Beyond aerobic glycolysis: Transformed cells can engage in glutamine metabolism that exceeds the requirement for protein and nucleotide synthesis

Abstract

Tumor cell proliferation requires rapid synthesis of macromolecules including lipids, proteins, and nucleotides. Many tumor cells exhibit rapid glucose consumption, with most of the glucose-derived carbon being secreted as lactate despite abundant oxygen availability (the Warburg effect). Here, we used 13 C NMR spectroscopy to examine the metabolism of glioblastoma cells exhibiting aerobic glycolysis. In these cells, the tricarboxylic acid (TCA) cycle was active but was characterized by an efflux of substrates for use in biosynthetic pathways, particularly fatty acid synthesis. The success of this synthetic activity depends on activation of pathways to generate reductive power (NADPH) and to restore oxaloacetate for continued TCA cycle function (anaplerosis). Surprisingly, both these needs were met by a high rate of glutamine metabolism. First, conversion of glutamine to lactate (glutaminolysis) was rapid enough to produce sufficient NADPH to support fatty acid synthesis. Second, despite substantial mitochondrial pyruvate metabolism, pyruvate carboxylation was suppressed, and anaplerotic oxaloacetate was derived from glutamine. Glutamine catabolism was accompanied by secretion of alanine and ammonia, such that most of the amino groups from glutamine were lost from the cell rather than incorporated into other molecules. These data demonstrate that transformed cells exhibit a high rate of glutamine consumption that cannot be explained by the nitrogen demand imposed by nucleotide synthesis or maintenance of nonessential amino acid pools. Rather, glutamine metabolism provides a carbon source that facilitates the cell's ability to use glucose-derived carbon and TCA cycle intermediates as biosynthetic precursors.cancer ͉ glioblastoma ͉ Warburg effect ͉ glutaminolysis ͉ anaplerosis I n mammals, cell proliferation is controlled by signal transduction pathways stimulated by lineage-specific growth factors or, in tumors, constitutive activation of these pathways through oncogenic mutations. A proximal effect of signaling pathways is a robust increase in nutrient uptake (1-3). Cells must also allocate these molecules into appropriate metabolic pathways to produce and maintain pools of intermediates needed to synthesize macromolecules. Therefore, a complete understanding of the biology of cell proliferation will require a comprehensive understanding of the regulation of metabolic fluxes.Glucose and glutamine are two of the most abundant nutrients in plasma, and together they account for most carbon and nitrogen metabolism in mammalian cells. Rapid cell proliferation has been associated with a robust but apparently wasteful metabolism of glucose. In the 1920s, Otto Warburg demonstrated that ascites tumor cells had high rates of glucose consumption and lactate production despite availability of sufficient oxygen to oxidize glucose completely (4). The ''Warburg effect'' is taken to be a metabolic hallmark of aggressive tumors; however, the phenotype is also observed in nontransformed cells during rapid proliferation (2, 5). Gl...

Metabolic reprogramming of the tumor

“…The progressive loss of glucose as mitochondrial substrate (Semenza, 2009) in tumor cells is often compensated by glutamine metabolism (DeBerardinis et al, 2007). In fact, ammonia levels in the interstitial fluid of xenografts were found to reach 5 mM, a 10-fold increase over physiological values (Eng et al, 2010), and it has long been known that neoplastic cells overexpress glutaminase along with tumor growth and dedifferentiation (Knox et al, 1969).…”

Sugar-free approaches to cancer cell killing

“…Indeed, glucose is not the only molecule that tumor cells require to grow. For instance, oncogenic myc promotes the use of the amino acid glutamine, which cells can use to produce not only proteins but also ATP and nucleic acids (DeBerardinis et al, 2007;Gao et al, 2009). Growing cells require to synthesize new lipids, nucleic acids and proteins, which means that inhibition of many metabolic pathways, such as fatty acid synthesis or nucleotide synthesis, could promote tumor cell death.…”

Glutamine at focus: versatile roles in cancer

“…The malate-aspartate shuttle is able to transfer electrons to mitochondrial complex I, generating ATP, and resupplying NAD + that drives glycolysis. Moreover, oxalacetate (OAA), another TCA cycle intermediary, in the presence of acetyl-CoA, can generate citrate, providing de novo fatty acid biosynthesis [17].…”

“…For instance, extracellular glutamine can donate carbon and nitrogen to supply anabolic pathways and energy production. It has the ability to replenish TCA cycle intermediaries (anaplerosis) and promote synthesis of nucleotides, proteins, and lipids [17]. 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.…”