Fine-tuning lipid metabolism by targeting mitochondria-associated Acetyl-CoA-Carboxylase 2 in BRAFV600E papillary thyroid carcinoma.

BACKGROUND BRAFV600E acts as an ATP-dependent cytosolic kinase. BRAFV600E inhibitors are widely available, but resistance to them is widely reported in the clinic. Lipid metabolism (fatty acids) is fundamental for energy and to control cell stress. Whether and how BRAFV600E impacts lipid metabolism regulation in papillary thyroid carcinoma (PTC) is still unknown. Acetyl-CoA Carboxylase (ACC) is a rate-limiting enzyme for de novo lipid synthesis and inhibition of fatty acid oxidation (FAO). ACC1 and ACC2 genes encode distinct isoforms of ACC. DESIGN Our study aims to determine the relationship between BRAFV600E and ACC in PTC. We applied RNA-seq and DNA copy number analyses to PTC and normal thyroid (NT) TCGA samples. Validations were performed using assays on PTC-derived cell lines of differing BRAF status and a xenograft mouse model derived from a heterozygous BRAFWT/V600E PTC-derived cell line with knockdown (sh) of ACC1 or ACC2. RESULTS ACC2 mRNA expression was significantly down-regulated in BRAFV600E-PTC vs. BRAFWT-PTC or NT clinical samples. ACC2 protein levels were downregulated in BRAFV600E-PTC cell lines vs. BRAFWT/WT PTC cell line. Vemurafenib increased ACC2 (and to a lesser extent ACC1) mRNA levels in PTC-derived cell lines in a BRAFV600E allelic dose-dependent manner. BRAFV600E inhibition increased de novo lipid synthesis rates, and decreased FAO due to oxygen consumption rate (OCR), and extracellular acidification rate (ECAR), after addition of palmitate. Only shACC2 significantly increased OCR rates due to FAO, while decreasing ECAR in BRAFV600E PTC-derived cells vs. controls. BRAFV600E inhibition synergized with shACC2 to increase intracellular reactive oxygen species (ROS) production, leading to increased cell proliferation and ultimately vemurafenib resistance. Mice implanted with a BRAFWT/V600E PTC-derived cell line with shACC2 showed significantly increased tumor growth after vemurafenib treatment, while vehicle-treated controls, or shGFP control cells treated with vemurafenib showed stable tumor growth. CONCLUSIONS These findings suggest a potential link between BRAFV600E and lipid metabolism regulation in PTC. BRAFV600E down-regulates ACC2 levels, which deregulates de novo lipid synthesis, FAO due to OCR, and ECAR rates. ShACC2 may contribute to vemurafenib resistance and increased tumor growth. ACC2 rescue may represent a novel molecular strategy for overcoming resistance to BRAFV600E inhibitors in refractory PTC.