The metabolic syndrome

Obesity is associated with an increased risk of developing insulin resistance and type 2 diabetes In obese individuals, adipose tissue releases increased amounts of non-esterified fatty acids, glycerol, hormones, pro-inflammatory cytokines and other factors that are involved in the development of insulin resistance When insulin resistance is accompanied by dysfunction of pancreatic islet beta-cells - the cells that release insulin - failure to control blood glucose levels results Abnormalities in beta-cell function are therefore critical in defining the risk and development of type 2 diabetes This knowledge is fostering exploration of the molecular and genetic basis of the disease and new approaches to its treatment and prevention

Mechanisms linking obesity to insulin resistance and type 2 diabetes

Methods for assessment, e.g., anthropometric indicators and imaging techniques, of several phenotypes of human obesity, with special reference to abdominal fat content, have been evaluated. The correlation of fat distribution with age, gender, total body fat, energy balance, adipose tissue lipoprotein lipase and lipolytic activity, adipose tissue receptors, and genetic characteristics are discussed. Several secreted or expressed factors in the adipocyte are evaluated in the context of fat tissue localization. The body fat distribution and the metabolic profile in nonobese and obese individuals is discussed relative to lipolysis, antilypolysis and lipogenesis, insulin sensitivity, and glucose, lipid, and protein metabolism. Finally, the endocrine regulation of abdominal visceral fat in comparison with the adipose tissue localized in other areas is presented.

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Subcutaneous and visceral adipose tissue : Their relation to the metabolic syndrome

Accumulation of depolarized mitochondria within β-cells has been associated with oxidative damage and development of diabetes. To determine the source and fate of depolarized mitochondria, individual mitochondria were photolabeled and tracked through fusion and fission. Mitochondria were found to go through frequent cycles of fusion and fission in a ‘kiss and run' pattern. Fission events often generated uneven daughter units: one daughter exhibited increased membrane potential (Δψm) and a high probability of subsequent fusion, while the other had decreased membrane potential and a reduced probability for a fusion event. Together, this pattern generated a subpopulation of non-fusing mitochondria that were found to have reduced Δψm and decreased levels of the fusion protein OPA1. Inhibition of the fission machinery through DRP1K38A or FIS1 RNAi decreased mitochondrial autophagy and resulted in the accumulation of oxidized mitochondrial proteins, reduced respiration and impaired insulin secretion. Pulse chase and arrest of autophagy at the pre-proteolysis stage reveal that before autophagy mitochondria lose Δψm and OPA1, and that overexpression of OPA1 decreases mitochondrial autophagy. Together, these findings suggest that fission followed by selective fusion segregates dysfunctional mitochondria and permits their removal by autophagy.

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Fission and selective fusion govern mitochondrial segregation and elimination by autophagy

In both type 1 and type 2 diabetes, the late diabetic complications in nerve, vascular endothelium, and kidney arise from chronic elevations of glucose and possibly other metabolites including free fatty acids (FFA). Recent evidence suggests that common stress-activated signaling pathways such as nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases underlie the development of these late diabetic complications. In addition, in type 2 diabetes, there is evidence that the activation of these same stress pathways by glucose and possibly FFA leads to both insulin resistance and impaired insulin secretion. Thus, we propose a unifying hypothesis whereby hyperglycemia and FFA-induced activation of the nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases stress pathways, along with the activation of the advanced glycosylation end-products/receptor for advanced glycosylation end-products, protein kinase C, and sorbitol stress pathways, plays a key role in causing late complications in type 1 and type 2 diabetes, along with insulin resistance and impaired insulin secretion in type 2 diabetes. Studies with antioxidants such as vitamin E, alpha-lipoic acid, and N-acetylcysteine suggest that new strategies may become available to treat these conditions.

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Oxidative Stress and Stress-Activated Signaling Pathways: A Unifying Hypothesis of Type 2 Diabetes

The kinetics of carnitine palmitoyltransferase I (CPT I; EC 2.3.1.21) were examined in mitochondria from rat liver, heart and skeletal muscle as a function of pH over the range 6.8-7.6. In all three tissues raising the pH resulted in a fall in the Km for carnitine, no change in the Km for palmitoyl-CoA or Octanoyl-CoA, and a marked decrease in the inhibitory potency of malonyl-CoA. Studies with skeletal-muscle mitochondria established that increasing pH was accompanied by an increase in the Kd of the malonyl-CoA binding site for this ligand, coupled with a decrease in the Kd for fatty acyl-CoA species to compete for malonyl-CoA binding. Three principal conclusions are drawn. (1) The pH-induced shift in malonyl-CoA sensitivity of CPT I is not a phenomenon restricted to liver mitochondria. (2) At any given pH within the range tested, the ability of malonyl-CoA (and closely related compounds) to inhibit enzyme activity is governed by the efficiency of their binding to the malonyl-CoA site. (3) The competitive interaction between fatty acyl-CoA substrates and malonyl-CoA as regards CPT I activity is exerted at the malonyl-CoA binding site. Finally, the possibility is strengthened that the malonyl-CoA binding site is distinct from the active site of CPT I.

Effects of pH on the interaction of substrates and malonyl-CoA with mitochondrial carnitine palmitoyltransferase I.

The effect of (+)-decanoylcarnitine, a potent inhibitor of long-chain acylcarnitine transferase, was tested for its ability to inhibit hepatic ketogenesis both in the isolated perfused liver and in vivo in severely ketotic alloxan diabetic rats. In vitro the inhibitor caused an almost complete block in ketone body production. In vivo (+)-decanoylcarnitine caused a rapid reversal of ketosis under conditions where large doses of insulin had little effect. A combination of the two agents produced an even more striking fall in plasma ketone levels.

While (+)-decanoylcarnitine alone had no effect on plasma glucose levels it enhanced the hypoglycemic effect of insulin in anesthetized animals. Loss of this effect was noted in nonanesthetized animals, possibly as a result of increased muscle activity.

Studies in the isolated perfused liver indicated that the blockade of fatty acid oxidation and ketogenesis produced by (+)-decanoylcarnitine was rapidly reversible upon removal of the inhibitor.

/pdf/acute-reversal-of-experimental-diabetic-ketoacidosis-in-the-1jxy0puiju.pdf

Acute Reversal of Experimental Diabetic Ketoacidosis in the Rat with (+)-Decanoylcarnitine

In insulin-deprived alloxan-induced diabetic dogs with severe hyperglycemia and marked hyperglucagonemia, glucagon was not suppressed by intravenous infusion of glucose at a progressively increasing rate up to 24 mg/kg of body weight per min. However, when the hyperglycemia was corrected by phloridzin, a blocker of renal tubular glucose reabsorption, the hyperglucagonemia was readily suppressed by as little as 2 mg of glucose per kg/min. Direct perfusion of phloridzin into the isolated pancreas of nondiabetic dogs had no effect on the in vitro glucagon response to increments in glucose. However, in pancreata isolated from dogs whose glucose levels had been lowered by phloridzin pretreatment, in vitro glucagon suppression in response to glucose increments was more than twice that of controls. This enhancing effect of phloridzin treatment was completely abolished by giving an intravenous infusion of glucose for the 5 hr prior to surgery for isolation of the pancreas. It is concluded that (i) alpha cells have a glucose-sensing system that is independent of insulin and beta cells, and (ii) this system is reversibly attenuated by hyperglycemia. Thus, hyperglycemia, a metabolic consequence of islet cell dysfunction, may be a self-exacerbating inducer of further islet cell dysfunction, a possibility with implications for human diabetes.

https://www.pnas.org/content/pnas/82/5/1544.full.pdf

Correction of hyperglycemia with phloridzin restores the glucagon response to glucose in insulin-deficient dogs: implications for human diabetes.

The relationship between the concentration of carnitine and the oxidation of oleate was examined in homogenates prepared from skeletal muscle, liver, kidney, and heart of the rat, and from canine and human skeletal muscle. the carnitine content of these tissues in situ spanned a wide range, from about 0.1 mumol per gram in rat liver to about 3.0 mumol per gram in human muscle. The concentration of carnitine required for half-maximal rates of fatty acid oxidation in vitro also varied greatly (10 to 15 microM for rat liver to 200 to 400 microM for human muscle), and in rough proportion to the normal carnitine content of the tissues. For any given tissue, the carnitine content seems to be set at a level necessary for optimal rates of fatty acid oxidation. The data provide a plausible explanation for the fact that muscle fatty acid metabolism is severely impaired in the syndrome of human carnitine deficiency, since measured carnitine levels are in the range expected to limit substantially the capacity for fatty acid oxidation.

Kinetics of carnitine‐dependent fatty acid oxidation: Implications for human carnitine deficiency

The mitochondrial outer membrane enzyme carnitine palmitoyltransferase I (CPT I) plays a major role in the regulation of fatty acid entry into the mitochondrial matrix for beta-oxidation by virtue of its inhibition by malonyl-CoA. Two isoforms of CPT I, the liver type (L) and muscle type (M), have been identified, the latter being 100 times more sensitive to malonyl-CoA and having a much higher Km for the substrate carnitine. Here we have examined the roles of different regions of the CPT I molecules in their response to malonyl-CoA, etomoxir (an irreversible inhibitor) and carnitine. To this end, we analysed the properties of engineered rat CPT I constructs in which (a) the N-terminal domain of L-CPT I was deleted, (b) the N-terminal domains of L- and M-CPT I were switched, or (c) each of three conserved histidine residues located towards the N-terminus in L-CPT I was mutated. Several novel points emerged: (1) whereas the N-terminal domain is critical for a normal malonyl-CoA response, it does not itself account for the widely disparate sensitivities of the liver and muscle enzymes to the inhibitor; (2) His-5 and/or His-140 probably play a direct role in the malonyl-CoA response, but His-133 does not; (3) the truncated, chimaeric and point- mutant variants of the enzyme all bound the covalent, active-site- directed ligand, etomoxir; and (4) only the most radical alteration of L-CPT I, i.e. deletion of the N-terminal 82 residues, affected the response to carnitine. We conclude that the N-terminal domain of CPT I plays an essential, but permissive, role in the inhibition of the enzyme by malonyl-CoA. By contrast, the larger C-terminal region dictates the degree of sensitivity to malonyl-CoA, as well as the response to carnitine; it is also sufficient for etomoxir binding. Additionally, further weight is added to the notion that one or more histidine residues may be involved in the CPT I-malonyl-CoA interaction.

Roles of the N- and C-terminal domains of carnitine palmitoyltransferase I isoforms in malonyl-CoA sensitivity of the enzymes: insights from expression of chimaeric proteins and mutation of conserved histidine residues.

J. D. McGarry

Papers

Effects of pH on the interaction of substrates and malonyl-CoA with mitochondrial carnitine palmitoyltransferase I.

Acute Reversal of Experimental Diabetic Ketoacidosis in the Rat with (+)-Decanoylcarnitine

Correction of hyperglycemia with phloridzin restores the glucagon response to glucose in insulin-deficient dogs: implications for human diabetes.

Kinetics of carnitine‐dependent fatty acid oxidation: Implications for human carnitine deficiency

Roles of the N- and C-terminal domains of carnitine palmitoyltransferase I isoforms in malonyl-CoA sensitivity of the enzymes: insights from expression of chimaeric proteins and mutation of conserved histidine residues.