Carcinogenicity tests of fecapentaene-12 in mice and rats

Fecapentaenes, a class of direct-acting bacterial mutagens, have been isolated from the feces and intestinal tract of humans on a Western meat-containing diet. Two bioassays to test pure fecapentaene-12 (FP-12) for carcinogenicity were performed. FP-12 in dimethylsulfoxide (DMSO) solution was injected i.p. into newborn ICR/MA mice on days 1, 3, 7, 10, 14 and 21. The mice killed after 21 months had neoplasms in liver, lung, glandular stomach and subcutaneous fibrosarcoma. Intrarectal (i.r.) infusion of FP-12 in an aqueous vehicle into male F344 rats for 71 weeks, and killing the rats after 21 weeks more, displayed no evidence of neoplasia associated with FP-12 exposure. The positive control, N-nitrosomethylurea (NMU), given i.r. as 4 2-mg doses in 2 weeks, as expected, yielded multiple colonic neoplasms in less than 11 months. Fecapentaene may exert its effect in bacteria and in newborn mice through the generation of hydroxy radicals. However, adult rodent and human colon may have adequate biochemical defense mechanisms against low level, even continuous exposures to chemicals like FP-12, and thus be at low risk of neoplasia, as was found.     

N-nitrosocimetidine as a modifier of chemically-initiated tumors in mice

N-Nitrosocimetidine (NCM) is a nitrosation product of cimetidine, a commonly-prescribed pharmaceutical agent. In spite of its known genotoxicity, NCM has failed to cause tumors in assays with rats and mice, but has given indications of enhancing or suppressive effects on tumor development. This possibility was tested by administering NCM topically to the skin or in the drinking water to mice in which tumors had been initiated by treatment with chemical carcinogens. Sencar mouse skin papillomas initiated by 7,12-dimethylbenzanthracene (DMBA) and promoted by 12-O-decanoylphorbol-13-acetate (TPA), progressed more rapidly to carcinoma on mice given treatment during stage 3 (after TPA) with NCM (1 mg/week) or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG, 120 micrograms/week) [corrected] than on stage 3 acetone controls. Oral NCM (1 g/l drinking water) did not have this effect but rather suppressed development of keratoacanthomas, as did stage 3 MNNG or TPA. Primary lung tumors initiated in BALB/c mice by i.p. injection of urethane; and tumors of forestomach, lung, mammary, lymphoid and skin tissues caused in (C57BL/6 X DBA/2)F1 mice by oral DMBA were not markedly affected by NCM given in drinking water (1000-1800 ppm) until 14-16 months of age. These results confirm NCM's general lack of activity as an in vivo toxicant, but show that under certain circumstances it may enhance or suppress tumor development.     

Strain differences in mouse skin tumorigenesis by the promotion of 12-o-tetradecanoylphorbol-13-acetate or benzoyl peroxide

Strain difference of mouse skin tumorigenesis was studied with 12-O-tetradecanoylphorbol-13-acetate (TPA) and benzoyl peroxide (BzP) both in 8-week-old hypocatalasemic C3H mice (C-s(b)) and in normal C3H/HeNCrj mice (C3H). Two weeks after initiation by 4 mu mole MNNG, both strains of female mice were promoted by 10 nmole TPA or 10 mg, 20 mg BzP twice weekly for 30 and 52 weeks. The incidence of skin tumors in C-b(s) was significantly increased as compared to that in C3H mice promoted by TPA for 30 weeks but after 52 weeks of TPA promotion there was no difference between the 2 strains. By the promotion with 10 mg or 20 mg BzP, the incidence of skin tumor in C3H was 5% at both levels after 30 weeks of promotion and 19% and 38%, respectively, after 52 weeks of promotion. No skin tumors appeared in C-b(s) mice with BzP treatment after 30 weeks of promotion. After 52 weeks of promotion by 10 mg BzP, skin tumors were induced in only 15% of C-b(s) mice. The results show that genetic factors, especially radical scavenging enzymes, controlled susceptibility to skin tumorigenesis by promotion with either TPA or BzP.     

Malignant progression of mouse skin papillomas treated with ethylnitrosourea, N-methyl-N''-nitro-N-nitrosoguanidine, or 12-O-tetradecanoylphorbol-13-acetate

Mouse skin tumors were induced by a single topical application of 7,12-dimethylbenzanthracene (DMBA), followed by biweekly promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). After 20 weeks of promotion, mice were treated twice weekly for 2 weeks with either ethylnitrosourea (ENU), N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) or TPA. Thereafter all groups were treated biweekly with TPA. The ENU-treated group had a higher percentage of animals with carcinomas and developed 217% more cumulative carcinomas per group than TPA-treated controls. The percentage of mice with carcinomas and the cumulative number of carcinomas per group in MNNG-treated mice was higher than TPA-treated controls but was less than ENU-treated mice. The ratio of cumulative carcinomas to cumulative papillomas in ENU treated, MNNG-treated and TPA-treated mice was 16%, 9% and 6%, respectively. Histological examination of tumors remaining at the termination of the experiment revealed the presence of keratoacanthomas, some of which stained positive for γ-glutamyltransferase (GGT), in the ENU-treated and MNNG-treated, but not the TPA-treated groups. The fact that no new papillomas developed during the progression stage indicated that enhanced carcinogenesis resulted from the progression of pre-existing tumors. Enhanced progression of benign skin tumors in mice by only a few treatments of an agent may serve as a potential model for studies into the mechanisms and the inhibition of malignant progression. The model also allows for a comparison of the potency of agents in enhancing malignant progression.     

Induction of murine fibrosarcomas by low dose treatment with 3-methylcholanthrene followed by promotion with 12-O-tetradecanoyl-phorbol-13-acetate

Repeated application of the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA) to the skin of mice previously treated with an initiating dose of the carcinogen 7,12-dimethylbenz[a] anthracene has been shown to lead to an increased incidence of papilloma. The studies presented here describe a modified murine two-stage carcinogenesis model in which a single subcutaneous administration of the carcinogen 3-methylcholanthrene (3-MC) is followed by multiple applications of TPA administered subcutaneously or intraperitoneally. TPA was observed to act as a promoter under these conditions when given either subcutaneously or intraperitoneally. When a carcinogenic dose of 3-MC was administered (0.5 mg/mouse) followed by regular treatment with TPA (10 micrograms/mouse) the percent of tumor-bearing mice increased and the length of time until tumors developed significantly shortened. At a subcarcinogenic dose of 3-MC (0.025 mg/mouse), repeated treatment with TPA led to tumor development whereas no tumors were observed in mice not treated with TPA. All tumors were found to be fibrosarcomas. Thus, TPA is capable of acting as a systemic promoter of mesenchymally derived tumors.     

7-Sulfooxymethyl-12-methylbenz[a]anthracene is an electrophilic mutagen, but does not appear to play a role in carcinogenesis by 7,12-dimethylbenz[a]anthracene or 7-hydroxymethyl-12-methylbenz[a]anthracene

Although a bay-region dihydrodiolepoxide metabolite has been considered as a principal ultimate electrophilic and carcinogenic form of 7,12-dimethylbenz[a]anthracene (DMBA), other reactive metabolites might also play a role in the activation of this hydrocarbon in vivo. Earlier studies suggested the hydroxylation of a meso-anthracenic methyl group with subsequent formation of a benzylic ester bearing a good leaving group (e.g. sulfate) as a metabolic activation pathway for DMBA. In support of this hypothesis, the formation of an electrophilic and mutagenic sulfuric acid ester of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA) by rat liver cytosolic sulfotransferase activity has previously been demonstrated, but no data have been reported on the carcinogenicity of this reactive ester. In the present study, we compared the carcinogenicity of chemically synthesized sodium 7-sulfooxymethyl-12-methylbenz[a]anthracene (SMBA) with that of the parent methyl and hydroxymethyl hydrocarbons. For this purpose, tests were made in several animal tumor models: induction of hepatomas in male B6C3F1 mice, lung adenoma induction in A/J mice, initiation of mouse skin tumors, development of sarcomas in rats at the injection sites, and initiation of preneoplastic enzyme-altered foci in rat liver. Data from all of these studies indicate that SMBA is not more carcinogenic than DMBA or HMBA. In addition, the carcinogenic activity of HMBA was not altered by dehydroepiandrosterone, a strong inhibitor of sulfotransferase activity for HMBA. DMBA produced only a low level of hepatic benzylic DNA adducts in rats when a relatively high dose was administered. These adducts constitute less than 5% of total DMBA residues bound to hepatic DNA. The rest of the adducts appear to be associated with other electrophilic intermediates including the dihydrodiol epoxide metabolites. Based on the results of our present study, it is unlikely that DMBA exerts its carcinogenic activity via metabolic activation to SMBA.     

Effect of a calcium-enriched diet on the colonic epithelial hyperproliferation induced by N-methyl-N'-nitro-N-nitrosoguanidine in rats on a low calcium and fat diet

We examined whether hyperproliferation of colonic crypt epithelium during cancer induction by N-methyl-N-nitro-N-nitrosoguanidine (MNNG), in rats on a low fat and calcium diet could be reduced by added calcium p.o. From the age of 4 weeks, 104 male Sprague-Dawley rats received a low fat (3.5%), low calcium (0.05% calcium ion), and low vitamin D (0.4 IU/g) diet. Sixty-four also had calcium salts, derived from either calcium lactate or solubilized calcium carbonate, added to their drinking water; therefore their total calcium intake was about 1% of daily diet. At age 12 weeks the rats were divided into 4 treatment groups: 8 rats, not receiving added calcium, had rectal saline instillations weekly (saline control group) and were sacrificed after a further 28 weeks; 3 groups of 32 rats each received intrarectal MNNG (1.5 mg) weekly. One group, not receiving added calcium, was the MNNG control group; while the second group also received added calcium lactate, and the third group received calcium carbonate. Groups of 24 were sacrificed periodically until 28 weeks of treatment. Rats were sacrificed and epithelial proliferation was estimated, 1 week after the last intrarectal instillation, by in vivo labeling with tritiated thymidine and measuring the ratio of labeled to total colonic crypt epithelial cells. The mean labeling index of the MNNG treated and added calcium groups were significantly higher (8.7-9.5%) than that of the saline controls (2.8%) only at week 28; however, it was then still significantly less than that of the MNNG controls not having added calcium (17.9%). Hyperproliferation, during induction of colonic cancer by MNNG in rats on a low calcium diet, can be reduced by a calcium enriched diet even in the presence of a low fat intake.     

Effects of dose and duration of treatment with the tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate on mouse skin carcinogenesis

The effects of dose and duration of treatment with the potenttumor-promoting agent 12-O-tetra-decanoylphorbol-13-acetate(TPA) on the formation of skin tumors in Charles River CD-1mice was studied. Mice were initiated with a single applicationof 0.2 µmol of 7, 12-dimethylbenz[a]anthracene (DMBA)in 0.2 ml acetone. Beginning two weeks after initiation, micewere treated twice weekly with various doses (0.01 – 20nmol) of TPA in 0.2ml acetone. Application of either 0.01 or0.1 nmol of TPA did not elcity tumors during the 50 weeks durationof treatment. A dose-dependent increase in the number of papillomaswas observed through the range of 1 to 10 nmol of TPA. Twiceweekly applications of 20nmol of TPA did not further enhancethe papilloma incidence. A good correlation was observed betweenthe induction of ornithine decarboxylase (ODC) activity andthe formation of skin tumors by various doses of TPA. To determine the effect of promotion duration on the incidenceof papillomas and carcinomas, mice were treated with 10 nmolof TPA for various durations (6,12,18,24,30, or 36 weeks) beginning2 weeks after initiation with 0.2 µmol of DMBA. Mice promotedfor only 6 weeks developed papilllomas and carcinomas afterpromotion had been discontinued. There was an intermediate incidenceof tumors in the group treated for 12 weeks. Promotion for 18,24, 30, or 36 weeks elicited virtually identical yields of papillomas.The incidence of carcinomas was proportional to promotion durationtimes of 6, 12, and 18 weeks, but carcinoma incidence was lessthan maximal in mice promoted for 24 weeks or longer. The results indicate that a) the incidence of papillomas servesas a rapid (18 weeks) index for subsequent appearance of carcinomas,b) twice weekly applications of 10 nmol of TPA for 18 weeksfollowing initiation of female CD-1 mice with 0.2 µmnolof DMBA is an appropriate protocol for maximum tumor yield ininitiation-promotion experiments, and c) ODC induction may bean important component of the mechanism of skin tumor promotionby TPA.     

Induction of cutaneous mast cell tumors by N-methyl-N'-nitro-N-nitrosoguanidine followed by TPA in female mice of 4 out of 5 strains tested

Appreciable yields of cutaneous mast cell tumors were induced in a two-stage skin carcinogenesis protocol comprising N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) initiation followed by 12-O-tetradecanoylphorbol-13-acetate (TPA) promotion in 4 of 5 strains of mice. Only female mice of each of the 5 strains were studied. The incidences of benign and/or malignant lesions differed considerably between strains; 27% in DBA/2, 22% in BDF1, 11% in BALB/c, 10% in CDF1 and 0% in C57BL/6 mice and no mast cell tumors were detected in any of the strains when treated with the initiator alone. First found in a DBA/2 mouse at week 50, most tumors were observed after 100 weeks of promotion, and were usually small in size (less than 2 mm in diameter) and predominantly located within the corium, although they occasionally extended into the subcutaneous tissue. Histologically, the benign mast cell tumors were composed of non-encapsulated, well circumscribed densely packed sheets of discrete cuboidal or rhomboid cells. Metachromatic granules were clearly visible in the cytoplasm by Toluidine Blue staining. Two of the tumors induced in DBA/2 mice were diagnosed as malignant mast cell tumors on the twin bases of cellular atypia and deep infiltration into the muscular layer. The cutaneous mast cell tumors were constantly accompanied by subepidermal mast cell aggregations which were also commonly observed in tumor-free skin of mice receiving the initiation-promotion procedure.     

Frequent codon 12 Ki-ras mutations in mouse skin tumors initiated by N-methyl-N'-nitro-N-nitrosoguanidine and promoted by mezerein

The skin tumor initiators N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 7,12-dimethylbenz[a]anthracene (DMBA) differ in effectiveness when tumor formation is promoted by 12-O-tetradecanoylphorbol-13-acetate (TPA). Even at high doses, MNNG is less effective, producing fewer benign and malignant tumors with a longer latent period. In DMBA-initiated skin, 10 wk of TPA promotion produced a maximal tumor response. With MNNG, 20 wk of TPA promotion was required, producing nearly four times as many papillomas as 10 wk of promotion. Promotion of MNNG-initiated skin with mezerein induced the appearance of very rapidly-growing papillomas within 5 wk, 3 wk earlier than the first TPA-promoted papillomas. Thus, MNNG may induce a novel mutation resulting in a population of initiated cells that respond especially well to mezerein. Since ras mutations are common in experimental tumors in many tissues, we determined the frequency of activating mutations in the Ha-ras, Ki-ras, and N-ras oncogenes. Activating Ha-ras mutations were present in essentially all DMBA-initiated tumors and about 70% of MNNG-initiated tumors. No N-ras mutations were found in tumors lacking other ras mutations. Surprisingly, 41% of the papillomas arising in the first 11 wk in MNNG-initiated, mezerein-promoted mice bore mutations in codon 12 of the Ki-ras oncogene. Activating Ki-ras mutations were also found in more than 60% of squamous cell carcinomas and 40% of keratoacanthomas. Although mutations in Ha-ras are frequently detected in mouse skin tumors, mutations in Ki-ras are rare. This is the first report of mutated Ki-ras in skin tumors from mice initiated by MNNG.     

Carcinogenic activity of acrylamide in the skin and lung of Swiss-ICR mice

Doses of acrylamide ranging from 12.5 to 50 mg/kg were administered orally to female ICR-Swiss mice over 3 days for each of 2 weeks (total doses of 75, 150 and 300 mg/kg). Two weeks later some of the animals were started on a promotion schedule involving the application of 2.5 micrograms TPA/mouse 3 times weekly. Development of tumors was observed weekly in the skin, and in the lungs at 1 year. Acrylamide was found to initiate squamous cell adenoma and carcinomas in the skin and increased the yield of adenomas and carcinomas in the lung. Skin tumor development was dependent upon 12-O-tetradecanoylphorbol-13-acetate (TPA) promotion whereas lung tumor induction was not. These data extend previous observations of carcinogenic activity of acrylamide in the skin of SENCAR mice and lungs of strain A/J mice to a third strain of mouse, the ICR-Swiss.     

Promotion by histamine of carcinogenesis in the forestomach and protection by histamine against carcinogenesis induced by N-nitroso-N-methylnitroguanidine in the glandular stomach in W rats

The effect of histamine on induction of tumors in the forestomach and the glandular stomach after N-nitroso-N-methylnitroguanidine (MNNG) administration was studied in male inbred W rats. Animals were given 50 micrograms MNNG solution/ml orally for 25 weeks and then 4 mg histamine dihydrochloride sc per day in depot form. Administration of histamine in depot form after MNNG significantly increased the incidence of tumors in the forestomach, but it significantly decreased the incidence of adenocarcinomas in the glandular stomach. All of the tumors induced in the forestomach were of the squamous cell type, and 50% of them were squamous cell carcinomas. Histamine alone had no apparent carcinogenicity in rats.     

Terminal differentiation-resistant epidermal cells in mice undergoing two-stage carcinogenesis

We have used an in vivo-in vitro approach to investigate the cellular aspects of two-stage skin carcinogenesis. Female SENCAR mice initiated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were promoted twice weekly with 12-O-tetradecanoylphorbol-13-acetate (TPA). Epidermal cultures from untreated or TPA-treated mice had few focus-forming cells resistant to calcium-induced terminal differentiation. Cultures from mice treated with MNNG alone formed numerous foci. Brief promotion (four TPA treatments) of MNNG-treated mice produced fewer but statistically larger foci, suggesting that TPA was selecting against more slowly growing cells. MNNG plus TPA-treated mice with very early papillomas produced more and larger foci than those due to MNNG treatment alone, suggesting that the papillomas may have comprised calcium-resistant cells. These cells may indeed be initiated cells since a permanent cell line arising after MNNG plus brief TPA treatment eventually formed histological papillomas in vivo. If calcium-resistant cells are initiated, then there were many more initiated cells in the skin (with or without TPA treatment) than papillomas expected, implying that either some initiated cells never formed papillomas, or that a significant accumulation of initiated cells had already occurred in the skin within 2 weeks of MNNG treatment. Subsequent TPA promotion of these cells apparently produced a toxic response that passively selected for more rapidly growing initiated cells, which eventually accumulated into papillomas.     

Carcinogenicities of quinoline derivatives in F344 rats

In Ames test, the quinoline derivatives, 6-nitroquinoline (6-NQ), 8-nitroquinoline (8-NQ), 6-methylquinoline (6-NQ), 8-methylquinoline (8-MQ) and 8-hydroxyquinoline (8-HQ) are mutagenic, whereas 5,7-dibromo-quinoline (5,7-DQ) is not. These compounds were administered to groups of male and female F344 rats in their diet for 104 weeks. 8-NQ induced forestomach tumors in male and female rats: it induced squamous cell papillomas in 28 (93.3%) of 30 males and in 36 (97.3%) of 37 females, and squamous cell carcinomas in 20 (66.7%) of 30 males and 24 (64.9%) of 37 females. Thus there was no sex difference in its carcinogenicity. 6-NQ induced only focal hyperplasia of the forestomach at an incidence of 30.0% in male rats and 52.8% in female rats. These results indicate a discrepancy between the mutagenicities of quinoline derivatives and their carcinogenicities to rats: only 8-NQ was carcinogenic to F344 rats.     

Systemic modulation by ultraviolet irradiation of cutaneous N-methyl-N'-nitro-N-nitrosoguanidine-induced carcinogenesis.

Ultraviolet irradiation can systemically enhance subsequent skin cancer induction by benzo[a]pyrene, methylcholanthrene, or UV radiation. The present study was designed to determine whether UVB irradiation influences host susceptibility to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Female C3H/HeJ mice were exposed dorsally to UVB radiation from banks of 6 Westinghouse FS40 sun lamps. The mice received a total UV dose of approximately 8.1 x 10(5) J m-2 over a 15-week period. After termination of UVB treatments, ventral tumors were induced by 4 applications of 30 mumol of MNNG at 8-day intervals. At 20 weeks after the first MNNG treatment, UVB-irradiated mice had 7-fold more MNNG-induced, ventral tumors than did the unirradiated control mice (P = 0.026, Wilcoxon rank sum test). Ventral application of MNNG after cessation of dorsal UVB exposure, but before UV tumor appearance, did not influence photocarcinogenesis. These results demonstrate that UV irradiation can systemically decrease host resistance to tumor induction by the methylating agent, MNNG.     

Promoting effect of sodium deoxycholate on colon adenocarcinomas in germfree rats.

The promoting effect of sodium deoxycholate (DC) on colon carcinogenesis was studied in female F344 germfree rats. Animals received intrarectal (ir) instillations of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) for 4 weeks (total dose, 16 mg/rat), then weekly ir doses of DC (total dose, 3 g/rat); the rats were autopsied 52 weeks after the first injection. DC increased the number of MNNG-induced colon adenocarcinomas. No tumors were in the colons of germfree rats given DC alone. It was concluded that DC (present in high concentrations in human stools) had a promoting effect on colon carcinogenesis in rats.     

Colon carcinogenesis in germ-free rats with intrarectal 1,2-dimethylhydrazine and subcutaneous azoxymethane.

The effect of intestinal microflora on colon carcinogenesis by 1,2-dimethylhydrazine and azoxymethane was studied, with the use of germ-free and conventional female Fischer rats. At 7 weeks of age, germ-free and conventional rats were treated with 20 weekly intrarectal 1,2-dimethylhydrazine (20 mg per kg body weight per week) or subcutaneous azoxymethane (10 mg per kg body weight per week) doses and were autopsied 15 weeks later. Tumors were induced in the small intestine and colon of germ-free and conventional rats treated with intrarectal 1,2-dimethylhydrazine; the number of rats with colon tumors and the multiplicity of tumors were decreased in germ-free rats, compared with conventional animals. Azoxymethane given subcutaneously increased the incidence and multiplicity of colon tumors in germ-free rats, compared with conventional controls. It is concluded that the intestinal microflora alter the carcinogenic and/or cocarcinogenic effect of different compounds in the large intestine.     

Carcinogenicity study of fecapentaene-12 diacetate on skin painting in SENCAR mice

To investigate the effects of both diol esterification and coadministration with antioxidant on the tumorigenicity of fecapentaene-12 (FP-12) preparations, diacetylfecapentaene-12 (DAFP-12) in dimethylsulfoxide (DMSO) was applied to SENCAR mouse skin with or without the stabilizer, vitamin E, twice/week for 5 weeks, following which all animals were promoted for up to 25 weeks by weekly applications of 12-O-tetradecanoylphorbol-13-acetate (TPA). While positive controls receiving 7,12-dimethylbenz[a]anthracene (DMBA) instead of DAFP-12 in a similar protocol all developed papillomas (average of 23/animal), papilloma incidence in mice given DAFP-12 did not differ significantly from that of the vehicle control. We conclude that DAFP-12 shows little or no tumor initiating activity for mouse skin even when coadministered with vitamin E.     

Transplacental and transgeneration carcinogenic effect of 7,12-dimethylbenz[a]anthracene: relationship with ras oncogene activation.

Transgeneration transmission of the carcinogenic action of 7,12-dimethylbenz[a]anthracene (DMBA) was studied in two generations of mice using transplacental DMBA initiation followed by postnatal skin tumor promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA) in the first generation (F0) and only promotion in the second generation (F1). Local application of TPA resulted in increased skin tumor yield in both the in utero DMBA-exposed mice and their progeny (P = 0.0002 and P = 0.0941 respectively compared to control). Similarly, lung tumor incidence was increased in the two generations of mice (P less than 0.0001 and P = 0.0080 respectively). The results suggest transgeneration transfer of the effect of DMBA. A to T mutation at the second base of codon 61 of the Ha-ras oncogene was found in skin tumors of DMBA-exposed mice, but not in tumors induced by TPA without initiation. Analysis of Ki-ras codon 61 in seven lung tumors from DMBA-treated mice revealed three types of mutation: two cases with CA[C or G or T], one case with CCA and one case with CTA (the remaining cases having only the wild type). Six of these mice also had skin tumors, which contained A to T mutation at the second base of codon 61 of the Ha-ras gene in five cases. Thus mutations of different ras genes were found in skin and lung tumors from the same animals. In the progeny (F1) of DMBA-exposed F0 mice, only skin tumor samples were available for oncogene analysis and none contained the Ha-ras mutation. The results confirm our previous finding that initiation of skin and lung tumorigenesis can be transmitted transgenerationally. On the other hand, our data from a limited number of skin tumors suggests that ras gene mutation may not be critically involved in this transmission.     

Urinary bladder carcinogenicity of dimethylarsinic acid in male F344 rats.

The present study was conducted to determine the carcinogenicity of dimethylarsinic acid (DMA) administered to male F344 rats in a 2 year bioassay. A total of 144 rats (10 weeks old at the start) were divided into four groups of 36 rats each. Groups 1-4 received DMA (purity 100%) at concentrations of 200, 50, 12.5 and 0 p.p.m. in the drinking water, respectively, for 104 weeks. From weeks 97 to 104, urinary bladder tumors were observed in 12 of 31, eight of 31 and none of 33 in groups 1-3, respectively. No bladder tumors were observed in group 4. The present study demonstrated that long-term p. o. administration of DMA induced urinary bladder carcinomas in male F344 rats. Therefore, the results indicate that DMA is carcinogenic for the rat urinary bladder, which may be related to the human carcinogenicity of arsenicals.