Most BRAF (V600E) mutant melanomas are sensitive to selective BRAF inhibitors, but BRAF mutant colon cancers are intrinsically resistant to these drugs because of feedback activation of EGFR. We performed an RNA-interference-based genetic screen in BRAF mutant colon cancer cells to search for phosphatases whose knockdown induces sensitivity to BRAF inhibition. We found that suppression of protein tyrosine phosphatase non-receptor type 11 (PTPN11) confers sensitivity to BRAF inhibitors in colon cancer. Mechanistically, we found that inhibition of PTPN11 blocks signaling from receptor tyrosine kinases (RTKs) to the RAS-MEK-ERK pathway. PTPN11 suppression is lethal to cells that are driven by activated RTKs and prevents acquired resistance to targeted cancer drugs that results from RTK activation. Our findings identify PTPN11 as a drug target to combat both intrinsic and acquired resistance to several targeted cancer drugs. Moreover, activated PTPN11 can serve as a biomarker of drug resistance resulting from RTK activation.

The RAS/RAF/MEK/MAPK and the PTEN/PI3K/AKT/mTOR pathways are key regulators of proliferation and survival in human cancer cells. Selective inhibitors of different transducer molecules in these pathways have been developed as molecular targeted anti‐cancer therapies. The in vitro and in vivo anti‐tumor activity of pimasertib, a selective MEK 1/2 inhibitor, alone or in combination with a PI3K inhibitor (PI3Ki), a mTOR inhibitor (everolimus), or with multi‐targeted kinase inhibitors (sorafenib and regorafenib), that block also BRAF and CRAF, were tested in a panel of eight human lung and colon cancer cell lines. Following pimasertib treatment, cancer cell lines were classified as pimasertib‐sensitive (IC₅₀ for cell growth inhibition of 0.001 µM) or pimasertib‐resistant. Evaluation of basal gene expression profiles by microarrays identified several genes that were up‐regulated in pimasertib‐resistant cancer cells and that were involved in both RAS/RAF/MEK/MAPK and PTEN/PI3K/AKT/mTOR pathways. Therefore, a series of combination experiments with pimasertib and either PI3Ki, everolimus, sorafenib or regorafenib were conducted, demonstrating a synergistic effect in cell growth inhibition and induction of apoptosis with sustained blockade in MAPK‐ and AKT‐dependent signaling pathways in pimasertib‐resistant human colon carcinoma (HCT15) and lung adenocarcinoma (H1975) cells. Finally, in nude mice bearing established HCT15 and H1975 subcutaneous tumor xenografts, the combined treatment with pimasertib and BEZ235 (a dual PI3K/mTOR inhibitor) or with sorafenib caused significant tumor growth delays and increase in mice survival as compared to single agent treatment. These results suggest that dual blockade of MAPK and PI3K pathways could overcome intrinsic resistance to MEK inhibition.

Purpose: BRAF^V600E mutations are associated with poor clinical prognosis in colorectal cancer (CRC). Although selective BRAF inhibitors are effective for treatment of melanoma, comparable efforts in CRC have been disappointing. Here, we investigated potential mechanisms underlying this resistance to BRAF inhibitors in BRAF^V600E CRC. Experimental Design: We examined phosphoinositide 3-kinase (PI3K)/mTOR signaling in BRAF^V600E CRC cell lines after BRAF inhibition and cell viability and apoptosis after combined BRAF and PI3K/mTOR inhibition. We assessed the efficacy of in vivo combination treatment using a novel genetically engineered mouse model (GEMM) for BRAF^V600E CRC. Results: Western blot analysis revealed sustained PI3K/mTOR signaling upon BRAF inhibition. Our BRAF^V600E GEMM presented with sessile serrated adenomas/polyps, as seen in humans. Combination treatment in vivo resulted in induction of apoptosis and tumor regression. Conclusions: We have established a novel GEMM to interrogate BRAF^V600E CRC biology and identify more efficacious treatment strategies. Combination BRAF and PI3K/mTOR inhibitor treatment should be explored in clinical trials. Clin Cancer Res; 19(10); 2688–98. ©2013 AACR.