Radiation response of mouse lymphoid and myeloid cell lines. Part I. Sensitivity to killing by ionizing radiation, rate of loss of viability, and cell type of origin

Normal Tissue Complication Probability Modeling of Severe Radiation-Induced Lymphopenia Using Blood Dose for Patients With Hepatocellular Carcinoma

PURPOSE

This study aimed to develop a normal tissue complication probability (NTCP) model to estimate the risk of severe radiation-induced lymphopenia (SRIL; absolute lymphocyte count [ALC] < 500/μL) by using the blood dose of patients with hepatocellular carcinoma (HCC).

METHODS AND MATERIALS

We retrospectively collected data from 75 patients with HCC who received radiation therapy (RT) between 2015 and 2018. The hematological dose framework calculated blood dose-volume histograms (DVHs) using a predefined blood flow model, organ DVHs, the number of treatment fractions, and beam delivery time. A Lyman-Kutcher-Burman model with a generalized equivalent dose was used to establish the NTCP model, reflecting the whole-blood DVHs. Optimization of the Lyman-Kutcher-Burman parameters was conducted by minimizing a negative log-likelihood function.

RESULTS

There were 6, 4, 18, 33, and 14 patients in the groups with radiation-induced lymphopenia grades 0, 1, 2, 3, and 4, respectively. The median pre- and post-RT ALC values were 1410/μL (range, 520-3710/μL) and 470/μL (range, 60-1760/μL), respectively. There was a correlation between mean blood dose and ALC depletion (Pearson r = -0.664; P < .001). The average mean blood doses in each radiation-induced lymphopenia group were 2.90 Gy (95% CI, 1.96-3.85 Gy) for grade 0 to 1, 5.29 Gy (95% CI, 4.12-6.45 Gy) for grade 2, 8.81 Gy (95% CI, 7.55-10.07 Gy) for grade 3, and 11.69 Gy (95% CI, 9.82-17.57 Gy) for grade 4. When applying the developed NTCP model to predict SRIL, the area under the receiver operating characteristic curve and Brier score values were 0.89 and 0.12, respectively.

CONCLUSIONS

We developed the first NTCP model based on whole-blood DVHs for estimating SRIL after abdominal RT in patients with HCC. Our results showed a strong correlation between blood dose and ALC depletion, suggesting the potential to predict the risk of SRIL occurrence using blood dose.

Lymphocyte radiosensitivity: An extension to the linear-quadratic model?

BACKGROUND AND PURPOSE

The linear-quadratic (LQ) model has been pivotal for evaluating the effects of radiation on cells, but it is primarily characterized by linear responses, which has exhibited limitations when applied to lymphocyte data. The present research aims to address these limitations and to explore an alternative model extended from the conventional LQ model.

MATERIALS AND METHODS

Literature providing lymphocyte counts from assays investigating apoptosis and survival after in vitro irradiation was selected. To address the nonlinearity in lymphocyte responses to radiation, we developed a saturation model characterized by a negative exponential relationship between radiation dose and cellular response. We compared the performance of this saturation model against that of conventional models, including the LQ model and its variants (linear model LM and linear-quadratic-cubic model LQC), as well as the repair-misrepair (RMR) model. The models were evaluated based on prediction-residual plots, residual standard errors, and the Akaike information criterion (AIC). We applied the saturation model to two additional datasets: (1) a dataset from the existing literature that assessed stimulated and unstimulated human lymphocytes exposed to gamma irradiation in vitro and (2) a novel dataset involving T lymphocytes from rodent spleens after exposure to various radiation types (X-rays and protons).

RESULTS

The literature (n = 15 out of 2342) showed that lymphocyte apoptosis varies with dose, time and experimental conditions. The saturation model had a lower AIC of 718 compared to the LM, LQ, LQC and RMR models (AIC of 728, 720, 720 and 734, respectively). The saturation model had a lower residual error and more consistent error distribution. Integrating time as a covariate, the saturation model also had a better AIC for demonstrating time-dependent variations in lymphocyte responses after irradiation. For datasets involving unstimulated lymphocytes before irradiation, the saturation model provided a more accurate fit than did the LM, LQ, and RMR models. In these cases, the fit of the saturation model was comparable to that of the LQC model but offered an advantage when extrapolating to higher doses, where the LQC model might underestimate survival. For stimulated lymphocytes, which are radioresistant, all the models approximated the LM. Both the LQ and saturation models indicated greater radiosensitivity to protons in vitro.

CONCLUSION

The new "saturation model" performed better than the LQ model in quantifying lymphocyte apoptosis and survival, estimating time dependency and assessing the role of radiation modalities or lymphocyte stimulation. Further experiments are warranted to experimentally explore the validity of the saturation model as a promising alternative in the clinical setting.

Beyond lymphopenia, unraveling radiation-induced leucocyte subpopulation kinetics and mechanisms through modeling approaches

Leucocyte subpopulations in both lymphoid and myeloid lineages have a significant impact on antitumor immune response. While radiation-induced lymphopenia is being studied extensively, radiation effects on lymphoid and myeloid subtypes have been relatively less addressed. Interactions between leucocyte subpopulations, their specific radiation sensitivity and the specific kinetics of each subpopulation can be modeled based on both experimental data and knowledge of physiological leucocyte depletion, production, proliferation, maturation and homeostasis. Modeling approaches of the leucocyte kinetics that may be used to unravel mechanisms underlying radiation induced-leucopenia and prediction of changes in cell counts and compositions after irradiation are presented in this review. The approaches described open up new possibilities for determining the influence of irradiation parameters both on a single-time point of acute effects and the subsequent recovery of leukocyte subpopulations. Utilization of these approaches to model kinetic data in post-radiotherapy states may be a useful tool for further development of new treatment strategies or for the combination of radiotherapy and immunotherapy.

Expanding the role of interventional oncology for advancing precision immunotherapy of solid tumors

Adoptive cell therapy with chimeric antigen receptors (CAR) T cells has proven effective for hematologic malignancies, but success in solid tumors has been impeded by poor intratumoral infiltration, exhaustion of effector cells from antigen burden, and an immunosuppressive tumor microenvironment. Results from recent clinical trials and preclinical studies lend promising evidence of locoregional approaches for CAR T cell delivery, priming the tumor microenvironment, and performing adjuvant therapies that sustain T cell activity. Interventional oncology is a subspeciality of interventional radiology where imaging guidance is used to perform percutaneous and catheter-directed procedures for localized, non-surgical therapy or interrogation of solid tumors. Interventional oncology provides unique synergies with immunotherapy, which has been well-studied to improve treatment efficacy while reducing toxicities associated with systemic treatment. Besides aiding in CAR T cell delivery, priming, or the stimulation of the tumor microenvironment to promote effector survival and function, interventional oncology can also aid in the monitoring of treatment response through selective, multiplex tumor sampling and catheter-based venous sampling. This review presents an overview of interventional oncology, its various procedures, and its potential for advancing CAR T cell immunotherapy of solid tumors.

Prediction of radiation-induced toxicity by in vitro radiosensitivity of lymphocytes in prostate cancer patients

Aim: To identify predictive assays for radiation-induced toxicity in prostate cancer patients. Patients & methods: Patients have been surveyed prospectively before and up to 16 months after radiotherapy using a validated questionnaire. Subgroups of 25 patients with minor and larger score changes, respectively, were selected for γ-H2AX, G2 and Annexin V assays. Results: A significantly higher spontaneous chromatid aberration yield (HR: 1.46 [95% CI: 1.02–2.09]; p = 0.04), higher levels of early apoptotic (HR: 1.12 [95% CI: 1.01–1.24]; p = 0.04) and late apoptotic and necrotic (HR: 1.10 [95% CI: 0.99–1.23]; p = 0.08) lymphocytes 24 h post-irradiation were found in patients with a bowel bother score decrease greater than 20 points more than 1 year after treatment. Conclusion: Chromatid aberration and apoptosis/necrosis assays appear to be suitable for the prediction of radiation-induced toxicity.

Ionizing Radiation Impairs T Cell Activation by Affecting Metabolic Reprogramming

Ionizing radiation has a variety of acute and long-lasting adverse effects on the immune system. Whereas measureable effects of radiation on immune cell cytotoxicity and population change have been well studied in human and animal models, little is known about the functional alterations of the surviving immune cells after ionizing radiation. The objective of this study was to delineate the effects of radiation on T cell function by studying the alterations of T cell receptor activation and metabolic changes in activated T cells isolated from previously irradiated animals. Using a global metabolomics profiling approach, for the first time we demonstrate that ionizing radiation impairs metabolic reprogramming of T cell activation, which leads to substantial decreases in the efficiency of key metabolic processes required for activation, such as glucose uptake, glycolysis, and energy metabolism. In-depth understanding of how radiation impacts T cell function highlighting modulation of metabolism during activation is not only a novel approach to investigate the pivotal processes in the shift of T cell homeostasis after radiation, it also may lead to new targets for therapeutic manipulation in the combination of radiotherapy and immune therapy. Given that appreciable effects were observed with as low as 10 cGy, our results also have implications for low dose environmental exposures.

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.

Chromosomal rearrangement as the basis for human tumourigenesis

PURPOSE

To develop a model for the initiation of human tumourigenesis that is consistent with various observations that are difficult to reconcile with current models.

CONCLUSIONS

A novel model of tumourigenesis was developed that includes three basic postulates: (1) tumourigenesis is initiated by recombinogenic DNA lesions, (2) potentially recombinogenic DNA lesions in transcribed regions of the genome can be converted into chromosomal rearrangements and (3) chromosomal rearrangements alone are insufficient for tumourigenesis but can initiate a mutator/recombinator phenotype.

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.

Phosphorylation of histone H2B at DNA double-strand breaks

Posttranslational modifications of histone tails regulate numerous biological processes including transcription, DNA repair, and apoptosis. Although recent studies suggest that structural alterations in chromatin are critical for triggering the DNA damage response, very little is known about the nature of DNA damage-induced chromatin perturbations. Here we show that the serine 14 residue in the NH₂-terminal tail of histone H2B is rapidly phosphorylated at sites of DNA double-strand breaks. At late time points after irradiation, the phosphorylated form of H2B, H2B-^Ser14P, accumulates into irradiation-induced foci. H2B-^Ser14P foci formation is not associated with the apoptotic phosphorylation of H2B but is strictly dependent on the phosphorylated isoform of H2AX. Our results broaden the spectrum of histone modifications that constitute the DNA damage “histone code” and suggest a model for the underlying chromatin structure within damage-induced foci.

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.

Evaluation of the histologic effect of chemoradiation therapy for squamous cell carcinomas of the esophagus by assessing morphologic features of surgical specimens

The histologic effects of chemoradiation therapy (CRT) for esophageal cancer, which determine the benefit obtained from a salvage operation, are difficult to evaluate preoperatively. We therefore investigated whether or not the morphologic features of esophageal cancer tissue after CRT can be correlated with the histologic features of the tissue. Seventy-six patients with advanced esophageal squamous cell carcinoma underwent CRT followed by esophagectomy. The effects of CRT were evaluated by histologic examination of the residual tumors in the surgical specimen and correlated with clinicopathologic factors, including postoperative prognosis. The histologic effects of CRT were used to classify tumors as grade 1 (CRT poorly effective; 23 cases, 30.3%); grade 2 (CRT moderately effective; 31 cases, 40.8%); or grade 3 (CRT completely effective with no residual tumors; 22 cases, 28.9%). Among the gross findings of the removed esophagus, significant correlation with the CRT effects was observed in the case of wall thickness and ulceration but not in the case of longitudinal tumor length. Tumors with no wall thickening or ulceration were never classified as grade 1, whereas tumors with both wall thickening and ulceration were frequently rated as grade 1 (18/30, 60%). Microscopic examination of grade 2 tumors (23/31, 74.1%) revealed residual tumor cells growing below the mucosal layer, whereas tumor cells were exposed to the esophageal surface in 22 out of 23 patients with grade 1 tumors. The morphologic features after CRT can be used to evaluate its histologic effect, especially in the case of grade 1 tumors. However, the detection and prediction of grade 2 tumors remains difficult because of the presence of small amounts of residual tumor underneath the mucosa.

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.

Expression of p53, p21/WAF/CIP, Bcl-2, Bax, Bcl-x, and Bak in radiation-induced apoptosis in testicular germ cell tumor lines

PURPOSE

Testicular germ cell tumors (TGCTs) represent one of the few tumor types that are curable by antineoplastic therapy, probably due to the high sensitivity of this neoplasm to induction of apoptosis by chemotherapeutic agents and/or ionizing radiation. Here, we tested cell susceptibility to radiation-induced apoptosis in a panel of TGCT cell lines and attempted to correlate this with the known potentially relevant molecular determinants (p53 gene status and Bcl-2 family proteins) of apoptosis.

METHODS AND MATERIALS

Induction of apoptosis by gamma-radiation was morphologically recognized in NT2, NCCIT, S2, and 2102 EP using Hoechst/PI staining and additionally confirmed by Western blot analysis of PARP cleavage. The p53 gene status was estimated by sequence analysis. Expression of p21/WAF/CIP was determined by Northern blot analysis and immunoblotting was used to monitor p53, Bax, Bcl-2, Bcl-x, and Bak protein levels. In vitro colony formation was studied to establish clonogenic survival curves.

RESULTS

NT2 and NCCIT appeared to be susceptible for radiation-induced apoptosis, contrasting 2102 EP and S2 which were highly resistant. Sequence analysis showed that NT2, S2, and 2102 EP are homozygous for wild-type p53 (wtp53), whereas NCCIT contains mutant p53 (mtp53). NT2 and 2102 EP cells showed radiation-induced p53 upregulation, while NCCIT (mtp53) and S2 (no p53 protein) cells did not. Consistently, gamma-radiation-induced DNA damage resulted in a p53-dependent transactivation of the p21/WAF/CIP gene in NT2 and 2102 EP, but not in mtp53-containing NCCIT cells and p53 nonexpressing S2 cells. Constitutive expression of Bax, Bcl-2, Bcl-x, and Bak was not affected by radiation and showed no correlation with cell susceptibility to radiation-induced apoptosis. A discrepancy was found between apoptosis and reproductive death.

CONCLUSIONS

The present study revealed that: i) the presence of wtp53 may not be absolutely required for the hypersensitivity for radiation-induced apoptosis in TGCT cell lines, ii) the molecular mechanism underlying the unique radiosensitivity was independent of the expression of Bcl-2 family proteins, and iii) cell susceptibility to apoptosis induction is not sufficiently informative to predict intrinsic radiosensitivity as determined by clonogenic survival.

Radiation-induced apoptosis in different pH environments in vitro

PURPOSE

The effect of environmental pH on the radiation-induced apoptosis in tumor cells in vitro was investigated.

METHODS AND MATERIALS

Mammary adenocarcinoma cells of A/J mice (SCK cells) were irradiated with gamma-rays using a 137Cs irradiator and incubated in media of different pHs. After incubation at 37 degrees C for 24-120 h the extent of apoptosis was determined using agarose gel electrophoresis, TdT-mediated dUTP-biotin nick end labeling (TUNEL) staining, flow cytometry, and release of 3H from 3H-thymidine labeled cells. The clonogenicity of the cells irradiated in different pH medium was determined, and the progression of cells through the cell cycle after irradiation in different pHs was also determined with flow cytometry.

RESULTS

Irradiation with 2-12 Gy of gamma-rays induced apoptosis in SCK cells in pH 7.5 medium within 48 h as judged from the results of four different assays mentioned. Radiation-induced apoptosis declined as the medium pH was lowered from 7.5 to 6.4. Specifically, the radiation-induced degradation of DNA including the early DNA breaks, as determined with the TUNEL method, progressively declined as the medium pH was lowered so that little DNA fragmentation occurred 48 h after irradiation with 12 Gy in pH 6.6 medium. When the cells were irradiated and incubated for 48 h in pH 6.6 medium and the medium was replaced with pH 7.5 medium, DNA fragmentation promptly occurred. DNA fragmentation also occurred even in pH 6.6 medium when the cells were irradiated and maintained in pH 7.5 medium for 8 h or longer post-irradiation before incubation in pH 6.6 medium. The radiation-induced G2 arrest in pH 6.6 medium lasted markedly longer than that in pH 7.5 medium.

CONCLUSION

Radiation-induced apoptosis in SCK cells in vitro is reversibly suppressed in an acidic environment. Taking the results of four different assays together, it was concluded that early step(s) in the apoptotic pathway, probably the DNA break or upstream of DNA break, is reversibly halted by an acidic environment in irradiated cells. Radiation-induced G2 arrest is prolonged in an acidic environment indicating that the suppression of radiation-induced apoptosis and prolongation of radiation-induced G2 arrest in an acidic environment are related.

Radiation induced apoptosis

The response to ionising radiation, in terms of level of cell killing, depends on a number of factors that may be grouped into those that are genetically controlled, radiation quality and dosage, and environmental factors. There is a range of genetically controlled cellular properties such as stage of differentiation, mutations in specific genes (such as p53 and bcl-2) and stage of transformation that will determine the ability of the target cell to enter apoptosis. The so-called normal cells, are usually more radiosensitive and the majority of the cell population will enter into an apoptotic death. However, in response to high doses of ionising radiation and complex DNA damage as produced by high-LET radiation, an increased fraction of these cells will die by necrosis. There are several examples of environmental factors with relevance for the combined action of radiation and xenobiotics on carcinogenesis and in tumour therapy. In the case of normal cells, agents such as growth factors and tumour promoters, may decrease radiosensitivity. For certain type of tumour cells, radiation sensitivity can be increased in the presence of agents such as hormones, and the cells may die an apoptotic death. Removal of heavily compromised cells is essential to prevent a potential spreading of mutated clones. However, if apoptosis is inhibited (e.g., by tumour promoter), an increased fraction of damaged cells carrying genotoxic lesions may survive. This would significantly increase the risk of proliferation of precancerous cells. As discussed above, it is probably incorrect to make predictions about relative radiosensitivity based solely on mode of death. Intrinsic characteristics deriving from the cell type of origin of a line may be more important in determining radiosensitivity. The rapidly increasing knowledge about the process of radiation induced apoptosis has opened new frontiers in radiation biology, genetic toxicology, and cancer therapy and strongly motivates further research in this field.

A role for apoptosis in the toxicity and mutagenicity of bleomycin in AHH-1 tk+/- human lymphoblastoid cells

The chromosomal mutagen, bleomycin, is also noted for its toxic properties, although the mechanism of cell death is not fully understood. In order to determine if cell death occurred by apoptosis or necrosis, AHH-1 tk+/- cells were exposed to bleomycin and the percentage of viable, apoptotic and necrotic cells quantified by flow cytometry. Logistic regression analysis indicated that the primary manner of cell death was through the apoptosis pathways, that apoptosis was delayed, and that apoptosis was accompanied by an arrest in the G2 phase of the cell cycle. Once apoptosis was established as a mechanism for cell death, the efficiency with which these pathways removed damaged cells from the population was evaluated with the use of specific-locus mutation assays (tk and hprt) as indicators of cells with DNA damage that maintained viability and clonogenicity. Linear regression analysis detected a significant, concentration-dependent increase in the numbers of TFTr clones with the slow-growth phenotype. This suggests that a proportion of cells with bleomycin-induced DNA damage did not undergo cell death by apoptosis and that apoptosis, a mechanism for the destruction of damaged cells, is not fully efficient in the AHH-1 tk +/- cell line.

Apoptosis--molecular mechanisms and biomedical implications

Apoptosis is a distinct form of cell death of importance in tissue development and homeostasis and in several diseases. This review summarizes current knowledge about the regulation and molecular mechanisms of apoptosis and discusses the potential role of disregulated apoptosis in several major diseases. Finally, we speculate that modulation of apoptosis may be a target in future drug therapy.

Cell cycle traverse in AHH-1 tk +/- human lymphoblastoid cells exposed to the chromosomal mutagen, m-amsa

AHH-1 tk +/- cells were exposed to the chemotherapeutic agent, m-amsa, both in complete medium and in medium without serum, subcultured in complete medium, and the effect on the traverse of the cell cycle determined by flow cytometric analysis of bromodeoxyuridine (BrdUrd)-labeled DNA. After exposure to m-amsa (day 0), the percentage of S-phase cells increased significantly (P < 0.0017) with increasing concentration. Cells also accumulated in G2/M as evidenced by the significant (P < 0.0026), concentration-dependent increase in the percentage of cells detected within this phase. Serum deprivation during exposure resulted in significantly (P = 0.024) more cells in S-phase than in cultures exposed to m-amsa in complete medium. After three days in culture, a significant (P = 0.0001) accumulation of cells in G2/M was present; the percentage of cells in G2/M did not differ significantly (P = 0.148) in cultures exposed to m-amsa in complete medium or in serum-free medium. However, a significant (P < 0.001) loss of S-phase cells was found in cultures exposed without serum. At day 7, no significant concentration effects were detected (GO/G1, P = 0.6026; S-phase, P = 0.9773; G2/M, P = 0.8401). These results demonstrate that exposure to m-amsa perturbs the traverse of the cell cycle, initially by inhibiting the completion of S-phase and followed by an accumulation of cells in G2/M. In addition, exposure to m-amsa under conditions of serum deprivation results in an increased percentage of cells in the initial S-phase after exposure, the loss of S-phase cells from the culture after three days, and the appearance of subdiploid peak, consistent with cells undergoing apoptosis.

Radiation-induced apoptosis: relevance to radiotherapy

Radiation-induced apoptosis is reviewed in terms of: (a) the identification of apoptotic and necrotic cells, (b) observations in vitro and in vivo of radiation-induced apoptosis, (c) genes controlling apoptosis, (d) evidence that the target may be the plasma membrane or nuclear DNA, (e) quantitative comparisons of apoptotic death and reproductive (clonogenic) death, (f) the importance of radiation-induced apoptosis in radiotherapy, and (g) studies of radiation-induced apoptosis that are needed. High priority should be placed on determining the molecular pathways that are important in the expression and modulation of radiation-induced apoptosis. Specifically, the events that modulate the apoptosis that occurs in interphase before the cell can divide should be distinguished from the events before division that modulate the misrepair of DNA damage, that results in chromosomal aberrations observed in mitotic cells, which in turn cause the progeny of the dividing cell with aberrations to die by either apoptosis or necrosis. Then, molecular events that determine whether a cell that divides with or without a chromosomal aberration will produce progeny that apoptose or necrose need to be identified. These considerations are important for determining how modulation of radiation-induced apoptosis will affect the ultimate clonogenic survival, and possibly genomic instability in the surviving progeny.

Comparative studies of induction and repair of DNA double-strand breaks in X-irradiated alveolar macrophages and resting peripheral blood lymphocytes using constant-field gel electrophoresis

Induction and repair of X-ray-induced DNA double-strand breaks (dsbs) was compared for normal broncho-alveolar macrophages and human peripheral blood lymphocytes, using CHO cells as a reference cell model. The cells, upon their separation, were processed in a similar manner. After X-irradiation, cell lysis and proteinase K treatment, the DNA samples were subjected to constant-field gel electrophoresis (CFGE) followed by fluorimetric densitometry for quantification of released DNA. Induction of dsbs after X-ray doses of 5-100 Gy was found to show no gross differences for all cell systems used. Repair of dsbs was studied after X-ray dose of 60 Gy for up to 24 h after irradiation. The repair curves obtained proved to be similar for bronchoalveolar macrophages and CHO cells (97% of all dsbs rejoined after 24 h). However, in blood lymphocytes from normal subjects and from bone marrow recipients, dsb repair proceeded rapidly only for 0.5-1 h post-irradiation, being followed by the gradual degradation of DNA at longer intervals. The kinetics of DNA degradation correlated with cytological features of pyknosis and necrosis.

Increasing the susceptibility of the rat 208F fibroblast cell line to radiation-induced apoptosis does not alter its clonogenic survival dose-response

Recent studies have suggested a correlation between the rate and incidence of apoptosis and the radiation response of particular cell lines. However, we found that increasing the rate of induction of apoptosis in the fibroblast line 208F, by transfecting it with human c-myc, did not lead to a change in its clonogenic survival dose-response for either gamma-irradiation or 125I-induced DNA damage. It was also found that expression of mutant (T24) Ha-ras in the 208F line appeared to decrease the level of apoptosis per mitosis after irradiation and inhibited the formation of nucleosomal ladders, but did not affect either the onset of the morphological features of apoptosis or the clonogenic survival dose-response of the cells to either gamma-irradiation or 125I-induced DNA damage. Our findings suggest that it may be incorrect to make predictions about the radiosensitivity of cells based only on knowledge of their mode of death.

Correlation of micronucleus and apoptosis assays with reproductive cell death

The relationship between ionizing radiation-induced cell killing and DNA damage measured by the micronucleus and apoptosis assays was determined in three established cell lines (L929, HL-60, and Chang). Irradiation experiments revealed a dose-dependent increase of micronucleated cells until a certain dose was reached. Above this dose no further increase of the micronucleus frequency was observed, but in HL-60 and Chang cells additional DNA fragmentation was detected by morphological criteria, characteristic of apoptosis. This change was detected at different doses for the three cell lines examined, suggesting the existence of a cell-type-dependent upper limit for the employment of the micronucleus assay. However, the sum of both kinds of cellular DNA damage (e.g. micronucleation and morphological-like apoptosis) led to a significant cell-type-independent correlation with cell survival, even above the dose where micronuclei levels saturated. Therefore, a total cell damage assay, involving the inclusion of micronuclei and morphological-like apoptotic events, should be considered when evaluating the use of a predictor assay for ionizing radiation-induced cell killing, especially in conditions when apoptosis (-like) processes may occur.

p53 status, DNA double-strand break repair proficiency, and radiation response of mouse lymphoid and myeloid cell lines

The p53 status of a panel of 10 mouse lymphoid or myeloid cell lines was determined by immunoprecipitation with mutant- and wild-type-specific antibodies and was compared with the radiation response of the lines. The more rapidly dying cell lines all contained p53 displaying the wild-type epitope. By contrast, four of six more slowly dying cell lines contained either no or mutant p53 protein. It was of interest that radiation-induced apoptosis occurred, albeit at a considerable time after irradiation, in cells ostensibly lacking p53 protein. DNA double-strand break (dsb) repair was examined in both a rapidly and more slowly dying cell line. The rapidly dying cell line was capable of DNA dsb rejoining, however this repair was interrupted by postirradiation DNA degradation.

DNA damage triggers a prolonged p53-dependent G1 arrest and long-term induction of Cip1 in normal human fibroblasts

The tumor suppressor p53 is a cell cycle checkpoint protein that contributes to the preservation of genetic stability by mediating either a G1 arrest or apoptosis in response to DNA damage. Recent reports suggest that p53 causes growth arrest through transcriptional activation of the cyclin-dependent kinase (Cdk)-inhibitor Cip1. Here, we characterize the p53-dependent G1 arrest in several normal human diploid fibroblast (NDF) strains and p53-deficient cell lines treated with 0.1-6 Gy gamma radiation. DNA damage and cell cycle progression analyses showed that NDF entered a prolonged arrest state resembling senescence, even at low doses of radiation. This contrasts with the view that p53 ensures genetic stability by inducing a transient arrest to enable repair of DNA damage, as reported for some myeloid leukemia lines. Gamma radiation administered in early to mid-, but not late, G1 induced the arrest, suggesting that the p53 checkpoint is only active in G1 until cells commit to enter S phase at the G1 restriction point. A log-linear plot of the fraction of irradiated G0 cells able to enter S phase as a function of dose is consistent with single-hit kinetics. Cytogenetic analyses combined with radiation dosage data indicate that only one or a small number of unrepaired DNA breaks may be sufficient to cause arrest. The arrest also correlated with long-term elevations of p53 protein, Cip1 mRNA, and Cip1 protein. We propose that p53 helps maintain genetic stability in NDF by mediating a permanent cell cycle arrest through long-term induction of Cip1 when low amounts of unrepaired DNA damage are present in G1 before the restriction point.

Phorbol esters can protect mouse pre-T cell lines from radiation-induced rapid interphase apoptosis

Protein kinase C stimulators were found to increase the radioresistance of the mouse pre-T cell-derived line ST4. Increased resistance to gamma-ray-induced killing could be produced by addition of 10 nM phorbol 12-myristate 13-acetate (PMA) to ST4 cultures either immediately before or up to 2 h after irradiation. Following PMA treatment, ST4 changed from a cell line that underwent rapid interphase apoptosis (i.e. DNA degradation and morphology characteristic of apoptosis were evident 2-3 h after irradiation) to a line that continued to cycle after irradiation and began to die by apoptosis after completing mitosis. Associated with these PMA-induced changes, the D0 of ST4 cells increased from 7.7 +/- 0.7 to 18.8 +/- 2.7 125I decays. Another mouse pre-T cell-derived line, ST1, which is susceptible to radiation-induced rapid interphase apoptosis, also showed radioprotection after PMA treatment. In contrast, PMA increased the radiosensitivity of the pre-T cell-derived W7 line, which undergoes radiation-induced delayed interphase apoptosis (i.e. death following blockage in G2 phase). PMA had no effect on the radiosensitivity of a pre-B cell-derived line, A8, which undergoes rapid interphase apoptosis, and on a pre-T cell-derived line, W22, which undergoes apoptosis after mitosis. These results suggest that the radiomodifying ability of PMA treatment is dependent upon the cell death pathway induced by irradiation and upon the cell lineage.

Radiation response of mouse lymphoid and myeloid cell lines. Part III. Different signals can lead to apoptosis and may influence sensitivity to killing by DNA double-strand breakage

We have examined the timing of gamma-irradiation-induced death in relation to cell cycle progression using a panel of mouse lymphoid or myeloid cell lines. Death was found to occur immediately after irradiation ('rapid interphase' death), or after arrest in G2 phase ('delayed interphase' death). In part II of this series of papers we demonstrated the occurrence of radiation-induced apoptosis in all these cell lines. This suggests that different signals can lead to apoptosis in these cell lines. DNA double-strand breakage appeared to directly stimulate the destruction of cell lines susceptible to rapid interphase death, whilst the signal for delayed interphase and mitotic death, whilst the signal for delayed interphase and mitotic death appeared to be chromosomal aberrations. Several of the cell lines showed different timing of death dependent upon the radiation dose used. These differences in the timing of radiation-induced death are shown to be useful indicators of the relative radiosensitivity of haematopoietic cell lines.

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

The mode of death induced by gamma-irradiation in a panel of 10 mouse lymphoid or myeloid cell lines was examined. Four of these lines were known to lose viability (membrane integrity) rapidly after irradiation, whilst the others were known to lose viability considerably more slowly. However, based on the criteria of morphology and DNA degradation pattern, all 10 lines showed apoptotic death. The occurrence of apoptosis after irradiation in rapid-dying lymphoid cell lines was consistent with published results, whilst the demonstration of apoptosis in slow-dying lines was unexpected. Cells of the slow-dying lymphoid lines underwent one or more mitoses prior to death, a feature also reported for fibroblastoid cell lines. However, the occurrence of radiation-induced necrosis in fibroblasts suggests that the pathways leading to 'mitotic death' differ between fibroblastoid and lymphoid cell lines.