Despite metastasis as an important cause of death in colorectal cancer patients, current animal models of this disease are scarcely metastatic. We evaluated whether direct orthotopic cell microinjection, between the mucosa and the muscularis layers of the cecal wall of nude mice, drives tumor foci to the most relevant metastatic sites observed in humans and/or improves its yield as compared with previous methods. We injected eight animals each tested human colorectal cancer cell line (HCT-116, SW-620, and DLD-1), using a especially designed micropipette under binocular guidance, and evaluated the take rate, local growth, pattern and rate of dissemination, and survival time. Take rates were in the 75 to 88% range. Tumors showed varying degrees of mesenteric and retroperitoneal lymphatic foci (57 to 100%), hematogenous dissemination to liver (29 to 67%) and lung (29 to 100%), and peritoneal carcinomatosis (29 to 100%). Tumor staging closely correlated with animal survival. Therefore, the orthotopic cell microinjection procedure induces tumor foci in the most clinically relevant metastatic sites: colon-draining lymphatics, liver, lung, and peritoneum. The replication of the clinical pattern of dissemination makes it a good model for advanced colorectal cancer. Moreover, this procedure also enhances the rates of hematogenous and lymphatic dissemination at relevant sites, as compared with previously described methods that only partially reproduce this pattern. Colorectal cancer cases represents 15% of all cancer types. Its poor prognosis and the consequence of its metastatic spread makes colorectal cancer the second most common cause of cancer death in western countries.1 However, genetically modified mouse models of colorectal cancer are scarcely or not metastatic.2,3 Moreover, more metastatic cancer models, such as surgical orthotopic implantation (SOI), experimental or spontaneous metastasis assays, and orthotopic cell injection also show limitations. Thus, although SOI of human colorectal cancer in nude mice yields liver metastasis, 4,5 it does not generate lung metastasis, nor mesenteric or retroperitoneal lymphatic metastasis, 6 and requires the previous expansion of the tumor in subcutaneous xenografts, [7][8][9] which may alter its growth and dissemination capacities. 10 On the other hand, the experimental metastasis assay or spontaneous metastasis assay, consisting of cell injection into the tail vein or footpad, are less physiological and usually generate tumor foci only at one single site. [11][12][13][14][15][16] Moreover, injection of colorectal cancer cells in the ileocolic vein or in the apical lymphoid follicle 12,17,18 limits metastases to liver and lymphatics, varying widely in their rate.We tested whether direct orthotopic cell microinjection (OCMI), between the mucosa and the muscularis externa

Colorectal adenocarcinoma is the second cause of cancer mortality in developed countries. Rac1 is a member of the family of Rho GTPases that regulates many intracellular signaling pathways, including those involved in tumorigenesis, invasion, and metastasis. We have investigated the role of Rac1 in colorectal tumor progression by genetic modification of the human colorectal adenocarcinoma cell line SW620 to either overexpress Rac1 or lack Rac1 expression. Tumor behavior was studied by orthotopic injection of stably modified cell lines into the cecal wall of athymic nude mice, a model that replicates the histopathological appearance and clinical behavior of human colorectal adenocarcinoma in humans. While overexpression of Rac1 resulted in an accelerated tumorigenic process, inducing a faster mortality rate, inhibition of Rac1 completely suppressed tumor formation. These results suggest that Rac1 plays a major role in colorectal adenocarcinoma progression. Finally, interference with Rac1 function may provide an important tool to block the malignant phenotype of colorectal adenocarcinoma cells.

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