Relationship between insulin sensitivity and gene expression in human skeletal muscle.

BACKGROUND: Insulin resistance (IR) in skeletal muscle is a key feature of the pre-diabetic state, hypertension, dyslipidemia, cardiovascular diseases and also predicts type 2 diabetes. However, the underlying molecular mechanisms are still poorly understood. METHODS: To explore these mechanisms, we related global skeletal muscle gene expression profiling of 38 non-diabetic men to a surrogate measure of insulin sensitivity, i.e. homeostatic model assessment of insulin resistance (HOMA-IR). RESULTS: We identified 70 genes positively and 110 genes inversely correlated with insulin sensitivity in human skeletal muscle, identifying autophagy-related genes as positively correlated with insulin sensitivity. Replication in an independent study of 9 non-diabetic men resulted in 10 overlapping genes that strongly correlated with insulin sensitivity, including SIRT2, involved in lipid metabolism, and FBXW5 that regulates mammalian target-of-rapamycin (mTOR) and autophagy. The expressions of SIRT2 and FBXW5 were also positively correlated with the expression of key genes promoting the phenotype of an insulin sensitive myocyte e.g. PPARGC1A. CONCLUSIONS: The muscle expression of 180 genes were correlated with insulin sensitivity. These data suggest that activation of genes involved in lipid metabolism, e.g. SIRT2, and genes regulating autophagy and mTOR signaling, e.g. FBXW5, are associated with increased insulin sensitivity in human skeletal muscle, reflecting a highly flexible nutrient sensing.

Increasing circulating levels of Tenascin C in response to the Wingate anaerobic test.

AIM: Tenascin C (TNC) is a large extracellular matrix glycoprotein. It is involved in development and upregulated both during tissue repair and in several pathological conditions, including cardiovascular disease. Extracellular matrix proteins play a role in promoting exercise responses, leading to adaptation, regeneration, and repair. The main goal of this study was to investigate whether a short anaerobic effort leads to increased levels of TNC in serum. METHODS: Thirty-nine healthy men performed a Wingate test followed by a muscle biopsy. Myoblasts were isolated from the muscle biopsies and differentiated to myotubes ex vivo. TNC RNA was quantified in the biopsies, myotubes and myoblasts using RNA sequencing. Blood samples were drawn before and 5 min after the Wingate test. Serum TNC levels were measured using enzyme-linked immunosorbent assay. RESULTS: After the Wingate test, serum TNC increased on average by 23% [15-33], median [interquartile range]; PWilcoxon < 0.0001. This increase is correlated with peak power output and power drop, but not with VO2max . TNC RNA expression is higher in myoblasts and myotubes compared to skeletal muscle tissue. CONCLUSION: TNC is secreted systemically as a response to the Wingate anaerobic test in healthy males. The response was positively correlated with peak power and power drop, but not with VO2max which implicates a relation to mechanical strain and/or blood flow. With higher expression in undifferentiated myoblast cells than muscle tissue, it is likely that TNC plays a role in muscle tissue remodelling in humans. Our findings open for research on how TNC contributes to exercise adaptation.

High-throughput muscle fiber typing from RNA sequencing data.

BACKGROUND: Skeletal muscle fiber type distribution has implications for human health, muscle function, and performance. This knowledge has been gathered using labor-intensive and costly methodology that limited these studies. Here, we present a method based on muscle tissue RNA sequencing data (totRNAseq) to estimate the distribution of skeletal muscle fiber types from frozen human samples, allowing for a larger number of individuals to be tested. METHODS: By using single-nuclei RNA sequencing (snRNAseq) data as a reference, cluster expression signatures were produced by averaging gene expression of cluster gene markers and then applying these to totRNAseq data and inferring muscle fiber nuclei type via linear matrix decomposition. This estimate was then compared with fiber type distribution measured by ATPase staining or myosin heavy chain protein isoform distribution of 62 muscle samples in two independent cohorts (n = 39 and 22). RESULTS: The correlation between the sequencing-based method and the other two were rATPas = 0.44 [0.13-0.67], [95% CI], and rmyosin = 0.83 [0.61-0.93], with p = 5.70 × 10-3 and 2.00 × 10-6, respectively. The deconvolution inference of fiber type composition was accurate even for very low totRNAseq sequencing depths, i.e., down to an average of ~ 10,000 paired-end reads. CONCLUSIONS: This new method ( https://github.com/OlaHanssonLab/PredictFiberType ) consequently allows for measurement of fiber type distribution of a larger number of samples using totRNAseq in a cost and labor-efficient way. It is now feasible to study the association between fiber type distribution and e.g. health outcomes in large well-powered studies.

Regulation of AMP-activated protein kinase by LKB1 and CaMKK in adipocytes.

AMP-activated protein kinase (AMPK) is a serine/threonine kinase that regulates cellular and whole body energy homeostasis. In adipose tissue, activation of AMPK has been demonstrated in response to a variety of extracellular stimuli. However, the upstream kinase that activates AMPK in adipocytes remains elusive. Previous studies have identified LKB1 as a major AMPK kinase in muscle, liver, and other tissues. In certain cell types, Ca(2+) /calmodulin-dependent protein kinase kinase ß (CaMKKß) has been shown to activate AMPK in response to increases of intracellular Ca(2+) levels. Our aim was to investigate if LKB1 and/or CaMKK function as AMPK kinases in adipocytes. We used adipose tissue and isolated adipocytes from mice in which the expression of LKB1 was reduced to 10-20% of that of wild-type (LKB1 hypomorphic mice). We show that adipocytes from LKB1 hypomorphic mice display a 40% decrease in basal AMPK activity and a decrease of AMPK activity in the presence of the AMPK activator phenformin. We also demonstrate that stimulation of 3T3L1 adipocytes with intracellular [Ca(2+) ]-raising agents results in an activation of the AMPK pathway. The inhibition of CaMKK isoforms, particularly CaMKKß, by the inhibitor STO-609 or by siRNAs, blocked Ca(2+) -, but not phenformin-, AICAR-, or forskolin-induced activation of AMPK, indicating that CaMKK activated AMPK in response to Ca(2+) . Collectively, we show that LKB1 is required to maintain normal AMPK-signaling in non-stimulated adipocytes and in the presence of phenformin. In addition, we demonstrate the existence of a Ca(2+) /CaMKK signaling pathway that can also regulate the activity of AMPK in adipocytes.

Rose hip exerts antidiabetic effects via a mechanism involving downregulation of the hepatic lipogenic program.

The aim of this study was to investigate the metabolic effects of a dietary supplement of powdered rose hip to C57BL/6J mice fed a high-fat diet (HFD). Two different study protocols were used; rose hip was fed together with HFD to lean mice for 20 wk (prevention study) and to obese mice for 10 wk (intervention study). Parameters related to obesity and glucose tolerance were monitored, and livers were examined for lipids and expression of genes and proteins related to lipid metabolism and gluconeogenesis. A supplement of rose hip was capable of both preventing and reversing the increase in body weight and body fat mass imposed by a HFD in the C57BL/6J mouse. Oral and intravenous glucose tolerance tests together with lower basal levels of insulin and glucose showed improved glucose tolerance in mice fed a supplement of rose hip compared with control mice. Hepatic lipid accumulation was reduced in mice fed rose hip compared with control, and the expression of lipogenic proteins was downregulated, whereas AMP-activated protein kinase and other proteins involved in fatty acid oxidation were unaltered. Rose hip intake lowered total plasma cholesterol as well as the low-density lipoprotein-to-high-density lipoprotein ratio via a mechanism not involving altered gene expression of sterol regulatory element-binding protein 2 or 3-hydroxymethylglutaryl-CoA reductase. Taken together, these data show that a dietary supplement of rose hip prevents the development of a diabetic state in the C57BL/6J mouse and that downregulation of the hepatic lipogenic program appears to be at least one mechanism underlying the antidiabetic effect of rose hip.

Hormone-sensitive lipase (HSL) is also a retinyl ester hydrolase: evidence from mice lacking HSL.

Here, we investigated the importance of hormone-sensitive lipase (HSL) as a retinyl ester hydrolase (REH). REH activity was measured in vitro using recombinant HSL and retinyl palmitate. The expression of retinoic acid (RA)-regulated genes and retinoid metabolites were measured in high-fat diet fed HSL-null mice using real-time quantitative PCR and triple-stage liquid chromatography/tandem mass spectrometry, respectively. Age- and gender-matched wild-type littermates were used as controls. The REH activity of rat HSL was found to be higher than that against the hitherto best known HSL substrate, i.e., diacylglycerols. REH activity in white adipose tissue (WAT) of HSL-null mice was completely blunted and accompanied by increased levels of retinyl esters and decreased levels of retinol, retinaldehyde and all-trans RA. Accordingly, genes known to be positively regulated by RA were down-regulated in HSL-null mice, including pRb and RIP140, key factors promoting differentiation into the white over the brown adipocyte lineage. Dietary RA supplementation partly restored WAT mass and the expression of RA-regulated genes in WAT of HSL-null mice. These findings demonstrate the importance of HSL as an REH of adipose tissue and suggest that HSL via this action provides RA and other retinoids for signaling events that are crucial for adipocyte differentiation and lineage commitment.

Circulating protein biomarkers predict incident hypertensive heart failure independently of N-terminal pro-B-type natriuretic peptide levels.

AIMS: Hypertension is the leading cause for the development of heart failure (HF). Here, we aimed to identify cardiomyocyte stretch-induced circulating biomarkers for predicting hypertension-associated HF. METHODS AND RESULTS: Circulating levels of 149 proteins were measured by proximity extension assay at baseline examination in 4742 individuals from the Malmö Diet and Cancer study. Protein levels were compared with stretch-activated gene expression changes in cultured neonatal rat ventricular myocytes (NRVMs) in response to 1-48 h of mechanical stretch. We also studied the association between protein levels and hypertension and HF incidence using respectively binary logistic and Cox regressions. Levels of 35 proteins were differentially expressed after Bonferroni correction in incident HF vs. control (P < 3.4E-4). Growth differentiation factor-15 (GDF-15), interleukin-6 (IL-6), IL-1 receptor type 1, and urokinase plasminogen activator surface receptor had corresponding mRNA levels up-regulated by stretch in NRVMs at all time points (P < 0.05). These four proteins were individually associated with increased risk of HF after age and sex adjustment [hazard ratio (HR) per standard deviation: 1.19 ≤ HR ≤ 1.49, P ≤ 4.90E-3]. GDF-15 and IL-6 were associated with HF independently of each other (1.22 ≤ HR ≤ 1.33, P ≤ 0.001). In subjects with hypertension, these associations remained significant after further adjustment for N-terminal pro-B-type natriuretic peptide (NT-proBNP) levels (1.23 ≤ HR ≤ 1.45, P ≤ 0.001). A higher fasting value of a GDF-15, IL-6 score aggregate was associated with increased risk of hypertensive HF after adjustment for all traditional risk factors for HF and NT-proBNP (HR = 1.31, P = 2.19E-4). CONCLUSIONS: Cardiomyocyte mRNA levels of GDF-15 and IL-6 are consistently up-regulated by stretch, and their circulating protein levels predict HF in hypertensive subjects independently of NT-proBNP during long-term follow-up. Our results encourage further studies on lower blood pressure goals in hypertensive subjects with high GDF-15 and IL-6, and interventions targeted at stretch-induced cardiomyocyte expressed biomarkers.

N1-methylnicotinamide is a signalling molecule produced in skeletal muscle coordinating energy metabolism.

Obesity is a major health problem, and although caloric restriction and exercise are successful strategies to lose adipose tissue in obese individuals, a simultaneous decrease in skeletal muscle mass, negatively effects metabolism and muscle function. To deeper understand molecular events occurring in muscle during weight-loss, we measured the expressional change in human skeletal muscle following a combination of severe caloric restriction and exercise over 4 days in 15 Swedish men. Key metabolic genes were regulated after the intervention, indicating a shift from carbohydrate to fat metabolism. Nicotinamide N-methyltransferase (NNMT) was the most consistently upregulated gene following the energy-deficit exercise. Circulating levels of N1-methylnicotinamide (MNA), the product of NNMT activity, were doubled after the intervention. The fasting-fed state was an important determinant of plasma MNA levels, peaking at ~18 h of fasting and being lowest ~3 h after a meal. In culture, MNA was secreted by isolated human myotubes and stimulated lipolysis directly, with no effect on glucagon or insulin secretion. We propose that MNA is a novel myokine that enhances the utilization of energy stores in response to low muscle energy availability. Future research should focus on applying MNA as a biomarker to identify individuals with metabolic disturbances at an early stage.

No Additional Benefits of Block- Over Evenly-Distributed High-Intensity Interval Training within a Polarized Microcycle.

Introduction: The current study aimed to investigate the responses to block- versus evenly-distributed high-intensity interval training (HIT) within a polarized microcycle. Methods: Twenty well-trained junior cross-country skiers (10 males, age 17.6 ± 1.5 and 10 females, age 17.3 ± 1.5) completed two, 3-week periods of training (EVEN and BLOCK) in a randomized, crossover-design study. In EVEN, 3 HIT sessions (5 × 4-min of diagonal-stride roller-skiing) were completed at a maximal sustainable intensity each week while low-intensity training (LIT) was distributed evenly around the HIT. In BLOCK, the same 9 HIT sessions were completed in the second week while only LIT was completed in the first and third weeks. Heart rate (HR), session ratings of perceived exertion (sRPE), and perceived recovery (pREC) were recorded for all HIT and LIT sessions, while distance covered was recorded for each HIT interval. The recovery-stress questionnaire for athletes (RESTQ-Sport) was completed weekly. Before and after EVEN and BLOCK, resting saliva and muscle samples were collected and an incremental test and 600-m time-trial (TT) were completed. Results: Pre- to post-testing revealed no significant differences between EVEN and BLOCK for changes in resting salivary cortisol, testosterone, or IgA, or for changes in muscle capillary density, fiber area, fiber composition, enzyme activity (CS, HAD, and PFK) or the protein content of VEGF or PGC-1α. Neither were any differences observed in the changes in skiing economy, [Formula: see text] or 600-m time-trial performance between interventions. These findings were coupled with no significant differences between EVEN and BLOCK for distance covered during HIT, summated HR zone scores, total sRPE training load, overall pREC or overall recovery-stress state. However, 600-m TT performance improved from pre- to post-training, irrespective of intervention (P = 0.003), and a number of hormonal and muscle biopsy markers were also significantly altered post-training (P < 0.05). Discussion: The current study shows that well-trained junior cross-country skiers are able to complete 9 HIT sessions within 1 week without compromising total work done and without experiencing greater stress or reduced recovery over a 3-week polarized microcycle. However, the findings do not support block-distributed HIT as a superior method to a more even distribution of HIT in terms of enhancing physiological or performance adaptions.

A novel atlas of gene expression in human skeletal muscle reveals molecular changes associated with aging.

BACKGROUND: Although high-throughput studies of gene expression have generated large amounts of data, most of which is freely available in public archives, the use of this valuable resource is limited by computational complications and non-homogenous annotation. To address these issues, we have performed a complete re-annotation of public microarray data from human skeletal muscle biopsies and constructed a muscle expression compendium consisting of nearly 3000 samples. The created muscle compendium is a publicly available resource including all curated annotation. Using this data set, we aimed to elucidate the molecular mechanism of muscle aging and to describe how physical exercise may alleviate negative physiological effects. RESULTS: We find 957 genes to be significantly associated with aging (p < 0.05, FDR = 5 %, n = 361). Aging was associated with perturbation of many central metabolic pathways like mitochondrial function including reduced expression of genes in the ATP synthase, NADH dehydrogenase, cytochrome C reductase and oxidase complexes, as well as in glucose and pyruvate processing. Among the genes with the strongest association with aging were H3 histone, family 3B (H3F3B, p = 3.4 × 10(-13)), AHNAK nucleoprotein, desmoyokin (AHNAK, p = 6.9 × 10(-12)), and histone deacetylase 4 (HDAC4, p = 4.0 × 10(-9)). We also discover genes previously not linked to muscle aging and metabolism, such as fasciculation and elongation protein zeta 2 (FEZ2, p = 2.8 × 10(-8)). Out of the 957 genes associated with aging, 21 (p < 0.001, false discovery rate = 5 %, n = 116) were also associated with maximal oxygen consumption (VO2MAX). Strikingly, 20 out of those 21 genes are regulated in opposite direction when comparing increasing age with increasing VO2MAX. CONCLUSIONS: These results support that mitochondrial dysfunction is a major age-related factor and also highlight the beneficial effects of maintaining a high physical capacity for prevention of age-related sarcopenia.

Smoking induces overexpression of immediate early genes in active Graves' ophthalmopathy.

BACKGROUND: Cigarette smoking is a risk factor for the development of Graves' ophthalmopathy (GO). In a previous study of gene expression in intraorbital fat, adipocyte-related immediate early genes (IEGs) were overexpressed in patients with GO compared to controls. We investigated whether IEGs are upregulated by smoking, and examined other pathways that may be affected by smoking. METHODS: Gene expression in intraorbital fat was studied in smokers (n=8) and nonsmokers (n=8) with severe active GO, as well as in subcutaneous fat in thyroid-healthy smokers (n=5) and nonsmokers (n=5) using microarray and real-time polymerase chain reaction (PCR). RESULTS: With microarray, eight IEGs were upregulated more than 1.5-fold in smokers compared to nonsmokers with GO. Five were chosen for confirmation and were also overexpressed with real-time PCR. Interleukin-1 beta/IL-1B/(2.3-fold) and interleukin-6/IL-6/(2.4-fold) were upregulated both with microarray and with real-time PCR in smokers with GO compared to nonsmokers. Major histocompatibility complex, class II, DR beta 1/HLA-DRB1/was upregulated with microarray (2.1-fold) and with borderline significance with real-time PCR. None of these genes were upregulated in smokers compared to nonsmokers in subcutaneous fat. CONCLUSIONS: IEGs, IL-1B, and IL-6 were overexpressed in smokers with severe active GO compared to nonsmokers, suggesting that smoking activates pathways associated with adipogenesis and inflammation. This study underlines the importance of IEGs in the pathogenesis of GO, and provides evidence for possible novel therapeutic interventions in GO. The mechanisms activated by smoking may be shared with other conditions such as rheumatoid arthritis.

Expression of phosphofructokinase in skeletal muscle is influenced by genetic variation and associated with insulin sensitivity.

Using an integrative approach in which genetic variation, gene expression, and clinical phenotypes are assessed in relevant tissues may help functionally characterize the contribution of genetics to disease susceptibility. We sought to identify genetic variation influencing skeletal muscle gene expression (expression quantitative trait loci [eQTLs]) as well as expression associated with measures of insulin sensitivity. We investigated associations of 3,799,401 genetic variants in expression of >7,000 genes from three cohorts (n = 104). We identified 287 genes with cis-acting eQTLs (false discovery rate [FDR] <5%; P < 1.96 × 10(-5)) and 49 expression-insulin sensitivity phenotype associations (i.e., fasting insulin, homeostasis model assessment-insulin resistance, and BMI) (FDR <5%; P = 1.34 × 10(-4)). One of these associations, fasting insulin/phosphofructokinase (PFKM), overlaps with an eQTL. Furthermore, the expression of PFKM, a rate-limiting enzyme in glycolysis, was nominally associated with glucose uptake in skeletal muscle (P = 0.026; n = 42) and overexpressed (Bonferroni-corrected P = 0.03) in skeletal muscle of patients with T2D (n = 102) compared with normoglycemic controls (n = 87). The PFKM eQTL (rs4547172; P = 7.69 × 10(-6)) was nominally associated with glucose uptake, glucose oxidation rate, intramuscular triglyceride content, and metabolic flexibility (P = 0.016-0.048; n = 178). We explored eQTL results using published data from genome-wide association studies (DIAGRAM and MAGIC), and a proxy for the PFKM eQTL (rs11168327; r(2) = 0.75) was nominally associated with T2D (DIAGRAM P = 2.7 × 10(-3)). Taken together, our analysis highlights PFKM as a potential regulator of skeletal muscle insulin sensitivity.

Green tea powder and Lactobacillus plantarum affect gut microbiota, lipid metabolism and inflammation in high-fat fed C57BL/6J mice.

BACKGROUND: Type 2 diabetes is associated with obesity, ectopic lipid accumulation and low-grade inflammation. A dysfunctional gut microbiota has been suggested to participate in the pathogenesis of the disease. Green tea is rich in polyphenols and has previously been shown to exert beneficial metabolic effects. Lactobacillus plantarum has the ability to metabolize phenolic acids. The health promoting effect of whole green tea powder as a prebiotic compound has not been thoroughly investigated previously. METHODS: C57BL/6J mice were fed a high-fat diet with or without a supplement of 4% green tea powder (GT), and offered drinking water supplemented with Lactobacillus plantarum DSM 15313 (Lp) or the combination of both (Lp + GT) for 22 weeks. Parameters related to obesity, glucose tolerance, lipid metabolism, hepatic steatosis and inflammation were examined. Small intestinal tissue and caecal content were collected for bacterial analysis. RESULTS: Mice in the Lp + GT group had significantly more Lactobacillus and higher diversity of bacteria in the intestine compared to both mice in the control and the GT group. Green tea strongly reduced the body fat content and hepatic triacylglycerol and cholesterol accumulation. The reduction was negatively correlated to the amount of Akkermansia and/or the total amount of bacteria in the small intestine. Markers of inflammation were reduced in the Lp + GT group compared to control. PLS analysis of correlations between the microbiota and the metabolic variables of the individual mice showed that relatively few components of the microbiota had high impact on the correlation model. CONCLUSIONS: Green tea powder in combination with a single strain of Lactobacillus plantarum was able to promote growth of Lactobacillus in the intestine and to attenuate high fat diet-induced inflammation. In addition, a component of the microbiota, Akkermansia, correlated negatively with several metabolic parameters known to be risk factors for the development of type 2 diabetes.

Altered desaturation and elongation of fatty acids in hormone-sensitive lipase null mice.

BACKGROUND: Hormone-sensitive lipase (HSL) is expressed predominantly in adipose tissue, where it plays an important role in catecholamine-stimulated hydrolysis of stored lipids, thus mobilizing fatty acids. HSL exhibits broad substrate specificity and besides acylglycerides it hydrolyzes cholesteryl esters, retinyl esters and lipoidal esters. Despite its role in fatty acid mobilization, HSL null mice have been shown to be resistant to diet-induced obesity. The aim of this study was to define lipid profiles in plasma, white adipose tissue (WAT) and liver of HSL null mice, in order to better understand the role of this multifunctional enzyme. METHODOLOGY/PRINCIPAL FINDINGS: This study used global and targeted lipidomics and expression profiling to reveal changed lipid profiles in WAT, liver and plasma as well as altered expression of desaturases and elongases in WAT and liver of HSL null mice on high fat diet. Decreased mRNA levels of stearoyl-CoA desaturase 1 and 2 in WAT were consistent with a lowered ratio of 16:1n7/16:0 and 18:1n9/18:0 in WAT and plasma. In WAT, increased ratio of 18:0/16:0 could be linked to elevated mRNA levels of the Elovl1 elongase. CONCLUSIONS: This study illustrates the importance of HSL for normal lipid metabolism in response to a high fat diet. HSL deficiency greatly influences the expression of elongases and desaturases, resulting in altered lipid profiles in WAT, liver and plasma. Finally, altered proportions of palmitoleate, a recently-suggested lipokine, in tissue and plasma of HSL null mice, could be an important factor mediating and contributing to the changed lipid profile, and possibly also to the decreased insulin sensitivity seen in HSL null mice.

Metabolic effects of whole grain wheat and whole grain rye in the C57BL/6J mouse.

OBJECTIVE: A diet rich in whole grain cereals is suggested to protect against type 2 diabetes and facilitate body weight regulation. However, little is known about the impact of different cereals and the underlying mechanisms. The objective of this study was to compare the long-term metabolic effects of diets supplemented with whole grain wheat or whole grain rye in the C57BL/6J mouse. METHODS: Mice were fed the whole grain supplements in a low-fat background diet for 22 wk. Oral and intravenous glucose tolerance tests were performed during the study and in vitro insulin secretion assays were performed at the end of the study. Body weight, energy intake, body fat content, and plasma parameters were measured during the study. RESULTS: A dietary supplement of whole grain rye suppressed body weight gain and resulted in significantly decreased adiposity, plasma leptin, total plasma cholesterol, and triacylglycerols compared with a supplement of whole grain wheat. Also, a slight improvement in insulin sensitivity was observed in the rye group compared with the wheat group. The decreases in body weight and adiposity were observed in the absence of differences in energy intake. CONCLUSION: Long-term administration of whole grain rye evokes a different metabolic profile compared with whole grain wheat in the C57BL/6J mouse, the primary difference being that whole grain rye reduces body weight and adiposity compared with whole grain wheat. In addition, whole grain rye slightly improves insulin sensitivity and lowers total plasma cholesterol.

Perilipin is present in islets of Langerhans and protects against lipotoxicity when overexpressed in the beta-cell line INS-1.

Lipids have been shown to play a dual role in pancreatic beta-cells: a lipid-derived signal appears to be necessary for glucose-stimulated insulin secretion, whereas lipid accumulation causes impaired insulin secretion and apoptosis. The ability of the protein perilipin to regulate lipolysis prompted an investigation of the presence of perilipin in the islets of Langerhans. In this study evidence is presented for perilipin expression in rat, mouse, and human islets of Langerhans as well as the rat clonal beta-cell line INS-1. In rat and mouse islets, perilipin was verified to be present in beta-cells. To examine whether the development of lipotoxicity could be prevented by manipulating the conditions for lipid storage in the beta-cell, INS-1 cells with adenoviral-mediated overexpression of perilipin were exposed to lipotoxic conditions for 72 h. In cells exposed to palmitate, perilipin overexpression caused increased accumulation of triacylglycerols and decreased lipolysis compared with control cells. Whereas glucose-stimulated insulin secretion was retained after palmitate exposure in cells overexpressing perilipin, it was completely abolished in control beta-cells. Thus, overexpression of perilipin appears to confer protection against the development of beta-cell dysfunction after prolonged exposure to palmitate by promoting lipid storage and limiting lipolysis.

Attainment of brown adipocyte features in white adipocytes of hormone-sensitive lipase null mice.

BACKGROUND: Hormone-sensitive lipase (HSL) is expressed predominantly in adipose tissue, where it plays an important role in catecholamine-stimulated hydrolysis of stored tri- and diglycerides, thus mobilizing fatty acids. HSL exhibits broad substrate specificity and besides acylglycerides it hydrolyzes cholesteryl esters, retinyl esters and lipoidal esters. Despite its role in fatty acid mobilization, HSL null mice have been shown to be resistant to diet-induced obesity. METHODOLOGY/PRINCIPAL FINDINGS: Following a high-fat diet (HFD) regimen, energy expenditure, measured using indirect calorimetry, was increased in HSL null mice. White adipose tissue of HSL null mice was characterized by reduced mass and reduced protein expression of PPARgamma, a key transcription factor in adipogenesis, and stearoyl-CoA desaturase 1, the expression of which is known to be positively correlated to the differentiation state of the adipocyte. The protein expression of uncoupling protein-1 (UCP-1), the highly specific marker of brown adipocytes, was increased 7-fold in white adipose tissue of HSL null mice compared to wildtype littermates. Transmission electron microscopy revealed an increase in the size of mitochondria of white adipocytes of HSL null mice. The mRNA expression of pRb and RIP140 was decreased in isolated white adipocytes, while the expression of UCP-1 and CPT1 was increased in HSL null mice compared to wildtype littermates. Basal oxygen consumption was increased almost 3-fold in white adipose tissue of HSL null mice and was accompanied by increased uncoupling activity. CONCLUSIONS: These data suggest that HSL is involved in the determination of white versus brown adipocytes during adipocyte differentiation The exact mechanism(s) underlying this novel role of HSL remains to be elucidated, but it seems clear that HSL is required to sustain normal expression levels of pRb and RIP140, which both promote differentiation into the white, rather than the brown, adipocyte lineage.

Glucose-stimulated insulin secretion correlates with beta-cell lipolysis.

BACKGROUND AND AIMS: Lipids are needed for optimal glucose-stimulated insulin secretion (GSIS), and long-chain acyl-CoA (LC-CoA) has been suggested as one candidate molecule active as a lipidic coupling factor. LC-CoAs may be available to the beta-cell via uptake of circulating free fatty acids or from hydrolysis of intracellularly stored triglycerides. Inhibition of lipolysis in rat islets using a non-specific lipase inhibitor (orlistat) resulted in blunted GSIS. The aim of this study was to investigate the relationship between GSIS and lipolysis in clonal beta-cells and in mouse islets. METHODS AND RESULTS: INS-1 cells, cultured overnight at 3.3 mM or 11.1 mM glucose, or freshly isolated islets were incubated with 3.3 mM or 16.7 mM glucose for 1 h. Medium samples were collected and analyzed for insulin and glycerol. Triglycerides were measured in both INS-1 cells and islets. There was a dose-dependent glucose-stimulated lipolysis in INS-1 cells, which strongly correlated with insulin secretion (r=0.85, P<0.0001). The same phenomenon was observed in mouse islets (r=0.9, P=0.013). Low levels of triglycerides, which were observed in INS-1 cells pre-cultured at 3.3 mM glucose, were associated with reduced GSIS. CONCLUSIONS: This study suggests that lipids obtained from lipolysis of intracellular triglycerides are involved in GSIS.

Inflammatory response in white adipose tissue in the non-obese hormone-sensitive lipase null mouse model.

In the present study, a local inflammatory response in white adipose tissue from the nonobese HSL-null mouse model is demonstrated. The protein levels of several well-known markers of inflammation, like TNFalpha and ferritin HC, were highly increased and accompanied by an activation of NFkappaB. A number of macrophage proteins, i.e., gal-3, Capg, and MCP-4, were expressed at increased levels and immunohistochemical analyses revealed an increased infiltration of F4/80+ cells.