Cytotoxic Potencies of Zinc Oxide Nanoforms in A549 and J774 Cells

Zinc oxide nanoparticles (NPs) are used in a wide range of consumer products and in biomedical applications, resulting in an increased production of these materials with potential for exposure, thus causing human health concerns. Although there are many reports on the size-related toxicity of ZnO NPs, the toxicity of different nanoforms of this chemical, toxicity mechanisms, and potency determinants need clarification to support health risk characterization. A set of well-characterized ZnO nanoforms (e.g., uncoated ca. 30, 45, and 53 nm; coated with silicon oil, stearic acid, and (3-aminopropyl) triethoxysilane) were screened for in vitro cytotoxicity in two cell types, human lung epithelial cells (A549), and mouse monocyte/macrophage (J774) cells. ZnO (bulk) and ZnCl₂ served as reference particles. Cytotoxicity was examined 24 h post-exposure by measuring CTB (viability), ATP (energy metabolism), and %LDH released (membrane integrity). Cellular oxidative stress (GSH-GSSG) and secreted proteins (targeted multiplex assay) were analyzed. Zinc oxide nanoform type-, dose-, and cell type-specific cytotoxic responses were seen, along with cellular oxidative stress. Cell-secreted protein profiles suggested ZnO NP exposure-related perturbations in signaling pathways relevant to inflammation/cell injury and corresponding biological processes, namely reactive oxygen species generation and apoptosis/necrosis, for some nanoforms, consistent with cellular oxidative stress and ATP status. The size, surface area, agglomeration state and metal contents of these ZnO nanoforms appeared to be physicochemical determinants of particle potencies. These findings warrant further research on high-content "OMICs" to validate and resolve toxicity pathways related to exposure to nanoforms to advance health risk-assessment efforts and to inform on safer materials.