Hepatic Triglyceride Production Research Articles

Abstract 2049: Regulation Of MTP-ATGL Interactions In Adipocytes And Their Impact On Hepatic Lipid Metabolism

Introduction: We have shown that microsomal triglyceride transfer protein (MTP) in adipocytes regulates lipolysis by inhibiting adipose triglyceride lipase (ATGL). Adipose MTP knockout mice (A- Mttp -/- ) exhibit smaller adipocytes, more thermogenesis, and less weight gain on an obesogenic diet. Aim: To investigate the MTP-ATGL interaction in adipocytes and its impact on hepatic lipid metabolism. Methods: We studied MTP-ATGL interactions using in silico modeling, ELISA, and co-IP. Lipoprotein profiles were analyzed using FPLC. Adipose-liver cross-talk was assessed through co-culture experiments, adipokine profiling and lipidomics. Hepatic triglyceride (TG) production and fatty acid (FA) oxidation were assessed using poloxamer-407 and 14 C-palmitate. Statistical analyses were done using GraphPad Prism (P <.05 significant). Results: Our study suggests MTP-ATGL interactions are hydrophobic involving C-terminal of ATGL and N-terminal β-barrel of MTP. These interactions vary in different metabolic states like fasting and adipocyte differentiation. A- Mttp -/- mice had higher plasma TG content in VLDL and LDL fractions. Despite increased movement of FAs from adipose to the liver as indicated by lipidomics of eWAT, plasma and liver, A- Mttp -/- mice liver showed less steatosis. Adipokine profiling showed significantly higher amounts of adiponectin in the plasma of A- Mttp -/- mice. Reduced ceramide levels and significantly increased AMPK phosphorylation suggest higher adiponectin activity in A- Mttp -/- livers. A- Mttp -/- livers showed higher expression of FA uptake and utilization related genes, but no change in lipid synthesis. Hepatocytes treated with Mttp -/- adipocytes conditioned media (CM) showed higher FA oxidation than those treated with Mttp f/f adipocytes CM. Mttp -/- adipocytes CM showed significantly higher PPARα reporter activity than Mttp f/f adipocytes CM. Conclusion: Our study reveals dynamic hydrophobic interactions between MTP and ATGL that vary with metabolic states. Enhanced movement of PPARα ligands (FAs) and adiponectin from A- Mttp -/- adipocytes boosts VLDL production and FA oxidation, to protect livers from hepatic steatosis. This highlights a novel role of adipose MTP in the regulation of hepatic lipid homeostasis.

Garcinia cambogia Extract Increased Hepatic Levels of Lipolysis-Stimulated Lipoprotein Receptor and Lipids in Mice on Normal Diet.

Garcinia cambogia extract (GCE) is a popular weight-loss supplement that also lowers plasma triglyceride (TG) levels. We hypothesized that GCE-mediated inhibition of ATP citrate lyase and thereby hepatic TG production could lead to compensatory mechanisms, including increased hepatic TG uptake via lipoprotein receptors. GCE (20 mg/day) administered 40 days orally to female C57BL/6Rj mice on a standard chow diet led to a decrease in both plasma fasting and post-prandial TG-rich lipoprotein levels, but with no significant change in body weight gain. Lipolysis stimulated lipoprotein receptor (LSR) protein levels, but not those of LDL-receptor, were increased as compared to controls. Mouse Hepa1-6 cells treated with the GCE active ingredient, hydroxycitrate, also led to increased LSR protein levels. Hepatic total cholesterol, TG, and muscle TG contents were higher in GCE-treated animals as compared to controls, whereas adipose TG levels were unchanged. LSR and LDL-receptor protein levels were correlated with liver total cholesterol, but only LDL-receptor was associated with liver TG. These results show that GCE treatment in mice on a standard chow diet led to significantly increased liver and muscle lipids, with no significant change in adipose tissue TG levels, which should be considered in the long-term use of GCE.

Abstract 118: Positive Adipose Liver Crosstalk Protects Adipose MTP Knockout Mice from Hepatic Steatosis on an Obesogenic Diet

Introduction: Our previous study demonstrated that adipocyte MTP regulates intracellular lipolysis by inhibiting ATGL activity. Adipose-specific MTP knockout mice (A- Mttp -/- ) had smaller adipocytes and showed increased thermogenesis and gained less weight on an obesogenic diet. They also exhibited moderately high plasma triglyceride levels and less hepatic steatosis compared to wild-type counterparts. In this study, we have elucidated the mechanism behind less hepatic lipid accumulation and moderately high plasma TG in A- Mttp -/- mice. Methods: We characterized lipoproteins and studied hepatic ApoB production. We have analyzed adipose-liver crosstalk using two strategies 1) lipidomics of adipose tissue, plasma, and liver. 2) adipokine profiling. Further, we have used western blotting and qRT-PCR to check signaling and gene expression. Results: Plasma lipid profiling revealed increased TG content in the VLDL and LDL fraction of A- Mttp -/- mice compared to Mttp f/f mice. Additionally, A- Mttp -/- mice showed significantly increased hepatic triglyceride production compared to the wild type. Lipidomic analysis revealed the movement of fatty acids from adipose tissue to the liver, resulting in significantly higher amounts of oleate, palmitate, linoleate, and stearate in the liver of A- Mttp -/- mice compared to Mttp f/f mice. A- Mttp -/- mice liver also showed significantly increased expression of genes involved in fatty acid uptake and utilization such as Cd36 , Mcad , Fatp2 , and Cpt1a compared to Mttp f/f mice. Further adipokine profiling showed a significantly higher amount of adiponectin and decreased amount of proinflammatory cytokines such as leptin in the plasma of A- Mttp -/- mice compared to Mttp f/f mice. We also found decreased ceramide levels and significantly increased phosphorylation of AMPK in the liver of A- Mttp -/- mice compared to Mttp f/f mice. Conclusion: We conclude that increased availability of substrate for TG production might contribute to increased ApoB secretion by the liver of A- Mttp -/- mice. In addition, positive cytokine profile and increased adiponectin signaling in the liver of A- Mttp -/- mice protects it from hepatic steatosis.

Abstract 9575: TOTUM-070 Hypocholesterolemic Effect Linked to Intestinal Cholesterol Absorption Inhibition. An in vivo and in vitro Study

TOTUM-070 (T070) is a patented blend of five plant extracts designed to act in combination to prevent hypercholesterolemia. Previous study in dyslipidemic hamster model fed a High Fat High Cholesterol (HFHC) diet for 12 weeks demonstrated that supplementation with T070 at 3,5% to 5% in the diet (n=16 per group) reduced serum non-HDL cholesterol compared to HFHC group (-38%, p<0,001 and -47%, p<0,001, respectively) as well as serum triglyceride levels (-30%, n.s and -46%, p<0,05, respectively). We aimed to decipher the mechanisms of action of T070 both in vivo and in vitro. First, the chemical characterization of T070 by HPLC-UV/MS analyses led to the identification and quantification of 37 biomolecules such as oleuropein, oleanolic acid or chlorogenic acid. Total RNA sequencing analysis of livers from the HFHC and T070 5% supplemented groups highlighted 511 genes with significant different expression level between groups. Among canonical pathways identified, 19 genes implicated in metabolism were differentially regulated with T070. Several functional tests were performed in hamster fed with T070 5% for 12 weeks in comparison to HFHC group (n=14 per group). Dosage of lipoprotein lipase activity ex vivo did not show any difference between groups. Results from hepatic triglyceride production test detected no effect of T070. However, an oral fat tolerance test showed that post-prandial hypertriglyceridemia was significantly reduced by 40% (p<0,01) in T070 group following olive oil gavage compared to the HFHC hamsters. Finally, in vivo intestinal [4- 14 C]-Cholesterol absorption test demonstrated a 12% reduction (n.s) with T070 5% compared to HFHC group. In vitro investigations were assessed on differentiated human enterocyte Caco2 cells. Incubation with T070 at 1g/l for 1h acutely reduced [1,2- 3 H(N)]-cholesterol uptake by 30% in Caco2 cells (p<0.01). Pharmacological inhibition of cholesterol transporter NPC1L1 abolished the effect of T070 on cholesterol uptake. There was no effect of T070 on NPC1L1 protein expression in Caco2 cells, suggesting T070 blocked directly NPC1L1 activity. Altogether, our results demonstrate that hypocholesterolemic effect of T070 is mediated by inhibition of intestinal cholesterol absorption.

Hepatic Carbohydrate Response Element Binding Protein Activation Limits Nonalcoholic Fatty Liver Disease Development in a Mouse Model for Glycogen Storage Disease Type 1a.

Background and AimsGlycogen storage disease (GSD) type 1a is an inborn error of metabolism caused by defective glucose‐6‐phosphatase catalytic subunit (G6PC) activity. Patients with GSD 1a exhibit severe hepatomegaly due to glycogen and triglyceride (TG) accumulation in the liver. We have shown that the activity of carbohydrate response element binding protein (ChREBP), a key regulator of glycolysis and de novo lipogenesis, is increased in GSD 1a. In the current study, we assessed the contribution of ChREBP to nonalcoholic fatty liver disease (NAFLD) development in a mouse model for hepatic GSD 1a.Approach and ResultsLiver‐specific G6pc–knockout (L‐G6pc −/−) mice were treated with adeno‐associated viruses (AAVs) 2 or 8 directed against short hairpin ChREBP to normalize hepatic ChREBP activity to levels observed in wild‐type mice receiving AAV8–scrambled short hairpin RNA (shSCR). Hepatic ChREBP knockdown markedly increased liver weight and hepatocyte size in L‐G6pc −/− mice. This was associated with hepatic accumulation of G6P, glycogen, and lipids, whereas the expression of glycolytic and lipogenic genes was reduced. Enzyme activities, flux measurements, hepatic metabolite analysis and very low density lipoprotein (VLDL)‐TG secretion assays revealed that hepatic ChREBP knockdown reduced downstream glycolysis and de novo lipogenesis but also strongly suppressed hepatic VLDL lipidation, hence promoting the storage of “old fat.” Interestingly, enhanced VLDL‐TG secretion in shSCR‐treated L‐G6pc −/− mice associated with a ChREBP‐dependent induction of the VLDL lipidation proteins microsomal TG transfer protein and transmembrane 6 superfamily member 2 (TM6SF2), the latter being confirmed by ChIP‐qPCR.ConclusionsAttenuation of hepatic ChREBP induction in GSD 1a liver aggravates hepatomegaly because of further accumulation of glycogen and lipids as a result of reduced glycolysis and suppressed VLDL‐TG secretion. TM6SF2, critical for VLDL formation, was identified as a ChREBP target in mouse liver. Altogether, our data show that enhanced ChREBP activity limits NAFLD development in GSD 1a by balancing hepatic TG production and secretion.

Maternal hypercholesterolemia programs dyslipidemia in adult male mouse progeny.

As a collection of metabolic abnormalities including inflammation, insulin resistance, hypertension, hormone imbalance, and dyslipidemia, maternal obesity has been well-documented to program disease risk in adult offspring. Although hypercholesterolemia is strongly associated with obesity, less work has examined the programming influence of maternal hypercholesterolemia (MHC) independent of maternal obesity or high-fat feeding. This study was conducted to characterize how MHC per se impacts lipid metabolism in offspring. Female (n = 6/group) C57BL/6J mice were randomly assigned to: (1.) a standard chow diet (Control, CON) or (2.) the CON diet supplemented with exogenous cholesterol (CH) (0.15%, w/w) throughout mating and the gestation and lactation periods. At weaning (postnatal day (PND) 21) and adulthood (PND 84), male offspring were characterized for blood lipid and lipoprotein profile and hepatic lipid endpoints, namely cholesterol and triglyceride (TG) accumulation, fatty acid profile, TG production, and mRNA expression of lipid-regulatory genes. Both newly weaned and adult offspring from CH mothers demonstrated increased very low-density lipoprotein (VLDL) particle number and size and hepatic TG and n-6 polyunsaturated fatty acid accumulation. Further, adult CH offspring exhibited reduced fatty acid synthase (Fasn) and increased diglyceride acyltransferase (Dgat1) mRNA expression. These programming effects appear to be independent of changes in hepatic TG production and postprandial lipid clearance. Study results suggest that MHC, independent of obesity or high-fat feeding, can induce early changes to serum VLDL distribution and hepatic lipid profile that persist into adulthood.

Obesity Reduces Maternal Blood Triglyceride Concentrations by Reducing Angiopoietin-Like Protein 4 Expression in Mice

To ensure fetal lipid supply, maternal blood triglyceride (TG) concentrations are robustly elevated during pregnancy. Interestingly, a lower increase in maternal blood TG concentrations has been observed in some obese mothers. We have shown that high-fat (HF) feeding during pregnancy significantly reduces maternal blood TG levels. Therefore, we performed this study to investigate if and how obesity alters maternal blood TG levels. Maternal obesity was established by prepregnant HF (ppHF) feeding, which avoided the dietary effect during pregnancy. We found not only that maternal blood TG concentrations in ppHF dams were remarkably lower than in control dams but also that the TG peak occurred earlier during gestation. Hepatic TG production and intestinal TG absorption were unchanged in ppHF dams, but systemic lipoprotein lipase (LPL) activity was increased, suggesting that increased blood TG clearance contributes to the decreased blood TG concentrations in ppHF dams. Although significantly higher levels of UCP1 protein were observed in interscapular brown adipose tissue (iBAT) of ppHF dams, Ucp1 gene deletion did not restore blood TG concentrations in ppHF dams. Expression of the angiopoietin-like protein 4 (ANGPTL4), a potent endogenous LPL inhibitor, was significantly increased during pregnancy. However, the pregnancy-induced elevation of blood TG was almost abolished in Angptl4 -/- dams. Compared with control dams, Angptl4 mRNA levels were significantly lower in iBAT, gonadal white adipose tissue, and livers of ppHF dams. Importantly, ectopic overexpression of ANGPTL4 restored maternal blood TG concentrations in ppHF dams. Together, these results indicate that ANGPTL4 plays a vital role in increasing maternal blood TG concentrations during pregnancy. Obesity impairs the rise of maternal blood TG concentrations by reducing ANGPTL4 expression in mice.

Abnormality of hepatic triglyceride metabolism in ApcMin/+ mice with colon cancer cachexia

AimsColorectal cancer syndrome has been one of the greatest concerns in the world. Although several epidemiological studies have shown that hepatic low lipoprotein lipase (LPL) mRNA expression may be associated with dyslipidemia and tumor progression, it is still not known whether the liver plays an essential role in hyperlipidemia of ApcMin/+ mice. Main methodsWe measured the expression of metabolic enzymes that involved fatty acid uptake, de novo lipogenesis (DNL), β-oxidation and investigated hepatic triglyceride production in the liver of wild-type and ApcMin/+ mice. Key findingsWe found that hepatic fatty acid uptake and DNL decreased, but there was no significant difference in fatty acid β-oxidation. Interestingly, the production of hepatic very low-density lipoprotein-triglyceride (VLDL-TG) decreased at 20 weeks of age, but marked steatosis was observed in the livers of the ApcMin/+ mouse. To further explore hypertriglyceridemia, we assessed the function of hepatic glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) for the first time. GPIHBP1 is governed by the transcription factor octamer-binding transcription factor-1 (Oct-1) which are involved in the nuclear factor-κB (NF-κB) signaling pathway in the liver of ApcMin/+ mice. Importantly, it was also confirmed that sn50 (100 μg/mL, an inhibitor of the NF-κB) reversed the tumor necrosis factor α (TNFα)-induced Oct-1 and GPIHBP1 reduction in HepG2 cells. SignificanceAltogether, these findings highlighted a novel role of GPIHBP1 that might be responsible for hypertriglyceridemia in ApcMin/+ mice. Hypertriglyceridemia in these mice may be associated with their hepatic lipid metabolism development.

Characterization of Hepatic Triglyceride Metabolism in the ApcMin/＋ Mouse

The ApcMin/＋ mouse exhibits an intestinal cachexia accompanied by hypertriglyceridemia (HTG). Because the liver governs systemic energy demands through lipid metabolism regulation, the liver is a likely pathological target of HTG in the ApcMin/＋ mouse. Methods: On the basal diet, at 8, 14 and 20 weeks of age, the mice were fasted for 4 h prior to undergoing a hepatic triglyceride production test. Several rate-limiting enzymes and transcription factors participate in hepatic lipid metabolism were determined by RT-qPCR and western blot analysis. Findings: Hepatic de novo lipogenesis (DNL) and β-oxidation were found to be regulated by the AMPK pathway in the ApcMin/＋ mouse at 20 weeks of age. Although hepatic triglyceride production was significantly decreased at 20 weeks of age (p<0.001), hepatic steatosis remained in the ApcMin/＋ mouse. Glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) is governed by the transcription factor octamer-binding transcription factor-1 (Oct-1), which is involved in the nuclear factor-κB (NF-κB) signaling pathway in the liver of the ApcMin/＋ mouse. Additionally, in HepG2 cells, sn50 (an inhibitor of the NF-κB signaling pathway) reversed the tumor necrosis factor α (TNFα)-induced Oct-1 and GPIHBP1 reduction. Interpretation: Our findings indicate that the liver might be responsible for the metabolic complications of HTG in individuals with APC defects and in the ApcMin/＋ mouse. Funding Statement: This work was financially supported by the National Natural Science Foundation of China (Grant No. 31472053 and 31572345) and Graduate Innovation Fund of Jilin University (Grant No. 2017094). The Program for JLU Science and Technology Innovative Research Team (JLUSTIRT, No. 2017TD497 28), the Fundamental Research Funds for the Central Universities. Declaration of Interests: The authors declare that they have no competing interests. Ethics Approval Statement: All animal welfare and experimental procedures were performed strictly according to the guidelines from the National Institutes of Health Guide for the Care and Use of Laboratory animals (NIH Publications No. 8023, revised 1978). Additionally, the procedures were approved by the Institutional Animal Care and Use Committee of Jilin University under the approved protocol number 201707025.

Abstract 390: A New Humanized Mouse Model to Study Lipid Abnormalities and HDL Function in Poorly Controlled Type 1 Diabetes Mellitus

Elevated plasma triglycerides (TG) and VLDL are often present in poorly controlled type 1 diabetes mellitus (T1DM). Furthermore, T1DM patients exhibit an increased risk of cardiovascular disease (CVD) despite normal or high HDL-cholesterol. There is a need for new mouse models to study T1DM lipid abnormalities because mice do not express cholesteryl ester transfer protein (CETP), which transfers cholesteryl ester (CE) from HDL to VLDL and other lipoproteins in exchange for TG, and because mice do not exhibit human-like HDL populations. To develop a physiologically relevant model, we crossed a virally-induced mouse model of T1DM ( Ldlr -/- ;Gp Tg ) deficient in the LDL receptor with human apolipoprotein A-I ( APOA1 ) transgenic mice. These mice (n=8-10) were injected with a liver-specific adeno-associated virus driving expression of human CETP, mimicking the human CETP activity/apoA-I ratio, and the 3 major human HDL subpopulations. Using this model ( APOA1 Tg ;Ldlr -/- ;Gp Tg ), we examined the impact of diabetes on dyslipidemia, HDL particle concentration (HDL-P), and serum HDL function. Induction of diabetes increased plasma TG and cholesterol, primarily due to an increase in VLDL, as compared with non-diabetic (ND) littermates. The dyslipidemia was associated with increases in hepatic TG content and hepatic TG production rate (2-fold over ND). Moreover, resident peritoneal macrophages from diabetic mice exhibited a 5-fold increase in CE loading, increased lipid raft content (20%), and increased Tnfa (3-fold) and Il1b mRNA (4-fold), as compared with ND littermates. In addition, T1DM mice had increased small, medium and large HDL-P (2-fold) concentrations compared with ND controls. Serum HDL from T1DM mice showed a significant increase in its cholesterol efflux capacity through both ABCA1 and ABCG1. Thus, in this humanized mouse model, poorly controlled T1DM leads to elevated plasma TG and VLDL due in part to increased hepatic TG production. This dyslipidemia is associated with higher HDL-P and an increase in serum HDL’s cholesterol efflux capacity, likely due to the increased plasma HDL-P. We have found similar changes in HDL concentration and function in humans with T1DM, suggesting that our mouse model might provide insights into CVD risk in T1DM patients.

Adiponectin Deficiency Impairs Maternal Metabolic Adaptation to Pregnancy in Mice.

Hypoadiponectinemia has been widely observed in patients with gestational diabetes mellitus (GDM). To investigate the causal role of hypoadiponectinemia in GDM, adiponectin gene knockout (Adipoq−/−) and wild-type (WT) mice were crossed to produce pregnant mouse models with or without adiponectin deficiency. Adenoviral vector–mediated in vivo transduction was used to reconstitute adiponectin during late pregnancy. Results showed that Adipoq−/− dams developed glucose intolerance and hyperlipidemia in late pregnancy. Increased fetal body weight was detected in Adipoq−/− dams. Adiponectin reconstitution abolished these metabolic defects in Adipoq−/− dams. Hepatic glucose and triglyceride production rates of Adipoq−/− dams were significantly higher than those of WT dams. Robustly enhanced lipolysis was found in gonadal fat of Adipoq−/− dams. Interestingly, similar levels of insulin-induced glucose disposal and insulin signaling in metabolically active tissues in Adipoq−/− and WT dams indicated that maternal adiponectin deficiency does not reduce insulin sensitivity. However, remarkably decreased serum insulin concentrations were observed in Adipoq−/− dams. Furthermore, β-cell mass, but not glucose-stimulated insulin release, in Adipoq−/− dams was significantly reduced compared with WT dams. Together, these results demonstrate that adiponectin plays an important role in controlling maternal metabolic adaptation to pregnancy.

The Fatty Acid Synthase Inhibitor Platensimycin Improves Insulin Resistance without Inducing Liver Steatosis in Mice and Monkeys.

ObjectivesPlatensimycin (PTM) is a natural antibiotic produced by Streptomyces platensis that selectively inhibits bacterial and mammalian fatty acid synthase (FAS) without affecting synthesis of other lipids. Recently, we reported that oral administration of PTM in mouse models (db/db and db/+) with high de novo lipogenesis (DNL) tone inhibited DNL and enhanced glucose oxidation, which in turn led to net reduction of liver triglycerides (TG), reduced ambient glucose, and improved insulin sensitivity. The present study was conducted to explore translatability and the therapeutic potential of FAS inhibition for the treatment of diabetes in humans.MethodsWe tested PTM in animal models with different DNL tones, i.e. intrinsic synthesis rates, which vary among species and are regulated by nutritional and disease states, and confirmed glucose-lowering efficacy of PTM in lean NHPs with quantitation of liver lipid by MRS imaging. To understand the direct effect of PTM on liver metabolism, we performed ex vivo liver perfusion study to compare FAS inhibitor and carnitine palmitoyltransferase 1 (CPT1) inhibitor.ResultsThe efficacy of PTM is generally reproduced in preclinical models with DNL tones comparable to humans, including lean and established diet-induced obese (eDIO) mice as well as non-human primates (NHPs). Similar effects of PTM on DNL reduction were observed in lean and type 2 diabetic rhesus and lean cynomolgus monkeys after acute and chronic treatment of PTM. Mechanistically, PTM lowers plasma glucose in part by enhancing hepatic glucose uptake and glycolysis. Teglicar, a CPT1 inhibitor, has similar effects on glucose uptake and glycolysis. In sharp contrast, Teglicar but not PTM significantly increased hepatic TG production, thus caused liver steatosis in eDIO mice.ConclusionsThese findings demonstrate unique properties of PTM and provide proof-of-concept of FAS inhibition having potential utility for the treatment of diabetes and related metabolic disorders.

Long-term Stress with Hyperglucocorticoidemia-induced Hepatic Steatosis with VLDL Overproduction Is Dependent on both 5-HT2 Receptor and 5-HT Synthesis in Liver.

Hepatic triglycerides production and adipose lipolysis are pivotal for long-term stress (LTS) or hyperglucocorticoidemia-induced insulin resistance. 5-hydroxytryptamine (5-HT) has been demonstrated to induce hepatic lipid metabolic abnormality by activating mammalian target of rapamycin (mTOR). In present study, we explored whether 5-HT is involved in LTS effects in liver using restraint stress-exposed rats and cultured primary rat hepatocytes and HepG2 cells. LTS with hyperglucocorticoidemia induced hepatic 5-HT synthetic increase with tryptophan hydroxylase 1 (Tph1) up-regulation, and 5-HT2 receptor (5-HT2R, including 5-HT2A, 2B receptor) up-regulation in liver and visceral adipose, as well as hepatic mTOR activation with triglycerides and VLDL overproduction with steatosis, and visceral adipose lipolytic increase with high blood free fatty acids (FFAs) level. 5-HT exposure exhibited LTS-like effects in both tissues, and both LTS and 5-HT effects could be abolished significantly by blocking 5-HT2R. In HepG2 cells dexamethasone or palmitate-induced mTOR activation with triglycerides and VLDL overproduction were accompanied by up-regulations of 5-HT synthesis and 5-HT2R, which were significantly abolished by gene silencing Tph1 or 5-HT2R and were almost fully abolished by co-silencing of both, especially on VLDL overproduction. Chemical inhibition of Tph1 or/and 5-HT2R in both hepatocytes exhibited similar abolishment with genetic inhibition on dexamethason-induced effects. 5-HT-stimulated effects in both hepatocytes were fully abolished by blocking 5-HT2R, while 5-HT itself also up-regulated 5-HT2R. In conclusion, up-regulated hepatic 5-HT synthesis and 5-HT2R induced by both glucocorticoid and FFAs are crucial for LTS-induced hepatic steatosis with VLDL overproduction, while 5-HT by acting on 5-HT2R mediates mTOR activation in liver.

Silencing CCR2 in Macrophages Alleviates Adipose Tissue Inflammation and the Associated Metabolic Syndrome in Dietary Obese Mice.

Adipose tissue macrophage (ATM)-mediated inflammation is a key feature contributing to the adverse metabolic outcomes of dietary obesity. Recruitment of macrophages to obese adipose tissues (AT) can occur through the engagement of CCR2, the receptor for MCP-1 (monocyte chemoattractant protein-1), which is expressed on peripheral monocytes/macrophages. Here, we show that i.p. administration of a rabies virus glycoprotein-derived acetylcholine receptor-binding peptide effectively delivers complexed siRNA into peritoneal macrophages and ATMs in a mouse model of high-fat diet-induced obesity. Treatment with siRNA against CCR2 inhibited macrophage infiltration and accumulation in AT and, therefore, proinflammatory cytokines produced by macrophages. Consequently, the treatment significantly improved glucose tolerance and insulin sensitivity profiles, and also alleviated the associated symptoms of hepatic steatosis and reduced hepatic triglyceride production. These results demonstrate that disruption of macrophage chemotaxis to the AT through cell-targeted gene knockdown strategies can provide a therapeutic intervention for obesity-related metabolic diseases. The study also highlights a siRNA delivery approach for targeting specific monocyte subsets that contribute to obesity-associated inflammation without affecting the function of other tissue-resident macrophages that are essential for host homeostasis and survival.

Discovery and Pharmacology of a Novel Class of Diacylglycerol Acyltransferase 2 Inhibitors.

DGAT2 plays a critical role in hepatic triglyceride production, and data suggests that inhibition of DGAT2 could prove to be beneficial in treating a number of disease states. This article documents the discovery and optimization of a selective small molecule inhibitor of DGAT2 as well as pharmacological proof of biology in a mouse model of triglyceride production.

Ectopic expression of N-acetylglucosaminyltransferase V accelerates hepatic triglyceride synthesis.

Glycosylation changes induce various types of biological phenomena in human diseases. N-Acetylglucosaminyltransferase V (GnT-V) is one of the most important glycosyltransferases involved in cancer biology. Recently, many researchers have challenged studies of lipid metabolism in cancer. To elucidate the relationships between cancer and lipid metabolism more precisely, we investigated the effects of GnT-V on lipid metabolism. In this study, we investigated the effects of aberrant glycosylation by GnT-V on hepatic triglyceride production. We compared lipid metabolism in GnT-V transgenic (Tg) mice with that of wild-type (WT) mice fed with normal chow or a choline-deficient amino acid-defined (CDAA) diet in vivo. HepG2 cells and GnT-V transfectants of Hep3B cells were used in an in vitro study. Serum triglyceride levels and hepatic very low-density lipoprotein (VLDL) secretion in Tg mice were significantly elevated compared with that of WT mice. Hepatic lipogenic genes (Lxrα, Srebp1, Fas and Acc) and VLDL secretion-related gene (Mttp1) were significantly higher in Tg mice. Expression of these genes was also significantly higher in GnT-V transfectants than in mock cells. Knockdown of GnT-V decreased, while both epidermal growth factor and transforming growth factor-β1 stimulation increased LXRα gene expression in HepG2 cells. Finally, we found that the blockade of VLDL secretion by CDAA diet induced massive hepatic steatosis in Tg mice. Our study demonstrates that enhancement of hepatic GnT-V activity accelerates triglyceride synthesis and VLDL secretion. Glycosylation modification by GnT-V regulation could be a novel target for a therapeutic approach to lipid metabolism.

Lipolysis, and not hepatic lipogenesis, is the primary modulator of triglyceride levels in streptozotocin-induced diabetic mice.

Diabetic hypertriglyceridemia is thought to be primarily driven by increased hepatic de novo lipogenesis. However, experiments in animal models indicated that insulin deficiency should decrease hepatic de novo lipogenesis and reduce plasma triglyceride levels. To address the discrepancy between human data and genetically altered mouse models, we investigated whether insulin-deficient diabetic mice had triglyceride changes that resemble those in diabetic humans. Streptozotocin-induced insulin deficiency increased plasma triglyceride levels in mice. Contrary to the mouse models with impaired hepatic insulin receptor signaling, insulin deficiency did not reduce hepatic triglyceride secretion and de novo lipogenesis-related gene expression. Diabetic mice had a marked decrease in postprandial triglycerides clearance, which was associated with decreased lipoprotein lipase and peroxisome proliferator-activated receptor α mRNA levels in peripheral tissues and decreased lipoprotein lipase activity in skeletal muscle, heart, and brown adipose tissue. Diabetic heterozygous lipoprotein lipase knockout mice had markedly elevated fasting plasma triglyceride levels and prolonged postprandial triglycerides clearance. Insulin deficiency causes hypertriglyceridemia by decreasing peripheral lipolysis and not by an increase in hepatic triglycerides production and secretion.

Western diet and/or lifestyle: is this a big health problem?

There is no doubt that the interaction between genetic and environmental (mainly dietary) factors plays a very significant role in the aetiology of many socially significant diseases. It should be noted that the human genome was originally selected in completely different living conditions and has remained unchanged since the agricultural revolution 10,000 years ago. Accumulating evidence suggests that the discrepancies between our Palaeolithic genome and ‘modern’ Western diet and lifestyle could play a significant role in the ongoing epidemics of obesity, hypertension, diabetes, atherosclerosis and other symptoms of metabolic syndrome (Kuneš & Zicha, 2009). This has been caused by the transition from wild and unprocessed food to a diet high in fat, sugar and salt. It has also been suggested that a sedentary lifestyle and overweight are major clinical and economic problems in modern societies. The worldwide epidemic of overweight is due to the imbalance between physical activity and dietary energy intake. A sedentary lifestyle, unhealthy diet and consequent overweight and obesity markedly increase the risk of subsequent cardiac events, such as non-fatal arrhythmia, myocardial infarction and sudden cardiac death. Regular physical activity (45–60 min per day) prevents unhealthy weight gain and obesity, whereas sedentary behaviours, such as watching television and/or working on a computer promote them. The best long-term results may be achieved when physical activity produces an energy expenditure of at least 200–500 kcal day−1. Regular moderate physical activity (mainly aerobic exercise), a healthy diet and avoidance of unhealthy weight gain are effective and safe ways to prevent and treat cardiovascular diseases. Moreover, there is plenty of evidence that exercise lowers blood pressure and has a beneficial influence on serum lipids. Recently, attention has been focused on the excessive accumulation of triglycerides (TGs) within the liver as a component of metabolic syndrome. It proves that fat accumulation in the liver is associated with several features of insulin resistance. Accumulation of TGs in hepatocytes is regulated by the integrated activities of cellular mechanisms that facilitate hepatic TG uptake, fatty acid synthesis and their esterification (input), which is balanced by hepatic fatty acid oxidation and TG export (output; den Boer et al. 2004). Steatosis occurs when input exceeds the capacity for output. Hepatic non-alcoholic steatosis is one of the most frequent liver diseases, occurring together with the plurimetabolic syndrome in overweight people. In this issue of Experimental Physiology, Crescenzo et al. (2014) clearly demonstrated that the short-term combined effects of dietary fat and fructose fed to adult rats, in proportions that mimic the Western diet, are similar to those induced by a high-fat diet alone. However, dual effects of this dietary regimen were apparent. Fructose supplementation did not worsen the deleterious effects of the high-fat diet on whole-body composition, but it worsened the plasma lipid profile by increasing circulating lipids, predominantly TGs and non-esterified fatty acids (NEFAs). These authors in their previous work (Crescenzo et al. 2008) have demonstrated that long-term feeding of a high-fat diet increases plasma NEFAs, which are associated with insulin resistance. Therefore, they hypothesized that increased plasma levels of NEFAs could contribute to hepatic insulin resistance during consumption of a high-fat diet. This hepatic insulin resistance is associated with steatosis, which means that the liver is less sensitive to the suppressive effects of insulin on hepatic glucose and TG production. Indeed, in their present paper (Crescenzo et al. 2014), the high-fat diet led to hepatic TG accumulation, and this was further increased by fructose supplementation. This observation indicates that the amount of liver TG content is not constant but can easily be modulated by nutritional conditions. The authors should be congratulated for this demonstration that short-term consumption of a Western diet, rich in saturated fat and sucrose, is more conducive to development of liver steatosis in adult rats than consumption of a high-fat diet alone. However, one should ask the question, what would be the situation in young animals in light of the fact that the number of overweight children is increasing? The answer is complicated because there is no information about developmental windows (critical developmental periods) regarding hepatic TG accumulation, hepatic insulin resistance and hepatic non-alcoholic steatosis. It is clear that the developing organism represents an ‘ideal terrain’ for the induction of many modifications, which could lead to the onset of certain diseases (Kuneš et al. 2012); therefore, detailed knowledge about the ontogenetic interactions of experimental animals with environmental factors might help us to understand the aetiology of many human diseases. Readers are invited to give their opinion on this article. To submit a comment, go to: http://ep.physoc.org/letters/submit/expphysiol;99/9/1180 None declared. The author is partially supported by grants AV0Z 50110509, 304/12/0259 (GA CR).

ATF4 Protein Deficiency Protects against High Fructose-induced Hypertriglyceridemia in Mice

Hypertriglyceridemia is the most common lipid disorder in obesity and type 2 diabetes. It results from increased production and/or decreased clearance of triglyceride-rich lipoproteins. To better understand the pathophysiology of hypertriglyceridemia, we studied hepatic regulation of triglyceride metabolism by the activating transcription factor 4 (ATF4), a member of the basic leucine zipper-containing protein subfamily. We determined the effect of ATF4 on hepatic lipid metabolism in Atf4(-/-) mice fed regular chow or provided with free access to fructose drinking water. ATF4 depletion preferentially attenuated hepatic lipogenesis without affecting hepatic triglyceride production and fatty acid oxidation. This effect prevented excessive fat accumulation in the liver of Atf4(-/-) mice, when compared with wild-type littermates. To gain insight into the underlying mechanism, we showed that ATF4 depletion resulted in a significant reduction in hepatic expression of peroxisome proliferator-activated receptor-γ, a nuclear receptor that acts to promote lipogenesis in the liver. This effect was accompanied by a significant reduction in hepatic expression of sterol regulatory element-binding protein 1c (SREBP-1c), acetyl-CoA carboxylase, and fatty-acid synthase, three key functions in the lipogenic pathway in Atf4(-/-) mice. Of particular significance, we found that Atf4(-/-) mice, as opposed to wild-type littermates, were protected against the development of steatosis and hypertriglyceridemia in response to high fructose feeding. These data demonstrate that ATF4 plays a critical role in regulating hepatic lipid metabolism in response to nutritional cues.

Potential mechanisms of atypical antipsychotic-induced hypertriglyceridemia

The development of atypical antipsychotic (AAP) drugs has brought about dramatic improvement in the function of many patients with schizophrenia and related mental disorders. However, prescription of AAPs is frequently associated with the emergence of weight gain, hypertriglyceridemia, and other metabolic disturbances. Although the mechanisms involved in AAP-induced hypertriglyceridemia remain to be fully elucidated, several studies have proposed that this side effect may be associated with weight gain and obesity. Recently, special emphasis has been placed on the evidence indicating a direct effect of AAPs on triglyceride metabolism. In this review, we highlight recent findings discussing the potential mechanisms by which AAPs may contribute to hypertriglyceridemia. In addition, we summarize the adjunctive pharmacologic treatments for AAP-associated dyslipidemia. There is evidence that AAPs may cause hypertriglyceridemia through several possible mechanisms: (1) a direct effect on triglyceride metabolism either by stimulation of hepatic triglyceride production and secretion or by inhibition of lipoprotein lipase-mediated triglyceride hydrolysis and (2) an indirect mechanism associated with obesity and insulin resistance. The practical applications of this manuscript provide new insights for the future investigation of AAPs.