Autoamplificatory singlet oxygen generation sensitizes tumor cells for intercellular apoptosis-inducing signaling

Inhibition of Membrane-Associated Catalase, Extracellular ROS/RNS Signaling and Aquaporin/H2O2-Mediated Intracellular Glutathione Depletion Cooperate during Apoptosis Induction in the Human Gastric Carcinoma Cell Line MKN-45

The human gastric carcinoma cell line MKN-45 is a prototype of bona fide tumor cells, as it is protected from the NADPH oxidase-1 (NOX-1)-driven HOCl- and nitric oxide (NO)/peroxynitrite apoptosis-inducing signaling pathways by a membrane-associated catalase. The use of inhibitors/scavengers shows that inhibition of membrane-associated catalase is sufficient for the activation of NO/peroxynitrite or HOCl signaling. However, this signaling is not sufficient for apoptosis induction, as intracellular glutathione peroxidase/glutathione counteracts these signaling effects. Therefore, intrusion of extracellular tumor cell-derived H₂O₂ through aquaporins is required for the full apoptosis-inducing effect of extracellular reactive oxygen/nitrogen species. This secondary step in apoptosis induction can be prevented by inhibition of aquaporins, inhibition of NOX1 and decomposition of H₂O₂. Pretreatment with inhibitors of glutathione synthase or the cysteine-glutamine antiporter (xC transporter) abrogate the requirement for aquaporin/H₂O₂-mediated glutathione depletion, thus demonstrating that intracellular glutathione is the target of intruding H₂O₂. These data allow definition of mechanistic interactions between ROS/RNS signaling after inhibition of membrane-associated catalase, the sensitizing effects of aquaporins/H₂O₂ and the counteraction of the xC transporter/glutathione synthase system. Knowledge of these mechanistic interactions is required for the understanding of selective apoptosis induction in tumor cells through reestablishment of apoptosis-inducing ROS/RNS signaling.

Selective Apoptotic Effect of Plasma Activated Liquids on Human Cancer Cell Lines

Plasma medicine is a new field focusing on biomedical and clinical applications of cold gas plasmas, including their anticancer effects. Cold plasmas can be applied directly or indirectly as plasma-activated liquids (PAL). The effects of plasma-activated cell growth medium (PAM) and plasma-activated phosphate buffered saline (PAPBS) were tested, using a plasma pen generating streamer corona discharge in ambient air, on different cancer cell lines (melanoma A375, glioblastoma LN229 and pancreatic cancer MiaPaCa-2) and normal cells (human dermal fibroblasts HDFa). The viability reduction and apoptosis induction were detected in all cancer cells after incubation in PAL. In melanoma cells we focused on detailed insights to the apoptotic pathways. The anticancer effects depend on the plasma treatment time or PAL concentration. The first 30 min of incubation in PAL were enough to start processes leading to cell death. In fibroblasts, no apoptosis induction was observed, and only PAPBS, activated for a longer time, slightly decreased their viability. Effects of PAM and PAPBS on cancer cells showed selectivity compared to normal fibroblasts, depending on correctly chosen activation time and PAL concentration, which is very promising for potential clinical applications. This selectivity effect of PAL is conceivably induced by plasma-generated hydrogen peroxide.

Plasma Treated Water Solutions in Cancer Treatments: The Contrasting Role of RNS

Plasma Treated Water Solutions (PTWS) recently emerged as a novel tool for the generation of Reactive Oxygen and Nitrogen Species (ROS and RNS) in liquids. The presence of ROS with a strong oxidative power, like hydrogen peroxide (H₂O₂), has been proposed as the main effector for the cancer-killing properties of PTWS. A protective role has been postulated for RNS, with nitric oxide (NO) being involved in the activation of antioxidant responses and cell survival. However, recent evidences proved that NO-derivatives in proper mixtures with ROS in PTWS could enhance rather than reduce the selectivity of PTWS-induced cancer cell death through the inhibition of specific antioxidant cancer defenses. In this paper we discuss the formation of RNS in different liquids with a Dielectric Barrier Discharge (DBD), to show that NO is absent in PTWS of complex composition like plasma treated (PT)-cell culture media used for in vitro experiments, as well as its supposed protective role. Nitrite anions (NO₂^-) instead, present in our PTWS, were found to improve the selective death of Saos2 cancer cells compared to EA.hy926 cells by decreasing the cytotoxic threshold of H₂O₂ to non-toxic values for the endothelial cell line.

On the Anti-Cancer Effect of Cold Atmospheric Plasma and the Possible Role of Catalase-Dependent Apoptotic Pathways

Cold atmospheric plasma (CAP) is a promising new agent for (selective) cancer treatment, but the underlying cause of the anti-cancer effect of CAP is not well understood yet. Among different theories and observations, one theory in particular has been postulated in great detail and consists of a very complex network of reactions that are claimed to account for the anti-cancer effect of CAP. Here, the key concept is a reactivation of two specific apoptotic cell signaling pathways through catalase inactivation caused by CAP. Thus, it is postulated that the anti-cancer effect of CAP is due to its ability to inactivate catalase, either directly or indirectly. A theoretical investigation of the proposed theory, especially the role of catalase inactivation, can contribute to the understanding of the underlying cause of the anti-cancer effect of CAP. In the present study, we develop a mathematical model to analyze the proposed catalase-dependent anti-cancer effect of CAP. Our results show that a catalase-dependent reactivation of the two apoptotic pathways of interest is unlikely to contribute to the observed anti-cancer effect of CAP. Thus, we believe that other theories of the underlying cause should be considered and evaluated to gain knowledge about the principles of CAP-induced cancer cell death.

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

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 H₂O_2.¹O₂ 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.

Anti-inflammatory and antioxidant effects of muscarinic acetylcholine receptor (mAChR) activation in the rat hippocampus

Recently we found that acute treatment with Oxotremorine (Oxo), a non-selective mAChRs agonist, up-regulates heat shock proteins and activates their transcription factor heat shock factor 1 in the rat hippocampus. Here we aimed to investigate: a) if acute treatment with Oxo may regulate pro-inflammatory or anti-inflammatory cytokines and oxidative stress in the rat hippocampus; b) if chronic restraint stress (CRS) induces inflammatory or oxidative alterations in the hippocampus and whether such alterations may be affected by chronic treatment with Oxo. In the acute experiment, rats were injected with single dose of Oxo (0.4 mg/kg) and sacrificed at 24 h, 48 h and 72 h. In the CRS experiment, the rats were exposed for 21 days to the CRS and then were treated with Oxo (0.2 mg/kg) for further 10 days. The acute Oxo treatment showed an ability to significantly reduce reactive oxygen species (ROS), singlet oxygen (¹O₂), pro-inflammatory cytokines levels (IL-1β and IL-6) and phosphorylated NF-κB-p65. Acute Oxo treatment also increased superoxide dismutase (SOD)-2 protein levels and stimulated SOD activity. No differences were detected in the anti-inflammatory cytokine levels, including IL-10 and TGF-β1. In the group of rats exposed to the CRS were found increased hippocampal IL-1β and IL-6 levels, together with a reduction of SOD activity level. These changes produced by CRS were counteracted by chronic Oxo treatment. In contrast, the upregulation of ROS and ¹O₂ levels in the CRS group was not counteracted by chronic Oxo treatment. The results revealed a hippocampal anti-inflammatory and antioxidant effect of Oxo treatment in both basal conditions and anti-inflammatory in the CRS rat model.

Dynamics of Singlet Oxygen-Triggered, RONS-Based Apoptosis Induction after Treatment of Tumor Cells with Cold Atmospheric Plasma or Plasma-Activated Medium

Treatment of tumor cells with cold atmospheric plasma (CAP) or with plasma-activated medium (PAM) leads to a biochemical imprint on these cells. This imprint is mediated by primary singlet oxygen, which is mainly generated through the interaction between CAP-derived H2O2 and NO2-. This imprint is induced with a low efficiency as local inactivation of a few membrane-associated catalase molecules. As sustained generation of secondary singlet oxygen by the tumor cells is activated at the site of the imprint, a rapid bystander effect-like spreading of secondary singlet oxygen generation and catalase inactivation within the cell population is thus induced. This highly dynamic process is essentially driven by NOX1 and NOS of the tumor cells, and finally leads to intercellular RONS-driven apoptosis induction. This dynamic process can be studied by kinetic analysis, combined with the use of specific inhibitors at defined time intervals. Alternatively, it can be demonstrated and quantified by transfer experiments, where pretreated cells are mixed with untreated cells and bystander signaling is determined. These studies allow to conclude that the specific response of tumor cells to generate secondary singlet oxygen is the essential motor for their self-destruction, after a singlet oxygen-mediated triggering process by CAP or PAM.

Intercellular singlet oxygen-mediated bystander signaling triggered by long-lived species of cold atmospheric plasma and plasma-activated medium

Treatment of tumor cells with H₂O₂ and nitrite, two long-lived species derived from cold atmospheric plasma, induces a complex autoamplificatory, singlet oxygen-mediated process, which leads to catalase inactivation and reactivation of intercellular apoptosis-inducing signaling. Experimental dissection and quantification of this process is described in this study. When tumor cells were pretreated with H₂O₂ and nitrite, and then were added to untreated tumor cells, they propaged singlet oxygen mediated catalase inactivation and generation of singlet oxygen to the untreated cell population. This bystander effect allowed to analyze the biochemical requirements of a) induction of the bystander effect-inducing potential, b) transmission of the bystander effect to untreated neighbouring cells, and c) the biochemical consequences of these signaling events. The induction of bystander effect-inducing potential requires the generation of “primary singlet oxygen” through the reactions following the interaction between nitrite and H₂O₂, followed by local inactivation of a few catalase molecules. This primary effect seems to be very rare, but is efficiently enhanced by the generation of "secondary singlet oxygen" through the interaction between H₂O₂ and peroxynitrite at the site of inactivated catalase. Transmission of bystander signaling between pretreated and untreated tumor cells depends on the generation of secondary singlet oxygen by the pretreated cells and singlet oxygen-mediated catalase inactivation of the untreated recipient cells. This induces autoamplificatory propagation of secondary singlet oxygen generation in the population. This experimental approach allowed to quantify the efficiencies of primary and secondary singlet oxgen generation after CAP and PAM action, to dissect the system and to study the underlying chemical biology in detail. Our data confirm that CAP and PAM-derived components are merely the trigger for the activation of autoamplificatory mechanisms of tumor cells, whereas the tumor cells efficiently propagate their cell death through their own ROS/RNS signaling potential.

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Primary ¹O₂ generated by H₂O₂ and NO₂^─ induces in tumor cells the potential for bystander signaling.

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Bystander signaling depends on inactivation of membrane-associated catalase.

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It is propagated by secondary singlet oxgen generated by cell-derived H₂O₂ and peroxynitrite.

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The action of primary singlet oxygen is a rare effect.

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Secondary singlet oxygen is generated in a sustained mode and acts efficiently.

The synergistic effect between hydrogen peroxide and nitrite, two long-lived molecular species from cold atmospheric plasma, triggers tumor cells to induce their own cell death

Nitrite and H₂O₂ are long-lived species in cold atmospheric plasma and plasma-activated medium. It is known that their synergistic interaction is required for selective apoptosis induction in tumor cells that are treated with plasma-activated medium. This study shows that the interaction between nitrite and H₂O₂ leads to the formation of peroxynitrite, followed by singlet oxygen generation through the interaction between peroxynitrite and residual H₂O₂. This primary singlet oxygen causes local inactivation of few catalase molecules on the surface of tumor cells. As a consequence, H₂O₂ and peroxynitrite that are constantly produced by tumor cells and are usually decomposed by their protective membrane-associated catalase, are surviving at the site of locally inactivated catalase. This leads to the generation of secondary singlet oxygen through the interaction between tumor cell-derived H₂O₂ and peroxynitrite. This selfsustained process leads to autoamplification of secondary singlet oxygen generation and catalase inactivation. Inactivation of catalase allows the influx of H₂O₂ through aquaporins, leading to intracellular glutathione depletion and sensitization of the cells for apoptosis induction through lipid peroxidation. It also allows to establish intercellular apoptosis-inducing HOCl signaling, driven by active NOX1 and finalized by lipid peroxidation through hydroxyl radicals that activates the mitochondrial pathway of apoptosis. This experimentally established model is based on a triggering function of CAP and PAM-derived H₂O₂/nitrite that causes selective cell death in tumor cells based on their own ROS and RNS. This model explains the selectivity of CAP and PAM action towards tumor cells and is in contradiction to previous models that implicated that ROS/RNS from CAP or PAM were sufficient to directly cause cell death of tumor cells.

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H₂O₂ and nitrite generate peroxynitrite, followed by primary singlet oxygen formation.

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Primary singlet oxygen causes local inactivation of tumor cell protective catalase.

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Amplificatory generation of secondary singlet oxygen and catalase inactivation are established.

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Inactivation of catalase allows aquaporin-mediated influx of H₂O₂ and glutathione depletion.

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In this way, CAP and PAM trigger tumor cells to contribute to their own cell death.

Emodin, Physcion, and Crude Extract of Rhamnus sphaerosperma var. pubescens Induce Mixed Cell Death, Increase in Oxidative Stress, DNA Damage, and Inhibition of AKT in Cervical and Oral Squamous Carcinoma Cell Lines

There have been few studies on the pharmacological properties of Rhamnus sphaerosperma var. pubescens, a native Brazilian species popularly known as “fruto-de-pombo.” The aim of this study was to investigate the scavenging capacity of emodin, physcion, and the ethanolic crude extract of Rhamnus sphaerosperma var. pubescens against reactive oxygen and nitrogen species, as well as their role and plausible mechanisms in prompting cell death and changes in AKT phosphorylation after cervical (SiHa and C33A) and oral (HSC-3) squamous cell carcinoma treatments. Emodin was shown to be the best scavenger of NO^• and O₂^•−, while all samples were equally effective in HOCl/OCl⁻ capture. Emodin, physcion, and the ethanolic extract all exhibited cytotoxic effects on SiHa, C33A, HSC-3, and HaCaT (immortalized human keratinocytes, nontumorigenic cell line), involving mixed cell death (apoptosis and necrosis) independent of the caspase activation pathway. Emodin, physcion, and the ethanolic extract increased intracellular oxidative stress and DNA damage. Emodin decreased the activation of AKT in all tumor cells, physcion in HSC-3 and HaCaT cells, and the ethanolic extract in C33A and HaCaT cells, respectively. The induction of cancer cell death by emodin, physcion, and the ethanolic crude extract of Rhamnus sphaerosperma var. pubescens was related to an increase in intracellular oxidative stress and DNA damage and a decrease in AKT activation. These molecules are therefore emerging as interesting candidates for further study as novel options to treat cervical and oral carcinomas.