Negative regulation of LRP6 function by casein kinase I ε phosphorylation

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.