30-60 fold higher in the pancreatic islet than in liver. If metformin actually inhibited the enzyme, why would it not inhibit insulin secretion and exacerbate diabetes? Total body knockout of the mitochondrial glycerol phosphate dehydrogenase does not inhibit insulin secretion because insulin cells and liver cells possess the malate aspartate shuttle that is redundant to the action of the glycerol phosphate shuttle. In view of these and other apparent inconsistencies we reassessed the idea that metformin inhibited the mitochondrial glycerol phosphate dehydrogenase. We measured the enzyme’s activity in whole cell homogenates and mitochondria of insulin cells and liver cells using four different enzyme assays and were unable to show that metformin directly inhibits the enzyme. We conclude that metformin does not actually inhibit the enzyme. If it did, it might not be efficacious as a diabetes medicine.

GW9508 inhibits insulin secretion by activating ATP-sensitive potassium channels in rat pancreatic β-cells

GW9508 is an agonist of G protein-coupled receptor 40 (GPR40) that is expressed in pancreatic β-cells and is reported to regulate insulin secretion. However, the effects of GW9508 on pancreatic β-cells in primary culture have not been well investigated. This study measured the acute effects of GW9508 on insulin secretion from rat pancreatic islets in primary culture, and the insulin secretion-related events such as the changes in membrane potential, ATP-sensitive potassium currents (KATP currents), and intracellular Ca2+ concentrations ([Ca2+]i) of rat islet β-cells were also recorded. GW9508 (10–40 μM) did not influence basal insulin levels at 2 mM glucose, but it (above 20 μM) significantly inhibited 5 and 15 mM glucose-stimulated insulin secretion (GSIS). GW9508 did not inhibit insulin secretion stimulated by tolbutamide, the closer of KATP channels. GW9508 activated KATP channels and blocked the membrane depolarization and the increase in [Ca2+]i that were stimulated by glucose. GW9508 itself stimulated a transient increase in [Ca2+]i, which was fully blocked by depletion of intracellular Ca2+ stores with thapsigargin or by inhibition of phospholipase C (PLC) activity with U73122. GW9508-induced activation of KATP channels was only partly inhibited by U73122 treatment. In conclusion, although it stimulates a transient release of Ca2+ from intracellular Ca2+ stores via activation of PLC, GW9508 inhibits GSIS by activating KATP channels probably in a distal step to GPR40 activation in rat β-cells.

Neuropeptide Y and somatostatin inhibit insulin secretion through different mechanisms

Pancreatic β-cells regulate glucose homeostasis by secreting insulin in response to glucose elevation and G protein-coupled receptor (GPCR) activation. Neuropeptide Y (NPY) and somatostatin (SST) attenuate insulin secretion through Gi activation of Y1 and SSTR1&5 receptors, respectively. The downstream pathways altered by NPY and SST are poorly understood. Thus, we investigated these underlying mechanisms. NPY and SST increase cellular redox potential, suggesting that their inhibitory effect may not be mediated through metabolic inhibition. NPY does not affect intracellular calcium ([Ca2+]i) activity upon glucose stimulation, whereas SST alters this response. Gβγ-subunit inhibition by gallein attenuates insulin secretion but does not alter metabolism or [Ca2+]i. mSIRK-induced Gβγ activation does not modulate glucose metabolism but increases [Ca2+]i activity and potentiates insulin release. Cotreatment with gallein and NPY or SST reduces insulin secretion to levels similar to that of gallein alone. mSIRK and NPY cotreatment potentiates insulin secretion similarly to mSIRK alone, whereas mSIRK and SST treatment decreases insulin release. The data support a model where SST attenuates secretion through Gβγ inhibition of Ca2+ activity, while NPY activates a Ca2+-independent pathway mediated by Gα. GPCR ligands signal through multiple pathways to inhibit insulin secretion, and determining these mechanisms could lead to novel diabetic therapies.