| BackgroundsThe incidence of colorectal cancer (CRC) is known to be increased in patients with ulcerative colitis (UC) and the risk of CRC increases with increasing extent and duration. The mechanisms underlying the frequent development of CRC in patients with UC are still unknown.Through the study of animal models which allow experiments not feasible in humans, we can better understand the cause and mechanisms of various diseases. There are many animal models for ulcerative colitis, but only a few of them are applicable to study the dysplasia-cancer sequence. Among them, the most widely used is a mouse model with dextran sulfate sodium (DSS). DSS can be used to induce ulcerative colitis in mice. Acute colitis can be observed by administration of 5%-10% DSS to mice for 4-9 days. A chronic colitis can be induced by feeding 3%-5% DSS in drinking water in cycles of 7 days of DSS followed by 7-14 days of water for several cycles. In recent years, some researchers have described the appearance of dysplasia and/or cacers in mice when they are subjected to repeated administration of DSS in a long term. Before the occurrence of cancer, the features of the colitis in this model are very similar to those in patients in terms of both clinical and histopathological characteristics, i.e. diarrhea, occult blood, melena, mucosal inflammatory cell infiltration, crypt abscess formation, and mucosal erosion. But, this kind of model needs a long period and the incidence of induced tumors are relatively low.1,2-dimethylhydrazine (DMH) is known as a toxic environmental pollutant which was reported also to be a specific colon procarcinogen. Evidence from animal studies showed that experimental colonic tumors induced by DMH are of epithelial origin with a similar histology, morphology and anatomy to human colonic neoplasms. This procarcinogen thus provides an adequate model for studies of the colorectal cancer. However, mutiple treatments with DMH (20-40mg/kg body weight) and a long-term experimental period are needed to induce large bowel neoplasms.MethodsStudy designAt 8 weeks of age, the animals were divided into 4 groups of 10-12 mice each. There was one experimental group (group A) and 3 control groups (groups B-D). The animals in group A were subjected to three cycles of alternating administration of distilled water containing 3% (w/v) synthetic dextran sulfate sodium for 7 days followed by distilled water for the subsequent 14 days after intraperitoneal pretreatment with 20 mg/kg 1,2-dimethylhydrazine. For comparison purposes, control groups B to D received each of the treatment alone or maintained as untreated control.Observation of colitisChanges in body weight were recorded every week throughout the experiment. Occult blood was examined during day 3 or 4 of DSS feeding, on the day DSS feeding was stopped and once a week thereafter. Presence of gross blood and stool consistency were observed daily.Histopathological evaluationAfter death, the entire colorectum from the colocecal junction to the anal verge was examined. Their length was measured , and then the specimen was opened longitudinally and washed with saline. After the colorectum was macroscopically inspected, it was immediately fixed in a 10% formaldehyde buffer solution (pH 7.2).The part of the colon was divided into three equal portions (proximal, middle and distal). The average five-six pieces/portion and 14-17 pieces/colorectum were stained with hematoxylin and eosin (H&E) for histological processing.The evaluation of β -catenin and p53To detect the expression of β -catenin and p53, we utilized the two step immunostaining technique.ResultsBody growth, fecal examination and colorectal lengthAt the end of the first DSS treatment period, 75.0% (18/24) mice in group A and B had diarrhea and occult blood or gross blood in the feces, and these signs disappeared after the mice drank distilled water for next 14 days. However, during the second and third administration of DSS, there are only 45.5% (10/22) and 36.4% (8/22) mice who had diarrhea or occult blood, respectively.Body... |
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