IN VIVO REACTIONS OF NITROSO COMPOUNDS

Systemic Carcinogens (N-Nitroso Compounds) and Synergistic or Additive Effects in Respiratory Carcinogenesis

Systemic carcinogens induce respiratory tract tumors in various animals species. Typical examples of systemic carcinogens are the N-nitroso compounds, which require metabolic conversion to exert their carcinogenic effect. In this review particular emphasis is put on the comparative studies performed in the same species with different nitrosamines, providing an experimental model in which is possible to examine and correlate the biochemical changes preceding the development of detectable tumors and the carcinogenic effect. In addition studies describing synergistic or additive effects in respiratory carcinogenesis are reviewed and discussed.

The mutagenicities of alkaloids and N-nitrosoguvacoline from betel quid

In Taiwan, betel quid is a natural masticatory, which is composed of fresh green areca fruit, Piper betle and slaked lime paste. Areca fruit contains some alkaloids, of which arecoline is the major one. N-Nitrosoguvacoline (NG), one of the N-nitrosation products of arecoline, is the only one N-nitrosamine found in Taiwanese betel quid chewing saliva. The mutagenic studies in Ames Salmonella microsome test showed that crude alkaloid extracts of areca fruit and arecoline were active in Salmonella typhimurium TA100, and NG was weakly active in TA98 and TA100. The activities in both arecoline and NG decreased further in the presence of rat liver S9 mix. Nitrite was significantly consumed during the N-nitrosation of arecoline and sodium nitrite at acidic condition (pH 3), whereas the formation of NG was favored at neutral condition (pH 7). Crude phenolic extracts of leaf and inflorescence of Piper betle inhibited the formation of NG by blocking the nitrite. However, a high amount of crude phenolic extracts of areca fruit enhanced the formation of NG.

Alteration of immune function in mice following carcinogen exposure

Treatment of mice with the direct-acting alkylating agent methyl methanesulfonate produced marked suppression of the humoral response to sheep erythrocytes and suppression of T cell responses to foreign antigens. These effects occurred without loss of spleen, thymus or body weight and in the absence of peripheral blood, splenic or bone marrow cytotoxicity. In comparison, exposure to urethan decreased spleen weights, number and viability of spleen cells, and numbers of circulating lymphocytes. Significant suppression of T cell mitogen responsiveness was observed at all dose levels of urethan. Thymus weights, proliferative responses to the B cell mitogen lipopolysaccharide and delayed hypersensitivity responses were decreased at the highest urethan dose. Cyclophosphamide treatment significantly depressed thymic weight, lymphoproliferative responses of T and B cells, antibody production and delayed hypersensitivity responses. These results suggest differential sensitivity in components of the host defense system to weak carcinogens.

In vivo formation of nitrosocarbamates in the stomach of rats and guinea pigs

The N-nitrosocarbamates are potent mutagens and carcinogens, and have been synthesized under acid conditions that prevail in the human stomach. However, it has never been documented that nitrosocarbamates are actually formed in vivo in the stomach of any mammalian species. Using 14C-labeled carbaryl and carbofuran, attempts were made to isolate the nitroso derivatives from the stomach contents of rats and guinea pigs treated orally with the carbamate and sodium nitrite. Only trace quantities of nitrocarbamate were recovered from the rat stomach, whereas 0.5 to 2.0% of the carbamate doses were isolated as the nitroso derivative from the contents of the guinea pig stomach. The rather low apparent yields resulted in part from the instability of the nitrosocarbamates and from absorption of the carbamate and/or nitrosocarbamate from the stomach. Higher rates of synthesis were indicated by incubating the carbamates with sodium nitrite in the presence of the stomach contents at 37 degrees C for 15 min. About 30% nitrosation occurred with the guinea pig and about 0.5% with the rat. The difference was attributed to the pH of the gastric contents. For the rat, the pH ranged from 3 to 5; gastric contents of the guinea pig had a pH between 1 and 2. Since the pH of the human stomach is also in the pH 1-2 range, it is likely that nitrosation of carbamates in humans would be very similar to that in the guinea pig.

Determination of N-nitrosamines and N-nitrosamine precursors in rubber nipples from baby pacifiers by gas chromatography-thermal energy analysis

N-Nitrosamines and precursors are present in rubber products in which the accelerators and stabilizers used in the vulcanization process were derived from dialkylamines. Research was performed to develop data concerning the presence of N-nitrosamines and precursors so that the health significance of the exposure problem related to infant ingestion of these chemicals could be properly assessed. Volatile N-nitrosamines were determined in cut-up pacifier nipples by extraction with dichloromethane followed by concentration in a Kuderna-Danish evaporator, high-temperature mineral oil purge and trap, and analysis by gas chromatography--thermal energy analysis (GC-TEA). N-nitrosodibutylamine (NDBA) was the principal N-nitrosamine found, with concentrations up to 427 ppb. N-Nitrosamines and precursors in cut-up and intact nipples were determined by GC-TEA after a single extraction with artificial saliva. NDBA was the principal nitrosamine found, at levels up to 1040 ppb, while dibutylamine (DBA) was the principal precursor found, at levels up to 3890 ppb. The persistence of these compounds in intact nipples was determined by multiple artificial saliva extractions. Amounts of NDBA and DBA found after 15 artificial saliva extractions of intact pacifier nipples totalled 824 ppb and 15.6 ppm, respectively. N-Nitrosamine levels generally showed a gradual decrease in concentration with each extraction, whereas no consistent trend could be determined for concentrations of precursors.

Inhibition of two rat hepatic microsomal drug-metabolizing enzymes by a carcinogenic N-nitrosated pesticide: N-nitrosocarbaryl

N-Nitrosocarbaryl (N-methyl-1-naphthyl N-nitrosocarbamate) was intraperitoneally administered to male and female rats on four consecutive days at the following doses: 6.25 mg, 12.5 mg, 25 mg and 50 mg/kg body weight/day in olive oil solution; the controls received just the oil. In a second experiment, a daily intraperitoneal dose of 25 mg/kg of N-nitrosocarbaryl was given for 1, 2, 3 or 4 days; the animals were killed 24 h after the last treatment. The two following microsomal enzymatic activities were assayed: aniline aromatic hydroxylase and p-nitroanisole O-demethylase; the levels of cytochrome P-450, proteins and RNA were measured in the hepatic microsomal fraction. N-Nitrosocarbaryl is an inhibitor of the two investigated microsomal monooxygenases at doses of 25 and 50 mg/kg when administered on 4 consecutive days. During the daily administration, enzyme inhibition is seen in females after one day of treatment whereas cytochrome P-450 only becomes lowered after 4 days of administration. In males, no modification of this parameter is observed whereas the activities of microsomal monooxygenases are inhibited. These results suggest that N-nitrosocarbaryl could act on the active sites of the enzymes which metabolize aniline and p-nitroanisole.

Biodistribution of 14C-lomustine in an animal tumor model

A formulation of 14C-lomustine in propylene glycol-ethanol (4:1) was administered intravenously to rats infiltrated with glioma tumors of the astrocytic series (RT6). The organ and tumor distribution of this agent was followed at 1, 4, 12, and 24 hr. Rapid blood disappearance (0-1 hr) of the label concomitant with an increase in all organs except the lung, muscle, and brain was observed. Only the blood, liver, and muscle contained greater than 1% of the dose after 24 hr. The bladder, liver, small bowel, and kidneys concentrated the highest percentages throughout the study. The distribution of 14C-lomustine in the tumor relative to the brain, muscle, and blood showed a maximum 4-12 hr after administration.

Transplacental action of sodium nitrite on embryonic cells of Syrian golden hamster

Hamster embryos were treated with various doses of NaNO2 in utero, by its oral administration to the mothers, and then the embryonic cells were examined for micronucleus formation, chromosomal aberrations, morphological or malignant transformation and drug-resistant mutations. For induction of resistant mutations, the cells were cultured in normal medium for 72 h, and then selected in media containing 8-azaguanine (10 or 20 microgram/ml) or 1 mM ouabain. This treatment with NaNO2 caused marked dose-dependent induction of 8-azaguanine- and ouabain-resistant mutations. Cultured embryonic fibroblasts in the resting state also showed a marked dose-dependent increase in micronucleus formation but not an increase in chromosomal aberrations. This treatment also caused morphological and neoplastic transformation of the cells. Transplacental oral treatment with DMN, as a positive control, caused changes of similar extent in biological effects of embryonic fibroblasts, and in addition it caused chromosomal aberrations in metaphase plates. On the contrary, transplacental oral application of NaNO2 did not induce any biological change in cultured embryonic fibroblasts.

Activation of the guanylate cyclase-guanosine 3'5' monophosphate system of colonic mucosa by n-methyl-n'-nitro-n-nitrosoguanidine

The effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on the guanylate cyclase (GC)-guanosine 3'5' monophosphate (cGMP) system of rat colonic mucosa were studied. MNNG (1 mM) increased colonic mucosal cGMP from 1.8 +/- 0.2 to 22.5 +/- 2.7 pmol/mg protein in 5 minutes. Increases in response to MNNG occurred in the presence or absence of extracellular Ca2+, whereas the two-fold increase in mucosal cGMP mediated by carbamylcholine was abolished by exclusion of Ca2+. Although GC activity of mucosal homogenates was found predominantly (90%) in the 100,000 g particulate fraction, the effects of MNNG on mucosal cGMP correlated with stimulation of 100,000 g soluble GC by this agonist. MNNG increased soluble GC 13-fold over the corresponding basal with 4 mM Mn2+, and 48-fold with 4 mM Mg2+ as the sole available divalent cation. Compared with unstimulated GC, the MNNG-activated soluble enzyme was less dependent upon Mn2+ availability and effectively utilized Mg2+ as metal co-factor. N-ethylmaleimide, a sulfhydryl group alkylator, inhibited MNNG stimulation of GC and cGMP. Thus, expression of these MNNG actions may involve drug interaction with tissue thiol groups. Prior incubation of MNNG with thiol antioxidants or ascorbate also suppressed MNNG stimulation of GC, possibly through direct drug reactions involving nucleophilic and electrophilic reactants. The ability of MNNG to stimulate the colonic mucosal GC-cGMP system could be linked to its carcinogenic action.

Suppression of dimethylnitrosamine mutagenicity by nitrososarcosine and other nitrosamines

Nitrososarcosine, not mutagenic itself in the host-mediated assay using Salmonella typhimurium G46 as indicator organism, lowered the mutant frequency produced by dimethylnitrosamine (DMN). Mutant frequency was significantly depressed when 1.0 g/kg nitrososarcosine was administered by gavage 0.5--2.0 h prior to intramuscular injection of 500 mg/kg DMN. Doses of nitrososarcosine as low as 37.2 mg/kg administered 45 min prior to dimethylnitrosamine treatment produced statistically significant reduction of mutant frequency. Dimethylnitrosamine, diethylnitrosamine and dibutylnitrosamine (500 mg/kg) also partially suppressed the mutant frequency produced by 500 mg/kg DMN when administered 45 min prior to dmn. diethylnitrosamine and dibutylnitrosamine were not found to be mutagenic in this test system.

Heterogeneous distribution of DNA alkylation products in rat liver chromatin after in vivo administration of N,N-di[14C]methylnitrosamine

Poly-l-lysine (PL) binds to about 50% of chromatin DNA, rendering it resistant to degradation by DNAase I. Separation of the unbound DNA as acid-soluble nucleotides allows the fractionation of chromatin DNA into two zones. After in vivo administration of N,N-di[14C]methylnitrosamine, the amount of alkylation in DNA was found to be lower in the polylysine-binding regions. Some possible reasons for this heterogeneous distribution are discussed.

Mutagenic selectivity of carcinogenic nitroso compounds 2. n-methyl-N-nitrosourethane

The mutagenicity of the carcinogen methylnitrosourethane (MNUr) was examined in Drosophila with a view to the determination of its activity on heterochromatic loci (especially rDNA) relative to those in the euchromatin. Assays were made of the yield of rDNA mutations (bobbed: bb) relative to other X-chromosome recessive lethals and visibles [X(l + v)] in the same male germ cells after treatment with different doses (1-10 mM) and at various stages in spermatogenesis. Dose dependence followed the same pattern for all genic loci and germ cell stages. In all instances, the regression of mutation frequency on injected molar dose was approximately linear, but could better be described by a quadratic dose curve. In contrast, the mutagenicity pattern during spermatogenesis varied according to the target genes. The response of the euchromatic loci reached a peak among the earlier germ cells (probably the spermatocytes), whereas that for the heterochromatic sites (including rDNA) was maximal in mature sperm. Mutagenic selectivity for rDNA with MNUr, as indicated by the percentage bb/X-mutations, was among the highest for the intrinsically reactive carcinogens (alkylating and arylating agents). This correlates with the strong carcinogenicity of MNUr and adds further support to the concept that rDNA mutations might well be a crucial step in cancer initiation.