Fine-Tuning Lipid Metabolism by Targeting Mitochondria-Associated Acetyl-CoA-Carboxylase 2 in <i> BRAF ^V600E </i> Papillary Thyroid Carcinoma

Abstract

Background: BRAF V600E acts as an ATP-dependent cytosolic kinase. BRAF V600E 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 BRAF V600E 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. The aim of our study was to determine the relationship between BRAF V600E and ACC in PTC. Methods: We performed RNA-seq and DNA copy number analyses in PTC and normal thyroid (NT) in The Cancer Genome Atlas samples. Validations were performed by using assays on PTC-derived cell lines of differing BRAF status and a xenograft mouse model derived from a heterozygous BRAF WT/V600E PTC-derived cell line with knockdown (sh) of ACC1 or ACC2 . Results: ACC2 mRNA expression was significantly downregulated in BRAF V600E -PTC vs. BRAF WT -PTC or NT clinical samples. ACC2 protein levels were downregulated in BRAF V600E -PTC cell lines vs. the BRAF WT/WT PTC cell line. Vemurafenib increased ACC2 (and to a lesser extent ACC1) mRNA levels in PTC-derived cell lines in a BRAF V600E allelic dose-dependent manner. BRAF V600E 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 sh ACC2 significantly increased OCR rates due to FAO, while it decreased ECAR in BRAF V600E PTC-derived cells vs. controls. BRAF V600E inhibition synergized with sh ACC2 to increase intracellular reactive oxygen species production, leading to increased cell proliferation and, ultimately, vemurafenib resistance. Mice implanted with a BRAF WT/V600E PTC-derived cell line with sh ACC2 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 BRAF V600E and lipid metabolism regulation in PTC. BRAF V600E downregulates ACC2 levels, which deregulates de novo lipid synthesis, FAO due to OCR, and ECAR rates. Sh ACC2 may contribute to vemurafenib resistance and increased tumor growth. ACC2 rescue may represent a novel molecular strategy for overcoming resistance to BRAF V600E inhibitors in refractory PTC.

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