Recent progress on the Chinese space programme and radiation research

Manned space exploration was initiated in China in 1992, and substantial progress has been made. The next step is to build the Chinese Space Station (CSS), which is planned to be launched in 2020. The CSS will provide an on-orbit laboratory for experimental studies including space radiation research. The health risk of space radiation, especially carcinogenesis, is a major concern for long-term space exploration. Establishing a risk assessment system suitable for Chinese astronauts and developing effective countermeasures are major tasks for Chinese space radiobiologists. The Institute of Space Life Sciences, Soochow University has focused on these topics for years. We established cancer models with low-dose-rate exposure of alpha particles, and elucidated a microRNA-TGFβ network regulating bystander effects and a lncRNA-cytoskeleton network regulating genomic instability induced by ionising radiation. We also confirmed the radioresistance of quiescent cells, which inspires a potential strategy to improve individual radioresistance during long-term space travel. However, we believe that a multi-disciplinary strategy must be developed to protect astronauts from highly energised space radiation.

Connexins and cyclooxygenase-2 crosstalk in the expression of radiation-induced bystander effects

Background:

Signalling events mediated by connexins and cyclooxygenase-2 (COX-2) have important roles in bystander effects induced by ionising radiation. However, whether these proteins mediate bystander effects independently or cooperatively has not been investigated.

Methods:

Bystander normal human fibroblasts were cocultured with irradiated adenocarcinoma HeLa cells in which specific connexins (Cx) are expressed in the absence of endogenous Cx, before and after COX-2 knockdown, to investigate DNA damage in bystander cells and their progeny.

Results:

Inducible expression of gap junctions composed of connexin26 (Cx26) in irradiated HeLa cells enhanced the induction of micronuclei in bystander cells (P<0.01) and reduced the coculture time necessary for manifestation of the effect. In contrast, expression of connexin32 (Cx32) conferred protective effects. COX-2 knockdown in irradiated HeLa Cx26 cells attenuated the bystander response due to connexin expression. However, COX-2 knockdown resulted in enhanced micronucleus formation in the progeny of the bystander cells (P<0.001). COX-2 knockdown delayed junctional communication in HeLa Cx26 cells, and reduced, in the plasma membrane, the physical interaction of Cx26 with MAPKKK, a controller of the MAPK pathway that regulates COX-2 and connexin.

Conclusions:

Junctional communication and COX-2 cooperatively mediate the propagation of radiation-induced non-targeted effects. Characterising the mediating events affected by both mechanisms may lead to new approaches that mitigate secondary debilitating effects of cancer radiotherapy.

Integrated interdisciplinary training in the radiological sciences

The radiation sciences are increasingly interdisciplinary, both from the research and the clinical perspectives. Beyond clinical and research issues, there are very real issues of communication between scientists from different disciplines. It follows that there is an increasing need for interdisciplinary training courses in the radiological sciences. Training courses are common in biomedical academic and clinical environments, but are typically targeted to scientists in specific technical fields. In the era of multidisciplinary biomedical science, there is a need for highly integrated multidisciplinary training courses that are designed for, and are useful to, scientists who are from a mix of very different academic fields and backgrounds. We briefly describe our experiences running such an integrated training course for researchers in the field of biomedical radiation microbeams, and draw some conclusions about how such interdisciplinary training courses can best function. These conclusions should be applicable to many other areas of the radiological sciences. In summary, we found that it is highly beneficial to keep the scientists from the different disciplines together. In practice, this means not segregating the training course into sections specifically for biologists and sections specifically for physicists and engineers, but rather keeping the students together to attend the same lectures and hands-on studies throughout the course. This structure added value to the learning experience not only in terms of the cross fertilization of information and ideas between scientists from the different disciplines, but also in terms of reinforcing some basic concepts for scientists in their own discipline.

Radiation-induced non-targeted response in vivo: role of the TGFβ-TGFBR1-COX-2 signalling pathway

Background:

Previous studies from our group and others have shown that cyclooxygenase-2 (COX-2) has an essential role in radiation-induced non-targeted responses and genomic instability in vivo. However, the signalling pathways involved in such effects remain unclear.

Methods:

A 1 cm² area (1 cm × 1 cm) in the lower abdominal region of gpt delta transgenic mice was irradiated with 5 Gy of 300 keV X-rays. Nimesulide, a selective COX-2 inhibitor, was given to mice for five consecutive days before irradiation. Changes in transforming growth factor-beta (TGF-β) and TGF-β receptor type-1 (TGFBR1) mediated signalling pathways, in the out of radiation field lung and liver tissues were examined.

Results:

While the plasma level of cytokines remained unchanged, the expression of TGF-β and its receptors was elevated in non-targeted lung tissues after partial body irradiation. In contrast to the predominant expression of TGF-β in stromal and alveolar cells, but not in bronchial epithelial cells, TGF-β receptors, especially TGFBR1 were significantly elevated in non-targeted bronchial epithelial cells, which is consistent with the induction of COX-2. The different expression levels of TGFBR1 between liver and lung resulted in a tissue specific induction of COX-2 in these two non-targeted tissues. Multiple TGF-β induced signalling pathways were activated in the non-targeted lung tissues.

Conclusion:

The TGFβ-TGFBR1-COX-2 Signalling Pathway has a critical role in radiation-induced non-targeted response in vivo.

Abstract PD09-09: The Akt inhibitor MK-2206 is an effective radio-sensitizer of p53 deficient triple negative breast cancer (TNBC) cells

Purpose: Triple negative breast cancer (TNBC) is an aggressive tumor with a higher locoregional recurrence compared to estrogen receptor (ER) positive breast cancer. Mutations in the PI3K/Akt/mTOR pathway leading to up-regulation of Akt occur with high frequency in TNBC. Hyperactive Akt is associated with increase radiation resistance, mediated through inhibition of apoptosis and up-regulation of DNA damage-induced G2 arrest. p53 plays a key role in mediating G1 cell cycle arrest following genotoxic stress such as radiation; p53-deficient cells however must rely on alternate mechanisms for cell cycle arrest in S- and G2-phases. Inhibition of Akt therefore would be expected to act in a synthetic lethal fashion to radiosensitize p53 deficient TNBC cells by decreasing their ability to arrest in G2. Given that p53 mutations occur in 44% of TNBC this may be an effective strategy for radiosensitizing TNBC.

Methods: Experiments were conducted using MDA-MB-468, MDA-MB-231 and SUM 149 cells, all well established models for p53-deficient TNBC. Cells were treated with MK-2206, a potent allosteric Akt inhibitor, alone or in combination with increasing doses of radiation from 0–8 Gy. In vitro studies preformed included; cell survival assays, MTT assay, immunoblot analysis of key proteins, FACS analysis for cell cycle, apoptosis, and gH2AX staining.

Results: We found that the combination of IR and Akt inhibition by MK-2206 is synergistic. MK-2206 inhibited both endogenous and radiation-induced Akt activation and phosphorylation of its down-stream targets. Decreased clonogenic survival was seen after 2 or 24 hours pretreatment with MK-2206 as well as decreased viability by MTT assay. Cell cycle analysis demonstrates MK-2206 decreases the proportion of cells in G2/M arrest following irradiation, as well as induced a greater proportion of apoptosis. Evaluation of the DNA damage response pathway demonstrated decreased levels of total DNA-PK, as well as a delayed DNA damage response.

Conclusions: These studies demonstrate that targeted inhibition of Akt effectively radiosensitizes p53 deficient TNBC cells lines, possibly through a synthetic lethal effect on the DNA damage response.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD09-09.

Phosphoproteomic profiling of arsenite-treated human small airway epithelial cells

Arsenic is well documented as a chemotherapeutic agent capable of inducing cell death; however, it is also considered as a human carcinogen. Although it has recently been shown that arsenite exposure can potentiate genotoxicity, little is known about its global effects exerted in cells at the proteome level. Immortalized human small airway epithelial cells exposed to arsenite were used to identify phosphoproteins of two major signaling cascades, such as the human phospho-receptor tyrosine kinase (Phospho-RTK) and the mitogen-activated protein kinases (MAPKs). These two arrays included several phosphoproteins, such as EGFR, ErbB2, ErbB4, InsulinR, Flt-3, extracellular signal-regulated kinases (ERK1/2), intracellular kinases such as AKT, GSK-3, c-Jun N-terminal kinases (JNK1-3) and different p38 isoforms (alpha/beta/delta/gamma). In arsenite-treated cells, phosphorylation of EGFR, InsulinR and Flt3R showed an increase when compared to their non-arsenite treated counterparts. Inhibitors of these proteins further confirmed the involvement of such proteins in the neoplasm transformation of arsenite-treated human small airway epithelial cells as seen in changes in plating efficiency, anchorage-independent growth and proliferation rate. It can be concluded that analysis of phosphoprotein by using phosphoproteomic profiling can be very useful to understand the mechanism of arsenite-induced carcinogenesis.

Constitutive nitric oxide acting as a possible intercellular signaling molecule in the initiation of radiation-induced DNA double strand breaks in non-irradiated bystander cells

The initiation and propagation of the early processes of bystander signaling induced by low-dose alpha-particle irradiation are very important for understanding the underlying mechanism of the bystander process. Our previous investigation showed that the medium collected from cell culture exposed to low-dose alpha-particle rapidly induced phosphorylated form of H2AX protein foci formation among the non-irradiated medium receptor cells in a time-dependent manner. Using N(G)-methyl-L-arginine, 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate and N(omega)-nitro-L-arginine (L-NNA) treatment before exposure to 1 cGy alpha-particle, we showed in the present study that nitric oxide (NO(*)) produced in the irradiated cells was important and necessary for the DNA double strand break inducing activity (DIA) of conditioned medium and the generation of NO(*) in irradiated confluent AG1522 cells is in a time-dependent manner and that almost all NO(*) was generated within 15 min post-irradiation. Concurrently, the kinetics of NO(*) production in the medium of irradiated cells after irradiation was rapid and in a time-dependent manner as well, with a maximum yield observed at 10 min after irradiation with electron spin resonance analysis. Furthermore, our results that 7-Nitroindazole and L-NNA, but not aminoguanidine hemisulfate, treatment before exposure to 1 cGy alpha-particle significantly decrease the DIA of the conditioned medium suggested that constitutive NO(*) from the irradiated cells possibly acted as an intercellular signaling molecule to initiate and activate the early process (<or=30 min) of bystander response after low-dose irradiation.

Allelic imbalance at 11q23-q24 chromosome associated with estrogen and radiation-induced breast cancer progression

Multiple genetic alterations are common in cancers including those of the breast. The mechanisms leading to these alterations such as point mutations, gene amplifications, deletions and replication error are often associated with frequent and consistent loss of heterozygosity (LOH) or microsatellite instability (MSI). Several cytological and molecular studies have shown high frequency loss of genetic information on the long arm of chromosome 11 (i.e., 11q) in various primary breast cancers. In the present study allelic alterations in a refined position on the long arm of chromosome 11 were studied to identify the spectrum of induced damage at different stages of malignant transformation of MCF-10F cell lines after exposure to high-LET radiation using alpha-particles and exposure to estradiol by using PCR-single strand conformation polymorphism (SSCP) and fluorescence in situ hybridization (FISH) analysis. Microsatellite markers were selected from chromosome 11 (11q23-q24 loci) and it was found that frequency of allelic imbalance occurs at different stages of tumor progression with a range of 15-45% depending on the marker studied. These results strongly suggested the presence of several tumor suppressor genes in this critical region of chromosome 11 (11q23-q24). It also represents the first indication of allele loss at these loci in human breast epithelial cells induced by radiation and estrogen treatment suggesting a potential interventional target in breast carcinogenesis.

Oncoprotein expression and morphological phenotypes of human breast epithelial cells transformed by the c-Ha-ras oncogene

Activated oncogenes have been detected in a variety of malignant tumors and altered expressions of certain genes are known to play a functional role in the cancer process. The chemical carcinogen, BP, and the insertion of c-Ha-ras, induced characteristics of transformed phenotypes in a suitable human breast epithelial cell line. Carcinogen-treated and Ha-ras-transfected cells showed a progression of changes in the morphology, anchorage independent growth, invasiveness and tumorigenicity in SCID mice. Tumor growth occurs after a series of molecular events that parallel morphological changes. The aim of this work was to determine the neoplastic phenotypes following treatment with benzo(a)pyrene (BP) and transfection with c-Ha-ras oncogene changes and PCNA, Neu, ErbB-3 and Cytokeratin 18 protein expression in MCF-10F cells, a spontaneously immortalized human breast epithelial cell line. Protein expression was determined by immunofluorescent staining coupled with confocal microscopy. An increased oncoprotein expression in comparison to MCF-10F cells was observed in PCNA, Neu, ErbB-3 and Cytokeratin 18 protein expression in breast epithelial cells transformed with a chemical carcinogen and/or oncogene transfected that are not present in the MCF-10F. This in vitro cancer model can be used as a valuable model in the study of breast carcinogenesis.

Growth factor biomarkers associated with estrogen- and radiation-induced breast cancer progression

Breast cancer is the most common cancer in women worldwide. Transformation of a normal cell to a malignant one results from mutations in genes that encode key regulatory proteins. Growth factors are proteins secreted by a variety of transformed cells and tumors and function as autocrine regulators of growth. Biomarkers associated with cancer were examined in human breast epithelial cells transformed by high-LET radiation in the presence of 17beta-estradiol. An established cancer model was used in these studies. The MCF-10F cells that were irradiated with double doses of alpha-particles in the presence of estrogen (60 cGy + E/60 cGy + E, named Alpha 5) showed gradual phenotypic changes relative to control, including tumorigenicity in heterologous animals. Protein expression was determined by quantification of immunofluorescence staining coupled with confocal microscopy. The transforming growth factor alpha, epidermal growth factor, ERK1 and fibroblast growth factor-1 (Int2) protein expression was analyzed. Increased protein expression was observed in non-tumorigenic and tumorigenic alpha-irradiated and estrogen-treated cells. However, Stat-1alpha and pS2 protein expression was only increased in the tumorigenic Alpha 5 and Tumor 2 cell lines. It can be concluded that high-LET radiation in the presence of estrogen-induced changes in the proteins associated with growth factors and their overexpression may be a critical step in the cascade of events that characterize progression in breast cancer.

Effect of retinol on radiation- and estrogen-induced neoplastic transformation of human breast epithelial cells

Clinical, epidemiological and experimental findings have provided evidence supporting a role of free radicals in the etiology of cancer. Free radical production is enhanced in many disease states, by carcinogen exposure, and under conditions of stress contributing widely to cancer development in humans. We have established an experimental breast cancer model to examine the effects of all-trans-retinol (retinol/vitamin A) on the production of free radicals in human breast epithelial cells induced by high linear energy transfer (LET)-radiation in the presence of 17beta estradiol. The following cell lines were used in these studies: the MCF-10F cell line, a spontaneously immortalized human breast epithelial cell line. Alpha 5 derived from MCF-10F cells irradiated with two separated doses of 60 cGy alpha particles in the presence of estrogens (60E/60E). Tumor 2, from a tumor formed in nude mice after injection with the cell line alpha 5. Tumor 3, from secondary tumor formed from injecting tumor 2 cells into nude mice. Each of the cell types examined had significantly elevated H(2)O(2) production levels compared to MCF-10F control cells (p<0.001). Retinol (1 microl/ml) significantly (p<0.05) decreased H(2)O(2) production in all cell types examined. Retinol significantly decreased (p<0.05) invasive capabilities of cells across matrigel coated invasion chambers and significantly reduced (p<0.05) PCNA, Fra-1, mutant p53 and increased Rb protein expression levels in comparison to non-retinol-treated ones when assayed using immunofluorescent staining coupled with confocal microscopy. The reduced H(2)O(2) production, decrease in cell invasive capabilities and alterations in protein expression levels suggest that retinol can be used as a chemopreventive agent in human breast cancer.

In SituVisualization of DSBs to Assess the Extranuclear/Extracellular Effects Induced by Low-Dose α-Particle Irradiation

Extranuclear/extracellular effects may have a significant effect on low-dose radiation risk assessment as well as on the shape of the dose-response relationship. Numerous studies using different end points such as sister chromatid exchanges, micronuclei and mutation have shown that this phenomenon exists in many cell types. However, these end points mostly reflect the late events after radiation damage, and little is known about the early response in this phenomenon. DNA double-strand breaks (DSBs) induced by ionizing radiation or carcinogenic chemicals can be visualized in situ using gamma-H2AX immunofluorescence staining, and there is evidence that the number of gamma-H2AX foci can be closely correlated with DSBs induced. Here we used gamma-H2AX as a biomarker to assess the extranuclear/extracellular effects induced by low-dose alpha particles in situ. The results show that a greater fraction of positive cells with DSBs (48.6%) was observed than the number of cells whose nuclei were actually traversed by the 1-cGy dose of alpha particles (9.2%). The fraction of DSB-positive cells was greatly reduced after treatment with either lindane or DMSO. These results suggest that in situ visualization of DSBs can be used to assess radiation-induced extranuclear/extracellular effects soon after irradiation. Moreover, the in situ DSB assay may provide a means to evaluate the spatial effect on unirradiated cells that are located in the neighboring region of cells irradiated by alpha particles.

Ionizing radiation induces alterations in cellular proliferation and c-myc, c-jun and c-fos protein expression in breast epithelial cells

The identification of genes involved in breast cancer is of critical importance in understanding the pathogenesis of the disease. Expression of the nuclear proto-oncogenes, c-myc, c-jun and c-fos, are indicative of early response events during cellular proliferation. Among them, the c-myc oncogene has been found frequently over-expressed in breast cancer. In vitro systems allow us to test the sensitivity of human breast epithelial cells to different carcinogens, including ionizing radiation. The aim of this work was to define whether these oncogenes play a functional role in radiation-induced transformation of human breast epithelial cells. We examined: a) the spontaneously immortalized MCF-10F cell line, b) clones derived from these cells treated with the carcinogen benzo(a)pyrene (BP) and then transfected with c-Ha-ras-oncogene, followed, c) by a single 3 Gy dose of gamma-rays. Protein expressions were analysed by Western immunoblot assays. Results indicated that 3 Gy dose of gamma-ray decreased the expression of these oncoproteins in the MCF-10F cells (ranging from 23 to 80%). In BP1, non-tumorigenic MCF-10F cells, radiation induced an even sharper decrease in the oncoprotein levels (ranging from 50 to 100%) relative to their non-irradiated controls. In contrast, in BP1-E tumorigenic cell line radiation increased the expression in 68-80% of c-myc, c-jun and c-fos protein expression relative to non-irradiated control. Furthermore, radiation increased c-my, c-jun and c-fos protein expression in the c-Ha-ras-3 Gy cell line relative to non-irradiated control cell line (ranging from 45 and 120%). Interesting, among the tumorigenic MCF-10F cells previously exposed to both BP and c-Ha-ras (BP1-Tras-3 Gy cell line), radiation increased the c-myc, c-jun, c-fos protein expression by more than 120% relative to the non-irradiated controls. In can be concluded that the MCF-10F model of breast carcinogenesis allows us to examine various aspects of regulations in gene expression and can provide us the basis for understanding the process of breast cancer.

Radiation-induced bystander effect and adaptive response in mammalian cells

Two conflicting phenomena, bystander effect and adaptive response, are important in determining the biological responses at low doses of radiation and have the potential to impact the shape of the dose-response relationship. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, we show here that non-irradiated cells acquire mutagenesis through direct contact with cells whose nuclei have been traversed with a single alpha particle each. Pretreatment of cells with a low dose of X-rays four hours before alpha particle irradiation significantly decreased this bystander mutagenic response. Results from the present study address some of the fundamental issues regarding both the actual target and radiation dose effect and can contribute to our current understanding in radiation risk assessment.

The “Pro-drug” RibCys Decreases the Mutagenicity of High-LET Radiation in Cultured Mammalian Cells

We are carrying out studies aimed at reducing the mutagenic effects of high-LET 56Fe ions and 12C ions (56Fe ions, 143 keV/microm; 12C ions, 100 keV/microm) with certain drugs, including RibCys [2-(R,S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)-thiazolidine-4(R)-carboxylic acid]. RibCys, formed by condensation of L-cysteine with D-ribose, is designed so that the sulfhydryl amino acid L-cysteine is released intracellularly through nonenzymatic ring opening and hydrolysis leading to increased levels of glutathione (GSH). RibCys (4 or 10 mM), which was present during irradiation and for a few hours after, significantly decreased the yield of CD59- mutants induced by radiation in AL human-hamster hybrid cells. RibCys did not affect the clonogenic survival of irradiated cells, nor was it mutagenic itself. These results, together with the minimal side effects reported in mice and pigs, indicate that RibCys may be useful, perhaps even when used prophylactically, in reducing the mutation load created by high-LET radiation in astronauts or other exposed individuals.

Tumor suppressor function of Betaig-h3 gene in radiation carcinogenesis

Interaction between cell and extracellular matrix (ECM) plays a crucial role in tumor invasiveness and metastasis. Using an immortalized human bronchial epithelial (BEP2D) cell model, we showed previously that expression of a list of genes including Betaig-h3 (induced by transforming growth factor-beta), DCC (deleted in colorectal cancer), p21(cipl), c-fos, Heat shock protein (HSP27) and cytokeratin 14 were differentially expressed in several independently generated, radiation-induced tumor cell lines (TL1-TL5) relative to parental BEP2D cells. Our previous data further demonstrated that loss of tumor suppressor gene(s) as a likely mechanism of radiation carcinogenesis. In the present study, we chose Betaig-h3 and DCC that were downregulated in tumorigenic cells for further study. Restored expression of Betaig-h3 gene, not DCC gene, by transfecting cDNA into tumor cells resulted in a significant reduction in tumor growth. While integrin receptor alpha 5 beta 1 was overexpressed in tumor cells, its expression was corrected to the level found in control BEP2D cells after Betaig-h3 transfection. These data suggest that Betaig-h3 gene is involved in tumor progression by regulating integrin alpha 5 beta 1 receptor. Furthermore, exogenous TGF- beta 1 induced expression of Betaig-h3 gene and inhibited the growth of both control and tumorigenic BEP2D cells. Therefore, downregulation of Betaig-h3 gene may results from the decreased expression of upstream mediators such as TGF-beta. The findings provide strong evidence that the Betaig-h3 gene has tumor suppressor function in radiation-induced tumorigenic human bronchial epithelial cells and suggest a potential target for interventional therapy.

Overexpression of Betaig-h3 gene downregulates integrin alpha5beta1 and suppresses tumorigenicity in radiation-induced tumorigenic human bronchial epithelial cells

Interaction between cell and extracellular matrix plays a crucial role in tumour invasion and metastasis. Using an immortalised human bronchial epithelial (BEP2D) cell model, the study here shows that expression of Betaig-h3 gene, which encodes a secreted adhesion molecule induced by transforming growth factor-beta, is markedly decreased in several independently generated, radiation-induced tumour cell lines (TL1-TL5) relative to parental BEP2D cells. Transfection of Betaig-h3 gene into tumour cells resulted in a significant reduction in tumour growth. While integrin receptor alpha5beta1 was overexpressed in tumour cells, its expression was corrected to the level found in control BEP2D cells after Betaig-h3 transfection. These data suggest that Betaig-h3 gene is involved in tumour progression by regulating integrin receptor alpha5beta1. The findings provide strong evidence that the Betaig-h3 gene has tumour suppressor function in human BEP2D cell model and suggest a potential target for interventional therapy.

Genotoxic damage in non-irradiated cells: contribution from the bystander effect

It has always been accepted dogma that the deleterious effects of ionising radiation such as mutagenesis and carcinogenesis are due mainly to direct damage to DNA. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, it is shown here that non-irradiated cells acquire the mutagenic phenotype through direct contact with cells whose nuclei are traversed with 2 alpha particles each. Pre-treatment of cells with lindane, a gap junction inhibitor, significantly decreased the mutant yield. Furthermore, when irradiated cells were mixed with control cells in a similar ratio as the in situ studies, no enhancement in bystander mutagenesis was detected. Our studies provide clear evidence that genotoxic damage can be induced in non-irradiated cells, and that gap junction mediated cell-cell communication plays a critical role in the bystander phenomenon.

Radiation risk to low fluences of alpha particles may be greater than we thought

Based principally on the cancer incidence found in survivors of the atomic bombs dropped in Hiroshima and Nagasaki, the International Commission on Radiation Protection (ICRP) and the United States National Council on Radiation Protection and Measurements (NCRP) have recommended that estimates of cancer risk for low dose exposure be extrapolated from higher doses by using a linear, no-threshold model. This recommendation is based on the dogma that the DNA of the nucleus is the main target for radiation-induced genotoxicity and, as fewer cells are directly damaged, the deleterious effects of radiation proportionally decline. In this paper, we used a precision microbeam to target an exact fraction (either 100% or < or =20%) of the cells in a confluent population and irradiated their nuclei with exactly one alpha particle each. We found that the frequencies of induced mutations and chromosomal changes in populations where some known fractions of nuclei were hit are consistent with non-hit cells contributing significantly to the response. In fact, irradiation of 10% of a confluent mammalian cell population with a single alpha particle per cell results in a mutant yield similar to that observed when all of the cells in the population are irradiated. This effect was significantly eliminated in cells pretreated with a 1 mM dose of octanol, which inhibits gap junction-mediated intercellular communication, or in cells carrying a dominant negative connexin 43 vector. The data imply that the relevant target for radiation mutagenesis is larger than an individual cell and suggest a need to reconsider the validity of the linear extrapolation in making risk estimates for low dose, high linear-energy-transfer (LET) radiation exposure.

Analysis of p16 and p21(Cip1) expression in tumorigenic human bronchial epithelial cells induced by asbestos

Although asbestos is carcinogenic to humans, the mechanism(s) by which it induces cancer is unknown. Using tumor cell lines generated previously by asbestos treatment of immortalized human bronchial epithelial (BEP2D) cells, we examined alterations in p16 and p21(Cip1) genes together with their protein levels. Results were compared with untreated BEP2D cells, normal human bronchial epithelial cells (NHBE), as well as non-tumorigenic fusion cell lines generated by fusing tumor cells with BEP2D cells. No deletion in the p16 gene was found in any of the tumor cell lines examined. Although p16 protein was expressed at a similar level in tumor and BEP2D cells, there was a fourfold decrease in its expression among NHBE cells. In contrast, both the protein and mRNA expression levels of p21(Cip1) were decreased by about threefold in tumor cell lines when compared with either BEP2D or NHBE cells, which had a similar expression level. Expression of p21(Cip1) mRNA was restored to the control level in all the fusion cell lines examined. The results suggested that down regulation of p21(Cip1) expression is linked to the tumorigenic conversion of BEP2D cells by asbestos.

Frequent allelic imbalance on chromosome 6 and 17 correlate with radiation-induced neoplastic transformation of human breast epithelial cells

The development of human breast cancer is a complex multi-step process that depends on various exogenous and endogenous factors that modulate the transformation of normal human breast epithelial cells into neoplastic ones. Using a spontaneously-immortalized human breast epithelial (MCF-10F) cell line, we have shown previously that radiation, in combination with estrogen, induces a stepwise neoplastic transformation of this cell line. In the present study, we investigate the incidence of microsatellite instability and loss of heterozygosity using a battery of markers on chromosomes 6 and 17, we correlate the genetic alteration with the malignant transformation of the MCF-10F cell line ranging from altered morphology to increase in proliferative rate, anchorage independent growth and tumorigenicity in nude mice. Microsatellite markers were selected from the hot spot regions (6q21-q27, 17p12-p13.3 and 17q12-q21) of both chromosomes. We found that the frequency of allelic imbalance occurs at the different stages of tumor progression with a range of 21 to 50% depending on the marker studied. The relatively high rate of allele imbalance at all these loci suggests the presence and inactivation of one or more tumor suppressor genes in these regions. Thus, the present data will be useful for systematic studies to identify the cellular and molecular changes associated with radiation-induced breast carcinogenesis.

Profiling of differentially expressed genes induced by high linear energy transfer radiation in breast epithelial cells

Methods to define patterns of gene expression have applications in a wide range of biological systems. Several molecular biological techniques are used to study expression patterns during the neoplastic progression of breast epithelial cells. In the present study, differential expression of human oncogenes/tumor suppressor genes in human breast epithelial cell lines irradiated with low doses of high linear energy transfer radiation and treated with estrogen was assessed with cDNA expression arrays. Transformed and tumorigenic cell lines were compared with the control cell line to identify differentially expressed genes during tumorigenic progression. Autoradiographic analysis showed that of the 190 genes analyzed, 49 genes showed a high level of altered expression, and 12 genes had minor differences in expression levels. Among these 49 genes, 17 genes were altered at all stages of transformation, 21 were altered only at the early stage, and the remaining 11 were at the late stage of transformation to the tumorigenic stage of progression. Among the 11 late stage-associated genes, seven genes were altered exclusively in the tumorigenic cell lines and in Tumor-T. Of the 17 all-stage genes, six were randomly selected, and we confirmed their altered expression by gene-specific semiquantitative reverse transcription polymerase chain reaction, followed by Northern blot analysis. The results showed that the mRNA expression patterns of all these genes were consistent with the expression pattern seen on the array. Among these six genes, five genes, including c-myc, puf, MNDA, c-yes, and Fra-1 showed upregulation, and the other gene, RBA/p48, showed downregulation in the transformed and tumorigenic cell lines compared with the control MCF-10F cell line. Investigation of these genes should help establish the molecular mechanisms of progression that are altered by radiation and estrogen treatment. A number of candidates reported here should be useful as biomarkers involved in breast carcinogenesis.

Karyotype analysis of tumorigenic human bronchial epithelial cells transformed by chrysolite asbestos using chemically induced premature chromosome condensation technique

We examined karyotypic changes of tumorigenic human bronchial epithelial cell lines transformed by asbestos fibers. Using Calyculin A mediated premature chromosome condensation (PCC) assay and Giemsa-trypsin banding, we showed that the common changes of all tumorigenic cell lines were the loss of one or two copies of chromosome 5, the monosomy of chromosome 19 and the increased trisomy of chromosome 8. The results indicate that the karyotypic change of chromosome 5, 8 and 19 could play an important role in asbestos-induced tumorigenic conversion of human bronchial epithelial cells from an immortalized to tumorigenic state.

Cell killing and chromatid damage in primary human bronchial epithelial cells irradiated with accelerated 56Fe ions

We examined cell killing and chromatid damage in primary human bronchial epithelial cells irradiated with high-energy 56Fe ions. Cells were irradiated with graded doses of 56Fe ions (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at Brookhaven National Laboratory. The survival curves for cells plated 1 h after irradiation (immediate plating) showed little or no shoulder. However, the survival curves for cells plated 24 h after irradiation (delayed plating) had a small initial shoulder. The RBE for 56Fe ions compared to 137Cs gamma rays was 1.99 for immediate plating and 2.73 for delayed plating at the D10. The repair ratio (delayed plating/immediate plating) was 1.67 for 137Cs gamma rays and 1.22 for 56Fe ions. The dose-response curves for initially measured and residual chromatid fragments detected by the Calyculin A-mediated premature chromosome condensation technique showed a linear response. The results indicated that the induction frequency for initially measured fragments was the same for 137Cs gamma rays and 56Fe ions. On the other hand, approximately 85% of the fragments induced by 137Cs gamma rays had rejoined after 24 h of postirradiation incubation; the corresponding amount for 56Fe ions was 37%. Furthermore, the frequency of chromatid exchanges induced by gamma rays measured 24 h after irradiation was higher than that induced by 56Fe ions. No difference in the amount of chromatid damage induced by the two types of radiations was detected when assayed 1 h after irradiation. The results suggest that high-energy 56Fe ions induce a higher frequency of complex, unrepairable damage at both the cellular and chromosomal levels than 137Cs gamma rays in the target cells for radiation-induced lung cancers.

Induction of oxyradicals by arsenic: implication for mechanism of genotoxicity

Although arsenic is a well-established human carcinogen, the mechanisms by which it induces cancer remain poorly understood. We previously showed arsenite to be a potent mutagen in human-hamster hybrid (A(L)) cells, and that it induces predominantly multilocus deletions. We show here by confocal scanning microscopy with the fluorescent probe 5',6'-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate that arsenite induces, within 5 min after treatment, a dose-dependent increase of up to 3-fold in intracellular oxyradical production. Concurrent treatment of cells with arsenite and the radical scavenger DMSO reduced the fluorescent intensity to control levels. ESR spectroscopy with 4-hydroxy-2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOL-H) as a probe in conjunction with superoxide dismutase and catalase to quench superoxide anions and hydrogen peroxide, respectively, indicates that arsenite increases the levels of superoxide-driven hydroxyl radicals in these cells. Furthermore, reducing the intracellular levels of nonprotein sulfhydryls (mainly glutathione) in A(L) cells with buthionine S-R-sulfoximine increases the mutagenic potential of arsenite by more than 5-fold. The data are consistent with our previous results with the radical scavenger DMSO, which reduced the mutagenicity of arsenic in these cells, and provide convincing evidence that reactive oxygen species, particularly hydroxyl radicals, play an important causal role in the genotoxicity of arsenical compounds in mammalian cells.

Characterization of ataxia telangiectasia fibroblasts with extended life-span through telomerase expression

Ataxia-telangiectasia (A-T) is an autosomal recessive disease characterized by progressive cerebellar degeneration, immunodeficiencies, genomic instability and gonadal atrophy. A-T patients are hypersensitive to ionizing radiation and have an elevated cancer risk. Cells derived from A-T patients require higher levels of serum factors, exhibit cytoskeletal defects and undergo premature senescence in culture. We show here that expression of the catalytic subunit of telomerase (hTERT) in primary A-T patient fibroblasts can rescue the premature senescence phenotype. Ectopic expression of hTERT does not rescue the radiosensitivity or the telomere fusions in A-T fibroblasts. The hTERT+AT cells also retain the characteristic defects in cell-cycle checkpoints, and show increased chromosome damage before and after ionizing radiation. Although A-T patients have an increased susceptibility to cancer, the expression of hTERT in A-T fibroblasts does not stimulate malignant transformation. These immortalized A-T cells provide a more stable cell system to investigate the molecular mechanisms underlying the cellular phenotypes of Ataxia-telangiectasia.

Molecular alterations in tumorigenic human bronchial and breast epithelial cells induced by high LET radiation

Carcinogenesis is a multi-stage process with sequence of genetic events governing the phenotypic expression of a series of transformation steps leading to the development of metastatic cancer. In the present study, immortalized human bronchial (BEP2D) and breast (MCF-10F) cells were irradiated with graded doses of either 150 keV/micrometer alpha particles or 1 GeV/nucleon 56Fe ions. Transformed cells developed through a series of successive steps before becoming tumorigenic in nude mice. Cell fusion studies indicated that radiation-induced tumorigenic phenotype in BEP2D cells could be completely suppressed by fusion with non-tumorigenic BEP2D cells. The differential expressions of known genes between tumorigenic bronchial and breast cells induced by alpha particles and their respective control cultures were compared using cDNA expression array. Among the 11 genes identified to be differentially expressed in BEP2D cells, three (DCC, DNA-PK and p21(CIP1)) were shown to be consistently down-regulated by 2 to 4 fold in all the 5 tumor cell lines examined. In contrast, their expressions in the fusion cell lines were comparable to control BEP2D cells. Similarly, expression levels of a series of genes were found to be altered in a step-wise manner among tumorigenic MCF-10F cells. The results are highly suggestive that functional alterations of these genes may be causally related to the carcinogenic process.

Gene amplification and microsatellite instability induced in tumorigenic human bronchial epithelial cells by alpha particles and heavy ions

Gene amplification and microsatellite alteration are useful markers of genomic instability in tumor and transformed cell lines. It has been suggested that genomic instability contributes to the progression of tumorigenesis by accumulating genetic changes. In this study, amplification of the carbamyl-P-synthetase, aspartate transcarbamylase, dihydro-orotase (CAD) gene in transformed and tumorigenic human bronchial epithelial (BEP2D) cells induced by either alpha particles or (56)Fe ions was assessed by measuring resistance to N-(phosphonacetyl)-l-aspartate (PALA). In addition, alterations of microsatellite loci located on chromosomes 3p and 18q were analyzed in a series of primary and secondary tumor cell lines generated in nude mice. The frequency of PALA-resistant colonies was 1-3 x 10(-3) in tumor cell lines, 5-8 x 10(-5) in transformed cells prior to inoculation into nude mice, and less than 10(-7) in control BEP2D cells. Microsatellite alterations were detected in all 11 tumor cell lines examined at the following loci: D18S34, D18S363, D18S877, D3S1038 and D3S1607. No significant difference in either PALA resistance or microsatellite instability was found in tumor cell lines that were induced by alpha particles compared to those induced by (56)Fe ions.

Oncoprotein expression in human breast epithelial cells transformed by high-LET radiation

PURPOSE

The aim of the present work was to analyze the expression of oncoproteins that are frequently altered in breast cancer with specific phenotypic stages in the neoplastic process.

MATERIALS AND METHODS

Expression of c-myc, c-jun, c-Ha-ras and the tumor suppressor gene p53 oncoproteins were examined by immunohistochemical staining coupled with confocal microscopy in transformed and tumorigenic human breast epithelial cells induced by high-LET alpha-particles (150 kcV/microm).

RESULTS

MCF-10F cells, irradiated with single and double doses of 60 cGy alpha-particles and subsequently treated with cstrogen, showed gradual phenotypic changes including altered morphology, increased cell proliferation relative to control, anchorage-independent growth, invasive capabilities and tumorigenicity in nude mice. MCF-10F cells irradiated with a second dose of 60 cGy alpha-particles after estrogen treatment (60 cGy+ E/60 cGy+E) showed tumorigenicity both in SCII) and nude mice. Alterations in the protein expression of several oncogenes including c-myc, c-jun, c-Ha-ras and the tumor suppressor gene p53 were detected in alpha-particle-irradiated cells and in those cells subsequently cultured in the presence of estrogen. The expression level of these oncoproteins correlated with the progressive nature of the neoplastic process.

CONCLUSION

These studies suggest that overexpression of several oncoproteins is important in the neoplastic transformation of human breast epithelial cells induced by high-LET radiation. In addition, use of endocrine factors such as estrogen allows the examination of various aspects of protein expression providing the basis for understanding the complex interactions of hormones and genes.

Mechanisms of radiation-induced neoplastic transformation of human bronchial epithelial cells

Carcinogenesis is a multistage process with sequences of genetic events that govern the phenotypic expression of a series of transformation steps that lead to the development of metastatic cancer. To better understand the mechanisms involved in human bronchial carcinogenesis induced by alpha particles from radon, we have developed a model of neoplastic transformation based on human papillomavirus-immortalized human bronchial epithelial (BEP2D) cells. Cells exposed to alpha particles become tumorigenic after progressing through a series of sequential stages including altered growth pattern, resistance to serum-induced terminal differentiation, agar-positive growth, tumorigenicity, and metastasis, with each step representing a necessary yet insufficient step toward the later, more malignant phase. Cell fusion studies indicated that the radiation-induced tumorigenic phenotype in BEP2D cells can be completely suppressed by fusion with nontumorigenic BEP2D cells. Several cellular differentiation and growth regulation genes such as DCC (deleted in colorectal cancer), CDKN1A (also known as p21(C1P1)) and the gene that encodes DNA-PK were frequently found to be modulated in tumorigenic BEP2D cells and may be related to the process of carcinogenesis.

Differentially expressed genes in asbestos-induced tumorigenic human bronchial epithelial cells: implication for mechanism

Although exposure to asbestos fibers is associated with the development of lung cancer, the underlying mechanism(s) remains unclear. Using human papillomavirus-immortalized human bronchial epithelial (BEP2D) cells, we previously showed that UICC chrysotiles can malignantly transform these cells in a stepwise fashion before they become tumorigenic in nude mice. In the present study we used cDNA expression arrays to screen differentially expressed genes among the tumorigenic cells. A total of 15 genes were identified, 11 of which were further confirmed by northern blot. Expression levels of these genes were then determined among transformed BEP2D cells at different stages of the neoplastic process, including non-tumorigenic cells that were resistant to serum-induced terminal differentiation, early and late passage transformed BEP2D cells, five representative tumor cell lines and fused tumorigenic-control cell lines which were no longer tumorigenic. A consistent 2- to 3-fold down-regulation of the DCC (deleted in colon cancer), Ku70 and heat shock protein 27 genes were detected in all the independently generated tumor cell lines while expression levels in early transformants as well as in the fusion cell lines remained normal. In contrast, all the tumor cell lines examined demonstrated 2- to 4-fold overexpression of the insulin receptor and its signal transduction genes. Differential expression of these genes was completely restored in the fusion cell lines examined. No alteration in c-jun or EGF receptor expression was found in any of the cell lines. Our data suggest that activation of the insulin receptor pathway and inactivation of DCC and Ku70 may cooperate in malignant transformation of BEP2D cells induced by asbestos.

Effects of 60 Hz extremely low frequency magnetic fields (EMF) on radiation- and chemical-induced mutagenesis in mammalian cells

There is considerable uncertainty of the potential biological effects of extremely low frequency magnetic fields (ELF-MF or EMF) because of mixed results in epidemiological and laboratory studies. In the present study, exponentially growing human-hamster hybrid A(L) cells were treated with a 100 microT alternating EMF powered at 60 Hz for either 24 h or 7 days. Exposure to EMF was conducted either alone or in combination with graded doses of a physical or chemical carcinogen. gamma-radiation was chosen as a form of ionizing radiation while N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was chosen as a form of chemical contaminant. Exposure of A(L) cells to EMF alone for a period up to 7 days was non-cytotoxic and non-mutagenic. Concurrent EMF treatment did not increase either the cytotoxicity or induction of CD59- mutants by graded doses of gamma-rays or MNNG in A(L) cells. This study shows conclusively that short-term or long-term exposure to EMF alone neither affects the survival of A(L) cells nor increases the mutagenic potency of other environmental carcinogens.

Establishment of a radiation- and estrogen-induced breast cancer model

It is well accepted that cancer arises in a multistep fashion in which exposure to environmental carcinogens is a major etiological factor. The aim of this work was to establish an experimental breast cancer model in order to understand the mechanism of neoplastic transformation induced by high LET radiation in the presence of 17beta-estradiol (E). Immortalized human breast cells (MCF-10F) were exposed to low doses of high LET alpha particles (150 keV/microm) and subsequently cultured in the presence or absence of E for periods of up to 10 months post-irradiation. MCF-10F cells irradiated with either a single 60 cGy dose or 60/60 cGy doses of alpha particles showed gradual phenotypic changes including altered morphology, increase in cell proliferation relative to the control, anchorage-independent growth and invasive capability before becoming tumorigenic in nude mice. In alpha particle-irradiated cells and in those cells subsequently cultured in the presence of E, increased BRCA1, BRCA2 and RAD51 expression were detected by immunofluorescence staining and quantified by confocal microscopy. These studies showed that high LET radiation such as that emitted by radon progeny, in the presence of estrogen, induced a cascade of events indicative of cell transformation and tumorigenicity in human breast epithelial cells.

Induction of a bystander mutagenic effect of alpha particles in mammalian cells

Ever since the discovery of X-rays was made by Röntgen more than a hundred years ago, it has always been accepted that the deleterious effects of ionizing radiation such as mutation and carcinogenesis are attributable mainly to direct damage to DNA. Although evidence based on microdosimetric estimation in support of a bystander effect appears to be consistent, direct proof of such extranuclear/extracellular effects are limited. Using a precision charged particle microbeam, we show here that irradiation of 20% of randomly selected A(L) cells with 20 alpha particles each results in a mutant fraction that is 3-fold higher than expected, assuming no bystander modulation effect. Furthermore, analysis by multiplex PCR shows that the types of mutants induced are significantly different from those of spontaneous origin. Pretreatment of cells with the radical scavenger DMSO had no effect on the mutagenic incidence. In contrast, cells pretreated with a 40 microM dose of lindane, which inhibits cell-cell communication, significantly decreased the mutant yield. The doses of DMSO and lindane used in these experiments are nontoxic and nonmutagenic. We further examined the mutagenic yield when 5-10% of randomly selected cells were irradiated with 20 alpha particles each. Results showed, likewise, a higher mutant yield than expected assuming no bystander effects. Our studies provide clear evidence that irradiated cells can induce a bystander mutagenic response in neighboring cells not directly traversed by alpha particles and that cell-cell communication process play a critical role in mediating the bystander phenomenon.

Localization of tumor suppressor gene candidates by cytogenetic and short tandem repeat analyses in tumorigenic human bronchial epithelial cells

Radon exposure is associated with increased risk for bronchogenic carcinoma. Mutagenesis analyses have revealed that radon induces mostly multi-locus chromosome deletions. Based on these findings, it was hypothesized that deletion analysis of multiple radon-induced malignant transformants would reveal common mutations in chromosomal regions containing tumor suppressor genes responsible for malignant transformation. This hypothesis was supported by a previous study in which tumorigenic derivatives of the human papillomavirus 18-immortalized human bronchial epithelial cell line BEP2D were established following irradiation with 30 cGy of high linear energy transfer radon-simulated alpha-particles. Herein, we describe the analyses of 10 additional tumorigenic derivative cell lines resulting from the irradiation of five additional independent BEP2D populations. The new transformants have common cytogenetic changes, including the loss of chromosome (ch)Y, one of three copies of ch8, one of two copies of ch11p15-pter and one of three copies of ch14. These changes are the same as those reported previously. Analysis of PCR-amplified short tandem repeats of informative loci confirmed the loss of heterozygosity (LOH) at 12 loci spanning the length of ch8 in cell lines from four of the total of eight irradiation treatments to date and the loss of chY in all cell lines (8 of 8). LOH analysis with a total of 17 informative loci confirmed loss on ch14 in transformants from seven of eight irradiation treatments and indicated a 0.5-1.7 cM region of common involvement centered around locus D14S306. No LOH was detected at any of the informative loci on ch11. The overall results support our stated hypothesis. Further studies are currently in progress to determine whether the ch8 and ch14 regions contain genes with tumor suppressor function in bronchial epithelial cells.

Role of oxyradicals in mutagenicity and DNA damage induced by crocidolite asbestos in mammalian cells

Crocidolite, one of the most carcinogenic forms of asbestos, is mutagenic in cultured mammalian cells when assayed using a system that can detect multilocus deletions. In the present study, we examined the effect of buthionine sulfoximine (BSO) on mutation frequency and the formation of 8-hydroxydeoxyguanosine (8-OHdG) in human-hamster hybrid (A(L)) cells induced by crocidolite fibers in an attempt to determine the role of oxyradicals in mediating fiber mutagenesis. BSO, a competitive inhibitor of the enzyme gamma-glutamyl cysteine synthetase, depleted nonprotein sulfhydryls to <5% of control within 24 h at a nonmutagenic dose of 25 microM. In cells pretreated with BSO for 24 h, the mutation yield at the CD59 locus induced by a 4 microg/cm2 dose of crocidolite fibers was increased by more than 3-fold (P < 0.05). Using immunoperoxidase staining with a monoclonal antibody specific for 8-OHdG, we demonstrated that crocidolite fibers induced a dose-dependent increase in oxidative DNA damage in A(L) cells. Furthermore, addition of DMSO, a well-established hydroxyl radical (OH*) scavenger, dramatically suppressed 8-OHdG induction (P < 0.005). Our results definitely demonstrate that reactive oxygen species mediate fiber-induced DNA damage mutagenesis in A(L) cells in a concentration-dependent manner.

Radon, tobacco-specific nitrosamine and mutagenesis in mammalian cells

The mutagenicity of 4-methylnitrosamine-1-3-pyridyl-1-butanone (NNK), either alone or in combination with low dose alpha particle irradiation, was examined using the human-hamster hybrid (A(L)) cell assay. NNK induced a dose-dependent toxicity in A(L) cells. In combination with a 25 cGy dose of alpha particles, the induced survival fraction fell within the statistical range of the calculated values assuming an additive interaction of the two agents. In addition, NNK is mutagenic in A(L) cells at the CD59 locus. Furthermore, a low dose of NNK, when combined with radon alpha particles, resulted in a combined mutagenic effect in A(L) cells that was consistent with an additive model but less than additive at higher NNK concentrations. The majority of NNK induced CD59(-) mutants (77.6%) lost at least one additional marker in addition to the CD59 which encodes the cell surface antigen. When combined with alpha particles, the proportion of mutants with additional marker loss increased with increasing dose of NNK. Our study further confirms that NNK is mutagenic in mammalian cells, induces mostly deletions, and provides an in vitro assessment of the combined genotoxic effects of NNK and alpha particles at low environmentally relevant doses. This finding should be helpful in understanding the molecular mechanism of the mutagenic process as a result of multi-agent interaction.

Alterations of p53 in tumorigenic human bronchial epithelial cells correlate with metastatic potential

The cellular and molecular mechanisms of radiation-induced lung cancer are not known. In the present study, alterations of p53 in tumorigenic human papillomavirus-immortalized human bronchial epithelial (BEP2D) cells induced by a single low dose of either alpha-particles or 1 GeV/nucleon (56)Fe were analyzed by PCR-single-stranded conformation polymorphism (SSCP) coupled with sequencing analysis and immunoprecipitation assay. A total of nine primary and four secondary tumor cell lines, three of which were metastatic, together with the parental BEP2D and primary human bronchial epithelial (NHBE) cells were studied. The immunoprecipitation assay showed overexpression of mutant p53 proteins in all the tumor lines but not in NHBE and BEP2D cells. PCR-SSCP and sequencing analysis found band shifts and gene mutations in all four of the secondary tumors. A G-->T transversion in codon 139 in exon 5 that replaced Lys with Asn was detected in two tumor lines. One mutation each, involving a G-->T transversion in codon 215 in exon 6 (Ser-->lle) and a G-->A transition in codon 373 in exon 8 (Arg-->His), was identified in the remaining two secondary tumors. These results suggest that p53 alterations correlate with tumorigenesis in the BEP2D cell model and that mutations in the p53 gene may be indicative of metastatic potential.

Antigen S1, encoded by the MIC1 gene, is characterized as an epitope of human CD59, enabling measurement of mutagen-induced intragenic deletions in the AL cell system

S1 cell membrane antigen is encoded by the MIC1 gene on human chromosome 11. This antigen has been widely used as a marker for studies in gene mapping or in analysis of mutagen-induced gene deletions/mutations, which utilized the human-hamster hybrid cell-line, AL-J1, carrying human chromosome 11. Evidence is presented here which identifies S1 as an epitope of CD59, a cell membrane complement inhibiting protein. E7.1 monoclonal antibody, specific for the S1 determinant, was found to react strongly with membrane CD59 in Western blotting, and to bind to purified, urinary form of CD59 in ELISAs. Cell membrane expression of S1 on various cell lines always correlated with that of CD59 when examined by immunofluorescent staining. In addition, E7.1 antibody inhibited the complement regulatory function of CD59. Identification of S1 protein as CD59 has increased the scope of the AL cell system by enabling analysis of intragenic mutations, and multiplex PCR analysis of mutated cells is described, showing variable loss of CD59 exons.

Targeted cytoplasmic irradiation with alpha particles induces mutations in mammalian cells

Ever since x-rays were shown to induce mutation in Drosophila more than 70 years ago, prevailing dogma considered the genotoxic effects of ionizing radiation, such as mutations and carcinogenesis, as being due mostly to direct damage to the nucleus. Although there was indication that alpha particle traversal through cellular cytoplasm was innocuous, the full impact remained unknown. The availability of the microbeam at the Radiological Research Accelerator Facility of Columbia University made it possible to target and irradiate the cytoplasm of individual cells in a highly localized spatial region. By using dual fluorochrome dyes (Hoechst and Nile Red) to locate nucleus and cellular cytoplasm, respectively, thereby avoiding inadvertent traversal of nuclei, we show here that cytoplasmic irradiation is mutagenic at the CD59 (S1) locus of human-hamster hybrid (AL) cells, while inflicting minimal cytotoxicity. The principal class of mutations induced are similar to those of spontaneous origin and are entirely different from those of nuclear irradiation. Furthermore, experiments with radical scavenger and inhibitor of intracellular glutathione indicated that the mutagenicity of cytoplasmic irradiation depends on generation of reactive oxygen species. These findings suggest that cytoplasm is an important target for genotoxic effects of ionizing radiation, particularly radon, the second leading cause of lung cancer in the United States. In addition, cytoplasmic traversal by alpha particles may be more dangerous than nuclear traversal, because the mutagenicity is accomplished by little or no killing of the target cells.

Biological effects of naturally occurring and man-made fibres: in vitro cytotoxicity and mutagenesis in mammalian cells

Cytotoxicity and mutagenicity of tremolite, erionite and the man-made ceramic (RCF-1) fibre were studied using the human-hamster hybrid A(L) cells. Results from these fibres were compared with those of UICC Rhodesian chrysotile fibres. The A(L) cell mutation assay, based on the S1 gene marker located on human chromosome 11, the only human chromosome contained in the hybrid cell, has been shown to be more sensitive than conventional assays in detecting deletion mutations. Tremolite, erionite and RCF-1 fibres were significantly less cytotoxic to A(L) cells than chrysotile. Mutagenesis studies at the HPRT locus revealed no significant mutant yield with any of these fibres. In contrast, both erionite and tremolite induced dose-dependent S1- mutations in fibre-exposed cells, with the former inducing a significantly higher mutant yield than the latter fibre type. On the other hand, RCF-1 fibres were largely non-mutagenic. At equitoxic doses (cell survival at approximately 0.7), erionite was found to be the most potent mutagen among the three fibres tested and at a level comparable to that of chrysotile fibres. These results indicate that RCF-1 fibres are non-genotoxic under the conditions used in the studies and suggest that the high mesothelioma incidence previously observed in hamster may either be a result of selective sensitivity of hamster pleura to fibre-induced chronic irritation or as a result of prolonged fibre treatment. Furthermore, the relatively high mutagenic potential for erionite is consistent with its documented carcinogenicity.

Asbestos and DNA double strand breaks

A radiosensitive DNA repair-deficient xrs-5 cell line was used to study asbestos cytotoxicity and DNA double strand breaks (DSBs). Although xrs-5 cells did not show any increase in sensitivity to chrysotile fibers in short-term (4-h) treatment when compared with wild-type CHO cells, longer-term exposure (24 h) gave significantly lower cell survival accompanied by a cell growth delay as well as a higher DNA DSB induction in this mutant cell line. These results suggest an important role played by DNA DSBs at the initial stage of asbestos injury.

Mutagenicity of arsenic in mammalian cells: role of reactive oxygen species

Arsenite, the trivalent form of arsenic present in the environment, is a known human carcinogen that lacked mutagenic activity in bacterial and standard mammalian cell mutation assays. We show herein that when evaluated in an assay (AL cell assay), in which both intragenic and multilocus mutations are detectable, that arsenite is in fact a strong dose-dependent mutagen and that it induces mostly large deletion mutations. Cotreatment of cells with the oxygen radical scavenger dimethyl sulfoxide significantly reduces the mutagenicity of arsenite. Thus, the carcinogenicity of arsenite can be explained at least in part by it being a mutagen that depends on reactive oxygen species for its activity.

Genomic instability and tumorigenic induction in immortalized human bronchial epithelial cells by heavy ions

Carcinogenesis is postulated to be a progressive multistage process characterized by an increase in genomic instability and clonal selection with each mutational event endowing a selective growth advantage. Genomic instability as manifested by the amplification of specific gene fragments is common among tumor and transformed cells. In the present study, immortalized human bronchial (BEP2D) cells were irradiated with graded doses of either 1GeV/nucleon 56Fe ions or 150 keV/micrometer alpha particles. Transformed cells developed through a series of successive steps before becoming tumorigenic in nude mice. Tumorigenic cells showed neither ras mutations nor deletion in the p16 tumor suppressor gene. In contrast, they harbored mutations in the p53 gene and over-expressed cyclin D1. Genomic instability among transformed cells at various stage of the carcinogenic process was examined based on frequencies of PALA resistance. Incidence of genomic instability was highest among established tumor cell lines relative to transformed, non-tumorigenic and control cell lines. Treatment of BEP2D cells with a 4 mM dose of the aminothiol WR-1065 significantly reduced their neoplastic transforming response to 56Fe particles. This model provides an opportunity to study the cellular and molecular mechanisms involved in malignant transformation of human epithelial cells by heavy ions.

Malignant transformation of immortalized human bronchial epithelial cells by asbestos fibers

Although asbestos is a well-established lung carcinogen, there currently is no suitable human cell model in which to examine the underlying cellular and molecular changes associated with fiber-mediated bronchial carcinogenesis. Using a recently established transformation model based on a human papillomavirus-immortalized human bronchial epithelial cell line, we successfully transformed these BEP2D cells after a single, 7-day treatment with a 20-microgram/ml (4 micrograms per cm2 area) dose of Union Internationale Contre le Cancer (UICC) Rhodesian chrysotile fibers. Asbestos treatment resulted in a surviving fraction of 0.18 compared to control cells. Transformed cells developed through a series of sequential steps, including altered growth kinetics, resistance to serum-induced terminal differentiation, and anchorage-independent growth, before becoming tumorigenic to form progressively growing tumors in nude mice. Seven tumorigenic cell lines were isolated and determined to be of human epithelial origin based on immunofluorescent staining of keratin and isozyme analysis. Analysis of tumor DNA revealed no mutations at either codon 12 or 13 in any the ras oncogenes. An independent role for K-ras mutation in fiber carcinogenesis, therefore, cannot be confirmed. This model provides a unique opportunity to study the cellular and molecular changes at the various stages in fiber-mediated neoplastic transformation of human bronchial epithelial cells.

Cytogenetic and molecular genetic analysis of tumorigenic human bronchial epithelial cells induced by radon alpha particles

To establish a cell culture model for lung carcinogenesis, independent populations of the human papillomavirus 18-immortalized human bronchial epithelial cell line BEP2D were treated with high linear energy transfer radon-simulated alpha-particles, expanded and xenotransplanted into Nu/Nu mice. Six independent cell lines were established from tumors that developed from three separate radiation treatments as follows: treatment (Tx) 1 (30 cGy--two doses), H2BT, Tx 2 (30 cGy--single dose), R30T1L, R30T2 and R30T3L, Tx 3 (30 cGy--single dose), H1ATN and H1ATBA1. Cytogenetic analysis revealed common changes in all tumor lines: loss of the Y chromosome (ch), one of three copies of ch8, one of three copies of ch14, and one of two copies of ch4p16-pter and ch11p15-pter. Analysis of polymerase chain reaction-amplified short tandem repeats of informative loci confirmed the loss of chY in all lines and loss of heterozygosity (LOH) at eight loci spanning the length of ch8 in all lines from Tx's 1 and 2. Our data support previous studies indicating the presence of tumor suppressor genes on ch8. LOH also was confirmed on ch14 at locus D14S306 in all cell lines from Tx 2 and in one of two lines from Tx 3. This region, 14q12-q13, may contain changes in one of the five known somatostatin receptor genes (SSTR1). No LOH was detected at any of the informative loci tested for on ch4 or ch11.

Mutagenic effects of a single and an exact number of alpha particles in mammalian cells

One of the main uncertainties in risk estimation for environmental radon exposure using lung cancer data from underground miners is the extrapolation from high- to low-dose exposure where multiple traversal is extremely rare. The biological effects of a single alpha particle are currently unknown. Using the recently available microbeam source at the Radiological Research Accelerator Facility at Columbia University, we examined the frequencies and molecular spectrum of S1- mutants induced in human-hamster hybrid (A(L)) cells by either a single or an exact number of alpha particles. Exponentially growing cells were stained briefly with a nontoxic concentration of Hoechst dye for image analysis, and the location of individual cells was computer-monitored. The nucleus of each cell was irradiated with either 1,2,4, or 8 alpha particles at a linear energy transfer of 90 keV/microm consistent with the energy spectrum of domestic radon exposure. Although single-particle traversal was only slightly cytotoxic to A(L) cells (survival fraction approximately 0.82), it was highly mutagenic, and the induced mutant fraction averaged 110 mutants per 10(5) survivors. In addition, both toxicity and mutant induction were dose-dependent. Multiplex PCR analysis of mutant DNA showed that the proportion of mutants with multilocus deletions increased with the number of particle traversals. These data provide direct evidence that a single a particle traversing a nucleus will have a high probability of resulting in a mutation and highlight the need for radiation protection at low doses.