Tissue-specific overexpression of lipoprotein lipase causes tissue-specific insulin resistance

Increased prevalence of insulin resistance and nonalcoholic fatty liver disease in Asian-Indian men

Type 2 diabetes mellitus (T2DM) is strongly associated with obesity in most, but not all, ethnic groups, suggesting important ethnic differences in disease susceptibility. Although it is clear that insulin resistance plays a major role in the pathogenesis of T2DM and that insulin resistance is strongly associated with increases in hepatic (HTG) and/or intramyocellular lipid content, little is known about the prevalence of insulin resistance and potential differences in intracellular lipid distribution among healthy, young, lean individuals of different ethnic groups. To examine this question, 482 young, lean, healthy, sedentary, nonsmoking Eastern Asians (n = 49), Asian-Indians (n = 59), Blacks (n = 48), Caucasians (n = 292), and Hispanics (n = 34) underwent an oral glucose tolerance test to assess whole-body insulin sensitivity by an insulin sensitivity index. In addition, intramyocellular lipid and HTG contents were measured by using proton magnetic resonance spectroscopy. The prevalence of insulin resistance, defined as the lower quartile of insulin sensitivity index, was approximately 2- to 3-fold higher in the Asian-Indians compared with all other ethnic groups, and this could entirely be attributed to a 3- to 4-fold increased prevalence of insulin resistance in Asian-Indian men. This increased prevalence of insulin resistance in the Asian-Indian men was associated with an approximately 2-fold increase in HTG content and plasma IL-6 concentrations compared with Caucasian men. These data demonstrate important ethnic and gender differences in the pathogenesis of insulin resistance in Asian-Indian men and have important therapeutic implications for treatment of T2DM and for the development of steatosis-related liver disease in this ethnic group.

Review: Non-alcoholic fatty liver disease: natural history, pathogenesis and treatment

Non-alcoholic fatty liver disease (NAFLD) is the term used to describe the alcohol-like liver injury that occurs in the absence of alcohol abuse. It embraces a range of histological abnormalities including simple steatosis or fatty liver, non-alcoholic steatohepatitis (NASH) and NAFLD induced cirrhosis. The predominant risk factor for NAFLD appears to be insulin resistance. Simple steatosis and NASH are generally asymptomatic and it is only the development of cirrhosis that has clinical consequence. At present, therapy in NAFLD concentrates on managing risk factors but in the future clinical trials may provide robust evidence for the use of insulin sensitising agents and other potential therapies.

AMP-activated protein kinase: Role in metabolism and therapeutic implications

AMP-activated protein kinase (AMPK) is an enzyme that works as a fuel gauge which becomes activated in situations of energy consumption. AMPK functions to restore cellular ATP levels by modifying diverse metabolic and cellular pathways. In the skeletal muscle, AMPK is activated during exercise and is involved in contraction-stimulated glucose transport and fatty acid oxidation. In the heart, AMPK activity increases during ischaemia and functions to sustain ATP, cardiac function and myocardial viability. In the liver, AMPK inhibits the production of glucose, cholesterol and triglycerides and stimulates fatty acid oxidation. Recent studies have shown that AMPK is involved in the mechanism of action of metformin and thiazolidinediones, and the adipocytokines leptin and adiponectin. These data, along with evidence that pharmacological activation of AMPK in vivo improves blood glucose homeostasis, cholesterol concentrations and blood pressure in insulin-resistant rodents, make this enzyme an attractive pharmacological target for the treatment of type 2 diabetes, ischaemic heart disease and other metabolic diseases.

Role of obesity, insulin resistance, and steatosis in hepatitis C virus infection

Hepatitis C, nonalcoholic fatty liver characterized by hepatic steatosis, and obesity inflict significant health and economic burdens on the Western world. Insulin resistance is the key player in these disease processes. Complex interplay between these conditions results in the ultimate phenotype of liver disease. This article focuses on the current understanding of host and viral interactions as well as on consequent clinical implications.

Hepatic steatosis and insulin resistance are associated with serum imbalance of adiponectin/tumour necrosis factor-alpha in chronic hepatitis C patients

BACKGROUND

Steatosis and insulin resistance (IR) have a pathogenic role in chronic hepatitis C (HCV). Adipocytokines balance modulates hepatic lipid content and IR.

AIM

To evaluate serum adipocytokines and relationship with virological, histological and metabolic parameters in chronic HCV.

METHODS

Adiponectin and tumour necrosis factor-alpha (TNF-alpha) levels, HCV genotypes, HCV-RNA, IR (HOMA-IR), body mass index and liver steatosis and fibrosis were assessed in 161 non-diabetic chronic HCV patients.

RESULTS

Chronic HCV patients with steatosis showed lower serum adiponectin levels and higher levels of TNF-alpha, HOMA, alanine aminotransferase, gamma-glutamiltransferase and Histological Activity Index (HAI) and fibrosis scores. Low adiponectin levels were independently associated with grades of steatosis and HOMA-IR. Higher tumour necrosis factor-alpha levels were observed in patients with low adiponectin levels. The extension of steatosis was inversely correlated with adiponectin levels. A correlation between grade of steatosis with HOMA-IR and fibrosis was observed. HCV genotype 3-infected patients showed lower adiponectin levels than those with other genotypes. An independent predictor of low adiponectin levels in genotype 3 infection was the extension of steatosis. Liver fibrosis score was associated with steatosis, HAI and age.

CONCLUSIONS

Chronic HCV patients with steatosis showed a serum adiponectin/TNF-alpha imbalance that is associated with IR. Reduced adiponectin levels may be involved in the pathogenesis of steatosis, which in turn accelerates progression of fibrosis in chronic HCV.

Fasting decreases free fatty acid turnover in mice overexpressing skeletal muscle lipoprotein lipase

Skeletal muscle lipoprotein lipase (LPL) overexpression in mice results in whole-body insulin resistance and increased intramuscular triglyceride stores, but decreased plasma triglyceride concentration and unchanged plasma free fatty acid (FFA) concentration. The effects of skeletal muscle LPL overexpression and fasting duration on FFA kinetics are unknown. Transgenic mice with muscle-specific LPL overexpression (MCKhLPL) and control mice (Con) were studied at rest during a 50-minute constant infusion of [9,10- 3H]palmitate to determine FFA kinetics after both 4 and 16 hours of fasting. FFA concentration was not different between groups after the 4-hour (Con, 0.80 +/- 0.06 mmol/L; MCKhLPL, 0.83 +/- 0.07 mmol/L) and 16-hour (Con, 0.83 +/- 0.04 mmol/L; MCKhLPL, 0.80 +/- 0.07 mmol/L) fast. FFA turnover (Ra) was not significantly different between MCKhLPL and Con groups after the 4-hour fast (Con Ra = 2.52 +/- 0.36 micromol/min; MCKhLPL Ra = 2.37 +/- 0.27 micromol/min). However, FFA turnover was significantly decreased after the 16-hour fast in MCKhLPL mice vs controls (Con Ra = 2.89 +/- 0.52 micromol/min; MCKhLPL Ra = 1.64 +/- 0.17 micromol/min; P < .05). The significantly lower FFA Ra in MCKhLPL vs control mice was due to a decrease in MCKhLPL FFA turnover from the 4- to 16-hour fast, whereas FFA turnover was unchanged in controls. The changes in FFA appearance after the 16-hour fast in MCKhLPL mice are most likely explained by increased reliance by skeletal muscle on plasma triglyceride as a fuel. These data suggest increased skeletal muscle LPL expression decreases dependence on plasma FFA during prolonged fasting in mice.

Protection against Western diet-induced obesity and hepatic steatosis in liver fatty acid-binding protein knockout mice

Liver fatty acid-binding protein (L-Fabp) regulates murine hepatic fatty acid trafficking in response to fasting. In this study, we show that L-Fabp(-/-) mice fed a high-fat Western diet for up to 18 weeks are less obese and accumulate less hepatic triglyceride than C57BL/6J controls. Paradoxically, both control and L-Fabp(-/-) mice manifested comparable glucose intolerance and insulin resistance when fed a Western diet. Protection against obesity in Western diet-fed L-Fabp(-/-) mice was not due to discernable changes in food intake, fat malabsorption, or heat production, although intestinal lipid secretion kinetics were significantly slower in both chow-fed and Western diet-fed L-Fabp(-/-) mice. By contrast, there was a significant increase in the respiratory exchange ratio in L-Fabp(-/-) mice, suggesting a shift in energy substrate use from fat to carbohydrate, findings supported by an approximately threefold increase in serum lactate. Microarray analysis revealed increased expression of genes involved in lipid synthesis (fatty acid synthase, squalene epoxidase, hydroxy-methylglutaryl coenzyme A reductase), while genes involved in glycolysis (glucokinase and glycerol kinase) were decreased in L-Fabp(-/-) mice. Fatty acid synthase expression was also increased in the skeletal muscle of L-Fabp(-/-) mice. In conclusion, L-Fabp may function as a metabolic sensor in regulating lipid homeostasis. We suggest that L-Fabp(-/-) mice are protected against Western diet-induced obesity and hepatic steatosis through a series of adaptations in both hepatic and extrahepatic energy substrate use. (HEPATOLOGY 2006;44:1191-1205.).

Reduction of plasma triglycerides in apolipoprotein C-II transgenic mice overexpressing lipoprotein lipase in muscle

LPL and its specific physiological activator, apolipoprotein C-II (apoC-II), regulate the hydrolysis of triglycerides (TGs) from circulating TG-rich lipoproteins. Previously, we developed a skeletal muscle-specific LPL transgenic mouse that had lower plasma TG levels. ApoC-II transgenic mice develop hypertriglyceridemia attributed to delayed clearance. To investigate whether overexpression of LPL could correct this apoC-II-induced hypertriglyceridemia, mice with overexpression of human apoC-II (CII) were cross-bred with mice with two levels of muscle-specific human LPL overexpression (LPL-L or LPL-H). Plasma TG levels were 319 +/- 39 mg/dl in CII mice and 39 +/- 5 mg/dl in wild-type mice. Compared with CII mice, apoC-II transgenic mice with the higher level of LPL overexpression (CIILPL-H) had a 50% reduction in plasma TG levels (P = 0.013). Heart LPL activity was reduced by approximately 30% in mice with the human apoC-II transgene, which accompanied a more modest 10% decrease in total LPL protein. Overexpression of human LPL in skeletal muscle resulted in dose-dependent reduction of plasma TGs in apoC-II transgenic mice. Along with plasma apoC-II concentrations, heart and skeletal muscle LPL activities were predictors of plasma TGs. These data suggest that mice with the human apoC-II transgene may have alterations in the expression/activity of endogenous LPL in the heart. Furthermore, the decrease of LPL activity in the heart, along with the inhibitory effects of excess apoC-II, may contribute to the hypertriglyceridemia observed in apoC-II transgenic mice.

Mechanism of glucose intolerance in mice with dominant negative mutation of CEACAM1

Mice with liver-specific overexpression of dominant negative phosphorylation-defective S503A-CEACAM1 mutant (L-SACC1) developed chronic hyperinsulinemia resulting from blunted hepatic clearance of insulin, visceral obesity, and glucose intolerance. To determine the underlying mechanism of altered glucose homeostasis, a 2-h hyperinsulinemic euglycemic clamp was performed, and tissue-specific glucose and lipid metabolism was assessed in awake L-SACC1 and wild-type mice. Inactivation of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) caused insulin resistance in liver that was mostly due to increased expression of fatty acid synthase and lipid metabolism, resulting in elevated intrahepatic levels of triglyceride and long-chain acyl-CoAs. Whole body insulin resistance in the L-SACC1 mice was further attributed to defects in insulin-stimulated glucose uptake in skeletal muscle and adipose tissue. Insulin resistance in peripheral tissues was associated with significantly elevated intramuscular fat contents that may be secondary to increased whole body adiposity (assessed by (1)H-MRS) in the L-SACC1 mice. Overall, these results demonstrate that L-SACC1 is a mouse model in which chronic hyperinsulinemia acts as a cause, and not a consequence, of insulin resistance. Our findings further indicate the important role of CEACAM1 and hepatic insulin clearance in the pathogenesis of obesity and insulin resistance.

Beneficial and deleterious effects of hydroxycitrate in rats fed a high-fructose diet

OBJECTIVE

The present study assessed whether long-term supplementation of a high-fructose diet with hydroxycitrate (HCA), an inhibitor of de novo lipogenesis that is widely used as a non-prescription dietary aid, decreases food intake, visceral fat accumulation, hypertriglyceridemia, and hyperinsulinemia in rats.

METHODS

We examined the effects of HCA (1.8% of diet) on food intake, body weight gain, visceral fat accumulation, hypertriglyceridemia, and hyperinsulinemia in rats during a 4-wk period of ad libitum access to a 50% fructose diet after a 3-wk period of food restriction in which they lost about 20% of their body weight.

RESULTS

HCA decreased food intake and weight gain throughout the test and reduced visceral fat accumulation compared with control rats fed ad libitum (CON). Rats that were pair-fed (PF) to the HCA rats showed similar decreases in weight gain and visceral fat. HCA did not ameliorate the hypertriglyceridemia induced by high-fructose feeding. HCA improved insulin sensitivity on day 10 in comparison with CON rats, but by day 27 insulin levels were similarly higher and liver glycogen levels were similarly lower in HCA and CON rats in comparison with PF rats. Liver lipid content was elevated in HCA rats compared with CON and PF rats.

CONCLUSION

These findings indicate that, although HCA attenuates body weight gain and visceral fat accumulation by reducing food intake under these conditions, it has no lasting beneficial effects on hypertriglyceridemia and hyperinsulinemia and leads to the accumulation of liver lipids.

Orphan nuclear receptors: therapeutic opportunities in skeletal muscle

Nuclear hormone receptors (NRs) are ligand-dependent transcription factors that bind DNA and translate physiological signals into gene regulation. The therapeutic utility of NRs is underscored by the diversity of drugs created to manage dysfunctional hormone signaling in the context of reproductive biology, inflammation, dermatology, cancer, and metabolic disease. For example, drugs that target nuclear receptors generate over $10 billion in annual sales. Almost two decades ago, gene products were identified that belonged to the NR superfamily on the basis of DNA and protein sequence identity. However, the endogenous and synthetic small molecules that modulate their action were not known, and they were denoted orphan NRs. Many of the remaining orphan NRs are highly enriched in energy-demanding major mass tissues, including skeletal muscle, brown and white adipose, brain, liver, and kidney. This review focuses on recently adopted and orphan NR function in skeletal muscle, a tissue that accounts for approximately 35% of the total body mass and energy expenditure, and is a major site of fatty acid and glucose utilization. Moreover, this lean tissue is involved in cholesterol efflux and secretes that control energy expenditure and adiposity. Consequently, muscle has a significant role in insulin sensitivity, the blood lipid profile, and energy balance. Accordingly, skeletal muscle plays a considerable role in the progression of dyslipidemia, diabetes, and obesity. These are risk factors for cardiovascular disease, which is the the foremost cause of global mortality (>16.7 million deaths in 2003). Therefore, it is not surprising that orphan NRs and skeletal muscle are emerging as therapeutic candidates in the battle against dyslipidemia, diabetes, obesity, and cardiovascular disease.

Molecular approaches to study control of glucose homeostasis

Type 2 diabetes is a polygenic disease that can lead to severe complications in multiple tissues. Rodent models have been used widely for investigating the pathophysiology underlying type 2 diabetes and for examining the potential link with obesity, largely due to the limitations of invasive testing and of studying detailed molecular mechanisms in human tissues. Among rodents, the mouse model is especially popular because mice are easy to manipulate genetically, have a short generation time, and are relatively inexpensive. The most commonly used inbred mouse strains are reviewed in addition to several genetically engineered mouse models that have been generated to study type 2 diabetes in the context of obesity, with a focus on insulin, leptin, and peroxisome proliferator-activated receptor (PPAR) signaling pathways.

Influence of hepatic steatosis (fatty liver) on severity and composition of dyslipidemia in type 2 diabetes

OBJECTIVE

The objective of this study was to examine the associations between the severity of hepatic steatosis and dyslipidemia in type 2 diabetes, including circulating apolipoprotein B100 (apoB) concentrations and lipoprotein particle size and numbers.

RESEARCH DESIGN AND METHODS

Computed tomography imaging was used to assess hepatic fat content and adipose tissue distribution in 67 men and women with type 2 diabetes, withdrawn from antidiabetic medications preceding the study. Fasting serum lipoprotein number and size was determined by nuclear magnetic resonance. Insulin sensitivity was measured with a glucose clamp and a [6,6-(2)H(2)]glucose isotope infusion.

RESULTS

Two-thirds of the cohort had fatty liver. Hepatic steatosis correlated with serum triglycerides (r = 0.40, P < 0.01) and lower HDL cholesterol (r = -0.31, P < 0.05). ApoB and LDL cholesterol did not, being virtually identical in those with or without steatosis. The association between serum triglycerides and hepatic steatosis was largely accounted for by greater triglyceride enrichment in VLDL particles, which were larger. Severe steatosis was also associated with 70% higher small, dense LDL concentrations. Visceral obesity did not fully explain these associations, and hepatic steatosis was better correlated with triglycerides than with hyperglycemia or hepatic insulin resistance (P > 0.05).

CONCLUSIONS

The presence of hepatic steatosis in type 2 diabetes does not appear to affect apoB levels, but potentially increases atherogenesis by increasing triglycerides, reducing HDL levels, and increasing small, dense LDL.

The impact of diabetes mellitus on fibrosis progression in patients transplanted for hepatitis C

Despite the recognition of numerous factors for aggressive hepatitis C virus (HCV) recurrence after liver transplantation (LT) our understanding of this phenomenon is incomplete. We tested the hypothesis that diabetes mellitus (DM) was implicated. One hundred sixty-three patients undergoing primary LT for HCV from 1990 to 2004 were evaluated and biopsies were scored according to the modified Ishak score. Severe recurrence of HCV was defined as a fibrosis score > or = 4 within 6 years of LT. Risk factors assessed included recipient, donor and transplant variables. Fifty-four patients (33.1%) had a fibrosis score > or = 4 at the end of the study period. Factors associated with progression to severe fibrosis was donor age (p = 0.008) especially donor age >55 (p = 0.038, HR 2.43), pre-LT DM (p = 0.039, HR 2.68) and DM post-LT (p = 0.004, HR 3.28). The combination of receiving a liver from a donor older than 55 years and having DM post-LT was associated with an 8.38-fold risk of progression to severe fibrosis (p = 0.000124) when compared to patients not diabetic post-LT who received livers from donors aged <55 years. These data indicate that diabetic status is one of the more important variables determining the severity of HCV recurrence and is synergistic with donor age. This observation may provide an additional management opportunity to modify the impact of HCV recurrence.

Effect of sucrose and saturated-fat diets on mRNA levels of genes limiting muscle fatty acid and glucose supply in rats

In this study, we examined whether the increased availability of lipids in blood resulting from two types of diet manipulation regulated metabolic gene expression in the skeletal muscle of rats. Feeding for 4 wk on an isocaloric-sucrose or a hypercaloric-fat diet increased plasma TAG in the fed condition by increments of 70 and 40%, respectively, and increased fasting insulinemia (approximately 3-fold) compared with a starch diet. The fat diet impaired glucose tolerance and caused obesity, whereas sucrose-fed rats maintained their normal weight. We analyzed the expression of genes that regulate the exogenous FA supply (LPL, FAT/CD36, FATP1), synthesis (ACC1), glucose (GLUT4, GLUT1, HK2, GFAT1, glycogen phosphorylase) or glycerol (glycerol kinase) provision, or substrate choice for oxidation (PDK4) in gastrocnemius and soleus muscles at the end of the glucose tolerance test. LPL, FAT/CD36, FATP1, PDK4, and GLUT4 mRNA as well as glycogen phosphorylase and glycerol kinase activity levels in both muscles were unchanged by the diets. Increased mRNA levels of GLUT1 (1.6- and 2.6-fold, respectively) and GFAT1 (about 1.7-fold) in gastrocnemius, and of ACC1 (about 1.5-fold) in soleus, were found in both the sucrose and fat groups. In the fat group, HK2 mRNA was also higher (1.8-fold) in the gastrocnemius. Both sucrose and saturated-fat diets prompted hyperinsulinemia and hyperlipemia in rats. These metabolic disturbances did not alter the expression of LPL, FAT/CD36, FATP1, PDK4, and GLUT4 genes or glycogen phosphorylase and glycerol kinase activity levels in either analyzed muscle. Instead, they were linked to the coordinated upregulation in gastrocnemius of genes that govern glucose uptake and the hexosamine pathway, namely, GLUT1 and GFAT1, which might contribute to insulin resistance.

Hypercytolipidemia-induced cellular lipoapoptosis: Cytostructural and endometabolic basis of progressive organo-involution following expression of diabetes (db/db) and obese (ob/ob) mutation syndromes

Onset expression of Type 2 (NIDDM) diabetes and obesity metabolic syndromes (DOS) are characterized by premature, progressive cytoatrophy and organo-involution induced by dysregulated cellular gluco- and lipo-metabolic cascades. The consequential systemic, interstitial and intracellular hyperlipidemia disrupts normal cytointegrity and metabolic responsivity to the established hypercaloric pericellular environments. The sequential cytostructural, metabolic and endocrine disturbances associated with the development of progressive DOS-associated hypercytolipidemia compromises cellular metabolic response cascades and promotes cytochemical disturbances which culminate with nuclear lipoapoptosis and cytoatrophy. The dramatic alterations in interstitial glucose and lipid (free fatty acids/triglycerides) concentrations are recognized to influence interstitial and cytoplasmic microchemical environments, which markedly alter cellular nutrient diffusion and active trans-membrane flux rates. The progressive exacerbation of interstitial and cytoplasmic lipid imbibition has been demonstrated to be associated with DNA fragmentation by lipo-infiltration into the chromatin matrix, inducing structural disruption and physical dissolution, indexed as nuclear lipoapoptosis. Therapeutic reduction of the severity of hypercytolipidemia-induced structural and cytochemical compromise promotes the restoration of homeostatic metabolic support for normalized cytostructural indices and supportive cellular gluco- and lipo-metabolic cascades. The re-establishment of a homeostatic interstitial microenvironment moderates the severity of cytolipidemic compromise within affected cell types, reduces nuclear lipo-infiltration and DNA lipo-dissolution, resulting in the preservation of cytostructural integrity. Through the therapeutic restoration of extra- and intra-cellular microchemical environments in genetically dysregulated metabolic syndrome models, the coincident cytochemical, endocrine and metabolic disturbances associated with progressive hypercytolipidemia, resulting from the expressed systemic hypercaloric environmental and hepato-pancreatic endometabolic disturbances which characterize Type 2 (NIDDM) diabetes-obesity and metabolic (X) syndromes, may be ameliorated.

Targeting foxo1 in mice using antisense oligonucleotide improves hepatic and peripheral insulin action

Fasting hyperglycemia, a prominent finding in diabetes, is primarily due to increased gluconeogenesis. The transcription factor Foxo1 links insulin signaling to decreased transcription of PEPCK and glucose-6-phosphatase (G6Pase) and provides a possible therapeutic target in insulin-resistant states. Synthetic, optimized antisense oligonucleotides (ASOs) specifically inhibit Foxo1 expression. Here we show the effect of such therapy on insulin resistance in mice with diet-induced obesity (DIO). Reducing Foxo1 mRNA expression with ASO therapy in mouse hepatocytes decreased levels of Foxo1 protein and mRNA expression of PEPCK by 48 +/- 4% and G6Pase by 64 +/- 3%. In mice with DIO and insulin resistance, Foxo1 ASO therapy lowered plasma glucose concentration and the rate of basal endogenous glucose production. In addition, Foxo1 ASO therapy lowered both hepatic triglyceride and diacylglycerol content and improved hepatic insulin sensitivity. Foxo1 ASO also improved adipocyte insulin action. At a tissue-specific level, this manifested as improved insulin-mediated 2-deoxyglucose uptake and suppression of lipolysis. On a whole-body level, the result was improved glucose tolerance after an intraperitoneal glucose load and increased insulin-stimulated whole-body glucose disposal during a hyperinsulinemic-euglycemic clamp. In conclusion, Foxo1 ASO therapy improved both hepatic insulin and peripheral insulin action. Foxo1 is a potential therapeutic target for improving insulin resistance.

A choline-deficient diet exacerbates fatty liver but attenuates insulin resistance and glucose intolerance in mice fed a high-fat diet

Liver fat accumulation is proposed to link obesity and insulin resistance. To dissect the role of liver fat in the insulin resistance of diet-induced obesity, we altered liver fat using a choline-deficient diet. C57Bl/6 mice were fed a low-fat (10% of calories) or high-fat (45% of calories) diet for 8 weeks; during the final 4 weeks, diets were either choline deficient or choline supplemented. In choline replete animals, high-fat feeding induced weight gain, elevated liver triglycerides (171%), hyperinsulinemia, and glucose intolerance. Choline deficiency did not affect body or adipose depot weights but amplified liver fat accumulation with high-fat diet (281%, P < 0.01). However, choline deficiency lowered fasting plasma insulin (from 983 +/- 175 to 433 +/- 36 pmol/l, P < 0.01) and improved glucose tolerance on a high-fat diet. In mice on 30% fat diet, choline deficiency increased liver mRNA levels of the rate-limiting enzyme in phosphatidylcholine synthesis and of enzymes involved in free fatty acid esterification, without affecting those of de novo lipogenesis or fatty acid oxidation. We conclude that liver fat accumulation per se does not cause insulin resistance during high-fat feeding and that choline deficiency may shunt potentially toxic free fatty acids toward innocuous storage triglyceride in the liver.

Computational disease gene identification: a concert of methods prioritizes type 2 diabetes and obesity candidate genes

Genome-wide experimental methods to identify disease genes, such as linkage analysis and association studies, generate increasingly large candidate gene sets for which comprehensive empirical analysis is impractical. Computational methods employ data from a variety of sources to identify the most likely candidate disease genes from these gene sets. Here, we review seven independent computational disease gene prioritization methods, and then apply them in concert to the analysis of 9556 positional candidate genes for type 2 diabetes (T2D) and the related trait obesity. We generate and analyse a list of nine primary candidate genes for T2D genes and five for obesity. Two genes, LPL and BCKDHA, are common to these two sets. We also present a set of secondary candidates for T2D (94 genes) and for obesity (116 genes) with 58 genes in common to both diseases.

Role of changes in cardiac metabolism in development of diabetic cardiomyopathy

In patients with diabetes, an increased risk of symptomatic heart failure usually develops in the presence of hypertension or ischemic heart disease. However, a predisposition to heart failure might also reflect the effects of underlying abnormalities in diastolic function that can occur in asymptomatic patients with diabetes alone (termed diabetic cardiomyopathy). Evidence of cardiomyopathy has also been demonstrated in animal models of both Type 1 (streptozotocin-induced diabetes) and Type 2 diabetes (Zucker diabetic fatty rats and ob/ob or db/db mice). During insulin resistance or diabetes, the heart rapidly modifies its energy metabolism, resulting in augmented fatty acid and decreased glucose consumption. Accumulating evidence suggests that this alteration of cardiac metabolism plays an important role in the development of cardiomyopathy. Hence, a better understanding of this dysregulation in cardiac substrate utilization during insulin resistance and diabetes could provide information as to potential targets for the treatment of cardiomyopathy. This review is focused on evaluating the acute and chronic regulation and dysregulation of cardiac metabolism in normal and insulin-resistant/diabetic hearts and how these changes could contribute toward the development of cardiomyopathy.

Fatty acid-induced inflammation and insulin resistance in skeletal muscle and liver

Plasma free fatty acid (FFA) levels are elevated in obesity. FFA, by causing insulin resistance in muscle, liver, and endothelial cells, contributes to the development of type 2 diabetes mellitus (T2DM), hypertension, dyslipidemia, and nonalcoholic fatty liver disease (NAFLD). The mechanism through which FFA induces insulin resistance involves intramyocellular and intrahepatocellular accumulation of triglycerides and diacylglycerol, activation of several serine/threonine kinases, reduction in tyrosine phosphorylation of the insulin receptor substrate (IRS)-1/2, and impairment of the IRS/phosphatidylinositol 3-kinase pathway of insulin signaling. FFA also produces low-grade inflammation in skeletal muscle and liver through activation of nuclear factor-kappaB, resulting in release of several proinflammatory and proatherogenic cytokines. Thus, elevated FFA levels (due to obesity or to high-fat feeding) cause insulin resistance in skeletal muscle and liver, which contributes to the development of T2DM, and produce low-grade inflammation, which contributes to the development of atherosclerotic vascular diseases and NAFLD.

Acetic acid activates hepatic AMPK and reduces hyperglycemia in diabetic KK-A(y) mice

Acetic acid (AcOH), which is a short-chain fatty acid, is reported to have some beneficial effects on metabolism. To test the hypothesis that feeding of AcOH exerts beneficial effects on glucose homeostasis in type 2 diabetes, we fed either a standard diet or one containing 0.3% AcOH to KK-A(y) mice for 8 weeks. Fasting plasma glucose and HbA1c levels were lower in mice fed AcOH for 8 weeks than in control mice. AcOH also reduced the expression of genes involved in gluconeogenesis and lipogenesis, which is in part regulated by 5'-AMP-activated protein kinase (AMPK) in the liver. Finally, sodium acetate, in the form of neutralized AcOH, directly activated AMPK and lowered the expression of genes such as for glucose-6-phosphatase and sterol regulatory element binding protein-1 in rat hepatocytes. These results indicate that the hypoglycemic effect of AcOH might be due to activation of AMPK in the liver.

FATP1 is an insulin-sensitive fatty acid transporter involved in diet-induced obesity

Fatty acid transport protein 1 (FATP1), a member of the FATP/Slc27 protein family, enhances the cellular uptake of long-chain fatty acids (LCFAs) and is expressed in several insulin-sensitive tissues. In adipocytes and skeletal muscle, FATP1 translocates from an intracellular compartment to the plasma membrane in response to insulin. Here we show that insulin-stimulated fatty acid uptake is completely abolished in FATP1-null adipocytes and greatly reduced in skeletal muscle of FATP1-knockout animals while basal LCFA uptake by both tissues was unaffected. Moreover, loss of FATP1 function altered regulation of postprandial serum LCFA, causing a redistribution of lipids from adipocyte tissue and muscle to the liver, and led to a complete protection from diet-induced obesity and insulin desensitization. This is the first in vivo evidence that insulin can regulate the uptake of LCFA by tissues via FATP1 activation and that FATPs determine the tissue distribution of dietary lipids. The strong protection against diet-induced obesity and insulin desensitization observed in FATP1-null animals suggests FATP1 as a novel antidiabetic target.

A double-blind randomized placebo-controlled trial of orlistat for the treatment of nonalcoholic fatty liver disease

BACKGROUND & AIMS

Few controlled studies have addressed the issue of effective medical treatment for nonalcoholic fatty liver disease (NAFLD). We herein assessed the effect of orlistat in patients with NAFLD.

METHODS

We performed a randomized, double-blind, placebo-controlled study on 52 patients with NAFLD diagnosed by ultrasound (US) and confirmed by liver biopsy (40 patients). The patients were randomized to receive either orlistat (120 mg 3 times daily for 6 months) or placebo. All patients participated in an identical behavioral weight loss program. All patients underwent monthly evaluation by abdominal US; liver enzyme levels, lipid profiles, insulin levels, and anthropometric parameters were monitored, and all patients underwent nutritional follow-up evaluation. Twenty-two patients underwent a second liver biopsy examination at the end of the study.

RESULTS

Fifty-two patients were recruited and 44 (mean age, 47.7 y; mean body mass index, 33) completed the study. Serum glucose and insulin levels (P<.03) were significantly higher in the orlistat group, which also presented a higher degree of fibrosis. Body mass index was reduced significantly in each group, with a nonsignificant difference between the groups. Serum alanine transaminase (ALT) levels decreased significantly in both groups, with an almost 2-fold reduction in the orlistat group (48% vs 26.4%). There was a statistically significant reversal of fatty liver by US only in the orlistat group (P<.05).

CONCLUSIONS

Orlistat improves serum ALT levels and steatosis on US in NAFLD patients, beyond its effect on weight reduction.

The evolution of severe steatosis after bariatric surgery is related to insulin resistance

BACKGROUND & AIMS

In severely obese patients, factors implicated in the evolution of severe steatosis after bariatric surgery remain unresolved. Our aim was to determine whether insulin resistance (IR) influences the histologic effects induced by bariatric surgery.

METHODS

We prospectively included 185 severely obese patients (body mass index >/=35 kg/m(2)) referred for bariatric surgery. The evolution of IR (IR index = 1/quantitative insulin sensitivity check index) and liver injury with consecutive biopsy was concomitantly assessed before and 1 year after surgery.

RESULTS

At preoperative biopsy, 27% of severely obese patients disclosed severe steatosis (>/=60%). The alanine aminotransferase (P = .01) and IR indexes (P = .04) were independent predictive factors of severe steatosis at baseline. One year after surgery, surgical treatment induced a decrease in body mass index (9.5 kg/m(2); P < .0001), steatosis score (8.5%; P < .0001), and IR index (0.29; P < .0001). The preoperative IR index (P = .01) and preoperative steatosis (P = .006) were independent predictive factors in the persistence of severe steatosis after surgery. Moderate or severe steatosis was more frequently observed in patients who had conserved a higher IR index after surgery than in patients who had improved their IR index (44% vs 20.2%; P = .04).

CONCLUSIONS

IR was independently associated with severe steatosis and predicted its persistence after surgery. The amelioration of IR after surgery is associated with a decrease in the amount of fat. Taken together, the results of this prospective study in severely obese patients demonstrate that severe steatosis and its evolution after surgery are intimately connected with IR.

Viral and metabolic factors influencing alanine aminotransferase activity in patients with chronic hepatitis C

BACKGROUND/AIMS

In chronic hepatitis C, disease progression and clinical manifestations are heterogenous. To clarify the role and interactions of viral and host factors in inducing liver cell injury, we examined the associations of several virological and metabolic variables with serum alanine aminotransferase levels.

METHODS

Patients with chronic hepatitis C enrolled in three phase III clinical trials of peginterferon alfa-2a (40KD) plus ribavirin (two studies analysing 'elevated' and one persistently 'normal' alanine aminotransferase) were included.

RESULTS

Multivariate analyses of 2,881 patients before treatment and of 1,403 patients with a sustained virological response indicated that gender, viral factors (genotype, HCV RNA titer) and indicators of metabolic syndrome (body mass index, blood pressure, blood glucose, cholesterol and triglyceride concentration) were associated with alanine aminotransferase levels. In addition, hepatitis C virus infection influenced serum lipids concentration according to a genotype-specific effect.

CONCLUSIONS

Heterogeneity in alanine aminotransferase levels in patients with chronic hepatitis C partially depends on the degree of derangement of fat and carbohydrate metabolism. As this is the result of an interaction of chronic hepatitis C infection with the patient's individual characteristics, treatment decisions should not be based on alanine aminotransferase level alone but rather on global evaluation of the patient.

Genetics and regulation of angiopoietin-like proteins 3 and 4

PURPOSE OF REVIEW

Lipoprotein lipase activity in a given tissue is the rate limiting step for the uptake of triglyceride-derived fatty acids. Imbalances in the partitioning of fatty acids have major metabolic consequences. Given the central role of lipoprotein lipase in energy metabolism, the discovery of new molecules that affect the activity of lipoprotein lipase holds great potential for novel therapeutic targets.

RECENT FINDINGS

Angiopoietin-like proteins 3 and 4 are two members of the angiopoietin-like family of proteins (Angptl). Unique within this family, Angptl3 and 4 inhibit lipoprotein metabolism via their ability to inhibit the activity of lipoprotein lipase. This review highlights recent studies on the biochemistry of Angptl3 and 4 as well as mouse models with selective overexpression of Angptl4 or global knockout of Angptl3, 4, or both.

SUMMARY

Both angiopoietins and angiopoietin-like proteins share similar domain structures. Angptl3 and 4 are the only two members of this superfamily that inhibit lipoprotein lipase activity. However, Angptl3 and 4 are differentially regulated at multiple levels, suggesting non-redundant functions in vivo. Angptl3 and 4 are proteolytically processed into two halves and are differentially regulated by nuclear receptors. Transgenic overexpression of Angptl4 as well as knockout of Angptl3 or 4 demonstrate that these two proteins play essential roles in lipoprotein metabolism: liver-derived Angptl3 inhibits lipoprotein lipase activity primarily in the fed state, while Angptl4 plays important roles in both fed and fasted states. In addition, Angptl4 regulates the tissue-specific delivery of lipoprotein-derived fatty acids. Angptl4 is thus an endocrine or autocrine/paracarine inhibitor of lipoprotein lipase depending on its sites of expression.

Nonalcoholic fatty liver disease

NAFLD likely is the most common liver disease in children and is responsible for significant progression to cirrhosis, portal hypertension, and the need for liver transplantation in adults and even in some adolescents. Early diagnosis and lifestyle interventions appear to be our best hope for controlling progression of disease. The pediatrician is responsible for screening all obese children with measurement of aminotransferases. Those with elevated enzymes (particularly ALT) for longer than 3 months, in the absence of markers of hepatitis B or C, autoimmune chronic active hepatitis, Wilson's disease, hemochromatosis, or alpha-1-antitrypsin deficiency, should follow up with an abdominal ultrasound. In patients with a BMI in the morbidly obese range, an ultrasound to search for a diffusely echogenic liver should be performed even if the liver enzymes are normal. Findings suggestive of NAFLD should prompt the institution of appropriate dietary and exercise regimens. If these are unsuccessful after a 3-month trial, the patient should be referred to a pediatric gastroenterologist and hepatologist for further work-up and treatment, preferably in the context of a controlled therapeutic trial. Only by aggressively engaging this current epidemic will we be able to decrease the mounting human cost of NAFLD.

Overexpression of Rad in muscle worsens diet-induced insulin resistance and glucose intolerance and lowers plasma triglyceride level

Rad is a low molecular weight GTPase that is overexpressed in skeletal muscle of some patients with type 2 diabetes mellitus and/or obesity. Overexpression of Rad in adipocytes and muscle cells in culture results in diminished insulin-stimulated glucose uptake. To further elucidate the potential role of Rad in vivo, we have generated transgenic (tg) mice that overexpress Rad in muscle using the muscle creatine kinase (MCK) promoter-enhancer. Rad tg mice have a 6- to 12-fold increase in Rad expression in muscle as compared to wild-type littermates. Rad tg mice grow normally and have normal glucose tolerance and insulin sensitivity, but have reduced plasma triglyceride levels. On a high-fat diet, Rad tg mice develop more severe glucose intolerance than the wild-type mice; this is due to increased insulin resistance in muscle, as exemplified by a rightward shift in the dose-response curve for insulin stimulated 2-deoxyglucose uptake. There is also a unexpected further reduction of the plasma triglyceride levels that is associated with increased levels of lipoprotein lipase in the Rad tg mice. These results demonstrate a potential synergistic interaction between increased expression of Rad and high-fat diet in creation of insulin resistance and altered lipid metabolism present in type 2 diabetes.

Increased malonyl-CoA and diacylglycerol content and reduced AMPK activity accompany insulin resistance induced by glucose infusion in muscle and liver of rats

Glucose infusion in rats for 1-4 days results in insulin resistance and increased triglyceride, whole tissue long-chain fatty acyl-CoA (LCA-CoA), and malonyl-CoA content in red skeletal muscle. Despite this, the relation between these alterations and the onset of insulin resistance has not been defined. We aimed to 1) identify whether the changes in these lipids and of diacylglycerol (DAG) precede or accompany the onset of insulin resistance in glucose-infused rats, 2) determine whether the insulin resistance is associated with alterations in AMP-activated protein kinase (AMPK), and 3) assess whether similar changes occur in liver and in muscle. Hyperglycemia (17-18 mM) was maintained by intravenous glucose infusion in rats for 3 or 5 h; then euglycemia was restored and a 2-h hyperinsulinemic clamp was performed. Significant (P < 0.01) muscle and liver insulin resistance first appeared in red quadriceps and liver of the glucose-infused group at 5 h and was associated with a twofold increase in DAG and malonyl-CoA content and a 50% decrease in AMPK and acetyl-CoA carboxylase (ACC) phosphorylation and AMPK activity. White quadriceps showed qualitatively similar changes but without decreases in AMPK or ACC phosphorylation. Triglyceride mass was increased at 5 h only in liver, and whole tissue LCA-CoA content was not increased in liver or either muscle type. We conclude that the onset of insulin resistance induced by glucose oversupply correlates temporally with increases in malonyl-CoA and DAG content in all three tissues and with reduced AMPK phosphorylation and activity in red muscle and liver. In contrast, it was not associated with increased whole tissue LCA-CoA content in any tissue or triglyceride in muscle, although both are observed at later times.

Reversal of diet-induced hepatic steatosis and hepatic insulin resistance by antisense oligonucleotide inhibitors of acetyl-CoA carboxylases 1 and 2

Hepatic steatosis is a core feature of the metabolic syndrome and type 2 diabetes and leads to hepatic insulin resistance. Malonyl-CoA, generated by acetyl-CoA carboxylases 1 and 2 (Acc1 and Acc2), is a key regulator of both mitochondrial fatty acid oxidation and fat synthesis. We used a diet-induced rat model of nonalcoholic fatty liver disease (NAFLD) and hepatic insulin resistance to explore the impact of suppressing Acc1, Acc2, or both Acc1 and Acc2 on hepatic lipid levels and insulin sensitivity. While suppression of Acc1 or Acc2 expression with antisense oligonucleotides (ASOs) increased fat oxidation in rat hepatocytes, suppression of both enzymes with a single ASO was significantly more effective in promoting fat oxidation. Suppression of Acc1 also inhibited lipogenesis whereas Acc2 reduction had no effect on lipogenesis. In rats with NAFLD, suppression of both enzymes with a single ASO was required to significantly reduce hepatic malonyl-CoA levels in vivo, lower hepatic lipids (long-chain acyl-CoAs, diacylglycerol, and triglycerides), and improve hepatic insulin sensitivity. Plasma ketones were significantly elevated compared with controls in the fed state but not in the fasting state, indicating that lowering Acc1 and -2 expression increases hepatic fat oxidation specifically in the fed state. These studies suggest that pharmacological inhibition of Acc1 and -2 may be a novel approach in the treatment of NAFLD and hepatic insulin resistance.

Considerations in the design of hyperinsulinemic-euglycemic clamps in the conscious mouse

Despite increased use of the hyperinsulinemic-euglycemic clamp to study insulin action in mice, the effects of experimental parameters on the results obtained have not been addressed. In our studies, we determined the influences of sampling sites, fasting duration, and insulin delivery on results obtained from clamps in conscious mice. Carotid artery and jugular vein catheters were implanted in C57BL/6J mice (n = 6-10/group) fed a normal diet for sampling and infusions. After a 5-day recovery period, mice underwent a 120-min clamp (2.5-mU . kg(-1) . min(-1) insulin infusion; approximately 120-130 mg/dl glucose) while receiving [3-(3)H]glucose to determine glucose appearance (endoR(a)) and disappearance (R(d)). Sampling large volumes (approximately 100 mul) from the cut tail resulted in elevated catecholamines and basal glucose compared with artery sampling. Catecholamines were not elevated when taking small samples ( approximately 5 mul) from the cut tail. Overnight (18-h) fasting resulted in greater loss of total body, lean, and fat masses and hepatic glycogen but resulted in enhanced insulin sensitivity compared with 5-h fasting. Compared with a 16-mU/kg insulin prime, a 300-mU/kg prime resulted in hepatic insulin resistance and slower acquisition of steady-state glucose infusion rates (GIR) after a 5-h fast. The steady-state GIR was expedited after the 300-mU/kg prime in 18-h-fasted mice. The GIR and R(d) rose with increasing insulin infusions (0.8, 2.5, 4, and 20 mU . kg(-1) . min(-1)), but endoR(a) was fully suppressed with doses higher than 0.8 mU . kg(-1) . min(-1). Thus, common variations in experimental factors yield different results and should be considered in designing and interpreting clamps.

Nonalcoholic fatty liver disease: from steatosis to cirrhosis

Nonalcoholic steatohepatitis (NASH), the lynchpin between steatosis and cirrhosis in the spectrum of nonalcoholic fatty liver disorders (NAFLD), was barely recognized in 1981. NAFLD is now present in 17% to 33% of Americans, has a worldwide distribution, and parallels the frequency of central adiposity, obesity, insulin resistance, metabolic syndrome and type 2 diabetes. NASH could be present in one third of NAFLD cases. Age, activity of steatohepatitis, and established fibrosis predispose to cirrhosis, which has a 7- to 10-year liver-related mortality of 12% to 25%. Many cases of cryptogenic cirrhosis are likely endstage NASH. While endstage NAFLD currently accounts for 4% to 10% of liver transplants, this may soon rise. Pathogenic concepts for NAFLD/NASH must account for the strong links with overnutrition and underactivity, insulin resistance, and genetic factors. Lipotoxicity, oxidative stress, cytokines, and other proinflammatory mediators may each play a role in transition of steatosis to NASH. The present "gold standard" management of NASH is modest weight reduction, particularly correction of central obesity achieved by combining dietary measures with increased physical activity. Whether achieved by "lifestyle adjustment" or anti-obesity surgery, this improves insulin resistance and reverses steatosis, hepatocellular injury, inflammation, and fibrosis. The same potential for "unwinding" fibrotic NASH is indicated by studies of the peroxisome proliferation activator receptor (PPAR)-gamma agonist "glitazones," but these agents may improve liver disease at the expense of worsening obesity. Future challenges are to approach NAFLD as a preventive public health initiative and to motivate affected persons to adopt a healthier lifestyle.

The farnesoid X receptor modulates adiposity and peripheral insulin sensitivity in mice

The farnesoid X receptor (FXR) is a bile acid (BA)-activated nuclear receptor that plays a major role in the regulation of BA and lipid metabolism. Recently, several studies have suggested a potential role of FXR in the control of hepatic carbohydrate metabolism, but its contribution to the maintenance of peripheral glucose homeostasis remains to be established. FXR-deficient mice display decreased adipose tissue mass, lower serum leptin concentrations, and elevated plasma free fatty acid levels. Glucose and insulin tolerance tests revealed that FXR deficiency is associated with impaired glucose tolerance and insulin resistance. Moreover, whole-body glucose disposal during a hyperinsulinemic euglycemic clamp is decreased in FXR-deficient mice. In parallel, FXR deficiency alters distal insulin signaling, as reflected by decreased insulin-dependent Akt phosphorylation in both white adipose tissue and skeletal muscle. Whereas FXR is not expressed in skeletal muscle, it was detected at a low level in white adipose tissue in vivo and induced during adipocyte differentiation in vitro. Moreover, mouse embryonic fibroblasts derived from FXR-deficient mice displayed impaired adipocyte differentiation, identifying a direct role for FXR in adipocyte function. Treatment of differentiated 3T3-L1 adipocytes with the FXR-specific synthetic agonist GW4064 enhanced insulin signaling and insulin-stimulated glucose uptake. Finally, treatment with GW4064 improved insulin resistance in genetically obese ob/ob mice in vivo. Although the underlying molecular mechanisms remain to be unraveled, these results clearly identify a novel role of FXR in the regulation of peripheral insulin sensitivity and adipocyte function. This unexpected function of FXR opens new perspectives for the treatment of type 2 diabetes.

Variation in the lipoprotein lipase gene influences exercise-induced left ventricular growth

The adult heart relies predominantly on fatty acids (FA) for energy generation, and defects in FA catabolism cause dramatic left ventricular (LV) growth in early age. Since lipoprotein lipase (LPL) is the key enzyme in plasma triglyceride catabolism and is highly expressed in the myocardium, we investigated an association between the functional LPL gene serine 447 stop (S447X) variant and exercise-induced LV growth. The S447X variant was genotyped in 146 British Army recruits undergoing a 10-week exercise programme. Over the training period, X447 allele carriers showed less LV growth than S447 homozygotes (SS, 5.8+/-0.7%; SX, 2.2+/-1.5%; P=0.03) and a decrease in systolic blood pressure (DeltaSBP: SS, 1.9+/-1.3 mmHg; SX, -5.7+/-2.2 mmHg; P=0.015). Although LPL genotype did not significantly predict LV growth with DeltaSBP in statistical modelling (LPL, P=0.14; DeltaSBP, P=0.06), regression analysis indicated that LPL S447X genotype effect on DeltaSBP accounted for only 20% of the effect on LV growth. In multivariate analysis, LPL, peroxisome-proliferator-activated receptor alpha and angiotensin-converting enzyme genotypes were independent predictors of cardiac growth. Thus, LPL S447X genotype influenced exercise-induced changes in LV mass and SBP. Change in blood pressure accounted for a proportion of LV growth. These data suggest that increased myocardial FA availability may reduce exercise-induced LV growth.

Skeletal Muscle Lipid Metabolism in Exercise and Insulin Resistance

Lipids as fuel for energy provision originate from different sources: albumin-bound long-chain fatty acids (LCFA) in the blood plasma, circulating very-low-density lipoproteins-triacylglycerols (VLDL-TG), fatty acids from triacylglycerol located in the muscle cell (IMTG), and possibly fatty acids liberated from adipose tissue adhering to the muscle cells. The regulation of utilization of the different lipid sources in skeletal muscle during exercise is reviewed, and the influence of diet, training, and gender is discussed. Major points deliberated are the methods utilized to measure uptake and oxidation of LCFA during exercise in humans. The role of the various lipid-binding proteins in transmembrane and cytosolic transport of lipids is considered as well as regulation of lipid entry into the mitochondria, focusing on the putative role of AMP-activated protein kinase (AMPK), acetyl CoA carboxylase (ACC), and carnitine during exercise. The possible contribution to fuel provision during exercise of circulating VLDL-TG as well as the role of IMTG is discussed from a methodological point of view. The contribution of IMTG for energy provision may not be large, covering ∼10% of total energy provision during fasting exercise in male subjects, whereas in females, IMTG may cover a larger proportion of energy delivery. Molecular mechanisms involved in breakdown of IMTG during exercise are also considered focusing on hormone-sensitive lipase (HSL). Finally, the role of lipids in development of insulin resistance in skeletal muscle, including possible molecular mechanisms involved, is discussed.

Different roles of liver X receptor alpha and beta in lipid metabolism: effects of an alpha-selective and a dual agonist in mice deficient in each subtype

Liver X receptor (LXR) alpha and LXRbeta are closely related nuclear receptors that respond to elevated levels of intracellular cholesterol by enhancing transcription of genes that control cholesterol efflux and fatty acid biosynthesis. The consequences of inactivation of either LXR isoform have been thoroughly studied, as have the effects of simultaneous activation of both LXRalpha and LXRbeta by synthetic compounds. We here describe the effects of selective activation of LXRalpha or LXRbeta on lipid metabolism. This was accomplished by treating mice genetically deficient in either LXRalpha or LXRbeta with an agonist with equal potency for both isoforms (Compound B) or a synthetic agonist selective for LXRalpha (Compound A). We also determined the effect of these agonists on gene expression and cholesterol efflux in peritoneal macrophages derived from wild-type and knockout mice. Both compounds raised HDL-cholesterol and increased liver triglycerides in wild-type mice; in contrast, in mice deficient in LXRalpha, Compound B increased HDL-cholesterol but did not cause hepatic steatosis. Compound B induced ATP-binding cassette transporter (ABC) A1 expression and stimulated cholesterol efflux in macrophages from both LXRalpha and LXRbeta-deficient mice. Our data lend further experimental support to the hypothesis that LXRbeta-selective agonists may raise HDL-cholesterol and stimulate macrophage cholesterol efflux without causing liver triglyceride accumulation.

Specific inhibition of hormone-sensitive lipase improves lipid profile while reducing plasma glucose

Elevation of plasma free fatty acids has been linked with insulin resistance and diabetes. Inhibition of lipolysis may provide a mechanism to decrease plasma fatty acids, thereby improving insulin sensitivity. Hormone-sensitive lipase (HSL) is a critical enzyme involved in the hormonally regulated release of fatty acids and glycerol from adipocyte lipid stores, and its inhibition may thus improve insulin sensitivity and blood glucose handling in type 2 diabetes. In rat adipocytes, forskolin-activated lipolysis was blocked by in vitro addition of a potent and selective HSL inhibitor or by prior treatment of the animals themselves. Antilipolytic effects also were demonstrated in overnight-fasted mice, rats, and dogs with species-dependent effects on plasma free fatty acid levels but with similar reductions in plasma glycerol being observed in all species. Inhibition of HSL also reduced hyperglycemia in streptozotocin-induced diabetic rats. The data support a connection between adipose tissue lipolysis and plasma glucose levels.

Link between obesity and type 2 diabetes

The relationship between obesity and diabetes is of such interdependence that the term 'diabesity' has been coined. The passage from obesity to diabetes is made by a progressive defect in insulin secretion coupled with a progressive rise in insulin resistance. Both insulin resistance and defective insulin secretion appear very prematurely in obese patients, and both worsen similarly towards diabetes. Thus, the classic 'hyperbolic relationship' between insulin resistance and insulin secretion and the 'glucose allostasis concept' remain prevailing concepts in this particular field of knowledge. An increase in overall fatness, preferentially of visceral as well as ectopic fat depots, is specifically associated with insulin resistance. The accumulation of intramyocellular lipids may be due to reduced lipid oxidation capacity. The ability to lose weight is related to the capacity to oxidize fat. Thus, a relative defect in fat oxidation capacity is responsible for energy economy and hampered weight loss.

Unraveling the temporal pattern of diet-induced insulin resistance in individual organs and cardiac dysfunction in C57BL/6 mice

Type 2 diabetes is a heterogeneous disease characterized by insulin resistance and altered glucose and lipid metabolism in multiple organs. To understand the complex series of events that occur during the development of obesity-associated diabetes, we examined the temporal pattern of changes in insulin action and glucose metabolism in individual organs during chronic high-fat feeding in C57BL/6 mice. Insulin-stimulated cardiac glucose metabolism was significantly reduced after 1.5 weeks of high-fat feeding, and cardiac insulin resistance was associated with blunted Akt-mediated insulin signaling and GLUT4 levels. Insulin resistance in skeletal muscle, adipose tissue, and liver developed in parallel after 3 weeks of high-fat feeding. Diet-induced whole-body insulin resistance was associated with increased circulating levels of resistin and leptin but unaltered adiponectin levels. High-fat feeding caused insulin resistance in skeletal muscle that was associated with significantly elevated intramuscular fat content. In contrast, diet-induced hepatic insulin resistance developed before a marked increase in intrahepatic triglyceride levels. Cardiac function gradually declined over the course of high-fat feeding, and after 20 weeks of high-fat diet, cardiac dysfunction was associated with mild hyperglycemia, hyperleptinemia, and reduced circulating adiponectin levels. Our findings demonstrate that cardiac insulin resistance is an early adaptive event in response to obesity and develops before changes in whole-body glucose homeostasis. This suggests that obesity-associated defects in cardiac function may not be due to insulin resistance per se but may be attributable to chronic alteration in cardiac glucose and lipid metabolism and circulating adipokines.

Fatty acids and insulin resistance in muscle and liver

Free fatty acids (FFAs) circulate round the body and represent important nutrients and the key oxidative fuel for the heart and resting skeletal muscle. In addition, FFAs are thought to be potent signalling molecules. Growing evidence indicates that FFAs may be involved in type 2 diabetes mellitus and obesity by mediating insulin resistance. In 1963, it was postulated that accumulated glucose-6-phosphate as a result of increased FFA oxidation leads to decreased glucose uptake. An alternative hypothesis is that increased concentrations of plasma FFA induce insulin resistance in humans through inhibition of glucose transport activity, which appears to be a consequence of decreased insulin receptor substrate-1-associated phosphatidyl inositol 3 kinase activity. Moreover, FFAs can arise locally, and increased intramyocellular and hepatocellular lipids have been shown to be associated with insulin resistance. This paper reviews the main aspects of FFA metabolism in the development of insulin resistance in skeletal muscle and liver, as well as the role of ectopic lipid deposits as a local source of FFAs. Finally, the role of thiazolidinediones as modulators of FFA-induced insulin resistance will be discussed.

High fat diet induced hepatic insulin resistance is not related to changes in hypothalamic mRNA expression of NPY, AgRP, POMC and CART in mice

The hypothalamic circuitry, apart from its impact on food intake, modulates insulin sensitivity to adapt metabolic conditions in the face of environmental fluctuations in nutrient availability. The purpose of the present study was to investigate the effects of 2 weeks high fat feeding in wildtype mice on (1) insulin sensitivity and triglyceride accumulation in liver and muscle in relation to (2) mRNA expression levels of Neuropeptide Y (NPY), Agouti-related protein (AgRP), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART) in the hypothalamus. Two weeks of high fat feeding induced hepatic insulin resistance in the presence of increased hepatic triglyceride accumulation. In muscle, however, 2 weeks of high fat feeding did not result in changes in insulin sensitivity or in triglyceride content. mRNA expression levels of NPY, AgRP, POMC, and CART in the hypothalamus were not different between the groups. This study shows that 2 weeks of high fat feeding in mice does not affect mRNA expression levels of NPY, AgRP, POMC or CART, in the whole hypothalamus, despite induction of hepatic, but not peripheral, insulin resistance. Therefore, a major physiological role of these neuroendocrine factors in the induction of hepatic insulin resistance during a high fat diet seems less likely.

NO-1886 (ibrolipim), a lipoprotein lipase activator, increases the expression of uncoupling protein 3 in skeletal muscle and suppresses fat accumulation in high-fat diet-induced obesity in rats

Although the lipoprotein lipase (LPL) activator NO-1886 shows antiobesity effects in high-fat-induced obese animals, the mechanism remains unclear. To clarify the mechanism, we studied the effects of NO-1886 on the expression of uncoupling protein (UCP) 1, UCP2, and UCP3 in rats. NO-1886 was mixed with a high-fat chow to supply a dose of 100 mg/kg to 8-month-old male Sprague-Dawley rats. The animals were fed the high-fat chow for 8 weeks. At the end of the administration period, brown adipose tissue (BAT), mesenteric fat, and soleus muscle were collected and levels of UCP1, UCP2, and UCP3 messenger RNA (mRNA) were determined. NO-1886 suppressed the body weight increase seen in the high-fat control group after the 8-week administration (585 +/- 39 vs 657 +/- 66 g, P < .05). NO-1886 also suppressed fat accumulation in visceral (46.9 +/- 10.4 vs 73.7 +/- 14.5 g, P < .01) and subcutaneous (43.1 +/- 18.1 vs 68.9 +/- 18.8 g, P < .05) tissues and increased the levels of plasma total cholesterol and high-density lipoprotein cholesterol in comparison to the high-fat control group. In contrast, NO-1886 decreased the levels of plasma triglycerides, nonesterified free fatty acid, glucose, and insulin. NO-1886 increased LPL activity in soleus muscle (0.082 +/- 0.013 vs 0.061 +/- 0.016 mumol of free fatty acid per minute per gram of tissue, P < .05). NO-1886 increased the expression of UCP3 mRNA in soleus muscle 3.14-fold (P < .01) compared with the high-fat control group without affecting the levels of UCP3 in mesenteric adipose tissue and BAT. In addition, NO-1886 did not affect the expression of UCP1 and UCP2 in BAT, mesenteric adipose tissue, and soleus muscle. In conclusion, NO-1886 increased the expression of UCP3 mRNA and LPL activity only in skeletal muscle. Therefore, a possible mechanism for NO-1886's antiobesity effects in rats may be the enhancement of LPL activity in skeletal muscle and the accompanying increase in UCP3 expression.

Reduced mitochondrial density and increased IRS-1 serine phosphorylation in muscle of insulin-resistant offspring of type 2 diabetic parents

To further explore the nature of the mitochondrial dysfunction and insulin resistance that occur in the muscle of young, lean, normoglycemic, insulin-resistant offspring of parents with type 2 diabetes (IR offspring), we measured mitochondrial content by electron microscopy and insulin signaling in muscle biopsy samples obtained from these individuals before and during a hyperinsulinemic-euglycemic clamp. The rate of insulin-stimulated muscle glucose uptake was approximately 60% lower in the IR offspring than the control subjects and was associated with an approximately 60% increase in the intramyocellular lipid content as assessed by H magnetic resonance spectroscopy. Muscle mitochondrial density was 38% lower in the IR offspring. These changes were associated with a 50% increase in IRS-1 Ser312 and IRS-1 Ser636 phosphorylation and an approximately 60% reduction in insulin-stimulated Akt activation in the IR offspring. These data provide new insights into the earliest defects that may be responsible for the development of type 2 diabetes and support the hypothesis that reductions in mitochondrial content result in decreased mitochondrial function, which predisposes IR offspring to intramyocellular lipid accumulation, which in turn activates a serine kinase cascade that leads to defects in insulin signaling and action in muscle.

Dysfunctional fat cells, lipotoxicity and type 2 diabetes

Type 2 diabetes is characterized by insulin resistance and impaired insulin secretion. Considerable evidence implicates altered fat topography and defects in adipocyte metabolism in the pathogenesis of type 2 diabetes. In individuals who develop type 2 diabetes, fat cells tend to be enlarged. Enlarged fat cells are resistant to the antilipolytic effects of insulin, leading to day-long elevated plasma free fatty acid (FFA) levels. Chronically increased plasma FFA stimulates gluconeogenesis, induces hepatic and muscle insulin resistance, and impairs insulin secretion in genetically predisposed individuals. These FFA-induced disturbances are referred to as lipotoxicity. Enlarged fat cells also have diminished capacity to store fat. When adipocyte storage capacity is exceeded, lipid 'overflows' into muscle and liver, and possibly the beta-cells of the pancreas, exacerbating insulin resistance and further impairing insulin secretion. In addition, dysfunctional fat cells produce excessive amounts of insulin resistance-inducing, inflammatory and atherosclerosis-provoking cytokines, and fail to secrete normal amounts of insulin-sensitizing cytokines. As more evidence emerges, there is a stronger case for targeting adipose tissue in the treatment of type 2 diabetes. Peroxisome-proliferator activated receptor gamma (PPARgamma) agonists, for example the thiazolidinediones, redistribute fat within the body (decrease visceral and hepatic fat; increase subcutaneous fat) and have been shown to enhance adipocyte insulin sensitivity, inhibit lipolysis, reduce plasma FFA and favourably influence the production of adipocytokines. This article examines in detail the role of adipose tissue in the pathogenesis of type 2 diabetes and highlights the potential of PPAR agonists to improve the management of patients with the condition.

Effect of a sustained reduction in plasma free fatty acid concentration on intramuscular long-chain fatty Acyl-CoAs and insulin action in type 2 diabetic patients

To investigate the effect of a sustained (7-day) decrease in plasma free fatty acid (FFA) concentrations on insulin action and intramyocellular long-chain fatty acyl-CoAs (LCFA-CoAs), we studied the effect of acipimox, a potent inhibitor of lipolysis, in seven type 2 diabetic patients (age 53 +/- 3 years, BMI 30.2 +/- 2.0 kg/m2, fasting plasma glucose 8.5 +/- 0.8 mmol/l, HbA 1c 7.5 +/- 0.4%). Subjects received an oral glucose tolerance test (OGTT) and 120-min euglycemic insulin (80 mU/m2 per min) clamp with 3-[3H]glucose/vastus lateralis muscle biopsies to quantitate rates of insulin-mediated whole-body glucose disposal (Rd) and intramyocellular LCFA-CoAs before and after acipimox (250 mg every 6 h for 7 days). Acipimox significantly reduced fasting plasma FFAs (from 563 +/- 74 to 230 +/- 33 micromol/l; P < 0.01) and mean plasma FFAs during the OGTT (from 409 +/- 44 to 184 +/- 22 micromol/l; P < 0.01). After acipimox, decreases were seen in fasting plasma insulin (from 78 +/- 18 to 42 +/- 6 pmol/l; P < 0.05), fasting plasma glucose (from 8.5 +/- 0.8 to 7.0 +/- 0.5 mmol/l; P < 0.02), and mean plasma glucose during the OGTT (from 14.5 +/- 0.8 to 13.0 +/- 0.8 mmol/l; P < 0.05). After acipimox, insulin-stimulated Rd increased from 3.3 +/- 0.4 to 4.4 +/- 0.4 mg x kg(-1) x min(-1) (P < 0.03), whereas suppression of endogenous glucose production (EGP) was similar and virtually complete during both insulin clamp studies (0.16 +/- 0.10 vs. 0.14 +/- 0.10 mg x kg(-1) x min(-1); P > 0.05). Basal EGP did not change after acipimox (1.9 +/- 0.2 vs. 1.9 +/- 0.2 mg x kg(-1) x min(-1)). Total muscle LCFA-CoA content decreased after acipimox treatment (from 7.26 +/- 0.58 to 5.64 +/- 0.79 nmol/g; P < 0.05). Decreases were also seen in muscle palmityl CoA (16:0; from 1.06 +/- 0.10 to 0.75 +/- 0.11 nmol/g; P < 0.05), palmitoleate CoA (16:1; from 0.48 +/- 0.05 to 0.33 +/- 0.05 nmol/g; P = 0.07), oleate CoA (18:1; from 2.60 +/- 0.11 to 1.95 +/- 0.31 nmol/g; P < 0.05), linoleate CoA (18:2; from 1.81 +/- 0.26 to 1.38 +/- 0.18 nmol/g; P = 0.13), and linolenate CoA (18:3; from 0.27 +/- 0.03 to 0.19 +/- 0.02 nmol/g; P < 0.03) levels after acipimox treatment. Muscle stearate CoA (18:0) did not decrease after acipimox treatment. The increase in R(d) correlated strongly with the decrease in muscle palmityl CoA (r = 0.75, P < 0.05), oleate CoA (r = 0.76, P < 0.05), and total muscle LCFA-CoA (r = 0.74, P < 0.05) levels. Plasma adiponectin did not change significantly after acipimox treatment (7.9 +/- 1.8 vs. 7.5 +/- 1.5 microg/ml). These data demonstrate that the reduction in intramuscular LCFA-CoA content is closely associated with enhanced insulin sensitivity in muscle after a chronic reduction in plasma FFA concentrations in type 2 diabetic patients despite the lack of an effect on plasma adiponectin concentration.

Insulin resistance: a metabolic pathway to chronic liver disease

Insulin resistance (IR) is the pathophysiological hallmark of nonalcoholic fatty liver disease (NAFLD), one of the most common causes of chronic liver disease in Western countries. We review the definition of IR, the methods for the quantitative assessment of insulin action, the pathophysiology of IR, and the role of IR in the pathogenesis of chronic liver disease. Increased free fatty acid flux from adipose tissue to nonadipose organs, a result of abnormal fat metabolism, leads to hepatic triglyceride accumulation and contributes to impaired glucose metabolism and insulin sensitivity in muscle and in the liver. Several factors secreted or expressed in the adipocyte contribute to the onset of a proinflammatory state, which may be limited to the liver or more extensively expressed throughout the body. IR is the common characteristic of the metabolic syndrome and its related features. It is a systemic disease affecting the nervous system, muscles, pancreas, kidney, heart, and immune system, in addition to the liver. A complex interaction between genes and the environment favors or enhances IR and the phenotypic expression of NAFLD in individual patients. Advanced fibrotic liver disease is associated with multiple features of the metabolic syndrome, and the risk of progressive liver disease should not be underestimated in individuals with metabolic disorders. Finally, the ability of insulin-sensitizing, pharmacological agents to treat NAFLD by reducing IR in the liver (metformin) and in the periphery (thiazolidinediones) are discussed.