Pancreatic β cells put the glutamine engine in reverse
Authors: Evan C. Lien, Matthew G. Vander Heiden
Journal: Cell Metabolism (2021)

The metabolism of nutrients other than glucose influences insulin secretion by pancreatic β cells, but the mechanisms involved are incompletely understood. In this issue of Cell Metabolism, Zhang et al., 2020Zhang G.F. Jensen M.V. Gray S.M. El K. Wang Y. Lu D. Becker T.C. Campbell J.E. Newgard C.B. Reductive TCA cycle metabolism fuels glutamine- and glucose-stimulated insulin secretion.Cell Metab. 2020; 33 (this issue): 833-844Abstract Full Text Full Text PDF Scopus (16) Google Scholar report that reductive glutamine metabolism generates cytosolic NADPH to promote insulin secretion by β cells. The metabolism of nutrients other than glucose influences insulin secretion by pancreatic β cells, but the mechanisms involved are incompletely understood. In this issue of Cell Metabolism, Zhang et al., 2020Zhang G.F. Jensen M.V. Gray S.M. El K. Wang Y. Lu D. Becker T.C. Campbell J.E. Newgard C.B. Reductive TCA cycle metabolism fuels glutamine- and glucose-stimulated insulin secretion.Cell Metab. 2020; 33 (this issue): 833-844Abstract Full Text Full Text PDF Scopus (16) Google Scholar report that reductive glutamine metabolism generates cytosolic NADPH to promote insulin secretion by β cells. Homeostasis in mammals requires the coordination of metabolism across tissues. This involves the integration of circulating metabolites with hormonal signals that regulate nutrient utilization in specific tissues. This paradigm for whole-body metabolic regulation is best understood with respect to the use of insulin and other hormones to maintain blood glucose levels within a narrow range. A major component of this system is the release of insulin by pancreatic β cells in response to changing blood glucose levels, and defects in this process underlie metabolic diseases including diabetes and obesity. Glucose-stimulated insulin secretion (GSIS) is well studied and is initiated by glucose metabolism through glycolysis, the tricarboxylic acid (TCA) cycle, and oxidative phosphorylation to generate ATP from ADP. The resulting increased ATP/ADP ratio inhibits ATP-sensitive potassium (KATP) channels in the plasma membrane, which depolarizes the membrane to trigger voltage-gated Ca2+ channels to increase Ca2+ influx into cells and stimulate release of insulin-containing secretory granules (Figure 1) (Newgard and Matschinsky, 2011Newgard C.B. Matschinsky F.M. Substrate control of insulin release.in: Terjung R. Comprehensive Physiology. Wiley Online Library, 2011: 125-151Crossref Google Scholar). Thus, an increased ATP/ADP ratio serves as the major metabolic output downstream of glucose metabolism that stimulates GSIS. While this core mechanism is critical for triggering GSIS, it is increasingly recognized that outputs of metabolism beyond ATP may also be involved in the control of insulin secretion and may underlie how levels of other nutrients modulate GSIS. Multiple nutrients other than glucose, including glutamine, alanine, branched-chain amino acids, and fatty acids, can modulate the β cell response to glucose, but the mechanism(s) involved are less understood (Newgard and Matschinsky, 2011Newgard C.B. Matschinsky F.M. Substrate control of insulin release.in: Terjung R. Comprehensive Physiology. Wiley Online Library, 2011: 125-151Crossref Google Scholar). In this issue of Cell Metabolism, Zhang et al. elucidate the metabolic route by which glutamine is metabolized by pancreatic β cells through the TCA cycle to generate the redox co-factor NADPH, which promotes insulin granule exocytosis (Zhang et al., 2020Zhang G.F. Jensen M.V. Gray S.M. El K. Wang Y. Lu D. Becker T.C. Campbell J.E. Newgard C.B. Reductive TCA cycle metabolism fuels glutamine- and glucose-stimulated insulin secretion.Cell Metab. 2020; 33 (this issue): 833-844Abstract Full Text Full Text PDF Scopus (16) Google Scholar). Upon entering cells, glutamine can be metabolized to glutamate through glutaminase (GLS), and further metabolism of glutamate by glutamate dehydrogenase (GDH) or transaminases produces ⍺-ketoglutarate (⍺KG) that can enter the TCA cycle (Figure 1). Prior work had demonstrated that citrate and isocitrate produced from the TCA cycle are exported out of the mitochondria and subsequently used by isocitrate dehydrogenase 1 (IDH1) to generate cytosolic NADPH as a coupling factor that promotes insulin secretion (Figure 1) (Ronnebaum et al., 2006Ronnebaum S.M. Ilkayeva O. Burgess S.C. Joseph J.W. Lu D. Stevens R.D. Becker T.C. Sherry A.D. Newgard C.B. Jensen M.V. A pyruvate cycling pathway involving cytosolic NADP-dependent isocitrate dehydrogenase regulates glucose-stimulated insulin secretion.J. Biol. Chem. 2006; 281: 30593-30602Abstract Full Text Full Text PDF PubMed Scopus (171) Google Scholar; Odegaard et al., 2010Odegaard M.L. Joseph J.W. Jensen M.V. Lu D. Ilkayeva O. Ronnebaum S.M. Becker T.C. Newgard C.B. The mitochondrial 2-oxoglutarate carrier is part of a metabolic pathway that mediates glucose- and glutamine-stimulated insulin secretion.J. Biol. Chem. 2010; 285: 16530-16537Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). Since the TCA cycle is canonically thought to proceed in an oxidative direction, it was assumed that oxidative metabolism of glutamine-derived ⍺KG contributed to citrate and isocitrate production. To test this, Zhang et al. used mass spectrometry to trace glutamine fate in a rat insulinoma cell line and in primary rat pancreatic islets. Surprisingly, they find that while both glucose- and glutamine-stimulated insulin secretion are indeed accompanied by increased oxidative TCA cycle flux, exposure to glucose or glutamine more prominently induces the reductive metabolism of ⍺KG to isocitrate and citrate (Figure 1). In this case, reductive carboxylation of ⍺KG to isocitrate in the “reverse” direction is mediated by mitochondrial isocitrate dehydrogenase 2 (IDH2) and contributes to both glucose- and glutamine-stimulated insulin secretion, since impairment of IDH2 with either siRNA or the IDH2 inhibitor AGI6780 inhibits reductive TCA cycle flux, the production of NADPH as a coupling factor, and insulin secretion. Consistent with these observations, treating mice with the IDH2 inhibitor AGI6780 also impairs insulin secretion in response to glucose challenge, suggesting that reductive TCA cycle flux can modulate GSIS in animals. Taken together, the authors propose a metabolic cycle in which glutamine-derived ⍺KG is reductively carboxylated by IDH2 in the mitochondria to produce isocitrate and citrate, which are then exported to the cytosol to support NADPH production and ⍺KG regeneration by IDH1. The cytosolic NADPH produced through this cycle then influences insulin secretion by activating insulin granule exocytosis (Figure 1) (Ronnebaum et al., 2006Ronnebaum S.M. Ilkayeva O. Burgess S.C. Joseph J.W. Lu D. Stevens R.D. Becker T.C. Sherry A.D. Newgard C.B. Jensen M.V. A pyruvate cycling pathway involving cytosolic NADP-dependent isocitrate dehydrogenase regulates glucose-stimulated insulin secretion.J. Biol. Chem. 2006; 281: 30593-30602Abstract Full Text Full Text PDF PubMed Scopus (171) Google Scholar; Odegaard et al., 2010Odegaard M.L. Joseph J.W. Jensen M.V. Lu D. Ilkayeva O. Ronnebaum S.M. Becker T.C. Newgard C.B. The mitochondrial 2-oxoglutarate carrier is part of a metabolic pathway that mediates glucose- and glutamine-stimulated insulin secretion.J. Biol. Chem. 2010; 285: 16530-16537Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). Reductive TCA cycle flux may also be active in some cancers, where it has been proposed that hypoxia or other perturbations that cause mitochondrial dysfunction stimulate the reductive carboxylation of ⍺KG to support citrate synthesis (Metallo et al., 2011Metallo C.M. Gameiro P.A. Bell E.L. Mattaini K.R. Yang J. Hiller K. Jewell C.M. Johnson Z.R. Irvine D.J. Guarente L. et al.Reductive glutamine metabolism by IDH1 mediates lipogenesis under hypoxia.Nature. 2011; 481: 380-384Crossref PubMed Scopus (1087) Google Scholar; Mullen et al., 2011Mullen A.R. Wheaton W.W. Jin E.S. Chen P.H. Sullivan L.B. Cheng T. Yang Y. Linehan W.M. Chandel N.S. DeBerardinis R.J. Reductive carboxylation supports growth in tumour cells with defective mitochondria.Nature. 2011; 481: 385-388Crossref PubMed Scopus (807) Google Scholar; Wise et al., 2011Wise D.R. Ward P.S. Shay J.E. Cross J.R. Gruber J.J. Sachdeva U.M. Platt J.M. DeMatteo R.G. Simon M.C. Thompson C.B. Hypoxia promotes isocitrate dehydrogenase-dependent carboxylation of α-ketoglutarate to citrate to support cell growth and viability.Proc. Natl. Acad. Sci. USA. 2011; 108: 19611-19616Crossref PubMed Scopus (654) Google Scholar). However, there is some debate over whether reductive TCA cycle flux exceeds oxidative flux in these settings (Fan et al., 2013Fan J. Kamphorst J.J. Rabinowitz J.D. Shlomi T. Fatty acid labeling from glutamine in hypoxia can be explained by isotope exchange without net reductive isocitrate dehydrogenase (IDH) flux.J. Biol. Chem. 2013; 288: 31363-31369Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar). As a result, it is controversial whether reductive carboxylation is a relevant net flux in tumors or other tissues, although labeling of citrate from 13C-labeled glutamine in mouse lung tumors is consistent with some reductive ⍺KG metabolism in this context (Davidson et al., 2016Davidson S.M. Papagiannakopoulos T. Olenchock B.A. Heyman J.E. Keibler M.A. Luengo A. Bauer M.R. Jha A.K. O’Brien J.P. Pierce K.A. et al.Environment impacts the metabolic dependencies of Ras-driven non-small cell lung cancer.Cell Metab. 2016; 23: 517-528Abstract Full Text Full Text PDF PubMed Scopus (429) Google Scholar). In this study, Zhang et al. provide clear evidence that stimulation of reductive TCA flux is important for glucose- and glutamine-stimulated insulin secretion. It remains to be determined whether defects in reductive TCA cycling contribute to the impairment of β cell function in type 2 diabetes. Why might nutrient excess stimulate reductive ⍺KG metabolism in pancreatic β cells? IDH2-mediated reductive carboxylation of ⍺KG to isocitrate is coupled to the conversion of NADPH to NADP+. Interestingly, the alternative mitochondrial isocitrate dehydrogenase IDH3 does not catalyze the reductive carboxylation of ⍺KG under physiological conditions, but does mediate oxidative production of ⍺KG from isocitrate while converting NAD+ to NADH (Figure 1). Thus, the IDH3 reaction will be favored in oxidizing conditions where cells have a higher NAD+/NADH ratio, while reductive conditions with a lower NADP+/NADPH ratio will favor reductive carboxylation by IDH2. One possibility is that when nutrients such as glucose or glutamine are in excess, oxidation of those nutrients drives a lower NAD+/NADH ratio in pancreatic β cells that both increases the ATP/ADP ratio to trigger insulin secretion and disfavors the oxidative IDH3 reaction. This more reduced redox state in mitochondria would then drive reductive carboxylation via IDH2. Indeed, cellular ATP in excess of demand can impair NAD+ regeneration by mitochondrial respiration to induce a more reduced cell state (Luengo et al., 2021Luengo A. Li Z. Gui D.Y. Sullivan L.B. Zagorulya M. Do B.T. Ferreira R. Naamati A. Ali A. Lewis C.A. et al.Increased demand for NAD+ relative to ATP drives aerobic glycolysis.Mol. Cell. 2021; 81: 691-707.e6Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar), and the metabolic cycle described by Zhang et al. could then serve as a redox shuttle that transfers excess electrons from mitochondria to generate the NADPH in the cytosol that stimulates insulin granule exocytosis. Thus, the same metabolic state that raises ATP/ADP is predicted to also raise cytosolic NADPH and promote insulin secretion. Other nutrients beyond glucose and glutamine have also been shown to stimulate insulin secretion by pancreatic β cells, including alanine, branched-chain amino acids, and fatty acids. It will be interesting to determine whether their metabolism by β cells also promotes a more reduced mitochondrial state that favors increased ATP/ADP while also increasing reductive metabolic reactions that raise cytosolic NADPH as an explanation for how they modulate insulin secretion. Importantly, better understanding the mechanisms by which β cells integrate the circulating availability of these nutrients to regulate insulin secretion could lend new insights into the pathogenesis of metabolic diseases and provide strategies to better treat insulin resistance and diabetes. E.C.L. is supported by the Damon Runyon Cancer Research Foundation ( DRG-2299-17 ). M.G.V.H. acknowledges support from the Emerald Foundation , the Lustgarten Foundation , the MIT Center for Precision Cancer Medicine , the Ludwig Center at MIT , a Faculty Scholar grant from the Howard Hughes Medical Institute , and the NIH ( R35CA242379 , R01CA201276 , and P30CA14051 ). M.G.V.H. discloses that he is on the scientific advisory board for Agios Pharmaceuticals, Aeglea Biotherapeutics, iTeos Therapeutics, Faeth Therapeutics, and Auron Therapeutics. Reductive TCA cycle metabolism fuels glutamine- and glucose-stimulated insulin secretionZhang et al.Cell MetabolismDecember 14, 2020In BriefZhang et al. report that reductive “counter-clockwise” TCA cycle flux via isocitrate dehydrogenase-2 (IDH2) is critical for glucose- and glutamine-stimulated insulin secretion. This pathway serves as a source of citrate and isocitrate to drive cytosolic NADPH production by IDH1, resulting in activation of insulin granule exocytosis. Full-Text PDF