The role of Ha-ras oncogenes in growth factor independence in rat mammary carcinoma cells

Suppression of 7,12-dimethylbenz[a]anthracene-induced mammary carcinogenesis by pre-initiation treatment of rats with beta-naphthoflavone coincides with decreased levels of the carcinogen-derived DNA adducts in the mammary gland

BACKGROUND

Mechanisms underlying prevention by beta-naphthoflavone (beta-NF) of mammary carcinogenesis initiated with 7,12-dimethylbenz[a]anthracene (DMBA) in the rat were elucidated.

METHODS AND RESULTS

Treatment of female Sprague-Dawley rats with beta-NF at 40 mg/kg b.wt. for 4 days by oral gavage in corn oil before a single oral dose of DMBA (112 mg/kg b.wt.) suppressed mammary gland carcinogenesis as shown by an increase in the median latent period from 10 to 24 weeks and a 60% decrease in the multiplicity of mammary adenocarcinomas. In contrast, a 20-day treatment with beta-NF starting 3 weeks after DMBA had no significant effects on mammary tumorigenesis. The activities of phase I and phase II enzymes were examined in the liver and mammary gland 24 h after treatment of rats with beta-NF, DMBA, or beta-NF followed by DMBA as in the first bioassay. Treatment with either beta-NF or DMBA increased the hepatic activities of cytochrome P450 (CYP)1A1, 1A2, and 2B1/2, and glutathione S-transferase, and the mammary activity of CYP1A1. The activity of mammary CYP2B1/2 induced by DMBA was decreased by beta-NF. In the liver, the increase of UDP-glucuronosyl transferase (GT) activity in rats treated with beta-NF and DMBA was 2.3-fold greater than in rats treated with DMBA alone. Thus, treatment with beta-NF likely increased the rate of glucuronidation of DMBA dihydrodiols leading to carcinogen detoxification. The levels of the DMBA adducts determined by 32P-postlabeling of the mammary gland DNA were decreased in the beta-NF-pretreated rats.

CONCLUSION

The beta-NF-induced increase in the hepatic UDP-GT activity and decrease in the mammary DNA-DMBA adducts occurred under the same treatment regimen that led to suppression of DMBA-induced mammary carcinogenesis.

K-ras codon 12 and 61 point mutations in bromodeoxyuridine- and N-nitrosomethylurea-induced rat renal mesenchymal tumors

The mutagenic thymidine analog bromodeoxyuridine (BrdUrd) may incorrectly incorporate opposite deoxyguanine in DNA, then pair with deoxyadenosine during subsequent replication. It appears to preferentially target the 3'-G of 5'-NGGN-3' sequences in mammalian cells in culture to induce G-->A transitions. Ras genes should therefore be vulnerable to activation by mutation at glycine codons 12 (GGT) and/or 13 (GGC) by misincorporation of BrdUrd. There is limited evidence that BrdUrd may be carcinogenic or co-carcinogenic in rats: three renal mesenchymal tumors, a tumor known to be associated with activating mutations in the c-K-ras-2 oncogene, were reported in 87 rats treated with BrdUrd alone, while N-nitrosomethylurea (NMU) alone or NMU + BrdUrd resulted in incidences of 12/52 and 26/76, respectively, against a zero incidence in untreated rats. We analyzed renal mesenchymal tumors from rats treated with BrdUrd for mutations in K-ras exons 1 and 2 and compared the prevalence and spectrum of mutations with those found in comparable tumors induced with NMU. DNAs from 22 paraffin-embedded renal mesenchymal tumors from rats treated 12-15 months earlier with BrdUrd (three specimens) or NMU (11 specimens) or both agents sequentially (eight specimens) were amplified by PCR. The base sequence of codons 12-13 and 59-63 of K-ras was determined by the dideoxynucleotide method. Sequencing results were confirmed by allele-specific oligonucleotide hybridization. Two of three tumors that appeared in rats given BrdUrd alone contained both a codon 12 GGT-->GAT transition and a codon 61 CAA-->CTA transversion. One tumor induced by NMU alone also showed a codon 12 GGT-->GAT mutation, while only wild type sequence could be demonstrated in the codon 12-13 region in the remaining ten such tumors. Three NMU-induced tumors also showed codon 61 CAA-->CTA mutations, while the remaining tumors had wild type sequence. While the GGT-->GAT transitions identified in tumors from BrdUrd-treated rats are consistent with BrdUrd mutagenesis by misincorporation, the co-occurrence of CAA-->CTA transversions, the overall low prevalence of mutations, and the lack of any difference in mutation spectrum between tumors induced by NMU and those that occurred in BrdUrd-treated rats suggests that in both groups the mutations that did occur did not result from a direct effect of either agent.

Mutation and altered expression of p53 genes in experimental rat bladder tumor cells

p53 genes were analyzed for mutations and expression in a series of 24 tumors or hyperplastic lesions of the urinary bladder induced in F344 rats by carcinogen treatment. Of these, 18 were analyzed as short-term urothelial cultures. Polymerase chain reaction-single-strand conformation polymorphism analysis and DNA sequencing were used to detect alterations in p53 genes or cDNAs, and the relative amounts of p53 protein per cell were estimated by immunohistochemical staining. Missense substitutions were found in the exon 5-9 region of two of five cell cultures analyzed from lesions induced by the bladder carcinogen N-butyl-N-(4-hydroxybutyl)nitrosamine. One of these was a papillary nodular hyperplasia, indicating that p53 mutations can be present in low- as well as high-stage/grade bladder lesions. p53 mutations were not found in the exon 5-9 region in cells of any of eight bladder lesions induced by N-[4-(5-nitro-2-furyl)-2- thiazoly]formamide (FANFT), including five transitional cell carcinomas (TCCs), or either of two TCCs induced by N-methylnitrosourea. Two of nine TCCs induced by the N-glucuronide of N-hydroxy-2-aminofluorene were found to have p53 mutations. One of these was evidently altered by three genetic events: a missense substitution in exon 8, a nonsense mutation in exon 6, and silencing of the "nonsense" allele (i.e., only the p53 missense mutation was detected). Immunohistochemical analysis with monoclonal antibody PAb240 (which preferentially binds to mutant p53 protein) detected p53 antigen only in those samples in which missense p53 mutations were found. With monoclonal antibody PAb421 (which detects mutant and wild-type p53), p53 antigen was also detected in cells from F542, a bladder tumor induced by FANFT in which no p53 mutations were found. Northern blot hybridization analysis showed that p53 transcripts were elevated twofold to threefold in several cases, including F542, suggesting that constitutive overexpression of wild-type p53 may occur in some bladder neoplasias. These data support the view that p53 may be involved in multiple rate-limiting steps in neoplastic transformation and may be a continuing target during bladder carcinogenesis. The data also contribute to evidence that certain chemical carcinogens may directly alter p53 genes during tumorigenesis.

Cell proliferation and prevalence of ras gene mutations in 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumors

Exposure of female inbred Sprague-Dawley Curl rats to intragastric 7,12-dimethylbenz(a)anthracene (DMBA) resulted in a variety of benign and malignant mammary tumors exhibiting a wide spectrum of proliferative activity as measured by [3H]-thymidine autoradiography. On the basis of earlier observations of 21% H-ras codon 61 mutations in DMBA-induced rat mammary tumors reported by Zarbl et al., the prevalence of this type of ras gene mutation was studied in relation to the rate of proliferation in individual benign and malignant tumors. In a series of 50 mammary tumors exhibiting highly different proliferation activities we did not detect any H-ras codon 61 mutations after allele-specific dot blot hybridization of PCR-amplified material obtained from paraffin sections. Neither mode of proliferation nor type of differentiation appears related to H-ras codon 61 mutations. The relevance of activated ras genes for rat mammary carcinogenesis appears restricted to model systems using N-methyl-N-nitro-sourea (MNU) as initiating carcinogen.

Insulin like growth factor-I independence in rat mammary carcinoma cells: a dominant phenotype in somatic cell hybrid experiments

Normal rat mammary epithelial cells have an absolute requirement for insulin (IN) or insulin-like growth factor-I (IGF-I) for proliferation in serum-free medium. By contrast, serially transplantable rat mammary carcinoma (RMC) cells are IGF-I-independent for continuous growth in vitro. Previously, we demonstrated that IGF-I independence is not mediated by an autocrine loop. Therefore, experiments were carried out to determine if IGF-I independence behaves as a dominant or recessive phenotype in somatic cell hybridization experiments. IGF-I-independent 1-9RMT cells were rendered hygromycin-resistant and IGF-I-dependent MCF-10A cells were rendered G418 resistant by infection with retroviral expression vectors. Cells of each line were co-incubated in 60 mm dishes and fused with polyethylene glycol. Hybrid cells were selected with media containing hygromycin and G418 in the presence or absence of IN. In three experiments, approximately the same number of colonies emerged in double selection media in the presence or absence of IN. Furthermore, colonies that emerged in IN-containing media, when subcultured, grew equally well in the presence or absence of IN. Thus, fusion of IGF-I-independent RMC cells with IGF-I-dependent human mammary epithelial cells yields hybrids that are IGF-I-independent for growth in serum-free medium.