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Obesity is a complex, multifactorial, chronic disease that acts as a gateway to a range of other diseases. Evidence from recent studies suggests that changes in oxygen availability in the microenvironment of metabolic organs may exert an important role in the development of obesity-related cardiometabolic complications. In this review, we will first discuss results from observational and controlled laboratory studies that examined the relationship between reduced oxygen availability and obesity-related metabolic derangements. Next, the effects of alterations in oxygen partial pressure (pO 2 ) in the adipose tissue, skeletal muscle and the liver microenvironment on physiological processes in these key metabolic organs will be addressed, and how this might relate to cardiometabolic complications. Since many obesity-related chronic diseases, including type 2 diabetes mellitus, cardiovascular diseases, chronic kidney disease, chronic obstructive pulmonary disease and obstructive sleep apnea, are characterized by changes in pO 2 in the tissue microenvironment, a better understanding of the metabolic impact of altered tissue oxygenation can provide valuable insights into the complex interplay between environmental and biological factors involved in the pathophysiology of metabolic impairments. This may ultimately contribute to the development of novel strategies to prevent and treat obesity-related cardiometabolic diseases., (© 2024. The Author(s).)

Chrono-nutrition (meal timing) aligns food consumption with one's circadian rhythm. The first meal (e.g., breakfast) likely promotes synchronization of peripheral circadian clocks, thereby supporting metabolic health. Time-restricted feeding (TRF) has been shown to reduce body weight (BW) and/or improve cardiovascular biomarkers. In this explorative literature assessment, 13 TRF randomized controlled trials (RCTs) were selected from PubMed and Scopus to evaluate the effects of early (eTRF: first meal before 10:30 a.m.) and late TRF (lTRF: first meal after 11:30 a.m.) on parameters of metabolic health. Although distinct variations in study design were evident between reports, TRF consistently decreased energy intake (EI) and BW, and improved insulin resistance as well as systolic blood pressure. eTRF seemed to have a greater beneficial effect than lTRF on insulin resistance (HOMA-IR). Importantly, most studies did not appear to consider chronotype in their evaluation, which may have underestimated TRF effects. TRF intervention may be a promising approach for risk reduction of human metabolic diseases. To conclusively determine benefits of TRF and identify clear differences between eTRF and lTRF, future studies should be longer-term (≥8 weeks) with well-defined (differences in) feeding windows, include participants chronotypically matching the intervention, and compare outcomes to those of control groups without any dietary limitations.

Objective: Fetuin B is a steatosis-responsive hepatokine that causes glucose intolerance in mice, but the underlying mechanisms remain incompletely described. This study aimed to elucidate the mechanisms of action of fetuin B by investigating its putative effects on white adipose tissue metabolism., Methods: First, fetuin B gene and protein expression was measured in multiple organs in mice and in cultured adipocytes. Next, the authors performed a hyperinsulinemic-euglycemic clamp in mice and in humans to examine the link between white adipose tissue fetuin B content and indices of insulin sensitivity. Finally, the effect of fetuin B on inflammation was investigated in cultured adipocytes by quantitative polymerase chain reaction and full RNA sequencing., Results: This study demonstrated in adipocytes and mice that fetuin B was produced and secreted by the liver and taken up by adipocytes and adipose tissue. There was a strong negative correlation between white adipose tissue fetuin B content and peripheral insulin sensitivity in mice and in humans. RNA sequencing and polymerase chain reaction analysis revealed that fetuin B induced an inflammatory response in adipocytes., Conclusions: Fetuin B content in white adipose tissue strongly associated with peripheral insulin resistance in mice and humans. Furthermore, fetuin B induced a proinflammatory response in adipocytes, which might drive peripheral insulin resistance., (© 2023 The Authors. Obesity published by Wiley Periodicals LLC on behalf of The Obesity Society.)

Dietary interventions to delay carbohydrate digestion or absorption can effectively prevent hyperglycaemia in the early postprandial phase. L-arabinose can specifically inhibit sucrase. It remains to be assessed whether co-ingestion of L-arabinose with sucrose delays sucrose digestion, attenuates subsequent glucose absorption and impacts hepatic glucose output. In this double-blind, randomised crossover study, we assessed blood glucose kinetics following ingestion of a 200-ml drink containing 50 g of sucrose with 7·5 g of L-arabinose (L-ARA) or without L-arabinose (CONT) in twelve young, healthy participants (24 ± 1 years; BMI: 22·2 ± 0·5 kg/m 2 ). Plasma glucose kinetics were determined by a dual stable isotope methodology involving ingestion of (U- 13 C 6 )-glucose-enriched sucrose, and continuous intravenous infusion of (6,6- 2 H 2 )-glucose. Peak glucose concentrations reached 8·18 ± 0·29 mmol/l for CONT 30 min after ingestion. In contrast, the postprandial rise in plasma glucose was attenuated for L-ARA, because peak glucose concentrations reached 6·62 ± 0·18 mmol/l only 60 min after ingestion. The rate of exogenous glucose appearance for L-ARA was 67 and 57 % lower compared with CONT at t = 15 min and 30 min, respectively, whereas it was 214 % higher at t = 150 min, indicating a more stable absorption of exogenous glucose for L-ARA compared with CONT. Total glucose disappearance during the first hour was lower for L-ARA compared with CONT (11 ± 1 v . 17 ± 1 g, P < 0·0001). Endogenous glucose production was not differentially affected at any time point ( P = 0·27). Co-ingestion of L-arabinose with sucrose delays sucrose digestion, resulting in a slower absorption of sucrose-derived glucose without causing adverse effects in young, healthy adults.

Maintaining good glycemic control to prevent complications is crucial in people with type 2 diabetes and in people with prediabetes and in the general population. Different strategies to improve glycemic control involve the prescription of blood glucose-lowering drugs and the modulation of physical activity and diet. Interestingly, lifestyle intervention may be more effective in lowering hyperglycemia than pharmaceutical intervention. Regulation of postprandial glycemia is complex, but specific nutritional strategies can be applied to attenuate postprandial hyperglycemia. These strategies include reducing total carbohydrate intake, consuming carbohydrates with a lower glycemic index, the addition of or substitution by sweeteners and fibers, using food compounds which delay or inhibit gastric emptying or carbohydrate digestion, and using food compounds which inhibit intestinal glucose absorption. Nevertheless, it must be noted that every individual may respond differently to certain nutritional interventions. Therefore, a personalized approach is of importance to choose the optimal nutritional strategy to improve postprandial glycemia for each individual, but this requires a better understanding of the mechanisms explaining the differential responses between individuals., (© 2022 The Authors. Obesity Reviews published by John Wiley & Sons Ltd on behalf of World Obesity Federation.)

Background: Cachexia-associated skeletal muscle wasting or 'sarcopenia' is highly prevalent in ovarian cancer and contributes to poor outcome. Drivers of cachexia-associated sarcopenia in ovarian cancer remain elusive, underscoring the need for novel and better models to identify tumour factors inducing sarcopenia. We aimed to assess whether factors present in ascites of sarcopenic vs. non-sarcopenic ovarian cancer patients differentially affect protein metabolism in skeletal muscle cells and to determine if these effects are correlated to cachexia-related patient characteristics., Methods: Fifteen patients with an ovarian mass and ascites underwent extensive physical screening focusing on cachexia-related parameters. Based on computed tomography-based body composition imaging, six cancer patients were classified as sarcopenic and six were not; three patients with a benign condition served as an additional non-sarcopenic control group. Ascites was collected, and concentrations of cachexia-associated factors were assessed by enzyme-linked immunosorbent assay. Subsequently, ascites was used for in vitro exposure of C2C12 myotubes followed by measurements of protein synthesis and breakdown by radioactive isotope tracing, qPCR-based analysis of atrophy-related gene expression, and NF-κB activity reporter assays., Results: C2C12 protein synthesis was lower after exposure to ascites from sarcopenic patients (sarcopenia 3.1 ± 0.1 nmol/h/mg protein vs. non-sarcopenia 5.5 ± 0.2 nmol/h/mg protein, P < 0.01), and protein breakdown rates tended to be higher (sarcopenia 31.2 ± 5.2% vs. non-sarcopenia 20.9 ± 1.9%, P = 0.08). Ascites did not affect MuRF1, Atrogin-1, or REDD1 expression of C2C12 myotubes, but NF-κB activity was specifically increased in cells exposed to ascites from sarcopenic patients (sarcopenia 2.2 ± 0.4-fold compared with control vs. non-sarcopenia 1.2 ± 0.2-fold compared with control, P = 0.01). Protein synthesis and breakdown correlated with NF-κB activity (r s  = -0.60, P = 0.03 and r s  = 0.67, P = 0.01, respectively). The skeletal muscle index of the ascites donors was also correlated to both in vitro protein synthesis (r s  = 0.70, P = 0.005) and protein breakdown rates (r s  = -0.57, P = 0.04)., Conclusions: Ascites of sarcopenic ovarian cancer patients induces pronounced skeletal muscle protein metabolism changes in C2C12 cells that correlate with clinical muscle measures of the patient and that are characteristic of cachexia. The use of ascites offers a new experimental tool to study the impact of both tumour-derived and systemic factors in various cachexia model systems, enabling identification of novel drivers of tissue wasting in ovarian cancer., (© 2021 The Authors. Journal of Cachexia, Sarcopenia and Muscle published by John Wiley & Sons Ltd on behalf of Society on Sarcopenia, Cachexia and Wasting Disorders.)

Individuals with hepatic steatosis often display several metabolic abnormalities including insulin resistance and muscle atrophy. Previously, we found that hepatic steatosis results in an altered hepatokine secretion profile, thereby inducing skeletal muscle insulin resistance via inter-organ crosstalk. In this study, we aimed to investigate whether the altered secretion profile in the state of hepatic steatosis also induces skeletal muscle atrophy via effects on muscle protein turnover. To investigate this, eight-week-old male C57BL/6J mice were fed a chow (4.5% fat) or a high-fat diet (HFD; 45% fat) for 12 weeks to induce hepatic steatosis, after which the livers were excised and cut into ~200-µm slices. Slices were cultured to collect secretion products (conditioned medium; CM). Differentiated L6-GLUT4myc myotubes were incubated with chow or HFD CM to measure glucose uptake. Differentiated C2C12 myotubes were incubated with chow or HFD CM to measure protein synthesis and breakdown, and gene expression via RNA sequencing. Furthermore, proteomics analysis was performed in chow and HFD CM. It was found that HFD CM caused insulin resistance in L6-GLUT4myc myotubes compared with chow CM, as indicated by a blunted insulin-stimulated increase in glucose uptake. Furthermore, protein breakdown was increased in C2C12 cells incubated with HFD CM, while there was no effect on protein synthesis. RNA profiling of C2C12 cells indicated that 197 genes were differentially expressed after incubation with HFD CM, compared with chow CM, and pathway analysis showed that pathways related to anatomical structure and function were enriched. Proteomics analysis of the CM showed that 32 proteins were differentially expressed in HFD CM compared with chow CM. Pathway enrichment analysis indicated that these proteins had important functions with respect to insulin-like growth factor transport and uptake, and affect post-translational processes, including protein folding, protein secretion and protein phosphorylation. In conclusion, the results of this study support the hypothesis that secretion products from the liver contribute to the development of muscle atrophy in individuals with hepatic steatosis., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Pasmans, Adriaens, Olinga, Langen, Rensen, Schaap, Olde Damink, Caiment, van Loon, Blaak and Meex.)

Non-Alcoholic fatty liver disease (NAFLD) is the most common form of liver disease and is characterized by fat accumulation in the liver. Hypercaloric diets generally increase hepatic fat accumulation, whereas hypocaloric diets decrease liver fat content. In addition, there is evidence to suggest that moderate amounts of unsaturated fatty acids seems to be protective for the development of a fatty liver, while consumption of saturated fatty acids (SFA) appears to predispose toward hepatic steatosis. Recent studies highlight a key role for mitochondrial dysfunction in the development and progression of NAFLD. It is proposed that changes in mitochondrial structure and function are key mechanisms by which SFA lead to the development and progression of NAFLD. In this review, it is described how SFA intake is associated with liver steatosis and decreases the efficiency of the respiratory transport chain. This results in the production of reactive oxygen species and damage to nearby structures, eventually leading to inflammation, apoptosis, and scarring of the liver. Furthermore, studies demonstrating that SFA intake affects the composition of mitochondrial membranes are presented, and this process accelerates the progression of NAFLD. It is likely that events are intertwined and reinforce each other, leading to a constant deterioration in health., (© 2020 The Authors. Molecular Nutrition & Food Research published by Wiley-VCH GmbH.)

Intertissue communication is a fundamental feature of metabolic regulation, and the liver is central to this process. We have identified sparc-related modular calcium-binding protein 1 (SMOC1) as a glucose-responsive hepatokine and regulator of glucose homeostasis. Acute intraperitoneal administration of SMOC1 improved glycemic control and insulin sensitivity in mice without changes in insulin secretion. SMOC1 exerted its favorable glycemic effects by inhibiting adenosine 3',5'-cyclic monophosphate (cAMP)-cAMP-dependent protein kinase (PKA)-cAMP response element-binding protein (CREB) signaling in the liver, leading to decreased gluconeogenic gene expression and suppression of hepatic glucose output. Overexpression of SMOC1 in the liver or once-weekly intraperitoneal injections of a stabilized SMOC1-FC fusion protein induced durable improvements in glucose tolerance and insulin sensitivity in db / db mice, without adverse effects on adiposity, liver histopathology, or inflammation. Furthermore, circulating SMOC1 correlated with hepatic and systemic insulin sensitivity and was decreased in obese, insulin-resistant humans. Together, these findings identify SMOC1 as a potential pharmacological target for the management of glycemic control in type 2 diabetes., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Insulin resistance and muscle mass loss often coincide in individuals with type 2 diabetes. Most patients with type 2 diabetes are overweight, and it is well established that obesity and derangements in lipid metabolism play an important role in the development of insulin resistance in these individuals. Specifically, increased adipose tissue mass and dysfunctional adipose tissue lead to systemic lipid overflow and to low-grade inflammation via altered secretion of adipokines and cytokines. Furthermore, an increased flux of fatty acids from the adipose tissue may contribute to increased fat storage in the liver and in skeletal muscle, resulting in an altered secretion of hepatokines, mitochondrial dysfunction, and impaired insulin signalling in skeletal muscle. Recent studies suggest that obesity and lipid derangements in adipose tissue can also lead to the development of muscle atrophy, which would make insulin resistance and muscle atrophy two sides of the same coin. Unfortunately, the exact relationship between lipid accumulation, type 2 diabetes, and muscle atrophy remains largely unexplored. The aim of this review is to discuss the relationship between type 2 diabetes and muscle loss and to discuss some of the joint pathways through which lipid accumulation in organs may affect peripheral insulin sensitivity and muscle mass., (© 2019 The Authors. Obesity Reviews published by John Wiley & Sons Ltd on behalf of World Obesity Federation.)

Evidence is accumulating that the gut microbiome is involved in the aetiology of obesity and obesity-related complications such as nonalcoholic fatty liver disease (NAFLD), insulin resistance and type 2 diabetes mellitus (T2DM). The gut microbiota is able to ferment indigestible carbohydrates (for example, dietary fibre), thereby yielding important metabolites such as short-chain fatty acids and succinate. Numerous animal studies and a handful of human studies suggest a beneficial role of these metabolites in the prevention and treatment of obesity and its comorbidities. Interestingly, the more distal colonic microbiota primarily ferments peptides and proteins, as availability of fermentable fibre, the major energy source for the microbiota, is limited here. This proteolytic fermentation yields mainly harmful products such as ammonia, phenols and branched-chain fatty acids, which might be detrimental for host gut and metabolic health. Therefore, a switch from proteolytic to saccharolytic fermentation could be of major interest for the prevention and/or treatment of metabolic diseases. This Review focuses on the role of products derived from microbial carbohydrate and protein fermentation in relation to obesity and obesity-associated insulin resistance, T2DM and NAFLD, and discusses the mechanisms involved.

Dysregulation of lipid homeostasis is a precipitating event in the pathogenesis and progression of hepatosteatosis and metabolic syndrome. These conditions are highly prevalent in developed societies and currently have limited options for diagnostic and therapeutic intervention. Here, using a proteomic and lipidomic-wide systems genetic approach, we interrogated lipid regulatory networks in 107 genetically distinct mouse strains to reveal key insights into the control and network structure of mammalian lipid metabolism. These include the identification of plasma lipid signatures that predict pathological lipid abundance in the liver of mice and humans, defining subcellular localization and functionality of lipid-related proteins, and revealing functional protein and genetic variants that are predicted to modulate lipid abundance. Trans-omic analyses using these datasets facilitated the identification and validation of PSMD9 as a previously unknown lipid regulatory protein. Collectively, our study serves as a rich resource for probing mammalian lipid metabolism and provides opportunities for the discovery of therapeutic agents and biomarkers in the setting of hepatic lipotoxicity.

Objective: Intramyocellular lipid (IMCL) storage negatively associates with insulin resistance, albeit not in endurance-trained athletes. We investigated the putative contribution of lipid droplet (LD) morphology and subcellular localization to the so-called athlete's paradox., Methods: We performed quantitative immunofluorescent confocal imaging of muscle biopsy sections from endurance Trained, Lean sedentary, Obese, and Type 2 diabetes (T2DM) participants (n = 8/group). T2DM patients and Trained individuals were matched for IMCL content. Furthermore we performed this analysis in biopsies of T2DM patients before and after a 12-week exercise program (n = 8)., Results: We found marked differences in lipid storage morphology between trained subjects and T2DM: the latter group mainly store lipid in larger LDs in the subsarcolemmal (SS) region of type II fibers, whereas Trained store lipid in a higher number of LDs in the intramyofibrillar (IMF) region of type I fibers. In addition, a twelve-week combined endurance and strength exercise program resulted in a LD phenotype shift in T2DM patients partly towards an 'athlete-like' phenotype, accompanied by improved insulin sensitivity. Proteins involved in LD turnover were also more abundant in Trained than in T2DM and partly changed in an 'athlete-like' fashion in T2DM patients upon exercise training., Conclusions: Our findings provide a physiological explanation for the athlete's paradox and reveal LD morphology and distribution as a major determinant of skeletal muscle insulin sensitivity., (Copyright © 2018 The Authors. Published by Elsevier GmbH.. All rights reserved.)

Nonalcoholic fatty liver disease (NAFLD) comprises fat-accumulating conditions within hepatocytes that can cause severe liver damage and metabolic comorbidities. Studies suggest that mitochondrial dysfunction contributes to its development and progression and that the hepatic lipidome changes extensively in obesity and in NAFLD. To gain insight into the relationship between lipid metabolism and disease progression through different stages of NAFLD, we performed lipidomic analysis of plasma and liver biopsy samples from obese patients with nonalcoholic fatty liver (NAFL) or nonalcoholic steatohepatitis (NASH) and from those without NAFLD. Congruent with earlier studies, hepatic lipid levels overall increased with NAFLD. Lipid species that differed with NAFLD severity were related to mitochondrial dysfunction; specifically, hepatic cardiolipin and ubiquinone accumulated in NAFL, and levels of acylcarnitine increased with NASH. We propose that increased levels of cardiolipin and ubiquinone may help to preserve mitochondrial function in early NAFLD, but that mitochondrial function eventually fails with progression to NASH, leading to increased acylcarnitine. We also found a negative association between hepatic odd-chain phosphatidylcholine and NAFLD, which may result from mitochondrial dysfunction-related impairment of branched-chain amino acid catabolism. Overall, these data suggest a close link between accumulation of specific hepatic lipid species, mitochondrial dysfunction, and the progression of NAFLD., (Copyright © 2018 Peng et al.)

Hepatic steatosis is an underlying feature of nonalcoholic fatty liver disease (NAFLD), which is the most common form of liver disease and is present in up to ∼70% of individuals who are overweight. NAFLD is also associated with hypertriglyceridaemia and low levels of HDL, glucose intolerance, insulin resistance and type 2 diabetes mellitus. Hepatic steatosis is a strong predictor of the development of insulin resistance and often precedes the onset of other known mediators of insulin resistance. This sequence of events suggests that hepatic steatosis has a causal role in the development of insulin resistance in other tissues, such as skeletal muscle. Hepatokines are proteins that are secreted by hepatocytes, and many hepatokines have been linked to the induction of metabolic dysfunction, including fetuin A, fetuin B, retinol-binding protein 4 (RBP4) and selenoprotein P. In this Review, we describe the factors that influence the development of hepatic steatosis, provide evidence of strong links between hepatic steatosis and insulin resistance in non-hepatic tissues, and discuss recent advances in our understanding of how steatosis alters hepatokine secretion to influence metabolic phenotypes through inter-organ communication.

Adipocytes are major regulators of metabolism, and endurance exercise training improves adipocyte function; however, the molecular mechanisms that regulate chronic adaptive responses remain unresolved. microRNAs (miRNAs) influence adipocyte differentiation and metabolism. Accordingly, we aimed to determine whether adipocyte miRNA expression is responsive to exercise training and to identify exercise-responsive miRNAs that influence adipocyte metabolism. Next-generation sequencing was used to profile miRNA expression of adipocytes that were isolated from abdominal subcutaneous (ABD) and gluteofemoral (GF) adipose tissue of overweight men before and after 6 wk of endurance exercise training. Differentially expressed miRNAs were overexpressed or silenced in 3T3-L1 adipocytes, and lipid metabolism was examined. Next-generation sequencing identified 526 miRNAs in adipocytes, and there were no statistical differences in miRNA expression when comparing pre- and post-training samples for ABD and GF adipocytes. miR-10b expression was increased in ABD compared with GF adipocytes, whereas miR-204, miR-3613, and miR-4532 were more highly expressed in GF compared with ABD adipocytes. Blocking miR-10b in adipocytes suppressed β-adrenergic lipolysis but generally had a minor effect on lipid metabolism. Thus, unlike their critical role in adipogenesis, stable changes in miRNA expression do not play a prominent role in the regulation of adipocyte function in response to endurance exercise training.-Tsiloulis, T., Pike, J., Powell, D., Rossello, F. J., Canny, B. J., Meex, R. C. R., Watt, M. J. Impact of endurance exercise training on adipocyte microRNA expression in overweight men., (© FASEB.)

Emerging evidence indicates that skeletal muscle lipid droplets are an important control point for intracellular lipid homeostasis and that regulating fatty acid fluxes from lipid droplets might influence mitochondrial capacity. We used pharmacological blockers of the major triglyceride lipases, adipose triglyceride lipase (ATGL) and hormone-sensitive lipase, to show that a large proportion of the fatty acids that are transported into myotubes are trafficked through the intramyocellular triglyceride pool. We next tested whether increasing lipolysis from intramyocellular lipid droplets could activate transcriptional responses to enhance mitochondrial and fatty acid oxidative capacity. ATGL was overexpressed by adenoviral and adenoassociated viral infection in C2C12 myotubes and the tibialis anterior muscle of C57Bl/6 mice, respectively. ATGL overexpression in C2C12 myotubes increased lipolysis, which was associated with increased peroxisome proliferator-activated receptor (PPAR)-∂ activity, transcriptional upregulation of some PPAR∂ target genes, and enhanced mitochondrial capacity. The transcriptional responses were specific to ATGL actions and not a generalized increase in fatty acid flux in the myotubes. Marked ATGL overexpression (20-fold) induced modest molecular changes in the skeletal muscle of mice, but these effects were not sufficient to alter fatty acid oxidation. Together, these data demonstrate the importance of lipid droplets for myocellular fatty acid trafficking and the capacity to modulate mitochondrial capacity by enhancing lipid droplet lipolysis in vitro; however, this adaptive program is of minor importance when superimposing the normal metabolic stresses encountered in free-moving animals., (Copyright © 2015 the American Physiological Society.)

Lipolysis involves the sequential breakdown of fatty acids from triacylglycerol and is increased during energy stress such as exercise. Adipose triglyceride lipase (ATGL) is a key regulator of skeletal muscle lipolysis and perilipin (PLIN) 5 is postulated to be an important regulator of ATGL action of muscle lipolysis. Hence, we hypothesized that non-genomic regulation such as cellular localization and the interaction of these key proteins modulate muscle lipolysis during exercise. PLIN5, ATGL and CGI-58 were highly (>60%) colocated with Oil Red O (ORO) stained lipid droplets. PLIN5 was significantly colocated with ATGL, mitochondria and CGI-58, indicating a close association between the key lipolytic effectors in resting skeletal muscle. The colocation of the lipolytic proteins, their independent association with ORO and the PLIN5/ORO colocation were not altered after 60 min of moderate intensity exercise. Further experiments in cultured human myocytes showed that PLIN5 colocation with ORO or mitochondria is unaffected by pharmacological activation of lipolytic pathways. Together, these data suggest that the major lipolytic proteins are highly expressed at the lipid droplet and colocate in resting skeletal muscle, that their localization and interactions appear to remain unchanged during prolonged exercise, and, accordingly, that other post-translational mechanisms are likely regulators of skeletal muscle lipolysis.

Catecholamine-stimulated lipolysis occurs by activating adenylate cyclase and raising cAMP levels, thereby increasing protein kinase A (PKA) activity. This results in phosphorylation and modulated activity of several key lipolytic proteins. Adipose triglyceride lipase (ATGL) is the primary lipase for the initial step in triacylglycerol hydrolysis, and ATGL activity is increased during stimulated lipolysis. Here, we demonstrate that murine ATGL is phosphorylated by PKA at several serine residues in vitro and identify Ser(406) as a functionally important site. ATGL null adipocytes expressing ATGL S406A (nonphosphorylatable) had reduced stimulated lipolysis. Studies in mice demonstrated increased ATGL Ser(406) phosphorylation during fasting and moderate intensity exercise, conditions associated with elevated lipolytic rates. ATGL Ser(404) (corresponding to murine Ser(406)) phosphorylation was increased by β-adrenergic stimulation but not 5'AMP-activated protein kinase activation in human subcutaneous adipose tissue explants, which correlated with lipolysis rates. Our studies suggest that β-adrenergic activation can result in PKA-mediated phosphorylation of ATGL Ser(406), to moderately increase ATGL-mediated lipolysis.

Intramyocellular triacylglycerol provides fatty acid substrate for ATP generation in contracting muscle. The protein adipose triglyceride lipase (ATGL) is a key regulator of triacylglycerol lipolysis and whole body energy metabolism at rest and during exercise, and ATGL activity is reported to be enhanced by 5'-AMP-activated protein kinase (AMPK)-mediated phosphorylation at Ser(406) in mice. This is a curious observation, because AMPK activation reduces lipolysis in several cell types. We investigated whether the phosphorylation of ATGL Ser(404) (corresponding to murine Ser(406)) was increased during exercise in human skeletal muscle and with pharmacological AMPK activation in myotubes in vitro. In human experiments, skeletal muscle and venous blood samples were obtained from recreationally active male subjects before and at 5 and 60 min during exercise. ATGL Ser(404) phosphorylation was not increased from rest during exercise, but ATGL Ser(404) phosphorylation correlated with myosin heavy chain 1 expression, suggesting a possible fiber type dependency. ATGL Ser(404) phosphorylation was not related to increases in AMPK activity, and immunoprecipitation experiments indicated no interaction between AMPK and ATGL. Rather, ATGL Ser(404) phosphorylation was associated with protein kinase A (PKA) signaling. ATGL Ser(406) phosphorylation in C(2)C(12) myotubes was unaffected by 5-aminoimidazole-4-carboxaminde-1-β-d-ribofuranoside, an AMPK activator, and the PKA activator forskolin. Our results demonstrate that ATGL Ser(404) phosphorylation is not increased in mixed skeletal muscle during moderate-intensity exercise and that AMPK does not appear to be an activating kinase for ATGL Ser(404/406) in skeletal muscle.

Background: Increased cardiac lipid content has been associated with diabetic cardiomyopathy. We recently showed that cardiac lipid content is reduced after 12 weeks of physical activity training in healthy overweight subjects. The beneficial effect of exercise training on cardiovascular risk is well established and the decrease in cardiac lipid content with exercise training in healthy overweight subjects was accompanied by improved ejection fraction. It is yet unclear whether diabetic patients respond similarly to physical activity training and whether a lowered lipid content in the heart is necessary for improvements in cardiac function. Here, we investigated whether exercise training is able to lower cardiac lipid content and improve cardiac function in type 2 diabetic patients., Methods: Eleven overweight-to-obese male patients with type 2 diabetes mellitus (age: 58.4 ± 0.9 years, BMI: 29.9 ± 0.01 kg/m2) followed a 12-week training program (combination endurance/strength training, three sessions/week). Before and after training, maximal whole body oxygen uptake (VO2max) and insulin sensitivity (by hyperinsulinemic, euglycemic clamp) was determined. Systolic function was determined under resting conditions by CINE-MRI and cardiac lipid content in the septum of the heart by Proton Magnetic Resonance Spectroscopy., Results: VO2max increased (from 27.1 ± 1.5 to 30.1 ± 1.6 ml/min/kg, p = 0.001) and insulin sensitivity improved upon training (insulin stimulated glucose disposal (delta Rd of glucose) improved from 5.8 ± 1.9 to 10.3 ± 2.0 μmol/kg/min, p = 0.02. Left-ventricular ejection fraction improved after training (from 50.5 ± 2.0 to 55.6 ± 1.5%, p = 0.01) as well as cardiac index and cardiac output. Unexpectedly, cardiac lipid content in the septum remained unchanged (from 0.80 ± 0.22% to 0.95 ± 0.21%, p = 0.15)., Conclusions: Twelve weeks of progressive endurance/strength training was effective in improving VO2max, insulin sensitivity and cardiac function in patients with type 2 diabetes mellitus. However, cardiac lipid content remained unchanged. These data suggest that a decrease in cardiac lipid content in type 2 diabetic patients is not a prerequisite for improvements in cardiac function., Trial Registration: ISRCTN: ISRCTN43780395.

Objective: Nonsteroidal antiinflammatory drugs appear to improve insulin sensitivity and are currently tested in clinical trials. Salsalate, however, may blunt mitochondrial function, an unwarranted side effect for type 2 diabetics. We examined the effect of salsalate on ex vivo mitochondrial function and lipid-induced insulin resistance., Design: In a crossover design, nine volunteers underwent a hyperinsulinemic-euglycemic clamp with simultaneous infusion of glycerol (control), Intralipid, or Intralipid preceded by 4 d of salsalate (4000 mg Disalsid). Oxidative glucose disposal and nonoxidative glucose disposal (NOGD), metabolic flexibility, energy expenditure (EE), and ex vivo muscle mitochondrial function were measured., Results: Lipid infusion reduced insulin-stimulated glucose disposal by approximately 40%, glucose oxidation (CHOox) by approximately 50%, and NOGD by approximately 35%. Lipid-induced whole-body insulin resistance and decreased NOGD were not ameliorated by salsalate. However, salsalate repressed lipid-induced reduction in CHOox and reduced insulin clearance, resulting in higher insulin levels under basal as well as under clamp conditions (∼25 and ∼39%, respectively). Intriguingly, EE was higher after administration of salsalate (∼18 and ∼16% under basal and clamp conditions, respectively) and was fueled by increased fat oxidation in the basal state and increased CHOox upon insulin stimulation. Salsalate did not affect mitochondrial function and coupling., Conclusion: We conclude that salsalate failed to improve whole-body insulin sensitivity but increased basal fat oxidation and insulin-stimulated CHOox, indicating improved metabolic flexibility. The beneficial effects of salsalate on CHOox can be attributed to elevated insulin levels. Mitochondrial respirometry revealed no indications that the changes in substrate selection and EE could be attributed to changes in skeletal muscle mitochondrial capacity or mitochondrial coupling.

Exercise training is advocated in insulin resistance and statins are used to treat hyperlipidaemia, two cardiometabolic risk factors often presenting concurrently. Statin intake may blunt mitochondrial function and the adaptive response to exercise training. Thus combining exercise training with statin administration may have adverse effects. We examined whether improvements in cardiometabolic risk factors, insulin sensitivity and mitochondrial function mediated by progressive exercise training are affected by statin use. A group of 14 obese elderly males on statins (ST) and 22 matched control subjects (C) were examined. Results on in vivo mitochondrial function [MRS (magnetic resonance spectroscopy)], mitochondrial density (Western blotting), insulin sensitivity (clamp) and metabolic flexibility (indirect calorimetry) were compared before and after a 12-week combined progressive exercise training programme (3 x per week; 45 min per session). Except for LDL (low-density lipoprotein) cholesterol, all pre-training values were comparable between statin users and control subjects. In vivo mitochondrial function and mitochondrial density improved by training in both groups. Interestingly, blood-lipid profile, insulin sensitivity (+72%), non-oxidative and oxidative glucose disposal (+38% and +112%) and insulin-mediated suppression of fat oxidation (-62%) improved only in the ST group. We conclude that statin treatment did not impede exercise performance or tolerance, mitochondrial function or mass. In addition, training-induced improvements in glucose homoeostasis were preserved in the ST group. Strikingly, the insulin-sensitizing effect of training was more prominent in the ST group than in the C group. The combined prescription of statins along with exercise training is safe and should be considered for subjects prone to develop insulin resistance.

Objective: Mitochondrial dysfunction and fat accumulation in skeletal muscle (increased intramyocellular lipid [IMCL]) have been linked to development of type 2 diabetes. We examined whether exercise training could restore mitochondrial function and insulin sensitivity in patients with type 2 diabetes., Research Design and Methods: Eighteen male type 2 diabetic and 20 healthy male control subjects of comparable body weight, BMI, age, and VO2max participated in a 12-week combined progressive training program (three times per week and 45 min per session). In vivo mitochondrial function (assessed via magnetic resonance spectroscopy), insulin sensitivity (clamp), metabolic flexibility (indirect calorimetry), and IMCL content (histochemically) were measured before and after training., Results: Mitochondrial function was lower in type 2 diabetic compared with control subjects (P = 0.03), improved by training in control subjects (28% increase; P = 0.02), and restored to control values in type 2 diabetic subjects (48% increase; P < 0.01). Insulin sensitivity tended to improve in control subjects (delta Rd 8% increase; P = 0.08) and improved significantly in type 2 diabetic subjects (delta Rd 63% increase; P < 0.01). Suppression of insulin-stimulated endogenous glucose production improved in both groups (-64%; P < 0.01 in control subjects and -52% in diabetic subjects; P < 0.01). After training, metabolic flexibility in type 2 diabetic subjects was restored (delta respiratory exchange ratio 63% increase; P = 0.01) but was unchanged in control subjects (delta respiratory exchange ratio 7% increase; P = 0.22). Starting with comparable pretraining IMCL levels, training tended to increase IMCL content in type 2 diabetic subjects (27% increase; P = 0.10), especially in type 2 muscle fibers., Conclusions: Exercise training restored in vivo mitochondrial function in type 2 diabetic subjects. Insulin-mediated glucose disposal and metabolic flexibility improved in type 2 diabetic subjects in the face of near-significantly increased IMCL content. This indicates that increased capacity to store IMCL and restoration of improved mitochondrial function contribute to improved muscle insulin sensitivity.

Storage of fatty acids as triacylglycerol (TAG) occurs in almost all mammalian tissues. Whereas adipose tissue is by far the largest storage site of fatty acids as TAG, subcellular TAG-containing structures--referred to as lipid droplets (LD)--are also present in other tissues. Until recently, LD were considered inert storage sites of energy dense fats. Nowadays, however, LD are increasingly considered dynamic functional organelles involved in many intracellular processes like lipid metabolism, vesicle trafficking, and cell signaling. Next to TAG, LD also contain other neutral lipids such as diacylglycerol. Furthermore, LD are coated by a monolayer of phospholipids decorated with a variety of proteins regulating the delicate balance between LD synthesis, growth, and degradation. Disturbances in LD-coating proteins may result in disequilibrium of TAG synthesis and degradation, giving rise to insulin-desensitizing lipid intermediates, especially in insulin-responsive tissues like skeletal muscle. For a proper and detailed understanding, more information on processes and players involved in LD synthesis and degradation is necessary. This, however, is hampered by the fact that research on LD dynamics in (human) muscle is still in its infancy. A rapidly expanding body of knowledge on LD dynamics originates from studies in other tissues and other species. Here, we aim to review the involvement of LD-coating proteins in LD formation and degradation (LD dynamics) and to extrapolate this knowledge to human skeletal muscle and to explore the role of LD dynamics in myocellular insulin sensitivity.

The aim of the present study was to assess the level of glycaemic control by the measurement of 24 h blood glucose profiles and standard blood analyses under identical nutritional and physical activity conditions in patients with Type II diabetes and healthy normoglycaemic controls. A total of 11 male patients with Type II diabetes and 11 healthy matched controls participated in a 24 h CGMS (continuous subcutaneous glucose-monitoring system) assessment trial under strictly standardized dietary and physical activity conditions. In addition, fasting plasma glucose, insulin and HbA(1c) (glycated haemoglobin) concentrations were measured, and an OGTT (oral glucose tolerance test) was performed to calculate indices of whole-body insulin sensitivity, oral glucose tolerance and/or glycaemic control. In the healthy control group, hyperglycaemia (blood glucose concentration >10 mmol/l) was hardly present (2+/-1% or 0.4+/-0.2/24 h). However, in the patients with Type II diabetes, hyperglycaemia was experienced for as much as 55+/-7% of the time (13+/-2 h over 24 h) while using the same standardized diet. Breakfast-related hyperglycaemia contributed most (46+/-7%; P<0.01 as determined by ANOVA) to the total amount of hyperglycaemia and postprandial glycaemic instability. In the diabetes patients, blood HbA(1c) content correlated well with the duration of hyperglycaemia and the postprandial glucose responses (P<0.05). In conclusion, CGMS determinations show that standard measurements of glycaemic control underestimate the amount of hyperglycaemia prevalent during real-life conditions in Type II diabetes. Given the macro- and micro-vascular damage caused by postprandial hyperglycaemia, CGMS provides an excellent tool to evaluate alternative therapeutic strategies to reduce hyperglycaemic blood glucose excursions.