Negative regulation of LRP6 function by casein kinase I ε phosphorylation

Abstract

Wnt signaling acts in part through the low density lipoprotein receptor-related transmembrane proteins LRP5 and LRP6 to regulate embryonic development and stem cell proliferation. Up-regulated signaling is associated with many forms of cancer. Casein kinase I ε (CKIε) is a known component of the Wnt-β-catenin signaling pathway. We find that CKIε binds to LRP5 and LRP6 in vitro and in vivo and identify three CKIε-specific phosphorylation sites in LRP6. Two of the identified phosphorylation sites, Ser1420 and Ser1430, influence Wnt signaling in vivo, since LRP6 with mutation of these sites is a more potent activator of both β-catenin accumulation and Lef-1 reporter activity. Whereas Wnt3a regulates CKIε kinase activity, LRP6 does not, placing CKIε upstream of LRP6. Mutation of LRP6 Ser1420 and Ser 1430 to alanine strengthens its interaction with axin, suggesting a mechanism by which CKIε may negatively regulate Wnt signaling. The role of CKIε is therefore more complex than was previously appreciated. Generation of active CKIε may induce a negative feedback loop by phosphorylation of sites on LRP5/6 that modulate axin binding and hence β-catenin degradation. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.