Negative Regulation of LRP6 Function by Casein Kinase I Phosphorylation

Department of Oncological Sciences, University of Utah, Salt Lake City, Utah, United States
Journal of Biological Chemistry (Impact Factor: 4.57). 06/2006; 281(18):12233-41. DOI: 10.1074/jbc.M510580200
Source: PubMed


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 Ser1430 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.

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    • "The Wg and Hh signaling pathways share several common elements, including members of the Casein kinase 1 (CK1) family of serine/threonine protein kinases. Cell culture and RNAi experiments have shown that CK1g and CK1e can phosphorylate the Wg co-receptor LRP6/Arrow (Arr) (Davidson et al. 2005; Swiatek et al. 2006; Zhang et al. 2006; Casagolda et al. 2010) and the downstream component Dsh (Klein et al. 2006; Bernatik et al. 2011). CK1a is thought to behave as a priming kinase for Arm, triggering its proteasomal degradation and inhibiting transcriptional output from the pathway (Liu et al. 2002; Yanagawa et al. 2002; Marin et al. 2003). "
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    ABSTRACT: The Wnt and Hedgehog signaling pathways are essential for normal development and are misregulated in cancer. The casein kinase family of serine/threonine kinases regulates both pathways at multiple levels. However, it has been difficult to determine whether individual members of this family have distinct functions in vivo, due to their overlapping substrate specificities. In Drosophila melanogaster, photoreceptor differentiation is induced by Hedgehog and inhibited by Wingless, providing a sensitive system in which to identify regulators of each pathway. We used a mosaic genetic screen in the Drosophila eye to identify mutations in genes on the X chromosome required for signal transduction. We recovered mutations affecting the transcriptional regulator CREB binding protein, the small GTPase dynamin, the cytoskeletal regulator Actin-related protein 2, and the protein kinase Casein kinase 1α. Consistent with its reported function in the β-Catenin degradation complex, Casein Kinase 1α mutant cells accumulate β-Catenin and ectopically induce Wingless target genes. In contrast to previous studies based on RNA interference, we could not detect any effect of the same Casein Kinase 1α mutation on Hedgehog signaling. We thus propose that Casein kinase 1α is essential to allow β-Catenin degradation and prevent inappropriate Wingless signaling, but its effects on the Hedgehog pathway are redundant with other Casein kinase 1 family members.
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    • "The STF3A cell line contains, in addition, stably integrated pPGK plus mWNT3A (a gift from Karl Willert, University of California San Diego, La Jolla, CA) and a co-transfected blasticidin resistance marker. STF LRP6N cells had been modified in a similar manner, stably integrating pCS2 + plus VSVG-tagged LRP6N linearized at NotI (Swiatek et al., 2006) into STF cells. STF -catenin S45A cells were made using the vector pCS2MT plus -catenin S45A linearized at SalI. "
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