Insulin signalling and regulation of glucokinase gene expression in cultured hepatocytes

Insights into the Hexose Liver Metabolism-Glucose versus Fructose

High-fructose intake in healthy men is associated with characteristics of metabolic syndrome. Extensive knowledge exists about the differences between hepatic fructose and glucose metabolism and fructose-specific mechanisms favoring the development of metabolic disturbances. Nevertheless, the causal relationship between fructose consumption and metabolic alterations is still debated. Multiple effects of fructose on hepatic metabolism are attributed to the fact that the liver represents the major sink of fructose. Fructose, as a lipogenic substrate and potent inducer of lipogenic enzyme expression, enhances fatty acid synthesis. Consequently, increased hepatic diacylglycerols (DAG) are thought to directly interfere with insulin signaling. However, independently of this effect, fructose may also counteract insulin-mediated effects on liver metabolism by a range of mechanisms. It may drive gluconeogenesis not only as a gluconeogenic substrate, but also as a potent inducer of carbohydrate responsive element binding protein (ChREBP), which induces the expression of lipogenic enzymes as well as gluconeogenic enzymes. It remains a challenge to determine the relative contributions of the impact of fructose on hepatic transcriptome, proteome and allosterome changes and consequently on the regulation of plasma glucose metabolism/homeostasis. Mathematical models exist modeling hepatic glucose metabolism. Future models should not only consider the hepatic adjustments of enzyme abundances and activities in response to changing plasma glucose and insulin/glucagon concentrations, but also to varying fructose concentrations for defining the role of fructose in the hepatic control of plasma glucose homeostasis.

Metabolic, anabolic, and mitogenic insulin responses: A tissue-specific perspective for insulin receptor activators

Insulin acts as the major regulator of the fasting-to-fed metabolic transition by altering substrate metabolism, promoting energy storage, and helping activate protein synthesis. In addition to its glucoregulatory and other metabolic properties, insulin can also act as a growth factor. The metabolic and mitogenic responses to insulin are regulated by divergent post-receptor signaling mechanisms downstream from the activated insulin receptor (IR). However, the anabolic and growth-promoting properties of insulin require tissue-specific inter-relationships between the two pathways, and the nature and scope of insulin-regulated processes vary greatly across tissues. Understanding the nuances of this interplay between metabolic and growth-regulating properties of insulin would have important implications for development of novel insulin and IR modulator therapies that stimulate insulin receptor activation in both pathway- and tissue-specific manners. This review will provide a unique perspective focusing on the roles of "metabolic" and "mitogenic" actions of insulin signaling in various tissues, and how these networks should be considered when evaluating selective pharmacologic approaches to prevent or treat metabolic disease.

Modulation of de novo purine biosynthesis leads to activation of AMPK and results in improved glucose handling and insulin sensitivity

BACKGROUND

AMP activated protein kinase (AMPK) regulates key metabolic reactions and plays a major role in glucose homeostasis. Activating the AMPK is considered as one of the potential therapeutic strategies in treating type-2 diabetes. However, targeting AMPK by small molecule mediated approach can be challenging owing to diverse isoforms of the enzyme and their varied combination in different tissues. In the current study we employ a novel strategy of achieving AMPK activation through increasing the levels of cellular AMP (an allosteric activator of AMPK) levels by activating the enzyme involved in AMP biosynthesis namely Adenylosuccinate lyase (ADSL).

METHODS

Rat primary hepatocytes were cultured under metabolic overload conditions (500 μM palmitate) to induce insulin resistance. ADSL was overexpressed in these hepatocytes and its effect on hepatic glucose output, and triglyceride accumulation was checked. In addition to this, ADSL was overexpressed in high fat diet induced obese mice by hydrodynamic tail vein injection and its effect on fasting glucose, glucose tolerance and pyruvate tolerance were checked.

RESULTS

Rat primary hepatocytes when cultured under metabolic overload conditions developed insulin resistance as measured in terms of failure of insulin to suppress the glucose output. Overexpressing the ADSL in these hepatocytes resulted in increased AMPK phosporylation and improved the insulin sensitivity and also resulted in reduced triglyceride accumulation and inflammatory cytokine levels. In addition to this, when ADSL was overexpressed in high fat diet induced obese mice, it resulted in reduced the fasting hyperglycemia (20% reduction), and increased glucose and pyruvate tolerance.

CONCLUSIONS

This study indicates that activating ADSL can be a potential mechanism to achieve the activation of AMPK in the cells. This leads to a novel idea of exploring the purine nucleotide metabolic pathway as a promising therapeutic target for diabetes and metabolic syndrome.

Characterization of the gene expression profile of heterozygous liver-specific glucokinase knockout mice at a young age

In the liver, glucokinase (GCK) facilitates hepatic glucose uptake during hyperglycemia and is essential for the regulation of a network of glucose-responsive genes involved in glycolysis, glycogen synthesis, and lipogenesis. To better understand the consequences of changes in response to a liver-specific deficiency of GCK function, we examined the expression profiles of genes involved in glucose metabolism in the liver, pancreas, muscle and adipose tissue in heterozygous liver-specific Gck knockout (Gck(w/-)) mice. Our results showed that with the development of a liver GCK deficiency, significant decreases in the mRNA levels for insulin receptor and Glut2 were observed in the liver, and HkII in muscle, while glucagon mRNA increased markedly in the pancreas. The levels of circulating glucagon hormone levels increased with increased mRNA levels. Depite a decrease in muscle HkII levels, the hexokinase activity level did not change. Our findings suggest that in liver-specific Gck(w/-) mice, peripheral tissues use different strategies to tackle with hyperglycemia even at a young age. By identifying the specific changes that occur in different tissues at an early stage of glucokinase deficiency, potentially we can develop interventions to prevent further progression to diabetes.

Sex steroids enhance insulin receptors and glucose oxidation in Chang liver cells

BACKGROUND

The present study was designed to assess the effect of sex steroids (testosterone and 17beta-estradiol) on insulin receptor expression, insulin binding and glucose oxidation in human liver cell line.

METHODS

Non-malignant Chang liver cells were treated with different concentrations of testosterone and 17beta-estradiol dissolved in serum free medium for 24 h to identify the effective dose of both steroids for further studies. Cells with 70-80% confluency were challenged with testosterone (0.1 micromol/l), 17beta-estradiol (0.1 micromol/l) and their combination along with insulin as a positive control for 24 h. After the treatment period, insulin receptor mRNA expression, cell surface insulin binding and (14)C-glucose oxidation were assessed.

RESULTS

Both testosterone and 17beta-estradiol significantly increased the insulin receptor mRNA expression, cell surface insulin binding and (14)C-glucose oxidation compared to basal, but the increase was not at par with the effect of insulin. Compared to individual effects of testosterone and 17beta-estradiol, their combination significantly increased the glucose oxidation similar to that of insulin.

CONCLUSION

It is concluded from the present study that testosterone and 17beta-estradiol can directly enhance insulin receptor mRNA expression, insulin binding and glucose oxidation in Chang liver cells and thereby glucose metabolism.

Molecular Coordination of Hepatic Glucose Metabolism by the 6-Phosphofructo-2-Kinase/Fructose-2,6- Bisphosphatase:Glucokinase Complex

Glucokinase (GK) and 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase (FBP-2) are each powerful regulators of hepatic carbohydrate metabolism that have been reported to influence each other's expression, activities, and cellular location. Here we present the first physical evidence for saturable and reversible binding of GK to the FBP-2 domain of PFK-2/FBP-2 in a 1:1 stoichiometric complex. We confirmed complex formation and stoichiometry by independent methods including affinity resin pull-down assays and fluorescent resonance energy transfer. All suggest that the binding of GK to PFK-2/FBP-2 is weak. Enzymatic assays of the GK:PFK-2/FBP-2 complex suggest a concomitant increase of the kinase-to-bisphosphatase ratio of bifunctional enzyme and activation of GK upon binding. The kinase-to-bisphosphatase ratio is increased by activation of the PFK-2 activity whereas FBP-2 activity is unchanged. This means that the GK-bound PFK-2/FBP-2 produces more of the biofactor fructose-2,6-bisphosphate, a potent activator of 6-phosphofructo-1-kinase, the committing step to glycolysis. Therefore, we conclude that the binding of GK to PFK-2/FBP-2 promotes a coordinated up-regulation of glucose phosphorylation and glycolysis in the liver, i.e. hepatic glucose disposal. The GK:PFK-2/FBP-2 interaction may also serve as a metabolic signal transduction pathway for the glucose sensor, GK, in the liver. Demonstration of molecular coordination of hepatic carbohydrate metabolism has fundamental relevance to understanding the function of the liver in maintaining fuel homeostasis, particularly in managing excursions in glycemia produced by meal consumption.

Regulation of hepatic insulin-like growth factor-binding protein-1 gene expression by insulin: central role for mammalian target of rapamycin independent of forkhead box O proteins

The expression of IGF-binding protein-1 (IGFBP-1) is induced in rat liver by dexamethasone and glucagon and is completely inhibited by 100 nM insulin. Various studies have implicated phosphatidylinositol 3-kinase, protein kinase B (Akt), phosphorylation of the transcription factors forkhead in rhabdomyosarcoma 1 (Foxo1)/Foxo3, and the mammalian target of rapamycin (mTOR) in insulin's effect. In this study we examined insulin regulation of IGFBP-1 in both subconfluent and confluent hepatocytes. In subconfluent hepatocytes, insulin inhibition of IGFBP-1 mRNA levels was blocked by inhibiting PI3 kinase activation, and there was a corresponding inhibition of Foxo1/Foxo3 phosphorylation. In these same cells, inhibition of the insulin effect by rapamycin occurred in the presence of insulin-induced Foxo1/Foxo3 phosphorylation. In confluent hepatocytes, insulin could not activate the phosphatidylinositol 3-kinase (PI3 kinase)-Akt-Foxo1/Foxo3 pathway, but still inhibited IGFBP-1 gene expression in an mTOR-dependent manner. In subconfluent hepatocytes, the serine/threonine phosphatase inhibitor okadaic acid (100 nM) partially inhibited IGFBP-1 gene expression by 40%, but did not produce phosphorylation of either Akt or Foxo proteins. In contrast, 1 nm insulin inhibited the IGFBP-1 mRNA level by 40% and correspondingly activated Akt and Foxo1/Foxo3 phosphorylation to a level comparable to that observed with 100 nM insulin. These results suggest a potential role for a serine/threonine phosphatase(s) in the regulation of IGFBP-1 gene transcription, which is not downstream of mTOR and is independent of Akt. In conclusion, we have found that in rat liver, insulin inhibition of IGFBP-1 mRNA levels can occur in the absence of the phosphorylation of Foxo1/Foxo3, whereas activation of the mTOR pathway is both necessary and sufficient.

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.

Analysis of the role of protein kinase B (cAKT) in insulin-dependent induction of glucokinase and sterol regulatory element-binding protein 1 (SREBP1) mRNAs in hepatocytes

Previous work showed that acute stimulation of a conditionally active protein kinase B (PKB or cAKT) was sufficient to elicit insulin-like induction of GCK (glucokinase) and SREBP1 (sterol regulatory element-binding protein 1) in hepatocytes [Iynedjian, Roth, Fleischmann and Gjinovci (2000) Biochem. J. 351, 621-627; Fleischmann and Iynedjian (2000) Biochem. J. 349, 13-17]. The objective of the present study was to determine whether activation of PKB during insulin stimulation of hepatocytes was a necessary condition for the induction of the two genes. Activation of PKB by insulin was inhibited by pretreatment of the hepatocytes with C2 ceramide. This resulted in the inhibition of insulin-dependent increases in GCK and SREBP1 mRNAs. A triple mutant of PKB failed to interfere with insulin activation of PKB in hepatocytes even at high overexpression levels achieved after adenovirus transduction. A PKB-CaaX fusion protein, which can act as a dominant-negative inhibitor of PKB activation in other cells, was shown to be constitutively activated in hepatocytes and to trigger insulin-like induction of GCK and SREBP1. In addition, constitutive PKB-CaaX activity caused refractoriness of the hepatocytes to insulin signalling at an upstream step resulting in the inhibition of both extracellular-signal-regulated kinase 1/2 and endogenous PKB activation. The stimulation of gene expression by constitutively active PKB-CaaX and inhibition of the insulin effect by ceramide are compatible with a role for PKB in the insulin-dependent induction of GCK and SREBP1.

Insulin-mediated modulation of cytochrome P450 gene induction profiles in primary rat hepatocyte cultures

In this investigation, we examined the effects of insulin on gene induction responsiveness in primary rat hepatocytes. Cells were cultured for 72 hours either in the absence or presence of 1 microM insulin and then exposed to increasing concentrations of phenobarbital (PB; 0.01-3.5 mM). Culturing in the absence of insulin produced 1.5-2-fold increases in the induction magnitude of CYP2B1 and CYP2B2 mRNA expression resulting from PB exposures, without altering the bell-shaped dose-response curve characteristic of this agent. However, for the CYP3A1 gene, insulin removal led to a pronounced shift in both the PB-induction magnitude and dose-response relationships of the induction response, with higher levels of CYP3A1 expression resulting from exposures to lower concentrations of inducer. Insulin removal also reduced the time required to attain maximal induction of CYP2B1/2 and CYP3A1 gene expression. The insulin effects were not specific for PB induction, as insulin deprivation similarly enhanced both dexamethasone- and beta-naphthoflavone-inducible CYP3A1 and CYP1A1 expression profiles, respectively. In contrast, the level of albumin mRNA expression was reduced considerably in cells deprived of insulin. We conclude that insulin is an important regulator of inducible and liver-specific gene expression in primary rat hepatocytes.

Effects of endothelin-1 and nitric oxide on glucokinase activity in isolated rat hepatocytes

To test the hypothesis that endothelin-1 (ET-1) and nitric oxide (NO) influence glucokinase (GK) activity in an opposite manner, we evaluated the effects of ET-1, L-NAME, an inhibitor of NO synthase, and L-arginine, a substrate for NO synthase, on GK activity and glycogen content in isolated rat hepatocytes. Moreover, to understand the receptor involved in the process, the effects of BQ 788, a specific antagonist of ETB receptor, and PD 142893, an antagonist of ETA-ETB receptors, were also evaluated. GK activity, cyclic guanosine monophosphate (cGMP), and glycogen intracellular content were measured on isolated hepatocytes, while glucose levels and NO as NO2-/NO3- were determined in the medium. High ET-1 levels induced a 20% decrease of NO2-/NO3- levels and cGMP intracellular content, followed by a 49% reduction of GK activity and a 15% decrease of glycogen. In parallel, a 10% increase of glucose in the medium was observed. In the presence of L-NAME, GK activity and glycogen levels showed analogous decrements as observed with ET-1. Also in this case, a significant decrease of the intracellular content of cGMP was observed. No synergistic effects of ET-1 and L-NAME were observed. L-Arginine was able to counteract the inhibitory effect of ET-1 on cGMP and GK activity. Glycogen content was slightly but not significantly reduced, and under those conditions, a significant decrease of glucose in the medium was observed. When hepatocytes were incubated with ET-1 plus BQ 788 or ET-1 plus PD 142893, GK activity was unchanged. Interestingly, no changes were observed in NO2-/NO3- levels and the intracellular content of cGMP was not modified when the antagonists of ET-1 receptors were added to the medium. In conclusion, the present study shows that the NO pathway seems to be an important regulator of GK activity and glycogen content through cGMP activity. In addition, ET-1 seems to be not active per se, but its activity seems mediated by a simultaneous decrease of NO levels.

Rapid reversal of the effects of the portal signal under hyperinsulinemic conditions in the conscious dog

Experiments were performed on two groups of 42-h-fasted conscious dogs (n = 6/group). Somatostatin was given peripherally with insulin (4-fold basal) and glucagon (basal) intraportally. In the first experimental period, glucose was infused peripherally to double the hepatic glucose load (HGL) in both groups. In the second experimental period, glucose (21.8 micromol. kg-1. min-1) was infused intraportally and the peripheral glucose infusion rate (PeGIR) was reduced to maintain the precreating HGL in the portal signal (PO) group, whereas saline was given intraportally in the control (CON) group and PeGIR was not changed. In the third period, the portal glucose infusion was stopped in the PO group and PeGIR was increased to sustain HGL. PeGIR was continued in the CON group. The glucose loads to the liver did not differ in the CON and PO groups. Net hepatic glucose uptake was 9.6 +/- 2.5, 11.6 +/- 2.6, and 15.5 +/- 3.2 vs. 10.8 +/- 1.8, 23.7 +/- 3.0, and 15.5 +/- 1.1 micromol. kg-1. min-1, and nonhepatic glucose uptake (non-HGU) was 29.8 +/- 1.1, 40.1 +/- 4.5, and 49.5 +/- 4.0 vs. 26.6 +/- 4.3, 23.2 +/- 4.0, and 40.4 +/- 3.1 micromol. kg-1. min-1 in the CON and PO groups during the three periods, respectively. Cessation of the portal signal shifted NHGU and non-HGU to rates similar to those evident in the CON group within 10 min. These results indicate that even under hyperinsulinemic conditions the effects of the portal signal on hepatic and peripheral glucose uptake are rapidly reversible.

Hepatic glucose uptake rapidly decreases after removal of the portal signal in conscious dogs

The aim of this study was to assess the decay of the effect of the portal signal on net hepatic glucose uptake (NHGU). Experiments were performed on five 42-h-fasted conscious dogs. After the 40-min basal period, somatostatin was given peripherally along with insulin (1.8 pmol. kg-1. min-1) and glucagon (0.65 ng. kg-1. min-1) intraportally. In the first experimental period (Pe-GLU-1; 90 min), glucose was infused into a peripheral vein to double the glucose load to the liver (HGL). In the second experimental period (Po-GLU; 90 min), glucose (20.1 micromol. kg-1. min-1) was infused intraportally and the peripheral glucose infusion was reduced to maintain the same HGL. In the third period (Pe-GLU-2; 120 min), the portal glucose infusion was stopped and the peripheral glucose infusion was increased to again sustain HGL. Arterial insulin levels (42 +/- 3, 47 +/- 3, 43 +/- 3 pmol/l) were basal and similar in the Pe-GLU-1, Po-GLU, and Pe-GLU-2 periods, respectively. Arterial glucagon levels were also basal and similar (51 +/- 3, 49 +/- 2, 46 +/- 2 ng/l) in the three experimental periods. The glucose loads to the liver were 251 +/- 11, 274 +/- 14, and 276 +/- 12 micromol. kg-1. min-1, respectively. NHGU was 6.3 +/- 2.4, 19.1 +/- 2.8, and 9.2 +/- 1.2 micromol. kg-1. min-1, and nonhepatic glucose uptake (non-HGU) was 23.6 +/- 3.0, 5.3 +/- 1.8, and 25.5 +/- 3.7 micromol. kg-1. min-1 in the three periods, respectively. Cessation of the portal signal for only 10 min shifted NHGU and non-HGU to 9.4 +/- 2.2 and 25.0 +/- 2.8 micromol. kg-1. min-1, respectively; thus the effect of the portal signal was rapidly reversed both at the liver and peripheral tissues.

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.

Rat primary hepatocytes and H4 hepatoma cells display differential sensitivity to cyclic AMP at the level of expression of tyrosine aminotransferase

We have shown that the sensitivity of isolated hepatocytes and H4 hepatoma cells to cyclic AMP is different. In terms of activation of tyrosine aminotransferase at mRNA and activity level in response to cyclic AMP, isolated hepatocytes are 10-fold more sensitive. Hepatocytes and H4 hepatoma cells show similar sensitivities to cyclic AMP at the level of protein kinase A activation and phosphorylation of cyclic AMP response element binding protein (CREB) and the differential sensitivity must reside at other sites. The consequences of these findings to studies of control phenomena at the transcriptional level is discussed.

Time course of enzyme changes after a switch from a high-fat to a low-fat diet

This study was conducted to determine the time course of metabolic changes associated with a switch from a high-fat to a low-fat diet in rats. Adult rats, maintained on a high-fat diet (42% of energy from fat) for 4-5 weeks were switched to a low-fat diet (11% of energy from fat), and the activities of several liver enzymes were followed. Three different phases could be distinguished. The early phase, complete by 2 days after the switch in diets, included an increase in the activity of glucose 6-phosphate dehydrogenase (pentose phosphate pathway), an increase in pyruvate kinase and pyruvate dehydrogenase activities (terminal end of the glycolytic pathway) and an increase in ATP-citrate lyase and fatty acid synthetase (fatty acid synthesis pathway). The early phase also included a decrease in the activity of phosphoenolpyruvate carboxykinase (PEPCK, gluconeogenesis) and a lower branched-chain amino acid dehydrogenase activity (BCAADH, branched-chain amino acid degradation). The concentration of the allosteric phosphofructokinase regulator, fructose 2,6-bisphosphate (Fru-2,6-P2, glycolysis), decreased during the early phase. An intermediate phase could also be discerned between 3 and 10 days after the switch in diets. In this phase, the decreased Fru-2,6-P2 concentration and the decreased PEPCK and BCAADH activities observed in the early phase were reversed. The late phase occurred 10 days after the dietary switch and was characterized by an increase in the activities of glucokinase (glycolytic pathway) and glycogen phosphorylase (associated with glycogenolysis) and by a decrease in glutamate dehydrogenase, PEPCK and BCAADH activities. These measurements indicate that at least 20 days are required before metabolic changes associated with a switch in diet are complete.

Evidence for a role of glucose-induced translocation of glucokinase in the control of hepatic glycogen synthesis

Glucokinase reversibly partitions between a bound and a free state in the hepatocyte in response to the metabolic status of the cell. Maximum binding occurs at low [glucose] (<5 mM) and minimum binding at high [glucose] or in the presence of sorbitol or fructose. In this study we determined the binding characteristics of glucokinase in the hepatocyte in situ, by adenovirus-mediated glucokinase overexpression combined with the digitonin-permeabilization technique. We also determined the sensitivity of glycogen synthesis to changes in either total glucokinase overexpression or in free glucokinase activity. Glucokinase overexpression is associated with an increase in both free and bound activity, with an overall decrease in the proportion of bound activity. In hepatocytes incubated at low [glucose] (0-5 mM), glucokinase binding involves a high-affinity binding site with a Kd of approximately 0.1 microM and a binding capacity of approximately 3 pmol/mg total cell protein and low-affinity binding with a Kd of approximately 1.6 microM. Increasing glucose concentration to 20 mM causes a dose-dependent increase in the Kd of the high- affinity site to approximately 0.6 microM, and this effect was mimicked by 50 microM sorbitol, a precursor of fructose 1-P, confirming that this site is the regulatory protein of glucokinase. Glycogen synthesis determined from the incorporation of [2-3H,U-14C]glucose into glycogen at 5 mM or 10 mM glucose was very sensitive to small increases in total glucokinase activity and correlated more closely with the increase in free glucokinase activity. The relation between glycogenic flux and glucokinase activity is sigmoidal. Expression of the sensitivity of glycogen synthesis to glucokinase activity as the control coefficient reveals that the coefficient is greater for the incorporation of 2-tritium (which occurs exclusively by the direct pathway) than for incorporation of 14C label (which involves direct and indirect pathways) and is greater at 5 mM glucose (when glucokinase is maximally sequestered at its high-affinity site) than at 10 mM glucose. The results support the hypothesis that compartmentation of glucokinase in the hepatocyte increases the sensitivity of glycogen synthesis to small changes in total glucokinase activity and that glucose-induced translocation of glucokinase has a major role in the acute control of glycogen synthesis.

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.

Insulin stimulation of intracellular free Ca2+ recovery and Ca(2+)-ATPase gene expression in cultured vascular smooth-muscle cells: role of glucose 6-phosphate

Wa have previously reported that insulin accelerates recovery of intracellular Ca2+ concentrations ([Ca2+]i) from pressor agonist-induced Ca2+ loads and stimulates both plasmalemmal and sarcoplasmic-reticulum Ca(2+)-ATPase gene expression in cultured and freshly isolated vascular smooth-muscle cells (VSMCs), suggesting that insulin attenuation of vascular tone may result from modulation of [Ca2+]i. Accordingly, we have now evaluated the linkage between this insulin-regulation of VSMC[Ca2+]i and classical actions of insulin (i.e. glucose transport and metabolism). Cultured VSMCs were incubated in the presence or absence of insulin in a medium containing either pyruvate, glucose, 3-O-methylglucose or 2-deoxyglycose. Insulin caused an 87% increase in [Ca2+]i recovery rate after stimulation with arginine-vasopressin (P < 0.01) and caused a marked increase in Ca(2+)-ATPase mRNA and protein levels in the presence of glucose. Comparable increases in both [Ca2+]i recovery and Ca2(+)-ATPase expression were found when glucose was replaced by 2-deoxyglucose. In contrast, no stimulation was found in either the glucose-free or 3-O-methylglucose-containing medium. As both glucose analogues are transported, but only 2-deoxyglucose is phosphorylated, this indicates that glucose transport and metabolism to glucose 6-phosphate is essential for insulin regulation of VSMC [Ca2+]i, possibly via a glucose-6-phosphate-dependent carbohydrate-response element in the Ca2(+)-ATPase gene.

Glucokinase and cytosolic phosphoenolpyruvate carboxykinase (GTP) in the human liver. Regulation of gene expression in cultured hepatocytes

Glucokinase and phosphoenolpyruvate carboxykinase are key enzymes of glucose metabolism in the rat liver. The former is considered to be instrumental in regulating glucose hepatic release/uptake according to the glycaemia level, and cytosolic phosphoenolpyruvate carboxykinase is a major flux-generating enzyme for gluconeogenesis. The level of expression of both enzymes and the regulation of their mRNAs in the human liver cell were investigated. Surgical biopsies of liver from patients undergoing partial hepatectomies and parenchymal hepatocytes derived from the biopsies were used to assay glucokinase, hexokinase and phosphoenolpyruvate carboxykinase activities. Hepatocytes were placed in culture and the actions of insulin, glucagon and cAMP on glucokinase and phosphoenolpyruvate carboxykinase mRNAs were studied. The main results are: (a) glucokinase accounts for 95% of the glucose phosphorylation activity of human hepatocytes, although this fact is masked in assays of total liver tissue; (b) glucokinase activity is set at a lower level in human hepatocytes than in rat hepatocytes, and vice-versa for the gluconeogenic enzyme phosphoenolpyruvate carboxykinase; and (c) as previously shown in rat liver, glucokinase and phosphoenolpyruvate carboxykinase mRNAs are regulated in a reciprocal fashion in human hepatocytes, insulin inducing the first enzyme and repressing the latter, whereas glucagon has opposite effects. These data have interesting implications with respect to metabolic regulation and intracellular hormone signaling in the human liver.

Requirement of tyrosine- and serine/threonine kinases in the transcriptional activation of the mammalian grp78/BiP promoter by thapsigargin

Depletion of endoplasmic reticulum (ER) Ca2+ store by thapsigargin (Tg) in mammalian cells induces a set of ER protein genes known as the glucose-regulated proteins. Recently, IRE1p, a transmembrane protein postulated to have a serine/threonine kinase activity, has been identified as required for the induction of ER resident proteins genes in Saccharomyces cerevisiae. To investigate whether IRE1p can stimulate mammalian grp transcription, a stable Chinese hamster ovary cell line containing amplified copies of IRE1p has been created. The IRE1p expressing transfectants exhibited a modest (2-fold) enhancement of both the basal and Tg induced level of grp78 and grp94, two coordinately regulated grp genes. Using okadaic acid as a specific inhibitor for the endogenous serine/threonine protein phosphatase activities, a mild (2-fold) stimulative effect was observed for Tg induction of grp78 transcription. The okadaic acid potentiating effect requires a 50-base pair region in the vicinity of the grp78 TATA element. In contrast, the transcriptional activation of grp78 by Tg is almost totally eliminated by genistein, a tyrosine kinase inhibitor. The grp core, the C3 and C1 elements which are major Tg response elements of the rat grp78 promoter, are also major targets of the inhibitive effects of genistein.

Inhibition by recombinant human interleukin-6 of the glucagon-dependent induction of phosphoenolpyruvate carboxykinase and of the insulin-dependent induction of glucokinase gene expression in cultured rat hepatocytes: regulation of gene transcription and messenger RNA degradation

The influence of recombinant human interleukin-6, the major mediator of the inflammatory response in liver, on the glucagon- and insulin-dependent induction of the phosphoenolpyruvate carboxykinase and glucokinase gene, respectively, was monitored on the level of gene transcription, mRNA abundance and enzyme activity in cultured rat hepatocytes. As control markers of the interleukin-6-induced acute-phase response the mRNA levels of the acute phase proteins alpha 2-macroglobulin and beta-fibrinogen were determined. In cultured rat hepatocytes, recombinant human interleukin-6, added simultaneously with glucagon and insulin, lowered the maximal increase in glucagon-induced phosphoenolpyruvate carboxykinase mRNA levels after 2 hr and the maximal increase in glucokinase mRNA levels after 3 hr to about 30%, respectively. It inhibited the glucagon-induced increase in phosphoenolpyruvate carboxykinase gene transcription and phosphoenolpyruvate carboxykinase enzyme activity, as well as the insulin-induced increases in glucokinase gene transcription and glucokinase enzyme activity. Recombinant human interleukin-6 increased the mRNA levels of the acute-phase proteins alpha 2-macroglobulin and beta-fibrinogen gradually over 4 to 6 hr. Recombinant human interleukin-6, added 2 hr after glucagon or 3 hr after insulin at the maximum of the hormone-induced enzyme mRNA levels, almost doubled the decay rate of phosphoenolpyruvate carboxykinase mRNA and glucokinase mRNA. The results show that interleukin-6 induced the expression of inflammatory proteins and simultaneously inhibited the hormone-induced expression of enzymes of intermediary metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochalisin D exerts stimulatory and inhibitory effects on insulin-induced glucokinase mRNA expression in hepatocytes

The microfilament cytoskeleton is postulated to have a role in the localization, transport and anchorage of certain specific mRNAs. We investigated the effects of cytochalasin D, a fungal metabolite that binds to actin and disrupts the microfilament structure, on insulin-induced expression of glucokinase mRNA in rat hepatocyte cultures. Cytochalasin-D significantly potentiates insulin-induced glucokinase mRNA expression at 100 nM concentration but counteracts glucokinase expression at 2-20 microM. The latter effect is at least in part due to an increase in glucokinase mRNA degradation. This effect of cytochalasin D cannot be accounted for by an increase in cAMP and is also not due to a non-specific effect on mRNA degradation since albumin mRNA levels were not affected by cytochalasin-D and actin mRNA and tubulin mRNA levels were increased. Measurement of glucokinase mRNA release from digitonin-permeabilized hepatocytes suggests that cytochalasin D does not cause acute dissociation of glucokinase mRNA from its binding site. The increased degradation of glucokinase mRNA suggests involvement of the cytoskeleton in glucokinase mRNA stability. However, an additional effect of cytochalasin D on the insulin signalling mechanism cannot be excluded.

Transcriptional control of genes that regulate glycolysis and gluconeogenesis in adult liver

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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.

Inhibitors of serine/threonine phosphatases enhance phosphorylation of the interferon-gamma receptor while selectively attenuating interferon-gamma-induced gene expression in human peripheral-blood monocytes

Since many events following ligand-induced receptor clustering are controlled by serine and threonine (Ser/Thr) phosphorylation, we initiated an investigation into the role of Ser/Thr phosphatases in both phosphorylation of the interferon-gamma (IFN-gamma) receptor and IFN gamma-induced gene expression in human peripheral-blood monocytes. Whereas IFN gamma alone did not enhance phosphorylation of the IFN gamma receptor, treatment of monocytes with the Ser/Thr phosphatase inhibitors, okadaic acid and calyculin A, resulted in increased phosphorylation of the IFN gamma receptor. However, when these cells were analysed for IFN gamma-induced IP-10 gene expression, there was profound inhibition. Using three IFN gamma-induced early-response genes, IP-10, the Fc gamma receptor type I (Fc gamma RI) and ISG-54, we found selective sensitivity to pretreatment with okadaic acid and calyculin A. Whereas IFN gamma induction of IP-10 was blocked by both inhibitors, only calyculin A prevented Fc gamma RI-gene expression. Neither inhibitor prevented ISG-54 induction by IFN gamma. IFN-gamma-activated formation of the DNA-binding-protein complex FcRF gamma (which binds to the promoter of the Fc gamma RI gene) remained unaffected by okadaic acid or calyculin A. Therefore these data suggest that Ser/Thr phosphatases have no major part in IFN gamma-initiated signal transduction across the membrane, but selectively control the ultimate transcription of a set of early-response genes.

Mammalian glucokinase and its gene

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