Low-density Lipoprotein Subclasses Research Articles

The NHLBI Twin Study: heritability of apolipoprotein A-I, B, and low density lipoprotein subclasses and concordance for lipoprotein(a)

Heritability of plasma apolipoprotein (apo) A-I, apo B, and low density lipoprotein (LDL) subclasses and concordance for lipoprotein(a) excess were assessed in 109 monozygotic (MZ) and 113 dizygotic (DZ) twin pairs participating in the third examination of the National Heart, Lung, and Blood Institute Twin Study. The intraclass correlation coefficient for apo A-I was significantly greater in MZ twins (0.56) than in DZ twins (0.37, P less than 0.05); however, apo A-I showed an unequal distribution in the two groups, with significantly greater total variance in DZ twins. Therefore the among-component estimate of genetic variance was applied, and the results indicated no significant heritability for apo A-I (P = 0.59). MZ and DZ twins had equal apo B variance. The intraclass correlation coefficient for apo B in MZ twins (0.71) was significantly higher than in DZ twins (0.25) (P less than 0.0001), indicating significant heritability for apo B. Plasma apo A-I levels were significantly correlated with alcohol intake (P less than 0.0001), body mass index (BMI, P less than 0.0001), and physical activity, while apo B levels were significantly correlated only with BMI (P less than 0.05). After plasma apo A-I and apo B concentrations were adjusted for all of these variables and for cigarette smoking, the analysis of variance and intraclass correlation coefficients remained virtually unchanged. The LDL type intraclass correlation coefficient was higher in MZ twins (0.58) than in DZ twins (0.32, P less than 0.005); however, greater total variance for this parameter in DZ twins was observed and after applying the among component estimate of genetic variance, no significant heritability of LDL type was observed. After adjustment for covariate effects the conclusions were not changed. Only 8.4% of MZ twin pairs, as compared with 26.7% of DZ twin pairs, were discordant for elevated lipoprotein(a) on gradient gels (P less than 0.0001). Our data indicate that there is a strong heritability for plasma apo B and lipoprotein(a), with only weak evidence for heritability of LDL type or plasma apo A-I levels within this population sample.

Low-density lipoprotein subclasses and risk of coronary artery disease

A heritable low-density lipoprotein (LDL) subclass profile associated with increased risk of coronary disease in humans (atherogenic lipoprotein phenotype) is characterized by predominance of small, dense, cholesterol depleted LDL particles, increased plasma levels of very-low- and intermediate- density lipoproteins and apolipoprotein B, and reduced concentrations of the high-density lipoprotein2 subclass.

The effect of exercise on plasma lipids and LDL subclass metabolism in miniature swine

The effects of exercise on plasma lipids and lipoproteins, high density lipoprotein (HDL) subclass cholesterol levels, and low density lipoprotein (LDL) subclass composition and metabolism were studied in Yucatan miniature swine following 2 yr of training. The exercise protocol produced significant training effects. Post-heparin lipolytic activity was also significantly increased. Although plasma cholesterol and triglycerides did not differ significantly (P = 0.08) between the exercised and control groups, multivariate analysis indicated a strong association between lipoprotein lipase (LPL) and HDL2-C (P less than 0.0001). Although HDL-C levels rose only slightly (P less than 0.09) with exercise, a significant shift was noted in the distribution of cholesterol from the HDL3 to the HDL2 fractions, perhaps mediated by the substantial increase in LPL activity. Exercise had little effect on the chemical composition of the major lipoprotein classes; however, the triglyceride content of the lighter LDL1 subclass was significantly reduced. In the more dense LDL2 subclass, exercise resulted in a significant decrease in triglycerides concomitant with a significant increase in free cholesterol levels. In contrast with the small reductions in fractional catabolic rates (FCR) in either subclass, production rates of the exercised group were reduced, which accounted for the reduction in LDL subclass pool size. These data indicate that exercise produces subtle but significant changes in lipoprotein metabolism that have been previously associated with reduced risk of atherosclerosis.

NIDDM患者におけるLDL亜分画pattern B (small, dense LDL)の検討

Low density lipoprotein (LDL) in plasma has been known to fractionate into subgroups, and two distinct LDL subclasses identified using gradient polyacrylamide gel electrophoresis were named pattern A and B by Austin and Krauss in 1986. They demonstrated that individuals with pattern B were characterized by a preponderance of small and dense LDL particles which significantly increased the risk of myocardial infarction. It has also been proposed that these LDL subclass patterns are influenced by a genetic trait.In this study, LDL patterns of 131 patients with non-insulin dependent diabetes mellitus (NIDDM) and 71 healthy controls were determined by gradient gel electrophoresis analyses. The results showed that the frequency of pattern B in the patients with NIDDM was greater (38.9%) than that in the controls (7.0%). Mean particle diameters for NIDDM patients with pattern A were 26.56±0.46nm and those with pattern B, 25.19±0.40nm. The interrelation between LDL subclass patterns and the clinical status showed that patients with pattern B had a higher body mass index (BMI) than those with pattern A. However, the mode of treatment and levels of blood glucose control were not different for the two groups of LDL subclass patterns. As far as plasma lipid and lipoprotein levels are concerned, total cholesterol, triglyceride and apolipoprotein B levels were significantly higher, but HDL-C and HDL2-C levels were significantly lower in patients with pattern B than those with pattern A. The study in the selected group of diabetic patients with normal triglyceridemia (<150mg/dl) also showed similar results.The majority of the diabetic patients with pattern B showed no change in their LDL pattern, though their blood glucose had been controlled and triglyceride levels had decreased.The families of 6 diabetic patients with pattern B were studied. As a result, a parent or a child in two families showed pattern B in their LDL. These findings suggest that LDL subclass pattern B is controlled by a genetic trait.

Lack of evidence for linkage between low‐density lipoprotein subclass phenotypes and the apolipoprotein B locus in familial combined hyperlipidemia

Low-density lipoprotein (LDL) subclass phenotype B, characterized by a predominance of small, dense LDL particles, appears to be a genetically influenced risk factor for coronary heart disease. Phenotype B, as determined by gradient gel electrophoresis, appears to be inherited in a manner consistent with the presence of a single major genetic locus, based on complex segregation analysis. Familial combined hyperlipidemia (FCHL) is a disorder characterized by elevations in total plasma cholesterol and/or triglyceride levels in probands and family members, variable lipoprotein phenotypes over time, and elevations in apolipoprotein B levels. Because apo B is the primary protein component of LDL particles, the present study was undertaken to determine whether LDL subclass phenotypes are controlled by the APOB locus in FCHL families. The evidence against linkage was very strong based on lod score analyses (total lod = -13.3), under assumptions that LDL subclass phenotypes are influenced by a major genetic locus and that the mode of inheritance and penetrance functions are known. Other methods requiring fewer assumptions also provided evidence against linkage, although the strength of this evidence was weaker. Thus the results demonstrate that the proposed gene responsible for LDL subclass phenotypes is unlikely to be the APOB gene in families with FCHL.

Familial cholesteryl ester transfer protein deficiency is associated with triglyceride-rich low density lipoproteins containing cholesteryl esters of probable intracellular origin.

The net transfer of core lipids between lipoproteins is facilitated by cholesteryl ester transfer protein (CETP). We have recently documented CETP deficiency in a family with hyperalphalipoproteinemia, due to a CETP gene splicing defect. The purpose of the present study was to characterize the plasma lipoproteins within the low density lipoprotein (LDL) density range and also the cholesteryl ester fatty acid distribution amongst lipoproteins in CETP-deficient subjects. In CETP deficiency, the conventional LDL density range contained both an apoE-rich enlarged high density lipoprotein (HDL) (resembling HDLc), and also apoB-containing lipoproteins. Native gradient gel electrophoresis revealed clear speciation of LDL subclasses, including a distinct population larger in size than normal LDL. Anti-apoB affinity-purified LDL from the CETP-deficient subjects were shown to contain an elevated triglyceride to cholesteryl ester ratio, and also a high ratio of cholesteryl oleate to cholesteryl linoleate, compared to their own HDL or to LDL from normal subjects. Addition of purified CETP to CETP-deficient plasma results in equilibration of very low density lipoprotein (VLDL) cholesteryl esters with those of HDL. These data suggest that, in CETP-deficient humans, the cholesteryl esters of VLDL and its catabolic product, LDL, originate predominantly from intracellular acyl-CoA:cholesterol acyltransferase (ACAT). The CETP plays a role in the normal formation of LDL, removing triglyceride and transferring LCAT-derived cholesteryl esters into LDL precursors.

Linkage analysis of low‐density lipoprotein subclass phenotypes and the apolipoprotein B gene

A common heritable phenotype has recently been identified which is characterized by a relative abundance of small, dense low-density lipoproteins (LDL), and mild elevations of plasma triglycerides and reductions in plasma high-density lipoproteins (HDL) cholesterol. This phenotype, designated LDL subclass phenotype B, has been associated with up to a three-fold increase in coronary disease risk. Complex segregation analysis in two large family studies has demonstrated that LDL subclass phenotype B is influenced by an allele at a single genetic locus with a population frequency of 0.25-0.3, and autosomal dominant inheritance, but with full penetrance only in males age 20 and over and in postmenopausal women. Since apolipoprotein B (apoB) is the principal protein component of LDL, linkage analysis was used to investigate possible linkage between the phenotype B phenotype and the apoB gene, using a variable number of tandem repeats site located 0.5 kb from the 3' end of the apoB gene. In 6 informative families including only family members in the penetrant classes, a total LOD score of -7.49 was found at a recombination fraction of 0.001. Thus, under the assumptions of the single gene model, it is unlikely that the apoB locus controls LDL subclass phenotype B.

Differences in carbohydrate content of low density lipoproteins associated with low density lipoprotein subclass patterns

The neutral carbohydrate content of both the protein (apoB) and lipid fractions of low density lipoproteins (LDL) from subjects with a predominance of small, dense LDL (subclass pattern B) was found to be lower than in subjects with larger LDL (subclass pattern A): 45 +/- 12 versus 64 +/- 13 mg/g apoLDL, and 58 +/- 8 versus 71 +/- 8 mg/g apoLDL (P less than 0.0005 for both). Sialic acid content of LDL lipids, but not apoB, was also reduced in subclass pattern B. ApoB and glycolipid carbohydrate content of total LDL and LDL density subfractions declined with increasing LDL density and decreasing particle diameter. Moreover, in LDL subfractions from pattern B subjects, carbohydrate content of LDL apoB, but not LDL glycolipid, was significantly lower in comparison with particles of similar size from pattern A subjects. Thus, in LDL subclass pattern B, reductions in LDL carbohydrate content are associated both with reduced concentrations of larger carbohydrate-enriched LDL subclasses, and with reduced glycosylation of apoB in all LDL particles. LDL glycolipids may vary with overall lipid content of LDL particles, but variation in apoB glycosylation may indicate differences in pathways for LDL production, and reduced apoB glycosylation may reflect the altered metabolic state responsible for LDL subclass pattern B.

Differential uptake of proteoglycan-selected subfractions of low density lipoprotein by human macrophages.

Macrophages and arterial chondroitin sulfate proteoglycans (CSPG) are probably associated with extracellular and intracellular lipoprotein deposition during atherogenesis. We found that human arterial CSPG can be used to select subclasses from low density lipoprotein (LDL) with different structural properties and capacities to interact with human monocyte-derived macrophages (HMDM). Four subclasses, LDL(PG)1 to LDL(PG)4, in order of decreasing CSPG-complexing capacity, were prepared and characterized in terms of their ability to interact with HMDM. The LDL subclasses with highest avidity for CSPG, LDL(PG)1 and LDL(PG)2, were bound, internalized, and degraded more efficiently than those of lower avidity for CSPG. From LDL(PG)1 to LDL(PG)4, the gradual decrease in uptake by HMDM and decreasing avidity for CSPG were associated with a gradual decrease in isoelectric point (from 5.93 to 5.68) and an augmented ratio of surface polar lipid to core nonpolar components (from 0.35 to 0.54). Competition experiments indicated that the proteoglycan-selected subfractions shared the binding sites and uptake mechanisms of native LDL. The results suggest the existence of a structurally related gradation in the avidity of LDL subpopulations for cells and matrix components. The presence within LDL subpopulations of a differential capacity to interact with intimal extracellular and cellular elements could be associated with a similar heterogeneity in their atherogenic potential.

Atherogenic lipoprotein phenotype. A proposed genetic marker for coronary heart disease risk.

In a community-based study of 301 subjects from 61 nuclear families, two distinct phenotypes (denoted A and B) were identified by nondenaturing gradient gel electrophoretic analysis of low density lipoprotein (LDL) subclasses. Phenotype A was characterized by predominance of large, buoyant LDL particles, and phenotype B consisted of a major peak of small, dense LDL particles. Previous analysis of the family data by complex segregation analysis demonstrated that these phenotypes appear to be inherited as a single-gene trait. In the present study, the phenotypes were found to be closely associated with variations in plasma levels of other lipid, lipoprotein, and apolipoprotein measurements. Specifically, phenotype B was associated with increases in plasma levels of triglyceride and apolipoprotein B, with mass of very low and intermediate density lipoproteins, and with decreases in high density lipoprotein (HDL) cholesterol, HDL2 mass, and plasma levels of apolipoprotein A-I. Thus, the proposed genetic locus responsible for LDL subclass phenotypes also results in an atherogenic lipoprotein phenotype.

Inheritance of low density lipoprotein subclass patterns in familial combined hyperlipidemia.

The inheritance of low density lipoprotein (LDL) subclass patterns was investigated in 234 members of seven large kindreds with familial combined hyperlipidemia (FCHL), a disorder characterized by elevated LDL cholesterol and/or triglyceride and increased coronary disease risk in families. Analysis of LDL subclasses by nondenaturing gradient gel electrophoresis showed a predominance of large, buoyant LDL particles (pattern A) in 71% of the family members and a predominance of small, dense LDL particles (pattern B) in 29% of family members. Based on complex segregation analysis, pattern B appeared to be inherited as an autosomal trait with either a dominant or an additive mode of inheritance and a small, but significant, multifactorial inheritance component. The proposed allele for pattern B was common (frequency = 0.3), and reduced penetrance was observed among men under age 20 and among women under age 50. These results in these FCHL families are consistent with those from a previously reported population-based sample of families, in which pattern B showed an apparent dominant mode of inheritance. In that study, reduced penetrance was observed for men under age 20 and for premenopausal women, but a somewhat lower allele frequency was found for pattern B (0.25). In the FCHL family members, LDL subclass pattern B was associated with significantly increased plasma levels of apolipoprotein B and triglyceride and decreased high density lipoprotein cholesterol. In comparison with a group of controls, the FCHL family members with pattern A had similar mean triglyceride levels, but higher mean apolipoprotein B. Thus, in families with FCHL, a predominance of small, dense LDL particles appears to be inherited as a common, single-gene trait, which is closely associated with the higher plasma triglyceride levels found in these families. The increased plasma apolipoprotein B levels found in FCHL cannot, however, be accounted for by this proposed locus.

Regulation of low density lipoprotein apoprotein B metabolism by lovastatin and cholestyramine in miniature pigs: Effects on LDL composition and synthesis of LDL subfractions

A major factor in the regulation of low density lipoprotein (LDL) apoprotein B (apo B) concentrations in miniature pigs is the direct synthesis of LDL apo B. LDL apo B derived from plasma very low density lipoproteins (VLDL) accounts for only 20% to 30% of total LDL synthesis. Treatment with lovastatin and cholestyramine can inhibit the direct synthesis pathway in this species, thereby lowering LDL apo B concentrations. The present study was carried out to determine if lovastatin alone was as effective as in combination with cholestyramine. The possibility that the direct synthesis pathway was confined to a specific subclass of LDL and the effect of lovastatin and cholestyramine on the metabolism of LDL subfractions were also investigated. Homologous 126I-VLDL and 131I-LDL were injected into miniature pigs during a control period and again following 18 days of treatment with lovastatin (1.2 mg/kg/d, n = 4) or in combination with cholestyramine (1.0 g/kg/d, n = 4). Kinetic analysis of apo B specific activity curves following lovastatin treatment indicated that LDL apo B pool size was decreased by 25% ( P < .025), which was due entirely to a 70% ( P < .025) decrease in the direct synthesis of LDL apo B, since VLDL-derived apo B, and LDL fractional catabolic rate (FCR) were not affected. Parameters of VLDL apo B metabolism were not affected. Lovastatin in combination with cholestyramine was more effective than lovastatin alone. LDL apo B pool size was reduced by 50% ( P < .05) due entirely to a 85% ( P < .025) reduction in the direct synthesis of LDL apo B. LDL apo B FCR was not affected. Combined treatment did not change VLDL apo B pool size, but increased the FCR by 74% ( P < .025) and increased total VLDL apo B flux by 50% ( P < .025), which was reflected primarily in an increased direct removal of VLDL. The direct synthesis of LDL apo B entered the circulation in the LDL 2 (density [d] 1.040 to 1.063 g/mL) fraction entirely, since LDL 1 (d 1.019 to 1.040 g/mL), which comprised approximately 10% of plasma LDL apo B, was completely derived from VLDL catabolism. Only the synthesis of LDL 2 was inhibited (by 60%) by treatment with both drugs. To determine if changes in lipid composition could explain the lack of effect of LDL apo B FCR, the lipid composition of LDL from animals treated with both drugs was analyzed by total lipid profiling. Small increases in the lipid surface to core ratio (0.8 ± 0.03 v 0.7 ± 0.02, P < .025) and decreases in the estimated particle diameter ( 184 ± 5 v 202 ± 6 A ̊ , P < .025 ) were observed. These results indicate that the direct synthesis of LDL 2 apo B can be regulated in this species and that a combination of lovastatin and cholestyramine is more effective than lovastatin alone. The regulation of the direct synthesis of LDL may have a greater effect on LDL concentrations than previously appreciated.

Low-Density Lipoprotein Subclass Patterns and Risk of Myocardial Infarction

The association of low-density lipoprotein (LDL) subclass patterns with coronary heart disease was investigated in a case-control study of nonfatal myocardial infarction. Subclasses of LDL were analyzed by gradient gel electrophoresis of plasma samples from 109 cases and 121 controls. The LDL subclass pattern characterized by a preponderance of small, dense LDL particles was significantly associated with a threefold increased risk of myocardial infarction, independent of age, sex, and relative weight. Plasma levels of high-density lipoprotein cholesterol were decreased, and levels of triglyceride, very low-density lipoproteins, and intermediate-density lipoproteins were increased in subjects with this LDL subclass pattern. Multivariate logistic regression analyses showed that both high-density lipoprotein cholesterol and triglyceride levels contributed to the risk associated with the small, dense LDL subclass pattern. Thus, the metabolic trait responsible for this LDL subclass pattern results in a set of interrelated lipoprotein changes that lead to increased risk of coronary heart disease.

Action of lecithin-cholesterol acyltransferase on low-density lipoproteins in native pig plasma.

The action of lecithin-cholesterol acyltransferase (LCAT, EC 2.3.1.43) on the different pig lipoprotein classes was investigated with emphasis on low-density lipoproteins (LDL). It was demonstrated previously that LDL can serve as substrate for LCAT, probably because they contain sufficient amounts of apoA-I and other non-apoB proteins, known as LCAT activators. Upon a 24-h incubation of pig plasma in vitro in the presence of active LCAT, both pig LDL subclasses, LDL-1 and LDL-2, fused together, forming one fraction, as revealed by analytical ultracentrifugation. This fusion was time dependent, becoming visible after 3 h and complete after 18 h of incubation. Concomitantly, free cholesterol and phospholipids decreased and cholesteryl esters increased. When isolated LDL-1 and LDL-2 were incubated with purified pig LCAT for 24 h, LDL-1 floated toward higher densities and LDL-2 toward lower densities, although this effect was not as pronounced as in incubations of whole serum. In further experiments, pig serum was incubated for various periods of time in the presence and absence of the LCAT inhibitor sodium iodoacetate. The individual lipoproteins then were separated by density gradient ultracentrifugation or by specific immunoprecipitation and chemically analyzed. Both methods revealed that in the absence of active LCAT there was a transfer of free cholesterol from LDL to high-density lipoproteins (HDL) and a small transfer of cholesteryl esters in the opposite direction. In the presence of LCAT the loss of free cholesterol started immediately in all three lipoprotein classes, was most prominent in LDL, and was proportional to the newly synthesized cholesteryl esters incorporated in each fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of intermediate and low-density lipoprotein subspecies to risk of coronary artery disease

Recent studies have shown that heterogeneity of human plasma low-density lipoproteins (LDL) is, in part, the result of production of different LDL products from two subspecies of intermediate-density lipoproteins (IDL). Cholesterol-enriched forms of both IDL species are found in plasma of patients with atherogenic dyslipidemias (familial hypercholesterolemia and type 3 hyperlipoproteinemia) and have physical properties similar to the major species in plasma of cholesterol-fed monkeys. Patients with familial combined hyperlipidemia have been shown to have increased plasma levels of IDL and of a smaller, denser LDL subclass (LDL-IIIA) that appears to be a metabolic product of the smaller IDL subspecies. Results from the NHLBI Type II Coronary intervention study have supported a link between the small IDL-LDL pathway and coronary disease, In that 2-year changes in levels of these species were associated with disease progression. Furthermore, therapeutic reductions in IDL levels were correlated with increases in high-density lipoprotein cholesterol. Thus variation in IDL levels might influence coronary disease risk by both a direct effect and indirectly by affecting LDL particle number and possibly high-density lipoprotein metabolism.

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 ( S f ⪢ 9) and a decrease in the mass of denser LDL. LDL (1.019 < d < 1.063) were subdivided into two subfractions LDL, (1.019 < d < 1.035) and LDL 2 (1.035 < d < 1.063) by ultracentrifugation. The lipoproteins in these these two fractions could be shown to have different floatation 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 LDL 1 prior to its appearance in LDL 2. [ 125I)LDL 1 injected into the monkey was removed from the LDL 1 density subclass with a half-life of 5.5–10.3 h. Much of the radioactivity injected as LDL 1 was converted to denser LDL (LDL 2). Labeled LDL 2 injected into the monkey was not converted to LDL 1. 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 (LDL 2).

Surface structure of the two porcine low-density lipoprotein subclasses — a spin labelling study

Porcine low-density lipoprotein (LDL) subclasses were spin-labelled at both protein and lipid sites. The surface structures of the two subclasses were compared. E.s.r. spectra of stearic acid spin-labels [I( m/ n)] and of androstanol spin-label indicated more restricted motion of the labels in LDL 2 ( d = 1.063−1.090 g/ml) than in LDL 1 ( d = 1.020−1.063 g/ml). Thermotropic change in the surface structure was found in both subclasses by both protein and lipid spin-labels, but at lower temperature in LDl 1, than in LDL 2. These results indicate the relationship between the size and the dynamics of the lipid components in the surface layer of the LDL subclasses.

Inhibition of macrophage-mediated tumor cell destruction by oxidized lipoproteins.

Lipoproteins (LP), isolated from human sera by column chromatography and density ultracentrifugation, were tested for their ability to inhibit macrophage (M phi)-mediated tumor cell destruction. None of the LP subclasses isolated by ultracentrifugation inhibited M phi-mediated cytolysis. Chromatography on a Sephadex G-200 column, prior to or following ultracentrifugation, resulted in the isolation of very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) that prevented tumor cell destruction by M phi. High-density lipoprotein did not acquire the ability to inhibit M phi-mediated tumor cell killing under any condition. The acquisition of inhibitory activity by VLDL and LDL subclasses could be prevented by incorporation of EDTA and the bubbling of nitrogen gas into the chromatography buffer. These conditions inhibited the formation of lipid peroxides and thus prevented the formation of LP that inhibit M phi-mediated cytotoxicity. The mechanism by which oxidized LP prevents M phi from destroying tumor targets is not known. However, the mechanism does not appear to be related to a decrease in M phi viability.

Abnormal suppression of 3-hydroxy-3-methylglutaryl-CoA reductase activity in cultured human fibroblasts by hypertriglyceridemic very low density lipoprotein subclasses.

AbstractOur previous studies showed that hypertriglyceridemic very low density lipoproteins (HTG VLDL) are functionally abnormal. HTG VLDL, but not normal VLDL, suppress HMG‐CoA reductase in cultured normal human fibroblasts. To determine if the suppression by HTG VLDL resulted from a subpopulation of smaller suppressive particles, more homogeneous subclasses of VLDL‐VLDL1 (Sf 100–400), VLDL2 (Sf 60–100), and VLDL3 (Sf 20–60) were obtained from the d&lt;1.006 (g°ml−1) fraction of normal and hypertriglyceridemic plasma by flotation through a discontinuous salt gradient and tested for suppression in normal human fibroblasts. VLDL1 and VLDL2 from each of the 12 normolipemic subjects tested failed to suppress HMG‐CoA reductase activity in normal fibroblasts. Eleven out of 12 preparations of normal VLDL3 suppressed HMG‐CoA reductase, but only one‐third as effectively as LDL. By contrast, the VLDL1, VLDL2 and VLDL3 from 15 out of 17 hypertriglyceridemic patients (hyperlipoproteinemia Types IIb, III, IV and V) were highly effective in suppression, with half‐maximal suppression at 0.1–2.0 μg VLDL protein/ml. The VLDL abnormality is apparently associated with hypertriglyceridemia and not hypercholesterolemia, since VLDL from a homozygous familial hypercholesterolemia patient with a Type IIa pattern did not suppress whereas each of the VLDL subclasses from a Type IIb patient suppressed. Suppression by HTG VLDL in normal cells is apparently a consequence of interaction of the protein portion of the VLDL with the specific LDL cell surface receptor since HTG VLDL1 treated with 0.1 M 1,2‐cyclohexanedione to block arginyl residues failed to suppress the enzyme. Moreover, hypertriglyceridemic Sf 60–400 VLDL failed to suppress HMG‐CoA reductase activity in LDL receptor‐negative fibroblasts. There were no consistent major compositional differences between comparable normal and hypertriglyceridemic VLDL subclasses which could account for differences in suppression. All VLDL subclasses from Type III subjects were enriched in cholesteryl esters and depleted in triglyceride, relative to the corresponding normal VLDL subclasses. However, Type IV and Type V VLDL subclasses were normal in this repect. We conclude from these studies that small particle diameter is not required for suppression, since HTG VLDL1 and VLDL2 which contained few, if any, small particles were effective in suppression.

Very low density lipoprotein subfractions in a subject with broad-β disease (type III hyperlipoproteinemia) and a subject with endogenous lipemia (type IV) chemical composition and electrophoretic mobility

1. 1. Three subfractions ( s f 20–60, 60–100, 100–400) of very low density lipoproteins have been isolated from a subject with broad-β disease (Type III hyperlipoproteinemia) and a subject with endogenous (Type IV) lipemia by preparative ultracentrifugation through a saline density gradient. The subfractions and the entire very low density lipoproteins, isolated in a single, separate ultracentrifugation, have been examined by analytical ultracentrifugation, paper and agarose electrophoresis, and chemical analysis. Recovery in the three subfractions as compared with the entire very low density lipoproteins (by both chemical analysis and analytical ultracentrifugation) confirmed the validity of the subfractionation procedure. 2. 2. The subject with broad-β disease exhibited a large quantity of s f > 400 lipoproteins and a very low density lipoprotein distribution with a higher peak S f value than his counterpart with endogenous lipemia. 3. 3. Electrophoresis revealed pre-β mobility for all subtractions in the subject with endogenous lipemia and s f 60–100 and 100–400 subfractions in the subject with broad-β disease; in the latter, β-mobility was confined to the S f 20–60 very low density lipoprotein subclass. 4. 4. In both subjects comparison of chemical composition among subfractions revealed progressive increases in protein, phospholipid, and cholesterol content and a parallel decrease in triglyceride as s f rate decreased. 5. 5. Chemical composition of all very low density lipoprotein subfractions appeared to be normal in the subject with endogenous lipemia. His counterpart with broad-β disease revealed abnormal increases in cholesterol ester and reciprocal decreases in triglyceride in all subfractions.