“ Adaptive response ”-Some underlying mechanisms and open questions

Organisms are affected by different DNA damaging agents naturally present in the environment or released as a result of human activity. Many defense mechanisms have evolved in organisms to minimize genotoxic damage. One of them is induced radioresistance or adaptive response. The adaptive response could be considered as a nonspecific phenomenon in which exposure to minimal stress could result in increased resistance to higher levels of the same or to other types of stress some hours later. A better understanding of the molecular mechanism underlying the adaptive response may lead to an improvement of cancer treatment, risk assessment and risk management strategies, radiation protection, e.g. of astronauts during long-term space flights. In this mini-review we discuss some open questions and the probable underlying mechanisms involved in adaptive response: the transcription of many genes and the activation of numerous signaling pathways that trigger cell defenses DNA repair systems, induction of proteins synthesis, enhanced detoxification of free radicals and antioxidant production.


The adaptive response
Organisms are affected by various different physical and chemical genotoxic agents, some of them natural (e.g.solar ultraviolet light, ionizing radiation) and others released in the environment as a result of human activity (anthropogenic environmental pollutants).Many defense mechanisms have evolved to minimize genotoxic damage.One of these is induced radioresistance or adaptive response (AR).The term "adaptive response" usually means that a relatively small "conditioning" radiation dose induces increased radioresistance when the cells are irradiated with higher doses several hours later (Hillova and Drasil, 1967).Thus, radioadaptive response induction expresses the ability of low dose radiation to induce cellular changes that alter the level of subsequent radiation-induced or spontaneous damage.The AR could be considered a nonspecific phenomenon -the exposure to minimal stress inducing a very low level of damage can trigger an AR resulting in increased resistance to higher levels of the same or of other types of stress (Joiner et al., 1996;Wolff, 1998;Joiner et al., 1999;Patra et al., 2003;Asad et al., 2004;Girigoswami and Ghosh, 2005;Yan et al., 2006).
An adaptive response to radiation also occurs in human lymphocytes (Shadley and Wolff, 1987;Wojewózka et al., 1996;Stoilov et al., 2007).This was first demonstrated by Olivieri et al. (1984) when peripheral blood lymphocytes were irradiated with low doses of X-rays or exposed to tritium labeled thymidine and a lower than expected frequency of chromosomal aberrations was found after a subsequent higher test (or challenge) dose.However, other authors reported a diversity in response of lymphocytes; in some cases showing additive effects or no response at all (e.g.Mortazavi et al., 2003c).Sawant et al. (2001) also found an adaptive response to low dose gamma irradiation of 10T1/2 cells that were subsequently exposed to microbeam alpha-particle irradiation.Other test systems under some experimental conditions may also not show an AR (Boreham and Mitchel, 1993;Colombi and Gomes, 1997;Zasukhina et al., 2000;Pelevina et al., 2003;Joksic and Petrovic, 2004).
A popular hypothesis presented in Figure 1 postulates that the AR could be induced by reactive oxygen species (ROS) (Feinendegen et al., 1996(Feinendegen et al., , 1999;;Jones et al., 1999;de Saint-Georges, 2004;Shankar et al., 2006).ROS are generated in organisms during metabolism and/or formed after exposure to different biotic and abiotic stimuli (UVirradiation, ionizing radiation, ozone exposure, heavy metals), damaging some cell constituents and producing oxidative stress (Joiner et al., 1996(Joiner et al., , 1999;;Mendez-Alvarez et al., 1999;Bolwell et al., 2002;Neill et al., 2002;Vranová et al., 2002;Babu et al., 2003;Asad et al., 2004;Verschooten et al., 2006;Wang et al., 2006).Ionizing radiation (IR) can damage DNA both by direct ionization and by indirect processes in which DNA is affected by numerous radiolytic reactive products.Free radicals can attack biomolecules such as DNA, proteins and lipids and initiate lipid peroxidation and generate intermediates that can react with DNA (Halliwell and Gutteridge, 1989;Marnett et al., 2003).ROS could also induce multiple localized lesions consisting in base damage, single-and double-strand breaks (SSBs and DSBs), DNA-DNA cross-links and DNA-protein crosslinks (Goldberg and Lehnert, 2002;Marnett et al., 2003;Asad et al., 2004).For example, it has been found that administration of heavy metals could reduce subsequent thy-mus lymphocyte DNA lesions and lipid peroxidation in gamma irradiated mice (Osipov et al., 2003) and many bacteria species have adaptive responses which protect them against the toxicity and mutagenicity of DNA alkylating agents (Sedgwick and Lindahl, 2002).

Molecular mechanisms of the adaptive response
Little is currently known about the precise mechanisms of AR.There is evidence that different stress conditions can activate similar defense mechanisms in various biological systems (Joiner et al., 1996(Joiner et al., , 1999;;Babu et al., 2003).The AR probably involves the transcription of many genes and the activation of numerous signaling pathways that trigger cell defenses (Figure 2): more efficient detoxification of free radicals, DNA repair systems, induction of new proteins in irradiated cells with a conditioning dose, and enhanced antioxidant production (Bryant, 1979;Wolff, 1998;Mendez-Alvarez et al., 1999;Pajovic et al., 2001;Assis et al., 2002;Chankova and Bryant, 2002;Neill et al., 2002;Sasiadek et al., 2002;Sedgwick and Lindahl, 2002;Dimova et al. 397 Figure 1 -Scheme of a popular hypothesis for the induction of the adaptive response (AR) via reactive oxygen species (ROS) (Feinendegen et al., 1996(Feinendegen et al., , 1999)).
Sakamoto-Hojo et al. (2003) showed that the cell responses to ionizing radiation in lymphocytes of radiation workers involved altered expression of genes associated with cell cycle regulation, DNA repair, signal transduction, apoptosis induction/tumorigenesis and damage response/ maintenance of genetic stability (P53-related functions).Similar results were obtained for human lymphoblastoid cells in vitro and the authors proposed that certain low dose-induced alterations in cellular functions could be predictive of the subsequent genomic damage risk (Coleman et al., 2005).Other recent molecular studies suggested that alternative dose-specific pathways of radioadaptive response could exist in mammalian cells: one response activated at low doses by the protein kinase C through p38 MAP kinase resulting in P53 activation and another activated at higher doses resulting in activation of ERK and JNK kinases and WIP phosphatase (Lanza et al., 2005).AR is known to require a certain minimal dose for activation (Leonard, 2007).Low levels of damage could be triggering events that signal the activation of DNA repair systems (Boreham and Mitchel, 1991;Wolff, 1998;Matsumoto et al., 2004).For example, the persistence of DNA strand discontinuities could serve as a triggering signal for the adaptation of human lymphocytes against ionizing radiation exposure (Stoilov et al., 2007).The magnitude of the AR has been shown to increase with the dose of radiation up to a certain threshold (Bryant, 1976).A specific dose of UVB was required to induce AR in Euglena (Takahashi et al., 2006).Induction of AR by methylating agents has been reported in eukaryotic cells as well.For example, Mahmood et al. (1996) reported that in murine cells the AR was induced by the methylating agent methyl methanesulfonate and was stronger than that induced by the ethylating agent ethyl methanesulfonate.Schlade-Bartusiak et al. (2002) showed a more pronounced AR in human lymphocytes after treatment with bleomycin, which generates DNA breaks, than with the alkylating agent mitomycin.
Experiments with restriction enzymes indicated that DNA DSBs with blunt or cohesive ends were capable of inducing an AR (Wolff, 1996(Wolff, , 1998)).Some radiosensitive DSB repair-deficient mutants were found to exhibit no induced radioresistance, suggesting the involvement of DSB rejoining (Skov et al., 1994).Changes in chromatin conformation could result in less sensitivity of chromatin to damage by indirect effect of a test dose or in increased accessibility of damaged sites to repair enzymes (Belyaev et al., 1996;Kleczkowska and Althaus, 1996).Experiments with repair inhibitors suggested that poly-ADP-Ribose Polymerase-1 (PARP) is also involved in the AR (Kleczkowska and Althaus, 1996;Wolff, 1998;Marples and Joiner, 2000;Patra et al., 2003) possibly interfering in the cell cycle control (Tang et al., 2005) or in the damage-sensing process (Marples et al., 2004).This was confirmed and it has been postulated that the AR can be interpreted in terms of increased non-homologous end-joining of DSB or increased homologous recombination (Vaganay-Juery et al., 2000;Marples et al., 2004;Raaphorst et al., 2006).On the other hand PARP may not be involved in the induction of AR after treatment with alkylating agents in mouse bone marrow cells (Guruprasad et al., 2002).An AR was found to be absent in some radiosensitive tumor lines and ataxia telangiectasia patients cells (Lambin et al., 1994).In our experiments the highly radioresistant Chlamydomonas reinhardtii strain H-3 surprisingly showed a clear adaptive response (Figure 3).These results show that the already enhanced ability of strain H-3 to repair radiation damage, evidenced by its radioresistance to single doses of radiation, does not prevent this strain from 'adapting' still further following a priming dose of radiation (Chankova et al., 2005).In contrast, the level of AR from Bloom syndrome (human autosomal recessive disorder, characterized by chromosomal instability and increased risk of malignancy at an early age) patients blood cells has been shown to be the same as that in control cells from healthy donors (Zasukhina et al., 2000).The hamster cell line EM9, which is SSB repair-deficient, can also develop an AR (Skov et al., 1994).CHO cells mutated for different components of the nucleotide excision repair pathway do not express mutation and/or survival AR (Hafer et al., 2007).In spite of the variation in AR in different systems, it has been postulated that AR analysis could be used for assaying DNA repair capacity (Sasiadek et al., 2002).It has also been suggested that membrane damage may switch on some of these responses (Skov et al., 1994) and/or be reduced as a result of AR (Girigoswami and Ghosh, 2005;El-Tayeb et al., 2006).

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Adaptive response: Mechanisms and questions Does the AR operate via up-regulation of DNA repair?
DNA damage due to hydroxyl radicals derived from the radiolytic decomposition of H 2 O produces lesions that strongly induce DNA repair mechanisms (Boreham and Mitchel, 1991).Experiments in various biological systems have been performed to test the hypothesis that DNA repair up-regulation could be involved in the AR.There is evidence that DNA repair underlies the AR induced by low radiation doses in human and plant cells (Lambin et al, 1994;Patra et al., 2003) by increasing the amount and rate of DNA repair (Joiner et al., 1996;Joiner et al., 1999).For example, when peripheral blood mononuclear cells from residents of Ramsar (a high natural background radiation area) were irradiated with a challenging dose of gamma rays, Mohammadi et al. (2006) detected lower levels of micronuclei, higher numbers of apoptotic cells and enhanced DNA repair.It has been proposed that these effects could be related to the induction of an AR.The AR induced by conditioning UVB exposures in Euglena may not be due to biosynthetic UV-absorbing compounds, but to the induction of photolyase enzymes (Takahashi et al., 2006).Activation of UVB-induced AR in human skin cells could involve a p53-dependent gene program with p53-induced cell cycle arrest and DNA repair (Decraene et al., 2005).The study of repair kinetics of DNA damage in Chinese hamster V79 cells showed that the radioadaptive response could be a result of DNA repair mechanisms which lead to less residual DNA damage, but not from the induction of protective mechanisms that reduce the initial DNA damage (Ikushima et al., 1996).An adaptive response was observed through micronuclei formation and neoplastic transformation in murine 10T1/2 cells and the authors postulated that this adaptive response resulted from an enhanced DSB repair (Azzam et al., 1994).This would be in agreement with our finding of accelerated DSB rejoining in C. reinhardtii following a conditioning dose of gamma rays (Chankova and Bryant, 2002) or radiomimetics (Chankova et al., 2007) presented in Figures 4 and 5.A reduction in deletion-type mutants in adapted cells may also be a result of DSB DNA repair in various cell systems (Rigaud and Moustacchi, 1996).However, as Szumiel (2005) pointed out, the view that DNA repair is stimulated in the 'primed' and challenged cell is not supported by all the available data.For instance, at least in some cases the AR may have no connection with modification of repair processes (Tskhovrebova and Makedonov, 2004).In such cases, the AR could be partly due to diminished fixation of DSBs (Szumiel, 2005).For example, induction of AR has been observed in terms of reduced initial DNA damage as well, which could be due to increased oxidative defense processes or to other undefined molecular processes, e.g.per-turbation of cell cycle progression (Atanasova et al., 2005;Cramers et al., 2005).
Some evidence indicates that the H 2 O 2 induced AR in cultured human retinal pigment epithelium (RPE) cells involves increased nuclear DNA protection but no adaptive benefit for mtDNA protection or repair (Jarrett and Boulton, 2005).Hence, it has been suggested that the AR could be an important antioxidant defense for cells located in inherently oxidizing microenvironments.However, mitochondria have been viewed as a weak link in this defense mechanism which would contribute to aging and agerelated disease (Jarrett and Boulton, 2005).

Does the AR involve induction of new proteins synthesis?
Little is known about the proteins and genes involved in adaptive responses in cells.Changes in gene transcriptional levels have been found after exposure to ionizing ra-  diation with low doses that result in the induction of AR (Wolff, 1998;Coleman et al., 2005;Lanza et al., 2005).A clue as to the nature of the underlying process was provided by results showing a dependence on de novo protein synthesis.Treatment of Oedogonium, Chlamydomonas and Closterium cells with protein synthesis inhibitors (cycloheximide and chloramphenicol) after the first 'conditioning' dose prevented the induced repair responses in these organisms (Horsley and Lazlo, 1971;Bryant, 1975;Howard and Cowie, 1978;Chankova and Bryant, 2002).
The synthesis of DNA-binding proteins (MWs 50, 74 and 130 kdal) was found in radiation-conditioned cells of C. reinhardtii (Bryant, 1979).Our previous work showed an up-regulation of DNA DSB rejoining four hours after irradiation of C. reinhardtii CW15 (a Cell-Wall-less mutant with WT radiation response) that was strongly reduced when cells were treated with the protein synthesis inhibitors cycloheximide and chloramphenicol (Chankova and Bryant, 2002) (Figure 6).The induction of new protein synthesis by low doses could be caused by an effect of low doses on chromatin conformation near genes coding for DNA repair proteins (Belyaev et al., 1996).The AR to alkylating agents in Escherichia coli is thought to be related to an increased expression of genes which encode DNA repair proteins (aidB, ada, alkA, alkB) (Rohankhedkar et al., 2006).The AidB component of E. coli AR to alkylating agents has been identified as a flavin-containing DNA-binding protein and has been predicted to catalyze the direct repair of alkylated DNA (Rohankhedkar et al., 2006).Robson et al. (1999) isolated a novel gene from L132 cells that is down-regulated in response to ionizing radia-tion (DIR1 gene) which they postulated played a regulatory role in the AR.Later research on cells showing low-dose hypersensitivity (V79, RT112 and UM-UC-3) showed that antisense oligonucleotides against the DIR1 gene resulted in an increased rate of rejoining of DNA single-strand breaks coupled with an increase in cell survival after a dose of 2Gy (Robson et al., 2000).However, ataxia telangiectasia cells (ATBIVA), which do not show low-dose hypersensitivity, did not show such enhanced repair and survival.The authors concluded that radiosensitive cells such as those from AT patients lack the ability to switch on the DIR1 gene.Not all examples of induced AR involve de novo protein synthesis.For example, there are earlier observations that hydrogen peroxide induced a cross-adaptive response to cumene hydroperoxide in E. coli which did not require novel gene products but involved modification of the small subunit of Ahp, a protein involved in the protection against alkyl hydroperoxides (Asad et al., 1998).

Could activation of antioxidant systems have a role in induced resistance?
Mendez-Alvarez et al. (1999) proposed that the cell ability to induce AR could be affected by altering cellular oxidative stress levels.Enzymes are considered as a very important component of cell defense mechanisms which protect organisms from the harmful action of ROS damaging DNA and other biomolecules.It is assumed that enzymatic, non-enzymatic and indirect antioxidant defense systems could be involved in the formation of AR to oxidative stress (Mendez-Alvarez et al., 1999;Chen et al., 2006;Yan et al., 2006;Tosello et al., 2007).Joksic et al. (2000) suggested that oxidative stress can trigger an antioxidant response that includes changes in the activity of enzymatic defense system, mainly SOD.Radioresistant variants isolated from MCF-7 human carcinoma cells following fractionated radiation or overexpression of MnSOD demonstrated dose-modified factors at 10% isosurvival (Guo et al., 2003).The authors speculated that maybe the induction of MnSOD after fractionated doses caused a redox alteration that resulted in the up-regulation of stress response genes and radiation induced AR.Similarly, enhancement of the antioxidative capacities (catalase and MnSOD) probably played an important role in the reduction of initial DNA damage by low-dose-rate radiation in mice spleen (Otsuka et al., 2006).DNA microarray analysis has revealed that GPX1, CAT, SOD1 and several other genes involved in peroxidase activity were up-regulated after low-dose X-ray exposure of HUVEC cells (Lanza et al., 2005).The AR of yeast cells induced by the oxidants H 2 O 2 , menadione and juglone was associated with an increase in the activity of cellular catalase, SOD, glucose-6-phosphate dehydrogenase, and glutathione reductase, the main enzymes involved in cell defense against oxidative stress (Biryukova et al., 2006).Enhancement of these antioxidant 400 Adaptive response: Mechanisms and questions activities could be involved in menadione-induced AR to menadione and to H 2 O 2 in Bacillus sp.F26 (Yan et al., 2006).Leisinger et al. (1999) described a glutathione peroxidase homologous gene in Chlamydomonas reinhardtii whose expression is up-regulated after treatment with different oxidative stress inducing agents (Leisinger et al., 2001).Other studies indicated that some non-enzymatic antioxidants could affect the AR through binding and detoxification of the genotoxic chemical.For example, it has been recently demonstrated that the AR induced by sublethal concentrations of some oxisterols and prostaglandins in PC12 cells is mediated through elevation of cellular glutathione contents (Chen et al., 2006).Similarly, pretreatment of murine and human cells with alkylating or 8-oxoG inducing agents prior to a test dose resulted in a twofold shift of cellular glutathione levels as an AR (Bercht et al., 2007).
Chlamydomonas reinhardtii has been reported to produce metal-binding peptides in response to stress induced by different heavy metals (Cd, Hg, Ag) (Howe and Merchant, 1992) and the cadmium-induced AR in Allium cepa was prevented after inhibition of phytochelatin synthesis (Panda et al., 1997).Similarly, inhibition of metallothionein synthesis prevented the AR after heavy metal (Cu, Pb) conditioning treatment in Vicia faba (Rieger et al., 1993).Inhibition of cytoplasmic protein synthesis prevented the AR induced by Cd in Allium cepa (Panda et al., 1997).Inhibition of de novo protein synthesis by cycloheximide probably inhibited the Cu 2+ -dependent metallothionein synthesis in human peripheral blood lymphocytes thereby eliminating the AR triggered by copper sulphate (Nikolova et al., 1999).
On the other hand, ROS (e.g.hydrogen peroxide and nitric oxide) could serve as signal transducers in plant and animal cells (Neill et al., 2002;Vranová et al., 2002;Babu et al., 2003;Matsumoto et al., 2004).As signaling molecules, ROS might affect the development of AR through participation in the damage-sensing process after conditioning dose exposure.For example, in UV-irradiation experiments with human skin fibroblasts the addition of antioxidants reduced the cellular oxidative stress and adaptive response in a concentration-dependent manner (Jones et al., 1999).Thus, it is possible that in some cases the increased scavenging of ROS by the antioxidant system might reduce the induced damage resulting in AR.The contribution of the antioxidant system for the development of AR could be further complicated by the fact that certain ROS are known to serve as signal transducers in plant and animal cells.Miura (2004) showed an insignificantly increased level of the activity of CAT, GPx, GR and glutathione content after low dose and subsequently given higher dose of X-rays irradiation in rat glial cells.He therefore concluded that antioxidant defense can contribute only partly to the radiation induced AR in tested cells.The AR in fibroblasts derived from transgenic mice overexpressing the Cu/ZnSOD gene appeared to be unrelated to the amount of SOD in the cells and, hence, independent of superoxide radicals (Wolff, 1996).

Kinetics of the adaptive response
As summarized by Feinendegen (2005), adaptive protection develops with a delay of hours, may last for days to months, decreases steadily at doses above about 100 mGy to 200 mGy and is not observed anymore after acute exposures to more than 500 mGy.Indeed, there is abundant evidence that the adaptive response depends on the experimental design.The adaptive response was shown to be both dose and time-dependent with a maximal effect occurring several hours later, for example between four to six hours after exposure for the unicellular green alga Chlamydomonas reinhardtii (Bryant, 1976;Chankova et al., 2005Chankova et al., , 2007)).The experimental design used by us for induction of AR by different genotoxic agents in the green alga Chlamydomonas reinhardtii as a test system is shown in Figure 7.We also observed a dose-dependence of the radiation-induced AR in C. reinhardtii when DSB DNA rejoining was used as an indicator (Chankova and Bryant, 2002).A small conditioning dose of gamma rays irradiation led to a small increase in the rate of DSB rejoining but when the magnitude of the conditioning dose was progressively increased there was a corresponding decrease in the fraction of damage remaining (Figure 8).Heat shock protection against induction of chromatid aberrations by clastogens in Vicia faba was found to be dependent on the time span (from less than ten minutes up to four hours) between the test dose and the adaptive treatment (Rieger and Michaelis, 1988).The time course of the adaptive response in lymphocytes was found to be similar to that in plant systems, reaching a plateau after about six hours (Shadley and Wolff, 1987).In murine leukocytes the minimum adaptive dose lies between 0.005 and 0.01 Gy of gamma rays and the early AR to a test dose of 1.0 Gy is induced as early as 30 min after the exposure and persists for at least 18 h (Morales-Ramírez and Mendiola-Cruz, 2004).Venkat et al. (2001) observed a maximum AR when a test dose of 100 cGy was given four hours after an adaptive dose, 30 h following the mitogenic stimulation of lymphocytes.The Dimova et al. 401 protective effect against N-methyl-N'-nitro-N-nitrosoguanidine induced by prior treatment with H 2 O 2 in E. coli is also time dependent, decreasing 15 min after the pretreatment and almost abolished after 30 min (Asad et al., 1997).
Besides being brief, the AR to irradiation could also be modulated (Joiner et al., 1996(Joiner et al., , 1999;;Raaphorst et al., 2000;Tiku and Kale, 2004), e.g.AR could be induced more effectively when a conditioning dose was given in small fractions (Tiku and Kale, 2004).It has been proposed that such fractionated irradiation of human fibroblasts could result in elevated survival due to repair of sublethal damage (Raaphorst et al., 2000).The efficiency of cellular defense reactions that are activated can vary and depend on the level and type of the impact.It has been more recently reported that in some instances the conditioning dose can act synergistically, thereby increasing the frequency of chromosomal aberrations seen following the test dose (Matsumoto et al., 2004;Zhou et al., 2004).Different agents may have different impacts which could result in adaptive reactions of variable efficiency (Boreham and Mitchel, 1991;Marples and Joiner, 1993;Joiner, 1994;Schlade-Bartusiak et al., 2002;Marples et al., 2004;Matsumoto et al., 2004).For example, acute doses of alpha-particles and other high Linear Energy Transfer (LET) radiations, such as neutrons, appear to be less efficient in eliciting an adaptive response than low LET radiations (e.g.X-rays) (Boreham and Mitchel, 1991;Marples and Joiner, 1993;Joiner, 1994) presumably since the level of local DNA damage is more severe and therefore immediately activates the G2 sensing systems (Marples et al., 2004).However, some reports indicated that AR following neutron exposure could be observed in V79 Chinese hamster cells (Marples and Skov, 1996) or human lymphocytes in vitro (Gajendiran et al., 2001).
Remarkably, it has recently been reported that in prostate cells of pKZ1 transgenic mice X-ray-induced 'reverse' AR could be observed when the high damaging dose preceded the low dose (Day et al., 2007).This 'reverse' AR was of similar magnitude to the AR observed when the low dose was given first.These results may indicate that the mechanisms underlying AR may not be due to prevention of damage induced by the high dose but to modulation of the cellular response to this damage.

Relevance of the adaptive response
Low-dose radiation may have some beneficial effects, e.g.adaptive protection causing DNA damage prevention and repair and immune stimulation (Upton , 2001;Liu, 2003;Ghiassi-nejad et al., 2004;Feinendegen, 2005).Low to intermediate doses of ionizing radiation have been observed to enhance growth and survival, augment the immune response, and increase resistance to the mutagenic and clastogenic effects of further irradiation in plants, bacteria, insects and mammals (Upton, 2001).Stimulation of immunity has been observed in human populations after long-term exposure to high level natural background radiation (Ghiassi-nejad et al., 2004).Ghiassi-nejad et al. (2004) observed a significant increase of CD69 expression on TCD4+ stimulated cells and a significant increase of total serum IgE in Ramsar residents.Removal of damaged cells occurred in vivo by way of a low dose-induced immune competence (reviewed in Feinendegen, 2005).Nevertheless, some scientists argue that the AR does not appear to be a relevant mechanism for radiation protection because the worst outcome for the cell (cell death) is probably the best outcome for the organism as a whole since the low (conditioning) dose could also generate a risk of cellular transformation (Hofseth, 2004;de Saint-Georges, 2004).Indeed, there is abundant experimental evidence that chronic lowdose occupational exposure to ionizing radiation could result in adverse health effects (Lin and Mao, 2004) and increased DNA damage (Joksic and Spasojevic-Tisma, 1998;Cardoso et al., 2001;Maffei et al., 2002;Sari-Minodier et al., 2002;Hadjidekova et al., 2003;Zakeri and Assaei, 2004;Güerci et al., 2006;Sari-Minodier et al., 2007).Similarly, increased levels of DNA damage have been obtained for chromium platers (Benova et al., 2002), offset printing workers (Aksoy et al., 2005), welders (Iarmarcovai et al., 2005;2007), pathologists/anatomists exposed to formaldehyde (Iarmarcovai et al., 2007), and patients taking certain phytopharmaceuticals (Lazutka and Mierauskiene, 2001) or after long-term low-dose antimicrobial prophylaxis (Slapyte et al., 2002).An increased frequency of micronuclei has also been detected after in vitro irradiation of domestic animals lymphocytes (Danika and Dunja, 2007) but not in the lymphocytes of cattle raised in the vicinity of a nuclear power plant (Lee et al., 2007).Conversely, very low-dose rate chronic gamma irradiation induced a marked cytogenetic adaptive response to a subsequent higher dose in mouse germ cells and probably did not cause any risk of damaging effects to the offspring of the irradiated male animals (Cai and Wang, 1995).Some authors consider that it is misleading to conceive the AR in terms of radioprotection because the AR is highly dependent on the genetic constitution and its measurement depends on the experimental design (Salone et al., 1996).For instance, the genotoxic effect of radiation in occupationally exposed persons may or may not vary depending on alcohol consumption, age and gender (Maffei et al., 2002;Hadjidekova et al., 2003;Zakeri and Assaei, 2004;Iarmarcovai et al., 2007;Sari-Minodier et al., 2007).The increased frequencies of chromosomal aberrations and sister chromatid exchanges in radiation workers could indicate a cumulative effect of low level chronic exposure to ionizing radiation, pointing to the relevance of conducting cytogenetic analysis in addition to physical dosimetry in such cases (Cardoso et al., 2001).Although cytokinetic and cytostatic effects have been detected in heavy and moderate smokers (Calderón-Ezquerro et al., 2007), smoking habits may or may not affect the genotoxic effect of chronic radiation exposure (Maffei et al., 2002;Hadjidekova et al., 2003;Sari-Minodier et al., 2007).
Nevertheless, a better understanding of the molecular mechanisms underlying the AR may lead to an improvement in radiation protection, e.g. of astronauts during long-term space flights, risk assessment and management, and cancer treatment strategies (Delone et al., 1991;Wang and Cai, 2000;Upton, 2001;Takahashi et al., 2002;Bonner, 2003;Mortazavi et al., 2003aMortazavi et al., , 2003b;;Liu, 2003;Schaffer et al., 2004;Preston, 2005).In preliminary cytogenetic studies, after in vitro irradiation with a test dose of gamma rays, a strong cytogenetic AR was induced in the lymphocytes of residents of the very high background radiation areas of Ramsar (Ghiassi-nejad et al., 2002).A study of the residents living in radioactively contaminated buildings in Taiwan suggested that chronic irradiation could actually be an effective prophylaxis against cancer (Chen et al., 2004).In support of this idea, lifelong low-dose irradiation accompanied by immune activation was shown to result in suppression of thymic lymphoma induction in mice (Ina et al., 2005).Low-dose irradiation has been used successfully for cancer therapy without causing significant symptoms or presenting significant risk (Cuttler and Pollycove, 2003).In line with these observations, some authors proposed additional coefficients reflecting the protective role of low-dose radiation to be introduced in the mathematical dose-response model for estimation of radiation risk (Scott, 2004;Feinendegen and Neumann, 2006).
In conclusion, a better understanding of AR could open up new approaches for protection of cells.The AR and the ability to modify it may play important roles in fractionated radiotherapy and can help to verify whether this phe-nomenon affects the estimation of the risk of low level radiation exposure.In order to more fully understand the AR, at least two hypothesis need to be tested: firstly, that radiation-induced AR may differ depending on the cell type and genotype; and secondly, that altering cellular oxidative stress levels have an impact on the ability of the cells to initiate the AR.

Figure 3 -
Figure 3 -Adaptive response measured as cell surviving fraction in the radio resistant strain Chlamydomonas reinhardtii H-3.The effect of a conditioning dose (150 Gy) on the response of cells given a series of test doses four hours later is shown.(❏ ) with conditioning dose (150 Gy); (■) without conditioning dose.Modified from Chankova et al. (2005).

Dimova et al. 399 Figure 4 -
Figure 4 -Rejoining kinetics of DNA double-strand breaks in Chlamydomonas reinhardtii CW15 following a test dose of 500 Gy, with (❏ ) or without (■) a conditioning dose (50 Gy), given four hours before the test dose.Error bars are not visible because the standard error is too small.Modified from Chankova and Bryant (2002).

Figure 7 -
Figure 7 -Experimental design used for induction of the AR in Chlamydomonas reinhardtii as a model system.

Figure 8 -
Figure8-The effect of increasing the magnitude of the conditioning dose on the fraction of DNA double-strand breaks remaining after a test dose of 500 Gy and four hours incubation in Chlamydomonas reinhardtii CW15.Error bars are not visible because the standard error is too small.Modified fromChankova and Bryant (2002).