Transferrin: A potential source of iron for oxygen free radical-mediated endothelial cell injury

Abstract

The ability of transferrin to potentiate oxygen free radical-mediated endothelial cell injury was assessed. 51Cr-labeled endothelial cells derived from rat pulmonary arteries (RPAECs) were incubated with hydrogen peroxide (H 2O 2) in the presence and absence of holosaturated human transferrin, and the effect of transferrin on H 2O 2-mediated endothelial cell toxicity was determined. Addition of holosaturated transferrin potentiated H 2O 2-mediated RPAEC cytotoxicity at concentrations of H 2O 2 greater than 10 μ m, suggesting that transferrin may provide a source of iron for free radical-mediated endothelial cell injury. Free radical-mediated injury is dependent on non-protein-bound iron. The ability of RPAECs to facilitate the release of iron from transferrin was assessed. We determined that RPAECs facilitate the release of transferrin-derived iron by reduction of transferrinbound ferric iron (Fe 3+) to ferrous iron (Fe 2+). The reduction and release of transferrin-derived Fe 2+ were inhibited by apotransferrin and chloroquine, indicating a dependence on receptor-specific binding of transferrin to the RPAEC cell surface, with subsequent endocytosis, acidification, and reduction of transferrin-bound Fe 3+ to Fe 2+. The release of transferrin-derived Fe 2+ was potentiated by diethyldithiocarbamate, an inhibitor of intracellular superoxide dismutase (SOD). In contrast, exogenous SOD did not alter iron release, suggesting that intracellular superoxide anion (O 2 −) may play an important role in mediating the reduction and release of transferrin-derived iron. Results of this study suggest that transferrin may provide a source of iron for oxygen free radical-mediated endothelial cell injury and identify a novel mechanism by which endothelial cells may mediate the reduction and release of transferrin-derived iron.