Effects of Increased Glucokinase Gene Copy Number on Glucose Homeostasis and Hepatic Glucose Metabolism
1997; Elsevier BV; Volume: 272; Issue: 36 Linguagem: Inglês
10.1074/jbc.272.36.22570
ISSN1083-351X
AutoresKevin D. Niswender, Masakazu Shiota, Catherine Postic, Alan D. Cherrington, Mark A. Magnuson,
Tópico(s)Genetics and Neurodevelopmental Disorders
ResumoThe relationship between glucokinase (GK) gene copy number and glucose homeostasis was studied in transgenic mice with additional copies of the entire GK gene locus (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem.272, 22564–22569). The plasma glucose concentration was reduced by 25 ± 3% and 37 ± 4% in mice with one or two extra copies of the gene locus, respectively. The basis for the hypoglycemic phenotype was determined using metabolic tracer techniques in chronically cannulated, conscious mice with one extra GK gene copy. Under basal conditions (6-h fasted) transgenic mice had a lower blood glucose concentration (−12 ± 1%) and a slightly higher glucose turnover rate (+8 ± 3%), resulting in a significantly higher glucose clearance rate (+21 ± 2%). Plasma insulin levels were not different, suggesting that increased glucose clearance was due to augmented hepatic, not islet, GK gene expression. Under hyperglycemic clamp conditions the transgenic mice had glucose turnover and clearance rates similar to the controls, but showed a lower plasma insulin response (−48 ± 5%). Net hepatic glycogen synthesis was markedly elevated (+360%), whereas skeletal muscle glycogen synthesis was decreased (−40%). These results indicate that increased GK gene dosage leads to increased hepatic glucose metabolism and, consequently, a lower plasma glucose concentration. Increased insulin secretion was not observed, even though the transgene is expressed in islets, because hypoglycemia causes a down-regulation in islet GK content (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem. 272, in press). The relationship between glucokinase (GK) gene copy number and glucose homeostasis was studied in transgenic mice with additional copies of the entire GK gene locus (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem.272, 22564–22569). The plasma glucose concentration was reduced by 25 ± 3% and 37 ± 4% in mice with one or two extra copies of the gene locus, respectively. The basis for the hypoglycemic phenotype was determined using metabolic tracer techniques in chronically cannulated, conscious mice with one extra GK gene copy. Under basal conditions (6-h fasted) transgenic mice had a lower blood glucose concentration (−12 ± 1%) and a slightly higher glucose turnover rate (+8 ± 3%), resulting in a significantly higher glucose clearance rate (+21 ± 2%). Plasma insulin levels were not different, suggesting that increased glucose clearance was due to augmented hepatic, not islet, GK gene expression. Under hyperglycemic clamp conditions the transgenic mice had glucose turnover and clearance rates similar to the controls, but showed a lower plasma insulin response (−48 ± 5%). Net hepatic glycogen synthesis was markedly elevated (+360%), whereas skeletal muscle glycogen synthesis was decreased (−40%). These results indicate that increased GK gene dosage leads to increased hepatic glucose metabolism and, consequently, a lower plasma glucose concentration. Increased insulin secretion was not observed, even though the transgene is expressed in islets, because hypoglycemia causes a down-regulation in islet GK content (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem. 272, in press). Glucokinase (GK) 1The abbreviations used are: GK, glucokinase;MODY-2, maturity onset diabetes of the young, type 2; GKRP, GK regulatory protein; L-PK, L-type pyruvate kinase; NEFA, non-esterified fatty acids; PEPCK, phosphoenolpyruvate carboxykinase.1The abbreviations used are: GK, glucokinase;MODY-2, maturity onset diabetes of the young, type 2; GKRP, GK regulatory protein; L-PK, L-type pyruvate kinase; NEFA, non-esterified fatty acids; PEPCK, phosphoenolpyruvate carboxykinase. is thought to be the principal determinant of both hepatic and islet glucose utilization (2Matschinsky F.M. Diabetes. 1990; 39: 647-652Crossref PubMed Google Scholar). The prediction, over a decade ago, that diminished GK activity would cause diabetes (3Meglasson M.D. Matschinsky F.M. Am. J. Physiol. 1984; 246: E1-E13Crossref PubMed Google Scholar) has been validated with the discovery of GK gene mutations as the cause of maturity onset diabetes of the young, type 2 (MODY-2) (4Froguel P. Vaxillaire M. Sun F. Velho G. Zouali H. Butel M.O. Lesage S. Vionnet N. Clëment K. Fougerousse F. Tanizawa Y. Weissenbach J. Beckmann J.S. Lathrop G.M. Passa P. Permutt M.A. Cohen D. Nature. 1992; 356: 162-164Crossref PubMed Scopus (561) Google Scholar, 5Vionnet N. Stoffel M. Takeda J. Yasuda K. Bell G.I. Zouali H. Sesage S. Lesage S. Velho G. Iris F. Passa P. Froguel P. Cohen D. Nature. 1992; 356: 721-722Crossref PubMed Scopus (554) Google Scholar) and by the generation of GK gene knock-out mice (6Bali D. Svetlanov A. Lee H.-W. Fusco-DeMane D. Leiser M. Li B. Barzilai N. Surana M. Hou H. Fleischer N. DePinho R. Rossetti L. Efrat S. J. Biol. Chem. 1995; 270: 21464-21467Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, 7Terauchi Y. Sakura H. Yasuda K. Iwamoto K. Takahashi N. Ito K. Kasai H. Suzuki H. Ueda O. Kamada N. Jishage K. Komeda K. Noda M. Kanazawa Y. Taniguchi S. Miwa I. Akanuma Y. Kodama T. Yazaki Y. Kadowaki T. J. Biol. Chem. 1995; 270: 30253-30256Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar, 8Grupe A. Hultgren B. Ryan A. Ma Y.H. Bauer M. Stewart T.A. Cell. 1995; 83: 69-78Abstract Full Text PDF PubMed Scopus (226) Google Scholar). The loss of one functional GK allele leads to an early onset, but relatively mild hyperglycemia that is due, at least in part, to decreased activity of GK in the pancreatic β cell. This causes an increase in the glucose set point and attenuates glucose-stimulated insulin secretion (6Bali D. Svetlanov A. Lee H.-W. Fusco-DeMane D. Leiser M. Li B. Barzilai N. Surana M. Hou H. Fleischer N. DePinho R. Rossetti L. Efrat S. J. Biol. Chem. 1995; 270: 21464-21467Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, 7Terauchi Y. Sakura H. Yasuda K. Iwamoto K. Takahashi N. Ito K. Kasai H. Suzuki H. Ueda O. Kamada N. Jishage K. Komeda K. Noda M. Kanazawa Y. Taniguchi S. Miwa I. Akanuma Y. Kodama T. Yazaki Y. Kadowaki T. J. Biol. Chem. 1995; 270: 30253-30256Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar, 8Grupe A. Hultgren B. Ryan A. Ma Y.H. Bauer M. Stewart T.A. Cell. 1995; 83: 69-78Abstract Full Text PDF PubMed Scopus (226) Google Scholar, 9Froguel P. Zouali H. Vionnet N. Velho G. Vaxillaire M. Sun F. Lesage S. Stoffel M. Takeda J. Passa P. Permutt M.A. Beckmann J.S. Bell G.I. Cohen D. N. Engl. J. Med. 1993; 328: 697-702Crossref PubMed Scopus (707) Google Scholar, 10Velho G. Froguel P. Clement K. Pueyo M.E. Rakotoambinina B. Zouali H. Passa P. Cohen D. Robert J.-J. Lancet. 1992; 340: 444-448Abstract PubMed Scopus (211) Google Scholar). Less is known about the role of altered hepatic GK gene expression in MODY-2, even though GK activity has long been thought to be a key determinant of hepatic glucose usage (2Matschinsky F.M. Diabetes. 1990; 39: 647-652Crossref PubMed Google Scholar). Studies using heterozygous GK null mice have been helpful, but only provide insights into the effects of diminished GK expression. For this reason, the effects of increased GK gene expression in the liver have also been studied in both transgenic mice and primary hepatocytes. In transgenic mice, increased hepatic GK causes hypoglycemia (11Hariharan N. Farrelly D. Hagan D. Hillyer D. Arbeeny C. Sabrah T. Treloar A. Brown K. Kalinowski S. Mookhtiar K. Diabetes. 1997; 46: 11-16Crossref PubMed Google Scholar, 12Ferre T. Riu E. Bosch F. Valera A. FASEB J. 1996; 10: 1213-1218Crossref PubMed Scopus (164) Google Scholar), whereas in primary heptocytes increased GK leads to both increased glycolysis and glycogen synthesis (13O'Doherty R.M. Lehman D.L. Seoane J. Gomex-Foix A.M. Guinovart J.J. Newgard C.B. J. Biol. Chem. 1996; 271: 20524-20530Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar,14Agius L. Peak M. Newgard C.B. Gomez-Foix A.M. Guinovart J.J. J. Biol. Chem. 1996; 271: 30479-30486Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar).Both the liver and islet, key sites of GK gene expression, participate in a feedback loop that is necessary for maintaining euglycemia (15Meglasson M.D. Matschinsky F.M. Diabetes Metab. Rev. 1986; 2: 163-214Crossref PubMed Scopus (412) Google Scholar,16Magnuson M. Diabetes. 1990; 39: 523-527Crossref PubMed Google Scholar). The differential, tissue-specific regulation of GK gene expression is thought to be an essential component of this feedback loop (16Magnuson M. Diabetes. 1990; 39: 523-527Crossref PubMed Google Scholar). The liver can be either a net consumer or net producer of glucose, as determined in large part by the concentrations of insulin and glucagon in the portal blood (17Hers H.-G. J. Inher. Metab. Dis. 1990; 13: 395-410Crossref PubMed Scopus (51) Google Scholar, 18Pagliassotti M.J. Horton T.J. Pagliassotti M.J. Davis S.N. Cherrington A.D. The Role of the Liver in Maintaining Glucose Homeostasis. R. G. Landes Co., Georgetown, TX1994: 45-62Google Scholar, 19Pilkis S.J. Granner D.K. Annu. Rev. Physiol. 1992; 54: 885-909Crossref PubMed Scopus (698) Google Scholar), as well as the plasma glucose level itself. In turn, the secretion of pancreatic hormones depends on the plasma glucose concentration, which is affected in part by the status of the liver. In liver, GK gene transcription is stimulated by insulin and inhibited by glucagon (20Iynedjian P.B. Pilot P.R. Nouspikel T. Milburn J.L. Quaade C. Hughes S. Ucla C. Newgard C.B. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 7838-7842Crossref PubMed Scopus (143) Google Scholar, 21Magnuson M.A. Andreone T.L. Printz R.L. Koch S. Granner D.K. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 4838-4842Crossref PubMed Scopus (175) Google Scholar). In addition, GK activity in liver can be acutely inhibited through interactions with the GK regulatory protein (GKRP) (22Toyoda Y. Miwa I. Kamiya M. Ogiso S. Nonogaki T. Aoki S. Okuda J. Biochem. Biophys. Res. Commun. 1994; 204: 252-256Crossref PubMed Scopus (79) Google Scholar, 23Van Schaftingen E. Eur. J. Biochem. 1989; 179: 179-184Crossref PubMed Scopus (160) Google Scholar, 24Van Schaftingen E. Vandercammen A. Detheux M. Davies D.R. Adv. Enzyme Regul. 1992; 32: 133-148Crossref PubMed Scopus (69) Google Scholar). In the islet, GK gene transcription is thought to be largely constitutive, although glucose modulates islet GK content, probably by directly affecting the half-life of the enzyme (25Liang Y. Najafi H. Matschinsky F.M. J. Biol. Chem. 1990; 265: 16863-16866Abstract Full Text PDF PubMed Google Scholar, 26Liang Y. Najafi H. Smith R.M. Zimmerman E.C. Magnuson M.A. Tal M. Matschinsky F.M. Diabetes. 1992; 41: 792-806Crossref PubMed Scopus (147) Google Scholar).Metabolic flux in the liver reflects the net activities of several pathways. Some of the rate-determining enzymes in these pathways are regulated at the transcriptional level by the metabolism of glucose itself, presumably through a glycolytic intermediate (27Towle H.C. Kaytor E.N. Shih H.-M. Biochem. Soc. Trans. 1996; 24: 364-368Crossref PubMed Scopus (16) Google Scholar). Carbohydrate response elements have been identified in the genes for L-type pyruvate kinase (L-PK) (28Liu Z. Thompson K.S. Towle H.C. J. Biol. Chem. 1993; 268: 12787-12795Abstract Full Text PDF PubMed Google Scholar, 29Cuif M.-H. Porteu A. Kahn A. Vaulont S. J. Biol. Chem. 1993; 268: 13769-13772Abstract Full Text PDF PubMed Google Scholar, 30Vaulont S. Kahn A. FASEB J. 1994; 8: 28-35Crossref PubMed Scopus (174) Google Scholar), S14 (31Shih H. Towle H.C. J. Biol. Chem. 1994; 269: 9380-9387Abstract Full Text PDF PubMed Google Scholar, 32Shih H. Towle H.C. J. Biol. Chem. 1992; 267: 13222-13228Abstract Full Text PDF PubMed Google Scholar), fatty acid synthase (33Foufelle F. Lepetit N. Bosc D. Delzenne N. Morin J. Raymondjean M. Ferre P. Biochem. J. 1995; 308: 521-527Crossref PubMed Scopus (39) Google Scholar, 34Prip-Buus C. Perdereau D. Foufelle F. Maury J. Ferre P. Girard J. Eur. J. Biochem. 1995; 230: 309-315Crossref PubMed Scopus (88) Google Scholar), and GLUT2 (35Rencurel F. Waeber G. Antoine B. Rocchiccioli F. Maulard P. Girard J. Leturque A. Biochem. J. 1996; 314: 903-909Crossref PubMed Scopus (95) Google Scholar). The induction of hepatic genes by glucose has been found to require GK, suggesting that GK gene expression has both catalytic and regulatory functions in the liver. Given the complex and interdependent factors involved in the regulation of both islet and hepatic GK, it is difficult to accurately predict the in vivo ramifications of altered GK gene expression from in vitro model systems.To further understand the effect of physiologically relevant variations in GK gene copy number on glucose homeostasis, we studied whole body glucose metabolism in mice containing a GK gene locus transgene (1Niswender K.D. Postic C. Jetton T.L. Bennett B.D. Piston D.W. Efrat S. Magnuson M.A. J. Biol. Chem. 1997; 272: 22564-22569Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). Using this experimental design, we were able to determine the effect on the plasma glucose concentration and hepatic glucose metabolism of one and two additional GK gene copies. These studies clearly revealed the reciprocal relationship between GK gene copy number and blood glucose concentration, the high control strength that GK gene expression has on blood glucose homeostasis, and the importance of feedback interactions between the liver and islet.EXPERIMENTAL PROCEDURESGK Gene Locus Transgenic MiceThe animals used in this study contain an 83-kilobase pair GK gene locus transgene inserted by pronuclear DNA microinjection into B6D2 F1 hybrid mice. Previous studies have shown there is one copy of the transgene per haploid genome, located on the X chromosome (1Niswender K.D. Postic C. Jetton T.L. Bennett B.D. Piston D.W. Efrat S. Magnuson M.A. J. Biol. Chem. 1997; 272: 22564-22569Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). All studies were performed using female animals from line 37 that were either heterozygous or homozygous for the transgene allele. Controls were non-transgenic animals of similar genetic background generated from within the same mouse colony. Only non-obese animals weighing 25–30 g were studied. Mice were housed either in a barrier facility or individual filtered units, fed standard mouse chow (Purina Mills, Inc.), and maintained on a 12-h light/dark cycle.Glucose Tolerance TestMice were anesthetized with sodium pentobarbital (70 μg/g body weight), and blood was sampled from the retro-orbital sinus for glucose tolerance testing in a simple two-point protocol. Base-line samples were taken ∼45 min after induction of anesthesia to allow metabolic fluctuations to return to base-line levels. Glucose was then administered at 1.5 mg/g body weight by intraperitoneal injection, and the second blood sample was obtained 20 min later, corresponding to the peak glucose excursion. Plasma glucose concentrations were determined with a Beckman Glucose Analyzer 2.In Vivo Glucose KineticsAnimal PreparationThe kinetics of glucose metabolism were studied in chronically cannulated, conscious mice. Cannulae were surgically implanted 4–5 days prior to experimentation. Mice were anesthetized with sodium pentobarbital (70 μg/g body weight, intraperitoneal). Cannulae were surgically implanted into the right jugular vein and the left carotid artery, externalized in the interscapulum and sealed (details available on request). The arterial cannula was used for sampling well mixed arterial blood from which tracer and metabolite measurements were made. The venous cannula was used to infuse tracer and glucose. Body weight, hematocrit, general appearance, and intra-experimental physiological responses were used as indices of health.Experimental ProceduresControl and heterozygous transgenic (3 GK gene copies) mice were fasted for 6 h prior to experimentation. The experimental protocol was 220 min in duration and consisted of a 100-min equilibration period (−100 to 0 min) followed by a 120-min experimental period (0–120 min). A 2-μCi bolus of tracer ([3-3H]glucose, NEN Life Science Products) was given at −100 min, followed by a constant infusion at 0.04 μCi/min for the duration of the study. Samples for tracer and insulin determinations were taken every 30 min starting at −30 min. The blood glucose concentration was measured at −30, 0, 30, and every 15 min thereafter using a Hemocue blood glucose meter (Hemocue, Mission Viejo, CA). The values obtained from the Hemocue device were 20% higher than values of a plasma glucose assay. Red cells were washed with saline and infused back into the animals to prevent anemia. A 50% glucose solution, infused at a variable rate, was used to maintain hyperglycemia in the hyperglycemic clamp studies. In total, ∼400 μl of blood/animal was sampled and 300–800 μl of solutions infused. The experiments were terminated at 120 min, when the animals were anesthetized with sodium pentobarbital and the abdomen was opened. Livers were removed within 30 s, and hindlimb muscle was removed within 2 min of anesthesia; both tissues were snap frozen in liquid nitrogen immediately upon removal. A separate set of animals were killed in the same fashion at the 0-min time point for base-line glycogen assessment. Plasma samples for [3H]glucose determinations were deproteinized with Ba(OH)2 and ZnSO4 and dried to eliminate tritiated water. The glucose turnover rate (mg/kg-min) was calculated as the rate of tracer infusion (dpm/min) divided by the corrected plasma glucose specific activity (dpm/mg) per kg body weight of the mouse. The concentration of plasma insulin was determined by radioimmunoassay using a rat insulin radioimmunoassay kit from Linco Research (St. Louis, MO). The binding reactions were modified to perform the assay on 10 μl of plasma. Liver and skeletal muscle glycogen content were assayed as described (36Keppler D. Decker K. Bergmeyer H.U. Methods of Enzymatic Analysis. Verlag Chemie, Weinheim, Germany1984: 11-18Google Scholar). Liver glycogen depletion was determined by subtracting the glycogen content at the end of the study from the basal hepatic glycogen content of a separate set of animals. Lactate was measured as described (37Noll F. Bergmeyer H.U. Methods of Enzymatic Analysis. Verlag Chemie, Weinheim, Germany1984: 582-588Google Scholar) using 20 μl of deproteinized plasma solution (2 μl of plasma). Plasma non-esterified fatty acids were measured using 1–3 μl of plasma with a NEFA C kit from Wako Pure Chemical Industries.Northern AnalysisNorthern blot analysis was performed as described (1Niswender K.D. Postic C. Jetton T.L. Bennett B.D. Piston D.W. Efrat S. Magnuson M.A. J. Biol. Chem. 1997; 272: 22564-22569Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). A 1.8-kilobase pair PstI fragment from the rat cDNA PK G4 (38Lone Y.-C. Simon M.-P. Kahn A. Marie J. FEBS Lett. 1986; 195: 97-100Crossref PubMed Scopus (53) Google Scholar) was used as a probe for L-PK mRNA. The phosphoenolpyruvate carboxykinase (PEPCK) probe was a 569-bp fragment spanning exons 1–4 of the mouse PEPCK cDNA provided by R. Chalkley (Department of Molecular Physiology and Biophysics, Vanderbilt University). The cyclophilin control probe was a ∼700-bpHindIII-EcoRI fragment from a rat cDNA plasmid (39Danielson P.E. Forss-Petter S. Brow M.A. Calavetta L. Douglass J. Milner R.J. Sutcliffe J.G. DNA. 1988; 7: 261-267Crossref PubMed Scopus (1032) Google Scholar).Statistical AnalysisAll results are presented as the mean ± standard error of the mean. Statistical significance was determined by Student'st test. Differences are reported as statistically significant at an α value of 0.05. For the metabolic studies, values for the last 60 min of each protocol were averaged to summarize each experiment. Glucokinase (GK) 1The abbreviations used are: GK, glucokinase;MODY-2, maturity onset diabetes of the young, type 2; GKRP, GK regulatory protein; L-PK, L-type pyruvate kinase; NEFA, non-esterified fatty acids; PEPCK, phosphoenolpyruvate carboxykinase.1The abbreviations used are: GK, glucokinase;MODY-2, maturity onset diabetes of the young, type 2; GKRP, GK regulatory protein; L-PK, L-type pyruvate kinase; NEFA, non-esterified fatty acids; PEPCK, phosphoenolpyruvate carboxykinase. is thought to be the principal determinant of both hepatic and islet glucose utilization (2Matschinsky F.M. Diabetes. 1990; 39: 647-652Crossref PubMed Google Scholar). The prediction, over a decade ago, that diminished GK activity would cause diabetes (3Meglasson M.D. Matschinsky F.M. Am. J. Physiol. 1984; 246: E1-E13Crossref PubMed Google Scholar) has been validated with the discovery of GK gene mutations as the cause of maturity onset diabetes of the young, type 2 (MODY-2) (4Froguel P. Vaxillaire M. Sun F. Velho G. Zouali H. Butel M.O. Lesage S. Vionnet N. Clëment K. Fougerousse F. Tanizawa Y. Weissenbach J. Beckmann J.S. Lathrop G.M. Passa P. Permutt M.A. Cohen D. Nature. 1992; 356: 162-164Crossref PubMed Scopus (561) Google Scholar, 5Vionnet N. Stoffel M. Takeda J. Yasuda K. Bell G.I. Zouali H. Sesage S. Lesage S. Velho G. Iris F. Passa P. Froguel P. Cohen D. Nature. 1992; 356: 721-722Crossref PubMed Scopus (554) Google Scholar) and by the generation of GK gene knock-out mice (6Bali D. Svetlanov A. Lee H.-W. Fusco-DeMane D. Leiser M. Li B. Barzilai N. Surana M. Hou H. Fleischer N. DePinho R. Rossetti L. Efrat S. J. Biol. Chem. 1995; 270: 21464-21467Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, 7Terauchi Y. Sakura H. Yasuda K. Iwamoto K. Takahashi N. Ito K. Kasai H. Suzuki H. Ueda O. Kamada N. Jishage K. Komeda K. Noda M. Kanazawa Y. Taniguchi S. Miwa I. Akanuma Y. Kodama T. Yazaki Y. Kadowaki T. J. Biol. Chem. 1995; 270: 30253-30256Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar, 8Grupe A. Hultgren B. Ryan A. Ma Y.H. Bauer M. Stewart T.A. Cell. 1995; 83: 69-78Abstract Full Text PDF PubMed Scopus (226) Google Scholar). The loss of one functional GK allele leads to an early onset, but relatively mild hyperglycemia that is due, at least in part, to decreased activity of GK in the pancreatic β cell. This causes an increase in the glucose set point and attenuates glucose-stimulated insulin secretion (6Bali D. Svetlanov A. Lee H.-W. Fusco-DeMane D. Leiser M. Li B. Barzilai N. Surana M. Hou H. Fleischer N. DePinho R. Rossetti L. Efrat S. J. Biol. Chem. 1995; 270: 21464-21467Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, 7Terauchi Y. Sakura H. Yasuda K. Iwamoto K. Takahashi N. Ito K. Kasai H. Suzuki H. Ueda O. Kamada N. Jishage K. Komeda K. Noda M. Kanazawa Y. Taniguchi S. Miwa I. Akanuma Y. Kodama T. Yazaki Y. Kadowaki T. J. Biol. Chem. 1995; 270: 30253-30256Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar, 8Grupe A. Hultgren B. Ryan A. Ma Y.H. Bauer M. Stewart T.A. Cell. 1995; 83: 69-78Abstract Full Text PDF PubMed Scopus (226) Google Scholar, 9Froguel P. Zouali H. Vionnet N. Velho G. Vaxillaire M. Sun F. Lesage S. Stoffel M. Takeda J. Passa P. Permutt M.A. Beckmann J.S. Bell G.I. Cohen D. N. Engl. J. Med. 1993; 328: 697-702Crossref PubMed Scopus (707) Google Scholar, 10Velho G. Froguel P. Clement K. Pueyo M.E. Rakotoambinina B. Zouali H. Passa P. Cohen D. Robert J.-J. Lancet. 1992; 340: 444-448Abstract PubMed Scopus (211) Google Scholar). Less is known about the role of altered hepatic GK gene expression in MODY-2, even though GK activity has long been thought to be a key determinant of hepatic glucose usage (2Matschinsky F.M. Diabetes. 1990; 39: 647-652Crossref PubMed Google Scholar). Studies using heterozygous GK null mice have been helpful, but only provide insights into the effects of diminished GK expression. For this reason, the effects of increased GK gene expression in the liver have also been studied in both transgenic mice and primary hepatocytes. In transgenic mice, increased hepatic GK causes hypoglycemia (11Hariharan N. Farrelly D. Hagan D. Hillyer D. Arbeeny C. Sabrah T. Treloar A. Brown K. Kalinowski S. Mookhtiar K. Diabetes. 1997; 46: 11-16Crossref PubMed Google Scholar, 12Ferre T. Riu E. Bosch F. Valera A. FASEB J. 1996; 10: 1213-1218Crossref PubMed Scopus (164) Google Scholar), whereas in primary heptocytes increased GK leads to both increased glycolysis and glycogen synthesis (13O'Doherty R.M. Lehman D.L. Seoane J. Gomex-Foix A.M. Guinovart J.J. Newgard C.B. J. Biol. Chem. 1996; 271: 20524-20530Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar,14Agius L. Peak M. Newgard C.B. Gomez-Foix A.M. Guinovart J.J. J. Biol. Chem. 1996; 271: 30479-30486Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar). Both the liver and islet, key sites of GK gene expression, participate in a feedback loop that is necessary for maintaining euglycemia (15Meglasson M.D. Matschinsky F.M. Diabetes Metab. Rev. 1986; 2: 163-214Crossref PubMed Scopus (412) Google Scholar,16Magnuson M. Diabetes. 1990; 39: 523-527Crossref PubMed Google Scholar). The differential, tissue-specific regulation of GK gene expression is thought to be an essential component of this feedback loop (16Magnuson M. Diabetes. 1990; 39: 523-527Crossref PubMed Google Scholar). The liver can be either a net consumer or net producer of glucose, as determined in large part by the concentrations of insulin and glucagon in the portal blood (17Hers H.-G. J. Inher. Metab. Dis. 1990; 13: 395-410Crossref PubMed Scopus (51) Google Scholar, 18Pagliassotti M.J. Horton T.J. Pagliassotti M.J. Davis S.N. Cherrington A.D. The Role of the Liver in Maintaining Glucose Homeostasis. R. G. Landes Co., Georgetown, TX1994: 45-62Google Scholar, 19Pilkis S.J. Granner D.K. Annu. Rev. Physiol. 1992; 54: 885-909Crossref PubMed Scopus (698) Google Scholar), as well as the plasma glucose level itself. In turn, the secretion of pancreatic hormones depends on the plasma glucose concentration, which is affected in part by the status of the liver. In liver, GK gene transcription is stimulated by insulin and inhibited by glucagon (20Iynedjian P.B. Pilot P.R. Nouspikel T. Milburn J.L. Quaade C. Hughes S. Ucla C. Newgard C.B. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 7838-7842Crossref PubMed Scopus (143) Google Scholar, 21Magnuson M.A. Andreone T.L. Printz R.L. Koch S. Granner D.K. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 4838-4842Crossref PubMed Scopus (175) Google Scholar). In addition, GK activity in liver can be acutely inhibited through interactions with the GK regulatory protein (GKRP) (22Toyoda Y. Miwa I. Kamiya M. Ogiso S. Nonogaki T. Aoki S. Okuda J. Biochem. Biophys. Res. Commun. 1994; 204: 252-256Crossref PubMed Scopus (79) Google Scholar, 23Van Schaftingen E. Eur. J. Biochem. 1989; 179: 179-184Crossref PubMed Scopus (160) Google Scholar, 24Van Schaftingen E. Vandercammen A. Detheux M. Davies D.R. Adv. Enzyme Regul. 1992; 32: 133-148Crossref PubMed Scopus (69) Google Scholar). In the islet, GK gene transcription is thought to be largely constitutive, although glucose modulates islet GK content, probably by directly affecting the half-life of the enzyme (25Liang Y. Najafi H. Matschinsky F.M. J. Biol. Chem. 1990; 265: 16863-16866Abstract Full Text PDF PubMed Google Scholar, 26Liang Y. Najafi H. Smith R.M. Zimmerman E.C. Magnuson M.A. Tal M. Matschinsky F.M. Diabetes. 1992; 41: 792-806Crossref PubMed Scopus (147) Google Scholar). Metabolic flux in the liver reflects the net activities of several pathways. 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To further understand the effect of physiologically relevant variations in GK gene copy number on glucose homeostasis, we studied whole body glucose metabolism in mice containing a GK gene locus transgene (1Niswender K.D.
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