MEK inhibitors activate Wnt signalling and induce stem cell plasticity in colorectal cancer

Background:

Colorectal cancer (CRC) is characterized by the aberrant activation of cellular signalling pathways, which are frequently caused by genetic alterations. Among those affected pathways, Wnt signalling plays an essential role for tumorigenesis and maintenance of cancer stem cells. However, it is unclear how other oncogenic pathways converge on Wnt signalling to modulate stem cell homeostasis in CRC.

Objective:

We investigated the interactions between Wnt and Ras signalling and their impact on stem cell plasticity in CRC.

Methods:

We performed large scale compound screens to discover novel small molecule modulators of Wnt signalling. The effect of the identified candidate was characterized in CRC cell lines, mouse models and murine and patient derived intestinal cancer organoids.

Results:

Using large-scale compound screens in CRC cell lines, we identify MEK1/2 inhibitors as potent activators of Wnt/b-catenin signalling. Targeting MEK increases Wnt activity in different CRC cell lines and in murine intestinal and colon organoid models. Furthermore, in vivo application of MEK inhibitors increase Wnt target gene expression in the intestines of mice. Truncating mutations of APC generated by CRISPR/Cas9 strongly synergize with MEK inhibitors in enhancing Wnt responses in isogenic CRC cell line models. Mechanistically, we show that MEK inhibition induces a rapid downregulation of AXIN1 by transcriptional repression. Using patient-derived CRC organoids, we demonstrate that MEK inhibition leads to increased Wnt activity, elevated LGR5 levels and enrichment of gene signatures associated with stemness and cancer relapse. This shift in stemness could be reversed by cotreatment with a pharmacological Wnt inhibitor. Co-targeting MEK and Wnt signalling resulted in enhanced antiproliferative effects in a CRC organoid xenograft model.

Conclusion:

Our study demonstrates that clinically used MEK inhibitors inadvertently induce stem cell plasticity, revealing an unknown side effect of RAS pathway inhibition.