Mechanisms and biological importance of photon-induced bystander responses: do they have an impact on low-dose radiation responses

Factors Influencing Effects of Low-dose Radiation Exposure

ABSTRACT

It is now well accepted that the mechanisms induced by low-dose exposures to ionizing radiation (LDR) are different from those occurring after high-dose exposures. However, the downstream effects of these mechanisms are unclear as are the quantitative relationships between exposure, effect, harm, and risk. In this paper, we will discuss the mechanisms known to be important with an overall emphasis on how so-called "non-targeted effects" (NTE) communicate and coordinate responses to LDR. Targeted deposition of ionizing radiation energy in cells causing DNA damage is still regarded as the dominant trigger leading to all downstream events whether targeted or non-targeted. We regard this as an over-simplification dating back to formal target theory. It ignores that last 100 y of biological research into stress responses and signaling mechanisms in organisms exposed to toxic substances, including ionizing radiation. We will provide evidence for situations where energy deposition in cellular targets alone cannot be plausible as a mechanism for LDR effects. An example is where the energy deposition takes place in an organism not receiving the radiation dose. We will also discuss how effects after LDR depend more on dose rate and radiation quality rather than actual dose, which appears rather irrelevant. Finally, we will use recent evidence from studies of cataract and melanoma induction to suggest that after LDR, post-translational effects, such as protein misfolding or defects in energy metabolism or mitochondrial function, may dominate the etiology and progression of the disease. A focus on such novel pathways may open the way to successful prophylaxis and development of new biomarkers for better risk assessment after low dose exposures.

Low-dose radiation from CT examination induces DNA double-strand breaks and detectable changes of DNA methylation in peripheral blood cells

BACKGROUND

Radiation burden from CT examinations increases rapidly with the increased clinical use frequency. Previous studies have disclosed the association between radiation exposure and increased double-strand breaks (DSBs) and changes in DNA methylation. However, whether the induced DSBs by CT examination recover within 24h and whether a CT examination induces detectable gene-specific methylation changes are still unclear. The aim of the present study was to analyze γ-H2AX in the peripheral blood lymphocyte (PBL) of healthy adults before and after CT examination and to discover the differentially methylated positions (DMPs) along with an analysis of DNA methylation changes caused by CT examination.

MATERIALS AND METHODS

Peripheral blood samples of 4 ml were drawn from 20 healthy volunteers at three time points: before CT examination, after CT examination 1h, and after CT examination 24h. γ-H2AX immunofluorescence and Illumina Infinium Human Methylation EPIC BeadChip (850k BeadChip) were used respectively for the test of DSBs and the epigenome-wide DNA methylation analysis. Linear mixed-effect (LME) models were used to evaluate the impacts of doses represented by different parameters and foci on genome-wide DNA methylation.

RESULTS

The number of γ-H2AX foci per cell at 1h showed linear dose-responses for the radiation doses represented by CT index volume (CTDIvol), dose length product (DLP), and blood absorbed dose, respectively. Residual γ-H2AX foci was observed after CT examination at 24h (p < .001). DMPs and γ-H2AX foci changes could be found within 1h. One CpG site related to PAX5 was significantly changed by using most of the parameters in LME models and did not recover till 24h.

CONCLUSIONS

Residual γ-H2AX foci exist after CT examination at 24h. The DNA methylation changes induced by CT examination may not recover within 24h. The DNA methylation had been changed as early as at 1h. The PAX5-related CpG site may be a potential biomarker of low-dose radiation.

CLINICAL RELEVANCE

The biological effects and the cancer risks of CT examination are still unclear. The present study is an effort to document the CT scan-induced events in 24h in vivo. The CT scanning area should be strictly limited, and non-essential repeated operations shouldn't be performed within 24h.

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.

Pathogenesis and therapy of radiation enteritis with gut microbiota

Radiotherapy is widely used in clinic due to its good effect for cancer treatment. But radiotherapy of malignant tumors in the abdomen and pelvis is easy to cause radiation enteritis complications. Gastrointestinal tract contains numerous microbes, most of which are mutualistic relationship with the host. Abdominal radiation results in gut microbiota dysbiosis. Microbial therapy can directly target gut microbiota to reverse microbiota dysbiosis, hence relieving intestinal inflammation. In this review, we mainly summarized pathogenesis and novel therapy of the radiation-induced intestinal injury with gut microbiota dysbiosis and envision the opportunities and challenges of radiation enteritis therapy.

Preclinical and Clinical Status of PSMA-Targeted Alpha Therapy for Metastatic Castration-Resistant Prostate Cancer

Bone, lymph node, and visceral metastases are frequent in castrate-resistant prostate cancer patients. Since such patients have only a few months' survival benefit from standard therapies, there is an urgent need for new personalized therapies. The prostate-specific membrane antigen (PSMA) is overexpressed in prostate cancer and is a molecular target for imaging diagnostics and targeted radionuclide therapy (theragnostics). PSMA-targeted α therapies (PSMA-TAT) may deliver potent and local radiation more selectively to cancer cells than PSMA-targeted β- therapies. In this review, we summarize both the recent preclinical and clinical advances made in the development of PSMA-TAT, as well as the availability of therapeutic α-emitting radionuclides, the development of small molecules and antibodies targeting PSMA. Lastly, we discuss the potentials, limitations, and future perspectives of PSMA-TAT.

Radiation-induced bystander effects may contribute to radiation-induced cognitive impairment

PURPOSE

Despite being a major treatment modality for brain cancer due to its efficiency in achieving cancer control, radiotherapy has long been known to cause long-term side effects, including radiation-induced cognitive impairment (RICI). Neurogenesis inhibition due to radiation-induced damage in neural stem cells (NSCs) has been demonstrated to be an important mechanism underlying RICI. Radiation-induced bystander effects (RIBEs) denote the biological responses in non-targeted cells after their neighboring cells are irradiated. We have previously demonstrated that RIBEs could play an important role in the skin wound healing process. Therefore, we aimed to investigate whether RIBEs contribute to RICI in this study.

MATERIALS AND METHODS

The transwell co-culture method was used to investigate bystander effects in mouse NSCs induced by irradiated GL261 mouse glioma cells in vitro. The proliferation, neurosphere-forming capacity and differentiation potential of NSCs were determined as the bystander endpoints. The exosomes were extracted from the media used to culture GL261 cells and were injected into the hippocampus of C57BL/6 mice. Two months later, the neurogenesis of mice was assessed using BrdU incorporation and immunofluorescence microscopy, and cognitive function was evaluated by the Morris Water Maze.

RESULTS

After co-culture with GL261 glioma cells, mouse NSCs displayed inhibited proliferation and reduced neurosphere-forming capacity and differentiation potential. The irradiated GL261 cells caused greater inhibition and reduction in NSCs than unirradiated GL261 cells. Moreover, adding the exosomes secreted by GL261 cells into the culture of NSCs inhibited NSC proliferation, suggesting that the cancer cell-derived exosomes may be critical intercellular signals. Furthermore, injection of the exosomes from GL261 cells into the hippocampus of mice caused significant neurogenesis inhibition and cognitive impairment two month later, and the exosomes from irradiated GL261 cells induced greater inhibitory effects.

CONCLUSION

RIBEs mediated by the exosomes from irradiated cancer cells could contribute to RICI and, therefore, could be a novel mechanism underlying RICI.

Imaging prior to radiotherapy impacts in-vitro survival

Background and purpose

Cone Beam Computed Tomography (CBCT) is routinely used in radiotherapy to identify the position of the target volume. The aim of this study was to determine whether the CBCT dose, when followed by the treatment, influences the therapeutic outcomes as determined by in-vitro clonogenic cell survival in a radiobiological experiment.

Materials and methods

Human cell lines, four cancer and one normal, were exposed to a 6 MV photon beam, produced by a linear accelerator. For half of each sample, a prior imaging dose was delivered using the on-board CBCT. A sample size of n = 103 was used to achieve statistical power.

Results

The experimental group of cell lines exposed to CBCT imaging prior to treatment exhibited a reduction in mean cancer cell survival of ~17 times (p = 0.02) greater than predicted from the average dose response and equivalent to more than 5% of the therapeutic dose, compared to 11 times greater than predicted for normal cells (n.s.).

Conclusion

The greater than predicted reduction in survival resulting from the additional CBCT dose is consistent with radiation-induced bystander effects.

Enhanced Cell Inactivation and Double-Strand Break Induction in V79 Chinese Hamster Cells by Monochromatic X-Rays at Phosphorus K-Shell Absorption Peak

The cell inactivation and DNA double-strand break (DSB) induction by K-shell ionization of phosphorus atoms and Auger electrons were investigated. Monochromatic X-rays of on and below the phosphorus K-shell absorption peak, 2.153 keV and 2.147 keV were exposed to Chinese hamster lung fibroblast V79 cells. Survival fractions were plotted against exposure, Ψ [nC/kg] and the linear-quadratic model was adapted to estimate the parameters, α and β, of the survival curves. DSB induction rate [DSB/cell/Ψ] was estimated from the measured fractions of induced DNA fragments below 4.6 Mbp (Find(k < 4.6)), which were determined using pulse field gel electrophoresis. As results, cell inactivation and DSB induction rate of on the peak were significantly higher compared to that of the below. However, when converting Ψ to absorbed dose (Gy) of cell nucleus, the enhanced effect was only observed for parameter α, and not for a survival dose (Gy) of 37%, 10%, and 1% nor for a DSB induction rate. Our findings indicate that enhancement of cell inactivation and DSB induction were due to the additional dose delivered to the DNA and more complex DSB lesions were induced due to the release of phosphorus K-shell photoelectrons and Auger electrons.

Streptococcus mutans bystander-induced bioeffects following sonodynamic antimicrobial chemotherapy through sonocatalytic performance of Curcumin-Poly (Lactic-co-Glycolic Acid) on off-target cells

To assessed the Streptococcus mutans bystander-induced bioeffects following sonodynamic antimicrobial chemotherapy (SACT) by Curcumin-Poly (Lactic-co-Glycolic Acid) nanoparticles (Cur-PLGA-NPs). Cur-PLGA-NPs were synthesized and characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and Attenuated Total Reflection Fourier Transform IR (ATR-FTIR) spectroscopy, as well as, determination of in vitro drug release. Following the successful synthesis and characterization of Cur-PLGA-NPs, the cell survival, intracellular ROS production, apoptotic effects, DNA fragmentation, and gene expression levels of pro-inflammatory cytokines were investigated on human gingival fibroblast (HGF) cells as off-target cells through S. mutans bystander-induced bioeffects following SACT (BCS^S). No significant cytotoxic and damage caused by the release of ROS from BCS^S were observed in HGF cells (P > 0.05). There was no DNA fragmentation and anti-proliferation effects on HGF cells. The expression levels of bFGF, TNF-α, and IL-8 genes were increased after exposure to BCS^S to 15.4-, 13.5-, and 8.7-fold, respectively (P < 0.05), while TGF-ß and IL-10 were downregulated to -4.1- and -6.8-fold, respectively (P < 0.05). It could be concluded that there were no bystander bioeffects of targeted sonocatalytic stress on off-target cells.

RADIATION-INDUCED BYSTANDER EFFECT - MODELING, MANIFESTATION, MECHANISMS, PERSISTENCE, CANCER RISKS (literature review)

The review summarizes and analyzes the data of world scientific literature and the results of the own research con- cerning one of the main non-targeted effects of ionizing radiation - the radiation induced bystander effect (RIBE) - the ability of irradiated target cells to induce secondary biological changes in non-irradiated receptor cells. The his- tory of studies of this phenomenon is presented - it described under various names since 1905, began to study from the end of the twentieth century when named as RIBE and caused particular interest in the scientific community during recent decades. It is shown that the development of biological science and the improvement of research methods allowed to get new in-depth data on the development of RIBE not only at the level of the whole organism, but even at the genome level. The review highlights the key points of numerous RIBE investigations including mod- eling; methodological approaches to studying; classification; features of interaction between irradiated and intact cells; the role of the immune system, oxidative stress, cytogenetic disorders, changes in gene expression in the mechanism of development of RIBE; rescue effect, abscopal effect, persistence, modification, medical effects. It is emphasized that despite the considerable amount of research concerning the bystander response as the universal phenomenon and RIBE as one of its manifestations, there are still enough «white spots» in determining the mech- anisms of the RIBE formation and assessing the possible consequences of its development for human health.

Environmental and industrial developments in radiation cataractogenesis

Purpose: This review discusses recent developments in our understanding of biological and physiological mechanisms underlying radiation cataractogenesis. The areas discussed include effects of low-dose exposures to the lens including potential relevance of non-targeted effects, the development of new personal-protective equipment (PPE) and standards in clinical and nuclear settings motivated by the updated ICRP recommendations to mitigate exposures to the lens of the eye. The review also looks at evidence from the field linking cataracts in birds and mammals to low dose exposures.Conclusions: The review suggests that there is evidence that cataractogenesis is not a tissue reaction (deterministic effect) but rather is a low dose effect which shows a saturable dose response relationship similar to that seen for non-targeted effects in general. The review concludes that new research is needed to determine the dose response relationship in environmental studies where field data are contradictory and lab studies confined to rodent models for human exposure studies.

Targeted and Off-Target (Bystander and Abscopal) Effects of Radiation Therapy: Redox Mechanisms and Risk/Benefit Analysis

SIGNIFICANCE

Radiation therapy (from external beams to unsealed and sealed radionuclide sources) takes advantage of the detrimental effects of the clustered production of radicals and reactive oxygen species (ROS). Research has mainly focused on the interaction of radiation with water, which is the major constituent of living beings, and with nuclear DNA, which contains the genetic information. This led to the so-called target theory according to which cells have to be hit by ionizing particles to elicit an important biological response, including cell death. In cancer therapy, the Poisson law and linear quadratic mathematical models have been used to describe the probability of hits per cell as a function of the radiation dose. Recent Advances: However, in the last 20 years, many studies have shown that radiation generates "danger" signals that propagate from irradiated to nonirradiated cells, leading to bystander and other off-target effects.

CRITICAL ISSUES

Like for targeted effects, redox mechanisms play a key role also in off-target effects through transmission of ROS and reactive nitrogen species (RNS), and also of cytokines, ATP, and extracellular DNA. Particularly, nuclear factor kappa B is essential for triggering self-sustained production of ROS and RNS, thus making the bystander response similar to inflammation. In some therapeutic cases, this phenomenon is associated with recruitment of immune cells that are involved in distant irradiation effects (called "away-from-target" i.e., abscopal effects).

FUTURE DIRECTIONS

Determining the contribution of targeted and off-target effects in the clinic is still challenging. This has important consequences not only in radiotherapy but also possibly in diagnostic procedures and in radiation protection.

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.

Old Data-New Concepts: Integrating "Indirect Effects" Into Radiation Protection

PURPOSE

To address the following key question, what are the consequences of nontargeted and delayed effects for linear nonthreshold models of radiation risk? This paper considers low-dose "indirect" or nontargeted effects and how they might impact radiation protection, particularly at the level of the environment. Nontargeted effects refer to effects in cells, tissues, or organisms that were not targeted by irradiation and that did not receive direct energy deposition. They include genomic instability and lethal mutations in progeny of irradiated cells and bystander effects in neighboring cells, tissues, or organisms. Low-dose hypersensitivity and adaptive responses are sometimes included under the nontargeted effects umbrella, but these are not considered in this paper. Some concepts emerging in the nontargeted effects field that could be important include historic dose. This suggests that the initial exposure to radiation initiates the instability phenotype which is passed to progeny leading to a transgenerational radiation-response phenotype, which suggests that the system response rather than the individual response is critical in determining outcome.

CONCLUSION

Nontargeted effects need to be considered, and modeling, experimental, and epidemiological approaches could all be used to determine the impact of nontargeted effects on the currently used linear nonthreshold model in radiation protection.

Molecular mechanism of bystander effects and related abscopal/cohort effects in cancer therapy

Cancer cells subjected to ionizing radiation may release signals which can influence nearby non-irradiated cells, termed bystander effects. The transmission of bystander effects among cancer cells involves the activation of inflammatory cytokines, death ligands, and reactive oxygen/nitrogen species. In addition to bystander effects, two other forms of non-target effects (NTEs) have been identified in radiotherapy, as one is called cohort effects and the other is called abscopal effects. Cohort effects represent the phenomenon where irradiated cells can produce signals that reduce the survival of neighboring cells within an irradiated volume. The effects suggest the importance of cellular communication under irradiation with non-uniform dose distribution. In contrast, abscopal effects describe the NTEs that typically occur in non-irradiated cells distant from an irradiated target. These effects can be mediated primarily by immune cells such as T cells. Clinical trials have shown that application of radiation along with immunotherapy may enhance abscopal effects and improve therapeutic efficacy on non-target lesions outside an irradiated field. According to NTEs, cell viability is reduced not only by direct irradiation effects, but also due to signals emitted from nearby irradiated cells. A clinical consideration of NTEs could have a revolutionary impact on current radiotherapy via the establishment of more efficient and less toxic radiobiological models for treatment planning compared to conventional models. Thus, we will review the most updated findings about these effects and outline their mechanisms and potential applications in cancer treatment with a special focus on the brain, lung, and breast cancers.

Effects of Nrf2 knockdown on the properties of irradiated cell conditioned medium from A549 human lung cancer cells

The nuclear factor erythroid 2-related factor 2 (Nrf2) plays an important role in cellular defense against oxidative stress. Recent studies have demonstrated that Nrf2 is a useful target for cancer treatment, including radiation therapy. Ionizing radiation affects, not only the irradiated cells, but also the non-irradiated neighboring cells, and this effect is known as radiation-induced bystander effect. Upon exposure to radiation, the irradiated cells transmit signals to the non-irradiated cells via gap junctions or soluble factors. These signals in turn cause biological effects, such as a decrease in the clonogenic potential and cell death, in the non-irradiated neighboring cells. Nrf2 inhibition enhances cellular radiosensitivity. However, whether this modification of radiosensitivity by Nrf2 inhibition affects the radiation-induced bystander effects is unknown. In this study, we prepared an Nrf2 knockdown human lung cancer cell A549 and investigated whether the effects of irradiated cell conditioned medium (ICCM) on cell growth and cell death induction of non-irradiated cells vary depending on the Nrf2 knockdown. We found that Nrf2 knockdown resulted in a decrease in the cell growth and an increase in the radiosensitivity of A549 cells. When non-irradiated A549 cells were transfected with control siRNA and treated with ICCM, no significant difference was observed in the cell growth and proportion of Annexin V⁺ dead cells between ICCM from non-irradiated cells and that from 2 or 8 Gy-irradiated cells. Similarly, no significant difference was observed in the cell growth and cell death induction upon treatment with ICCM in the Nrf2 knockdown A549 cells. Taken together, these results suggest that Nrf2 knockdown decreases cell growth and enhances the radiosensitivity of A549 cells; however, it does not alter the effect of ICCM on cell growth.

Second Primary Neoplasms in Patients With Uveal Melanoma: A SEER Database Analysis

PURPOSE

To determine the risk of second primary neoplasms (SPNs) in subjects previously diagnosed with uveal melanoma (UM), including an analysis on whether radiotherapy is a risk factor to develop these SPNs.

DESIGN

Retrospective cohort study.

METHODS

Using the Surveillance, Epidemiology, and End Results (SEER) 9 database, we identified patients diagnosed with UM as their first malignancy between 1973 and 2011 (n = 3976). We obtained standardized incidence ratios (SIR) and excess absolute risks of SPNs on patients with UM compared to a reference population. Multivariate Cox regression models were used to evaluate the effect of radiotherapy in SPN risk.

RESULTS

Sixteen percent (n = 641) of the patients developed SPNs during a median follow-up of 83 months (range, 1-463 months). This represented an 11% excess risk compared to the reference population, mainly owing to a significantly increased risk of skin melanomas (SIR = 2.93, 95% CI: 2.23-3.78) and kidney tumors (SIR = 1.91, 95% CI: 1.27-2.76), primarily in those diagnosed between 30 and 59 years of age. The occurrence of second UM was also increased (SIR = 16.90, 95% CI: 9.00-28.90), which likely includes recurrences misclassified as a second cancer. Radiotherapy was performed in 39% (n = 1538) of the patients. Multivariate analysis revealed that this treatment was not an independent risk factor for SPNs (hazard ratio = 1.06, 95% CI: 0.88-1.26, P = .54).

CONCLUSIONS

Patients with UM presented an 11% higher risk of SPNs compared to the reference population. Radiotherapy does not seem to be a risk factor. SPNs should be considered in the surveillance of UM.

Delayed Numerical Chromosome Aberrations in Human Fibroblasts by Low Dose of Radiation

Radiation-induced genomic instability refers to a type of damage transmitted over many generations following irradiation. This delayed impact of radiation exposure may pose a high risk to human health and increases concern over the dose limit of radiation exposure for both the public and radiation workers. Therefore, the development of additional biomarkers is still needed for the detection of delayed responses following low doses of radiation exposure. In this study, we examined the effect of X-irradiation on delayed induction of numerical chromosomal aberrations in normal human fibroblasts irradiated with 20, 50 and 100 cGy of X-rays using the micronucleus-centromere assay. Frequencies of centromere negative- and positive-micronuclei, and aneuploidy of chromosome 1 and 4 were analyzed in the surviving cells at 28, 88 and 240 h after X-irradiation. X-irradiation increased the frequency of micronuclei (MN) in a dose-dependent manner in the cells at all measured time-points, but no significant differences in MN frequency among cell passages were observed. Aneuploid frequency of chromosomes 1 and 4 increased with radiation doses, and a significantly higher frequency of aneuploidy was observed in the surviving cells analyzed at 240 h compared to 28 h. These results indicate that low-dose of X-irradiation can induce delayed aneuploidy of chromosomes 1 and 4 in normal fibroblasts.

Possible scenarios of the influence of low-dose ionizing radiation on neural functioning

Possible scenarios of the influence of ionizing radiation on neural functioning and the CNS are suggested. We argue that the radiation-induced bystander mechanisms associated with Ca(2+) flows, reactive nitrogen and oxygen species, and cytokines might lead to modulation of certain neuronal signaling pathways. The considered scenarios of conjugation of the bystander signaling and the neuronal signaling might result in modulation of certain synaptic receptors, neurogenesis, neurotransmission, channel conductance, synaptic signaling, different forms of neural plasticity, memory formation and storage, and learning. On this basis, corresponding new possible strategies for treating neurodegenerative deceases and mental disorders are proposed. The mechanisms considered might also be associated with neuronal survival and relevant to the treatment for brain injuries. At the same time, these mechanisms might be associated with detrimental effects and might facilitate the development of some neurological and psychiatric disorders.

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

In general, a radiation-induced bystander response is known to be a cellular response induced in non-irradiated cells after receiving bystander signaling factors released from directly irradiated cells within a cell population. Bystander responses induced by high-linear energy transfer (LET) heavy ions at low fluence are an important health problem for astronauts in space. Bystander responses are mediated via physical cell-cell contact, such as gap-junction intercellular communication (GJIC) and/or diffusive factors released into the medium in cell culture conditions. Nitric oxide (NO) is a well-known major initiator/mediator of intercellular signaling within culture medium during bystander responses. In this study, we investigated the NO-mediated bystander signal transduction induced by high-LET argon (Ar)-ion microbeam irradiation of normal human fibroblasts. Foci formation by DNA double-strand break repair proteins was induced in non-irradiated cells, which were co-cultured with those irradiated by high-LET Ar-ion microbeams in the same culture plate. Foci formation was suppressed significantly by pretreatment with an NO scavenger. Furthermore, NO-mediated reproductive cell death was also induced in bystander cells. Phosphorylation of NF-κB and Akt were induced during NO-mediated bystander signaling in the irradiated and bystander cells. However, the activation of these proteins depended on the incubation time after irradiation. The accumulation of cyclooxygenase-2 (COX-2), a downstream target of NO and NF-κB, was observed in the bystander cells 6 h after irradiation but not in the directly irradiated cells. Our findings suggest that Akt- and NF-κB-dependent signaling pathways involving COX-2 play important roles in NO-mediated high-LET heavy-ion-induced bystander responses. In addition, COX-2 may be used as a molecular marker of high-LET heavy-ion-induced bystander cells to distinguish them from directly irradiated cells, although this may depend on the time after irradiation.

Radiation quality-dependence of bystander effect in unirradiated fibroblasts is associated with TGF-β1-Smad2 pathway and miR-21 in irradiated keratinocytes

Traditional radiation biology states that radiation causes damage only in cells traversed by ionizing radiation. But radiation-induced bystander effect (RIBE), which refers to the biological responses in unirradiated cells when the neighboring cells are exposed to radiation, challenged this old dogma and has become a new paradigm of this field. By nature, RIBEs are the consequences of intercellular communication between irradiated and unirradiated cells. However, there are still some important questions remain unanswered such as whether RIBE is dependent on radiation quality, what are the determining factors if so, etc. Using a transwell co-culture system, we found that HaCaT keratinocytes irradiated with α-particles but not X-rays could induce bystander micronucleus formation in unirradiated WS1 fibroblasts after co-culture. More importantly, the activation of TGF-β1-Smad2 pathway and the consistent decrease of miR-21 level in α-irradiated HaCaT cells were essential to the micronucleus induction in bystander WS1 cells. On the other hand, X-irradiation did not induce bystander effect in unirradiated WS1 cells, accompanied by lack of Smad2 activation and consistent decrease of miR-21 in X-irradiated HaCaT cells. Taken together, these results suggest that the radiation quality-dependence of bystander effect may be associated with the TGF-β1-Smad2 pathway and miR-21 in irradiated cells.

The different radiation response and radiation-induced bystander effects in colorectal carcinoma cells differing in p53 status

Radiation-induced bystander effect, appearing as different biological changes in cells that are not directly exposed to ionizing radiation but are under the influence of molecular signals secreted by irradiated neighbors, have recently attracted considerable interest due to their possible implication for radiotherapy. However, various cells present diverse radiosensitivity and bystander responses that depend, inter alia, on genetic status including TP53, the gene controlling the cell cycle, DNA repair and apoptosis. Here we compared the ionizing radiation and bystander responses of human colorectal carcinoma HCT116 cells with wild type or knockout TP53 using a transwell co-culture system. The viability of exposed to X-rays (0-8 Gy) and bystander cells of both lines showed a roughly comparable decline with increasing dose. The frequency of micronuclei was also comparable at lower doses but at higher increased considerably, especially in bystander TP53-/- cells. Moreover, the TP53-/- cells showed a significantly elevated frequency of apoptosis, while TP53+/+ counterparts expressed high level of senescence. The cross-matched experiments where irradiated cells of one line were co-cultured with non-irradiated cells of opposite line show that both cell lines were also able to induce bystander effects in their counterparts, however different endpoints revealed with different strength. Potential mediators of bystander effects, IL-6 and IL-8, were also generated differently in both lines. The knockout cells secreted IL-6 at lower doses whereas wild type cells only at higher doses. Secretion of IL-8 by TP53-/- control cells was many times lower than that by TP53+/+ but increased significantly after irradiation. Transcription of the NFκBIA was induced in irradiated TP53+/+ mainly, but in bystanders a higher level was observed in TP53-/- cells, suggesting that TP53 is required for induction of NFκB pathway after irradiation but another mechanism of activation must operate in bystander cells.