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

Impact of DNA repair and reactive oxygen species levels on radioresistance in pancreatic cancer

PURPOSE

Radioresistance in pancreatic cancer patients remains a critical obstacle to overcome. Understanding the molecular mechanisms underlying radioresistance may achieve better response to radiotherapy and thereby improving the poor treatment outcome. The aim of the present study was to elucidate the mechanisms leading to radioresistance by detailed characterization of isogenic radioresistant and radiosensitive cell lines.

METHODS

The human pancreatic cancer cell lines, Panc-1 and MIA PaCa-2 were repeatedly exposed to radiation to generate radioresistant (RR) isogenic cell lines. The surviving cells were expanded, and their radiosensitivity was measured using colony formation assay. Tumor growth delay after irradiation was determined in a mouse pancreatic cancer xenograft model. Gene and protein expression were analyzed using RNA sequencing and Western blot, respectively. Cell cycle distribution and apoptosis (Caspase 3/7) were measured by FACS analysis. Reactive oxygen species generation and DNA damage were analyzed by detection of CM-H₂DCFDA and γH2AX staining, respectively. Transwell chamber assays were used to investigate cell migration and invasion.

RESULTS

The acquired radioresistance of RR cell lines was demonstrated in vitro and validated in vivo. Ingenuity pathway analysis of RNA sequencing data predicted activation of cell viability in both RR cell lines. RR cancer cell lines demonstrated greater DNA repair efficiency and lower basal and radiation-induced reactive oxygen species levels. Migration and invasion were differentially affected in RR cell lines.

CONCLUSIONS

Our data indicate that repeated exposure to irradiation increases the expression of genes involved in cell viability and thereby leads to radioresistance. Mechanistically, increased DNA repair capacity and reduced oxidative stress might contribute to the radioresistant phenotype.

Biological determinants of radioresistance and their remediation in pancreatic cancer

Despite recent advances in radiotherapy, a majority of patients diagnosed with pancreatic cancer (PC) do not achieve objective responses due to the existence of intrinsic and acquired radioresistance. Identification of molecular mechanisms that compromise the efficacy of radiation therapy and targeting these pathways is paramount for improving radiation response in PC patients. In this review, we have summarized molecular mechanisms associated with the radio-resistant phenotype of PC. Briefly, we discuss the reversible and irreversible biological consequences of radiotherapy, such as DNA damage and DNA repair, mechanisms of cancer cell survival and radiation-induced apoptosis following radiotherapy. We further describe various small molecule inhibitors and molecular targeting agents currently being tested in preclinical and clinical studies as potential radiosensitizers for PC. Notably, we draw attention towards the confounding effects of cancer stem cells, immune system, and the tumor microenvironment in the context of PC radioresistance and radiosensitization. Finally, we discuss the need for examining selective radioprotectors in light of the emerging evidence on radiation toxicity to non-target tissue associated with PC radiotherapy.

Radioresistance in a tumour cell line correlates with radiation inducible Ku 70/80 end-binding activity

PURPOSE

The aims of the present study were to better understand the role of Ku 80, which is involved in double-strand break repair in mammalian cells in the mechanism of radiation resistance and to verify the possibility of increasing cell radiosensitivity by targeted inhibition of Ku autoantigen 80 (Ku 80).

MATERIALS AND METHODS

Western blot and electrophoretic mobility shift assay (EMSA) were performed on the human bladder carcinoma cell line RT112 (radioresistant) and on the human colorectal carcinoma cell line SW48 (radiosensitive) to assess the expression levels of DNA-dependent protein kinase (DNA-PK) components and the DNA-binding activity of the Ku 70/80 heterodimer after exposure to radiation, respectively. Ku 80 silencing was carried out with the use of small interfering RNA (siRNA).

RESULTS

Greater differences in the DNA-binding activity of Ku 70/80 and Ku 80 phosphorylation level were observed in RT112 as compared to SW48 after X-ray treatment. There is no correlation between Ku expression and DNA-binding activity at lower doses. A significant increase in nuclear Ku 80 expression was observed one hour after the exposure, only at the higher doses, while the DNA-PK catalytic subunits (DNA-PKcs) and Ku 70 levels did not change significantly. Inhibition of Ku 80 expression by siRNA induced radiosensitivity in the RT112 cell line.

CONCLUSIONS

Our data demonstrate that in a bladder tumour cell line up-regulation of Ku end-binding activity without any marked change in Ku expression underlie radiation 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.

Split dose recovery studies using homologous recombination deficient gene knockout chicken B lymphocyte cells

To understand the role of proteins involved in DSB repair modulating SLD recovery, chicken B lymphoma (DT 40) cell lines either proficient or deficient in RAD52, XRCC2, XRCC3, RAD51C and RAD51D were subjected to fractionated irradiation and their survival curves charted. Survival curves of both WT DT40 and RAD52 (-/-) cells had a big shoulder while all the other cells exhibited small shoulders. However, at the higher doses of radiation, RAD51C(-/-) cells displayed hypersensitivity comparable to the data obtained for the homologous recombination deficient RAD54(-/-) cells. Repair of SLD was measured as an increase in survival after a split dose irradiation with an interval of incubation between the radiation doses. All the cell lines (parental DT40 and genetic knockout cell lines viz., RAD52(-/-), XRCC2(-/-), XRCC3(-/-) RAD51C(-/-) and RAD51D(-/-)) used in this study demonstrated a typical split-dose recovery capacity with a specific peak, which varied depending on the cell type. The maximum survival of WT DT40 and RAD52(-/-) was reached at about 1-2 hours after the first dose of radiation and then decreased to a minimum thereafter (5h). The increase in the survival peaked once again by about 8 hours. The survival trends observed in XRCC2 (-/-), XRCC3(-/-), RAD51C (-/-) and RAD51D(-/-) knockout cells were also similar, except for the difference in the initial delay of a peak survival for RAD51D(-/-) and lower survival ratios. The second phase of increase in the survival in these cell lines was much slower in XRCC2(-/-) , XRCC3(-/-), RAD51C(-/-) and RAD51D(-/-) and further delayed when compared with that of RAD52(-/-) and parental DT40 cells suggesting a dependence on their cell cycle kinetics. This study demonstrates that the participation of RAD52, XRCC2, XRCC3, RAD51C and RAD51D in the DSB repair via homologous recombination is of less importance in comparison to RAD54, as RAD54 deficient cells demonstrated complete absence of SLD recovery.

Quantified relationship between cellular radiosensitivity, DNA repair defects and chromatin relaxation: a study of 19 human tumour cell lines from different origin

BACKGROUND AND PURPOSE

There is still confusion in the choice of the molecular assays to predict the radiation response of human cells. The case of tumours appears to be particularly complex, may be because of their instability and heterogeneity. The aim of this study was to investigate quantitatively the relationships between DNA double-strand breaks (DSB) repair, chromatin relaxation and cellular radiosensitivity. Nineteen human tumour cell lines, representing a large spectrum of radiation responses and tissues, were examined.

MATERIALS AND METHODS

Intrinsic radiosensitivity was quantified with surviving fraction at 2 Gy (SF2) as an endpoint. Standard and modified pulsed-field gel electrophoresis techniques were employed to assess DSB repair rate and chromatin relaxation. A cell-free assay was chosen to estimate DSB repair activity, independently of chromatin impairment.

RESULTS AND CONCLUSIONS

Surviving fraction at 2 Gy (SF2) decreases linearly with the amount of unrepaired DSB and the extent of chromatin relaxation: one additional unrepaired DSB per cell or 1% chromatin decondensation produce a loss of about 1.5% surviving fraction. However, all the cell lines did not obey both correlations, suggesting that DSB repair and chromatin impairments contribute separately to increase the severity of DNA damage involved in cell lethality. Four cell lines groups showing different DSB repair and/or chromatin impairments were defined. Cell lines exhibiting both DSB repair defect and chromatin relaxation are the most radiosensitive.

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.

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.

Heat effects on DNA repair after ionising radiation: hyperthermia commonly increases the number of non-repaired double-strand breaks and structural rearrangements

After ionising radiation double-strand breaks (dsb) are lethal if not repaired or misrepaired. Cell killing is greatly enhanced by hyperthermia and it is questioned here whether heat not only affects dsb repair capacity but also fidelity in a chromosomal context. dsb repair experiments were designed so as to mainly score non-homologous end joining, while homologous recombination was largely precluded. Human male G(0) fibroblasts were either preheated (45 degrees C, 20 min) or not before X-irradiation. dsb induction and repair were measured by conventional gel electrophoresis and an assay combining restriction digestion using a rare cutting enzyme (NotI) and Southern hybridisation, which detects large chromosomal rearrangements (>100 kb). dsb induction rate in an X-chromosomal NotI fragment was 4.8 x 10(-3) dsb/Gy/MB: Similar values were found for the genome overall and also when cells were preheated. After 50 Gy, fibroblasts were competent to largely restore the original restriction fragment size. Five per cent of dsb remained non-rejoined and 14% were misrejoined. Correct restitution of restriction fragments occurred preferably during the first hour but continued at a slow rate for 12-16 h. In addition, dsb appeared to misrejoin throughout the entire repair period. After hyperthermia the fractions of non-rejoined and misrejoined dsb were similarly increased to 13 and 51%, respectively. It is suggested that heat increases the probability of dsb being incorrectly rejoined but it is not likely to interfere with one dsb repair pathway in particular.

Induction and rejoining of DNA double-strand breaks in bladder tumor cells

The induction and rejoining of radiation-induced double-strand breaks (DSBs) in cells of six bladder tumor cell lines (T24, UM-UC-3, TCC-SUP, RT112, J82, HT1376) were measured using the neutral comet assay. Radiation dose-response curves (0-60 Gy) showed damage (measured as mean tail moment) for five of the cell lines in the same rank order as cell survival (measured over 0-10 Gy), with the least damage in the most radioresistant cell line. Damage induction correlated well with clonogenic survival at high doses (SF10) for all six cell lines. At the clinically relevant dose of 2 Gy, correlation was good for four cell lines but poor for two (TCC-SUP and T24). The rejoining process had a fast and slow component for all cell lines. The rate of these two components of DNA repair did not correlate with cell survival. However, the time taken to reduce the amount of DNA damage to preirradiated control levels correlated positively with cell survival at 10 Gy but not 2 Gy; radioresistant cells rejoined the induced DSBs to preirradiation control levels more quickly than the radiosensitive cells. Although the results show good correlation between SF10 and DSBs for all six cell lines, the lack of correlation with SF2 for TCC-SUP and T24 cells would suggest that a predictive test should be carried out at the clinically relevant dose. At present the neutral comet assay cannot achieve this.

Repair of DNA double-strand breaks and radiosensitivity to killing in an isogenic group of p53 mutant cell lines

PURPOSE

Accumulation of the p53 protein can result in G1 arrest that may facilitate DNA repair, or alternatively, it may lead to apoptosis. Mutations that alter p53's ability to mediate these responses are expected to alter cell radiosensitivity to killing. However, the relationship between p53 status and cell radiosensitivity has proven to be complex. Several studies have suggested that p53 mutations are associated with increased radioresistance to killing, while others have shown no such correlation. These differences may be derived from the fact that different mutations of p53 exert different effects on cell radiosensitivity.

METHODS AND MATERIALS

To address this question, we examined a group of isogenic cell lines that express different "hot spot" mutant forms of p53. These cells were generated from human osteosarcoma (SAOS) cells, a p53 null cell line, by transfection with vectors expressing different p53 mutants. Vectors with the following p53 mutations were utilized: 143Ala, 175His, 248Try, 273His, and 281Gly. As controls, we used the original SAOS cells and cells transfected with the vector alone. Results were compared to those obtained with a cell line expressing wild-type p53 (wt p53). Radiosensitivity to killing was determined in the exponential phase of growth by measuring loss of colony-forming ability. Induction and repair of DNA double-strand breaks (dsb) was measured in irradiated cells using pulsed-field gel electrophoresis. Apoptosis was assessed using morphologic evaluation of DAPI-stained cells after treatment either with radiation or paclitaxel.

RESULTS

Transfected SAOS-2 cell lines expressed a mutant form of p53 that could not be induced by radiation, and which was transcriptionally inactive. Among the 7 cell lines studied, we observed no difference in cellular radiosensitivity to killing (p = NS). When examining DNA repair, no difference in either the induction or repair of DNA dsb was noted in any of the cell lines studied (p = NS). Also, induction of apoptosis, either after exposure to radiation or paclitaxel, was low, and similar in all cell lines (p = NS). Non-isogenic cells expressing wt p53 were more radioresistant to killing by radiation, but showed similar kinetics of dsb rejoining.

CONCLUSION

The results suggest that expression of different p53 mutants does not alter the yields of radiation-induced dsb, or the ability of cells to repair this type of lesion. In addition, the same p53 mutants do not affect cellular radiosensitivity to killing, or the induction of apoptosis after exposure to radiation or paclitaxel.

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.

The ratio of initial/residual DNA damage predicts intrinsic radiosensitivity in seven cervix carcinoma cell lines

The single-cell gel electrophoresis (comet) assay was used to measure radiation-produced DNA double-strand breaks (dsbs) in a series of seven cervical tumour cell lines (ME180, HT3, C33A, C41, SiHa, MS751 and CaSki). The proportion of DNA dsbs was measured immediately after radiation treatment (initial damage) and 16 h later after incubation at 37 degrees C (residual damage). Linear dose-response curves were seen for initial (slopes 0.23-0.66) and residual (slopes 0.16-0.87) DNA dsbs. Neither of the slopes of the linear regression analysis on the initial and on the residual DNA dsbs dose-response curves (range 0-80 Gy) correlated with SF2 (surviving fraction at 2 Gy) measured after high- (HDR) or low-dose-rate (LDR) irradiation. An association was evident between SF2 after HDR and LDR irradiation and the ratio of the absolute level of initial and residual damage after a single dose of 60 Gy. However, a significant correlation was found between HDR (r= -0.78, P = 0.04) and LDR (r = -0.86, P = 0.03) SF2 values and the ratio of the slopes of the initial and residual DNA dsbs dose-response curves (range 0.47-0.99), representing the fraction of DNA damage remaining. These results indicate that the neutral comet assay can be used to predict radiosensitivity of cervical tumour cell lines by assessing the ratio of initial and residual DNA dsbs.

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.

Differential level of DSB repair fidelity effected by nuclear protein extracts derived from radiosensitive and radioresistant human tumour cells

A cell-free plasmid reactivation assay was used to determine the fidelity of DNA double-strand break (DSB) repair in a panel of eight DSB repair-proficient human tumour cell lines. Nuclear protein extracts derived from radiosensitive tumour cells were less capable of correctly rejoining EcoRI-induced DSBs than were similar extracts from radioresistant tumour cells. Linear regression analysis suggests that there was a significant (r2 = 0.84, P = 0.001, d.f. = 6) correlation between the fidelity of DSB rejoining and the SF2 values of the cell lines studied. This cell-free assay is clearly sensitive to differences in the nuclear protein composition that reflect the clinically relevant radiosensitivity of these cell lines. The fact that our cell-free assay yielded similar results to previous studies that used intracellular plasmid reactivation assays suggests that those differences in DSB mis-rejoining frequencies in radiosensitive and radioresistant cell lines may be due to inherent differences in nuclear protein composition and are not directly attributable to differences in proliferation rates between cell lines. The underlying cause for this association between DSB mis-rejoining frequencies and radiosensitivity is presently unknown, however restriction endonuclease mapping and polymerase chain reaction (PCR) amplification analysis revealed that approximately 40% of the mis-rejoined DSBs arose as a result of the deletion of between 40 and 440 base pairs. These data raise the possibility that the radiosensitivity of DSB repair-proficient human tumour cell lines may be partly determined by the predisposition of these cell lines to activate non-conservative DSB rejoining pathways.

Hypersensitivity to very-low single radiation doses: its relationship to the adaptive response and induced radioresistance

There is now little doubt of the existence of radioprotective mechanisms, or stress responses, that are upregulated in response to exposure to small doses of ionizing radiation and other DNA-damaging agents. Phenomenologically, there are two ways in which these induced mechanisms operate. First, a small conditioning dose (generally below 30 cGy) may protect against a subsequent, separate, exposure to radiation that may be substantially larger than the initial dose. This has been termed the adaptive response. Second, the response to single doses may itself be dose-dependent so that small acute radiation exposures, or exposures at very low dose rates, are more effective per unit dose than larger exposures above the threshold where the induced radioprotection is triggered. This combination has been termed low-dose hypersensitivity (HRS) and induced radioresistance (IRR) as the dose increases. Both the adaptive response and HRS/IRR have been well documented in studies with yeast, bacteria, protozoa, algae, higher plant cells, insect cells, mammalian and human cells in vitro, and in studies on animal models in vivo. There is indirect evidence that the HRS/IRR phenomenon in response to single doses is a manifestation of the same underlying mechanism that determines the adaptive response in the two-dose case and that it can be triggered by high and low LET radiations as well as a variety of other stress-inducing agents such as hydrogen peroxide and chemotherapeutic agents although exact homology remains to be tested. Little is currently known about the precise nature of this underlying mechanism, but there is evidence that it operates by increasing the amount and rate of DNA repair, rather than by indirect mechanisms such as modulation of cell-cycle progression or apoptosis. Changed expression of some genes, only in response to low and not high doses, may occur within a few hours of irradiation and this would be rapid enough to explain the phenomenon of induced radioresistance although its specific molecular components have yet to be identified.

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.

Repair of chromosome and DNA breaks versus cell survival in Chinese hamster cells

Clonogenic and non-clonogenic parameters of cell survival were compared in irradiated Chinese hamster cells. Clonogenic survival, chromatid break and repair kinetics, as well as DNA damage and repair, were assessed in synchronized cells in different parts of the cell cycle. C2 chromatid damage and repair was examined in metaphase chromosomes of cells irradiated during S and G2 phase, treated with or without inhibitors of DNA repair. Bromodeoxyuridine labelling of S phase cells starting at the time of irradiation made it possible to determine precisely, while scoring metaphase chromosomes, whether cells were irradiated in mid S, late S, or G2 phases of the cycle. The results showed that chromatid breaks induced in S phase are efficiently repaired until the moment cells progress into G2, when repair stops abruptly. Chromatid damage in G2 phase is not repaired. On the other hand, DNA double-strand breaks are repaired in all phases of the cycle, even during G2 phase which has no concurrent chromatid break repair. Finally, there is no consistent correlation between chromatid damage and repair, DNA damage and repair, and cell survival, thus indicating that the interaction of different parameters of radiosensitivity must be better understood for them to be useful predictors of cell survival.

Cell-cycle variation in DNA migration in pulsed-field gel electrophoresis

Pulsed-field electrophoresis is being used extensively in the gene mapping studies and in the analysis of DNA strand breakage by ionizing radiation. We have evaluated the relationship between the fraction of S phase DNA in a cell population and its ability to modify the migration of DNA in pulsed-field gel electrophoresis. We have shown that increasing the proportion of S phase DNA reduced the effective rate of migration of MGH-U1 cellular DNA. This effect was observed after treatment with ionizing radiation or the restriction enzyme Not I. However, when radiation-induced damage was studied using intact cells, only the DNA with 70 percent S phase showed apparent differences in damage induction. These studies therefore provide data to indicate the percentage of S phase cells at which overall DNA migration might be affected significantly.

Relationship between DNA damage, rejoining and cell killing by radiation in mammalian cells

The prevailing hypothesis on the mechanism of radiation-induced cell killing identifies the genetic material deoxyribonucleic acid (DNA) as the most important subcellular target at biologically relevant doses. In this review we present new data and summarize the role of the DNA double-strand breaks (dsb) induced by ionizing radiation and DNA dsb rejoining as determinants of cellular radiosensitivity. When cells were irradiated at high dose-rate, two molecular end-points were identified which often correlated with radiosensitivity: (1) the apparent number of DNA dsb induced per Gy per DNA unit and (2) the half-time of the fast component of the DNA dsb rejoining kinetics. These two molecular determinants, not mutually exclusive, may be linked through a common factor such as the conformation of DNA.

Might intrinsic radioresistance of human tumour cells be induced by radiation?

Survival measurements were made on six human tumour cell lines in vitro after irradiation with single doses of X rays. Doses up to 5 Gy were used giving surviving fractions down to 20%, but the majority of the measurements were made at doses < 1 Gy. These six cell lines have very different intrinsic radiosensitivities: HT29, Be11, and RT112 are radioresistant with surviving fractions at 2 Gy (SF2) between 60 and 74%, while MeWo, SW48, and HX142 are radiosensitive (SF2 = 3-29%). For all the cell lines, response over the dose range 2-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. However, HT29, Be11, and RT112 cells showed a significant increase in X-ray radiosensitivity at doses below < 1 Gy compared with the prediction extrapolated from a LQ model fitted to the data at higher doses. The LQ model also slightly underpredicted the effect of low-dose X rays in MeWo cells, but the response of SW48 and HX142 cells was well described by the LQ model at all doses, with no evidence of increased low-dose effectiveness. The most plausible explanation for this phenomenon is that it reflects an induced radioresistance so that low doses of X-rays in vitro are more effective per Gy than higher doses, because only at higher doses is there sufficient damage to trigger repair systems or other radioprotective mechanisms. It follows that variation in the amount of inducible radioresistance might explain, in part, differences in intrinsic radiosensitivity above > 1 Gy between cell lines: cells would be intrinsically radiosensitive because they have a diminished inducible response.

Radiation-induced DNA double-strand break rejoining in human tumour cells

Five established human breast cancer cell lines and one established human bladder cancer cell line of varying radiosensitivity have been used to determine whether the rejoining of DNA double-strand breaks (dsbs) shows a correlation with radiosensitivity. The kinetics of dsb rejoining was biphasic and both components proceeded exponentially with time. The half-time (t1/2) of rejoining ranged from 18.0 +/- 1.4 to 36.4 +/- 3.2 min (fast rejoining process) and from 1.5 +/- 0.2 to 5.1 +/- 0.2 h (slow rejoining process). We found a statistically significant relationship between the survival fraction at 2 Gy (SF2) and the t1/2 of the fast rejoining component (r = 0.949, P = 0.0039). Our results suggest that cell lines which show rapid rejoining are more radioresistant. These results support the view that, as well as the level of damage induction that we have reported previously, the repair process is a major determinant of cellular radiosensitivity. It is possible that the differences found in DNA dsb rejoining and the differences in DNA dsb induction are related by a common mechanism, e.g. conformation of chromatin in the cell.

The spectrum of in vitro radiosensitivity in four human melanoma cell lines is not accounted for by differential induction or rejoining of DNA double strand breaks

PURPOSE

Radioresistance is a significant clinical problem in advanced malignant melanoma and many melanoma cell lines show a radioresistant acute x-ray survival response in vitro. Given that the DNA double strand break is the lesion most closely correlated with x-ray induced cell lethality, differences in the induction and rejoining of these lesions may account for the radioresistance of some human melanoma cell lines.

METHODS AND MATERIALS

The above hypothesis was tested using pulsed field gel electrophoresis to measure x-ray induced DNA double strand break induction and rejoining in four human melanoma cell lines: MM138, MM170, MM96-L and HT 144.

RESULTS

The MM138, MM170 and MM96-L cell lines were characterized in vitro by low alpha/beta ratios and broad x-ray survival curve shoulders. MM138 and MM170 were the most radioresistant and MM96-L had intermediate sensitivity. In contrast, HT144 was markedly x-ray sensitive, despite retaining a shoulder and like the other lines, having a low alpha/beta ratio. There were no significant differences in DNA double strand break induction between the cell lines, and thus no correlation existed between DNA double strand break induction and radiosensitivity. Consistent with the shoulders on the x-ray survival curves, all four cell lines showed efficient DNA double strand break rejoining. Highly efficient DNA double strand break rejoining could account for the radioresistance of one of the melanoma lines (MM138). For example, MM138 had rejoined 50% of the induced DNA double strand breaks by 5.5 min compared to 13-17 min for the other three cell lines. The development of postirradiation apoptosis was effectively excluded as the cause of the marked radiosensitivity of the HT144 cell line.

CONCLUSION

Other factors (such as lesion repair fidelity or differential lesion tolerance) underlie the differences in the intrinsic radiosensitivity between these melanoma cell lines.

DNA double-strand break induction and rejoining as determinants of human tumour cell radiosensitivity. A pulsed-field gel electrophoresis study

Measurement of the surviving fraction after 2 Gy (SF2) may predict for local control of the tumour and patients cure, but clonogenic assays are unsuitable for wider clinical application. Promising results have been obtained using DNA damage assays such as pulsed-field gel electrophoresis, PFGE. In the current study, nine human tumour cell lines (SF2, range 0.08-0.62) were studied for DNA double-strand break (dsb) induction and six of these for dsb rejoining using PFGE. Differences in dsb induction, as the slope (+/- SEM) of DNA release per Gy, varied from 1.30 (0.05) to 2.42 (0.17). The dsb induction frequency varied from 3.55 (0.33) to 9.69 (2.18) dsb x 10(-9)/bp/Gy (21-56 dsb/Gy/cell). Variations in the half-time for fast phase (18-60 min) and slow phase (38-445 min) dsb rejoining were observed. Statistically significant correlations were found between SF2 and the slope of the DNA release curve (p = 0.003), DNA release after 10 Gy (p = 0.029) and 20 Gy (p = 0.011) and slow phase dsb rejoining (p = 0.012). While the underlying mechanisms of cell killing remain unclear, PFGF measurement of dsb induction and rejoining shows great potential as a predictive assay for intrinsic cellular radiosensitivity.

The effect of radiation on G2 blocks, cyclin B expression and cdc2 expression in human squamous carcinoma cell lines with different radiosensitivities

The purpose of the present study was to investigate the role of cyclin B and cdc2 in the G2 delay and to test whether the magnitude of the G2 delay correlated with sensitivity to ionizing radiation in two human cell lines. Cell cycle delays were measured by flow cytometry after pulse labeling with bromodeoxyuridine, and expression of cell cycle control genes were measured in Western blots in radiosensitive SCC61 and radioresistant SQ20B cell lines. Flow cytometry data demonstrated that the duration of the G2 arrest was dose dependent in both cell lines, amounting to approximately 1.1 h/Gy. No difference was found between the cell lines in the length of the G2 block. Radiation exposure did not result in a decrease of cyclin B. Cyclin B protein levels in both asynchronous and synchronized populations in fact showed a dose dependent increase, concomitant with the rise in the fraction of cells in G2/M. Similarly, the cdc2 protein levels did not decrease after irradiation. However, it was found that the levels of hyperphosphorylated, and therefore inactive, kinase were significantly higher in irradiated cells than in unirradiated cells. The accumulation of this hyperphosphorylated form correlated with the arrest of cells in the G2 phase. Finally, immunocytochemical staining of cyclin B revealed an increase of this protein in the cytoplasm after irradiation and a decrease in nuclear staining. This differential localization could possibly account for the reduced nuclear phosphorylation of cdc2 kinase leading to the G2 arrest.

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.

Use of fluorescence in situ hybridization to measure chromosome aberrations as a predictor of radiosensitivity in human tumour cells

Fluorescence in situ hybridization (FISH) is a potential assay for determining cellular radiosensitivity based on the detection of chromosome damage. This approach was chosen because of its relative simplicity and short assay time. Two radiosensitive and two radioresistant human tumour cell lines were used. The radiosensitive lines were an ovarian carcinoma line (A1847) and a squamous carcinoma line (SCC61). The radioresistant cells were a lung adenocarcinoma line (A549) and a second squamous line (SQ20B). Whole chromosome-specific probes were used to detect radiation-induced chromosome aberrations in mitotic cells. Available probes were first screened to characterize the intrinsic chromosome aberrations before irradiation and the appropriate probes (minimum fluorescent spots) were selected for each cell line. Maximum radiation-induced aberrations were found 24 h after irradiation. Dose-response curves corrected for target size (proportion of genome probed) differed for all cell lines. The radiosensitive A1847 cell line showed more induced aberrations compared with the radioresistant A549 cell line, in agreement with the survival data. In contrast, the SQ20B cell line showed more induced chromosome aberrations than the more radiosensitive SCC61 cell line, leading to the hypothesis that the SQ20B cells could tolerate more aberrations. Dose-response curves obtained in surviving cells 14 days postirradiation indeed showed elevated levels of chromosome aberrations for SQ20B cells. The difference in chromosome aberrations between 1 and 14 days showed a good correlation with the survival data for all four cell lines. In conclusion, FISH of mitotic cells with whole chromosome probes appears to be a suitable assay to predict radiosensitivity. It seems necessary, however, to determine both induced and remaining chromosome aberrations, since different processing or tolerance of radiation-induced aberrations, including stable types, could lead to different correlations with cell survival.