β-Cell Adaptation to Insulin Resistance

Self Cite

It remains unclear how ␣-ketoisocaproate (KIC) and leucine are metabolized to stimulate insulin secretion. Mitochondrial BCATm (branched-chain aminotransferase) catalyzes reversible transamination of leucine and ␣-ketoglutarate to KIC and glutamate, the first step of leucine catabolism. We investigated the biochemical mechanisms of KIC and leucine-stimulated insulin secretion (KICSIS and LSIS, respectively) using Although leucine oxidation and KIC transamination were blocked in BCATm ؊/؊ islets, KIC oxidation was unaltered.These data indicate that KICSIS requires transamination of KIC and glutamate to leucine and ␣-ketoglutarate, respectively. LSIS does not require leucine catabolism and may be through leucine activation of glutamate dehydrogenase. Thus, KICSIS and LSIS occur by enhancing the metabolism of glutamine/glutamate to ␣-ketoglutarate, which, in turn, is metabolized to produce the intracellular signals such as ATP and NADPH for insulin secretion.To maintain glucose homeostasis in response to a meal, insulin secretion is precisely stimulated by nutrients such as glucose, amino acids, and free fatty acids as well as incretin hormones such as glucagon-like peptide-1. Nutrients are thought to stimulate insulin secretion through metabolic secretion coupling to generate metabolic signals, i.e. second messengers or coupling factors. Extensive research has been conducted to determine how nutrients are metabolized to generate these coupling factors, e.g. ATP and NADPH. Although leucine and ␣-ketoisocaproate (KIC) 3 are potent insulin secretagogues (1), the underlying mechanisms of leucine and KIC-stimulated insulin secretion (LSIS and KICSIS, respectively) remain elusive. A key question is whether their oxidative decarboxylation is required for induction of insulin secretion.

Section: Discussionsupporting

confidence: 81%

Transamination Is Required for α-Ketoisocaproate but Not Leucine to Stimulate Insulin Secretion*

Zhou

Jetton

Goshorn

et al. 2010

Self Cite

“…Most interesting, these alterations occur in the absence of any significant changes in glucose oxidation indicating a compensatory adaptation that maintains overall CO 2 production. Consistent with this observation, repressed pyruvate dehydrogenase activity in rat islets exhibited no alterations in CO 2 production because of a compensatory increase in pyruvate carboxylase (52). The observation that a subunit of the mitochondrially encoded enzyme NADH dehydrogenase is inhibited by repression of a nuclear transcription factor led us to postulate that PDX1 may regulate expression of the mitochondrial transcription factor TFAM.…”

Section: Discussionsupporting

confidence: 55%

Oligonucleotide Microarray Analysis Reveals PDX1 as an Essential Regulator of Mitochondrial Metabolism in Rat Islets

Gauthier¹,

Brun²,

Sarret³

et al. 2004

Mutations in the transcription factor IPF1/PDX1 have been associated with type 2 diabetes. To elucidate ␤-cell dysfunction, PDX1 was suppressed by transduction of rat islets with an adenoviral construct encoding a dominant negative form of PDX1. After 2 days, there was a marked inhibition of insulin secretion in response to glucose, leucine, and arginine. Increasing cAMP levels with forskolin and isobutylmethylxanthine restored glucose-stimulated insulin secretion, indicating normal capacity for exocytosis. To identify molecular targets implicated in the altered metabolism secretion coupling, DNA microarray analysis was performed on PDX1-deficient and control islets. Of the 2640 detected transcripts, 70 were up-regulated and 56 were down-regulated. Transcripts were subdivided into 12 clusters; the most prevalent were associated with metabolism. Quantitative reverse transcriptase-PCR confirmed increases in succinate dehydrogenase and ATP synthase mRNAs as well as pyruvate carboxylase and the transcript for the malate shuttle. In parallel there was a 50% reduction in mRNA levels for the mitochondrially encoded nd1 gene, a subunit of the NADH dehydrogenase comprising complex I of the mitochondrial respiratory chain. As a consequence, total cellular ATP concentration was drastically decreased by 75%, and glucose failed to augment cytosolic ATP, explaining the blunted glucose-stimulated insulin secretion. Rotenone, an inhibitor of complex I, mimicked this effect. Surprisingly, TFAM, a nuclear-encoded transcription factor important for sustaining expression of mitochondrial genes, was down-regulated in islets expressing DN79PDX1. In conclusion, loss of PDX1 function alters expression of mitochondrially encoded genes through regulation of TFAM leading to impaired insulin secretion.Type 2 diabetes mellitus is a common severe disease of intermediary metabolism usually caused by both ␤-cell dysfunction and resistance to the biological actions of insulin on its main target tissues (liver, muscle, and fat). The susceptibility for type 2 diabetes is inherited, but single diabetes genes have only been identified in about 5% of cases. Mutations in one of these genes, the homeodomain transcription factor ipf1 (also known as, pdx1, idx1, or stf1), have been associated with a rare form of maturity onset diabetes of the young (MODY4) 1 as well as predisposing individuals to late onset type 2 diabetes (1-5). Furthermore, homozygous null mutations in the ipf1 gene result in pancreas agenesis indicating that this transcription factor is indispensable for both pancreas development and subsequent ␤-cell function (6, 7).As in humans, inactivation of both pdx1 alleles in murine models results in pancreas agenesis, whereas heterozygous mice or animals carrying a ␤-cell-specific deletion of the gene exhibit impaired glucose tolerance (8 -11). Several of these models also develop overt diabetes with age indicating a progressive deterioration of ␤-cell function and/or mass. A recent study concluded that reduction in PDX1 expression impedes p...

“…Sustained glucose stimulation in islet ␤-cells results in export of intermediates such as malate, citrate, isocitrate, and ␣KG from the mitochondria to the cytosol (33)(34)(35). Recent work has begun to provide evidence that this export of mitochondrial substrates is involved in the regulation of insulin secretion (3,15,20,21,32,36,37). For example, we previously demonstrated a strong correlation between PC-mediated pyruvate exchange with tricarboxylic acid cycle intermediates (pyruvate cycling) and GSIS in INS-1-derived cell lines with varying capacities for glucose response (15), and we also showed that lipid-induced impairment of GSIS results in ablation of the normal glucoseinduced rise in pyruvate cycling activity (21).…”

Section: Discussionmentioning

confidence: 99%

A Pyruvate Cycling Pathway Involving Cytosolic NADP-dependent Isocitrate Dehydrogenase Regulates Glucose-stimulated Insulin Secretion

Ronnebaum

Ilkayeva

Burgess

et al. 2006

210

305

Glucose-stimulated insulin secretion (GSIS) from pancreatic islet ␤-cells is central to control of mammalian fuel homeostasis.Glucose metabolism mediates GSIS in part via ATP-regulated K ؉ (K ATP ) channels, but multiple lines of evidence suggest participation of other signals. Here we investigated the role of cytosolic NADP-dependent isocitrate dehydrogenase (ICDc) in control of GSIS in ␤-cells. Delivery of small interfering RNAs specific for ICDc caused impairment of GSIS in two independent robustly glucose-responsive rat insulinoma (INS-1-derived) cell lines and in primary rat islets. Suppression of ICDc also attenuated the glucose-induced increments in pyruvate cycling activity and in NADPH levels, a predicted by-product of pyruvate cycling pathways, as well as the total cellular NADP(H) content. Metabolic profiling of eight organic acids in cell extracts revealed that suppression of ICDc caused increases in lactate production in both INS-1-derived cell lines and primary islets, consistent with the attenuation of pyruvate cycling, with no significant changes in other intermediates. Based on these studies, we propose that a pyruvate cycling pathway involving ICDc plays an important role in control of GSIS.Glucose metabolism in pancreatic islet ␤-cells generates signals for acute stimulation of insulin secretion, and it is widely accepted that an increase in the ATP:ADP ratio brought about by glucose flux through glycolysis and the tricarboxylic acid cycle is central to this process. The rise in the ATP:ADP ratio results in closure of ATP-regulated K ϩ (K ATP ) 2 channels, plasma membrane depolarization, activation of voltage-dependent Ca 2ϩ channels, and subsequent influx of Ca 2ϩ to stimulate insulin release (1, 2). This K ATP channel-dependent pathway has been suggested to be especially important for the acute first phase of glucose-stimulated insulin secretion (GSIS). In the second and more prolonged phase of GSIS, glucose-derived factors in addition to ATP have been implicated (3, 4), including glutamate (5), malonyl-CoA/cytosolic long chain acyl-CoA esters (6 -9), and transport of NADH reducing equivalents into the mitochondria via hydrogen shuttles (10), but data arguing against a role for some of these events have also been presented (11-13).Pyruvate carboxylase (PC), which catalyzes the conversion of pyruvate to oxaloacetate, is highly active in ␤-cells and accounts for ϳ40 -50% of pyruvate entry into mitochondrial metabolism at stimulatory glucose concentrations (14 -17). The high PC activity in ␤-cells is remarkable in light of the absence of gluconeogenesis (18) and relatively low lipogenic activity (19) in these cells. Furthermore, it has been estimated that only 25% of the glucose-derived carbons entering the tricarboxylic acid cycle via PC are channeled into protein synthesis (17). These findings suggest that PC-catalyzed entry of metabolites into mitochondrial metabolic pathways (anaplerosis) may play other roles in ␤-cell function.Previously, we examined glucose flux in a set of INS-derived ce...