Elimination of cholesterol ester from macrophage foam cells by adenovirus-mediated gene transfer of hormone-sensitive lipase

Hormone-sensitive lipase: sixty years later

Hormone-sensitive lipase (HSL) was initially characterized as the hormonally regulated neutral lipase activity responsible for the breakdown of triacylglycerols into fatty acids in adipose tissue. This review aims at providing up-to-date information on structural properties, regulation of expression, activity and function as well as therapeutic potential. The lipase is expressed as different isoforms produced from tissue-specific alternative promoters. All isoforms are composed of an N-terminal domain and a C-terminal catalytic domain within which a regulatory domain containing the phosphorylation sites is embedded. Some isoforms possess additional N-terminal regions. The catalytic domain shares similarities with bacteria, fungus and vascular plant proteins but not with other mammalian lipases. HSL singularity is provided by regulatory and N-terminal domains sharing no homology with other proteins. HSL has a broad substrate specificity compared to other neutral lipases. It hydrolyzes acylglycerols, cholesteryl and retinyl esters among other substrates. A novel role of HSL, independent of its enzymatic function, has recently been described in adipocytes. Clinical studies revealed dysregulations of HSL expression and activity in disorders, such as lipodystrophy, obesity, type 2 diabetes and cancer-associated cachexia. Development of specific inhibitors positions HSL as a pharmacological target for the treatment of metabolic complications.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

Role of Hormone-sensitive Lipase in Leptin-Promoted Fat Loss and Glucose Lowering

Aim: Myriad biological effects of leptin may lead to broad therapeutic applications for various metabolic diseases, including diabetes and its complications; however, in contrast to its anorexic effect, the molecular mechanisms underlying adipopenic and glucose-lowering effects of leptin have not been fully understood. Here we aim to clarify the role of hormone-sensitive lipase (HSL) in leptin's action.

Methods: Wild-type (WT) and HSL-deficient (HSLKO) mice were made hyperleptinemic by two commonly-used methods: adenovirus-mediated overexpression of leptin and continuous subcutaneous infusion of leptin by osmotic pumps. The amount of food intake, body weights, organ weights, and parameters of glucose and lipid metabolism were measured.

Results: Hyperleptinemia equally suppressed the food intake in WT and HSLKO mice. On the other hand, leptin-mediated fat loss and glucose-lowering were significantly blunted in the absence of HSL when leptin was overexpressed by recombinant adenovirus carrying leptin. By osmotic pumps, the fat-losing and glucose-lowering effects of leptin were milder due to lower levels of hyperleptinemia; although the difference between WT and HSLKO mice did not reach statistical significance, HSLKO mice had a tendency to retain more fat than WT mice in the face of hyperleptinemia.

Conclusions: We clarify for the first time the role of HSL in leptin's effect using a genetic model: leptin-promoted fat loss and glucose-lowering are at least in part mediated via HSL-mediated lipolysis. Further studies to define the pathophysiological role of adipocyte lipases in leptin action may lead to a new therapeutic approach to circumvent leptin resistance.

Absence of Nceh1 augments 25-hydroxycholesterol-induced ER stress and apoptosis in macrophages

An excess of cholesterol and/or oxysterols induces apoptosis in macrophages, contributing to the development of advanced atherosclerotic lesions. In foam cells, these sterols are stored in esterified forms, which are hydrolyzed by two enzymes: neutral cholesterol ester hydrolase 1 (Nceh1) and hormone-sensitive lipase (Lipe). A deficiency in either enzyme leads to accelerated growth of atherosclerotic lesions in mice. However, it is poorly understood how the esterification and hydrolysis of sterols are linked to apoptosis. Remarkably, Nceh1-deficient thioglycollate-elicited peritoneal macrophages (TGEMs), but not Lipe-deficient TGEMs, were more susceptible to apoptosis induced by oxysterols, particularly 25-hydroxycholesterol (25-HC), and incubation with 25-HC caused massive accumulation of 25-HC ester in the endoplasmic reticulum (ER) due to its defective hydrolysis, thereby activating ER stress signaling such as induction of CCAAT/enhancer-binding protein-homologous protein (CHOP). These changes were nearly reversed by inhibition of ACAT1. In conclusion, deficiency of Nceh1 augments 25-HC-induced ER stress and subsequent apoptosis in TGEMs. In addition to reducing the cholesteryl ester content of foam cells, Nceh1 may protect against the pro-apoptotic effect of oxysterols and modulate the development of atherosclerosis.

Critical role of neutral cholesteryl ester hydrolase 1 in cholesteryl ester hydrolysis in murine macrophages

Hydrolysis of intracellular cholesteryl ester (CE) is the rate-limiting step in the efflux of cholesterol from macrophage foam cells. In mouse peritoneal macrophages (MPMs), this process is thought to involve several enzymes: hormone-sensitive lipase (Lipe), carboxylesterase 3 (Ces3), neutral CE hydrolase 1 (Nceh1). However, there is some disagreement over the relative contributions of these enzymes. To solve this problem, we first compared the abilities of several compounds to inhibit the hydrolysis of CE in cells overexpressing Lipe, Ces3, or Nceh1. Cells overexpressing Ces3 had negligible neutral CE hydrolase activity. We next examined the effects of these inhibitors on the hydrolysis of CE and subsequent cholesterol trafficking in MPMs. CE accumulation was increased by a selective inhibitor of Nceh1, paraoxon, and two nonselective inhibitors of Nceh1, (+)-AS115 and (-)-AS115, but not by two Lipe-selective inhibitors, orlistat and 76-0079. Paraoxon inhibited cholesterol efflux to apoA-I or HDL, while 76-0079 did not. These results suggest that Nceh1 plays a dominant role over Lipe in the hydrolysis of CE and subsequent cholesterol efflux in MPMs.

Adeno-associated virus serotype 2 mediated transduction and coexpression of the human apoAI and SR-BI gene in HepG2 cells

Cholesterol efflux is the first step in the reverse cholesterol transport (RCT) pathway, removing excess cholesterol from tissues, including the arterial wall, thus preventing the development of atherosclerosis. Adeno-associated virus (rAAV) has demonstrated significant promise as a DNA-delivery vector to treat serious human diseases. In this study, we constructed recombinant adeno-associated viruses coexpressing apoAI and SR-BI successfully, the double gene mRNA and protein were both strongly expressed in transduced HepG2 cells. A novel safe and efficient method of promoting the reverse cholesterol transport (RCT) may be established. These results may provide a new method for gene therapy of Arteriosclerosis.

Lipid droplets: size matters

The lipid droplet (LD), an organelle that exists ubiquitously in various organisms, from bacteria to mammals, has attracted much attention from both medical and cell biology fields. The LD in white adipocytes is often treated as the prototype LD, but is rather a special example, considering that its size, intracellular localization and molecular composition are vastly different from those of non-adipocyte LDs. These differences confer distinct properties on adipocyte and non-adipocyte LDs. In this article, we address the current understanding of LDs by discussing the differences between adipocyte and non-adipocyte LDs.

Cholesteryl ester hydrolase activity is abolished in HSL-/- macrophages but unchanged in macrophages lacking KIAA1363

Cholesteryl ester (CE) accumulation in macrophages represents a crucial event during foam cell formation, a hallmark of atherogenesis. Here we investigated the role of two previously described CE hydrolases, hormone-sensitive lipase (HSL) and KIAA1363, in macrophage CE hydrolysis. HSL and KIAA1363 exhibited marked differences in their abilities to hydrolyze CE, triacylglycerol (TG), diacylglycerol (DG), and 2-acetyl monoalkylglycerol ether (AcMAGE), a precursor for biosynthesis of platelet-activating factor (PAF). HSL efficiently cleaved all four substrates, whereas KIAA1363 hydrolyzed only AcMAGE. This contradicts previous studies suggesting that KIAA1363 is a neutral CE hydrolase. Macrophages of KIAA1363(-/-) and wild-type mice exhibited identical neutral CE hydrolase activity, which was almost abolished in tissues and macrophages of HSL(-/-) mice. Conversely, AcMAGE hydrolase activity was diminished in macrophages and some tissues of KIAA1363(-/-) but unchanged in HSL(-/-) mice. CE turnover was unaffected in macrophages lacking KIAA1363 and HSL, whereas cAMP-dependent cholesterol efflux was influenced by HSL but not by KIAA1363. Despite decreased CE hydrolase activities, HSL(-/-) macrophages exhibited CE accumulation similar to wild-type (WT) macrophages. We conclude that additional enzymes must exist that cooperate with HSL to regulate CE levels in macrophages. KIAA1363 affects AcMAGE hydrolase activity but is of minor importance as a direct CE hydrolase in macrophages.

LXR-SREBP-1c-phospholipid transfer protein axis controls very low density lipoprotein (VLDL) particle size

Liver X receptors (LXRs) activate triglyceride synthesis in liver directly and indirectly by inducing sterol regulatory element-binding protein-1c (SREBP-1c). When administered to wild-type mice, the LXR activator T0901317 produces a mild and transient hypertriglyceridemia. Here, we show that T0901317 produces massive hypertriglyceridemia when given to mice lacking low density lipoprotein (LDL) receptors (Ldlr(-/-) mice). Triglycerides ranged from 4000 to 6000 mg/dl, and the plasma turned milky. The median diameter of VLDL particles, measured by electron microscopy, increased from 43 to 112 nm, 87% exceeding 80 nm, the size of chylomicrons. Hypertriglyceridemia was prevented in Ldlr(-/-) recipient mice that lacked SREBP-1c (Ldlr(-/-);Srebp-1c(-/-) double knock-out mice). In Ldlr(-/-) mice, T0901317 increased mRNAs not only for enzymes of fatty acid and triglyceride synthesis, but also for phospholipid transfer protein (PLTP), which transfers phospholipids into nascent VLDL, allowing particle expansion. The PLTP increase was blunted in Ldlr(-/-);Srebp-1c(-/-) animals. When Ldlr(-/-);Srebp-1c(-/-) mice received an adenovirus encoding Pltp, the hypertriglyceridemic response to T0901317 was partially restored and the VLDL size increased. We conclude that LXR agonists activate triglyceride synthesis and Pltp transcription by activating Srebp-1c. In concert with the increase in TG synthesis, the increased PLTP permits triglyceride incorporation into abnormally large VLDL, which are removed from plasma by LDL receptors. In the absence of LDL receptors, the large VLDLs accumulate and produce massive hypertriglyceridemia.

Ablation of neutral cholesterol ester hydrolase 1 accelerates atherosclerosis

Cholesterol ester (CE)-laden macrophage foam cells are the hallmark of atherosclerosis, and the hydrolysis of intracellular CE is one of the key steps in foam cell formation. Although hormone-sensitive lipase (LIPE) and cholesterol ester hydrolase (CEH), which is identical to carboxylsterase 1 (CES1, hCE1), were proposed to mediate the neutral CE hydrolase (nCEH) activity in macrophages, recent evidences have suggested the involvement of other enzymes. We have recently reported the identification of a candidate, neutral cholesterol ester hydrolase 1(Nceh1). Here we demonstrate that genetic ablation of Nceh1 promotes foam cell formation and the development of atherosclerosis in mice. We further demonstrate that Nceh1 and Lipe mediate a comparable degree of nCEH activity in macrophages and together account for most of the activity. Mice lacking both Nceh1 and Lipe aggravated atherosclerosis in an additive manner. Thus, Nceh1 is a promising target for the treatment of atherosclerosis.

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.

Identification of neutral cholesterol ester hydrolase, a key enzyme removing cholesterol from macrophages

Unstable lipid-rich plaques in atherosclerosis are characterized by the accumulation of macrophage foam cells loaded with cholesterol ester (CE). Although hormone-sensitive lipase and cholesteryl ester hydrolase (CEH) have been proposed to mediate the hydrolysis of CE in macrophages, circumstantial evidence suggests the presence of other enzymes with neutral cholesterol ester hydrolase (nCEH) activity. Here we show that the murine orthologue of KIAA1363, designated as neutral cholesterol ester hydrolase (NCEH), is a microsomal nCEH with high expression in murine and human macrophages. The effect of various concentrations of NaCl on its nCEH activity resembles that on endogenous nCEH activity of macrophages. RNA silencing of NCEH decreases nCEH activity at least by 50%; conversely, its overexpression inhibits the CE formation in macrophages. Immunohistochemistry reveals that NCEH is expressed in macrophage foam cells in atherosclerotic lesions. These data indicate that NCEH is responsible for a major part of nCEH activity in macrophages and may be a potential therapeutic target for the prevention of atherosclerosis.

Induction of ABCA1 by overexpression of hormone-sensitive lipase in macrophages

Initial step toward the reverse-cholesterol transport is cholesterol efflux that is mediated by the ATP-binding cassette transporter A1 (ABCA1). However, it is unknown how the cholesteryl ester (CE) hydrolysis induces the expression of the ABCA1 gene. Overexpression of hormone-sensitive lipase (HSL) increased the hydrolysis of CE and stimulated the expression of ABCA1 gene at the transcriptional level in RAW 264.7 macrophages. The stimulatory effects of the HSL overexpression and cholesterol loading on the ABCA1 promoter activity were additive. Mutational analyses of the promoter of ABCA1 identified the responsible element as the direct repeat-4 (DR-4) that binds LXR/RXR heterodimers. In conclusion, stimulation of hydrolysis of CE in macrophages induces the expression of ABCA1 gene primarily via the LXR-dependent pathway and can be useful for the prevention of atherosclerosis.

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.

Overexpression of lysosomal acid lipase and other proteins in atherosclerosis

Atherosclerosis is one of the major causes of morbidity and mortality in the western world. The existing data of elevated expression levels of proteins like DNA damage and DNA repair enzymes in human atherosclerotic plaques are reviewed. From the literature, the effect of overexpression of different proteins using adenoviral vectors or the model of transgenic mice on the development of atherosclerosis will be discussed. Special focus is placed on the lysosomal acid lipase (LAL), because LAL connects extra-cellular with intra-cellular lipid metabolism and is the only hydrolase for cleavage of cholesteryl esters delivered to the lysosomes. Patients with a deficiency of LAL show an accumulation of lipids in the cells and develop pre-mature atherosclerosis. To answer the question of the influence of LAL in atherosclerosis if overexpressed, we show for the first time data of transgenic mice overexpressing LAL and the effect on the lipid level.

Identification of a novel member of the carboxylesterase family that hydrolyzes triacylglycerol: a potential role in adipocyte lipolysis

Molecular mechanisms underlying lipolysis, as defined by mobilization of fatty acids from adipose tissue, are not fully understood. A database search for enzymes with alpha/beta hydrolase folds, the GXSXG motif for serine esterase and the His-Gly dipeptide motif, has provided a previously unannotated gene that is induced during 3T3-L1 adipocytic differentiation. Because of its remarkable structural resemblance to triacylglycerol hydrolase (TGH) with 70.4% identity, we have tentatively designated this enzyme as TGH-2 and the original TGH as TGH-1. TGH-2 is also similar to TGH-1 in terms of tissue distribution, subcellular localization, substrate specificity, and regulation. Both enzymes are predominantly expressed in liver, adipose tissue, and kidney. In adipocytes, they are localized in microsome and fatcake. Both enzymes hydrolyzed p-nitophenyl butyrate, triolein, and monoolein but not diolein, cholesteryl oleate, or phospholipids; hydrolysis of short-chain fatty acid ester was 30,000-fold more efficient than that of long-chain fatty acid triacylglycerol. Fasting increased the expression of both genes in white adipose tissue, whereas refeeding suppressed their expression. RNA silencing of TGH-2 reduced isoproterenol-stimulated glycerol release by 10% in 3T3-L1 adipocytes, while its overexpression increased the glycerol release by 20%. Thus, TGH-2 may make a contribution to adipocyte lipolysis during period of increased energy demand.

Increased cholesterol biosynthesis and hypercholesterolemia in mice overexpressing squalene synthase in the liver

Squalene synthase (SS) is the first committed enzyme for cholesterol biosynthesis, located at a branch point in the mevalonate pathway. To examine the role of SS in the overall cholesterol metabolism, we transiently overexpressed mouse SS in the livers of mice using adenovirus-mediated gene transfer. Overexpression of SS increased de novo cholesterol biosynthesis with increased 3-hydroxy-3-methyglutaryl-CoA (HMG-CoA) reductase activity, in spite of the downregulation of its own mRNA expression. Furthermore, overexpression of SS increased plasma concentrations of LDL, irrespective of the presence of functional LDL receptor (LDLR). Thus, the hypercholesterolemia is primarily caused by increased hepatic production of cholesterol-rich VLDL, as demonstrated by the increases in plasma cholesterol levels after intravenous injection of Triton WR1339. mRNA expression of LDLR was decreased, suggesting that defective LDL clearance contributed to the development of hypercholesterolemia. Curiously, the liver was enlarged, with a larger number of Ki-67-positive cells. These results demonstrate that transient upregulation of SS stimulates cholesterol biosynthesis as well as lipoprotein production, providing the first in vivo evidence that SS plays a regulatory role in cholesterol metabolism through modulation of HMG-CoA reductase activity and cholesterol biosynthesis.

17-Beta estradiol enhances prostaglandin E2 production in human U937-derived macrophages

Prostaglandins (PGs) are potent eicosanoid lipid mediators that have been implicated in numerous homeostatic functions and inflammation. Estrogens have been shown to regulate the expression of genes in lipid metabolism in many cellular systems. In this study, the activation of macrophages and the modulation of PG release by estrogens were examined. The effects of 17-alpha and 17-beta estradiols, phytoestrogen Genistein and several selective estrogen receptor modulators on the release of PGE2 were investigated in human U937-derived macrophages. 17-Beta estradiol caused an enhancement of PGE2 production in a time- and dose-dependent manner. Treatment of macrophages with 17-beta estradiol elicited an increased arachidonic acid (AA) release and an up-regulation of both cyclooxygenesis-1 and cyclooxygenesis-2 enzymes at both the transcript and protein levels. In addition, immunostaining of nuclear estrogen receptor alpha and the observation of ICI182 780 blockade of PGE2 production indicated that 17-beta estradiol-induced PGE2 release was mainly through nuclear estrogen receptor alpha.

Continuous monitoring of cholesterol oleate hydrolysis by hormone-sensitive lipase and other cholesterol esterases

Hormone-sensitive lipase (HSL) contributes importantly to the hydrolysis of cholesteryl ester in steroidogenic tissues, releasing the cholesterol required for adrenal steroidogenesis. HSL has broad substrate specificity, because it hydrolyzes triacylglycerols (TAGs), diacylglycerols, monoacylglycerols, and cholesteryl esters. In this study, we developed a specific cholesterol esterase assay using cholesterol oleate (CO) dispersed in phosphatidylcholine and gum arabic by sonication. To continuously monitor the hydrolysis of CO by HSL, we used the pH-stat technique. For the sake of comparison, the hydrolysis of CO dispersion was also tested using other cholesteryl ester-hydrolyzing enzymes. The specific activities measured on CO were found to be 18, 100, 27, and 3 micromol/min/mg for HSL, cholesterol esterase from Pseudomonas species, Candida rugosa lipase-3, and cholesterol esterase from bovine pancreas, respectively. The activity of HSL on CO is approximately 4- to 5-fold higher than on long-chain TAGs. In contrast, with all other enzymes tested, the rates of TAG hydrolysis were higher than those of CO hydrolysis. The relatively higher turnover of HSL on CO observed in vitro adds further molecular insight on the physiological importance of HSL in cholesteryl ester catabolism in vivo. Thus, HSL could be considered more as a cholesteryl ester hydrolase than as a TAG lipase.

Over-expression of hepatic neutral cytosolic cholesteryl ester hydrolase in mice increases free cholesterol and reduces expression of HMG-CoAR, CYP27, and CYP7A1

Hepatic neutral cytosolic cholesteryl ester hydrolase (hncCEH) is a key enzyme in the regulation of hepatic free cholesterol (FC). In examining the effects of over-expression of this enzyme on cholesterol homeostasis, mice were infected with a recombinant adenovirus construct (AdCEH) of the rat hncCEH cDNA driven by the human cytomegalovirus promoter. Cholesteryl esterase and p-nitrophenylcaprylate (PNPC) esterase activities were measured in liver postmitochondrial supernatants at 1, 3, 7, and 11 d after infection with AdCEH or a control virus expressing beta-galactosidase (AdbetaGAL). The PNPC esterase activity of AdCEH mice peaked threefold higher than controls on day 2, declining on subsequent days. In contrast, cholesteryl esterase peaked eightfold higher than controls on day 3, indicating a shift in substrate selectivity of hncCEH. Hepatic FC peaked at 144% of controls, 7 d postinfection. The mRNAs for cholesterol 7alpha-hydroxylase, sterol 27-hydroxylase, and HMG-CoA reductase decreased to 47, 46, and 58% of controls, respectively, on day 7, coinciding with peak FC concentrations. Coinciding with increased cholesteryl esterase activity, hepatic esterified cholesterol dropped precipitously from day 3 onward, to 11% of controls by day 11. Hepatic TAG levels also declined, consistent with the reported TAG lipase activity of hncCEH. These results demonstrate elevation of FC and depletion of cholesteryl esters by over-expression of hncCEH, which were resistant to compensatory responses by other enzymes of cholesterol homeostasis.

Sterol-mediated regulation of hormone-sensitive lipase in 3T3-L1 adipocytes

We previously reported that intracellular free cholesterol at physiological concentrations regulates the activity of neutral cholesterol esterase (N-CEase) in macrophages. The objective of the present study is to investigate whether the regulation of N-CEase by cholesterol is generally observed in other types of cells such as adipocytes with high activity of hormone-sensitive lipase (HSL), the same gene product as N-CEase. 3T3-L1 adipocytes were cultured with and without cholesterol (1-30 microg/mL) or 25-hydroxycholesterol (0.1-10 microg/mL), and changes in the N-CEase activity, expression of HSL mRNA, and protein were examined. Incubation (24 h) of cells with cholesterol did not change N-CEase activity, but incubation with 25-hydroxycholesterol decreased the activity in a concentration-dependent manner by 24 (24 h) and 54% (36 h). Quantitative reverse transcription-PCR indicated that 25-hydroxycholesterol (10 microg/mL) did not influence expression of HSL mRNA. However, Western blot analysis showed that this sterol reduced HSL protein by 72 (24 h) and by 93% (36 h), respectively. It was concluded that sterol-mediated regulation of HSL/N-CEase occurs not only in macrophages but also in adipocytes, and regulation appears to occur not at a transcriptional level but by a post-transcriptional process. Sterol-mediated proteolysis may be involved in the loss of HSL protein.

Expression of Caveolin-1 Enhances Cholesterol Efflux in Hepatic Cells

HepG2 cells were stably transfected with human caveolin-1 (HepG2/cav cells). Transfection resulted in expression of caveolin-1 mRNA, a high abundance of caveolin-1 protein, and the formation of caveolae on the plasma membrane. Cholesterol efflux from HepG2/cav cells was 280 and 45% higher than that from parent HepG2 cells when human plasma and human apoA-I, respectively, were used as acceptors. The difference in efflux was eliminated by treatment of cells with progesterone. There was no difference in cholesterol efflux to cyclodextrin. Cholesterol efflux from plasma membrane vesicles was similar for the two cell types. Transfection led to a 40% increase in the amount of plasma membrane cholesterol in cholesterol-rich domains (caveolae and/or rafts) and a 67% increase in the rate of cholesterol trafficking from intracellular compartments to these domains. Cholesterol biosynthesis in HepG2/cav cells was increased by 2-fold, and cholesterol esterification was reduced by 50% compared with parent HepG2 cells. The proliferation rate of transfected cells was significantly lower than that of non-transfected cells. Transfection did not affect expression of ABCA1 or the abundance of ABCA1 protein, but decreased secretion of apoA-I. We conclude that overexpression of caveolin-1 in hepatic cells stimulates cholesterol efflux by enhancing transfer of cholesterol to cholesterol-rich domains in the plasma membrane.

Adipophilin Enhances Lipid Accumulation and Prevents Lipid Efflux From THP-1 Macrophages

Objective— Uptake of modified low-density lipoprotein (LDL) by macrophages through scavenger receptors results in lipid droplets accumulation and foam cell formation. Excess lipid deposition in macrophages has been reported to modulate expression of several genes including adipophilin. In this study, we investigated the function of adipophilin in lipid accumulation and cholesterol efflux in THP-1 macrophages.

Methods and Results— Adipophilin mRNA expression was 3.5-fold higher in human atherosclerotic plaques compared with healthy areas of the same arteries. Moreover, in the presence of acetylated LDL (AcLDL), triglycerides and cholesteryl esters were increased in macrophages overexpressing adipophilin by 40% and 67%, respectively, whereas their accumulation was reduced when endogenous cellular adipophilin was depleted using siRNA approach. In addition, neither overexpression nor downregulation of adipophilin altered expression of genes involved in lipid efflux. However, the affinity and the number of AcLDL receptors were not affected. After 24-hour incubation of lipid-loaded macrophages with apolipoprotein A-I, cholesterol efflux was reduced by 47% in adipophilin transfected cells versus control cells.

Conclusion— Our results showed that stimulation of adipophilin expression in macrophages by modified LDL promotes triglycerides and cholesterol storage and reduces cholesterol efflux. Therefore, adipophilin might contribute, in vivo, to lipid accumulation in the intima of the arterial wall.

Reduced atherosclerosis in hormone-sensitive lipase transgenic mice overexpressing cholesterol acceptors

Macrophage-specific overexpression of cholesteryl ester hydrolysis in hormone-sensitive lipase transgenic (HSL Tg) female mice paradoxically increases cholesterol esterification and cholesteryl ester accumulation in macrophages, and thus susceptibility to diet-induced atherosclerosis compared to nontransgenic C57BL/6 mice. The current studies suggest that whereas increased cholesterol uptake could contribute to transgenic foam cell formation, there are no differences in cholesterol synthesis and the expression of cholesterol efflux mediators (ABCA1, ABCG1, apoE, PPARgamma, and LXRalpha) compared to wild-type macrophages. HSL Tg macrophages exhibit twofold greater efflux of cholesterol to apoA-I in vitro, suggesting the potential rate-limiting role of cholesteryl ester hydrolysis in efflux. However, macrophage cholesteryl ester levels appear to depend on the relative efficacy of alternate pathways for free cholesterol in either efflux or re-esterification. Thus, increased atherosclerosis in HSL Tg mice appears to be due to the coupling of the efficient re-esterification of excess free cholesterol to its limited removal mediated by the cholesterol acceptors in these mice. The overexpression of cholesterol acceptors in HSL-apoA-IV double-transgenic mice increases plasma HDL levels and decreases diet-induced atherosclerosis compared to HSL Tg mice, with aortic lesions reduced to sizes in nontransgenic littermates. The results in vivo are consistent with the effective efflux from HSL Tg macrophages supplemented with HDL and apoA-I in vitro, and highlight the importance of cholesterol acceptors in inhibiting atherosclerosis caused by imbalances in the cholesteryl ester cycle.

Letting lipids go: hormone-sensitive lipase

PURPOSE OF REVIEW

Despite their pathophysiological importance, the molecular mechanisms and enzymatic components of lipid mobilization from intracellular storage compartments are insufficiently understood. The aim of this review is to evaluate the role of hormone-sensitive lipase in this process.

RECENT FINDINGS

Hormone-sensitive lipase exhibits a broad specificity for lipid substrates such as triglycerides, diglycerides, cholesteryl esters, and retinyl esters and the enzyme is in a wide variety of tissues. The high enzyme activity in adipose tissue was considered rate-limiting in the degradation of stored triglycerides. This view of a single enzyme controlling the catabolism of stored fat was challenged by recent findings that in hormone-sensitive lipase deficient mice adipose tissue triglycerides were still hydrolyzed and that these animals were leaner than normal mice. These results indicated that in adipose tissue hormone-sensitive lipase cooperates with other yet unidentified lipases to control the mobilization of fatty acids from cellular depots and that this process is coordinately regulated with lipid synthesis. Induced mutant mouse lines that overexpress or lack hormone-sensitive lipase also provided evidence that hormone-sensitive lipase-mediated cholesteryl ester hydrolysis is involved in steroid-hormone production in adrenals and affects testis function. Finally, hormone-sensitive lipase deficiency in mice results in a lipoprotein profile characterized by low triglyceride and VLDL levels and increased HDL cholesterol concentrations.

SUMMARY

The 'anti-atherosclerotic' plasma lipoprotein profile and the fact that hormone-sensitive lipase deficient animals become lean identifies the inhibition of hormone-sensitive lipase as a potential target for the treatment of lipid disorders and obesity.

Glucose-dependent regulation of cholesterol ester metabolism in macrophages by insulin and leptin

Insulin resistance, obesity, and diabetes are characterized by hyperglycemia, hyperinsulinemia, and hyperleptinemia and are associated with increased risk of atherosclerosis. In an effort to understand how this occurs, we have investigated whether these factors cause disregulation of cholesterol ester metabolism in J774.2 macrophages. Raising glucose levels alone was sufficient to increase uptake of acetylated low density lipoprotein but did not stimulate synthesis of cholesterol esters. In the presence of high glucose, both insulin and leptin increased the rate of cholesterol ester synthesis, although they did not further increase uptake of acetylated low density lipoprotein. However, in the presence of high glucose both insulin and leptin caused a significant increase in the activity of acyl-CoA: cholesterol O-acyltransferase (ACAT) combined with a significant reduction in the level of hormone-sensitive lipase (HSL). Because ACAT is the main enzyme responsible for cholesterol ester synthesis and HSL contributes significantly to neutral cholesterol ester hydrolase activity, this suggests that glucose primes the J774.2 cells so that in the presence of high insulin or leptin they will store cholesterol esters. This contrasts with 3T3-L1 adipocytes, where HSL activity and expression are increased by insulin in high glucose conditions. These findings may provide an explanation for the observation that in conditions characterized by hyperglycemia, hyperleptinemia, and hyperinsulinemia, triglyceride lipolysis in adipocytes is increased while hydrolysis of cholesterol esters in macrophages is decreased, contributing to foam cell formation.

Hormone-sensitive lipase: control of intracellular tri-(di-)acylglycerol and cholesteryl ester hydrolysis

Hormone-sensitive lipase (HSL) is an intracellular neutral lipase that is capable of hydrolyzing triacylglycerols, diacylglycerols, monoacylglycerols, and cholesteryl esters, as well as other lipid and water soluble substrates. HSL activity is regulated post-translationally by phosphorylation and also by pretranslational mechanisms. The enzyme is highly expressed in adipose tissue and steroidogenic tissues, with lower amounts expressed in cardiac and skeletal muscle, macrophages, and islets. Studies of the structure of HSL have identified several amino acids and regions of the molecule that are critical for enzymatic activity and regulation of HSL. This has led to important insights into its function, including the interaction of HSL with other intracellular proteins, such as adipocyte lipid binding protein. Accumulating evidence has defined important functions for HSL in normal physiology, affecting adipocyte lipolysis, steroidogenesis, spermatogenesis, and perhaps insulin secretion and insulin action; however, direct links between abnormal expression or genetic variations of HSL and human disorders, such as obesity, insulin resistance, type 2 diabetes, and hyperlipidemia, await further clarification. The published reports examining the regulation, and function of HSL in normal physiology and disease are reviewed in this paper.