Adaptive response to ionizing radiation-induced chromosome aberrations in rabbit lymphocytes: effect of pre-exposure to zinc, and copper salts

The protective effects of trace elements against side effects induced by ionizing radiation

Trace elements play crucial role in the maintenance of genome stability in the cells. Many endogenous defense enzymes are containing trace elements such as superoxide dismutase and metalloproteins. These enzymes are contributing in the detoxification of reactive oxidative species (ROS) induced by ionizing radiation in the cells. Zinc, copper, manganese, and selenium are main trace elements that have protective roles against radiation-induced DNA damages. Trace elements in the free salt forms have protective effect against cell toxicity induced by oxidative stress, metal-complex are more active in the attenuation of ROS particularly through superoxide dismutase mimetic activity. Manganese-complexes in protection of normal cell against radiation without any protective effect on cancer cells are more interesting compounds in this topic. The aim of this paper to review the role of trace elements in protection cells against genotoxicity and side effects induced by ionizing radiation.

Role of DNA methylation in the adaptive responses induced in a human B lymphoblast cell line by long-term low-dose exposures to γ-rays and cadmium

The possible involvement of epigenetic factors in health risks due to exposures to environmental toxicants and ionizing radiation is poorly understood. We have tested the hypothesis that DNA methylation contributes to the adaptive response (AR) to ionizing radiation or Cd. Human B lymphoblast cells HMy2.CIR were irradiated (0.032 Gy γ-rays) three times per week for 4 weeks or exposed to CdCl2 (0.005, 0.01, or 0.1 μM) for 3 months, and then challenged with a high dose of Cd (50 or 100 μM) or γ-rays (2 Gy). Long-term low-dose radiation (LDR) or long-term low-dose Cd exposure induced AR against challenging doses of Cd and irradiation, respectively. When the primed cells were treated with 5-aza-2'-deoxycytidine (5-aza-dC), a DNA methyltransferase inhibitor, the ARs were eliminated. These results indicate that DNA methylation is involved in the induction of AR in HMy2.CIR cells.

Radiation-induced bystander effects in the Atlantic salmon (salmo salar L.) following mixed exposure to copper and aluminum combined with low-dose gamma radiation

Very little is known about the combined effects of low doses of heavy metals and radiation. However, such "multiple stressor" exposure is the reality in the environment. In the work reported in this paper, fish were exposed to cobalt 60 gamma irradiation with or without copper or aluminum in the water. Doses of radiation ranged from 4 to 75 mGy delivered over 48 or 6 h. Copper doses ranged from 10 to 80 μg/L for the same time period. The aluminum dose was 250 μg/L. Gills and skin were removed from the fish after exposure and explanted in tissue culture flasks for investigation of bystander effects of the exposures using a stress signal reporter assay, which has been demonstrated to be a sensitive indicator of homeostatic perturbations in cells. The results show complex synergistic interactions of radiation and copper. Gills on the whole produce more toxic bystander signals than skin, but the additivity scores show highly variable results which depend on dose and time of exposure. The impacts of low doses of copper and low doses of radiation are greater than additive, medium levels of copper alone have a similar level of effect of bystander signal toxicity to the low dose. The addition of radiation stress, however, produces clear protective effects in the reporters treated with skin-derived medium. Gill-derived medium from the same fish did not show protective effects. Radiation exposure in the presence of 80 μg/L led to highly variable results, which due to animal variation were not significantly different from the effect of copper alone. The results are stressor type, stressor concentration and time dependent. Clearly co-exposure to radiation and heavy metals does not always lead to simple additive effects.

Different responses of tumor and normal cells to low-dose radiation

AIM OF THE STUDY

We demonstrated stimulation of both erythrocyte immune function and superoxide dismutase activity in tumor-bearing mice in response to whole-body 75 mGy X-rays. In addition, we enhanced the chemotherapeutic effect by exposing tumor-bearing mice to low-dose radiation (LDR). This study aims to investigate the different responses of tumor cells and normal cells to LDR.

MATERIAL AND METHODS

Survival fraction, micronucleus frequency, and cell cycle of Lewis cells and primary human fibroblast AG01522 cells were measured. S180 sarcoma cells were implanted in mice, and tumor sizes were measured in vivo.

RESULTS

In response to LDR exposure in vitro, a stimulating effect was observed in AG01522 cells but not in Lewis cells. Low-dose radiation did not cause an adaptive response in the Lewis cell cycle. Lack of an LDR-induced radioadaptive response in tumor cells was observed in tumor-bearing mouse models. Furthermore, a higher apoptotic effect and lower expression of the anti-apoptosis gene Bcl-2 were found in tumor cells of tumor-bearing mice exposed to D1 + D2 than those in tumor cells of tumor-bearing mice exposed to D2 alone.

CONCLUSIONS

Different responses of tumor cells and normal cells to LDR were found. Low-dose radiation was found to stimulate the growth of normal cells but not of tumor cells in vitro and in vivo, which is a very important and clinically relevant phenomenon.

Zinc homeostasis in the metabolic syndrome and diabetes

Zinc (Zn) is an essential mineral that is required for various cellular functions. Zn dyshomeostasis always is related to certain disorders such as metabolic syndrome, diabetes and diabetic complications. The associations of Zn with metabolic syndrome, diabetes and diabetic complications, thus, stem from the multiple roles of Zn: (1) a constructive component of many important enzymes or proteins, (2) a requirement for insulin storage and secretion, (3) a direct or indirect antioxidant action, and (4) an insulin-like action. However, whether there is a clear cause-and-effect relationship of Zn with metabolic syndrome, diabetes, or diabetic complications remains unclear. In fact, it is known that Zn deficiency is a common phenomenon in diabetic patients. Chronic low intake of Zn was associated with the increased risk of diabetes and diabetes also impairs Zn metabolism. Theoretically Zn supplementation should prevent the metabolic syndrome, diabetes, and diabetic complications; however, limited available data are not always supportive of the above notion. Therefore, this review has tried to summarize these pieces of available information, possible mechanisms by which Zn prevents the metabolic syndrome, diabetes, and diabetic complications. In the final part, what are the current issues for Zn supplementation were also discussed.

The new radiobiology: returning to our roots

In 2005, two expert advisory bodies examined the evidence on the effects of low doses of ionizing radiation. The U.S. National Research Council concluded that current scientific evidence is consistent with the linear no-threshold dose-response relationship (NRCNA 2005) while the French National Academies of Science and Medicine concluded the opposite (Aurengo et al. 2005). These contradictory conclusions may stem in part from an emphasis on epidemiological data (a "top down" approach) versus an emphasis on biological mechanisms (a "bottom up" approach). In this paper, the strengths and limitations of the top down and bottom up approaches are discussed, and proposals for strengthening and reconciling them are suggested. The past seven years since these two reports were published have yielded increasing evidence of nonlinear responses of biological systems to low radiation doses delivered at low dose-rates. This growing body of evidence is casting ever more doubt on the extrapolation of risks observed at high doses and dose-rates to estimate risks associated with typical environmental and occupational exposures. This paper compares current evidence on low dose, low dose-rate effects against objective criteria of causation. Finally, some questions for a post-LNT world are posed.

Adaptive response in frogs chronically exposed to low doses of ionizing radiation in the environment

Using the micronucleus assay, decreased levels of DNA damage were found after high dose ionizing radiation exposure of liver cells taken from frogs inhabiting a natural environment with above-background levels of ionizing radiation, compared to cells taken from frogs inhabiting background areas. The data obtained from a small number of animals suggest that stress present in the above-background environment could induce an adaptive response to ionizing radiation. This study did not reveal harmful effects of exposure to low levels of radioactivity. On the contrary, stress present in the above-background area may serve to enhance cellular defense mechanisms.

Low-dose radiation and its clinical implications: diabetes

Induction of hormesis and adaptive response by low-dose radiation (LDR) has been extensively indicated. Adaptive response induced by LDR was not only resistant to damage caused by a subsequently high-dose radiation, but also cross-resistant to other non-radiation challenges, such as chemicals. Mechanisms by which LDR induces the preventive effect on radiation- or chemical-induced tissue damage include induced or up-regulated expression of protective proteins, such as heat shock proteins and antioxidants. Since oxidative damage to tissues is a major pathogenesis of many human diseases including diabetes, this review will summarize the available data with an emphasis of the preventive effect of LDR on the development of diabetes and the therapeutic effect of LDR on diabetic cardiovascular complications. The available data indicated that pre-exposure of mice to LDR reduced the incidence of alloxan-induced diabetes, and also delayed the onset of hyperglycaemia in diabetes-prone non-obese diabetic mice. Experiments with animals indicated the effectively therapeutic effect of low-intensity or power laser (LIL or LPL) radiation on skin wound healing, which has stimulated clinical use of LIL to cure skin ulcer in diabetic patients. Mechanisms by which LDR prevents diabetes, though are unclear now, may include the induction of pancreatic antioxidants to prevent beta cell from oxidative damage and immunomodulation to preserve pancreatic function. For LIL therapeutic effect on diabetic wound healing, mechanisms may include its antioxidant action, immunomodulation, cell proliferation stimulation as well as improvement of systemic and wound-regional microcirculation. Therefore, although only a few studies indicating LDR prevention of the development of diabetes, many studies have demonstrated LDR, specifically LIL, therapeutic effectiveness of diabetic wound healing. These preliminary results are really encouraging for us to further pursue the clinical implication of LDT to diabetes-related areas.

Cardiac metallothionein induction plays the major role in the prevention of diabetic cardiomyopathy by zinc supplementation

BACKGROUND

Our previous studies showed that transgenic mice that overexpress cardiac-specific metallothionein (MT) are highly resistant to diabetes-induced cardiomyopathy. Zinc is the major metal that binds to MT under physiological conditions and is a potent inducer of MT. The present study therefore explored whether zinc supplementation can protect against diabetic cardiomyopathy through cardiac MT induction.

METHODS AND RESULTS

Diabetes was induced in mice (C57BL/6J strain) by a single injection of streptozotocin. Half were supplemented intraperitoneally with zinc sulfate (5 mg/kg) every other day for 3 months. After zinc supplementation, mice were maintained for 3 more months and then examined for cardiomyopathy by functional and morphological analysis. Significant increases in cardiac morphological impairment, fibrosis, and dysfunction were observed in diabetic mice but not in diabetic mice supplemented with zinc. Zinc supplementation also induced a significant increase in cardiac MT expression. The role of MT in cardiac protection by zinc supplementation was examined in cultured cardiac cells that were directly exposed to high levels of glucose (HG) and free fatty acid (FFA) (palmitate), treatment that mimics diabetic conditions. Cell survival rate was significantly decreased for cells exposed to HG/FFA but did not change for cells exposed to HG/FFA and pretreated with zinc or low-dose cadmium, each of which induces significant MT synthesis. When MT expression was silenced with the use of MT small-interfering RNA, the preventive effect of pretreatment with zinc or low-dose cadmium was abolished.

CONCLUSIONS

These results suggest that the prevention of diabetic cardiomyopathy by zinc supplementation is predominantly mediated by an increase in cardiac MT.

Kinetics of the early adaptive response to gamma rays: induction of a cellular radioprotective mechanism in murine leukocytes in vivo

The aims of the study were to establish the kinetics of the early adaptive response and to determine the minimum adaptive dose of gamma rays capable of inducing this response. The minimum adaptive dose was determined by exposing groups of five BALB/c male mice to an adaptive dose of 0.005 or of 0.02 Gy gamma rays from a 137Cs source and challenge with 1.0 Gy 60 min later. The kinetics of adaptive response induction was established by exposing mice to an adaptive dose of 0.01 Gy, and subsequently to a challenge dose of 1.0 Gy at different times. Blood samples were collected from the tail immediately after exposure to the challenge dose, and the percentage of DNA-damaged cells and the extent of damaged were determined by single cell gel electrophoresis in 300 leukocytes per animal in five mice. The results confirms the capability of an in vivo induction of an early radioprotective process against the DNA-damage produced by gamma rays in murine leukocytes, and allows us to conclude that the minimum adaptive dose lies between 0.005 and 0.01 Gy of gamma rays, and the early adaptive response is induced as early as 30 min after the exposure and persists for at least 18 hr.

Assessing ecological effects of radionuclides: data gaps and extrapolation issues

By inspection of the FASSET database on radiation effects on non-human biota, one of the major difficulties in the implementation of ecological risk assessments for radioactive pollutants is found to be the lack of data for chronic low-level exposure. A critical review is provided of a number of extrapolation issues that arise in undertaking an ecological risk assessment: acute versus chronic exposure regime; radiation quality including relative biological effectiveness and radiation weighting factors; biological effects from an individual to a population level, including radiosensitivity and lifestyle variations throughout the life cycle; single radionuclide versus multi-contaminants. The specificities of the environmental situations of interest (mainly chronic low-level exposure regimes) emphasise the importance of reproductive parameters governing the demography of the population within a given ecosystem and, as a consequence, the structure and functioning of that ecosystem. As an operational conclusion to keep in mind for any site-specific risk assessment, the present state-of-the-art on extrapolation issues allows us to grade the magnitude of the uncertainties as follows: one species to another > acute to chronic = external to internal = mixture of stressors > individual to population > ecosystem structure to function.

Metallothionein-III prevents gamma-ray-induced 8-oxoguanine accumulation in normal and hOGG1-depleted cells

Metallothioneins (MT) play an important biological role in preventing oxidative damage to cells. We have previously demonstrated that the efficiency of the protective effect of MT-III against the DNA degradation from oxidative damage was much higher than that of MT-I/II. As an extension of the latter investigation, this study aimed to assess the ability of MT-III to suppress 8-oxoguanine (8-oxoG), which is one of the major base lesions formed after an oxidative attack to DNA and the mutant frequency of the HPRT gene in human fibroblast GM00637 cells upon exposure to gamma-rays. We found that human MT-III expression decreased the level of 8-oxoG and mutation frequency in the gamma-irradiated cells. Using an 8-oxoguanine DNA glycosylase (OGG1)-specific siRNAs, we also found that MT-III expression resulted in the suppression of the gamma-radiation-induced 8-oxoG accumulation and mutation in the OGG1-depleted cells. Moreover, the down-regulation of MT in human neuroblastoma SKNSH cells induced by MT-specific siRNA led to a significant increase in the 8-oxoG level, after exposure to gamma-irradiation. These results suggest that under the conditions of gamma-ray oxidative stress, MT-III prevents the gamma-radiation-induced 8-oxoG accumulation and mutation in normal and hOGG1-depleted cells, and this suppression might, at least in part, contribute to the anticarcinogenic and neuroprotective role of MT-III.

Zinc- or cadmium-pre-induced metallothionein protects human central nervous system cells and astrocytes from radiation-induced apoptosis

We have shown the protection of human central nervous system (CNS) cultures by zinc (Zn) or cadmium (Cd)-pre-induced metallothionein (MT) synthesis from radiation-induced cytotoxicity (lactate dehydrogenase (LDH) release and neuronal dendritic injury). The present study is to further define the types of cell death induced by different dose levels of radiation and investigate the effect of MT induction (by Zn or Cd) on radiation-induced apoptosis in primary human CNS and astrocyte cultures. Apoptosis was detected by fragmented DNA electrophoresis, TUNEL technique, and propidium iodide staining. Expression of MT protein was examined by immunofluorescent staining. Results showed that exposure of primary human CNS cultures to 15 and 30 Gy gamma-radiation predominantly induced apoptotic cell death, while exposure to 60 Gy gamma-radiation predominantly induced necrotic cell death. Normal primary human CNS cultures showed weak MT staining, while primary human CNS cultures exposed to Zn or Cd showed intense MT staining. The induced apoptotic cell death by exposure to 30 Gy gamma-radiation increased to a maximum level at 12 and 24 h, and was reduced significantly by Zn or Cd pre-induced MT. Using primary human astrocytes, the induction of MT protein by Zn or Cd was further confirmed. The enhanced MT expression also afforded a significant protection from 30 Gy gamma-ray-induced apoptosis in the primary human astrocytes. These results suggest that MT protected human CNS cells from apoptosis following ionizing radiation, probably through its antioxidant property.

Cytogenetic dose-response and adaptive response in cells of ungulate species exposed to ionizing radiation

In the studies reported here, the micronucleus assay, a common cytogenetic technique, was used to examine the dose-responses in fibroblasts from three ungulate species (white-tailed deer, woodland caribou, and Indian muntjac) exposed to high doses of ionizing radiation (1-4 Gy of (60)Co gamma radiation). This assay was also used to examine the effects of exposure to low doses (1-100 mGy) typical of what these species experience in a year from natural and anthropogenic environmental sources. An adaptive response, defined as the induction of resistance to a stressor by a prior exposure to a small "adapting" stress, was observed after exposure to low doses. This work indicates that very small doses are protective for the endpoint examined. The same level of protection was seen at all adapting doses, including 1 radiation track per cell, the lowest possible cellular dose. These results are consistent with other studies in a wide variety of organisms that demonstrate a protective effect of low doses at both cellular and whole-organism levels. This implies that environmental regulations predicated on the idea that even the smallest dose of radiation carries a quantifiable risk of direct adverse consequences to the exposed organism require further examination. Cytogenetic assays provide affordable and feasible biological effects-based alternatives that are more biologically relevant than traditional contaminant concentration-based radioecological risk assessment.

Zinc-metallothionein protects from DNA damage induced by radiation better than glutathione and copper- or cadmium-metallothioneins

Protection of radiation-induced DNA damage by metallothionein (MT) has been documented, but there is no detailed information about its efficiency compared to other antioxidants or the effect of metals which bind to MT on the protective effect of MT in radiation-induced DNA damage. In this study, we used a cell-free system to investigate the effect of MT with other antioxidants, such as albumin and glutathione and we compared the efficiency of MT bound to different metals on radiation-induced DNA damage. DNA damage was measured by loss in ethidium bromide/DNA fluorescence and increased mobility of DNA on gel electrophoresis. Gamma rays at 30 Gy induced significant DNA damage and zinc-MT showed a significant higher protection from radiation-induced DNA damage than both glutathione and albumin. Metallothionein bound to other metals, such as copper and cadmium, also showed protection of radiation-induced DNA damage, but the protective effect by zinc-MT was the highest. These results suggest that MT, in particular bound to zinc, is a high-capacity antioxidant to protect radiation-induced DNA damage.

Repeated UVR exposures cause keratocyte resistance to apoptosis and hyaluronan accumulation in the rabbit cornea

PURPOSE

To evaluate hyaluronan (HA) production and level of apoptosis of corneal cells after repeated UVR exposures.

METHODS

Fifteen albino rabbit corneas were exposed to 310 nm UVR at a dose that causes biomicroscopically significant keratitis (0.47 J/cm2). Nine rabbits received a single dose of UVR. Six rabbits were irradiated 3 times at 7-day intervals. Rabbits exposed to the single dose of UVR, were sacrificed 24 hours, 7 and 14 days after irradiation. Rabbits exposed to the repeated doses of UVR, were sacrificed 24 hours and 14 days after the last irradiation. The corneal tissue specimens were processed for histological analysis using specific staining for HA, and the TdT-dUTP terminal nick-end labeling (TUNEL) assay.

RESULTS

Corneas exposed to a single UVR dose showed extensive positive TUNEL staining 24 hours after exposure. Almost all basal epithelial cells, keratocytes throughout the entire thickness of the stroma, and endothelial cells were TUNEL-positive. No HA was found 24 hours after exposure. Extracellular HA staining of high intensity was found at day 7 throughout the entire central stroma, except the anterior one-fourth. At day 14 only a faint HA staining was detected in the posterior stroma, close to Descemet's membrane. Corneas exposed to repeated UVR doses showed at 24 hours positive TUNEL staining only in epithelial cells and in very few stromal cells. The majority of stromal cells and endothelial cells were unaffected. At the same time HA staining of very high intensity was found both at 24 hours and day 14, and it was evenly distributed throughout the entire thickness of the stroma.

CONCLUSION

Repeated UVR exposures lead to increased production and accumulation of HA in the corneal stroma. The repopulated keratocytes are much more resistant to apoptosis than the native ones. HA accumulation may be a sign of long-term changes in the cornea.

Zinc and the gene

A significant portion of cellular zinc is found in the nucleus where it appears to be critically involved in maintaining genetic stability and in the process of gene expression. With regard to gene expression zinc functions mechanistically at several levels but recent interest has focussed especially on the involvement of zinc in DNA transcription through the activity of transcription factors which contain specific zinc-finger regions which bind to DNA and, in conjunction with other families of transcription factors, control cell proliferation, differentiation and cell death. Because of the central importance of zinc in cell division and growth, considerable attention is paid to zinc as an essential trace element and much has been written concerning dietary sources of zinc and recommended dietary intakes of the metal.

Infiltrating lymphocytes in undifferentiated nasopharyngeal cancer lack metallothionein expression

Metallothionein (MT) is a metal-binding protein with functional roles in cell growth, repair and differentiation. MT is reported to be differentially expressed in lymphocytes of malignant gastrointestinal lesions. The level of MT protein was examined by immunohistochemical analysis at light microscopic and ultrastructural level in infiltrating lymphocytes from 20 cases of undifferentiated nasopharyngeal carcinoma (NPC). MT expression was found to be absent in the infiltrating lymphocytes of NPC and in reactive lymphocytes of lymphoid hyperplasia in nasopharyngeal tissues. Ultrastructural examination confirmed the absence of MT immunoreactivity in the lymphoid infiltrate of NPC. On the other hand, malignant lymphoblasts of diffuse large cell lymphoma, showed MT-immunopositivity by immunoelectron microscopy. This study demonstrates a lack of MT expression in the lymphoid stroma of undifferentiated NPC, a further characteristic of its non-neoplastic nature.

Measurement of breast skin viscoelasticity and a pilot study on the potential radioprotective effect of a zinc-based cream

Radiation-induced late skin effects were studied in patients with breast cancer in relation to different protocols of fractionated radiotherapy in three different medical centres, in Israel, the UK and the USA. The mechanical properties of skin were evaluated in breasts of healthy volunteers, and non-irradiated and irradiated breasts of patients, using a newly developed viscoelasticity skin analyser (VESA). The increase of the dose of radiation per fraction was found to have more impact on the development of radiation-induced late skin effects than the elevation of the total dose given. In addition, a pilot study on the possible radioprotective effect of external application of a cream containing zinc oxide on radiation-induced early skin changes in patients with breast cancer was initiated. Non-invasive measurement of trace elements and zinc pharmacokinetics in the skin of healthy controls following the application of the zinc oxide cream were performed by unique diagnostic X-ray spectrometry (DXS). Application of the cream, followed by thorough skin cleansing, significantly increased the amount of residual zinc in the skin, but continuous daily treatment did not cause further build-up of the dermal zinc level. The radioprotective effect of the zinc oxide cream on the skin is now being studied.

Increased radiation-induced apoptosis in mouse thymus in the absence of metallothionein

Metallothionein (MT) has been shown to protect cells from free radical induced DNA damage after exposure to copper, hydrogen peroxide and also radiation. In order to study the role of MT in radiation induced apoptosis, age-matched male control mice (C57BL/6J), MT-I overexpressing (MT-I*) and MT-null transgenic mice were exposed to whole body cobalt 60 gamma-irradiation at 0, 5, or 10 Gy, and their thymus were removed 24 h later. The basal levels of MT and zinc concentrations in the thymus were measured by 109Cadmium-heme assay and atomic absorption spectrophotometry, respectively. The MT expression after radiation was determined by immunohistochemical staining using a polyclonal antibody to MT. The extent of apoptosis in thymocytes was determined by histology (H&E stain). DNA was isolated from the thymus, and DNA fragmentation was determined by agarose gel electrophoresis. The results showed that the basal level of MT protein in MT-I* thymus was 2.4-fold higher than control mice, and that MT was inducible in both MT-I* and control C57BL6 thymus after radiation exposure. Minimal MT protein was detected in MT-null mice thymus before or after radiation, while, a significantly higher number of apoptotic cells and DNA fragmentation were found in MT-null thymus after whole body irradiation. These data demonstrated a protective role for MT in radiation-induced apoptosis in mouse thymus.

Low-dose whole-body irradiation induces alteration of protein expression in mouse splenocytes

In previous studies, the stimulatory effect on immunological function and adaptive response in splenocytes were found after exposure of mice to low-dose whole-body irradiation (LD-WBI). Protein synthesis was found to be required for these responses. The present study, therefore, attempts to investigate the protein changes in mouse splenocytes after exposure to LD-WBI. Two-dimensional gel electrophoresis was used to show the alterations of proteins in nuclei, cytoplasm and extracellular fluid of the splenocytes at 4 h after mice exposed to 75 mGy X-rays. As compared to control, changed expressions of 20, 15 and 6 proteins, in sizes ranging from 10 to 69 kDa with pI of 5.0-8.2, were found in the nuclei, cytoplasm and extracellular fluid of splenocytes, respectively. One protein, shown in the cytoplasm of splenocytes of control mice, appeared in extracellular fluid of splenocytes after LD-WBI. Two proteins, shown in cytoplasm, and one protein, shown in extracellular fluid of splenocytes in control mice, appeared in the nuclei after LD-WBI. Time-course of protein synthesis varied in different proteins after LD-WBI. These results suggest that alterations of protein expressions and redistribution of proteins between intracellular and extracellular compartments occurred in the splenocytes after LD-WBI. Protein extract from LD-WBI splenocytes was fractionated by Sephadex G-100. Fractions 61-80 contained proteins able to protect lymphocytes in vitro from radiation-induced chromosome aberrations. These findings are of importance in elucidating mechanisms of immuno-enhancement and adaptive response induced by low-dose radiation. although the features and the functions of these proteins remain to be elucidated in future studies.

Metallothionein in radiation exposure: its induction and protective role

Since its discovery about 40 years ago, there has been a wide interdisciplinary research interest in metallothionein (MT) on its physiological and toxicological aspects. Functionally, MT is involved not only in metal detoxification and homeostasis, but also in scavenging free radicals during oxidative damage. Among over 4500 publications which can be retrieved by Medline search, only about 50 reports have been published on the relationship of MT with ionizing and UV radiation. In this review, we have evaluated critically the published data on the induced synthesis of MT by radiation, and the potential functions of MT in radiation induced cell damage. MT mRNA expression or MT synthesis was found to be induced by exposure of cells in vitro or tissues in vivo to ionizing or UV radiation. In most of the studies in animals and tissue cultures, high doses of ionizing radiation were used to induce MT, and, therefore, it is difficult to extrapolate these results to low level of repeated exposures to radiation in humans. Induced synthesis of MT is considered as one of the mechanisms involved in the adaptive response to low dose radiation exposure. The presence of MT in normal cells may provide protective effects from radiation-induced genotoxicity and cytotoxicity. However, in tumor cells, the presence of MT can result in drug and radiation resistance as well. These effects are modulated by other cellular factors, besides MT, such as antioxidants, and by the cell cycle stages in cell proliferation and differentiation.

Protective role of zinc-metallothionein on DNA damage in vitro by ferric nitrilotriacetate (Fe-NTA) and ferric salts

Oxidative DNA damage can be caused by radicals generated by transitional metals like iron in Fenton reaction. Metallothionein (MT) may play an important role in preventing oxidative DNA damage. Therefore, after comparing the effects of ferric salts (Fe), and complexes of ferric salts with nitrilotriacetic acid (Fe-NTA) on DNA damage, the protective effects of zinc-MT (Zn-MT) on DNA damage of Fe salts or Fe-NTA were investigated in vitro. DNA damage was measured by loss of fluorescence of DNA binding to ethidium bromide, and also by increased DNA mobility in agarose gel electrophoresis. Both Fe salts and Fe-NTA could induce calf thymus DNA damage in presence of hydrogen peroxide and ascorbate. However, the degree of DNA damage was lower with Fe salts than that with Fe-NTA complex. Addition of 50 microM Zn-MT could only protect DNA from Fe-NTA, but not from Fe salt induced damage. The protective effect of MT was about five times better than that of glutathione (GSH). These results suggest a potential role for MT in protection from Fe-NTA-induced DNA damage.

Protective effects of extracted human-liver RNA, a known interferon inducer, against radiation-induced cytogenetic damage in male mice

Cells in vitro or in vivo pre-exposed to low-dose radiation (LDR) or low concentrations of chemical mutagens became more resistant to large-dose radiation-induced DNA or chromosome damage. This was known as radio-adaptive response, for which the exact mechanism was unclear. However, multiple cellular and molecular responses to LDR have been documented, for instance, the induction of some cytokines such as interferon (IFN). Administration of exogenous IFN to cultured cells or mice showed marked radio-protection. In the present study, we investigated the in vivo radio-protective effects of extracted human liver RNA (HL-RNA), a known IFN inducer, indirectly to determine the radio-protective action of endogenous IFN. First, mice were administered with 6.25 mg/kg HL-RNA at different times before exposure to radiation and the 24 h pretreatment offered the optimal protective action for HL-RNA on cytogenetic effects in bone marrow cells. When the mice were treated with different concentrations of HL-RNA for 24 h, a wide dose-range (25-100 mg/kg) of HL-RNA resulted in a marked protection from X-ray-induced chromosome aberrations in both bone marrow cells and germ cells. In subsequent experiments, a protective effect of pretreatment with 25 mg/kg HL-RNA for 24 h was also found for radiation-induced micronuclei in polychromatic erythrocytes (PCE), and inhibition of DNA repair ability (unscheduled DNA synthesis, UDS). These results demonstrated that HL-RNA, an IFN inducer, is able to offer significant cytogenetic protection from radiation, implying indirectly that the induction of IFN by LDR may also play a protective role as one of the mechanisms in the induction of the cytogenetic adaptive response.