Oxidized low density lipoprotein inhibits macrophage apoptosis through activation of the PI 3-kinase/PKB pathway

Oxidized LDL (oxLDL) is known to induce endothelial adhesion molecule and monocyte chemoattractant protein 1 expression and this is thought to be involved in monocyte recruitment into atherosclerotic lesions. oxLDL has also been found to induce macrophage proliferation. The purpose of the present study was to determine whether oxLDL might also have the ability to increase macrophage populations by inhibiting apoptosis. We found that oxLDL caused a dose-dependent inhibition of the apoptosis that occurs in cultured bone marrow-derived macrophages after macrophage colony-stimulating factor (M-CSF) withdrawal without inducing proliferation. Incubation of macrophages with either native LDL or acetylated LDL had no effect on apoptosis. The prosurvival effect of oxLDL was not inhibited by neutralizing antibodies to granulocyte-macrophage colony-stimulating factor, was maintained in mice homozygous for a mutation in the M-CSF gene, and was not due to other secreted cytokines or growth factors. oxLDL caused activation of the mitogen-activated protein kinases ERK1/2 (extracellular signal-regulated kinases 1 and 2) as well as protein kinase B (PKB), a target of phosphatidylinositol 3-kinase (PI 3-kinase). Furthermore, there was phosphorylation of two important prosurvival PKB targets, I-kappaBalpha(Ser-32) and Bad(Ser-136). The MEK inhibitors PD 98059 and U0126 blocked ERK1/2 activation but did not diminish survival. Conversely, the PI 3-kinase inhibitors LY 294002 and wortmannin blocked PKB activation, and the ability of oxidized LDL to promote macrophage survival. Taken together, these results indicate that oxLDL can directly activate a PI 3-kinase/PKB-dependent pathway that permits macrophage survival in the absence of growth factors.