Mitochondrial Calcium Signaling Regulates Branched-Chain Amino Acid Catabolism in Fibrolamellar Carcinoma

Abstract

Metabolic adaptations in response to changes in energy supply and demand are essential for survival. The mitochondrial calcium uniporter coordinates metabolic homeostasis by regulating TCA cycle activation, mitochondrial fatty acid oxidation and cellular calcium signaling. However, a comprehensive analysis of uniporter-regulated mitochondrial metabolic pathways has remained unexplored. Here, we investigate the metabolic consequences of uniporter loss- and gain-of-function and find that mitochondrial calcium signaling regulates the branched-chain amino acid (BCAA) catabolism pathway. Loss of uniporter function activates the pathway through two mechanisms: increased expression of pathway proteins and stimulation of the enzyme that catalyzes the committed step of the pathway by dephosphorylation. Conversely, in the liver cancer fibrolamellar carcinoma (FLC) – which we demonstrate to have increased mitochondrial calcium levels – expression of BCAA catabolism enzymes is suppressed. The transcription factor KLF15, a master regulator of liver metabolic gene expression, is also regulated by mitochondrial calcium signaling. KLF15 is downregulated in FLC patient tumors and cellular models. The KLF15 target gene ornithine transcarbamylase (OTC) is downregulated in FLC, which is thought to contribute to urea cycle defects and hyperammonemia observed in some FLC patients, suggesting a role for mitochondrial calcium in hyperammonemia. Inhibition of uniporter activity in FLC cellular models upregulates KLF15, BCAA catabolism genes, and OTC. Collectively, we identify an important role for mitochondrial calcium signaling in metabolic adaptation through transcriptional and post-translational regulation of metabolism and elucidate its importance for BCAA and ammonia regulation in FLC.