Nitric oxide-mediated bystander signal transduction induced by heavy-ion microbeam irradiation

Primary and Secondary Bystander Effects of Proton Microbeam Irradiation on Human Lung Cancer Cells under Hypoxic Conditions

Tumor hypoxia is the most common feature of radioresistance to the radiotherapy (RT) of lung cancer and results in poor clinical outcomes. High-linear energy transfer (LET) radiation is a novel RT technique to overcome this problem. However, a limited number of studies have been elucidated on the underlying mechanism(s) of RIBE and RISBE in cancer cells exposed to high-LET radiation under hypoxia. Here, we developed a new method to investigate the RIBE and RISBE under hypoxia using the SPICE-QST proton microbeams and a layered tissue co-culture system. Normal lung fibroblast (WI-38) and lung cancer (A549) cells were exposed in the range of 06 Gy of proton microbeams, wherein only ~0.04-0.15% of the cells were traversed by protons. Subsequently, primary bystander A549 cells were co-cultured with secondary bystander A549 cells in the presence or absence of a GJIC and NO inhibitor using co-culture systems. Studies show that there are differences in RIBE in A549 and WI-38 primary bystander cells under normoxia and hypoxia. Interestingly, treatment with a GJIC inhibitor showed an increase in the toxicity of primary bystander WI-38 cells but a decrease in A549 cells under hypoxia. Our results also show the induction of RISBE in secondary bystander A549 cells under hypoxia, where GJIC and NO inhibitors reduced the stressful effects on secondary bystander A549 cells. Together, these preliminary results, for the first time, represented the involvement of intercellular communications through GJIC in propagation of RIBE and RISBE in hypoxic cancer cells.

Observation of Histone H2AX Phosphorylation by Radiation-Induced Bystander Response Using Titanium Characteristic X-ray Microbeam

Radiation-induced bystander response (RIBR) is a response induced in non-irradiated cells that receive bystander signals from directly irradiated cells. X-ray microbeams are useful tools for elucidating the mechanisms underlying RIBR. However, previous X-ray microbeams used low-energy soft X-rays with higher biological effects, such as aluminum characteristic X-rays, and the difference from conventional X-rays and γ-rays has often been discussed. The microbeam X-ray cell irradiation system at the Central Research Institute of Electric Power Industry has been upgraded to generate higher energy titanium characteristic X-rays (Ti_K X-rays), which have a longer penetration distance sufficient to irradiate 3D cultured tissues. Using this system, we irradiated the nuclei of HeLa cells with high precision and found that the pan-nuclear induction of phosphorylated histone H2AX on serine 139 (γ-H2AX) in the non-irradiated cells increased 180 and 360 min after irradiation. We established a new method to quantitatively evaluate bystander cells, using the fluorescence intensity of γ-H2AX as an indicator. The percentage of bystander cells increased significantly to 23.2% ± 3.2% and 29.3% ± 3.5% at 180 and 360 min after irradiation, respectively. Our irradiation system and the obtained results may be useful for studies of cell competition as well as non-targeted effects.

Predominant contribution of the dose received from constituent heavy-ions in the induction of gastrointestinal tumorigenesis after simulated space radiation exposure

Gastrointestinal (GI) cancer risk among astronauts after encountering galactic cosmic radiation (GCR) is predicted to exceed safe permissible limits in long duration deep-space missions. Current predictions are based on relative biological effectiveness (RBE) values derived from in-vivo studies using single-ion beams, while GCR is essentially a mixed radiation field composed of protons (H), helium (He), and heavy ions. Therefore, a sequentially delivered proton (H) → Helium (He) → Oxygen (O) → Silicon (Si) beam was designed to simulate simplified-mixed-field GCR (Smf-GCR), and Apc^1638N/+ mice were total-body irradiated to sham or γ (¹⁵⁷Cs) or Smf-GCR followed by assessment of GI-tumorigenesis at 150 days post-exposure. Further, GI-tumor data from equivalent doses of heavy-ions (i.e., 0.05 Gy of O and Si) in 0.5 Gy of Smf-GCR were compared to understand the contributions of heavy-ions in GI-tumorigenesis. The Smf-GCR-induced tumor and carcinoma count were significantly greater than γ-rays, and male preponderance for GI-tumorigenesis was consistent with our earlier findings. Comparison of tumor data from Smf-GCR and equivalent doses of heavy ions revealed an association between higher GI-tumorigenesis where dose received from heavy-ions contributed to > 95% of the total GI-tumorigenic effect observed after Smf-GCR. This study provides the first experimental evidence that cancer risk after GCR exposure could largely depend on doses received from constituent heavy-ions.

Radiation-Induced Bystander Effect and Cytoplasmic Irradiation Studies with Microbeams

Simple Summary

Microbeams are useful tools in studies on non-target effects, such as the radiation-induced bystander effect, and responses related to cytoplasmic irradiation. A micrometer or even sub-micrometer-level beam size enables the precise delivery of radiation energy to a specific target. Here we summarize the observations of the bystander effect and the cytoplasmic irradiation-related effect using different kinds of microbeam irradiators as well as discuss the cellular and molecular mechanisms that are involved in these responses. Non-target effects may increase the detrimental effect caused by radiation, so a more comprehensive knowledge of the process will enable better evaluation of the damage resulting from irradiation.

Abstract

Although direct damage to nuclear DNA is considered as the major contributing event that leads to radiation-induced effects, accumulating evidence in the past two decades has shown that non-target events, in which cells are not directly irradiated but receive signals from the irradiated cells, or cells irradiated at extranuclear targets, may also contribute to the biological consequences of exposure to ionizing radiation. With a beam diameter at the micrometer or sub-micrometer level, microbeams can precisely deliver radiation, without damaging the surrounding area, or deposit the radiation energy at specific sub-cellular locations within a cell. Such unique features cannot be achieved by other kinds of radiation settings, hence making a microbeam irradiator useful in studies of a radiation-induced bystander effect (RIBE) and cytoplasmic irradiation. Here, studies on RIBE and different responses to cytoplasmic irradiation using microbeams are summarized. Possible mechanisms related to the bystander effect, which include gap-junction intercellular communications and soluble signal molecules as well as factors involved in cytoplasmic irradiation-induced events, are also discussed.

The intercellular communications mediating radiation-induced bystander effects and their relevance to environmental, occupational, and therapeutic exposures

PURPOSE

The assumption that traversal of the cell nucleus by ionizing radiation is a prerequisite to induce genetic damage, or other important biological responses, has been challenged by studies showing that oxidative alterations extend beyond the irradiated cells and occur also in neighboring bystander cells. Cells and tissues outside the radiation field experience significant biochemical and phenotypic changes that are often similar to those observed in the irradiated cells and tissues. With relevance to the assessment of long-term health risks of occupational, environmental and clinical exposures, measurable genetic, epigenetic, and metabolic changes have been also detected in the progeny of bystander cells. How the oxidative damage spreads from the irradiated cells to their neighboring bystander cells has been under intense investigation. Following a brief summary of the trends in radiobiology leading to this paradigm shift in the field, we review key findings of bystander effects induced by low and high doses of various types of radiation that differ in their biophysical characteristics. While notable mechanistic insights continue to emerge, here the focus is on the many means of intercellular communication that mediate these effects, namely junctional channels, secreted molecules and extracellular vesicles, and immune pathways.

CONCLUSIONS

The insights gained by studying radiation bystander effects are leading to a basic understanding of the intercellular communications that occur under mild and severe oxidative stress in both normal and cancerous tissues. Understanding the mechanisms underlying these communications will likely contribute to reducing the uncertainty of predicting adverse health effects following exposure to low dose/low fluence ionizing radiation, guide novel interventions that mitigate adverse out-of-field effects, and contribute to better outcomes of radiotherapeutic treatments of cancer. In this review, we highlight novel routes of intercellular communication for investigation, and raise the rationale for reconsidering classification of bystander responses, abscopal effects, and expression of genomic instability as non-targeted effects of radiation.

Revisiting the Radiobiology of Targeted Alpha Therapy

Targeted alpha therapy (TAT) using alpha particle-emitting radionuclides is in the spotlight after the approval of ²²³RaCl₂ for patients with metastatic castration-resistant prostate cancer and the development of several alpha emitter-based radiopharmaceuticals. It is acknowledged that alpha particles are highly cytotoxic because they produce complex DNA lesions. Hence, the nucleus is considered their critical target, and many studies did not report any effect in other subcellular compartments. Moreover, their physical features, including their range in tissues (<100 μm) and their linear energy transfer (50–230 keV/μm), are well-characterized. Theoretically, TAT is indicated for very small-volume, disseminated tumors (e.g., micrometastases, circulating tumor cells). Moreover, due to their high cytotoxicity, alpha particles should be preferred to beta particles and X-rays to overcome radiation resistance. However, clinical studies showed that TAT might be efficient also in quite large tumors, and biological effects have been observed also away from irradiated cells. These distant effects are called bystander effects when occurring at short distance (<1 mm), and systemic effects when occurring at much longer distance. Systemic effects implicate the immune system. These findings showed that cells can die without receiving any radiation dose, and that a more complex and integrated view of radiobiology is required. This includes the notion that the direct, bystander and systemic responses cannot be dissociated because DNA damage is intimately linked to bystander effects and immune response. Here, we provide a brief overview of the paradigms that need to be revisited.

Cx43 channels and signaling via IP3/Ca2+, ATP, and ROS/NO propagate radiation-induced DNA damage to non-irradiated brain microvascular endothelial cells

Radiotherapeutic treatment consists of targeted application of radiation beams to a tumor but exposure of surrounding healthy tissue is inevitable. In the brain, ionizing radiation induces breakdown of the blood-brain barrier by effects on brain microvascular endothelial cells. Damage from directly irradiated cells can be transferred to surrounding non-exposed bystander cells, known as the radiation-induced bystander effect. We investigated involvement of connexin channels and paracrine signaling in radiation-induced bystander DNA damage in brain microvascular endothelial cells exposed to focused X-rays. Irradiation caused DNA damage in the directly exposed area, which propagated over several millimeters in the bystander area. DNA damage was significantly reduced by the connexin channel-targeting peptide Gap26 and the Cx43 hemichannel blocker TAT-Gap19. ATP release, dye uptake, and patch clamp experiments showed that hemichannels opened within 5 min post irradiation in both irradiated and bystander areas. Bystander signaling involved cellular Ca²⁺ dynamics and IP₃, ATP, ROS, and NO signaling, with Ca²⁺, IP₃, and ROS as crucial propagators of DNA damage. We conclude that bystander effects are communicated by a concerted cascade involving connexin channels, and IP₃/Ca²⁺, ATP, ROS, and NO as major contributors of regenerative signal expansion.

Melatonin Modulates Regulation of NOX2 and NOX4 Following Irradiation in the Lung

Background:

Exposure to ionizing radiation may lead to chronic upregulation of inflammatory mediators and pro-oxidant enzymes, which give rise to continuous production of reactive oxygen species (ROS). NADPH oxidases are among the most important ROS producing enzymes. Their upregulation is associated with DNA damage and genomic instability. In the present study, we sought to determine the expressions of NADPH oxidases; NOX2 and NOX4, in rat’s lung following whole body or pelvis irradiation. In addition, we evaluated the protective effect of melatonin on the expressions of NOX2 and NOX4, as well as oxidative DNA injury.

Materials and Methods:

35 male rats were divided into 7 groups, G1: control; G2: melatonin (100 mg/kg) treatment; G3: whole body irradiation (2 Gy); G4: melatonin plus whole body irradiation; G5: local irradiation to pelvis area; G6: melatonin treatment plus 2 Gy gamma rays to pelvis area; G7: scatter group. All the rats were sacrificed after 24 h. afterwards, the expressions of TGFβR1, Smad2, NF- κB, NOX2 and NOX4 were detected using real-time PCR. Also, the level of 8-OHdG was detected by ELISA, and NOX2 and NOX4 protein levels were detected by western blot.

Results:

Whole body irradiation led to the upregulation of all genes, while local pelvis irradiation caused upregulation of TGFβR1, NF-κB, NOX2 and NOX4, as well as protein levels of NOX2 and NOX4. Treatment with melatonin reduced the expressions of these genes and also alleviated oxidative injury in both targeted and non-targeted lung tissues. Results also showed no significant reduction for NOX2 and NOX4 in bystander tissues following melatonin treatment.

Conclusion:

It is possible that upregulation of NOX2 and NOX4 is involved in radiation-induced targeted and non-targeted lung injury. Melatonin may reduce oxidative stress following upregulation of these enzymes in directly irradiated lung tissues but not for bystander.

Genomic Instability and Carcinogenesis of Heavy Charged Particles Radiation: Clinical and Environmental Implications

One of the uses of ionizing radiation is in cancer treatment. The use of heavy charged particles for treatment has been introduced in recent decades because of their priority for deposition of radiation energy in the tumor, via the Bragg peak phenomenon. In addition to medical implications, exposure to heavy charged particles is a crucial issue for environmental and space radiobiology. Ionizing radiation is one of the most powerful clastogenic and carcinogenic agents. Studies have shown that although both low and high linear energy transfer (LET) radiations are carcinogenic, their risks are different. Molecular studies have also shown that although heavy charged particles mainly induce DNA damage directly, they may be more potent inducer of endogenous generation of free radicals compared to the low LET gamma or X-rays. It seems that the severity of genotoxicity for non-irradiated bystander cells is potentiated as the quality of radiation increases. However, this is not true in all situations. Evidence suggests the involvement of some mechanisms such as upregulation of pro-oxidant enzymes and change in the methylation of DNA in the development of genomic instability and carcinogenesis. This review aimed to report important issues for genotoxicity of carcinogenic effects of heavy charged particles. Furthermore, we tried to explain some mechanisms that may be involved in cancer development following exposure to heavy charged particles.

Heavy-Ion Microbeams for Biological Science: Development of System and Utilization for Biological Experiments in QST-Takasaki

Target irradiation of biological material with a heavy-ion microbeam is a useful means to analyze the mechanisms underlying the effects of heavy-ion irradiation on cells and individuals. At QST-Takasaki, there are two heavy-ion microbeam systems, one using beam collimation and the other beam focusing. They are installed on the vertical beam lines of the azimuthally-varying-field cyclotron of the TIARA facility for analyzing heavy-ion radiation effects on biological samples. The collimating heavy-ion microbeam system is used in a wide range of biological research not only in regard to cultured cells but also small individuals, such as silkworms, nematode C. elegans, and medaka fish. The focusing microbeam system was designed and developed to perform more precise target irradiation that cannot be achieved through collimation. This review describes recent updates of the collimating heavy ion microbeam system and the research performed using it. In addition, a brief outline of the focusing microbeam system and current development status is described.

Intercellular communications-redox interactions in radiation toxicity; potential targets for radiation mitigation

Nowadays, using ionizing radiation (IR) is necessary for clinical, agricultural, nuclear energy or industrial applications. Accidental exposure to IR after a radiation terror or disaster poses a threat to human. In contrast to the old dogma of radiation toxicity, several experiments during the last two recent decades have revealed that intercellular signaling and communications play a key role in this procedure. Elevated level of cytokines and other intercellular signals increase oxidative damage and inflammatory responses via reduction/oxidation interactions (redox system). Intercellular signals induce production of free radicals and inflammatory mediators by some intermediate enzymes such as cyclooxygenase-2 (COX-2), nitric oxide synthase (NOS), NADPH oxidase, and also via triggering mitochondrial ROS. Furthermore, these signals facilitate cell to cell contact and increasing cell toxicity via cohort effect. Nitric oxide is a free radical with ability to act as an intercellular signal that induce DNA damage and changes in some signaling pathways in irradiated as well as non-irradiated adjacent cells. Targeting of these mediators by some anti-inflammatory agents or via antioxidants such as mitochondrial ROS scavengers opens a window to mitigate radiation toxicity after an accidental exposure. Experiments which have been done so far suggests that some cytokines such as IL-1β, TNF-α, TGF-β, IL-4 and IL-13 are some interesting targets that depend on irradiated organs and may help mitigate radiation toxicity. Moreover, animal experiments in recent years indicated that targeting of toll like receptors (TLRs) may be more useful for radioprotection and mitigation. In this review, we aimed to describe the role of intercellular interactions in oxidative injury, inflammation, cell death and killing effects of IR. Moreover, we described evidence on potential mitigation of radiation injury via targeting of these mediators.

Radiation quality effects alteration in COX-2 pathway to trigger radiation-induced bystander response in A549 lung carcinoma cells

This study aimed to determine whether the radiation-induced bystander effect (RIBE) is affected by radiation quality. To mimic the different radiation qualities of the direct action (D)/indirect action (ID) ratio, A549 cells were exposed to X-rays, with either 100 mM of the radical scavenger, thio-urea (TU+), or null (TU-). Biological responses in irradiated and bystander cells were compared at equal lethal effects of a 6% survival dose, which was estimated from the survival curves to be 8 Gy and 5 Gy for TU+ and TU-, respectively. Cyclooxygenase-2 (COX-2) expression in TU- irradiated cells increased up to 8 h post-irradiation, before decreasing towards 24 h. The concentration of prostaglandin E2 (PGE2), a primary product of COX-2 and known as a secreted inducible factor in RIBE, increased over 3-fold compared with that in the control at 8 h post-irradiation. Conversely, COX-2 expression and PGE2 production of TU+ irradiated cells were drastically suppressed. These results show that the larger D/ID suppressed COX-2 expression and PGE2 production in irradiated cells. However, in contrast to the case in the irradiated cells, COX-2 expression was equally observed in the TU- and TU+ co-cultured bystander cells, which showed the highest expression levels at 24 h post-irradiation. Taken together, these findings demonstrate that radiation quality, such as the D/ID ratio, may be an important factor in the alteration of signalling pathways involved in RIBE.

Molecular Signaling in Response to Charged Particle Exposures and its Importance in Particle Therapy

Energetic, charged particles elicit an orchestrated DNA damage response (DDR) during their traversal through healthy tissues and tumors. Complex DNA damage formation, after exposure to high linear energy transfer (LET) charged particles, results in DNA repair foci formation, which begins within seconds. More protein modifications occur after high-LET, compared with low-LET, irradiation. Charged-particle exposure activates several transcription factors that are cytoprotective or cytodestructive, or that upregulate cytokine and chemokine expression, and are involved in bystander signaling. Molecular signaling for a survival or death decision in different tumor types and healthy tissues should be studied as prerequisite for shaping sensitizing and protective strategies. Long-term signaling and gene expression changes were found in various tissues of animals exposed to charged particles, and elucidation of their role in chronic and late effects of charged-particle therapy will help to develop effective preventive measures.

Region-specific irradiation system with heavy-ion microbeam for active individuals of Caenorhabditis elegans

Radiation may affect essential functions and behaviors such as locomotion, feeding, learning and memory. Although whole-body irradiation has been shown to reduce motility in the nematode Caenorhabditis elegans, the detailed mechanism responsible for this effect remains unknown. Targeted irradiation of the nerve ring responsible for sensory integration and information processing would allow us to determine whether the reduction of motility following whole-body irradiation reflects effects on the central nervous system or on the muscle cells themselves. We therefore addressed this issue using a collimating microbeam system. However, radiation targeting requires the animal to be immobilized, and previous studies have anesthetized animals to prevent their movement, thus making it impossible to assess their locomotion immediately after irradiation. We developed a method in which the animal was enclosed in a straight, microfluidic channel in a polydimethylsiloxane chip to inhibit free motion during irradiation, thus allowing locomotion to be observed immediately after irradiation. The head region (including the central nervous system), mid region around the intestine and uterus, and tail region were targeted independently. Each region was irradiated with 12 000 carbon ions (12C; 18.3 MeV/u; linear energy transfer = 106.4 keV/μm), corresponding to 500 Gy at a φ20 μm region. Motility was significantly decreased by whole-body irradiation, but not by irradiation of any of the individual regions, including the central nervous system. This suggests that radiation inhibits locomotion by a whole-body mechanism, potentially involving motoneurons and/or body-wall muscle cells, rather than affecting motor control via the central nervous system and the stimulation response.

Transcriptomics, NF-κB Pathway, and Their Potential Spaceflight-Related Health Consequences

In space, living organisms are exposed to multiple stress factors including microgravity and space radiation. For humans, these harmful environmental factors have been known to cause negative health impacts such as bone loss and immune dysfunction. Understanding the mechanisms by which spaceflight impacts human health at the molecular level is critical not only for accurately assessing the risks associated with spaceflight, but also for developing effective countermeasures. Over the years, a number of studies have been conducted under real or simulated space conditions. RNA and protein levels in cellular and animal models have been targeted in order to identify pathways affected by spaceflight. Of the many pathways responsive to the space environment, the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) network appears to commonly be affected across many different cell types under the true or simulated spaceflight conditions. NF-κB is of particular interest, as it is associated with many of the spaceflight-related health consequences. This review intends to summarize the transcriptomics studies that identified NF-κB as a responsive pathway to ground-based simulated microgravity or the true spaceflight condition. These studies were carried out using either human cell or animal models. In addition, the review summarizes the studies that focused specifically on NF-κB pathway in specific cell types or organ tissues as related to the known spaceflight-related health risks including immune dysfunction, bone loss, muscle atrophy, central nerve system (CNS) dysfunction, and risks associated with space radiation. Whether the NF-κB pathway is activated or inhibited in space is dependent on the cell type, but the potential health impact appeared to be always negative. It is argued that more studies on NF-κB should be conducted to fully understand this particular pathway for the benefit of crew health in space.

Systemic mechanisms and effects of ionizing radiation: A new 'old' paradigm of how the bystanders and distant can become the players

Exposure of cells to any form of ionizing radiation (IR) is expected to induce a variety of DNA lesions, including double strand breaks (DSBs), single strand breaks (SSBs) and oxidized bases, as well as loss of bases, i.e., abasic sites. The damaging potential of IR is primarily related to the generation of electrons, which through their interaction with water produce free radicals. In their turn, free radicals attack DNA, proteins and lipids. Damage is induced also through direct deposition of energy. These types of IR interactions with biological materials are collectively called 'targeted effects', since they refer only to the irradiated cells. Earlier and sometimes 'anecdotal' findings were pointing to the possibility of IR actions unrelated to the irradiated cells or area, i.e., a type of systemic response with unknown mechanistic basis. Over the last years, significant experimental evidence has accumulated, showing a variety of radiation effects for 'out-of-field' areas (non-targeted effects-NTE). The NTE involve the release of chemical and biological mediators from the 'in-field' area and thus the communication of the radiation insult via the so called 'danger' signals. The NTE can be separated in two major groups: bystander and distant (systemic). In this review, we have collected a detailed list of proteins implicated in either bystander or systemic effects, including the clinically relevant abscopal phenomenon, using improved text-mining and bioinformatics tools from the literature. We have identified which of these genes belong to the DNA damage response and repair pathway (DDR/R) and made protein-protein interaction (PPi) networks. Our analysis supports that the apoptosis, TLR-like and NOD-like receptor signaling pathways are the main pathways participating in NTE. Based on this analysis, we formulate a biophysical hypothesis for the regulation of NTE, based on DNA damage and apoptosis gradients between the irradiation point and various distances corresponding to bystander (5mm) or distant effects (5cm). Last but not least, in order to provide a more realistic support for our model, we calculate the expected DSB and non-DSB clusters along the central axis of a representative 200.6MeV pencil beam calculated using Monte Carlo DNA damage simulation software (MCDS) based on the actual beam energy-to-depth curves used in therapy.