Abstract SY20-01: Role of Notch signaling in colon cancer invasion and metastasis

Abstract

Abstract While there have been substantial advances in our understanding of the mechanisms of cancer metastasis, efficient remedies for prevention and treatment of metastasis are still missing. The invasion-metastasis cascade consists of local invasion, intravasation, transport, extravasation, formation of micrometastases, and colonization (1,2). This sequence is completed only infrequently, causing metastatic inefficiency, and the least efficient of these steps appears to be colonization (3). Spread of metastatic cancer cells via blood circulation is responsible for the majority of distant metastases although they may travel also through the lymph ducts to nodes (4). In colorectal cancer, metastatic tropism to the liver and lungs can be explained largely by the organization of venous circulation of the intestines through portal vein to the liver, and further to the lungs through pulmonary artery. Among endogenous colon tumor models, the widely used Apc (Adenomatous polyposis coli) mutant mice form adenomas in the small intestine, with various multiplicities (3᠄300 per animal) depending on the mutational allele, although several adenomas are also found in the colon (5). Additional mutations introduced into Apc mutant mice can modify the tumor phenotype. For example, knocking out Smad4 gene in the TGF- family signaling converts the benign intestinal adenomas to very invasive adenocarcinomas (6). Even in such a model, the adenocarcinomas are only locally invasive, and neither intravasation nor distant metastasis is observed during the short life span of these mice. Accordingly, we have screened for candidate genes whose inactivation can stimulate metastasis of mouse colon cancer cells from the rectum to the liver, the commonest site of metastasis using an orthotopic transplantation model. We found such a metastasis suppressor of CRC; Aes that turned out to be an endogenous inhibitor of Notch signaling transcription that is carried out by a complex of Notch intracellular domain (NICD), Maml and Rbpj (7, 8). Upon introduction of a conditional Aes knockout mutation into adenomatous intestinal epithelium of Apc+/∆716 polyposis mice, their tumors become malignant due to Notch signaling activation, showing marked submucosal invasion and intravasation. Consistently, transendothelial migration (TEM) is increased significantly, when CRC cells are activated for Notch signaling and placed on an endothelial cell (EC) layer in culture. When human CRC cells that were inhibited for Aes expression were transplanted to the nude mouse rectum, the number of metastatic foci increased significantly. Thus, reduced level of Aes and stimulation of Notch signaling are implicated in the invasion and intravasation of CRC cells during metastasis. The molecular mechanism of Notch inhibition by Aes is through transcriptional repression by sequestering Rbpj, NICD and Maml1 to the nuclear matrix. The cellular mode of metastasis suppression includes inhibition of transendothelial migration (TEM) of tumor cells, which blocks intravasation. Heterotypic interaction between cancer epithelium and host stromal cells activates Notch signaling and promotes TEM of metastasizing cancer cells. Because TEM in metastasis is common to many solid tumors, this mechanism is important in various types of cancers. As a model for colon cancer invasion and intravasation, compound mutant mice for the Apc and Aes genes should be useful to evaluate new therapeutics including Notch signaling inhibitors against colon cancer metastasis. Some more recent advances will be presented and discussed regarding the molecular mechanism responsible for metastatic invasion downstream of Notch signaling activation.