Modulation of hOGG1 DNA repair enzyme in human cultured cells in response to pro-oxidant and antioxidant challenge

The 24-h profile of the DNA repair enzyme 8-oxoguanine glycosylase 1 (OGG1) is associated with age, TNF-α, and waist circumference in healthy adults

It is unknown how the DNA repair enzyme OGG1 relates to healthy aging in humans, in particular to inflammaging, that is associated with increased levels of TNF-α. This study aimed (1) to investigate how 24-h profiles for OGG1 change during healthy aging and (2) to analyze the relationship of OGG1 with TNF-α, central body fat, cortisol and oxidative DNA/RNA damage. In a cross-sectional study in 20 healthy older and 20 young women, salivary levels of OGG1, TNF-α, cortisol and oxidative DNA/RNA damage were quantified by ELISAs every 4 h for a 24-h period. Trunk circumferences were taken as measures of central body fat. Older women, compared to young women, exhibited significantly lower protein levels of OGG1 throughout the whole 24-h period, a 2.5 times lower 24-h mean level for OGG1 (P < 0.00001) and loss of 24-h variation of OGG1. Both age groups demonstrated significant 24-h variation for TNF-alpha, cortisol and oxidative damage. The 24-h mean level for TNF-α was more than twice as high in older compared to young women (P = 0.011). Regression analysis detected that age, TNF-α and waist circumference were negative significant predictors of OGG1, explaining 56% of variance of OGG1 (P < 0.00001), while levels of cortisol and oxidative damage were no predictors of OGG1. Results indicate a strong decrease of protein levels of OGG1 and a loss of 24-h variation during natural cellular aging. The negative relationship, found between OGG1 and TNF-α and between OGG1 and waist circumference, suggests involvement of proinflammatory processes in DNA repair.

Circadian Modulation of 8-Oxoguanine DNA Damage Repair

The DNA base excision repair pathway is the main system involved in the removal of oxidative damage to DNA such as 8-Oxoguanine (8-oxoG) primarily via the 8-Oxoguanine DNA glycosylase (OGG1). Our goal was to investigate whether the repair of 8-oxoG DNA damage follow a circadian rhythm. In a group of 15 healthy volunteers, we found a daily variation of Ogg1 expression and activity with higher levels in the morning compared to the evening hours. Consistent with this, we also found lower levels of 8-oxoG in morning hours compared to those in the evening hours. Lymphocytes exposed to oxidative damage to DNA at 8:00 AM display lower accumulation of 8-oxoG than lymphocytes exposed at 8:00 PM. Furthermore, altered levels of Ogg1 expression were also observed in a group of shift workers experiencing a deregulation of circadian clock genes compared to a control group. Moreover, BMAL1 knockdown fibroblasts with a deregulated molecular clock showed an abolishment of circadian variation of Ogg1 expression and an increase of OGG1 activity. Our results suggest that the circadian modulation of 8-oxoG DNA damage repair, according to a variation of Ogg1 expression, could render humans less susceptible to accumulate 8-oxoG DNA damage in the morning hours.

Pro-oxidant induced DNA damage in human lymphoblastoid cells: homeostatic mechanisms of genotoxic tolerance

Oxidative stress contributes to many disease etiologies including ageing, neurodegeneration, and cancer, partly through DNA damage induction (genotoxicity). Understanding the i nteractions of free radicals with DNA is fundamental to discern mutation risks. In genetic toxicology, regulatory authorities consider that most genotoxins exhibit a linear relationship between dose and mutagenic response. Yet, homeostatic mechanisms, including DNA repair, that allow cells to tolerate low levels of genotoxic exposure exist. Acceptance of thresholds for genotoxicity has widespread consequences in terms of understanding cancer risk and regulating human exposure to chemicals/drugs. Three pro-oxidant chemicals, hydrogen peroxide (H(2)O(2)), potassium bromate (KBrO(3)), and menadione, were examined for low dose-response curves in human lymphoblastoid cells. DNA repair and antioxidant capacity were assessed as possible threshold mechanisms. H(2)O(2) and KBrO(3), but not menadione, exhibited thresholded responses, containing a range of nongenotoxic low doses. Levels of the DNA glycosylase 8-oxoguanine glycosylase were unchanged in response to pro- oxidant stress. DNA repair-focused gene expression arrays reported changes in ATM and BRCA1, involved in double-strand break repair, in response to low-dose pro-oxidant exposure; however, these alterations were not substantiated at the protein level. Determination of oxidatively induced DNA damage in H(2)O(2)-treated AHH-1 cells reported accumulation of thymine glycol above the genotoxic threshold. Further, the H(2)O(2) dose-response curve was shifted by modulating the antioxidant glutathione. Hence, observed pro- oxidant thresholds were due to protective capacities of base excision repair enzymes and antioxidants against DNA damage, highlighting the importance of homeostatic mechanisms in "genotoxic tolerance."

Solar radiation induced skin damage: review of protective and preventive options

PURPOSE

Solar energy has a number of short- and long-term detrimental effects on skin that can result in several skin disorders. The aim of this review is to summarise current knowledge on endogenous systems within the skin for protection from solar radiation and present research findings to date, on the exogenous options for such skin photoprotection.

RESULTS

Endogenous systems for protection from solar radiation include melanin synthesis, epidermal thickening and an antioxidant network. Existing lesions are eliminated via repair mechanisms. Cells with irreparable damage undergo apoptosis. Excessive and chronic sun exposure however can overwhelm these mechanisms leading to photoaging and the development of cutaneous malignancies. Therefore exogenous means are a necessity. Exogenous protection includes sun avoidance, use of photoprotective clothing and sufficient application of broad-spectrum sunscreens as presently the best way to protect the skin. However other strategies that may enhance currently used means of protection are being investigated. These are often based on the endogenous protective response to solar light such as compounds that stimulate pigmentation, antioxidant enzymes, DNA repair enzymes, non-enzymatic antioxidants.

CONCLUSION

More research is needed to confirm the effectiveness of new alternatives to photoprotection such as use of DNA repair and antioxidant enzymes and plant polyphenols and to find an efficient way for their delivery to the skin. New approaches to the prevention of skin damage are important especially for specific groups of people such as (young) children, photosensitive people and patients on immunosuppressive therapy. Changes in public awareness on the subject too must be made.

Establishment of a non-radioactive cleavage assay to assess the DNA repair capacity towards oxidatively damaged DNA in subcellular and cellular systems and the impact of copper

Oxidative stress is involved in many diseases, and the search for appropriate biomarkers is one major focus in molecular epidemiology. 8-Oxoguanine (8-oxoG), a potentially mutagenic DNA lesion, is considered to be a sensitive biomarker for oxidative stress. Another approach consists in assessing the repair capacity towards 8-oxoG, mediated predominantly by the human 8-oxoguanine DNA glycosylase 1 (hOGG1). With respect to the latter, during the last few years so-called cleavage assays have been described, investigating the incision of (32)P-labelled and 8-oxoG damaged oligonucleotides by cell extracts. Within the present study, a sensitive non-radioactive test system based on a Cy5-labelled oligonucleotide has been established. Sources of incision activity are isolated proteins or extracts prepared from cultured cells and peripheral blood mononuclear cells (PBMC). After comparing different oligonucleotide structures, a hairpin-like structure was selected which was not degraded by cell extracts. Applying this test system the impact of copper on the activity of isolated hOGG1 and on hOGG activity in A549 cells was examined, showing a distinct inhibition of the isolated protein at low copper concentration as compared to a modest inhibition of hOGG activity in cells at beginning cytotoxic concentrations. For investigating PBMC, all reaction conditions, including the amounts of oligonucleotide and cell extract as well as the reaction time have been optimized. The incision activities of PBMC protein extracts obtained from different donors have been investigated, and inter-individual differences have been observed. In summary, the established method is as sensitive and even faster than the radioactive technique, and additionally, offers the advantage of reduced costs and low health risk.

Membrane gamma-glutamyl transferase activity promotes iron-dependent oxidative DNA damage in melanoma cells

A number of recent observations have suggested a potential role for membrane-bound gamma-glutamyltransferase (GGT) in tumor progression and appearance of more aggressive and resistant phenotypes, through redox interactions leading to production of reactive oxygen species. The present study was aimed to evaluate whether such pro-oxidant activity of GGT can promote oxidative DNA damage, thus contributing to cancer genomic instability. Human GGT-transfected melanoma cells were studied, and DNA damage was measured by using the alkaline comet assay. Our results indicate that higher levels of GGT activity are associated with higher levels of background DNA damage and oxidized bases. This association cannot be explained by differences in cell cycle distribution or apoptotic rates. GGT-over-expressing cells also presented with a markedly higher glucose uptake, a phenomenon potentially leading to higher metabolic rate and oxidative DNA damage. Anyway, when GGT-over-expressing cells were incubated in the presence of GGT substrates and a source of catalytic iron, increased levels of DNA damage and oxidized bases were observed, an effect completely prevented in the presence of GGT inhibitors or various antioxidants.The findings reported indicate that GGT activity is able to promote iron-dependent DNA oxidative damage, thus potentially representing an important mechanism in initiation/progression of neoplastic transformation.

Oxidative DNA base damage in MCF-10A breast epithelial cells at clinically achievable concentrations of doxorubicin

The cellular metabolism of doxorubicin generates reactive oxygen species with significant potential to damage DNA. Such DNA damage can result in mutations if not adequately repaired by cellular DNA repair pathways. Secondary malignancies have been reported in patients who have received doxorubicin-containing chemotherapeutic regimens; however, the underlying molecular mechanism(s) to explain the development of these tumors remains under active investigation. We have previously demonstrated the presence of DNA bases modified by oxidation in the peripheral blood mononuclear cells of patients with breast cancer following treatment with doxorubicin. In those studies, doxorubicin was administered by continuous infusion over 96 h to minimize the risk of cardiac toxicity. To evaluate potential mechanisms underlying doxorubicin-induced DNA base oxidation in non-malignant tissues, MCF-10A breast epithelial cells were cultured for 96 h with the same doxorubicin concentration achieved in vivo (0.1 microM). During doxorubicin exposure, MCF-10A cells underwent growth arrest and apoptosis, developed elevated levels of reactive oxygen species, and demonstrated a time-dependent and significant increase in the levels of 11 oxidized DNA bases, as determined by gas chromatography/mass spectroscopy. Diminished expression of DNA repair enzymes was also observed over the same time course. Thus, clinically achievable concentrations of doxorubicin induce a level of oxidative stress in MCF-10A cells that is capable of oxidizing DNA bases and significantly altering cellular proliferation.

Acetylation of human 8-oxoguanine-DNA glycosylase by p300 and its role in 8-oxoguanine repair in vivo

The human 8-oxoguanine-DNA glycosylase 1 (OGG1) is the major DNA glycosylase responsible for repair of 7,8-dihydro-8-oxoguanine (8-oxoG) and ring-opened fapyguanine, critical mutagenic DNA lesions that are induced by reactive oxygen species. Here we show that OGG1 is acetylated by p300 in vivo predominantly at Lys338/Lys341. About 20% of OGG1 is present in acetylated form in HeLa cells. Acetylation significantly increases OGG1's activity in vitro in the presence of AP-endonuclease by reducing its affinity for the abasic (AP) site product. The enhanced rate of repair of 8-oxoG in the genome by wild-type OGG1 but not the K338R/K341R mutant, ectopically expressed in oxidatively stressed OGG1-null mouse embryonic fibroblasts, suggests that acetylation increases OGG1 activity in vivo. At the same time, acetylation of OGG1 was increased by about 2.5-fold after oxidative stress with no change at the polypeptide level. OGG1 interacts with class I histone deacetylases, which may be responsible for its deacetylation. Based on these results, we propose a novel regulatory function of OGG1 acetylation in repair of its substrates in oxidatively stressed cells.

Mitochondrial DNA maintenance and bioenergetics

Oxidative phosphorylation requires assembly of the protein products of both mitochondrial and of nuclear genomes into functional respiratory complexes. Cellular respiration can be compromised when mitochondrial DNA (mtDNA) sequences are corrupted. Oxidative damage resulting from reactive oxygen species (ROS) produced during respiration is probably a major source of mitochondrial genomic instability leading to respiratory dysfunction. Here, we review mechanisms of mitochondrial ROS production, mtDNA damage and its relationship to mitochondrial dysfunction. We focus particular attention on the roles of mtDNA repair enzymes and processes by which the integrity of the mitochondrial genome is maintained and dysfunction prevented.

REDOX REGULATION BY INTRINSIC SPECIES AND EXTRINSIC NUTRIENTS IN NORMAL AND CANCER CELLS

Cells in multicellular organisms are exposed to both endogenous oxidative stresses generated metabolically and to oxidative stresses that originate from neighboring cells and from other tissues. To protect themselves from oxidative stress, cells are equipped with reducing buffer systems (glutathione/GSH and thioredoxin/thioredoxin reductase) and have developed several enzymatic mechanisms against oxidants that include catalase, superoxide dismutase, and glutathione peroxidase. Other major extrinsic defenses (from the diet) include ascorbic acid, beta-carotene and other carotenoids, and selenium. Recent evidence indicates that in addition to their antioxidant function, several of these redox species and systems are involved in regulation of biological processes, including cellular signaling, transcription factor activity, and apoptosis in normal and cancer cells. The survival and overall well-being of the cell is dependent upon the balance between the activity and the intracellular levels of these antioxidants as well as their interaction with various regulatory factors, including Ref-1, nuclear factor-kappaB, and activating protein-1.

Dietary supplementation with different vitamin C doses: no effect on oxidative DNA damage in healthy people

BACKGROUND

Antioxidants are believed to prevent many types of disease. Some previous studies suggest that dietary supplementation with vitamin C results in a decrease in the level of one of the markers of oxidative damage-8-oxoguanine in the DNA of peripheral blood mononuclear cells (PBMC). AIM OF TRIAL: To investigate the effect of different dose levels of dietary supplementation with vitamin C on oxidative DNA damage.

METHODS

A randomised double-blind placebo-controlled trial was carried out using three different levels (80, 200 and 400 mg) of dietary vitamin C supplementation in a healthy population of 160 volunteers; supplementation was for a period of 15 weeks followed by a 10 week washout period. Peripheral blood samples were obtained every 5 weeks from baseline to 25 weeks.

RESULTS

An increase in PBMC vitamin C levels was not observed following supplementation in healthy volunteers. There was no effect found on 8-oxoguanine measured using HPLC with electrochemical detection for any of the three supplemented groups compared to placebo. 8-oxoadenine levels were below the limit of detection of the HPLC system used here.

CONCLUSIONS

Supplementation with vitamin C had little effect on cellular levels in this group of healthy individuals, suggesting their diets were replete in vitamin C. The dose range of vitamin C used did not affect oxidative damage in PBMC DNA.

DNA base excision repair activities and pathway function in mitochondrial and cellular lysates from cells lacking mitochondrial DNA

Mitochondrial DNA (mtDNA) contains higher steady-state levels of oxidative damage and mutates at rates significantly greater than nuclear DNA. Oxidative lesions in mtDNA are removed by a base excision repair (BER) pathway. All mtDNA repair proteins are nuclear encoded and imported. Most mtDNA repair proteins so far discovered are either identical to nuclear DNA repair proteins or isoforms of nuclear proteins arising from differential splicing. Regulation of mitochondrial BER is therefore not expected to be independent of nuclear BER, though the extent to which mitochondrial BER is regulated with respect to mtDNA amount or damage is largely unknown. Here we have measured DNA BER activities in lysates of mitochondria isolated from human 143B TK(-) osteosarcoma cells that had been depleted of mtDNA (rho(0)) or not (wt). Despite the total absence of mtDNA in the rho(0) cells, a complete mitochondrial BER pathway was present, as demonstrated using an in vitro assay with synthetic oligonucleotides. Measurement of individual BER protein activities in mitochondrial lysates indicated that some BER activities are insensitive to the lack of mtDNA. Uracil and 8-oxoguanine DNA glycosylase activities were relatively insensitive to the absence of mtDNA, only about 25% reduced in rho(0) relative to wt cells. Apurinic/apyrimidinic (AP) endonuclease and polymerase gamma activities were more affected, 65 and 45% lower, respectively, in rho(0) mitochondria. Overall BER activity in lysates was also about 65% reduced in rho(0) mitochondria. To identify the limiting deficiencies in BER of rho(0) mitochondria we supplemented the BER assay of mitochondrial lysates with pure uracil DNA glycosylase, AP endonuclease and/or the catalytic subunit of polymerase gamma. BER activity was stimulated by addition of uracil DNA glycosylase and polymerase gamma. However, no addition or combination of additions stimulated BER activity to wt levels. This suggests that an unknown activity, factor or interaction important in BER is deficient in rho(0) mitochondria. While nuclear BER protein levels and activities were generally not altered in rho(0) cells, AP endonuclease activity was substantially reduced in nuclear and in whole cell extracts. This appeared to be due to reduced endogenous reactive oxygen species (ROS) production in rho(0) cells, and not a general dysfunction of rho(0) cells, as exposure of cells to ROS rapidly stimulated increases in AP endonuclease activities and APE1 protein levels.

Expression of hOGG1 mRNA and protein in hepatocellular carcinoma

AIM: To study the expression of DNA repair enzyme hOGG1 mRNA and protein in nomal liver cell, hepatoma cell lines and hepatocellular carcinoma (HCC) tissues, and to investigate their function in the progress of HCC.

METHODS: Expression of hOGG1 in normal liver cell L-02, hepatoma cell lines HepG2, SMMC7721 and HCC tissues (26 cases) as well as surrouding tissues of HCC (21 cases) were detected by semi-quantitative reverse-transcription polymerase chain reaction. hOGG1 protein was detected in corresponding HT tissues (17 cases HT) and their surrouding liver tissues (15 cases HST) by immunohistochemistry.

RESULTS: The expression level of hOGG1 mRNA in nomal liver cell line was lower than that in two hepatoma cell lines (0.270±0.014 vs 0.662±0.011, 0.624±0.020, P < 0.05). The expression of hOGG1 mRNA in HepG2 was similar to that in SMMC7721. The expression of hOGG1 mRNA in HT was lower than that in HST (P < 0.05). hOGG1 protein was 87.2% (41 of 47) positive in HT and HST and was mainly distributed in liver cells. The protein level of hOGG1 in HCC tissues was correspondingly lower than in their surrouding tissues (P < 0.05).

CONCLUSION: Overexpression of hOGG1 in hepatoma cell lines and the surrouding liver tissuses of HCC may be one of the key events which promote the malignant growth. These results suggest a role for hOGG1 expression in the course of the multistage process of carcinogenesis in hepatocellular carcinoma.