SUMMARY
N-acetyl-aspartyl-glutamate (NAAG) is a peptide-based neurotransmitter that has been extensively studied in many neurological diseases. In this study, we show a specific role of NAAG in cancer. We found that NAAG is more abundant in higher grade cancers and is a source of glutamate in cancers expressing glutamate carboxypeptidase II (GCPII), the enzyme that hydrolyzes NAAG to glutamate and N-acetyl-aspartate (NAA). Knocking down GCPII expression through genetic alteration or pharmacological inhibition of GCPII results in a reduction of both glutamate concentrations and cancer growth. Moreover, targeting GCPII in combination with glutaminase inhibition accentuates these effects. These findings suggest that NAAG serves as an important reservoir to provide glutamate to cancer cells through GCPII when glutamate production from other sources is limited. Thus, GCPII is a viable target for cancer therapy, either alone or in combination with glutaminase inhibition.
The targeting of glutamine metabolism specifically via pharmacological inhibition of glutaminase 1 (GLS1) has been translated into clinical trials as a novel therapy for several cancers. The results, though encouraging, show room for improvement in terms of tumor reduction. In this study, the glutaminase II pathway is found to be upregulated for glutamate production upon GLS1 inhibition in pancreatic tumors. Moreover, genetic suppression of glutamine transaminase K (GTK), a key enzyme of the glutaminase II pathway, leads to the complete inhibition of pancreatic tumorigenesis in vivo unveiling GTK as a new metabolic target for cancer therapy. These results suggest that current trials using GLS1 inhibition as a therapeutic approach targeting glutamine metabolism in cancer should take into account the upregulation of other metabolic pathways that can lead to glutamate production; one such pathway is the glutaminase II pathway via GTK.
DOI: https://doi.org/10.1002/pmic.201800451
Much like the classic arcade game “Whack‐a‐Mole”, cancer cells are able to upregulate a different metabolic pathway when one pathway is inhibited. Specifically, using metabolomics technologies with isotopic labelling, Sunag Udupa et al. demonstrate in article number 1800451 the production of glutamate from glutamine via the glutaminase II pathway when the glutaminase 1 pathway is inhibited. This metabolic adaptability of tumors justifies the need for targeting multiple metabolic pathways, aiming at both main and adaptive metabolic networks.
Measurement and Methods: In this study, we sought to expand our knowledge of glutamine metabolism beyond glutaminolysis. Using mass spectroscopy-based stable isotope-resolved metabolomics (SIRM) with 13C515N2-labeled-glutamine, we precisely identified the metabolites produced from glutamine both in vitro and in vivo. Tumors were harvested 2 hours post first glutamine injection. Metabolites were then extracted from tumors, and analyzed using Agilent 6520 Q-TOF mass spectrometer and 1H-NMR. Metabolite intensities were later normalized to protein concentration following analysis.
Results: Our results showed that total glutamate levels were lower in BPTES-NP treated tumors as compared to vehicle control ones. Interestingly, we found an increase in (m+5) labeled glutamate (mass of the parent ions (m) and 5 more mass units due to 13C415N1-glutamate or 13C5-glutamate labelling) in BPTES-NP treated tumors as compared to the vehicle control tumors. Moreover, we found that (m+5) glutamate is a product of (m+7) glutamine being converted to (m+5) alpha-ketoglutaramate (KGM) via glutamine-pyruvate transaminase and further on into alpha-ketoglutarate (aKG) by omega-amidase, which can finally produce the identified (m+5) glutamate through glutamate dehydrogenase. Further analysis using 1H-NMR detailed a significant increase in overall KGM intermediate intensity in treatment groups compared to the control groups, confirming the upregulation of compensatory pathway (glutamine-KGM- aKG-glutamate) to produce glutamate upon treatment of GLS1 inhibitor.
Conclusion: These results explain the reasons behind the limited clinical outcomes for single therapy with a GLS1 inhibitor, and provide potential therapeutic targets: glutamine-pyruvate transaminase, for combination treatments with GLS1 inhibitors to prevent the compensation for glutamate production amid GLS1 inhibition.
Citation Format: Ryoichi Asaka, Tu Nguyen, Brian J. Krisch, Karim Nabi, Addison Quinones, Jessica Tan, Marjorie J. Antonio, Ting Li, Felipe Camelo, Kiet Nguyen, Sunag Udupa, Edward Gabrielson, Anne Le. Identification of compensatory pathway for glutamate production [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 803.
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