Sialic-acid content of low-density lipoproteins controls their binding and uptake by cultured cells

Sphingolipid-induced enhancement of receptor-mediated uptake of low density lipoproteins in normal and receptor-deficient human skin fibroblasts

(1) The receptor mediated endocytosis of homologous LDL by human skin fibroblasts can be significantly enhanced by prior incubation of the cells with sphingolipids. Gangliosides GM1 or GD1a, their desialylated derivatives and sphingosine stimulate binding and uptake to LDL by up to 40% of normal values. The effect is observed in normal fibroblasts, LDL receptor deficient fibroblasts or in tunicamycin-treated cells with a reduced number of functional receptors but is dependent on the time of preincubation of the cells and the concentration of the sphingolipid in the medium. (2) Detailed studies on the ganglioside effect revealed, that cell bound gangliosides intensify the LDL-induced suppression of [14C] acetate incorporation into cholesterol. (3) The receptor dependence and relative receptor specificity of the sphingolipid effect is evident from the fact that (a) after complete suppression of receptor synthesis gangliosides fail to stimulate uptake of LDL, that (b) fatty acids or lipids not containing sphingosine are without effect and that (c) the receptor specific internalisation of alpha 2-macroglobulin or epidermal growth factor is not influenced by exogenous sphingolipids.

Action of low-density lipoprotein and compactin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase, on the synthesis of dolichol-linked oligosaccharides and low-density-lipoprotein receptor in human skin fibroblasts

To explore whether there is an inter-relationship between the rate of low-density (LD) lipoprotein binding to its receptor and the formation of dolichol-linked oligosaccharides, experiments were performed with human fibroblasts where the synthesis of lipoprotein receptor and dolichyl saccharides was under control of LD lipoprotein and compactin. Pretreatment of the cells with nonlabelled LD lipoprotein resulted in a suppression of both the binding of 125I-labelled LD lipoprotein to the receptor and the synthesis of dolichyl saccharides from [14C]acetate and [3H]mannose, but not from [3H]mevalonolactone. Compactin, in contrast, inhibited only the formation of dolichol-linked oligosaccharides. Mevalonolactone (1 microM) abolished the inhibitory effect of LD lipoprotein on dolichyl saccharide formation, but was not able to restore the receptor-binding capacity, thus suggesting that the synthesis of lipoprotein receptor is not coupled to the formation of dolichyl saccharides.

Effect of sialic acid removal on human low density lipoprotein catabolism in vivo

This study was undertaken to determine whether sialic acid removal alters the catabolism of low density lipoprotein in humans. Human low density lipoproteins labeled in vitro with 125I were incubated in the presence (termed desialylated) or absence (sialylated) of neuraminidase. The treatment with neuraminidase from Clostridium perfringens removed 90% of the sialic acid residues which did not change the chemical composition of the lipoproteins. Sialylated or desialylated LDL were injected intravenously into normal human subjects. The mathematical analysis of the plasma radioactivity decay curves (followed for 220 h) of desialylated low density lipoproteins, when compared with sialylated LDL, showed a shorter mean transit time (51 h vs 60 h), a 52% faster metabolic catabolic rate and an increased volume of distribution. The data are consistent with a proposed metabolism of low density lipoproteins: in humans, desialylation appears to accelerate the first step of the low density lipoprotein conversion but not to alter its final catabolism.

Effect of tunicamycin on the metabolism of low-density lipoproteins by control and low-density-lipoprotein-receptor-deficient human skin fibroblasts

Preincubation of normal human skin fibroblasts with tunicamycin, which inhibits N-glycosylation of glycoproteins, resulted in a dose-dependent and reversible inhibition of binding and internalization of homologous low-density lipoproteins by the cells. The degradation of the internalized lipoproteins was not affected by the drug. Comparative studies with fibroblasts deficient in low-density-lipoprotein receptors indicated that tunicamycin exerts its inhibitory effect only via the receptor-mediated high-affinity binding and uptake of lipoproteins. These results suggest that expression of low-density-lipoprotein receptors on the cell surface of human skin fibroblasts depends on intact N-glycosylation.