Reductive glutamine metabolism by IDH1 mediates lipogenesis under hypoxia

Abstract

Acetyl coenzyme A (AcCoA) is the central biosynthetic precursor for fatty acid synthesis and protein acetylation. In the conventional view of mammalian cell metabolism, AcCoA is primarily generated from glucose-derived pyruvate through the citrate shuttle and adenosine triphosphate citrate lyase (ACL) in the cytosol1-3. However, proliferating cells that exhibit aerobic glycolysis and those exposed to hypoxia convert glucose to lactate at near stoichiometric levels, directing glucose carbon away from the tricarboxylic acid cycle (TCA) and fatty acid synthesis4. Although glutamine is consumed at levels exceeding that required for nitrogen biosynthesis5, the regulation and utilization of glutamine metabolism in hypoxic cells is not well understood. Here we show that human cells employ reductive metabolism of alpha-ketoglutarate (αKG) to synthesize AcCoA for lipid synthesis. This isocitrate dehydrogenase 1 (IDH1) dependent pathway is active in most cell lines under normal culture conditions, but cells grown under hypoxia rely almost exclusively on the reductive carboxylation of glutamine-derived αKG for de novo lipogenesis. Furthermore, renal cell lines deficient in the von Hippel-Lindau (VHL) tumor suppressor protein preferentially utilize reductive glutamine metabolism for lipid biosynthesis even at normal oxygen levels. These results identify a critical role for oxygen in regulating carbon utilization in order to produce AcCoA and support lipid synthesis in mammalian cells.

L-2-Hydroxyglutarate production arises from noncanonical enzyme function at acidic pH

“…The relative contributions of LDHA and MDH2 to hypoxia-induced L-2HG appear to vary depending on the type of cell, which may be related to enzyme levels and subcellular pools of substrate. Hypoxic cells undergo a variety of metabolic changes that might favor L-2HG production 18,22–25,45 . Here we identify acidification and increased αKG concentration as factors that are sufficient to induce cellular production of L-2HG in normoxia.…”

Mitochondrial metabolic remodeling in response to genetic and environmental perturbations

“…This is a potentially important pathway in hypoxia, as pyruvate oxidation is inhibited by HIF1‐dependent upregulation of PDK1, producing a deficit of the metabolites in the proximal portion of the TCA cycle (citrate and isocitrate). Indeed, as previously suggested, a reduction in oxygen tension may also be limiting for oxidation of glutamine, making reductive carboxylation more relied upon for anabolic metabolism 30. The enzymes responsible for the reductive metabolism of glutamine can be found both in the mitochondrial matrix and the cytosol (ACO2 and IDH2; ACO1 and IDH1, respectively), and may be able to compensate for this deficit in these conditions 29, 31.…”

“…As outlined above, it has previously been shown that hypoxic cells may rely more on reductive glutamine metabolism to produce citrate (and thereby lipids) than normoxic cells 30. Both IDH1 and 2 have potential roles in hypoxia‐induced reductive carboxylation,29, 31 suggesting that this pathway may be perturbed in IDH1‐mutated gliomas.…”

“…Both IDH1 and 2 have potential roles in hypoxia‐induced reductive carboxylation,29, 31 suggesting that this pathway may be perturbed in IDH1‐mutated gliomas. Given that the cytosolic pathway has been shown to be important in hypoxic reductive carboxylation for lipogenesis,30, 32, 33, 34 and that gliomas appear to ‘favor’ IDH1 mutations, there may be a requirement for IDH2 wild‐type activity in hypoxic IDH1 mutant gliomas. However, a recent paper also showed that IDH1 ‐mutated cells retained more of their oxidative TCA cycle metabolism than their wild‐type counterparts 117.…”

Non-targeted Tracer Fate Detection

“…Stable isotope labeled tracers have been successfully applied to study metabolic fluxes in cell culture and in vivo in animals and humans [2][3][4][5][6][7].…”