No involvement of beta-catenin gene mutation in gastric carcinomas induced by N-methyl-N-nitrosourea in male F344 rats

N-Methyl-N-nitrosourea as a mammary carcinogenic agent

The administration of chemical carcinogens is one of the most commonly used methods to induce tumors in several organs in laboratory animals in order to study oncologic diseases of humans. The carcinogen agent N-methyl-N-nitrosourea (MNU) is the oldest member of the nitroso compounds that has the ability to alkylate DNA. MNU is classified as a complete, potent, and direct alkylating compound. Depending on the animals' species and strain, dose, route, and age at the administration, MNU may induce tumors' development in several organs. The aim of this manuscript was to review MNU as a carcinogenic agent, taking into account that this carcinogen agent has been frequently used in experimental protocols to study the carcinogenesis in several tissues, namely breast, ovary, uterus, prostate, liver, spleen, kidney, stomach, small intestine, colon, hematopoietic system, lung, skin, retina, and urinary bladder. In this paper, we also reviewed the experimental conditions to the chemical induction of tumors in different organs with this carcinogen agent, with a special emphasis in the mammary carcinogenesis.

O modelo experimental de carcinogênese gástrica induzido por n-methyl-n-nitrosourea em ratos F344 e camundongos C3H é válido para os ratos Wistar?

INTRODUÇÃO: O N-metil-N-nitrosourea (MNU) tem ação cancerígena direta, induzindo tumores em várias espécies em uma variedade de órgãos, incluindo o estômago de ratos. Tratamento do MNU na água de beber por 25-42 semanas, seletivamente, induz carcinoma gástrico glandular de ratos F344 e camundongos C3H. OBJETIVO: Estabelecer um modelo experimental para indução seletiva de câncer no estômago glandular de ratos Wistar com MNU. MÉTODOS: Um total de 48 ratos Wistar machos com oito semanas, foram utilizados no presente estudo. MNU (Sigma-Aldrich) foi dissolvido em DMSO e liberada água potável ad libitum por um período variando de 16 a 70 semanas. Após 16 semanas, quatro ratos foram selecionados aleatoriamente e mortos. Depois, de seis em seis semanas, quatro animais também foram mortos até 70 semanas. RESULTADOS: A taxa de sobrevivência foi superior a 90%. Ocorreu a indução de dois adenocarcinomas, um carcinoma espinocelular e um sarcoma. A incidência de adenocarcinoma gástrico foi de 4,5% (0,5 a 15). CONCLUSÕES: O modelo experimental de carcinogênese gástrica em ratos Wistar, utilizando MNU dissolvido na água, não mostrou viabilidade prática neste estudo, devido à baixa taxa de adenocarcinoma gástrico que ocorreu.

Roles of cyclooxygenase-2 and microsomal prostaglandin E synthase-1 expression and beta-catenin activation in gastric carcinogenesis in N-methyl-N-nitrosourea-treated K19-C2mE transgenic mice

K19-C2mE transgenic (Tg) mice, simultaneously expressing cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES-1) in the gastric mucosa under the cytokeratin 19 gene promoter, were here treated with N-methyl-N-nitrosourea (MNU) and inoculated with Helicobacter pylori (H. pylori) to investigate gastric carcinogenesis. Wild-type (WT) and Tg mice undergoing MNU treatment frequently developed tumors in the pyloric region (100% and 94.7%, respectively); multiplicity in Tg was higher than that in WT (P < 0.05) with H. pylori infection. Larger pyloric tumors were more frequently observed in Tg than in WT (P < 0.05). In addition, Tg developed fundic tumors, where WT did not. No gastric tumors were observed without MNU treatment. Transcripts of TNF-alpha, iNOS, IL-1beta, and CXCL14 were up-regulated with H. pylori infection in both genotypes and were also increased more in Tg than in WT within H. pylori-inoculated animals. Immunohistochemical analysis demonstrated significantly greater beta-catenin accumulation in pyloric tumors, compared with those in the fundus (P < 0.01) with mutations of exon 3; 18.2% and 31.6% in MNU-alone and MNU + H. pylori-treated WT, whereas 21.4% and 62.5% was observed in the Tg, respectively; the latter significantly higher (P < 0.05), suggesting the role of H. pylori in Wnt activation. In conclusion, K19-C2mE mice promoted gastric cancer in both fundic and pyloric regions. Furthermore beta-catenin activation may play the important role of pyloric carcinogenesis especially in H. pylori-infected Tg. Induction of various inflammatory cytokines in addition to overexpression of COX-2/mPGES-1 could be risk factors of gastric carcinogenesis and may serve as a better gastric carcinogenesis model.

Development of gastric tumors in Apc(Min/+) mice by the activation of the beta-catenin/Tcf signaling pathway

Although several lines of evidence suggest the involvement of the Wnt pathway in the development of gastric cancers, the functional significance of the pathway in gastric carcinogenesis is still poorly defined. To examine the role of the Apc/beta-catenin signaling pathway in the development of gastric cancers, we investigated the gastric mucosa of the Apc(Min/+) mouse, which is a murine model for familial adenomatous polyposis, carrying a germ-line mutation at codon 850 of Apc. We found that aged Apc(Min/+) mice spontaneously develop multiple tumors in the stomach, which are accompanied by loss of heterozygosity of Apc. Such tumors consisted of adenomatous glands with strong nuclear accumulation of beta-catenin. Even a single adenomatous gland already showed nuclear accumulation of beta-catenin, suggesting that Apc/beta-catenin pathway is an initiating event in gastric tumorigenesis in Apc(Min/+) mice. Myc and cyclin D1 expressions, which are transcriptional targets of beta-catenin/Tcf, increased in the adenomatous lesions. Furthermore, beta-catenin/Tcf reporter transgenic mice with Apc(Min) allele showed higher levels of the transcriptional activity of beta-catenin/Tcf in the gastric tumors. We also treated Apc(Min/+) and wild-type mice with N-methyl-N-nitrosourea (MNU), an alkylating agent that induces adenomas and adenocarcinomas in the stomach. Consequently, MNU-treated Apc(Min/+) mice significantly enhanced the tumor development in comparison with Apc(Min/+) mice or MNU-treated wild-type mice. Several gastric tumors in MNU-treated Apc(Min/+) mice showed invasion into the submucosal layer. These results indicate that the Apc/beta-catenin pathway may play an important role in at least subset of gastric carcinomas. In addition, Apc(Min/+) mice combined with MNU could be a useful short-term model to investigate multistage carcinogenesis in the stomach.

Correlation of Wnt-2 expression and beta-catenin intracellular accumulation in Chinese gastric cancers: relevance with tumour dissemination

Wnt/beta-catenin signalling pathway is integrally associated with human tumour development and progression. Aberrant beta-catenin intracellular distribution has been found in gastric cancer, but the pattern of Wnt expression in stepwise gastrocarcinogenesis and its potential influence in beta-catenin distribution are still lesser known. By the methods of frozen tissue array-based immunohistochemistry, Western blot analysis and RT-PCR, a paralleled study was conducted to check Wnt2 expression and beta-catenin intracellular distribution in two major subtypes of gastric cancers (intestinal gastric cancer, i-GC and diffuse gastric cancer, d-GC) and their premalignant (intestinal metaplasia, IM and chronic gastritis, CG) and noncancerous counterparts. According to the results obtained and the clinical data collected, correlation of Wnt2 expression with beta-catenin translocalisation and their links with tumour dissemination were elucidated. The results demonstrated (1) that Wnt2 expression and cytoplasmic/nuclear beta-catenin accumulations appeared in most gastric cancers irrespective to their morphological phenotypes, (2) that over-expressed Wnt and nuclear translocalisation of beta-catenin were found in 68 and 58% of i-GCs and in 47 and 47% of d-GCs in a closely related pattern (P<0.01) and (3) that co-existence of Wnt2 up-regulation/beta-catenin nuclear translocalisation were positively associated with lymph node metastasis (P<0.05) as well as T-stage. These data indicate that Wnt/beta-catenin signalling pathway is activated in most of gastric cancers, which may play pivotal roles either in gastric cancer formation or in tumour invasion and dissemination.

Beta-catenin gene alteration in glandular stomach adenocarcinomas in N-methyl-N-nitrosourea-treated and Helicobacter pylori-infected Mongolian gerbils

The goal of this study was to elucidate whether beta-catenin gene mutations might contribute to glandular stomach carcinogenesis in Helicobacter pylori (H.pylori)-infected Mongolian gerbils. Firstly, exon 3 of gerbil beta-catenin cDNA, a mutation hot spot, was cloned and sequenced and found to have 89.3% homology with the human form and 95.5% with the rat and mouse forms. Peptide sequence in this region was shown to be 100% conserved in these mammals. Then, 45 stomach adenocarcinomas induced with N-methyl-N-nitrosourea (MNU) plus H. pylori infection and 7 induced with MNU alone were examined for beta-catenin expression by immunohistochemistry and for DNA mutations using a combination of microdissection and PCR-single strand conformation polymorphism analysis. One gastric cancer in the MNU + H. pylori group (2.2%) displayed nuclear (N) beta-catenin localization, 3 (6.7%) showed cytoplasmic (C) distribution in local regions, and 41 (91.1%) demonstrated cell membrane (M) localization. Tumors induced by MNU alone showed only membranous beta-catenin localization (7/7). Analysis of exon 3 of the beta-catenin gene dem-onstrated all tumors with membrane or cytoplasmic staining as well as surrounding normal mucosa (S) to feature wild-type beta-catenin. In contrast, the lesion with nuclear staining had a missense mutation at codon 34 [GAC (Gly) --> GAA (Glu)] in exon 3 (1/1 = 100%, N vs. M, P < 0.05; and N vs. S, P < 0.05). In conclusion, these results suggest that beta-catenin may not be a frequent target for mutation in stomach carcinogenesis in MNU + H. pylori-treated gerbils.