gms | German Medical Science

Objective: Tumor metabolism has been the focus of oncological research for several years now. An altered metabolism is often the basis of malignancy and increased proliferation of tumors. In gliomas there is a close connection between metabolism and microenvironment. In particular, the balance of uptake and release of neurotransmitter metabolites is crucial for a variety of functions. In this work we aim to investigate the various regulatory functions of glutamate, an important neurotransmitter which has also been found to be strongly released by tumor cells.

Methods: Extracellular glutamate release and transcriptional data from 33 patients were analyzed by weighted correlation network analysis. We used glutamate stimulation combined with several antagonists of glutamate receptors and BRdU proliferation assay to identify responsible glutamate receptors on tumor cells. RNA sequencing and large-scale proteomic data were performed to investigate the down-stream pathway activation of glutamate stimulation. We simulated different metabolic states of tumor cells and validated the glutamate release by metabolic profiling.

Results: First, a network-based analysis of glutamate release and transcriptomic datasets, showing a significant correlation of extracellular glutamate and cell cycle activation. We investigated the impact of glutamate on cell migration and proliferation, which was significantly induced by activation of the GRM3, a G-coupled glutamate receptor forcing a MAPK pathway activity validated by western blot and RNA-sequencing data. In order to define the role of glutamate as an extracellular stimulator of proliferation, and not only as an energy resource, we treated the cells with glutamine, which revealed no differences in cell migration or proliferation. We further showed that the extracellular glutamate release could temporally change based on the differentiation status and metabolic state of the tumor cells by RNA sequencing and immunostainings. Lower levels of glutamate release were associated with increased stemness, while differentiated cells had an increased glutamate release.

Conclusion: Our analysis revealed a novel role of glutamate release which is dependent on the metabolic state of the tumor cell and its differentiation status. This autocrine-loop drives proliferation mediated by the GRM3 glutamate receptor. We further plan to investigate the role of GRM3 in normal brain and validate GRM3 as a potential target for tumor therapy