Monitoring the histogenesis of colonic tumors in the Sprague-Dawley rat

Traditional serrated adenomas and serrated carcinomas in carcinogen-treated rats

AIMS

A recent review of archived sections from early experiments in rats showed neoplasias exhibiting serrated configurations. The aim was to assess the frequency of serrated neoplasias in the colon and small intestine of carcinogen-treated rats.

METHODS

While reviewing archival sections from early experiments in Sprague-Dawley (SD) and Fisher-344 (F-344) rats, we recently detected colonic and intestinal traditional serrated adenomas (displaying serrated or microtubular patterns) and serrated carcinomas. SD rats were injected 1,2-dimethylhydrazine (DMH) for 27 weeks whereas F-344 rats were fed with a pyrolysate (GLU-1) for 24 months. Filed sections from 358 colonic and small intestinal neoplasias were re-evaluated.

RESULTS

DMH-treated SD rats had 215 colonic neoplasias (1.4% were serrated adenomas, 7.9% microtubular adenomas, 2.8% serrated carcinomas and 2.8% microtubular carcinomas). GLU1-treated F-344 rats had 53 colonic neoplasias (1.9% were serrated adenomas and 20.8% microtubular adenomas), and 89 small intestinal neoplasias (1.1% were serrated adenomas, 42.7% microtubular adenomas and 6.7%, microtubular carcinomas).

CONCLUSIONS

DMH/SD-rats develop serrated and microtubular adenomas and carcinomas in the colon, whereas GLU1/F-344 rats develop microtubular adenomas in the colon and microtubular adenomas and carcinomas in the small intestine. The two rat-settings emerge as suitable models to study the molecular attributes of serrated and microtubular neoplasias under the standard conditions of the laboratory. This study is the first showing that a substantial number of serrated and particularly microtubular adenomas and carcinomas develop in the colon and small intestine of experimental rats. Importantly, serrated and microtubular neoplasias in rats recreate the histology of duodenal and colonic traditional serrated neoplasias in human beings.

Inhibition of tumor progression and neoangiogenesis using cyclic RGD-peptides in a chemically induced colon carcinoma in rats

Integrins are cell surface molecules that mediate cell adhesion, but are also important regulators of tumor cell interactions with their microenvironment, tumor cell survival and growth. In addition, the alpha(v)beta3-integrins appear to be critical for microvessel formation in tumor-induced neoangiogenesis. The present study is the first to investigate the effects of therapeutic alpha(v)beta3-integrin inhibition in a chemically induced tumor model that largely resembles human colon carcinomas. Tumor induction was performed in 47 male Sprague-Dawley rats using 1,2 dimethylhydrazin (21 mg/kg) twice a week. After 20 weeks of tumor induction, 100% of the animals developed adenocarcinomas with a median of 13.5 macroscopic tumor nodules (range 12-17), but no distant metastases. During further tumor induction for an additional 10 weeks, rats were treated three times/week with (a) 15 mg/kg RGDfV-peptide that can block vitronectin and fibronectin receptors; (b) an equimolar amount of an ineffective cyclic control peptide; or (c) with equimolar amounts of a linear RGDS-peptide. At the end of this treatment period, rats were sacrificed, and tumor load was quantified macroscopically and confirmed by histological examination. For investigation of the involvement of tumor-induced neoangiogenesis microvessel, density was determined using CD31-immunostaining. After 30 weeks, control animals (group B) had 5-18 tumors (median 14.5). If rats were treated with RGDfV-peptide (group A), the number of tumor nodules was significantly reduced (P < 0.005) to a median of seven macroscopic tumors (range 2-10 tumors), which also represented a significant reduction (P < 0.005) compared with prior to treatment. Application of noncylic RGDS-peptides (group C) did not affect the number of tumor nodules (median 18; range 10-30 tumors). The diameters of tumor nodules were comparable (3.2-6.1 mm) in animals of all groups. In addition, microvessel density was significantly (P < 0.05) reduced in tumors in group A compared to control rats. The major side effect in the treatment group was increased susceptibility to respiratory infections. Our results demonstrate that alpha(v)beta3-integrin-receptor inhibition appears to be a therapeutic strategy for colorectal cancer. In our therapeutic model, late onset of treatment with integrin-blocking peptides resulted in an inhibition of tumor growth and a reduced tumor load which appeared to be mediated, at least in part, by inhibition of neoangiogenesis.

Flat adenoma-adenocarcinoma sequence in the colon of rats

PURPOSE

As there is an increased awareness of the existence of a "flat adenoma-adenocarcinoma sequence" in the colonic mucosa of human subjects, the aims of the study were to assess whether flat colonic adenocarcinomas in rats are also preceded by flat adenomas, as is reported in humans, and to determine the frequency of flat lesions compared with exophytic lesions in the colon of rats.

METHOD

The colonotropic carcinogen 1,2-dimethylhydrazine was injected subcutaneously in 300 Sprague-Dawley rats for 27 weeks.

RESULTS

A total of 358 tumors developed in 278 of the 300 rats. Of the 60 adenomas found at histology, 25 percent were flat adenomas. Of the 298 adenocarcinomas, 12.7 percent had originated in a flat adenoma. Of the 180 colonic neoplasias (adenomas or adenocarcinomas), 29.4 percent were flat neoplasias (flat adenomas or adenocarcinomas arising in a flat adenoma), and the remaining 70.6 percent were exophytic neoplasias (tubulo or villous adenomas or adenocarcinomas arising in exophytic adenomas). From the 298 colonic adenocarcinomas, 1 was a intramucosal adenocarcinoma, 87 were overt adenocarcinomas, and 90 were lymphoid-associated carcinomas; in those 298 adenocarcinomas, no preneoplastic lesion could be recorded. In 208 animals, biopsies were taken from macroscopically visible colonic lesions, and, in the remaining 70 animals, the entire colon was processed for histologic examination. Flat adenomas were found in 3.8 percent of the 208 biopsy specimens and in 10 percent of the 70 colectomy specimens. Further, of the 40 adenomas found in biopsy specimens, 20 percent were flat adenomas, and, of the 20 adenomas found in colectomy specimens, 35 percent were flat adenomas.

CONCLUSIONS

The study reported herein indicates the existence of a "flat adenoma-adenocarcinoma sequence" in the colonic mucosa of Sprague-Dawley rats. The flat lesions of the colon constituted approximately one-third of the total neoplastic lesions seen in the rat following injections of 1,2-dimethylhydrazine. More flat adenomas were detected at histologic examination of the entire colon than in biopsies obtained from the macroscopically visible colonic lesions. Consequently, flat adenomas may be overlooked by naked-eye examination.

Quantification of acid mucins in the descending colon of rats having simultaneously growing colonic tumors

The acid mucins contained in goblet cells of the descending colon of 34 male Sprague-Dawley rats were histochemically labeled by Alcian blue pH 2.5 and quantified in an image analyzer (Cortex Controller). Twenty-two of these 34 rats were treated with 1,2-dimethylhydrazine (DMH) suspended in EDTA solution as a stabilizing agent and the remaining 12 rats with EDTA only. Of the 22 DMH-treated rats, 11 had concomitantly an adenocarcinoma elsewhere in the colon and the remaining 11 rats had no colonic tumors despite DMH treatment. The results indicated that Alcian blue-positive areas occupied 34.5% of the mucosa of the descending colon in tumor-bearing rats, and 35.2% in non-tumor-bearing DMH-treated rats. For EDTA-treated rats the percentage of mucosa occupied by Alcian blue-positive cells was 48.1%. The difference between DMH-treated rats (with or without tumors) and EDTA-treated rats was significant (p less than 0.001). These results suggest that the decrease of Alcian blue areas is related to the protracted treatment with DMH. Whether the decrease in acid mucins is induced by the carcinogen per se or whether it represents a true biochemical premalignant change at the cytoplasmic level remains to be elucidated.

Effect of indomethacin in autotransplanted colonic tumors

Seventy-five male Sprague-Dawley rats received weekly injections of dimethylhydrazine (DMH) for six months. Indomethacin was given orally to 40 of the rats. At the end of the allotted period, colonic tumors were autotransplanted into the subcapsular space of the left kidney. The autopsy demonstrated colonic tumors in 32 of 34 rats treated with DMH (94.1 percent), but only in 15 of 40 rats (37.5 percent) treated with DMH-indomethacin. Successful kidney autotransplantation was accomplished in 20 of 32 DMH-treated rats (63 percent) and in 5 of 15 DMH-indomethacin-treated rats (33.3 percent). Thus, the induction of colonic tumors by DMH and successful kidney autotransplants can be substantially abrogated by synchronous treatment with indomethacin.

A model for the study of metastases from colonic tumors by autotransplantation

Thirty male Sprague-Dawley rats received weekly injections of dimethylhydrazine for three months. They were subsequently laparotomized and tumors were palpated in 21 rats. The colonic tumor was removed and a fraction was autotransplanted into the wall of the transverse colon and a second fraction into the subcapsular space of the left kidney. The results demonstrated successful autotransplantation of colonic tumors in 13 of 21 rats (i.e., 62 percent). In 11 of the 21 rats (52 percent) the tumor grew in the left kidney. Of the various experimental methods proposed for the study of tumor metastases, the one described herein appears to be the first in which primary and secondary tumors were studied in the same individual, since dimethylhydrazine-treated rats usually develop more than one colonic tumor per individual. The model may be useful to monitor the effect of various drugs known to affect tumor growth, not only at the primary site, but also at the site of metastasis.