Adaptive response in embryogenesis. III. Relationship to radiation-induced apoptosis and Trp53 gene status

We reported previously that a radiation-induced adaptive response existed in the late period of embryogenesis, and that radiation-induced apoptosis in the predigital regions was responsible for digital defects in embryonic ICR mice. To investigate the possible involvement of the Trp53 gene and radiation-induced apoptosis in radiation-induced adaptive responses in embryogenesis, the present study was conducted using Trp53 wild-type (Trp53(+/+)) and Trp53 heterozygous (Trp53(+/-)) embryonic mice of the C57BL/6 strain. The existence of a radioadaptive response in the Trp53(+/+) embryonic mice was demonstrated by irradiating the embryos with 5 or 30 cGy on embryonic day 11 prior to a challenging irradiation at 3 Gy on embryonic day 12. The two conditioning doses at 5 and 30 cGy significantly suppressed the induction of apoptosis by the challenging dose in the predigital regions of limb buds in the Trp53(+/+) embryonic mice, while no such effect was found in the Trp53(+/-) embryonic mice. These findings indicate that induction of a radioadaptive response in embryogenesis is related to Trp53 gene status and the occurrence of radiation-induced apoptosis.     

Adaptive response in embryogenesis: V. Existence of two efficient dose-rate ranges for 0.3 Gy of priming irradiation to adapt mouse fetuses

The adaptive response is an important phenomenon in radiobiology. A study of the conditions essential for the induction of an adaptive response is of critical importance to understanding the novel biological defense mechanisms against the hazardous effects of radiation. In our previous studies, the specific dose and timing of radiation for induction of an adaptive response were studied in ICR mouse fetuses. We found that exposure of the fetuses on embryonic day 11 to a priming dose of 0.3 Gy significantly suppressed prenatal death and malformation induced by a challenging dose of radiation on embryonic day 12. Since a significant dose-rate effect has been observed in a variety of radiobiological phenomena, the effect of dose rate on the effectiveness of induction of an adaptive response by a priming dose of 0.3 Gy administered to fetuses on embryonic day 11 was investigated over the range from 0.06 to 5.0 Gy/min. The occurrence of apoptosis in limb buds, incidences of prenatal death and digital defects, and postnatal mortality induced by a challenging dose of 3.5 Gy given at 1.8 Gy/min to the fetuses on embryonic day 12 were the biological end points examined. Unexpectedly, effective induction of an adaptive response was observed within two dose-rate ranges for the same dose of priming radiation, from 0.18 to 0.98 Gy/ min and from 3.5 to 4.6 Gy/min, for reduction of the detrimental effect induced by a challenging dose of 3.5 Gy. In contrast, when the priming irradiation was delivered at a dose rate outside these two ranges, no protective effect was observed, and at some dose rates elevation of detrimental effects was observed. In general, neither a normal nor a reverse dose- rate effect was found in the dose-rate range tested. These results clearly indicated that the dose rate at which the priming irradiation was delivered played a crucial role in the induction of an adaptive response. This paper provides the first evidence for the existence of two dose-rate ranges for the same dose of priming radiation to successfully induce an adaptive response in mouse fetuses.     

Bone morphogenetic protein-2 modulation of chondrogenic differentiation in vitro involves gap junction-mediated intercellular communication

Undifferentiated mesenchymal cells in the limb bud integrate a complex array of local and systemic signals during the process of cell condensation and chondrogenic differentiation. To address the relationship between bone morphogenetic protein (BMP) signaling and gap junction-mediated intercellular communication, we examined the effects of BMP-2 and a gap junction blocker 18 alpha glycyrrhetinic acid (18alpha-GCA) on mesenchymal cell condensation and chondrogenic differentiation in an in vitro chondrogenic model. We find that connexin43 protein expression significantly correlates with early mesenchymal cellular condensation and chondrogenesis in high-density limb bud cell culture. The level of connexin43 mRNA is maximally upregulated 48 h after treatment with recombinant human BMP-2 with corresponding changes in protein expression. Inhibition of gap junction-mediated intercellular communication with 2.5 microM 18alpha-GCA decreases chondrogenic differentiation by 50% at 96 h without effects on housekeeping genes. Exposure to 18alpha-GCA for only the first 24-48 h after plating does not affect condensation or later chondrogenic differentiation suggesting that gap junction-mediated intercellular communication is not critical for the initial phase of condensation but is important for the onset of differentiation. 18alpha-GCA can also block the chondrogenic effects of BMP-2 without effects on cell number or connexin43 expression. These observations demonstrate 18alpha-GCA-sensitive regulation of intercellular communication in limb mesenchymal cells undergoing chondrogenic differentiation and suggest that BMP-2 induced chondrogenic differentiation may be mediated in part through the modulation of connexin43 expression and gap junction-mediated intercellular communication.     

Increase in endogenous spleen colonies without recovery of blood cell counts in radioadaptive survival response in C57BL/6 mice

The radioadaptive survival response induced by a conditioning exposure to 0.45 Gy and measured as an increase in 30-day survival after mid-lethal X irradiation was studied in C57BL/6N mice. The acquired radioresistance appeared on day 9 after the conditioning exposure, reached a maximum on days 12-14, and disappeared on day 21. The conditioning exposure 14 days prior to the challenge exposure increased the number of endogenous spleen colonies (CFU-S) on days 12-13 after the exposure to 5 Gy. On day 12 after irradiation, the conditioning exposure also increased the number of endogenous CFU-S to about five times that seen in animals exposed to 4.25-6.75 Gy without preirradiation. The effect of the interval between the preirradiation and the challenge irradiation on the increase in endogenous CFU-S was also examined. A significant increase in endogenous CFU-S was observed when the interval was 14 days, but not 9 days. This result corresponded to the increase in survival observed on day 14 after the challenge irradiation. Radiation-inducted resistance to radiation-induced lethality in mice appears to be closely related to the marked recovery of endogenous CFU-S in the surviving hematopoietic stem cells that acquired radioresistance by preirradiation. Preirradiation enhanced the recovery of the numbers of erythrocytes, leukocytes and thrombocytes very slightly in mice exposed to a sublethal dose of 5 Gy, a dose that does not cause bone marrow death. There appears to be no correlation between the marked increase in endogenous CFU-S and the slight increase or no increase in peripheral blood cells induced by the radioadaptive response. The possible contribution by some factor, such as Il4 or Il11, that has been reported to protect irradiated animals without stimulating hematopoiesis is discussed.     

Induction of radio-adaptive response in colony formation by low dose X-ray irradiation.

In this study, we examined the induction of a radio-adaptive response to cell death using a colony formation test in m5S, G401.2/6TG.1 and HeLa cells. When m5S cells were subjected to priming irradiation of 0.05 to approximately 0.15 Gy 4 hr before being irradiated with 4.5 Gy, the survival ratios increased significantly to 39 to approximately 42%. The priming irradiation effect was also observed when G401.2/6TG.1 cells were subjected to priming irradiation of 0.025 to approximately 0.1 Gy 4 hr before being irradiated with 0.8 Gy. This effect showed a two-phasic characteristic, where the first peak was reached at 0.025 Gy, and the second peak was reached at 0.075 Gy. The first peak showed a survival ratio of 56%, while the second peak was at 55%. However, in HeLa cells, this priming irradiation effect was not observed. These results indicated that induction of the radio-adaptive response did not depend on whether cells are normal or cancerous. One of the differences in these cells is that m5S and G401.2/6TG.1 cells have gap-junctional intercellular communication, but HeLa cells do not. Induction of the radio-adaptive response may be related to gap-junctional intercellular communication.     

Low-dose binary behavior of bystander cell killing after microbeam irradiation of a single cell with focused c(k) x rays

Although conclusive evidence has been obtained for the presence of radiation-induced bystander effects, the mechanisms that trigger and regulate these processes are still largely unknown. The bystander effect may play a critical role in determining the biological effectiveness of low-dose exposures, but questions on how to incorporate it into current models and extrapolate the risks of radiation-induced carcinogenesis are still open. The Gray Cancer Institute soft X-ray microbeam has been used to investigate the dose-response relationship of the bystander effect below 0.5 Gy. The survival response of V79 cells was assessed after the irradiation of a single cell within a population with a submicrometer-size beam of carbon K X rays (278 eV). Above 0.3 Gy, the measured bystander cell killing was in agreement with previously published data; however, a significant increase in the scatter of the data was observed in the low-dose region (<0.3 Gy). The data distribution observed indicates a binary behavior for triggering of the bystander response. According to our hypothesis, the probability of triggering a bystander response increases approximately linearly with the dose delivered to the single selected cell, reaching 100% above about 0.3 Gy. The magnitude of the bystander effect, when triggered, is approximately constant with the dose and results in an overall approximately 10% reduction in survival in our system. This suggests that the event that triggers the emission of the bystander signal by the hit cell is an all-or-nothing process. Extrapolation of the data indicates that when a single fast electron traverses a V79 cell, there is a probability of approximately 0.3% that the cell will emit the bystander signal. The data presented in this paper have also been analyzed statistically to test the possibility that complex DNA double-strand breaks may be the initial critical event.     

Radioadaptive response: Efficient repair of radiation-induced DNA damage in adapted cells

To verify the hypothesis that the induction of a novel, efficient repair mechanism for chromosomal DNA breaks may be involved in the radioadaptive response, the repair kinetics of DNA damage has been studied in cultured Chinese hamster V79 cells with single-cell gel electrophoresis. The cells were adapted by priming exposure with 5 cGy of γ-rays and 4-h incubation at 37°C. There were no indication of any difference in the initial yields of DNA double-strand breaks induced by challenging doses from non-adapted cells and from adapted cells. The rejoining of DNA double-strand breaks was monitored over 120 min after the adapted cells were challenged with 5 or 1.5 Gy, doses at the same level to those used in the cytogenetical adaptive response. The rate of DNA damage repair in adapted cells was higher than that in non-adapted cells, and the residual damage was less in adapted cells than in non-adapted cells. These results indicate that the radioadaptive response may result from the induction of a novel, efficient DNA repair mechanism which leads to less residual damage, but not from the induction of protective functions that reduce the initial DNA damage.     

Spatially fractionated radiation induces cytotoxicity and changes in gene expression in bystander and radiation adjacent murine carcinoma cells

Radiation-induced bystander effects have been extensively studied at low doses, since evidence of bystander induced cell killing and other effects on unirradiated cells were found to be predominant at doses up to 0.5 Gy. Therefore, few studies have examined bystander effects induced by exposure to higher doses of radiation, such as spatially fractionated radiation (GRID) treatment. In the present study, we evaluate the ability of GRID treatment to induce changes in GRID adjacent (bystander) regions, in two different murine carcinoma cell lines following exposure to a single irradiation dose of 10 Gy. Murine SCK mammary carcinoma cells and SCCVII squamous carcinoma cells were irradiated using a brass collimator to create a GRID pattern of nine circular fields 12 mm in diameter with a center-to-center distance of 18 mm. Similar to the typical clinical implementation of GRID, this is approximately a 50:50 ratio of direct and bystander exposure. We also performed experiments by irradiating separate cultures and transferring the medium to unirradiated bystander cultures. Clonogenic survival was evaluated in both cell lines to determine the occurrence of radiation-induced bystander effects. For the purpose of our study, we have defined bystander cells as GRID adjacent cells that received approximately 1 Gy scatter dose or unirradiated cells receiving conditioned medium from irradiated cells. We observed significant bystander killing of cells adjacent to the GRID irradiated regions compared to sham treated controls. We also observed bystander killing of SCK and SCCVII cells cultured in conditioned medium obtained from cells irradiated with 10 Gy. Therefore, our results confirm the occurrence of bystander effects following exposure to a high-dose of radiation and suggest that cell-to-cell contact is not required for these effects. In addition, the gene expression profile for DNA damage and cellular stress response signaling in SCCVII cells after GRID exposure was studied. The occurrence of GRID-induced bystander gene expression changes in significant numbers of DNA damage and cellular stress response signaling genes, providing molecular evidence for possible mechanisms of bystander cell killing.     

Bystander effects, adaptive response and genomic instability induced by prenatal irradiation

The developing human embryo and fetus undergo very radiosensitive stages during the prenatal development. It is likely that the induction of low dose related effects such as bystander effects, the adaptive response, and genomic instability would have profound effects on embryonic and fetal development. In this paper, I review what has been reported on the induction of these three phenomena in exposed embryos and fetuses. All three phenomena have been shown to occur in murine embryonic or fetal cells and structures, although the induction of an adaptive response (and also likely the induction of bystander effects) are limited in terms of when during development they can be induced and the dose or dose-rate used to treat animals in utero. In contrast, genomic instability can be induced throughout development, and the effects of radiation exposure on genome instability can be observed for long times after irradiation including through pre- and postnatal development and into the next generation of mice. There are clearly strain-specific differences in the induction of these phenomena and all three can lead to long-term detrimental effects. This is true for the adaptive response as well. While induction of an adaptive response can make fetuses more resistant to some gross developmental defects induced by a subsequent high dose challenge with ionizing radiation, the long-term effects of this low dose exposure are detrimental. The negative effects of all three phenomena reflect the complexity of fetal development, a process where even small changes in the timing of gene expression or suppression can have dramatic effects on the pattern of biological events and the subsequent development of the mammalian organism.     

A computational model for radiation-induced cellular transformation to in vitro irradiation of cells by acute doses of X-rays

This research incorporates new biological concepts to improve the predictive ability of a state-vector model with respect to dose-response data on in vitro oncogenic transformation, including mechanisms of DNA damage, DNA repair, cell death, cell proliferation and intercellular communication. Experimentally recognized biological processes, including background transformation, compensatory proliferation and bystander cell-killing effect were formulated mathematically and included as model parameters. These were then adjusted with an optimization method to reproduce in vitro transformation frequency data from C3H10T1/2 mouse cells exposed to acute doses of X-rays. A plateau observed in the data at low doses is reproduced well and a dose-dependent increase above 1Gy is predicted almost precisely. Extension of the model predictions to the dose range 0-100mGy indicates that transformation frequencies are practically constant over this low dose region. Results suggest a protective, rather than detrimental, bystander cell-killing effect. Further analysis of model sensitivity to this bystander parameter, though, revealed uncertainties with respect to its biological plausibility in the model.     

Alpha-particle-induced increases in the radioresistance of normal human bystander cells.

Numerous investigators have reported that direct exposure of cells to a low dose of ionizing radiation can induce a condition of enhanced radioresistance, i.e. a "radioadaptive" response. In this report, we investigated the hypothesis that a radioadaptive bystander effect may be induced in unirradiated cells by a transmissible factor(s) present in the supernatants of cells exposed to a low dose of alpha particles. Normal human lung fibroblasts (HFL-1) were irradiated with 1 cGy of alpha particles and their supernatants were transferred to unirradiated HFL-1 cells as a bystander cell model. Compared to directly irradiated cells that were not treated with supernatants from HFL-1 cells exposed to low-dose radiation, such treatment resulted in increased clonogenic survival after subsequent exposure to 10 and 19 cGy of alpha particles. Increases in protein levels of AP-endonuclease, a redox and DNA base excision repair protein, were found in the bystander cells, but not in directly irradiated cells. Supernatants from alpha-particle-irradiated cells were also found to increase the clonogenicity of unirradiated cells. These results, in conjunction with our earlier findings that supernatants from cells exposed to a low dose of alpha particles contain growth-promoting activity, suggest that this new bystander effect may be related to an increase in DNA repair and cell growth/cell cycle regulation.     

Radiation-induced bystander effect in healthy G(o) human lymphocytes: biological and clinical significance

To study the bystander effects, G(0) human peripheral blood lymphocytes were X-irradiated with 0.1, 0.5 and 3 Gy. After 24h, cell-free conditioned media from irradiated cultures were transferred to unexposed lymphocytes. Following 48 h of medium transfer, viability, induction of apoptosis, telomere shortening, reactive oxygen species (ROS) levels and micronuclei (after stimulation) were analyzed. A statistically significant decrement in cell viability, concomitant with the loss of mitochondrial membrane potential, telomere shortening, increases in hydrogen peroxide (H(2)O(2)) and superoxide anion (O(2)(-)) with depletion of intracellular glutathione (GSH) level, and higher frequencies of micronuclei, were observed in bystander lymphocytes incubated with medium from 0.5 and 3 Gy irradiated samples, compared to lymphocytes unexposed. Furthermore, no statistically significant difference between the response to 0.5 and 3 Gy of irradiation in bystander lymphocytes, was found. However, when lymphocytes were irradiated with 0.1 Gy, no bystander effect with regard to viability, apoptosis, telomere length, and micronuclei was observed, although a high production of ROS level persisted. Radiation in the presence of the radical scavenger dimethyl sulfoxide (DMSO) suppressed oxidative stress induced by 3 Gy of X-rays with the effective elimination of bystander effects, suggesting a correlation between ROS and bystander signal formation in irradiated cells. The data propose that bystander effect might be mostly due to the reactions of radiation induced free radicals on DNA, with the existence of a threshold at which the bystander signal is not operative (0.1 Gy dose of X-rays). Our results may have clinical implications for health risk associated with radiation exposure.     

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.     

Sequence and structural characterization of tbnat gene in isoniazid-resistant Mycobacterium tuberculosis: identification of new mutations

One of the key issues of current radiation research is the biological effect of low doses. Unfortunately, low dose science is hampered by the unavailability of easily performable, reliable and sensitive quantitative biomarkers suitable detecting low frequency alterations in irradiated cells. We applied a quantitative real time polymerase chain reaction (qRT-PCR) based protocol detecting common deletions (CD) in the mitochondrial genome to assess direct and non-targeted effects of radiation in human fibroblasts. In directly irradiated (IR) cells CD increased with dose and was higher in radiosensitive cells. Investigating conditioned medium-mediated bystander effects we demonstrated that low and high (0.1 and 2Gy) doses induced similar levels of bystander responses and found individual differences in human fibroblasts. The bystander response was not related to the radiosensitivity of the cells. The importance of signal sending donor and signal receiving target cells was investigated by placing conditioned medium from a bystander response positive cell line (F11-hTERT) to bystander negative cells (S1-hTERT) and vice versa. The data indicated that signal sending cells are more important in the medium-mediated bystander effect than recipients. Finally, we followed long term effects in immortalized radiation sensitive (S1-hTERT) and normal (F11-hTERT) fibroblasts up to 63 days after IR. In F11-hTERT cells CD level was increased until 35 days after IR then reduced back to control level by day 49. In S1-hTERT cells the increased CD level was also normalized by day 42, however a second wave of increased CD incidence appeared by day 49 which was maintained up to day 63 after IR. This second CD wave might be the indication of radiation-induced instability in the mitochondrial genome of S1-hTERT cells. The data demonstrated that measuring CD in mtDNA by qRT-PCR is a reliable and sensitive biomarker to estimate radiation-induced direct and non-targeted effects.     

Evidence for induction of DNA double strand breaks in the bystander response to targeted soft X-rays in CHO cells

This study investigated the role of DNA double strand breaks and DNA base damage in radiation-induced bystander responses in Chinese hamster ovary (CHO) cell lines. Two CHO repair-deficient clones, xrs5 (DNA double strand break repair-deficient) and EM9 (DNA base excision repair-deficient) were used in addition to the wild type (CHO). The Gray Cancer Institute ultrasoft X-ray microprobe is a powerful tool for investigating the bystander response, because it permits the irradiation of only a single nucleus of a cell, as reported previously. In order to investigate the bystander effect in each repair-deficient cell line, we irradiated a single cell within a population and scored the formation of micronuclei. When a single nucleus in the population was targeted with 1 Gy, elevated numbers of micronuclei were induced in the neighbouring unirradiated cells in the EM9 and xrs5 cell lines, whereas induction was not observed in CHO. The induction of micronuclei in xrs5 was significantly higher than that in EM9. Under these conditions, the surviving fraction in the neighbouring cells was significantly lower in xrs5 than in the other cell lines, showing a higher cell killing effect in xrs5. To confirm that bystander factors secreted from irradiated cells caused these effects, we carried out medium transfer experiments using conventional X-irradiation. Medium conditioned for 24 h with irradiated cells was transferred to unirradiated cells and elevated induction of micronuclei was observed in xrs5. These results suggest that DNA double strand breaks rather than base damage are caused by factors secreted in the medium from irradiated cells.     

Cell cycle-related bystander responses are not increased with LET after heavy-ion irradiation

Evidence has accumulated that irradiated cells affect their unirradiated neighbors, so that they in turn display cellular responses typically associated with direct radiation exposure. These responses are generally known as bystander effects. In this study, cell cycle-related bystander responses were investigated in three strains of human fibroblasts after exposure to densely ionizing radiation. Varying the linear energy transfer (LET) from 11 to 15,000 keV microm(-1) allowed a study of the impact of the complexity of DNA damage in the inducing cells on the responses of bystander cells. Using both broad-beam and microbeam irradiation, transient bystander responses were obtained for the induction of CDKN1A (p21). The latter was also observed when the transmission of bystander signals was limited to soluble factors. Targeted irradiation of single cells in confluent cell monolayers revealed no correlation between the amount of CDKN1A protein in the bystander cells and the radial distance to the targeted cells. In line with the induction of CDKN1A in bystander cells after irradiation with different LETs, a transient delay in the first G1 phase after irradiation of G0/G1 cells was observed. However, the CDKN1A induction revealed no significant effect on premature terminal differentiation considered to underlie fibrosis in irradiated tissue. Thus the unchanged differentiation pattern in bystander cells does not indicate pronounced, long-lasting effects.     

A bystander effect in alpha-particle irradiations of human prostate tumor cells

Alpha-particle exposures were used to determine whether cells of the human prostate carcinoma cell line DU-145 can produce and respond to a bystander effect signal. An apparatus for alpha-particle irradiation of cells growing as a monolayer on a 1.4-microm-thick Mylar membrane directly above an 241Am alpha-particle source was constructed and calibrated. At the cell irradiation position, the alpha-particle fluence was 998 counts/mm2 s(-1), the average alpha-particle energy was 3.14 MeV, and the average linear energy transfer was 128 keV/microm. The average dose rate to the cells growing on the Mylar surface was 1.2 Gy/min. A co-culture system was used to examine bystander effects transmitted through the medium from the directly targeted cells to tumor cells growing on an insert well beyond the range of the alpha particles. Alpha-particle doses from 0.1 to 6.0 Gy to the targeted cells on the Mylar membrane, followed by a 2-h co-incubation of the cells on the insert in the irradiated medium above the irradiated cells, all caused an approximately 50% increase in micronucleus formation in the nontargeted co-cultured cells. Addition of the radical scavenger DMSO to the medium during the irradiation and the 2-h postirradiation incubation period completely blocked the bystander effect, whereas addition of a nitric oxide scavenger had no effect. Irradiation of medium containing serum, followed by a 2-h incubation, caused no bystander effect in the co-cultured cells. When the co-cultured cells on the insert were placed into the irradiated medium above the directly targeted cells immediately (approximately 1 min) after the irradiation and co-incubated for 2 h, there was no bystander effect. These data indicate that the observed bystander effect requires that the co-cultured cells be present in the medium during the irradiation of the directly targeted cells and suggest the involvement of a short-lived radical species.     

Cellular radiation effects and the bystander response

This report reviews briefly some of the findings reported over the past 2 years that enhance our understanding of the radiation-induced bystander effect. These developments include: technicological advances; the role of oxidative stress; the effect of cytoplasmic irradiation; cell-to-cell communication; and evidence that Connexin 43 mediated intercellular communication is induced by radiation exposure. A few overriding unanswered questions are discussed. These include: what is the signal(s) transmitted from irradiated to bystander cells; what is the relationship between the bystander response and other non-targeted effects of radiation; are there beneficial effects associated with the bystander response; and what is the significance of the bystander effect for radiation protection?