Differential genotoxicity of chemical properties and particle size of rare metal and metal oxide nanoparticles.

Nanoparticles of rare metal compounds are used in various products. However, their carcinogenicity and genotoxicity have not been sufficiently evaluated. The tumor-initiating and -promoting potentials of four rare metals, indium oxide (In2O3), dysprosium oxide (Dy2O3), tungsten oxide (WO3) and molybdenum (Mo), with a well-defined particle diameter were evaluated. The mutagenicity of these rare metals was investigated by Ames test using five bacteria strains, and transformability of these rare metals was investigated by cell-transformation assay using v-Ha-ras-transfected BALB/c 3T3 cells (Bhas 42 cells). Nano-sized Dy2O3 showed strong mutagenesis in all five bacteria strains tested with and without metabolic activation, while micro-sized particles showed weak mutagenesis in two bacterial strains. Dy2O3 induced transformation colonies of Bhas 42 cell dose-dependently, although there was no difference in the number of transformed foci between nano-sized and micro-sized particles. Nano-sized In2O3 and WO3 showed positive mutagenic response in TA1537 and TA98, respectively, whereas the micro-sized metal oxide particles showed no mutagenesis in the test bacterial strains. Both nano-sized and micro-sized In2O3 showed similar levels of transformability. However, nano-sized and micro-sized WO3 did not show any transformability. Both nano-sized and micro-sized Mo particles showed neither mutagenesis nor transformability. These results suggest that mutagenicity of rare metals depends on their particle size, although transformability depends on their chemical components but not on their particle size.