Glucose administration induces the premature expression of liver glucokinase gene in newborn rats. Relation with DNase-I-hypersensitive sites

Integrated control of hepatic lipogenesis versus glucose production requires FoxO transcription factors

Insulin integrates hepatic glucose and lipid metabolism, directing nutrients to storage as glycogen and triglyceride. In type 2 diabetes, levels of the former are low and the latter are exaggerated, posing a pathophysiologic and therapeutic conundrum. A branching model of insulin signaling, with FoxO1 presiding over glucose production and Srebp–1c regulating lipogenesis, provides a potential explanation. Here we illustrate an alternative mechanism that integrates glucose production and lipogenesis under the unifying control of FoxO. Liver–specific ablation of three FoxOs (L–FoxO1,3,4) prevents the induction of glucose–6–phosphatase and the repression of glucokinase during fasting, thus increasing lipogenesis at the expense of glucose production. We document a similar pattern in the early phases of diet-induced insulin resistance, and propose that FoxOs are required to enable the liver to direct nutritionally derived carbons to glucose vs. lipid metabolism. Our data underscore the heterogeneity of hepatic insulin resistance during progression from the metabolic syndrome to overt diabetes, and the conceptual challenge of designing therapies that curtail glucose production without promoting hepatic lipid accumulation.

Expression of the human glucokinase gene: important roles of the 5' flanking and intron 1 sequences

Background

Glucokinase plays important tissue-specific roles in human physiology, where it acts as a sensor of blood glucose levels in the pancreas, and a few other cells of the gut and brain, and as the rate-limiting step in glucose metabolism in the liver. Liver-specific expression is driven by one of the two tissue-specific promoters, and has an absolute requirement for insulin. The sequences that mediate regulation by insulin are incompletely understood.

Methodology/Principal Findings

To better understand the liver-specific expression of the human glucokinase gene we compared the structures of this gene from diverse mammals. Much of the sequence located between the 5′ pancreatic beta-cell-specific and downstream liver-specific promoters of the glucokinase genes is composed of repetitive DNA elements that were inserted in parallel on different mammalian lineages. The transcriptional activity of the liver-specific promoter 5′ flanking sequences were tested with and without downstream intronic sequences in two human liver cells lines, HepG2 and L-02. While glucokinase liver-specific 5′ flanking sequences support expression in liver cell lines, a sequence located about 2000 bases 3′ to the liver-specific mRNA start site represses gene expression. Enhanced reporter gene expression was observed in both cell lines when cells were treated with fetal calf serum, but only in the L-02 cells was expression enhanced by insulin.

Conclusions/Significance

Our results suggest that the normal liver L-02 cell line may be a better model to understand the regulation of the liver-specific expression of the human glucokinase gene. Our results also suggest that sequences downstream of the liver-specific mRNA start site have important roles in the regulation of liver-specific glucokinase gene expression.

Hypermethylation of hepatic Gck promoter in ageing rats contributes to diabetogenic potential

AIMS/HYPOTHESIS

Hepatic glucokinase (GCK) is a key enzyme in glucose utilisation. Downregulation of its activity is associated with insulin resistance and type 2 diabetes mellitus. However, it is unknown whether hepatic Gck expression is influenced by age and is involved in ageing-mediated diabetes, and whether the degree of methylation of the hepatic Gck promoter is correlated with the transcription of Gck. To address the question, we evaluated hepatic Gck transcription and promoter methylation in young (14 weeks), adult (40 weeks) and aged (80 weeks) rats.

METHODS

Hepatic glycogen, Gck expression and the kinase activity of GCK were measured in three age groups. The CpG methylation status was determined by both bisulphite direct sequencing and clone sequencing of the PCR amplificates of Gck promoter. The causal relationship between Gck methylation and mRNA expression was confirmed by treating rat primary hepatocytes with 5-aza-2'-deoxycytidine (5-Aza-CdR).

RESULTS

We have shown an age-associated decline in hepatic glycogen, Gck expression levels and the kinase activity of hepatic GCK. The eleven CpG sites studied displayed age-related progressive methylation changes in hepatic Gck promoter, which were confirmed by two methods: direct and clone sequencing. After 5-Aza-CdR treatment of rat primary hepatocytes, there was a fourfold increase in Gck expression.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that an age-related increase in methylation is negatively associated with hepatic Gck expression, suggesting that DNA methylation could be involved in increasing age-dependent susceptibility to hepatic insulin resistance and diabetes. Thus, the epigenetic modification of the hepatic Gck promoter may represent an important marker for diabetogenic potential during the ageing process.

Identification of a novel rat hepatic gene induced early by insulin, independently of glucose

We used mRNA differential display to identify new genes induced early after exposure to insulin. Our screening strategy was based on the comparison of gene expression during the time course of insulin induction in the liver of 12-day-old suckling rats both in vivo and in vitro. A novel, early induced transcript, EIIH, was identified that encodes a 353-amino-acid protein with several features suggesting that it may be secreted or bound to membranes. EIIH is also distantly related to a variety of LRR (leucine-rich repeat) proteins. Insulin treatment increased EIIH mRNA levels in the hepatocytes of suckling, fasted adult and STZ (streptozotocin)-treated diabetic rats, where insulin was required to maintain the basal level of EIIH expression. EIIH expression was induced during the suckling/weaning transition, and remained detectable thereafter. Tissue distribution analysis in adult rats revealed a pattern of expression mainly in the liver, intestine and islets of Langerhans, closely following that of the Glut2 (glucose transporter 2), suggesting that it may play a role in carbohydrate metabolism. EIIH may be a primary target of the transcriptional regulation by insulin, and may therefore constitute a new model to study the mechanisms by which insulin acts on gene transcription.

Differential regulation of sterol regulatory element-binding protein 1c transcriptional activity by insulin and liver X receptor during liver development

Sterol regulatory element-binding proteins (SREBPs) are transcription factors involved in the synthesis of cholesterol and fatty acids. In adults, the isoform SREBP-1c is the predominant transcript in the liver of fed animals, and it activates triglyceride production from glucose when diet is enriched in carbohydrates. Studies have shown that SREBP-1c expression is dependent on insulin but also on the availability of oxysterols, ligands of the nuclear liver X receptor (LXR). The aim of this study was to investigate the regulation of the hepatic SREBP-1c expression in vivo in situations where drastic nutritional and hormonal changes occur, from the gestation to the weaning period. In this paper, we report the discovery of LXR-independent SREBP-1c transcriptional activity during late gestation. In utero insulin injection prior to the natural rise in insulin in late gestation triggers SREBP-1c mRNA elevation, nuclear SREBP-1c binding activity, and expression of its target genes independently of LXR transactivation. On the other hand, during suckling, we observed strong SREBP-1c mRNA expression despite very low plasma insulin, an expression that may be due to LXR transactivation. In contrast to insulin, LXR is not sufficient to trigger nuclear SREBP-1c binding activity and target gene induction. This could be due to the concomitant induction of INSIG-2a by LXR and subsequent retention of SREBP-1c in the endoplasmic reticulum.

Insulin resistance of gluconeogenic pathways in neonatal rats after prenatal ethanol exposure

Alcohol exposure during pregnancy is associated with fetal growth restriction and programs the offspring to insulin resistance later in life. The underlying mechanisms are still uncertain, but a dysregulation of gluconeogenesis and adipose hormones may be contributory. Newborn rats from dams that had been given ethanol (EtOH) or water (controls) during pregnancy were studied. Adiponectin mRNA was determined in subcutaneous fat by RT-PCR, and serum adiponectin was measured by RIA. Subsets of rats were killed before and after intraperitoneal administration of insulin, to determine, by RT-PCR, the hepatic expression of gluconeogenic enzymes and that of the transcription factor peroxisome proliferator-activated receptor-coactivator (PGC)-1, which promotes gluconeogenesis. EtOH offspring had delayed hypoglycemic response to insulin but normal adiponectin mRNA and serum levels compared with controls. The inhibitory response of the gluconeogenic enzyme phospho enol- pyruvate carboxykinase (PEPCK) and PGC-1 mRNAs to insulin was blunted in EtOH offspring compared with controls. The data suggest that intrauterine EtOH exposure causes insulin resistance of genes for PGC-1 and PEPCK early in life.

Investigation of the role of epigenetic modification of the rat glucokinase gene in fetal programming

Fetal malnutrition is associated with development of impaired glucose tolerance, diabetes and hypertension in later life in humans and several mammalian species. The mechanisms that underlie this phenomenon of fetal programming are unknown. We hypothesize that adverse effects in utero and early life may influence the basal expression levels of certain genes such that they are re-set with long-term consequences for the organism. An excellent candidate mechanism for this re-setting process is DNA methylation, since post-natal methylation patterns are largely established in utero. We have sought to test this hypothesis by investigating the glucokinase gene (Gck) in rat offspring programmed using a maternal low protein diet model (MLP). Northern blot reveals that fasting levels of Gck expression are reduced after programming, although this distinction disappears after feeding. Bisulphite sequencing of the hepatic Gck promoter indicates a complete absence of methylation at the 12 CpG sites studied in controls and MLP animals. Non-expressing cardiac tissue also showed no DNA methylation in this region, whereas brain and all fetal tissues were fully methylated. These findings are not consistent with the hypothesis that programming results from differential methylation of Gck. However, it remains possible that programming may influence methylation patterns in Gck at a distance from the promoter, or in genes encoding factors that regulate basal Gck expression.

Activation of glucokinase gene expression by hepatic nuclear factor 4alpha in primary hepatocytes

Glucokinase (GK) is a key enzyme for glucose utilization in liver and shows a higher expression in the perivenous zone. In primary rat hepatocytes, the GK gene expression was activated by HNF (hepatic nuclear factor)-4alpha via the sequence -52/-39 of the GK promoter. Venous pO2 enhanced HNF-4 levels and HNF-4 binding to the GK-HNF-4 element. Thus, HNF-4alpha could play the role of a regulator for zonated GK expression.

Long-chain fatty acids regulate liver carnitine palmitoyltransferase I gene (L-CPT I) expression through a peroxisome-proliferator-activated receptor alpha (PPARalpha)-independent pathway

Liver carnitine palmitoyltransferase I (L-CPT I) catalyses the transfer of long-chain fatty acid (LCFA) for translocation across the mitochondrial membrane. Expression of the L-CPT I gene is induced by LCFAs as well as by lipid-lowering compounds such as clofibrate. Previous studies have suggested that the peroxisome-proliferator-activated receptor alpha (PPARalpha) is a common mediator of the transcriptional effects of LCFA and clofibrate. We found that free LCFAs rather than acyl-CoA esters are the signal metabolites responsible for the stimulation of L-CPT I gene expression. Using primary culture of hepatocytes we found that LCFAs failed to stimulate L-CPT I gene expression both in wild-type and PPARalpha-null mice. These results suggest that the PPARalpha-knockout mouse does not represent a suitable model for the regulation of L-CPT I gene expression by LCFAs in the liver. Finally, we determined that clofibrate stimulates L-CPT I through a classical direct repeat 1 (DR1) motif in the promoter of the L-CPT I gene while LCFAs induce L-CPT I via elements in the first intron of the gene. Our results demonstrate that LCFAs can regulate gene expression through PPARalpha-independent pathways and suggest that the regulation of gene expression by dietary lipids is more complex than previously proposed.

Sterol regulatory element binding protein-1c is a major mediator of insulin action on the hepatic expression of glucokinase and lipogenesis-related genes

Hepatic glucokinase plays a key role in glucose metabolism as underlined by the anomalies associated with glucokinase mutations and the consequences of tissue-specific knock-out. In the liver, glucokinase transcription is absolutely dependent on the presence of insulin. The cis-elements and trans-acting factors that mediate the insulin effect are presently unknown; this is also the case for most insulin-responsive genes. We have shown previously that the hepatic expression of the transcription factor sterol regulatory element binding protein-1c (SREBP-1c) is activated by insulin. We show here in primary cultures of hepatocytes that the adenovirus-mediated transduction of a dominant negative form of SREBP-1c inhibits the insulin effect on endogenous glucokinase expression. Conversely, in the absence of insulin, the adenovirus-mediated transduction of a dominant positive form of SREBP-1c overcomes the insulin dependency of glucokinase expression. Hepatic fatty acid synthase and Spot-14 are insulin/glucose-dependent genes. For this latter class of genes, the dominant positive form of SREBP-1c obviates the necessity for the presence of insulin, whereas glucose potentiates the effect of SREBP-1c on their expression. In addition, the insulin dependency of lipid accumulation in cultured hepatocytes is overcome by the dominant positive form of SREBP-1c. We propose that SREBP-1c is a major mediator of insulin action on hepatic gene expression and a key regulator of hepatic glucose/lipid metabolism.

Expression and regulation of glucokinase in rat islet beta- and alpha-cells during development

Glucokinase (GK) is the rate-limiting enzyme in the glycolytic pathway of the beta-cell and, even in the rat fetus at 22-days gestation, immediately before birth, acts as a sensor of glucose influencing the rate of glucose utilization. However, when GK first appears in islets during beta-cell development is unknown. Whether GK is expressed in fetal glucagon-producing cells is also unknown. To determine this information, fetal rat islets were examined at 16-, 18-, and 22-days gestation. GK was identified immunocytochemically in both beta- and alpha-cells at all these ages, with the number of GK immunoreactive cells positively correlated to the fetal age from 16-22 days. Western blot analysis of islet protein extracts demonstrated the presence of GK, at 52 kDa, at 16 days and thereafter. To determine whether glucose had any effect on regulation of GK biosynthesis, fetal islets were cultured in medium containing a wide range of concentrations of glucose for 7 days. The amount of GK protein was significantly decreased in low concentrations of glucose and augmented at high concentrations. In conclusion, GK was expressed in both beta- and alpha-cells in fetal rat islets during development. GK is an integral part of the function of both of these cells at all stages in the development of the fetal islet.

Fetal growth and long-term consequences in animal models of growth retardation

Perturbations of the maternal environment involve an abnormal intrauterine milieu for the developing fetus. The altered fuel supply (depends on substrate availability, placental transport of nutrients and uteroplacental blood flow) from mother to fetus induces alterations in the development of the fetal endocrine pancreas and adaptations of the fetal metabolism to the altered intrauterine environment, resulting in intrauterine growth retardation. The alterations induced by maternal diabetes or maternal malnutrition (protein-calorie or protein deprivation) have consequences for the offspring, persisting into adulthood and into the next generation.

Variable expression of hepatic glucokinase in mice is due to a regulational locus that cosegregates with the glucokinase gene

The Gk activity locus affects expression of hepatic glucokinase (GK) in mice. Analysis of microsatellites within the mouse GK gene locus revealed two major haplotypes in 19 of 22 inbred strains predictive of either high or low hepatic GK gene expression. C3H/HeJ mice, a high-activity strain, and two other wild-derived strains contain less common haplotypes. No coding sequence differences were found in hepatic GK-coding sequences from representative high and low Gk activity strains, thereby excluding kinetic abnormalities as the basis for hepatic GK activity differences. Screening of approximately 10 kb of potential regulatory DNA, including all eight known and three of four newly identified DNase I-hypersensitive sites, by restriction enzyme fingerprinting-single-strand conformation analysis revealed a tetranucleotide microsatellite, the length of which was also predictive of the Gk activity phenotype. This tetranucleotide repeat is located in the first intron of the hepatic transcription unit and lies close to a newly identified liver-specific DNase I-hypersensitive site. These results indicate that the Gk activity alleles are a regulational locus associated with the GK gene locus.

Liver-specific enhancer of the glucokinase gene

Glucokinase gene regions that are important for liver specific expression of the enzyme have been functionally identified using transient transfection of rat hepatocytes. Maximal luciferase activity was elicited by a reporter plasmid with 3.4 kilobase pairs of genomic DNA flanking the liver glucokinase promoter. Deletion of a gene fragment between -1000 and -600 with respect to the start of transcription resulted in a 60% decrease in luciferase activity. Further reduction, close to background level, occurred upon deletion of a 90-base pair sequence between -123 and -34. Reporter plasmids with the liver glucokinase promoter and any length of flanking sequence were minimally active in INS-1 insulinoma cells, and conversely reporters with the beta-cell-specific promoter were ineffective in primary hepatocytes. In FTO-2B hepatoma cells, a differentiated line expressing many liver-specific traits but not the endogenous glucokinase gene, the promoter proximal region between -123 and -34 markedly stimulated the expression of transfected plasmids above background. However, addition of the flanking region up to -1000 inhibited luciferase expression. The gene fragment from -1003 to -707 was shown to be a bona fide, hepatocyte-specific enhancer by the following criteria: 1) it stimulated reporter expression by more than 10- and 5-fold when inserted directly upstream of the glucokinase TATA box or complete promoter, respectively, regardless of orientation; 2) it stimulated gene expression from the heterologous SV 40 promoter 4-fold; 3) it was also effective from a downstream position; and 4) in contrast to the enhancer effect in primary hepatocytes, the sequence acted as a silencer in FTO-2B cells and was neutral in INS-1 cells. Both the promoter proximal and the enhancer regions were marked by DNase I hypersensitive sites in the chromatin of primary hepatocytes but not hepatoma or insulinoma cells. Seven footprinted elements termed A through G were mapped in the enhancer by the in vitro DNase I protection assay. Elements A-C may bind liver enriched factors, because they were not protected by spleen nuclear extract. In hepatocyte transfection, the downstream half of the enhancer containing elements A-C was about half as effective as the complete enhancer in stimulating glucokinase promoter activity. Site-directed mutagenesis of element A virtually abrogated the activity of the half-enhancer, whereas mutation of element C had a more moderate effect. The sequence between -732 and -578 upstream of the liver start of transcription in the human glucokinase gene displays 79% sequence identity with the downstream half of the rat enhancer. The human gene fragment ligated to the minimal rat liver glucokinase promoter was shown to work as an enhancer in the hepatocyte transfection system.

Induction of the glucokinase gene by insulin in cultured neonatal rat hepatocytes. Relationship with DNase-I hypersensitive sites and functional analysis of a putative insulin-response element

Previous, in vivo experiments have shown that an appropriate hormonal environment (high plasma insulin, low plasma glucagon) was unable to induce the accumulation of glucokinase mRNA in term fetal rat liver, whereas it was very efficient in the newly born rat. We have confirmed in the present study that insulin induced the accumulation of glucokinase mRNA in cultured hepatocytes from 1-day-old newborn rats, but not in cultured hepatocytes from 21-day-old fetuses. To identify regulatory regions of the glucokinase gene involved in the insulin response, we have scanned the glucokinase locus for DNase I hypersensitive sites in its in vivo conformation. We confirmed the presence of four liver-specific DNase I hypersensitive sites located in the 5' flanking region of the gene. Moreover, two additional hypersensitive sites, located at 2.5 kb and 3.5 kb upstream of the cap site were found but none of these new sites displayed inducibility by insulin. Finally, an increase of the sensitivity of hypersensitive site-1 and hypersensitive site-2 to DNase I correlates with the ability of insulin to induce glucokinase gene expression in cultured hepatocytes from 1-day-old rats, as observed in previous in vivo studies. This suggests that neither a prior exposure to insulin nor a simple aging of the fetal cells in the presence of the hormone in culture are instrumental for the full DNase-I hypersensitivity of the two proximal sites necessary for the neonatal response of the glucokinase gene to insulin. The proximal hypersensitive site-1, which is close to the transcription start site in the liver, does coincide with a sequence (designated IRSL) that is 80% identical to the phosphoenolpyruvate carboxykinase IRS and with a DNase-I footprint that has been identified overlapping this sequence. Nevertheless, functional analysis of this sequence suggested that it is unlikely that the insulin-response sequence like alone is sufficient to mediate the transcriptional effect of insulin on the hepatic glucokinase gene.

Changes in subcellular and zonal distribution of glucokinase in rat liver during postnatal development

Subcellular and zonal distribution of glucokinase in rat liver during postnatal development was examined immunohistochemically. Before day 11 after birth, only some hepatocytes were immunostained, and a positive immunostaining was found in the cytoplasm but not in the nucleus. No zonal distribution of glucokinase was observed in livers of such pups. From day 15, at which time a dietary change from milk to laboratory chow begins to take place, glucokinase immunoreactivity increased; this increase was associated with increases in glucokinase activity and in glucokinase protein, and also the immunostaining was observed mainly in the nuclei. At day 21, the glucokinase immunoreactivity was found almost exclusively in the perivenous zone. At day 30, an intense immunostaining was seen both in the perivenous zone and in the periportal zone, being slightly predominant in the former. The present results indicate that dramatic changes in the distribution of glucokinase in developing rat liver may be related to dietary change.

Initial expression of glucokinase gene in cultured hepatocytes from suckling rats is linked to the synthesis of an insulin-dependent protein

The initial accumulation of glucokinase mRNA in response to insulin in cultured hepatocytes from 10-day-old suckling rats was characterized by a delay of 18-24 h with a maximal level reached after 48 h. This delay is not observed in cultured adult rat hepatocytes. When hepatocytes from 10-day-old suckling rats were cultured for 48 h in the presence of insulin (to obtain a maximal accumulation of glucokinase mRNA) and then deprived of insulin for 18 h, glucokinase mRNA returned to very low levels. Reexposure of these cultured hepatocytes to insulin allowed a rapid accumulation of glucokinase mRNA, with a maximal level reached after 8 h, as in adult rat hepatocytes. The aim of the present study was to investigate the factors responsible for the delay in insulin action during first exposure to insulin. The difference in the kinetics of glucokinase mRNA accumulation after the first and secondary exposure to insulin was due to differences in the rate of transcriptional activity of the glucokinase gene, as shown by a run-on assay on isolated nuclei. The half-life of glucokinase mRNA was similar after the first and second exposure to insulin. The delay in the initial accumulation of glucokinase mRNA in response to the first exposure to insulin was not due to elevated levels of cAMP (a potent inhibitor of glucokinase gene expression) or to a defect in insulin signalling (insulin inhibited without delay phosphoenolpyruvate carboxykinase gene expression). In contrast, it was markedly dependent upon whether glucokinase has been already expressed in vivo. Hepatocytes from rats that had already expressed glucokinase in vivo (suckling rats force-fed with glucose or rats weaned to a high-carbohydrate diet) showed no delay in their response to insulin in culture, whereas hepatocytes from rats that have never expressed glucokinase in vivo (suckling rats or rats weaned to a high-fat diet) showed a delay of 24 h. Two different inhibitors of protein synthesis (cycloheximide and puromycin) prevented the initial accumulation of glucokinase mRNA in response to the first exposure to insulin but not to the secondary accumulation of glucokinase mRNA in response to reexposure to insulin. This suggests that the synthesis of one or several insulin-dependent proteins is necessary for the first activation of glucokinase gene transcription in response to the first exposure to insulin.