Small-molecule inhibition of Wnt signaling through activation of casein kinase 1α

Wnt/beta-catenin signaling is critically involved in metazoan development, stem cell maintenance and human disease. Using Xenopus laevis egg extract to screen for compounds that both stabilize Axin and promote beta-catenin turnover, we identified an FDA-approved drug, pyrvinium, as a potent inhibitor of Wnt signaling (EC(50) of similar to 10 nM). We show pyrvinium binds all casein kinase 1 (CK1) family members in vitro at low nanomolar concentrations and pyrvinium selectively potentiates casein kinase 1 alpha (CK1 alpha) kinase activity. CK1 alpha knockdown abrogates the effects of pyrvinium on the Wnt pathway. In addition to its effects on Axin and beta-catenin levels, pyrvinium promotes degradation of Pygopus, a Wnt transcriptional component. Pyrvinium treatment of colon cancer cells with mutation of the gene for adenomatous polyposis coli (APC) or beta-catenin inhibits both Wnt signaling and proliferation. Our findings reveal allosteric activation of CK1 alpha as an effective mechanism to inhibit Wnt signaling and highlight a new strategy for targeted therapeutics directed against the Wnt pathway.