Cytosolic and Mitochondrial Malic Enzyme Isoforms Differentially Control Insulin Secretion

In islet β-cells and INS-1 cells both the high activity of malic enzyme and the correlation of insulin secretion rates with pyruvate carboxylase (PC) flux suggest that a pyruvate-malate cycle is functionally relevant to insulin secretion. Expression of the malic enzyme isoforms in INS-1 cells and rat islets was measured, and small interfering RNA was used to selectively reduce isoform mRNA expression in INS-1 cells to evaluate its impact on insulin secretion. The cytosolic NADP+-specific isoform (ME1) was the most abundant, with the mitochondrial isoforms NAD+-preferred (ME2) expressed at ∼50%, and the NADP+-specific (ME3) at ∼10% compared with ME1. Selective reduction (89 ± 2%) of cytosolic ME1 mRNA expression and enzyme activity significantly reduced glucose (15 mm:41 ± 6%, p < 0.01) and amino acid (4 mm glutamine ± 10 mm leucine: 39 ± 6%, p < 0.01)-stimulated insulin secretion. Selective small interfering RNA reduction (51 ± 6%) of mitochondrial ME2 mRNA expression did not impact glucose-induced insulin secretion, but decreased amino acid-stimulated insulin secretion by 25 ± 4% (p < 0.01). Modeling of the metabolism of U-13Cglucose by its isotopic distribution in glutamate indicates a second pool of pyruvate distinct from glycolytically derived pyruvate in INS-1 cells. ME1 knockdown decreased flux of both pools of pyruvate through PC. In contrast, ME2 knockdown affected only PC flux of the pyruvate derived from glutamate metabolism. These results suggest a physiological basis for two metabolically and functionally distinct pyruvate cycles. The cycling of pyruvate by ME1 generates cytosolic NADPH, whereas mitochondrial ME2 responds to elevated amino acids and serves to supply sufficient pyruvate for increased Krebs cycle flux when glucose is limiting. In islet β-cells and INS-1 cells both the high activity of malic enzyme and the correlation of insulin secretion rates with pyruvate carboxylase (PC) flux suggest that a pyruvate-malate cycle is functionally relevant to insulin secretion. Expression of the malic enzyme isoforms in INS-1 cells and rat islets was measured, and small interfering RNA was used to selectively reduce isoform mRNA expression in INS-1 cells to evaluate its impact on insulin secretion. The cytosolic NADP+-specific isoform (ME1) was the most abundant, with the mitochondrial isoforms NAD+-preferred (ME2) expressed at ∼50%, and the NADP+-specific (ME3) at ∼10% compared with ME1. Selective reduction (89 ± 2%) of cytosolic ME1 mRNA expression and enzyme activity significantly reduced glucose (15 mm:41 ± 6%, p < 0.01) and amino acid (4 mm glutamine ± 10 mm leucine: 39 ± 6%, p < 0.01)-stimulated insulin secretion. Selective small interfering RNA reduction (51 ± 6%) of mitochondrial ME2 mRNA expression did not impact glucose-induced insulin secretion, but decreased amino acid-stimulated insulin secretion by 25 ± 4% (p < 0.01). Modeling of the metabolism of U-13Cglucose by its isotopic distribution in glutamate indicates a second pool of pyruvate distinct from glycolytically derived pyruvate in INS-1 cells. ME1 knockdown decreased flux of both pools of pyruvate through PC. In contrast, ME2 knockdown affected only PC flux of the pyruvate derived from glutamate metabolism. These results suggest a physiological basis for two metabolically and functionally distinct pyruvate cycles. The cycling of pyruvate by ME1 generates cytosolic NADPH, whereas mitochondrial ME2 responds to elevated amino acids and serves to supply sufficient pyruvate for increased Krebs cycle flux when glucose is limiting. It is now generally accepted that in addition to the key role of the ratio of ATP to ADP on closure of the ATP-dependent K+-channel, other metabolically derived second messengers are needed to promote insulin secretion from pancreatic islet β-cells. Many of these second messengers are believed to result from the export of the Krebs cycle intermediates from the mitochondria to the cytosol (1MacDonald M.J. Fahien L.A. Brown L.J. Hasan N.M. Buss J.D. Kendrick M.A. Am. J. Physiol. Metab. 2005; 288: E1-E15Crossref PubMed Scopus (201) Google Scholar, 2Wollheim C.B. Maechler P. Diabetes. 2002; 51: S37-S42Crossref PubMed Google Scholar, 3Prentki M. Vischer S. Glennon M.C. Regazzi R. Deeney J.T. Corkey B.E. J. Biol. Chem. 1992; 267: 5802-5810Abstract Full Text PDF PubMed Google Scholar, 4Brun T. Roche E. Assimacopoulos-Jeannet F. Corkey B.E. Kim K.-H. Prentki M. Diabetes. 1996; 45: 190-198Crossref PubMed Scopus (135) Google Scholar, 5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar). By necessity the export of these second messengers from the mitochondria must be matched by an equivalent input of substrates (anaplerosis) to prevent the depletion of the Krebs cycle intermediates and detrimental effects on ATP generation. Approximately one-half of the glucose (or pyruvate) metabolized by the mitochondria of islet β-cells enters the Krebs cycle through pyruvate dehydrogenase (PDH), 2The abbreviations used are: PDH, pyruvate dehydrogenase; PC, pyruvate carboxylase; ME1, malic enzyme 1; ME2, malic enzyme 2; ME3, malic enzyme 3; siRNA, small interfering RNA; KRB, Krebs-Ringer bicarbonate buffer; LC, liquid chromatography; MS, mass spectrometry. with the other half entering through the anaplerotic pathway of pyruvate carboxylase (PC) (5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar, 6MacDonald M.J. Arch. Biochem. Biophys. 1993; 300: 201-205Crossref PubMed Scopus (43) Google Scholar, 7MacDonald M.J. Arch. Biochem. Biophys. 1993; 305: 205-214Crossref PubMed Scopus (79) Google Scholar, 8MacDonald M.J. Metabolism. 1993; 42: 1229-1231Abstract Full Text PDF PubMed Scopus (48) Google Scholar, 9Schuit F. De Vos A. Farfari S. Moens K. Pipeleers D. Brun T. Prentki M. J. Biol. Chem. 1997; 272: 18572-18579Abstract Full Text Full Text PDF PubMed Scopus (358) Google Scholar, 10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar). In rat clonal INS-1 cells, rates of glucose-stimulated insulin secretion were shown to directly correlate with the rates of PC flux (10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar, 11Lu D. Mulder H. Zhao P. Burgess S.C. Jensen M.V. Kamzolova S. Newgard C.B. Sherry A.D. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 2708-2713Crossref PubMed Scopus (220) Google Scholar). In addition to replenishing Krebs cycle intermediates by PC, the input of glutamate into the Krebs cycle by glutamate dehydrogenase and/or transaminase represents another significant and physiologically important anaplerotic pathway (10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar, 12Sener A. Malaisse W.J. Nature. 1980; 288: 187-189Crossref PubMed Scopus (283) Google Scholar, 13Li C. Najafi H. Daikhin Y. Nissim I.B. Collins H.W. Yudkoff M. Matschinsky F.M. Stanley C.A. J. Biol. Chem. 2003; 278: 2853-2858Abstract Full Text Full Text PDF PubMed Scopus (129) Google Scholar). We recently that whereas glutamate is increased in the of is the significant of PC flux from glutamine metabolism that with insulin secretion (10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar). metabolism of pyruvate is directly with insulin secretion. insulin secretion second messengers with in of insulin (1MacDonald M.J. Fahien L.A. Brown L.J. Hasan N.M. Buss J.D. Kendrick M.A. Am. J. Physiol. Metab. 2005; 288: E1-E15Crossref PubMed Scopus (201) Google Scholar, 2Wollheim C.B. Maechler P. Diabetes. 2002; 51: S37-S42Crossref PubMed Google Scholar, 3Prentki M. Vischer S. Glennon M.C. Regazzi R. Deeney J.T. Corkey B.E. J. Biol. Chem. 1992; 267: 5802-5810Abstract Full Text PDF PubMed Google Scholar, 4Brun T. Roche E. Assimacopoulos-Jeannet F. Corkey B.E. Kim K.-H. Prentki M. Diabetes. 1996; 45: 190-198Crossref PubMed Scopus (135) Google Scholar, 5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar). a role for PC flux be The of insulin secretion and glucose from the is on the directly in flux F.M. Y. P. P. D. M.A. Y. K. M.A. J. 1993; PubMed Scopus Google Scholar, F.M. M.A. Diabetes. PubMed Scopus Google Scholar). the to the glucose the increased flux to at the input F. De Vos A. Farfari S. Moens K. Pipeleers D. Brun T. Prentki M. J. Biol. Chem. 1997; 272: 18572-18579Abstract Full Text Full Text PDF PubMed Scopus (358) Google Scholar, H. H. C.B. J. PubMed Scopus Google Scholar). cycling through PC an to the increased and the to be metabolically to in glucose represents of the Krebs cycle intermediates from the mitochondria to the malic enzyme pyruvate from for cycling to the mitochondria (5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar). in to the not and pyruvate cycling only the result of malic enzyme activity M.J. Diabetes. PubMed Scopus Google Scholar, M.J. Metab. 1992; PubMed Scopus Google Scholar). The high activity of both PC and cytosolic malic enzyme a of pyruvate and cycle to be a of glucose-stimulated insulin secretion (5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar, 11Lu D. Mulder H. Zhao P. Burgess S.C. Jensen M.V. Kamzolova S. Newgard C.B. Sherry A.D. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 2708-2713Crossref PubMed Scopus (220) Google Scholar, J. Biol. Chem. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). pyruvate cycle by PC and cytosolic malic enzyme at the of but to the of from mitochondrial to cytosolic The in increased of with increased pyruvate cycling increased mitochondrial activity with in the cytosol to insulin secretion H. H. C.B. J. PubMed Scopus Google Scholar, Biochem. J. PubMed Scopus Google Scholar, R. R. S. K. P. E. Diabetes. 2005; PubMed Scopus Google Scholar). the for an pyruvate cycle the mitochondrial a mitochondrial isoform of malic In cells, isoforms of malic enzyme are In addition to the cytosolic malic enzyme two mitochondrial isoforms malic enzyme (ME2) with a for and an malic enzyme is the for the mitochondrial ME2 and ME3, to a role in insulin secretion mitochondrial malic enzyme activity not an for of other substrates from the mitochondria to the and L.A. J. Biol. Chem. Full Text PDF PubMed Google the of the mitochondrial and that the high of ME2 for and the of to pyruvate of pyruvate but an of pyruvate from glutamine when flux is L.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar). In and cells, mitochondrial malic enzyme glutamine a by the of pyruvate and J. Biol. Chem. Full Text PDF PubMed Google Scholar, U. 1997; PubMed Scopus Google Scholar). The of glutamine and is to glucose for insulin in A. Malaisse W.J. Nature. 1980; 288: 187-189Crossref PubMed Scopus (283) Google Scholar, 13Li C. Najafi H. Daikhin Y. Nissim I.B. Collins H.W. Yudkoff M. Matschinsky F.M. Stanley C.A. J. Biol. Chem. 2003; 278: 2853-2858Abstract Full Text Full Text PDF PubMed Scopus (129) Google and mitochondrial malic enzyme be by the to amino acid metabolism with insulin secretion. into the of PC flux and pyruvate cycling in insulin secretion, the expression of the cytosolic and mitochondrial isoforms of malic enzyme in INS-1 cells and rat pyruvate cycling a key of insulin secretion from of of the from small of D. Mulder H. Zhao P. Burgess S.C. Jensen M.V. Kamzolova S. Newgard C.B. Sherry A.D. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 2708-2713Crossref PubMed Scopus (220) Google Scholar, M.J. Arch. Biochem. Biophys. PubMed Scopus (135) Google Scholar). these not the of PC flux from pyruvate cytosolic from mitochondrial pyruvate In the results of reduction of cytosolic ME1 and mitochondrial ME2 and pyruvate cycling in INS-1 cells. These of the impact on both insulin secretion and rates of anaplerotic flux from the reduction in pyruvate cycling cytosol and mitochondria by ME1, distinct from reduced pyruvate cycling to the mitochondria by of clonal INS-1 cells were by Newgard Mulder H. R. Prentki M. Newgard C.B. Diabetes. PubMed Scopus Google Scholar). were to be and in with mm with and 10 10 mm mm and were from were into at a of cells and at in the of for to for The INS-1 cells were for to for mRNA and insulin secretion INS-1 cells from pancreatic β-cells in of glucose metabolism H. H. C.B. J. PubMed Scopus Google and shown that of the metabolism with insulin secretion are to islet β-cells. for mitochondrial and malic enzyme activity are in INS-1 cells and rat pancreatic islets M. Vischer S. Glennon M.C. Regazzi R. Deeney J.T. Corkey B.E. J. Biol. Chem. 1992; 267: 5802-5810Abstract Full Text PDF PubMed Google Scholar, 4Brun T. Roche E. Assimacopoulos-Jeannet F. Corkey B.E. Kim K.-H. Prentki M. Diabetes. 1996; 45: 190-198Crossref PubMed Scopus (135) Google Scholar, 10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar, 11Lu D. Mulder H. Zhao P. Burgess S.C. Jensen M.V. Kamzolova S. Newgard C.B. Sherry A.D. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 2708-2713Crossref PubMed Scopus (220) Google Scholar). of to the malic enzyme for of the rat malic enzyme isoforms were the and The used to the and are rat ME1, rat ME2, and rat ME3, for two of isoform were for of mRNA was used to that significant to other The of the and its to a at were by with acid with of the and for malic enzyme and and were and the results were with for of were the the INS-1 cells in a was with of with was with the of of siRNA, of and of and at for The were in secretion in to glucose amino acid was on glucose-stimulated insulin secretion, mRNA expression of ME1, ME2, the effects of ME1 and ME2 knockdown were not to effects of was from and was from to with at was to be not of were the with from that for RNA and were not used for the RNA was to on a and and were for malic enzyme isoform and to for and used ME1, and ME2, and ME3, and and was the by were to be for ME1, ME2, and INS-1 from the malic enzyme isoform were for malic enzyme malic enzyme was at in a The was the mm mm mm mm mm and 10 mm were in with for and were to of the and the for activity was by the activity of the for from the with The of were and to were by a of A. PubMed Scopus Google Scholar). INS-1 cells were to on 10 in The cells were on with and by the in mitochondria mm mm mm mm and mm The of the was at were at to the cells. The was and the was in of mitochondria The was to a and was for at to and in The from both were for at to the The was by in mitochondria and a second The of mitochondria was in of mitochondria were for and the the of the was used for the The was on and mitochondria to mitochondrial was to be ± of was by dehydrogenase activity in mm mm The mitochondria were in mitochondrial with ADP in the of a the was with acid and to the The was with and the was and in for by of pyruvate isotopic of of the distribution of was with of from the was used to of the and and and pyruvate mass of the mitochondrial was a C.B. Maechler P. Diabetes. 2002; 51: S37-S42Crossref PubMed Google and a with to 10 was with mm and was with mm These for the of pyruvate from to was on the of and and and INS-1 cells were in Krebs-Ringer with mm glucose for and in of the mm (15 mm mm 10 mm mm 10 mm for an were and in the for in the were on with and with to of of anaplerotic was by the and INS-1 cells to the effects of malic enzyme knockdown on insulin secretion, that U-13Cglucose was for the of the the amino acids were and the of and were (10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar). The of and substrates into the Krebs cycle were from the distribution of and the (10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar, Sherry A.D. Am. J. Physiol. PubMed Google Scholar). were for insulin the and on a at were to a Expression of of of the malic enzyme cytosolic ME1, mitochondrial ME2, and mitochondrial ME3, were for both INS-1 cells and rat and islet mRNA expression for of the malic enzyme isoforms were to expression and compared with the for were mRNA expression from isoforms of malic enzyme were in the INS-1 cells was with the two ME1 and ME2 for of malic enzyme Expression of mitochondrial isoforms ME2 and are shown to cytosolic ME1 expression were in rat islets when compared with the INS-1 cells, ME1 and ME2 for of malic enzyme expression in both the mitochondrial isoform was a on the of cytosolic ME1 and mitochondrial ME2 in and amino acid-stimulated insulin secretion. were on the INS-1 in the of and in the to ME1 was in malic enzyme activity in cells with the compared with the cells (p in a reduction in ME1 activity in cells with ± p < in and ± p < in compared with the cells. NAD+-preferred to the activity of ME2 in mitochondria that are on the rates of in reduction the of malic enzyme and We used an isotopic to mitochondrial malic enzyme mitochondria were from INS-1 cells and in the of ADP and (4 these pyruvate only malic enzyme is the and metabolism of both the and the to the of the pyruvate was of the isotopic into mitochondrial used to the mass distribution of pyruvate The distribution of mass only be derived from the of The other and of pyruvate are derived cycling through the Krebs The mass distribution of the pyruvate that was to the distribution by the Sherry A.D. Am. J. Physiol. PubMed Google Scholar). These results the for the activity of mitochondrial malic enzyme in INS-1 mass ± ± ± and and and in a of and the on of the for mRNA expression and were two for malic enzyme isoform two distinct on the to pathway effects of RNA We significant and in the for mRNA expression effects on insulin secretion, the results for ME1 and ME2 ME1 decreased ME1 mRNA expression ± (p with on ME2 secretion was in at mm glucose and mm glucose with mm glutamine and 10 mm a (p in the cells was when compared with of ME1 in the INS-1 cells insulin secretion by ± (p In a ME1 knockdown reduced amino acid-stimulated insulin secretion by 39 ± (p acid was at glucose to in the INS-1 rat secretion by the of high glucose and amino acids was to a ± (p the result of ME1 ME2 role in insulin secretion, the expression of ME2 was decreased a to the ME2 mRNA expression of ME2 was reduced by ± 4% (p ME1 In to the reduction in insulin secretion with knockdown of the cytosolic isoform ME1, the knockdown in ME2 on insulin secretion by mm glucose (p that reduction in ME2 expression to a 25 ± 4% (p reduction in amino acid-stimulated insulin secretion shown in of on shown in insulin secretion in to was by knockdown of malic enzyme by the These that the decreased rates of insulin secretion from the cytosolic and mitochondrial malic enzyme knockdown are a result of reduced pyruvate of and the on impact on the were of the cells with mm with and glutamine and and and of the of glutamate the shown in and the that are two distinct pools of pyruvate entering into the Krebs derived from and the other from an knockdown of ME1 ME2 on the anaplerotic of PC glutamate at mm ME1 knockdown decreased PC flux by with equivalent in flux of the and but significant in PC flux was when the INS-1 cells were with glutamine and In contrast, reduced activity of ME2 flux of the pyruvate from the pyruvate pool when by by glutamine and but significant on PC flux of the pyruvate The reduction in the pyruvate pool with decreased expression of the mitochondrial isoform of malic enzyme a physiological basis for two pyruvate a cytosolic pool and a mitochondrial are these results are of of glutamine through Krebs cycle to malic of cytosolic and mitochondrial malic enzyme in INS-1 cells. In INS-1 cells, two pyruvate insulin secretion. ME1 a cytosolic pyruvate cycle that is for the of insulin secretion by of glucose ME2 anaplerotic metabolism of glutamate with insulin secretion by sufficient pyruvate for ATP and for export of for ME1 of of in and secretion rates are with in mitochondrial metabolism (1MacDonald M.J. Fahien L.A. Brown L.J. Hasan N.M. Buss J.D. Kendrick M.A. Am. J. Physiol. Metab. 2005; 288: E1-E15Crossref PubMed Scopus (201) Google Scholar, 2Wollheim C.B. Maechler P. Diabetes. 2002; 51: S37-S42Crossref PubMed Google Scholar, F.M. Y. P. P. D. M.A. Y. K. M.A. J. 1993; PubMed Scopus Google Scholar, F.M. M.A. Diabetes. PubMed Scopus Google Scholar). In addition to the for increased mitochondrial ATP is that other of mitochondrial metabolism are for insulin secretion. The of of these second messengers is the result of the pyruvate carboxylase flux to an in the of Krebs cycle intermediates and the export of and into the cytosol (1MacDonald M.J. Fahien L.A. Brown L.J. Hasan N.M. Buss J.D. Kendrick M.A. Am. J. Physiol. Metab. 2005; 288: E1-E15Crossref PubMed Scopus (201) Google Scholar, 2Wollheim C.B. Maechler P. Diabetes. 2002; 51: S37-S42Crossref PubMed Google Scholar, 3Prentki M. Vischer S. Glennon M.C. Regazzi R. Deeney J.T. Corkey B.E. J. Biol. Chem. 1992; 267: 5802-5810Abstract Full Text PDF PubMed Google Scholar, 4Brun T. Roche E. Assimacopoulos-Jeannet F. Corkey B.E. Kim K.-H. Prentki M. Diabetes. 1996; 45: 190-198Crossref PubMed Scopus (135) Google Scholar, 5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar). In mitochondria from pancreatic islets export at rates with the of pyruvate by PC, is with the of pyruvate by (5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar). The of these to the of insulin is and is an of (1MacDonald M.J. Fahien L.A. Brown L.J. Hasan N.M. Buss J.D. Kendrick M.A. Am. J. Physiol. Metab. 2005; 288: E1-E15Crossref PubMed Scopus (201) Google Scholar, 2Wollheim C.B. Maechler P. Diabetes. 2002; 51: S37-S42Crossref PubMed Google Scholar, 3Prentki M. Vischer S. Glennon M.C. Regazzi R. Deeney J.T. Corkey B.E. J. Biol. Chem. 1992; 267: 5802-5810Abstract Full Text PDF PubMed Google Scholar, 4Brun T. Roche E. Assimacopoulos-Jeannet F. Corkey B.E. Kim K.-H. Prentki M. Diabetes. 1996; 45: 190-198Crossref PubMed Scopus (135) Google Scholar, 5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar, 9Schuit F. De Vos A. Farfari S. Moens K. Pipeleers D. Brun T. Prentki M. J. Biol. Chem. 1997; 272: 18572-18579Abstract Full Text Full Text PDF PubMed Scopus (358) Google Scholar). of is the to pyruvate by cytosolic malic enzyme (5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar, M.J. Diabetes. PubMed Scopus Google Scholar). The high activity of pyruvate carboxylase and malic enzyme in the to the a key role in the of the increased in to increased glucose and to a in insulin secretion (1MacDonald M.J. Fahien L.A. Brown L.J. Hasan N.M. Buss J.D. Kendrick M.A. Am. J. Physiol. Metab. 2005; 288: E1-E15Crossref PubMed Scopus (201) Google Scholar, 5MacDonald M.J. J. Biol. Chem. 1995; 270: 20051-20058Abstract Full Text Full Text PDF PubMed Google Scholar, 11Lu D. Mulder H. Zhao P. Burgess S.C. Jensen M.V. Kamzolova S. Newgard C.B. Sherry A.D. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 2708-2713Crossref PubMed Scopus (220) Google Scholar). In the rates of insulin secretion in compared with was with in the of pyruvate carboxylase and malic enzyme in the islets J. Biol. Chem. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). in contrast, malic enzyme and the of cycle was to the of insulin secretion from islets compared with rat islets M.J. Am. J. Physiol. 2002; PubMed Scopus Google Scholar). in clonal rat INS-1 the of glucose-stimulated insulin secretion to in pyruvate cycling from D. Mulder H. Zhao P. Burgess S.C. Jensen M.V. Kamzolova S. Newgard C.B. Sherry A.D. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 2708-2713Crossref PubMed Scopus (220) Google Scholar). for the role of malic enzyme in insulin secretion is to small that the export from the mitochondria to the cytosol M.J. Arch. Biochem. Biophys. PubMed Scopus (135) Google Scholar). an to small on malic enzyme isoforms are to be expressed in these isoforms in and and cytosolic and two that in the of second messengers of insulin secretion. In to only cytosolic malic enzyme be in rat significant mRNA expression of the cytosolic and the mitochondrial malic enzyme isoforms in both a rat and in rat The of in that the cytosolic isoform is for both glucose and amino acid-stimulated insulin secretion and the that a second to insulin secretion. glucose flux is sufficient to and PC flux at rates sufficient to ATP and rates to insulin secretion. shown insulin secretion by the addition of glutamine and mm the with rates of increased PC flux (10Cline G.W. LePine R. Papas K.K. Shulman G.I. J. Biol. Chem. 2004; 279: 44370-44375Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar). the of pyruvate is the of pyruvate that is and that is to other knockdown of ME1 only the of from pyruvate The and export of other mitochondrial second the of Krebs cycle and mitochondrial ATP rates is to insulin be from these The of to directly insulin secretion rates by its to the of the by M. S. F.M. J. Physiol. 2003; Scopus Google Scholar). the a of the high of pyruvate cycling that a of is needed for the of insulin the second of insulin R. R. S. K. P. E. Diabetes. 2005; PubMed Scopus Google shown that increased activity of the to increased by of β-cells R. R. S. K. P. E. Diabetes. 2005; PubMed Scopus Google Scholar). from to be used to reduce in the of The of by these be to in rates and of insulin secretion The of in to the role of cytosolic malic enzyme to NADPH, is that the mitochondrial isoform a to flux through PC and in to increased amino acid and of in the rat the of substrates on ME2 activity and high activity of the enzyme with the addition of substrates glutamine and other L.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar). on these was that ME2 flux through the Krebs cycle by pyruvate when glucose is not the L.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar). glucose the flux of pyruvate into the mitochondria be sufficient only to flux for ATP and pyruvate cycling needed to a of glutamine and are the increased flux through mitochondrial malic enzyme and PC sufficient mitochondrial pyruvate to both ATP rates and the of cytosolic from pyruvate The increased rates of ATP and to increased rates of insulin secretion. is the to a role for the mitochondrial isoform of malic enzyme in amino acid-stimulated insulin secretion. of the correlation and insulin secretion a suggest the of two metabolically distinct pools of It was that pool was derived from and was used to whereas a second pool for pyruvate cycling D. Mulder H. Zhao P. Burgess S.C. Jensen M.V. Kamzolova S. Newgard C.B. Sherry A.D. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 2708-2713Crossref PubMed Scopus (220) Google Scholar). The results a physiological basis for of the of Krebs cycle intermediates from entering at the of shown to Sherry A.D. PubMed Scopus Google and a to into mitochondrial pyruvate of glycolytically derived We that the of glutamine to mitochondrial malic enzyme a pool of mitochondrial pyruvate that is distinct from pyruvate from cycling through cytosolic malic and that for two pools of In the of malic enzyme to a cytosolic pyruvate cycle and a mitochondrial pyruvate both functionally important for the of to insulin secretion The cytosolic pyruvate cycle by ME1 is for the of insulin secretion by insulin secretion mitochondrial malic enzyme to sufficient pyruvate for increased mitochondrial ATP and the export of to the cytosol to In in and to functionally distinct for the cytosolic and mitochondrial isoforms of malic enzyme and for metabolically pools of pyruvate in insulin cells. We and for with and