Abstract 726: Nox4 Expression in Human Monocyte-Derived Macrophages: Upregulation by Oxidized LDL via a MEK-ERK1/2-dependent Pathway
Nox4 is member of the family of NADPH oxidase expressed in endothelium and smooth muscle cells, but has not previously been reported in human monocytes or monocyte-derived macrophages. We now identified Nox4 as a new Nox member in human monocytes and monocyte-derived macrophages. Nox4 and the phagocytic NADPH oxidase gp91/Nox2 were differentially regulated during monocyte differentiation. In contrast to Nox2, Nox4 protein expression was upregulated in mature human macrophages, and confocal microscopy revealed that Nox4 colocalized primarily with mitochondria. Exposure of macrophages to OxLDL for 24 h further increased Nox4 expression in a concentration-dependent manner. OxLDL-induced Nox4 expression was blocked by inhibitors of MEK1/2, UO126 and PD98059, but not by SB203580 and SP600125, inhibitors of p38MAPK or JNK, respectively. Furthermore, MEK inhibitors, but not inhibitors of p38MAPK or JNK, protected macrophages from OxLDL-induced cell injury, identifying a MEK/ERK1/2-dependent signaling pathway upstream of OxLDL-induced Nox4 expression that is involved in macrophage death. Previously, we showed that OxLDL stimulates the intracellular formation of reactive oxygen species (ROS) in human macrophages, but the source of these ROS remained elusive. We also demonstrated that the peroxyl radical scavenger Trolox protects macrophages from OxLDL-induced cell injury. Interestingly, Trolox did not prevent Nox4 expression induced by OxLDL, suggesting that Trolox acts downstream of OxLDL-induced Nox4 expression. These results not only identify Nox4 as a new intracellular source of ROS in macrophages, but also implicate Nox4 in OxLDL-induced ROS formation and macrophage death, which may have important implications for the pathogenesis of atherosclerosis.