Glyceroneogenesis and the Triglyceride/Fatty Acid Cycle

During fasting in all mammals, triglyceride stored in adipose tissue is hydrolyzed by a hormone-sensitive lipase to produce free fatty acids (FFA) 1The abbreviations used are: FFA, free fatty acid(s); WAT, white adipose tissue; BAT, brown adipose tissue; PPAR, peroxisome proliferator-activated receptor; PPARE, PPAR response element; PEPCK, phosphoenolpyruvate carboxykinase; C/EBP, CCAAT/enhancer-binding protein; CREB, cAMP-response element-binding protein.1The abbreviations used are: FFA, free fatty acid(s); WAT, white adipose tissue; BAT, brown adipose tissue; PPAR, peroxisome proliferator-activated receptor; PPARE, PPAR response element; PEPCK, phosphoenolpyruvate carboxykinase; C/EBP, CCAAT/enhancer-binding protein; CREB, cAMP-response element-binding protein. and glycerol. Detailed studies of the balance of glycerol and FFA released from white adipose tissue (WAT) during starvation have noted considerable re-esterification of the FFA in adipose tissue during periods of active lipolysis. For example, in rats fasted for 24 h, about 30% of the FFA is recycled back to triglyceride in WAT (1Vaughan M.J. J. Biol. Chem. 1962; 237: 3354-3358Abstract Full Text PDF PubMed Google Scholar). In humans, the recycling in this tissue has been estimated to be as high as 40% (2Jensen M.D. Ekberg K. Landau B.R. Am. J. Physiol. 2001; 281: E789-E793Crossref PubMed Google Scholar). The recycling of FFA also occurs in the liver as part of a triglyceride/fatty acid cycle that accounts for a considerable quantity of fatty acid recycling. Thus the triglyceride/fatty acid cycle includes local intracellular cycling within the adipose tissue and extracellular or systemic recycling, i.e. the formation of triglycerides in the liver and possibly skeletal muscle (Fig. 1). Almost 30 years ago, Newsholme and Crabtree (3Newsholme E.A. Crabtree B. Biochem. Soc. Symp. 1976; 41: 61-109PubMed Google Scholar) discussed the importance of this cycle in metabolic regulation and heat production. Quantitative estimates of the triglyceride/fatty acid cycle in human adults and newborn infants and studies in animals show that only a small fraction of the FFA released as a result of lipolysis in the WAT are oxidized, and the majority are re-esterified to triglycerides in various tissues (2Jensen M.D. Ekberg K. Landau B.R. Am. J. Physiol. 2001; 281: E789-E793Crossref PubMed Google Scholar, 3Newsholme E.A. Crabtree B. Biochem. Soc. Symp. 1976; 41: 61-109PubMed Google Scholar, 4Wolfe R.R. Herndon D.N. Jahoor F. Miyoshi H. Wolfe M. N. Engl. J. Med. 1987; 317: 403-408Crossref PubMed Scopus (351) Google Scholar, 5Klein S. Wolfe R.R. J. Clin. Invest. 1990; 86: 1403-1408Crossref PubMed Scopus (76) Google Scholar, 6Reidy S.P. Weber J.M. Am. J. Physiol. 2002; 282: E312-E317Crossref PubMed Scopus (51) Google Scholar, 7Bahr R. Hansson P. Sejersted O.M. Metabolism. 1990; 39: 993-999Abstract Full Text PDF PubMed Scopus (76) Google Scholar, 8Elia M. Zed C. Neale G. Livesey G. Metabolism. 1987; 36: 251-255Abstract Full Text PDF PubMed Scopus (86) Google Scholar, 9Patel D. Kalhan S. Pediatr. Res. 1992; 31: 52-58Crossref PubMed Scopus (62) Google Scholar). The quantitative estimates of triglyceride/fatty acid cycling vary in different studies in humans, depending upon the methodology employed. Intracellular recycling (primarily fatty acid re-esterification in WAT) appears to represent ∼20–30% of the total, whereas non-adipose tissue recycling (primarily hepatic) accounts for ∼50% of re-esterification of fatty acids in healthy adults after an overnight fast (Table I). It is important to note that the fraction of FFA released (lipolysis) that is recycled back to triglyceride remains relatively constant (∼75%), despite marked changes in the rate of total triglyceride/fatty acid cycling during different metabolic states (Table I). The metabolic significance of this fixed fractional rate of triglyceride/fatty acid recycling remains to be determined. However, it is clear that triglyceride/fatty acid recycling requires the constant generation of glycerol 3-phosphate for triglyceride synthesis, particularly in situations when cycling is increased.Table IMeasurement of the triglyceride/fatty acid cycle in vivoFFA RaaFFA Ra is the rate of appearance of FFA from adipose tissue. The amount of FFA recycled is calculated as described in detail in the specific references listed. Data from the following publications were used to construct this table: Ref. 8, fasting adults; Ref. 4, severe burn patients; Ref. 9, newborn humans; Ref. 6, effect of leptin on fatty acid recycling.RecycledAdipose tissueSystemicTotalμmol·kg-1·min-1%%%AdultsFast (12 h)7.28.340.348Burn24.641.123.264Newborn infantsHealthy28.576Malnourished36.678Effect of leptin (rabbits)Basal29.1285685Leptin44.4344984a FFA Ra is the rate of appearance of FFA from adipose tissue. The amount of FFA recycled is calculated as described in detail in the specific references listed. Data from the following publications were used to construct this table: Ref. 8Elia M. Zed C. Neale G. Livesey G. Metabolism. 1987; 36: 251-255Abstract Full Text PDF PubMed Scopus (86) Google Scholar, fasting adults; Ref. 4Wolfe R.R. Herndon D.N. Jahoor F. Miyoshi H. Wolfe M. N. Engl. J. Med. 1987; 317: 403-408Crossref PubMed Scopus (351) Google Scholar, severe burn patients; Ref. 9Patel D. Kalhan S. Pediatr. Res. 1992; 31: 52-58Crossref PubMed Scopus (62) Google Scholar, newborn humans; Ref. 6Reidy S.P. Weber J.M. Am. J. Physiol. 2002; 282: E312-E317Crossref PubMed Scopus (51) Google Scholar, effect of leptin on fatty acid recycling. Open table in a new tab Quantitative changes in the triglyceride/fatty acid cycle have been related to the increased thermogenesis after burns in humans (4Wolfe R.R. Herndon D.N. Jahoor F. Miyoshi H. Wolfe M. N. Engl. J. Med. 1987; 317: 403-408Crossref PubMed Scopus (351) Google Scholar), the increased metabolic rate of cachectic patients with esophageal cancer (5Klein S. Wolfe R.R. J. Clin. Invest. 1990; 86: 1403-1408Crossref PubMed Scopus (76) Google Scholar), and increased oxygen consumption following leptin administration (6Reidy S.P. Weber J.M. Am. J. Physiol. 2002; 282: E312-E317Crossref PubMed Scopus (51) Google Scholar),and to the amplification of substrate flux during acute exercise (7Bahr R. Hansson P. Sejersted O.M. Metabolism. 1990; 39: 993-999Abstract Full Text PDF PubMed Scopus (76) Google Scholar). In addition, triglyceride/fatty acid cycle flux is markedly increased following an 87-h fast in humans (8Elia M. Zed C. Neale G. Livesey G. Metabolism. 1987; 36: 251-255Abstract Full Text PDF PubMed Scopus (86) Google Scholar). Data from studies using healthy human newborn infants have also shown that 75% of the fatty acids released by lipolysis are recycled back to triglycerides (9Patel D. Kalhan S. Pediatr. Res. 1992; 31: 52-58Crossref PubMed Scopus (62) Google Scholar). In undernourished, intrauterine growth-retarded infants, higher rates of fatty acid oxidation were associated with an increased rate of lipolysis and fatty acid cycling, so that the magnitude of recycling (∼76%) was similar to that in normal infants.What is the metabolic source of glyceride-glycerol needed to support the triglyceride/fatty acid cycle? There are three major possibilities: glucose, via glycolysis, glycerol after phosphorylation by glycerol kinase, or the conversion of pyruvate to glyceride-glycerol via a pathway to be discussed, termed glyceroneogenesis.The Role of Glyceroneogenesis in WATMore than 30 years ago we (10Ballard F.J. Hanson R.W. Leveille G.A. J. Biol. Chem. 1967; 242: 2746-2750Abstract Full Text PDF PubMed Google Scholar, 11Reshef L. Hanson R.W. Ballard F.J. J. Biol. Chem. 1970; 245: 5979-5984Abstract Full Text PDF PubMed Google Scholar) and others (12Gorin E. Tal-Or Z. Shafrir E. Eur. J. Biochem. 1969; 8: 370-375Crossref PubMed Scopus (49) Google Scholar) described a pathway for the re-esterification of FFA during fasting in WAT that involved the generation of 3-glycerol phosphate from precursors other than glucose. This pathway, termed glyceroneogenesis, is an abbreviated version of gluconeogenesis. Glyceroneogenesis is defined as the conversion of precursors other than glycerol or glucose to 3-glycerol phosphate for the synthesis of glyceride-glycerol. The discovery of this pathway resulted from our finding that WAT contains both pyruvate carboxylase and the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK-C) (10Ballard F.J. Hanson R.W. Leveille G.A. J. Biol. Chem. 1967; 242: 2746-2750Abstract Full Text PDF PubMed Google Scholar), two enzymes thought at the time to be involved only in gluconeogenesis. In addition, glyceroneogenesis was predicted to be important in lipid metabolism in ruminants (13Reshef L. Niv J. Shapiro B. J. Lipid Res. 1967; 8: 688-691Abstract Full Text PDF PubMed Google Scholar), because these animals do not derive glucose from the digestion of dietary carbohydrate, because of the activity of rumen microflora. Rather, they rely on the synthesis of glucose by hepatic and renal gluconeogenesis; thus glucose is at a premium in all ruminants, especially during pregnancy and lactation (14Ballard F.J. Hanson R.W. Kronfeld D.S. Fed. Proc. 1969; 112: 195-202Google Scholar). On this basis, we sought an alternative pathway for the generation of the 3-glycerol phosphate that is required for triglyceride synthesis during fasting in ruminant animals. In fact, in all mammals glucose is a critical fuel for the metabolism of a number of tissues, such as the brain and red blood cells. Thus the importance of an alternative source of 3-glycerol phosphate during fasting, other than glucose, is evident.In the initial studies that established the existence of the pathway of glyceroneogenesis, we demonstrated that the addition of pyruvate to rat epididymal adipose tissue, incubated in vitro, reduced FFA release by ∼65%, while not altering lipolysis (as determined by the amount of glycerol released into the medium) because of increased FFA re-esterification (11Reshef L. Hanson R.W. Ballard F.J. J. Biol. Chem. 1970; 245: 5979-5984Abstract Full Text PDF PubMed Google Scholar, 15Reshef L. Ballard F.J. Hanson R.W. J. Biol. Chem. 1969; 244: 5577-5581Abstract Full Text PDF PubMed Google Scholar, 16Reshef L. Hanson R.W. Ballard F.J. J. Biol. Chem. 1969; 244: 1994-2001Abstract Full Text PDF PubMed Google Scholar). These findings suggested a potential physiological role for glyceroneogenesis in the re-esterification of FFA in WAT during fasting, thereby controlling both the release of FFA and subsequently ketogenesis (11Reshef L. Hanson R.W. Ballard F.J. J. Biol. Chem. 1970; 245: 5979-5984Abstract Full Text PDF PubMed Google Scholar). Despite these early studies, the pathway has remained largely ignored (17Guan H.P. Li Y. Jensen M.V. Newgard C.B. Steppan C.M. Lazar M.A. Nat Med. 2002; 8: 1122-1128Crossref PubMed Scopus (325) Google Scholar) or cited only sparingly in the literature over the past 35 years. A recent perusal of the archives of PubMed indicated that only 28 published papers referred to glyceroneogenesis in their titles; the subject has yet to be introduced into text books of general biochemistry.This situation has slowly changed over the past several years (18Beale E.G. Hammer R.E. Antoine B. Forest C. FASEB J. 2002; 16: 1695-1696Crossref PubMed Scopus (54) Google Scholar) due in part to the availability of genetically modified mice in which the gene for PEPCK-C has been either specifically deleted (19She P. Shiota M. Shelton K.D. Chalkley R. Postic C. Magnuson M.A. Mol. Cell. Biol. 2000; 20: 6508-6517Crossref PubMed Scopus (188) Google Scholar, 20Olswang Y. Cohen H. Papo O. Cassuto H. Croniger C.M. Hakimi P. Tilghman S.M. Hanson R.W. Reshef L. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 625-630Crossref PubMed Scopus (116) Google Scholar) or overexpressed in adipose tissue (21Franckhauser S. Munoz S. Pujol A. Casellas A. Riu E. Otaegui P. Su B. Bosch F. Diabetes. 2002; 51: 624-630Crossref PubMed Scopus (177) Google Scholar). Modifying the expression of the gene for PEPCK-C is critical because this enzyme, which catalyzes the synthesis of phosphoenolpyruvate from oxalacetate, is generally considered to be the pace-setting step in both gluconeogenesis and glyceroneogenesis. PEPCK-C is encoded by a single copy gene, which is expressed to the greatest extent in the liver and kidney cortex and both WAT and brown adipose tissue (BAT). Both the activity of PEPCK-C and the number of enzyme molecules is acutely regulated by alterations in the rate of transcription of the PEPCK-C gene (22Hanson R.W. Reshef L. Annu. Rev. Biochem. 1997; 66: 581-611Crossref PubMed Scopus (624) Google Scholar), in response to various dietary, hormonal, and environmental stimuli.The tissue-specific expression of the gene for PEPCK-C is due to regulatory elements in the gene promoter that bind tissue-specific transcription activators to direct its expression (22Hanson R.W. Reshef L. Annu. Rev. Biochem. 1997; 66: 581-611Crossref PubMed Scopus (624) Google Scholar). Because deletion of the gene for PEPCK-C is neonatal lethal (19She P. Shiota M. Shelton K.D. Chalkley R. Postic C. Magnuson M.A. Mol. Cell. Biol. 2000; 20: 6508-6517Crossref PubMed Scopus (188) Google Scholar), we have generated a targeted mutation in embryonic stem cells of the PPARγ2 binding site in the promoter of the PEPCK-C gene in mice to assess the role of this enzyme in adipose tissue in vivo (20Olswang Y. Cohen H. Papo O. Cassuto H. Croniger C.M. Hakimi P. Tilghman S.M. Hanson R.W. Reshef L. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 625-630Crossref PubMed Scopus (116) Google Scholar). The PPARγ2 binding site is required for the adipose tissue-specific expression of the gene (23Devine J.H. Eubank D.W. Clouthier D.E. Tontonoz P. Spiegelman B.M. Hammer R.E. Beale E.G. J. Biol. Chem. 1999; 274: 13604-13612Abstract Full Text Full Text PDF PubMed Scopus (77) Google Scholar). The mutation abolished PEPCK-C gene expression only in WAT and to a smaller extent (50%) in BAT of the homozygous descendent mice (PEPCK-PPARE–/– mice). The mutation virtually abolished glyceroneogenesis in WAT of PEPCK-PPARE–/– mice, as determined by their inability to synthesize glyceride-glycerol from pyruvate; this establishes PEPCK-C as a key enzyme in glyceroneogenesis. The PEPCK-PPARE–/– mice also lost considerable triglyceride from their adipose tissue, and about 30% of the animals became lipodystrophic as adults (20Olswang Y. Cohen H. Papo O. Cassuto H. Croniger C.M. Hakimi P. Tilghman S.M. Hanson R.W. Reshef L. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 625-630Crossref PubMed Scopus (116) Google Scholar). The lipodystrophy noted in the PEPCK-PPARE–/– mice differs considerably from that found in other models of this disorder (24Reue K. Peterfy M. Curr. Atheroscler. Rep. 2000; 2: 390-396Crossref PubMed Scopus (28) Google Scholar) because the animals do not have altered glucose metabolism, except for a very moderate hyperglycemia noted in older mice.Further support for the importance of glyceroneogenesis in adipose tissue was provided by the experiments of Franckhauser et al. (21Franckhauser S. Munoz S. Pujol A. Casellas A. Riu E. Otaegui P. Su B. Bosch F. Diabetes. 2002; 51: 624-630Crossref PubMed Scopus (177) Google Scholar) who overexpressed a chimeric transgene containing the PEPCK-C structural gene linked to the aP2 promoter in transgenic mice. The gene was expressed at high levels, specifically in WAT because of the specificity of the aP2 gene promoter. Adult transgenic mice had greatly enhanced rates of glyceroneogenesis and higher levels of triglyceride synthesis in their WAT; the animals were also markedly obese and did not exhibit signs of altered glucose metabolism. These findings, together with those of Olswang et al. (20Olswang Y. Cohen H. Papo O. Cassuto H. Croniger C.M. Hakimi P. Tilghman S.M. Hanson R.W. Reshef L. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 625-630Crossref PubMed Scopus (116) Google Scholar), provide strong support for the pathway of glyceroneogenesis in WAT and the key role that is played by PEPCK-C in controlling the turnover of triglyceride during fasting.More recently an analysis of 16,757 genes in Caenorhabditis elegans using the RNA-mediated interference technique indicated that disruption of the gene for PEPCK results in a reduced or disorganized pattern of fat deposition in the worm (25Ashrafi K. Chang F.Y. Watts J.L. Fraser A.G. Kamath R.S. Ahringer J. Ruvkun G. Nature. 2003; 421: 268-272Crossref PubMed Scopus (823) Google Scholar).The importance of glyceroneogenesis in controlling triglyceride turnover in WAT is supported by other lines of evidence. Glucose is the major precursor of 3-glycerol phosphate for triglyceride synthesis in this tissue in the fed state. During diabetes, there is both an elevated level of lipolysis and a greatly diminished rate of transport of glucose into the adipocyte, resulting in mobilization of triglyceride from adipose tissue. Adipocyte-specific deletion of the gene for the transporter that is required for glucose entry into the adipocyte (GLUT4) generated mice that were insulin-resistant. Despite this, these mice did not have a loss of triglycerides from WAT (26Abel E.D. Peroni O. Kim J.K. Kim Y.B. Boss O. Hadro E. Minnemann T. Shulman G.I. Kahn B.B. Nature. 2001; 409: 729-733Crossref PubMed Scopus (938) Google Scholar). Thus, there must be an alternative source of 3-glycerol phosphate for triglyceride synthesis in WAT in the GLUT4-deficient mice. It is likely that these mice will have an enhanced activity of glyceroneogenesis in their WAT in order to maintain triglyceride homeostasis.Finally, it is of interest that the gene for the mitochondrial dicarboxylate transporter is highly expressed in WAT and the level of its mRNA is induced by fatty acids and inhibited by insulin (27Das K. Lewis R.Y. Combatsiaris T.P. Lin Y. Shapiro L. Charron M.J. Scherer P.E. Biochem. J. 1999; 344: 313-320Crossref PubMed Scopus (25) Google Scholar). The dicarboxylate transporter is required for the movement of malate in exchange for other anions, such as α-ketoglutarate, from the mitochondria to the cytosol. This pathway would provide the major route for the generation of cytosolic oxalacetate, a substrate of PEPCK-C in glyceroneogenesis.Glyceroneogenesis in Brown Adipose TissueIt has been known for many years that BAT contains considerable activity of PEPCK-C (about 10 times that of WAT) (28Hahn P. Novak M. J. Lipid Res. 1975; 16: 79-91Abstract Full Text PDF PubMed Google Scholar) yet the physiological function of the enzyme in that tissue has not been formally established. The activity of PEPCK-C can be induced in BAT by corticosteroids (29Feldman D. Hirst M. Am. J. Physiol. 1978; 235: E197-E202PubMed Google Scholar), by the administration of norepinephrine (30Hahn P. Kirby L.T. Can. J. Biochem. 1974; 52: 739-743Crossref PubMed Scopus (12) Google Scholar) and thyroid hormone (31Hahn P. Hassanali S. Biol. Neonate. 1982; 41: 1-7Crossref PubMed Scopus (13) Google Scholar), by a diet high in protein and devoid of carbohydrate (30Hahn P. Kirby L.T. Can. J. Biochem. 1974; 52: 739-743Crossref PubMed Scopus (12) Google Scholar) and can be inhibited by insulin (31Hahn P. Hassanali S. Biol. Neonate. 1982; 41: 1-7Crossref PubMed Scopus (13) Google Scholar). Brito et al. (32Brito M.N. Brito N.A. Brito S.R. Moura M.A. Kawashita N.H. Kettelhut I.C. Migliorini R.H. Am. J. Physiol. 1999; 276: R1003-R1009PubMed Google Scholar), using isotopic tracers, have shown that feeding rats a high protein, carbohydrate-free diet will markedly induce the level of glyceroneogenesis from alanine, pyruvate, and lactate in BAT and increase the activity of PEPCK-C 4-fold, leading to increased re-esterification of FFA. This pathway may play a critical role in determining the rate of delivery of fatty acids to the mitochondria for energy generation required for nonshivering thermogenesis. In support of this concept, a high protein, carbohydrate-free diet also reduces the thermogenic capacity of BAT in rats (33Brito M.N. Brito N.A. Migliorini R.H. J. Nutr. 1992; 122: 2081-2086Crossref PubMed Scopus (32) Google Scholar). Taken together, these findings suggest that glyceroneogenesis plays a critical role in thermogenesis in BAT by controlling the rate of formation of 3-glycerol phosphate required for triglyceride synthesis in that tissue. There is one surprising aspect of this story; unlike WAT, BAT has considerable activity of glycerol that can form the 3-glycerol phosphate for triglyceride synthesis from glycerol. However, of the fatty acids are re-esterified to triglyceride using 3-glycerol phosphate generated via glyceroneogenesis (32Brito M.N. Brito N.A. Brito S.R. Moura M.A. Kawashita N.H. Kettelhut I.C. Migliorini R.H. Am. J. Physiol. 1999; 276: R1003-R1009PubMed Google Scholar). It is also likely that in these mice fed a carbohydrate-free glyceroneogenesis plays a key role in the 3-glycerol phosphate required to triglyceride synthesis from dietary fatty acids in BAT during the fed state. Glyceroneogenesis of also critical in controlling the rate of triglyceride re-esterification after norepinephrine due to In this and Hirst (29Feldman D. Hirst M. Am. J. Physiol. 1978; 235: E197-E202PubMed Google Scholar) that rats to the a marked in PEPCK-C in This would a in the rate of FFA re-esterification and an increase in the delivery of fatty acid to the mitochondria to maintain a published in and Novak (28Hahn P. Novak M. J. Lipid Res. 1975; 16: 79-91Abstract Full Text PDF PubMed Google Scholar) that BAT has 10 times the activity of PEPCK-C as with WAT on protein the rate of glyceroneogenesis, as by the rate of of pyruvate into is times in WAT than in that the PEPCK-C activity is involved in a cycle in which the enzyme the generated in the acid cycle by to form phosphoenolpyruvate from oxalacetate, which is to pyruvate via pyruvate the pyruvate is to or to to the acid The of the acid cycle will malate that the mitochondria to maintain the of cytosolic for The result is a cycle in which one of is used by PEPCK-C and one of by pyruvate with only one of generated by pyruvate This cycle in the generation of heat by the The of expression of the gene for PEPCK-C in BAT result in a rate of cycling of pyruvate as as a of triglyceride fatty acid cycling leading to a of triglyceride in the tissue. This was in the in which the binding site in the PEPCK-C gene promoter was thereby expression of the gene in WAT and BAT in other the mice had a marked loss of triglyceride from both tissues (20Olswang Y. Cohen H. Papo O. Cassuto H. Croniger C.M. Hakimi P. Tilghman S.M. Hanson R.W. Reshef L. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 625-630Crossref PubMed Scopus (116) Google Scholar). Thus, glyceroneogenesis in BAT is important for the level of triglycerides in the Because fatty acid re-esterification is a cycle molecules of are required to molecules of fatty acid to for triglyceride active rates of triglyceride synthesis in BAT to the heat generated by fatty via for the of of WAT metabolism is that the glycerol released during lipolysis be and used for triglyceride synthesis because this tissue has a activity of glycerol of the glycerol released during fasting is by the liver and to glucose, glycerol can be used by BAT (28Hahn P. Novak M. J. Lipid Res. 1975; 16: 79-91Abstract Full Text PDF PubMed Google Scholar) and muscle M.V. D. F. J. D.W. H. Am. J. Physiol. Google Scholar, Z. Jensen M.D. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). Because FFA is re-esterified back to triglyceride by WAT during fasting, the rate of glycerol release by that tissue is used as a of lipolysis. the years there have been several of glycerol activity in WAT, the activity is not to support the high rates of FFA re-esterification that would be required for triglyceride synthesis when glucose is such as occurs in this tissue during et al. (17Guan H.P. Li Y. Jensen M.V. Newgard C.B. Steppan C.M. Lazar M.A. Nat Med. 2002; 8: 1122-1128Crossref PubMed Scopus (325) Google Scholar) that WAT contains glycerol to play a role in triglyceride in this tissue in the of they very levels of glycerol in upon the addition of of of they noted the of glycerol In addition, the of of for to mice an of glycerol in the WAT of the animals. mice had glycerol after and animals had levels of glycerol in their adipose tissue. to the addition of by greatly the of glycerol into glyceride-glycerol and by FFA release of can from this that the can induce the expression of the gene for glycerol in WAT and can thus FFA re-esterification in that tissue. However, the levels of glycerol in WAT are in are for a of the of transcription the marked and of PEPCK-C gene expression in WAT P. E. J. Beale E.G. Spiegelman B.M. Mol. Cell. Biol. PubMed Google Scholar) and has been shown to bind to specific in the PEPCK-C gene promoter P. E. J. Beale E.G. Spiegelman B.M. Mol. Cell. Biol. PubMed Google Scholar, D.W. E. Forest C. Beale E.G. J. Biol. Chem. 2001; 276: Full Text Full Text PDF PubMed Scopus Google Scholar, P. H. M. E. P. Spiegelman B.M. Res. PubMed Scopus (325) Google Scholar). In addition, is required for the tissue-specific expression of the gene for PEPCK-C in WAT (23Devine J.H. Eubank D.W. Clouthier D.E. Tontonoz P. Spiegelman B.M. Hammer R.E. Beale E.G. J. Biol. Chem. 1999; 274: 13604-13612Abstract Full Text Full Text PDF PubMed Scopus (77) Google Scholar). Thus the administration of would transcription of the gene for PEPCK-C M. E. Beale E.G. Forest C. 2001; PubMed Scopus Google Scholar), as as the gene for glycerol kinase, that the increase in FFA re-esterification noted by et al. (17Guan H.P. Li Y. Jensen M.V. Newgard C.B. Steppan C.M. Lazar M.A. Nat Med. 2002; 8: 1122-1128Crossref PubMed Scopus (325) Google Scholar) be due as to the of PEPCK-C gene transcription (18Beale E.G. Hammer R.E. Antoine B. Forest C. FASEB J. 2002; 16: 1695-1696Crossref PubMed Scopus (54) Google Scholar, M. E. Beale E.G. Forest C. 2001; PubMed Scopus Google Scholar), which in would be by an increase in glyceroneogenesis.The importance of glyceroneogenesis glycerol in the in the release of fatty acid from WAT has been in a recent by et al. J. G. J. Beale E.G. Forest C. Antoine B. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). noted that induced glycerol activity in from a very activity of of protein to of protein after In the activity of PEPCK-C in adipose tissue was of protein in the and of protein after the addition of The rate of fatty acid re-esterification resulting from glyceroneogenesis, as determined in this was times higher than the rates for glycerol conversion to glyceride-glycerol in incubated with These that the major effect of in the levels of