Oxidized low density lipoprotein-mediated macrophage survival : a role for calcium and regulation of cellular energy

Oxidized low density lipoprotein (oxLDL) as well as specific macrophage populations, play a pivotal role in the initiation and progression of atherosclerotic lesions. Our group previously reported that oxLDL inhibits apoptosis in bone marrow-derived macrophages (BMDM) deprived of macrophage colony-stimulating factor (M-CSF). In this study, I showed that oxLDL induces an oscillatory increase in intracellular calcium ([Ca²+]i) that is mediated by the activation of sphingosine kinase. This leads to activation of the Ca²+/calmodulin dependent kinase, eukaryotic elongation factor-2 kinase (eEF2 kinase). Both the increase in [Ca²+]i and activation of eEF2 kinase can be blocked by BAPTA-AM, an intracellular Ca²+ chelator. Addition of oxLDL also results in the phosphorylation of eEF2, the only known substrate of eEF2 kinase. The eEF2 kinase selective inhibitors, TS-4 and TX-1918, blocked oxLDL-mediated phosphorylation of eEF2 as well as oxLDL’s ability to promote survival of BMDMs. eEF2 kinase can be negatively regulated by p38 MAPK. Withdrawal of M-CSF results in the activation of p38 MAPK, an effect that is blocked with the addition of oxLDL. This indicates that oxLDL can positively regulate eEF2 kinase activity by both (1) generating an oscillatory increase in [Ca²+]i and (2) inhibiting its negative regulation by p38 MAPK. eEF2 is a monomeric GTPase that regulates peptide chain elongation and its phosphorylation inhibits its activity. Addition of oxLDL results in a decrease in overall protein synthesis and induction of autophagy in BMDMs. This suggests that under conditions of metabolic stress (e.g., growth factor withdrawal), there may be an eEF2 kinase-dependent protective mechanism that can be activated by the presence of oxLDL, thus linking energy conservation (e.g., via inhibition of protein synthesis) and the replenishment of energy supplies by digestion of cellular organelles through the induction of autophagy.Medicine, Faculty ofMedicine, Department ofExperimental Medicine, Division ofGraduat