Plasma apolipoprotein B metabolism in familial type III dysbetalipoproteinaemia

Effect of apolipoprotein E genotype on apolipoprotein B-100 metabolism in normolipidemic and hyperlipidemic subjects

The effect of apolipoprotein (apo) E genotype on apoB-100 metabolism was examined in three normolipidemic apoE2/E2, five type III hyperlipidemic apoE2/E2, and five hyperlipidemic apoE3/E2 subjects using simultaneous administration of (131)I-VLDL and (125)I-LDL, and multi-compartmental modeling. Compared with normolipidemic apoE2/E2 subjects, type III hyperlipidemic E2/E2 subjects had increased plasma and VLDL cholesterol, plasma and VLDL triglycerides, and VLDL and intermediate density lipoprotein (IDL) apoB concentrations (P < 0.05). These abnormalities were chiefly a consequence of decreased VLDL and IDL apoB fractional catabolic rate (FCR). Compared with hyperlipidemic E3/E2 subjects, type III hyperlipidemic E2/E2 subjects had increased IDL apoB concentration and decreased conversion of IDL to LDL particles (P < 0.05). In a pooled analysis, VLDL cholesterol was positively associated with VLDL and IDL apoB concentrations and the proportion of VLDL apoB in the slowly turning over VLDL pool, and was negatively associated with VLDL apoB FCR after adjusting for subject group. VLDL triglyceride was positively associated with VLDL apoB concentration and VLDL and IDL apoB production rates after adjusting for subject group. A defective apoE contributes to altered lipoprotein metabolism but is not sufficient to cause overt hyperlipidemia. Additional genetic mutations and environmental factors, including insulin resistance and obesity, may contribute to the development of type III hyperlipidemia.

The effect of PPAR-alpha agonism on apolipoprotein metabolism in humans

Metabolic syndrome, diabetes and obesity are frequently associated with hypertriglyceridemia, hypercholesterolemia and low HDL levels, a phenotype known as atherogenic dyslipidemia. Atherogenic dyslipidemia and hypertriglyceridemia are frequently treated with fibric acid derivatives which activate the nuclear receptor PPAR-alpha leading to reduce plasma triglycerides and an increase in HDL cholesterol levels. The mechanism by which activation of PPAR-alpha with fibrates improves the plasma lipid profile in patients with atherogenic dyslipidemia and hypertriglyceridemia has been examined in several small studies measuring lipoprotein kinetics. The results of these studies indicate that the changes in lipoprotein metabolism observed in response to fibrate treatment vary according to lipoprotein phenotype. In general, fibrates act to reduce VLDL apoB-100 through enhanced fractional catabolism (clearance) of VLDL apoB-100 with additional effects on reducing VLDL apoB-100 production. LDL apoB-100 levels generally decrease in response to fibrates due to increased LDL fractional catabolism except in those patients with high to very high plasma triglyceride levels (>400mg/dL). Fibrates also increase HDL apoA-I and apoA-II levels by enhancing apoA-I and apoA-II production, although this is partially counteracted by increasing fractional catabolism of these apolipoproteins. The potent and specific PPAR-alpha agonist LY518674, reduced VLDL apoB-100 levels through enhanced fractional catabolism similar to what is seen with fibrates. In contrast to fibrates, LY518674 did not change HDL apoA-I levels in response to due to an increased turnover of apoA-I where an increased fractional catabolic rate entirely counteracted the increase in apoA-I production. The changes in apoB metabolism in response to PPAR-alpha activation with fibrates and specific PPAR-alpha agonists would be expected to reduce the risk of cardiovascular disease. However, the benefit of the enhanced turnover of HDL apoA-I in response to PPAR-alpha activation remains to be determined.

Effects of fenofibrate on apolipoprotein kinetics in patients with coexisting dysbetalipoproteinemia and heterozygous familial hypercholesterolemia

Dysbetalipoproteinemia (dysb) and familial hypercholesterolemia (FH) are two genetic disorders giving rise to severe disturbances of lipid homeostasis and premature atherosclerosis. The co-occurrence of both metabolic abnormalities is very rare and is estimated to affect 1 individual per 2,500,000 in the general population. However, the relative contribution of these two dyslipidemias to the combined lipoprotein phenotype is unknown. The two objectives of this study were (1) to compare the in vivo kinetics of triglyceride-rich lipoprotein (TRL) apolipoprotein (apo) B48, VLDL, IDL and LDL apo B100 as well as plasma apo A-l labelled with a stable isotope (l-(5,5,5-D3) leucine) in two subjects presenting both heterozygous FH and dysbetalipoproteinemia (FH+/dysb+), in six FH heterozygotes and in five normolipidemic controls, and (2) to examine the impact of a 6-week treatment with micronized fenofibrate 200 mg/d on apolipoprotein kinetics in FH+/dysb+. As compared with FH heterozygotes and controls, the two FH+/dysb+ subjects showed elevated TRL apo B48 and VLDL, IDL apo B100 pool sizes (PS) mainly due to lower fractional catabolic rates (FCR). Moreover, as compared with FH heterozygotes, FH+/dysb+ subjects presented lower LDL apo B100 PS due to a higher FCR. Pool size, FCR and production rate (PR) of apo A-l were higher in FH+/dysb+ subjects than in FH heterozygotes. In FH+/dysb+ subjects, fenofibrate treatment was associated with a decreased TRL apo B48 PS (-52 and -61%), VLDL apo B100 (-61 and -63%) and IDL apo B100 (-37 and -16%) and an increased FCR of TRL apo B48 (10 and 67%), VLDL apo B100 (123 and 57%) and IDL apo B100 (29 and 10%). Fenofibrate also increased LDL apo B100 PS (3 and 57%) due to an increase in PR (80 and 26%) but had divergent effects on LDL apo B100 FCR. These results indicate that the coexistence of dysbetalipoproteinemia and heterozygous FH results in a mixed lipoprotein phenotype that is intermediate between the two pure phenotypes and that fenofibrate treatment partially reverses lipoprotein abnormalities, mostly through changes in PR and FCR of apo B48- and B100-containing lipoproteins.

Thematic review series: patient-oriented research. What we have learned about VLDL and LDL metabolism from human kinetics studies

Lipoprotein metabolism is the result of a complex network of many individual components. Abnormal lipoprotein concentrations can result from changes in the production, conversion, or catabolism of lipoprotein particles. Studies in hypolipoproteinemia and hyperlipoproteinemia have elucidated the processes that control VLDL secretion as well as VLDL and LDL catabolism. Here, we review the current knowledge regarding apolipoprotein B (apoB) metabolism, focusing on selected clinically relevant conditions. In hypobetalipoproteinemia attributable to truncations in apoB, the rate of secretion is closely linked to the length of apoB. On the other hand, in patients with the metabolic syndrome, it appears that substrate, in the form of free fatty acids, coupled to the state of insulin resistance can induce hypersecretion of VLDL-apoB. Studies in patients with familial hypercholesterolemia, familial defective apoB, and mutant forms of proprotein convertase subtilisin/kexin type 9 show that mutations in the LDL receptor, the ligand for the receptor, or an intracellular chaperone for the receptor are the most important determinants in regulating LDL catabolism. This review also demonstrates the variance of results within similar, or even the same, phenotypic conditions. This underscores the sensitivity of metabolic studies to methodological aspects and thus the importance of the inclusion of adequate controls in studies.

The effect of apolipoprotein E polymorphism on lipid levels in Korean adults

The aim of this study was to determine the effects of polymorphisms in the apolipoprotein E gene (APOE) on lipid levels in Korean adults and to investigate the interactions between these polymorphisms and environmental factors in determining lipid levels. We performed a cross-sectional study of 1,900 subjects (668 men and 1,232 women; 45-74 yr old) in Namwon, Korea, in 2004. APOE polymorphisms were determined by polymerase chain reaction and restriction enzyme analysis. Carriers of the APOE*E2 (E2) allele had significantly lower total cholesterol and low-density lipoprotein cholesterol (LDL-C) concentrations than did carriers of the APOE*E3 (E3) or APOE*E4 (E4) alleles, regardless of gender. The APOE allele type had significant effect on high-density lipoprotein cholesterol (HDL-C) and triglyceride levels in women, but not in men. The effect of APOE allele type on HDL-C levels was modified by age in women. In addition, in men, the effect of APOE allele type on triglyceride levels was modified by smoking. These findings highlight the important effect of gene-environment interactions on lipid levels.

Abnormal in vivo metabolism of apoB-containing lipoproteins in human apoE deficiency

The present study was undertaken to elucidate the metabolic basis for the increased remnants and lipoprotein(a) [Lp(a)] and decreased LDL apolipoprotein B (apoB) levels in human apoE deficiency. A primed constant infusion of (13)C(6)-phenylalanine was administered to a homozygous apoE-deficient subject. apoB-100 and apoB-48 were isolated, and tracer enrichments were determined by gas chromatography-mass spectrometry, then kinetic parameters were calculated by multicompartmental modeling. In the apoE-deficient subject, fractional catabolic rates (FCRs) of apoB-100 in VLDL and intermediate density lipoprotein and apoB-48 in VLDL were 3x, 12x, and 12x slower than those of controls. On the other hand, the LDL apoB-100 FCR was increased by 2.6x. The production rate of VLDL apoB-100 was decreased by 45%. In the Lp(a) kinetic study, two types of Lp(a) were isolated from plasma with apoE deficiency: buoyant and normal Lp(a). (125)I-buoyant Lp(a) was catabolized at a slower rate in the patient. However, (125)I-buoyant Lp(a) was catabolized at twice as fast as (131)I-normal Lp(a) in the control subjects. In summary, apoE deficiency results in: 1) a markedly impaired catabolism of VLDL/chylomicron and their remnants due to lack of direct removal and impaired lipolysis; 2) an increased rate of catabolism of LDL apoB-100, likely due to upregulation of LDL receptor activity; 3) reduced VLDL apoB production; and 4) a delayed catabolism of a portion of Lp(a).

Apolipoprotein E polymorphism in men and women from a Spanish population: allele frequencies and influence on plasma lipids and apolipoproteins

The apolipoprotein (apo) E phenotype and its influence on plasma lipid and apolipoprotein levels were determined in men and women from a working population of Madrid, Spain. The relative frequencies of alleles epsilon(2), epsilon(3) and epsilon(4) for the study population (n=614) were 0.080, 0.842 and 0.078, respectively. In men, apo E polymorphism was associated with variations in plasma triglyceride and very low-density lipoprotein (VLDL) lipid levels. It was associated with the proportion of apo C-II in VLDL, and explained 5.5% of the variability in the latter parameter. In women apo E polymorphism was associated with the concentrations of plasma cholesterol and low-density lipoprotein (LDL) and high-density lipoprotein (HDL) related variables. The allelic effects were examined taking allele epsilon(3) homozygosity as reference. In men, allele epsilon(2) significantly increased VLDL triglyceride and VLDL cholesterol concentrations, and this was accompanied by an increase of the apo C-II content in these particles. Allele epsilon(4) did not show any significant influence on men's lipoproteins. In women, allele epsilon(2) lowered LDL cholesterol and apo B levels, while allele epsilon(4) increased LDL cholesterol and decreased the concentrations of HDL cholesterol, HDL phospholipid and apo A-I. These effects were essentially maintained after excluding postmenopausal women and oral contraceptive users from the analysis.

IN CONCLUSION

(1) the population of Madrid, similar to other Mediterranean populations, exhibits an underexpression of apo E4 compared to the average prevalence in Caucasians, (2) gender interacts with the effects of apo E polymorphism: in women, it influenced LDL and HDL levels, whereas in men it preferentially affected VLDL, and (3) allele epsilon(2) decreased LDL levels in women, while it increased both VLDL lipid levels and apo C-II content in men, but, in contrast to allele epsilon(4), it did not show an impact on HDL in either sex.

Postprandial vitamin A and squalene clearances and cholesterol synthesis off and on lovastatin treatment in type III hyperlipoproteinemia

Postprandial fat clearance and absorption, fecal elimination and synthesis of cholesterol, bile acid synthesis, and cholesterol precursors and plant sterols in serum were studied in five patients with type III dyslipoproteinemia off and on lovastatin. The basal values were related to those in nontreated normolipidemic control subjects with apolipoprotein E3/3 phenotype (apo E3 controls, n = 16). On regular home diets, cholesterol precursor concentrations and cholesterol precursor/cholesterol ratios were high in the type III group. However, cholesterol absorption efficiency, bile acid and cholesterol synthesis measured with sterol balance technique and the precursor sterol/plant sterol ratios in serum were similar to the control values, suggesting that cholesterol absorption and metabolism was normal in these subjects. Lovastatin normalized the increased lipoprotein concentrations and reduced biliary cholesterol secretion, absolute absorption of cholesterol, precursor sterol/cholesterol and precursor sterol/plant sterol ratios in serum, fecal neutral and total sterol outputs and cholesterol synthesis. Lovastatin had no effect on cholesterol absorption efficiency or bile acid synthesis. Despite normalization of the triglyceride-rich lipoprotein levels by lovastatin, the postprandial vitamin A and squalene peak concentrations and the areas under the curves remained above the control ranges. The findings show that in type III hyperlipidemia, the precursor sterol/cholesterol ratios do not predict cholesterol synthesis. The latter, bile acid synthesis, precursor sterol/plant sterol ratios in serum, and cholesterol absorption are normal under basal conditions. The normalization of increased lipids by lovastatin is mainly due to reduced synthesis and absolute absorption of cholesterol, while the retarded postprandial fat clearance was not normalized by the drug.

Changes of lipoprotein profile in familial dysbetalipoproteinemia with gemfibrozil

PURPOSE

This prospective study was undertaken to evaluate the effects of gemfibrozil on the lipoprotein profile of patients with familial dysbetalipoproteinemia (type III hyperlipoproteinemia).

PATIENTS AND METHODS

Eight patients with well-defined familial dysbetalipoproteinemia associated with the apolipoprotein (apo) E2/2 phenotype were treated with gemfibrozil (Lopid) at a dose of 600 mg twice daily for a period of 10 months. Blood samples were taken at baseline, after 4 and 5 weeks, after 3 months, and after 10 months. The separation of serum lipoprotein (sub)fractions was performed by a recently developed density gradient ultracentrifugation technique.

RESULTS

After 4 weeks of gemfibrozil therapy, the concentrations of serum total cholesterol and serum total triglyceride had decreased by 45% (from 11.87 to 6.51 mmol/L, p < 0.01) and by 63% (from 6.08 to 2.23 mmol/L, p < 0.001), respectively. The cholesterol concentrations of very-low-density lipoprotein-1 (VLDL1) (large VLDL), VLDL2 (small VLDL), and intermediate-density lipoprotein (IDL) had decreased significantly by 73%, 74%, and 34%, respectively. The low-density lipoprotein (LDL)-cholesterol level remained unchanged, whereas the particle size of LDL showed a small but significant increase (from 24.09 nm to 24.43 nm, p < 0.01). The concentrations of high-density lipoprotein (HDL)-cholesterol, apo A-I, and apo A-II had increased significantly by 23%, 13%, and 29%, respectively. Only minor changes in the composition of the lipoprotein (sub)fractions were observed. After 3 months of treatment with gemfibrozil, the concentrations of serum total cholesterol and serum total triglyceride were 5.95 mmol/L and 2.06 mmol/L, respectively, and after 10 months of treatment with gemfibrozil, the concentrations of serum total cholesterol and serum total triglyceride were 6.19 mmol/L and 2.27 mmol/L, respectively.

CONCLUSION

Gemfibrozil treatment in patients with familial dysbetalipoproteinemia resulted in a marked reduction of the concentrations of large VLDL, small VLDL, and IDL, and an increase in the levels of HDL, apo A-I, and apo A-II. These changes are considered to exert an antiatherosclerotic effect in these patients.

Different hydrolytic efficiencies of adipose tissue lipoprotein lipase on very-low-density lipoprotein subfractions separated by heparin-Sepharose chromatography

Human very-low-density lipoproteins (VLDL) were subfractionated by heparin-Sepharose chromatography into an unbound (A) and three bound (B, C and D) populations at increasing ionic strengths. Subfractions were characterized regarding their chemical composition and efficiency of triacylglycerol hydrolysis by rat adipose tissue LPL. The triacylglycerol content decreased, whereas the cholesterol and protein contents increased from subfractions A and B to subfraction D. VLDL-D showed the highest apo E/apo C ratio, though all the subfractions contained appreciable apo E. Appearance of VLDL-A resulted from exceeding the binding capacity of the column, since practically all its particles eluted at positions of bound VLDL under re-chromatography. Subfractions B, C and D stimulated LPL activity on emulsified tri[14C]oleoylglycerol to a similar extent, indicating that their apo C-II content was equally effective activating LPL. Incubation of tri[14C]oleoylglycerol labeled VLDL subfractions with fat pad pieces in the presence or absence of heparin resulted in greater hydrolysis and fatty acid uptake for VLDL-B and -C than for VLDL-D, a pattern observed over a wide range of LPL activities in the media. We conclude: (1) any VLDL particle can interact with heparin, which is consistent with the presence of apo E in all the subfractions, and (2) triacylglycerols in apo E-rich VLDL are less efficiently hydrolyzed by LPL than those in apo E-poor particles. We propose that richness in apo E impairs LPL action upon VLDL and decreases the rate of delivery of fatty acids to peripheral tissues.

Hypo- and hyperresponse of serum cholesterol level and low density lipoprotein production and degradation to dietary cholesterol in man

Serum cholesterol in man rises when cholesterol intake increases, but the extent of the elevation varies between subjects. Part of the variation between subjects is spurious and not reproducible; it is caused by random diet-independent fluctuations of serum lipid levels. Part is due to consistent metabolic differences between subjects. We have earlier found that responsiveness was associated with higher initial total and HDL cholesterol, lower habitual cholesterol consumption, and lower body mass index, and unrelated to gender, age, or apo E phenotype. We have now investigated the metabolic basis of variability by measuring turnover rates of low density lipoprotein (LDL) apolipoprotein B (apo B) on a low-cholesterol diet (140 mg/day) and a high-cholesterol diet (900 mg/day) in 8 volunteers with well-defined differences in the responsiveness of their serum cholesterol to diet. Autologous 125I-LDL was injected on day 23 of each diet period. Its fractional catabolic rate (FCR) was estimated from the ratio of 125I in urine over that in plasma, seven days after injection. FCR (mean +/- SD) increased from 0.24 +/- 0.02 pools/day on the low- to 0.31 +/- 0.20 on the high-cholesterol diet. LDL-apo B concentration rose from 49 +/- 13 to 63 +/- 12 mg/dl, and LDL-apo B production rate, calculated as FCR x concentration/body weight, from 4.8 +/- 1.2 to 8.0 +/- 1.4 mg/kg/day. The individual rise in production rate was significantly correlated with the rise in the serum concentration of LDL-apo B (r = 0.90) or LDL-cholesterol (r = 0.75), and also with the rise in total serum cholesterol measured in these same subjects in similar experiments 3-4 years earlier (r = 0.74). Degradation of LDL by freshly isolated blood mononuclear cells and by mononuclear cells incubated for 72 h in lipoprotein-deficient medium (derepressed cells) was measured on both diets in these and in additional volunteers. The rate of degradation (mean +/- SD) of standard human LDL by fresh cells was 336 +/- 166 ng LDL protein/mg cell protein per 8 h on the low-cholesterol diet, and decreased by 147 +/- 180 ng/mg per 8 h or 44% on the high-cholesterol diet (n = 23, p < 0.01). The catabolic activity of derepressed cells obtained when subjects were on the low-cholesterol diet was negatively related to the LDL cholesterol response (r = -0.57, n = 18, p < 0.05), and to the total cholesterol response in earlier experiments (r = -0.45, n = 18, p < 0.10).(ABSTRACT TRUNCATED AT 400 WORDS)

Lipoproteins in familial dysbetalipoproteinemia. Variation of serum cholesterol level associated with VLDL concentration

Patients with familial dysbetalipoproteinemia (FD) associated with the apo E2/2 phenotype exhibit a marked interindividual variability in serum cholesterol and triglyceride concentrations. It has been proposed that this variability is due to a combination of the apo E2/2 phenotype and additional genetic factors implicated in diseases like familial hypercholesterolemia, familial combined hyperlipoproteinemia, and familial hypertriglyceridemia. To further explore the nature of this variability, the lipoprotein profiles of 17 patients with FD associated with the apo E2/2 phenotype were analyzed by a density-gradient ultracentrifugation technique and by 2-16% polyacrylamide gel electrophoresis. It was found that all patients with FD were characterized by 1) markedly increased cholesterol concentrations of large very low density lipoprotein (VLDL) (VLDL1) (2.98 +/- 3.08 versus 0.08 +/- 0.03 mmol/L), small VLDL (VLDL2) (4.68 +/- 1.93 versus 0.27 +/- 0.13 mmol/L), and intermediate density lipoprotein (IDL) (2.25 +/- 0.72 versus 0.39 +/- 0.16 mmol/L); 2) decreased low density lipoprotein (LDL) cholesterol level (1.84 +/- 0.54 versus 3.36 +/- 0.53 mmol/L); and 3) altered composition (enrichment by cholesteryl ester) of VLDL1 and VLDL2 compared with normolipidemic control subjects. The cholesterol levels of IDL and LDL showed minor interindividual variabilities and were not correlated with serum cholesterol and triglyceride levels. The compositions of VLDL1 and VLDL2 were independent of the concentrations of lipids in serum. However, the cholesterol concentrations of VLDL1 and VLDL2 showed considerable interindividual variabilities and were positively correlated with the serum cholesterol concentration (r = 0.84 and r = 0.95, respectively, both p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of apolipoprotein E polymorphism on apolipoprotein B-100 metabolism in normolipemic subjects

This study examined apolipoprotein (apo) B metabolism in normolipemic subjects homozygous for the apo E2 (n = 4), apo E3 (n = 5), or apo E4 (n = 5) phenotype. Radioiodinated very low density lipoprotein (VLDL1) (ultracentrifuge flotation rate [Sf] 60-400) and VLDL2 (Sf 20-60) were injected into volunteers and the conversion of apo B was followed through intermediate density lipoprotein (IDL) to low density lipoprotein (LDL). Subjects homozygous for E3 converted approximately 50% of LVDL2 to LDL, the remainder being lost by direct catabolism. Those with the E2 phenotype produced less VLDL1, but converted more of it to VLDL2 (compared to E3 subjects). They displayed a characteristic dyslipidemia with the presence of slowly catabolized VLDL1 and VLDL2 remnants. LDL levels were low owing to increased direct catabolism of VLDL2 and IDL and a reduced efficiency of delipidation; only 25% of VLDL2 apo B was directed to LDL production. In contrast, E4 subjects converted more VLDL2 apo B to LDL than E3 subjects. About 70% of VLDL2 apo B was found in LDL; direct catabolism of VLDL and IDL was reduced as was the fractional catabolic rate of LDL (0.2 vs. 0.26 in E3 subjects). These changes in the VLDL----IDL----LDL metabolic cascade can in part be explained by alterations in hepatic LDL receptors with E2 subjects having higher and E4 subjects lower activities than those in E3 homozygotes.

Lipoprotein lipase modulates net secretory output of apolipoprotein B in vitro. A possible pathophysiologic explanation for familial combined hyperlipidemia

We showed previously that net secretory output of apolipoprotein B (apo B) from cultured human hepatoma cells (HepG2) is regulated by rapid reuptake of nascent lipoproteins before they have diffused away from the vicinity of the cells. We now sought to determine if the nascent lipoproteins could be remodeled to enhance or impede reuptake. We found that lipoprotein lipase (LpL), an enzyme that hydrolyzes lipoprotein triglyceride, reduced HepG2 output of apo B to one-quarter to one-half of control. The reduction was apparent during co-incubations as short as 2 h and as long as 24 h. Heparin, which blocks receptor-mediated binding of lipoproteins, abolished the effect of LpL on apo B output, without causing enzyme inhibition. To assess uptake directly, we prepared labeled nascent lipoproteins. LpL tripled the cellular uptake of labeled nascent lipoproteins, from 15.2% +/- 0.7% to 48.7% +/- 0.3% of the total applied to the cells. Cellular uptake of 125I-labeled anti-LDL receptor IgG was unaffected by LpL; thus, LpL enhanced reuptake by altering lipoproteins, not receptors. Because LpL is present in the space of Disse in the liver, we conclude that LpL may act on newly secreted lipoproteins to enhance reuptake in vivo. LpL deficiency would reduce local reuptake of apo B, which would appear as overproduction, thereby providing a mechanistic link between partial LpL deficiency and familial combined hyperlipidemia.

Lathosterol level in plasma is elevated in type III hyperlipoproteinemia, but not in non-type III subjects with apolipoprotein E2/2 phenotype, nor in type IIa or IIb hyperlipoproteinemia

We measured the serum lathosterol level, a reflection of the rate of whole body cholesterol synthesis, in 15 patients with manifest type III hyperlipoproteinemia (HLP), in 20 subjects with apolipoprotein (apo) E2/2 phenotype, but without type III HLP, in 21 patients with type IIA and 10 patients with type IIB HLP. A group of 100 subjects with apo E3/3 phenotype served as reference. Using ANCOVA, lathosterol was adjusted for serum cholesterol and triglyceride concentrations, since these parameters were found to independently correlate with lathosterol. The adjusted means (+/- SEM), in mumol/L, in these groups were 12.9 +/- 1.1, 8.2 +/- 1.1, 4.8 +/- 0.9, 9.8 +/- 1.4, and 7.8 +/- 0.4, respectively. Type III HLP patients had significantly higher lathosterol levels than all other groups except type IIB HLP. In addition, lathosterol was significantly lower in type IIA patients than in all other groups. The serum levels of plant sterols, used as a reflection of cholesterol absorption, did not differ among the various groups after adjustment for serum cholesterol. These findings suggest that an overproduction of cholesterol is one factor discriminating E2/2 homozygotes with type III HLP from those without the disease.

Metabolic studies on the hypolipidaemic effect of guar gum

The hypolipidaemic effect of guar gum (30 g/day) was examined in a double blind placebo-controlled crossover study in 9 patients with primary hyperlipidaemia. The treatment periods were of six weeks duration. Cholesterol levels in low density lipoprotein (LDL) were decreased by 11.5% and in intermediate density lipoprotein (IDL) by 10.7%. Plasma cholesterol levels were reduced by 9.6% (P less than 0.05). Kinetic studies using autologous 125I-labelled LDL showed a decrease of 21.6% in plasma LDL apo B pool size (P less than 0.05) that resulted from a 39.1% increase in its fractional rate of catabolism. The kinetic effects of guar gum on LDL metabolism appear similar to that of bile acid binding resins in that LDL apo B fractional catabolism is greatly increased while there is a slight increase in production rate.

Very low density lipoprotein apolipoprotein B metabolism in humans

The human plasma lipoproteins encompass a broad spectrum of particles of widely varying physical and chemical properties whose metabolism is directed by their protein components. Apolipoprotein B100 (apo B100) is the major structural protein resident in particles within the Svedberg flotation range 0-400. The largest of these, the very low density lipoprotein (VLDL), rich in triglyceride, are metabolised by sequential delipidation through a transient intermediate density lipoprotein (IDL) to cholesterol-rich low density lipoproteins (LDL). Several components contribute to the regulation of this process, including (a) the lipolytic enzymes lipoprotein lipase and hepatic lipase (b), apolipoproteins B, CII, CIII and E, and (c) the apolipoprotein B/E or LDL receptor. Lipoprotein lipase acts primarily on large VLDL of Sf 60-400. Hepatic lipase on the other hand seems to be critical for the conversion of smaller particles (Sf 12-60) to LDL (Sf 0-12). Although most apo B100 flux is directed to the production of the delipidation end product LDL, along the length of the cascade there is potential for direct removal of particles from the system, probably via the actions of cell membrane receptors. This alternative pathway is particularly evident in hypertriglyceridaemic subjects, in whom the delipidation process is retarded. VLDL metabolism shows inter subject variability even in normal individuals. In this regard, apolipoprotein E plays an important role. Normolipidaemic individuals homozygous for the apo E2 variant exhibit gross disturbances in the transit of B protein through the VLDL-IDL-LDL chain.

Cholesterol in the plasma very low density lipoprotein fraction in patients with type III hyperlipoproteinemia: analysis of factors which modulate its concentration

Anthropometric data, plasma lipoprotein lipid levels, and post-heparin lipoprotein lipase (PHLPL) activity were measured in nine patients with type III hyperlipoproteinemia (HLP) and two hypocholesterolemic subjects with the apo-E2/2 phenotype. Five type III HLP patients were treated with clofibrate. Log PHLPL activity was inversely correlated (r = -0.667, p less than 0.05) and age was positively correlated (r = 0.706, p less than 0.05) with cholesterol levels in the VLDL fraction of plasma from type III HLP patients. The correlation between log PHLPL and VLDL cholesterol levels remained significant when age was held constant in partial correlation analysis. Together age and log PHLPL activity accounted for 77% of individual variation in VLDL cholesterol levels in the type III patients. Clofibrate treatment raised PHLPL activity (+48%, p less than 0.05) and reduced the levels of VLDL cholesterol (-67%, P less than 0.05), VLDL triglycerides (-40%, P less than 0.02), and the ratio cholesterol/triglyceride in VLDL (-50%, P less than 0.05) in five type III HLP patients. Mean PHLPL activity was higher in the hypocholesterolemic subjects with the apo-E2/2 phenotype compared to the type III HLP patients. These results suggest that lipoprotein lipase activity and factors associated with age modulate the levels of abnormal and atherogenic remnant particles (beta-VLDL) in the VLDL plasma fraction of type III HLP patients.

Sheehan's syndrome presenting with type III hyperlipoproteinaemia

A patient is described in whom the presenting feature of hypopituitarism was the development of palmar xanthomata associated with type III hyperlipoproteinaemia. Treatment of her secondary hypothyroidism with thyroxine caused resolution of the xanthomata and hyperlipidaemia, but the underlying compositional abnormality of the lipoproteins could still be observed.

Helsinki Heart Study: primary-prevention trial with gemfibrozil in middle-aged men with dyslipidemia. Safety of treatment, changes in risk factors, and incidence of coronary heart disease

In a randomized, double-blind five-year trial, we tested the efficacy of simultaneously elevating serum levels of high-density lipoprotein (HDL) cholesterol and lowering levels of non-HDL cholesterol with gemfibrozil in reducing the risk of coronary heart disease in 4081 asymptomatic middle-aged men (40 to 55 years of age) with primary dyslipidemia (non-HDL cholesterol greater than or equal to 200 mg per deciliter [5.2 mmol per liter] in two consecutive pretreatment measurements). One group (2051 men) received 600 mg of gemfibrozil twice daily, and the other (2030 men) received placebo. Gemfibrozil caused a marked increase in HDL cholesterol and persistent reductions in serum levels of total, low-density lipoprotein (LDL), and non-HDL cholesterol and triglycerides. There were minimal changes in serum lipid levels in the placebo group. The cumulative rate of cardiac end points at five years was 27.3 per 1,000 in the gemfibrozil group and 41.4 per 1,000 in the placebo group--a reduction of 34.0 percent in the incidence of coronary heart disease (95 percent confidence interval, 8.2 to 52.6; P less than 0.02; two-tailed test). The decline in incidence in the gemfibrozil group became evident in the second year and continued throughout the study. There was no difference between the groups in the total death rate, nor did the treatment influence the cancer rates. The results are in accord with two previous trials with different pharmacologic agents and indicate that modification of lipoprotein levels with gemfibrozil reduces the incidence of coronary heart disease in men with dyslipidemia.

Metabolic consequences of genetic heterogeneity of lipoprotein composition (lipoprotein heterogeneity)

Lipoprotein composition varies among different genetic forms of hyperlipidemia. An increase in hepatic triglyceride (TG) synthesis in subjects with familial hypertriglyceridemia (FHTG) is associated with secretion of large, TG-enriched, very low-density lipoproteins (VLDL), which have an increased affinity for lipoprotein lipase (LPL) in vivo as compared with VLDL from subjects with familial combined hyperlipidemia (FCHL) or from normal subjects. Elevated levels of plasma low-density lipoprotein (LDL) apoprotein B in FCHL are associated with high apoprotein B production rates. The LDL in FCHL is heterogeneous, with a preponderance of an LDL subfraction, which is denser, smaller, and lipid poor as compared with LDL from normal subjects. The more buoyant LDL subfraction in FCHL seems to be catabolized more rapidly than this dense LDL subfraction.

Abnormal in vivo metabolism of apolipoprotein E4 in humans

Apolipoprotein E (apoE) is important in modulating the catabolism of remnants of triglyceride-rich lipoprotein particles. It is a polymorphic protein with the three common alleles coding for apoE2, apoE3, and apoE4. ApoE3 is considered the normal isoform, while apoE4 is associated both with hypercholesterolemia and type V hyperlipoproteinemia. We quantitated the kinetics of metabolism of apoE4 in 19 normolipidemic apoE3 homozygotes and 1 normolipidemic apoE4 homozygote, and compared this with the metabolism of apoE3 in 12 normolipidemic apoE3 homozygotes. In the apoE3 homozygous subjects, apoE4 was catabolized twice as fast as apoE3, with a mean plasma residence time of 0.37 +/- 0.01 d (+/- SEM) and 0.73 +/- 0.05 (P less than 0.001), respectively. When plasma was fractionated into the lipoprotein subclasses, the greatest amount of labeled apoE4 was present on very low density lipoproteins, while the largest fraction of labeled apoE3 was associated with high density lipoproteins. The plasma apoE concentration was decreased in an apoE4 homozygote compared with the apoE3 homozygotes (3.11 mg/dl vs. 4.83 +/- 0.35 mg/dl). The reduced apoE4 concentration was entirely due to a decreased apoE4 residence time in the apoE4 homozygote (0.36 d vs. 0.73 +/- 0.05 d for apoE3 in apoE3 homozygotes). These results indicate that apoE4 is kinetically different than apoE3, and suggest that the presence of apoE4 in hypercholesterolemic and type V hyperlipoproteinemic individuals may play an important pathophysiological role in the development of these dyslipoproteinemias.