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Xylulose 5-Phosphate Mediates Glucose-Induced Lipogenesis by Xylulose 5-Phosphate-Activated Protein Phosphatase in Rat Liver

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Xylulose 5-Phosphate Mediates Glucose-Induced Lipogenesis by Xylulose 5-Phosphate-Activated Protein Phosphatase in Rat Liver Xylulose 5-phosphate mediates glucose-induced lipogenesis by xylulose 5-phosphate-activated protein phosphatase in rat liver Tsutomu Kabashima*, Takumi Kawaguchi*†, Brian E. Wadzinski‡, and Kosaku Uyeda*§ *Dallas Veterans Affairs Medical Center and Department of Biochemistry, University of Texas Southwestern Medical Center, 4500 South Lancaster Road, Dallas, TX 75216; and ‡Department of Pharmacology, Vanderbilt University Medical Center, Nashville, TN 37232 Communicated by Steven L. McKnight, University of Texas Southwestern Medical Center, Dallas, TX, February 11, 2003 (received for review December 19, 2002) Carbohydrate-responsive element binding protein (ChREBP) is a tran- While insulin and glucagon play crucial roles in homeostasis of scription factor that activates lipogenic genes in liver in response to carbohydrate and fatty acids, glucose also has been implicated as an excess carbohydrate in the diet. ChREBP is regulated in a reciprocal independent signal for activating the synthesis of glycolytic and manner by glucose and cAMP. cAMP-dependent protein kinase (pro- lipogenic enzymes. The mechanism by which transcription is acti- tein kinase A) phosphorylates two physiologically important sites in vated by high glucose feeding has been poorly understood (4–8). It ChREBP, Ser-196, which is located near nuclear localization signal is known that not glucose itself but its metabolites activate the gene sequence (NLS), and Thr-666, within the basic helix–loop–helix (bHLH) transcription. Among the glucose metabolites, glucose 6-phosphate site, resulting in inactivation of nuclear translocation of ChREBP and (9), a metabolite of pyruvate (10), and Xu5P (11) have been of the DNA-binding activity, respectively. We demonstrate here that suggested as a glucose signal that regulates a glucose-responsive crude cytosolic extracts from livers of rats fed a high carbohydrate transcription factor. Because a mixture of the various intermediates diet contained protein phosphatase (PPase) activity that dephospho- is formed in vivo, it has been difficult to identify the true signaling rylated a peptide containing Ser-196, whereas a PPase in the nuclear compound, especially when the target glucose-responsive transcrip- extract catalyzed dephosphorylation of Ser-568 and Thr-666. All these tion factor is not known. PPases are activated specifically by xylulose 5-phosphate (Xu5P), but More recently, we purified and identified a previously unchar- not by other sugar phosphates. Furthermore, addition of Xu5P ele- acterized hepatic transcription factor, designated carbohydrate- vated PPase activity to the level observed in extracts of fed liver cells. responsive element binding protein (ChREBP) (12). This factor ␦ These partially purified PPases were characterized as PP2A-AB Cby meets a number of requirements for carbohydrate-responsive ac- immunoblotting with specific antibodies. These results suggest that tivation of transcription of the L-type pyruvate kinase (LPK) as well (ia) Xu5P-dependent PPase is responsible for activation of transcrip- as the lipogenic enzyme genes such as acetyl-CoA carboxylase and tion of the L-type pyruvate kinase gene and lipogenic enzyme genes, fatty acid synthase. and (ii) Xu5P is the glucose signaling compound. Thus, we propose ChREBP is a member of the basic helix–loop–helix͞leucine that the same Xu5P-activated PPase controls both acute and long- ͞ ϭ zipper (bHLH ZIP) family of transcription factors with Mr term regulation of glucose metabolism and fat synthesis. 94,600 and contains a nuclear localization signal (NLS), proline-rich stretches (PRO), a bHLH͞ZIP, and a ZIP-like domain (Fig. 1). We volutionary pressures favor the ability to store nutrients as fat demonstrated that the NLS site is inhibited by phosphorylation of Eduring times of abundant food supply as a safeguard against Ser-196 by PKA and that ChREBP is localized in the cytosol of starvation during periods of food shortage in which energy intake hepatocytes. However, when an animal is fed a diet high in is not sufficient to meet demands. In mammals the predominant carbohydrate, ChREBP is activated by being imported into the storage form of energy is triglycerides synthesized from fatty acids nucleus (13), and inside the nucleus the DNA-binding activity of derived from the diet or synthesized from carbohydrates through ChREBP is activated by dephosphorylation of Thr-666, which is the intermediate acetyl-CoA. Glucose is the essential energy situated within the basic amino acids essential for DNA binding source. The pathways involved in the conversion of excess glucose (Fig. 1). Thus, excess glucose stimulates ChREBP activity by two to triglycerides include glycolysis and lipogenesis, which occur mechanisms: dephosphorylation and activation of the NLS in primarily in liver. Feeding a diet high in carbohydrate to rats causes cytosol and DNA binding in the nucleus. In addition, fatty acid rapid activation of a number of key enzymes in glycolysis and metabolism produces AMP, which activates AMP-activated pro- ͞ lipogenesis. These enzymes include fructose-6-P 2-kinase fructose- tein kinase (AMPK). AMPK inhibits the DNA-binding activity of 2,6-P2 bisphosphatase, pyruvate kinase, fatty acid synthase, and ChREBP by phosphorylation of Ser-568 of ChREBP (14). acetyl-CoA carboxylase (reviewed in refs. 1 and 2). Fructose The remaining questions are (i) how does glucose (or a glucose 2,6-bisphosphate (Fru-2,6-P2), the most potent activator of phos- metabolite) activate ChREBP, and (ii) what is the glucose signaling phofructokinase (PFK), increases rapidly in liver in response to high compound? Here we find that high glucose activates ChREBP by glucose. Fru-2,6-P2 is synthesized and degraded by a bifunctional dephosphorylation of ChREBP by PPases in the cytoplasm and the ͞ enzyme termed 6-phosphofructo-2-kinase Fru-2,6-P2 bisphos- nuclei. These PPases are specifically activated by a metabolite in the phatase, which is inhibited by phosphorylation catalyzed by cAMP- extracts of cytoplasm and the metabolite was identified as Xu5P, dependent protein kinase (PKA). Glucose activates the enzyme by dephosphorylation catalyzed by xylulose 5-phosphate (Xu5P)- activated protein phosphatase (PP2A) (3), resulting in increased Abbreviations: AMPK, AMP-activated protein kinase; bHLH, basic helix–loop–helix; ChRE, carbohydrate-responsive element; ChREBP, ChRE-binding protein; Fru-2,6-P , fructose 2,6- Fru-2,6-P2. Thus, the glucose signaling mechanism in the activation 2 bisphosphate; LPK, L-type pyruvate kinase; NLS, nuclear localization signal; PFK, phospho- of PFK and glycolysis is mediated by increased Xu5P, which acts as fructokinase; PKA, cAMP-dependent protein kinase; PP2A, xylulose 5-phosphate-activated a glucose signaling compound and recruits and activates a specific protein phosphatase; PPase, protein serine͞threonine phosphatase; Xu5P, xylulose 5-phos- protein serine͞threonine phosphatase (PPase). As a long-term phate. response, high-carbohydrate diet induces expression of many of the †Permanent address: Second Department of Medicine, Kurume University School of Med- genes encoding the glycolytic and lipogenic enzymes, thereby icine, Fukuoka 830-0011, Japan. BIOCHEMISTRY promoting conversion of excess carbohydrate to storage fat in liver. §To whom correspondence should be addressed. E-mail: [email protected]. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0730817100 PNAS ͉ April 29, 2003 ͉ vol. 100 ͉ no. 9 ͉ 5107–5112 Downloaded by guest on September 29, 2021 The reaction mixture was mixed thoroughly and centrifuged. An aliquot of the top layer was then added to 5 ml of scintillation fluid and the radioactivity was measured. One unit of protein phospha- tase is defined as that amount which catalyzes the release of 1.0 ␮mol of 32P per min at 30°C. Fig. 1. The domain structure of ChREBP. The locations of the NLS, proline-rich Construction of Plasmids. stretch, bHLH͞leucine zipper (bHLH͞ZIP), and ZIP-like domains are indicated. The Full-length rat ChREBP cDNA (ref. 14, locations of three phosphorylation sites of PKA are designated as P1, P2, and P3. GenBank accession no. AB074517) was ligated into the mammalian The AMP-activated protein kinase (AMPK) site is indicated as P4. expression vector pcDNA3 (Invitrogen) or the pEGFP-N3 vector encoding enhanced green fluorescent protein (GFP; CLON- TECH), as previously described (13). suggesting that Xu5P is the glucose signaling compound. The PPases were partially purified and characterized as PP2A, the Primary Hepatocyte Culture and Transfection. Primary hepatocytes Xu5P-activated PPase described previously (15). were prepared from male Sprague–Dawley rats (250–300 g) by using the collagenase perfusion method (13) and plated in collagen- Materials and Methods coated 35-mm tissue culture plates (Primaria Falcon͞Franklin ␥ 32 ͞ ϭ Materials. [ - P]ATP (3,000 Ci mmol; 1 Ci 37 GBq) was Lakes, NJ) at a density of 1.0 ϫ 106 cells per well in glucose-free purchased from Amersham Pharmacia Biosciences. The PKA DMEM (Life Technologies) supplemented with 10 nM dexameth- catalytic subunit was from New England Biolabs. AMPK was asone, 0.1 unit͞ml insulin, 100 units͞ml penicillin, 100 ␮g͞ml purchased from Upstate Biotechnology (Lake Placid, NY). Oka- streptomycin, 10% dialyzed FBS, and 5.5 mM glucose. After a 6-h daic acid and other inhibitors were from Calbiochem. DEAE- attachment period, hepatocytes were transfected by using synthetic cellulose (DE52) was purchased from Whatman, and Mono Q liposomes (Lipofectamine 2000 from Life Technologies) in Opti-
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