Evidence for reduced capacity for damage accumulation and repair in plateau-phase C3H 10T1/2 cells following multiple-dose irradiation with gamma rays

The effects of multiple-dose gamma irradiation on the shape of survival curves were studied with mouse C3H 10T1/2 cells maintained in contact-inhibited plateau phase. The dose-fractionation intervals included 3, 6, and 24 h. Following three fractionated doses (5 Gy per dose) of exposures, cells responded to further irradiation by displaying a survival curve with a much reduced shoulder width (Dq) compared to that of the survival curve measured in cells irradiated with single-graded doses alone. The effect on the mean lethal dose (D0) was small and appeared to be significant. The effect on reduction of Dq could not be completely overcome by lengthening the fractionation intervals from 3 to 6 h or 24 h, times in which repair of sublethal damage (SLD) measured by simple split-dose scheme and potentially lethal damage (PLD) measured by postirradiation incubation was completed. Other experiments showed that pretreatments of cells with fractionated irradiation appeared to slow down the cellular repair processes of SLD and PLD. Therefore, the observed change in the shape of survival curves after fractionation treatments may be attributed to a reduction of the cells' capacity for damage accumulation by an enhancement of the lethal expression of SLD and PLD. Although the molecular mechanism(s) is not known, the results of this study indicate that the acute graded dose-survival curve cannot be used a priori to extrapolate and reliably predict results of hyperfractionation. It is probable that for a nondividing or slowly dividing cell population, such an extrapolation may lead to an underestimation of cell killing. Furthermore, the findings of this investigation appear to support an interpretation, alternative to the high-linear energy transfer (LET) track-end postulate, for the effects on cell survival seen at low doses or low dose rates.     

Split-dose recovery is due to the repair of DNA double-strand breaks

DNA double-strand breaks are the molecular lesions the repair of which leads to the reappearance of the shoulder observed in split-dose experiments. This conclusion is based on results obtained with the help of a diploid yeast mutant rad 54-3 which is temperature-conditional for the repair of DNA double-strand breaks. Two repair steps must be met to yield the reappearance of the shoulder on a split-dose survival curve: the repair of double-strand breaks during the interval between two doses and on the nutrient agar plate after the second dose. In yeast lethality may be attributable to either an unrepaired double-strand break (i.e. a double-strand break is a potentially lethal lesion) or to the interaction of two double-strand breaks (misrepair of double-strand breaks). Evidence is presented that the two cellular phenomena of liquid holding recovery (repair of potentially lethal damage) and of split-dose recovery (repair of sublethal damage) are based on the repair of the same molecular lesion, the DNA double-strand break.     

Radiation damage repair capacity of a human germ-cell tumour cell line: inhibition by 3-aminobenzamide

The capacity of a human germ-cell tumour line to repair radiation damage has been investigated by means of a clonogenic assay. Dose-rate dependence studies, split-dose experiments and experiments designed to measure repair of potentially lethal damage have been performed. The cells showed some ability to repair radiation-induced damage in all three types of experiment. An attempt has been made to understand the possible cellular mechanisms of these repair processes by the use of 3-aminobenzamide (3-AB), an agent thought to act by inhibition of ADP-ribosylation. 3-AB added 2 h prior to and removed 18 h after irradiation at a non-toxic dose to unirradiated cells caused a small but consistent increase in cell kill with acute (150 cGy min-1) irradiation, largely involving a reduction in the shoulder region of the survival curve, but had a greater effect in increasing cell kill at a dose rate of 7.6 cGy min-1 and an even greater effect at a dose rate of 1.6 cGy min-1. When 3-Ab was present 2 h prior to the first dose and between two equal doses in a split-dose experiment, inhibition of split-dose recovery was observed. In addition, some inhibition of potentially lethal damage recovery was observed with 3-AB. A possible role for poly(ADP-ribosylation) is thus implicated in the repair of radiation-induced damage of this human tumour cell line during continuous low dose rate or fractionated radiation schedules, although other effects of 3-AB on respiratory metabolism and/or purine synthesis cannot be eliminated as the cause of the observed inhibitory effects.     

Enhanced recovery from ionizing radiation damage in a lepidopteran insect cell line

TN-368 lepidopteran insect cells display a pronounced resistance to the lethal effects of ionizing radiation and exhibit superior DNA repair capabilities. When a TN-368 cell population entering stationary growth phase is irradiated with 137Cs gamma rays and then incubated for several hours before cell dilution and plating for colony formation, the surviving fraction is increased several-fold over cells diluted and plated immediately after irradiation. Similarly, the survival of cells plated immediately following the second of two equivalent doses separated by several hours is greater than the survival of cells plated immediately following a single dose equal to the sum of the split doses. Both processes exhibit similar biphasic repair kinetics and reach maximal levels by 6 h. The phenomena appear initially to be analogous to confluent-holding and split-dose recovery as described for mammalian cells. However, the survival levels obtained for doses of 61-306 Gy after allowing for these recovery processes to occur are quite high and greatly exceed survival levels for all but relatively low doses less than 50 Gy. For example, while the survival of cells irradiated with 150 Gy is near 0.15, the survival of cells receiving 306 Gy in two equivalent split doses is approximately 0.77. Even if damage induced by the first of the split doses was completely repaired, it might be expected that the survival would be near the level of the second dose alone, or near 0.15. Instead the survival is approximately five times greater, suggesting that the first split dose stimulated a repair system not present in unirradiated cells. The situation for confluent-holding recovery is similar to that for split-dose recovery.     

Protection by WR-3689 against gamma-ray-induced intestinal damage: comparative effect on clonogenic cell survival, mouse survival, and DNA damage

The aminophosphorothioate WR-3689 was characterized for its ability to protect mouse jejunal cells in vivo from single doses of X or gamma radiation. First, the effect of the drug on the survival of jejunal stem cells was examined using a clonogenic end point, the crypt microcolony assay. When WR-3689 was administered 30 min prior to whole-body irradiation, the number of surviving crypt cells was markedly increased at all doses of the drug, although protection began to level out at doses larger than 600 mg/kg. Protection was maximal when the drug was given 30 min before whole-body irradiation and declined rapidly with both shorter and longer intervals. Protection factors (PFs) were obtained by measuring survival curves for clonogenic crypt cells as a function of radiation dose; WR-3689 given 30 min before whole-body irradiation protected jejunum in the microcolony assay with a PF of 1.26 +/- 0.02, 1.50 +/- 0.10, and 1.65 +/- 0.10 at doses of 200, 400, and 800 mg/kg, respectively. Next, the effect of WR-3689 on the survival of jejunal stem cells was determined by assaying the survival of mice given X-ray doses to the whole abdomen in the range leading to death from the gastrointestinal syndrome. The PFs based on the LD50 values for 11-day survival were 1.31 +/- 0.05 (200 mg/kg) and 1.48 +/- 0.05 (400 mg/kg). Crypt-cell survival and animal survival were thus modified to a similar extent by this agent. Finally, the effect of WR-3689 on the induction of DNA single-strand breaks (SSBs) in jejunal cells was measured using an adaptation of the alkaline elution methodology. In mice treated with WR-3689 (400 or 800 mg/kg) 30 min prior to whole-body irradiation with 10 Gy there was no significant reduction in the number of DNA SSBs induced either in samples of the jejunum or in the cycling crypt cells, providing further evidence that there is no simple relationship between the modification of DNA SSBs and the survival of jejunal stem cells.     

Split-dose exposure to N-methyl-N'-nitro-N-nitrosoguanidine in BALB/3T3 C1 a31-1-1 cells: evidence of DNA repair by alkaline elution without changes in cell survival, mutation and transformation rates

Dose fractionation of a direct-acting chemical carcinogen, the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), was studied for its concurrent effects on survival, DNA damage and repair, ouabain resistance (Ouar) mutations and neoplastic transformation, in the mouse embryo cell line BALB/3T3 C1A31-1-1. MNNG doses of 0.5, 1 and 2 micrograms/ml were added to the cells either as a single exposure or in two equal fractions separated by 1, 3 or 5 h intervals. No significant difference in cytotoxicity was found when single and split-dose treatments were compared. No recovery from sublethal damage was therefore found in this cell line by split-dose administration of MNNG, although such an effect was found when the same cell line was treated with single and split doses of X-rays. Repair of DNA damage as measured by alkaline elution was studied up to 24 h after a single MNNG exposure (0.5 micrograms/ml). DNA repair was rapid during the first 5 h after treatment and slow thereafter. DNA damage detected after split doses of MNNG at 1 and 5 h intervals was significantly lower than after a corresponding single dose. With both single and split doses, rejoining of single-strand breaks (ssb) was nearly complete after 24 h of repair time. Ouar mutation and neoplastic transformation frequencies were determined for single and split doses of MNNG with the second treatment being given during (1 h) or after (5 h) the period of rapid DNA repair. No significant differences in either effect were detected for dose splitting at any tested dose.     

DNA double-strand breaks in human lymphocytes after single irradiation by low doses of pulsed X-rays: non-linear dose-response relationship

Effects of ionizing radiation registered in cells after low dose irradiation are still poorly understood. A pulsed mode of irradiation is even more problematic in terms of predicting the radiation-induced response in cells. Thus, the aim of this paper was to study and analyze the effects of dose and frequency of pulsed X-rays on the frequency of radiation-induced DNA double-strand breaks and their repair kinetics in human peripheral blood lymphocytes in vitro. Analysis of radiation-induced gammaH2AX and 53BP1 repair foci was used to assess the DNA damage in these cells. The dose-response curve of radiation-induced foci of both proteins has shown deviations from linearity to a higher effect in the 12-32 mGy dose range and a lower effect at 72 mGy. The dose-response curve was linear at doses higher than 100 mGy. The number of radiation-induced gammaH2AX and 53BP1 foci depended on the frequency of X-ray pulses: the highest effect was registered at 13 pulses per second. Moreover, slower repair kinetics was observed for those foci induced by very low doses with a nonlinear dose-response relationship.     

Differences in the acute intestinal syndrome after partial and total abdominal irradiation in mice

The acute intestinal syndrome in mice was analysed after partial (PAI) and total abdominal irradiation (TAI). The LD50/15 was significantly higher after PAI (16.3 Gy) than after TAI (14.3 Gy). The dose-response curve for maximal weight loss also showed a shift of 1.8-2 Gy to higher doses after PAI compared with TAI. The X-ray survival curve for duodenal crypt cells was shifted by only 0.6 Gy for PAI and TAI. In order to assess the possible role of radiation-induced leucopenia and the influence of irradiating the spleen (shielded with PAI), lethality, weight loss and blood leucocyte counts were compared after PAI and TAI in splenectomized and non-splenectomized mice. No major difference in leucopenia was found between the different treatment groups, whereas the differences in lethality and weight loss between PAI and TAI remained the same. Shielding the spleen in the partial abdominal field therefore did not contribute to the difference in LD50/15. These findings imply that the increased LD50/15 after PAI compared with TAI was mainly due to shielding of a small part of the bowel (about 13 per cent of the abdominal area).     

Radiosensitization of GL261 glioma cells by tetraiodothyroacetic acid (tetrac)

We studied effects of tetrac (tetraiodothyroacetic acid) on survival of GL261, a murine brain tumor cell line, following single doses of 250 kVp x-rays and on repair of damage (sublethal and potentially lethal damage repair; SLDR, PLDR) in both exponential and plateau phase cells. Cells were exposed to 2 μM tetrac (1 h at 37oC) prior to x-irradiation. At varying times after irradiation, cells were re-plated in medium without tetrac. Two weeks later, colonies were counted and results analyzed using either the linear-quadratic (LQ) or single-hit, multitarget (SHMT) formalisms. Tetrac sensitized both exponential and plateau phase cells to x-irradiation, as shown by a decrease in the quasi-threshold dose (Dq), leading to an average tetrac enhancement factor (ratio of SF2 values) of 2.5. Tetrac reduced SLDR in exponential cells by a factor of 1.8. In plateau phase cells there was little expression of SLDR, but tetrac produced additional cell killing at 1-4 h after the first dose. For PLDR expression in exponential cells, tetrac inhibited PLDR by a factor of 1.9, and in plateau phase cells, tetrac decreased PLDR expression by a factor of 3.4. These data show that the decreased Dq value seen after single doses of x-rays with tetrac treatment is also accompanied by a significant decrease in recovery from sublethal and potentially lethal damage.     

Kernel estimates of dose response

A nonparametric method for analyzing quantal response data from an indirect bioassay experiment is proposed. Kernel estimates of the dose-response curve are used to develop approximate confidence intervals for (i) the optimal combination dose of a drug with therapeutic effects at low doses and toxic effects at high doses, and (ii) the lethal dose levels of a toxic chemical. This nonparametric procedure was implemented on real and simulated data. The confidence interval for problem (i) has high coverage probabilities when the dose-response curve is symmetric about the optima. However, the coverage probabilities are adversely affected by asymmetry about the optima and consequently are not reliable unless the sample sizes are large. The use of kernel estimators with higher-order kernels may alleviate this sensitivity to asymmetry. The confidence interval for problem (ii) has high coverage probabilities robust with respect to the shape or symmetry of the underlying dose-response curve.     

Cell survival and recovery processes in Chinese hamster AA8 cells and in two radiosensitive clones

Cell survival and recovery after gamma irradiation were investigated in a Chinese hamster ovary cell line (AA8) and in two radiosensitive clones (EM9 and NM2) derived from it. When analyzed by the multitarget and linear-quadratic equations, the dose-response curves for survival of both EM9 and NM2 cells, compared with AA8 cells, were characterized by a decreased magnitude of the shoulder or single-hit region (as reflected by Dq or alpha, respectively) but no difference in the terminal slope or double-hit region (as reflected by DO or beta, respectively). Recovery from sublethal damage (SLD) and potentially lethal damage (PLD) was measured in the three cell lines to examine the relationship between the shoulder width of the survival curve and the magnitude of cellular recovery. NM2 cells exhibited a reduced shoulder on their survival curve and a reduced capacity for SLD recovery, compared with AA8 cells, after equitoxic doses of radiation. EM9 cells, which also had a reduced shoulder on their survival curve, displayed the same rate and extent of recovery as AA8 cells for both SLD and PLD. PLD recovery, as assayed in fed plateau-phase NM2 cells by delayed plating, occurred with slower initial kinetics but to the same final extent as that in AA8 cells, resulting in modification of both the shoulder and the slope of the survival curve. However, PLD recovery, as assayed in log-phase NM2 cells by postirradiation treatment with hypertonic salt, was normal and affected predominantly the slope of the survival curve. These data demonstrate that although both SLD and PLD recovery play a role in determining cell survival, cell-survival curve parameters may not always be useful in predicting cellular recovery capacity.     

Radiosensitization by cisplatin of RIF1 tumour cells in vitro

The ability of cis-diamminedichloroplatinum (II) (c-DDP) to enhance radiation-induced cell killing was tested on oxic RIF1 tumour cells in monolayer culture. Marked radiosensitization of the survivors of a 1 h drug treatment was found with all c-DDP doses tested, with enhancement ratios increasing from 1.2 to 2.2 with increasing drug dose. Isobologram analyses showed that the interactions of c-DDP with X-rays were supra-additive. To test whether part of the enhancement was due to a selection of subpopulations, the diploid and tetraploid RIF1 cells, which normally coexist in culture, were separated by unit gravity velocity sedimentation, and by developing diploid and tetraploid clones. Both methods showed that there was little difference in either drug sensitivity or radiation sensitivity between diploid and tetraploid cells. DNA histograms obtained by flow cytometry showed little or no cycle progression during the 1 h drug treatment. These data indicate that the radiosensitization was not the result of the drug exposure leaving cells in a radiosensitive phase. The observed radiosensitization, therefore, appears to have resulted from a true drug/X-ray interaction.     

The influence of conditions affecting repair and fixation of potentially lethal damage on the induction of 6-thioguanine resistance after exposure of mammalian cells to X-rays

We have studied the influence of postirradiation conditions resulting in repair or fixation of X-ray-induced potentially lethal damage (PLD) on the induction of 6-thioguanine-resistant mutants in plateau phase Ehrlich ascites tumour cells. For repair of PLD cells were incubated under plateau-phase conditions for 6–8 hours after irradiation. For fixation of PLD we used either a 4-h treatment with 120 μM β-araA or a 50-min treatment in hypertonic medium (2.5 times the normal tonicity). These treatment are known to effectively reduce or eliminate the shoulder of the X-ray survival care. The mutants were allowed to form colonies in agar medium containing 1.5 μg/ml 6-thioguanine, after expression times of 6–12 days.We observed a decrease in the number of mutants induced (per 10⁵ cells) when the cells were allowed to repair PLD, as compared with that of cells processed immediately after irradiation, and an increase in their number after treatment either with β-araA or in hypertonic medium. The curves obtained for the induction of mutants as a function of the radiation dose were usually upward bending.After irradiation at low dose rate we obtained an exponential survival curve and a linear induction of mutants as a function of the dose.Based on these results we suggest that potentially lethal lesions resulting in the formation of the shoulder of the survival curve are not identical with those lesions responsible for the induction of mutants.     

Dose fractionation effects in low dose rate irradiation of jejunal crypt stem cells

Jejunal crypt survival after fractionated total body irradiation of C3H mice given at dose rates of 1.2 or 0.08 Gy/min was studied. The fractionation effect was more pronounced at the high dose rate than at the low dose rate. Analysis of the data according to the linear-quadratic survival curve model yielded an alpha/beta value at 1.2 Gy/min of 13.3 Gy and at 0.08 Gy/min of 96 Gy.     

Radiation-induced renal damage: the effects of hyperfractionation

The response of mouse kidneys to multifraction irradiation was assessed using three nondestructive functional end points. A series of schedules was investigated giving 1, 2, 4, 8, 16, 32, or 64 equal X-ray doses, using doses per fraction in the range of 0.9 to 16 Gy. The overall treatment time was kept constant at 3 weeks. Kidney function was assessed from 19 to 48 weeks after irradiation by measuring changes in isotope clearance, urine output, and hematocrit. The degree of anemia (assessed from the hematocrit measurements) is a newly developed assay which is an early indicator of the extent of renal damage after irradiation. All three assays yielded steep dose-effect curves from which the repair capacity of kidney could be estimated by comparing the isoeffective doses in different schedules. There was a marked influence of fractionation, with increasing dose being required to achieve the same level of damage for increasing fraction number, even between 32 and 64 fractions. The data are well fitted by a linear quadratic dose-response equation, and analysis of the data in this way yields low values (approximately 3.0 Gy) for the ratio alpha/beta. This would suggest that hyperfractionation , using extremely small X-ray doses per fraction, would spare kidneys relative to tumors and acutely responding tissues.     

Discrepancies between patterns of potentially lethal damage repair in the RIF-1 tumor system in vitro and in vivo

Repair of potentially lethal damage (PLD) was studied in the RIF-1 tumor system in several different growth states in vivo and in vitro. Exponentially growing, fed plateau, and unfed plateau cells in cell culture as well as small and large subcutaneous or intramuscular tumors were investigated. Large single doses of radiation followed by variable repair times as well as graded doses of radiation to generate survival curves immediately after irradiation or after full repair were investigated. All repair-promoting conditions studied in vitro (delayed subculture, exposure of cells to depleted growth medium after irradiation) increased surviving fraction after a single dose. The D0 of the cell survival curve was also increased by these procedures. No PLD repair was observed for any tumors irradiated in vivo and maintained in the animal for varying times prior to assay in vitro. The nearly 100% cell yield obtained when this tumor is prepared as a single-cell suspension for colony formation, the representative cell sample obtained, and the constant cell yield per gram as a function of time postirradiation suggest that this discrepancy is not an artifact of the assay system. The most logical explanation of these data and information on radiocurability of this neoplasm is that PLD repair, which is so frequently demonstrated in vitro, may not be a major factor in the radioresponse of this tumor when left in situ.     

The clonogenic response of bovine aortic endothelial cells in culture to radiation

The effect of ionizing radiation on the survival of bovine aortic endothelial (BAE) cells was determined by the in vitro colony formation method. The BAE cells were cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% calf serum, antibiotics, and growth factors obtained from the culture of mouse S-180 cells. The cultured BAE cells were positive to the staining of antibodies against human factor VIII and formed clones in plastic culture flasks with a plating efficiency of about 11%. The survival curve of the BAE cells following an exposure to a single dose of X rays was characterized by D0 = 101 rad, Dq = 65 rad, and an extrapolation number (n) of 1.9. These parameters were not modified by the absence of growth factors at the time of irradiation. The response of BAE cells to radiation was dose-rate dependent. The split-dose studies demonstrated that the BAE cells were able to repair sublethal radiation damage within 1 h after irradiation.     

Reirradiation at long time intervals in mouse kidney: a comparison between experimental results and the predictions of the F-type tissue model

The tolerance of a late-responding tissue to reirradiation after long time intervals has been analysed using the F-type tissue model. In this model the tissue is composed of identical cells, each of which is capable of extensive proliferation and of tissue-specific function. The model was adapted to calculate the response to two fractions of radiation given in a variable overall time. For two equal doses of radiation the repair of tissue damage after the first fraction could be detected theoretically by a change in the rate of cell depletion after retreatment and by an increase in the minimum cell number attained. For an 'experimental set-up', in which a constant first dose was followed by a range of retreatment doses in a variable overall time, the repair of tissue damage theoretically could be detected most sensitively by a shift of the dose-response curves to higher retreatment doses as the time interval between the two doses was increased. A prerequisite for a proper comparison of these dose-response curves was that the responses were evaluated at times after the first dose determined by the minimal latency times after high retreatment doses. From a comparison of these theoretical results with experimental findings for mouse kidneys it was concluded that no recovery of tissue function took place over a 6-month period. Instead it appeared that the kidneys had become more sensitive to irradiation over this period.     

Gamma-rays kill grasshopper primary spermatocytes in groups

Primary spermatocyte killing by gamma-rays was studied in the grasshopper Heteracris littoralis in which spermatogenic development occurs in cysts containing a maximum of 64 cells during the first meiotic division. Cell killing at this stage is not random and mainly involves the death of whole cysts. The dose-response curve for cell killing has complex kinetics with at least two components but lacks any shoulder at low doses, thus indicating no repair of the lethal damage. Cell loss is apparent from surviving cysts as early as 45 min post irradiation but loss of greater than 24 cells is incompatible with cyst survival. Loss of fewer than 24 cells also is not random since certain values for cell loss are frequently observed while other, interspersed values are not seen at all.