Nuclear overexpression of the oncoprotein β-catenin in colorectal cancer is localized predominantly at the invasion front

Sixty to eighty percent of all colorectal cancers are characterized by mutations in the APC tumor suppressor gene. Recently, it was shown that these mutations lead to a nuclear overexpression of beta-Catenin by disruption of the wingless/WNT signal pathway. Since nuclear beta-Catenin functions as a transcriptional activator of hitherto unknown tumor genes, this form of beta-Catenin is now considered a major oncoprotein in colorectal cancer. Using immunohistochemistry, we investigated the distribution of overexpressed beta-Catenin within individual colorectal carcinomas. In the majority of the tumors, we found no homogeneous staining, but a strong nuclear expression of beta-Catenin predominantly localized at the invasion front with strongest nuclear staining of isolated, scattered tumor cells. In contrast, cells in the tumor center often showed no nuclear staining, but retained a membranous expression of beta-Catenin, comparable to normal colon epithelium. It is, therefore, likely that in addition to the overexpression of beta-Catenin caused by defects in the APC locus, regulatory events in the tumor itself lead to a different distribution of this oncoprotein. Possibly, surrounding tissue at the invasion front can give signals to the tumor cells, leading to a nuclear translocation of beta-Catenin, where it may play a direct role in tumor invasion processes.