sign in sign up
<<
Home , Lipogenesis

A humble hexose monophosphate pathway metabolite regulates short- and long-term control of lipogenesis

Richard L. Veech* Laboratory of and Molecular Biology, Department of Health and Human Services, National Institute of Alcohol Abuse and Alcoholism, National Institutes of Health, 12501 Washington Avenue, Rockville, MD 20852

ost modern biologists and Feeding of a high- diet nuclear entry of ChREBP, whereas contemporary textbooks of has long been known to stimulate the phosphorylation of Thr-666 inhibited its biochemistry present the synthesis of fatty acids and to cause the binding to the LPK promoter site. This glycolytic pathway and the coordinate induction not only of the en- inhibition by ChREBP of nuclear entry M zymes of the synthesis path- and DNA binding gave a further mecha- synthesis of fats as history where all of the relevant facts are known. An article way but also of glycolytic re- nism to the well known observation that in a recent issue of PNAS by Kosaku quired for the supply of pyruvate, the , a stimulator of cAMP produc- Uyeda and his group (1) combines new precursor of acetyl-CoA, and the en- tion, inhibits . insights into the regulation of lipogene- zymes required for the synthesis of Just as feeding a high- diet sis with heroic chemistry and an NADPH, the essential for all inhibits cholesterol synthesis, Uyeda’s elegant combination of enzymology and biosynthesis. Further studies of the group next analyzed the mechanisms molecular biology. This paper describes whereby feeding of fat inhibits carbohy- how a small and ignored metabolite of drate metabolism (6). Activation of fatty acids produces AMP in a reaction of the the hexose monophosphate pathway, ϩ ϩ 3 xylulose 5-phosphate (Xu-5-P), activates Biochemistry type fatty acid ATP CoA acyl- CoA ϩ AMP ϩ PP . Using protein phosphatase 2A to mediate the i textbooks present transfected with ChREBP, they showed acute effects of carbohydrate feeding on that administration of the fatty acids the glycolytic pathway, as well as the and fat acetate, octanoate, or palmitate resulted coordinate long-term control of the en- in a 30-fold increase in the concentra- zymes required for fatty acid and triglyc- synthesis as history, tion of free cytosolic AMP and a 2-fold eride synthesis. increase in the activity of the AMP- It has long been known that feeding with all of the stimulated protein kinase. AMP kinase of cholesterol suppresses cholesterol syn- phosphorylated ChREBP at a specific thesis. In the early 1990s a transcription facts known. site on the molecule and inhibited its factor designated as SREBP, a sterol binding to DNA. response element binding protein, was The unification of the long-term con- identified that regulated the transcrip- signaling involved in lipogenesis in iso- trol of fat synthesis by the transcription tion of the genes encoding a number of lated hepatocytes showed that two dis- factor ChREBP and the short-term con- the key enzymes of cholesterol biosyn- tinct transcription factors were involved. trol on glycolysis and NADPH genera- thesis, including hydroxymethylglutaryl One, SREBP-1c, was stimulated by insu- tion at the phosphofructokinase (PFK; (HMG)-CoA synthase (EC 4.1.3.5), lin. However, another DNA-binding site EC 2.7.1.11) was explained by changes HMG-CoA reductase (EC 1.1.1.88), promoting the transcription of lipogenic in the concentration of the hexose farnesyl-diphosphate synthase (EC enzymes after stimulation by high glu- monophosphate pathway metabolite Xu- 2.5.1.10), and the low-density cose in the absence of was iden- 5-P (1). During carbohydrate feeding, receptor protein (for review, see ref. 2). tified and designated the carbohydrate- pyruvate produced in glycolysis enters SREBP also regulates the transcription response element, ChoRE (3). In a the mitochondria, where pyruvate dehy- of the gene encoding (EC heroic feat of protein isolation, Uyeda drogenase multienzyme complex con- 2.7.1.12), an responsible for ca- and his group (4) managed to purify 100 verts it to acetyl-CoA. The acetyl-CoA talysis of the first step of hepatic glycol- ␮g of ChREBP, the protein that bound produced is condensed with oxaloac- ysis. The processing of SREBP is regu- to the ChoRE of the promoter region of etate by citrate synthase (EC 4.1.3.7) to lated by proteolysis by mechanisms the gene encoding pyruvate kinase form citrate, the first step in the tricar- analogous to those involved in the pro- (LPK; EC 2.7.1.40) from the of boxylic acid (TCA) cycle. Both fatty ac- cessing of amyloid precursor protein. 800 rats that had been fasted and then ids and cholesterol are synthesized from Cleavage of SREBP requires an activat- refed a high-carbohydrate diet. The pro- citrate produced in the mitochondrial ing protein designated SCAP and is re- tein isolated was a 100-kDa polypeptide TCA cycle and exported to the cytosol sponsible for the feedback inhibition of of the basic helix–loop–helix leucine zip- on the tricarboxylate carrier. There ci- cholesterol on sterol synthesis. It was per (bHLH-ZIP) family of transcription trate is converted to cytosolic acetyl- CoA by citrate-cleavage enzyme (EC also found that, in addition to affecting factors which bind to E-box motifs, Ϫ 4.1.3.8). HCO is added to acetyl-CoA sterol synthesis, SREBP also modulates CACGTG, within their target promot- 3 by the biotin enzyme acetyl-CoA car- the expression of some of the genes en- ers. DNA-binding activity assays, so boxylase (EC 6.4.1.2) to form malonyl- coding enzymes necessary for fatty acid called gel shifts, disclosed its presence in CoA, the first committed step of fatty biosynthesis. These findings led to the kidney and small intestine in addition to acid synthesis. The synthesis of fatty ac- conclusion that SREBP coordinates the liver. In another paper (5) Uyeda’s synthesis of the two major building group reported that ChREBP contained blocks of membranes, fatty acids and three consensus sequences for protein See companion article on page 5107 in issue 9 of volume cholesterol. Soon, however, more data kinase A phosphorylation sites and that 100. were to emerge. phosphorylation of Ser-196 inhibited *E-mail: [email protected].

5578–5580 ͉ PNAS ͉ May 13, 2003 ͉ vol. 100 ͉ no. 10 www.pnas.org͞cgi͞doi͞10.1073͞pnas.1132039100 Downloaded by guest on September 24, 2021 COMMENTARY

ids requires both malonyl-CoA, pro- duced largely from pyruvate, and reduc- ing power. That reducing power is supplied from the reactions of the malic enzyme (EC 1.1.1.39) and the first two enzymes of the hexose monophosphate pathway, -6-phosphate dehydro- genase (EC 1.1.1.49) and 6-phosphoglu- conate dehydrogenase (EC 1.1.1.43). The malic enzyme and 6-phosphoglu- conate dehydrogenase reactions are both in near equilibrium with the cyto- solic free [NADPϩ]͞[NADPH] system and as such are sensitive to changes in the concentrations of their products and reactants. The reactions catalyzed by these dehydrogenases create the low- potential cytosolic free NADPϩ system, whose redox potential is Ϫ0.41 V. In comparison, the free cytosolic NADϩ system, formed by the glycolytic dehy- drogenases, -3-phosphate dehy- drogenase (EC 1.2.1.12), whose redox potential is around Ϫ0.19 V, is unable to drive the reduction of the ␤-oxoacyl- CoA to ␤-hydroxyacyl-CoA, whose half- reduction potential is Ϫ0.24 V, during fatty acid synthesis (7). The concentra- tion of malonyl-CoA and the Vmax of complex (EC 2.3.1.85) are major rate controlling steps in the process of fatty acid synthesis (8). Malonyl-CoA molecules are repeatedly added to the lengthening fatty acid chain until the complete fatty acid is released from the surface of the mul- tienzyme complex. How the disparate pathways of glycol- ysis, fatty acid synthesis, and gene tran- scription are integrated has been de- scribed in several recent papers by the Uyeda group. The enzyme PFK sits astride the intersection of the glycolytic pathway and the termination of the hex- Fig. 1. Xu-5-P is the signal for the coordinated control of lipogenesis. Feeding carbohydrate causes levels ose monophosphate pathway at the me- of liver glucose, Glc-6-P, and Fru-6-P to rise. Elevation of [Fru-6-P] leads to elevation of [Xu-5-P] in reactions tabolites fructose 6-phosphate and glyc- catalyzed by the near-equilibrium isomerases of the nonoxidative portion of the hexose monophosphate eraldehyde 3-phosphate (see Fig. 1). pathway. The elevation of [Xu-5-P] is the coordinating signal that both acutely activates PFK in glycolysis PFK activity is controlled in liver by the and promotes the action of the ChREBP to increase transcription of the genes for the concentration of Fru-2,6-P , which is enzymes of lipogenesis, the hexose monophosphate shunt, and glycolysis, all of which are required for the 2 of fat. The figure depicts the increase in enzyme transcription caused by the carbohy- produced and destroyed by a bifunc- drate response element binding protein, ChREBP, in green dashed lines. Stimulation of the Fru-2,6-kinase tional enzyme called 6-phosphofructo-2- reaction by protein phosphatase 2A (PP2A) and its stimulation by Xu-5-P are by indicated green dotted ͞ kinase Fru-2,6-P2 phosphatase (EC lines. Metabolic reactions are indicated by solid black lines. Those reactions that are reversible in vivo are 2.7.1.105͞3.1.3.46). The kinase activity is indicated with double arrows, and those catalyzing unidirectional reactions have only a single arrowhead. inhibited, and the phosphatase is acti- [ATP]͞[ADP][Pi] represents the free cytosolic phosphorylation potential catalyzed by the combined vated (9), by phosphorylation by cAMP- glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and 3-phosphoglycerate kinase reactions, and dependent protein kinase. Approxi- [AMP] represents the free cytosolic value catalyzed by the myokinase reaction. The names and EC numbers mately 10 years later it was shown that of the enzymes in green are given in the text. Inhibitions by AMP-stimulated protein kinase and dephosphorylation of the bifunctional cAMP-stimulated protein kinase are indicted by red dotted lines. enzyme, and hence activation of 6-phos- phofructo-2-kinase, was stimulated, and (11). The Ka for the activation of the that the transport of ChREBP into the the Fru-2,6-P2 phosphatase was inhib- ited, in response to high-carbohydrate PP2A by Xu-5-P was shown both in cel- nucleus and DNA binding are inhibited feeding by a specific protein phospha- lular preparations and in the isolated by phosphorylation by cAMP-dependent Ϸ ␮ tase that was activated by Xu-5-P (10). enzyme, to be 9 M (12), in the protein kinase, and that activation of The Xu-5-P-stimulated protein phospha- midrange for concentrations of Xu-5-P nuclear transport and binding is stimu- tase responsible for the activation of the in liver in vivo (13). lated in response to high-carbohydrate enzyme was identified as an isozyme of In their recent paper (1), the Uyeda feeding by a xylulose-stimulated PP2A the protein phosphatase 2A class, PP2A group completes the circle. They show of the AB␦C class. Transcription of the

Veech PNAS ͉ May 13, 2003 ͉ vol. 100 ͉ no. 10 ͉ 5579 Downloaded by guest on September 24, 2021 genes of the enzymes of glycolysis pro- ketolase (13). What this equation says is rate-controlling steps, which vary de- ducing malonyl-CoA, the enzymes of that, as [Fru-6-P] increases during car- pending upon hormonal and substrate NADPH production, and the enzymes bohydrate feeding, the concentration of concentration (16). The principle of dis- of the fatty acid synthesis pathway are Xu-5-P must also increase, with the re- tributive control is well illustrated by activated by PP2A in response to sultant effects of the stimulation of Uyeda’s findings on the enzymes in- changes in the cellular levels of Xu-5-P. PP2A cited above. Furthermore, the creased by the action of ChREBP. In Similar acute effects catalyzed by the product of the PFK reaction, Fru-1,6-P2, the glycolytic pathway there are in- same Xu-5-P stimulated PP2A by in- is in near equilibrium through aldolase creases in PFK, but also in pyruvate ki- creasing the activity of the 6-phospho- (EC 4.1.2.13) with the near-equilibrium nase. Were PFK the only ‘‘limiting’’ step fructo-2-kinase (EC 2.7.1.105). system catalyzed by glyceraldehyde-3-P in the pathway, such increases would be Why should an obscure metabolite dehydrogenase and 3-phosphoglycerate hard to explain. Coordinate control of such as Xu-5-P be such an important kinase (EC 2.7.2.3) (14). Hence changes transcription is also illustrated by the signal? The products of the four isomer- in [glyceraldehyde-3-P] must conform to ases of the nonoxidative portion of the finding that both glucose-6-phosphate changes in both the free cytoplasmic dehydrogenase and the malic enzyme, hexose monophosphate pathway, ribu- [NADϩ]͞[NADH] ratio and the free lose 5-phosphate (Ru5P) epimerase (EC ͞ producers of the NADPH needed for fat cytoplasmic [ATP] [ADP][Pi] ratio. Ad- synthesis, are increased by ChREBP. 5.1.3.1), ribose 5-phosphate (Rib5P) ditionally, the product of the 6-phospho- Finally, all of the enzymes required for isomerase (EC 5.3.1.6), transaldolase gluconate reaction, Ru5P, is in near fat synthesis, starting with ATP citrate (EC 2.2.1.2), and transketolase (EC equilibrium with 6-phosphogluconate ϩ lyase, are increased. The coordinate 2.2.1.1) produce Fru-6-P and glyceralde- and the free cytosolic [NADP ]͞ hyde-3-P, with their substrates and prod- [NADPH] ratio (15). Ru5P, a compo- changes in the disparate pathways re- ucts being in approximate near equilib- nent of the nonoxidative portion of the quired to achieve fat synthesis represent rium in liver in vivo (13). One may hexose monophosphate pathway, is thus an elegant example of distributive con- therefore write sensitive to changes in the concentra- trol. It is remarkable that such complex control mechanisms are all sensitive to ͓Xu-5-P͔ tions of the combined PFK and aldolase reactions in liver. The substrates of this the simple hexose monophosphate shunt ͑K ͒1/3͑K ͒1/3 system are thus responsive to changes in metabolite Xu-5-P. I know of no other ϭ Ru5P epimerase transketolase-E4P way that one could come to such unex- ͑ ͒1/3͑ ͒1/3 all of the great systems of the KRib5P isomerase Ktransaldolase ϩ ͞ 1/3 cell, the free [NAD ] [NADH], the pected, insightful conclusions other than ϫ ͑K ͒ ϩ transketolase-S7P [NADP ]͞[NADPH], and the [ATP]͞ through the application of protein chem- 1/3 2/3 ical, biochemical, and molecular biologi- ϫ ͓glyceraldehyde-3-P͔ ͓Fru-6-P͔ , [ADP][Pi] systems. It is widely stated in textbooks that cal techniques informed by a global in which E4P represent erythrose PFK is ‘‘the rate-controlling step’’ in knowledge of metabolic systems and 4-phosphate and S7P represents se- glycolysis. More systematic analysis of their enormous plasticity made possible doheptulose 7-phosphate, representing the control of fluxes in glycolysis sug- by distributed, context-dependent two of the reactions catalyzed by trans- gests that there are instead multiple control.

1. Kabashima, T., Kawaguchi, T., Wadzinski, B. E. & 6. Kawaguchi, T., Osatomi, K., Yamashita, H., 11. Nishimura, M. & Uyeda, K. (1995) J. Biol. Chem. Uyeda, K. (2003) Proc. Natl. Acad. Sci. USA 100, Kabashima, T. & Uyeda, K. (2002) J. Biol. Chem. 270, 26341–26346. 5107–5112. 277, 3829–3835. 12. Liu, Y. Q. & Uyeda, K. (1996) J. Biol. Chem. 271, 2. Brown, M. S. & Goldstein, J. L. (1997) Cell 89, 331–340. 7. Krebs, H. A. & Veech, R. L. (1969) in The 8824–8830. 3. Bergot, M. O., Diaz-Guerra, M. J., Puzenat, N., Energy Level and Metabolic Control in Mitochon- 13. Casazza, J. P. & Veech, R. L. (1986) J. Biol. Chem. Raymondjean, M. & Kahn, A. (1992) Nucleic dria, eds. Papa, S., Tager, J. M., Quagliariello, 261, 690–698. Acids Res. 20, 1871–1877. E. & Slater, E. C. (Adriatica, Bari, Italy), pp. 14. Veech, R. L., Lawson, J. W. R., Cornell, N. W. 4. Yamashita, H., Takenoshita, M., Sakurai, M., 329–382. & Krebs, H. A. (1979) J. Biol. Chem. 254, Bruick, R. K., Henzel, W. J., Shillinglaw, W., 8. Guynn, R. W., Veloso, D. & Veech, R. L. (1972) 6538–6547. Arnot, D. & Uyeda, K. (2001) Proc. Natl. Acad. J. Biol. Chem. 247, 7325–7331. 15. Veech, R. L., Eggleston, L. V. & Krebs, H. A. Sci. USA 98, 9116–9121. 9. Furuya, E., Yokoyama, M. & Uyeda, K. (1982) (1969) Biochem. J. 115, 609–619. 5. Kawaguchi, T., Takenoshita, M., Kabashima, T. & Proc. Natl. Acad. Sci. USA 79, 325–329. 16. Kashiwaya, Y., Sato, K., Tsuchiya, N., Thomas, S., Uyeda, K. (2001) Proc. Natl. Acad. Sci. USA 98, 10. Nishimura, M., Fedorov, S. & Uyeda, K. (1994) Fell, D. A., Veech, R. L. & Passonneau, J. V. 13710–13715. J. Biol. Chem. 269, 26100–26106. (1994) J. Biol. Chem. 269, 25502–25514.

5580 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.1132039100 Veech Downloaded by guest on September 24, 2021