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1.  Regulation of Insulin and Leptin Signaling by Muscle Suppressor of Cytokine Signaling 3 (SOCS3) 
PLoS ONE  2012;7(10):e47493.
Skeletal muscle resistance to the key metabolic hormones, leptin and insulin, is an early defect in obesity. Suppressor of cytokine signaling 3 (SOCS3) is a major negative regulator of both leptin and insulin signaling, thereby implicating SOCS3 in the pathogenesis of obesity and associated metabolic abnormalities. Here, we demonstrate that SOCS3 mRNA expression is increased in murine skeletal muscle in the setting of diet-induced and genetic obesity, inflammation, and hyperlipidemia. To further evaluate the contribution of muscle SOCS3 to leptin and insulin resistance in obesity, we generated transgenic mice with muscle-specific overexpression of SOCS3 (MCK/SOCS3 mice). Despite similar body weight, MCK/SOCS3 mice develop impaired systemic and muscle-specific glucose homeostasis and insulin action based on glucose and insulin tolerance tests, hyperinsulinemic-euglycemic clamps, and insulin signaling studies. With regards to leptin action, MCK/SOCS3 mice exhibit suppressed basal and leptin-stimulated activity and phosphorylation of alpha2 AMP-activated protein kinase (α2AMPK) and its downstream target, acetyl-CoA carboxylase (ACC). Muscle SOCS3 overexpression also suppresses leptin-regulated genes involved in fatty acid oxidation and mitochondrial function. These studies demonstrate that SOC3 within skeletal muscle is a critical regulator of leptin and insulin action and that increased SOCS may mediate insulin and leptin resistance in obesity.
PMCID: PMC3480378  PMID: 23115649
2.  Paradoxical Coupling of Triglyceride Synthesis and Fatty Acid Oxidation in Skeletal Muscle Overexpressing DGAT1 
Diabetes  2009;58(11):2516-2524.
Transgenic expression of diacylglycerol acyltransferase-1 (DGAT1) in skeletal muscle leads to protection against fat-induced insulin resistance despite accumulation of intramuscular triglyceride, a phenomenon similar to what is known as the “athlete paradox.” The primary objective of this study is to determine how DGAT1 affects muscle fatty acid oxidation in relation to whole-body energy metabolism and insulin sensitivity.
We first quantified insulin sensitivity and the relative tissue contributions to the improved whole-body insulin sensitivity in muscle creatine kisase (MCK)-DGAT1 transgenic mice by hyperinsulinemic-euglycemic clamps. Metabolic consequences of DGAT1 overexpression in skeletal muscles were determined by quantifying triglyceride synthesis/storage (anabolic) and fatty acid oxidation (catabolic), in conjunction with gene expression levels of representative marker genes in fatty acid metabolism. Whole-body energy metabolism including food consumption, body weights, oxygen consumption, locomotor activity, and respiration exchange ratios were determined at steady states.
MCK-DGAT1 mice were protected against muscle lipoptoxicity, although they remain susceptible to hepatic lipotoxicity. While augmenting triglyceride synthesis, DGAT1 overexpression also led to increased muscle mitochondrial fatty acid oxidation efficiency, as compared with wild-type muscles. On a high-fat diet, MCK-DGAT1 mice displayed higher basal metabolic rates and 5–10% lower body weights compared with wild-type littermates, whereas food consumption was not different.
DGAT1 overexpression in skeletal muscle led to parallel increases in triglyceride synthesis and fatty acid oxidation. Seemingly paradoxical, this phenomenon is characteristic of insulin-sensitive myofibers and suggests that DGAT1 plays an active role in metabolic “remodeling” of skeletal muscle coupled with insulin sensitization.
PMCID: PMC2768165  PMID: 19675136
3.  Myeloid Cell-Restricted Insulin Receptor Deficiency Protects Against Obesity-Induced Inflammation and Systemic Insulin Resistance 
PLoS Genetics  2010;6(5):e1000938.
A major component of obesity-related insulin resistance is the establishment of a chronic inflammatory state with invasion of white adipose tissue by mononuclear cells. This results in the release of pro-inflammatory cytokines, which in turn leads to insulin resistance in target tissues such as skeletal muscle and liver. To determine the role of insulin action in macrophages and monocytes in obesity-associated insulin resistance, we conditionally inactivated the insulin receptor (IR) gene in myeloid lineage cells in mice (IRΔmyel-mice). While these animals exhibit unaltered glucose metabolism on a normal diet, they are protected from the development of obesity-associated insulin resistance upon high fat feeding. Euglycemic, hyperinsulinemic clamp studies demonstrate that this results from decreased basal hepatic glucose production and from increased insulin-stimulated glucose disposal in skeletal muscle. Furthermore, IRΔmyel-mice exhibit decreased concentrations of circulating tumor necrosis factor (TNF) α and thus reduced c-Jun N-terminal kinase (JNK) activity in skeletal muscle upon high fat feeding, reflecting a dramatic reduction of the chronic and systemic low-grade inflammatory state associated with obesity. This is paralleled by a reduced accumulation of macrophages in white adipose tissue due to a pronounced impairment of matrix metalloproteinase (MMP) 9 expression and activity in these cells. These data indicate that insulin action in myeloid cells plays an unexpected, critical role in the regulation of macrophage invasion into white adipose tissue and in the development of obesity-associated insulin resistance.
Author Summary
Obesity represents a major health burden with steadily increasing incidence. While it is associated with numerous co-morbidities, type 2 diabetes mellitus represents one of the major life-threatening, obesity-related conditions. Over the last years, it has become clear that during the course of obesity development not only does fat mass increase, but also fat composition changes qualitatively, leading to an influx of inflammatory cells, such as macrophages, into adipose tissue. Macrophages in turn secrete inflammatory mediators, which inhibit insulin action in skeletal muscle, liver, and even the central nervous system to ultimately cause insulin-resistant diabetes mellitus. However, the effect of insulin action and resistance in these inflammatory cell types themselves has not been addressed. To this end, we have generated and analyzed mice with inactivation of the insulin receptor specifically in myeloid cell-derived, inflammatory cells. Surprisingly, these animals are protected from the development of obesity-associated deterioration of glucose metabolism, thereby defining insulin action in inflammatory cells as a novel and promising target for therapeutic intervention against obesity-associated diabetes mellitus.
PMCID: PMC2865520  PMID: 20463885
4.  Alteration of JNK-1 Signaling in Skeletal Muscle Fails to Affect Glucose Homeostasis and Obesity-Associated Insulin Resistance in Mice 
PLoS ONE  2013;8(1):e54247.
Obesity and associated metabolic disturbances, such as increased circulating fatty acids cause prolonged low grade activation of inflammatory signaling pathways in liver, skeletal muscle, adipose tissue and even in the CNS. Activation of inflammatory pathways in turn impairs insulin signaling, ultimately leading to obesity-associated type 2 diabetes mellitus. Conventional JNK-1 knock out mice are protected from high fat diet-induced insulin resistance, characterizing JNK-1-inhibition as a potential approach to improve glucose metabolism in obese patients. However, the cell type-specific role of elevated JNK-1 signaling as present during the course of obesity has not been fully elucidated yet. To investigate the functional contribution of altered JNK-1 activation in skeletal muscle, we have generated a ROSA26 insertion mouse strain allowing for Cre-activatable expression of a JNK-1 constitutive active construct (JNKC). To examine the consequence of skeletal muscle-restricted JNK-1 overactivation in the development of insulin resistance and glucose metabolism, JNKC mice were crossed to Mck-Cre mice yielding JNKSM-C mice. However, despite increased muscle-specific JNK activation, energy homeostasis and glucose metabolism in JNKSM-C mice remained largely unaltered compared to controls. In line with these findings, obese mice with skeletal muscle specific disruption of JNK-1, did not affect energy and glucose homeostasis. These experiments indicate that JNK-1 activation in skeletal muscle does not account for the major effects on diet-induced, JNK-1-mediated deterioration of insulin action and points towards a so far underappreciated role of JNK-1 in other tissues than skeletal muscle during the development of obesity-associated insulin resistance.
PMCID: PMC3547909  PMID: 23349837
5.  12/15-Lipoxygenase Is Required for the Early Onset of High Fat Diet-Induced Adipose Tissue Inflammation and Insulin Resistance in Mice 
PLoS ONE  2009;4(9):e7250.
Recent understanding that insulin resistance is an inflammatory condition necessitates searching for genes that regulate inflammation in insulin sensitive tissues. 12/15-lipoxygenase (12/15LO) regulates the expression of proinflammatory cytokines and chemokines and is implicated in the early development of diet-induced atherosclerosis. Thus, we tested the hypothesis that 12/15LO is involved in the onset of high fat diet (HFD)-induced insulin resistance.
Methodology/Principal Findings
Cells over-expressing 12/15LO secreted two potent chemokines, MCP-1 and osteopontin, implicated in the development of insulin resistance. We assessed adipose tissue inflammation and whole body insulin resistance in wild type (WT) and 12/15LO knockout (KO) mice after 2–4 weeks on HFD. In adipose tissue from WT mice, HFD resulted in recruitment of CD11b+, F4/80+ macrophages and elevated protein levels of the inflammatory markers IL-1β, IL-6, IL-10, IL-12, IFNγ, Cxcl1 and TNFα. Remarkably, adipose tissue from HFD-fed 12/15LO KO mice was not infiltrated by macrophages and did not display any increase in the inflammatory markers compared to adipose tissue from normal chow-fed mice. WT mice developed severe whole body (hepatic and skeletal muscle) insulin resistance after HFD, as measured by hyperinsulinemic euglycemic clamp. In contrast, 12/15LO KO mice exhibited no HFD-induced change in insulin-stimulated glucose disposal rate or hepatic glucose output during clamp studies. Insulin-stimulated Akt phosphorylation in muscle tissue from HFD-fed mice was significantly greater in 12/15LO KO mice than in WT mice.
These results demonstrate that 12/15LO mediates early stages of adipose tissue inflammation and whole body insulin resistance induced by high fat feeding.
PMCID: PMC2746280  PMID: 19787041
6.  Enhanced Lipid Oxidation and Maintenance of Muscle Insulin Sensitivity Despite Glucose Intolerance in a Diet-Induced Obesity Mouse Model 
PLoS ONE  2013;8(8):e71747.
Diet-induced obesity is a rising health concern which can lead to the development of glucose intolerance and muscle insulin resistance and, ultimately, type II diabetes mellitus. This research investigates the associations between glucose intolerance or muscle insulin resistance and tissue specific changes during the progression of diet-induced obesity.
C57BL/6J mice were fed a normal or high-fat diet (HFD; 60% kcal fat) for 3 or 8 weeks. Disease progression was monitored by measurements of body/tissue mass changes, glucose and insulin tolerance tests, and ex vivo glucose uptake in intact muscles. Lipid metabolism was analyzed using metabolic chambers and ex vivo palmitate assays in intact muscles. Skeletal muscle, liver and adipose tissues were analyzed for changes in inflammatory gene expression. Plasma was analyzed for insulin levels and inflammatory proteins. Histological techniques were used on muscle and liver cryosections to assess metabolic and morphological changes.
Principal Findings/Conclusions
A rapid shift in whole body metabolism towards lipids was observed with HFD. Following 3 weeks of HFD, elevated total lipid oxidation and an oxidative fiber type shift had occurred in the skeletal muscle, which we propose was responsible for delaying intramyocellular lipid accumulation and maintaining muscle’s insulin sensitivity. Glucose intolerance was present after three weeks of HFD and was associated with an enlarged adipose tissue depot, adipose tissue inflammation and excess hepatic lipids, but not hepatic inflammation. Furthermore, HFD did not significantly increase systemic or muscle inflammation after 3 or 8 weeks of HFD suggesting that early diet-induced obesity does not cause inflammation throughout the whole body. Overall these findings indicate skeletal muscle did not contribute to the development of HFD-induced impairments in whole-body glucose tolerance following 3 weeks of HFD.
PMCID: PMC3741110  PMID: 23951235
7.  Osteopontin Is Required for the Early Onset of High Fat Diet-Induced Insulin Resistance in Mice 
PLoS ONE  2010;5(11):e13959.
Insulin resistance is manifested in muscle, adipose tissue, and liver and is associated with adipose tissue inflammation. The cellular components and mechanisms that regulate the onset of diet-induced insulin resistance are not clearly defined.
Methodology and Principal Findings
We initially observed osteopontin (OPN) mRNA over-expression in adipose tissue of obese, insulin resistant humans and rats which was normalized by thiazolidinedione (TZD) treatment in both species. OPN regulates inflammation and is implicated in pathogenic maladies resulting from chronic obesity. Thus, we tested the hypothesis that OPN is involved in the early development of insulin resistance using a 2–4 week high fat diet (HFD) model. OPN KO mice fed HFD for 2 weeks were completely protected from the severe skeletal muscle, liver and adipose tissue insulin resistance that developed in wild type (WT) controls, as determined by hyperinsulinemic euglycemic clamp and acute insulin-stimulation studies. Although two-week HFD did not alter body weight or plasma free fatty acids and cytokines in either strain, HFD-induced hyperleptinemia, increased adipose tissue inflammation (macrophages and cytokines), and adipocyte hypertrophy were significant in WT mice and blunted or absent in OPN KO mice. Adipose tissue OPN protein isoform expression was significantly altered in 2- and 4-week HFD-fed WT mice but total OPN protein was unchanged. OPN KO bone marrow stromal cells were more osteogenic and less adipogenic than WT cells in vitro. Interestingly, the two differentiation pathways were inversely affected by HFD in WT cells in vitro.
The OPN KO phenotypes we report reflect protection from insulin resistance that is associated with changes in adipocyte biology and adipose tissue inflammatory status. OPN is a key component in the development of HFD-induced insulin resistance.
PMCID: PMC2980483  PMID: 21103061
8.  Endurance interval training in obese mice reduces muscle inflammation and macrophage content independently of weight loss 
Physiological Reports  2014;2(5):e12012.
Obesity is associated with chronic low‐grade inflammation that involves infiltration of macrophages into metabolic organs such as skeletal muscle. Exercise enhances skeletal muscle insulin sensitivity independently of weight loss; but its role in regulating muscle inflammation is not fully understood. We hypothesized that exercise training would inhibit skeletal muscle inflammation and alter macrophage infiltration into muscle independently of weight loss. Wild type C57BL/6 male mice were fed a chow diet or a high‐fat diet (HFD, 45% calories fat) for 6 weeks. Then, mice maintained on the HFD either remained sedentary (HFD Sed) or exercised (HFD Ex) on a treadmill for another 6 weeks. The exercise training protocol involved conducting intervals of 2 min in duration followed by 2 min of rest for 60 min thrice weekly. Chow‐fed control mice remained sedentary for the entire 12 weeks. Muscle cytokine and macrophage gene expression analysis were conducted using qRT‐PCR, and muscle macrophage content was also measured using immunohistochemistry. Muscle cytokine protein content was quantified using a cytokine array. The HFD increased adiposity and weight gain compared to chow‐fed controls. HFD Sed and HFD Ex mice had similar body mass as well as total and visceral adiposity. However, despite similar adiposity, exercise reduced inflammation and muscle macrophage infiltration. We conclude that Endurance exercise training modulates the immune‐metabolic crosstalk in obesity independently of weight loss, and may have potential benefits in reducing obesity‐related muscle inflammation.
Obesity is associated with chronic low‐grade inflammation that involves infiltration of macrophages into metabolic organs such as skeletal muscle. Exercise enhances skeletal muscle insulin sensitivity independently of weight loss; but its role in regulating muscle inflammation is not fully understood. In this article, we show that endurance interval training inhibited skeletal muscle inflammation and reduced macrophage infiltration into muscle independently of weight loss in mice.
PMCID: PMC4098740  PMID: 24843075
Cytokines; endurance exercise; inflammation; macrophage
9.  Loss-of-Function Mutation in Myostatin Reduces Tumor Necrosis Factor α Production and Protects Liver Against Obesity-Induced Insulin Resistance 
Diabetes  2009;58(5):1133-1143.
Insulin resistance develops in tandem with obesity. Ablating myostatin (Mstn) prevents obesity, so we investigated if Mstn deficiency could improve insulin sensitivity. A loss-of-function mutation (MstnLn) in either one or both alleles of the Mstn gene shows how Mstn deficiency protects whole-body insulin sensitivity.
MstnLn/Ln mice were weaned onto a high-fat diet (HFD) or standard diet. HFD-fed MstnLn/Ln mice exhibited high lean, low-fat body compositions compared with wild types. Wild-type and heterozygous and homozygous mutant mice were bled to determine basal levels of insulin, glucose, and homeostasis model assessment of insulin resistance. To evaluate postprandial insulin sensitivity between animals of a similar size, glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamp studies were performed with heterozygous and homozygous mutant mice. Quantitative RT-PCR quantified TNF∝, IL-6, IL-1β, F4/80, GPR43, and CD36 expression in muscle, fat, and liver. Histological analysis measured hepatosteatosis.
Homozygous mutants were glucose tolerant and protected against overall insulin resistance compared with heterozygous mice. Hyperinsulinemic-euglycemic clamp studies revealed a dramatically improved glucose infusion rate, glucose disposal rate, and hepatic glucose production in 11-month-old MstnLn/Ln mice on an HFD. Improvements to muscle and liver insulin sensitivity (∼200–400%) correlated with 50–75% decreased tumor necrosis factor (TNF)α production and coincided with severe Mstn deficiency. Hepatosteatosis appeared to be ameliorated. Short-term treatment of MstnLn/Ln mice with recombinant Mstn led to increased plasma TNFα and insulin resistance.
We find that severe Mstn deficiency caused by Ln (lean) mutations in HFD-fed mice protects muscle and liver against obesity-induced insulin resistance.
PMCID: PMC2671051  PMID: 19208906
10.  Tumor Progression Locus 2 (TPL2) Regulates Obesity-Associated Inflammation and Insulin Resistance 
Diabetes  2011;60(4):1168-1176.
Obesity-associated low-grade systemic inflammation resulting from increased adipose mass is strongly related to the development of insulin resistance and type 2 diabetes as well as other metabolic complications. Recent studies have demonstrated that the obese metabolic state can be improved by ablating certain inflammatory signaling pathways. Tumor progression locus 2 (TPL2), a kinase that integrates signals from Toll receptors, cytokine receptors, and inhibitor of κ-B kinase-β is an important regulator of inflammatory pathways. We used TPL2 knockout (KO) mice to investigate the role of TPL2 in mediating obesity-associated inflammation and insulin resistance.
Male TPL2KO and wild-type (WT) littermates were fed a low-fat diet or a high-fat diet to investigate the effect of TPL2 deletion on obesity, inflammation, and insulin sensitivity.
We demonstrate that TPL2 deletion does not alter body weight gain or adipose depot weight. However, hyperinsulinemic euglycemic clamp studies revealed improved insulin sensitivity with enhanced glucose uptake in skeletal muscle and increased suppression of hepatic glucose output in obese TPL2KO mice compared with obese WT mice. Consistent with an improved metabolic phenotype, immune cell infiltration and inflammation was attenuated in the adipose tissue of obese TPL2KO mice coincident with reduced hepatic inflammatory gene expression and lipid accumulation.
Our results provide the first in vivo demonstration that TPL2 ablation attenuates obesity-associated metabolic dysfunction. These data suggest TPL2 is a novel target for improving the metabolic state associated with obesity.
PMCID: PMC3064090  PMID: 21346175
11.  Muscle Mitochondrial ATP Synthesis and Glucose Transport/Phosphorylation in Type 2 Diabetes 
PLoS Medicine  2007;4(5):e154.
Muscular insulin resistance is frequently characterized by blunted increases in glucose-6-phosphate (G-6-P) reflecting impaired glucose transport/phosphorylation. These abnormalities likely relate to excessive intramyocellular lipids and mitochondrial dysfunction. We hypothesized that alterations in insulin action and mitochondrial function should be present even in nonobese patients with well-controlled type 2 diabetes mellitus (T2DM).
Methods and Findings
We measured G-6-P, ATP synthetic flux (i.e., synthesis) and lipid contents of skeletal muscle with 31P/1H magnetic resonance spectroscopy in ten patients with T2DM and in two control groups: ten sex-, age-, and body mass-matched elderly people; and 11 younger healthy individuals. Although insulin sensitivity was lower in patients with T2DM, muscle lipid contents were comparable and hyperinsulinemia increased G-6-P by 50% (95% confidence interval [CI] 39%–99%) in all groups. Patients with diabetes had 27% lower fasting ATP synthetic flux compared to younger controls (p = 0.031). Insulin stimulation increased ATP synthetic flux only in controls (younger: 26%, 95% CI 13%–42%; older: 11%, 95% CI 2%–25%), but failed to increase even during hyperglycemic hyperinsulinemia in patients with T2DM. Fasting free fatty acids and waist-to-hip ratios explained 44% of basal ATP synthetic flux. Insulin sensitivity explained 30% of insulin-stimulated ATP synthetic flux.
Patients with well-controlled T2DM feature slightly lower flux through muscle ATP synthesis, which occurs independently of glucose transport /phosphorylation and lipid deposition but is determined by lipid availability and insulin sensitivity. Furthermore, the reduction in insulin-stimulated glucose disposal despite normal glucose transport/phosphorylation suggests further abnormalities mainly in glycogen synthesis in these patients.
Michael Roden and colleagues report that even patients with well-controlled insulin-resistant type 2 diabetes have altered mitochondrial function.
Editors' Summary
Diabetes mellitus is an increasingly common chronic disease characterized by high blood sugar (glucose) levels. In normal individuals, blood sugar levels are maintained by the hormone insulin. Insulin is released by the pancreas when blood glucose levels rise after eating (glucose is produced by the digestion of food) and “instructs” insulin-responsive muscle and fat cells to take up glucose from the bloodstream. The cells then use glucose as a fuel or convert it into glycogen, a storage form of glucose. In type 2 diabetes, the commonest type of diabetes, the muscle and fat cells become nonresponsive to insulin (a condition called insulin resistance) and consequently blood glucose levels rise. Over time, this hyperglycemia increases the risk of heart attacks, kidney failure, and other life-threatening complications.
Why Was This Study Done?
Insulin resistance is often an early sign of type 2 diabetes, sometimes predating its development by many years, so understanding its causes might provide clues about how to stop the global diabetes epidemic. One theory is that mitochondria—cellular structures that produce the energy (in the form of a molecule called ATP) needed to keep cells functioning—do not work properly in people with insulin resistance. Mitochondria change (metabolize) fatty acids into energy, and recent studies have revealed that fat accumulation caused by poorly regulated fatty acid metabolism blocks insulin signaling, thus causing insulin resistance. Other studies using magnetic resonance spectroscopy (MRS) to study mitochondrial function noninvasively in human muscle indicate that mitochondria are dysfunctional in people with insulin resistance by showing that ATP synthesis is impaired in such individuals. In this study, the researchers have examined both baseline and insulin-stimulated mitochondrial function in nonobese patients with well-controlled type 2 diabetes and in normal controls to discover more about the relationship between mitochondrial dysfunction and insulin resistance.
What Did the Researchers Do and Find?
The researchers determined the insulin sensitivity of people with type 2 diabetes and two sets of people (the “controls”) who did not have diabetes: one in which the volunteers were age-matched to the people with diabetes, and the other containing younger individuals (insulin resistance increases with age). To study insulin sensitivity in all three groups, the researchers used a “hyperinsulinemic–euglycemic clamp.” For this, after an overnight fast, the participants' insulin levels were kept high with a continuous insulin infusion while blood glucose levels were kept normal using a variable glucose infusion. In this situation, the glucose infusion rate equals glucose uptake by the body and therefore measures tissue sensitivity to insulin. Before and during the clamp, the researchers used MRS to measure glucose-6-phosphate (an indicator of how effectively glucose is taken into cells and phosphorylated), ATP synthesis, and the fat content of the participants' muscle cells. Insulin sensitivity was lower in the patients with diabetes than in the controls, but muscle lipid content was comparable and hyperinsulinemia increased glucose-6-phosphate levels similarly in all the groups. Patients with diabetes and the older controls had lower fasting ATP synthesis rates than the young controls and, whereas insulin stimulation increased ATP synthesis in all the controls, it had no effect in the patients with diabetes. In addition, fasting blood fatty acid levels were inversely related to basal ATP synthesis, whereas insulin sensitivity was directly related to insulin-stimulated ATP synthesis.
What Do These Findings Mean?
These findings indicate that the impairment of muscle mitochondrial ATP synthesis in fasting conditions and after insulin stimulation in people with diabetes is not due to impaired glucose transport/phosphorylation or fat deposition in the muscles. Instead, it seems to be determined by lipid availability and insulin sensitivity. These results add to the evidence suggesting that mitochondrial function is disrupted in type 2 diabetes and in insulin resistance, but also suggest that there may be abnormalities in glycogen synthesis. More work is needed to determine the exact nature of these abnormalities and to discover whether they can be modulated to prevent the development of insulin resistance and type 2 diabetes. For now, though, these findings re-emphasize the need for people with type 2 diabetes or insulin resistance to reduce their food intake to compensate for the reduced energy needs of their muscles and to exercise to increase the ATP-generating capacity of their muscles. Both lifestyle changes could improve their overall health and life expectancy.
Additional Information.
Please access these Web sites via the online version of this summary at
The MedlinePlus encyclopedia has pages on diabetes
The US National Institute of Diabetes and Digestive and Kidney Diseases provides information for patients on diabetes and insulin resistance
The US Centers for Disease Control and Prevention has information on diabetes for patients and professionals
American Diabetes Association provides information for patients on diabetes and insulin resistance
Diabetes UK has information for patients and professionals on diabetes
PMCID: PMC1858707  PMID: 17472434
12.  Telmisartan Improves Insulin Resistance of Skeletal Muscle Through Peroxisome Proliferator–Activated Receptor-δ Activation 
Diabetes  2013;62(3):762-774.
The mechanisms of the improvement of glucose homeostasis through angiotensin receptor blockers are not fully elucidated in hypertensive patients. We investigated the effects of telmisartan on insulin signaling and glucose uptake in cultured myotubes and skeletal muscle from wild-type and muscle-specific peroxisome proliferator–activated receptor (PPAR) δ knockout (MCK-PPARδ−/−) mice. Telmisartan increased PPARδ expression and activated PPARδ transcriptional activity in cultured C2C12 myotubes. In palmitate-induced insulin-resistant C2C12 myotubes, telmisartan enhanced insulin-stimulated Akt and Akt substrate of 160 kDa (AS160) phosphorylation as well as Glut4 translocation to the plasma membrane. These effects were inhibited by antagonizing PPARδ or phosphatidylinositol-3 kinase, but not by PPARγ and PPARα inhibition. Palmitate reducing the insulin-stimulated glucose uptake in C2C12 myotubes could be restored by telmisartan. In vivo experiments showed that telmisartan treatment reversed high-fat diet–induced insulin resistance and glucose intolerance in wild-type mice but not in MCK-PPARδ−/− mice. The protein levels of PPARδ, phospho-Akt, phospho-AS160, and Glut4 translocation to the plasma membrane in the skeletal muscle on insulin stimulation were reduced by high-fat diet and were restored by telmisartan administration in wild-type mice. These effects were absent in MCK-PPARδ−/− mice. These findings implicate PPARδ as a potential therapeutic target in the treatment of hypertensive subjects with insulin resistance.
PMCID: PMC3581229  PMID: 23238297
13.  Inflammation Is Necessary for Long-Term but Not Short-Term High-Fat Diet–Induced Insulin Resistance 
Diabetes  2011;60(10):2474-2483.
Tissue inflammation is a key factor underlying insulin resistance in established obesity. Several models of immuno-compromised mice are protected from obesity-induced insulin resistance. However, it is unanswered whether inflammation triggers systemic insulin resistance or vice versa in obesity. The purpose of this study was to assess these questions.
We fed a high-fat diet (HFD) to wild-type mice and three different immuno-compromised mouse models (lymphocyte-deficient Rag1 knockout, macrophage-depleted, and hematopoietic cell-specific Jun NH2-terminal kinase–deficient mice) and measured the time course of changes in macrophage content, inflammatory markers, and lipid accumulation in adipose tissue, liver, and skeletal muscle along with systemic insulin sensitivity.
In wild-type mice, body weight and adipose tissue mass, as well as insulin resistance, were clearly increased by 3 days of HFD. Concurrently, in the short-term HFD period inflammation was selectively elevated in adipose tissue. Interestingly, however, all three immuno-compromised mouse models were not protected from insulin resistance induced by the short-term HFD. On the other hand, lipid content was markedly increased in liver and skeletal muscle at day 3 of HFD.
These data suggest that the initial stage of HFD-induced insulin resistance is independent of inflammation, whereas the more chronic state of insulin resistance in established obesity is largely mediated by macrophage-induced proinflammatory actions. The early-onset insulin resistance during HFD feeding is more likely related to acute tissue lipid overload.
PMCID: PMC3178297  PMID: 21911747
14.  Hepatic Cannabinoid Receptor-1 Mediates Diet-Induced Insulin Resistance via Inhibition of Insulin Signaling and Clearance in Mice 
Gastroenterology  2012;142(5):1218-1228.e1.
Obesity-related insulin resistance contributes to cardiovascular disease. Cannabinoid receptor-1 (CB1) blockade improves insulin sensitivity in obese animals and people, suggesting endocannabinoid involvement. We explored the role of hepatic CB1 in insulin resistance and inhibition of insulin signaling pathways.
Wild-type mice and mice with disruption of CB1 (CB1−/− mice) or with hepatocyte-specific deletion or transgenic overexpression of CB1 were maintained on regular chow or a high-fat diet (HFD) to induce obesity and insulin resistance. Hyperinsulinemic-euglycemic clamp analysis was used to analyze the role of the liver and hepatic CB1 in HFD-induced insulin resistance. The cellular mechanisms of insulin resistance were analyzed in mouse and human isolated hepatocytes using small interfering or short hairpin RNAs and lentiviral knockdown of gene expression.
The HFD induced hepatic insulin resistance in wild-type mice, but not in CB1−/− mice or mice with hepatocyte-specific deletion of CB1. CB1−/− mice that overexpressed CB1 specifically in hepatocytes became hyperinsulinemic as a result of reduced insulin clearance due to down-regulation of the insulin-degrading enzyme. However, they had increased hepatic glucose production due to increased glycogenolysis, indicating hepatic insulin resistance; this was further increased by the HFD. In mice with hepatocytes that express CB1, the HFD or CB1 activation induced the endoplasmic reticulum stress response via activation of the Bip-PERK-eIF2α protein translation pathway. In hepatocytes isolated from human or mouse liver, CB1 activation caused endoplasmic reticulum stress-dependent suppression of insulin-induced phosphorylation of akt-2 via phosphorylation of IRS1 at serine-307 and by inducing the expression of the serine and threonine phosphatase Phlpp1. Expression of CB1 was up-regulated in samples from patients with nonalcoholic fatty liver disease.
Endocannabinoids contribute to diet-induced insulin resistance in mice via hepatic CB1-mediated inhibition of insulin signaling and clearance.
PMCID: PMC3482511  PMID: 22307032
NASH; Signal Transduction; Mouse Model; Liver Disease
15.  Grp78 Heterozygosity Promotes Adaptive Unfolded Protein Response and Attenuates Diet-Induced Obesity and Insulin Resistance 
Diabetes  2009;59(1):6-16.
To investigate the role of the endoplasmic reticulum (ER) chaperone glucose-regulated protein (GRP) 78/BiP in the pathogenesis of obesity, insulin resistance, and type 2 diabetes.
Male Grp78+/− mice and their wild-type littermates were subjected to a high-fat diet (HFD) regimen. Pathogenesis of obesity and type 2 diabetes was examined by multiple approaches of metabolic phenotyping. Tissue-specific insulin sensitivity was analyzed by hyperinsulinemic-euglycemic clamps. Molecular mechanism was explored via immunoblotting and tissue culture manipulation.
Grp78 heterozygosity increases energy expenditure and attenuates HFD-induced obesity. Grp78+/− mice are resistant to diet-induced hyperinsulinemia, liver steatosis, white adipose tissue (WAT) inflammation, and hyperglycemia. Hyperinsulinemic-euglycemic clamp studies revealed that Grp78 heterozygosity improves glucose metabolism independent of adiposity and following an HFD increases insulin sensitivity predominantly in WAT. As mechanistic explanations, Grp78 heterozygosity in WAT under HFD stress promotes adaptive unfolded protein response (UPR), attenuates translational block, and upregulates ER degradation-enhancing α-mannosidase–like protein (EDEM) and ER chaperones, thus improving ER quality control and folding capacity. Further, overexpression of the active form of ATF6 induces protective UPR and improves insulin signaling upon ER stress.
HFD-induced obesity and type 2 diabetes are improved in Grp78+/− mice. Adaptive UPR in WAT could contribute to this improvement, linking ER homeostasis to energy balance and glucose metabolism.
PMCID: PMC2797945  PMID: 19808896
16.  Metabolic Actions of Estrogen Receptor Beta (ERβ) are Mediated by a Negative Cross-Talk with PPARγ 
PLoS Genetics  2008;4(6):e1000108.
Estrogen receptors (ER) are important regulators of metabolic diseases such as obesity and insulin resistance (IR). While ERα seems to have a protective role in such diseases, the function of ERβ is not clear. To characterize the metabolic function of ERβ, we investigated its molecular interaction with a master regulator of insulin signaling/glucose metabolism, the PPARγ, in vitro and in high-fat diet (HFD)-fed ERβ -/- mice (βERKO) mice. Our in vitro experiments showed that ERβ inhibits ligand-mediated PPARγ-transcriptional activity. That resulted in a blockade of PPARγ-induced adipocytic gene expression and in decreased adipogenesis. Overexpression of nuclear coactivators such as SRC1 and TIF2 prevented the ERβ-mediated inhibition of PPARγ activity. Consistent with the in vitro data, we observed increased PPARγ activity in gonadal fat from HFD-fed βERKO mice. In consonance with enhanced PPARγ activation, HFD-fed βERKO mice showed increased body weight gain and fat mass in the presence of improved insulin sensitivity. To directly demonstrate the role of PPARγ in HFD-fed βERKO mice, PPARγ signaling was disrupted by PPARγ antisense oligonucleotide (ASO). Blockade of adipose PPARγ by ASO reversed the phenotype of βERKO mice with an impairment of insulin sensitization and glucose tolerance. Finally, binding of SRC1 and TIF2 to the PPARγ-regulated adiponectin promoter was enhanced in gonadal fat from βERKO mice indicating that the absence of ERβ in adipose tissue results in exaggerated coactivator binding to a PPARγ target promoter. Collectively, our data provide the first evidence that ERβ-deficiency protects against diet-induced IR and glucose intolerance which involves an augmented PPARγ signaling in adipose tissue. Moreover, our data suggest that the coactivators SRC1 and TIF2 are involved in this interaction. Impairment of insulin and glucose metabolism by ERβ may have significant implications for our understanding of hormone receptor-dependent pathophysiology of metabolic diseases, and may be essential for the development of new ERβ-selective agonists.
Author Summary
In the present study, we demonstrate for the first time a pro-diabetogenic function of the ERβ. Our experiments indicate that ERβ impairs insulin sensitivity and glucose tolerance in mice challenged with a high fat diet (HFD). Loss of ERβ, studied in ERβ -/- mice (βERKO mice), results in increased body weight gain and fat deposition under HFD-treatment. Conversely, absence of ERβ averted accumulation of triglycerides and preserved regular insulin signaling in liver and skeletal muscle. This observation was associated with improved whole-body insulin sensitivity and glucose tolerance. Increased adipose tissue mass in the presence of improved insulin sensitivity and glucose tolerance is usually observed under chronic stimulation of the nuclear hormone receptor PPARγ. In consonance, we show that activation of PPARγ was markedly induced in gonadal fat from βERKO mice and blockade of adipose PPARγ signaling by antisense oligonucleotide injection reversed the metabolic phenotype. Moreover, our cell culture experiments indicate that ERβ is a negative regulator of ligand-induced PPARγ activity in vitro. Finally, we identify SRC1 and TIF2 as key players in the ERβ-PPARγ interaction. In summary, the present study demonstrates that ERβ impairs insulin and glucose metabolism, which may, at least in part, result from a negative cross-talk with adipose PPARγ.
PMCID: PMC2432036  PMID: 18584035
17.  The C3a Anaphylatoxin Receptor Is a Key Mediator of Insulin Resistance and Functions by Modulating Adipose Tissue Macrophage Infiltration and Activation 
Diabetes  2009;58(9):2006-2017.
Significant new data suggest that metabolic disorders such as diabetes, obesity, and atherosclerosis all posses an important inflammatory component. Infiltrating macrophages contribute to both tissue-specific and systemic inflammation, which promotes insulin resistance. The complement cascade is involved in the inflammatory cascade initiated by the innate and adaptive immune response. A mouse genomic F2 cross biology was performed and identified several causal genes linked to type 2 diabetes, including the complement pathway.
We therefore sought to investigate the effect of a C3a receptor (C3aR) deletion on insulin resistance, obesity, and macrophage function utilizing both the normal-diet (ND) and a diet-induced obesity mouse model.
We demonstrate that high C3aR expression is found in white adipose tissue and increases upon high-fat diet (HFD) feeding. Both adipocytes and macrophages within the white adipose tissue express significant amounts of C3aR. C3aR−/− mice on HFD are transiently resistant to diet-induced obesity during an 8-week period. Metabolic profiling suggests that they are also protected from HFD-induced insulin resistance and liver steatosis. C3aR−/− mice had improved insulin sensitivity on both ND and HFD as seen by an insulin tolerance test and an oral glucose tolerance test. Adipose tissue analysis revealed a striking decrease in macrophage infiltration with a concomitant reduction in both tissue and plasma proinflammatory cytokine production. Furthermore, C3aR−/− macrophages polarized to the M1 phenotype showed a considerable decrease in proinflammatory mediators.
Overall, our results suggest that the C3aR in macrophages, and potentially adipocytes, plays an important role in adipose tissue homeostasis and insulin resistance.
PMCID: PMC2731537  PMID: 19581423
18.  Hepatocyte-Specific Ptpn6 Deletion Protects From Obesity-Linked Hepatic Insulin Resistance 
Diabetes  2012;61(8):1949-1958.
The protein-tyrosine phosphatase Shp1 negatively regulates insulin action on glucose homeostasis in liver and muscle, but its potential role in obesity-linked insulin resistance has not been examined. To investigate the role of Shp1 in hepatic insulin resistance, we generated hepatocyte-specific Shp1 knockout mice (Ptpn6H-KO), which were subjected to extensive metabolic monitoring throughout an 8-week standard chow diet (SD) or high-fat diet (HFD) feeding. We report for the first time that Shp1 expression is upregulated in metabolic tissues of HFD-fed obese mice. When compared with their Shp1-expressing Ptpn6f/f littermates, Ptpn6H-KO mice exhibited significantly lowered fasting glycemia and heightened hepatic insulin sensitivity. After HFD feeding, Ptpn6H-KO mice developed comparable levels of obesity as Ptpn6f/f mice, but they were remarkably protected from liver insulin resistance, as revealed by euglycemic clamps and hepatic insulin signaling determinations. Although Ptpn6H-KO mice still acquired diet-induced peripheral insulin resistance, they were less hyperinsulinemic during a glucose tolerance test because of reduced insulin secretion. Ptpn6H-KO mice also exhibited increased insulin clearance in line with enhanced CC1 tyrosine phosphorylation in liver. These results show that hepatocyte Shp1 plays a critical role in the development of hepatic insulin resistance and represents a novel therapeutic target for obesity-linked diabetes.
PMCID: PMC3402325  PMID: 22698917
19.  Carnitine Palmitoyltransferase 1b Deficient Mice Develop Severe Insulin Resistance After Prolonged High Fat Diet Feeding 
Carnitine palmitoyltransferase 1 (CPT1) is the rate-limiting enzyme governing the entry of long-chain acyl-CoAs into mitochondria. Treatments with CPT1 inhibitors protect against insulin resistance in short-term preclinical animal studies. We recently reported that mice with muscle isoform CPT1b deficiency demonstrated improved insulin sensitivity when fed a High Fat-Diet (HFD) for up to 5 months. In this follow up study, we further investigated whether the insulin sensitizing effects of partial CPT1b deficiency could be maintained under a prolonged HFD feeding condition.
We investigated the effects of CPT1b deficiency on HFD-induced insulin resistance using heterozygous CPT1b deficient (Cpt1b+/−) mice compared with Wild Type (WT) mice fed a HFD for a prolonged period of time (7 months). We assessed insulin sensitivity using hyperinsulinemic-euglycemic clamps. We also examined body composition, skeletal muscle lipid profile, and changes in the insulin signaling pathways of skeletal muscle, liver, and adipose tissue.
We found that Cpt1b+/− mice became severely insulin resistant after 7 months of HFD feeding. Cpt1b+/− mice exhibited a substantially reduced glucose infusion rate and skeletal muscle glucose uptake. While Cpt1b+/− mice maintained a slower weight gain with less fat mass than WT mice, accumulation of lipid intermediates became evident in the muscle of Cpt1b+/− but not WT mice after 7 months of HFD feeding. Insulin signaling was impaired in the Cpt1b+/− as compared to the WT muscles.
Partial CPT1b deficiency, mimicking CPT1b inhibition, may lead to impaired insulin signaling and insulin sensitivity under a prolonged HFD feeding condition. Therefore, further studies on the potential detrimental effects of prolonged therapy with CPT1 inhibition are necessary in the development of this potential therapeutic strategy.
PMCID: PMC4286342  PMID: 25580367
CPT1b; Insulin sensitivity; Skeletal muscle
20.  NOD1 Activators Link Innate Immunity to Insulin Resistance 
Diabetes  2011;60(9):2206-2215.
Insulin resistance associates with chronic inflammation, and participatory elements of the immune system are emerging. We hypothesized that bacterial elements acting on distinct intracellular pattern recognition receptors of the innate immune system, such as bacterial peptidoglycan (PGN) acting on nucleotide oligomerization domain (NOD) proteins, contribute to insulin resistance.
Metabolic and inflammatory properties were assessed in wild-type (WT) and NOD1/2−/− double knockout mice fed a high-fat diet (HFD) for 16 weeks. Insulin resistance was measured by hyperinsulinemic euglycemic clamps in mice injected with mimetics of meso-diaminopimelic acid–containing PGN or the minimal bioactive PGN motif, which activate NOD1 and NOD2, respectively. Systemic and tissue-specific inflammation was assessed using enzyme-linked immunosorbent assays in NOD ligand–injected mice. Cytokine secretion, glucose uptake, and insulin signaling were assessed in adipocytes and primary hepatocytes exposed to NOD ligands in vitro.
NOD1/2−/− mice were protected from HFD-induced inflammation, lipid accumulation, and peripheral insulin intolerance. Conversely, direct activation of NOD1 protein caused insulin resistance. NOD1 ligands induced peripheral and hepatic insulin resistance within 6 h in WT, but not NOD1−/−, mice. NOD2 ligands only modestly reduced peripheral glucose disposal. NOD1 ligand elicited minor changes in circulating proinflammatory mediators, yet caused adipose tissue inflammation and insulin resistance of muscle AS160 and liver FOXO1. Ex vivo, NOD1 ligand caused proinflammatory cytokine secretion and impaired insulin-stimulated glucose uptake directly in adipocytes. NOD1 ligand also caused inflammation and insulin resistance directly in primary hepatocytes from WT, but not NOD1−/−, mice.
We identify NOD proteins as innate immune components that are involved in diet-induced inflammation and insulin intolerance. Acute activation of NOD proteins by mimetics of bacterial PGNs causes whole-body insulin resistance, bolstering the concept that innate immune responses to distinctive bacterial cues directly lead to insulin resistance. Hence, NOD1 is a plausible, new link between innate immunity and metabolism.
PMCID: PMC3161332  PMID: 21715553
21.  Exercise Protects against Diet-Induced Insulin Resistance through Downregulation of Protein Kinase Cβ in Mice 
PLoS ONE  2013;8(12):e81364.
Physical exercise is an important and effective therapy for diabetes. However, its underlying mechanism is not fully understood. Protein kinase Cβ (PKCβ) has been suggested to be involved in the pathogenesis of obesity and insulin resistance, but the role of PKCβ in exercise-induced improvements in insulin resistance is completely unknown. In this study, we evaluated the involvement of PKCβ in exercise-attenuated insulin resistance in high-fat diet (HFD)-fed mice. PKCβ-/- and wild-type mice were fed a HFD with or without exercise training. PKC protein expression, body and tissue weight change, glucose and insulin tolerance, metabolic rate, mitochondria size and number, adipose inflammation, and AKT activation were determined to evaluate insulin sensitivity and metabolic changes after intervention. PKCβ expression decreased in both skeletal muscle and liver tissue after exercise. Exercise and PKCβ deficiency can alleviate HFD-induced insulin resistance, as evidenced by improved insulin tolerance. In addition, fat accumulation and mitochondrial dysfunction induced by HFD were also ameliorated by both exercise and PKCβ deficiency. On the other hand, exercise had little effect on PKCβ-/- mice. Further, our data indicated improved activation of AKT, the downstream signal molecule of insulin, in skeletal muscle and liver of exercised mice, whereas PKCβ deficiency blunted the difference between sedentary and exercised mice. These results suggest that downregulation of PKCβ contributes to exercise-induced improvement of insulin resistance in HFD-fed mice.
PMCID: PMC3857188  PMID: 24349059
22.  Selective Inactivation of c-Jun NH2-Terminal Kinase in Adipose Tissue Protects Against Diet-Induced Obesity and Improves Insulin Sensitivity in Both Liver and Skeletal Muscle in Mice 
Diabetes  2011;60(2):486-495.
Obesity is associated with increased activation of the c-Jun NH2-terminal kinase (JNK) in several metabolic organs, including adipose tissue, liver, and skeletal muscle. In this study, we aimed to define the role of JNK activation in adipose tissue in the development of obesity-related insulin resistance.
Transgenic mice with adipose tissue–specific overexpression of dominant-negative JNK (ap2-dn-JNK) under the transcriptional control of the aP2 gene promoter were generated and subjected to metabolic characterization together with the wild-type littermates.
On a high-fat diet (HFD), the ap2-dn-JNK mice displayed a marked suppression of both JNK1 and JNK2 activation in their adipose tissue, accompanied by a marked reduction in weight gain, fat mass, and size of the adipocytes. The transgenic mice were resistant to the deleterious impact of an HFD on systemic insulin sensitivity, glucose tolerance, and hepatic steatosis. Reduced hepatic gluconeogenesis was evident in in vivo and ex vivo studies and showed greater insulin-induced glucose uptake in skeletal muscles. These changes were accompanied by reduced macrophage infiltration in adipose tissue, decreased production of proinflammatory adipokines, and increased expression of adiponectin. Indirect calorimetry analysis showed that the transgenic mice had significant increases in oxygen consumption and reductions in respiration exchange rates compared with their wild-type littermates.
Selective suppression of JNK activation in adipose tissue alone is sufficient to counteract HFD-induced obesity and its associated metabolic dysregulations, in part through an increase in energy expenditure and a decrease in systemic inflammation.
PMCID: PMC3028348  PMID: 21270260
23.  The Dual-Specificity Phosphatase 2 (DUSP2) Does Not Regulate Obesity-Associated Inflammation or Insulin Resistance in Mice 
PLoS ONE  2014;9(11):e111524.
Alterations in the immune cell profile and the induction of inflammation within adipose tissue are a hallmark of obesity in mice and humans. Dual-specificity phosphatase 2 (DUSP2) is widely expressed within the immune system and plays a key role promoting immune and inflammatory responses dependent on mitogen-activated protein kinase (MAPK) activity. We hypothesised that the absence of DUSP2 would protect mice against obesity-associated inflammation and insulin resistance. Accordingly, male and female littermate mice that are either wild-type (wt) or homozygous for a germ-line null mutation of the dusp2 gene (dusp2−/−) were fed either a standard chow diet (SCD) or high fat diet (HFD) for 12 weeks prior to metabolic phenotyping. Compared with mice fed the SCD, all mice consuming the HFD became obese, developed glucose intolerance and insulin resistance, and displayed increased macrophage recruitment and markers of inflammation in epididymal white adipose tissue. The absence of DUSP2, however, had no effect on the development of obesity or adipose tissue inflammation. Whole body insulin sensitivity in male mice was unaffected by an absence of DUSP2 in response to either the SCD or HFD; however, HFD-induced insulin resistance was slightly, but significantly, reduced in female dusp2−/− mice. In conclusion, DUSP2 plays no role in regulating obesity-associated inflammation and only a minor role in controlling insulin sensitivity following HFD in female, but not male, mice. These data indicate that rather than DUSP2 being a pan regulator of MAPK dependent immune cell mediated inflammation, it appears to differentially regulate inflammatory responses that have a MAPK component.
PMCID: PMC4222916  PMID: 25375135
24.  Time-Resolved and Tissue-Specific Systems Analysis of the Pathogenesis of Insulin Resistance 
PLoS ONE  2010;5(1):e8817.
The sequence of events leading to the development of insulin resistance (IR) as well as the underlying pathophysiological mechanisms are incompletely understood. As reductionist approaches have been largely unsuccessful in providing an understanding of the pathogenesis of IR, there is a need for an integrative, time-resolved approach to elucidate the development of the disease.
Methodology/Principal Findings
Male ApoE3Leiden transgenic mice exhibiting a humanized lipid metabolism were fed a high-fat diet (HFD) for 0, 1, 6, 9, or 12 weeks. Development of IR was monitored in individual mice over time by performing glucose tolerance tests and measuring specific biomarkers in plasma, and hyperinsulinemic-euglycemic clamp analysis to assess IR in a tissue-specific manner. To elucidate the dynamics and tissue-specificity of metabolic and inflammatory processes key to IR development, a time-resolved systems analysis of gene expression and metabolite levels in liver, white adipose tissue (WAT), and muscle was performed. During HFD feeding, the mice became increasingly obese and showed a gradual increase in glucose intolerance. IR became first manifest in liver (week 6) and then in WAT (week 12), while skeletal muscle remained insulin-sensitive. Microarray analysis showed rapid upregulation of carbohydrate (only liver) and lipid metabolism genes (liver, WAT). Metabolomics revealed significant changes in the ratio of saturated to polyunsaturated fatty acids (liver, WAT, plasma) and in the concentrations of glucose, gluconeogenesis and Krebs cycle metabolites, and branched amino acids (liver). HFD evoked an early hepatic inflammatory response which then gradually declined to near baseline. By contrast, inflammation in WAT increased over time, reaching highest values in week 12. In skeletal muscle, carbohydrate metabolism, lipid metabolism, and inflammation was gradually suppressed with HFD.
HFD-induced IR is a time- and tissue-dependent process that starts in liver and proceeds in WAT. IR development is paralleled by tissue-specific gene expression changes, metabolic adjustments, changes in lipid composition, and inflammatory responses in liver and WAT involving p65-NFkB and SOCS3. The alterations in skeletal muscle are largely opposite to those in liver and WAT.
PMCID: PMC2809107  PMID: 20098690
25.  Resistance to High-Fat Diet–Induced Obesity but Exacerbated Insulin Resistance in Mice Overexpressing Preadipocyte Factor-1 (Pref-1) 
Diabetes  2008;57(12):3258-3266.
OBJECTIVE—White adipose tissue is a critical regulator of whole-body glucose metabolism. Preadipocyte factor-1 (Pref-1) is a secreted protein that inhibits adipocyte differentiation, both in vitro and in vivo. In this study, we have investigated the effects of Pref-1 overexpression on whole-body glucose homeostasis and its contribution to the development of insulin resistance.
RESEARCH DESIGN AND METHODS—To gain insight into the role of Pref-1 on the onset of insulin resistance and type 2 diabetes, we measured body composition and whole-body insulin-stimulated glucose metabolism during a hyperinsulinemic-euglycemic clamp in Pref-1 transgenic and wild-type control mice fed a high-fat diet.
RESULTS—Mice overexpressing Pref-1 were resistant to high-fat diet–induced obesity, as reflected by a marked reduction in adipose tissue mass. However, Pref-1–overexpressing mice were severely insulin resistant, mainly because of a reduction in insulin-stimulated glucose uptake in skeletal muscle and adipose tissue. The aggravated insulin resistance was associated with impaired insulin signaling and increased diacylglycerol content in skeletal muscle.
CONCLUSIONS—Mice overexpressing Pref-1 are insulin resistant despite being protected from diet-induced obesity and may provide a new rodent model for the study of lipodystrophic disorders.
PMCID: PMC2584131  PMID: 18835937

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