Synthesis of saturated, unsaturated, spin-labeled, and fluorescent cholesteryl esters: Acylation of cholesterol using fatty acid anhydride and 4-pyrrolidinopyridine

A simple and efficient approach for the synthesis of cholesterol esters of long-chain saturated fatty acids by using Ph3P·SO3 as a versatile organocatalyst

Cholesteryl esters are more nonpolar compounds formed by the conjugation of cholesterol with fatty acids containing a long hydrocarbon chain. These ester molecules do not participate in the composition of the cell membranes and are found in lipid droplets. Because this form of cholesterol can be packaged in a larger amount in lipoproteins, the transport of cholesterol is more effective, and also, these esters play an important role in cholesterol metabolism. Moreover, some of their hydroperoxide derivatives are biologically active components of minimally modified low-density lipoprotein (mmLDL). In recent years, these esters have attracted attention in many industrial and medical applications. In this study, a triphenylphosphine-sulfur trioxide adduct efficiently catalyzed the esterification reactions between homologous long-chain saturated monocarboxylic or dicarboxylic fatty acids and cholesterol molecule in toluene. In these reactions, the triphenylphosphine-based adduct acts as a source of sulfur trioxide. Reactions were performed at 110 °C with equimolar amounts of the reagents except in the case of dicarboxylic acids, and two equimolar of cholesterol was used to afford the corresponding homologous cholesteryl esters in good to excellent yields. The present developed method has advantages such as being simpler, practical, and less toxic than the existing ones as well as enabling the synthesis of the related esters with higher yields.

Fluorescent Sterols and Cholesteryl Esters as Probes for Intracellular Cholesterol Transport

Cholesterol transport between cellular organelles comprised vesicular trafficking and nonvesicular exchange; these processes are often studied by quantitative fluorescence microscopy. A major challenge for using this approach is producing analogs of cholesterol with suitable brightness and structural and chemical properties comparable with those of cholesterol. This review surveys currently used fluorescent sterols with respect to their behavior in model membranes, their photophysical properties, as well as their transport and metabolism in cells. In the first part, several intrinsically fluorescent sterols, such as dehydroergosterol or cholestatrienol, are discussed. These polyene sterols (P-sterols) contain three conjugated double bonds in the steroid ring system, giving them slight fluorescence in ultraviolet light. We discuss the properties of P-sterols relative to cholesterol, outline their chemical synthesis, and explain how to image them in living cells and organisms. In particular, we show that P-sterol esters inserted into low-density lipoprotein can be tracked in the fibroblasts of Niemann–Pick disease using high-resolution deconvolution microscopy. We also describe fluorophore-tagged cholesterol probes, such as BODIPY-, NBD-, Dansyl-, or Pyrene-tagged cholesterol, and eventual esters of these analogs. Finally, we survey the latest developments in the synthesis and use of alkyne cholesterol analogs to be labeled with fluorophores by click chemistry and discuss the potential of all approaches for future applications.

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.

Fluorescent sterols as tools in membrane biophysics and cell biology

Cholesterol is an important constituent of cellular membranes playing a fundamental role in many biological processes. This sterol affects membrane permeability, lateral lipid organization, signal transduction and membrane trafficking. Intracellular sterol transport modes and pathways as well as the regulation of sterol metabolism and disposition in various tissues are areas of intense research. Progress is intimately linked to development and use of appropriate analogs, which closely mimic the properties of cholesterol while allowing to be detected by spectroscopic or microscopic methods. This review provides an overview of various fluorescent sterols used in membrane biophysics and cell biology including analogs of cholesterol and cholesteryl esters. Attention is paid to the natural fluorescent sterol dehydroergosterol (DHE). A survey of the many applications of DHE in biological research is presented. Special emphasis is on recent developments in fluorescence microscopy instrumentation to visualize DHE as an intrinsically fluorescent analog of cholesterol in living cells.

Synthesis of the fatty sterol bound protein for a new sterol antibody

For the purpose of applying the particular antibodies as a new diagnostic procedure for atherosclerosis and related diseases, we successfully achieved the synthesis of the fatty sterol with a linker, then linked the target protein to this sterol. Synthesis was started from pregnenolone and achieved by the Grignard reaction with pentenyl magnesium bromide, regioselective photoaddition of thiolacetic acid toward the 25-double bond, esterification of 3-OH with linoleic anhydride, in situ conjunction of the cross-linker (MBS) to the thiol group after selective deprotection from its acetyl ester, and finally by the reaction with protein such as KLH or albumin through this linker.

Microphase separation in low density lipoproteins. Evidence for a fluid triglyceride core below the lipid melting transition

The structural organization of the neutral lipid core in human low density lipoproteins (LDL) was investigated in physicochemically defined, distinct human LDL subspecies in the density range of 1. 0244-1.0435 g/ml by evaluation of the core lipid transition temperature, chemical composition, and the behavior of spin-labeled core lipids. Calorimetric studies were performed on more than 60 LDL preparations, and the transition temperature, which varied between 19 and 32 degreesC, was correlated to the chemical composition and revealed a discontinuity at a critical cholesteryl ester to triglyceride ratio of approximately 7:1. For electron spin resonance studies, several LDL preparations were probed with spin-labeled cholesteryl esters and triglycerides, respectively. In LDL with a high triglyceride content, both labels exhibited similar mobility behavior. In contrast, in LDL with only small concentrations of triglycerides, the behavior of labeled cholesteryl esters and labeled triglycerides differed distinctly. The cholesteryl esters were strongly immobilized below the transition temperature, whereas the triglycerides remained fluid throughout the measured temperatures. These results suggest that the critical cholesteryl ester to triglyceride mass ratio of 7:1 corresponds to two concentric compartments with a radial ratio of 2:1, where the liquid triglycerides occupy the core, and the cholesteryl esters form the frozen shell. At higher triglyceride contents, the triglyceride molecules insert into the cholesteryl ester shell and depress the peak transition temperature of the LDL core, whereas at lower triglyceride contents, excess cholesteryl esters are dissolved in the core.

Synthesis of the cholesteryl ester prodrugs cholesteryl ibuprofen and cholesteryl flufenamate and their formulation into phospholipid microemulsions

Phospholipid micoremulsions have been suggested as a drug-delivery system for hydrophobic compounds. In this study hydrophobicity was achieved by derivatizing with cholesterol. Cholesteryl ibuprofen (3) and cholesteryl flufenamate (4) were synthesized. 3 was isolated as an amorphous, white solid with a melting range of 114-120 degrees C. 4 was isolated as a crystalline, white solid with a melting range of 145-148 degrees C. The proposed structures of 3 and 4 were supported by IR, NMR, MS, and organic microanalysis. Phospholipid:cholesteryl ester microemulsions were prepared by the addition of a 1-propanol solution of the cholesteryl ester, other lipids, and phospholipid to a rapidly mixing KCl/KBr solution. The hydrophobic phase was modified by the addition of cholesteryl oleate or triolein to study the effect of the fluidity of the hydrophobic core on the formation of the microemulsions. The results indicated that a molar ratio of 75:25 and a total lipid concentration of 60 mg/mL consistently gave microemulsions with a mean size of 100-150 nm. In addition, the formation of eutectic mixtures of 3 and 4 with cholesteryl oleate were determined to be 16% (w/w) for 3 and 12% (w/w) for 4; melting points were 35.2 and 45.2 degrees C, respectively. The solubilities of 3 and 4 in triolein were determined to be 13.2% (w/w) and 11.5% (w/w), respectively. Other investigators have shown that if the core of a phospholipid:cholesteryl estermicroemulsion exists in a liquid state at physiologic temperature, the turnover of the cholesteryl esters from these microemulsions occurs at a faster rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of the atherosclerotic core region. Chemical and ultrastructural analysis of microdissected atherosclerotic lesions from human aorta

Lipid deposits in human atherosclerotic fibrous plaques exhibit marked differences in chemistry and ultrastructure from lipid deposits in fatty streaks, leading some investigators to question whether fibrous plaques originate from fatty streaks. To examine lesion transition, we employed lipid microanalysis, electron microscopy, and immunohistochemistry on fatty streaks, fibrolipid lesions (small raised lesions), and fibrous plaques from human aorta. Both fatty streaks and caps of fibrolipid lesions were high in esterified cholesterol content (mean, 62% of total cholesterol) and high in cholesteryl oleate content compared with cholesteryl linoleate content. Fatty streaks and fibrolipid lesion caps also showed similar morphology, characterized mostly by macrophage-derived foam cells in the superficial intima. Core lipids in both small and large raised lesions differed markedly from this pattern. Fibrolipid lesion cores showed mostly vesicular extracellular deposits, sometimes accompanied by cholesterol clefts, while fibrous plaque core deposits were also extracellular but had a variable appearance. Compared with fatty streaks, fibrolipid lesion cores showed significantly increased free/total cholesterol fractions (63%) and decreased fractional contents of cholesteryl oleate. Fibrous plaque cores had variable distributions of free and esterified cholesterol but significantly decreased cholesteryl oleate fractions compared with fatty streaks. The results support the concept of lesion transition, which is marked by deep intimal, extracellular deposition of cholesterol-rich, vesicular lipid deposits in small raised lesions. In the core region of larger raised lesions, both cholesterol-rich and cholesteryl ester-rich lipid deposits appear to form in the extracellular space.

Characterization of the core and surface of human plasma lipoproteins. A study based on the use of five fluorophores

The physical properties of the core and the surface of five classes of human plasma lipoproteins were investigated using five fluorescent probes. The location of the fluorescence probes in the lipoprotein assembly was determined using collisional quenching and resonance energy transfer. The fluorophores monitor different regions of the lipoproteins, as shown by fluorescence quenching. Diphenylhexatriene (DPH) and methyl trans-parinaric acid (MTPA), which are apolar molecules, are localized mainly in the lipoprotein core. Their distribution into the surface is dependent upon the volume ratio of the hydrophobic part of the envelope and the core. The polar fluorophores, trimethylaminodiphenylhexatriene (TMADPH), hydroxycoumarin (HC) and trans-parinaric acid (TPA) are anchored in the glycerol skeleton region of the surface monolayer with the fluorophore group of HC in the headgroup region of the phospholipids. We determined the temperature-dependent steady-state fluorescence anisotropy (r) of these fluorophores in the four major classes of lipoproteins: VLDL, LDL, HDL2, HDL3 and in abnormal HDL from abetalipoproteinemia patients (HDLab). The hydrophobic probes, DPH and MTPA, reported the r values in the lipoproteins in the following order: LDL greater than HDL2 greater than HDL3 much greater than VLDL. This order correlates with the triglyceride-to-cholesterol ester (TG/CE) ratio in the core of lipoproteins. The polar probes HC, TPA and TMADPH reported the r value in a different order: HDL2, HDL3 greater than or equal to LDL much greater than VLDL. This is compatible with the decreasing order of the protein to lipid ratio in the envelope of these lipoproteins. HDLab was investigated by three fluorescent probes: DPH, TMADPH and HC. The anisotropy of DPH in HDLab was larger than that of either HDL2 or HDL3 in normal donors, probably due to the smaller TG/CE ratio in HDLab. The lower r values reported by HC and TMADPH for HDLab are not fully understood and may be related to other factors such as acyl chains composition. The characterization of lipoproteins by fluorescence depolarization using probes of known location in the lipoprotein assembly is very sensitive and may be used to report deviation from the norm.

Hormone-sensitive cholesterol ester hydrolase in adrenal tumor cells: activation by corticotropin and tetradecanoyl phorbol acetate

The effects of corticotropin (ACTH) and tetradecanoyl phorbol acetate (TPA) on cholesterol ester hydrolase, intracellular cholesteryl ester concentration and steroid hormone formation were studied in mouse adrenal tumor cells (Y-1) in monolayer culture. Cholesterol ester hydrolase activity increased about 2-fold during 7 min incubation with ACTH, dibutyryl 3',5'-cyclic AMP (dbcAMP) and TPA at maximally effective concentrations; whereas, incubation with phorbol monoacetate had no effect. Long-term exposure to ACTH and dbcAMP markedly lowered intracellular cholesteryl [3H]-oleate concentration and highly increased steroid hormone output, while TPA treatment resulted in lowering cholesteryl [3H]-oleate content without affecting steroid hormone formation. Calcium activated phospholipid-dependent protein kinase C was detected in Y-1 cell cytosol. It is concluded that the mouse adrenal tumor cells in monolayer culture respond to ACTH in a fashion similar to normal adrenocortical cells; whereas, the response to the phorbol ester TPA (possibly mediated through protein kinase C) involves activation of cholesterol ester hydrolase and cholesteryl ester depletion, however, without affecting steroid hormone secretion.

A novel spectrophotometric assay for lipase activity utilizing cis-parinaric acid

A new spectrophotometric assay for determining the activity of acylglycerol hydrolases (lipases, E.C. 3.1.1.3) was developed and optimized for yeast lipase (Candida cylindracea). Studies with porcine pancreatic lipase were also conducted and the influence of various detergents and divalent cations on the assay was evaluated. The assay uses cis-parinaric acid (PnA), a naturally occurring fatty acid that has unique spectroscopic properties, and takes advantage of the reversible binding of fatty acids to bovine serum albumin (BSA). Free PnA has an ultraviolet absorption peak at 321.2 nm. When PnA is bound to BSA, however, the peak shifts to 324.2 nm. The assay mixture contains 6 microM PnA, 1 microM BSA, 75 microM triolein, and 0.3 mM taurocholate in a 50 mM tris-HCl buffer with 1 microM EDTA. The release of oleic acid from triolein is monitored over time by measuring the ratio of optical densities (OD) at 319.0 and 329.0 nm. Initially, there is maximum binding of PnA to BSA, and the OD ratio is approximately 1.0. Upon addition of lipase, PnA is displaced from the BSA by oleic acid released from triolein, and the OD ratio increases to a maximum of about 1.8. However, when calcium is present in the reaction mixture an insoluble calcium-PnA complex forms, resulting in a progressive decrease in OD at both 319.0 and 329.0 nm. The kinetic assay described here is simple, rapid, sensitive, reproducible, inexpensive, and it can be adapted to measure the activity of a variety of calcium-independent lipases. Under similar assay conditions, activities for Candida cylindracea lipase obtained with this assay are similar to those obtained with 14C-labelled triolein.

Pyrene-labeled lipids: versatile probes of membrane dynamics in vitro and in living cells

Pyrene-labeled analogs of fatty acids have been studied as probes of lipid metabolism in vitro and in cultured cells. Procedures for the synthesis of complex pyrenyl lipids and the analytical methods for their separation and quantification are described. Pyrenyl-lipids have been used to quantify the relationship between lipid structure and the rates of spontaneous lipid transfer. Modifications of these methods have also been used to monitor protein-mediated lipid transfer, lipolysis and lipid translocation across bilayer membranes. According to several criteria, pyrene dodecanoic acid has been identified as a good analog of some naturally occurring fatty acids. Digital imaging microscopy has been used to monitor the rate of accumulation of pyrenyl lipids in living cells.

Molecular dynamics of antitumor ether-linked phospholipids in model membranes: a spin-label study

We have synthesized a spin-labeled derivative of ET-18-OCH3, a known antitumor ether-linked phospholipid. The spin-labeled analog was shown to be as potent as ET-18-OCH3 in inhibiting 3H-thymidine uptake of HL60 leukemic cells. Electron spin resonance (ESR) studies showed that the mobility of this ether-linked phospholipid in the membrane is more restricted when compared to its ester-linked counterparts. It is probable that the absence of the bulky carbonyl oxygens allows closer packing of the two alkyl chains in the ether-linked phospholipid, thereby reducing the angular amplitude of the motion of the alkyl chains. These findings may be of importance in elucidating mechanisms by which the antitumor ether-linked phospholipids perturb the structure of cellular membranes.

Synthesis of single- and double-13C-labeled cholesterol oleate

Cholesterol oleate with the 13C-label in oleic acid at the carbonyl and/or in the sterol ring at position 4 was synthesized by two methods: (1) cholesterol was condensed with oleic anhydride, prepared from [1-13C] oleic acid, in the presence of dimethylaminopyridine (DMAP) in anhydrous chloroform at room temperature for 4--5 h; (2) cholesterol or 13C-enriched cholesterol at position 4 were reacted with 90% [1-13C]-oleic acid in the presence of dicyclohexylcarbodiimide (DCC) and DMAP at room temperature in anhydrous chloroform for 1.25 h. The single-13C and double-13C-labeled cholesterol oleate were obtained in 90% yields after purification by silicic acid column chromatography. Their purity was assessed by thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC) and 13C-NMR spectroscopy. Tritium-labeled cholesterol oleate was also synthesized by method 1 using the fatty acid anhydride.

Metabolism of normal and modified low-density lipoproteins by macrophage cell lines of murine and human origin

Four murine macrophage-like continuous cell lines (P388D1, J774.1, RAW 264.7, and PU5-1.8) and two human cell lines displaying macrophage-monocyte characteristics (HL-60, U-937) have been examined for their ability to degrade both normal and acetylated low-density lipoproteins. All of these cell lines, except PU5-1.8, were demonstrated to have LDL receptors that were induced 2-5-fold by preincubation in lipoprotein-deficient serum. Metabolism of dextran sulfate-LDL complexes by all lines except PU5-1.8 was observed. Three cell lines, P388D1, J774.1 and RAW 264.7, while exhibiting individual differences in their metabolism of acetyl-LDL, all processed acetyl-LDL in a fashion qualitatively analogous to that by murine peritoneal macrophages and human monocytes. Cell lines PU5-1.8, U-937 and HL-60 did not bind or degrade significant quantities of acetyl-LDL. In P388D1 cells, metabolism of acetyl-LDL exhibited time and concentration dependence, was reversibly inhibited by chloroquine, blocked by fucoidan and dextran sulfate, and was calcium independent. Approximately 4 X 10(5) receptors, with an apparent Kd of 3 X 10(-8) M, were present on P388D1 cells. P388D1 cells metabolized 30% as much acetyl-LDL as murine peritoneal macrophages at 37 degrees C and bound 60% as much at 4 degrees C. Chemical measurement demonstrated a 250-fold increase in the cholesteryl ester content of P388D1 cells over 96 h. The accumulation of cholesteryl esters was reversible in the presence of HDL3 and involved continuous hydrolysis and reesterification. These lines represent a convenient resource for examining the metabolism of chemically modified lipoproteins, for isolation of cell mutants, and for isolation of specific lipoprotein receptors.

Serum lipoprotein structure: resonance energy transfer localization of fluorescent lipid probes

The location of several fluorescent chromophores in lipoproteins has been determined by using resonance energy transfer. The primary acceptor is 5-(N-hexadecanoylamino)fluorescein whose chromophore is shown to reside at the lipoprotein surface at pH 7.4. Polar donors include cis-parinaric acid (cis,trans,trans,cis-9,11,13,15-octadecatetraenoic acid), trans-parinaric acid (all-trans-9,11,13,15-octadecatetraenoic acid), and 16-(9-anthroyloxy)palmitic acid; nonpolar donors are parinaric acid methyl ester, parinaric acid cholesteryl ester, and 1,6-diphenyl-1,3,5-hexatriene. The polar donors transfer more efficiently than the nonpolar donors in several classes of lipoprotein particles. The data are analyzed by a simple mathematical model from which it is concluded that the polar donors are localized in the putative lipoprotein surface monolayer; the possibility that nonpolar donors are partitioned between the surface and core of lipoproteins is considered.