Induction and prevention of carcinogenesis in rat skin exposed to space radiation

Drug Discovery Targeting Post-Translational Modifications in Response to DNA Damages Induced by Space Radiation

DNA damage in astronauts induced by cosmic radiation poses a major barrier to human space exploration. Cellular responses and repair of the most lethal DNA double-strand breaks (DSBs) are crucial for genomic integrity and cell survival. Post-translational modifications (PTMs), including phosphorylation, ubiquitylation, and SUMOylation, are among the regulatory factors modulating a delicate balance and choice between predominant DSB repair pathways, such as non-homologous end joining (NHEJ) and homologous recombination (HR). In this review, we focused on the engagement of proteins in the DNA damage response (DDR) modulated by phosphorylation and ubiquitylation, including ATM, DNA-PKcs, CtIP, MDM2, and ubiquitin ligases. The involvement and function of acetylation, methylation, PARylation, and their essential proteins were also investigated, providing a repository of candidate targets for DDR regulators. However, there is a lack of radioprotectors in spite of their consideration in the discovery of radiosensitizers. We proposed new perspectives for the research and development of future agents against space radiation by the systematic integration and utilization of evolutionary strategies, including multi-omics analyses, rational computing methods, drug repositioning, and combinations of drugs and targets, which may facilitate the use of radioprotectors in practical applications in human space exploration to combat fatal radiation hazards.

Contemporary review of dermatologic conditions in space flight and future implications for long-duration exploration missions

BACKGROUND

Future planned exploration missions to outer space will almost surely require the longest periods of continuous space exposure by the human body yet. As the most external organ, the skin seems the most vulnerable to injury. Therefore, discussion of the dermatological implications of such extended-duration missions is critical.

OBJECTIVES

In order to help future missions understand the risks of spaceflight on the human skin, this review aims to consolidate data from the current literature pertaining to the space environment and its physiologic effects on skin, describe all reported dermatologic manifestations in spaceflight, and extrapolate this information to longer-duration mission.

METHODS AND MATERIALS

The authors searched PubMed and Google Scholar using keywords and Mesh terms. The publications that were found to be relevant to the objectives were included and described.

RESULTS

The space environment causes changes in the skin at the cellular level by thinning the epidermis, altering wound healing, and dysregulating the immune system. Clinically, dermatological conditions represented the most common medical issues occurring in spaceflight. We predict that as exploration missions increase in duration, astronauts will experience further physiological changes and an increased rate and severity of adverse events.

CONCLUSION

Maximizing astronaut safety requires a continued knowledge of the human body's response to space, as well as consideration and prediction of future events. Dermatologic effects of space missions comprise the majority of health-related issues arising on missions to outer space, and these issues are likely to become more prominent with increasing time spent in space. Improvements in hygiene may mitigate some of these conditions.

Countermeasure efficacy of apigenin for silicon-ion-induced early damage in blood and bone marrow of exposed C57BL/6J mice

We investigated the countermeasure efficacy of apigenin (AP), given as a diet supplement, for radiation-induced damage in the hematopoietic tissues collected on day 7 after a total-body exposure of male C57BL/6J mice to 0 or 0.5 Gy of 260 MeV/n silicon (²⁸Si) ions. We gave food with AP at the concentration of 20 mg/kg body weight (bw) (AP20) or without AP (AP0) to mice before and after irradiation. There were four groups of mice (six mice in each): Group 1- Control, i.e. No Radiation (0 Gy) with AP0; Group 2 - Radiation (0.5 Gy) with AP0; Group 3 - No Radiation (0 Gy) with AP20; and Group 4 - Radiation (0.5 Gy) with AP20. The complete blood count (CBC) and differential blood count were performed for each mouse. In the same mouse, an anti-clastogenic activity of AP was evaluated using the in vivo blood-erythrocyte micronucleus (MN) assay. Further in each mouse, bone marrow (BM) cells were collected and used for measuring the levels of activated nuclear factor-kappa B (NF-κB), and pro-inflammatory cytokines (i.e. tumor necrotic factor-alpha (TNF-α), interleukin-1α (IL-1α), IL-1 beta (IL-1β), and IL-6). We used the colony-forming unit assay (CFU-A) as a tool to study the countermeasure efficacy of AP against the harmful effects of ²⁸Si ions on the proliferation of the hematopoietic stem/progenitor cells (HSPCs). Our results showed that AP is highly effective not only in the prevention of leukopenia and thrombocytopenia but also in the enhancement of erythropoiesis and the proliferation of HSPCs. We also observed the potent anti-clastogenic activity of AP given to mice as a diet supplement. Further, we found that AP is very effective in the suppression of activated NF-κB and pro-inflammatory cytokines, suggesting that AP given as a diet supplement protects mice from ²⁸Si-ion-induced damage in the hematopoietic tissues of irradiated male C57BL/6J mice via its anti-inflammation activity.

Genomic Changes Driven by Radiation-Induced DNA Damage and Microgravity in Human Cells

The space environment consists of a complex mixture of different types of ionizing radiation and altered gravity that represents a threat to humans during space missions. In particular, individual radiation sensitivity is strictly related to the risk of space radiation carcinogenesis. Therefore, in view of future missions to the Moon and Mars, there is an urgent need to estimate as accurately as possible the individual risk from space exposure to improve the safety of space exploration. In this review, we survey the combined effects from the two main physical components of the space environment, ionizing radiation and microgravity, to alter the genetics and epigenetics of human cells, considering both real and simulated space conditions. Data collected from studies on human cells are discussed for their potential use to estimate individual radiation carcinogenesis risk from space exposure.

Fundamental Biological Features of Spaceflight: Advancing the Field to Enable Deep-Space Exploration

Research on astronaut health and model organisms have revealed six features of spaceflight biology that guide our current understanding of fundamental molecular changes that occur during space travel. The features include oxidative stress, DNA damage, mitochondrial dysregulation, epigenetic changes (including gene regulation), telomere length alterations, and microbiome shifts. Here we review the known hazards of human spaceflight, how spaceflight affects living systems through these six fundamental features, and the associated health risks of space exploration. We also discuss the essential issues related to the health and safety of astronauts involved in future missions, especially planned long-duration and Martian missions.

Relative Biological Effectiveness of HZE Particles for Chromosomal Exchanges and Other Surrogate Cancer Risk Endpoints

The biological effects of high charge and energy (HZE) particle exposures are of interest in space radiation protection of astronauts and cosmonauts, and estimating secondary cancer risks for patients undergoing Hadron therapy for primary cancers. The large number of particles types and energies that makeup primary or secondary radiation in HZE particle exposures precludes tumor induction studies in animal models for all but a few particle types and energies, thus leading to the use of surrogate endpoints to investigate the details of the radiation quality dependence of relative biological effectiveness (RBE) factors. In this report we make detailed RBE predictions of the charge number and energy dependence of RBE’s using a parametric track structure model to represent experimental results for the low dose response for chromosomal exchanges in normal human lymphocyte and fibroblast cells with comparison to published data for neoplastic transformation and gene mutation. RBE’s are evaluated against acute doses of γ-rays for doses near 1 Gy. Models that assume linear or non-targeted effects at low dose are considered. Modest values of RBE (<10) are found for simple exchanges using a linear dose response model, however in the non-targeted effects model for fibroblast cells large RBE values (>10) are predicted at low doses <0.1 Gy. The radiation quality dependence of RBE’s against the effects of acute doses γ-rays found for neoplastic transformation and gene mutation studies are similar to those found for simple exchanges if a linear response is assumed at low HZE particle doses. Comparisons of the resulting model parameters to those used in the NASA radiation quality factor function are discussed.

Harderian Gland Tumorigenesis: Low-Dose and LET Response

Increased cancer risk remains a primary concern for travel into deep space and may preclude manned missions to Mars due to large uncertainties that currently exist in estimating cancer risk from the spectrum of radiations found in space with the very limited available human epidemiological radiation-induced cancer data. Existing data on human risk of cancer from X-ray and gamma-ray exposure must be scaled to the many types and fluences of radiations found in space using radiation quality factors and dose-rate modification factors, and assuming linearity of response since the shapes of the dose responses at low doses below 100 mSv are unknown. The goal of this work was to reduce uncertainties in the relative biological effect (RBE) and linear energy transfer (LET) relationship for space-relevant doses of charged-particle radiation-induced carcinogenesis. The historical data from the studies of Fry et al. and Alpen et al. for Harderian gland (HG) tumors in the female CB6F1 strain of mouse represent the most complete set of experimental observations, including dose dependence, available on a specific radiation-induced tumor in an experimental animal using heavy ion beams that are found in the cosmic radiation spectrum. However, these data lack complete information on low-dose responses below 0.1 Gy, and for chronic low-dose-rate exposures, and there are gaps in the LET region between 25 and 190 keV/μm. In this study, we used the historical HG tumorigenesis data as reference, and obtained HG tumor data for 260 MeV/u silicon (LET ∼70 keV/μm) and 1,000 MeV/u titanium (LET ∼100 keV/μm) to fill existing gaps of data in this LET range to improve our understanding of the dose-response curve at low doses, to test for deviations from linearity and to provide RBE estimates. Animals were also exposed to five daily fractions of 0.026 or 0.052 Gy of 1,000 MeV/u titanium ions to simulate chronic exposure, and HG tumorigenesis from this fractionated study were compared to the results from single 0.13 or 0.26 Gy acute titanium exposures. Theoretical modeling of the data show that a nontargeted effect model provides a better fit than the targeted effect model, providing important information at space-relevant doses of heavy ions.

HZE Radiation Non-Targeted Effects on the Microenvironment That Mediate Mammary Carcinogenesis

Clear mechanistic understanding of the biological processes elicited by radiation that increase cancer risk can be used to inform prediction of health consequences of medical uses, such as radiotherapy, or occupational exposures, such as those of astronauts during deep space travel. Here, we review the current concepts of carcinogenesis as a multicellular process during which transformed cells escape normal tissue controls, including the immune system, and establish a tumor microenvironment. We discuss the contribution of two broad classes of radiation effects that may increase cancer: radiation targeted effects that occur as a result of direct energy deposition, e.g., DNA damage, and non-targeted effects (NTE) that result from changes in cell signaling, e.g., genomic instability. It is unknown whether the potentially greater carcinogenic effect of high Z and energy (HZE) particle radiation is a function of the relative contribution or extent of NTE or due to unique NTE. We addressed this problem using a radiation/genetic mammary chimera mouse model of breast cancer. Our experiments suggest that NTE promote more aggressive cancers, as evidenced by increased growth rate, transcriptomic signatures, and metastasis, and that HZE particle NTE are more effective than reference γ-radiation. Emerging evidence suggest that HZE irradiation dampens antitumor immunity. These studies raise concern that HZE radiation exposure not only increases the likelihood of developing cancer but also could promote progression to more aggressive cancer with a greater risk of mortality.

Linking Gamma-H2AX Foci and Cancer in Rat Skin Exposed to Heavy Ions and Electron Radiation

This study uses acute doses of three test radiations, [Ar ions (L = 125 keVμ), Ne ions (L = 25 keVμ) and electron radiation] to examine a potential quantitative link between rat skin cancer induction and gamma-H2AX foci in rat keratinocytes exposed in vitro to radiations with comparable L values. Theory provided a testable link between cancer yield and gamma-H2AX foci yields: YCa(D,L)rat = (NF)2YAX(D,L)keratinocyte (eqn 1), where YCa(D,L) is cancers(rat) at 1.0 y, YAX(D,L) is in vitro gamma-H2AX foci(keratinocyte) , D is radiation dose, L is linear energy transfer, N is irradiated keratinocytes in vivo, and F is the error rate of end joining. An explicit expression for cancer yield was derived based on cancers arising in the ion track region in proportion to D and L (first term) and independently in proportion to D in the delta ray region in between the ion tracks (second term): YCa(D,L) = CCaLD + BCaD (eqn 1a). Parameters quantified include: CCa = 0.000589 ± 0.000150 cancers-micron[rat(kev)Gy]; BCa = 0.0088 ± 0.0035 cancers(ratGy), F = (8.18 ± 0.91) × 10; N = (8.8 ± 1.2) × 10 and (NF)2 = 0.036 ± 0.006 cancer keratinocyte(rat H2AX foci). Verification of eqns (1) and (1a) and the constancy of F support the hypothesis that end-rejoining errors play a major role in radiation carcinogenesis in rat skin. Cancer yields per rat were consistently predictable based on gamma-H2AX foci yields in keratinocytes in vitro such that 27.8 H2AXfoci(keratinocyte) predicted 1.0 cancer(rat) at 1 y.

Concepts and challenges in cancer risk prediction for the space radiation environment

Cancer is an important long-term risk for astronauts exposed to protons and high-energy charged particles during travel and residence on asteroids, the moon, and other planets. NASA's Biomedical Critical Path Roadmap defines the carcinogenic risks of radiation exposure as one of four type I risks. A type I risk represents a demonstrated, serious problem with no countermeasure concepts, and may be a potential "show-stopper" for long duration spaceflight. Estimating the carcinogenic risks for humans who will be exposed to heavy ions during deep space exploration has very large uncertainties at present. There are no human data that address risk from extended exposure to complex radiation fields. The overarching goal in this area to improve risk modeling is to provide biological insight and mechanistic analysis of radiation quality effects on carcinogenesis. Understanding mechanisms will provide routes to modeling and predicting risk and designing countermeasures. This white paper reviews broad issues related to experimental models and concepts in space radiation carcinogenesis as well as the current state of the field to place into context recent findings and concepts derived from the NASA Space Radiation Program.

Understanding cancer development processes after HZE-particle exposure: roles of ROS, DNA damage repair and inflammation

During space travel astronauts are exposed to a variety of radiations, including galactic cosmic rays composed of high-energy protons and high-energy charged (HZE) nuclei, and solar particle events containing low- to medium-energy protons. Risks from these exposures include carcinogenesis, central nervous system damage and degenerative tissue effects. Currently, career radiation limits are based on estimates of fatal cancer risks calculated using a model that incorporates human epidemiological data from exposed populations, estimates of relative biological effectiveness and dose-response data from relevant mammalian experimental models. A major goal of space radiation risk assessment is to link mechanistic data from biological studies at NASA Space Radiation Laboratory and other particle accelerators with risk models. Early phenotypes of HZE exposure, such as the induction of reactive oxygen species, DNA damage signaling and inflammation, are sensitive to HZE damage complexity. This review summarizes our current understanding of critical areas within the DNA damage and oxidative stress arena and provides insight into their mechanistic interdependence and their usefulness in accurately modeling cancer and other risks in astronauts exposed to space radiation. Our ultimate goals are to examine potential links and crosstalk between early response modules activated by charged particle exposure, to identify critical areas that require further research and to use these data to reduced uncertainties in modeling cancer risk for astronauts. A clearer understanding of the links between early mechanistic aspects of high-LET response and later surrogate cancer end points could reveal key nodes that can be therapeutically targeted to mitigate the health effects from charged particle exposures.

ALDH1A1 Deficiency in Gorlin Syndrome Suggests a Central Role for Retinoic Acid and ATM Deficits in Radiation Carcinogenesis

We hypothesize that aldehyde dehydrogenase 1A1 (ALDH1A1) deficiency will result in impaired ataxia-telangiectasia mutated (ATM) activation in a retinoic acid-sensitive fashion. Data supporting this hypothesis include (1) reduced ATM activation in irradiated primary dermal fibroblasts from ALDH1A1-deficient Gorlin syndrome patients (GDFs), relative to ALDH1A1-positive normal human dermal fibroblasts (NHDFs) and (2) increased ATM activation by X-radiation in GDFs pretreated with retinoic acid, however, the impact of donor variability on ATM activation in fibroblasts was not assessed and is a prudent consideration in future studies. Clonogenic survival of irradiated cells showed differential responses to retinoic acid as a function of treatment time. Long-term (5 Day) retinoic acid treatment functioned as a radiosensitizer and was associated with downregulation of ATM protein levels. Short-term (7 h) retinoic acid treatment showed a trend toward increased survival of irradiated cells and did not downregulate ATM protein levels. Using a newly developed IncubATR technology, which defines changes in bulk chemical bond patterns in live cells, we can discriminate between the NHDF and GDF phenotypes, but treatment of GDFs with retinoic acid does not induce reversion of bulk chemical bond patterns associated with GDFs toward the NHDF phenotype. Collectively, our preliminary investigation of the Gorlin phenotype has identified deficient ALDH1A1 expression associated with deficient ATM activation as a possible susceptibility factor that is consistent with the high incidence of spontaneous and radiation-induced carcinogenesis in these patients. The IncubATR technology exhibits sufficient sensitivity to detect phenotypic differences in live cells that may be relevant to radiation health effects.

ERK oscillation-dependent gene expression patterns and deregulation by stress response

Studies were undertaken to determine whether extracellular signal regulated kinase (ERK) oscillations regulate a unique subset of genes in human keratinocytes and subsequently whether the p38 stress response inhibits ERK oscillations. A DNA microarray identified many genes that were unique to ERK oscillations, and network reconstruction predicted an important role for the mediator complex subunit 1 (MED1) node in mediating ERK oscillation-dependent gene expression. Increased ERK-dependent phosphorylation of MED1 was observed in oscillating cells compared to nonoscillating counterparts as validation. Treatment of keratinocytes with a p38 inhibitor (SB203580) increased ERK oscillation amplitudes and MED1 and phospho-MED1 protein levels. Bromate is a probable human carcinogen that activates p38. Bromate inhibited ERK oscillations in human keratinocytes and JB6 cells and induced an increase in phospho-p38 and a decrease in phospho-MED1 protein levels. Treatment of normal rat kidney cells and primary salivary gland epithelial cells with bromate decreased phospho-MED1 levels in a reversible fashion upon treatment with p38 inhibitors (SB202190; SB203580). Our results indicate that oscillatory behavior in the ERK pathway alters homeostatic gene regulation patterns and that the cellular response to perturbation may manifest differently in oscillating vs nonoscillating cells.

Mitigating the risk of radiation-induced cancers: limitations and paradigms in drug development

The United States radiation medical countermeasures (MCM) programme for radiological and nuclear incidents has been focusing on developing mitigators for the acute radiation syndrome (ARS) and delayed effects of acute radiation exposure (DEARE), and biodosimetry technologies to provide radiation dose assessments for guiding treatment. Because a nuclear accident or terrorist incident could potentially expose a large number of people to low to moderate doses of ionising radiation, and thus increase their excess lifetime cancer risk, there is an interest in developing mitigators for this purpose. This article discusses the current status, issues, and challenges regarding development of mitigators against radiation-induced cancers. The challenges of developing mitigators for ARS include: the long latency between exposure and cancer manifestation, limitations of animal models, potential side effects of the mitigator itself, potential need for long-term use, the complexity of human trials to demonstrate effectiveness, and statistical power constraints for measuring health risks (and reduction of health risks after mitigation) following relatively low radiation doses (<0.75 Gy). Nevertheless, progress in the understanding of the molecular mechanisms resulting in radiation injury, along with parallel progress in dose assessment technologies, make this an opportune, if not critical, time to invest in research strategies that result in the development of agents to lower the risk of radiation-induced cancers for populations that survive a significant radiation exposure incident.

Biological Effects of Space Radiation and Development of Effective Countermeasures

As part of a program to assess the adverse biological effects expected from astronaut exposure to space radiation, numerous different biological effects relating to astronaut health have been evaluated. There has been major focus recently on the assessment of risks related to exposure to solar particle event (SPE) radiation. The effects related to various types of space radiation exposure that have been evaluated are: gene expression changes (primarily associated with programmed cell death and extracellular matrix (ECM) remodeling), oxidative stress, gastrointestinal tract bacterial translocation and immune system activation, peripheral hematopoietic cell counts, emesis, blood coagulation, skin, behavior/fatigue (including social exploration, submaximal exercise treadmill and spontaneous locomotor activity), heart functions, alterations in biological endpoints related to astronaut vision problems (lumbar puncture/intracranial pressure, ocular ultrasound and histopathology studies), and survival, as well as long-term effects such as cancer and cataract development. A number of different countermeasures have been identified that can potentially mitigate or prevent the adverse biological effects resulting from exposure to space radiation.

Health risks of space exploration: targeted and nontargeted oxidative injury by high-charge and high-energy particles

SIGNIFICANCE

During deep space travel, astronauts are often exposed to high atomic number (Z) and high-energy (E) (high charge and high energy [HZE]) particles. On interaction with cells, these particles cause severe oxidative injury and result in unique biological responses. When cell populations are exposed to low fluences of HZE particles, a significant fraction of the cells are not traversed by a primary radiation track, and yet, oxidative stress induced in the targeted cells may spread to nearby bystander cells. The long-term effects are more complex because the oxidative effects persist in progeny of the targeted and affected bystander cells, which promote genomic instability and may increase the risk of age-related cancer and degenerative diseases.

RECENT ADVANCES

Greater understanding of the spatial and temporal features of reactive oxygen species bursts along the tracks of HZE particles, and the availability of facilities that can simulate exposure to space radiations have supported the characterization of oxidative stress from targeted and nontargeted effects.

CRITICAL ISSUES

The significance of secondary radiations generated from the interaction of the primary HZE particles with biological material and the mitigating effects of antioxidants on various cellular injuries are central to understanding nontargeted effects and alleviating tissue injury.

FUTURE DIRECTIONS

Elucidation of the mechanisms underlying the cellular responses to HZE particles, particularly under reduced gravity and situations of exposure to additional radiations, such as protons, should be useful in reducing the uncertainty associated with current models for predicting long-term health risks of space radiation. These studies are also relevant to hadron therapy of cancer.

Systems biology perspectives on the carcinogenic potential of radiation

This review focuses on recent experimental and modeling studies that attempt to define the physiological context in which high linear energy transfer (LET) radiation increases epithelial cancer risk and the efficiency with which it does so. Radiation carcinogenesis is a two-compartment problem: ionizing radiation can alter genomic sequence as a result of damage due to targeted effects (TE) from the interaction of energy and DNA; it can also alter phenotype and multicellular interactions that contribute to cancer by poorly understood non-targeted effects (NTE). Rather than being secondary to DNA damage and mutations that can initiate cancer, radiation NTE create the critical context in which to promote cancer. Systems biology modeling using comprehensive experimental data that integrates different levels of biological organization and time-scales is a means of identifying the key processes underlying the carcinogenic potential of high-LET radiation. We hypothesize that inflammation is a key process, and thus cancer susceptibility will depend on specific genetic predisposition to the type and duration of this response. Systems genetics using novel mouse models can be used to identify such determinants of susceptibility to cancer in radiation sensitive tissues following high-LET radiation. Improved understanding of radiation carcinogenesis achieved by defining the relative contribution of NTE carcinogenic effects and identifying the genetic determinants of the high-LET cancer susceptibility will help reduce uncertainties in radiation risk assessment.

Second malignant neoplasms and cardiovascular disease following radiotherapy

Second malignant neoplasms (SMNs) and cardiovascular disease (CVD) are among the most serious and life-threatening late adverse effects experienced by the growing number of cancer survivors worldwide and are due in part to radiotherapy. The National Council on Radiation Protection and Measurements (NCRP) convened an expert scientific committee to critically and comprehensively review associations between radiotherapy and SMNs and CVD, taking into account radiobiology; genomics; treatment (i.e., radiotherapy with or without chemotherapy and other therapies); type of radiation; and quantitative considerations (i.e., dose-response relationships). Major conclusions of the NCRP include: (1) the relevance of older technologies for current risk assessment when organ-specific absorbed dose and the appropriate relative biological effectiveness are taken into account and (2) the identification of critical research needs with regard to newer radiation modalities, dose-response relationships, and genetic susceptibility. Recommendation for research priorities and infrastructural requirements include (1) long-term large-scale follow-up of extant cancer survivors and prospectively treated patients to characterize risks of SMNs and CVD in terms of radiation dose and type; (2) biological sample collection to integrate epidemiological studies with molecular and genetic evaluations; (3) investigation of interactions between radiotherapy and other potential confounding factors, such as age, sex, race, tobacco and alcohol use, dietary intake, energy balance, and other cofactors, as well as genetic susceptibility; (4) focusing on adolescent and young adult cancer survivors, given the sparse research in this population; and (5) construction of comprehensive risk prediction models for SMNs and CVD to permit the development of follow-up guidelines and prevention and intervention strategies.

Deficient expression of aldehyde dehydrogenase 1A1 is consistent with increased sensitivity of Gorlin syndrome patients to radiation carcinogenesis

Human phenotypes that are highly susceptible to radiation carcinogenesis have been identified. Sensitive phenotypes often display robust regulation of molecular features that modify biological response, which can facilitate identification of the pathways/networks that contribute to pathophysiological outcomes. Here we interrogate primary dermal fibroblasts isolated from Gorlin syndrome patients (GDFs), who display a pronounced inducible tumorigenic response to radiation, in comparison to normal human dermal fibroblasts (NHDFs). Our approach exploits newly developed thiol reactive probes to define changes in protein thiol profiles in live cell studies, which minimizes artifacts associated with cell lysis. Redox probes revealed deficient expression of an apparent 55 kDa protein thiol in GDFs from independent Gorlin syndrome patients, compared with NHDFs. Proteomics tentatively identified this protein as aldehyde dehydrogenase 1A1 (ALDH1A1), a key enzyme regulating retinoic acid synthesis, and ALDH1A1 protein deficiency in GDFs was confirmed by Western blot. A number of additional protein thiol differences in GDFs were identified, including radiation responsive annexin family members and lamin A/C. Collectively, candidates identified in our study have plausible implications for radiation health effects and cancer susceptibility.

Animal studies of charged particle-induced carcinogenesis

The distribution of energy deposition in cells and tissues by high-charge, high-energy (HZE) nuclei differs considerably from that of low linear energy transfer (LET) radiation, raising concerns that charged particle exposure may be more efficient in inducing radiogenic cancers or may induce a different spectrum of tumors. The authors have performed a review of charged particle carcinogenesis in animals with the following observations. A limited number of animal studies with carcinogenesis endpoints have been performed to evaluate the effectiveness of HZE ions. These include the induction of skin and mammary tumors in the rat and Harderian gland tumors, acute myeloid leukemia (AML), and hepatocellular carcinomas in the mouse. In general, high relative biological effectiveness (RBE) has been reported for solid tumor induction. RBE dependence on HZE radiation quality has been most extensively characterized in studies of mouse Harderian gland tumorigenesis. In this model, the RBE increases with LET and plateaus in the 193-953 keV μm(-1) range. Unlike the results of solid tumor studies, a leukemogenesis study found 1 GeV nucleon(-1) 56Fe ions no more efficient than gamma-rays for AML induction. No novel tumor types have been observed in HZE irradiated animals as compared with those that occur spontaneously or following low-LET radiation exposures. Genetic background of the irradiated animals is critical; the tumor types induced in HZE irradiated mice depend on their strain background, and the incidence of HZE ion-induced mammary carcinogenesis in the rat is also strain dependent.

Genetic susceptibility: radiation effects relevant to space travel

Genetic variation in the capacity to repair radiation damage is an important factor influencing both cellular and tissue radiosensitivity variation among individuals as well as dose rate effects associated with such damage. This paper consists of two parts. The first part reviews some of the available data relating to genetic components governing such variability among individuals in susceptibility to radiation damage relevant for radiation protection and discusses the possibility and extent to which these may also apply for space radiations. The second part focuses on the importance of dose rate effects and genetic-based variations that influence them. Very few dose rate effect studies have been carried out for the kinds of radiations encountered in space. The authors present here new data on the production of chromosomal aberrations in noncycling low passage human ATM+/+ or ATM+/- cells following irradiations with protons (50 MeV or 1 GeV), 1 GeV(-1) n iron ions and gamma rays, where doses were delivered at a high dose rate of 700 mGy(-1) min, or a lower dose rate of 5 mGy min(-1). Dose responses were essentially linear over the dose ranges tested and not significantly different for the two cell strains. Values of the dose rate effectiveness factor (DREF) were expressed as the ratio of the slopes of the dose-response curves for the high versus the lower (5 mGy min(-1)) dose rate exposures. The authors refer to this as the DREF5. For the gamma ray standard, DREF5 values of approximately two were observed. Similar dose rate effects were seen for both energies of protons (DREF5 ≈ 2.2 in both cases). For 1 GeV(-1) n iron ions [linear energy transfer (LET) ≈ 150 keV μ(-1)], the DREF5 was not 1 as might have been expected on the basis of LET alone but was approximately 1.3. From these results and conditions, the authors estimate that the relative biological effectiveness for 1 GeV(-1) n iron ions for high and low dose rates, respectively, were about 10 and 15 rather than around 20 for low dose rates, as has been assumed by most recommendations from radiation protection organizations for charged particles of this LET. The authors suggest that similar studies using appropriate animal models of carcinogenesis would be valuable.

Eighth Warren K. Sinclair keynote address: Heavy ions in therapy and space: benefits and risks

Heavy charged particles produce biological damage that is different from that normally produced by sparsely ionizing radiation, such as x- or gamma-rays, which are a large component of the natural radiation background. In fact, as a result of the different spatial distribution of the energy deposited along the core and penumbra of the track, DNA lesions are exquisitely complex and difficult to repair. Relative biological effectiveness (RBE) factors are normally used to scale from x-rays to heavy ion damage, but it should be kept in mind that RBE depends on several factors (dose, dose rate, endpoint, particle energy, and charge, etc.), and sometimes heavy ions produce special damages that just cannot be scaled by x-ray damage alone. These special characteristics of heavy ions can be used to treat tumors efficiently, as it is currently done in Japan and Germany, but they represent a threat for human space exploration.

A novel mechanism of filaggrin induction and sunburn prevention by β-damascenone in Skh-1 mice

Understanding how oral administration of aroma terpenes can prevent sunburn or skin cancer in mice could lead to more effective and safer ways of blocking sun damage to human skin. To establish sunburn preventive activity, female Skh-1 mice were given oral β-damascenone followed by irradiation with UVR from fluorescent 'sunlamps'. The following endpoints were evaluated versus controls at various times between 1 and 12 days after the terpene: whole genome gene expression and in situ immunohistochemistry of PCNA, keratin 10, filaggrin and caspase 14, and sunburn was evaluated at 5 days. UVR-induced sunburn was prevented by a single oral β-damascenone dose as low as 20 μL (0.95 mg/g body weight). Microarray analysis showed sunburn prevention doses of β-damascenone up-regulated several types of cornification genes, including keratins 1 and 10, filaggrin, caspase 14, loricrin, hornerin and 6 late cornified envelope genes. Immunohistochemical studies of PCNA labeling showed that β-damascenone increased the proliferation rates of the following cell types: epidermal basal cells, follicular outer root sheath cells and sebaceous gland cells. Keratin 10 was not affected by β-damascenone in epidermis, and filaggrin and caspase 14 were increased in enlarged sebaceous glands. The thickness of the cornified envelope plus sebum layer nearly doubled within 1 day after administration of the β-damascenone and remained at or above double thickness for at least 12 days. β-Damascenone protected against sunburn by activating a sebaceous gland-based pathway that fortified and thickened the cornified envelope plus sebum layer in a way that previously has been observed to occur only in keratinocytes.

Second malignant neoplasms and cardiovascular disease following radiotherapy

Second malignant neoplasms (SMNs) and cardiovascular disease (CVD) are among the most serious and life-threatening late adverse effects experienced by the growing number of cancer survivors worldwide and are due in part to radiotherapy. The National Council on Radiation Protection and Measurements (NCRP) convened an expert scientific committee to critically and comprehensively review associations between radiotherapy and SMNs and CVD, taking into account radiobiology; genomics; treatment (ie, radiotherapy with or without chemotherapy and other therapies); type of radiation; and quantitative considerations (ie, dose-response relationships). Major conclusions of the NCRP include: 1) the relevance of older technologies for current risk assessment when organ-specific absorbed dose and the appropriate relative biological effectiveness are taken into account and 2) the identification of critical research needs with regard to newer radiation modalities, dose-response relationships, and genetic susceptibility. Recommendation for research priorities and infrastructural requirements include 1) long-term large-scale follow-up of extant cancer survivors and prospectively treated patients to characterize risks of SMNs and CVD in terms of radiation dose and type; 2) biological sample collection to integrate epidemiological studies with molecular and genetic evaluations; 3) investigation of interactions between radiotherapy and other potential confounding factors, such as age, sex, race, tobacco and alcohol use, dietary intake, energy balance, and other cofactors, as well as genetic susceptibility; 4) focusing on adolescent and young adult cancer survivors, given the sparse research in this population; and 5) construction of comprehensive risk prediction models for SMNs and CVD to permit the development of follow-up guidelines and prevention and intervention strategies.

CDDO-Me protects against space radiation-induced transformation of human colon epithelial cells

Radiation-induced carcinogenesis is a major concern both for astronauts on long-term space missions and for cancer patients being treated with therapeutic radiation. Exposure to radiation induces oxidative stress and chronic inflammation, which are critical initiators and promoters of carcinogenesis. Many studies have demonstrated that non-steroidal anti-inflammatory drugs and antioxidants can reduce the risk of radiation-induced cancer. In this study, we found that a synthetic triterpenoid, CDDO-Me (bardoxolone methyl), was able to protect human colon epithelial cells (HCECs) against radiation-induced transformation. HCECs that were immortalized by ectopic expression of hTERT and cdk4 and exhibit trisomy for chromosome 7 (a non-random chromosome change that occurs in 37% of premalignant colon adenomas) can be transformed experimentally with one combined exposure to 2 Gy of protons at 1 GeV/nucleon followed 24 h later by 50 cGy of (56)Fe ions at 1 GeV/nucleon. Transformed cells showed an increase in proliferation rate and in both anchorage-dependent and independent colony formation ability. A spectrum of chromosome aberrations was observed in transformed cells, with 40% showing loss of 17p (e.g. loss of one copy of p53). Pretreatment of cells with pharmacological doses of CDDO-Me, which has been shown to induce antioxidative as well as anti-inflammatory responses, prevented the heavy-ion-induced increase in proliferation rate and anchorage-dependent and independent colony formation efficiencies. Taken together, these results demonstrate that experimentally immortalized human colon epithelial cells with a non-random chromosome 7 trisomy are valuable premalignant cellular reagents that can be used to study radiation-induced colorectal carcinogenesis. The utility of premalignant HCECs to test novel compounds such as CDDO-Me that can be used to protect against radiation-induced neoplastic transformation is also demonstrated.

Cancer research in rat models

Rat has been the major model species used in several biomedical fields, notably in drug development and toxicology, including carcinogenicity testing. Rat is also a useful model in basic cancer research. Several rat models of monogenic (Mendelian) human hereditary cancers are available. Some were obtained spontaneously, while others were generated either by mutagenesis of tumor suppressor genes or by transgenesis of activated oncogenes (transgenesis can be performed efficiently in the rat). In addition, among the hundreds of inbred rat strains that have been isolated, some are highly susceptible or resistant to certain types of cancer, and these divergent phenotypes were shown to be polygenic. Numerous quantitative trait loci (QTLs) controlling cancer susceptibility/resistance have been defined in linkage analyses, and several of these QTLs were physically demonstrated in congenic strains. These studies led, in particular, to rapid translation to the human, with the identification of loci controlling susceptibility to a form of multiple endocrine neoplasia (monogenic trait) and to breast cancer (polygenic disease). The biology of cancer resistance has also been analyzed, and in some (but not all) cases, it was linked to regression of preneoplasic lesions. Rat tumors have been the subject of various types of analyses, and these studies led to important conclusions, including that tumors can be classified on the basis of the identity of the inducing agent, thereby suggesting that analyses of human tumors may be valuable in determining retrospectively the role of specific carcinogens in the formation of human cancers, and of human breast cancer in particular.

Comparative analysis of cell killing and autosomal mutation in mouse kidney epithelium exposed to 1 GeV/nucleon iron ions in vitro or in situ

Astronauts receive exposures to high-energy heavy ions from galactic cosmic radiation. Although high-energy heavy ions are mutagenic and carcinogenic, their mutagenic potency in epithelial cells, where most human cancers develop, is poorly understood. Mutations are a critical component of human cancer, and mutations involving autosomal loci predominate. This study addresses the cytotoxic and mutagenic effects of 1 GeV/nucleon iron ions in mouse kidney epithelium. Mutant fractions were measured for an endogenous autosomal locus (Aprt) that detects all types of mutagenic events contributing to human cancer. Results for kidneys irradiated in situ are compared with results for kidney cells from the same strain exposed in vitro. The results demonstrate dose-dependent cell killing in vitro and for cells explanted 3-4 months postirradiation in situ, but in situ exposures were less likely to result in cell death than in vitro exposures. Prolonged incubation in situ (8-9 months) further attenuated cell killing at lower doses. Iron ions were mutagenic to cells in vitro and for irradiated kidneys. No sparing was seen for mutant frequency with a long incubation period in situ. In addition, the degree of mutation induction (relative increase over background) was similar for cells exposed in vitro or in situ. We speculate that the latent effects of iron-ion exposure contribute to the maintenance of an elevated mutation burden in an epithelial tissue.

Factors that modify radiation-induced carcinogenesis

It is known that numerous factors can influence radiation carcinogenesis in animals; these factors include the specific characteristics of the radiation (radiation type and dose, dose-rate, dose-fractionation, dose distribution, etc.) as well as many other contributing elements that are not specific to the radiation exposure, such as animal genetic characteristics and age, the environment of the animal, dietary factors and whether specific modifying agents for radiation carcinogenesis have been utilized in the studies. This overview focuses on the modifying factors for radiation carcinogenesis, in both in vivo and in vitro systems, and includes a discussion of agents that enhance (e.g., promoting agents) or suppress (e.g., cancer preventive agents) radiation-induced carcinogenesis. The agents that enhance or suppress radiation carcinogenesis in experimental model systems have been shown to lead to effects equally as large as other known modifying factors for radiation-induced carcinogenesis (e.g., dose-rate, dose-fractionation, linear energy transfer). It is known that dietary factors play an important role in determining the yields of radiation-induced cancers in animal model systems, and it is likely that they also influence radiation-induced cancer risks in human populations.

Musculoskeletal changes in mice from 20-50 cGy of simulated galactic cosmic rays

On a mission to Mars, astronauts will be exposed to a complex mix of radiation from galactic cosmic rays. We have demonstrated a loss of bone mass from exposure to types of radiation relevant to space flight at doses of 1 and 2 Gy. The effects of space radiation on skeletal muscle, however, have not been investigated. To evaluate the effect of simulated galactic cosmic radiation on muscle fiber area and bone volume, we examined mice from a study in which brains were exposed to collimated iron-ion radiation. The collimator transmitted a complex mix of charged secondary particles to bone and muscle tissue that represented a low-fidelity simulation of the space radiation environment. Measured radiation doses of uncollimated secondary particles were 0.47 Gy at the proximal humerus, 0.24-0.31 Gy at the midbelly of the triceps brachii, and 0.18 Gy at the proximal tibia. Compared to nonirradiated controls, the proximal humerus of irradiated mice had a lower trabecular bone volume fraction, lower trabecular thickness, greater cortical porosity, and lower polar moment of inertia. The tibia showed no differences in any bone parameter. The triceps brachii of irradiated mice had fewer small-diameter fibers and more fibers containing central nuclei. These results demonstrate a negative effect on the skeletal muscle and bone systems of simulated galactic cosmic rays at a dose and LET range relevant to a Mars exploration mission. The presence of evidence of muscle remodeling highlights the need for further study.

Effects of dietary antioxidant supplementation on the development of malignant lymphoma and other neoplastic lesions in mice exposed to proton or iron-ion radiation

Malignancy is considered to be a particular risk associated with exposure to the types of ionizing radiation encountered during extended space flight. In the present study, two dietary preparations were evaluated for their ability to prevent carcinogenesis in CBA mice exposed to different forms of space radiation: protons and highly energetic heavy particles (HZE particles). One preparation contained a mixture of antioxidant agents. The other contained the soybean-derived Bowman-Birk protease inhibitor (BBI), used in the form of BBI Concentrate (BBIC). The major finding was that there was a reduced risk of developing malignant lymphoma in animals exposed to space radiation and maintained on diets containing the antioxidant formulation or BBIC compared to the irradiated animals maintained on the control diet. In addition, the two different dietary countermeasures also reduced the yields of a variety of different rare tumor types observed in the animals exposed to space radiation. These results suggest that dietary supplements could be useful in the prevention of malignancies and other neoplastic lesions developing from exposure to space radiation.

Heavy ion carcinogenesis and human space exploration

Before the human exploration of Mars or long-duration missions on the Earth's moon, the risk of cancer and other diseases from space radiation must be accurately estimated and mitigated. Space radiation, comprised of energetic protons and heavy nuclei, has been shown to produce distinct biological damage compared with radiation on Earth, leading to large uncertainties in the projection of cancer and other health risks, and obscuring evaluation of the effectiveness of possible countermeasures. Here, we describe how research in cancer radiobiology can support human missions to Mars and other planets.