Oxidized Low Density Lipoproteins Alter Macrophage Lipid Uptake, Apoptosis, Viability and Nitric Oxide Synthesis

Abstract

Uptake of oxidized low density lipoproteins (LDL) by monocyte macrophages to form “foam” cells occurs during formation of atherosclerotic lesions. Inducible nitric oxide synthase (iNOS) has been identified in foam cells. To investigate interactions between oxidized LDL, monocyte macrophage viability and iNOS, studies were performed with J774.A1 macrophages. iNOS mRNA, protein and enzyme activity were induced in J774.A1 macrophages by IFN-γ and lipopolysaccharide (LPS). Neither iNOS induction nor inhibition of nitric oxide (NO) formation was associated with significant alterations in the binding, uptake or degradation of native or oxidized LDL. Nontoxic doses of native LDL or of oxidized LDL did not influence iNOS mRNA or protein in macrophages. However, oxidized LDL, but not native LDL or acetyl LDL, inhibited NO production by macrophages in a dose- and time-dependent fashion. Inhibition of iNOS was not correlated with cholesteryl ester formation but with the degree of LDL oxidation. Inhibition of iNOS did not require the scavenger receptor or directed endocytosis and exhibited noncompetitive kinetics. Inhibition of iNOS activity in J774.A1 macrophages was produced by lipid from oxidized LDL but not by lipid from native LDL and by PC vesicles containing LPC but not by PC vesicles alone. Inhibition of NO formation diminished apoptosis of the activated macrophages. The data suggest NO production by iNOS and inhibition of the enzyme by oxidized LDL lipid may influence cell viability, cell-cell interactions and vasomotor tone during atherogenesis.