Covalent regulation of ULVWF string formation and elongation on endothelial cells under flow conditions

BACKGROUND AND OBJECTIVES

The adhesion ligand von Willebrand factor (VWF) is a multimeric glycoprotein that mediates platelet adhesion to exposed subendothelium. On endothelial cells, freshly released ultra-large (UL) VWF multimers form long string-like structures to which platelets adhere.

METHODS

The formation and elongation of ULVWF strings were studied in the presence of the thiol-blocking N-ethylmaleimide (NEM). The presence of thiols in ULVWF and plasma VWF multimers was determined by maleimide-PEO(2)-Biotin labeling and thiol-chromatography. Finally, covalent re-multimerization of ULVWF was examined in a cell- and enzyme-free system.

RESULTS

We found that purified plasma VWF multimers adhere to and elongate ULVWF strings under flow conditions. The formation and propagation of ULVWF strings were dose-dependently reduced by blocking thiols on VWF with NEM, indicating that ULVWF strings are formed by the covalent association of perfused VWF to ULVWF anchored to endothelial cells. The association is made possible by the presence of free thiols in VWF multimers and by the ability of (UL) VWF to covalently re-multimerize.

CONCLUSION

The data provide a mechanism by which the thrombogenic ULVWF strings are formed and elongated on endothelial cells. This mechanism suggests that the thiol-disulfide state of ULVWF regulates the adhesion properties of strings on endothelial cells.

Energy translocation across cell membranes and membrane models

Fatty acid transport is an important process in cellular energy distribution and storage in both normal and pathological states, especially obesity-linked type 2 diabetes mellitus. Fatty acid transport has been studied by the complementary approaches of cell biology and biophysics. According to the latter approach, specific proteins that enhance the uptake and storage of fatty acids are posited as fatty acid translocases, which facilitate fatty acid movement from the outer to inner leaflets of the plasma membrane. According to biophysical studies conducted in vitro, fatty acid translocation occurs by a rapid diffusive process that does not require a protein. Herein, we critically review these two mechanisms and their importance in the regulation of fatty acid uptake in vivo.

Isolation, quantitation, and characterization of a stable complex formed by Lp[a] binding to triglyceride-rich lipoproteins

Lipoprotein [a] (Lp[a]) is a cholesterol-rich lipoprotein resembling LDL to which a large polymorphic glycoprotein, apolipoprotein [a] (apo[a]), is covalently coupled. Lp[a] usually exists as a free-standing particle in normolipidemic subjects; however, it can associate noncovalently with triglyceride-rich lipoproteins in hypertriglyceridemic (HTG) subjects. In this study, 10-78% of the Lp[a] present in five HTG subjects was found in the triglyceride-rich lipoprotein (TRL) fraction. The Lp[a]-TRL complex was resistant to dissociation by ultracentrifugation (UCF) alone, but was quantitatively dissociated by UCF in the presence of 100 mM proline. Of this dissociated Lp[a], 70-88% was in the form of a lipoprotein resembling conventional Lp[a]. Incubation of Lp[a]-depleted TRL with native Lp[a] resulted in a reconstituted Lp[a]-TRL complex that closely resembled the native isolates in all examined properties. Complex formation was inhibited by several compounds in the order proline > tranexamate > epsilon-aminocaproate >> arginine > lysine. Neither plasminogen nor LDL inhibited binding of Lp[a] to TRL. We observed the preferential binding of Lp[a] containing higher apparent molecular weight apo[a] polymorphs to TRL both in native and reconstituted Lp[a]-TRL complexes. A disproportionate amount of Lp[a] was bound to the larger TRL particles. Although most apo[a] bound to TRL was in the form of conventional Lp[a] particles, lipid-free recombinant apo[a] was observed to bind TRL. These results provide unequivocal evidence of the existence of an Lp[a]-TRL complex under pathophysiologic conditions. The metabolic fate of the Lp[a]-TRL complex, which is more abundant in hypertriglyceridemia, may be different from that of conventional Lp[a], and may contribute uniquely to the progression or severity of cardiovascular disease.

Effect of sirolimus on the metabolism of apoB100- containing lipoproteins in renal transplant patients

BACKGROUND

Sirolimus (Rapamune, rapamycin, RAPA) is a potent immunosuppressive drug that has reduced the rate of acute rejection episodes by more than 40% in phase III trials when added to an immunosuppression regimen of cyclosporine (CsA) and prednisone. However, RAPA treatment tends to increase lipid levels, particularly among patients with pre-existing hyperlipidemia.

METHODS

To identify the metabolic pathway(s) leading to RAPA-mediated hyperlipidemia, five patients with renal transplants maintained on CsA+/-prednisone+/- azathioprine (AZA) were studied before and after 6 weeks of treatment with RAPA (off RAPA and on RAPA, respectively). Each study patient was infused with a single bolus of [2H4]-lysine to derive metabolic parameters for apoB100-containing lipoproteins by using kinetic analysis based upon quantitation of isotopic enrichment by gas chromatography-mass spectrometry.

RESULTS

Serial lipid measurements revealed that four patients displayed increased plasma triglyceride levels after RAPA treatment, which coincided with significantly higher plasma VLDL-apoB100 concentrations (21.7+/-12.1 mg/dl off RAPA vs. 38.7+/-14.8 mg/dl on RAPA, mean+/-SD, P<0.05). Kinetic analysis showed that the RAPA-induced increase in VLDL-apoB100 concentrations was due to a significant reduction in the fractional catabolic rate (FCR) of very low-density lipoprotein (VLDL) apoB100 (0.83+/-0.65 off RAPA vs. 0.24+/-0.10 on RAPA, mean+/-SD, P<0.05), rather than an enhanced VLDL-apoB100 synthesis. In one patient, RAPA treatment induced hypercholesterolemia but not hypertriglyceridemia. This hypercholesterolemia was due to elevated low-density lipoprotein (LDL) cholesterol levels, which coincided with a decreased FCR of LDL-apoB100. Heparin-induced lipoprotein lipase activity was significantly lower in the immunosuppressed hyperlipidemic patients than in normolipidemic controls. However, RAPA treatment did not significantly alter basal lipoprotein lipase activity in renal transplant patients in this study.

CONCLUSIONS

This study indicates that for renal transplant patients in whom RAPA treatment induces hyperlipidemia, this effect is the result of reduced catabolism of apoB100-containing lipoproteins.

Cellular transport of nonesterified fatty acids

Transport of nonesterified fatty acids (NEFA) is an important component of whole-body energy metabolism, and derangements in NEFA transport have been linked to several diseases. NEFA are transferred from their sites of production to cells in hepatic and peripheral tissues by mechanisms that are regulated in part by cell status and as determined by the covalent structure of the NEFA species. Major barriers to physical transport are transfer from the hydrophobic surfaces on cell membranes and NEFA-binding proteins, such as albumin, into the surrounding aqueous phase and translocation across a membrane that contains a very hydrophobic interior; this process could be purely diffusive or require specific protein cofactors. Herein evidence is provided suggesting that this step is driven by intracellular metabolism that supports a NEFA gradient across the cell membrane. According to current models of NEFA transfer, the rate-limiting step is likely to be desorption of NEFA from the inner leaflet of the cell membrane or intracellular metabolism; for very long chain NEFA, the former is more likely.

Overexpression of 1-acyl-glycerol-3-phosphate acyltransferase-alpha enhances lipid storage in cellular models of adipose tissue and skeletal muscle

Plasma nonesterified fatty acids (NEFA) at elevated concentrations antagonize insulin action and thus may play a critical role in the development of insulin resistance in type 2 diabetes. Plasma NEFA and glucose concentrations are regulated, in part, by their uptake into peripheral tissues. Cellular energy uptake can be increased by enhancing either energy transport or metabolism. The effects of overexpression of 1-acylglycerol-3-phosphate acyltransferase (AGAT)-alpha, which catalyzes the second step in triglyceride formation from glycerol-3-phosphate, was studied in 3T3-L1 adipocytes and C2C12 myotubes. In myotubes, overexpression of AGAT-alpha did not affect total [14C]glucose uptake in the presence or absence of insulin, whereas insulin-stimulated [14C]glucose conversion to cellular lipids increased significantly (33%, P = 0.004) with a concomitant decrease (-30%, P = 0.005) in glycogen formation. [3H]oleic acid (OA) uptake in AGAT-overexpressing myotubes increased 34% (P = 0.027) upon insulin stimulation. AGAT-alpha overexpression in adipocytes increased basal (130%, P = 0.04) and insulin-stimulated (27%, P = 0.01) [3H]OA uptake, increased insulin-stimulated glucose uptake (56%, P = 0.04) and conversion to cellular lipids (85%, P = 0.007), and suppressed basal (-44%, P = 0.01) and isoproterenol-stimulated OA release (-45%, P = 0.03) but not glycerol release. Our data indicate that an increase in metabolic flow to triglyceride synthesis can inhibit NEFA release, increase NEFA uptake, and promote insulin-mediated glucose utilization in 3T3-L1 adipocytes. In myotubes, however, AGAT-alpha overexpression does not increase basal cellular energy uptake, but can enhance NEFA uptake and divert glucose from glycogen synthesis to lipogenesis upon insulin stimulation.

Effects of high-density lipoprotein(2) on cholesterol transport and acyl-coenzyme A:cholesterol acyltransferase activity in P388D1 macrophages

High-density lipoproteins are the putative vehicles for cholesterol removal from monocyte-derived macrophages, which are an important cell type in all stages of atherosclerosis. The role of HDL(2), an HDL subclass that accounts for most variation in plasma HDL-cholesterol concentration, in cholesterol metabolism in monocyte-derived macrophages is not known. In this study, the dose-dependent effects of HDL(2) on cellular cholesterol mass, efflux, and esterification, and on cellular cholesteryl ester (CE) hydrolysis using the mouse macrophage P388D1 cell line was investigated. HDL(2) at low concentrations (40 microg protein/ml) decreased CE content without affecting cellular free cholesterol content (FC), CE hydrolysis, or cholesterol biosynthesis. In addition, HDL(2) at low concentrations reduced cellular acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity and increased FC efflux from macrophages. Thus, HDL(2) has two potential roles in reverse cholesterol transport. In one, HDL(2) is an acceptor of macrophage FC. In the other, more novel role, HDL(2) increases the availability of macrophage FC through the inhibition of ACAT. Elucidation of the mechanism by which HDL(2) inhibits ACAT could identify new therapeutic targets that enhance the transfer of cholesterol from macrophages to the liver.

Regulation of acyl-coenzyme A:cholesterol acyltransferase (ACAT) synthesis, degradation, and translocation by high-density lipoprotein(2) at a low concentration

(,Although plasma HDL(2) cholesterol concentration stands in inverse relation to risk for atherosclerotic disease, little is known about the mechanism of the apparent cardioprotection. In mouse P388D1 macrophages, HDL(2) at a low concentration (< or = 40 microg/mL) inhibits macrophage acyl-coenzyme A:cholesterol acyltransferase (ACAT), the enzyme that catalyzes esterification of intracellular cholesterol. The effects of HDL(2) on ACAT synthesis, degradation, and intracellular translocation were investigated in mouse P388D1 macrophages. HDL(2) at a low concentration enhanced ACAT synthesis but not total ACAT mass. Immunocytochemical studies showed that in the absence of lipoproteins, ACAT associated primarily with the perinuclear region of the cell. The addition of HDL(2), however, induced the transfer of ACAT to vesicular structures and the cell periphery adjacent to the plasma membrane. Subfractionation combined with immunoprecipitation complemented these observations and showed that HDL(2) promoted the transfer of ACAT to the plasma membrane fraction. Brefeldin A, which inhibits vesicular protein transport from the endoplasmic reticulum to the Golgi compartment in mammalian cells, blocked ACAT translocation and partially restored ACAT activity. These results suggest that HDL(2) is an initiating factor in a signal transduction pathway that leads to intracellular ACAT translocation and inactivation.

Efficient nuclear delivery of antisense oligodeoxynucleotides and selective inhibition of CETP expression by apo E peptide in a human CETP-stably transfected CHO cell line

N,N-Dipalmitylglycyl-apolipoprotein E (129-169) peptide (dpGapoE) is an efficient gene delivery system for both plasmids and antisense oligodeoxynucleotides (ODNs). To develop a new and efficient approach to the regulation of cholesteryl ester transfer protein (CETP) expression, we used dpGapoE to transfect phosphorothioate antisense ODNs against nucleotides 329 to 349 of human CETP cDNA into a human CETP-stably transfected Chinese hamster ovary (CHO) cell line (hCETP-CHO). After transfection, translocation to the nuclei and concentration in nuclear structures were observed in >95% of the cells at 6 and 12 hours by fluorescence microscopy. No membrane disruption was observed after transfection of ODNs by dpGapoE. Although the translocation stability of phosphorothioate ODNs in the nuclei continued for >48 hours, it had weakened after 24 hours. Cellular CETP mRNA levels gradually declined, and the maximum reduction in the mRNA level (>50%) was observed at 36 hours, after which the mRNA level started to recover. CETP activity in the culture medium declined over 72 hours. The maximum reduction in CETP activity was observed at 36 hours (53.8% of control). Neither CETP mRNA nor CETP activities changed throughout the experiment after the transfection of sense phosphorothioate ODNs delivered by dpGapoE complex or naked antisense ODNs. We conclude that (1) the novel synthetic dpGapoE was a highly effective and nontoxic vehicle for the nuclear delivery of antisense ODNs into hCETP-CHO cells and (2) antisense ODNs selectively inhibited both CETP expression and activity in an hCETP-CHO cell line. This approach may enable gene regulation in vivo and could possibly be used as an antiatherosclerotic agent to alter high density lipoprotein metabolism.

Lipoprotein lipase gene mutations, plasma lipid levels, progression/regression of coronary atherosclerosis, response to therapy, and future clinical events. Lipoproteins and Coronary Atherosclerosis Study

Mutations in human lipoprotein lipase (LPL) gene are potential risk factors for susceptibility to coronary artery disease (CAD). The objectives of this study were to determine the influence LPL mutations Asn291Ser and Ser447Ter on plasma lipid levels, regression and progression of CAD, clinical events rate, and response to fluvastatin therapy in the Lipoprotein and Coronary Atherosclerosis Study (LCAS) population. LCAS is a double blind, randomized, placebo-controlled study designed to test the influence of fluvastatin on progression or regression of CAD. The Asn291Ser and Ser447Ter genotypes were determined by polymerase chain reaction (PCR) and restriction enzyme digestion. Fasting plasma lipid profiles were measured and quantitative coronary angiography was performed at baseline and 2.5 years following randomization. Fatal and non-fatal cardiovascular events during the follow-up period were recorded. A total of 4% (14/363) and 18% (62/352) of the subjects had the Asn291Ser and Ser447Ter mutations, respectively. Overall, there was no statistically association between the Asn291Ser and Ser447Ter mutations and the baseline or final mean plasma levels of lipids, number of coronary lesions, total occlusions, the mean minimal lumen diameter (MLD) stenoses and the clinical events rate. However, patients with the Ser447Ter variant had a slightly higher baseline high density lipoprotein-cholesterol (HDL-C) level (46.2 +/- 12 vs 43.2 +/- 11, P = 0.057), less increase in plasma HDL levels in response to fluvastatin therapy (3 vs 11%, P = 0.056) and a higher cardiovascular events rate (23 vs 13%, P = 0.056). Thus, the Ser447Ter variant had a modest influence on plasma HDL levels and the rate of cardiovascular events. These changes were of borderline statistical significance. Neither the Ser447Ter nor the Asn291Ser mutation had a major impact on susceptibility to CAD, progression or regression of CAD, clinical events rate or response to fluvastatin therapy in LCAS population.

Effect of moderate alcohol consumption on hypertriglyceridemia: a study in the fasting state

BACKGROUND

Patients with hypertriglyceridemia (HTG) are generally advised to avoid alcohol, even though moderate alcohol consumption is cardioprotective. Alcohol increases plasma triglyceride concentration transiently in normolipidemic subjects, but whether alcohol consumption per se increases triglyceride concentrations in patients with HTG is unclear.

OBJECTIVE

To assess whether baseline fasting triglyceride concentration determines plasma triglyceride concentration after acute oral alcohol intake.

METHODS

Twelve persons with fasting triglyceride concentrations of 2.3 to 8.5 mmol/L (200-750 mg/dL) and 12 persons as a non-HTG group were enrolled. Obesity, current smoking, and history of hypertension, diabetes, or excessive alcohol use were exclusionary. Fasted subjects consumed 38 mL of ethanol in water (equivalent, 2 alcoholic drinks); blood samples were collected at baseline and at intervals thereafter for 10 hours. No less than 1 week later, the subjects consumed water alone in a control test.

RESULTS

Mean triglyceride values were 4.04+/-0.41 mmol/L (358+/-36.9 mg/dL) and 1.00+/-0.11 mmol/L (89+/-10.2 mg/dL) for the HTG and non-HTG groups, respectively. Despite similar changes with alcohol feeding in plasma ethanol, nonesterified fatty acid, and acetate concentrations, the groups differed in triglyceride response. At 6 hours (peak) compared with baseline, triglyceride concentration increased only 3% in the HTG group but 53% in the non-HTG group. The former change was not significantly different from the effect with water alone (-9.2% from baseline; P = .43), whereas the latter was (-8.0%; P = .003).

CONCLUSIONS

Acute alcohol intake alone is not an important determinant of plasma triglyceride concentration in individuals with HTG. Other factors, such as the contemporaneous consumption of fat and alcohol, known to increase triglyceride concentrations synergistically in non-HTG individuals, may be more important.

Correlation of serum triglyceride and its reduction by omega-3 fatty acids with lipid transfer activity and the neutral lipid compositions of high-density and low-density lipoproteins

Serum triglyceride (TG) and high-density lipoprotein cholesterol (HDL-C) concentrations are inversely correlated and mechanistically linked by means of lipid transfer activities. Phospholipid transfer activity (PLTA) moves phospholipids among serum lipoproteins; cholesteryl ester transfer activity (CETA), which exchanges cholesteryl esters (CE) and TG among lipoproteins, is stimulated by nonesterified fatty acids (NEFA). The aims of this study were (a) to develop a quantitative model that correlates the neutral lipid (NL = CE + TG) compositions of HDL and LDL with serum TG concentration; (b) identify the serum lipid determinants of CETA and PLTA, and; (c) identify the effects of serum TG reductions on the neutral lipid compositions of HDL and LDL, serum NEFA concentrations, and on PLTA and CETA. These aims were addressed in 40 hypertriglyceridemic subjects before and after treatment with an 85% concentrate of omega-3 fatty acids (Omacor) and in 16 untreated normolipidemic subjects. In vivo, the NL compositions of LDL and HDL were described by a mathematical model having the form of adsorption isotherms: HDL - (TG/NL) = (0.90 +/- 0.07) serum TG/(7.0 +/- 1.2 mmol/l + serum TG) and LDL - (TG/NL) = (0.65 +/- 0.08) serum TG/(4.9 +/- 1.5 mmol/l + serum TG). Reduction of serum TG was associated with reductions in HDL - (TG/NL), serum NEFA concentration, and serum CETA but not PLTA. These data suggest that both hypertriglyceridemia and the attendant elevated serum CETA but not PLTA are determinants of HDL and LDL composition and structure and that serum TG concentrations are good predictors of the NL compositions of HDL and LDL.

Role of cysteine residues in human plasma phospholipid transfer protein

Phospholipid transfer protein (PLTP) belongs to a family of human plasma lipid transfer proteins that bind to small amphophilic molecules. PLTP contains cysteines at residues 5, 129, 168, and 318. Bactericidal/permeability-increasing protein, which is a member of the same gene family, contains an essential disulfide bond between Cys135 and Cys175; these residues, which correspond to Cys129 and Cys168 in PLTP, are conserved among all known members of the gene family. To identify the importance of these and the remaining cysteine residues to PLTP secretion and activity, each was replaced by a glycine by site-directed mutagenesis. The mutant as well as wild-type PLTP cDNAs were cloned into the mammalian expression vector pSV.SPORT1, and the PLTP cDNAs were transfected to COS-6 cells for expression. PLTP Cys129 --> Gly and PLTP Cys168 --> Gly were secretion incompetent. Neither PLTP mass nor activity was detectable in cell lysates and culture medium. Relative to wild-type PLTP, PLTP Cys5 --> Gly and PLTP Cys318 --> Gly exhibited similar specific activities but partially impaired PLTP synthesis and secretion. Intracellular PLTP appeared as two bands of 75 and 51 kDa corresponding to reported molecular masses for the glycosylated and nonglycosylated forms. The specific activities of PLTP Cys5 --> Gly and PLTP Cys318 --> Gly were similar in the cell lysates and medium, suggesting that glycosylation does not affect transfer activity.

Interaction of alpha-tocopherol with model human high-density lipoproteins

The effects of alpha-tocopherol on the properties of model high-density lipoproteins (HDLs), composed of human apolipoprotein A-I and dimyristoylphosphatidylcholine, were investigated by physicochemical methods. The intrinsic fluorescence of alpha-tocopherol and its effects on the polarization of fluorescence of 1,6-diphenyl-1,3,5-hexatriene, which probes the hydrocarbon region of the lipids, and 4-heptadecyl-7-hydroxycoumarin, which is a probe of lipid surfaces, suggest that alpha-tocopherol is located at the lipid-water interface. Relative to cholesterol, alpha-tocopherol in lipid surfaces is virtually inert physicochemically. Incorporation of alpha-tocopherol into HDLs induces only a modest increase in particle size, no change in the transition temperature, and little change in lipid polarity and lipid-lipid interactions. Moreover, alpha-tocopherol has only a negligible effect on the kinetic parameters of the lipophilic enzyme lecithin:cholesterol acyltransferase, which binds to phosphatidylcholine surfaces and forms cholesteryl esters. However, alpha-tocopherol has a dramatic inhibitory effect on the rate of association of apolipoprotein A-I with dimyristoylphosphatidylcholine, a process that occurs through the insertion of the protein into preformed defects in the lipid surface. It is proposed that alpha-tocopherol inhibits the rate of association of apolipoprotein A-I with dimyristoylphosphatidylcholine by inserting into defects within the lipid surface, thereby reducing the size and/or number of sites for insertion of apolipoprotein A-I.

Mechanism of action of probucol on cholesteryl ester transfer protein (CETP) mRNA in a Chinese hamster ovary cell line that had been stably transfected with a human CETP gene

Probucol, a widely used lipid-lowering agent, is associated with a significant reduction of plasma high density lipoprotein (HDL)-cholesterol levels. To examine the mechanism of probucol HDL-lowering and probucol's effects on cholesteryl ester transfer protein (CETP) and cholesterol metabolism in cells, we used a Chinese hamster ovary (CHO) cell line that had been stably transfected with a human CETP gene (hCETP-CHO). After this cell line was incubated with various concentrations of probucol (5, 10 and 50 microM) for 24 h, mean intracellular probucol concentrations reached 0.47, 0.67, and 1.52 microg/mg cell protein, respectively. Northern blot analysis showed that cellular CETP mRNA was increased by probucol in a dose-dependent manner (137%, 162%, and 221% of the control, respectively). The specific CET activity in the culture medium, measured as the percentage of [3H]cholesterol oleate transferred from discoidal bilayer particles (which mimic HDL) to LDL, also increased in a dose-dependent manner. Intracellular total cholesterol levels were decreased to 87.5%, 74.9%, and 52.5% of the control, respectively. Probucol had no effects on HMG-CoA reductase activity or cholesterol synthesis from [14C]acetate in hCETP-CHO. However, 14C-incorporated cholesterol secretion into the culture medium from hCETP-CHO was increased to 181%, 256% and 354% of the control by 5, 10 and 50 microM probucol, respectively. We concluded that (1) treatment with probucol increased the CETP mRNA level and specific CET activity in the hCETP-CHO cell line, and (2) probucol promoted cholesterol efflux from hCETP-CHO, which resulted in a decrease in intracellular cholesterol levels.

Increased sphingomyelin content of plasma lipoproteins in apolipoprotein E knockout mice reflects combined production and catabolic defects and enhances reactivity with mammalian sphingomyelinase

Apolipoprotein E knockout (apoE0) mice accumulate atherogenic remnant lipoproteins in plasma. We now provide evidence that these particles are enriched in sphingomyelin (SM), and explore the mechanisms and possible pathophysiological consequences of this finding. The phosphatidylcholine/sphingomyelin (PC/SM) ratio was reduced in all lipoproteins in apoE0 mice compared with wild-type (Wt) mice (2.0+/-0.2 vs. 4.7+/-0.5; 2.8+/-0.5 vs. 5.5+/-0.9; 1.9+/-0. 5 vs. 4.6+/-0.5 for VLDL, LDL, and HDL), reflecting 400 and 179% increases in plasma pools of SM and PC, respectively. Turnover studies using [14C]PC/[3H]SM VLDL or HDL showed that the fractional catabolic rate (FCR) of VLDL-SM and HDL-SM were markedly reduced in the apoE0 mice compared with Wt mice, while the FCRs of VLDL-PC and HDL-PC were similar. By contrast, the FCRs of [3H]PC ether and [14C]SM were identical in apoE0 and Wt mice. The production rates of VLDL-SM and HDL-SM in apoE0 mice were much higher than in Wt mice, while the production rates of lipoprotein PC were similar. To assess the underlying mechanisms, we also measured the PC/SM ratio in VLDL and LDL of LDL receptor knockout (LDLr0) and hepatic LDL receptor-related protein knockout/LDLr0 mice, but found no difference with Wt mice. Using S-sphingomyelinase, an enzyme secreted by macrophages and endothelial cells, we found that VLDL and LDL from apoE0, but not from Wt or LDLr0 mice, were significantly aggregated, and that aggregation was not prevented by adding back apoE. We then enriched the apoE0-VLDL and Wt-VLDL with different amounts of SM, and found that VLDL aggregation was enhanced. Thus, the increased SM content of lipoproteins in apoE0 mice is due to combined synthesis and clearance defects. Impaired SM clearance reflects resistance to intravascular enzymes and delayed removal by a non-LDLr, non-LDLr related protein pathway. The increased SM content in slowly cleared remnant lipoproteins may enhance their susceptibility to arterial wall SMase and increase their atherogenic potential.

Surface properties of native human plasma lipoproteins and lipoprotein models

Plasma lipoprotein surface properties are important but poorly understood determinants of lipoprotein catabolism. To elucidate the relation between surface properties and surface reactivity, the physical properties of surface monolayers of native lipoproteins and lipoprotein models were investigated by fluorescent probes of surface lipid fluidity, surface lateral diffusion, and interfacial polarity, and by their reactivity to Naja melanoleuca phospholipase A2 (PLA2). Native lipoproteins were human very low, low-, and subclass 3 high-density lipoproteins (VLDL, LDL, and HDL3); models were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or its ether analog in single-bilayer vesicles, large and small microemulsions of POPC and triolein, and reassembled HDL (apolipoprotein A-I plus phospholipid). Among lipoproteins, surface lipid fluidity increased in the order HDL3 < LDL < VLDL, varying inversely with their (protein + cholesterol)/phospholipid ratios. Models resembled VLDL in fluidity. Both lateral mobility in the surface monolayer and polarity of the interfacial region were lower in native lipoproteins than in models. Among native lipoproteins and models, increased fluidity in the surface monolayer was associated with increased reactivity to PLA2. Addition of cholesterol (up to 20 mol%) to models had little effect on PLA2 activity, whereas the addition of apolipoprotein C-III stimulated it. Single-bilayer vesicles, phospholipid-triolein microemulsions, and VLDL have surface monolayers that are quantitatively similar, and distinct from those of LDL and HDL3. Surface property and enzymatic reactivity differences between lipoproteins and models were associated with differences in surface monolayer protein and cholesterol contents. Thus differences in the surface properties that regulate lipolytic reactivity are a predictable function of surface composition.

Molecular basis of fish-eye disease in a patient from Spain. Characterization of a novel mutation in the LCAT gene and lipid analysis of the cornea

The genetic and biochemical basis of fish-eye disease (FED) was investigated in a 63-year-old female proband with low plasma HDL cholesterol. Analyses of corneal and plasma lipids of the proband were consistent with impaired lecithin:cholesterol acyltransferase (LCAT) activity. Free cholesterol and phospholipid levels were elevated relative to control values, whereas cholesteryl ester levels were greatly reduced. Fatty acid compositions of corneal lipids from the proband and control subjects differ from the respective fatty acid compositions of their plasma lipids. This suggests that the metabolic pathways and acyl chain specificities for phospholipid, cholesteryl ester, and triglyceride metabolism within the cornea are distinct from those of plasma. Sequencing of the LCAT gene from the proband revealed a novel mutation at nucleotide 399, corresponding to an Arg99-->Cys substitution. Secretion of LCAT (Arg99-->Cys) by transfected COS-6 cells was approximately 50% of that of the wild type, but its specific activity against reassembled HDL was 93% lower than that of wild-type LCAT. The specific activities of wild-type and LCAT (Arg99-->Cys) against LDL were reduced similarly, suggesting that the appearance of the FED phenotype does not require enhanced activity against LDL. Our data support the hypothesis that FED is a partial LCAT deficiency in which poor esterification in specific types of HDL particles may contribute to the appearance of the corneal opacities.

Spontaneous transfer of monoacyl amphiphiles between lipid and protein surfaces

The kinetics of transfer of natural and fluorescent nonesterified fatty acids (NEFA) and lysolecithins (lysoPC) from phospholipid and protein surfaces were measured. The kinetics of transfer of 12-(1-pyrenyl)dodecanoic acid, from liquid crystalline and gel phase single unilamellar phospholipid vesicles, very low, low, and high density lipoproteins, human serum albumin, and rat liver fatty acid-binding protein, were first-order and characterized by similar rate constants. The halftimes (t1/2) of NEFA transfer from lipids and proteins were dependent on the acyl chain structure according to log t1/2 = -0.62n + 0.59m + 12.0, where n and m, respectively, are the numbers of carbon atoms and double bonds. The structure of the donor surface had a measurable but smaller effect on transfer rates. The kinetics of NEFA and lysoPC transfer are slow relative to the lipolytic processes that liberate them. Therefore, one would predict a transient accumulation of NEFA and lysoPC during lipolysis and an attendant modulation of many metabolic processes within living cells and within the plasma compartment of blood. These data will be useful in the refinement of current models of membrane and lipoprotein function and in the selection of fluorescent NEFA analogs for studying transport in living cells.

Molecular and macromolecular specificity of human plasma phospholipid transfer protein

Phospholipid transfer protein (PLTP), also known as lipid transfer protein 2 (LTP-2), mediates a transfer of phospholipids between high-density lipoproteins (HDL). The molecular and macromolecular specificities of recombinant human PLTP were studied using a fluorometric assay based on the excimer fluorescence of pyrenyl lipids. To determine lipoprotein specificity of PLTP, donor very low density lipoproteins (VLDL), low-density lipoproteins (LDL), and HDL were labeled with 1-palmitoyl-2-[10-(1-pyrenyl)decanoyl]phosphatidylcholine (PPyDPC) and incubated with unlabeled acceptor VLDL, LDL, and HDL in every pairwise combination. The highest rate of PPyDPC transfer mediated by PLTP occurred between donor HDL and acceptor HDL. Reassembled HDL (rHDL) consisting of 1-palmitoyl-2-oleoylphosphatidylcholine, apolipoprotein A-I, and pyrene lipids (100:1:4) were used to demonstrate that PLTP transfers diacylglyceride > phosphatidic acid > sphingomyelin > phosphatidylcholine (PC) > phosphatidylglycerol > cerobroside > phosphatidylethanolamine. Thus, PLTP transfers a variety of lipids with two carbon chains and a polar head group. Unsaturation of one PC acyl chain greatly increased transfer rate, whereas increasing chain length and exchanging sn-1/sn-2 position had only small effects. The rate of PPyDPC transfer by PLTP decreases with increasing free cholesterol content in rHDL and with decreasing HDL size. In contrast to spontaneous transfer, PLTP mediates the accumulation of PC in small rHDL particles. PLTP may be important in vivo in the recycling of PC from mature HDL to nascent HDL, the latter of which are the initial acceptors of cholesterol from peripheral tissue for reverse cholesterol transport to the liver.

Alterations in serum phosphatidylcholine fatty acyl species by eicosapentaenoic and docosahexaenoic ethyl esters in patients with severe hypertriglyceridemia

A new and sensitive method has been developed to analyze the molecular species of glycerophospholipids. This method was used to examine the effects of hypolipidemic intervention with n-3 fatty acids on the serum phosphatidylcholine species in severely hypertriglyceridemic patients. The drug treated group (n = 19) received 4 g/day of an 85% concentrate of the ethyl esters of eicosapentenoic and docosahexaenoic acids for 6 weeks. Control patients (n = 21) received 4 g/day of ethyl esters of corn oil fatty acids. To evaluate the effects of n-3 fatty acids upon serum phosphatidylcholines (PCs), sera from treated and control patients were analyzed before and after 6 weeks of intervention. PCs isolated from sera were digested with phospholipase C to diglycerides, derivatized with 7-methoxycoumarin-3-carbonyl azide, and analyzed by reverse phase high performance liquid chromatography (HPLC) with fluorescence detection. Pre-intervention serum PC species were, in order of decreasing concentration C16:0,18:2, C16:0,18:1, C18:0,18:2, C16:0,20:1, C16:0,22:0, C18:0,20:4, C16:0,16:0, C18:0,18:1, C18:1,18:2, C16:0,20:5, and C18:1,20:5. In the treated patients, mean increases of 300% in C16:0,20:5 and of 160% in C16:0,22:6 species were observed. There were no significant changes in the molecular species of the serum phosphatidylcholines in the group receiving the corn oil ethyl esters. The cumulative relative percentages for each of the individual fatty acids measured by HPLC were comparable to those determined by gas-liquid chromatography (GLC). In the treated group plasma triglycerides were reduced 26%, while they were increased by 7% in the placebo group. Our data showed that incorporation of eicosapentaenoic and docosahexaenoic acid into the serum PCs occurred within 6 weeks primarily in the C16:0,20:5 and C16:0,22:6 species and were usually accompanied by a reduction in plasma triglyceride.

The Lipoprotein and Coronary Atherosclerosis Study (LCAS): design, methods, and baseline data of a trial of fluvastatin in patients without severe hypercholesterolemia

Few direct clinical data are available regarding whether cholesterol-lowering therapy should be extended to patients with coronary heart disease (CHD) and normal or only slightly elevated plasma cholesterol concentrations. The one published angiographic trial designed to examine this question found no benefit. Additional prospective data will be provided by the Lipoprotein and Coronary Atherosclerosis Study (LCAS), a randomized, double-blind, placebo-controlled trial of fluvastatin therapy (20 mg twice daily) monitored by both quantitative coronary angiography (QCA) and, in a subset of patients, positron-emission tomography (PET). Eligible subjects in LCAS were men and women 35-75 years of age with low-density lipoprotein (LDL) cholesterol of 115-190 mg/dL on stable dietary therapy and with angiographic evidence by caliper measurement of at least one coronary atherosclerotic lesion causing 30-75% diameter stenosis. Among the 429 patients randomized (mean age 58.8, 81% male), mean baseline LDL cholesterol was only 145.6 mg/dL. Any patient with mean prerandomization LDL cholesterol of 160 mg/dL or higher also received open-label adjunctive cholestyramine. The primary endpoint is within-patient per-lesion change in minimum lumen diameter (MLD) as measured by QCA at baseline and 2.5-year follow-up. All evaluable lesions had MLD at least 0.8mm less than the reference lumen diameter at either baseline or follow-up and MLD at least 25% of the reference lumen diameter at baseline. Data obtained on myocardial perfusion changes (99 patients underwent initial PET), special lipid particles, and coagulation factors may help define which patients with CHD and relatively low LDL cholesterol will benefit from lipid-lowering treatment.

Effect of gemfibrozil on levels of lipoprotein[a] in type II hyperlipoproteinemic subjects

Plasma lipoprotein[a] (Lp[a]) levels are highly correlated with angiographically demonstrable coronary heart disease, and elevated Lp[a] is an independent risk factor for atherosclerosis. Previous studies have provided evidence that the levels of Lp[a] and triglyceride are related, suggesting that Lp[a] might be altered by gemfibrozil, a drug well known for its efficacy in reducing plasma triglycerides. Accordingly, 18 type IIa and 16 type IIb hyperlipoproteinemic males aged 35-58 were treated for 3 months with 600 mg of gemfibrozil twice daily. The efficacy of the drug in altering lipid and lipoprotein levels was different in the two type groups. In type IIa and IIb subjects the respective changes in median levels were: total cholesterol, -7.5 and -8.5% triglycerides, -35.6 and -54.4%; HDL-cholesterol, +9.0 and +11.0%; and Lp[a], -17.2 and +6.1%. Before and after gemfibrozil treatment, 7 type IIa and 10 type IIB subjects were given a 100 g/2 m2 oral-fat load; triglycerides and Lp[a] were measured post-prandially at 0, 2, 4, 6, 8, and 10 h. The differences between before- and after-gemfibrozil post-prandial curve integrated areas (PPCIA) were compared for triglycerides and Lp[a]. The changes in median PPCIA for triglycerides in types IIa and IIB were -54% and -53%, and for Lp[a] were -8% and +8%, respectively. These results indicate i) that the levels of Lp[a] are about 2 times higher in type IIa than IIb subjects, and ii) that although gemfibrozil elicits a rather uniform decrease in fasting and post-prandial triglyceride levels in type IIa and IIb patients, the drug causes heterogeneous changes in Lp[a], suggesting that different metabolic mechanisms may be dominant in subjects showing opposing effects.

In vitro expression of natural mutants of human lecithin:cholesterol acyltransferase

Fish-eye disease (FED) and familial lecithin:cholesterol acyltransferase (LCAT) deficiency (FLD) are rare disorders of lipid metabolism linked to mutations in the LCAT gene. Eleven LCAT cDNA constructs associated with FED and FLD were prepared by site-directed mutagenesis and expressed in COS-6 cells. Analysis of total RNA from wild-type, FED, and FLD transfectants revealed that all contained LCAT-specific mRNA. Western blot analysis demonstrated that all LCAT transfectants synthesized LCAT. Mean LCAT secretion by FED transfectants was slightly lower than secretion by wild-type transfectants, whereas secretion by FLD transfectants was much lower. The specific activities of FED and FLD LCAT against model high density lipoproteins were 6% and 11%, respectively, of wild-type activity. The ratios of the LCAT activities against low density lipoproteins to those against model high density lipoproteins decreased in the order FED mutants > FLD mutants approximately wild type. FED and FLD LCAT mutants are different: the former are more active against low density lipoproteins, and the latter are less secretion-competent. The greater reactivity of FED LCAT against low density lipoproteins may explain the relative mildness of the clinical manifestations of FED compared to those of FLD.

Plasma factors affecting the in vitro conversion of high-density lipoproteins labeled with a non-transferable marker

We studied the in vitro conversion of HDL3 labeled with a radioiodinated diacyl lipid associating peptide (diLAP). DiLAP was previously shown to be nontransferable, which permitted its' use as a reliable marker of HDL particles. DiLAP-labeled HDL3 was incubated for 23 h at 37 degrees C in human or rat plasma or in reconstituted media containing delipidated plasma and/or lipoproteins and/or partially purified CETP. At the end of the incubations, the samples were adjusted to a density of 1.125 g/ml and ultracentrifuged. The two resulting fractions containing HDL2 and HDL3, respectively, were analyzed by gradient gel electrophoresis. Depending upon experimental conditions, diLAP-labeled HDL3 was converted into HDL2b- and/or small HDL3c-like particles. LCAT inhibition and to a lesser extent CETP promoted the formation of small HDL3c. Reactivation of LCAT led to the disappearance of small HDL3c. No HDL3c formed from HDL2 even in the absence of LCAT activity. When the incubations were performed in the presence of 100 mM thimerosal, which inhibited PLTP but not CETP activity, the conversion of diLAP-labeled HDL3 into HDL2 was almost completely blocked. Collective consideration of these data indicates that the formation of small HDL is moderately facilitated by CETP; that small HDL are converted to larger HDL species by LCAT and that the transformation of HDL3 into HDL2 is a process which largely depends upon PLTP activity.

Dietary ethanol is associated with reduced lipolysis of intestinally derived lipoproteins

An acute oral fat load produces transient lipemia that is enhanced by the simultaneous ingestion of ethanol. This phenomenon has been reinvestigated in six normal subjects who consumed three different fat loads with and without ethanol. The fat loads consisted of saturated fat, polyunsaturated fat, or polyunsaturated fat enriched with omega-3 (n-3) fatty acids. Each fat load contained retinol as a marker for intestinally derived lipoproteins. Plasma levels of triglycerides, retinyl palmitate, and nonesterified fatty acid species were determined at several time points after consumption. Increasing the size of a saturated fat load increased postprandial lipemia and delayed retinyl palmitate clearance. Postprandial lipemia and plasma retinyl palmitate concentrations were lower when omega-3 or omega-6 (n-6) polyunsaturated fat was substituted for saturated fat. Preprandial ethanol increased postprandial lipemia, an effect that was most profound with the saturated fat load. Concurrently, oral ethanol also increased postprandial retinyl palmitate. Addition of ethanol to the fat loads had no effect on the plasma concentrations of fatty acids derived from peripheral tissue but appeared to decrease the plasma concentration of free fatty acids of dietary origin. These data support the hypothesis that preprandial ethanol is associated with impaired chylomicron hydrolysis that may be due to inhibition of plasma lipoprotein lipase. These findings are important to the search for the molecular mechanism of ethanolic hypertriglyceridemia and to the development of dietary guidelines for its control.

Effects of site-directed mutagenesis on the serine residues of human lecithin:cholesterol acyltransferase

Lecithin:cholesterol acyltransferase (LCAT) is a serine protease-type enzyme that esterifies cholesterol in human plasma and is activated by apolipoprotein A-I in high-density lipoproteins. LCAT contains 22 serine residues, including Ser181, which is thought to be part of the catalytic site. In order to determine the importance of these serine residues in LCAT, we prepared six LCAT mutants: LCAT (Ser19-->Ala), LCAT (Ser181-->Gly), LCAT (Ser208-->Ala), LCAT (SEr216-->Ala), LCAT (Ser225-->Ala) and LCAT (Ser383-->Ala). We also replaced the adjacent asparagine residues in two additional mutants, LCAT (Ser19-->Ala, Asn20-->Thr) and LCAT (Ser383-->Ala, Asn384-->Thr), in order to ascertain the effect of the serines on N-glycosylation. The mutant complementary DNA (cDNA) were subcloned into a eukaryotic expression vector (pSG5) and expressed in COS-6 cells. By polymerase chain reaction analysis, LCAT-specific messenger RNA (mRNA) was found in all mutant and wild-type transfectants. Western blot analysis revealed LCAT-specific bands in media and lysates of the transfected cells. With two exceptions, the amounts of LCAT mass secreted by the transfectants were similar to that of the wild type (mean, 90% mass of wild type; range, 34-138%). Except for LCAT (Ser181-->Gly), which was inactive, the specific activities of the remainder of the mutant enzymes were also similar (mean 95% activity of wild type; range, 65-169%). These results indicate that Ser181 is part of the catalytic site and that stereoconservative substitutions for serines have minor effects on the synthesis, secretion and specific activities of human LCAT.

Structure of human apolipoprotein D: locations of the intermolecular and intramolecular disulfide links

We have determined the primary structure of human apolipoprotein D (apoD) by aligning peptides derived from digestions by cyanogen bromide, trypsin, and chymotrypsin. Our results confirm the primary structure derived from cDNA [Drayna et al. (1986) J. Biol. Chem. 261, 16535-16539]. ApoD consists of 169 amino acid residues, including 5 cysteines. Tryptic peptide analysis indicated that Cys41 and Cys16 are joined by a disulfide bridge. Using a combination of manual Edman degradations and mass spectrometric analysis on a purified cluster of chymotryptic fragments, we identified an intramolecular disulfide bridge between Cys8 and Cys114 and an intermolecular bridge between Cys116 of apoD and Cys6 of apoA-II. In addition, sites of N-glycosylation were found at Asn45 and Asn78. Because apoD contains two intramolecular disulfide linkages and has a high content of proline to disrupt alpha-helical structures, formation of the amphipathic helical regions that characterize the other soluble apolipoproteins is unlikely. We conclude that apoD binds to lipoprotein surfaces through structures other than alpha-helices, such as disulfide links.

A nonexchangeable apolipoprotein E peptide that mediates binding to the low density lipoprotein receptor

ApoE is a 34-kDa apoprotein that mediates lipoprotein binding to the low density lipoprotein (LDL) receptor and to the LDL receptor-related protein. Receptor binding is mediated by a highly basic, alpha-helical sequence of approximately 15 amino acids that interacts with cysteine-rich repeat regions of the receptor. To determine the relationship between the receptor binding and lipid associating properties of apoE, we have synthesized a series of apoE peptides containing all (residues 129-169) or part (residues 139-169, 144-169, and 148-169) of the receptor-binding domain. The lipophilicity of these peptides was increased by modification of their N termini by acylation with either palmitic acid (C16-apoE peptide) or the N,N-distearyl derivative of glycine (diC18-Gly-apoE peptide). The unmodified peptides demonstrated low affinity for lipid surfaces (Kd > 10(-5) M) and moderate alpha-helicity in the presence of lipid (40%) and had no effect on LDL uptake by fibroblasts. N-Palmitoyl peptides had increased affinity for lipid (Kd approximately 10(-6) M) and increased alpha-helicity (55%) in the presence of lipid. The addition of the C16-apoE-(129-169)-peptide to 125I-LDL enhanced its uptake and degradation by fibroblasts 8-10-fold; however, < 50% of the degradation was mediated by the LDL receptor. By contrast, the diC18-Gly-apoE-(129-169)-peptide was essentially nonexchangeable (Kd < or = 10(-9) M) and highly helical (78%) in the presence of lipid. The addition of the diC18-Gly-apoE-(129-169)-peptide to 125I-LDL enhanced the specific uptake and degradation of LDL by both LDL receptor-mediated and non-LDL receptor-mediated mechanisms. Uptake and degradation of methylated LDL containing diC18-Gly-apoE-(129-169) revealed that the lipoprotein-bound peptide is the active agent. In agreement with this finding, a mutant diC18-Gly-apoE peptide (Arg142-->Gln) was much less effective than the wild-type peptide in potentiating binding, uptake, and degradation of 125I-LDL. Complexes of diC18-Gly-apoE-(129-169), apoA-I, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine containing four to six copies of the peptide/particle displayed an affinity for the LDL receptor similar to that of apoE-L-alpha-dimyristoylphosphatidylcholine discs containing four copies of apoE.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of beef and chicken consumption on plasma lipid levels in hypercholesterolemic men

BACKGROUND

The recommendation to lower saturated fat intake is often interpreted as requiring the elimination of beef to control or lower serum cholesterol levels. The study hypothesis was that the Step I Diet (8% to 10% of energy intake from saturated fatty acids) containing beef would have the same effect on plasma lipid levels of hypercholesterolemic men as a like diet containing chicken.

METHODS

Thirty-eight free-living hypercholesterolemic (otherwise healthy) men completed a 13-week dietary intervention study. Subjects consumed their usual diets for 3 weeks, followed by a 5-week stabilization diet (18% of energy intake from saturated fatty acids), before randomization to one of two test diets for 5 weeks. The test diets contained either 85 g of cooked beef (8% fat) or 85 g of cooked chicken (7% fat) per 4184 kJ and had 7% to 8% of energy from saturated fatty acids. All food was supplied during the stabilization and test diets.

RESULTS

The beef and chicken test diets both produced significant decreases in average plasma total cholesterol level (0.54 mmol/L [7.6%] for beef and 0.70 mmol/L [10.2%] for chicken) and low-density lipoprotein cholesterol level (0.46 mmol/L [9%] for beef and 0.55 mmol/L [11%] for chicken). Changes in average levels of plasma total cholesterol, high-density lipoprotein cholesterol, triglyceride, and low-density lipoprotein cholesterol were not statistically different (smallest P = .26) between the beef and chicken test diets. The average triglyceride level did not change for either test diet group.

CONCLUSIONS

In this short-term study, comparably lean beef and chicken had similar effects on plasma levels of total, low-density lipoprotein, and high-density lipoprotein cholesterol and triglyceride. We concluded that lean beef and chicken are interchangeable in the Step I Diet.

Free radical-induced alterations in endothelial cell function

Free radical-induced injury to the arterial wall has been implicated in the pathogenesis and progression of atherosclerosis. To model the in vitro effects of free radicals on endothelial cell function, protein and lipid synthesis were measured after exposing cells to a superoxide generating system of xanthine (X = 100 microM) and xanthine oxidase (XO = 0.2 units). Total protein synthesis, measured by [35S]methionine uptake, decreased by 87.65 +/- 2.04% over 4 hr compared to controls (P < 0.05). Examination of lipid synthesis by high-performance liquid chromatography in cells prelabeled with either [3H]oleic acid or [3H]sodium acetate revealed alterations in all lipid classes. Phospholipid and neutral glyceride synthesis significantly decreased in a time- and dose-dependent fashion compared to controls (two-way ANOVA). In contrast, cholesterol synthesis and lipid peroxidation increased in a time- and dose-dependent fashion. When X = 200 microM and XO = 0.3 units, there was a statistically significant increase in cholesterol synthesis and lipid peroxidation within 24 hr (Tukey's HSD). We conclude that there is evidence of endothelial cell injury as measured by decreases in protein, glyceride, and phospholipid synthesis. The concurrent increases in lipid peroxidation and cholesterol synthesis may explain the relationship between free radical injury and the pathogenesis of atherosclerosis.

Mechanism of cellular phospholipid efflux

Plasma phospholipid binding to cell-derived cholesterol is important in reverse cholesterol transport, a key step in the regression of atherosclerosis. However, the mechanism by which phospholipids are transferred from cells to plasma remains unclear. [3H]Choline-labeled phospholipid efflux from fibroblasts has been studied using plasma and its components as acceptors. The kinetics were resolved into a fast component (k1 = 0.119 +/- 0.23 min-1) that corresponded to high-affinity binding of high-density lipoproteins (HDL) to the cell surface and a slow component (k2 = 0.0047 +/- 0.0009 min-1) due to protein-mediated desorption (n = 3). Altering the donor charge with heparinase or the acceptor charge by acetylation abolished the fast component, while the slow phase was unchanged. Only HDL displayed biexponential kinetics, comparable to whole plasma. Half-lives for low-density lipoprotein and very-low-density lipoprotein were t1/2 = 278 +/- 22 min and t1/2 = 1003 +/- 147 min, respectively. In the absence of transfer factor, HDL alone significantly reduced phospholipid efflux (t1/2 = 663 min). Phospholipid transfer protein restored biexponential kinetics. We conclude that cell membranes are a potentially important source of plasma phospholipids and that protein-mediated transfer to HDL is the major route for cell-to-plasma transfer. This step represents a locus for anti-atherosclerotic intervention.

Disulfide linked dimers of apolipoprotein D in urine

Apolipoprotein (apo) D is a glycoprotein that contains at least one free cysteine. This allows the formation of disulfide linked dimers of apoD, a phenomenon that could interfere with the study of the isoforms of apoD. Consequently, it is important to consider the effects of hetero- and homodimer formation on the molecular heterogeneity of apoD as well as on the evaluation of the specificity of antibodies to this glycoprotein. The identification of apoD in urine has provided a potential new marker of tubular proteinuria. Thus, we have studied the specificity of our polyclonal antibodies to apoD against the proteins present in normal urine, and at the same time, the existence of dimeric species of apoD linked by disulfide bonds in urine. The specimens were obtained from apparently healthy individuals and analyzed by Western blot. The results showed that apoD in urine exists as a mixture of monomers and dimers, the latter having apparent molecular weights different from those occurring in plasma. Only monomeric apoD was observed under reducing conditions, proving the monospecificity of the polyclonal apoD antibodies.

Effects of site-directed mutagenesis on the N-glycosylation sites of human lecithin:cholesterol acyltransferase

There are four potential N-glycosylation site (Asn-X-Ser/Thr) in human lecithin:cholesterol acyltransferase (LCAT, residues 20, 84, 272, and 384). To study the role of the N-linked sugars, the codon for Asn at these positions was replaced with one for Thr (AAC to ACC). The wild-type and mutant LCAT cDNAs were used to transfect COS-6 cells from which RNA was isolated; cDNAs were synthesized by reverse transcription and subjected to the polymerase chain reaction, which showed that all transfectants synthesized LCAT-specific mRNA. No intracellular or secreted LCAT was detected with the Asn272-->Thr transfectants, indicating that this residue is essential for intracellular processing. All other single-point transfectants were secretion-competent. Although there was detectable LCAT protein inside the cells and in the media of the transfectant, Asn84-->Thr, its specific activity and secreted amount were only 26% and 58% of the wild type, respectively. This implies that Asn84 is critical for full activity but not for intracellular processing. The amount secreted, specific activity, and Vmax of LCAT (Asn20-->Thr) were similar to those of the wild-type LCAT. LCAT (Asn384-->Thr) differed from the wild-type LCAT only by a lower Km. These results suggest that glycosylation at residues 20 and 384 is not essential for intracellular processing, secretion, or activity.

Human very low density lipoprotein structure: interaction of the C apolipoproteins with apolipoprotein B-100

Very low density lipoproteins (VLDL) are a heterogenous population of particles differing in size and composition. Heparin-Sepharose chromatography yields three VLDL subfractions. Two subfractions, VLDLNR-1 and VLDLNR-2, which are not retained by heparin, contain little or no detectable apolipoprotein (apo)E. According to negative stain electron microscopy, VLDLNR-1 is slightly larger than VLDLNR-2. The third fraction, VLDLR, is composed of smaller particles that are retained by the heparin-Sepharose and contain apoE. The C apolipoproteins of the respective VLDL subfractions transfer to 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) single bilayer vesicles giving three subfractions designated VLDLNR-1-C, VLDLNR-2-C, and VLDLR-C. The protein, phospholipid, and cholesterol (free + esterified) contents decrease in the order VLDLR > VLDLNR-2 > VLDLNR-1. Triglyceride content decreases in the opposite order. POPC treatment of each VLDL subfraction increases the phospholipid and decreases the protein, triglyceride, and cholesteryl ester contents, while free cholesterol remains unchanged. According to immunological analysis of each subfraction with well-characterized monoclonal antibodies, the accessibility of some epitopes of apoB-100 on VLDL is changed by POPC treatment. Electron-microscopic analysis of POPC-treated VLDL subfraction reveals vacancies on the surfaces of each particle. VLDLNR-1, VLDLNR-2, and VLDLR are resistant to thrombin cleavage, whereas the lipoproteins lacking C apolipoproteins are not. Thrombin cleavage (8 h) of apoB-100 of VLDLNR-2-C and VLDLR-C gives two fragments, T1 and T2, that are converted to smaller fragments only after prolonged treatment. In contrast, apoB-100 of VLDLNR-1-C is converted into small fragments after 8 h thrombin treatment. These results suggest that removal of apoCs affects the accessibility and conformation of apoB-100 in the individual VLDL subfractions in the region near residue 3249, which is the primary thrombin cleavage site and the epitope of monoclonal antibody 4C11.

Plasma triglycerides determine low density lipoprotein composition, physical properties, and cell-specific binding in cultured cells

The relationship between the plasma triglycerides and the LDL triglycerides of 30 normal and 48 hypertriglyceridemic subjects has been quantified; the data fit a simple adsorption isotherm, LDL triglyceride/(LDL triglyceride+LDL cholesterol ester) = 0.65 plasma triglyceride/(464 + plasma triglyceride). In vitro transfer of triglyceride from concentrated VLDL to VLDL-depleted plasma produced triglyceride-rich LDL that had similar properties. LDL uptake by HepG2 cells increased with LDL triglyceride content whereas the reverse was found with skin fibroblasts. At 37 degrees C, the cores of both normal and hypertriglyceridemic LDL were isotropic liquids. Circular dichroic spectra revealed no difference in the secondary structure of normal and triglyceride-rich LDL. The affinity of monoclonal antibody MB47, which binds to the receptor ligand of apo B-100 was independent of LDL triglyceride content. MB3, which binds near residue 1022 of apo B-100, showed a triglyceride-dependent decrease in affinity for LDL from hypertriglyceridemic subjects and from in vitro incubations. LDL with an elevated triglyceride content formed in vitro had reduced proteolytic cleavage of apo B-100 by Staphylococcus aureus V8 protease. From these data, we infer that (a) LDL triglyceride is a predictable function of plasma triglyceride, (b) triglyceride induces subtle changes in apo B-100 structure at a site that is remote from the putative receptor binding ligand, and (c) the triglyceride-dependent receptor-binding determinants of apo B-100 are recognized differently by fibroblasts and HepG2 cells.

Roles of cysteines in human lecithin:cholesterol acyltransferase

Human lecithin:cholesterol acyltransferase (LCAT, E.C.2.3.1.43) is a serine-type esterase that contains six cysteines, two of which, Cys31 and Cys184, are free. The remaining cysteines form disulfide links. One of these is between Cys50 and Cys74 and the other is between Cys313 and Cys356. The cDNA of LCAT and mutants in which one or two of the six cysteines were replaced by glycine was expressed in COS-6 cells. Polymerase chain reactions and Northern blot analysis indicated that LCAT mRNA was produced by all transfectants. Western blots of all transfected cells probed with a polyclonal antibody revealed intracellular LCAT. Substitution of glycine for either Cys50, Cys74, Cys313, or Cys356 was associated with a nearly total absence of activity in the medium. No protein was secreted when glycine replaced either of the amino acid residues that link Cys313 and Cys356. The small amounts of the Cys50-->Gly and Cys74-->Gly mutants found in the medium had specific activities that were much lower than that of the wild-type LCAT. All other transfectants secreted immunologically measurable amounts of active enzyme. Mutants in which one or both free cysteines, Cys31 and Cys184, were replaced with glycine were less active than the wild type and only partially inhibited by a sulfhydryl blocking reagent. The substrate specificities of the Cys31-->Gly and Cys184-->Gly mutants differed from that of the wild type.(ABSTRACT TRUNCATED AT 250 WORDS)

High density lipoprotein interconversions in rat and man as assessed with a novel nontransferable apolipopeptide

A nontransferable peptide analog of a plasma apolipoprotein diacyl lipid-associating peptide (diLAP) incorporates into model reassembled high density lipoproteins (R-HDL). In whole plasma in vitro, diLAP irreversibly transfers to native rat HDL2 and human HDL3, but not to rat HDL1 or human HDL2. The rate of transfer is dependent on the physical state of the lipid in the R-HDL. Exogenous cholesterol promotes the formation of larger HDL. When diLAP-labeled R-HDL were injected into rats, the diLAP that initially associated with HDL2 transferred to HDL1 over a period of 48 h. The rate of clearance of diLAP-labeled HDL was slower than that of apoA-I. The liver was the preferred site for diLAP-labeled HDL1 uptake. In contrast, diLAP-labeled HDL2 were associated with liver, ovaries, and adrenal glands, with the adrenal grands exhibiting the highest specific association. DiLAP was not found in the kidneys. These data show that 1) rat HDL is cleared more slowly than rat apoA-I; 2) HDL is removed from the plasma compartment as a particle; 3) there are tissue-specific differences in the removal of rat HDL1 and HDL2; 4) HDL2 is a precursor to HDL1; and 5) cholesterol and the activity of lecithin:cholesterol acyltransferase are essential to HDL remodeling.

The calcium uptake of the rat heart sarcoplasmic reticulum is altered by dietary lipid

Small amounts of dietary n-3 fatty acids can have dramatic physiological effects, including the reduction of plasma triglycerides and an elevation of cellular eicosapentanoic (EPA) and docosahexanoic acids (DHA) at the expense of arachidonic acid (AA). We investigated the effects of alterations in the fatty acid compositions of cardiac sarcoplasmic reticulum (CSR) produced by dietary manipulation on the calcium pump protein that is required for energy dependent calcium transport. CSR was isolated from rats fed menhaden oil, which is rich in n-3 fatty acids, and from control animals that were given corn oil. Relative to control membranes, those isolated from rats fed menhaden oil, had a lower content of saturated phospholipids, an increased DHA/AA ratio, and an increased ratio of n-3 to n-6 fatty acids. These changes were associated with a 30% decrease in oxalate-facilitated, ATP-dependent calcium uptake and concomitant decreased Ca-ATPase activity in the membranes from the animals fed menhaden oil. In contrast, there was no alteration in active pump sites as measured by phosphoenzyme formation. Thus, the CSR Ca-ATPase function can be altered by dietary interventions that change the composition, and possibly structure, of the phospholipid membranes thereby affecting enzyme turnover.

Characterization of disulfide-linked heterodimers containing apolipoprotein D in human plasma lipoproteins

Human plasma apolipoprotein (apo) D is a glycoprotein with an apparent molecular weight of 29,000 M(r). It is present, mainly, in high density lipoproteins (HDL) and very high density lipoproteins (VHDL). Western blot analysis of HDL and VHDL using rabbit antibodies to human apoD revealed major immunoreactive bands at 29,000 and 38,000 M(r), with minor bands ranging from 50,000 to and 80,000 M(r). Only the 29,000 M(r) band corresponding to apoD remained when the electrophoresis was conducted under reducing conditions, demonstrating that apoD is cross-linked to other proteins via disulfide bonds. The broad pattern of immunoreactivity was also observed under nonreducing conditions when the blood was collected into a solution of sulfhydryl-trapping reagents, or when these reagents were added to the isolated lipoproteins. These results indicated that the disulfide bonds were not the result of disulfide exchange during the experimental procedures. On the basis of amino acid sequencing and reactions to antibodies, the 38,000 M(r) band was identified as an apoD-apoA-II heterodimer. The apoD-apoA-II was also demonstrated in plasma. In both HDL and plasma, the apoD-apoA-II heterodimer constituted the major form of apoD. Disulfide-linked heterodimers of apoD and apoB-100 were also found in low and very low density lipoproteins, and in whole plasma. It is concluded that a fraction of human apoD, like other cysteine-containing apolipoproteins, exists as a disulfide-linked heterodimer with other apolipoproteins in all major human lipoprotein fractions.

Spontaneous phospholipid transfer: development of a quantitative model

The effects of lipid structure on the kinetics of spontaneous transfer of a series of phosphatidylcholines have been determined. Donors, which were model-reassembled high-density lipoproteins composed of apo A-I, 1-palmitoyl-2-oleoylphosphatidylcholine, and a trace of a radiolabeled lipid, were mixed with acceptors, which were human low-density lipoproteins. Within a series of phosphatidylcholines, the addition of double bonds and methylene units, respectively, increased and decreased the rate of transfer in a predictable way. An equation that predicts the rates of transfer of a large number of diacylglycerides and phosphoglycerides from any lipoprotein has been empirically derived from these data. The transfers of phosphatidylcholines that contain superpolyunsaturated fatty acids (four or more double bonds) do not obey the derived equation, probably due to limitations on the number of conformational degrees of freedom in these lipids. The range of measured transfer halftimes extends from less than 2 h to more than 12 days. Thus, the spontaneous transfer halftimes of some (but not all) lipids are short compared to the lifetime of lipoproteins in plasma. These results suggest that some lipids transfer among lipoproteins and cells via a spontaneous mechanism while others require specific transfer factors or hydrolysis to achieve this within a physiologically significant time frame.

Comparative specificity of plasma lecithin:cholesterol acyltransferase from ten animal species

The molecular specificities of plasma lecithin:cholesterol acyltransferase (LCAT) from ten animal species have been compared. Using a reassembled high density lipoprotein containing a mixture of phosphatidylcholines, the relative rates of liberation of different species of cholesteryl ester were measured. All but two species of LCAT clustered according to one of three patterns of substrate specificity. The LCAT from six species, including human, did not transfer highly polyunsaturated fatty acyl chains. In addition, human LCAT transesterified saturated fatty acyl chains more effectively than unsaturated fatty acyl chains. We conclude that the structures of the active sites of the enzymes differ, and that this may be related to size constraints that prevent efficient binding of large bulky phosphatidylcholines.

Effects of fluorophore structure and hydrophobicity on the uptake and metabolism of fluorescent lipid analogs

Cellular transport and metabolism of fatty acids are integral components of lipid metabolism, but the mechanisms and regulation involved are poorly understood. A variety of commercially available fluorescent analogs of fatty acids, are potentially useful probes for the study of lipid metabolism by such techniques as cell sorting and fluorescence microscopy. We have screened a series of fluorescent fatty acids to identify analogs that would reliably simulate the metabolic behavior of natural fatty acids; i.e., similar kinetics of transport, of intracellular movement, and of metabolic fate. The metabolic behavior of these analogs was compared with those of some naturally occurring fatty acids in HepG2 cells, which are a good model of some aspects of hepatic function. Fluorescent analogs containing polar fluorophores yielded the lowest rates of cellular uptake and conversion to acylated lipid products. Similarly, fluorescent analogs with the fluorophore located near the carboxylic acid group were poorly metabolized. Fatty acid analogs containing anthracene or pyrene at the n-terminus of the acyl chain were the most extensively incorporated into cellular lipids. The types and amounts of labeled lipid products formed from these analogs and from natural fatty acids were similar. Pyrene-labeled analogs have spectral properties that can be measured fluorometrically at very low concentrations. Therefore, we compared the cellular metabolism of 12-(1-pyrenyl)dodecanoic acid with those of palmitic and oleic acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Unsaturated aminophospholipids are preferentially retained by the fast skeletal muscle CaATPase during detergent solubilization. Evidence for a specific association between aminophospholipids and the calcium pump protein

When fast twitch skeletal muscle vesicles (SR) and purified calcium pump protein are stripped with the nonionic detergent C12E8 (octaethylene glycol dodecyl ether), not all the membrane phospholipids are removed from the calcium pump protein. Maximal extraction produces a remnant of 6-8 mol of phospholipid/mole of calcium ATPase (CaATPase). In contrast to native SR and the prestripped purified CaATPase, the remaining phospholipid is markedly enriched in phosphatidylethanolamine (PE) and phosphatidylserine (PS) in both preparations; the remaining lipid is also enriched in phospholipid that is predominantly unsaturated. In addition, virtually all of the associated PE is plasmalogenic (96% as opposed to 63% in the native SR). The amino-specific cross-linking reagent DFDNB (1,5-difluoro-2,4-dinitrobenzene sulfonic acid) and the amino binding reagent TNBS (2,4,6-trinitrobenzene sulfonic acid) were utilized to identify the monolayer of the native preparation where these phospholipids reside, and to determine which phospholipids are closely associated with the calcium pump protein following detergent treatment. These studies demonstrate that PE and PS are closely associated with the pump protein, PE residing almost exclusively in the outer monolayer of SR, while PS resides in the inner monolayer. Nonspecific phospholipid exchange protein was shown to be capable of exchanging phospholipids from donor vesicles into those phospholipids associated with the CaATPase; stripping of lipid-exchanged vesicles with C12E8 exhibited the same specificity with regard to head-group species (i.e., PE is markedly enriched in the extracted protein associated fraction). The results suggest that specific protein-lipid interactions exist, favoring the association of plasmalogenic aminophospholipids with the calcium pump protein.