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 upregulation of sterol regulatory element binding protein 1c-mediated lipogenesis, which can be reversed with dietary x-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. (HEPATOLOGY 2012;56:2188-2198 T he liver is the central metabolic organ that regulates key pathways in lipid metabolism including fatty acid (FA) b-oxidation, lipogenesis, as well as lipoprotein uptake and secretion in response to nutritional and hormonal signals. 1 Dysregulation of hepatic lipid metabolic pathways results in the development of hepatic steatosis, which in turn contributes to the development of chronic hepatic inflammation, insulin resistance, and liver damage. 2,3 We have previously demonstrated that carboxylesterase 3/triacylglycerol hydrolase (Ces3/TGH) (recently annotated Ces1d) 4 plays an important role in the provision of lipid substrates for the assembly of apolipoprotein B (apoB)-containing lipoproteins. 5 Ablation of Ces3/Tgh/ Ces1d expression in mice leads to decreased plasma lipids, including triacylglycerol (TG) and nonesterified fatty acids (NEFA), along with decreased apoB100 secretion. 6 Mouse Ces1/Es-x (recently annotated Ces1g) 4 shares $76% protein sequence identity with mouse Ces3/ TGH/Ces1d, including conserved lipase/esterase catalytic triad amino acid residues and regions of the lid domain. 7,8 Hepatic Ces1/Es-x expression was found to be augmented in mice fed either a ketogenic 9 or a cholate supplemented 8 diet and is reduced in stearoylCoA desaturase-1 (SCD-1)-deficient mice fed a verylow-fat diet. 10 It has also been reported that ectopic expression of Ces1/Es-x in McArdle-RH7777 attenuates

Carboxylesterase 3/triacylglycerol hydrolase (Ces3/TGH) participates in hepatic very lowdensity 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. (HEPATOLOGY 2012;56:2154-2162 I ncreased plasma triacylglycerol (TG) concentration represents an independent risk factor for cardiovascular disease.1,2 Elevated levels of circulating TGrich, apolipoprotein B (apoB)-containing lipoproteins (very low-density lipoprotein [VLDL] and chylomicrons) accompany insulin resistance and visceral obesity.1,3-5 Secretion of VLDL depends on the availability of lipids for apoB lipidation, and therefore, intracellular VLDL assembly presents a potential pharmacological target for the treatment of hypertriglyceridemia and associated cardiovascular and metabolic diseases. 6,7 Accumulating evidence suggests that the formation of apoB-containing lipoproteins is accomplished by a two-step process, where a primordial lipoprotein with low amounts of TG is initially synthesized, followed by the addition of TG from preexisting lipid storage pools. [8][9][10] Preformed intrahepatic TG was shown to be the source of 60%-80% of VLDL-TG; however, this TG is not transferred to the primordial lipoprotein en bloc but is delivered via a process involving lipolysis and re-esterification.11-15 Triacylglycerol hydrolase (TGH), also termed carboxylesterase 3 (Ces3) in mice and carboxylesterase 1 in humans, has been suggested to play an important role in the provision of TG for the assembly of apoB-containing lipoproteins.

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. an indirect transfer of triacylglycerol for VLDL maturation. 11 It has been established that the majority (60%-80%) of triacylglycerol in VLDL is derived from re-esterification of lipolytic products in hepatocytes. [12][13][14] This sinuous supply of triacylglycerol for VLDL maturation overcomes the inability of triacylglycerol stored in cytosolic lipid droplets (LDs) to cross the lipid bilayer of the endoplasmic reticulum (ER) and provides a mechanism for the regulation of VLDL secretion independently of plasma FA and intracellular triacylglycerol concentration. 15 We have previously shown that lipases involved in the hydrolysis of preformed triacylglycerol include ER-localized carboxylesterase 1d/triacylglycerol hydrolase and arylacetamide deacetylase [16][17][18] ; however, it has not been determined which DGAT catalyzes the re-esterification of diacylglycerol to support VLDL maturation.Topological studies separated DGAT activities in hepatic microsomes into overt, cytosolic side-localized, and latent, lumenal side-localized. [19][20][21] This led to a hypothesis that the overt DGAT activity might be responsible for the synthesis of tria...

SummaryThe known link between obesity and cancer suggests an important interaction between the host lipid metabolism and tumorigenesis. Here, we used a syngeneic tumor graft model to demonstrate that tumor development influences the host lipid metabolism. BCR-Abl-transformed precursor B cell tumors induced hyperlipidemia by stimulating very low-density lipoprotein (VLDL) production and blunting VLDL and low-density lipoprotein (LDL) turnover. To assess whether tumor progression was dependent on tumor-induced hyperlipidemia, we utilized the VLDL production-deficient mouse model, carboxylesterase3/triacylglycerol hydrolase (Ces3/TGH) knockout mice. In Ces3/Tgh–/– tumor-bearing mice, plasma triglyceride and cholesterol levels were attenuated. Importantly tumor weight was reduced in Ces3/Tgh–/– mice. Mechanistically, reduced tumor growth in Ces3/Tgh–/– mice was attributed to reversal of tumor-induced PCSK9-mediated degradation of hepatic LDLR and decrease of LDL turnover. Our data demonstrate that tumor-induced hyperlipidemia encompasses a feed-forward loop that reprograms hepatic lipoprotein homeostasis in part by providing LDL cholesterol to support tumor growth.