Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis

Cellular Mechanisms of Singlet Oxygen in Photodynamic Therapy

In this review, we delve into the realm of photodynamic therapy (PDT), an established method for combating cancer. The foundation of PDT lies in the activation of a photosensitizing agent using specific wavelengths of light, resulting in the generation of reactive oxygen species (ROS), notably singlet oxygen (1O2). We explore PDT's intricacies, emphasizing its precise targeting of cancer cells while sparing healthy tissue. We examine the pivotal role of singlet oxygen in initiating apoptosis and other cell death pathways, highlighting its potential for minimally invasive cancer treatment. Additionally, we delve into the complex interplay of cellular components, including catalase and NOX1, in defending cancer cells against PDT-induced oxidative and nitrative stress. We unveil an intriguing auto-amplifying mechanism involving secondary singlet oxygen production and catalase inactivation, offering promising avenues for enhancing PDT's effectiveness. In conclusion, our review unravels PDT's inner workings and underscores the importance of selective illumination and photosensitizer properties for achieving precision in cancer therapy. The exploration of cellular responses and interactions reveals opportunities for refining and optimizing PDT, which holds significant potential in the ongoing fight against cancer.

Influence of ionizing radiation and cell density on the kinetics of autocrine destruction and intercellular induction of apoptosis in precancerous cells

Intercellular induction of apoptosis (IIA) represents a well-defined signaling model by which precancerous cells are selectively eradicated through reactive oxygen/nitrogen species and cytokine signaling from neighbour normal cells. Previously, we demonstrated that the IIA process could be enhanced by exposure of normal cells to very low doses of ionizing radiation as a result of perturbing the intercellular signaling. In this study, we investigate the kinetic behaviour of both autocrine destruction (AD) and IIA as a function of cell density of both precancerous and normal cells using an insert co-culture system and how exposure of normal cells to ionizing radiation influence the kinetics of apoptosis induction in precancerous cells. Increasing the seeding density of transformed cells shifts the kinetics of AD towards earlier times with the response plateauing only at high seeding densities. Likewise, when co-culturing precancerous cells with normal cells, increasing the seeding density of either normal or precancerous cells also shifts the kinetics of IIA response towards earlier times and plateau only at higher seeding densities. Irradiation of normal cells prior to co-culture further enhances the kinetics of IIA response, with the degree of enhancement dependent on the relative cell densities. These results demonstrate the pivotal role of the cell seeding density of normal and precancerous cells in modulating both AD and IIA. These results further support the proposition that ionizing radiation could result in an enhancement in the rate of removal of precancerous cells through the IIA process.

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.

Acacetin exerts antioxidant potential against atherosclerosis through Nrf2 pathway in apoE-/- Mice

Oxidative stress has a considerable influence on endothelial cell dysfunction and atherosclerosis. Acacetin, an anti-inflammatory and antiarrhythmic, is frequently used in the treatment of myocarditis, albeit its role in managing atherosclerosis is currently unclear. Thus, we evaluated the regulatory effects of acacetin in maintaining endothelial cell function and further investigated whether the flavonoid could attenuate atherosclerosis in apolipoprotein E deficiency (apoE-/- ) mice. Different concentrations of acacetin were tested on EA.hy926 cells, either induced or non-induced by human oxidized low-density lipoprotein (oxLDL), to clarify its influence on cell viability, cellular reactive oxidative stress (ROS) level, apoptotic ratios and other regulatory effects. In vivo, apoE-/- mice were fed either a Western diet or a chow diet. Acacetin pro-drug (15 mg/kg) was injected subcutaneously two times a day for 12 weeks. The effects of acacetin on the atherosclerotic process, plasma inflammatory factors and lipid metabolism were also investigated. Acacetin significantly increased EA.hy926 cell viability by reducing the ratios of apoptotic and necrotic cells at 3 μmol/L. Moreover, 3 μmol/L acacetin clearly decreased ROS levels and enhanced reductase protein expression through MsrA and Nrf2 pathway through phosphorylation of Nrf2 and degradation of Keap1. In vivo, acacetin treatment remarkably attenuated atherosclerosis by increasing reductase levels in circulation and aortic roots, decreasing plasma inflammatory factor levels as well as accelerating lipid metabolism in Western diet-fed apoE-/- mice. Our findings demonstrate the anti-oxidative and anti-atherosclerotic effects of acacetin, in turn suggesting its potential therapeutic value in atherosclerotic-related cardiovascular diseases (CVD).

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.

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

Tumours are complex systems formed by cellular (malignant, immune, and endothelial cells, fibroblasts) and acellular components (extracellular matrix (ECM) constituents and secreted factors). A close interplay between these factors, collectively called the tumour microenvironment, is required to respond appropriately to external cues and to determine the treatment outcome. Cold plasma (here referred as ‘plasma’) is an emerging anticancer technology that generates a unique cocktail of reactive oxygen and nitrogen species to eliminate cancerous cells via multiple mechanisms of action. While plasma is currently regarded as a local therapy, it can also modulate the mechanisms of cell-to-cell and cell-to-ECM communication, which could facilitate the propagation of its effect in tissue and distant sites. However, it is still largely unknown how the physical interactions occurring between cells and/or the ECM in the tumour microenvironment affect the plasma therapy outcome. In this review, we discuss the effect of plasma on cell-to-cell and cell-to-ECM communication in the context of the tumour microenvironment and suggest new avenues of research to advance our knowledge in the field. Furthermore, we revise the relevant state-of-the-art in three-dimensional in vitro models that could be used to analyse cell-to-cell and cell-to-ECM communication and further strengthen our understanding of the effect of plasma in solid tumours.

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.

Interaction of saffron carotenoids with catalase: in vitro, in vivo and molecular docking studies

The effects of saffron carotenoids, crocetin (Crt) and crocin (Cro) on the structure, function and kinetics of catalase (CAT) were investigated. Both Crt and Cro quenched the fluorescence emission of CAT through the dynamic mechanism, but Crt (Ksv= 8.1 × 10⁴ mol^-1) was more effective than Cro (Ksv= 0.6 × 10⁴ mol^-1) at 300 °K. The UV-vis and circular dichroism spectra showed conformational changes of CAT in the presence of both carotenoids, but with different degrees. Kinetic studies showed strong inhibition of CAT by Crt, while, different concentrations of Cro showed different effects. Our in vitro data showed that Crt treatment significantly (p = 0.002) reduced the CAT activity in MCF-7, up to 24 h. The in vivo results showed that both Crt and Cro significantly increased the CAT activity in the tumor (p = 0.000 for both), and liver (p = 0.000 and p = 0.026 for Crt and Cro, respectively) tissues of 4T1-induced breast cancer in BALB/c mice, after 4 weeks of treatment. These findings are consistent with the binding, thermodynamic and molecular docking data. In conclusion, Crt and Cro with some differences in the structure affect CAT structure, function and activity, but in a slightly different manner.Communicated by Ramaswamy H. Sarma.

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.

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.

The LNT model for cancer induction is not supported by radiobiological data

The hallmarks of cancer have been the focus of much research and have influenced the development of risk models for radiation-induced cancer. However, natural defenses against cancer, which constitute the hallmarks of cancer prevention, have largely been neglected in developing cancer risk models. These natural defenses are enhanced by low doses and dose rates of ionizing radiation, which has aided in the continuation of human life over many generations. Our natural defenses operate at the molecular, cellular, tissue, and whole-body levels and include epigenetically regulated (epiregulated) DNA damage repair and antioxidant production, selective p53-independent apoptosis of aberrant cells (e.g. neoplastically transformed and tumor cells), suppression of cancer-promoting inflammation, and anticancer immunity (both innate and adaptive components). This publication reviews the scientific bases for the indicated cancer-preventing natural defenses and evaluates their implication for assessing cancer risk after exposure to low radiation doses and dose rates. Based on the extensive radiobiological evidence reviewed, it is concluded that the linear-no-threshold (LNT) model (which ignores natural defenses against cancer), as it relates to cancer risk from ionizing radiation, is highly implausible. Plausible models include dose-threshold and hormetic models. More research is needed to establish when a given model (threshold, hormetic, or other) applies to a given low-dose-radiation exposure scenario.

Mitochondrial dysfunction and oxidative stress in aging and cancer

Aging and cancer are the most important issues to research. The population in the world is growing older, and the incidence of cancer increases with age. There is no doubt about the linkage between aging and cancer. However, the molecular mechanisms underlying this association are still unknown. Several lines of evidence suggest that the oxidative stress as a cause and/or consequence of the mitochondrial dysfunction is one of the main drivers of these processes. Increasing ROS levels and products of the oxidative stress, which occur in aging and age-related disorders, were also found in cancer. This review focuses on the similarities between ageing-associated and cancer-associated oxidative stress and mitochondrial dysfunction as their common phenotype.

Lobocrassin B Induces Apoptosis of Human Lung Cancer and Inhibits Tumor Xenograft Growth

Lobocrassin B, a natural cembrane-type compound isolated from the soft coral Lobophytum crassum, has been shown to have significant biological effects, including anticancer activity. As the most common cause of cancer mortality worldwide, lung cancer remains a major concern threatening human health. In the current study, we conducted in vitro experiments to demonstrate the inhibiting effect of Lobocrassin B on CL1-5 and H520 human lung cancer cells growth and to explore the underlying mechanisms, as well as in nude mice bearing CL1-5 tumor xenografts. Lobocrassin B exerted cytotoxic effects on lung cancer cells, as shown by decreasing cell viability, and inducing apoptosis, oxidative stress and mitochondrial dysfunction. In addition, the increased level of Bax, cleaved caspase-3, -9 and -8, and the suppression of Bcl-2 were observed in the Lobocrassin B treated cells. Moreover, in vivo assays verified the significance of these results, revealing that Lobocrassin B inhibited CL1-5 tumor xenograft growth and that inhibitory effects were accompanied by a marked increase in tumor cell apoptosis. In conclusion, the results suggested that Lobocrassin B could be a potential anticancer compound for its propensity to inhibit growth and induce apoptosis in human lung cancer cells.

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

Tumor cells express NADPH oxidase-1 (NOX1) in their membrane and control NOX1-based intercellular reactive oxygen and nitrogen species (ROS/RNS)-dependent apoptosis-inducing signaling through membrane-associated catalase and superoxide dismutase.

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of tumor cells with high concentrations of H₂O₂, peroxnitrite, HOCl, or increasing the concentration of cell-derived NO causes initial generation of singlet oxygen and local inactivation of membrane-associated catalase. As a result, free peroxynitrite and H₂O₂ interact and generate secondary singlet oxygen. Inactivation of further catalase molecules by secondary singlet oxygen leads to auto-amplification of singlet oxygen generation and catalase inactivation. This allows reactivation of intercellular ROS/RNS-signaling and selective apoptosis induction in tumor cells. The initial singlet oxygen generation seems to be the critical point in this complex biochemical multistep mechanism. Initial singlet oxygen generation requires the interaction between distinct tumor cell-derived ROS and RNS and may also depend on either the induction of NO synthase expression or NOX1 activation through the FAS receptor. FAS receptor activation can be achieved by singlet oxygen. Autoamplificatory generation of singlet oxygen through the interaction between peroxynitrite and hydrogen peroxide inherits a rich potential for the establishment of synergistic effects that may be instrumental for novel approaches of tumor therapy with high selectivity towards malignant cells.

Lactobacilli enhance reactive oxygen species-dependent apoptosis-inducing signaling

H₂O₂-producing lactobacilli in the vaginal fluid have been suggested to play a potential tumor-preventive role in addition to the control of undesirable microorganisms. As the vaginal fluid also contains a significant concentration of peroxidase that might utilize lactobacilli-derived H₂O₂ as substrate for HOCl synthesis, a dominant biological role of HOCl in both natural defence systems has been postulated.

Our study shows that lactobacillus-derived H₂O₂ per se is not likely to be beneficial for the vaginal epithelium, as it causes apoptosis nonselectively in nontransformed as well as transformed cells. However, the combination of lactobacilli and peroxidase, i.e. the situation that is actually found in vivo, leads to the conversion of H₂O₂ to HOCl which does not affect non-malignant cells, as these do not generate extracellular superoxide anions. In contrast, malignant cells, due to their abundant extracellular superoxide anion generation allow the generation of apoptosis-inducing hydroxyl radicals through HOCl/superoxide anion interaction. In total, our data show that the combination of H₂O₂ -generating lactobacilli and peroxidase causes the selective elimination of malignant cells and thus might contribute to the tumorpreventive potential of lactobacilli. These findings are in good agreement with epidemiological data. The contribution of lactobacilli in this system can be completely mimicked by H₂O₂-generating glucose oxidase, indicating that it is fully explained by bacterial generation of H₂O₂.

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Lactobacillus-derived H₂O₂ induces apoptosis in nontransformed and transformed cells.

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MPO converts H₂O₂ into HOCl that exerts a selective apoptosis-inducing effect on malignant cells.

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Extracellular superoxide anions of malignant cells are crucial for selective apoptosis induction.

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A model for the tumor protective role of lactobacilli in the presence of peroxidase is presented.

Enhanced release of primary signals may render intercellular signalling ineffective due to spatial aspects

Detailed mechanistic modelling has been performed of the intercellular signalling cascade between precancerous cells and their normal neighbours that leads to a selective removal of the precancerous cells by apoptosis. Two interconnected signalling pathways that were identified experimentally have been modelled, explicitly accounting for temporal and spatial effects. The model predicts highly non-linear behaviour of the signalling. Importantly, under certain conditions, enhanced release of primary signals by precancerous cells renders the signalling ineffective. This counter-intuitive behaviour arises due to spatial aspects of the underlying signalling scheme: Increased primary signalling by precancerous cells does, upon reaction with factors derived from normal cells, produce higher yields of apoptosis-triggering molecules. However, the apoptosis-triggering signals are formed farther from the precancerous cells, so that these are attacked less efficiently. Spatial effects thus may represent a novel analogue of negative feedback mechanisms.

Extracellular localization of catalase is associated with the transformed state of malignant cells

Oncogenic transformation is dependent on activated membrane-associated NADPH oxidase (NOX). However, the resultant extracellular superoxide anions are also driving the NO/peroxynitrite and the HOCl pathway, which eliminates NOX-expressing transformed cells through selective apoptosis induction. Tumor progression is dependent on dominant interference with intercellular apoptosis-inducing ROS signaling through membrane-associated catalase, which decomposes H2O2 and peroxynitrite and oxidizes NO. Particularly, the decomposition of extracellular peroxynitrite strictly requires membrane-associated catalase. We utilized small interfering RNA (siRNA)-mediated knockdown of catalase and neutralizing antibodies directed against the enzyme in combination with challenging H2O2 or peroxynitrite to determine activity and localization of catalase in cells from three distinct steps of multistage oncogenesis. Nontransformed cells did not generate extracellular superoxide anions and only showed intracellular catalase activity. Transformed cells showed superoxide anion-dependent intercellular apoptosis-inducing ROS signaling in the presence of suboptimal catalase activity in their membrane. Tumor cells exhibited tight control of intercellular apoptosis-inducing ROS signaling through a high local concentration of membrane-associated catalase. These data demonstrate that translocation of catalase to the outside of the cell membrane is already associated with the transformation step. A strong local increase in the concentration of membrane-associated catalase is achieved during tumor progression and is controlled by tumor cell-derived H2O2 and by transglutaminase.

Enhancement of Peroxidase Release from Non-Malignant and Malignant Cells through Low-Dose Irradiation with Different Radiation Quality

The release of peroxidase by nontransformed or transformed fibroblasts or epithelial cells (effector cells) triggers apoptosis induction selectively in transformed fibroblasts or transformed epithelial cells (target cells) through intercellular apoptosis-inducing signaling. The release of peroxidase can be induced either by treatment with transforming growth factor beta 1 or by low doses of alpha particles, gamma rays or ultrasoft X rays. In addiation, data indicates that radiation quality does not determine the overall efficiency of peroxidase release and the effects among a wide range of radiation doses are indistinguishable. These findings suggested that peroxidase release might be being triggered through intercellular bystander signaling. We show here that maximal peroxidase release does indeed occur after coculture of a small number of irradiated cells with an excess of unirradiated cells and demonstrate an enhanced effector function of nontransformed cells after the addition of a small number of irradiated cells. These data strongly indicate that peroxidase release is indeed triggered through bystander signaling mechanisms in mammalian cells.

Impact of intercellular induction of apoptosis on low-dose radiation carcinogenesis

In vitro data indicate that selective removal of oncogenic transformed cells by apoptosis induced via signalling by neighbouring cells may represent an important anti-carcinogenic process. Mechanistic modelling supports this concept and predicts that the phenomenon can stop the growth of a transformed cell population, forming a dormant pre-neoplastic lesion, or even remove the transformed clone completely. Radiation has been shown to enhance the underpinning signalling and increase the extent and rate of apoptosis induction in precancerous cells. Implications for low-dose radiation carcinogenesis are discussed based on in vitro data and mechanistic modelling. The possibility is outlined for radiation to act in a pro-carcinogenic manner, i.e. to reduce rather than enhance the removal of transformed cells by apoptosis. The effects of radiation exposure during early or late carcinogenesis are discussed.

Increasing the endogenous NO level causes catalase inactivation and reactivation of intercellular apoptosis signaling specifically in tumor cells

Tumor cells generate extracellular superoxide anions and are protected against intercellular apoptosis-inducing HOCl- and NO/peroxynitrite signaling through the expression of membrane-associated catalase. This enzyme decomposes H₂O₂ and thus prevents HOCl synthesis. It efficiently interferes with NO/peroxynitrite signaling through oxidation of NO and decomposition of peroxynitrite. The regulatory potential of catalase at the crosspoint of ROS and RNS chemical biology, as well as its high local concentration on the outside of the cell membrane of tumor cells, establish tight control of intercellular signaling and thus prevent tumor cell apoptosis. Therefore, inhibition of catalase or its inactivation by singlet oxygen reactivate intercellular apoptosis-inducing signaling. Nitric oxide and peroxynitrite are connected with catalase in multiple and meaningful ways, as (i) NO can be oxidated by compound I of catalase, (ii) NO can reversibly inhibit catalase, (iii) peroxynitrite can be decomposed by catalase and (iv) the interaction between peroxynitrite and H₂O₂ leads to the generation of singlet oxygen that inactivates catalase. Therefore, modulation of the concentration of free NO through addition of arginine, inhibition of arginase, induction of NOS expression or inhibition of NO dioxygenase triggers an autoamplificatory biochemical cascade that is based on initial formation of singlet oxygen, amplification of superoxide anion/H₂O₂ and NO generation through singlet oxygen dependent stimulation of the FAS receptor and caspase-8. Finally, singlet oxygen is generated at sufficiently high concentration to inactivate protective catalase and to reactivate intercellular apoptosis-inducing ROS signaling. This regulatory network allows to establish several pathways for synergistic interactions, like the combination of modulators of NO metabolism with enhancers of superoxide anion generation, modulators of NO metabolism that act at different targets and between modulators of NO metabolism and direct catalase inhibitors. The latter aspect is explicitely studied for the interaction between catalase inhibiting acetylsalicylic acid and an NO donor. It is also shown that hybrid molecules like NO-aspirin utilize this synergistic potential. Our data open novel approaches for rational tumor therapy based on specific ROS signaling and its control in tumor cells.

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Membrane-associated catalase protects tumor cells against ROS/RNS signaling.

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NO can be oxidated by catalase, but can also reversibly inhibit the enzyme.

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ONOO⁻ is decomposed by catalase but also drives its inactivation through singlet oxygen.

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Modulation of the NO level triggers singlet oxygen generation and catalase inactivation.

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This signaling network allows to establish synergistic antitumor effects.

Singlet oxygen treatment of tumor cells triggers extracellular singlet oxygen generation, catalase inactivation and reactivation of intercellular apoptosis-inducing signaling

Intracellular singlet oxygen generation in photofrin-loaded cells caused cell death without discrimination between nonmalignant and malignant cells. In contrast, extracellular singlet oxygen generation caused apoptosis induction selectively in tumor cells through singlet oxygen-mediated inactivation of tumor cell protective catalase and subsequent reactivation of intercellular ROS-mediated apoptosis signaling through the HOCl and the NO/peroxynitrite signaling pathway. Singlet oxygen generation by extracellular photofrin alone was, however, not sufficient for optimal direct inactivation of catalase, but needed to trigger the generation of cell-derived extracellular singlet oxygen through the interaction between H2O2 and peroxynitrite. Thereby, formation of peroxynitrous acid, generation of hydroxyl radicals and formation of perhydroxyl radicals (HO2(.)) through hydroxyl radical/H2O2 interaction seemed to be required as intermediate steps. This amplificatory mechanism led to the formation of singlet oxygen at a sufficiently high concentration for optimal inactivation of membrane-associated catalase. At low initial concentrations of singlet oxygen, an additional amplification step needed to be activated. It depended on singlet oxygen-dependent activation of the FAS receptor and caspase-8, followed by caspase-8-mediated enhancement of NOX activity. The biochemical mechanisms described here might be considered as promising principle for the development of novel approaches in tumor therapy that specifically direct membrane-associated catalase of tumor cells and thus utilize tumor cell-specific apoptosis-inducing ROS signaling.

Revealing mechanisms of selective, concentration-dependent potentials of 4-hydroxy-2-nonenal to induce apoptosis in cancer cells through inactivation of membrane-associated catalase

Tumor cells generate extracellular superoxide anions and are protected against superoxide anion-mediated intercellular apoptosis-inducing signaling by the expression of membrane-associated catalase. 4-Hydroxy-2-nonenal (4-HNE), a versatile second messenger generated during lipid peroxidation, has been shown to induce apoptosis selectively in malignant cells. The findings described in this paper reveal the strong, concentration-dependent potential of 4-HNE to specifically inactivate extracellular catalase of tumor cells both indirectly and directly and to consequently trigger apoptosis in malignant cells through superoxide anion-mediated intercellular apoptosis-inducing signaling. Namely, 4-HNE caused apoptosis selectively in NOX1-expressing tumor cells through inactivation of their membrane-associated catalase, thus reactivating subsequent intercellular signaling through the NO/peroxynitrite and HOCl pathways, followed by the mitochondrial pathway of apoptosis. Concentrations of 4-HNE of 1.2 µM and higher directly inactivated membrane-associated catalase of tumor cells, whereas at lower concentrations, 4-HNE triggered a complex amplificatory pathway based on initial singlet oxygen formation through H2O2 and peroxynitrite interaction. Singlet-oxygen-dependent activation of the FAS receptor and caspase-8 increased superoxide anion generation by NOX1 and amplification of singlet oxygen generation, which allowed singlet-oxygen-dependent inactivation of catalase. 4-HNE and singlet oxygen cooperate in complex autoamplificatory loops during this process. The finding of these novel anticancer pathways may be useful for understanding the role of 4-HNE in the control of malignant cells and for the optimization of ROS-dependent therapeutic approaches including antioxidant treatments.

Helicobacter pylori protects oncogenically transformed cells from reactive oxygen species-mediated intercellular induction of apoptosis

Malignant transformation of gastric epithelial cells by chronic Helicobacter pylori infection is caused by several mechanisms including attraction of reactive oxygen species (ROS)-producing neutrophils and cytotoxin-associated antigen A-mediated dysplastic alterations. Here we show that H.pylori protects transformed cells from ROS-mediated intercellular induction of apoptosis. This potential control step in oncogenesis depends on the HOCl and NO/peroxynitrite (PON) signaling pathways. Helicobacter pylori-associated catalase and superoxide dismutase (SOD) efficiently cooperate in the inhibition of HOCl and the NO/PON signaling pathways. Helicobacter pylori catalase prevents HOCl synthesis through decomposition of hydrogen peroxide. Helicobacter pylori-associated SOD interferes with the crucial interactions between superoxide anions and HOCl, as well as superoxide anions and NO. The ratio of bacteria to malignant cells is critical for sufficient protection of transformed cells. Low concentrations of H.pylori more efficiently inhibited ROS-mediated destruction of transformed cells when compared with high concentrations of bacteria. Our data demonstrate the critical role of H.pylori antioxidant enzymes in the survival of transformed cells, modulating an early step of oncogenesis that is distinct from the transformation process per se.

Redox signaling in cardiovascular health and disease

Spatiotemporal regulation of the activity of a vast array of intracellular proteins and signaling pathways by reactive oxygen species (ROS) governs normal cardiovascular function. However, data from experimental and animal studies strongly support that dysregulated redox signaling, resulting from hyperactivation of various cellular oxidases or mitochondrial dysfunction, is integral to the pathogenesis and progression of cardiovascular disease (CVD). In this review, we address how redox signaling modulates the protein function, the various sources of increased oxidative stress in CVD, and the labyrinth of redox-sensitive molecular mechanisms involved in the development of atherosclerosis, hypertension, cardiac hypertrophy and heart failure, and ischemia-reperfusion injury. Advances in redox biology and pharmacology for inhibiting ROS production in specific cell types and subcellular organelles combined with the development of nanotechnology-based new in vivo imaging systems and targeted drug delivery mechanisms may enable fine-tuning of redox signaling for the treatment and prevention of CVD.

Low-dose gamma irradiation enhances superoxide anion production by nonirradiated cells through TGF-β1-dependent bystander signaling

We show here that low-dose gamma irradiation substantially increase in extracellular superoxide anion production in oncogenically transformed cells and tumor cells but not by nontransformed cells. The transfer of only a few cells from an irradiated culture to nonirradiated control cells was sufficient for the transmission of a signal to induce superoxide anion production in the nonirradiated cells. The number of irradiated cells that was necessary for the successful induction of superoxide anion production in the nonirradiated cells depended on radiation dose. When irradiated cells were allowed to incubate for 1 h before transmission to the nonirradiated cultures, nearly all of the cells from the irradiated cell population were able to communicate the inducing signal to nonirradiated cells. siRNA-dependent knockdown and reconstitution experiments showed that TGF-β1 was sufficient to mediate the bystander effect triggered by low-dose radiation in this experimental system. A kinetic analysis demonstrated that the enhanced superoxide anion production was substantially reduced before the release of the bystander signal by activated TGF-β.

Effect of hydrogen gas on the survival rate of mice following global cerebral ischemia

Global cerebral ischemia and reperfusion (I/R) often result in high mortality. Free radicals have been reported to play an important role in global cerebral I/R, and therefore, reduction of these might improve the outcome. Here, we investigated the effect of hydrogen gas (H2) (a strong free radical scavenger) on the survival rate of mice following global cerebral I/R. We further examined the histopathological outcome and also the brain water content (as a possible determinant of mortality). Male C57BL/6J mice were subjected to global cerebral I/R by means of 45-min bilateral common carotid artery occlusion (BCCAO). A total of 160 mice were divided into three groups: sham surgery (sham group), BCCAO without H2 (BCCAO group), and BCCAO treated with 1.3% H2 (BCCAO + H2 group). We observed that H2 treatment significantly (P = 0.0232) improved the 7-day survival rate of mice, from 8.3% (BCCAO group, n = 12) to 50% (BCCAO + H2 group, n = 10). Histopathological analysis revealed that H2 treatment significantly attenuated neuronal injury and autophagy in the hippocampal cornu ammonis 1 sector and also brain edema, after 24 h of reperfusion. The beneficial effects of H2 treatment on brain injury were associated with significantly lower levels of oxidative stress markers (8-hydroxy-2'-deoxyguanosine and malondialdehyde) in the brain tissue. Thus, we believe that H2 may be an effective treatment for global cerebral I/R.

Mechanistic modelling suggests that the size of preneoplastic lesions is limited by intercellular induction of apoptosis in oncogenically transformed cells

Selective removal of oncogenically transformed cells by apoptosis induced via signalling by surrounding cells has been suggested to represent a natural anticarcinogenic process. To investigate its potential effect in detail, a mechanistic model of this process is proposed. The model is calibrated against in vitro data on apoptosis triggered in transformed cells by defined external inducers as well as through signalling by normal cells under coculture conditions. The model predicts that intercellular induction of apoptosis is capable of balancing the proliferation of oncogenically transformed cells and limiting the size of their populations over long times, even if their proliferation per se were unlimited. Experimental research is desired to verify whether the predicted stable population of transformed cells corresponds to a kind of dormancy during early-stage carcinogenesis (dormant preneoplastic lesions), and how this process relates to other anticarcinogenic mechanisms taking place under in vivo conditions.

The effect of non-targeted cellular mechanisms on lung cancer risk for chronic, low level radon exposures

PURPOSE

The goal of the present study was to investigate the effect of non-targeted mechanisms on the shape of the lung cancer risk function at chronic, low level radon exposures relative to direct cellular radiation effects. This includes detrimental and protective bystander effects, radio-adaptive bystander response, genomic instability and induction of apoptosis by surrounding cells.

METHODS

To quantify the dependence of these mechanisms on dose, analytical functions were derived from the experimental evidence presently available. Alpha particle intersections of bronchial target cells during a given exposure period were simulated by a Transformation Frequency-Tissue Response (TF-TR) model, formulated in terms of cellular hits within the cycle time of the cell and then integrated over the whole exposure period.

RESULTS

In general, non-targeted effects like genomic instability and bystander effects amplify the biological effectiveness of a given radiation dose, while induction of apoptosis and adaptive response will decrease the risk values. While these observations are related to the absolute number of lung cancer cases, normalization to the epidemiologically observed risk at 0.675 Gy suggests that the effect of such mechanisms on the shape of the dose-response relationship may be different. Indeed, genomic instability and adaptive response cause a substantial reduction of the risk at low doses, while induction of apoptosis and detrimental bystander effects slightly increase the risk.

CONCLUSIONS

Predictions of lung cancer risk, including these mechanisms, exhibit a distinct sublinear dose-response relationship at low exposures, particularly for very low exposure rates. However, the relatively large error bars of the epidemiological data do not currently allow the prediction of a statistically significant deviation from the Linear - No Threshold (LNT) assumption.

Modelling of intercellular induction of apoptosis in oncogenic transformed cells and radiation effects on the phenomenon

The removal of transformed cells via induction of apoptosis through intercellular signalling by surrounding cells is supposed to represent an important control mechanism limiting carcinogenesis. Low doses of radiation influence the efficiency of this anti-carcinogenesis process, indicating possible beneficial effects of low doses of radiation mediated by intercellular communication ('non-targeted effects'). To quantitatively understand the signalling system involved and the effects of radiation and to assess the role of this phenomenon in radiation-induced carcinogenesis, multi-scale modelling studies have been started. The proposed kinetic model takes into account (i) triggering of the effector function in cells in the vicinity of transformed cells, (ii) intercellular signalling between effector and transformed cells and (iii) execution of apoptosis in attacked cells. The systems model without radiation perturbance is reviewed. First results accounting for radiation-induced modulations of the signalling schemes are presented.

Intercellular induction of apoptosis signalling pathways

Intercellular signalling plays an important role in the progression of a transformed cell to a tumour. In order to characterise the underlying mechanisms, a well-defined model cell system of intercellular induction of apoptosis was used where neighbouring normal cells can selectively eliminate transformed cells. In the absence of non-transformed cells, the induction of apoptosis in transformed 208Fsrc3 cells occurs via autocrine destruction and is dominated by peroxidase (PO), which initiates the PO/hypochlorous acid signalling pathway at high local cell densities. However, when the transformed cells are co-cultured with the non-transformed 208F cells, apoptosis in transformed cells additionally occurs as a result of intercellular signalling with the non-transformed cells and is predominantly due to the production of nitric oxide (NO(•)), which initiates the NO(•)/peroxynitrite pathway.

Review

Production of superoxide anion O2*- by the membrane-bound enzyme NADPH oxidase of phagocytes is a long-known phenomenon; it is generally assumed that O2*-helps phagocytes kill bacterial intruders. The details and the chemistry of the killing process have, however, remained a mystery. Isoforms of NADPH oxidase exist in membranes of nearly every cell, suggesting that reactive oxygen species (ROS) participate in intra- and intercellular signaling processes. What the nature of the signal is exactly, how it is transmitted, and what structural characteristics a receptor of a "radical message" must have, have not been addressed convincingly. This review discusses how the action of messengers is in agreement with radical-specific behavior. In search for the smallest common denominator of cellular free radical activity we hypothesize that O2*- and its conjugate acid, HO2*, may have evolved under primordial conditions as regulators of membrane mechanics and that isoprostanes, widely used markers of "oxidative stress", may be an adventitious correlate of this biologic activity of O2*-/HO2*. An overall picture is presented that suggests that O2*-/HO2* radicals, by modifying cell membranes, help other agents gain access to the hydrophobic region of phospholipid bilayers and hence contribute to lipid-dependent signaling cascades. With this, O2*-/HO2* are proposed as indispensable adjuvants for the generation of cellular signals, for membrane transport, channel gating and hence, in a global sense, for cell viability and growth. We also suggest that many of the allegedly O2*- dependent bacterial pathologies and carcinogenic derailments are due to membrane-modifying activity rather than other chemical reactions of O2*-/HO2*. A consequence of this picture is the potential evolution of the "radical theory of ageing" to a "lipid theory of aging".

Fibroblast-mediated in vivo and in vitro growth promotion of tumorigenic rat thyroid carcinoma cells but not normal Fisher rat thyroid follicular cells

BACKGROUND

It is known that genetic abnormalities in oncogenes and/or tumor suppressor genes promote carcinogenesis. Numerous recent articles, however, have demonstrated that epithelial-stromal interaction also plays a critical role for initiation and progression of carcinoma cells. Furthermore, ionizing radiation induces alterations in the tissue microenvironments that promote carcinogenesis. There is little or no information on epithelial-stromal interaction in thyroid carcinoma cells. The objective of this study was to determine if epithelial-stromal interaction influenced the growth of thyroid carcinoma cells in vivo and in vitro and to determine if radiation had added or interacting effects.

METHODS

Normal Fisher rat thyroid follicular cells (FRTL5 cells) and tumorigenic rat thyroid carcinoma cells (FRTL-Tc cells) derived from FRTL5 cells were employed. The cells were injected into thyroids or subcutaneously into left flanks of rats alone or in combination with skin-derived fibroblasts. In groups of rats, fibroblasts were irradiated with 0.1 or 4 Gy x-ray 3 days before inoculation. In vitro growth of FRTL-Tc and FRTL-5 cells were evaluated using the fibroblast-conditioned medium and in a co-culture system with fibroblasts.

RESULTS

The in vivo experiments demonstrated that FRTL-Tc cells injected intrathyroidally grew faster than those injected subcutaneously, and that admixed fibroblasts enhanced growth of subcutaneous FRTL-Tc tumors, indicating that the intrathyroidal milieu, particularly in the presence of fibroblasts, confer growth-promoting advantage to thyroid carcinoma cells. This in vivo growth-promoting effect of fibroblasts on FRTL-Tc cells was duplicated in the in vitro experiments using the fibroblast-conditioned medium. Thus, our data demonstrate that this effect is mediated by soluble factor(s), is reversible, and is comparable to that of 10% fetal bovine serum. However, normal FRTL5 cells did not respond to the fibroblast-conditioned medium. Furthermore, high- and low-dose irradiation enhanced and suppressed, respectively, the in vivo fibroblast-mediated growth promotion. This effect was, however, not observed in the in vitro experiment with conditioned medium or even that allowing cell-cell contact.

CONCLUSIONS

The intrathyroidal stromal microenvironments, particularly fibroblasts, appear to enhance the growth of thyroid carcinomas through soluble factor(s), which is modulated differently by high- and low-dose irradiation. To our knowledge this is the first study to show epithelial-stromal interaction in thyroid carcinoma.

Mitochondrial pathophysiology, reactive oxygen species, and cardiovascular diseases

This article discusses mitochondrial pathophysiology, reactive oxygen species, and cardiovascular diseases. Mitochondrial respiratory chains are responsible for energy metabolism/ATP production through the tricyclic antidepressant cycle, coupling of oxidative phosphorylation, and electron transfer. The mitochondrion produces reactive oxygen species as "side products" of respiration. The mitochondrial derived reactive oxygen species is involved in the pathogenesis of various clinical disorders including heart failure, hypoxia, ischemia/reperfusion injury, diabetes, neurodegenerative diseases, and the physiologic process of aging. Observational and mechanistical studies of these pathologic roles of mitochondria are discussed in depth in this article.

Low dose radiation and intercellular induction of apoptosis: potential implications for the control of oncogenesis

PURPOSE

This review is focused on the potential impact of low dose radiation effects on intercellular induction of apoptosis and the underlying reactive-oxygen species (ROS)-mediated signaling pathways.

RESULTS

Transformed cells are subject to ROS-mediated apoptosis induction by non-transformed cells ('intercellular induction of apoptosis') and by ROS-mediated autocrine self-destruction. Sensitivity to intercellular induction of apoptosis and autocrine self-destruction are strictly correlated to the expression of the transformed state. Extracellular superoxide anions generated by transformed target cells drive the selectivity and sensitivity of this signaling system which is based on four different signaling pathways. Low dose irradiation of non-transformed cells enhances intercellular induction of apoptosis in transformed cells. This process is controlled by TGF-beta and seems to depend on the induction of peroxidase release. In addition, low dose radiation enhances superoxide anion generation of transformed target cells.

CONCLUSIONS

Low dose radiation-triggered enhancement of intercellular induction of apoptosis and autocrine self-destruction might represent a potential control system during carcinogenesis. It might be the underlying mechanism for the well-known inhibitory effect of low dose radiation on detectable transformation events. However, modifications of the complex intercellular ROS-based signaling system may also lead to configurations in which low dose radiation attenuates ROS-mediated apoptosis induction.

Low-dose irradiation of nontransformed cells stimulates the selective removal of precancerous cells via intercellular induction of apoptosis

An important stage in tumorigenesis is the ability of a precancerous cell to escape natural anticancer signals imposed on it by neighboring cells and its microenvironment. We have previously characterized a system of intercellular induction of apoptosis whereby nontransformed cells selectively remove transformed cells from coculture via cytokine and reactive oxygen/nitrogen species (ROS/RNS) signaling. We report that irradiation of nontransformed cells with low doses of either high linear energy transfer (LET) alpha-particles or low-LET gamma-rays leads to stimulation of intercellular induction of apoptosis. The use of scavengers and inhibitors confirms the involvement of ROS/RNS signaling and of the importance of transformed cell NADPH oxidase in the selectivity of the system. Doses as low as 2-mGy gamma-rays and 0.29-mGy alpha-particles were sufficient to produce an observable increase in transformed cell apoptosis. This radiation-stimulated effect saturates at very low doses (50 mGy for gamma-rays and 25 mGy for alpha-particles). The use of transforming growth factor-beta (TGF-beta) neutralizing antibody confirms a role for the cytokine in the radiation-induced signaling. The system may represent a natural anticancer mechanism stimulated by extremely low doses of ionizing radiation.