The multifunctional protein E4F1 links P53 to lipid metabolism in adipocytes.

Growing evidence supports the importance of the p53 tumor suppressor in metabolism but the mechanisms underlying p53-mediated control of metabolism remain poorly understood. Here, we identify the multifunctional E4F1 protein as a key regulator of p53 metabolic functions in adipocytes. While E4F1 expression is upregulated during obesity, E4f1 inactivation in mouse adipose tissue results in a lean phenotype associated with insulin resistance and protection against induced obesity. Adipocytes lacking E4F1 activate a p53-dependent transcriptional program involved in lipid metabolism. The direct interaction between E4F1 and p53 and their co-recruitment to the Steaoryl-CoA Desaturase-1 locus play an important role to regulate monounsaturated fatty acids synthesis in adipocytes. Consistent with the role of this E4F1-p53-Steaoryl-CoA Desaturase-1 axis in adipocytes, p53 inactivation or diet complementation with oleate partly restore adiposity and improve insulin sensitivity in E4F1-deficient mice. Altogether, our findings identify a crosstalk between E4F1 and p53 in the control of lipid metabolism in adipocytes that is relevant to obesity and insulin resistance.

Atrial Natriuretic Peptide Orchestrates a Coordinated Physiological Response to Fuel Non-shivering Thermogenesis.

Atrial natriuretic peptide (ANP) is a cardiac hormone controlling blood volume and pressure in mammals. It is still unclear whether ANP controls cold-induced thermogenesis in vivo. Here, we show that acute cold exposure induces cardiac ANP secretion in mice and humans. Genetic inactivation of ANP promotes cold intolerance and suppresses half of cold-induced brown adipose tissue (BAT) activation in mice. While white adipocytes are resistant to ANP-mediated lipolysis at thermoneutral temperature in mice, cold exposure renders white adipocytes fully responsive to ANP to activate lipolysis and a thermogenic program, a physiological response that is dramatically suppressed in ANP null mice. ANP deficiency also blunts liver triglycerides and glycogen metabolism, thus impairing fuel availability for BAT thermogenesis. ANP directly increases mitochondrial uncoupling and thermogenic gene expression in human white and brown adipocytes. Together, these results indicate that ANP is a major physiological trigger of BAT thermogenesis upon cold exposure in mammals.

Interaction between hormone-sensitive lipase and ChREBP in fat cells controls insulin sensitivity.

Impaired adipose tissue insulin signalling is a critical feature of insulin resistance. Here we identify a pathway linking the lipolytic enzyme hormone-sensitive lipase (HSL) to insulin action via the glucose-responsive transcription factor ChREBP and its target, the fatty acid elongase ELOVL6. Genetic inhibition of HSL in human adipocytes and mouse adipose tissue results in enhanced insulin sensitivity and induction of ELOVL6. ELOVL6 promotes an increase in phospholipid oleic acid, which modifies plasma membrane fluidity and enhances insulin signalling. HSL deficiency-mediated effects are suppressed by gene silencing of ChREBP and ELOVL6. Mechanistically, physical interaction between HSL, independent of lipase activity, and the isoform activated by glucose metabolism ChREBPα impairs ChREBPα translocation into the nucleus and induction of ChREBPß, the isoform with high transcriptional activity that is strongly associated with whole-body insulin sensitivity. Targeting the HSL-ChREBP interaction may allow therapeutic strategies for the restoration of insulin sensitivity.

Natriuretic peptides promote glucose uptake in a cGMP-dependent manner in human adipocytes.

Robust associations between low plasma level of natriuretic peptides (NP) and increased risk of type 2 diabetes (T2D) have been recently reported in humans. Adipose tissue (AT) is a known target of NP. However it is unknown whether NP signalling in human AT relates to insulin sensitivity and modulates glucose metabolism. We here show in two European cohorts that the NP receptor guanylyl cyclase-A (GC-A) expression in subcutaneous AT was down-regulated as a function of obesity grade while adipose NP clearance receptor (NPRC) was up-regulated. Adipose GC-A mRNA level was down-regulated in prediabetes and T2D, and negatively correlated with HOMA-IR and fasting blood glucose. We show for the first time that NP promote glucose uptake in a dose-dependent manner. This effect is reduced in adipocytes of obese individuals. NP activate mammalian target of rapamycin complex 1/2 (mTORC1/2) and Akt signalling. These effects were totally abrogated by inhibition of cGMP-dependent protein kinase and mTORC1/2 by rapamycin. We further show that NP treatment favoured glucose oxidation and de novo lipogenesis independently of significant gene regulation. Collectively, our data support a role for NP in blood glucose control and insulin sensitivity by increasing glucose uptake in human adipocytes. This effect is partly blunted in obesity.

Mitochondrial fission is associated with UCP1 activity in human brite/beige adipocytes.

OBJECTIVE: Thermogenic adipocytes (i.e. brown or brite/beige adipocytes) are able to burn large amounts of lipids and carbohydrates as a result of highly active mitochondria and enhanced uncoupled respiration, due to UCP1 activity. Although mitochondria are the key organelles for this thermogenic function, limited human data are available. METHODS/RESULTS: We characterized changes in the mitochondrial function of human brite adipocytes, using hMADS cells as a model of white- to brite-adipocyte conversion. We found that profound molecular modifications were associated with morphological changes in mitochondria. The fission process was partly driven by the DRP1 protein, which also promoted mitochondrial uncoupling. CONCLUSION: Our data demonstrate that white-to-brite conversion of human adipocytes relies on molecular, morphological and functional changes in mitochondria, which enable brite/beige cells to carry out thermogenesis.

Perilipin 5 fine-tunes lipid oxidation to metabolic demand and protects against lipotoxicity in skeletal muscle.

Lipid droplets (LD) play a central role in lipid homeostasis by controlling transient fatty acid (FA) storage and release from triacylglycerols stores, while preventing high levels of cellular toxic lipids. This crucial function in oxidative tissues is altered in obesity and type 2 diabetes. Perilipin 5 (PLIN5) is a LD protein whose mechanistic and causal link with lipotoxicity and insulin resistance has raised controversies. We investigated here the physiological role of PLIN5 in skeletal muscle upon various metabolic challenges. We show that PLIN5 protein is elevated in endurance-trained (ET) subjects and correlates with muscle oxidative capacity and whole-body insulin sensitivity. When overexpressed in human skeletal muscle cells to recapitulate the ET phenotype, PLIN5 diminishes lipolysis and FA oxidation under basal condition, but paradoxically enhances FA oxidation during forskolin- and contraction- mediated lipolysis. Moreover, PLIN5 partly protects muscle cells against lipid-induced lipotoxicity. In addition, we demonstrate that down-regulation of PLIN5 in skeletal muscle inhibits insulin-mediated glucose uptake under normal chow feeding condition, while paradoxically improving insulin sensitivity upon high-fat feeding. These data highlight a key role of PLIN5 in LD function, first by finely adjusting LD FA supply to mitochondrial oxidation, and second acting as a protective factor against lipotoxicity in skeletal muscle.

G0/G1 Switch Gene 2 controls adipose triglyceride lipase activity and lipid metabolism in skeletal muscle.

OBJECTIVE: Recent data suggest that adipose triglyceride lipase (ATGL) plays a key role in providing energy substrate from triglyceride pools and that alterations of its expression/activity relate to metabolic disturbances in skeletal muscle. Yet little is known about its regulation. We here investigated the role of the protein G0/G1 Switch Gene 2 (G0S2), recently described as an inhibitor of ATGL in white adipose tissue, in the regulation of lipolysis and oxidative metabolism in skeletal muscle. METHODS: We first examined G0S2 protein expression in relation to metabolic status and muscle characteristics in humans. We next overexpressed and knocked down G0S2 in human primary myotubes to assess its impact on ATGL activity, lipid turnover and oxidative metabolism, and further knocked down G0S2 in vivo in mouse skeletal muscle. RESULTS: G0S2 protein is increased in skeletal muscle of endurance-trained individuals and correlates with markers of oxidative capacity and lipid content. Recombinant G0S2 protein inhibits ATGL activity by about 40% in lysates of mouse and human skeletal muscle. G0S2 overexpression augments (+49%, p < 0.05) while G0S2 knockdown strongly reduces (-68%, p < 0.001) triglyceride content in human primary myotubes and mouse skeletal muscle. We further show that G0S2 controls lipolysis and fatty acid oxidation in a strictly ATGL-dependent manner. These metabolic adaptations mediated by G0S2 are paralleled by concomitant changes in glucose metabolism through the modulation of Pyruvate Dehydrogenase Kinase 4 (PDK4) expression (5.4 fold, p < 0.001). Importantly, downregulation of G0S2 in vivo in mouse skeletal muscle recapitulates changes in lipid metabolism observed in vitro. CONCLUSION: Collectively, these data indicate that G0S2 plays a key role in the regulation of skeletal muscle ATGL activity, lipid content and oxidative metabolism.

Stress of endoplasmic reticulum modulates differentiation and lipogenesis of human adipocytes.

BACKGROUND: Adipocytes are cells specialized for storage of neutral lipids. This storage capacity is dependent on lipogenesis and is diminished in obesity. The reason for the decline in lipogenic activity of adipocytes in obesity remains unknown. Recent data show that lipogenesis in liver is regulated by pathways initiated by endoplasmic reticulum stress (ERS). Thus, we aimed at investigating the effect of ERS on lipogenesis in adipose cells. METHODS: Preadipocytes were isolated from subcutaneous abdominal adipose tissue from obese volunteers and in vitro differentiated into adipocytes. ERS was induced pharmacologically by thapsigargin (TG) or tunicamycin (TM). Activation of Unfolded Protein Response pathway (UPR) was monitored on the level of eIF2α phosphorylation and mRNA expression of downstream targets of UPR sensors. Adipogenic and lipogenic capacity was evaluated by Oil Red O staining, measurement of incorporation of radio-labelled glucose or acetic acid into lipids and mRNA analysis of adipogenic/lipogenic markers. RESULTS: Exposition of adipocytes to high doses of TG (100 nM) and TM (1 µg/ml) for 1-24 h enhanced expression of several UPR markers (HSPA5, EDEM1, ATF4, XBP1s) and phosphorylation of eIF2α. This acute ERS substantially inhibited expression of lipogenic genes (DGAT2, FASN, SCD1) and glucose incorporation into lipids. Moreover, chronic exposure of preadipocytes to low dose of TG (2.5 nM) during the early phases of adipogenic conversion of preadipocytes impaired both, lipogenesis and adipogenesis. On the other hand, chronic low ERS had no apparent effect on lipogenesis in mature adipocytes. CONCLUSIONS: Acute ERS weakened a capacity of mature adipocytes to store lipids and chronic ERS diminished adipogenic potential of preadipocytes.

Positive interaction between prebiotics and thiazolidinedione treatment on adiposity in diet-induced obese mice.

OBJECTIVES: To investigate whether inulin-type fructan (ITF) prebiotics could counteract the thiazolidinedione (TZD, PPARγ activator) induced-fat mass gain, without affecting its beneficial effect on glucose homeostasis, in high-fat (HF) diet fed mice. METHODS: Male C57bl6/J mice were fed a HF diet alone or supplemented with ITF prebiotics (0.2 g/day × mouse) or TZD (30 mg pioglitazone (PIO)/kg body weight × day) or both during 4 weeks. An insulin tolerance test was performed after 3 weeks of treatment. RESULTS: As expected, PIO improved glucose homeostasis and increased adiponectinaemia. Furthermore, it induced an over-expression of several PPARγ target genes in white adipose tissues. ITF prebiotics modulated the PIO-induced PPARγ activation in a tissue-dependent manner. The co-treatment with ITF prebiotics and PIO maintained the beneficial impact of TZD on glucose homeostasis and adiponectinaemia. Moreover, the combination of both treatments reduced fat mass accumulation, circulating lipids and hepatic triglyceride content, suggesting an overall improvement of metabolism. Finally, the co-treatment favored induction of white-to-brown fat conversion in subcutaneous adipose tissue, thereby leading to the development of brite adipocytes that could increase the oxidative capacity of the tissue. CONCLUSIONS: ITF prebiotics decrease adiposity and improve the metabolic response in HF fed mice treated with TZD.

High-fat diet-mediated lipotoxicity and insulin resistance is related to impaired lipase expression in mouse skeletal muscle.

Elevated expression/activity of adipose triglyceride lipase (ATGL) and/or reduced activity of hormone-sensitive lipase (HSL) in skeletal muscle are causally linked to insulin resistance in vitro. We investigated here the effect of high-fat feeding on skeletal muscle lipolytic proteins, lipotoxicity, and insulin signaling in vivo. Five-week-old C3H mice were fed normal chow diet (NCD) or 45% kcal high-fat diet (HFD) for 4 weeks. Wild-type and HSL knockout mice fed NCD were also studied. Whole-body and muscle insulin sensitivity, as well as lipolytic protein expression, lipid levels, and insulin signaling in skeletal muscle, were measured. HFD induced whole-body insulin resistance and glucose intolerance and reduced skeletal muscle glucose uptake compared with NCD. HFD increased skeletal muscle total diacylglycerol (DAG) content, protein kinase Cθ and protein kinase Cε membrane translocation, and impaired insulin signaling as reflected by a robust increase of basal Ser1101 insulin receptor substrate 1 phosphorylation (2.8-fold, P < .05) and a decrease of insulin-stimulated v-Akt murine thymoma viral oncogene homolog Ser473 (-37%, P < .05) and AS160 Thr642 (-47%, P <.01) phosphorylation. We next showed that HFD strongly reduced HSL phosphorylation at Ser660. HFD significantly up-regulated the muscle protein content of the ATGL coactivator comparative gene identification 58 and triacylglycerol hydrolase activity, despite a lower ATGL protein content. We further show a defective skeletal muscle insulin signaling and DAG accumulation in HSL knockout compared with wild-type mice. Together, these data suggest a pathophysiological link between altered skeletal muscle lipase expression and DAG-mediated insulin resistance in mice.

Partial inhibition of adipose tissue lipolysis improves glucose metabolism and insulin sensitivity without alteration of fat mass.

When energy is needed, white adipose tissue (WAT) provides fatty acids (FAs) for use in peripheral tissues via stimulation of fat cell lipolysis. FAs have been postulated to play a critical role in the development of obesity-induced insulin resistance, a major risk factor for diabetes and cardiovascular disease. However, whether and how chronic inhibition of fat mobilization from WAT modulates insulin sensitivity remains elusive. Hormone-sensitive lipase (HSL) participates in the breakdown of WAT triacylglycerol into FAs. HSL haploinsufficiency and treatment with a HSL inhibitor resulted in improvement of insulin tolerance without impact on body weight, fat mass, and WAT inflammation in high-fat-diet-fed mice. In vivo palmitate turnover analysis revealed that blunted lipolytic capacity is associated with diminution in FA uptake and storage in peripheral tissues of obese HSL haploinsufficient mice. The reduction in FA turnover was accompanied by an improvement of glucose metabolism with a shift in respiratory quotient, increase of glucose uptake in WAT and skeletal muscle, and enhancement of de novo lipogenesis and insulin signalling in liver. In human adipocytes, HSL gene silencing led to improved insulin-stimulated glucose uptake, resulting in increased de novo lipogenesis and activation of cognate gene expression. In clinical studies, WAT lipolytic rate was positively and negatively correlated with indexes of insulin resistance and WAT de novo lipogenesis gene expression, respectively. In obese individuals, chronic inhibition of lipolysis resulted in induction of WAT de novo lipogenesis gene expression. Thus, reduction in WAT lipolysis reshapes FA fluxes without increase of fat mass and improves glucose metabolism through cell-autonomous induction of fat cell de novo lipogenesis, which contributes to improved insulin sensitivity.

Regulation of skeletal muscle lipolysis and oxidative metabolism by the co-lipase CGI-58.

We investigated here the specific role of CGI-58 in the regulation of energy metabolism in skeletal muscle. We first examined CGI-58 protein expression in various muscle types in mice, and next modulated CGI-58 expression during overexpression and knockdown studies in human primary myotubes and evaluated the consequences on oxidative metabolism. We observed a preferential expression of CGI-58 in oxidative muscles in mice consistent with triacylglycerol hydrolase activity. We next showed by pulse-chase that CGI-58 overexpression increased by more than 2-fold the rate of triacylglycerol (TAG) hydrolysis, as well as TAG-derived fatty acid (FA) release and oxidation. Oppositely, CGI-58 silencing reduced TAG hydrolysis and TAG-derived FA release and oxidation (-77%, P < 0.001), whereas it increased glucose oxidation and glycogen synthesis. Interestingly, modulations of CGI-58 expression and FA release are reflected by changes in pyruvate dehydrogenase kinase 4 gene expression. This regulation involves the activation of the peroxisome proliferator activating receptor-δ (PPARδ) by lipolysis products. Altogether, these data reveal that CGI-58 plays a limiting role in the control of oxidative metabolism by modulating FA availability and the expression of PPARδ-target genes, and highlight an important metabolic function of CGI-58 in skeletal muscle.

Altered skeletal muscle lipase expression and activity contribute to insulin resistance in humans.

OBJECTIVE: Insulin resistance is associated with elevated content of skeletal muscle lipids, including triacylglycerols (TAGs) and diacylglycerols (DAGs). DAGs are by-products of lipolysis consecutive to TAG hydrolysis by adipose triglyceride lipase (ATGL) and are subsequently hydrolyzed by hormone-sensitive lipase (HSL). We hypothesized that an imbalance of ATGL relative to HSL (expression or activity) may contribute to DAG accumulation and insulin resistance. RESEARCH DESIGN AND METHODS: We first measured lipase expression in vastus lateralis biopsies of young lean (n = 9), young obese (n = 9), and obese-matched type 2 diabetic (n = 8) subjects. We next investigated in vitro in human primary myotubes the impact of altered lipase expression/activity on lipid content and insulin signaling. RESULTS: Muscle ATGL protein was negatively associated with whole-body insulin sensitivity in our population (r = -0.55, P = 0.005), whereas muscle HSL protein was reduced in obese subjects. We next showed that adenovirus-mediated ATGL overexpression in human primary myotubes induced DAG and ceramide accumulation. ATGL overexpression reduced insulin-stimulated glycogen synthesis (-30%, P < 0.05) and disrupted insulin signaling at Ser1101 of the insulin receptor substrate-1 and downstream Akt activation at Ser473. These defects were fully rescued by nonselective protein kinase C inhibition or concomitant HSL overexpression to restore a proper lipolytic balance. We show that selective HSL inhibition induces DAG accumulation and insulin resistance. CONCLUSIONS: Altogether, the data indicate that altered ATGL and HSL expression in skeletal muscle could promote DAG accumulation and disrupt insulin signaling and action. Targeting skeletal muscle lipases may constitute an interesting strategy to improve insulin sensitivity in obesity and type 2 diabetes.

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.

Skeletal muscle lipase content and activity in obesity and type 2 diabetes.

CONTEXT: The obese insulin-resistant state is characterized by elevated lipid storage in skeletal muscle tissue. OBJECTIVE: We tested whether differences in muscle triacylglycerol (TAG) and diacylglycerol (DAG) lipase content and activity are associated with incomplete in vivo lipolysis and lipid accumulation. DESIGN AND PATIENTS: Two case-control studies were conducted on skeletal muscle biopsies from lean (n=13) and obese (n=10) men (study 1) and from 11 nonobese type 2 diabetic (T2D), obese T2D, and healthy normoglycemic men (study 2). MAIN OUTCOME MEASURES: Skeletal muscle lipase protein content and activity and muscle lipid content (TAG and DAG) were determined. RESULTS: Skeletal muscle hormone-sensitive lipase protein content was lower (0.39±0.07 vs. 1.00±0.19 arbitrary units; P=0.004) and adipose triglyceride lipase protein content was higher in obese men compared with lean controls (2.17±0.40 vs. 0.42±0.23 arbitrary units; P=0.008). This apparent difference in lipase content was accompanied by a 60% lower ratio of DAG to TAG hydrolase activity in the obese men (11.4±2.3 vs. 26.5±7.3 nmol/h·mg; P=0.045), implying incomplete lipolysis. Lower hormone-sensitive lipase and higher adipose triglyceride lipase content was confined to obesity per se, because it was observed solely in obese T2D men but not in healthy normoglycemic controls and nonobese T2D men. Muscle total DAG content was not higher in obese men but was even lower (6.2±0.7 vs. 9.4±0.9 µmol/mg dry weight; P=0.017). TAG content did not differ between groups (84.7±18.9 vs. 70.4±12.4 µmol/mg dry weight; P=0.543). CONCLUSIONS: Our data do not support an important role of total muscle DAG content in the development of insulin resistance in obese men.

Peroxisome proliferator-activated receptor-alpha control of lipid and glucose metabolism in human white adipocytes.

This work aimed at characterizing the role of peroxisome proliferator-activated receptors (PPAR)alpha in human white adipocyte metabolism and at comparing PPAR alpha and PPAR gamma actions in these cells. Primary cultures of human fat cells were treated with the PPAR alpha agonist GW7647 or the PPAR gamma agonist rosiglitazone. Changes in gene expression were determined using DNA microarrays and quantitative RT-PCR. Western blot and metabolic studies were performed to identify the biological effects elicited by PPAR agonist treatments. GW7647 induced an up-regulation of beta-oxidation gene expression and increased palmitate oxidation. Unexpectedly, glycolysis was strongly reduced at transcriptional and functional levels by GW7647 leading to a decrease in pyruvate and lactate production. Glucose oxidation was decreased. Triglyceride esterification and de novo lipogenesis were inhibited by the PPAR alpha agonist. GW7647-induced alterations were abolished by a treatment with a PPAR alpha antagonist. Small interfering RNA-mediated extinction of PPAR alpha gene expression in hMADS adipocytes attenuated GW7647 induction of palmitate oxidation. Rosiglitazone had no major impact on glycolysis and beta-oxidation. Altogether these results show that PPAR alpha can selectively up-regulate beta-oxidation and decrease glucose utilization in human white adipocytes.

Chronic TNFalpha and cAMP pre-treatment of human adipocytes alter HSL, ATGL and perilipin to regulate basal and stimulated lipolysis.

We examined the effects of chronic TNFalpha and dibutyryl-cAMP (Db-cAMP) pre-treatment on the lipolytic machinery of human hMADS adipocytes. TNFalpha decreased adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) protein content and triglycerides (TG)-hydrolase activity but increased basal lipolysis due to a marked reduction in perilipin (PLIN) protein content. Conversely, Db-cAMP increased ATGL and HSL protein content but prevented PLIN phosphorylation, the net result being accentuated basal lipolysis. In forskolin-stimulated conditions, TNFalpha and Db-cAMP pre-treatment decreased stimulated TG-hydrolase activity and impaired PLIN phosphorylation. Together, this resulted in a severely attenuated response to forskolin-stimulated lipolysis.

Influence of gender, obesity, and muscle lipase activity on intramyocellular lipids in sedentary individuals.

CONTEXT: Obesity and type 2 diabetes are associated with elevated intramyocellular lipids (IMCLs) and insulin resistance. OBJECTIVE: We tested the hypothesis that skeletal muscle lipases activity could influence IMCL content (including diacylglycerol and ceramides). DESIGN AND PATIENTS: The present study included 48 subjects with a wide range of age (19-68 yr) and body mass index (20-45 kg/m(2)) who underwent skeletal muscle biopsy, dual-energy x-ray absorptiometry and a hyperinsulinemic euglycemic clamp. MAIN OUTCOME MEASURES: Insulin sensitivity by hyperinsulinemic clamp, and intramyocellular triacylglycerol (IMTG), diacylglycerol (DAG), and ceramides content, and triacylglycerol and diacylglycerol hydrolase activities were measured in biopsies of vastus lateralis. IMCL was measured by (1)H-magnetic resonance spectroscopy in a subgroup of 25 subjects. Multivariate regression analyses were performed to identify the main predictors of IMCL. RESULTS: Body fat was the main predictor of IMTG independently of the method and the type of muscle; IMTG concentration was higher in females vs. males and obese vs. nonobese subjects. Muscle DAG and ceramides concentrations were elevated in obese and type 2 diabetic subjects and were not related to body fat and fasting free fatty acids, whereas a direct association with the ratio of diacylglycerol hydrolase to triacylglycerol hydrolase activity (an index of incomplete triacylglycerol hydrolysis) was observed, which explained 54 and 38% of the variance in DAG and ceramides (P < 0.001), respectively. DAG content was the main determinant of insulin resistance. CONCLUSIONS: These data suggest that intramyocellular DAG is an independent predictor of insulin resistance in humans and that its levels correlate with lipolytic enzymes activity in skeletal muscle but not with markers of adiposity.

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.

ApoB100-LDL acts as a metabolic signal from liver to peripheral fat causing inhibition of lipolysis in adipocytes.

BACKGROUND: Free fatty acids released from adipose tissue affect the synthesis of apolipoprotein B-containing lipoproteins and glucose metabolism in the liver. Whether there also exists a reciprocal metabolic arm affecting energy metabolism in white adipose tissue is unknown. METHODS AND FINDINGS: We investigated the effects of apoB-containing lipoproteins on catecholamine-induced lipolysis in adipocytes from subcutaneous fat cells of obese but otherwise healthy men, fat pads from mice with plasma lipoproteins containing high or intermediate levels of apoB100 or no apoB100, primary cultured adipocytes, and 3T3-L1 cells. In subcutaneous fat cells, the rate of lipolysis was inversely related to plasma apoB levels. In human primary adipocytes, LDL inhibited lipolysis in a concentration-dependent fashion. In contrast, VLDL had no effect. Lipolysis was increased in fat pads from mice lacking plasma apoB100, reduced in apoB100-only mice, and intermediate in wild-type mice. Mice lacking apoB100 also had higher oxygen consumption and lipid oxidation. In 3T3-L1 cells, apoB100-containing lipoproteins inhibited lipolysis in a dose-dependent fashion, but lipoproteins containing apoB48 had no effect. ApoB100-LDL mediated inhibition of lipolysis was abolished in fat pads of mice deficient in the LDL receptor (Ldlr(-/-)Apob(100/100)). CONCLUSIONS: Our results show that the binding of apoB100-LDL to adipocytes via the LDL receptor inhibits intracellular noradrenaline-induced lipolysis in adipocytes. Thus, apoB100-LDL is a novel signaling molecule from the liver to peripheral fat deposits that may be an important link between atherogenic dyslipidemias and facets of the metabolic syndrome.