Radiation response of mouse lymphoid and myeloid cell lines. Part II. Apoptotic death is shown by all lines examined

Dynamics of Chromosome Aberrations and Cell Death in Zebrafish Embryos Exposed to 137Cs Total-Body Irradiation

Ionizing radiation exposure induces significant DNA damage and cell death in aquatic species. Accurate sensing and quantification play pivotal roles in environmental monitoring and surveillance. Zebrafish (Danio rerio) is a well-suited animal model for research into this aspect, especially with recent development of cytogenetic and transgenic tools. In this study, we present time-course studies of chromosome aberrations and cell death in zebrafish embryos exposed to 2 Gy 137Cs total-body irradiation. Using a cytogenetic approach, we quantified chromosome and chromatid aberrations in irradiated embryos at 6, 14, 20, and 24 h postirradiation. Metaphases with aberrations showed rapid declining kinetics, accompanied by incomplete karyotypes and irregular chromatin contents. Using an apoptosis-reporting transgenic zebrafish, we found increasing cell death along these time points, with the embryonic eyes and brain contributing the majority of the cell death volumes. We provide evidence that self-proliferating progenitor cells form the underlying linkage between the two kinetics and their positions define radiosensitive niches in zebrafish embryos. Our results provide detailed chromosome aberration and cell death dynamics in 137Cs-irradiated zebrafish embryos and unveil the appropriate timeline and tissue positions for accurate sensing and quantification of radiation-induced damages in zebrafish embryos.

In Vivo Imaging of Radiation-Induced Apoptosis at Single-Cell Resolution in Transgenic Zebrafish Embryos

Among the various types of cell death induced by ionizing radiation, apoptosis is a highly regulated and well-characterized form. Investigating radiation-induced apoptosis in an intact organism offers advantages in capturing the dynamics of apoptosis under preserved physiology, although high resolution imaging remains challenging. Owing to their optical transparency and genetic amenability, zebrafish is an ideal animal model for research into this aspect. In this study, we present a secA5 transgenic zebrafish expressing genetically encoded secreted ANNEXIN V fused with mVenus, a yellow fluorescent protein that enables reporting of radiation-induced apoptosis. Using in vivo imaging approach, we show that after 2 Gy total-body irradiation, apoptosis could be visualized at single-cell resolution in different cell types throughout the embryo. Elevated apoptosis could be imaged and quantified in the neuroepithelium of the embryonic brain, as well as the proliferative zone and parenchyma of the larval brain. In addition, clearance of apoptotic cells by microglia, the professional phagocytes residing in the brain, could be imaged at single-cell resolution in irradiated larvae. These results establish transgenic secA5 zebrafish as a useful and versatile in vivo system for investigating the dynamic process of radiation-induced apoptosis.

Key biological mechanisms involved in high-LET radiation therapies with a focus on DNA damage and repair

DNA damage and repair studies are at the core of the radiation biology field and represent also the fundamental principles informing radiation therapy (RT). DNA damage levels are a function of radiation dose, whereas the type of damage and biological effects such as DNA damage complexity, depend on radiation quality that is linear energy transfer (LET). Both levels and types of DNA damage determine cell fate, which can include necrosis, apoptosis, senescence or autophagy. Herein, we present an overview of current RT modalities in the light of DNA damage and repair with emphasis on medium to high-LET radiation. Proton radiation is discussed along with its new adaptation of FLASH RT. RT based on α-particles includes brachytherapy and nuclear-RT, that is proton-boron capture therapy (PBCT) and boron-neutron capture therapy (BNCT). We also discuss carbon ion therapy along with combinatorial immune-based therapies and high-LET RT. For each RT modality, we summarise relevant DNA damage studies. Finally, we provide an update of the role of DNA repair in high-LET RT and we explore the biological responses triggered by differential LET and dose.

Isorhamnetin Promotes 53BP1 Recruitment through the Enhancement of ATM Phosphorylation and Protects Mice from Radiation Gastrointestinal Syndrome

Flavonoids are a subclass of polyphenols which are attractive, due to possessing various physiological activities, including a radioprotective effect. Tumor suppressor p53 is a primary regulator in the radiation response and is involved in the pathogenesis of radiation injuries. In this study, we revealed that isorhamnetin inhibited radiation cell death, and investigated its action mechanism focusing on DNA damage response. Although isorhamnetin moderated p53 activity, it promoted phosphorylation of ataxia telangiectasia mutated (ATM) and enhanced 53BP1 recruitment in irradiated cells. The radioprotective effect of isorhamnetin was not observed in the presence of ATM inhibitor, indicating that its protective effect was dependent on ATM. Furthermore, isorhamnetin-treated mice survived gastrointestinal death caused by a lethal dose of abdominal irradiation. These findings suggested that isorhamnetin enhances the ATM-dependent DNA repair process, which is presumably associated with the suppressive effect against GI syndrome.

Leveraging Radiobiology for Arrhythmia Management: A New Treatment Paradigm?

Radiation therapy is a well-established approach for safely and non-invasively treating solid tumours and benign diseases with high precision and accuracy. Cardiac radiation therapy has recently emerged as a non-invasive treatment option for the management of refractory ventricular tachycardia. Here we summarise existing clinical and preclinical literature surrounding cardiac radiobiology and discuss how these studies may inform basic and translational research, as well as clinical treatment paradigms in the management of arrhythmias.

Evaluation of sodium orthovanadate as a radioprotective agent under total-body irradiation and partial-body irradiation conditions in mice

PURPOSE

Our previous study indicated that sodium orthovanadate (vanadate), a strong inhibitor of p53, effectively suppressed the lethality from the hematopoietic (HP) and gastrointestinal (GI) syndromes after 12 Gy total-body irradiation (TBI) in mice. This conclusion, however, was inconsistent with the fact that p53 plays a radioprotective role in the intestinal epithelium. The death after TBI of around 12 Gy was attributed to a combined effect of HP and GI syndromes. To verify the effect from prophylactic administration of p53 inhibitor on protection of HP and GI syndromes, in this study, the radioprotective effects from vanadate were investigated in TBI and lower half-body irradiation (partial-body irradiation: PBI) mouse models.

METHODS

Female ICR mice were given a single injection of vanadate or vehicle, followed by a lethal dose of TBI or PBI. Radioprotective effects of vanadate against the irradiations were evaluated by analyzing survival rate, body weight, hematopoietic parameters, and histological changes in the bone marrow and intestinal epithelium.

RESULTS

TBI-induced HP syndrome was effectively suppressed by vanadate treatment. After TBI, the vanadate-treated mice retained better bone marrow cellularity and showed markedly higher survival rate compared to the vehicle-treated animals. In contrast, vanadate did not relieve loss of intestinal crypts and failed to rescue mice from GI death after PBI.

CONCLUSION

Vanadate is a p53 inhibitor that has been shown to be beneficial as a radiation protective agent against HP but was not effective in protecting against acute GI radiation injury.

Clinical Progress in Proton Radiotherapy: Biological Unknowns

Simple Summary

Proton radiation therapy is a more recent type of radiotherapy that uses proton beams instead of classical photon or X-rays beams. The clinical benefit of proton therapy is that it allows to treat tumors more precisely. As a result, proton radiotherapy induces less toxicity to healthy tissue near the tumor site. Despite the experience in the clinical use of protons, the response of cells to proton radiation, the radiobiology, is less understood. In this review, we describe the current knowledge about proton radiobiology.

Abstract

Clinical use of proton radiation has massively increased over the past years. The main reason for this is the beneficial depth-dose distribution of protons that allows to reduce toxicity to normal tissues surrounding the tumor. Despite the experience in the clinical use of protons, the radiobiology after proton irradiation compared to photon irradiation remains to be completely elucidated. Proton radiation may lead to differential damages and activation of biological processes. Here, we will review the current knowledge of proton radiobiology in terms of induction of reactive oxygen species, hypoxia, DNA damage response, as well as cell death after proton irradiation and radioresistance.

The protective effect of oleuropein against radiation-induced cytotoxicity, apoptosis, and genetic damage in cultured human lymphocytes

PURPOSE

The aim of this study was to evaluate the effects of oleuropein radiation protection and to find an effective radioprotector.

MATERIALS AND METHOD

Human mononuclear cells were treated with oleuropein at the concentration of 100 μM (optimum concentration), incubated for 24 h, and then exposed to 2 Gy gamma-rays. The anti-radiation effect of oleuropein was assessed by MTT assay, flow cytometry, comet assay, and micronucleus (MN) assay.

RESULTS

It was found that pretreatment with oleuropein (25, 50, 75, 100, 200, 400, and 800 nM, and 1, 5, 10, 15, 20, 25, 30, 40, 50, 75, 100, 125, 150, 175, and 200 µM) significantly increased the percentage of cell viability compared to the irradiated group (p < .001). Moreover, oleuropein treatment with the above concentrations defined without gamma-ray did not show any cytotoxicity effect in human mononuclear cells. The LD_50/24h dose was calculated as 2.9 Gy, whereas by 200, 150, 50, and 100 µM oleuropein prior to radiation (1, 2,and 4 Gy), radiation LD_50/24h increased to 3.36, 3.54, 3.81, and >4 Gy, in that order. A very noticeable dose-modifying factor (DMF) of 1.16, 1.23, 1.31, and 1.72 was observed for 200, 150, 50, and 100 µM, in order. Therefore, 100 µM of oleuropein was selected as the desirable dose for radio-protection trial, and 2 Gy gamma-rays were used for further research. Human mononuclear cells treatment with oleuropein (100 µM) prior to 2 Gy gamma-rays significantly decreased apoptosis, genomic damage, and MN occurrence in human mononuclear caused by gamma-radiation (p < .001). Furthermore, treatment with oleuropein (100 µM) without radiation did not lead to apoptosis, genotoxicity, or clastogenic effects caused by oleuropein in human mononuclear cells.

CONCLUSION

The results revealed that oleuropein is able to significantly reduce cytotoxicity, apoptosis, genotoxic, and clastogenic effects of gamma-rays.

Response and outcome following radiation therapy of macroscopic canine plasma cell tumours

Thirty dogs with macroscopic plasma cell tumours (PCTs) were treated with radiation therapy (RT). Twelve patients were treated with palliative-intent prescriptions (range, 4-10 Gy/fraction (median, 7 Gy/fraction) for a total dose of 20 to 35 Gy (median total dose 30 Gy). Eighteen patients received definitive-intent prescriptions (range, 3.0-4.2 Gy/fraction (median, 3 Gy/fraction) for a total dose of 42 to 54 Gy (median total dose 48 Gy). Involved sites included the oral cavity, skin, multiple myeloma (MM)-associated lytic bone lesions, bone (solitary osseous plasmacytoma; SOP), nasal cavity, larynx, retrobulbar space, lymph node and rectum. Ninety-five percent of evaluable dogs had a complete (CR; 16/22) or partial response (PR; 5/22). Patients with MM experienced significant analgesia. The median progression-free survival (PFS) was 611 days (range: 36-2001 days). Events in the non-MM cases included in-field progression (5/26, 19%) and disseminated disease (5/26, 19%). The median survival time (MST) for all dogs was 697 days (range: 71-2075 days), and when only non-MM cases were considered, MST was 771 days (range: 71-2075 days). Fourteen patients were alive without disease progression or had died of unrelated causes. Achievement of a PR was associated with an inferior PFS and MST as compared with CR. Palliative-intent RT was associated with inferior MST as compared with definitive-intent RT. RT is a useful therapeutic modality for PCTs and tumour responses are often complete and durable, with protracted survivals. The optimal radiation dose and schedule are yet to be defined.

Radiation resistance in head and neck squamous cell carcinoma: dire need for an appropriate sensitizer

Radiation is a significant treatment for patients with head and neck cancer. Despite advances to improve treatment, many tumors acquire radiation resistance resulting in poor survival. Radiation kills cancer cells by inducing DNA double-strand breaks. Therefore, radiation resistance is enhanced by efficient repair of damaged DNA. Head and neck cancers overexpress EGFR and have a high frequency of p53 mutations, both of which enhance DNA repair. This review discusses the clinical criteria for radiation resistance in patients with head and neck cancer and summarizes how cancer cells evade radiation-mediated apoptosis by p53- and epidermal growth factor receptor (EGFR)-mediated DNA repair. In addition, we explore the role of cancer stem cells in promoting radiation resistance, and how the abscopal effect provides rationale for combination strategies with immunotherapy.

Molecular Mechanisms of Radiation-Induced Cancer Cell Death: A Primer

Radiation therapy (RT) is responsible for at least 40% of cancer cures, however treatment resistance remains a clinical problem. There have been recent advances in understanding the molecular mechanisms of radiation-induced cell death. The type of cell death after radiation depends on a number of factors including cell type, radiation dose and quality, oxygen tension, TP53 status, DNA repair capacity, cell cycle phase at time of radiation exposure, and the microenvironment. Mitotic catastrophe (a pathway preceding cell death that happens in mitosis or as a consequence of aberrant mitotic progression) is the primary context of radiation-induced cell death in solid cancers, although in a small subset of cancers such as haematopoietic malignancies, radiation results in immediate interphase apoptosis, occurring within hours after exposure. There is intense therapeutic interest in using stereotactic ablative body radiotherapy (SABR), a precise, high-dose form of RT given in a small number of fractions, to prime the immune system for cancer cell killing, but the optimal radiation dose and fractionation remain unclear. Additionally, promising novel radiosensitisers targeting the cell cycle and DNA repair pathways are being trialled. In the context of the increasing use of SABR and such novel agents in the clinic, we provide an updated primer on the major types of radiation-induced cell death, focussing on their molecular mechanisms, factors affecting their initiation, and their implications on immunogenicity.

Comparison of 30 mCi and 50 mCi I-131 doses for ablation of thyroid remnant in papillary thyroid cancer patients

PURPOSE OF THE STUDY

To compare efficacy of thyroid remnant ablation using 30 mCi or 50 mCi 131-I in papillary thyroid cancer patients.

MATERIALS AND METHODS

Five hundred and fifteen consecutive patients with Tumor-Node-Metastasis (TNM) stages T1-T3 N1/N0/NX receiving either 30 mCi or 50 mCi I-131 were analyzed for the effectiveness of remnant ablation using rhTSH-stimulated serum thyroglobulin. One hundred and five consecutive patients receiving 100 mCi I-131 were analyzed for the incidence of radiation thyroiditis and sialadenitis.

RESULTS AND CONCLUSIONS

Doses of 30 mCi and 50 mCi were equally effective for low- and moderate-risk disease but 30 mCi was less effective for T1T2NX disease, and 50 mCi was less effective for T3 compared to T1T2 disease. Low dose radiation hypersensitivity or unknown more extensive disease may have accounted for observed differences. Radiation thyroiditis and sialadenitis were more common in a comparison series of 100 mCi dose compared to 30 mCi, but not more common than in 50 mCi doses.

Regulation of Cancer Cell Responsiveness to Ionizing Radiation Treatment by Cyclic AMP Response Element Binding Nuclear Transcription Factor

Cyclic AMP response element binding (CREB) protein is a member of the CREB/activating transcription factor (ATF) family of transcription factors that play an important role in the cell response to different environmental stimuli leading to proliferation, differentiation, apoptosis, and survival. A number of studies highlight the involvement of CREB in the resistance to ionizing radiation (IR) therapy, demonstrating a relationship between IR-induced CREB family members' activation and cell survival. Consistent with these observations, we have recently demonstrated that CREB and ATF-1 are expressed in leukemia cell lines and that low-dose radiation treatment can trigger CREB activation, leading to survival of erythro-leukemia cells (K562). On the other hand, a number of evidences highlight a proapoptotic role of CREB following IR treatment of cancer cells. Since the development of multiple mechanisms of resistance is one key problem of most malignancies, including those of hematological origin, it is highly desirable to identify biological markers of responsiveness/unresponsiveness useful to follow-up the individual response and to adjust anticancer treatments. Taking into account all these considerations, this mini-review will be focused on the involvement of CREB/ATF family members in response to IR therapy, to deepen our knowledge of this topic, and to pave the way to translation into a therapeutic context.

Ionizing radiation modulates human macrophages towards a pro-inflammatory phenotype preserving their pro-invasive and pro-angiogenic capacities

In order to improve the efficacy of conventional radiotherapy, attention has been paid to immune cells, which not only modulate cancer cell response to therapy but are also highly recruited to tumours after irradiation. Particularly, the effect of ionizing radiation on macrophages, using therapeutically relevant doses, is not well understood. To evaluate how radiotherapy affects macrophage behaviour and macrophage-mediated cancer cell activity, human monocyte derived-macrophages were subjected, for a week, to cumulative ionizing radiation doses, as used during cancer treatment (2 Gy/fraction/day). Irradiated macrophages remained viable and metabolically active, despite DNA damage. NF-kappaB transcription activation and increased Bcl-xL expression evidenced the promotion of pro-survival activity. A significant increase of pro-inflammatory macrophage markers CD80, CD86 and HLA-DR, but not CCR7, TNF and IL1B was observed after 10 Gy cumulative doses, while anti-inflammatory markers CD163, MRC1, VCAN and IL-10 expression decreased, suggesting the modulation towards a more pro-inflammatory phenotype. Moreover, ionizing radiation induced macrophage morphological alterations and increased their phagocytic rate, without affecting matrix metalloproteases (MMP)2 and MMP9 activity. Importantly, irradiated macrophages promoted cancer cell-invasion and cancer cell-induced angiogenesis. Our work highlights macrophage ability to sustain cancer cell activities as a major concern that needs to be addressed to improve radiotherapy efficacy.

Tumor-Absorbed Dose Predicts Progression-Free Survival Following (131)I-Tositumomab Radioimmunotherapy

The study aimed at identifying patient-specific dosimetric and nondosimetric factors predicting outcome of non-Hodgkin lymphoma patients after (131)I-tositumomab radioimmunotherapy for potential use in treatment planning.

METHODS

Tumor-absorbed dose measures were estimated for 130 tumors in 39 relapsed or refractory non-Hodgkin lymphoma patients by coupling SPECT/CT imaging with the Dose Planning Method (DPM) Monte Carlo code. Equivalent biologic effect was calculated to assess the biologic effects of nonuniform absorbed dose including the effects of the unlabeled antibody. Evaluated nondosimetric covariates included histology, presence of bulky disease, and prior treatment history. Tumor level outcome was based on volume shrinkage assessed on follow-up CT. Patient level outcome measures were overall response (OR), complete response (CR), and progression-free survival (PFS), determined from clinical assessments that included PET/CT.

RESULTS

The estimated mean tumor-absorbed dose had a median value of 275 cGy (range, 94-711 cGy). A high correlation was observed between tracer-predicted and therapy-delivered mean tumor-absorbed doses (P < 0.001; r = 0.85). In univariate tumor-level analysis, tumor shrinkage correlated significantly with almost all of the evaluated dosimetric factors, including equivalent biologic effect. Regression analysis showed that OR, CR, and PFS were associated with the dosimetric factors and equivalent biologic effect. Both mean tumor-absorbed dose (P = 0.025) and equivalent biologic effect (P = 0.035) were significant predictors of PFS whereas none of the nondosimetric covariates were found to be statistically significant factors affecting PFS. The most important finding of the study was that in Kaplan-Meier curves stratified by mean dose, longer PFS was observed in patients receiving mean tumor-absorbed doses greater than 200 cGy than in those receiving 200 cGy or less (median PFS, 13.6 vs. 1.9 mo for the 2 dose groups; log-rank P < 0.0001).

CONCLUSION

A higher mean tumor-absorbed dose was significantly predictive of improved PFS after (131)I-tositumomab radioimmunotherapy. Hence tumor-absorbed dose, which can be estimated before therapy, can potentially be used to design radioimmunotherapy protocols to improve efficacy.

Analysis of DNA repair gene polymorphisms and survival in low-grade and anaplastic gliomas

The purpose of this study was to explore the variation in DNA repair genes in adults with WHO grade II and III gliomas and their relationship to patient survival. We analysed a total of 1,458 tagging single-nucleotide polymorphisms (SNPs) that were selected to cover DNA repair genes, in 81 grade II and grade III gliomas samples, collected in Sweden and Denmark. The statistically significant genetic variants from the first dataset (P < 0.05) were taken forward for confirmation in a second dataset of 72 grade II and III gliomas from northern UK. In this dataset, eight gene variants mapping to five different DNA repair genes (ATM, NEIL1, NEIL2, ERCC6 and RPA4) which were associated with survival. Finally, these eight genetic variants were adjusted for treatment, malignancy grade, patient age and gender, leaving one variant, rs4253079, mapped to ERCC6, with a significant association to survival (OR 0.184, 95% CI 0.054-0.63, P = 0.007). We suggest a possible novel association between rs4253079 and survival in this group of patients with low-grade and anaplastic gliomas that needs confirmation in larger datasets.

Comparative investigations on the protective effects of rhodioside, ciwujianoside-B and astragaloside IV on radiation injuries of the hematopoietic system in mice

The aim of this study was to investigate the protective effects of three glycosides (rhodioside, ciwujianoside-B and astragaloside IV) on the hematopoietic system in the mice exposed to γ-rays, and to examine the possible mechanisms involved. Mice were pretreated with the glycosides (40 mg/kg, i.g.) daily for 7 days prior to radiation. The survival of mice pretreated with three glycosides after total body irradiation (6.0 Gy) was examined. Peripheral blood leucocytes and endogenous spleen colony counts, colony-forming unit-granulocyte macrophage assay, analysis of DNA content and apoptosis rate determination were performed to evaluate the effects of the three glycosides on hematogenesis. The fragmentation of double-stranded DNA in lymphocytes was detected by the comet assay. The changes in cell cycle were analysed by flow cytometry. Furthermore, the expression levels of Bcl-2, Bax and nuclear factor-kappa B (NF-κB) were measured by western blot and the electrophoretic mobility shift assay. The results showed that pretreatment with all of the glycosides improved survival time and increased the number of leucocytes, spleen colonies and granulocyte-macrophage colonies in mice exposed to 6.0 Gy γ-radiation. Rhodioside showed more protective efficacy than both ciwujianoside-B and astragaloside IV. All three glycosides significantly increased the proliferation abilities of bone marrow cells, and decreased the ratio of cells in G(0)/G(1) phase. Further analysis showed that these three glycosides were able to decrease DNA damage and the increment in the Bax/Bcl-2 ratio induced by radiation. In summary, the three glycosides showed radioprotective effects on the hematopoietic system in mice, which was associated with changes in the cell cycle, a reduction in DNA damage, and down-regulation of the ratio of Bax/Bcl-2 in bone marrow cells exposed to radiation.

Radiation-induced cell death mechanisms

The main goal when treating malignancies with radiation therapy is to deprive tumor cells of their reproductive potential. One approach to achieve this is by inducing tumor cell apoptosis. Accumulating evidences suggest that induction of apoptosis alone is insufficient to account for the therapeutic effect of radiotherapy. It has become obvious in the last few years that inhibition of the proliferative capacity of malignant cells following irradiation, especially with solid tumors, can occur via alternative cell death modalities or permanent cell cycle arrests, i.e., senescence. In this review, apoptosis and mitotic catastrophe, the two major cell deaths induced by radiation, are described and dissected in terms of activating mechanisms. Furthermore, treatment-induced senescence and its relevance for the outcome of radiotherapy of cancer will be discussed. The importance of p53 for the induction and execution of these different types of cell deaths is highlighted. The efficiency of radiotherapy and radioimmunotherapy has much to gain by understanding the cell death mechanisms that are induced in tumor cells following irradiation. Strategies to use specific inhibitors that will manipulate key molecules in these pathways in combination with radiation might potentiate therapy and enhance tumor cell kill.

Ionizing radiation induces apoptotic signal through protein kinase Cdelta (delta) and survival signal through Akt and cyclic-nucleotide response element-binding protein (CREB) in Jurkat T cells

Although ionizing radiation induces a loss of proliferative capacity as well as cell death by apoptosis and necrosis, cells can oppose the damaging effects by activating survival signal pathways. Here we report the effect of 1.5- and 6-Gy doses of ionizing radiation on apoptotic protein kinase Cdelta (PKCdelta) and survival cyclic-nucleotide response element-binding protein (CREB) signal in Jurkat T cells. Cell cycle analysis, performed by flow cytometry, showed a significant G2M arrest 24 h after exposure to 6 Gy. This arrest was accompanied by dead cells, which increased in number up to 7 days, when cell viability was further reduced. The response was apparently promoted by caspase-3-mediated PKCdelta activation, and thus apoptosis. Moreover, the presence of viable cells up to 7 days in samples exposed to 6 Gy is explained by Akt activation, which may influence the nuclear transcription factor CREB, leading to resistance to ionizing radiation. Thus, the knowledge of apoptotic and survival pathways activated in tumor cells may help in establishing specific therapies by combining selective inhibitors or stimulators of key signaling proteins with conventional chemotherapy, hormone therapy, and radiotherapy.

Induction of apoptosis in human tumor cells after exposure to Auger electrons: comparison with gamma-ray exposure

To clarify the contribution of apoptosis to cell death in four human solid tumor cell lines, clonogenic cell survival (indicator of radiosensitivity) and induction of caspase-3 (CASP-3)/caspase-3-like proteases (CASP-3LP) and the production of DNA fragmentation (markers for apoptosis) were studied in RKO, LS174T, MCF7 and TE671 cells exposed to DNA-incorporated Auger-electron-emitting (125)I (5-[(125)I]iodo-2'-deoxyuridine) or gamma-radiation. Clonogenic survival was assessed by colony-forming assay, CASP-3/CASP-3LP induction with a fluorogenic substrate and DNA fragmentation by ligation-mediated polymerase chain reaction. For (125)I, log dose-survival curves had no shoulder [high-linear-energy-transfer (LET)-like] and decreased exponentially at different rates in various cell lines. Induction of CASP-3/CASP-3LP in radiosensitive RKO and LS174T cells was threefold greater than that in radioresistant TE671 and MCF7 cells. Nucleosomal laddering in (125)I-radiosensitive cell lines was dose-dependent, and no laddering was detected in radioresistant lines. For gamma-radiation, the survival curve for LS174T cells was monoexponential and that for the other lines exhibited a distinct shoulder (low-LET-like). The most radiosensitive cell line, LS174T, showed the highest induction of CASP-3/CASP-3LP, and the most radioresistant line, TE671, showed the lowest induction. Although DNA laddering was not detectable in TE671 cells, it was observed in other lines, being most prominent in LS174T cells. We conclude that apoptosis initiated by DNA-incorporated (125)I is dose-dependent, correlates with cell radiosensitivity and takes place through a CASP-3-mediated pathway, whereas that after gamma-irradiation probably occurs via a CASP-3-independent pathway and/or a CASP-3-mediated pathway and does not correlate with cell radiosensitivity.

Promoter methylation and response to chemotherapy and radiation in esophageal cancer

BACKGROUND & AIMS

Multiple studies have shown that promoter methylation of tumor suppressor genes underlies esophageal carcinogenesis. Hypothetically, methylation resulting in tumor suppressor gene inactivation might result in tumors that are unresponsive to chemotherapy and radiation. Accordingly, our aim was to find methylation markers that could be used to predict response to chemoradiation.

METHODS

Tumor specimens were obtained before treatment from 35 patients enrolled in a uniform chemoradiation treatment protocol. Methylation-specific quantitative polymerase chain reaction was performed on all samples. Pathology reports from esophagectomy specimens were used to define response to treatment.

RESULTS

Thirteen (37%) of 35 patients were responders, and 22 (63%) of 35 patients were nonresponders. The number of methylated genes per patient was significantly lower in responders than in nonresponders (1.4 vs 2.4 genes per patient; Student t test, P = .026). The combined mean level of promoter methylation of p16, Reprimo, p57, p73, RUNX-3, CHFR, MGMT, TIMP-3, and HPP1 was also lower in responders than in nonresponders (Student t test, P = .003; Mann-Whitney test, P = .001). The frequency (15% of responders vs 64% of nonresponders; Fisher exact test, P = .01) and level (0.078 in responders vs 0.313 in nonresponders; Mann-Whitney test, P = .037) of Reprimo methylation was significantly lower in responders than in nonresponders.

CONCLUSIONS

Reprimo methylation occurred at significantly lower levels and less frequently in chemoradioresponsive than in nonresponsive esophageal cancer patients, suggesting potential clinical application of this single-gene biomarker in defining prognosis and management. In addition, increased methylation of a 9-gene panel correlated significantly with poor responsiveness to chemoradiation.

Cell death induced by 131I in a differentiated thyroid carcinoma cell line in vitro: Necrosis or apoptosis?

AIM

The apoptotic and necrotic dose-response of thyroid carcinoma cells following irradiation with I was evaluated.

METHODS

In our in-vitro model, cells of well-differentiated papillary thyroid carcinoma (B-CPAP) were incubated with increasing activity concentrations of I for 2 days. Changes in cell viability and the extents of necrosis and apoptosis were evaluated both immediately and 2 days after irradiation.

RESULTS

Viability of B-CPAP cells diminished with increasing I activity concentration. No apoptosis was detectable immediately after irradiation. Two days after irradiation significant apoptosis was found. The lowest I activity concentration at which apoptosis was detectable corresponds to about 1 MBq . ml. At higher activity concentrations a larger percentage of cells became apoptotic but the proportion decreased again at activity concentrations >10 MBq . ml. Likewise, necrosis was minimal at low activity concentrations and showed an exponential increase with rising I activity concentrations (>5-10 MBq . ml). Necrosis was already detectable immediately after irradiation and was the predominant form of cell death at high activity concentrations.

CONCLUSION

The data suggest that the nature of the cytotoxic effect of I and whether it leads to apoptotic or necrotic cell death is dose-dependent. High I doses seem to produce mainly necrotic phenomena, whereas at low I activity concentrations apoptotic phenomena prevail. The predominance of delayed apoptosis could explain why radioiodine therapy at lower doses is often linked to delayed onset and possible continuation of thyroid volume reduction over some months and even up to a year.

Role of cell cycle in mediating sensitivity to radiotherapy

Multiple pathways are involved in maintaining the genetic integrity of a cell after its exposure to ionizing radiation. Although repair mechanisms such as homologous recombination and nonhomologous end-joining are important mammalian responses to double-strand DNA damage, cell cycle regulation is perhaps the most important determinant of ionizing radiation sensitivity. A common cellular response to DNA-damaging agents is the activation of cell cycle checkpoints. The DNA damage induced by ionizing radiation initiates signals that can ultimately activate either temporary checkpoints that permit time for genetic repair or irreversible growth arrest that results in cell death (necrosis or apoptosis). Such checkpoint activation constitutes an integrated response that involves sensor (RAD, BRCA, NBS1), transducer (ATM, CHK), and effector (p53, p21, CDK) genes. One of the key proteins in the checkpoint pathways is the tumor suppressor gene p53, which coordinates DNA repair with cell cycle progression and apoptosis. Specifically, in addition to other mediators of the checkpoint response (CHK kinases, p21), p53 mediates the two major DNA damage-dependent cellular checkpoints, one at the G(1)-S transition and the other at the G(2)-M transition, although the influence on the former process is more direct and significant. The cell cycle phase also determines a cell's relative radiosensitivity, with cells being most radiosensitive in the G(2)-M phase, less sensitive in the G(1) phase, and least sensitive during the latter part of the S phase. This understanding has, therefore, led to the realization that one way in which chemotherapy and fractionated radiotherapy may work better is by partial synchronization of cells in the most radiosensitive phase of the cell cycle. We describe how cell cycle and DNA damage checkpoint control relates to exposure to ionizing radiation.

Effect of Antioxidant Vitamins on Radiation-Induced Apoptosis in Cells of a Human Lymphoblastic Cell Line

Modulating the amount of radiation-induced apoptosis by administering antioxidant vitamins offers a possible way to influence radiation-induced side effects in normal tissues. Therefore, we investigated the effect of beta-carotene, vitamin C and alpha-tocopherol on radiation-induced apoptosis in cells in culture. Human T-lymphoblastic MOLT-3 cells were irradiated with a dose of 3 Gy 1 h after or immediately prior to the addition of vitamins in three concentrations (0.01 microM, 1 microM and 100 microM). Eight hours later, apoptosis was scored morphologically by staining the nuclear DNA with Hoechst 33342. When given prior to irradiation, beta-carotene and vitamin E reduced the amount of radiation-induced apoptosis significantly at concentrations of 0.01 microM and 1 microM. In contrast, vitamin C did not show any protective effect when given at these two concentrations and caused a slight but significant radiosensitization at 100 microM. At 0.01 microM, all combinations of two vitamins showed a protective effect. This was also observed for the combination of all three vitamins at concentrations of 0.01 and 1 microM. When given immediately after irradiation, each of the three vitamins showed a protective effect at 0.01 microM. In addition, the combination of alpha-tocopherol and vitamin C reduced radiation-induced apoptosis slightly when given at 1 microM. In all other cases, no statistically significant modulation of radiation-induced apoptosis was observed. In our experimental system, the protective effect of beta-carotene and vitamin E was dependent on concentration and occurred only in the micromolar and sub-micromolar concentration range, while vitamin C alone, but not in combinations, had a sensitizing effect, thus arguing for a careful consideration of vitamin concentrations in clinical settings.

Radiosensitizer sanazole (AK-2123) enhances gamma-radiation-induced apoptosis in murine fibrosarcoma

Sanazole (AK-2123) (N-2'-methoxy ethyl)-2-(3"-nitro-1"-triazolyl)acetamide, which has completed phase III clinical trials as a radiosensitizer, enhanced gamma-radiation induced apoptosis in murine fibrosarcoma upon i.p. administration at 40 mg/kg body weight one hour prior to irradiation. A microscopic examination of Giemsa-May-Grunwald stained cells has shown a higher frequency of condensed nuclei and fragmented nuclei in the tumor cells. The administration of sanazole to tumor-bearing animals enhanced the radiation-induced internucleosomal fragmentation in the nuclear genome of tumor cells. Higher levels of caspase-3 activity were also observed in the cell extracts of tumours from AK-2123 administered mice. Exposure to gamma-radiation of AK-2123-treated mouse further enhanced the caspase-3 activity, indicating the induction of apoptosis. The radiation sensitization property of sanazole was discernible by comparing the relative tumor diameter following irradiation after i.p. administration of AK-2123 and irradiation alone; it was higher during the first few days followed by the treatment.

Functional genomics as a window on radiation stress signaling

Exposure to ionizing radiation, as well as other stresses, results in the activation of complex signal transduction pathways, which eventually shape the response of cells and organisms. Some of the important pathways responding to radiation include the ATM/P53 pathway, MAPK cascades and NF-kappaB activation, as well as signaling events initiated at the cell membrane and within the cytoplasm. Alterations in gene expression play roles both as intermediaries in signaling and as downstream effector genes. Differences in cell type, interindividual genetic differences and crosstalk occurring between signaling pathways may help to channel radiation stress signals between cell cycle delay, enhanced DNA repair, and apoptosis. These differences may in turn help determine the likelihood of late effects of radiation exposure, including carcinogenesis and fibrosis. The tools of the postgenomic era enable high-throughput studies of the multiple changes resulting from the interplay of radiation signaling pathways. Gene expression profiling, in particular shows great promise, both in terms of insight into basic molecular mechanisms and for the future hope of biomarker development and individual tailoring of cancer therapy.

Monitoring human radiation exposure by gene expression profiling: possibilities and pitfalls

Advances in high throughput analysis of mRNA expression have made it possible to establish gene expression profiles for different cells, tissues, diseases and exposure states. For instance, recent studies have demonstrated the utility of such an approach to classify sub-types of cancers with more detail than was previously possible. In addition, gene expression studies of ionizing radiation exposure both in vitro and in vivo are affording insight into the molecular mechanisms of mammalian radiation response. We have demonstrated that radiation expression profiles are a good predictor of p53 function in cell lines, and such profiles also indicate a major role for p53-regulated genes in the in vivo radiation response. Gene expression can be a sensitive indicator of radiation response as we have shown linear dose-responses for induction of several genes down to doses as low as 2 cGy. As profiles are established from radiation studies, it is hoped that they may be useful for identifying individuals with specific exposures or predisposition to negative outcome of exposure. Although this technology holds great promise, some obstacles remain to be overcome before it can be successfully applied to population studies.

A novel p53 transcriptional repressor element (p53TRE) and the asymmetrical contribution of two p53 binding sites modulate the response of the placental transforming growth factor-beta promoter to p53

Previous studies in our laboratory and others identified placental transforming growth factor-beta (PTGF-beta) as an important downstream mediator of DNA damage signaling and a transcriptional target of p53. Here we show that accumulation of PTGF-beta mRNA in response to p53 overexpression is delayed relative to p21(WAF1), whereas the promoters of these genes respond to p53 with similar kinetics. Mutational analyses of two p53 binding sites within the PTGF-beta promoter revealed that site p53-1 (+29 bp) is responsible for as much as 80% of the transcriptional response to p53. This is consistent with electrophoretic mobility shift assays showing that site p53-1 binds p53 with a much higher affinity than site p53-2 (-850 bp). We also describe for the first time a novel 21-bp element (-222 to -242 bp) that acts to down-regulate the PTGF-beta promoter response to p53. Termed the p53 transcriptional repressor element (p53TRE), this sequence was shown to suppress p53 transactivation in a position- and promoter-independent fashion and to associate with a 28-kDa protein expressed in several tumor cell lines. A p53 suppressor element and asymmetric p53 binding sites may participate determining the activation thresholds of p53-responsive promoters in a cell- and context-specific manner.

Mechanism of action for N-substituted benzamide-induced apoptosis

We have analysed the mechanism of action for induction of apoptosis by N-substituted benzamides using declopramide as a lead compound. We show here that declopramide at doses above 250 microM in the mouse 70Z/3 pre-B cell line or in the human promyeolocytic cancer cell line HL60 induced cytochrome c release into the cytosol and caspase-9 activation. The broad spectrum caspase inhibitor zVADfmk and caspase-9 inhibitor zLEDHfmk inhibited apoptosis and improved cell viability when administrated to cells 1 h before exposure to declopramide, whereas the caspase-8 inhibitor zIEDHfmk had less effect. Also, the over expression of Bcl-2 by transfection in 70Z/3 cells inhibited declopramide-induced apoptosis. Prior to the induction of apoptosis, a G(2)/M cell cycle block was induced by declopramide. The cell cycle block was also observed in the presence of broad spectrum caspase inhibitor zVADfmk and in a transfectant expressing high levels of Bcl-2. Furthermore, while p53 was induced in 70Z/3 cells by declopramide, neither the apoptotic mechanism nor the G(2)/M cell cycle block were dependent on p53 activation since both effects were also seen in p53 deficient HL60 cells after addition of declopramide.

A high and sustained response rate in refractory or relapsing low-grade lymphoma masses after low-dose radiation: analysis of predictive parameters of response to treatment

PURPOSE

To determine the efficacy of small doses of radiation in patients with recurrent or refractory low-grade lymphoma masses.

METHODS AND MATERIALS

Patients with refractory or relapsing low-grade lymphoma masses. The two largest diameters of the tumor mass were measured, whenever possible, before and after treatment. A dose of 4 Gy of radiotherapy was delivered to tumor sites in 2 fractions. Patients were evaluated for response 1-4 months later and at regular follow-up visits.

RESULTS

Forty-eight patients with low-grade lymphomas according to the working formulation received low-dose radiotherapy between March 1987 and November 1998. Most patients had advanced disease at the time of radiation treatment, and 80% had received at least two chemotherapy regimens before treatment. The median interval between the initial diagnosis and radiotherapy was 2.7 years (range 0-22 years). Low-dose radiation was delivered to 135 tumor sites. Nodal and extranodal tumor sites represented 80% and 20% of masses, respectively. An objective response was obtained in 81% of the sites, with 57% attaining a complete remission. The 2-year actuarial freedom from local progression (FFLP) rate was 56% (95% CI, 46-66%). Tumor masses </=5 cm in diameter had a significantly higher 2-year FFLP rate than larger masses (51% vs. 27%). It is noteworthy that the 2-year FFLP rate for patients treated with less than 2 chemotherapy regimens before radiotherapy was significantly higher than the 2-year FFLP rate for more heavily treated patients (96% vs. 48%). The 2-year FFLP rates for extranodal tumor sites and nodal sites were not significantly different. The tumor size (< or =5 cm vs. > 5 cm), the number of chemotherapy regimens (0-1 vs. more), and age at time of radiation treatment (< or =65 years or > 65 years) were significant predictive parameters of response to treatment.

CONCLUSIONS

In this retrospective study, low-dose radiation proved efficient, with long-lasting effects in the majority of patients with recurrent or refractory low-grade lymphomas. This simple and nontoxic treatment should be investigated prospectively in patients with advanced disease and a low tumor burden not immediately warranting chemotherapy.

New concepts in radiation-induced apoptosis: 'premitotic apoptosis' and 'postmitotic apoptosis'

Formerly, the mechanisms responsible for the killing of cells by ionizing radiation were regarded as being divided into two distinct forms, interphase death and reproductive death. Since they were defined based on the classical radiobiological concepts using a clonogenic cell survival assay, biochemical and molecular biological mechanisms involved in the induction of radiation-induced cell death were not fully understood in relation to the modes of cell death. Recent multidisciplinary approaches to cell death mechanism have revealed that radiation-induced cell death is divided into several distinct pathways by the time course and cell-cycle position, and that apoptotic cell death plays a key role in almost every mode of cell death. This review discusses the mechanisms of radiation-induced apoptosis in relation to cell-cycle progression and highlights a new concept of the mode of cell death: 'premitotic apoptosis' and 'postmitotic apoptosis'. The former is a rapid apoptotic cell death associated with a prompt activation of caspase-3, a key enzyme of intracellular signaling of apoptosis. A rapid execution of cell killing in premitotic apoptosis is presumably due to the prompt activation of a set of pre-existed molecules following DNA damages. In contrast, the latter is a delayed apoptotic cell death after cell division, and unlike premitotic apoptosis, it neither requires a rapid activation of caspase-3 nor is inhibited by a specific inhibitor, Ac-DEVD-CHO. A downregulation of anti-apoptotic genes such as MAPK and Bcl-2 may play a key role in this mode of cell death. Characterization of these two types of apoptotic cell death regarding the cell cycle regulation and intracellular signaling will greatly help to understand the mechanisms of radiation-induced apoptosis.

High levels of delayed radiation-induced apoptosis observed in lymphoblastoid cell lines from ataxia-telangiectasia patients

PURPOSE

Cells from ataxia-telangiectasia (A-T) patients are extremely sensitive to radiation but display decreased apoptosis, as measured during the first 3 days following radiation. To explain this apparent contradiction, we examined apoptosis in normal and A-T cells at late time points following radiation, under the assumption that radiation-induced apoptosis is delayed in the A-T cells.

METHODS AND MATERIALS

Blood cells and lymphoblastoid cell lines from A-T patients, as well as healthy donors, were irradiated with X-rays. Apoptosis was measured at different time points (up to 7 and 30 days for lymphocytes and lymphoblastoid cells, respectively) using a flow cytometric method based on the reduction of intracellular DNA content (sub-G1 population).

RESULTS

Compared to normal cells, CD4 and CD8 A-T lymphocytes displayed constantly reduced levels of radiation-induced apoptosis for up to 7 days after treatment. A-T lymphoblastoid cells, however, displayed a delayed and prolonged apoptosis.

CONCLUSION

A-T lymphoblastoid cells show high levels of delayed radiation-induced apoptosis, which may contribute to the high cellular radiosensitivity displayed by the A-T phenotype. ATM (the gene mutated in A-T) plays different roles in the apoptotic response to ionizing radiation in quiescent lymphocytes and proliferative lymphoblastoid cells.

Growth arrest and cell death in the breast tumor cell in response to ionizing radiation and chemotherapeutic agents which induce DNA damage

Breast tumor cells are relatively refractory to apoptosis in response to modalities which induce DNA damage such as ionizing radiation and the topoisomerase II inhibitor, adriamycin. Various factors which may modulate the apoptotic response to DNA damage include the p53 status of the cell, levels and activity of the Bax and Bcl-2 families of proteins, activation of NF-kappa B, relative levels of insulin like growth factor and insulin-like growth factor binding proteins, activation of MAP kinases and PI3/Akt kinases, (the absence of) ceramide generation and the CD95 (APO1/Fas) signaling pathway. Prolonged growth arrest associated with replicative senescence may represent an alternative and reciprocal response to DNA-damage induced apoptosis that is p53 and/or p21waf1/cip1 dependent while delayed apoptosis may occur in p53 mutant breast tumor cells which fail to maintain the growth-arrested state. Clearly, the absence of an immediate apoptotic response to DNA damage does not eliminate other avenues leading to cell death and loss of self-renewal capacity in the breast tumor cell. Nevertheless, prolonged growth arrest (even if ultimately succeeded by apoptotic or necrotic cell death) could provide an opportunity for subpopulations of breast tumor cells to recover proliferative capacity and to develop resistance to subsequent clinical intervention.

Computerized video time-lapse microscopy studies of ionizing radiation-induced rapid-interphase and mitosis-related apoptosis in lymphoid cells

Computerized video time-lapse (CVTL) microscopy of X-irradiated cultures of cells of the murine lymphoma cell lines ST4 and L5178Y-S and the human lymphoid cell line MOLT-4 demonstrated that these cells exhibit a wide disparity in the timing of induction and execution of radiation-induced cell death that included rapid-interphase apoptosis, delayed apoptosis, and postmitotic apoptosis. ST4 cells that received 2.5 or 4 Gy of X radiation underwent rapid-interphase apoptosis within 2 h. Apoptosis commenced with a 10-20-min burst of membrane blebbing followed by swelling for 2-4 h and cell collapse. No apoptotic bodies were formed. After a dose of 1 Gy, approximately 90% of ST4 cells died by rapid-interphase apoptosis, while the remainder completed several rounds of cell division prior to cell death. Postmitotic death of ST4 cells occurred with the same morphological sequence of events as during rapid-interphase apoptosis induced by doses of 1-4 Gy. In contrast, L5178Y-S and MOLT-4 cells that received 4 Gy underwent apoptosis more slowly, with a complex series of events occurring over 30-60 h. Only 3% of L5178Y-S cells and 24% of MOLT-4 cells underwent apoptosis without attempting cell division. The cells became abnormally large during a long G(2)-phase delay, and then most of the cells (76-97%) attempted to divide for the first or second time at approximately 18-30 h postirradiation. However, either mitosis failed or division was aberrant; i.e., the large cells divided into three or four fragments which eventually fused together. This process was followed by several rounds of complex and unpredictable membrane blebbing, gross distortions of shape, fragmentation-refusion events, and formation of apoptotic bodies, after which the cells collapsed at 36-60 h postirradiation.

Altered apoptotic profiles in irradiated patients with increased toxicity

PURPOSE

A retrospective study of radiation-induced apoptosis in CD4 and CD8 T-lymphocytes, from 12 cancer patients who displayed enhanced toxicity to radiation therapy and 9 ataxia telangiectasia patients, was performed to test for altered response compared to healthy blood-donors and normal cancer patients.

METHODS AND MATERIALS

Three milliliters of heparinized blood from each donor was sent via express post to the Paul Scherrer Institute (PSI) for subsequent examination. The blood was diluted 1:10 in RPMI medium, irradiated with 0-, 2-, or 9-Gy X-rays, and incubated for 48 h. CD4 and CD8 T-lymphocytes were then labeled using FITC-conjugated antibodies, erythrocytes were lysed, and the DNA stained with propidium iodide. Subsequently, cells were analyzed using a Becton Dickinson FACScan flow cytometer. Radiation-induced apoptosis was recognized in leukocytes as reduced DNA content attributed to apoptosis-associated changes in chromatin structure. Apoptosis was confirmed by light microscopy, electron microscopy, and by the use of commercially available apoptosis detection kits (in situ nick translation and Annexin V). Data from hypersensitive individuals were compared to a standard database of 105 healthy blood-donors, and a database of 48 cancer patient blood donors who displayed normal toxicity to radiation therapy. To integrate radiosensitivity results from CD4 and CD8 T-lymphocytes after 2 and 9 Gy, z-score analyses were performed.

RESULTS

A cohort of 12 hypersensitive patients was evaluated; 8 showed enhanced early toxicity, 3 showed enhanced late toxicity, and 1 showed both. The cohort displayed less radiation-induced apoptosis (-1.8 sigma) than average age-matched donors. A cohort of 9 ataxia telangiectasia homozygotes displayed even less apoptosis (-3.6 sigma).

CONCLUSION

The leukocyte apoptosis assay appears to be a useful predictor of individuals likely to display increased toxicity to radiation therapy; however, validation of this requires a prospective study.

How does radiation kill cells?

Recent advances in the understanding of intracellular signaling after genotoxic injury have led to a better understanding of the pathways that influence radiation-induced cell death. Particular progress has been made in defining molecular controls of apoptosis and radiation-induced cell cycle arrest, as well as the possible role of telomerase activity in stabilizing DNA breaks.

Reduction of radiation-induced chromosome aberration and apoptosis by dithiothreitol

We have examined in vitro and in vivo radioprotective effects of a well-known thiol-containing compound, dithiothreitol (DTT). The treatment of both 0.5 and 1 mM of DTT significantly increased clonogenic survival of gamma-ray irradiated Chinese hamster (V79-4) cells. In order to investigate the possible radioprotective mechanism of DTT, we measured gamma-ray induced chromosome aberration by micronucleus assay. In the presence of 0.5 mM or 1 mM DTT, the frequencies of micronuclei were greatly reduced in all dose range examined (1.5-8 Gy). Slightly higher reduction in micronucleus formation was observed in 1 mM DTT-treated cells than in 0.5 mM DTT-treated cells. In addition, incubation with both 0.5 and 1 mM of DTT prior to gamma-ray irradiation reduced nucleosomal DNA fragmentation at about same extent, this result suggests that treatment of DTT at concentrations of 0.5 and 1 mM reduced radiation-induced apoptosis. In vivo experiments, we also observed that DTT treatment reduced the incidence of apoptotic cells in mouse small intestine crypts. In irradiated control group 4.4 +/- 0.5 apoptotic cells per crypt were observed. In DTT-administered and irradiated mice, only 2.1 +/- 0.4 apoptotic cells per crypt was observed. In vitro and in vivo data obtained in this study showed that DTT reduced radiation-induced damages and it seems that the possible radioprotective mechanisms of action of DTT are prevention of chromosome aberration.

Gamma-irradiation does not impair ATRA-induced maturation of myeloid leukaemic cells: implication for combined radiation and differentiation therapy

In the present study we investigated the effects of various doses of gamma-irradiation, followed by induction of granulocytic differentiation with all-trans-retinoic acid (ATRA), on proliferative rate, differentiation capability and oxidative metabolism of leukaemic cells from two different myeloid leukaemia cell lines, HL-60 and PLB-985. Regarding the effects of such combined treatment on the proliferative capabilities of HL-60 and PLB-985 cell lines, we showed that their growth kinetics were similar after 2 Gy gamma-irradiation combined with ATRA. However, with doses >2 Gy, the behaviour of the cell lines differed largely. Indeed, HL-60 appeared to be more radiosensitive than PLB-985 regarding cell viability and proliferation. Besides, whatever dose of irradiation (2, 5 or 10 Gy) was applied, ATRA was still able to induce differentiation of HL-60 and PLB-985 into granulocytes that retained the capacity to produce superoxide anion. The results of these in vitro studies suggest that leukaemia cell lines retain their ability to respond to ATRA, a granulocytic-differentiating inducer following high doses of irradiation. This may have implications for the use of radiation therapy in combination with ATRA for the treatment of extramedullary infiltrations of myeloid leukaemias in humans.

Influence of ionizing radiation on proliferation, c-myc expression and the induction of apoptotic cell death in two breast tumour cell lines differing in p53 status

PURPOSE

To determine the capacity of ionizing radiation to inhibit proliferation, to suppress c-myc expression and to induce apoptotic cell death in the p53 wild-type MCF-7 cell line and the p53 mutated MDA-MB231 cell line.

MATERIALS AND METHODS

Growth inhibition and cell killing were determined by cell number and trypan blue exclusion. Apoptosis was assessed through cell morphology and fluorescent end-labelling. c-myc expression was monitored by Northern blotting.

RESULTS

Inhibition of cell proliferation by ionizing radiation was similar in both cell lines. MDA-MB231 cells accumulated in G2 while MCF-7 cells accumulated in both the G1 and G2 phases of the cell cycle after irradiation. There was no evidence of apoptosis in either cell line. In MCF-7 cells, growth inhibition correlated closely with an early dose-dependent suppression of c-myc expression; in MDA-MB231 cells, there was no correspondence between growth inhibition and a transient, dose-independent reduction in c-myc message.

CONCLUSIONS

These findings suggest that in the absence of classical apoptotic cell death, radiosensitivity is not predictably related to the p53 status of the cell. While both p53 and c-myc may be linked to the DNA damage response pathway, neither p53 nor c-myc are essential for growth arrest in response to ionizing radiation.