Sialic acid-containing components of lipoproteins influence lipoprotein-proteoglycan interactions.

Sialic acid is a negatively charged sugar associated with the protein and lipid portions of lipoproteins. Sialic acid has been hypothesised to play an anti-atherogenic role in lipoprotein metabolism through the electrostatic inhibition of lipoprotein interactions with chondroitin-6-sulphate-rich arterial proteoglycans (APG). We conducted a series of studies using native and modified lipoproteins (VLDL1 Sf 60-400, VLDL2 Sf 20-60, IDL1 Sf 16-20, IDL2 Sf 12-16, LDL(A) Sf 8-12, and LDL(B) Sf0-8) that vary in their sialic acid content to examine the relationship between lipoprotein sialic acid content and its interaction with APG. Lipoprotein sialic acid was greatest in VLDL1 and decreased progressively with particle density until the IDL2 fraction (VLDL1 > VLDL2 > IDL1 > IDL2 = LDL(A) = LDL(B)). The pattern of reactivity of each fraction with APG was different from the pattern observed for lipoprotein sialic acid content (IDL2 > LDL(A) > LDL(B) > IDL1 > VLDL2 > VLDL1). Levels of sialic acid were lower in subjects with CHD as compared to control subjects but the presence of CHD had no effect on lipoprotein-APG complex formation when sex and plasma triglyceride levels were taken into account. There was also no significant relationship between the lipoprotein sialic acid content and the reactivity with APG within each lipoprotein fraction. Treatment of hypertriglyceridaemic subjects with ciprofibrate decreased lipoprotein-APG complex formation in all lipoprotein fractions. This was associated with a decrease in the total sialic acid content of apo B100-containing lipoproteins suggesting that the total sialic acid content of apo B100-containing lipoproteins has no influence on lipoprotein-APG complex formation. We next conducted in vitro experiments to manipulate LDL sialic acid content. Enzymatic removal of sialic acid from LDL with neuraminidase resulted in an increase in LDL-APG complex formation. This was accompanied by an increase in the exposure of free amino groups on LDL possibly due to disruption of interactions between free amino groups and sialic acid-containing components on LDL. Increasing LDL sialic acid content through incubation with ganglioside resulted in a decrease in lipoprotein-APG complex formation without any changes in the exposure of free amino groups on LDL. We conclude that total sialic acid content of lipoproteins is not a major determinant of their binding to APG. However, specific sialic acid-containing components on lipoproteins can affect their interaction with APG.

Interaction of very-low-density, intermediate-density, and low-density lipoproteins with human arterial wall proteoglycans.

The specific interaction of lipoproteins with arterial wall constituents, particularly proteoglycans (APG), is believed to play an important role in the development of atherosclerosis. The objective of this study was to examine the interaction of apolipoprotein B (apoB) containing lipoprotein subfractions (VLDL1, Sf 60 to 400; VLDL2, Sf 20 to 60; IDL1, Sf 16 to 20; IDL2, Sf 12 to 16; LDLA, Sf 8 to 12; and LDLB, Sf 0 to 8) prepared by cumulative density gradient centrifugation with chondroitin sulfate-rich APG. Eighteen subjects were studied, and a similar pattern of interaction between the lipoprotein species and APG was found in all. The order of reactivity (as measured by increased turbidity due to insoluble complex formation) was IDL Sf 12 to 16 > or = LDL Sf 8 to 12 > LDL Sf 0 to 8 > IDL Sf 16 to 20 >> VLDL Sf 20 to 60 > VLDL Sf 60 to 400. When the subjects were divided on the basis of their LDL subfraction profile, the extent of insoluble complex formation was highest in the group in which small, dense LDLIII was predominant; intermediate in the group whose LDL was mainly LDLII; and lowest in the group with a high proportion of LDLI (the mean reactivity, AU at 600 nm. of APG with IDL Sf 12 to 16 and LDL Sf 8 to 12 was 0.66; 0.62 and 0.46, 0.43 and 0.20, and 0.21 for the three groups, respectively). Fibrate lipid-lowering treatment decreased the percentage of LDLIII and increased the percentage of LDLI within total LDL and reduced the reactivity of all apoB-containing lipoprotein fractions toward APG. Sialic acid content varied in different lipoprotein subfractions, being the highest in VLDL and lowest in LDL. However, across lipoprotein species, it did not significantly correlate with APG-binding reactivity, suggesting that other factors are important in determining the interaction of lipoproteins with APG. Modification of LDL arginine and lysine residues abolished the ability of the lipoprotein to interact with APG, a finding that supports the hypothesis that the interaction is dependent on key positively charged amino acids on apoB. These findings demonstrate that (1) the overall reactivity of apoB-containing lipoproteins is greatest in individuals with small, dense LDL and (2) within an individual, IDL of Sf 12 to 16 is the most reactive species, and this may in part explain the positive correlation between IDL and risk of coronary heart disease.

Influence of plasma lipid and LDL-subfraction profile on the interaction between low density lipoprotein with human arterial wall proteoglycans.

Low density lipoprotein (LDL) is known to bind to arterial wall proteoglycans (APG), an interaction which may initiate cholesterol deposition in the arterial wall. The objective of this study was to determine whether a predominance of small, dense LDL (LDL-III, d = 1.044-1.063 g/ml) in the circulation in association with an atherogenic lipoprotein phenotype (ALP) (i.e. LDL-III > 100 mg/dl, an elevated plasma triglyceride and a low high density lipoprotein cholesterol) alters LDL reactivity towards APG. Total LDL (d = 1.019-1.063 g/ml) was isolated from 59 patients undergoing coronary angiography (39 males and 20 females) and the LDL subfraction profile determined by non-equilibrium density gradient centrifugation. A binding assay was developed in which total LDL (0.1 mg/ml apo LDL) was mixed with a standard preparation of APG containing 2.5 micrograms/ml chondroitin sulphate and the extent of APG-LDL complex formation followed by absorbance measurement and the amount of precipitated LDL cholesterol. APG-LDL complex formation was positively associated with (a) the percentage of LDL-III within total LDL (r = 0.48, P < 0.0001); (b) the plasma triglyceride level (r = 0.27, P < 0.04); and negatively associated with (a) the percentage of the buoyant LDL-I (d = 1.019-1.033 g/ml)(r = -0.47, P < 0.0001); and (b) the HDL cholesterol concentration (r = -0.37, P < 0.004). There was no association with the percentage of the major LDL species LDL-II. When the patients were divided according to the presence or absence of an ALP i.e. LDL-III greater or less than 100 mg/dl respectively, proteoglycan-LDL complex formation was significantly higher in the former compared to the latter group of patients (P < 0.0001). This study therefore provides evidence that the extent of the interaction of LDL with APG varies considerably between individuals and is enhanced in the presence of ALP. It is postulated that the increased atherogenicity associated with ALP may in part be due to prolonged and enhanced retention of LDL by the arterial wall.