Failure of leptin to affect basal and insulin-stimulated glucose metabolism of rat skeletal muscle in vitro

Leptin reduces the expression and increases the phosphorylation of the negative regulators of GLUT4 traffic TBC1D1 and TBC1D4 in muscle of ob/ob mice

Leptin improves insulin sensitivity in skeletal muscle. Our goal was to determine whether proteins controlling GLUT4 traffic are altered by leptin deficiency and in vivo leptin administration in skeletal muscle of wild type and ob/ob mice. Leptin-deficient ob/ob mice were divided in three groups: control, leptin-treated (1 mg/kg/d) and leptin pair-fed ob/ob mice. Microarray analysis revealed that 1,546 and 1,127 genes were regulated by leptin deficiency and leptin treatment, respectively. Among these, we identified 24 genes involved in intracellular vesicle-mediated transport in ob/ob mice. TBC1 domain family, member 1 (Tbc1d1), a negative regulator of GLUT4 translocation, was up-regulated (P = 0.001) in ob/ob mice as compared to wild types. Importantly, leptin treatment reduced the transcript levels of Tbc1d1 (P<0.001) and Tbc1d4 (P = 0.004) in the leptin-treated ob/ob as compared to pair-fed ob/ob animals. In addition, phosphorylation levels of TBC1D1 and TBC1D4 were enhanced in leptin-treated ob/ob as compared to control ob/ob (P = 0.015 and P = 0.023, respectively) and pair-fed ob/ob (P = 0.036 and P = 0.034, respectively) mice. Despite similar GLUT4 protein expression in wild type and ob/ob groups a different immunolocalization of this protein was evidenced in muscle sections. Leptin treatment increased GLUT4 immunoreactivity in gastrocnemius and extensor digitorum longus sections of leptin-treated ob/ob mice. Moreover, GLUT4 protein detected in immunoprecipitates from TBC1D4 was reduced by leptin replacement compared to control ob/ob (P = 0.013) and pair-fed ob/ob (P = 0.037) mice. Our findings suggest that leptin enhances the intracellular GLUT4 transport in skeletal muscle of ob/ob animals by reducing the expression and activity of the negative regulators of GLUT4 traffic TBC1D1 and TBC1D4.

Novel factors as therapeutic targets to treat diabetes. Focus on leptin and ghrelin

BACKGROUND

Obesity is the major cause of type 2 diabetes. In the mid 1990s interest in adipose tissue was revived by the discovery of leptin. The association of obesity and diabetes emphasizes their shared physiopathological features. At the end of the 1990s, ghrelin, a potent gastric orexigenic factor, was found to be involved in obesity. Leptin and ghrelin have opposite actions in several tissues including the regulation of feeding in the brain.

OBJECTIVE/METHODS

To survey the role of leptin and ghrelin in glucose metabolism. We summarize the current state of research and discuss the roles of ghrelin and leptin in glucose homeostases and the potential application of drugs targeting leptin and ghrelin signalling to prevent and treat diabetes.

RESULTS/CONCLUSIONS

A pressing challenge is to determine how leptin, ghrelin and other adipokines or gastric factors are involved in metabolic disorders. Answering these questions will require the development of new pharmacological tools that target specific adipokine systems. Hopefully, new therapeutic targets will be identified.

Investigation of the acute effect of leptin on the inhibition of glycogen catabolism by insulin in rat liver perfused in situ

Leptin, a cytokine secreted by adipose tissue, has been implicated in the insulin resistance associated with obesity. Here we examined the acute influence of leptin at physiological (10 ng/ml) and supraphysiological (50 ng/ml and 100 ng/ml) concentrations on the inhibition of glycogen catabolism promoted by insulin in rat liver perfusion experiments. Perfusion of the liver with insulin (20 microU/ml) decreased the activation of glucose production (p < 0.05) and glycogenolysis by cAMP (3 microM). However, the infusion of leptin, at concentrations similar to those found in non-obese (10 ng/ml), obese (50 ng/ml), and morbidly obese (100 ng/ml) individuals did not influence the acute inhibitory effect of insulin (20 microU/ml) on glucose production and glycogenolysis stimulated by cAMP (p > 0.05).We conclude that neither physiological nor supraphysiological concentrations of leptin directly influence the inhibition of glycogen catabolism promoted by insulin in rat liver perfused in situ.

Leptin inhibits glycogen catabolism but does not modify acutely the suppressive effect of insulin on glucose production and glycogenolysis stimulated by 8-Br-cAMP in rat liver perfused in situ

Leptin, a hormone secreted by the adipocytes, plays a central role in glucose metabolism and the action of insulin. Here we assessed, by means of rat-liver perfusion, the direct influence of physiological (10 ng/ml) and supraphysiological (50 or 100 ng/ml) concentrations of leptin on the suppressive effect of insulin on the glucose production and glycogenolysis stimulated by 8-bromoadenosine-3':5'-monophosphate (8-Br-cAMP). Portal infusion of insulin (20 microU/ml) or leptin (10 ng/ml) reduced (p<0.05) the glucose production and glycogenolysis induced by 8-Br-cAMP (0.3 microM). However, portal infusion of physiological (10 ng/ml) and supraphysiological (50 or 100 ng/ml) concentrations of leptin together with the insulin did not modify the suppressive effect of the latter on the glucose production and glycogenolysis stimulated by 8-Br-cAMP. Moreover, prolonging the period of leptin infusion from 20 to 40 min also failed to influence the liver response to insulin. Thus, we conclude that: (a) leptin, at physiological levels, has a direct and acute effect, inhibiting the glucose production and glycogenolysis stimulated by 8-Br-cAMP; (b) leptin, at either physiological or supraphysiological concentrations, has no short-term influence on the suppressive effect of insulin on glycogen catabolism stimulated by 8-Br-cAMP.

Circulating leptin levels predict the development of metabolic syndrome in middle-aged men: an 8-year follow-up study

BACKGROUND

Because high circulating plasma leptin is associated with many features of the metabolic syndrome (MS), such as abdominal obesity, insulin resistance and high blood pressure (BP), we analysed the ability of plasma leptin concentration to predict the risk of developing MS in a prospective investigation of adult male participants of the Olivetti Heart Study (OHS).

METHODS AND RESULTS

Three hundred and sixty out of 907 men participating in the 1994-95 and 2002-04 OHS examinations (mean age at baseline 50.4 years, range 25-73 years) were free of MS at first visit according to NCEP-ATP III criteria (modified for the lack of high-density lipoprotein cholesterol measurement at baseline). During an average follow-up period of 8 years, there were 52 incident cases of MS (14.5%) due, in particular, to a rise in the prevalence of high BP (+42.4%), abdominal obesity (+16.4%) and impaired fasting glucose (IFG, +6.1%). In multivariate analyses, a one standard deviation difference in baseline plasma leptin concentration was associated with a 1.58-fold greater risk of developing MS (95% confidence interval = 1.10-2.30, P = 0.016) accounting for age, waist circumference, homeostatic assessment model index, smoking, alcohol consumption and physical activity. In particular, plasma leptin was positively associated with the risk of developing high BP (0.006) and IFG (0.014), after adjustment for confounders.

CONCLUSION

In this sample of an adult male population free of MS at baseline, circulating plasma leptin was a significant predictor of the risk of MS and, in particular, of its high BP and IFG components, independently of potential confounders.

Adipocytes, myofibers, and cytokine biology: new horizons in the regulation of growth and body composition

Muscle growth in meat animals is a complex process governed by integrated signals emanating from multiple endocrine and immune cells. A generalized phenomenon among meat animal industries is that animals commonly fail to meet their genetic potential for growth in commercial production settings. Therefore, understanding the impact of stress and disease on muscle growth is essential to improving production efficiency. The adipocyte in particular seems to be well positioned as an interface between energy status and immune function, and may thus influence nutrient partitioning and growth through a combination of signals that influence fat metabolism, glucose uptake, and insulin sensitivity. Adipocytes and myofibers are active participants in the innate immune response, and as such, produce a number of metabolic regulators, including leptin, adiponectin, and proinflammatory cytokines. Specifically, adipocytes and muscle cells respond directly to bacterial lipopolysaccharide (LPS) by producing interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNFalpha). However, adipocytes are also the predominant source of the antiinflammatory hormone adiponectin, which regulates the nuclear factor kappa-B transcription factor. The ability to recognize antigens and produce regulatory molecules strategically positions adipocytes and myofibers to regulate growth locally, and to reciprocally regulate metabolism peripherally.

Rethinking leptin and insulin action: Therapeutic opportunities for diabetes

Leptin is an adipocyte-derived hormone that primarily acts in the hypothalamus and plays a key role in the regulation of food intake, body weight, energy expenditure and neuroendocrine function. Leptin has direct peripheral effects on several tissues, and it may be independently involved in insulin secretion and action besides its effects on body weight regulation. Basal plasma leptin and insulin concentrations correlate with each other. Insulin and glucose appear to increase leptin secretion. In turn, leptin increases peripheral insulin sensitivity while decreasing insulin secretion from pancreatic beta cells. Leptin increases skeletal muscle glucose uptake and oxidation, and suppresses hepatic glucose output. Effects of leptin on lipid metabolism might reduce lipotoxicity and therefore contribute to the improvement of hepatic, skeletal and whole body insulin sensitivity. Leptin is the first adipokine used in the treatment of hypoleptinemic clinical disorders. Although leptin therapy has limited success in common obesity, it has impressive effects in congenital leptin deficiency, lipoatrophic diabetes and syndromes of severe insulin resistance. Leptin has been reported to ameliorate hyperinsulinemia and diabetes in the clinical setting of congenital leptin deficiency. It also improves hyperglycemia, insulin resistance, hyperinsulinemia, dyslipidemia and hepatic steatosis in lipoatrophic diabetes. These promising results warrant clinical trials to test the hypothesis that leptin alone or with classical antidiabetic agents may potentially be beneficial in the treatment of hypoleptinemic non-obese individuals with glucose intolerance and diabetes. This review summarizes the clinical applications of leptin, particularly emphasizing the effects of leptin on glucose homeostasis.

Intercellular signaling between adipose tissue and muscle tissue

Adipose and muscle tissues undergo regulated growth and differentiation processes that are modulated by a wide range of factors. The interactions between myogenic cells and adipocytes play a significant role in growth and development, including the rate and extent of myogenesis, muscle growth, adipogenesis, lipogenesis/lipolysis, and in the utilization of energy substrates. Important hormones and growth factors involved in the regulation of these processes include glucocorticoids, insulin-like growth factors, various cytokines, insulin, and leptin. Interactions among these axes have important implications in their influence on relative fat and lean deposition and the efficiency of energy utilization in growth and development. As research progresses to better clarify the interactions among adipose tissue depots and muscle of different fiber types, pathways will become better understood, ultimately leading to the optimized management of fat and lean growth in domestic livestock species. This review will focus on elements of intercellular signaling, using data from cell culture studies to illustrate specific examples of signaling pathways between cells.

Acute and chronic leptin treatment mediate contrasting effects on signaling, glucose uptake, and GLUT4 translocation in L6-GLUT4myc myotubes

We have previously shown that in L6-GLUT4myc rat skeletal muscle cells, acute treatment with leptin reduced insulin-stimulated glucose uptake without altering insulin-stimulated GLUT4 translocation. In contrast, we show here that the ability of leptin to increase phosphorylation of its receptor and to reduce insulin-stimulated glucose uptake was lost in cells that were continuously exposed to leptin for 24 h. This desensitization correlated with an increase in expression of suppressor of cytokine signaling-3 (SOCS-3). Time course analysis demonstrated that the transition from acute to chronic effects of leptin occurs after 2 h. The desensitization of leptin action at 24 h was not reversed by 30 min washout period prior to re-exposing cells to leptin. However, despite insulin-stimulated glucose uptake being unaffected upon 24 h preincubation with leptin, a small but significant decrease (37%) in insulin-stimulated GLUT4 translocation and phosphorylation of Akt on T308 was detected. Insulin-stimulated phosphorylation of Akt on S473 or of p38 MAPK were unaffected. These results suggest that the chronic leptin treatment leads to desensitization of leptin signaling yet can simultaneously decrease the ability of insulin to phosphorylate Akt on T308 and translocate GLUT4. However, this does not manifest as a reduction in total glucose uptake into L6 myotubes.

Analysis of paradoxical observations on the association between leptin and insulin resistance

Obesity is commonly associated with the development of insulin resistance and diabetes in humans and rodents. Insulin resistance and diabetes are observed in lipoatrophic individuals or rodent models of lipoatrophy. Here we focus on the role of leptin, the product of the obesity (ob) gene, in the development of insulin resistance and diabetes associated with obesity and lipoatrophy. We review the reported effects of leptin on whole body glucose metabolism and compare and contrast these with direct effects on skeletal muscle, fat and liver. This summary of paradoxical observations on the effects of leptin on glucose homeostasis and the ability of leptin to induce or improve insulin resistance suggests that a complex interplay exists between direct peripheral and centrally mediated effects of the hormone. Evidence suggesting that leptin acts as a mediator of insulin release from pancreatic beta cells is reviewed. Finally, intracellular signaling mechanisms stimulated by both leptin and insulin are discussed, with potential points of cross-talk suggested.

Increased muscle proteolysis after local trauma mainly reflects macrophage-associated lysosomal proteolysis

Rat gastrocnemius showed increased protein degradation (+75-115%) at 48 h after traumatic injury. Injured muscle showed increased cathepsin B activity (+327%) and mRNA encoding cathepsin B (+670%), cathepsin L (+298%), cathepsin H (+159%), and cathepsin C (+268%). In in situ hybridization, cathepsin B mRNA localized to the mononuclear cell infiltrate in injured muscle, and only background levels of hybridization were observed either over muscle cells in injured tissue or in uninjured muscle. Immunogold/electron microscopy showed specific staining for cathepsin B only in lysosome-like structures in cells of the mononuclear cell infiltrate in injured muscle. Muscle cells were uniformly negative in the immunocytochemistry. Matrix metalloproteinase-9 (granulocyte-macrophage gelatinase) mRNA and activity were not present in uninjured muscle but were expressed after trauma. There was no activation of the ATP-ubiquitin-proteasome-dependent proteolytic pathway in injured muscle, by contrast to diverse forms of muscle wasting where the activity of this system and the expression of genes encoding ubiquitin and proteasome elements rise. These results suggest that proteolytic systems of the muscle cells remain unstimulated after local injury and that lysosomal enzymes of the inflammatory infiltrated cells are likely to be the major participant in protein catabolism associated with local trauma.

High leptin levels acutely inhibit insulin-stimulated glucose uptake without affecting glucose transporter 4 translocation in l6 rat skeletal muscle cells

Obesity is a major risk factor for the development of insulin resistance, characterized by impaired stimulation of glucose disposal into muscle. The mechanisms underlying insulin resistance are unknown. Here we examine the direct effect of leptin, the product of the obesity gene, on insulin-stimulated glucose uptake in cultured rat skeletal muscle cells. Preincubation of L6 myotubes with leptin (2 or 100 nM, 30 min) had no effect on basal glucose uptake but reduced insulin-stimulated glucose uptake. However, leptin had no effect on the insulin-induced gain in myc-tagged glucose transporter 4 (GLUT4) appearance at the cell surface of L6 myotubes. Preincubation of cells with leptin also had no effect on insulin-stimulated tyrosine phosphorylation of insulin receptor, IRS-1 and IRS-2, phosphatidylinositol 3-kinase activity, or Akt phosphorylation. We have previously shown that insulin regulates glucose uptake via a signaling pathway sensitive to inhibitors of p38 MAP kinase. Here, leptin pretreatment reduced the extent of insulin-stimulated p38 MAP kinase phosphorylation and phosphorylation of cAMP response element binder, a downstream effector of p38 MAP kinase. These results show that high leptin levels can directly reduce insulin-stimulated glucose uptake in L6 muscle cells despite normal GLUT4 translocation. The mechanism of this effect could involve inhibition of insulin-stimulated p38 MAP kinase and GLUT4 activation.

Independent influences of central fat and skeletal muscle lipids on insulin sensitivity

OBJECTIVE

Insulin resistance is closely associated with two disparate aspects of lipid storage: the intracellular lipid content of skeletal muscle and the magnitude of central adipose beds. Our aim was to determine their relative contribution to impaired insulin action.

RESEARCH METHODS AND PROCEDURES

Eighteen older (56 to 75 years of age) men were studied before elective knee surgery. Insulin sensitivity (M/Delta I) was determined by hyperinsulinemic-euglycemic clamp. Central abdominal fat (CF) was assessed by DXA. Skeletal muscle was excised at surgery and assayed for content of metabolically active long-chain acyl-CoA esters (LCAC).

RESULTS

Significant inverse relationships were observed between LCAC and M/Delta I (R(2) = 0.34, p = 0.01) and between CF and M/Delta I (R(2) = 0.38, p = 0.006), but not between CF and LCAC (R(2) = 0.0005, p = 0.93). In a multiple regression model (R(2) = 0.71, p < 0.0001), both CF (p = 0.0006) and LCAC (p = 0.0009) were independent statistical predictors of M/Delta I. Leptin levels correlated inversely with M/Delta I (R(2) = 0.60, p = 0.0002) and positively with central (R(2) = 0.41, p = 0.006) and total body fat (R(2) = 0.63, p = 0.0001).

DISCUSSION

The mechanisms by which altered lipid metabolism in skeletal muscle influences insulin action may not be related directly to those linking central fat and insulin sensitivity. In particular, it is unlikely that muscle accumulation of lipids directly derived from labile central fat depots is a principal contributor to peripheral insulin resistance. Instead, our results imply that circulating factors, other than nonesterified fatty acids or triglyceride, mediate between central fat depots and skeletal muscle tissue. Leptin was not exclusively associated with central fat, but other factors, secreted specifically from central fat cells, could modulate muscle insulin sensitivity.

Leptin stimulates glucose uptake in C2C12 muscle cells by activation of ERK2

Leptin regulates energy homeostasis via binding to receptors in the hypothalamus and peripheral tissues. We have investigated the signaling pathways and effects of leptin on glucose transport in C2C12 muscle cells. Long and short forms of leptin receptor are expressed in differentiated C2C12 myotubes. Leptin enhanced the DNA-binding activity of the transcription factor STAT3 and extracellular signal-regulated kinase 2 (ERK2) activity was stimulated by leptin after 15 min. Leptin increased glucose uptake and GLUT4 recruitment to the cell surface after 30 min, whereas no changes in GLUT1 was observed. PD98059, an ERK2 kinase-1 inhibitor, and wortmannin, an inhibitor of phosphatidylinositol 3-kinase blocked the leptin-induced increase in glucose uptake and GLUT4 recruitment to the cell surface. In contrast, insulin-stimulated glucose transport and GLUT4 translocation was inhibited by wortmannin, but not by PD98059. Our results suggest that leptin may regulate glucose metabolism by acting directly on skeletal muscle and that the signaling pathways involved may be different from that activated by insulin.

Leptin has acute effects on glucose and lipid metabolism in both lean and gold thioglucose-obese mice

Leptin is reported to have effects in peripheral tissues that are independent of its central effects on food intake and body weight. In this study, the acute effects of a single dose of recombinant mouse leptin on lipid and glucose metabolism in lean and gold thioglucose-injected obese mice were examined. Changes were measured 2 h after leptin injection. In lean mice, liver and white adipose tissue (WAT) lipogenesis was inhibited. The activity of the pyruvate dehydrogenase complex (PDHCa), the rate-determining step for glucose oxidation, was reduced in heart, liver, quadriceps muscle, and both brown and white adipose tissues. Muscle and liver glycogen and liver triglyceride (TG) content was unchanged, but muscle TG was decreased. In obese mice, liver and WAT lipogenesis was inhibited and PDHCa reduced in heart and quadriceps muscle. Muscle and liver glycogen was decreased but not TG. Serum insulin was reduced in obese but not lean mice. These results are consistent with a role for leptin in the maintenance of steady-state energy stores by decreasing lipid synthesis and increasing fat mobilization, with decreased glucose oxidation occurring as a result of increased fatty acid oxidation.

Sparing effect of leptin on liver glycogen stores in rats during the fed-to-fasted transition

The effect of moderate hyperleptinemia ( approximately 20 ng/ml) on liver and skeletal muscle glycogen metabolism was examined in Wistar rats. Animals were studied approximately 90 h after receiving recombinant adenoviruses encoding rat leptin (AdCMV-leptin) or beta-galactosidase (AdCMV-betaGal). Liver and skeletal muscle glycogen levels in the fed and fasted (18 h) states were similar in AdCMV-leptin- and AdCMV-betaGal-treated rats. However, after delivery of a glucose bolus, liver glycogen levels were significantly greater in AdCMV-leptin compared with AdCMV-betaGal rats (P < 0.05). To investigate the mechanism(s) of these differences, glycogen levels were measured immediately after the cessation of a 3- or 6-h glucose infusion or 3, 6, and 9 h after the cessation of a 6-h glucose infusion. Similar increases in liver and skeletal muscle glycogen occurred in hyperleptinemic and control rats in response to glucose infusions. However, 3 and 6 h after the cessation of a glucose infusion, liver glycogen levels were approximately twofold greater (P < 0.05) in AdCMV-leptin-treated compared with AdCMV-betaGal-treated animals. Skeletal muscle glycogen levels were similar in AdCMV-leptin-treated and AdCMV-betaGal-treated animals at the same time points. Glycogen phosphorylase, phosphodiesterase 3B, and glycogen synthase activities were unaltered by hyperleptinemia. We conclude that moderate increases in plasma leptin levels decrease liver glycogen degradation during the fed-to-fasted transition.

Obesity and diabetes

Obesity, particularly truncal obesity, is closely correlated to the prevalence of diabetes and cardiovascular disease. Plasma leptin, tumour necrosis factor-alpha and non-esterified fatty acid levels are all elevated in obesity and play a role in causing insulin resistance. Diabetic glycaemic control and insulin resistance improve with reductions in obesity, but the treatment of obesity is difficult, and sustained weight reduction rarely occurs with dietary management alone. Hypocaloric diets should be combined with education and low-impact exercise, as well as behavioural techniques used to encourage long-term changes. Weight-reducing drugs have a role in the management of obesity but only as part of such a total package. Newer anti-obesity drugs such as orlistat and sibutramine are well tolerated and have been shown to improve glycaemic control in diabetes. It is probable that drugs developed in the future will act at different sites in the pathways regulating body weight, but they may have to be used in combination.