Wilson, Diane L; (1995) Lipoprotein oxidation and cholesterol esterification in a human monocytic cell line. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The pathogenesis of atherosclerosis is a complex process involving the interaction of a variety of components with cells of the arterial wall. Central to an understanding of the pathogenesis of this disease is the role of lipoproteins and their metabolism by foam cells of the developing atherosclerotic lesion. These cells are mainly monocyte-derived macrophages which have accumulated large quantities of cholesteryl ester in their cytoplasm due to the activity of the enzyme acylCoA:cholesterol acyltransferase (ACAT). There is now considerable evidence to suggest that the formation of foam cells may occur as a result of uptake of modified LDL, probably oxidised LDL (oxLDL), via the scavenger on the surface of macrophages. In the present study the effects of native and modified lipoproteins on cholesterol esterification by ACAT were investigated in a human macrophage cell line THP-1. Lower cholesteryl ester accumulation was observed when oxLDL or native LDL were incubated with the cells when compared to that using acetylated LDL (acLDL). This is probably not due to inhibition of ACAT by oxidation products of LDL because five different oxysterols, lysophosphatidylcholine and two hydroperoxides (13-HPODE and 15-HPETE) did not inhibit the enzyme. In fact oxysterols acted as substrates for ACAT and the hydroperoxides caused slight inhibition of ACAT but only at high concentrations. Although oxLDL themselves showed cytotoxic effects to the cells, none of the tested oxysterols showed any significant cytotoxicity except for 25-OH cholesterol. The presence of an ACAT inhibitor had no effect on the cytotoxicity indicating that a potential build up of oxysterols was no more cytotoxic to the cells. Measurement of the accumulation within the cells of fluorescently-labelled lipoproteins by flow cytometry revealed that both acLDL and oxLDL were taken up into these cells to the same extent. Use of the scavenger receptor ligands, polyinosinic acid and fucoidan, indicated that both acLDL and oxLDL were taken up via the scavenger receptor. Taken together, these results suggest that the lower cholesteryl ester accumulation observed with oxLDL relative to acLDL may be the result of differential intracellular processing rather than a difference in lipoprotein uptake or in inhibition of ACAT by lipoprotein-derived oxidation products.

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