Molecular mechanisms of necrosis in glioblastoma: the role of glutamate excitotoxicity

Abstract

Glioblastomas continue to rank among the most lethal primary human tumors. Despite treatment with the most rigorous surgical interventions along with the most optimal chemotherapeutic and radiation regimens, the median survival is just 12-15 mo for patients with glioblastoma. Among the histological hallmarks of glioblastoma, necrosis has been demonstrated to be a powerful predictor of poor patient prognosis. Over the years, there have been many advances in our understanding of the molecular mechanisms underlying glioblastoma formation, yet the mechanisms that lead to tumor necrosis remain unclear. One pathway that may lead to necrosis in glioblastoma involves the neurotransmitter, glutamate, which has been shown to accumulate in the peritumoral fluid as a result of decreased cellular uptake by glioblastoma cells. This accumulation leads to subsequent glutamate excitotoxicity and probable necrosis through a massive elevation of intracellular Ca(2+) and reduction in cellular ATP levels. We propose that a pathway involving tumor necrosis factor-alpha (TNFalpha), astrocyte-elevated gene-1 (AEG-1) and nuclear factor-kappaB (NFkappaB) leads to decreased glutamate uptake through coordinated downregulation of the excitatory amino acid transporter 2 (EAAT2), the glutamate transporter responsible for the majority of glutamate uptake in the human brain. In addition, we suggest that AEG-1 signaling, loss of phosphatase and tensin homolog (PTEN), and ionotropic glutamate receptor activity lead to AKT pathway activation, which results in nutrient overconsumption and necrosis. Together, these pathways provide a new perspective on glioblastoma necrosis involving the process of glutamate excitotoxicity. Future research should address the components of these molecular pathways in order to better understand the mechanism of necrosis in glioblastoma and to begin to develop targeted therapies that may improve patient prognosis in the future.

Figure 1. Mechanism of glutamate excitotoxicity-induced necrosis in glioblastoma

Glutamate accumulation as a result of decreased functional EAAT2 activity triggers ionotropic glutamate receptor activation, which causes an increase in the [Ca²⁺]_i. At the same time, elevated levels of extracellular glutamate inhibit system ${\mathrm{x}}_{\mathrm{c}}^{-}$ activity. This inhibition leads to decreased intracellular cysteine and subsequent impairment of glutathione production, culminating in the inability to neutralize reactive oxygen species. Reactive oxygen species cause membrane oxidation and ATP-dependent Ca²⁺ release from the endoplasmic reticulum (ER), leading to mitochondrial damage and further ATP depletion. With severe reduction in ATP levels, Na⁺ and water enter the cell and precipitate a massive cell volume increase. K⁺ efflux fails to maintain ionic homeostasis, which results in further increases in Na⁺ and water influx. As a result, the cell swells, and its plasma membrane ruptures, leading to cellular collapse. In the final stage, the leakage of intracellular contents results in the activation of extracellular proteases, which induces inflammation and eventual necrosis. NMDA-R, N-methyl D-aspartic acid receptor; AMPA-R, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; EAAT2, excitatory amino acid transporter 2; ROOH, reactive oxygen species; ROH, neutralized reactive oxygen species.

Figure 2. Molecular pathway leading to extracellular glutamate accumulation in glioblastoma

AEG-1, astrocyte-elevated gene 1; TNF-α, tumor necrosis factor-alpha; PI3K, phosphatidylinositol-3-kinase; PTEN, phosphatase and tensin homolog; NMDA-R, N-methyl D-aspartic acid receptor; AMPA-R, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; EAAT2, excitatory amino acid transporter 2; NF-κB, nuclear factor-kappa B.