Mutagenicity and DNA adduct formation by aristolochic acid in the spleen of Big Blue® rats

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

Aristolochic Acid-Induced Genotoxicity and Toxicogenomic Changes in Rodents

Aristolochic acid (AA) is a group of structurally related nitrophenanthrene carboxylic acids found in many plants that are widely used by many cultures as traditional herbal medicines. AA is a causative agent for Chinese herbs nephropathy, a term replaced later by AA nephropathy. Evidence indicates that AA is nephrotoxic, genotoxic, and carcinogenic in humans; and it also induces tumors in the forestomach, kidney, renal pelvis, urinary bladder, and lung of rats and mice. Therefore, plants containing AA have been classified as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer. In our laboratories, we have conducted a series of genotoxicity and toxicogenomic studies in the rats exposed to AA of 0.1–10 mg/kg for 12 weeks. Our results demonstrated that AA treatments induced DNA adducts and mutations in the kidney, liver, and spleen of rats, as well as significant alteration of gene expression in both its target and nontarget tissues. AA treatments altered mutagenesis- or carcinogenesis-related microRNA expression in rat kidney and resulted in significant changes in protein expression profiling. We also applied benchmark dose (BMD) modeling to the 3-month AA-induced genotoxicity data. The obtained BMDL₁₀ (the lower 95% confidence interval of the BMD₁₀ that is a 10% increase over the background level) for AA-induced mutations in the kidney of rats was about 7 μg/kg body weight per day. This review constitutes an overview of our investigations on AA-induced genotoxicity and toxicogenomic changes including gene expression, microRNA expression, and proteomics; and presents updated information focused on AA-induced genotoxicity in rodents.

Recent developments in detoxication techniques for aristolochic acid-containing traditional Chinese medicines

Aristolochic acids (AAs) have attracted significant attention because they have been proven to be the culprits in the mass incidents of AA nephropathy that occurred in Belgium in 1993. From then on, the door to sales of medicines containing AAs has been closed. As aristolochic acid (AA)-containing traditional Chinese medicine (TCM) has a potent therapeutic effect on some diseases, research into detoxication techniques for AA-containing traditional Chinese medicines (TCMs) should be considered to be absolutely essential. Therefore, in this paper, the use of AA-containing TCMs has been investigated and detoxication techniques, such as, processing (Paozhi, Chinese name), compatibility (Peiwu, Chinese name), pressurized liquid extraction (PLE) and supercritical fluid extraction (SFE), have been reviewed in detail. A large number of relevant studies have been reviewed and it was found that processing with honey or alkaline salts is the most widely used method in practical production. As the AAs are a group of weak acids, relatively speaking, processing with alkaline salts can achieve a high rate of reduction of the AAs. Meanwhile, it is necessary to consider the compatibility of AA-containing TCMs and other herbal medicines. In addition, PLE and SFE can also achieve an excellent reducing rate for AAs in a much shorter processing time. Therefore, the promotion of alkaline salt processing technology should be strengthened in the future. At the same time, some advanced modern extraction technologies also have good potential and should be further studied.

The Lambda Select cII Mutation Detection System

A number of transgenic animal models and mutation detection systems have been developed for mutagenicity testing of carcinogens in mammalian cells. Of these, transgenic mice and the Lambda (λ) Select cII Mutation Detection System have been employed for mutagenicity experiments by many research groups worldwide. Here, we describe a detailed protocol for the Lambda Select cII mutation assay, which can be applied to cultured cells of transgenic mice/rats or the corresponding animals treated with a chemical/physical agent of interest. The protocol consists of the following steps: (1) isolation of genomic DNA from the cells or organs/tissues of transgenic animals treated in vitro or in vivo, respectively, with a test compound; (2) recovery of the lambda shuttle vector carrying a mutational reporter gene (i.e., cII transgene) from the genomic DNA; (3) packaging of the rescued vectors into infectious bacteriophages; (4) infecting a host bacteria and culturing under selective conditions to allow propagation of the induced cII mutations; and (5) scoring the cII-mutants and DNA sequence analysis to determine the cII mutant frequency and mutation spectrum, respectively.

Transgenic rat models for mutagenesis and carcinogenesis

Rats are a standard experimental animal for cancer bioassay and toxicological research for chemicals. Although the genetic analyses were behind mice, rats have been more frequently used for toxicological research than mice. This is partly because they live longer than mice and induce a wider variety of tumors, which are morphologically similar to those in humans. The body mass is larger than mice, which enables to take samples from organs for studies on pharmacokinetics or toxicokinetics. In addition, there are a number of chemicals that exhibit marked species differences in the carcinogenicity. These compounds are carcinogenic in rats but not in mice. Such examples are aflatoxin B1 and tamoxifen, both are carcinogenic to humans. Therefore, negative mutagenic/carcinogenic responses in mice do not guarantee that the chemical is not mutagenic/carcinogenic to rats or perhaps to humans. To facilitate research on in vivo mutagenesis and carcinogenesis, several transgenic rat models have been established. In general, the transgenic rats for mutagenesis are treated with chemicals longer than transgenic mice for more exact examination of the relationship between mutagenesis and carcinogenesis. Transgenic rat models for carcinogenesis are engineered mostly to understand mechanisms underlying chemical carcinogenesis. Here, we review papers dealing with the transgenic rat models for mutagenesis and carcinogenesis, and discuss the future perspective.

Evaluation for a mutagenicity of aristolochic acid by Pig-a and PIGRET assays in rats

The Pig-a assay, which uses the endogenous phosphatidylinositol glycan, class A gene (Pig-a) as a reporter of mutation, has been developed as a method for evaluating in vivo mutagenicity. Pig-a gene mutation can be detected by identifying the presence of CD59, the glycosylphosphatidylinositol anchor protein, on the surface of erythrocytes (RBC Pig-a assay) and reticulocytes (PIGRET assay). The International Workshop on Genotoxicity Testing (IWGT) showed the usefulness of the RBC Pig-a assay through the evaluation of several compounds. Aristolochic acid (AA), one of the evaluated compounds in the IWGT workgroup, is a carcinogenic plant toxin that is a relatively strong gene mutagen both in vitro and in vivo, but a weak inducer of micronuclei in vivo. In the present study, we examined the mutagenicity of AA in the peripheral blood of rats treated orally with a single dose of AA using Pig-a assays. Furthermore, we evaluated the advantages of the PIGRET assay compared with the RBC Pig-a assay. The results showed that a statistically significant increase in mutant frequency of the Pig-a gene was detected at day 28 by the RBC Pig-a assay, and at days 7, 14 and 28 by the PIGRET assay. In addition, the mutant frequency by the PIGRET assay was higher than that by the RBC Pig-a assay. These results indicate that the mutagenicity of AA can be detected using the Pig-a assays, as reported by the IWGT, and the PIGRET assay can detect Pig-a mutants at an early time point compared with the RBC Pig-a assay.

Tissue-specific microRNA responses in rats treated with mutagenic and carcinogenic doses of aristolochic acid

Aristolochic acid (AA) is an active component in herbal drugs derived from the Aristolochia species. Although these drugs have been used since antiquity, AA is both genotoxic and carcinogenic in animals and humans, resulting in kidney tumours in rats and upper urinary tract tumours in humans. In the present study, we conducted microarray analysis of microRNA (miRNA) expression in tissues from transgenic Big Blue rats that were treated for 12 weeks with 0.1-10mg/kg AA, using a protocol that previous studies indicate eventually results in kidney tumours and mutations in kidney and liver. Global analysis of miRNA expression of rats treated with 10 mg/kg AA indicated that 19 miRNAs were significantly dysregulated in the kidney, with most of the miRNAs related to carcinogenesis. Only one miRNA, miR-34a (a tumour suppressor), was differentially expressed in the liver. The expression of the two most responsive kidney miRNAs (miR-21, an oncomiR and miR-34a) was further examined in the kidney, liver and testis of rats exposed to 0, 0.1, 1.0 and 10mg/kg AA. Expression of miR-21 was up-regulated in the kidney only, while miR-34a was dose-dependently up-regulated in both the kidney and liver; the expression of miR-21 and miR-34a was unaltered by the AA treatment in the testis. Analysis of cII mutations in the testis of treated rats also was negative. Our results indicate that AA treatment of rats produced dysregulation of a large number of miRNAs in the tumour target tissue and that the up-regulation of miR-21 correlated with the carcinogenicity of AA while the up-regulation of miR-34a correlated with its mutagenicity.

Toxicogenomics and cancer susceptibility: advances with next-generation sequencing

The aim of this review is to comprehensively summarize the recent achievements in the field of toxicogenomics and cancer research regarding genetic-environmental interactions in carcinogenesis and detection of genetic aberrations in cancer genomes by next-generation sequencing technology. Cancer is primarily a genetic disease in which genetic factors and environmental stimuli interact to cause genetic and epigenetic aberrations in human cells. Mutations in the germline act as either high-penetrance alleles that strongly increase the risk of cancer development, or as low-penetrance alleles that mildly change an individual's susceptibility to cancer. Somatic mutations, resulting from either DNA damage induced by exposure to environmental mutagens or from spontaneous errors in DNA replication or repair are involved in the development or progression of the cancer. Induced or spontaneous changes in the epigenome may also drive carcinogenesis. Advances in next-generation sequencing technology provide us opportunities to accurately, economically, and rapidly identify genetic variants, somatic mutations, gene expression profiles, and epigenetic alterations with single-base resolution. Whole genome sequencing, whole exome sequencing, and RNA sequencing of paired cancer and adjacent normal tissue present a comprehensive picture of the cancer genome. These new findings should benefit public health by providing insights in understanding cancer biology, and in improving cancer diagnosis and therapy.

Evaluation of the cytotoxicity and genotoxicity of aristolochic acid I - a component of Aristolochiaceae plant extracts used in homeopathy

The medicinal plants Aristolochia clematitis L. as well as Asarum europaeum L., representatives of the plant family Aristolochiaceae and mentioned in the German Homeopathic Pharmacopeia, contain aristolochic acid. We found that the mother tinctures of A. clematitis and A. europaeum inhibited DNA synthesis in human hepatoma HepG2 cells in a dose-dependent manner. One of the components of the plant extract, aristolochic acid I (AAI), is linked to the development of nephropathy and urothelial cancer in humans. Therefore, we also evaluated the cytotoxicity and genotoxicity of AAI in HepG2 cells. Cell proliferation was inhibited concentration-dependently by AAI using BrdU-ELISA and colony forming assay. AAI formed DNA adducts (measured by (32)P-postlabeling), induced chromosomal aberrations (micronuclei) and DNA strand breaks. DNA damage induced by AAI led to an arrest of cells in the S-phase which was associated with the increased expression of p53 and p21 proteins. The results are discussed under consideration of former studies.

Analysis of TP53 mutation spectra reveals the fingerprint of the potent environmental carcinogen, aristolochic acid

Genetic alterations in cancer tissues may reflect the mutational fingerprint of environmental carcinogens. Here we review the pieces of evidence that support the role of aristolochic acid (AA) in inducing a mutational fingerprint in the tumor suppressor gene TP53 in urothelial carcinomas of the upper urinary tract (UUT). Exposure to AA, a nitrophenathrene carboxylic acid present in certain herbal remedies and in flour prepared from wheat grain contaminated with seeds of Aristolochia clematitis, has been linked to chronic nephropathy and UUT. TP53 mutations in UUT of individuals exposed to AA reveal a unique pattern of mutations characterized by A to T transversions on the non-transcribed strand, which cluster at hotspots rarely mutated in other cancers. This unusual pattern, originally discovered in UUTs from two different populations, one in Taiwan, and one in the Balkans, has been reproduced experimentally by treating mouse cells that harbor human TP53 sequences with AA. The convergence of molecular epidemiological and experimental data establishes a clear causal association between exposure to the human carcinogen AA and UUT. Despite bans on the sale of herbs containing AA, their use continues, raising global public health concern and an urgent need to identify populations at risk.