SLC25A32 sustains cancer cell proliferation by regulating flavin adenine nucleotide (FAD) metabolism

// Valeria Santoro 1 , * , Ilya Kovalenko 1 , 2 , * , Kim Vriens 3 , 4 , Stefan Christen 3 , 4 , Andreas Bernthaler 1 , Andrea Haegebarth 1 , Sarah-Maria Fendt 3 , 4 and Sven Christian 1 1 Bayer AG, Drug Discovery, Berlin 13353, Germany 2 Current address: University of Michigan, Cancer Center, Ann Arbor, MI 48108, USA 3 Laboratory of Cellular Metabolism and Metabolic Regulation, VIB Center for Cancer Biology, VIB, Leuven 3000, Belgium 4 Laboratory of Cellular Metabolism and Metabolic Regulation, Department of Oncology, KU Leuven and Leuven Cancer Institute (LKI), Leuven 3000, Belgium * These authors contributed equally to this work Correspondence to: Sven Christian, email: sven.christian@bayer.com Keywords: transporter; mitochondria; metabolism; ROS; FAD Received: August 12, 2019     Accepted: December 16, 2019     Published: February 25, 2020 ABSTRACT SLC25A32 is a member of the solute carrier 25 family of mitochondrial transporters. SLC25A32 transports tetrahydrofolate (THF) as well as FAD into mitochondria and regulates mitochondrial one-carbon metabolism and redox balance. While it is known that cancer cells require one-carbon and FAD-dependent mitochondrial metabolism to sustain cell proliferation, the role of SLC25A32 in cancer cell growth remains unexplored. Our results indicate that the SLC25A32 gene is highly amplified in different tumors and that amplification correlates with increased mRNA expression and reduced patients´ survival. siRNA-mediated knock-down and CRISPR-mediated knock-out of SLC25A32 in cancer cells of different origins, resulted in the identification of cell lines sensitive and resistant to SLC25A32 inhibition. Mechanistically, tracing of deuterated serine revealed that SLC25A32 knock-down does not affect the mitochondrial/cytosolic folate flux as measured by Liquid Chromatography coupled Mass Spectrometry (LC-MS). Instead, SLC25A32 inhibition results in a respiratory chain dysfunction at the FAD-dependent complex II enzyme, induction of Reactive Oxygen Species (ROS) and depletion of reduced glutathione (GSH), which impairs cancer cell proliferation. Moreover, buthionine sulfoximine (BSO) treatment further sensitizes cells to ROS-mediated inhibition of cell proliferation upon SLC25A32 knock-down. Treatment of cells with the FAD precursor riboflavin and with GSH rescues cancer cell proliferation upon SLC25A32 down-regulation. Our results indicate that the reduction of mitochondrial FAD concentrations by targeting SLC25A32 has potential clinical applications as a single agent or in combination with approved cancer drugs that lead to increased oxidative stress and reduced tumor growth.