Redox control of glutamine utilization in cancer

Abstract

Glutamine utilization promotes enhanced growth of cancer cells. We propose a new concept map of cancer metabolism in which mitochondrial NADH and NADPH, in the presence of a dysfunctional electron transfer chain, promote reductive carboxylation from glutamine. We also discuss why nicotinamide nucleotide transhydrogenase (NNT) is required in vivo for glutamine utilization by reductive carboxylation. Moreover, NADPH, generated by both the pentose phosphate pathway and the cancer-specific serine glycolytic diversion, appears to sustain glutamine utilization for amino-acid synthesis, lipid synthesis, and for ROS quenching. The fact that the supply of NAD(+) precursors reduces tumor aggressiveness suggests experimental approaches to clarify the role of the NADH-driven redox network in cancer.

Figure 1

Schematic representation of glutamine metabolic rewiring. Glutamine imported via Slc1a5 enters in a complex metabolic pathway, described in the text, so that both its carbon and nitrogen are utilized to promote growth and survival of cancer cells. AcCoA, acetyl-CoA; ACL, ATP citrate lyase; Akg, α-ketoglutarate; Ala, alanine; Arg, arginine; Asn, asparagine; Asp, aspartate; Cit, citrate; Cys, cysteine; FFA, fatty acids; Fum, Fumarate; GDH, glutamate dehydrogenase; Gln, glutamine; Glu, glutamate; Gly, glycine; GOT, glutamic-oxaloacetic transaminase; GSH, glutathione reduced; GSSG, glutathione oxidized; Isocit, isocitrate; Mal, malate; MDH 1 and 2, malate dehydrogenase; ME, malic enzyme; NEAA, non-essential amino acids; NNT, nicotinamide nucleotide transhydrogenase; Oaa, oxalacetate; PDH, pyruvate dehydrogenase; PDHKs, pyruvate dehydrogenase kinases; Pyr, pyruvate; ROS, reactive species oxygen; Succ, succinate

Figure 2

Concept map of the cancer metabolic rewiring. Large glycolytic flux and complex I dysfunction are required to sustain glutamine reductive carboxylation. Yellow arrows identify glycolysis. Blue arrows identify glutamine metabolism, green arrows identify pentose phosphate pathway (PPP), bright Lilac identifies serine/glycine/one-carbon-folate-metabolism, and red arrows identify an NAD(P)H electron transfer flux (ETF). NAD(P)H ETF originates from serine diversion pathway, sustains lipid synthesis, ROS quenching by GSH and reductive steps of non-essential amino-acid (NEAA) synthesis. Abbreviations: 1,3BPGA, bisphosphoglycerate; 3PGA, 3phosphoglycerate; 6PGL, 6-phosphogluconolactone; AcCoA, acetyl-CoA; ACL, ATP citrate lyase; Akg, α-ketoglutarate; Ala, alanine; Arg, arginine; Asn, asparagine; Asp, aspartate; CH⁺-Thf, 5-methylenetetrahydrofolate; CH₂-Thf, 5,10-methylenetetrahydrofolate; Cit, citrate; Cys, cysteine; F1,6BP, fructose 1,6 biphosphate; FFA, fatty acids; G6P, glucose 6-phosphate; GA3P, glyceraldehyde 3-phosphate; Glc, glucose; Gln, glutamine; Glu, glutamate; Gly, glycine; GOT, glutamic-oxaloacetic transaminase; GSH, glutathione reduced; GSSG, glutathione oxidized; Lac, lactate; LDHA, lactate dehydrogenase; MDH 1, malate dehydrogenase; ME, malic enzyme; MTHFD1 and 2, methylenetetrahydrofolate dehydrogenase (NADP⁺ dependent); NEAA, non-essential amino acids; Oaa, oxalacetate; PPP, pentose phosphate pathway; P-Pyr, 3-phosphohydroxypyruvate; Pyr, pyruvate; R5P, ribose 5-phosphate; ROS, reactive species oxygen; Ser, serine; SHMT1 and 2, serine hydroxymethyltransferase

Figure 3

Schematic representation of NEAA synthesis. (a) Schematic fractional contribution of each amino acid in total protein and the corresponding number of molecules calculated for each NEAA that have to be obtained from metabolism. (b) Schematic representation of NEAA synthesis derived from glutamine utilization in cancer cells

Figure 4

Map of the NAD±biosynthetic pathway. To clarify the interpretation of the ‘in vivo' experiments described in the test, the NAD⁺ biosynthetic pathway is schematically reported: NAD⁺ can be synthesized from tryptophan (de novo synthesis) or nicotinic acid -NicA- or nicotinamide -Nam- (salvage pathway). NAD⁺ synthesis via tryptophan is performed by several reactions that lead to quinolinic acid (Quin) synthesis: this is next converted into nicotinic acid mononucleotide (NAMN) and NAMN to desamido-NAD (NAAD) and eventually into NAD⁺. Diversely, in the major salvage pathway of NAD⁺ synthesis, NAM can be converted into nicotinamide mononucleotide (NMN) by nicotinamide phosphoribosyltransferase (NAMPT) and NMN is further converted into NAD⁺ by NMN adenylyltransferase (NMNAT 1 and 2 nuclear and cytoplasmic isoforms respectively, NMNAT3 mitochodrial isoform). The asterisk indicates a reaction, catalyzed by NAD⁺ glycohydrolase, which bears close similarity with the family of ADP ribosyl cyclases. However, all the NAD⁺-transforming enzyme families mentioned above (i.e., Mono ADP-ribosyl transferases, PARP_s, sirtuins) share with ADP ribosyl cyclases/NAD⁺ glycohydrolases the property of releasing NAM from NAD⁺