Autophagy sustains mitochondrial respiration and determines resistance to BRAFV600E inhibition in thyroid carcinoma cells.

ABSTRACT

BRAFV600E is the most prevalent mutation in thyroid cancer and correlates with poor prognosis and therapy resistance. Although selective inhibitors of BRAFV600E have been developed, more advanced tumors such as anaplastic thyroid carcinomas show a poor response in clinical trials. Therefore, the study of alternative survival mechanisms is needed. Since metabolic changes have been related to malignant progression, in this work we explore metabolic dependencies of thyroid tumor cells to exploit them therapeutically. Our results show that respiration of thyroid carcinoma cells is highly dependent on fatty acid oxidation and, in turn, fatty acid mitochondrial availability is regulated through macroautophagy/autophagy. Furthermore, we show that both lysosomal inhibition and the knockout of the essential autophagy gene, ATG7, lead to enhanced lipolysis; although this effect is not essential for survival of thyroid carcinoma cells. We also demonstrate that following inhibition of either autophagy or fatty acid oxidation, thyroid tumor cells compensate oxidative phosphorylation deficiency with an increase in glycolysis. In contrast to lipolysis induction, upon autophagy inhibition, glycolytic boost in autophagy-deficient cells is essential for survival and, importantly, correlates with a higher sensitivity to the BRAFV600E selective inhibitor, vemurafenib. In agreement, downregulation of the glycolytic pathway results in enhanced mitochondrial respiration and vemurafenib resistance. Our work provides new insights into the role of autophagy in thyroid cancer metabolism and supports mitochondrial targeting in combination with vemurafenib to eliminate BRAFV600E-positive thyroid carcinoma cells.Abbreviations AMP adenosine monophosphate; ATC anaplastic thyroid carcinoma; ATG autophagy related; ATP adenosine triphosphate; BRAF B-Raf proto-oncogene, serine/threonine kinase; Cas9 CRISPR-associated protein; CREB cAMP responsive element binding protein; CRISPR clustered regularly interspaced short palindromic repeats; 2DG 2-deoxyglucose; FA fatty acid; FAO fatty acid oxidation; FASN fatty acid synthase; FCCP trifluoromethoxy carbonyl cyanide phenylhydrazone; LAMP1 lysosomal associated membrane protein 1; LIPE/HSL lipase E, hormone sensitive type; MAP1LC3/LC3 microtubule associated protein 1 light chain 3; OCR oxygen consumption rate; OXPHOS oxidative phosphorylation; PRKA/PKA protein kinase cAMP-activated; PTC papillary thyroid carcinoma; SREBF1/SREBP1 sterol regulatory element binding transcription factor 1.