Contribution Of Intracellular Mobile Zinc To Anti-Cancer Effects Of Plasma-Activated Medium

Non-thermal plasma (NTP) irradiation has been reported to have a broad array of biological effects, and has been found to preferentially kill cancer cells. Therefore, NTP is a promising approach for cancer therapy. The indirect NTP irradiation method using plasma-activated medium (PAM) induces cancer cell death to the same extent as the direct NTP irradiation method. The anti-cancer effects of PAM are thought to be attributed to reactive oxygen species (ROS), mainly H₂O₂. On the other hand, oxidative stress caused by ROS has been demonstrated to induce the liberation of zinc (Zn²⁺) from intracellular Zn²⁺ pools and Zn²⁺-dependent cell death. Zn²⁺ is thought to function as a second messenger activated by oxidative stress. We previously reported that PAM rapidly triggered intracellular Zn²⁺ liberation in human neuroblastoma SH-SY5Y cells, and that the liberated Zn²⁺ induced activation of poly(ADP-ribose) polymerase 1 (PARP-1) and subsequent loss of cellular NAD⁺ and ATP, resulting in cell death associated with energy exhaustion. Recently, we also found that normal human fibroblasts were less sensitive to PAM cytotoxicity compared with SH-SY5Y cells. PAM decreased intracellular NAD⁺ levels in both cell types, whereas the depletion of ATP and mitochondrial ROS generation were hardly observed in fibroblasts. The addition of Zn²⁺ augmented PAM-induced cell death and ATP depletion in fibroblasts. The levels of PAM-induced Zn²⁺ liberation were lower in fibroblasts than in SH-SY5Y cells. These findings suggest the possibility that cells with high intracellular mobile Zn²⁺ are susceptible to PAM cytotoxicity. Moreover, PAM promoted G2/M growth arrest of lung adenocarcinoma A549 cells via activation of the ATM-p53 pathway and the Zn²⁺ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) suppressed these events. Thus, we consider intracellular mobile Zn²⁺ to play a key role in the anti-cancer effects of PAM.