Abstract PO-028: Investigating the contribution of CHAC1 to persistent mitochondrial respiratory function in NSCLC under cysteine deprivation
Authors: Ward Nathan P. , DeNicola Gina M. 
Published: December 2020

Abstract
Solid tumors exhibit metabolic rewiring to enhance their energetic capacity while meeting the biosynthetic demands for rapid proliferation. Though this reprogramming supports robust tumor growth, it often imposes bottlenecks and metabolic dependencies that can be exploited clinically. Among the metabolic vulnerabilities that is of therapeutic interest is the sensitivity of many cancers to sustained deprivation of the non-essential amino acid cysteine (Cys). In certain malignant contexts, ferroptosis has been linked to mitochondrial dysfunction, where oxidative metabolism drives ROS production, the loss of mitochondrial membrane integrity and respiratory function, leading to the propagation of lipid radicals that cripple cell viability. We found that while non-small cell lung cancer (NSCLC) cells are susceptible to Cys-starvation induced ferroptosis, mitochondrial respiratory function robustly persists under prolonged Cys deprivation in these NSCLC cells. We also find that mitochondrial iron-sulfur (Fe-S) protein function (e.g. aconitase, lipoic acid synthase, and respiratory complexes I-III) critical to oxidative metabolism is unaltered by Cys deprivation despite the need for Cys-derived sulfur in the synthesis of Fe-S clusters. Under these Cys-restricted conditions, we observe an increase in the expression of glutathione-specific gamma-glutamylcyclotransferase 1 (CHAC1), a mitochondrial protein that catalyzes the degradation of the antioxidant glutathione (GSH). Because Cys is a constituent of the tripeptide antioxidant, enhanced mitochondrial GSH turnover could provide Cys for the generation of Fe-S clusters. Consistent with this, CRISPR-mediated knockout of CHAC1 diminished Fe-S protein function in Cys-starved NSCLC cells. Despite this decrease in oxidative capacity, CHAC1 knockout prolonged survival under Cys-deprivation, suggesting that intact mitochondrial function may contribute to Cys-starvation induced cell death. Enhanced GSH degradation by CHAC1 would also restrict GSH-based antioxidant capacity imposing a greater requirement for the NADPH-dependent thioredoxin system. Interestingly, we find that Cys deprivation also induces the expression of nicotinamide nucleotide transhydrogenase (NNT), a critical contributor of mitochondrial NADPH in lung cancer. Moreover, we find that the activity of the mitochondrial NADPH-producing isocitrate dehydrogenase (IDH2) is enhanced under Cys deprivation independent of a change in expression. Collectively, these data indicate a potential rewiring of mitochondrial antioxidant defenses for the maintenance of robust mitochondrial respiratory function in the absence of extracellular Cys. This ultimately may constitute a self-defeating response to nutrient stress in NSCLC, where mitochondria selfishly sustain functionality at the expense of cell viability.
Citation Format: Nathan P. Ward, Gina M. DeNicola. Investigating the contribution of CHAC1 to persistent mitochondrial respiratory function in NSCLC under cysteine deprivation [abstract]. In: Abstracts: AACR Special Virtual Conference on Epigenetics and Metabolism; October 15-16, 2020; 2020 Oct 15-16. Philadelphia (PA): AACR; Cancer Res 2020;80(23 Suppl):Abstract nr PO-028.