Mechanisms involved in the immortalization of mammalian cells by ionizing radiation and chemical carcinogens

Stem cell-based strategies and challenges for production of cultivated meat

Cultivated meat scale-up and industrial production will require multiple stable cell lines from different species to recreate the organoleptic and nutritional properties of meat from livestock. In this Review, we explore the potential of stem cells to create the major cellular components of cultivated meat. By using developments in the fields of tissue engineering and biomedicine, we explore the advantages and disadvantages of strategies involving primary adult and pluripotent stem cells for generating cell sources that can be grown at scale. These myogenic, adipogenic or extracellular matrix-producing adult stem cells as well as embryonic or inducible pluripotent stem cells are discussed for their proliferative and differentiation capacity, necessary for cultivated meat. We examine the challenges for industrial scale-up, including differentiation and culture protocols, as well as genetic modification options for stem cell immortalization and controlled differentiation. Finally, we discuss stem cell-related safety and regulatory challenges for bringing cultivated meat to the marketplace.

DNA methylation changes from primary cultures through senescence-bypass in Syrian hamster fetal cells initially exposed to benzo[a]pyrene

Current chemical testing strategies are limited in their ability to detect non-genotoxic carcinogens (NGTxC). Epigenetic anomalies develop during carcinogenesis regardless of whether the molecular initiating event is associated with genotoxic (GTxC) or NGTxC events; therefore, epigenetic markers may be harnessed to develop new approach methodologies that improve the detection of both types of carcinogens. This study used Syrian hamster fetal cells to establish the chronology of carcinogen-induced DNA methylation changes from primary cells until senescence-bypass as an essential carcinogenic step. Cells exposed to solvent control for 7 days were compared to naïve primary cultures, to cells exposed for 7 days to benzo[a]pyrene, and to cells at the subsequent transformation stages: normal colonies, morphologically transformed colonies, senescence, senescence-bypass, and sustained proliferation in vitro. DNA methylation changes identified by reduced representation bisulphite sequencing were minimal at day-7. Profound DNA methylation changes arose during cellular senescence and some of these early differentially methylated regions (DMRs) were preserved through the final sustained proliferation stage. A set of these DMRs (e.g., Pou4f1, Aifm3, B3galnt2, Bhlhe22, Gja8, Klf17, and L1l) were validated by pyrosequencing and their reproducibility was confirmed across multiple clones obtained from a different laboratory. These DNA methylation changes could serve as biomarkers to enhance objectivity and mechanistic understanding of cell transformation and could be used to predict senescence-bypass and chemical carcinogenicity.

Perspectives on scaling production of adipose tissue for food applications

With rising global demand for food proteins and significant environmental impact associated with conventional animal agriculture, it is important to develop sustainable alternatives to supplement existing meat production. Since fat is an important contributor to meat flavor, recapitulating this component in meat alternatives such as plant based and cell cultured meats is important. Here, we discuss the topic of cell cultured or tissue engineered fat, growing adipocytes in vitro that could imbue meat alternatives with the complex flavor and aromas of animal meat. We outline potential paths for the large scale production of in vitro cultured fat, including adipogenic precursors during cell proliferation, methods to adipogenically differentiate cells at scale, as well as strategies for converting differentiated adipocytes into 3D cultured fat tissues. We showcase the maturation of knowledge and technology behind cell sourcing and scaled proliferation, while also highlighting that adipogenic differentiation and 3D adipose tissue formation at scale need further research. We also provide some potential solutions for achieving adipose cell differentiation and tissue formation at scale based on contemporary research and the state of the field.

Integration of Epigenetic Mechanisms into Non-Genotoxic Carcinogenicity Hazard Assessment: Focus on DNA Methylation and Histone Modifications

Epigenetics involves a series of mechanisms that entail histone and DNA covalent modifications and non-coding RNAs, and that collectively contribute to programing cell functions and differentiation. Epigenetic anomalies and DNA mutations are co-drivers of cellular dysfunctions, including carcinogenesis. Alterations of the epigenetic system occur in cancers whether the initial carcinogenic events are from genotoxic (GTxC) or non-genotoxic (NGTxC) carcinogens. NGTxC are not inherently DNA reactive, they do not have a unifying mode of action and as yet there are no regulatory test guidelines addressing mechanisms of NGTxC. To fil this gap, the Test Guideline Programme of the Organisation for Economic Cooperation and Development is developing a framework for an integrated approach for the testing and assessment (IATA) of NGTxC and is considering assays that address key events of cancer hallmarks. Here, with the intent of better understanding the applicability of epigenetic assays in chemical carcinogenicity assessment, we focus on DNA methylation and histone modifications and review: (1) epigenetic mechanisms contributing to carcinogenesis, (2) epigenetic mechanisms altered following exposure to arsenic, nickel, or phenobarbital in order to identify common carcinogen-specific mechanisms, (3) characteristics of a series of epigenetic assay types, and (4) epigenetic assay validation needs in the context of chemical hazard assessment. As a key component of numerous NGTxC mechanisms of action, epigenetic assays included in IATA assay combinations can contribute to improved chemical carcinogen identification for the better protection of public health.

Canine corneal epithelial cells possess a sustained proliferative capacity and generate a spontaneously derived cell line

We have previously reported characteristics of canine corneal epithelial cells in vitro and found that canine corneal epithelial cells could maintain their proliferative capacity even after continuous culture without the use of feeder cells and growth promoting additives. The objective of this study was to elucidate proliferative characteristics of canine corneal epithelial cells independent of feeder cells and growth promoting additives, with the aim of developing a spontaneously derived corneal epithelial cell line. Canine and rabbit corneal epithelial cells were harvested from the limbus and cultured with, or without, feeder cells and growth promoting additives, and both were passaged continuously until growth arrest. Canine corneal epithelial cells could proliferate independently, and could be passaged more times than rabbit cells. A canine corneal epithelial cell line, cCEpi, which could be passaged more than 100 times without using feeder cells and growth promoting additives, was established. cCEpi cells maintained a cell morphology close to the primary culture and expressed p63, cytokeratin 15 (K15), and K3. Although changes in colony morphology, shortening of the population doubling time and a heteroploid karyotype were observed, cCEpi was not tumorigenic. Stratified cell sheets cultured from cCEpi were morphologically and immunohistologically similar to sheets cultivated from early passage cells. In conclusion, canine corneal epithelial cells can proliferate independent of feeder cells and growth promoting additives. cCEpi maintains properties similar to normal corneal epithelial cells and could be a useful source for studies in cellular biology and for developing novel therapies.

A mechanistic evaluation of the Syrian hamster embryo cell transformation assay (pH 6.7) and molecular events leading to senescence bypass in SHE cells

The implementation of the Syrian hamster embryo cell transformation assay (SHE CTA) into test batteries and its relevance in predicting carcinogenicity has been long debated. Despite prevalidation studies to ensure reproducibility and minimise the subjective nature of the assay's endpoint, an underlying mechanistic and molecular basis supporting morphological transformation (MT) as an indicator of carcinogenesis is still missing. We found that only 20% of benzo(a)pyrene-induced MT clones immortalised suggesting that, alone, the MT phenotype is insufficient for senescence bypass. From a total of 12 B(a)P- immortalised MT lines, inactivating p53 mutations were identified in 30% of clones, and the majority of these were consistent with the potent carcinogen's mode of action. Expression of p16 was commonly silenced or markedly reduced with extensive promoter methylation observed in 45% of MT clones, while Bmi1 was strongly upregulated in 25% of clones. In instances where secondary events to MT appeared necessary for senescence bypass, as evidenced by a transient cellular crisis, clonal growth correlated with monoallelic deletion of the CDKN2A/B locus. The findings further implicate the importance of p16 and p53 pathways in regulating senescence while providing a molecular evaluation of SHE CTA -derived variant MT clones induced by benzo(a)pyrene.

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Low-dose exposures to common environmental chemicals that are deemed safe individually may be combining to instigate carcinogenesis, thereby contributing to the incidence of cancer. This risk may be overlooked by current regulatory practices and needs to be vigorously investigated.

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology.

Disruptive chemicals, senescence and immortality

Carcinogenesis is thought to be a multistep process, with clonal evolution playing a central role in the process. Clonal evolution involves the repeated 'selection and succession' of rare variant cells that acquire a growth advantage over the remaining cell population through the acquisition of 'driver mutations' enabling a selective advantage in a particular micro-environment. Clonal selection is the driving force behind tumorigenesis and possesses three basic requirements: (i) effective competitive proliferation of the variant clone when compared with its neighboring cells, (ii) acquisition of an indefinite capacity for self-renewal, and (iii) establishment of sufficiently high levels of genetic and epigenetic variability to permit the emergence of rare variants. However, several questions regarding the process of clonal evolution remain. Which cellular processes initiate carcinogenesis in the first place? To what extent are environmental carcinogens responsible for the initiation of clonal evolution? What are the roles of genotoxic and non-genotoxic carcinogens in carcinogenesis? What are the underlying mechanisms responsible for chemical carcinogen-induced cellular immortality? Here, we explore the possible mechanisms of cellular immortalization, the contribution of immortalization to tumorigenesis and the mechanisms by which chemical carcinogens may contribute to these processes.

Cell size and cancer: a new solution to Peto's paradox?

Cancer, one of the leading health concerns for humans, is by no means a human-unique malady. Accumulating evidence shows that cancer kills domestic and wild animals at a similar rate to humans and can even pose a conservation threat to certain species. Assuming that each physiologically active and proliferating cell is at risk of malignant transformation, any evolutionary increase in the number of cells (and thus body mass) will lead to a higher cancer frequency, all else being equal. However, available data fail to support the prediction that bigger animals are affected by cancer more than smaller ones. The unexpected lack of correlation between body size (and life span) and cancer risk across taxa was dubbed Peto's paradox. In this perspective, several plausible explanations of Peto's paradox are presented, with the emphasis on a largely underappreciated relation of cell size to both metabolism and cell division rates across species, which we believe are key factors underlying the paradox. We conclude that larger organisms have bigger and slowly dividing cells with lower energy turnover, all significantly reducing the risk of cancer initiation. Solving Peto's paradox will enhance our understanding the evolution of cancer and may provide new implications for cancer prevention and treatment.

Chemometric methods for studying the relationships between trace elements in laryngeal cancer and healthy tissues

A quick and reliable method for the evaluation and classification of two types of tissues is presented. Several chemometric methods were applied to evaluate multivariate data of the tissue samples with respect to the content of trace elements. The content of Pb, Al, Zn, Cd, Cu, Ni and Co was determined in samples of healthy and cancerous tissue obtained from 26 patients. Determination was done at milligram/kilogram level with inductively coupled plasma optical emission spectrometry (ICP-OES) and atomic absorption spectroscopy (AAS) techniques. Contents of trace metals in studied tissues are not normally distributed; however, normal distribution was confirmed for log values. There is a statistically significant difference in the content of Zn, Cd, Cu and Al (p<0.01) and Ni and Co (p<0.05) when healthy tissue is compared to cancerous one. Correlation between contents of trace elements for studied tissues was positive; the highest was found between Zn and Cu. A chemometric methodology seems to be a promising tool for classifications of the tissue samples.

Environmental epigenetics in metal exposure

Although it is widely accepted that chronic exposure to arsenite, nickel, chromium and cadmium increases cancer incidence in individuals, the molecular mechanisms underlying their ability to transform cells remain largely unknown. Carcinogenic metals are typically weak mutagens, suggesting that genetic-based mechanisms may not be primarily responsible for metal-induced carcinogenesis. Growing evidence shows that environmental metal exposure involves changes in epigenetic marks, which may lead to a possible link between heritable changes in gene expression and disease susceptibility and development. Here, we review recent advances in the understanding of metal exposure affecting epigenetic marks and discuss establishment of heritable gene expression in metal-induced carcinogenesis.

Effects of nickel, chromate, and arsenite on histone 3 lysine methylation

Occupational exposure to nickel (Ni), chromium (Cr), and arsenic (As) containing compounds has been associated with lung cancer and other adverse health effects. Their carcinogenic properties may be attributable in part, to activation and/or repression of gene expression induced by changes in the DNA methylation status and histone tail post-translational modifications. Here we show that individual treatment with nickel, chromate, and arsenite all affect the gene activating mark H3K4 methylation. We found that nickel (1 mM), chromate (10 microM), and arsenite (1 microM) significantly increase tri-methyl H3K4 after 24 h exposure in human lung carcinoma A549 cells. Seven days of exposure to lower levels of nickel (50 and 100 microM), chromate (0.5 and 1 microM) or arsenite (0.1, 0.5 and 1 microM) also increased tri-methylated H3K4 in A549 cells. This mark still remained elevated and inherited through cell division 7 days following removal of 1 microM arsenite. We also demonstrate by dual staining immunofluorescence microscopy that both H3K4 tri-methyl and H3K9 di-methyl marks increase globally after 24 h exposure to each metal treatment in A549 cells. However, the tri-methyl H3K4 and di-methyl H3K9 marks localize in different regions in the nucleus of the cell. Thus, our study provides further evidence that a mechanism(s) of carcinogenicity of nickel, chromate, and arsenite metal compounds may involve alterations of various histone tail modifications that may in turn affect the expression of genes that may cause transformation.

Neoplastic transformation induced by carbon ions

PURPOSE

The objective of this experiment was to compare the oncogenic potential of carbon ion beams and conventional photon beams for use in radiotherapy.

METHODS AND MATERIALS

The HeLa X human skin fibroblast cell line CGL1 was irradiated with carbon ions of three different energies (270, 100, and 11.4 MeV/u). Inactivation and transformation data were compared with those for 15 MeV photons.

RESULTS

Inactivation and transformation frequencies for the 270 MeV/u carbon ions were similar to those for 15-MeV photons. The maximal relative biologic effectiveness (RBE(alpha)) values for 100MeV/u and 11.4 MeV/u carbon ions, respectively, were as follows: inactivation, 1.6 +/- 0.2 and 6.7 +/- 0.7; and transformation per surviving cell, 2.5 +/- 0.6 and 12 +/- 3. The curve for dose-transformation per cell at risk exhibited a maximum that was shifted toward lower doses at lower energies.

CONCLUSIONS

Transformation induction per cell at risk for carbon ions in the entrance channel was comparable to that for photons, whereas for the lower energies, 100 MeV/u and 11 MeV/u, which are representative of the energies delivered to the tumor margins and volume, respectively, the probability of transformation in a single cell was greater than it was for photons. In addition, at isoeffective doses with respect to cell killing, the 11.4-MeV/u beam was more oncogenic than were photons.

Epigenetics in metal carcinogenesis: nickel, arsenic, chromium and cadmium

Although carcinogenic metals have been known to disrupt a wide range of cellular processes the precise mechanism by which these exert their carcinogenic effects is not known. Over the last decade or two, studies in the field of metal carcinogenesis suggest that epigenetic mechanisms may play a role in metal-induced carcinogenesis. In this review we summarize the evidence demonstrating that exposure to carcinogenic metals such as nickel, arsenic, chromium, and cadmium can perturb DNA methylation levels as well as global and gene specific histone tail posttranslational modification marks. We also wish to emphasize the importance in understanding that gene expression can be regulated by both genetic and epigenetic mechanisms and both these must be considered when studying the mechanism underlying the toxicity and cell-transforming ability of carcinogenic metals and other toxicants, and aberrant changes in gene expression that occur during disease states such as cancer.

Soluble and insoluble nickel compounds exert a differential inhibitory effect on cell growth through IKKalpha-dependent cyclin D1 down-regulation

It is well-known that insoluble nickel compounds possess much more potent carcinogenic activities as compared with soluble nickel compounds. Although it is assumed that the different entry and clearance rate are responsible for the difference, the mechanisms underlying the different carcinogenic activities are still not well understood yet. In the present study, we found that exposure to soluble, but not insoluble nickel compounds, caused a significant inhibition of cell growth and G1/G0 cell cycle arrest, which was concomitant with a marked down-regulation of cylin D1, an essential nuclear protein for controlling G1/S transition, while both soluble and insoluble nickel compounds showed similar effects on NFkappaB activation, HIF-1alpha protein accumulation and TNF-alpha transcription and CAP43 protein expression at same doses range. The down-regulation of cyclin D1 is due to protein degradation rather than inhibition of transcription, because the nickel compounds treatment did not change cyclin D1 mRNA level, while MG132, the proteasome inhibitor, can rescue the degradation of cyclin D1 caused by soluble nickel compound. Moreover, the soluble nickel-induced cyclin D1 degradation is dependent on its Thr286 residue and requires IKKalpha, but not HIF-1alpha, which are both reported to be involved in cyclin D1 down-regulation. Taken together, we demonstrate that soluble, but not insoluble nickel compound, is able to cause cyclin D1 degradation and a cell growth arrest in an IKKalpha-dependent manner. Given the role of cyclin D1 and cell proliferation in carcinogenesis, we anticipate that the different effects of soluble and insoluble nickel compounds on cyclin D1 degradation and cell growth arrest may at least partially account for their different carcinogenic activities.

Nickel compounds induce phosphorylation of histone H3 at serine 10 by activating JNK-MAPK pathway

Nickel (Ni) is a known carcinogen, although the mechanism of its carcinogenicity is not clear. Here, we provide evidence that Ni can induce phosphorylation of histone H3 at its serine 10 residue in a c-jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK)-dependent manner. Ni induces the phosphorylation of JNK, with no effect on the phosphorylation states of the extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein kinases. An inhibitor of JNK eliminated the Ni-initiated JNK-mediated induction of histone H3 phosphorylation at serine 10, whereas inhibitors specific for ERK or p38 kinases had no effect on the phosphorylation levels of histone H3 at serine 10 (P-H3S10) in Ni-treated cells. A complete loss of Ni ion-induced phosphorylation of H3S10 was observed when JNK was specifically knocked down with RNAi. These results are the first to show the specific JNK-mediated phosphorylation of histone H3 at its serine 10 residue. We show that addition of Ni to an in vitro P-H3S10 dephosphorylation reaction does not change the loss of phosphorylation in the reaction, supporting the notion that Ni causes H3S10 phosphorylation via the JNK/SAPK pathway. It is likely that modification of H3S10 is one of a growing number of epigenetic changes believed to be involved in the carcinogenesis caused by Ni.

Genetic and epigenetic mechanisms in metal carcinogenesis and cocarcinogenesis: nickel, arsenic, and chromium

Chronic exposure to nickel(II), chromium(VI), or inorganic arsenic (iAs) has long been known to increase cancer incidence among affected individuals. Recent epidemiological studies have found that carcinogenic risks associated with chromate and iAs exposures were substantially higher than previously thought, which led to major revisions of the federal standards regulating ambient and drinking water levels. Genotoxic effects of Cr(VI) and iAs are strongly influenced by their intracellular metabolism, which creates several reactive intermediates and byproducts. Toxic metals are capable of potent and surprisingly selective activation of stress-signaling pathways, which are known to contribute to the development of human cancers. Depending on the metal, ascorbate (vitamin C) has been found to act either as a strong enhancer or suppressor of toxic responses in human cells. In addition to genetic damage via both oxidative and nonoxidative (DNA adducts) mechanisms, metals can also cause significant changes in DNA methylation and histone modifications, leading to epigenetic silencing or reactivation of gene expression. In vitro genotoxicity experiments and recent animal carcinogenicity studies provided strong support for the idea that metals can act as cocarcinogens in combination with nonmetal carcinogens. Cocarcinogenic and comutagenic effects of metals are likely to stem from their ability to interfere with DNA repair processes. Overall, metal carcinogenesis appears to require the formation of specific metal complexes, chromosomal damage, and activation of signal transduction pathways promoting survival and expansion of genetically/epigenetically altered cells.

Dietary chromium and nickel enhance UV-carcinogenesis in skin of hairless mice

The skin cancer enhancing effect of chromium (in male mice) and nickel in UVR-irradiated female Skh1 mice was investigated. The dietary vitamin E and selenomethionine were tested for prevention of chromium-enhanced skin carcinogenesis. The mice were exposed to UVR (1.0 kJ/m(2) 3 x weekly) for 26 weeks either alone, or combined with 2.5 or 5.0 ppm potassium chromate, or with 20, 100 or 500 ppm nickel chloride in drinking water. Vitamin E or selenomethionine was added to the lab chow for 29 weeks beginning 3 weeks before the start of UVR exposure. Both chromium and nickel significantly increased the UVR-induced skin cancer yield in mice. In male Skh1 mice, UVR alone induced 1.9+/-0.4 cancers/mouse, and 2.5 or 5.0 ppm potassium chromate added to drinking water increased the yields to 5.9+/-0.8 and 8.6+/-0.9 cancers/mouse, respectively. In female Skh1 mice, UVR alone induced 1.7+/-0.4 cancers/mouse, and the addition of 20, 100 or 500 ppm nickel chloride increased the yields to 2.8+/-0.9, 5.6+/-0.7 and 4.2+/-1.0 cancers/mouse, respectively. Neither vitamin E nor selenomethionine reduced the cancer yield enhancement by chromium. These results confirm that chromium and nickel, while not good skin carcinogens per se, are enhancers of UVR-induced skin cancers in Skh1 mice. Data also suggest that the enhancement of UVR-induced skin cancers by chromate may not be oxidatively mediated since the antioxidant vitamin E as well as selenomethionine, found to prevent arsenite-enhanced skin carcinogenesis, failed to suppress enhancement by chromate.

Carcinogenic effect of nickel compounds

Nickel is a widely distributed metal that is industrially applied in many forms. Accumulated epidemiological evidence confirms that exposures to nickel compounds are associated with increased nasal and lung cancer incidence, both in mostly occupational exposures. Although the molecular mechanisms by which nickel compounds cause cancer are still under intense investigation, the carcinogenic actions of nickel compounds are thought to involve oxidative stress, genomic DNA damage, epigenetic effects, and the regulation of gene expression by activation of certain transcription factors related to corresponding signal transduction pathways. The present review summarizes our current knowledge on the molecular mechanisms of nickel carcinogenesis, with special emphasis on the role of nickel induced reactive oxygen species (ROS) and signal transduction pathways.

Fluorescent tracking of nickel ions in human cultured cells

The carcinogenic activity of various nickel (Ni) compounds is likely dependent upon their ability to enter cells and elevate intracellular levels of Ni ions. Water-insoluble Ni compounds such as NiS and Ni(3)S(2) were shown in vitro to enter cells by phagocytosis and potently induce tumors in experimental animals at the site of exposure. These water-insoluble nickel compounds are generally considered to be more potent carcinogens than the water-soluble forms. However, recent in vitro studies have shown similar effects for insoluble and soluble Ni compounds. Using a dye that fluoresces when intracellular Ni ion binds to it, we showed that both soluble and insoluble Ni compounds were able to elevate the levels of Ni ions in the cytoplasmic and nuclear compartments. However, when the source of Ni ions was removed from the culture dish, the intracellular Ni ions derived from soluble Ni compound were lost from the cells at a significantly faster rate than those derived from the insoluble Ni compound. Within 10 h after NiCl(2) removal from the culture medium, Ni ions disappeared from the nucleus and were not detected in the cells by 16 h, while insoluble Ni(3)S(2) yielded Ni ions that persisted in the nucleus after 16 h and were detected in the cytoplasm even after 24 h following Ni removal. These effects are discussed in terms of whole body exposure to water-soluble and -insoluble Ni compounds and consistency with animal carcinogenicity studies.

Induction of anchorage-independent growth in primary human cells exposed to protons or HZE ions separately or in dual exposures

Travelers on space missions will be exposed to a complex radiation environment that includes protons and heavy charged particles. Since protons are present at much higher levels than are heavy ions, the most likely scenario for cellular radiation exposure will be proton exposure followed by a hit by a heavy ion. Although the effects of individual ion species on human cells are being investigated extensively, little is known about the effects of exposure to both radiation types. One useful measure of mammalian cell damage is induction of the ability to grow in a semi-solid agar medium highly inhibitory to the growth of normal human cells, termed neoplastic transformation. Using primary human cells, we evaluated induction of soft-agar growth and survival of cells exposed to protons only or to heavy charged particles (600 MeV/nucleon silicon) only as well as of cells exposed to protons followed after a 4-day interval by silicon ions. Both ions alone efficiently transformed the human cells to anchorage-independent growth. Initial experiments indicate that the dose responses for neoplastic transformation of cells exposed to protons and then after 4 days to silicon ions appear similar to that of cells exposed to silicon ions alone.

Downregulation of Cap43 gene by von Hippel-Lindau tumor suppressor protein in human renal cancer cells

We previously identified 9 genes (i.e., thymosin beta4, secreted protein acidic and rich in cysteine, Cap43, ceruloplasmin, serum amyloid A, heat shock protein 90, LOT1, osteopontin and casein kinase Igamma) that are more highly expressed in cancerous regions than in noncancerous regions in human renal cancers. In our study, we considered the possibility that the von Hippel-Lindau (VHL) tumor suppressor gene might be able to affect the expression of these 9 genes in renal cancer cells. We first established 2 VHL-positive cell lines, 786/VHL-1 and 786/VHL-2, after the introduction of wild-type VHL into VHL-negative renal cancer 786-O cells. Of these 9 genes, expression of the Cap43 gene was specifically downregulated by VHL. Expression of Cap43 was also much lower in 4 other VHL-positive renal cancer cell lines than in VHL-negative 786-O cells. Cap43 promoter assays with several deletion or mutation constructs demonstrated that the Sp1 site in the element from -286 base pairs (bp) to -62 bp was partly responsible for VHL-induced suppression of the Cap43 gene. Immunostaining analysis with human specimens of renal cancers demonstrated that the Cap43 protein was expressed in most cancer cells and macrophages. We also observed a marked and specific increase of Cap43 mRNA levels in response to hypoxia or nickel in all VHL-positive cell lines. Cellular expression of Cap43 mRNA in response to hypoxia or nickel thus is closely associated with VHL gene expression in renal cancer cells. Although the function of the Cap43 protein remains unclear, the expression of Cap43 protein could be a molecular marker closely associated with VHL in renal cancer.

In vitro aging of macaque adherent cells: similar pattern of cellular aging between human and macaque

To explore new models for human cellular aging as well as to evaluate aging of the macaques, profiles of cellular aging in macaques were studied. Adherent cells were obtained from five Japanese macaques (Macaca fuscata), 14 long-tailed macaques (Macaca fascicularis), two bonnet monkeys (Macaca radiata) and a rhesus monkey (Macaca mulatta). A total of 35 cultures were performed and cell morphology, doubling time, telomere length and telomerase activity were studied. They were classified into three groups; group I: cell strains with a definite replicative life-span (-41 PDLs) (presence of M1), group II: cell strains with a limited extension of replicative life-span (79-106 PDLs) with p53 mutation(s) (presence of M2), and group III: a cell strain with an indefinite replicative life-span (>150 PDLs) with characteristics of transformation. Except for the last group, telomerase activity was not observed. Macaque cells demonstrated three chronological patterns comprising both human and rodent patterns, however, presence of the two limits of proliferation in vitro grants macaque cells to be more appropriate than rodents in both studying human aging and oncogenesis.

DNA methylation and epigenetic inheritance

Mammalian cell lines silence genes at low frequency by the methylation of promoter sequences. These silent genes can be reactivated at high frequency by the demethylating agent 5-azacytidine (5-aza-CR). The inactive and active epigenetic states of such genes are stably inherited. A method for silencing genes is now available. It involves treatment of permeabilized cells with 5-methyl deoxycytidine triphosphate (5-methyl dCTP) which is incorporated into DNA. The methylation of promoter sequences has been confirmed using the bisulfite genomic sequencing procedure. Methylated oligonucleotides homologous to promoter sequences might be used to specifically target and silence given genes, but results so far have not been conclusive. Treatments that silence or reactivate genes by changing DNA methylation can be referred to as epimutagens, as distinct from mutagens that act by changing DNA sequences. The epimutagen 5-aza-CR reactivates genes but has little mutagenic activity, whereas standard mutagens (such as ethyl methane sulfonate and ultraviolet light) have little reactivation activity. Nevertheless, much more information is required about the effects of DNA-damaging agents in changing DNA methylation and gene activity and also about the role of epimutations in tumor progression.

Nickel essentiality, toxicity, and carcinogenicity

The increasing utilization of heavy metals in modern industries leads to an increase in the environmental burden. Nickel represents a good example of a metal whose use is widening in modern technologies. As the result of accelerated consumption of nickel-containing products nickel compounds are released to the environment at all stages of production and utilization. Their accumulation in the environment may represent a serious hazard to human health. Among the known health related effects of nickel are skin allergies, lung fibrosis, variable degrees of kidney and cardiovascular system poisoning and stimulation of neoplastic transformation. The mechanism of the latter effect is not known and is the subject of detailed investigation. This review provides an analysis of the current state in the field.

A perspective on current and future uses of alternative models for carcinogenicity testing

This perspective is based upon the data presented at the International Life Sciences Institute (ILSI), Health and Environmental Sciences Institute Workshop on the Evaluation of Alternative Methods for Carcinogenicity Testing (ILSI Workshop). It is important to understand that all models discussed at the Workshop have limitations and that they are not designed to be employed as stand-alone assays. Although they may have other, appropriate applications. I do not recommend use of the SHE cell assay and the Tg.AC model for the regulatory purposes of a safety assessment. In my view, the neonatal mouse, p53+/-, XPA-/-, XPA-/- and p53+/-, and the rasH2 models can, as a component of an overall assessment, provide information on potential carcinogenicity of a chemical that is appropriate for consideration in a regulatory context. Generally, these models exhibit the ability to detect genotoxic compounds. In most cases these compounds would be detected in a standard battery of genotoxicity tests and, therefore, quite often the use of an alternative is not necessary. Actually, I believe that a bioassay in rats will suffice most of the time, that is, in my view, a routine bioassay in mice is not necessary. Specific circumstances where data obtained from one of the "recommended" alternative models might be helpful are discussed. With regard to lessons for the future, there is a particular need for models that are responsive to chemicals that exhibit a nongenotoxic mode of action. Additionally, new models will continue to be developed and their half-life will likely be substantially shorter than the time required for traditional validation. The development of enhanced paradigms for validation should be a priority so that improved safety assessment decisions can be made more quickly. However, while evaluating and validating such models, it is important to consider the fundamental issues, for example, rational dose selection, evaluation of mode of action in the context of dose-response relationships including the existence of thresholds and secondary mechanisms, and species-to-species extrapolation. The alternatives to carcinogenicity testing project was a very major undertaking. In addition to the valuable information provided, it serves to illustrate the value of cooperation between academia, government, and industry. Furthermore, the involvement of the International Life Sciences Institute as the overall organizing, facilitating umbrella was crucial for the success of the project.

Free radicals-mediated induction of oxidized DNA bases and DNA-protein cross-links by nickel chloride

Using the comet assay, we showed that nickel chloride at 250-1000 microM induced DNA damage in human lymphocytes, measured as the change in comet tail moment, which increased with nickel concentration up to 500 microM and then decreased. Observed increase might follow from the induction of strand breaks or/and alkali-labile sites (ALS) by nickel, whereas decrease from its induction of DNA-DNA and/or DNA-protein cross-links. Proteinase K caused an increase in the tail moment, suggesting that nickel chloride at 1000 microM might cross-link DNA with nuclear proteins. Lymphocytes exposed to NiCl(2) and treated with enzymes recognizing oxidized and alkylated bases: endonuclease III (Endo III), formamidopyrimidine-DNA glycosylase (Fpg) and 3-methyladenine-DNA glycosylase II (AlkA), displayed greater extent of DNA damage than those not treated with these enzymes, indicating the induction of oxidized and alkylated bases by nickel. The incubation of lymphocytes with spin traps, 5,5-dimethyl-pyrroline N-oxide (DMPO) and PBN decreased the extent of DNA damage, which might follow from the production of free radicals by nickel. The pre-treatment with Vitamin C at 10 microM and Vitamin E at 25 microM decreased the tail moment of the cells exposed to NiCl(2) at the concentrations of the metal causing strand breaks or/and ALS. The results obtained suggest that free radicals may be involved in the formation of strand breaks or/and ALS in DNA as well as DNA-protein cross-links induced by NiCl(2). Nickel chloride can also alkylate DNA bases. Our results support thesis on multiple, free radicals-based genotoxicity pathways of nickel.

Tumor progression of skin carcinoma cells in vivo promoted by clonal selection, mutagenesis, and autocrine growth regulation by granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor

Tumor microenvironment is crucial for cancer growth and progression as evidenced by reports on the significance of tumor angiogenesis and stromal cells. Using the HaCaT/HaCaT-ras human skin carcinogenesis model, we studied tumor progression from benign tumors to highly malignant squamous cell carcinomas. Progression of tumorigenic HaCaT-ras clones to more aggressive and eventually metastatic phenotypes was reproducibly achieved by their in vivo growth as subcutaneous tumors in nude mice. Their enhanced malignant phenotype was stably maintained in recultured tumor cells that represented, identified by chromosomal analysis, a distinct subpopulation of the parental line. Additional mutagenic effects were apparent in genetic alterations involving chromosomes 11 and 2, and in amplification and overexpression of the H-ras oncogene. Importantly, in vitro clonal selection of benign and malignant cell lines never resulted in late-stage malignant clones, indicating the importance of the in vivo environment in promoting an enhanced malignant phenotype. Independently of their H-ras status, all in vivo-progressed tumor cell lines (five of five) exhibited a constitutive and stable expression of the hematopoietic growth factors granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, which may function as autocrine/paracrine mediators of tumor progression in vivo. Thus, malignant progression favored by the in vivo microenvironment requires both clonal selection of subpopulations adapted to in vivo growth and mutational events leading to stable functional alterations.

Cell transformation assays

Morphological transformation of mammalian cells following exposure to a chemical agent is frequently used as a preliminary screen for the carcinogenic potential of the agent. This unit describes a protocol using Syrian hamster embryo (SHE) cells to assay transformation.

Culture condition-dependent senescence-like growth arrest and immortalization in rodent embryo cells

Culture Condition-Dependent Senescence-Like Growth Arrest and Immortalization in Rodent Embryo Cells. We investigated the telomerase activity, telomere length, and replicative life span of cells from human embryos and rodent embryos (mouse, rat and Syrian hamster). We used two culture conditions for rodent embryo cells whereby the cells were plated at a density of 2 x 10(5) into a 25-cm(2) flask and subcultured every 3 days or every 10 days. We found that nearly 100% of the cultures of rodent embryo cells become immortal when they are subcultured using the 10-day culture protocol. These rodent embryo cells retain telomerase activity and long telomeres (19-50 kb) in the long-term cultures, whereas human embryo cells rapidly deplete telomerase activity associated with significant shortening of telomeres, and then they senesce. In contrast to the results from 10-day cultures, we found that some mouse cell cultures and most Syrian hamster cell cultures arrest cell growth after 13 and 29 population doublings, respectively, while retaining substantial levels of telomerase activity and experiencing no significant loss of telomeres when the cells were subcultured using the 3-day culture protocol. This growth arrest is phenotypically indistinguishable from cellular senescence. The present results suggest that in rodent cells the onset of senescence-like arrest can be activated without repression of telomerase, and that this activation pathway can be bypassed easily under certain culture conditions, such as the 10-day culture protocol.

Aneuploidy precedes and segregates with chemical carcinogenesis

A century ago, Boveri proposed that cancer is caused by aneuploidy, an abnormal balance of chromosomes, because aneuploidy correlates with cancer and because experimental aneuploidy generates "pathological" phenotypes. Half a century later, when cancers were found to be nonclonal for aneuploidy, but clonal for somatic gene mutations, this hypothesis was abandoned. As a result, aneuploidy is now generally viewed as a consequence, and mutated genes as a cause of cancer. However, we have recently proposed a two-stage mechanism of carcinogenesis that resolves the discrepancy between clonal mutation and nonclonal karyotypes. The proposal is as follows: in stage 1, a carcinogen "initiates" carcinogenesis by generating a preneoplastic aneuploidy; in stage 2, aneuploidy causes asymmetric mitosis because it biases balance-sensitive spindle and chromosomal proteins and alters centrosomes both numerically and structurally (in proportion to the degree of aneuploidy). Therefore, the karyotype of an initiated cell evolves autocatalytically, generating ever-new chromosome combinations, including neoplastic ones. Accordingly, the heterogeneous karyotypes of "clonal" cancers are an inevitable consequence of the karyotypic instability of aneuploid cells. The notorious long latent periods, of months to decades, from carcinogen to carcinogenesis, would reflect the low probability of evolving by chance karyotypes that compete favorably with normal cells, in principle analagous to natural evolution. Here, we have confirmed experimentally five predictions of the aneuploidy hypothesis: (1) the carcinogens dimethylbenzanthracene and cytosine arabinoside induced aneuploidy in a fraction of treated Chinese hamster embryo cells; (2) aneuploidy preceded malignant transformation; (3) transformation of carcinogen-treated cells occurred only months after carcinogen treatment, i.e., autocatalytically; (4) preneoplastic aneuploidy segregated with malignant transformation in vitro and with 14 of 14 tumors in animals; and (5) karyotypes of tumors were heterogeneous. We conclude that, with the carcinogens studied, aneuploidy precedes cancer and is necessary for carcinogenesis.

Syrian hamster dermal cell immortalization is not enhanced by power line frequency electromagnetic field exposure

Several epidemiological studies have suggested associations between exposure to residential power line frequency electromagnetic fields and childhood leukaemia, and between occupational exposure and adult leukaemia. A variety of in vitro studies have provided limited supporting evidence for the role of such exposures in cancer induction in the form of acknowledged cellular end points, such as enhanced mutation rate and cell proliferation, though the former is seen only with extremely high flux density exposure or with co-exposure to ionizing radiation. However, in vitro experiments on a scale large enough to detect rare cancer-initiating events, such as primary cell immortalization following residential level exposures, have not thus far been reported. In this study, large cultures of primary Syrian hamster dermal cells were continuously exposed to power line frequency electromagnetic fields of 10 100 and 1000 microT for 60 h, with and without prior exposure to a threshold (1.5 Gy), or sub-threshold (0.5 Gy), immortalizing dose of ionizing radiation. Electromagnetic field exposure alone did not immortalize these cells at a detectable frequency (> or = 1 x 10(-7)); furthermore, such exposure did not enhance the frequency of ionizing radiation-induced immortalization.

Effects of nickel on DNA methyltransferase activity and genomic DNA methylation levels

Methylation of DNA plays an important role in organizing the genome into transcriptionally active and inactive zones. Nickel compounds cause chromatin condensation and DNA methylation in the transgenic gpt+ Chinese hamster cell line (G12). Here we show that nickel is an inhibitor of cytosine 5-methyltransferase activity in vivo and in vitro. In living cells, this inhibition is transient and following a recovery period after nickel treatment, Mtase activity slightly rebounds. Genomic DNA methylation levels are also somewhat decreased following nickel treatment, but with time, there is an elevation of total DNA methylation above basal levels and before any rebound of methyltransferase activity. These results suggest that nickel exposure can elevate total genomic DNA methylation levels even when DNA methyltransferase activity is depressed. These findings may explain the hypermethylation of senescence and tumor suppressor genes found during nickel carcinogenesis and support the model of a direct effect of Ni2+ on chromatin leading to de novo DNA methylation.

Genetic control of telomerase and replicative senescence in human and rodent cells

The ribonucleoprotein telomerase is detectable in most human cancer cells and immortalized cells but is absent or inactive in the vast majority of normal counterparts. Repression of telomerase activity in human somatic cells, which leads to telomere shortening and replicative senescence, may have evolved as a protective mechanism against immortalization, unfettered clonal evolution and cancer. Rodent cells in culture are far more susceptible to immortalization and malignant progression than human cells. This can be explained by our observation that normal diploid rodent (hamster) fibroblasts possess active telomerase throughout their proliferative life span, and therefore they do not require a telomerase activation step during immortalization. Monochromosome transfer techniques have enabled us to identify powerful telomerase repressive activity specifically associated with the introduction of a single copy of human chromosome 3 into human carcinoma cells. Fine-structure deletion analysis of non-repressed hybrids has permitted us to map the position of the candidate telomerase repressor gene to 3p21.1-3p21.3. A strategy for isolating the gene has been developed involving a combination of fine-structure deletion mapping and functional gene transfer approaches. The availability of cloned telomerase repressor genes will advance our understanding of telomerase regulation in the human soma and its disruption during human cancer development.

Bypass of senescence after disruption of p21CIP1/WAF1 gene in normal diploid human fibroblasts

Most somatic cells die after a finite number of cell divisions, a phenomenon described as senescence. The p21(CIP1/WAF1) gene encodes an inhibitor of cyclin-dependent kinases. Inactivation of p21 by two sequential rounds of targeted homologous recombination was sufficient to bypass senescence in normal diploid human fibroblasts. At the checkpoint between the prereplicative phase of growth and the phase of chromosome replication, cells lacking p21 failed to arrest the cell cycle in response to DNA damage, but their apoptotic response and genomic stability were unaltered. These results establish the feasibility of using gene targeting for genetic studies of normal human cells.

Sustained nontumorigenic phenotype correlates with a largely stable chromosome content during long-term culture of the human keratinocyte line HaCaT

Altered growth and differentiation and a highly abnormal karyotype are generally believed to be indicators for tumorigenic conversion of human cells. Inactivation of TP53 is supposedly one possible mechanism for accelerated genetic aberrations via reduced control of the genetic integrity. To examine the significance of this functional relationship, we investigated the long-term development of the spontaneously immortalized human skin keratinocyte line HaCaT, carrying UV-specific mutations in both alleles of the TP53 tumor suppressor gene. During > 300 passages, proliferation, clonogenicity, and serum-independent growth potential increased. In addition, HaCaT cells gained anchorage independence and at late passages showed reduced differentiation. Karyotypic analysis up to passage 225 revealed a high frequency of translocations and deletions, with a particular increase during passages 30 and 50. Nevertheless, the HaCaT cells remained nontumorigenic when injected subcutaneously, and noninvasive in surface transplants in nude mice. By comparative genomic hybridization, we confirmed the karyotypically identified phase of increased chromosomal aberrations between passages 30 and 50. However, before and thereafter, the CGH profiles of the individual chromosomes were largely unchanged, demonstrating that those translocations-also maintained in later passages-did not cause a gross chromosomal imbalance. Thus, our data suggest that multiple changes often correlated with a "transformed phenotype," including extensive karyotypic alterations and mutational inactivation of TP53, are well compatible with a nontumorigenic phenotype of the HaCaT cells, and that preserved chromosomal balance may be crucial for this stability during long-term propagation.

DNA methylation, heterochromatin and epigenetic carcinogens

This paper will explore emerging concepts related to alternative carcinogenic mechanisms of 'non-mutagenic,' and hence epigenetic, carcinogens that may heritably alter DNA methylation without changing the underlying DNA sequence. In this review, we will touch on the basic concepts of DNA methylation, and will elaborate in greater detail on related topics including chromatin condensation, and heterochromatin spreading that is well known to induce gene silencing by position effect variegation in Drosophila and other species. Data from our model transgenic G12 cell system will be presented to support our hypothesis that certain carcinogens, such as nickel, may be carcinogenic not primarily because of their overt mutability, but rather as the result of their ability to promote DNA hypermethylation of important cancer-related genes. We will conclude with a discussion of the broader relevance of our findings and its application to other so-called 'epigenetic' carcinogens.

Carcinogenicity assessment of selected nickel compounds

The early epidemiological data indicated different carcinogenic risks from inhalation of different nickel compounds, but it was not clear what characteristics governed the intrinsic carcinogenic hazard of the various nickel compounds. Based on the earlier results, all soluble and insoluble nickel compounds were assumed to have the same carcinogenic mechanism albeit different potencies. Recent in vivo and in vitro studies challenged this assumption. In this paper an attempt is made to integrate the most relevant human, animal, and in vitro data into a general model that can help understand the different carcinogenic potentials of the various nickel compounds. In this perspective, it is recognized that there are two main components that could contribute to the development of lung cancer via exposure to certain nickel compounds. The first component corresponds to the heritable changes (genetic or epigenetic) derived from the direct or indirect actions of nickel compounds. The second component may be the promotion of cell proliferation elicited by certain nickel compounds. The different contributions of three nickel compounds to these two components are presented. This paper emphasizes the importance of recognizing the individuality of the different nickel species in reaching regulatory decisions and the fact that different risk assessment considerations may apply for compounds that appear to produce immortality and cancer by genetic/epigenetic mechanisms (like nickel subsulfide), compounds that may present a threshold for the induction of tumors in rats (like high-temperature nickel oxide), or compounds that may only have an enhancing effect on carcinogenicity (like nickel sulfate).

Nongenotoxic carcinogens: development of detection methods based on mechanisms: a European project

While the accumulation of genetic changes in a somatic cell is considered essential for the genesis of a cancer, it has become clear that not all carcinogens are genotoxic, suggesting that some carcinogens indirectly participate in the generation of genetic changes during carcinogenesis. A European project funded by the European Community was thus conceived to study mechanisms of nongenotoxic aspects of carcinogenesis. Two main strategical approaches were adapted: (i) to study whether and how Syrian hamster embryo (SHE), Syrian hamster dermal (SHD) and BALB/c 3T3 cell transformation systems simulate in vivo carcinogenesis, and to examine whether they can detect nongenotoxic carcinogens; (ii) to study, refine and validate mechanisms-based end-points for detection of nongenotoxic carcinogens. For mechanisms-based research, the proposed end-points included gap junctional intercellular communication (GJIC) inhibition, altered expression of critical genes, immortalization and aberrant cell proliferation. We also selected model compounds commonly usable for various endpoints. Our major results can be summarized as follows: (1) SHE and BALB/c 3T3 transformation systems reflect both genotoxic and nongenotoxic carcinogenic events; they detect not only genotoxic but also many although not all, nongenotoxic carcinogens. This is further supported by the fact that both genotoxic and nongenotoxic carcinogens were able to immortalize SHD cells. (2) Many nongenotoxic carcinogens, although not all, inhibit GJIC in vitro as well as in vivo. Mechanistic studies suggest an important role of blocked GJIC in carcinogenesis and that different mechanisms are involved in inhibition of the communication by different agents used. However, inhibition of GJIC is not a prerequisite for the enhancement (or induction) of transformation of SHE or BALB/c 3T3 cells. (3) Among compounds examined, there was a good correlation between induction of micronuclei and cell transformation in SHE cells while no such correlation was found between the induction of cell transformation and ornithine decarboxylase activity. (4) Two transgenic mouse mutation assays (lacI and lacZ) were established and validated. The genotoxin dimethylnitrosamine was shown to be mutagenic to the liver in both assays. Ortho-anisidine, a bladder-specific carcinogen that was inactive in standard rodent genetic toxicity assays was uniquely mutagenic to the bladder of the transgenic mice. The peroxisome proliferator methyl clofenipate was established as nonmutagenic to the liver of both transgenic mice. That eliminated DNA damage as a cause of the liver tumours produced by this chemical and weakened the idea that induced cell division leads to mutation induction. (5) With an in vitro DNA replication model, it was found that DNA damage induced by genotoxic agents can be responsible for inhibition of DNA replication, while certain nongenotoxic agents such as phorbol esters increase DNA replication. (6) An attempt to use structure-activity relationship for subfamilies of nongenotoxic carcinogens, e.g., receptor-mediated carcinogens, has been initiated with some promising results. Our results support the idea that there are multiple nongenotoxic mechanisms in carcinogenesis, and that working hypothesis-oriented approaches are encouraged rather than simple screening of chemicals in developing test systems for the detection of nongenotoxic carcinogens.

Novel use of a selectable fusion gene as an "in-out" marker for studying genetic loss in mammalian cells

Recent demonstrations of loss of heterozygosity in a wide variety of human cancers suggest that large multilocus genetic deletions (presumably including tumor suppressor genes) constitute a major class of genetic alteration in human carcinogenesis. Here we show that a bifunctional fusion gene (Hytk), suitable for both positive and negative selection, is an effective marker for studying genetic loss in mammalian cells with minimal interference from point-mutational changes. Studies with a transgenic V79 cell line in which a single functional copy of Hytk was stably inserted into the genome in a retroviral vector showed that loss of the marker (and presumably flanking cellular genetic material) could be induced efficiently by ionizing radiation (gamma-rays and fast neutrons) but only weakly by the powerful point-mutagen benzo[a]pyrene diol epoxide. In a first application of the system, we provide evidence that radiation-induced loss can occur through an indirect mechanism after a high-frequency event. Collectively, our results suggest that the Hytk marker should be a valuable tool for studying genome position effects on the tolerance of genetic loss in cultured human cells that represent different stages in clonal evolution and tumor progression.