title: Cofactor balance by nicotinamide nucleotide transhydrogenase (NNT) coordinates reductive carboxylation and glucose catabolism in the tricarboxylic acid (TCA) cycle.
abstract: Cancer and proliferating cells exhibit an increased demand for glutamine-derived carbons to support anabolic processes. In addition, reductive carboxylation of α-ketoglutarate by isocitrate dehydrogenase 1 (IDH1) and 2 (IDH2) was recently shown to be a major source of citrate synthesis from glutamine. The role of NAD(P)H/NAD(P)(+) cofactors in coordinating glucose and glutamine utilization in the tricarboxylic acid (TCA) cycle is not well understood, with the source(s) of NADPH for the reductive carboxylation reaction remaining unexplored. Nicotinamide nucleotide transhydrogenase (NNT) is a mitochondrial enzyme that transfers reducing equivalents from NADH to NADPH. Here, we show that knockdown of NNT inhibits the contribution of glutamine to the TCA cycle and activates glucose catabolism in SkMel5 melanoma cells. The increase in glucose oxidation partially occurred through pyruvate carboxylase and rendered NNT knockdown cells more sensitive to glucose deprivation. Importantly, knocking down NNT inhibits reductive carboxylation in SkMel5 and 786-O renal carcinoma cells. Overexpression of NNT is sufficient to stimulate glutamine oxidation and reductive carboxylation, whereas it inhibits glucose catabolism in the TCA cycle. These observations are supported by an impairment of the NAD(P)H/NAD(P)(+) ratios. Our findings underscore the role of NNT in regulating central carbon metabolism via redox balance, calling for other mechanisms that coordinate substrate preference to maintain a functional TCA cycle.
authors.affiliation: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Paulo A
authors.initials: PA
authors.lastname: Gameiro
authors.forename: Laura A
authors.initials: LA
authors.lastname: Laviolette
authors.forename: Joanne K
authors.initials: JK
authors.lastname: Kelleher
authors.forename: Othon
authors.initials: O
authors.lastname: Iliopoulos
authors.forename: Gregory
authors.initials: G
authors.lastname: Stephanopoulos
mesh_terms: D000818:Animals; D002460:Cell Line; D002952:Citric Acid Cycle / Q000502:physiology*; D055785:Gene Knockdown Techniques; D005947:Glucose / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008819:Mice, Nude; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism; D009243:NAD / Q000235:genetics / Q000378:metabolism*; D009249:NADP / Q000235:genetics / Q000378:metabolism*; D061087:NADP Transhydrogenase, AB-Specific / Q000235:genetics / Q000378:metabolism*; D010084:Oxidation-Reduction
chemical_list: D024101:Mitochondrial Proteins; D009243:NAD; D009249:NADP; D061087:NADP Transhydrogenase, AB-Specific; C556048:Nnt protein, mouse; D005947:Glucose
keywords: Glutamine; Metabolic Tracers; Mitochondria; NADPH; NNT; Pyruvate Carboxylase; Redox Regulation; Reductive Carboxylation