Wnt/β-Catenin Pathway Activation Mediates Adaptive Resistance to BRAF Inhibition in Colorectal Cancer

Abstract

One of the most encouraging developments in oncology has been the success of BRAF inhibitors in -mutant melanoma. However, in contrast to its striking efficacy in-mutant melanomas, BRAF inhibitor monotherapy is ineffective in -mutant colorectal cancer. Although many studies on BRAF inhibitor resistance in colorectal cancer 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 colorectal cancer 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-mutant colorectal cancer cell lines through activating the cytoplasmic tyrosine kinase focal adhesion kinase (FAK). The results showed that FAK activation upon BRAF inhibitor treatment did not require EGFR or ERK1/2 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 -mutant colorectal cancer, including cell line xenograft model and a 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 although complete vertical pathway blockade is pivotal for effective and durable control of-mutant colorectal cancer, cotargeting parallel adaptive signaling-the Wnt/β-catenin pathway-is also essential. .

The most common RNF43 mutant G659Vfs41 is fully functional in inhibiting Wnt signaling and unlikely to play a role in tumorigenesis

“…6A), indicating that BRAF-V600E had distinct effect in colon cancer and melanoma cells. In cell growth tests of the two colon cancer cell lines with BRAF-V600E mutation, HT29 (MSI-low, not hypermutated) and RKO (MSI-HI) 32 , HT29 cells were highly sensitive to PLX4720 whereas RKO cells were rather resistant which is consistent with previous report 31 . Instead, we found RKO cells is highly sensitive to a Wnt/βcatenin signaling activator CHIR-99021 which was not known to be cytotoxic to any cell line.…”

“…Inhibition of BRAF (PLX4720) was highly effective in HT29 cells (BRAF-V600E, RNF43-WT, APC mutant) but had little activity in RKO cells (BRAF-V600E, RNF43-G659Vfs41, APC-WT) ( Fig. 5a), consistent with previously published results by others 31 . In LoVo and DLD1 cells (no RNF43 nor BRAF mutation), PLX4720 had intermediate effect ( Fig.…”

Direct targeting of oncogenic RAS mutants with a tumor-specific cytosol-penetrating antibody inhibits RAS mutant–driven tumor growth

“…6A). When evaluated in mouse CDX models by means of the Bliss independence model (41), consistent with the in vitro results, the combination of inRas37 and PI3Ki synergized to inhibit the in vivo growth of PI3K MUT -carrying inRas37resistant CDXs, but not PI3K WT inRas37-sensitive CDXs, when compared with either single agent alone (Fig. 6, C and D).…”

FAK activity sustains intrinsic and acquired ovarian cancer resistance to platinum chemotherapy

“…7F). Interestingly, a FAK to b-catenin signaling linkage has been identified as an adaptive chemotherapy resistance pathway in BRAF mutated colon cancer (Chen, Gao et al, 2018, Taylor & Schlaepfer, 2018. Although stabilized b-catenin DGSK expression in KMF FAK-/-cells activated a number of canonical Wnt target genes, this was unexpectedly insufficient to rescue KMF FAK-/-growth as tumors.…”