Differences in intracellular mobile zinc levels affect susceptibility to plasma-activated medium-induced cytotoxicity.

There is growing evidence that plasma-activated medium (PAM), which is prepared by non-thermal plasma (NTP) irradiation of cell-free medium, is a beneficial tool for cancer therapy. PAM has been reported to preferentially kill cancer cells; however, its mechanism is not fully understood. Since PAM contains reactive oxygen species (ROS) and reactive nitrogen species, the anti-cancer effects of PAM are thought to be attributed to oxidative stress induced by these reactive molecules. Oxidative stress has been shown to release zinc (Zn) from intracellular Zn stores and provoke Zn-dependent cell death. We have previously demonstrated that intracellular free Zn plays a critical role in PAM-induced cell death in human neuroblastoma SH-SY5Y cells. In this study, we found that normal human fibroblasts were less susceptible to PAM cytotoxicity compared with SH-SY5Y cells. PAM decreased intracellular NAD levels in both cells, whereas the depletion of ATP and mitochondrial ROS generation was hardly observed in fibroblasts. Intracellular mobile Zn contents of fibroblasts were lower than those of SH-SY5Y cells. PAM suppressed the activity of aconitase, which is a tricarboxylic acid cycle enzyme, only in SH-SY5Y cells, and N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), a Zn chelator, counteracted the suppression. The combination treatment with PAM and Zn augmented PAM-induced ATP depletion, mitochondrial ROS generation, and cytotoxicity in fibroblasts. These findings suggest the possibility that cells with high intracellular mobile Zn are susceptible to PAM cytotoxicity. Therefore, we concluded that the differences in mobile Zn levels affect PAM-induced cellular responses.