Cell-biological effects of zinc oxide spheres and rods from the nano- to the microscale at sub-toxic levels

Zinc oxide particles were synthesized in various sizes and shapes, i.e., spheres of 40-nm, 200-nm, and 500-nm diameter and rods of 40 center dot 100 nm(2) and 100 center dot 400 nm(2) (all PVP-stabilized and well dispersed in water and cell culture medium). Crystallographically, the particles consisted of the hexagonal wurtzite phase with a primary crystallite size of 20 to 100 nm. The particles showed a slow dissolution in water and cell culture medium (both neutral; about 10% after 5 days) but dissolved within about 1 h in two different simulated lysosomal media (pH 4.5 to 4.8). Cells relevant for respiratory exposure (NR8383 rat alveolar macrophages) were exposed to these particles in vitro. Viability, apoptosis, and cell activation (generation of reactive oxygen species, ROS, release of cytokines) were investigated in an in vitro lung cell model with respect to the migration of inflammatory cells. All particle types were rapidly taken up by the cells, leading to an increased intracellular zinc ion concentration. The nanoparticles were more cytotoxic than the microparticles and comparable with dissolved zinc acetate. All particles induced cell apoptosis, unlike dissolved zinc acetate, indicating a particle-related mechanism. Microparticles induced a stronger formation of reactive oxygen species than smaller particles probably due to higher sedimentation (cell-to-particle contact) of microparticles in contrast to nanoparticles. The effect of particle types on the cytokine release was weak and mainly resulted in a decrease as shown by a protein microarray. In the particle-induced cell migration assay (PICMA), all particles had a lower effect than dissolved zinc acetate. In conclusion, the biological effects of zinc oxide particles in the sub-toxic range are caused by zinc ions after intracellular dissolution, by cell-to-particle contacts, and by the uptake of zinc oxide particles into cells. Graphical headlights center dot The cytotoxicity of zinc oxide particles is mainly due to the intracellular release of zinc ions. center dot The size and shape of zinc oxide micro- and nanoparticles has only small effects on lung cells in the sub-toxic range. center dot Zinc oxide particles are rapidly taken up by cells, regardless of their size and shape. center dot Zinc oxide particles rapidly dissolve after cellular uptake in endolysosomes.

Automated summary

The zinc oxide particles synthesized in various sizes and shapes showed slow dissolution in water and cell culture medium but dissolved rapidly in simulated lysosomal media. All particle types were quickly taken up by cells and induced cell apoptosis. The cytotoxicity of zinc oxide particles is mainly attributed to the intracellular release of zinc ions, regardless of size and shape.