Cholesterol-mediated changes of neutral cholesterol esterase activity in macrophages. Mechanism for mobilization of cholesteryl esters in lipid droplets by HDL

Role of Lipoproteins in the Microenvironment of Hormone-Dependent Cancers

The tumor microenvironment (TME) is an attractive target to develop novel strategies for hormone-dependent cancers. Several molecules in the TME can favor tumor development and progression, including lipoproteins. Lipoproteins are taken up by cancer cells, providing them with cholesterol and fatty acids. Cholesterol regulates cell signaling and it is converted into a series of bioactive metabolites, including hormones. The conflicting results of epidemiological and interventional studies suggest that the local availability of lipoproteins in the TME is more relevant for cancer biology than their circulating levels. Thus, reducing lipoprotein uptake and stimulating cell cholesterol efflux to high-density lipoproteins (HDLs) can represent a novel adjuvant strategy for cancer management. HDL-like particles can also act as drug delivery systems for tumor targeting.

Involvement of cholesterol efflux pathway in the control of cardiomyocytes cholesterol homeostasis

Although cholesterol-rich microdomains are highly involved in the functions of cardiomyocytes, the cholesterol homeostasis is largely unknown in these cells. We developed experimental procedures to assess cholesterol synthesis, cholesterol masses and cholesterol efflux from primary cultures of cardiac myocytes obtained from 2 to 4 days old Wistar rats. We first observed that cardiomyocytes poorly internalized exogenously supplied native or modified LDL and that free cholesterol (FC) efflux to free apolipoprotein AI (apo AI) and to HDL was mediated by ATP binding cassette transporter A1 (ABCA1) and likely by ATP binding cassette transporter G1 (ABCG1), respectively, which are both upregulated by liver X receptor/retinoid X receptor (LXR/RXR) activation. We then investigated the consequences of cholesterol synthesis inhibition on cholesterol homeostasis using an HMGCoA reductase inhibitor (pravastatin, 90% effective concentration (EC90): 0.11 mM, 18 h). We observed no impact of cholesterol synthesis inhibition on the FC or cholesteryl ester (CE) masses. Consistently with no FC mass changes, pravastatin treatment had no notable impact on LDL receptors mRNA expression or on the capacity of cardiomyocytes to uptake radiolabeled LDL. Conversely, pravastatin treatment induced a significant decrease of cholesterol efflux to both apo AI and HDL whereas the passive aqueous diffusion remained unchanged. The cholesterol efflux pathway reductions induced by cholesterol synthesis inhibition were not caused by a reduction of ABC transporter expression (mRNA or protein). These results show that cardiac myocytes down-regulate active cholesterol efflux processes when endogenous cholesterol synthesis is inhibited, allowing them to preserve cholesterol homeostasis.

Probing stereoselective inhibition of the acyl binding site of cholesterol esterase with four diastereomers of 2'-N-alpha-methylbenzylcarbamyl-1, 1'-bi-2-naphthol

Background

Recently there has been increased interest in pancreatic cholesterol esterase due to correlation between enzymatic activity in vivo and absorption of dietary cholesterol. Cholesterol esterase plays a role in digestive lipid absorption in the upper intestinal tract, though its role in cholesterol absorption in particular is controversial. Serine lipases, acetylcholinesterase, butyrylcholinesterase, and cholesterol esterase belong to a large family of proteins called the α/β-hydrolase fold, and they share the same catalytic machinery as serine proteases in that they have an active site serine residue which, with a histidine and an aspartic or glutamic acid, forms a catalytic triad. The aim of this work is to study the stereoselectivity of the acyl chain binding site of the enzyme for four diastereomers of an inhibitor.

Results

Four diastereomers of 2'-N-α-methylbenzylcarbamyl-1, 1'-bi-2-naphthol (1) are synthesized from the condensation of R-(+)- or S-(-)-1, 1'-bi-2-naphthanol with R-(+)- or S-(-)-α-methylbenzyl isocyanate in the presence of a catalytic amount of pyridine in CH₂Cl₂. The [α]²⁵_D values for (1R, αR)-1, (1R, αS)-1, (1S, αR)-1, and (1S, αS)-1 are +40, +21, -21, and -41°, respectively. All four diastereomers of inhibitors are characterized as pseudo substrate inhibitors of pancreatic cholesterol esterase. Values of the inhibition constant (K_i), the carbamylation constant (k₂), and the bimolecular rate constant (k_i) for these four diastereomeric inhibitors are investigated. The inhibitory potencies for these four diastereomers are in the descending order of (1R, αR)-1, (1R, αS)-1, (1S, αR)-1, and (1S, αS)-1. The k₂ values for these four diastereomers are about the same. The enzyme stereoselectivity for the 1, 1'-bi-2-naphthyl moiety of the inhibitors (R > S, ca. 10 times) is the same as that for 2'-N-butylcarbamyl-1, 1'-bi-2-naphthol (2). The enzyme stereoselectivity for the α-methylbenzylcarbamyl moiety of the inhibitors is also R > S (2–3 times) due to the constraints in the acyl binding site.

Conclusion

We are the first to report that the acyl chain binding site of cholesterol esterase shows stereoselectivity for the four diastereomers of 1.

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.

Oxidative tyrosylation of high density lipoproteins impairs cholesterol efflux from mouse J774 macrophages: role of scavenger receptors, classes A and B

Studies were designed to test whether tyrosylation of high-density lipoprotein (HDL(T)) modifies its metabolic features. HDL(T) was less effective than native HDL in promoting cholesterol efflux from J774-AI macrophages. Cell association with fluorescent HDL(T)-apolipoprotein and the uptake of HDL(T)-[(3)H]cholesteryl hexadecyl ether were enhanced by 50% in comparison with native HDL. In addition, neutral cholesterol ester hydrolase (nCEH) activity in J774-AI, which controls the hydrolysis of cholesteryl ester stores to provide free cholesterol for cellular release, declined in the presence of HDL(T). In vitro displacement experiments revealed the ability of HDL(T) to compete with oxidized and acetylated LDL, known as ligands of scavenger receptor (SR) class B type I/II. Similarly, treatment with a blocking antibody to SR-BI/II reduced the cell association of HDL(T) and native HDL by 50%. The addition of polyinosinic acid, an inhibitor of SR class A, reduced the cell association of HDL(T) without affecting that of native HDL. These findings provide evidence that HDL(T) can compete with modified LDL, bind SR-BI/BII and internalize cholesterol ester. Furthermore, the impaired capacity of HDL(T) in promoting cholesterol efflux from J774-AI was accompanied by diminished nCEH and enhanced recognition by SR-AI/II, which appears to involve the transport of cholesterol into cells.

The in-vitro influence of serum amyloid A isoforms on enzymes that regulate the balance between esterified and un-esterified cholesterol

The intracellular balance between un-esterified and esterified cholesterol is regulated by two enzyme activities, cholesterol ester hydrolases, which drive the balance in favor of un-esterified cholesterol, and acyl-CoA:cholesterol acyl transferase (ACAT) which acts in the opposite direction. During acute inflammation apo-serum amyloid A (apoSAA) isoforms 1.1 and 2.1 become major constituents of high density lipoprotein and this complex is internalized by macrophages. Mixtures of the two isoforms have been shown to enhance cholesterol esterase activity. Using a purified form of the pancreatic enzyme we have explored the mechanism by which apoSAA may accomplish this stimulation. The pancreatic esterase cleaves cholesteryl-oleate with a Km of 0.255 mM, releasing both cholesterol and oleate. Cholesterol exhibits a product inhibition which is relieved by isoform 2.1 but not 1.1 nor apolipoprotein A-I. The NH2-terminal 16 residues of isoform 2.1 had no effect on the esterase, but the 80 residue peptide constituting its COOH-terminus possessed the stimulatory property. Purified isoforms 1.1, 2.1, 2.2, apolipoprotein A-I, the NH2-terminal 16 residues and COOH-terminal 80 residues of isoform 2.1 were also examined for their effects on macrophage ACAT activity. Isoforms 2.1 and 2.2 produced dose dependent inhibitions of up to 50%, (p<0.001). Isoform 1.1, and apoA-I had no effect on ACAT activity. The NH2-terminal 16 residue peptide of isoform 2.1 reduced the ACAT activity in a dose dependent manner by 74% (p<0.001), whereas the COOH-terminal 80 residues, in contrast to its enhancing effect on the esterase, had no inhibitory effect on ACAT. Such complementary but opposite effects of isoform 2.1 on ACAT and the esterase are consistent with a role for this protein in shifting the balance between unesterified (transportable) and esterified (storage) forms of cholesterol in favor of the latter. They suggest that apoSAA2.1 may mediate cholesterol mobilization at sites of tissue injury.

Insulin and leptin acutely regulate cholesterol ester metabolism in macrophages by novel signaling pathways

Leptin is produced in adipose tissue and acts in the hypothalamus to regulate food intake. However, recent evidence also indicates a potential for direct roles for leptin in peripheral tissues, including those of the immune system. In this study, we provide direct evidence that macrophages are a target tissue for leptin. We found that J774.2 macrophages express the functional long form of the leptin receptor (ObRb) and that this becomes tyrosine-phosphorylated after stimulation with low doses of leptin. Leptin also stimulates both phosphoinositide 3-kinase (PI 3-kinase) activity and tyrosine phosphorylation of JAK2 and STAT3 in these cells. We investigated the effects of leptin on hormone-sensitive lipase (HSL), which acts as a neutral cholesterol esterase in macrophages and is a rate-limiting step in cholesterol ester breakdown. Leptin significantly increased HSL activity in J774.2 macrophages, and these effects were additive with the effects of cAMP and were blocked by PI 3-kinase inhibitors. Conversely, insulin inhibited HSL in macrophages, but unlike adipocytes, this effect did not require PI 3-kinase. These results indicate that leptin and insulin regulate cholesterol-ester homeostasis in macrophages and, therefore, defects in this process caused by leptin and/or insulin resistance could contribute to the increased incidence of atherosclerosis found associated with obesity and type 2 diabetes.

The influence of simvastatin on lipase and cholesterol esterase activity in the serum of men with coronary heart disease

Several studies have demonstrated that any beneficial effects of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors (statins), of which simvastatin (Merck Sharp & Dohme) is an example, on coronary events are linked to their hypocholesterolemic properties. The in vivo effects of simvastatin treatment on lipase (GEH = glycerol ester hydrolase) and cholesterol esterase (CEase) activity in the serum of men with coronary heart disease (CHD) were examined. GEH and CEase activity in the serum of men with CHD, before simvastatin treatment, was lower than in the control subjects. In our study we have provided evidence that simvastatin increases GEH activity in a time-dependent manner, but has no effect on CEase activity. This suggests that simvastatin can directly affect acylglycerol metabolism by an increase in GEH activity and may therefore be suitable for the treatment of combined lipoprotein disorders characterized by elevation of triacylglycerols.

Effects of probucol on cholesterol metabolism in mouse peritoneal macrophages: inhibition of HDL-mediated cholesterol efflux

Macrophage-derived foam cells are known to play an essential role in the development and progression of atherosclerotic lesions. Probucol prevents oxidative modification of low-density lipoprotein (LDL) and lowers plasma contents of LDL and high-density lipoprotein (HDL). A recent report using apoE -/- mice demonstrated that probucol treatment enhanced atherosclerosis in apoE -/- mice more rapidly than that in untreated apoE -/- mice, and a reduction in plasma cholesterol by probucol was not the cause of enhancement of atherosclerotic lesions in probucol-treated apoE -/- mice. Moreover, probucol was reported to inhibit apoA-I mediated cholesterol efflux from mouse macrophages. These reports suggested that probucol might directly affect cholesterol metabolism in mouse macrophages. Thus, we investigated the effects of probucol on cholesterol metabolism in mouse resident peritoneal macrophages. Probucol did not affect degradation of acetylated LDL (Ac-LDL), degradation of LDL and endogenous cholesterol synthesis in mouse macrophages. However, it significantly inhibited HDL-mediated cholesterol efflux. Moreover, probucol partially (30%) inhibited the binding of HDL to mouse macrophages, and significantly activated acyl-coenzyme A:cholesterol acyltransferase (ACAT). Our results suggested that probucol inhibited HDL-mediated cholesterol efflux by inhibiting the binding of HDL to mouse macrophages and reducing HDL-accessible free cholesterol content by ACAT activation, thereby worsening atherosclerotic lesions in apoE -/- mice. However, it remains unclear whether probucol inhibits HDL-mediated cholesterol efflux from human macrophages.

Hormone sensitive lipase mRNA in both monocyte and macrophage forms of the human THP-1 cell line

The identity of the neutral cholesteryl ester hydrolase (CEH) in human monocyte/macrophages is uncertain. Prior studies indicate that hormone sensitive lipase (HSL) is a major CEH in mouse macrophages, and that HSL mRNA is present in human THP-1 monocytes. In the present study, HSL mRNA expression was examined in THP-1 cells as a function of differentiation status and cholesterol enrichment. By RT-PCR with primer pairs that span exon boundaries, HSL mRNA was demonstrated in THP-1 monocytes and phorbol-ester differentiated THP-1 macrophages. cDNA identities were confirmed by sequencing. By Northern blotting, with HSL cDNA as probe, THP-1 monocytes were found to contain HSL mRNA of approximately 3 and 3.9 kb. In THP-1 macrophages, the 3 kb mRNA was greatly diminished, while the level of the 3.9 kb mRNA was maintained. mRNA of approximately 3 and 3.9 kb are those expected of the 86-kDa (adipocyte) and 117-kDa (testicular) HSL isoforms, respectively. The presence of the testicular isoform mRNA was confirmed in THP-1 cells by amplification and sequencing of an isoform-specific cDNA. Additionally, Northern-blot comparisons showed that the 3 and 3.9 kb mRNA in THP-1 comigrated with the HSL mRNA in 3T3-L1 adipocytes and rat testis, respectively. The level of the 3.9 kb mRNA did not vary greatly with cholesterol enrichment. Thus, the HSL gene is transcribed in THP-1 cells both before and after differentiation into macrophages; after differentiation, the predominant mRNA is that for the 117-kDa isoform. This isoform is a CEH, and may mediate some CE turnover in THP-1 cells.

Low level expression of hormone-sensitive lipase in arterial macrophage-derived foam cells: potential explanation for low rates of cholesteryl ester hydrolysis

Conversion of arterial macrophages into foam cells is a key process involved in both the initiation and progression of atherosclerotic lesions. Foam cell formation involves the progressive accumulation and storage of lipoprotein-derived cholesteryl esters. The resulting imbalance in cholesterol metabolism in arterial foam cells may be due in part to an inadequately low level of cytoplasmic neutral cholesteryl ester hydrolase (NCEH) activity. In this study, we have demonstrated that hormone-sensitive lipase (HSL) mRNA is expressed at very low levels in macrophage-derived foam cells, using the unique approach of extracting mRNA from macrophage-derived foam cells purified from human and rabbit atherosclerotic plaques coupled with reverse transcriptase polymerase chain reaction (RT-PCR). We also demonstrate that macrophage-derived foam cells isolated from rabbit atherosclerotic lesions exhibit a resistance to high density lipoprotein (HDL)-mediated cholesterol efflux along with reduced levels of NCEH activity compared to lipid-loaded mouse peritoneal macrophages. Thus, low level expression of HSL may partially account for the reduced NCEH activity observed in arterial foam cells isolated from atherosclerosis-susceptible species.

Effects of phosphatidylcholine/phosphatidylethanolamine composition in cholesteryl ester-micellar substrates on neutral cholesterol esterase activity

The effect of phospholipid composition in cholesteryl ester (CE)-micellar substrates on neutral cholesterol esterase (N-CEase) activity was examined. N-CEase preparation was incubated with micelles composed of cholesteryl-[1-14C]-oleate, sodium taurocholate, and phosphatidylcholine (PC)/phosphatidylethanolamine (PE) at varying ratios (%PE:0 = PC only, 17, 33, 50, 66, 83). The activity increased dependently with the increase in PE content; the activity with the micelles containing the highest ratio of PE was 2.5-fold compared with the micelles consisting of PC only. Vmax with the micelles of 83, 66, and 50% PE was 3.1-, 2.7-, and 1.9-fold, respectively, compared with the micelles of PC only. Each micellar preparation was chromatographed through a Superose 6 column by the FPLC system. In 66 and 83% PE-containing micelles, PC, PE, CE, and part of sodium taurocholate eluted completely together in a single peak, whereas in micelles with 33 and 50% PE they eluted loosely together. The micelles with PC only or 17% PE formed PC-micelles without including CE and PE. It is concluded that PE plays a critical role in the formation of CE micelles with PC, and in the interaction with N-CEase. The CE-micelles with 66-83% PE serve as substrates for sensitive and reproducible N-CEase assay.