Activating mutations in the BRAF oncogene are found in 8-15% of colorectal cancer (CRC) patients and have been associated with poor survival. In contrast to BRAF mutant (MT) melanoma, inhibition of the MAPK pathway is ineffective in the majority of BRAFMT CRC patients. Therefore, identification of novel therapies for BRAFMT CRC is urgently needed.

BRAFMT and WT CRC models were assessed in vitro and in vivo. Small molecule inhibitors of MEK1/2, MET and HDAC were employed, over-expression and siRNA approaches were applied, and cell death was assessed by flow cytometry, Western blotting, cell viability and caspase activity assays.

Increased c-MET-STAT3 signalling was identified as a novel adaptive resistance mechanism to MEK inhibitors (MEKi) in BRAFMT CRC models in vitro and in vivo. Moreover, MEKi treatment resulted in acute increases in transcription of the endogenous caspase-8 inhibitor c-FLIPL in BRAFMT cells, but not in BRAFWT cells, and inhibition of STAT3 activity abrogated MEKi-induced c-FLIPL expression. In addition, treatment with c-FLIP-specific siRNA or HDAC inhibitors abrogated MEKi-induced upregulation of c-FLIPL expression and resulted in significant increases in MEKi-induced cell death in BRAFMT CRC cells. Notably, combined HDAC inhibitor/MEKi treatment resulted in dramatically attenuated tumor growth in BRAFMT xenografts.

Our findings indicate that c-MET/STAT3-dependent upregulation of c-FLIPL expression is an important escape mechanism following MEKi treatment in BRAFMT CRC. Thus, combinations of MEKi with inhibitors of c-MET or c-FLIP (eg. HDAC inhibitors) could be potential novel treatment strategies for BRAFMT CRC.