Wnt secretion is required to maintain high levels of Wnt activity in colon cancer cells

Aberrant regulation of the Wnt/beta-catenin pathway has an important role during the onset and progression of colorectal cancer, with over 90% of cases of sporadic colon cancer featuring mutations in APC or beta-catenin. However, it has remained a point of controversy whether these mutations are sufficient to activate the pathway or require additional upstream signals. Here we show that colorectal tumours express elevated levels of Wnt3 and Evi/Wls/GPR177. We found that in colon cancer cells, even in the presence of mutations in APC or beta-catenin, downstream signalling remains responsive to Wnt ligands and receptor proximal signalling. Furthermore, we demonstrate that truncated APC proteins bind beta-catenin and key components of the destruction complex. These results indicate that cells with mutations in APC or beta-catenin depend on Wnt ligands and their secretion for a sufficient level of beta-catenin signalling, which potentially opens new avenues for therapeutic interventions by targeting Wnt secretion via Evi/Wls.