Regulation of Casein Kinase Iε Activity by Wnt Signaling

Abstract

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.