V600EBRAF Inhibition Induces Cytoprotective Autophagy through AMPK in Thyroid Cancer Cells

The dysregulation of autophagy is important in the development of many cancers, including thyroid cancer, where (V600E)BRAF is a main oncogene. Here, we analyse the effect of (V600E)BRAF inhibition on autophagy, the mechanisms involved in this regulation and the role of autophagy in cell survival of thyroid cancer cells. We reveal that the inhibition of (V600E)BRAF activity with its specific inhibitor PLX4720 or the depletion of its expression by siRNA induces autophagy in thyroid tumour cells. We show that (V600E)BRAF downregulation increases LKB1-AMPK signalling and decreases mTOR activity through a MEK/ERK-dependent mechanism. Moreover, we demonstrate that PLX4720 activates ULK1 and increases autophagy through the activation of the AMPK-ULK1 pathway, but not by the inhibition of mTOR. In addition, we find that autophagy blockade decreases cell viability and sensitize thyroid cancer cells to (V600E)BRAF inhibition by PLX4720 treatment. Finally, we generate a thyroid xenograft model to demonstrate that autophagy inhibition synergistically enhances the anti-proliferative and pro-apoptotic effects of (V600E)BRAF inhibition in vivo. Collectively, we uncover a new role of AMPK in mediating the induction of cytoprotective autophagy by (V600E)BRAF inhibition. In addition, these data establish a rationale for designing an integrated therapy targeting (V600E)BRAF and the LKB1-AMPK-ULK1-autophagy axis for the treatment of (V600E)BRAF-positive thyroid tumours.