The metabolic syndrome

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

The major focus of this Review is on the mechanisms of islet beta cell failure in the pathogenesis of obesity-associated type 2 diabetes (T2D). As this demise occurs within the context of beta cell compensation for insulin resistance, consideration is also given to the mechanisms involved in the compensation process, including mechanisms for expansion of beta cell mass and for enhanced beta cell performance. The importance of genetic, intrauterine, and environmental factors in the determination of "susceptible" islets and overall risk for T2D is reviewed. The likely mechanisms of beta cell failure are discussed within the two broad categories: those with initiation and those with progression roles.

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Islet β cell failure in type 2 diabetes

The "metabolic syndrome" (MetS) is a clustering of components that reflect overnutrition, sedentary lifestyles, and resultant excess adiposity. The MetS includes the clustering of abdominal obesity, insulin resistance, dyslipidemia, and elevated blood pressure and is associated with other comorbidities including the prothrombotic state, proinflammatory state, nonalcoholic fatty liver disease, and reproductive disorders. Because the MetS is a cluster of different conditions, and not a single disease, the development of multiple concurrent definitions has resulted. The prevalence of the MetS is increasing to epidemic proportions not only in the United States and the remainder of the urbanized world but also in developing nations. Most studies show that the MetS is associated with an approximate doubling of cardiovascular disease risk and a 5-fold increased risk for incident type 2 diabetes mellitus. Although it is unclear whether there is a unifying pathophysiological mechanism resulting in the MetS, abdominal adiposity and insulin resistance appear to be central to the MetS and its individual components. Lifestyle modification and weight loss should, therefore, be at the core of treating or preventing the MetS and its components. In addition, there is a general consensus that other cardiac risk factors should be aggressively managed in individuals with the MetS. Finally, in 2008 the MetS is an evolving concept that continues to be data driven and evidence based with revisions forthcoming.

The Metabolic Syndrome

Readers of this journal do not need to be reminded of the explosive increase in obesity/type 2 diabetes syndromes and their attendant staggering public health costs that are currently afflicting developed countries. Because of this alarming development and what it portends for the future if left unchecked, the American Diabetes Association, together with its counterparts around the world, as well as the World Health Organization, are faced with the daunting challenge of finding answers to two huge questions. First, what is the pathophysiological basis for these disorders? Second, why is their rate of appearance accelerating so rapidly at this particular juncture of human history, and what, if anything, can we do to intervene? The primary focus of this lecture will be on the first of these issues. I want to emphasize at the outset that I do not plan to dwell exclusively on our own studies in this area. Instead, I shall try to incorporate recent developments from a variety of other laboratories working in the field. Although these will of necessity have to be highly selective, I hope that collectively they will illustrate how we have come to a relatively new and exciting way of thinking about the etiology of type 2 diabetes.

It has been said in literary circles that James Joyce’s Ulysses is probably the most frequently opened and least read book that has ever been published. In a similar vein, I sometimes think that the question of what causes type 2 diabetes might be one of the most frequently asked and least satisfactorily answered in the history of diabetes research. This is not meant to be an arrogant statement, nor indeed does it imply that I have a magical solution to the question at hand. Rather, it simply reflects the fact that at the dawn of …

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Banting Lecture 2001: Dysregulation of Fatty Acid Metabolism in the Etiology of Type 2 Diabetes

Hyperinsulinemia, loss of glucose-stimulated insulin secretion (GSIS), and peripheral insulin resistance coexist in non-insulin-dependent diabetes mellitus (NIDDM). Because free fatty acids (FFA) can induce these same abnormalities, we studied their role in the pathogenesis of the NIDDM of obese Zucker diabetic fatty (ZDF-drt) rats from 5 weeks of age (before the onset of hyperglycemia) until 14 weeks. Two weeks prior to hyperglycemia, plasma FFA began to rise progressively, averaging 1.9 +/- 0.06 mM at the onset of hyperglycemia (P < 0.001 vs. controls). At this time GSIS was absent and beta-cell GLUT-2 glucose transporter was decreased. The triacylglycerol content of prediabetic islets rose to 10 times that of controls and was correlated with plasma FFA (r = 0.825; P < 0.001), which, in turn, was correlated with the plasma glucose concentration (r = 0.873; P < 0.001). Reduction of hyperlipacidemia to 1.3 +/- 0.07 mM by pair feeding with lean littermates reduced all beta-cell abnormalities and prevented hyperglycemia. Normal rat islets that had been cultured for 7 days in medium containing 2 mM FFA exhibited increased basal insulin secretion at 3 mM glucose, and first-phase GSIS was reduced by 68%; in prediabetic islets, first-phase GSIS was reduced by 69% by FFA. The results suggest a role for hyperlipacidemia in the pathogenesis of NIDDM; resistance to insulin-mediated antilipolysis is invoked to explain the high FFA despite hyperinsulinemia, and sensitivity of beta cells to hyperlipacedemia is invoked to explain the FFA-induced loss of GSIS.

https://www.pnas.org/content/pnas/91/23/10878.full.pdf

Beta-cell lipotoxicity in the pathogenesis of non-insulin-dependent diabetes mellitus of obese rats: impairment in adipocyte-beta-cell relationships

Obesity is associated with both insulin resistance and hyperinsulinemia. Initially hyperinsulinemia compensates for the insulin resistance and thereby maintains normal glucose homeostasis. Obesity ...

Tissue triglycerides, insulin resistance, and insulin production: implications for hyperinsulinemia of obesity

Leptin was overexpressed in the liver of normal Wistar rats by infusing recombinant adenovirus containing the cDNA encoding leptin. Plasma leptin levels rose to 12-24 ng/ml (vs. <2 ng/ml in control rats), and food intake and body weight fell. Visible fat disappeared within 7 days. Plasma insulin fell to <50% of normal in association with hypoglycemia, suggesting enhanced insulin sensitivity. Although beta-cells appeared histologically normal, the pancreases were unresponsive to perfusion with stimulatory levels of glucose and arginine. Since islet triglyceride content was 0, compared with 14 ng/islet in pair-fed control rats, we coperfused a 2:1 oleate:palmitate mixture (0.5 mmol/l). This restored insulin responses to supranormal levels. When normal islets were cultured with 20 ng/ml of leptin, they too became triglyceride-depleted and failed to respond when perifused with glucose or arginine. Perifusion of fatty acids restored both responses. We conclude that in normal rats, hyperleptinemia for 2 weeks causes reversible beta-cell dysfunction by depleting tissue lipids, thereby depriving beta-cells of a lipid-derived signal required for the insulin response to other fuels.

β-Cell Function in Normal Rats Made Chronically Hyperleptinemic by Adenovirus-Leptin Gene Therapy

The present invention is directed to methods and compositions for preventing β-cell destruction and for ameliorating obesity related NIDDM. More particularly the present inventors have found that β-cell destruction occurs in obesity related NIDDM through the action of ceramides. The present invention suggests that ceramides mediate their cytotoxic influence through increased NO production. More particularly, the active agents for preventing β-cell destruction in obesity related NIDDM are troglitazone, leptin, triacsin and fumosinins, and no-production inhibitors, like aminoguanidines.

Roger H Unger

Papers

Beta-cell lipotoxicity in the pathogenesis of non-insulin-dependent diabetes mellitus of obese rats: impairment in adipocyte-beta-cell relationships

Tissue triglycerides, insulin resistance, and insulin production: implications for hyperinsulinemia of obesity

β-Cell Function in Normal Rats Made Chronically Hyperleptinemic by Adenovirus-Leptin Gene Therapy

Methods for inhibiting βcell apoptosis