Induction of structural chromosome aberrations and sister chromatid exchanges in human lymphocytes in vitro by aristolochic acid

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.

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

Aristolochic acid (AA) is a potent human nephrotoxin and carcinogen. We previously reported that AA treatment resulted in DNA damage and mutation in the kidney and liver of rats. In this study, we have determined the DNA adducts and mutations induced by AA in rat spleen. Big Blue® transgenic rats were gavaged with 0, 0.1, 1.0, and 10.0 mg AA/kg body weight five-times/week for 3 months. Three DNA adducts, [7-(deoxyadenosin-N(6)-yl)-aristolactam I, 7-(deoxyadenosin-N(6)-yl)-aristolactam II and 7-(deoxyguanosin-N(2)-yl)-aristolactam I], were identified by (32)P-postlabeling. Over the dose range studied, there were strong linear dose-responses for AA-DNA adduct formation in the treated rat spleens, ranging from 4.6 to 217.6 adducts/10(8) nucleotides. Spleen cII mutant frequencies also increased in a dose-dependent manner, ranging from 32.7 to 286.2 × 10(-6) in the treated animals. Mutants isolated from the different treatment groups were sequenced; analysis of the resulting spectra indicated that there was a significant difference between the pattern of mutation in the 10 mg/kg AA-treated and the vehicle control rats. A:T → T:A transversion was the major type of mutation in AA-treated rats, whereas G:C → A:T transition was the main type of mutation in the vehicle controls. These results indicate that AA is genotoxic in the spleen of rats exposed under conditions that result in DNA adduct formation and mutation induction in kidney and liver.

Genetic loci that affect aristolochic acid-induced nephrotoxicity in the mouse

Aristolochic acids (AA) are plant-derived nephrotoxins and carcinogens found in traditional medicines and herbal remedies. AA causes aristolochic acid nephropathy (AAN) and is a suspected environmental agent in Balkan endemic nephropathy (BEN) and its associated upper urothelial cancer. Approximately 5-10% of individuals exposed to AA develop renal insufficiency and/or cancer; thus a genetic predisposition to AA sensitivity has been proposed. The mouse is an established animal model of AAN, and inbred murine strains vary in AA sensitivity, confirming the genetic predisposition. We mapped quantitative trait loci (QTL) correlated with proximal tubule dysfunction after exposure to AA in an F2 population of mice, derived from breeding an AA-resistant strain (C57BL/6J) and an AA-sensitive strain (DBA/2J). A single main QTL was identified on chromosome 4 (Aanq1); three other interacting QTLs, (Aanq2-4) also were detected. The Aanq1 region was also detected in untreated mice, raising the possibility that preexisting differences in proximal tubule function may affect the severity of AA-elicited toxicity. This study lays the groundwork for identifying the genetic pathways contributing to AA sensitivity in the mouse and will further our understanding of human susceptibility to AA found widely in traditional medicines.

Aristolochic acid I induced oxidative DNA damage associated with glutathione depletion and ERK1/2 activation in human cells

Aristolochic acid I (AAI) has been widely found in herbal remedies and linked to the development of nephropathy and urothelial carcinoma in humans. This study elucidated the mechanism of oxidative stress and DNA damage mediated by AAI in human cells. Treatment of human promyelocytic leukemia cells (HL-60) and human renal proximal tubular cells (HK-2) with AAI led to a dose-dependent increase of reactive oxygen species (ROS). AAI also elevated the levels of DNA strand breaks and 8-hydroxy guanosine in HL-60 and HK-2 cells. Antioxidants, including Tiron, N-acetyl-l-cysteine (NAC) and glutathione (GSH), effectively suppressed the AAI-induced ROS and AAI-elicited genotoxicity, indicating that AAI induced the DNA damage through oxidative stress. GSH depletion was also found in AAI-treated cultures and proceeded prior to ROS formation. Exposure of HL-60 cells with AAI activated both ERK1/2 and p38 kinase phosphorylation, while only MEK1/2 inhibitor, U0126, significantly decreased AAI-mediated ROS. Preincubation of cells with thiol-containing compounds (NAC and GSH) inhibited the caspase 3 activity triggered by AAI, but non-thiol Tiron did not show a similar effect. This study demonstrated that AAI treatment results in oxidative stress-related DNA damage through GSH depletion and ERK1/2 activation; AAI-induced apoptosis is associated with GSH loss, but is independent of ROS generation.

Metabolic activation of carcinogenic aristolochic acid, a risk factor for Balkan endemic nephropathy

Aristolochic acid (AA), a naturally occurring nephrotoxin and carcinogen, is associated with tumor development in patients suffering from Chinese herbs nephropathy (now termed aristolochic acid nephropathy, AAN) and may also be a cause for the development of a similar type of nephropathy, the Balkan endemic nephropathy (BEN). Major DNA adducts [7-(deoxyadenosin-N6-yl)-aristolactam and 7-(deoxyguanosin-N2-yl)aristolactam] formed from AA after reductive metabolic activation were found in renal tissues of patients with both diseases. Understanding which human enzymes are involved in AA activation and/or detoxication is important in the assessment of an individual's susceptibility to this plant carcinogen. This paper reviews major hepatic and renal enzymes responsible for AA-DNA adduct formation in humans. Phase I biotransformation enzymes play a crucial role in the metabolic activation of AA to species forming DNA adducts, while a role of phase II enzymes in this process is questionable. Most of the activation of AA in human hepatic microsomes is mediated by cytochrome P450 (CYP) 1A2 and, to a lower extent, by CYP1A1; NADPH:CYP reductase plays a minor role. In human renal microsomes NADPH:CYP reductase is more effective in AA activation. Prostaglandin H synthase (cyclooxygenase, COX) is another enzyme activating AA in human renal microsomes. Among the cytosolic reductases, NAD(P)H:quinone oxidoreductase (NQO1) is the most efficient in the activation of AA in human liver and kidney. Studies with purified enzymes confirmed the importance of CYPs, NADPH:CYP reductase, COX and NQO1 in the AA activation. The orientation of AA in the active sites of human CYP1A1, -1A2 and NQO1 was predicted from molecular modeling and explains the strong reductive potential of these enzymes for AA detected experimentally. We hypothesized that inter-individual variations in expressions and activities of enzymes activating AA may be one of the causes responsible for the different susceptibilities to this carcinogen reflected in the development of AA-induced nephropathies and associated urothelial cancer.

Genotoxic effect and nitrative DNA damage in HepG2 cells exposed to aristolochic acid

Aristolochic acid (AA), extensively used as a traditional herbal medicine, was withdrawn from the market in the last century because it was found to be a potent carcinogen in humans and animals. The aim of this study was to evaluate the genotoxic effect of AA and obtain further insight into whether the nitrative DNA damage can be induced by reactive nitrogen species (RNS), including nitric oxide (NO) and its derivative peroxynitrite (ONOO(-)) using human hepatoma HepG2 cells. To identify the genotoxic effect, the comet assay and micronucleus test (MNT) were performed. In the comet assay, 25-200microM of AA caused a significant increase of DNA migration in a dose-dependent manner. A significant increase of the frequency of micronuclei was found in the range between 12.5 and 50microM in the MNT. The results showed that AA caused DNA and chromosome damages. To elucidate the nitrative DNA damage mechanism, the level of nitrite and 8-hydroxydeoxyguanosine (8-OHdG), which can be generated by ONOO(-), were monitored with the 2,3-diaminonaphthalene (DAN) assay and immunoperoxidase staining, respectively. The results showed that AA causes a significant increase in the levels of NO and formation of 8-OHdG at concentrations >/=50microM. This observation supports the assumption that AA could exert genotoxicity probably via NO and its derivatives at higher concentrations in HepG2 cells.

Application of simplified in vitro screening tests to detect genotoxicity of aristolochic acid

Aristolochic acid (AA), the active compound found in Aristolochia extracts, has been used as a traditional medicine. However, products containing AA were withdrawn from the market in the early 1980s because AA was found to be a potent carcinogen. Some genotoxicity studies of AA were conducted after the carcinogenicity of AA was reported. The purpose of this study was to check the ability of simplified, screening tests for genotoxicity to indicate the genotoxic activities of AA. Four commonly used in vitro genotoxicity endpoints were examined. In a bacterial mutation screening test, AA was mutagenic to tester strains TA98 and TA100 with and without rat liver S9. In the L5178Y mouse lymphoma cell gene mutation test, mutagenic activity was observed at > or = 25 microg/ml with or without S9. A concentration-dependent increase in structural chromosome aberrations was observed in CHO cells, with significant increases at 50 microg/ml without S9 and at 25 microg/ml with S9. Significant increases in micronucleated binucleated cells were observed in CHO cells treated with AA at > or = 25 microg/ml with or without S9. These results demonstrated that the genotoxicity of AA would have been easily detected if simple screening versions of in vitro genotoxicity assays had been used during early product development. It is suggested that simplified screening tests such as those used in this study would be a rapid and economical way of obtaining the preliminary genotoxicity profiles of new substances or products as an aid to decision-making for further development.

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

Renal toxicity of aristolochic acid in rats as an example of nephrotoxicity testing in routine toxicology

The nephrotoxic action of aristolochic acid (AA) was investigated in female Wistar rats given single doses of 10, 50 or 100 mg/kg by gastric tube. Renal lesions developed within 3 days, the effect being dose-dependent. Histologically, there was evidence of necrosis of the epithelium of the renal tubules, and functionally, there were rises in plasma creatinine and urea together with increases in urinary glucose, protein, N-acetyl-beta-glucosaminidase, gammaglutamyl transferase and malate dehydrogenase. Taking AA as an example, the aim of the present study was to consider the suitability of this model, based on a combination of histology and laboratory investigations, as a short-term test for the detection of nephrotoxic agents.

Chapter 1 Allelochemical Properties or the Raison D'être of Alkaloids

This chapter provides evidence that alkaloids are not waste products or functionless molecules as formerly assumed, but rather defense compounds employed by plants for survival against herbivores and against microorganisms and competing plants. These molecules were developed during evolution through natural selection in that they fit many important molecular targets, often receptors, of cells, which are seen in molecules that mimic endogenous neurotransmitters. The chapter discusses that microorganisms and herbivores rely on plants as a food source. Since both have survived, there must be mechanisms of adaptations toward the defensive chemistry of plants. Many herbivores have evolved strategies to avoid the extremely toxic plants and prefer the less toxic ones. Many herbivores have potent mechanisms to detoxify xenobiotics, which allow the exploitation of at least the less toxic plants. In insects, many specialists evolved that are adapted to the defense chemicals of their host plant, in that they accumulate these compounds and exploit them for their own defense. Alkaloids function as defense molecules against insect predators in the examples studied, and this is further support for the hypothesis that the same compound also serves for chemical defense in the host plant. It needs more experimental data to understand fully the intricate interconnections between plants, their alkaloids, and herbivores, microorganisms, and other plants.

The clastogenic effect of 5-methoxypsoralen plus UV-A in human lymphocytes in vitro and its modification by the anticlastogen beta-aminoethylisothiouronium

The treatment of human lymphocytes in vitro with 5-methoxypsoralen (5-MOP) plus UV-A induces a dose-dependent increase in the SCE rate and in structural chromosome aberrations. We carried out tests to see whether the clastogenic effect of 5-MOP plus UV-A could be reduced by the anticlastogen beta-aminoethylisothiouronium (AET). The occurrence of a protective effect proved to be dependent upon the conditions of treatment. When AET was present over a long period of time (22 h) in cultures with 5-MOP, the number of breaks was reduced compared with such cultures without AET (reduction factor 0.5-0.6). On the other hand, a short period of action by AET (1.5 h) in the presence of 5-MOP produced no reduction of breaks. Posttreatment with AET (20 h) yielded an obvious protective effect (reduction factor 0.2-0.4). The possible mechanisms of the protective effect of AET are discussed.

Acute toxicity of aristolochic acid in rodents

The acute toxic effects of aristolochic acid (AA) were tested in rats and mice of both sexes. Oral or intravenous administration in high doses was followed by death from acute renal failure within 15 days. Histologically, the predominant features were severe necrosis affecting the renal tubules, atrophy of the lymphatic organs and large areas of superficial ulceration in the forestomach, followed by hyperplasia and hyperkeratosis of the squamous epithelium. The LD50 ranged from 56 to 203 mg/kg orally or 38 to 83 mg/kg intravenously, depending on species and sex.

Aristolochic acid is mutagenic and recombinogenic in Drosophila genotoxicity tests

Aristolochic acid (AA) has been tested for genotoxic activity in three different assays with Drosophila melanogaster (i-iii). AA induced sex-linked recessive lethals (i) and chromosome losses (ii) in male germ cells. In a newly developed fast assay with somatic cells of larvae (iii), AA induced mutant single spots as well as twin spots. The data indicate that in addition to the mutagenic activity, AA also possesses recombinogenic activity leading to somatic recombination in mitotically active cells. The experimental labor involved to detect the genotoxic activity of AA was lowest with the somatic cell assay.