Interleukin-15 increases glucose uptake in skeletal muscle An antidiabetogenic effect of the cytokine

Interleukin-15 treatment improves glucose homeostasis and insulin sensitivity in obese mice

The prevalence of metabolic diseases associated with obesity, such as type 2 diabetes, continues to rise along with obesity rates. Recently, obesity has been described as an inflammatory condition, suggesting a link between the dysregulation in proinflammatory cytokine production and the aetiology of these metabolic diseases. While known as an immunomodulatory cytokine, Interleukin-15 (IL-15) has been shown to have effects on adipose tissue and induce weight loss in diet-induced obese mice. As weight loss improves glucose homeostasis, the goal of this study was to determine whether IL-15 impacts glucose regulation in a mouse model of diet-induced obesity. Our data demonstrate that IL-15 treatment significantly improves insulin sensitivity and glucose and insulin responses to an oral glucose challenge compared to obese counterparts and/or lean controls. These results show that IL-15 may be a novel therapeutic target for the treatment of obesity and its associated abnormal glucose regulation.

Overexpression of interleukin-15 in mice promotes resistance to diet-induced obesity, increased insulin sensitivity, and markers of oxidative skeletal muscle metabolism

Interleukin-15 (IL-15) is a cytokine that is highly expressed in skeletal muscle. In addition to its well-characterized effects on innate immunity, IL-15 has been proposed to modulate skeletal muscle and adipose tissue mass, as well as insulin sensitivity. In the present study, an IL-15 gain-of-function model, transgenic mice with skeletal muscle-specific oversecretion of IL-15 (IL-15 Tg mice), was utilized to test the hypotheses that IL-15 promotes insulin sensitivity and resistance to diet-induced obesity (DIO) by increasing circulating adiponectin levels, and that IL-15 regulates skeletal muscle metabolism without inducing overt muscle hypertrophy. Compared to closely related control mice, IL-15 Tg mice exhibited lower total body fat following high-fat feeding, lower intra-abdominal fat following both low- and high-fat feeding, and greater insulin sensitivity. However, this was not accompanied by increased total or high molecular weight serum adiponectin levels in IL-15 Tg mice. While overall lean body mass did not differ, IL-15 Tg mice exhibited increased mass of the oxidative soleus muscle, and increased expression of mRNA encoding the slow isoform of troponin I (TnnI 1) in the predominately glycolytic extensor digitorum longus muscle. Skeletal muscle tissue from IL-15 Tg mice also exhibited alterations in the expression of several genes associated with fatty acid metabolism, such as SIRT1, SIRT4, and uncoupling protein 2 (UCP2). These findings suggest changes in oxidative metabolism, rather than induction of adiponectin expression, appear to be responsible for the DIO-resistant and more insulin-sensitive phenotype of IL-15 Tg mice.

Distinct effects of oleic acid and itstrans-isomer elaidic acid on the expression of myokines and adipokines in cell models

Trans-fatty acids (TFA) andcis-monounsaturated fat appear to exert detrimental and beneficial effects, respectively, on glucose metabolism and insulin sensitivity. Adipose tissue and skeletal muscle are a source of signalling proteins (adipokines and myokines), some of which have been related to the control of insulin sensitivity. Here, we investigated the possible differential effects of elaidic acid (EA;trans-9-18 : 1) – the major component in industrially produced TFA – and oleic acid (OA;cis-9-18 : 1) – itscis-isomer naturally present in food – on cellular glucose uptake and the expression of selected myokines and adipokines using cell models. Differentiated C2C12 myotubes and 3T3-L1 adipocytes were pretreated with the vehicle (control cells) or fatty acids for 24 h, after which basal and insulin-stimulated 2-deoxyglucose uptake and the expression of selected signalling proteins were measured. In C2C12 myotubes, pretreatment with OA, but not with EA, led to increased insulin-stimulated 2-deoxyglucose uptake and IL-6 expression levels, while pretreatment with EA, but not with OA, led to reduced IL-15 mRNA levels and increased TNF-α expression levels. In 3T3-L1 adipocytes, exposure to OA, but not to EA, resulted in reduced resistin gene expression and increased adiponectin gene expression. The results show evidence of distinct, direct effects of OA and EA on muscle glucose uptake and the expression of target myokines and adipokines, thus suggesting novel mechanisms by whichcis- andtrans-monounsaturated fat may differentially affect systemic functions.

Identification of secreted proteins associated with obesity and type 2 diabetes in Psammomys obesus

OBJECTIVE

Skeletal muscle produces a variety of secreted proteins that have important roles in intercellular communication and affects processes such as glucose homoeostasis. The objective of this study was to develop a novel Signal Sequence Trap (SST) in conjunction with cDNA microarray technology to identify proteins secreted from skeletal muscle of Psammomys obesus that were associated with obesity and type 2 diabetes (T2D).

DESIGN

Secreted proteins that were differentially expressed between lean, normal glucose tolerant (NGT), overweight and impaired glucose tolerant (IGT) and obese, T2D P. obesus were isolated using SST in conjunction with cDNA microarray technology. Subsequent gene expression was measured in tissues from P. obesus by real-time PCR (RT-PCR).

RESULTS

The SST yielded 1600 positive clones, which were screened for differential expression. A total of 91 (approximately 6%) clones were identified by microarray to be differentially expressed between NGT, IGT and T2D P. obesus. These clones were sequenced to identify 51 genes, of which only 27 were previously known to encode secreted proteins. Three candidate genes not previously associated with obesity or type 2 diabetes, sushi domain containing 2, collagen and calcium-binding EGF domains 1 and periostin (Postn), as well as one gene known to be associated, complement component 1, were shown by RT-PCR to be differentially expressed in skeletal muscle of P. obesus. Further characterization of the secreted protein Postn revealed it to be predominantly expressed in adipose tissue, with higher expression in visceral compared with subcutaneous adipose depots.

CONCLUSION

SST in conjunction with cDNA microarray technology is a powerful tool to identify differentially expressed secreted proteins involved in complex diseases such as obesity and type 2 diabetes. Furthermore, a number of candidate genes were identified, in particular, Postn, which may have a role in the development of obesity and type 2 diabetes.

Potential mechanisms of muscle mitochondrial dysfunction in aging and obesity and cellular consequences

Mitochondria play a key role in the energy metabolism in skeletal muscle. A new concept has emerged suggesting that impaired mitochondrial oxidative capacity in skeletal muscle may be the underlying defect that causes insulin resistance. According to current knowledge, the causes and the underlying molecular mechanisms at the origin of decreased mitochondrial oxidative capacity in skeletal muscle still remain to be elucidated. The present review focuses on recent data investigating these issues in the area of metabolic disorders and describes the potential causes, mechanisms and consequences of mitochondrial dysfunction in the skeletal muscle.

Oversecretion of interleukin-15 from skeletal muscle reduces adiposity

Obesity is a risk factor for development of insulin resistance, type 2 diabetes, cardiovascular disease, osteoarthritis, and some forms of cancer. Many of the adverse health consequences of excess fat deposition are caused by increased secretion of proinflammatory adipokines by adipose tissue. Reciprocal muscle-to-fat signaling factors, or myokines, are starting to be identified. Interleukin-15 (IL-15) is a cytokine that is highly expressed in muscle tissue and that, on the basis of cell culture experiments, has been proposed to act as a circulating myokine that inhibits adipose tissue deposition. To test this hypothesis in vivo, two lines of transgenic mice that overexpressed IL-15 mRNA and protein in skeletal muscle tissue were constructed. By substitution of the inefficient native IL-15 signal peptide with a more efficient signal peptide, one of the transgenic mouse lines also exhibited elevated secretion of IL-15 in the circulation. Overexpression of IL-15 in muscle tissue without secretion in the bloodstream resulted in no differences in body composition. Elevated circulating levels of IL-15 resulted in significant reductions in body fat and increased bone mineral content, without appreciably affecting lean body mass or levels of other cytokines. Elevated circulating levels of IL-15 also inhibited adiposity induced by consumption of a high-fat/high-energy diet in male, but not female, transgenic mice. Female mice with elevated serum IL-15 exhibited increased deposition of lean body mass on a low-fat/low-energy diet and a high-fat/high-energy diet. These findings indicate that muscle-derived circulating IL-15 can modulate adipose tissue deposition and support addition of IL-15 to the growing list of potential myokines that are increasingly being implicated in regulation of body composition.

Therapeutic potential of interleukin-15: a myokine involved in muscle wasting and adiposity

Since the discovery of IL-15 and its role in T-cell proliferation in 1994, different studies on the effects of the cytokine on metabolic effects have been performed. These studies have mainly been involved with the metabolic pathways involved in lipid and protein metabolism. The present review summarises the metabolic effects of IL-15 at different target tissues and the possibilities and potential for therapeutic interventions based on the cytokine's roles in obesity and wasting.

Association between interleukin-15 and obesity: interleukin-15 as a potential regulator of fat mass

OBJECTIVE

IL-15 decreases lipid deposition in preadipocytes and decreases the mass of white adipose tissue in rats, indicating that IL-15 may take part in regulating this tissue. IL-15 is expressed in human skeletal muscle and skeletal muscle may be a source of plasma IL-15 and in this way regulate adipose tissue mass.

DESIGN

The relation between skeletal muscle IL-15 mRNA expression, plasma IL-15, and adipose tissue mass was studied in 199 humans divided into four groups on the basis of obesity and type 2 diabetes. Furthermore, using a DNA electrotransfer model, we assessed the effect of IL-15 overexpression in skeletal muscle of mice.

RESULTS

In humans, multiple regression analysis showed a negative association between plasma IL-15 and total fat mass (P<0.05), trunk fat mass (P<0.01), and percent fat mass (P<0.05), independent of type 2 diabetes. Negative associations were also found between muscle IL-15 mRNA and obesity parameters. IL-15 overexpression in skeletal muscle of mice reduced trunk fat mass but not sc fat mass.

CONCLUSIONS

Our results indicate that IL-15 may be a regulator of trunk fat mass.

Important genetic checkpoints for insulin resistance in salt-sensitive (S) Dahl rats

Despite the marked advances in research on insulin resistance (IR) in humans and animal models of insulin resistance, the mechanisms underlying high salt-induced insulin resistance remain unclear. Insulin resistance is a multifactorial disease with both genetic and environmental factors (such as high salt) involved in its pathogenesis. High salt triggers insulin resistance in genetically susceptible patients and animal models of insulin resistance. One of the mechanisms by which high salt might precipitate insulin resistance is through its ability to enhance an oxidative stress-induced inflammatory response that disrupts the insulin signaling pathway. The aim of this hypothesis is to discuss two complementary approaches to find out how high salt might interact with genetic defects along the insulin signaling and inflammatory pathways to predispose to insulin resistance in a genetically susceptible model of insulin resistance. The first approach will consist of examining variations in genes involved in the insulin signaling pathway in the Dahl S rat (an animal model of insulin resistance and salt-sensitivity) and the Dahl R rat (an animal model of insulin sensitivity and salt-resistance), and the putative cellular mechanisms responsible for the development of insulin resistance. The second approach will consist of studying the over-expressed genes along the inflammatory pathway whose respective activation might be predictive of high salt-induced insulin resistance in Dahl S rats.

Variations in genes encoding the insulin receptor substrates -1 and/or -2 (IRS-1, -2) and/or genes encoding the glucose transporter (GLUTs) proteins have been found in patients with insulin resistance. To better understand the combined contribution of excessive salt and genetic defects to the etiology of the disease, it is essential to investigate the following question:

Question 1: Do variations in genes encoding the IRS -1 and -2 and/or genes encoding the GLUTs proteins predict high salt-induced insulin resistance in Dahl S rats?

A significant amount of evidence suggested that salt-induced oxidative stress might predict an inflammatory response that upregulates mediators of inflammation such as the nuclear factor- kappa B (NF-kappa B), the tumor necrosis factor-alpha (TNF-α) and the c-Jun Terminal Kinase (JNK). These inflammatory mediators disrupt the insulin signaling pathway and predispose to insulin resistance. Therefore, the following question will be thoroughly investigated:

Question 2: Do variations in genes encoding the NF-kappa B, the TNF-α and the JNK, independently or in synergy, predict an enhanced inflammatory response and subsequent insulin resistance in Dahl S rats in excessive salt environment?

Finally, to better understand the combined role of these variations on glucose metabolism, the following question will be addressed:

Question 3: What are the functional consequences of gene variations on the rate of glucose delivery, the rate of glucose transport and the rate of glucose phosphorylation in Dahl S rats?

The general hypothesis is that "high-salt diet in combination with defects in candidate genes along the insulin signaling and inflammatory pathways predicts susceptibility to high salt-induced insulin resistance in Dahl S rats".

Interleukin-15 and interleukin-15R alpha SNPs and associations with muscle, bone, and predictors of the metabolic syndrome

The aims of this study were to examine associations between two SNPs in the human IL-15 gene and three SNPs in the IL-15Ralpha gene with predictors of metabolic syndrome and phenotypes in muscle, strength, and bone at baseline and in response to resistance training (RT). Subjects were Caucasians who had not performed RT in the previous year and consisted of a strength cohort (n=748), volumetric cohort (n=722), and serum cohort (n=544). Subjects completed 12 weeks of unilateral RT of the non-dominant arm, using their dominant arm as an untrained control. ANCOVA analyses revealed gender-specific associations with: (1) IL-15 SNP (rs1589241) and cholesterol (p=0.04), LDL (p=0.02), the homeostasis model assessment (HOMA; p=0.03), and BMI (p=0.002); (2) IL-15 SNP (rs1057972) and the pre- to post-training absolute difference in 1RM strength (p=0.02), BMI (p=0.008), and fasting glucose (p=0.03); (3) IL-15Ralpha SNP (rs2296135) and baseline total bone volume (p=0.04) and the pre- to post-training absolute difference in isometric strength (p=0.01); and 4) IL-15Ralpha SNP (rs2228059) and serum triglycerides (p=0.04), baseline whole muscle volume (p=0.04), baseline cortical bone volume (p=0.04), and baseline muscle quality (p=0.04). All associations were consistent in showing a potential involvement of the IL-15 pathway with muscle and bone phenotypes and predictors of metabolic syndrome.

Abnormal glucose homeostasis in skeletal muscle-specific PGC-1alpha knockout mice reveals skeletal muscle-pancreatic beta cell crosstalk

The transcriptional coactivator PPARgamma coactivator 1alpha (PGC-1alpha) is a strong activator of mitochondrial biogenesis and oxidative metabolism. While expression of PGC-1alpha and many of its mitochondrial target genes are decreased in the skeletal muscle of patients with type 2 diabetes, no causal relationship between decreased PGC-1alpha expression and abnormal glucose metabolism has been established. To address this question, we generated skeletal muscle-specific PGC-1alpha knockout mice (MKOs), which developed significantly impaired glucose tolerance but showed normal peripheral insulin sensitivity. Surprisingly, MKOs had expanded pancreatic beta cell mass, but markedly reduced plasma insulin levels, in both fed and fasted conditions. Muscle tissue from MKOs showed increased expression of several proinflammatory genes, and these mice also had elevated levels of the circulating IL-6. We further demonstrated that IL-6 treatment of isolated mouse islets suppressed glucose-stimulated insulin secretion. These data clearly illustrate a causal role for muscle PGC-1alpha in maintenance of glucose homeostasis and highlight an unexpected cytokine-mediated crosstalk between skeletal muscle and pancreatic islets.

Skeletal muscle insulin resistance: role of inflammatory cytokines and reactive oxygen species

The cardiometabolic syndrome (CMS), with its increased risk for cardiovascular disease (CVD), nonalcoholic fatty liver disease (NAFLD), and chronic kidney disease (CKD), has become a growing worldwide health problem. Insulin resistance is a key factor for the development of the CMS and is strongly related to obesity, hyperlipidemia, hypertension, type 2 diabetes mellitus (T2DM), CKD, and NAFLD. Insulin resistance in skeletal muscle is particularly important since it is normally responsible for more than 75% of all insulin-mediated glucose disposal. However, the molecular mechanisms responsible for skeletal muscle insulin resistance remain poorly defined. Accumulating evidence indicates that low-grade chronic inflammation and oxidative stress play fundamental roles in the development of insulin resistance, and inflammatory cytokines likely contribute to the link between inflammation, oxidative stress, and skeletal muscle insulin resistance. Understanding the mechanisms by which skeletal muscle tissue develops resistance to insulin will provide attractive targets for interventions, which may ultimately curb this serious problem. This review is focused on the effects of inflammatory cytokines and oxidative stress on insulin signaling in skeletal muscle and consequent development of insulin resistance.

Interleukin-15: a muscle-derived cytokine regulating fat-to-lean body composition

An increasing body of literature links immune and inflammatory factors to modulation of growth and control of fat:lean body composition. Recent progress in understanding the control of body composition has been made through identification of inflammatory cytokines and other factors produced by adipose tissue that affect body composition, often by direct effects on skeletal muscle tissue. Adipose-derived factors such as leptin, tumor necrosis factor-alpha, resistin, and adiponectin have been shown to affect muscle metabolism, protein dynamics, or both, by direct actions. This review summarizes recent results that support the existence of a reciprocal muscle-to-fat signaling pathway involving release of the cytokine IL-15 from muscle tissue. Cell culture studies, short-term in vivo studies, and human genotype association studies all support the model that muscle-derived IL-15 can decrease fat deposition and adipocyte metabolism via a muscle-to-fat endocrine pathway. Fat:lean body composition is an important factor determining the efficiency of meat production, as well as the fat content of meat products. Modulation of the IL-15 signaling axis may be a novel mechanism to affect body composition in meat animal production.

Gene doping: a review of performance-enhancing genetics

Unethical athletes and their mentors have long arrogated scientific and medical advances to enhance athletic performance, thus gaining a dishonest competitive advantage. Building on advances in genetics, a new threat arises from athletes using gene therapy techniques in the same manner that some abused performance-enhancing drugs were used. Gene doping, as this is known, may produce spectacular physiologic alterations to dramatically enhance athletic abilities or physical appearance. Furthermore, gene doping may present pernicious problems for the regulatory agencies and investigatory laboratories that are entrusted to keep sporting events fair and ethical. Performance-enhanced genetics will likewise present unique challenges to physicians in many spheres of their practice.