We investigated the relationship between DNA damage detected as single strand breaks using a Comet assay and the expression of glutathione S-transferase P-form (GST-P) as a marker of the early stage of carcinogenesis. In this study, 4-week-old rats were exposed to 10 or 40 ppm diethylnitrosamine (DEN) in drinking water for 14 days, then both DNA damage and GST-P expression were evaluated in the three primary lobes [left (L), right median (RM) and right anterior (RA)] of the liver to clarify the correlation between two endpoints within the organ over time. Both DNA damage and GST-P expression increased almost in parallel in a dose- and time-dependent manner in all lobes, and those had a good relationship with heterogeneous response, L≥RM>RA. Therefore, it was confirmed that the result of the Comet assay could correspond to the histopathological changes appearing in the initiation stage in rat hepatocytes.
The Ames Salmonella/microsome assay (Ames test) is a convenient method for screening mutagens in our diet including those in drinking water. In this study, we assayed the mutagenicity levels of tap water in Kitakyushu-city and of humic acid solutions treated with chlorine. The amount of chlorine added was calculated to maintain the residual chlorine level constant at 20°C as in city office tap water. The samples were concentrated with adsorbent (CSP800) and the mutagenic activity was assayed with Salmonella typhimurium TA100 and TA98 strains with or without S9 mix. The tap water was analyzed for volatile organic compounds and some factors in conventional water quality monitoring every two months from March 1998 to January 2000. The tap water samples tested showed mutagenicity on strain TA100 without S9 mix. The mutagenicity of the samples tended to be higher from winter to spring than that from summer to fall. Chlorine-treated humic acid solutions were used as a model to examine the effect of the concentrations of humic acid and chlorine, and the temperature on the mutagenicity. The descending order of sensitivity to mutagenicity was TA100 without S9 mix, TA98 without S9 mix, and TA100 with S9 mix; no mutagenicity was observed for TA98 with S9 mix. Mutagenicity seemed to increase with increasing concentrations of humic acid and chlorine, and with lowering the water temperature. The observed seasonal variation of mutagenicity of the tap water may be partly explained by the rainfall and the water temperature during the rainy season, from summer to fall, because the organic substances of river water decreased and the water temperature increased over that season.
Aneuploidy can have significant impact upon human health including reproductive problems and cancer. However, there is no test method currently that is widely used in routine screening assay for aneuploidy detection. In this study, in order to develop a routinely available screening assay, we evaluated the efficacy of our in vitro micronucleus (MN) test protocol. The MN test with a Chinese hamster lung cell line (CHL/IU) without cytochalasin B was conducted on seven chemicals, namely, five potential aneugens which induced polyploidy in the in vitro chromosomal aberration (CA) test and whose mechanism of action is unknown, and two well-known aneugens (colcemid and diethylstilbestrol). Besides micronucleated (MN) cells (parameter widely used for detecting clastogens and aneugens in the existing protocols), polynuclear (PN) cells and mitotic (M) cells were used for assessment of aneuploidy induction. In vitro CA tests were also performed to compare with the MN test. All seven chemicals induced polyploidy in the CA tests. While three chemicals significantly increased the frequency of MN, PN, and M cells in the MN tests, the other four chemicals increased only the frequency of PN cells. Based on the patterns of appearance of aberrant cells induced by test chemicals, the seven chemicals could be classified into two groups. It is thought that the difference in the patterns between the two groups reflects the difference in mechanisms of action. Our results indicate a possibility that aneuploidy and polyploidy can be distinguished by the analysis of patterns of appearance of aberrant cells in the MN test. We have concluded that our MN test protocol, easier-to-perform, and more sensitive for detecting potential aneugens as compared with the existing protocols, may be useful for routine screening.
In the present study, we investigated the mechanism for the decrease in levels of serum thyroid hormones, especially thyroxine (T4) , by polychlorinated biphenyls (PCBs) such as Kanechlor-500 (KC500) , 2,2’,4’,5,5’-pentachlorobiphenyl (PentaCB) , and 2,2’,3’,4’,5,6-hexachlorobiphenyl (HexaCB) , and studied species differences among mice, hamsters, rats, and guinea pigs in the PCB effect. Significant decrease in serum total T4 level by KC500 was observed in all four species. On the other hand, there were differences in the level of decrease of serum total T4 level by PentaCB and HexaCB. Differences in the level of hepatic methylsulfonyl-PCB metabolites of KC500, PentaCB and HexaCB, which were thought to be associated with the PCB-toxic effects, did not necessarily correlate with the magnitude of decrease in serum total T4 level. Likewise, the induction of UDP-glucuronosyltransferases (T4-UDP-GT) toward T4 by PCB did not necessarily correlate with the decrease in serum T4 level in the animals used. Further studies on transthyretin (TTR) and serum T4-transporter suggested that decrease in serum total T4 level induced by PCB occurred not only by induction of T4-UDP-GT but also by the alteration of levels of T4-TTR binding and hepatic T4-transporter. In addition, species difference in the decrease of serum total T4 was associated with various PCB-induced total effects, including induction of T4-UDP-GT, decrease in T4-TTR binding level, the increase of hepatic thyroid hormone transporter, and other thyroid function correlates.
Levels of cholesterol 7α- and 7β-hydroperoxides (Ch 7-OOHs) in the skin of rats were investigated under different conditions in vivo such as aging or UVB-irradiation, and the results suggest that Ch 7-OOHs are good markers for lipid peroxidation. In the rat of any age, liver had no detectable level of Ch 7-OOHs. The Ch 7-OOHs were glutathione (GSH)-dependently reduced to the corresponding alcohols by GSH peroxidase (GSH Px) and only by the Alpha-class GSH S-transferases (GSTs) A1-1 and A1-3 among all isoforms of the GSTs occurring in the liver. On the other hand, rat skin had a very low level of GSH Px compared with the liver and lacked the subunit A1-bearing GSTs. Therefore, the absence or the presence of low levels of the enzyme systems which scavenge the cholesterol hydroperoxides in the skin of rats appears to be one of the main reasons for 1) the presence of Ch 7-OOHs in the skin of rats, 2) the increase in the levels of Ch 7-OOHs with age of rats or by UVB-irradiation to rats. Enzyme, Western blot, and immunohistochemical analyses indicated that rat skin cytosol contained no detectable level of the Alpha-class GSTA4-4 which catalyzes the GSH conjugation of the toxic product, 4-hydroxy-2(E)-nonenal (HNE) , nonenzymatically formed from n-6 polyunsaturated fatty acid residues of lipids by lipid peroxidation. Rats irradiated with single doses (4,000-24,000 mJ/cm2) of UVB markedly expressed rGSTA4-4 in the skin at a level one-fifth that of the liver; at a single dose of 24,000 mJ/cm2, there was apparent specific activity toward HNE. The glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase was irreversibly and (S)-selectively inactivated by the enantiomers of racemic HNE. Also, (S)-HNE was a much more potent agent than (R)-HNE for cell growth inhibition and induction of apoptosis. In rat liver cytosol, the HNE was detoxified 2.5- fold more (S)-selectively by GSH conjugation. The minor GST isoforms, A4-4, had a major role in the cytosolic (S)-selective GSH conjugation in the rat liver.
Chk1 is required for arrest of the mammalian cell cycle before mitosis in response to DNA damage or replication block. It is also implicated in regulation of cell cycle progression because it is essential for embryonic cell viability. With the use of mouse embryonic stem cells conditionally deficient in Chk1, we now show that this kinase is indispensable for the timing of mitotic initiation. Chk1 deficiency resulted in premature onset of mitosis through reduction of Cdc2 phosphorylation on Tyr15 as a result of increased Cdc25 activity and decreased Wee1 activity. Our results suggest that Chk1 regulates Cdc2 to establish proper timing of mitotic initiation during the mammalian embryonic cell cycle.
Reactive oxygen species generated by environmental factors, such as radiation, UV and chemicals can cause sequence-specific DNA damage and play important roles in mutagenesis and carcinogenesis. We have investigated sequence specificity of oxidative stress-mediated DNA damage by using 32P-labeled DNA fragments obtained from the human c-Ha-ras-1, p53 and p16 genes. Free hydroxyl radicals cause DNA damage with no marked site specificity. Copper-hydroperoxo complex caused DNA damage at thymine, cytosine and guanine residues. 1O2 preferentially induces lesions at guanine residues. Benzoyloxyl radical specifically causes damage to the 5’-G in GG sequence; this sequence is easily oxidized because a large part of the highest occupied molecular orbital of this radical is distributed on this site. Recently, we demonstrated that BP-7,8-dione, a metabolite of carcinogenic benzo [a] pyrene (BP) , strongly damaged the G and C of the 5’-ACG-3’ sequence complementary to codon 273 of the p53 gene in the presence of NADH and Cu (II) . BP-7,8-dione also caused preferential double base lesion at 5’-TG-3’ sequences. Since clustered DNA damage is poorly repaired, it is speculated that induction of the double base lesions in DNA might lead to activation of proto-oncogene or inactivation of the tumor suppressor gene. Therefore, oxidative DNA damage induced by BP-7,8-dione, especially double base lesions, may participate in the expression of carcinogenicity of BP in addition to DNA adduct formation. Here, we discuss the mechanisms of sequence-specific DNA damage including clustered DNA damage in relation to mutagenesis and carcinogenesis.
Frameshift (deletion) is induced by many types of DNA damage in cells. However, the mechanism by which deletions are generated has not been extensively explored. The number of deletions during DNA synthesis catalyzed by the 3’→5’ exonuclease-free (exo-) Klenow fragment of Escherichia coli DNA polymerase I (pol I) was determined systematically on dG-acetylaminofluorene (dG-AAF)-modified oligodeoxynucleotides templates with different bases 3’ and/or 5’ to the lesion. Under conditions where the dNMP (deoxynucleoside 3’-monophosphate) positions opposite dG-AAF can pair with its complementary base at the 5’flanking position, one-base deletions are produced. Since the relative frequency of base insertion opposite the lesion followed the order: dCMP>dAMP>dGMP>dTMP, frequency of generating deletions paralleled to the insertion frequency of dNTP opposite the lesion. Inhibition of chain extension from the dC:dG-AAF pair also be involved in the formation of deletions. These results were supported by steady-state kinetic studies. Two and more base deletions were formed in a similar manner to that observed for one-base deletions. When the dG-AAF-modified templates containing iterated bases 5’ to the lesion were used, shorter deletions predominated. The formation of deletions was reduced when exo+ Klenow fragment was used, suggesting that the proofreading function of the enzyme minimizes the deletion formation. Thus, the ability of generating deletions depends on the (a) the nature of base inserted opposite the lesion, (b) sequence context to the lesion, and (c) the overall rate of translesion DNA synthesis past the lesion. The mechanism for deletions by E. coli DNA pol I may be applied to predict the nature of deletions generated by a variety of DNA adducts and by other prokaryotic and eukaryotic DNA polymerases.
Since the discovery of 8-OH-dG formation, various aspects of oxidative DNA damage have been studied. In the present decade, we have studied the chemistry of oxidative DNA damage, its mutagenic effects, and its repair mechanisms. We are also analyzing 8-OH-dG levels in human samples, such as leukocytes and urine, as a marker of oxidative DNA damage, to evaluate the oxidative stress of individuals. In the meantime, many groups have initiated studies of the biological significance of 8-OH-dG, and more than 2000 papers about 8-OH-dG have been published thus far. In this review, we will summarize our resent research progress on oxidative damage.
Reactive oxygen species from endogenous and exogenous sources induce varieties of oxidative damage to DNA. The response of DNA glycosylases and DNA polymerases to such damage is best studied by using defined oligonucleotide substrates containing unique lesions. Recently, chemical synthesis of oligonucleotides containing the stereoisomers of thymine glycol and the formamidopyrimidine derivatives of guanine and adenine has been established. NEIL1, NEIL2, and SMUG1 are the new members of mammalian DNA glycosylases involved in the repair of oxidative DNA damage. Their repair function has been assessed by the analyses of damage specificity of purified proteins and cellular activity. The lack of overt phenotypes of mice deficient in DNA glycosylases for oxidative damage (NTH1, OGG1, and MYH) suggest more elaborate repair networks and functional redundancy of participating DNA glycosylases in mammals than in prokaryotes.
Altered oxidative metabolism is a property of many tumor cells. Oxidation of dNTP pool as well as DNA is a source of genome instability. We report here that two Y-family DNA polymerases of the archaeon Sulfolobus solfataricus strains P1 and P2 incorporate oxidized dNTPs into nascent DNA in an erroneous manner: the polymerases exclusively incorporate 8-hydroxy-dGTP opposite template A, and incorporate 2-hydroxy-dATP opposite G and T. Extension onto the incorporated analogs is only slightly reduced. Human DNA polymerase η, a member of human Y-family DNA polymerases, incorporates the oxidized dNTPs in a similar erroneous manner. These DNA polymerases are shown to bypass a variety of DNA lesions. Thus, our results suggest the Y-family DNA polymerases promote mutagenesis through the erroneous incorporation of the oxidized dNTPs during DNA synthesis in addition to facilitating translesion DNA synthesis.
Ethylene oxide (EO) gas sterilization is one of the most frequently used sterilization methods in medical device manufacture. However, considering that EO is a genotoxic carcinogen, the allowable limits of the EO residue in medical devices after sterilization have been discussed in the international standardization working group, ISO/TC194/WG11, and adopted as an ISO standard, ISO 10993-7 in 1995. This article describes the rationale of these allowable limits and is especially focused on risk assessment procedures.
A quantitative genetic risk assessment has been recognized as one of the goals of the research of environmental mutagens. To establish the genetic risk assessment as a practice routinely applicable to daily tasks, a collaborative study was carried out from 1996 to 1999 in the Mammalian Mutagenesis Study Group. Mitomycin C was selected as a model compound and its genetic risk was quantitatively assessed for autosomal dominant and X linked genetic diseases. In this paper, outcomes of the study are presented together with the recent progress in a genetic risk assessment on radiation.
An updated short-review of the carcinogenicity and the genotoxicity studies with acrylamide (AA) is provided. The review includes: (i) chemical and physical data of AA. (ii) occurrence of AA in foods. (iii) carcinogenicity of AA in rats and mice. (iv) AA exposure (occupational and dietary) and cancer in human. (v) genotoxicity of AA in vivo and in vitro. (vi) metabolism of AA. (vii) genotoxicity of glycidamide, an epoxide metabolite of AA. (viii) binding of AA to proteins and nucleic acids. (ix) risk assessment of AA in food, including lifetime cancer hazards after exposure to AA. (x) threshold of carcinogenicity of AA (Application of Rozman scale to AA carcinogenicity). In addition to the description of data reported, data gaps and priority research needs are discussed.
Kojic acid (KA) , belonging to existing food additives for which compositions or usages are not clarified, had been used for prevention of enzymatic browning. In 1995, the food sanitation law was largely revised to harmonize with JECFA, OECD and FDA. Under the new law, reevaluation of existing food additives was required. In 1998, it was found that KA induced tumors in the thyroid and liver of mice. KA also showed genotoxicities; gene mutations in S. typhimurium, chromosome aberrations in CHO-K1 and CHL/IU cells in vitro, and micronuclei in the liver of mice and hematopoietic cells in rats. Although it has not been clarified whether liver or thyroid tumors were induced by genotoxic effects of KA or not, use of KA as a food additive was banned in 2003, based on the fact that KA was not used in any country at that time. The ad hoc committee which was set-up for a three-year task from 2003-2005 considered that KA was an appropriate model compound to re-evaluate the strategies presently used to detect genotoxicity in vitro and in vivo, and to re-evaluate the regulatory rules (use of genotoxic carcinogens as food additives should be totally avoided; genotoxic non-carcinogens in rodents can be used as food additives). First of all, we confirmed the genotoxicity of KA; we demonstrated that genotoxicity in S. typhimurium was due to KA itself, but not due to contaminants, KA induced TK mutations, micronuclei and DNA damage (Comet) in human lymphoblastoid cells, TK6 and WTK-1. These results support the finding that KA is genotoxic in vivo, although it is not clear yet whether KA induces tumors by its genotoxicity or not. Speculating that liver tumors induced by KA were due to its genotoxicity, human risks to KA to which humans are exposed by taking fermented food products was calculated to be 2×10-7 by the linearized multistage model.