Determination of the relative contribution of the non-dissolved fraction of ZnO NP on membrane permeability and cytotoxicity

Background: While the role of lysosomal membrane permeabilization (LMP) in NP-induced inflammatory responses has been recognized, the underlying mechanism of LMP is still unclear. The assumption has been that zinc oxide (ZnO)-induced LMP is due to Zn2+; however, little is known about the role of ZnO nanoparticles (NP) in toxicity. Methods: We examined the contribution of intact ZnO NP on membrane permeability using red blood cells (RBC) and undifferentiated THP-1 cells as models of particle-membrane interactions to simulate ZnO NP-lysosomal membrane interaction. The integrity of plasma membranes was evaluated by transmission electron microscopy (TEM) and confocal microscopy. ZnO NP dissolution was determined using ZnAF-2F, Zn2+ specific probe. The stability of ZnO NP inside the phagolysosomes of phagocytic cells, differentiated THP-1, alveolar macrophages, and bone marrow-derived macrophages, was determined. Results: ZnO NP caused significant hemolysis and cytotoxicity under conditions of negligible dissolution. Fully ionized Zn2SO4 caused slight hemolysis, while partially ionized ZnO induced significant hemolysis. Confocal microscopy and TEM images did not reveal membrane disruption in RBC and THP-1 cells, respectively. ZnO NP remained intact inside the phagolysosomes after a 4 h incubation with phagocytic cells. Conclusions: These studies demonstrate the ability of intact ZnO NP to induce membrane permeability and cytotoxicity without the contribution of dissolved Zn2+, suggesting that ZnO NP toxicity does not necessarily depend upon Zn2+. The stability of ZnO NP inside the phagolysosomes suggests that LMP is the result of the toxic effect of intact ZnO NP on phagolysosomal membranes.