ONCOGENIC C-KI-RAS BUT NOT ONCOGENIC C-HA-RAS UP-REGULATES CEA EXPRESSION AND DISRUPTS BASOLATERAL POLARITY IN COLON EPITHELIAL-CELLS

Colon carcinomas commonly contain mutations in Ki-ras4B, but very rare ly in Ha-ras, suggesting that different Ras isoforms may have distinct functions in colon epithelial cell biology, In an earlier study we ha d demonstrated that oncogenic Ki-ras4B(Val-12), but not oncogenic Ha-r as(Val-12), blocks the apicobasal polarization of colon epithelial cel ls by preventing normal glycosylation of the integrin beta 1 chain of the collagen receptor, As a result, only the Ki-ras mutated cells exhi bited altered cell to substratum attachment, whereas mutation of eithe r Res isoform activated mitogen-activated protein kinases. We have now asked whether intercellular adhesion proteins implicated in establish ing basolateral polarity in colon epithelial cells are modulated by on cogenic Ki-Ras4B(Val-12) proteins but not oncogenic Ha-Ras(Val-12) pro teins, The embryonic adhesion protein carcinoembryonic antigen (CEA) w as up-regulated on the mRNA and protein levels in each of three stable Ki-ras(Val-12) transfectant lines but in none of three stable Ha-ras( Val-12) transfectant lines. The elevated protein levels of CEA in Ki-r as4B(Val-12) transfectant cells were decreased by blocking expression of Ki-ras4B(Val-12) with antisense oligonucleotides. N-cadherin levels were decreased in only the Ki-ras transfectants, whereas E-cadherin l evels were unchanged. Immunohistochemical analysis demonstrated that K i-ras4B(Val-12) transfectant cells did not polarize into cells with di screte apical and basal regions and so could not restrict expression o f CEA to the apical region, These unpolarized cells displayed elevated levels of CEA all along their surface membrane where CEA mediated ran dom, multilayered associations of tumor cells, This aggregation was bo th calcium-independent and blocked by Fab' fragments of anti-CEA monoc lonal antibody col-1. Trafficking of the lysosomal cysteine protease c athepsin B may also be altered when cell polarity cannot be establishe d. Ki-ras4B(Val-12) transfectant cells expressed 2-fold elevated prote in levels of the lysosomal cysteine protease cathepsin B but did not u p-regulate cathepsin B mRNA expression. One function of oncogenic c-Ki -Ras proteins in colon cancer progression may be to up-regulate CEA an d thus to prevent the lateral adhesion of adjacent colon epithelial ce lls that normally form a monolayer in vivo.