Chibby, a nuclear β-catenin-associated antagonist of the Wnt/Wingless pathway

Abstract

Inappropriate activation of downstream target genes by the oncoprotein beta-catenin is implicated in development of numerous human cancers. beta-catenin and its fruitfly counterpart Armadillo act as a coactivator in the canonical Wnt/Wingless pathway by binding to Tcf/Lef transcription factors. Here we report a conserved nuclear protein, named Chibby, which was identified in a screen for proteins that directly interact with the C-terminal region of beta-catenin. In mammalian cultured cells we demonstrate that Chibby inhibits beta-catenin-mediated transcriptional activation by competing with Lef-1 to bind to beta-catenin. Inhibition of Drosophila Chibby by RNA interference results in segment polarity defects that mimick a wingless gain-of-function phenotype, and overexpression of the wingless target genes engrailed and Ultrabithorax. In addition, epistasis experiments indicate that chibby acts downstream of wingless and upstream of armadillo.

Wnt signaling and skeletal development

“…Chibby is a nuclear antagonist that binds to the C terminus of β-catenin [20]. Another β-catenin binding protein, ICAT, can block the binding of β-catenin to TCF [21] and also can lead to the dissociation of complexes between β-catenin, LEF, and CBP/p300 [22,23].…”

Chibby1 knockdown promotes mesenchymal-to-epithelial transition-like changes

“…Cby1 is known to physically interact with b-catenin. 13,14 We did not detect a physical interaction of Cby1 with E-cadherin, nor a competition of Cby1 with Ecadherin for b-catenin binding ( Figure S3). Therefore, it is possible that the morphological changes induced by Cby1 K D are exerted, at least in part, through its ability to interact with b-catenin.…”

“…Acute Cby1 K D using morpholino oligonucleotides in HEK293T cells stimulated TopFlash activity. 13 It was therefore surprising to find that stable Cby1-K D (Fig. 2E).…”

“…31,32 The cytoplasmic domain of E-cadherin binds b-catenin at adherens junctions, which in turn interacts with a-catenin to link the complex to the actin cytoskeleton. Since Cby1 was initially isolated as a b-catenin-binding protein, 13 we analyzed the expression of b-catenin and E-cadherin in Cby1-K D cells. Strikingly, immunofluorescence (IF) staining revealed a significant increase in b-catenin levels at adherens junctions in Cby1-K D cells in comparison with control scrambled shRNA cells ( Fig.…”

“…11,12 Chibby1 (Cby1) was originally isolated as a b-catenin binding partner in a Ras recruitment screen in yeast. 13 It is a 15-kDa coiled-coil protein that is conserved throughout evolution. We previously reported that Cby1 represses canonical Wnt/b-catenin signaling via 2 distinct molecular mechanisms.…”