The Cdc42/Rac Nucleotide Exchange Factor Protein β1 Pix (Pak-interacting Exchange Factor) Modulates β-Catenin Transcriptional Activity in Colon Cancer Cells EVIDENCE FOR DIRECT INTERACTION OF β1 PIX WITH β-CATENIN

Wnt/beta-catenin signaling is highly regulated and critical for intestinal epithelial development and repair; aberrant -catenin signaling is strongly associated with colon cancer. The small GTPase Rac1 regulates -catenin nuclear translocation and signaling. Here we tested the hypothesis that (1)Pix, a Cdc42/Rac guanine nucleotide exchange factor (GEF), regulates -catenin-dependent transcriptional activity and cell function. We report the novel observations that (1)Pix binds directly to -catenin, an action requiring both the (1)Pix DH and dimerization domains but not (1)Pix GEF activity. In human colon cancer cells, activation of -catenin signaling with LiCl decreased (1)Pix/-catenin association in the cytosol and increased nuclear binding of -catenin to (1)Pix. Nuclear association of (1)Pix and -catenin was independent of Rac1 expression and activation; down- and up-regulating Rac1 expression levels did not alter nuclear (1)Pix/-catenin association. Ectopic (1)Pix expression enhanced LiCl-induced -catenin transcriptional activity. Conversely, siRNA knockdown of (1)Pix attenuated both LiCl-induced -catenin transcriptional activity and colon cancer cell proliferation. Ectopic expression of (1)Pix stimulated -catenin transcriptional activity, whereas (1)Pix(602-611), which is unable to bind -catenin, had no effect. Altogether, these findings suggest that (1)Pix functions as a transcriptional regulator of -catenin signaling through direct interaction with -catenin, an action that may be functionally relevant to colon cancer biology.