Acute overexpression of lactate dehydrogenase-A perturbs beta-cell mitochondrial metabolism and insulin secretion

Islet beta-cells express low levels of lactate dehydrogenase and have high glycerol phosphate dehydrogenase activity. To determine whether this config uration favors oxidative glucose metabolism via mitochondria in the beta-ce ll and is important for beta-cell metabolic signal transduction, we have de termined the effects on glucose metabolism and insulin secretion of acute o verexpression of the skeletal muscle isoform of lactate dehydrogenase (LDH) -A. Monitored in single MIN6 beta-cells, LDH hyperexpression (achieved by i ntranuclear cDNA microinjection or adenoviral infection) diminished the res ponse to glucose of both phases of increases in mitochondrial NAD(P)H, as w ell as increases in mitochondrial membrane potential, cytosolic free ATP, a nd cystolic free Ca2+. These effects were observed at all glucose concentra tions, but were most pronounced at submaximal glucose levels. Corresponding ly, adenoviral vector-mediated LDH-A overexpression reduced insulin secreti on stimulated by 11 mmol/l glucose and the subsequent response to stimulati on with 30 mmol/l glucose, but it was without significant effect when the c oncentration of glucose was raised acutely from 3 to 30 mmol/l. Thus, overe xpression of LDH activity interferes with normal glucose metabolism and ins ulin secretion in the islet beta-cell type, and it may therefore be directl y responsible for insulin secretory defects in some forms of type 2 diabete s. The results also reinforce the view that glucose-derived pyruvate metabo lism in the mitochondrion is critical for glucose-stimulated insulin secret ion in the beta-cell.