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

Spontaneous p53 activation in middle-aged C57BL/6 mice mitigates the lifespan-extending adaptive response induced by low-dose ionizing radiation

Understanding the biological effects of low-dose (<100 mGy) ionizing radiation (LDR) is technically challenging. We investigated age-dependent LDR effects using adaptive response experiments in young (7-to 12-week-old) and middle-aged (40-to 62-week-old) C57BL/6 mice. Compared with 3 Gy irradiation, 0.02 Gy preirradiation followed by 3 Gy irradiation prolonged life in young mice but not middle-aged mice. Preirradiation also suppressed irradiation-induced 53BP1 repair foci in the small intestines, splenic apoptosis, and p53 activity in young mice but not middle-aged mice. Young p53+/- C57BL/6 mice did not show these adaptive responses, indicating that insufficient p53 function in young mice mitigated the adaptive responses. Interestingly, p53 activation in middle-aged mice spontaneously became approximately 4.5-fold greater than that in young mice, possibly masking LDR stresses. Furthermore, adaptive responses in young mice, but not in middle-aged mice, suppressed some senescence-associated secretory phenotype (SASP) factors (IL-6, CCL2, CCL5, CXCL1). Thus, LDR-induced adaptive responses associated with specific SASP factors may be attenuated by a combination of reduced DNA damage sensor/transducer function and chronic p53 activation in middle-aged mice.

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.

Differential molecular response in mice and human thymocytes exposed to a combined-dose radiation regime

In the quest for more effective radiation treatment options that can improve both cell killing and healthy tissue recovery, combined radiation therapies are lately in the spotlight. The molecular response to a combined radiation regime where exposure to an initial low dose (priming dose) of ionizing radiation is administered prior to a subsequent higher radiation dose (challenging dose) after a given latency period have not been thoroughly explored. In this study we report on the differential response to either a combined radiation regime or a single challenging dose both in mouse in vivo and in human ex vivo thymocytes. A differential cell cycle response including an increase in the subG1 fraction on cells exposed to the combined regime was found. Together with this, a differential protein expression profiling in several pathways including cell cycle control (ATM, TP53, p21^CDKN1A), damage response (γH2AX) and cell death pathways such as apoptosis (Cleaved Caspase-3, PARP1, PKCδ and H3T45ph) and ferroptosis (xCT/GPX4) was demonstrated. This study also shows the epigenetic regulation following a combined regime that alters the expression of chromatin modifiers such as DNMTs (DNMT1, DNMT2, DNMT3A, DNMT3B, DNMT3L) and glycosylases (MBD4 and TDG). Furthermore, a study of the underlying cellular status six hours after the priming dose alone showed evidence of retained modifications on the molecular and epigenetic pathways suggesting that the priming dose infers a “radiation awareness phenotype” to the thymocytes, a sensitization key to the differential response seen after the second hit with the challenging dose. These data suggest that combined-dose radiation regimes could be more efficient at making cells respond to radiation and it would be interesting to further investigate how can these schemes be of use to potential new radiation therapies.

Comparison of the fertility of tumor suppressor gene-deficient C57BL/6 mouse strains reveals stable reproductive aging and novel pleiotropic gene

Tumor suppressor genes are involved in maintaining genome integrity during reproduction (e.g., meiosis). Thus, deleterious alleles in tumor suppressor-deficient mice would exhibit higher mortality during the perinatal period. A recent aging model proposes that perinatal mortality and age-related deleterious changes might define lifespan. This study aimed to quantitatively understand the relationship between reproduction and lifespan using three established tumor suppressor gene (p53, APC, and RECQL4)-deficient mouse strains with the same C57BL/6 background. Transgenic mice delivered slightly reduced numbers of 1st pups than wild-type mice [ratio: 0.81–0.93 (p = 0.1–0.61)] during a similar delivery period, which suggest that the tumor suppressor gene-deficient mice undergo relatively stable reproduction. However, the transgenic 1st pups died within a few days after birth, resulting in a further reduction in litter size at 3 weeks after delivery compared with that of wild-type mice [ratio: 0.35–0.68 (p = 0.034–0.24)] without sex differences, although the lifespan was variable. Unexpectedly, the significance of reproductive reduction in transgenic mice was decreased at the 2nd or later delivery. Because mice are easily affected by environmental factors, our data underscore the importance of defining reproductive ability through experiments on aging-related reproduction that can reveal a trade-off between fecundity and aging and identify RECQL4 as a novel pleiotropic gene.

The Phenotypic and Transcriptomic Response of the Caenorhabditis elegans Nematode to Background and Below-Background Radiation Levels

Studies of the biological effects of low-level and below-background radiation are important in understanding the potential effects of radiation exposure in humans. To study this issue we exposed the nematode Caenorhabditis elegans to average background and below-background radiation levels. Two experiments were carried-out in the underground radiation biology laboratory at the Waste Isolation Pilot Plant (WIPP) in New Mexico USA. The first experiment used naïve nematodes with data collected within 1 week of being placed underground. The second experiment used worms that were incubated for 8 months underground at below background radiation levels. Nematode eggs were placed in two incubators, one at low radiation (ca.15.6 nGy/hr) and one supplemented with 2 kg of natural KCl (ca. 67.4 nGy/hr). Phenotypic variables measured were: (1) egg hatching success (2) body size from larval development to adulthood, (3) developmental time from egg to egg laying adult, and (4) egg laying rate of young adult worms. Transcriptome analysis was performed on the first experiment on 72 h old adult worms. Within 72 h of being underground, there was a trend of increased egg-laying rate in the below-background radiation treatment. This trend became statistically significant in the group of worms exposed to below-background radiation for 8 months. Worms raised for 8 months in these shielded conditions also had significantly faster growth rates during larval development. Transcriptome analyses of 72-h old naïve nematode RNA showed significant differential expression of genes coding for sperm-related proteins and collagen production. In the below-background radiation group, the genes for major sperm protein (msp, 42% of total genes) and sperm-related proteins (7.5%) represented 49.5% of the total genes significantly up-regulated, while the majority of down-regulated genes were collagen (col, 37%) or cuticle-related (28%) genes. RT-qPCR analysis of target genes confirmed transcriptomic data. These results demonstrate that exposure to below-background radiation rapidly induces phenotypic and transcriptomic changes in C. elegans within 72 h of being brought underground.

Immunological mechanism of low-dose priming radiation resistance in walker-256 tumor model mice

The aim of the present study was to investigate whether low-dose priming radiation induces antitumor immunity that can be augmented by the modulation of natural killer (NK) cell and cytokine activity using a mouse tumor model. Walker-256 cells were injected into the right flank of male BALB/c mice. At 7 days after inoculation, mice were divided into three groups, including group 1,2,3. In group 1 the mice were without radiation, in group 2 the mice were received 2 Gy radiation only, and in group 3 the mice were radiated with a priming dose of 75 mGy followed by 2 Gy radiation after 24 h. On day 21 following the radiation, the tumors were removed and the tumor index (tumor weight as a percentage of body weight) was calculated. At 1, 7, 14 and 21 days following the 2 Gy radiation, mouse splenocytes were isolated to analyze the NK activity and measure the production of the cytokines interleukin-1β, interferon-γ and tumor necrosis factor-α by ELISA. Apoptosis was also measured by flow cytometry. The results demonstrated that priming radiation significantly delayed the tumor growth and prolonged the median survival time to 38 days compared with the 31-day survival in the 2 Gy radiation group. The percentage of apoptotic cells was significantly higher in the mice that received 75 mGy + 2 Gy radiation compared with that in the mice that received 2 Gy alone; by contrast, mice that were not irradiated exhibited a relatively low level of apoptosis. The primed mice had a higher level of NK activity as compared with the mice exposed to 2 Gy radiation only or mice that were not irradiated. Furthermore, cytokine expression remained at a higher level in mice receiving priming dose of radiation compared that in the mice receiving only 2 Gy radiation. In conclusion, the results indicated that low-dose priming X-ray radiation may enhance the NK activity and the levels of cytokines, and that the immune response serves an important role in anticancer therapy, including radiotherapy.

Parthenolide Selectively Sensitizes Prostate Tumor Tissue to Radiotherapy while Protecting Healthy Tissues In Vivo

Radiotherapy is widely used in cancer treatment, however the benefits can be limited by radiation-induced damage to neighboring normal tissues. Parthenolide (PTL) exhibits anti-inflammatory and anti-tumor properties and selectively induces radiosensitivity in prostate cancer cell lines, while protecting primary prostate epithelial cell lines from radiation-induced damage. Low doses of radiation have also been shown to protect from subsequent high-dose-radiation-induced apoptosis as well as DNA damage. These properties of PTL and low-dose radiation could be used to improve radiotherapy by killing more tumor cells and less normal cells. Sixteen-week-old male Transgenic Adenocarcinoma of the Mouse Prostate (TRAMP) and C57BL/6J mice were treated with PTL (40 mg/kg), dimethylaminoparthenolide (DMAPT, a PTL analogue with increased bioavailability) (100 mg/kg), or vehicle control three times over one week prior to combinations of low (10 mGy) and high (6 Gy) doses of whole-body X-irradiation. Tissues were analyzed for apoptosis at a range of time points up to 72 h postirradiation. Both PTL and DMAPT protected normal tissues, but not prostate tumor tissues, from a significant proportion of high-dose-radiation-induced apoptosis. DMAPT provided superior protection compared to PTL in normal dorsolateral prostate (71.7% reduction, P = 0.026), spleen (48.2% reduction, P = 0.0001) and colorectal tissue (38.0% reduction, P = 0.0002), and doubled radiation-induced apoptosis in TRAMP prostate tumor tissue (101.3% increase, P = 0.039). Both drugs induced the greatest radiosensitivity in TRAMP prostate tissue in areas with higher grade prostatic intraepithelial neoplasia (PIN) lesions. A 10 mGy dose delivered 3 h prior to a 6 Gy dose induced a radioadaptive apoptosis response in normal C57Bl/6J prostate (28.4% reduction, P = 0.045) and normal TRAMP spleen (13.6% reduction, P = 0.047), however the low-dose-adaptive radioprotection did not significantly add to the PTL/DMAPT-induced protection in normal tissues, nor did it affect tumor kill. These results support the use of the more bioavailable DMAPT and low-dose radiation, alone or in combination as useful radioprotectors of normal tissues to alleviate radiotherapy-induced side-effects in patients. The enhanced radiosensitisation in prostate tissues displaying high-grade PIN suggests that DMAPT also holds promise for targeted therapy of advanced prostate cancer, which may go on to become metastatic. The redox mechanisms involved in the differential radioprotection observed here suggest that increased radiotherapy efficacy by DMAPT is more broadly applicable to a range of cancer types.

Transcriptomic analysis reveals sex-specific differences in the expression of Dcl1 and Fis1 genes in the radio-adaptive response of thymocytes to TRP53-mediated apoptosis

BACKGROUND

Radio-Adaptive Response (RAR) is a biological defense mechanism whereby exposure to low dose ionizing radiation (IR) mitigates the detrimental effects of high dose irradiation. RAR has been widely observed in vivo using as endpoint less induction of apoptosis. However, sex differences associated with RAR and variations between males and females on global gene expression influenced by RAR have not been still investigated. In addition, the response to radiation-induced apoptosis is associated with phosphorylation of TRP53 at both the serine 15 (ser-18 in the mouse) and serine 392 (ser-389 in mice) residues, but the role of these two phosphorylated forms in male and female RAR remains to be elucidated.

RESULTS

We analyzed the effect of administering priming low dose radiation (0.075 Gy of X-rays) prior to high dose radiation (1.75 Gy of γ-rays) on the level of caspase-3-mediated apoptosis and on global transcriptional expression in thymocytes of male and female mice. Here, we provide the first evidence of a differential sex effect of RAR on the reduction of thymocyte apoptosis with males showing lesser levels of caspase-3-mediated apoptosis than females. Analysis of transcriptomic profiles of 1944 genes involved in apoptosis signaling in radio-adapted thymocytes identified 17 transcripts exhibiting differential expression between both sexes. Among them, Dlc1 and Fis1 are closely related to the apoptosis mediated by the TRP53 protein. Our data demonstrate that overexpression of Dlc1 and Fis1 occur concomitantly with a highest accumulation of phosphoserine-18-TRP53 and caspase-3 in radio-adapted thymocytes of female mice. In an opposite way, both down-modulation of Fis1 and phosphoserine-389-TRP53 accumulation appear to be associated with protection from thymocyte apoptosis mediated by caspase-3 in males.

CONCLUSIONS

Transcriptomic analysis performed in this work reveals for the first time sex-specific differences in gene expression influenced by RAR. Our results also suggest a sex-dependent dual role for phosphoserine-18-TRP53 and phosphoserine-389-TRP53 in the regulation of the radio-adaptive response in mouse thymocytes.

Stress induction in the bacteria Shewanella oneidensis and Deinococcus radiodurans in response to below-background ionizing radiation

PURPOSE

The 'Linear no-threshold' (LNT) model predicts that any amount of radiation increases the risk of organisms to accumulate negative effects. Several studies at below background radiation levels (4.5-11.4 nGy h(-1)) show decreased growth rates and an increased susceptibility to oxidative stress. The purpose of our study is to obtain molecular evidence of a stress response in Shewanella oneidensis and Deinococcus radiodurans grown at a gamma dose rate of 0.16 nGy h(-1), about 400 times less than normal background radiation.

MATERIALS AND METHODS

Bacteria cultures were grown at a dose rate of 0.16 or 71.3 nGy h(-1) gamma irradiation. Total RNA was extracted from samples at early-exponential and stationary phases for the rt-PCR relative quantification (radiation-deprived treatment/background radiation control) of the stress-related genes katB (catalase), recA (recombinase), oxyR (oxidative stress transcriptional regulator), lexA (SOS regulon transcriptional repressor), dnaK (heat shock protein 70) and SOA0154 (putative heavy metal efflux pump).

RESULTS

Deprivation of normal levels of radiation caused a reduction in growth of both bacterial species, accompanied by the upregulation of katB, recA, SOA0154 genes in S. oneidensis and the upregulation of dnaK in D. radiodurans. When cells were returned to background radiation levels, growth rates recovered and the stress response dissipated.

CONCLUSIONS

Our results indicate that below-background levels of radiation inhibited growth and elicited a stress response in two species of bacteria, contrary to the LNT model prediction.

Radiobiological effects of multiple vs. single low-dose pre-irradiation on the HT29 cell line

AIM OF THE STUDY

Aim of the study was to compare radiobiological effects of multiple vs. single low-dose pre-irradiation on the HT29 cell line. This regime is designed to be as similar as possible to fractionated tumour radiotherapy treatment, and to provide data on radiobiological effects on human tumour cells.

MATERIAL AND METHODS

The cell line used in the study was HT29 (human colorectal adenocarcinoma, American Type Culture Collection HTB-38™). Also, for comparison, the MRC5 cell line (human foetal lung fibroblasts, American Type Culture Collection CCL 171) was used. Four-day treatment in a 4 × 2 Gy regime was performed. Cell viability was evaluated by tetrazolium colorimetric MTT assay.

RESULTS

Multiple low-dose pre-irradiation induced a stronger radioadaptive response compared to single low-dose application in the HT29 cell line. Multiple pre-irradiation with 0.03 Gy and 0.05 Gy caused radioadaptive effects, while in both single and multiple low-dose pre-irradiation regimes 0.07 Gy led to radiosensitivity. Radiobiological effects induced in the HT29 cell line by low-dose pre-irradiation were evidently weak during the treatment time, because a single low-dose applied only on the first day gave no radioadaptive effects. In the MRC5 cell line different effects were registered, since radioadaptive response has not been observed after multiple or single pre-irradiation.

CONCLUSIONS

The obtained data are interesting, especially for the possible application of low-dose pre-irradiation in radiotherapy.

Relieved residual damage in the hematopoietic system of mice rescued by radiation-induced adaptive response (Yonezawa Effect)

Existence of adaptive response (AR) was previously demonstrated in C57BL/6J mice. Irradiations were performed by delivering a priming low dose of X-rays (0.50 Gy) in combination with a challenge high dose of accelerated carbon or neon ion particles. AR was characterized by significantly decreased mortality in the 30-day survival test. This mouse AR model (‘Yonezawa Effect’) was originally established by using X-rays as both the priming and challenge irradiations. The underlying mechanism was due to radio-resistance occurring in blood-forming tissues. In this study, we verified the existence of AR and further investigated residual damage in the hematopoietic system in surviving animals. Results showed that the priming low dose of X-rays could relieve the detrimental effects on the hematopoietic system. We observed both an improvement in the blood platelet count and the ratio of polychromatic erythrocytes (PCEs) to the sum of PCEs and normochromatic erythrocytes (NCEs) and a marked reduction of the incidences of micronucleated PCEs and micronucleated NCEs. These findings suggest that the priming low dose of low linear energy transfer (LET) X-rays induced a protective effect on the hematopoietic system, which may play an important role in both rescue from acute lethal damage (mouse killing) and prevention of late detrimental consequences (residual anhematopoiesis and delayed genotoxic effects) caused by exposure to a high challenge dose from low-LET (X-ray) or high-LET (carbon and neon ion) irradiations. These findings provide new knowledge of the characterization of the Yonezawa Effect by providing new insight into the mechanistic study of AR in vivo.

Low-dose γ-rays modify CD4(+) T cell signalling response to simulated solar particle event protons in a mouse model

PURPOSE

Astronauts on missions are exposed to low-dose/low-dose-rate (LDR) radiation and could receive high doses during solar particle events (SPE). This study investigated T cell function in response to LDR radiation and simulated SPE (sSPE) protons, alone and in combination.

MATERIALS AND METHODS

C57BL/6 mice received LDR γ-radiation (⁵⁷Co) to a total dose of 0.01 Gray (Gy) at 0.179 mGy/h, either with or without subsequent exposure to 1.7 Gy sSPE protons delivered over 36 h. Mice were euthanised on days 4 and 21 post-exposure. T cells with cluster of differentiation 4 (CD4(+)) were negatively isolated from spleens and activated with anti-CD3 antibody. Cells and supernatants were evaluated for survival/signalling proteins and cytokines.

RESULTS

The most striking effects were noted on day 21. In the survival pathway, nuclear factor-kappaB (NF-κB; total and active forms) and p38 mitogen activated protein kinase (p38MAPK; total) were significantly increased and cJun N-terminal kinase (JNK; total and active) was decreased when mice were primed with LDR γ-rays prior to sSPE exposure (P < 0.001). Evaluation of the T cell antigen receptor (TCR) signalling pathway revealed that LDR γ-ray exposure normalised the high sSPE proton-induced level of lymphocyte specific protein tyrosine kinase (Lck; total and active) on day 21 (P < 0.001 for sSPE vs. LDR + sSPE), while radiation had no effect on active zeta-chain-associated protein kinase 70 (Zap-70). There was increased production of interleukin-2 (IL-2) and IL-4 and decreased transforming growth factor-β1 in the LDR + sSPE group compared to the sSPE group.

CONCLUSION

The data demonstrate, for the first time, that protracted exposure to LDR γ-rays can significantly modify the effects of sSPE protons on critical survival/signalling proteins and immunomodulatory cytokines produced by CD4(+) T cells.

Low-dose Photon and Simulated Solar Particle Event Proton Effects on Foxp3+ T Regulatory Cells and other Leukocytes

Radiation is a major factor in the spaceflight environment that has carcinogenic potential. Astronauts on missions are continuously exposed to low-dose/low-dose-rate (LDR) radiation and may receive relatively high doses during a solar particle event (SPE) that consists primarily of protons. However, there are very few reports in which LDR photons were combined with protons. In this study, C57BL/6 mice were exposed to 1.7 Gy simulated SPE (sSPE) protons over 36 h, both with and without pre-exposure to 0.01 Gray (Gy) LDR γ-rays at 0.018 cGy/h. Apoptosis in skin samples was determined by immunohistochemistry immediately post-irradiation (day 0). Spleen mass relative to body mass, white blood cells (WBC), major leukocyte populations, lymphocyte subsets (T, Th, Tc, B, NK), and CD4+ CD25+ Foxp3+ T regulatory (Treg) cells were analyzed on days 4 and 21. Apoptosis in skin samples was evident in all irradiated groups; the LDR+sSPE mice had the greatest expression of activated caspase-3. On day 4 post-irradiation, the sSPE and LDR+sSPE groups had significantly lower WBC counts in blood and spleen compared to non-irradiated controls ( p < 0.05 vs. 0 Gy). CD4+ CD25+ Foxp3+ Treg cell numbers in spleen were decreased at day 4, but proportions were increased in the sSPE and LDR+sSPE groups ( p < 0.05 vs. 0 Gy). By day 21, lymphocyte counts were still low in blood from the LDR+sSPE mice, especially due to reductions in B, NK, and CD8+ T cytotoxic cells. The data demonstrate, for the first time, that pre-exposure to LDR photons did not protect against the adverse effects of radiation mimicking a large solar storm. The increased proportion of immunosuppressive CD4+ CD25+ Foxp3+ Treg and persistent reduction in circulating lymphocytes may adversely impact immune defenses that include removal of sub-lethally damaged cells with carcinogenic potential, at least for a period of time post-irradiation.

If bystander effects for apoptosis occur in spleen after low-dose irradiation in vivo then the magnitude of the effect falls within the range of normal homeostatic apoptosis

To test whether bystander effects occur in vivo after low doses of radiation relevant to occupational and population exposure, we exposed mice to whole-body X-radiation doses (0.01 and 1 mGy) where only a proportion of cells would receive an electron track. We used a precise method to analyze the apoptosis frequency in situ in spleen tissue sections at 7 h and 1, 3 and 7 days after irradiation to determine whether an increase in apoptosis above that predicted by direct effects was observed. No significant changes in the apoptosis frequency at any time after low-dose irradiation were detected. Apoptosis was induced above endogenous levels by five- to sevenfold 7 h after 1000 mGy. Using these data, the expected increases in apoptosis 7 h after a dose of 1 mGy or 0.01 mGy were calculated based on the assumption that induction of apoptosis would decrease linearly with dose. The magnitude of potential bystander effects for apoptosis that could be detected above homeostatic levels after these low doses of radiation was determined. A substantial bystander effect for apoptosis (>50-fold above direct effects) would be required before such proposed effects would be identified using 10 animals/treatment group as studied here. These data demonstrate that amplification of apoptosis even due to a substantial bystander effect would fall within the homeostatic range.

Molecular mechanisms involved in adaptive responses to radiation, UV light, and heat

Viable organisms recognize and respond to environmental changes or stresses. When these environmental changes and their responses by organisms are extreme, they can limit viability. However, organisms can adapt to these different stresses by utilizing different possible responses via signal transduction pathways when the stress is not lethal. In particular, prior mild stresses can provide some aid to prepare organisms for subsequent more severe stresses. These adjustments or adaptations for future stresses have been called adaptive responses. These responses are present in bacteria, plants and animals. The following review covers recent research which can help describe or postulate possible mechanisms which may be active in producing adaptive responses to radiation, ultraviolet light, and heat.

The effect of radio-adaptive doses on HT29 and GM637 cells

BACKGROUND

The shape of the dose-response curve at low doses differs from the linear quadratic model. The effect of a radio-adaptive response is the centre of many studies and well known inspite that the clinical applications are still rarely considered.

METHODS

We studied the effect of a low-dose pre-irradiation (0.03 Gy - 0.1 Gy) alone or followed by a 2.0 Gy challenging dose 4 h later on the survival of the HT29 cell line (human colorectal cancer cells) and on the GM637 cell line (human fibroblasts).

RESULTS

0.03 Gy given alone did not have a significant effect on both cell lines, the other low doses alone significantly reduced the cell survival. Applied 4 h before the 2.0 Gy fraction, 0.03 Gy led to a significant induced radioresistance in GM637 cells, but not in HT29 cells, and 0.05 Gy led to a significant hyperradiosensitivity in HT29 cells, but not in GM637 cells.

CONCLUSION

A pre-irradiation with 0.03 Gy can protect normal fibroblasts, but not colorectal cancer cells, from damage induced by an irradiation of 2.0 Gy and the application of 0.05 Gy prior to the 2.0 Gy fraction can enhance the cell killing of colorectal cancer cells while not additionally damaging normal fibroblasts. If these findings prove to be true in vivo as well this may optimize the balance between local tumour control and injury to normal tissue in modern radiotherapy.

Hdm2 and nitric oxide radicals contribute to the p53-dependent radioadaptive response

PURPOSE

The aim of this work was to characterize the radioadaptive response at the molecular level.

METHODS AND MATERIALS

We used wild-type (wt) p53 and mutated (m) p53-containing cells derived from the human lung cancer H1299 cell line, which is p53-null. Cellular radiation sensitivities were determined with a colony-forming assay. The accumulations of p53, the human homolog of endogenous murine double minute 2 (Hdm2), and inducible nitric oxide synthase were analyzed with Western blotting. Quantification of chromosomal aberrations was estimated by scoring dicentrics per cell.

RESULTS

In wtp53 cells, it was demonstrated that the lack of p53 accumulation was coupled with the activation of Hdm2 after low-dose irradiation (0.02 Gy). Although NO radicals were only minimally induced in wtp53 cells irradiated with a challenging irradiation (6 Gy) alone, NO radicals were seen to increase about two- to fourfold after challenging irradiation subsequent to a priming irradiation (0.02 Gy). Under similar irradiation conditions with a priming and challenging irradiation in wtp53 cells, induction of radioresistance and a depression of chromosomal aberrations were observed only in the absence of 5, 5'-(2, 5-Furanidiyl)bis-2-thiophenemethanol (RITA) or Nutlin-3 (p53-Hdm2 interaction inhibitors), aminoguanidine (an inducible nitric oxide synthase inhibitor), and c-PTIO (an NO radical scavenger). On the other hand, in p53 dysfunctional cells, a radioadaptive response was not observed in the presence or absence of those inhibitors. Moreover radioresistance developed when wtp53 cells were treated with isosorbide dinitrate (an NO-generating agent) alone.

CONCLUSIONS

These findings suggest that NO radicals are initiators of the radioadaptive response, acting through the activation of Hdm2 and the depression of p53 accumulations.