The repair of large DNA adducts in mammalian cells

Assessment of a panel of cellular biomarkers and the kinetics of their induction in comparing genotoxic modes of action in HepG2 cells

One major challenge for in vitro genotoxicology is the determination of the genotoxic mode of action of tested compounds. The quantification of the phosphorylation of the histones H3 (pH3) and H2AX (γH2AX) allows an efficient discrimination between aneugenic and clastogenic compounds. However, these two biomarkers do not permit to deduct the specific mechanisms involved in the action of clastogenic compounds. The aim of this study was to investigate other possible cellular biomarkers allowing differentiating clastogenic properties. For this purpose, we analyzed γH2AX and pH3 plus six other biomarkers involved in the DNA damage signaling pathway in HepG2 cells treated with nine clastogens exhibiting different mechanisms of action, as well as one aneugen. All compounds were tested at various concentrations and with kinetics of 2, 6, 24 and 48 hr. Our results demonstrate the activation of the investigated biomarkers by the tested compounds in a time and concentration dependent manner. Notably, we observed for some nondirect genotoxic clastogens, notably dNTPs pool imbalance inducers, a different kinetic of DNA damage induction compared with direct genotoxins (oxidative stress). However, no specific biomarker signature of mechanisms of clastogenic action could be specified. Multiparametric analysis demonstrates a strong correlation between γH2AX and p-p53(S15) for clastogen compounds. Environ. Mol. Mutagen. 59:516-528, 2018. © 2018 Wiley Periodicals, Inc.

Interactions between hepatitis B virus and aflatoxin B1: effects on p53 induction in HepaRG cells

Infection by hepatitis B virus (HBV) and dietary exposure to aflatoxin B1 (AFB1) are the main risk factors for the development of chronic liver disease and hepatocellular carcinoma (HCC). How these factors cooperate is still largely unknown. AFB1 activation leads to DNA adduction and mutagenesis, with a specific mutation at codon 249 in TP53 (p.R249S). So far, only limited studies have addressed the effects of AFB1 on HBV replication. We have analysed the effects of both risk factors on p53 induction during HBV infection in HepaRG, a cell line with hepatocyte-like morphology that metabolizes AFB1 and supports HBV infection. Exposure to AFB1 up to 5 µM induced a downregulation of HBV replication after 48 h, as measured by a decrease in viral antigens in the culture medium (HBsAg, HBeAg and large envelope protein) and in intracellular levels of HBV transcripts, DNA and HBsAg. Conversely, HBV infection did not significantly modify AFB1-DNA adduct formation or repair as assessed by immunodot-blot assay, and the induction of p53 in response to AFB1 was similar in infected and non-infected HepaRG cells. Overall, our results suggest that AFB1 exposure decreases HBV replication, whereas DNA damage by AFB1 and subsequent p53 induction is not affected by the presence of the virus. Thus, in HepaRG cell line, AFB1 and HBV do not cooperate to increase DNA damage by AFB1. Further studies on the effects of both factors in a context of chronicity are needed to better understand synergistic effects.

4-nitroquinoline-1-oxide induced experimental oral carcinogenesis

Human oral cancer is the sixth largest group of malignancies worldwide and single largest group of malignancies in the Indian subcontinent. Seventy percent of premalignant cancers appear from premalignant lesions. Only 8-10% of these lesions finally turn into malignancy. The appearance of these premalignant lesions is one distinct feature of human oral cancer. At present there is dearth of biomarkers to identify which of these lesions will turn into malignancy. Regional lymph node metastasis and locoregional recurrence are the major factors responsible for the limited survival of patients with oral cancer. Paucity of early diagnostic and prognostic markers is one of the contributory factors for higher mortality rates. Cancer is a multistep process and because of constrain in availability of human tissues from multiple stages of oral carcinogenesis including normal tissues, animal models are being widely used, aiming for the development of diagnostic and prognostic markers. A number of chemical carcinogens like coal tar, 20 methyl cholanthrene (20MC), 9,10-dimethyl-1,2-benzanthracene (DMBA) and 4-nitroquinoline-1-oxide (4NQO) have been used in experimental oral carcinogenesis. However, 4NQO is the preferred carcinogen apart from DMBA in the development of experimental oral carcinogenesis. 4NQO is a water soluble carcinogen, which induces tumors predominantly in the oral cavity. It produces all the stages of oral carcinogenesis and several lines of evidences suggest that similar histological as well as molecular changes are observed in the human system. In the present review an attempt has been made to collate the information available on mechanisms of action of 4NQO, studies carried out for the development of biomarkers and chemopreventives agents using 4NQO animal models.

Expression of a human cytochrome p450 in yeast permits analysis of pathways for response to and repair of aflatoxin-induced DNA damage

Aflatoxin B1 (AFB1) is a human hepatotoxin and hepatocarcinogen produced by the mold Aspergillus flavus. In humans, AFB1 is primarily bioactivated by cytochrome P450 1A2 (CYP1A2) and 3A4 to a genotoxic epoxide that forms N7-guanine DNA adducts. A series of yeast haploid mutants defective in DNA repair and cell cycle checkpoints were transformed with human CYP1A2 to investigate how these DNA adducts are repaired. Cell survival and mutagenesis following aflatoxin B1 treatment was assayed in strains defective in nucleotide excision repair (NER) (rad14), postreplication repair (PRR) (rad6, rad18, mms2, and rad5), homologous recombinational repair (HRR) (rad51 and rad54), base excision repair (BER) (apn1 apn2), nonhomologous end-joining (NHEJ) (yku70), mismatch repair (MMR) (pms1), translesion synthesis (TLS) (rev3), and checkpoints (mec1-1, mec1-1 rad53, rad9, and rad17). Together our data suggest the involvement of homologous recombination and nucleotide excision repair, postreplication repair, and checkpoints in the repair and/or tolerance of AFB1-induced DNA damage in the yeast model. Rev3 appears to mediate AFB1-induced mutagenesis when error-free pathways are compromised. The results further suggest unique roles for Rad5 and abasic endonuclease-dependent DNA intermediates in regulating AFB1-induced mutagenicity.

Inhibitory effects of paracetamol on DNA repair in mammalian cells

Paracetamol blocks DNA replication by inhibiting deoxyribonucleotide (dNTP) synthesis and may therefore also interfere with DNA repair. In the present work various mammalian cell types were treated with genotoxic agents and allowed to repair in the presence or absence of paracetamol. Alkaline elution was used to assay DNA single-strand breaks plus alkali-labile sites (= SSBs). Resting human mononuclear blood cells (MNC) exposed to 4-nitroquinoline N-oxide (NQO, 3 microM) plus 0.3 mM paracetamol contained twice as many DNA SSBs compared to MNC exposed to NQO alone, and the level of SSBs decreased more slowly during repair in the presence of paracetamol. Deoxyribonucleosides reversed the effects of paracetamol. SSBs induced by MMS or X-rays (2.6 Gy) were not increased by paracetamol. Resting and growth-stimulated MNC, HL-60 cells, rat hepatocytes and human fibroblasts exposed to UV-C (3-12 J/m2) showed varying levels of transient SSBs formed during repair but these were consistently higher in the presence of paracetamol (0.3-1 mM). In rat testicular cells SSBs were induced by NQO and the levels were further increased in the presence of paracetamol, whereas after UV almost no SSBs were detected during repair. The cell-type specific levels of transient SSBs after UV did not correlate with the rate of incision of DNA lesions, measured as the rate of SSB accumulation in the presence of repair inhibitors Ara C plus hydroxyurea. Transient SSBs were present in resting MNC for at least 24 h after UV and paracetamol increased these breaks 4-fold however the overall rate of removal of excisable photodamage during repair did not appear to be reduced by the presence of paracetamol. The present data indicate that paracetamol interferes with nucleotide excision repair in several mammalian cell types. This constitutes a mechanism by which paracetamol may contribute to genotoxicity in humans.

Cellular interactions and metabolism of aflatoxin: an update

The aflatoxins are a group of closely related mycotoxins that are widely distributed in nature. The most important of the group is aflatoxin B1 (AFB1), which has a range of biological activities, including acute toxicity, teratogenicity, mutagenicity and carcinogenicity. In order for AFB1 to exert its effects, it must be converted to its reactive epoxide by the action of the mixed function mono-oxygenase enzyme systems (cytochrome P450-dependent) in the tissues (in particular, the liver) of the affected animal. This epoxide is highly reactive and can form derivatives with several cellular macromolecules, including DNA, RNA and protein. Cytochrome P450 enzymes may additionally catalyse the hydroxylation (to AFQ1 and AFM1) and demethylation (to AFP1) of the parent AFB1 molecule, resulting in products less toxic than AFB1. Conjugation of AFB1 to glutathione (mediated by glutathione S-transferase) and its subsequent excretion is regarded as an important detoxification pathway in animals. Resistance to AFB1 toxicity has been interpreted in terms of levels and activities of these detoxifying pathways. This article reviews the multiple reactions and effects attributed to aflatoxin, with particular reference to the interaction of aflatoxin with nucleic acids and proteins, and the contribution this mycotoxin has in disease development and in the promotion of hepatocellular carcinoma (HCC). The anti-mutagenic properties of several dietary factors are also considered in this article. Undoubtedly, the most important aspect of aflatoxin action is its putative role in the development of human cancer, in particular, HCC. Recently, there has been a renewed interest in this aspect and experimental evidence is rapidly accumulating at the molecular level, indicating aflatoxin as an important consideration in the aetiology of human HCC.