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

Effects of exposing Japanese quail eggs to a low dose of gamma radiation and in ovo feeding by two sources of trace elements on embryonic development activities

The present study investigated the influence of exposing quail eggs to low-dose gamma radiation (GR) and in ovo feeding with 2 sources of a mixture of trace elements (Zn, Fe, and Cu), including sulfate (TES) and loaded with montmorillonite (TEM), on embryonic development activities and prehatch quality. A total of 960 eggs on the seventh day of incubation were randomly divided into 6 groups (160 eggs/group) with 4 replicate of 40 eggs in each. A 3 × 2 factorial arrangement experiment was performed and included 3 sources in ovo feeding with a mixture of trace elements (Zn, Fe, and Cu), including 0 mg/egg, 50 mg TES/egg, and 50 mg TEM/egg with egg irradiation using 0 and 0.2 Gy from GR. Eggs injected with 50 mg TEM/egg and exposed to 0.2 Gy from GR (TEM/GR) was significantly (P ≤ 0.05 and 0.01) higher in hatchability, hatch body weight, and relative organ weight (liver, gizzard, proventriculus, heart, and intestine). The obtained results indicated significant (P ≤ 0.05) decreased in the serum concentration of malondialdehyde (MDA) in TEM/GR group. There was significant (P ≤ 0.05) increased of catalase (CAT) activity and the concentrations of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) in TEM/GR group; however; total antioxidant capacity (T-AOC) was significant (P ≤ 0.05) increased in CT/GR group. Serum concentrations of immunoglobulin M (IgM) (P ≤ 0.05) and tumor necrosis factor-alpha (TNF-α) were increased in the TEM/CR group; the concentration of transforming growth factor beta (TGF-β) significant (P ≤ 0.05) increased in the TEM/GR group; and interleukins (IL6 and IL10) showed no significant differences among the groups. Our results showed increase in thyroxine and myostatin concentrations with TES/CR and CT/GR of our study groups, respectively. The relative mRNA expression levels of the GH, IGF-1, and Fas cell surface death receptor (FAS) genes were significantly (P ≤ 0.05 and 0.01) upregulated in the liver tissue of the TEM/GR group compared with the other groups. In conclusion, TEM/GR was the best treatment for improving prehatch quality, increasing serum antioxidant enzyme activities, and promoting the expression of growth and immune genes in fertilized quail eggs.

Role of p53 in Regulating Radiation Responses

p53 is known as the guardian of the genome and plays various roles in DNA damage and cancer suppression. The p53 gene was found to express multiple p53 splice variants (isoforms) in a physiological, tissue-dependent manner. The various genes that up- and down-regulated p53 are involved in cell viability, senescence, inflammation, and carcinogenesis. Moreover, p53 affects the radioadaptive response. Given that several studies have already been published on p53, this review presents its role in the response to gamma irradiation by interacting with MDM2, NF-κB, and miRNA, as well as in the inflammation processes, senescence, carcinogenesis, and radiation adaptive responses. Finally, the potential of p53 as a biomarker is discussed.

Low-Dose Gamma Radiation Modulates Liver and Testis Tissues Response to Acute Whole Body Irradiation

Aim

This work aims to investigate whether the pre-exposure to low dose/low dose rate (40 mGy, 2.2 mGy/hour) γ-radiation as a priming dose can produce a protective effect against the subsequent high one (4 Gy, .425 Gy/minute).

Methods

Rats were divided into Group I (control), Group II (L); exposed to 40 mGy, Group III (H); exposed to 4 Gy, and Group IV (L+H); exposed to 40 mGy 24 hours before the exposure to 4Gy. The molecular and biochemical changes related to oxidative stress, DNA damage, apoptosis, and mitochondrial activity in the liver and testis were studied 4 hours after irradiation.

Results

Exposure to 40 mGy before 4 Gy induced a significant increase in the levels of Nrf2, Nrf2 mRNA, TAC, and mitochondrial complexes I & II accompanied by a significant decrease in the levels of LPO, 8-OHdG, DNA fragmentation, TNF-α, caspase-3, and caspase-3 mRNA compared with H group.

Conclusion

Exposure to low-dose γ-radiation before a high dose provides protective mechanisms that allow the body to survive better after exposure to a subsequent high one via reducing the oxidative stress, DNA damage, and apoptosis-induced early after irradiation. However, further studies are required to identify the long-term effects of this low dose.

The paradox of adaptive responses and iso-effect per fraction

PURPOSE

Despite decades of research into radiation-induced adaptive responses, where prior irradiation changes the response to subsequent irradiations, the field of radiation oncology relies upon models of tumor control that assume that each radiation therapy fraction reproduces the same effect, known as iso-effect per fraction. Can these radiobiology principles both be true, forming a paradox or is only one of them right? Here, the apparent coexistence of these two contradictory observations is considered, examining how adaptive responses might apply in radiotherapy scenarios that are inconsistent with the majority of adaptive response experimental designs.

CONCLUSION

While the iso-effect per fraction assumption would preclude the observation of adaptive responses for cells survival after radiotherapy fractions, this does not preclude the observation of adaptive responses for other endpoints. Adaptive responses for cell survival might also manifest without invalidating the iso-effect principle in practical terms. It may also be the case that instances of both phenomena can be observed under different conditions, but not at the same time.

Novel effect of Daflon and low-dose γ-radiation in modulation of thioacetamide-induced hepatic encephalopathy in male albino rats

This study was designed to evaluate the hepato and neuroprotective activity of Daflon and low-dose γ radiation on thioacetamide (TAA)-induced liver damage and hepatic encephalopathy (HE) in rats. Effect of daily Daflon treatment (100 mg/kg body weight, Per OS (p.o.) for consecutive 3 days) and/or fractionated low-dose γ-radiation (LDR; 0.25 Gy, twice the total dose of 0.5 Gy at the 1st and 3rd day, respectively) was evaluated against TAA (300 mg/kg, intraperitoneal × 3) induced liver damage and HE in rats. Serum aspartate transaminase, alanine transaminase, γ-glutamyltransferase, total bilirubin, ammonia, and manganese were estimated to evaluate liver function. In addition, malondialdehyde (MDA) as well as reduced glutathione (GSH), glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT) were determined to assess antioxidant capacity in liver tissue. Moreover, hepatic apoptotic markers (cysteine-dependent aspartate-directed proteases 3, 8 (caspase-3, 8) and cytochrome C) were estimated to indicate hepatic apoptosis. HE was evaluated through the determination of whole brain ammonia, manganese, MDA, GSH, GPX, SOD, CAT, and caspase-3. The cognitive and locomotor deficits were assessed via step through passive avoidance test, activity cage (actophotometer), γ-aminobutyric acid, and N-methyl-d-aspartate/adenosine triphosphate-neuronal nitric oxide synthase/nitric oxide-cyclic guanosine monophosphate axis in rats' cerebella and hippocampi. The involvement of hypoxia inducible factor-1α, aquaporine-4, and matrix metalloproteinase 9 in association with the brain water content (%) in the whole brain as an index for brain edema was also evaluated. The obtained results showed a marked amelioration of the aforementioned biochemical parameters and behavioral tasks which is supported by histopathological and immunohistochemical examination. It could be concluded that Daflon and LDR afforded hepatoprotection and neuroprotection against TAA-induced acute liver damage and HE.

Gene profiling characteristics of radioadaptive response in AG01522 normal human fibroblasts

Radioadaptive response (RAR) in mammalian cells refers to the phenomenon where a low-dose ionizing irradiation alters the gene expression profiles, and protects the cells from the detrimental effects of a subsequent high dose exposure. Despite the completion of numerous experimental studies on RAR, the underlying mechanism has remained unclear. In this study, we aimed to have a comprehensive investigation on the RAR induced in the AG01522 human fibroblasts first exposed to 5 cGy (priming dose) and then followed by 2 Gy (challenge dose) of X-ray through comparisons to those cells that had only received a single 2 Gy dose. We studied how the priming dose affected the expression of gene transcripts, and to identify transcripts or pathways that were associated with the reduced chromosomal damages (in terms of the number of micronuclei) after application of the challenging dose. Through the mRNA and microRNA microarray analyses, the transcriptome alteration in AG01522 cells was examined, and the significantly altered genes were identified for different irradiation procedures using bioinformatics approaches. We observed that a low-dose X-ray exposure produced an alert, triggering and altering cellular responses to defend against subsequent high dose-induced damages, and accelerating the cell repair process. Moreover, the p53 signaling pathway was found to play critial roles in regulating DNA damage responses at the early stage after application of the challenging dose, particularly in the RAR group. Furthermore, microRNA analyses also revealed that cell communication and intercellular signaling transduction played important roles after low-dose irradiation. We conclude that RAR benefits from the alarm mechanisms triggered by a low-dose priming radation dose.

Bystander effect between zebrafish embryos in vivo induced by high-dose X-rays

We employed embryos of the zebrafish, Danio rerio, for our studies on the in vivo bystander effect between embryos irradiated with high-dose X-rays and naive unirradiated embryos. The effects on the naive whole embryos were studied through quantification of apoptotic signals at 25 h post fertilization (hpf) through the terminal dUTP transferase-mediated nick end-labeling (TUNEL) assay followed by counting the stained cells under a microscope. We report data showing that embryos at 5 hpf subjected to a 4-Gy X-ray irradiation could release a stress signal into the medium, which could induce a bystander effect in partnered naive embryos sharing the same medium. We further demonstrated that this bystander effect (induced through partnering) could be successfully suppressed through the addition of the nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) into the medium but not through the addition of the CO liberator tricarbonylchloro(glycinato)ruthenium(II) (CORM-3). This shows that NO was involved in the bystander response between zebrafish embryos induced through X-ray irradiation. We also report data showing that the bystander effect could be successfully induced in naive embryos by introducing them into the irradiated embryo conditioned medium (IECM) alone, i.e., without partnering with the irradiated embryos. The IECM was harvested from the medium that had conditioned the zebrafish embryos irradiated at 5 hpf with 4-Gy X-ray until the irradiated embryos developed into 29 hpf. NO released from the irradiated embryos was unlikely to be involved in the bystander effect induced through the IECM because of the short life of NO. We further revealed that this bystander effect (induced through IECM) was rapidly abolished through diluting the IECM by a factor of 2× or greater, which agreed with the proposal that the bystander effect was an on/off response with a threshold.

Adaptive response to ionising radiation induced by cadmium in zebrafish embryos

An adaptive response is a biological response where the exposure of cells or animals to a low priming exposure induces mechanisms that protect the cells or animals against the detrimental effects of a subsequent larger challenging exposure. In realistic environmental situations, living organisms can be exposed to a mixture of stressors, and the resultant effects due to such exposures are referred to as multiple stressor effects. In the present work we demonstrated, via quantification of apoptosis in the embryos, that embryos of the zebrafish (Danio rerio) subjected to a priming exposure provided by one environmental stressor (cadmium in micromolar concentrations) could undergo an adaptive response against a subsequent challenging exposure provided by another environmental stressor (alpha particles). We concluded that zebrafish embryos treated with 1 to 10 μM Cd at 5 h postfertilisation (hpf) for both 1 and 5 h could undergo an adaptive response against subsequent ~4.4 mGy alpha-particle irradiation at 10 hpf, which could be interpreted as an antagonistic multiple stressor effect between Cd and ionising radiation. The zebrafish has become a popular vertebrate model for studying the in vivo response to ionising radiation. As such, our results suggested that multiple stressor effects should be carefully considered for human radiation risk assessment since the risk may be perturbed by another environmental stressor such as a heavy metal.

Hormetic effect induced by alpha-particle-induced stress communicated in vivo between zebrafish embryos

We report data showing that embryos of the zebrafish, Danio rerio, at 1.5 h post fertilization (hpf) subjected to a low-dose alpha-particle irradiation can release a stress signal into the water, which can be communicated to unirradiated bystander zebrafish embryos sharing the same water medium to induce a hormetic effect in the bystander embryos. Hormetic responses are characterized as biphasic dose-response relationships exhibiting a low-dose stimulation and a high-dose inhibition. The effects on the whole embryos were studied through quantification of apoptotic signals at 24 hpf through staining with the vital dye acridine orange, followed by counting the stained cells under a microscope. The results show that, for low alpha-particle dose, the number of apoptotic signals decreases in the irradiated embryos and also in the unirradiated bystander embryos having partnered with the irradiated embryos. These suggested that alpha-particle-irradiated zebrafish embryos could release a stress signal into the water, which could be communicated to unirradiated bystander zebrafish embryos sharing the same water medium to induce a hormetic effect in the bystander embryos.

Alpha radiation exposure decreases apoptotic cells in zebrafish embryos subsequently exposed to the chemical stressor, Cd

The aim of this study was to demonstrate that zebrafish embryos subjected to a priming exposure provided by one environmental stressor (low-dose alpha particles) can induce an adaptive response against a subsequent challenging exposure provided by another environmental stressor (heavy metal Cd). The effect thus identified would be an antagonistic multiple stressor effect. The effects of alpha particle radiation and/or Cd on whole embryos were studied through quantification of apoptotic signals at 24 h post-fertilization (hpf). Embryos were stained with the vital dye acridine orange, followed by counting the stained cells. For each set of experiments, 30 dechorionated embryos were divided into three groups, each having ten embryos. The three groups of embryos were referred to as (A) the control group, which received no more further treatments after dechorionation, (B) Cd-treated group, which did not receive any priming exposure and would receive a challenging exposure at 10 hpf and (C) (alpha + Cd)-treated group, which would receive both priming and challenging exposures. We defined the normalized net number of apoptotic signals in the (alpha + Cd)-treated group as N (C) * = [(apoptotic signals for (alpha + Cd)-treated group - average apoptotic signals for the corresponding control group)/average apoptotic signals for the corresponding control group] and that in the Cd-treated group as N (B)* = [(apoptotic signals for Cd-treated group - average apoptotic signals for the corresponding control group)/ average apoptotic signals for the corresponding control group]. By using the non-parametric Mann-Whitney U statistic, we were able to show that N (C) * was significantly smaller than N (B) *(p = 0.006). These demonstrated an antagonistic multiple stressor effect between ionizing radiation and Cd through the induction of an adaptive response by the ionizing radiation against subsequent exposures to Cd.

Adaptive response of low linear energy transfer X-rays for protection against high linear energy transfer accelerated heavy ion-induced teratogenesis

BACKGROUND

Adaptive response (AR) of low linear energy transfer (LET) irradiations for protection against teratogenesis induced by high LET irradiations is not well documented. In this study, induction of AR by X-rays against teratogenesis induced by accelerated heavy ions was examined in fetal mice.

METHODS

Irradiations of pregnant C57BL/6J mice were performed by delivering a priming low dose from X-rays at 0.05 or 0.30 Gy on gestation day 11 followed one day later by a challenge high dose from either X-rays or accelerated heavy ions. Monoenergetic beams of carbon, neon, silicon, and iron with the LET values of about 15, 30, 55, and 200 keV/μm, respectively, were examined. Significant suppression of teratogenic effects (fetal death, malformation of live fetuses, or low body weight) was used as the endpoint for judgment of a successful AR induction.

RESULTS

Existence of AR induced by low-LET X-rays against teratogenic effect induced by high-LET accelerated heavy ions was demonstrated. The priming low dose of X-rays significantly reduced the occurrence of prenatal fetal death, malformation, and/or low body weight induced by the challenge high dose from either X-rays or accelerated heavy ions of carbon, neon or silicon but not iron particles.

CONCLUSIONS

Successful AR induction appears to be a radiation quality event, depending on the LET value and/or the particle species of the challenge irradiations. These findings would provide a new insight into the study on radiation-induced AR in utero.

Effects of different gamma exposure regimes on reproduction in the earthworm Eisenia fetida (Oligochaeta)

Ecological risk assessment of ionising radiation requires knowledge about the responses of individuals and populations to chronic exposures, including situations when exposure levels change over time. The present study investigated processes such as recovery and the adaptive response with respect to reproduction endpoints in the earthworm Eisenia fetida exposed to (60)Co γ-radiation. Furthermore, a crossed experiment was performed to investigate the influence of F0 parental and F1 embryonic irradiation history on the response of irradiated or non-irradiated F1 offspring. Recovery: The sterility induced by sub-chronic exposure at 17 m Gy/h (accumulated dose: 25 Gy) was temporary, and 8 weeks after irradiation the worms had regained their reproductive capacity (number of viable offspring produced per adult per week). Adaptive response: Adult worms were continuously exposed at a low priming dose rate of 0.14 mGy/h for 12 weeks (accumulated dose: 0.24 Gy), followed by 14 weeks exposure at a challenge dose rate of 11 mGy/h. The results suggest a lack of adaptive response, since there were no significant differences in the effects on reproduction capacity between the primed and the unprimed groups after challenge doses ranging from 7.6 to 27 Gy. Crossed experiment: The effects of exposure at 11 mGy/h for 21 weeks on growth, sexual maturation and reproduction of offspring, derived either from parent worms and cocoons both exposed at 11 mGy/h, or from non-irradiated parents and cocoons (total accumulated dose 44 and 38 Gy, respectively) were compared. There were no significant differences between the two exposed offspring groups for any of the endpoints. The reproduction capacity was very low for both groups compared to the controls, but the reproduction seemed to be maintained at the reduced level, which could indicate acclimatisation or stabilisation. Finally, parental and embryonic exposures at 11 mGy/h did not affect reproduction in the F1 offspring as adults.

Radioadaptive response induced by alpha-particle-induced stress communicated in vivo between zebrafish embryos

We report data demonstrating that zebrafish embryos irradiated by alpha particles can release a stress signal into the water, which can be communicated to the unirradiated zebrafish embryos sharing the same water medium and thereby inducing a radioadaptive response in these unirradiated zebrafish embryos. The effects of radiation on the whole embryos were studied through quantification of apoptotic signals at 24 h post fertilization through staining with the vital dye acridine orange, followed by counting the stained cells under a microscope. In these experiments, dechorionated embryos were irradiated and then partnered with two other groups of unirradiated embryos, namely the bystander group (no more further treatments) and adaptive group (subjected to a further challenging dose) of embryos. The adaptive group of embryos were then separately further irradiated with a challenging dose. The results show that the number of apoptotic signals for the adaptive group is smaller than that for the corresponding control group, while that for the bystander group is larger than that for the corresponding control group. These suggest that the stress communicated in vivo between the irradiated zebrafish embryos and those unirradiated embryos sharing the same medium will induce radioadaptive response in the unirradiated embryos.

Immunohistochemical study of postnatal neurogenesis after whole-body exposure to electromagnetic fields: evaluation of age- and dose-related changes in rats

It is well established that strong electromagnetic fields (EMFs) can give rise to acute health effects, such as burns, which can be effectively prevented by respecting exposure guidelines and regulations. Current concerns are instead directed toward the possibility that long-term exposure to weak EMF might have detrimental health effects due to some biological mechanism, to date unknown. (1) The possible risk due to pulsed EMF at frequency 2.45 GHz and mean power density 2.8 mW/cm(2) on rat postnatal neurogenesis was studied in relation to the animal's age, duration of the exposure dose, and post-irradiation survival. (2) Proliferating cells marker, BrdU, was used to map age- and dose-related immunohistochemical changes within the rostral migratory stream (RMS) after whole-body exposure of newborn (P7) and senescent (24 months) rats. (3) Two dose-related exposure patterns were performed to clarify the cumulative effect of EMF: short-term exposure dose, 2 days irradiation (4 h/day), versus long-term exposure dose, 3 days irradiation (8 h/day), both followed by acute (24 h) and chronic (1-4 weeks) post-irradiation survival. (4) We found that the EMF induces significant age- and dose-dependent changes in proliferating cell numbers within the RMS. Our results indicate that the concerns about the possible risk of EMF generated in connection with production, transmission, distribution, and the use of electrical equipment and communication sets are justified at least with regard to early postnatal neurogenesis.

Low dose gamma-irradiation differentially modulates antioxidant defense in liver and lungs of Balb/c mice

PURPOSE

To evaluate the effect of low-dose (<50 cGy) whole body ?-irradiation on the antioxidant defense system in the liver and the lungs of mice at various post-irradiation intervals.

MATERIALS AND METHODS

Male Balb/c mice, 5 - 6 weeks of age, were divided into irradiated and non-irradiated groups. Whole body irradiation was done with gamma-rays from a (60)Co source at doses of 10, 25 and 50 cGy (48.78 cGy/min). Lipid peroxidation and antioxidant status were measured in the liver and the lungs at 4, 12 and 24 h after irradiation.

RESULTS

Lipid peroxidation increased by 1.38 and 2.0 fold in lung and liver respectively at 12 h after exposure to 25 cGy. Whole body exposure to 25 and 50 cGy significantly (p < 0.05) increased the hepatic reduced glutathione at 4 h. Reduced glutathione continued to rise until 12 h and returned to the basal level at 24 h, whereas in the lungs it remained elevated until 24 h at 10 and 25 cGy. Antioxidant enzymes activities for superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase increased by 1.22, 1.13, 1.22 and 1.11 fold respectively (p < 0.05) in the liver at 4 h after exposure to 50 cGy and remained elevated at almost the same level up to 12 h after exposure. Surprisingly these antioxidant defense enzymes remained unaltered in the lung at the above radiation doses.

CONCLUSIONS

Low-dose whole body gamma-irradiation differentially modulates the antioxidant defense system in the liver and lungs of mice. The induction of endogenous glutathione, immediately after exposure to low-dose -irradiation, may be beneficial in protecting the cells from reactive oxygen species (ROS) induced oxidative stress.

Adaptive response in embryogenesis: vi. Comparative microarray analysis of gene expressions in mouse fetuses

PURPOSE

Exposure of sublethal doses of ionizing radiation can induce protective mechanisms against a subsequent higher dose irradiation. This phenomenon, called radiation-induced adaptive response (AR), has been described in a wide range of biological models. We previously demonstrated the existence of AR in mice during late organogenesis. In this study, we investigated molecular mechanisms underlying AR in this model.

MATERIALS AND METHODS

Using DNA microarrays, we performed a global analysis of transcriptome regulations in adapted and non-adapted cells collected from whole mouse fetuses, after in utero exposure to priming irradiation.

RESULTS

We identified AR-specific gene modulations. Our results suggested the involvement of signal transduction and Tumor protein (p53)-related pathways in the induction of AR.

CONCLUSIONS

Our results are in agreement with previous investigations showing that AR could be dependant on p53 activity. The observed gene modulations may also have possible consequences for subsequent developmental process of the fetus. This is the first report of AR-specific modulations at the molecular level in utero, which could serve as a basis for subsequent studies aimed at understanding AR in this model and possible long-term effects.

Rapid myeloid recovery as a possible mechanism of whole-body radioadaptive response

We investigated the mechanism underlying the radioadaptive response that rescues mice from hematopoietic failure. C57BL/6 mice were irradiated with low-dose acute X rays (0.5 Gy) for priming 2 weeks prior to a high-dose (6 Gy) challenge irradiation. Bone marrow cells, erythrocytes and platelets in low-dose-preirradiated mice showed earlier recovery after the challenge irradiation than those in mice subjected only to the challenge irradiation. This suggests that hematopoiesis is enhanced after a challenge irradiation in preirradiated mice. The rapid recovery of bone marrow cells after the challenge irradiation was consistent with the proliferation of hematopoietic progenitors expressing the cell surface markers Lin-, Sca-1- and c-Kit+ in low-dose-preirradiated mice. A subpopulation of myeloid (Mac-1+/Gr-1+) cells, which were descendants of Lin-, Sca-1- and c-Kit+ cells, rapidly recovered in the bone marrow of low-dose-preirradiated mice, whereas the number of B-lymphoid (CD19+/B220+) cells did not show a statistically significant increase. Plasma cytokine profiles were analyzed using antibody arrays, and results indicated that the concentrations of several growth factors for myelopoiesis after the challenge irradiation were considerably increased by low-dose preirradiation. The rapid recovery of erythrocytes and platelets but not leukocytes was observed in the peripheral blood of preirradiated mice, suggesting that low-dose preirradiation triggered the differentiation to myelopoiesis. Thus the adaptive response induced by low-dose preirradiation in terms of the recovery kinetics of the number of hematopoietic cells may be due to the rapid recovery of the number of myeloid cells after high-dose irradiation.

Lack of adaptive response of gamma radiation for protection against neutron-induced teratogenesis

BACKGROUND

Although there are some reports on neutron teratology, there is little information on the adaptive response of gamma radiation for protection against neutron-induced teratogenesis. This study examined whether or not a low dose of gamma radiation can induce an adaptive response in mouse fetuses exposed to a subsequent dose of neutrons in vivo.

METHODS

Pregnant ICR mice were exposed to a priming dose of 0.3 Gy (0.9 Gy/min) of gamma rays on day 10.5 of gestation and challenged with 0.8 Gy (0.94 Gy/minute) of neutrons 24 h later. The mice were sacrificed on day 18.5 of gestation. The fetuses were examined for mortality, growth retardation, and other morphologic abnormalities.

RESULTS

The tail length in the 0.3 Gy of gamma rays + 0.8 Gy of neutrons group was significantly shorter than in the 0.8 Gy of neutrons group. Although there was no significant difference compared with the 0.8 Gy of neutrons group, the number of live fetuses in the 0.3 Gy of gamma rays +0.8 Gy of neutrons group was lower. There was no evidence of primed exposure-related reductions in the malformed fetuses. Although there was no significant difference compared with the unprimed group, the number of malformed offspring in the primed group was higher. Furthermore, the incidence of kinked tail and adactyly was significantly higher in the primed mice than in the unprimed mice.

CONCLUSIONS

Overall, this study shows that exposure to 0.3 Gy of gamma rays failed to induce an adaptive response of fetogenesis to a neutron challenge dose.

Adaptive response for chromosomal inversions in pKZ1 mouse prostate induced by low doses of X radiation delivered after a high dose

Adaptive responses are induced by stress such as X radiation and result in a lower than expected biological response. Two-dose adaptive response experiments typically involve a low priming dose followed by a subsequent high radiation dose. Here, we used a sensitive in vivo chromosomal inversion assay to demonstrate for the first time an adaptive response when a low dose (0.01-1 mGy) was given several hours after a high 1000-mGy radiation dose. The adaptive responses in this study were of similar magnitude to the two-dose adaptive responses previously observed in this test system when the low dose was given first. A chromosomal inversion adaptive response was also induced by two 1000-mGy doses and when a 1-mGy dose was preceded or followed by a dose of 0.01 mGy, but not by two 4000-mGy doses. This is also the first example of an adaptive response when both doses are low. Our data agree with previous reports of an on-off mechanism of adaptive response. The induction of an adaptive response by a low dose after a high damaging dose provides evidence that the mechanisms underlying radiation adaptive responses are not due to prevention of damage induced by the high dose but to modulation of the cellular response to this damage.

Effect of paternal exposure to gamma rays on juvenile rat forebrain

We studied the transgenerational effect of the sublethal dose of gamma radiation on the proliferative activity of cells in the rostral migratory stream (RMS) in the brain of neonatal and young progeny of male rats exposed to the dose of 3 Gy 25 days before conception with intact control females. To label proliferating cells, the progeny received bromodeoxyuridine (BrdU) injection before sacrifice. The number of BrdU-positive cells was counted in three parts of the forebrain RMS, i.e. in the vertical arm, elbow and horizontal arm, at the 3rd, 7th, 14th, 21st and 28th postnatal days (P3-P28). In the RMS of control rats' progeny, the number of BrdU(+) cells transiently decreased at the 7th-14th postnatal days. In the progeny of irradiated rats, the dynamics of the changes was similar to that in the corresponding control groups, however, the number of BrdU-positive cells was significantly higher along the whole RMS at all intervals of investigation. These results suggest that paternal exposure to ionizing radiation induced the genome instability, which manifested itself in the progeny by alteration in proliferative activity or slackening of cell migration in the RMS.

Reduction of background mutations by low-dose X irradiation of Drosophila spermatocytes at a low dose rate

A sex-linked recessive lethal mutation assay was performed in Drosophila melanogaster using immature spermatocytes and spermatogonia irradiated with X rays at a high or low dose rate. The mutation frequency in the sperm irradiated with a low dose at a low dose rate was significantly lower than that in the sham-irradiated group, whereas irradiation with a high dose resulted in a significant increase in the mutation frequency. It was obvious that the dose-response relationship was not linear, but rather was U-shaped. When mutant germ cells defective in DNA excision repair were used instead of wild-type cells, low-dose irradiation at a low dose rate did not reduce the mutation frequency. These observations suggest that error-free DNA repair functions were activated by low dose of low-dose-rate radiation and that this repaired spontaneous DNA damage rather than the X-ray-induced damage, thus producing a practical threshold.

Whole body exposure to low-dose gamma radiation promotes kidney antioxidant status in Balb/c mice

We examined the effect of whole body low-dose gamma-irradiation on the status of the antioxidant defense system in the rodent kidneys at different time intervals. Young male Balb/c mice were exposed to whole body radiation from a (60)Co source at doses of 10, 25 and 50 cGy (48.78 cGy/min). Antioxidant status and lipid peroxidation were estimated in the kidneys at 4, 12 and 24 h after irradiation. Lipid peroxidation increased between 33% and 49% and reduced glutathione between 12% and 47% at 12 h at different radiation doses. Reduced glutathione level remained significantly (p < 0.05) elevated even at 24 h after irradiation to 25 cGy. Superoxide dismutase activity also increased by 37% at 12 h on exposure of animals to all the doses up to 50 cGy. Catalase activity increased significantly at 12 h on exposure to 10 cGy and 50 cGy. Interestingly, glutathione peroxidase activity increased by 31% at 4 h and subsequently returned to control levels at 24 h after exposure to 50 cGy. Glutathione reductase activity increased by 10-12% at 12 h after exposure to 25 cGy and 50 cGy. The results suggest that the whole body exposure of animals to gamma radiation stimulates the antioxidant defense system in the kidneys within 4 to 24 h after irradiation, at doses of 25 cGy and 50 cGy.

TP53 and TP53-Related Genes Associated with Protection from Apoptosis in the Radioadaptive Response

We investigated the effect of administering priming low-dose radiation prior to high-dose radiation on the level of apoptosis and on the expression of TP53 and TP53-related genes in mouse splenocytes. The percentage of apoptotic cells was significantly lower in TP53(+/+) mice receiving priming radiation 2 to 168 h before the high-dose irradiation, compared to TP53(+/+) mice exposed to 2 Gy alone. In contrast, TP53(+/-) mice exhibited a reduced level of apoptosis only when priming was performed for 2 or 4 h prior to the high-dose irradiation. In TP53(+/+) mice, primed mice had higher TP53 expression than mice exposed to 2 Gy. Phospho-TP53 (ser15/18) expression was the highest in mice exposed to 2 Gy and intermediate in primed mice. Expression of p21 (CDKN1A) was higher in primed mice compared with mice exposed to 2 Gy. MDM2 expression remained at a high level in all mice receiving 2 Gy. Elevated phospho-ATM expression was observed only in mice exposed to 2 Gy. We conclude that TP53 plays a critical role in the radioadaptive response and that TP53 and TP53-related genes might protect cells from apoptosis through activation of the intracellular repair system.

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.

Radioadaptive response for protection against radiation-induced teratogenesis

To clarify the characteristics of the radioadaptive response in mice, we compared the incidence of radiation-induced malformations in ICR mice. Pregnant ICR mice were exposed to a priming dose of 2 cGy (667 muGy/min) on day 9.5 of gestation and to a challenging dose of 2 Gy (1.04 Gy/min) 4 h later and were killed on day 18.5 of gestation. The incidence of malformations and prenatal death and fetal body weights were studied. The incidence of external malformations was significantly lower (by approximately 10%) in the primed (2 cGy + 2 Gy) mice compared to the unprimed (2 Gy alone) mice. However, there were no differences in the incidence of prenatal death or the skeletal malformations or the body weights between primed and unprimed mice. These results suggest that primary conditioning with low doses of radiation suppresses radiation-induced teratogenesis.

Nonlinear response for neoplastic transformation following low doses of low let radiation

There are now several independent studies that indicate that the dose-response for the endpoint of radiation-induced neoplastic transformation in vitro is non-linear for low linear energy transfer (LET) radiation. At low doses (<10 cGy) the transformation frequency drops below that seen spontaneously. Importantly, this observation has been made using fluoroscopic energy x-rays, a commonly used modality in diagnostic radiology, the practice of which is responsible for the majority of radiation exposure to the general public. Since the transformation frequency is reduced over a large dose range (0.1 to 10cGy) it is likely that multiple mechanisms are involved and that the relative contribution of these may vary with dose. These include the killing of a subpopulation of cells prone to spontaneous transformation at the lowest doses, and the induction of DNA repair at somewhat higher doses. Protective effects of low doses of low LET radiation on other cancer-relevant endpoints in vitro and in vivo have also been observed by several independent laboratories. These observations strongly suggest that the linear-nonthreshold dose-response model is unlikely to apply to the induction of cancer by low doses of low LET radiation in humans.