Low-density lipoprotein modification occurring in human plasma: Possible mechanism of in vivo lipoprotein desialylation as a primary step of atherogenic modification

Abstract

We previously found in human blood a fraction of low-density lipoprotein (LDL) that is characterized by a reduced content of sialic acid. Desialylated LDL also has a low neutral carbohydrate level, decreased content of major lipids, small size, high density, increased electronegative charge and altered tertiary apolipoprotein B structure. Unlike native LDL, this fraction of desialylated (multiple-modified) LDL induces the accumulation of lipids in smooth muscle cells cultured from unaffected human aortic intima, i.e. it exhibits atherogenic properties. In this study, we attempted to elucidate the mechanism of desialylation and other changes in the multiple-modified LDL by investigating the possibility of LDL modification by different cells and the blood plasma. A 24-h incubation at 37°C of lipoprotein with intact endotheliocytes, hepatocytes, macrophages and smooth muscle cells or cell homogenates did not cause alterations either in the physical properties or in the chemical composition of native LDL. On the other hand, a significant fall in the lipoprotein sialic acid level was observed already after a 1-h incubation of native LDL with an autologous plasma-derived serum. While LDL sialic acid level continuously decreased, LDL became capable of inducing the accumulation of total cholesterol in the smooth muscle cells cultured from unaffected human aortic intima after 3 h of incubation. Starting from the sixth hour of LDL incubation with serum, a steady decrease in the lipoprotein lipid content was observed as well as the related reduction of LDL size. Following 36 h of incubation, an increase in the negative charge of lipoprotein particles was also seen. Prolonged incubation of LDL with plasma-derived serum (48 and 72 h) leads to the loss of α-tocopherol by the LDL as well as to an increase in LDL susceptibility to copper oxidation and to accumulation of cholesterol covalently bound to apolipoprotein B, a marker of lipoperoxidation. Degradation of apolipoprotein B starts within the same period of time. Hence, desialylation of LDL particles represents one of the first or the primary act of modification which is, apparently, a sufficient prerequisite for the development of atherogenic properties. Subsequent modifications just enhance the atherogenic potential of LDL. The loss of sialic acid by LDL occurred at neutral pH and was not inhibited by the sialidase inhibitor 2,3-dehydro-2-deoxy- N-acetylneuraminic acid. The [ 3H]sialic acid removed from LDL was not found in free form, but in the plasma fraction precipitated by trichloroacetic acid. These data along with the fact that cytidine-5′-triphosphate inhibited LDL desialylation suggest that enzymes close to sialyltransferases play a role in this process. Thus, this study demonstrated that the LDL modification processes imparting atherogenic properties to this lipoprotein can take place in human blood plasma. Multiple modification of LDL is a cascade of successive changes in the lipoprotein particle: desialylation, loss of lipids, reduction in particle size, increase of its electronegative charge and peroxidation of lipids.