Zinc oxide nanoparticles modulate the gene expression of ZnT(1) and ZIP(8) to manipulate zinc homeostasis and stress-induced cytotoxicity in human neuroblastoma SH-SY5Y cells

Zinc ions (Zn(2+)) are important messenger molecules involved in various physiological functions. To maintain the homeostasis of cytosolic Zn(2+) concentration ([Zn(2+)](c)), Zrt/Irt-related proteins (ZIPs) and Zn(2+) transporters (ZnTs) are the two families of proteins responsible for decreasing and increasing the [Zn(2+)](c), respectively, by fluxing Zn(2+) across the membranes of the cell and intracellular compartments in opposite directions. Most studies focus on the cytotoxicity incurred by a high concentration of [Zn(2+)](c) and less investigate the [Zn(2+)](c) at physiological levels. Zinc oxide-nanoparticle (ZnO-NP) is blood brain barrier-permeable and elevates the [Zn(2+)](c) to different levels according to the concentrations of ZnO-NP applied. In this study, we mildly elevated the [Zn(2+)](c) by ZnO-NP at concentrations below 1 μg/ml, which had little cytotoxicity, in cultured human neuroblastoma SH-SY5Y cells and characterized the importance of Zn(2+) transporters in 6-hydroxy dopamine (6-OHDA)-induced cell death. The results show that ZnO-NP at low concentrations elevated the [Zn(2+)](c) transiently in 6 hr, then declined gradually to a basal level in 24 hr. Knocking down the expression levels of ZnT(1) (located mostly at the plasma membrane) and ZIP(8) (present in endosomes and lysosomes) increased and decreased the ZnO-NP-induced elevation of [Zn(2+)](c), respectively. ZnO-NP treatment reduced the basal levels of reactive oxygen species and Bax/Bcl-2 mRNA ratios; in addition, ZnO-NP decreased the 6-OHDA-induced ROS production, p53 expression, and cell death. These results show that ZnO-NP-induced mild elevation in [Zn(2+)](c) activates beneficial effects in reducing the 6-OHDA-induced cytotoxic effects. Therefore, brain-delivery of ZnO-NP can be regarded as a potential therapy for neurodegenerative diseases.