Fasting energy homeostasis in mice with adipose deficiency of desnutrin/adipose triglyceride lipase

Adipose triglyceride lipase (ATGL) catalyzes the first step of lipolysis of cytoplasmic triacylglycerols in white adipose tissue (WAT) and several other organs. We created adipose-specific ATGL-deficient (ATGLAKO) mice. In these mice, in vivo lipolysis, measured as the increase of plasma nonesterified fatty acid and glycerol levels after injection of a β3-adrenergic agonist, was undetectable. In isolated ATGLAKO adipocytes, β3-adrenergic-stimulated glycerol release was 10-fold less than in controls. Under fed conditions, ATGLAKO mice had normal viability, mild obesity, low plasma nonesterified fatty acid levels, increased insulin sensitivity, and increased daytime food intake. After 5 h of fasting, ATGLAKO WAT showed phosphorylation of the major protein kinase A-mediated targets hormone-sensitive lipase and perilipin A and ATGLAKO liver showed low glycogen and triacylglycerol contents. During a 48-h fast, ATGLAKO mice developed striking and complex differences from controls: progressive reduction of oxygen consumption, high respiratory exchange ratio, consistent with reduced fatty acid availability for energy production, lethargy, hypothermia, and undiminished fat mass, but greater loss of lean mass than controls. Plasma of 48 h-fasted ATGLAKO mice had a unique pattern: low 3-hydroxybutyrate, insulin, adiponectin, and fibroblast growth factor 21 with elevated leptin and corticosterone. ATGLAKO WAT, liver, skeletal muscle, and heart showed increased levels of mRNA related to autophagy and proteolysis. In murine ATGL deficiency, adipose lipolysis is critical for fasting energy homeostasis, and fasting imposes proteolytic stress on many organs, including heart and skeletal muscle.     

脂肪组织甘油三酯脂肪酶的研究进展

脂肪组织甘油三酯脂肪酶(ATGL)是新近发现的启动脂肪动员的另一关键酶,其主要催化脂肪组织中甘油三酯生成甘油二酯,与长期认为的经典限速酶激素敏感性脂肪酶(HSL)在脂肪分解途径中共同承担着重要作用.鉴于ATGL与脂肪分解密切相关,并在脂肪代谢紊乱性疾病中发挥着重要的作用,本文就ATGL的研究进展及与脂肪分解、肥胖和2型糖尿病等脂代谢紊乱性疾病的关系作一综述.     

Inhibiting triglyceride synthesis improves hepatic steatosis but exacerbates liver damage and fibrosis in obese mice with nonalcoholic steatohepatitis

In the early stages of nonalcoholic fatty liver disease (NAFLD), triglycerides accumulate in hepatocytes. Diacylglycerol acyltransferase 2 (DGAT2) catalyzes the final step in hepatocyte triglyceride biosynthesis. DGAT2 antisense oligonucleotide (ASO) treatment improved hepatic steatosis dramatically in a previous study of obese mice. According to the 2-hit hypothesis for progression of NAFLD, hepatic steatosis is a risk factor for nonalcoholic steatohepatitis (NASH) and fibrosis. To evaluate this hypothesis, we inhibited DGAT2 in a mouse model of NASH induced by a diet deficient in methionine and choline (MCD). Six-week-old genetically obese and diabetic male db/db mice were fed either the control or the MCD diet for 4 or 8 weeks. The MCD diet group was treated with either 25 mg/kg DGAT2 ASO or saline intraperitoneally twice weekly. Hepatic steatosis, injury, fibrosis, markers of lipid peroxidation/oxidant stress, and systemic insulin sensitivity were evaluated. Hepatic steatosis, necroinflammation, and fibrosis were increased in saline-treated MCD diet-fed mice compared to controls. Treating MCD diet-fed mice with DGAT2 ASO for 4 and 8 weeks decreased hepatic steatosis, but increased hepatic free fatty acids, cytochrome P4502E1, markers of lipid peroxidation/oxidant stress, lobular necroinflammation, and fibrosis. Progression of liver damage occurred despite reduced hepatic expression of tumor necrosis factor alpha, increased serum adiponectin, and striking improvement in systemic insulin sensitivity. CONCLUSION: Results from this mouse model would suggest accumulation of triglycerides may be a protective mechanism to prevent progressive liver damage in NAFLD.     

Hemodynamic changes in hepatic sinusoids of hepatic steatosis mice

BACKGROUND Fatty liver(FL) is now a worldwide disease. For decades, researchers have been kept trying to elucidate the mechanism of FL at the molecular level, but rarely involve the study of morphology and medical physics. Traditionally, it was believed that hemodynamic changes occur only when fibrosis occurs, but it has been proved that these changes already show in steatosis stage, which may help to reveal the pathogenesis and its progress. Because the pseudolobules are not formed during the steatosis stage, this phenomenon may be caused by the compression of the liver microcirculation and changes in the hemodynamics.AIM To understand the pathogenesis of hepatic steatosis and to study the hemodynamic changes associated with hepatic steatosis.METHODS Eight-week-old male C57 BL/6 mice were divided into three groups randomly(control group, 2-wk group, and 4-wk group), with 16 mice per group. A hepatic steatosis model was established by subcutaneous injection of carbon tetrachloride in mice. After establishing the model, liver tissue from mice was stained with hematoxylin and eosin(HE), and oil red O stains. Blood was collected from the angular vein, and hemorheological parameters were estimated. A two-photon fluorescence microscope was used to examine the flow properties of red blood cells in the hepatic sinusoids.RESULTS Oil red O staining indicated lipid accumulation in the liver after CCl_4 treatment.HE staining indicated narrowing of the hepatic sinusoidal vessels. No significant difference was observed between the 2-wk and 4-wk groups of mice onmorphological examination. Hemorheological tests included whole blood viscosity(mPas, γ = 10 s-1/γ = 100 s-1)(8.83 ± 2.22/4.69 ± 1.16, 7.73 ± 2.46/4.22 ±1.32, and 8.06 ± 2.88/4.22 ± 1.50), red blood cell volume(%)(51.00 ± 4.00, 42.00 ±5.00, and 40.00 ± 3.00), the content of plasma fibrinase(g/L)(3.80 ± 0.50, 2.90 ±0.80, and 2.30 ± 0.70), erythrocyte deformation index(%)(44.49 ± 5.81, 48.00 ±15.29, and 44.36 ± 15.01), erythrocyte electrophoresis rate(mm/s per V/m)(0.55 ±0.11, 0.50 ± 0.11, and 0.60 ± 0.20), revealing pathological changes in plasma components and red blood cells of hepatic steatosis. Assessment of blood flow velocity in the hepatic sinusoids with a laser Doppler flowmeter(mL/min per100 g)(94.43 ± 14.64, 80.00 ± 12.12, and 67.26 ± 5.92) and two-photon laser scanning microscope(μm/s)(325.68 ± 112.66, 213.53 ± 65.33, and 173.26 ± 44.02)revealed that as the modeling time increased, the blood flow velocity in the hepatic sinusoids decreased gradually, and the diameter of the hepatic sinusoids became smaller(μm)(10.28 ± 1.40, 6.84 ± 0.93, and 5.82 ± 0.79).CONCLUSION The inner diameter of the hepatic sinusoids decreases along with the decrease in the blood flow velocity within the sinusoids and the changes in the systemic hemorheology.     

Growth-hormone-induced changes in postheparin plasma lipoprotein lipase and hepatic triglyceride lipase activities

A comparison of treated and untreated patients with growth hormone deficiency revealed that administration of growth hormone reduced lipoprotein lipase and hepatic lipase activities, total cholesterol, and high-density lipoprotein cholesterol concentrations. The possible significance of these results is discussed.     

Diet associated hepatic steatosis sensitizes to Fas mediated liver injury in mice

BACKGROUND/AIMS: Hepatic steatosis sensitizes the liver to injury and inflammation by unclear mechanisms. Because Fas has been linked to liver injury and inflammation, Fas expression and sensitization to Fas signaling was examined in models of hepatic steatosis. METHODS: Mice were fed a carbohydrate diet while control animals received standard chow. Sensitization to Fas was examined following administration of Jo2 antibody. For the in vitro experiments, HepG2 cells were incubated with or without a mixture of long chain fatty acids (2:1 oleate:palmitate). Sensitization of the cells to Fas was examined using the CH11 antibody. RESULTS: Mice fed a high caloric diet developed hepatic steatosis, hyperlipidemia, insulin resistance, and hyperleptinemia, all features of the human syndrome. Fas expression in hepatocytes was increased as compared to lean animals and was coupled to cytotoxic signaling. Indeed, hepatocyte apoptosis, liver injury and chemokine generation were all accentuated in obese animals following administration of Jo-2, a Fas agonist. Hep G2 cells cultured in the presence of free fatty acids also developed 'cellular steatosis', upregulated Fas expression and were more sensitive to apoptosis by a Fas agonist. CONCLUSIONS: Collectively, these data implicate Fas as a link between obesity associated fatty liver and increased susceptibility to liver damage.     

Distinct roles of adipose triglyceride lipase and hormone-sensitive lipase in the catabolism of triacylglycerol estolides

Branched esters of palmitic acid and hydroxy stearic acid are antiinflammatory and antidiabetic lipokines that belong to a family of fatty acid (FA) esters of hydroxy fatty acids (HFAs) called FAHFAs. FAHFAs themselves belong to oligomeric FA esters, known as estolides. Glycerol-bound FAHFAs in triacylglycerols (TAGs), named TAG estolides, serve as metabolite reservoir of FAHFAs mobilized by lipases upon demand. Here, we characterized the involvement of two major metabolic lipases, adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL), in TAG estolide and FAHFA degradation. We synthesized a library of 20 TAG estolide isomers with FAHFAs varying in branching position, chain length, saturation grade, and position on the glycerol backbone and developed an in silico mass spectra library of all predicted catabolic intermediates. We found that ATGL alone or coactivated by comparative gene identification-58 efficiently liberated FAHFAs from TAG estolides with a preference for more compact substrates where the estolide branching point is located near the glycerol ester bond. ATGL was further involved in transesterification and remodeling reactions leading to the formation of TAG estolides with alternative acyl compositions. HSL represented a much more potent estolide bond hydrolase for both TAG estolides and free FAHFAs. FAHFA and TAG estolide accumulation in white adipose tissue of mice lacking HSL argued for a functional role of HSL in estolide catabolism in vivo. Our data show that ATGL and HSL participate in the metabolism of estolides and TAG estolides in distinct manners and are likely to affect the lipokine function of FAHFAs.

Branched esters of palmitic acid and hydroxy stearic acid (PAHSAs) are antiinflammatory and antidiabetic lipokines (1–3). PAHSA serum and adipose tissue levels correlate with insulin sensitivity and are decreased in insulin-resistant humans (2, 4). PAHSAs increase glucose-stimulated insulin secretion by enhancing the production of the gut-derived incretin glucagon-like peptide-1 (5, 6). The antiinflammatory effects of PAHSA isomers (2, 7, 8) are mediated via free fatty acid receptor 4 (FFAR4, GPR120) and modulate both innate and adaptive immune responses in a mouse colitis model (1) and type-1 diabetes (6). Therefore, PAHSAs have beneficial effects on both metabolism and the immune system (9).PAHSAs belong to the family of fatty acid (FA) esters of hydroxy FAs (HFAs) called FAHFAs, which are part of a much larger family of mono- or oligomeric FAHFA esters named estolides. Since FAHFAs contain only a single ester bond of one FA with one HFA (the estolide bond), they represent monoestolides (10). The position of the branching carbon atom defines the regioisomer (e.g., 5-PAHSA or 9-PAHSA). PAHSAs and other less-well-studied FAHFAs such as the oleic acid esters of hydroxy palmitic acid (OAHPAs) or the docosahexaenoic acid ester of 13-hydroxy linoleic acid (13-DHAHLA) derive from either dietary sources or de novo synthesis in adipose tissue and other organs (2, 11, 12). Nonesterified, free FAHFAs (free mono-estolides) can be esterified to glycerol to form FAHFA acylglycerols, which in combination with other FAs result in the formation of triacylglycerol (TAG) estolides, diacylglycerol (DAG) estolides, or monoacylglycerol (MAG) estolides. TAG estolides represent a major storage form of bioactive free FAHFAs and are present in plant oils (e.g., castor oil) (13, 14) and adipose tissue of mice (3, 15) and humans (16).Both the synthetic and catabolic pathways of FAHFAs and TAG estolides are insufficiently understood. The hydrolytic catabolism of FAHFAs and TAG estolides results in the generation of highly bioactive and physiologically relevant FAHFAs, HFAs, FAs, and DAGs. Given the structural and metabolic similarity between TAGs and TAG estolides, it seemed reasonable to suspect that canonical TAG lipases will be involved in FAHFA and TAG estolide degradation. Generally, the catabolism of TAGs in cells occurs in the cytosol (neutral lipolysis) or in lysosomes (acidic lipolysis). Neutral lipolysis represents the predominant pathway for the hydrolysis of lipid droplet-associated TAGs in adipocytes involving three major enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase (MGL). ATGL catalyzes the initial step of TAG hydrolysis, generating DAG and one FA (3, 17, 18). The enzyme belongs to the patatin-like phospholipase domain-containing (PNPLA) family of proteins comprising a number of lipid hydrolases (3, 15). ATGL is the most potent TAG hydrolase within this family but also exhibits some phospholipase, retinylesterase, and transacylase activities of undefined physiological relevance (17, 19, 20). For full TAG hydrolase activity, ATGL requires a coactivator named comparative gene identification-58 (CGI-58; also called α/β-hydrolase domain containing 5, ABHD5) (21–23). CGI-58 features α/β-hydrolase folds and also exerts some acyltransferase and protease activities (23–25). Yet, the physiological role of these activities remains elusive. ATGL exhibits unique regioselectivity for TAG substrates and preferentially hydrolyzes the sn-2 position of the glycerol chain of TAGs (26). Upon stimulation of ATGL by CGI-58 this regioselectivity broadens to the sn-1 but not the sn-3 position (26).HSL is rate-limiting for the second step of TAG lipolysis converting DAG to one FA and MAG (27). The enzyme preferentially catalyzes DAGs at the sn-3 position and cholesteryl esters (28, 29) but also cleaves TAGs (sn-1 and sn-3 position), retinylesters (30), or medium- and short-chain carboxylic acid glycerol esters (29). The enzyme is structurally unrelated to ATGL and does not require enzyme coactivators. Hormonal stimulation of neutral lipolysis by β-adrenoreceptor agonists such as catecholamines activates both ATGL and HSL by promoting the molecular interaction of ATGL with CGI-58 on the surface of TAG-containing lipid droplets (21, 31) and the translocation of HSL from the cytoplasm to lipid droplets. These processes involve the protein kinase A-dependent phosphorylation of perilipin-1, CGI-58, and HSL (31–33).Previous studies by Tan et al. (15) and our laboratory (3) demonstrated that ATGL and HSL are both able to hydrolyze FAHFA–glycerol ester bonds of TAG estolides. However, enzyme preferences for this reaction, the substrate requirements, or the contribution of these enzymes to hydrolyze the FA–HFA ester bond (estolide bond) in TAG estolides as well as in free FAHFAs remained unaddressed. Using a newly generated library of TAG estolides, we now show that ATGL and HSL play distinct roles in the formation of TAG estolides by transesterification reactions and the degradation of (TAG) estolides by hydrolysis reactions.     

Lipid overloading during liver regeneration causes delayed hepatocyte DNA replication by increasing ER stress in mice with simple hepatic steatosis

Background and aim

Impaired fatty liver regeneration has already been reported in many genetic modification models. However, in diet-induced simple hepatic steatosis, which showed similar phenotype with clinical pathology, whether liver regeneration is impaired or not remains unclear. In this study, we evaluated liver regeneration in mice with diet-induced simple hepatic steatosis, and focused on excess lipid accumulation occurring during liver regeneration.

Methods

Mice were fed high fat diet (HFD) or control diet for 9–10 weeks. We analyzed intrahepatic lipid accumulation, DNA replication, and various signaling pathways including cell proliferation and ER stress during liver regeneration after partial hepatectomy. In addition, some of mice were pretreated with tauroursodeoxycholic acid (TUDCA), a chemical chaperone which alleviates ER stress, and then we estimated TUDCA effects on liver regeneration.

Results

The peak of hepatocyte BrdU incorporation, the expression of proliferation cell nuclear antigen (PCNA) protein, and the expressions of cell cycle-related genes were observed in delayed time in HFD mice. The expression of phosphorylated Erk1/2 was also delayed in HFD mice. The amounts of liver triglyceride were at least twofold higher in HFD mice at each time point. Intrahepatic palmitic acid was increased especially in HFD mice. ER stress induced during liver regeneration was significantly higher in HFD mice. In HFD mice, pretreatment with TUDCA reduced ER stress and resulted in improvement of delayed liver regeneration.

Conclusion

In simple hepatic steatosis, lipid overloading occurring during liver regeneration might be caused ER stress and results in delayed hepatocyte DNA replication.     

Decreased activities of lipoprotein lipase and hepatic triglyceride lipase in patients with gout

Postheparin plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activities were measured in 30 male primary gout patients as well as in control subjects. The activities of these lipolytic enzymes were significantly decreased in the patients as compared with the controls (gout v control; LPL, 5.4 +/- 0.4 v 7.9 +/- 0.9 U; HTGL, 14.6 +/- 2.0 v 17.9 +/- 3.4 U) when matched with serum triglyceride concentration. Further, LPL activity was negatively correlated with serum- and very-low-density lipoprotein (VLDL)-triglyceride in gout patients, while that of HTGL was negatively correlated with low-density lipoprotein (LDL)-triglyceride in both gout patients and control subjects. These results suggest that decreased activities of LPL and HTGL may contribute, in part, to the increased concentrations of serum-, VLDL-, and LDL-triglyceride seen in gout patients, leading to a higher risk for coronary atherosclerotic diseases in gout.     

Black-white differences in postprandial triglyceride response and postheparin lipoprotein lipase and hepatic triglyceride lipase among young men.

Black-white differences in serum triglycerides and high-density lipoprotein (HDL) cholesterol concentrations are known. However, the metabolic basis for these differences is not clear. This study determined the magnitude of postprandial triglyceride concentrations, lipoprotein lipase and hepatic triglyceride lipase activities in postheparin plasma, and serum lipid and lipoprotein cholesterol concentrations in healthy young adult black men (n = 22) and white men (n = 28). Postprandial triglyceride concentrations were measured at 2, 3, 4, 5, 6, and 8 hours after a standardized test meal. Serum lipid and lipoprotein cholesterol concentrations were similar between the races in this study sample. However, incremental (above basal) increases in triglycerides were significantly greater in white men versus black men at 2 hours (P = .01) and tended to be greater at 3 hours (P = .12) and 4 hours (P = .06) after the fat load. In a multivariate analysis that included age, race, apolipoprotein E (apoE) genotype, fasting triglycerides, obesity measures, alcohol intake, and cigarette use, fasting triglycerides (P = .04) and, to a lesser extent, race (P = .07) were associated independently with the 2-hour incremental increase in triglycerides. The incremental triglyceride response correlated inversely with HDL cholesterol in both whites (r = -.38, P = .04) and blacks (r = -.59, P = .004). Lipoprotein lipase activity was higher (P = .049) and hepatic triglyceride lipase activity lower (P = .0001) in black men compared with white men; racial differences persisted after adjusting for the covariates. While lipoprotein lipase activity tended to associate inversely with the postprandial triglyceride concentration in both races, hepatic triglyceride lipase activity tended to correlate positively in whites and inversely in blacks. These results suggest that compared with whites, blacks may have an efficient lipid-clearing mechanism that could explain the black-white differences in lipoproteins found in the population at large.     

Anti-angiogenic pigment epithelium-derived factor regulates hepatocyte triglyceride content through adipose triglyceride lipase (ATGL)

BACKGROUND/AIMS: Anti-angiogenic pigment epithelium-derived factor (PEDF) is a 50 kDa secreted glycoprotein that is highly expressed in hepatocytes. Adipose triglyceride lipase (ATGL), a novel lipase critical for triglyceride metabolism, is a receptor for PEDF. We postulated that hepatocyte triglyceride metabolism was dependent on interactions between PEDF and ATGL, and loss of PEDF would impair mobilization of triglycerides in the liver. METHODS: Immunoprecipitation studies were performed in PEDF null and control hepatocytes with recombinant PEDF (rPEDF) as bait. Immunofluorescent microscopy was used to localize ATGL. Triglyceride content was analyzed in hepatocytes and in whole liver with and without rPEDF. ATGL was blocked using an inhibitor, (R)-bromoenol lactone. RESULTS: PEDF co-immunoprecipitated with ATGL in hepatic and HCC lysates. All PEDF deficient livers demonstrated steatosis. Triglyceride content was significantly increased in PEDF null livers compared to wildtype (p<0.05) and in isolated hepatocytes (p<0.01). Treatment of PEDF null hepatocytes with rPEDF decreased TG content (p<0.05) and this activity was dependent on ATGL. CONCLUSIONS: Our results identify a novel role for PEDF in hepatic triglyceride homeostasis through binding to ATGL and demonstrate that rPEDF and ATGL localize to adiposomes in hepatocytes. Dysregulation of this pathway may be one mechanism underlying fatty liver disease.     

吡格列酮对肥胖大鼠脂肪甘油三酯脂酶表达的调控

观察吡格列酮对高脂饮食诱导胰岛素抵抗大鼠脂肪、肝脏、肌肉三大组织脂肪甘油三酯脂酶(adipose triglyceride lipase,ATGL)表达的变化.结果显示吡格列酮促进胰岛素抵抗大鼠脂肪、肝脏、肌肉三大代谢组织中ATGL的表达.