Effect of oxidative DNA damage in promoter elements on transcription factor binding

Oxidative stress and antioxidant defense

Reactive oxygen species (ROS) are produced by living organisms as a result of normal cellular metabolism and environmental factors, such as air pollutants or cigarette smoke. ROS are highly reactive molecules and can damage cell structures such as carbohydrates, nucleic acids, lipids, and proteins and alter their functions. The shift in the balance between oxidants and antioxidants in favor of oxidants is termed “oxidative stress.” Regulation of reducing and oxidizing (redox) state is critical for cell viability, activation, proliferation, and organ function. Aerobic organisms have integrated antioxidant systems, which include enzymatic and nonenzymatic antioxidants that are usually effective in blocking harmful effects of ROS. However, in pathological conditions, the antioxidant systems can be overwhelmed. Oxidative stress contributes to many pathological conditions and diseases, including cancer, neurological disorders, atherosclerosis, hypertension, ischemia/perfusion, diabetes, acute respiratory distress syndrome, idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, and asthma. In this review, we summarize the cellular oxidant and antioxidant systems and discuss the cellular effects and mechanisms of the oxidative stress.

Biochemical evidence on the potential role of organophosphates in hepatic glucose metabolism toward insulin resistance through inflammatory signaling and free radical pathways

Several studies show that organophosphate pesticides exert several effects on glucose homeostasis. The current study investigates the influence of subchronic exposure to malathion (MT) on hepatic gluconeogenesis in relation to acetyl cholinesterase (AChE) inhibition, oxidative stress and inflammatory response in the rat. MT was administered by gavage at doses of 25, 50 and 100 mg/kg for 32 days. Fasting hyperglycemia was seen in line with an increased activity of hepatic phosphoenolpyruvate carboxykinase, glucose 6-phosphatase and tumor necrosis factor α. In addition to the impaired glucose tolerance and inhibition of AChE in a dose-dependent manner, there were significant increases in hepatic lipid peroxidation, carbonyl groups and 8-deoxyguanosine as the biomarkers of reactive oxygen species–mediated damage to lipid, protein and DNA, respectively. Altered quality of the liver in glucose production especially gluconeogenesis could be a compensatory mechanism against MT toxicity or even result in tissue damage. MT-induced insulin resistance in the liver occurs through oxidative and inflammatory signaling pathways.

Impaired integrity of DNA after recovery from inflammation causes persistent dysfunction of colonic smooth muscle

BACKGROUND & AIMS

Patients with inflammatory bowel disease who are in remission and those who developed irritable bowel syndrome after enteric infection continue to have symptoms of diarrhea or constipation in the absence of overt inflammation, indicating motility dysfunction. We investigated whether oxidative stress during inflammation impairs integrity of the promoter of Cacna1c, which encodes the pore-forming α1C subunit of Ca(v)1.2b calcium channels.

METHODS

We used long-extension polymerase chain reaction to evaluate DNA integrity in tissues from distal colons of rats; trinitrobenzene sulfonic acid was used to induce inflammation.

RESULTS

The H2O2 increased in the muscularis externa 1-7 days after inflammation was induced with trinitrobenzene sulfonic acid. The oxidative stress significantly impaired DNA integrity in 2 specific segments of the Cacna1c promoter: -506 to -260 and -2193 to -1542. The impairment peaked at day 3 and recovered partially by day 7 after induction of inflammation; expression of the products of Cacna1c followed a similar time course. Oxidative stress suppressed the expression of nuclear factor-erythroid-2-related factor 2 (Nrf2), an important regulator of anti-oxidant proteins. Intraperitoneal administration of sulforaphane significantly reversed the suppression of Nrf2, oxidative damage in the promoter of Cacna1c, and suppression of Cacna1c on day 7 of inflammation. The inflammation subsided completely by 56 days after inflammation was induced; however, impairment of DNA integrity, expression of Nrf2 and Cacna1c, and smooth muscle reactivity to acetylcholine remained suppressed at this time point.

CONCLUSIONS

Oxidative stress during inflammation impairs the integrity of the promoter of Cacna1c; impairment persists partially after inflammation has subsided. Reduced transcription of Cacna1c contributes to smooth muscle dysfunction in the absence of inflammation.

Is neurodegenerative disease a long-latency response to early-life genotoxin exposure?

Western Pacific amyotrophic lateral sclerosis and parkinsonism-dementia complex, a disappearing neurodegenerative disease linked to use of the neurotoxic cycad plant for food and/or medicine, is intensively studied because the neuropathology (tauopathy) is similar to that of Alzheimer's disease. Cycads contain neurotoxic and genotoxic principles, notably cycasin and methylazoxymethanol, the latter sharing chemical relations with nitrosamines, which are derived from nitrates and nitrites in preserved meats and fertilizers, and also used in the rubber and leather industries. This review includes new data that influence understanding of the neurobiological actions of cycad and related genotoxins and the putative mechanisms by which they might trigger neurodegenerative disease.

Interactions between DNA damage, repair, and transcription

This review addresses a variety of mechanisms by which DNA repair interacts with transcription and vice versa. Blocking of transcriptional elongation is the best studied of these mechanisms. Transcription recovery after damage therefore has often been used as a surrogate marker of DNA repair in cells. However, it has become evident that relationships between DNA damage, repair, and transcription are more complex due to various indirect effects of DNA damage on gene transcription. These include inhibition of transcription by DNA repair intermediates as well as regulation of transcription and of the epigenetic status of the genes by DNA repair-related mechanisms. In addition, since transcription is emerging as an important endogenous source of DNA damage in cells, we briefly summarise recent advances in understanding the nature of co-transcriptionally induced DNA damage and the DNA repair pathways involved.

The cycad genotoxin MAM modulates brain cellular pathways involved in neurodegenerative disease and cancer in a DNA damage-linked manner

Methylazoxymethanol (MAM), the genotoxic metabolite of the cycad azoxyglucoside cycasin, induces genetic alterations in bacteria, yeast, plants, insects and mammalian cells, but adult nerve cells are thought to be unaffected. We show that the brains of adult C57BL6 wild-type mice treated with a single systemic dose of MAM acetate display DNA damage (O⁶-methyldeoxyguanosine lesions, O⁶-mG) that remains constant up to 7 days post-treatment. By contrast, MAM-treated mice lacking a functional gene encoding the DNA repair enzyme O⁶-mG DNA methyltransferase (MGMT) showed elevated O⁶-mG DNA damage starting at 48 hours post-treatment. The DNA damage was linked to changes in the expression of genes in cell-signaling pathways associated with cancer, human neurodegenerative disease, and neurodevelopmental disorders. These data are consistent with the established developmental neurotoxic and carcinogenic properties of MAM in rodents. They also support the hypothesis that early-life exposure to MAM-glucoside (cycasin) has an etiological association with a declining, prototypical neurodegenerative disease seen in Guam, Japan, and New Guinea populations that formerly used the neurotoxic cycad plant for food or medicine, or both. These findings suggest environmental genotoxins, specifically MAM, target common pathways involved in neurodegeneration and cancer, the outcome depending on whether the cell can divide (cancer) or not (neurodegeneration). Exposure to MAM-related environmental genotoxins may have relevance to the etiology of related tauopathies, notably, Alzheimer's disease.

Divergent effects of oxidatively induced modification to the C8 of 2'-deoxyadenosine on transcription factor binding: 8,5'(S)-cyclo-2'-deoxyadenosine inhibits the binding of multiple sequence specific transcription factors, while 8-oxo-2'-deoxyadenosine increases binding of CREB and NF-kappa B to DNA

DNA is exposed to endogenous and environmental factors that can form stable lesions. If not repaired, these lesions can lead to transcription/replication blocking or mutagenic bypass. Our previous work has focused on 8,5'-cyclopurine 2'-deoxyribonucleosides, a unique class of oxidatively induced DNA lesions that are specifically repaired by the NER pathway (see Brooks PJ [2008]: DNA Repair 7:1168-1179). Here we used EMSA to monitor the ability of sequence-specific transcription factors, HSF1, CREB, and NF-kappaB and "architectural" transcription factor, HMGA, to bind to their target sequences when 8, 5'(S)-cyclo-2'-deoxyadenosine (cyclo-dAdo) is present within their recognition sequences. For comparison, we also tested the effect of 8-oxo-7,8-dihydro-2'-deoxyadenosine (8-oxo-dAdo) in the same recognition sequences. The presence of a cyclo-dAdo lesion in the target sequence essentially eliminated the binding activity of HSF1, CREB, and NF-kappa B whereas HMGA retained some of its binding activity. In contrast, 8-oxo-dAdo had no obvious effect on the binding activity of HSF1 and HMGA in comparison to lesion-free DNA. Notably, though, CREB and NFκB binding increased when an 8-oxo-dAdo lesion was present in their target sequence. Competition EMSA showed about 2-3-fold increased affinity of both proteins for the 8-oxo-dAdo containing target sequence compared to lesion-free DNA. Molecular modeling of the lesions in the NF-kappaB sequence indicated that 8-oxo-dAdo may form an additional hydrogen bond with the protein, thereby strengthening the binding of NF-kappa B to its DNA target. The cyclo-dAdo lesion, in contrast, distorted the DNA structure, providing an explanation for the inhibition of NF-kappaB binding.

Neurotoxic mechanisms of DNA damage: focus on transcriptional inhibition

Although DNA damage-induced neurotoxicity is implicated in various pathologies of the nervous system, its underlying mechanisms are not completely understood. Transcription is a DNA transaction that is highly active in the nervous system. In addition to its direct role in expression of the genetic information, transcription contributes to DNA damage detection and repair as well as chromatin organization including biogenesis of the nucleolus. Transcription is inhibited by DNA single-strand breaks and DNA adducts. Hence, transcription inhibition may be an important contributor to the neurotoxic consequences of such types of DNA damage. This review discusses the existing evidence in support of the latter hypothesis. The presented literature suggests that neuronal DNA damage interferes with the RNA-Polymerase-2-dependent transcription of genes encoding proteins with critical functions in neurotransmission and intracellular signaling. The latter category includes extracellular signal-regulated kinase-1/2 mitogen-activated protein kinase phosphatases whose lowered expression results in chronic activation of extracellular signal-regulated kinase-1/2 and its reduced responsiveness to physiological stimuli. Conversely, DNA damage-induced inhibition of RNA-Polymerase-1 and the subsequent disruption of the nucleolus induce p53-mediated apoptosis of developing neurons. Finally, decreasing nucleolar transcription may link DNA damage to chronic neurodegeneration in adults.

Base excision repair in a glucocorticoid response element: effect of glucocorticoid receptor binding

DNA repair takes place in the context of chromatin. Previous studies showed that histones impair base excision repair (BER) of modified bases at both the excision and synthesis steps. We examined BER of uracil in a glucocorticoid response element (GRE) complexed with the glucocorticoid receptor DNA binding domain (GR-DBD). Five substrates were designed, each containing a unique C→U substitution within the mouse mammary tumor virus promoter, one located within each GRE half-site and the others located outside the GRE. To examine distinct steps of BER, DNA cleavage by uracil-DNA glycosylase and Ape1 endonuclease was used to assess initiation, dCTP incorporation by DNA polymerase (pol) β was used to measure repair synthesis, and DNA ligase I was used to seal the nick. For uracil sites within the GRE, there was a reduced rate of uracil-DNA glycosylase/Ape1 activity following GR-DBD binding. Cleavage in the right half-site, with higher GR-DBD binding affinity, was reduced ∼5-fold, whereas cleavage in the left half-site was reduced ∼3.8-fold. Conversely, uracil-directed cleavage outside the GRE was unaffected by GR-DBD binding. Surprisingly, there was no reduction in the rate of pol β synthesis or DNA ligase activity on any of the fragments bound to GR-DBD. Indeed, we observed a small increase (∼1.5–2.2-fold) in the rate of pol β synthesis at uracil residues in both the GRE and one site six nucleotides downstream. These results highlight the potential for both positive and negative impacts of DNA-transcription factor binding on the rate of BER.

Gene silencing induced by oxidative DNA base damage: association with local decrease of histone H4 acetylation in the promoter region

Oxidized DNA bases, particularly 7,8-dihydro-8-oxoguanine (8-oxoG), are endogenously generated in cells, being a cause of carcinogenic mutations and possibly interfering with gene expression. We found that expression of an oxidatively damaged plasmid DNA is impaired after delivery into human host cells not only due to decreased retention in the transfected cells, but also due to selective silencing of the damaged reporter gene. To test whether the gene silencing was associated with a specific change of the chromatin structure, we determined the levels of histone modifications related to transcriptional activation (acetylated histones H3 and H4) or repression (methylated K9 and K27 of the histone H3, and histone H1) in the promoter region and in the downstream transcribed DNA. Acetylation of histone H4 was found to be specifically decreased by 25% in the proximal promoter region of the damaged gene, while minor quantitative changes in other tested chromatin components could not be proven as significant. Treatment with an inhibitor of histone deacetylases, trichostatin A, partially restored expression of the damaged DNA, suggesting a causal connection between the changes of histone acetylation and persistent gene repression. Based on these findings, we propose that silencing of the oxidatively damaged DNA may occur in a chromatin-mediated mechanism.

DNA-mediated charge transport in redox sensing and signaling

The transport of charge through the DNA base-pair stack offers a route to carry out redox chemistry at a distance. Here we describe characteristics of this chemistry that have been elucidated and how this chemistry may be utilized within the cell. The shallow distance dependence associated with these redox reactions permits DNA-mediated signaling over long molecular distances in the genome and facilitates the activation of redox-sensitive transcription factors globally in response to oxidative stress. The long-range funneling of oxidative damage to sites of low oxidation potential in the genome also may provide a means of protection within the cell. Furthermore, the sensitivity of DNA charge transport to perturbations in base-pair stacking, as may arise with base lesions and mismatches, may be used as a route to scan the genome for damage as a first step in DNA repair. Thus, the ability of double-helical DNA in mediating redox chemistry at a distance provides a natural mechanism for redox sensing and signaling in the genome.

Impact of 7,8-dihydro-8-oxoguanine on methylation of the CpG site by Dnmt3a

7,8-Dihydro-8-oxoguanine (8-oxoG) is a ubiquitous oxidative DNA lesion resulting from injury to DNA via reactive oxygen species. 8-oxoG lesions may play a role in the formation of aberrant DNA methylation patterns during carcinogenesis. In this study, we assessed the effects of 8-oxoG on methylation and complex formation of nine 30-mer oligodeoxynucleotide duplexes by the catalytic domain of murine Dnmt3a DNA methyltransferase (Dnmt3a-CD). The effects of 8-oxoG on the methylation rate of hemimethylated duplexes varied from a 25-fold decrease to a 1.8-fold increase, depending on the position of the lesion relative to the Dnmt3a-CD recognition site (CpG) and target cytosine (C). The most significant effect was observed when 8-oxoG replaced guanine within the recognition site immediately downstream of the target cytosine. Fluorescence polarization experiments with fluorescein-labeled duplexes revealed that two molecules of Dnmt3a-CD bind per duplex, generating sigmoid binding curves. Duplexes exhibiting the highest apparent binding cooperativity formed the least stable 1:2 complexes with Dnmt3a-CD and were methylated at the lowest rate. Kinetic analyses disclosed the formation of very stable nonproductive enzyme-substrate complexes with hemimethylated duplexes that act as suicide substrates of Dnmt3a-CD. The presence of 8-oxoG within the CpG site downstream of the target cytosine markedly diminished productive versus nonproductive binding. We propose that 8-oxoG located adjacent to the target cytosine interferes with methylation by weakening the affinity of DNA for Dnmt3a-CD, thereby favoring a nonproductive binding mode.

Oxidative damage to the promoter region of SQSTM1/p62 is common to neurodegenerative disease

Recently we reported that declined SQSTM1/p62 expression in Alzheimer disease brain was age-correlated with oxidative damage to the p62 promoter. The objective of this study was to examine whether oxidative damage to the p62 promoter is common to DNA recovered from brain of individuals with neurodegenerative disease. Increased 8-OHdG staining was observed in brain sections from Alzheimer's disease (AD), Parkinson disease (PD), Huntington disease (HD), Frontotemporal dementia (FTD), and Pick's disease compared to control subjects. In parallel, the p62 promoter exhibited elevated oxidative damage in samples from various diseases compared to normal brain, and damage was negatively correlated with p62 expression in FTD samples. Oxidative damage to the p62 promoter induced by H2O2 treatment decreased its transcriptional activity. In keeping with this observation, the transcriptional activity of a Sp-1 element deletion mutant displayed reduced stimulus-induced activity. These findings reveal that oxidative damage to the p62 promoter decreased its transcriptional activity and might therefore account for decreased expression of p62. Altogether these results suggest that pharmacological means to increase p62 expression may be beneficial in delaying the onset of neurodegeneration.

Epigenetics, oxidative stress, and Alzheimer disease

Alzheimer disease (AD) is a progressive neurodegenerative disorder whose clinical manifestations appear in old age. The sporadic nature of 90% of AD cases, the differential susceptibility to and course of the illness, as well as the late age onset of the disease suggest that epigenetic and environmental components play a role in the etiology of late-onset AD. Animal exposure studies demonstrated that AD may begin early in life and may involve an interplay between the environment, epigenetics, and oxidative stress. Early life exposure of rodents and primates to the xenobiotic metal lead (Pb) enhanced the expression of genes associated with AD, repressed the expression of others, and increased the burden of oxidative DNA damage in the aged brain. Epigenetic mechanisms that control gene expression and promote the accumulation of oxidative DNA damage are mediated through alterations in the methylation or oxidation of CpG dinucleotides. We found that environmental influences occurring during brain development inhibit DNA-methyltransferases, thus hypomethylating promoters of genes associated with AD such as the beta-amyloid precursor protein (APP). This early life imprint was sustained and triggered later in life to increase the levels of APP and amyloid-beta (Abeta). Increased Abeta levels promoted the production of reactive oxygen species, which damage DNA and accelerate neurodegenerative events. Whereas AD-associated genes were overexpressed late in life, others were repressed, suggesting that these early life perturbations result in hypomethylation as well as hypermethylation of genes. The hypermethylated genes are rendered susceptible to Abeta-enhanced oxidative DNA damage because methylcytosines restrict repair of adjacent hydroxyguanosines. Although the conditions leading to early life hypo- or hypermethylation of specific genes are not known, these changes can have an impact on gene expression and imprint susceptibility to oxidative DNA damage in the aged brain.

Age-associated oxidative damage to the p62 promoter: implications for Alzheimer disease

The absence of the p62 gene in mouse brain leads to biochemical and cognitive deficits that resemble Alzheimer disease (AD). In this context, the objective of this study was to examine the relationship between age-induced oxidative damage to the p62 promoter and AD. Increased 8-OHdG staining, a marker of oxidative stress, was observed in brain sections from mice deficient in the p62 gene compared to control. Treatment of MEF cells deficient in p62 with H(2)O(2) resulted in decreased cell survival and an absence of Nrf2 nuclear translocation. The mouse p62 promoter exhibited elevated oxidative damage with increasing age, and the degree of p62 promoter damage was also age-correlated in human brain samples. In human subjects, the expression of p62 was decreased in AD brain relative to age-matched controls, and likewise decreased p62 expression correlated with oxidative damage to the promoter. Treatment of HEK cells with H(2)O(2) resulted in decreased p62 expression concomitant with increased promoter damage. Consistent with these findings, a transgenic AD mouse model also exhibited increased p62 promoter damage and reduced p62 levels in brain. Altogether, our results reveal that oxidative damage to the p62 promoter correlates with decreased expression of p62 and may contribute to age-associated neurodegenerative disease such as AD and others.

Reactive nitrogen and oxygen species in interleukin-1-mediated DNA damage associated with osteoarthritis

OBJECTIVE

Osteoarthritis (OA) is associated with increased levels of reactive nitrogen and oxygen species and pro-inflammatory cytokines, such as interleukin-1 (IL-1). Nitric oxide (NO) can mediate a number of the catabolic effects of IL-1 in articular cartilage. The aims of this study were to determine if OA cartilage shows evidence of DNA damage, and if IL-1 could induce DNA damage in non-OA cartilage by increasing NO or superoxide.

METHODS

Articular chondrocytes were isolated from porcine femoral condyles and embedded in 1.2% alginate. The effects of 24h incubation with IL-1, the nitric oxide synthase 2 (NOS2)-selective inhibitor, the free radical scavenger superoxide dismutase (SOD), the NO donor NOC18, or the combined NO and peroxynitrite donor SIN-1 on DNA damage were tested, using the "comet" assay. NO production was measured using the Griess assay. The type of oxidative damage present was assessed using a modified comet assay.

RESULTS

OA cartilage had significantly more DNA damage than non-OA cartilage (P<0.001). IL-1 caused an increase in DNA damage (P<0.01), which was associated with increased NO production (P<0.01). Both oxidative DNA strand breaks and base modifications of purines and pyrimidines were observed. IL-1-induced DNA damage was inhibited by an NOS2 inhibitor or by SOD (P<0.01). Furthermore, NOC18 or SIN-1 caused DNA damage (P<0.001).

CONCLUSION

Our work shows chondrocytes in osteoarthritic cartilage exhibit DNA damage, and that IL-1 induces DNA damage and reactive oxygen and nitrogen species in non-OA chondrocytes in alginate.

Cytotoxicity and mutagenicity of endogenous DNA base lesions as potential cause of human aging

Endogenous factors constitute a substantial source of damage to the genomic DNA. The type of damage includes a number of different base lesions and single- and double-strand breaks. Unrepaired DNA damage can give rise to mutations and may cause cell death. A number of studies have demonstrated an association between aging and the accumulation of DNA damage. This may be attributed to reduced DNA repair with age, although this is apparently not a general feature for all types of damage and repair mechanisms. Therefore, detailed studies that improve our knowledge of DNA repair systems as well as mutagenic and toxic effects of DNA lesions will help us to gain a better insight into the mechanisms of aging. The aim of this review is to provide a brief description of cytotoxic and mutagenic endogenous DNA lesions that are mainly repaired by base excision repair and single-strand break repair pathways and to discuss the potential role of DNA lesions and DNA repair dysfunction in the onset of human aging.

Oxidative DNA damage in mild cognitive impairment and late-stage Alzheimer's disease

Increasing evidence supports a role for oxidative DNA damage in aging and several neurodegenerative diseases including Alzheimer's disease (AD). Attack of DNA by reactive oxygen species (ROS), particularly hydroxyl radicals, can lead to strand breaks, DNA–DNA and DNA–protein cross-linking, and formation of at least 20 modified bases adducts. In addition, α,β-unsaturated aldehydic by-products of lipid peroxidation including 4-hydroxynonenal and acrolein can interact with DNA bases leading to the formation of bulky exocyclic adducts. Modification of DNA bases by direct interaction with ROS or aldehydes can lead to mutations and altered protein synthesis. Several studies of DNA base adducts in late-stage AD (LAD) brain show elevations of 8-hydroxyguanine (8-OHG), 8-hydroxyadenine (8-OHA), 5-hydroxycytosine (5-OHC), and 5-hydroxyuracil, a chemical degradation product of cytosine, in both nuclear and mitochondrial DNA (mtDNA) isolated from vulnerable regions of LAD brain compared to age-matched normal control subjects. Previous studies also show elevations of acrolein/guanine adducts in the hippocampus of LAD subjects compared to age-matched controls. In addition, studies of base excision repair show a decline in repair of 8-OHG in vulnerable regions of LAD brain. Our recent studies show elevated 8-OHG, 8-OHA, and 5,6-diamino-5-formamidopyrimidine in both nuclear and mtDNA isolated from vulnerable brain regions in amnestic mild cognitive impairment, the earliest clinical manifestation of AD, suggesting that oxidative DNA damage is an early event in AD and is not merely a secondary phenomenon.

Detection of UV-induced activation of NF-kappaB in a recombinant human cell line by means of Enhanced Green Fluorescent Protein (EGFP)

The cellular protection reaction known as ultraviolet (UV) response leads to increased transcription of several genes. Parts of this transcriptional response are transmitted via activation of the Nuclear factor kappaB (NF-kappaB). The contribution of different UV radiation qualities to this process is not yet known. In a previous work, a stably transfected human cell line was developed which indicates activation of the NF-kappaB pathway by fluorescence of the reporters Enhanced Green Fluorescent Protein (EGFP) and its destabilized variant (d2EGFP) thereby allowing a fast and reliable monitoring of UV effects on the NF-kappaB pathway. Cells were exposed to a mercury low-pressure lamp or to simulated sunlight of different wavelength ranges and subjected to flow cytometric analysis after different post-irradiation periods. Growth capacity of cells after UV irradiation was quantified using a luminance measurement of crystal violet stained cell layers. In contrast to UVC and UVB, UVA radiation induced d2EGFP expression and NF-kappaB activation in a non-cytotoxic dose range. These results show that NF-kappaB plays a role in the UVA-induced gene activation in a non-cytotoxic dose range in a human epithelial cell line.

Regular exercise reduces 8-oxodG in the nuclear and mitochondrial DNA and modulates the DNA repair activity in the liver of old rats

Exercise is often said to increase the generation of reactive oxygen species that are potentially harmful. On the other hand, regular exercise has various health benefits even late in life. The specific aim of this study was to explore effects of regular exercise on oxidative status of DNA in aged animals. We report that 2 months of regular treadmill running of aged rats (21 month old) significantly reduced 8-oxodG content to the level of young adult animals (11 month old) in both nuclear and mitochondrial DNA of the liver. The mitochondrial DNA showed 10-fold higher content of the oxidative lesion than the nuclear DNA. The levels in old animals were 2- and 1.5-fold higher than that in young adults for the nucleus and mitochondria, respectively. The activity of the repair enzyme OGG1 was upregulated significantly in the nucleus but not in mitochondria by the exercise. To our knowledge, this is the first report demonstrating that regular exercise can reduce significantly oxidative damage to both the nuclear and mitochondrial DNA. We suggest that the apparent beneficial outcomes in reducing the DNA damage by regular exercise can be interpreted in terms of hormetic effect by moderate oxidative stress and potential adaptation to stronger stresses.

Mechanisms of genotoxin-induced transcription and hypermutation in p53

It is widely assumed that genotoxin-induced damage (e.g., G-to-T transversions) to the tumor suppressor gene, p53, is a direct cause of cancer. However, genotoxins also induce the stress response, which upregulates p53 transcription and the formation of secondary structures from ssDNA. Since unpaired bases are thermodynamically unstable and intrinsically mutable, increased transcription could be the cause of hypermutation, and thus cancer. Support for this hypothesis has been obtained by analyzing 6662 mutations in all types of cancer compared to lung and colon cancers, using the p53 mutation database. The data suggest that genotoxins have two independent effects: first, they induce p53 transcription, which increases the number of mutable bases that determine the incidence of cancer. Second, genotoxins may alter the fate, or ultimate mutation of a mutable base, for example, by causing more of the available mutable Gs to mutate to T, leaving fewer to mutate to A. Such effects on the fate of mutable bases have no impact on the incidence of cancer, as both types of mutations lead to cancer.

Does measurement of oxidative damage to DNA have clinical significance?

Oxidative damage to DNA is the seemingly inevitable consequence of cellular metabolism. Furthermore, despite protective mechanisms, cellular levels of damage may increase under conditions of oxidative stress, arising from exposure to a variety of physical or chemical insults. Elevated levels of oxidatively damaged DNA have been measured in numerous diseases, and as a result, it has been hypothesised that such damage plays an integral role in the aetiology of that disease. This review examines the validity of this hypothesis, exploring the mechanisms by which oxidative DNA damage may lead to disease. We conclude that further validation of biomarkers of oxidative DNA damage, along with further elucidation of the role of damage in disease, may allow these biomarkers to become potentially useful clinical tools.

Phosphorylation of the retinoid x receptor at the omega loop, modulates the expression of retinoic-acid-target genes with a promoter context specificity

The retinoid response is mediated by two classes of nuclear receptors, the retinoic acid receptors (RARalpha, beta, and gamma) and the retinoid X receptors (RXRalpha, beta, and gamma) which act as ligand-dependent heterodimeric RAR/RXR transcription activators. Like most transcription factors, RARs and RXRs are regulated by phosphorylation processes. Here, we report that stress agents induce RXRalpha phosphorylation, subsequently to the activation of the stress-activated protein kinases cascade (JNKs). This phosphorylation process concerns three residues located in the N-terminal AF-1 domain of RXRalpha and one located in the omega loop of the Ligand Binding Domain. To decipher how stress-induced RXRalpha phosphorylation influences the transcription of RA-target genes, we used a ribotoxic stress agent, anisomycin, which activates signaling kinases without promoting DNA or protein damages, at subinhibitory concentrations. Taking advantage of vectors expressing recombinant RXRalpha mutated at its phosphorylation sites and of F9 cell lines re-expressing the same RXRalpha mutants in an RXRalpha null background, we provide evidence that stress signaling modulates RAR/RXRalpha-mediated transcription, through the phosphorylation of RXRalpha at the residue located in the Omega loop, in a promoter context-dependent manner.

Cancer dose–response assessment for acrylonitrile based upon rodent brain tumor incidence: Use of epidemiologic, mechanistic, and pharmacokinetic support for nonlinearity

A cancer dose-response assessment was conducted for acrylonitrile (AN) using updated information on mechanism of action, epidemiology, toxicity, and pharmacokinetics. Although more than 10 chronic bioassays indicate that AN produces multiple tumors in rats and mice, a number of large, well-conducted epidemiology studies provide no evidence of a causal association between AN exposure and cancer mortality of any type. The epidemiological data include early industry exposures that are far higher than occur today and that approach or exceed levels found to be tumorigenic in animals. Despite the absence of positive findings in the epidemiology data, a dose-response assessment was conducted for AN based on brain tumors in rats. Mechanistic studies implicate the involvement of oxidative stress in rat brain due to a metabolite (2-cyanoethylene oxide or CEO, cyanide), but do not conclusively rule out a potential role for the direct genotoxicity of CEO. A PBPK model was used to predict internal doses (peak CEO in brain) for 12 data sets, which were pooled together to provide a consistent characterization of the dose-response relationship for brain tumor incidence in the rat. The internal dose corresponding to a 5% increase in extra risk (ED 05=0.017 mg/L brain) and its lower confidence limit (LED 05=0.014 mg/L brain) was used as the point of departure. The weight-of-evidence supports the use of a nonlinear extrapolation for the cancer dose-response assessment. A quantitative comparison of the epidemiology exposure-response data (lung and brain cancer mortality) to the rat brain tumor data in terms of internal dose adds to the confidence in the nonlinear extrapolation. Uncertainty factors of 200 and 220 (for the oral and inhalation routes, respectively) were applied to the LED 05 to account for interspecies variation, intraspecies variation, and the severity of the response measure. Accordingly, oral doses below 0.009 mg/kg-day and air concentrations below 0.1mg/m(3) are not expected to pose an appreciable risk to human populations exposed to AN.

Host cell reactivation of plasmids containing oxidative DNA lesions is defective in Cockayne syndrome but normal in UV-sensitive syndrome fibroblasts

UV-sensitive syndrome (UV(S)S) is a human DNA repair-deficient disease with mild clinical manifestations. No neurological or developmental abnormalities or predisposition to cancer have been reported. In contrast, Cockayne syndrome (CS) patients exhibit severe developmental and neurological defects, in addition to photosensitivity. The cellular and biochemical responses of UV(S)S and CS cells to UV are indistinguishable, and result from defective transcription-coupled repair (TCR) of photoproducts in expressed genes. We propose that UV(S)S patients develop normally because they are proficient in repair of oxidative base damage. Consistent with our model, we show that Cockayne syndrome cells from complementation groups A and B (CS-A, CS-B) are more sensitive to treatment with hydrogen peroxide than wild type or UV(S)S cells. Using a host cell reactivation assay with plasmids containing UV-induced photoproducts, we find that expression of the plasmid-encoded lacZ gene is reduced in the TCR-deficient CS-B and UV(S)S cells. When the plasmids contain the oxidative base lesion thymine glycol, CS-B cells are defective in recovery of expression, whereas UV(S)S cells show levels of expression similar to those in wild type cells. 8-oxoguanine in the plasmids result in similarly defective host cell reactivation in CS-A and CS-B cells; abasic sites or single strand breaks in the plasmids cause similar decreases in expression in all the cell lines examined. Repair of thymine glycols in the lacZ gene was measured in plasmids extracted from transfected cells; removal of the lesions is efficient and without strand bias in all the cell lines tested.

Aging and gene expression in the primate brain

It is well established that gene expression levels in many organisms change during the aging process, and the advent of DNA microarrays has allowed genome-wide patterns of transcriptional changes associated with aging to be studied in both model organisms and various human tissues. Understanding the effects of aging on gene expression in the human brain is of particular interest, because of its relation to both normal and pathological neurodegeneration. Here we show that human cerebral cortex, human cerebellum, and chimpanzee cortex each undergo different patterns of age-related gene expression alterations. In humans, many more genes undergo consistent expression changes in the cortex than in the cerebellum; in chimpanzees, many genes change expression with age in cortex, but the pattern of changes in expression bears almost no resemblance to that of human cortex. These results demonstrate the diversity of aging patterns present within the human brain, as well as how rapidly genome-wide patterns of aging can evolve between species; they may also have implications for the oxidative free radical theory of aging, and help to improve our understanding of human neurodegenerative diseases.

Transcriptional profiles in human and chimpanzee reveal a diversity of aging patterns present within the human brain, as well as how rapidly genome-wide patterns of aging can evolve between species.

Genotoxicity and anti-genotoxicity of some traditional medicinal herbs

Six herbal infusions used worldwide (Matricaria chamomilla, Tilia cordata, Mentha piperita, Mentha pulegium, Uncaria tomentosa and Valeriana officinalis) were assayed for anti-genotoxicity using the Somatic Mutation And Recombination Test (SMART) in Drosophila melanogaster. All these infusions are traditionally used for various medical purposes, including anti-inflammatory processes. Hydrogen peroxide was used as an oxidative genotoxicant to test the anti-genotoxic potency of the medicinal infusions. None of these infusions showed a significant genotoxicity, quite the reverse they were able to behave as desmutagens, detoxifying the mutagen hydrogen peroxide. The phenolic content of such herbal infusions is argued to be the possible scavenger of reactive oxygen radicals produced by the hydrogen peroxide.

Phase IIa chemoprevention trial of green tea polyphenols in high-risk individuals of liver cancer: modulation of urinary excretion of green tea polyphenols and 8-hydroxydeoxyguanosine

Modulation of urinary excretion of green tea polyphenols (GTPs) and oxidative DNA damage biomarker, 8-hydroxydeoxyguanosine (8-OHdG), were assessed in urine samples collected from a randomized, double-blinded and placebo-controlled phase IIa chemoprevention trial with GTP in 124 individuals. These individuals were sero-positive for both HBsAg and aflatoxin-albumin adducts, and took GTP capsules daily at doses of 500 mg, 1000 mg or a placebo for 3 months. Twenty-four hour urine samples were collected before the intervention and at the first and third month of the study. Urinary excretion of 8-OHdG and GTP components was measured by HPLC-CoulArray electrochemical detection. The baseline levels of 8-OHdG and GTP components among the three groups showed homogeneity (P > 0.70), and a non-significant fluctuation was observed in the placebo group over the 3 months (P > 0.30). In GTP-treated groups, epigallocatechin (EGC) and epicatechin (EC) levels displayed significant and dose-dependent increases in both the 500 mg group and 1000 mg group (P < 0.05). The 8-OHdG levels did not differ between the three groups at the 1 month collection, with medians of 1.83, 2.08 and 1.86 ng/mg-creatinine for placebo, 500 and 1000 mg group, respectively (P = 0.999). At the end of the 3 months' intervention, 8-OHdG levels decreased significantly in both GTP-treated groups, with medians of 2.02, 1.03 and 1.15 ng/mg-creatinine for placebo, 500 mg and 1000 mg group, respectively (P = 0.007). These results suggest that urinary excretions of EGC and EC can serve as practical biomarkers for green tea consumption in human populations. The results also suggest that chemoprevention with GTP is effective in diminishing oxidative DNA damage.

Retinoic acid and arsenic trioxide cooperate for apoptosis through phosphorylated RXR alpha

Arsenite trioxide (As2O3) induces apoptosis in several cell lines by disturbing key signal transduction pathways through its oxidative properties. Here, we report that As2O3 also induces the phosphorylation of the retinoid receptor RXRalpha, subsequent to oxidative damages and the activation of the stress-activated protein kinases cascade (JNKs). We also report that RA amplifies both As2O3-induced phosphorylation of RXRalpha and apoptosis. Taking advantage of 'rescue' F9 cell lines expressing RXRalpha mutated at its phosphorylation sites, in an RXRalpha null background, we provide evidence that RXRalpha is a key element involved in that potentiating effect. Finally, we demonstrate that As2O3 also abrogates the transactivation of RA-target genes.

Role of selenium in regulation of spermatogenesis: involvement of activator protein 1

Selenium (Se) is involved in the process of male reproduction. Several studies have been carried out to find the mechanism of Se action through identified selenoproteins. Especially selenoenzyme phospholipid glutathione peroxidase (PHGPx, GPx-4) plays a pivotal role in regulating spermatogenesis. However, the action of selenium is best known as an antioxidant which acts through various selenoproteins viz. glutathione peroxidase, thioredoxin reductase and selenoprotein P. Oxidative stress is currently being considered a leading cause of male infertility. Presently, the involvement of redox active transcription factor, AP1 (Activator protein1) in testicular function was studied. AP1 is redox sensitive and also controls cell proliferation. The effects of Se might be mediated through it. Different Se status - deficient, adequate and excess Se - were generated in male Balb/c mice by feeding yeast based selenium deficient diet and deficient diet supplemented with Se as sodium selenite (0.2 and 1 ppm Se), respectively, for a period of 4 and 8 weeks. Se status was checked by measuring the Se levels and glutathione peroxidase (GSH-Px) activity in testis and liver. The reproductive potential of mice was affected at these changed Se levels. Changes in the activity of superoxide dismutase (SOD), levels of reduced glutathione (GSH) and oxidized glutathione (GSSG) were observed indicating increased oxidative stress at both the levels. Further, changes in the mRNA expression of GSH-Px, gamma-glutamylcysteine synthetase gammaGCS) and Mn superoxide dismutase (MnSOD) were observed. Decrease in cjun and cfos mRNA levels were observed at both the Se status (deficient and excess) which might be responsible for decreased germ cell number, differentiation and reduced fertility observed at the altered Se levels.

Oxidative DNA damage and disease: induction, repair and significance

The generation of reactive oxygen species may be both beneficial to cells, performing a function in inter- and intracellular signalling, and detrimental, modifying cellular biomolecules, accumulation of which has been associated with numerous diseases. Of the molecules subject to oxidative modification, DNA has received the greatest attention, with biomarkers of exposure and effect closest to validation. Despite nearly a quarter of a century of study, and a large number of base- and sugar-derived DNA lesions having been identified, the majority of studies have focussed upon the guanine modification, 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-OH-dG). For the most part, the biological significance of other lesions has not, as yet, been investigated. In contrast, the description and characterisation of enzyme systems responsible for repairing oxidative DNA base damage is growing rapidly, being the subject of intense study. However, there remain notable gaps in our knowledge of which repair proteins remove which lesions, plus, as more lesions identified, new processes/substrates need to be determined. There are many reports describing elevated levels of oxidatively modified DNA lesions, in various biological matrices, in a plethora of diseases; however, for the majority of these the association could merely be coincidental, and more detailed studies are required. Nevertheless, even based simply upon reports of studies investigating the potential role of 8-OH-dG in disease, the weight of evidence strongly suggests a link between such damage and the pathogenesis of disease. However, exact roles remain to be elucidated.

Gene-promoter hypermethylation as a biomarker in lung cancer

Silencing of genes by aberrant promoter hypermethylation is now recognized as a crucial component in cancer initiation and progression. Highly sensitive assays have been developed to assess gene-promoter methylation in biological fluids. The detection of methylated genes in sputum could lead to the development of a screening test to non-invasively identify early cancer in high-risk people.

Biological Relevance of Adduct Detection to the Chemoprevention of Cancer

Adducts arise from the chemical modification of bases in DNA or amino acids in proteins by toxic chemicals. Many chemicals known to be carcinogenic in humans have been shown to form adducts or to cause oxidative damage to genomic DNA in model systems. Biomarkers of carcinogenesis reflect biological events that take place between exposure to external or endogenous carcinogens and the subsequent development of cancer. Therapeutic intervention for the purpose of cancer chemoprevention may modify these biomarkers. In this article, the potential efficacy of DNA adducts as biomarkers of carcinogenesis and chemoprevention is discussed using criteria defined for phases of biomarker development. The sensitivity of adduct detection in histologically normal tissue offers opportunities for the early detection of carcinogenesis. Extensive evidence for aflatoxin B(1) adducts as biomarkers of risk and progression of hepatic carcinogenesis and for oxidative DNA adducts as biomarkers of the development of prostate carcinogenesis is reviewed together with the clinical trials measuring these adducts as biomarkers of the efficacy of chemoprevention. Favorable modification of oxidative DNA adducts by dietary intervention and chemoprevention has been demonstrated in preclinical and clinical studies. Protein adducts and DNA adducts in blood constituents or urine may act as useful surrogates for the target organ. Additional information regarding reliability, reproducibility, specificity, and confounding variables are required at the clinical level to validate adducts as suitable biomarkers of chemoprevention. "We do not administer antihypertensive drugs to patients in clinical trials without checking their blood pressure, so why should we give antioxidants without checking that they have decreased oxidant status.

Aberrant promoter hypermethylation of the death-associated protein kinase gene is early and frequent in murine lung tumors induced by cigarette smoke and tobacco carcinogens

Loss of expression of the death-associated protein (DAP)-kinase gene by aberrant promoter methylation may play an important role in cancer development and progression. The purpose of this investigation was to determine the commonality for inactivation of the DAP-kinase gene in adenocarcinomas induced in mice by chronic exposure to mainstream cigarette smoke, the tobacco carcinogens 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and vinyl carbamate, and the occupational carcinogen methylene chloride. The timing for inactivation was also determined in alveolar hyperplasias that arise in lung cancer induced in the A/J mouse by NNK. The DAP-kinase gene was not expressed in three of five NNK-induced lung tumor-derived cell lines or in a spontaneously arising lung tumor-derived cell line. Treatment with 5-aza-2'-deoxycytidine restored expression; dense methylation throughout the DAP-kinase CpG island detected by bisulfite sequencing supported methylation as the inactivating event in these cell lines. Methylation-specific PCR detected inactivation of the DAP-kinase gene in 43% of tumors associated with cigarette smoke, a frequency similar to those reported in human non-small cell lung cancer. In addition, DAP-kinase methylation was detected in 52%, 60%, and 50% of tumors associated with NNK, vinyl carbamate, and methylene chloride, respectively. Methylation was observed at similar prevalence in both NNK-induced hyperplasias and adenocarcinomas (46% versus 52%), suggesting that inactivation of this gene is one pathway for tumor development in the mouse lung. Bisulfite sequencing of both premalignant and malignant lesions revealed dense methylation, substantiating that this gene is functionally inactivated at the earliest histological stages of adenocarcinoma development. This study is the first to use a murine model of cigarette smoke-induced lung cancer and demonstrate commonality for inactivation by promoter hypermethylation of a gene implicated in the development of this disease in humans.

Factors contributing to the outcome of oxidative damage to nucleic acids

Oxidative damage to DNA appears to be a factor in cancer, yet explanations for why highly elevated levels of such lesions do not always result in cancer remain elusive. Much of the genome is non-coding and lesions in these regions might be expected to have little biological effect, an inference supported by observations that there is preferential repair of coding sequences. RNA has an important coding function in protein synthesis, and yet the consequences of RNA oxidation are largely unknown. Some non-coding nucleic acid is functional, e.g. promoters, and damage to these sequences may well have biological consequences. Similarly, oxidative damage to DNA may promote microsatellite instability, inhibit methylation and accelerate telomere shortening. DNA repair appears pivotal to the maintenance of genome integrity, and genetic alterations in repair capacity, due to single nucleotide polymorphisms or mutation, may account for inter-individual differences in cancer susceptibility. This review will survey these aspects of oxidative damage to nucleic acids and their implication for disease.

Gene regulation and DNA damage in the ageing human brain

The ageing of the human brain is a cause of cognitive decline in the elderly and the major risk factor for Alzheimer's disease. The time in life when brain ageing begins is undefined. Here we show that transcriptional profiling of the human frontal cortex from individuals ranging from 26 to 106 years of age defines a set of genes with reduced expression after age 40. These genes play central roles in synaptic plasticity, vesicular transport and mitochondrial function. This is followed by induction of stress response, antioxidant and DNA repair genes. DNA damage is markedly increased in the promoters of genes with reduced expression in the aged cortex. Moreover, these gene promoters are selectively damaged by oxidative stress in cultured human neurons, and show reduced base-excision DNA repair. Thus, DNA damage may reduce the expression of selectively vulnerable genes involved in learning, memory and neuronal survival, initiating a programme of brain ageing that starts early in adult life.

Stress-directed adaptive mutations and evolution

Comparative biochemistry demonstrates that the metabolites, complex biochemical networks, enzymes and regulatory mechanisms essential to all living cells are conserved in amazing detail throughout evolution. Thus, in order to evolve, an organism must overcome new adverse conditions without creating different but equally dangerous alterations in its ongoing successful metabolic relationship with its environment. Evidence suggests that stable long-term acquisitive evolution results from minor increases in mutation rates of genes related to a particular stress, with minimal disturbance to the balanced and resilient metabolism critical for responding to an unpredictable environment. Microorganisms have evolved specific biochemical feedback mechanisms that direct mutations to genes derepressed by starvation or other stressors in their environment. Transcription of the activated genes creates localized supercoiling and DNA secondary structures with unpaired bases vulnerable to mutation. The resulting mutants provide appropriate variants for selection by the stress involved, thus accelerating evolution with minimal random damage to the genome. This model has successfully predicted mutation frequencies in genes of E. coli and humans. Stressed cells observed in the laboratory over hundreds of generations accumulate mutations that also arise by this mechanism. When this occurs in repair-deficient mutator strains with high rates of random mutation, the specific stress-directed mutations are also enhanced.

Ultraviolet radiation-induced DNA damage in promoter elements inhibits gene expression

Repair of DNA damage in gene promoters is slower than in actively transcribed genes. Persistent damage in gene promoters though transient can have significant biological effects on regulated gene expression. In this study we investigated the effect of ultraviolet radiation on gene promoter-associated functions when DNA damage is located within and outside transcription factor binding sites. Our results show that both cyclobutane pyrimidine dimers and (6-4) photoproducts inhibit DNA-protein interaction, in vitro transcript production and transactivation of reporter genes. The biological significance of transient DNA damage as a mechanism in carcinogenesis is discussed.

Oxidative DNA damage: mechanisms, mutation, and disease

Oxidative DNA damage is an inevitable consequence of cellular metabolism, with a propensity for increased levels following toxic insult. Although more than 20 base lesions have been identified, only a fraction of these have received appreciable study, most notably 8-oxo-2'deoxyguanosine. This lesion has been the focus of intense research interest and been ascribed much importance, largely to the detriment of other lesions. The present work reviews the basis for the biological significance of oxidative DNA damage, drawing attention to the multiplicity of proteins with repair activities along with a number of poorly considered effects of damage. Given the plethora of (often contradictory) reports describing pathological conditions in which levels of oxidative DNA damage have been measured, this review critically addresses the extent to which the in vitro significance of such damage has relevance for the pathogenesis of disease. It is suggested that some shortcomings associated with biomarkers, along with gaps in our knowledge, may be responsible for the failure to produce consistent and definitive results when applied to understanding the role of DNA damage in disease, highlighting the need for further studies.

Adriamycin activates NF-kappaB in human lung carcinoma cells by IkappaBalpha degradation

The aim of this study was to investigate the effect of adriamycin (ADR) in signaling activation of NF-kappaB in ADR-sensitive and -resistant GLC(4) human small-cell lung carcinoma. ADR activated NF-kappaB only in ADR-sensitive GLC(4) cells in a time- and dose-dependant manner by stimulating IkappaBalpha degradation after 4h. Activation of NF-kappaB in response to tumor necrosis factor was intact in both cell lines. Topoisomerase II, a target for a number of chemotherapeutic agents, was depleted in both types of GLC(4) cells after ADR treatment, suggesting the stabilization of transient DNA-topoisomerase II complexes. Another transcription factor, Sp1, was activated by ADR, demonstrating the nonspecificity of NF-kappaB activation in ADR-sensitive GLC(4) cells. These findings indicated that resistance to ADR in ADR-sensitive GLC(4) cells did not involve the NF-kappaB transcription factor.

Hydrogen peroxide- and cell-density-regulated expression of NADH-cytochrome b5 reductase in HeLa cells

Environmental conditions regulate the expression of different antioxidant enzymes in cell culture. We have studied the effect of cell density and hydrogen peroxide on the expression of NADH-cytochrome b5 reductase in HeLa cells. Polypeptide levels of the NADH-cytochrome b5 reductase increased about three fold in confluent HeLa cells compared to sparse cells. Addition of H2O2 to HeLa cells altered expression levels of the NADH-cytochrome b5 reducatase in a concentration-dependent way, being sparse cells more sensitive to H2O2 addition than confluent cells. The presence of pyruvate, a H2O2 scavenger, produced a significant increment (200%) in the levels of NADH-cytochrome b5 reductase in sparse cells, but less increase (25%) in confluent cells, suggesting that generation of endogenous H2O2 could repress NADH-cytochrome b5 reductase expression, particularly in sparse cultures. Accordingly, confluent HeLa cells showed significantly lower levels of reactive oxygen species than cells in sparse cultures. Addition of tert-butylhydroquinone, a compound which generates reactive oxygen species through redox cycling, also reduced expression of the NADH-cytochrome b5 reductase. Increments in several antioxidant enzymes taking place during confluency could participate in the increase of NADH-cytochrome b5 reductase expression by reducing reactive oxygen species levels in cells. Overall, our results support that acute oxidative stress caused by H2O2 inhibits the expression levels of NADH-cytochrome b5 reductase, most likely due to inhibition of SP1 transcriptional activity. On the other hand, adaptation to H2O2 involved increased expression of the cytochrome b5 reductase, supporting the existence of additional regulatory mechanisms.

Effects of bile acids on base hydroxylation in a model of human colonic mucosal DNA

BACKGROUND AND AIMS

Increased intestinal bile acids as a possible consequence of a high fat/meat, low fiber diet are believed to play an important role in the formation of colon cancer. Interactions of bile salts particularly secondary bile acids with different cell components including DNA may contribute to carcinogenesis. To further investigate DNA damage by bile salts, we assessed the effects of a bile salt mixture containing deoxycholate and chenodeoxycholate on base hydroxylation in Chelex-treated DNA from calf thymus as a model of human colonic mucosal DNA in the presence and absence of reactive oxygen metabolites (ROM).

METHODS

Chelex-treated DNA from calf thymus (to remove residual iron impurities) was incubated with different bile salt concentrations (4 microM, 4.0 mM) (20.0% deoxycholate, 21.0% chenodeoxycholate) in the presence and absence of an OH generating system (25 microM FeCl3, 50 microM H2O2, 100 microM nitrilotriacetic acid) for 18 h (37 degrees C). After hydrolyzation, lyophilization and derivatization hydroxylated DNA bases were characterised and quantitated with gas chromatography-mass spectrometry (GS-MS) and SIM analysis. Two concentration ranges of bile salts were used, micromolar concentrations being present in plasma, millimolar in the gut lumen.

RESULTS

In the absence of ROM Chelex-treated DNA preparations contain only small amounts of hydroxylated base products. Bile salts at 4.0 mM significantly increased the amounts of 5-OH uracil and cis-thymine glycol. In the presence of ROM bile salts at 4.0 microM increased the production of 8-OH adenine and 8-OH guanine whereas bile salts at 4.0 mM inhibited ROM-induced base hydroxylation.

DISCUSSION

In the absence of ROM millimolar concentrations of a bile salt mixture with deoxycholate and chenodeoxycholate increase basal (spontaneous) DNA hydroxylation, whereas, they are without effects at micromolar concentrations. In the presence of ROM micromolar concentrations enhance oxidative DNA damage and millimolar concentrations were inhibitory. These results support the view that bile acids may cause oxidative DNA damage depending on their concentrations and the surrounding conditions both directly (enhancement of basal hydroxylation) and indirectly (enhancement of ROM-induced hydroxylation).

A single 8,5'-cyclo-2'-deoxyadenosine lesion in a TATA box prevents binding of the TATA binding protein and strongly reduces transcription in vivo

8,5'-Cyclo-2'-deoxypurine (cPu) lesions result from the action of the hydroxyl radical on DNA. These lesions represent a unique class of oxidative DNA lesions in that they are repaired by the nucleotide excision repair (NER) pathway but not by base excision repair (BER) or direct repair. Previous work has shown that cyclopurines can block mammalian DNA and RNA polymerases. Thus, these lesions are of interest because of their potential role in the neurodegeneration as well as internal cancers observed in patients with xeroderma pigmentosum (XP) who lack the capacity to carry out NER. In the present work, we found that the S-isomer of 8,5'-cyclo-2'-deoxyadenosine (cA) can prevent binding of the TATA binding protein (TBP) to the TATA box from the CMV promoter. To assess the functional importance of this effect in living cells, we transfected constructs containing a single cA in the CMV TATA box into XP cells to determine the effect of the lesion on gene expression in vivo. Using this approach, we found that the lesion reduced gene expression by approximately 75%. This effect was comparable to the effect of an inactivating mutation of the TATA box in the same promoter. These findings identify an additional biological effect of cyclopurine lesions in mammalian cells, which is the ability to interfere with transcription by preventing transcription factor binding to cognate recognition sequences. In addition, the approach we used in this study represents a novel method for assessing the effects of DNA lesions in non-transcribed sequences on gene expression in living cells.

Elevated oxidative stress in patients with ataxia telangiectasia

Ataxia telangiectasia (AT) is a pleiotropic genetic disorder characterized by progressive neurodegeneration, especially of cerebellar Purkinje cells, immunodeficiency, increased incidence of cancer, and premature aging. The disease is caused by functional inactivation of the ATM (AT-mutated) gene product, which is thought to act as a sensor of reactive oxygen species and oxidative damage of cellular macromolecules and DNA. The compound phenotype of AT might thus be linked to a continuous state of oxidative stress leading to an increase of programmed cell death (apoptosis). To assess this hypothesis, we analyzed lipid peroxidation products and the oxidative stress associated DNA base damage 8-hydroxy-2-deoxyguanosine in patients with AT. Oxidative damage to lipids and DNA was found to be markedly increased in AT patients. These results indicate that ATM might play an important role in the maintenance of cell homeostasis in response to oxidative damage. In this context, a better control of levels of reactive oxygen species could be a rational foundation of therapeutic intervention to help alleviate some of the symptoms associated with AT.