Oxidized LDL triggers phosphatidylserine exposure in human monocyte cell lines by both caspase-dependent and -independent mechanisms.

Monocytic cell lines have been extensively used to characterize and model various features of the atherogenic process. We found striking differences in the apoptotic pathways of U937 cells and THP-1 cells exposed to copper-oxidized LDL. While phosphatidylserine exposure occurred in both lines, caspase activation was only apparent in the THP-1 cells. OxLDL caused caspase activity to decrease below that seen in untreated U937 cells, and this corresponded with a loss in intracellular thiols. In conclusion, exposure of U937 cells to oxLDL did not trigger a conventional apoptosis response, but still resulted in phosphatidylserine externalization.