Cellular signal adaptation with damage control at low doses versus the predominance of DNA damage at high doses

Linking lipid peroxidation and neuropsychiatric disorders: focus on 4-hydroxy-2-nonenal

4-hydroxy-2-nonenal (HNE) is considered to be a strong marker of oxidative stress; the interaction between HNE and cellular proteins leads to the formation of HNE-protein adducts able to alter cellular homeostasis and cause the development of a pathological state. By virtue of its high lipid concentration, oxygen utilization, and the presence of metal ions participating to redox reactions, the brain is highly susceptible to the formation of free radicals and HNE-related compounds. A variety of neuropsychiatric disorders have been associated with elevations of HNE concentration. For example, increased levels of HNE were found in the cortex of bipolar and schizophrenic patients, while HNE plasma concentrations resulted high in patients with major depression. On the same line, high brain concentrations of HNE were found associated with Huntington's inclusions. The incidence of high HNE levels is relevant also in the brain and cerebrospinal fluid of patients suffering from Parkinson's disease. Intriguingly, in this case the increase of HNE was associated with an accumulation of iron in the substantia nigra, a brain region highly affected by the pathology. In the present review we recapitulate the findings supporting the role of HNE in the pathogenesis of different neuropsychiatric disorders to highlight the pathogenic mechanisms ascribed to HNE accumulation. The aim of this review is to offer novel perspectives both for the understanding of etiopathogenetic mechanisms that remain still unclear and for the identification of new useful biological markers. We conclude suggesting that targeting HNE-driven cellular processes may represent a new more efficacious therapeutical intervention.

Obesity, Persistent Organic Pollutants and Related Health Problems

The present review aims to delve into persistent organic pollutants (POPs) , as xenobiotics, in correlation to human health. POPs exhibit a group of common characteristics, including lipophilicity, persistence to decomposition and bioaccumulation in tissues. POPs have been thoroughly studied by former researchers, as they offer a particular interest in the elucidation of metabolic, endocrine and immune perturbation caused by their synergy with intracellular mechanisms. Herein particular focus is attributed to the relationship of POPs with obesity provocation. Obesity nowadays receives epidemic dimensions, as its prevalence elevates in an exponential degree. POPs-induced obesity rotates around interfering in metabolic and endocrinal procedures and interacting with peroxisome-proliferator and retinoic receptors. Moreover, polymorphisms in CYP gene families exert a negative result, as they incapacitate detoxification of POPs. Obesity could be deemed as a multidimensional condition, as various factors interact to lead to an obesogenic result. Therefore, concomitant disorders may occur, from mild to lethal, and get intensified due to POPs exposure. POPs exact function mechanisms remain rather enigmatic, thus further investigation should be prospectively performed, for a more lucid picture of this issue, and, consequently for the establishment of alternative solutions.

CDK4-mediated MnSOD activation and mitochondrial homeostasis in radioadaptive protection

Mammalian cells are able to sense environmental oxidative and genotoxic conditions such as the environmental low-dose ionizing radiation (LDIR) present naturally on the earth's surface. The stressed cells then can induce a so-called radioadaptive response with an enhanced cellular homeostasis and repair capacity against subsequent similar genotoxic conditions such as a high dose radiation. Manganese superoxide dismutase (MnSOD), a primary mitochondrial antioxidant in mammals, has long been known to play a crucial role in radioadaptive protection by detoxifying O2(•-) generated by mitochondrial oxidative phosphorylation. In contrast to the well-studied mechanisms of SOD2 gene regulation, the mechanisms underlying posttranslational regulation of MnSOD for radioprotection remain to be defined. Herein, we demonstrate that cyclin D1/cyclin-dependent kinase 4 (CDK4) serves as the messenger to deliver the stress signal to mitochondria to boost mitochondrial homeostasis in human skin keratinocytes under LDIR-adaptive radioprotection. Cyclin D1/CDK4 relocates to mitochondria at the same time as MnSOD enzymatic activation peaks without significant changes in total MnSOD protein level. The mitochondrial-localized CDK4 directly phosphorylates MnSOD at serine-106 (S106), causing enhanced MnSOD enzymatic activity and mitochondrial respiration. Expression of mitochondria-targeted dominant negative CDK4 or the MnSOD-S106 mutant reverses LDIR-induced mitochondrial enhancement and adaptive protection. The CDK4-mediated MnSOD activation and mitochondrial metabolism boost are also detected in skin tissues of mice receiving in vivo whole-body LDIR. These results demonstrate a unique CDK4-mediated mitochondrial communication that allows cells to sense environmental genotoxic stress and boost mitochondrial homeostasis by enhancing phosphorylation and activation of MnSOD.

Does short-term exposure to elevated levels of natural gamma radiation in Ramsar cause oxidative stress?

BACKGROUND

Ramsar, a city in northern Iran, has areas with some of the highest recorded levels of natural radiation among inhabited areas measured on the earth.

AIMS

To determine whether short-term exposure to extremely high levels of natural radiation induce oxidative stress.

MATERIALS AND METHODS

In this study, 53 Wistar rats were randomly divided into five groups of 10-12 animals. Animals in the 1(st) group were kept for 7 days in an outdoor area with normal background radiation while the 2(nd) , 3(rd) , 4(th) and 5(th) groups were kept in four different outdoor areas with naturally elevated levels of gamma radiation in Ramsar. A calibrated RDS-110 survey meter, mounted on a tripod approximately 1 m above the ground, was used to measure exposure rate at each location. On days 7 and 9 blood sampling was performed to assess the serum levels of catalase (CAT) and malondialdehyde (MDA). On day 8, all animals were exposed to a lethal dose of 8 Gy gamma radiations emitted by a Theratron Phoenix (Theratronics, Canada) Cobalt-60 (55 cGy/min) at Radiotherapy Department of Razi Hospital in Rasht, Iran.

RESULTS

Findings obtained in this study indicate that high levels of natural radiation cannot induce oxidative stress. CAT and MDA levels in almost all groups were not significantly different (P = 0.69 and P = 0.05, respectively). After exposure to the lethal dose, CAT and MDA levels in all groups were not significantly different (P = 0.054 and P = 0.163, respectively).

CONCLUSIONS

These findings indicate that short-term exposure to extremely high levels of natural radiation (up to 196 times higher than the normal background) does not induce oxidative stress.

Cell cycle regulators guide mitochondrial activity in radiation-induced adaptive response

SIGNIFICANCE

There are accruing concerns on potential genotoxic agents present in the environment including low-dose ionizing radiation (LDIR) that naturally exists on earth's surface and atmosphere and is frequently used in medical diagnosis and nuclear industry. Although its long-term health risk is being evaluated and remains controversial, LDIR is shown to induce temporary but significant adaptive responses in mammalian cells and animals. The mechanisms guiding the mitochondrial function in LDIR-induced adaptive response represent a unique communication between DNA damage and cellular metabolism. Elucidation of the LDIR-regulated mitochondrial activity may reveal new mechanisms adjusting cellular function to cope with hazardous environmental stress.

RECENT ADVANCES

Key cell cycle regulators, including Cyclin D1/CDK4 and Cyclin B1/cyclin-dependent kinase 1 (CDK1) complexes, are actively involved in the regulation of mitochondrial functions via phosphorylation of their mitochondrial targets. Accumulating new evidence supports a concept that the Cyclin B1/CDK1 complex acts as a mediator in the cross talk between radiation-induced DNA damage and mitochondrial functions to coordinate cellular responses to low-level genotoxic stresses.

CRITICAL ISSUES

The LDIR-mediated mitochondrial activity via Cyclin B1/CDK1 regulation is an irreplaceable network that is able to harmonize vital cellular functions with adjusted mitochondrial metabolism to enhance cellular homeostasis.

FUTURE DIRECTIONS

Further investigation of the coordinative mechanism that regulates mitochondrial activities in sublethal stress conditions, including LDIR, will reveal new insights of how cells cope with genotoxic injury and will be vital for future targeted therapeutic interventions that reduce environmental injury and cancer risk.

Protective properties of radio-chemoresistant glioblastoma stem cell clones are associated with metabolic adaptation to reduced glucose dependence

Glioblastoma stem cells (GSC) are a significant cell model for explaining brain tumor recurrence. However, mechanisms underlying their radiochemoresistance remain obscure. Here we show that most clonogenic cells in GSC cultures are sensitive to radiation treatment (RT) with or without temozolomide (TMZ). Only a few single cells survive treatment and regain their self-repopulating capacity. Cells re-populated from treatment-resistant GSC clones contain more clonogenic cells compared to those grown from treatment-sensitive GSC clones, and repeated treatment cycles rapidly enriched clonogenic survival. When compared to sensitive clones, resistant clones exhibited slower tumor development in animals. Upregulated genes identified in resistant clones via comparative expression microarray analysis characterized cells under metabolic stress, including blocked glucose uptake, impaired insulin/Akt signaling, enhanced lipid catabolism and oxidative stress, and suppressed growth and inflammation. Moreover, many upregulated genes highlighted maintenance and repair activities, including detoxifying lipid peroxidation products, activating lysosomal autophagy/ubiquitin-proteasome pathways, and enhancing telomere maintenance and DNA repair, closely resembling the anti-aging effects of caloric/glucose restriction (CR/GR), a nutritional intervention that is known to increase lifespan and stress resistance in model organisms. Although treatment–introduced genetic mutations were detected in resistant clones, all resistant and sensitive clones were subclassified to either proneural (PN) or mesenchymal (MES) glioblastoma subtype based on their expression profiles. Functional assays demonstrated the association of treatment resistance with energy stress, including reduced glucose uptake, fatty acid oxidation (FAO)-dependent ATP maintenance, elevated reactive oxygen species (ROS) production and autophagic activity, and increased AMPK activity and NAD⁺ levels accompanied by upregulated mRNA levels of SIRT1/PGC-1α axis and DNA repair genes. These data support the view that treatment resistance may arise from quiescent GSC exhibiting a GR-like phenotype, and suggest that targeting stress response pathways of resistant GSC may provide a novel strategy in combination with standard treatment for glioblastoma.

Non-linear adaptive phenomena which decrease the risk of infection after pre-exposure to radiofrequency radiation

Substantial evidence indicates that adaptive response induced by low doses of ionizing radiation can result in resistance to the damage caused by a subsequently high-dose radiation or cause cross-resistance to other non-radiation stressors. Adaptive response contradicts the linear-non-threshold (LNT) dose-response model for ionizing radiation. We have previously reported that exposure of laboratory animals to radiofrequency radiation can induce a survival adaptive response. Furthermore, we have indicated that pre-exposure of mice to radiofrequency radiation emitted by a GSM mobile phone increased their resistance to a subsequent Escherichia coli infection. In this study, the survival rates in animals receiving both adapting (radiofrequency) and challenge dose (bacteria) and the animals receiving only the challenge dose (bacteria) were 56% and 20%, respectively. In this light, our findings contribute to the assumption that radiofrequency-induced adaptive response can be used as an efficient method for decreasing the risk of infection in immunosuppressed irradiated individuals. The implication of this phenomenon in human's long term stay in the space is also discussed.

Hormesis in aging and neurodegeneration-a prodigy awaiting dissection

Hormesis describes the drug action of low dose stimulation and high dose inhibition. The hormesis phenomenon has been observed in a wide range of biological systems. Although known in its descriptive context, the underlying mode-of-action of hormesis is largely unexplored. Recently, the hormesis concept has been receiving increasing attention in the field of aging research. It has been proposed that within a certain concentration window, reactive oxygen species (ROS) or reactive nitrogen species (RNS) could act as major mediators of anti-aging and neuroprotective processes. Such hormetic phenomena could have potential therapeutic applications, if properly employed. Here, we review the current theories of hormetic phenomena in regard to aging and neurodegeneration, with the focus on its underlying mechanism. Facilitated by a simple mathematical model, we show for the first time that ROS-mediated hormesis can be explained by the addition of different biomolecular reactions including oxidative damage, MAPK signaling and autophagy stimulation. Due to their divergent scales, the optimal hormetic window is sensitive to each kinetic parameter, which may vary between individuals. Therefore, therapeutic utilization of hormesis requires quantitative characterizations in order to access the optimal hormetic window for each individual. This calls for a personalized medicine approach for a longer human healthspan.

Manganese superoxide dismutase interacts with a large scale of cellular and mitochondrial proteins in low-dose radiation-induced adaptive radioprotection

The cellular adaptive response to certain low-level genotoxic stresses, including exposure to low-dose ionizing radiation (LDIR), shows promise as a tool to enhance radioprotection in normal cells but not in tumor cells. Manganese superoxide dismutase (MnSOD), a fundamental mitochondrial antioxidant in mammalian cells, plays a key role in the LDIR-induced adaptive response. In this study, we aimed to elucidate the signaling network associated with MnSOD-induced radiation protection. A MnSOD-interacting protein profile was established in LDIR-treated human skin cells. Human skin keratinocytes (HK18) were irradiated with a single dose of LDIR (10 cGy X-ray) and the cell lysates were immunoprecipitated using α-MnSOD and applied to two different gel-based proteomic experiments followed by mass spectrometry for protein identification. Analysis of the profiles of MnSOD-interacting partners before and after LDIR detected various patterns of MnSOD protein-protein interactions in response to LDIR. Interestingly, many of the MnSOD-interacting proteins are known to have functions related to mitochondrial regulation of cell metabolism, apoptosis, and DNA repair. These results provide evidence indicating that in addition to the enzymatic action of detoxifying superoxide, the antioxidant MnSOD may function as a signaling regulator in stress-induced adaptive protection through cell survival pathways.

Increased Radioresistance to Lethal Doses of Gamma Rays in Mice and Rats after Exposure to Microwave Radiation Emitted by a GSM Mobile Phone Simulator

The aim of this study was to investigate the effect of pre-irradiation with microwaves on the induction of radioadaptive response. In the 1(st) phase of the study, 110 male mice were divided into 8 groups. The animals in these groups were exposed/sham-exposed to microwave, low dose rate gamma or both for 5 days. On day six, the animals were exposed to a lethal dose (LD). In the 2(nd) phase, 30 male rats were divided into 2 groups of 15 animals. The 1(st) group received microwave exposure. The 2(nd) group (controls) received the same LD but there was no treatment before the LD. On day 5, all animals were whole-body irradiated with the LD. Statistically significant differences between the survival rate of the mice only exposed to lethal dose of gamma radiation before irradiation with a lethal dose of gamma radiation with those of the animals pre-exposed to either microwave (p=0.02), low dose rate gamma (p=0.001) or both of these physical adapting doses (p=0.003) were observed. Likewise, a statistically significant difference between survival rates of the rats in control and test groups was observed. Altogether, these experiments showed that exposure to microwave radiation may induce a significant survival adaptive response.

Changing paradigms in radiobiology

The last 25 years have seen a major shift in emphasis in the field of radiobiology from a DNA-centric view of how radiation damage occurs to a much more biological view that appreciates the importance of macro-and micro-environments, hierarchical organization, underlying genetics, evolution, adaptation and signaling at all levels from atoms to ecosystems. The new view incorporates concepts of hormesis, nonlinear systems, bioenergy field theory, uncertainty and homeodynamics. While the mechanisms underlying these effects and responses are still far from clear, it is very apparent that their implications are much wider than the field of radiobiology. This reflection discusses the changing views and considers how they are influencing thought in environmental and medical science and systems biology.

Computational modeling of signaling pathways mediating cell cycle checkpoint control and apoptotic responses to ionizing radiation-induced DNA damage

The shape of dose response of ionizing radiation (IR) induced cancer at low dose region, either linear non-threshold or J-shaped, has been a debate for a long time. This dose response relationship can be influenced by built-in capabilities of cells that minimize the fixation of IR-mediated DNA damage as pro-carcinogenic mutations. Key capabilities include sensing of damage, activation of cell cycle checkpoint arrests that provide time needed for repair of the damage as well as apoptosis. Here we describe computational modeling of the signaling pathways that link sensing of DNA damage and checkpoint arrest activation/apoptosis to investigate how these molecular-level interactions influence the dose response relationship for IR induced cancer. The model provides qualitatively accurate descriptions of the IR-mediated activation of cell cycle checkpoints and the apoptotic pathway, and of time-course activities and dose response of relevant regulatory proteins (e.g. p53 and p21). Linking to a two-stage clonal growth cancer model, the model described here successfully captured a monotonically increasing to a J-shaped dose response curve and identified one potential mechanism leading to the J-shape: the cell cycle checkpoint arrest time saturates with the increase of the dose.

Delayed effects of exposure to a moderate radiation dose on transcription profiles in human primary fibroblasts

Ionizing radiation (IR) is used in a wide variety of medical and nonmedical applications and poses a potential threat to human health. Knowledge of changes in gene expression in irradiated cells may be helpful for the establishment of effective paradigms for radiation protection. IR-induced DNA damage triggers a complex cascade of signal transduction. Recently, genome-wide approaches have allowed the detection of alterations in gene expression across a wide range of radiation doses. However, the delayed or long-term biological effects of mild-doses of IR remain largely unknown. The main objective of the present study was to investigate the effects of a moderate dose of gamma-rays (50 cGy) on gene expression 6 days post-irradiation. Gene expression using cDNA microarrays revealed statistically significant changes in the expression of 59 genes (FDR < 0.07), whose functions are related to cell-cycle control, protein trafficking, ubiquitin cycle, Rho-GTPAse pathway, protein phosphatase signalization, oxidoreductase control, and stress response. A set of 464 genes was also selected by a less stringent approach, and we demonstrate that this broader set of genes can efficiently distinguish the irradiated samples from the unirradiated, defining a long-term IR signature in human primary fibroblasts. Our findings support the existence of persistent responses to mild doses of IR detectable by changes in gene expression profiles. These results provide insight into delayed effects observed in human primary cells as well as the role of long-term response in neoplastic transformation. Environ.

Low-dose cancer risk modeling must recognize up-regulation of protection

IONIZING RADIATION PRIMARILY PERTURBS THE BASIC MOLECULAR LEVEL PROPORTIONAL TO DOSE, WITH POTENTIAL DAMAGE PROPAGATION TO HIGHER LEVELS: cells, tissues, organs, and whole body. There are three types of defenses against damage propagation. These operate deterministically and below a certain impact threshold there is no propagation. Physical-static defenses precede metabolic-dynamic defenses acting immediately: scavenging of toxins; - molecular repair, especially of DNA; - removal of damaged cells either by apoptosis, necrosis, phagocytosis, cell differentiation-senescence, or by immune responses, - followed by replacement of lost elements. Another metabolic-dynamic defense arises delayed by up-regulating immediately operating defense mechanisms. Some of these adaptive protections may last beyond a year and all create temporary protection against renewed potentially toxic impacts also from non-radiogenic endogenous sources. Adaptive protections have a maximum after single tissue absorbed doses around 100 to 200 mSv and disappear with higher doses. Low dose rates initiate maximum protection likely at lower cell doses delivered repetitively at certain time intervals. Adaptive protection preventing only about 2 - 3 % of endogenous life-time cancer risk would fully balance a calculated induced cancer risk at about 100 mSv, in agreement with epidemiological data and concordant with an hormetic effect. Low-dose-risk modeling must recognize up-regulation of protection.

Coactivation of ATM/ERK/NF-kappaB in the low-dose radiation-induced radioadaptive response in human skin keratinocytes

Elucidating the molecular mechanism of the low-dose radiation (LDR)-mediated radioadaptive response is crucial for inventing potential therapeutic approaches to improving normal tissue protection in radiation therapy. ATM, a DNA-damage sensor, is known to activate the stress-sensitive transcription factor NF-kappaB upon exposure to ionizing radiation. This study provides evidence of the cooperative functions of ATM, ERK, and NF-kappaB in inducing a survival advantage through a radioadaptive response as a result of LDR treatment (10 cGy X-rays). By using p53-inhibited human skin keratinocytes, we show that phosphorylation of ATM, MEK, and ERK (but not JNK or p38) is enhanced along with a twofold increase in NF-kappaB luciferase activity at 24 h post-LDR. However, NF-kappaB reporter gene transactivation without a significant enhancement of p65 or p50 protein level suggests that NF-kappaB is activated as a rapid protein response via ATM without involving the transcriptional activation of NF-kappaB subunit genes. A direct interaction between ATM and NF-kappaB p65 is detected in the resting cells and this interaction is significantly increased with LDR treatment. Inhibition of ATM with caffeine, KU-55933, or siRNA or inhibition of the MEK/ERK pathway can block the LDR-induced NF-kappaB activation and eliminate the LDR-induced survival advantage. Altogether, these results suggest a p53-independent prosurvival network involving the coactivation of the ATM, MEK/ERK, and NF-kappaB pathways in LDR-treated human skin keratinocytes, which is absent from mutant IkappaB cells (HK18/mIkappaB), which fail to express NF-kappaB activity.

Systems biology and its potential role in radiobiology

About a century ago, Conrad Röentgen discovered X-rays, and Henri Becquerel discovered a new phenomenon, which Marie and Pierre Curie later coined as radio-activity. Since their seminal work, we have learned much about the physical properties of radiation and its effects on living matter. Alas, the more we discover, the more we appreciate the complexity of the biological processes that are triggered by radiation exposure and eventually lead (or do not lead) to disease. Equipped with modern biological methods of high-throughput experimentation, imaging, and vastly increased computational prowess, we are now entering an era where we can piece some of the multifold aspects of radiation exposure and its sequelae together, and develop a more systemic understanding of radiogenic effects such as radio-carcinogenesis than has been possible in the past. It is evident from the complexity of even the known processes that such an understanding can only be gained if it is supported by mathematical models. At this point, the construction of comprehensive models is hampered both by technical inadequacies and a paucity of appropriate data. Nonetheless, some initial steps have been taken already and the generally increased interest in systems biology may be expected to speed up future progress. In this context, we discuss in this article examples of relatively small, yet very useful models that elucidate selected aspects of the effects of exposure to ionizing radiation and may shine a light on the path before us.

Induction of DNA double-strand breaks by zeocin in Chlamydomonas reinhardtii and the role of increased DNA double-strand breaks rejoining in the formation of an adaptive response

This study aimed to test the potential of the radiomimetic chemical zeocin to induce DNA double-strand breaks (DSB) and "adaptive response" (AR) in Chlamydomonas reinhardtii strain CW15 as a model system. The AR was measured as cell survival using a micro-colony assay, and by changes in rejoining of DSB DNA. The level of induced DSB was measured by constant field gel electrophoresis based on incorporation of cells into agarose blocks before cell lysis. This avoids the risk of accidental induction of DSB during the manipulation procedures. Our results showed that zeocin could induce DSB in C. reinhardtii strain CW15 in a linear dose-response fashion up to 100 microg ml(-1) which marked the beginning of a plateau. The level of DSB induced by 100 microg ml(-1) zeocin was similar to that induced by 250 Gy of gamma-ray irradiation. It was also found that, similar to gamma rays, zeocin could induce AR measured as DSB in C. reinhardtii CW15 and this AR involved acceleration of the rate of DSB rejoining, too. To our knowledge, this is the first demonstration that zeocin could induce AR in some low eukaryotes such as C. reinhardtii.

Adaptive responses to low-dose/low-dose-rate gamma rays in normal human fibroblasts: the role of growth architecture and oxidative metabolism

To investigate low-dose/low-dose-rate effects of low-linear energy transfer (LET) ionizing radiation, we used gamma-irradiated cells adapted to grow in a three-dimensional architecture that mimics cell growth in vivo. We determined the cellular, molecular and biochemical changes in these cells. Quiescent normal human fibroblasts were irradiated with single acute or chronic doses (1-10 cGy) of (137)Cs gamma rays. Whereas exposure to an acute dose of 10 cGy increased micronucleus formation, protraction of the dose over 48 h reduced micronucleus frequency to a level similar to or lower than what occurs spontaneously. The protracted treatment also up-regulated the cellular content of the antioxidant glutathione. These changes correlated with modulation of phospho-TP53 (serine 15), a stress marker that was regulated by doses as low as 1 cGy. The DNA damage that occurred after exposure to an acute dose of 10 cGy was protected against in two ways: (1) up-regulation of cellular antioxidant enzyme activity by ectopic overexpression of MnSOD, catalase or glutathione peroxidase, and (2) inhibition of superoxide anion generation by flavin-containing oxidases. These results support a significant role for oxidative metabolism in mediating low-dose radiation effects and demonstrate that cell culture in three dimensions is ideal to investigate radiation-induced adaptive responses. Expression of connexin 43, a constitutive protein of gap junctions, and the G(1) checkpoint were more sensitive to regulation by gamma rays in cells maintained in a three-dimensional than in a two-dimensional configuration.

What are the risks from medical X-rays and other low dose radiation?

The magnitude of the risks from low doses of radiation is one of the central questions in radiological protection. It is particularly relevant when discussing the justification and optimization of diagnostic medical exposures. Medical X-rays can undoubtedly confer substantial benefits in the healthcare of patients, but not without exposing them to effective doses ranging from a few microsieverts to a few tens of millisieverts. Do we have any evidence that these levels of exposure result in significant health risks to patients? The current consensus held by national and international radiological protection organizations is that, for these comparatively low doses, the most appropriate risk model is one in which the risk of radiation-induced cancer and hereditary disease is assumed to increase linearly with increasing radiation dose, with no threshold (the so-called linear no threshold (LNT) model). However, the LNT hypothesis has been challenged both by those who believe that low doses of radiation are more damaging than the hypothesis predicts and by those who believe that they are less harmful, and possibly even beneficial (often referred to as hormesis). This article reviews the evidence for and against both the LNT hypothesis and hormesis, and explains why the general scientific consensus is currently in favour of the LNT model as the most appropriate dose-response relationship for radiation protection purposes at low doses. Finally, the impact of the LNT model on the assessment of the risks from medical X-rays and how this affects the justification and optimization of such exposures is discussed.

Patient dose in multislice CT: why is it increasing and does it matter?

A brief review is presented of the reasons why multislice spiral/helical CT is associated with a higher radiation dose burden to the patient even than incremental CT. These include both intrinsic technological and geometric factors as well as simply a growing use of CT in an increasing number of applications. The typical magnitude of this dose burden is indicated and the basis for the anxiety that underpins it, namely the linear no-threshold (LNT) hypothesis, is discussed, together with the countervailing hypothesis that there is indeed a threshold for radiation harm in man and that the radiation doses associated with CT may lie below this threshold and may even be beneficial (radiation hormesis). There are as yet no certainties in this important area but it is argued that it is not a given that the doses associated with CT are harmful.

Low-dose radiation and genotoxic chemicals can protect against stochastic biological effects

A protective apoptosis-mediated (PAM) process that is turned on in mammalian cells by low-dose photon (X and gamma) radiation and appears to also be turned on by the genotoxic chemical ethylene oxide is discussed. Because of the PAM process, exposure to low-dose photon radiation (and possibly also some genotoxic chemicals) can lead to a reduction in the risk of stochastic effects such as problematic mutations, neoplastic transformation (an early step in cancer occurrence), and cancer. These findings indicate a need to revise the current low-dose risk assessment paradigm for which risk of cancer is presumed to increase linearly with dose (without a threshold) after exposure to any amount of a genotoxic agent such as ionizing radiation. These findings support a view seldom mentioned in the past, that cancer risk can actually decrease, rather than increase, after exposure to low doses of photon radiation and possibly some other genotoxic agents. The PAM process (a form of natural protection) may contribute substantially to cancer prevention in humans and other mammals. However, new research is needed to improve our understanding of the process. The new research could unlock novel strategies for optimizing cancer prevention and novel protocols for low-dose therapy for cancer. With low-dose cancer therapy, normal tissue could be spared from severe damage while possibly eliminating the cancer.

p53 activation in chronic radiation-treated breast cancer cells: regulation of MDM2/p14ARF

Mammalian cells chronically exposed to ionizing radiation (IR) induce stress response with a tolerance to the subsequent cytotoxicity of IR. Although p53 is well documented in IR response, the signaling network causing p53 activation in chronic IR remains to be identified. Using breast carcinoma MCF+FIR cells that showed a transient radioresistance after exposure chronically to fractionated IR (FIR), the present study shows that the basal DNA binding and transcriptional activity of p53 was elevated by FIR. p53-controlled luciferase activity was strikingly induced ( approximately 7.9-fold) with little enhancement of p53/DNA binding activity ( approximately 1.3-fold). The phosphorylated p53 (Thr 55) was increased in the cytoplasm and nucleus of MCF+FIR but not in the sham-FIR control cells. On the contrary, the sham-FIR control MCF-7 cells showed a low p53 luciferase transcription ( approximately 3-fold) but a striking enhancement of p53/DNA binding (12-fold) after 5 Gy of IR. To determine the signaling elements regulating p53 activity, DNA microarray of MCF+FIR using sham-FIR MCF-7 cells as a reference demonstrated that the mRNA of p21, MDM2, and p14ARF was up-regulated. Time course Western blot analysis, however, showed no difference in p21 induction. In contrast, MDM2 that was absent in control cells and was predominantly induced by IR was not induced in MCF+FIR cells. In agreement with MDM2 inhibition, MDM2-inhibitory protein p14ARF was increased in MCF+FIR cells. In summary, these results demonstrate that up-regulation of p14ARF paralleled with MDM2 inhibition contributes to p53 accumulation in the nucleus and causes a high responsiveness of p53 in chronic IR-treated breast cancer cells.

P16INK4A and p15INK4B gene alteration associated with oxidative stress in renal cell carcinomas after the chernobyl accident (pilot study)

Our study was undertaken to better understand the role of G1/S transition abnormalities in the malignant progression of renal cell carcinomas (RCCs), exposed to long-term low doses of ionizing radiation (IR), from patients living in radiocontaminated areas of the Ukraine after the Chernobyl accident. We studied p16 and p15 gene alteration in association with oxidative stress markers, including inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX2). We analyzed 88 samples collected from 22 patients with RCCs and with different exposure to IR. Homozygous deletion of the p16 and p15 genes, as well as hypermethylation of the 5CpG island in the promoter region of the same genes, were analyzed by differential PCR and Methylation-Specific PCR respectively, in association with histopathology and immunohistochemical analysis of p16 and p15 proteins. COX2 and iNOS expression in the same tumors were likewise analyzed. Aberrant hypermethylation was observed in 7 (32%) and 5 (23%) cases accompanied, by immunohistochemical loss of expression for p16 and p15 genes respectively, in both high stage and grade tumors from patients living in radiocontaminated areas, this being especially outstanding for the p16 gene. An association with COX2 and less iNOS overexpression in the same tumors was observed. Our data suggest that inactivation of p16 gene, but not p15, induced by increased oxidative stress generated by persistent chronic exposure to IR, could be one of the major pathways responsible for RCCs malignant progression.

Reactive oxygen species in cell responses to toxic agents

This review first summarizes experimental data on biological effects of different concentrations of ROS in mammalian cells and on their potential role in modifying cell responses to toxic agents. It then attempts to link the role of steadily produced metabolic ROS at various concentrations in mammalian cells to that of environmentally derived ROS bursts from exposure to ionizing radiation. The ROS from both sources are known to both cause biological damage and change cellular signaling, depending on their concentration at a given time. At low concentrations signaling effects of ROS appear to protect cellular survival and dominate over damage, and the reverse occurs at high ROS concentrations. Background radiation generates suprabasal ROS bursts along charged particle tracks several times a year in each nanogram of tissue, i.e., average mass of a mammalian cell. For instance, a burst of about 200 ROS occurs within less than a microsecond from low-LET irradiation such as X-rays along the track of a Compton electron (about 6 keV, ranging about 1 microm). One such track per nanogram tissue gives about 1 mGy to this mass. The number of instantaneous ROS per burst along the track of a 4-meV alpha-particle in 1 ng tissue reaches some 70000. The sizes, types and sites of these bursts, and the time intervals between them directly in and around cells appear essential for understanding low-dose and low dose-rate effects on top of effects from endogenous ROS. At background and low-dose radiation exposure, a major role of ROS bursts along particle tracks focuses on ROS-induced apoptosis of damage-carrying cells, and also on prevention and removal of DNA damage from endogenous sources by way of temporarily protective, i.e., adaptive, cellular responses. A conclusion is to consider low-dose radiation exposure as a provider of physiological mechanisms for tissue homoeostasis.

[Ionizing radiation]

Everyone is exposed to radiation from natural, man-made and medical sources, and world-wide average annual exposure can be set at about 3.5 mSv. Exposure to natural sources is characterised by very large fluctuations, not excluding a range covering two orders of magnitude. Millions of inhabitants are continuously exposed to external doses as high as 10 mSv per year, delivered at low dose rates, very few workers are exposed above the legal limit of 50 mSv/year, and referring to accidental exposures, only 5% of the 116,000 people evacuated following the Chernobyl disaster encountered doses above 100 mSv. Epidemiological survey of accidentally, occupationally or medically exposed groups have revealed radio-induced cancers, mostly following high dose-rate exposure levels, only above 100 mSv. Risk coefficients were derived from these studies and projected into linear models of risk (linear non-threshold hypothesis: LNT), for the purpose of risk management following exposures at low doses and low dose-rates. The legitimacy of this approach has been questioned, by the Academy of sciences and the Academy of medicine in France, arguing: that LNT was not supported by Hiroshima and Nagasaki studies when neutron dose was revisited; that linear modelling failed to explain why so many site-related cancers were obviously non-linearly related to the dose, and especially when theory predicted they ought to be; that no evidence could be found of radio-induced cancers related to natural exposures or to low exposures at the work place; and that no evidence of genetic disease could be shown from any of the exposed groups. Arguments were provided from cellular and molecular biology helping to solve this issue, all resulting in dismissing the LNT hypothesis. These arguments included: different mechanisms of DNA repair at high and low dose rate; influence of inducible stress responses modifying mutagenesis and lethality; bystander effects allowing it to be considered that individual cellular responses reflected in fact the results of multiple cellular interactions. Following the conclusion of the French Academy of medicine, LNT modelling resulted in public anxiety by changing an hypothetical residual risk at low doses into a real one, calling on regulators, continuously, for a more and more severe control of tiny sources which may result in considerable collective doses when considered as being exposed to billions of people for hundreds of years. Examples were provided that showed that the perception of risk of radioactive sources was not related to the severity of the risk itself but to the importance attributed to the situation by the media. In some instances, such as those resulting from the loss of gammagraphy sources, it resulted in a dangerous underestimate of the necessary remedial actions.