Induction of ouabain-resistance mutation and cycle-dependent transformation of C3H/10T1/2 cells by N-nitroso-2-acetylaminofluorene

Lack of mismatch repair enhances resistance to methylating agents for cells deficient in oxidative demethylation

The human alkylation B (AlkB) homologs, ALKBH2 and ALKBH3, respond to methylation damage to maintain genomic integrity and cellular viability. Both ALKBH2 and ALKBH3 are direct reversal repair enzymes that remove 1-methyladenine (1meA) and 3-methylcytosine (3meC) lesions commonly generated by alkylating chemotherapeutic agents. Thus, the existence of deficiencies in ALKBH proteins can be exploited in synergy with chemotherapy. In this study, we investigated possible interactions between ALKBH2 and ALKBH3 with other proteins that could alter damage response and discovered an interaction with the mismatch repair (MMR) system. To test whether the lack of active MMR impacts ALKBH2 and/or ALKBH3 response to methylating agents, we generated cells deficient in ALKBH2, ALKBH3, or both in addition to Mlh homolog 1 (MLH1), another MMR protein. We found that MLH1koALKBH3ko cells showed enhanced resistance toward SN1- and SN2-type methylating agents, whereas MLH1koALKBH2ko cells were only resistant to SN1-type methylating agents. Concomitant loss of ALKBH2 and ALKBH3 (ALKBH2ko3ko) rendered cells sensitive to SN1- and SN2-agents, but the additional loss of MLH1 enhanced resistance to both types of damage. We also showed that ALKBH2ko3ko cells have an ATR-dependent arrest at the G2/M checkpoint, increased apoptotic signaling, and replication fork stress in response to methylation. However, these responses were not observed with the loss of functional MLH1 in MLH1koALKBH2ko3ko cells. Finally, in MLH1koALKBH2ko3ko cells, we observed elevated mutant frequency in untreated and temozolomide treated cells. These results suggest that obtaining a more accurate prognosis of chemotherapeutic outcome requires information on the functionality of ALKBH2, ALKBH3, and MLH1.

Activation of aminoimidazole carcinogens by nitrosation: mutagenicity and nucleotide adducts

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) are heterocyclic amines (HCAs) derived from high temperature cooking of meat and thought to cause colon cancer in humans. Reactive nitrogen oxygen species, which are mediators of the inflammatory response, can convert these amines to the corresponding N-nitrosamines, N-NO-IQ and N-NO-MeIQx. This study was designed to evaluate whether these N-nitrosamines are genotoxic and could be responsible, in part, for the high incidence of colon cancer in individuals with colitis. Such an association would counsel reduced intake of well-done red meat by colitis patients. Mutagenicity was evaluated by reversion of a lacZ frameshift allele in three different E. coli strains. Strains DJ701 and DJ702 express recombinant(S. typhimurium) aromatic amine N-acetyltransferase (NAT); DJ702 also expresses recombinant human cytochrome P450 1A2 and NADPH-P450 reductase; and DJ2002 served as an N-acetyltransferase negative control. In strain DJ701, N-NO-IQ and N-NO-MeIQx elicited dose-dependent mutagenicity,which was not further increased in DJ702. Neither nitrosamine was mutagenic in strain DJ2002. While both N-nitrosamines are stable for >4 h (pH 7.4, 37 degrees C), they react with DNA or 2'-deoxyguanosine 3'-monophosphate at lower pH (5.5) to form adducts. HOCl, a component of the inflammatory response,increased adduct formation, as measured by 32P-postlabeling. Following treatment with nuclease P1and separation by two-dimensional thin-layer chromatography and then HPLC, N-NO-IQ and N-NOMeIQxwere shown to form the same adducts as those formed by N-OH-MeIQx or N-OH-IQ, namely N-(deoxyguanosin-8-yl) adducts. In summary, these N-nitrosamines are genotoxic and might be alternatives to their hydroxylamine analogues as activated intermediates leading to initiation of colon cancer in individuals with colitis.

Tumor promotion of N-nitroso-N-(3-keto-1, 2-butanediol)-3'-nitrotyramine derived from nitrosation of Maillard reaction product in CD-1 mice

N-Nitroso-N-(3-keto-1,2-butanediol)-3'-nitrotyramine (NO-NTA) is a product of a model browning system generated in the presence of sodium nitrite. Our previous study showed that NO-NTA had genotoxicity and proved to be an initiator and promoter on mouse C3H10T1/2 cells. In this study, a two-stage skin carcinogenesis protocol was used to promote CD-1 mouse skin carcinogenesis using NO-NTA. Twice weekly, for 38 weeks, topical application of NO-NTA at the concentration of 250 nmol to mice previously initiated with benzo(a)pyrene (BaP) caused 90% tumor incidence. However, no tumors were observed in mice treated with BaP or treated with NO-NTA alone. The NO-NTA-promoted tumors that were observed histologically in mice showed well-differentiated squamous cell carcinoma with invasion into the subcutaneous region. Application of the same amount of NO-NTA not only caused significant induction of hyperplasia but also epidermal ornithine decarboxylase (ODC) activity. Treatment of mouse skin (1 cm(2)) with various amounts of NO-NTA (10, 50, or 250 nmol) caused production of hydrogen peroxide by 1.63-, 1.91-, and 2. 38-fold, respectively, and marked induction of myeloperoxidase (MPO) by 21-, 39-, and 61-fold. These results indicate that NO-NTA is a new tumor promoter and may induce tumor promotion by oxidant stress in CD-1 mouse skin.

Promotional effect of N-nitroso-N-(3-keto-1,2-butanediol)-3'-nitrotyramine (a nitrosated Maillard reaction product) in mouse fibroblast cells

N-Nitroso-N-(3-keto-1,2-butanediol)-3'-nitrotyramine (NO-NTA) is a product of model browning system generated in the presence of sodium nitrite. The chemical structure of this compound has been confirmed by UV, mass and nuclear magnetic resonance, and infrared spectroscopy in our previous study. A two-stage transformation protocol was used to chemically transform the mouse embryo fibroblasts C3H10T1/2 cells. To initiate transformation, the cells were treated with benzo[a]pyrene (BaP) (0.1 mg/ml), and NO-NTA (0.01, 0.1 and 1 mg/ml) was employed subsequently to complete the transformation process. Malignant transformed foci were formed in BaP-initiated and NO-NTA promoted C3H10T1/2 cells after 8 wk. Cells treated with NO-NTA alone failed to induce transformation. However, cells initiated with BaP and promoted by cells initiated with BaP and promoted by NO-NTA demonstrated oncogenic properties. Cell lines transformed with NO-NTA-transformed colonies exhibited enhanced growth rate, anchorage independence and tumorigenicity in animals relative to parent cells. These results indicate that NO-NTA is a new tumour promoter and may induce tumour promotion by two-stage oncogenesis. Further studies on the mechanism of action of NO-NTA are now in progress.

Potential effect of sodium nitrite on the expression of nuclear proto-oncogenes during 2-acetyl aminofluorene-induced hepatocarcinogenesis in rats

2-acetyl aminofluorene (AAF) reacts in acidic conditions with nitrous fume yielding N-nitroso-AAF (N-NO-AAF), as previously described, that exerts more toxic and mutagenic effects than its parental compound. In this study, the effect of sodium nitrite (NaNO2) on the tumorigenicity of AAF in rats fed with AAF and NaNO2 was observed. Wistar rats were divided into five groups: group I served as control; group II were treated with NaNO2 (0.3%); group III was given 0.02% AAF alone; groups IV and V received both AAF and NaNO2 (0.2 and 0.3% respectively) in their diet for 12 weeks. At the end of the experiment, all rats in groups III, IV and V developed early stage phenomena of hepatocellular carcinoma, including hepatomegaly with variable-sized foci and neoplastic nodules. Severe damage was observed in the rats treated with AAF and NaNO2. Feeding of AAF (0.02%) for 3 months elevated the levels of c-Fos, c-Jun and c-Myc proteins in the rat livers. The AAF-induced c-Jun, c-Fos and c-Myc expressions were significantly magnified (P < 0.001) by NaNO2. These data confirmed that the strengthening of AAF-induced hepatocarcinogenesis by NaNO2 should be associated with its enhancing effect on the AAF-induced increases in the expressions of c-Jun, c-Fos and c-Myc.

Hepatotoxicity of 1,3,5-trinitro-2-acetyl pyrrole derived from nitrosation of Maillard reaction product in BALB/C mouse

1,3,5-Trinitro-2-acetyl pyrrole (TNAP) is a product derived from the reaction of 2-acetyl pyrrole with nitrite in the model of Maillard browning systems. This compound is moderately mutagenic to the Salmonella strains TA98 and TA100 and is markedly cytotoxic to mouse C3H10T1/2 cells. Experiments are performed to investigate the effects of TNAP on the hepatic toxicity in mouse. Male BALB/C mice were subjected to a dose of 7.2 mg/kg body weight twice a week by i.p. injection for 24 weeks, then followed by a feeding diet for 21 weeks. TNAP-treated mice showed an increase in mortality and time-dependent appearance of lesions in the liver. TNAP is hepatotoxic as demonstrated by a marked increase in the activities of serum alanine transaminase (ALT) and aspartic transaminase (AST). TNAP-related lesions observed histologically in mice, included hapatic atrophy, mild fatty metamorphosis with multilocular cysts in the liver. In conclusion, TNAP was considered to be a toxic compound in mice as evidenced by increased incidences of mortality, and lesions of liver.

N-Nitroso-N-(3-keto-1,2-butanediol)-3'-nitrotyramine. A new genotoxic agent derived from the reaction of tyrosine and glucose in the presence of sodium nitrite

N-Nitroso-N-(3-keto-1,2-butanediol)-3'-nitrotyramine (NO-NTA) is a product of a model browning system in the presence of sodium nitrite. In this study, the chemical structure is confirmed by spectral studies, including UV, mass spectrometry, nuclear magnetic resonance and infrared spectroscopy. NO-NTA is strongly genotoxic to the rat hepatocyte and is moderately cytotoxic to mouse C3H10T1/2 cells. Results obtained in this study indicate that NO-NTA inflicted DNA damage through the formation of a DNA adduct. In addition, C3H10T1/2 cells were treated with NO-NTA and, following addition of 12-O-tetradecanoylphorbol-13-acetate (TPA) as promotor, the increase of transformed foci indicated that NO-NTA could possibly be an initiator [corrected] of TPA tumor promotion. A transformed cell line from NO-NTA initiated and TPA promoted foci increased saturation density and growth ability in soft agar reactive to the control line. These results suggest that the formation of a genotoxic agent of nitroso-derivatives may take place in a nitrite-containing food system during processing and cooking.

Preferential promutagenic lesions at exons 7-8 of human p53 genomic DNA induced by the direct-acting hepatocarcinogens N-nitroso-2-acetylaminofluorene and N-acetoxy-2-acetylaminofluorene

DNA polymerase fingerprint analysis (DPFA) was employed for identifying DNA-carcinogen adduct formation in the human p53 and lac gene sequence. Two 'hot regions' at codons 223-250 and 257-283 of the p53 gene were easily attacked by nitroso-2-acetylaminofluorene or acetoxy-2-acetylaminofluorene. However, the promutational lesions in lac gene were rather randomly distributed. The chemical treated plasmid (pUC 19) which contains lac gene were transfected into Escherichia coli JM109 cells and the induced lac gene mutants were selected with X-Gal plate as indicated by the appearance of white colonies. No mutational hot regions were found in the lac gene.

Mutagenicity and cytotoxicity of nitropyrrole compounds derived from the reaction of 2-acetyl pyrrole with nitrite

2-Acetylpyrrole (AP) is a product of model browning systems, and has been isolated as a major flavour component of many foods. Reaction of AP with nitrite produces two N-nitropyrrole compounds, 1-nitro-2-acetyl-pyrrole (NAP) and 1,3,5-trinitro-2-acetylpyrrole (TNAP), the chemical structures of which have been confirmed by spectral studies, including UV mass, nuclear magnetic resonance, infra-red and elemental analysis (EA). NAP and TNAP are moderately mutagenic to the Salmonella strains TA98 and TA100 in the absence of a mammalian activation system and are markedly cytotoxic to mouse C3H10T1/2 cells. These results suggest that the formation of direct-acting mutagens of nitro-derivatives may take place in nitrite-containing food or in vivo by nitrosation following ingestion of AP.

N-nitroso-2-acetylaminofluorene: a direct-acting carcinogen inducing hepatocellular carcinoma in Sprague-Dawley rats

To compare the hepatotoxicity and hepatocarcinogenicity of N-nitroso-2-acetylaminofluorene (NO-AAF) and its parent compound, 2-acetylaminofluorene (AAF), male Sprague-Dawley rats were given intraperitoneal (i.p.) or subcutaneous (s.c.) injections of AAF or NO-AAF (60 mg/kg body weight/week) for ten months. In the AAF group, morphological changes were produced which involved gross distortions of the liver with multiple nodule formations. The rat livers in the NO-AAF group appeared to be smooth with a blunt-thick superior segment of the lateral lobe. The serum gamma-glutamyl transpeptidase activity in both the AAF group and the NO-AAF group was significantly elevated (P < 0.0005). The present study shows that i.p. and s.c. injections of NO-AAF resulted in a high incidence of well-differentiated hepatocellular carcinomas (HCC) (7/9 and 4/6, respectively), while poorly differentiated HCCs were induced by i.p. or s.c. administration of AAF (6/9 or 2/6, respectively). Subcutaneous lesions consisting of an inflammatory reaction and fibroadenoma formation were observed in the NO-AAF-treated rats, whereas no such skin lesions were detected in the AAF-treated animals. These results suggest that NO-AAF is a new direct-acting carcinogen which may be useful for investigating hepatocarcinogenesis.

Chromosomal aberrations and sister-chromatid exchanges induced by N-nitroso-2-acetylaminofluorene and their modifications by arsenite and selenite in Chinese hamster ovary cells

The frequencies of chromosomal aberrations (CA) and sister-chromatid exchanges (SCE) in Chinese hamster cells were significantly increased by the direct-acting mutagen N-nitroso-2-acetylaminofluorene (N-NO-AAF) at the concentration of 0.1 mM. N-NO-AAF was prepared by nitrosation of the protohepatocarcinogen 2-acetylaminofluorene. The induced CA, which included chromatid breaks, chromatid exchanges, chromosome breaks, and chromosome ring formation were significantly potentiated by the presence of sodium arsenite (10 microM), but not by hydroxyurea (20 mM) or cytosine arabinoside (25 microM). On the other hand, the clastogenic effect of N-NO-AAF was effectively inhibited by sodium selenite (100 microM). Arsenite (10 microM) was shown to be moderately active in CA induction which was partially blocked by the presence of selenite (10 nM). N-Nitroso compounds such as N-nitroso-N-methylurea, N-nitroso-N-ethylurea and N-methyl-N'-nitro-N-nitrosoguanidine were equally or more active in the induction of CA and SCE in CHO cells when compared with N-NO-AAF. The cell cycle was significantly delayed by the intervention of N-NO-AAF.

Cell-cycle dependent mutation of human lymphoblasts: bromodeoxyuridine and butyl methanesulfonate

Cells of the human lymphoblast line WI-L2 and its derivative TK-6 were synchronized by centrifugal elutriation and cell-cycle dependent mutation to 6TGR (HPRT) and OUAR (Na+, K+ ATPase) measured. Bromodeoxyuridine induced 6TGR and OUAR mutations within S phase while butylmethyl-sulfonate induced mutation displayed no cell-cycle dependence. The data indicate that centrifugal elutriation is a facile means to obtain a useful degree of synchrony for these cell lines.