methylazoxymethanol and Adenocarcinoma

Colon cancer is the third most common cancer and third most common cause of cancer-related death in the USA according to 2008 American Cancer Society statistics. The carcinogenesis of colon cancer has been associated with both genetics and environmental factors. It has been found that several signal pathways, including K-ras, Src/PI3K/Akt, beta-catenin, TGFbeta and p53 play critical roles in its pathogenesis. The 5 y survival rate of metastatic colon cancer is below 10%. Thus, it is necessary to further understand its biology and search for effective therapy. Azoxymethane (AOM) is a common model for colon cancer. It can specifically induce colon cancer similar to the pathogenesis of human sporadic colon cancer. Thus, it has been extensively used in the study of the molecular biology, prevention and treatment of colon cancer. After administration, AOM is metabolised into methylazoxymethanol by CYP2E1, which causes DNA mutations. Mutation of K-ras activates this pathway and its downstream PI3K/Akt pathway and MAPK pathway. Mutation of beta-catenin also prevents it from being degraded by GSK-3 and accumulation of beta-catenin leads to cell proliferation. TGFbeta, a pro-apoptotic protein, is inhibited. All of these changes form the basis of AOM carcinogenesis. This model has been used in the study of the genetic deficiencies of colon cancer and in the prevention and treatment of the disease. For example, TGF-betaR2 and adiponectin knockout mice are more susceptible to AOM, while high amylose cornstarch, green tea and artemisia have protective effects.

Topics: Adenocarcinoma; Adenoma; Adiponectin; Animals; Anticarcinogenic Agents; Apoptosis; Azoxymethane; Carcinogens; Colonic Neoplasms; Cytochrome P-450 CYP2E1; Diet; DNA Damage; Genes, ras; Humans; MAP Kinase Signaling System; Methylazoxymethanol Acetate; Mice; Mice, Knockout; Phosphatidylinositol 3-Kinases; Proto-Oncogene Proteins c-akt; Rats; Signal Transduction; Transforming Growth Factor beta; Transforming Growth Factor beta2

p53 is one the most frequently mutated genes found in human colonic tumors. Because colonic neoplasms induced in rats by certain chemical carcinogens are similar to human colonic tumors in their histological features and proliferation characteristics, the rat has been used as an experimental model to study the pathogenesis of colon cancer. However, p53 mutations were not detected in the chemically induced colonic tumors analyzed for p53 mutations. X-irradiation has also been shown to induce colonic neoplasms in rats that resemble human colonic tumors histopathologically. Because the incidence of colonic tumors induced by methylazoxymethanol (MAM) in rats was shown to be enhanced by X-irradiation, we immunohistochemically analyzed these colonic carcinomas for the presence of p53 gene mutations. The immunohistochemical analyses clearly showed the absence of nuclear immunoreactivity in all ten tumors examined. The results from the present study indicate that point mutations in p53, at least in the coding region, are not involved in the development of colon cancer induced by the combination of MAM and X-irradiation. Our observations, together with the data from previous studies, further suggest that rat colon carcinogenesis, unlike human colon cancer, may not involve p53 mutation as an obligatory event.

Topics: Adenocarcinoma; Animals; Carcinogens; Colonic Neoplasms; Disease Models, Animal; Immunohistochemistry; Male; Methylazoxymethanol Acetate; Neoplasms, Radiation-Induced; Point Mutation; Rats; Tumor Suppressor Protein p53; Whole-Body Irradiation

Cisplatin resistance was developed in sublines of the CC531 rat colon adenocarcinoma cell line by continued low level drug exposure. Two relatively stable lines were obtained (RL2 and RL4) which were 6- and 20-fold more resistant to cisplatin. In addition, a subline more sensitive than the parent line by a factor of 2 (RLS) was obtained by subculture from a treated tumor. Mechanisms of resistance to cisplatin were investigated in these four lines, with the aim of determining the relative contributions of different resistance mechanisms at various resistance levels. Drug accumulation linearly decreased with increasing drug resistance. A 20-fold resistance was associated with only a 5-fold decrease in accumulation, suggesting that other resistance mechanisms may be involved in the total degree of resistance. Intracellular glutathione, measured fluorometrically, also increased with increasing resistance, varying by a factor of 4 between the most and least resistant lines. Reduction of glutathione levels by buthionine sulfoximine to parent line levels increased sensitivity but the cells remained considerably more resistant than parent cells. Resistant lines cultured in the absence of drug became progressively more sensitive, without accompanying changes in total glutathione levels. DNA-drug adducts, the presumed toxic lesion, were measured immunocytochemically. Initial levels decreased with increasing platinum resistance, although not proportional to resistance (factor of 5 decrease for 20-fold resistance). Drug dose ratios for equal initial adducts were similar to dose ratios for equal drug accumulation, implying that intracellular concentrations solely determine DNA adduction and that differences in glutathione level had little influence on the proportion of drug which eventually formed adducts. After 48 h, a better correlation between remaining adducts and resistance was found (factor 12 less adducts for 20-fold resistance). This implies that repair of adducts was important in determining survival. These data indicate that decreased drug accumulation played a proportionally greater role in the moderately resistant cell line and that adduct repair played a progressively greater role in the highly resistant cell line.

Topics: Adenocarcinoma; Animals; Buthionine Sulfoximine; Cisplatin; Colonic Neoplasms; DNA, Neoplasm; Drug Resistance; Glutathione; Methionine Sulfoximine; Methylazoxymethanol Acetate; Rats; Tumor Cells, Cultured; Tumor Stem Cell Assay

Exocrine pancreatic neoplasms developed in the guppy Poecilia reticulata following a single brief exposure to methylazoxymethanol acetate [(MAM-Ac) CAS: 592-62-1]. Fish 6-10 days old were exposed to concentrations of MAM-Ac up to 100 mg/liter for 2 hours. Exposed specimens were transferred to carcinogen-free water and sampled periodically for tumor development. Pancreatic neoplasms occurred in approximately 9% of histologically examined individuals exposed to 10 mg MAM-Ac/liter or less. Neoplastic lesions were not found in 122 control specimens. The neoplasms included 6 cases diagnosed as adenoma, 7 cases diagnosed as acinar cell carcinoma, and 2 cases diagnosed as adenocarcinoma. Adenomas consisted mainly of well-differentiated acinar cells that were filled with zymogen granules. Two adenomas also contained foci of atypical, less-differentiated acinar cells possessing basophilic, fibrillar cytoplasm. Acinar cell carcinomas occurred in several cellular patterns that ranged from well-differentiated to more anaplastic lesions; however, none exhibited areas of ductular proliferation. Adenocarcinomas, on the other hand, exhibited a glandular growth pattern and contained numerous ductlike structures. Both types of carcinomas appear to arise from primary acinar cells. Thus lesions probably progress from adenomas to acinar cell carcinomas and adenocarcinomas. The findings of carcinogen-induced pancreatic neoplasms in guppies further strengthen the usefulness of small fish species in carcinogen testing and provide an additional model for pancreatic tumors.

Topics: Adenocarcinoma; Adenoma; Animals; Azo Compounds; Methylazoxymethanol Acetate; Pancreas; Pancreatic Neoplasms; Poecilia