Identification and characterization of a human cDNA and gene encoding a ubiquitously expressed glucose-6-phosphatase catalytic subunit-related protein

A syndrome with congenital neutropenia and mutations in G6PC3

BACKGROUND

The main features of severe congenital neutropenia are the onset of severe bacterial infections early in life, a paucity of mature neutrophils, and an increased risk of leukemia. In many patients, the genetic causes of severe congenital neutropenia are unknown.

METHODS

We performed genomewide genotyping and linkage analysis on two consanguineous pedigrees with a total of five children affected with severe congenital neutropenia. Candidate genes from the linkage interval were sequenced. Functional assays and reconstitution experiments were carried out.

RESULTS

All index patients were susceptible to bacterial infections and had very few mature neutrophils in the bone marrow; structural heart defects, urogenital abnormalities, and venous angiectasia on the trunk and extremities were additional features. Linkage analysis of the two index families yielded a combined multipoint lod score of 5.74 on a linkage interval on chromosome 17q21. Sequencing of G6PC3, the candidate gene encoding glucose-6-phosphatase, catalytic subunit 3, revealed a homozygous missense mutation in exon 6 that abolished the enzymatic activity of glucose-6-phosphatase in all affected children in the two families. The patients' neutrophils and fibroblasts had increased susceptibility to apoptosis. The myeloid cells showed evidence of increased endoplasmic reticulum stress and increased activity of glycogen synthase kinase 3beta (GSK-3beta). We identified seven additional, unrelated patients who had severe congenital neutropenia with syndromic features and distinct biallelic mutations in G6PC3.

CONCLUSIONS

Defective function of glucose-6-phosphatase, catalytic subunit 3, underlies a severe congenital neutropenia syndrome associated with cardiac and urogenital malformations.

Foxa2 and MafA regulate islet-specific glucose-6-phosphatase catalytic subunit-related protein gene expression

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP/G6PC2) is a major autoantigen in both mouse and human type 1 diabetes. IGRP is selectively expressed in islet beta cells and polymorphisms in the IGRP gene have recently been associated with variations in fasting blood glucose levels and cardiovascular-associated mortality in humans. Chromatin immunoprecipitation (ChIP) assays have shown that the IGRP promoter binds the islet-enriched transcription factors Pax-6 and BETA2. We show here, again using ChIP assays, that the IGRP promoter also binds the islet-enriched transcription factors MafA and Foxa2. Single binding sites for these factors were identified in the proximal IGRP promoter, mutation of which resulted in decreased IGRP fusion gene expression in betaTC-3, Hamster insulinoma tumor (HIT), and Min6 cells. ChiP assays have shown that the islet-enriched transcription factor Pdx-1 also binds the IGRP promoter, but mutational analysis of four Pdx-1 binding sites in the proximal IGRP promoter revealed surprisingly little effect of Pdx-1 binding on IGRP fusion gene expression in betaTC-3 cells. In contrast, in both HIT and Min6 cells mutation of these four Pdx-1 binding sites resulted in a approximately 50% reduction in fusion gene expression. These data suggest that the same group of islet-enriched transcription factors, namely Pdx-1, Pax-6, MafA, BETA2, and Foxa2, directly or indirectly regulate expression of the two major autoantigens in type 1 diabetes.

Mutations in the glucose-6-phosphatase-alpha (G6PC) gene that cause type Ia glycogen storage disease

Glucose-6-phosphatase-alpha (G6PC) is a key enzyme in glucose homeostasis that catalyzes the hydrolysis of glucose-6-phosphate to glucose and phosphate in the terminal step of gluconeogenesis and glycogenolysis. Mutations in the G6PC gene, located on chromosome 17q21, result in glycogen storage disease type Ia (GSD-Ia), an autosomal recessive metabolic disorder. GSD-Ia patients manifest a disturbed glucose homeostasis, characterized by fasting hypoglycemia, hepatomegaly, nephromegaly, hyperlipidemia, hyperuricemia, lactic acidemia, and growth retardation. G6PC is a highly hydrophobic glycoprotein, anchored in the membrane of the endoplasmic reticulum with the active center facing into the lumen. To date, 54 missense, 10 nonsense, 17 insertion/deletion, and three splicing mutations in the G6PC gene have been identified in more than 550 patients. Of these, 50 missense, two nonsense, and two insertion/deletion mutations have been functionally characterized for their effects on enzymatic activity and stability. While GSD-Ia is not more prevalent in any ethnic group, mutations unique to Caucasian, Oriental, and Jewish populations have been described. Despite this, GSD-Ia patients exhibit phenotypic heterogeneity and a stringent genotype-phenotype relationship does not exist.

Sequence variation between the mouse and human glucose-6-phosphatase catalytic subunit gene promoters results in differential activation by peroxisome proliferator activated receptor gamma coactivator-1alpha

AIMS/HYPOTHESIS

The glucose-6-phosphatase catalytic subunit (G6PC) plays a key role in hepatic glucose production by catalysing the final step in gluconeogenesis and glycogenolysis. Peroxisome proliferator activated receptor gamma coactivator-1alpha (PGC-1alpha) stimulates mouse G6pc-luciferase fusion gene expression through hepatocyte nuclear factor-4alpha (HNF-4alpha), which binds an element located between -76 and -64 in the promoter. The aim of this study was to compare the regulation of mouse G6pc and human G6PC gene expression by PGC-1alpha.

METHODS

PGC-1alpha action was analysed by transient transfection and gel retardation assays.

RESULTS

In H4IIE cells, PGC-1alpha alone failed to stimulate human G6PC-luciferase fusion gene expression even though the sequence of the -76 to -64 HNF-4alpha binding site is perfectly conserved in the human promoter. This difference could be explained, in part, by a 3 bp sequence variation between the mouse and human promoters. Introducing the human sequence into the mouse G6pc promoter reduced PGC-1alpha-stimulated fusion gene expression, whereas the inverse experiment, in which the mouse sequence was introduced into the human G6PC promoter, resulted in the generation of a G6PC-luciferase fusion gene that was now induced by PGC-1alpha. This critical 3 bp region is located immediately adjacent to a consensus nuclear hormone receptor half-site that is perfectly conserved between the mouse G6pc and human G6PC promoters. Gel retardation experiments revealed that this 3 bp region influences the affinity of HNF-4alpha binding to the half-site.

CONCLUSIONS/INTERPRETATION

These observations suggest that PGC-1alpha may be more important in the control of mouse G6pc than human G6PC gene expression.

Long-range enhancers are required to maintain expression of the autoantigen islet-specific glucose-6-phosphatase catalytic subunit-related protein in adult mouse islets in vivo

OBJECTIVE

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is selectively expressed in islet beta-cells and is a major autoantigen in both mouse and human type 1 diabetes. This study describes the use of a combination of transgenic and transfection approaches to characterize the gene regions that confer the islet-specific expression of IGRP.

RESEARCH DESIGN AND METHODS

Transgenic mice were generated containing the IGRP promoter sequence from -306, -911, or -3911 to +3 ligated to a LacZ reporter gene. Transgene expression was monitored by 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside staining of pancreatic tissue.

RESULTS

In all the transgenic mice, robust LacZ expression was detected in newborn mouse islets, but expression became mosaic as animals aged, suggesting that additional elements are required for the maintenance of IGRP gene expression. VISTA analyses identified two conserved regions in the distal IGRP promoter and one in the third intron. Transfection experiments demonstrated that all three regions confer enhanced luciferase reporter gene expression in beta TC-3 cells when ligated to a minimal IGRP promoter. A transgene containing all three conserved regions was generated by using a bacterial recombination strategy to insert a LacZ cassette into exon 5 of the IGRP gene. Transgenic mice containing a 15-kbp fragment of the IGRP gene were then generated. This transgene conferred LacZ expression in newborn mouse islets; however, expression was still suppressed as animals aged.

CONCLUSIONS

The data suggest that long-range enhancers 5' or 3' of the IGRP gene are required for the maintenance of IGRP gene expression in adult mice.

Specific reduction of hepatic glucose 6-phosphate transporter-1 ameliorates diabetes while avoiding complications of glycogen storage disease

D-Glucose-6-phosphatase is a key regulator of endogenous glucose production, and its inhibition may improve glucose control in type 2 diabetes. Herein, 2'-O-(2-methoxy)ethyl-modified phosphorothioate antisense oligonucleotides (ASOs) specific to the glucose 6-phosphate transporter-1 (G6PT1) enabled reduction of hepatic D-Glu-6-phosphatase activity in diabetic ob/ob mice. Treatment with G6PT1 ASOs decreased G6PT1 expression, reduced G6PT1 activity, blunted glucagon-stimulated glucose production, and lowered plasma glucose concentration in a dose-dependent manner. In contrast to G6PT1 knock-out mice and patients with glycogen storage disease, excess hepatic and renal glycogen accumulation, hyperlipidemia, neutropenia, and elevations in plasma lactate and uric acid did not occur. In addition, hypoglycemia was not observed in animals during extended periods of fasting, and the ability of G6PT1 ASO-treated mice to recover from an exogenous insulin challenge was not impaired. Together, these results demonstrate that effective glucose lowering by G6PT1 inhibitors can be achieved without adversely affecting carbohydrate and lipid metabolism.

Liver glyconeogenesis: a pathway to cope with postprandial amino acid excess in high-protein fed rats?

This paper provides molecular evidence for a liver glyconeogenic pathway, that is, a concomitant activation of hepatic gluconeogenesis and glycogenesis, which could participate in the mechanisms that cope with amino acid excess in high-protein (HP) fed rats. This evidence is based on the concomitant upregulation of phosphoenolpyruvate carboxykinase (PEPCK) gene expression, downregulation of glucose 6-phosphatase catalytic subunit (G6PC1) gene expression, an absence of glucose release from isolated hepatocytes and restored hepatic glycogen stores in the fed state in HP fed rats. These effects are mainly due to the ability of high physiological concentrations of portal blood amino acids to counteract glucagon-induced liver G6PC1 but not PEPCK gene expression. These results agree with the idea that the metabolic pathway involved in glycogen synthesis is dependent upon the pattern of nutrient availability. This nonoxidative glyconeogenic disposal pathway of gluconeogenic substrates copes with amino excess and participates in adjusting both amino acid and glucose homeostasis. In addition, the pattern of PEPCK and G6PC1 gene expression provides evidence that neither the kidney nor the small intestine participated in gluconeogenic glucose production under our experimental conditions. Moreover, the main glucose-6-phosphatase (G6Pase) isoform expressed in the small intestine is the ubiquitous isoform of G6Pase (G6PC3) rather than the G6PC1 isoform expressed in gluconeogenic organs.

Deletion of the gene encoding the ubiquitously expressed glucose-6-phosphatase catalytic subunit-related protein (UGRP)/glucose-6-phosphatase catalytic subunit-beta results in lowered plasma cholesterol and elevated glucagon

In liver, glucose-6-phosphatase catalyzes the hydrolysis of glucose-6-phosphate (G6P) to glucose and inorganic phosphate, the final step in the gluconeogenic and glycogenolytic pathways. Mutations in the glucose-6-phosphatase catalytic subunit (G6Pase) give rise to glycogen storage disease (GSD) type 1a, which is characterized in part by hypoglycemia, growth retardation, hypertriglyceridemia, hypercholesterolemia, and hepatic glycogen accumulation. Recently, a novel G6Pase isoform was identified, designated UGRP/G6Pase-beta. The activity of UGRP relative to G6Pase in vitro is disputed, raising the question as to whether G6P is a physiologically important substrate for this protein. To address this issue we have characterized the phenotype of UGRP knock-out mice. G6P hydrolytic activity was decreased by approximately 50% in homogenates of UGRP(-/-) mouse brain relative to wild type tissue, consistent with the ability of UGRP to hydrolyze G6P. In addition, female, but not male, UGRP(-/-) mice exhibit growth retardation as do G6Pase(-/-) mice and patients with GSD type 1a. However, in contrast to G6Pase(-/-) mice and patients with GSD type 1a, UGRP(-/-) mice exhibit no change in hepatic glycogen content, blood glucose, or triglyceride levels. Although UGRP(-/-) mice are not hypoglycemic, female UGRP(-/-) mice have elevated ( approximately 60%) plasma glucagon and reduced ( approximately 20%) plasma cholesterol. We hypothesize that the hyperglucagonemia prevents hypoglycemia and that the hypocholesterolemia is secondary to the hyperglucagonemia. As such, the phenotype of UGRP(-/-) mice is mild, indicating that G6Pase is the major glucose-6-phosphatase of physiological importance for glucose homeostasis in vivo.

Correlation between FOXO1a (FKHR) and FOXO3a (FKHRL1) binding and the inhibition of basal glucose-6-phosphatase catalytic subunit gene transcription by insulin

Insulin inhibits transcription of the genes encoding the glucose-6-phosphatase catalytic subunit (G6Pase), phosphoenolpyruvate carboxykinase, and IGF binding protein-1 through insulin response sequences (IRSs) that share the same core sequence, T(G/A)TTTT(G/T). The transcription factors FOXO1a and FOXO3a have been shown to bind these elements, but there are conflicting reports as to whether this binding correlates with the action of insulin on gene transcription. Some researchers concluded, from overexpression experiments using FOXO1a, that binding correlated with the insulin response, whereas others concluded, mainly from gel retardation competition experiments using FOXO3a, that it did not. We show here that, although these factors can differentially activate gene transcription in a context-dependent manner, these conflicting data are not explained by a difference in FOXO1a and FOXO3a binding specificity. Instead, we find that gel retardation competition and binding experiments give different results; the latter reveal a correlation between FOXO1a/3a binding and the inhibition of basal G6Pase gene transcription by insulin. In addition, these data show that the binding of FOXO1a/3a to two adjacent IRSs in the G6Pase promoter is cooperative and that promoter context alters the specific IRS base requirements for FOXO1a-stimulated fusion gene expression. Surprisingly, an analysis of insulin action mediated through the G6Pase and IGF binding protein-1 IRSs in the context of a heterologous thymidine kinase promoter reveals that signaling through the latter does not support the accepted model for insulin-stimulated FOXO nuclear exclusion.

Silencing of the human microsomal glucose-6-phosphate translocase induces glioma cell death: Potential new anticancer target for curcumin

G6P translocase (G6PT) is thought to play a crucial role in transducing intracellular signaling events in brain tumor-derived cancer cells. In this report, we investigated the contribution of G6PT to the control of U-87 brain tumor-derived glioma cell survival using small interfering RNA (siRNA)-mediated suppression of G6PT. Three siRNA constructs were generated and found to suppress up to 91% G6PT gene expression. Flow cytometry analysis of propidium iodide/annexin-V-stained cells indicated that silencing the G6PT gene induced necrosis and late apoptosis. The anticancer agent curcumin, also inhibited G6PT gene expression by more than 90% and triggered U-87 glioma cells death. Overexpression of recombinant G6PT rescued the cells from curcumin-induced cell death. Targeting G6PT expression may provide a new mechanistic rationale for the action of chemopreventive drugs and lead to the development of new anti-cancer strategies.

In islet-specific glucose-6-phosphatase-related protein, the beta cell antigenic sequence that is targeted in diabetes is not responsible for the loss of phosphohydrolase activity

AIMS/HYPOTHESIS

There are three members of the glucose-6-phosphatase (G6Pase) family: (1) the liver/kidney/intestine G6Pase-alpha (encoded by G6PC), which is a key enzyme in glucose homeostasis; (2) the ubiquitous G6Pase-beta (encoded by G6PC3); and (3) the islet-specific G6Pase-related protein (IGRP, encoded by /G6PC2). While G6Pase-alpha and G6Pase-beta are functional glucose-6-phosphate hydrolases, IGRP possesses almost no hydrolase activity. This was unexpected since G6Pase-alpha is more closely related to IGRP than G6Pase-beta. Recently, amino acids 206-214 in IGRP were identified as a beta cell antigen targeted by a prevalent population of pathogenic CD8+ T cells in autoimmune diabetes, suggesting that this peptide confers functional specificity to IGRP. We therefore investigated the molecular events that inactivate IGRP activity and the effects of the beta cell antigen sequence on the stability and enzymatic activity of G6Pase-alpha.

METHODS

Studies were performed using site-directed mutagenesis and transient expression assays. Protein stability was evaluated by Western blotting, proteasome inhibitor studies and in vitro transcription-translation.

RESULTS

We showed that the residues responsible for G6Pase activity are more extensive than previously recognised. Introducing the IGRP antigenic motif into G6Pase-alpha does not completely destroy activity, although it does destabilise the protein. The low hydrolytic activity in IGRP is due to the combination of multiple independent mutations.

CONCLUSIONS/INTERPRETATION

The loss of catalytic activity in IGRP arises from the sum of many sequence differences. G6Pase-alpha mutants containing the beta cell antigen sequence are preferentially degraded in cells, which prevents targeting by pathogenic CD8+ T cells. It is possible that IGRP levels in beta cells could dictate susceptibilities to diabetes.

The stages of type 1A diabetes: 2005

Type 1A diabetes is a chronic autoimmune disease usually preceded by a long prodrome during which autoantibodies to islet autoantigens are present. These antibodies are directed to a variety of antigens, but the best characterized are glutamic acid decarboxylase-65, insulinoma-associated antigen-2, and insulin. We hypothesize that the natural history of type 1A diabetes can be represented by several stages, starting from genetic susceptibility and ending in complete beta-cell destruction and overt diabetes. Type 1A diabetes probably results from a balance between genetic susceptibility and environmental influences. In both humans and animal models, the major determinants of the disease are genes within the major histocompatibility complex. The next best-characterized susceptibility locus is the insulin gene, the variable nucleotide tandem repeat locus. This gene affects the expression of insulin in the thymus and thus may play a role in the modulation of tolerance to this molecule. In a subset of genetically susceptible individuals, the activation of autoimmunity may be triggered by environmental factors such as viruses and/or diet. However, no conclusive association has been established between type 1A diabetes and specific environmental triggers. In this review, we provide evidence that insulin has a fundamental role in anti-islet autoimmunity.

Brain contains a functional glucose-6-phosphatase complex capable of endogenous glucose production

Glucose is absolutely essential for the survival and function of the brain. In our current understanding, there is no endogenous glucose production in the brain, and it is totally dependent upon blood glucose. This glucose is generated between meals by the hydrolysis of glucose-6-phosphate (Glc-6-P) in the liver and the kidney. Recently, we reported a ubiquitously expressed Glc-6-P hydrolase, glucose-6-phosphatase-beta (Glc-6-Pase-beta), that can couple with the Glc-6-P transporter to hydrolyze Glc-6-P to glucose in the terminal stages of glycogenolysis and gluconeogenesis. Here we show that astrocytes, the main reservoir of brain glycogen, express both the Glc-6-Pase-beta and Glc-6-P transporter activities and that these activities can couple to form an active Glc-6-Pase complex, suggesting that astrocytes may provide an endogenous source of brain glucose.

A Potential New Role for Muscle in Blood Glucose Homeostasis

The breakdown of tissue glycogen into glucose is critical for blood glucose homeostasis between meals. In the final steps of glycogenolysis, intracellular glucose 6-phosphate (Glc-6-P) is transported into the endoplasmic reticulum where it is hydrolyzed to glucose by glucose-6-phosphatase (Glc-6-Pase). Although the majority of body glycogen is stored in the muscle, the current dogma holds that Glc-6-Pase (now named Glc-6-Pase-alpha) is expressed only in the liver, kidney, and intestine, implying that muscle glycogen cannot contribute to interprandial blood glucose homeostasis. Recently we reported a second Glc-6-P hydrolase, Glc-6-Pase-beta. Glc-6-Pase-beta shares kinetic and structural similarities to Glc-6-Pase-alpha and couples with the Glc-6-P transporter to form an active Glc-6-Pase complex (Shieh, J.-J., Pan, C.-J., Mansfield, B. C., and Chou, J. Y. (2003) J. Biol. Chem. 278, 47098-47103). Here we demonstrate that muscle expresses both Glc-6-Pase-beta and Glc-6-P transporter and that they can couple to form an active Glc-6-Pase complex. Our data suggest that muscle may have a previously unrecognized role in interprandial glucose homeostasis.

Selective stimulation of G-6-Pase catalytic subunit but not G-6-P transporter gene expression by glucagon in vivo and cAMP in situ

We recently compared the regulation of glucose-6-phosphatase (G-6-Pase) catalytic subunit and glucose 6-phosphate (G-6-P) transporter gene expression by insulin in conscious dogs in vivo (Hornbuckle LA, Edgerton DS, Ayala JE, Svitek CA, Neal DW, Cardin S, Cherrington AD, and O'Brien RM. Am J Physiol Endocrinol Metab 281: E713-E725, 2001). In pancreatic-clamped, euglycemic conscious dogs, a 5-h period of hypoinsulinemia led to a marked increase in hepatic G-6-Pase catalytic subunit mRNA; however, G-6-P transporter mRNA was unchanged. Here, we demonstrate, again using pancreatic-clamped, conscious dogs, that glucagon is a candidate for the factor responsible for this selective induction. Thus glucagon stimulated G-6-Pase catalytic subunit but not G-6-P transporter gene expression in vivo. Furthermore, cAMP stimulated endogenous G-6-Pase catalytic subunit gene expression in HepG2 cells but had no effect on G-6-P transporter gene expression. The cAMP response element (CRE) that mediates this induction was identified through transient transfection of HepG2 cells with G-6-Pase catalytic subunit-chloramphenicol acetyltransferase fusion genes. Gel retardation assays demonstrate that this CRE binds several transcription factors including CRE-binding protein and CCAAT enhancer-binding protein.

The islet-specific glucose-6-phosphatase-related protein, implicated in diabetes, is a glycoprotein embedded in the endoplasmic reticulum membrane

The islet-specific glucose-6-phosphatase-related protein (IGRP) has no known catalytic activity, but is of interest because it is the source of the peptide autoantigen targeted by a prevalent population of pathogenic CD8(+) T cells in non-obese diabetic mice. To better understand the potential roles of this protein in diabetes mellitus, we examine the subcellular localization and membrane topography of human IGRP. We show that IGRP is a glycoprotein, held in the endoplasmic reticulum by nine transmembrane domains, which is degraded in cells predominantly through the proteasome pathway that generates the major histocompatibility complex class I-presented peptides.

Histidine 167 is the phosphate acceptor in glucose-6-phosphatase-beta forming a phosphohistidine enzyme intermediate during catalysis

The glucose-6-phosphatase (Glc-6-Pase) family comprises two active endoplasmic reticulum (ER)-associated isozymes: the liver/kidney/intestine Glc-6-Pase-alpha and the ubiquitous Glc-6-Pase-beta. Both share similar kinetic properties. Sequence alignments predict the two proteins are structurally similar. During glucose 6-phosphate (Glc-6-P) hydrolysis, Glc-6-Pase-alpha, a nine-transmembrane domain protein, forms a covalently bound phosphoryl enzyme intermediate through His(176), which lies on the lumenal side of the ER membrane. We showed that Glc-6-Pase-beta is also a nine-transmembrane domain protein that forms a covalently bound phosphoryl enzyme intermediate during Glc-6-P hydrolysis. However, the intermediate was not detectable in Glc-6-Pase-beta active site mutants R79A, H114A, and H167A. Using [(32)P]Glc-6-P coupled with cyanogen bromide mapping, we demonstrated that the phosphate acceptor in Glc-6-Pase-beta is His(167) and that it lies inside the ER lumen with the active site residues, Arg(79) and His(114). Therefore Glc-6-Pase-alpha and Glc-6-Pase-beta share a similar active site structure, topology, and mechanism of action.

A glucose-6-phosphate hydrolase, widely expressed outside the liver, can explain age-dependent resolution of hypoglycemia in glycogen storage disease type Ia

A fine control of the blood glucose level is essential to avoid hyper- or hypo-glycemic shocks associated with many metabolic disorders, including diabetes mellitus and type I glycogen storage disease. Between meals, the primary source of blood glucose is gluconeogenesis and glycogenolysis. In the final step of both pathways, glucose-6-phosphate (G6P) is hydrolyzed to glucose by the glucose-6-phosphatase (G6Pase) complex. Because G6Pase (renamed G6Pase-alpha) is primarily expressed only in the liver, kidney, and intestine, it has implied that most other tissues cannot contribute to interprandial blood glucose homeostasis. We demonstrate that a novel, widely expressed G6Pase-related protein, PAP2.8/UGRP, renamed here G6Pase-beta, is an acid-labile, vanadate-sensitive, endoplasmic reticulum-associated phosphohydrolase, like G6Pase-alpha. Both enzymes have the same active site structure, exhibit a similar Km toward G6P, but the Vmax of G6Pase-alpha is approximately 6-fold greater than that of G6Pase-beta. Most importantly, G6Pase-beta couples with the G6P transporter to form an active G6Pase complex that can hydrolyze G6P to glucose. Our findings challenge the current dogma that only liver, kidney, and intestine can contribute to blood glucose homeostasis and explain why type Ia glycogen storage disease patients, lacking a functional liver/kidney/intestine G6Pase complex, are still capable of endogenous glucose production.

Identification and characterisation of a new human glucose-6-phosphatase isoform

The liver endoplasmic reticulum glucose-6-phosphatase catalytic subunit (G6PC1) catalyses glucose 6-phosphate hydrolysis during gluconeogenesis and glycogenolysis. The highest glucose-6-phosphatase activities are found in the liver and the kidney; there have been many reports of glucose 6-phosphate hydrolysis in other tissues. We cloned a new G6Pase isoform (G6PC3) from human brain encoded by a six-exon gene (chromosome 17q21). G6PC3 protein was able to hydrolyse glucose 6-phosphate in transfected Chinese hamster ovary cells. The optimal pH for glucose 6-phosphate hydrolysis was lower and the K(m) higher relative to G6PC1. G6PC3 preferentially hydrolyzed other substrates including pNPP and 2-deoxy-glucose-6-phosphate compared to the liver enzyme.

The three insulin response sequences in the glucose-6-phosphatase catalytic subunit gene promoter are functionally distinct

Glucose-6-phosphatase catalyzes the terminal step in the gluconeogenic and glycogenolytic pathways. In HepG2 cells, the maximum repression of basal glucose-6-phosphatase catalytic subunit (G6Pase) gene transcription by insulin requires two distinct promoter regions, designated A (located between -231 and -199) and B (located between -198 and -159), that together form an insulin response unit. Region A binds hepatocyte nuclear factor-1, which acts as an accessory factor to enhance the effect of insulin, mediated through region B, on G6Pase gene transcription. We have previously shown that region B binds the transcriptional activator FKHR (FOXO1a) in vitro. Chromatin immunoprecipitation assays demonstrate that FKHR also binds the G6Pase promoter in situ and that insulin inhibits this binding. Region B contains three insulin response sequences (IRSs), designated IRS 1, 2, and 3, that share the core sequence T(G/A)TTTT. However, detailed analyses reveal that these three G6Pase IRSs are functionally distinct. Thus, FKHR binds IRS 1 with high affinity and IRS 2 with low affinity but it does not bind IRS 3. Moreover, in the context of the G6Pase promoter, IRS 1 and 2, but not IRS 3, are required for the insulin response. Surprisingly, IRS 3, as well as IRS 1 and IRS 2, can each confer an inhibitory effect of insulin on the expression of a heterologous fusion gene, indicating that, in this context, a transcription factor other than FKHR, or its orthologs, can also mediate an insulin response through the T(G/A)TTTT motif.

Hepatocyte nuclear factor-4 alpha mediates the stimulatory effect of peroxisome proliferator-activated receptor gamma co-activator-1 alpha (PGC-1 alpha) on glucose-6-phosphatase catalytic subunit gene transcription in H4IIE cells

It has recently been shown that adenoviral-mediated expression of peroxisome proliferator-activated receptor gamma co-activator-1 alpha (PGC-1 alpha) in hepatocytes stimulates glucose-6-phosphatase catalytic subunit (G6Pase) gene expression. A combination of fusion gene, gel retardation and chromatin immunoprecipitation assays revealed that, in H4IIE cells, PGC-1 alpha mediates this stimulation through an evolutionarily conserved region of the G6Pase promoter that binds hepatocyte nuclear factor-4 alpha.