Immunocytochemical analysis of O6-alkylguanine shows tissue specific formation in and removal from esophageal and liver DNA in rats treated with methylbenzylnitrosamine, dimethylnitrosamine, diethylnitrosamine and ethylnitrosourea

Targeted deletion of GSNOR in hepatocytes of mice causes nitrosative inactivation of O6-alkylguanine-DNA alkyltransferase and increased sensitivity to genotoxic diethylnitrosamine

S-nitrosoglutathione reductase (GSNOR), a ubiquitously expressed protein central to the control of protein S-nitrosylation, plays critical roles in many biological systems. We showed recently that GSNOR is often deficient in human hepatocellular carcinoma and that germ line deletion of the GSNOR gene in mice causes hepatocellular carcinoma through S-nitrosylation and proteasomal degradation of the key DNA repair protein O⁶-alkylguanine-DNA alkyltransferase (AGT). We report here the generation of mice with targeted deletion of GSNOR in hepatocytes or in cells of the hematopoietic lineage. We found that during inflammatory responses induced by intraperitoneal injection of diethylnitrosamine (DEN) or lipopolysaccharide, the amount of liver AGT was not changed in mice with GSNOR deletion in hematopoietic cells but was almost completely depleted in mice with GSNOR deletion in hepatocytes. In livers of DEN-challenged mice, GSNOR deletion in hepatocytes but not hematopoietic cells resulted in an increase in phosphorylated histone H2AX, a well-established marker of DNA double-strand breaks. Hepatocyte deletion of GSNOR increased DEN-induced mortality, which was abolished in mice deficient in both GSNOR and inducible nitric oxide synthase. Thus, protection of AGT and resistance to nitrosamine-induced genotoxicity critically depends on GSNOR in hepatocytes. In addition, our findings suggest that nitrosative inactivation of AGT from GSNOR deficiency might sensitize cancerous cells to alkylating drugs in cancer treatment.

Antioxidant and antimutagenic effect of quercetin against DEN induced hepatotoxicity in rat

Diethylnitrosamine (DEN), a potent hepatocarcinogen, is found in tobacco smoke, processed meat as well as in different food products. Quercetin (QC), a naturally occurring flavonoid has excellent antioxidant properties. The present study was aimed to investigate the chemoprotective potential of QC against DEN induced hepatotoxicity in Sprague-Dawley (SD) rats. Quercetin was administered (10, 30 and 100 mg/kg) for 5 consecutive days after DEN (200 mg/kg) treatment. The animals were killed 24 h after the last dose of QC/saline treatment. The DEN induced hepatotoxicity was evident by elevated malondialdehyde (MDA) and decreased glutathione (GSH) levels in the liver. A significant increase in the levels of plasma aspartate transaminase (AST) and plasma alanine transaminase (ALT) was observed in the DEN treated group. The DEN induced DNA damage was evaluated using a single cell gel electrophoresis (SCGE) assay. A significant increase in the number of TUNEL positive cells was observed in the DEN treated group. Quercetin restored AST, ALT and GSH levels at all the tested doses. Restoration of the MDA level and cellular morphology was observed at doses of 10 and 30 mg/kg of QC. Further, DEN induced DNA damage and apoptosis was ameliorated by QC. The results indicate that QC ameliorates the DEN induced hepatotoxicity in rats and can be a candidate for a good chemoprotectant.

N-nitrososdimethylamine is activated in microsomes from hepatocytes to reactive metabolites which damage DNA of non-parenchymal cells in rat liver

The liver carcinogen N-nitrosodimethylamine (NDMA) has to be metabolically activated by specific cytochromes before it can react with cellular macromolecules (e.g. proteins or DNA). Although hepatocytes are believed to be responsible for this activation, the liver tumours originate mainly from non-parenchymal cells (NPC). To investigate their activation capacity we determined NDMA-demethylase activity in isolated microsomes from both liver cell types. The results demonstrate that only hepatocytes have activation capacity. Additional experiments were performed with hepatocytes and NPC using the single cell microgel electrophoresis assay (MGE). DNA damage appears in both cell types following in vivo exposure. Tested in vitro, however, the carcinogens induce DNA damages only in hepatocytes (the cells which activate these compounds). N-nitroso-hydroxymethyl-methylamine could be the responsible metabolite as it is stable enough to be transported from hepatocytes to NPC in an intact liver.

Presystemic intestinal metabolism of N-nitrosodimethylamine in mouse intestine

N-Nitrosodimethylamine (NDMA), a common food contaminant, is a potent liver carcinogen in rodents. A high presystemic intestinal metabolism has been shown for several nitrosamines including environmentally important compounds. We determined the metabolism of 1 micron [14C]-NDMA in isolated perfused mouse intestinal segments. We found NDMA to be equally distributed between the absorbed fluid and the perfusate. During a 2-h perfusion period, 0.13% of the radioactivity was converted to CO2. The formation of CO2 was decreased by pretreatment with diallylsulfide or addition of SKF 525A, and slightly increased by phenobarbital. Hydrophilic metabolites were found in the absorbate (0.9%) and perfusate (3.8%) of untreated mice. The amount of metabolites in the absorbate was increased by treatment with acetone or phenobarbital (8-fold), but not after starvation, with formaldehyde being present only in phenobarbital-treated animals. Treatment with diallylsulfide or addition of SKF 525A reduced the amount of metabolites in acetone-treated animals to control values. In conclusion, intestinal turnover does not significantly reduce the body burden of orally ingested NDMA and thus is not a first-line defense against this carcinogenic nitrosamine. NDMA metabolism has been attributed to the presence of cytochrome P450IIE1, which has not been detected in the intestine of untreated animals. The low turnover of NDMA, the induction by acetone and phenobarbital treatment, and the inhibition by diallylsulfide suggest the presence of low amounts of this or related cytochrome P450 isozyme(s) in mouse intestine.

Zonal location of compensatory hepatocyte proliferation following chemically induced hepatotoxicity in rats and humans

Hepatocyte proliferation stimulated by partial hepatectomy occurs first in periportal cells, with midlobular and then perivenous cell division occurring later. We have previously shown that this pattern of compensatory cell proliferation also occurs following the hepatotoxicity of N-nitrosodimethylamine. We examined the generality of this pattern in livers of rats given a minimally toxic dose of an hepatotoxin and in liver biopsy samples from patients who had taken overdoses of acetaminophen. Proliferating hepatocytes were detected immunohistochemically (5'-bromodeoxyuridine or Ki-67 antigens). The perivenous necrogens N-nitrosodiethylamine, carbon tetrachloride (CCl4), bromobenzene, and acetaminophen all induced periportal hepatocyte proliferation. With CCl4, bromobenzene, and acetaminophen, the initial appearance of proliferating periportal hepatocytes was followed 12-24 hr later by division in the midlobular region, with a few cells dividing adjacent to the perivenous region of necrosis. The periportal necrogen allyl alcohol also induced periportal cell division. In the human studies, perivenous necrosis was accompanied by periportal and midlobular hepatocyte proliferation. These results suggest that regardless of its lobular location chemically induced hepatotoxicity stimulates cell proliferation that begins in the periportal zone and then moves in an orchestrated response into the midlobular and perivenous zones.

Formation and persistence of the miscoding DNA alkylation product O6-ethylguanine in male germ cells of the hamster

The cellular parameters which modulate trans germ-line carcinogenesis by DNA-reactive agents have not yet been studied in detail. Therefore, we have measured in this study the formation and repair kinetics of the miscoding alkylation product O6-ethylguanine (O6-EtGua) in nuclear DNA of spermatogonial cells of the Syrian golden hamster (SGH) after exposure to either of two potent N-nitroso carcinogens, ethylnitrosourea (ENU) or diethylnitrosamine (DEN). Both compounds, the spontaneously decomposing ENU, and DEN, which has to be converted by cellular enzymes to the reactive ethyl diazonium ion, induce the same pattern of alkylation products in nuclear DNA. Adduct analyses were performed at the single-cell level by using a quantitative immunocytological assay and anti-(O6-EtGua) monoclonal antibodies. 1.5 h after intraperitoneal application of ENU (100 microg/g body weight) O6-EtGua levels in the nuclear DNA of spermatogonia were similar to those in other cell types of the same hamster. About 30% of the initially formed DNA adducts were still persistent in spermatogonial cells even 4 days after ENU exposure. The presence of O6-EtGua in DNA after exposure to DEN (100 microg/g body weight) implies the capability of hamster spermatogonial cells to convert nitrosamines into DNA-alkylating metabolites. This capability of male germ cells in combination with their limited repair capacity for a critical DNA adduct and their high rate of proliferation may be considered as a major risk factor for genetic effects including carcinogenesis in subsequent generation(s).

N-Nitrosobenzylmethylamine is activated to a DNA benzylating agent in rats

The ability of rat tissues to activate the esophageal carcinogen, N-nitrosobenzylmethylamine (NBzMA), to a DNA benzylating intermediate was investigated. [3-3H]NBzMA was prepared and given to male F344 rats. Tissues were harvested 4 h after treatment, and DNA was isolated. HPLC analysis with radiochemical detection of chemical and enzymatic hydrolysates of DNA from liver and lung revealed the formation of benzyl adducts. Benzyl alcohol, N2-benzylguanine, 3-benzyladenine, N6-benzyladenine, and 7-benzylguanine were the major radioactive components in the hydrolysates. An unknown adduct was also observed. The adduct distribution was similar to that observed in [3-3H]benzylnitrosourea ([3-3H]BzNU)-treated calf thymus DNA. However, enzymatic hydrolysates of [3-3H]BzNU-treated DNA also contained significant levels of O6-benzyl-2'-deoxyguanosine (O6-BzdG). This radioactive adduct disappeared upon incubation of the DNA with a crude preparation of the repair protein, O6-alkylguanine-DNA alkyltransferase isolated from rat liver. These data provide evidence that O6-BzdG is probably rapidly repaired in vivo. No benzylation of esophageal mucosal DNA was detected. The level of DNA benzylation observed in tissues from [3-3H]NBzMA-treated rats was several orders of magnitude lower than the level of DNA methylation in these same tissues. Therefore, these data indicate that DNA benzylation plays a minor role, if any, in the carcinogenic activity of NBzMA.

Alkyltransferase transgenic mice: probes of chemical carcinogenesis

Transgenic mice expressing DNA-repair genes are an instructive model with which to study the protective role of DNA-repair pathways in both spontaneous and chemical carcinogenesis. Of particular interest in chemical carcinogenesis is the DNA-repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) which repairs O6-alkylguanine-DNA adducts. Transgenic mice carrying expression constructs for the alkyltransferase gene--either the human MGMT cDNA or the bacterial ada gene--express increased levels of alkyltransferase and have increased capacity to remove O6-methylguanine-DNA adducts. Protection from the DNA damaging effects of N-nitroso compounds occurs specifically in the cells and tissues in which the alkyltransferase transgene is expressed. For instance, mice carrying the PEPCKada construct have increased alkyltransferase in the liver and more rapid removal of O6methylguanine-DNA adducts. The protective effect is noted in hepatocytes, which express PEPCK-linked genes, not in nonparenchymal cells of the liver, which do not. Other tissues that express the transgene in the various models include the thymus, spleen, testes, muscle, stomach and brain. Mice expressing the human alkyltransferase in the thymus have a reduced incidence of thymic lymphomas following exposure to methyl nitrosourea (MNU), evidence of a role for this DNA-repair protein in protection from carcinogenesis due to N-nitroso compounds. Protection has also been observed in the induction of hepatic tumors by N-nitroso-dimethylamine (NDMA). These models will be used to identify whether overexpression of a single DNA-repair gene can block the carcinogenic process of N-nitroso compounds in many different tissues.

Repair of O6-alkylguanines in the nuclear DNA of human lymphocytes and leukaemic cells: analysis at the single-cell level

Inter-individual and cell-cell variability of repair of O6-alkylguanines (O6-AlkGua) in nuclear DNA was studied at the single-cell level in peripheral lymphocytes from healthy donors and in leukaemic cells isolated from patients with chronic lymphatic leukaemia (CLL) or acute myeloid leukaemia (AML). Cells were pulse exposed to N-ethyl- or N-(n-)butyl-N-nitrosourea in vitro, and O6-AlkGua residues in DNA were quantified using an anti-(O6-AlkGua) monoclonal antibody and electronically intensified fluorescence. The kinetics of O6-AlkGua elimination revealed considerable inter-individual differences in O6-ethylguanine (O6-EtGua) half-life (t1/2) values in DNA, ranging from 1.5 to 4.5 h (five AML patients), from 0.8 to 2.8 h (five CLL patients) and from 1.2 to 7.3 h (five healthy donors). The elimination from DNA of equimolar amounts of O6-butylguanine was generally 3-5 times slower in comparison with O6-EtGua. The t1/2 values of individual samples varied in parallel for both DNA alkylation products. Upon preincubation with O6-benzylguanine, the activity of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AT) in both lymphocytes and leukaemic blasts was reduced to < or = 1%. However, while the rate of O6-EtGua elimination from DNA was decelerated it was not abolished, suggesting the possible involvement of additional repair systems that might be co-regulated with AT. Within individual samples, no major cell subpopulations were observed whose repair kinetics would differ significantly from the remaining cells.

Oncogene activation: c-raf-1 gene mutations in experimental and naturally occurring tumors

We demonstrate here consistent point mutations of the c-raf-1 proto-oncogene, within a small region of the kinase domain, in a mouse model for chemical tumor induction. This is the first demonstration of point mutated raf genes in vivo, and the first isolation of activating in vivo point mutations in the kinase domain of a proto-oncogene. The specific region where these mutations are clustered also has biological significance. This is precisely the region where 5/5 independently generated monoclonal antibodies raised against Raf-1 map to [29], and predictions based upon the crystal structure of A kinase identify this as the substrate pocket. The tumors examined show a selective specificity for Raf-1 mutations in that another family of genes, the ras proto-oncogenes which are frequently activated by point mutation in both animal and human tumors [15-21,26], is not involved. Our consistent finding of Raf-1 mutations in a mouse tumor model also has consequences for further evaluation of the role of Raf-1 in human tumor development, as it emphasizes the need to examine c-raf-1 at the sequence level. In fact preliminary screening of human lung tumors indicates point mutations at amino acid 533 (John Lyons, personal communication). Finally, the cumulative data on the critical role of Raf-1 in signal transduction and the occurrence of oncogenic Raf-1 in tumors [32-41] highlight this enzyme as an attractive target for development of novel anticancer regimens.

Immunocytochemical analysis of in vivo DNA modification

In the past decades a large number of DNA adducts induced in the intact animal by alkylating agents have been identified. The formation and repair of these adducts are important determinants, not only of mutagenesis, tumor initiation and DNA-mediated toxicity but probably also of tumor progression. Most studies on in vivo DNA modification have been performed on isolated bulk DNA. More recently, methods have been developed to study the distribution of DNA adducts at the level of either the individual gene or the individual cell. This paper reviews immunocytochemical methods to study the formation and repair of DNA adducts and other DNA modifications at the level of the individual cell. DNA modifications induced by alkylating agents and a variety of other agents including ultraviolet radiation, aromatic amines, polycyclic aromatic hydrocarbons and platinum anti-cancer drugs will be discussed. Up to now, immunocytochemical analysis of in vivo modified DNA has largely concentrated on experimental animals. These studies have revealed striking heterogeneities with regard to formation and/or repair of DNA adducts in tissues from rat, hamster and mouse. Immunocytochemical adduct analysis can be used to identify in a convenient, fast and detailed way cell types, cell stages and sites in which biological effects of the adducts might be expressed. More recently, immunocytochemical analysis of DNA adducts also proved to be feasible on in situ exposed human samples. A number of existing and potential applications in the field of chemical carcinogenesis, experimental chemotherapy and molecular epidemiology are discussed.