Preferential alteration of inducible gene expression in vivo by carcinogens that induce bulky DNA lesions

Methionine alleviates aflatoxinb1-induced broiler chicks embryotoxicity through inhibition of caspase-dependent apoptosis and enhancement of cellular antioxidant status

Practical methods for preventing embryotoxicity in chickens that are caused by aflatoxin-B1 (AFB1) are currently rare. Binding absorbers are commonly used in feeding stuff to reduce laying hens' exposure to off-contaminated diets, thus reducing residue exposure to fertilized eggs. Nonetheless, several adsorbents have been shown to affect the use of nutrients and the absorption of minerals in poultry. Thus, seeking an effective strategy to counter or control embryotoxicity in broiler chicks caused by AFB1 is a problem. A total of 180 embryonated eggs were injected with 36 ng AFB1 with or without 5.90 mg L-methionine (Met) 30 embryonated eggs each, followed by incubation in an incubator until hatching time. The in ovo injection of Met significantly reduced toxicity caused by AFB1 in broiler embryos by enhancing the liver and kidney functions, lipid profiles, and alleviated oxidative stress during the incubation period. Furthermore, the relative gene expressions (SSTR5, TSH-β, Bcl-2, GSH-Px, GST-a, and SOD in the liver) were up-regulated with in ovo injection of AFB1+Met compared to AFB1 alone. Moreover, there was a dowin-regulated trend in Bax, Caspases-3, Caspases-7, Caspases-9, CYP1A1, CYP2H1, and P53 gene expression with in ovo injection of AFB1+Met compared to AFB1 alone. The in ovo injection of Met led to less apoptotic cells in liver tissues. Such results might be necessary for the poultry industry as it is focused on managing the embryotoxicity of AFB1, which affecting poultry production and welfare. Results from this study demonstrated that in ovo Met injection could alleviate AF-induced toxicity in chicken embryos.

The efficiency of synthetic polymers to ameliorate the adverse effects of Aflatoxin on plasma biochemistry, immune responses, and hepatic genes expression in ducklings

To evaluate the effect of molecularly imprinted polymers as a synthetic polymer (TMU95) and commercial toxin binder (CTB) on aflatoxins (AFs) toxic effects on hepatic gene expression, and the biochemical and immunological parameters in ducklings, 240 four-day-old ducklings were randomly allocated into six groups with four replicates of 10 ducklings per each. The experimental groups were as follows: Negative control (basal diet without any additive or AFs), Negative control + TMU95 (5 g/kg feed), Negative control + CTB (Zarinbinder, Vivan Group, Mashhad, Iran. 5 g/kg feed), Positive control (0.2 mg AFs/kg feed), Positive control + TMU95 (5 g/kg feed), and Positive control + CTB (5 g/kg feed). On day 14, livers were collected (8 per treatment) to evaluate change in the expression of genes involved in AFs biotransformation (cytochrome P450 1A1 and 2H1) and antioxidant function (glutathione S-transferase). Several biochemical biomarkers and immune responses were also recorded. Compared with the negative control group AFs treatment significantly decreased plasma total cholesterol, triglyceride and increased the aspartate-aminotransferase (AST), alanine-aminotransferase (ALT) and alkaline phosphatase (ALP) activity (P ≤ 0.01). Cellular immune responses to the phytohemagglutinin-and 2, 4-dinitro 1-chlorobenzene skin test were significantly influenced by dietary aflatoxins (P ≤ 0.01) but a humoral immune response to Newcastle disease virus/vaccine was not affected (P ≥ 0.01). Compared with negative control group, the genes associated with AFs biotransformation were downregulated, whereas the gene associated with the antioxidant function was upregulated in birds fed AFs. The CTB supplement in contaminated feed could alleviate AFs adverse effects on cellular immunity, ALT concentration, and cytochrome P450 2H1 gene expression partially, whereas TMU95 could not ameliorate the adverse effects of AFs on the traits studied, except for ALP. The data suggest that TMU95 may alleviate some of the toxic effects of aflatoxins in duckling and it might prove to be beneficial in the reduction of aflatoxicosis adverse effect in poultry when used in combination with other aflatoxin management practices.

Involvement of transcription factor HNF3gamma in the effect of o-aminoazotoluene on glucocorticoid induction of tyrosine aminotransferase in mice sensitive to its hepatocarcinogenic action

In the rodent liver, hepatocarcinogens inhibit the glucocorticoid induction of several liver-specific genes, including tyrosine aminotransferase (TAT). A distinct positive correlation exists in mice between the extent of inhibition of TAT induction after acute administration of o-aminoazotoluene (OAT) and the frequency of liver tumors after chronic exposure to the carcinogen. To elucidate the mechanism of the carcinogenic action, the effects of OAT on the DNA-binding activity of several transcription factors participating in the glucocorticoid regulation of TAT gene expression were studied. The experimental inbred male mice were sensitive (A/He and SWR/J, tumor induction frequency of 75-100%, TAT induction inhibition of 35-50%) and resistant (CC57BR/Mv and AKR/J, 0-6% and 10-15%, respectively) to OAT. Gel retardation experiments showed that hepatocyte nuclear factor 3 (HNF3)gamma DNA-binding activity was strongly reduced in nuclear extracts from the livers of OAT-treated A/He and SWR/J mice but only slightly reduced in CC57Br/Mv and AKR/J mice. The DNA-binding activities of Ets, AP1 family members, and GME binding proteins were unaffected. HNF3gamma DNA-binding activity was reduced by 1 h after OAT administration and remained low for 1 mo, as did inhibition of TAT induction in the liver. These results suggested that the inhibitory effect of OAT on the glucocorticoid induction of TAT is mediated by reduced HNF3gamma DNA-binding activity.

Molecular basis for effects of carcinogenic heavy metals on inducible gene expression

Certain forms of the heavy metals arsenic and chromium are considered human carcinogens, although they are believed to act through very different mechanisms. Chromium(VI) is believed to act as a classic and mutagenic agent, and DNA/chromatin appears to be the principal target for its effects. In contrast, arsenic(III) is considered nongenotoxic, but is able to target specific cellular proteins, principally through sulfhydryl interactions. We had previously shown that various genotoxic chemical carcinogens, including chromium (VI), preferentially altered expression of several inducible genes but had little or no effect on constitutive gene expression. We were therefore interested in whether these carcinogenic heavy metals might target specific but distinct sites within cells, leading to alterations in gene expression that might contribute to the carcinogenic process. Arsenic(III) and chromium(VI) each significantly altered both basal and hormone-inducible expression of a model inducible gene, phosphoenolpyruvate carboxykinase (PEPCK), at nonovertly toxic doses in the chick embryo in vivo and rat hepatoma H411E cells in culture. We have recently developed two parallel cell culture approaches for examining the molecular basis for these effects. First, we are examining the effects of heavy metals on expression and activation of specific transcription factors known to be involved in regulation of susceptible inducible genes, and have recently observed significant but different effects of arsenic(III) and chromium(VI) on nuclear transcription factor binding. Second, we have developed cell lines with stably integrated PEPCK promoter-luciferase reporter gene constructs to examine effects of heavy metals on promoter function, and have also recently seen profound effects induced by both chromium(VI) and arsenic(III) in this system. These model systems should enable us to be able to identify the critical cis (DNA) and trans (protein) cellular targets of heavy metal exposure leading to alterations in expression of specific susceptible genes. It is anticipated that such information will provide valuable insight into the mechanistic basis for these effects as well as provide sensitive molecular biomarkers for evaluating human exposure.

Downregulated expression of the signaling molecules Nck, c-Crk, Grb2/Ash, PI 3-kinase p110 alpha and WRN during fibroblast aging in vitro

An RT-PCR analysis was performed to examine changes in intracellular signal transducing molecules during in-vitro aging of human fibroblasts. Expression of Nck, c-Crk, Grb2/Ash, phosphoinositide (PI) 3-kinase p110 alpha and Werner's syndrome gene product WRN was noticeably reduced in late passage cells, showing a concurrent downregulation of a set of signaling molecules accompanying aging.

Synthesis and structural characterization of the N2G-mitomycin C-N2G interstrand cross-link in a model synthetic 23 base pair oligonucleotide DNA duplex

Mitomycin C (MMC) is a genotoxic cancer chemotherapeutic agent that reacts principally at the N2 position of guanine to form one of two predominant monoadducts, or a G-G interstrand cross-link at CpG sites, or a G-G intrastrand cross-link at GpG sites. Previous studies of MMC adduction have principally used very short duplex oligonucleotides (5-15 bp) or very long native duplex DNAs. We examined the formation and structural features of the MMC CpG interstrand cross-link on a model 23 bp synthetic oligonucleotide duplex having the (upper strand) sequence 5'-ATAAATACGTATTTATTTATAAA-3'. MMC was reacted with the duplex oligonucleotide in the presence of sodium dithionite at ratios of 6 mM dithionite: 1.5 mM MMC:0.03 mM duplex. The yield of cross-link in the reaction was determined to be approximately 4.8% by denaturing gel electrophoresis, which represented approximately 75% of the total bound MMC. The cross-linked DNA was isolated to greater than 97% purity in a single step by high temperature size exclusion column chromatography. Characterization of the purified product confirmed that the complex contained exclusively the N2G-MMC-N2G cross-link at the single central CpG site. CD spectroscopy demonstrated a negative band at approximately 290-320 nm which has previously been shown to be characteristic of the MMC cross-link. The relative intensity of this band compared to those reported for shorter duplexes suggested that the majority of the duplex is in a normal B-DNA helical configuration. Base-specific chemical footprinting techniques also indicated that there were subtle but distinct structural perturbations principally within the central four to six base pairs containing and adjacent to the cross-link.

Programmed cell death and N-acetoxy-2-acetylaminofluorene-induced apoptosis in the rat embryo

N-acetoxy-2-acetylaminofluorene (N-Ac-AAF) is an alkylating agent that forms DNA adducts at C-8 in guanine and causes single strand breaks. It has previously been shown to be embryotoxic, but the mechanisms by which it causes abnormal development have not been investigated. Previous studies have indicated that other DNA alkylating agents cause cell death during embryonic development although the types of cell death were not characterized. Using a whole embryo culture system, gestation day 10 rat embryos were exposed to several concentrations (5, 50, and 200 micrograms/ml) of N-Ac-AAF. At several time points after exposure was begun (5, 10, and 24 hours), the embryos were removed from culture and examined to identify location, type and quantity of cell death, relative to programmed cell death observed in control embryos. Vital staining with Nile blue sulphate revealed that the location of N-Ac-AAF-induced cell death included the forebrain region, tail, and areas of programmed cell death. Examination of tissue sections from both control and treated embryos indicated that the location of apoptotic cell death revealed by in situ DNA nick end-labelling was generally consistent with the cell death pattern observed by vital staining of whole embryos. Agarose gel analyses indicated that all concentrations of N-Ac-AAF caused DNA fragmentation, and quantification demonstrated a dose response. Examination of treated embryos (50 and 200 micrograms/ml) by transmission electron microscopy revealed that, by 5 hours after exposure, cells with classic, ultrastructural features of apoptosis were present. In conclusion, multiple methods have all indicated that, regardless of exposure level, apoptosis was the predominant form of cell death. Because apoptosis also occurs in developmental cell death, it is possible that apoptosis induced by N-Ac-AAF is due to an alteration in cell fate via premature or ectopic induction of the cell death program.

Characterization of the basal and carcinogen regulatory elements of the rat mdr1b promoter

In this report we characterized the transcriptional regulation of the rat mdr1b gene by xenobiotics. The expression of this gene was increased in primary rat hepatocytes and in the H4-II-E hepatoma cell line by exposure to carcinogens such as aflatoxin B1, N-acetoxy-2-acetylaminofluorene, and methyl methanesulfonate. Nuclear run-on experiments indicated that the higher steady-state levels of mdr1b mRNA were due to an increase in transcription. The 5'-flanking region of the mdr1b gene was isolated, sequenced, and functionally characterized in transient and stable transfection assays. A single transcription start site was identified for this gene; no alternate start sites were used after induction with aflatoxin B1. Deletion analysis of this promoter demonstrated that the sequence between nt -214 and -178 was critical for basal promoter activity. This region did not contain any consensus-binding sites for previously identified transcription factors. A negative regulatory region was also identified between nt -940 and -250. No specific carcinogen-responsive element was identified; the xenobiotic response required a large part of the promoter. These data suggest that the carcinogen induction of mdr1b expression is mediated through sequences that overlap or that are identical to the basal promoter element.

International Commission for Protection against Environmental Mutagens and Carcinogens. An evaluation of the genetic toxicity of paracetamol

During the last years, several reports have indicated genotoxic effects of paracetamol, a widely used non-prescription analgesic and antipyretic drug. Thus, a careful evaluation of a possible genotoxic effect related to paracetamol use is warranted. Studies in vitro and in vivo indicate that the reactive metabolite of paracetamol can bind irreversibly to DNA and cause DNA strand breaks. Paracetamol inhibits both replicative DNA synthesis and DNA repair synthesis in vitro and in experimental animals. Paracetamol does not cause gene mutations, either in bacteria or in mammalian cells. On the other hand, a co-mutagenic effect of paracetamol has been reported. Furthermore, paracetamol increases the frequency of chromosomal damage in mammalian cell lines, isolated human lymphocytes and experimental animals. Two independent studies have shown an increase in chromosomal damage in lymphocytes of human volunteers after intake of therapeutic doses of paracetamol, whereas a third study was negative. Paracetamol-induced chromosomal damage appears to be caused by an inhibition of ribonucleotide reductase. This indicates that a threshold level for the paracetamol-induced chromosomal damage may exist. Genotoxic effects of paracetamol have, however, been demonstrated both in vitro and in vivo at or near therapeutic concentrations. The data indicate that the use of paracetamol may contribute to an increase in the total burden of genotoxic damage in man. Thus, there may be a need to evaluate the therapeutic benefit of paracetamol, taking into consideration not only its potential to induce acute and chronic organ damage, but also genotoxic effects.

Preferential effects of the chemotherapeutic DNA crosslinking agent mitomycin C on inducible gene expression in vivo

The immediate effects of a single dose of the chemotherapeutic DNA crosslinking agent, mitomycin C (MMC), on the expression of several constitutive and drug-inducible genes were examined in a simple in vivo system, the 14 day chick embryo. We observed no effect of MMC on the steady-state mRNA expression of the constitutively expressed beta-actin, transferrin, or albumin genes. In contrast, MMC treatment significantly altered both the basal and drug-inducible mRNA expression of two glutethimide-inducible genes, 5-aminolevulinic acid (ALA) synthase and cytochrome P450 CYP2H1. The basal expression of these genes was transiently but significantly increased over a 24 hr period following a single dose of MMC. Conversely, MMC significantly suppressed the glutethimide-inducible expression of these genes when administered 1 to 24 hr prior to the inducing drug. The effects of MMC on both basal and drug-inducible ALA synthase and CYP2H1 mRNA expression were principally a result of changes in the transcription rates of these genes. In contrast, MMC treatment had little or no effect on glutethimide-induced expression of ALA synthase or CYP2H1 when administered 1 hr after the inducing drug, suggesting that a very early event in the induction process represents the target for these MMC effects. Covalent binding studies demonstrated that the effects of MMC on gene expression were closely correlated temporally with formation of [3H]-porfiromycin-DNA adducts. These results support the hypothesis that genotoxic chemicals specifically target their effects to inducible genes in vivo.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Effects of the genotoxic carcinogen chromium(VI) on basal and hormone-inducible phosphoenolpyruvate carboxykinase gene expression in vivo: correlation with glucocorticoid- and developmentally regulated expression

Previous studies have shown that a number of different genotoxic carcinogens that induce different types of DNA damage preferentially alter the expression of inducible genes in vivo. To investigate further the mechanistic basis for these effects, we examined the effects of the human lung carcinogen chromium(VI) on expression of the hormone-inducible cytosolic phosphoenolpyruvate carboxykinase (PEPCK) gene in chick embryo liver. Chromium(VI) pretreatment had significant effects on both basal and glucocorticoid-inducible PEPCK expression in 14-d-old embryo liver. These effects were principally a result of changes in PEPCK transcription. In contrast, treatment with chromium(VI) 1 h after treatment with glucocorticoid had no effect on PEPCK induction, suggesting that an early event in the induction process is the target for carcinogen effects. In 16-d-old liver, in which PEPCK expression is no longer responsive to glucocorticoid induction, both basal and inducible PEPCK expression were also refractory to chromium(VI) effects, indicating that carcinogen responsiveness is a phenotypic rather than an inherent property of inducible genes and is related to their competence for induction. Chromium(VI) had no effect on cAMP induction of PEPCK expression, demonstrating that carcinogens target their effects to specific regulatory pathways. Comparison of the effects of chromium(VI) with those of cycloheximide suggests that chromium(VI) targets its effects to a labile, constitutively expressed repressor involved in PEPCK gene regulation.

Comparison of effects of direct-acting DNA methylating and ethylating agents on inducible gene expression in vivo

Our laboratory is interested in whether chemical carcinogen-induced DNA damage is non-randomly distributed in the genome, i.e., "targeted," at the level of individual genes. As one means of investigating this, we have examined whether carcinogen treatment differentially alters the expression of specific genes in vivo. In this study, we have compared the effects of four direct-acting simple alkylating agents (methyl methanesulfonate, ethyl methanesulfonate, methylnitrosourea, and ethylnitrosourea) on the steady-state mRNA expression of a model inducible gene, phosphoenolpyruvate carboxykinase (PEPCK), using the chick embryo as a simple in vivo test system. We observed no effect of any of these four carcinogens on the steady-state mRNA expression of the constitutively expressed beta-actin, transferrin, or albumin genes in chick embryo liver following a single dose of carcinogen. In contrast, these same treatments significantly altered both the basal and inducible expression of the glucocorticoid-inducible PEPCK gene. These results support the hypothesis that inducible gene expression is a target for the effects of chemical carcinogens in vivo. In addition, the direction, magnitude, and time course of these effects were agent-specific. Qualitative and quantitative differences in effects between the methylating and ethylating agents and between the methanesulfonates and nitrosoureas were correlated with differences in their specific patterns of DNA adduct formation, suggesting that different DNA lesions have different effects on inducible gene expression.