Abnormalities of composition and of in vitro lipolysis products of human small very low density lipoproteins in hypertriglyceridemia

In vitro and in vivo lipolysis of plasma triglycerides increases the resistance to oxidative modification of low-density lipoproteins

BACKGROUND

The majority of studies on low-density lipoprotein (LDL) particle size and susceptibility to oxidative modification have been either descriptive or interventional, but there are few mechanistic studies.

MATERIALS AND METHODS

Effects of exhaustive in vitro and in vivo lipolysis of serum and plasma triglycerides, respectively, by lipoprotein lipase (LPL) were investigated in healthy normotriglyceridemic men. The LDL end-product of lipolysis of very low-density lipoprotein (VLDL) underwent compositional analysis, gradient gel electrophoresis and an assessment of resistance to copper-induced oxidative modification.

RESULTS

The LDL particle contents of free fatty acid and alpha-tocopherol increased, whereas the contents of free and esterified cholesterol, alpha-carotene and coenzyme Q10 decreased upon incubation of serum with LPL in vitro. The LDL particle size decreased and the resistance to the oxidative modification of LDL increased. Lipolysis of plasma triglycerides in vivo, achieved by intravenous injection of heparin, did not alter the LDL particle size but increased the resistance to the oxidative modification of LDL. This change was accompanied by an increase in the LDL particle content of alpha-tocopherol, whereas the free fatty acid content was unaltered.

CONCLUSIONS

The results show that the increased resistance to oxidative modification of LDL after lipolysis of plasma triglycerides was concomitant with an increased LDL particle content of alpha-tocopherol, and that free fatty acids did not seem to contribute to the increased resistance to oxidative modification of LDL in vivo. Furthermore, our data indicate that the resistance of LDL to oxidative modification is not dependent on particle size.

Relation of plasma levels and composition of apolipoprotein B-containing lipoproteins to angiographically defined coronary artery disease in young patients with myocardial infarction

BACKGROUND

Hypertriglyceridemia is a common metabolic disturbance in men < 45 years old with myocardial infarction. To further investigate the relation between triglyceride-rich lipoproteins and severity of coronary atherosclerosis in this subset of postinfarction patients, apolipoprotein B-containing lipoproteins of 64 consecutive patients were subfractionated in connection with coronary angiography.

METHODS AND RESULTS

Density-gradient ultracentrifugation of plasma and coronary angiography were performed 4 to 6 months after the myocardial infarction. Global coronary atherosclerosis and the number and severity of distinct stenoses were evaluated by semiquantitative analysis of 15 proximal coronary segments. The majority of the patients (60%) were hypertriglyceridemic and had higher coronary scores than normotriglyceridemic patients. Of the major plasma lipoproteins, triglycerides and cholesterol in the low-density lipoprotein (LDL) fraction were associated with global coronary atherosclerosis, whereas LDL triglycerides and high-density lipoprotein (HDL) cholesterol correlated directly and inversely, respectively, with the coronary stenosis score. Plasma apolipoprotein B correlated with both coronary scores. The plasma concentrations of lipid and protein in the very-low-density lipoprotein (VLDL) subfractions (VLDL1 through VLDL3) and intermediate-density lipoprotein (IDL) did not correlate with either of the coronary scores, whereas the concentration of triglycerides in dense LDL (density > 1.040 kg/L) was strongly associated with both coronary scores. Compositional analysis of the smallest VLDL particles (VLDL3) and IDL revealed a correlation between the number of cholesteryl ester molecules in small VLDL and global coronary atherosclerosis in hypertriglyceridemic patients.

CONCLUSIONS

Global coronary atherosclerosis and distinct stenoses in young postinfarction patients are associated with the number of apolipoprotein B-containing particles in plasma and the concentration of LDL triglyceride. Specifically, dense triglyceride-rich LDL particles and, in hypertriglyceridemic patients, small cholesteryl ester-rich VLDL particles relate to coronary artery disease severity.

Reduced lipolysis of large apo E-poor very-low-density lipoprotein subfractions from type IV hypertriglyceridemic subjects in vitro and in vivo

Heparin-Sepharose chromatography was used to separate Sf 60-400 very-low-density lipoproteins (VLDL) from type IV hypertriglyceridemic subjects into apolipoprotein (apo) E-poor and apo E-rich subfractions. Since we have previously demonstrated that the apo E-poor fraction accumulates in plasma of type IV subjects, the aim of the present studies was to determine whether it was resistant to lipolysis in comparison to the apo E-rich fraction. The apo E-rich fraction was found to be 30% more effective than the apo E-poor fraction at competing with a glycerol tri[1-14C]oleate emulsion for in vitro lipolysis by normolipidemic human post-heparin plasma (P < .01), when assayed under conditions in which both lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) were active. Similar results were obtained when bovine milk LPL was used as the source of lipolytic activity (P < .025 for apo E-rich relative to apo E-poor VLDL), while neither fraction competed effectively with the synthetic substrate for lipolysis by HTGL only. When equal amounts of triglyceride from VLDL subfractions were incubated with bovine milk LPL, 25% more free fatty acid was released from the apo E-rich fraction than from the apo E-poor fraction (P < .025). The effects of heparin-induced lipolysis in vivo in type IV subjects on the relative amounts and composition of these VLDL subfractions were also assessed. Heparin infusion was associated with a 50% reduction in plasma Sf 60-400 VLDL triglyceride concentration. In addition, heparin-induced lipolysis resulted in a marked decrease in the relative amount of apo E-rich VLDL, while the relative amount of apo E-poor VLDL was increased. These results demonstrate that the apo E-poor VLDL subfraction is resistant to lipolysis by LPL relative to its apo E-rich counterpart, suggesting that reduced lipolytic efficiency may contribute to its observed accumulation in plasma of type IV subjects.

Normalisation of the composition of very low density lipoprotein in hypertriglyceridemia by nicotinic acid

Large (Sf greater than 100) and small (Sf 100-20) very low density lipoprotein (VLDL) particles were isolated by density gradient ultracentrifugation and characterized chemically in 8 patients with primary hypertriglyceridemia before and after 6 weeks treatment with 4 grammes daily of nicotinic acid (NA). Concomitant changes in high density lipoprotein (HDL) subclass distribution were determined by gradient gel electrophoresis. Small VLDL was subjected to lipolysis in vitro by incubation with bovine lipoprotein lipase before and after NA, and the change in the lipolytic end-product isolated in the low density lipoprotein (LDL) fraction was investigated. Reductions were achieved in the plasma levels of triglycerides, free and esterified cholesterol, phospholipids and proteins in the two VLDL subfractions. In all, the composition of both large and small VLDL particles changed towards potentially less atherogenic particles that were poorer in cholesteryl esters. The HDL cholesterol concentration increased and the HDL protein distribution on gradient gel electrophoresis changed towards larger particles. The mechanism behind the change in cholesterol distribution between VLDL and HDL after NA treatment is unclear, but it could possibly relate to decreased lipid transfer activity. NA reduced the content of apolipoprotein B in both VLDL subclasses and did not decrease the calculated particle size or the number of triglyceride molecules per particle, indicating a reduction of VLDL particle number rather than of particle size. The LDL density fraction isolated after lipolysis in vitro of small VLDL contained less total cholesterol and phospholipids and had a density profile more similar to native LDL after the patients had been treated with NA.