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].

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1. Calf arterial tissue incorporated [ l-14C]acetate into fatty acids of triglycerides when incubated under a gas phase of either 95O/, 0,/5°/0 CO, or 95O/, N,/5°/0C0,, but in hypoxia the specific radioactivity of fatty acids was 3-5 times higher.2. Gas liquid chromatography of the C8-C18: , fatty acid methylesters and analysis of the amount of radioactivity present in the carboxyl carbon of the fatty acids were consistent with the assumption of a chain elongation mechanism of preformed acyl units which is active mainly with shorter chain (C8-Cl, carbon) fatty acids and appeared to be the predominant synthetic system in the presence and in the absence of oxygen.3. Compared to incubation experiments under a gas phase of O,/CO,, the rate of chain elongation rises markedly in hypoxia, resulting in a 37-fold increase of the specific radioactivity of lauric acid and a 3 to 11 -fold increase of the specific radioactivity of the other fatty acids investigated.4. I n hypoxic arterial tissue minor changes in the amount and in the specific radioactivity of acetyl-CoA were observed, while the NADH/NAD ratio rose to high levels.Arterial tissue has been shown to metabolize [Wlacetate and to incorporate 1%-radioactivity into total lipids and their subfractions under appropiate conditions [1,2]. On incubation in vitro the incorporation rate depends on the partial pressure of oxygen in the gas phase used for the experiments. Previous studies revealed a 5-fold increase of the 14C-incorporation rate into the triglyceride fraction of total lipids and a concomitant rise of NADH/NAD ratio in the absence of oxygen [3]. Since oxygen supply and NADHINAD ratio were found to be factors controlling the mitochondria1 fatty acid synthesis in arterial tissue [4] and many other organs [5][6][7]171, this paper presents a more detailed analysis of the fatty acid synthesis of arterial tissue in the presence and absence of oxygen. The underlying process is a chain elongation mechanism which is stimulated by impaired respiration. The results obtained may be of significance for the interpretation of the known lipid accumulation in arteriosclerosis. MATERIALS AND METHODS MaterialsCalf arterial tissue (aorta thoracica) was obtained from the abattoir immediately after slaughter, kept in ice-cold 155 mM NaCl aolution and was used for the experiments within 1 h after previous careful removal of the tunica adventitia and adherent connective tissue. Acetyl-CoA, chemicals used for enzymatic analyses and organic solvents were of analytical grade and were purchased from Boehringer Mannheim GmbH (Mannheim, Germany) and E. Merck (Darmstadt, Germany). Na[l-14C]acetate (specific activity 40.0 mCi/mmole) and sodium [32P]phosphate (specific radioactivity 31 mCi/mmole phosphate) were obtained from Radiochemical Centre (Amersham). Although a total absence of oxygen was not secured, the experiments under a gas phase of N,/CO, are termed "anaerobic". For comparative experiments in aerobiosis and anaerobiosis, samples of the same aorta were used. The experiments were ...

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Binding of homologous low density lipoproteins (LDL) to specific cell-membrane receptors initiates regulation of cholesterol metabolism in normal human fibroblasts, but minor regulatory effects are observed in receptor-deficient cells. After enzymic removal of sialic acid residues, however, the desialized LDL aquires the ability to regulate cholesterol biosynthesis and cholesterol esterification in receptor-deficient cells as effective as native LDL in normal cells. This is indicated by a suppression of hydroxymethylglutaryl-CoA reductase activity, by a reduced incorporation of [14C]acetate into cholesterol and by a stimulation of the [3H]oleate incorporation into cholesterol esters in both normal and receptor deficient cells. Competitive binding and internalisation experiments revealed that the action of desialized LDL in normal and receptor-deficient cells is receptorspecific.The results suggest that desialisation of LDL facilitates its binding to cell membrane receptors, its internalisation and subsequent regulation of cellular cholesterol metabolism, this effect becoming significant in fibroblasts with reduced number of LDL receptors.The receptor-mediated high affinity binding and internalisation of LDL by cultured human skin fibroblasts or aortic smooth-muscle cells initiates a series of events that regulate cholesterol metabolism (for reviews see [l -3]), including (a) suppression of hydroxymethylglutdryl-CoA reductase activity and hence the reduction of cellular cholesterol synthesis, (b) stimulation of acyl-CoA : cholesterol acetyltransferase activity resulting in enhanced cholesterol esterification and (c) inhibition of LDL receptor synthesis thus protecting the cell against overaccumulation of cholesterol.In fibroblasts from patients with the homozygous form of familial hypercholesterolaemia in which the LDL receptor is absent, the LDL enters the cell by a non-specific pinocytotic process indepent of the LDL receptor. 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.

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