Stimulation of glucose-6-phosphatase gene expression by glucose and fructose-2,6-bisphosphate

Precision-cut liver slices as an ex vivo model to assess impaired hepatic glucose production

Fasting hypoglycemia is a severe and incompletely understood symptom of various inborn errors of metabolism (IEM). Precision-cut liver slices (PCLS) represent a promising model for studying glucose production ex vivo. This study quantified the net glucose production of human and murine PCLS in the presence of different gluconeogenic precursors. Dihydroxyacetone-supplemented slices from the fed mice yielded the highest rate, further stimulated by forskolin and dibutyryl-cAMP. Moreover, using 13C isotope tracing, we assessed the contribution of glycogenolysis and gluconeogenesis to net glucose production over time. Pharmacological inhibition of the glucose 6-phosphate transporter SLC37A4 markedly reduced net glucose production and increased lactate secretion and glycogen storage, while glucose production was completely abolished in PCLS from glycogen storage disease type Ia and Ib patients. In conclusion, this study identifies PCLS as an effective ex vivo model to study hepatic glucose production and opens opportunities for its future application in IEM research and beyond.

The role of ChREBP in carbohydrate sensing and NAFLD development

Excessive sugar consumption and defective glucose sensing by hepatocytes contribute to the development of metabolic diseases including type 2 diabetes mellitus (T2DM) and nonalcoholic fatty liver disease (NAFLD). Hepatic metabolism of carbohydrates into lipids is largely dependent on the carbohydrate-responsive element binding protein (ChREBP), a transcription factor that senses intracellular carbohydrates and activates many different target genes, through the activation of de novo lipogenesis (DNL). This process is crucial for the storage of energy as triglycerides in hepatocytes. Furthermore, ChREBP and its downstream targets represent promising targets for the development of therapies for the treatment of NAFLD and T2DM. Although lipogenic inhibitors (for example, inhibitors of fatty acid synthase, acetyl-CoA carboxylase or ATP citrate lyase) are currently under investigation, targeting lipogenesis remains a topic of discussion for NAFLD treatment. In this Review, we discuss mechanisms that regulate ChREBP activity in a tissue-specific manner and their respective roles in controlling DNL and beyond. We also provide in-depth discussion of the roles of ChREBP in the onset and progression of NAFLD and consider emerging targets for NAFLD therapeutics.

Elevated lactate in Mauriac syndrome: still a mystery

BACKGROUND

The Mauriac syndrome was described in 1930 as a peculiar combination of poorly controlled diabetes mellitus type 1, stunted growth and glycogenic hepatopathy. More recently, lactic acidosis was recognized as an additional feature, often induced by insulin treatment.

CASE PRESENTATION

A 17-year old girl known for diabetes type 1A and Mauriac syndrome was admitted to the emergency room with hyperglycemia of > 41 mmol/l without ketoacidosis. Under a standard insulin regimen, hyperglycemia was rapidly corrected but marked hyperlactatemia occurred.

CONCLUSIONS

The mechanism of impaired glucose utilization and lactate elevation independent of ketoacidosis in Mauriac syndrome is intriguing. The rarity of Mauriac syndrome and its resemblance to glycogen storage diseases suggest the presence of a specific metabolic or genetic predisposition that remains to be identified.

Postpartum supplementation with fermented ammoniated condensed whey altered nutrient partitioning to support hepatic metabolism

Our previously published paper demonstrated that fermented ammoniated condensed whey (FACW) supplementation improved feed efficiency and metabolic profile in postpartum dairy cows. The objective of this study was to further explore the effects of FACW supplementation on liver triglyceride content, hepatic gene expression and protein abundance, and plasma biomarkers related to liver function, inflammation, and damage. Individually fed multiparous Holstein cows were blocked by calving date and randomly assigned to postpartum (1 to 45 d in milk, DIM) isonitrogenous treatments: control diet (n = 20) or diet supplemented with FACW (2.9% dry matter of diet as GlucoBoost; Fermented Nutrition, Luxemburg, WI, replacing soybean meal; n = 19). Liver biopsies were performed at 14 and 28 DIM for analysis of mRNA expression, protein abundance, and liver triglyceride content. There was marginal evidence for a reduction in liver triglyceride content at 14 DIM in FACW-supplemented cows compared with the control group. Cows supplemented with FACW had greater mRNA expression of glucose-6-phosphatase at 14 DIM relative to control. Supplementation with FACW increased mRNA expression of pyruvate carboxylase (PC), but did not alter cytosolic phosphoenolpyruvate carboxykinase (PCK1), resulting in a 2.4-fold greater PC:PCK1 ratio for FACW-supplemented cows compared with control. There was no evidence for a FACW effect on mRNA expression of propionyl-CoA carboxylase nor on mRNA expression or protein abundance of lactate dehydrogenase A or B. Cows supplemented with FACW had lower plasma urea nitrogen compared with control. Plasma l-lactate was greater for FACW-supplemented cows compared with control at 2 h before feeding time at 21 DIM. There was no evidence for altered expression of IL1B or IL10, or blood biomarkers related to liver function and damage. Greater glucose-6-phosphatase and PC gene expression, together with greater blood glucose and similar milk lactose output, suggests that FACW increased the supply of glucose precursors, resulting in greater gluconeogenesis between 3 and 14 DIM. Greater hepatic PC:PCK1 ratio, together with previously reported decreased plasma β-hydroxybutyrate and the marginal evidence for lower liver triglyceride content at 14 DIM, suggests greater hepatic capacity for complete oxidation of fatty acids in FACW-supplemented cows compared with control. Overall, improvements in metabolite profile and feed efficiency observed with postpartum supplementation of FACW may be attributed to increased gluconeogenic and anaplerotic precursors, most likely propionate, due to modulated rumen fermentation.

Systemic oscillator-driven and nutrient-responsive hormonal regulation of daily expression rhythms for gluconeogenic enzyme genes in the mouse liver

Gluconeogenesis is de novo glucose synthesis from substrates such as amino acids and is vital when glucose is lacking in the diurnal nutritional fluctuation. Accordingly, genes for hepatic gluconeogenic enzymes exhibit daily expression rhythms, whose detailed regulations under nutritional variations remain elusive. As a first step, we performed general systematic characterization of daily expression profiles of gluconeogenic enzyme genes for phosphoenolpyruvate carboxykinase (PEPCK), cytosolic form (Pck1), glucose-6-phosphatase (G6Pase), catalytic subunit (G6pc), and tyrosine aminotransferase (TAT) (Tat) in the mouse liver. On a standard diet fed ad libitum, mRNA levels of these genes showed robust daily rhythms with a peak or an elevation phase during the late sleep-fasting period in the diurnal feeding/fasting (wake/sleep) cycle. The rhythmicity was preserved in constant darkness, modulated with prolonged fasting, attenuated by Clock mutation, and entrained to varied photoperiods and time-restricted feedings. These results are concordant with the notion that gluconeogenic enzyme genes are under the control of the intrinsic circadian oscillator, which is entrained by the light/dark cycle, and which in turn entrains the feeding/fasting cycle and also drives systemic signaling pathways such as the hypothalamic-pituitary-adrenal axis. On the other hand, time-restricted feedings also showed that the ingestion schedule, when separated from the light/dark cycle, can serve as an independent entrainer to daily expression rhythms of gluconeogenic enzyme genes. Moreover, nutritional changes dramatically modified expression profiles of the genes. In addition to prolonged fasting, a high-fat diet and a high-carbohydrate (no-protein) diet caused modification of daily expression rhythms of the genes, with characteristic changes in profiles of glucoregulatory hormones such as corticosterone, glucagon, and insulin, as well as their modulators including ghrelin, leptin, resistin, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-1 (GLP-1). Remarkably, high-protein (60% casein or soy-protein) diets activated the gluconeogenic enzyme genes atypically during the wake-feeding period, with paradoxical up-regulation of glucagon, which frequently formed correlation networks with other humoral factors. Based on these results, we propose that daily expression rhythms of gluconeogenic enzyme genes are under the control of systemic oscillator-driven and nutrient-responsive hormones.

Perfusion CT and PET with 18F-FDG and 18F-FCh in the complex diagnosis of hepatocellular carcinoma

Background

The purpose of the study to evaluate possibilities of CT-perfusion and PET methods with 18F-FDG and 18F-fluorocholine in the complex diagnosis of hepatocellular carcinoma. The study included the results of PET/CT with 18F-FDG, 18F-FCh and CT-perfusion of the liver in 18 patients with histologically confirmed diagnosis of hepatocellular carcinoma (HCC). Depending on the degree of tumor differentiation, all patients were divided into 3 groups - patients with highly differentiated (6 patients), moderately differentiated (4 patients), and low-differentiated HCC (8 patients).

Results

Average values of maxSUV in the group of patients with highly differentiated HCC in PET/CT with 18F-FDG and 18F- fluorocholine in a solid component of tumor reached 3.51 and 18.24, respectively; in patients with moderately differentiated HCC - 3.91 and 12.32, respectively; in patients with low-differentiated HCC - 9.58 and 9.70, respectively. Average values of CT perfusion imaging in a solid component of the tumor in the group of patients with highly differentiated HCC were the following: BF - 55,33 ml/100 ml/min, BV - 13,71 ml/100 ml, ALP - 52,41 ml/100 ml/min, PVP - 10.81 ml/100 ml/min (p ≤ 0,05), in the group of patients with moderately differentiated HCC: BF - 52,78 ml/100 m /min, BV - 12,23 ml/100 ml, ALP - 47,26 ml/100 ml/min, PVP - 9,10 ml/100 ml/min (p ≤ 0.05), in the solid component of low-differentiated HCC: BF - 46,96 ml/100 ml/min, BV - 9,49 ml/100 ml, ALP - 40.54 ml/100 ml/min, PVP - 7,66 ml/100 ml/min (p ≤ 0,05).

Conclusions

The diagnostic capabilities of the complex of PET/CT techniques with 18F-FDG and 18F-FCh and CT perfusion in a single-scan mode for hepatocellular carcinoma were evaluated for the first time. The obtained data allow to assume that the integrated use of PET with 18F-FDG and 18F-FCh and CT perfusion in a single scan improves the differential diagnostic possibilities of PET/CT diagnostics, which can find application in planning and prognosis of the disease. Due to the small number of patients further study of the problem is required.

Metabolomic characterisation of the effects of oncogenic PIK3CA transformation in a breast epithelial cell line

Somatic mutations in PIK3CA are frequently found in a number of human cancers, including breast cancer, altering cellular physiology and tumour sensitivity to chemotherapy. This renders PIK3CA an attractive molecular target for early detection and personalised therapy. Using ¹H Nuclear Magnetic Resonance spectroscopy (NMR) and Gas Chromatography – Mass Spectrometery (GC-MS) together with ¹³C stable isotope-labelled glucose and glutamine as metabolic tracers, we probed the phenotypic changes in metabolism following a single copy knock-in of mutant PIK3CA (H1047R) in the MCF10A cell line, an important cell model for studying oncogenic transformation in breast tissues. We observed effects in several metabolic pathways, including a decrease in glycerophosphocholine level together with increases in glutaminolysis, de novo fatty acid synthesis and pyruvate entry into the tricarboxylic acid cycle. Our findings highlight altered glyceroplipid metabolism and lipogenesis, as key metabolic phenotypes of mutant PIK3CA transformation that are recapitulated in the MCF10A cellular model.

Liver glucose-6-phosphatase proteins in suckling and weaned grey seal pups: structural similarities to other mammals and relationship to nutrition, insulin signalling and metabolite levels

Phocid seals have been proposed as models for diabetes because they exhibit limited insulin response to glucose, high blood glucose and increasing insulin resistance when fasting. Liver glucose-6-phosphatase (G6Pase) catalyses the final step in glucose production and is central to glucose regulation in other animals. G6Pase comprises a translocase (SLC37A4) and a catalytic subunit (G6PC). G6PC and SLC37A4 expression and activity are normally regulated by nutritional state and glucostatic hormones, particularly insulin, and are elevated in diabetes. We tested the hypotheses that (1) grey seal G6PC and SLC37A4 cDNA and predicted protein sequences differ from other species' at functional sites, (2) relative G6Pase protein abundances are lower during feeding than fasting and (3) relative G6Pase protein abundances are related to insulin, insulin receptor phosphorylation and key metabolite levels. We show that G6PC and partial SLC37A4 cDNA sequences encode proteins sharing 82-95 % identity with other mammals. Seal G6PC contained no differences in sites responsible for activity, stability or subcellular location. Several substitutions in seal SLC37A4 were predicted to be tolerated with low probability, which could affect glucose production. Suckling pups had higher relative abundance of both subunits than healthy, postweaned fasting pups. Furthermore, relative G6PC abundance was negatively related to glucose levels. These findings contrast markedly with the response of relative hepatic G6Pase abundance to feeding, fasting, insulin, insulin sensitivity and key metabolites in other animals, and highlight the need to understand the regulation of enzymes involved in glucose control in phocids if these animals are to be informative models of diabetes.

The selective control of glycolysis, gluconeogenesis and glycogenesis by temporal insulin patterns

The regulation of glucose metabolism by pulse stimulations of insulin is compared with the effect of ramp stimulations. Specific network motifs mediate the differential response to these temporal patterns of stimulations that mimic in vivo patterns of insulin secretion.

Temporal patterns and absolute concentration of insulin selectively control glycolysis, gluconeogenesis and glycogenesis through the different network motif in FAO hepatoma cells.

Step stimulation of insulin induces the transient responses and adaptations of glycolysis (via F16P) and glycogenesis through a feedforward with substrate depletion and though an incoherent feedforward loop, respectively, and induces the sustained response of gluconeogenesis (via PEPCK) through a feedforward inhibition.

Pulse stimulation of insulin, like additional secretory pattern in vivo, induces responses of glycolysis (via F16P), gluconeogenesis (via PEPCK) and glycogenesis.

Ramp stimulation of insulin, like basal secretory pattern in vivo, induces only the response of gluconeogenesis (via PEPCK), but not the responses of glycolysis (via F16P) and glycogenesis.

Insulin governs systemic glucose metabolism, including glycolysis, gluconeogenesis and glycogenesis, through temporal change and absolute concentration. However, how insulin-signalling pathway selectively regulates glycolysis, gluconeogenesis and glycogenesis remains to be elucidated. To address this issue, we experimentally measured metabolites in glucose metabolism in response to insulin. Step stimulation of insulin induced transient response of glycolysis and glycogenesis, and sustained response of gluconeogenesis and extracellular glucose concentration (GLC_ex). Based on the experimental results, we constructed a simple computational model that characterises response of insulin-signalling-dependent glucose metabolism. The model revealed that the network motifs of glycolysis and glycogenesis pathways constitute a feedforward (FF) with substrate depletion and incoherent feedforward loop (iFFL), respectively, enabling glycolysis and glycogenesis responsive to temporal changes of insulin rather than its absolute concentration. In contrast, the network motifs of gluconeogenesis pathway constituted a FF inhibition, enabling gluconeogenesis responsive to absolute concentration of insulin regardless of its temporal patterns. GLC_exwas regulated by gluconeogenesis and glycolysis. These results demonstrate the selective control mechanism of glucose metabolism by temporal patterns of insulin.

Glucotoxicity induces glucose-6-phosphatase catalytic unit expression by acting on the interaction of HIF-1α with CREB-binding protein

The activation of glucose-6-phosphatase (G6Pase), a key enzyme of endogenous glucose production, is correlated with type 2 diabetes. Type 2 diabetes is characterized by sustained hyperglycemia leading to glucotoxicity. We investigated whether glucotoxicity mechanisms control the expression of the G6Pase catalytic unit (G6pc). We deciphered the transcriptional regulatory mechanisms of the G6pc promoter by glucotoxicity in a hepatoma cell line then in primary hepatocytes and in the liver of diabetic mice. High glucose exposure induced the production of reactive oxygen species (ROS) and, in parallel, induced G6pc promoter activity. In hepatocytes, glucose induced G6pc gene expression and glucose release. The decrease of ROS concentrations by antioxidants eliminated all the glucose-inductive effects. The induction of G6pc promoter activity by glucose was eliminated in the presence of small interfering RNA, targeting either the hypoxia-inducible factor (HIF)-1α or the CREB-binding protein (CBP). Glucose increased the interaction of HIF-1α with CBP and the recruitment of HIF-1 on the G6pc promoter. The same mechanism might occur in hyperglycemic mice. We deciphered a new regulatory mechanism induced by glucotoxicity. This mechanism leading to the induction of HIF-1 transcriptional activity may contribute to the increase of hepatic glucose production during type 2 diabetes.

Fructose 2,6-bisphosphate is essential for glucose-regulated gene transcription of glucose-6-phosphatase and other ChREBP target genes in hepatocytes

Glucose metabolism in the liver activates the transcription of various genes encoding enzymes of glycolysis and lipogenesis and also G6pc (glucose-6-phosphatase). Allosteric mechanisms involving glucose 6-phosphate or xylulose 5-phosphate and covalent modification of ChREBP (carbohydrate-response element-binding protein) have been implicated in this mechanism. However, evidence supporting an essential role for a specific metabolite or pathway in hepatocytes remains equivocal. By using diverse substrates and inhibitors and a kinase-deficient bisphosphatase-active variant of the bifunctional enzyme PFK2/FBP2 (6-phosphofructo-2-kinase-fructose-2,6-bisphosphatase), we demonstrate an essential role for fructose 2,6-bisphosphate in the induction of G6pc and other ChREBP target genes by glucose. Selective depletion of fructose 2,6-bisphosphate inhibits glucose-induced recruitment of ChREBP to the G6pc promoter and also induction of G6pc by xylitol and gluconeogenic precursors. The requirement for fructose 2,6-bisphosphate for ChREBP recruitment to the promoter does not exclude the involvement of additional metabolites acting either co-ordinately or at downstream sites. Glucose raises fructose 2,6-bisphosphate levels in hepatocytes by reversing the phosphorylation of PFK2/FBP2 at Ser32, but also independently of Ser32 dephosphorylation. This supports a role for the bifunctional enzyme as the phosphometabolite sensor and for its product, fructose 2,6-bisphosphate, as the metabolic signal for substrate-regulated ChREBP-mediated expression of G6pc and other ChREBP target genes.

Elevated glucose represses liver glucokinase and induces its regulatory protein to safeguard hepatic phosphate homeostasis

OBJECTIVE

The induction of hepatic glucose 6-phosphatase (G6pc) by glucose presents a paradox of glucose-induced glucose intolerance. We tested whether glucose regulation of liver gene expression is geared toward intracellular homeostasis.

RESEARCH DESIGN AND METHODS

The effect of glucose-induced accumulation of phosphorylated intermediates on expression of glucokinase (Gck) and its regulator Gckr was determined in hepatocytes. Cell ATP and uric acid production were measured as indices of cell phosphate homeostasis.

RESULTS

Accumulation of phosphorylated intermediates in hepatocytes incubated at elevated glucose induced rapid and inverse changes in Gck (repression) and Gckr (induction) mRNA concomitantly with induction of G6pc, but had slower effects on the Gckr-to-Gck protein ratio. Dynamic metabolic labeling in mice and liver proteome analysis confirmed that Gckr and Gck are low-turnover proteins. Involvement of Max-like protein X in glucose-mediated Gck-repression was confirmed by chromatin immunoprecipitation analysis. Elevation of the Gck-to-Gckr ratio in hepatocytes was associated with glucose-dependent ATP depletion and elevated urate production confirming compromised phosphate homeostasis.

CONCLUSIONS

The lowering by glucose of the Gck-to-Gckr ratio provides a potential explanation for the impaired hepatic glucose uptake in diabetes. Elevated uric acid production at an elevated Gck-to-Gckr ratio supports a role for glucose regulation of gene expression in hepatic phosphate homeostasis.

A primary defect in glucose production alone cannot induce glucose intolerance without defects in insulin secretion

Increased glucose production is associated with fasting hyperglycaemia in type 2 diabetes but whether or not it causes glucose intolerance is unclear. This study sought to determine whether a primary defect in gluconeogenesis (GNG) resulting in elevated glucose production is sufficient to induce glucose intolerance in the absence of insulin resistance and impaired insulin secretion. Progression of glucose intolerance was assessed in phosphoenolpyruvate carboxykinase (PEPCK) transgenic rats, a genetic model with a primary increase in GNG. Young (4-5 weeks of age) and adult (12-14 weeks of age) PEPCK transgenic and Piebald Virol Glaxo (PVG/c) control rats were studied. GNG, insulin sensitivity, insulin secretion and glucose tolerance were assessed by intraperitoneal and intravascular substrate tolerance tests and hyperinsulinaemic/euglycaemic clamps. Despite elevated GNG and increased glucose appearance, PEPCK transgenic rats displayed normal glucose tolerance due to adequate glucose disposal and robust glucose-mediated insulin secretion. Glucose intolerance only became apparent in the PEPCK transgenic rats following the development of insulin resistance (both hepatic and peripheral) and defective glucose-mediated insulin secretion. Taken together, a single genetic defect in GNG leading to increased glucose production does not adversely affect glucose tolerance. Insulin resistance and impaired glucose-mediated insulin secretion are required to precipitate glucose intolerance in a setting of chronic glucose oversupply.

Functional analysis of the 5' flanking region of the human G6PC3 gene: regulation of promoter activity by glucose, pyruvate, AMP kinase and the pentose phosphate pathway

G6PC3 is a widely expressed isoform of glucose-6-phosphatase, found in many foetal and adult tissues. Mutations in this gene cause developmental abnormalities and severe neutropenia due to abolition of glucose recycling between the cytoplasm and endoplasmic reticulum. Low G6PC3 expression as a result of promoter polymorphisms or dysregulation could produce similar outcomes. Here we investigated the regulation of human G6PC3 promoter activity. HeLa and H4IIE cells were transiently transfected with G6PC3 promoter coupled to the firefly luciferase gene, and promoter activity was measured by dual luciferase assay. Activity was highest in a 453 bp segment of the G6PC3 promoter, from -455 to -3 relative to the transcriptional start site. This promoter was unresponsive to glucostatic hormones. Its activity increased significantly between 1 and 5.5 mM glucose, and was not elevated further by glucose concentrations up to 25 mM. Pyruvate increased its activity, but β-hydroxybutyrate and sodium acetate did not. Promoter activity was reduced by inhibitors of hexokinase, glyceraldehyde phosphate dehydrogenase and the oxidative branch of the pentose phosphate pathway, but not by a transketolase inhibitor. Deletion of two adjacent Enhancer-boxes (-274 to -279 and -299 to -304) reduced promoter activity and abolished the glucose effect, suggesting they could function as a glucose response element. Deletion of an additional downstream 140 bp (-140 to -306) restored activity, but not the glucose response, suggesting the presence of repressor elements in this region. 5-Aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) reduced promoter activity, showing dependence on AMP-kinase. Regulation of the G6PC3 promoter is thus radically different to that of the hepatic isoform, G6PC. It is sensitive to carbohydrate, but not to fatty acid metabolites, and at much lower physiological concentrations. Based on these findings, we speculate that reduced G6PC3 expression could occur during hypoglycemic episodes in vivo, which are common in utero and in the postnatal period. If such episodes lower G6PC3 expression they could place the foetus or infant at risk of impaired immune function and development, and this possibility requires further examination both in vitro and in vivo.

Peroxisome proliferator-activated receptor {alpha} is responsible for the up-regulation of hepatic glucose-6-phosphatase gene expression in fasting and db/db Mice

Glucose-6-phosphatase (G6Pase) is a key enzyme that is responsible for the production of glucose in the liver during fasting or in type 2 diabetes mellitus (T2DM). During fasting or in T2DM, peroxisome proliferator-activated receptor α (PPARα) is activated, which may contribute to increased hepatic glucose output. However, the mechanism by which PPARα up-regulates hepatic G6Pase gene expression in these states is not well understood. We evaluated the mechanism by which PPARα up-regulates hepatic G6Pase gene expression in fasting and T2DM states. In PPARα-null mice, both hepatic G6Pase and phosphoenolpyruvate carboxykinase levels were not increased in the fasting state. Moreover, treatment of primary cultured hepatocytes with Wy14,643 or fenofibrate increased the G6Pase mRNA level. In addition, we have localized and characterized a PPAR-responsive element in the promoter region of the G6Pase gene. Chromatin immunoprecipitation (ChIP) assay revealed that PPARα binding to the putative PPAR-responsive element of the G6Pase promoter was increased in fasted wild-type mice and db/db mice. These results indicate that PPARα is responsible for glucose production through the up-regulation of hepatic G6Pase gene expression during fasting or T2DM animal models.

O-GlcNAc modification, insulin signaling and diabetic complications

O-GlcNAc glycosylation (O-GlcNAcylation) corresponds to the addition of N-acetylglucosamine on serine and threonine residues of cytosolic and nuclear proteins. O-GlcNAcylation is a dynamic post-translational modification, analogous to phosphorylation, that regulates the stability, the activity or the subcellular localisation of target proteins. This reversible modification depends on the availability of glucose and therefore constitutes a powerful mechanism by which cellular activities are regulated according to the nutritional environment of the cell. O-GlcNAcylation has been implicated in important human pathologies including Alzheimer disease and type-2 diabetes. Only two enzymes, OGT and O-GlcNAcase, control the O-GlcNAc level on proteins. Therefore, O-GlcNAcylations cannot organize in signaling cascades as observed for phosphorylations. O-GlcNAcylations should rather be considered as a "rheostat" that controls the intensity of the signals traveling through different pathways according to the nutritional status of the cell. Thus, OGT attenuates insulin signal by O-GlcNAcylation of proteins involved in proximal and distal steps in the PI-3 kinase signaling pathway. This negative feedback may be exacerbated when cells are chronically exposed to elevated glucose concentrations and could thereby contribute to alterations in insulin signaling observed in diabetic patients. O-GlcNAcylation also appears to contribute to the deleterious effects of hyperglycaemia on excessive glucose production by the liver and deterioration of β-cell pancreatic function, resulting in worsening of hyperglycaemia (glucotoxicity). Moreover, O-GlcNAcylations directly participate in several diabetic complications. O-GlcNAcylation of eNOS in endothelial cells have been involved in micro- and macrovascular complications. In addition, O-GlcNAcylations activate the expression of profibrotic and antifibrinolytic factors, contributing to vascular and renal dysfunctions.

A retrospective review of the roles of multifunctional glucose-6-phosphatase in blood glucose homeostasis: Genesis of the tuning/retuning hypothesis

In a scientific career spanning from 1955 to 2000, my research focused on phosphoenolpyruvate carboxykinase and glucose-6-phosphatase. Grounded in basic enzymology, and initially pursuing the steady-state rate behavior of isolated preparations of these critically important gluconeogenic enzymes, our key findings were confirmed and extended by in situ enzyme rate experiments exploiting isolated liver perfusions. These efforts culminated in the discovery of the liver cytosolic isozyme of carboxykinase, known today as (GTP)PEPCK-C (EC4.1.1.32) and also revealed a biosynthetic function and multicomponent nature of glucose-6-phosphatase (EC3.1.3.9). Discovery that glucose-6-phosphatase possessed an intrinsically biosynthetic activity, now known as carbamyl-P:glucose phosphotransferase - along with a deeper consideration of the enzyme's hydrolytic activity as well as the action of liver glucokinase resulted in the evolution of Tuning/Retuning Hypothesis for blood glucose homeostasis in health and disease. This THEN & NOW review shares with the reader the joy and exhilaration of major scientific discovery and also contrasts the methodologies and approaches on which I relied with those currently in use.

Phosphorylation of IRS proteins Yin-Yang regulation of insulin signaling

Growing evidence reveals that insulin signal pathway is not static, but is rather a dynamic, flexible, and fed in by negative (Yin) and positive (Yang) regulation in response to environmental changes. Normal insulin response reflects the balance between Yin and Yang regulation acting upon insulin signaling pathway. Conceivably, imbalance between the Yin and Yang results in abnormal insulin sensitivity such as insulin resistance. IRS-proteins are insulin receptor substrates that mediate insulin signaling via multiple tyrosyl phosphorylations. However, they are also substrates for many serine/threonine kinases downstream of other signaling network and become serine phosphorylated in response to various conditions such as inflammation, stress and over nutrients. The serine phosphorylation of IRS-proteins alters the capacities of IRS-proteins to be phosphorylated on tyrosyl, therefore, able to mediate insulin signaling. The unique structure of IRS-proteins render them idea molecules to fulfill the task to sense the environmental cues and integrate them into insulin sensitivity through serine/threonine phosphorylation. This review intends to summarize the role of IRS-proteins in insulin signaling with focuses on the role of Yin and Yang regulation of insulin signaling pathway. Understanding the dynamic of these complicated regulation net work not only provide us a complete picture of what happens in the normal conditions, but also pathaphysiological conditions such as obesity and insulin resistance.

Insulin regulates the expression of several metabolism-related genes in the liver and primary hepatocytes of rainbow trout (Oncorhynchus mykiss)

Rainbow trout have a limited ability to use dietary carbohydrates efficiently and are considered to be glucose intolerant. Administration of carbohydrates results in persistent hyperglycemia and impairs post-prandial down regulation of gluconeogenesis despite normal insulin secretion. Since gluconeogenic genes are mainly under insulin control, we put forward the hypothesis that the transcriptional function of insulin as a whole may be impaired in the trout liver. In order to test this hypothesis, we performed intraperitoneal administration of bovine insulin to fasted rainbow trout and also subjected rainbow trout primary hepatocytes to insulin and/or glucose stimulation. We demonstrate that insulin was able to activate Akt, a key element in the insulin signaling pathway, and to regulate hepatic metabolism-related target genes both in vivo and in vitro. In the same way as in mammals, insulin decreased mRNA expression of gluconeogenic genes, including glucose 6-phosphatase (G6Pase),fructose 1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK). Insulin also limited the expression of carnitine palmitoyltransferase 1 (CPT1), a limiting enzyme of fatty acid β-oxidation. In vitro studies revealed that, as in mammals,glucose is an important regulator of some insulin target genes such as the glycolytic enzyme pyruvate kinase (PK) and the lipogenic enzyme fatty acid synthase (FAS). Interestingly, glucose also stimulates expression of glucokinase (GK), which has no equivalent in mammals. This study demonstrates that insulin possesses the intrinsic ability to regulate hepatic gene expression in rainbow trout, suggesting that other hormonal or metabolic factors may counteract some of the post-prandial actions of insulin.

Primary Cancer of the Liver and Biliary Duct

F-18-FDG PET and PET/CT are useful for staging, predicting the prognosis, and evaluating recurrence and treatment response in hepatocellular carcinomas (HCCs) and cholangiocarcinomas. Increased F-18-FDG uptake within tumors could be a surrogate marker of aggressive behavior and poor clinical outcome, despite high false-negative rates in detecting primary intrahepatic low-grade HCCs and periductal-infiltrating cholangiocarcinomas. Dualtracer PET or PET/CT using carbon-11-acetate and F-18-FDG will increase diagnostic performance in HCC.

O-GlcNAc modification of FoxO1 increases its transcriptional activity: a role in the glucotoxicity phenomenon?

O-GlcNAc glycosylations on serines or threonines are reversible post-translational modifications that control the localisation, the activity or the stability of cytosolic and nuclear proteins. These dynamic modifications are tightly dependent on the availability of glucose and on its flux through the hexosamine biosynthetic pathway. We recently showed that treatments that increase protein O-GlcNAc glycosylation (high-glucose concentrations, glucosamine) or inhibit their deglycosylation (PUGNAc), induced O-GlcNAc modification of FoxO1 in HEK293 cells. O-GlcNAc glycosylation of FoxO1 resulted in an increased of its activity towards a glucose 6-phosphatase promoter-luciferase reporter gene (G6Pase-luc). This effect appeared to be independent of FoxO1 sub-cellular re-localisation, since it was also observed with the constitutively nuclear FoxO1-AAA mutant. In liver-derived HepG2 cells, glucosamine and PUGNAc increased the expression of G6Pase mRNA, and synergistic effects were observed when both agents were present together. In addition, the expression of PGC1 alpha gene, which is known to be under the control of FoxO1, was also increased by glucosamine and PUGNAc. In HepG2 cells stably expressing the G6Pase-luc reporter gene, glucosamine and PUGNAc also increased the activity of the G6Pase promoter. The stimulation of the G6Pase reporter gene by these agents was abolished by two different FoxO1 siRNAs, thereby demonstrating the involvement of endogenous FoxO1 in the observed effects. Since G6Pase plays a key role in glucose production by the liver, increased in its expression through FoxO1 O-GlcNAc modification may be of considerable importance in the context of glucotoxicity associated with chronic hyperglycaemia. Moreover, since FoxO1 also plays important roles in several aspects of cell biology, including cell proliferation, survival and apoptosis, the regulation of FoxO1 activity by O-GlcNAc modification may have implications for other crucial biological processes.

Hepatic glucose sensing via the CREB coactivator CRTC2

Chronic hyperglycemia contributes to the development of diabetes-associated complications. Increases in the concentration of circulating glucose activate the hexosamine biosynthetic pathway (HBP) and promote the O-glycosylation of proteins by O-glycosyl transferase (OGT). We show that OGT triggered hepatic gluconeogenesis through the O-glycosylation of the transducer of regulated cyclic adenosine monophosphate response element-binding protein (CREB) 2 (TORC2 or CRTC2). CRTC2 was O-glycosylated at sites that normally sequester CRTC2 in the cytoplasm through a phosphorylation-dependent mechanism. Decreasing amounts of O-glycosylated CRTC2 by expression of the deglycosylating enzyme O-GlcNAcase blocked effects of glucose on gluconeogenesis, demonstrating the importance of the HBP in the development of glucose intolerance.

O-glycosylation of FoxO1 increases its transcriptional activity towards the glucose 6-phosphatase gene

Mono-O-glycosylations post-translationally regulate the activity of nucleocytoplasmic proteins. We showed that glucosamine and an inhibitor of deglycosylation (PUGNAc) induced O-glycosylation of FoxO1, resulting in increased expression of a glucose-6-phosphatase reporter gene. This effect was independent of FoxO1 re-localisation, since it was also observed with constitutively nuclear FoxO1-AAA mutant. Moreover, in HepG2 cells, glucosamine and PUGNAc have a synergistic effect on the glucose-6-phosphatase reporter gene, and this effect was inhibited by FoxO1 siRNAs. Since glucose-6-phosphatase plays a key role in hepatic glucose production, our observation may be of importance with regard to glucotoxicity associated with chronic hyperglycaemia in diabetes.

The promoter for the gene encoding the catalytic subunit of rat glucose-6-phosphatase contains two distinct glucose-responsive regions

Glucose homeostasis requires the proper expression and regulation of the catalytic subunit of glucose-6-phosphatase (G-6-Pase), which hydrolyzes glucose 6-phosphate to glucose in glucose-producing tissues. Glucose induces the expression of G-6-Pase at the transcriptional and posttranscriptional levels by unknown mechanisms. To better understand this metabolic regulation, we mapped the cis-regulatory elements conferring glucose responsiveness to the rat G-6-Pase gene promoter in glucose-responsive cell lines. The full-length (-4078/+64) promoter conferred a moderate glucose response to a reporter construct in HL1C rat hepatoma cells, which was dependent on coexpression of glucokinase. The same construct provided a robust glucose response in 832/13 INS-1 rat insulinoma cells, which are not glucogenic. Glucose also strongly increased endogenous G-6-Pase mRNA levels in 832/13 cells and in rat pancreatic islets, although the induced levels from islets were still markedly lower than in untreated primary hepatocytes. A distal promoter region was glucose responsive in 832/13 cells and contained a carbohydrate response element with two E-boxes separated by five base pairs. Carbohydrate response element-binding protein bound this region in a glucose-dependent manner in situ. A second, proximal promoter region was glucose responsive in both 832/13 and HL1C cells, with a hepatocyte nuclear factor 1 binding site and two cAMP response elements required for glucose responsiveness. Expression of dominant-negative versions of both cAMP response element-binding protein and CAAT/enhancer-binding protein blocked the glucose response of the proximal region in a dose-dependent manner. We conclude that multiple, distinct cis-regulatory promoter elements are involved in the glucose response of the rat G-6-Pase gene.

Perturbation of glucose flux in the liver by decreasing F26P2 levels causes hepatic insulin resistance and hyperglycemia

Hepatic insulin resistance is one of the characteristics of type 2 diabetes and contributes to the development of hyperglycemia. How changes in hepatic glucose flux lead to insulin resistance is not clearly defined. We determined the effects of decreasing the levels of hepatic fructose 2,6-bisphosphate (F26P(2)), a key regulator of glucose metabolism, on hepatic glucose flux in the normal 129J mice. Upon adenoviral overexpression of a kinase activity-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, the enzyme that determines F26P(2) level, hepatic F26P(2) levels were decreased twofold compared with those of control virus-treated mice in basal state. In addition, under hyperinsulinemic conditions, hepatic F26P(2) levels were much lower than those of the control. The decrease in F26P(2) leads to the elevation of basal and insulin-suppressed hepatic glucose production. Also, the efficiency of insulin to suppress hepatic glucose production was decreased (63.3 vs. 95.5% suppression of the control). At the molecular level, a decrease in insulin-stimulated Akt phosphorylation was consistent with hepatic insulin resistance. In the low hepatic F26P(2) states, increases in both gluconeogenesis and glycogenolysis in the liver are responsible for elevations of hepatic glucose production and thereby contribute to the development of hyperglycemia. Additionally, the increased hepatic gluconeogenesis was associated with the elevated mRNA levels of peroxisome proliferator-activated receptor-gamma coactivator-1alpha and phosphoenolpyruvate carboxykinase. This study provides the first in vivo demonstration showing that decreasing hepatic F26P(2) levels leads to increased gluconeogenesis in the liver. Taken together, the present study demonstrates that perturbation of glucose flux in the liver plays a predominant role in the development of a diabetic phenotype, as characterized by hepatic insulin resistance.

ChREBP*Mlx is the principal mediator of glucose-induced gene expression in the liver

In mammals, glucose-regulated gene expression has been best characterized in the liver, where increased glucose metabolism induces transcription of genes encoding enzymes involved in de novo lipogenesis. ChREBP and Mlx dimerize and function together as a glucose-responsive transcription factor to regulate target genes, such as liver-type pyruvate kinase, acetyl-CoA carboxylase 1, and fatty acid synthase. To identify additional glucose-responsive genes in the liver, we used microarray analysis to compare gene expression patterns in low and high glucose conditions in hepatocytes. Target genes of ChREBP.Mlx were simultaneously identified by gene profiling in the presence or absence of a dominant negative Mlx. Of 224 genes that are induced by glucose, 139 genes (62%) were also inhibited by the dominant negative Mlx. Lipogenic enzyme genes involved in the entire pathway of de novo lipogenesis were found to be glucose-responsive target genes of ChREBP.Mlx. Genes encoding enzymes in other metabolic pathways and numerous regulators of metabolism were also identified. To determine if any of these genes are direct targets of ChREBP.Mlx, we searched for ChoRE-like sequences in the 5'-flanking regions of several genes that responded rapidly to glucose. ChoRE sequences that bound to ChREBP.Mlx and supported a glucose response were identified in two additional genes. Combining all of the known ChoRE sequences, we generated a modified ChoRE consensus sequence, CAYGNGN(5)CNCRTG. In summary, ChREBP.Mlx is the principal transcription factor regulating glucose-responsive genes in the liver and coordinately regulates a family of genes required for glucose utilization and energy storage.

Acetic acid activates hepatic AMPK and reduces hyperglycemia in diabetic KK-A(y) mice

Acetic acid (AcOH), which is a short-chain fatty acid, is reported to have some beneficial effects on metabolism. To test the hypothesis that feeding of AcOH exerts beneficial effects on glucose homeostasis in type 2 diabetes, we fed either a standard diet or one containing 0.3% AcOH to KK-A(y) mice for 8 weeks. Fasting plasma glucose and HbA1c levels were lower in mice fed AcOH for 8 weeks than in control mice. AcOH also reduced the expression of genes involved in gluconeogenesis and lipogenesis, which is in part regulated by 5'-AMP-activated protein kinase (AMPK) in the liver. Finally, sodium acetate, in the form of neutralized AcOH, directly activated AMPK and lowered the expression of genes such as for glucose-6-phosphatase and sterol regulatory element binding protein-1 in rat hepatocytes. These results indicate that the hypoglycemic effect of AcOH might be due to activation of AMPK in the liver.

Roles for fructose-2,6-bisphosphate in the control of fuel metabolism: Beyond its allosteric effects on glycolytic and gluconeogenic enzymes

Fructose-2,6-bisphosphate (F26P2) was identified as a regulator of glucose metabolism over 25 years ago. A truly bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PFK2/FBP2), with two active sites synthesizes F26P2 from fructose-6-phosphate (F6P) and ATP or degrades F26P2 to F6P and Pi. In the classic view, F26P2 regulates glucose metabolism by allosteric effects on 6-phosphofructo-1-kinase (6PFK1, activation) and fructose-1,6-bisphosphatase (FBPase, inhibition). When levels of F26P2 are high, glycolysis is enhanced and gluconeogenesis is inhibited. In this regard, altering levels of F26P2 via 6PFK2/FBP2 overexpression has been used for metabolic modulation, and has been shown capable of restoring euglycemia in rodent models of diabetes. Recently, a number of novel observations have suggested that F26P2 has much broader effects on the enzymes of glucose metabolism. This is evidenced by the effects of F26P2 on the gene expression of two key glucose metabolic enzymes, glucokinase (GK) and glucose-6-phosphatase (G6Pase). When levels of F26P2 are elevated in the liver, the gene expression and protein amount of GK is increased whereas G6Pase is decreased. These coordinated changes in GK and G6Pase protein illustrate how F26P2 regulates glucose metabolism. F26P2 also affects the gene expression of enzymes related to lipid metabolism. When F26P2 levels are elevated in liver, the expression of two key lipogenic enzymes, acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FAS) is reduced, contributing to a unique coordinated decrease in lipogenesis. When combined, F26P2 effects on glucose and lipid metabolism provide cooperative regulation of fuel metabolism. The regulatory roles for F26P2 have also expanded to transcription factors, as well as certain key proteins (enzymes) of signaling and/or energy sensoring. Although some effects may be secondary to changes in metabolite levels, high levels of F26P2 have been shown to regulate protein amount and/or phosphorylation state of hepatic nuclear factor 1-alpha (HNF1alpha), carbohydrate response element binding protein (ChREBP), peroxisome proliferators-activated receptor alpha (PPARalpha), and peroxisome proliferators-activated receptor gamma co-activator 1beta (PGC1beta), as well as Akt and AMP-activated protein kinase (AMPK). Importantly, changes in these transcription factors, signaling proteins, and sensor proteins are produced in a way that appropriately coordinates whole body fuel metabolism.

Dual role of phosphofructokinase-2/fructose bisphosphatase-2 in regulating the compartmentation and expression of glucokinase in hepatocytes

Hepatic glucokinase is regulated by a 68-kDa regulatory protein (GKRP) that is both an inhibitor and nuclear receptor for glucokinase. We tested the role of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) in regulating glucokinase compartmentation in hepatocytes. PFK2 catalyzes formation or degradation of the regulator of glycolysis fructose 2,6-bisphosphate (fructose 2,6-P2), depending on its phosphorylation state (ser-32), and is also a glucokinase-binding protein. Incubation of hepatocytes at 25 mmol/l glucose causes translocation of glucokinase from the nucleus to the cytoplasm and an increase in fructose 2,6-P2. Glucagon caused phosphorylation of PFK2-ser-32, lowered the fructose 2,6-P2 concentration, and inhibited glucose-induced translocation of glucokinase. These effects of glucagon were reversed by expression of a kinase-active PFK2 mutant (S32A/H258A) that overrides the suppression of fructose 2,6-P2 but not by overexpression of wild-type PFK2. Overexpression of PFK2 potentiated glucokinase expression in hepatocytes transduced with an adenoviral vector-encoding glucokinase by a mechanism that does not involve stabilization of glucokinase protein from degradation. It is concluded that PFK2 has a dual role in regulating glucokinase in hepatocytes: it potentiates glucokinase protein expression by posttranscriptional mechanisms and favors its cytoplasmic compartmentation. Thus, it acts in a complementary mechanism to GKRP, which also regulates glucokinase protein expression and compartmentation.

The farnesoid X receptor modulates hepatic carbohydrate metabolism during the fasting-refeeding transition

The liver plays a central role in the control of blood glucose homeostasis by maintaining a balance between glucose production and utilization. The farnesoid X receptor (FXR) is a bile acid-activated nuclear receptor. Hepatic FXR expression is regulated by glucose and insulin. Here we identify a role for FXR in the control of hepatic carbohydrate metabolism. When submitted to a controlled fasting-refeeding schedule, FXR(-/-) mice displayed an accelerated response to high carbohydrate refeeding with an accelerated induction of glycolytic and lipogenic genes and a more pronounced repression of gluconeogenic genes. Plasma insulin and glucose levels were lower in FXR(-/-) mice upon refeeding the high-carbohydrate diet. These alterations were paralleled by decreased hepatic glycogen content. Hepatic insulin sensitivity was unchanged in FXR(-/-) mice. Treatment of isolated primary hepatocytes with a synthetic FXR agonist attenuated glucose-induced mRNA expression as well as promoter activity of L-type pyruvate kinase, acetyl-CoA carboxylase 1, and Spot14. Moreover, activated FXR interfered negatively with the carbohydrate response elements regions. These results identify a novel role for FXR as a modulator of hepatic carbohydrate metabolism.

Intestinal glucose-dependent expression of glucose-6-phosphatase: involvement of the aryl receptor nuclear translocator transcription factor

Glucose-6-phosphatase (G6Pase) catalyzes the release of glucose from glucose 6-phosphate. This enzyme was mainly studied in the liver, but while detected in the small intestine little is known about the regulation of its intestinal expression. This study describes the mechanisms of the glucose-dependent regulation of G6Pase expression in intestinal cells. Results obtained in vivo and in Caco-2/TC7 enterocytes showed that glucose increases the G6Pase mRNA level. In Caco-2/TC7 cells, glucose stabilized G6Pase mRNA and activated the transcription of the gene, meaning that glucose-dependent G6Pase expression involved both transcriptional and post-transcriptional mechanisms. Reporter-gene studies showed that, although the -299/+57 region of the human G6Pase promoter was sufficient to trigger the glucose response in the hepatoma cell line HepG2, the -1157/-1133 fragment was required for maximal activation of glucose-6-phosphatase gene transcription in Caco-2/TC7 cells. This fragment binds the aryl receptor nuclear translocator (ARNT), cAMP-responsive element-binding protein, and upstream stimulatory factor transcription factors. The DNA binding activity of these transcription factors was increased in nuclear extracts of differentiated cells from the intestinal villus of mice fed sugar-rich diets as compared with mice fed a no-sugar diet. A direct implication of ARNT in the activation of G6Pase gene transcription by glucose has been observed in Caco-2/TC7 cells using RNA interference experiments. These results support a physiological role for G6Pase in the control of nutrient absorption in the small intestine.

Molecular cloning of hepatic glucose-6-phosphatase catalytic subunit from gilthead sea bream (Sparus aurata): response of its mRNA levels and glucokinase expression to refeeding and diet composition

To examine the relationship between structure and function of glucose-6-phosphatase (G6Pase) in fish, we undertook molecular cloning and modulation of G6Pase expression by starvation and refeeding on diets with different nutrient composition in the liver of the carnivorous fish, Sparus aurata. A cDNA encoding the full-length G6Pase catalytic subunit from the liver of S. aurata was isolated. This cDNA encodes a 350-amino acid protein, with low homology to the mammalian G6Pase, although it contains most of the key residues required for catalysis. Based on hydrophobicity and membrane structure prediction, we propose a model containing nine-transmembrane regions for S. aurata G6Pase. Northern blots showed that refeeding after a prolonged starvation rapidly reverses the glucose/glucose-6-phosphate substrate cycle flux in the fish liver through decreased G6Pase expression and strong glucokinase (GK) induction. The effect of refeeding different diets on G6Pase and GK expression, indicated that hepatic intermediary metabolism of fish fed diets with low protein/high carbohydrate diets is impelled towards utilization of dietary carbohydrates, by means of modulation of GK mRNA levels rather than G6Pase expression. These findings challenge the role attributed to dysregulation of G6Pase or GK expression in the low ability of carnivorous fish to metabolise glucose.

A modest glucokinase overexpression in the liver promotes fed expression levels of glycolytic and lipogenic enzyme genes in the fasted state without altering SREBP-1c expression

Hepatic genes crucial for carbohydrate and lipid homeostasis are regulated by insulin and glucose metabolism. However, the relative contributions of insulin and glucose to the regulation of metabolic gene expression are poorly defined in vivo. To address this issue, adenovirus-mediated hepatic overexpression of glucokinase was used to determine the effects of increased hepatic glucose metabolism on gene expression in fasted or ad libitum fed rats. In the fasted state, a 3 fold glucokinase overexpression was sufficient to mimic feeding-induced increases in pyruvate kinase and acetyl CoA carboxylase mRNA levels, demonstrating a primary role for glucose metabolism in the regulation of these genes in vivo. Conversely, glucokinase overexpression was unable to mimic feeding-induced alterations of fatty acid synthase, glucose-6-phosphate dehydrogenase, carnitine palmitoyl transferase I or PEPCK mRNAs, indicating insulin as the primary regulator of these genes. Interestingly, glucose-6-phosphatase mRNA was increased by glucokinase overexpression in both the fasted and fed states, providing evidence, under these conditions, for the dominance of glucose over insulin signaling for this gene in vivo. Importantly, glucokinase overexpression did not alter sterol regulatory element binding protein 1-c mRNA levels in vivo and glucose signaling did not alter the expression of this gene in primary hepatocytes. We conclude that a modest hepatic overexpression of glucokinase is sufficient to alter expression of metabolic genes without changing the expression of SREBP-1c.

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.

Interaction of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) with glucokinase activates glucose phosphorylation and glucose metabolism in insulin-producing cells

The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) was recently identified as a new intracellular binding partner for glucokinase (GK). Therefore, we studied the importance of this interaction for the activity status of GK and glucose metabolism in insulin-producing cells by overexpression of the rat liver and pancreatic islet isoforms of PFK-2/FBPase-2. PFK-2/FBPase-2 overexpression in RINm5F-GK cells significantly increased the GK activity by 78% in cells expressing the islet isoform, by 130% in cells expressing the liver isoform, and by 116% in cells expressing a cAMP-insensitive liver S32A/H258A double mutant isoform. Only in cells overexpressing the wild-type liver PFK-2/FBPase-2 isoform was the increase of GK activity abolished by forskolin, apparently due to the regulatory site for phosphorylation by a cAMP-dependent protein kinase. In cells overexpressing any isoform of the PFK-2/FBPase-2, the increase of the GK enzyme activity was antagonized by treatment with anti-FBPase-2 antibody. Increasing the glucose concentration from 2 to 10 mmol/l had a significant stimulatory effect on the GK activity in RINm5F-GK cells overexpressing any isoform of PFK-2/FBPase-2. The interaction of GK with PFK-2/FBPase-2 takes place at glucose concentrations that are physiologically relevant for the activation of GK and the regulation of glucose-induced insulin secretion. This new mechanism of posttranslational GK regulation may also represent a new site for pharmacotherapeutic intervention in type 2 diabetes treatment.

A potential role for fructose-2,6-bisphosphate in the stimulation of hepatic glucokinase gene expression

The effects of fructose-2,6-bisphosphate (F-2,6-P(2)) on hepatic glucokinase (GK) and glucose-6-phosphatase (G-6-Pase) gene expression were investigated in streptozotocin-treated mice, which exhibited undetectable levels of insulin. Hepatic F-2,6-P(2) levels were manipulated by adenovirus-mediated overexpression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Streptozotocin treatment alone or with infusion of control adenovirus leads to a dramatic decrease in hepatic F-2,6-P(2) content compared with normal nondiabetic mice. This is accompanied by a 14-fold decrease in GK and a 3-fold increase in G-6-Pase protein levels, consistent with a diabetic phenotype. Streptozotocin-treated mice that were infused with adenovirus-overexpressing an engineered 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase with high kinase activity and little bisphosphatase activity showed high levels of hepatic F-2,6-P(2). Surprisingly, these mice had a 13-fold increase in GK protein and a 2-fold decrease in G-6-Pase protein compared with diabetic controls. The restoration of GK is associated with increases in the phosphorylation of Akt upon increasing hepatic F-2,6-P(2) content. Moreover, the changes in levels of F-2,6-P(2) and Akt phosphorylation revealed a pattern similar to that of streptozotocin mice treated with insulin, indicating that increasing hepatic content of F-2,6-P(2) mimics the action of insulin. Because G-6-Pase gene expression was down-regulated only after the restoration of euglycemia, the effect of F-2,6-P(2) was indirect. Also, the lowering of blood glucose by high F-2,6-P(2) was associated with an increase in hepatic nuclear factor 1-alpha protein, a transcription factor involved in G-6-Pase gene expression. In conclusion, F-2,6-P(2) can stimulate hepatic GK gene expression in an insulin-independent manner and can secondarily affect G-6-Pase gene expression by lowering the level of plasma glucose.

DEVELOPMENT OF RAINBOW TROUT HEPATOMA CELL LINES: EFFECT OF PRO-IGF-I EA4-PEPTIDE ON MORPHOLOGICAL CHANGES AND ANCHORAGE-INDEPENDENT GROWTH

Our laboratory has shown previously that recombinant rainbow trout Ea4 (rtEa4)-peptide of pro-insulin-like growth factor-I (pro-IGF-I) exhibited antitumor activities against cancer cell lines derived from various human cancer tissues (Chen et al., 2002; Kuo and Chen, 2002). To confirm that rtEa4-peptide can exhibit the same spectrum of antitumor activities in fish tumor cells, we had developed permanent single-cell clones (RTH1B1A, RTH1B1D, RTH1B2A, and RTH1B2C) from a rainbow trout liver tumor induced by dibenzo[a,l]pyrene treatment. At 135 passages, the doubling time of these single-cell clones in CO2-independent medium at 20 degrees C was 3.9, 3.5, 3.0, and 4.5 d, respectively. Reverse transcription-polymerase chain reaction analysis showed that the expression of liver signature genes (e.g., aldolase B, glucose-6-phosphatase [G-6-Pase], phosphoenolpyruvate carboxykinase [PEPCK], hepatic nuclear factor-1 [HNF-I], IGF-I, IGF-II, and growth hormone [GH] receptor-2 genes) and CYP1A1 and CYP1A3 genes was detected in these four single-cell clones. Furthermore, results of in vitro colony formation assay in a soft-agar medium showed different degrees of colony formation activities among them. These results confirmed that the single-cell clones were derived from the rainbow trout liver. Treatment of RTH1B1D with recombinant trout Ea4-peptide resulted in the induction of a dose-dependent morphological change and the suppression of colony formation in a soft-agar medium. In addition, both morphological change and reduction of colony formation were also observed in permanent transfectants of RTH1B1D cells carrying a trout Ea4-peptide gene or its human counterpart, hEb-peptide gene. These results confirm our earlier observations that trout pre-IGF-I Ea4-peptide and hEb possess activities counteracting malignant properties of cancer cells in vitro.

c-Myc is required for the glucose-mediated induction of metabolic enzyme genes

Glucose exerts powerful effects on hepatocyte gene transcription by mechanisms that are incompletely understood. c-Myc regulates hepatic glucose metabolism by increasing glycolytic enzyme gene transcription while concomitantly decreasing gluconeogenic and ketogenic enzyme gene expression. However, the molecular mechanisms by which c-Myc exerts these effects is not known. In this study, the glucose-mediated induction of L-type pyruvate kinase and glucose-6-phosphatase mRNA levels was diminished by maneuvers involving recombinant adenoviral vectors that interfere with (i) c-Myc protein levels by antisense expression or (ii) c-Myc function through a dominant-negative Max protein. These results were obtained using both HL1C rat hepatoma cells and primary rat hepatocytes. Furthermore, a decrease in c-Myc abundance reduced glucose production in HL1C cells, presumably by decreasing glucose-6-phosphatase activity. The repression of hormone-activated phosphoenolpyruvate carboxykinase gene transcription by glucose was not affected by a reduction in c-Myc levels. The basal mRNA levels for L-pyruvate kinase and glucose-6-phosphatase were not altered to any significant degree by adenoviral treatment. Furthermore, adenoviral overexpression of the c-Myc protein induced glucose-6-phosphatase mRNA in the absence of glucose stimulation. We conclude that multiple mechanisms exist to communicate the glucose-derived signal and that c-Myc has a key role in the hepatic glucose signaling pathway.

Glucose-6-phosphatase activity is not suppressed but the mRNA level is increased by a sucrose-enriched meal in rats

The expression of glucose-6-phosphatase (G6Pase) mRNA is repressed by insulin and stimulated by cAMP and dexamethasone, with the insulin effect dominant. Both lipids and glucose increase the expression of G6Pase mRNA under conditions in which insulin is either absent or at basal levels. The aim of the present study was to investigate dietary nutrient regulation of G6Pase mRNA and protein under postprandial conditions. Male rats (n = 6-8/group) were deprived of food for 48 h and then either remained food deprived (FD) or were refed diets containing 68% cornstarch and 12% corn oil (ST; % energy), 68% sucrose and 12% corn oil (SU) or 35% cornstarch and 45% corn oil (HF) for 3 h. Rats were anesthetized, blood was drawn from the portal vein, and the liver was removed and immediately processed for subsequent analyses. Energy intake over the 3-h refeeding period did not differ among groups (209 +/- 25 kJ). Portal vein glucose and insulin were 5.0 +/- 0.2 mmol/L and 90 +/- 18 pmol/L, respectively, in FD rats and were not significantly different among the refed groups (14.5 +/- 1.2 mmol/L and 1302 +/- 154 pmol/L, respectively). Compared with the FD rats, G6Pase mRNA was approximately 50% lower in ST and HF groups, whereas it was approximately 1.6 fold higher in SU-refed rats (P < 0.05). G6Pase activity in whole liver homogenates was lower in ST and HF rats than in FD and SU rats. Insulin receptor substrate (IRS) phosphorylation, IRS-association with phosphatidylinositol 3 (PI3)-kinase and activation of protein kinase B (PKB) were not significantly different among the refed groups. However, glycogen synthase kinase-3alpha phosphorylation was lower and cAMP response element binding protein (CREB) phosphorylation was higher in SU rats than in ST and HF refed groups. Thus, the postprandial environment after ingestion of sucrose appears to overcome the dominant effects of insulin on G6Pase mRNA, perhaps via cellular changes that reduce phosphorylation of, and therefore activate, glycogen synthase kinase-3alpha.

Nutritional regulation of glucose-6-phosphatase gene expression in liver of the gilthead sea bream (Sparus aurata)

To examine the role of glucose-6-phosphatase (G6Pase) in glucose homeostasis in the diabetes-like experimental model of carnivorous fish, we analysed postprandial variations and the effect of starvation, ration size and diet composition on the regulation of G6Pase expression at the enzyme activity and mRNA level in the liver of gilthead sea bream (Sparus aurata). G6Pase expression increased in long-term starved or energy-restricted fish. In contrast to data reported for other fish species, short-term regulation of G6Pase expression was found in regularly fed S. aurata. G6Pase mRNA levels were lowest between 4 and 15 h after food intake, whereas minimal enzyme activity was observed 10-15 h postprandially. Alterations of plasma glucose levels affect G6Pase in mammals. However, the carbohydrate content of the diet did not affect hepatic expression of G6Pase in S. aurata, suggesting that a different molecular mechanism is involved in the control of G6Pase expression in fish. Although G6Pase was unaffected, high-carbohydrate low-protein diets increased glucokinase (GK) expression and thus allowed a metabolic adaptation favouring glycolysis over gluconeogenesis. Interestingly, only the nutritional conditions that promoted variations in the blood glucose levels resulted in changes in the hepatic expression of G6Pase. These findings indicate a concerted regulation of G6Pase and GK expression and suggest that the direction and rate of the glucose-glucose-6-phosphate substrate cycle flux is finely regulated in the liver of S. aurata, challenging the role attributed to deficient regulation of G6Pase or GK expression in the low ability of carnivorous fish to metabolize glucose.

Diets enriched in sucrose or fat increase gluconeogenesis and G-6-Pase but not basal glucose production in rats

High-fat (HFD) and high-sucrose diets (HSD) reduce insulin suppression of glucose production in vivo, increase the capacity for gluconeogenesis in vitro, and increase glucose-6-phosphatase (G-6-Pase) activity in whole cell homogenates. The present study examined the effects of HSD and HFD on in vivo gluconeogenesis, the catalytic and glucose-6-phosphate translocase subunits of G-6-Pase, glucokinase (GK) translocation, and glucose cycling. Rats were fed a high-starch control diet (STD; 68% cornstarch), HSD (68% sucrose), or HFD (45% fat) for 7-13 days. The ratio of 3H in C6:C2 of glucose after 3H2O injection into 6- to 8-h-fasted rats was significantly increased in HSD (0.68 +/- 0.07) and HFD (0.71 +/- 0.08) vs. STD (0.40 +/- 0.10). G-6-Pase activity was significantly higher in HSD and HFD vs. STD in both intact and disrupted liver microsomes. HSD and HFD significantly increased the amount of the p36 catalytic subunit protein, whereas the p46 glucose-6-phosphate translocase protein was increased in HSD only. Despite increased nonglycerol gluconeogenesis and increased G-6-Pase, basal glucose and insulin levels as well as glucose production were not significantly different among groups. Hepatocyte cell suspensions were used to ascertain whether diet-induced adaptations in glucose phosphorylation and GK might serve to compensate for upregulation of G-6-Pase. Tracer-estimated glucose phosphorylation and glucose cycling (glucose <--> glucose 6-phosphate) were significantly higher in cells isolated from HSD only. After incubation with either 5 or 20 mM glucose and no insulin, GK activity (nmol. mg protein(-1). min(-1)) in digitonin-treated eluates (translocated GK) was significantly higher in HSD (32 +/- 4 and 146 +/- 6) vs. HFD (4 +/- 1 and 83 +/- 10) and STD (9 +/- 2 and 87 +/- 9). Thus short-term, chronic exposure to HSD and HFD increase in vivo gluconeogenesis and the G-6-Pase catalytic subunit. Exposure to HSD diet also leads to adaptations in glucose phosphorylation and GK translocation.

Glycemic control determines hepatic and peripheral glucose effectiveness in type 2 diabetic subjects

Glucose effectiveness is impaired in type 2 diabetes. We hypothesize that chronic hyperglycemia and hyperlipidemia contribute importantly to this defect. To test this hypothesis, we compared the effect of acute hyperglycemia on glucose turnover in type 2 diabetic subjects in good control (GC) (n = 14, age 51.7 +/- 3.7 years, BMI 28.4 +/- 1.0 kg/ m(2), HbA(1c) 5.9 +/- 0.2%) and poor control (PC) (n = 10, age 50.0 +/- 2.5 years, BMI 27.9 +/- 1.5 kg/m(2), HbA(1c) 9.9 +/- 0.6%) with age- and weight-matched nondiabetic subjects (ND) (n = 11, age 47.0 +/- 4.4 years, BMI 28.5 +/- 1.0 kg/m(2), HbA(1c) 5.1 +/- 0.2%). Fixed hormonal conditions were attained by infusing somatostatin for 6 h with replacement of basal insulin, glucagon, and growth hormone. Glucose fluxes ([3-(3)H]glucose) were compared during euglycemic (5 mmol/l, t = 180-240 min) and hyperglycemic (Hy) (10 mmol/l, t = 300-360 min, variable glucose infusion) clamp intervals. Acute hyperglycemia suppressed hepatic glucose production (GP) by 43% and increased peripheral glucose uptake (GU) by 86% in the ND subjects. Conversely, GP failed to suppress (-7%) and GU was suboptimally increased (+34%) in response to Hy in the PC group. However, optimal glycemic control was associated with normal glucose effectiveness in GC subjects (GP -38%, GU +72%; P > 0.05 for GC vs. ND). To determine whether short-term correction of hyperglycemia and/or hyperlipidemia is sufficient to reverse the impairment in glucose effectiveness, five PC subjects were restudied after 72 h of normoglycemia ( approximately 100 mg/dl; variable insulin infusions). These subjects regained normal effectiveness of glucose to suppress GP and stimulate GU and in response to Hy (GP -47%, GU + 71%; P > 0.05 vs. baseline studies). Thus, chronic hyperglycemia and/or hyperlipidemia contribute to impaired effectiveness of glucose in regulating glucose fluxes in type 2 diabetes and hence to worsening of the overall metabolic condition. Short-term normalization of plasma glucose might break the vicious cycle of impaired glucose effectiveness in type 2 diabetes.

Phosphoenolpyruvate carboxykinase overexpression selectively attenuates insulin signaling and hepatic insulin sensitivity in transgenic mice

The ability of insulin to suppress gluconeogenesis in type II diabetes mellitus is impaired; however, the cellular mechanisms for this insulin resistance remain poorly understood. To address this question, we generated transgenic (TG) mice overexpressing the phosphoenolpyruvate carboxykinase (PEPCK) gene under control of its own promoter. TG mice had increased basal hepatic glucose production (HGP), but normal levels of plasma free fatty acids (FFAs) and whole-body glucose disposal during a hyperinsulinemic-euglycemic clamp compared with wild-type controls. The steady-state levels of PEPCK and glucose-6-phosphatase mRNAs were elevated in livers of TG mice and were resistant to down-regulation by insulin. Conversely, GLUT2 and glucokinase mRNA levels were appropriately regulated by insulin, suggesting that insulin resistance is selective to gluconeogenic gene expression. Insulin-stimulated phosphorylation of the insulin receptor, insulin receptor substrate (IRS)-1, and associated phosphatidylinositol 3-kinase were normal in TG mice, whereas IRS-2 protein and phosphorylation were down-regulated compared with control mice. These results establish that a modest (2-fold) increase in PEPCK gene expression in vivo is sufficient to increase HGP without affecting FFA concentrations. Furthermore, these results demonstrate that PEPCK overexpression results in a metabolic pattern that increases glucose-6-phosphatase mRNA and results in a selective decrease in IRS-2 protein, decreased phosphatidylinositol 3-kinase activity, and reduced ability of insulin to suppress gluconeogenic gene expression. However, acute suppression of HGP and glycolytic gene expression remained intact, suggesting that FFA and/or IRS-1 signaling, in addition to reduced IRS-2, plays an important role in downstream insulin signal transduction pathways involved in control of gluconeogenesis and progression to type II diabetes mellitus.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

Fructose improves the ability of hyperglycemia per se to regulate glucose production in type 2 diabetes

The ability of hyperglycemia per se to suppress endogenous glucose production (GP) is blunted in type 2 diabetes. This could be due in part to decreased glucose-induced flux through glucokinase (GK). Because fructose activates hepatic GK, we examined whether catalytic amounts of fructose could restore inhibition of GP by hyperglycemia in humans with type 2 diabetes. Glucose fluxes ([3-(3)H]glucose) were measured during euglycemia (5 mmol/l) and after abrupt onset of hyperglycemia (10 mmol/l; variable dextrose infusion) under fixed hormonal conditions (somatostatin infusion for 6 h with basal insulin/glucagon/growth hormone replacement). A total of 10 subjects with moderately controlled type 2 diabetes and 7 age- and BMI-matched nondiabetic subjects were studied on up to three separate occasions under the following conditions: without fructose (F(-)) or with infusion of fructose at two dosages: 0.6 mg/kg center dot min (low F) and 1.8 mg/kg center dot min (high F). Although GP failed to decrease in response to hyperglycemia in type 2 diabetes, the coinfusion of both doses of fructose was associated with comparable decreases in GP in response to hyperglycemia (low F = -27%, high F = -33%; P < 0.01 vs. F(-) at both dosages), which approached the 44% decline in GP observed without fructose in the nondiabetic subjects. GP responses to hyperglycemia were not altered by the addition of fructose in the nondiabetic group (low F = -47%, high F = -42%; P > 0.05 vs. F(-)). Thus, the administration of small amounts of fructose to type 2 diabetic subjects partially corrected the regulation of GP by hyperglycemia per se, yet did not affect this regulation in the nondiabetic subjects. This suggests that the liver's inability to respond to hyperglycemia in type 2 diabetes, likely caused by impaired GK activity, contributes substantially to the increased GP in these individuals.