Glutamine synthetase activity fuels nucleotide biosynthesis and supports growth of glutamine-restricted glioblastoma

Abstract

L-Glutamine (Gln) functions physiologically to balance tissue requirements of carbon and nitrogen. It has been proposed that in cancer cells undergoing aerobic glycolysis, accelerated anabolism is sustained by Gln-derived carbons, which replenish the tricarboxylic acid (TCA) cycle (anaplerosis). However, it is shown here that in glioblastoma (GBM) cells, almost half of the Gln-derived glutamate (Glu) is secreted and does not enter the TCA cycle and, that inhibiting glutaminolysis does not affect proliferation. Moreover, Gln-starved cells are not rescued by TCA cycle replenishment. Instead, the conversion of Glu to Gln by Glutamine Synthetase (GS) (cataplerosis) confers Gln prototrophy, and fuels de novo purine biosynthesis. In both orthotopic GBM models and in patients, 13C-glucose tracing showed that GS produces Gln from TCA cycle-derived carbons. Finally, while it is contributed only marginally by the circulation, the Gln required for the growth of GBM tumours is either autonomously synthesized by GS-positive glioma cells, or supplied by astrocytes.

Glutamine at focus: versatile roles in cancer

“…Targeting glutamine metabolism has demonstrated therapeutic efficacy in a number of models of glutamine-addicted cancer cell types. Innumerous reports have used 18 F-labeled analogs of glutamine that serves as a metabolic marker for the localization of tumor tissue (see below)…”

“…As mentioned above, the clinical procedure of glucosebased imaging by 18 F-FDG PET has been extensively applied for cancer diagnostic purposes [4]. Since emerging data has pointed out the dependence of certain tumor cell types to glutamine consumption, there is renewed attention to measure glutamine in cancer.…”

“…In this regard, innumerous studies have identified some analogs of glutamine as potential tracer to identify and characterize tumor cell types that rely on glutamine metabolism. For example, the new tracer L-[5-11 C]-glutamine was developed to serve as a metabolic marker for probing glutamine-addicted tumors not detected by 18 F-FDG PET [161]. Studies with glioblastoma cells with enhanced Myc activity identified another analog of glutamine (8F-(2S,4R)4-fluoroglutamine).…”

“…Although this term is coined Bthe Warburg effect,^it is a misinterpretation of damage in mitochondrial respiration instead of damage to the regulation of glycolysis. It has governed basic research and clinical implications over decades, leading, for instance, to the development of the clinical procedure known as 18 fluoro-2-deoxyglucose imaging by positron emission tomography (PET), which has been used as a diagnostic test [3,4]. The metabolic state of tumor cells have recently reemerged and are more complicated than previously thought.…”

“…Although glutamine contributes to anaplerosis in many tumor cells, in glioblastomas, it has been recently described as a cataplerotic mechanism, where glutamine-derived glutamate is secreted and does not enter the TCA cycle. This mechanism relies on glutamine synthetase (GS) activity, the enzyme that catalyzes, using ATP, the synthesis of glutamine from glutamate and ammonia (NH 4+ ), leading to nucleotide biosynthesis and support glioblastoma growth [18]. Moreover, glutamine might be robustly correlated, rather than glucose, to maintain cancer invasion in ovarian tumor growth and in overall patient survival [19].…”