Comparing Redox and Intracellular Signalling Responses to Cold Plasma in Wound Healing and Cancer

Abstract

Cold plasma (CP) is an ionised gas containing excited molecules and ions, radicals, and free electrons, and which emits electric fields and UV radiation. CP is potently antimicrobial, and can be applied safely to biological tissue, birthing the field of plasma medicine. Reactive oxygen and nitrogen species (RONS) produced by CP affect biological processes directly or indirectly via the modification of cellular lipids, proteins, DNA, and intracellular signalling pathways. CP can be applied at lower levels for oxidative eustress to activate cell proliferation, motility, migration, and antioxidant production in normal cells, mainly potentiated by the unfolded protein response, the nuclear factor-erythroid factor 2-related factor 2 (Nrf2)-activated antioxidant response element, and the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) pathway, which also activates nuclear factor-kappa B (NFκB). At higher CP exposures, inactivation, apoptosis, and autophagy of malignant cells can occur via the degradation of the PI3K/Akt and mitogen-activated protein kinase (MAPK)-dependent and -independent activation of the master tumour suppressor p53, leading to caspase-mediated cell death. These opposing responses validate a hormesis approach to plasma medicine. Clinical applications of CP are becoming increasingly realised in wound healing, while clinical effectiveness in tumours is currently coming to light. This review will outline advances in plasma medicine and compare the main redox and intracellular signalling responses to CP in wound healing and cancer.

Keywords: MAPK; Nrf2; PI3K/Akt; cancer; cold plasma; endoplasmic reticulum stress; plasma-activated water; redox signalling; wound healing.

Figure 1

Chemical reactions of cold plasma (CP)-derived reactive oxygen and nitrogen species (RONS) at the gas–liquid interface and through diffusion in the liquid phase. The depths at which reactions occur are indicated on the vertical axis, with important and negligible reactions denoted by solid and dashed lines, respectively. Large orange (ROS) and green (RNS) vertical bars with “diffuse” arrows indicate diffusion as the dominant factor in the movement of RONS in the depth of liquid. Redistributed from [29] (CC BY 4.0).

Figure 2

CP-derived RONS promote wound healing by dissociating Kelch-like ECH-associated protein 1 (Keap1)/nuclear factor erythroid-2 related factor 2 (Nrf2) to activate the antioxidant response element (ARE), rearranging cytoskeletal architecture to promote cell motility, recruiting inflammatory cells, and autocrine signalling to promote cell survival, proliferation, and angiogenesis. The Jun N-terminal kinase (JNK) pathway has been shown to potentiate Nrf2 activation, but the mechanisms are still unclear (indicated by “?”), while Akt decreases p53 expression to promote Nrf2. Keap1 activity may also go beyond the canonical function of inhibiting Nrf2, but these functions are greyed out to indicate the unexplored nature of these potential responses to CP.

Figure 3

CP exposure causes endoplasmic reticulum stress (ERS) due to redox stress and activates the inositol-requiring protein 1α (IRE1α) and protein kinase RNA-like ER kinase (PERK) arms of the UPR. Activating transcription factor 6 (ATF6) activity has not been observed (indicated by “?”). Lower CP exposure allows cells to recover, while higher CP exposure can cause prolonged C/EBP-homologous protein (CHOP) activity, leading to apoptosis.

Figure 4

CP-derived RONS promote cancer cell autophagy and death by activating the mitogen-activated protein kinase (MAPK) family signalling pathways. RONS activating JNK and extracellular signal-related kinases 1/2 (ERK1/2) pathways leads to autophagosome formation and autophagy, while JNK and p38 also potentiate apoptosis via both intrinsic and extrinsic pathways of p53. Cancer cells that highly express gasdermin E (GSDME) also undergo pyroptosis.

Figure 5

Phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway stimulation by CP leads to multifactorial stimulation of cell survival and proliferation by indirectly stimulating the β-catenin pathway (indicated by dashed green arrow), activating nuclear factor-kappa B (NFκB) via ERK1/2 and inhibiting p53.

Figure 6

Complex physiological responses to deactivation of PI3K/Akt with high CP exposure. Excessive CP-derived RONS (indicated by “↑↑”) leads to E3 ligase-dependent degradation of Akt, resulting in reduced hypoxia-inducible factor 1-alpha (HIF1α) signalling (antiproliferation), compensation by activation of signal transducer and activator of the transcription 3 (STAT3), particularly in PTEN-deficient cancer cells, activation of autophagic proteins beclin 1 and LC3-II, and inhibition of NFκB (apoptosis).

Figure 7

Overview of the effect of CP on cell signalling pathway activity, interactions between pathways, and cell fate decisions. The black cross indicates degradation of Akt. Black and red arrows indicate the direction of activation, being (↑) upregulated and (↑↑) prolonged or excessive upregulation.

Figure 8

Conceptual hormesis response curves for cancer (red) and normal (green) cells, underpinned by the different responses in cell signalling pathway regulation between redox stress and eustress. The red and green text boxes represent cancer and normal cells, respectively. Arrows indicate the direction of activation, being (↑) upregulation, (↓) downregulation, and (↑↑) prolonged upregulation.