Fenofibrate reverses the decline in HDL cholesterol in mice overexpressing human phospholipid transfer protein

In humans, fibrates are used to treat dyslipidemia, because these drugs lower plasma triglycerides and raise HDL cholesterol. Treatment with fibrates lowers plasma phospholipid transfer protein (PLTP) activity in humans, but increases PLTP activity in mice, without a consistent effect on HDL-cholesterol concentration. Earlier, we found that PLTP overexpression in transgenic mice results in decreased plasma HDL levels and increased diet-induced atherosclerosis. So it seems that the interplay between fibrates, PLTP and HDL is different in mice and man, which may be important for atherosclerosis development. In the present study, we measured the effects of fibrates on PLTP expression in cultured human hepatocytes and effects of fibrate treatment on human PLTP expression, plasma PLTP activity and HDL levels in human PLTP transgenic mice. Fibrate treatment did not influence PLTP mRNA levels in human hepatocytes. Hepatic human PLTP mRNA levels and PLTP activity were both moderately elevated by fenofibrate treatment in human PLTP transgenic mice. In wild-type mice, however, feeding fenofibrate resulted in a strong induction of PLTP mRNA in the liver and a more than 4-fold increase of plasma PLTP activity. Plasma triglycerides were reduced in all mice by 48% or more by fenofibrate treatment. HDL-cholesterol concentrations were substantially increased by fenofibrate in PLTP overexpressing mice (+72%), but unaffected in wild-type mice. We conclude that fenofibrate treatment reverses the HDL-lowering effect of PLTP overexpression in human PLTP transgenic mice.     

Down-regulation of hepatic lipase gene expression and activity by fenofibrate.

The influence of the hypolipidemic drug, fenofibrate, on hepatic lipase (HL) gene expression and activity was investigated in the rat. Fenofibrate treatment provoked a dose-dependent decrease in HL mRNA levels. At a dose of 0.5% (w/w), HL mRNA levels were reduced to nearly 50% the levels in untreated controls. This decrease was parallelled by a comparable reduction in liver HL activity. The decrease in HL mRNA levels was already observed after 1 day of fenofibrate treatment. Whole liver perfusion experiments showed that the heparin-releasable HL activity in fenofibrate-treated livers dropped to 10% the activity in control livers. In conclusion, treatment with fenofibrate decreases HL gene expression, leading to a lowered activity of endothelium bound HL in fenofibrate-treated livers.     

Activation of the constitutive androstane receptor decreases HDL in wild-type and human apoA-I transgenic mice

The nuclear hormone receptor constitutive androstane receptor (CAR, NR1I3) regulates detoxification of xenobiotics and endogenous molecules, and has been shown to be involved in the metabolism of hepatic bile acids and cholesterol. The goal of this study was to address potential effects of CAR on the metabolism of HDL particles, key components in the reverse transport of cholesterol to the liver. Wild-type (WT) mice, transgenic mice expressing human apolipoprotein A-I (HuAITg), and CAR-deficient (CAR(-/-)) mice were treated with the specific CAR agonist 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). CAR activation decreased HDL cholesterol and plasma apolipoprotein A-I (apoA-I) levels in both WT and HuAITg mice, but not CAR(-/-) mice. Both mouse apoA-I and human apoA-I were decreased by more than 40% after TCPOBOP treatment, and kinetic studies revealed that the production rate of HDL is reduced in TCPOBOP-treated WT mice. In transient transfections, TCPOBOP-activated CAR decreased the activity of the human apoA-I promoter. Although loss of CAR function did not alter HDL levels in normal chow-fed mice, HDL cholesterol, apoA-I concentration, and apoA-I mRNA levels were increased in CAR(-/-) mice relative to WT mice when both were fed a high-fat diet. We conclude that CAR activation in mice induces a pronounced decrease in circulating levels of plasma HDL, at least in part through downregulation of apoA-I gene expression.     

Osbpl8 Deficiency in Mouse Causes an Elevation of High-Density Lipoproteins and Gender-Specific Alterations of Lipid Metabolism

OSBP-related protein 8 (ORP8) encoded by Osbpl8 is an endoplasmic reticulum sterol sensor implicated in cellular lipid metabolism. We generated an Osbpl8^−/− (KO) C57Bl/6 mouse strain. Wild-type and Osbpl8KO animals at the age of 13-weeks were fed for 5 weeks either chow or high-fat diet, and their plasma lipids/lipoproteins and hepatic lipids were analyzed. The chow-fed Osbpl8KO male mice showed a marked elevation of high-density lipoprotein (HDL) cholesterol (+79%) and phospholipids (+35%), while only minor increase of apolipoprotein A-I (apoA-I) was detected. In chow-fed female KO mice a less prominent increase of HDL cholesterol (+27%) was observed, while on western diet the HDL increment was prominent in both genders. The HDL increase was accompanied by an elevated level of HDL-associated apolipoprotein E in male, but not female KO animals. No differences between genotypes were observed in lecithin:cholesterol acyltransferase (LCAT) or hepatic lipase (HL) activity, or in the fractional catabolic rate of fluorescently labeled mouse HDL injected in chow-diet fed animals. The Osbpl8KO mice of both genders displayed reduced phospholipid transfer protein (PLTP) activity, but only on chow diet. These findings are consistent with a model in which Osbpl8 deficiency results in altered biosynthesis of HDL. Consistent with this hypothesis, ORP8 depleted mouse hepatocytes secreted an increased amount of nascent HDL into the culture medium. In addition to the HDL phenotype, distinct gender-specific alterations in lipid metabolism were detected: Female KO animals on chow diet showed reduced lipoprotein lipase (LPL) activity and increased plasma triglycerides, while the male KO mice displayed elevated plasma cholesterol biosynthetic markers cholestenol, desmosterol, and lathosterol. Moreover, modest gender-specific alterations in the hepatic expression of lipid homeostatic genes were observed. In conclusion, we report the first viable OsbplKO mouse model, demonstrating a HDL elevating effect of Osbpl8 knock-out and additional gender- and/or diet-dependent impacts on lipid metabolism.     

Phospholipid transfer protein deficiency ameliorates diet-induced hypercholesterolemia and inflammation in mice

Phospholipid transfer protein (PLTP) facilitates the transfer of phospholipids from triglyceride-rich lipoproteins into HDL. PLTP has been shown to be an important factor in lipoprotein metabolism and atherogenesis. Here, we report that chronic high-fat, high-cholesterol diet feeding markedly increased plasma cholesterol levels in C57BL/6 mice. PLTP deficiency attenuated diet-induced hypercholesterolemia by dramatically reducing apolipoprotein E-rich lipoproteins (-88%) and, to a lesser extent, LDL (-40%) and HDL (-35%). Increased biliary cholesterol secretion, indicated by increased hepatic ABCG5/ABCG8 gene expression, and decreased intestinal cholesterol absorption may contribute to the lower plasma cholesterol in PLTP-deficient mice. The expression of proinflammatory genes (intercellular adhesion molecule-1 and vascular cell adhesion molecule-1) is reduced in aorta of PLTP knockout mice compared with wild-type mice fed either a chow or a high-cholesterol diet. Furthermore, plasma interleukin-6 levels are significantly lower in PLTP-deficient mice, indicating reduced systemic inflammation. These data suggest that PLTP appears to play a proatherogenic role in diet-induced hyperlipidemic mice.     

Fenofibrate increases HDL-cholesterol by reducing cholesteryl ester transfer protein expression

In addition to efficiently decreasing VLDL-triglycerides (TGs), fenofibrate increases HDL-cholesterol levels in humans. We investigated whether the fenofibrate-induced increase in HDL-cholesterol is dependent on the expression of the cholesteryl ester transfer protein (CETP). To this end, APOE*3-Leiden (E3L) transgenic mice without and with the human CETP transgene, under the control of its natural regulatory flanking regions, were fed a Western-type diet with or without fenofibrate. Fenofibrate (0.04% in the diet) decreased plasma TG in E3L and E3L.CETP mice (-59% and -60%; P < 0.001), caused by a strong reduction in VLDL. Whereas fenofibrate did not affect HDL-cholesterol in E3L mice, fenofibrate dose-dependently increased HDL-cholesterol in E3L.CETP mice (up to +91%). Fenofibrate did not affect the turnover of HDL-cholesteryl ester (CE), indicating that fenofibrate causes a higher steady-state HDL-cholesterol level without altering the HDL-cholesterol flux through plasma. Analysis of the hepatic gene expression profile showed that fenofibrate did not differentially affect the main players in HDL metabolism in E3L.CETP mice compared with E3L mice. However, in E3L.CETP mice, fenofibrate reduced hepatic CETP mRNA (-72%; P < 0.01) as well as the CE transfer activity in plasma (-73%; P < 0.01). We conclude that fenofibrate increases HDL-cholesterol by reducing the CETP-dependent transfer of cholesterol from HDL to (V)LDL, as related to lower hepatic CETP expression and a reduced plasma (V)LDL pool.     

Fibrate treatment induced quantitative and qualitative HDL changes associated with an increase of SR-BI cholesterol efflux capacities in rabbits

Fibrates are widely used as lipid lowering drugs acting as peroxisome proliferator-activated receptors α (PPARα) agonists and modulating the expression of several genes involved in lipid and lipoprotein metabolism. Much less is known on the effect of fibrates in HDL structure and composition. Therefore, we examined whether fenofibrate induces quantitative and/or qualitative modifications in HDL metabolism in the rabbit, an animal that, contrary to rodents and similar to humans, is less sensitive to peroxisome proliferators. We first demonstrated that 3-week treatment with fenofibrate (250 mg/kg/day) induced an important increase in serum apolipoprotein A-I, HDL-cholesterol and HDL-phospholipids concentrations and a relative enrichment in HDL cholesteryl ester content. Moreover, the fatty acid profiles from fenofibrate-treated rabbits displayed a dramatic increase in the serum or HDL C18:3 ω6 to C18:2 ω6 ratio suggesting higher Δ6 desaturase activity. In addition, HDL from fenofibrate-treated animals exhibited higher relative proportions of sphingomyelin, phosphatidylinositol and phosphatidylethanolamine. We then reported that fenofibrate induced major changes in the physical characteristics of HDL, mainly a higher size and a faster mobility on agarose gel electrophoresis. Finally, serum or HDL from treated rabbits exhibited higher capacity to promote cholesterol efflux from Scavenger receptor class B type I (SR-BI)-rich Fu5AH cells compared to controls. Our findings demonstrate that fenofibrate has beneficial effects in rabbits by increasing the mass of the circulating HDL pool and by modifying their composition transforming them as better acceptors of cellular cholesterol through SR-BI pathway. These effects of fenofibrate might contribute to its benefits on the prevention and treatment of atherosclerosis.     

Fish Oil and Fenofibrate Prevented Phosphorylation-dependent Hepatic Sortilin 1 Degradation in Western Diet-fed Mice

Obesity and diabetes are associated with hepatic triglyceride overproduction and hypertriglyceridemia. Recent studies have found that the cellular trafficking receptor sortilin 1 (Sort1) inhibits hepatic apolipoprotein B secretion and reduces plasma lipid levels in mice, and its hepatic expression was negatively associated with plasma lipids in humans. This study investigated the regulation of hepatic Sort1 under diabetic conditions and by lipid-lowering fish oil and fenofibrate. Results showed that hepatic Sort1 protein, but not mRNA, was markedly lower in Western diet-fed mice. Knockdown of hepatic Sort1 increased plasma triglyceride in mice. Feeding mice a fish oil-enriched diet completely restored hepatic Sort1 levels in Western diet-fed mice. Fenofibrate also restored hepatic Sort1 protein levels in Western diet-fed wild type mice, but not in peroxisome proliferator-activated receptor α (PPARα) knock-out mice. PPARα ligands did not induce Sort1 in hepatocytes in vitro. Instead, fish oil and fenofibrate reduced circulating and hepatic fatty acids in mice, and n-3 polyunsaturated fatty acids prevented palmitate inhibition of Sort1 protein in HepG2 cells. LC/MS/MS analysis revealed that Sort1 phosphorylation at serine 793 was increased in obese mice and in palmitate-treated HepG2 cells. Mutations that abolished phosphorylation at Ser-793 increased Sort1 stability and prevented palmitate inhibition of Sort1 ubiquitination and degradation in HepG2 cells. In summary, therapeutic strategies that prevent posttranslational hepatic Sort1 down-regulation in obesity and diabetes may be beneficial in improving dyslipidemia.     

Reduction of atherosclerosis by the peroxisome proliferator-activated receptor alpha agonist fenofibrate in mice

Several clinical and angiographic intervention trials have shown that fibrate treatment leads to a reduction of the coronary events associated to atherosclerosis. Fibrates are ligands for peroxisome proliferator-activated receptor alpha (PPARalpha) that modulate risk factors related to atherosclerosis by acting at both systemic and vascular levels. Here, we investigated the effect of treatment with the PPARalpha agonist fenofibrate (FF) on the development of atherosclerotic lesions in apolipoprotein (apo) E-deficient mice and human apoA-I transgenic apoE-deficient (hapoA-I Tg x apoE-deficient) mice fed a Western diet. In apoE-deficient mice, plasma lipid levels were increased by FF treatment with no alteration in the cholesterol distribution profile. FF treatment did not reduce atherosclerotic lesion surface area in the aortic sinus of 5-month-old apoE-deficient mice. By contrast, FF treatment decreased total cholesterol and esterified cholesterol contents in descending aortas of these mice, an effect that was more pronounced in older mice exhibiting more advanced lesions. Furthermore, FF treatment reduced MCP-1 mRNA levels in the descending aortas of apoE-deficient mice, whereas ABCA-1 expression levels were maintained despite a significant reduction of aortic cholesterol content. In apoE-deficient mice expressing a human apoA-I transgene, FF increased human apoA-I plasma and hepatic mRNA levels without affecting plasma lipid levels. This increase in human apoA-I expression was accompanied by a significant reduction in the lesion surface area in the aortic sinus. These data indicate that the PPARalpha agonist fenofibrate reduces atherosclerosis in these animal models of atherosclerosis.     

Influence of insulin sensitivity and the TaqIB cholesteryl ester transfer protein gene polymorphism on plasma lecithin:cholesterol acyltransferase and lipid transfer protein activities and their response to hyperinsulinemia in non-diabetic men.

Lecithin:cholesteryl acyl transferase (LCAT), cholesteryl ester transfer protein (CETP), phospholipid transfer protein (PLTP), and lipoprotein lipases are involved in high density lipoprotein (HDL) metabolism. We evaluated the influence of insulin sensitivity and of the TaqIB CETP gene polymorphism (B1B2) on plasma LCAT, CETP, and PLTP activities (measured with exogenous substrates) and their responses to hyperinsulinemia. Thirty-two non-diabetic men without hyperlipidemia were divided in quartiles of high (Q(1)) to low (Q(4)) insulin sensitivity. Plasma total cholesterol, very low + low density lipoprotein cholesterol, triglycerides, and apolipoprotein (apo) B were higher in Q(4) compared to Q(1) (P < 0.05 for all), whereas HDL cholesterol and apoA-I were lowest in Q(4) (P < 0.05 for both). Plasma LCAT activity was higher in Q(4) than in Q(1) (P < 0. 05) and PLTP activity was higher in Q(4) than in Q(2) (P < 0.05). Insulin sensitivity did not influence plasma CETP activity. Postheparin plasma lipoprotein lipase activity was highest and hepatic lipase activity was lowest in Q(1). Insulin infusion decreased PLTP activity (P < 0.05), irrespective of the degree of insulin sensitivity. The CETP genotype exerted no consistent effects on baseline plasma lipoproteins and LCAT, CETP, and PLTP activities. The decrease in plasma PLTP activity after insulin was larger in B1B1 than in B2B2 homozygotes (P < 0.05). These data suggest that insulin sensitivity influences plasma LCAT, PLTP, lipoprotein lipase, and hepatic lipase activities in men. As PLTP, LCAT, and hepatic lipase may enhance reverse cholesterol transport, it is tempting to speculate that high levels of these factors in association with insulin resistance could be involved in an antiatherogenic mechanism. A possible relationship between the CETP genotype and PLTP lowering by insulin warrants further study.     

Opposite regulation of hepatic lipase and lecithin: cholesterol acyltransferase by glucocorticoids in rats.

Rats were treated with hydrocortisone, dexamethasone or triamcinolone for 4 days. The effect of treatment on hepatic lipase and lecithin:cholesterol acyltransferase (LCAT) mRNA levels and catalytic activities was determined. Hepatic lipase mRNA was not affected by hydrocortisone, but was decreased after dexamethasone (-28%) and triamcinolone (-54%). Hepatic lipase activity followed the same pattern, it was not affected by hydrocortisone and lowered by dexamethasone (-38%) and triamcinolone (-70%). The LCAT mRNA level in the liver was also not affected by hydrocortisone, but increased upon treatment with dexamethasone (+22%) and triamcinolone (+72%). Plasma LCAT, determined with an excess exogenous substrate (designated LCAT-II), tended to decrease after hydrocortisone treatment (-11%) and was higher after dexamethasone (+21%) and triamcinolone (+22%). The plasma cholesterol esterification rate (designated LCAT-I), determined by incubation of the plasma at 37 degrees C, followed the same pattern. The activity ratio of hepatic lipase/LCAT-II decreased from 1 in the controls to 0.51 after dexamethasone and 0.25 in the triamcinolone-treated animals. The plasma HDL cholesterol concentration in the different groups changed oppositely to the hepatic lipase/LCAT activity ratio. It is concluded that HDL cholesterol is raised by synthetic glucocorticoids due, among other factors, to a lowered hepatic lipase and an increased plasma LCAT activity. The influence of glucocorticoids on these enzymes is, at least partly, explained by the effects on the hepatic mRNA contents.     

Fenofibrate Increases Very Low Density Lipoprotein Triglyceride Production Despite Reducing Plasma Triglyceride Levels in APOE*3-Leiden.CETP Mice

The peroxisome proliferator-activated receptor alpha (PPARα) activator fenofibrate efficiently decreases plasma triglycerides (TG), which is generally attributed to enhanced very low density lipoprotein (VLDL)-TG clearance and decreased VLDL-TG production. However, because data on the effect of fenofibrate on VLDL production are controversial, we aimed to investigate in (more) detail the mechanism underlying the TG-lowering effect by studying VLDL-TG production and clearance using APOE*3-Leiden.CETP mice, a unique mouse model for human-like lipoprotein metabolism. Male mice were fed a Western-type diet for 4 weeks, followed by the same diet without or with fenofibrate (30 mg/kg bodyweight/day) for 4 weeks. Fenofibrate strongly lowered plasma cholesterol (−38%) and TG (−60%) caused by reduction of VLDL. Fenofibrate markedly accelerated VLDL-TG clearance, as judged from a reduced plasma half-life of glycerol tri[³H]oleate-labeled VLDL-like emulsion particles (−68%). This was associated with an increased post-heparin lipoprotein lipase (LPL) activity (+110%) and an increased uptake of VLDL-derived fatty acids by skeletal muscle, white adipose tissue, and liver. Concomitantly, fenofibrate markedly increased the VLDL-TG production rate (+73%) but not the VLDL-apolipoprotein B (apoB) production rate. Kinetic studies using [³H]palmitic acid showed that fenofibrate increased VLDL-TG production by equally increasing incorporation of re-esterified plasma fatty acids and liver TG into VLDL, which was supported by hepatic gene expression profiling data. We conclude that fenofibrate decreases plasma TG by enhancing LPL-mediated VLDL-TG clearance, which results in a compensatory increase in VLDL-TG production by the liver.     

Conversion of apolipoprotein-specific high-density lipoprotein populations during incubation of human plasma

Incubation studies were performed on plasma obtained from subjects selected for relatively low levels of high-density lipoprotein cholesterol (HDL-C) (no greater than 30 mg/dl) and particle size distributions enriched in the HDL3 subclass. Incubation (12 h, 37 degrees C) of plasma in the presence or absence of lecithin: cholesterol acyltransferase activity produces marked alteration in size profiles of both major apolipoprotein-specific HDL3 populations (HDL3(AI w AII), HDL3 species containing both apolipoprotein A-I and apolipoprotein A-II, and HDL3(AI w/o AII), HDL3 species containing apolipoprotein A-I) as isolated by immunoaffinity chromatography. In the presence or absence of lecithin: cholesterol acyltransferase activity, plasma incubation results in a shift of HDL3(AI w AII) species (initial mean sizes of major components, approx. 8.8 and 8.0 nm) predominantly to larger particles (mean size, 9.8 nm). A less prominent shift to smaller particles (mean size, 7.8 nm) accompanies the conversion to larger particles only when the enzyme is active. Combined shifts to larger (mean size, 9.8 nm) and smaller (mean size, 7.4 nm) particles are observed for HDL3(AI w/o AII) particles (mean size, 8.3 nm) also only in the presence of enzyme activity. However, in the absence of enzyme activity, HDL3(AI w/o AII) species, unlike the HDL3(AI w AII) species, are converted to smaller (mean size 7.4 nm) rather than to larger particles. Like native HDL2b(AI w/o AII) particles, the larger HDL3(AI w/o AII) conversion products exhibit a protein moiety with molecular weight equivalent to four apolipoprotein A-I molecules per particle; small HDL3(AI w/o AII) products are comprised predominantly of particles with two apolipoprotein A-I per particle. Incubation-induced conversion of HDL3 particles in the presence of lecithin: cholesterol acyltransferase activity is associated with increased binding of both apolipoprotein-specific HDL populations to low-density lipoproteins (LDL). The present studies indicate that, in the absence of lecithin: cholesterol acyltransferase activity, the two HDL3 populations follow different conversion pathways, possibly due to apolipoprotein-specific activities of lipid transfer protein or conversion protein in plasma. Our studies also suggest that lecithin: cholesterol acyltransferase activity may play a role in the origins of large HDL2b(AI w/o AII) species in human plasma by participating in the conversion of HDL3(AI w/o AII) particles, initially with three apolipoprotein A-I, to larger particles with four apolipoprotein A-I per particle.     

PLTP activity in premenopausal women. Relationship with lipoprotein lipase, HDL, LDL, body fat, and insulin resistance

Plasma phospholipid transfer protein (PLTP) is thought to play a major role in the facilitated transfer of phospholipids between lipoproteins and in the modulation of high density lipoprotein (HDL) particle size and composition. However, little has been reported concerning the relationships of PLTP with plasma lipoprotein parameters, lipolytic enzymes, body fat distribution, insulin, and glucose in normolipidemic individuals, particularly females. In the present study, 50 normolipidemic healthy premenopausal females were investigated. The relationships between the plasma PLTP activity and selected variables were assessed. PLTP activity was significantly and positively correlated with low density lipoprotein (LDL) cholesterol (r(s) = 0.53), apoB (r(s) = 0.44), glucose (r(s) = 0.40), HDL cholesterol (r(s) = 0.38), HDL(3) cholesterol (r(s) = 0.37), lipoprotein lipase activity (r(s) = 0.36), insulin (r(s) = 0.33), subcutaneous abdominal fat (r(s) = 0.36), intra-abdominal fat (r(s) = 0.29), and body mass index (r(s) = 0.29). HDL(2) cholesterol, triglyceride, and hepatic lipase were not significantly related to PLTP activity. As HDL(2) can be decreased by hepatic lipase and hepatic lipase is increased in obesity with increasing intra-abdominal fat, the participants were divided into sub-groups of non-obese (n = 35) and obese (n = 15) individuals and the correlation of PLTP with HDL(2) cholesterol was re-examined. In the non-obese subjects, HDL(2) cholesterol was found to be significantly and positively related to PLTP activity (r(s) = 0.44). Adjustment of the HDL(2) values for the effect of hepatic lipase activity resulted in a significant positive correlation between PLTP and HDL(2) (r(s) = 0.41), indicating that the strength of the relationship between PLTP activity and HDL(2) can be reduced by the opposing effect of hepatic lipase on HDL(2) concentrations. We conclude that PLTP-facilitated lipid transfer activity is related to HDL and LDL metabolism, as well as lipoprotein lipase activity, adiposity, and insulin resistance.     

Selective and independent associations of phospholipid transfer protein and hepatic lipase with the LDL subfraction distribution

Phospholipid transfer protein (PLTP), hepatic lipase (HL), and lipoprotein lipase (LPL) have all been reported to be intricately involved in HDL metabolism but the effect of PLTP on the apolipoprotein B-containing lipoproteins relative to that of HL and LPL has not been established. Due to our previous observation of a positive correlation of PLTP activity with plasma apoB and LDL cholesterol, the relationship of PLTP with the LDL subfractions was investigated and compared with that of HL and LPL. Plasma lipoproteins from 50 premenopausal women were fractionated by density gradient ultracentrifugation. Correlations were calculated between the cholesterol concentration of each fraction and plasma PLTP, HL, and LPL activity. Plasma PLTP activity was highly, positively, and selectively correlated with the cholesterol concentration of the buoyant LDL/dense IDL fractions, yet demonstrated a complete absence of an association with the dense LDL fractions. In contrast, HL was positively correlated with the dense LDL fractions but showed no association with buoyant LDL. LPL was also positively correlated with several buoyant LDL fractions; however, the correlations were weaker than those of PLTP. PLTP and LPL were positively correlated and HL was negatively correlated with HDL fractions. The results suggest that PLTP and HL may be important and independent determinants of the LDL subpopulation density distributions.