DNA replication-blocking properties of adducts formed by aflatoxin B1-2,3-dichloride and aflatoxin B1-2,3-oxide

Zinc inhibits aflatoxin B1-induced cytotoxicity and genotoxicity in human hepatocytes (HepG2 cells)

Aflatoxin B1 (AFB1) has strong carcinogenicity. Consumption of AFB1-contaminated agricultural products and the occurrence of hepatocellular carcinoma have received widespread attention. The aim of this paper was to investigate whether zinc supplementation could inhibit AFB1-induced cytotoxicity and genotoxicity in HepG2 cells and the mechanism of this inhibition. Our data suggest that zinc sources can relieve a certain degree of AFB1-induced cytotoxicity and genotoxicity by protecting against apoptotic body formation and DNA strand breaks, affecting S phase cell cycle arrest, reducing 8-OHdG formation, inhibiting global DNA hypomethylation and regulating gene expression in antioxidation, zinc-association and apoptosis processes. Consequently, zinc stabilizes the integrity of DNA and improves cell survival. These data provides new insights into the protective role of zinc in alleviating AFB1-induced cytotoxicity and mediating epigenetic changes in hepatocytes, demonstrating that zinc sources have detoxification properties in mycotoxin-induced toxicity.

Susceptibility to aflatoxin B1-induced carcinogenesis correlates with tissue-specific differences in DNA repair activity in mouse and in rat

To investigate the mechanisms responsible for species- and tissue-specific differences in susceptibility to aflatoxin B(1) (AFB(1))-induced carcinogenesis, DNA repair activities of nuclear extracts from whole mouse lung and liver and rat liver were compared, and the ability of in vivo treatment of mice with AFB(1) to alter repair of AFB(1)-DNA damage was determined. Plasmid DNA containing AFB(1)-N(7)-guanine or AFB(1)-formamidopyrimidine adducts were used as substrates for the in vitro determination of DNA repair synthesis activity, detected as incorporation of radiolabeled nucleotides. Liver extracts from CD-1 mice repaired AFB(1)-N(7)-guanine and AFB(1)-formamidopyrimidine adducts 5- and 30-fold more effectively than did mouse lung, and approximately 6- and 4-fold more effectively than did liver extracts from Sprague-Dawley rats. The susceptibility of mouse lung and rat liver to AFB(1)-induced carcinogenesis correlated with lower DNA repair activity of these tissues relative to mouse liver. Lung extracts prepared from mice treated with a single tumorigenic dose of 50 mg/kg AFB(1) i.p. and euthanized 2 hours post-dosing showed minimal incision and repair synthesis activities relative to extracts from vehicle-treated mice. Conversely, repair activity towards AFB(1)-N(7)-guanine damage was approximately 3.5-fold higher in liver of AFB(1)-treated mice relative to control. This is the first study to show that in vivo treatment with AFB(1) can lead to a tissue-specific induction in DNA repair. The results suggest that lower DNA repair activity, sensitivity of mouse lung to inhibition by AFB(1), and selective induction of repair in liver contribute to the susceptibility of mice to AFB(1)-induced lung tumorigenesis relative to hepatocarcinogenesis.

Low hepatic glutathione S-transferase and increased hepatic DNA adduction contribute to increased tumorigenicity of aflatoxin B1 in newborn and partially hepatectomized mice

Low levels of hepatic glutathione S-transferase and increased formation of aflatoxin B1 (AFB1)-DNA adducts correlate with hepatocyte proliferation and increased hepatocarcinogenesis in both newborn mice and partially hepatectomized adult mice, as compared to normal, adult C57BL/6J mice. Newborn mice, which are highly susceptible to the hepatocarcinogenic effects of AFB1, have active proliferation of hepatocytes until 3 weeks of age, when hepatocyte proliferation abruptly ceases. At about this time, the mice become highly resistant to AFB1. In adult mice, AFB1 carcinogenicity is increased after stimulation of liver proliferation by partial hepatectomy. To become carcinogenic, AFB1 is activated in the liver by the P450 enzyme system to electrophilic intermediates, some of which form DNA adducts believed to be responsible for mutations leading to cancer. The most carcinogenic intermediate, AFB(1)-8,9-epoxide, is detoxified by glutathione S-transferase-mediated conjugation to glutathione. Glutathione levels, glutathione S-transferase levels, and AFB1-DNA adduct formation were measured at 4, 10, 30, 120, 245 and 365 days of age in C57BL/6J mice. There was a 5-fold increase in hepatic glutathione S-transferase levels and 13-fold decrease in hepatic AFB1-DNA adduct formation over these ages. Induction of hepatocyte proliferation following partial hepatectomy of 120-day-old mice lowered hepatic glutathione S-transferase levels and increased the extent of hepatic AFB1-DNA formation to levels similar to those measured in 4-day-old mice. These results indicate that increased susceptibility to AFB1 hepatocarcinogenesis in newborn mice, and in adult mice following partial hepatectomy, is due to decreased GST and increased adduct formation in proliferating liver.

A survey of the sequence-specific interaction of damaging agents with DNA: emphasis on antitumor agents

This article reviews the literature concerning the sequence specificity of DNA-damaging agents. DNA-damaging agents are widely used in cancer chemotherapy. It is important to understand fully the determinants of DNA sequence specificity so that more effective DNA-damaging agents can be developed as antitumor drugs. There are five main methods of DNA sequence specificity analysis: cleavage of end-labeled fragments, linear amplification with Taq DNA polymerase, ligation-mediated polymerase chain reaction (PCR), single-strand ligation PCR, and footprinting. The DNA sequence specificity in purified DNA and in intact mammalian cells is reviewed for several classes of DNA-damaging agent. These include agents that form covalent adducts with DNA, free radical generators, topoisomerase inhibitors, intercalators and minor groove binders, enzymes, and electromagnetic radiation. The main sites of adduct formation are at the N-7 of guanine in the major groove of DNA and the N-3 of adenine in the minor groove, whereas free radical generators abstract hydrogen from the deoxyribose sugar and topoisomerase inhibitors cause enzyme-DNA cross-links to form. Several issues involved in the determination of the DNA sequence specificity are discussed. The future directions of the field, with respect to cancer chemotherapy, are also examined.

Interaction of aflatoxin with L-ascorbic acid: a kinetic and mechanistic approach

Aflatoxins containing B(1), B(2), G(1) and G(2) obtained by growing Aspergillus parasiticus on SMKY liquid medium were tested for cytotoxicity (hemolysis) on RBC suspension in the presence and absence of L-ascorbic acid (AA). The results revealed that hemolysis was significantly increased on increasing the concentration of aflatoxin (0.5-3 microg ml(-1)). It was also found that pretreatment with AA (5-100 microg ml(-1)) significantly decreased aflatoxin-induced hemolysis. The solution chemistry of the interaction of aflatoxin with AA in aqueous solutions showed enhanced conversion of AFB(1) and AFG(1) to AFB(2) and AFG(2), respectively. Hemolytic, kinetic and mechanistic aspects of the interactions of aflatoxins and AA are discussed.

Mutations of the p53 tumor suppressor gene and ras oncogenes in aflatoxin hepatocarcinogenesis

Aflatoxin B1 (AFB1) is classified as a group I carcinogen in humans by IARC. However, the exact mechanisms of AFB1 hepatocarcinogenesis have not been fully elucidated. Recent studies have suggested that oncogenes are critical molecular targets for AFB1, and AFB1 causes characteristic genetic changes in the p53 tumor suppressor gene and ras protooncogenes. Up to date, more than 1500 human hepatocellular carcinoma (HCC) samples have been examined for p53 mutations with respect to different AFB1 exposure levels. The most significant finding is that more than 50% of HCC patients from high aflatoxin exposure areas such as southern Africa and Qidong, China harboured a codon 249 G to T transversion in the p53 tumor suppressor gene, which is found to be consistent with the mutagenic specificity of AFB1 observed in vitro. In contrast, this mutational pattern is not found in HCC samples from moderate or low aflatoxin exposure countries or regions. Therefore, this hot-spot mutation is believed to be a molecular fingerprint linking the initial event of AFB1-DNA adduct formation with the ultimate development and progress of human HCC. However, some important points still remain to be explicated. First, in many of these studies, the systematic evaluation of AFB1 exposure is rather limited and the classification of AFB1 exposure level is speculative and confusing, without the definite evidence for the actual aflatoxin exposure level. Second, the role of hepadnaviral infection has to be considered in the induction of this unique mutational spectrum. On the other hand, ras oncogene mutations are frequently found in AFB1-induced HCC samples in experimental animals, while the frequency of ras mutation in human HCC in contrast is much lower than that of p53. Recent studies have provided additional evidence that reactive oxygen species (ROS) and oxidative DNA damage may be involved in AFB1-induced p53 and ras mutations. In future, follow-up cohorts exposed to different levels of AFB1 combined with the determination of putative gene markers are much needed.

Mutagenic and recombinagenic effects of diethylstilbestrol quinone

Estrogens are believed to be major contributors to many cancers of the human female genital tract, but the mechanism of their carcinogenic action is not well-understood. While a tumor-promoting role for estrogens is well-supported, whether they also act as tumor initiators has remained controversial. Here, we have sought to examine the mutagenic potential of diethylstilbestrol, a synthetic estrogen that is a powerful carcinogen in hamsters, and is suspected to be a human carcinogen. Phage M13 single-stranded DNA was treated in vitro with diethylstilbestrol quinone (DES Q: 1.25 mM) and transfected into Escherichia coli cells. DES Q treatment resulted in an apparent enhancement of mutagenesis in the LacZ(alpha) gene segment. DNA sequence analysis of LacZ(alpha) mutants obtained by transfection of DES Q-treated DNA revealed that the major effect of DES Q treatment has been a 6-fold elevation of recombination between the phage-borne LacZ(alpha) sequence and the LacZ delta M15 sequence on the E. coli fertility plasmid F. To confirm whether DES Q treatment is recombinagenic, we used an experimental system that allows the detection of recombination between a defective E. coli chromosomal LacY gene and a normal counterpart borne on a plasmid. Transfection of DES Q (0.06-12 mM) treated plasmid DNA showed significant enhancement (2-100-fold) in recombination, but not in mutagenesis. These results raise the possibility that estrogen quinones may induce recombinagenic DNA damage.

Germline selection: population genetic aspects of the sexual/asexual life cycle

Population geneticists make a distinction between sexual and asexual organisms depending on whether individuals inherit genes from one or two parents. When individual genes are considered, this distinction becomes less satisfactory for multicellular sexual organisms. Individual genes pass through numerous asexual mitotic cell divisions in the germline prior to meiosis and sexual recombination. The processes of mitotic mutation, mitotic crossing over, and mitotic gene conversion create genotypic diversity between diploid cells in the germline. Genes expressed in the germline whose products affect cell viability (such as many "housekeeping" enzymes) may be subjected to natural selection acting on this variability resulting in a non-Mendelian output of gametes. Such genes will be governed by the population genetics of the sexual/asexual life cycle rather than the conventional sexual/Mendelian life cycle. A model is developed to investigate some properties of the sexual/asexual life cycle. When appropriate parameter values were included in the model, it was found that mutation rates per locus per gamete may vary by a factor of up to 100 if selection acts in the germline. Sexual/asexual populations appear able to evolve to a genotype of higher fitness despite intervening genotypes of lower fitness, reducing the problems of underdominance and Wright's adaptive landscape encountered by purely sexual populations. As might be expected this ability is chiefly determined by the number of asexual mitotic cell divisions within the germline. The evolutionary consequences of "housekeeping" loci being governed by the dynamics of the sexual/asexual life cycle are considered.

Genetic requirements for frameshift reversion induced by bulky DNA adducts in M13 DNA

In order to analyze the genetic requirements and mechanisms of frameshift mutagenesis by activated aflatoxin B1 (AFB1), in vitro-modified phage M13 replicative form (RF) DNA was transfected into appropriate Escherichia coli cells and +1 or -1 frameshift revertants in the lacZ(alpha) gene were isolated. This analysis shows that both +1 and -1 frameshift mutagenesis by AFB1 is significantly reduced in a umuC- background. On the other hand, in the absence of RecA, +1 frameshift mutagenesis is partially reduced, but -1 frameshift mutagenesis is unaffected. DNA sequence analysis of +1 frameshifts induced by AFB1 in recA- cells suggests that the mutations occur at the same sites as in recA+ cells, but that there are significant differences in the specificity of the observed base changes. A model consistent with the observed effects in the absence of RecA suggests that an appreciable fraction of AFB1-adducted guanines can correctly template for a cytosine.

Mutagenesis by aflatoxin in M13 DNA: base-substitution mechanisms and the origin of strand bias

The two goals of the experiments described here are: (a) to examine whether there is a strand bias in mutagenic processing of bulky lesions in M13 replicative form (RF) DNA, and (b) to examine the mutational mechanisms of metabolically activated aflatoxin. For these experiments, two types of nicked heteroduplex M13 RF DNA molecules (+WT/-am1 and +am1/-WT) in which either the minus (-) or the plus (+) strand carried a gene 1 amber nonsense codon, were constructed. Heteroduplex DNAs were modified in vitro with aflatoxin B1 activated by hamster liver S9 enzymes, and transfected into SOS(UV)-induced Escherichia coli (Supo/uvrA-/mucAB+). Forward mutations in the lacZ alpha-complementing gene segment were scored and sequenced. Results indicated that aflatoxin-induced mutation frequencies in the +WT/-am1 heteroduplex were significantly greater than those in the +am1/-WT heteroduplex, suggesting more efficient mutagenic processing of lesions in the plus strand. These results permit specific suggestions for improved mutation detection in the extensively used M13 forward mutagenesis system. Sequence analysis of point mutations from the +WT/-am1 experiments showed that most substitutions were targeted to plus-strand guanines. Both G-to-A transitions and G-to-T transversions were induced with equal efficiency. Since activated aflatoxin B1 is known to react almost exclusively with DNA guanines at the N7 position, these results suggest that bulky lesions at guanine N7 position may have the properties of mis-instructional as well as non-instructional lesions.

Structure-activity relationships among mycotoxins

Relationships between structural features and biological effects of mycotoxins are reviewed. Structure-activity relationships are characterized at the molecular, subcellular, cellular, or supracellular level. Major chemical and physicochemical factors responsible for bioactivity of mycotoxins are stressed. A variety of chemical families of mycotoxins are then discussed from the point of view of structure-activity relationships. The structurally related families comprise small lactones, macrocyclic lactones, isocoumarin derivatives, aflatoxins and related compounds trichothecenes, anthraquinones, indole-derived tremorgens and selected amino acid-derived mycotoxins such as sporidesmins and cyclosporines. Biological effects of mycotoxins include acute and chronic toxicity, antimicrobial activity, mutagenicity and genotoxicity, carcinogenicity and biochemical modes of action.

Effects of SOS and MucAB functions on reactivation and mutagenesis of M13 replicative form DNA bearing bulky lesions

We have previously determined the specificity of -1 frameshifts induced by aflatoxin-B1-2,3-dichloride (AFB1C12) in phage M13 double-strand replicative form (RF) DNA. The system consists of: (i) in vitro adduction of RF DNA of BK8, a lacZ + 1 frameshift derivative of phage M13mp8; (ii) transfection into unirradiated or UV-irradiated bacterial host cells; (iii) scoring and sequencing of revertants (i.e., -1 frameshifts). The previous data had shown that induction of SOS functions enhanced mutagenesis. However, this increase in mutagenesis is not accompanied by enhanced survival in a majority of the strains tested. Here, we present evidence to show that the lack of SOS reactivation is a specific property of the RF DNA system rather than a specific property of the lesion. A model mechanism based on the replicative strategy of transfected RF DNA can account for these observations. In addition, we have calculated individual Weigle mutagenesis factors at 8 major mutagen induced sites reported previously. Analysis of these data indicates that, within a restricted subset of possible mutational events (i.e., -1 frameshifts), Weigle mutagenesis is affected by both the DNA sequence environment of the mutation site as well as the repair phenotype of the cell.