Intercellular and intracellular signaling pathways mediating ionizing radiation-induced bystander effects

Radiation-induced bystander effects in cultured human stem cells

Background

The radiation-induced “bystander effect” (RIBE) was shown to occur in a number of experimental systems both in vitro and in vivo as a result of exposure to ionizing radiation (IR). RIBE manifests itself by intercellular communication from irradiated cells to non-irradiated cells which may cause DNA damage and eventual death in these bystander cells. It is known that human stem cells (hSC) are ultimately involved in numerous crucial biological processes such as embryologic development; maintenance of normal homeostasis; aging; and aging-related pathologies such as cancerogenesis and other diseases. However, very little is known about radiation-induced bystander effect in hSC. To mechanistically interrogate RIBE responses and to gain novel insights into RIBE specifically in hSC compartment, both medium transfer and cell co-culture bystander protocols were employed.

Methodology/Principal Findings

Human bone-marrow mesenchymal stem cells (hMSC) and embryonic stem cells (hESC) were irradiated with doses 0.2 Gy, 2 Gy and 10 Gy of X-rays, allowed to recover either for 1 hr or 24 hr. Then conditioned medium was collected and transferred to non-irradiated hSC for time course studies. In addition, irradiated hMSC were labeled with a vital CMRA dye and co-cultured with non-irradiated bystander hMSC. The medium transfer data showed no evidence for RIBE either in hMSC and hESC by the criteria of induction of DNA damage and for apoptotic cell death compared to non-irradiated cells (p>0.05). A lack of robust RIBE was also demonstrated in hMSC co-cultured with irradiated cells (p>0.05).

Conclusions/Significance

These data indicate that hSC might not be susceptible to damaging effects of RIBE signaling compared to differentiated adult human somatic cells as shown previously. This finding could have profound implications in a field of radiation biology/oncology, in evaluating radiation risk of IR exposures, and for the safety and efficacy of hSC regenerative-based therapies.

Emerging role of radiation induced bystander effects: Cell communications and carcinogenesis

Ionizing radiation is an invaluable diagnostic and treatment tool used in various clinical applications. On the other hand, radiation is a known cytotoxic with a potential DNA damaging and carcinogenic effects. However, the biological effects of low and high linear energy transfer (LET) radiations are considerably more complex than previously thought. In the past decade, evidence has mounted for a novel biological phenomenon termed as "bystander effect" (BE), wherein directly irradiated cells transmit damaging signals to non-irradiated cells thereby inducing a response similar to that of irradiated cells. BE can also be induced in various cells irrespective of the type of radiation, and the BE may be more damaging in the longer term than direct radiation exposure. BE is mediated either through gap-junctions or via soluble factors released by irradiated cells. DNA damage response mechanisms represent a vital line of defense against exogenous and endogenous damage caused by radiation and promote two distinct outcomes: survival and the maintenance of genomic stability. The latter is critical for cancer avoidance. Therefore, efforts to understand and modulate the bystander responses will provide new approaches to cancer therapy and prevention. This review overviews the emerging role of BE of low and high LET radiations on the genomic instability of bystander cells and its possible implications for carcinogenesis.

Enhanced micronucleus formation in the descendants of gamma-ray-irradiated tobacco cells: evidence for radiation-induced genomic instability in plant cells

Ionizing radiation-induced genomic instability has been documented in various end points such as chromosomal aberrations and mutations, which arises in the descendants of irradiated mammalian or yeast cells many generations after the initial insult. This study aimed at addressing radiation-induced genomic instability in higher plant tobacco cells. We thus investigated micronucleus (MN) formation and cell proliferation in tobacco cells irradiated with gamma-rays and their descendants. In gamma-irradiated cells, cell cycle was arrested at G2/M phase at around 24 h post-irradiation but released afterward. In contrast, MN frequency peaked at 48 h post-irradiation. Almost half of 40 Gy-irradiated cells had MN at 48 h post-irradiation, but proliferated as actively as sham-irradiated cells up to 120 h post-irradiation. Moreover, the descendants that have undergone at least 22 generations after irradiation still showed a two-fold MN frequency compared to sham-irradiated cells. This is the direct evidence for radiation-induced genomic instability in tobacco cells.

Heavy ion irradiation induces autophagy in irradiated C2C12 myoblasts and their bystander cells

Autophagy is one of the major processes involved in the degradation of intracellular materials. Here, we examined the potential impact of heavy ion irradiation on the induction of autophagy in irradiated C2C12 mouse myoblasts and their non-targeted bystander cells. In irradiated cells, ultrastructural analysis revealed the accumulation of autophagic structures at various stages of autophagy (i.e. phagophores, autophagosomes and autolysosomes) within 20 min after irradiation. Multivesicular bodies (MVBs) and autolysosomes containing MVBs (amphisomes) were also observed. Heavy ion irradiation increased the staining of microtubule-associated protein 1 light chain 3 and LysoTracker Red (LTR). Such enhanced staining was suppressed by an autophagy inhibitor 3-methyladenine. In addition to irradiated cells, bystander cells were also positive with LTR staining. Altogether, these results suggest that heavy ion irradiation induces autophagy not only in irradiated myoblasts but also in their bystander cells.

Non-targeted effects as a paradigm breaking evidence

The finding that mammalian cells and tissues and whole organisms react differently at high than at low doses of ionizing radiation questions the scientific validity of the linear no-threshold concept for low-dose exposures. Indeed, the classical paradigm of radiobiology was based on the concept that all radiation effects on living matter are due to the direct action of radiation. Meanwhile, the discovery of non-targeted and delayed radiation effects has challenged this concept, and one might ask whether a new paradigm has to be developed to provide more realistic protection against low radiation doses. The present overview summarizes recent findings on the low-dose radiation-induced bystander effect, genomic instability, radiation hypersensitivity, hormesis, radioadaptive and transgenerational responses. For these, some common features can be recognized. Most of these phenomena include (1) intra- and intercellular signaling, involving reactive oxygen species (ROS). This signaling may be transient or persistent, and may involve the release of cytokines (bystander effect, genomic instability) or epigenetic changes (translesional responses), (2) a large variability of responses depending on the type of radiation, genotype (DNA repair capacity) and physiological state of the cells and tissues. Many more parameters are involved in responses at low doses than at high doses, and different pathways are activated. At low doses, non-linear responses are obtained that are not compatible with the LNT concept. At present, more work is needed to identify the essential parameters involved and to provide a basis for proper modelling of low-dose radiation health effects for radiation protection purposes.

Involvement of purinergic signaling in cellular response to gamma radiation

Recent studies have suggested a bystander effect in nonirradiated cells adjacent to irradiated cells; however, the mechanism is poorly understood. In this study, we investigated the involvement of both extracellular nucleotides and activation of P2 receptors in cellular responses to gamma radiation using human HaCaT keratinocytes. The concentration of ATP in culture medium was increased after gamma irradiation (0.1-1.0 Gy), suggesting that radiation induces ATP release from cells. Intracellular Ca(2+) concentration was elevated when conditioned medium from irradiated cells was transferred to nonirradiated cells, and this elevation was suppressed by apyrase (ecto-nucleotidase), indicating the involvement of extracellular nucleotides in this event. Further, we examined the activation of ERK1/2 by gamma radiation and nucleotides (ATP and UTP). Both gamma radiation and nucleotides induced activation of ERK1/2. Next, the effect of inhibitors of P2 receptors on radiation-induced activation of ERK1/2 was examined. The activation of ERK1/2 was blocked by suramin (P2Y inhibitor), MRS2578 (P2Y(6) antagonist) and apyrase. These results suggest that both released nucleotides and activation of P2Y receptors are involved in gamma-radiation-induced activation of ERK1/2. We conclude that ionizing radiation induces release of nucleotides from cells, leading to activation of P2Y receptors, which in turn would result in a variety of biological effects.

Cytogenetic damage in cells exposed to ionizing radiation under conditions of a changing dose rate

The current international paradigm on the biological effects of radiation is based mainly on the effects of dose with some consideration for the dose rate. No allowance has been made for the potential influence of a changing dose rate (second derivative of dose), and the biological effects of exposing cells to changing dose rates have never been analyzed. This paper provides evidence that radiation effects in cells may depend on temporal changes in the dose rate. In these experiments, cells were moved toward or away from an X-ray source. The speed of movement, the time of irradiation, and the temperature during exposure were controlled. Here we report the results of the first experiments with TK6 cells that were exposed at a constant dose rate, at an increasing dose rate, or at a decreasing dose rate. The average dose rate and the total dose were same for all samples. Micronuclei were scored as the end point. The results show that the level of cytogenetic damage was higher in cells exposed to a decreasing dose rate compared to both an increasing and a constant dose rate. This finding may suggest that the second derivative of dose may influence radiation risk estimates, and the results should trigger further studies on this issue.

Bystander cell killing in normal human fibroblasts is induced by synchrotron X-ray microbeams

Abstract The radiation-induced bystander response is defined as a response in cells that have not been directly targeted by radiation but that are in the neighborhood of cells that have been directly exposed. In the work described here, it is shown that bystander cell killing of normal human fibroblast WI-38 cells was induced by synchrotron microbeam X radiation. Cell nuclei in confluent WI-38 cells were irradiated with the microbeam. All of the cells on the dish were harvested and plated 24 h after irradiation. It was found that the bystander cell killing effect showed a parabolic relationship to the radiation dose when five cells were irradiated. At doses above 1.9 Gy, the surviving fraction increased to approximately 1.0. This suggests that induction of bystander cell killing may require some type of activity in the targeted cells, because the dose resulting in 37% cell survival was about 2.0 Gy. Bystander cell killing was suppressed by a pretreatment with aminoguanidine [an inhibitor of inducible nitric oxide (NO) synthase] or carboxy-PTIO (a scavenger of NO). These results suggest that NO is the chief initiator/mediator of bystander cell killing induced by X-ray microbeams.

Year-long upregulation of connexin43 in rabbit hearts by heavy ion irradiation

A previous study from our laboratory has shown that a single targeted heavy ion irradiation (THIR; 15 Gy) to rabbit hearts increases connexin43 (Cx43) expression for 2 wk in association with an improvement of conduction, a decrease of the spatial inhomogeneity of repolarization, and a reduction of vulnerability to ventricular arrhythmias after myocardial infarction. This study investigated the time- and dose-dependent effects of THIR (5–15 Gy) on Cx43 expression in normal rabbit hearts ( n = 45). Five rabbits without THIR were used as controls. A significant upregulation of Cx43 protein and mRNA in the ventricular myocardium was recognized by immunohistochemistry, Western blotting, and real-time PCR from 2 wk up to 1 yr after a single THIR at 15 Gy. THIR ≥10 Gy caused a significant dose-dependent increase of Cx43 protein and mRNA 2 wk after THIR. Anterior, lateral, and posterior free wall of the left ventricle, interventricular septum, and right ventricular free wall were affected similarly by THIR in terms of Cx43 upregulation. The radiation-induced increase of immunolabeled Cx43 was observed not only at the intercalated disk region but also at the lateral surface of ventricular myocytes. The increase of immunoreactive Cx43 protein was predominant in the membrane fraction insoluble in Triton X-100, that is the Cx43 in the sarcolemma. In vivo examinations of the rabbits 1 yr after THIR (15 Gy) revealed no significant changes in ECGs and echocardiograms (left ventricular dimensions, contractility, and diastolic function), indicating no apparent late radiation injury. A single application of THIR causes upregulation and altered cellular distribution of Cx43 in the ventricles lasting for at least 1 yr. This long-lasting remodeling effect on gap junctions may open the pathway to novel therapy against life threatening ventricular arrhythmias in structural heart disease.

Recent advances in the biology of heavy-ion cancer therapy

Superb biological effectiveness and dose conformity represent a rationale for heavy-ion therapy, which has thus far achieved good cancer controllability while sparing critical normal organs. Immediately after irradiation, heavy ions produce dense ionization along their trajectories, cause irreparable clustered DNA damage, and alter cellular ultrastructure. These ions, as a consequence, inactivate cells more effectively with less cell-cycle and oxygen dependence than conventional photons. The modes of heavy ion-induced cell death/inactivation include apoptosis, necrosis, autophagy, premature senescence, accelerated differentiation, delayed reproductive death of progeny cells, and bystander cell death. This paper briefly reviews the current knowledge of the biological aspects of heavy-ion therapy, with emphasis on the authors' recent findings. The topics include (i) repair mechanisms of heavy ion-induced DNA damage, (ii) superior effects of heavy ions on radioresistant tumor cells (intratumor quiescent cell population, TP53-mutated and BCL2-overexpressing tumors), (iii) novel capacity of heavy ions in suppressing cancer metastasis and neoangiogenesis, and (iv) potential of heavy ions to induce secondary (especially breast) cancer.

Injection of resperpine into zebrafish, prevents fish to fish communication of radiation-induced bystander signals: confirmation in vivo of a role for serotonin in the mechanism

Serotonin (5-HT) has been implicated as a potential modulator of the bystander effect in cell cultures. To assess the relevance of serotonin in vivo experiments were done with the zebrafish (Danio rerio). This species, when irradiated, transmits bystander signals to non-irradiated fish. The animals were injected with reserpine, an inhibitor of serotonin at a dose of 80mg/kg of body mass. The results show that reserpine treated fish had only 27% of the serotonin in non-treated fish. Skin tissue samples were collected from the fish and assayed for bystander signal production using a reporter bioassay. Reserpine prevented the production and communication of signals between fish. Intracellular calcium flux, identified as a bystander response in the reporter cells confirmed this. Medium harvested from tissues of X-rayed fish and their bystanders, showed an oscillating pattern of calcium flux. Samples from X-rayed fish pretreated with reserpine produced a chaotic pattern of random fluctuations in the reporter cells, while their bystander fish led to increased calcium, but no oscillations. These results suggest that 5-HT is involved in bystander signalling in zebrafish, and by decreasing the amount of available 5-HT the bystander effect can be blocked.

Oral PEG 15-20 protects the intestine against radiation: role of lipid rafts

Intestinal injury following abdominal radiation therapy or accidental exposure remains a significant clinical problem that can result in varying degrees of mucosal destruction such as ulceration, vascular sclerosis, intestinal wall fibrosis, loss of barrier function, and even lethal gut-derived sepsis. We determined the ability of a high-molecular-weight polyethylene glycol-based copolymer, PEG 15-20, to protect the intestine against the early and late effects of radiation in mice and rats and to determine its mechanism of action by examining cultured rat intestinal epithelia. Rats were exposed to fractionated radiation in an established model of intestinal injury, whereby an intestinal segment is surgically placed into the scrotum and radiated daily. Radiation injury score was decreased in a dose-dependent manner in rats gavaged with 0.5 or 2.0 g/kg per day of PEG 15-20 (n = 9-13/group, P < 0.005). Complementary studies were performed in a novel mouse model of abdominal radiation followed by intestinal inoculation with Pseudomonas aeruginosa (P. aeruginosa), a common pathogen that causes lethal gut-derived sepsis following radiation. Mice mortality was decreased by 40% in mice drinking 1% PEG 15-20 (n = 10/group, P < 0.001). Parallel studies were performed in cultured rat intestinal epithelial cells treated with PEG 15-20 before radiation. Results demonstrated that PEG 15-20 prevented radiation-induced intestinal injury in rats, prevented apoptosis and lethal sepsis attributable to P. aeruginosa in mice, and protected cultured intestinal epithelial cells from apoptosis and microbial adherence and possible invasion. PEG 15-20 appeared to exert its protective effect via its binding to lipid rafts by preventing their coalescence, a hallmark feature in intestinal epithelial cells exposed to radiation.

Protein kinase C epsilon is involved in ionizing radiation induced bystander response in human cells

Our earlier study demonstrated the induction of PKC isoforms (betaII, PKC-alpha/beta, PKC-theta) by ionizing radiation induced bystander response in human cells. In this study, we extended our investigation to yet another important member of PKC family, PKC epsilon (PKCepsilon). PKCepsilon functions both as an anti-apoptotic and pro-apoptotic protein and it is the only PKC isozyme implicated in oncogenesis. Given the importance of PKCepsilon in oncogenesis, we wished to determine whether or not PKCepsilon is involved in bystander response. Gene expression array analysis demonstrated a 2-3-fold increase in PKCepsilon expression in the bystander human primary fibroblast cells that were co-cultured in double-sided Mylar dishes for 3h with human primary fibroblast cells irradiated with 5Gy of alpha-particles. The elevated PKCepsilon expression in bystander cells was verified by quantitative real time PCR. Suppression of PKCepsilon expression by small molecule inhibitor Bisindolylmaleimide IX (Ro 31-8220) considerably reduced the frequency of micronuclei (MN) induced both by 5Gy of gamma-rays (low LET) and alpha-particles (high LET) in bystander cells. Similar cytoprotective effects were observed in bystander cells after siRNA mediated silencing of PKCepsilon suggestive of its critical role in mediating some of the bystander effects (BE). Our novel study suggests the possibility that PKC signaling pathway may be a critical molecular target for suppression of ionizing radiation induced biological effects in bystander cells.

gamma-Irradiation induces P2X(7) receptor-dependent ATP release from B16 melanoma cells

BACKGROUND

Ionizing irradiation causes not only growth arrest and cell death, but also release of growth factors or signal transmitters, which promote cancer malignancy. Extracellular ATP controls cancer growth through activation of purinoceptors. However, there is no report of radiation-induced ATP release from cancer cells. Here, we examined gamma-irradiation-induced ATP release and its mechanism in B16 melanoma.

METHODS

Extracellular ATP was measured by luciferin-luciferase assay. To investigate mechanism of radiation-induced ATP release, we pharmacologically inhibited the ATP release and established stable P2X(7) receptor-knockdown B16 melanoma cells using two short hairpin RNAs targeting P2X(7) receptor.

RESULTS

Cells were exposed to 0.5-8 Gy of gamma-rays. Extracellular ATP was increased, peaking at 5 min after 0.5 Gy irradiation. A selective P2X(7) receptor channel antagonist, but not anion transporter inhibitors, blocked the release of ATP. Further, radiation-induced ATP release was significantly decreased in P2X(7) receptor-knockdown cells. Our results indicate that gamma-irradiation evokes ATP release from melanoma cells, and P2X(7) receptor channel plays a significant role in mediating the ATP release.

GENERAL SIGNIFICANCE

We suggest that extracellular ATP could be a novel intercellular signaling molecule released from cancer cells when cells are exposed to ionizing radiation.

No evidence for DNA and early cytogenetic damage in bystander cells after heavy-ion microirradiation at two facilities

The occurrence of bystander effects has challenged the evaluation of risk for heavy ions, mainly in the context of space exploration and the increasing application of carbon ions in radiotherapy. In the present study, we addressed whether heavy-ion-induced DNA and cytogenetic damage is detectable in bystander cells. The formation of gamma-H2AX foci, sister chromatid exchanges and micronuclei were used as markers of damage to DNA. Normal human fibroblasts were exposed to low fluences of carbon and uranium ions, and alternatively single cells were targeted with heavy ions using the GSI microbeam. We did not observe a significant increase in the bystander formation of gamma-H2AX foci, sister chromatid exchanges or micronuclei. In addition, we performed for the first time parallel experiments at two microbeam facilities (GSI, JAEA) using the same cell line, culture conditions and irradiation protocols. No significant enhancement of the micronucleus frequencies in bystander cells was detected after targeted carbon-ion irradiation, confirming the results. Details regarding the history, culture conditions or support of the cells might be affecting the detection of bystander effects. On the other hand, the potential X-ray- and heavy-ion-induced bystander effects investigated herein clearly do not exceed the experimental error and thus are either lacking or are less pronounced than the effects reported in the literature for similar end points after alpha-particle and X-ray exposure.

Cellular and sub-cellular responses to UVA in relation to carcinogenesis

PURPOSE

UVA radiation (315-400 nm) contributes to skin aging and carcinogenesis. The aim of this review is to consider the mechanisms that underlie UVA-induced cellular damage, how this damage may be prevented or repaired and the signal transduction processes that are elicited in response to it.

RESULTS

Exposure to ultraviolet (UV) light is well-established as the causative factor in skin cancer. Until recently, most work on the mechanisms that underlie skin carcinogenesis focused on shorter wavelength UVB radiation (280-315 nm), however in recent years there has been increased interest in the contribution made by UVA. UVA is able to cause a range of damage to cellular biomolecules including lipid peroxidation, oxidized protein and DNA damage, such as 8-oxoguanine and cyclobutane pyrimidine dimers. Such damage is strongly implicated in both cell death and malignant transformation and cells have a number of mechanisms in place to mitigate the effects of UVA exposure, including antioxidants, DNA repair, and stress signalling pathways.

CONCLUSIONS

The past decade has seen a surge of interest in the biological effects of UVA exposure as its significance to the process of photo-carcinogenesis has become increasingly evident. However, unpicking the unique complexity of the cellular response to UVA, which is only now becoming apparent, will be a major challenge for the field of photobiology in the 21st century.

Twilight effects of low doses of ionizing radiation on cellular systems: a bird's eye view on current concepts and research

The debate about the health risks from low doses of radiation is vigorous and often acrimonious since many years and does not appear to weaken. Being far from completeness, this review presents only a bird's eye view on current concepts and research in the field. It is organized and divided in two parts. The first is dedicated to molecular responses determined by radiation-induced DNA ruptures. It focuses its attention on molecular pathways that are activated by ATM and tries to describe the variegated functions and specific roles of Chk2 and p53 and other proteins in sensing, promoting and executing DNA repair. The second part is more concerned with the risk associated with exposure to low dose radiation and possible effects that the radiation-affected cell may undergo. These effects include induction of apoptosis and mitotic catastrophe, bystander effect and genomic instability, senescence and hormetic response. Current hypotheses and research on these issues are briefly discussed.

Radiation-induced bystander signalling in cancer therapy

Our understanding of how radiation kills normal and tumour cells has been based on an intimate knowledge of the direct induction of DNA damage and its cellular consequences. What has become clear is that, as well as responses to direct DNA damage, cell-cell signalling -- known as the bystander effect -- mediated through gap junctions and inflammatory responses may have an important role in the response of cells and tissues to radiation exposure and also chemotherapy agents. This Review outlines the key aspects of radiation-induced intercellular signalling and assesses its relevance for existing and future radiation-based therapies.

Propagation distance of the alpha-particle-induced bystander effect: the role of nuclear traversal and gap junction communication

When cell populations are exposed to low-dose alpha-particle radiation, a significant fraction of the cells will not be traversed by a radiation track. However, stressful effects occur in both irradiated and bystander cells in the population. Characterizing these effects, and investigating their underlying mechanism(s), is critical to understanding human health risks associated with exposure to alpha particles. To this end, confluent normal human fibroblast cultures were grown on polyethylene terephthalate foil grafted to an ultrathin solid-state nuclear track detector and exposed under non-perturbing conditions to low-fluence alpha particles from a broadbeam irradiator. Irradiated and affected bystander cells were localized with micrometer precision. The stress-responsive protein p21(Waf1) (also known as CDKN1A) was induced in bystander cells within a 100-microm radius from an irradiated cell. The mean propagation distance ranged from 20 to 40 microm around the intranuclear alpha-particle impact point, which corresponds to a set of approximately 30 cells. Nuclear traversal, induced DNA damage, and gap junction communication were critical contributors to propagation of this stressful effect. The strategy described here may be ideal to investigate the size of radiation-affected target and the relative contribution of different cellular organelles to bystander effects induced by energetic particles, which is relevant to radioprotection and cancer radiotherapy.

Communication of radiation-induced signals in vivo between DNA repair deficient and proficient medaka (Oryzias latipes)

Radiation-induced bystander effects are established consequences of exposure to ionizing radiation. The operation of this mechanism has been seen in vitro and also between fish, mammals, and plants in vive where stress signals from treated organisms induce responses in neighbors. In vitro research shows that DNA repair deficient cells produce more toxic bystander responses. To test this in vivo two strains of Japanese medaka were tested. One is a mutant, repair deficient strain (ric2) and the other, the wildtype repair proficient strain (CAB). Irradiated fish swam with unirradiated partners in a strain mix and match protocol. The data suggest that medaka produce signals, when exposed to radiation, that induce unirradiated fish ofthe same strain swimming with them to produce an altered response to that seen in bystanders to sham irradiated fish. More apoptosis was seen in bystanders to repair deficient fish. When the strains are mixed, the bystanders of either strain respond like the donor strain. Measurements of Bcl-2 and cmyc proteins in the explants confirmed these observations. A possible role for p53 was also identified in that the use of reporters with mutant p53 demonstrated that CAB signals killed all the reporter cells by apoptosis. Use of a similar but p53 wildtype cell line had no such effect. The data add to the body of knowledge showing that bystander signals operate at hierarchical levels of organization greater than the individual and may therefore have relevance in radioecology and (eco)systems biology.

The range of the bystander effect signal in three-dimensional tissue and estimation of the range in human lung tissue at low radon levels

It is shown in the measurements of Belyakov et al. (Proc. Natl. Acad. Sci. USA 102, 14203-14208, 2005) that the distribution of microbeam-irradiated cells represents a finite line source distribution for the emission of bystander effect signals. By assuming a plane source configuration for the propagation of the bystander effect deleterious signal, in the modeling analysis of Shuryak et al. (Radiat. Res. 168, 741-749, 2007) there is an overestimate of the range to effectiveness. A correction is provided to this analysis to estimate the bystander signal propagation distance in the case of low domestic radon levels where single isolated cells are "hit" by alpha particles and emit the bystander signals as a "point" source to neighboring bystander effect receptor cells. The result is an estimate, but it shows that with the spatial correction the deleterious propagation range would be of the order of 210 microm, a factor of about 5 less than prior predictions.

Influence of the bystander phenomenon on the chromosome aberration pattern in human lymphocytes induced by in vitro alpha-particle exposure

A recent publication on both chromosome-type and chromatid-type aberrations in lymphocytes of patients during treatment with radium-224 for ankylosing spondilitis has revived the question of whether the chromatid-type aberrations may be the consequence of factors released by irradiated cells. Therefore, the aim of the present study was to investigate the influence of such a bystander phenomenon on the chromosome aberration pattern of lymphocytes. Monolayers of human lymphocytes were irradiated with 1 Gy of alpha-particles from an americium-241 source in the absence or presence of whole blood, autologous plasma or culture medium. In the presence of any liquid covering the monolayer during irradiation, the chromatid-type aberrations were, contrary to expectation, elevated. Whereas the intercellular distribution of dicentrics was significantly overdispersed, the chromatid-type aberrations showed a regular dispersion. It can be concluded that the enhanced frequency of chromatid aberrations is the result of a damage signal or a bystander phenomenon released by irradiated cells.

Heavy-ion-induced bystander killing of human lung cancer cells: role of gap junctional intercellular communication

The aim of the present study was to clarify the mechanisms of cell death induced by heavy-ion irradiation focusing on the bystander effect in human lung cancer A549 cells. In microbeam irradiation, each of 1, 5, and 25 cells under confluent cell conditions was irradiated with 1, 5, or 10 particles of carbon ions (220 MeV), and then the surviving fraction of the population was measured by a clonogenic assay in order to investigate the bystander effect of heavy-ions. In this experiment, the limited number of cells (0.0001-0.002%, 5-25 cells) under confluent cell conditions irradiated with 5 or 10 carbon ions resulted in an exaggerated 8-14% increase in cell death by clonogenic assay. However, these overshooting responses were not observed under exponentially growing cell conditions. Furthermore, these responses were inhibited in cells treated with an inhibitor of gap junctional intercellular communication (GJIC), whereas they were markedly enhanced by the addition of a stimulator of GJIC. The present results suggest that bystander cell killing by heavy-ions was induced mainly by direct cell-to-cell communication, such as GJIC, which might play important roles in bystander responses.

Different involvement of radical species in irradiated and bystander cells

PURPOSE

To examine whether nitric oxide (NO) and other radical species are involved in radiation-induced bystander effects in normal human fibroblasts.

MATERIALS AND METHODS

Bystander effects were modeled by co-culture of non-irradiated cells with X-irradiated cells, and induction levels of micronuclei in co-cultured non-irradiated cells were examined. Three types of radical scavenger, 2-(4-carboxyphenyl)-4,4,5,5- tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), dimethylsulfoxide (DMSO) and ascorbic acid phosphoric ester magnesium salt (APM), were used to discover which types of radicals are involved in bystander responses.

RESULTS

When irradiated cells were treated with c-PTIO, known to be an NO scavenger, the induction of micronuclei in non-irradiated bystander cells was suppressed. On the other hand, bystander effects were most effectively suppressed when non-irradiated bystander cells were treated with ascorbic acid, known to be a scavenger of long lived radicals.

CONCLUSION

These results suggest that NO participates in bystander signal formation in irradiated cells but not in bystander cells that are receiving bystander signals.

Recent insights into the biological action of heavy-ion radiation

Biological effectiveness varies with the linear energy transfer (LET) of ionizing radiation. During cancer therapy or long-term interplanetary manned explorations, humans are exposed to high-LET energetic heavy ions that inactivate cells more effectively than low-LET photons like X-rays and gamma-rays. Recent biological studies have illustrated that heavy ions overcome tumor radioresistance caused by Bcl-2 overexpression, p53 mutations and intratumor hypoxia, and possess antiangiogenic and antimetastatic potential. Compared with heavy ions alone, the combination with chemical agents (a Bcl-2 inhibitor HA14-1, an anticancer drug docetaxel, and a halogenated pyrimidine analogue 5-iodo-2'-deoxyuridine) or hyperthermia further enhances tumor cell killing. Beer, its certain constituents, or melatonin ameliorate heavy ion-induced damage to normal cells. In addition to effects in cells directly targeted with heavy ions, there is mounting evidence for nontargeted biological effects in cells that have not themselves been directly irradiated. The bystander effect of heavy ions manifests itself as the loss of clonogenic potential, a transient apoptotic response, delayed p53 phosphorylation, alterations in gene expression profiles, and the elevated frequency of gene mutations, micronuclei and chromosome aberrations, which arise in nonirradiated cells having received signals from irradiated cells. Proposed mediating mechanisms involve gap junctional intercellular communication, reactive oxygen species and nitric oxide. This paper reviews briefly the current knowledge of the biological effects of heavy-ion irradiation with a focus on recent findings regarding its potential benefits for therapeutic use as well as on the bystander effect.

Irradiated fibroblast-induced bystander effects on invasive growth of squamous cell carcinoma under cancer-stromal cell interaction

The irradiated fibroblast-induced response of non-irradiated neighboring cells is called 'radiation-induced bystander effect', but it is unclear in non-irradiated human squamous cell carcinoma (SCC) cells. The present study shows that irradiated fibroblasts promoted the invasive growth of T3M-1 SCC cells, but not their apoptosis, more greatly than non-irradiated fibroblasts, using collagen gel invasion assay, immunohistochemistry and Western blot. The number of irradiated fibroblasts decreased to about 30% of that of non-irradiated fibroblasts, but irradiated fibroblasts increased the growth marker ki-67 display of SCC cells more greatly than non-irradiated fibroblasts. Irradiated fibroblasts did not affect the apoptosis marker ss-DNA expression of SCC cells. Irradiated fibroblasts enhanced the display of the following growth-, invasion- and motility-related molecules in SCC cells more greatly than non-irradiated fibroblasts: c-Met, Ras, mitogen-activated protein kinase (MAPK) cascade (Raf-1, MEK-1 and ERK-1/2), matrix metalloproteinase-1 and -9, laminin 5 and filamin A. Irradiated fibroblasts, but not non-irradiated ones, formed irradiation-induced foci (IRIF) of the genomic instability marker p53-binding protein 1 (53BP1) and expressed transforming growth factor-beta1 (TGF- beta1). Irradiated fibroblasts in turn enabled SCC cells to enhance 53BP1 IRIF formation more extensively than non-irradiated fibroblasts. Finally, effects of irradiated fibroblasts on growth and apoptosis of another HEp-2 SCC cell type were similar to those of T3M-1. These results suggest that irradiated fibroblasts promotes invasion and growth of SCC cells by enhancement of invasive growth-related molecules above through TGF- beta1-mediated bystander mechanism, in which irradiated fibroblast-induced genomic instability of SCC cells may be involved.

FLT-PET imaging of radiation responses in murine tumors

BACKGROUND

3'-[F-18]fluoro-3'-deoxythymidine (FLT) traces thymidine phosphorylation catalyzed by thymidine kinase during cell proliferation. Knowing the rate of cell proliferation during cancer treatment, such as radiation therapy, would be valuable in assessing whether tumor recurrence is likely and might indicate the need for additional treatments. However, the relationship between FLT kinetics and the effects of radiation is not well-understood. Nor has the method for optimal quantification of FLT uptake within the irradiated tumor microenvironment been extensively examined.

MATERIALS AND METHODS

We performed dynamic FLT-positron emission tomography (PET) studies (60 min) on 22 mice implanted subcutaneously with syngeneic mammary MCaK tumors bilaterally in the shoulder area. A day before the FLT-PET imaging, the tumor on the right side was irradiated with a single dose (0, 2.5, 5, 10, or 20 Gy) or with fractionated exposures (4x2.5 Gy given in 12 h intervals). Standardized uptake value (SUVs) of FLT on tumors at 10 and 60 min post injection were calculated; model fitting was used to estimate the kinetic parameters. Significant radiation-induced changes were shown by comparing the irradiated tumor with the control tumor in the same animal and by comparing it to nonirradiated mice. The effect of radiation on MCaK cell cycle parameters and FLT uptake was also examined in vitro.

RESULTS

In vivo FLT kinetics were sensitive to radiation doses of 5 Gy and higher (administered 1 day earlier), as judged by SUV semiquantitative measures and by modeling. Single irradiation with 10 Gy had greater impact on SUVs and kinetic parameters than fractionated exposures. Overall, the uptake constant Ki appeared to be the best marker for these radiation effects. FLT uptake by irradiated cells in vitro at various doses gave similar findings, and the in vitro FLT uptake correlated well with Ki. Radiation-induced G2/M arrest appeared to influence FLT uptake, and this was more pronounced after single than fractionated doses.

CONCLUSION

The kinetics of FLT uptake into murine mammary tumors was altered 1 day after radiation treatment. The dose-dependent response correlated well with in vitro FLT cellular uptake. Parameters (e.g., Ki) derived from FLT kinetics are expected to be useful for assessing the efficacy of irradiation treatment of tumors.

Bystander/abscopal effects induced in intact Arabidopsis seeds by low-energy heavy-ion radiation

To date, radiation-induced bystander effects have been observed largely in in vitro single-cell systems; verification of both the effects and the mechanisms in multicellular systems in vivo is important. Previously we showed that bystander/ abscopal effects can be induced by irradiating the shoot apical meristem cells in Arabidopsis embryos. In this study, we investigated the in vivo effects induced by 30 keV 40Ar ions in intact Arabidopsis seeds and traced the postembryonic development of both irradiated and nonirradiated shoot apical meristem and root apical meristem cells. Since the range of 30 keV 40Ar ions in water is about 0.07 microm, which is less than the distance from the testa to shoot apical meristem and root apical meristem in Arabidopsis seeds (about 100 microm), the incident low-energy heavy ions generally stop in the proximal surface. Our results showed that, after the 30 keV 40Ar-ion irradiation of shielded and nonshielded Arabidopsis seeds at a fluence of 1.5 x 10(17) ions/cm2, short- and long-term postembryonic development, including germination, root hair differentiation, primary root elongation, lateral root initiation and survival, was significantly inhibited. Since shoot apical meristem and root apical meristem cells were not damaged directly by radiation, the results suggested that a damage signal(s) is transferred from the irradiated cells to shoot apical meristem and root apical meristem cells and causes the ultimate developmental alterations, indicating that long-distance bystander/ abscopal effects exist in the intact seed. A further study of mechanisms showed that the effects are associated with either enhanced generation of reactive oxygen species (ROS) or decreased auxin-dependent transcription in postembryonic development. Treatment with the ROS scavenger dimethyl sulfoxide (DMSO) or synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) can significantly reverse both the alterations in postembryonic development and auxin-dependent transcription, suggesting that ROS and auxin-dependent transcription processes play essential roles in the low-energy heavy-ion radiation-induced long-distance bystander/abscopal effects in the intact organism.

NADPH oxidase mediates radiation-induced oxidative stress in rat brain microvascular endothelial cells

The need to both understand and minimize the side effects of brain irradiation is heightened by the ever-increasing number of patients with brain metastases that require treatment with whole brain irradiation (WBI); some 200,000 cancer patients/year receive partial or WBI. At the present time, there are no successful treatments for radiation-induced brain injury, nor are there any known effective preventive strategies. Data support a role for chronic oxidative stress in radiation-induced late effects. However, the pathogenic mechanism(s) involved remains unknown. One candidate source of reactive oxygen species (ROS) is nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase, which converts molecular oxygen (O(2)) to the superoxide anion (O(2)(-)) on activation. We hypothesize that brain irradiation leads to activation of NADPH oxidase. We report that irradiating rat brain microvascular endothelial cells in vitro leads to increased (i) intracellular ROS generation, (ii) activation of the transcription factor NFkappaB, (iii) expression of ICAM-1 and PAI-1, and (iv) expression of Nox4, p22(phox), and p47(phox). Pharmacologic and genetic inhibition of NADPH oxidase blocked the radiation-mediated upregulation of intracellular ROS, activation of NFkappaB, and upregulation of ICAM-1 and PAI-1. These results suggest that activation of NADPH oxidase may play a role in radiation-induced oxidative stress.

Radiation-induced bystander effects in vivo are sex specific

Ionizing radiation (IR) effects span beyond the area of direct exposure and can be observed in neighboring and distant naïve cells and organs. This phenomenon is termed a 'bystander effect'. IR effects in directly exposed tissue in vivo are epigenetically mediated and distinct in males and females. Yet, IR-induced bystander effects have never been explored in a sex-specificity domain. We used an in vivo mouse model, whereby the bystander effects are studied in spleen of male and female animals subjected to head exposure when the rest of the body is protected by a medical-grade lead shield. We analyzed the induction of DNA damage and alterations in global DNA methylation. Molecular parameters were correlated with cellular proliferation and apoptosis levels. The changes observed in bystander organs are compared to the changes in unexposed animals and animals exposed to predicted and measured scatter doses. We have found the selective induction of DNA damage levels, global DNA methylation, cell proliferation and apoptosis in exposed and bystander spleen tissue of male and female mice. Sex differences were significantly diminished in animals subjected to a surgical removal of gonads. These data constitute the first evidence of sex differences in radiation-induced bystander effects in mouse spleen in vivo. We show the role of sex hormones in spleen bystander responses and discuss implications of the observed changes.

A role for endogenous and radiation-induced DNA double-strand breaks in p53-dependent apoptosis during cortical neurogenesis

Prenatal exposure to low-dose radiation increases the risk of microcephaly and/or mental retardation. Microcephaly is also associated with genetic mutations that affect the non-homologous end-joining pathway of DNA double-strand break repair. To examine the link between these two causal factors, we characterized the neural developmental effects of acute radiation exposure in mouse littermate embryos harboring mutations in the Ku70 and p53 genes. Both low-dose radiation exposure and Ku70 deficiency induced morphologically indistinguishable cortical neuronal apoptosis. Irradiated Ku70-deficient embryos displayed anatomical damage indicative of increased radiosensitivity in the developing cerebral cortex. Deleting the p53 gene not only rescued cortical neuronal apoptosis at all levels but also restored the in vitro growth of Ku70-deficient embryonic fibroblasts despite the presence of unrepaired DNA/chromosomal breaks. The results confirm the role of DNA double-strand breaks as a common causative agent of apoptosis in the developing cerebral cortex. Furthermore, the findings suggest a disease mechanism by which the presence of endogenous DNA double-strand breaks in the newly generated cortical neurons becomes radiomimetic when DNA end joining is defective. This in turn activates p53-dependent neuronal apoptosis and leads to microcephaly and mental retardation.

Expression profiles are different in carbon ion-irradiated normal human fibroblasts and their bystander cells

Evidence has accumulated that ionizing radiation induces biological effects in non-irradiated bystander cells having received signals from directly irradiated cells; however, energetic heavy ion-induced bystander response is incompletely characterized. Here we performed microarray analysis of irradiated and bystander fibroblasts in confluent cultures. To see the effects in bystander cells, each of 1, 5 and 25 sites was targeted with 10 particles of carbon ions (18.3 MeV/u, 103 keV/microm) using microbeams, where particles traversed 0.00026, 0.0013 and 0.0066% of cells, respectively. diated cells, cultures were exposed to 10% survival dose (D), 0.1D and 0.01D of corresponding broadbeams (108 keV/microm). Irrespective of the target numbers (1, 5 or 25 sites) and the time (2 or 6h postirradiation), similar expression changes were observed in bystander cells. Among 874 probes that showed more than 1.5-fold changes in bystander cells, 25% were upregulated and the remainder downregulated. These included genes related to cell communication (PIK3C2A, GNA13, FN1, ANXA1 and IL1RAP), stress response (RAD23B, ATF4 and EIF2AK4) and cell cycle (MYCN, RBBP4 and NEUROG1). Pathway analysis revealed serial bystander activation of G protein/PI-3 kinase pathways. Instead, genes related to cell cycle or death (CDKN1A, GADD45A, NOTCH1 and BCL2L1), and cell communication (IL1B, TCF7 and ID1) were upregulated in irradiated cells, but not in bystander cells. Our results indicate different expression profiles in irradiated and bystander cells, and imply that intercellular signaling between irradiated and bystander cells activate intracellular signaling, leading to the transcriptional stress response in bystander cells.

The survival of heavy ion-irradiated Bcl-2 overexpressing radioresistant tumor cells and their progeny

Here, we investigated the cell killing effectiveness of heavy-ion radiation in Bcl-2 overexpressing radioresistant tumor cells. First, irradiated cells underwent primary colony formation. Radioresistance decreased with increasing linear energy transfer (LET), indicating that heavy ions may be a promising therapeutic modality for Bcl-2 overexpressing tumors. Second, cells in primary colonies were reseeded for secondary colony formation. The incidence of delayed reproductive death increased with LET irrespective of Bcl-2 overexpression, suggesting that Bcl-2 overexpression may not facilitate heavy ion-induced genomic instability.

Hyperoxia exposure induced hormesis decreases mitochondrial superoxide radical levels via Ins/IGF-1 signaling pathway in a long-lived age-1 mutant of Caenorhabditis elegans

The hormetic effect, which extends the lifespan by various stressors, has been confirmed in Caenorhabditis elegans (C. elegans). We have previously reported that oxidative stress resistance in a long-lived mutant age-1 is associated with the hormesis. In the age-1 allele, which activates an insulin/insulin-like growth factor-1 (Ins/IGF-1) signaling pathway, the superoxide dismutase (SOD) and catalase activities increased during normal aging. We now demonstrate changes in the mitochondrial superoxide radical (*O(2)(-)) levels of the hormetic conditioned age-related strains. The *O(2)(-) levels in age-1 strain significantly decreased after intermittent hyperoxia exposure. On the other hand, this phenomenon was not observed in a daf-16 null mutant. This hormesis-dependent reduction of the *O(2)(-) levels was observed even if the mitochondrial Mn-SOD was experimentally reduced. Therefore, it is indicated that the hormesis is mediated by events that suppress the mitochondrial *O(2)(-) production. Moreover, some SOD gene expressions in the hormetic conditioned age-1 mutant were induced over steady state mRNA levels. These data suggest that oxidative stress-inducible hormesis is associated with a reduction of the mitochondrial *O(2)(-) production by activation of the antioxidant system via the Ins/IGF-1 signaling pathway.

Dilution of irradiated cell conditioned medium and the bystander effect

While nontargeted and low-dose effects such as the bystander effect are now accepted, the mechanisms underlying the response have yet to be elucidated. It has been shown that the transfer of irradiated cell conditioned medium (ICCM) can kill cells that are not directly irradiated; however, to date the effect of ICCM concentration on cell killing has not been reported. The occurrence of a bystander effect was determined by measuring cell survival after exposure to various ICCM dilutions, using the colony-forming assay, in cells of six human cell lines with varied bystander responses and tumor/ p53 status. Autologous ICCM transfer for these cell lines induced a bystander effect as reported previously. ICCM from these cell lines was transferred to cells of a common reporter cell line (HPV-G) to investigate whether the lack of an induced bystander effect was due to their inability to generate or to respond to a bystander signal(s). ICCM from cells of four cell lines induced a bystander effect in HPV-G reporter cells, confirming that signal production is a critical factor. A saturation response was observed when ICCM was diluted. Survival was found to increase linearly until a plateau was reached and the bystander effect was abolished at 2x dilution. The effect of ICCM from the different cell lines reached a plateau at different dilutions, which were found to correlate with the cell line's radiosensitivity.

Energetic heavy ions overcome tumor radioresistance caused by overexpression of Bcl-2

BACKGROUND AND PURPOSE

Overexpression of Bcl-2 is frequent in human cancers and has been associated with radioresistance. Here we investigated the potential impact of heavy ions on Bcl-2 overexpressing tumors.

MATERIALS AND METHODS

Bcl-2 cells (Bcl-2 overexpressing HeLa cells) and Neo cells (neomycin resistant gene-expressing HeLa cells) exposed to gamma-rays or heavy ions were assessed for the clonogenic survival, apoptosis and cell cycle distribution.

RESULTS

Whereas Bcl-2 cells were more resistant to gamma-rays (0.2keV/microm) and helium ions (16.2keV/microm) than Neo cells, heavy ions (76.3-1610keV/microm) yielded similar survival regardless of Bcl-2 overexpression. Carbon ions (108keV/microm) decreased the difference in the apoptotic incidence between Bcl-2 and Neo cells, and prolonged G(2)/M arrest that occurred more extensively in Bcl-2 cells than in Neo cells.

CONCLUSIONS

High-LET heavy ions overcome tumor radioresistance caused by Bcl-2 overexpression, which may be explained at least in part by the enhanced apoptotic response and prolonged G(2)/M arrest. Thus, heavy-ion therapy may be a promising modality for Bcl-2 overexpressing radioresistant tumors.

Stress-induced premature senescence (SIPS)--influence of SIPS on radiotherapy

Replicative senescence is a fundamental feature in normal human diploid cells and results from dysfunctional telomeres at the Hayflick cell division limit. Ionizing radiation (IR) prematurely induces the same phenotypes as replicative senescence prior to the Hayflick limit. This process is known as stress-induced premature senescence (SIPS). Since the cell cycle is irreversibly arrested in SIPS-induced cells, even if they are stimulated by various growth factors, it is thought that SIPS is a form of cell death, irreversibly eliminating replicating cells. IR-induced-focus formation of DNA repair proteins, a marker of DNA damage, is detected in SIPS as well as replicative senescent cells. Furthermore, both processes persistently induce cell cycle checkpoint mechanisms, indicating DNA damage created by ionizing radiation induces SIPS in normal cells, possibly by the same mechanisms as those occurring in replicative senescence. Interestingly, IR induces SIPS not only in normal cells, but also in tumor cells. Due to the expression of telomerase in tumor cells, telomere-dependent replicative senescence does not occur. However, SIPS is induced under certain conditions after IR exposure. Thus, cell death triggered by IR can be attributed to apoptosis or SIPS in tumor cells. However, metabolic function remains intact in SIPS-induced cancer cells, and recent studies show that senescence eliminate cells undergoing SIPS secrete various kinds of factors outside the cell, changing the microenvironment. Evidence using co-culture systems containing normal senescent stromal cells and epithelial tumor cells show that factors secreted from senescent stroma cells promote the growth of tumor epithelial cells both in vitro and in vivo. Thus, regulation of factors secreted from SIPS-induced stromal cells, as well as tumor cells, may affect radiotherapy.

Temporally distinct response of irradiated normal human fibroblasts and their bystander cells to energetic heavy ions

Ionizing radiation-induced bystander effects have been documented for a multitude of endpoints such as mutations, chromosome aberrations and cell death, which arise in nonirradiated bystander cells having received signals from directly irradiated cells; however, energetic heavy ion-induced bystander response is incompletely characterized. To address this, we employed precise microbeams of carbon and neon ions for targeting only a very small fraction of cells in confluent fibroblast cultures. Conventional broadfield irradiation was conducted in parallel to see the effects in irradiated cells. Exposure of 0.00026% of cells led to nearly 10% reductions in the clonogenic survival and twofold rises in the apoptotic incidence regardless of ion species. Whilst apoptotic frequency increased with time up to 72 h postirradiation in irradiated cells, its frequency escalated up to 24h postirradiation but declined at 48 h postirradiation in bystander cells, indicating that bystander cells exhibit transient commitment to apoptosis. Carbon- and neon-ion microbeam irradiation similarly caused almost twofold increments in the levels of serine 15-phosphorylated p53 proteins, irrespective of whether 0.00026, 0.0013 or 0.0066% of cells were targeted. Whereas the levels of phosphorylated p53 were elevated and remained unchanged at 2h and 6h postirradiation in irradiated cells, its levels rose at 6h postirradiation but not at 2h postirradiation in bystander cells, suggesting that bystander cells manifest delayed p53 phosphorylation. Collectively, our results indicate that heavy ions inactivate clonogenic potential of bystander cells, and that the time course of the response to heavy ions differs between irradiated and bystander cells. These induced bystander responses could be a defensive mechanism that minimizes further expansion of aberrant cells.

Low-dose hypersensitivity in nucleus-irradiated V79 cells studied with synchrotron X-ray microbeam

This study aims at elucidating the cellular responses induced by energy deposition in the cell nucleus or cytoplasm in the low-dose (< 1 Gy) region. We compared the survival fraction of V79 cells irradiated with X-ray microbeams of different sizes. Entire cells or cell nuclei were targeted with 5.35 keV monochromatic X-ray microbeams using a synchrotron radiation (SR) X-ray microbeam irradiation apparatus. Using a threshold of 30 cells/colony after 60 h of incubation, conditions that had been proven to give results equivalent to those of the conventional method, we determined the survival fraction of the microbeam-irradiated cells. When cell nuclei were irradiated with 10 x 10 microm (2) X-ray beams, the survival fraction was almost the same as that obtained with 50 x 50 microm (2) beams except in the low-dose region. In the low-dose region irradiated with 10 x 10 microm (2) beams, hyper-radiosensitivity (HRS) was clearly observed in the nucleus-irradiated cells, and the survival curve exhibited a minimum of about 60% at 0.5 Gy. This may be the most distinct HRS reported thus far when an asynchronous population is used. Difference in observed HRS phenomena is solely due to the irradiated part in the cell. These results suggest that energy deposition in the cytoplasm might suppress the HRS.

Proliferation and cell death of human glioblastoma cells after carbon-ion beam exposure: morphologic and morphometric analyses

Histological analyses of glioblastoma cells after carbon-ion exposure are still limited and ultrastructural characteristics have not been investigated in detail. Here we report the results of morphological and morphometric analyses of a human glioblastoma cell line, CGNH-89, after ionizing radiation to characterize the effect of a carbon-beam on glioblastoma cells. Using CGNH-89 cells exposed to 0-10 Gy of X-ray (140 kVp) or carbon-ions (18.3 MeV/nucleon, LET=108 keV/microm), we performed conventional histology and immunocytochemistry with MIB-1 antibody, transmission electron microscopy, and computer-assisted, nuclear size measurements. CGNH-89 cells with a G to A transition in codon 280 in exon 8 of the TP53 gene had nuclei with pleomorphism, marked nuclear atypia and brisk mitotic activity. After carbon-ion and X-ray exposure, living cells showed decreased cell number, nuclear condensation, increased atypical mitotic figures, and a tendency of cytoplasmic enlargement at the level of light microscopy. The deviation of the nuclear area size increased during 48 h after irradiation, while the small cell fraction increased in 336 h. In glioblastoma cells of the control, 5 Gy carbon-beam, and 10 Gy carbon-beam, and MIB-1 labeling index decreased in 24 h (12%, 11%, 7%, respectively) but increased in 48 h (10%, 20%, 21%, respectively). Ultrastructurally, cellular enlargement seemed to depend on vacuolation, swelling of mitochondria, and increase of cellular organelles, such as the cytoskeleton and secondary lysosome. We could not observe apoptotic bodies in the CGNH-89 cells under any conditions. We conclude that carbon-ion irradiation induced cell death and senescence in a glioblastoma cell line with mutant TP53. Our results indicated that the increase of large cells with enlarged and bizarre nuclei, swollen mitochondria, and secondary lysosome occurred in glioblastoma cells after carbon-beam exposure.

Heavy-ion microbeam system at JAEA-Takasaki for microbeam biology

Research concerning cellular responses to low dose irradiation, radiation-induced bystander effects, and the biological track structure of charged particles has recently received particular attention in the field of radiation biology. Target irradiation employing a microbeam represents a useful means of advancing this research by obviating some of the disadvantages associated with the conventional irradiation strategies. The heavy-ion microbeam system at JAEA-Takasaki, which was planned in 1987 and started in the early 1990's, can provide target irradiation of heavy charged particles to biological material at atmospheric pressure using a minimum beam size 5 mum in diameter. A variety of biological material has been irradiated using this microbeam system including cultured mammalian and higher plant cells, isolated fibers of mouse skeletal muscle, silkworm (Bombyx mori) embryos and larvae, Arabidopsis thaliana roots, and the nematode Caenorhabditis elegans. The system can be applied to the investigation of mechanisms within biological organisms not only in the context of radiation biology, but also in the fields of general biology such as physiology, developmental biology and neurobiology, and should help to establish and contribute to the field of "microbeam biology".

Abdominal radiation initiates apoptotic mechanism in rat femur bone marrow cells in vivo that is reversed by IGF-1 administration

PURPOSE

Radiation induces apoptosis as a result of damage to cellular DNA and RNA. The aim of our work was to study the effect of radiation on rat bone marrow cells (as a neighboring tissue) in the context of a model of experimental radiation enteritis in rats. The effect of systematic administration in irradiated animals of r-IGF-1 and GH was also studied.

MATERIALS AND METHODS

Wistar type, normal rats, were divided in 4 groups. One control group and the other 3 groups were irradiated in the abdomen. The measured scattered irradiation in the femur ranged from 16.5 to 47.3 cGy. In 2 groups of irradiated animals, rIGF-1 (0.1 microg/g of body weight twice/d) and rGH (0.25 microg/g of body weight /d) were administered. Bone marrow cells were harvested from both femurs. DNA and RNA were analyzed in specific gels. The m-RNA was hybridized for c-fos proto-oncogene expression.

RESULTS

The calculated low dose of radiation that affected the femurs of the animals induced reduction in bone marrow cell numbers and endonuclease activation manifested by subsequent fragmentation of DNA and RNA. This phenomenon was reversed by rGH and rIGF-1 administration. The c-fos proto-oncogene expression was upregulated by irradiation.

CONCLUSION

These observations indicate that scattered low dose radiation is capable of initiating apoptosis in rat bone marrow cells and rGH and rIGF-1 administration reverse this process.

Epigenetic changes and nontargeted radiation effects--is there a link?

It is now well accepted that the effects of ionizing radiation (IR) exposure can be noticed far beyond the borders of the directly irradiated tissue. IR can affect neighboring cells in the proximity, giving rise to a bystander effect. IR effects can also span several generations and influence the progeny of exposed parents, leading to transgeneration effects. Bystander and transgeneration IR effects are linked to the phenomenon of the IR-induced genome instability that manifests itself as chromosome aberrations, gene mutations, late cell death, and aneuploidy. While the occurrence of the above-mentioned phenomena is well documented, the exact mechanisms that lead to their development have still to be delineated. Evidence suggests that the IR-induced genome instability, bystander, and transgeneration effects may be epigenetically mediated. The epigenetic changes encompass DNA methylation, histone modification, and RNA-associated silencing. Recent studies demonstrated that IR exposure alters epigenetic parameters in the directly exposed tissues and in the distant bystander tissues. Transgeneration radiation effects were also proposed to be of an epigenetic nature. We will discuss the role of the epigenetic mechanisms in radiation responses, bystander effects, and transgeneration effects.