title: Negative regulation of LRP6 function by casein kinase I epsilon 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 epsilon (CKIepsilon) is a known component of the Wnt-beta-catenin signaling pathway. We find that CKIepsilon binds to LRP5 and LRP6 in vitro and in vivo and identify three CKIepsilon-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 beta-catenin accumulation and Lef-1 reporter activity. Whereas Wnt3a regulates CKIepsilon kinase activity, LRP6 does not, placing CKIepsilon upstream of LRP6. Mutation of LRP6 Ser1420 and Ser1430 to alanine strengthens its interaction with axin, suggesting a mechanism by which CKIepsilon may negatively regulate Wnt signaling. The role of CKIepsilon is therefore more complex than was previously appreciated. Generation of active CKIepsilon may induce a negative feedback loop by phosphorylation of sites on LRP5/6 that modulate axin binding and hence beta-catenin degradation.
authors.affiliation: Department of Oncological Sciences, Huntsman Cancer Institute, University of Utah, Salt Lake City, Utah 84112-5550, USA.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Swiatek
authors.forename: Heeseog
authors.initials: H
authors.lastname: Kang
authors.forename: Benjamin A
authors.initials: BA
authors.lastname: Garcia
authors.forename: Jeffery
authors.initials: J
authors.lastname: Shabanowitz
authors.forename: Gary S
authors.initials: GS
authors.lastname: Coombs
authors.forename: Donald F
authors.initials: DF
authors.lastname: Hunt
authors.forename: David M
authors.initials: DM
authors.lastname: Virshup
mesh_terms: D000595:Amino Acid Sequence; D048149:Casein Kinase 1 epsilon / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D026502:LDL-Receptor Related Proteins / Q000378:metabolism* / Q000502:physiology*; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D051821:Lymphoid Enhancer-Binding Factor 1 / Q000378:metabolism; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D051153:Wnt Proteins / Q000378:metabolism; D060508:Wnt3 Protein; D060509:Wnt3A Protein; D051176:beta Catenin / Q000378:metabolism*
chemical_list: D026502:LDL-Receptor Related Proteins; C555482:LRP5 protein, human; C555475:LRP6 protein, human; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D051821:Lymphoid Enhancer-Binding Factor 1; C555510:WNT3A protein, human; D051153:Wnt Proteins; D060508:Wnt3 Protein; D060509:Wnt3A Protein; D051176:beta Catenin; D048149:Casein Kinase 1 epsilon