Acute reversal of lipid-induced muscle insulin resistance is associated with rapid alteration in PKC-theta localization

Sterol Regulatory Element Binding Protein 1: A Mediator for High-Fat Diet-Induced Hepatic Gluconeogenesis and Glucose Intolerance in Fish

BACKGROUND

Sterol regulatory element binding protein (SREBP) 1 is considered to be a crucial regulator for lipid synthesis in vertebrates. However, whether SREBP1 could regulate hepatic gluconeogenesis under high-fat diet (HFD) condition is still unknown, and the underlying mechanism is also unclear.

OBJECTIVES

This study aimed to determine gluconeogenesis-related gene and protein expressions in response to HFD in large yellow croaker and explore the role and mechanism of SREBP1 in regulating the related transcription and signaling.

METHODS

Croakers (mean weight, 15.61 ± 0.10 g) were fed with diets containing 12% crude lipid [control diet (ND)] or 18% crude lipid (HFD) for 10 weeks. The glucose tolerance, insulin tolerance, hepatic gluconeogenesis-related genes, and proteins expressions were determined. To explore the role of SREBP1 in HFD-induced gluconeogenesis, SREBP1 was inhibited by pharmacologic inhibitor (fatostatin) or genetic knockdown in croaker hepatocytes under palmitic acid (PA) condition. To explore the underlying mechanism, luciferase reporter and chromatin immunoprecipitation assays were conducted in HEK293T cells. Data were analyzed using analysis of variance or Student t test.

RESULTS

Compared with ND, HFD increased the mRNA expressions of gluconeogenesis genes (2.40-fold to 2.60-fold) (P < 0.05) and reduced protein kinase B (AKT) phosphorylation levels (0.28-fold to 0.34-fold) (P < 0.05) in croakers. However, inhibition of SREBP1 by fatostatin addition or SREBP1 knockdown reduced the mRNA expressions of gluconeogenesis genes (P < 0.05) and increased AKT phosphorylation levels (P < 0.05) in hepatocytes, compared with that by PA treatment. Moreover, fatostatin addition or SREBP1 knockdown also increased the mRNA expressions of irs1 (P < 0.05) and reduced serine phosphorylation of IRS1 (P < 0.05). Furthermore, SREBP1 inhibited IRS1 transcriptions by binding to its promoter and induced IRS1 serine phosphorylation by activating diacylglycerol-protein kinase Cε signaling.

CONCLUSIONS

This study reveals the role of SREBP1 in hepatic gluconeogenesis under HFD condition in croakers, which may provide a potential strategy for improving HFD-induced glucose intolerance.

Role of Sphingosine Kinase in Type 2 Diabetes Mellitus

Sphingolipids are a class of essential lipids, functioning as both cell membrane constituents and signaling messengers. In the sphingolipid metabolic network, ceramides serve as the central hub that is hydrolyzed to sphingosine, followed by phosphorylation to sphingosine 1-phosphate (S1P) by sphingosine kinase (SphK). SphK is regarded as a "switch" of the sphingolipid rheostat, as it catalyzes the conversion of ceramide/sphingosine to S1P, which often exhibit opposing biological roles in the cell. Besides, SphK is an important signaling enzyme that has been implicated in the regulation of a wide variety of biological functions. In recent years, an increasing body of evidence has suggested a critical role of SphK in type 2 diabetes mellitus (T2D), although a certain level of controversy remains. Herein, we review recent findings related to SphK in the field of T2D research with a focus on peripheral insulin resistance and pancreatic β-cell failure. It is expected that a comprehensive understanding of the role of SphK and the associated sphingolipids in T2D will help to identify druggable targets for future anti-diabetes therapy.

Reassessing the Role of Diacylglycerols in Insulin Resistance

Skeletal muscle (SM) insulin resistance (IR) plays an important role in the burden of obesity, particularly because it leads to glucose intolerance and type 2 diabetes. Among the mechanisms thought to link IR to obesity is the accumulation, in muscle cells, of different lipid metabolites. Diacylglycerols (DAGs) are subject of particular attention due to reported interactions with the insulin signaling cascade. Given that SM accounts for the majority of insulin-stimulated glucose uptake, this review integrates recent observational and mechanistic works with the sole focus on questioning the role of DAGs in SM IR. Particular attention is given to the subcellular distributions and specific structures of DAGs, highlighting future research directions towards reaching a consensus on the mechanistic role played by DAGs.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Modeling insulin resistance in rodents by alterations in diet: what have high-fat and high-calorie diets revealed?

For over half a century, researchers have been feeding different diets to rodents to examine the effects of macronutrients on whole body and tissue insulin action. During this period, the number of different diets and the source of macronutrients employed have grown dramatically. Because of the large heterogeneity in both the source and percentage of different macronutrients used for studies, it is not surprising that different high-calorie diets do not produce the same changes in insulin action. Despite this, diverse high-calorie diets continue to be employed in an attempt to generate a "generic" insulin resistance. The high-fat diet in particular varies greatly between studies with regard to the source, complexity, and ratio of dietary fat, carbohydrate, and protein. This review examines the range of rodent dietary models and methods for assessing insulin action. In almost all studies reviewed, rodents fed diets that had more than 45% of dietary energy as fat or simple carbohydrates had reduced whole body insulin action compared with chow. However, different high-calorie diets produced significantly different effects in liver, muscle, and whole body insulin action when insulin action was measured by the hyperinsulinemic-euglycemic clamp method. Rodent dietary models remain an important tool for exploring potential mechanisms of insulin resistance, but more attention needs to be given to the total macronutrient content and composition when interpreting dietary effects on insulin action.

Lipotoxic lethal and sublethal stress signaling in hepatocytes: relevance to NASH pathogenesis

The accumulation of lipids is a histologic and biochemical hallmark of obesity-associated nonalcoholic fatty liver disease (NAFLD). A subset of NALFD patients develops progressive liver disease, termed nonalcoholic steatohepatitis, which is characterized by hepatocellular apoptosis and innate immune system-mediated inflammation. These responses are orchestrated by signaling pathways that can be activated by lipids, directly or indirectly. In this review, we discuss palmitate- and lysophosphatidylcholine (LPC)-induced upregulation of p53-upregulated modulator of apoptosis and cell-surface expression of the death receptor TNF-related apoptosis-inducing ligand receptor 2. Next, we review the activation of stress-induced kinases, mixed lineage kinase 3, and c-Jun N-terminal kinase, and the activation of endoplasmic reticulum stress response and its downstream proapoptotic effector, CAAT/enhancer binding homologous protein, by palmitate and LPC. Moreover, the activation of these stress signaling pathways is linked to the release of proinflammatory, proangiogenic, and profibrotic extracellular vesicles by stressed hepatocytes. This review discusses the signaling pathways induced by lethal and sublethal lipid overload that contribute to the pathogenesis of NAFLD.

The Novel PKCθ from Benchtop to Clinic

The protein kinases C (PKCs) are a family of serine/threonine kinases involved in regulating multiple essential cellular processes such as survival, proliferation, and differentiation. Of particular interest is the novel, calcium-independent PKCθ which plays a central role in immune responses. PKCθ shares structural similarities with other PKC family members, mainly consisting of an N-terminal regulatory domain and a C-terminal catalytic domain tethered by a hinge region. This isozyme, however, is unique in that it translocates to the immunological synapse between a T cell and an antigen-presenting cell (APC) upon T cell receptor-peptide MHC recognition. Thereafter, PKCθ interacts physically and functionally with downstream effectors to mediate T cell activation and differentiation, subsequently leading to inflammation. PKCθ-specific perturbations have been identified in several diseases, most notably autoimmune disorders, and hence the modulation of its activity presents an attractive therapeutic intervention. To that end, many inhibitors of PKCs and PKCθ have been developed and tested in preclinical and clinical studies. And although selectivity remains a challenge, results are promising for the future development of effective PKCθ inhibitors that would greatly advance the treatment of several T-cell mediated diseases.

Adiposity and insulin resistance in humans: the role of the different tissue and cellular lipid depots

Human adiposity has long been associated with insulin resistance and increased cardiovascular risk, and abdominal adiposity is considered particularly adverse. Intra-abdominal fat is associated with insulin resistance, possibly mediated by greater lipolytic activity, lower adiponectin levels, resistance to leptin, and increased inflammatory cytokines, although the latter contribution is less clear. Liver lipid is also closely associated with, and likely to be an important contributor to, insulin resistance, but it may also be in part the consequence of the lipogenic pathway of insulin action being up-regulated by hyperinsulinemia and unimpaired signaling. Again, intramyocellular triglyceride is associated with muscle insulin resistance, but anomalies include higher intramyocellular triglyceride in insulin-sensitive athletes and women (vs men). Such issues could be explained if the "culprits" were active lipid moieties such as diacylglycerol and ceramide species, dependent more on lipid metabolism and partitioning than triglyceride amount. Subcutaneous fat, especially gluteofemoral, appears metabolically protective, illustrated by insulin resistance and dyslipidemia in patients with lipodystrophy. However, some studies suggest that deep sc abdominal fat may have adverse properties. Pericardial and perivascular fat relate to atheromatous disease, but not clearly to insulin resistance. There has been recent interest in recognizable brown adipose tissue in adult humans and its possible augmentation by a hormone, irisin, from exercising muscle. Brown adipose tissue is metabolically active, oxidizes fatty acids, and generates heat but, because of its small and variable quantities, its metabolic importance in humans under usual living conditions is still unclear. Further understanding of specific roles of different lipid depots may help new approaches to control obesity and its metabolic sequelae.

Mechanisms underlying the onset of oral lipid-induced skeletal muscle insulin resistance in humans

Several mechanisms, such as innate immune responses via Toll-like receptor-4, accumulation of diacylglycerols (DAG)/ceramides, and activation of protein kinase C (PKC), are considered to underlie skeletal muscle insulin resistance. In this study, we examined initial events occurring during the onset of insulin resistance upon oral high-fat loading compared with lipid and low-dose endotoxin infusion. Sixteen lean insulin-sensitive volunteers received intravenous fat (iv fat), oral fat (po fat), intravenous endotoxin (lipopolysaccharide [LPS]), and intravenous glycerol as control. After 6 h, whole-body insulin sensitivity was reduced by iv fat, po fat, and LPS to 60, 67, and 48%, respectively (all P < 0.01), which was due to decreased nonoxidative glucose utilization, while hepatic insulin sensitivity was unaffected. Muscle PKCθ activation increased by 50% after iv and po fat, membrane Di-C18:2 DAG species doubled after iv fat and correlated with PKCθ activation after po fat, whereas ceramides were unchanged. Only after LPS, circulating inflammatory markers (tumor necrosis factor-α, interleukin-6, and interleukin-1 receptor antagonist), their mRNA expression in subcutaneous adipose tissue, and circulating cortisol were elevated. Po fat ingestion rapidly induces insulin resistance by reducing nonoxidative glucose disposal, which associates with PKCθ activation and a rise in distinct myocellular membrane DAG, while endotoxin-induced insulin resistance is exclusively associated with stimulation of inflammatory pathways.

PKCθ expression in the amygdala regulates insulin signaling, food intake and body weight

OBJECTIVE

To investigate the signaling mechanisms that might underlie the loss of anorectic response to insulin injections into the central nucleus of the amygdala (CeA) within 3 days of feeding a high fat diet.

DESIGN AND METHODS

Protein samples from amygdala and hypothalamus of rats fed high or low fat diets were subjected to a phosphorylation screening assay. The effects of dietary fat intake on the expression and activation of protein kinase C theta (PKCθ) in brain regions was studied. Finally, lentiviral vectors were used to overexpress rat PKCθ unilaterally or bilaterally into the CeA of rats and the effects on food intake, body weight and insulin stimulation of Akt phosphorylation were studied.

RESULTS

The level of pMARCKS (Myristoylated alanine-rich C-kinase substrate), a major substrate of PKCθ, was increased 116% in amygdala of high fat diet fed rats but reduced in the hypothalamus. High fat diets increased the level of PKCθ in a region specific manner in the brain and this PKCθ was activated by membrane association. Overexpressing rat PKCθ either unilaterally or bilaterally into the CeA inhibited insulin stimulation of Akt signaling and blocked the anorectic response to insulin injected into the amygdala. Bilaterally injected PKCθ rats gained more weight and body fat and had increased food intake when fed a high fat diet compared to the control rats that received a lentiviral-Green Fluorescent Protein construct.

CONCLUSION

The data suggest that insulin may have a physiological role within the amygdala to regulate energy balance.

AMPK regulation of fatty acid metabolism and mitochondrial biogenesis: implications for obesity

Skeletal muscle plays an important role in regulating whole-body energy expenditure given it is a major site for glucose and lipid oxidation. Obesity and type 2 diabetes are causally linked through their association with skeletal muscle insulin resistance, while conversely exercise is known to improve whole body glucose homeostasis simultaneously with muscle insulin sensitivity. Exercise activates skeletal muscle AMP-activated protein kinase (AMPK). AMPK plays a role in regulating exercise capacity, skeletal muscle mitochondrial content and contraction-stimulated glucose uptake. Skeletal muscle AMPK is also thought to be important for regulating fatty acid metabolism; however, direct genetic evidence in this area is currently lacking. This review will discuss the current paradigms regarding the influence of AMPK in regulating skeletal muscle fatty acid metabolism and mitochondrial biogenesis at rest and during exercise, and highlight the potential implications in the development of insulin resistance.

Inhibitory effect on protein kinase Ctheta by Crocetin attenuates palmitate-induced insulin insensitivity in 3T3-L1 adipocytes

Epidemiologic and experimental studies have pointed to an etiologic role of elevated plasma free fatty acids in insulin resistance, which is frequently associated with a state of low-grade inflammation. In this study, we investigated the effects of Crocetin, a unique carotenoid, on insulin resistance induced by palmitate in 3T3-L1 adipocytes. Exposure of palmitate led to an increase in insulin receptor substrate-1 (IRS-1) serine(307) phosphorylation as well as activation of c-Jun NH(2)-terminal kinase (JNK) and inhibitor kappaB kinase beta (IKKbeta), concomitantly with reductions of IRS-1 function and glucose metabolism. Interestingly, pretreatment with Crocetin almost reversed all of these abnormalities in a dose-dependent manner. IRS-1 serine(307) phosphorylation was significantly reduced by JNK or IKKbeta inhibitor, especially by combination of these two inhibitors. Moreover, palmitate treatment induced activation of protein kinase Ctheta (PKCtheta) while blocking PKCtheta significantly inhibited JNK and IKKbeta activation induced by palmitate or phorbol 12-myristate 13-acetate (PKC activator, PMA), and attenuated the palmitate-induced defects in insulin action. Crocetin demonstrated an impressive suppression in the activation of PKCtheta induced not only by palmitate but also by PMA in a dose-dependent manner. Taken together, Crocetin inhibited JNK and IKKbeta activation via suppression of PKCtheta phosphorylation, attenuating insulin insensitivity induced by palmitate in 3T3-L1 adipocytes.

Is "fat-induced" muscle insulin resistance rapidly reversible?

Elevated plasma free fatty acids (FFA) cause insulin resistance and are thought to play a key role in mediating insulin resistance in patients with the metabolic syndrome (MTS) and type 2 diabetes mellitus (DM). Two experimental models used to study the mechanisms responsible for insulin resistance in patients are high-fat diet-fed rodents and administration of triglycerides and heparin to raise plasma FFA. As evidence that insulin resistance in high-fat diet-fed rats is due to high FFA, it has been reported that the insulin resistance is rapidly reversed by an overnight fast, a high-glucose meal, and an exercise bout. If true, these findings would invalidate the high-fat diet-fed rodent as a model for MTS or type 2 DM, because insulin resistance is not rapidly reversed by these treatments in patients. The purpose of this study was to determine whether diet-induced insulin resistance is, in fact, rapidly reversible. Incubation of muscles in vitro rapidly reversed insulin resistance induced by administration of triglycerides and heparin, but not by a high-fat diet. An overnight fast and a high-glucose meal were followed by a large increase in insulin-stimulated muscle glucose transport. However, these are adaptive responses, rather than reversals of insulin resistance, because they also occurred in muscles of insulin-sensitive, chow-fed control rats. Our results show that insulin resistance induced by high FFA, i.e., Randle glucose-fatty acid cycle, is transient. In contrast, the insulin resistance induced by a high-fat diet does not reverse rapidly.

Rescuing 3T3-L1 adipocytes from insulin resistance induced by stimulation of Akt-mammalian target of rapamycin/p70 S6 kinase (S6K1) pathway and serine phosphorylation of insulin receptor substrate-1: effect of reduced expression of p85alpha subunit of phosphatidylinositol 3-kinase and S6K1 kinase

Phosphorylation of insulin receptor substrate-1 (IRS-1) on serine residues has been recognized as a mechanism responsible for a diminution of insulin action and insulin resistance. Potential approaches to improve insulin sensitivity may include interference with and/or reduction in expression of certain signaling intermediates that participate in the pathogenesis of insulin resistance. In this study, we transduced fully differentiated 3T3-L1 adipocytes with a constitutively active myristoylated Akt that led to hyperactivation of mammalian target of rapamycin and p70 S6 kinase (S6K1), increased serine phosphorylation of IRS-1, and reduction in insulin-stimulated phosphatidylinositol (PI) 3-kinase activity and glucose transport. We then reduced expression of the PI 3-kinase regulatory subunit, p85alpha, or expression of S6K1 kinase using small interfering RNA transfections, which led to a reduction in p85alpha expression of 70% at 48 h (P < 0.05) and S6K1 of 49% (P < 0.05). Reduction in expression of either p85alpha or S6K1 achieved with small interfering RNA in the presence of myristoylated Akt rescued 3T3-L1 adipocytes from the insulin resistance induced by serine phosphorylation of IRS-1 and completely restored insulin-stimulated activation of PI 3-kinase and glucose uptake. We conclude that reduction in expression of p85alpha or S6K1 could represent therapeutic targets to mitigate insulin resistance.

Saturated fatty acids inhibit hepatic insulin action by modulating insulin receptor expression and post-receptor signalling

Free fatty acids (FFAs) are proposed to play a pathogenic role in both peripheral and hepatic insulin resistance. We have examined the effect of saturated FFA on insulin signalling (100 nM) in two hepatocyte cell lines. Fao hepatoma cells were treated with physiological concentrations of sodium palmitate (0.25 mM) (16:0) for 0.25-48 h. Palmitate decreased insulin receptor (IR) protein and mRNA expression in a dose- and time-dependent manner (35% decrease at 12 h). Palmitate also reduced insulin-stimulated IR and IRS-2 tyrosine phosphorylation, IRS-2-associated PI 3-kinase activity, and phosphorylation of Akt, p70 S6 kinase, GSK-3 and FOXO1A. Palmitate also inhibited insulin action in hepatocytes derived from wild-type IR (+/+) mice, but was ineffective in IR-deficient (-/-) cells. The effects of palmitate were reversed by triacsin C, an inhibitor of fatty acyl CoA synthases, indicating that palmitoyl CoA ester formation is critical. Neither the non-metabolized bromopalmitate alone nor the medium chain fatty acid octanoate (8:0) produced similar effects. However, the CPT-1 inhibitor (+/-)-etomoxir and bromopalmitate (in molar excess) reversed the effects of palmitate. Thus, the inhibition of insulin signalling by palmitate in hepatoma cells is dependent upon oxidation of fatty acyl-CoA species and requires intact insulin receptor expression.

Muscular diacylglycerol metabolism and insulin resistance

Failure of insulin to elicit an increase in glucose uptake and metabolism in target tissues such as skeletal muscle is a major characteristic of non-insulin dependent type 2 diabetes mellitus. A strong correlation between intramyocellular triacylglycerol concentrations and the severity of insulin resistance has been found and led to the assumption that lipid oversupply to skeletal muscle contributes to reduced insulin action. However, the molecular mechanism that links intramyocellular lipid content with the generation of muscle insulin resistance is still unclear. It appears unlikely that the neutral lipid metabolite triacylglycerol directly impairs insulin action. Hence it is believed that intermediates in fatty acid metabolism, such as fatty acyl-CoA, ceramides or diacylglycerol (DAG) link fat deposition in the muscle to compromised insulin signaling. DAG is identified as a potential mediator of lipid-induced insulin resistance, as increased DAG levels are associated with protein kinase C activation and a reduction in both insulin-stimulated IRS-1 tyrosine phosphorylation and PI3 kinase activity. As DAG is an intermediate in the synthesis of triacylglycerol from fatty acids and glycerol, its level can be lowered by either improving the oxidation of cellular fatty acids or by accelerating the incorporation of fatty acids into triacylglycerol. This review discusses the evidence that implicates DAG being central in the development of muscular insulin resistance. Furthermore, we will discuss if and how modulation of skeletal muscle DAG levels could function as a possible therapeutic target for the treatment of type 2 diabetes mellitus.

Silymarin as an adjunct to glibenclamide therapy improves long-term and postprandial glycemic control and body mass index in type 2 diabetes

Oxidative stress is increased postprandially and during long-term hyperglycemia in type 2 diabetic patients who present with poor response to glibenclamide. This study was designed to evaluate the effects of the antioxidant flavonoid silymarin in improving long-term and postprandial glycemic and weight control in type 2 diabetic patients treated with glibenclamide. Using a randomized, double-blind, placebo-controlled design, 59 type 2 diabetic patients, previously maintained on 10 mg/day glibenclamide and diet control, with poor glycemic control, were randomized into three groups: the first two groups were treated with either 200 mg/day silymarin or placebo as adjuncts to glibenclamide, and the third group was maintained on glibenclamide alone for 120 days. Fasting and 4-hour postprandial plasma glucose, glycated hemoglobin (HbA(1c)), and body mass index (BMI) were evaluated at baseline and after 120 days. Compared with placebo, silymarin treatment significantly reduced both fasting and postprandial plasma glucose excursions, in addition to significantly reducing HbA(1c) levels and BMI after 120 days. No significantly different effects were observed for placebo compared to glibenclamide alone. In conclusion, adjunct use of silymarin with glibenclamide improves the glycemic control targeted by glibenclamide, during both fasting and postprandially, an effect that may be related to increased insulin sensitivity in peripheral tissues.

Metformin and exercise reduce muscle FAT/CD36 and lipid accumulation and blunt the progression of high-fat diet-induced hyperglycemia

Derangements in skeletal muscle fatty acid (FA) metabolism associated with insulin resistance in obesity appear to involve decreased FA oxidation and increased accumulation of lipids such as ceramides and diacylglycerol (DAG). We investigated potential lipid-related mechanisms of metformin (Met) and/or exercise for blunting the progression of hyperglycemia/hyperinsulinemia and skeletal muscle insulin resistance in female Zucker diabetic fatty rats (ZDF), a high-fat (HF) diet-induced model of diabetes. Lean and ZDF rats consumed control or HF diet (48 kcal %fat) alone or with Met (500 mg/kg), with treadmill exercise, or with both exercise and Met interventions for 8 wk. HF-fed ZDF rats developed hyperglycemia (mean: 24.4 +/- 2.1 mM), impairments in muscle insulin-stimulated glucose transport, increases in the FA transporter FAT/CD36, and increases in total ceramide and DAG content. The development of hyperglycemia was significantly attenuated with all interventions, as was skeletal muscle FAT/CD36 abundance and ceramide and DAG content. Interestingly, improvements in insulin-stimulated glucose transport and increased GLUT4 transporter expression in isolated muscle were seen only in conditions that included exercise training. Reduced FA oxidation and increased triacylglycerol synthesis in isolated muscle were observed with all ZDF rats compared with lean rats (P < 0.01) and were unaltered by therapeutic intervention. However, exercise did induce modest increases in peroxisome proliferator-activated receptor-gamma coactivator-1alpha, citrate synthase, and beta-hydroxyacyl-CoA dehydrogenase activity. Thus reduction of skeletal muscle FAT/CD36 and content of ceramide and DAG may be important mechanisms by which exercise training blunts the progression of diet-induced insulin resistance in skeletal muscle.

Leptin, skeletal muscle lipids, and lipid-induced insulin resistance

Leptin-induced increases in insulin sensitivity are well established and may be related to the effects of leptin on lipid metabolism. However, the effects of leptin on the levels of lipid metabolites implicated in pathogenesis of insulin resistance and the effects of leptin on lipid-induced insulin resistance are unknown. The current study addressed in rats the effects of hyperleptinemia (HL) on insulin action and markers of skeletal muscle (SkM) lipid metabolism in the absence or presence of acute hyperlipidemia induced by an infusion of a lipid emulsion. Compared with controls (CONT), HL increased insulin sensitivity, as assessed by hyperinsulinemic-euglycemic clamp ( approximately 15%), and increased SkM Akt ( approximately 30%) and glycogen synthase kinase 3 alpha ( approximately 52%) phosphorylation. These improvements in insulin action were associated with decreased SkM triglycerides (TG; approximately 61%), elevated ceramides ( approximately 50%), and similar diacylglycerol (DAG) levels in HL compared with CONT. Acute hyperlipidemia in CONT decreased insulin sensitivity ( approximately 25%) and increased SkM DAG ( approximately 33%) and ceramide ( approximately 60%) levels. However, hyperlipidemia did not induce insulin resistance or SkM DAG and ceramide accumulation in HL. SkM total fatty acid transporter CD36, plasma membrane fatty acid binding protein, acetyl Co-A carboxylase phosphorylation, and fatty acid oxidation were similar in HL compared with CONT. However, HL decreased SkM protein kinase C theta (PKC theta), a kinase implicated in mediating the detrimental effects of lipids on insulin action. We conclude that increases in insulin sensitivity induced by HL are associated with decreased levels of SkM TG and PKC theta and increased SkM insulin signaling, but not with decreases in other lipid metabolites implicated in altering SkM insulin sensitivity (DAG and ceramide). Furthermore, insulin resistance induced by an acute lipid infusion is prevented by HL.

Kinase-dependent pathways and the development of insulin resistance in hepatocytes

Hepatic insulin resistance is considered to be a dominant component in the pathogenesis of fasting hyperglycemia in Type 2 diabetes. The role of nutrients, free fatty acids and secretory inflammatory factors released by visceral fat in the pathogenesis of liver insulin resistance requires clarification, but a number of signaling pathways and serine kinases have been implicated. These include the discovery of c-Jun N-terminal kinase, I κβ kinase, protein kinase C θ, δ and ε, and ribosomal protein S6 kinase 1 as critical regulators of insulin action and steatosis in liver. In this article, the causes and mechanisms involved in the development of hepatic insulin resistance, and the signaling pathways and kinases involved, will be discussed. Elucidation of the molecular mechanisms underlying regulation and specificity may prompt novel approaches to the pharmacological modulation of protein kinase activities involved in hepatic insulin resistance. This review will detail recent discoveries and highlight emerging kinase targets that hold potential to reduce hepatic insulin resistance and normalize blood glucose.

The relationship between the metabolic syndrome and energy-utilization deficit in the pathogenesis of obesity-induced osteoarthritis

We propose that the pathogenesis of obesity-induced osteoarthritis may be explained by the metabolic changes in the striated muscle induced by the interaction of insulin resistance and systemic inflammation in obese individuals with metabolic syndrome being osteoarthritis the latest consequence by the physiological changes seen in the metabolic syndrome. Increased levels of TH1 cytokines are produced by activated macrophages in the presence of an acute or chronic infectious disease and suppress the sensitivity of insulin receptors on the membrane of muscle cell and adipocytes. Both cells are activated by inflammatory cytokines and contribute to enhance acute inflammation and to maintain a state of chronic, low-grade inflammation in apparently healthy obese individuals. The increased number of macrophage in the adipose tissue of obese individuals acts as an amplifier of inflammation. Patients with osteoarthritis and metabolic syndrome frequently are complaining about hotness and recurrent edema of feet and hands. It is probable that hyperinsulinemia in the presence of insulin resistance and inflammation, induce vasodilation through the TNF mediated-iNOS overexpression. Patients with metabolic syndrome express clinically the consequence of a poor uptake, storage and energy expenditure by the muscle and any other insulin dependent tissue and the consequence of high insulin plasma levels are vasodilation and increased protein synthesis. The fatigue and muscle weakness induced by insulin resistance and inflammation in obese patients with metabolic syndrome increase the frequency and the intensity of traumatic events of peripheral or axial joints that result in stretch and breaking of tenoperiosteal junction and abrasive damage of cartilage and therefore in these patients with metabolic syndrome and pro-inflammatory state the reparative process of cartilage and periarticular tissues would be severely modified by the growth factor activity in presence of high levels of insulin.

Selective PPAR modulators, dual and pan PPAR agonists: multimodal drugs for the treatment of Type 2 diabetes and atherosclerosis

More than 70% of patients with Type 2 diabetes mellitus (T2DM) die because of cardiovascular diseases. Current therapeutic strategies are based on separate treatment of insulin resistance and dyslipidaemia. Development of drugs with multimodal activities should improve management of the global cardiovascular risk of T2DM patients and result in better patient compliance. New therapeutic strategies are aimed at targeting the entire spectrum of dysfunctioning organs, cells and regulatory pathways implicated in the pathogenesis of T2DM, dyslipidaemia and atherosclerosis. PPAR family members play major roles in the regulation of lipid metabolism, glucose homeostasis and inflammatory processes, making these transcription factors ideal targets for therapeutic strategies against these diseases. This review discusses why PPARs and development of novel selective PPAR modulators, dual and pan PPAR agonists constitute promising approaches for the treatment of diabetes, dyslipidaemia and atherosclerosis.

Molecular mechanisms of insulin resistance: serine phosphorylation of insulin receptor substrate-1 and increased expression of p85alpha: the two sides of a coin

Initial attempts to unravel the molecular mechanism of insulin resistance have strongly suggested that a defect responsible for insulin resistance in the majority of patients lies at the postreceptor level of insulin signaling. Subsequent studies in insulin-resistant animal models and humans have consistently demonstrated a reduced strength of insulin signaling via the insulin receptor substrate (IRS)-1/phosphatidylinositol (PI) 3-kinase pathway, resulting in diminished glucose uptake and utilization in insulin target tissues. However, the nature of the triggering event(s) remains largely enigmatic. Two separate, but likely, complementary mechanisms have recently emerged as a potential explanation. First, it became apparent that serine phosphorylation of IRS proteins can reduce their ability to attract PI 3-kinase, thereby minimizing its activation. A number of serine kinases that phosphorylate serine residues of IRS-1 and weaken insulin signal transduction have been identified. Additionally, mitochondrial dysfunction has been suggested to trigger activation of several serine kinases, leading to a serine phosphorylation of IRS-1. Second, a distinct mechanism involving increased expression of p85alpha has also been found to play an important role in the pathogenesis of insulin resistance. Conceivably, a combination of both increased expression of p85alpha and increased serine phosphorylation of IRS-1 is needed to induce clinically apparent insulin resistance.

PKCtheta is a key player in the development of insulin resistance

Activation of PKCtheta is associated with lipid-induced insulin resistance and PKCtheta knockout mice are protected from the lipid-induced defects. However, the exact mechanism by which PKCtheta contributes to insulin resistance is not known. To investigate whether an increase in PKCtheta expression leads to insulin resistance, C2C12 skeletal muscle cells were transfected with PKCtheta DNA and treated with different concentrations of insulin for 10 min. PKCtheta overexpression induced reduction of IRS-1 protein levels with a decrease in insulin-induced p85 binding to IRS-1, phosphorylation of PKB and its substrates, p70 and GSK3. Pretreatment of these cells with GF-109203X (a non-specific PKC inhibitor, IC50 for PKCtheta = 10 nM) recovered insulin signaling. PKCtheta was found to be expressed in liver and treatment of human hepatoma cells (HepG2) with high insulin and glucose resulted in an increase in PKCtheta expression that correlated with a decrease in IRS-1 protein levels and the development of insulin resistance. Reduction of PKCtheta expression using RNAi technology significantly inhibited the degradation of IRS-1 and enhanced insulin-induced IRS-1 tyrosine phosphorylation, p85 association to IRS-1 and PKB phosphorylation. In conclusion, by overexpressing PKCtheta or using RNAi technology to downregulate PKCtheta, we have demonstrated that PKCtheta has a key role in the development of insulin resistance. These findings suggest that PKCtheta mediates not only insulin resistance in muscle but also in liver, which may contribute to the development of whole body insulin resistance and diabetes.

Mechanisms of Disease: pathway-selective insulin resistance and microvascular complications of diabetes

Resistance to the actions of insulin is strongly associated with the microvascular complications of diabetes. To the extent that insulin resistance leads to hyperglycemia, dyslipidemia and hypertension, this association is not surprising. It is now clear that insulin also has direct actions in the microvasculature that influence the development and progression of microvascular disease. In the healthy state, insulin appears to have only minor effects on vascular function, because of the activation of opposing mediators such as nitric oxide and endothelin-1. Diabetes and obesity, however, are associated with selective insulin resistance in the phosphatidylinositol-3-kinase signaling pathway, which leads to reduced synthesis of nitric oxide, impaired metabolic control and compensatory hyperinsulinemia. By contrast, insulin signaling via extracellular signal-regulated kinase dependent pathways is relatively unaffected in diabetes, tipping the balance of insulin's actions so that they favor abnormal vasoreactivity, angiogenesis, and other pathways implicated in microvascular complications and hypertension. In addition, preferential impairment of nonoxidative glucose metabolism leads to increased intracellular formation of advanced glycation end products, oxidative stress and activation of other pathogenic mediators. Despite a strong temporal association, a causal link between pathway-selective insulin resistance and microvascular damage remains to be established. It is possible that this association reflects a common genotype or phenotype. Nonetheless, insulin resistance remains an important marker of risk and a key target for intervention, because those patients who achieve a greater improvement of insulin sensitivity achieve better microvascular outcomes.

Free fatty acids produce insulin resistance and activate the proinflammatory nuclear factor-kappaB pathway in rat liver

To study mechanisms by which free fatty acids (FFAs) cause hepatic insulin resistance, we have used euglycemic-hyperinsulinemic clamping with and without infusion of lipid/heparin (to raise or to lower plasma FFAs) in alert male rats. FFA-induced hepatic insulin resistance was associated with increased hepatic diacylglycerol content (+210%), increased activities of two serine/threonine kinases (protein kinase C-delta and inhibitor of kappaB [IkappaB] kinase-beta), increased activation of the proinflammatory nuclear factor-kappaB (NF-kappaB) pathway (IkappaB kinase-beta, +640%; IkappaB-alpha, -54%; and NF-kappaB, +73%), and increased expression of inflammatory cytokines (tumor necrosis factor-alpha, +1,700% and interleukin-1beta, +440%) and plasma levels of monocyte chemoattractant protein-1 (+220%). We conclude that FFAs caused hepatic insulin resistance, which can produce overproduction of glucose and hyperglycemia, and initiated inflammatory processes in the liver that could potentially result in the development of steatohepatitis.

Increased p85/55/50 expression and decreased phosphotidylinositol 3-kinase activity in insulin-resistant human skeletal muscle

Insulin resistance is predominantly characterized by decreased insulin-stimulated glucose uptake into skeletal muscle. In the current study, we have assessed various aspects of the phosphatidylinositol (PI) 3-kinase pathway in skeletal muscle biopsies obtained from normal, obese nondiabetic, and type 2 diabetic subjects, before and after a 5-h insulin infusion. We found a highly significant inverse correlation between in vivo insulin sensitivity (as measured by the glucose infusion rate) and increased protein expression of p85/55/50, protein kinase C (PKC)-theta activity, levels of pSer307 insulin receptor substrate (IRS)-1 and p-Jun NH2-terminal kinase (JNK)-1, and myosin heavy chain IIx fibers. Increased basal phosphorylation of Ser307 IRS-1 in the obese and type 2 diabetic subjects corresponds with decrease in insulin-stimulated IRS-1 tyrosine phosphorylation, PI 3-kinase activity, and insulin-induced activation of Akt and, more prominently, PKC-zeta/lambda. In summary, increased expression of the PI 3-kinase adaptor subunits p85/55/50, as well as increased activity of the proinflammatory kinases JNK-1, PKC-theta, and, to a lesser extent, inhibitor of kappaB kinase-beta, are associated with increased basal Ser307 IRS-1 phosphorylation and decreased PI 3-kinase activity and may follow a common pathway to attenuate in vivo insulin sensitivity in insulin-resistant subjects. These findings demonstrate interacting mechanisms that can lead to impaired insulin-stimulated PI 3-kinase activity in skeletal muscle from obese and type 2 diabetic subjects.

Potential utility of natural polyphenols for reversing fat-induced insulin resistance

There is intriguing recent evidence that the beta subunit of the signalsome--IKKbeta, a crucial catalyst of NF-kappaB activation--is an obligate mediator of the disruption of insulin signaling induced by excessive exposure of tissues to free fatty acids and by hypertrophy of adipocytes. Thus, agents which safely inhibit or suppress the activation of IKKbeta may have utility for reversing insulin resistance syndrome and aiding control of type 2 diabetes. Two natural agents which can achieve this effect in vitro--and which may have clinical potential in this regard--are the polyphenols resveratrol and silibinin. To date, limited absorbability and/or rapid glucuronidation have prevented these agents from achieving full therapeutic utility, but, by administering these agents in optimally absorbable forms, and co-administering inhibitors of glucuronidation such as probenecid, it may prove feasible to make these agents more clinically viable. Oral silibinin, in the guise of the milk thistle extract silymarin, already has documented clinical utility in a range of hepatic disorders, and recent evidence that dietary silibinin can inhibit the growth of certain cancers in rodents suggests that this agent may indeed have clinical potential as an IKKbeta inhibitor. A report that silymarin has a favorable impact on glycemic and lipidemic control in type 2 diabetics with cirrhosis, may or may not be indicative of IKKbeta inhibition in skeletal muscle and adipocytes. In light of the fact that IKKbeta plays a crucial role, not only in the induction of insulin resistance, but also atherogenesis, a host of inflammatory disorders, and the survival and spread of cancer, the development of pharmaceutical agents that could safely and feasibly achieve a down-regulation of IKKbeta activity would have broad therapeutic and preventive implications.

Inhibition of Insulin Sensitivity by Free Fatty Acids Requires Activation of Multiple Serine Kinases in 3T3-L1 Adipocytes

Insulin receptor substrate (IRS) has been suggested as a molecular target of free fatty acids (FFAs) for insulin resistance. However, the signaling pathways by which FFAs lead to the inhibition of IRS function remain to be established. In this study, we explored the FFA-signaling pathway that contributes to serine phosphorylation and degradation of IRS-1 in adipocytes and in dietary obese mice. Linoleic acid, an FFA used in this study, resulted in a reduction in insulin-induced glucose uptake in 3T3-L1 adipocytes. This mimics insulin resistance induced by high-fat diet in C57BL/6J mice. The reduction in glucose uptake is associated with a decrease in IRS-1, but not IRS-2 or GLUT4 protein abundance. Decrease in IRS-1 protein was proceeded by IRS-1 (serine 307) phosphorylation that was catalyzed by serine kinases inhibitor kappaB kinase (IKK) and c-JUN NH2-terminal kinase (JNK). IKK and JNK were activated by linoleic acid and inhibition of the two kinases led to prevention of IRS-1 reduction. We demonstrate that protein kinase C (PKC) theta is expressed in adipocytes. In 3T3-L1 adipocytes and fat tissue, PKCtheta was activated by fatty acids as indicated by its phosphorylation status, and by its protein level, respectively. Activation of PKCtheta contributes to IKK and JNK activation as inhibition of PKCtheta by calphostin C blocked activation of the latter kinases. Inhibition of either PKCtheta or IKK plus JNK by chemical inhibitors resulted in protection of IRS-1 function and insulin sensitivity in 3T3-L1 adipocytes. These data suggest that: 1) activation of PKCtheta contributes to IKK and JNK activation by FFAs; 2) IKK and JNK mediate PKCtheta signals for IRS-1 serine phosphorylation and degradation; and 3) this molecular mechanism may be responsible for insulin resistance associated with hyperlipidemia.

The role of intramuscular lipid in insulin resistance

There is interest in how altered lipid metabolism could contribute to muscle insulin resistance. Many animal and human states of insulin resistance have increased muscle triglyceride content, and there are now plausible mechanistic links between muscle lipid accumulation and insulin resistance, which go beyond the classic glucose-fatty acid cycle. We postulate that muscle cytosolic accumulation of the metabolically active long-chain fatty acyl CoAs (LCACoA) is involved, leading to insulin resistance and impaired insulin signalling or impaired enzyme activity (e.g. glycogen synthase or hexokinase) either directly or via chronic translocation/activation of mediators such as a protein kinase C (particularly PKC theta and epsilon ). Ceramides and diacylglycerols (DAGs) have also been implicated in forms of lipid-induced muscle insulin resistance. Dietary lipid-induced muscle insulin resistance in rodents is relatively easily reversed by manipulations that lessen cytosolic lipid accumulation (e.g. diet change, exercise or fasting). PPAR agonists (both gamma and alpha) also lower muscle LCACoA and enhance insulin sensitivity. Activation of AMP-activated protein kinase (AMPK) by AICAR leads to muscle enhancement (especially glycolytic muscle) of insulin sensitivity, but involvement of altered lipid metabolism is less clear cut. In rodents there are similarities in the pattern of muscle lipid accumulation/PKC translocation/altered insulin signalling/insulin resistance inducible by 3-5-h acute free fatty acid elevation, 1-4 days intravenous glucose infusion or several weeks of high-fat feeding. Recent studies extend findings and show relevance to humans. Muscle cytosolic lipids may accumulate either by increased fatty acid flux into muscle, or by reduced fatty acid oxidation. In some circumstances muscle insulin resistance may be an adaptation to optimize use of fatty acids when they are the predominant available energy fuel. The interactions described here are fundamental to optimizing therapy of insulin resistance based on alterations in muscle lipid metabolism.

Exercise and insulin signaling: a historical perspective

Over the past 30 years, a considerable body of evidence has revealed that a prior bout of exercise can increase the ability of insulin to stimulate glucose transport and glycogen synthesis in skeletal muscle. Apart from its clinical implications, this work has led to a considerable effort to determine at a molecular level how exercise causes this effect and, in particular, whether it does so by enhancing specific events in the insulin-signaling cascade. The objective of this review is to discuss from a historical perspective how our current thinking in this area has evolved and the people responsible for it. Areas to be discussed include the effect or lack of effect of prior exercise on the insulin-signaling pathway, effects of exercise on the regulation by insulin of the GLUT-4 glucose transporter in muscle, and the emerging role of AMP-activated protein kinase as a mediator of exercise-induced signaling events. In addition, we will discuss briefly some of the avenues that research in this area is likely to follow.

Lipid-induced insulin resistance in human muscle is associated with changes in diacylglycerol, protein kinase C, and IkappaB-alpha

The possibility that lipid-induced insulin resistance in human muscle is related to alterations in diacylglycerol (DAG)/protein kinase C (PKC) signaling was investigated in normal volunteers during euglycemic-hyperinsulinemic clamping in which plasma free fatty acid (FFA) levels were increased by a lipid/heparin infusion. In keeping with previous reports, rates of insulin-stimulated glucose disappearance (G(Rd)) were normal after 2 h but were reduced by 43% (from 52.7 +/- 8.2 to 30.0 +/- 5.3 micromol. kg(-1). min(-1), P < 0.05) after 6 h of lipid infusion. No changes in PKC activity or DAG mass were seen in muscle biopsy samples after 2 h of lipid infusion; however, at approximately 6 h, PKC activity and DAG mass were increased approximately fourfold, as were the abundance of membrane-associated PKC-betaII and -delta. A threefold increase in membrane-associated PKC-betaII was also observed at approximately 2 h but was not statistically significant (P = 0.058). Ceramide mass was not changed at either time point. To evaluate whether the fatty acid-induced insulin activation of PKC was associated with a change in the IkB kinase (IKK)/nuclear factor (NF)-kappaB pathway, we determined the abundance in muscle of IkappaB-alpha, an inhibitor of NF-kappaB that is degraded after its phosphorylation by IKK. In parallel with the changes in DAG/PKC, no change in IkappaB-alpha mass was observed after 2 h of lipid infusion, but at approximately 6 h, IkappaB-alpha was diminished by 70%. In summary, the results indicated that the insulin resistance observed in human muscle when plasma FFA levels were elevated during euglycemic-hyperinsulinemic clamping was associated with increases in DAG mass and membrane-associated PKC-betaII and -delta and a decrease in IkappaB-alpha. Whether acute FFA-induced insulin resistance in human skeletal muscle is caused by the activation of these specific PKC isoforms and the IKK-beta/IkappaB/NFkappaB pathway remains to be established.

Insulin increases FA uptake and esterification but reduces lipid utilization in isolated contracting muscle

We examined the effect of insulin on the synthesis and degradation of muscle lipid pools [phospholipid (PL), diacylglycerol (DG), triacylglycerol (TG)] and palmitate oxidation in isolated resting and contracting (20 tetani/min) soleus muscles. Lipid metabolism was monitored using the previously defined pulse-chase procedure. At rest, insulin significantly increased total palmitate uptake into soleus muscle (+49%, P < 0.05), corresponding to enhanced DG (+60%, P < 0.05) and TG (+61%, P < 0.05) esterification, but blunted palmitate oxidation (-38%, P < 0.05) and TG hydrolysis (-34%, P < 0.05). During muscle contraction, when total palmitate uptake was increased, insulin further enhanced uptake (+21%, P < 0.05) and esterification of fatty acids (FA) to PL (+73%, P < 0.05), DG (+19%, P < 0.05), and TG (+161%, P < 0.01). Despite a profound shift in the relative partitioning of FA away from esterification and toward oxidation during contraction, the increase in palmitate oxidation and TG hydrolysis was significantly blunted by insulin [oxidation, -24% (P = 0.05); hydrolysis, -83% (P < 0.01)]. The effects of insulin on FA esterification (stimulation) and oxidation (inhibition) during contraction were reduced in the presence of the phosphatidylinositol 3-kinase inhibitor LY-294002. In summary, the effects of insulin and contraction on palmitate uptake and esterification are additive, while insulin opposes the stimulatory effect of contraction on FA oxidation and TG hydrolysis. Insulin's modulatory effects on muscle FA metabolism during contraction are mediated at least in part through phosphatidylinositol 3-kinase.