Molecular evidence supporting the portal theory: a causative link between visceral adiposity and hepatic insulin resistance

The portal theory supported by venous drainage-selective fat transplantation

OBJECTIVE

The "portal hypothesis" proposes that the liver is directly exposed to free fatty acids and cytokines increasingly released from visceral fat tissue into the portal vein of obese subjects, thus rendering visceral fat accumulation particularly hazardous for the development of hepatic insulin resistance and type 2 diabetes. In the present study, we used a fat transplantation paradigm to (artificially) increase intra-abdominal fat mass to test the hypothesis that venous drainage of fat tissue determines its impact on glucose homeostasis.

RESEARCH DESIGN AND METHODS

Epididymal fat pads of C57Bl6/J donor mice were transplanted into littermates, either to the parietal peritoneum (caval/systemic venous drainage) or, by using a novel approach, to the mesenterium, which confers portal venous drainage.

RESULTS

Only mice receiving the portal drained fat transplant developed impaired glucose tolerance and hepatic insulin resistance. mRNA expression of proinflammatory cytokines was increased in both portally and systemically transplanted fat pads. However, portal vein (but not systemic) plasma levels of interleukin (IL)-6 were elevated only in mice receiving a portal fat transplant. Intriguingly, mice receiving portal drained transplants from IL-6 knockout mice showed normal glucose tolerance.

CONCLUSIONS

These results demonstrate that the metabolic fate of intra-abdominal fat tissue transplantation is determined by the delivery of inflammatory cytokines to the liver specifically via the portal system, providing direct evidence in support of the portal hypothesis.

Insulin-regulated Srebp-1c and Pck1 mRNA expression in primary hepatocytes from zucker fatty but not lean rats is affected by feeding conditions

Insulin regulates the transcription of genes for hepatic glucose and lipid metabolism. We hypothesized that this action may be impaired in hepatocytes from insulin resistant animals. Primary hepatocytes from insulin sensitive Zucker lean (ZL) and insulin resistant Zucker fatty (ZF) rats in ad libitum or after an overnight fasting were isolated, cultured and treated with insulin and other compounds for analysis of gene expression using real-time PCR. The mRNA levels of one insulin-induced (Srebp-1c) and one insulin-suppressed (Pck1) genes in response to insulin, glucagon, and compactin treatments in hepatocytes from ad libitum ZL and ZF rats were analyzed. Additionally, the effects of insulin and T1317 on their levels in hepatocytes from ad libitum or fasted ZL or ZF rats were compared. The mRNA levels of Srebp-1c, Fas, and Scd1, but not that of Insr, Gck and Pck1, were higher in freshly isolated hepatocytes from ad libitum ZF than that from ZL rats. These patterns of Srebp-1c and Pck1 mRNA levels remained in primary hepatocyte cultured in vitro. Insulin's ability to regulate Srebp-1c and Pck1 expression was diminished in hepatocytes from ad libitum ZF, but not ZL rats. Glucagon or compactin suppressed Srebp-1c mRNA expression in lean, but not fatty hepatocytes. However, glucagon induced Pck1 mRNA expression similarly in hepatocytes from ad libitum ZL and ZF rats. Insulin caused the same dose-dependent increase of Akt phosphorylation in hepatocytes from ad libitum ZL and ZF rats. It synergized with T1317 to induce Srebp-1c, and suppressed Pck1 mRNA levels in hepatocytes from fasted, but not that from ad libitum ZF rats. We demonstrated that insulin was unable to regulate its downstream genes' mRNA expression in hepatocytes from ad libitum ZF rats. This impairment can be partially restored in hepatocytes from ZF rats after an overnight fasting, a phenomenon that deserves further investigation.

Dietary milled flaxseed and flaxseed oil improve N-3 fatty acid status and do not affect glycemic control in individuals with well-controlled type 2 diabetes

OBJECTIVE

To determine the effects of dietary consumption of milled flaxseed or flaxseed oil on glycemic control, n-3 fatty acid status, anthropometrics, and adipokines in individuals with type 2 diabetes.

DESIGN

Thirty-four participants were randomized into a parallel, controlled trial.

SUBJECTS

The participants were adults with type 2 diabetes (age 52.4 +/- 1.5 years, body mass index 32.4 +/- 1.0 kg/m(2), n = 17 men and 17 women).

INTERVENTIONS

Participants consumed a selection of bakery products containing no flax (control group [CTL], n = 9), milled flaxseed (FXS, n = 13; 32 g/d), or flaxseed oil (FXO, n = 12; 13 g/d) daily for 12 weeks. The FXS and FXO groups received equivalent amounts of alpha-linolenic acid (ALA; 7.4 g/day).

MEASURES OF OUTCOME

The primary outcome measures were fasting plasma hemoglobin A(1c), glucose, insulin, and phospholipid fatty acid composition. The secondary outcome measures were fasting circulating leptin and adiponectin, as well as body weight, body mass index, and waist circumference. Dietary intake assessment and calculations for homeostasis model assessment for insulin resistance and quantified insulin sensitivity check were also completed.

RESULTS

The FXS and FXO groups had increases in plasma phospholipid n-3 fatty acids (ALA, eicosapentaenoic acid [EPA], or decosapentaenoic acid [DPA], but not docosahexaenoic acid), and the FXO group had more EPA and DPA in plasma phospholipids compared to the FXS group. All groups had similar caloric intakes; however, the CTL group experienced a 4% weight gain compared to baseline (p < 0.05), while both flax groups had constant body weights during the study period. All other parameters, including glycemic control, were unchanged by dietary treatment.

CONCLUSIONS

Milled FXS and FXO intake does not affect glycemic control in adults with well-controlled type 2 diabetes. Possible prevention of weight gain by flax consumption warrants further investigation.

Npc1 haploinsufficiency promotes weight gain and metabolic features associated with insulin resistance

A recent population-based genome-wide association study has revealed that the Niemann-Pick C1 (NPC1) gene is associated with early-onset and morbid adult obesity. Concurrently, our candidate gene-based mouse growth study performed using the BALB/cJ NPC1 mouse model (Npc1) with decreased Npc1 gene dosage independently supported these results by suggesting an Npc1 gene-diet interaction in relation to early-onset weight gain. To further investigate the Npc1 gene in relation to weight gain and metabolic features associated with insulin resistance, we interbred BALB/cJ Npc1(+/-) mice with wild-type C57BL/6J mice, the latter mouse strain commonly used to study aspects of diet-induced obesity and insulin resistance. This breeding produced a hybrid (BALB/cJ-C57BL/6J) Npc1(+/-) mouse model with increased susceptibility to weight gain and insulin resistance. The results from our study indicated that these Npc1(+/-) mice were susceptible to increased weight gain characterized by increased whole body and abdominal adiposity, adipocyte hypertrophy and hepatic steatosis in the absence of hyperphagia. Moreover, these Npc1(+/-) mice developed abnormal metabolic features characterized by impaired fasting glucose, glucose intolerance, hyperinsulinemia, hyperleptinemia and dyslipidemia marked by an increased concentration of cholesterol and triacylglycerol associated with low-density lipoprotein and high-density lipoprotein. The overall results are consistent with a unique Npc1 gene-diet interaction that promotes both weight gain and metabolic features associated with insulin resistance. Therefore, the NPC1 gene now represents a previously unrecognized gene involved in maintaining energy and metabolic homeostasis that will contribute to our understanding concerning the current global epidemic of obesity and type 2 diabetes mellitus.

Hepatic steatosis, low-grade chronic inflammation and hormone/growth factor/adipokine imbalance

Non-alcoholic fatty liver disease (NAFLD), a further expression of metabolic syndrome, strictly linked to obesity and diabetes mellitus, is characterized by insulin resistance (IR), elevated serum levels of free fatty acids and fatty infiltration of the liver, which is known as hepatic steatosis. Hepatocyte apoptosis is a key feature of this disease and correlates with its severity. Free-fatty-acid-induced toxicity represents one of mechanisms for the pathogenesis of NAFLD and hormones, growth factors and adipokines influence also play a key role. This review highlights the various pathways that contribute to the development of hepatic steatosis. Circulating concentrations of inflammatory cytokines are reckoned to be the most important factor in causing and maintaining IR. Low-grade chronic inflammation is fundamental in the progression of NAFLD toward higher risk cirrhotic states.

Fructose: metabolic, hedonic, and societal parallels with ethanol

Rates of fructose consumption continue to rise nationwide and have been linked to rising rates of obesity, type 2 diabetes, and metabolic syndrome. Because obesity has been equated with addiction, and because of their evolutionary commonalities, we chose to examine the metabolic, hedonic, and societal similarities between fructose and its fermentation byproduct ethanol. Elucidation of fructose metabolism in liver and fructose action in brain demonstrate three parallelisms with ethanol. First, hepatic fructose metabolism is similar to ethanol, as they both serve as substrates for de novo lipogenesis, and in the process both promote hepatic insulin resistance, dyslipidemia, and hepatic steatosis. Second, fructosylation of proteins with resultant superoxide formation can result in hepatic inflammation similar to acetaldehyde, an intermediary metabolite of ethanol. Lastly, by stimulating the "hedonic pathway" of the brain both directly and indirectly, fructose creates habituation, and possibly dependence; also paralleling ethanol. Thus, fructose induces alterations in both hepatic metabolism and central nervous system energy signaling, leading to a "vicious cycle" of excessive consumption and disease consistent with metabolic syndrome. On a societal level, the treatment of fructose as a commodity exhibits market similarities to ethanol. Analogous to ethanol, societal efforts to reduce fructose consumption will likely be necessary to combat the obesity epidemic.

Ethnic differences in lipoprotein subclasses in obese adolescents: importance of liver and intraabdominal fat accretion

BACKGROUND

Recently, the deleterious metabolic effects of visceral fat [visceral adipose tissue (VAT)] deposition were challenged, and liver fat emerged as having a key independent role in the modulation of cardiometabolic risk factors.

OBJECTIVE

We explored the relation between liver fat content and VAT in 3 ethnic groups and evaluated whether the ethnic differences in the distributions of lipoprotein concentrations and sizes were associated with the hepatic fat fraction (HFF), VAT, or both.

DESIGN

In a multiethnic group of 33 white, 33 African American, and 33 Hispanic obese adolescents with normal glucose tolerance, we measured VAT and HFF by using magnetic resonance imaging. Fasting lipoprotein particle number and size were measured by using nuclear magnetic resonance spectroscopy. To assess the association between VAT and HFF, we categorized VAT into tertiles.

RESULTS

In each ethnic group, HFF values increased between successive tertiles of VAT. After multivariate adjustment and in comparison with the 2 other groups, African Americans showed lower triglyceride (P = 0.001) and higher HDL (P = 0.03) concentrations, lower concentrations of total (P = 0.007), large (P = 0.005), and medium (P lt 0.0001) VLDL, but higher concentrations of large HDL particles (P = 0.01) and larger HDL (P = 0.005). In multivariate linear models, independent of ethnicity, VAT was a significant predictor for large HDL (P = 0.003) and total small LDL (P = 0.001) concentrations, whereas HFF significantly predicted large VLDL (P = 0.03) concentrations.

CONCLUSION

Liver fat accretion, independent of VAT, may play a role in the ethnic differences seen in large VLDL particles. This trial was registered at clinicaltrials.gov as NCT00536250.

A family history of type 2 diabetes increases risk factors associated with overfeeding

AIMS/HYPOTHESIS

The purpose of the study was to test prospectively whether healthy individuals with a family history of type 2 diabetes are more susceptible to adverse metabolic effects during experimental overfeeding.

METHODS

We studied the effects of 3 and 28 days of overfeeding by 5,200 kJ/day in 41 sedentary individuals with and without a family history of type 2 diabetes (FH+ and FH- respectively). Measures included body weight, fat distribution (computed tomography) and insulin sensitivity (hyperinsulinaemic-euglycaemic clamp).

RESULTS

Body weight was increased compared with baseline at 3 and 28 days in both groups (p < 0.001), FH+ individuals having gained significantly more weight than FH- individuals at 28 days (3.4 +/- 1.6 vs 2.2 +/- 1.4 kg, p < 0.05). Fasting serum insulin and C-peptide were increased at 3 and 28 days compared with baseline in both groups, with greater increases in FH+ than in FH- for insulin at +3 and +28 days (p < 0.01) and C-peptide at +28 days (p < 0.05). Fasting glucose also increased at both time points, but without a significant group effect (p = 0.1). Peripheral insulin sensitivity decreased in the whole cohort at +28 days (54.8 +/- 17.7 to 50.3 +/- 15.6 micromol min(-1) [kg fat-free mass](-1), p = 0.03), and insulin sensitivity by HOMA-IR decreased at both time points (p < 0.001) and to a greater extent in FH+ than in FH- (p = 0.008). Liver fat, subcutaneous and visceral fat increased similarly in the two groups (p < 0.001).

CONCLUSIONS

Overfeeding induced weight and fat gain, insulin resistance and hepatic fat deposition in healthy individuals. However, individuals with a family history of type 2 diabetes gained more weight and greater insulin resistance by HOMA-IR. The results of this study suggest that healthy individuals with a family history of type 2 diabetes are predisposed to adverse effects of overfeeding.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00562393

FUNDING

The study was funded by the National Health and Medical Research Council (NHMRC), Australia (no. #427639).

[Resting metabolic rate and body composition in postmenopausal women]

OBJECTIVE

The present study evaluated the relationship between resting metabolic rate (RMR) and body composition of postmenopausal women.

METHODS

Thirty physically inactive women participated in the study, and their age average was 54,33 +/- 5,20 years old. Oxygen consumption was measured by indirect calorimetry after 12 hours of fasting and the values were calculated according to the equation of Weir. Body composition was obtained by the method of skinfolds and the measurement of waist circumference (WC) was used to assess abdominal fat. The linear correlation of Pearson was used to establish correlations between the variables.

RESULTS

We found significant correlations of TMR with the CC (0.42) and the lean mass (LM) (r = 0.48).

CONCLUSIONS

The variables of body composition that can be involved in the determination of the RMR are LM and WC.

Potential additional effect of omentectomy on metabolic syndrome, acute-phase reactants, and inflammatory mediators in grade III obese patients undergoing laparoscopic Roux-en-Y gastric bypass: a randomized trial

OBJECTIVE

To assess the additional effect of sudden visceral fat reduction by omentectomy on metabolic syndrome, acute-phase reactants, and inflammatory mediators in patients with grade III obesity (G-III O) undergoing laparoscopic Roux-en-Y gastric bypass (LRYGB).

RESEARCH DESIGN AND METHODS

Twenty-two patients were randomized into two groups, LRYGB alone or with omentectomy. Levels of interleukin-6, C-reactive protein, tumor necrosis factor-alpha, leptin, adiponectin, glucose, total cholesterol, HDL cholesterol, LDL cholesterol, and triglycerides, as well as clinical characteristics, were evaluated before surgery and at 1, 3, 6, and 12 months after surgery. Results were compared between groups.

RESULTS

Baseline characteristics were comparable in both groups. Mean operative time was significantly higher in the group of patients who underwent omentectomy (P < 0.001). Median weight of the omentum was 795 +/- 341 g. In one patient, a duodenal perforation occurred at the time of omentectomy. BMI, blood pressure, glucose, total cholesterol, LDL, and triglycerides significantly improved in both groups at 1, 3, 6, and 12 months of follow-up when compared with basal values. However, there were no consistent statistically significant differences among the groups in terms of metabolic syndrome components, acute-phase reactants, and inflammatory mediators.

CONCLUSIONS

Omentectomy does not have an ancillary short-term significant impact on the components of metabolic syndrome and does not induce important changes in the inflammatory mediators in patients undergoing LRYGB. Operative time is more prolonged when omentectomy is performed.

The role of the autonomic nervous liver innervation in the control of energy metabolism

Despite a longstanding research interest ever since the early work by Claude Bernard, the functional significance of autonomic liver innervation, either sympathetic or parasympathetic, is still ill defined. This scarcity of information not only holds for the brain control of hepatic metabolism, but also for the metabolic sensing function of the liver and the way in which this metabolic information from the liver affects the brain. Clinical information from the bedside suggests that successful human liver transplantation (implying a complete autonomic liver denervation) causes no life threatening metabolic derangements, at least in the absence of severe metabolic challenges such as hypoglycemia. However, from the benchside, data are accumulating that interference with the neuronal brain-liver connection does cause pronounced changes in liver metabolism. This review provides an extensive overview on how metabolic information is sensed by the liver, and how this information is processed via neuronal pathways to the brain. With this information the brain controls liver metabolism and that of other organs and tissues. We will pay special attention to the hypothalamic pathways involved in these liver-brain-liver circuits. At this stage, we still do not know the final destination and processing of the metabolic information that is transferred from the liver to the brain. On the other hand, in recent years, there has been a considerable increase in the understanding which brain areas are involved in the control of liver metabolism via its autonomic innervation. However, in view of the ever rising prevalence of type 2 diabetes, this potentially highly relevant knowledge is still by far too limited. Thus the autonomic innervation of the liver and its role in the control of metabolism needs our continued and devoted attention.

Visceral obesity and insulin resistance as risk factors for colorectal adenoma: a cross-sectional, case-control study

OBJECTIVES

Colorectal adenoma is known to be associated with obesity, but the association between colorectal adenoma and visceral adipose tissue (VAT) area measured by abdominal computed tomography (CT) has not been documented clearly. In addition, the relationship between insulin resistance and colorectal adenomas, which underlies the mechanism that links obesity and colorectal adenoma, has not been studied extensively. The aim of this study was to examine VAT area and insulin resistance as risk factors of colorectal adenoma.

METHODS

A cross-sectional, case-control study was conducted in Koreans that presented for health check-ups. Subjects underwent various laboratory tests, abdominal CT, and colonoscopy. VAT, subcutaneous adipose tissue (SAT), and homeostatic metabolic assessment (HOMA) index were evaluated as potential risk factors of colorectal adenoma in 2,244 age- and sex-matched subjects.

RESULTS

According to univariate analysis, the prevalences of smoking, hypertension, metabolic syndrome, and family history of colorectal cancer were higher in the adenoma group than in the normal control group. In addition, body mass index, waist circumference, triglyceride, high-density lipoprotein cholesterol, and VAT and SAT areas were significantly different in the two groups. According to the multivariate analysis adjusted for multiple confounders, VAT area was independently associated with the risk of colorectal adenoma (odds ratio (OR)=3.09, 95% confidence interval (CI): 2.19-4.36, highest quintile vs. lowest quintile). Mean HOMA index was higher in the adenoma group than in the control group (OR=1.99, 95% CI: 1.35-2.92, highest vs. lowest quintile).

CONCLUSIONS

Visceral obesity was found to be an independent risk factor of colorectal adenoma, and insulin resistance was associated with the presence of colorectal adenoma.

Molecular mechanisms of obesity and diabetes: at the intersection of weight regulation, inflammation, and glucose homeostasis

Obesity is a major health crisis, and diabetes is one of its most serious sequelae. Obesity is associated with a state of chronic systemic inflammation that is a primary etiologic factor in the development of insulin resistance and diabetes. This inflammatory state is based in adipose tissue and mediated in large part by tissue macrophages and their cytokine and adipokine products. Recent research has identified specific molecular mediators of the link between inflammation and insulin resistance in obesity. Study of these mediators and the specific mechanisms underlying inflammation and insulin resistance in obesity holds the promise for novel pharmacotherapy for obesity-related metabolic disease.

Exercise therapy in type 2 diabetes

Structured exercise is considered an important cornerstone to achieve good glycemic control and improve cardiovascular risk profile in Type 2 diabetes. Current clinical guidelines acknowledge the therapeutic strength of exercise intervention. This paper reviews the wide pathophysiological problems associated with Type 2 diabetes and discusses the benefits of exercise therapy on phenotype characteristics, glycemic control and cardiovascular risk profile in Type 2 diabetes patients. Based on the currently available literature, it is concluded that Type 2 diabetes patients should be stimulated to participate in specifically designed exercise intervention programs. More attention should be paid to cardiovascular and musculoskeletal deconditioning as well as motivational factors to improve long-term treatment adherence and clinical efficacy. More clinical research is warranted to establish the efficacy of exercise intervention in a more differentiated approach for Type 2 diabetes subpopulations within different stages of the disease and various levels of co-morbidity.

Disruption of inducible 6-phosphofructo-2-kinase ameliorates diet-induced adiposity but exacerbates systemic insulin resistance and adipose tissue inflammatory response

Adiposity is commonly associated with adipose tissue dysfunction and many overnutrition-related metabolic diseases including type 2 diabetes. Much attention has been paid to reducing adiposity as a way to improve adipose tissue function and systemic insulin sensitivity. PFKFB3/iPFK2 is a master regulator of adipocyte nutrient metabolism. Using PFKFB3(+/-) mice, the present study investigated the role of PFKFB3/iPFK2 in regulating diet-induced adiposity and systemic insulin resistance. On a high-fat diet (HFD), PFKFB3(+/-) mice gained much less body weight than did wild-type littermates. This was attributed to a smaller increase in adiposity in PFKFB3(+/-) mice than in wild-type controls. However, HFD-induced systemic insulin resistance was more severe in PFKFB3(+/-) mice than in wild-type littermates. Compared with wild-type littermates, PFKFB3(+/-) mice exhibited increased severity of HFD-induced adipose tissue dysfunction, as evidenced by increased adipose tissue lipolysis, inappropriate adipokine expression, and decreased insulin signaling, as well as increased levels of proinflammatory cytokines in both isolated adipose tissue macrophages and adipocytes. In an in vitro system, knockdown of PFKFB3/iPFK2 in 3T3-L1 adipocytes caused a decrease in the rate of glucose incorporation into lipid but an increase in the production of reactive oxygen species. Furthermore, knockdown of PFKFB3/iPFK2 in 3T3-L1 adipocytes inappropriately altered the expression of adipokines, decreased insulin signaling, increased the phosphorylation states of JNK and NFkappaB p65, and enhanced the production of proinflammatory cytokines. Together, these data suggest that PFKFB3/iPFK2, although contributing to adiposity, protects against diet-induced insulin resistance and adipose tissue inflammatory response.

Discrete localization of various fatty-acid-binding proteins in various cell populations of mouse retina

Various fatty acids (FAs) are involved as an energy source in many different functions in the organism. They are also essential ingredients of membranous lipids and act as intracellular signaling molecules. Intracellular fatty-acid-binding proteins (FABPs) comprise a family of soluble lipid-binding proteins with low molecular masses and solubilize long-chain FAs to allow intracellular translocation in the aqueous cytosol. To clarify the functions of FABPs in the retina, which is remarkably rich in polyunsaturated FAs, we have investigated the localization of B (brain type)-, H (heart type)-, E (epidermal type)-, and A (adipocyte type)-FABPs in adult mouse retinae by immunohistochemistry. In order to determine the possible involvement of FABPs in retinal degenerative diseases, we have also examined changes in FABP expression in light-induced photoreceptor cell degeneration (photic injury). The discrete localization of B-, H-, E-, and A-FABP species in various cell populations of the retina has been clarified: B-FABP is mainly localized in the cone photoreceptor cells, H-FABP in some populations of amacrine/bipolar/horizontal interneurons, and E-FABP in ganglion cells, with A-FABP-like immunoreactivity being located in resident microglia of normal retinae. E-FABP has further been localized in invasive macrophages in damaged retinae following photic injury, allowing discrete identification of the resident microglia and invasive macrophages by A- and E-FABP immunoreactivity, respectively.

Adipocytokines: a bridge connecting obesity and insulin resistance

Overweight or obesity has become a critical health problem in the world. The association of obesity with type 2 diabetes mellitus (T2D) has been recognized for decades, and the major basis for this link is the ability of obesity to engender insulin resistance (IR). Adipose tissue is not only an energy depot but also an active endocrine organ. Furthermore, fat distribution in the body is important for the progress of IR. Many studies show that visceral fat is more important in relation to IR than subcutaneous fat. Circulating free fatty acids (FFAs) derived from adipocytes are elevated in many IR states and have been suggested to be a main underlying mechanism of IR in obesity-associated T2D. However, compelling evidence demonstrates that adipocytokines including several adipocyte-derived cytokines or hormones are also involved in obesity-induced IR. Therefore, we hypothesise that adipocytokines may be a bridge connecting obesity and IR, and abnormal fat depot distribution or visceral fat/subcutaneous fat ratio (V/S ratio) in obesity also could be a primer for IR. When visceral fat accumulates and V/S ratio deteriorates , just like a primer,in visceral obesity it should begin to display unhealthy effect begin to take place in the body. In addition to it, as one of physiological regulation mechanisms of the body, most of the adipocytokines from the visceral fat reduce the visceral fat volume or normalize the V/S ratio. Actually, on the contrary, with serum a change in the serum adipocytokine level and an imbalance of them in the body for a long term, it will become a pathological condition and various kinds of effects may contribute to the development of IR. If confirmed, this hypothesis may lead to the formulation of new pathogenesis and new therapeutic approaches to IR. For example, an effective slimming pill will be assessed in future on the basis of the decrease of V/S and serum adipocytokines level rather than of body weight.

Genetic disruption of myostatin reduces the development of proatherogenic dyslipidemia and atherogenic lesions in Ldlr null mice

OBJECTIVE

Insulin-resistant states, such as obesity and type 2 diabetes, contribute substantially to accelerated atherogenesis. Null mutations of myostatin (Mstn) are associated with increased muscle mass and decreased fat mass. In this study, we determined whether Mstn disruption could prevent the development of insulin resistance, proatherogenic dyslipidemia, and atherogenesis.

RESEARCH DESIGN AND METHODS

C57BL/6 Ldlr(-/-) mice were cross-bred with C57BL/6 Mstn(-/-) mice for >10 generations to generate Mstn(-/-)/Ldlr(-/-) double-knockout mice. The effects of high-fat/high-cholesterol diet on body composition, plasma lipids, systemic and tissue-specific insulin sensitivity, hepatic steatosis, as well as aortic atheromatous lesion were characterized in Mstn(-/-)/Ldlr(-/-) mice in comparison with control Mstn(+/+)/Ldlr(-/-) mice.

RESULTS

Compared with Mstn(+/+)/Ldlr(-/-) controls, Mstn(-/-)/ Ldlr(-/-) mice were resistant to diet-induced obesity, and had greatly improved insulin sensitivity, as indicated by 42% higher glucose infusion rate and 90% greater muscle [(3)H]-2-deoxyglucose uptake during hyperinsulinemic-euglycemic clamp. Mstn(-/-)/Ldlr(-/-) mice were protected against diet-induced hepatic steatosis and had 56% higher rate of hepatic fatty acid beta-oxidation than controls. Mstn(-/-)/Ldlr(-/-) mice also had 36% lower VLDL secretion rate and were protected against diet-induced dyslipidemia, as indicated by 30-60% lower VLDL and LDL cholesterol, free fatty acids, and triglycerides. Magnetic resonance angiography and en face analyses demonstrated 41% reduction in aortic atheromatous lesions in Ldlr(-/-) mice with Mstn deletion.

CONCLUSIONS

Inactivation of Mstn protects against the development of insulin resistance, proatherogenic dyslipidemia, and aortic atherogenesis in Ldlr(-/-) mice. Myostatin may be a useful target for drug development for prevention and treatment of obesity and its associated type 2 diabetes and atherosclerosis.

c-Jun NH2-terminal kinase activity in subcutaneous adipose tissue but not nuclear factor-kappaB activity in peripheral blood mononuclear cells is an independent determinant of insulin resistance in healthy individuals

OBJECTIVE

Chronic low-grade activation of the immune system (CLAIS) predicts type 2 diabetes via a decrease in insulin sensitivity. Our study investigated potential relationships between nuclear factor-kappaB (NF-kappaB) and c-Jun NH(2)-terminal kinase (JNK) pathways-two pathways proposed as the link between CLAIS and insulin resistance.

RESEARCH DESIGN AND METHODS

Adiposity (dual-energy X-ray absorptiometry), waist-to-hip ratio (WHR), and insulin sensitivity (M, hyperinsulinemic-euglycemic clamp) were measured in 22 healthy nondiabetic volunteers (aged 29 +/- 11 years, body fat 28 +/- 11%). NF-kappaB activity (DNA-binding assay) and JNK1/2 activity (phosphorylated JNK) were assessed in biopsies of the vastus lateralis muscle and subcutaneous adipose tissue and in peripheral blood mononuclear cell (PBMC) lysates.

RESULTS

NF-kappaB activities in PBMCs and muscle were positively associated with WHR after adjustment for age, sex, and percent body fat (both P < 0.05). NF-kappaB activity in PBMCs was inversely associated with M after adjustment for age, sex, percent body fat, and WHR (P = 0.02) and explained 16% of the variance of M. There were no significant relationships between NF-kappaB activity and M in muscle or adipose tissue (both NS). Adipose-derived JNK1/2 activity was not associated with obesity (all P> 0.1), although it was inversely related to M (r = -0.54, P < 0.05) and explained 29% of its variance. When both NF-kappaB and JNK1/2 were examined statistically, only JNK1/2 activity in adipose tissue was a significant determinant of insulin resistance (P = 0.02).

CONCLUSIONS

JNK1/2 activity in adipose tissue but not NF-kappaB activity in PBMCs is an independent determinant of insulin resistance in healthy individuals.

Getting the message across: mechanisms of physiological cross talk by adipose tissue

Obesity is associated with resistance of skeletal muscle to insulin-mediated glucose uptake, as well as resistance of different organs and tissues to other metabolic and vascular actions of insulin. In addition, the body is exquisitely sensitive to nutrient imbalance, with energy excess or a high-fat diet rapidly increasing insulin resistance, even before noticeable changes occur in fat mass. There is a growing acceptance of the fact that, as well as acting as a storage site for surplus energy, adipose tissue is an important source of signals relevant to, inter alia, energy homeostasis, fertility, and bone turnover. It has also been widely recognized that obesity is a state of low-grade inflammation, with adipose tissue generating substantial quantities of proinflammatory molecules. At a cellular level, the understanding of the signaling pathways responsible for such alterations has been intensively investigated. What is less clear, however, is how alterations of physiology, and of signaling, within one cell or one tissue are communicated to other parts of the body. The concepts of cell signals being disseminated systemically through a circulating "endocrine" signal have been complemented by the view that local signaling may similarly occur through autocrine or paracrine mechanisms. Yet, while much elegant work has focused on the alterations in signaling that are found in obesity or energy excess, there has been less attention paid to ways in which such signals may propagate to remote organs. This review of the integrative physiology of obesity critically appraises the data and outlines a series of hypotheses as to how interorgan cross talk takes place. The hypotheses presented include the "fatty acid hypothesis,", the "portal hypothesis,", the "endocrine hypothesis,", the "inflammatory hypothesis,", the "overflow hypothesis,", a novel "vasocrine hypothesis," and a "neural hypothesis," and the strengths and weaknesses of each hypothesis are discussed.

Relationship between alanine aminotransferase levels and metabolic syndrome in nonalcoholic fatty liver disease

OBJECTIVE

To investigate the relationship between alanine aminotransferase (ALT) levels and metabolic syndrome (MS) in nonalcoholic fatty liver disease (NAFLD).

METHODS

A total of 26527 subjects who received medical health checkup in our hospital from January 2005 to July 2007 were enrolled in the study. The diagnosis of fatty liver was based on ultrasound imaging. MS was defined according to the criteria of the Adult Treatment Panel III. ALT, triglyceride (TG), high density lipoprotein cholesterol (HDL-c), fasting plasma glucose (FPG), height, weight, waist circumference (WC), systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured in each subject to analyze the relationship between MS and ALT activity.

RESULTS

(1) The prevalence of NAFLD in men (30.94%) was significantly higher than that in women (15.65%); (2) The incidence of MS in NAFLD (33.83%) was significantly greater than that in non-NAFLD (10.62%); (3) Of the 6470 subjects with NAFLD, in the age-adjusted partial correlation analysis, there were statistically significant correlations between the ALT levels and most metabolic risk factors in each sex (P<0.01), except that ALT levels had no correlation with HDL-c in women. Moreover, in the multiple stepwise regression analysis, SBP lost its significance, and WC, body mass index (BMI), age, DBP, TG and FPG were independently associated with ALT levels in both sexes (P<0.05). HDL-c remained significant and was independently related to ALT levels in men; (4) ALT levels were significantly higher in subjects with MS compared to those without MS (P<0.001). Mean ALT levels increased with the number of MS components in each sex (P<0.05 for trend).

CONCLUSION

We found a strong relationship between ALT levels and MS in NAFLD and revealed that the cluster of MS components might be the predictor for ALT elevations.

Depot-specific adipocyte cell lines reveal differential drug-induced responses of white adipocytes--relevance for partial lipodystrophy

Intra-abdominal (IA) fat functionally differs from subcutaneous (SC) adipose tissue, likely contributing to its stronger association with obesity-induced morbidity and to differential response to medications. Drug-induced partial lipodystrophy, like in response to antiretroviral agents, is an extreme manifestation of the different response of different fat depots, with loss of SC but not IA. Investigating depot-specific adipocyte differences is limited by the low accessibility to IA fat and by the heterogenous cell population comprising adipose tissue. Here, we aimed at utilizing immortalized preadipocyte cell lines from IA (epididymal) or SC (inguinal) fat to investigate whether they differentially respond to the HIV protease inhibitor nelfinavir. Preadipocytes were readily amenable to adipogenesis, as evidenced by lipid accumulation, expression of adipose-specific genes, measurable lipolysis, and insulin responsiveness. Leptin secretion was higher by the SC line, consistent with known differences between IA and SC fat. As previously reported, nelfinavir inhibited adipogenesis downstream of C/EBPbeta, but similarly in both cell lines. In contrast, nelfinavir's capacity to diminish insulin signaling, decrease leptin secretion, enhance basal lipolysis, and decrease expression of the lipid droplet-associated protein perilipin occurred more robustly and/or at lower nelfinavir concentrations in the SC line. This was despite similar intracellular concentrations of nelfinavir (23.8 +/- 5.6 and 33.6 +/- 12.2 microg/mg protein for inguinal and epididymal adipocytes, respectively, P = 0.46). The cell lines recapitulated depot-differential effects of nelfinavir observed in differentiated primary preadipocytes and with whole tissue explants. Thus, we report the use of fat depot-specific adipocyte cell lines for unraveling depot-differential responses to a drug causing partial lipodystrophy.

Adipose tissue and skeletal muscle plasticity modulates metabolic health

Obesity, accumulation of adipose tissue, develops when energy intake exceeds energy expenditure. Adipose tissue is essential for buffering the differences between energy intake and expenditure by accumulating lipids while skeletal muscle is the energy burning machine. Here we adopted the concept that (i) adipose tissue ability to regulate the storage capacity for lipids as well as (ii) dynamic regulation of muscle and adipose tissue secretory and metabolic activity is important for maintaining the metabolic health. This might be at least in part related to tissue plasticity, a phenomenon enabling dynamic modulation of the tissue phenotype in different physiological and pathophysiological situations. Recent advances in our understanding of the complex endocrine function of adipose tissue in regulating lipid metabolism, adipogenesis, angiogenesis, extracellular matrix remodelling, inflammation and oxidative stress prompted us to review the role of tissue plasticity--dynamic changes in adipose tissue and skeletal muscle metabolic and endocrine phenotype--in determining the difference between metabolic health and disease.

Visceral adiposity and its anatomical distribution as predictors of the metabolic syndrome and cardiometabolic risk factor levels

BACKGROUND

Despite the recognition that central obesity plays a critical role in chronic disease, few large-scale imaging studies have documented human variation in abdominal adipose tissue patterning.

OBJECTIVE

We aimed to compare the associations between abdominal subcutaneous adipose tissue (ASAT) and visceral abdominal tissue (VAT), which were measured at different locations across the abdomen, and the presence of the metabolic syndrome (MS; National Cholesterol Education Program Adult Treatment Panel III definition) and individual cardiometabolic risk factors.

DESIGN

This study included 713 non-Hispanic whites aged 18-86 y, in whom VAT and ASAT were assessed by using multiple-image magnetic resonance imaging. The anatomical position of the magnetic resonance image containing the maximum VAT area for each subject was used as a measure of VAT patterning. Multivariate linear and logistic regression analyses were used to examine the relation of VAT, ASAT, and VAT patterning to cardiometabolic risk.

RESULTS

VAT mass was a stronger predictor of the MS than was ASAT mass, but ASAT mass (and other measures of subcutaneous adiposity) had signification interactions with VAT mass, whereby elevated ASAT reduced the probability of MS among men with high VAT (P = 0.0008). There was variation across image locations in the association of VAT area with the MS in men, and magnetic resonance images located 4-8 cm above L4-L5 provided the strongest correlations between VAT area and cardiometabolic risk factors. Subjects whose maximum VAT area was higher in the abdomen had higher LDL-cholesterol concentrations (R(2) = 0.07, P < 0.0001), independent of age and adiposity.

CONCLUSION

Further studies are needed to confirm the effects of VAT patterning on cardiometabolic risk.

Visceral fat accumulation induced by a high-fat diet causes the atrophy of mesenteric lymph nodes in obese mice

OBJECTIVE

A high intake of fat in the diet plays a crucial role in promoting obesity and obesity-related pathologies, and especially visceral obesity is closely associated with obesity-related complications. Because adipose tissue is anatomically associated with lymph nodes, the secondary lymphoid organ, we hypothesized that fat tissue-derived factors may influence the cellularity of lymphoid tissue embedded in fat.

METHODS AND PROCEDURES

Mesenteric and inguinal lymph nodes were isolated from obese mice fed a high-fat diet and control mice fed a regular diet. T-cell population, activation state, and the extent of apoptosis were determined by flow cytometric analysis or terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) assay.

RESULTS

The weight of mesenteric lymph nodes and the total number of lymphoid cells in the obese mice significantly decreased compared with those in the control mice; however, no change was observed in the weight of inguinal lymph nodes. The numbers of CD4(+) and CD8(+) T cells in the mesenteric lymph nodes of obese mice significantly decreased compared with those of the control. Enhanced T-cell activation and apoptosis were observed in the mesenteric lymph node cells of the obese mice. The treatment of lymph node cells with free fatty acids, oxidative stress, and chylomicrons, which are obesity-related factors, resulted in lymph node T-cell activation and apoptosis.

DISCUSSION

These results suggest that visceral fat accumulation with a high-fat diet can cause the atrophy of mesenteric lymph nodes by enhancing activation-induced lymphoid cell apoptosis. Dietary fat-induced visceral obesity may be crucial for obesity-related immune dysfunction.

Fatty liver and insulin resistance in obese Zucker rats: no role for mitochondrial dysfunction

The relationship between insulin resistance and mitochondrial function is of increasing interest. Studies looking for such interactions are usually made in muscle and only a few studies have been done in liver, which is known to be a crucial partner in whole body insulin action. Recent studies have revealed a similar mechanism to that of muscle for fat-induced insulin resistance in liver. However, the exact mechanism of lipid metabolites accumulation in liver leading to insulin resistance is far from being elucidated. One of the hypothetical mechanisms for liver steatosis development is an impairment of mitochondrial function. We examined mitochondrial function in fatty liver and insulin resistance state using isolated mitochondria from obese Zucker rats. We determined the relationship between ATP synthesis and oxygen consumption as well as the relationship between mitochondrial membrane potential and oxygen consumption. In order to evaluate the quantity of mitochondria and the oxidative capacity we measured citrate synthase and cytochrome c oxidase activities. Results showed that despite significant fatty liver and hyperinsulinemia, isolated liver mitochondria from obese Zucker rats display no difference in oxygen consumption, ATP synthesis, and membrane potential compared with lean Zucker rats. There was no difference in citrate synthase and cytochrome c oxidase activities between obese and lean Zucker rats in isolated mitochondria as well as in liver homogenate, indicating a similar relative amount of hepatic mitochondria and a similar oxidative capacity. Adiponectin, which is involved in bioenergetic homeostasis, was increased two-fold in obese Zucker rats despite insulin resistance. In conclusion, isolated liver mitochondria from lean and obese insulin-resistant Zucker rats showed strictly the same mitochondrial function. It remains to be elucidated whether adiponectin increase is involved in these results.

Beneficial effects of subcutaneous fat transplantation on metabolism

Subcutaneous (SC) and visceral (VIS) obesity are associated with different risks of diabetes and the metabolic syndrome. To elucidate whether these differences are due to anatomic location or intrinsic differences in adipose depots, we characterized mice after transplantation of SC or VIS fat from donor mice into either SC or VIS regions of recipient mice. The group with SC fat transplanted into the VIS cavity exhibited decreased body weight, total fat mass, and glucose and insulin levels. These mice also exhibited improved insulin sensitivity during hyperinsulinemic-euglycemic clamps with increased whole-body glucose uptake, glucose uptake into endogenous fat, and insulin suppression of hepatic glucose production. These effects were observed to a lesser extent with SC fat transplanted to the SC area, whereas VIS fat transplanted to the VIS area was without effect. These data suggest that SC fat is intrinsically different from VIS fat and produces substances that can act systemically to improve glucose metabolism.

Fatty liver, insulin resistance, and dyslipidemia

After recently being recognized as a feature of the metabolic syndrome, fatty liver has evolved as a key player in the pathogenesis of dyslipidemia. Development of nonalcoholic fatty liver disease comes from an imbalance between the influx and production of fatty acids and the use of fatty acids for oxidation or secretion as very low density lipoprotein (VLDL) triglycerides. Previously, we have shown a strong relationship between increased liver fat and overproduction of large VLDL particles. We observed recently that in patients with high liver fat, insulin was unable to regulate VLDL production. The result is increased concentrations of VLDL particles in the circulation. Consequently, changes are seen in the metabolism of other lipoproteins that interact with VLDL particles, the net result being decreased high-density lipoprotein cholesterol and increased formation of small, dense low-density lipoprotein. In this article, we review recent findings on the development of fatty liver and its role in the diabetic dyslipidemia pathogenesis.

Factors associated with percent change in visceral versus subcutaneous abdominal fat during weight loss: findings from a systematic review

BACKGROUND

Visceral adipose tissue (VAT) is associated with greater obesity-related metabolic disturbance. Many studies have reported preferential loss of VAT with weight loss.

OBJECTIVE

This systematic review looks for factors associated with preferential loss of VAT relative to subcutaneous abdominal fat (SAT) during weight loss.

DESIGN

Medline and Embase were searched for imaging-based measurements of VAT and subcutaneous abdominal adipose tissue (SAT) before and after weight loss interventions. We examine for factors that influences the percentage change in VAT versus SAT (%deltaV/%deltaS) with weight loss. Linear regression analyses were performed on the complete data set and on subgroups of studies. Factors examined included percentage weight loss, degree of caloric restriction, exercise, initial body mass index (BMI), gender, time of follow-up and baseline VAT/SAT.

RESULTS

There were 61 studies with a total of 98 cohort time points extracted. Percentage weight loss was the only variable that influenced %deltaV/%deltaS (r=-0.29, P=0.005). Modest weight loss generated preferential loss of VAT, but with greater weight loss this effect was attenuated. The method of weight loss was not an influence with one exception. Very-low-calorie diets (VLCDs) provided exceptional short-term (<4 weeks) preferential VAT loss. But this effect was lost by 12-14 weeks.

CONCLUSIONS

Visceral adipose tissue is lost preferentially with modest weight loss, but the effect is attenuated with greater weight loss. Acute caloric restriction, using VLCD, produces early preferential loss of VAT. These observations may help to explain the metabolic benefits of modest weight loss.

Anatomical patterning of visceral adipose tissue: race, sex, and age variation

OBJECTIVE

We tested sex, race, and age differences in the patterning of visceral adipose tissue (VAT) and subcutaneous adipose tissue.

RESEARCH METHODS AND PROCEDURES

Contiguous 1-cm-thick magnetic resonance (MR) images of the abdomen were collected from 820 African-American and white adults. Repeated-measures ANOVA was used to examine the effects of image location, sex, race, and age (>or=50 vs. <50 years) on adipose tissue areas. Maximum VAT area was identified for each subject from the raw data.

RESULTS

Compared to women, men had greater total VAT volume (p < 0.0001), and their maximum VAT area occurred higher in the abdomen (p < 0.0001). Among white men, maximim VAT area most frequently occurred 5 to 10 cm above L4-L5, whereas in the other groups, maximim VAT area most frequently occurred 1 to 4 cm above L4-L5 (p < 0.0001). African-American men had greater total VAT volume than African-American women (p < 0.01), but this sex difference was only significant using single images cranial to L4-L5 + 2 cm. Age-related increases in VAT tended to be greatest 5 to 10 cm above L4-L5 in men and near L4-L5 in women.

DISCUSSION

A single MR image 5 to 10 cm above L4-L5 may allow more accurate conclusions than the L4-L5 image regarding group differences in visceral adiposity.

Acute suppression of VLDL1 secretion rate by insulin is associated with hepatic fat content and insulin resistance

AIMS/HYPOTHESIS

Overproduction of VLDL(1) seems to be the central pathophysiological feature of the dyslipidaemia associated with type 2 diabetes. We explored the relationship between liver fat and suppression of VLDL(1) production by insulin in participants with a broad range of liver fat content.

METHODS

A multicompartmental model was used to determine the kinetic parameters of apolipoprotein B and TG in VLDL(1) and VLDL(2) after a bolus of [(2)H(3)]leucine and [(2)H(5)]glycerol during a hyperinsulinaemic-euglycaemic clamp in 20 male participants: eight with type 2 diabetes and 12 control volunteers. The participants were divided into two groups with low or high liver fat. All participants with diabetes were in the high liver-fat group.

RESULTS

The results showed a rapid drop in VLDL(1)-apolipoprotein B and -triacylglycerol secretion in participants with low liver fat during the insulin infusion. In contrast, participants with high liver fat showed no significant change in VLDL(1) secretion. The VLDL(1) suppression following insulin infusion correlated with the suppression of NEFA, and the ability of insulin to suppress the plasma NEFA was impaired in participants with high liver fat. A novel finding was an inverse response between VLDL(1) and VLDL(2) secretion in participants with low liver fat: VLDL(1) secretion decreased acutely after insulin infusion whereas VLDL(2) secretion increased.

CONCLUSIONS/INTERPRETATION

Insulin downregulates VLDL(1) secretion and increases VLDL(2) secretion in participants with low liver fat but fails to suppress VLDL(1) secretion in participants with high liver fat, resulting in overproduction of VLDL(1). Thus, liver fat is associated with lack of VLDL(1) suppression in response to insulin.

Impact of different fat depots on insulin sensitivity: predominant role of liver fat

BACKGROUND

Overall obesity and, as it is increasingly appreciated, body fat distribution and ectopic fat deposition in liver and skeletal muscle, determine insulin resistance in humans. However, little is known about the independence of these relationships. Therefore, we determined the impact of different fat depots as well as fat accumulation in ectopic tissues such as liver and skeletal muscle in the prediction of insulin resistance in healthy humans.

METHODS

Visceral and subcutaneous abdominal fat were determined by magnetic resonance (MR) tomography and liver fat and intramyocellular fat in the tibialis anterior muscle by (1)H-MR spectroscopy in 220 subjects. Insulin sensitivity was estimated from the oral glucose tolerance test (OGTT) and measured by a euglycemic hyperinsulinemic clamp in a subgroup (n = 157).

RESULTS

Insulin sensitivity estimated from the OGTT correlated negatively with total body fat (r = -0.27, p < 0.0001), subcutaneous abdominal fat (r = -0.35, p < 0.0001), and visceral fat (r = -0.43, p < 0.0001). Furthermore, insulin sensitivity correlated negatively with liver fat (r = -0.53, p < 0.0001) and intramyocellular fat (r = -0.26, p < 0.0001). In multivariate regression models, high liver and visceral fat emerged as the strongest predictors of low insulin sensitivity.

CONCLUSION

Among various fat compartments, high liver fat and high visceral fat are the strongest determinants of insulin sensitivity in humans.

Metabolic syndrome, hyperinsulinemia, and cancer

The term metabolic syndrome describes the association between obesity, insulin resistance, and the risk of several prominent chronic diseases, including cancer. The causal link between many of these components remains unexplained, however. What is clear are the events that precede the development of the syndrome itself. In animal models, a fat-supplemented diet causes 1) lipid deposition in adipose depots, 2) insulin resistance of liver and skeletal muscle, and 3) hyperinsulinemia. One hypothesis relating fat deposition and insulin resistance involves enhanced lipolysis in the visceral depot, which leads to an increase in free fatty acid (FFA) flux. Increased mass of stored lipid and insulin resistance of visceral adipocytes favors lipolysis. Additionally, hypersensitivity of visceral adipose cells to sympathetic nervous system stimulation leads to increased lipolysis in the obese state. However, little evidence is available for enhanced plasma FFA concentrations in the fasting state. We measured FFA concentrations over a 24-h day in obese animals and found that plasma FFAs are elevated in the middle of the night, peaking at 0300. Therefore, it is possible that nocturnal lipolysis increases exposure of liver and muscle to FFAs at night, thus causing insulin resistance, which may play a role in hyperinsulinemic compensation to insulin resistance. Nocturnal lipolysis secondary to sympathetic stimulation may not only cause insulin resistance but also be responsible for hyperinsulinemia by stimulating secretion and reducing clearance of insulin by the liver. The resulting syndrome-elevated nocturnal FFAs and elevated insulin-may synergize and increase the risk of some cancers. This possible scenario needs further study.

The importance of free fatty acids in the development of Type 2 diabetes

The recent increase in the prevalence of obesity has been associated with a coincident rise in the prevalence of Type 2 diabetes, whereas weight loss has been shown to decrease the risk of Type 2 diabetes. The pathophysiological mechanisms that have been proposed to explain this link are fundamentally concerned with insulin resistance and the decline in pancreatic B-cell function that accompanies an increase in visceral obesity. They involve the rise in the plasma concentrations of free fatty acids (FFAs) that are associated with an increase in fat mass. Elevated levels of FFAs can lead to insulin resistance, and evidence is growing that B-cell function is impaired through lipotoxicity. Factors such as tumour necrosis factor-alpha (TNF-alpha) and adiponectin, released from adipose tissue, can also modulate insulin resistance. Many interventions that are helpful in treating or preventing Type 2 diabetes, such as weight loss and certain pharmacological interventions, reduce circulating FFA concentrations to a greater or lesser extent. Recent study results suggest that peroxisome proliferator-activated receptor (PPAR)gamma agonists have an effect on the development of Type 2 diabetes. However, in light of concerns over the apparent increase in congestive heart failure with PPARgamma agonists, their place in the prevention of Type 2 diabetes remains to be determined.

Mechanisms of obesity-associated insulin resistance: many choices on the menu

Obesity-associated insulin resistance is a major risk factor for type 2 diabetes and cardiovascular disease. In the past decade, a large number of endocrine, inflammatory, neural, and cell-intrinsic pathways have been shown to be dysregulated in obesity. Although it is possible that one of these factors plays a dominant role, many of these factors are interdependent, and it is likely that their dynamic interplay underlies the pathophysiology of insulin resistance. Understanding the biology of these systems will inform the search for interventions that specifically prevent or treat insulin resistance and its associated pathologies.

Insulin resistance, insulin response, and obesity as indicators of metabolic risk

CONTEXT

Insulin resistance (IR) and obesity, especially abdominal obesity, are regarded as central pathophysiological features of a cluster of cardiovascular risk factors (CVRFs), but their relative roles remain undefined. Moreover, the differential impact of IR viz. insulin response has not been evaluated.

OBJECTIVE

The objective of this study was to dissect out the impact of obesity, abdominal obesity, and IR/insulin response on CVRF.

DESIGN

This was a cross-sectional study.

SETTING

The study was conducted at 21 research centers in Europe.

SUBJECTS

The study included a cohort of 1308 nondiabetic subjects [718 women and 590 men, age 30-60 yr, body mass index (BMI) 17-44 kg.m(-2)].

MAIN OUTCOME MEASURES

We measured IR (by a standardized euglycemic insulin clamp), waist girth, insulin response to an oral glucose tolerance test, and major CVRF, and analyzed their associations by multivariate models and factor analysis.

RESULTS

BMI was positively related to all CVRFs. Waist circumference was related to higher blood pressure and serum triglycerides and lower high-density lipoprotein-cholesterol, IR to reduced glucose tolerance, higher free fatty acids, triglyceride and low-density lipoprotein-cholesterol, and lower high-density lipoprotein-cholesterol, and insulin response to higher heart rate, blood pressure and fasting glucose, and the same dyslipidemic profile as IR (P < or = 0.05 for all). By factor analysis, three main factors (related to IR, age, and fatness, respectively) appeared to underlie this pattern of associations. Each of BMI, waist girth, IR, and insulin response was independently associated with total CVRF load (all P < 0.001).

CONCLUSIONS

When IR, fat mass and distribution, and insulin response are measured simultaneously in a large cohort, no one factor stands out as the sole driving force of the CVRF cluster, each being associated with one or more physiological pathways according to known cause-effect relationships.

Beta-cell "rest" accompanies reduced first-pass hepatic insulin extraction in the insulin-resistant, fat-fed canine model

During insulin resistance, glucose homeostasis is maintained by an increase in plasma insulin via increased secretion and/or decreased first-pass hepatic insulin extraction. However, the relative importance of insulin secretion vs. clearance to compensate for insulin resistance in obesity has yet to be determined. This study utilizes the fat-fed dog model to examine longitudinal changes in insulin secretion and first-pass hepatic insulin extraction during development of obesity and insulin resistance. Six dogs were fed an isocaloric diet with an approximately 8% increase in fat calories for 12 wk and evaluated at weeks 0, 6, and 12 for changes in 1) insulin sensitivity by euglycemic-hyperinsulinemic clamp, 2) first-pass hepatic insulin extraction by direct assessment, and 3) glucose-stimulated insulin secretory response by hyperglycemic clamp. We found that 12 wk of a fat diet increased subcutaneous and visceral fat as assessed by MR imaging. Consistent with increased body fat, the dogs exhibited a approximately 30% decrease in insulin sensitivity and fasting hyperinsulinemia. Although insulin secretion was substantially increased at week 6, beta-cell sensitivity returned to prediet levels by week 12. However, peripheral hyperinsulinemia was maintained because of a significant decrease in first-pass hepatic insulin extraction, thus maintaining hyperinsulinemia, despite changes in insulin release. Our results indicate that when obesity and insulin resistance are induced by an isocaloric, increased-fat diet, an initial increase in insulin secretion by the beta-cells is followed by a decrease in first-pass hepatic insulin extraction. This may provide a secondary physiological mechanism to preserve pancreatic beta-cell function during insulin resistance.

Nocturnal free fatty acids are uniquely elevated in the longitudinal development of diet-induced insulin resistance and hyperinsulinemia

Obesity is strongly associated with hyperinsulinemia and insulin resistance, both primary risk factors for type 2 diabetes. It has been thought that increased fasting free fatty acids (FFA) may be responsible for the development of insulin resistance during obesity, causing an increase in plasma glucose levels, which would then signal for compensatory hyperinsulinemia. But when obesity is induced by fat feeding in the dog model, there is development of insulin resistance and a marked increase in fasting insulin despite constant fasting FFA and glucose. We examined the 24-h plasma profiles of FFA, glucose, and other hormones to observe any potential longitudinal postprandial or nocturnal alterations that could lead to both insulin resistance and compensatory hyperinsulinemia induced by a high-fat diet in eight normal dogs. We found that after 6 wk of a high-fat, hypercaloric diet, there was development of significant insulin resistance and hyperinsulinemia as well as accumulation of both subcutaneous and visceral fat without a change in either fasting glucose or postprandial glucose. Moreover, although there was no change in fasting FFA, there was a highly significant increase in the nocturnal levels of FFA that occurred as a result of fat feeding. Thus enhanced nocturnal FFA, but not glucose, may be responsible for development of insulin resistance and fasting hyperinsulinemia in the fat-fed dog model.

Inhibition of protein kinase Cepsilon prevents hepatic insulin resistance in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance and type 2 diabetes mellitus, but the molecular signals linking hepatic fat accumulation to hepatic insulin resistance are unknown. Three days of high-fat feeding in rats results specifically in hepatic steatosis and hepatic insulin resistance. In this setting, PKCepsilon, but not other isoforms of PKC, is activated. To determine whether PKCepsilon plays a causal role in the pathogenesis of hepatic insulin resistance, we treated rats with an antisense oligonucleotide against PKCepsilon and subjected them to 3 days of high-fat feeding. Knocking down PKCepsilon expression protects rats from fat-induced hepatic insulin resistance and reverses fat-induced defects in hepatic insulin signaling. Furthermore, we show that PKCepsilon associates with the insulin receptor in vivo and impairs insulin receptor kinase activity both in vivo and in vitro. These data support the hypothesis that PKCepsilon plays a critical role in mediating fat-induced hepatic insulin resistance and represents a novel therapeutic target for type 2 diabetes.

Inhibition of protein kinase Cepsilon prevents hepatic insulin resistance in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance and type 2 diabetes mellitus, but the molecular signals linking hepatic fat accumulation to hepatic insulin resistance are unknown. Three days of high-fat feeding in rats results specifically in hepatic steatosis and hepatic insulin resistance. In this setting, PKCepsilon, but not other isoforms of PKC, is activated. To determine whether PKCepsilon plays a causal role in the pathogenesis of hepatic insulin resistance, we treated rats with an antisense oligonucleotide against PKCepsilon and subjected them to 3 days of high-fat feeding. Knocking down PKCepsilon expression protects rats from fat-induced hepatic insulin resistance and reverses fat-induced defects in hepatic insulin signaling. Furthermore, we show that PKCepsilon associates with the insulin receptor in vivo and impairs insulin receptor kinase activity both in vivo and in vitro. These data support the hypothesis that PKCepsilon plays a critical role in mediating fat-induced hepatic insulin resistance and represents a novel therapeutic target for type 2 diabetes.

Inhibition of protein kinase Cepsilon prevents hepatic insulin resistance in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance and type 2 diabetes mellitus, but the molecular signals linking hepatic fat accumulation to hepatic insulin resistance are unknown. Three days of high-fat feeding in rats results specifically in hepatic steatosis and hepatic insulin resistance. In this setting, PKCepsilon, but not other isoforms of PKC, is activated. To determine whether PKCepsilon plays a causal role in the pathogenesis of hepatic insulin resistance, we treated rats with an antisense oligonucleotide against PKCepsilon and subjected them to 3 days of high-fat feeding. Knocking down PKCepsilon expression protects rats from fat-induced hepatic insulin resistance and reverses fat-induced defects in hepatic insulin signaling. Furthermore, we show that PKCepsilon associates with the insulin receptor in vivo and impairs insulin receptor kinase activity both in vivo and in vitro. These data support the hypothesis that PKCepsilon plays a critical role in mediating fat-induced hepatic insulin resistance and represents a novel therapeutic target for type 2 diabetes.

Metabolic syndrome pathophysiology: the role of adipose tissue

Several pathophysiological explanations for the metabolic syndrome have been proposed involving insulin resistance, chronic inflammation and ectopic fat accumulation following adipose tissue saturation. However, current concepts create several paradoxes, including limited cardiovascular risk reduction with intensive glucose control in diabetics, therapies that result in weight gain (PPAR agonists), and presence of some of the metabolic traits among some lipodystrophies. We propose the functional failure of an organ, in this case, the adipose tissue as a model to interpret its manifestations and to reconcile some of the apparent paradox. A cornerstone of this model is the failure of the adipose tissue to buffer postprandial lipids. In addition, homeostatic feedback loops guide physiological and pathological adipose tissue activities. Fat turnover is determined by a complex equilibrium in which insulin is a main factor but not the only one. Chronically inadequate energy balance may be a key factor, stressing the system. In this situation, an adipose tissue functional failure occurs resulting in changes in systemic energy delivery, impaired glucose consumption and activation of self-regulatory mechanisms that extend their influence to whole body homeostasis system. These include changes in adipokines secretion and vascular effects. The functional capacity of the adipose tissue varies among subjects explaining the incomplete overlapping among the metabolic syndrome and obesity. Variations at multiple gene loci will be partially responsible for these interindividual differences. Two of those candidate genes, the adiponectin (APM1) and the perilipin (PLIN) genes, are discussed in more detail.

High visceral fat mass and high liver fat are associated with resistance to lifestyle intervention

OBJECTIVE

High visceral adipose tissue (VAT) and high liver fat (LF) are associated with the metabolic syndrome and diabetes. We studied changes in these two fat depots during weight loss and analyzed whether VAT and LF at baseline predict the response to lifestyle intervention.

RESEARCH METHODS AND PROCEDURES

One hundred twelve subjects (48 men and 64 women; age, 46 +/- 11 years; BMI, 29.2 +/- 4.4 kg/m(2)) were studied after a follow up-time of 264 +/- 60 (SD) days. Insulin sensitivity was estimated from the oral glucose tolerance test. Body fat depots were quantified using magnetic resonance imaging and spectroscopy.

RESULTS

Cross-sectionally high VAT (r = -0.22, p = 0.02) and high LF (r = -0.36, p < 0.0001) were independently associated with low insulin sensitivity. With intervention, BMI (-3.0%), VAT (-12.0%), and LF (-33.0%) were reduced (all p < 0.001). Insulin sensitivity was improved (+17%, p < 0.01). The changes in BMI (r = -0.41), VAT (r = -0.28), and LF (r = -0.39) were associated with the increase in insulin sensitivity (all p < 0.01). High VAT (r = -0.28, p = 0.01) and high LF (r = -0.38, p < 0.01) at baseline were associated with a lesser increase in insulin sensitivity.

DISCUSSION

Baseline values and changes in BMI, VAT, and LF are related to changes in insulin sensitivity during lifestyle intervention. Subjects with high VAT and LF have a lower chance of profiting from lifestyle intervention and may require intensified lifestyle prevention strategies or even pharmacological approaches to improve insulin sensitivity.