Thiol-dependent and non-thiol-dependent stimulations of insulin release

Protective role for nitric oxide during the endoplasmic reticulum stress response in pancreatic beta-cells

Higher requirements for disulfide bond formation in professional secretory cells may affect intracellular redox homeostasis, particularly during an endoplasmic reticulum (ER) stress response. To assess this hypothesis, we investigated the effects of the ER stress response on the major redox couple (GSH/GSSG), endogenous ROS production, expression of genes involved in ER oxidative protein folding, general antioxidant defense, and thiol metabolism by use of the well-validated MIN6 beta-cell as a model and mouse islets. The data revealed that glucose concentration-dependent decreases in the GSH/GSSG ratio were further decreased significantly by ER-derived oxidative stress induced by inhibiting ER-associated degradation with the specific proteasome inhibitor lactacystin (10 microM) in mouse islets. Notably, minimal cell death was observed during 12-h treatments. This was likely attributed to the upregulation of genes encoding the rate limiting enzyme for glutathione synthesis (gamma-glutamylcysteine ligase), as well as genes involved in antioxidant defense (glutathione peroxidase, peroxiredoxin-1) and ER protein folding (Grp78/BiP, PDI, Ero1). Gene expression and reporter assays with a NO synthase inhibitor (Nomega-nitro-L-arginine methyl ester, 1-10 mM) indicated that endogenous NO production was essential for the upregulation of several ER stress-responsive genes. Specifically, gel shift analyses demonstrate NO-independent binding of the transcription factor NF-E2-related factor to the antioxidant response element Gclc-ARE4 in MIN6 cells. However, endogenous NO production was necessary for activation of Gclc-ARE4-driven reporter gene expression. Together, these data reveal a distinct protective role for NO during the ER stress response, which helps to dissipate ROS and promote beta-cell survival.

The chemical form of selenium affects insulinomimetic properties of the trace element: investigations in type II diabetic dbdb mice

The objective of the present study was to investigate the effects of oral selenate application in comparison to selenium deficiency and selenite treatment on the development of the diabetic status (glucose tolerance, insulin resistance and activities of glycolytic and gluconeogenic marker enzymes) in dbdb mice, representing a type II diabetic animal model. Therefore 21 adult male dbdb mice were assigned to 3 experimental groups of 7 animals each and put on a selenium deficient diet (< 0.03 mg/kg diet) based on torula yeast. Group 0Se was kept on selenium deficiency for 10 weeks while the mice of the groups SeIV and SeVI were supplemented daily with 15% of their individual LD(50) of sodium selenite or sodium selenate in addition to the diet. After 10 weeks a distinct melioration of the diabetic status indicated by a corrected glucose tolerance and a lowered insulin resistance was measured in selenate treated mice (group SeVI) in comparison to their selenium deficient and selenite treated companions and to their initial status. Activities of the glycolytic marker enzymes hexokinase, phosphofructokinase and pyruvate kinase were increased 1.7 to 3-fold in liver and/or adipose tissue by selenate treatment as compared to mice on selenium deficiency and mice with selenite administration. In contrast selenate treatment (SeVI) repressed the activity of liver pyruvate carboxylase the first enzyme in gluconeogenesis by about 33% in comparison to the selenium deficient (0Se) and selenite treated mice (SeIV). However the current study revealed an insulinomimetic role for selenate (selenium VI) also in type II diabetic animals due to a melioration of insulin resistance. In contrast selenium deficiency and especially selenite (selenium IV) impaired the diabetic status of dbdb mice, demonstrating the need for investigations on the insulinomimetic action of selenium due to the metabolism of different selenium compounds.

Metabolic consequence of long-term exposure of pancreatic beta cells to free fatty acid with special reference to glucose insensitivity

Long-term exposure of the pancreatic beta cells to free fatty acid (FFA) reportedly inhibits glucose-stimulated insulin secretion. We here studied the impact of FFA on glucose and lipid metabolism in pancreatic beta cells with special reference to insulin secretion. Pancreatic beta-cell line MIN6 was exposed to various concentrations of palmitate for 3 days. Glucose-stimulated insulin secretion and insulin content were decreased corresponding to the concentration of the palmitate exposed. Glycolytic flux and ATP synthesis was unchanged, but pyruvate-stimulated change in NAD(P)H concentration was decreased. Pyruvate carboxylase was decreased at the protein level, which was restored by the removal of palmitate or the inhibition of beta-oxidation. Intracellular content of triglyceride and FFA were elevated, beta-oxidation was increased, and de novo lipogenesis from glucose was decreased. NADPH content and citrate output into the medium, which reflected pyruvate malate shuttle flux, were decreased, but malic enzyme activity was unaffected. The malic enzyme inhibitor alone inhibited insulin response to glucose. In conclusion, long-term exposure of FFA to beta cells inhibits glucose-stimulated insulin secretion via the decreased NADPH contents due to the inhibition of pyruvate carboxylase and malate pyruvate shuttle flux.

A role for potassium channels in smooth muscle cells and platelets in the etiology of primary pulmonary hypertension

Plasma serotonin levels are markedly elevated in patients with primary pulmonary hypertension (PPH) and platelet levels of serotonin are low. Furthermore, plasma serotonin levels remain elevated after bilateral lung transplantation, in the absence of any pulmonary hypertension. Dexfenfluramine can cause the anorexigen-induced form of PPH that is clinically and histologically indistinguishable from PPH. We find that dexfenfluramine releases serotonin from platelets and inhibits its reuptake. These observations suggest that serotonin might be involved in, or be a marker for, the mechanism responsible for both forms of PPH. Dexfenfluramine causes inhibition of voltage-sensitive potassium (Kv) channels, membrane depolarization, and calcium entry in pulmonary artery smooth muscle cells and vasoconstriction in isolated perfused rat lungs. We have recently found that dexfenfluramine also inhibits Kv channels in megakaryocytes, the stem cell for platelets. In smooth muscle cells, taken from the pulmonary arteries of PPH patients, Kv channels appear to be dysfunctional. The underlying defect in PPH is likely to be an abnormality of one or more Kv channels in both pulmonary artery smooth muscle cells and platelets. Relatively few patients exposed to dexfenfluramine develop PPH. The factors responsible for susceptibility might be a difference in expression of potassium channels and/or a decrease in the endogenous production of nitric oxide.

Significance of gamma-glutamyl transpeptidase and thiols in amino acid uptake by rat pancreatic islets: studies with glutamine and leucine

The importance of gamma-glutamyl transpeptidase, the key enzyme of the gamma-glutamyl cycle and of thiols for the uptake of amino acids into rat pancreatic islets was investigated. Both serine-borate, an inhibitor of gamma-glutamy transpeptidase, and serine which does not inhibit this enzyme, but probably is a competitive inhibitor of amino acid uptake, inhibited the uptake of glutamine. The inhibitory effect of serine-borate was not greater than that of serine alone. The uptake of glutamine was not affected by either GSH (reduced glutathione) or diamide (a thiol oxidant). Neither substances affected the uptake of leucine. The results indicate that the uptake of glutamine by rat pancreatic islets is not dependent on the functioning of gamma-glutamyl transpeptidase and that thiols are not important for the uptake of the amino acids glutamine and leucine.

Effect of thioloxidant diazene dicarboxylic acid bis-(N'-methylpiperazide) (DIP) on 45Ca2+ net uptake into rat pancreatic islets

The effect of DIP (an oxidant of glutathione) on 45Ca2+ net uptake induced by a variety of stimulators of insulin secretion was studied in rat pancreatic islets. In addition the effect of exogenous glutathione (GSH) on 45Ca2+ net uptake in response to glucose was tested. DIP (0.1 mM) inhibited the increase of 45Ca2+ net uptake in the presence of glucose (16.7 mM) and glyceraldehyde (10 mM). A similar inhibitory effect could be demonstrated, when 45Ca2+ net uptake was enhanced by tolbutamide (100 micrograms/ml), glibenclamide (0.5 micrograms/ml), b-BCH (20 mM), 2-ketoisocaproate (20 mM), arginine (20 mM) in the presence of 3 mM glucose or by high extracellular potassium (20 mM). The increase of 45Ca2+ net uptake stimulated by leucine (20 mM) plus glucose (3 mM) was further augmented by DIP. Exogenous GSH did not affect 45Ca2+ net uptake in the presence of (5.6-16.7 mM) glucose. It is suggested that 45Ca2+ net uptake of pancreatic islets depends on the redox state of islet thiols regardless of whether uptake is promoted via inhibition of potassium efflux (nutrients, sulfonylureas) or by high potassium and arginine. The voltage sensitive calcium-channel is the site of action of critical thiols. It is possible that these thiols are localized at the inner side of the plasma membrane.

Thiols and pancreatic beta-cell function: a review

In pancreatic islets insulin secretion in response to a variety of stimulators is sensitive to the redox state of extracellular and intracellular thiols. In this connection variations of plasma glutathione (GSH) may also be of importance. In the process of stimulus-secretion coupling, membrane thiols play an important role. One major localization of critical thiols appears to be related to the influx of calcium through the voltage-dependent channel. Other transmembranal ion movements and the cAMP system seem to be less sensitive to thiol oxidation than calcium influx via voltage-dependent Ca channels.

Failure of glucose to affect 86rubidium efflux and 45calcium uptake of fetal rat pancreatic islets

Ion movements and insulin secretion of pancreatic islets of adult and fetal rats have been studied at three glucose concentrations. In islets of adult rats, 86Rb efflux is maximally decreased by 5.6 mM-glucose. 16.7 mM-glucose caused a biphasic efflux pattern which may be due to glucose-stimulated Ca uptake. In islets of fetal rats elevation of the glucose concentration from 3 to 5.6 or 16.7 mM does not cause a change of 86Rb efflux, and the fractional efflux from fetal islets in the presence of 3 mM-glucose is similar to that from adult rat islets in the presence of 5.6 mM-glucose. Elevation of the glucose concentration from 3 to 16.7 mM is not associated with an increase in 45Ca uptake into fetal islets, although this change in glucose concentration doubles 45Ca uptake into adult islets. When challenged with 16.7 mM-glucose, fetal islets exhibit no insulin secretory response; however, they do respond to theophylline. It is concluded that the failure of fetal islets to exhibit an insulin-secretory response when challenged with glucose might be related to the inability of glucose to affect 86Rb efflux and Ca uptake. The present data are discussed in light of differences between pancreatic islets of fetal and adult rats with respect to the redox state of pyridine nucleotides, thiols and glucose metabolism.