One of the most encouraging developments in oncology has been the success of BRAF inhibitors in BRAF-mutant melanoma. However, in contrast to its striking efficacy in BRAF-mutant melanomas, BRAF inhibitor monotherapy is ineffective in BRAF-mutant colorectal cancer (CRC). While many studies on BRAF inhibitor resistance in CRC have focused on mechanisms underlying the reactivation of the EGFR/RAS/RAF/MEK/ERK pathway, the current study focuses on identifying novel adaptive signaling mechanisms, a fresh angle on CRC resistance to BRAF inhibition. We found that treatment with BRAF inhibitors (both current and next generation BRAF inhibitors) upregulated the Wnt/β-catenin pathway in BRAFV600E-mutant CRC cell lines through activating the cytoplasmic tyrosine kinase FAK (focal adhesion kinase). The results showed that FAK activation upon BRAF inhibitor treatment did not require EGFR (Epidermal Growth Factor Receptor) or ERK1/2 (extracellular-signal-regulated kinases1/2) activation, implying that BRAF inhibitor treatment-induced hyperactivation of Wnt signaling is "pathway reactivation"-independent. BRAF inhibition-induced Wnt pathway activation was further validated in preclinical models of BRAFV600E-mutant CRC including cell line xenograft model and a PDX (patient-derived xenograft) model. Combined inhibition of BRAF/Wnt pathways or BRAF/FAK pathways exerted strong synergistic antitumor effects in cell culture model and mouse xenograft model. Overall, the current study has identified activation of the Wnt/β-catenin pathway as a novel fundamental cause of colon cancer resistance to BRAF inhibition. Our results suggest that while complete vertical pathway blockade is pivotal for effective and durable control of BRAF-mutant CRC, co-targeting parallel adaptive signaling-the Wnt/β-catenin pathway-is also essential.
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