Formaldehyde inhibits UV-induced phosphorylation of histone H2AX.

Formaldehyde (FA) is widely known to cause DNA damage. Recently, our study showed that FA can also inhibit a repair process of DNA damage, nucleotide excision repair (NER). DNA damage response (DDR) involving activation of phosphorylation pathways is important for the accuracy of the repair process, and the inhibition of the accurate repair would raise mutation rate, leading to cancer. We herein investigated whether FA influences phosphorylation of histone H2AX (γ-H2AX), an intermediate player of DDR signaling pathways. Human keratinocytes HaCaT were treated with FA and then exposed to UV known to generate clear γ-H2AX signal. UV-induced γ-H2AX was inhibited by FA in a dose-dependent manner. The repair of pyrimidine dimers was inhibited by FA, while the recruitments of γ-H2AX-related proteins, Mre11 and 53BP1, to damaged sites were also delayed. Mre11, Nbs-1, H2AX and ATM were not degraded after treatment with FA as opposed to NER-related protein, TFIIH. On the other hand, FA inhibited phosphorylation of ATM which acts upstream of γ-H2AX. These results suggest that FA can affect the repair of DNA damage via inhibition of the phosphorylation pathways of H2AX.

The PIGRET assay, a method for measuring Pig-a gene mutation in reticulocytes, is reliable as a short-term in vivo genotoxicity test: Summary of the MMS/JEMS-collaborative study across 16 laboratories using 24 chemicals.

The in vivo mutation assay using the X-linked phosphatidylinositol glycan class A gene (Pig-a in rodents, PIG-A in humans) is a promising tool for evaluating the mutagenicity of chemicals. Approaches for measuring Pig-a mutant cells have focused on peripheral red blood cells (RBCs) and reticulocytes (RETs) from rodents. The recently developed PIGRET assay is capable of screening >1×106 RETs for Pig-a mutants by concentrating RETs in whole blood prior to flow cytometric analysis. Additionally, due to the characteristics of erythropoiesis, the PIGRET assay can potentially detect increases in Pig-a mutant frequency (MF) sooner after exposure compared with a Pig-a assay targeting total RBCs (RBC Pig-a assay). In order to test the merits and limitations of the PIGRET assay as a short-term genotoxicity test, an interlaboratory trial involving 16 laboratories was organized by the Mammalian Mutagenicity Study Group of the Japanese Environmental Mutagenicity Society (MMS/JEMS). First, the technical proficiency of the laboratories and transferability of the assay were confirmed by performing both the PIGRET and RBC Pig-a assays on rats treated with single doses of N-nitroso-N-ethylurea. Next, the collaborating laboratories used the PIGRET and RBC Pig-a assays to assess the mutagenicity of a total of 24 chemicals in rats, using a single treatment design and mutant analysis at 1, 2, and 4 weeks after the treatment. Thirteen chemicals produced positive responses in the PIGRET assay; three of these chemicals were not detected in the RBC Pig-a assay. Twelve chemicals induced an increase in RET Pig-a MF beginning 1 week after dosing, while only 3 chemicals positive for RBC Pig-a MF produced positive responses 1 week after dosing. Based on these results, we conclude that the PIGRET assay is useful as a short-term test for in vivo mutation using a single-dose protocol.

Measuring reproducibility of dose response data for the Pig-a assay using covariate benchmark dose analysis.

The reproducibility of the in vivo Pig-a gene mutation test system was assessed across 13 different Japanese laboratories. In each laboratory rats were exposed to the same dosing regimen of N-nitroso-N-ethylurea (ENU), and red blood cells (RBCs) and reticulocytes (RETs) were collected for mutant phenotypic analysis using flow cytometry. Mutant frequency dose response data were analysed using the PROAST benchmark dose (BMD) statistical package. Laboratory was used as a covariate during the analysis to allow all dose responses to be analysed at the same time, with conserved shape parameters. This approach has recently been shown to increase the precision of the BMD analysis, as well as providing a measure of equipotency. This measure of equipotency was used here to demonstrate a reasonable level of interlaboratory reproducibility. Increased reproducibility could have been achieved by increasing the number of cells scored, as this would reduce the number of zero values within the mutant frequency data. Overall, the interlaboratory trial was successful, and these findings support the transferability of the in vivo Pig-a gene mutation assay.

Pyrene did not induce gene mutation in red blood cell Pig-a assay and PIGRET assay in rats.

A new in vivo gene mutation assay has been developed based on the phosphatidylinositol glycan anchor biosynthesis, Class A gene (Pig-a in rodents) as an endogenous reporter. Although a large number of chemicals have been evaluated in the rat Pig-a assay in 28-day repeat dose regimens, there was limited reporting of rat Pig-a assay after a single dose. A collaborative study by the Mammalian Mutagenicity Study group, which is a subgroup of the Japanese Environmental Mutagen Society, was conducted to verify the usefulness of the rat Pig-a assay after a single dose as a short-term genotoxicity test. As a part of this collaborative study, the in vivo mutagenicity of a single dose of pyrene (Pyr) was investigated in the red blood cell (RBC Pig-a assay) and in reticulocytes (PIGRET) of rats. Eight-week old male rats were orally dosed with Pyr at 500, 1000, and 2000 mg/kg or ethylnitrosourea (ENU) at 10 and 40 mg/kg as a positive control. The animals in each group were examined for Pig-a mutant frequencies (MF) except for animals in the 2000mg/kg group because of mortality or severe toxicity. The Pig-a MF in RBCs and reticulocytes, as CD59 negative cells, were evaluated once a week for 4 weeks after the dosing. With a single exposure to ENU, the Pig-a MF in both RBCs and reticulocytes increased in a time- and dose-dependent manner. In contrast, no statistically significant effect was observed in rats dosed with Pyr at 500 and 1000 mg/kg. Therefore, Pyr was concluded to be negative in the RBC Pig-a assay and the PIGRET assay after a single oral administration in rats. The result was consistent with previously reported Pig-a assays with repeat dose regimens.

Red blood cell Pig-a assay and PIGRET assay in rats with azathioprine.

A new in vivo gene mutation assay has been developed based on the phosphatidylinositol glycan anchor biosynthesis, Class A gene (Pig-a in rodents) as an endogenous reporter. Using this Pig-a assay, the in vivo mutagenicity of a single dose of azathioprine (Aza) was investigated in red blood cells (RBC Pig-a assay) and reticulocytes (PIGRET) of rats. Eight-week old male rats were orally dosed once with Aza at 50, 100 and 200mg/kg or ethylnitrosourea (ENU) at 10 and 40mg/kg as a positive control. Because 4 out of 6 animals at 200mg/kg of Aza died 3days after the dosing, this dose group was excluded for analyses. The frequencies of Pig-a mutants in RBCs and reticulocytes (RET) were evaluated once a week for 4 weeks after the treatment. With a single exposure to ENU, the frequencies of Pig-a mutants in both RBCs and RETs increased in a time- and dose-dependent manner. In contrast, with Aza small effects that were not statistically significant were observed in rats at 21 and 14days in the RBC Pig-a and PIGRET assays respectively. Based on the present results, the mutagenic potential of Aza is negligible after single oral administration in rats.

Cigarette sidestream smoke induces histone H3 phosphorylation via JNK and PI3K/Akt pathways, leading to the expression of proto-oncogenes.

Post-translational modifications in histones have been associated with cancer. Although cigarette sidestream smoke (CSS) as well as mainstream smoke are carcinogens, the relationship between carcinogenicity and histone modifications has not yet been clarified. Here, we demonstrated that CSS induced phosphorylation of histones, involving a carcinogenic process. Treatment with CSS markedly induced the phosphorylation of histone H3 at serine 10 and 28 residues (H3S10 and H3S28), which was independent from the cell cycle, in the human pulmonary epithelial cell model, A549 and normal human lung fibroblasts, MRC-5 and WI-38. Using specific inhibitors and small interfering RNA, the phosphorylation of H3S10 was found to be mediated by c-jun N-terminal kinase (JNK) and phosphoinositide 3-kinase (PI3K)/Akt pathways. These pathways were different from that of the CSS-induced phosphorylation of histone H2AX (γ-H2AX) mediated by Ataxia telangiectasia-mutated (ATM) and ATM-Rad3-related (ATR) protein kinases. A chromatin immunoprecipitation assay revealed that the phosphorylation of H3S10 was increased in the promoter sites of the proto-oncogenes, c-fos and c-jun, which indicated that CSS plays a role in tumor promotion. Because the phosphorylation of H3S10 was decreased in the aldehyde-removed CSS and was significantly induced by treatment with formaldehyde, aldehydes are suspected to partially contribute to this phosphorylation. These findings suggested that any chemicals in CSS, including aldehydes, phosphorylate H3S10 via JNK and PI3K/Akt pathways, which is different from the DNA damage response, resulting in tumor promotion.

Formaldehyde-induced histone H3 phosphorylation via JNK and the expression of proto-oncogenes.

Formaldehyde (FA) is a very reactive compound that forms DNA adducts and DNA-protein crosslinks, which are known to contribute to FA-induced mutations and carcinogenesis. Post-translational modifications to histones have recently attracted attention due to their link with cancer. In the present study, we examined histone modifications following a treatment with FA. FA significantly phosphorylated histone H3 at serine 10 (H3S10), and at serine 28 (H3S28), the time-course of which was similar to the phosphorylation of H2AX at serine 139 (γ-H2AX), a marker of DNA double strand breaks. The temporal deacetylation of H3 was observed due to the reaction of FA with the lysine residues of histones. The phosphorylation mechanism was then analyzed by focusing on H3S10. The nuclear distribution of the phosphorylation of H3S10 and γ-H2AX did not overlap, and the phosphorylation of H3S10 could not be suppressed with an inhibitor of ATM/ATR, suggesting that the phosphorylation of H3S10 was independent of the DNA damage response. ERK and JNK in the MAPK pathways were phosphorylated by the treatment with FA, in which the JNK pathway was the main target for phosphorylation. The phosphorylation of H3S10 increased at the promoter regions of c-fos and c-jun, indicating a relationship between FA-induced tumor promotion activity and phosphorylation of H3S10. These results suggested that FA both initiates and promotes cancer, as judged by an analysis of histone modifications.

Distinct DNA methylation patterns of lysophosphatidic acid receptor genes during rat hepatocarcinogenesis induced by a choline-deficient L-amino acid-defined diet.

Altered expressions of lysophosphatidic acid (LPA) receptor genes have been reported in tumor cells of human and rats. Recently, we detected the frequent mutations of LPA receptor-1 (LPA1) gene in rat hepatocellular carcinomas (HCCs) induced by a choline-deficient L-amino acid-defined (CDAA) diet. In this study, the DNA methylation patterns of LPA receptor genes and their expression levels during rat hepatocarcinogenesis induced by the CDAA diet were investigated. Six-week-old F344 male rats were continuously fed with the CDAA diet, and animals were then killed at 7 days and 2, 12, 20, and 75 weeks, respectively. Genomic DNAs were extracted from livers and HCCs for the assessment of methylation status by bisulfite sequencing, comparing to normal livers. The livers of rats fed the CDAA diet were unmethylated in LPA1 and LPA2 genes as well as normal livers. In LPA3 gene, although normal livers were unmethylated, the livers at 7 days and 2 and 12 weeks weakly or moderately methylated and those at 20 weeks markedly methylated. Moreover, 4 HCCs were completely methylated in LPA3 gene. Expression levels of LPA receptor genes in the livers of rats fed the CDAA diet and HCCs were correlating with DNA methylation status. These results indicate that DNA methylation status of the LPA3 gene was disturbed in the livers of rats fed the CDAA diet and established HCCs, suggesting that alterations of the LPA receptor genes might be involved during rat hepatocarcinogenesis induced by the CDAA diet.