A genotoxicity test system based on p53R2 gene expression in human cells: Assessment of its reactivity to various classes of genotoxic chemicals

Correlation of Occupational Exposure to Carcinogenic Polycyclic Aromatic Hydrocarbons (cPAHs) and Blood Levels of p53 and p21 Protiens

Carcinogenic polycyclic aromatic hydrocarbons (cPAHs) are considered the most serious cancer risk. This study was conducted to assess the effect of acute exposure to cPAHs on cancer biomarker proteins p53 and p21 in occupational workers during the hajj season in Makkah. One hundred five participants were recruited, including occupational workers and apparently healthy individuals; air samples were collected using personal sample monitors to identify the subjects’ exposure to cPAHs. Quantitative analyses of benzo(a)anthracene (BaA), benzo(b)fluoranthene (BbF), benzo(a)pyrene (BaP), dibenzo(a,h)fluronathene (DBA), indeno(1,2,3-c,d)pyrene (IND) and chyresene (CRY) were carried out using the GC/Mass technique. Serum p53 and p21 proteins were analyzed using ELISA. The ambient air samples collected by the occupationally exposed group were more highly polluted by cPAHs, (90.25 ± 14.1) ng/m³, than those of the unexposed control groups, (30.12 ± 5.56) ng/m³. The concentration of distributive cPAHs was markedly more elevated in the air samples of the exposed group than in those taken from the non-exposed group. The study results demonstrated significant links between short-term exposure to cPAHs and serum p53 and p21 levels. Serum p53 and p21 proteins potentially influence biomarkers when exposed to ambient air cPAHs.

Airborne polycyclic aromatic compounds contribute to the induction of the tumour-suppressing P53 pathway in wild double-crested cormorants

Polycyclic aromatic compounds (PACs), including polycyclic aromatic hydrocarbons (PAHs) and PAH-like compounds are known or probable environmental carcinogens released into the environment as a by-product of incomplete combustion of fossil fuels and other organic materials. Studies have shown that exposure to PACs in the environment can induce both genotoxicity and epigenetic toxicity, but few studies have related PAC exposure to molecular changes in free ranging wildlife. Previous work has suggested that double-crested cormorants (Phalacrocorax auritus; DCCO) exhibited a higher incidence of genetic mutations when their breeding sites were located in heavily industrialized areas (e.g., Hamilton Harbour, Hamilton, ON, Canada) as compared to sites located in more pristine environments, such as in Lake Erie. The aim of this study was to determine if airborne PACs from Hamilton Harbour alter the tumour-suppressing P53 pathway and/or global DNA methylation in DCCOs. Airborne PACs were measured using passive air samplers in the Hamilton Harbour area and low-resolution mass spectrometry analysis detected PACs in livers of DCCOs living in Hamilton Harbour. Further hepatic and lung transcriptional analysis demonstrated that the expression of the genes involved in the DNA repair and cellular apoptosis pathway were up-regulated in both tissues of DCCOs exposed to PACs, while genes involved in p53 regulation were down-regulated. However, global methylation levels did not differ between reference- and PAC-exposed DCCOs. Altogether, data suggest that PACs activate the P53 pathway in free-ranging DCCOs living nearby PAC-contaminated areas.

The predominant role of apoptosis in γH2AX formation induced by aneugens is useful for distinguishing aneugens from clastogens

The phosphorylated form of the histone protein H2AX, called γH2AX, is recognized as a useful biomarker not only for DNA double-strand breaks but also for a wide range of other DNA damage. An increasing number of publications propose γH2AX to be measured when determining genotoxicity, phototoxicity, and the effectiveness of cancer therapy. Because γH2AX is also generated by apoptosis, a γH2AX-assay might assess genotoxic risk incorrectly. The aim of this study was to elucidate the influence of apoptosis on measurements of γH2AX by flow cytometry, with the clastogens mitomycin C (MMC) and etoposide (ETP), and the aneugens vinblastine (VB) and paclitaxel (PT), which do not react directly with DNA. TK6 human lymphoblastoid cells were treated with the clastogens and the aneugens, stained for the apoptotic biomarker caspase-3 and for γH2AX, and then analyzed by flow cytometry. All the test compounds caused a dose-dependent increase of γH2AX-positive (γH2AX+) cells. The γH2AX+ cell population included both caspase-3-positive (γH2AX+/caspase-3+) and caspase-3-negative (γH2AX+/caspase-3-) cells. The increase in γH2AX+ cells after treatment with the aneugens corresponded to the increase in caspase-3+ cells. The increase in γH2AX+/caspase-3- cells after treatment with the clastogens was significant, but there was only a slight increase after treatment with the aneugens. This reflects the fact that the apoptotic pathway of a clastogen starts from DNA damage, whereas that of an aneugen starts from cell-cycle arrest in the M-phase. Therefore, the two pathways contribute differently to apoptosis. Double staining for γH2AX and caspase-3 provided helpful information for the different mechanistic effects of aneugens and clastogens that induce γH2AX.

Synthetic memory circuits for tracking human cell fate

A variety of biological phenomena, from disease progression to stem cell differentiation, are typified by a prolonged cellular response to a transient environmental cue. While biologically relevant, heterogeneity in these long-term responses is difficult to assess at the population level, necessitating the development of biological tools to track cell fate within subpopulations. Here we present a novel synthetic biology approach for identifying and tracking mammalian cell subpopulations. We constructed three genomically integrated circuits that use bistable autoregulatory transcriptional feedback to retain memory of exposure to brief stimuli. These "memory devices" are used to isolate and track the progeny of cells that responded differentially to doxycycline, hypoxia, or DNA-damaging agents. Following hypoxic or ultraviolet radiation exposure, strongly responding cells activate the memory device and exhibit changes in gene expression, growth rates, and viability for multiple generations after the initial stimulus. Taken together, these results indicate that a heritable memory of hypoxia and DNA damage exists in subpopulations that differ in long-term cell behavior.

Development of a Fish Cell Biosensor System for Genotoxicity Detection Based on DNA Damage-Induced Trans-Activation of p21 Gene Expression

p21^CIP1/WAF1 is a p53-target gene in response to cellular DNA damage. Here we report the development of a fish cell biosensor system for high throughput genotoxicity detection of new drugs, by stably integrating two reporter plasmids of pGL₃-p21-luc (human p21 promoter linked to firefly luciferase) and pRL-CMV-luc (CMV promoter linked to Renilla luciferase) into marine flatfish flounder gill (FG) cells, referred to as p21FGLuc. Initial validation of this genotoxicity biosensor system showed that p21FGLuc cells had a wild-type p53 signaling pathway and responded positively to the challenge of both directly acting genotoxic agents (bleomycin and mitomycin C) and indirectly acting genotoxic agents (cyclophosphamide with metabolic activation), but negatively to cyclophosphamide without metabolic activation and the non-genotoxic agents ethanol and D-mannitol, thus confirming a high specificity and sensitivity, fast and stable response to genotoxic agents for this easily maintained fish cell biosensor system. This system was especially useful in the genotoxicity detection of Di(2-ethylhexyl) phthalate (DEHP), a rodent carcinogen, but negatively reported in most non-mammalian in vitro mutation assays, by providing a strong indication of genotoxicity for DEHP. A limitation for this biosensor system was that it might give false positive results in response to sodium butyrate and any other agents, which can trans-activate the p21 gene in a p53-independent manner.

Development of a High-Throughput Gaussia Luciferase Reporter Assay for the Activation of the GADD45a Gene by Mutagens, Promutagens, Clastogens, and Aneugens

Exposure to genotoxic carcinogens leads to increased expression of the GADD45a gene in mammalian cells. This signature of genotoxic hazard has previously been exploited in the GreenScreen HC assay, in which GADD45a expression is linked to green fluorescent protein (GFP) expression in the human TK6 lymphoblastoid cell line. This article describes the development and validation of an alternative assay (“BlueScreen HC”), in which expression is linked to Gaussia luciferase (GLuc) expression, yielding a luminescent reporter, the preferred optical output in high-throughput screening. The coelentrazine substrate of GLuc is relatively unstable, and a new buffer is reported that improves its stability. A more sensitive method is demonstrated for the measurement of cell densities in the assay, using the fluorescent cyanine dye thiazole orange. A protocol amendment also allows the assessment of pro-genotoxicity using S9 liver extracts. Compounds from the European Centre for the Validation of Alternative Methods (ECVAM) recommended list for the assessment of new or improved genotoxicity assays were evaluated with and without S9 in the new assay. The new GLuc assay was as effective as the GFP assay in producing positive results for all classes of genotoxic carcinogen and negative results for all nongenotoxins tested.

Review of current and "omics" methods for assessing the toxicity (genotoxicity, teratogenicity and nephrotoxicity) of herbal medicines and mushrooms

ETHNOPHARMACOLOGICAL RELEVANCE

The increasing use of traditional herbal medicines around the world requires more scientific evidence for their putative harmlessness. To this end, a plethora of methods exist, more or less satisfying. In this post-genome era, recent reviews are however scarce, not only on the use of new "omics" methods (transcriptomics, proteomics, metabonomics) for genotoxicity, teratogenicity, and nephrotoxicity assessment, but also on conventional ones.

METHODS

The present work aims (i) to review conventional methods used to assess genotoxicity, teratogenicity and nephrotoxicity of medicinal plants and mushrooms; (ii) to report recent progress in the use of "omics" technologies in this field; (iii) to underline advantages and limitations of promising methods; and lastly (iv) to suggest ways whereby the genotoxicity, teratogenicity, and nephrotoxicity assessment of traditional herbal medicines could be more predictive.

RESULTS

Literature and safety reports show that structural alerts, in silico and classical in vitro and in vivo predictive methods are often used. The current trend to develop "omics" technologies to assess genotoxicity, teratogenicity and nephrotoxicity is promising but most often relies on methods that are still not standardized and validated.

CONCLUSION

Hence, it is critical that toxicologists in industry, regulatory agencies and academic institutions develop a consensus, based on rigorous methods, about the reliability and interpretation of endpoints. It will also be important to regulate the integration of conventional methods for toxicity assessments with new "omics" technologies.

Quantitative and subcellular localization analysis of the nuclear isoform dUTP pyrophosphatase in alkylating agent-induced cell responses

Our previous proteome analysis showed that the nuclear isoform of dUTP pyrophosphatase (DUT-N) was identified in the culture medium of human amnion FL cells after exposure to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). These results suggest that DUT-N may be a potential early biomarker to assess the risk of alkylating agents exposure. DUT-N is one of the two isoforms of deoxyuridine triphosphate nucleotidohydrolase (dUTPase). Our current knowledge of DUT-N expression in human cells is very limited. In the current study, we first investigated the appearance of DUT-N in the culture medium of different human cell lines in response to a low concentration of MNNG exposure. We verified that the MNNG-induced appearance of DUT-N in the extracellular environment is cell-specific. Western blot analysis confirmed that the intracellular DUT-N changes responded to MNNG in a concentration-dependent and cell-specific manner. Furthermore, subcellular fraction experiments showed that 0.25μM MNNG treatment dramatically increased the DUT-N expression levels in the cytoplasmic extracts prepared from both FL and HepG2 cells, increased DUT-N levels in nuclear extracts prepared from HepG2 cells, and decreased DUT-N levels in nuclear extracts from FL cells. Morphological studies using immunofluorescence showed that a low concentration of MNNG could alter the distribution of DUT-N in FL and HepG2 cells in different ways. Taken together, these studies indicate a role of DUT-N in alkylating agent-induced cell responses.

The dual role of p53: DNA protection and antioxidant

The classical functions of p53 protein are those related to its role on DNA damage, cell growth arrest, senescence and apoptosis. For this reason it is called 'the guardian of the genome' and is considered one of the most important players in the development of cancer. However, more recently it has been show that p53 is not only involved in cancer, but also in ageing. p53 is stimulated by stress, which in turn results in the activation of a wide range of transcriptional targets. Low-intensity stress will activate p53 in a manner which results in antioxidant response, thus protecting against ageing because of its antioxidant function. On the contrary, high-intensity activation of p53 will result in an increase of oxidative stress by activation of p53-mediated pro-oxidant targets, thus increasing the rate of ageing, but protecting against cancer.

Molecular analysis and functions of p53R2 in zebrafish

p53R2 is a newly identified small subunit of ribonucleotide reductase and plays a pivotal role in the supply of dNTPs for genomic DNA repair and mitochondrial DNA synthesis, but little is known about its functions in zebrafish. Herein, we obtained the cDNA of zebrafish p53R2 that shares 72.8% and 72.5% amino acid identities with human p53R2 and zebrafish R2, respectively. Residues crucial for enzymatic activity are highly conserved among p53R2 proteins from different species. p53R2 in zebrafish was maternally expressed, its transcripts were detected in developing embryos and all adult tissues examined. A 250-bp minimal promoter upstream of the translational initiation site was identified to drive basal expression of p53R2 in a p53-independent manner. Expression of p53R2 was induced by DNA-damaging reagents CPT or MMS, but suppressed by p53-knockdown in zebrafish embryos. Moreover, p53R2 was mainly distributed in the cytoplasm of cells under normal condition and upon DNA damage. Furthermore, overexpression of p53R2 attenuated apoptosis of embryonic cells caused by CPT or MMS treatment and protected developing embryos from death. Therefore, functions of p53R2 in zebrafish are closely associated with its activity in DNA repair and synthesis.

Development of human cell biosensor system for genotoxicity detection based on DNA damage-induced gene expression

Background

Human exposure to genotoxic agents in the environment and everyday life represents a serious health threat. Fast and reliable assessment of genotoxicity of chemicals is of main importance in the fields of new chemicals and drug development as well as in environmental monitoring. The tumor suppressor gene p21, the major downstream target gene of activated p53 which is responsible for cell cycle arrest following DNA damage, has been shown to be specifically up-regulated by genotoxic carcinogens. The aim of our study was to develop a human cell-based biosensor system for simple and fast detection of genotoxic agents.

Methods

Metabolically active HepG2 human hepatoma cells were transfected with plasmid encoding Enhanced Green Fluorescent Protein (EGFP) under the control of the p21 promoter (p21HepG2GFP). DNA damage was induced by genotoxic agents with known mechanisms of action. The increase in fluorescence intensity, due to p21 mediated EGFP expression, was measured with a fluorescence microplate reader. The viability of treated cells was determined by the colorimetric MTS assay.

Results

The directly acting alkylating agent methylmethane sulphonate (MMS) showed significant increase in EGFP production after 48 h at 20 μg/mL. The indirectly acting carcinogen benzo(a)pyren (BaP) and the cross-linking agent cisplatin (CisPt) induced a dose- dependent increase in EGFP fluorescence, which was already significant at concentrations 0.13 μg/mL and 0.41 μg/mL, respectively. Vinblastine (VLB), a spindle poison that does not induce direct DNA damage, induced only a small increase in EGFP fluorescence intensity after 24 h at the lowest concentration (0.1 μg/mL), while exposure to higher concentrations was associated with significantly reduced cell viability.

Conclusions

The results of our study demonstrated that this novel assay based on the stably transformed cell line p21HepG2GFP can be used as a fast and simple biosensor system for detection of genetic damage caused by chemical agents.

Cytotoxicity and genotoxicity reporter systems based on the use of mammalian cells

With the dramatic increase in the number of new agents arising from the chemical, pharmaceutical, and agricultural industries, there is an urgent need to develop assays for rapid evaluation of potential risks to man and environment. The panel of conventional tests used for cytotoxicity and genotoxicity and the strategies to progress from small scale assays to high content screening in toxicology are discussed. The properties of components necessary as sensors and reporters for new reporter assays, and the application of genetic strategies to design assays are reviewed. The concept of cellular reporters is based on the use of promoters of chemical stress-regulated genes ligated to a suitable luminescent or fluorescent reporter gene. Current reporter assays designed from constructs transferred into suitable cell lines are presented.

The development of RAD51C, Cystatin A, p53 and Nrf2 luciferase-reporter assays in metabolically competent HepG2 cells for the assessment of mechanism-based genotoxicity and of oxidative stress in the early research phase of drug development

Four different mechanism-based high-throughput luciferase-reporter assays were developed in human HepG2 cells, which contain phase I and II metabolic activity and a functionally active p53 protein. The promoter regions of RAD51C and Cystatin A, as well as the responsive element of the p53 protein, were selected for the generation of the genotoxicity reporter assays. Moreover, a luciferase-based reporter assay was generated that measures the activation of the Nrf2 oxidative stress pathway. Validation with respect to the ECVAM compound list [D. Kirkland, P. Kasper, L. Muller, R. Corvi, G. Speit, Recommended lists of genotoxic and non-genotoxic chemicals for assessment of the performance of new or improved genotoxicity tests: a follow-up to an ECVAM workshop, Mutat. Res. 653 (2008) 99-108] resulted in an overall sensitivity of the HepG2 genotoxicity reporter assays for genotoxicity of 85% (17/20). The specificity and predictivity were high with 81% (34/42) and 82% (51/62), respectively. Various compounds had a positive score although metabolic activation was needed. The HepG2 reporter data were also compared with the available data on bacterial mutagenicity (Ames test), in vitro clastogenicity and in vivo clastogenicity for an additional set of 192 compounds. The predictivity for mutagenicity results was 74% (sensitivity, 61%, 30/49; specificity, 80%, 77/96) and for in vitro clastogenicity 59% (sensitivity, 45%, 35/78; specificity 83%, 38/46). The correlation between results from the HepG2 genotoxicity reporter assays and in vivo clastogenicity was much higher with 77% (sensitivity, 74%, 28/38; specificity 81%, 26/32). Results from the Nrf2 reporter assay showed that a large number of genotoxic compounds activated the Nrf2 oxidative stress pathway. In conclusion, four high-throughput mechanism-based reporter assays in the HepG2 cell line were developed, which can be applied for screening in the early research phase of drug development. The use of these assays in combination with the previously validated Vitotox and RadarScreen assays will certainly reduce the attrition rate due to genotoxicity in the developmental phase of drug development.

Eukaryotic systems broaden the scope of synthetic biology

Synthetic biology aims to engineer novel cellular functions by assembling well-characterized molecular parts (i.e., nucleic acids and proteins) into biological "devices" that exhibit predictable behavior. Recently, efforts in eukaryotic synthetic biology have sprung from foundational work in bacteria. Designing synthetic circuits to operate reliably in the context of differentiating and morphologically complex cells presents unique challenges and opportunities for progress in the field. This review surveys recent advances in eukaryotic synthetic biology and describes how synthetic systems can be linked to natural cellular processes in order to manipulate cell behavior and to foster new discoveries in cell biology research.

Evaluation of high-throughput genotoxicity assays used in profiling the US EPA ToxCast chemicals

Three high-throughput screening (HTS) genotoxicity assays-GreenScreen HC GADD45a-GFP (Gentronix Ltd.), CellCiphr p53 (Cellumen Inc.) and CellSensor p53RE-bla (Invitrogen Corp.)-were used to analyze the collection of 320 predominantly pesticide active compounds being tested in Phase I of US. Environmental Protection Agency's ToxCast research project. Between 9% and 12% of compounds were positive for genotoxicity in the assays. However, results of the varied tests only partially overlapped, suggesting a strategy of combining data from a battery of assays. The HTS results were compared to mutagenicity (Ames) and animal tumorigenicity data. Overall, the HTS assays demonstrated low sensitivity for rodent tumorigens, likely due to: screening at a low concentration, coverage of selected genotoxic mechanisms, lack of metabolic activation and difficulty detecting non-genotoxic carcinogens. Conversely, HTS results demonstrated high specificity, >88%. Overall concordance of the HTS assays with tumorigenicity data was low, around 50% for all tumorigens, but increased to 74-78% (vs. 60% for Ames) for those compounds producing tumors in rodents at multiple sites and, thus, more likely genotoxic carcinogens. The aim of the present study was to evaluate the utility of HTS assays to identify potential genotoxicity hazard in the larger context of the ToxCast project, to aid prioritization of environmentally relevant chemicals for further testing and assessment of carcinogenicity risk to humans.