| For medical practitioners and the general public - Cholesterol Journal Article Catalog. |
| First Page | Previous Page | Next Page | Last Page |
| Apolipoprotein E receptor LR11: intersections between neurodegeneration and cholesterol metabolism Wolozin, B. (2004), Arch Neurol 61(8): 1178-80. |
| Apolipoprotein E regulates dietary cholesterol absorption and biliary cholesterol excretion: studies in C57BL/6 apolipoprotein E knockout mice Sehayek, E., S. Shefer, et al. (2000), Proc Natl Acad Sci U S A 97(7): 3433-7. Abstract: The present study examined the role of apolipoprotein E (apoE) in the regulation of dietary cholesterol absorption and biliary cholesterol excretion. Increasing dietary cholesterol from 0.02% to 0.5% in C57BL/6 wild-type mice decreased the percentage of dietary cholesterol that is absorbed by 25%, and this decrease was associated with a 2-fold increase in gallbladder biliary cholesterol concentration. In contrast, increasing dietary cholesterol from 0. 02% to 0.5% in C57BL/6 apoE knockout mice produced no significant suppression of the percentage dietary cholesterol absorption and increased gallbladder biliary cholesterol concentration only 16%. Whereas in wild-type mice, the increase in dietary cholesterol increased the hepatic excretion of biliary cholesterol 4-fold, there was only a 2-fold increase in apoE knockout mice. On both the low- and the high-cholesterol diets, whole liver and isolated hepatocyte cholesterol content was higher in the apoE knockout mice. These results suggest that, in response to dietary cholesterol, apoE may play a critical role in decreasing the percentage absorption of dietary cholesterol and increasing biliary cholesterol excretion. These observations suggest a mechanism whereby the absence of apoE contributes to the propensity for tissue cholesterol deposition and accelerated atherogenesis in apoE knockout mice. |
| Apolipoprotein E synthesis by cultured human ovarian granulosa cells: regulation by human chorionic gonadotropin and cholesterol Beckmann, M. W., L. M. Olson, et al. (1991), Fertil Steril 55(6): 1118-25. Abstract: OBJECTIVE: Apolipoprotein E (Apo E) is a surface component of several classes of plasma lipoproteins and functions as a receptor ligand for the low-density lipoprotein (LDL) (B/E) receptor, the hepatic E receptor, and the LDL receptor related protein. In continuing studies of Apo E synthesis by extrahepatic steroidogenic tissue, we have examined Apo E production and its control in cultured human granulosa cells (GC). DESIGN: Human GC obtained at the time of oocyte aspiration for in vitro fertilization were cultured, and the following parameters were measured: progesterone secretion into the medium, Apo E synthesis and secretion, and Apo E messenger ribonucleic acid (mRNA) content of the GC. RESULTS: Human chorionic gonadotropin induces a decrease in Apo E protein synthesis and Apo E mRNA content. The addition of aminoglutethimide (which blocks cholesterol conversion to pregnenolone) and/or LDL (which provides the cell with cholesterol), both result in an increase in Apo E protein synthesis and Apo E mRNA content. These latter findings are in agreement with studies in nonsteroidogenic cells, which show that Apo E production is positively correlated with cell cholesterol content. |
| Apolipoprotein E, cholesterol transport and synthesis in sporadic Alzheimer's disease Poirier, J. (2005), Neurobiol Aging 26(3): 355-61. Abstract: The discovery that the apolipoprotein E4 (apoE4) allele is genetically linked to both sporadic and familial late onset Alzheimer's disease (AD) raises the possibility that a dysfunction of the lipid transport system could seriously affect lipid homeostasis in the brain of AD subjects. The presence of the E4 allele has been associated with lower levels of apoE in both serum and brain tissues of normal and AD subjects. In an attempt to reverse the apoE deficit in AD, we identified and characterized several apoE inducer agents using a low throughput-screening assay. The most promising of these compounds is called probucol. Administration of probucol, an old cholesterol lowering drug, in mild to moderate sporadic AD led to significant increases in CSF apoE levels and a decrease of CSF beta amyloid 1-42 without significant modifications of CSF tau concentration or CSF lipid peroxides levels. These results are consistent with recent reports suggesting that the long term use of cholesterol lowering drugs that block 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity in the fourth and fifth decade of life may help reduce the risk of developing AD at later age. These results indicate that, in addition to lipid transport mediated by apoE, cholesterol homeostasis in the brain is markedly altered in response to changes in the HMGR pathway; suggesting a possible explanation for the potentially beneficial effect of statins in common AD. |
| Apolipoprotein E: a cholesterol transport protein with lipid transport-independent cell signaling properties Swertfeger, D. K. and D. Y. Hui (2001), Front Biosci 6: D526-35. Abstract: Apolipoprotein (apo) E is a 34-kDa glycoprotein associated with plasma lipoproteins. In contrast to other apolipoproteins, apoE is expressed ubiquitously in all tissues and appears to have a wide variety of functions in addition to lipid transport. Although the lipid transport properties of apoE are well characterized, the appreciation apoE functions other than lipid transport is still in an infancy stage. Nevertheless, there is increasing evidence that apoE regulates cell functions in a variety of tissues and organs. This review article summarizes briefly the current status of our understanding on the lipid transport-independent properties of this apolipoprotein. |
| Apolipoprotein E-4 gene dose in clinically diagnosed Alzheimer's disease: prevalence, plasma cholesterol levels and cerebrovascular change Czech, C., H. Forstl, et al. (1994), Eur Arch Psychiatry Clin Neurosci 243(5): 291-2. Abstract: The prevalence of the apolipoprotein E-4 allele (ApoE-4) was significantly higher in a referral population of 40 patients with clinically diagnosed Alzheimer's disease than in a sample of non-demented elderly controls (P < 0.01). The highest plasma cholesterol levels were found in demented patients homozygotic for Apo E-4, but no significant increases of glucose, triglycerides and thyroxine or of leuko-araiosis and brain infarcts were verified in this preliminary study. |
| Apolipoprotein E4 homozygosity predisposes to serum cholesterol elevation during high fat diet Tikkanen, M. J., J. K. Huttunen, et al. (1990), Arteriosclerosis 10(2): 285-8. Abstract: The hypothesis that apolipoprotein E (apo E)-isoform-related differences in plasma and LDL cholesterol concentrations are due to differential responses to dietary lipids was explored in 110 North Karelian subjects who had previously participated in dietary intervention studies. This was accomplished by collecting fresh blood samples for apo E phenotyping and by re-analysis of the original plasma lipid data according to apo E phenotypes. During high fat, high cholesterol baseline (p = 0.003) and switchback diets (p = 0.002), plasma cholesterol correlated inversely with the sum of subscript numbers (e.g., apo E3/4 = 7). Thus, subjects with the apo E4/4 phenotype had the highest (7.63 +/- 1.32 mmol/l), and subjects with the apo E3/2 phenotype had the lowest baseline levels of plasma cholesterol (5.85 +/- 1.48 mmol/l). This association became weaker during a low fat, low cholesterol diet intervention (p = 0.069). Greater reductions in plasma cholesterol occurred in subjects homozygous for the apo epsilon 4 allele (-1.84 mmol/l) as compared to subjects with other genotypes (-1.13 mmol/l) (p = 0.0097). Moreover, these subjects responded to the switchback diet by greater increases in plasma cholesterol (1.52 mmol/l) than others (0.92 mmol/l, p = 0.0141). The results suggest that the effect of apo epsilon genotype on plasma cholesterol is modulated by dietary fat and cholesterol intake. |
| Apolipoprotein E4 induces neuronal cell death under conditions of suppressed de novo cholesterol synthesis Michikawa, M. and K. Yanagisawa (1998), J Neurosci Res 54(1): 58-67. Abstract: The presence of the apolipoprotein E (apoE) allele epsilon4 is a major risk factor for the development of Alzheimer's disease (AD); however, the molecular mechanism underlying the acceleration of AD development in individuals with epsilon4 remains to be determined. To investigate the isoform-specific effects of apoE on neurons, primary neuron cultures were prepared from fetal rat cerebral cortices. Inhibition by compactin, a 3-hydroxyl-3-methylglutaryl coenzyme A reductase inhibitor of de novo cholesterol synthesis, induced premature neuronal cell death in a dose-dependent manner. In the presence of compactin at a sublethal dose to the cells, rabbit beta-migrating very low density lipoprotein (beta-VLDL) with human apoE4 (the product of epsilon4) induced premature neuronal cell death, while that with apoE3 (the product of epsilon3) did not. Neurons cultured in the presence of apoE4, beta-VLDL, and compactin were shrunken and spherical, containing condensed chromatin and fragmented DNA, features characteristic of apoptosis. The addition of intermediate metabolites of the cholesterol biosynthetic pathway, including mevalonate and squalene, rescued neuronal cells incubated with apoE4 and beta-VLDL, in the presence of compactin. These results strongly suggest that a reduction in the level of endogenously synthesized cholesterol is a prerequisite for apoE4-induced neuronal cell death. |
| Apolipoprotein E4: another risk factor for cholesterol gallstone formation? Van Erpecum, K. J. and M. C. Carey (1996), Gastroenterology 111(6): 1764-7. |
| Apolipoprotein E-dependent cholesterol efflux from macrophages: kinetic study and divergent mechanisms for endogenous versus exogenous apolipoprotein E Lin, C. Y., H. Duan, et al. (1999), J Lipid Res 40(9): 1618-27. Abstract: In these studies, we have utilized a J774 macrophage model in order to compare phospholipid and cholesterol efflux kinetics in macrophage cells that do not express endogenous apoE to cells transfected to express physiologic levels of human apoE. This model was also used to compare the effect of exogenously added versus endogenously expressed apoE on cholesterol efflux kinetics from macrophages. ApoE expression increased free cholesterol and phospholipid efflux into the medium, but did not change the free cholesterol/phospholipid molar ratio of secreted lipids. Kinetic examination showed that free cholesterol and phospholipid appeared simultaneously in the medium, and that cholesterol loading widened the difference in the rate of cholesterol efflux between apoE-expressing and non-expressing macrophages. Addition of exogenous lipid-free apoE added to non-expressing cells, at a >2-fold higher medium concentration than that produced by endogenous expression, produced less cholesterol efflux than that observed from apoE-expressing cells. The addition of phosphatidylcholine liposomes substantially increased cholesterol efflux from apoE-expressing and non-expressing J774 cells. Addition of these liposomes eliminated the enhanced cholesterol efflux produced by addition of exogenous apoE. On the other hand, even in the presence of phosphatidylcholine liposomes, cholesterol efflux rates remained significantly higher from apoE-expressing macrophages than non-expressing cells. Similar results were obtained when efflux was studied in the presence of cyclodextrin. These results suggest that endogenous expression of apoE by macrophages alters cell cholesterol balance via mechanisms distinct from those utilized by the extracellular addition of apoE, and may involve intracellular or pericellular mechanisms. |
| Apolipoprotein gene polymorphisms as cause of cholesterol QTLs in mice Suto, J. (2005), J Vet Med Sci 67(6): 583-9. Abstract: Quantitative trait locus (QTL) analyses of plasma cholesterol levels were carried out in three sets of F(2) mice that were formed in a 'round-robin' manner from C57BL/6J, KK (-A(y)), and RR strains. Six QTLs were identified on chromosomes 1 (Cq1, Cq2, and Cq6), 3 (Cq3), and 9 (Cq4 and Cq5); of these, Cq2 colocalized with Cq6, and Cq4 colocalized with Cq5. The major candidate gene for Cq2 and Cq6 is Apoa2, and that for Cq4 and Cq5 is Apoa4. The adequacy of polymorphisms in candidate genes as cause of QTLs was investigated in this study. For Apoa2, three different alleles (Apoa2(a), Apoa2(b), and Apoa2(c)) are known. Since there was no significant physiologic difference between Apoa2(a) and Apoa2(c) alleles, previous hypothesis that Apoa2(b) was different from Apoa2(a) and Apoa2(c) in the ability to increase cholesterol levels was further supported. Presumably, G-to-A substitution at nucleotide 84 and/or C-to-T substitution at nucleotide 182 are crucial to make the Apoa2(b) unique. On the other hand, for Apoa4, the most striking polymorphism was the number of Glu-Gln-Ala/Val-Gln repeats in carboxyl end; however, this might not be responsible for QTLs. Instead, a silent mutation, C-to-T substitution at nucleotide 771, was shown to be completely correlated with the occurrence of QTLs in a total of six F(2) intercrosses. Provisionally, but reasonably, these base substitutions are qualified as primary causes that constitute QTL effect. The potential strategy for identifying genes and base substitutions underlying QTLs is discussed. |
| Apolipoprotein H levels in diabetic subjects: correlation with cholesterol levels Cassader, M., G. Ruiu, et al. (1997), Metabolism 46(5): 522-5. Abstract: To assess the relationship between apolipoprotein H (apo H) plasma levels and lipid metabolism in diabetes mellitus, we have examined the correlation between apo H plasma concentration and the main plasma lipid levels in 127 non-insulin-dependent (NIDDM) and 118 insulin-dependent (IDDM) diabetes mellitus patients. The data are compared with those in 286 nondiabetics. Our data show a significant increase in plasma apo H in diabetic as opposed to nondiabetic subjects (NIDDM, 29.9 +/- 10.8 mg/dL; IDDM, 31.3 +/- 9.9; controls, 22.5 +/- 7.7; F = 53.3, P =.0001). The relation between plasma lipids and apo H was simultaneously evaluated in the three groups with inclusion of diabetes, sex, body mass index (BMI), and age as covariates in the model. This analysis showed a strong positive correlation (P =.0009) between apo H and total cholesterol, and a weaker positive correlation with triglycerides (TGs P =.016). The correlation between apo H and hemoglobin A1c (HbA1c) levels in diabetics (P =.03) highlights the importance of glycemic control for plasma levels of this apoprotein, which is highly glycated. Although the role of apo H in lipid metabolism is still uncertain, recent investigations on the possible relation between plasma apo H levels and increased plasma lipids and thrombotic risk could explain the increased atherosclerotic risk in diabetic patients. |
| Apolipoprotein J (clusterin) induces cholesterol export from macrophage-foam cells: a potential anti-atherogenic function? Gelissen, I. C., T. Hochgrebe, et al. (1998), Biochem J 331 (Pt 1): 231-7. Abstract: Apolipoprotein J (apo J) is a secreted glycoprotein of which the exact function remains a matter for speculation. Apo J has been implicated in such diverse processes as sperm maturation, regulation of complement activation, programmed cell death, tissue remodelling and lipid transport. In this study a possible role for apo J in lipid transport was explored. Mouse peritoneal macrophages were incubated with acetylated low-density lipoprotein (AcLDL) to produce foam cells containing cholesterol and cholesteryl esters. Incubation of the foam cells with physiological concentrations of purified apo J led to a dose-dependent export of cholesterol. The appearance of cholesterol in the medium was associated predominantly with a decline in intracellular cholesteryl esters rather than intracellular free cholesterol. The kinetics of cholesterol release to apo J were similar to apo A-I, an established promoter of cholesterol efflux. Apo J was also shown to induce phospholipid efflux from cells, whereas the cholesterol exported to the medium was associated with the apo J. Studies using foam cells from apo E-null mice showed that the cholesterol exported to the medium was independent of apo E production by the cells. These results present the first evidence that apo J can promote cholesterol efflux from foam cells and indicates that it might have a function in cellular cholesterol homoeostasis in both normal and pathological situations. |
| Apolipoprotein J polymorphisms and serum HDL cholesterol levels in African blacks Nestlerode, C. S., C. H. Bunker, et al. (1999), Hum Biol 71(2): 197-218. Abstract: Apolipoprotein J (apoJ, protein; APOJ, gene) is found in serum associated with high-density lipoprotein (HDL) subfractions, which also contain apolipoprotein A-I (apoA1) and cholesteryl ester transfer protein. ApoJ has been shown to be involved in a variety of physiological functions, including lipid transport. In earlier studies we reported the existence of a common genetic polymorphism (APOJ*1 and APOJ*2 alleles) using isoelectric focusing (IEF) and immunoblotting. In this study we determined the molecular basis of this polymorphism and together with another polymorphism at codon 328 (G-->A) evaluated its relationship with serum HDL cholesterol and apoA1 levels in 767 African blacks stratified by staff level: junior (less affluent, n = 450) and senior (more affluent, n = 317). The molecular analysis of the cathodally shifted APOJ*2 allele on IEF gels revealed an amino acid substitution of asparagine by histidine resulting from a missense mutation (A-->C) at codon 317 in exon 7. The frequency of the APOJ*2 (C) allele of codon 317 in the total sample was 0.267, whereas that of the less common allele A of codon 328 was 0.04. Despite their close proximity, no linkage disequilibrium was observed between the 2 polymorphisms. The impact of the codon 317 polymorphic variation was significant on serum HDL cholesterol (p = 0.003) and HDL3 cholesterol (p = 0.001) in junior staff. The adjusted mean values of these traits were higher in the codon 317 APOJ*2/*2 genotype than in the *1/*1 and *1/*2 genotypes. Overall, the APOJ codon 317 polymorphism explained 10.2% and 8.3% of the phenotypic variation in HDL cholesterol and HDL3 cholesterol, respectively, in junior staff. The codon 328 polymorphism showed a significant effect on HDL2 cholesterol (p = 0.039) and apoA1 (p = 0.007) only in junior women and accounted for 2.5% and 4.2% of the phenotypic variation in HDL2 cholesterol and apoA1, respectively. We also analyzed the combined effects of these genotypes at the 2 polymorphic sites. Significant effects on HDL cholesterol (p = 0.004) and HDL3 cholesterol (p = 0.008) in junior men and on HDL2 cholesterol (p = 0.003) in junior women were observed in the combined genotype data. The 2-locus genotypes explained 6.0% and 5.3% of the residual phenotypic variation of HDL cholesterol and HDL3 cholesterol in junior men and 10.4% of HDL2 cholesterol in junior women. These data indicate that the effect of the APOJ polymorphism on HDL cholesterol levels is modulated by socioeconomic status, as measured by staff level. Given the association of HDL and its subfractions with cardiovascular disease, these polymorphisms may lead to a better understanding of interracial differences in the risk of cardiovascular disease. |
| Apolipoprotein M is required for prebeta-HDL formation and cholesterol efflux to HDL and protects against atherosclerosis Wolfrum, C., M. N. Poy, et al. (2005), Nat Med 11(4): 418-22. Abstract: High-density lipoproteins (HDLs) are considered antiatherogenic because they mediate reverse cholesterol transport from the periphery to the liver for excretion and degradation. Here we show that mice deficient in apolipoprotein M (apoM), a component of the HDL particle, accumulated cholesterol in large HDL particles (HDL(1)) while the conversion of HDL to prebeta-HDL was impaired. Accordingly, apoM-deficient mice lacked prebeta-HDL, a subclass of lipid-poor apolipoproteins that serves as a key acceptor of peripheral cellular cholesterol. This deficiency led to a markedly reduced cholesterol efflux from macrophages to apoM-deficient HDL compared to normal HDL in vitro. Overexpression of apoM in Ldlr(-/-) mice protected against atherosclerosis when the mice were challenged with a cholesterol-enriched diet, showing that apoM is important for the formation of prebeta-HDL and cholesterol efflux to HDL, and thereby inhibits formation of atherosclerotic lesions. |
| Apolipoprotein-E phenotype and basal activity of low-density lipoprotein receptor are independent of changes in plasma lipoprotein subfractions after cholesterol ingestion in Japanese subjects Homma, Y., T. Kobayashi, et al. (2001), Nutrition 17(4): 310-4. Abstract: We investigated whether the apolipoprotein-E (apoE) phenotype and the basal activity of low-density lipoprotein (LDL) receptor, which were reported to be the major determinants for increase in plasma LDL levels by cholesterol ingestion, have the same role in Japanese subjects whose diet is low in fat and cholesterol. Cholesterol (750 mg/d) was added to the ordinary diet as a dried egg-yolk supplement for 4 wk to 110 subjects. Plasma levels of lipids, apolipoproteins, and cholesterol in lipoprotein subfractions were measured at the beginning and end of the test period. Phenotyping of apoE was determined by an isoelectric focusing-immunoblotting method, and LDL receptor activity in lymphocytes was determined by flow cytometry. Plasma levels of cholesterol in less-dense LDL (LDL(1)) and less-dense high-density lipoprotein (HDL(2)) were slightly but significantly increased, 3.4% and 4.1%, respectively, by cholesterol ingestion, but the increases were not statistically significant in any of E2, E3, and E4 groups. The distribution of the apoE phenotype was equivalent in all three LDL-cholesterol groups (no change, increase, and decrease by cholesterol ingestion). Plasma levels of LDL, LDL(1), and LDL(2) cholesterol were not significantly increased in the three groups of subjects with lymphocyte LDL-receptor activities (low, medium, and high). As with apoE phenotype, LDL-receptor activities were the same in all three LDL-cholesterol groups. In addition, there were no significant correlations between LDL-receptor activity and changes in plasma levels of lipids, apolipoproteins, and cholesterol in lipoprotein subfractions. Therefore, we concluded that cholesterol ingestion significantly increases plasma levels of less-dense LDL and HDL, but neither apoE phenotype nor basal LDL-receptor activity explain the variability in changes in plasma lipoprotein subfractions by cholesterol ingestion in Japanese subjects. |
| Apolipoprotein-mediated cellular cholesterol and phospholipid efflux depend on a functional Golgi apparatus Mendez, A. J. and L. Uint (1996), J Lipid Res 37(12): 2510-24. Abstract: Several studies have demonstrated that lipid-free apolipoproteins can promote cholesterol and phospholipid efflux from cells; however, the mechanisms and the role of cell-mediated pathways involved remain incompletely elucidated. We have recently demonstrated that brefeldin A or monensin, agents that disrupt Golgi apparatus structure and function, inhibit intracellular cholesterol efflux from cells to high density lipoproteins. In the present study we examined the effects of those agents on cell cholesterol and phospholipid efflux to purified apolipoprotein A-I (apoA-I) and apolipoprotein-depleted acceptors from cholesterol-loaded fibroblasts. Brefeldin A or monensin treatment of cells during incubation with apoA-I inhibited efflux of cellular cholesterol by greater than 80% compared with control cells, measured by changes in cellular cholesterol radioactivity, mass, and the substrate pool of cholesterol available for esterification by acyl coenzyme A:cholesterol acyltransferase. Inhibition of cholesterol efflux by these agents could not be overcome by increasing the apoA-I concentration and persisted during incubations up to 24 h. Similarly, brefeldin A and monensin inhibited up to 80% of apoA-I-mediated efflux of labeled phospholipids from cholesterol-loaded cells relative to controls. In contrast, lipid efflux mediated by apolipoprotein-depleted acceptors (trypsin-modified HDL and sonicated phospholipid vesicles) was not sensitive to these drugs. On the basis the known effects of brefeldin A and monensin on Golgi apparatus structure and function, these results are consistent with the notion that efflux of cell lipids by apolipoprotein-dependent mechanisms, but not by apolipoprotein-independent mechanisms, require active cellular processes involving an intact and functional Golgi apparatus. |
| Apolipoprotein-mediated cellular cholesterol efflux Yokoyama, S. (1998), Biochim Biophys Acta 1392(1): 1-15. |
| Apolipoprotein-mediated cellular cholesterol/phospholipid efflux and plasma high density lipoprotein level in mice Tsujita, M., S. Tomimoto, et al. (2000), Biochim Biophys Acta 1485(2-3): 199-213. Abstract: Helical apolipoprotein(apo)s generate pre-beta-high density lipoprotein (HDL) by removing cellular cholesterol and phospholipid upon the interaction with cells. To investigate its physiological relevance, we studied the effect of an in vitro inhibitor of this reaction, probucol, in mice on the cell-apo interaction and plasma HDL levels. Plasma HDL severely dropped in a few days with probucol-containing chow while low density protein decreased more mildly over a few weeks. The peritoneal macrophages were assayed for apoA-I binding, apoA-I-mediated release of cellular cholesterol and phospholipid and the reduction by apoA-I of the ACAT-available intracellular cholesterol pool. All of these parameters were strongly suppressed in the probucol-fed mice. In contrast, the mRNA levels of the potential regulatory proteins of the HDL level such as apoA-I, apoE, LCAT, PLTP, SRB1 and ABC1 did not change with probucol. The fractional clearance rate of plasma HDL-cholesteryl ester was uninfluenced by probucol, but that of the HDL-apoprotein was slightly increased. No measurable CETP activity was detected either in the control or probucol-fed mice plasma. The change in these functional parameters is consistent with that observed in the Tangier disease patients. We thus concluded that generation of HDL by apo-cell interaction is a major source of plasma HDL in mice. |
| Apolipoprotein-mediated plasma membrane microsolubilization. Role of lipid affinity and membrane penetration in the efflux of cellular cholesterol and phospholipid Gillotte, K. L., M. Zaiou, et al. (1999), J Biol Chem 274(4): 2021-8. Abstract: Lipid-free apolipoprotein (apo) A-I contributes to the reverse transport of cholesterol from the periphery to the liver by solubilizing plasma membrane phospholipid and cholesterol. The features of the apolipoprotein required for this process are not understood and are addressed in the current study. Membrane microsolubilization of human fibroblasts is not specific for apo A-I; unlipidated apos A-II, C, and E incubated with the fibroblast monolayers at a saturating concentration of 50 micrograms/ml are all able to release cholesterol and phospholipid similarly. To determine the properties of the apolipoprotein that drive the process, apo A-I peptides spanning the entire sequence of the protein were utilized; the peptides correspond to the 11- and 22-residue amphipathic alpha-helical segments, as well as adjacent combinations of the helices. Of the 20 helical peptides examined, only peptides representing the N-and C-terminal portions of the protein had the ability to solubilize phospholipid and cholesterol. Cholesterol efflux to the most effective peptides, 44-65 and 209-241, was approximately 50 and 70%, respectively, of that to intact apo A-I. Deletion mutants of apo E and apo A-I were constructed that have reduced lipid binding affinities as compared with the intact molecule. The proteins, apo A-I (Delta222-243), apo A-I (Delta190-243), apo E3 (Delta192-299) and apo E4 (Delta192-299) all exhibited a decreased ability to remove cellular cholesterol and phospholipid. These decreases correlated with the reduced ability of these proteins to penetrate into a phospholipid monomolecular film. Overall, the results indicate that insertion of amphipathic alpha-helices between the plasma membrane phospholipid molecules is a required step in the mechanism of apolipoprotein-mediated cellular lipid efflux. Therefore the lipid binding ability of the apolipoprotein is critical for efficient membrane microsolubilization. |