Acquired resistance to BRAF inhibition induces epithelial-to-mesenchymal transition in BRAF (V600E) mutant thyroid cancer by c-Met-mediated AKT activation

// Hyung Kwon Byeon 1 , Hwi Jung Na 1 , Yeon Ju Yang 1 , Sooah Ko 1 , Sun Och Yoon 2 , Minhee Ku 3, 4 , Jaemoon Yang 3, 5 , Jae Wook Kim 6 , Myung Jin Ban 6 , Ji-Hoon Kim 7 , Da Hee Kim 1 , Jung Min Kim 1 , Eun Chang Choi 1 , Chang-Hoon Kim 1, 8 , Joo-Heon Yoon 1, 8, 9 , Yoon Woo Koh 1, 8 1 Department of Otorhinolaryngology, Yonsei University College of Medicine, Seoul, Republic of Korea 2 Department of Pathology, Yonsei University College of Medicine, Seoul, Republic of Korea 3 Department of Radiology, Yonsei University College of Medicine, Seoul, Republic of Korea 4 Brain Korea 21 Plus Project for Medical Science, Yonsei University College of Medicine, Seoul, Republic of Korea 5 YUHS-KRIBB Medical Convergence Research Institute, Seoul, Republic of Korea 6 Department of Otorhinolaryngology, Soonchunhyang University College of Medicine, Republic of Korea 7 Department of Otorhinolaryngology-Head and Neck Surgery, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea 8 The Airway Mucus Institute, Yonsei University College of Medicine, Seoul, Republic of Korea 9 Research Center for Human Natural Defense System, Yonsei University College of Medicine, Seoul, Republic of Korea Correspondence to: Yoon Woo Koh, email: ywkohent@yuhs.ac Keywords: thyroid cancer, molecular targeted therapy, drug resistance, BRAF mutation, epithelial-mesenchymal transition Received: September 23, 2016 Accepted: November 12, 2016 Published: November 21, 2016 ABSTRACT Previously, the authors have identified that c-Met mediates reactivation of the PI3K/AKT pathway following BRAF inhibitor treatment in BRAF (V600E) mutant anaplastic thyroid cancer, thereby contributing to the acquired drug resistance. Therefore dual inhibition of BRAF and c-Met led to sustained treatment response, thereby maximizing the specific anti-tumor effect of targeted therapy. The present study goes one step further and aims to investigate the effect of acquired resistance of BRAF inhibitor on epithelial-to-mesenchymal transition (EMT) in BRAF mutant thyroid cancer cells and the effect of dual inhibition from combinatorial therapy. Two thyroid cancer cell lines, 8505C and BCPAP were selected and treated with BRAF inhibitor, PLX4032 and its effect on EMT were examined and compared. Further investigation was carried out in orthotopic xenograft mouse models. Unlike BCPAP cells, the BRAF inhibitor resistant 8505C cells showed increased expressions of EMT related markers such as vimentin, β-catenin, and CD44. The combinatorial treatment of PLX4032 and PHA665752, a c-Met inhibitor reversed EMT. Similar results were confirmed in vivo . c-Met-mediated reactivation of the PI3K/AKT pathway contributes to the drug resistance to PLX4032 in BRAF (V600E) mutant anaplastic thyroid cancer cells and further promotes tumor cell migration and invasion by upregulated EMT mechanism. Dual inhibition of BRAF and c-Met leads to reversal of EMT, suggesting a maximal therapeutic response.