Fatty acid labeling from glutamine in hypoxia can be explained by isotope exchange without net reductive isocitrate dehydrogenase (IDH) flux - PubMed (original) (raw)

Fatty acid labeling from glutamine in hypoxia can be explained by isotope exchange without net reductive isocitrate dehydrogenase (IDH) flux

Jing Fan et al. J Biol Chem. 2013.

Abstract

Acetyl-CoA is an important anabolic precursor for lipid biosynthesis. In the conventional view of mammalian metabolism, acetyl-CoA is primarily derived by the oxidation of glucose-derived pyruvate in mitochondria. Recent studies have employed isotope tracers to show that in cancer cells grown in hypoxia or with defective mitochondria, a major fraction of acetyl-CoA is produced via another route, reductive carboxylation of glutamine-derived α-ketoglutarate (catalyzed by reverse flux through isocitrate dehydrogenase, IDH). Here, we employ a quantitative flux model to show that in hypoxia and in cells with defective mitochondria, oxidative IDH flux persists and may exceed the reductive flux. Therefore, IDH flux may not be a net contributor to acetyl-CoA production, although we cannot rule out net reductive IDH flux in some compartments. Instead of producing large amounts of net acetyl-CoA reductively, the cells adapt by reducing their demand for acetyl-CoA by importing rather than synthesizing fatty acids. Thus, fatty acid labeling from glutamine in hypoxia can be explained by spreading of label without net reductive IDH flux.

Keywords: Cancer; Hypoxia; Isotopic Tracers; Metabolism; Mitochondrial Metabolism.

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Figures

FIGURE 1.

FIGURE 1.

Network diagram. OAA, oxaloacetate; AKG, α-ketoglutarate.

FIGURE 2.

FIGURE 2.

Glutamine intake to TCA cycle and acetyl-CoA demand. a, glutamine influx into TCA cycle inferred based on measurement of total glutamine uptake from media, minus glutamate and proline secretion, glutamine demand for protein biosynthesis, and dilution of glutamine and glutamate pools. nmol/uL-cells/h, nmol/μl of cells/h. b, labeling pattern of palmitate in 143B-WT and 143B-CYTB when fed with both [U-13C]glucose and [U-13C]glutamine. Error bars designate mean ± S.D. c, acetyl-CoA demand for fatty acid biosynthesis and protein acetylation.

FIGURE 3.

FIGURE 3.

Extensive labeling of acetyl-CoA from glutamine can occur without net reductive IDH flux. a, fraction of cytosolic acetyl-CoA m+2 labeling from [U-13C]glutamine (y axis) for various combinations of net IDH flux (represented by color) and unidirectional reductive IDH flux (x axis) (fluxes shown in nmol/μl of cells/h (nmol/uL-cells/h)). Oxidative IDH net flux is shown in blue, whereas reductive IDH net flux is shown in green. The solid red line represents an upper bound on acetyl-CoA m+2 labeling when the net IDH flux is oxidative. The analysis was done based on measurements in 143B-CYTB. b, measured acetyl-CoA m+2 labeling (blue) versus the feasible upper bound (red) assuming oxidative IDH net flux. Error bars designate mean ± S.D.

FIGURE 4.

FIGURE 4.

Bounding oxidative and reductive IDH fluxes via steady-state metabolite labeling patterns. a, measured fractional labeling of α-ketoglutarate m+5, citrate m+5, and malate m+3 from [U-13C]glutamine. Error bars designate mean ± S.D. b, a lower bound on IDH oxidative flux (blue) versus an upper bound on IDH reductive flux (red) calculated based on Equations 8 and 10. Error bars designate mean ± S.D. c, a lower bound on IDH net flux (calculated by subtracting the upper bound on IDH reductive flux from the lower bound on IDH oxidative flux), assuming potentially lower glutamine flux into TCA cycle via α-ketoglutarate (_v_4). The lower bound on net oxidative IDH flux is marked with a straight line, whereas the standard deviation is marked with a dashed line. nmol/uL-cells/h, nmol/μl of cells/h.

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