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Tumor cell budding and laminin‐5 expression in colorectal carcinoma can be modulated by the tissue micro‐environment

Expression of laminin‐5 α3, β3 and γ2 protein subunits was investigated in colorectal adenocarcinomas using immunostaining and confocal microscopy. The laminin‐5 heterotrimer was found in basement membranes and as extracellular deposits in tumor stroma. In contrast to the α3 subunit, which was under‐expressed, the γ2 and β3 subunits were detected in the cytoplasm of carcinoma cells dissociating (budding) from neoplastic tubules, suggestive of focal alterations in laminin‐5 assembly and secretion. Laminin‐5 γ2 or β3 subunit‐reactive budding carcinoma cells expressed cytokeratins but not vimentin; they did not proliferate and were not apoptotic. Furthermore, expression of laminin‐5 γ2 and β3 subunits in budding cells was associated with focal under‐expression of the E‐cadherin–β‐catenin complex. Results from xenograft experiments showed that budding activity in colorectal adenocarcinomas could be suppressed when these tumors grew at ectopic s.c. sites in nude mice. In vitro, cultured colon carcinoma cells, but not adenoma‐derived tumor cells, shared the laminin‐5 phenotype expressed by carcinoma cells in vivo. Using colon carcinoma cell lines implanted orthotopically and invading the cecum of nude mice, the laminin‐5–associated budding was restored, indicating that this phenotype is not only determined by tumor cell properties but also dependent on the tissue micro‐environment. Our results indicate that both laminin‐5 α3 subunit expression and cell–cell cohesiveness are altered in budding carcinoma cells, which we consider to be actively invading. We propose that the local tissue micro‐environment contributes to these events. Int. J. Cancer 88:708–717, 2000. © 2000 Wiley‐Liss, Inc.