Macrophage-enhanced formation of cholesteryl ester-core aldehydes during oxidation of low density lipoprotein

Oxidation of low density lipoproteins (LDL) results in changes to the lipoprotein particle that are potentially pro-atherogenic. To investigate mechanisms contributing to the formation of cholesteryl ester (CE)-core aldehydes (9-oxononanoyl- and 5-oxovaleroyl-cholesterol; 9-ONC and 5-OVC, respectively) LDL was incubated in the presence of mouse macrophages (J774 cells) under different culture conditions. Here we demonstrate that the formation of core aldehydes occurs only in transition metal-containing HAM's F10 medium but not in Dulbecco's modified Eagle's medium (DMEM), independent of supplementation with iron and copper at concentrations up to ten times higher than present in HAM's F10. The antioxidative properties of DMEM could be ascribed to the higher amino acid and vitamin content as compared to HAM's F10 medium. Supplementation with these components efficiently inhibited LDL oxidation in HAM's F10. Stimulation of J774 cells with phorbol ester (PMA) resulted in significantly enhanced 9-ONC and 5-OVC formation rates that were accompanied by increased consumption of LDL cholesteryl linoleate (Ch18:2) and cholesteryl arachidonate (Ch20:4) in the cellular supernatant. In PMA (10 ng/ml) activated cells, approximately 5% of Ch18:2 contained in LDL was converted to 9-ONC and 4% of Ch20:4 was converted to 5-OVC. With respect to core aldehyde formation, lipopolysaccharide (LPS, 10 μg/ml) was a less effective stimulant as compared to PMA. Part of the core aldehydes accumulated within the cells. Our study demonstrates that i) J774 macrophages are able to promote/accelerate core aldehyde formation in HAM's F10 medium, and ii) that core aldehyde formation rates can be increased by stimulation of the cells with PMA, and, although to a lesser extent, with LPS. Finally we could show that iii) a small amount of the core aldehydes is internalized by J774 macrophages.