Karyotypic instability and centrosome aberrations in the progeny of finite life-span human mammary epithelial cells exposed to sparsely or densely ionizing radiation

The intercellular communications mediating radiation-induced bystander effects and their relevance to environmental, occupational, and therapeutic exposures

PURPOSE

The assumption that traversal of the cell nucleus by ionizing radiation is a prerequisite to induce genetic damage, or other important biological responses, has been challenged by studies showing that oxidative alterations extend beyond the irradiated cells and occur also in neighboring bystander cells. Cells and tissues outside the radiation field experience significant biochemical and phenotypic changes that are often similar to those observed in the irradiated cells and tissues. With relevance to the assessment of long-term health risks of occupational, environmental and clinical exposures, measurable genetic, epigenetic, and metabolic changes have been also detected in the progeny of bystander cells. How the oxidative damage spreads from the irradiated cells to their neighboring bystander cells has been under intense investigation. Following a brief summary of the trends in radiobiology leading to this paradigm shift in the field, we review key findings of bystander effects induced by low and high doses of various types of radiation that differ in their biophysical characteristics. While notable mechanistic insights continue to emerge, here the focus is on the many means of intercellular communication that mediate these effects, namely junctional channels, secreted molecules and extracellular vesicles, and immune pathways.

CONCLUSIONS

The insights gained by studying radiation bystander effects are leading to a basic understanding of the intercellular communications that occur under mild and severe oxidative stress in both normal and cancerous tissues. Understanding the mechanisms underlying these communications will likely contribute to reducing the uncertainty of predicting adverse health effects following exposure to low dose/low fluence ionizing radiation, guide novel interventions that mitigate adverse out-of-field effects, and contribute to better outcomes of radiotherapeutic treatments of cancer. In this review, we highlight novel routes of intercellular communication for investigation, and raise the rationale for reconsidering classification of bystander responses, abscopal effects, and expression of genomic instability as non-targeted effects of radiation.

Genetic variation and radiation quality impact cancer promoting cellular phenotypes in response to HZE exposure

There exists a wide degree of genetic variation within the normal human population which includes disease free individuals with heterozygote defects in major DNA repair genes. A lack of understanding of how this genetic variation impacts cellular phenotypes that inform cancer risk post heavy ion exposure poses a major limitation in developing personalized cancer risk assessment astronauts. We initiated a pilot study with Human Mammary Epithelial Cell strains (HMEC) derived from wild type, a p16 silenced derivative of wild type, and various genetic variants that were heterozygote for DNA repair genes; BRCA1, BRCA2 and ATM. Cells strains were exposed to different high and low LET radiation qualities to generate both simple and complex lesions and centrosome aberrations were examined as a surrogate marker of genomic instability and cancer susceptibility post different exposures. Our results indicate that centrosome aberration frequency is higher in the genetic variants under study. The aberration frequency increases with dose, complexity of the lesion generated by different radiation qualities and age of the individual. This increase in genomic instability correlates with elevated check-point activation post radiation exposure. These studies suggest that the influence of individual genetics on cell cycle regulation could modify the degree of early genomic instability in response to complex lesions and potentially define cancer predisposition in response to HZE exposure. These results will have significant implications in estimating cancer susceptibility in genetically variant individuals exposed to HZE particles.

Exposure to galactic cosmic radiation compromises DNA repair and increases the potential for oncogenic chromosomal rearrangement in bronchial epithelial cells

Participants in deep space missions face protracted exposure to galactic cosmic radiation (GCR). In this setting, lung cancer is a significant component of the overall risk of radiation-exposure induced death. Here we investigate persistent effects of GCR exposure on DNA repair capacity in lung-derived epithelial cells, using an enzyme-stimulated chromosomal rearrangement as an endpoint. Replicate cell cultures were irradiated with energetic ⁴⁸Ti ions (a GCR component) or reference γ-rays. After a six-day recovery, they were challenged by expression of a Cas9/sgRNA pair that creates double-strand breaks simultaneously in the EML4 and ALK loci, misjoining of which creates an EML4-ALK fusion oncogene. Misjoining was significantly elevated in ⁴⁸Ti-irradiated populations, relative to the baseline rate in mock-irradiated controls. The effect was not seen in γ-ray irradiated populations exposed to equal or higher radiation doses. Sequence analysis of the EML4-ALK joints from ⁴⁸Ti-irradiated cultures showed that they were far more likely to contain deletions, sometimes flanked by short microhomologies, than equivalent samples from mock-irradiated cultures, consistent with a shift toward error-prone alternative nonhomologous end joining repair. Results suggest a potential mechanism by which a persistent physiological effect of GCR exposure may increase lung cancer risk.

The Lowest Radiation Dose Having Molecular Changes in the Living Body

We herein attempted to identify the lowest radiation dose causing molecular changes in the living body. We investigated the effects of radiation in human cells, animals, and humans. DNA double-strand breaks (DSBs) formed in cells at γ- or X-ray irradiation doses between 1 mGy and 0.5 Gy; however, the extent of DSB formation differed depending on the cell species. The formation of micronuclei (MNs) and nucleoplasmic bridges (NPBs) was noted at radiation doses between 0.1 and 0.2 Gy. Stress-responsive genes were upregulated by lower radiation doses than those that induced DNA DSBs or MN and NPBs. These γ- or X-ray radiation doses ranged between approximately 10 and 50 mGy. In animals, chromosomal aberrations were detected between 50 mGy and 0.1 Gy of low linear energy transfer radiation, 0.1 Gy of metal ion beams, and 9 mGy of fast neutrons. In humans, DNA damage has been observed in children who underwent computed tomography scans with an estimated blood radiation dose as low as 0.15 mGy shortly after examination. The frequencies of chromosomal translocations were lower in residents of high background areas than in those of control areas. In humans, systemic adaptive responses may have been prominently expressed at these radiation doses.

Lesion complexity drives age related cancer susceptibility in human mammary epithelial cells

Exposures to various DNA damaging agents can deregulate a wide array of critical mechanisms that maintain genome integrity. It is unclear how these processes are impacted by one's age at the time of exposure and the complexity of the DNA lesion. To clarify this, we employed radiation as a tool to generate simple and complex lesions in normal primary human mammary epithelial cells derived from women of various ages. We hypothesized that genomic instability in the progeny of older cells exposed to complex damages will be exacerbated by age-associated deterioration in function and accentuate age-related cancer predisposition. Centrosome aberrations and changes in stem cell numbers were examined to assess cancer susceptibility. Our data show that the frequency of centrosome aberrations proportionately increases with age following complex damage causing exposures. However, a dose-dependent increase in stem cell numbers was independent of both age and the nature of the insult. Phospho-protein signatures provide mechanistic clues to signaling networks implicated in these effects. Together these studies suggest that complex damage can threaten the genome stability of the stem cell population in older people. Propagation of this instability is subject to influence by the microenvironment and will ultimately define cancer risk in the older population.

A Single Exposure to Low- or High-LET Radiation Induces Persistent Genomic Damage in Mouse Epithelial Cells In Vitro and in Lung Tissue

Exposures to low- and high-linear energy transfer (LET) radiation induce clustered damage in DNA that is difficult to repair. These lesions are manifested as DNA-associated foci positive for DNA repair proteins and have been shown to persist in vitro and in vivo for days in several cell types and tissues in response to low-LET radiation. Although in some experimental conditions these residual foci have been linked with genomic instability and chromosomal aberrations, it remains poorly understood what type of damage they represent. Because high-LET radiation induces complex DNA lesions more efficiently than low-LET radiation, we compared the efficacy of several heavy ions (oxygen, silicon and iron) in a range (17 , 70 and 175 keV/μm, respectively) of LET and X rays at a 1 Gy dose. Persistent genomic damage was measured by γ-H2AX-53BP1-positive residual foci and micronucleus levels during the first three days and up to a week after in vitro and in vivo irradiation in lung cells and tissue. We demonstrate that in an in vitro irradiated mouse bronchial epithelial cell line, the expression of residual foci is readily detectable at 24 h with levels declining in the following 72 h postirradiation, but still persisting elevated over background at day 7. At this time, foci numbers are low but significant and proportional to the dose and quality of the radiation. The expression of residual foci in vitro was mirrored by increased micronuclei generation measured in cytokinesis-blocked cells, indicating long-term, persistent effects of genomic damage in this cell type. We also tested the expression of residual foci in lung tissue of C57BL/6 mice that received whole-body X-ray or heavy-ion irradiation. We found that at day 7 postirradiation, Clara/Club cells, but not pro-SPC-positive pneumocytes, contained a subpopulation of cells expressing γ-H2AX-53BP1-positive foci in a radiation quality-dependent manner. These findings suggest that in vivo persistent DNA repair foci reflect the initial genotoxic damage induced by radiation and a differential vulnerability among cells in the lung.

Simulated space radiation-induced mutants in the mouse kidney display widespread genomic change

Exposure to a small number of high-energy heavy charged particles (HZE ions), as found in the deep space environment, could significantly affect astronaut health following prolonged periods of space travel if these ions induce mutations and related cancers. In this study, we used an in vivo mutagenesis assay to define the mutagenic effects of accelerated 56Fe ions (1 GeV/amu, 151 keV/μm) in the mouse kidney epithelium exposed to doses ranging from 0.25 to 2.0 Gy. These doses represent fluences ranging from 1 to 8 particle traversals per cell nucleus. The Aprt locus, located on chromosome 8, was used to select induced and spontaneous mutants. To fully define the mutagenic effects, we used multiple endpoints including mutant frequencies, mutation spectrum for chromosome 8, translocations involving chromosome 8, and mutations affecting non-selected chromosomes. The results demonstrate mutagenic effects that often affect multiple chromosomes for all Fe ion doses tested. For comparison with the most abundant sparsely ionizing particle found in space, we also examined the mutagenic effects of high-energy protons (1 GeV, 0.24 keV/μm) at 0.5 and 1.0 Gy. Similar doses of protons were not as mutagenic as Fe ions for many assays, though genomic effects were detected in Aprt mutants at these doses. Considered as a whole, the data demonstrate that Fe ions are highly mutagenic at the low doses and fluences of relevance to human spaceflight, and that cells with considerable genomic mutations are readily induced by these exposures and persist in the kidney epithelium. The level of genomic change produced by low fluence exposure to heavy ions is reminiscent of the extensive rearrangements seen in tumor genomes suggesting a potential initiation step in radiation carcinogenesis.

Role of Pro-inflammatory Cytokines in Radiation-Induced Genomic Instability in Human Bronchial Epithelial Cells

Inflammatory cytokines have been implicated in the regulation of radiation-induced genomic instability in the hematopoietic system and have also been shown to induce chronic DNA damage responses in radiation-induced senescence. We have previously shown that human bronchial epithelial cells (HBEC3-KT) have increased genomic instability and IL-8 production persisting at day 7 after exposure to high-LET (600 MeV/nucleon (56)Fe ions) compared to low-LET (320 keV X rays) radiation. Thus, we investigated whether IL-8 induction is part of a broader pro-inflammatory response produced by the epithelial cells in response to damage, which influences genomic instability measured by increased micronuclei and DNA repair foci frequencies. We found that exposure to radiation induced the release of multiple inflammatory cytokines into the media, including GM-CSF, GROα, IL-1α, IL-8 and the inflammation modulator, IL-1 receptor antagonist (IL-1RA). Our results suggest that this is an IL-1α-driven response, because an identical signature was induced by the addition of recombinant IL-1α to nonirradiated cells and functional interference with recombinant IL-1RA (Anakinra) or anti-IL-1α function-blocking antibody, decreased IL-8 production induced by radiation exposure. However, genomic instability was not influenced by this pathway as addition of recombinant IL-1α to naive or irradiated cells or the presence of IL-1 RA under the same conditions as those that interfered with the function of IL-8, did not affect micronuclei or DNA repair foci frequencies measured at day 7 after exposure. While dose-response studies revealed that genomic instability and IL-8 production are the consequences of targeted effects, experiments employing a co-culture transwell system revealed the propagation of pro-inflammatory responses but not genomic instability from irradiated to nonirradiated cells. Collectively, these results point to a cell-autonomous mechanism sustaining radiation-induced genomic instability in this model system and suggest that while molecules associated with these mechanisms could be markers for persisting damage, they reflect two different outcomes.

Numerical chromosomal instability mediates susceptibility to radiation treatment

The exquisite sensitivity of mitotic cancer cells to ionizing radiation (IR) underlies an important rationale for the widely used fractionated radiation therapy. However, the mechanism for this cell cycle-dependent vulnerability is unknown. Here we show that treatment with IR leads to mitotic chromosome segregation errors in vivo and long-lasting aneuploidy in tumour-derived cell lines. These mitotic errors generate an abundance of micronuclei that predispose chromosomes to subsequent catastrophic pulverization thereby independently amplifying radiation-induced genome damage. Experimentally suppressing whole-chromosome missegregation reduces downstream chromosomal defects and significantly increases the viability of irradiated mitotic cells. Further, orthotopically transplanted human glioblastoma tumours in which chromosome missegregation rates have been reduced are rendered markedly more resistant to IR, exhibiting diminished markers of cell death in response to treatment. This work identifies a novel mitotic pathway for radiation-induced genome damage, which occurs outside of the primary nucleus and augments chromosomal breaks. This relationship between radiation treatment and whole-chromosome missegregation can be exploited to modulate therapeutic response in a clinically relevant manner.

Biological response of cancer cells to radiation treatment

Cancer is a class of diseases characterized by uncontrolled cell growth and has the ability to spread or metastasize throughout the body. In recent years, remarkable progress has been made toward the understanding of proposed hallmarks of cancer development, care, and treatment modalities. Radiation therapy or radiotherapy is an important and integral component of cancer management, mostly conferring a survival benefit. Radiation therapy destroys cancer by depositing high-energy radiation on the cancer tissues. Over the years, radiation therapy has been driven by constant technological advances and approximately 50% of all patients with localized malignant tumors are treated with radiation at some point in the course of their disease. In radiation oncology, research and development in the last three decades has led to considerable improvement in our understanding of the differential responses of normal and cancer cells. The biological effectiveness of radiation depends on the linear energy transfer (LET), total dose, number of fractions and radiosensitivity of the targeted cells or tissues. Radiation can either directly or indirectly (by producing free radicals) damages the genome of the cell. This has been challenged in recent years by a newly identified phenomenon known as radiation induced bystander effect (RIBE). In RIBE, the non-irradiated cells adjacent to or located far from the irradiated cells/tissues demonstrate similar responses to that of the directly irradiated cells. Understanding the cancer cell responses during the fractions or after the course of irradiation will lead to improvements in therapeutic efficacy and potentially, benefitting a significant proportion of cancer patients. In this review, the clinical implications of radiation induced direct and bystander effects on the cancer cell are discussed.

Effect of radiation quality on mutagenic joining of enzymatically-induced DNA double-strand breaks in previously irradiated human cells

Previous work has shown that high charge and energy particle irradiation of human cells evokes a mutagenic repair phenotype, defined by increased mutagenic repair of new double-strand breaks that are introduced enzymatically, days or weeks after the initial irradiation. The effect was seen originally with 600 MeV/u (56)Fe particles, which have a linear energy transfer (LET) value of 174 keV/μm, but not with X rays or γ rays (LET ≤ 2 keV/μm). To better define the radiation quality dependence of the phenomenon, we tested two ions with intermediate LET values, 1,000 MeV/u (48)Ti (LET = 108 keV/μm) and 300 MeV/u (28)Si (LET = 69 keV/μm). These experiments used a previously validated assay, where a rare-cutting nuclease introduces double-strand breaks in two reporter transgene cassettes, which are located on different chromosomes. Deletions of a block of sequence in one of the cassettes, or translocations between cassettes, are measured independently using a multicolor fluorescence assay. The results showed that (48)Ti was a potent, but transient, inducer of mutagenic repair, based on increased frequency of nuclease-induced translocations. The (48)Ti ions did not affect the frequency of nuclease-induced deletions. The (28)Si ions had no measurable effect on either endpoint. There was a close correlation between the induction of the mutagenic repair phenomenon and the frequency of micronuclei in the targeted population (R(2) = 0.74), whereas there was no apparent correlation with radiation-induced cell inactivation. Together, these results better define the radiation quality dependence of the mutagenic repair phenomenon and establish its correlation, or lack of correlation, with other endpoints.

Autosomal mutants of proton-exposed kidney cells display frequent loss of heterozygosity on nonselected chromosomes

High-energy protons found in the space environment can induce mutations and cancer, which are inextricably linked. We hypothesized that some mutants isolated from proton-exposed kidneys arose through a genome-wide incident that causes loss of heterozygosity (LOH)-generating mutations on multiple chromosomes (termed here genomic LOH). To test this hypothesis, we examined 11 pairs of nonselected chromosomes for LOH events in mutant cells isolated from the kidneys of mice exposed to 4 or 5 Gy of 1 GeV protons. The mutant kidney cells were selected for loss of expression of the chromosome 8-encoded Aprt gene. Genomic LOH events were also assessed in Aprt mutants isolated from isogenic cultured kidney epithelial cells exposed to 5 Gy of protons in vitro. Control groups were spontaneous Aprt mutants and clones isolated without selection from the proton-exposed kidneys or cultures. The in vivo results showed significant increases in genomic LOH events in the Aprt mutants from proton-exposed kidneys when compared with spontaneous Aprt mutants and when compared with nonmutant (i.e., nonselected) clones from the proton-exposed kidneys. A bias for LOH events affecting chromosome 14 was observed in the proton-induced Aprt mutants, though LOH for this chromosome did not confer increased radiation resistance. Genomic LOH events were observed in Aprt mutants isolated from proton-exposed cultured kidney cells; however the incidence was fivefold lower than in Aprt mutants isolated from exposed intact kidneys, suggesting a more permissive environment in the intact organ and/or the evolution of kidney clones prior to their isolation from the tissue. We conclude that proton exposure creates a subset of viable cells with LOH events on multiple chromosomes, that these cells form and persist in vivo, and that they can be isolated from an intact tissue by selection for a mutation on a single chromosome.

NQO1 prevents radiation-induced aneuploidy by interacting with Aurora-A

Aneuploidy is the most common characteristic of human cancer cells. It also causes genomic instability, which is involved in the initiation of cancer development. Various lines of evidence indicate that nicotinamide adenine dinucleotide(P)H quinone oxidoreductase 1 (NQO1) plays an important role in cancer prevention, but the molecular mechanisms underlying this effect have not yet been fully elucidated. Here, we report that ionizing radiation (IR) induces substantial aneuploidy and centrosome amplification in NQO1-deficient cancer cells, suggesting that NQO1 plays a crucial role in preventing aneuploidy. NQO1 deficiency markedly increased the protein stability of Aurora-A in irradiated cancer cells. Small interfering RNA targeting Aurora-A effectively attenuated IR-induced centrosome amplification concerned with aneuploidy in NQO1-deficient cancer cells. Furthermore, we found that NQO1 specifically binds to Aurora-A via competing with the microtubule-binding protein, TPX2 (targeting protein for Xklp2), and contributes to the degradation of Aurora-A. Our results collectively demonstrate that NQO1 plays a key role in suppressing IR-induced centrosome amplification and aneuploidy through a direct interaction with Aurora-A.

Autosomal mutations in mouse kidney epithelial cells exposed to high-energy protons in vivo or in culture

Proton exposure induces mutations and cancer, which are presumably linked. Because protons are abundant in the space environment and significant uncertainties exist for the effects of space travel on human health, the purpose of this study was to identify the types of mutations induced by exposure of mammalian cells to 4-5 Gy of 1 GeV protons. We used an assay that selects for mutations affecting the chromosome 8-encoded Aprt locus in mouse kidney cells and selected mutants after proton exposure both in vivo and in cell culture. A loss of heterozygosity (LOH) assay for DNA preparations from the in vivo-derived kidney mutants revealed that protons readily induced large mutational events. Fluorescent in situ hybridization painting for chromosome 8 showed that >70% of proton-induced LOH patterns resembling mitotic recombination were in fact the result of nonreciprocal chromosome translocations, thereby demonstrating an important role for DNA double-strand breaks in proton mutagenesis. Large interstitial deletions, which also require the formation and resolution of double-strand breaks, were significantly induced in the cell culture environment (14% of all mutants), but to a lesser extend in vivo (2% of all mutants) suggesting that the resolution of proton-induced double-strand breaks can differ between the intact tissue and cell culture microenvironments. In total, the results demonstrate that double-strand break formation is a primary determinant for proton mutagenesis in epithelial cell types and suggest that resultant LOH for significant genomic regions play a critical role in proton-induced cancers.

High LET radiation amplifies centrosome overduplication through a pathway of γ-tubulin monoubiquitination

PURPOSE

Radiation induces centrosome overduplication, leading to mitotic catastrophe and tumorigenesis. Because mitotic catastrophe is one of the major tumor cell killing factors in high linear energy transfer (LET) radiation therapy and long-term survivors from such treatment have a potential risk of secondary tumors, we investigated LET dependence of radiation-induced centrosome overduplication and the underlying mechanism.

METHODS AND MATERIALS

Carbon and iron ion beams (13-200 keV/μm) and γ-rays (0.5 keV/μm) were used as radiation sources. To count centrosomes after IR exposure, human U2OS and mouse NIH3T3 cells were immunostained with antibodies of γ-tubulin and centrin 2. Similarly, Nbs1-, Brca1-, Ku70-, and DNA-PKcs-deficient mouse cells and their counterpart wild-type cells were used for measurement of centrosome overduplication.

RESULTS

The number of excess centrosome-containing cells at interphase and the resulting multipolar spindle at mitosis were amplified with increased LET, reaching a maximum level of 100 keV/μm, followed by sharp decrease in frequency. Interestingly, Ku70 and DNA-PKcs deficiencies marginally affected the induction of centrosome overduplication, whereas the cell killings were significantly enhanced. This was in contrast to observation that high LET radiation significantly enhanced frequencies of centrosome overduplication in Nbs1- and Brca1-deficient cells. Because NBS1/BRCA1 is implicated in monoubiquitination of γ-tubulin, we subsequently tested whether it is affected by high LET radiation. As a result, monoubiquitination of γ-tubulin was abolished in 48 to 72 hours after exposure to high LET radiation, although γ-ray exposure slightly decreased it 48 hours postirradiation and was restored to a normal level at 72 hours.

CONCLUSIONS

High LET radiation significantly reduces NBS1/BRCA1-mediated monoubiquitination of γ-tubulin and amplifies centrosome overduplication with a peak at 100 keV/μm. In contrast, Ku70 and DNA-PKcs deficiencies mitigate centrosome overduplication, although deficiencies of both NBS1/BRCA1 and Ku70/DNA-PKcs markedly enhance cell killing.

CD24(-/low) stem-like breast cancer marker defines the radiation-resistant cells involved in memorization and transmission of radiation-induced genomic instability

A growing body of evidence attributes properties of chemo- and/or radiation-resistance to cancer stem cells (CSCs). Moreover, non-targeted delayed effects such as genomic instability, transmitted through many generations, can be observed in the progeny of surviving irradiated cells. As a consequence, we propose that radiation-resistance properties associated to CSCs could confer a key role to this subpopulation in the transmission of genomic instability. To test this hypothesis, we searched the CSC markers associated to radiation-resistance in breast cancer cell lines and studied the role of the resistant cells in the transmission of genomic instability. First, we show that irradiation induces a 2-4 weeks period of intense cell death leading to the emergence of chromosomal unstable cells during more than 35 population doublings. Then, among seven breast CSC markers, we identify CD24(-/low) labelling as a marker of radiation-resistance. We demonstrate that CD24(+) progeny of irradiated cells exclusively descends from CD24(-/low) cells. Finally, we show that delayed chromosomal instability is only expressed by CD24(+) cells, but is transmitted by stable surviving CD24(-/low) cells. So, for the first time a CSC marker, CD24, is associated with the transmission of genomic instability. This work may assign a new deleterious role to breast CSCs in aggressive recurrence after radiotherapy, as the transmitted genomic instability potentially leads tumour cells to acquire more aggressive characteristics.

Persistence of γ-H2AX and 53BP1 foci in proliferating and non-proliferating human mammary epithelial cells after exposure to γ-rays or iron ions

PURPOSE

To investigate γ-H2AX (phosphorylated histone H2AX) and 53BP1 (tumour protein 53 binding protein No. 1) foci formation and removal in proliferating and non-proliferating human mammary epithelial cells (HMEC) after exposure to sparsely and densely ionising radiation under different cell culture conditions.

MATERIAL AND METHODS

HMEC cells were grown either as monolayers (2D) or in extracellular matrix to allow the formation of acinar structures in vitro (3D). Foci numbers were quantified by image analysis at various time points after exposure.

RESULTS

Our results reveal that in non-proliferating cells under 2D and 3D cell culture conditions, iron-ion induced γ-H2AX foci were still present at 72 h after exposure, although 53BP1 foci returned to control levels at 48 h. In contrast in proliferating HMEC, both γ-H2AX and 53BP1 foci decreased to control levels during the 24-48 h time interval after irradiation under 2D conditions. Foci numbers decreased faster after γ-ray irradiation and returned to control levels by 12 h regardless of marker, cell proliferation status, and cell culture condition.

CONCLUSIONS

The disappearance of radiation-induced γ-H2AX and 53BP1 foci in HMEC has different dynamics that depend on radiation quality and proliferation status. Notably, the general patterns do not depend on the cell culture condition (2D versus 3D). We speculate that the persistent γ-H2AX foci in iron-ion irradiated non-proliferating cells could be due to limited availability of double-strand break (DSB) repair pathways in G0/G1-phase, or that repair of complex DSB requires replication or chromatin remodelling.

Heavy ions, radioprotectors and genomic instability: implications for human space exploration

The risk associated with space radiation exposure is unique from terrestrial radiation exposures due to differences in radiation quality, including linear energy transfer (LET). Both high- and low-LET radiations are capable of inducing genomic instability in mammalian cells, and this instability is thought to be a driving force underlying radiation carcinogenesis. Unfortunately, during space exploration, flight crews cannot entirely avoid radiation exposure. As a result, chemical and biological countermeasures will be an important component of successful extended missions such as the exploration of Mars. There are currently several radioprotective agents (radioprotectors) in use; however, scientists continue to search for ideal radioprotective compounds-safe to use and effective in preventing and/or reducing acute and delayed effects of irradiation. This review discusses the agents that are currently available or being evaluated for their potential as radioprotectors. Further, this review discusses some implications of radioprotection for the induction and/or propagation of genomic instability in the progeny of irradiated cells.

Radiation-induced premature senescence is associated with specific cytogenetic changes

In the present study, we set out to investigate cytogenetic changes in the progeny of two normal human fibroblast cell strains after exposure to sparsely or densely ionizing irradiation (X-rays or 9.8 MeV u(-1) carbon ions). The cells were regularly subcultured up to senescence. The transition to senescence was determined by measurement of population doubling numbers and senescence associated (SA) beta-galactosidase activity. Chromosomal changes (structural aberrations, tetraploidy) were investigated by solid staining. In temporal proximity to senescence, we observed for all populations of the two fibroblasts cell strains an increase in the fraction of cells with structural and numerical aberrations. The observed changes in the yield of structural chromosomal aberrations were similar for the progeny of controls and irradiated cells, except that a previous irradiation with a high, fractionated X-ray dose resulted in a stronger increase. Noteworthy, delayed tetraploidy in the descendants of irradiated cells exceeded the level in control cells. In addition, tetraploidy and the time of onset of senescence were significantly correlated for all populations, regardless of a preceding radiation exposure. However, the time of the onset of senescence depends on previous exposure to radiation. We conclude that the occurrence of tetraploidy is associated with senescence independently of exposure to radiation.

Promotion of variant human mammary epithelial cell outgrowth by ionizing radiation: an agent-based model supported by in vitro studies

Introduction

Most human mammary epithelial cells (HMEC) cultured from histologically normal breast tissues enter a senescent state termed stasis after 5 to 20 population doublings. These senescent cells display increased size, contain senescence associated β-galactosidase activity, and express cyclin-dependent kinase inhibitor, p16^INK4A (CDKN2A; p16). However, HMEC grown in a serum-free medium, spontaneously yield, at low frequency, variant (v) HMEC that are capable of long-term growth and are susceptible to genomic instability. We investigated whether ionizing radiation, which increases breast cancer risk in women, affects the rate of vHMEC outgrowth.

Methods

Pre-stasis HMEC cultures were exposed to 5 to 200 cGy of sparsely (X- or γ-rays) or densely (1 GeV/amu ⁵⁶Fe) ionizing radiation. Proliferation (bromodeoxyuridine incorporation), senescence (senescence-associated β-galactosidase activity), and p16 expression were assayed in subcultured irradiated or unirradiated populations four to six weeks following radiation exposure, when patches of vHMEC became apparent. Long-term growth potential and p16 promoter methylation in subsequent passages were also monitored. Agent-based modeling, incorporating a simple set of rules and underlying assumptions, was used to simulate vHMEC outgrowth and evaluate mechanistic hypotheses.

Results

Cultures derived from irradiated cells contained significantly more vHMEC, lacking senescence associated β-galactosidase or p16 expression, than cultures derived from unirradiated cells. As expected, post-stasis vHMEC cultures derived from both unirradiated and irradiated cells exhibited more extensive methylation of the p16 gene than pre-stasis HMEC cultures. However, the extent of methylation of individual CpG sites in vHMEC samples did not correlate with passage number or treatment. Exposure to sparsely or densely ionizing radiation elicited similar increases in the numbers of vHMEC compared to unirradiated controls. Agent-based modeling indicated that radiation-induced premature senescence of normal HMEC most likely accelerated vHMEC outgrowth through alleviation of spatial constraints. Subsequent experiments using defined co-cultures of vHMEC and senescent cells supported this mechanism.

Conclusions

Our studies indicate that ionizing radiation can promote the outgrowth of epigenetically altered cells with pre-malignant potential.

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.

Radiation carcinogenesis in context: how do irradiated tissues become tumors?

It is clear from experimental studies that genotype is an important determinant of cancer susceptibility in general, and for radiation carcinogenesis specifically. It has become increasingly clear that genotype influences not only the ability to cope with DNA damage but also influences the cooperation of other tissues, like the vasculature and immune system, necessary for the establishment of cancer. Our experimental data and that of others suggest that the carcinogenic action of ionizing radiation (IR) can also be considered a two-compartment problem: while IR can alter genomic sequence as a result of DNA damage, it can also induce signals that alter multicellular interactions and phenotypes that underpin carcinogenesis. Rather than being accessory or secondary to genetic damage, we propose that such non-targeted radiation effects create the critical context that promotes cancer development. This review focuses on experimental studies that clearly define molecular mechanisms by which cell interactions contribute to cancer in different organs, and addresses how non-targeted radiation effects may similarly act though the microenvironment. The definition of non-targeted radiation effects and their dose dependence could modify the current paradigms for radiation risk assessment since radiation non-targeted effects, unlike DNA damage, are amenable to intervention. The implications of this perspective in terms of reducing cancer risk after exposure are discussed.

Targeted and nontargeted effects of ionizing radiation that impact genomic instability

Radiation-induced genomic instability, in which the progeny of irradiated cells display a high frequency of nonclonal genomic damage, occurs at a frequency inconsistent with mutation. We investigated the mechanism of this nontargeted effect in human mammary epithelial cells (HMEC) exposed to low doses of radiation. We identified a centrosome-associated expression signature in irradiated HMEC and show here that centrosome deregulation occurs in the first cell cycle after irradiation, is dose dependent, and that viable daughters of these cells are genomically unstable as evidenced by spontaneous DNA damage, tetraploidy, and aneuploidy. Clonal analysis of genomic instability showed a threshold of >10 cGy. Treatment with transforming growth factor beta1 (TGFbeta), which is implicated in regulation of genomic stability and is activated by radiation, reduced both the centrosome expression signature and centrosome aberrations in irradiated HMEC. Furthermore, TGFbeta inhibition significantly increased centrosome aberration frequency, tetraploidy, and aneuploidy in nonirradiated HMEC. Rather than preventing radiation-induced or spontaneous centrosome aberrations, TGFbeta selectively deleted unstable cells via p53-dependent apoptosis. Together, these studies show that radiation deregulates centrosome stability, which underlies genomic instability in normal human epithelial cells, and that this can be opposed by radiation-induced TGFbeta signaling.