Familial aggregation of lipids and lipoproteins in families ascertained through random and nonrandom probands in the Minnesota Lipid Research Clinic Family Study

The familial aggregation of lipids [total cholesterol (CH) and triglyceride (TG)] and lipoproteins [high-density lipoprotein cholesterol (HDL) and low-density lipoprotein cholesterol (LDL)] was investigated in families ascertained through both random and nonrandom probands in the Minnesota Lipid Research Clinic Family Study. Nonrandom proband ascertainment was based on single selection through truncation for hyperlipidemia at an earlier screening. A path model was used to investigate the nature of familial resemblance using appropriate adjustments for ascertainment and to determine whether random and hyperlipidemic samples are heterogeneous with regard to the multifactorial model. The results suggest that parameter estimates are consistent with those from previous studies in which only random families were used and that random and nonrandom samples are homogeneous with regard to the path model for CH and LDL. However, for TG and HDL the random and hyperlipidemic samples are significantly heterogeneous. This heterogeneity would be observed if familial hypertriglyceridemia and/or familial hypoalphalipoproteinemia segregates predominantly in the hyperlipidemic rather than in the random sample, as on might expect.     

Free cholesterol is a potent regulator of lipid transfer protein function

This study investigates the effect of altered lipoprotein free cholesterol (FC) content on the transfer of cholesteryl ester (CE) and triglyceride (TG) from very low- (VLDL), low- (LDL), and high-(HDL) density lipoproteins by the plasma-derived lipid transfer protein (LTP). The FC content of VLDL and HDL was selectively altered by incubating these lipoproteins with FC/phospholipid dispersions of varying composition. FC-modified lipoproteins were then equilibrated with [3H] TG, [14C]CE-labeled lipoproteins of another class to facilitate the subsequent modification of the radiolabeled donor lipoproteins. LTP was added and the extent of radiolabeled TG and CE transfer determined after 1 h. With either LDL or VLDL as lipid donor, an increase in the FC content of these lipoproteins caused a concentration-dependent inhibition (up to 50%) of CE transfer from these particles, without any significant effect on TG transfer. In contrast, with HDL as donor, increasing the HDL FC content had little effect on CE transfer from HDL, but markedly stimulated (up to 2.5-fold) the transfer of TG. This differential effect of FC on the unidirectional transfer of radiolabeled lipids from VLDL and HDL led to marked effects on LTP-facilitated net mass transfer of lipids. During long-term incubation of a constant amount of LTP with FC-modified VLDL and HDL, the extent of net mass transfer was linearly related to lipoprotein FC content; a 4-fold increase in FC content resulted in a 3-fold stimulation of the CE mass transferred to VLDL, which was coupled to an equimolar, reciprocal transfer of TG mass to HDL. Since lipid transfer between lipoproteins is integral to the process of reverse cholesterol transport, we conclude that lipoprotein FC levels are a potent, positive regulator of the pathways involved in sterol clearance. FC may modulate lipid transfer by altering the availability of CE and TG to LTP at the lipoprotein surface.     

A rapid single-step centrifugation method for determination of HDL, LDL, and VLDL cholesterol, and TG, and identification of predominant LDL subclass

Determination of the circulating levels of plasma lipoproteins HDL, LDL, and VLDL is critical in the assessment of risk of coronary heart disease. More recently it has become apparent that the LDL subclass pattern is a further important diagnostic parameter. The reference method for separation of plasma lipoproteins is ultracentrifugation. However, current methods often involve prolonged centrifugation steps and use high salt concentrations, which can modify the lipoprotein structure and must be removed before further analysis. To overcome these problems we have now investigated the use of rapid self-generating gradients of iodixanol for separation and analysis of plasma lipoproteins. A protocol is presented in which HDL, LDL, and VLDL, characterized by electron microscopy and agarose gel electophoresis, separate in three bands in a 2.5 h centrifugation step. Recoveries of cholesterol and TG from the gradients were close to 100%. The distribution profiles of cholesterol and TG in the gradient were used to calculate the concentrations of individual lipoprotein classes. The values correlated with those obtained using commercial kits for HDL and LDL cholesterol. The position of the LDL peak in the gradient and its shape varied between plasma samples and was indicative of the density of the predominant LDL class. The novel protocol offers a rapid, reproducible and accurate single-step centrifugation method for the determination of HDL, LDL, and VLDL cholesterol, and TG, and identification of LDL subclass pattern.     

Lipid and fatty acid composition of canine lipoproteins

Lipid classes and their fatty acids were studied in the major lipoprotein fractions from canine, in comparison with human, plasma. In dogs, high-density-lipoprotein (HDL), the main carrier of plasma phospholipid (PL), cholesterol ester (CE) and free cholesterol, was the most abundant lipoprotein, followed by low and very-low density lipoproteins (LDL and VLDL). Notably, LDL and VLDL contributed similarly to the total dog plasma triacylglycerol (TG). The PL composition was similar in all three lipoproteins, dominated by phosphatidylcholine (PC). Even though the content and composition of lipids within and among lipoproteins differed markedly between dog and man, the total amount of circulating lipid was similar. All canine lipoproteins were relatively richer than those from humans in long-chain (C20-C22) n-6 and n-3 polyunsaturated fatty acids (PUFA) but had comparable proportions of total saturated and monoenoic fatty acids, with 18:2n-6 being the main PUFA in both mammals. The fatty acid profile of canine and human lipoproteins differed because they had distinct proportions of their major lipids. There were more n-3 and n-6 long-chain PUFA in canine than in human plasma, because dogs had more HDL, their HDL had more PC and CE, and both these lipids were richer in such PUFA.     

Oxidation of cholesterol in low density and high density lipoproteins by cholesterol oxidase

The cholesterol oxidase-catalyzed oxidation of cholesterol in native low density (LDL) and high density lipoproteins (HDL3) as well as in monolayers prepared from surface lipids of these particles, has been examined. The objective of the study was to compare the oxidizability of cholesterol, and to examine the effects of lipid packing on oxidation rates. When [3H]cholesterol-labeled lipoproteins were exposed to cholesterol oxidase (Streptomyces sp.), it was observed that LDL [3H]cholesterol was oxidized much faster than HDL3 [3H]cholesterol. This was true both at equal cholesterol concentration per enzyme unit, and at equal amounts of lipoprotein particles per enzyme unit. About 95% of lipoprotein [3H]cholesterol was available for oxidation. The complete degradation of lipoprotein sphingomyelin by sphingomyelinase (Staphylococcus aureus) resulted in a 10-fold increase in the rate of LDL [3H]cholesterol oxidation, whereas the effects on rates of HDL3 [3H]cholesterol oxidation were less dramatic. A monolayer study with LDL surface lipids indicated that degradation of sphingomyelin loosened the lipid packing, because the ceramide formed occupied a smaller surface area than the parent sphingomyelin, and since the condensing effect of cholesterol on sphingomyelin packing was lost. The effects of sphingomyelin degradation on lipid packing in monolayers of HDL3-derived surface lipids were difficult to determine from monolayer experiments. Based on the finding that cholesterol oxidases are surface pressure-sensitive with regard to their catalytic activity, these were used to estimate the surface pressure of intact LDL and HDL3. The cut-off surface pressure of a Brevibacterium enzyme was 25 mN/m and 20 mN/m in monolayers of LDL and HDL3-derived surface lipids, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Method for quantitating cholesterol in subfractions of serum lipoproteins separated by gradient gel electrophoresis

Extensive heterogeneity in particle size distribution of serum lipoproteins of baboons was resolved by a procedure that combined Sudan black B prestaining, polyacrylamide gradient gel electrophoresis (GGE), and quantitative densitometry. Each densitometric scan represented a continuous distribution of the relative amount of cholesterol in a serum sample, as a function of the lipoprotein particle size. For analytical purposes, each scan was divided into 12 fractions, representing 12 particle size ranges. The relationship between the estimated cholesterol concentrations in the summed GGE/densitometric fractions corresponding to very low-density lipoproteins (VLDL) + low-density lipoproteins (LDL) and those corresponding to high-density lipoproteins (HDL) and concentrations measured by the heparin-Mn2+ precipitation/enzymatic procedure was linear over a broad range. However, a systematic overestimation of HDL cholesterol concentration and an underestimation of VLDL + LDL cholesterol concentration was apparent. Therefore, correction factors were developed for adjusting the estimates of VLDL + LDL and HDL cholesterol concentrations obtained by the GGE/densitometric method. This analytical method is rapid, repeatable, economical, and useful for genetic and dietary research in which cholesterol concentrations in multiple particle size ranges of lipoproteins must be measured in large numbers of samples. It also is adaptable to immunoblotting procedures for detecting the distribution of specific apolipoproteins among the size-resolved lipoproteins.     

Long-term hypolipaemic effect of apple pulp on golden hamsters with alimentary hyperlipoproteinaemia

Golden hamsters with alimentary hyperlipoproteinaemia (fed on a diet in which 44% of the energy was accounted for by fats, 52 mg cholesterol (CH)/100 g) were given apple pulp (10% of the diet) for six months. From the end of the first month up to the end of the experiment, elevation of serum CH, triacylglycerol (TG) and phospholipid (PL) levels and CH and TG accumulation in the liver were effectively inhibited. At the close of the experiment, hyperlipoproteinaemia was characterized by an increase in the amount of very low density lipoproteins (VLDL), which carried 60-80% of all circulating lipids, accounted for over 70% of the total lipoprotein pool and transported almost 60% of the serum CH (high density lipoproteins--HDL--only 16%). The apple diet markedly reduced (by 50-80%) the amount of all lipids in the lipoproteins within density limits of 1,006-1,063 kg/m3 and lowered the total lipid concentration in these density classes and in the lipoprotein pool as a whole (by 50%). The proportion of the individual lipoprotein classes in the lipoprotein pool was not significantly affected. The apple diet did not affect either the chemical composition of HDL or their participation in the lipoprotein pool, but raised the proportion of serum CH transported in this lipoprotein class. Neither the lipoprotein lipase activity of the adipose tissue nor lecithin:cholesterol acyltransferase activity were influenced by the apple diet.     

Increased atherosclerosis in diabetic dyslipidemic swine: protection by atorvastatin involves decreased VLDL triglycerides but minimal effects on the lipoprotein profile

Male Yucatan swine were allocated to four groups (n = 5-6 pigs per group): low fat (3%) fed control, high fat/2% cholesterol (CH) fed (HF), high fat/CH fed with alloxan-induced diabetes (DF) and DF pigs that were treated with atorvastatin (80 mg/day; DF+A). Pigs were fed two meals per day and daily insulin injections were used in diabetic pigs to maintain plasma glucose between 250 and 350 mg/dl. Diabetic dyslipidemic (DF) pigs exhibited greater coronary atherosclerosis and increased collagen deposition in internal mammary artery compared with normoglycemic hyperlipidemic pigs. Although total and LDL CH concentrations did not differ, triglyceride (TG) were increased in DF pigs and FPLC analysis indicated that the LDL/HDL CH ratio was significantly increased in DF compared with HF pigs. The LDL fraction of DF pigs contained larger, lipid enriched particles resembling IDL. Consumption of the high fat/CH diet caused a moderate increase in the percentage of 14:0 fatty acids in plasma lipids and this was compensated by small-moderate declines in several unsaturated fatty acids. There was a significant increase in phospholipid arachidonic acid in DF compared with HF pigs. Atorvastatin protected diabetic pigs from atherosclerosis and decreased total and VLDL TG, but exerted minimal effects on the FPLC lipoprotein and plasma fatty acid profiles and plasma concentrations of total and LDL CH, vitamin A, vitamin E, and lysophosphatidylcholine. Across all groups the plasma CH concentration was positively correlated with hepatic CH concentration. These findings suggest that atorvastatin's protection against coronary artery atherosclerosis in diabetes may involve effects on plasma VLDL TG concentration. Lack of major effects on other lipid parameters, including the LDL/HDL ratio, suggests that atorvastatin may have yet other anti-atherogenic effects, possibly directly in the vessel wall.     

Evidence for a Trypanosoma brucei lipoprotein scavenger receptor

African trypanosomes are lipid auxotrophs that live in the bloodstream of their human and animal hosts. Trypanosomes require lipoproteins in addition to other serum components in order to multiply under axenic culture conditions. Delipidation of the lipoproteins abrogates their capacity to support trypanosome growth. Both major classes of serum lipoproteins, LDL and HDL, are primary sources of lipids, delivering cholesterol esters, cholesterol, and phospholipids to trypanosomes. We show evidence for the existence of a trypanosome lipoprotein scavenger receptor, which facilitates the endocytosis of both native and modified lipoproteins, including HDL and LDL. This lipoprotein scavenger receptor also exhibits selective lipid uptake, whereby the uptake of the lipid components of the lipoprotein exceeds that of the protein components. Trypanosome lytic factor (TLF1), an unusual HDL found in human serum that protects from infection by lysing Trypanosoma brucei brucei, is also bound and endocytosed by this lipoprotein scavenger receptor. HDL and LDL compete for the binding and uptake of TLF1 and thereby attenuate the trypanosome lysis mediated by TLF1. We also show that a mammalian scavenger receptor facilitates lipid uptake from TLF1 in a manner similar to the trypanosome scavenger receptor. Based on these results we propose that HDL, LDL, and TLF1 are all bound and taken up by a lipoprotein scavenger receptor, which may constitute the parasite's major pathway mediating the uptake of essential lipids.     

Dose and duration effects of estradiol valerate on serum and lipoprotein lipids

Non-alkylated estrogens, like estradiol valerate (F2V), are widely used in the treatment of the postmenopausal hormonal deficiency syndrome. Their effects on serum and lipoprotein lipids are characterized by an increase in the lipid constituents of high density lipoproteins (HDL) and, usually, a decrease in low density lipoproteins (LDL). These effects are considered beneficial as regards atherogenesis and the risk for cardiovascular diseases. Unlike the effects of alkylated estrogens, no concomitant increase in triglycerides (TG) in serum and very low density lipoproteins (VLDL) - adverse effects - are seen in doses of up to 2 mg E2V. In order to compare the effects of 2 and 4 mg of E2V on serum and lipoprotein lipids, 19 bilaterally oophorectomized women participated in a cross-over study after a 4 week long wash-out period. To evaluate the influence of the time factor, 10 of the women continued taking 2 mg and 9 taking 4 mg of E2V respectively for an additional period of 12 weeks, resulting in a total treatment period of 24 weeks per group. The serum lipoproteins were separated by preparative ultracentrifugation, the serum and lipoprotein lipids being assessed using commercially available kits. In the cross-over part of the study, total (TC) and free cholesterol (FC) and phospholipids (PL) increased in HDL and decreased in LDL. Neither dose increased TG in serum or VLDL. These changes in the lipoprotein pattern persisted at the end of the entire study. Consequently, within the range of commonly used doses (2 and 4 mg) E2V seems to have a constant and, in terms of cardiovascular disease, favourable influence on lipoprotein metabolism irrespective of doses and periods studied.     

LDL and HDL enriched in triglyceride promote abnormal cholesterol transport

Hypertriglyceridemia induces multiple changes in lipoprotein composition. Here we investigate how one of these modifications, triglyceride (TG) enrichment, affects HDL and LDL function when this alteration occurs under conditions in which more polar components can naturally re-equilibrate. TG-enriched lipoproteins were produced by co-incubating VLDL, LDL, and HDL with cholesteryl ester (CE) transfer protein. The resulting 2.5-fold increase in TG/CE ratio did not measurably alter the apoprotein composition of LDL or HDL, or modify LDL size. HDL mean diameter increased slightly from 9.1 to 9.4 nm. Modified LDL was internalized by fibroblasts normally, but its protein was degraded much less efficiently. This likely reflects an aberrant apolipoprotein B (apoB) conformation, as suggested by its resistance to V8 protease digestion and altered LDL electrophoretic mobility. TG-enriched LDL ineffectively down-regulated cholesterol biosynthesis compared with control LDL at the same protein concentration, but was equivalent in sterol regulation when compared on a cholesterol basis. TG-enriched HDL promoted greater net cholesterol efflux from cholesterol-loaded J774 cells. However, cholesterol associated with TG-enriched HDL was inefficiently esterified by lecithin:cholesterol acyltransferase, and TG-enriched HDLs were poor donors of CE to HepG2 hepatocytes by selective uptake. We conclude that TG-enrichment, in the absence of other significant alterations in lipoprotein composition, is sufficient to alter both cholesterol delivery and removal mechanisms. Some of these abnormalities may contribute to increased coronary disease in hypertriglyceridemia.     

Transfer of free and esterified cholesterol from low-density lipoproteins and high-density lipoproteins to human adipocytes

Cholesterol stored in human adipose tissue is derived from circulating lipoproteins. To delineate the cholesterol transport function of LDL and HDL, the movement of radiolabelled esterified cholesterol and free cholesterol from labelled LDL and HDL to human adipocytes was examined in the present study. LDL and HDL were enriched and labelled in esterified cholesterol with [14C]cholesterol by the action of plasma lipid transfer proteins and lecithin-cholesterol acyltransferase. Doubly labelled (3H,14C) LDL and HDL were prepared by exchanging free [3H]cholesterol into the 14C-labelled lipoproteins. 14C-labelled lipoprotein and 3H-labelled lipoprotein were also prepared separately and mixed to yield a mixed doubly labelled lipoprotein. Relative to the total amount added, proportionally more free than esterified cholesterol was transferred to the adipocytes upon incubation with any doubly labelled LDL and HDL. The calculated mass of free and esterified cholesterol transferred, however, varied with different labelled lipoproteins. 3H- and 14C-labelled LDL or HDL transferred 2-3-fold more esterified than free cholesterol while the reverse occurred with the mixed doubly labelled LDL or HDL. Thus, free cholesterol-depleted particles preferentially transferred cholesterol ester to the fat cells. In the presence of the homologous unlabelled native lipoprotein, the transfers of free and esterified cholesterol from labelled LDL or HDL were specifically inhibited. Selective transfer of esterified cholesterol relative to apoprotein was also observed when esterified cholesterol uptake from both LDL and HDL was assayed along with the binding of 125I-labelled lipoprotein. The cellular accumulation of cholesterol ether-labelled HDL (a non-hydrolyzable analogue of cholesterol ester) exceeded that of cholesterol ester consistent with significant hydrolysis of the latter physiological substrate. These results demonstrate preferential transfer of free cholesterol and esterified cholesterol over apoprotein for both LDL and HDL in human adipocytes. Furthermore, the data suggest that the cholesterol ester transport function of LDL and HDL can be enhanced by free cholesterol depletion and cholesterol ester enrichment of the particles, and affirms a role for adipose tissue in the metabolism of lipid-modified lipoproteins.     

1H NMR investigation on interaction between ibuprofen and lipoproteins

A large number of studies indicate that oxidative modification of plasma lipoproteins, especially low-density lipoprotein (LDL), is a critical factor in initiation and progression of atherosclerosis. We have previously found that ibuprofen (IBP), a potential antioxidant drug to inhibit LDL oxidation, interacted with lipoproteins in intact human plasma. In the present study, we compare the binding affinities of IBP to LDL and HDL (high-density lipoprotein) by (1)H NMR spectroscopy. When IBP is added into the HDL and LDL samples, the - N(+)(CH(3))(3) moieties of phosphatidylcholine (PC) and sphingomyelin (SM) in lipoprotein particles experience the chemical shift up-field drift. Intermolecular cross-peaks observed in NOESY spectra imply that there are direct interactions between ibuprofen and lipoproteins at both hydrophobic and hydrophilic (ionic) regions. These interactions are likely to be important in the solubility of ibuprofen into lipoprotein particles. Ibuprofen has higher impact on the PC and SM head group ( - N(+)(CH(3))(3)) and - (CH(2))(n) - group in HDL than that in LDL. This could be explained by either IBP has higher binding affinity to HDL than to LDL, or IBP induces orientation of the phospholipid head group at the surface of the lipoprotein particles.     

Effect of bromocriptine on lipid plasma levels in rats

Results show that bromocriptine induced marked alterations in plasma levels of cholesterol and lipids in response to acute and chronic administrations in rats. Two hours after an I.P. dose of 10 mg/kg, bromocriptine mesylate caused significant reductions in plasma levels of total high density lipoprotein (HDL) and high density lipoprotein cholesterol (HDL cholesterol). At a dose of 20 mg/kg, bromocriptine mesylate induced significant elevations in plasma levels of total cholesterol, total HDL, HDL cholesterol, total low density lipoproteins (LDL), and low density lipoprotein cholesterol (LDL cholesterol). Injected at a dose of 4 or 10 mg/kg daily for 14 consecutive days, bromocriptine mesylate caused significant increases in plasma levels of total cholesterol, LDL cholesterol and total LDL whereas the levels of HDL cholesterol, total HDL triglycerides (TG) were reduced. At a dose of 20 mg/kg all parameters were significantly increased. Marked hyperglycaemia was noticed in response to doses of 10, 15 and 20 mg/kg injected daily for 14 consecutive days or 2 hrs after a single administration of 15 mg/kg. Plasma insulin activity was reduced 2 hours after injection of bromocriptine at a dose of 15 mg/kg Likewise, a significant reduction in plasma insulin activity was observed in response to daily I.P. injections of bromocriptine at a dose of 15 mg/kg. Hyperglycaemic and hypoinsulinaemic effects of bromocriptine (acute and chronic) were markedly decreased when sulpiride, a dopaminergic D2 antagonist, was injected at an I.P. dose of 10 mg/kg before bromocriptine. Plasma ACTH activity was significantly increased in response to bromocriptine (15 mg/kg I.P.) in acute and chronic experiments. This effect was markedly diminished when sulpiride was injected prior to bromocriptine. In conclusion, bromocriptine induced marked elevations in plasma levels of total cholesterol and lipids which are likely to be related to hyperglycaemic and hypoinsulinaemic effects.     

蝇蛆壳聚糖对实验性糖尿病大鼠脂代谢的影响

目的测定实验糖尿病大鼠血脂,探讨蝇蛆壳聚糖改善脂代谢以降低血糖的机制。方法用全自动生化分析仪测定各组大鼠血清总胆固醇(TC)、甘油三酯(TG)、高密度脂蛋白(HDL)、低密度脂蛋白(LDL)的含量。结果经蝇蛆壳聚糖治疗后,高剂量组TC,TG含量明显低于模型对照组(P<0.01),其中TG含量与拜糖平组相比有明显差异(P<0.05)。中剂量组TC含量明显低于模型对照组(P<0.01),TG含量低于模型对照组(P<0.05)。低剂量组HDL含量最高,与模型对照组相比有显著差异(P<0.01)。结论蝇蛆壳聚糖有改善实验糖尿病大鼠血脂的作用。     

Presence of CuZn superoxide dismutase in human serum lipoproteins

It has previously been demonstrated that CuZn-superoxide dismutase (SOD) is secreted by several human cell lines. This suggests that the circulating enzyme derives from both hemolysis and peripheral tissues as a result of cellular secretion. In the present report, we evaluated the presence of CuZn-SOD in human serum lipoproteins by both enzyme-linked immunosorbent assay and Western blot analysis of immunoprecipitated lipoprotein samples. The distribution of CuZn-SOD activity among the different lipoprotein fractions was also determined by the xanthine/xanthine oxidase method. The results demonstrated that CuZn-SOD is noticeably present in serum lipoproteins and mainly in low and high density lipoproteins (LDL and HDL). Moreover, experiments performed by incubating CuZn-SOD with a lipid emulsion and subsequent separation of the lipid fraction by ultracentrifugation showed that this enzyme associates in a saturable manner with lipids. The CuZn-SOD bound to LDL and HDL could exert a physiological protective role against oxidative damage of these lipoprotein classes that carry out a crucial role in the cholesterol transport.     

Absorption and lipoprotein transport of sphingomyelin

Dietary sphingomyelin (SM) is hydrolyzed by intestinal alkaline sphingomyelinase and neutral ceramidase to sphingosine, which is absorbed and converted to palmitic acid and acylated into chylomicron triglycerides (TGs). SM digestion is slow and is affected by luminal factors such as bile salt, cholesterol, and other lipids. In the gut, SM and its metabolites may influence TG hydrolysis, cholesterol absorption, lipoprotein formation, and mucosal growth. SM accounts for approximately 20% of the phospholipids in human plasma lipoproteins, of which two-thirds are in LDL and VLDL. It is secreted in chylomicrons and VLDL and transferred into HDL via the ABCA1 transporter. Plasma SM increases after periods of large lipid loads, during suckling, and in type II hypercholesterolemia, cholesterol-fed animals, and apolipoprotein E-deficient mice. SM is thus an important amphiphilic component when plasma lipoprotein pools expand in response to large lipid loads or metabolic abnormalities. It inhibits lipoprotein lipase and LCAT as well as the interaction of lipoproteins with receptors and counteracts LDL oxidation. The turnover of plasma SM is greater than can be accounted for by the turnover of LDL and HDL particles. Some SM must be degraded via receptor-mediated catabolism of chylomicron and VLDL remnants and by scavenger receptor class B type I receptor-mediated transfer into cells.     

Transport of lipids from high and low density lipoproteins via scavenger receptor-BI.

The scavenger receptor-BI (SR-BI) delivers sterols from circulating lipoproteins to tissues, but the relative potency of individual lipoproteins and the transported cholesterol has not been studied in detail. In this study, we used Chinese hamster ovary cells that express recombinant mouse SR-BI but have no functional low density lipoprotein (LDL) receptors (ldlA7-SRBI cells) to compare the fate of lipids transferred from high or low density lipoproteins to cells by SR-BI. HDL and LDL were equally effective in mediating the transfer of [(3)H]cholesterol to cells. Only 5% of the free cholesterol transferred to cells was esterified, in direct contrast to the findings in the cells that express LDL receptors in which 50% of the transported cholesterol was esterified. Almost all the free cholesterol transferred from lipoproteins to cells was rapidly excreted when the ldlA7-SRBI cells were switched to media containing unlabeled lipoproteins. SR-BI expression was associated with an increase in selective cholesteryl ester uptake from both lipoproteins, but HDL was a more effective donor. HDL and LDL were equally effective in delivering cholesterol to the intracellular regulatory pool via SR-BI. These data indicate that SR-BI is able to exchange cholesterol rapidly between lipoproteins and cell membranes and can mediate the uptake of cholesteryl esters from both classes of lipoproteins.     

Lipid transfer inhibitor protein defines the participation of high density lipoprotein subfractions in lipid transfer reactions mediated by cholesterol ester transfer protein (CETP)

Cholesterol ester transfer protein (CETP) moves triglyceride (TG) and cholesteryl ester (CE) between lipoproteins. CETP has no apparent preference for high (HDL) or low (LDL) density lipoprotein as lipid donor to very low density lipoprotein (VLDL), and the preference for HDL observed in plasma is due to suppression of LDL transfers by lipid transfer inhibitor protein (LTIP). Given the heterogeneity of HDL, and a demonstrated ability of HDL subfractions to bind LTIP, we examined whether LTIP might also control CETP-facilitated lipid flux among HDL subfractions. CETP-mediated CE transfers from [3H]CE VLDL to various lipoproteins, combined on an equal phospholipid basis, ranged 2-fold and followed the order: HDL3 > LDL > HDL2. LTIP inhibited VLDL to HDL2 transfer at one-half the rate of VLDL to LDL. In contrast, VLDL to HDL3 transfer was stimulated, resulting in a CETP preference for HDL3 that was 3-fold greater than that for LDL or HDL2. Long-term mass transfer experiments confirmed these findings and further established that the previously observed stimulation of CETP activity on HDL by LTIP is due solely to its stimulation of transfer activity on HDL3. TG enrichment of HDL2, which occurs during the HDL cycle, inhibited CETP activity by approximately 2-fold and LTIP activity was blocked almost completely. This suggests that LTIP keeps lipid transfer activity on HDL2 low and constant regardless of its TG enrichment status. Overall, these results show that LTIP tailors CETP-mediated remodeling of HDL3 and HDL2 particles in subclass-specific ways, strongly implicating LTIP as a regulator of HDL metabolism.     

Oxidized cholesterol in the diet is a source of oxidized lipoproteins in human serum

The aim of this study was to determine in humans whether oxidized cholesterol in the diet is absorbed and contributes to the pool of oxidized lipids in circulating lipoproteins. When a meal containing 400 mg cholestan-5alpha,6alpha-epoxy-3beta-ol (alpha-epoxy cholesterol) was fed to six controls and three subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol in serum was found in chylomicron/chylomicron remnants (CM/RM) and endogenous (VLDL, LDL, and HDL) lipoproteins. In controls, alpha-epoxy cholesterol in CM/RM was decreased by 10 h, whereas in endogenous lipoproteins it remained in the circulation for 72 h. In subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol was mainly in CM/RM. In vitro incubation of the CM/RM fraction containing alpha-epoxy cholesterol with human LDL and HDL that did not contain alpha-epoxy cholesterol resulted in a rapid transfer of oxidized cholesterol from CM/RM to both LDL and HDL. In contrast, no transfer was observed when human serum was substituted with rat serum, suggesting that cholesteryl ester transfer protein is mediating the transfer. Thus, alpha-epoxy cholesterol in the diet is incorporated into the CM/RM fraction and then transferred to LDL and HDL, contributing to lipoprotein oxidation. Moreover, LDL containing alpha-epoxy cholesterol displayed increased susceptibility to further copper oxidation in vitro. It is possible that oxidized cholesterol in the diet accelerates atherosclerosis by increasing oxidized cholesterol levels in circulating LDL and chylomicron remnants.