Phospholipase A2-modified low density lipoprotein induces mitochondrial uncoupling and lowers reactive oxygen species in phagocytes

Low density lipoprotein modified by secretory phospholipase A(2) (PLA-LDL) protects monocytes against oxidative stress. In this study we investigated possible direct effects of PLA-LDL on mitochondrial membrane potential and reactive oxygen species generation. Mitochondrial membrane potential in human monocytic THP-1 cells or primary human monocytes was monitored by flow cytometry using the fluorescent dye 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide or respirometry. Formation of reactive oxygen species was determined by flow cytometric measuring 2',7'-dichlorofluorescin oxidation. Cell death was assessed using Annexin V/propidium iodide staining. We observed that PLA-LDL caused mitochondrial uncoupling in monocyte/macrophage cell lines as well as in primary human monocytes. PLA-LDL-associated non-esterified fatty acids provoked uncoupling. Uncoupling attenuated reactive oxygen species formation induced by hydrogen peroxide, 2,3-dimethoxy-1,4-naphthoquinone or oxidized LDL. Knock-down of uncoupling protein UCP2 affected neither PLA-LDL-induced uncoupling, nor reactive oxygen species generation. Furthermore, we observed that the chemical uncoupler carbonyl cyanide m-chlorophenylhydrazone increased THP-1 cell survival after hydrogen peroxide treatment. Thus, PLA-LDL-induced uncoupling attenuates reactive oxygen species generation, which may contribute to increased monocyte survival in atherosclerotic plaques and support pro-atherogenic effects of LDL modified by PLA(2).