Survival of cells in mammalian tissues after low doses of irradiation: a short review

Expression of blood serum proteins and lymphocyte differentiation clusters after chronic occupational exposure to ionizing radiation

This study aimed to assess effects of chronic occupational exposure on immune status in Mayak workers chronically exposed to ionizing radiation (IR). The study cohort consists of 77 workers occupationally exposed to external gamma-rays at total dose from 0.5 to 3.0 Gy (14 individuals) and workers with combined exposure (external gamma-rays at total dose range 0.7-5.1 Gy and internal alpha-radiation from incorporated plutonium with a body burden of 0.3-16.4 kBq). The control group consists of 43 age- and sex-matched individuals who never were exposed to IR, never involved in any cleanup operations following radiation accidents and never resided at contaminated areas. Enzyme-linked immunoassay and flow cytometry were used to determine the relative concentration of lymphocytes and proteins. The concentrations of T-lymphocytes, interleukin-8 and immunoglobulins G were decreased in external gamma-exposed workers relative to control. Relative concentrations of NKT-lymphocytes, concentrations of transforming growth factor-β, interferon gamma, immunoglobulins A, immunoglobulins M and matrix proteinase-9 were higher in this group as compared with control. Relative concentrations of T-lymphocytes and concentration of interleukin-8 were decreased, while both the relative and absolute concentration of natural killers, concentration of immunoglobulins A and M and matrix proteinase-9 were increased in workers with combined exposure as compared to control. An inverse linear relation was revealed between absolute concentration of T-lymphocytes, relative and absolute concentration of T-helpers cells, concentration of interferon gamma and total absorbed dose from external gamma-rays in exposed workers. For workers with incorporated plutonium, there was an inverse linear relation of absolute concentration of T-helpers as well as direct linear relation of relative concentration of NKT-lymphocytes to total absorbed red bone marrow dose from internal alpha-radiation. In all, chronic occupational IR exposure of workers induced a depletion of immune cells in peripheral blood of the individuals involved.

Effect of total body X-ray irradiation on lymph node in Tibet minipig

The purpose of this study was to determine the time-dose-effect of total body X-ray irradiation on lymphocytes in lymph nodes and peripheral blood in Tibet minipigs. Forty-eight Tibet minipigs were assigned into 6 groups including 5 experimental groups with 9 and the control group with 3. The minipigs in experimental groups were subjected to a total body X-ray irradiation of 2, 5, 8, 11, and 14 Gy respectively. Lymph nodes and peripheral blood samples were collected at 6, 24, and 72 hours after X-ray exposure and received histological microscopy examination and apoptosis analysis. Histology observation showed that the number of lymphocytes decreased within the lymph nodes with the increase of radiation doses and exposure time. The observation of transmission electron microscopy (TEM) showed typical apoptotic cells below 11 Gy while at 14 Gy necrotic cells were dominant. The apoptotic rate of lymphocytes in the lymph nodes was positively correlated with radiation dose in the range of 2-11 Gy, and reached the maximal level (39.4 ± 2.8) at 24 hours after 11 Gy irradiation, followed by a decrease in the apoptotic rate, but still higher than that of the control group. The number of lymphocytes in the peripheral blood samples was decreased significantly by increasing of the radiation dose and exposure time. We conclude that early damage of lymphocytes by total body X-ray irradiation is dose and time dependent below 11 Gy and before 24 hours post irradiation, and that the dosage of irradiation less than 11 Gy induced apoptosis, whereas the dose at 14 Gy resulted in necrosis in lymphocytes of the lymph nodes.

A small-scale anatomic model for testicular radiation dosimetry for radionuclides localized in the human testes

The testis is a radiosensitive tissue. It contains a large number of lobules, which in turn are composed of convoluted seminiferous tubules. The epithelium inside each tubule consists of a complex mosaic of supporting cells and germ cells of different sizes and degrees of maturation. These cells are known to have diverse sensitivity to radiation, those with the highest sensitivity being the spermatogonia, which form part of the basal cell layer, and those with the lowest sensitivity being the mature sperm cells closest to the lumen of the tubule. For many years, the internal dosimetry community has discussed the need for improvements to bring about more detailed, cell-level testicular dosimetry. This paper presents a small-scale dosimetry model for calculation of S factors for several different source-target configurations within the testicular tissue.

METHODS

A model of the testis was designed in which the lobules were approximated by a cross-section of seminiferous tubules arranged in a hexagonal pattern, with interstitial tissue between them. The seminiferous tubules were divided into concentric layers representing spermatogenic development in the seminiferous epithelium. S factors were calculated for electrons, photons, α-particles, and for (18)F, (90)Y, (99m)Tc, (111)In, (125)I, (131)I, (177)Lu, and (211)At using Monte Carlo simulations.

RESULTS

For electrons with low energies the range was small, compared with the diameter of the seminiferous tubules, resulting in high energy deposition close to the source, whereas for higher electron energies more uniform energy deposition was seen, as expected. The same trend was seen for low-energy photons, whose mean free paths are small, compared with the diameter of the seminiferous tubules, resulting in high energy deposition close to the source, whereas for higher photon energies the location of the activity in the testis is less important.

CONCLUSION

The model presented in this paper is a simplification of the organized chaos that constitutes the structure of the actual testis. However, it provides a relevant, small-scale anatomic model to help us understand the significance of the heterogeneity of radioactivity in this important radiosensitive tissue.

Response kinetics of radiation-induced micronucleated reticulocytes in human bone marrow culture

The frequency of micronucleated reticulocytes (MN-RETs) in the bone marrow or peripheral blood is a sensitive indicator of cytogenetic damage. While the kinetics of MN-RET induction in rodent models following irradiation has been investigated and reported, information about MN-RET induction of human bone marrow after radiation exposure is sparse. In this report, we describe a human long-term bone marrow culture (LTBMC), established in three-dimensional (3D) bioreactors, which sustains long-term erythropoiesis. Using this system, we measured the kinetics of human bone marrow red blood cell (RBC) and reticulocyte (RET) production, as well as the kinetics of human MN-RET induction following radiation exposure up to 6Gy. Human bone marrow established in the 3D bioreactor demonstrated an average percentage of RBCs among total viable cells peaking at 21% on day 21. The average percentage of RETs among total viable cells reached a maximum of 11% on day 14, and remained above 5% by day 28, suggesting that terminal erythroid differentiation was still active. Time- and dose-dependent induction of MN-RET by gamma radiation was observed in the human 3D LTBMC, with peak values occurring at approximately 3 days following 1Gy irradiation. A trend towards delayed peak to 3-5 days post-radiation was observed with radiation doses ≥2Gy. Our data reveal valuable information on the kinetics of radiation-induced MN-RET of human bone marrow cultured in the 3D bioreactor, a synthetic bioculture system, and suggest that this model may serve as a promising tool for studying MN-RET formation in human bone marrow, thereby providing opportunities to study bone marrow genotoxicity testing, mitigating agent effects, and other conditions that are not ordinarily feasible to experimental manipulation in vivo.

Mutations to Ku reveal differences in human somatic cell lines

NHEJ (non-homologous end joining) is the predominant mechanism for repairing DNA double-stranded breaks in human cells. One essential NHEJ factor is the Ku heterodimer, which is composed of Ku70 and Ku86. Here we have generated heterozygous loss-of-function mutations for each of these genes in two different human somatic cell lines, HCT116 and NALM-6, using gene targeting. Previous work had suggested that phenotypic differences might exist between the genes and/or between the cell lines. By providing a side-by-each comparison of the four cell lines, we demonstrate that there are indeed subtle differences between loss-of-function mutations for Ku70 versus Ku86, which is accentuated by whether the mutations were derived in the HCT116 or NALM-6 genetic background. Overall, however, the phenotypes of the four lines are quite similar and they provide a compelling argument for the hypothesis that Ku loss-of-function mutations in human somatic cells result in demonstrable haploinsufficiencies. Collectively, these studies demonstrate the importance of proper biallelic expression of these genes for NHEJ and telomere maintenance and they provide insights into why these genes are uniquely essential for primates.

Rat testis as a radiobiological in vivo model for radionuclides

The radiobiological effect of intracellularly localised radionuclides emitting low energy electrons (Auger electrons) has received much attention. Most in vivo studies reported have been performed in the mouse testis. We have investigated the rat testis as an in vivo radiobiological model, with sperm-head survival, testis weight loss and also alteration in the blood plasma hormone levels of FSH and LH as radiobiological endpoints. Validation of the rat testis model was evaluated by using mean absorbed doses of up to 10 Gy from intratesticularly (i.t.) injected (111)In oxine or local X-ray irradiation. Biokinetics of the i.t. injected radionuclide was analysed by scintillation camera imaging and used in the absorbed dose estimation. By the analysis of the autoradiographs, the activity distribution was revealed. Cell fractionation showed (111)In to be mainly associated with the cell nuclei. External irradiations were monitored by thermoluminescence dosimeters. The sperm-head survival was the most sensitive radiobiological parameter correlated to the mean absorbed dose, with a D(37) of 2.3 Gy for (111)In oxine and 1.3 Gy for X rays. The levels of plasma pituitary gonadal hormones FSH and LH were elevated for absorbed doses >7.7 Gy. This investigation shows that the radiobiological model based on the rat testis has several advantages compared with the previously commonly used mouse testis model. The model is appropriate for further investigations of basic phenomena such as radiation geometry, intracellular kinetics and heterogeneity, crucial for an understanding of the biological effect of low-energy electrons.

Changes in epidermal radiosensitivity with time associated with increased colony numbers

Epidermal clonogenic cell survival and colony formation following irradiation were investigated and related to radiosensitivity. A rapid in vivo/in vitro assay was developed for the quantification of colonies arising from surviving clonogenic cells in pig epidermis after irradiation. Bromodeoxyuridine (BrdU)-labelled cells in full thickness epidermal sheets were visualized using standard immunohistochemistry. In unirradiated skin, approximately 900 BrdU-positive cells mm(-2) were counted. In a time sequence experiment, BrdU-positive cell numbers increased from an average of 900 cells mm(-2) to approximately 1400 cells mm(-2) after BrdU-labelling for 2-24 h. In irradiated skin, colonies containing >/=16 BrdU-positive cells were seen for the first time at days 14/15 after irradiation. The number of these colonies per cm(2) as a function of skin surface dose yielded a cell survival curve with a D(0)-value (+/-SE) of 3.9+/-0.6 Gy. This relatively high D(0)-value is possibly due to a rapid fall off in depth dose distribution for the iridium-192 source and consequently a substantial contribution of hair follicular epithelium to colony formation. At 14/15 days after irradiation, the ED(50) level of 33.6 Gy for the in vivo response of moist desquamation corresponded with 2.7 colonies cm(-2). Surprisingly, the number of colonies increased with time after irradiation with an estimated doubling time of approximately 4 days, while the D(0)-value remained virtually unchanged. This increase in colony numbers could be due to migration of clonogenic cells, to the recruitment of dormant clonogenic cell survivors by elevated levels of cytokines, or to both. Although frequent biopsying caused increased cytokine levels, which had a systemic effect on unirradiated skin, it had no influence on colony formation in irradiated skin. Smaller colonies, containing 4-8 cells or 9-15 cells, were abundant, particularly after higher doses, which resulted in higher D(0)-values. The majority of these small colonies were abortive and did not progress to larger colonies. There was no statistical evidence for significant variations in the interanimal responses.

Biological energy from the igneous rock enhances cell growth and enzyme activity

Some effects from natural resources might be ignored and unused by humans. Environmental hormesis could be a phenomena necessary to bio-organism existence on earth. Since 1919, radiation and some heavy metal hormesis from the environment were proved in various reports. In this study, igneous rock with very low radioactivity and high ferrous activity was measured by multichannel analyzer and inductively coupled plasma analyzer. The water treated by igneous rock, both directly soaked or indirectly in contact, induced increased activities of glucose oxidase, catalase, peroxidase, and superoxide dismutase. It also increased cell growth of SC-M1, HCT-15, Raji, and fibroblast cell lines. The water after treatment of igneous rock had no change in pH values, but displayed decreased conductivity values. We assume that the igneous rock could transfer energy to water to change the molecular structure or conformation of water cluster, or by radiation hormesis effect could then induce increased enzyme activity and cell growth. It is also possible that the energy from rock may combine radiation hormesis with other transferable biological energy forms to change water cluster conformation.

The effects, risks, and guidelines for radiation use in orthopaedic surgery

Radiation is used during orthopaedic surgery in more than 15 million studies performed yearly. The biologic effects of radiation have been shown to inhibit mitosis by producing irrepairable deoxyribonucleic acid double strand breaks or create structural changes by damaging the nucleus, thereby producing potential genetic transmissions. Although human cells are thought to be resistant to malignant change and no studies have shown toxic effects resulting from long-term exposure to low-dose radiation, risks still are assumed. To decrease all risks, radiographic units should undergo periodic calibration, surgeons should wear protective devices, increase their working distance from the x-ray beam, and limit their duration of radiation exposure by making certain that they follow the guidelines set forth by the National Council for Radiation Protection and Measurement.

Determination of colony numbers in pig epidermis as an estimate for radiosensitivity. A rapid assay based on in vitro BrdU-labelling

A rapid assay has been developed for the quantitation of colonies arising from surviving clonogenic cells in pig epidermis after irradiation. The number of surviving clonogenic cells per unit area was related to the epidermal in vivo response of moist desquamation. After irradiation with single doses, ranging from 20 to 36 Gy, skin biopsies were taken and incubated in dispase for enzymatic separation of the epidermis and dermis. Full thickness epidermal sheets were labelled with bromodeoxyuridine (BrdU) in vitro. Proliferating cells were visualized using standard immunohistochemical procedures. Cell groups containing > or = 16 cells were counted as colonies. These colonies were first seen on day 14/15 after irradiation. The number of colonies per cm2, as a function of skin surface dose, yielded a cell survival curve with a D0 (+/- SE) of 3.87 +/- 0.57 Gy. The ED50 for the epidermal in vivo reaction of moist desquamation corresponded with a colony density of 2.7 colonies per cm2. After higher doses, abundant smaller colonies of 4-8 BrdU-positive cells were seen and these were more radioresistant, as represented by higher D0 values.

Evidence for a G2 checkpoint in p53-independent apoptosis induction by X-irradiation

The p53 tumor suppressor gene is thought to be required for the induction of programmed cell death (apoptosis) initiated by DNA damage. We show here, however, that the human promyelocytic leukemia cell line HL-60, which is known to be deficient in p53 because of large deletions in the p53 gene, can be induced to undergo apoptosis following X-irradiation. We demonstrate that the decision to undergo apoptosis in this cell line appears to be made at a G2 checkpoint. In addition, we characterize an HL-60 variant, HCW-2, which is radioresistant. HCW-2 cells display DNA damage induction and repair capabilities identical to those of the parental HL-60 cell line. Thus, the difference between the two cell lines appears to be that X-irradiation induces apoptosis in HL-60, but not in HCW-2, cells. Paradoxically, HCW-2 cells display high levels of expression of bax, which enhances apoptosis, and no longer express bcl-2, which blocks apoptosis. HCW-2 cells' resistance to apoptosis may be due to the acquisition of expression of bcl-XL, a bcl-2-related inhibitor of apoptosis. In summary, apoptosis can be induced in X-irradiated HL-60 cells by a p53-independent mechanism at a G2 checkpoint, despite the presence of endogenous bcl-2. The resistance shown by HCW-2 cells suggests that bcl-XL can block this process.

Radioresponsiveness of human glioma, sarcoma, and breast cancer spheroids depends on tumor differentiation

PURPOSE

Differences in the intrinsic radiosensitivity within and between different tumor classes have been noticed for human tumor cell lines using the clonogenic assay. By far, most of the cell lines studied up to now were derived from poorly differentiated tumors. In this study, the influence of tumor differentiation on the radiation doses necessary to control 50% of small oxic spheroids (SCD50) was determined. Evidence of a distinct dependence of radioresponsiveness on tumor progression provides a background for an investigation of the underlying mechanisms.

METHODS AND MATERIALS

Spheroids were aggregated from 1000-1500 cells in agarose coated 24 multi-well plates. Their diameters ranged from 156 to 405 microns, depending on the cell line. Spheroids were irradiated with graded 60Co single doses using spheroid control as end point and a minimum follow-up period of 3 months.

RESULTS

Cell lines from three low grade gliomas and 10 malignant gliomas were studied in the spheroid control assay. The group mean SCD50 values were 6.1 +/- 1.6 Gy and 13.1 +/- 3.3 Gy, respectively. Four cell lines from grade 2 soft tissue sarcomas had a mean SCD50 value of 6.2 +/- 0.5 Gy and one undifferentiated sarcoma line of 11.0 Gy. Three well-differentiated breast cancer lines expressed the cell adhesion molecule E-cadherin, had an epithelioid morphology in monolayer culture, were estrogen receptor positive or contact inhibited in multicellular spheroids. Two undifferentiated breast cancer lines had a fibroblastoid morphology and were marker negative. The mean SCD50 value of the former was 10.5 +/- 1.0 Gy while that of the undifferentiated lines was 14.8 +/- 2.8 Gy. Analysis of variance revealed a significant effect of the tumor type as well as the grade of dedifferentiation on the SCD50 after irradiation with one fraction or 2Gy/fraction. The surviving fractions at 2 Gy (SF2), obtained from the spheroid control rates after different fractionation schedules by approximation of the linear quadratic model assuming Poisson statistics were significantly dependent on tumor type (p = 0.001, ANOVA F-test) but not on tumor differentiation (p = 0.27). The alpha/beta ratios did not depend on tumor type (p = 0.08, ANOVA F-Test) but significantly increased with the grade of tumor cell dedifferentiation (p = 0.03).

CONCLUSION

The spheroid model is suitable for measuring the radioresponsiveness of differentiated cell lines with very low colony forming efficiencies. Tumor cell differentiation is an important factor for the radioresponsiveness and recovery capacity of human tumor cells.

Mouse lymphoma cells that undergo interphase death show markedly increased sensitivity to radiation-induced DNA double-strand breakage as compared with cells that undergo mitotic death

The relationship between radiation-induced DNA double-strand breakage (dsb) and reproductive death (clonogenicity) for two mouse lymphoma cell lines was compared with that for the fibroblast-like hamster cell line V79. One of the lymphoma lines (STRij-4-2.2), which undergoes rapid disintegration following cytotoxic insult, showed extreme sensitivity to gamma-ray or DNA-associated 125I decay-induced DNA dsb (7 +/- 1 125I decays per clonogenic lethal event). Surprisingly, the other lymphoma line (WEHI-22.1), which does not undergo rapid disintegration, was also much more sensitive to DNA dsb than were V79 cells (17 +/- 1 versus 61 +/- 2 125I decays per clonogenic lethal event). Ultrastructure, DNA degradation, and flow cytometric cell cycle data suggested that both lymphoma cell lines may undergo interphase death, but that the induction of this process in WEHI-22.1 may depend upon blockage in the G2 phase. It is concluded that there are marked differences between the radiation responses of lymphoma and fibroblast lines, that there may be different forms of radiation-induced interphase death, and that the low number of DNA dsb required to produce a clonogenic lethal event in cells undergoing interphase death could explain the radiosensitivity of organs such as ovary, testis and thymus.

Biochemical and cellular mechanisms of low-dose effects

Low-dose irradiation is usually considered to be rather ineffective in producing biologically relevant effects. Yet, individual radiation absorption events within cell nuclei or whole cells interact stochastically with subcellular structures due to the multiple ionizations along primary or secondary particle tracks, depending on ionization density. Whereas radiation effects are usually seen in the context of structure and function of DNA, other cellular effects, perhaps influencing DNA by secondary biochemical mechanisms, also warrant attention. Thus, previous work from this laboratory with bone marrow that was obtained from whole-body exposed mice, has shown that single or few instantaneous radiation absorption events per cell from gamma-rays produce an acute and temporary partial inhibition of the enzyme thymidine kinase; the effect appears within about 1 h after the event, reaches its maximum at approximately 4 h and disappears completely within another 6 h. This pattern of enzyme inhibition is fully concordant with the pattern of inhibition of uptake of tritiated thymidine or 125I-labelled deoxyuridine into the DNA; also concordant is a temporary increase in the concentration of free thymidine in the blood serum of the exposed mice. The particular response of thymidine kinase was considered to relate to some, thus far unknown, repair systems and/or to a defence mechanism of the hit cells. In order to further elucidate the role of the acute and temporary partial inhibition of thymidine kinase in cellular metabolism, experiments were carried out in which mice were acutely exposed to 0.01 or 0.1 Gy and again exposed to the same dose at different times up to 12 h after the first exposure. At regular time intervals after the second exposure bone marrow cells were obtained and thymidine kinase activity was studied by various assays. The results indicate that the first acute irradiation conditioned the cells in such a way that the second acute irradiation produced either an enhanced inhibition and recovery of thymidine kinase activity, or no effect at all was seen, when the second irradiation was given between about 3 and 8 h after the first irradiation. From 8 to 12 h after the first irradiation the cells apparently resumed their original state, so that the second irradiation produced effects quite similar to those seen after a single irradiation in unconditioned cells.(ABSTRACT TRUNCATED AT 400 WORDS)