Editors' ChoiceCancer

Inhibiting Wnt Signaling

Science Signaling  06 Oct 2009:
Vol. 2, Issue 91, pp. ec326
DOI: 10.1126/scisignal.291ec326

Wnt signaling leads to destabilization of the scaffolding protein axin and dissociation of the axin-containing β-catenin destruction complex, allowing β-catenin to accumulate in the nucleus and regulate the transcription of target genes. Crucial in development, deregulated signaling through the Wnt pathway has been implicated in the development of various cancers, notably colon cancer; however, its components have proved elusive to targeted therapy (see Peterson). Huang et al. conducted a high-throughput screen using a Wnt-responsive luciferase reporter in human embryonic kidney (HEK) 293 cells and identified XAV939 as a small-molecule inhibitor of Wnt signaling. Analysis of its effects on components of the Wnt signaling pathway in colorectal cancer cell lines indicated that XAV939 increased the abundance of the β-catenin destruction complex. Using a quantitative chemical proteomics approach in which proteins that bound immobilized XAV939 were competed by solubilized XAV939, the authors identified 18 proteins that specifically bound XAV939, including several poly(ADP-ribose) polymerases (PARPs). Like XAV939, the combined knockdown of the PARPs tankyrase 1 (TNKS1) and TNKS2 increased the abundance of axin1 and axin2, as well as increasing β-catenin phosphorylation, decreasing β-catenin abundance, and inhibiting activity of a Wnt-responsive reporter. TNKS1 and TNKS2 coimmunoprecipitated with axin2 in SW480 cells, and analyses of the interaction between axin1 fragments and TNKS1 in a yeast two-hybrid assay identified an N-terminal region of axin1 that was necessary and sufficient for TNKS binding [the “tankyrase-binding domain” (TBD)]. The ability of TNKS1 and TNKS2 to decrease axin abundance depended on the presence in axin of the TBD and on their poly-ADP-ribosylation (PARsylation) activity. TNKS2 PARsylated an axin1 fragment containing the TBD in vitro; in cells, XAV939 inhibited axin PARsylation, ubiquitination, and degradation. XAV939 inhibited the proliferation of a line of colorectal cancer cells with dysregulated Wnt signaling, an effect abolished by knockdown of axin1 and axin2. Thus, the authors conclude that tankyrases represent previously unknown components of the Wnt signaling pathway that may provide new approaches to therapeutic intervention in Wnt-dependent cancers.

S.-M. A. Huang, Y. M. Mishina, S. Liu, A. Cheung, F. Stegmeier, G. A. Michaud, O. Charlat, E. Wiellette, Y. Zhang, S. Wiessner, M. Hild, X. Shi, C. J. Wilson, C. Mickanin, V. Myer, A. Fazal, R. Tomlinson, F. Serluca, W. Shao, H. Cheng, M. Shultz, C. Rau, M. Schirle, J. Schlegl, S. Ghidelli, S. Fawell, C. Lu, D. Curtis, M. W. Kirschner, C. Lengauer, P. M. Finan, J. A. Tallarico, T. Bouwmeester, J. A. Porter, A. Bauer, F. Cong, Tankyrase inhibition stabilizes axin and antagonizes Wnt signalling. Nature 461, 614–620 (2009). [PubMed]

R. T. Peterson, Propping up a destructive regime. Nature 461, 599–600 (2009). [PubMed]