Inhibitory effects of paracetamol on DNA repair in mammalian cells

A comprehensive weight of evidence assessment of published acetaminophen genotoxicity data: Implications for its carcinogenic hazard potential

In 2019, the California Office of Environmental Health Hazard Assessment initiated a review of the carcinogenic hazard potential of acetaminophen, including an assessment of its genotoxicity. The objective of this analysis was to inform this review process with a weight-of-evidence assessment of more than 65 acetaminophen genetic toxicology studies that are of widely varying quality and conformance to accepted standards and relevance to humans. In these studies, acetaminophen showed no evidence of induction of point or gene mutations in bacterial and mammalian cell systems or in in vivo studies. In reliable, well-controlled test systems, clastogenic effects were only observed in unstable, p53-deficient cell systems or at toxic and/or excessively high concentrations that adversely affect cellular processes (e.g., mitochondrial respiration) and cause cytotoxicity. Across the studies, there was no clear evidence that acetaminophen causes DNA damage in the absence of toxicity. In well-controlled clinical studies, there was no meaningful evidence of chromosomal damage. Based on this weight-of-evidence assessment, acetaminophen overwhelmingly produces negative results (i.e., is not a genotoxic hazard) in reliable, robust high-weight studies. Its mode of action produces cytotoxic effects before it can induce the stable, genetic damage that would be indicative of a genotoxic or carcinogenic hazard.

Animal products, diseases and drugs: a plea for better integration between agricultural sciences, human nutrition and human pharmacology

Eicosanoids are major players in the pathogenesis of several common diseases, with either overproduction or imbalance (e.g. between thromboxanes and prostacyclins) often leading to worsening of disease symptoms. Both the total rate of eicosanoid production and the balance between eicosanoids with opposite effects are strongly dependent on dietary factors, such as the daily intakes of various eicosanoid precursor fatty acids, and also on the intakes of several antioxidant nutrients including selenium and sulphur amino acids. Even though the underlying biochemical mechanisms have been thoroughly studied for more than 30 years, neither the agricultural sector nor medical practitioners have shown much interest in making practical use of the abundant high-quality research data now available. In this article, we discuss some specific examples of the interactions between diet and drugs in the pathogenesis and therapy of various common diseases. We also discuss, using common pain conditions and cancer as specific examples, how a better integration between agricultural science, nutrition and pharmacology could lead to improved treatment for important diseases (with improved overall therapeutic effect at the same time as negative side effects and therapy costs can be strongly reduced). It is shown how an unnaturally high omega-6/omega-3 fatty acid concentration ratio in meat, offal and eggs (because the omega-6/omega-3 ratio of the animal diet is unnaturally high) directly leads to exacerbation of pain conditions, cardiovascular disease and probably most cancers. It should be technologically easy and fairly inexpensive to produce poultry and pork meat with much more long-chain omega-3 fatty acids and less arachidonic acid than now, at the same time as they could also have a similar selenium concentration as is common in marine fish. The health economic benefits of such products for society as a whole must be expected vastly to outweigh the direct costs for the farming sector.

Nephrotoxicity assessments of acetaminophen during zebrafish embryogenesis

We used a green fluorescent kidney line, Tg(wt1b:GFP), as a model to access the acetaminophen (AAP)-induced nephrotoxicity dynamically. Zebrafish (Danio rerio) embryos at different developmental stages (12-60hpf) were treated with different dosages of AAP (0-45mM) for different time courses (12-60h). Results showed that zebrafish embryos exhibited no evident differences in survival rates and morphological changes between the mock-treated control (0mM) and 2.25mM AAP-exposure (12-72hpf) groups. In contrast, after higher doses (22.5 and 45mM) of exposure, embryos displayed malformed kidney phenotypes, such as curved, cystic pronephric tube, pronephric duct, and a cystic and atrophic glomerulus. The percentages of embryos with malformed kidney phenotypes increased as the exposure dosages of AAP increased. Interestingly, under the same exposure time course (12h) and dose (22.5mM), embryos displayed higher percentages of severe defects at earlier developmental stage of exposure (12-24hpf), whereas embryos displayed higher percentages of mild defects at later exposure (60-72hpf). With an exposure time course less than 24h of 45mM AAP, no embryo survived by the developmental stage of 72hpf. These results indicated that AAP-induced nephrotoxicity depended on the exposure dose, time course and developmental stages. Immunohistochemical experiments showed that the cells' morphologies of the pronephric tube, pronephric duct and glomerulus were disrupted by AAP, and consequently caused cell death. Real-time RT-PCR revealed embryos after AAP treatment decreased the expression of cox2 and bcl2, but increased p53 expression. In conclusion, AAP-induced defects on glomerulus, pronephric tube and pronephric duct could be easily and dynamically observed in vivo during kidney development in this present model.

Genotoxic Activities of Aniline and its Metabolites and Their Relationship to the Carcinogenicity of Aniline in the Spleen of Rats

Aniline (in the form of its hydrochloride) has been shown to induce a rather rare spectrum of tumors in the spleen of Fischer 344 rats. The dose levels necessary for this carcinogenic activity were in a range where also massive effects on the blood and non-neoplastic splenotoxicity as a consequence of methemoglobinemia were to be observed. This review aimed at clarifying if aniline itself or one of its metabolites has a genotoxic potential which would explain the occurrence of the spleen tumors in rats as a result of a primary genetic activity. The database for aniline and its metabolites is extremely heterogeneous. With validated assays it ranges from a few limited Ames tests (o- and m-hydroxyacetanilide, phenylhydroxylamine, nitrosobenzene) to a broad range of studies covering all genetic endpoints partly with several studies of the same or different test systems (aniline, p-aminophenol, p-hydroxyacetanilide). This makes a direct comparison rather difficult. In addition, a varying number of results with as yet not validated systems are available for aniline and its metabolites. Most results, especially those with validated and well performed/documented studies, did not indicate a potential of aniline to induce gene mutations. In five different mouse lymphoma tests, where colony sizing was performed only in one test, aniline was positive. If this indicates a peculiar feature of a point mutagenic potential or does represent a part of the clastogenic activity for which there is evidence in vitro as well as in vivo remains to be investigated. There is little evidence for a DNA damaging potential of aniline. The clastogenic activity in vivo is confined to dose levels, which are close to lethality essentially due to hematotoxic effects. The quantitatively most important metabolites for experimental animals as well as for humans (p-aminophenol, p-hydroxyacetanilide) seem to have a potential for inducing chromosomal damage in vitro and, at relatively high dose levels, also in vivo. This could be the explanation for the clastogenic effects that have been observed after high doses/concentrations with aniline. They do not induce gene mutations and there is little evidence for a DNA damaging potential. None of these metabolites revealed a splenotoxic potential comparable to that of aniline in studies with repeated or long-term administration to rats. The genotoxicity database on those metabolites with a demonstrated and marked splenotoxic potential, i.e. phenylhydroxylamine, nitrosobenzene, is unfortunately very limited and does not allow to exclude with certainty primary genotoxic events in the development of spleen tumors. But quite a number of considerations by analogy from other investigations support the conclusion that the effects in the spleen do not develop on a primary genotoxic basis. The weight of evidences suggests that the carcinogenic effects in the spleen of rats are the endstage of a chronic high-dose damage of the blood leading to a massive overload of the spleen with iron, which causes chronic oxidative stress. This conclusion, based essentially on pathomorphological observations, and analogy considerations thereof by previous authors, is herewith reconfirmed under consideration of the more recently reported studies on the genotoxicity of aniline and its metabolites, on biochemical measurements indicating oxidative stress, and on the metabolism of aniline. It is concluded that there is no relationship between the damage to the chromosomes at high, toxic doses of aniline and its major metabolites p-aminophenol/p-hydroxyacetanilide and the aniline-induced spleen tumors in the rat.

Cytogenetic Evaluation of Paracetamol Effects in Human Lymphocytes Culture

Paracetamol is a common analgesic and antipyretic drug. It has been recognized as one of the most ordinary medications taken in overdoses. We examined the possible genotoxic effects of high paracetamol concentrations expected to occur after overdose. Paracetamol was added to the cultures at the beginning of the cultivation period. Separate cultures for three tested concentrations of paracetamol (50 microg/mL, 100 microg/mL, and 200 microg/mL) were set. Effects of paracetamol were evaluated by micronucleus cytokinesis-block assay, chromosome aberration analysis, and nuclear division index. Results demonstrate that paracetamol concentration of 200 microg/mL expresses certain genotoxic effects in human peripheral blood lymphocytes.

Effect of crude sulphated polysaccharide from brown algae against acetaminophen-induced toxicity in rats

This study was conducted to examine the protective role of crude polysaccharide from brown seaweed Sargassum polycystum against acetaminophen-induced abnormality in blood glucose, serum albumin/globulin ratio, and liver glycogen, lactate, and pyruvate. Liver and renal tissue histology was performed to confirm the efficacy of Sargassum polysaccharide. A toxic dose of acetaminophen (800 mg/kg body weight intraperitoneally) induced severe abnormality in all basic parameters with apparent toxicity in liver (enlargement of hepatocytes, loss of cytoplasmic content with disruption in the hepatic plates and sinusoidal dilation) and renal tissue (glomerular damage with congestion of tubules). The isolated liver cells were stained with acridine orange and examined under fluorescence microscope, which revealed that the acetaminophen induced significant damage. In contrast, the rats pretreated with Sargassum polysaccharide (200 mg/kg body weight) daily for 3 weeks did not show liver and renal tissue with these severe aberrations induced by acetaminophen. Histology results were also consistent with analyzed basic biochemical parameters, which confirmed the effectiveness of the crude polysaccharide against acetaminophen-induced abnormality in rats.

Nucleotide excision repair in differentiated cells

Nucleotide excision repair (NER) is the principal pathway for the removal of a wide range of DNA helix-distorting lesions and operates via two NER subpathways, i.e. global genome repair (GGR) and transcription-coupled repair (TCR). Although detailed information is available on expression and efficiency of NER in established mammalian cell lines, little is known about the expression of NER pathways in (terminally) differentiated cells. The majority of studies in differentiated cells have focused on repair of UV-induced cyclobutane pyrimidine dimers (CPD) and 6-4-photoproducts (6-4PP) because of the high frequency of photolesions at low level of toxicity and availability of sensitive technologies to determine photolesions in defined regions of the genome. The picture that emerges from these studies is blurred and rather complex. Fibroblasts and terminally differentiated myocytes of the rat heart display equally efficient GGR of 6-4PP but poor repair of CPD due to the absence of p48 expression. This repair phenotype is clearly different from human terminal differentiated neurons. Furthermore, both cell types were found to carry out TCR of CPD, thus mimicking the repair phenotype of established rodent cell lines. In contrast, in intact rat spermatogenic cells repair was very inefficient at the genome overall level and in transcriptionally active genes indicating that GGR and TCR are non-functional. Also, non-differentiated mouse embryonic stem (ES) cells exhibit low levels of NER after UV irradiation. However, the mechanisms that lead to low NER activity are clearly different: in differentiated spermatogenic cells differences in chromatin compaction and sequestering of NER proteins may underlie the lack of NER activity in pre-meiotic cells, whereas in non-differentiated ES cells NER is impaired by a strong apoptotic response.

Preliminary Evaluation of DNA Damage Related with the Smoking Habit Measured by the Comet Assay in Whole Blood Cells

The alkaline single-cell gel electrophoresis (SCGE) assay, also called the comet assay, is a rapid and simple method for the detection of DNA damage in individual cells. The objective of this study was to establish if the alkaline SCGE assay in whole blood cells gives similar results as the same method in isolated lymphocytes, because whole blood cells are simpler and more economical to use, specifically in human genotoxic biomonitoring. To validate the method, we first used mouse blood cells, because mouse is one of the most commonly used animals in genetic toxicology testing. Groups of seven CF1 male mice were given i.p. injections of relatively low doses of methyl methanesulfonate (25 mg/kg body weight), a direct acting genotoxic agent, or cyclophosphamide (50 mg/kg body weight), which requires metabolic activation. Three, 6, 8, 12, 16, 20, and 65 hours after treatment, 5 μL of blood were collected from each animal and were processed for the alkaline SCGE assay. On the basis of an analysis of tail moment, the results showed that this assay can detect DNA damage induced by both kinds of alkylating mutagens. We then did a preliminary study to assess the status of DNA damage in a young (19 to 23 years old) healthy population of male smokers (n = 6) and nonsmokers (n = 6) using the comet assay in whole blood cells. A significant difference was observed between the two groups, showing that the method is able to detect DNA damage in the smoking group despite the short time that the volunteers had actually been smoking.

Antioxidants protect primary rat hepatocyte cultures against acetaminophen-induced DNA strand breaks but not against acetaminophen-induced cytotoxicity

Acetaminophen, a safe analgesic when dosed properly but hepatotoxic at overdoses, has been reported to induce DNA strand breaks but it is unclear whether this event preceeds hepatocyte toxicity or is only obvious in case of overt cytotoxicity. Moreover, it is not known whether the formation of reactive oxygen species (ROS) is involved in the formation of the DNA strand breaks. In the present study, the dose-response curves for cytotoxicity and DNA strand breaks and the response to antioxidant protection have been compared. In primary hepatocytes from untreated male rats, cytotoxicity as measured by the MTT test and by Neutral Red accumulation was obvious at 10 mM acetaminophen but DNA strand breaks as measured by the comet assay were only found at 25-30 mM acetaminophen. Non-cytotoxic concentrations of three compounds with antioxidant activity, the glutathione precursor N-acetylcysteine (100 micro M), the plant polyphenol silibin (25 micro M) and the antioxidant vitamin alpha-tocopherol (50 micro M), were not able to inhibit acetaminophen toxicity at any acetaminophen concentration, while they completely prevented the formation of DNA strand breaks at 25-30 mM acetaminophen. The occurrence of oxidative stress in our experiments was indicated by a slight increase of malondialdehyde formation at 40 mM acetaminophen and by an adaptive increase in catalase mRNA concentration. We conclude that in acetaminophen-treated hepatocytes ROS-independent cell death and ROS-dependent DNA strand breaks occur which appear not to be causally related as judged from their dose dependency and their response to antioxidants.

Toxicity of acetaminophen, salicylic acid, and caffeine for first-passage rat renal inner medullary collecting duct cells

Chronic excess ingestion of nonsteroid anti-inflammatory drugs causes renal medullary necrosis. Previously, using an immortalized line of mouse inner medullary collecting ducts cells (mIMCD3), we found that acetaminophen, salicylic acid, and caffeine are toxic, and the effects of acetaminophen and caffeine are strongly additive. Furthermore, toxicity was greater in proliferating than in nonproliferating cells. Important limitations were that mIMCD3 cells do not readily tolerate the high concentrations of salt and urea normally present in renal inner medullas and proliferate much more rapidly than inner medullary cells in vivo. Thus, these cells may not serve as an appropriate model for the in vivo IMCD. The present studies address these limitations by using passage-1 rat inner medullary collecting duct (p1rIMCD) cells, which tolerate high salt and urea and become contact inhibited when confluent. At 640 mOsmol/kg (the lowest normal inner medullary osmolality), the drugs, singly and in combination, reduce the number of proliferating (i.e., subconfluent) p1rIMCD cells more than they do confluent cells. Effects of acetaminophen and caffeine are strongly additive. Addition of as little as 0.1 mM caffeine significantly enhances the toxicity of acetaminophen plus salicylic acid. With confluent cells at 640 mOsmol/kg and very slowly growing cells at 1370 mOsmol/kg, combinations of drugs that include acetaminophen increase proliferation, accompanied by DNA damage and apoptosis. We conclude that these drugs are toxic to renal inner medullary collecting duct cells under the conditions of high osmolality normally present in the inner medulla, that combinations of the drugs are more toxic than are the drugs individually, and that the toxicity includes induction of proliferation of these cells that are otherwise quiescent in the presence of high osmolality.

Limited repair of 8-hydroxy-7,8-dihydroguanine residues in human testicular cells

Oxidative damage in testicular DNA is associated with poor semen quality, reduced fertility and increased risk of stillbirths and birth defects. These DNA lesions are predominantly removed by base excision repair. Cellular extracts from human and rat testicular cells and three enriched populations of rat male germ cells (primary spermatocytes, round spermatids and elongating/elongated spermatids) all showed proficient excision/incision of 5-hydroxycytosine, thymine glycol and 2,6-diamino-4-hydroxy-5-formamidopyrimidine. DNA containing 8-oxo-7,8-dihydroguanine was excised poorly by human testicular cell extracts, although 8-oxoguanine-DNA glycosylase-1 (hOGG1) was present in human testicular cells, at levels that varied markedly between 13 individuals. This excision was as low as with human mononuclear blood cell extracts. The level of endonuclease III homologue-1 (NTH1), which excises oxidised pyrimidines, was higher in testicular than in somatic cells of both species. Cellular repair studies of lesions recognised by formamidopyrimidine-DNA glycosylase (Fpg) or endonuclease III (Nth) were assayed with alkaline elution and the Comet assay. Consistent with the enzymatic activities, human testicular cells showed poor removal of Fpg-sensitive lesions but efficient repair of Nth-sensitive lesions. Rat testicular cells efficiently repaired both Fpg- and Nth-sensitive lesions. In conclusion, human testicular cells have limited capacity to repair important oxidative DNA lesions, which could lead to impaired reproduction and de novo mutations.

Rat testicular structure and ultrastructure after paracetamol treatment

The morphological modifications of Wistar rat testicles were investigated for animals treated with paracetamol. The rats (n = 6) received a single dose of 4.4 mmol/kg paracetamol, while controls (n = 6) received the same volume of physiological solution. After 5, 10 and 50 days, the rats were perfused with Karnovsky's fixative and processed by routine methods for light and electron microscopy. Some altered seminiferous tubules were found, as well as a few degenerating tubules. Within the tubules, Sertoli cells appeared fragmented while the spermatids showed unusually well-developed rough endoplasmic reticulum and Golgi complexes and had irregularly compacted chromatin. Morphologically altered late spermatids were found retaining a larger volume of residual cytoplasm than spermatids of controls. An indirect cause of alterations was also suggested based on the occurrence of dilated blood vessels and edema.

Direct toxicity of nonsteroidal antiinflammatory drugs for renal medullary cells

Antipyretic analgesics, taken in large doses over a prolonged period, cause a specific form of kidney disease, characterized by papillary necrosis and interstitial scarring. Epidemiological evidence incriminated mixtures of drugs including aspirin (ASA), phenacetin, and caffeine. The mechanism of toxicity is unclear. We tested the effects of ASA, acetaminophen (APAF, the active metabolite of phenacetin), caffeine, and other related drugs individually and in combination on mouse inner medullary collecting duct cells (mIMCD3). The number of rapidly proliferating cells was reduced by approximately 50% by 0.5 mM ASA, salicylic acid, or APAF. The drugs had less effect on confluent cells, which proliferate slowly. Thus, the slow in vivo turnover of IMCD cells could explain why clinical toxicity requires very high doses of these drugs over a very long period. Caffeine greatly potentiated the effect of acetaminophen, pointing to a potential danger of the mixture. Cyclooxygenase (COX) inhibitors, indomethacin and NS-398, did not reduce cell number except at concentrations greatly in excess of those that inhibit COX. Therefore, COX inhibition alone is not toxic. APAF arrests most cells in late G(1) and S and produces a mixed form of cell death with both oncosis (swollen cells and nuclei) and apoptosis. APAF is known to inhibit the synthesis of DNA and cause chromosomal aberrations due to inhibition of ribonucleotide reductase. Such effects of APAF might account for renal medullary cell death in vivo and development of uroepithelial tumors from surviving cells that have chromosomal aberrations.

Highly efficient base excision repair (BER) in human and rat male germ cells

The quality of germ cell DNA is critical for the fate of the offspring, yet there is limited knowledge of the DNA repair capabilities of such cells. One of the main DNA repair pathways is base excision repair (BER) which is initiated by DNA glycosylases that excise damaged bases, followed by incision of the generated abasic (AP) sites. We have studied human and rat methylpurine-DNA glycosylase (MPG), uracil-DNA glycosylase (UNG), and the major AP endonuclease (HAP1/APEX) in male germ cells. Enzymatic activities and western analyses indicate that these enzymes are present in human and rat male germ cells in amounts that are at least as high as in somatic cells. Minor differences were observed between different cellular stages of rat spermatogenesis and spermiogenesis. Repair of methylated DNA was also studied at the cellular level using the Comet assay. The repair was highly efficient in both human and rat male germ cells, in primary spermatocytes as well as round spermatids, compared to rat mononuclear blood cells or hepatocytes. This efficient BER removes frequently occurring DNA lesions that arise spontaneously or via environmental agents, thereby minimising the number of potential mutations transferred to the next generation.

Nucleotide excision repair in rat male germ cells: low level of repair in intact cells contrasts with high dual incision activity in vitro

The acquisition of genotoxin-induced mutations in the mammalian germline is detrimental to the stable transfer of genomic information. In somatic cells, nucleotide excision repair (NER) is a major pathway to counteract the mutagenic effects of DNA damage. Two NER subpathways have been identified, global genome repair (GGR) and transcription-coupled repair (TCR). In contrast to somatic cells, little is known regarding the expression of these pathways in germ cells. To address this basic question, we have studied NER in rat spermatogenic cells in crude cell suspension, in enriched cell stages and within seminiferous tubules after exposure to UV or N-acetoxy-2-acetylaminofluorene. Surprisingly, repair in spermatogenic cells was inefficient in the genome overall and in transcriptionally active genes indicating non-functional GGR and TCR. In contrast, extracts from early/mid pachytene cells displayed dual incision activity in vitro as high as extracts from somatic cells, demonstrating that the proteins involved in incision are present and functional in premeiotic cells. However, incision activities of extracts from diplotene cells and round spermatids were low, indicating a stage-dependent expression of incision activity. We hypothesize that sequestering of NER proteins by mispaired regions in DNA involved in synapsis and recombination may underlie the lack of NER activity in premeiotic cells.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Micronucleus formation in human amnion cells after exposure to 50 Hz MF applied horizontally and vertically

Micronucleus (MN) induction as a genotoxic effect of extremely-low-frequency electromagnetic fields (ELF-EMF, 50 Hz, 1 mT) was studied in human amniotic fluid cells (AFC) after continuous exposure to magnetic fields (MF), oriented horizontally and vertically with respect to the surface of the culture medium, at different time points. To compare the effectiveness of different exposure systems, a Helmholtz-coil system and a so-called Merritt-coil system was used. A statistically significant increase in MN frequency could be detected in exposed cells compared to controls after 72 h continuous exposure to MF applied vertically in the Merritt-coil system, while no effect was found after exposure in the Helmholtz-coil system. Furthermore, a significant increase in MN induction occurred after 24, 48 and 72 h exposure to MF applied horizontally in the Helmholtz-coil system in comparison to controls, whereas horizontally MF generated in the Merritt-coil system induced no genotoxic effects. To exclude suppression of indirect EMF-induced DNA-lesions, we studied MN formation in the presence of N-Acetyl-p-aminophenol (APAP, Paracetamol(R)), which is an inhibitor of DNA-repair mechanisms. We found a dose-dependent increase of MN formation in APAP-treated AFC cells, but no significant further increase in MN frequency after additional MF exposure. Therefore we conclude, that EMF-induced MN formation is not caused by directly or indirectly induced clastogenic mechanisms. The obtained results show that the orientation of MF with respect to the cell culture dish and the physical condition of the exposure system is of major importance for the induction of micronuclei in certain cell types. Therefore, the reason for inconsistent results published in the literature may be caused by the variability of exposure systems, the exposure conditions and the cell types used.

Paracetamol increases sensitivity to ultraviolet (UV) irradiation, delays repair of the UNG-gene and recovery of RNA synthesis in HaCaT cells

We have studied the effect of low levels of paracetamol (0.3 and 1.0 mM) on gene-specific DNA repair, recovery of total RNA synthesis and cytotoxicity after exposure of human keratinocyte cells (HaCaT) to ultraviolet (UV) irradiation. Repair of cyclobutane pyrimidine dimers (CPDs) was measured in the transcriptionally active uracil-DNA glycosylase (UNG) and c-MYC loci. Repair of both strands in the UNG gene was consistently lower in the presence of paracetamol, but this reduction reached significance only at 8 h after irradiation and no dose-response was observed. For the c-MYC gene, we found no significant effect of paracetamol on the repair of CPDs, possibly because UV-irradiation is known to induce transcription of the c-MYC gene and enhanced transcription coupled repair might counteract a negative effect of paracetamol on global genome repair. A dose-dependent delay in the recovery of total RNA synthesis after UV exposure was observed in the presence of paracetamol, which also caused a 20% increase in UV-induced cytotoxicity after 24 h. Paracetamol had no significant effect on either RNA synthesis or cell survival in the absence of UV after 24 h, but reduced cell survival by approximately 10% (at 0.3 mM) and 50%, (at 1.0 mM) after 96 h exposure. Our results demonstrate that paracetamol may inhibit gene-specific repair of CPDs by affecting global genome repair and that different genes may be differentially affected.

DNA damage, gadd153 expression, and cytotoxicity in plateau-phase renal proximal tubular epithelial cells treated with a quinol thioether

2-Bromo-bis-(glutathion-S-yl)hydroquinone [2-Br-bis-(GSyl)HQ] causes DNA single-strand breaks (SSB), causes growth arrest, induces the expression of gadd153 (a gene inducible by growth arrest and DNA damage), and decreases histone H2B mRNA in log-phase renal proximal tubular epithelial cells (LLC-PK1). Renal epithelial cells in vivo normally exhibit a low mitotic index, therefore experiments in both plateau- and log-phase cells are necessary for a comprehensive understanding of the stress response to 2-Br-bis-(GSyl)HQ. In the present article we demonstrate that not all features of the stress response in log-phase cells are reproduced in plateau-phase cells. Thus, although 2-Br-bis-(GSyl)HQ causes concentration and time-dependent increases in DNA SSB, and increases the expression of gadd153, histone H2B mRNA levels are unaltered in plateau-phase cells. The relationship between reactive oxygen species, DNA damage, gene expression, and cytotoxicity was also investigated. Our findings suggest that (i) 2-Br-bis-(GSyl)HQ-mediated DNA damage in LLC-PK1 cells is mediated by the generation of H2O2; (ii) DNA damage, either directly or indirectly, contributes to cell death; and (iii) DNA damage, either directly or indirectly, provides the initial signal for gadd153 expression. In addition, DNA repair is rapid in LLC-PK1 cells, and the DNA-repair inhibitors 1-beta-D-arabinofuranosylcytosine and hydroxyurea have no effect on the amount of DNA SSB. Although the addition of 3-aminobenzamide following 2-Br-bis-(GSyl)HQ exposure has no effect on the removal of DNA SSB, it causes a slight but significant increase in gadd153 expression and cell viability, indicating that activation of poly(ADP-ribose)polymerase may exacerbate toxicity. Finally, aurintricarboxylic acid did not prevent DNA SSB or cytotoxicity in 2-Br-bis-(GSyl) HQ-treated LLC-PK1 cells, implying that activation of endonucleases does not play a role in these processes.

DNA damage induced by 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) in HL-60 cells and purified DNA in vitro

Chlorinated tap water often contains 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), which is a potent directly acting bacterial mutagen. We have investigated the induction of DNA damage by MX in a promyelocytic human leukaemia cell line (HL-60 cells). Exposure of HL-60 cells to 100-300 microM MX resulted in increased levels of DNA single-strand breaks and/or alkali-labile sites (SSBs) as detected by alkaline filter elution. When adding inhibitors of DNA break repair (AraC plus hydroxyurea), increased levels of DNA SSBs were observed at very low concentrations (1-3 microM) of MX, as observed by both alkaline filter elution and the single-cell gel electrophoresis assay. Increased DNA SSBs could also be observed if DNA repair inhibitors were added immediately after exposure to 10 microM MX, indicating that low concentrations of MX cause a relatively stable modification of DNA that may be recognized and incised by DNA repair enzyme activities. Further studies with DNA break repair inhibitors indicated that HL-60 cells exposed to 10 microM MX for 1 h repaired 50% of their initial DNA damage during a 2-h period and the repair appeared to be complete at 22 h. Analysis of MX-treated DNA by sequencing methods indicated that MX preferentially reacts with guanines in DNA.

DNA damage induced by etoposide; a comparison of two different methods for determination of strand breaks in DNA

Etoposide induces DNA damage to cells by interacting with the nuclear enzyme topoisomerase II. In this investigation the human lymphoblastic leukemia cell line (CEM) was used to study induction of DNA-strand breaks and cellular drug uptake after treatment with etoposide at a concentration of 0.5-2 micrograms/ml. High performance liquid chromatography was used for determination of etoposide concentrations. The alkaline elution assay and the DNA unwinding technique were compared for quantifying strand breaks in DNA induced by etoposide. The concentrations required to increase the level of DNA damage significantly was as follows: the DNA unwinding technique, 0.20 microgram/ml; the alkaline elution assay with proteinase K, 0.45 microgram/ml; the alkaline elution assay without proteinase K, 0.60 microgram/ml. When the half-life was adjusted, considering the efflux time of etoposide from cells, it was found to be only a few minutes. The present data show that the DNA unwinding technique is to be preferred for the screening of DNA damage. This technique is easier and quicker to perform than the alkaline elution technique.

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

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