Sialic‐Acid Content of Low‐Density Lipoproteins Controls Their Binding and Uptake by Cultured Cells

Abstract

The (high-affinity receptor)-mediated uptake of homologous low-density (low-g) lipoproteins by cultured human arterial smooth muscle cells or human skin fibroblasts is controlled by the sialic acid content of low-g lipoprotein particles. This conclusion is derived from the following results .1. Gangliosides incubated with native low-@ lipoproteins associate with low-e lipoprotein particles.Low-Q lipoproteins modified by associated GL,,l, GGtetl, and GGtet2b + Gctet3 gangliosides are internalized by arterial smooth muscle cells at a rate up to 80 % lower than native 1ow-Q lipoproteins or those preincubated with desialized gangliosides.2. The inhibitory effect of gangliosides is specific for high affinity uptake and not detectable on skin fibroblasts deficient in low-@-lipoprotein receptor.3. Desialyzed 1ow-Q lipoproteins are internalized by smooth muscle cells up to 100 % faster than native 1ow-Q lipoproteins, the enhancement of uptake corresponding to the degree of desialization.Homologous 1ow-Q lipoproteins are internalized by cultured human fibroblasts through an endocytotic process that requires high-affinity binding of lipoproteins to specific receptors located in coated pits on the cell surface [1,2] and a recognition site on the apoprotein component of lipoprotein, Arginine residues within apoprotein B and an arginine-rich protein have been identified as significant recognition markers of low-@ lipoproteins [3].The apoprotein component of low-g lipoproteins has been shown to contain 5-97! carbohydrates consisting of galactose, mannose, N-acetyl glucosamine and sialic acid, which form branched oligosaccharides with N-acetylneuraminic acid as the nonreducing terminus [4].The function of the carbohydrate moiety in low-g lipoprotein is not clear at present, but the assumption that the uptake of 1ow-Q lipoproteins by cultured cells might involve the interaction of the carbohydrate moiety of 1ow-g lipoproteins with cell membranes has been excluded [5].

Desialized Low‐Density Lipoprotein Regulates Cholesterol Metabolism in Receptor‐Deficient Fibroblasts

“…Recent studies [8] have pointed to the role of the net charge of LDL in its binding, internalisation and degradation by cultured skin fibroblasts. Removal of the sialic acid residues from LDL enhances its binding and uptake up to 100% relative to native LDL.…”

“…Normal human fibroblasts were grown from the skin explant of a normal infant as described previously [8] and were used for incubation experiments between the third and eight passages. Human skin fibroblasts deficient in LDL receptor (GM 1915) were obtained from the Institute for Medical Research (Camden, N.J., U.S.A.).…”

“…This process, however, does not cause regulation of cholesterol synthesis or cholesterol esterification [4 -71. Previous studies [8,9] revealed that the sialic acid content of LDL controls their receptor-mediated binding and uptake. Removal of sialic acid residues from the LDL results in an internalisation rate up to 100 % faster than that of native LDL.…”

“…Removal of sialic acid residues from the LDL results in an internalisation rate up to 100 % faster than that of native LDL. Native LDL were desialized as described previously [8].…”

Chemical composition of circulating native and desialylated low density lipoprotein: what is the difference?

“…[10] An early study by Filipovic [11] and co-authors showed that LDL modification enhanced cholesterol intake by cultured cells. Subsequently, naturally modified LDL types were found in human blood.…”