Modification of copper-catalyzed oxidation of low density lipoprotein by proteoglycans and glycosaminoglycans.

G Camejo,G Bondjers,O Wiklund,F López,B Rosengren,E Hurt-Camejo

doi:10.1016/s0022-2275(20)41901-7

Abstract

Chondroitin sulfate proteoglycans (CSPG) appear to contribute to retention of low density lipoproteins (LDL) in atherosclerotic lesions. In vitro, CSPG and glycosaminoglycans (GAG) modify LDL structure and increase its uptake by macrophages. This latter effect appears related to increased exposure of arginine- and lysine-rich segments of apoB-100. We explored whether alterations of LDL induced by human arterial CSPG and purified GAG alter the lipoprotein susceptibility to transition metals-catalyzed oxidation. Human LDL was complexed with human arterial CSPG and dissociated by raising the ionic strength. The nonaggregated, CSPG- and GAG-treated LDL was subjected to oxidation by micromolar amounts of Cu+, Cu2+, Fe2+, and Fe3+. This treatment increased LDL susceptibility to Cu2+ oxidation 3- to 5-times, as indicated by the degradation rate of phospholipids and cholesteryl esters and formation rates of dienes and thiobarbituric acid-reacting substances (TBARS). Also, human macrophages degraded the CSPG-treated, Cu2+-oxidized LDL 3- to 6-times faster than native LDL similarly treated. No enhancement of oxidation was observed with Fe2+, Fe3+, and Cu+. Quenching of the LDL intrinsic fluorescence by Cu2+ showed that heparin, CSPG, and chondroitin-6-SO4 pretreatment increased the access of Cu2+ to hydrophobic chromophores, probably tryptophan, 6- to 7-, 3- to 4-, and 2- to 3-fold, respectively. Also, the affinity constant (Ka) of LDL for Cu2+ was increased from 0.12 microM to 0.20 microM by the treatment with CSPG and GAG. These results and evaluation of the fraction of surface-accessible LDL chromophores to acrylamide quenching suggest that the increased susceptibility to oxidation may be associated with an increase in the access of Cu2+ to hydrophobic regions in LDL caused by treatment with CSPG and GAG. This effect was not detected with Cu+, Fe2+, or Fe3+. The phenomenon may contribute to acceleration of the oxidative modifications of LDL in cell culture models and in vivo.

Highlights

Chondroitin sulfate proteoglycans (CSPG) appear to contribute to retention of low density lipoproteins (LDL) in atherosclerotic lesions
Low density lipoprotein that had been insolubilized with arterial CSPG, C6S, and heparin at low ionic strength and dissociated by raising the concentration of NaCl to physiological levels, when exposed to Cu2+was oxidized at a faster rate than native LDL
The conversion of the lipids in LDL to products that were not extracted by chloroform-methanol 2:l and that remained at the origin during thin-layer chromatography took place markedly faster in LDL that had been complexed and dissociated from arterial CSPG

Summary

Introduction

Chondroitin sulfate proteoglycans (CSPG) appear to contribute to retention of low density lipoproteins (LDL) in atherosclerotic lesions. In vitro experiments show that human LDL that has been complexed to chondroitin sulfate-rich proteoglycans (CSPG) and subsequently dissociated is taken u p more avidly by human macrophages with the eventual formation of foam cells [12, 13] These effects are probably caused both by structural alterations of LDL induced by the formation of reversible complexes with the intimal proteoglycans and by selection of LDL particles with a high affinity for the CSPG [14]. Such LDL alterations appear to involve a disruption of the lipid-protein organization, which was detected by low angle X-ray analysis [15], differential scanning calorimetry, proton nuclear magnetic resonance [16], and increased susceptibility to tryptic fragmentation [12, 17]. This could cause rapid internalization of LDL by resident macrophages that accumulate at sites of lesion progression in response to chemo-attractants produced during lipid peroxidation

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Conclusion