LRP6 transduces a canonical Wnt signal independently of Axin degradation by inhibiting GSK3's phosphorylation of beta-catenin.
Wnt/beta-catenin signaling controls various cell fates in metazoan development and is misregulated in several cancers and developmental disorders. Binding of a Wnt ligand to its transmembrane coreceptors inhibits phosphorylation and degradation of the transcriptional coactivator beta-catenin, which then translocates to the nucleus to regulate target gene expression. To understand how Wnt signaling prevents beta-catenin degradation, we focused on the Wnt coreceptor low-density lipoprotein receptor-related protein 6 (LRP6), which is required for signal transduction and is sufficient to activate Wnt signaling when overexpressed. LRP6 has been proposed to stabilize beta-catenin by stimulating degradation of Axin, a scaffold protein required for beta-catenin degradation. In certain systems, however, Wnt-mediated Axin turnover is not detected until after beta-catenin has been stabilized. Thus, LRP6 may also signal through a mechanism distinct from Axin degradation. To establish a biochemically tractable system to test this hypothesis, we expressed and purified the LRP6 intracellular domain from bacteria and show that it promotes beta-catenin stabilization and Axin degradation in Xenopus egg extract. Using an Axin mutant that does not degrade in response to LRP6, we demonstrate that LRP6 can stabilize beta-catenin in the absence of Axin turnover. Through experiments in egg extract and reconstitution with purified proteins, we identify a mechanism whereby LRP6 stabilizes beta-catenin independently of Axin degradation by directly inhibiting GSK3's phosphorylation of beta-catenin.
Department of Cell and Developmental Biology, Vanderbilt University Medical Center, 465 21st Avenue South, U-4200 Learned Laboratory, Medical Research Building III, Nashville, TN 37232-8240, USA.
Christopher S
CS
Cselenyi
Kristin K
KK
Jernigan
Emilios
E
Tahinci
Curtis A
CA
Thorne
Laura A
LA
Lee
Ethan
E
Lee
D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000818:Animals; D060466:Axin Protein; D002417:Cattle; D002457:Cell Extracts; D000072261:Dishevelled Proteins; D004625:Embryo, Nonmammalian / Q000201:enzymology / Q000378:metabolism; D038362:Glycogen Synthase Kinase 3 / Q000037:antagonists & inhibitors* / Q000378:metabolism; D042541:Intracellular Space; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D051379:Mice; D010063:Ovum / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D011973:Receptors, LDL / Q000737:chemistry / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism; D015398:Signal Transduction; D013816:Thermodynamics; D025801:Ubiquitin / Q000378:metabolism; D051153:Wnt Proteins / Q000378:metabolism*; D014981:Xenopus / Q000196:embryology / Q000378:metabolism*; D029867:Xenopus Proteins; D051176:beta Catenin / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; D060466:Axin Protein; D002457:Cell Extracts; D000072261:Dishevelled Proteins; D060488:Low Density Lipoprotein Receptor-Related Protein-6; C555476:Lrp6 protein, mouse; D010750:Phosphoproteins; D011973:Receptors, LDL; D011994:Recombinant Proteins; D012097:Repressor Proteins; D025801:Ubiquitin; D051153:Wnt Proteins; D029867:Xenopus Proteins; C555454:axin1 protein, Xenopus; D051176:beta Catenin; D038362:Glycogen Synthase Kinase 3; D046988:Proteasome Endopeptidase Complex