Redistribution of intracellular calcium and its effect on apoptosis in macrophages: Induction by oxidized LDL

Calcium signaling, as a key to early step of the elementary intracellular events, has been implicated in controlling the development of atherosclerosis. We have shown previously that oxidized low density lipoprotein OxLDL-induced spatiotemporal increases of intracellular free calcium ([Ca2+]i) in the early formation of macrophage foam cells. Here, we evaluated how spatiotemporal redistribution of intracellular calcium occurs and would affect OxLDL-induced apoptosis. Confocal laser scanning microscopy and flow cytometry showed the time-dependent increase of mitochondrial Ca2+ ([Ca2+]m) in acute and chronic exposure of U937-derived macrophages to OxLDL (100 μg/ml). Independent of the presence or absence of external Ca2+, OxLDL-induced a peak of [Ca2+]m in acute exposure, whose amplitude in the absence of extracellular Ca2+ was obviously lower than the presence of extracellular Ca2+. In addition, the thapsigargin-mediated increase of [Ca2+]i, through endoplasmic reticulum (ER) Ca2+ pump depletion, was obviously reduced by 1-h pretreatment of OxLDL. OxLDL also caused a time-dependent opening of mitochondrial permeability transition pores (PTPs). EGTA/AM, an intracellular Ca2+ chelator, significantly reduced OxLDL-induced apoptosis and failed to prevent OxLDL-induced necrosis at 6 h. In contrast to control cells, chelation of cytosolic Ca2+ by EGTA/AM at 6 h did not completely reverse OxLDL-induced apoptosis. OxLDL stimulated depolarization of mitochondrial membrane potential (Δψ) in time-dependent manner. Our data demonstrated that OxLDL-induced spatiotemporal Ca2+ redistribution in appropriate organelles and mediated Ca2+-dependent apoptosis in relation to depolarization of Δψ. These findings suggested that manipulation of the intracellular calcium balance may be a useful strategy to limit the loss of macrophages in early atherosclerosis.