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

Abstract

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.