# Cold Atmospheric Pressure Plasma in Wound Healing and Cancer Treatment
## 4 Properties of CAP Stimulate Tissue Regeneration but May also Induce Cell Death
In the same study, the authors report significantly accelerated wound re-epithelialization after CAP treatment of a murine model of dermal full-thickness ear wounds [70]. A decrease of E-cadherin on the protein level after treatment of keratinocytes with CAP conditioned medium was also observed in another recent study [71]. This decrease was accompanied by an increase in N-cadherin, cyclin D1, Ki-67, Cdk2, and p-ERK levels, and together this is evidence for CAP-induced epithelial-mesenchymal transition (EMT) and cell cycle progression supporting proliferation and migration. Collectively, CAP improves wound healing by modulating inflammation, proliferation, angiogenesis/revascularization, migration, and re-epithelialization using pre-existing signaling pathway partially explored in redox biology studies already. Processes, such as proliferation, angiogenesis, or migration, are important for wound healing, but unwanted with respect to cancer. Therefore, CAP intensities have to be significantly increased for cancer therapy compared to wound treatment to inhibit growth and induce cell death. Higher intensities of CAP lead to higher concentrations of RONS, and supraphysiological levels of oxidative stress disrupt redox signaling and/or cause damage to biomolecules, such as membrane lipids, proteins, and DNA [72]. Such a supraphysiological level of oxidative stress is termed oxidative distress instead of physiological low-level oxidative stress, termed oxidative eustress [73]. While oxidative eustress, as described above, triggers signaling pathways enhancing cell proliferation and tissue regeneration, oxidative distress triggers cell cycle arrest and cell death. The adaptive response to oxidative stress is regulated by modulating the nuclear E2-related factor (Nrf2) pathway, which maintains redox homeostasis and defends cells from damage [45]. In response to increased ROS levels, Nrf2 translocates from the cytosol to the nucleus where it binds to antioxidant responsive elements on DNA and thus promotes the upregulation of antioxidant genes [74]. These genes encode, for example, for glutathione (GSH), glutathione reductase (GSR), glutathione S-transferase (GST), superoxide dismutase (SOD), heme oxygenase 1 (HMOX-1), and NADPHquine oxidoreductase 1 (NQO1) [45]. Nrf signaling has also been identified as a critical event in CAP-induced wound healing [75]. While temporarily increased oxidative stress is beneficial in wound healing, chronically increased supraphysiological levels of RONS are also supporting carcinogenesis and tumor progression [76]. Although cancer cells adapt to the elevated levels of RONS, they also depend on increased activity of the antioxidant system to keep redox homeostasis. It is hypothesized that the increased cellular levels of RONS in cancer cells render them especially sensitive to further exposure to RONS in comparison to their non-malignant counterparts [48; 77], although contrary evidence exists as well [78]. Besides plasma treatment time and intensity, the extent of the response to CAP also depends on genetic background, cell type, tumor microenvironment, as well as the different classes of RONS generated [33; 79]. Prolonged CAP treatment mediates apoptosis or necrosis, but the induction of cellular senescence and autophagy have also been reported. It is hypothesized that the parameters of treatment time and probably also the type of plasma device govern the biological outcome [80; 81]. While senescence, a well-known irreversible growth arrest in response to oxidative stress and DNA damage [82], may be induced by relatively short treatments (regarding the treatment setting used in this study) with CAP, apoptosis, and necrosis are induced by prolonged treatment times. The dose-dependent induction of senescence depends on the cytosolic influx of calcium [83; 84]. Cell death induced by RONS involves several apoptosis-related signaling pathways, such as MAPK and ERK pathways [85].