Overexpression Of Adipose Triglyceride Lipase Research Articles

Effects of Chrysoeriol on Adipogenesis and Lipolysis in 3T3-L1 Adipocytes

We examined the effect of chrysoeriol on adipogenesis and lipolysis and elucidated the underlying molecular mechanisms. Chrysoeriol inhibited fat deposition in adipocytes. Treatment with chrysoeriol suppressed the expression of peroxisome proliferator-activated receptor γ, fatty acid synthase, fatty acid-binding protein, CCAAT/enhancer-binding proteins (C/EBP) α, C/EBPβ, and sterol regulatory element-binding protein-1. In addition, chrysoeriol significantly elevated the activation of 5'-adenosine monophosphate-activated protein kinase. Moreover, chrysoeriol increased free glycerol and fatty acid levels and promoted lipolysis in adipocytes. Overexpression of adipose triglyceride lipase and hormone-sensitive lipase by chrysoeriol led to increased lipolysis in 3T3-L1 adipocytes. Taken together, chrysoeriol showed anti-adipogenic and lipolytic properties in adipocytes.

Overexpression of ATGL impairs lipid droplet accumulation by accelerating lipolysis in goat mammary epithelial cells

Adipose triglyceride lipase (ATGL) is the key enzyme for the degradation of triacylglycerols (TAGs). It functions in concert with other enzymes to mobilize TAG and supply fatty acids (FAs) for energy production. Dysregulated lipolysis leads to excess concentrations of circulating FAs, which may lead to destructive and lipotoxic effects to the organism. To understand the role of ATGL in mammary lipid metabolism, ATGL was overexpressed in goat mammary epithelial cells (GMECs) by using a recombinant adenovirus system. ATGL overexpression decreased lipid droplet (LD) accumulation and cellular TG content (p < 0.05) along with a decrease in the expression of the key enzyme that catalyzes the final step of TG synthesis (DGAT). Significant increases were observed in the expression of genes related to lipolysis (hormone-sensitive lipase [HSL]) and FA desaturation (SCD) by ATGL overexpression. Genes responsible for FA oxidation (PPARα), LD formation and secretion (ADRP and BTN1A1), and long-chain FA uptake (CD36) were all decreased by ATGL overexpression (p < 0.05). The primary products of TAG lipolysis, free FAs (FFAs), were notably increased in the ATGL-overexpressing cells. Taken together, our results demonstrated that ATGL activation impairs lipid formation partially through accelerating lipolysis in GMECs.

Adipose triglyceride lipase protects renal cell endocytosis in a Drosophila dietary model of chronic kidney disease

Obesity-related renal lipotoxicity and chronic kidney disease (CKD) are prevalent pathologies with complex aetiologies. One hallmark of renal lipotoxicity is the ectopic accumulation of lipid droplets in kidney podocytes and in proximal tubule cells. Renal lipid droplets are observed in human CKD patients and in high-fat diet (HFD) rodent models, but their precise role remains unclear. Here, we establish a HFD model in Drosophila that recapitulates renal lipid droplets and several other aspects of mammalian CKD. Cell type-specific genetic manipulations show that lipid can overflow from adipose tissue and is taken up by renal cells called nephrocytes. A HFD drives nephrocyte lipid uptake via the multiligand receptor Cubilin (Cubn), leading to the ectopic accumulation of lipid droplets. These nephrocyte lipid droplets correlate with endoplasmic reticulum (ER) and mitochondrial deficits, as well as with impaired macromolecular endocytosis, a key conserved function of renal cells. Nephrocyte knockdown of diglyceride acyltransferase 1 (DGAT1), overexpression of adipose triglyceride lipase (ATGL), and epistasis tests together reveal that fatty acid flux through the lipid droplet triglyceride compartment protects the ER, mitochondria, and endocytosis of renal cells. Strikingly, boosting nephrocyte expression of the lipid droplet resident enzyme ATGL is sufficient to rescue HFD-induced defects in renal endocytosis. Moreover, endocytic rescue requires a conserved mitochondrial regulator, peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC1α). This study demonstrates that lipid droplet lipolysis counteracts the harmful effects of a HFD via a mitochondrial pathway that protects renal endocytosis. It also provides a genetic strategy for determining whether lipid droplets in different biological contexts function primarily to release beneficial or to sequester toxic lipids.

ROS-dependent HIF1\u03b1 activation under forced lipid catabolism entails glycolysis and mitophagy as mediators of higher proliferation rate in cervical cancer cells

BackgroundIn the last decades, the concept of metabolic rewiring as a cancer hallmark has been expanded beyond the “Warburg effect” and the importance of other metabolic routes, including lipid metabolism, has emerged. In cancer, lipids are not only a source of energy but are also required for the formation of membranes building blocks, signaling and post-translational modification of proteins. Since lipid metabolism contributes to the malignancy of cancer cells, it is an attractive target for therapeutic strategies.MethodsOver-expression of the adipose triglyceride lipase (ATGL) was used to boost lipid catabolism in cervical cancer cells. The cervical cancer cell line HeLa was employed as the primary experimental model for all subsequent studies. The lipolytic activity of ATGL was mimicked by caproate, a short-chain fatty acid that is efficiently oxidized in mitochondria.ResultsHere, we provide evidence of the association between boosted lipid catabolism and the increased proliferation and migration capability of cervical cancer cells. These pro-tumoral effects were ascribed to the reactive oxygen species (ROS)-mediated induction of hypoxia-inducible factor-1α (HIF1α) triggered by the increased mitochondrial fatty acids (FAs) oxidation. HIF1α activation increases glycolytic flux and lactate production, promoting cell proliferation. At the same time, HIF1α increases protein and mRNA levels of its known target BCL2 and adenovirus E1B 19-kDa-interacting protein 3 (BNIP3), which in turn activates mitophagy as a pro-survival process, as demonstrated by the induction of apoptosis upon inhibition of mitophagy. These effects were mimicked by the short-chain fatty acid caproate, confirming that forcing lipid catabolism results in HIF1α induction.ConclusionsBoosting lipid catabolism by ATGL over-expression has a pro-tumor role in cervical cancer cells, dependent on ROS production and HIF1α induction. Together with the bioinformatics evidence of the correlation of ATGL activity with the aggressiveness of cervical cancer cells, our data suggest that ATGL could be a promising prognostic marker for cervical cancer and highlight the need of further investigations on the role of this lipase in cancer cells. This evidence could be exploited to develop new personalized therapy, based on the functionality of the antioxidant equipment of cancer cells, considering that ROS content could affect ATGL role.

Adipose triglyceride lipase promotes the proliferation of colorectal cancer cells via enhancing the lipolytic pathway.

Abnormal lipid metabolism is the sign of tumour cells. Previous researches have revealed that the lipolytic pathway may contribute to the progression of colorectal cancer (CRC). However, adipose triglyceride lipase (ATGL) role in CRC cells remains unclear. Here, we find that elevated ATGL positively correlates with CRC clinical stages and negatively associates with overall survival. Overexpression of ATGL significantly promotes CRC cell proliferation, while knockdown of ATGL inhibits the proliferation and promotes the apoptosis of CRC cells in vitro. Moreover, in vivo experiments, ATGL promotes the growth of CRC cells. Mechanistically, ATGL enhances the carcinogenic function of CRC cells via promoting sphingolipid metabolism and CoA biosynthesis pathway‐related gene levels by degrading triglycerides, which provides adequate nutrition for the progression of CRC. Our researches clarify for the first time that ATGL is a novel oncogene in CRC and may provide an important prognostic factor and therapeutic target for CRC.

ATGL activity regulates GLUT1-mediated glucose uptake and lactate production via TXNIP stability in adipocytes

Traditionally, lipolysis has been regarded as an enzymatic activity that liberates fatty acids as metabolic fuel. However, recent work has shown that novel substrates, including a variety of lipid compounds such as fatty acids and their derivatives, release lipolysis products that act as signaling molecules and transcriptional modulators. While these studies have expanded the role of lipolysis, the mechanisms underpinning lipolysis signaling are not fully defined. Here, we uncover a new mechanism regulating glucose uptake, whereby activation of lipolysis, in response to elevated cAMP, leads to the stimulation of thioredoxin-interacting protein (TXNIP) degradation. This, in turn, selectively induces glucose transporter 1 surface localization and glucose uptake in 3T3-L1 adipocytes and increases lactate production. Interestingly, cAMP-induced glucose uptake via degradation of TXNIP is largely dependent upon adipose triglyceride lipase (ATGL) and not hormone-sensitive lipase or monoacylglycerol lipase. Pharmacological inhibition or knockdown of ATGL alone prevents cAMP-dependent TXNIP degradation and thus significantly decreases glucose uptake and lactate secretion. Conversely, overexpression of ATGL amplifies the cAMP response, yielding increased glucose uptake and lactate production. Similarly, knockdown of TXNIP elicits enhanced basal glucose uptake and lactate secretion, and increased cAMP further amplifies this phenotype. Overexpression of TXNIP reduces basal and cAMP-stimulated glucose uptake and lactate secretion. As a proof of concept, we replicated these findings in human primary adipocytes and observed TXNIP degradation and increased glucose uptake and lactate secretion upon elevated cAMP signaling. Taken together, our results suggest a crosstalk between ATGL-mediated lipolysis and glucose uptake.

Adipose triglyceride lipase activity regulates cancer cell proliferation via AMP-kinase and mTOR signaling

Aberrant fatty acid (FA) metabolism is a hallmark of proliferating cells, including untransformed fibroblasts or cancer cells. Lipolysis of intracellular triglyceride (TG) stores by adipose triglyceride lipase (ATGL) provides an important source of FAs serving as energy substrates, signaling molecules, and precursors for membrane lipids. To investigate if ATGL-mediated lipolysis impacts cell proliferation, we modified ATGL activity in murine embryonic fibroblasts (MEFs) and in five different cancer cell lines to determine the consequences on cell growth and metabolism. Genetic or pharmacological inhibition of ATGL in MEFs causes impaired FA oxidation, decreased ROS production, and a substrate switch from FA to glucose leading to decreased AMPK-mTOR signaling and higher cell proliferation rates. ATGL expression in these cancer cells is low when compared to MEFs. Additional ATGL knockdown in cancer cells did not significantly affect cellular lipid metabolism or cell proliferation whereas the ectopic overexpression of ATGL increased lipolysis and reduced proliferation. In contrast to ATGL silencing, pharmacological inhibition of ATGL by Atglistatin© impeded the proliferation of diverse cancer cell lines, which points at an ATGL-independent effect. Our data indicate a crucial role of ATGL-mediated lipolysis in the regulation of cell proliferation. The observed low ATGL activity in cancer cells may represent an evolutionary selection process and mechanism to sustain high cell proliferation rates. As the increasing ATGL activity decelerates proliferation of five different cancer cell lines this may represent a novel therapeutic strategy to counteract uncontrolled cell growth.

Effect and mechanism of cardiac adipose triglyceride lipase overexpression on burn-induced cardiac injury

Objective: To explore the effect and potential mechanism of cardiac adipose triglyceride lipase (ATGL) overexpression on burn-induced cardiac injury. Methods: Eight-week-old C57BL/6J mice with cardiac ATGL overexpression driven by the myosin heavy chain (MHC) promoter (MHC-ATGL burn group) and wild-type (wild-type burn group) mice were randomly chose to the following experiments with burn injury after 24 h (n=8/group), MHC-ATGL mice and wild-type mice with corresponded age and sex were included as control. Cardiac ATGL protein expression, serum levels of cardiac troponin T and cardiac kinase-MB (CK-MB), cardiac free fatty acid and reactive oxygen species were detected. The wild-type and MHC-ATGL burn groups were not only compared with their corresponded control groups, but also compared between each other. Results: The hair color and development were shown little difference between each group. ATGL protein expression is elevated in wild-type burn group (1.00±0.68 vs 3.09±0.93, P=0.023) and decreased in MHC-ATGL burn group (17.84±2.41 vs 10.36±2.22, P<0.001), while ATGL protein expression is still increased in MHC-ATGL burn group compared with wild-type burn group (P<0.001). Serum levels of cardiac troponin T and CK-MB were both elevated in wild-type burn group and MHC-ATGL burn group [(0.456±0.131) vs (0.076±0.019) μg/L and (0.219±0.089) vs (0.060±0.019) μg/L, (1 421±162) vs (221±67) U/L and (761±142) vs (221±41) U/L] (all P<0.001), while serum levels of cardiac troponin T and CK-MB was still decreased in MHC-ATGL burn group compared with wild-type burn group (P<0.001). In addition, cardiac free fatty acid was increased in wild-type burn group and little difference was found in MHC-ATGL burn group [(2.54±0.51) vs (0.46±0.27) mmol/L, P<0.001, and (0.81±0.38) vs (0.59±0.25) mmol/L, P=0.251], while cardiac free fatty acid was significant reduction in MHC-ATGL burn group compared with wild-type burn group (P<0.001). Levels of cardiac reactive oxygen species was both elevated in wild-type burn group and MHC-ATGL burn group [(1.89±0.23) vs (1.00±0.18) and (1.38±0.17) vs (0.95±0.13)] (both P<0.001), while levels of cardiac reactive oxygen was reduction in MHC-ATGL burn group compared with wild-type burn group (P<0.001). Conclusion: Cardiac ATGL overexpression may protect against burn-induced cardiac injury through reducing free fatty acid and reactive oxygen species production.

Oroxylin A regulates the turnover of lipid droplet via downregulating adipose triglyceride lipase (ATGL) in hepatic stellate cells

Proliferation and differentiation of hepatic stellate cells (HSCs) are the most noticeable events in hepatic fibrosis, in which the loss of lipid droplets (LDs) is the most important feature. However, the complex mechanisms of LD disappearance have not been fully elucidated. In the current study, we investigated whether oroxylin A has the pharmacological activity of reversing LDs in activated HSCs, and further examined its potential molecular mechanisms. Using genetic, pharmacological, and molecular biological measure, we found that LD content significantly decreased during HSC activation, whereas oroxylin A markedly reversed LD content in activated HSCs. Interestingly, oroxylin A treatment observably decreased the expression of adipose triglyceride lipase (ATGL) without large differences in classical LD synthesis pathway, LD-related transcription factors, and autophagy pathway. ATGL overexpression could completely impair the effect of oroxylin A on reversing LD content. Importantly, reactive oxygen species (ROS) signaling pathway mediated oroxylin A-induced ATGL downregulation and LD revision in activated HSCs. ROS specific stimulant buthionine sulfoximine (BSO) could dramatically diminish the antioxidant effect of oroxylin A, and in turn, abolish reversal effect of oroxylin A on LD content. Conversely, ROS specific scavenger N-acetyl cystenine (NAC) can significantly enhance the pharmacological effect of oroxylin A on LD revision. Taken together, our study reveals the important molecular mechanism of anti-fibrosis effect of oroxylin A, and also suggests that ROS-ATGL pathway is a potential target for reversing LDs.

ATGL promotes the proliferation of hepatocellular carcinoma cells via the p-AKT signaling pathway.

Abnormal metabolism, including abnormal lipid metabolism, is a hallmark of cancer cells. Some studies have demonstrated that the lipogenic pathway might promote the development of hepatocellular carcinoma (HCC). However, the role of adipose triglyceride lipase (ATGL) in hepatocellular carcinoma cells has not been elucidated. We evaluated the function of ATGL in hepatocellular carcinoma using methyl azazolyl blue and migration assay through overexpression of ATGL in HepG2 cells. Quantitative reverse-transcription polymerase chain reaction and Western blot analyses were used to assess the mechanisms of ATGL in hepatocellular carcinoma. In the current study, we first constructed and transiently transfected ATGL into hepatocellular carcinoma cells. Secondly, we found that ATGL promoted the proliferation of hepatoma cell lines via upregulating the phosphorylation of AKT, but did not affect the metastatic ability of HCC cells. Moreover, the p-AKT inhibitor significantly eliminated the effect of ATGL on the proliferation of hepatoma carcinoma cells. Taken together, our results indicated that ATGL promotes hepatocellular carcinoma cells proliferation through upregulation of the AKT signaling pathway.

Myocardial Adipose Triglyceride Lipase Overexpression Protects against Burn-Induced Cardiac Lipid Accumulation and Injury

Maladaptive cardiac metabolism is a common trigger of cardiac lipid accumulation and cardiac injury under serious burn challenge. Adipose triglyceride lipase (ATGL) is the key enzyme that catalyzes triglyceride hydrolysis; however, its alteration and impact on cardiac function following serious burn injury are still unknown. Here, we found that the cardiac fatty acid (FA) metabolism increased, accompanied by augmented FA accumulation and ATGL expression, after serious burn injury. We generated heterozygous ATGL knockout and heterozygous cardiac-specific ATGL overexpression thermal burn mice. The results demonstrated that partial loss of ATGL could not relieve burn-induced cardiac lipid accumulation and cardiac injury, possibly due to the suppression of cardiac FA metabolism plus insufficient compensatory glucose utilization. In contrast, cardiac-specific overexpression of ATGL alleviated cardiac lipid accumulation and cardiac injury following burn challenge by switching the substrate preference from FA towards increased glucose utilization. The underlying mechanism was possibly related to increased glucose transporter-1 expression and reduced cardiac lipid accumulation induced by ATGL overexpression. Our data first demonstrated that elevated cardiac ATGL expression after serious burn injury is an adaptive, albeit insufficient, response to compensate for the increase in energy consumption and that further overexpression of ATGL is beneficial for ameliorating cardiac injury, indicating its therapeutic potential.

PNPLA3, CGI-58, and Inhibition of Hepatic Triglyceride Hydrolysis in Mice.

A variant (148M) in patatin‐like phospholipase domain‐containing protein 3 (PNPLA3) is a major risk factor for fatty liver disease. Despite its clinical importance, the pathogenic mechanism linking the variant to liver disease remains poorly defined. Previously, we showed that PNPLA3(148M) accumulates to high levels on hepatic lipid droplets (LDs). Here we examined the effect of that accumulation on triglyceride (TG) hydrolysis by adipose triglyceride lipase (ATGL), the major lipase in the liver. As expected, overexpression of ATGL in cultured hepatoma (HuH‐7) cells depleted the cells of LDs, but unexpectedly, co‐expression of PNPLA3(wild type [WT] or 148M) with ATGL inhibited that depletion. The inhibitory effect of PNPLA3 was not caused by the displacement of ATGL from LDs. We tested the hypothesis that PNPLA3 interferes with ATGL activity by interacting with its cofactor, comparative gene identification‐58 (CGI‐58). Evidence supporting such an interaction came from two findings. First, co‐expression of PNPLA3 and CGI‐58 resulted in LD depletion in cultured cells, but expression of PNPLA3 alone did not. Second, PNPLA3 failed to localize to hepatic LDs in liver‐specific Cgi‐58 knockout (KO) mice. Moreover, overexpression of PNPLA3(148M) increased hepatic TG levels in WT, but not in Cgi‐58 KO mice. Thus, the pro‐steatotic effects of PNPLA3 required the presence of CGI‐58. Co‐immunoprecipitation and pulldown experiments in livers of mice and in vitro using purified proteins provided evidence that PNPLA3 and CGI‐58 can interact directly. Conclusion: Taken together, these findings are consistent with a model in which PNPLA3(148M) promotes steatosis by CGI‐58‐dependent inhibition of ATGL on LDs.

Forcing ATGL expression in hepatocarcinoma cells imposes glycolytic rewiring through PPAR-\u03b1/p300-mediated acetylation of p53

Metabolic reprogramming is a typical feature of cancer cells aimed at sustaining high-energetic demand and proliferation rate. Here, we report clear-cut evidence for decreased expression of the adipose triglyceride lipase (ATGL), the first and rate-limiting enzyme of triglyceride hydrolysis, in both human and mouse-induced hepatocellular carcinoma (HCC). We identified metabolic rewiring as major outcome of ATGL overexpression in HCC-derived cell lines. Indeed, ATGL slackened both glucose uptake/utilization and cell proliferation in parallel with increased oxidative metabolism of fatty acids and enhanced mitochondria capacity. We ascribed these ATGL—downstream events to the activity of the tumor-suppressor p53, whose protein levels—but not transcript—were upregulated upon ATGL overexpression. The role of p53 was further assessed by abrogation of the ATGL-mediated effects upon p53 silencing or in p53-null hepatocarcinoma Hep3B cells. Furthermore, we provided insights on the molecular mechanisms governed by ATGL in HCC cells, identifying a new PPAR-α/p300 axis responsible for p53 acetylation/accumulation. Finally, we highlighted that ATGL levels confer different susceptibility of HCC cells to common therapeutic drugs, with ATGL overexpressing cells being more resistant to glycolysis inhibitors (e.g., 2-deoxyglucose and 3-bromopyruvate), compared to genotoxic compounds. Collectively, our data provide evidence for a previously uncovered tumor-suppressor function of ATGL in HCC, with the outlined molecular mechanisms shedding light on new potential targets for anticancer therapy.

PPARα-ATGL pathway improves muscle mitochondrial metabolism: implication in aging.

Adipose triglyceride lipase (ATGL) maintains an optimum mitochondrial function putatively by generating cognate ligands for peroxisome proliferator-activated receptor α (PPARα), which, together with PPARγ coactivator-1α (PGC1α), regulate muscle mitochondrial biogenesis. However, the cross-talk between ATGL and PPARα in skeletal muscle mitochondrial metabolism and its implication in chronological aging is poorly understood. The role of ATGL in muscle mitochondrial metabolism was studied by overexpressing and depleting the gene and studying its downstream effect in cultured myotubes and in murine skeletal muscle. We found that PPARα directly induces ATGL expression during myogenesis. Overexpression of ATGL significantly enhanced while depletion of ATGL attenuated mitochondrial oxidative phosphorylation and fatty acid oxidation without alteration in mitochondrial content, and it rendered PPARα and PGC1α redundant in promoting mitochondrial oxidative function. However, ATGL did not alter PPARα-dependent lipid accumulation and insulin sensitivity. In middle-aged rats, ATGL expression was higher and correlated with PPARα expression and sustained fatty acid oxidation in oxidative soleus muscle. Fenofibrate feeding further induced ATGL expression selectively in this muscle compartment. These findings illustrate that PPARα and ATGL constitute a regulatory pathway in skeletal muscle, suggesting their role as a mitochondrial metabolic reserve.-Biswas, D., Ghosh, M., Kumar, S., Chakrabarti, P. PPARα-ATGL pathway improves muscle mitochondrial metabolism: implication in aging.

Hepatic ATGL manipulation regulates lipid metabolism through altered autophagy

Adipose triglyceride lipase (ATGL) catalyzes the rate‐limiting step of triacylglycerol (TAG) hydrolysis in multiple tissues. Our laboratory has previously shown that hepatic ATGL preferentially channels hydrolyzed fatty acids (FAs) to β‐oxidation with a concomitant induction of peroxisome proliferator activated receptor‐α (PPAR‐α), independent of a FA carrier. Additionally, our work describes a novel signaling axis involving cAMP/PKA, ATGL‐catalyzed lipolysis and sirtuin1 (SIRT1) activation that governs transcriptional regulation of oxidative metabolism and mitochondrial biogenesis. Recent studies have also shown SIRT1 promotes autophagy, which also contributes to lipid droplet catabolism (lipophagy). Thus, the objective of these studies is to determine the effects of ATGL on autophagy in the liver. Hepatic ATGL overexpression promotes the expression of autophagy genes, whereas ATGL knockdown reduces the expression of the same genes. ATGL regulates autophagosome biogenesis and lysosomal levels through alterations in the microtubule‐associated protein 1A/1B‐light chain 3 (LC3), sequestome1 (p62) and lysosomal‐associated membrane protein 1 (LAMP1). Use of a chemical inhibitor of ATGL, ATGListatin (Astat) in hepatocytes shows a marked suppression of autophagy, similar to the effects observed with ATGL knockdown in vivo. Finally, ATGL requires SIRT1 to govern its effects on autophagy as evidenced by studies showing that ATGL overexpression in liver‐specific SIRT1 KO mice is unable to induce autophagy. Thus, our findings identify ATGL‐mediated lipolysis as an important and potent regulator of autophagy that could have important implications for NAFLD etiology and its comorbidities.Support or Funding InformationFunding sources include the NIDDK (DK085008 to DGM) and the Minnesota Obesity Center (DK050456).

ATGL-mediated triglyceride turnover and the regulation of mitochondrial capacity in skeletal muscle.

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.

Downregulation of adipose triglyceride lipase promotes cardiomyocyte hypertrophy by triggering the accumulation of ceramides

Adipose triglyceride lipase (ATGL), the rate-limiting enzyme of triglyceride (TG) hydrolysis, plays an important role in TG metabolism. ATGL knockout mice suffer from TG accumulation and die from heart failure. However, the mechanisms underlying cardiac hypertrophy caused by ATGL dysfunction remain unknown. In this study, we found that ATGL expression declined in pressure overload-induced cardiac hypertrophy in vivo and phenylephrine (PE)-induced cardiomyocyte hypertrophy in vitro. ATGL knockdown led to cardiomyocyte hypertrophy, while ATGL overexpression prevented PE-induced hypertrophy. In addition, ATGL downregulation increased but ATGL overexpression reduced the contents of ceramide, which has been proved to be closely associated with cardiac hypertrophy. Moreover, the accumulation of ceramide was due to elevation of free fatty acids in ATGL-knockdown cardiomyocytes, which could be explained by the reduced activity of peroxisome proliferator-activated receptor (PPAR) α leading to imbalance of fatty acid uptake and oxidation. These observations suggest that downregulation of ATGL causes the decreased PPARα activity which results in the imbalance of FA uptake and oxidation, elevating intracellular FFA contents to promote the accumulation of ceramides, and finally inducing cardiac hypertrophy. Upregulation of ATGL could be a strategy for ameliorating lipotoxic damage in cardiac hypertrophy.

Role of the ubiquitin–proteasome system in cardiac dysfunction of adipose triglyceride lipase-deficient mice

Systemic deletion of the gene encoding for adipose triglyceride lipase (ATGL) in mice leads to severe cardiac dysfunction due to massive accumulation of neutral lipids in cardiomyocytes. Recently, impaired peroxisome proliferator-activated receptor α (PPARα) signaling has been described to substantially contribute to the observed cardiac phenotype. Disturbances of the ubiquitin–proteasome system (UPS) have been implicated in numerous cardiac diseases including cardiomyopathy, ischemic heart disease, and heart failure. The objective of the present study was to investigate the potential role of UPS in cardiac ATGL deficiency. Our results demonstrate prominent accumulation of ubiquitinated proteins in hearts of ATGL-deficient mice, an effect that was abolished upon cardiomyocyte-directed overexpression of ATGL. In parallel, cardiac protein expression of the ubiquitin-activating enzyme E1a, which catalyzes the first step of the ubiquitination cascade, was significantly upregulated in ATGL-deficient hearts. Dysfunction of the UPS was accompanied by activation of NF-κB signaling. Moreover, the endoplasmic reticulum (ER)-resident chaperon protein disulfide isomerase was significantly upregulated in ATGL-deficient hearts. Chronic treatment of ATGL-deficient mice with the PPARα agonist Wy14,643 improved proteasomal function, prevented NF-κB activation and decreased oxidative stress. In summary, our data point to a hitherto unrecognized link between proteasomal function, PPARα signaling and cardiovascular disease.

Skeletal Muscle Triacylglycerol Hydrolysis Does Not Influence Metabolic Complications of Obesity

Intramyocellular triacylglycerol (IMTG) accumulation is highly associated with insulin resistance and metabolic complications of obesity (lipotoxicity), whereas comparable IMTG accumulation in endurance-trained athletes is associated with insulin sensitivity (the athlete’s paradox). Despite these findings, it remains unclear whether changes in IMTG accumulation and metabolism per se influence muscle-specific and systemic metabolic homeostasis and insulin responsiveness. By mediating the rate-limiting step in triacylglycerol hydrolysis, adipose triglyceride lipase (ATGL) has been proposed to influence the storage/production of deleterious as well as essential lipid metabolites. However, the physiological relevance of ATGL-mediated triacylglycerol hydrolysis in skeletal muscle remains unknown. To determine the contribution of IMTG hydrolysis to tissue-specific and systemic metabolic phenotypes in the context of obesity, we generated mice with targeted deletion or transgenic overexpression of ATGL exclusively in skeletal muscle. Despite dramatic changes in IMTG content on both chow and high-fat diets, modulation of ATGL-mediated IMTG hydrolysis did not significantly influence systemic energy, lipid, or glucose homeostasis, nor did it influence insulin responsiveness or mitochondrial function. These data argue against a role for altered IMTG accumulation and lipolysis in muscle insulin resistance and metabolic complications of obesity.

Endothelial dysfunction in a mouse model of human neutral lipid storage disease

Background Systemic knockout of adipose triglyceride lipase (ATGL), the rate-limiting enzyme of triglyceride catabolism, results in a murine phenotype characterized by progressive accumulation of lipids in the heart finally leading to lethal cardiac dysfunction. Since cardiac and vascular dysfunction are closely related we investigated endothelium-dependent and -independent vessel function of ATGL knockout (ATGL(-/-)) mice. Using mice with cardiomyocyterestricted overexpression of ATGL (cardiac-rescued phenotype; ATGL(-/-)/MHC-A35) we were able to differentiate between heart-related and -unrelated effects. Results Aortic relaxation studies and Langendorff perfusion experiments of isolated hearts demonstrated that ATGL (-/-) mice suffer from pronounced microand macrovascular endothelial dysfunction. Experiments with DEA/NO revealed the functional integrity of the smooth muscle cell layer. Since loss in vascular reactivity was restored by ~50 % in cardiac-rescued mice, this phenomenon seems partly a consequence of impaired cardiac contractility. Biochemical analysis revealed that aortic eNOS protein was down-regulated by more than 60% in aortas of ATGL (-/-) mice. As consequence, phosphorylation of VASP at Ser 239 was almost abolished. Both parameters were