Potentially lethal damage repair in human cells

A review on lymphocyte radiosensitivity and its impact on radiotherapy

It is well known that radiation therapy causes lymphopenia in patients and that this is correlated with a negative outcome. The mechanism is not well understood because radiation can have both immunostimulatory and immunosuppressive effects. How tumor dose conformation, dose fractionation, and selective lymph node irradiation in radiation therapy does affect lymphopenia and immune response is an active area of research. In addition, understanding the impact of radiation on the immune system is important for the design and interpretation of clinical trials combining radiation with immune checkpoint inhibitors, both in terms of radiation dose and treatment schedules. Although only a few percent of the total lymphocyte population are circulating, it has been speculated that their increased radiosensitivity may contribute to, or even be the primary cause of, lymphopenia. This review summarizes published data on lymphocyte radiosensitivity based on human, small animal, and in vitro studies. The data indicate differences in radiosensitivity among lymphocyte subpopulations that affect their relative contribution and thus the dynamics of the immune response. In general, B cells appear to be more radiosensitive than T cells and NK cells appear to be the most resistant. However, the reported dose-response data suggest that in the context of lymphopenia in patients, aspects other than cell death must also be considered. Not only absolute lymphocyte counts, but also lymphocyte diversity and activity are likely to be affected by radiation. Taken together, the reviewed data suggest that it is unlikely that radiation-induced cell death in lymphocytes is the sole factor in radiation-induced lymphopenia.

Potential lethal damage repair in glioblastoma cells irradiated with ion beams of various types and levels of linear energy transfer

Glioblastoma (GBM), a Grade IV brain tumour, is a well-known radioresistant cancer. To investigate one of the causes of radioresistance, we studied the capacity for potential lethal damage repair (PLDR) of three altered strains of GBM: T98G, U87 and LN18, irradiated with various ions and various levels of linear energy transfer (LET). The GBM cells were exposed to 12C and 28Si ion beams with LETs of 55, 100 and 200 keV/μm, and with X-ray beams of 1.7 keV/μm. Mono-energetic 12C ions and 28Si ions were generated by the Heavy Ion Medical Accelerator at the National Institute of Radiological Science, Chiba, Japan. Clonogenic assays were used to determine cell inactivation. The ability of the cells to repair potential lethal damage was demonstrated by allowing one identical set of irradiated cells to repair for 24 h before subplating. The results show there is definite PLDR with X-rays, some evidence of PLDR at 55 keV/μm, and minimal PLDR at 100 keV/μm. There is no observable PLDR at 200 keV/μm. This is the first study, to the authors' knowledge, demonstrating the capability of GBM cells to repair potential lethal damage following charged ion irradiations. It is concluded that a GBM's PLDR is dependent on LET, dose and GBM strain; and the more radioresistant the cell strain, the greater the PLDR.

Hibernation for space travel: Impact on radioprotection

Hibernation is a state of reduced metabolic activity used by some animals to survive in harsh environmental conditions. The idea of exploiting hibernation for space exploration has been proposed many years ago, but in recent years it is becoming more realistic, thanks to the introduction of specific methods to induce hibernation-like conditions (synthetic torpor) in non-hibernating animals. In addition to the expected advantages in long-term exploratory-class missions in terms of resource consumptions, aging, and psychology, hibernation may provide protection from cosmic radiation damage to the crew. Data from over half century ago in animal models suggest indeed that radiation effects are reduced during hibernation. We will review the mechanisms of increased radioprotection in hibernation, and discuss possible impact on human space exploration.

Biologically effective dose in fractionated molecular radiotherapy--application to treatment of neuroblastoma with (131)I-mIBG

In this work, the biologically effective dose (BED) is investigated for fractionated molecular radiotherapy (MRT). A formula for the Lea-Catcheside G-factor is derived which takes the possibility of combinations of sub-lethal damage due to radiation from different administrations of activity into account. In contrast to the previous formula, the new G-factor has an explicit dependence on the time interval between administrations. The BED of tumour and liver is analysed in MRT of neuroblastoma with (131)I-mIBG, following a common two-administration protocol with a mass-based activity prescription. A BED analysis is also made for modified schedules, when due to local regulations there is a maximum permitted activity for each administration. Modifications include both the simplistic approach of delivering this maximum permitted activity in each of the two administrations, and also the introduction of additional administrations while maintaining the protocol-prescribed total activity. For the cases studied with additional (i.e. more than two) administrations, BED of tumour and liver decreases at most 12% and 29%, respectively. The decrease in BED of the tumour is however modest compared to the two-administration schedule using the maximum permitted activity, where the decrease compared to the original schedule is 47%.

Potentially lethal damage repair in drug arrested G2-phase cells after radiation exposure

Potentially lethal damage (PLD) repair has been defined as that property conferring the ability of cells to recover from DNA damage depending on the postirradiation environment. Using a novel cyclin dependent kinase 1 inhibitor RO-3306 to arrest cells in the G2 phase of the cell cycle, examined PLD repair in G2 in cultured Chinese hamster ovary (CHO) cells. Several CHO-derived DNA repair mutant cell lines were used in this study to elucidate the mechanism of DNA double-strand break repair and to examine PLD repair during the G2 phase of the cell cycle. While arrested in G2 phase, wild-type CHO cells displayed significant PLD repair and improved cell survival compared with cells released immediately from G2 after irradiation. Both the radiation-induced chromosomal aberrations and the delayed entry into mitosis were also reduced by G2-holding PLD recovery. The PLD repair observed in G2 was observed in nonhomologous end-joining (NHEJ) mutant cell lines but absent in homologous recombination mutant cell lines. From the survival curves, G2-NHEJ mutant cell lines were found to be very sensitive to gamma-ray exposure when compared to G2/homologous recombination mutant cell lines. Our findings suggest that after exposure to ionizing radiation during G2, NHEJ is responsible for the majority of non-PLD repair, and conversely, that the homologous recombination is responsible for PLD repair in G2.

Modeling the biological response of normal human cells, including repair processes, to fractionated carbon beam irradiation

To understand the biological response of normal cells to fractionated carbon beam irradiation, the effects of potentially lethal damage repair (PLDR) and sublethal damage repair (SLDR) were both taken into account in a linear-quadratic (LQ) model. The model was verified by the results of a fractionated cell survival experiment with normal human fibroblast cells. Cells were irradiated with 200-kV X-rays and monoenergetic carbon ion beams (290 MeV/u) at two irradiation depths, corresponding to linear energy transfers (LETs) of approximately 13 keV/μm and 75 keV/μm, respectively, at the Heavy Ion Medical Accelerator in Chiba of the National Institute of Radiological Sciences. When we only took into account the repair factor of PLDR, γ, which was derived from the delayed assay, the cell survival response to fractionated carbon ion irradiation was not fully explained in some cases. When both the effects of SLDR and PLDR were taken into account in the LQ model, the cell survival response was well reproduced. The model analysis suggested that PLDR occurs in any type of radiation. The γ factors ranged from 0.36-0.93. In addition, SLD was perfectly repaired during the fraction interval for the lower LET irradiations but remained at about 30% for the high-LET irradiation.

Evaluation of the gamma-H2AX assay for radiation biodosimetry in a swine model

There is a paucity of large animal models to study both the extent and the health risk of ionizing radiation exposure in humans. One promising candidate for such a model is the minipig. Here, we evaluate the minipig for its potential in γ-H2AX-based biodosimetry after exposure to ionizing radiation using both Cs137 and Co60 sources. γ-H2AX foci were enumerated in blood lymphocytes and normal fibroblasts of human and porcine origin after ex vivo γ-ray irradiation. DNA double-strand break repair kinetics in minipig blood lymphocytes and fibroblasts, based on the γ-H2AX assay, were similar to those observed in their human counterparts. To substantiate the similarity observed between the human and minipig we show that minipig fibroblast radiosensitivity was similar to that observed with human fibroblasts. Finally, a strong γ-H2AX induction was observed in blood lymphocytes following minipig total body irradiation. Significant responses were detected 3 days after 1.8 Gy and 1 week after 3.8 and 5 Gy with residual γ-H2AX foci proportional to the initial radiation doses. These findings show that the Gottingen minipig provides a useful in vivo model for validation of γ-H2AX biodosimetry for dose assessment in humans.

Potentially lethal damage repair and its inhibitory effect of caffeine in two yolk sac tumor cell lines with different radiosensitivities

PURPOSE

In order to investigate the role of potentially lethal damage repair (PLDR) in cellular radiosensitivity, PLDR and its inhibitory effect by caffeine was examined. In addition, cell cycle distribution was also examined.

MATERIALS AND METHODS

Two rat yolk sac tumor cell lines, NMT-1 and NMT-1R, with different radiosensitivities in vitro were used. The capacity for PLDR was examined using confluent-phase cells, and evaluated by calculating the recovery ratio. Inhibitory effect of caffeine on PLDR was examined with doses of 1, 5 and 10 mM.

RESULTS

The capacity of PLDR in two cell lines reflected radiosensitivity. The recovery ratio after irradiation of 5 Gy was 2.8 in the radiosensitive NMT-1 and 5.2 in the radioresistant NMT-1R, and recovery reached its peak level at 6 h in both cell lines. The degree of inhibition of PLDR was weaker in NMT-1R than that in NMT-1 at the same dose level, and was correlated with reduction of G2-arrested cells by caffeine.

CONCLUSIONS

The results of this study suggest that the capacity of PLDR may be one of the determinant factors for radiosensitivity in the two cell lines used, and the inhibitory effect of caffeine on PLDR was in part attributable to the modification of the cell cycle progression.

A review of human cell radiosensitivity in vitro

The survival curves of 694 human cell lines irradiated in exponentially growing phase in vitro were collected from the literature. Among them, 271 were derived from tumors, 423 were nontransformed fibroblasts and other normal cell strains from healthy people or people with some genetic disorders. Seventy-six different cell types are identified, and a specific radiosensitivity could be associated with each, using D and surviving fraction at 2 Gy. Technical factors such as culture medium, feeder cells, and scoring method were found to affect intrinsic radiosensitivity. In particular, the cell type is not a discriminating factor when cells are studied in agar. Results obtained with cells irradiated in agar must be used cautiously, depending on how the cells were prepared for the experiments. The use of feeder cells narrows the range of radiosensitivity of human cells. For cells irradiated as monolayer, it was possible to build a scale of radiosensitivity according to cell type, ranging, in terms of D from 0.6 Gy for the most sensitive cell lines to more than 4 Gy for the most resistant. Considering that, in most cases, we could estimate the variation of radiosensitivity within each cell type, our classification among cell types can be used by researchers to place their results in the context of the literature.

Comparison between in vitro radiosensitivity and in vivo radioresponse in murine tumor cell lines. II: In vivo radioresponse following fractionated treatment and in vitro/in vivo correlations

Survival in the low-dose region of in vitro radiation survival curves for human tumor cell lines may be correlated with the expected clinical radiocurability of tumors of similar histopathological type. The present investigation examined this hypothesis using a series of transplantable murine solid tumors. The in vivo radioresponse of the tumors following fractionated dose (10 fractions of 2 Gy given at 4 hr intervals) and single dose (20 Gy) radiation treatment was measured by growth delay and tumor cell survival assays. The measurements of tumor cell survival were initiated either immediately or 8 hr after the end of radiation treatment. There was no evidence for repair of potentially lethal damage in vivo in any of the tumors. The measurements of in vivo response were compared to parameters of in vitro radiation survival curves for the same tumor cell lines, which were measured concurrently. The tumor cell survival following 10 fractions of 2 Gy correlated best with the measured in vitro survival at 2 Gy, although good correlations were also observed with two parameters calculated from the in vitro data; the value of alpha from fitting the linear-quadratic (LQ) model and the mean inactivation dose (MID). Correlations were also observed between specific growth delay measured in vivo following 10 fractions of 2 Gy and these in vitro parameters. Despite the correlations observed, the measured survival values following 10 fractions of 2 Gy in vivo did not agree quantitatively with the theoretical values predicted from the in vitro survival data assuming equal effect for each fraction. This discrepancy indicates that other factors also contribute to the overall response of a tumor to fractionated irradiation. Nevertheless, these findings support the idea that intrinsic radiosensitivity plays a significant role in determining the overall response of a tumor to fractionated irradiation and provides strong support for the concept of testing the in vitro radiation sensitivity of biopsy specimens as a predictive assay of clinical radiocurability.

Comparison between in vitro radiosensitivity and in vivo radioresponse of murine tumor cell lines. I: Parameters of in vitro radiosensitivity and endogenous cellular glutathione levels

Recent studies have suggested that differences in the initial low-dose region of the radiation survival curves for human tumor cells might explain the differences in clinical response of tumors to fractionated radiation treatment. In this study, which is described in two companion papers, we investigated this hypothesis directly using animal model systems. In the present paper we determined in vitro radiation survival curves for eight murine tumor cell lines of varying histopathological type and: (a) measured survival at the 2 Gy and 8 Gy dose levels, (b) fitted parameters to the linear quadratic and two component multi-target equation models of cellular survival and (c) calculated mean inactivation doses. We found that the choice of the data fitting procedure affected the absolute value, relative ranking, and power to discriminate between the cell lines of these calculated parameters. A detailed statistical study indicated that the measured surviving fraction at 2 Gy (SF2) was the best discriminant of intrinsic radiosensitivity between the eight tumor cell lines. When these same cell lines were assayed for intracellular glutathione (GSH) levels, no correlation was found between levels of GSH and the SF2 value. Determining the SF2 value may be the method of choice to describe the low-dose region of the radiation survival curve, as it precludes the necessity of choosing a model to fit the survival data, it has excellent discriminatory powers, and it represents the survival in the radiotherapeutically relevant region of the in vitro radiation survival curve. Furthermore, as demonstrated in the companion paper, it correlates with cell survival in the tumors following 10 fractions of 2 Gy given in vivo.

High intrinsic radiosensitivity of a newly established and characterised human embryonal rhabdomyosarcoma cell line

A new human rhabdomyosarcoma cell line (HX170c) has been established from a paratesticular embryonal tumour in a 5-year-old male. The cells grew as an adherent monolayer with a doubling time of 32 h and showed pleomorphic features. Intermediate filament analysis revealed the line to be mesenchymal in origin (reactivity to vimentin and desmin antibodies). The line was tumorigenic in nude mice, possessed elevated levels of creatine phosphokinase (mainly of the MM isoenzyme form) and had a near diploid mean chromosome number of 50. In vitro cell cloning determinations gave colony forming efficiencies of 0.01% in soft agar and 24% in a monolayer anchorage-dependent assay. Radiosensitivity determinations using a monolayer clonogenic assay with feeder layer support showed the cells to be among the more radiosensitive human tumour cell types (surviving fraction at 2 Gy of 0.26) that have been investigated. Furthermore, experiments utilising continuous low dose rate radiation at 3.2 cGy min-1, showed that, under these experimental conditions, the cells possessed only a very low capacity to recover from radiation-induced damage (dose reduction factor at 1% cell survival of 1.07 for 150 versus 3.2 cGy min-1). As other human tumour cells of an embryonal cell origin (e.g. neuroblastoma and germ cell tumours of the testis) have also been shown to be radiosensitive it appears that sensitivity to radiation may be a common property of this group of tumours.

Inhibition of recovery from damage induced by ionizing radiation in mammalian cells

Recent studies indicate the importance for radiotherapy of the inherent radiosensitivity of tumour cells in the low-dose region; a region where recovery is probably almost complete. Improvements in radiotherapy may therefore depend on the search for specific inhibitors of cellular recovery. This review summarizes data from studies in which use was made of a variety of mammalian, including human cell systems, where attempts have been made to inhibit recovery using chemical agents. Inhibition of sublethal damage repair, potentially lethal damage repair and low dose-rate sparing has been observed to varying extents by several agents including those thought to act by interfering with DNA repair processes (e.g. ara-C, 3-aminobenzamide) differentiation-inducing agents (e.g. N-methylformamide), halogenated pyrimidines (e.g. iododeoxyuridine), caffeine and chemotherapeutic agents (e.g. adriamycin). No individual agent stands out as exerting an exceptionally dramatic effect on recovery. However, of the agents used clinically, cis-platinum appears to hold some promise, while iododeoxyuridine, N-methylformamide, beta ara-A and caffeine all appear to inhibit to some extent the recovery of ionizing radiation-induced damage. There is an urgent need for further study to determine, in particular, relative effects in tumour versus normal cell types and whether any agents found to be effective in vitro show similar effects in vivo.