title: The Cdc42/Rac nucleotide exchange factor protein β1Pix (Pak-interacting exchange factor) modulates β-catenin transcriptional activity in colon cancer cells: evidence for direct interaction of β1PIX with β-catenin.
abstract: Wnt/β-catenin signaling is highly regulated and critical for intestinal epithelial development and repair; aberrant β-catenin signaling is strongly associated with colon cancer. The small GTPase Rac1 regulates β-catenin nuclear translocation and signaling. Here we tested the hypothesis that β1Pix, a Cdc42/Rac guanine nucleotide exchange factor (GEF), regulates β-catenin-dependent transcriptional activity and cell function. We report the novel observations that β1Pix binds directly to β-catenin, an action requiring both the β1Pix DH and dimerization domains but not β1Pix GEF activity. In human colon cancer cells, activation of β-catenin signaling with LiCl decreased β1Pix/β-catenin association in the cytosol and increased nuclear binding of β-catenin to β1Pix. Nuclear association of β1Pix and β-catenin was independent of Rac1 expression and activation; down- and up-regulating Rac1 expression levels did not alter nuclear β1Pix/β-catenin association. Ectopic β1Pix expression enhanced LiCl-induced β-catenin transcriptional activity. Conversely, siRNA knockdown of β1Pix attenuated both LiCl-induced β-catenin transcriptional activity and colon cancer cell proliferation. Ectopic expression of β1Pix stimulated β-catenin transcriptional activity, whereas β1PixΔ(602-611), which is unable to bind β-catenin, had no effect. Altogether, these findings suggest that β1Pix functions as a transcriptional regulator of β-catenin signaling through direct interaction with β-catenin, an action that may be functionally relevant to colon cancer biology.
authors.affiliation: Division of Gastroenterology and Hepatology, Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland 21201-1595.
authors.forename: Ahmed
authors.initials: A
authors.lastname: Chahdi
authors.affiliation: Division of Gastroenterology and Hepatology, Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland 21201-1595; Marlene and Stewart Greenebaum Cancer Center, University of Maryland School of Medicine, Baltimore, Maryland 21201-1595; Veterans Affairs Maryland Health Care System, Baltimore, Maryland 21201-1595. Electronic address: jraufman@medicine.umaryland.edu.
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Raufman
mesh_terms: D000276:Adjuvants, Immunologic / Q000494:pharmacology; D018938:Caco-2 Cells; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D049109:Cell Proliferation; D003110:Colonic Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D003600:Cytosol / Q000378:metabolism* / Q000473:pathology; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000235:genetics; D055785:Gene Knockdown Techniques; D006801:Humans; D018021:Lithium Chloride / Q000494:pharmacology; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism*; D055503:Protein Multimerization / Q000187:drug effects / Q000235:genetics; D017434:Protein Structure, Tertiary; D064067:Rho Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism*
chemical_list: C576606:ARHGEF7 protein, human; D000276:Adjuvants, Immunologic; D009363:Neoplasm Proteins; D064067:Rho Guanine Nucleotide Exchange Factors; D051176:beta Catenin; D018021:Lithium Chloride
keywords: Cell Proliferation; Colon Cancer; Guanine Nucleotide Exchange Factor (GEF); Small GTPases; β-Catenin