Deficiency of pyruvate dehydrogenase activity in pancreatic islets of diabetic GK rats.

Hypoglycemic sulfonylureas such as glibenclamide have been widely used to treat type 2 diabetic patients for 40 yr, but controversy remains about their mode of action. The widely held view is that they promote rapid insulin exocytosis by binding to and blocking pancreatic β-cell ATP-dependent K+ (KATP) channels in the plasma membrane. This event stimulates Ca2+ influx and sets in motion the exocytotic release of insulin. However, recent reports show that >90% of glibenclamide-binding sites are localized intracellularly and that the drug can stimulate insulin release independently of changes in KATP channels and cytoplasmic free Ca2+. Also, glibenclamide specifically and progressively accumulates in islets in association with secretory granules and mitochondria and causes long-lasting insulin secretion. It has been proposed that nutrient insulin secretagogues stimulate insulin release by increasing formation of malonyl-CoA, which, by blocking carnitine palmitoyltransferase 1 (CPT-1), switches fatty acid (FA) catabolism to synthesis of PKC-activating lipids. We show that glibenclamide dose-dependently inhibits β-cell CPT-1 activity, consequently suppressing FA oxidation to the same extent as glucose in cultured fetal rat islets. This is associated with enhanced diacylglycerol (DAG) formation, PKC activation, and KATP-independent glibenclamide-stimulated insulin exocytosis. The fat oxidation inhibitor etomoxir stimulated KATP-independent insulin secretion to the same extent as glibenclamide, and the action of both drugs was not additive. We propose a mechanism in which inhibition of CPT-1 activity by glibenclamide switches β-cell FA metabolism to DAG synthesis and subsequent PKC-dependent and KATP-independent insulin exocytosis. We suggest that chronic CPT inhibition, through the progressive islet accumulation of glibenclamide, may explain the prolonged stimulation of insulin secretion in some diabetic patients even after drug removal that contributes to the sustained hypoglycemia of the sulfonylurea.

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“…6) and in previous reports from other groups (8,44). Moreover, etomoxir elevates DAG contents (19), adding further credence to our concept.…”

Section: Discussionsupporting

confidence: 86%

Glibenclamide inhibits islet carnitine palmitoyltransferase 1 activity, leading to PKC-dependent insulin exocytosis

Lehtihet

Welsh

Berggren

et al. 2003

American Journal of Physiology-Endocrinology and Metabolism

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“…In fact, a decreased PDH activity has already been observed in vivo, in GK rats, a genetic model of type 2 diabetes (34). In this article, we first demonstrate a direct effect of chronic hyperglycemia on PDH activity and the association with a reduced ATP synthesis and impaired insulin secretion.…”

Section: Discussionsupporting

confidence: 54%

“…Preparation of islet homogenate extracts and radiochemical assay of PDH activity. Islet PDH complex was extracted and measured as previously described (34). With this method, we measured active, not total, PDH activity.…”

mentioning

confidence: 99%

Role of ATP Production and Uncoupling Protein-2 in the Insulin Secretory Defect Induced by Chronic Exposure to High Glucose or Free Fatty Acids and Effects of Peroxisome Proliferator-Activated Receptor-γ Inhibition

et al. 2002

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In rat pancreatic islets chronically exposed to high glucose or high free fatty acid (FFA) levels, glucoseinduced insulin release and mitochondrial glucose oxidation are impaired. These abnormalities are associated with high basal ATP levels but a decreased glucoseinduced ATP production (⌬ of increment over baseline 0.7 ؎ 0.5 or 0.5 ؎ 0.3 pmol/islet in islets exposed to glucose or FFA vs. 12.0 ؎ 0.6 in control islets, n ‫؍‬ 3; P < 0.01) and, as a consequence, with an altered ATP/ADP ratio. To investigate further the mechanism of the impaired ATP formation, we measured in rat pancreatic islets glucose-stimulated pyruvate dehydrogenase (PDH) activity, a key enzyme for pyruvate metabolism and for the subsequent glucose oxidation through the Krebs cycle, and also the uncoupling protein-2 (UCP-2) content by Western blot. In islets exposed to high glucose or FFA, glucose-stimulated PDH activity was impaired and UCP-2 was overexpressed. Because UCP-2 expression is modulated by a peroxisome proliferatoractivated receptor (PPAR)-dependent pathway, we measured PPAR-␥ contents by Western blot and the effects of a PPAR-␥ antagonist. PPAR-␥ levels were overexpressed in islets cultured with high FFA levels but unaffected in islets exposed to high glucose. In islets exposed to high FFA concentration, a PPAR-␥ antagonist was able to prevent UCP-2 overexpression and to restore insulin secretion and the ATP/ADP ratio. These data indicate that in rat pancreatic islets chronically exposed to high glucose or FFA, glucose-induced impairment of insulin secretion is associated with (and might be due to) altered mitochondrial function, which results in impaired glucose oxidation, overexpression of the UCP-2 protein, and a consequent decrease of ATP production. This alteration in FFA cultured islets is mediated by the PPAR-␥ pathway. Diabetes 51: 2749 -2756, 2002 P atients with type 2 diabetes are characterized by a progressive decline of insulin secretion that becomes more severe with the increasing duration of the disease (1-4). In these patients, the mechanisms that cause the progressive ␤-cell failure are currently under investigation: the altered insulin secretory pattern depends, at least in part, on the negative influence of chronic high glucose (5-8) and/or high free fatty acid (FFA) (9 -12) plasma concentrations (gluco-or lipotoxicity). These metabolites are believed to affect pancreatic ␤-cell function by chronic ␤-cell stimulation and consequent "desensitization" to glucose. However, the molecular mechanisms of glucose desensitization induced by hyperglycemia or hypernefemia are still unclear (13,14).Rat pancreatic islets that are chronically exposed to high glucose or FFAs have an impaired glucose oxidation (15). Because glucose oxidation generates ATP and the rise of ATP and ATP/ADP ratio plays a central role in glucose-induced insulin release by causing K ϩ -ATP channel closure, membrane depolarization, increased calcium influx, and insulin granule exocytosis, we first measured ATP and ADP levels in islets that were chronic...

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“…Deficient expression of pyruvate dehydrogenase (29), increased secretion of islet amyloid polypeptide (30), alterations in lysosomal enzymes (31), lowered cAMP levels (32), and low levels of mitochondrial glycerol phosphate dehydrogenase (33) all have been suggested in this context. Glycerol phosphate dehydrogenase was mapped to rat chromosome 3 in the vicinity of a region linked to diabetes in GK rats (34).…”

Section: Discussionmentioning

confidence: 99%

Phenotyping of Individual Pancreatic Islets Locates Genetic Defects in Stimulus Secretion Coupling to Niddm1i Within the Major Diabetes Locus in GK Rats

Lin

Ortsäter²,

Fakhrai-Rad³

et al. 2001

Diabetes

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The major diabetes quantitative trait locus (Niddm1), which segregates in crosses between GK rats affected with spontaneous type 2-like diabetes and normoglycemic F344 rats, encodes at least two different diabetes susceptibility genes. Congenic strains for the two subloci (Niddm1f and Niddm1i) have been generated by transfer of GK alleles onto the genome of F344 rats. Whereas the Niddm1f phenotype implicated insulin resistance, the Niddm1i phenotype displayed diabetes related to insulin deficiency. Individual islets from 16-week-old congenic rats were characterized for insulin release and oxygen tension (pO 2 ). In the presence of 3 mmol/l glucose, insulin release from Niddm1f and Niddm1i islets was ϳ5 pmol ⅐ g ؊1 ⅐ s ؊1 and pO 2 was 120 mmHg. Similar recordings were obtained from GK and F344 islets. When glucose was raised to 11 mmol/l, insulin release increased significantly in Niddm1f and F344 islets but was essentially unchanged in islets from GK and Niddm1i. The high glucose concentration lowered pO 2 to the same extent in islets from all strains. Addition of 1 mmol/l tolbutamide to the perifusion medium further increased pulsatile insulin release threefold in all islets. The pulse frequency was ϳ0.4 min ؊1 . ␣-Ketoisocaproate (11 mmol/l) alone increased pulsatile insulin release eightfold in islets from Niddm1f, Niddm1i, and control F344 rats but had no effect on insulin release from GK islets. These secretory patterns in response to ␣-ketoisocaproate were paralleled by reduction of pO 2 in Niddm1f, Niddm1i, and control F344 islets and no change of pO 2 in GK islets. The results demonstrate that Niddm1i carries alleles of gene(s) that reduce glucose-induced insulin release and that are amenable to molecular identification by genetic fine mapping.

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Deficiency of pyruvate dehydrogenase activity in pancreatic islets of diabetic GK rats.

Cited by 47 publications

References 24 publications

Glibenclamide inhibits islet carnitine palmitoyltransferase 1 activity, leading to PKC-dependent insulin exocytosis

Glibenclamide inhibits islet carnitine palmitoyltransferase 1 activity, leading to PKC-dependent insulin exocytosis

Role of ATP Production and Uncoupling Protein-2 in the Insulin Secretory Defect Induced by Chronic Exposure to High Glucose or Free Fatty Acids and Effects of Peroxisome Proliferator-Activated Receptor-γ Inhibition

Phenotyping of Individual Pancreatic Islets Locates Genetic Defects in Stimulus Secretion Coupling to Niddm1i Within the Major Diabetes Locus in GK Rats

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