ZnO Particulate Matter Requires Cell Contact for Toxicity in Human Colon Cancer Cells

There is ongoing concern regarding the toxicity of nanoparticles with sizes less than 100 nm as compared to larger particles of the same nominal substance. Two commercial ZnO types, one sold as a 8-10 nm powder and the other described as 325 mesh (<44 mu m) powder, were evaluated in human colon-derived RKO cells. The powders had a volume-to-surface area ratio equivalent to 40 and 330 nm spheres, respectively. Both materials formed micrometer-sized agglomerates in cell culture media. The nanosized ZnO was more cytotoxic than the micrometer-sized ZnO with LC50 values of 15 +/- 1 and 29 +/- 4 mu g/cm(2), respectively. Transfer of Zn from the solid phase to the cell culture media in the presence of RKO cells was time- and concentration-dependent. However, direct particle cell contact was required for RKO cell cytotoxicity, and the toxicity of particles was independent of the amount of soluble Zn in the cell culture media. The mechanism of cell death includes the disruption of mitochondrial function. Robust markers of apoptosis, Annexin V staining, loss of mitochondrial potential, and increased generation of superoxide were observed when cells were treated with ZnO particulate matter but not when treated with comparable concentration of a soluble Zn salt. Both ZnO samples induced similar mechanisms of toxicity, but there was a statistically significant increase in potency per unit mass with the smaller particles.