The Wnt/β-catenin signaling pathway is important in both development and cancer. Casein kinase Iϵ (CKIϵ) is a positive regulator of the canonical Wnt pathway. CKIϵ itself can be regulated in vitro by inhibitory autophosphorylation, and recent data suggest that in vivo kinase activity can be regulated by extracellular stimuli. We show here that the phosphorylation state and kinase activity of CKIϵ are directly regulated by Wnt signaling. Coexpression of XWnt-8 or addition of soluble Wnt-3a ligand led to a significant and rapid increase in the activity of endogenous CKIϵ. The increase in CKIϵ activity is the result of decreased inhibitory autophosphorylation because it is abolished by preincubation of immunoprecipitated kinase with ATP. Furthermore, mutation of CKIϵ inhibitory autophosphorylation sites creates a kinase termed CKIϵ(MM2) that is significantly more active than CKIϵ and is not activated further upon Wnt stimulation. Autoinhibition of CKIϵ is biologically relevant because CKIϵ(MM2) is more effective than CKIϵ at activating transcription from a Lef1-dependent promoter. Finally, CKIϵ(MM2) expression in Xenopus embryos induces both axis duplication and additional developmental abnormalities. The data suggest that Wnt signaling activates CKIϵ by causing transient dephosphorylation of critical inhibitory sites present in the carboxyl-terminal domain of the kinase. Activation of the Wnt pathway may therefore stimulate a cellular phosphatase to dephosphorylate and activate CKIϵ
This work was supported by the Huntsman Cancer Foundation and National Institutes of Health Grant R01CA80809 (to D. M. V.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.