malonyl-coenzyme-a and Hyperglycemia

Hepatic steatosis is a hallmark of nonalcoholic fatty liver disease (NAFLD) and a key component of obesity-associated metabolic dysfunctions featuring dyslipidemia, insulin resistance, and loss of glycemic control. It has yet to be completely understood how much dysregulated de novo lipogenesis contributes to the pathogenic development of hepatic steatosis and insulin resistance. ATP-citrate lyase (ACL) is a lipogenic enzyme that catalyzes the critical reaction linking cellular glucose catabolism and lipogenesis, converting cytosolic citrate to acetyl-coenzyme A (CoA). Acetyl-CoA is further converted to malonyl-CoA, the essential precursor for fatty acid biosynthesis. We investigated whether dysregulation of hepatic ACL is metabolically connected to hepatic steatosis, insulin resistance, and hyperglycemia. We found that in leptin receptor-deficient db/db mice, the expression of ACL was selectively elevated in the liver but not in the white adipose tissue. Liver-specific ACL abrogation via adenovirus-mediated RNA interference prominently reduced the hepatic contents of both acetyl-CoA and malonyl-CoA, markedly inhibited hepatic de novo lipogenesis, and protected against hepatic steatosis in db/db mice. Surprisingly, liver-specific ACL abrogation markedly inhibited the expression of peroxisome proliferator-activated receptor-gamma and the entire lipogenic program in the liver. Moreover, hepatic ACL deficiency resulted in significantly down-regulated expression of gluconeogenic genes in the liver as well as enhanced insulin sensitivity in the muscle, leading to substantially improved systemic glucose metabolism.. These findings establish a crucial role of hepatic ACL in lipid and glucose metabolism; therefore, hepatic ACL may serve as a potential target to treat NAFLD and type 2 diabetes.

Topics: Acetyl Coenzyme A; Animals; ATP Citrate (pro-S)-Lyase; Fatty Acids, Nonesterified; Fatty Liver; Glucose; Homeostasis; Hyperglycemia; Insulin Resistance; Lipid Metabolism; Lipogenesis; Liver; Male; Malonyl Coenzyme A; Mice; Mice, Inbred C57BL; Mice, Knockout; Receptors, Leptin; RNA Interference

Myocardial fatty acid oxidation is regulated by carnitine palmitoyltransferase I (CPT I), which is inhibited by malonyl-CoA. Increased cardiac power causes a fall in malonyl-CoA content and accelerated fatty acid oxidation; however, the mechanism for the decrease in malonyl-CoA is unclear. Malonyl-CoA is formed by acetyl-CoA carboxylase (ACC) and degraded by malonyl-CoA decarboxylase (MCD); thus a fall in malonyl-CoA could be due to activation of MCD, inhibition of ACC, or both. This study assessed the effects of increased cardiac power on malonyl-CoA content and ACC and MCD activities. Anesthetized pigs were studied under control conditions and during increased cardiac power in response to dobutamine infusion and aortic constriction alone, under hyperglycemic conditions, or with the CPT I inhibitor oxfenicine. An increase in cardiac power was accompanied by increased myocardial O(2) consumption, decreased malonyl-CoA concentration, and increased fatty acid oxidation. There were no differences among groups in activity of ACC or AMP-activated protein kinase (AMPK), which physiologically inhibits ACC. There also were no differences in V(max) or K(m) of MCD. Previous studies have demonstrated that AMPK can be inhibited by protein kinase B (PKB); however, PKB was activated by dobutamine and the elevated insulin that accompanied hyperglycemia, but there was no effect on AMPK activity. In conclusion, the fall in malonyl-CoA and increase in fatty acid oxidation that occur with increased cardiac work were not due to inhibition of ACC or activation of MCD, suggesting alternative regulatory mechanisms for the work-induced decrease in malonyl-CoA concentration.

Topics: AMP-Activated Protein Kinase Kinases; AMP-Activated Protein Kinases; Animals; Blood Pressure; Cardiotonic Agents; Dobutamine; Fatty Acids; Heart Rate; Hyperglycemia; Malonyl Coenzyme A; Multienzyme Complexes; Myocardial Contraction; Myocardium; Oxidation-Reduction; Oxygen Consumption; Phosphorylation; Protein Kinases; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; Sus scrofa

During diabetes, impaired glucose transport and utilization by the heart switches energy production to exclusive beta-oxidation of fatty acid (FA). In the current study, we examined the contribution of cardiac lipoprotein lipase (LPL) towards providing FA to the diabetic heart. Streptozotocin (STZ) caused an augmentation of LPL activity at the coronary lumen, an effect duplicated by diazoxide (DZ). With DZ, the amplification of LPL at the coronary luminal surface was determined to be exceptionally rapid. Interestingly, unlike DZ, the capability of hearts from STZ animals to maintain this amplified LPL activity was sustained in vitro. This increased enzyme in the hyperglycemic heart is likely unrelated to an increase in the number of capillary endothelial LPL-binding sites. Our data imply that binding sites for LPL in the control rat heart are only partly occupied by the enzyme and diabetes rapidly initiates filling of all of these sites. Phloridzin treatment of STZ animals normalized plasma glucose with no effect on luminal LPL suggesting that the effects of diabetes on LPL are also largely independent of changes in blood glucose. Both 2 and 8 U of insulin normalized plasma glucose in DZ-treated animals but only 8 U reversed DZ-induced augmentation of cardiac luminal LPL. Our data suggest that impaired intracellular glucose utilization allows rapid vectorial transfer of LPL to unoccupied binding sites to supply the diabetic heart with excess FA. The persistence of increased coronary luminal LPL even in a setting of normoglycemia may provide excessive FA to the diabetic heart with deleterious consequences over the long term.

Topics: Acute Disease; Animals; Binding Sites; Blood Glucose; Diabetes Mellitus, Experimental; Dichloroacetic Acid; Endothelial Cells; Endothelium, Vascular; Fatty Acids, Nonesterified; Gene Expression Regulation, Enzymologic; Hyperglycemia; Insulin; Lipolysis; Lipoprotein Lipase; Male; Malonyl Coenzyme A; Milk; Perfusion; Phlorhizin; Pyruvate Dehydrogenase Complex; Rats; Rats, Wistar; Streptozocin; Time Factors

Physiological hyperglycemia with hyperinsulinemia reduces fat oxidation in skeletal muscle. The mechanism responsible for this decrease in fat oxidation in human muscle is not known and may contribute to the development of insulin resistance. We hypothesized that the transfer of long-chain fatty acids (LCFAs) into the mitochondria via carnitine palmitoyltransferase-1 (CPT-1) is inhibited by increased malonyl coenzyme A (malonyl-CoA) (a known potent inhibitor of CPT-1) in human muscle during hyperglycemia with hyperinsulinemia. We studied six healthy subjects after an overnight fast and during an induced 5-hour period of hyperglycemia with hyperinsulinemia. Muscle fatty acid oxidation was calculated using stable isotope methodology combined with blood sampling from the femoral artery and vein of one leg. Muscle functional CPT-1 activity was assessed by concurrently infusing an LCFA tracer and a CPT-independent medium-chain fatty acid tracer. Muscle biopsies were obtained from the vastus lateralis after the periods of fasting and hyperglycemia with hyperinsulinemia. Hyperglycemia with hyperinsulinemia decreased LCFA oxidation, but had no effect on LCFA uptake or medium-chain fatty acid oxidation across the leg. Malonyl-CoA concentration significantly increased from 0.13 +/- 0.01 to 0.35 +/- 0.07 nmol/g during hyperglycemia with hyperinsulinemia. We conclude that hyperglycemia with hyperinsulinemia increases malonyl-CoA, inhibits functional CPT-1 activity, and shunts LCFA away from oxidation and toward storage in human muscle.

Topics: Adult; Blood Glucose; Body Constitution; Calorimetry, Indirect; Carnitine O-Palmitoyltransferase; Cholesterol; Fatty Acids; Fatty Acids, Nonesterified; Female; Glucose; Glucose Tolerance Test; Humans; Hyperglycemia; Hyperinsulinism; Male; Malonyl Coenzyme A; Muscle, Skeletal; Reference Values; Triglycerides

Apoptosis has been observed in vascular cells, nerve, and myocardium of diabetic humans and experimental animals, although whether it contributes to or is a marker of complications in these tissues is unclear. Previous studies have shown that incubation of human umbilical vein endothelial cells (HUVECs) with 30 vs. 5 mmol/l glucose for 72 h causes a significant increase in apoptosis, possibly related to an increase in oxidative stress. We report here that this increase in apoptosis (assessed morphologically by TdT-mediated dUTP nick- end labeling staining) is preceded (24 h of incubation) by inhibition of fatty acid oxidation, by increases in diacylglycerol synthesis, the concentration of malonyl CoA, and caspase-3 activity, and by decreases in mitochondrial membrane potential and cellular ATP content. In addition, the phosphorylation of Akt in the presence of 150 microU/ml insulin was impaired. No increases in ceramide content or its de novo synthesis were observed. AMP-activated protein kinase (AMPK) activity was not diminished; however, incubation with the AMPK activator 5-aminoimidazole-4-carboxamide-riboside increased AMPK activity twofold and completely prevented all of these changes. Likewise, expression of a constitutively active AMPK in HUVEC prevented the increase in caspase-3 activity. The results indicate that alterations in fatty-acid metabolism, impaired Akt activation by insulin, and increased caspase-3 activity precede visible evidence of apoptosis in HUVEC incubated in a hyperglycemic medium. They also suggest that AMPK could play an important role in protecting the endothelial cell against the adverse effects of sustained hyperglycemia.

Topics: Aminoimidazole Carboxamide; AMP-Activated Protein Kinases; Apoptosis; Cells, Cultured; Diglycerides; Endothelium, Vascular; Enzyme Activation; Fatty Acids, Nonesterified; Glucose; Humans; Hyperglycemia; In Situ Nick-End Labeling; Intracellular Membranes; Malonyl Coenzyme A; Membrane Potentials; Mitochondria; Multienzyme Complexes; Oxidation-Reduction; Protein Serine-Threonine Kinases; Ribonucleotides; Umbilical Veins

Chronic glucose infusion results in hyperinsulinemia and causes lipid accumulation and insulin resistance in rat muscle. To examine possible mechanisms for the insulin resistance, alterations in malonyl-CoA and long-chain acyl-CoA (LCA-CoA) concentration and the distribution of protein kinase C (PKC) isozymes, putative links between muscle lipids and insulin resistance, were determined. Cannulated rats were infused with glucose (40 mg. kg(-1). min(-1)) for 1 or 4 days. This increased red quadriceps muscle LCA-CoA content (sum of 6 species) by 1.3-fold at 1 day and 1.4-fold at 4 days vs. saline-infused controls (both P < 0.001 vs. control). The concentration of malonyl-CoA was also increased (1.7-fold at 1 day, P < 0.01, and 2.2-fold at 4 days, P < 0.001 vs. control), suggesting an even greater increase in cytosolic LCA-CoA. The ratio of membrane to cytosolic PKC-epsilon was increased twofold in the red gastrocnemius after both 1 and 4 days, suggesting chronic activation. No changes were observed for PKC-alpha, -delta, and -theta. We conclude that LCA-CoAs accumulate in muscle during chronic glucose infusion, consistent with a malonyl-CoA-induced inhibition of fatty acid oxidation (reverse glucose-fatty acid cycle). Accumulation of LCA-CoAs could play a role in the generation of muscle insulin resistance by glucose oversupply, either directly or via chronic activation of PKC-epsilon.

Topics: Acyl Coenzyme A; Animals; Blood Glucose; Glucose; Hyperglycemia; Hyperinsulinism; Insulin; Insulin Resistance; Isoenzymes; Lipid Metabolism; Male; Malonyl Coenzyme A; Muscle, Skeletal; Protein Kinase C; Protein Kinase C-alpha; Protein Kinase C-delta; Protein Kinase C-epsilon; Protein Kinase C-theta; Rats; Rats, Wistar; Subcellular Fractions