Abstract 1839: Concurrent inhibition of glutaminase (GLS) and metabotropic receptor 1 (GRM1) reduced glutamate bioavailability and led to a decrease in GRM1-expressing melanoma cell proliferation in vitro and tumor progression in vivo
Authors: Shah Raj , Singh Simar , Eddy Kevinn , Filipp Fabian , Chen Suzie 
Published: August 2019

Abstract
Aberrant glutamatergic signaling has been implicated in multiple metabolic processes and several types of cancer. Our laboratory has previously illustrated the role of metabotropic glutamate receptor 1 (GRM1), a G-protein coupled receptor, in neoplastic transformation of melanocytes in vitro and spontaneous development of metastatic melanoma in vivo. Here, we demonstrate significant overexpression of glutaminase (GLS) in GRM1 expressing melanoma cells, resulting in excess production of GRM1’s natural ligand, glutamate, and the establishment of autocrine loops. GLS catalyzes the first step in glutamine metabolism–the conversion of glutamine to glutamate before entering the TCA cycle. Comparison of glutamate levels in circulating blood plasma between melanoma prone GRM1-transgenic and wild type mice showed elevated glutamate levels in melanoma prone GRM1-transgenic mice, suggesting that aberrant GRM1 expression also promotes an increase in circulating glutamate, to ensure constitutive activation of the GRM1 receptor and its downstream signaling effecters in vivo. We also discovered that manipulation of GRM1 expression by genetic means led to robust parallel changes in metabolite expression levels and intracellular glutamate concentrations by mass spectrometry analysis, as well as the endogenous levels of GLS in melanoma cells, suggesting a strong association between GRM1 and GLS. Inclusion of CB-839, a potent, selective, and orally bioavailable inhibitor of GLS, resulted in significant inhibition of GRM1+ melanoma cell proliferation compared to GRM1- ones. The sensitivity of GRM1+ melanoma cells to modulation of GLS points to the dependency of these cells to glutamate amounts via GLS activity. We also demonstrated that treatment with Riluzole, an inhibitor of glutamatergic signaling by decreasing the available ligand, glutamate, led to significantly reduced melanoma cell proliferation in vitro and tumor progression in vivo. We found that simultaneous targeting of glutamate production (CB-839) and release (Riluzole) in GRM1+ melanoma cells led to additive suppression of cell proliferation in vitro. Most importantly, disruption of GRM1 signaling through combined actions of CB-839 and Riluzole significantly suppressed tumor growth in two independent xenograft mouse models of melanoma, with no obvious symptoms of toxicity detected in these mice. Taken together, these results describe a novel pathway for GRM1-mediated tumorigenesis through rewiring of cellular metabolic networks. We are continuing to investigate the mechanisms and regulation of GLS in our experimental system, with the goal of developing a rational strategy for the treatment of GRM1-expressing cancers.
Citation Format: Raj Shah, Simar Singh, Kevinn Eddy, Fabian Filipp, Suzie Chen. Concurrent inhibition of glutaminase (GLS) and metabotropic receptor 1 (GRM1) reduced glutamate bioavailability and led to a decrease in GRM1-expressing melanoma cell proliferation in vitro and tumor progression in vivo [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 1839.