Effects of TNF-alpha and IL-1 beta on iron metabolism by A549 cells and influence on cytotoxicity

Extracellular iron, which is predominantly bound by transferrin, is present in low concentrations within alveolar structures, and concentrations are i ncreased in various pulmonary disorders. Iron accumulation by cells can pro mote oxidative injury. However, the synthesis of ferritin stimulated by met al exposure for intracellular iron storage is normally protective. The cyto kines tumor necrosis factor (TNF)-alpha and interleukin (IL)-1 beta may alt er iron metabolism by alveolar cells. In this study, we assessed the effect s of TNF-alpha and IL-1 beta on iron metabolism with a cell line with prope rties of type 2 alveolar epithelial cells (A549) exposed to non-transferrin -bound (NTBI; FeSO4) or transferrin-bound (TBI) iron. In addition, we asses sed the cytotoxicity of these exposures by measuring the cell accumulation of malondialdehyde (MDA), a product of lipid peroxidation, and cell death ( MTT assay and lactate dehydrogenase release). A549 cells treated with NTBI or TBI in concentrations up to 40 mu M accumulated iron and synthesized pre dominantly L-type ferritin without accumulation of MDA or cell death. Treat ment of A549 cells with TNF-alpha (20 ng) or IL-1 beta (20 ng) decreased ce ll transferrin-receptor expression and induced synthesis of H-type ferritin . TNF-alpha and IL-1 beta decreased the uptake of TBI; however, the uptake of NTBI was increased. Both cytokines enhanced total ferritin synthesis (H plus L types) in response to iron treatments due to enhanced synthesis of H -type ferritin. Coexposure to TNF-alpha and NTBI, but not to TBI, induced M DA accumulation and greater cytotoxicity (MTT and lactate dehydrogenase rel ease) than TNF-alpha alone. These findings indicate that TNF-alpha and IL-1 beta modulate iron uptake by A549 cells, with differing effects on TBI and NTBI, as well as on H-ferritin synthesis. Enhanced iron uptake induced by TNF-alpha and NTBI was also associated with increased cytotoxicity to A549 cells.