Johan W.E. Jocken

Short-chain fatty acids in control of body weight and insulin sensitivity.

The connection between the gut microbiota and the aetiology of obesity and cardiometabolic disorders is increasingly being recognized by clinicians. Our gut microbiota might affect the cardiometabolic phenotype by fermenting indigestible dietary components and thereby producing short-chain fatty acids (SCFA). These SCFA are not only of importance in gut health and as signalling molecules, but might also enter the systemic circulation and directly affect metabolism or the function of peripheral tissues. In this Review, we discuss the effects of three SCFA (acetate, propionate and butyrate) on energy homeostasis and metabolism, as well as how these SCFA can beneficially modulate adipose tissue, skeletal muscle and liver tissue function. As a result, these SCFA contribute to improved glucose homeostasis and insulin sensitivity. Furthermore, we also summarize the increasing evidence for a potential role of SCFA as metabolic targets to prevent and counteract obesity and its associated disorders in glucose metabolism and insulin resistance. However, most data are derived from animal and in vitro studies, and consequently the importance of SCFA and differential SCFA availability in human energy and substrate metabolism remains to be fully established. Well-controlled human intervention studies investigating the role of SCFA on cardiometabolic health are, therefore, eagerly awaited.

Increased Adipose Tissue Oxygen Tension in Obese Compared With Lean Men Is Accompanied by Insulin Resistance, Impaired Adipose Tissue Capillarization, and Inflammation

Background— Adipose tissue (AT) dysfunction in obesity contributes to chronic, low-grade inflammation that predisposes to type 2 diabetes mellitus and cardiovascular disease. Recent in vitro studies suggest that AT hypoxia may induce inflammation. We hypothesized that adipose tissue blood flow (ATBF) regulates AT oxygen partial pressure (AT PO2), thereby affecting AT inflammation and insulin sensitivity. Methods and Results— We developed an optochemical measurement system for continuous monitoring of AT PO2 using microdialysis. The effect of alterations in ATBF on AT PO2 was investigated in lean and obese subjects with both pharmacological and physiological approaches to manipulate ATBF. Local administration of angiotensin II (vasoconstrictor) in abdominal subcutaneous AT decreased ATBF and AT PO2, whereas infusion of isoprenaline (vasodilator) evoked opposite effects. Ingestion of a glucose drink increased ATBF and AT PO2 in lean subjects, but these responses were blunted in obese individuals. However, AT PO2 was higher (hyperoxia) in obese subjects despite lower ATBF, which appears to be explained by lower AT oxygen consumption. This was accompanied by insulin resistance, lower AT capillarization, lower AT expression of genes encoding proteins involved in mitochondrial biogenesis and function, and higher AT gene expression of macrophage infiltration and inflammatory markers. Conclusions— Our findings establish ATBF as an important regulator of AT PO2. Nevertheless, obese individuals exhibit AT hyperoxia despite lower ATBF, which seems to be explained by lower AT oxygen consumption. This is accompanied by insulin resistance, impaired AT capillarization, and higher AT gene expression of inflammatory cell markers. Clinical Trial Registration— URL: http://www.trialregister.nl. Unique identifier: NTR2451.

Contribution of adipose triglyceride lipase and hormone-sensitive lipase to lipolysis in hMADS adipocytes

Lipolysis is the catabolic pathway by which triglycerides are hydrolyzed into fatty acids. Adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) have the capacity to hydrolyze in vitro the first ester bond of triglycerides, but their respective contributions to whole cell lipolysis in human adipocytes is unclear. Here, we have investigated the roles of HSL, ATGL, and its coactivator CGI-58 in basal and forskolin-stimulated lipolysis in a human white adipocyte model, the hMADS cells. The hMADS adipocytes express the various components of fatty acid metabolism and show lipolytic capacity similar to primary cultured adipocytes. We show that lipolysis and fatty acid esterification are tightly coupled except in conditions of stimulated lipolysis. Immunocytochemistry experiments revealed that acute forskolin treatment promotes HSL translocation from the cytosol to small lipid droplets and redistribution of ATGL from the cytosol and large lipid droplets to small lipid droplets, resulting in enriched colocalization of the two lipases. HSL or ATGL overexpression resulted in increased triglyceride-specific hydrolase capacity, but only ATGL overexpression increased whole cell lipolysis. HSL silencing had no effect on basal lipolysis and only partially reduced forskolin-stimulated lipolysis. Conversely, silencing of ATGL or CGI-58 significantly reduced basal lipolysis and essentially abolished forskolin-stimulated lipolysis. Altogether, these results suggest that ATGL/CGI-58 acts independently of HSL and precedes its action in the sequential hydrolysis of triglycerides in human hMADS adipocytes.

Catecholamine-induced lipolysis in adipose tissue and skeletal muscle in obesity

Increased fat storage in adipose and non-adipose tissue (e.g. skeletal muscle) characterizes the obese insulin resistant state. Disturbances in pathways of lipolysis may play a role in the development and maintenance of these increased fat stores. A reduced catecholamine-induced lipolysis may contribute to the development and maintenance of increased adipose tissue stores. To data, a reduced hormone-sensitive lipase (HSL) expression is the best characterized defect contributing to this catecholamine resistance. The recently discovered adipose triglyceride lipase (ATGL) seems not to be involved in the catecholamine resistance of lipolysis observed in abdominal subcutaneous adipose tissue of obese subjects, which contrasts with findings in mice studies. So far, little is known on the regulation of skeletal muscle lipolysis. There is evidence of both HSL and ATGL activity and/or expression in skeletal muscle. A blunted fasting and/or catecholamine-induced lipolysis has been reported in skeletal muscle, but data require confirmation. It is tempting to speculate that an imbalance between ATGL and HSL expression results in incomplete lipolysis and increased accumulation of lipid intermediates in skeletal muscle of obese insulin resistant subjects. The latter may inhibit insulin signalling and play a role in the development of type 2 diabetes. This review summarizes the current knowledge on (intracellular) adipose tissue and skeletal muscle lipolysis in obesity, discusses the underlying mechanisms of these disturbances and will finally address the question whether disturbances in the lipolytic pathways may be primary factors in the etiology of obesity or adaptational responses to the obese insulin resistant state.

The effects of 30 days resveratrol supplementation on adipose tissue morphology and gene expression patterns in obese men

Polyphenolic compounds, such as resveratrol, have recently received widespread interest because of their ability to mimic effects of calorie restriction. The objective of the present study was to gain more insight into the effects of 30 days resveratrol supplementation on adipose tissue morphology and underlying processes. Eleven healthy obese men were supplemented with placebo and resveratrol for 30 days (150 mg per day), separated by a 4-week washout period in a double-blind randomized crossover design. A postprandial abdominal subcutaneous adipose tissue biopsy was collected to assess adipose tissue morphology and gene expression using microarray analysis. Resveratrol significantly decreased adipocyte size, with a shift toward a reduction in the proportion of large and very-large adipocytes and an increase in small adipocytes. Microarray analysis revealed downregulation of Wnt and Notch signaling pathways and upregulation of pathways involved in cell cycle regulation after resveratrol supplementation, suggesting enhanced adipogenesis. Furthermore, lysosomal/phagosomal pathway and transcription factor EB were upregulated reflecting an alternative pathway of lipid breakdown by autophagy. Further research is necessary to investigate whether resveratrol improves adipose tissue function.

Liver X Receptor (LXR) Regulates Human Adipocyte Lipolysis

The Liver X receptor (LXR) is an important regulator of carbohydrate and lipid metabolism in humans and mice. We have recently shown that activation of LXR regulates cellular fuel utilization in adipocytes. In contrast, the role of LXR in human adipocyte lipolysis, the major function of human white fat cells, is not clear. In the present study, we stimulated in vitro differentiated human and murine adipocytes with the LXR agonist GW3965 and observed an increase in basal lipolysis. Microarray analysis of human adipocyte mRNA following LXR activation revealed an altered gene expression of several lipolysis-regulating proteins, which was also confirmed by quantitative real-time PCR. We show that expression and intracellular localization of perilipin1 (PLIN1) and hormone-sensitive lipase (HSL) are affected by GW3965. Although LXR activation does not influence phosphorylation status of HSL, HSL activity is required for the lipolytic effect of GW3965. This effect is abolished by PLIN1 knockdown. In addition, we demonstrate that upon activation, LXR binds to the proximal regions of the PLIN1 and HSL promoters. By selective knock-down of either LXR isoform, we show that LXRα is the major isoform mediating the lipolysis-related effects of LXR. In conclusion, the present study demonstrates that activation of LXRα up-regulates basal human adipocyte lipolysis. This is at least partially mediated through LXR binding to the PLIN1 promoter and down-regulation of PLIN1 expression.

Endocrine Role of the Renin-Angiotensin System in Human Adipose Tissue and Muscle. Effect of {beta}-Adrenergic Stimulation

The renin-angiotensin system has been implicated in obesity-related hypertension and insulin resistance. We examined whether locally produced components of the renin-angiotensin system in adipose tissue and skeletal muscle play an endocrine role in vivo in humans. Furthermore, the effects of &bgr;-adrenergic stimulation on plasma concentrations and tissue release of renin-angiotensin system components were investigated. Systemic renin-angiotensin system components and arteriovenous differences of angiotensin II (Ang II) and angiotensinogen (AGT) across abdominal subcutaneous adipose tissue and skeletal muscle were assessed in combination with measurements of tissue blood flow before and during systemic &bgr;-adrenergic stimulation in 13 lean and 10 obese subjects. Basal plasma Ang II and AGT concentrations were not significantly different between lean and obese subjects. Ang II concentrations were increased in obese compared with lean subjects during &bgr;-adrenergic stimulation (12.6±1.5 versus 8.1±1.0 pmol/L; P=0.04), whereas AGT concentrations remained unchanged. Plasma renin activity increased to a similar extent in lean and obese subjects during &bgr;-adrenergic stimulation (both P<0.01). No net Ang II release across adipose tissue and skeletal muscle could be detected in both groups of subjects. However, AGT was released from adipose tissue and muscle during &bgr;-adrenergic stimulation in obese subjects (both P<0.05). In conclusion, locally produced Ang II in adipose tissue and skeletal muscle exerts no endocrine role in lean and obese subjects. In contrast, AGT is released from adipose tissue and muscle in obese subjects during &bgr;-adrenergic stimulation, which may contribute to the increased plasma Ang II concentrations during &bgr;-adrenergic stimulation in obese subjects.

Activation of liver X receptor regulates substrate oxidation in white adipocytes

Liver X receptors (LXRs) are nuclear receptors with established roles in cholesterol, lipid, and carbohydrate metabolism, although their function in adipocytes is not well characterized. Increased adipose tissue mass in obesity is associated with increased adipocyte lipolysis. Fatty acids (FA) generated by lipolysis can be oxidized by mitochondrial beta-oxidation, reesterified, or released from the adipocyte. The latter results in higher circulating levels of free FAs, in turn causing obesity-related metabolic complications. However, mitochondrial beta-oxidation can at least in part counteract an increased output of FA into circulation. In this study, we provide evidence that activation of LXRs up-regulates mitochondrial beta-oxidation in both human and murine white adipocytes. We also show that the expression of a kinase regulating the cellular fuel switch, pyruvate dehydrogenase kinase 4 (PDK4), is up-regulated by the LXR agonist GW3965 in both in vitro differentiated human primary adipocytes and differentiated murine 3T3-L1 cells. Moreover, activation of LXR causes PDK4-dependent phosphorylation of the pyruvate dehydrogenase complex, thereby decreasing its activity and attenuating glucose oxidation. The specificity of the GW3965 effect on oxidation was confirmed by RNA interference targeting LXRs. We propose that LXR has an important role in the regulation of substrate oxidation and the switch between lipids and carbohydrates as cellular fuel in both human and murine white adipocytes.

Targeting fatty acid metabolism to improve glucose metabolism

Disturbances in fatty acid metabolism in adipose tissue, liver, skeletal muscle, gut and pancreas play an important role in the development of insulin resistance, impaired glucose metabolism and type 2 diabetes mellitus. Alterations in diet composition may contribute to prevent and/or reverse these disturbances through modulation of fatty acid metabolism.

Short-term supplementation with a specific combination of dietary polyphenols increases energy expenditure and alters substrate metabolism in overweight subjects

Background and Objectives:Impaired regulation of lipid oxidation (metabolic inflexibility) is associated with obesity and type 2 diabetes mellitus. Recent evidence has indicated that dietary polyphenols may modulate mitochondrial function, substrate metabolism and energy expenditure in humans. The present study investigated the effects of short-term supplementation of two combinations of polyphenols on energy expenditure (EE) and substrate metabolism in overweight subjects.Subjects and Methods:Eighteen healthy overweight volunteers (9 women, 9 men; age 35±2.5 years; body mass index 28.9±0.4 kg m−2) participated in a randomized, double-blind cross-over trial. Combinations of epigallocatechin-gallate (E, 282 mg day−1)+resveratrol (R, 200 mg day−1) and E+R+80 mg day−1 soy isoflavones (S) or placebo capsules (PLA) were supplemented twice daily for a period of 3 days. On day 3, circulating metabolite concentrations, EE and substrate oxidation (using indirect calorimetry) were measured during fasting and postprandial conditions for 6 h (high-fat-mixed meal (2.6 MJ, 61.2 E% fat)).Results:Short-term supplementation of E+R increased resting EE (E+R vs PLA: 5.45±0.24 vs 5.23±0.25 kJ min−1, P=0.039), whereas both E+R (699±18 kJ 120 min−1 vs 676±20 kJ 120 min−1, P=0.028) and E+R+S (704±18 kJ 120 min−1 vs 676±20 kJ 120 min−1, P=0.014) increased 2–4 h-postprandial EE compared with PLA. Metabolic flexibility, calculated as the postprandial increase to the highest respiratory quotient achieved, tended to be improved by E+R compared with PLA and E+R+S only in men (E+R vs PLA: 0.11±0.02 vs 0.06±0.02, P=0.059; E+R+S: 0.03±0.02, P=0.009). E+R+S significantly increased fasting plasma free fatty acid (P=0.064) and glycerol (P=0.021) concentrations compared with PLA.Conclusions:We demonstrated for the first time that combined E+R supplementation for 3 days significantly increased fasting and postprandial EE, which was accompanied by improved metabolic flexibility in men but not in women. Addition of soy isoflavones partially reversed these effects possibly due to their higher lipolytic potential. The present findings may imply that long-term supplementation of these dosages of epigallocatechin-gallate combined with resveratrol may improve metabolic health and body weight regulation.