Caveolin is necessary for Wnt-3a-dependent internalization of LRP6 and accumulation of β-catenin

beta-catenin-mediated Writ signaling is critical in animal development and tumor progression. The single-span transmembrane Writ receptor, low-density lipoprotein receptor-related protein 6 (LRP6), interacts with Axin to promote the Wnt-dependent accumulation of beta-catenin. However, the molecular mechanism of receptor internalization and its impact on signaling are unclear. Here, we present evidence that LRP6 is internalized with caveolin and that the components of this endocytic pathway are required not only for Wnt-3a-induced internalization of LRP6 but also for accumulation of P-catenin. Overall, our data suggest that Wnt-3a triggers the interaction of LRP6 with caveofin and promotes recruitment of Axin to LRP6 phosphorylated by glycogen synthase kinase-3 beta and that caveolin thereby inhibits the binding of beta-catenin to Axin. Thus, caveolin plays critical roles in inducing the internalization of LRP6 and activating the Wnt/beta-catenin pathway. We also discuss the idea that distinct endocytic pathways correlate with the specificity of Writ signaling events.