# Low-Dose Oxidant Toxicity and Oxidative Stress in Human Papillary Thyroid Carcinoma Cells K1
## 4 Discussion
oxDMEM affected metabolic activity, proliferation, and viability of K1 cells in a dose-dependent manner, which was associated with a significant increase in lipid peroxidation. Similar tendencies were observed after exposure to H\({}_{2}\)O\({}_{2}\) and SNP, however, high concentrations were needed to elicit significant effects. Gas plasma-oxidized liquids are considered to act mainly via H\({}_{2}\)O\({}_{2}\)[33]. Important to note, thyroid tissues are capable of coping with high levels of H\({}_{2}\)O\({}_{2}\), as it is essentially needed in iodide oxidation in thyroglobulin synthesis. Hence, thyroid carcinomas evolve in a highly oxidative environment, and their antioxidant defense system is naturally prepared to protect them from the high ROS levels to which they are continuously exposed [22]. For instance, the thyrocyte apical membranes exhibit low permeability of H\({}_{2}\)O\({}_{2}\) into the cell [34]. Within the cells, redox homeostasis is ensured by antioxidant enzymes, including catalase, glutathione peroxidase, and glutathione reductase, as well as peroxiredoxin/th thioredoxin reductase systems [35], corroborating our findings that the TARP was largely unaffected after exposure to oxDMEM and H\({}_{2}\)O\({}_{2}\). In fact, recent research indicated SOD overexpression downstream _RAS_ oncogene signaling to be associated with oncogenesis in thyroid carcinoma [36; 37], indicating an even higher antioxidant capacity in malignant cells. However, the adaptive response of K1 cells was not enough to secure the cells from irreversible damage in the present study, as indicated by induction of cell death at high dosages. Despite H\({}_{2}\)O\({}_{2}\), plasma yields deposition of NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) in liquids [38]. The former is considered to act in synergy with H\({}_{2}\)O\({}_{2}\), as both can form highly reactive peroxuritites (ONOO\({}^{-}\)) [39]. The present study used sodium nitroprusside as an RNS donor, a chemical compound that generates mainly nitric oxide (NO), yielding NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) as secondary oxidation products [40]. In addition, NO itself competes for the same cytochrome oxidase c binding site as oxygen with higher affinity, inhibiting ATP production oxygen in the mitochondrial respiratory chain [41]. Furthermore, NO reacts with anion superoxide radical (O\({}_{2}\)\({}^{-}\)) in mitochondria to form ONOO\({}^{-}\). ONOO\({}^{-}\), in turn, can damage biomolecules, generate pores in the mitochondrial membrane, resulting in Ca\({}^{2+}\) and depolarization of the membrane and induction of pro-apoptotic signaling [42]. The increased reactivity of NO likely accounts for the high efficacy of SNP in the present study, affecting metabolic activity, proliferation, and viability of K1 cells, as well as the total antioxidant capacity of cells. However, anti-tumor effects were observed mainly at concentrations considered highly toxic in the literature [43]. Underlying mechanisms determining the susceptibility of cells to undergo plasma-induced cell death are only beginning to be understood and many studies focused on direct plasma applications, driving mainly short-lived species chemistries, so far. Despite the robustness of the antioxidant defense system, expression of membrane aquaporins [44], cholesterol content [45], and the baseline metabolic activity [46] are discussed as important factors. Overall, these findings might translate to indirect plasma treatments alike, despite conceivable differences due to the impact of long-lived oxidants. Here, a recent screening of 35 cell lines identified cell cycle-related genes to correlate with resistance toward H\({}_{2}\)O\({}_{2}\)[47]. Recent studies further indicate oscillations in oxygen consumption, energy metabolism, and redox state tightly integrated in cell cycle progression [48], which might link to observed differences in the TARP of untreated thyroid carcinoma cells in the present study.