| Objective Previous studies indicated that glioma cells could release large amount ofglutamate, which could promote the proliferation of glioma cells, induce neuronalinjuries and epilepsy. In normal brain, astrocytes have large capacity of glutamateuptake, which may limit the glutamate-stimulated glioma growth and protect neuronsfrom excitotoxicity. Understanding the interaction and glutamate/glutaminemetabolism between astrocytes and glioma is pivotal to preserving the endogenousanti-tumor function of normal astrocytes. Furthermore, commonly used glioma celllines in the literature were mostly established many decades ago, likely haveaccumulated mutations making them inadequate to properly represent in vivosituations. To resolve this problem, we will establish new cell lines and use cellsin early passage for most experiments.Methods Employing techniques including cell culturing, cell tracking, apoptosisdetection, immunocytochemistry, western blot, pharmacological modulation, highperformance liquid chromatography, and ion imaging, we will test the followinghypotheses:1) Glioma cells express abundant glutaminase rather than glutaminesynthetase, while astrocytes have prominent glutamine synthetase expression.2)Glioma cells release a large amount of glutamate, in a manner depend on extracellularglutamine and cystine.3) In the early stage of glioma proliferation, glioma cells wereoutnumbered by surrounding astrocytes and extracellular glutamate could be clampedat low level. However, in the late stage of glioma progression, glutamate uptake willbe compromised by glioma cells:1) Accompanying glutamate release, glioma cellsrelease large amount of ammonia and the resulting oxidative stress may damageastrocyte glutamate uptake. Furthermore, excessive extracellular glutamate can induceneuronal death.2) In addition, glioma cells may induce apoptosis of astrocytes viaoxidative stress. Results Glutamate/Glutamine metabolism coupling existed between glioma cells andastrocytes based on the following results:1) High expression levels of glutaminaseand mislocalization of glutamine synthetase were detected in glioma cells.2) Gliomacells released extraordinary concentration of glutamate via cystine-glutamateexchanger in the presence of extracellular glutamine and cystine.3) Astrocytes couldclear extracellular glutamate released by glioma cells in co-culture system. However,glutamate/glutamine metabolism coupling was compromised in the late stage ofglioma progression:1) Glutamate uptake of astrocytes was damaged by ammonia andoxidative stress produced by glioma cells and extracellular glutamate increased inco-culture of astrocytes and glioma cells.2) Excitotoxic concentration of glutamatereleased by glioma cells induced the death of neurons.3) Apoptosis of astrocytes weredetected in the presence of overwhelming amount of glioma cells in co-culturesystem.Conclusion In the initial stage of glioma development, glutamate/glutaminemetabolic coupling existed between glioma cells and astrocytes. Following theproliferation of glioma, decreased glutamate of astrocytes, increased extracellularglutamate, apoptosis of astrocytes and neurons were detected and glutamate/glutaminemetabolism coupling was broken. Death of astrocytes and neurons will promote theinvasion of glioma. Furthermore, increased extracellular glutamate may promote rapidproliferation of glioma. Thus, integrity of glutamate/glutamine metabolism couplingdemonstrates the endogenous anti-tumor and neuron-protection activity. |
