Apoptosis induced at different dose rates: implication for the shoulder region of cell survival curves

[Radiobiology in brachytherapy]

Low-dose rate brachytherapy has some radiobiological advantages compared to external beam radiotherapy: subletal damages repair during irradiation leading to a relative protection of healthy tissues; no tumor cell repopulation, cell cycle redistribution and a low oxygen enhancement ratio. High dose rate and pulsed dose rate modalities allow an optimization of dose distribution by varying the dwell times over the different dwell positions. Because of the use of afterloaders, they also offer a better radioprotection of the staff. High dose rate and pulsed dose rate treatments seem to offer the same results as low-dose rate brachytherapy, particularly in cervix carcinoma. For high dose rate brachytherapy, schedules must be designed according to the linear-quadratic model. In pulsed dose rate brachytherapy, pulse dose and time intervals must also be derived from the linear-quadratic model, but half-time repair must be taken into account.

Immunotherapy of murine prostate cancer using whole tumour cells killed ex vivo by cytosine deaminase/5-fluorocytosine suicide-gene therapy

OBJECTIVE

To evaluate the efficacy of antitumour vaccines comprising irradiated allogeneic or autologous whole cells expressing cytosine deaminase (CD) which are first killed ex vivo by prodrug activation using 5-fluorocytosine (5-FC), as the immunogenicity of tumour cells used as irradiated vaccines depends both on antigen expression and on the mode of their death.

MATERIALS AND METHODS

The PA3 rat prostate cell line and MATLyLu, an androgen-insensitive subline, were grown and transfected with CD (designated PCD and MCD). In vitro drug-sensitivity was assessed in the cell lines using a viability assay, and the mode of cell death quantified by assessing apoptosis. Bax and bcl-2 expression were assessed by Western blot analysis. For in vivo experiments, male 8-10-week-old Lobund-Wistar rats were vaccinated (using vehicle in control groups) with 5 x 10(6) cells, all cells being irradiated before injection, to give groups with PA3, PCD, PCD killed with 5-FC, MatLyLu, MCD, and MCD killed with 5-FC. After 7 days all animals were given a subcutaneous tumour challenge of PA3 cells, and tumour volume measured subsequently. Immune responses were assessed in splenocytes.

RESULTS

The efficiency of cell kill varied between the cell lines assessed, but cell death was by induced apoptosis. Single doses of vaccine were most effective in the allogeneic setting, causing significantly slower growth of syngeneic tumour challenge (P < 0.01), and 25% better survival at 50 days (P < 0.02) than irradiated untransfected cells. This was consistent with the greater proliferative response after allogeneic than autologous vaccination.

CONCLUSION

The immunogenicity of irradiated tumour cells is enhanced when they are killed ex-vivo using suicide-gene therapy. This approach would be clinically applicable in terms of ease of vaccine production, safety, storage and avoidance of potential toxicities of in vivo gene transfer.

Alpha-tocopheryl succinate, the most effective form of vitamin E for adjuvant cancer treatment: a review

In 1982, it was established that alpha-tocopheryl succinate (alpha-TS) was the most effective form of vitamin E in comparison to alpha-tocopherol, alpha-tocopheryl acetate and alpha-tocopheryl nicotinate in inducing differentiation, inhibition of proliferation and apoptosis in cancer cells, depending upon its concentration. During the last two decades, several studies have confirmed this observation in rodent and human cancer cells in culture and in vivo (animal model). The most exciting aspect of this alpha-TS effect is that it does not affect the proliferation of most normal cells. In spite of several studies published on the anti-cancer properties of alpha-TS, the value of this form of vitamin E has not drawn significant attention from researchers and clinicians. Therefore, a critical review on the potential role of alpha-TS in the management of cancer is needed. In addition, such a review can also provide in-depth analysis of existing literature on this subject. alpha-TS treatment causes extensive alterations in gene expression; however, only some can be attributed to differentiation, inhibition of proliferation and apoptosis. alpha-TS also enhances the growth-inhibitory effect of ionizing radiation, hyperthermia, some chemotherapeutic agents and biological response modifiers on tumor cells, while protecting normal cells against some of their adverse effects. Thus, alpha-TS alone or in combination with dietary micronutrients can be useful as an adjunct to standard cancer therapy by increasing tumor response and possibly decreasing some of the toxicities to normal cells.

Biologically effective dose (BED) for interstitial seed implants containing a mixture of radionuclides with different half-lives

PURPOSE

To develop a tool for evaluating interstitial seed implants that contain a mixture of radionuclides with different half-lives and to demonstrate its utility by examining the clinical implications of prescribing to an isodose surface for such an implant.

METHODS AND MATERIALS

A linear-quadratic model for continuous low dose rate irradiation was developed for permanent implants containing a mixture of radionuclides. Using a generalized equation for the biologically effective dose (BED), the effects of cell proliferation and sublethal damage repair were examined systematically for implants containing a mixture of radionuclides. A head-and-neck permanent seed implant that contained a mixture of (125)I and (103)Pd seeds was used to demonstrate the utility of the generalized BED.

RESULTS

An equation of BED for implants containing a mixture of radionuclides with different half-lives was obtained. In such an implant, the effective cell kill was shown to depend strongly on the relative dose contributions from each radionuclide type; dose delivered by radionuclides with shorter half-life always resulted in more cell kill for any given sublethal damage repair and cell proliferation rates. Application of the BED formula to an implant containing a mixture of (125)I and (103)Pd seeds demonstrates that the conventional dose prescription to an isodose surface is not unique for such an implant. When the prescription dose was based on existing clinical experience of using (125)I seeds alone, mixing (103)Pd seeds with (125)I seeds would increase the cell kill. On the other hand, if the prescription dose were based on existing clinical experience of using (103)Pd seeds alone, mixing (125)I seeds with (103)Pd seeds in the same implant would create radiobiologically "cold" spots (i.e., an increase in cell survival) at locations where a major portion of the prescription dose is contributed by the (125)I seeds. For fast-growing tumors, these "cold" spots can become significant.

CONCLUSIONS

Total dose alone is no longer sufficient for a complete characterization of a permanent seed implant containing a mixture of radionuclides with different half-lives due to the presence of cell proliferation and sublethal damage repair in the protracted dose delivery. BED provides a tool for evaluating the radiobiologic effects of mixing different type of radionuclides in the same implant. When radionuclides of different half-lives are mixed in a permanent implant, using the dose prescription established from existing clinical experience of implants with the longer half-life radionuclide would help to avoid radiobiologic "cold" spots.

Interferon-alpha combined with radiotherapy in the treatment of unresectable melanoma

A case of recurrent and twice resected sinonasal melanoma is presented. The large recurrent tumor was found to regress by a concurrent combination of 6660 cGy photon radiation and subcutaneous interferon-alpha injections given for a period of 8 weeks. Possible mechanisms of potentiation between interferon and radiation are discussed. The unexpected result in this case report raises interesting questions about this treatment combination.

Activation of a nuclear sphingomyelinase in radiation-induced apoptosis

The subcellular origin of ceramide signaling in ionizing radiation-triggered apoptosis was investigated using two previously described subclones of the autonomous erythro-myeloblastic cell line TF-1, radio-resistant and -sensitive TF-1-34 and TF-1-33, respectively. We show in nuclei-free lysates and cytoplasts that both cell lines failed to generate ceramide in response to ionizing radiation. Moreover, whereas cytoplasts did respond to anti-Fas stimulation through phosphatidylserine externalization, no effect was observed with ionizing radiation. Only in highly purified nuclei preparations did we observe ceramide generation, neutral sphingomyelinase activation, and apoptotic features (PARP cleavage, nuclear fragmentation, DNA laddering) in TF-1-33, but not in TF-1-34 cells. These observations suggest that nuclear sphingomyelinase and ceramide formation may contribute to ionizing radiation-triggered apoptosis.

Glucose-6-phosphate dehydrogenase and the oxidative pentose phosphate cycle protect cells against apoptosis induced by low doses of ionizing radiation

The initial and rate-limiting enzyme of the oxidative pentose phosphate shunt, glucose-6-phosphate dehydrogenase (G6PD), is inhibited by NADPH and stimulated by NADP(+). Hence, under normal growth conditions, where NADPH levels exceed NADP(+) levels by as much as 100-fold, the activity of the pentose phosphate cycle is extremely low. However, during oxidant stress, pentose phosphate cycle activity can increase by as much as 200-fold over basal levels, to maintain the cytosolic reducing environment. G6PD-deficient (G6PD(-)) cell lines are sensitive to toxicity induced by chemical oxidants and ionizing radiation. Compared to wild-type CHO cells, enhanced sensitivity to ionizing radiation was observed for G6PD(-) cells exposed to single-dose or fractionated radiation. Fitting the single-dose radiation response data to the linear-quadratic model of radiation-induced cytotoxicity, we found that the G6PD(-) cells exhibited a significant enhancement in the alpha component of radiation-induced cell killing, while the values obtained for the beta component were similar in both the G6PD(-) and wild-type CHO cell lines. Here we report that the enhanced alpha component of radiation-induced cell killing is associated with a significant increase in the incidence of ionizing radiation-induced apoptosis in the G6PD(-) cells. These data suggest that G6PD and the oxidative pentose phosphate shunt protect cells from ionizing radiation-induced cell killing by limiting the incidence of radiation-induced apoptosis. The sensitivity to radiation-induced apoptosis was lost when the cDNA for wild-type G6PD was transfected into the G6PD(-) cell lines. Depleting GSH with l-BSO enhanced apoptosis of K1 cells while having no effect in the G6PD(-) cell line

Issues in risk assessment from solar particle events

Uncertainties in risk assessment from solar particle events (SPE) include the role of high linear energy transfer (LET) secondary ions, the assessment of dose-rate effects as they relate to acute injury, the risk of cancer mortality and the modification of health effects due to the stress of spaceflight. We discuss several issues where new knowledge is required for improving estimates of radiation risk from SPE's. Secondary particles such as neutrons and low energy and charge ions (LZE) may dominate radiation risk behind a storm shelter and their biological effects are poorly understood, especially at low dose-rate. Dose-rate modulation of radiation response is also related to genetic pre-disposition an important determinant of radiation sensitivity. Molecular pathways that control cell death and tissue response have been elucidated in recent years and should provide new understanding of dose-rate effects for risk assessment. We consider some of these factors and discuss calculations using radiation transport codes, track structure models of energy deposition, and a molecular kinetics approach to model radiation response.

Distinct mathematical behavior of apoptotic versus non-apoptotic tumor cell death

PURPOSE

The presence or absence of a p53-dependent apoptosis response has previously been shown to greatly influence radiosensitivity in tumor cells. Here, we examine clonogenic survival curves for two genetically related oncogene transformed cell lines differing in the presence or absence of p53 and apoptosis. Solid tumor radiosensitivity patterns have been previously described for these lines.

MATERIALS AND METHODS

Oncogene-transformed fibroblasts derived from E1A + Ras transfection of p53-wild-type or p53-null mouse embryonic fibroblasts were plated as single cells and irradiated at increasing radiation doses in single fractions from 1.5 to 11 Gy. Clonogenic cell survival assays were obtained. Survival data are fit to a linear-quadratic relationship: S = e(-alphaD-betaD2). Apoptosis was assessed and quantitated morphologically by staining with the fluorescent nuclear dye DAPI, by TUNEL assay for DNA fragmentation, and by measurement of apoptotic cysteine protease cleavage activity in cytosolic extracts.

RESULTS

Whereas radiation triggers massive apoptosis in the presence of p53, it produces no measurable DNA fragmentation, apoptotic cysteine protease cleavage activity, or morphological changes of apoptosis in the cells lacking p53. These contrasting mechanisms of death display dramatically different quantitative behavior: log-survival of apoptotic cells is linearly proportional to dose (S = e(-alphaD)), whereas survival of non-apoptotic (p53 null) is linear-quadratic with a significant quadratic contribution. The surviving fraction at 2 Gy (SF-2) for p53-null cells was 70% verses 12% for p53-intact cells.

CONCLUSIONS

In this system, apoptosis appears to exhibit a dominance of single-event which produces a very high alpha/beta ratio, and no significant shoulder; whereas non-apoptotic death in this system exhibits a comparatively small linear component, a low alpha/beta ratio, and a larger shoulder.

Towards prediction and modulation of treatment response

The purpose of this paper is to evaluate new predictive assays and their potential to modulate treatment response. Their impact is presented in the context of three EORTC clinical trials in head and neck, lung and breast cancer, showing an improvement in survival by accelerated fractionation, concomitant use of cisplatin and radiotherapy and adjuvant hormonal treatment, respectively. Assays have been developed to predict the response to treatment by measuring tumor characteristics, such as the growth potential by the labeling index after i.v. injection of IdUrd, the extent of radiation-induced stable and unstable chromosome aberrations and the induction of apoptosis. These assays could guide us in the adaptation of the individual radiation doses and fractionation schedules. The measurement of the effect of cisplatin on DNA has become feasible with the development of antibodies against DNA adducts. In a recently completed phase II dose escalation trial with concomitant radiotherapy and daily cisplatin in lung cancer, we found that patients with high DNA adduct levels measured in the buccal mucosa, had a much better survival rate than patients with a low or undetectable amount of cisplatin DNA adducts. A better understanding of the signal transduction pathways involved in radiation-induced apoptosis may help to design studies aimed at modulating the apoptotic response. We and others have recently shown that alkylphospholipids, which inhibit mitogenic signaling, induce apoptosis in a variety of tumor cell lines. In combination with ionizing radiation, these compounds cause an enhancement of apoptotic cell kill. This type of signaling-based intervention study may form the basis for new therapeutic strategies. Pretreatment levels of apoptosis may be helpful in predicting treatment outcome, although the data so far show inconsistent results. The importance of evaluating other tumor-biological parameters, including cell kinetics should be stressed. Based on assays predicting reliably the response to hormonal therapy, a more appropriate choice can be made for therapeutic intervention with hormonal therapy and for selecting the appropriate adjuvant therapy in breast cancer patients. The development of a functional estrogen receptor assay (ER-FASAY), based on a yeast growth-assay, provides a way of estimating abnormal function of the receptor in tumors with a positive estrogen receptor score as measured by a classical immuno-histochemistry assay. This yeast assay can also detect different DNA mutations of the estrogen receptor existing in an individual tumor specimen.

Apoptosis after gamma irradiation. Is it an important cell death modality?

Apoptosis and necrosis are two different forms of cell death that can be induced by cytotoxic stress, such as ionizing radiation. We have studied the importance of apoptotic death induced after treatment with 6 Gy of gamma-irradiation in a panel of eight human tumour cell lines of different radiosensitivities. Three different techniques based on the detection of DNA fragmentation have been used, a qualitative one--DNA ladder formation --and two quantitative approaches--in situ tailing and comet assay. No statistically significant relationship between the two quantitative assays was found (r= 0.327, P = 0.159) so these methods seem to show different aspects of the process of cell death. The presence of the DNA ladder related well to the end-labelling method in that the least amount of end labelling was seen in samples in which necrotic degradation rather than apoptotic ladders were seen. However, as the results obtained by the comet assay are not in agreement with the DNA ladder experiments, we suggest that the distinction between the degraded DNA produced by apoptosis and necrosis may be difficult by this technique. Finally, although apoptosis has been proposed to be dependent on p53 functionality, and this may explain differences in cellular radiosensitivity, no statistically significant relationship was found between these parameters and apoptosis in the eight cell lines studied.

Lack of correlation between apoptosis and DNA single-strand breaks in X-irradiated human peripheral blood mononuclear cells in the course of ageing

The dependence on age of both the basal and the X-radiation-induced levels of apoptosis was examined in human peripheral blood mononuclear cells (PBMC). In the same samples, the base value and the extent of induced DNA single-strand breaks were determined, using a sensitive and fast microplate assay. PBMC were isolated from blood of donors of various age groups (20-30, 40-60 and > 70 years of age) and X-irradiated ex vivo using a 6 MV linear accelerator to give a total exposure of 4 Gy. The mean basal levels of apoptosis in PBMC from donors in the 40-60 year age group and the > 70 year age group were found to be only slightly higher (by 20-10%) compared to that of the 20-30 year age group, whereas the extent of DNA damage strongly and significantly (P < 0.01) increased with age by up to 2-fold. In contrast to the extent of induced DNA damage, which steadily increased in the course of ageing by up to 1.8-fold, there was only a transient increase in the level of induced apoptosis to 1.5-fold in PBMC from X-irradiated blood (4 Gy photons) from donors aged 40-60 followed by a decrease to 0.9-fold in PBMC from old donors (>70), compared to age group 20-30. The results show that X-ray-induced apoptosis and DNA damage in PBMC are not correlated during ageing.

The clinical radiobiology of brachytherapy

The unique geometrical features of brachytherapy, together with the wide variety of temporal patterns of dose delivery, result in important interactions between physics and radiobiology. These interactions exert a major influence on the way in which brachytherapy treatments should be evaluated, both in absolute and comparative terms. This article reviews the main physical and radiobiological aspects of brachytherapy and considers examples of their influence on specific types of treatment. The issues relating to the optimization of high dose rate brachytherapy are presented, together with the implications of multiphasic repair kinetics for low dose-rate and pulsed high dose rate brachytherapy. The opportunities for application of radiobiological principles to improve various brachytherapy techniques, together with the integration of brachytherapy with teletherapy, are also outlined. Equations for the numerical evaluation of brachytherapy treatments are presented in the Appendices.

[Significance of apoptotic processes in radiotherapy. II]

Apoptosis is known as an active process of cell death forced by radio- and chemotherapy. Therefore, established concepts (terms, therapy schemes) will reflect a picture different from that usually seen, when examined under the apoptotic point of view. Furthermore, the development of new concepts for innovative diagnosis, prognosis and therapy could be accomplished. This is an attempt to reveal actual features of both aspects.

The molecular regulation of apoptosis and implications for radiation oncology

One of the major goals of cancer research is to identify and understand the causes of cellular proliferation. The role of cell death, or lack thereof, in carcinogenesis, tumour growth, metastatic spread and response to treatment has been largely overlooked even though the morphology of apoptosis (programmed cell death) was clearly described over 20 years ago, and its importance in cancer speculated on at that time. Over the last 5 years, however, an explosion of research has focused on delineating the molecular components of the apoptotic pathways and examining the role of apoptosis in a tumour's growth and response to treatment. This review highlights the aspects of apoptosis most relevant to radiation oncologists and radiobiologists. The apoptotic pathways will be described, with attention to the stimuli that initiate apoptosis, the oncogenes and tumour suppressor genes that mediate apoptosis, and the effector enzymes (proteases and endonucleases) responsible for the execution of apoptosis. In addition, we review the effect of classically described radiobiology cell survival parameters-cell cycle stage, dose rate, linear energy transfer, oxygen, total dose, and fractionation-on radiation induced apoptosis.

Implications of radiation induced apoptosis on calculated radial cell inactivation for a linear 192Ir source

The cell inactivation probability as a function of the radial distance from a 4 cm linear, low dose rate (LDR) 192Ir source was calculated using a modified linear-quadratic (LQ) equation, taking into account the effect of radiation induced apoptosis. As a measure of the therapeutic range of the source, the radial distance from the midpoint of the central axis of the source to the point of 50% inactivation probability was calculated. It was found that the therapeutic range increased with increasing values of the maximal radiation induced apoptotic fraction, Fa; but only as long as the ratio between the apoptotic cell kill susceptibility, zeta, and the alpha value is larger than unity. If the ratio was smaller than unity, a decrease in therapeutic range was found. The slope of the radial cell inactivation probability curve was measured as the distance between the point of 10% and 90% inactivation probability. The slope was found to be fairly independent of the repair half-time, T(1/2), but became steeper with decreasing values of Fa.

Radiation-induced apoptosis in microvascular endothelial cells

The response of the microvasculature to ionizing radiation is thought to be an important factor in the overall response of both normal tissues and tumours. It has recently been reported that basic fibroblast growth factor (bFGF), a potent mitogen for endothelial cells, protects large vessel endothelial cells from radiation-induced apoptosis in vitro. Microvessel cells are phenotypically distinct from large vessel cells. We studied the apoptotic response of confluent monolayers of capillary endothelial cells (ECs) to ionizing radiation and bFGF. Apoptosis was assessed by identifying changes in nuclear morphology, recording cell detachment rates and by detecting internucleosomal DNA fragmentation. Withdrawal of bFGF alone induces apoptosis in these monolayers. The magnitude of this apoptotic response depends upon the duration of bFGF withdrawal. Irradiation (2-10 Gy) induces apoptosis in a dose-dependent manner. Radiation-induced apoptosis occurs in a discrete wave 6-10 h after irradiation, and radiation-induced apoptosis is enhanced in cultures that are simultaneously deprived of bFGF. For example, 6 h after 10 Gy, 44.3% (s.e. 6.3%) of cells in the monolayer simultaneously deprived of bFGF exhibit apoptotic morphology compared with 19.8% (s.e. 3.8%) in the presence of bFGF. These studies show that either bFGF withdrawal or ionizing radiation can induce apoptosis in confluent monolayers of capillary endothelial cells and that radiation-induced apoptosis can be modified by the presence of bFGF.

The p53 gene as a modifier of intrinsic radiosensitivity: implications for radiotherapy

BACKGROUND AND PURPOSE

Experimental studies have implicated the normal or "wild type' p53 protein (i.e. WTp53) in the cellular response to ionizing radiation and other DNA damaging agents. Whether altered WTp53 protein function can lead to changes in cellular radiosensitivity and/or clinical radiocurability remains an area of ongoing study. In this review, we describe the potential implications of altered WTp53 protein function in normal and tumour cells as it relates to clinical radiotherapy, and describe novel treatment strategies designed to re-institute WTp53 protein function as a means of sensitizing cells to ionizing radiation.

METHODS AND MATERIALS

A number of experimental and clinical studies are critically reviewed with respect to the role of the p53 protein as a determinant of cellular oncogenesis, genomic stability, apoptosis, DNA repair and radioresponse in normal and transformed mammalian cells.

RESULTS

In normal fibroblasts, exposure to ionizing radiation leads to a G1 cell cycle delay (i.e. a "G1 checkpoint') as a result of WTp53 mediated inhibition of G1-cyclin-kinase and retinoblastoma (pRb) protein function. The G1 checkpoint response is absent in tumour cells which express a mutant form of the p53 protein (i.e. MTp53), leading to acquired radioresistance in vitro. Depending on the cell type studied, this increase in cellular radiation survival can be mediated through decreased radiation-induced apoptosis, or altered kinetics of the radiation-induced G1 checkpoint. Recent biochemical studies support an indirect role for the p53 protein in both nucleotide excision and recombinational DNA repair pathways. However, based on clinicopathologic data, it remains unclear as to whether WTp53 protein function can predict for human tumour radiocurability and normal tissue radioresponse.

CONCLUSIONS

Alterations in cell cycle control secondary to aberrant WTp53 protein function may be clinically significant if they lead to the acquisition of mutant cellular phenotypes, including the radioresistant phenotype. Pre-clinical studies suggest that these phenotypes may be reversed using adenovirus-mediated gene therapy or pharmacologic strategies designed to re-institute WTp53 protein function. Our analysis of the published data strongly argues for the use of functional assays for the determination of WTp53 protein function in studies which attempt to correlate normal and tumour tissue radioresponse with p53 genotype, or p53 protein expression.

p53-dependent G1 arrest and p53-independent apoptosis influence the radiobiologic response of glioblastoma

PURPOSE

Loss of the p53 tumor suppressor gene has been associated with tumor progression, disease relapse, poor response to antineoplastic therapy, and poor prognosis in many malignancies. We have investigated the contribution of p53-mediated radiation-induced apoptosis and G1 arrest to the well described radiation resistance of glioblastoma multiforme (GM) cells.

METHODS AND MATERIALS

Radiation survival in vitro was quantitated using linear quadratic and repair-saturation mathematical models. Isogenic derivatives of glioblastoma cells differing only in their p53 status were generated using a retroviral vector expressing a dominant negative mutant of p53. Radiation-induced apoptosis was assayed by Fluorescence-activated cell sorter (FACS) analysis, terminal deoxynucleotide transferase labeling technique, and chromatin morphology. Cells were synchronized in early G1 phase and mitotic and labeling indices were measured.

RESULTS

Radiation-induced apoptosis of GM cells was independent of functional wild-type p53 (wt p53). Decreased susceptibility to radiation-induced apoptosis was associated with lower alpha values characterizing the shoulder of the clonogenic radiation survival curve. Using isogenic GM cells differing only in their p53 activity, we found that a p53-mediated function, radiation-induced G1 arrest, could also influence the value of alpha and clonogenic radiation resistance. Inactivation of wt p53 function by a dominant negative mutant of p53 resulted in a significantly diminished alpha value with no alteration in cellular susceptibility to radiation-induced apoptosis. The clonal derivative U87-LUX.8 expressing a functional wt p53 had an alpha (Gy-1) value of 0.609, whereas the isogenic clonal derivative U87-175.4 lacking wt p53 function had an alpha (Gy-1) value of 0.175.

CONCLUSION

We conclude that two distinct cellular responses to radiation, p53-independent apoptosis and p53-dependent G1-arrest, influence radiobiological parameters that characterize the radiation response of glioblastoma cells. Further understanding of the molecular basis of GM radiation resistance will lead to improvement in existing therapeutic modalities and to the development of novel treatment approaches.

Evaluation of a modified micronucleus assay

The relationship between ionizing radiation-induced cell killing and DNA damage measured by the micronucleus assay was determined in three established cell lines (L929, HL-60, and Chang). Our data revealed a dose-dependent increase of cells bearing multiple micronuclei. Cells with the same number of micronuclei were counted separately up to 50 h after irradiation. The counts of these subsets showed a parallel increase and decrease throughout the study. In order to transform the peak of the micronucleus frequency, occurring over only a brief time period into a less time dependent value, we calculated ratios between the different subsets of micronucleated cells. These ratios converged to values which were almost constant beyond 30 h after irradiation. The values showed correlations with cell survival (clonogenic assay) and radiation dose which were comparable with the correlations with the peak of the micronucleus frequency (maximum micronucleus yield) when utilizing the conventional evaluation of the micronucleus assay performed without cytochalasin B. This means that large-scale time kinetics and additional drugs like cytochalasin B can be avoided by changing the evaluation procedure of the conventional micronucleus assay. The modified assay described in this manuscript revealed apoptosis-induced limitations as recently detected for the maximum micronucleus yield assay.

Reducing the radiation-induced G2 delay causes HeLa cells to undergo apoptosis instead of mitotic death

Cells exposed to radiation may undergo death through apoptosis or mitotic death. HeLa cells predominantly undergo mitotic death after irradiation. Treatment of these cells with caffeine has been shown to shorten the G2 delay after irradiation, and to decrease their survival. The kinase inhibitor staurosporine also decreases the radiation-induced G2 delay in HeLa cells. Here we extend these findings to show that the decrease in radiation-induced G2 delay mediated by caffeine or staurosporine is accompanied by a shift in the pathway of cell death from mitotic death to apoptotic death. The increase in apoptosis is further accompanied by decreased clonogenic survival after irradiation. Based on these findings we propose the hypothesis that one mechanism of enhancing cell killing by radiation is to trigger apoptosis by decreasing the G2 delay induced by irradiation.

RBE of fast neutrons for apoptosis in mouse thymocytes

We compared apoptosis in mouse thymocytes following exposure to low doses of high linear energy transfer (LET), 62.5-MeV (p-->Be+) fast neutrons and low LET, 4-MeV photons by flow cytometric analysis of hypodiploid cells. The incidence of apoptotic cell death rose steeply at very low radiation doses reaching a plateau of 3 Gy. Both the time course and the radiation dose-response curves were similar for high and low LET radiation modalities. The relative biological effectiveness (RBE) of 1.0 for apoptosis in the mouse thymocyte system contrasts with the much higher value typically seen in many classical systems of clonogenic cell survival and tissue response. This difference suggests that while radiation-induced apoptosis may contribute significantly to loss of susceptible cells at doses of < or = 2 Gy, it appears to have a questionable role in determining the relative intrinsic radiosensitivity of mammalian cells to high and low LET irradiation at clinically relevant levels of cell kill.

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.

An equation for the dose response of radiation-induced apoptosis: possible incorporation with the LQ model

Based on in vitro and in vivo data, we proposed an equation to describe the dose response of radiation-induced apoptosis. This equation can be incorporated into the LQ model to account for the contribution of apoptosis-mediated cell death in clonogenic survival curve. The modified LQ equation is a composite of the dose responses of two subpopulations, one susceptible and the other resistant to apoptotic cell death. For the apoptosis-resistant fraction, the size of which varies for different biological systems, the conventional LQ relation applies. For the susceptible subpopulation, the LQ equation is modified by e-zeta D, a functional form consistent with the observed dose-rate independence of this mode of cell death.