Tobacco-specific nitrosamine 1-(N-methyl-N-nitrosamino)-1-(3-pyridinyl)-4-butanal (NNA) causes DNA damage and impaired replication/transcription in human lung cells

Thirdhand smoke (THS) is a newly described health hazard composed of toxicants, mutagens and carcinogens, including nicotine-derived tobacco specific nitrosamines (TSNAs), one of which is 1-(N-methyl-N-nitrosamino)-1-(3-pyridinyl)-4-butanal (NNA). Although TSNAs are generally potent carcinogens, the risk of NNA, which is specific to THS, is poorly understood. We recently reported that THS exposure-induced adverse impact on DNA replication and transcription with implications in the development of cancer and other diseases. Here, we investigated the role of NNA in THS exposure-induced harmful effects on fundamental cellular processes. We exposed cultured human lung epithelial BEAS-2B cells to NNA. The formation of DNA base damages was assessed by Long Amplicon QPCR (LA-QPCR); DNA double-strand breaks (DSBs) and NNA effects on replication and transcription by immunofluorescence (IF); and genomic instability by micronuclei (MN) formation. We found increased accumulation of oxidative DNA damage and DSBs as well as activation of DNA damage response pathway, after exposure of cells to NNA. Impaired S phase progression was also evident. Consistent with these results, we found increased MN formation, a marker of genomic instability, in NNA-exposed cells. Furthermore, ongoing RNA synthesis was significantly reduced by NNA exposure, however, RNA synthesis resumed fully after a 24h recovery period only in wild-type cells but not in those deficient in transcription-coupled nucleotide excision repair (TC-NER). Importantly, these cellular effects are common with the THS-exposure induced effects. Our findings suggest that NNA in THS could be a contributing factor for THS exposure-induced adverse health effect.

Thirdhand smoke exposure causes replication stress and impaired transcription in human lung cells

Thirdhand cigarette smoke (THS) is a newly described toxin that lingers in the indoor environment long after cigarettes have been extinguished. Emerging results from both cellular and animal model studies suggest that THS is a potential human health hazard. DNA damage derived from THS exposure could have genotoxic consequences that would lead to the development of diseases. However, THS exposure-induced interference with fundamental DNA transactions such as replication and transcription, and the role of DNA repair in ameliorating such effects, remain unexplored. Here, we found that THS exposure increased the percentage of cells in S-phase, suggesting impaired S-phase progression. Key DNA damage response proteins including RPA, ATR, ATM, CHK1, and BRCA1 were activated in lung cells exposed to THS, consistent with replication stress. In addition, THS exposure caused increased 53BP1 foci, indicating DNA double-strand break induction. Consistent with these results, we observed increased micronuclei formation, a marker of genomic instability, in THS-exposed cells. Exposure to THS also caused a significant increase in phosphorylated RNA Polymerase II engaged in transcription elongation, suggesting an increase in transcription-blocking lesions. In agreement with this conclusion, ongoing RNA synthesis was very significantly reduced by THS exposure. Loss of nucleotide excision repair exacerbated the reduction in RNA synthesis, suggesting that bulky DNA adducts formed by THS are blocks to transcription. The adverse impact on both replication and transcription supports genotoxic stress as a result of THS exposure, with important implications for both cancer and other diseases.

Mutational signatures in tumours induced by high and low energy radiation in Trp53 deficient mice

Ionising radiation (IR) is a recognised carcinogen responsible for cancer development in patients previously treated using radiotherapy, and in individuals exposed as a result of accidents at nuclear energy plants. However, the mutational signatures induced by distinct types and doses of radiation are unknown. Here, we analyse the genetic architecture of mammary tumours, lymphomas and sarcomas induced by high (56Fe-ions) or low (gamma) energy radiation in mice carrying Trp53 loss of function alleles. In mammary tumours, high-energy radiation is associated with induction of focal structural variants, leading to genomic instability and Met amplification. Gamma-radiation is linked to large-scale structural variants and a point mutation signature associated with oxidative stress. The genomic architecture of carcinomas, sarcomas and lymphomas arising in the same animals are significantly different. Our study illustrates the complex interactions between radiation quality, germline Trp53 deficiency and tissue/cell of origin in shaping the genomic landscape of IR-induced tumours.

Non-catalytic Roles for XPG with BRCA1 and BRCA2 in Homologous Recombination and Genome Stability

XPG is a structure-specific endonuclease required for nucleotide excision repair, and incision-defective XPG mutations cause the skin cancer-prone syndrome xeroderma pigmentosum. Truncating mutations instead cause the neurodevelopmental progeroid disorder Cockayne syndrome, but little is known about how XPG loss results in this devastating disease. We identify XPG as a partner of BRCA1 and BRCA2 in maintaining genomic stability through homologous recombination (HRR). XPG depletion causes DNA double-strand breaks, chromosomal abnormalities, cell-cycle delays, defective HRR, inability to overcome replication fork stalling, and replication stress. XPG directly interacts with BRCA2, RAD51, and PALB2, and XPG depletion reduces their chromatin binding and subsequent RAD51 foci formation. Upstream in HRR, XPG interacts directly with BRCA1. Its depletion causes BRCA1 hyper-phosphorylation and persistent chromatin binding. These unexpected findings establish XPG as an HRR protein with important roles in genome stability and suggest how XPG defects produce severe clinical consequences including cancer and accelerated aging.

Micronucleus formation in human keratinocytes is dependent on radiation quality and tissue architecture

The cytokinesis-block micronucleus (MN) assay was used to assess the genotoxicity of low doses of different types of space radiation. Normal human primary keratinocytes and immortalized keratinocytes grown in 2D monolayers each were exposed to graded doses of 0.3 or 1.0 GeV/n silicon ions or similar energies of iron ions. The frequencies of induced MN were determined and compared to γ-ray data. RBE(max) values ranged from 1.6 to 3.9 for primary keratinocytes and from 2.4 to 6.3 for immortalized keratinocytes. At low radiation doses ≤ 0.4 Gy, 0.3 GeV/n iron ions were the most effective at inducing MN in normal keratinocytes. An "over-kill effect" was observed for 0.3 GeV/n iron ions at higher doses, wherein 1.0 GeV/n iron ions were most efficient in inducing MN. In immortalized keratinocytes, 0.3 GeV/n iron ions produced MN with greater frequency than 1.0 GeV/n iron ions, except at the highest dose tested. MN formation was higher in immortalized keratinocytes than in normal keratinocytes for all doses and radiation qualities investigated. MN induction was also assessed in human keratinocytes cultured in 3D to simulate the complex architecture of human skin. RBE values for MN formation in 3D were reduced for normal keratinocytes exposed to iron ions, but were elevated for immortalized keratinocytes. Overall, MN induction was significantly lower in keratinocytes cultured in 3D than in 2D. Together, the results suggest that tissue architecture and immortalization status modulate the genotoxic response to space radiation, perhaps via alterations in DNA repair fidelity.

Opposite roles for p38MAPK-driven responses and reactive oxygen species in the persistence and resolution of radiation-induced genomic instability

We report the functional and temporal relationship between cellular phenotypes such as oxidative stress, p38MAPK-dependent responses and genomic instability persisting in the progeny of cells exposed to sparsely ionizing low-Linear Energy Transfer (LET) radiation such as X-rays or high-charge and high-energy (HZE) particle high-LET radiation such as ⁵⁶Fe ions. We found that exposure to low and high-LET radiation increased reactive oxygen species (ROS) levels as a threshold-like response induced independently of radiation quality and dose. This response was sustained for two weeks, which is the period of time when genomic instability is evidenced by increased micronucleus formation frequency and DNA damage associated foci. Indicators for another persisting response sharing phenotypes with stress-induced senescence, including beta galactosidase induction, increased nuclear size, p38MAPK activation and IL-8 production, were induced in the absence of cell proliferation arrest during the first, but not the second week following exposure to high-LET radiation. This response was driven by a p38MAPK-dependent mechanism and was affected by radiation quality and dose. This stress response and elevation of ROS affected genomic instability by distinct pathways. Through interference with p38MAPK activity, we show that radiation-induced stress phenotypes promote genomic instability. In contrast, exposure to physiologically relevant doses of hydrogen peroxide or increasing endogenous ROS levels with a catalase inhibitor reduced the level of genomic instability. Our results implicate persistently elevated ROS following exposure to radiation as a factor contributing to genome stabilization.

Uncomfortable issues in radiation protection posed by low-dose radiobiology

This paper aims to stimulate discussion about the relevance for radiation protection of recent findings in low-dose radiobiology. Issues are raised which suggest that low-dose effects are much more complex than has been previously assumed. These include genomic instability, bystander effects, multiple stressor exposures and chronic exposures. To date, these have been accepted as being relevant issues, but there is no clear way to integrate knowledge about these effects into the existing radiation protection framework. A further issue which might actually lead to some fruitful approaches for human radiation protection is the need to develop a new framework for protecting non-human biota. The brainstorming that is being applied to develop effective and practical ways to protect ecosystems widens the debate from the narrow focus of human protection which is currently about protecting humans from radiation-induced cancers.

DNA double strand breaks induced by the indirect effect of radiation are more efficiently repaired by non-homologous end joining compared to homologous recombination repair

The aim of this study was to investigate the relative involvement of three major DNA repair pathways, i.e., non-homologous end joining (NHEJ), homologous recombination (HRR) and base excision (BER) in repair of DNA lesions of different complexity induced by low- or high-LET radiation with emphasis on the contribution of the indirect effect of radiation for these radiation qualities. A panel of DNA repair-deficient CHO cell lines was irradiated by (137)Cs γ-rays or radon progeny α-particles. Irradiation was also performed in the presence of 2M DMSO to reduce the indirect effect of radiation and the complexity of the DNA damage formed. Clonogenic survival and micronucleus assays were used to estimate efficiencies of the different repair pathways for DNA damages produced by direct and indirect effects. Removal of the indirect effect of low-LET radiation by DMSO increased clonogenic survival and decreased MN formation for all cell lines investigated. A direct contribution of the indirect effect of radiation to DNA base damage was suggested by the significant protection by DMSO seen for the BER deficient cell line. Lesions formed by the indirect effect are more readily repaired by the NHEJ pathway than by HRR after irradiation with γ-rays or α-particles as evaluated by cell survival and the yields of MN. The results obtained with BER- and NHEJ-deficient cells suggest that the indirect effect of radiation contributes significantly to the formation of repair substrates for these pathways.

Ionizing radiation increases adhesiveness of human aortic endothelial cells via a chemokine-dependent mechanism

Exposure to radiation from a variety of sources is associated with increased risk of heart disease and stroke. Since radiation also induces inflammation, a possible mechanism is a change in the adhesiveness of vascular endothelial cells, triggering pro-atherogenic accumulation of leukocytes. To investigate this mechanism at the cellular level, the effect of X rays on adhesiveness of cultured human aortic endothelial cells (HAECs) was determined. HAECs were grown as monolayers and exposed to 0 to 30 Gy X rays, followed by measurement of adhesiveness under physiological shear stress using a flow chamber adhesion assay. Twenty-four hours after irradiation, HAEC adhesiveness was increased, with a peak effect at 15 Gy. Radiation had no significant effect on surface expression of the endothelial adhesion molecules ICAM-1 and VCAM-1. Antibody blockade of the leukocyte integrin receptors for ICAM-1 and VCAM-1, however, abolished the radiation-induced adhesiveness. Since these leukocyte integrins can be activated by chemokines presented on the endothelial cell surface, the effect of pertussis toxin (PTX), an inhibitor of chemokine-mediated integrin activation, was tested. PTX specifically inhibited radiation-induced adhesiveness, with no significant effect on nonirradiated cells. Therefore, radiation induces increased adhesiveness of aortic endothelial cells through chemokine-dependent signaling from endothelial cells to leukocytes, even in the absence of increased expression of the adhesion molecules involved.

Effects of low-dose heavy ions on the postnatal development of mice and the yield of white spots in the mid-ventrum and tail-tips

Effects of prenatal low-dose irradiations of heavy ions on the postnatal development of mice and of melanocytes have not been well studied. Pregnant females of C57BL/10J mice were irradiated whole-body at 9 days of gestation with a single acute dose of γ-rays, silicon (Si, 57 keV/µm), argon (Ar, 100 keV/µm) and iron (Fe, 220 keV/µm) ions. The effects were studied by scoring changes in the postnatal development of mice as well as in the pigmentation of cutaneous coats and tail-tips of their offspring 22 days after birth. The survival to day 22 decreased from the offspring exposed to 0.4 Gy of argon and iron ions and to 0.75 Gy of silicon ions. White spots were found in the mid-ventrum and tail-tips of irradiated offspring. The frequency and size of the white spots in the mid-ventrum in mice exposed to silicon, argon and iron ions were greater than those of γ-rays. Even in the low dose (0.1 Gy), γ-rays and heavy ions increased the frequency of the ventral spots. The RBE estimated by the frequency of the ventral spots was 2.3 (Si), 3.1 (Ar) and 4.5 (Fe). These results suggest that prenatal exposure to heavy ions possesses a greater effect on the postnatal development of mice as well as melanocyte development than does exposure to γ-rays.

Lipids, mitochondria and cell death: implications in neuro-oncology

Polyunsaturated fatty acids (PUFAs) are known to inhibit cell proliferation of many tumour types both in vitro and in vivo. Their capacity to interfere with cell proliferation has been linked to their induction of reactive oxygen species (ROS) production in tumour tissues leading to cell death through apoptosis. However, the exact mechanisms of action of PUFAs are far from clear, particularly in brain tumours. The loss of bound hexokinase from the mitochondrial voltage-dependent anion channel has been directly related to loss of protection from apoptosis, and PUFAs can induce this loss of bound hexokinase in tumour cells. Tumour cells overexpressing Akt activity, including gliomas, are sensitised to ROS damage by the Akt protein and may be good targets for chemotherapeutic agents, which produce ROS, such as PUFAs. Cardiolipin peroxidation may be an initial event in the release of cytochrome c from the mitochondria, and enriching cardiolipin with PUFA acyl chains may lead to increased peroxidation and therefore an increase in apoptosis. A better understanding of the metabolism of fatty acids and eicosanoids in primary brain tumours such as gliomas and their influence on energy balance will be fundamental to the possible targeting of mitochondria in tumour treatment.

Gamma-linolenic acid alters Ku80, E2F1, and bax expression and induces micronucleus formation in C6 glioma cells in vitro

Gamma-linolenic acid (GLA) is an inhibitor of tumor cell proliferation in both in vitro and in vivo conditions. The aim of this study was to investigate the effects of 150 muM GLA on the expression of E2F1, cyclin D1, bax, bcl2, Ku70, and Ku80 in C6 rat glioma cells. The Ku proteins were chosen as previous studies have shown that loss or reduction in their expression causes increased DNA damage and micronucleus formation in the presence of radiation. The fact that GLA exposure is known to enhance the efficacy of radiation treatment raised the question whether the Ku proteins could be involved in this effect as seen for other molecules such as roscovitine and flavopiridol. GLA altered the mRNA expression of E2F1, cyclin D1, and bax, but no changes were found for bcl2, Ku70, and Ku80. Alterations in protein expression were observed for bax, Ku80, and E2F1. The 45% decrease in E2F1 expression was proportional to decreased cell proliferation (44%). Morphological analysis found a 25% decrease in mitotic activity in the GLA-treated cells, which was accompanied by a 49% decrease in S-phase by FACS analysis. A 39% increase in the number of micronuclei detected by Hoechst fluorescence points to GLA's effects on cell division even at concentrations that do not produce significant increases in apoptosis. Most important was the finding that Ku80 expression, a critical protein involved in DNA repair as a heterodimer with Ku70, was decreased by 71%. It is probable that reduced Ku80 is responsible for the increase in micronucleus formation in GLA-treated cells in a similar manner to that found in Ku80 null cells exposed to radiation. The decreased expression of Ku80 and E2F1 could make cells more susceptible to radiotherapy and chemotherapy.

Modeling radiation-induced cell cycle delays

Ionizing radiation is known to delay the cell cycle progression. In particular after particle exposure significant delays have been observed and it has been shown that the extent of delay affects the expression of damage, such as chromosome aberrations. Thus, to predict how cells respond to ionizing radiation and to derive reliable estimates of radiation risks, information about radiation-induced cell cycle perturbations is required. In the present study we describe and apply a method for retrieval of information about the time-course of all cell cycle phases from experimental data on the mitotic index only. We study the progression of mammalian cells through the cell cycle after exposure. The analysis reveals a prolonged block of damaged cells in the G2 phase. Furthermore, by performing an error analysis on simulated data valuable information for the design of experimental studies has been obtained. The analysis showed that the number of cells analyzed in an experimental sample should be at least 100 to obtain a relative error <20%.

Differential effects of x-rays and high-energy 56Fe ions on human mesenchymal stem cells

PURPOSE

Stem cells hold great potential for regenerative medicine, but they have also been implicated in cancer and aging. How different kinds of ionizing radiation affect stem cell biology remains unexplored. This study was designed to compare the biological effects of X-rays and of high-linear energy transfer (LET) (56)Fe ions on human mesenchymal stem cells (hMSC).

METHODS AND MATERIALS

A multi-functional comparison was carried out to investigate the differential effects of X-rays and (56)Fe ions on hMSC. The end points included modulation of key markers such as p53, cell cycle progression, osteogenic differentiation, and pathway and networks through transcriptomic profiling and bioinformatics analysis.

RESULTS

X-rays and (56)Fe ions differentially inhibited the cell cycle progression of hMSC in a p53-dependent manner without impairing their in vitro osteogenic differentiation process. Pathway and network analyses revealed that cytoskeleton and receptor signaling were uniquely enriched for low-dose (0.1 Gy) X-rays. In contrast, DNA/RNA metabolism and cell cycle regulation were enriched for high-dose (1 Gy) X-rays and (56)Fe ions, with more significant effects from (56)Fe ions. Specifically, DNA replication, DNA strand elongation, and DNA binding/transferase activity were perturbed more severely by 1 Gy (56)Fe ions than by 1 Gy X-rays, consistent with the significant G2/M arrest for the former while not for the latter.

CONCLUSIONS

(56)Fe ions exert more significant effects on hMSC than X-rays. Since hMSC are the progenitors of osteoblasts in vivo, this study provides new mechanistic understandings of the relative health risks associated with low- and high-dose X-rays and high-LET space radiation.