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

A RARE of hepatic Gck promoter interacts with RARα, HNF4α and COUP-TFII that affect retinoic acid- and insulin-induced Gck expression

The expression of hepatic glucokinase gene (Gck) is regulated by hormonal and nutritional signals. How these signals integrate to regulate the hepatic Gck expression is unclear. We have shown that the hepatic Gck expression is affected by Vitamin A status and synergistically induced by insulin and retinoids in primary rat hepatocytes. We hypothesized that this is mediated by a retinoic acid responsive element (RARE) in the hepatic Gck promoter. Here, we identified the RARE in the hepatic Gck promoter using standard molecular biology techniques. The single nucleotide mutations affecting the promoter activation by retinoic acid (RA) were also determined for detail analysis of protein and DNA interactions. We have optimized experimental conditions for performing electrophoresis mobility shift assay and demonstrated the interactions of the retinoic acid receptor α (RARα), retinoid X receptor α (RXRα), hepatocyte nuclear factor 4α (HNF4α) and chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) in the rat nuclear extract with this RARE, suggesting their roles in the regulation of Gck expression. Chromatin immunoprecipitation assays demonstrated that recombinant adenovirus-mediated overexpression of RARα, HNF4α and COUP-TFII, but not RXRα, significantly increased their occupancy in the hepatic Gck promoter in primary rat hepatocytes. Overexpression of RARα, HNF4α and COUP-TFII, but not RXRα, also affected the RA- and insulin-mediated Gck expression in primary rat hepatocytes. In summary, this hepatic Gck promoter RARE interacts with RARα, HNF4α and COUP-TFII to integrate Vitamin A and insulin signals.

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.

Insulin represses phosphoenolpyruvate carboxykinase gene transcription by causing the rapid disruption of an active transcription complex: a potential epigenetic effect

Insulin represses gluconeogenesis, in part, by inhibiting the transcription of genes that encode rate-determining enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G-6-Pase). Glucocorticoids stimulate expression of the PEPCK gene but the repressive action of insulin is dominant. Here, we show that treatment of H4IIE hepatoma cells with the synthetic glucocorticoid, dexamethasone (dex), induces the accumulation of glucocorticoid receptor, as well as many transcription factors, coregulators, and RNA polymerase II, on the PEPCK gene promoter. The addition of insulin to dex-treated cells causes the rapid dissociation of glucocorticoid receptor, polymerase II, and several key transcriptional regulators from the PEPCK gene promoter. These changes are temporally related to the reduced rate of PEPCK gene transcription. A similar disruption of the G-6-Pase gene transcription complex was observed. Additionally, insulin causes the rapid demethylation of arginine-17 on histone H3 of both genes. This rapid, insulin-induced, histone demethylation is temporally related to the disruption of the PEPCK and G-6-Pase gene transcription complex, and may be causally related to the mechanism by which insulin represses transcription of these genes.

Insulin regulation of glucokinase gene expression: evidence against a role for sterol regulatory element binding protein 1 in primary hepatocytes

Liver key genes for carbohydrate and lipid homeostasis are regulated by insulin and glucose. The sterol regulatory-element binding protein-1c (SREBP-1c) has emerged as a mediator of insulin effects on gene transcription, particularly on glucokinase (GK). In this paper, we show that despite stimulation of GK promoter transcription by overexpression of mature SREBP-1c, insulin induced GK transcription at least 4h ahead of accumulation of mature SREBP-1c in the nucleus. Importantly, the knockdown of SREBP-1 mRNA using a RNA-interference technique reduced the level of nuclear SREBP-1 protein, diminished fatty acid synthase mRNA level, but did not affect GK and L-pyruvate kinase mRNA levels. We concluded that SREBP-1 is unlikely to be the mediator of the early insulin effect on GK gene transcription.

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.

Physiological concentrations of insulin promote binding of nuclear proteins to the insulin-like growth factor I gene

Limitations in understanding the mechanism of transcriptional regulation by insulin are due in part to lack of models in which there is insulin-responsive binding of nuclear factors to critical promoter regions. The insulin-like growth factor I (IGF-I) gene responds to diabetes status via a footprinted sequence, region V, which contains an AT-rich element and a GC-rich site. We tested the hypothesis that insulin regulates nuclear factor binding to the AT-rich site. Gel shift analysis with liver nuclear extracts and a region V probe showed binding of Sp1, Sp3, and B(1), which persisted despite the presence of antibodies against Sp1 and Sp3. B(1) was detected by a probe mutated in the GC-rich site (VmSp1), but not by a probe mutated at the AT-rich site (VmAT). We then asked whether B(1) was responsive to insulin. For both region V and VmSp1 probes, nuclear extracts from normal rat hepatocytes, H4IIE cells, and CHO-IR cells exposed to 10(-6) M insulin exhibited an increase in binding, designated insulin-responsive binding protein (IRBP); IRBP comigrated with B(1) from liver extracts. IRBP binding to region V was competed by VmSp1, but not by VmAT, indicating specific interactions with the AT-rich sequence; insulin response elements from other genes also failed to compete. After addition of insulin, IRBP began to increase by 1 h and rose further at 24 h, suggesting involvement of both posttranslational and transcriptional mechanisms. IRBP responded to as little as 10(-10) M insulin, indicating physiological relevance. Induction of IRBP was blunted by the phosphatidylinositol 3'-kinase inhibitor LY294002, whereas other signal transduction inhibitors had little effect. IRBP interacts with an important sequence in the IGF-I gene and may participate in the metabolic regulation of IGF-I expression. As most insulin-responsive genes do not exhibit insulin-responsive nuclear factor binding, further studies of IRBP may also contribute to understanding of the mechanism of insulin action on gene transcription.

High-dose biotin, an inducer of glucokinase expression, may synergize with chromium picolinate to enable a definitive nutritional therapy for type II diabetes

Glucokinase (GK), expressed in hepatocyte and pancreatic beta cells, has a central regulatory role in glucose metabolism. Efficient GK activity is required for normal glucose-stimulated insulin secretion, postprandial hepatic glucose uptake, and the appropriate suppression of hepatic glucose output and gluconeogenesis by elevated plasma glucose. Hepatic GK activity is subnormal in diabetes, and GK may also be decreased in the beta cells of type II diabetics. In supraphysiological concentrations, biotin promotes the transcription and translation of the GK gene in hepatocytes; this effect appears to be mediated by activation of soluble guanylate cyclase. More recent evidence indicates that biotin likewise increases GK activity in islet cells. On the other hand, high-dose biotin suppresses hepatocyte transcription of phosphoenolpyruvate carboxykinase, the rate-limiting enzyme for gluconeogenesis. Administration of high-dose biotin has improved glycemic control in several diabetic animals models, and a recent Japanese clinical study concludes that biotin (3 mg t.i.d. orally) can substantially lower fasting glucose in type II diabetics, without side-effects. The recently demonstrated utility of chromium picolinate in type II diabetes appears to reflect improved peripheral insulin sensitivity--a parameter which is unlikely to be directly influenced by biotin. Thus, the joint administration of supranutritional doses of biotin and chromium picolinate is likely to combat insulin resistance, improve beta-cell function, enhance postprandial glucose uptake by both liver and skeletal muscle, and inhibit excessive hepatic glucose production. Conceivably, this safe, convenient, nutritional regimen will constitute a definitive therapy for many type II diabetics, and may likewise be useful in the prevention and management of gestational diabetes. Biotin should also aid glycemic control in type I patients.

Effects of triiodothyronine and retinoic acid on glucokinase gene expression in neonatal rat hepatocytes

Glucokinase (EC 2.7.1.2) first appears in rat liver two weeks after birth and increases rapidly after weaning on to a high-carbohydrate diet. We investigated the role of triiodothyronine and retinoic acid in the absence of insulin on the first expression of the glucokinase gene in primary cultures of hepatocytes from 10 day-old rats. These two hormones were able to induce a rapid accumulation of liver glucokinase mRNA, secondarily to a stimulation of gene transcription during the first 24 h of culture. Moreover, the effects of individual hormones were not additive. Finally, glucokinase mRNA stability was not modified by these hormones. This suggests that triiodothyronine and retinoic acid act on glucokinase gene at the transcriptional.

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.