Similarities in the repair kinetics of sublethal and potentially lethal X-ray damage in log phase Chinese hamster V79 cells

Investigation of dose-rate effects and cell-cycle distribution under protracted exposure to ionizing radiation for various dose-rates

During exposure to ionizing radiation, sub-lethal damage repair (SLDR) competes with DNA damage induction in cultured cells. By virtue of SLDR, cell survival increases with decrease of dose-rate, so-called dose-rate effects (DREs). Here, we focused on a wide dose-rate range and investigated the change of cell-cycle distribution during X-ray protracted exposure and dose-response curves via hybrid analysis with a combination of in vitro experiments and mathematical modelling. In the course of flow-cytometric cell-cycle analysis and clonogenic assays, we found the following responses in CHO-K1 cells: (1) The fraction of cells in S phase gradually increases during 6 h exposure at 3.0 Gy/h, which leads to radio-resistance. (2) Slight cell accumulation in S and G2/M phases is observed after exposure at 6.0 Gy/h for more than 10 hours. This suggests that an increase of SLDR rate for cells in S phase during irradiation may be a reproducible factor to describe changes in the dose-response curve at dose-rates of 3.0 and 6.0 Gy/h. By re-evaluating cell survival for various dose-rates of 0.186-60.0 Gy/h considering experimental-based DNA content and SLDR, it is suggested that the change of S phase fraction during irradiation modulates the dose-response curve and is possibly responsible for some inverse DREs.

Investigation of dose-rate effects and cell-cycle distribution under protracted exposure to ionizing radiation for various dose-rates

During exposure to ionizing radiation, sub-lethal damage repair (SLDR) competes with DNA damage induction in cultured cells. By virtue of SLDR, cell survival increases with decrease of dose-rate, so-called dose-rate effects (DREs). Here, we focused on a wide dose-rate range and investigated the change of cell-cycle distribution during X-ray protracted exposure and dose-response curves via hybrid analysis with a combination of in vitro experiments and mathematical modelling. In the course of flow-cytometric cell-cycle analysis and clonogenic assays, we found the following responses in CHO-K1 cells: (1) The fraction of cells in S phase gradually increases during 6 h exposure at 3.0 Gy/h, which leads to radio-resistance. (2) Slight cell accumulation in S and G₂/M phases is observed after exposure at 6.0 Gy/h for more than 10 hours. This suggests that an increase of SLDR rate for cells in S phase during irradiation may be a reproducible factor to describe changes in the dose-response curve at dose-rates of 3.0 and 6.0 Gy/h. By re-evaluating cell survival for various dose-rates of 0.186-60.0 Gy/h considering experimental-based DNA content and SLDR, it is suggested that the change of S phase fraction during irradiation modulates the dose-response curve and is possibly responsible for some inverse DREs.

[Repeated radiation dose effect and DNA repair: Importance of the individual factor and the time interval between the doses]

The dose fractionation effect is a recurrent question of radiation biology research that remains unsolved since no model predicts the clinical effect only with the cumulated dose and the radiobiology of irradiated tissues. Such an important question is differentially answered in radioprotection, radiotherapy, radiology or epidemiology. A better understanding of the molecular response to radiation makes possible today a novel approach to identify the parameters that condition the fractionation effect. Particularly, the time between doses appears to be a key factor since it will permit, or not, the repair of certain radiation-induced DNA damages whose repair rates are of the order of seconds, minutes or hours: the fractionation effect will therefore vary according to the functionality of the different repair pathways, whatever for tumor or normal tissues.

The kinetics of cellular recovery in exponential and plateau growth phase human glioma cells following gamma-irradiation

PURPOSE

In this study the kinetics of recovery following irradiation was examined in a human glioma cell line. Specific objectives were: to determine whether recovery is mono- or biexponential in nature; to determine if recovery half-times are different in exponential and plateau growth phase cells; to compare recovery half-times as a function of dose or recovery levels; and finally, to compare the kinetics of sublethal damage recovery and potentially lethal damage recovery in plateau growth phase cells.

METHODS AND MATERIALS

U-87MG cells were irradiated in exponential and plateau growth phases and then subjected to incubation at 37 degrees C for various periods of time following or between doses prior to assaying for survival. Survival recovery curves were fit to a sum of exponential terms.

RESULTS

Potentially lethal damage recovery was monoexponential in both exponential and plateau growth phase cells and occurred at the same rate when isorecovery values were compared. Recovery half-times increased in an exponential manner within the observed dose range. Recovery between doses of radiation (sublethal damage recovery) proceeded at a slower rate than recovery following a single dose of radiation (potentially lethal damage recovery).

CONCLUSIONS

This study suggests that potentially lethal damage recovery is a saturated process and that the recovery half-time may increase in a linear-quadratic exponential function of dose similar to the absolute recovery level. In addition, if iso-recovery levels are compared, the recovery half-time is similar in rapidly and slowly proliferating cell populations.

Cell recovery kinetics for split-dose, multifractionated and continuous irradiation in the DSB model

The recovery kinetics for split-dose, multifractionated and continuous irradiations were considered in the framework of the DSB model (Ostashevsky 1989) with the assumption of a cooperative type of DSB repair. Two types of in vitro split-dose experiments can be distinguished on the basis of the time of cell plating: (i) IP, where cells are plated immediately after the second dose; and (ii) DP, where cells are incubated for a long period after the second dose before plating. The model predicts that the IP split-dose recovery kinetics depend mainly on the first dose and are not described by a single exponential curve. The rate of these recovery kinetics is faster than that of DSB repair. In contrast, the DP split-dose recovery kinetics are dose-independent and described by a single exponential curve, the time constant of which coincides with that for DSB repair. The equation for cell survival after multifractionated and continuous irradiations, derived in this work, are different from those in other models (e.g. incomplete repair (IR)) and represent an alternative which is worth testing. The values of the repair time constant estimated from these equations are at least 1.5-2-fold longer than those estimated from the IR equations applied to the same experimental data.

Trypsin-induced changes in cell shape and chromatin structure result in radiosensitization of monolayer Chinese hamster V79 cells

Trypsin is the enzyme commonly used to prepare single cell suspensions from monolayer and spheroid cultures, both to determine survival and to assay DNA damage. Trypsin induces rounding, dissociation and radiosensitization of anchorage-dependent cells. Radiosensitivity and chromatin structure were compared between trypsin-treated (0.05%) round V79 cells from monolayers and spheroids vs. untreated spread monolayer cells in situ. The fluorescent halo technique was used to measure the changes in DNA supercoiling in nucleoids isolated from control and irradiated round and spread cells. Maximal halo diameters, the amount of initial and residual radiation-induced DNA damage (estimated from nucleoid halo diameter changes), and the radiosensitivity were higher in round cells than in spread monolayer V79 cells. The effects on cellular radiosensitivity and maximal halo diameter of other agents which also round and dissociate cells, e.g. 0.25% trypsin, pronase E and a non-enzymatic cell-dissociation solution, were similar to those of 0.05% trypsin. In LY-S cells, which are anchorage-independent, DNA loop size, the initial amount of DNA damage and radiosensitivity were not affected by trypsin. We suggest that the higher radiosensitivity of anchorage-dependent cells under immediate trypsinization and plating conditions, compared to cells with postirradiation in situ repair incubation, is due to correlated changes in cell shape and chromatin structure.

A linear-quadratic model of cell survival considering both sublethal and potentially lethal radiation damage

We assessed the dose-dependence of repair of potentially lethal damage in Chinese hamster ovary cells x-irradiated in vitro. The recovery ratio (RR) by which survival (SF) of the irradiated cells was enhanced increased exponentially with a linear and a quadratic component, namely xi and psi: RR = e xi D + psi D2. Survival of irradiated cells can thus be expressed by a combined linear-quadratic model considering four variable, namely alpha and beta for the capacity of the cells to accumulate sublethal damage, and xi and psi for their capacity to repair potentially lethal damage: SF = e(xi - alpha)D + (psi - beta)D2.

Does melanin affect the low LET radiation response of Cloudman S91 mouse melanoma cell lines?

Melanin contains melanin-free radicals and can both absorb and produce additional free radicals and active oxygen species on exposure to various stimuli. Yet its role in the radiation responses of malignant melanoma has been little studied. In this report, three subclones of Cloudman S91 mouse melanoma clone PC1A varying in constitutive melanin content were compared with respect to killing by gamma irradiation. Radiation responses correlated with melanin content. The least melanotic line, S91/amel, was most sensitive and the most melanotic line, S91/I3, was most resistant. Curve fitting using the linear-quadratic model suggests that S91/amel is killed only by single event inactivations; S91/I3, only by double event inactivations; and S91/M1B, with intermediate melanin and radiation response, by both types of inactivations. Split dose experiments confirmed a lack of immediate split dose recovery in S91/amel and its existence in S91/I3. Potentially lethal damage and its repair could be demonstrated in both S91/amel and S91/I3. Double strand break (DSB) induction was evaluated as a function of gamma ray dose in DNA of S91/I3 and S91/amel, as well as in EMT6, a mouse mammary cancer line that lacks tyrosinase and melanin. The rates of induction were proportional to cellular melanization, i.e., the rate of DSB induction was greatest in S91/I3, least in EMT6. Levels of thioredoxin reductase (TR), glutathione reductase (GR), superoxide dismutase (SOD), and catalase (CAT) were determined in S91/amel and S91/I3. TR was the same in both cell lines, while the other three enzymes were 3- to 4-fold lower in S91/amel.(ABSTRACT TRUNCATED AT 250 WORDS)