Human liver cholesteryl ester hydrolase: cloning, molecular characterization, and role in cellular cholesterol homeostasis

Role of carboxylesterase and arylacetamide deacetylase in drug metabolism, physiology, and pathology

Carboxylesterases (CES1 and CES2) and arylacetamide deacetylase (AADAC), which are expressed primarily in the liver and/or gastrointestinal tract, hydrolyze drugs containing ester and amide bonds in their chemical structure. These enzymes often catalyze the conversion of prodrugs, including the COVID-19 drugs remdesivir and molnupiravir, to their pharmacologically active forms. Information on the substrate specificity and inhibitory properties of these enzymes, which would be useful for drug development and toxicity avoidance, has accumulated. Recently,in vitroandin vivostudies have shown that these enzymes are involved not only in drug hydrolysis but also in lipid metabolism. CES1 and CES2 are capable of hydrolyzing triacylglycerol, and the deletion of their orthologous genes in mice has been associated with impaired lipid metabolism and hepatic steatosis. Adeno-associated virus-mediated human CES overexpression decreases hepatic triacylglycerol levels and increases fatty acid oxidation in mice. It has also been shown that overexpression of CES enzymes or AADAC in cultured cells suppresses the intracellular accumulation of triacylglycerol. Recent reports indicate that AADAC can be up- or downregulated in tumors of various organs, and its varied expression is associated with poor prognosis in patients with cancer. Thus, CES and AADAC not only determine drug efficacy and toxicity but are also involved in pathophysiology. This review summarizes recent findings on the roles of CES and AADAC in drug metabolism, physiology, and pathology.

Integrated lipidomic and transcriptomic analyses reveal the mechanism of large yellow croaker roe phospholipids on lipid metabolism in normal-diet mice

Large yellow croaker roe phospholipids (LYCRPLs) could regulate the accumulation of triglycerides and blood lipid levels. However, there exists little research on the mechanism of LYCRPLs on lipid metabolism in normal-diet mice. In this work, the mice on a normal diet were given low-dose, medium-dose, and high-dose LYCRPLs by intragastric administration for 6 weeks. At the same time, the physiological and biochemical indicators of the mice were determined, and the histomorphological observation of the liver and epididymal fat was carried out. In addition, we examined the gene expression and lipid metabolites in the liver of mice using transcriptomic and lipidomic and performed a correlation analysis. The results showed that LYCRPLs regulated the lipid metabolism of normal-diet mice by affecting the expression of the glycerolipid metabolism pathway, insulin resistance pathway, and cholesterol metabolism pathway. This study not only elucidated the main pathway by which LYCRPLs regulate lipid metabolism, but also laid a foundation for exploring LYCRPLs as functional food supplements.

Effect and related mechanism of Yinchenhao decoction on mice with lithogenic diet-induced cholelithiasis

The aim of the present study was to investigate the effects and the underlying mechanisms of Yinchenhao Decoction (YCHD), a traditional Chinese medicine formulation, on C57BL/6 mice with lithogenic diet (LD)-induced cholelithiasis. The condition of cholelithiasis was evaluated using a six-level criteria. Levels of alanine aminotransferase (ALT), alkaline phosphatase (ALP), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) in the serum and liver tissue were measured using enzyme colorimetry. Concentrations of TC, phospholipids (PL) and total bile acids (TBA) in the bile were measured to calculate the cholesterol saturation index. Liver histopathology was microscopically observed and mRNA expression levels of ABCG5, ABCG8, SRBI, ABCB4, ABCB11 and NPC1L1 involved in cholesterol metabolism were measured using reverse transcription-quantitative PCR. The results showed that feeding mice the LD induced cholelithiasis, along with abnormal serum biochemical indices and imbalances in biliary cholesterol homeostasis. Increased ALT and ALP levels in the serum and ALT, ALP, TC and LDL-C levels in the serum and liver indicated the existence of hepatocyte injury, which were consistent with the pathological changes. YCHD treatment ameliorated the serum and hepatic biochemical abnormalities and adjusted the biliary imbalance. In addition, elevated expression of ATP-binding cassette subfamily G member 5/8, scavenger receptor class B type I and Niemann-Pick C1 Like 1 in the liver and small intestine were observed at the onset of cholelithiasis but were reversed by YCHD. Taken together, results from the present study suggest that YCHD ameliorated LD-induced cholelithiasis mice, which may be caused by improvements in biliary cholesterol supersaturation and regulation of cholesterol metabolism.

Impaired LXRα Phosphorylation Attenuates Progression of Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a very common indication for liver transplantation. How fat-rich diets promote progression from fatty liver to more damaging inflammatory and fibrotic stages is poorly understood. Here, we show that disrupting phosphorylation at Ser196 (S196A) in the liver X receptor alpha (LXRα, NR1H3) retards NAFLD progression in mice on a high-fat-high-cholesterol diet. Mechanistically, this is explained by key histone acetylation (H3K27) and transcriptional changes in pro-fibrotic and pro-inflammatory genes. Furthermore, S196A-LXRα expression reveals the regulation of novel diet-specific LXRα-responsive genes, including the induction of Ces1f, implicated in the breakdown of hepatic lipids. This involves induced H3K27 acetylation and altered LXR and TBLR1 cofactor occupancy at the Ces1f gene in S196A fatty livers. Overall, impaired Ser196-LXRα phosphorylation acts as a novel nutritional molecular sensor that profoundly alters the hepatic H3K27 acetylome and transcriptome during NAFLD progression placing LXRα phosphorylation as an alternative anti-inflammatory or anti-fibrotic therapeutic target.

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LXRαS196A induces liver steatosis and prevents cholesterol accumulation

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LXRαS196A reduces progression to hepatic inflammation and fibrosis

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LXRαS196A modulates hepatic chromatin acetylation

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LXRαS196A reveals unique dual LXRα phosphorylation and diet-responsive genes

Real-time measurement of cholesterol secreted by human hepatocytes using a novel microfluidic assay

The use of microfluidics has become widespread in recent years because of the use of lesser resources such as small size, low volume of reagents, and physiological representation of mammalian cells. One of the advantages of microfluidic-based cell culture is the ability to perfuse culture media which tends to improve cellular health and function. Although measurement of cellular function conventionally is carried out using well-plates and plate readers, these approaches are insufficient to carry out in-line analysis of perfused cell cultures because of mismatch between volumes and sensitivity. We report the development of a novel microfluidic device and assay that is carried out under perfusion, in-line to measure the cholesterol secreted from a human hepatocyte tissue-chip. The heart of the assay is the unique implementation of enzymatic chemistry that is carried out on a polystyrene bead. Using this approach, we successfully measured cholesterol secreted by the perfused human hepatocytes.

Bolstering cholesteryl ester hydrolysis in liver: A hepatocyte-targeting gene delivery strategy for potential alleviation of atherosclerosis

Current atherosclerosis treatment strategies primarily focus on limiting further cholesteryl esters (CE) accumulation by reducing endogenous synthesis of cholesterol in the liver. No therapy is currently available to enhance the removal of CE, a crucial step to reduce the burden of the existing disease. Given the central role of hepatic cholesteryl ester hydrolase (CEH) in the intrahepatic hydrolysis of CE and subsequent removal of the resulting free cholesterol (FC), in this work, we applied galactose-functionalized polyamidoamine (PAMAM) dendrimer generation 5 (Gal-G5) for hepatocyte-specific delivery of CEH expression vector. The data presented herein show the increased specific uptake of Gal-G5/CEH expression vector complexes (simply Gal-G5/CEH) by hepatocytes in vitro and in vivo. Furthermore, the upregulated CEH expression in the hepatocytes significantly enhanced the intracellular hydrolysis of high density lipoprotein-associated CE (HDL-CE) and subsequent conversion/secretion of hydrolyzed FC as bile acids (BA). The increased CEH expression in the liver significantly increased the flux of HDL-CE to biliary as well as fecal FC and BA. Meanwhile, Gal-G5 did not induce hepatic or renal toxicity. It was also not immunotoxic. Because of these encouraging pre-clinical testing results, using this safe and highly efficient hepatocyte-specific gene delivery platform to enhance the hepatic processes involved in cholesterol elimination is a promising strategy for the alleviation of atherosclerosis.

Sortilin 1 Modulates Hepatic Cholesterol Lipotoxicity in Mice via Functional Interaction with Liver Carboxylesterase 1

The liver plays a key role in cholesterol metabolism. Impaired hepatic cholesterol homeostasis causes intracellular free cholesterol accumulation and hepatocyte injury. Sortilin 1 (SORT1) is a lysosomal trafficking receptor that was identified by genome-wide association studies (GWAS) as a novel regulator of cholesterol metabolism in humans. Here we report that SORT1 deficiency protected against cholesterol accumulation-induced liver injury and inflammation in mice. Using an LC-MS/MS-based proteomics approach, we identified liver carboxylesterase 1 (CES1) as a novel SORT1-interacting protein. Mechanistic studies further showed that SORT1 may regulate CES1 lysosomal targeting and degradation and that SORT1 deficiency resulted in higher liver CES1 protein abundance. Previous studies have established an important role of hepatic CES1 in promoting intracellular cholesterol mobilization, cholesterol efflux, and bile acid synthesis. Consistently, high cholesterol atherogenic diet-challenged Sort1 knock-out mice showed less hepatic free cholesterol accumulation, increased bile acid synthesis, decreased biliary cholesterol secretion, and the absence of gallstone formation. SORT1 deficiency did not alter hepatic ceramide and fatty acid metabolism in high cholesterol atherogenic diet-fed mice. Finally, knockdown of liver CES1 in mice markedly increased the susceptibility to high cholesterol diet-induced liver injury and abolished the protective effect against cholesterol lipotoxicity in Sort1 knock-out mice. In summary, this study identified a novel SORT1-CES1 axis that regulates cholesterol-induced liver injury, which provides novel insights that improve our current understanding of the molecular links between SORT1 and cholesterol metabolism. This study further suggests that therapeutic inhibition of SORT1 may be beneficial in improving hepatic cholesterol homeostasis in metabolic and inflammatory liver diseases.

Intracellular cholesterol transport proteins enhance hydrolysis of HDL-CEs and facilitate elimination of cholesterol into bile

While HDL-associated unesterified or free cholesterol (FC) is thought to be rapidly secreted into the bile, the fate of HDL-associated cholesteryl esters (HDL-CEs) that represent >80% of HDL-cholesterol, is only beginning to be understood. In the present study, we examined the hypothesis that intracellular cholesterol transport proteins [sterol carrier protein 2 (SCP2) and fatty acid binding protein-1 (FABP1)] not only facilitate CE hydrolase-mediated hydrolysis of HDL-CEs, but also enhance elimination of cholesterol into bile. Adenovirus-mediated overexpression of FABP1 or SCP2 in primary hepatocytes significantly increased hydrolysis of HDL-[(3)H]CE, reduced resecretion of HDL-CE-derived FC as nascent HDL, and increased its secretion as bile acids. Consistently, the flux of [(3)H]cholesterol from HDL-[(3)H]CE to biliary bile acids was increased by overexpression of SCP2 or FABP1 in vivo and reduced in SCP2(-/-) mice. Increased flux of HDL-[(3)H]CE to biliary FC was noted with FABP1 overexpression and in SCP2(-/-) mice that have increased FABP1 expression. Lack of a significant decrease in the flux of HDL-[(3)H]CE to biliary FC or bile acids in FABP1(-/-) mice indicates the likely compensation of its function by an as yet unidentified mechanism. Taken together, these studies demonstrate that FABP1 and SCP2 facilitate the preferential movement of HDL-CEs to bile for final elimination.

Quantitative proteomics analysis reveals glutamine deprivation activates fatty acid β-oxidation pathway in HepG2 cells

Glutamine, a multifunctional amino acid, functions in nutrient metabolism, energy balance, apoptosis, and cell proliferation. Lipid is an important nutrient and controls a broad range of physiological processes. Previous studies have demonstrated that glutamine can affect lipolysis and lipogenesis, but the effect of glutamine on the detailed lipid metabolism remains incompletely understood. Here, we applied the quantitative proteomics approach to estimate the relative abundance of proteins in HepG2 cells treated by glutamine deprivation. The results showed that there were 212 differentially abundant proteins in response to glutamine deprivation, including 150 significantly increased proteins and 62 significantly decreased proteins. Interestingly, functional classification showed that 43 differentially abundant proteins were related to lipid metabolism. Further bioinformatics analysis and western blotting validation revealed that lipid accumulation may be affected by β-oxidation of fatty acid induced by glutamine deprivation in HepG2 cells. Together, our results may provide the potential for regulating lipid metabolism by glutamine in animal production and human nutrition. The MS data have been deposited to the ProteomeXchange Consortium with identifier PXD003387.

Streptococcal serum opacity factor promotes cholesterol ester metabolism and bile acid secretion in vitro and in vivo

Plasma high density lipoprotein-cholesterol (HDL-C) concentrations negatively correlate with atherosclerotic cardiovascular disease. HDL is thought to have several atheroprotective functions, which are likely distinct from the epidemiological inverse relationship between HDL-C levels and risk. Specifically, strategies that reduce HDL-C while promoting reverse cholesterol transport (RCT) may have therapeutic value. The major product of the serum opacity factor (SOF) reaction versus HDL is a cholesteryl ester (CE)-rich microemulsion (CERM), which contains apo E and the CE of ~400,000 HDL particles. Huh7 hepatocytes take up CE faster when delivered as CERM than as HDL, in part via the LDL-receptor (LDLR). Here we compared the final RCT step, hepatic uptake and subsequent intracellular processing to cholesterol and bile salts for radiolabeled HDL-, CERM- and LDL-CE by Huh7 cells and in vivo in C57BL/6J mice. In Huh7 cells, uptake from LDL was greater than from CERM (2-4X) and HDL (5-10X). Halftimes for [(14)C]CE hydrolysis were 3.0±0.2, 4.4±0.6 and 5.4±0.7h respectively for HDL, CERM and LDL-CE. The fraction of sterols secreted as bile acids was ~50% by 8h for all three particles. HDL, CERM and LDL-CE metabolism in mice showed efficient plasma clearance of CERM-CE, liver uptake and metabolism, and secretion as bile acids into the gall bladder. This work supports the therapeutic potential of the SOF reaction, which diverts HDL-CE to the LDLR, thereby increasing hepatic CE uptake, and sterol disposal as bile acids.

CES1A -816C as a genetic marker to predict greater platelet clopidogrel response in patients with percutaneous coronary intervention

This study was designed to determine whether CES1A -816A/C polymorphism could be associated with altered clopidogrel response. Recruited patients were pretreated with 300 mg clopidogrel loading dose before undergoing percutaneous coronary intervention for stenting and genotyped with CYP2C19 *2, *3, or *17, and CES1A -816A/C, respectively. Adenosine diphosphate-induced maximum platelet aggregation (MPA) was determined on day 3 after initiation of daily clopidogrel maintenance doses. The clinical primary end point was the 1-year incidence of definite stent thrombosis (ST). Multivariable linear regression revealed that the CES1A -816A/C polymorphism was independently associated with MPA measures with an absolute β value of 6.76. Of 617 patients, a subcohort of 249 patients not carrying CYP2C19 *2, *3, or *17 were categorized into 3 groups based on the -816A/C genotype. The median MPA value was lower in 125 carriers of the -816C variant than in 124 noncarriers (21.5% vs. 31.7%, P = 0.001). The 1-year definite ST occurred in 7 patients in that subcohort, and only 1 ST case was one of carriers of the -816 A/A that was associated with higher MPA values. The CES1A -816C would be used to predict greater platelet response to clopidogrel than the CES1A -816A in percutaneous coronary intervention-treated patients not carrying CYP2C19 variants.

Hepatic carboxylesterase 1 is essential for both normal and farnesoid X receptor-controlled lipid homeostasis

Nonalcoholic fatty liver disease (NAFLD) is one of the major health concerns worldwide. Farnesoid X receptor (FXR) is considered a therapeutic target for treatment of NAFLD. However, the mechanism by which activation of FXR lowers hepatic triglyceride (TG) levels remains unknown. Here we investigated the role of hepatic carboxylesterase 1 (CES1) in regulating both normal and FXR-controlled lipid homeostasis. Overexpression of hepatic CES1 lowered hepatic TG and plasma glucose levels in both wild-type and diabetic mice. In contrast, knockdown of hepatic CES1 increased hepatic TG and plasma cholesterol levels. These effects likely resulted from the TG hydrolase activity of CES1, with subsequent changes in fatty acid oxidation and/or de novo lipogenesis. Activation of FXR induced hepatic CES1, and reduced the levels of hepatic and plasma TG as well as plasma cholesterol in a CES1-dependent manner.

CONCLUSION

Hepatic CES1 plays a critical role in regulating both lipid and carbohydrate metabolism and FXR-controlled lipid homeostasis.

Liver-specific transgenic expression of cholesteryl ester hydrolase reduces atherosclerosis in Ldlr-/- mice

The liver plays a central role in the final elimination of cholesterol from the body either as bile acids or as free cholesterol (FC), and lipoprotein-derived cholesterol is the major source of total biliary cholesterol. HDL is the major lipoprotein responsible for removal and transport of cholesterol, mainly as cholesteryl esters (CEs), from the peripheral tissues to the liver. While HDL-FC is rapidly secreted into bile, the fate of HDL-CE remains unclear. We have earlier demonstrated the role of human CE hydrolase (CEH, CES1) in hepatic hydrolysis of HDL-CE and increasing bile acid synthesis, a process dependent on scavenger receptor BI expression. In the present study, we examined the hypothesis that by enhancing the elimination of HDL-CE into bile/feces, liver-specific transgenic expression of CEH will be anti-atherogenic. Increased CEH expression in the liver significantly increased the flux of HDL-CE to bile acids. In the LDLR(-/-) background, this enhanced elimination of cholesterol led to attenuation of diet-induced atherosclerosis with a consistent increase in fecal sterol secretion primarily as bile acids. Taken together with the observed reduction in atherosclerosis by increasing macrophage CEH-mediated cholesterol efflux, these studies establish CEH as an important regulator in enhancing cholesterol elimination and also as an anti-atherogenic target.

Cooperation between hepatic cholesteryl ester hydrolase and scavenger receptor BI for hydrolysis of HDL-CE

Liver is the sole organ responsible for the final elimination of cholesterol from the body either as biliary cholesterol or bile acids. High density lipoprotein (HDL)-derived cholesterol is the major source of biliary sterols and represents a mechanism for the removal of cholesterol from peripheral tissues including artery wall-associated macrophage foam cells. Via selective uptake through scavenger receptor BI (SR-BI), HDL-cholesterol is thought to be directly secreted into bile, and HDL cholesteryl esters (HDL-CEs) enter the hepatic metabolic pool and need to be hydrolyzed prior to conversion to bile acids. However, the identity of hepatic CE hydrolase (CEH) as well as the role of SR-BI in bile acid synthesis remains elusive. In this study we examined the role of human hepatic CEH (CES1) in facilitating hydrolysis of SR-BI-delivered HDL-CEs. Over-expression of CEH led to increased hydrolysis of HDL-[³H]CE in primary hepatocytes and SR-BI expression was required for this process. Intracellular CEH associated with BODIPY-CE delivered by selective uptake via SR-BI. CEH and SR-BI expression enhanced the movement of [³H]label from HDL-[³H]CE to bile acids in vitro and in vivo. Taken together, these studies demonstrate that SR-BI-delivered HDL-CEs are hydrolyzed by hepatic CEH and utilized for bile acid synthesis.

Liver-specific cholesteryl ester hydrolase deficiency attenuates sterol elimination in the feces and increases atherosclerosis in ldlr-/- mice

OBJECTIVE

Liver is the major organ responsible for the final elimination of cholesterol from the body either as biliary cholesterol or as bile acids. Intracellular hydrolysis of lipoprotein-derived cholesteryl esters (CEs) is essential to generate the free cholesterol required for this process. Earlier, we demonstrated that overexpression of human CE hydrolase (Gene symbol CES1) increased bile acid synthesis in human hepatocytes and enhanced reverse cholesterol transport in mice. The objective of the present study was to demonstrate that liver-specific deletion of its murine ortholog, Ces3, would decrease cholesterol elimination from the body and increase atherosclerosis.

APPROACH AND RESULTS

Liver-specific Ces3 knockout mice (Ces3-LKO) were generated, and Ces3 deficiency did not affect the expression of genes involved in cholesterol homeostasis and free cholesterol or bile acid transport. The effects of Ces3 deficiency on the development of Western diet-induced atherosclerosis were examined in low density lipoprotein receptor knock out(-/-) mice. Despite similar plasma lipoprotein profiles, there was increased lesion development in low density lipoprotein receptor knock out(-/-)Ces3-LKO mice along with a significant decrease in the bile acid content of bile. Ces3 deficiency significantly reduced the flux of cholesterol from [(3)H]-CE-labeled high-density lipoproteins to feces (as free cholesterol and bile acids) and decreased total fecal sterol elimination.

CONCLUSIONS

Our results demonstrate that hepatic Ces3 modulates the hydrolysis of lipoprotein-delivered CEs and thereby regulates free cholesterol and bile acid secretion into the feces. Therefore, its deficiency results in reduced cholesterol elimination from the body, leading to significant increase in atherosclerosis. Collectively, these data establish the antiatherogenic role of hepatic CE hydrolysis.

Inhibition of porcine liver carboxylesterase by phosphorylated flavonoids

We have recently synthesized a series of phosphorylated flavonoids and identified some of them as potent inhibitors of pancreatic cholesterol esterase (CEase) with excellent selectivity for CEase over acetylcholinesterase (AChE). In the present paper, we investigated the inhibitory activities of these compounds against porcine liver carboxylesterase (CE) since carboxylesterases (CEs) are another family of serine esterases responsible for the metabolism and detoxification of many ester-containing xenobiotics and clinical esterified drugs, and there exists much structural similarity between CEase and CEs. The results indicated that phosphorylated flavonoids exhibited significantly improved inhibition potency toward CE than their parent compounds, and six of them had IC50 values less than 5.0nM. Among all compounds tested, compounds 3d and 3e are the two most potent inhibitors of CE, giving IC50 values of 1.79nM and 1.58nM, respectively. Interestingly, these compounds inhibited CEase and CE with similar structure activity correlations, and those with high inhibitory activities toward CEase could also inhibit CE efficiently. The presences of a free hydroxyl group at position 5 and a phosphate group at position 7 of the phosphorylated flavonoids are favorable to the inhibition of CE. The inhibition mechanism and kinetic characterization studies of the most potent inhibitors revealed that they are irreversible competitive inhibitors.

Screening for drug-induced hepatotoxicity in primary mouse hepatocytes using acetaminophen, amiodarone, and cyclosporin a as model compounds: an omics-guided approach

Drug-induced hepatotoxicity is a leading cause of attrition for candidate pharmaceuticals in development. New preclinical screening methods are crucial to predict drug toxicity prior to human studies. Of all in vitro hepatotoxicity models, primary human hepatocytes are considered as 'the gold standard.' However, their use is hindered by limited availability and inter-individual variation. These barriers may be overcome by using primary mouse hepatocytes. We used differential in gel electrophoresis (DIGE) to study large-scale protein expression of primary mouse hepatocytes. These hepatocytes were exposed to three well-defined hepatotoxicants: acetaminophen, amiodarone, and cyclosporin A. Each hepatotoxicant induces a different hepatotoxic phenotype. Based on the DIGE results, the mRNA expression levels of deregulated proteins from cyclosporin A-treated cells were also analyzed. We were able to distinguish cyclosporin A from controls, as well as acetaminophen and amiodarone-treated samples. Cyclosporin A induced endoplasmic reticulum (ER) stress and altered the ER-Golgi transport. Moreover, liver carboxylesterase and bile salt sulfotransferase were differentially expressed. These proteins were associated with a protective adaptive response against cyclosporin A-induced cholestasis. The results of this study are comparable with effects in HepG2 cells. Therefore, we suggest both models can be used to analyze the cholestatic properties of cyclosporin A. Furthermore, this study showed a conserved response between primary mouse hepatocytes and HepG2 cells. These findings collectively lend support for use of omics strategies in preclinical toxicology, and might inform future efforts to better link preclinical and clinical research in rational drug development.

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

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.

Antioxidant sulforaphane and sensitizer trinitrobenzene sulfonate induce carboxylesterase-1 through a novel element transactivated by nuclear factor-E2 related factor-2

Carboxylesterase-1 (CES1), the most versatile human carboxylesterase, plays critical roles in drug metabolism and lipid mobilization. This enzyme is highly induced by antioxidants and sensitizers in various cell lines. These compounds are known to activate nuclear factor-E2 related factor-2 (Nrf2) by reacting to kelch-like ECH-associated protein-1 (Keap1). The aims of this study were to determine whether antioxidant sulforaphane (SFN) and sensitizer trinitrobenzene sulfonate (TNBS) target Keap1 similarly and whether they use the same element for CES1 induction. Cells over-expressing Keap1 were treated with TNBS or SFN and the formation of disulfide bonds among Keap1 molecules were determined. SFN promoted intramolecular disulfide formation whereas TNBS promoted intermolecular disulfide formation of Keap1. Two elements, sensitizing/antioxidant response element (S/ARE) and ARE4, were identified to support Nrf2 in the regulated expression of CES1A1. Both elements were bound by Nrf2, however, the S/ARE element supported, whereas the ARE4 element repressed Nrf2 transactivation. The repression required higher amounts of Nrf2, suggesting that the transactivation through the S/ARE element dominates the trans-repression through the ARE4 element under normal antioxidative condition. These findings conclude that compounds, although triggering the Keap1-Nrf2 pathway, may differ in the mode of reacting with Keap1. These findings also conclude that both positive and negative Nrf2 elements exist even within the same gene, and such opposing mechanisms provide fine-tuning in transcriptional regulation by the Keap1-Nrf2 pathway. High levels of CES1 are linked to lipid retention. Excessive induction of CES1 by antioxidants and sensitizers likely provides a mechanism for potential detrimental effect on human health.

Identification of a novel intracellular cholesteryl ester hydrolase (carboxylesterase 3) in human macrophages: compensatory increase in its expression after carboxylesterase 1 silencing

Cholesteryl ester (CE) hydrolysis is the rate-limiting step in the removal of free cholesterol (FC) from macrophage foam cells, and several enzymes have been identified as intracellular CE hydrolases in human macrophages. We have previously reported the antiatherogenic role of a carboxylesterase [carboxylesterase 1 (CES1)], and the objective of the present study was to determine the contribution of CES1 to total CE hydrolytic activity in human macrophages. Two approaches, namely, immune depletion and short hairpin (sh)RNA-mediated knockdown, were used. Immuneprecipitation by a CES1-specific antibody resulted in a 70-80% decrease in enzyme activity, indicating that CES1 is responsible for >70% of the total CE hydrolytic activity. THP1-shRNA cells were generated by stably transfecting human THP1 cells with four different CES1-specific shRNA vectors. Despite a significant (>90%) reduction in CES1 expression both at the mRNA and protein levels, CES1 knockdown neither decreased intracellular CE hydrolysis nor decreased FC efflux. Examination of the underlying mechanisms for the observed lack of effects of CES1 knockdown revealed a compensatory increase in the expression of a novel CES, CES3, which is only expressed at <30% of the level of CES1 in human macrophages. Transient overexpression of CES3 led to an increase in CE hydrolytic activity, mobilization of intracellular lipid droplets, and a reduction in cellular CE content, establishing CES3 as a bona fide CE hydrolase. This study provides the first evidence of functional compensation whereby increased expression of CES3 restores intracellular CE hydrolytic activity and FC efflux in CES1-deficient cells. Furthermore, these data support the concept that intracellular CE hydrolysis is a multienzyme process.

Examination of the carboxylesterase phenotype in human liver

Carboxylesterases (CES) metabolize esters. Two CES isoforms are expressed in human liver (CES1 and CES2) and liver extracts are used in reaction phenotyping studies to discern interindividual metabolic variation. We tested the hypothesis that an individual's CES phenotype can be characterized by reporter substrates/probes that interrogate native CES1 and CES2 activities in liver and immunoblotting methods. We obtained 25 livers and found that CES1 is the main hydrolytic enzyme. Moreover, although CES1 protein levels were similar, we observed large interindividual variation in bioresmethrin hydrolysis rates (17-fold), a pyrethroid metabolized by CES1 but not CES2. Bioresmethrin hydrolysis rates did not correlate with CES1 protein levels. In contrast, procaine hydrolysis rates, a drug metabolized by CES2 but not CES1, were much less variant (3-fold). Using activity-based fluorophosphonate probes (FP-biotin), which covalently reacts with active serine hydrolases, CES1 protein was the most active enzyme in the livers. Finally, using bioorthogonal probes and click chemistry methodology, the half-life of CES 1 and 2 in cultured HepG2 cells was estimated at 96 h. The cause of the differential CES1 activities is unknown, but the underlying factors will be important to understand because several carboxylic acid ester drugs and environmental toxicants are metabolized by this enzyme.

Age-related inducibility of carboxylesterases by the antiepileptic agent phenobarbital and implications in drug metabolism and lipid accumulation

Carboxylesterases (CES) constitute a class of hydrolytic enzymes that play critical roles in drug metabolism and lipid mobilization. Previous studies with a large number of human liver samples have suggested that the inducibility of carboxylesterases is inversely related with age. To directly test this possibility, neonatal (10 days of age) and adult mice were treated with the antiepileptic agent phenobarbital. The expression and hydrolytic activity were determined on six major carboxylesterases including ces1d, the ortholog of human CES1. Without exception, all carboxylesterases tested were induced to a greater extent in neonatal than adult mice. The induction was detected at mRNA, protein and catalytic levels. Ces1d was greatly induced and found to rapidly hydrolyze the antiplatelet agent clopidogrel and support the accumulation of neutral lipids. Phenobarbital represents a large number of therapeutic agents that induce drug metabolizing enzymes and transporters in a species-conserved manner. The higher inducibility of carboxylesterases in the developmental age likely represents a general phenomenon cross species including human. Consequently, individuals in the developmental age may experience greater drug-drug interactions. The greater induction of ces1d also provides a molecular explanation to the clinical observation that children on antiepileptic drugs increase plasma lipids.

Early steps in reverse cholesterol transport: cholesteryl ester hydrolase and other hydrolases

PURPOSE OF REVIEW

Several controversies exist related to the molecular identity and subcellular localization of the enzyme catalyzing macrophage cholesteryl ester hydrolysis. Some of these issues have been reviewed earlier and this review summarizes new developments that describe effects of overexpression or gene ablation. The main objective is to highlight the disagreement between lack of gene expression and incomplete abolition of macrophage cholesteryl ester hydrolytic activity and to emphasize the importance of redundancy.

RECENT FINDINGS

New information resulting from the continuing characterization of the various cholesteryl ester hydrolases (hormone-sensitive lipase, HSL; cholesteryl ester hydrolase, CEH; and KIAA1363/NCEH1) is reviewed. Whereas CEH overexpression leads to beneficial effects such as decreased inflammation, improved glucose tolerance/insulin sensitivity, and attenuation of atherosclerotic lesion progression, deficiency/ablation of HSL or KIAA1363/NCEH1 results in incomplete loss of macrophage cholesteryl ester hydrolysis/turnover. New paradigms challenging the classical view of cytoplasmic cholesteryl ester hydrolysis and reverse cholesterol transport are also presented.

SUMMARY

The observed beneficial effects of CEH overexpression identify macrophage cholesteryl ester hydrolysis as an important therapeutic target and future studies will determine whether similar effects are obtained with overexpression of HSL or KIAA1363/NCEH1. It is imperative that, for clinical benefit, mechanisms to enhance endogenous cholesteryl ester hydrolase(s) are established.

C57Bl/6 N mice on a western diet display reduced intestinal and hepatic cholesterol levels despite a plasma hypercholesterolemia

Background

Small intestine and liver greatly contribute to whole body lipid, cholesterol and phospholipid metabolism but to which extent cholesterol and phospholipid handling in these tissues is affected by high fat Western-style obesogenic diets remains to be determined.

Methods

We therefore measured cholesterol and phospholipid concentration in intestine and liver and quantified fecal neutral sterol and bile acid excretion in C57Bl/6 N mice fed for 12 weeks either a cholesterol-free high carbohydrate control diet or a high fat Western diet containing 0.03% (w/w) cholesterol. To identify the underlying mechanisms of dietary adaptations in intestine and liver, changes in gene expression were assessed by microarray and qPCR profiling, respectively.

Results

Mice on Western diet showed increased plasma cholesterol levels, associated with the higher dietary cholesterol supply, yet, significantly reduced cholesterol levels were found in intestine and liver. Transcript profiling revealed evidence that expression of numerous genes involved in cholesterol synthesis and uptake via LDL, but also in phospholipid metabolism, underwent compensatory regulations in both tissues. Alterations in glycerophospholipid metabolism were confirmed at the metabolite level by phospolipid profiling via mass spectrometry.

Conclusions

Our findings suggest that intestine and liver react to a high dietary fat intake by an activation of de novo cholesterol synthesis and other cholesterol-saving mechanisms, as well as with major changes in phospholipid metabolism, to accommodate to the fat load.

Hypolipidemic agent Z-guggulsterone: metabolism interplays with induction of carboxylesterase and bile salt export pump

Z-Guggulsterone is a major ingredient in the Indian traditional hypolipidemic remedy guggul. A study in mice has established that its hypolipidemic effect involves the farnesoid X receptor (FXR), presumably by acting as an antagonist of this receptor. It is generally assumed that the antagonism leads to induction of cytochrome P450 7A1 (CYP7A1), the rate-limiting enzyme converting free cholesterol to bile acids. In this study, we tested whether Z-guggulsterone indeed induces human CYP7A1. In addition, the expression of cholesteryl ester hydrolase CES1 and bile salt export pump (BSEP) was monitored. Contrary to the general assumption, Z-guggulsterone did not induce CYP7A1. Instead, this phytosterol significantly induced CES1 and BSEP through transactivation. Z-Guggulsterone underwent metabolism by CYP3A4, and the metabolites greatly increased the induction potency on BSEP but not on CES1. BSEP induction favors cholesterol elimination, whereas CES1 involves both elimination and retention (probably when excessively induced). Interestingly, clinical trials reported the hypolipidemic response rates from 18% to 80% and showed that higher dosages actually increased VLDL cholesterol. Our findings predict that better hypolipidemic outcomes likely occur in individuals who have a relatively higher capacity of metabolizing Z-guggulsterone with moderate CES1 induction, a scenario possibly achieved by lowering the dosing regimens.

Genomics and proteomics of vertebrate cholesterol ester lipase (LIPA) and cholesterol 25-hydroxylase (CH25H)

Cholesterol ester lipase (LIPA; EC 3.1.1.13) and cholesterol 25-hydroxylase (CH25H; EC 1.14.99.48) play essential role in cholesterol metabolism in the body by hydrolysing cholesteryl esters and triglycerides within lysosomes (LIPA) and catalysing the formation of 25-hydroxycholesterol from cholesterol (CH25H) which acts to repress cholesterol biosynthesis. Bioinformatic methods were used to predict the amino acid sequences, structures and genomic features of several vertebrate LIPA and CH25H genes and proteins, and to examine the phylogeny of vertebrate LIPA. Amino acid sequence alignments and predicted subunit structures enabled the identification of key sequences previously reported for human LIPA and CH25H and transmembrane structures for vertebrate CH25H sequences. Vertebrate LIPA and CH25H genes were located in tandem on all vertebrate genomes examined and showed several predicted transcription factor binding sites and CpG islands located within the 5′ regions of the human genes. Vertebrate LIPA genes contained nine coding exons, while all vertebrate CH25H genes were without introns. Phylogenetic analysis demonstrated the distinct nature of the vertebrate LIPA gene and protein family in comparison with other vertebrate acid lipases and has apparently evolved from an ancestral LIPA gene which predated the appearance of vertebrates.

Reverse cholesterol transport is elevated in carboxyl ester lipase-knockout mice

Mechanisms to increase reverse cholesterol transport (RCT) and biliary sterol disposal are currently sought to prevent atherosclerosis. Previous work with HepG2 cells and primary hepatocytes showed that carboxyl ester lipase (CEL), a broad-spectrum lipase secreted by pancreas and liver, plays an important role in hydrolysis of high-density lipoprotein (HDL) cholesteryl esters (CEs) after selective uptake by hepatocytes. The effect of CEL on RCT of HDL cholesterol was assessed by measuring biliary and fecal disposal of radiolabeled HDL-CE in control and Cel(-/-) mice. Radiolabeled CE was increased 3-fold in hepatic bile of Cel(-/-) mice, and the mass of CE in gall bladder bile was elevated. Total radiolabeled transport from plasma to hepatic bile was more rapid in Cel(-/-) mice. Fecal disposal of radiolabel from HDL-CE, as well as total sterol mass, was markedly elevated for Cel(-/-) mice, primarily due to more CE. RCT of macrophage CE was also increased in Cel(-/-) mice, as measured by excretion of radiolabel from injected J774 cells. Increased sterol loss was compensated by increased cholesterol synthesis in Cel(-/-) mice. Together, the data demonstrate significantly increased RCT in the absence of CEL and suggest a novel mechanism by which to manipulate plasma cholesterol flux.

Activity-based protein profiling identifies a host enzyme, carboxylesterase 1, which is differentially active during hepatitis C virus replication

Hepatitis C virus (HCV) relies on many interactions with host cell proteins for propagation. Successful HCV infection also requires enzymatic activity of host cell enzymes for key post-translational modifications. To identify such enzymes, we have applied activity-based protein profiling to examine the activity of serine hydrolases during HCV replication. Profiling of hydrolases in Huh7 cells replicating HCV identified CES1 (carboxylesterase 1) as a differentially active enzyme. CES1 is an endogenous liver protein involved in processing of triglycerides and cholesterol. We observe that CES1 expression and activity were altered in the presence of HCV. The knockdown of CES1 with siRNA resulted in lower levels of HCV replication, and up-regulation of CES1 was observed to favor HCV propagation, implying an important role for this host cell protein. Experiments in HCV JFH1-infected cells suggest that CES1 facilitates HCV release because less intracellular HCV core protein was observed, whereas HCV titers remained high. CES1 activity was observed to increase the size and density of lipid droplets, which are necessary for the maturation of very low density lipoproteins, one of the likely vehicles for HCV release. In transgenic mice containing human-mouse chimeric livers, HCV infection also correlates with higher levels of endogenous CES1, providing further evidence that CES1 has an important role in HCV propagation.

Carboxylesterases: Dual roles in lipid and pesticide metabolism

Carboxylesterases (CES, EC 3.1.1.1) are members of a superfamily of serine hydrolases that hydrolyze ester, amide, and carbamate bonds. Several different CES genes exist in mammalian species with evidence of multiple gene duplication events occurring throughout evolutionary history. There are five CES genes reported in the Human Genome Organization database, although CES1 and CES2 are the two best characterized human genes. An emerging picture of the CES family suggests that these enzymes have dual roles in the metabolism of xenobiotic and endobiotic compounds. Pesticides, such as the pyrethroids, are important xenobiotic substrates that are metabolized by CES, whereas cholesteryl esters, triacylglycerols, and 2-arachidonoylglycerol are examples of endobiotics known to be substrates for CES. Functional studies using selective chemical inhibitors, siRNA, and gene knockout models are providing valuable insights into the physiological functions of CES, and suggest that CES may be a novel target for the treatment of diseases such as diabetes and atherosclerosis. This review will examine the known physiological functions of CES, the interactions between xenobiotics (primarily pesticides) and lipids that occur with CES enzymes, and where possible the implications that these findings may have in terms of health and disease.

Inhibition of carboxylesterase activity of THP1 monocytes/macrophages and recombinant human carboxylesterase 1 by oxysterols and fatty acids

Two major isoforms of human carboxylesterases (CEs) are found in metabolically active tissues, CES1 and CES2. These hydrolytic enzymes are involved in xenobiotic and endobiotic metabolism. CES1 is abundantly expressed in human liver and monocytes/macrophages, including the THP1 cell line; CES2 is expressed in liver but not in monocytes/macrophages. The cholesteryl ester hydrolysis activity in human macrophages has been attributed to CES1. Here, we report the direct inhibitory effects of several endogenous oxysterols and fatty acids on the CE activity of THP1 monocytes/macrophages and recombinant human CES1 and CES2. Using THP1 whole-cell lysates we found: (1) 27-hydroxycholesterol (27-HC) is a potent inhibitor of carboxylesterase activity (IC50=33 nM); (2) 24(S),25-epoxycholesterol had moderate inhibitory activity (IC(50)=8.1 microM); and (3) cholesterol, 7-ketocholesterol, 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol, and 25-hydroxycholesterol each had little inhibitory activity. 27-HC was a partially noncompetitive inhibitor of recombinant CES1 (K(iapp)=10 nM) and impaired intracellular CES1 activity following treatment of intact THP1 cells. In contrast, recombinant CES2 activity was not inhibited by 27-HC, suggesting isoform-selective inhibition by 27-HC. Furthermore, unsaturated fatty acids were better inhibitors of CES1 activity than saturated fatty acids, while CES2 activity was unaffected by any fatty acid. Arachidonic acid (AA) was the most potent fatty acid inhibitor of recombinant CES1 and acted by a noncompetitive mechanism (K(iapp)=1.7 microM); when not complexed to albumin, exogenous AA penetrated intact THP1 cells and inhibited CES1. Inhibition results are discussed in light of recent structural models for CES1 that describe ligand binding sites separate from the active site. In addition, oxysterol-mediated inhibition of CES1 activity was demonstrated by pretreatment of human liver homogenates or intact THP1 cells with exogenous 27-HC, which resulted in significantly reduced hydrolysis of the pyrethroid insecticide bioresmethrin, a CES1-specific xenobiotic substrate. Collectively, these findings suggest that CE activity of recombinant CES1, cell lysates, and intact cells can be impaired by naturally occurring lipids, which may compromise the ability of CES1 to both detoxify environmental pollutants and metabolize endogenous compounds in vivo.

Human plasma carboxylesterase 1, a novel serologic biomarker candidate for hepatocellular carcinoma

To identify and characterize a serologic glycoprotein biomarker for hepatocellular carcinoma (HCC), multi-lectin affinity chromatography was used to isolate intracellular N-linked glycoprotein fractions from five paired non-tumor and tumor tissues. From the series of 2-D DIGE targeted differentially expressed N-linked glycoproteins, we identified human liver carboxylesterase 1 (hCE1), which was remarkably down-regulated in tumor tissues, a finding confirmed by Western blot, a quantitative real-time RT-PCR, and immunohistochemical staining of non-tumor and tumor tissues from total 58 HCC patients. To investigate whether hCE1 is also present in human plasma, we employed a magnetic bead-based immunoprecipitation followed by nano-LC-MS/MS analysis, and we found for the first time that hCE1 is present in human plasma as opposed to that in liver tissues. That is, from normalization of hCE1 signal by the immunoprecipitation and Western blot analysis, hCE1 levels were increased in plasma specimens from HCC patients than in plasma from other disease patient groups (e.g. liver cirrhosis, chronic hepatitis, cholangiocarcinoma, stomach cancer, and pancreatic cancer). From the receiver operating characteristic analysis in HCC, both sensitivity and specificity were shown to be greater than 70.0 and 85.0%, respectively. Thus, the high-resolution proteomic approach demonstrates that hCE1 is a good candidate for further validation as a serologic glycoprotein biomarker for HCC.

Macrophage cholesteryl ester mobilization and atherosclerosis

Accumulation of cholesteryl esters (CE) stored as cytoplasmic lipid droplets is the main characteristic of macrophage foam cells that are central to the development of atherosclerotic plaques. Since only unesterified or free cholesterol (FC) can be effluxed from the cells to extracellular cholesterol acceptors, hydrolysis of CE is the obligatory first step in CE mobilization from macrophages. This reaction, catalyzed by neutral cholesteryl ester hydrolase (CEH), is increasingly being recognized as the rate-limiting step in FC efflux. CEH, therefore, regulates the process of reverse cholesterol transport and ultimate elimination of cholesterol from the body. In this review, we summarize the earlier controversies surrounding the identity of CEH in macrophages, discuss the characteristics of the various candidates recognized to date and examine their role in mobilizing cellular CE and thus regulating atherogenesis. In addition, physiological requirements to hydrolyze lipid droplet-associated substrate and complexities of interfacial catalysis are also discussed to emphasize the importance of evaluating the biochemical characteristics of candidate enzymes that may be targeted in the future to attenuate atherosclerosis.

Transcriptional regulation of human carboxylesterase 1A1 by nuclear factor-erythroid 2 related factor 2 (Nrf2)

Human carboxylesterase (CES) 1A, which is predominantly expressed in liver and lung, plays an important role in the hydrolysis of endogenous compounds and xenobiotics. CES1A is reported to be induced in human hepatocytes by butylated hydroxyanisole, ticlopidine and diclofenac, and the induction is assumed to be caused by oxidative stress. However, the molecular mechanism remains to be determined. In this study, we sought to investigate whether CES1A is regulated by nuclear factor-erythroid 2 related factor 2 (Nrf2), which is a transcriptional factor activated by oxidative stress, and clarify the molecular mechanism. Real-time reverse transcription-PCR assays revealed that CES1A1 mRNA was significantly induced by tert-butylhydroquinone (tBHQ) and sulforaphane (SFN), which are representative activators of Nrf2 in HepG2, Caco-2 and HeLa cells. The induction was completely suppressed with small interfering RNA for Nrf2. In HepG2 cells, the CES1A protein level and imidapril hydrolase activity, which is specifically catalyzed by CES1A, were also significantly induced by tBHQ and SFN. Luciferase assays revealed that the antioxidant response element (ARE) at -2025 in the CES1A1 gene was responsible for the transactivation by Nrf2. In addition, electrophoretic mobility shift assays and chromatin immunoprecipitation assays revealed that Nrf2 binds to the ARE in the CES1A1 gene. These findings clearly demonstrated that human CES1A1 is induced by Nrf2. This is the first study to demonstrate the molecular mechanism of the inducible regulation of human CES1A1.

Disturbed cholesterol homeostasis in hormone-sensitive lipase-null mice

Transcriptomics analysis revealed that genes involved in hepatic de novo cholesterol synthesis were downregulated in fed HSL-null mice that had been on a high-fat diet (HFD) for 6 mo. This finding prompted a further analysis of cholesterol metabolism in HSL-null mice, which was performed in fed and 16-h-fasted mice on a normal chow diet (ND) or HFD regimen. Plasma cholesterol was elevated in HSL-null mice, in all tested conditions, as a result of cholesterol enrichment of HDL and VLDL. Hepatic esterified cholesterol content and ATP-binding cassette transporter A1 (ABCA1) mRNA and protein levels were increased in HSL-null mice regardless of the dietary regimen. Unsaturated fatty acid composition of hepatic triglycerides was modified in fasted HSL-null mice on ND and HFD. The increased ABCA1 expression had no major effect on cholesterol efflux from HSL-null mouse hepatocytes. Taken together, the results of this study suggest that HSL plays a critical role in the hydrolysis of cytosolic cholesteryl esters and that increased levels of hepatic cholesteryl esters, due to lack of action of HSL in the liver, are the main mechanism underlying the imbalance in cholesterol metabolism in HSL-null mice.

Inhibition of carboxylesterase 1 is associated with cholesteryl ester retention in human THP-1 monocyte/macrophages

Cholesteryl esters are hydrolyzed by cholesteryl ester hydrolase (CEH) yielding free cholesterol for export from macrophages. Hence, CEH has an important regulatory role in macrophage reverse cholesterol transport (RCT). CEH and human carboxylesterase 1 (CES1) appear to be the same enzyme. CES1 is inhibited by oxons, the bioactive metabolites of organophosphate (OP) pesticides. Here, we show that CES1 protein is robustly expressed in human THP-1 monocytes/macrophages and its biochemical activity inhibited following treatment of cell lysates and intact cells with chlorpyrifos oxon, paraoxon, or methyl paraoxon (with nanomolar IC(50) values) or after immunodepletion of CES1 protein. CES1 protein expression in cells is unaffected by a 24-h paraoxon treatment, suggesting that the reduced hydrolytic activity is due to covalent inhibition of CES1 by oxons and not down-regulation of expression. Most significantly, treatment of cholesterol-loaded macrophages with either paraoxon (a non-specific CES inhibitor) or benzil (a specific CES inhibitor) caused enhanced retention of intracellular cholesteryl esters and a "foamy" phenotype, consistent with reduced cholesteryl ester mobilization. Thus, exposure to OP pesticides, which results in the inhibition of CES1, may also inhibit macrophage RCT, an important process in the regression of atherosclerosis.

Regulation of cholesterol efflux from macrophages

PURPOSE OF REVIEW

The lipid efflux pathway is important for both HDL formation and the reverse cholesterol transport pathway. This review is focused on recent findings on the mechanism of lipid efflux and its regulation, particularly in macrophages.

RECENT FINDINGS

Significant progress has been made on understanding the sequence of events that accompany the interaction of apolipoproteins A-I with cell surface ATP-binding cassette transporter A1 and its subsequent lipidation. Continued research on the regulation of ATP-binding cassette transporter A1 and ATP-binding cassette transporter G1 expression and traffic has also generated new paradigms for the control of lipid efflux from macrophages and its contribution to reverse cholesterol transport. In addition, the mobilization of cholesteryl esters from lipid droplets represents a new step in the control of cholesterol efflux.

SUMMARY

The synergy between lipid transporters is a work in progress, but its importance in reverse cholesterol transport is clear. The regulation of efflux implies both the regulation of relevant transporters and the cellular trafficking of cholesterol.

Hepatic overexpression of cholesteryl ester hydrolase enhances cholesterol elimination and in vivo reverse cholesterol transport

Neutral cholesteryl ester hydrolase (CEH)-mediated hydrolysis of cellular cholesteryl esters (CEs) is required not only to generate free cholesterol (FC) for efflux from macrophages but also to release FC from lipoprotein-delivered CE in the liver for bile acid synthesis or direct secretion into the bile. We hypothesized that hepatic expression of CEH would regulate the hydrolysis of lipoprotein-derived CE and enhance reverse cholesterol transport (RCT). Adenoviral-mediated CEH overexpression led to a significant increase in bile acid output. To assess the role of hepatic CEH in promoting flux of cholesterol from macrophages to feces, cholesterol-loaded and [3H]cholesterol-labeled J774 macrophages were injected intraperitoneally into mice and the appearance of [3H]cholesterol in gallbladder bile and feces over 48 h was quantified. Mice overexpressing CEH had significantly higher [3H]cholesterol radiolabel in bile and feces, and it was associated with bile acids. This CEH-mediated increased movement of [3H]cholesterol from macrophages to bile acids and feces was significantly attenuated in SR-BI(-/-) mice. These studies demonstrate that similar to macrophage CEH that rate-limits the first step, hepatic CEH regulates the last step of RCT by promoting the flux of cholesterol entering the liver via SR-BI and increasing hepatic bile acid output.

Clair RW, Rudel LL, Ghosh S. Macrophage-specific transgenic expression of cholesteryl ester hydrolase significantly reduces atherosclerosis and lesion necrosis in Ldlr mice

Accumulation of cholesteryl esters (CEs) in macrophage foam cells, central to atherosclerotic plaque formation, occurs as a result of imbalance between the cholesterol influx and efflux pathways. While the uptake, or influx, of modified lipoproteins is largely unregulated, extracellular acceptor-mediated free cholesterol (FC) efflux is rate limited by the intracellular hydrolysis of CE. We previously identified and cloned a neutral CE hydrolase (CEH) from human macrophages and demonstrated its role in cellular CE mobilization. In the present study, we examined the hypothesis that macrophage-specific overexpression of CEH in atherosclerosis-susceptible Ldlr(-/-) mice will result in reduction of diet-induced atherosclerosis. Transgenic mice overexpressing this CEH specifically in the macrophages (driven by scavenger receptor promoter/enhancer) were developed and crossed into the Ldlr(-/-) background (Ldlr(-/-)CEHTg mice). Macrophage-specific overexpression of CEH led to a significant reduction in the lesion area and cholesterol content of high-fat, high-cholesterol diet-induced atherosclerotic lesions. The lesions from Ldlr(-/-)CEHTg mice did not have increased FC, were less necrotic, and contained significantly higher numbers of viable macrophage foam cells. Higher CEH-mediated FC efflux resulted in enhanced flux of FC from macrophages to gall bladder bile and feces in vivo. These studies demonstrate that by enhancing cholesterol efflux and reverse cholesterol transport, macrophage-specific overexpression of CEH is antiatherogenic.

Human carboxylesterases and their role in xenobiotic and endobiotic metabolism

Carboxylesterases (CEs) are traditionally regarded as xenobiotic metabolizing enzymes that hydrolyze esterified xenobiotics to alcohol and carboxylic acid products. However, there is a growing appreciation for the role of CEs in the processing of endobiotics, including cholesteryl esters and triacylglycerols. Human liver microsomes (HLMs) are often used in reaction phenotyping studies to discern interindividual variability in xenobiotic metabolism. The two major CE isoforms expressed in human liver are hCE1 and hCE2. These two isoforms are different gene products. We have begun studies to evaluate the CE phenotype'' of human liver samples, i.e. to determine both the levels of hCE1 and hCE2 protein and the hydrolytic activity of each. We have previously shown that there is little variation in hCE1 protein expression in HLM samples from 11 individuals [a 1.3-fold difference between the highest and lowest individuals; coefficient of variation (CV), 9%]. hCE2 protein expression in individual HLMs is only slightly more variable than hCE1 (2.3-fold difference between the highest and lowest individuals; CV, 36%). However, hCE1 protein is found in 46-fold higher amounts in HLMs than hCE2 protein (64.4 +/- 16.5 microg hCE1/mg microsomal protein compared to 1.4 +/- 0.2 microg hCE2/mg microsomal protein). The hydrolytic activity specifically attributable to hCE1 and hCE2 in individual HLMs was measured using bioresmethrin (a pyrethroid insecticide hydrolyzed specifically by hCE1, but not by hCE2) and procaine (an analgesic drug hydrolyzed by hCE2, but not by hCE1). The hydrolytic activity of individual HLMs toward bioresmethrin and procaine did not correlate with the protein content of hCE1 and hCE2. Thus, the mere abundance of CE proteins is not a good predictor of CE activity in HLMs. Identification of the factors that lead to altered CE activities in HLMs will be important to characterize since several pharmaceutical agents, environmental toxicants, and endobiotics are metabolized by these enzymes.

Stable overexpression of human macrophage cholesteryl ester hydrolase results in enhanced free cholesterol efflux from human THP1 macrophages

Reduction of the lipid burden of atherosclerotic lesion-associated macrophage foam cells is a logical strategy to reduce the plaque volume. Since extracellular cholesterol acceptor-mediated cholesterol efflux is the only recognized mechanism of cholesterol removal from foam cells and this process is rate limited at the level of intracellular cholesterol ester hydrolysis, a reaction catalyzed by neutral cholesteryl ester hydrolase (CEH), we examined the hypothesis that CEH overexpression in the human macrophage monocyte/macrophage cell line THP1 results in increased cholesterol efflux, as well as decreased cellular cholesterol ester accumulation. We generated THP1-CEH cells with stable integration of human macrophage CEH cDNA driven by the cytomegalovirus promoter. Compared with wild-type THP1 cells (THP1-WT), THP1-CEH cells showed increased CEH mRNA expression and increased CEH activity. Efflux of free or unesterified cholesterol by acetylated LDL-loaded THP1-CEH cells to ApoA-I by an ABCA1-dependent pathway or to HDL by an ABCG1-dependent pathway was significantly higher than that in THP1-WT cells. In addition, THP1-CEH cells accumulated significantly lower amount of esterified cholesterol. CEH overexpression, therefore, not only enhances cholesterol efflux but also reduces cellular accumulation of cholesteryl esters. Taken together, these data provide evidence for evaluating CEH expression in human macrophages as a potential target for attenuation of foam cell formation and regression of atherosclerotic plaques.

Anti-influenza prodrug oseltamivir is activated by carboxylesterase human carboxylesterase 1, and the activation is inhibited by antiplatelet agent clopidogrel

Oseltamivir is the main medicine recommended by the World Health Organization in anticipation of next influenza pandemic. This anti-influenza viral agent is an ester prodrug, and the antiviral activity is achieved by its hydrolytic metabolite: oseltamivir carboxylate. In this study, we report that the hydrolytic activation is catalyzed by carboxylesterase human carboxylesterase (HCE) 1. Liver microsomes rapidly hydrolyzed oseltamivir, but no hydrolysis was detected with intestinal microsomes or plasma. The overall rate of the hydrolysis varied among individual liver samples and was correlated well with the level of HCE1. Recombinant HCE1 but not HCE2 hydrolyzed this prodrug and produced similar kinetic parameters as the liver microsomes. Several HCE1 natural variants differed from the wild-type enzyme on the hydrolysis of oseltamivir. In the presence of antiplatelet agent clopidogrel, the hydrolysis of oseltamivir was inhibited by as much as 90% when the equal concentration was assayed. Given the fact that hydrolysis of oseltamivir is required for its therapeutic activity, concurrent use of both drugs would inhibit the activation of oseltamivir, thus making this antiviral agent therapeutically inactive. This is epidemiologically of significance because people who receive oseltamivir and clopidogrel simultaneously may maintain susceptibility to influenza infection or a source of spreading influenza virus if already infected.

Antiplatelet agents aspirin and clopidogrel are hydrolyzed by distinct carboxylesterases, and clopidogrel is transesterificated in the presence of ethyl alcohol

Aspirin (acetylsalicylic acid) and clopidogrel are two major antithrombogenic agents that are widely used for the treatment and prevention of cerebro- and cardiovascular conditions such as stroke. Combined use produces enhanced therapeutic effect. Aspirin and clopidogrel both are esters, and hydrolysis leads to decreased or inactivated therapeutic activity. The aim of the study was to determine whether aspirin and clopidogrel are hydrolyzed by the same enzyme(s), thus reciprocally prolonging the antithrombogenic activity. To test this possibility, microsomes from the liver and intestine were assayed for the hydrolysis of aspirin and clopidogrel. In contrary to the hypothesis, aspirin and clopidogrel were hydrolyzed in a tissue-differential manner. Liver microsomes hydrolyzed both drugs, whereas intestinal microsomes hydrolyzed aspirin only. Consistent with the tissue distribution of two carboxylesterases human carboxylesterase (HCE) 1 and HCE2, recombinant HCE1 hydrolyzed clopidogrel, whereas recombinant HCE2 hydrolyzed aspirin. In addition, hydrolysis of clopidogrel among liver samples was correlated well with the level of HCE1, and hydrolysis of aspirin with HCE2. Certain natural variants differed from the wild-type enzymes on the hydrolysis of aspirin or clopidogrel. In the presence of ethyl alcohol, clopidogrel is converted to ethyl clopidogrel. Carboxylesterases are important pharmacological determinants for drugs containing ester linkages and exhibit a large interindividual variation. The isoform-specific hydrolysis of aspirin and clopidogrel suggests that these two antithrombogenic agents may have pharmacokinetic interactions with different sets of ester drugs, and the altered hydrolysis by polymorphic mutants provides a molecular explanation to the interindividual variation.

Multisite promiscuity in the processing of endogenous substrates by human carboxylesterase 1

Human carboxylesterase 1 (hCE1) is a drug and endobiotic-processing serine hydrolase that exhibits relatively broad substrate specificity. It has been implicated in a variety of endogenous cholesterol metabolism pathways including the following apparently disparate reactions: cholesterol ester hydrolysis (CEH), fatty acyl Coenzyme A hydrolysis (FACoAH), acyl-Coenzyme A:cholesterol acyltransfer (ACAT), and fatty acyl ethyl ester synthesis (FAEES). The structural basis for the ability of hCE1 to perform these catalytic actions involving large substrates and products has remained unclear. Here we present four crystal structures of the hCE1 glycoprotein in complexes with the following endogenous substrates or substrate analogues: Coenzyme A, the fatty acid palmitate, and the bile acids cholate and taurocholate. While the active site of hCE1 was known to be promiscuous and capable of interacting with a variety of chemically distinct ligands, these structures reveal that the enzyme contains two additional ligand-binding sites and that each site also exhibits relatively non-specific ligand-binding properties. Using this multisite promiscuity, hCE1 appears structurally capable of assembling several catalytic events depending, apparently, on the physiological state of the cellular environment. These results expand our understanding of enzyme promiscuity and indicate that, in the case of hCE1, multiple non-specific sites are employed to perform distinct catalytic actions.