Lead/cytokine-mediated oxidative DNA damage in cultured mouse hepatocytes

Hepatocytes exposed to the cytokines tumor necrosis factor alpha (TNFalpha) and interferon gamma (IFNgamma), in combination with a nontoxic dose of lead (Pb), exhibited significantly greater cytotoxicity compared to cytokine-treated hepatocytes after 24 hr treatment. Concentrations of Pb which interact with the cytokines to cause cytotoxicity stimulated hepatocytes cultured at this high cell density (63,830 cells/cm2) to undergo active DNA synthesis. DNA synthesis was also observed in hepatocytes plated at a low cell densities (21,276 cells/cm2) and these cells were sensitive to the toxic effects of the cytokines in the absence of Pb. Under conditions of active DNA synthesis, induced by either Pb or cell density, cytokine-induced toxicity appeared to be the result of extensive DNA fragmentation. Both DNA fragmentation and cytotoxicity were inhibited by treatment with an antioxidant mixture. Pb potentiated cytokine-induced oxidative stress within hepatocytes as indicated by decreased intracellular glutathione (GSH) and increased efflux of oxidized glutathione (GSSG) prior to cytotoxicity. The combination of cytokines and Pb also caused a significant decline in intracellular ATP concentrations prior to the onset of cytotoxicity. ATP concentrations were not altered by treatment with Pb or cytokines alone. Pb treatment did not alter total protein synthesis within hepatocytes. These results indicate that the cytotoxic interaction between Pb and the cytokines TNFalpha and IFNgamma may be mediated by oxidative DNA damage resulting from cytokine-induced oxidative stress and stimulation of mitogenic signals.