Abstract 5486: The role of the GABA shunt in prostate cancer metabolic reprogramming and aggressive phenotype
Authors: Knott Erika L. , Miriyala Sumitra , Nam Hyung , Panchatcharam Manikandan , Leidenheimer Nancy 
Published: August 2018

Abstract
The GABA (γ-aminobutyric acid) shunt, a bypass mechanism for a portion of the tricarboxylic acid (TCA) cycle, is responsible in the mammalian central nervous system for the synthesis and degradation of the inhibitory neurotransmitter GABA. Glutamic acid decarboxylase (GAD67 and GAD65; gene names GAD1 and GAD2, respectively) synthesizes GABA from glutamate, while GABA-transaminase (GABA-T; gene name ABAT) metabolizes GABA into succinic semialdehyde, which is dehydrogenated into succinate. Although poorly studied in mammals, the GABA shunt plays a protective role against pH, thermo-, and oxidative stresses in plants and bacteria. We hypothesize that both cell non-autonomous GABA from the tumor microenvironment (TME), and cell-autonomous production of GABA contribute to cancer progression. Cell types within the TME such as endothelial and T-cells, are known to secrete GABA. To investigate the utilization of cell non-autonomous GABA by prostate cancer cells, we determined via LC-MS/MS that although the prostate adenocarcinoma LNCaP cell line does not contain significant amounts of GABA, it is capable of taking up exogenous GABA. The effect of exogenous GABA treatment on the metabolic capacity of LNCaP cells was examined using the SEAhorse XF24 Bioanalyzer. GABA treatment increased oxygen consumption rate while decreasing the rate of glycolysis and glycolytic reserve. This effect was blocked by co-treatment with GABA-T inhibitors, suggesting the ability of GABA to increase mitochondrial respiration occurs through the GABA shunt. GABA treatment did not affect the metabolic phenotype of a normal epithelial cell line control. Regarding cell-autonomous production of GABA, recent studies show dysregulation of genes encoding GABA shunt enzymes in various cancers; specifically relevant to this study, GAD1 is upregulated in castration-resistant prostate cancer cell lines (Ippolito 2014). Consistent with the involvement of GAD1 in aggressive prostate cancers, our analysis of the TCGA Provisional prostate cancer dataset reveals that GAD1 mRNA expression level increases with increasing Gleason Score (Pearson r = 0.19; p &lt; 0.00005). Additionally, analysis of the Beltran (2016) data set reveals that GAD1 transcripts are upregulated in treatment-induced neuroendocrine prostate cancer compared to prostate adenocarcinoma (unpaired t-test, p &lt; 0.0005). We hypothesize that both cell non-autonomous and autonomous GABA play a role in cancer progression. Uptake of GABA from the TME may support metabolic reprogramming of primary tumor cells, while upregulation of GAD1 and subsequent increase in cell-autonomous GABA production may provide a malignant advantage to late-stage cancer. The GABA shunt may offer a novel therapeutic target for prostate cancer treatment by exploiting a poorly understood metabolic pathway linked to the TCA cycle. We thank the LSUHSC-S Feist-Weiller Cancer Center for financial support.
Citation Format: Erika L. Knott, Sumitra Miriyala, Hyung Nam, Manikandan Panchatcharam, Nancy Leidenheimer. The role of the GABA shunt in prostate cancer metabolic reprogramming and aggressive phenotype [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5486.