Project Summary/Abstract Triple negative breast cancer (TNBC) is an aggressive breast cancer subtype in which limited targeted therapies are available. Therefore, the standard of care treatment for TNBC patients is neoadjuvant chemotherapy where ~50% of patients have residual tumor burden and poor prognosis after treatment. Recently, it has been demonstrated that mitochondrial oxidative phosphorylation (oxphos) is both upregulated and a therapeutic vulnerability in chemoresistant TNBC, however, the mechanism behind this finding is not understood. The tricarboxylic acid cycle (TCA), which produces reducing equivalents necessary for oxphos, is requires the molecule acetyl-CoA (AcCoA). AcCoA can be derived from the breakdown of long chain fatty acids during fatty acid oxidation (FAO). Because heightened fatty acid metabolism has been associated with improved survival in TNBC, it is possible that chemoresistant TNBC derives AcCoA from fatty acids, fueling the TCA and oxphos. The gene carnitine acetyl transferase (CRAT) produces an enzyme (CrAT) that catalyzes the reversible transfer of an acetyl group between CoA and carnitine within the mitochondria. It is thought that CrAT buffers the pool of free AcCoA to maximize the energetic needs of the TCA cycle and to prevent pyruvate dehydrogenase inhibition via excess AcCoA. Within chemoresistant TNBC, greater transcription and/or translation of CRAT may maximize TCA-derived reducing equivalents needed for oxphos, aiding in chemoresistance. We have identified the fatty acid metabolism pathway and CRAT as significantly enriched in chemotherapy (docetaxel combined with carboplatin, standard NACT for TNBC)-resistant versus chemo- sensitive TNBC PDXs and patient biopsies (NCT02547987) at the RNA and protein levels. In a preliminary analysis of post-versus pre-NACT TNBC patient derived xenograft (PDX) tumors, I also found increased cytosolic lipid droplets (LDs) in carboplatin treated PDXs compared to vehicle by transmission electron microscopy (TEM), suggestive of enhanced fatty acid metabolism. Therefore, I hypothesize that elevated expression of CRAT in chemoresistant TNBC provides enhanced metabolic plasticity, buffering lipid derived accumulation of AcCoA to maximize TCA cycle flux and oxphos, aiding in chemoresistance. I will address this hypothesis by determining if AcCoA is preferentially derived from increased fatty acid oxidation in chemoresistant TNBC. I will investigate if CRAT ablation impairs chemosensitivity and survival using both in vitro and in vivo models. I will also determine which isoform of CRAT is necessary and sufficient to drive chemoresistance. Together, these studies will improve our mechanistic understanding of increased oxphos in residual TNBC and provide rationale for therapies targeting CrAT function to improve prognosis for chemoresistant patients.