Radiation-induced DNA double-strand break frequencies in human squamous cell carcinoma cell lines of different radiation sensitivities

Cellular lethal damage of 64Cu incorporated in mammalian genome evaluated with Monte Carlo methods

Purpose

Targeted Radionuclide Therapy (TRT) with Auger Emitters (AE) is a technique that allows targeting specific sites on tumor cells using radionuclides. The toxicity of AE is critically dependent on its proximity to the DNA. The aim of this study is to quantify the DNA damage and radiotherapeutic potential of the promising AE radionuclide copper-64 (64Cu) incorporated into the DNA of mammalian cells using Monte Carlo track-structure simulations.

Methods

A mammalian cell nucleus model with a diameter of 9.3 μm available in TOPAS-nBio was used. The cellular nucleus consisted of double-helix DNA geometrical model of 2.3 nm diameter surrounded by a hydration shell with a thickness of 0.16 nm, organized in 46 chromosomes giving a total of 6.08 giga base-pairs (DNA density of 14.4 Mbp/μm3). The cellular nucleus was irradiated with monoenergetic electrons and radiation emissions from several radionuclides including 111In, 125I, 123I, and 99mTc in addition to 64Cu. For monoenergetic electrons, isotropic point sources randomly distributed within the nucleus were modeled. The radionuclides were incorporated in randomly chosen DNA base pairs at two positions near to the central axis of the double-helix DNA model at (1) 0.25 nm off the central axis and (2) at the periphery of the DNA (1.15 nm off the central axis). For all the radionuclides except for 99mTc, the complete physical decay process was explicitly simulated. For 99mTc only total electron spectrum from published data was used. The DNA Double Strand Breaks (DSB) yield per decay from direct and indirect actions were quantified. Results obtained for monoenergetic electrons and radionuclides 111In, 125I, 123I, and 99mTc were compared with measured and calculated data from the literature for verification purposes. The DSB yields per decay incorporated in DNA for 64Cu are first reported in this work. The therapeutic effect of 64Cu (activity that led 37% cell survival after two cell divisions) was determined in terms of the number of atoms incorporated into the nucleus that would lead to the same DSBs that 100 decays of 125I. Simulations were run until a 2% statistical uncertainty (1 standard deviation) was achieved.

Results

The behavior of DSBs as a function of the energy for monoenergetic electrons was consistent with published data, the DSBs increased with the energy until it reached a maximum value near 500 eV followed by a continuous decrement. For 64Cu, when incorporated in the genome at evaluated positions (1) and (2), the DSB were 0.171 ± 0.003 and 0.190 ± 0.003 DSB/decay, respectively. The number of initial atoms incorporated into the genome (per cell) for 64Cu that would cause a therapeutic effect was estimated as 3,107 ± 28, that corresponds to an initial activity of 47.1 ± 0.4 × 10-3 Bq.

Conclusion

Our results showed that TRT with 64Cu has comparable therapeutic effects in cells as that of TRT with radionuclides currently used in clinical practice.

Biological Predictors of Response to Radiotherapy in Head and Neck Cancer: Recent Advances and Emerging Perspectives

The study of new biological parameters has received considerable attention in radiotherapy during the last decade due to their potential value in predicting treatment response in squamous cell carcinoma of the head and neck (SCC-HN) and the foreseen possibility of selecting altered fractionation radiotherapy for the individual patient. Although there are established clinical parameters in SCC-HN patients that relate to radiation response (extent of disease, hemoglobin level), recent advances with direct measurement of tumor oxygenation, inherent radiosensitivity and proliferation rate have increased the promise of individualization of treatment strategy according to these radiobiologically based parameters. Molecular research has now identified a host of new biological parameters with potential predictive utility; oncogenes, tumor suppressor genes, cell-cycle control genes, apoptosis genes and angiogenesis genes have been extensively studied and correlated with radiation response. Moreover, study of the epidermal growth factor receptor signal-transduction system as a possible response modulator has recently fostered molecular strategies which employ blockade of the receptor to down-regulate tumor growth. This article briefly reviews and analyzes the main controversial issues and drawbacks that hinder the general use of biological parameters for predicting tumor response to radiotherapy. It highlights the future perspectives of radiotherapy predictive assay research and the need to shift from single-parameter analysis to multiparametric studies which take into account several potential predictors that together are involved in different biological and clinical pathways.

Induction and repair of DNA double-strand breaks using constant-field gel electrophoresis and apoptosis as predictive markers for sensitivity of cancer cells to cisplatin

This study was designed to evaluate some parameters that may play a role in the prediction of cancer cells sensitivity to cisplatin (CIS). Sensitivity, induction and repair of DNA double-strand breaks (DSB), cell cycle regulation and induction of apoptosis were measured in four cancer cell lines with different sensitivities to CIS. Using a sulphorhodamine-B assay, the cervical carcinoma cells (HeLa) were found to be the most sensitive to CIS followed by breast carcinoma cells (MCF-7) and liver carcinoma cells (HepG2). Colon carcinoma HCT116 cells were the most resistant. As measured by constant-field gel electrophoresis (CFGE), DSB induction, but not residual DSB exhibited a significant correlation with the sensitivity of cells to CIS. Flow cytometric DNA ploidy analysis revealed that 67% of HeLa cells and 10% of MCF-7 cells shift to sub-G1 phase after incubation with CIS. Additionally, CIS induced the arrest of MCF-7 cells in S-phase and the arrest of HepG2 and HCT116 cells in both S phase and G2/M phase. Determination of the Fas-L level and Caspase-9 activity indicated that CIS-induced apoptosis results from the mitochondrial (intrinsic) pathway. These results, if confirmed using clinical samples, indicate that the induction of DNA DSB as measured by CFGE and the induction of apoptosis should be considered, along with other predictive markers, in future clinical trials to develop predictive assays for platinum -based therapy.

DNA fragmentation by gamma radiation and electron beams using atomic force microscopy

Double-stranded pBS plasmid DNA was irradiated with gamma rays at doses ranging from 1 to 12 kGy and electron beams from 1 to 10 kGy. Fragment-size distributions were determined by direct visualization, using atomic force microscopy with nanometer-resolution operating in non-tapping mode, combined with an improved methodology. The fragment distributions from irradiation with gamma rays revealed discrete-like patterns at all doses, suggesting that these patterns are modulated by the base pair composition of the plasmid. Irradiation with electron beams, at very high dose rates, generated continuous distributions of highly shattered DNA fragments, similar to results at much lower dose rates found in the literature. Altogether, these results indicate that AFM could supplement traditional methods for high-resolution measurements of radiation damage to DNA, while providing new and relevant information.

Response of human HTB140 melanoma cells to conventional radiation and hadrons

Conventional radiotherapy with X- and gamma-rays is one of the common and effective treatments of cancer. High energy hadrons, i.e., charged particles like protons and (12)C ions, due to their specific physics and radiobiological advantages are increasingly used. In this study, effectiveness of different radiation types is evaluated on the radio-resistant human HTB140 melanoma cells. The cells were irradiated with gamma-rays, the 62 MeV protons at the Bragg peak and in the middle of the spread-out Bragg peak (SOBP), as well as with the 62 MeV/u (12)C ions. The doses ranged from 2 to 24 Gy. Cell survival and proliferation were assessed 7 days after irradiation, whereas apoptosis was evaluated after 48 h. The acquired results confirmed the high radio-resistance of cells, showing better effectiveness of protons than gamma-rays. The best efficiency was obtained with (12)C ions due to higher linear energy transfer. All analyzed radiation qualities reduced cell proliferation. The highest proliferation was detected for (12)C ions because of their large killing capacity followed by small induction of reparable lesions. This enabled unharmed cells to preserve proliferative activity. Irradiations with protons and (12)C ions revealed similar moderate pro-apoptotic ability that is in agreement with the level of cellular radio-resistance.

Radiation sensitivity of leukocytes from healthy individuals and breast cancer patients as measured by the alkaline and neutral comet assay

Initial radiation-induced DNA damage, dose-response curves and kinetics of DNA repair in leukocytes from healthy volunteers and breast cancer patients, was assessed using alkaline and neutral comet assay after exposure to (60)Co gamma rays. Both versions of comet assay showed higher levels of baseline DNA damage in leukocytes of breast cancer cases than in controls. Gamma ray induced initial DNA damage in leukocytes of cancer cases was not significantly different from that of healthy donors. A similar dose-response was obtained with both versions of comets for two groups. After a repair time of 24h, following irradiation, samples from the healthy individuals showed no residual DNA damage in their leukocytes, whereas breast cancer patients revealed more than 20%. Although similar initial radiosensitivity was observed for both groups but the repair kinetics of radiation-induced DNA damage of leukocytes from breast cancer cases and healthy subjects was statistically different.

Spatial distribution of radiation-induced double-strand breaks in plasmid DNA as resolved by atomic force microscopy

Atomic force microscopy (AFM) has been used to directly visualize, size and compare the DNA fragments resulting from exposure to low- and high-LET radiation. Double-stranded pUC-19 plasmid ("naked") DNA samples were irradiated by electron-beam or reactor neutron fluxes with doses ranging from 0.9 to 10 kGy. AFM scanning in the tapping mode was used to image and measure the DNA fragment lengths (ranging from a few bp up to 2864 bp long). Double-strand break (DSB) distributions resulting from high-LET neutron and lower-LET electron irradiation revealed a distinct difference between the effects of these two types of radiation: Low-LET radiation-induced DSBs are distributed more uniformly along the DNA, whereas a much larger proportion of neutron-induced DSBs are distributed locally and densely. Furthermore, comparisons with predictions of a random DSB model of radiation damage show that neutron-induced DSBs deviate more from the model than do electron-induced DSBs. In summary, our high-resolution AFM measurements of radiation-induced DNA fragment-length distributions reveal an increased number of very short fragments and hence clustering of DSBs induced by the high-LET neutron radiation compared with low-LET electron radiation and a random DSB model prediction.

Radiosensitivity of human tumour cells is correlated with the induction but not with the repair of DNA double-strand breaks

Nine human tumour cell lines (four mammary, one bladder, two prostate, one cervical, and one squamous cell carcinoma) were studied as to whether cellular radiosensitivity is related to the number of initial or residual double-strand breaks (dsb). Cellular sensitivity was measured by colony assay and dsb by means of constant- and graded-field gel electrophoresis (CFGE and GFGE, respectively). The nine tumour cell lines showed a broad variation in cellular sensitivity (SF2 0.17-0.63). The number of initial dsb as measured by GFGE ranged between 14 and 27 dsb/Gy/diploid DNA content. In contrast, normal fibroblasts raised from skin biopsies of seven individuals showed only a marginal variation with 18-20 dsb/Gy/diploid DNA content. For eight of the nine tumour cell lines, there was a significant correlation between the number of initial dsb and the cellular radiosensitivity. The tumour cells showed a broad variation in the amount of dsb measured 24 h after irradiation by CFGE, which, however, was not correlated with the cellular sensitivity. This residual damage was found to be influenced not only by the actual number of residual dsb, but also by apoptosis and cell cycle progression which had impact on CFGE measurements. Some cell line strains were able to proliferate even after exposure to 150 Gy while others were found to degrade their DNA. Our results suggest that for tumour cells, in contrast to normal cells, the variation in sensitivity is mainly determined by differences in the initial number of dsb induced.

Molecular targets in cellular response to ionizing radiation and implications in space radiation protection

DNA repair systems and cell cycle checkpoints closely co-operate in the attempt of maintaining the genomic integrity of cells damaged by ionizing radiation. DNA double-strand breaks (DSB) are considered as the most biologically important radiation-induced damage. Their spatial distribution and association with other types of damage depend on radiation quality. It is believed these features affect damage reparability, thus explaining the higher efficiency for cellular effects of densely ionizing radiation with respect to gamma-rays. DSB repair systems identified in mammalian cells are homologous recombination (HR), single-strand annealing (SSA) and non-homologous end-joining (NHEJ). Some enzymes may participate in more than one of these repair systems. DNA damage also triggers biochemical signals activating checkpoints responsible for delay in cell cycle progression that allows more time for repair. Those at G1/S and S phases prevent replication of damaged DNA and those at G2/M phase prevent segregation of changed chromosomes. Individuals with lack or alterations of genes involved in DNA DSB repair and cell cycle checkpoints exhibit syndromes characterized by genome instability and predisposition to cancer. Information reviewed in this paper on the basic mechanisms of cellular response to ionizing radiation indicates their importance for a number of issues relevant to protection of astronauts from space radiation.

Modulation of radiation-induced protein kinase C activity by phenolics

Natural phenolic compounds were tested in vitro for their effect on the activity of protein kinase C (PKC) isolated from the liver cytosol and the particulate fraction of unirradiated mice and mice irradiated at 5 Gy. Following irradiation, the PKC activity was found to be increased in both cytosolic and particulate fractions. Curcumin, ellagic acid and quercetin were effective in inhibiting radiation-induced PKC activity. Curcumin and ellagic acid were found to be more inhibitory towards radiation-induced PKC activity, while quercetin was the least effective. Curcumin was found to inhibit the activated cytosolic and particulate PKC at very low concentrations. Activation of PKC is one of the means of conferring radioresistance on a tumour cell. Suppression of PKC activity by phenolics may be one of the means of preventing the development of radioresistance following radiotherapy.

Heterogeneity in 2-deoxy-D-glucose-induced modifications in energetics and radiation responses of human tumor cell lines

PURPOSE

The glucose analog and glycolytic inhibitor, 2-deoxy-D-glucose (2-DG), has been shown to differentially enhance the radiation damage in tumor cells by inhibiting the postirradiation repair processes. The present study was undertaken to examine the relationship between 2-DG-induced modification of energy metabolism and cellular radioresponses and to identify the most relevant parameter(s) for predicting the tumor response to the combined treatment of radiation + 2-DG.

METHODS AND MATERIALS

Six human tumor cell lines (glioma: BMG-1 and U-87, squamous cell carcinoma: 4451 and 4197, and melanoma: MeWo and Be-11) were investigated. Cells were exposed to 2 Gy of Co-60 gamma-rays or 250 kVP X-rays and maintained under liquid-holding conditions 2-4 h to facilitate repair. 2-DG (5 mM, equimolar with glucose) that was added at the time of irradiation was present during the liquid holding. Glucose utilization, lactate production (enzymatic assays), and adenine nucleotides (high performance liquid chromatography and capillary isotachophoresis) were investigated as parameters of energy metabolism. Induction and repair of DNA damage (comet assay), cytogenetic damage (micronuclei formation), and cell death (macrocolony assay) were analyzed as parameters of radiation response.

RESULTS

The glucose consumption and lactate production of glioma cell lines (BMG-1 and U-87) were nearly 2-fold higher than the squamous carcinoma cell lines (4197 and 4451). The ATP content varied from 3.0 to 6.5 femto moles/cell among these lines, whereas the energy charge (0.86-0.90) did not show much variation. Presence of 2-DG inhibited the rate of glucose usage and glycolysis by 30-40% in glioma cell lines and by 15-20% in squamous carcinoma lines, while ATP levels reduced by nearly 40% in all the four cell lines. ATP:ADP ratios decreased to a greater extent ( approximately 40%) in glioma cells than in squamous carcinoma 4451 and MeWo cells; in contrast, presence of 2-DG reduced ADP:AMP ratios by 3-fold in the squamous carcinoma 4451, whereas an increase was noted in the glioma cell line BMG-1. 2-DG significantly reduced the initial rates of DNA repair in all cells, resulting in an excess residual damage after 2 h of repair in BMG-1, U-87, and 4451 cell lines, whereas no significant differences could be observed in the other cell lines. Recovery from potentially lethal damage was also significantly inhibited in BMG-1 cells. 2-DG increased the radiation-induced micronuclei formation in the melanoma line (MeWo) by nearly 60%, while a moderate (25-40%) increase was observed in the glioma cell lines (BMG-1 and U-87). Presence of 2-DG during liquid holding (4 h) enhanced the radiation-induced cell death by nearly 40% in both the glioma cell lines, while significant effects were not observed in others.

CONCLUSIONS

The modifications in energetics and radiation responses by 2-DG vary considerably among different human tumor cell lines, and the relationships between energy metabolism and various radiobiologic parameters are complex in nature. The 2-DG-induced modification of radiation response does not strictly correlate with changes in the levels of ATP. However, a significant enhancement of the radiation damage by 2-DG was observed in cells with high rates of glucose usage and glycolysis, which appear to be the two most important factors determining the tumor response to the combined treatment of 2-DG + radiation therapy.

Targets for radiation-induced cell death: when DNA damage doesn't kill

Chinese hamster ovary (CHO) K1 and radiosensitive CHO irs-20 cells were synchronized in S phase and labeled for 10 min with 5-[(125)I]-iodo-2'-deoxyuridine ((125)IdU). The cells were washed, incubated in fresh medium for 1 h for incorporation of the intracellular radionucleotides into DNA, and then frozen (-80 degrees C) for accumulation of (125)I decays. At intervals after freezing, when the cells had accumulated the desired number of decays, aliquots of the frozen cells were thawed and plated to determine survival. The survival curves for K1 and irs-20 cells were similar from 100% to 30% survival. At higher (125)I doses (more decays/cell), the survival of K1 cells continued to decline exponentially, but the survival of X-ray-sensitive irs-20 cells remained at approximately 30% even after the cells had accumulated 1265 decays/cell. The results contradict the notion that increased DNA damage inevitably causes increased cell death. To account for these findings, we propose a model that postulates the existence of a second radiation target. According to this model, radiation damage to DNA may be necessary to induce cell death, but DNA damage alone is not sufficient to kill cells. We infer from the survival response of irs-20 cells that damage to a second (non-DNA) structure is involved in cell death, and that this structure directly affects the repair of DNA and cell survival.

Combined RAF1 protein expression and p53 mutational status provides a strong predictor of cellular radiosensitivity

The tumour suppressor gene, p53, and genes coding for positive signal transduction factors can influence transit through cell-cycle checkpoints and modulate radiosensitivity. Here we examine the effects of RAF1 protein on the rate of exit from a G2/M block induced by gamma-irradiation in relation to intrinsic cellular radiosensitivity in human cell lines expressing wild-type p53 (wtp53) protein as compared to mutant p53 (mutp53) protein. Cell lines which expressed mutp53 protein were all relatively radioresistant and exhibited no relationship between RAF1 protein and cellular radiosensitivity. Cell lines expressing wtp53 protein, however, showed a strong relationship between RAF1 protein levels and the radiosensitivity parameter SF2. In addition, when post-irradiation perturbation of G2/M transit was compared using the parameter T50 (time after the peak of G2/M delay at which 50% of the cells had exited from a block induced by 2 Gy of irradiation), RAF1 was related to T50 in wtp53, but not mutp53, cell lines. Cell lines which expressed wtp53 protein and high levels of RAF1 had shorter T50s and were also more radiosensitive. These results suggest a cooperative role for wtp53 and RAF1 protein in determining cellular radiosensitivity in human cells, which involves control of the G2/M checkpoint.

The increment of micronucleus frequency in cervical carcinoma during irradiation in vivo and its prognostic value for tumour radiocurability

A potential usefulness of micronucleus assay for prediction of tumour radiosensitivity has been tested in 64 patients with advanced stage (II B-IV B) cervical carcinoma treated by radiotherapy. The study of cellular radiosensitivity in vitro was conducted in parallel with the study of cellular damage after tumour irradiation in vivo. Radiosensitivity of in vitro cultured primary cells isolated from tumour biopsies taken before radiotherapy was evaluated using cytokinesis-block micronucleus assay. Frequency of micronuclei per binucleated cell (MN/BNC) at 2 Gy was used as a measure of radiosensitivity. Radiation sensitivity in vivo was expressed as per cent increment of micronucleus frequency in cells isolated from biopsy taken after 20 Gy (external irradiation, 10 x 2 Gy) over the pre-treatment spontaneous micronucleus level and was called MN20. Very low correlation (r = 0.324) was observed between micronucleus frequency in vitro and in vivo. Although micronucleus frequency at 2 Gy differed widely between tumours evaluated (mean MN/BNC was 0.224; range 0.08-0.416), no significant correlation was observed between this parameter and clinical outcome. The average increment of micronucleus frequency after 20 Gy amounted to 193% of spontaneous level (range 60-610%) and was independent of spontaneous micronucleation before radiotherapy. In contrast to in vitro results, these from in vivo assay seem to have a predictive value for radiotherapy of cervix cancer. The micronucleus increment in vivo that reached at least 117.5% of pretreatment value (first quartile for MN20 data set) correlated significantly with better tumour local control (P < 0.008) and overall survival (P < 0.045). Our results suggest that evaluation of increment of micronucleus frequency during radiotherapy (after fixed tested dose of 20 Gy) offers a potentially valuable approach to predicting individual radioresponsiveness and may be helpful for individualization of treatment strategy in advanced stage cervical cancer.

DNA double-strand break repair, DNA-PK, and DNA ligases in two human squamous carcinoma cell lines with different radiosensitivity

PURPOSE

Variation in sensitivity to radiotherapy among tumors has been related to the capacity of cells to repair radiation-induced DNA double-strand breaks (DSBs). DNA-dependent protein kinase (DNA-PK) and DNA ligases may affect DNA dsb rejoining. This study was performed to compare rate of rejoining of radiation-induced DSBs, DNA-PK, and DNA ligase activities in two human squamous carcinoma cell lines with different sensitivity to ionizing radiation.

METHODS AND MATERIALS

Cell survival of two human squamous carcinoma cell lines, UM-SCC-1 and UM-SCC-14A, was determined by an in vitro clonogenic assay. DSB rejoining was studied using pulsed field gel electrophoresis (PFGE). DNA-PK activity was determined using BIOTRAK DNA-PK enzyme assay system (Amersham). DNA ligase activity in crude cell extracts was measured using [5'-33P] Poly (dA) x (oligo (dT) as a substrate. Proteolytic degradation of proteins was analyzed by means of Western blotting.

RESULTS

Applying the commonly used linear-quadratic equation to describe cell survival, S = e-alphaD-betaD2, the two cell lines roughly have the same alpha value (approximately 0.40 Gy(-1)) whereas the beta value was considerably higher in UM-SCC-14A (0.067 Gy(-2)+/-0.007 Gy(-2) [SEM]) as compared to UM-SCC-1 (0.013 Gy(-2)+/-0.004 Gy(-2) [SEM]). Furthermore, UM-SCC-1 was more proficient in rejoining of X-ray-induced DSBs as compared to UM-SCC-14A as quantified by PFGE. The constitutive level of DNA-PK activity was 1.6 times higher in UM-SCC-1 as compared to UM-SCC-14A ( < 0.05). The constitutive level of DNA ligase activity was similar in the two cell lines.

CONCLUSIONS

The results suggest that the proficiency in rejoining of DSBs is associated with DNA-PK activity but not with total DNA ligase activity.

Evaluation of radiation-induced DNA damage and DNA repair in human lung cancer cell lines with different radiosensitivity using alkaline and neutral single cell gel electrophoresis

Using the comet assay, radiation-induced DNA strand breaks were evaluated in human lung cancer cell lines with different radiosensitivity (U-1285, U-1906E, U-1752 and U-1810). Single strand breaks were more sensitive indicators of the radiation-induced damage than double strand breaks. However, there was no consistent pattern in the way the various cell lines responded to 1-5 Gy of gamma-irradiation and all cell lines showed a remarkably efficient DNA repair after 1 h. In a separate study of the repair kinetics of DNA double strand breaks, the radioresistant cell line U-1810 showed a more efficient initial strand rejoining than the radiosensitive cell line U-1285 after irradiation at 2 Gy. The latter finding suggests that the detection of early DNA repair may be useful when monitoring the intrinsic radiosensitivity of human lung cancer cells.

Radiosensitive human tumour cell lines show misrepair of DNA termini

Physical measures of the rejoining of radiation-induced breaks in DNA strands are limited in terms of sensitivity and the fact that they do not assess the fidelity with which the rejoining occurs. In this report, transfection of cleaved plasmid has been used as a probe for repair in three radiosensitive tumour cell lines and shown them to have low repair fidelity compared with resistant cells. Errors in the repair of linear plasmid were found by Southern analysis, in keeping with the measured repair fidelity. Radiosensitive tumour cells showed few errors in the uptake and integration of circular plasmid, in contrast to ataxia-telangiectasia (A-T) cells. In the neuroblastoma HX142, the repair of blunt-ended linear plasmid was associated with deletions of > 1 kb; staggered-ended linear plasmid was repaired with small insertions and circular plasmid integration was intact in > 60% of the copies. The neuroblastoma SKN.SH, processed staggered-ended plasmid by insertions of a variety of sizes, but processed circular plasmid largely error-free. In contrast, A-T cells (AT5BIVA) had the same spectrum of errors irrespective of the form of plasmid transfected. Cell fusion between HX142 and AT5BIVA showed complementation to a resistant phenotype, suggesting that misrepair in the tumour cell did not result from somatic mutation in the ATM gene. In conclusion, radiosensitive tumours show evidence of misrepair of DNA termini, with a mechanism which is functionally and genetically distinct from that in A-T cells.

Late G1 accumulation after 2 Gy of gamma-irradiation is related to endogenous Raf-1 protein expression and intrinsic radiosensitivity in human cells

We have previously reported a correlation between high endogenous expression of the protein product of the RAF-1 proto-oncogene, intrinsic cellular radiosensitivity and rapid exit from a G2/M delay induced by 2 Gy of gamma-irradiation. Raf1 is a positive serine/threonine kinase signal transduction factor that relays signals from the cell membrane to the MAP kinase system further downstream and is believed to be involved in an ionizing radiation signal transduction pathway modulating the G1/S checkpoint. We therefore extended our flow cytometric studies to investigate relationships between radiosensitivity, endogenous expression of the Raf1 protein and perturbation of cell cycle checkpoints, leading to alterations in the G1, S and G2/M populations after 2 Gy of gamma-irradiation. Differences in intrinsic radiosensitivity after modulation of the G1/S checkpoint have generally been understood to involve p53 function up to the present time. A role for dominant oncogenes in control of G1/S transit in radiation-treated cells has not been identified previously. Here, we show in 12 human in vitro cancer cell lines that late G1 accumulation after 2 Gy of radiation is related to both Raf1 expression (r = 0.91, P = 0.0001) and the radiosensitivity parameter SF2 (r = -0.71, P = 0.009).

Radiation-induced DNA double-strand breaks and the radiosensitivity of human cells: a closer look

A large number of reports suggest that DNA double-strand breaks (DSB) play a major role in the radiation-induced killing of mammalian cells. However, the arguments supporting the relationship between DSB and radiosensitivity are generally indirect. Furthermore, care must be taken to allow for the possible impact of the techniques and of the experimental protocols on the relationship between DSB and cell death. The recent data on DSB induction, repair and misrepair in human cell lines and their correlation with intrinsic radiosensitivity are reviewed.

Radiosensitivity and double-strand break rejoining in tumorigenic and non-tumorigenic human epithelial cell lines

Radiosensitivity and repair of DNA damage induced by ionizing radiation and restriction enzymes were investigated in three human epithelial cell lines: two tumorigenic squamous carcinoma cell lines (SCC-4 and SCC-25), and a non-tumorigenic epidermal keratinocyte cell line (RHEK-1). Sensitivity to ionizing radiation was determined using a clonogenic cell survival assay, which showed SCC-4 to be more radiosensitive than SCC-25 and RHEK-1, which in turn displayed about equal sensitivity. Using DNA precipitation under alkaline conditions for the analysis of induction and repair of DNA single-strand breaks (ssb), an increased level of ssb induction was found for SCC-4 while the efficiency of ssb repair was about equal in all three cell lines. Using pulsed-field gel electrophoresis (PFGE) for the measurement of induction and repair of DNA double-strand breaks (dsb), no consistent differences were detected between the three cell lines. A plasmid reconstitution assay was used to determine the capacity to rejoin restriction enzyme-induced dsb in whole-cell extracts prepared from the three cell lines. In these experiments, dsb rejoining was shown to be significantly reduced in the most radiosensitive SCC-4 cell line while it was about equal in RHEK-1 and SCC-25. The results indicate that plasmid reconstitution in cell-free extracts is a sufficiently sensitive assay to detect differences in repair capacity among tumour cell lines of different radiosensitivity which remain undetectable by DNA precipitation and PFGE.

Response to radiotherapy of human uterine cervix carcinoma is not correlated with rearrangements of the Ha-ras-1 and/or c-myc genes

An association between the presence of the activated form of Ha-ras-1 and c-myc genes and increased cellular radioresistance has been shown in several cell lines. The aim of this study was to determine whether such an association could be observed in clinical tumour biopsies. We examined 70 tumour specimens and 51 samples of peripheral blood obtained from untreated patients with carcinoma of the uterine cervix mainly stage II and III. In addition to initial clinical tumour response to radiotherapy, radiosensitivity was also analysed by the subrenal capsule assay (SRCA). Mutations in exons 1 and 2 of the Ha-ras-1 gene were examined using PCR single-strand conformation polymorphism (PCR-SSCP) and direct sequencing; and restriction fragment length polymorphism of the Ha-ras-1 gene was analysed using Southern hybridisation. Eight (11%) out of 70 tumours contained mutations in exons 1 and 2 of the Ha-ras-1 gene. Eleven (22%) out of the 51 tumours displayed rearrangements of the Ha-ras-1 gene (six tumours (12%) showed alterations of allele length, one amplification and four (8%) loss of one Ha-ras-1 allele). In addition, 12 (17%) out of 70 patients demonstrated the presence of rare alleles. Only one of the 70 tumours contained an amplified c-myc gene. There was no significant correlation between either rearrangements of the structure of the Ha-ras-1 and/or c-myc genes or presence of rare alleles in tumours and tumour response to radiotherapy.

DNA double-strand break rejoining rates, inherent radiation sensitivity and human tumour response to radiotherapy

The relationship between DNA double-strand break rejoining rates, inherent radiation sensitivity and tumour response to radiation therapy was determined for a group of 25 squamous cell carcinoma (SCC) and eight sarcoma (SAR) tumours. DNA double-strand break frequencies were measured by neutral filter elution in first passage following explant tumour samples after in vitro exposure to 100 Gy of 60Co gamma-rays. There was no significant difference between SCC and SAR tumour cells in their sensitivity to break induction, but in a 1 h time period SAR tumour cells rejoined significantly fewer breaks than SCC tumour cells, consistent with the greater sensitivity of SAR and suggesting that differences in rates of break rejoining account for the different distributions of radiosensitivities seen when different tumour types are compared. The percentage of breaks rejoined in 1 h in these tumour samples correlated well with D(o) and with the beta component of the survival curve, measured in vitro by clonogenic assay in tumour cell lines established from the tumour samples, but not with SF2 or the alpha component of the survival curve. The rates of DNA double-strand break rejoining therefore appear to influence the exponential portion of survival curves and probably the interactions between breaks. The percentage of breaks rejoined in 1 h was higher in SCC tumours that subsequently failed radiotherapy and, although the differences were not significant, they suggest that rates of break rejoining are an important component of tumour response to radiation therapy.

Radiation-induced DNA damage in canine hemopoietic cells and stromal cells as measured by the comet assay

Stromal cell progenitors (fibroblastoid colony-forming unit; CFU-Fs) are representative of the progenitor cell population of the hemopoietic microenvironment in bone marrow (BM). Previous studies of the radiation dose-effect relationships for colony formation have shown that canine CFU-Fs are relatively radioresistant as characterized by a D0 value of about 2.4 Gy. In contrast, hemopoietic progenitors are particularly radiosensitive (D0 values= 0.12-0.60 Gy. In the present study, the alkaline single-cell gel electrophoresis technique for the in situ quantitation of DNA strand breaks and alkali-labile sites was employed. Canine buffy coat cells from BM aspirates and cells harvested from CFU-F colonies or from mixed populations of adherent BM stomal cell (SC) layers were exposed to increasing doses of X-rays, embedded in agarose gel on slides, lysed with detergents, and placed in an electric field. DNA migrating from single cells in the gel was made visible as "comets" by ethidium bromide staining. Immediate DNA damage was much less in cultured stromal cells than in hemopoietic cells in BM aspirates. These results suggest that the observed differences in clonogenic survival could be partly due to differences in the type of the initial DNA damage between stromal cells and hemopoietic cells.

DNA double strand break production and rejoining in V79 cells irradiated with light ions

Low energy protons and other densely ionizing light ions are known to have RBE>1 for cellular end points relevant for stochastic and deterministic effects. The occurrence of a close relationship between them and induction of DNA dsb is still a matter of debate. We studied the production of DNA dsb in V79 cells irradiated with low energy protons having LET values ranging from 11 to 31 keV/micrometer, i.e. in the energy range characteristic of the Bragg peak, using the sedimentation technique. We found that the initial yield of dsb is quite insensitive to proton LET and not significantly higher than that observed with X-rays, in agreement with recent data on V79 cells irradiated with alpha particles of various LET up to 120 keV/micrometer. By contrast, RBE for cell inactivation and for mutation induction rises with the proton LET. In experiments aimed at evaluating the rejoining of dsb after proton irradiation we found that the amount of dsb left unrepaired after 120 min incubation is higher for protons than for sparsely ionizing radiation. These results indicate that dsb are not homogeneous with respect to repair and give support to the hypothesis that increasing LET leads to an increase in the complexity of DNA lesions with a consequent decrease in their repairability.

Carboplatin- and cisplatin-induced potentiation of moderate-dose radiation cytotoxicity in human lung cancer cell lines

The interaction between moderate-dose radiation and cisplatin or carboplatin was studied in a cisplatin-sensitive (GLC4) and -resistant (GLC4-CDDP) human small-cell lung cancer cell line. Cellular toxicity was analysed under oxic conditions with the microculture tetrazolium assay. For the platinum and radiation toxicity with the clinically relevant dose ranges applied, this assay was used to obtain information on cell survival after the treatments. Apart from effects on cell survival effects on DNA were also investigated. Configurational DNA changes could be induced by platinum drugs and thereby these drugs might change the frequency of DNA double-strand breaks (dsbs). DNA fragmentation assayed with the clamped homogeneous electric field (CHEF) technique was used as a measure for dsbs in DNA. The radiosensitising effect of the platinum drugs was expressed as enhancement ratio (ER) calculated directly from survival levels of the initial slope of the curve. The highest ER for cisplatin in GLC4 was 1.39 and in GLC4-CDDP 1.38. These were all at 75% cell survival. Carboplatin showed increased enhancement with prolonged incubation up to 1.21 in GLC4 and was equally effective as cisplatin in GLC4-CDDP. According to isobologram analysis, prolonged incubation with both platinum drugs showed at least additivity with radiation for both cell lines at clinically achievable doses. GLC4-CDDP showed cross-resistance to radiation. The radiosensitising capacity of both lung cancer cell lines was not dependent on their platinum sensitivity. The formation of dsbs in DNA directly after radiation was not influenced by pretreatment of either drug in the sensitive or in the resistant cell line. Drug treatment resulted in decreased DNA extractability in control as well as in irradiated cells. Modest enhancement ratio for radiosensitisation by platinum drugs cannot be explained on the level of dsb formation in DNA in both cell lines. Interaction of radiation with the clinically less toxic carboplatin can be improved by prolonged low-dose carboplatin exposure before irradiation and is as potent as cisplatin in the resistant lung cancer cell line. This suggests an advantage in combining radiation and carboplatin in lung cancer patients.

Deletions at short direct repeats and base substitutions are characteristic mutations for bleomycin-induced double- and single-strand breaks, respectively, in a human shuttle vector system

Using the radiomimetic drug, bleomycin, we have determined the mutagenic potential of DNA strand breaks in the shuttle vector pZ189 in human fibroblasts. The bleomycin treatment conditions used produce strand breaks with 3'-phosphoglycolate termini as > 95% of the detectable dose-dependent lesions. Breaks with this end group represent 50% of the strand break damage produced by ionizing radiation. We report that such strand breaks are mutagenic lesions. The type of mutation produced is largely determined by the type of strand break on the plasmid (i.e. single versus double). Mutagenesis studies with purified DNA forms showed that nicked plasmids (i.e. those containing single-strand breaks) predominantly produce base substitutions, the majority of which are multiples, which presumably originate from error-prone polymerase activity at strand break sites. In contrast, repair of linear plasmids (i.e. those containing double-strand breaks) mainly results in deletions at short direct repeat sequences, indicating the involvement of illegitimate recombination. The data characterize the nature of mutations produced by single- and double-strand breaks in human cells, and suggests that deletions at direct repeats may be a 'signature' mutation for the processing of DNA double-strand breaks.

Induction of DNA double-strand breaks in CHO-K1 cells by carbon ions

Radiation-induced DNA double-strand breaks (dsbs) were measured in CHO-K1 cells by means of an experimental approach involving constant-field gel electrophoresis and densitometric scanning of ethidium bromide stained gels. For X-irradiation, an induction efficiency of 36 +/- 5 dsbs (Gy x cell)-1 was determined. With this set-up, the induction of dsbs was investigated in CHO-K1 cells after irradiation with accelerated carbon ions with specific energies ranging from 2.7 to 261 MeV/u. This set of particle beams covers the important linear energy transfer (LET) range between 17 and 400 keV/microns, where maximum efficiencies have been reported for other cellular endpoints like inactivation or mutation induction. For LETs up to 100 keV/microns, RBEs of approximately 1 have been determined, while efficiencies per unit dose decline for higher LETs. No RBE maximum > 1 was found. Data are compared with published results on dsb induction in mammalian cells by radiations of comparable LET.

A role for genomic instability in cellular radioresistance?

Inherent cellular radioresistance plays a critical role in the failure of radiotherapy. Although the consequences of radioresistance are well known, the molecular, biological, and cellular bases of radioresistance remain a mystery. We propose that genomic instability, the increased rate of acquisition of alterations in the mammalian genome, can directly modulate cells' sensitivity to radiation. In particular, destabilization of chromosomes occurring as a consequence of genomic instability may result in enhanced 'plasticity of the genome'. This increased plasticity of the genome allows cells to better adapt to changes in local environment(s) during tumor progression, or improve cell survival following exposure to DNA damage encountered during radiotherapy protocols, thereby contributing to radioresistant cell populations found in tumors both before and after radiotherapy.

Cellular radiosensitivity and DNA damage in primary human fibroblasts

PURPOSE

To evaluate the relationship between radiation-induced cell survival and DNA damage in primary human fibroblasts to decide whether the initial or residual DNA damage levels are the more predictive of normal tissue cellular radiosensitivity.

METHODS AND MATERIALS

Five primary human nonsyndromic and two primary ataxia telangiectasia fibroblast strains grown in monolayer were studied. Cell survival was assessed by clonogenic assay. Irradiation was given at high dose rate (HDR) 1-2 Gy/min. DNA damage was measured in stationary phase cells and expressed as fraction released from the well by pulsed-field gel electrophoresis (PFGE). For initial damage, cells were embedded in agarose and irradiated at HDR on ice. Residual DNA damage was measured in monolayer by allowing a 4-h repair period after HDR irradiation.

RESULTS

Following HDR irradiation, cell survival varied between SF2 0.025 to 0.23. Measurement of initial DNA damage demonstrated linear induction up to 30 Gy, with small differences in the slope of the dose-response curve between strains. No correlation between cell survival and initial damage was found. Residual damage increased linearly up to 80 Gy with a variation in slope by a factor of 3.2. Cell survival correlated with the slope of the dose-response curves for residual damage of the different strains (p = 0.003).

CONCLUSION

The relationship between radiation-induced cell survival and DNA damage in primary human fibroblasts of differing radiosensitivity is closest with the amount of DNA damage remaining after repair. If assays of DNA damage are to be used as predictors of normal tissue response to radiation, residual DNA damage provides the most likely correlation with cell survival.

Molecular determinants of radiosensitivity in mammalian cells

Knowledge of the biochemical and molecular basis of sensitivity to ionizing radiation will provide useful information regarding carcinogenesis, cancer proneness and patient responses to radiotherapy. Cellular endpoints following irradiation are primarily the product of the induction, processing and manifestation of DNA damage. There are therefore several points in the postirradiation sequelae that can be altered to modify the sensitivity of a cell. At the present time there is no consensus as to the single most important determinant of radiosensitivity, but maybe this is because it does not exist. There could be a basic cellular characteristic, such as DNA conformation, which can influence every aspect of the cellular response to radiation, but it is likely that the critical controlling steps differ in different cell systems.

Use of radiation quality as a probe for DNA lesion complexity

A number of studies have examined the possible relationships between either initial levels of DNA double-strand break (dsb) induction, their rejoining kinetics, or residual dsb and lethality in mammalian cells. With radiations of differing linear energy transfer (LET), the relative biological effectiveness (RBE) for dsb induction (20-100 keV/microns) has been lower than the RBEs measured for cell survival, and in many cases are around unity. Several studies have shown differences in the rejoining of dsb with less dsb rejoined after high than after low LET irradiation. These results suggest that there may be differences in the types of lesions being induced by different radiations and scored as DNA dsbs using current techniques. From modelling studies it is known that there is a range of energy deposition event sizes likely to occur in DNA, and there may also be uniquely large energy depositions associated with high LET radiations, particularly for large target sizes associated with the higher levels of chromatin structure. Many lesions induced will be clustered at the sites of these energy depositions. Assays need to be developed to measure complex lesions in both model DNA and cellular systems. Different levels of complexity need to be considered such as clustering of radicals close to DNA, localized areas of DNA damage (1-15 bp) and lesions which may be induced over larger distances related to higher-order structure. The use of radiations of differing LET will be an important probe in understanding DNA lesion complexity.

Radiation-induced DNA damage and repair in radiosensitive and radioresistant human tumour cells measured by field inversion gel electrophoresis

Radiation-induced DNA damage induction and repair was measured in two human squamous carcinoma cell lines with differing radiosensitive. Experiments were carried out with field inversion gel electrophoresis (FIGE), adapted to measure DNA double strand break (DSB) induction and repair in unlabelled cells. The sensitivity of the method was increased by introducing a hybridization membrane into the agarose gel. Damaged DNA accumulated on one spot on the membrane resulting in high local concentrations. This DNA was quantified using radioactively-labelled total human DNA as a probe. Dose response experiments for damage induction correlated well with the results using prelabelled cells. Linear DNA damage induction curves were observed with a sensitivity for the post-labelling method of 1 Gy. No differences in DSB induction were found, however, between the radiosensitive SCC61 and the radioresistant SQ20B cell line. Repair experiments were carried out with trypsinized cells with different doses and repair temperatures. The 10, 25 and 50 Gy doses resulted in 6, 13 and 50% of the DNA migrating out of the plug at 0 h. For both the cell lines 75-85% of the initial damage was repaired within 1 h at 37 degrees C at all three radiation doses, i.e. no significant differences were observed in repair rates or extent between the two cell lines. At 24 degrees C repair was slower than at 37 degrees C, and at 0 degree C no repair was observed. In summary, radiosensitivity differences at physiological temperatures could not be explained by differences in either induction or repair of DNA damage as measured by pulsed field gel electrophoresis.

X-ray-induced DNA double-strand breaks as lethal lesions in diploid human fibroblasts compared to Chinese hamster ovary cells

Calibration of the non-denaturing (pH 9.6) filter elution technique by 125I decays has been used to examine the relationship between X-ray-induced DNA double-strand breaks (dsb) and cellular survival of CHO cells and human fibroblasts. DNA dsb induction varies markedly between the two cell lines, reflecting the different survival levels. The relationships between lethal lesions (-ln S) and DNA dsb are also significantly different for CHO cells and human fibroblasts, suggesting either different probabilities of dsb conversion into lethal lesions or different repair capacities.

N-acetylcysteine and captopril protect DNA and cells against radiolysis by fast neutrons

N-Acetylcysteine and captopril, respectively mucolytic and antihypertensive drugs, contain free sulfhydryl groups. Since in general thiols have well-established radioprotective abilities, we sought putative radioprotective effects of these drugs against therapeutic fast neutrons. We show that pBR322 plasmid DNA is indeed protected against radiolytic strand breakage by both drugs. The oxygen independent protection is consistent with a hydroxyl radical scavenging mechanism. A clonogenicity assay reveals an increase of the survival of SCL-1 cultured keratinocytes irradiated in the presence of the drugs compared with cells irradiated without drugs. Our results suggest possible interferences between treatment with drugs bearing-SH groups and radiotherapy.

A correlation between DNA-nuclear matrix binding and relative radiosensitivity in two human squamous cell carcinoma cell lines

Three aspects of DNA topology were examined in two human squamous cell carcinoma lines of differing radiosensitivity (SQ-9G, D0 = 1.46 Gy; and SQ-20B, D0 = 2.36 Gy). High-salt-extracted nuclei (nucleoids) were taken from gamma-irradiated cells, stained with ethidium bromide and examined by flow cytometry. After 5 Gy, nucleoids from SQ-9G cells became 30% less efficient at adopting positive DNA supercoils than were unirradiated controls. In contrast, only a 4% difference was found with the radioresistant SQ-20B line. Both lines produced positive supercoils more efficiently after irradiation if first exposed to the topoisomerase II inhibitor VP16. Ethidium bromide titration of nucleoids was consistent with each containing similar numbers and sizes of DNA loops. In each line approximately 30-35% of DNA was accessible to trioxsalen, as shown by inter-strand crosslinking after UV photo-activation. Exhaustive digestion of nuclear DNA by DNase I removed more DNA from the radiosensitive than from the radioresistant cell line (12% vs 28% remaining). This difference was thought to be due to the increased accessibility of SQ-9G DNA in vitro. We suggest that a looser association of SQ-9G DNA with the nuclear matrix both promotes DNase I digestion and affects the ability of SQ-9G nucleoids to maintain positive DNA supercoils after irradiation. These data implicate the DNA matrix attachment region in the expression of radiation sensitivity in the cell lines studied.

The use of asymmetric-field inversion gel electrophoresis to predict tumor cell radiosensitivity

The success of a predictive assay for radiotherapy relies on the use of one or more tumor cell traits that equate with tumor radioresistance or radiosensitivity. These traits can be divided into intrinsic (genetic) and extrinsic (epi-genetic) factors. Most probably, a tumor's response to radiotherapy will be influenced by both of these sets of traits. Radiobiological analysis of cultured cells derived from explanted tumors of head and neck patients has shown that in vitro survival of tumor cells is not the only factor affecting tumor radiocurability. Two possible reasons are the high degree of selection involved in growing the cells in vitro and the inability to assess the contribution of the cell-cell contact effect with cultured cells. A possible means of overcoming both of these problems would be an assessment of the radiosensitivity of the cell population immediately after removal from the tumor. Since a good correlation exists between intrinsic cellular radioresistance and DNA double-strand break repair (DSBR) as assayed by the Neutral Elution technique [21], we have investigated the feasibility of using asymmetric field inversion gel electrophoresis (AFIGE) in identifying resistant tumor cells in vitro. AFIGE has several advantages over neutral elution in that it is faster (approximately 60-80 samples can be run on the same agarose gel) and, most importantly, one can visualize DNA damage and repair by staining the DNA with ethidium bromide.(ABSTRACT TRUNCATED AT 250 WORDS)