Cold Atmospheric Plasma and Plasma-Activated Medium Trigger RONS-Based Tumor Cell Apoptosis

Abstract

The selective in vitro anti-tumor mechanisms of cold atmospheric plasma (CAP) and plasma-activated media (PAM) follow a sequential multi-step process. The first step involves the formation of primary singlet oxygen (1O2) through the complex interaction between NO2− and H2O2.1O2 then inactivates some membrane-associated catalase molecules on at least a few tumor cells. With some molecules of their protective catalase inactivated, these tumor cells allow locally surviving cell-derived, extracellular H2O2 and ONOO─ to form secondary 1O2. These species continue to inactivate catalase on the originally triggered cells and on adjacent cells. At the site of inactivated catalase, cell-generated H2O2 enters the cell via aquaporins, depletes glutathione and thus abrogates the cell’s protection towards lipid peroxidation. Optimal inactivation of catalase then allows efficient apoptosis induction through the HOCl signaling pathway that is finalized by lipid peroxidation. An identical CAP exposure did not result in apoptosis for nonmalignant cells. A key conclusion from these experiments is that tumor cell-generated RONS play the major role in inactivating protective catalase, depleting glutathione and establishing apoptosis-inducing RONS signaling. CAP or PAM exposure only trigger this response by initially inactivating a small percentage of protective membrane associated catalase molecules on tumor cells.

Modifying the Tumour Microenvironment: Challenges and Future Perspectives for Anticancer Plasma Treatments

“…Depending on the plasma treatment conditions, 1 O 2 could either be generated directly and transferred from the gaseous phase to the liquid phase of the ECM or generated indirectly from H 2 O 2 and NO 2 -(which are transferred from the gaseous phase to the ECM). The latter scenario, where so-called "primary" 1 O 2 is generated in the ECM, has been elucidated to be the most likely [59,60]. Indeed, the formation of such primary 1 O 2 from a solution of H 2 O 2 and NO 2 has been experimentally verified [96,97].…”

“…Catalase decomposes H 2 O 2 as well as ONOOand thus, removes the substrates for production of •OH in both the HOCl pathway and ONOOpathway ( Figure 1). In addition, when H 2 O 2 is not decomposed, it may, as already mentioned, also enter the intracellular compartment through aquaporins in the cell membrane, where it causes depletion of glutathione [59,96]. The depletion of intracellular glutathione renders the cells more sensitive for •OH attacking the membrane.…”

“…The origin of the ROS triggering apoptosis in cancer cells after plasma treatment has been under debate: Although it would be logical to assume that the ROS were generated by plasma and added exogenously, it is true that some of these ROS have a very short life time and diffusion length due to their highly reactive nature and will not be able to reach the ECM, particularly not in the bulk of a cancer tumour. A paradigm first proposed in [58] and recently experimentally verified [59,60], states that the ROS acting in the ECM of cancer cells after plasma treatment instead are generated by the cells themselves and are part of a system of signalling pathways used by cancer cells to promote proliferation. In this scenario, the effect that plasma exerts on cancer tumour is constituted by the transmission and/or creation of one species, 1 O 2 , to the ECM.…”

“…The generation of secondary 1 O 2 occurs in an exponential manner and creates an amplified apoptosis signal reaching the bulk of the tumour. The whole set of events-from the generation of primary 1 O 2 , catalase-inactivation, and subsequently, tumour-generated secondary 1 O 2 (followed by inactivation of more catalase), to apoptosis-induction-has been experimentally investigated and verified both in the case of tumour cells in a solution of H 2 O 2 and NO 2 - [96,97] and for plasma-treated tumour cells [59,60]. The selectivity of the treatment, given that the concentration of H 2 O 2 is below a certain limit, has been confirmed by control experiments with non-malignant cells [59].…”

Cold Atmospheric Plasma: A Powerful Tool for Modern Medicine

“…This is then used in cell cultivation or for direct injection into tested subjects, such as mouse xenografts [38]. In addition, long-term species including nitrates, nitrites, and H 2 O 2 are usually preserved in plasma-activated medium (PAM) [39].…”