Research Information

Mutations in the tumor suppressor Adenomatous Polyposis Coli (APC) initiate the formation of most colon tumors. APC is a key regulator of the canonical Wnt-pathway, a signal transduction pathway involved in a number of cellular processes including proliferation and cell fate decisions. The key regulated step in the Wnt-pathway is the regulation of the transcriptional co-activator ß-catenin. In the “Off”-state, ß-catenin is constitutively targeted for degradation in the proteasome by a multi-protein complex called the “destruction complex”. release of ß-catenin from the destruction complex. Another model proposed that binding of APC protects ß-catenin from dephosphorylation in the cytoplasm, which would block proteasomal degradation. Other models suggest APC acts in the mucleus to export ß-catenin or repress Wnt-regulated genes

I am using the human colorectal cancer cell line SW480 to test these models of APC function, I am expressing in this cell line mutant versions of the Drosophila APC family member APC2. These either lack individual protein domains or mis-localize APC2 to particular intracellular locations. In parallel, we are analyzing these same mutants in vivo in the fruit fly. Our data are inconsistent with current models suggesting high-affinity ßcatenin-binding sites on APC play key roles. Instead, they suggest multiple ßcatenin-binding sites act additively to fine-tune signaling via cytoplasmic retention. We identify essential roles for two putative binding sites for new partners—20-amino-acid repeat 2 and conserved sequence B—in destruction complex action. Finally, we demonstrate APC interacts with Axin by two different modes and provide evidence that conserved sequence B helps ensure release of APC from Axin, with disassembly critical in regulating ßcatenin levels. Using these data, we suggest a new model for destruction complex action in development, which also provides new insights into functions of truncated APC proteins in cancer.