Restoration of Epithelial Cell Polarity in a Colorectal Cancer Cell Line by Suppression of β-catenin/T-Cell Factor 4-mediated Gene Transactivation

β-Catenin acts as a transcriptional coactivator by forming a complex with T-cell factor/lymphoid enhancer factor (TCF/LEF) DNA-binding proteins. Aberrant transactivation of a certain set of target genes by β-catenin and TCF4 complexes has been implicated in familial and sporadic colorectal tumorigenesis. A colorectal cancer cell line, DLD-1, becomes irregularly multilayered, when maintained confluent for 2–3 weeks, and forms numerous dome-like polypoid foci piled-up over the surface of cell sheets. By the use of a strict tetracycline-regulation system, we found that the continuous suppression of β-catenin/TCF4-mediated gene transactivation by dominant-negative TCF4B (ΔN30) reduced these piled-up foci and restored a simple monolayer of polarized columnar cells resembling normal intestinal epithelium. The restoration of epithelial cell polarity was evident in two ways: ( a ) the formation of microvilli over the apical surface; and ( b ) the distribution of a tight junction protein, ZO-1, to the lateral plasma membrane. Retroviral expression of stabilized β-catenin (ΔN89) induced the formation of similar piled-up foci in untransformed IEC6 intestinal epithelial cells. Sulindac, a nonsteroidal anti-inflammatory drug effective against colorectal tumorigenesis in familial adenomatous polyposis syndrome, suppressed the formation of foci. The loss of epithelial cell polarity may be a critical cellular event driving β-catenin/TCF4-mediated intestinal tumorigenesis.