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

Abstract

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 ([Ca 2+] 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 Ca 2+ ([Ca 2+] m) in acute and chronic exposure of U937-derived macrophages to OxLDL (100 μg/ml). Independent of the presence or absence of external Ca 2+, OxLDL-induced a peak of [Ca 2+] m in acute exposure, whose amplitude in the absence of extracellular Ca 2+ was obviously lower than the presence of extracellular Ca 2+. In addition, the thapsigargin-mediated increase of [Ca 2+] i, through endoplasmic reticulum (ER) Ca 2+ 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 Ca 2+ chelator, significantly reduced OxLDL-induced apoptosis and failed to prevent OxLDL-induced necrosis at 6 h. In contrast to control cells, chelation of cytosolic Ca 2+ by EGTA/AM at 6 h did not completely reverse OxLDL-induced apoptosis. OxLDL stimulated depolarization of mitochondrial membrane potential (Δ ψ) in time-dependent manner. Our data demonstrated that OxLDL-induced spatiotemporal Ca 2+ redistribution in appropriate organelles and mediated Ca 2+-dependent apoptosis in relation to depolarization of Δ ψ. These findings suggested that manipulation of the intracellular calcium balance may be a useful strategy to limit the loss of macrophages in early atherosclerosis.