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

A framework for automated time-resolved analysis of cell colony growth after irradiation

Understanding dose-dependent survival of irradiated cells is a pivotal goal in radiotherapy and radiobiology. To this end, the clonogenic assay is the standard in vitro method, classifying colonies into either clonogenic or non-clonogenic based on a size threshold at a fixed time. Here we developed a methodological framework for the automated analysis of time course live-cell image data to examine in detail the growth dynamics of large numbers of colonies that occur during such an experiment. We developed a segmentation procedure that exploits the characteristic composition of phase-contrast images to identify individual colonies. Colony tracking allowed us to characterize colony growth dynamics as a function of dose by extracting essential information: (a) colony size distributions across time; (b) fractions of differential growth behavior; and (c) distributions of colony growth rates across all tested doses. We analyzed three data sets from two cell lines (H3122 and RENCA) and made consistent observations in line with already published results: (i) colony growth rates are normally distributed with a large variance; (ii) with increasing dose, the fraction of exponentially growing colonies decreases, whereas the fraction of delayed abortive colonies increases; as a novel finding, we observed that (iii) mean exponential growth rates decrease linearly with increasing dose across the tested range (0-10 Gy). The presented method is a powerful tool to examine live colony growth on a large scale and will help to deepen our understanding of the dynamic, stochastic processes underlying the radiation response in vitro.

Raman Spectroscopy: An Exploratory Study to Identify Post-Radiation Cell Survival

Resistance to radiotherapy has been an impediment in the treatment of cancer, and the inability to detect it at an early stage further exacerbates the prognosis. We have assessed the feasibility of Raman spectroscopy as a rapid assay for predicting radiosensitivity of cancer cells in comparison to the conventional biological assays. Cell lines derived from breast adenocarcinoma (MCF7), gingivobuccal squamous cell carcinoma (ITOC-03), and human embryonic kidney (HEK293) were subjected to varying doses of ionizing radiation. Cell viability of irradiated cells was assessed at different time points using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and Raman spectroscopy, and colony-forming capability was evaluated by clonogenic assay. Radiosensitivity observed using MTT assay was limited by the finding of similar cell viability in all the three cell lines 24 h post-irradiation. However, cell survival assessed using clonogenic assay and principal component linear discriminant analysis (PC-LDA) classification of Raman spectra showed correlating patterns. Irradiated cells showed loss of nucleic acid features and enhancement of 750 cm^-1 peak probably attributing to resonance Raman band of cytochromes in all three cell lines. PC-LDA analysis affirmed MCF7 to be a radioresistant cell line as compared to ITOC-03 and HEK293 to be the most radiosensitive cell line. Raman spectroscopy is shown to be a rapid and alternative assay for identification of radiosensitivity as compared to the gold standard clonogenic assay.

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.

Raman spectroscopy detects metabolic signatures of radiation response and hypoxic fluctuations in non-small cell lung cancer

BACKGROUND

Radiation therapy is a standard form of treating non-small cell lung cancer, however, local recurrence is a major issue with this type of treatment. A better understanding of the metabolic response to radiation therapy may provide insight into improved approaches for local tumour control. Cyclic hypoxia is a well-established determinant that influences radiation response, though its impact on other metabolic pathways that control radiosensitivity remains unclear.

METHODS

We used an established Raman spectroscopic (RS) technique in combination with immunofluorescence staining to measure radiation-induced metabolic responses in human non-small cell lung cancer (NSCLC) tumour xenografts. Tumours were established in NOD.CB17-Prkdcscid/J mice, and were exposed to radiation doses of 15 Gy or left untreated. Tumours were harvested at 2 h, 1, 3 and 10 days post irradiation.

RESULTS

We report that xenografted NSCLC tumours demonstrate rapid and stable metabolic changes, following exposure to 15 Gy radiation doses, which can be measured by RS and are dictated by the extent of local tissue oxygenation. In particular, fluctuations in tissue glycogen content were observed as early as 2 h and as late as 10 days post irradiation. Metabolically, this signature was correlated to the extent of tumour regression. Immunofluorescence staining for γ-H2AX, pimonidazole and carbonic anhydrase IX (CAIX) correlated with RS-identified metabolic changes in hypoxia and reoxygenation following radiation exposure.

CONCLUSION

Our results indicate that RS can identify sequential changes in hypoxia and tumour reoxygenation in NSCLC, that play crucial roles in radiosensitivity.

A high-throughput microscopy method for single-cell analysis of event-time correlations in nanoparticle-induced cell death

The temporal context of cell death decisions remains generally hidden in ensemble measurements with endpoint readouts. Here, we describe a method to extract event times from fluorescence time traces of cell death-related markers in automated live-cell imaging on single-cell arrays (LISCA) using epithelial A549 lung and Huh7 liver cancer cells as a model system. In pairwise marker combinations, we assess the chronological sequence and delay times of the events lysosomal membrane permeabilization, mitochondrial outer membrane permeabilization and oxidative burst after exposure to 58 nm amino-functionalized polystyrene nanoparticles (PS-NH2 nanoparticles). From two-dimensional event-time scatter plots we infer a lysosomal signal pathway at a low dose of nanoparticles (25 µg mL-1) for both cell lines, while at a higher dose (100 µg mL-1) a mitochondrial pathway coexists in A549 cells, but not in Huh7. In general, event-time correlations provide detailed insights into heterogeneity and interdependencies in signal transmission pathways.

Does chronomodulated radiotherapy improve pathological response in locally advanced rectal cancer?

The predominant mode of radiation-induced cell death for solid tumours is mitotic catastrophe, which is in part dependent on sublethal damage repair being complete at around 6 h. Circadian variation appears to play a role in normal cellular division, and this could influence tumour response of radiation treatment depending on the time of treatment delivery. We tested the hypothesis that radiation treatment later in the day may improve tumour response and nodal downstaging in rectal cancer patients treated neoadjuvantly with radiation therapy. Recruitment was by retrospective review of 267 rectal cancer patients treated neoadjuvantly in the Department of Radiation Oncology at the Canberra Hospital between January 2010 and November 2015. One hundred and fifty-five patients met the inclusion criteria for which demographic, pathological and imaging data were collected, as well as the time of day patients received treatment with each fraction of radiotherapy. Data analysis was performed using the Statistical Package R with nonparametric methods of significance for all tests set at p < 0.05. Of the 45 female and 110 male patients, the median age was 64. Seventy-three percent had cT3 disease and there was a mean tumour distance from the anal verge of 7 cm. Time to surgical resection following radiotherapy ranged from 4 to 162 days with a median of 50 days, with a complete pathological response seen in 21% of patients. Patients exhibiting a favourable pathological response had smaller median pre- and postradiotherapy tumour size and had a greater change in tumour size following treatment (p < 0.01). Patients who received the majority of their radiotherapy fractions after 12:00 pm were more likely to show a complete or moderate pathological response (p = 0.035) and improved nodal downstaging. There were also more favourable responses amongst patients with longer time to surgical resection postradiotherapy (p < 0.004), although no relationship was seen between response and tumour distance from the anal verge. Females were less likely to exhibit several of the above responses. Neoadjuvant radiotherapy for locally advanced rectal cancer performed later in the day coupled with a longer time period to surgical resection may improve pathological tumour response rates and nodal downstaging. A prospective study in chronomodulated radiotherapy in this disease is warranted.

Radiation-Induced Organizing Pneumonia: A Characteristic Disease that Requires Symptom-Oriented Management

Radiation-induced organizing pneumonia (RIOP) is an inflammatory lung disease that is occasionally observed after irradiation to the breast. It is a type of secondary organizing pneumonia that is characterized by infiltrates outside the irradiated volume that are sometimes migratory. Corticosteroids work acutely, but relapse of pneumonia is often experienced. Management of RIOP should simply be symptom-oriented, and the use of corticosteroids should be limited to severe symptoms from the perspective not only of cost-effectiveness but also of cancer treatment. Once steroid therapy is started, it takes a long time to stop it due to frequent relapses. We review RIOP from the perspective of its diagnosis, epidemiology, molecular pathogenesis, and patient management.

Radiotherapy scheduling using prime numbers

Background

The optimal delivery of radiation therapy to achieve maximum tumour cell kill while limiting damage to normal tissues underlies any radiation therapy treatment protocol. The biological effectiveness of radiation therapy is closely related to cellular reproductive activity. The scheduling of dose fraction to a time where actively dividing cells are at their most radiosensitive stage (RS) has potential to enhance therapeutic efficacy.

Materials and methods

A prime number is a natural number >1 whose only divisors are 1 and the number itself.

Purpose

We propose that the use of prime numbers in the scheduling of radiotherapy treatments could maximise biological effectiveness by facilitating the irradiation of the greatest number of cells at their most RS stage, and ultimately improve the therapeutic ratio of radiation therapy.

Conclusions

The theoretical clinical implementation of this concept into the scheduling of radiation therapy is discussed.

Ionizing irradiation not only inactivates clonogenic potential in primary normal human diploid lens epithelial cells but also stimulates cell proliferation in a subset of this population

Over the past century, ionizing radiation has been known to induce cataracts in the crystalline lens of the eye, but its mechanistic underpinnings remain incompletely understood. This study is the first to report the clonogenic survival of irradiated primary normal human lens epithelial cells and stimulation of its proliferation. Here we used two primary normal human cell strains: HLEC1 lens epithelial cells and WI-38 lung fibroblasts. Both strains were diploid, and a replicative lifespan was shorter in HLEC1 cells. The colony formation assay demonstrated that the clonogenic survival of both strains decreases similarly with increasing doses of X-rays. A difference in the survival between two strains was actually insignificant, although HLEC1 cells had the lower plating efficiency. This indicates that the same dose inactivates the same fraction of clonogenic cells in both strains. Intriguingly, irradiation enlarged the size of clonogenic colonies arising from HLEC1 cells in marked contrast to those from WI-38 cells. Such enhanced proliferation of clonogenic HLEC1 cells was significant at ≥2 Gy, and manifested as increments of ≤2.6 population doublings besides sham-irradiated controls. These results suggest that irradiation of HLEC1 cells not only inactivates clonogenic potential but also stimulates proliferation of surviving uniactivated clonogenic cells. Given that the lens is a closed system, the stimulated proliferation of lens epithelial cells may not be a homeostatic mechanism to compensate for their cell loss, but rather should be regarded as abnormal. This is because these findings are consistent with the early in vivo evidence documenting that irradiation induces excessive proliferation of rabbit lens epithelial cells and that suppression of lens epithelial cell divisions inhibits radiation cataractogenesis in frogs and rats. Thus, our in vitro model will be useful to evaluate the excessive proliferation of primary normal human lens epithelial cells that may underlie radiation cataractogenesis, warranting further investigations.

A branching process model for the analysis of abortive colony size distributions in carbon ion-irradiated normal human fibroblasts

A single cell can form a colony, and ionizing irradiation has long been known to reduce such a cellular clonogenic potential. Analysis of abortive colonies unable to continue to grow should provide important information on the reproductive cell death (RCD) following irradiation. Our previous analysis with a branching process model showed that the RCD in normal human fibroblasts can persist over 16 generations following irradiation with low linear energy transfer (LET) γ-rays. Here we further set out to evaluate the RCD persistency in abortive colonies arising from normal human fibroblasts exposed to high-LET carbon ions (18.3 MeV/u, 108 keV/µm). We found that the abortive colony size distribution determined by biological experiments follows a linear relationship on the log-log plot, and that the Monte Carlo simulation using the RCD probability estimated from such a linear relationship well simulates the experimentally determined surviving fraction and the relative biological effectiveness (RBE). We identified the short-term phase and long-term phase for the persistent RCD following carbon-ion irradiation, which were similar to those previously identified following γ-irradiation. Taken together, our results suggest that subsequent secondary or tertiary colony formation would be invaluable for understanding the long-lasting RCD. All together, our framework for analysis with a branching process model and a colony formation assay is applicable to determination of cellular responses to low- and high-LET radiation, and suggests that the long-lasting RCD is a pivotal determinant of the surviving fraction and the RBE.

Cell cycle regulators guide mitochondrial activity in radiation-induced adaptive response

SIGNIFICANCE

There are accruing concerns on potential genotoxic agents present in the environment including low-dose ionizing radiation (LDIR) that naturally exists on earth's surface and atmosphere and is frequently used in medical diagnosis and nuclear industry. Although its long-term health risk is being evaluated and remains controversial, LDIR is shown to induce temporary but significant adaptive responses in mammalian cells and animals. The mechanisms guiding the mitochondrial function in LDIR-induced adaptive response represent a unique communication between DNA damage and cellular metabolism. Elucidation of the LDIR-regulated mitochondrial activity may reveal new mechanisms adjusting cellular function to cope with hazardous environmental stress.

RECENT ADVANCES

Key cell cycle regulators, including Cyclin D1/CDK4 and Cyclin B1/cyclin-dependent kinase 1 (CDK1) complexes, are actively involved in the regulation of mitochondrial functions via phosphorylation of their mitochondrial targets. Accumulating new evidence supports a concept that the Cyclin B1/CDK1 complex acts as a mediator in the cross talk between radiation-induced DNA damage and mitochondrial functions to coordinate cellular responses to low-level genotoxic stresses.

CRITICAL ISSUES

The LDIR-mediated mitochondrial activity via Cyclin B1/CDK1 regulation is an irreplaceable network that is able to harmonize vital cellular functions with adjusted mitochondrial metabolism to enhance cellular homeostasis.

FUTURE DIRECTIONS

Further investigation of the coordinative mechanism that regulates mitochondrial activities in sublethal stress conditions, including LDIR, will reveal new insights of how cells cope with genotoxic injury and will be vital for future targeted therapeutic interventions that reduce environmental injury and cancer risk.

Dynamic rendering of the heterogeneous cell response to anticancer treatments

The antiproliferative response to anticancer treatment is the result of concurrent responses in all cell cycle phases, extending over several cell generations, whose complexity is not captured by current methods. In the proposed experimental/computational approach, the contemporary use of time-lapse live cell microscopy and flow cytometric data supported the computer rendering of the proliferative process through the cell cycle and subsequent generations during/after treatment. The effects of treatments were modelled with modules describing the functional activity of the main pathways causing arrest, repair and cell death in each phase. A framework modelling environment was created, enabling us to apply different types of modules in each phase and test models at the complexity level justified by the available data. We challenged the method with time-course measures taken in parallel with flow cytometry and time-lapse live cell microscopy in X-ray-treated human ovarian cancer cells, spanning a wide range of doses. The most suitable model of the treatment, including the dose-response of each effect, was progressively built, combining modules with a rational strategy and fitting simultaneously all data of different doses and platforms. The final model gave for the first time the complete rendering in silico of the cycling process following X-ray exposure, providing separate and quantitative measures of the dose-dependence of G₁, S and G₂M checkpoint activities in subsequent generations, reconciling known effects of ionizing radiations and new insights in a unique scenario.

The antiproliferative response to anticancer treatment is the result of concurrent effects in all cell cycle phases, where molecular control pathways (checkpoints) are activated and cells may be arrested to repair DNA damage or killed if not able to succeed in the repair process. The complexity and inter-cell variability of these phenomena are not captured by the available methods, and the origin of the dose-dependence of the response remains elusive. In this work, we present an experimental-computational method that discloses and measures the individual responses of cell cycle controls in each phase and generation. We demonstrate that the method, exploiting jointly data sets obtained by flow cytometry and time-lapse in vivo imaging with a suitable experimental design, is able to achieve a full reconstruction in silico of the actual movement of cell cohorts following X-ray exposure, providing separate and quantitative measures of the dose-dependence of G₁, S and G₂M checkpoint activities in subsequent generations. Best fit parameters values are actual measures of the probability of activation of the specific pathways of arrest, repair or death within the cell population, linking the molecular scale to the “macroscopic” response, with full appreciation of its dynamics and inter-cell heterogeneity.

A framework for analysis of abortive colony size distributions using a model of branching processes in irradiated normal human fibroblasts

Background

Clonogenicity gives important information about the cellular reproductive potential following ionizing irradiation, but an abortive colony that fails to continue to grow remains poorly characterized. It was recently reported that the fraction of abortive colonies increases with increasing dose. Thus, we set out to investigate the production kinetics of abortive colonies using a model of branching processes.

Methodology/Principal Findings

We firstly plotted the experimentally determined colony size distribution of abortive colonies in irradiated normal human fibroblasts, and found the linear relationship on the log-linear or log-log plot. By applying the simple model of branching processes to the linear relationship, we found the persistent reproductive cell death (RCD) over several generations following irradiation. To verify the estimated probability of RCD, abortive colony size distribution (≤15 cells) and the surviving fraction were simulated by the Monte Carlo computational approach for colony expansion. Parameters estimated from the log-log fit demonstrated the good performance in both simulations than those from the log-linear fit. Radiation-induced RCD, i.e. excess probability, lasted over 16 generations and mainly consisted of two components in the early (<3 generations) and late phases. Intriguingly, the survival curve was sensitive to the excess probability over 5 generations, whereas abortive colony size distribution was robust against it. These results suggest that, whereas short-term RCD is critical to the abortive colony size distribution, long-lasting RCD is important for the dose response of the surviving fraction.

Conclusions/Significance

Our present model provides a single framework for understanding the behavior of primary cell colonies in culture following irradiation.

Radiation up-regulates the expression of VEGF in a canine oral melanoma cell line

To evaluate radiosensitivity and the effects of radiation on the expression of vascular endothelial growth factor (VEGF) and VEGF receptors in the canine oral melanoma cell line, TLM 1, cells were irradiated with doses of 0, 2, 4, 6, 8 and 10 Gray (Gy). Survival rates were then determined by a MTT assay, while vascular endothelial growth factor receptor (VEGFR)-1 and -2 expression was measured by flow cytometry and apoptotic cell death rates were investigated using an Annexin assay. Additionally, a commercially available canine VEGF ELISA kit was used to measure VEGF. Radiosensitivity was detected in TLM 1 cells, and mitotic and apoptotic cell death was found to occur in a radiation dose dependent manner. VEGF was secreted constitutively and significant up-regulation was observed in the 8 and 10 Gy irradiated cells. In addition, a minor portion of TLM 1 cells expressed vascular endothelial growth factor receptor (VEGFR)-1 intracellularly. VEGFR-2 was detected in the cytoplasm and was down-regulated following radiation with increasing dosages. In TLM 1 cells, apoptosis plays an important role in radiation induced cell death. It has also been suggested that the significantly higher VEGF production in the 8 and 10 Gy group could lead to tumour resistance.

Outside the box--novel therapeutic strategies for glioblastoma

Standard approaches to therapy for malignant glioma provide modest improvement of progression-free survival and overall survival. Almost all patients experiencing glioblastoma eventually progress, and no cure is currently available. During the last decade, we have witnessed a 30% improvement in 2-year overall survival rates, yet glioblastoma continues to cause approximately 13,000 cancer-related deaths in the United States annually. Thus, novel therapies need to be investigated alongside continued development of currently available radiotherapy and chemotherapy options. Because glioblastoma does not typically metastasize outside the brain, development of unique local therapies that are not available for other cancers is feasible. Experimental agents, like scorpion venom-derived chlorotoxin, have been successfully applied in local therapy for glioblastoma. In addition, multiple new gene therapy approaches are emerging for both local and systemic glioblastoma therapy. Lastly, alternating electric fields are being introduced to cancer therapy. This review will discuss these "nonstandard"--outside the box--modalities for therapy for malignant glioma.

Exposure to hypoxia following irradiation increases radioresistance in prostate cancer cells

BACKGROUND

Tumor hypoxia is a common feature of prostate tumors associated with the stabilization of hypoxia-inducible-factor 1alpha (HIF-1α) and poor prognosis following radiation therapy. Lack of oxygen at the time of irradiation is associated with radioresistance, but recent reports suggest radioresponse is also modulated by the dynamic nature of tumor hypoxia.

OBJECTIVE

We proposed to evaluate the effect of post-irradiation hypoxic exposure on the radioresponse of 2 prostate cancer (CaP) cell lines (22Rv1, DU145) and to examine whether it correlates with modified cellular responses induced by hypoxia.

METHODS AND RESULTS

Aerobic and hypoxic CaP cells exposed to hypoxia (24 h) after irradiation (4 Gy) gained a survival advantage compared with cells fully oxygenated after irradiation. This survival advantage was associated with induction of a G2/M cell cycle arrest, reduced induction of apoptosis and decreased amount of senescent cells. These modified cellular responses appeared mediated by HIF-1α.

CONCLUSION

Our data suggest that targeting hypoxia after irradiation may benefit patients with aggressive hypoxic prostate tumors.

In vivo real-time imaging of the liver with confocal endomicroscopy permits visualization of the temporospatial patterns of hepatocyte apoptosis

Apoptosis is a dynamic process of programmed cell death and is involved in multiple diseases. However, its mechanisms and sequence of events are still incompletely understood, partly because of the inability to visualize single cells continuously in vivo. The aim of the present study was to monitor hepatocyte apoptosis with confocal endomicroscopy in living rodents. In 73 anaesthetized mice, apoptotic liver injury was induced by injection of the CD95-agonistic antibody Jo2. Individual hepatocytes were followed for up to 240 min with a handheld confocal probe (FIVE1; Optiscan) providing 0.7 μm resolution (1,000-fold magnification). Different fluorescence staining protocols were used for cellular staining, vascular and cellular barrier function imaging, and caspase activation visualization. The time course of apoptosis could be visualized in vivo while liver perfusion and tissue integrity were maintained. In contrast to most ex vivo studies, initial cell swelling was observed that coincided with early defects in barrier function of sinusoids and hepatocytes. Cytoplasmic vesicle formation, nuclear condensation, cellular disintegration, and macrophage infiltration were captured sequentially. Labeling of caspases allowed molecular imaging. Our study allowed for the first time to continuously follow distinct morphological, functional, and molecular features of apoptosis in a solid organ in vivo and at high resolution. Intravital confocal microscopy may be a valuable tool to study the effects of therapeutic intervention on apoptosis in animal models and humans.

Enhancement of autophagy is a potential modality for tumors refractory to radiotherapy

Radiotherapy is a well-established treatment for cancer. However, the existence of radioresistant cells is one of the major obstacles in radiotherapy. In order to understand the mechanism of cellular radioresistance and develop more effective radiotherapy, we have established clinically relevant radioresistant (CRR) cell lines, which continue to proliferate under daily exposure to 2 Gray (Gy) of X-rays for >30 days. X-ray irradiation significantly induced autophagic cells in parental cells, which was exiguous in CRR cells, suggesting that autophagic cell death is involved in cellular radiosensitivity. An autophagy inducer, rapamycin sensitized CRR cells to the level of parental cells and suppressed cell growth. An autophagy inhibitor, 3-methyladenine induced radioresistance of parental cells. Furthermore, inhibition of autophagy by knockdown of Beclin-1 made parental cells radioresistant to acute radiation. These suggest that the suppression of autophagic cell death but not apoptosis is mainly involved in cellular radioresistance. Therefore, the enhancement of autophagy may have a considerable impact on the treatment of radioresistant tumor.

Phototoxicity of Hoechst 33342 in time-lapse fluorescence microscopy

Dyes that bind to DNA, such as Hoechst 33342, are commonly used to visualize chromatin in live cells by fluorescence microscopy. A caveat is that the probes themselves should not perturb cellular responses and under normal conditions the dyes are generally non-toxic. However, researchers are increasingly using computerized time-lapse microscopy (CTLM), where cells stained with fluorescent dyes are often imaged frequently over a period of several days, to follow cellular responses in real time. Little is currently known about possible toxicity of fluorescent DNA dyes under CTLM conditions. In this study we demonstrate that the common live-cell DNA stain Hoechst 33342 can cause apoptosis under CTLM conditions. Although toxicity is evident at long times in the absence of imaging at high dye concentrations, phototoxicity from repeated excitation of the dye in the imaging process is dominant. We show that phototoxicity is a function of the product of light fluence and dye concentration, irrespective of irradiance, frequency and total number of scans. Thus, phototoxicity can be prevented by a combination of dye concentration and imaging procedure that is below this threshold. These quantitative data can be used as a guide to others performing time-lapse microscopy studies with this common live-cell DNA stain and serves as a caution for researchers when using other fluorescent stains under CTLM conditions.

Effect of radiation on vascular endothelial growth factor expression in the C2 canine mastocytoma cell line

OBJECTIVE

To establish the radiosensitivity and effect of irradiation on vascular endothelial growth factor (VEGF) and VEGF receptor (VEGFR) expression in the canine mastocytoma cell line C2.

SAMPLE POPULATION

Canine mastocytoma cell line C2.

PROCEDURES

C2 cells were irradiated with single doses of 2, 4, 6, and 8 Gy. The 3-(4, 5-di-methyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide assay and proliferation assays with (methyl-hydrogen 3) thymidine were used for radiosensitivity experiments. Expression of VEGFR was determined via flow cytometry and apoptotic rate via annexin assay. Human and canine VEGF ELISA kits were evaluated in crossover assay experiments, and the canine kit was used thereafter.

RESULTS

C2 cells secreted VEGF constitutively. Radiation did not induce a significant increase in VEGF secretion, regardless of radiation dose. Consistently, radiation did not up-regulate VEGFR. Cell survival rates decreased in a dose-dependent manner. The apoptotic cell fraction had a dose-dependent increase that reached its maximum 24 to 48 hours after radiation.

CONCLUSIONS AND CLINICAL RELEVANCE

The C2 cell line was radiosensitive, and a fraction (up to 40%) of cells died via apoptosis in a dose-dependent manner. In response to radiation, C2 cells did not upregulate VEGF production or VEGFR. Further studies are needed to determine whether tumor control could be improved by combining radiotherapy with VEGFR inhibitors or apoptosis-modulating agents.

A novel cell segmentation method and cell phase identification using Markov model

Optical microscopy is becoming an important technique in drug discovery and life science research. The approaches used to analyze optical microscopy images are generally classified into two categories: automatic and manual approaches. However, the existing automatic systems are rather limited in dealing with large volume of time-lapse microscopy images because of the complexity of cell behaviors and morphological variance. On the other hand, manual approaches are very time-consuming. In this paper, we propose an effective automated, quantitative analysis system that can be used to segment, track, and quantize cell cycle behaviors of a large population of cells nuclei effectively and efficiently. We use adaptive thresholding and watershed algorithm for cell nuclei segmentation followed by a fragment merging method that combines two scoring models based on trend and no trend features. Using the context information of time-lapse data, the phases of cell nuclei are identified accurately via a Markov model. Experimental results show that the proposed system is effective for nuclei segmentation and phase identification.

The role of apoptosis in radiation oncology

PURPOSE

Apoptosis, as a mode of cell death in irradiated cell populations, has been the subject of literarily hundreds if not thousands of published reports over the past few years. However, in spite of the large body of knowledge related to this subject, the role of apoptosis in determining tumor response to radiotherapy has been and remains poorly understood and controversial. Indeed, some previous reviews have suggested that apoptosis may not be important in this context. The purpose of the present review is to provide some examples of recently reported laboratory investigations that indicate that there is a reasonable expectation that the radiation-induced apoptosis observed has contributed to the tumor response.

CONCLUSIONS

We review reports in four areas of research: Molecularly targeted agents, in vivo imaging, Bcl-2 and cancer stem cells. Examples are provided in each of these areas that we believe justify a reassessment of the role that apoptosis plays in radiation oncology.

Is defect in phosphorylation of Nbs1 responsible for high radiosensitivity of T-lymphocyte leukemia cells MOLT-4?

Mutations in NBS1 gene are related to higher occurrence of malignancies. In this work we studied response of T-lymphocyte leukemia cells MOLT-4 to ionizing radiation. We detected IRIF (ionizing radiation forming foci) containing histone gammaH2A.X, protein 53BP1, and Nbs1, which were formed around double-strand breaks of DNA. We found dose-dependent increase in foci number (colocalization of gammaH2A.X and 53BP1) and gammaH2A.X amount (integral optical density) 1h after irradiation. After the dose of 1.5 Gy the number of foci decreases with time, but 72 h after irradiation 9% of live cells still contained big foci around unrepaired DNA damage. Western blot method revealed massive phosphorylation of H2A.X during apoptosis induction, 6-24 h after irradiation by the doses 1.5 and 3 Gy. Cells with apoptotic morphology showed strong phosphorylation of H2A.X, but it was not accompanied by 53BP1. 1h after irradiation by the lethal doses 5 and 10 Gy we detected by Western blot a decrease in repair proteins Mre11, Rad50, and Nbs1. While phosphorylation of H2A.X 1h after irradiation was detected by both confocal microscopy and Western blot, phosphorylation of Nbs1 on serine 343 was not detectable in MOLT-4 cells. Despite functional ATM and p53 the phosphorylation of Nbs1 on serine 343 was impaired in these cells, and might be responsible for high radiosensitivity of MOLT-4 cells.

Mammalian cell killing by ultrasoft X rays and high-energy radiation: an extension of the MK model

An alternate formulation of the microdosimetric-kinetic (MK) model is presented that applies to irradiation of mammalian cells with ultrasoft X rays as well as high-energy radiations of variable linear energy transfer (LET). Survival and DNA double-strand break measurements for V79 cells from the literature are examined to illustrate application of the model. It is demonstrated that the linear component of the linear-quadratic survival relationship (alpha) is enhanced because repairable potentially lethal lesions formed from a single ultrasoft X-ray energy deposition event, when closer on average than for a single high-energy radiation event, are more likely to combine to form a lethal lesion. The quadratic component (beta) of the linear-quadratic survival relationship is increased because the potentially lethal lesions formed by ultrasoft X rays are created with greater efficiency than those of high-energy radiation. In addition, potentially lethal lesions from very low-energy carbon K-shell X rays may be enriched in structural forms that favor combination to form lethal lesions instead of repair. These features account for the increased effectiveness of killing of V79 cells by ultrasoft X rays compared to cobalt-60 gamma radiation. The importance of pairwise combination of potentially lethal lesions to form exchange chromosome aberrations that become lethal lesions is discussed. The extended MK model explains and reconciles differences between the MK model and the theory of dual radiation action on the one hand, and on the other, the view that variation in the RBE with radiation quality is explained by differences in energy deposition in nanometer- rather than micrometer-size volumes.

The influence of concentration of DNA on the radiosensitivity of mammalian cells

PURPOSE

To develop relations that explicitly show the dependence of the linear-quadratic survival parameters alpha and beta on the nuclear volume, average DNA concentration, and degree to which the chromatin is condensed in zones of the nucleus.

METHODS AND MATERIALS

The microdosimetric-kinetic model of mammalian cell killing is used.

RESULTS

The relations indicate an increased tendency for lethal lesions to form by pairwise combination of potentially lethal lesions in regions of relatively greater local DNA concentration. In particular, the value of beta is proportional to the average nuclear concentration of DNA and to a parameter that reflects the condensation of DNA into a relatively concentrated phase, such as occurs in mitotic chromosomes and heterochromatin. These relations indicate local DNA concentration in the nucleus is a significant determinant of three magnitude of beta. Because alpha is composed of a term that is proportional to beta, it is also affected by DNA concentration, generally to equal or less degree.

CONCLUSION

It may be possible to establish a correlation between nuclear size and morphology, as determined from microscope slides prepared from biopsy specimens, with radiation sensitivity and the alpha/beta ratio that could be useful in radiation treatment planning.

L5178Y sublines: a look back from 40 years. Part 2: response to ionizing radiation

The aim was to review and summarize the results of 40 years of study concerning the response to ionizing radiation of the pair of L5178Y (LY) sublines, LY-R and LY-S, that differ in sensitivity to various DNA-damaging agents, among them X- and gamma-rays. The reviewed data indicate the key importance of DNA damage repair and fixation for the ultimate fate of the irradiated LY cell. The cause of slow double-strand break (DSB) repair in LY-S cells is not identified, but a defect in non-homologous end-joining (NHEJ) would explain most features of the cellular response of LY-S cells to irradiation, as compared with repair-competent LY-R cells. The most prominent features are the very high radiosensitivity of G1 cells, extensive poly(ADP-ribose)-dependent damage fixation, long G2 arrest, considerable chromosomal damage seen as premature chromatin condensation (PCC) fragments and aberrations in metaphase cells. The main cause of radiosensitivity difference between LY sublines is in DNA repair/damage fixation ability. At the level of damage corresponding to a comparable lethal effect, the type of death differs between LY sublines; LY-S cells die in considerably greater proportion by apoptosis than LY-R cells, whereas the latter die in greater proportion by necrosis. This observation is consistent with differential expression of proteins that are pro- or anti-apoptotic. The prominent role of poly(ADP-ribosylation) in the response of LY-S cells apparently is connected with damage fixation, but is in contrast to other cell lines hypersensitive to X- or gamma-radiation with DSB repair defects.

Hypoxia-induced BAX overexpression and radiation killing of hypoxic glioblastoma cells

One major challenge in treating glioblastoma multiforme (GBM) has been the presence of radiation-resistant hypoxic cells. The pro-apoptosis protein BAX has been reported to be a possible radiation sensitizer of cancer cells; however, to our knowledge, no studies have reported on the effects of BAX on radiation sensitivity under hypoxic conditions. Therefore, in this study, we specifically overexpressed murine Bax in hypoxic cells in an attempt to enhance radiation-induced cell killing. We have previously stably transfected U-251 MG and U-87 MG human GBM cells with constructs containing murine Bax under the regulation of nine copies of hypoxia-responsive elements (HREs). During hypoxia, the transcriptional complex hypoxia-inducible factor 1 (HIF1) forms and binds to HRE; this binding facilitates the transcription of downstream genes. In the experiments reported here, two protocols were used. In the first protocol, parent and clone cells were exposed to graded doses of X rays under hypoxic conditions, left hypoxic for 0, 4, 16 or 24 h, and then assayed for clonogenic cell survival. In the second protocol, cells were incubated under hypoxic conditions for 20 h, irradiated with graded doses under hypoxia, then left in hypoxic conditions for 4 h before being assayed for clonogenic cell survival. Western blots showed that we had successfully increased Bax expression in both U-251 MG and U-87 MG Bax clone cells after 16 h of hypoxic incubation, yet dose-response curves showed no difference in radiation-induced cell killing between control non-Bax-expressing pNeo clone cells and the U-251 MG Bax clone cells using either protocol. In U-87 MG cells, the first protocol showed no difference in radiation response between control pNeo clone cells and the Bax clone, similar to the results obtained in U-251 cells. However, the second protocol revealed that Bax overexpression did render these cells more sensitive to radiation under hypoxic conditions. Therefore, we conclude that whether Bax is a radiation enhancer under hypoxia not only is cell line-dependent but also depends on when the Bax overexpression occurs.

The role of apoptosis in cancer development and treatment response

The inactivation of programmed cell death, or apoptosis, is central to the development of cancer. This disabling of apoptotic responses might be a major contributor both to treatment resistance and to the observation that, in many tumours, apoptosis is not the main mechanism for the death of cancer cells in response to common treatment regimens. Importantly, this suggests that other modes of cell death are involved in the response to therapy.

Disruption of cancer cell replication by alternating electric fields

Low-intensity, intermediate-frequency (100-300 kHz), alternating electric fields, delivered by means of insulated electrodes, were found to have a profound inhibitory effect on the growth rate of a variety of human and rodent tumor cell lines (Patricia C, U-118, U-87, H-1299, MDA231, PC3, B16F1, F-98, C-6, RG2, and CT-26) and malignant tumors in animals. This effect, shown to be nonthermal, selectively affects dividing cells while quiescent cells are left intact. These fields act in two modes: arrest of cell proliferation and destruction of cells while undergoing division. Both effects are demonstrated when such fields are applied for 24 h to cells undergoing mitosis that is oriented roughly along the field direction. The first mode of action is manifested by interference with the proper formation of the mitotic spindle, whereas the second results in rapid disintegration of the dividing cells. Both effects, which are frequency dependent, are consistent with the computed directional forces exerted by these specific fields on charges and dipoles within the dividing cells. In vivo treatment of tumors in C57BL/6 and BALB/c mice (B16F1 and CT-26 syngeneic tumor models, respectively), resulted in significant slowing of tumor growth and extensive destruction of tumor cells within 3-6 days. These findings demonstrate the potential applicability of the described electric fields as a novel therapeutic modality for malignant tumors.

Bcl2-Independent Chromatin Cleavage is a Very Early Event during Induction of Apoptosis in Mouse Thymocytes after Treatment with Either Dexamethasone or Ionizing Radiation

We have quantified the emergence of early chromatin breaks during the signal transduction phase of apoptosis in mouse thymocytes after treatment with either ionizing radiation or dexamethasone. Dexamethasone at 1 microM can induce significant levels of DNA breaks (equivalent to the amount induced directly by 7.5 Gy ionizing radiation) within 0.5 h of treatment. The execution phase of apoptosis was not observed until 4-6 h after the same treatment. The presence of the Bcl2 transgene under the control of the p56lck promoter almost completely inhibited apoptosis up to 24 h after treatment, but it had virtually no effect on the early chromatin cleavage occurring in the first 6 h. Ionizing radiation induced chromatin cleavage both directly by damaging DNA and indirectly with kinetics similar to the induction of chromatin cleavage by dexamethasone. The presence of the Bcl2 transgene had no effect on the direct or indirect radiation-induced cleavage in the first 6 h, but after the first 6 h, the Bcl2 gene inhibited further radiation-induced chromatin cleavage. These results suggest that endonucleases are activated within minutes of treatment with either dexamethasone or ionizing radiation as part of the very early signal transduction phase of apoptosis, and prior to the irreversible commitment to cell death.

Computerized video time-lapse (CVTL) analysis of the fate of giant cells produced by X-irradiating EJ30 human bladder carcinoma cells

This study was designed to examine the viability and proliferation of uninucleated and multinucleated giant cells formed after 6 Gy X irradiation. The pedigrees of 102 individual EJ30 giant cells present 5 days after irradiation were analyzed from time-lapse movies captured over 6.3 days from 100 fields (100x). Pedigree analysis enabled us to study the proliferation of giant cells. The average starting size (area) of the giant cells (14500 +/- 9100 microm(2)) was approximately 25 times larger than the normal-sized cells observed after irradiation (560 +/- 200 microm(2)). From a total of 76 pedigrees of uninucleated giant cells, 42 had giant cells that either died or were arrested, while 34 divided at least once and produced progeny that divided again (five three times and three four times) before the progeny died or were arrested. Twenty-four pedigrees contained progeny that were lost from observation after dividing at least once. While most progeny continued to have giant cell morphology, two uninucleated giant cells ultimately produced progeny that contained two normal-sized cells. From a total of 26 multinucleated giant cells, only three divided. Two divided only once, but one produced progeny that divided two times. In all, 37 out of 102 giant cells divided at least once; eight of these divided four or five times with two of these pedigrees ultimately producing two normal-sized daughter cells. These results suggest that a small fraction of giant cells might be potentially clonogenic.

Identification of transcriptome profiles for the DNA-damaging agents bleomycin and hydrogen peroxide in L5178Y mouse lymphoma cells

It is believed that some aspects of genotoxicity are associated with changes in the transcription levels of certain genes, especially those involved in DNA repair and cell cycle control. Additionally, it is hypothesized that chemicals sharing a common mode of genotoxicity should exhibit similar changes in gene expression. We have evaluated these hypotheses by analyzing transcriptome profiles of mouse lymphoma L5178Y/TK(+/-) cells treated with bleomycin and hydrogen peroxide, two mutagens that produce genotoxicity by generating reactive free radicals. The cells were treated for 4 hr and RNA was isolated at the end of the treatment and after a 20 hr recovery. Transcriptome analyses were performed using the Clontech Mouse 1.2K cDNA microarray (1,185 genes) and hybridization with a (32)[P]-labeled probe. Of the genes examined, each mutagen altered the expression (1.5-fold or greater) of only two genes after the 4 hr treatment. In cells allowed to recover for 20 hr after treatment, bleomycin and hydrogen peroxide altered the expression of 8 and 5 genes, respectively. Many of the altered genes have some association with apoptosis. Of these genes, three (the genes encoding granzyme A, integrin beta 7, and 45 kDa calcium-binding protein precursor) were in common between chemical treatments. The expression of DNA repair and cell cycle controlling genes present on the array was not affected by the treatments. These results show that bleomycin and hydrogen peroxide both have unique and commonly regulated genes that have the potential to serve as biomarkers of exposure to agents causing DNA damage by free radical mechanisms.

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

Down regulation of p53 with HPV E6 delays and modifies cell death in oxidant response of human diploid fibroblasts: an apoptosis-like cell death associated with mitosis

The tumor suppressor p53 protein is known to play a critical role in apoptosis. In normal human diploid fibroblasts (HDFs), expression of the human papillomaviral (HPV) E6 gene results in a reduction of p53 protein and an inhibition of oxidant induced apoptosis within 24 h. In comparison, expression of the HPV E7 gene causes down-regulation of Rb protein without inhibiting apoptosis. Here we determine whether HDFs expressing E6 undergo cell death with a delayed time course following H2O2 exposure. Appearances of caspase-3 activity, cell detachment, trypan blue uptake and aberrant nuclei were all delayed in E6 cells compared to wild type (wt) or E7 cells. A mutant E6 gene that failed to reduce p53 could not delay cell death. Morphological examination revealed nuclear condensation in dying wt or E7 cells but nuclear fragmentation in E6 cells. Flow cytometry analysis indicated an S phase distribution of dying wt or E7 cells but a G2/M phase distribution of dying E6 cells. An elevation of cyclin B was observed in dying E6 cells but not in apoptotic E7 cells. Dying E6 cells also had elevated levels of cdc-2 protein and histone kinase activity, suggesting that the cells died at mitosis. Electron microscopy studies showed that E6 cells may die at prophase or prometaphase. Overexpression of bcl-2 resulted in an inhibition of both caspase-3 and death of E7 or E6 cells. Inactivating caspases with zVAD-fmk also reduced the death rate of E7 and E6 cells. Our data indicate that expression of HPV E6 causes a delay and morphological modification of cell death induced by oxidants. E6 cells die at mitosis, which can be inhibited by bcl-2 overexpression or caspase inhibition.

The generation and characterization of a radiation-resistant model system to study radioresistance in human breast cancer cells

To systematically study the selection of radioresistant cells in clinically advanced breast cancer, a model system was generated by treating MDA-MB231 breast cancer cells with fractionated gamma radiation. A clonogenic assay of the surviving cell populations showed that 2-6 Gy per fraction resulted in a rapid selection of radioresistant populations, within three to five fractions. Irradiation with additional fractions after this initial increase did not increase the radioresistance of the surviving population significantly. Doses of 0.5 and 8 Gy per fraction were not effective in selecting radioresistant cells. To further determine the cause of the changes in radiosensitivity, 15 clones were isolated from the cell populations treated with 40 or 60 Gy with 2 or 4 Gy per fraction, respectively, and were analyzed for radiosensitivity. The average D(10) for these clones was 6.75 +/- 0.36 Gy, which was higher than that for the parental cell population (D(10) = 6.0 +/- 0.2 Gy). The operation of cell cycle checkpoints and the doubling time were similar for both the nonirradiated parental population and the isolated radioresistant subclones. In contrast, a decrease in the apoptotic potential was correlated (r = 0.7, P < 0.01) with increased survival after irradiation, suggesting that apoptosis is an important factor in determining radioresistance under our experimental conditions. We also isolated several subclones from the nonirradiated parental cell population and analyzed them to determine their radiosensitivity after fractionated irradiation. Ten fractions of 4 Gy (40 Gy in total) did not result in a significant increase in the radioresistance of these subclones compared to the irradiated cell populations. The possible mechanisms of the increased radioresistance after fractionated irradiation are discussed.

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.

Computerized video time-lapse analysis of apoptosis of REC:Myc cells X-irradiated in different phases of the cell cycle

Asynchronous rat embryo cells expressing Myc were followed in 50 fields by computerized video time lapse (CVTL) for three to four cycles before irradiation (4 Gy) and then for 6-7 days thereafter. Pedigrees were constructed for single cells that had been irradiated in different parts of the cycle, i.e. at different times after they were born. Over 95% of the cell death occurred by postmitotic apoptosis after the cells and their progeny had divided from one to six times. The duration of the process of apoptosis once it was initiated was independent of the phase in which the cell was irradiated. Cell death was defined as cessation of movement, typically 20-60 min after the cell rounded with membrane blebbing, but membrane rupture did not occur until 5 to 40 h later. The times to apoptosis and the number of divisions after irradiation were less for cells irradiated late in the cycle. Cells irradiated in G(1) phase divided one to six times and survived 40-120 h before undergoing apoptosis compared to only one to two times and 5-40 h for cells irradiated in G(2) phase. The only cells that died without dividing after irradiation were irradiated in mid to late S phase. Essentially the same results were observed for a dose of 9.5 Gy, although the progeny died sooner and after fewer divisions than after 4 Gy. Regardless of the phase in which they were irradiated, the cells underwent apoptosis from 2 to 150 h after their last division. Therefore, the postmitotic apoptosis did not occur in a predictable or programmed manner, although apoptosis was associated with lengthening of both the generation time and the duration of mitosis immediately prior to the death of the daughter cells. After the non-clonogenic cells divided and yielded progeny entering the first generation after irradiation with 4 Gy, 60% of the progeny either had micronuclei or were sisters of cells that had micronuclei, compared to none of the progeny of clonogenic cells having micronuclei in generation 1. However, another 20% of the non-clonogenic cells had progeny with micronuclei appearing first in generation 2 or 3. As a result, 80% of the non-clonogenic cells had progeny with micronuclei. Furthermore, cells with micronuclei were more likely to die during the generation in which the micronuclei were observed than cells not having micronuclei. Also, micronuclei were occasionally observed in the progeny from clonogenic cells in later generations at about the same time that lethal sectoring was observed. Thus cell death was associated with formation of micronuclei. Most importantly, cells irradiated in late S or G(2) phase were more radiosensitive than cells irradiated in G(1) phase for both loss of clonogenic survival and the time of death and number of divisions completed after irradiation. Finally, the cumulative percentage of apoptosis scored in whole populations of asynchronous or synchronous populations, without distinguishing between the progeny of individually irradiated cells, underestimates the true amount of apoptosis that occurs in cells that undergo postmitotic apoptosis after irradiation. Scoring cell death in whole populations of cells gives erroneous results since both clonogenic and non-clonogenic cells are dividing as non-clonogenic cells are undergoing apoptosis over a period of many days.