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 epsilon (CKI epsilon) is a known component of the Wnt-beta-catenin signaling pathway. We find that CKI epsilon binds to LRP5 and LRP6 in vitro and in vivo and identify three CKI epsilon-specific phosphorylation sites in LRP6. Two of the identified phosphorylation sites, Ser(1420) and Ser(1430), influence Wnt signaling in vivo, since LRP6 with mutation of these sites is a more potent activator of both beta-catenin accumulation and Lef-1 reporter activity. Whereas Wnt3a regulates CKI epsilon kinase activity, LRP6 does not, placing CKI epsilon upstream of LRP6. Mutation of LRP6 Ser(1420) and Ser(1430) to alanine strengthens its interaction with axin, suggesting a mechanism by which CKI epsilon may negatively regulate Wnt signaling. The role of CKI epsilon is therefore more complex than was previously appreciated. Generation of active CKI epsilon may induce a negative feedback loop by phosphorylation of sites on LRP5/6 that modulate axin binding and hence beta-catenin degradation.