Abstract 1842: A novel therapeutic approach to overcome resistance to glutaminase inhibition in NF1 driven malignant peripheral nerve sheath tumors MPNST
Authors: Sheikh Tahir , Patwardhan Parag , Lu Chao , Schwartz Gary K. 
Published: August 2019

Abstract
Glutamine, one of the most abundant intracellular amino acids, plays an important role in satisfying the biosynthetic needs of proliferating cancer cells by providing carbons to produce tricarboxylic acid (TCA) cycle intermediates, glutathione, fatty acids, and nucleotides. Though recent research has focused on targeting novel pathways in cellular metabolism, targeting of dysregulated metabolic pathways in therapeutic resistance, a highly clinical relevant area in cancer research, has not been specifically addressed. In this study we hypothesized that combinatorial targeting of metabolism and epigenetics may overcome therapeutic resistance to glutaminase (GLS1) inhibitor, CB-839, in NF1 driven MPNST cells. To test this, we cultured NF1-null MPNST cell line (ST8814) sensitive to growth inhibition by CB-839 with the drug in a dose-escalating manner until the emergence of resistant cell populations. Cell viability assays and western blot analysis demonstrated that CB-839-induced inhibition of cell growth and apoptosis (PARP cleavage) were indeed abrogated in resistant ST8814 cells. To rule out the possibility that acquired resistance was due to changes in the uptake, metabolism or target inhibition of CB-839, we also confirmed that the resistant cells were no longer sensitive to growth inhibition and cell death induced by siRNA knockdown of GLS1. We profiled global landscape of histone modifications in ST8814 parental and resistant cells and found that there was a marked decrease in the repressive histone methylation marks H3K9me3 and H3K27me3 and an increase in H3K9ac, a modification associated with active transcription. Accordingly, levels of c-Myc were significantly elevated in resistant cells. RNAi-mediated c-Myc knockdown selectively inhibited growth factor signaling and cell proliferation in CB-839 resistant cells, suggesting that these cells are addicted to the reprogrammed epigenetic and transcriptional state. Expression levels of c-Myc and histone acetylation are connected by the Bromo-domain protein BRD4. Consistently, CB-839 resistant cells demonstrated augmented sensitivity towards treatment with JQ1, a small molecule inhibitor of BRD4. We also carried out receptor tyrosine kinase (RTK) array and immunoblotting analysis which demonstrated strong overexpression and activation of c-Met (HGF-R) in CB-839 resistant cells, compared to the parental cells. Conversely, combination treatment with inhibitors of c-Myc and c-MET resulted in decreased cell viability, and induction of apoptotic markers such as cleaved PARP and cleaved Caspase3 in the resistant ST8814 cells. Taken together, our data strongly indicates that combinatorial targeting of metabolism and epigenetics is a novel potential approach to overcome CB-839 resistance and merits evaluation in the treatment of drug resistant NF1 driven soft tissue sarcoma.
Citation Format: Tahir Sheikh, Parag Patwardhan, Chao Lu, Gary K. Schwartz. A novel therapeutic approach to overcome resistance to glutaminase inhibition in NF1 driven malignant peripheral nerve sheath tumors MPNST [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1842.