Endocannabinoid System Network (ECSN) - ADA SYMPOSIUM: METABOLIC CONTROL OF INSULIN SENSITIVITY

Insulin Action/Peroxide Formation

Barry J. Goldstein, MD, PhD

Hyperglycemia sharply increases the production of reactive oxygen species (ROS), which play a key role in endothelial dysfunction in diabetes. However, oxidants, including hydrogen peroxide (H₂O₂), mimic insulin action on glucose transport in adipose cells. Dr. Goldstein’s group recently reported that insulin-stimulated H₂O₂modulates proximal and distal insulin signaling, at least in part through the oxidative inhibition of protein tyrosine phosphatases that negatively regulate the insulin-action pathway.

Insulin stimulation elicits the rapid production of H₂O₂, which causes the oxidative inhibition of protein-tyrosine phosphatases and enhances the tyrosine phosphorylation of proteins in the early insulin-action cascade. The role of insulin-induced H₂O₂generation on downstream insulin signaling was investigated using diphenyleneiodonium (DPI), an inhibitor of cellular NADPH oxidase that blocks insulin-stimulated cellular H₂O₂production. DPI completely inhibited the activation of phosphatidylinositol (PI) 3-kinase activity by insulin and reduced the insulin-induced activation of the serine kinase Akt by up to 49%; these activities were restored when H₂O₂was added back to cells that had been pretreated with DPI. The H₂O₂-induced activation of Akt was entirely mediated by upstream PI 3-kinase activity, since treatment of cells with the PI 3-kinase inhibitors wortmannin or LY294002 completely blocked the subsequent activation of Akt by exogenous H₂O₂. Preventing oxidant generation with DPI also blocked insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane, providing further evidence for an oxidant signal in the regulation of the distal insulin-signaling cascade. In contrast to the cellular mechanism of H₂O₂generation by other growth factors, such as platelet-derived growth factor, insulin-stimulated cellular production of H₂O₂may occur through a unique pathway, independent of cellular PI 3-kinase activity. These data provide insight into the physiologic role of insulin-dependent H₂O₂generation, which is not only involved in the regulation of tyrosine phosphorylation events in the early insulin signaling cascade but also has important effects on the regulation of downstream insulin signaling, involving the activation of PI 3-kinase, Akt, and ultimately cellular glucose transport in response to insulin.

Glyceroneogenic Pathway

Richard W. Hanson, PhD

The concept of glyceroneogenesis began in 1966 with the discovery that the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK-C) is present in fat cells. Besides its role in gluconeogenesis, PEPCK-C is involved in glycerogenesis in adipose tissue and liver. Glycerogenesis is an abbreviated version of gluconeogenesis and provides the glycerol 3-phosphate necessary for fatty acid re-esterification.

During a fast, gluconeogenic precursors such as pyruvate are converted into the glycerol backbone of triacylglycerol, the major storage form of fat. Dr. Hanson and his colleagues coined the term glyceroneogenesis to describe this pathway. They proposed that glyceroneogenesis modulates fatty acid release from adipose tissue during fasting when the diet cannot provide sufficient glycerol-3-phosphate for triacylglycerol synthesis; further they showed that fasting induces PEPCK-C and glyceroneogenesis and that fatty acid release from fat tissue is reduced.

Recent studies raise the possibility that aberrant regulation of PEPCK-C in fat tissue may contribute to the development of type 2 diabetes. Any disorder that leads to a PEPCK-C deficiency in adipose tissue can enhance the rate of fatty acid release into the blood, which can lead to insulin resistance. Transgenic mice with increased adipose tissue glyceroneogenesis (caused by overexpression of PEPCK-C), show increased free fatty acid re-esterifiction and develop obesity but are insulin sensitive. However, overexpression of PEPCK-C in adipose tissue of transgenic mice fed a high-fat diet leads to high susceptibility to insulin resistance and obesity.

Mice administered rosiglitazone had a significant relative increase in glyceroneogenesis (from 17% to 53%) in adipose tissue. Similar results were obtained when mice were fed a low-carbohydrate diet. In addition, stimulation of glyceroneogenesis by rosiglitazone in adipose tissue may be an important factor in the antilipolytic actions of thiazolidinediones. There is strong evidence that glyceroneogenesis occurs to a significant degree in the liver, as liver-specific ablation of the PEPCK-C gene results in a fatty liver. In summary, these data support an important role for PEPCK-C in glycemic control.

Paracrine Effects in Adipose Tissue

Meredith Hawkins, MD

Obesity is a key factor in the development of insulin resistance, which in turn confers a heightened risk of diabetes mellitus and atherosclerosis. “Nutrient excess” refers to increased oral intake and circulating levels of glucose and free fatty acids. The hexosamine biosynthetic pathway (HBP) provides cellular "satiety" signals with increased availability of such nutrients as glucose and free fatty acids, and provides a potential mechanistic link between increased nutrients and the metabolic syndrome. Increased nutrient availability, by increasing HBP flux, enhances glycosylation of important intracellular factors, including transcription factors, and thereby affects the expression of many genes, including PAI-1. Increased circulating levels of PAI-1 accompany insulin resistance and probably contribute to the pathogenesis of atherothrombosis. The HBP is active in adipocytes, which are known to secrete PAI-1. Increasing plasma free fatty acid levels in lean, nondiabetic persons results in whole-body insulin resistance, about 2-fold increases in plasma PAI-1 levels, and dramatic increases in adipose PAI-1 gene expression. Thus, increases in plasma free fatty acid levels may contribute importantly to insulin resistance by exerting both direct metabolic effects and by altering circulating levels of certain adipocyte-derived proteins.

Thiazolidinediones can slow and may even prevent the progression of type 2 diabetes. Dr. Hawkin’s group showed in humans that pioglitazone induces 1) rapid and marked effects on hepatic insulin action after only 3 weeks, with more heterogeneous effects on peripheral insulin action and 2) marked increases in adiponectin, which correlated strongly with the improved hepatic insulin action. These effects preceded significant changes in plasma glucose, free fatty acid, or adipokine levels. The striking correlation of increased adiponectin with enhanced suppression of endogenous glucose production, but not with increased insulin-stimulated glucose uptake, suggests different insulin-sensitizing mechanisms of pioglitazone on liver and muscle. These findings could have important implications for the therapeutic use of thiazolidinediones in people with type 2 diabetes and for better understanding their mechanisms of action.

This website is sponsored by The University of Kansas School of Medicine-Wichita. The University of Kansas School of Medicine-Wichita is accredited by the ACCME to provide continuing medical education for physicians.


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		ADA SYMPOSIUM: METABOLIC CONTROL OF INSULIN SENSITIVITY

Insulin Action/Peroxide Formation Glyceroneogenic Pathway Paracrine Effects in Adipose Tissue	Print Version Insulin Action/Peroxide Formation Barry J. Goldstein, MD, PhD Hyperglycemia sharply increases the production of reactive oxygen species (ROS), which play a key role in endothelial dysfunction in diabetes. However, oxidants, including hydrogen peroxide (H₂O₂), mimic insulin action on glucose transport in adipose cells. Dr. Goldstein’s group recently reported that insulin-stimulated H₂O₂modulates proximal and distal insulin signaling, at least in part through the oxidative inhibition of protein tyrosine phosphatases that negatively regulate the insulin-action pathway. Insulin stimulation elicits the rapid production of H₂O₂, which causes the oxidative inhibition of protein-tyrosine phosphatases and enhances the tyrosine phosphorylation of proteins in the early insulin-action cascade. The role of insulin-induced H₂O₂generation on downstream insulin signaling was investigated using diphenyleneiodonium (DPI), an inhibitor of cellular NADPH oxidase that blocks insulin-stimulated cellular H₂O₂production. DPI completely inhibited the activation of phosphatidylinositol (PI) 3-kinase activity by insulin and reduced the insulin-induced activation of the serine kinase Akt by up to 49%; these activities were restored when H₂O₂was added back to cells that had been pretreated with DPI. The H₂O₂-induced activation of Akt was entirely mediated by upstream PI 3-kinase activity, since treatment of cells with the PI 3-kinase inhibitors wortmannin or LY294002 completely blocked the subsequent activation of Akt by exogenous H₂O₂. Preventing oxidant generation with DPI also blocked insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane, providing further evidence for an oxidant signal in the regulation of the distal insulin-signaling cascade. In contrast to the cellular mechanism of H₂O₂generation by other growth factors, such as platelet-derived growth factor, insulin-stimulated cellular production of H₂O₂may occur through a unique pathway, independent of cellular PI 3-kinase activity. These data provide insight into the physiologic role of insulin-dependent H₂O₂generation, which is not only involved in the regulation of tyrosine phosphorylation events in the early insulin signaling cascade but also has important effects on the regulation of downstream insulin signaling, involving the activation of PI 3-kinase, Akt, and ultimately cellular glucose transport in response to insulin. Glyceroneogenic Pathway Richard W. Hanson, PhD The concept of glyceroneogenesis began in 1966 with the discovery that the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK-C) is present in fat cells. Besides its role in gluconeogenesis, PEPCK-C is involved in glycerogenesis in adipose tissue and liver. Glycerogenesis is an abbreviated version of gluconeogenesis and provides the glycerol 3-phosphate necessary for fatty acid re-esterification. During a fast, gluconeogenic precursors such as pyruvate are converted into the glycerol backbone of triacylglycerol, the major storage form of fat. Dr. Hanson and his colleagues coined the term glyceroneogenesis to describe this pathway. They proposed that glyceroneogenesis modulates fatty acid release from adipose tissue during fasting when the diet cannot provide sufficient glycerol-3-phosphate for triacylglycerol synthesis; further they showed that fasting induces PEPCK-C and glyceroneogenesis and that fatty acid release from fat tissue is reduced. Recent studies raise the possibility that aberrant regulation of PEPCK-C in fat tissue may contribute to the development of type 2 diabetes. Any disorder that leads to a PEPCK-C deficiency in adipose tissue can enhance the rate of fatty acid release into the blood, which can lead to insulin resistance. Transgenic mice with increased adipose tissue glyceroneogenesis (caused by overexpression of PEPCK-C), show increased free fatty acid re-esterifiction and develop obesity but are insulin sensitive. However, overexpression of PEPCK-C in adipose tissue of transgenic mice fed a high-fat diet leads to high susceptibility to insulin resistance and obesity. Mice administered rosiglitazone had a significant relative increase in glyceroneogenesis (from 17% to 53%) in adipose tissue. Similar results were obtained when mice were fed a low-carbohydrate diet. In addition, stimulation of glyceroneogenesis by rosiglitazone in adipose tissue may be an important factor in the antilipolytic actions of thiazolidinediones. There is strong evidence that glyceroneogenesis occurs to a significant degree in the liver, as liver-specific ablation of the PEPCK-C gene results in a fatty liver. In summary, these data support an important role for PEPCK-C in glycemic control. Paracrine Effects in Adipose Tissue Meredith Hawkins, MD Obesity is a key factor in the development of insulin resistance, which in turn confers a heightened risk of diabetes mellitus and atherosclerosis. “Nutrient excess” refers to increased oral intake and circulating levels of glucose and free fatty acids. The hexosamine biosynthetic pathway (HBP) provides cellular "satiety" signals with increased availability of such nutrients as glucose and free fatty acids, and provides a potential mechanistic link between increased nutrients and the metabolic syndrome. Increased nutrient availability, by increasing HBP flux, enhances glycosylation of important intracellular factors, including transcription factors, and thereby affects the expression of many genes, including PAI-1. Increased circulating levels of PAI-1 accompany insulin resistance and probably contribute to the pathogenesis of atherothrombosis. The HBP is active in adipocytes, which are known to secrete PAI-1. Increasing plasma free fatty acid levels in lean, nondiabetic persons results in whole-body insulin resistance, about 2-fold increases in plasma PAI-1 levels, and dramatic increases in adipose PAI-1 gene expression. Thus, increases in plasma free fatty acid levels may contribute importantly to insulin resistance by exerting both direct metabolic effects and by altering circulating levels of certain adipocyte-derived proteins. Thiazolidinediones can slow and may even prevent the progression of type 2 diabetes. Dr. Hawkin’s group showed in humans that pioglitazone induces 1) rapid and marked effects on hepatic insulin action after only 3 weeks, with more heterogeneous effects on peripheral insulin action and 2) marked increases in adiponectin, which correlated strongly with the improved hepatic insulin action. These effects preceded significant changes in plasma glucose, free fatty acid, or adipokine levels. The striking correlation of increased adiponectin with enhanced suppression of endogenous glucose production, but not with increased insulin-stimulated glucose uptake, suggests different insulin-sensitizing mechanisms of pioglitazone on liver and muscle. These findings could have important implications for the therapeutic use of thiazolidinediones in people with type 2 diabetes and for better understanding their mechanisms of action.

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