Diesel exhaust particle-induced cell death of human leukemic promyelocytic cells HL-60 and their variant cells HL-NR6.

The cytotoxicity of diesel exhaust particles (DEPs) toward human leukemic promyelocytic cells HL-60 was examined. DEPs were toxic and cytotoxicity increased in a dose-dependent manner. All cells died with 750 microg/ml DEPs in culture media. Apoptosis occurred in HL-60 cells exposed to DEPs. The cytotoxicity of DEP extracts with organic solvents was much lower than those of DEPs and organic solvent-washed residual DEPs. HL-NR6 cells, an HL-60 variant cell line, having higher superoxide dismutase and catalase activities than HL-60 cells, were more resistant to DEP cytotoxicity. When preincubated with the fluorescent probe diacetoxymethyl 6-carboxy-2',7'-dichlorodihydrofluorescinate diacetate and then exposed to DEPs, HL-60 cells emitted green fluorescence under blue illumination, indicating that reactive oxygen species were generated within the cells. The DEP cytotoxicity correlated inversely with the cellular concentration of reduced glutathione (GSH), which had been attenuated with L-buthionine-(R,S)-sulfoximine, a gamma-glutamylcysteine synthetase inhibitor, and was lowered with ethyl reduced glutathionate, a GSH carrier across biomembranes. Further, DEPs themselves decreased the cellular concentration of GSH in a dose-dependent manner. The alpha-tocopherol model compound 2,2,5,7,8-pentamethylchroman-6-ol decreased DEP cytotoxicity, while alpha-tocopherol had no effect. In addition, quinacrine, an endocytosis inhibitor, decreased DEP cytotoxicity. These results show that DEPs are cytotoxic and suggest that the cytotoxicity results from generation of reactive oxygen species by DEPs which have been incorporated into cells.