The autoradiographic test for unscheduled DNA synthesis: a sensitive assay for the detection of DNA repair in the HepG2 cell line

A critical review of advances in tumor metabolism abnormalities induced by nitrosamine disinfection by-products in drinking water

Intensified sanitation practices amid the recent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak might result in the increased release of chloramine disinfectants into surface water, significantly promoting the formation of nitrosamine disinfection by-products (DBPs) in drinking water. Unfortunately, these nitrosamine DBPs exhibit significant genotoxic, carcinogenic, and mutagenic properties, whereas chlorinating disinfectants remain in global practice. The current review provides valuable insights into the occurrence, identification, contamination status, exposure limits, and toxicity of the new unregulated disinfection by-products (nitrosamine DBPs) in drinking water. As a result, concentrations of nitrosamine DBPs far exceed allowable limits in drinking water, and prolonged exposure has the potential to cause metabolic disorders, a critical step in tumor initiation and progression. Importantly, based on recent research, we have concluded the role of nitrosamines DBPs in different metabolic pathways. Remarkably, nitrosamine DBPs can induce chronic inflammation and initiate tumors by activating sphingolipid and polyunsaturated fatty acid metabolism. Regarding amino acid and nucleotide metabolism, nitrosamine DBPs can inhibit tryptophan metabolism and de novo nucleotide synthesis. Moreover, inhibition of de novo nucleotide synthesis fails to repair DNA damage induced by nitrosamines. Additionally, the accumulation of lactate induced by nitrosamine DBPs may act as a pivotal signaling molecule in communication within the tumor microenvironment. However, with the advancement of tumor metabolomics, understanding the role of nitrosamine DBPs in causing cancer by inducing metabolic abnormalities significantly lags behind, and specific mechanisms of toxic effects are not clearly defined. Urgently, further studies exploring this promising area are needed.

Metabolic conjugation reduces in vitro toxicity of the flavonoid nevadensin

The present study focuses on the association between metabolic capacity and toxicity of the natural occurring flavonoid nevadensin in vitro. Human colon (HT29), liver (HepG2) and bone marrow (KG1) carcinoma cells were used and strong cell line dependent differences in toxic effect strength were found. HepG2 and KG1 cells were more sensitive against nevadensin treatment in comparison to HT29 cells. High resolution mass spectrometry experiments showed that nevadensin is rapidly glucuronidated in HT29 cells, whereas KG1 cells do not metabolize nevadensin, thus glucuronidation was supposed to be a crucial metabolic pathway in vitro. To proof this suggestion, nevadensin glucuronides were isolated from pig liver microsomes und structurally elucidated via NMR spectroscopy. In HepG2 cells a cellular enrichment of nevadensin itself as well as nevadensin-7-O-glucuronide was determined by tandem mass spectrometry. A proteomic screening of uridine 5'-diphospho (UDP)-glucuronosyltransferase (UGT) in HT29 and HepG2 cells provided first hints that the isoforms UGT1A6 and UGT1A1 are responsible for nevadensin glucuronidation. Additionally, nevadensin was found to be a potent SULT inhibitor in HepG2 cells. In sum, the present study clearly illustrates the importance of obtaining detailed information about metabolic competence of cell lines which should be considered in the evaluation of toxic endpoints.

Development of a High-Throughput Human HepG2 Dual Luciferase Assay for Detection of Metabolically Activated Hepatotoxicants and Genotoxicants

Hepatic toxicity remains a major concern for drug failure; therefore, a thorough examination of chemically induced liver toxicity is essential for a robust safety evaluation. Current hypotheses suggest that the metabolic activation of a drug to a reactive intermediate is an important process. In this article, we describe a new high-throughput GADD45β reporter assay developed for assessing potential liver toxicity. Most importantly, this assay utilizes a human cell line and incorporates metabolic activation and thus provides significant advantage over other comparable assays used to determine hepatotoxicity. Our assay has low compound requirement and relies upon 2 reporter genes cotransfected into the HepG2 cells. The gene encoding Renilla luciferase is fused to the CMV promoter and provides a control for cell numbers. The firefly luciferase gene is fused to the GADD45β promoter and used to report an increase in DNA damage. A dual luciferase assay is performed by measuring the firefly and Renilla luciferase activities in the same sample. Results are expressed as the ratio of the 2 luciferase activities; increases over the control are interpreted as evidence of stress responses. This mammalian dual luciferase reporter has been characterized with, and without, metabolic activation using positive and negative control agents. Our data demonstrate that this assay provides for an assessment of potential toxic metabolites, is adaptable to a high-throughput platform, and yields data that accurately and reproducibly detect hepatotoxicants.

Mdm2 as a sensitive and mechanistically informative marker for genotoxicity induced by benzo[a]pyrene and dibenzo[a,l]pyrene

Mdm2 is an oncoprotein interacting with p53 and maintaining low p53 levels in unstressed cells. Here we investigated the effect of genotoxic compounds on Mdm2 phosphorylation levels. Employing the Mdm2 2A10 antibody and phosphatase treatment we found that Mdm2 accumulated in HepG2 cells when exposed to low concentrations of genotoxic compounds such as mitomycin C, etoposide, 5-fluorouracil, and benzo[a]pyrene (BP). The low-dose responses were not accompanied by p53 accumulation and the effect of low concentrations of BP on Mdm2 was not affected by small interfering RNA for p53. In human lymphoblasts 10nM BP induced an Mdm2 response. Low concentrations of BP also induced binding of Mdm2 to chromatin in HepG2 cells, but no p53 binding or H2AX phosphorylation. The more mutagenic dibenzo[a,l]pyrene as well as higher BP concentrations instead induced gammaH2AX and p53 Ser15 association with chromatin. Acrolein potentiated the effect of BP on p53 stabilization and chromatin binding. Taken together, these data suggest that (1) Mdm2 is a sensitive biomarker for certain types of genotoxicity, and (2) that polycyclic aromatic hydrocarbons-induced Mdm2 binding to chromatin reflects repairable damage, whereas chromatin binding of p53 Ser15 and gammaH2AX indicates more persistent DNA damage. The analysis of Mdm2 and related endpoints might be useful for evaluating mutagenic potentials of DNA damages. It is suggested that patterns documented here can be used for separating BP doses that induce readily repaired DNA adducts from doses that overwhelm this capacity.

Application of human hepatoma cell line HepG2 and its progress in the detection of genotoxicants

Genotoxicity test is widely used in the detection of various carcinogens and mutagens. HepG2 is derived from human hepatoblastoma, and it retains the activities of drug-metabolizing enzymes. It has been demonstrated that various carcinogens can be detected in genotoxicity test with HepG2 cells at several endpoints, whereas negative results have been obtained with non-carcinogens.

2,4-Diaminotoluene (2,4-DAT)-induced DNA damage, DNA repair and micronucleus formation in the human hepatoma cell line HepG2

2,4-Diaminotoluene (2,4-DAT) is a widely used industrial intermediate and human exposure is possible in the dye and plastics industries. We investigated the genotoxicity of the environmental pollutant, 2,4-DAT, in human HepG2 cells using the unscheduled DNA synthesis (UDS) test, the micronucleus (MN) assay and single-cell gel electrophoresis (SCGE). 2,4-DAT was first tested by the RNA synthesis inhibition test as a cytotoxicity assay: the IC(50) of 2,4-DAT was 5.2 mM after 20 h of exposure. The compound had a genotoxic effect at concentrations from 1.45 to 6.80 mM in both micronucleus and comet assays. In the micronucleus assay, the number of MN/1000 BNC was 3.5 times higher at a concentration of 6.80 mM 2,4-DAT than in the negative control. At the same concentration, DNA migration (SCGE) showed an Olive tail moment (OTM) of 3.56+/-0.45, as compared to 0.19+/-0.02 for the negative control. The UDS test detected genotoxic effects at lower concentrations than did the other assays (0.01-5 mM). The percentage of cells in repair increased in a concentration-dependent manner to a maximum of 57% at 1mM. At the highest concentration tested (5 mM), the NNG/cell score was 13.6+/-0.5 whereas it was -2.7+/-0.5 for the negative control. These data, based on various endpoints, show a midly genotoxic effect of 2,4-DAT in the HepG2 cells and confirm that this cell line is a suitable model to study the toxic effects of aromatic amines.