Macrophages Can Decrease the Level of Cholesteryl Ester Hydroperoxides in Low Density Lipoprotein

Anna Baoutina,Roger T Dean,Wendy Jessup

doi:10.1074/jbc.275.3.1635

Abstract

Murine and human macrophages rapidly decreased the level of cholesteryl ester hydroperoxides in low density lipoprotein (LDL) when cultured in media non-permissive for LDL oxidation. This process was proportional to cell number but could not be attributed to the net lipoprotein uptake. Macrophage-mediated loss of lipid hydroperoxides in LDL appears to be metal ion-independent. Degradation of cholesteryl linoleate hydroperoxides was accompanied by accumulation of the corresponding hydroxide as the major product and cholesteryl keto-octadecadienoate as a minor product, although taken together these products could not completely account for the hydroperoxide consumption. Cell-conditioned medium possessed a similar capacity to remove lipid hydroperoxides as seen with cellular monolayers, suggesting that the activity is not an integral component of the cell but is secreted from it. The activity of cell-conditioned medium to lower the level of LDL lipid hydroperoxides is associated with its high molecular weight fraction and is modulated by the availability of free thiol groups. Cell-mediated loss of LDL cholesteryl ester hydroperoxides is facilitated by the presence of alpha-tocopherol in the lipoprotein. Together with our earlier reports on the ability of macrophages to remove peroxides rapidly from oxidized amino acids, peptides, and proteins as well as to clear selectively cholesterol 7-beta-hydroperoxide, results presented in this paper provide evidence of a potential protective activity of the cell against further LDL oxidation by removing reactive peroxide groups in the lipoprotein.

Highlights

Oxidation of low density lipoprotein (LDL)1 in the arterial intima is believed to play a key role in atherogenesis [1, 2]
When LDL was incubated with cells plated at different densities, the decrease in CEOOH concentration occurred at different rates, with higher rate corresponding to the higher cell number condition
Parallel to reduction of CEOOH level, incubation of MPM with LDL in Dulbecco’s minimum essential medium (DMEM) resulted in a decrease in the concentration of TOH in LDL (Fig. 1C), whereas no significant loss of TOH was observed in cell-free conditions (Fig. 1C)

Summary

Introduction

Oxidation of low density lipoprotein (LDL) in the arterial intima is believed to play a key role in atherogenesis [1, 2]. In addition to pro-oxidant action of cells toward LDL, it has been demonstrated that under certain conditions the cells could have potential antioxidant activity toward the lipoprotein. Endothelial cells have been shown to prevent formation of lipid hydroperoxides in LDL [7]; this activity switched to a pro-oxidant action of the cells upon increasing the initial concentrations of hydroperoxides in LDL or metal ions in media. A potential protective role has been demonstrated for human hepatic cells, which selectively take up and detoxify cholesteryl ester hydroperoxides from high density lipoprotein particles [8]. We demonstrated the ability of cells to extracellularly detoxify cholesteryl ester hydroperoxides in LDL in a culture medium that did not support cell-mediated LDL oxidation, and we studied possible mechanisms of this process. We tested different cell types for their ability to reduce levels of lipid hydroperoxides in lipoprotein and regulation of this process

Methods

Results

Conclusion