Abstract 4379: Prostate cancer cells utilize the GABA shunt to enhance oxidative metabolism

Abstract

Although an upregulation of aerobic glycolysis (the Warburg effect) is a canonical hallmark of cancer, it is now recognized that most cancerous cells continue to rely on the TCA cycle as a source of both biosynthetic precursors (DeBarardinis & Chandel, 2016) and the majority of ATP production (Zu & Guppy, 2004). Drug-resistant cancers have been associated with metabolic reprogramming (Rahman & Hasan, 2015) involving an upregulation in oxidative phosphorylation (Wolf, 2014). Two advanced, treatment-resistant variants of prostate cancer are castration-resistant adenocarcinoma (CRPC-adeno) and neuroendocrine prostate cancer (NEPC). Emerging evidence suggests that the γ-aminobutyric acid (GABA) shunt—an evolutionarily conserved pathway for energy production, biosynthetic precursors, and ROS management—may be used by both CRPC-adeno and NEPC. The GABA shunt is a bypass for two steps of the TCA cycle, wherein GABA is synthesized from glutamate by a GAD1-encoded decarboxylase, transaminated into succinic semialdehyde, and metabolized into the TCA cycle intermediate succinate. Analysis of publicly available transcript datasets (via cBioPortal) indicates that GAD1 mRNA expression increases with increasing Gleason score (p < 0.00005, prostate adenocarcinoma TCGA provisional dataset), with higher expression in both CRPC-adeno compared to primary adenocarcinoma (Ippolito and Piwnica-Worms, 2014), and NEPC compared to CRPC-adeno (p < 0.005, Beltran, et al., 2016). Expression of GAD1 transcripts appears to be contingent on low levels of AR transcripts (Trento/Cornell/Broad data set), consistent with the AR independence of the NEPC phenotype. Similarly, GABA levels increase as AR expression is lost during transdifferentiation to NEPC (Solorzano, et al., 2018). GAD67 protein levels in prostate tumors are a more accurate predictor of patient risk for metastasis than Gleason score (Azuma, et al., 2003), and flux through the GABA shunt is critical for the growth of brain tumors in mouse xenograft models (Schnepp, et al., 2017; Neman, et al., 2014). Our experiments show that treatment of an early-stage prostate cancer cell line model (LNCaP) with GABA (100 µM, 24h) induces a shift from aerobic glycolysis to oxidative metabolism. This metabolic reprogramming toward increased oxidative phosphorylation requires at least 6 hours to emerge, and is maximal by 18 hours. During this time period, the presence of GABA in the mitochondrial matrix steadily increases, as shown by our newly-developed GABA-sensing fluorescent receptor, mitoGABASnFR. The observed increases in oxygen consumption are accompanied by GABA-induced decreases in extracellular acidification and lactate levels. In light of these findings, we propose that the GABA shunt represents an understudied pathway for inducing a metabolic shift towards oxidative phosphorylation in advanced prostate cancer, and therefore may be a target for future anticancer therapies. Citation Format: Erika L. Knott, Sumitra Miriyala, Manikandan Panchatcharam, Nancy Leidenheimer. Prostate cancer cells utilize the GABA shunt to enhance oxidative metabolism [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 4379.