Long-term exposure to oxidized low-density lipoprotein enhances tumor necrosis factor-α-stimulated endothelial adhesiveness of monocytes by activating superoxide generation and redox-sensitive pathways

This study was conducted to investigate the role of oxidized low-density lipoprotein (LDL) in monocyte/mononuclear cell (MNC) activation during atherogenesis. First, the activity of MNCs was studied in patients with coronary artery disease (CAD). Compared to normal subjects, phorbol 12-myristate 13-acetate (PMA)-stimulated reactive oxygen species (ROS) production and the adhesiveness to endothelial cells were increased in MNCs from CAD patients. After 24-h coculture with oxidized LDL, ROS elaboration of MNCs was significantly increased in CAD patients. It was further correlated to the endothelial adhesiveness of MNCs (r = 0.561, P < 0.05). Secondly, in an in vitro model for long-term, direct effects of oxidized LDL on murine monocytoid cells (MMCs), oxidized LDL, but not native LDL, induced ROS production of MMCs in a time-dependent manner up to a 4-day coincubation (261% elevation, P < 0.05). Four-day coincubation with ox-LDL enhanced cytoplasmic IκB phosphorylation and nuclear factor kappa B (NF-κB) translocation and increased endothelial adhesiveness of MMCs. The long-term exposure to oxidized LDL also significantly enhanced tumor necrosis factor-α (TNF-α)-stimulated ROS production and endothelial adhesiveness of MMCs, which could be completely abolished by the short-term existence of pyrrolidine dithiocarbamate (PDTC), an antioxidant and NF-κB blocker and by long-term coincubation with N-acetylcysteine, a nonspecific antioxidant. Accordingly, circulating MNCs were activated with increased endothelial adhesiveness in CAD patients. Long-term exposure to oxidized LDL could directly activate MNCs ex vivo and MMCs in vitro and enhance TNF-α-stimulated endothelial adhesiveness through the redox-dependent NF-κB transcriptional pathway. The findings suggest the pivotal role of oxidized LDL-induced oxidative stress in monocyte activation during atherogenesis.