Carboxylesterase 1d (Ces1d) does not contribute to cholesteryl ester hydrolysis in the liver.

The liver is the central organ regulating cholesterol synthesis, storage, transport, and elimination. Mouse carboxylesterase 1d (Ces1d) and its human ortholog CES1 have been described to possess lipase activity and play roles in hepatic triacylglycerol metabolism and VLDL assembly. It has been proposed that Ces1d/CES1 might also catalyze cholesteryl ester (CE) hydrolysis in the liver and thus be responsible for the hydrolysis of HDL-derived CE; this could contribute to the final step in the reverse cholesterol transport (RCT) pathway, wherein cholesterol is secreted from the liver into bile and feces, either directly or after conversion to water-soluble bile salts. However, the proposed function of Ces1d/CES1 as a CE hydrolase is controversial. In this study, we interrogated the role hepatic Ces1d plays in cholesterol homeostasis using liver-specific Ces1d-deficient mice. We rationalized that if Ces1d is a major hepatic CE hydrolase, its absence would (1) reduce in vivo RCT flux and (2) provoke liver CE accumulation after a high-cholesterol diet challenge. We found that liver-specific Ces1d-deficient mice did not show any difference in the flux of in vivo HDL-to-feces RCT nor did it cause additional liver CE accumulation after high-fat, high-cholesterol Western-type diet feeding. These findings challenge the importance of Ces1d as a major hepatic CE hydrolase.

Effects of Chlorpyrifos on Serine Hydrolase Activities, Lipid Mediators, and Immune Responses in Lungs of Neonatal and Adult Mice.

Chlorpyrifos (CPF) is an organophosphate (OP) pesticide that causes acute toxicity by inhibiting acetylcholinesterase (AChE) in the nervous system. However, endocannabinoid (eCB) metabolizing enzymes in brain of neonatal rats are more sensitive than AChE to inhibition by CPF, leading to increased levels of eCBs. Because eCBs are immunomodulatory molecules, we investigated the association between eCB metabolism, lipid mediators, and immune function in adult and neonatal mice exposed to CPF. We focused on lung effects because epidemiologic studies have linked pesticide exposures to respiratory diseases. CPF was hypothesized to disrupt lung eCB metabolism and alter lung immune responses to lipopolysaccharide (LPS), and these effects would be more pronounced in neonatal mice due to an immature immune system. We first assessed the biochemical effects of CPF in adult mice (≥8 weeks old) and neonatal mice after administering CPF (2.5 mg/kg, oral) or vehicle for 7 days. Tissues were harvested 4 h after the last CPF treatment and lung microsomes from both age groups demonstrated CPF-dependent inhibition of carboxylesterases (Ces), a family of xenobiotic and lipid metabolizing enzymes, whereas AChE activity was inhibited in adult lungs only. Activity-based protein profiling (ABPP)-mass spectrometry of lung microsomes identified 31 and 32 individual serine hydrolases in neonatal lung and adult lung, respectively. Of these, Ces1c/Ces1d/Ces1b isoforms were partially inactivated by CPF in neonatal lung, whereas Ces1c/Ces1b and Ces1c/BChE were partially inactivated in adult female and male lungs, respectively, suggesting age- and sex-related differences in their sensitivity to CPF. Monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) activities in lung were unaffected by CPF. When LPS (1.25 mg/kg, i.p.) was administered following the 7-day CPF dosing period, little to no differences in lung immune responses (cytokines and immunophenotyping) were noted between the CPF and vehicle groups. However, a CPF-dependent increase in the amounts of dendritic cells and certain lipid mediators in female lung following LPS challenge was observed. Experiments in neonatal and adult Ces1d-/- mice yielded similar results as wild type mice (WT) following CPF treatment, except that CPF augmented LPS-induced Tnfa mRNA in adult Ces1d-/- mouse lungs. This effect was associated with decreased expression of Ces1c mRNA in Ces1d-/- mice versus WT mice in the setting of LPS exposure. We conclude that CPF exposure inactivates several Ces isoforms in mouse lung and, during an inflammatory response, increases certain lipid mediators in a female-dependent manner. However, it did not cause widespread altered lung immune effects in response to an LPS challenge.

Ces1d deficiency protects against high-sucrose diet-induced hepatic triacylglycerol accumulation.

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease. Triacylglycerol accumulation in the liver is a hallmark of NAFLD. Metabolic studies have confirmed that increased hepatic de novo lipogenesis (DNL) in humans contributes to fat accumulation in the liver and to NAFLD progression. Mice deficient in carboxylesterase (Ces)1d expression are protected from high-fat diet-induced hepatic steatosis. To investigate whether loss of Ces1d can also mitigate steatosis induced by over-activated DNL, WT and Ces1d-deficient mice were fed a lipogenic high-sucrose diet (HSD). We found that Ces1d-deficient mice were protected from HSD-induced hepatic lipid accumulation. Mechanistically, Ces1d deficiency leads to activation of AMP-activated protein kinase and inhibitory phosphorylation of acetyl-CoA carboxylase. Together with our previous demonstration that Ces1d deficiency attenuated high-fat diet-induced steatosis, this study suggests that inhibition of CES1 (the human ortholog of Ces1d) might represent a novel pharmacological target for prevention and treatment of NAFLD.

Genetic variation in human carboxylesterase CES1 confers resistance to hepatic steatosis.

Obesity often leads non-alcoholic fatty liver disease, insulin resistance and hyperlipidemia. Expression of carboxylesterase CES1 is positively correlated with increased lipid storage and plasma lipid concentration. Here we investigated structural and metabolic consequences of a single nucleotide polymorphism in CES1 gene that results in p.Gly143Glu amino acid substitution. We generated a humanized mouse model expressing CES1WT (control), CES1G143E and catalytically dead CES1S221A (negative control) in the liver in the absence of endogenous expression of the mouse orthologous gene. We show that the CES1G143E variant exhibits only 20% of the wild-type lipolytic activity. High-fat diet fed mice expressing CES1G143E had reduced liver and plasma triacylglycerol levels. The mechanism by which decreased CES1 activity exerts this hypolipidemic phenotype was determined to include decreased very-low density lipoprotein secretion, decreased expression of hepatic lipogenic genes and increased fatty acid oxidation as determined by increased plasma ketone bodies and hepatic mitochondrial electron transport chain protein abundance. We conclude that attenuation of human CES1 activity provides a beneficial effect on hepatic lipid metabolism. These studies also suggest that CES1 is a potential therapeutic target for non-alcoholic fatty liver disease management.

Oxidative Stress Attenuates Lipid Synthesis and Increases Mitochondrial Fatty Acid Oxidation in Hepatoma Cells Infected with Hepatitis C Virus.

Cytopathic effects are currently believed to contribute to hepatitis C virus (HCV)-induced liver injury and are readily observed in Huh7.5 cells infected with the JFH-1 HCV strain, manifesting as apoptosis highly correlated with growth arrest. Reactive oxygen species, which are induced by HCV infection, have recently emerged as activators of AMP-activated protein kinase. The net effect is ATP conservation via on/off switching of metabolic pathways that produce/consume ATP. Depending on the scenario, this can have either pro-survival or pro-apoptotic effects. We demonstrate reactive oxygen species-mediated activation of AMP-activated kinase in Huh7.5 cells during HCV (JFH-1)-induced growth arrest. Metabolic labeling experiments provided direct evidence that lipid synthesis is attenuated, and ß-oxidation is enhanced in these cells. A striking increase in nuclear peroxisome proliferator-activated receptor α, which plays a dominant role in the expression of ß-oxidation genes after ligand-induced activation, was also observed, and we provide evidence that peroxisome proliferator-activated receptor α is constitutively activated in these cells. The combination of attenuated lipid synthesis and enhanced ß-oxidation is not conducive to lipid accumulation, yet cellular lipids still accumulated during this stage of infection. Notably, the serum in the culture media was the only available source for polyunsaturated fatty acids, which were elevated (2-fold) in the infected cells, implicating altered lipid import/export pathways in these cells. This study also provided the first in vivo evidence for enhanced ß-oxidation during HCV infection because HCV-infected SCID/Alb-uPA mice accumulated higher plasma ketones while fasting than did control mice. Overall, this study highlights the reprogramming of hepatocellular lipid metabolism and bioenergetics during HCV infection, which are predicted to impact both the HCV life cycle and pathogenesis.

Liver-specific expression of carboxylesterase 1g/esterase-x reduces hepatic steatosis, counteracts dyslipidemia and improves insulin signaling.

Ces1g/Es-x deficiency in mice results in weight gain, insulin resistance, fatty liver and hyperlipidemia through upregulation of de novo lipogenesis and oversecretion of triacylglycerol (TG)-rich lipoproteins. Here, we show that restoration of Ces1g/Es-x expression only in the liver significantly reduced hepatic TG concentration accompanied by decreased size of lipid droplets, reduced secretion of very low-density lipoproteins and improved insulin-mediated signal transduction in the liver. Collectively, these results demonstrate that hepatic Ces1g/Es-x plays a critical role in limiting hepatic steatosis, very low-density lipoprotein assembly and in augmenting insulin sensitivity.

Subcutaneous adiposity is an independent predictor of mortality in cancer patients.

BACKGROUND: Prognostic significance of adiposity, at the time of cancer diagnosis, on survival is not clear. Body mass index (kg m-2) does not provide an appropriate assessment of body composition; therefore, the concept of the 'obesity paradox' needs to be investigated based on the prognostic significance of fat and muscle. Independent prognostic significance of adipose tissue in predicting mortality, importance of visceral and subcutaneous adiposity in the presence and absence of sarcopenia on survival, was investigated. METHODS: Adiposity markers including total adipose index (TATI), visceral adipose tissue index (VATI) and subcutaneous adipose tissue index (SATI) were estimated for 1473 gastrointestinal and respiratory cancer patients and 273 metastatic renal cell carcinoma patients using computed tomography. Univariate and multivariate analysis to determine mortality hazard ratios (HR) were conducted using cox proportional hazard models. RESULTS: Low SATI (SATI <50.0 cm2 m-2 in males and <42.0 cm2 m-2 in females) independently associated with increased mortality (HR: 1.26; 95% CI: 1.11-1.43; P<0.001) and shorter survival (13.1 months; 95% CI, 11.4-14.7) compared to patients with high SATI (19.3 months; 95% CI, 17.6-21.0; P<0.001). In the presence of sarcopenia, the longest survival was observed in patients with high subcutaneous adiposity. CONCLUSIONS: Subcutaneous adipose tissues appear to associate with reduction in mortality risk demonstrating the prognostic importance of fat distribution. The effect of sarcopenia on survival was more pronounced in patients with low subcutaneous adiposity.

Chemotherapy diminishes lipid storage capacity of adipose tissue in a preclinical model of colon cancer.

BACKGROUND: Accelerated loss of adipose tissue in cancer is associated with shorter survival, and reduced quality of life. Evidence is emerging suggesting tumour association with alterations in adipose tissue, but much less is known about drug-related mechanisms contributing to adipose atrophy. Identification of mechanisms by which tumour and cancer treatments, such as chemotherapy, affect adipose tissue are required to develop appropriate therapeutic interventions to prevent fat depletion in cancer. This pre-clinical study aimed to assess alterations in adipose tissue during the clinical course of cancer. METHODS: Fischer 344 rats bearing the Ward colorectal tumour were euthanized before chemotherapy, after 1- cycle, or 2-cycles of a combination chemotherapy consisting of Irinotecan (CPT-11) combined with 5-fluorouracil (5-FU), which recapitulates first line treatment for human colorectal cancer. Periuterine adipose tissue was isolated. Healthy rats served as a reference group. Histological analysis (hematoxylin and eosin), Real-time PCR (TaqMan) and proteomic analysis (LC-MS/MS) were performed. RESULTS: Larger adipocytes (3993.7 ± 52.6 µm2) in tumour-bearing animals compared to the reference group (3227.7 ± 36.7 µm2; p < 0.001) was associated with reduced expression of proteins involved in mitochondrial fatty acid oxidation. The presence of a tumour has a significant effect on phospholipid but not triglyceride fatty acid composition. There were greater proportions of saturated fatty acids concurrent with lower monounsaturated fatty acids within the PL fraction of adipocytes in tumour-bearing animals. Chemotherapy treatment decreased the size of adipocytes (2243.9 ± 30.4 µm2; p < 0.001) and led to depletion of n-3 polyunsaturated fatty acids in adipose tissue triglyceride. Evaluation of the proteome profile revealed decreased expression of proteins involved in ATP generation, ß-oxidation, and lipogenesis. Overall, adipose tissue may not be able to efficiently oxidize fatty acids to provide energy to maintain energy demanding pathways like lipogenesis inside the tissue. CONCLUSIONS: In conclusion, metabolic adaptations to mitochondrial impairment may contribute to diminished lipid storage capacity of adipose tissue following chemotherapy delivery.

Pioglitazone attenuates hepatic inflammation and fibrosis in phosphatidylethanolamine N-methyltransferase-deficient mice.

Phosphatidylethanolamine N-methyltransferase (PEMT) is an important enzyme in hepatic phosphatidylcholine (PC) biosynthesis. Pemt(-/-) mice are protected against high-fat diet (HFD)-induced obesity and insulin resistance; however, these mice develop nonalcoholic fatty liver disease (NAFLD). We hypothesized that peroxisomal proliferator-activated receptor-γ (PPARγ) activation by pioglitazone might stimulate adipocyte proliferation, thereby directing lipids from the liver toward white adipose tissue. Pioglitazone might also act directly on PPARγ in the liver to improve NAFLD. Pemt(+/+) and Pemt(-/-) mice were fed a HFD with or without pioglitazone (20 mg·kg(-1)·day(-1)) for 10 wk. Pemt(-/-) mice were protected from HFD-induced obesity but developed NAFLD. Treatment with pioglitazone caused an increase in body weight gain in Pemt(-/-) mice that was mainly due to increased adiposity. Moreover, pioglitazone improved NAFLD in Pemt(-/-) mice, as indicated by a 35% reduction in liver weight and a 57% decrease in plasma alanine transaminase levels. Livers from HFD-fed Pemt(-/-) mice were steatotic, inflamed, and fibrotic. Hepatic steatosis was still evident in pioglitazone-treated Pemt(-/-) mice; however, treatment with pioglitazone reduced hepatic fibrosis, as evidenced by reduced Sirius red staining and lowered mRNA levels of collagen type Iα1 (Col1a1), tissue inhibitor of metalloproteinases 1 (Timp1), α-smooth muscle actin (Acta2), and transforming growth factor-ß (Tgf-ß). Similarly, oxidative stress and inflammation were reduced in livers from Pemt(-/-) mice upon treatment with pioglitazone. Together, these data show that activation of PPARγ in HFD-fed Pemt(-/-) mice improved liver function, while these mice were still protected against diet-induced obesity and insulin resistance.

Roles of Acyl-CoA:Diacylglycerol Acyltransferases 1 and 2 in Triacylglycerol Synthesis and Secretion in Primary Hepatocytes.

OBJECTIVE: Very low-density lipoprotein assembly and secretion are regulated by the availability of triacylglycerol. Although compelling evidence indicates that the majority of triacylglycerol in very low-density lipoprotein is derived from re-esterification of lipolytic products released by endoplasmic reticulum-associated lipases, little is known about roles of acyl-CoA:diacylglycerol acyltransferases (DGATs) in this process. We aimed to investigate the contribution of DGAT1 and DGAT2 in lipid metabolism and lipoprotein secretion in primary mouse and human hepatocytes. APPROACH AND RESULTS: We used highly selective small-molecule inhibitors of DGAT1 and DGAT2, and we tracked storage and secretion of lipids synthesized de novo from [(3)H]acetic acid and from exogenously supplied [(3)H]oleic acid. Inactivation of individual DGAT activity did not affect incorporation of either radiolabeled precursor into intracellular triacylglycerol, whereas combined inactivation of both DGATs severely attenuated triacylglycerol synthesis. However, inhibition of DGAT2 augmented fatty acid oxidation, whereas inhibition of DGAT1 increased triacylglycerol secretion, suggesting preferential channeling of separate DGAT-derived triacylglycerol pools to distinct metabolic pathways. Inactivation of DGAT2 impaired cytosolic lipid droplet expansion, whereas DGAT1 inactivation promoted large lipid droplet formation. Moreover, inactivation of DGAT2 attenuated expression of lipogenic genes. Finally, triacylglycerol secretion was significantly reduced on DGAT2 inhibition without altering extracellular apolipoprotein B levels. CONCLUSIONS: Our data suggest that DGAT1 and DGAT2 can compensate for each other to synthesize triacylglycerol, but triacylglycerol synthesized by DGAT1 is preferentially channeled to oxidation, whereas DGAT2 synthesizes triacylglycerol destined for very low-density lipoprotein assembly.

Endoplasmic reticulum-localized hepatic lipase decreases triacylglycerol storage and VLDL secretion.

Hepatic triacylglycerol levels are governed through synthesis, degradation and export of this lipid. Here we demonstrate that enforced expression of hepatic lipase in the endoplasmic reticulum in McArdle RH7777 hepatocytes resulted in a significant decrease in the incorporation of fatty acids into cellular triacylglycerol and cholesteryl ester accompanied by attenuation of secretion of apolipoprotein B-containing lipoproteins. Hepatic lipase-mediated depletion of intracellular lipid storage increased the expression of peroxisome proliferator-activated receptor α and its target genes and augmented oxidation of fatty acids. These data show that 1) hepatic lipase is active in the endoplasmic reticulum and 2) intracellular hepatic lipase modulates cellular lipid metabolism and lipoprotein secretion.

Identifying molecular features that distinguish fluvastatin-sensitive breast tumor cells.

Statins, routinely used to treat hypercholesterolemia, selectively induce apoptosis in some tumor cells by inhibiting the mevalonate pathway. Recent clinical studies suggest that a subset of breast tumors is particularly susceptible to lipophilic statins, such as fluvastatin. To quickly advance statins as effective anticancer agents for breast cancer treatment, it is critical to identify the molecular features defining this sensitive subset. We have therefore characterized fluvastatin sensitivity by MTT assay in a panel of 19 breast cell lines that reflect the molecular diversity of breast cancer, and have evaluated the association of sensitivity with several clinicopathological and molecular features. A wide range of fluvastatin sensitivity was observed across breast tumor cell lines, with fluvastatin triggering cell death in a subset of sensitive cell lines. Fluvastatin sensitivity was associated with an estrogen receptor alpha (ERα)-negative, basal-like tumor subtype, features that can be scored with routine and/or strong preclinical diagnostics. To ascertain additional candidate sensitivity-associated molecular features, we mined publicly available gene expression datasets, identifying genes encoding regulators of mevalonate production, non-sterol lipid homeostasis, and global cellular metabolism, including the oncogene MYC. Further exploration of this data allowed us to generate a 10-gene mRNA abundance signature predictive of fluvastatin sensitivity, which showed preliminary validation in an independent set of breast tumor cell lines. Here, we have therefore identified several candidate predictors of sensitivity to fluvastatin treatment in breast cancer, which warrant further preclinical and clinical evaluation.

Ces3/TGH deficiency improves dyslipidemia and reduces atherosclerosis in Ldlr(-/-) mice.

RATIONALE: Carboxylesterase 3/triacylglycerol hydrolase (TGH) has been shown to participate in hepatic very low-density lipoprotein (VLDL) assembly. Deficiency of TGH in mice lowers plasma lipids and atherogenic lipoproteins without inducing hepatic steatosis. OBJECTIVE: To investigate the contribution of TGH to atherosclerotic lesion development in mice that lack low-density lipoprotein receptor (LDLR). METHODS AND RESULTS: Mice deficient in LDL receptor (Ldlr(-/-)) and mice lacking both TGH and LDLR (Tgh(-/-)/Ldlr(-/-)) were fed with a Western-type diet for 12 weeks. Analysis of Tgh(-/-)/Ldlr(-/-) plasma showed an atheroprotective lipoprotein profile with decreased cholesterol in the VLDL and the LDL fractions, concomitant with elevated high-density lipoprotein cholesterol. Significantly reduced plasma apolipoprotein B levels were also observed in Tgh(-/-)/Ldlr(-/-) mice. Consequently, Tgh(-/-)/Ldlr(-/-) mice presented with a significant reduction (54%, P<0.01) of the high-fat, high-cholesterol dieteninduced atherosclerotic plaques when compared with Tgh(+/+)/Ldlr(-/-) mice in the cross-sectional aortic root analysis. TGH deficiency did not further increase liver steatosis despite lowering plasma lipids, mainly due to reduced hepatic lipogenesis. The ameliorated dyslipidemia in Tgh(-/-)/Ldlr(-/-) mice was accompanied with significantly improved insulin sensitivity. CONCLUSIONS: Inhibition of TGH activity ameliorates atherosclerosis development and improves insulin sensitivity in Ldlr(-/-) mice.

Liver triacylglycerol lipases.

The hallmark of obesity and one of the key contributing factors to insulin resistance, type 2 diabetes and cardiovascular disease is excess triacylglycerol (TG) storage. In hepatocytes, excessive accumulation of TG is the common denominator of a wide range of clinicopathological entities known as nonalcoholic fatty liver disease, which can eventually progress to cirrhosis and associated complications including hepatic failure, hepatocellular carcinoma and death. A tight regulation between TG synthesis, hydrolysis, secretion and fatty acid oxidation is required to prevent lipid accumulation as well as lipid depletion from hepatocytes. Therefore, understanding the pathways that regulate hepatic TG metabolism is crucial for development of therapies to ameliorate pathophysiological conditions associated with excessive hepatic TG accumulation, including dyslipidemias, viral infection and atherosclerosis. This review highlights the physiological roles of liver lipases that degrade TG in cytosolic lipid droplets, endoplasmic reticulum, late endosomes/lysosomes and along the secretory route. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Arylacetamide deacetylase: a novel host factor with important roles in the lipolysis of cellular triacylglycerol stores, VLDL assembly and HCV production.

BACKGROUND & AIMS: Very low density lipoproteins (VLDLs) are triacylglycerol (TG)-rich lipoproteins produced by the human liver. VLDLs derive the majority of their TG cargo from the lipolysis of TG stored in hepatocellular lipid droplets (LDs). Important roles for LDs and the VLDL secretory pathway in the cell culture production of infectious hepatitis C virus (HCV) have been established. We hypothesized that TG lipolysis and VLDL production are impaired during HCV infection so that these cellular processes can be diverted towards HCV production. METHODS: We used an HCV permissive cell culture system (JFH-1/HuH7.5 cells) to examine the relationship between TG lipolysis, VLDL assembly, and the HCV lifecycle using standard biochemical approaches. RESULTS: Lipolysis of cellular TG and VLDL production were impaired in HCV infected cells during the early peak of viral infection. This was partially explained by an apparent deficiency of a putative TG lipase, arylacetamide deacetylase (AADAC). The re-introduction of AADAC to infected cells restored cellular TG lipolysis, indicating a role for HCV-mediated downregulation of AADAC in this process. Defective lipolysis of cellular TG stores and VLDL production were also observed in HuH7.5 cells stably expressing a short hairpin RNA targeting AADAC expression, proving AADAC deficiency contributes to these defective pathways. Finally, impaired production of HCV was observed with AADAC knockdown cells, demonstrating a role for AADAC in the HCV lifecycle. CONCLUSIONS: This insight into the biology of HCV infection and possibly pathogenesis identifies AADAC as a novel and translationally relevant therapeutic target.

Deficiency of carboxylesterase 1/esterase-x results in obesity, hepatic steatosis, and hyperlipidemia.

UNLABELLED: Increased lipogenesis, together with hyperlipidemia and increased fat deposition, contribute to obesity and associated metabolic disorders including nonalcoholic fatty liver disease. Here we show that carboxylesterase 1/esterase-x (Ces1/Es-x) plays a regulatory role in hepatic fat metabolism in the mouse. We demonstrate that Ces1/Es-x knockout mice present with increased hepatic lipogenesis and with oversecretion of apolipoprotein B (apoB)-containing lipoproteins (hepatic very-low density lipoproteins), which leads to hyperlipidemia and increased fat deposition in peripheral tissues. Consequently, Ces1/Es-x knockout mice develop obesity, fatty liver, hyperinsulinemia, and insulin insensitivity on chow diet without change in food intake and present with decreased energy expenditure. Ces1/Es-x deficiency prevents the release of polyunsaturated fatty acids from triacylglycerol stores, leading to an up-regulation of sterol regulatory element binding protein 1c-mediated lipogenesis, which can be reversed with dietary ω-3 fatty acids. CONCLUSION: These studies support a role for Ces1/Es-x in the partitioning of regulatory fatty acids and concomitant control of hepatic lipid biosynthesis, secretion, and deposition.

Liver specific inactivation of carboxylesterase 3/triacylglycerol hydrolase decreases blood lipids without causing severe steatosis in mice.

UNLABELLED: Carboxylesterase 3/triacylglycerol hydrolase (Ces3/TGH) participates in hepatic very low-density lipoprotein (VLDL) assembly and in adipose tissue basal lipolysis. Global ablation of Ces3/Tgh expression decreases serum triacylglycerol (TG) and nonesterified fatty acid levels and improves insulin sensitivity. To understand the tissue-specific role of Ces3/TGH in lipid and glucose homeostasis, we generated mice with a liver-specific deletion of Ces3/Tgh expression (L-TGH knockout [KO]). Elimination of hepatic Ces3/Tgh expression dramatically decreased plasma VLDL TG and VLDL cholesterol concentrations but only moderately increased liver TG levels in mice fed a standard chow diet. Significantly reduced plasma TG and cholesterol without hepatic steatosis were also observed in L-TGH KO mice challenged with a high-fat, high-cholesterol diet. L-TGH KO mice presented with increased plasma ketone bodies and hepatic fatty acid oxidation. Intrahepatic TG in L-TGH KO mice was stored in significantly smaller lipid droplets. Augmented hepatic TG levels in chow-fed L-TGH KO mice did not affect glucose tolerance or glucose production from hepatocytes, but impaired insulin tolerance was observed in female mice. CONCLUSION: Our data suggest that ablation of hepatic Ces3/Tgh expression decreases plasma lipid levels without causing severe hepatic steatosis.

Lumenal lipid metabolism: implications for lipoprotein assembly.

Overproduction of apolipoprotein B (apoB)-containing lipoproteins by the liver and the intestine is 1 of the hallmarks of insulin resistance and type 2 diabetes and a well-established risk factor of cardiovascular disease. The assembly of apoB lipoproteins is regulated by the availability of lipids that form the neutral lipid core (triacylglycerol and cholesteryl ester) and the limiting lipoprotein monolayer (phospholipids and cholesterol). Although tremendous advances have been made over the past decade toward understanding neutral lipid and phospholipid biosynthesis and neutral lipid storage in cytosolic lipid droplets (LDs), little is known about the mechanisms that govern the transfer of lipids to the lumen of the endoplasmic reticulum for apoB lipidation. ApoB-synthesizing organs can deposit synthesized neutral lipids into at least 3 different types of LDs, each decorated with a subset of specific proteins: perilipin-decorated cytosolic LDs, and 2 types of LDs formed in the lumen of the endoplasmic reticulum, the secretion-destined LDs containing apoB, and resident lumenal LDs coated with microsomal triglyceride transfer protein and exchangeable apolipoproteins. This brief review will address the current knowledge of lumenal lipid metabolism in the context of apoB assembly and lipid storage.

Preferential lipolysis of DGAT1 over DGAT2 generated triacylglycerol in Huh7 hepatocytes.

Two distinct diacylglycerol acyltransferases (DGAT1 and DGAT2) catalyze the final committed step of triacylglycerol (TG) synthesis in hepatocytes. After its synthesis in the endoplasmic reticulum (ER) TG is either stored in cytosolic lipid droplets (LDs) or is assembled into very low-density lipoproteins in the ER lumen. TG stored in cytosolic LDs is hydrolyzed by adipose triglyceride lipase (ATGL) and the released fatty acids are converted to energy by oxidation in mitochondria. We hypothesized that targeting/association of ATGL to LDs would differ depending on whether the TG stores were generated through DGAT1 or DGAT2 activities. Individual inhibition of DGAT1 or DGAT2 in Huh7 hepatocytes incubated with oleic acid did not yield differences in TG accretion while combined inhibition of both DGATs completely prevented TG synthesis suggesting that either DGAT can efficiently esterify exogenously supplied fatty acid. DGAT2-made TG was stored in larger LDs, whereas TG formed by DGAT1 accumulated in smaller LDs. Inactivation of DGAT1 or DGAT2 did not alter expression (mRNA or protein) of ATGL, the ATGL activator ABHD5/CGI-58, or LD coat proteins PLIN2 or PLIN5, but inactivation of both DGATs increased PLIN2 abundance despite a dramatic reduction in the number of LDs. ATGL was found to preferentially target to LDs generated by DGAT1 and fatty acids released from TG in these LDs were also preferentially used for fatty acid oxidation. Combined inhibition of DGAT2 and ATGL resulted in larger LDs, suggesting that the smaller size of DGAT1-generated LDs is the result of increased lipolysis of TG in these LDs.

Interfering with lipid metabolism through targeting CES1 sensitizes hepatocellular carcinoma for chemotherapy.

Hepatocellular carcinoma (HCC) is the most common lethal form of liver cancer. Apart from surgical removal and transplantation, other treatments have not yet been well established for patients with HCC. In this study, we found that carboxylesterase 1 (CES1) is expressed at various levels in HCC. We further revealed that blockage of CES1 by pharmacological and genetical approaches leads to altered lipid profiles that are directly linked to impaired mitochondrial function. Mechanistically, lipidomic analyses indicated that lipid signaling molecules, including polyunsaturated fatty acids (PUFAs), which activate PPARα/γ, were dramatically reduced upon CES1 inhibition. As a result, the expression of SCD, a PPARα/γ target gene involved in tumor progression and chemoresistance, was significantly downregulated. Clinical analysis demonstrated a strong correlation between the protein levels of CES1 and SCD in HCC. Interference with lipid signaling by targeting the CES1-PPARα/γ-SCD axis sensitized HCC cells to cisplatin treatment. As a result, the growth of HCC xenograft tumors in NU/J mice was potently slowed by coadministration of cisplatin and CES1 inhibition. Our results, thus, suggest that CES1 is a promising therapeutic target for HCC treatment.