Identification of ICAT as an APC inhibitor, revealing Wnt-dependent inhibition of APC-Axin interaction

Adenomatous polyposis coli (APC) and Axin are core components of the beta-catenin destruction complex. How APC’s function is regulated and whether Wnt signaling influences the direct APC-Axin interaction to inhibit the beta-catenin destruction complex and not clear. Through a CRISPR screen of beta-catenin stability, we have identified ICAT, a polypeptide previously known to block beta-catenin-TCF interaction, as a natural inhibitor of APC. ICAT blocks beta-catenin-APC interaction and prevents beta-catenin-mediated APC-Axin interaction, enhancing stabilization of beta-catenin in cells harboring truncated APC or stimulated with Wnt, but not in cells deprived of a Wnt signal. Using ICAT as a tool to disengage beta-catenin-mediated APC-Axin interaction, we demonstrate that Wnt quickly inhibits the direct interaction between APC and Axin. Our study highlights an important scaffolding function of beta-catenin in the assembly of the destruction complex and suggests Wnt-inhibited APC-Axin interaction as a novel mechanism of Wnt-dependent inhibition of the destruction complex.

腺瘤性结肠息肉病蛋白（Adenomatous polyposis coli, APC）与轴蛋白（Axin）是β-连环蛋白破坏复合物（beta-catenin destruction complex）的核心组成成分。目前关于APC的功能调控机制，以及Wnt信号通路是否通过影响APC与Axin的直接相互作用以抑制β-连环蛋白破坏复合物，仍尚不明确。我们通过针对β-连环蛋白稳定性的CRISPR（Clustered Regularly Interspaced Short Palindromic Repeats）筛选，鉴定出ICAT——一种此前被报道可阻断β-连环蛋白与TCF相互作用的多肽——作为APC的天然抑制剂。ICAT可阻断β-连环蛋白与APC的结合，并抑制β-连环蛋白介导的APC-Axin相互作用，从而在携带截短型APC的细胞或经Wnt信号刺激的细胞中增强β-连环蛋白的稳定性，但在缺失Wnt信号的细胞中无此效应。以ICAT作为解离β-连环蛋白介导的APC-Axin相互作用的研究工具，我们证实Wnt可快速抑制APC与Axin之间的直接结合。本研究揭示了β-连环蛋白在破坏复合物组装过程中的重要支架功能，并提出Wnt抑制APC-Axin相互作用作为依赖Wnt的破坏复合物抑制新机制。