Tyrosine-based Signal Mediates LRP6 Receptor Endocytosis and Desensitization of Wnt/β-Catenin Pathway Signaling

Background: LRP6 is an essential co-receptor for Wnt/-catenin signaling, which controls various biological pathways. Results: LRP6 tyrosine mutant exhibited longer half-life and increased lipid raft distribution, phosphorylation, and signaling. Conclusion: Clathrin-mediated endocytosis of LRP6 mediates its degradation, whereas caveolae-dependent pathway promotes Wnt/-catenin signaling. Significance: Understanding the molecular mechanisms underlying LRP6 endocytosis and Wnt/-catenin signaling will help the development of therapeutic methods targeting this pathway. Wnt/-catenin signaling orchestrates a number of critical events including cell growth, differentiation, and cell survival during development. Misregulation of this pathway leads to various human diseases, specifically cancers. Endocytosis and phosphorylation of the LDL receptor-related protein 6 (LRP6), an essential co-receptor for Wnt/-catenin signaling, play a vital role in mediating Wnt/-catenin signal transduction. However, its regulatory mechanism is not fully understood. In this study, we define the mechanisms by which LRP6 endocytic trafficking regulates Wnt/-catenin signaling activation. We show that LRP6 mutant with defective tyrosine-based signal in its cytoplasmic tail has an increased cell surface distribution and decreased endocytosis rate. These changes in LRP6 endocytosis coincide with an increased distribution to caveolae, increased phosphorylation, and enhanced Wnt/-catenin signaling. We further demonstrate that treatment of Wnt3a ligands or blocking the clathrin-mediated endocytosis of LRP6 leads to a redistribution of wild-type receptor to lipid rafts. The LRP6 tyrosine mutant also exhibited an increase in signaling activation in response to Wnt3a stimulation when compared with wild-type LRP6, and this activation is suppressed when caveolae-mediated endocytosis is blocked. Our results reveal molecular mechanisms by which LRP6 endocytosis routes regulate its phosphorylation and the strength of Wnt/-catenin signaling, and have implications on how this pathway can be modulated in human diseases.