Adipose insulin

Dyslipidemia is a common accompaniment of the lipodystrophy syndrome observed in HIV-infected patients. This syndrome presents as a combination of peripheral lipoatrophy and the metabolic syndrome (central adiposity, insulin resistance, and dyslipidemia). The term lipodystrophy syndrome was first used in two case reports to describe a clinical picture of subcutaneous fat wasting in the face and limbs of HIV infected patients treated with indinavir, reminiscent of the rare congenital lipodystrophy syndromes (138,139). In addition, benign symmetric lipomatoses on the trunk and neck were described. A systematic study of this syndrome in the Australian HIV cohort showed co-existence of peripheral lipoatrophy with abdominal visceral obesity, dyslipidemia, and insulin resistance in HIV-infected patients with or without treatment with protease inhibitors (140). 

The absorption of sulfonylureas from the upper gastrointestinal tract is faidy rapid and complete. The agents are transported in the blood as protein-bound complexes. As they are released from protein-binding sites, the free (unbound) form becomes available for diffusion into tissues and to sites of action. Specific receptors are present on pancreatic islet P-ceU surfaces which bind sulfonylureas with high affinity. Binding of sulfonylureas to these receptors appears to be coupled to an ATP-sensitive channel to stimulate insulin secretion. These agents may also potentiate insulin-stimulated glucose transport in adipose tissue and skeletal muscle. 

Insulin is a peptide hormone, secreted by the pancreas, that regulates glucose metabolism in the body. Insufficient production of insulin or failure of insulin to stimulate target sites in liver, muscle, and adipose tissue leads to the serious metabolic disorder known as diabetes mellitus. Diabetes afflicts millions of people worldwide. Diabetic individuals typically exhibit high levels of glucose in the blood, but insulin injection therapy allows diabetic individuals to maintain normal levels of blood glucose. 

The first hormonal signal found to comply with the characteristics of both a satiety and an adiposity signal was insulin [1]. Insulin levels reflect substrate (carbohydrate) intake and stores, as they rise with blood glucose levels and fall with starvation. In addition, they may reflect the size of adipose stores, because a fatter person secretes more insulin than a lean individual in response to a given increase of blood glucose. This increased insulin secretion in obesity can be explained by the reduced insulin sensitivity of liver, muscle, and adipose tissue. Insulin is known to enter the brain, and direct administration of insulin to the brain reduces food intake. The adipostatic role of insulin is supported by the observation that mutant mice lacking the neuronal insulin receptor (NDRKO mice) develop obesity. 

PPARy is a transcription factor which controls the expression of enzymes and proteins involved in fat and glucose metabolism. More importantly, stimulation of this receptor induces differentiation of preadipocytes to adipose cells. It is believed that the formation of additional, small fat cells lowers free fatty acids and hepatic triglycerides, thereby collecting insulin resistance. 

At present, the only available drug that stimulates glucose transport is insulin. Insulin increases the abundance of the GLUT4 in plasma membranes of adipose and muscle cells by its recruitment from intracellular storage sites (for a detailed description of its mechanism, see Chapter Diabetes Mellitus). 

Insulin resistance occurs when the normal response to a given amount of insulin is reduced. Resistance of liver to the effects of insulin results in inadequate suppression of hepatic glucose production insulin resistance of skeletal muscle reduces the amount of glucose taken out of the circulation into skeletal muscle for storage and insulin resistance of adipose tissue results in impaired suppression of lipolysis and increased levels of free fatty acids. Therefore, insulin resistance is associated with a cluster of metabolic abnormalities including elevated blood glucose levels, abnormal blood lipid profile (dyslipidemia), hypertension, and increased expression of inflammatory markers (inflammation). Insulin resistance and this cluster of metabolic abnormalities is strongly associated with obesity, predominantly abdominal (visceral) obesity, and physical inactivity and increased risk for type 2 diabetes, cardiovascular and renal disease, as well as some forms of cancer. In addition to obesity, other situations in which insulin resistance occurs includes... 

Adipose Adipocyte differentiation f Proportion of small, insulin sensitive adipocytes... 

Adipose remodeling increase in subcutaneous adipose depots decrease in visceral depots l Proportion of hypertrophic, insulin resistant adipocytes... 

Altered expression and secretion of adipose tissue-derived factors thought to affect insulin sensitivity adiponectin, TNF-a, IL-6, etc. t Insulin sensitivity and glucose-lowering... 

PPARy White adipose tissue, atherosclerotic lesions Insulin-sensitizing and glucoselowering re-directs TG from non-adipose tissues and visceral adipose depots for storage in subcutaneous adipose tissue slowed progression of atherosclerosis Fatty acids, eico-sanoids Th iazolid i ned iones pioglitazone (Actos ), rosiglita-zone (Avandia ) Type 2 diabetes, (insulin resistance, metabolic syndrome)... 

Insulin appears to activate a process that helps glucose molecules enter the cells of striated muscle and adipose tissue Figure 49-1 depicts normal glucose metabolism. Insulin also stimulates die synthesis of glycogen by die liver. In addition, insulin promotes protein syntiiesis and helps the body store fat by preventing its breakdown for energy. 

The rate of mitochondrial oxidations and ATP synthesis is continually adjusted to the needs of the cell (see reviews by Brand and Murphy 1987 Brown, 1992). Physical activity and the nutritional and endocrine states determine which substrates are oxidized by skeletal muscle. Insulin increases the utilization of glucose by promoting its uptake by muscle and by decreasing the availability of free long-chain fatty acids, and of acetoacetate and 3-hydroxybutyrate formed by fatty acid oxidation in the liver, secondary to decreased lipolysis in adipose tissue. Product inhibition of pyruvate dehydrogenase by NADH and acetyl-CoA formed by fatty acid oxidation decreases glucose oxidation in muscle. 

GLUT 4 Heart and skeletal muscle, adipose tissue Insulin-stimulated uptake of glucose... 

Fatty acids are synthesized by an extramitochondrial system, which is responsible for the complete synthesis of palmitate from acetyl-CoA in the cytosol. In the rat, the pathway is well represented in adipose tissue and liver, whereas in humans adipose tissue may not be an important site, and liver has only low activity. In birds, lipogenesis is confined to the liver, where it is particularly important in providing lipids for egg formation. In most mammals, glucose is the primary substrate for lipogenesis, but in ruminants it is acetate, the main fuel molecule produced by the diet. Critical diseases of the pathway have not been reported in humans. However, inhibition of lipogenesis occurs in type 1 (insulin-de-pendent) diabetes mellitus, and variations in its activity may affect the nature and extent of obesity. 

Insulin stimulates lipogenesis by several other mechanisms as well as by increasing acetyl-CoA carboxylase activity. It increases the transport of glucose into the cell (eg, in adipose tissue), increasing the availability of both pyruvate for fatty acid synthesis and glycerol 3-phosphate for esterification of the newly formed fatty acids, and also converts the inactive form of pyruvate dehydrogenase to the active form in adipose tissue but not in liver. Insulin also—by its ability to depress the level of intracellular cAMP—inhibits lipolysis in adipose tissue and thereby reduces the concentration of... 

Figure 25-7. Metabolism of adipose tissue. Hormone-sensitive lipase is activated by ACTH, TSH, glucagon, epinephrine, norepinephrine, and vasopressin and inhibited by insulin, prostaglandin E, and nicotinic acid. Details of the formation of glycerol 3-phosphate from intermediates of glycolysis are shown in Figure 24-2. (PPP, pentose phosphate pathway TG, triacylglycerol FFA, free fatty acids VLDL, very low density lipoprotein.)...

A principal action of insufin in adipose tissue is to inhibit the activity of hormone-sensitive lipase, reducing the release not only of free fatty acids but of glycerol as well. Adipose tissue is much more sensitive to insulin than are many other tissues, which points to adipose tissue as a major site of insufin action in vivo. 

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