Low-density lipoproteins modified by lipid transfer protein have altered biological activity

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.

The surface cholesteryl ester content of donor and acceptor particles regulates CETP: a liposome-based approach to assess the substrate properties of lipoproteins

Cholesteryl ester transfer protein (CETP) activity is regulated, in part, by lipoprotein composition. We previously demonstrated that CETP activity follows saturation kinetics as cholesteryl ester (CE) levels in the phospholipid surface of donor particles are increased. We propose here that the plateau of CETP activity occurs because the surface concentration of CE in the acceptor becomes rate limiting. This hypothesis was tested in CETP assays between synthetic liposomes whose CE content was varied independently. As donor CE increased, CETP activity followed saturable kinetics, but the slope of the first-order portion of the curve and the maximum achievable CE transfer rate were linearly related to the acceptor's surface CE concentration. These findings, plus studies with free cholesterol-modified LDL, strongly suggest that CE-rich donor liposomes can measure the CETP-accessible CE in acceptor lipoproteins. CETP activity from CE-rich liposomes to multiple control LDLs ranged 1.8-fold despite equivalent CETP binding capacity, suggesting that LDLs vary widely in their capacity to present CE to CETP. Thus, CETP activity depends on the surface availability of substrate lipids in the donor and acceptor. Donor liposomes with high CE content can be used to assess how subtle changes in composition alter the substrate potential of plasma lipoproteins.

Lipid transfer inhibitor protein defines the participation of lipoproteins in lipid transfer reactions: CETP has no preference for cholesteryl esters in HDL versus LDL

Cholesteryl ester transfer protein (CETP) catalyzes the net transfer of cholesteryl ester (CE) between lipoproteins in exchange for triglyceride (heteroexchange). It is generally held that CETP primarily associates with HDL and preferentially transfers lipids from this lipoprotein fraction. This is illustrated in normal plasma where HDL is the primary donor of the CE transferred to VLDL by CETP. However, in plasma deficient in lipid transfer inhibitor protein (LTIP) activity, HDL and LDL are equivalent donors of CE to VLDL (Arterioscler Thromb Vasc Biol. 1997;17:1716-1724). Thus, we have hypothesized that the preferential transfer of CE from HDL in normal plasma is a consequence of LTIP activity and not caused by a preferential CETP-HDL interaction. We have tested this hypothesis in lipid mass transfer assays with partially purified CETP and LTIP, and isolated lipoproteins. With a physiological mixture of lipoproteins, the preference ratio (PR, ratio of CE mass transferred from a lipoprotein to VLDL versus its CE content) for HDL and LDL in the presence of CETP alone was approximately 1 (ie, no preference). Fourfold variations in the LDL/HDL ratio or in the levels of HDL in the assay did not result in significant preferential transfer from any lipoprotein. On addition of LTIP, the PR for HDL was increased up to 2-fold and that for LDL decreased in a concentration-dependent manner. Under all conditions where LDL and HDL levels were varied, LTIP consistently resulted in a PR >1 for CE transfer from HDL. Short-term experiments with radiolabeled lipoproteins and either partially purified or homogenous CETP confirmed these observations and further demonstrated that CETP has a strong predilection to mediate homoexchange (bidirectional transfer of the same lipid) rather than heteroexchange (CE for TG); LTIP had no effect on the selection of CE or TG by CETP or its mechanism of action. We conclude, in contrast to current opinion, that CETP has no preference for CE in HDL versus LDL, suggesting that the previously reported stable binding of CETP to HDL does not result in selective transfer from this lipoprotein. These data suggest that LTIP is responsible for the preferential transfer of CE from HDL that occurs in plasma. CETP and LTIP cooperatively determine the extent of CETP-mediated remodeling of individual lipoprotein fractions.

ApoA-IHelsinki (Lys107-->0) associated with reduced HDL cholesterol and LpA-I:A-II deficiency

A Finnish kindred with premature coronary heart disease and decreased HDL cholesterol levels was identified as having an apoA-I variant, apoA-I (Lys107-->0), caused by a 3-bp deletion of nucleotides 1396 through 1398 in exon 4 of the apoA-I gene. These subjects (n = 10) were heterozygous for this mutation. The mean serum HDL cholesterol concentration (26.7 +/- 9.7 mg/dL) of affected family members was 36%, lower than that of unaffected family members (P < .05). Mean serum apoA-I and apoA-II concentrations in heterozygotes were reduced by 18% and 22%, respectively, compared with normal family members (P < .05). In heterozygotes the mean concentration of lipoprotein containing both apoA-I and apoA-II (LpA-I:A-II) was 31% lower than in those with normal apoA-I (P < .001), while the mean level of lipoproteins containing apoA-I without apoA-II was similar in the two groups. HDL density-gradient ultracentrifugation showed a lack of HDL2 and small dense HDL3 in heterozygotes compared with unaffected family members. The HDL particle size distribution, as analyzed by nondenaturing gradient gel electrophoresis of heterozygotes, revealed one major peak at 8.0 to 9.7 nm, a minor peak at 7.8 to 8.5 nm, and an absence of HDL2b and HDL2a peaks. These latter peaks were observed in unaffected family members. Serum levels of LDL cholesterol, triglycerides, VLDL, IDL, and LDL subclasses were similar in the two groups. However, in heterozygotes the cholesterol-to-triglyceride ratios in VLDL2, LDL1, LDL3, HDL2b, HDL2a, and HDL3a were 8% to 54% lower than in unaffected family members (P < .05). Cholesteryl ester transfer protein activity in heterozygotes was reduced by 25% compared with unaffected family members (P < .05), while the plasma lecithin:cholesterol acyltransferase (LCAT) activity did not differ between heterozygotes and unaffected family members. The ability of isolated variant apoA-I to serve as a cofactor for LCAT in vitro did not differ from that of normal apoA-I. Our data are consistent with the concept that a low HDL cholesterol level in subjects heterozygous for the apoA-IHelsinki mutation (Lys107-->0) having normal LCAT activity is a consequence of decreased concentration of LpA-I:A-II particles and of a smaller size and reduced cholesterol content of HDL particles.

Decreased affinity of low density lipoprotein (LDL) particles for LDL receptors in patients with cholesteryl ester transfer protein deficiency

We have reported that the disorder of lipoprotein metabolism in hyperalphalipoproteinaemic patients with a deficiency of cholesteryl ester transfer protein (CETP) is characterized by the polydisperse low density lipoprotein (LDL) particles and the accumulation of cholesteryl ester (CE) in high density lipoprotein (HDL) particles, forming cholesterol-induced HDL (HDLc)-like particles. In the present study we have investigated the interaction of these abnormal LDL with LDL receptors of normal human fibroblasts. Since the ultracentrifugally separated LDL fraction (1.019 < d < 1.063 g mL-1) from the CETP-deficient patients contained HDLc-like particles, these particles were removed by anti-apolipoprotein (apo) A-I immunoaffinity column chromatography. The lipoproteins eluted in the unbound fraction of this column did not contain apo A-I, so this fraction was considered to be authentic LDL. The authentic LDL of the patients were deficient in CE and rich in triglycerides and apo B. The authentic LDL itself showed polydispersity, ranging in size from 23 nm to 30 nm. The affinity of these abnormal LDL particles for LDL receptors was analysed by a competitive assay in which cold LDL from the patients or control compete with 125I-labelled LDL for fibroblast LDL receptors. The concentration of LDL particles at which 50% of 125I-labelled normal LDL was replaced was two to three times higher for the patients than for the normal control. Therefore, the affinity of patient LDL was thought to be reduced compared to that of control LDL. These results demonstrate that CETP may play an important role in making LDL particles homogeneous and rich in CE.(ABSTRACT TRUNCATED AT 250 WORDS)

Low density lipoprotein for cytotoxic drug targeting: improved activity of elliptinium derivative against B16 melanoma in mice

Significant low density lipoprotein (LDL) uptake by tumour cells led to the use of LDL as a discriminatory vehicle for the delivery of cytotoxic drugs. In the current study, the lipophilic elliptinium derivative, elliptinium-oleate (OL-NME), was incorporated into LDL to reach an incorporation level of 400 molecules per LDL particle. The OL-NME-LDL complex showed cytotoxic effects on normal human fibroblasts while the cytotoxicity was not observed on receptor-defective human fibroblasts, indicating the ability of the complex to be preferentially metabolised by the LDL receptor. In vivo metabolism of the complex was related to the LDL receptor pathway. The metabolic clearance was the same for native LDL (17.1 ml h-1) and OL-NME-LDL complex (16.2 ml h-1). LDL incorporated OL-NME enhanced the anti-tumour activity against murine B16 melanoma model; this resulted from increased efficacy for OL-NME-LDL at doses equal to free 9-OH-NME (157 vs 76 of Increase Life Span (ILS) (%) values after intraperitoneal (i.p.) drug injection on i.p. implanted tumour model and 45 vs -2 ILS (%) values after intravenous drug injection on subcutaneous implanted tumour model). These data suggest that LDL improves the potency of lipophilic cytotoxic drugs against tumours that express LDL receptor activity.

Modulation of the expression of human LDL-Apo B-100 epitopes by lipids and apolipoproteins

The purpose of the present study was to determine the immunochemical properties of apolipoprotein (apo) B-100 associated with low density lipoprotein (LDL) in relation to lipid and apolipoprotein composition. LDLs were isolated by sequential ultracentrifugation (1.019 < d < 1.050 g/mL) from two healthy volunteers and 21 dyslipidemic patients to obtain heterogeneous samples of LDL. Lipid (free cholesterol, cholesteryl esters, triglycerides, and phospholipids) and apolipoprotein contents (apo B, apo C-III, apo E) were determined in each LDL sample. Immunoreactivities of apo B were tested in solid-phase competitive-binding radioimmunoassays using seven monoclonal anti-LDL antibodies that reacted with defined epitopes of apo B-100. The relation between lipid and/or protein variables and the immunoreactivity of apo B was evaluated by successive use of Spearman's rank simple correlation, partial correlation, and canonical correlation analyses. The canonical correlation analysis showed that apo B-100 immunoreactivity on LDL is highly dependent on lipid and apolipoprotein composition simultaneously. The results confirmed the influence of surface and core lipids on the expression of the apo B-100 epitopes, independent of their location on the molecule. However, the lipid requirement of LDL strongly influences the expression of epitopes mapped in the LDL receptor-recognition domain. In contrast to apo E, apo C-III does not seem to influence the expression of the apo B-100 epitopes in the LDL range studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure. In patients with chronic renal failure, cardiovascular disease accounts for a significant proportion of all deaths. Several factors contribute to the "accelerated" atherosclerosis in this population, including hyperlipidemia, the pathogenesis of which is multifactorial. We investigated low-density lipoprotein (LDL) metabolism in a remnant model of chronic renal failure in the guinea pig. After one and two-thirds nephrectomy, creatinine clearance decreased to one-sixth normal. Plasma cholesterol and triglyceride (TG) levels increased with induction of renal failure. Analysis of lipoprotein composition disclosed significant TG enrichment of both uremic very-low-density lipoprotein (VLDL) and uremic LDL compared with control lipoproteins. Plasma clearance of homologous LDL was evaluated in turnover studies in control and uremic guinea pigs. To discriminate between differences in catabolism related to the uremic lipoprotein particle and to the uremic host milieu, a crossover protocol was used comparing the fractional catabolic rate (FCR) after simultaneous injection into control and uremic animals of 125I-control LDL and 131I-uremic LDL. The FCR of native LDL was slower in uremic animals than in controls. In addition, FCR of uremic LDL was significantly less than that of control LDL in both groups. Degradation studies in cultured fibroblasts indicated substantially reduced degradation of uremic LDL compared with control LDL. These results suggest dual abnormalities of LDL catabolism in renal failure that are not only related to alterations in clearance mechanisms in the uremic environment, but also suggest important functional differences in the LDL particle itself isolated from uremic animals.

The effect of low density lipoprotein composition on the regulation of cellular cholesterol synthesis: a comparison in diabetic and non-diabetic subjects

This study investigates compositional differences in low density lipoprotein (LDL) subfractions and their relationship to cellular cholesterol synthesis. We examined ten normocholesterolaemic (serum cholesterol < 6.5 mM) non-diabetic subjects (group 1) and compared them with ten normocholesterolaemic (group 2) and ten hypercholesterolaemic (group 3) (serum cholesterol > 6.5 mM) type 2 (non-insulin-dependent) diabetic patients. Serum cholesterol levels for groups 1, 2 and 3 were 5.19 +/- 0.27, 5.20 +/- 0.27 and 7.51 +/- 0.31 mM. LDL (density 1.006-1.028 g/l) and LDL2 (1.028-1.063 g/l) were isolated by density gradient ultracentrifugation. A significantly greater proportion of cholesterol was carried in LDL2 than LDL1 in all groups. There was a significantly lower cholesterol/protein ratio in LDL1 from the hypercholesterolaemic diabetic patients compared with controls. The LDL esterified/free cholesterol ratio was significantly greater in both LDL1 and LDL2 in the hypercholesterolaemic diabetic patients compared with the other two groups. There was a negative correlation between inhibition of cholesterol synthesis and the esterified/free cholesterol ratio of both LDL1 (r = 0.56, P < 0.002) and LDL2 (r = 0.63, P < 0.001). Cellular cholesterol of 41.0 +/- 0.3 microgram/mg cell protein in the hypercholesterolaemic diabetic patients was also significantly higher compared with values of 30.32 +/- 2.0 and 34.1 +/- 4.2 micrograms/mg cell protein for the normocholesterolaemic non-diabetic and diabetic groups. In vitro LDL esterification led to a decrease in LDL receptor-mediated binding and resulted in a 40% reduction in the ability of the LDL to suppress cholesterol synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary fatty acids, lipoproteins, and cardiovascular disease

Dietary fat quality and quantity significantly affect the metabolism of all the plasma lipoproteins and probably constitute the most significant dietary determinants of plasma lipoprotein levels. Since the major role of the plasma lipoproteins is the transport of exogenous and endogenous fat, this would be expected of a highly regulated, metabolically homeostatic system. The data clearly show that dietary fat saturation affects all aspects of lipoprotein metabolism, from synthesis to intravascular remodeling and exchanges to receptor-mediated and nonspecific catabolism. The experimental data regarding dietary fatty acid effects on lipoprotein metabolism are complicated and at times contradictory due to the large degree of metabolic heterogeneity in the population, which, when coupled with the known abnormalities of lipoprotein metabolism associated with certain types of hyperlipoproteinemia, can present responses from A to Z. It is clear that the same dietary pattern has different effects in different individuals and that complicating factors of individuality raise some concerns regarding generalized dietary recommendations. As new knowledge of the role of dietary factors and CVD risk develops, and our abilities to characterize the individual patient's response to dietary interventions become more refined, it may be possible to specify dietary fat intervention from a patient-oriented concept rather than a single all-purpose diet approach. Thus it would be possible to design dietary interventions to match patient needs and gain both efficacy and compliance. With the spectrum of approaches possible--low fat, moderate fat with MUFA, n-3 PUFA, etc.--we should be able to approach dietary interventions to reduce CVD risk at both a population-based level and a patient-specific level. There remains much to learn regarding the effects of dietary fatty acids on the synthesis, intravascular modifications, and eventual catabolism of the plasma lipoproteins. The area of lipoprotein metabolism in health and disease, of its modifications by diets and drugs, and of the contributions of genetic heterogeneity to these processes is one of notable advances over the past two decades and continues to be an area of intense investigation.

Differential low density lipoprotein hydrated density distribution in female and male patients with familial hypercholesterolemia

The low density lipoprotein (LDL) hydrated density distribution and composition was studied by using density gradient ultracentrifugation in 26 heterozygous familial hypercholesterolemia patients (FH) (13 females and 13 males) and 28 normolipidemic subjects (14 females and 14 males). The average peak hydrated density of LDL mass in female FH patients was 1.0301 g/ml as compared with 1.0333 g/ml in male FH patients (P less than 0.01) indicating less dense LDL particles in females. A similar difference in the average peak density was observed between normolipidemic females and males (1.0315 g/ml and 1.0342 g/ml, respectively; P less than 0.001). The FH males had a significantly lower mean triglyceride (Tg) content in their LDL (4.8%), lower Tg to apolipoprotein B (Apo B) weight ratio (0.24) and higher cholesteryl ester (CE) to triglyceride weight ratio (9.11) in comparison to FH females (Tg 6.2%; Tg/Apo B 0.31; CE/Tg 5.99), P less than 0.05 in all. Similar LDL composition differences were also observed between normolipidemic males and females.

Dietary fat and plasma lipid physical properties in swine

Changes in the lipid composition of swine lipoprotein were induced by feeding highly saturated or highly unsaturated diets. The diet higher in oleic acid, even when this was a highly unsaturated diet, resulted in a lower order parameter (more fluid) than in the other diets, as has been observed by others. This difference could be detected in whole unfractionated lipoprotein and in isolated high-density lipoprotein. Changes in order parameter with changes in cholesterol/triglyceride, reported by others, were not observed, possibly because of the weighted average effect of measuring whole plasma lipoprotein.

The effect of lovastatin treatment on low-density lipoprotein hydrated density distribution and composition in patients with intermittent claudication and primary hypercholesterolemia

The effects of lovastatin treatment on density distribution and composition of low-density lipoproteins (LDL) were studied using a density gradient ultracentrifugation method in 35 hypercholesterolemic patients with severe peripheral vascular disease. Lovastatin caused a 32% mean reduction in LDL particle mass and a 36% reduction in LDL cholesterol. The cholesteryl ester to apolipoprotein (apo) B, free cholesterol to apo B, and phospholipid to apo B weight ratios in LDL decreased significantly during treatment (P less than .01, P less than .01, and P less than .001, respectively). The effect on plasma triglycerides (Tg) was not uniform. Plasma Tg levels decreased in 25 patients, but increased in 10 patients. Since plasma Tg level influences the LDL density distribution and composition, the patients were also subgrouped and analyzed according to change in plasma Tg. In those with increased plasma Tg, the light LDL particles were reduced more and the dense particles less compared with patients with decreased Tg. The mean Tg content of LDL increased (from 7.7% to 9.3%; P less than .05) and the weight ratio of core lipids (cholesteryl ester/Tg) in LDL decreased (from 4.57 to 3.44; P less than .01) in patients with increased plasma Tg during treatment. The results indicate that the change in plasma Tg (decrease or increase) determined the qualitative changes in LDL observed during lovastatin treatment.

Effects of continuous insulin infusion therapy on lipoprotein surface and core lipid composition in insulin-dependent diabetes mellitus

To determine whether intensive insulin therapy has the same beneficial effects on lipoprotein composition that it has been shown to have in insulin-dependent diabetes mellitus (IDDM) on the routinely measured plasma lipids, we studied 10 patients after 6 months of conventional therapy (CIT) and again after 6 months of therapy with continuous subcutaneous insulin infusion (CSII). While the mean of home blood glucose levels (8.1 +/- 0.5 v 7.9 +/- 0.5 mmol/L) decreased no further, plasma triglycerides (TG) (CIT, 102.7 +/- 25.0; CSII, 89.6 +/- 27.1 mg/dL; P less than .001) decreased after CSII, and high-density lipoprotein cholesterol (HDL-C) increased significantly, primarily as a consequence of an increase in HDL2 (CIT, 12.2 +/- 6.0; CSII, 18.1 +/- 6.3 mg/dL; P less than .02). Low-density lipoprotein cholesterol (LDL-C) was unchanged (CIT, 82.2 +/- 32; CSII, 84.0 +/- 27.8 mg/dL). After CIT, two indices of lipoprotein surface composition were altered: (1) the free cholesterol (FC) to lecithin ratio, which is a new cardiovascular risk factor, was abnormally increased in plasma, very-low-density lipoprotein (VLDL) + LDL, and HDL, and (2) the sphingomyelin to lecithin ratio, an index of the surface rigidity of lipoproteins, was increased in the HDL subfractions. While CSII treatment resulted in favorable changes in whole plasma lipids, it failed to correct these disturbances in composition. Since the participation of lipoproteins in certain steps in reverse cholesterol transport appears to be impaired when their surface constituents are altered, persistence of these disturbances may sustain the increased cardiovascular risk of IDDM patients, even when their clinical control is very good and their plasma lipids are normal.

Low density lipoprotein density and composition in hypercholesterolaemic men treated with HMG CoA reductase inhibitors and gemfibrozil

The effects of HMG CoA reductase inhibitor (lovastatin or simvastatin) and gemfibrozil treatment on low density lipoprotein (LDL) density distribution and composition were studied in male patients with heterozygous familial hypercholesterolaemia (FH) (n = 17) or non-familial hypercholesterolaemia (non-FH) (n = 14). In FH patients the HMG CoA reductase inhibitors reduced 'light' LDL particles (density 1.022-1.033 g ml-1) significantly more than 'heavy' LDL (density 1.033-1.059 g ml-1), while a more uniform reduction of 'light' and 'heavy' LDL occurred in non-FH patients. HMG CoA reductase inhibitor treatment increased the apolipoprotein B (apoB) content and decreased the cholesterol/apoB ratio of LDL in FH patients. Gemfibrozil significantly reduced 'heavy' LDL but not the 'light' LDL fraction in both FH and non-FH patients, and the mean cholesteryl ester content of LDL increased, while the Tg content decreased, in both FH and non-FH patients. The results indicate that HMG CoA reductase inhibitor and gemfibrozil treatment have distinctly different effects on the physico-chemical properties of LDL, reflecting their different modes of action on lipoprotein metabolism.

Hypercholesterolaemia: simvastatin and pravastatin alter cholesterol metabolism by different mechanisms

The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor simvastatin, reduced low-density-lipoprotein (LDL) cholesterol in hypercholesterolaemic patients by 40% (P less than 0.001). The reduction in LDL cholesterol was accompanied by a significant decrease in the esterified/free cholesterol ratio of the patients' LDL from 2.51 +/- 0.13 to 2.06 +/- 0.14 (P less than 0.01). This change led to a significant increase (P less than 0.05) in the capacity of the LDL to suppress [14C]acetate incorporation into cholesterol in mononuclear leucocytes. Furthermore, [14C]acetate incorporation into the patients mononuclear leucocytes was significantly lower (P less than 0.02) following drug treatment (117 +/- 22 vs. 162 +/- 29 nmol/mg cell protein). Comparison of simvastatin with another HMG-CoA reductase inhibitor pravastatin, showed similar reduction in LDL cholesterol. Pravastatin treatment however, did not result in a reduction in the LDL esterified/free cholesterol ratio or in the changes in cellular cholesterol synthesis and its regulation by LDL which accompanied simvastatin treatment. The activity of the enzyme acyl-coenzyme A: cholesterol acyltransferase (ACAT) in patients' mononuclear cells remained unchanged after treatment with either drug. Results of the study show that while the drugs are equally effective in lowering LDL cholesterol, simvastatin has additional compositional effects on LDL which increase its capacity to regulate mononuclear leucocyte cholesterologenesis.

Accelerated cholesteryl ester transfer in patients with insulin-dependent diabetes mellitus

Abnormalities in cholesteryl ester transfer (CET) may play a role in the development of diabetic arterial vascular complications. To assess this important step systematically in reverse cholesterol transport, we have studied 20 treated, clinically stable, normolipidaemic patients. Contrary to the impairment in CET described previously in NIDDM, the mass of CE transferred from HDL to VLDL + LDL was significantly greater in IDDM patients than in controls at 1,2, and 4 h (P less than 0.001). When the d less than 1.063 plasma fractions from IDDM subjects were combined with controls d less than 1.063 fractions, an accelerated CET response was observed which was identical to that found in intact IDDM plasma. This finding, which indicates that this disturbance in CET was associated with the acceptor lipoproteins, was confirmed when we found that it was reproduced by the addition of IDDM VLDL and not LDL to control d greater than 1.063 fractions. Changes observed in lipoprotein core lipid composition were consistent with accelerated CET occurring in IDDM in vivo: the TG/CE core lipid ratio was decreased in VLDL from six subjects (diabetic 9.5 +/- 0.8 vs control 12.9 +/- 3.4; P less than 0.1) and increased in their HDL (diabetic 0.55 +/- 0.11 vs control 0.42 +/- 0.04; P less than 0.025). No correlation was demonstrable between estimates of diabetic control (glycoalbumin, fasting glucose) and CET. These data indicate that CET may be abnormally increased in normolipidaemic IDDM patients. A defect of this type may be atherogenic because it increases the number of lipoprotein particles in plasma which resemble cholesteryl ester-enriched chylomicron and VLDL remnants but whose normal receptor-mediated catabolism may be altered.

Improvement in the regulation of cellular cholesterologenesis in diabetes: the effect of reduction in serum cholesterol by simvastatin

Cellular cholesterol homeostasis was examined in 11 hypercholesterolaemic Type 2 diabetic patients prior to and following reduction of serum cholesterol using simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase. Following 12 weeks of treatment with simvastatin (10-40 mg day-1), serum cholesterol decreased by 30 +/- 3% from 7.8 +/- 0.2 mmol l-1 to 5.5 +/- 0.2 mmol l-1 (p less than 0.001) and LDL-cholesterol by 35 +/- 4% from 5.7 +/- 0.2 to 3.6 +/- 0.1 mmol l-1 (p less than 0.001). The esterified/free cholesterol ratio in LDL also decreased from 2.75 +/- 0.18 to 1.94 +/- 0.10 (p less than 0.01) after treatment. Cellular cholesterol synthesis, measured by [14C] acetate incorporation into mononuclear leucocytes, decreased by 39 +/- 11% from 231 +/- 13 to 140 +/- 25 mumol g-protein-1 (p less than 0.01). The degree of suppression of [14C]acetate incorporation into cholesterol in normal mononuclear cells by diabetic patients' LDL increased from 32.1 +/- 4.0% to 48.8 +/- 2.5% (p less than 0.001) following simvastatin. The activity of acyl coenzyme A:cholesterol-0-acyltransferase (ACAT) increased significantly by 55 +/- 18% (p less than 0.05) after treatment. Cholesterol synthesis in patients' mononuclear cells correlated positively (r = 0.66, p less than 0.05) with the esterified/free cholesterol ratio of their LDL, while suppression of cholesterol synthesis by patients' LDL correlated negatively (r = -0.64, p less than 0.05) with the esterified/free cholesterol ratio of the LDL following treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in metabolism of three low density lipoprotein subfractions in Hep G2 cells

The metabolism of low density lipoprotein (LDL) subfractions was investigated in the human hepatoma cell line Hep G2. By means of a density gradient ultracentrifugation method three LDL subfractions were isolated from pooled serum of normolipidemic subjects: very light LDL-1A, light LDL-1B and heavy LDL-2, differing in size, relative lipid and protein content. Cell specific association, stimulation of cholesterol esterification and inhibition of sterol synthesis were determined in parallel after incubation of Hep G2 cells with increasing amounts of LDL-protein of the three LDL subfractions. These parallel experiments were repeated four times with freshly prepared LDL subfractions. Response curves were parametrized with the function y = a square root of x, depicting the relation between the cellular metabolic event (y) and the LDL-protein (x) or LDL-cholesterol (x) levels. An analysis of covariance model was used to test differences between parameters of the three LDL subfractions. When the results of all four experiments were taken into account, the cell specific association increased more with increasing LDL-protein concentration for LDL-1A than for LDL-2 (P less than 0.05). At the LDL-protein level of 80 micrograms/ml the cell specific association for LDL-2 amounted to 85.5% of that for LDL-1A. Results for LDL-1B were intermediate between those for LDL-1A and LDL-2. The corresponding cholesteryl ester formation increased more with increasing LDL-protein concentration for LDL-1A than for LDL-1B (P less than 0.001), and for LDL-1B more than for LDL-2 (P less than 0.001). At the LDL-protein level of 70 micrograms/ml the cholesterol ester accumulation for LDL-2 and LDL-1B was 48.4% and 70.3%, respectively, of that for LDL-1A. These differences between LDL subfractions in cholesteryl esterification were independent of the cholesterol content of the subfractions. Consistent with these findings, the [14C]acetate incorporation to sterols decreased more with increasing LDL-protein concentration for LDL-1A and LDL-1B than for LDL-2 (P less than 0.05). At the LDL-protein concentration of 70 micrograms/ml the decrease in sterol synthesis after incubation with LDL-2 and LDL-1B was 79.2% and 98.0%, respectively, relative to that for LDL-1A. When adjusted for differences in cholesterol content of the LDL subfractions these differences with regard to the 14C-acetate incorporation were not significant. The metabolic differences between LDL subfractions in vitro may have implications for the metabolism and atherogenic potential of the distinct LDL subfractions in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative study of the incorporation of ellipticine-esters into low density lipoprotein (LDL) and selective cell uptake of drug--LDL complex via the LDL receptor pathway in vitro

Esters of elliptinium with stearic (ST-NME), palmitic (PAL-NME) or oleic (OL-NME) acids, a series of lipophilic derivatives of ellipticine, were synthetized, in order to evaluate their incorporation into Low Density Lipoprotein (LDL). Among the three derivatives, OL-NME shows the most potent incorporation (83 micrograms/mg protein LDL) compared to ST-NME (37 micrograms/mg protein LDL) and PAL-NME (58 micrograms/mg protein LDL). The size of OL-NME-LDL was determined by size distribution particles, showing their homogeneity compared to native LDL. When culture normal human fibroblasts were incubated with [125I]LDL incorporated drug, they bound to the LDL receptor with the same affinity as native LDL and were internalized and degraded intracellularly. The presence of excess native LDL inhibited the cellular uptake and degradation of [125I]drug-LDL. We have used [125I]acetyl-LDL as a probe for a binding site on macrophages that mediated the uptake and degradation of chemically altered or denatured LDL. Mouse peritoneal macrophages were shown to take up and degrade [125I]acetyl-LDL at rates that were greater than those for the uptake and degradation of native [125I]LDL and [125I]drug-LDL. The in vitro cytotoxic test on L1210 murine leukemic cells demonstrated that the complex was cytotoxic and was more effective than the free drug. This cytotoxic activity of the drug-LDL complex depends on the LDL high affinity receptor since the addition of native LDL reduces the killing power. In contrast, methylated LDL, which does not bind to the LDL receptor, has no effect on it. We conclude that it is possible to incorporate a large amount of cytotoxic drug into LDL without modifying their cellular metabolism via the high affinity LDL receptor pathway. It indicates also that the delivery of lipophilic drugs using LDL might provide distinct advantages over the use of synthetic carriers.

Cellular cholesterol regulation--a defect in the type 2 (non-insulin-dependent) diabetic patient in poor metabolic control

This study investigates the relationship between Type 2 (non-insulin-dependent) diabetes mellitus and hypercholesterolaemia with regard to delivery of cholesterol to cells and regulation of endogenous cholesterol synthesis. The ability of LDL, from hypercholesterolaemic and Type 2 diabetic patients, to suppress cellular cholesterologenesis and to enhance mitogen-stimulated lymphocyte proliferation was compared. Cholesterol synthesis was estimated by measuring [14C]-acetate incorporation into cholesterol and lymphocyte proliferation was assessed by [3H]-thymidine incorporation into mitogen-stimulated normal lymphocytes. The results indicate that LDL from both Type 2 diabetic patients in poor metabolic control and hypercholesterolaemic patients was significantly less effective (p less than 0.001) than LDL from non-diabetic normocholesterolaemic subjects in suppressing cholesterol synthesis in lymphocytes. LDL from all hypercholesterolaemic patients enhanced lymphocyte proliferation to a greater extent than LDL from normocholesterolaemic subjects and this effect was significantly increased using LDL from Type 2 diabetic, hypercholesterolaemic patients. Both suppression of [14C]-acetate incorporation and enhancement of [3H]-thymidine uptake could be related to an increased esterified/free cholesterol ratio in the LDL particle. The fact that cholesterol synthesis and cell proliferation were markedly altered by the above changes in LDL composition suggests a mechanism for cellular cholesterol accumulation in the Type 2 diabetic patient, even in the absence of elevated serum cholesterol levels.

Receptor-mediated binding and degradation of subfractions of human plasma low-density lipoprotein by cultured fibroblasts

The receptor-mediated metabolism of human plasma low-density lipoprotein (LDL) subfractions was studied. LDL was isolated from healthy donors and further fractionated by density gradient ultracentrifugation into three subfractions: (I) d = 1.031-1.037, (II) d = 1.037-1.041 and (III) d = 1.041-1.047 g/ml, comprising 24 +/- 7%, 46 +/- 8% and 30 +/- 9% of the total LDL protein, respectively. As assessed by electron microscopy and gradient gel electrophoresis, the LDL particle size decreased and the relative protein content increased from fraction I towards fraction III. Fraction II had the highest (Kd 2.6 micrograms/ml) and fraction I the lowest (Kd 5.8 micrograms/ml) binding affinity to LDL receptors of human fibroblasts at 4 degrees C. The rate of receptor-mediated degradation of fraction II was also higher than that of the other two fractions at 37 degrees C. These results suggest that LDL subfractions have different rates of receptor-mediated catabolism depending on particle size or composition, and therefore their metabolic fate and atherogenic properties may also differ.

Neutral lipid transfer activities in the plasma of patients with abetalipoproteinemia

Plasma lipid transfer proteins stimulate transfer and molecular exchange of cholesteryl esters, phospholipids and triglycerides between individual plasma lipoproteins. To assess whether transfer protein activities are influenced by the inherent absence of apo B-containing lipoproteins, we determined cholesteryl ester and triglyceride transfer activities in the plasma of patients with abetalipoproteinemia (ABL). Transfer activities were measured in plasma fractions of d greater than 1.21 g/ml in 2 patients with abetalipoproteinemia and 12 normal volunteers and were expressed as a percent transfer of labeled lipid from donor high density lipoproteins to acceptor very low density lipoproteins. Cholesteryl ester and triglyceride transfer activities were reduced respectively by 50% and 66% in the plasma of patients with ABL. The addition of the plasma fraction d greater than 1.21 g/ml proteins from abetalipoproteinemic subjects resulted in progressive decreases in cholesteryl ester and triglyceride transfer activities. The reduced activities of these transfer proteins may reflect (at least in part) the presence of an inhibitor(s) which is heat-stable and trypsin-sensitive.

Stimulation of cholesteryl ester synthesis in human monocyte-derived macrophages by low-density lipoproteins from type 1 (insulin-dependent) diabetic patients: the influence of non-enzymatic glycosylation of low-density lipoproteins

Diabetes mellitus is an independent risk factor in the development of atherosclerosis. In this study we aimed to demonstrate whether there is an abnormal interaction between low-density lipoproteins from diabetic patients and human macrophages. We measured cholesteryl ester synthesis and cholesteryl ester accumulation in human monocyte-derived macrophages (obtained from non-diabetic donors) incubated with low density lipoproteins from Type 1 (insulin-dependent) diabetic patients in good or fair glycaemic control. Low density lipoproteins from the diabetic patients stimulated more cholesteryl ester synthesis than low density lipoproteins from non-diabetic control subjects (7.19 +/- 1.19 vs 6.11 +/- 0.94 nmol/mg cell protein/20 h, mean +/- SEM, p less than 0.05). The stimulation of cholesteryl ester synthesis by low density lipoproteins isolated from diabetic patients was paralleled by a significant increase in intracellular cholesteryl ester accumulation (p less than 0.02). There were no significant differences in the lipid composition of low density lipoproteins between the diabetic and control groups. Non-enzymatic glycosylation of low density lipoproteins was higher in the diabetic group (p less than 0.01) and correlated significantly with cholesteryl ester synthesis (r = 0.58). Similarly, low-density lipoproteins obtained from non-diabetic subjects and glycosylated in vitro stimulated more cholesteryl ester synthesis in macrophages than control low density lipoproteins. The increase in cholesteryl ester synthesis and accumulation by cells exposed to low density lipoproteins from diabetic patients seems to be mediated by an increased uptake of these lipoproteins by macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]cholesterol transfer from microemulsion particles of different sizes to human fibroblasts

A new technique for preparing microemulsion particles of well-defined sizes and compositions is presented. Utilization of these microemulsions is advocated as lipoprotein models in studies of lipid transport and metabolism, rather than the currently used phospholipid-cholesterol vesicles. The emulsion particles consisted of egg phosphatidylcholine and cholesterol as surface lipids and cholesteryl oleate as core lipid. They were prepared by a combined injection and sonication technique and size-separated by a two-step procedure of gel filtration chromatography and density gradient centrifugation. By varying the ratios of core and surface material, particles covering a size range of 20-200 nm in diameter could be produced. The adequacy of these microemulsions as lipoprotein models was tested by studying the transfer of [3H]cholesterol and [14C]cholesterol oleate from the particles to cultured human fibroblasts. Up to a particle size of 100 nm, there was a slight increase of [3H]cholesterol transfer. The transfer of [14C]cholesteryl oleate was very slow, yet measurable. Studies of the exchangeability of cholesterol between the microemulsion core and surface phases indicated that all cholesterol can be transferred from microemulsions to cultured cells as a single pool.

Evidence for heterogeneity of low-density lipoprotein metabolism in the cynomolgus monkey

Preliminary studies were performed to establish whether there was kinetic heterogeneity in the metabolism of subclasses of low-density lipoproteins (LDL) in the cynomolgus monkey. Previous studies of the effects of inhibition of hepatic triglyceride lipase in this species had shown an increase in the mass of lighter LDL (Sf greater than 9) and a decrease in the mass of denser LDL. LDL (1.019 less than d less than 1.063) were subdivided into two subfractions LDL1 (1.019 less than d less than 1.035) and LDL2 (1.035 less than d less than 1.063) by ultracentrifugation. The lipoproteins in these two fractions could be shown to have different flotation by analytic and isopycnic ultracentrifugation. When tracer amounts of homologous 125I-labeled very-low-density lipoproteins (VLDL) were injected into chow-fed cynomolgus monkeys, apoB radioactivity appeared in LDL1 prior to its appearance in LDL2. [125I]LDL1 injected into the monkey was removed from the LDL1 density subclass with a half-life of 5.5-10.3 h. Much of the radioactivity injected as LDL1 was converted to denser LDL (LDL2). Labeled LDL2 injected into the monkey was not converted to LDL1. Thus, at least two kinetically distinct subpopulations of LDL circulate in the plasma of this species. The lighter LDL is to a large extent a metabolic precursor of the more dense LDL (LDL2).

Scavenger activity in monocyte-derived macrophages from atherothrombotic strokes

Foam cells are lipid-laden macrophages derived primarily from circulating mononuclear cells and are a characteristic feature of atheromatous lesions. The exact role of these foam cells in the pathogenesis of atherosclerotic lesions remains uncertain, but one potential function is to take-up and process excess interstitial arterial lipoproteins, suggested by their extraordinary ability to engulf enormous quantities of modified low density lipoproteins by the so-called "scavenging pathway." To test this possibility, monocytes from 15 atherothrombotic brain infarct patients and age and sex matched controls were isolated and cultured for 7-8 days in 20% normal serum. The monocyte-derived macrophages were investigated for their ability to bind, internalize and degrade both native and modified (acetylated) LDL labelled with 125Iodine. While native LDL was metabolized similarly, stroke macrophages displayed significantly reduced ability to scavenge modified LDL. These findings suggest that insufficient processing of interstitial arterial cholesterol by monocyte-derived macrophages may contribute to the aggravation of atheroma formation. This inadequacy is likely further compromised by reduced levels of serum high density lipoprotein since the absence of a cholesterol-acceptor will promote the slow but continued accumulation of lipids and the formation of foam cells.

Lipoprotein modifications during dietary treatment in patients with primary type V hyperlipoproteinaemia

Type V hyperlipoproteinaemia is a disorder of lipid transport characterized by the accumulation in serum of chylomicrons and very low density lipoproteins. The purpose of the study was the analysis of serum lipids and lipoproteins by ultracentrifugation in nine patients with primary type V hyperlipoproteinaemia before and during dietary treatment. After 30 days of balanced isocaloric diet mean serum triglycerides fell from 25.4 +/- 15.0 (mean +/- SD) to 2.8 +/- 1.7 mmol l-1. At the same time the chylomicrons and the very low density lipoproteins of flotation rate higher than 100 disappeared from the serum while the remaining very low density lipoproteins maintained unaltered their normal protein-lipid composition. After 30 days the low density lipoproteins increased significantly in concentration (from 1.6 +/- 0.8 to 4.1 +/- 1.1 mmol l-1 cholesterol) and their percentage content of cholesterol and triglyceride was increased and reduced, respectively. The highest concentration of intermediate density lipoprotein cholesterol was observed after 15 days of treatment (1.2 +/- 0.6 mmol l-1. The abnormally low concentrations and the physicochemical properties of the high density lipoproteins remained unchanged throughout the study (from 0.6 +/- 0.2 to 0.8 +/- 0.2 mmol l-1 cholesterol concentration) and no high density lipoproteins two (HDL2) were observed at any time. The effects of this treatment were an increase in low density and marginal change in high density lipoproteins which are considered, respectively, a positive and a negative risk factor for atherosclerosis.

Defective metabolism of hypertriglyceridemic low density lipoprotein in cultured human skin fibroblasts. Normalization with bezafibrate therapy

The metabolism of hypertriglyceridemic low density lipoprotein (HTG-LDL) was investigated in upregulated cultured human skin fibroblasts. Low density lipoprotein (LDL) was isolated by zonal centrifugation from the plasma of seven HTG subjects, before and 2 wk after the initiation of bezafibrate (BZ) therapy. HTG-LDL is a cholesterol-poor, triglyceride-rich lipoprotein of smaller diameter than BZ-LDL or normal LDL (N-LDL). Binding, cell association, and proteolytic degradation of HTG-LDL were compared with that of BZ-LDL and N-LDL and were found to be significantly lower by a paired t test analysis (P less than 0.001). After 6 h preincubation with unlabeled HTG-LDL, the incorporation of [14C]acetate to sterols was significantly higher than with BZ-LDL or N-LDL (577 +/- 43.7; 330 +/- 41.5; 262 +/- 47, mean +/- SE, picomoles sterols per milligram cell protein per 2 h, respectively; P less than 0.001 by paired t test). To determine the effectiveness of HTG-LDL and BZ-LDL on the down-regulation of LDL receptor activity, up-regulated cells were incubated for 48 h with HTG-LDL and BZ-LDL. LDL receptor activity was significantly higher after preincubation with HTG-LDL compared with BZ-LDL, and the rates of sterol synthesis were similarly increased. These results demonstrate that HTG-LDL does not down-regulate the LDL receptor activity as efficiently as BZ-LDL and that its cholesterol content is not enough to adequately suppress cellular sterol synthesis. Significant correlation between LDL composition and cholesterol synthesis by cultured cells was found with all LDL preparations over a wide range of cholesteryl ester to protein ratio (0.8-2.2). This correlation indicates that the compositional and structural abnormalities of HTG-LDL, and especially the low cholesterol content of the lipoprotein, alter LDL metabolism and cellular cholesterol formation.