An IFNγ-dependent immune-endocrine circuit lowers blood glucose to potentiate the innate antiviral immune response.

Viral infection makes us feel sick as the immune system alters systemic metabolism to better fight the pathogen. The extent of these changes is relative to the severity of disease. Whether blood glucose is subject to infection-induced modulation is mostly unknown. Here we show that strong, nonlethal infection restricts systemic glucose availability, which promotes the antiviral type I interferon (IFN-I) response. Following viral infection, we find that IFNγ produced by γδ T cells stimulates pancreatic ß cells to increase glucose-induced insulin release. Subsequently, hyperinsulinemia lessens hepatic glucose output. Glucose restriction enhances IFN-I production by curtailing lactate-mediated inhibition of IRF3 and NF-κB signaling. Induced hyperglycemia constrained IFN-I production and increased mortality upon infection. Our findings identify glucose restriction as a physiological mechanism to bring the body into a heightened state of responsiveness to viral pathogens. This immune-endocrine circuit is disrupted in hyperglycemia, possibly explaining why patients with diabetes are more susceptible to viral infection.

Postprandial macrophage-derived IL-1ß stimulates insulin, and both synergistically promote glucose disposal and inflammation.

The deleterious effect of chronic activation of the IL-1ß system on type 2 diabetes and other metabolic diseases is well documented. However, a possible physiological role for IL-1ß in glucose metabolism has remained unexplored. Here we found that feeding induced a physiological increase in the number of peritoneal macrophages that secreted IL-1ß, in a glucose-dependent manner. Subsequently, IL-1ß contributed to the postprandial stimulation of insulin secretion. Accordingly, lack of endogenous IL-1ß signaling in mice during refeeding and obesity diminished the concentration of insulin in plasma. IL-1ß and insulin increased the uptake of glucose into macrophages, and insulin reinforced a pro-inflammatory pattern via the insulin receptor, glucose metabolism, production of reactive oxygen species, and secretion of IL-1ß mediated by the NLRP3 inflammasome. Postprandial inflammation might be limited by normalization of glycemia, since it was prevented by inhibition of the sodium-glucose cotransporter SGLT2. Our findings identify a physiological role for IL-1ß and insulin in the regulation of both metabolism and immunity.

Virus-Induced Interferon-γ Causes Insulin Resistance in Skeletal Muscle and Derails Glycemic Control in Obesity.

Pro-inflammatory cytokines of a T helper-1-signature are known to promote insulin resistance (IR) in obesity, but the physiological role of this mechanism is unclear. It is also unknown whether and how viral infection induces loss of glycemic control in subjects at risk for developing diabetes mellitus type 2 (DM2). We have found in mice and humans that viral infection caused short-term systemic IR. Virally-induced interferon-γ (IFN-γ) directly targeted skeletal muscle to downregulate the insulin receptor but did not cause loss of glycemic control because of a compensatory increase of insulin production. Hyperinsulinemia enhanced antiviral immunity through direct stimulation of CD8+ effector T cell function. In pre-diabetic mice with hepatic IR caused by diet-induced obesity, infection resulted in loss of glycemic control. Thus, upon pathogen encounter, the immune system transiently reduces insulin sensitivity of skeletal muscle to induce hyperinsulinemia and promote antiviral immunity, which derails to glucose intolerance in pre-diabetic obese subjects. VIDEO ABSTRACT.

Interleukin-33-Activated Islet-Resident Innate Lymphoid Cells Promote Insulin Secretion through Myeloid Cell Retinoic Acid Production.

Pancreatic-islet inflammation contributes to the failure of ß cell insulin secretion during obesity and type 2 diabetes. However, little is known about the nature and function of resident immune cells in this context or in homeostasis. Here we show that interleukin (IL)-33 was produced by islet mesenchymal cells and enhanced by a diabetes milieu (glucose, IL-1ß, and palmitate). IL-33 promoted ß cell function through islet-resident group 2 innate lymphoid cells (ILC2s) that elicited retinoic acid (RA)-producing capacities in macrophages and dendritic cells via the secretion of IL-13 and colony-stimulating factor 2. In turn, local RA signaled to the ß cells to increase insulin secretion. This IL-33-ILC2 axis was activated after acute ß cell stress but was defective during chronic obesity. Accordingly, IL-33 injections rescued islet function in obese mice. Our findings provide evidence that an immunometabolic crosstalk between islet-derived IL-33, ILC2s, and myeloid cells fosters insulin secretion.

Insights into energy balance dysregulation from a mouse model of methylmalonic aciduria.

Inherited disorders of mitochondrial metabolism, including isolated methylmalonic aciduria, present unique challenges to energetic homeostasis by disrupting energy-producing pathways. To better understand global responses to energy shortage, we investigated a hemizygous mouse model of methylmalonyl-CoA mutase (Mmut)-type methylmalonic aciduria. We found Mmut mutant mice to have reduced appetite, energy expenditure and body mass compared with littermate controls, along with a relative reduction in lean mass but increase in fat mass. Brown adipose tissue showed a process of whitening, in line with lower body surface temperature and lesser ability to cope with cold challenge. Mutant mice had dysregulated plasma glucose, delayed glucose clearance and a lesser ability to regulate energy sources when switching from the fed to fasted state, while liver investigations indicated metabolite accumulation and altered expression of peroxisome proliferator-activated receptor and Fgf21-controlled pathways. Together, these shed light on the mechanisms and adaptations behind energy imbalance in methylmalonic aciduria and provide insight into metabolic responses to chronic energy shortage, which may have important implications for disease understanding and patient management.

Thiazides Attenuate Insulin Secretion Through Inhibition of Mitochondrial Carbonic Anhydrase 5b in ß -Islet Cells in Mice.

SIGNIFICANCE STATEMENT: Thiazide diuretics (thiazides) are among the most widely prescribed drugs worldwide, but their use is associated with glucose intolerance and new-onset diabetes mellitus. The molecular mechanisms remain elusive. Our study reveals that thiazides attenuate insulin secretion through inhibition of the mitochondrial carbonic anhydrase isoform 5b (CA5b) in pancreatic ß cells. We furthermore discovered that pancreatic ß cells express only one functional carbonic anhydrase isoform, CA5b, which is critical in replenishing oxaloacetate in the mitochondrial tricarboxylic acid (TCA) cycle (anaplerosis). These findings explain the mechanism for thiazide-induced glucose intolerance and reveal a fundamental role of CA5b in TCA cycle anaplerosis and insulin secretion in ß cells. BACKGROUND: Thiazide diuretics are associated with glucose intolerance and new-onset diabetes mellitus. Previous studies demonstrated that thiazides attenuate insulin secretion, but the molecular mechanisms remain elusive. We hypothesized that thiazides attenuate insulin secretion via one of the known molecular thiazide targets in ß cells. METHODS: We performed static insulin secretion experiments with islets of wild-type, Sodium/chloride co-transporter (NCC) (SLC12A3), and sodium-driven chloride/bicarbonate exchanger (NDCBE) (SLC4A8) knock-out (KO) mice and with murine Min6 cells with individual knockdown of carbonic anhydrase (CA) isoforms to identify the molecular target of thiazides in ß cells. CA isoform 5b (CA5b) KO mice were then used to assess the role of the putative thiazide target CA5b in ß -cell function and in mediating thiazide sensitivity in vitro and in vivo . RESULTS: Thiazides inhibited glucose- and sulfonylurea-stimulated insulin secretion in islets and Min6 cells at pharmacologically relevant concentrations. Inhibition of insulin secretion by thiazides was CO 2 /HCO 3- -dependent, not additive to unselective CA inhibition with acetazolamide, and independent of extracellular potassium. By contrast, insulin secretion was unaltered in islets of mice lacking the known molecular thiazide targets NCC or NDCBE. CA expression profiling with subsequent knockdown of individual CA isoforms suggested mitochondrial CA5b as a molecular target. In support of these findings, thiazides significantly attenuated Krebs cycle anaplerosis through reduction of mitochondrial oxaloacetate synthesis. CA5b KO mice were resistant to thiazide-induced glucose intolerance, and thiazides did not alter insulin secretion in CA5b KO islets. CONCLUSIONS: Thiazides attenuate insulin secretion via inhibition of the mitochondrial CA5b isoform in ß cells of mice.

CSF1R inhibition with PLX5622 affects multiple immune cell compartments and induces tissue-specific metabolic effects in lean mice.

AIMS/HYPOTHESIS: Colony stimulating factor 1 (CSF1) promotes the proliferation, differentiation and survival of macrophages, which have been implicated in both beneficial and detrimental effects on glucose metabolism. However, the physiological role of CSF1 signalling in glucose homeostasis and the potential therapeutic implications of modulating this pathway are not known. We aimed to study the composition of tissue macrophages (and other immune cells) following CSF1 receptor (CSF1R) inhibition and elucidate the metabolic consequences of CSF1R inhibition. METHODS: We assessed immune cell populations in various organs by flow cytometry, and tissue-specific metabolic effects by hyperinsulinaemic-euglycaemic clamps and insulin secretion assays in mice fed a chow diet containing PLX5622 (a CSF1R inhibitor) or a control diet. RESULTS: CSF1R inhibition depleted macrophages in multiple tissues while simultaneously increasing eosinophils and group 2 innate lymphoid cells. These immunological changes were consistent across different organs and were sex independent and reversible after cessation of the PLX5622. CSF1R inhibition improved hepatic insulin sensitivity but concomitantly impaired insulin secretion. In healthy islets, we found a high frequency of IL-1ß+ islet macrophages. Their depletion by CSF1R inhibition led to downregulation of macrophage-related pathways and mediators of cytokine activity, including Nlrp3, suggesting IL-1ß as a candidate insulin secretagogue. Partial restoration of physiological insulin secretion was achieved by injecting recombinant IL-1ß prior to glucose stimulation in mice lacking macrophages. CONCLUSIONS/INTERPRETATION: Macrophages and macrophage-derived factors, such as IL-1ß, play an important role in physiological insulin secretion. A better understanding of the tissue-specific effects of CSF1R inhibition on immune cells and glucose homeostasis is crucial for the development of targeted immune-modulatory treatments in metabolic disease. DATA AVAILABILITY: The RNA-Seq dataset is available in the Gene Expression Omnibus (GEO) under the accession number GSE189434 ( http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE189434 ).

Deoxyguanosine kinase mutation F180S is associated with a lean phenotype in mice.

BACKGROUND: Deoxyguanosine kinase (DGUOK) deficiency is one of the genetic causes of mitochondrial DNA depletion syndrome (MDDS) in humans, leading to the hepatocerebral or the isolated hepatic form of MDDS. Mouse models are helpful tools for the improvement of understanding of the pathophysiology of diseases and offer the opportunity to examine new therapeutic options. METHODS: Herein, we describe the generation and metabolic characterization of a mouse line carrying a homozygous DguokF180S/F180S mutation derived from an N-ethyl-N-nitrosourea-mutagenesis screen. Energy expenditure (EE), oxygen consumption (VO2) and carbon dioxide production (VCO2) were assessed in metabolic cages. LC-MS/MS was used to quantify plasma adrenal steroids. Plasma insulin and leptin levels were quantified with commercially available assay kits. RESULTS: Mutant animals displayed significantly lower body weights and reduced inguinal fat pad mass, in comparison to unaffected littermates. Biochemically, they were characterized by significantly lower blood glucose levels, accompanied by significantly lower insulin, total cholesterol, high density lipoprotein and triglyceride levels. They also displayed an almost 2-fold increase in transaminases. Moreover, absolute EE was comparable in mutant and control mice, but EE in mutants was uncoupled from their body weights. Histological examination of inguinal white adipose tissue (WAT) revealed adipocytes with multilocular fat droplets reminiscent of WAT browning. In addition, mRNA and protein expression of Ucp1 was increased. Mutant mice also presented differing mitochondrial DNA content in various tissues and altered metabolic activity in mitochondria, but no further phenotypical or behavioral abnormalities. Preliminary data imply normal survival of DguokF180S/F180S mutant animals. CONCLUSION: Taken together, DGUOK mutation F180S leads to a lean phenotype, with lower glucose, insulin, and lipid levels rendering this mouse model not only useful for the study of MDDS forms but also for deciphering mechanisms resulting in a lean phenotype.

The controversial role of IL-6 in adipose tissue on obesity-induced dysregulation of glucose metabolism.

Interleukin (IL)-6 is a pleotropic cytokine with various physiological and pathophysiological functions in different cells and tissues. In cells residing within white adipose tissue, several, and sometimes conflicting, IL-6 actions have been described in the development of obesity-associated derangements of glucose metabolism. Herein, we aim to summarize opposing findings and discuss recent evidence that IL-6 signaling in adipose tissue is regulated in a depot and cell-specific manner.

Adipocyte-specific gp130 signalling mediates exercise-induced weight reduction.

BACKGROUND: Repetitive physical activity is a well-established intervention to reduce obesity and to prevent weight regain. Besides increased energy expenditure, reduced caloric intake may contribute to exercise-induced weight loss in obesity. Using adipocyte-specific glycoprotein 130 knockout (gp130Δadipo) mice, we recently unravelled that obesity-induced interleukin-6 (IL-6) signalling in adipose tissue contributes to circulating levels of the two anorectic hormones leptin and insulin. Herein, we aimed to investigate the role of adipocyte-specific IL-6 signalling in exercise-mediated appetite control and, hence, weight reduction in obesity. METHODS: gp130Δadipo and control littermate mice (gp130F/F) were repetitively exercised during a 12-week period of HFD-feeding. Thermogenesis was determined using thermography and food intake as well as energy expenditure were assessed in metabolic cages. Circulating IL-6, insulin and leptin levels were measured using immunoassays. Protein levels of phosphorylated STAT3, JAK2 and Akt were determined in the hypothalamus by Western blot technique. RESULTS: Repetitive physical activity reduced food intake and HFD-induced weight gain in gp130F/F but not gp130Δadipo mice. In contrast, energy expenditure was not different between the genotypes. Circulating insulin and leptin levels were significantly reduced in gp130Δadipo mice. Moreover, hypothalamic leptin and insulin signalling were enhanced in exercised gp130F/F but not gp130Δadipo mice as demonstrated by elevated pSTAT3, pJAK2 and pAkt protein levels. CONCLUSION: Adipocyte-specific IL-6 signalling is involved in exercise-mediated regulation of food intake and weight reduction in HFD-fed mice.

Intermittent fasting improves metabolic flexibility in short-term high-fat diet-fed mice.

Four days of high-fat diet (HFD) feeding are sufficient to induce glucose intolerance and hepatic steatosis in mice. While prolonged HFD-induced metabolic complications are partly mediated by increased food intake during the light (inactive) phase, such a link has not yet been established in short-term HFD-fed mice. Herein, we hypothesized that a short bout of HFD desynchronizes feeding behavior, thereby contributing to glucose intolerance and hepatic steatosis. To this end, 12-wk-old C57BL/6J littermates were fed a HFD for 4 days either ad libitum or intermittently. Intermittent-fed mice were fasted for 8 h during their inactive phase. Initiation of HFD led to an immediate increase in food intake already during the first light phase. Moreover, glucose tolerance was significantly impaired in ad libitum- but not in intermittent HFD-fed mice, indicating that desynchronized feeding behavior contributes to short-term HFD-induced glucose intolerance. Of note, overall food intake was similar between the groups, as was body weight. However, intermittent HFD-fed mice revealed higher fat depot weights. Phosphorylation of hormone sensitivity lipase and free fatty acid release from isolated adipocytes were significantly elevated, suggesting increased lipolysis in intermittent HFD-fed mice. Moreover, hepatic mRNA expression of lipogenetic enzymes and liver triglyceride levels were significantly increased in intermittent HFD-fed mice. Importantly, food deprivation decreased respiratory exchange ratio promptly in intermittent- but not in ad libitum HFD-fed mice. In conclusion, retaining a normal feeding pattern prevented HFD-induced impairment of metabolic flexibility in short-term HFD-fed mice.

Short-term feeding of a ketogenic diet induces more severe hepatic insulin resistance than an obesogenic high-fat diet.

KEY POINTS: A ketogenic diet is known to lead to weight loss and is considered metabolically healthy; however there are conflicting reports on its effect on hepatic insulin sensitivity. KD fed animals appear metabolically healthy in the fasted state after 3 days of dietary challenge, whereas obesogenic high-fat diet (HFD) fed animals show elevated insulin levels. A glucose challenge reveals that both KD and HFD fed animals are glucose intolerant. Glucose intolerance correlates with increased lipid oxidation and lower respiratory exchange ratio (RER); however, all animals respond to glucose injection with an increase in RER. Hyperinsulinaemic-euglycaemic clamps with double tracer show that the effect of KD is a result of hepatic insulin resistance and increased glucose output but not impaired glucose clearance or tissue glucose uptake in other tissues. ABSTRACT: Despite being a relevant healthcare issue and heavily investigated, the aetiology of type 2 diabetes (T2D) is still incompletely understood. It is well established that increased endogenous glucose production (EGP) leads to a progressive increase in glucose levels, causing insulin resistance and eventual loss of glucose homeostasis. The consumption of high carbohydrate, high-fat, western style diet (HFD) is linked to the development of T2D and obesity, whereas the consumption of a low carbohydrate, high-fat, ketogenic diet (KD) is considered healthy. However, several days of carbohydrate restriction are known to cause selective hepatic insulin resistance. In the present study, we compare the effects of short-term HFD and KD feeding on glucose homeostasis in mice. We show that, even though KD fed animals appear to be healthy in the fasted state, they exhibit decreased glucose tolerance to a greater extent than HFD fed animals. Furthermore, we show that this effect originates from blunted suppression of hepatic glucose production by insulin, rather than impaired glucose clearance and tissue glucose uptake. These data suggest that the early effects of HFD consumption on EGP may be part of a normal physiological response to increased lipid intake and oxidation, and that systemic insulin resistance results from the addition of dietary glucose to EGP-derived glucose.

Iron metabolism in patients with Graves' hyperthyroidism.

OBJECTIVES: Graves' hyperthyroidism (GH) interferes with iron metabolism and elevates ferritin. The precise mechanisms remain unclear. The influence of thyroid hormones on the synthesis/regulation of hepcidin, an important regulator of iron metabolism, remains uncharacterized. DESIGN: Prospective observational study. PATIENTS: We included patients (n = 31) with new-onset and untreated GH. MEASUREMENTS: Laboratory parameters indicative of iron metabolism (ferritin, transferrin, hepcidin), inflammatory markers/cytokines and smoking status were assessed at the diagnosis of GH (T0) and at euthyroidism (T1) in the same patients using multivariable analyses. Hepcidin was measured by mass spectrometry (hepcidinMS ) and ELISA (hepcidinEL ). The impact of T3 on hepatic hepcidin expression was studied in a cell culture model using HepG2 cells. RESULTS: Median ferritin levels were significantly lower and transferrin significantly higher at T1 than at T0. HepcidinMS levels were lower in males and females at T1 (statistically significant in males only). No statistically significant difference in hepcidinEL was detected between T0 and T1. Plasma levels of inflammatory markers (high-sensitive CRP, procalcitonin) and cytokines (interleukin 6, interleukin 1ß, tumour necrosis factor α) were not different between T0 and T1. Smokers tended to have lower fT3 and fT4 at T0 than nonsmoking GH patients. T3 significantly induced hepcidin mRNA expression in HepG2 cells. CONCLUSIONS: Iron metabolism in patients with GH undergoes dynamic changes in patients with GH that resemble an acute-phase reaction. Inflammatory parameters and cytokines were unaffected by thyroid status. Gender and smoking status had an impact on ferritin, hepcidin and thyroid hormones.

Brain catecholamine depletion and motor impairment in a Th knock-in mouse with type B tyrosine hydroxylase deficiency.

Tyrosine hydroxylase catalyses the hydroxylation of L-tyrosine to l-DOPA, the rate-limiting step in the synthesis of catecholamines. Mutations in the TH gene encoding tyrosine hydroxylase are associated with the autosomal recessive disorder tyrosine hydroxylase deficiency, which manifests phenotypes varying from infantile parkinsonism and DOPA-responsive dystonia, also termed type A, to complex encephalopathy with perinatal onset, termed type B. We generated homozygous Th knock-in mice with the mutation Th-p.R203H, equivalent to the most recurrent human mutation associated with type B tyrosine hydroxylase deficiency (TH-p.R233H), often unresponsive to l-DOPA treatment. The Th knock-in mice showed normal survival and food intake, but hypotension, hypokinesia, reduced motor coordination, wide-based gate and catalepsy. This phenotype was associated with a gradual loss of central catecholamines and the serious manifestations of motor impairment presented diurnal fluctuation but did not improve with standard l-DOPA treatment. The mutant tyrosine hydroxylase enzyme was unstable and exhibited deficient stabilization by catecholamines, leading to decline of brain tyrosine hydroxylase-immunoreactivity in the Th knock-in mice. In fact the substantia nigra presented an almost normal level of mutant tyrosine hydroxylase protein but distinct absence of the enzyme was observed in the striatum, indicating a mutation-associated mislocalization of tyrosine hydroxylase in the nigrostriatal pathway. This hypomorphic mouse model thus provides understanding on pathomechanisms in type B tyrosine hydroxylase deficiency and a platform for the evaluation of novel therapeutics for movement disorders with loss of dopaminergic input to the striatum.

The gut-adipose-liver axis in the metabolic syndrome.

Obesity is associated with altered gut microbiota composition and impaired gut barrier function. These changes, together with interrelated mesenteric adipose tissue inflammation, result in increased release of pro-inflammatory cytokines, bacteria-derived factors, and lipids into the portal circulation, promoting the development of (hepatic) insulin resistance. Herein, the potential impact of obesity-related changes in gut and visceral adipose tissue biology on the development of insulin resistance and Type 2 diabetes is reviewed.

Interleukin-6 contributes to early fasting-induced free fatty acid mobilization in mice.

Contracting muscle releases interleukin-6 (IL-6) enabling the metabolic switch from carbohydrate to fat utilization. Similarly, metabolism is switched during transition from fed to fasting state. Herein, we examined a putative role for IL-6 in the metabolic adaptation to normal fasting. In lean C57BL/6J mice, 6 h of food withdrawal increased gene transcription levels of IL-6 in skeletal muscle but not in white adipose tissue. Concomitantly, circulating IL-6 and free fatty acid (FFA) levels were significantly increased, whereas respiratory quotient (RQ) was reduced in 6-h fasted mice. In white adipose tissue, phosphorylation of hormone-sensitive lipase (HSL) was increased on fasting, indicating increased lipolysis. Intriguingly, fasting-induced increase in circulating IL-6 levels and parallel rise in FFA concentration were absent in obese and glucose-intolerant mice. A causative role for IL-6 in the physiological adaptation to fasting was further supported by the fact that fasting-induced increase in circulating FFA levels was significantly blunted in lean IL-6 knockout (KO) and lean C57BL/6J mice treated with neutralizing IL-6 antibody. Consistently, phosphorylation of HSL was significantly reduced in adipose tissue of IL-6-depleted mice. Hence, our findings suggest a novel role for IL-6 in energy supply during early fasting.

Fas (CD95) expression in adipocytes contributes to diet-induced obesity.

OBJECTIVE: Induction of browning in white adipose tissue (WAT) increases energy expenditure and may be an attractive target for the treatment of obesity. Since activation of Fas (CD95) induces pathways known to blunt expression of uncoupling protein 1 (UCP1), we hypothesized that Fas expression in adipocytes inhibits WAT browning and thus contributes to the development of obesity. METHODS: Adipocyte-specific Fas knockout (FasΔadipo) and control littermate (FasF/F) mice were fed a regular chow diet or a high-fat diet (HFD) for 20 weeks. Energy expenditure was assessed by indirect calorimetry, and browning was determined in subcutaneous WAT. In vitro, UCP1 was analyzed in subcutaneous murine adipocytes treated with or without Fas ligand. Moreover, FAS expression in WAT was correlated to UCP1 and percentage of body fat in human individuals. RESULTS: HFD-fed FasΔadipo mice displayed reduced body weight gain and blunted adiposity compared to control littermates. Concomitantly, whole-body energy expenditure and WAT browning were elevated. In cultured adipocytes, Fas ligand treatment blunted isoproterenol-induced UCP1 protein levels. In support of these findings in rodents, FAS expression in WAT correlated negatively with UCP1 but positively with adiposity in human individuals. CONCLUSIONS: Fas activation in adipocytes contributes to HFD-associated adiposity in rodents and may be a therapeutic target to reduce obesity and associated diseases.

Fas activates lipolysis in a Ca2+-CaMKII-dependent manner in 3T3-L1 adipocytes.

Fas (CD95) is a member of the tumor necrosis factor (TNF) receptor superfamily and plays a crucial role in the induction of apoptosis. However, like TNF, Fas can induce nonapoptotic signaling pathways. We previously demonstrated that mice lacking Fas specifically in adipocytes are partly protected from diet-induced insulin resistance, potentially via decreased delivery of FAs to the liver, as manifested by lower total liver ceramide content. In the present study, we aimed to delineate the signaling pathway involved in Fas-mediated adipocyte lipid mobilization. Treatment of differentiated 3T3-L1 adipocytes with membrane-bound Fas ligand (FasL) significantly increased lipolysis after 12 h without inducing apoptosis. In parallel, Fas activation increased phosphorylation of ERK1/2, and FasL-induced lipolysis was blunted in the presence of the ERK-inhibitor U0126 or in ERK1/2-depleted adipocytes. Furthermore, Fas activation increased phosphorylation of the Ca(2+)/calmodulin-dependent protein kinases II (CaMKII), and blocking of the CaMKII-pathway (either by the Ca(2+) chelator BAPTA or by the CaMKII inhibitor KN62) blunted FasL-induced ERK1/2 phosphorylation and glycerol release. In conclusion, we propose a novel role for CaMKII in promoting lipolysis in adipocytes.

Adipose tissue inflammation contributes to short-term high-fat diet-induced hepatic insulin resistance.

High-fat feeding for 3-4 days impairs glucose tolerance and hepatic insulin sensitivity. However, it remains unclear whether the evolving hepatic insulin resistance is due to acute lipid overload or the result of induced adipose tissue inflammation and consequent dysfunctional adipose tissue-liver cross-talk. In the present study, feeding C57Bl6/J mice a fat-enriched diet [high-fat diet (HFD)] for 4 days induced glucose intolerance, hepatic insulin resistance (as assessed by hyperinsulinemic euglycemic clamp studies), and hepatic steatosis as well as adipose tissue inflammation (i.e., TNFα expression) compared with standard chow-fed mice. Adipocyte-specific depletion of the antiapoptotic/anti-inflammatory factor Fas (CD95) attenuated adipose tissue inflammation and improved glucose tolerance as well as hepatic insulin sensitivity without altering the level of hepatic steatosis induced by HFD. In summary, our results identify adipose tissue inflammation and resulting dysfunctional adipose tissue-liver cross-talk as an early event in the development of HFD-induced hepatic insulin resistance.

IL-27 increases energy storage in white adipocytes by enhancing glucose uptake and fatty acid esterification.

The cytokine interleukin (IL)-27 has been reported to induce thermogenesis in white adipocytes. However, it remains unknown whether IL-27-mediated adipocyte energy dissipation is paralleled by an elevated energy supply from lipids and/or carbohydrates. We hypothesized that IL-27 increases lipolysis and glucose uptake in white adipocytes, thereby providing substrates for thermogenesis. Unexpectedly, we found that treatment of 3T3-L1 adipocytes with IL-27 reduced intra- and extracellular free fatty acid (FFA) concentrations and that phosphorylation of hormone-sensitive lipase (HSL) was not affected by IL-27. These results were confirmed in subcutaneous white adipocytes. Further, application of IL-27 to 3T3-L1 adipocytes increased intracellular triglyceride (TG) content but not mitochondrial ATP production nor expression of enzymes involved in beta-oxidation indicating that elevated esterification rather than oxidation causes FFA disappearance. In addition, IL-27 significantly increased GLUT1 protein levels, basal glucose uptake as well as glycolytic ATP production, suggesting that increased glycolytic flux due to IL-27 provides the glycerol backbone for TG synthesis. In conclusion, our findings suggest IL-27 increases glucose uptake and TG deposition in white adipocytes.