IQGAP1-mediated stimulation of transcriptional co-activation by beta-catenin is modulated by calmodulin.
Beta-catenin, an oncoprotein integral to cell-cell adhesion and proliferative signaling, is increased in several malignancies. The recently discovered calmodulin-binding protein IQGAP1 binds stoichiometrically to beta-catenin and regulates the association of beta-catenin with the cell-cell adhesion complex. In the present work, we investigated the role of IQGAP1 on the transcriptional co-activator function of beta-catenin, and whether calmodulin modulated the functional interaction between IQGAP1 and beta-catenin. In vitro competition assays revealed that both Ca(2+)/calmodulin and apo-calmodulin, when pre-bound to IQGAP1, prevented binding of beta-catenin to IQGAP1, and partially displaced beta-catenin pre-bound to IQGAP1 when added subsequently. Conversely, beta-catenin partially displaced apo-calmodulin, but not Ca(2+)/calmodulin, from IQGAP1. Overexpression of IQGAP1 in SW480 colon carcinoma cells enhanced beta-catenin-mediated transcriptional co-activation by 1.72-fold, and this stimulation was significantly attenuated upon antagonism of calmodulin using the cell-permeable antagonist CGS9343B. Moreover, an IQGAP1 mutant that does not bind calmodulin was unable to stimulate beta-catenin transcriptional function. Results of pulse-chase analyses suggested that IQGAP1 slowed the turnover of soluble, but not total, beta-catenin. Immunocytochemistry revealed that IQGAP1 overexpression increased the amount of beta-catenin located in the nucleus, whereas incubation of cells with CGS9343B blocked this accumulation. Together, our results imply that IQGAP1 enhances the function of beta-catenin in the nucleus and that calmodulin regulates this stimulation.
Briggs
Michael W
MW
Department of Pathology, Brigham and Women's Hospital and Harvard Medical School, 75 Francis Street, Boston, MA 02115, USA.
Li
Zhigang
Z
Sacks
David B
DB
D000930:Antidiarrheals / Q000494:pharmacology; D001562:Benzimidazoles / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D002147:Calmodulin / Q000378:metabolism*; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000378:metabolism*; D005982:Glutathione Transferase / Q000378:metabolism; D049452:Green Fluorescent Proteins; D006801:Humans; D007150:Immunohistochemistry; D008156:Luciferases / Q000378:metabolism; D008164:Luminescent Proteins / Q000378:metabolism; D008715:Methionine / Q000378:metabolism; D008840:Microfilament Proteins / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D010957:Plasmids / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D013997:Time Factors; D015534:Trans-Activators; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D014162:Transfection; D014407:Tumor Cells, Cultured; D015854:Up-Regulation; D051176:beta Catenin; D020703:ras GTPase-Activating Proteins
D000930:Antidiarrheals; D001562:Benzimidazoles; C495270:CTNNB1 protein, human; D002147:Calmodulin; D003598:Cytoskeletal Proteins; C101203:IQ motif containing GTPase activating protein 1; D008164:Luminescent Proteins; D008840:Microfilament Proteins; D015534:Trans-Activators; D051176:beta Catenin; D020703:ras GTPase-Activating Proteins; C052087:CGS 9343B; D049452:Green Fluorescent Proteins; D008715:Methionine; D008156:Luciferases; D005982:Glutathione Transferase