Ouabain suppresses ATP elevation in response to fuel secretagogues in pancreatic islets

Role of endogenous ROS production in impaired metabolism-secretion coupling of diabetic pancreatic β cells

One of the characteristics of type 2 diabetes is that the insulin secretory response of β cells is selectively impaired to glucose. In the Goto-Kakizaki (GK) rat, a genetic model of type 2 diabetes mellitus, glucose-induced insulin secretion is selectively impaired due to deficient ATP production derived from impaired glucose metabolism. In addition, islets in GK rat and human type 2 diabetes are oxidatively stressed. In this issue, role of endogenous reactive oxygen species (ROS) production in impaired metabolism-secretion coupling of diabetic pancreatic β cells is reviewed. In β cells, ROS is endogenously produced by activation of Src, a non-receptor tyrosine kinase. Src inhibitors restore the impaired insulin release and impaired ATP elevation by reduction in ROS production in diabetic islets. Src is endogenously activated in diabetic islets, since the level of Src pY416 in GK islets is higher than that in control islets. In addition, exendin-4, a glucagon-like peptide-1 (GLP-1) receptor agonist, decreases Src pY416 and glucose-induced ROS production and ameliorates impaired ATP production dependently on Epac in GK islets. These results indicate that GLP-1 signaling regulates endogenous ROS production due to Src activation and that incretin has unique therapeutic effects on impaired glucose metabolism in diabetic β cells.

Sodium-potassium ATPase 1 subunit is a molecular partner of Wolframin, an endoplasmic reticulum protein involved in ER stress

Wolfram syndrome, an autosomal recessive disorder characterized by diabetes mellitus and optic atrophy, is caused by mutations in the WFS1 gene encoding an endoplasmic reticulum (ER) membrane protein, Wolframin. Although its precise functions are unknown, Wolframin deficiency increases ER stress, impairs cell cycle progression and affects calcium homeostasis. To gain further insight into its function and identify molecular partners, we used the WFS1-C-terminal domain as bait in a yeast two-hybrid screen with a human brain cDNA library. Na+/K+ ATPase beta1 subunit was identified as an interacting clone. We mapped the interaction to the WFS1 C-terminal and transmembrane domains, but not the N-terminal domain. Our mapping data suggest that the interaction most likely occurs in the ER. We confirmed the interaction by co-immunoprecipitation in mammalian cells and with endogenous proteins in JEG3 placental cells, neuroblastoma SKNAS and pancreatic MIN6 beta cells. Na+/K+ ATPase beta1 subunit expression was reduced in plasma membrane fractions of human WFS1 mutant fibroblasts and WFS1 knockdown MIN6 pancreatic beta-cells compared with wild-type cells; Na+/K+ ATPase alpha1 subunit expression was also reduced in WFS-depleted MIN6 beta cells. Induction of ER stress in wild-type cells only partly accounted for the reduced Na+/K+ ATPase beta1 subunit expression observed. We conclude that the interaction may be important for Na+/K+ ATPase beta1 subunit maturation; loss of this interaction may contribute to the pathology seen in Wolfram syndrome via reductions in sodium pump alpha1 and beta1 subunit expression in pancreatic beta-cells.

Impaired metabolism-secretion coupling in pancreatic beta-cells: role of determinants of mitochondrial ATP production

Glucose-induced insulin secretion from beta-cells is often impaired in diabetic condition and by exposure to diabetogenic pharmacological agents. In pancreatic beta-cells, intracellular glucose metabolism regulates exocytosis of insulin granules, according to metabolism-secretion coupling in which glucose-induced mitochondrial ATP production plays an essential role. Impaired glucose-induced insulin secretion often results from impaired glucose-induced ATP elevation in beta-cells. Mitochondrial ATP production is driven by the proton-motive force including mitochondrial membrane potential (DeltaPsi(m)) generated by the electron transport chain. These electrons are derived from reducing equivalents, generated in the Krebs cycle and transferred from cytosol by the shuttles. Here, roles of the determinants of mitochondrial ATP production in impaired glucose-induced insulin secretion are discussed. Cytosolic alkalization, H(+) leak in the inner membrane by uncoupler (e.g. free fatty acid exposure), decrease in the supply of electron donors including NADH and FADH(2) to the respiratory chain, and endogenous mitochondrial ROS (e.g. Na(+)/K(+)-ATPase inhibition) all reduce hyperpolarlization of DeltaPsi(m) and ATP production, causing decresed glucose-induced insulin release. The decrease in the supply of NADH and FADH(2) to the respiratory chain derives from impairments in glucose metabolism including glycolysis (e.g. MODY2 and exposure to NO) and the shuttles (e.g. diabetic state and exposure to ketone body).

Ouabain suppresses glucose-induced mitochondrial ATP production and insulin release by generating reactive oxygen species in pancreatic islets

We examined the effects of reduced Na(+)/K(+)-ATPase activity on mitochondrial ATP production and insulin release from rat islets. Ouabain, an inhibitor of Na(+)/K(+)-ATPase, augmented 16.7 mmol/l glucose-induced insulin release in the early period but suppressed it after a delay of 20-30 min. Unexpectedly, the ATP content in an islet decreases in the presence of 16.7 mmol/l glucose when Na(+)/K(+)-ATPase activity is diminished by ouabain, despite the reduced consumption of ATP by the enzyme. Ouabain also suppressed the increment of ATP content produced by glucose even in Ca(2+)-depleted or Na(+)-depleted conditions. That mitochondrial membrane hyperpolarization and O(2) consumption in islets exposed to 16.7 mmol/l glucose were suppressed by ouabain indicates that the glycoside inhibits mitochondrial respiration but does not produce uncoupling. Ouabain induced mitochondrial reactive oxygen species (ROS) production that was blocked by myxothiazol, an inhibitor of site III of the mitochondrial respiratory chain. An antioxidant, alpha-tocopherol, also blocked ouabain-induced ROS production as well as the suppressive effect of ouabain on ATP production and insulin release. However, ouabain did not directly affect the mitochondrial ATP production originating from succinate and ADP. These results indicate that ouabain suppresses mitochondrial ATP production by generating ROS via transduction, independently of the intracellular cationic alternation that may account in part for the suppressive effect on insulin secretion.

Modulation of islet ATP content by inhibition or stimulation of the Na(+)/K(+) pump

High (30 mM) K(+), known to cause beta-cell membrane depolarisation, significantly decreased the islet total ATP content, supporting the view that beta-cell membrane depolarisation can activate the ATP-consuming Na(+)/K(+) pump. Ouabain (1 mM) did not change the islet ATP content after 5-15 min of incubation in the absence or presence of 3 mM glucose but reduced it after 30 min, and in the presence of 20 mM glucose, the reduction by ouabain occurred already after 15 min. Incubation of islets with ouabain for 60 min decreased the islet ATP content in the presence of 3, 10 or 20 mM glucose or 30 mM K(+). Also, the islet glucose oxidation rate was decreased by ouabain. When K(+) deficiency was used to inhibit the Na(+)/K(+) pump, no change in ATP content was observed irrespective of glucose concentration, although K(+) deficiency caused a slight inhibition of the glucose oxidation rate. Diazoxide reduced the islet glucose oxidation rate and increased the islet ATP content in the presence of 20 mM glucose. There may exist a feedback mechanism decreasing the flow of glucose metabolism in response to reduced ATP consumption by the Na(+)/K(+) pump.

Regulation of intracellular ATP concentration under conditions of reduced ATP consumption in pancreatic islets

ATP is the most important factor in glucose-induced insulin secretion in pancreatic beta-cells, but examination of intracellular differences in ATP concentration is difficult because ATP production and consumption occur simultaneously. In the present study, we measured the ATP concentration under the condition of a reduced ATP requirement by omitting extracellular Ca(2+) and inhibiting Na-K ATPase. The ATP concentration in islets incubated with 16.7 mM glucose in the absence of Ca(2+) for 30 min was increased by about 1. 9-fold more than in the presence of Ca(2+). The increment was extracellular Ca(2+)-dependent, and was completely abolished by the metabolic inhibitors dinitrophenol and iodoacetic acid. The Ca channel blockers including nitrendipine and Ni(2+) did not affect the ATP concentration in islets incubated with 16.7 mM glucose in the presence of Ca(2+). However, when thapsigargin and suramin, inhibitors of Ca-ATPase at the endoplasmic reticulum, were added to Ca channel blockers in the presence of ambient Ca(2+), the intraislet ATP content was increased, similarly to that under Ca-free conditions. But thapsigargin did not further augment the ATP concentration in the islet with 16.7 mM glucose in the absence of Ca(2+). On the other hand, the suppression of Na-K ATPase by ouabain rather reduced the ATP concentration augmented by omission of extracellular Ca(2+). In addition, vanadate, a blocker of Ca-ATPase at the plasma membrane, failed to increase the ATP concentration in the islets. These data suggest that the increment of ATP concentration in the absence of Ca(2+) is attributable to the reduced ATP requirement due to stopping of the Ca-ATPase activity at the endoplasmic reticulum, and that the intracellular ATP concentration is differently regulated by Na-K ATPase at plasma membrane and by Ca-ATPase at endoplasmic reticulum.