Attenuated AMPA Receptor Expression Allows Glioblastoma Cell Survival in Glutamate-Rich Environment

Abstract

BackgroundGlioblastoma multiforme (GBM) cells secrete large amounts of glutamate that can trigger AMPA-type glutamate receptors (AMPARs). This commonly results in Na+ and Ca2+-permeability and thereby in excitotoxic cell death of the surrounding neurons. Here we investigated how the GBM cells themselves survive in a glutamate-rich environment.Methods and Findings In silico analysis of published reports shows down-regulation of all ionotropic glutamate receptors in GBM as compared to normal brain. In vitro, in all GBM samples tested, mRNA expression of AMPAR subunit GluR1, 2 and 4 was relatively low compared to adult and fetal total brain mRNA and adult cerebellum mRNA. These findings were in line with primary GBM samples, in which protein expression patterns were down-regulated as compared to the normal tissue. Furthermore, mislocalized expression of these receptors was found. Sequence analysis of GluR2 RNA in primary and established GBM cell lines showed that the GluR2 subunit was found to be partly unedited.ConclusionsTogether with the lack of functional effect of AMPAR inhibition by NBQX our results suggest that down-regulation and afunctionality of AMPARs, enable GBM cells to survive in a high glutamate environment without going into excitotoxic cell death themselves. It can be speculated that specific AMPA receptor inhibitors may protect normal neurons against the high glutamate microenvironment of GBM tumors.

Quantitative Phosphoproteomics Reveals Wee1 Kinase as a Therapeutic Target in a Model of Proneural Glioblastoma

“…Reduced glutamate receptor phosphorylation, previously observed in GBM (35, 36), along with decreased phosphorylation of Dlg 2 (PSD-93), 3, and 4 (PSD-95) (37), and other neuron-specific proteins in our tumors likely reflect the differences in cellular composition between glioma and normal brain tissue (21). By comparison, phosphorylation of proteins involved in canonical oncogenic signaling networks, including ERK1/2 MAP kinases (T183/Y185 and T203/Y205 in the activation loop), PI3K regulatory subunit P85a, PDGFRα, and RTK scaffolds GAB1 and SHC, and cell cycle regulation (CDK1) were all increased in the tumor samples.…”

Dissection of mitogenic and neurodegenerative actions of cystine and glutamate in malignant gliomas

“…Relative to GluR1-GluR4 expression in primary astrocytes, quantitative analysis of GluR1-GluR4 transcript levels revealed the AMPA receptor subunits to be substantially downregulated in SMA-560, U87-MG and U373-MG glioma cells (Figure 4a), which might explain their lack of glutamate/AMPA responsiveness. A recent study revealed a complete lack of inward current in U87-MG cells following glutamate treatment, supporting this hypothesis (van Vuurden et al, 2009). In contrast, the transcript levels of GluR2 and GluR3 were severely increased in rat F98 and C6 glioma cells in comparison to primary astrocytes.…”

“…Conversely, overexpression of the unedited, Ca 2 þ permeable GluR2(Q) subunit augmented CGNH-89 glioma cell proliferation (Ishiuchi et al, 2002). In clear contrast to these studies, it has been recently suggested that glioma cells survive glutamate-rich, excitotoxic environment due to intrinsic glutamate receptor inactivation, which is supported by the finding that GluR subunits are transcriptionally downregulated in glioma specimens (van Vuurden et al, 2009). Owing to this caveat and the apparent importance of glutamate signaling for potential glioma therapy, we analyzed in detail the effects of genetic and pharmaco logical interference with glutamate secretion and cystine incorporation mediated by system X c À as well as glutamate receptor agonism and antagonism on glioma growth in vitro and ex vivo. We show that neither glutamate nor AMPA possesses any mitogenic effects on glioma cells even under growth factor-reduced or low-cystine conditions.…”

Brain tumour-associated status epilepticus

“…Importantly, Aronica et al demonstrated increased NR2A and NR2B (stimulatory NMDA receptor subunits) expression in 41 patients with LGG compared to controls [46]. Further, van Vuurden et al showed that oligodendrogliomas and astrocytomas exhibited greater AMPA-R expression than glioblastoma multiforme (GBM): that is, AMPA receptor expression appeared negatively correlated with WHO tumour grade [48]. In similar fashion, van Vuurden et al demonstrated significant down-regulation of NMDA (NR1, NR2A-C, NR3A), AMPA (GluR1-4) and kainite (GluR5,7; KA2) receptors (ionotropic receptors) in GBM compared to controls [48].…”

“…Further, van Vuurden et al showed that oligodendrogliomas and astrocytomas exhibited greater AMPA-R expression than glioblastoma multiforme (GBM): that is, AMPA receptor expression appeared negatively correlated with WHO tumour grade [48]. In similar fashion, van Vuurden et al demonstrated significant down-regulation of NMDA (NR1, NR2A-C, NR3A), AMPA (GluR1-4) and kainite (GluR5,7; KA2) receptors (ionotropic receptors) in GBM compared to controls [48]. Downregulation of AMPA-R, for example, enables GBM cells to survive a high glutamate environment without suffering excitotoxic cell death [48].…”