Small molecule inhibition of MEK activates Wnt signalling and leads to reprogramming of colon cancer stem cells [Affymetrix]

Resistance to Ras pathway inhibition is a major challenge in the treatment of colorectal cancer (CRC), but the underlying mechanisms are incompletely understood. Here we performed large-scale small molecule screens in CRC and identified inhibitors of MEK1/2 as potent activators of Wnt/beta-catenin signalling. Targeting MEK increased Wnt activity in different CRC cell lines and in the murine intestine in vivo. The MEK-induced Wnt response was strongly enhanced by truncating mutations in APC and proteomic experiments identified that AXIN1 levels are depleted upon MEK inhibition. We generated patient-derived colon cancer organoids and showed that a clinically approved MEK inhibitor leads to increased Wnt activity, elevated LGR5 levels and enrichment of gene signatures associated with stem cells and cancer relapse. This reprogramming was reverted by co-treatment with Wnt inhibitors. Our study demonstrates that MEK inhibition affects cellular plasticity and induces an intestinal stem cell program which constitutes a novel mechanism of drug resistance. Total RNA from human colorectal cancer organoids treated with trametinib for 72 h.