Control of β-Catenin Phosphorylation/Degradation by a Dual-Kinase Mechanism

␤-catenin protein level and Wnt signaling. and S45) at the amino-terminal region of ␤-catenin (Figure 1A) are conserved from Drosophila to human and La Jolla, California 92037 conform to the consensus GSK-3 phosphorylation site (Peifer et al., 1994). Indeed, ␤-catenin can be phosphory-lated by GSK-3 in vitro (Yost et al., 1996), and these Summary phospho-S/T residues are critical for ␤-catenin recognition by the F box protein ␤-Trcp, which is the specificity Wnt regulation of ␤-catenin degradation is essential for development and carcinogenesis. ␤-catenin degradation is initiated upon amino-terminal serine/threo-nine phosphorylation, which is believed to be per-and S45 phosphorylation in ␤-catenin degra-formed by glycogen synthase kinase-3 (GSK-3) in complex with tumor suppressor proteins Axin and ad-dation is underscored by the observation that mutations at these S/T residues frequently occur in human colo-nomatous polyposis coli (APC). Here we describe another Axin-associated kinase, whose phosphorylation rectal cancer and several other malignancies (Figure 1A), which are associated with and most likely caused of ␤-catenin precedes and is required for subsequent GSK-3 phosphorylation of ␤-catenin. This " priming " by deregulated accumulation of ␤-catenin kinase is casein kinase I␣ (CKI␣). Depletion of CKI␣ inhibits ␤-catenin phosphorylation and degradation 2000). However, despite the critical importance of these S/T phosphorylation events in regulating ␤-catenin sta-and causes abnormal embryogenesis associated with excessive Wnt/␤-catenin signaling. Our study uncov-bility, surprisingly little is known about how ␤-catenin is phosphorylated in vivo. ers distinct roles and steps of ␤-catenin phosphoryla-tion, identifies CKI␣ as a component in Wnt/␤-catenin Using a panel of antibodies that specifically recognize ␤-catenin phosphorylated at different S/T residues, we signaling, and has implications to pathogenesis/thera-peutics of human cancers and diabetes. found, unexpectedly, that ␤-catenin phosphorylation in vivo requires CKI␣, whose phosphorylation of ␤-catenin precedes and is obligatory for subsequent GSK-3 phos-Introduction phorylation of ␤-catenin. Depletion of CKI␣ prevents ␤-catenin phosphorylation and degradation. These find-Wnt signaling via ␤-catenin plays a central role in development and homeostasis (Wodarz and Nusse, 1998; ings have important implications to the understanding of Wnt/␤-catenin signaling, GSK-3 and CKI␣ function, Moon et al., 1997; Polakis, 2000). A key output of this pathway is the level of cytosolic ␤-catenin, which deter-and the pathogenesis and therapeutics of diseases. mines the activation of Wnt responsive genes. Without Wnt stimulation, ␤-catenin is constantly degraded by Results the proteosome (Orford et al., 1997; Aberle et al., 1997). This degradation strictly depends upon ␤-catenin phos-Distinct Roles and Steps of ␤-Catenin phorylation, which …