Colon
carcinomas commonly contain mutations in Ki-ras4B, but very rarely in Ha-ras, suggesting that different Ras
isoforms may have distinct functions in colon epithelial cell biology. In an earlier study we had demonstrated that oncogenic Ki-ras4BVal-12, but not oncogenic Ha-rasVal-12, blocks the apicobasal polarization of colon epithelial cells by preventing normal glycosylation of the
integrin beta1 chain of the
collagen receptor. As a result, only the Ki-ras mutated cells exhibited altered cell to substratum attachment, whereas mutation of either Ras
isoform activated
mitogen-activated protein kinases. We have now asked whether intercellular adhesion
proteins implicated in establishing basolateral polarity in colon epithelial cells are modulated by oncogenic Ki-Ras4BVal-12
proteins but not oncogenic Ha-RasVal-12
proteins. The embryonic adhesion
protein carcinoembryonic antigen (CEA) was up-regulated on the
mRNA and
protein levels in each of three stable Ki-rasVal-12 transfectant lines but in none of three stable Ha-rasVal-12 transfectant lines. The elevated
protein levels of CEA in Ki-ras4BVal-12 transfectant cells were decreased by blocking expression of Ki-ras4BVal-12 with
antisense oligonucleotides.
N-cadherin levels were decreased in only the Ki-ras transfectants, whereas
E-cadherin levels were unchanged. Immunohistochemical analysis demonstrated that Ki-ras4BVal-12 transfectant cells did not polarize into cells with discrete apical and basal regions and so could not restrict expression of CEA to the apical region. These unpolarized cells displayed elevated levels of CEA all along their surface membrane where CEA mediated random, multilayered associations of
tumor cells. This aggregation was both
calcium-independent and blocked by
Fab' fragments of anti-CEA
monoclonal antibody col-1. Trafficking of the lysosomal
cysteine protease cathepsin B may also be altered when cell polarity cannot be established. Ki-ras4BVal-12 transfectant cells expressed 2-fold elevated
protein levels of the lysosomal
cysteine protease cathepsin B but did not up-regulate
cathepsin B mRNA expression. One function of oncogenic c-Ki-
Ras proteins in
colon cancer progression may be to up-regulate CEA and thus to prevent the lateral adhesion of adjacent colon epithelial cells that normally form a monolayer in vivo.