Mitochondrial GTP regulates glucose-stimulated insulin secretion.

Nucleotide-specific isoforms of the tricarboxylic acid (TCA) cycle enzyme succinyl-CoA synthetase (SCS) catalyze substrate-level synthesis of mitochondrial GTP (mtGTP) and ATP (mtATP). While mtATP yield from glucose metabolism is coupled with oxidative phosphorylation and can vary, each molecule of glucose metabolized within pancreatic beta cells produces approximately one mtGTP, making mtGTP a potentially important fuel signal. In INS-1 832/13 cells and cultured rat islets, siRNA suppression of the GTP-producing pathway (DeltaSCS-GTP) reduced glucose-stimulated insulin secretion (GSIS) by 50%, while suppression of the ATP-producing isoform (DeltaSCS-ATP) increased GSIS 2-fold. Insulin secretion correlated with increases in cytosolic calcium, but not with changes in NAD(P)H or the ATP/ADP ratio. These data suggest a role for mtGTP in controlling pancreatic GSIS through modulation of mitochondrial metabolism, possibly involving mitochondrial calcium. Furthermore, in light of its tight coupling to TCA oxidation rates, mtGTP production may serve as an important molecular signal of TCA-cycle activity.

Prevention of hepatic steatosis and hepatic insulin resistance in mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 knockout mice.

In order to investigate the role of mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 (mtGPAT1) in the pathogenesis of hepatic steatosis and hepatic insulin resistance, we examined whole-body insulin action in awake mtGPAT1 knockout (mtGPAT1(-/-)) and wild-type (wt) mice after regular control diet or three weeks of high-fat feeding. In contrast to high-fat-fed wt mice, mtGPAT1(-/-) mice displayed markedly lower hepatic triacylglycerol and diacylglycerol concentrations and were protected from hepatic insulin resistance possibly due to a lower diacylglycerol-mediated PKC activation. Hepatic acyl-CoA has previously been implicated in the pathogenesis of insulin resistance. Surprisingly, compared to wt mice, mtGPAT1(-/-) mice exhibited increased hepatic insulin sensitivity despite an almost 2-fold elevation in hepatic acyl-CoA content. These data suggest that mtGPAT1 might serve as a novel target for treatment of hepatic steatosis and hepatic insulin resistance and that long chain acyl-CoA's do not mediate fat-induced hepatic insulin resistance in this model.

n-3 Fatty acids preserve insulin sensitivity in vivo in a peroxisome proliferator-activated receptor-alpha-dependent manner.

Recent studies have suggested that n-3 fatty acids, abundant in fish oil, protect against high-fat diet-induced insulin resistance through peroxisome proliferator-activated receptor (PPAR)-alpha activation and a subsequent decrease in intracellular lipid abundance. To directly test this hypothesis, we fed PPAR-alpha null and wild-type mice for 2 weeks with isocaloric high-fat diets containing 27% fat from either safflower oil or safflower oil with an 8% fish oil replacement (fish oil diet). In both genotypes the safflower oil diet blunted insulin-mediated suppression of hepatic glucose production (P < 0.02 vs. genotype control) and PEPCK gene expression. Feeding wild-type mice a fish oil diet restored hepatic insulin sensitivity (hepatic glucose production [HGP], P < 0.002 vs. wild-type mice fed safflower oil), whereas in contrast, in PPAR-alpha null mice failed to counteract hepatic insulin resistance (HGP, P = NS vs. PPAR-alpha null safflower oil-fed mice). In PPAR-alpha null mice fed the fish oil diet, safflower oil plus fish oil, hepatic insulin resistance was dissociated from increases in hepatic triacylglycerol and acyl-CoA but accompanied by a more than threefold increase in hepatic diacylglycerol concentration (P < 0.0001 vs. genotype control). These data support the hypothesis that n-3 fatty acids protect from high-fat diet-induced hepatic insulin resistance in a PPAR-alpha-and diacylglycerol-dependent manner.

Targeting foxo1 in mice using antisense oligonucleotide improves hepatic and peripheral insulin action.

Fasting hyperglycemia, a prominent finding in diabetes, is primarily due to increased gluconeogenesis. The transcription factor Foxo1 links insulin signaling to decreased transcription of PEPCK and glucose-6-phosphatase (G6Pase) and provides a possible therapeutic target in insulin-resistant states. Synthetic, optimized antisense oligonucleotides (ASOs) specifically inhibit Foxo1 expression. Here we show the effect of such therapy on insulin resistance in mice with diet-induced obesity (DIO). Reducing Foxo1 mRNA expression with ASO therapy in mouse hepatocytes decreased levels of Foxo1 protein and mRNA expression of PEPCK by 48 +/- 4% and G6Pase by 64 +/- 3%. In mice with DIO and insulin resistance, Foxo1 ASO therapy lowered plasma glucose concentration and the rate of basal endogenous glucose production. In addition, Foxo1 ASO therapy lowered both hepatic triglyceride and diacylglycerol content and improved hepatic insulin sensitivity. Foxo1 ASO also improved adipocyte insulin action. At a tissue-specific level, this manifested as improved insulin-mediated 2-deoxyglucose uptake and suppression of lipolysis. On a whole-body level, the result was improved glucose tolerance after an intraperitoneal glucose load and increased insulin-stimulated whole-body glucose disposal during a hyperinsulinemic-euglycemic clamp. In conclusion, Foxo1 ASO therapy improved both hepatic insulin and peripheral insulin action. Foxo1 is a potential therapeutic target for improving insulin resistance.

Phosphoinositide profiling in complex lipid mixtures using electrospray ionization mass spectrometry.

Phosphoinositides (phosphorylated derivatives of phosphatidylinositol, PI) are versatile intracellular signaling lipids whose occurrence in low concentrations complicates direct mass measurements. Here we present a sensitive method to detect, identify and quantify phosphatidylinositol phosphate (PIP) and phosphatidylinositol bisphosphate (PIP(2)) with different fatty acid compositions (phosphoinositide profiles) in total lipid extracts by electrospray ionization mass spectrometry (ESI-MS). Using this method, we detected elevated concentrations of PIP2 in human fibroblasts from patients with Lowe syndrome, a genetic disorder that affects phosphoinositide metabolism. Saccharomyces cerevisiae cells deficient in enzymes involved in PIP metabolism--Sac1p, a phosphoinositide phosphatase, and Vps34p and Pik1p, a PI 3-kinase and PI 4-kinase, respectively--showed not only different PIP concentrations but also differential changes in PIP profiles indicating metabolic and/or subcellular pooling. Mass spectrometric analysis of phosphoinositides offers unique advantages over existing approaches and may represent a powerful diagnostic tool for human diseases that involve defective phosphoinositide metabolism.

Mitochondrial glycerol-3-phosphate acyltransferase-1 is essential in liver for the metabolism of excess acyl-CoAs.

In vitro studies suggest that the mitochondrial glycerol-3-phosphate acyltransferase-1 (mtGPAT1) isoform catalyzes the initial and rate-controlling step in glycerolipid synthesis and aids in partitioning acyl-CoAs toward triacylglycerol synthesis and away from degradative pathways. To determine whether the absence of mtGPAT1 would increase oxidation of acyl-CoAs and restrict the development of hepatic steatosis, we fed wild type and mtGPAT1-/- mice a diet high in fat and sucrose (HH) for 4 months to induce the development of obesity and a fatty liver. Control mice were fed a diet low in fat and sucrose (LL). With the HH diet, absence of mtGPAT1 resulted in increased partitioning of acyl-CoAs toward oxidative pathways, demonstrated by 60% lower hepatic triacylglycerol content and 2-fold increases in plasma beta-hydroxybutyrate, acylcarnitines, and hepatic mRNA expression of mitochondrial HMG-CoA synthase. Despite the increase in fatty acid oxidation, liver acyl-CoA levels were 3-fold higher in the mtGPAT1-/- mice fed both diets. A lack of difference in CPT1 and FAS mRNA expression between genotypes suggested that the increased acyl-CoA content was not because of increased de novo synthesis, but instead, to an impaired ability to use long-chain acyl-CoAs derived from the diet, even when the dietary fat content was low. Hyperinsulinemia and reduced glucose tolerance on the HH diet was greater in the mtGPAT1-/- mice, which did not suppress the expression of the gluconeogenic genes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. This study demonstrates that mtGPAT1 is essential for normal acyl-CoA metabolism, and that the absence of hepatic mtGPAT1 results in the partitioning of fatty acids away from triacylglycerol synthesis and toward oxidation and ketogenesis.

Mechanism of hepatic insulin resistance in non-alcoholic fatty liver disease.

Short term high fat feeding in rats results specifically in hepatic fat accumulation and provides a model of non-alcoholic fatty liver disease in which to study the mechanism of hepatic insulin resistance. Short term fat feeding (FF) caused a approximately 3-fold increase in liver triglyceride and total fatty acyl-CoA content without any significant increase in visceral or skeletal muscle fat content. Suppression of endogenous glucose production (EGP) by insulin was diminished in the FF group, despite normal basal EGP and insulin-stimulated peripheral glucose disposal. Hepatic insulin resistance could be attributed to impaired insulin-stimulated IRS-1 and IRS-2 tyrosine phosphorylation. These changes were associated with activation of PKC-epsilon and JNK1. Ultimately, hepatic fat accumulation decreased insulin activation of glycogen synthase and increased gluconeogenesis. Treatment of the FF group with low dose 2,4-dinitrophenol to increase energy expenditure abrogated the development of fatty liver, hepatic insulin resistance, activation of PKC-epsilon and JNK1, and defects in insulin signaling. In conclusion, these data support the hypothesis hepatic steatosis leads to hepatic insulin resistance by stimulating gluconeogenesis and activating PKC-epsilon and JNK1, which may interfere with tyrosine phosphorylation of IRS-1 and IRS-2 and impair the ability of insulin to activate glycogen synthase.