5-Methylcytosine attack by hydroxyl free radicals and during carbon tetrachloride promoted liver microsomal lipid peroxidation: structure of reaction products

Chemical and enzymatic modifications of 5-methylcytosine at the intersection of DNA damage, repair, and epigenetic reprogramming

The DNA of all living organisms is persistently damaged by endogenous reactions including deamination and oxidation. Such damage, if not repaired correctly, can result in mutations that drive tumor development. In addition to chemical damage, recent studies have established that DNA bases can be enzymatically modified, generating many of the same modified bases. Irrespective of the mechanism of formation, modified bases can alter DNA-protein interactions and therefore modulate epigenetic control of gene transcription. The simultaneous presence of both chemically and enzymatically modified bases in DNA suggests a potential intersection, or collision, between DNA repair and epigenetic reprogramming. In this paper, we have prepared defined sequence oligonucleotides containing the complete set of oxidized and deaminated bases that could arise from 5-methylcytosine. We have probed these substrates with human glycosylases implicated in DNA repair and epigenetic reprogramming. New observations reported here include: SMUG1 excises 5-carboxyuracil (5caU) when paired with A or G. Both TDG and MBD4 cleave 5-formyluracil and 5caU when mispaired with G. Further, TDG not only removes 5-formylcytosine and 5-carboxycytosine when paired with G, but also when mispaired with A. Surprisingly, 5caU is one of the best substrates for human TDG, SMUG1 and MBD4, and a much better substrate than T. The data presented here introduces some unexpected findings that pose new questions on the interactions between endogenous DNA damage, repair, and epigenetic reprogramming pathways.

Mechanistic Study on Oxidative DNA Damage and Modifications by Haloquinoid Carcinogenic Intermediates and Disinfection Byproducts

Haloquinones (XQs) are a group of carcinogenic intermediates of the haloaromatic environmental pollutants and newly identified chlorination disinfection byproducts (DBPs) in drinking water. The highly reactive hydroxyl radicals/alkoxyl radicals and quinone enoxy/ketoxy radicals were found to arise in XQs and H₂O₂ or organic hydroperoxides system, independent of transition-metal ions. However, it was not clear whether these haloquinoid carcinogens and hydroperoxides can cause oxidative DNA damage and modifications, and if so, what are the underlying molecular mechanisms. We found that 8-oxodeoxyguanosine (8-oxodG), DNA strand breaks, and three methyl oxidation products could arise when DNA was treated with tetrachloro-1,4-benzoquinone and H₂O₂ via a metal-independent and intercalation-enhanced oxidation mechanism. Similar effects were observed with other XQs, which are generally more efficient than the typical Fenton system. We further extended our studies from isolated DNA to genomic DNA in living cells. We also found that potent oxidation of DNA to the more mutagenic imidazolone dIz could be induced by XQs and organic hydroperoxides such as t-butylhydroperoxide or the physiologically relevant hydroperoxide 13S-hydroperoxy-9Z,11E-octadecadienoic acid via an unprecedented quinone-enoxy radical-mediated mechanism. These findings should provide new perspectives to explain the potential genotoxicity, mutagenesis, and carcinogenicity for the ubiquitous haloquinoid carcinogenic intermediates and DBPs.

Environmental Epigenetics and Genome Flexibility: Focus on 5-Hydroxymethylcytosine

Convincing evidence accumulated over the last decades demonstrates the crucial role of epigenetic modifications for mammalian genome regulation and its flexibility. DNA methylation and demethylation is a key mechanism of genome programming and reprogramming. During ontogenesis, the DNA methylome undergoes both programmed changes and those induced by environmental and endogenous factors. The former enable accurate activation of developmental programs; the latter drive epigenetic responses to factors that directly or indirectly affect epigenetic biochemistry leading to alterations in genome regulation and mediating organism response to environmental transformations. Adverse environmental exposure can induce aberrant DNA methylation changes conducive to genetic dysfunction and, eventually, various pathologies. In recent years, evidence was derived that apart from 5-methylcytosine, the DNA methylation/demethylation cycle includes three other oxidative derivatives of cytosine-5-hydroxymethylcytosine (5hmC), 5-formylcytosine, and 5-carboxylcytosine. 5hmC is a predominantly stable form and serves as both an intermediate product of active DNA demethylation and an essential hallmark of epigenetic gene regulation. This makes 5hmC a potential contributor to epigenetically mediated responses to environmental factors. In this state-of-the-art review, we consolidate the latest findings on environmentally induced adverse effects on 5hmC patterns in mammalian genomes. Types of environmental exposure under consideration include hypnotic drugs and medicines (i.e., phenobarbital, diethylstilbestrol, cocaine, methamphetamine, ethanol, dimethyl sulfoxide), as well as anthropogenic pollutants (i.e., heavy metals, particulate air pollution, bisphenol A, hydroquinone, and pentachlorophenol metabolites). We put a special focus on the discussion of molecular mechanisms underlying environmentally induced alterations in DNA hydroxymethylation patterns and their impact on genetic dysfunction. We conclude that DNA hydroxymethylation is a sensitive biosensor for many harmful environmental factors each of which specifically targets 5hmC in different organs, cell types, and DNA sequences and induces its changes through a specific metabolic pathway. The associated transcriptional changes suggest that environmentally induced 5hmC alterations play a role in epigenetically mediated genome flexibility. We believe that knowledge accumulated in this review together with further studies will provide a solid basis for new approaches to epigenetic therapy and chemoprevention of environmentally induced epigenetic toxicity involving 5hmC patterns.

Effect of sex differences in donor foetal fibroblast on the early development and DNA methylation status of buffalo (Bubalus bubalis) nuclear transfer embryos

Low efficiency of somatic cell nuclear transfer (SCNT) embryos is largely attributable to imperfect reprogramming of the donor nucleus. The differences in epigenetic reprogramming between female and male buffalo cloned embryos remain unclear. We explored the effects of donor cell sex differences on the development of SCNT embryos. We and then compared the expression of DNA methylation (5-methylcytosine-5mC and 5-hydroxymethylcytosine-5hmC) and the expression level of relevant genes, and histone methylation (H3K9me2 and H3K9me3) level in SCNT-♀ and SCNT-♂ preimplantation embryos with in vitro fertilization (IVF) counterparts. In the study, we showed that developmental potential of SCNT-♀ embryos was greater than that of SCNT-♂ embryos (p < 0.05). 5mC was mainly expressed in SCNT-♀ embryos, whereas 5hmC was majorly expressed in SCNT-♂ embryos (p < 0.05). The levels of DNA methylation (5mC and 5hmC), Dnmt3b, TET1 and TET3 in the SCNT-♂ embryos were higher than those of SCNT-♀ embryos (p < 0.05). In addition, there were no significant differences in the expression of H3K9me2 at eight-stage of the IVF, SCNT-♀ and SCNT-♂embryos (p < 0.05). However, H3K9me3 was upregulated in SCNT-♂ embryos at the eight-cell stage (p < 0.05). Thus, KDM4B ectopic expression decreased the level of H3K9me3 and significantly improved the developmental rate of two-cell, eight-cell and blastocysts of SCNT-♂ embryos (p < 0.05). Overall, the lower levels of DNA methylation (5mC and 5hmC) and H3K9me3 may introduce the greater developmental potential in buffalo SCNT-♀ embryos than that of SCNT-♂ embryos.

Elevated 5-hydroxymethylcytosine in the Engrailed-2 (EN-2) promoter is associated with increased gene expression and decreased MeCP2 binding in autism cerebellum

Epigenetic mechanisms regulate programmed gene expression during prenatal neurogenesis and serve as a mediator between genetics and environment in postnatal life. The recent discovery of 5-hydroxymethylcytosine (5-hmC), with highest concentration in the brain, has added a new dimension to epigenetic regulation of neurogenesis and the development of complex behavior disorders. Here, we take a candidate gene approach to define the role 5-hmC in Engrailed-2 (EN-2) gene expression in the autism cerebellum. The EN-2 homeobox transcription factor, previously implicated in autism, is essential for normal cerebellar patterning and development. We previously reported EN-2 overexpression associated with promoter DNA hypermethylation in the autism cerebellum but because traditional DNA methylation methodology cannot distinguish 5-methylcytosine (5-mC) from 5-hmC, we now extend our investigation by quantifying global and gene-specific 5-mC and 5-hmC. Globally, 5-hmC was significantly increased in the autism cerebellum and accompanied by increases in the expression of de novo methyltransferases DNMT3A and DNMT3B, ten-eleven translocase genes TET1 and TET3, and in 8-oxo-deoxyguanosine (8-oxo-dG) content, a marker of oxidative DNA damage. Within the EN-2 promoter, there was a significant positive correlation between 5-hmC content and EN-2 gene expression. Based on reports of reduced MeCP2 affinity for 5-hmC, MeCP2 binding studies in the EN-2 promoter revealed a significant decrease in repressive MeCP2 binding that may contribute to the aberrant overexpression of EN-2. Because normal cerebellar development depends on perinatal EN-2 downregulation, the sustained postnatal overexpression suggests that a critical window of cerebellar development may have been missed in some individuals with autism with downstream developmental consequences. Epigenetic regulation of the programmed on-off switches in gene expression that occur at birth and during early brain development warrants further investigation.

TET enzymatic oxidation of 5-methylcytosine, 5-hydroxymethylcytosine and 5-formylcytosine

5-Methylcytosine and methylated histones have been considered for a long time as stable epigenetic marks of chromatin involved in gene regulation. This concept has been recently revisited with the detection of large amounts of 5-hydroxymethylcytosine, now considered as the sixth DNA base, in mouse embryonic stem cells, Purkinje neurons and brain tissues. The dioxygenases that belong to the ten eleven translocation (TET) oxygenase family have been shown to initiate the formation of this methyl oxidation product of 5-methylcytosine that is also generated although far less efficiently by radical reactions involving hydroxyl radical and one-electron oxidants. It was found as additional striking data that iterative TET-mediated oxidation of 5-hydroxymethylcytosine gives rise to 5-formylcytosine and 5-carboxylcytosine. This survey focuses on chemical and biochemical aspects of the enzymatic oxidation reactions of 5-methylcytosine that are likely to be involved in active demethylation pathways through the implication of enzymatic deamination of 5-methylcytosine oxidation products and/or several base excision repair enzymes. The high biological relevance of the latter modified bases explains why major efforts have been devoted to the design of a broad range of assays aimed at measuring globally or at the single base resolution, 5-hydroxymethylcytosine and the two other oxidation products in the DNA of cells and tissues. Another critical issue that is addressed in this review article deals with the assessment of the possible role of 5-methylcytosine oxidation products, when present in elevated amounts in cellular DNA, in terms of mutagenesis and interference with key cellular enzymes including DNA and RNA polymerases.

Epigenetic changes in the progression of Alzheimer's disease

The formation of 5-hydroxymethylcytosine (5hmC), a key intermediate of DNA demethylation, is driven by the ten eleven translocation (TET) family of proteins that oxidize 5-methylcytosine (5mC) to 5hmC. To determine whether methylation/demethylation status is altered during the progression of Alzheimer's disease (AD), levels of TET1, 5mC and subsequent intermediates, including 5hmC, 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC) were quantified in nuclear DNA from the hippocampus/parahippocampal gyrus (HPG) and the cerebellum of 5 age-matched normal controls, 5 subjects with preclinical AD (PCAD) and 7 late-stage AD (LAD) subjects by immunochemistry. The results showed significantly (p < 0.05) increased levels of TET1, 5mC, and 5hmC in the HPG of PCAD and LAD subjects. In contrast, levels of 5fC and 5caC were significantly (p < 0.05) decreased in the HPG of PCAD and LAD subjects. Overall, the data suggest altered methylation/demethylation patterns in vulnerable brain regions prior to the onset of clinical symptoms in AD suggesting a role in the pathogenesis of the disease.

Hydroquinone increases 5-hydroxymethylcytosine formation through ten eleven translocation 1 (TET1) 5-methylcytosine dioxygenase

DNA methylation regulates gene expression throughout development and in a wide range of pathologies such as cancer and neurological disorders. Pathways controlling the dynamic levels and targets of methylation are known to be disrupted by chemicals and are therefore of great interest in both prevention and clinical contexts. Benzene and its metabolite hydroquinone have been shown to lead to decreased levels of DNA methylation, although the mechanism is not known. This study employs a cell culture model to investigate the mechanism of hydroquinone-mediated changes in DNA methylation. Exposures that do not affect HEK293 cell viability led to genomic and methylated reporter DNA demethylation. Hydroquinone caused reactivation of a methylated reporter plasmid that was prevented by the addition of N-acetylcysteine. Hydroquinone also caused an increase in Ten Eleven Translocation 1 activity and global levels of 5-hydroxymethylcytosine. 5-Hydroxymethylcytosine was found enriched at LINE-1 prior to a decrease in both 5-hydroxymethylcytosine and 5-methylcytosine. Ten Eleven Translocation-1 knockdown decreased 5-hydroxymethylcytosine formation following hydroquinone exposure as well as the induction of glutamate-cysteine ligase catalytic subunit and 14-3-3σ. Finally, Ten Eleven Translocation 1 knockdown decreased the percentage of cells accumulating in G2+M following hydroquinone exposure, indicating that it may have a role in cell cycle changes in response to toxicants. This work demonstrates that hydroquinone exposure leads to active and functional DNA demethylation in HEK293 cells in a mechanism involving reactive oxygen species and Ten Eleven Translocation 1 5-methylcytosine dioxygenase.

Potent methyl oxidation of 5-methyl-2'-deoxycytidine by halogenated quinoid carcinogens and hydrogen peroxide via a metal-independent mechanism

Halogenated quinones are a class of carcinogenic intermediates and are newly identified chlorination disinfection by-products in drinking water. We found recently that the highly reactive and biologically important hydroxyl radical ((•)OH) can be produced by halogenated quinones and H2O2 independent of transition metal ions. However, it is not clear whether these quinoid carcinogens and H2O2 can oxidize the nucleoside 5-methyl-2'-deoxycytidine (5mdC) to its methyl oxidation products and, if so, what the underlying molecular mechanism is. Here we show that three methyl oxidation products, 5-(hydroperoxymethyl)-, 5-(hydroxymethyl)-, and 5-formyl-2'-deoxycytidine, could be produced when 5mdC was treated with tetrachloro-1,4-benzoquinone (TCBQ) and H2O2. The formation of the oxidation products was markedly inhibited by typical (•)OH scavengers and under anaerobic conditions. Analogous effects were observed with other halogenated quinones and the classic Fenton system. Based on these data, we propose that the oxidation of 5mdC by TCBQ/H2O2 might be through the following mechanism: (•)OH produced by TCBQ/H2O2 may first abstract hydrogen from the methyl group of 5mdC, leading to the formation of 5-(2'-deoxycytidylyl)methyl radical, which may combine with O2 to form the peroxyl radical. The unstable peroxyl radical transforms into the corresponding hydroperoxide 5-(hydroperoxymethyl)-2'-deoxycytidine, which reacts with TCBQ and results in the formation of 5-(hydroxymethyl)-2'-deoxycytidine and 5-formyl-2'-deoxycytidine. This is the first report that halogenated quinoid carcinogens and H2O2 can induce potent methyl oxidation of 5mdC via a metal-independent mechanism, which may partly explain their potential carcinogenicity.

Effect of aging on 5-hydroxymethylcytosine in the mouse hippocampus

PURPOSE

Aging is believed to affect epigenetic marking of brain DNA with 5-methylcytosine (5mC) and possibly via the 5mC to 5-hydroxymethylcytosine (5hmC) conversion by TET (ten-eleven translocation) enzymes. We investigated the impact of aging on hippocampal DNA 5-hydroxymethylation including in the sequence of aging-susceptible 5-lipoxygenase (5-LOX) gene.

METHODS

Hippocampal samples were obtained from C57BL6 mice. Cellular 5hmC localization was determined by immunofluorescence. The global 5mC and 5hmC contents were measured with the corresponding ELISA. The 5-LOX 5hmC content was measured using a glucosyltransferase/enzymatic restriction digest assay. TET mRNA was measured using qRT-PCR.

RESULTS

Global hippocampal 5hmC content increased during aging as did the 5hmC content in the 5-LOX gene. This occurred without alterations of TET1-3 mRNAs and without changes in the content of 8-hydroxy-2-deoxy-guanosine, a marker of non-enzymatic DNA oxidation.

CONCLUSIONS

The aging-associated increase of hippocampal 5hmC content (global and 5-LOX) appears to be unrelated to oxidative stress. It may be driven by an altered activity but not by the increased expression of the three TET enzymes. Global 5hmC content was increased during aging in the absence of 5mC decrease, suggesting that 5hmC could act as an epigenetic marker and not only as an intermediary in DNA demethylation. Further research is needed to elucidate the functional implications of the impact of aging on hippocampal cytosine hydroxymethylation.

Redox regulation of the epigenetic landscape in cancer: a role for metabolic reprogramming in remodeling the epigenome

Cancer arises from normal cells that acquire a series of molecular changes; however, the founding events that create the clonogens from which a tumor will arise and progress have been the subject of speculation. Through the efforts of several generations of cancer biologists it has been established that the malignant phenotype is an amalgamation of genetic and metabolic alterations. Numerous theories have suggested that either, or both, of these elements might serve as the impetus for cancer formation. Recently, the epigenetic origins of cancer have been suggested as an additional mechanism giving rise to the malignant phenotype. When the discovery that the enzymes responsible for initiating and perpetuating epigenetic events is linked to metabolism by their cofactors, a new paradigm for the origins of cancer can be created. Here, we summarize the foundation of such a paradigm on the origins of cancer, in which metabolic alterations create an epigenetic progenitor that clonally expands to become cancer. We suggest that metabolic alterations disrupt the production and availability of cofactors such as S-adenosylmethionine, α-ketoglutarate, NAD(+), and acetyl-CoA to modify the epigenotype of cells. We further speculate that redox biology can change epigenetic events through oxidation of enzymes and alterations in metabolic cofactors that affect epigenetic events such as DNA methylation. Combined, these metabolic and redox changes serve as the foundation for altering the epigenotype of normal cells and creating the epigenetic progenitor of cancer.

The mammalian de novo DNA methyltransferases DNMT3A and DNMT3B are also DNA 5-hydroxymethylcytosine dehydroxymethylases

For cytosine (C) demethylation of vertebrate DNA, it is known that the TET proteins could convert 5-methyl C (5-mC) to 5-hydroxymethyl C (5-hmC). However, DNA dehydroxymethylase(s), or enzymes able to directly convert 5-hmC to C, have been elusive. We present in vitro evidence that the mammalian de novo DNA methyltransferases DNMT3A and DNMT3B, but not the maintenance enzyme DNMT1, are also redox-dependent DNA dehydroxymethylases. Significantly, intactness of the C methylation catalytic sites of these de novo enzymes is also required for their 5-hmC dehydroxymethylation activity. That DNMT3A and DNMT3B function bidirectionally both as DNA methyltransferases and as dehydroxymethylases raises intriguing and new questions regarding the structural and functional aspects of these enzymes and their regulatory roles in the dynamic modifications of the vertebrate genomes during development, carcinogenesis, and gene regulation.

Effect of aging on 5-hydroxymethylcytosine in brain mitochondria

Nuclear epigenetics of the mammalian brain is modified during aging. Little is known about epigenetic modifications of mitochondrial DNA (mtDNA). We analyzed brain samples of 4- and 24-month-old mice and found that aging decreased mtDNA 5-hydroxymethylcytosine (5hmC) but not 5-methylcytosine (5mC) levels in the frontal cortex but not the cerebellum. Transcript levels of selected mtDNA-encoded genes increased during aging in the frontal cortex only. Aging affected the expression of enzymes involved in 5-methylcytosine and 5-hydroxymethylcytosine synthesis (mitochondrial DNA methyltransferase 1 [mtDNMT1] and ten-eleven-translocation [TET]1-TET3, respectively). In the frontal cortex, aging decreased mtDNMT1 messenger RNA (mRNA) levels without affecting TET1-TET3 mRNAs. In the cerebellum, TET2 and TET3 mRNA content was increased but mtDNMT1 mRNA was unaffected. Using Western immunoblotting of samples from primary neuronal cultures, we found TET immunoreactivity in the mitochondrial fraction. At the single cell level, TET immunoreactivity was detected in the nucleus and in the perinuclear/intraneurite areas where it frequently colocalized with a mitochondrial marker. Our results demonstrated the presence and susceptibility to aging of mitochondrial epigenetic mechanisms in the mammalian brain.

Introduction--Epiphanies in epigenetics

The combinatorial pattern of DNA and histone modifications and their associated histone variants constitute an epigenetic code that shapes gene expression patterns by increasing or decreasing the transcriptional potential of genomic domains. The epigenetic coding status, at any given chromosomal location, is subject to modulation by noncoding RNAs and remodeling complexes. DNA methylation is associated with histone modifications, particularly the absence of histone H3 lysine 4 methylation (H3K4me0) and the presence of histone H3 lysine 9 methylation (H3K9m). We briefly discuss four protein domains (ADD, CXXC, MBD, and SRA), and the functional implications of their architecture in linking histone methylation to that of DNA in mammalian cells. We also consider the domain structure of the DNA methyltransferase DNMT1, its accessory protein UHRF1, and their associated proteins. Finally, we discuss a mechanism by which methylation of DNA and of histones may be coordinately maintained during mitotic cell division, allowing for the transmission of parental methylation patterns to newly replicated chromatin.

Metabolic defects provide a spark for the epigenetic switch in cancer

Cancer is a pathology that is associated with aberrant gene expression and an altered metabolism. Whereas changes in gene expression have historically been attributed to mutations, it has become apparent that epigenetic processes also play a critical role in controlling gene expression during carcinogenesis. Global changes in epigenetic processes, including DNA methylation and histone modifications, have been observed in cancer. These epigenetic alterations can aberrantly silence or activate gene expression during the formation of cancer; however, the process leading to this epigenetic switch in cancer remains unknown. Carcinogenesis is also associated with metabolic defects that increase mitochondrially derived reactive oxygen species, create an atypical redox state, and change the fundamental means by which cells produce energy. Here, we summarize the influence of these metabolic defects on epigenetic processes. Metabolic defects affect epigenetic enzymes by limiting the availability of cofactors like S-adenosylmethionine. Increased production of reactive oxygen species alters DNA methylation and histone modifications in tumor cells by oxidizing DNMTs and HMTs or through direct oxidation of nucleotide bases. Last, the Warburg effect and increased glutamine consumption in cancer influence histone acetylation and methylation by affecting the activity of sirtuins and histone demethylases.

*H atom and *OH radical reactions with 5-methylcytosine

The reactions between either a hydrogen atom or a hydroxyl radical and 5-methylcytosine (5-MeCyt) are studied by using the hybrid kinetic energy meta-GGA functional MPW1B95. *H atom and *OH radical addition to positions C5 and C6 of 5-MeCyt, or *OH radical induced H-abstraction from the C5 methyl group, are explored. All systems are optimized in bulk solvent. The data presented show that the barriers to reaction are very low: ca. 7 kcal/mol for the *H atom additions and 1 kcal/mol for the reactions involving the *OH radical. Thermodynamically, the two C6 radical adducts and the *H-abstraction product are the most stable ones. The proton hyperfine coupling constants (HFCC), computed at the IEFPCM/MPW1B95/6-311++G(2d,2p) level, agree well with B3LYP results and available experimental and theoretical data on related thymine and cytosine radicals.

Garlic oil and DDB, comprised in a pharmaceutical composition for the treatment of patients with viral hepatitis, prevents acute liver injuries potentiated by glutathione deficiency in rats

A pharmaceutical composition PENNEL comprising garlic oil (GO) and dimethyl-4,4'-dimethoxy-5,6,5',6'-dimethylene dioxybiphenyl-2,2'-dicarboxylate (DDB) as ingredients active for phase II enzyme induction and liver protection, respectively, has been used as a curative preparation for patients with acute or chronic viral hepatitis. In spite of the wide clinical use of PENNEL in Asian and Middle Eastern countries, whether GO+DDB treatment synergistically protects the liver from injuries potentiated by GSH deficiency compared to the individual treatment has not been determined. This study investigated the effects of GO+DDB in comparison with each ingredient alone on chemical-induced liver injury potentiated by a GSH depleting agent. Rats that had been daily pretreated with GO+DDB, GO, DDB, ursodesoxycholic acid or silymarin for 6 days were exposed to buthionine sulfoximine (BSO) and then injected with a single dose of CCl4. The effects of the agents on acute liver toxicities induced by BSO, CCl4 or BSO+CCl4 were assessed by blood biochemistry and histopathology. GO+DDB pretreatment effectively prevented increases in plasma aminotransferases or lactate dehydrogenase activities in rats exposed to BSO+CCl4, compared to GO or DDB treatment alone. Whereas BSO potentiated CCl4-induced liver injuries as evidenced by elevations in central necrosis, hepatocyte degeneration and inflammation, pretreatment with GO+DDB abrogated BSO+CCl4-induced liver injuries more efficaciously than did that with GO or DDB. The hepatoprotective effect of GO+DDB was superior to that of ursodesoxycholic acid or silymarin. Also, blood biochemistry indicated that GO+DDB pretreatment prevented increases in plasma triglyceride contents in rats insulted with CCl4 or BSO+CCl4. The present study demonstrated that GO+DDB, when daily pretreated for six consecutive days, exerted synergistic protection of the liver from chemical-induced injury potentiated by the condition of GSH deficiency, and has additional advantages in lowering the plasma lipids.

Interactions in the tumor-promoting activity of carbon tetrachloride, trichloroacetate, and dichloroacetate in the liver of male B6C3F1 mice

Interactions between carcinogens in mixtures found in the environment have been a concern for several decades. In the present study, male B6C3F1 mice were used to study the responses to mixtures of dichloroacetate (DCA), trichloroacetate (TCA), and carbon tetrachloride (CT). TCA produces liver tumors in mice with the phenotypic characteristics common to peroxisome proliferators. DCA increases the growth of liver tumors with a phenotype that is distinct in several respects from those produced by TCA. These chemicals are effective as carcinogens at doses that do not produce cytotoxicity. Thus, they encourage clonal expansion of initiated cells through subtle, selective mechanisms. CT is well known for its ability to promote the growth of liver tumors through cytotoxicity that produces a generalized growth stimulus in the liver that is reflected in a reparative hyperplasia. Thus, CT is relatively non-specific in its promotion of initiated cells within the liver. The objective of this study was to determine how the differing modes of action of these chemicals might interact when given as mixed exposures. The hypothesis was that the effects of two selective promoters would not be more than additive. On the other hand, CT would be selective only to cells not sensitive to its effects as a cytotoxin. Thus, it was hypothesized that neither DCA nor TCA would add significantly to the effects produced by CT. Mice were initiated by vinyl carbamate (VC), and then promoted by DCA, TCA, CT, or the pair-wised combinations of the three compounds. The effect of each treatment or treatment combination on tumor number per animal and mean tumor volume was assessed in each animal. Dose-related increases in mean tumor volume were observed with 20 and 50mg/kg CT, but each produced equal numbers of tumors at 36 weeks. As the dose of CT was increased to >/=100mg/kg substantial increases in the number of tumors per animal were observed, but the mean tumor size decreased. This finding suggests that initiation occurs as doses of CT increase to >/=100mg/kg, perhaps as a result of the inflammatory response that is known to occur with high doses of CT. When administered alone in the drinking water at 0.1, 0.5 and 2g/l, DCA increased both tumor number and tumor size in a dose-related manner. With TCA treatment at 2g/l in drinking water a maximum tumor number was reached by 24 weeks and was maintained until 36 weeks of treatment. DCA treatment did not produce a plateau in tumor number within the experimental period, but the numbers observed at the end of the experimental period were similar to TCA and doses of 50mg/kg CT. The tumor numbers observed at the end of the experiment are consistent with the assumption that the administered dose of the tumor initiator, vinyl carbamate, was the major determinant of tumor number and that treatments with CT, DCA, and TCA primarily affected tumor size. The results with mixtures of these compounds were consistent with the basic hypotheses that the responses to tumor promoters with differing mechanisms are limited to additivity at low effective doses. More complex, mutually inhibitory activity was more often observed between the three compounds. At 24 weeks, DCA produced a decrease in tumor numbers promoted by TCA, but the numbers were not different from TCA alone at 36 weeks. The reason for this result became apparent at 36 weeks of treatment where a dose-related decrease in the size of tumors promoted by TCA resulted from DCA co-administration. On the other hand, the low dose of TCA (0.1g/l) decreased the number of tumors produced by a high dose of DCA (2g/l), but higher doses of TCA (2g/l) produced the same number as observed with DCA alone. DCA inhibited the growth rate of CT-induced tumors (CT dose = 50mg/kg). TCA substantially increased the numbers of tumors observed at early time points when combined with CT, but this was not observed at 36 weeks. The lack of an effect at 36 weeks was attributable to the fact that more than 90% of the livers consisted of tumors and the earlier effect was masked by coalescence of tumors. Thus, the ability of TCA to significantly increase tumor numbers in CT-treated mice was probably real and contrary to our original hypothesis that CT was non-specific in its effects on initiated cells. It is probable that the interaction between CT and TCA is explained through stimulation of the growth of cells with differing phenotypes. These data suggest that the outcome of interactions between the mechanisms of tumor promotion vary based on the characteristics of the initiated cells. The interactions may result in additive or inhibitory effects, but no significant evidence of synergy was observed.

Synthesis of stable-isotope enriched 5-methylpyrimidines and their use as probes of base reactivity in DNA

A specific and efficient method is presented for the conversion of 2'-deoxyuridine to thymidine via formation and reduction of the intermediate 5-hydroxymethyl derivative. The method has been used to generate both thymidine and 5-methyl-2'-deoxycytidine containing the stable isotopes 2H, 13C and 15N. Oligodeoxyribonucleotides have been constructed with these mass-tagged bases to investigate sequence-selectivity in hydroxyl radical reactions of pyrimidine methyl groups monitored by mass spectrometry. Studying the reactivity of 5-methylcytosine (5mC) is difficult as the reaction products can deaminate to the corresponding thymine derivatives, making the origin of the reaction products ambiguous. The method reported here can distinguish products derived from 5mC and thymine as well as investigate differences in reactivity for either base in different sequence contexts. The efficiency of formation of 5-hydroxymethyluracil from thymine is observed to be similar in magnitude in two different sequence contexts and when present in a mispair with guanine. The oxidation of 5mC proceeds slightly more efficiently than that of thymine and generates both 5-hydroxymethylcytosine and 5-formylcytosine but not the deaminated products. Thymine glycol is generated by both thymine and 5mC, although with reduced efficiency for 5mC. The method presented here should be widely applicable, enabling the examination of the reactivity of selected bases in DNA.

The usefulness of antioxidative enzymes for the estimation of synthetic effects of PGE1 analogue

The aim of our research was estimation of the activity of superoxide dysmutase (SOD) and glutathione peroxidase (GPx) in rabbits' blood after administration of single dose of carbon tetrachloride (CCl4) and synthetic analogue PGE1 - Cytotec. The research was carried out on 56 Chinchilla rabbits. By means of stomach tube rabbits were administered respectively: CCl4, Cytotec, CCl4 with Cytotec, indomethacin, Cytotec with indomethacin and CCl4 with Cytotec and indomethacin simultneously. In comparison with control group almost double increase of SOD activity was stated at rabbits which were administered CCl4 and CCl4 with indomethacin. At rabbits which were administered CCl4 with Cytotec and CCl4 with Cytotec and with indomethacin, the SOD activity increased by only 50% in comparison with control group. At rabbits in whose blood the SOD activity increased, the activity of GPx decreased on average by respectively 40% and 15%. The stated changes of SOD and GPx activity in rabbits' blood which were administered prostaglandin apart from CCl4 indicates prostaglandin's indirect interference into hepatocytes antioxidative system and thus its hepatoprotective activity.