Richard S. Surwit

A Branched-Chain Amino Acid-Related Metabolic Signature that Differentiates Obese and Lean Humans and Contributes to Insulin Resistance

Metabolomic profiling of obese versus lean humans reveals a branched-chain amino acid (BCAA)-related metabolite signature that is suggestive of increased catabolism of BCAA and correlated with insulin resistance. To test its impact on metabolic homeostasis, we fed rats on high-fat (HF), HF with supplemented BCAA (HF/BCAA), or standard chow (SC) diets. Despite having reduced food intake and a low rate of weight gain equivalent to the SC group, HF/BCAA rats were as insulin resistant as HF rats. Pair-feeding of HF diet to match the HF/BCAA animals or BCAA addition to SC diet did not cause insulin resistance. Insulin resistance induced by HF/BCAA feeding was accompanied by chronic phosphorylation of mTOR, JNK, and IRS1Ser307 and by accumulation of multiple acylcarnitines in muscle, and it was reversed by the mTOR inhibitor, rapamycin. Our findings show that in the context of a dietary pattern that includes high fat consumption, BCAA contributes to development of obesity-associated insulin resistance.

Disruption of the uncoupling protein-2 gene in mice reveals a role in immunity and reactive oxygen species production

The gene Ucp2 is a member of a family of genes found in animals and plants, encoding a protein homologous to the brown fat uncoupling protein Ucp1 (refs 1–3). As Ucp2 is widely expressed in mammalian tissues, uncouples respiration and resides within a region of genetic linkage to obesity, a role in energy dissipation has been proposed. We demonstrate here, however, that mice lacking Ucp2 following targeted gene disruption are not obese and have a normal response to cold exposure or high-fat diet. Expression of Ucp2 is robust in spleen, lung and isolated macrophages, suggesting a role for Ucp2 in immunity or inflammatory responsiveness. We investigated the response to infection with Toxoplasma gondii in Ucp2−/− mice, and found that they are completely resistant to infection, in contrast with the lethality observed in wild-type littermates. Parasitic cysts and inflammation sites in brain were significantly reduced in Ucp2−/− mice (63% decrease, P<0.04). Macrophages from Ucp2 −/− mice generated more reactive oxygen species than wild-type mice (80% increase, P<0.001) in response to T. gondii, and had a fivefold greater toxoplasmacidal activity in vitro compared with wild-type mice (P<0.001 ), which was absent in the presence of a quencher of reactive oxygen species (ROS). Our results indicate a role for Ucp2 in the limitation of ROS and macrophage-mediated immunity.

Diet-Induced Type II Diabetes in C57BL/6J Mice

We investigated the effects of diet-induced obesity on glucose metabolism in two strains of mice, C57BL/6J and A/J. Twenty animals from each strain received ad libitum exposure to a high–fat high-simple-carbohydrate diet or standard Purina Rodent Chow for 6 mo. Exposure to the high-fat, high-simple-carbohydrate, low-fiber diet produced obesity in both A/J and C57BL/6J mice. Whereas obesity was associated with only moderate glucose intolerance and insulin resistance in A/J mice, obese C57BL/6J mice showed clear-cut diabetes with fasting blood glucose levels of >240 mg/dl and blood insulin levels of >150 μU/ml. C57BL/6J mice showed larger glycemic responses to stress and epinephrine in the lean state than AJ mice, and these responses were exaggerated by obesity. These data suggest that the C57BL/6J mouse carries a genetic predisposition to develop non-insulin-dependent (type II) diabetes. Futhermore, altered glycemic response to adrenergic stimulation may be a biologic marker for this genetic predisposition to develop type II diabetes.

Differential effects of fat and sucrose on the development of obesity and diabetes in C57BL/6J and A/J mice

We have previously demonstrated that the C57BL/6J (B/6J) mouse will develop severe obesity, hyperglycemia, and hyperinsulinemia if weaned onto a high-fat, high-sucrose (HH) diet. In the present study, we compared the effects of fat and sucrose separately and in combination on diabetes- and obesity-prone B/6J and diabetes- and obesity-resistant A/J mice. After 4 months, the feed efficiency ([FE] weight gained divided by calories consumed) did not differ across diets in A/J mice, but B/6J mice showed a significantly increased FE for fat. That is, B/6J mice gained more weight on high-fat diets without consuming more calories than A/J mice. The increase in FE was related to adipocyte hyperplasia in B/6J mice on high-fat diets. Fat-induced obesity in B/6J mice was unrelated to adrenal cortical activity. In the absence of fat, sucrose produced a decreased in FE in both strains. Animals fed a low-fat, high-sucrose (LH) diet were actually leaner than animals fed a high-complex-carbohydrate diet. Fat was also found to be the critical stimulus for hyperglycemia and hyperinsulinemia in B/6J mice. In the absence of fat, sucrose had no effect on plasma glucose or insulin. These data clearly show that across these two strains of mice, genetic differences in the metabolic response to fat are more important in the development of obesity and diabetes than the increased caloric content of a high-fat diet.

Genetic vulnerability to diet-induced obesity in the C57BL/6J mouse: physiological and molecular characteristics

The development of the metabolic syndrome in an increasing percentage of the populations of Western societies, particularly in the United States, requires valid models for establishing basic biochemical changes and performing preclinical studies on potential drug targets. The C57BL/6J mouse has become an important model for understanding the interplay between genetic background and environmental challenges such as high-fat/high-calorie diets that predispose to the development of the metabolic syndrome. This review highlights metabolic and signal transduction features that are altered during the course of disease progression, many of which mirror the human situation.

Stress and Diabetes Mellitus

Stress is a potential contributor to chronic hyperglycemia in diabetes. Stress has long been shown to have major effects on metabolic activity. Energy mobilization is a primary result of the fight or flight response. Stress stimulates the release of various hormones, which can result in elevated blood glucose levels. Although this is of adaptive importance in a healthy organism, in diabetes, as a result of the relative or absolute lack of insulin, stress-induced increases in glucose cannot be metabolized properly. Furthermore, regulation of these stress hormones may be abnormal in diabetes. However, evidence characterizing the effects of stress in type I diabetes is contradictory. Although some retrospective human studies have suggested that stress can precipitate type I diabetes, animal studies have shown that stressors of various kinds can precipitate—or prevent—various experimental models of the disease. Human studies have shown that stress can stimulate hyperglycemia, hypoglycemia, or have no affect at all on glycemic status in established diabetes. Much of this confusion may be attributable to the presence of autonomic neuropathy, common in type I diabetes. In contrast, more consistent evidence supports the role of stress in type II diabetes. Although human studies on the role of stress in the onset and course of type II diabetes are few, a large body of animal study supports the notion that stress reliably produces hyperglycemia in this form of the disease. Furthermore, there is mounting evidence of autonomic contributions to the pathophysiology of this condition in both animals and humans.

Strain-Specific Response toβ 3-Adrenergic Receptor Agonist Treatment of Diet-Induced Obesity in Mice1

Fat intake has long been associated with the development of obesity. The studies described herein show that fat adversely affects adipocyte adrenergic receptor (AR) expression and function. As beta 3AR agonists have been shown to acutely reduce adipose tissue mass and improve thermogenesis in genetically obese rodents, we examined whether chronic supplementation of a high fat diet with a highly selective beta 3AR agonist, CL316,243 could prevent diet-induced obesity, and whether the effect could be sustained over prolonged treatment. C57BL/6J and A/J mice were weaned onto one of three diets: low fat (10.5% calories from fat), high fat (58% calories from fat), or high fat supplemented with 0.001% CL316,243. B/6J mice gained more weight on the high fat diet than A/J mice (at 16 weeks: B/6J, 36.6 +/- 1.4 g; A/J, 32.9 +/- 0.8 g; P 0.05; n = 10). CL316,243 prevented the development of diet-induced obesity in A/J animals, but not in B/6J animals. A/J mice weighed 26.0 +/- 0.5 g at 16 weeks, whereas B/6J animals on the same diet weighed 34.1 +/- 0.8 g (P < 0.00001; n = 10), but food intake was not different between the strains throughout the study. beta-Adrenergic stimulation of adenylyl cyclase in obese B/6J mice was decreased by more than 75% in white adipose tissue and by more than 90% in brown adipose tissue (BAT). In contrast, in fat-fed A/J mice, beta-agonist-stimulated adenylyl cyclase was decreased in white adipose tissue by about 10%, whereas the activity in interscapular BAT was decreased by 50%, indicating significant retention of beta AR-stimulated activity in A/J mice compared to B/6J mice. High fat feeding was associated with decreased expression of beta 3AR and beta 1AR in white adipose tissue of both strains. However, chronic CL316,243 treatment prevented both the obesity and the decline in beta 3AR and beta 1AR messenger RNA levels in all adipose depots from A/J mice, but not B/6J mice. As CL316,243-treated A/J mice, but not B/6J mice, also showed marked uncoupling protein expression in white adipose depots, the ability of chronic CL316,243 treatment to prevent diet-induced obesity is dependent upon the elaboration of functional BAT in these regions.

Reversal of diet-induced obesity and diabetes in C57BL/6J mice.

We have previously shown that C57BL/6J (B6) mice develop severe obesity and diabetes if weaned onto high-fat diets, whereas A/J mice tend to be obesity and diabetes-resistant. The purpose of this study was to determine if obesity and diabetes in the B6 mouse could be completely reversed by reducing dietary fat content. After 4 months, both strains consumed more calories on a high-fat diet than on a low-fat diet, and both strains showed a higher feed efficiency (FE=weight gained/calories consumed) on the high-fat diet versus the low-fat diet. However, relative to A/J mice, B6 mice demonstrated a significantly higher FE on the high-fat diet. Hyperglycemia, hyperinsulinemia, and increased adiposity were apparent in B6 mice after 4 months on the high-fat diet regardless of whether the diet was begun at weaning or 4 months later. Correlational analyses showed that adiposity was strongly related to both insulin and glucose levels in B6 mice, but only moderately related to insulin levels in A/J mice. In obese B6 mice that were switched to a low-fat diet, obesity and diabetes were completely reversed. Adiposity, fasting glucose, and fasting insulin values in these mice were equivalent to those in B6 mice of the same age that had spent 8 months on the low-fat diet. In summary, our data show that in the B6 mouse the severity of diabetes is a direct function of obesity and diabetes is completely reversible by reducing dietary fat.

Role of stress in the etiology and treatment of diabetes mellitus

&NA; Stress has long been suspected as having major effects on metabolic activity. The effects of stress on glucose metabolism are mediated by a variety of “counter‐regulatory” hormones that are released in response to stress and that result in elevated blood glucose levels and decreased insulin action. This energy mobilizing effect is of adaptive importance in a healthy organism. However, in diabetes, because of a relative or absolute lack of insulin, stress‐induced increases in blood glucose cannot be adequately metabolized. Thus, stress is a potential contributor to chronic hyperglycemia in diabetes, although its exact role is unclear. Although there is some suggestion from retrospective human studies that stress can precipitate type I diabetes, animal studies are contradictory with different stressors either having facilitatory or inhibitory effects upon the development of the disease. Human investigations in patients with established diabetes are equally confusing with some showing that stress can stimulate hyperglycemia, hypoglycemia or have no effect at all on glycemic status. There is more consistent evidence supporting the role of stress in animal models of type II diabetes. However, human studies on the role of stress on the course of established type II diabetes are few. Intervention studies suggest that behavioral or pharmacologic intervention to manage stress may contribute significantly to diabetes treatment, but more long‐term research is needed. It is concluded that further research is needed to establish the importance of behavioral factors in the etiology and management of diabetes, and several areas of methodologic improvement are suggested.

Depressed mood is a factor in glycemic control in type 1 diabetes.

Objective The diabetes literature contains conflicting evidence on the relationship between depression and glycemic control. This may be due, in part, to the fact that past studies failed to distinguish between patients with type 1 and type 2 diabetes. Because these are actually completely different diseases that are often treated differently and consequently make different demands on patients, the relationship between glycemic control and depressed mood in type 1 and type 2 diabetes was examined separately. Methods The relationship between Beck Depression Inventory (BDI) scores and HbA1c, as an index of long-term glycemic control, was measured in samples of 30 patients with type 1 and 34 patients with type 2 diabetes. Results Groups of patients with type 1 and type 2 diabetes did not differ in mean BDI score or HbA1c level. Correlation analysis revealed a significant positive relationship between BDI scores and HbA1c in the type 1 group (r = .44, p < .02) but not in the type 2 group (r = −0.06, p > .05). This relationship was evident throughout the entire range of BDI scores and was not restricted to scores indicative of clinical depression. Patients with type 1 diabetes who had higher HbA1c and BDI scores reported a lower frequency of home blood glucose monitoring. Conclusions Variations in depressive mood, below the level of clinical depression, are associated with meaningful differences in glycemic control in type 1 but not type 2 diabetes. Preliminary data analysis suggests that this effect may be mediated, at least in part, by decreased self-care behaviors in patients with more depressed mood.