Arterial oxygen partial pressures reduce the insulin-dependent induction of the perivenously located glucokinase in rat hepatocyte cultures: mimicry of arterial oxygen pressures by H2O2

Genomic alterations drive metastases formation in pancreatic ductal adenocarcinoma cancer: deciphering the role of CDKN2A and CDKN2B in mediating liver tropism

Metastases are often the direct cause of death from pancreatic ductal adenocarcinoma (PDAC). The role of genomic alterations (GA) in mediating tropism and metastasis formation by PDAC cells is currently unknown. We aimed to identify GAs predisposing colonization of PDAC cells to the liver and decipher mechanisms enabling this process. In order to reveal specific genes, we studied the frequency of GA in 8,880 local and 7,983 metastatic PDAC samples. We observed differential pattern of GA in the local tumor and specific metastatic sites, with liver metastases characterized by deletion of CDKN2A/B (encoding p16/p15, respectively). The role of CDKN2A/B in promoting liver metastasis was evidenced by enhanced tumorigenic phenotype of p15/p16-deleted PDAC cells when exposed to hepatocytes conditioned media. The liver is characterized by high-ammonia low-glutamine environment and transcriptomic assays indicated unique adaptation of PDAC cells to these conditions, including regulation of genes leading to reduced glutaminolysis, like overexpression of GLUL and reduction in GLS2. Furthermore, metabolic assays indicated an increase in glutamate derived from [U-13C]-glucose in p15/p16-deleted cells. Importantly, these cells thrived under high ammonia condition. These data suggest a unique role for genomic alterations in mediating tropism of PDAC. Among these alterations, p15/16 deletion was identified as a promoter of liver metastases. Further studies indicated a unique role for p15/16 in regulating glutaminolysis. These findings reveal vulnerabilities in PDAC cells, which may pave the way for the development of novel therapeutic strategies aiming at the prevention of liver metastases formation.

Dynamic Metabolic Zonation of the Hepatic Glucose Metabolism Is Accomplished by Sinusoidal Plasma Gradients of Nutrients and Hormones

Being the central metabolic organ of vertebrates, the liver possesses the largest repertoire of metabolic enzymes among all tissues and organs. Almost all metabolic pathways are resident in the parenchymal cell, hepatocyte, but the pathway capacities may largely differ depending on the localization of hepatocytes within the liver acinus-a phenomenon that is commonly referred to as metabolic zonation. Metabolic zonation is rather dynamic since gene expression patterns of metabolic enzymes may change in response to nutrition, drugs, hormones and pathological states of the liver (e.g., fibrosis and inflammation). This fact has to be ultimately taken into account in mathematical models aiming at the prediction of metabolic liver functions in different physiological and pathological settings. Here we present a spatially resolved kinetic tissue model of hepatic glucose metabolism which includes zone-specific temporal changes of enzyme abundances which are driven by concentration gradients of nutrients, hormones and oxygen along the hepatic sinusoids. As key modulators of enzyme expression we included oxygen, glucose and the hormones insulin and glucagon which also control enzyme activities by cAMP-dependent reversible phosphorylation. Starting with an initially non-zonated model using plasma profiles under fed, fasted and diabetic conditions, zonal patterns of glycolytic and gluconeogenetic enzymes as well as glucose uptake and release rates are created as an emergent property. We show that mechanisms controlling the adaptation of enzyme abundances to varying external conditions necessarily lead to the zonation of hepatic carbohydrate metabolism. To the best of our knowledge, this is the first kinetic tissue model which takes into account in a semi-mechanistic way all relevant levels of enzyme regulation.

Zonation of hepatic fat accumulation: insights from mathematical modelling of nutrient gradients and fatty acid uptake

Intrinsic of non-alcoholic fatty liver diseases is an aberrant accumulation of triglycerides (steatosis), which occurs inhomogeneously within lobules. To improve our understanding of the mechanisms involved in this zonation patterning, we developed a mathematical multicompartment model of hepatic fatty acid metabolism accompanied by blood flow simulations. A model analysis determines the influence of the uptake process of fatty acids, the porto-central gradient of plasma fatty acid concentration, and the oxygen supply via blood on the zonation of triglyceride accumulation. From this theoretical perspective, the plasma oxygen gradient, but not the fatty acid gradient, leads the way to a zonated triglyceride accumulation by its decisive role in oxidative processes. In addition, the uptake mechanism of fatty acids seems to be fundamental for a pericentral dominance of steatosis. However, the mechanism of cellular fatty acid uptake from the blood is still under debate. Our theoretical approach supports the transporter-mediated uptake mechanism and reveals that the maximal velocity of fatty acid uptake affects the switching between a periportal and a pericentral triglyceride accumulation. Further research on hepatic fatty acid uptake is needed to push forward our understanding of aberrant triglyceride accumulation in diet-induced steatosis.

An in vitro hepatic zonation model with a continuous oxygen gradient in a microdevice

In a hepatic lobule, different sets of metabolic enzymes are expressed in the periportal (PP) and pericentral (PC) regions, forming a functional zonation, and the oxygen gradient is considered a determinant of zone formation. It is desirable to reproduce lobular microenvironment in vitro, but incubation of primary hepatocytes in conventional culture dishes has been limited at fixed oxygen concentrations due to technical difficulties. We designed a cell culture microdevice with an oxygen gradient to reproduce the hepatic microenvironment in vitro. The oxygen gradient during cell culture was monitored using a laser-assisted phosphorescence quenching method, and the cellular oxygen consumption rate could be estimated from changes in the gradient. Culture medium was continuously exchanged through microchannels installed in the device to maintain the oxygen gradient for a long term without transient hyper-oxygenation. The oxygen consumption rates of hepatocytes at 70.0mmHg and 31.4mmHg of partial oxygen pressure, which correspond to PP and PC regions in the microdevice, were 3.67×10(-10) and 3.15×10(-10)mol/s/10(6) cells, respectively. Antimycin A changed the oxygen gradient profile, indicating that cellular respiration can be estimated during cell culture. RT-PCR analysis of hepatocytes cultured under the oxygen gradient showed that mRNA expression of PEPCK and GK significantly increased in culture areas corresponding to PP and PC regions, respectively. These results indicate that the developed microdevice can reproduce the hepatic lobular microenvironment. The oxygen gradient in the microdevice can be closely controlled by changing the sizes of gas channels and the ambient oxygen concentration around the device; therefore, it could be expected to mimic the oxygen gradient of various organs, and it may be applicable to other pathological models.

Nuclear accumulation of fructose 1,6-bisphosphatase is impaired in diabetic rat liver

Using a streptozotocin-induced type 1 diabetic rat model, we analyzed and separated the effects of hyperglycemia and hyperinsulinemia over the in vivo expression and subcellular localization of hepatic fructose 1,6-bisphosphatase (FBPase) in the multicellular context of the liver. Our data showed that FBPase subcellular localization was modulated by the nutritional state in normal but not in diabetic rats. By contrast, the liver zonation was not affected in any condition. In healthy starved rats, FBPase was localized in the cytoplasm of hepatocytes, whereas in healthy re-fed rats it was concentrated in the nucleus and the cell periphery. Interestingly, despite the hyperglycemia, FBPase was unable to accumulate in the nucleus in hepatocytes from streptozotocin-induced diabetic rats, suggesting that insulin is a critical in vivo modulator. This idea was confirmed by exogenous insulin supplementation to diabetic rats, where insulin was able to induce the rapid accumulation of FBPase within the hepatocyte nucleus. Besides, hepatic FBPase was found phosphorylated only in the cytoplasm, suggesting that the phosphorylation state is involved in the nuclear translocation. In conclusion, insulin and not hyperglycemia plays a crucial role in the nuclear accumulation of FBPase in vivo and may be an important regulatory mechanism that could account for the increased endogenous glucose production of liver of diabetic rodents.

Liver Glucokinase(A456V) Induces Potent Hypoglycemia without Dyslipidemia through a Paradoxical Induction of the Catalytic Subunit of Glucose-6-Phosphatase

Recent reports point out the importance of the complex GK-GKRP in controlling glucose and lipid homeostasis. Several GK mutations affect GKRP binding, resulting in permanent activation of the enzyme. We hypothesize that hepatic overexpression of a mutated form of GK, GK(A456V), described in a patient with persistent hyperinsulinemic hypoglycemia of infancy (PHHI) and could provide a model to study the consequences of GK-GKRP deregulation in vivo. GK(A456V) was overexpressed in the liver of streptozotocin diabetic mice. Metabolite profiling in serum and liver extracts, together with changes in key components of glucose and lipid homeostasis, were analyzed and compared to GK wild-type transfected livers. Cell compartmentalization of the mutant but not the wild-type GK was clearly affected in vivo, demonstrating impaired GKRP regulation. GK(A456V) overexpression markedly reduced blood glucose in the absence of dyslipidemia, in contrast to wild-type GK-overexpressing mice. Evidence in glucose utilization did not correlate with increased glycogen nor lactate levels in the liver. PEPCK mRNA was not affected, whereas the mRNA for the catalytic subunit of glucose-6-phosphatase was upregulated ~4 folds in the liver of GK(A456V)-treated animals, suggesting that glucose cycling was stimulated. Our results provide new insights into the complex GK regulatory network and validate liver-specific GK activation as a strategy for diabetes therapy.

Liver glucokinase: An overview on the regulatory mechanisms of its activity

Blood glucose is the primary cellular substrate and in vivo must be tightly maintained. The liver plays a key role in glucose homeostasis increasing or decreasing glucose output and uptake during fasting and feeding. Glucokinase (GCK) is central to this process. Its activity is modulated in a coordinated manner via a complex set of mechanisms: in the postprandial period, the simultaneous rise in glucose and insulin increases GCK activity by enhanced gene expression, changes in cellular location, and interaction with regulatory proteins. Conversely, in the fasting state, the combined decrease in glucose and insulin concentrations and increase in glucagon concentrations, halt GCK activity. Herein we summarize the current knowledge regarding the regulation of hepatic GCK activity.

Transcriptional regulation of glucose sensors in pancreatic β-cells and liver: an update

Pancreatic β-cells and the liver play a key role in glucose homeostasis. After a meal or in a state of hyperglycemia, glucose is transported into the β-cells or hepatocytes where it is metabolized. In the β-cells, glucose is metabolized to increase the ATP:ADP ratio, resulting in the secretion of insulin stored in the vesicle. In the hepatocytes, glucose is metabolized to CO(2), fatty acids or stored as glycogen. In these cells, solute carrier family 2 (SLC2A2) and glucokinase play a key role in sensing and uptaking glucose. Dysfunction of these proteins results in the hyperglycemia which is one of the characteristics of type 2 diabetes mellitus (T2DM). Thus, studies on the molecular mechanisms of their transcriptional regulations are important in understanding pathogenesis and combating T2DM. In this paper, we will review a recent update on the progress of gene regulation of glucose sensors in the liver and β-cells.

FoxO1 and HNF-4 are involved in regulation of hepatic glucokinase gene expression by resveratrol

Resveratrol, a polyphenol derived from grapes, exerts important effects on glucose and lipid metabolism, yet detailed mechanisms mediating these effects remain unknown. The liver plays a central role in energy homeostasis, and glucokinase (GK) is a key enzyme involved in glucose utilization. Resveratrol activates SIRT1 (sirtuin 1), which promotes deacetylation of the forkhead transcription factor FoxO1. Previously, we reported that FoxO1 can suppress and that HNF-4 can stimulate GK expression in the liver. Here, we examined the role of FoxO1 and HNF-4 in mediating resveratrol effects on liver GK expression. Resveratrol suppressed hepatic GK expression in vivo and in isolated hepatocytes, and knocking down FoxO1 with shRNAs disrupted this effect. Reporter gene, gel shift, supershift assay, and chromatin immunoprecipitation studies show that FoxO1 binds to the GK promoter and that the interplay between FoxO1 and HNF-4 within the GK promoter is essential for mediating the effects of resveratrol. Resveratrol promotes deacetylation of FoxO1 and enhances its recruitment to the FoxO-binding element. Conversely, resveratrol suppresses recruitment of HNF-4 to its binding site, and knockdown of FoxO1 blocks this effect of resveratrol. Coprecipitation and chromatin immunoprecipitation studies show that resveratrol enhances interaction between FoxO1 and HNF-4, reduces binding of HNF-4 to its own site, and promotes its recruitment to the FoxO site in a FoxO1-dependent manner. These results provide the first evidence that resveratrol represses GK expression via FoxO1 and that the interaction between FoxO1 and HNF-4 contributes to these effects of resveratrol.

Molecular physiology of mammalian glucokinase

The glucokinase (GCK) gene was one of the first candidate genes to be identified as a human “diabetes gene". Subsequently, important advances were made in understanding the impact of GCK in the regulation of glucose metabolism. Structure elucidation by crystallography provided insight into the kinetic properties of GCK. Protein interaction partners of GCK were discovered. Gene expression studies revealed new facets of the tissue distribution of GCK, including in the brain, and its regulation by insulin in the liver. Metabolic control analysis coupled to gene overexpression and knockout experiments highlighted the unique impact of GCK as a regulator of glucose metabolism. Human GCK mutants were studied biochemically to understand disease mechanisms. Drug development programs identified small molecule activators of GCK as potential antidiabetics. These advances are summarized here, with the aim of offering an integrated view of the role of GCK in the molecular physiology and medicine of glucose homeostasis.

Opposite expression of the antioxidant heme oxygenase-1 in primary cells and tumor cells: regulation by interaction of USF-2 and Fra-1

Heme oxygenase-1 is the rate-limiting enzyme for the degradation of the prooxidant heme. Previously, we showed that an E-box within the HO-1 promoter is crucial for the regulation of HO-1 expression in primary hepatocytes. Further to investigate the importance of this E-box, we determined the regulatory capacity of the E-box-binding factor USF-2 in primary cells in comparison with transformed cell lines. We found that HO-1 expression was inhibited by USF-2 in primary cells, whereas it was induced in tumor cell lines. Mutation of either the E-box or the AP-1 site within the HO-1 promoter only partially affected the USF-dependent regulation. However, this regulation was dramatically reduced in tumor cells and completely abolished in primary cells transfected with an HO-1 promoter construct containing mutations in both the E-box and the AP-1 site, suggesting that AP-1 factors and USF-2 may act in a cooperative manner. Indeed, protein-protein interaction studies revealed that USF proteins interacted with Fra-1. Further, the USF-dependent HO-1 promoter activity was not detectable with an USF-2 mutant lacking residues of the USF-specific region (USR) or the transactivation domain encoded by exon 4. Together, these data suggest that USF-2 has opposite regulatory roles for HO-1 gene expression in primary cells and tumor cell lines.

The MAPK pathway and HIF-1 are involved in the induction of the human PAI-1 gene expression by insulin in the human hepatoma cell line HepG2

Enhanced levels of plasminogen activator inhibitor-1 (PAI-1) are considered to be a risk factor for pathological conditions associated with hypoxia or hyperinsulinemia. The expression of the PAI-1 gene is increased by insulin in different cells, although, the molecular mechanisms behind insulin-induced PAI-1 expression are not fully known yet. Here, we show that insulin upregulates human PAI-1 gene expression and promoter activity in HepG2 cells and that mutation of the hypoxia-responsive element (HRE)-binding hypoxia-inducible factor-1 (HIF-1) abolished the insulin effects. Mutation of E-boxes E4 and E5 abolished the insulin-dependent activation of the PAI-1 promoter only under normoxia, but did not affect it under hypoxia. Furthermore, the insulin effect was associated with activation of HIF-1alpha via mitogen-activated protein kinases (MAPKs) but not PDK1 and PKB in HepG2 cells. Furthermore, mutation of a putative FoxO1 binding site which was supposed to be involved in insulin-dependent PAI-1 gene expression influenced the insulin-dependent activation only under normoxia. Thus, insulin-dependent PAI-1 gene expression might be regulated by the action of both HIF-1 and FoxO1 transcription factors.

Deficiency of manganese superoxide dismutase in hepatocytes disrupts zonated gene expression in mouse liver

The liver acinus displays a physiological periportal to perivenous oxygen gradient. This gradient was implicated to use reactive oxygen species (ROS) as mediators for the zonal gene expression. Mitochondria use oxygen and produce ROS, therefore they may contribute to the zonation of gene expression. To further elucidate this, we used the Cre-loxP system to generate a hepatocyte-specific null mutation of the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) in mice. We found that ROS levels were enhanced in livers of MnSOD(-/-) mice which were reduced in size and displayed signs of liver failure such as intracellular protein droplets, increased apoptotic bodies and Bax levels as well as multinuclear hepatocytes. Further, the zonation of glutamine synthetase, glucokinase and phosphoenolpyruvate carboxykinase was no longer preserved. We conclude that deficiency of mitochondrial MnSOD initiates a dysregulation of zonated gene expression in liver.

Reactive oxygen species in the control of hypoxia-inducible factor-mediated gene expression

Reactive oxygen species (ROS) have long been considered as cytotoxic. However, recent evidence indicates a prominent role of ROS as signaling molecules in the response to hormones, growth and coagulation factors, cytokines and other factors as well as to changes in oxygen tension. The hypoxia-inducible transcription factors (HIFs) are key players in the cellular response to changes in oxygen tension. Recently, HIFs have also been shown to respond to the above-mentioned non-hypoxic stimuli. In this article, the role of ROS in the regulation of HIF-1 under hypoxic and non-hypoxic conditions is summarized.

Cell type-dependent regulation of the hypoxia-responsive plasminogen activator inhibitor-1 gene by upstream stimulatory factor-2

Transcriptional regulation of the plasminogen activator inhibitor type-1 (PAI-1) gene is an important issue since PAI-1 plays a crucial role in various pathological conditions. The transcription factor USF-2 was shown to be a negative regulator for rat PAI-1 expression, and therefore it was the aim of this study to evaluate the role of USF-2 for human PAI-1 expression. We found in human hepatoma cells (HepG2) that USF-2 induced human PAI-1 expression via two classical E-boxes and the hypoxia-responsive element (HRE) within the promoter. Gel-shift analyses showed that E-box 4 and E-box 5 bound USFs, and although the HRE contributed to the USF-dependent effects, it did not bind them. By contrast, USF-2 inhibited PAI-1 promoter activity in primary rat hepatocytes suggesting that PAI-1 expression depends on either the promoter context or USF activity which might be cell type-specific. Cotransfection of human or rat PAI-1 promoter luciferase constructs with expression vectors for wild-type USF-2 or USF-2 mutants in human HepG2 and rat H4IIE cells as well as in primary rat hepatocytes revealed that the effects of USF on PAI-1 expression depend on the cell type rather than the promoter context and that the USF-specific region domain of USF accounts for the observed cell type-specific effects.

Redox-sensitive regulation of the HIF pathway under non-hypoxic conditions in pulmonary artery smooth muscle cells

Pulmonary hypertension and vascular remodeling processes are associated with oxidative stress, hypoxia and enhanced levels of thrombin and vascular endothelial growth factor (VEGF). The hypoxia-inducible transcription factor HIF regulates the expression of VEGF under hypoxia. The HIF pathway is also activated by thrombin or CoCl2, likely via reactive oxygen species (ROS). In this study we investigated whether the redox-modifying enzymes superoxide dismutase (SOD), glutathione peroxidase (GPX) and catalase affect HIF levels and the expression of VEGF mRNA in pulmonary artery smooth muscle cells (PASMC). Stimulation of PASMC with thrombin or CoCl2 increased ROS production and enhanced HIF-alpha protein and VEGF mRNA levels as well as HIF-dependent reporter gene activity. These responses were inhibited by vitamin C and by overexpression of GPX and catalase, whereas the opposite effects were observed in SOD-expressing cells. These findings suggest that an 'antioxidant' state with reduced levels of H2O2 limits the activation of the HIF pathway, whereas a 'prooxidant' state allowing elevated H2O2 levels promotes it. Thus, shifting the redox balance to a more reduced environment, thereby limiting VEGF expression, may be beneficial for treating remodeling processes during pulmonary hypertension.

A Fenton reaction at the endoplasmic reticulum is involved in the redox control of hypoxia-inducible gene expression

It has been proposed that hydroxyl radicals (.OH) generated in a perinuclear iron-dependent Fenton reaction are involved in O(2)-dependent gene expression. Thus, it was the aim of this study to localize the cellular compartment in which the Fenton reaction takes place and to determine whether scavenging of.OH can modulate hypoxia-inducible factor 1 (HIF-1)-dependent gene expression. The Fenton reaction was localized by using the nonfluorescent dihydrorhodamine (DHR) 123 that is irreversibly oxidized to fluorescent rhodamine 123 while scavenging.OH together with gene constructs allowing fluorescent labeling of mitochondria, endoplasmic reticulum (ER), Golgi apparatus, peroxisomes, or lysosomes. A 3D two-photon confocal laser scanning microscopy showed.OH generation in distinct hot spots of perinuclear ER pockets. This ER-based Fenton reaction was strictly pO(2)-dependent. Further colocalization experiments showed that the O(2)-sensitive transcription factor HIF-1alpha was present at the ER under normoxia, whereas HIF-1alpha was present only in the nucleus under hypoxia. Inhibition of the Fenton reaction by the.OH scavenger DHR attenuated HIF-prolyl hydroxylase activity and interaction with von Hippel-Lindau protein, leading to enhanced HIF-1alpha levels, HIF-1alpha transactivation, and activated expression of the HIF-1 target genes plasminogen activator inhibitor 1 and heme oxygenase 1. Further,.OH scavenging appeared to enhance redox factor 1 (Ref-1) binding and, thus, recruitment of p300 to the transactivation domain C because mutation of the Ref-1 binding site cysteine 800 abolished DHR-induced transactivation. Thus, the localized Fenton reaction appears to impact the expression of hypoxia-regulated genes by means of HIF-1alpha stabilization and coactivator recruitment.

Modulation of glucokinase expression by hypoxia-inducible factor 1 and upstream stimulatory factor 2 in primary rat hepatocytes

Glucokinase (GK) is the key enzyme of glucose utilization in liver and is localized in the less aerobic perivenous area. Until now, the O2-responsive elements in the liver-specific GK promoter are unknown, and therefore the aim of this study was to identify the O2-responsive element in this promoter. We found that the GK promoter sequence -87/-80 matched the binding site for hypoxia inducible factor 1 (HIF-1) and upstream stimulatory factor (USF). In primary rat hepatocytes we could show that venous pO2 enhanced HIF-1alpha and USF-2a levels, both of which activated GK expression. Furthermore, transfection experiments revealed that the GK sequence -87/-80 mediated the HIF-1alpha- or USF-2-dependent activation of the GK promoter. The binding of HIF-1 and USF to the GK-HRE was corroborated by electrophoretic mobility shift assay (EMSA). However, the maximal response to HIF-1alpha or USF was only achieved when constructs with the -87/-80 sequence in context with a 3'-36 bp native GK promoter sequence containing a hepatocyte nuclear factor 4 (HNF-4) binding site were used. HIF-1alpha and HNF-4 additively activated the GK promoter, while USF-2 and HNF-4 together did not show this additive activation. Thus, HIF-1 and USF may play differential roles in the modulation of GK expression in response to O2.

The Transcription Factors HIF-1 and HNF-4 and the Coactivator p300 Are Involved in Insulin-regulated Glucokinase Gene Expression via the Phosphatidylinositol 3-Kinase/Protein Kinase B Pathway

Glucokinase plays a key role in the regulation of glucose utilization in liver and its expression is strongly enhanced by insulin and modulated by venous pO(2). In primary rat hepatocytes, pO(2) modulated insulin-dependent glucokinase (GK) gene expression was abolished by wortmannin an inhibitor of phosphatidylinositol 3-kinase (PI3K). Transfection of vectors encoding the p110 catalytic subunit of PI3K or constitutively active protein kinase B (PKB) stimulated GK mRNA and protein expression. The transfection of GK promoter constructs together with expression vectors for p110 or constitutively active PKB revealed that the GK promoter region -87/-80 mediates the response to PI3K/PKB. Transfection experiments and gel shift assays show that this element is able to bind hypoxia-inducible factor-1 (HIF-1) in a hypoxia- and PKB-dependent manner. The ability of HIF-1alpha to activate the GK promoter was enhanced by hepatocyte nuclear factor-4alpha (HNF-4alpha), acting via the sequence -52/-39, and by the coactivator p300. Stimulation of the GK promoter by insulin was dependent on the intact -87/-80 region and maximal stimulation was achieved when HIF-1alpha, HNF-4, and p300 were cotransfected with the -1430 GK promoter Luc construct in primary hepatocytes. Maximal stimulation of GK promoter activity by insulin was inhibited when a p300 vector was used containing a mutation within a PKB phosphorylation site. Thus, a regulatory transcriptional complex consisting of HIF-1, HNF-4, and p300 appears to be involved in insulin-dependent GK gene activation.

Transcriptional regulation of heme oxygenase-1 gene expression by MAP kinases of the JNK and p38 pathways in primary cultures of rat hepatocytes

Heme oxygenase-1 (HO-1) gene expression is induced by various oxidative stress stimuli including sodium arsenite. Since mitogen-activated protein kinases (MAPKs) are involved in stress signaling we investigated the role of arsenite and MAPKs for HO-1 gene regulation in primary rat hepatocytes. The Jun N-terminal kinase (JNK) inhibitor SP600125 decreased sodium arsenite-mediated induction of HO-1 mRNA expression. HO-1 protein and luciferase activity of reporter gene constructs with -754 bp of the HO-1 promoter were induced by overexpression of kinases of the JNK pathway and MKK3. By contrast, overexpression of Raf-1 and ERK2 did not affect expression whereas overexpression of p38alpha, beta, and delta decreased and p38gamma increased HO-1 expression. Electrophoretic mobility shift assays (EMSA) revealed that a CRE/AP-1 element (-668/-654) bound c-Jun, a target of the JNK pathway. Deletion or mutation of the CRE/AP-1 obliterated the JNK- and c-Jun-dependent up-regulation of luciferase activity. EMSA also showed that an E-box (-47/-42) was bound by a putative p38 target c-Max. Mutation of the E-box strongly reduced MKK3, p38 isoform-, and c-Max-dependent effects on luciferase activity. Thus, the HO-1 CRE/AP-1 element mediates HO-1 gene induction via activation of JNK/c-Jun whereas p38 isoforms act through a different mechanism via the E-box.

Signaling cross-talk between hypoxia and glucose via hypoxia-inducible factor 1 and glucose response elements

The substrates oxygen and glucose are important for the appropriate regulation of metabolism, angiogenesis, tumorigenesis and embryonic development. The knowledge about an interaction between these two signals is limited. We demonstrated that the regulation of glucagon receptor, insulin receptor and L-type pyruvate kinase (L-PK) gene expression in liver is dependent upon a cross-talk between oxygen and glucose. The periportal to perivenous drop in O2 tension was proposed to be an endocrine key regulator for the zonated gene expression in liver. In primary rat hepatocyte cultures, the expression of the glucagon receptor and the L-PK mRNA was maximally induced by glucose under arterial pO2 whereas the insulin receptor was maximally induced under perivenous pO2. It was demonstrated for the L-PK gene that the modulation by O2 of the glucose-dependent induction occured at the glucose-responsive element (Glc(PK)RE) in the L-PK gene promoter. The reduction of the glucose-dependent induction of the L-PK gene expression under venous pO2 appeared to be mediated via an interference between hypoxia-inducible factor 1 (HIF-1) and the glucose-responsive transcription factors at the Glc(PK)RE. The glucose response element (GlcRE) also functioned as a hypoxia response element and, vice versa, a hypoxia-responsive element was functioning as a GlcRE. Thus, our findings implicate that the cross-talk between oxygen and glucose might have a fundamental role in the regulation of several physiological and pathophysiological processes.

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.

Cross-talk between the signals hypoxia and glucose at the glucose response element of the L-type pyruvate kinase gene

The signals oxygen and glucose play an important role in metabolism, angiogenesis, tumorigenesis, and embryonic development. Little is known about an interaction of these two signals. We demonstrate here the cross-talk between oxygen and glucose in the regulation of L-type pyruvate kinase (L-PK) gene expression in the liver. In the liver the periportal to perivenous drop in O(2) tension was proposed to be an endocrine key regulator for the zonated gene expression. In primary rat hepatocyte cultures the expression of the L-PK gene on mRNA and on protein level was induced by venous pO(2), whereas its glucose-dependent induction occurred predominantly under arterial pO(2). It was shown by transient transfection of L-PK promoter luciferase and glucose response element (Glc(PK)RE) SV40 promoter luciferase gene constructs that the modulation by O(2) of the glucose-dependent induction occurred at the Glc(PK)RE in the L-PK gene promoter. The reduction of the glucose-dependent induction of the L-PK gene expression under venous pO(2) appeared to be mediated via an interference between hypoxia inducible factor-1 (HIF-1) and upstream stimulating factor at the Glc(PK)RE. The glucose response element also functioned as an hypoxia response element which was confirmed in cotransfection assays with Glc(PK)RE luciferase gene constructs and HIF-1alpha expression vectors. Furthermore, it was found by gel shift and supershift assay that HIF-1alpha and USF-1 or USF-2 could bind to the Glc(PK)RE. Our findings implicate that the cross-talk between oxygen and glucose might have a fundamental role in the regulation of several physiological and pathophysiological processes.

The upstream stimulatory factor-2a inhibits plasminogen activator inhibitor-1 gene expression by binding to a promoter element adjacent to the hypoxia-inducible factor-1 binding site

Plasminogen activator inhibitor-1 (PAI-1) expression is induced by hypoxia (8% O(2)) via the PAI-1 promoter region -175/-159 containing a hypoxia response element (HRE-2) binding the hypoxia-inducible factor-1 (HIF-1) and an adjacent response element (HRE-1) binding a so far unknown factor. The aim of the present study was to identify this factor and to investigate its role in the regulation of PAI-1 expression. It was found by supershift assays that the upstream stimulatory factor-2a (USF-2a) bound mainly to the HRE-1 of the PAI-1 promoter and to a lesser extent to HRE-2. Overexpression of USF-2a inhibited PAI-1 messenger RNA and protein expression and activated L-type pyruvate kinase expression in primary rat hepatocytes under normoxia and hypoxia. Luciferase (Luc) gene constructs driven by 766 and 276 base pairs of the 5'-flanking region of the PAI-1 gene were transfected into primary hepatocytes together with expression vectors encoding wild-type USF-2a and a USF-2a mutant lacking DNA binding and dimerization activity (DeltaHU2a). Cotransfection of the wild-type USF-2a vector reduced Luc activity by about 8-fold, whereas cotransfection of DeltaHU2a did not influence Luc activity. Mutation of the HRE-1 (-175/-168) in the PAI-1 promoter Luc constructs decreased USF-dependent inhibition of Luc activity. Mutation of the HRE-2 (-165/-158) was less effective. Cotransfection of a HIF-1alpha vector could compete for the binding of USF at HRE-2. These results indicated that the balance between 2 transcriptional factors, HIF-1 and USF-2a, which can bind adjacent HRE sites, appears to be involved in the regulation of PAI-1 expression in many clinical conditions.

Oxygen regulation of gene expression: a study in opposites

Oxygen is crucial to aerobic metabolism, but excesses of oxygen or reactive oxygen species (ROS) can injure cells. This minireview addresses two transcription factors that regulate several cellular responses to oxygen tension. Hypoxia inducible factor-1 (HIF-1) is a heterodimeric protein activated by hypoxia. Levels of HIF-1 are regulated by removal of the HIF-1alpha subunit through ubiquination and proteasomal destruction under normoxic conditions. Hypoxia inhibits the ubiquination of HIF-1alpha, preventing its destruction and allowing it to bind to hypoxia-responsive elements in gene promoter, enhancer, and intronic sequences. HIF-1 induces the expression of the hypoxia responsive genes vascular endothelial growth factor and erythropoietin. Its dysregulation has been implicated in von Hippel-Lindau disease. Nuclear factor kappaB (NFkappaB) is a family of pleotropic, dimeric transcription factors, and has a complex pattern of regulation. Under normoxic conditions, NFkappaB is bound to one of several inhibitory proteins (e.g., IkappaB) that prevent its nuclear translocation. Hyperoxia or elevations of ROS cause the ubiquination and destruction of the inhibitory proteins, freeing NFkappaB and allowing it to bind to target gene promoters. Hyperoxia in cell and animal models and acute lung injury in humans induce the expression of multiple proinflammatory cytokines through NFkappaB-dependent mechanisms. Although HIF-1 and NFkappaB respond to changes in pO(2), the precise nature of the oxygen sensing and transduction pathways is unclear in both cases. Both heme-protein and redox-sensitive mechanisms have been proposed. Improved understanding of oxygen-sensitive gene regulation may suggest targeted therapies for human disease.

Physiological oxygen tensions modulate expression of the mdr1b multidrug-resistance gene in primary rat hepatocyte cultures

P-Glycoprotein transporters encoded by mdr1 (multidrug resistance) genes mediate extrusion of an array of lipophilic xenobiotics from the cell. In rat liver, mdr transcripts have been shown to be expressed mainly in hepatocytes of the periportal region. Since gradients in oxygen tension (pO(2)) may contribute towards zonated gene expression, the influence of arterial and venous pO(2) on mRNA expression of the mdr1b isoform was examined in primary rat hepatocytes cultured for up to 3 days. Maximal mdr1b mRNA levels (100%) were observed under arterial pO(2) after 72 h, whereas less than half-maximal mRNA levels (40%) were attained under venous pO(2). Accordingly, expression of mdr protein and extrusion of the mdr1 substrate rhodamine 123 were maximal under arterial pO(2) and reduced under venous pO(2). Oxygen-dependent modulation of mdr1b mRNA expression was prevented by actinomycin D, indicating transcriptional regulation. Inhibition of haem synthesis by 25 microM CoCl(2) blocked mdr1b mRNA expression under both oxygen tensions, whereas 80 microM desferrioxamine abolished modulation by O(2). Haem (10 microM) increased mdr1b mRNA levels under arterial and venous pO(2). In hepatocytes treated with 50 microM H(2)O(2), mdr1b mRNA expression was elevated by about 1.6-fold at venous pO(2) and 1.5-fold at arterial pO(2). These results support the conclusion that haem proteins are crucial for modulation of mdr1b mRNA expression by O(2) in hepatocyte cultures and that reactive oxygen species may participate in O(2)-dependent signal transduction. Furthermore, the present study suggests that oxygen might be a critical modulator for zonated secretion of mdr1 substrates into the bile.

Perivenous localization of insulin receptor protein in rat liver, and regulation of its expression by glucose and oxygen in hepatocyte cultures

Insulin stimulates glucose utilization in the liver, which occurs mainly in the less aerobic, perivenous, zone. Accordingly, the insulin receptor protein was predominantly expressed in this area, although the insulin receptor mRNA was homogeneously distributed. In hepatocyte cultures venous O(2) partial pressure (pO(2)) induced insulin receptor protein expression. High glucose concentrations enhanced insulin receptor protein under arterial and venous pO(2). The induction of insulin receptor protein by venous pO(2) would explain its zonated expression.

Mammalian oxygen sensing, signalling and gene regulation

Oxygen is essential to the life of all aerobic organisms. Virtually every cell type is able to sense a limited oxygen supply (hypoxia) and specifically to induce a set of oxygen-regulated genes. This review summarizes current concepts of mammalian oxygen-sensing and signal-transduction pathways. Since the discovery of the hypoxia-inducible factors (HIFs), a great deal of progress has been made in our comprehension of how hypoxia induces the expression of oxygen-regulated genes. The alpha subunit of the heterodimeric transcription factors HIF-1, 2 and 3 is unstable under normoxia but is rapidly stabilized upon exposure to hypoxic conditions. Following heterodimerization with the constitutively expressed beta subunit, HIFs activate the transcription of an increasing number of genes involved in maintaining oxygen homeostasis at the cellular, local and systemic levels.

Induction of the Plasminogen Activator Inhibitor-1 Gene Expression by Mild Hypoxia Via a Hypoxia Response Element Binding the Hypoxia-Inducible Factor-1 in Rat Hepatocytes

Plasminogen activator inhibitor-1 (PAI-1) is the primary physiological inhibitor of both tissue-type and urokinase-type plasminogen activators. The balance between plasminogen activators and PAI-1 plays an important role in several physiological and pathophysiological processes such as atherosclerosis or thrombosis. Because these conditions are associated with hypoxia, it was the aim of the present study to investigate the influence of low O2tension on the expression of PAI-1 mRNA and protein using primary cultured rat hepatocytes as a model system. We found that PAI-1 mRNA and protein were induced by mild hypoxia (8% O2). The hypoxia-dependent PAI-1 mRNA induction was transcriptionally regulated because it was inhibited by actinomycin D (ActD). Luciferase (LUC) reporter gene constructs driven by about 800 bp of the 5′-flanking region of the rat PAI-1 gene were transiently transfected into primary rat hepatocytes; mild hypoxia caused a 3-fold induction, which was mediated by the PAI-1 promoter region -175/-158 containing 2 putative hypoxia response elements (HRE) binding the hypoxia-inducible factor (HIF-1). Mutation of the HRE-1 (-175/-168) or HRE-2 (-165/-158) also abolished the induction by mild hypoxia. Cotransfection of a HIF-1 vector and the PAI-1–LUC constructs, as well as gel shift assays, showed that the HRE-2 of the PAI-1 promoter was most critical for induction by hypoxia and HIF-1 binding. Thus, PAI-1 induction by mild hypoxia via a HIF-1 binding HRE in the rat PAI-1 promoter appears to be the mechanism causing the increase in PAI-1 in many clinical conditions associated with O2deficiency.

Induction of the Plasminogen Activator Inhibitor-1 Gene Expression by Mild Hypoxia Via a Hypoxia Response Element Binding the Hypoxia-Inducible Factor-1 in Rat Hepatocytes

Plasminogen activator inhibitor-1 (PAI-1) is the primary physiological inhibitor of both tissue-type and urokinase-type plasminogen activators. The balance between plasminogen activators and PAI-1 plays an important role in several physiological and pathophysiological processes such as atherosclerosis or thrombosis. Because these conditions are associated with hypoxia, it was the aim of the present study to investigate the influence of low O2tension on the expression of PAI-1 mRNA and protein using primary cultured rat hepatocytes as a model system. We found that PAI-1 mRNA and protein were induced by mild hypoxia (8% O2). The hypoxia-dependent PAI-1 mRNA induction was transcriptionally regulated because it was inhibited by actinomycin D (ActD). Luciferase (LUC) reporter gene constructs driven by about 800 bp of the 5′-flanking region of the rat PAI-1 gene were transiently transfected into primary rat hepatocytes; mild hypoxia caused a 3-fold induction, which was mediated by the PAI-1 promoter region -175/-158 containing 2 putative hypoxia response elements (HRE) binding the hypoxia-inducible factor (HIF-1). Mutation of the HRE-1 (-175/-168) or HRE-2 (-165/-158) also abolished the induction by mild hypoxia. Cotransfection of a HIF-1 vector and the PAI-1–LUC constructs, as well as gel shift assays, showed that the HRE-2 of the PAI-1 promoter was most critical for induction by hypoxia and HIF-1 binding. Thus, PAI-1 induction by mild hypoxia via a HIF-1 binding HRE in the rat PAI-1 promoter appears to be the mechanism causing the increase in PAI-1 in many clinical conditions associated with O2deficiency.

Zonation of gluconeogenesis, ketogenesis and intracellular pH in livers from normal and diabetic ketoacidotic rats: evidence for intralobular redistribution of metabolic events in ketoacidosis

The intralobular distribution of metabolism was examined in the livers from rats with severe diabetic ketoacidosis (DKA), perfused at pH 6.8, and compared with that in livers from normal starved animals perfused at either pH 7.4 or 6.8. With lactate and palmitate as substrates, the perivenous uptake of periportally synthesized glucose seen in normal livers at pH 7.4 was abolished during DKA; indeed, gluconeogenesis was most active in the perivenous region. Whereas in normal livers perfused at pH 7.4 the periportal region showed a markedly elevated intracellular pH (pH(i)) compared with the perivenous zone, this distribution of pH(i) was reversed in DKA, with an intermediate distribution in normal livers perfused at pH 6. 8. 3-Hydroxybutyrate was generated throughout the lobule. Some acetoacetate generated periportally was converted to 3-hydroxybutyrate more perivenously. A steep gradient of oxygen uptake along the radius of the lobule was apparent in all three groups; oxygen uptake was greatly decreased perivenously despite adequate oxygen supply. These findings are consistent with our previous observations of the lobular co-location of high pH(i) and gluconeogenesis, and might offer an explanation of how high gluconeogenic rates can continue in spite of severe systemic acidosis in DKA. The findings provide direct evidence for a marked redistribution of intralobular metabolism in DKA.

Repression of gene expression by oxidative stress

Gene expression is modulated by both physiological signals (hormones, cytokines, etc.) and environmental stimuli (physical parameters, xenobiotics, etc.). Oxidative stress appears to be a key pleiotropic modulator which may be involved in either pathway. Indeed, reactive oxygen species (ROS) have been described as second messengers for several growth factors and cytokines, but have also been shown to rise following cellular insults such as xenobiotic metabolism or enzymic deficiency. Extensive studies on the induction of stress-response genes by oxidative stress have been reported. In contrast, owing to the historical focus on gene induction, less attention has been paid to gene repression by ROS. However, a growing number of studies have shown that moderate (i.e. non-cytotoxic) oxidative stress specifically down-regulates the expression of various genes. In this review, we describe the alteration of several physiological functions resulting from oxidative-stress-mediated inhibition of gene transcription. We will then focus on the repressive oxidative modulation of various transcription factors elicited by ROS.

Identification of an oxygen-responsive element in the 5'-flanking sequence of the rat cytosolic phosphoenolpyruvate carboxykinase-1 gene, modulating its glucagon-dependent activation

The glucagon-stimulated transcription of the cytosolic phosphoenolpyruvate carboxykinase-1 (PCK1) gene is mediated by cAMP and positively modulated by oxygen in primary hepatocytes. Rat hepatocytes were transfected with constructs containing the first 2500, 493 or 281 bp of the PCK1 5'-flanking region in front of the chloramphenicol acetyltransferase (CAT) reporter gene. With all three constructs glucagon induced CAT activity with decreasing efficiency maximally under arterial pO2 and to about 65% under venous pO2. Rat hepatocytes were then transfected with constructs containing the first 493 bp of the PCK1 5'-flanking region in front of the luciferase (LUC) reporter gene, which were block-mutated at the CRE1 (cAMP-response element-1; -93/-86), putative CRE2 (-146/-139), promoter element (P) 1 (-118/-104), P2 (-193/-181) or P4 (-291/-273) sites. Glucagon induced LUC activity strongly when the P1 and P2 sites were mutated and weakly when the P4 site was mutated; induction of the P1, P2 and P4 mutants was positively modulated by the pO2. Glucagon also induced LUC activity strongly when the putative CRE2 site was altered; however, induction of the CRE2 mutant was not modulated by the pO2. Glucagon did not induce LUC activity when the CRE1 site was modified. These experiments suggested that the CRE1 but not the putative CRE2 was an essential site necessary for the cAMP-mediated PCK1 gene activation by glucagon and that the putative CRE2 site was involved in the oxygen-dependent modulation of PCK1 gene activation. To confirm these conclusions rat hepatocytes were transfected with simian virus 40 (SV40)-promoter-driven LUC-gene constructs containing three CRE1 sequences (-95/-84), three CRE2 sequences (-148/-137) or three CRE1 sequences plus two CRE2 sequences of the PCK1 gene in front of the SV40 promoter. Glucagon induced LUC activity markedly when the CRE1, but not when the CRE2, sites were in front of the SV40-LUC gene; however, induction of the (CRE1)3SV40-LUC constructs was not modulated by the pO2. Glucagon also induced LUC activity very strongly when the CRE1 and CRE2 sites were combined; induction of the (CRE1)3(CRE2)2SV40-LUC constructs was positively modulated by the pO2. These findings corroborated that sequences of the putative CRE2 site were responsible for the modulation by oxygen of the CRE1-dependent induction by glucagon of PCK1 gene transcription.

A mechanistic model for the development and maintenance of portocentral gradients in gene expression in the liver

In the liver, genes are expressed along a portocentral gradient. Based on their adaptive behavior, a gradient versus compartment type, and a dynamic versus stable type of gradient have been recognized. To understand at least in principle the development and maintenance of these gradients in gene expression in relation to the limited number of signal gradients, we propose a simple and testable model. The model uses portocentral gradients of signal molecules as input, while the output depends on two gene-specific variables, viz., the affinity of the gene for its regulatory factors and the degree of cooperativity that determines the response in the signal-transduction pathways. As a preliminary validity test for its performance, the model was tested on control and hormonally induced expression patterns of phosphoenolpyruvate carboxykinase (PCK), carbamoylphosphate synthetase I (CPS), and glutamine synthetase (GS). Affinity was found to determine the overall steepness of the gradient, whereas cooperativity causes these gradients to steepen locally, as is necessary for a compartment-like expression pattern. Interaction between two or more different signal gradients is necessary to ensure a stable expression pattern under different conditions. The diversity in sequence and arrangement of related DNA-response elements of genes appears to account for the gene-specific shape of the portocentral gradients in expression. The feasibility of testing the function of hepatocyte-specific DNA-response units in vivo is demonstrated by integrating such units into a ubiquitously active promoter/enhancer and analyzing the pattern of expression of these constructs in transgenic mice.

Effects of modulators of the production and degradation of hydrogen peroxide on erythropoietin synthesis

Erythropoietin (Epo) synthesis is suppressed in normoxia and stimulated in hypoxia. To test the hypothesis that the cellular H2O2 level is important in the control of Epo synthesis, we have studied effects of modulators of H2O2 generation and degradation on Epo production in human hepatic cell cultures (hepatoma lines HepG2 and Hep3B). In addition, we measured the activities of antioxidant enzymes (catalase, superoxide dismutase, glutathione peroxidase) in cultures following hypoxia exposure or H2O2 treatment. The results show that the formation of immunoreactive Epo was stimulated in normoxic cultures by treatment with exogenous catalase thus mimicking the effect of hypoxia (24 h incubation periods). Epo production was also stimulated when scavengers of reactive O2 species (tetramethylthiourea, dihydrorhodamine) were added to the cells. On the other hand, stimulators of H2O2 generation (xanthine oxidase, glucose oxidase, NADH, NADPH) lowered Epo production in hypoxic cultures. Hypoxia exposure decreased superoxide dismutase activity and H2O2 treatment reduced catalase activity thus influencing the endogenous antioxidant defense system. These findings support the concept that reactive O2 species, primarily H2O2, act as messengers in the O2-dependent control of the hepatic production of Epo. Changes in the cellular activities of antioxidant enzymes appear to play only a minor role in this process.

Involvement of a local fenton reaction in the reciprocal modulation by O2 of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase gene and the insulin-dependent activation of the glucokinase gene in rat hepatocytes

H2O2 mimicked the action of periportal pO2 in the modulation by O2 of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase (PCK) gene and the insulin-dependent activation of the glucokinase (GK) gene. H2O2 can be converted in the presence of Fe2+ in a Fenton reaction into hydroxyl anions and hydroxyl radicals (.OH). The hydroxyl radicals are highly reactive and might interfere locally with transcription factors. It was the aim of the present study to investigate the role of and to localize such a Fenton reaction. Hepatocytes cultured for 24 h were treated under conditions mimicking periportal or perivenous pO2 with glucagon or insulin plus the iron chelator desferrioxamine (DSF) or the hydroxyl radical scavenger dimethylthiourea (DMTU) to inhibit the Fenton reaction. PCK mRNA was induced by glucagon maximally under conditions of periportal pO2 and half-maximally under venous pO2. GK mRNA was induced by insulin with reciprocal modulation by O2. DSF and DMTU reduced the induction of PCK mRNA to about half-maximal and increased the induction of GK mRNA to maximal under both O2 tensions. Hydroxyl radical formation was maximal under arterial pO2. Perivenous pO2, DSF and DMTU each decreased the formation of .OH to about 70% of control. The Fenton reaction could be localized in a perinuclear space by confocal laser microscopy and three-dimensional reconstruction techniques. In the same compartment, iron could be detected by electron-probe X-ray microanalysis. Thus a local Fenton reaction is involved in the O2 signalling, which modulated the glucagon- and insulin-dependent PCK gene and GK gene activation.

Identification of upstream stimulatory factor as transcriptional activator of the liver promoter of the glucokinase gene

A functionally important cis-acting element termed P2 was identified in the liver promoter of the glucokinase gene. Element P2 was delineated by footprinting in vitro with nuclear proteins from rat liver and spleen. Its core sequence in the rat gene is a canonical CACGTG E-box. In the electrophoretic mobility-shift assay with nuclear proteins from rat liver, hepatocytes and hepatoma cells, an oligonucleotide with P2 in the context of the glucokinase promoter sequence gave rise to a DNA-protein complex shown to contain the upstream stimulatory factor (USF) by specific competition experiments and by reactivity with anti-USF antibodies. Transient transfection of hepatoma HepG2 cells, combined with site-directed mutagenesis, demonstrated that the P2 element was important for liver glucokinase promoter activity. Co-transfection of an expression plasmid coding for USF1 activated reporter gene expression in a manner dependent on an intact P2 element, whereas an expression plasmid for c-Myc was ineffective. Expression of a truncated form of USF1 lacking the transcription activation domain and the basic region decreased reporter activity by a dominant-negative effect. The functional significance of the P2 element was also demonstrated in transient transfection of primary hepatocytes.

Periportal localization of glucagon receptor mRNA in rat liver and regulation of its expression by glucose and oxygen in hepatocyte cultures

Glucagon is the major hormone activating glycogenolysis and gluconeogenesis both localized in the periportal, more aerobic zone of the liver. Accordingly, the glucagon receptor (GcgR) mRNA was found to be predominantly expressed in this area. In hepatocyte cultures high glucose concentrations as reached after a meal induced GcgR mRNA under arterial but not venous pO2. The induction by glucose was partially antagonized by insulin and unaffected by glucagon. The modulation by 02 of the glucose-dependent induction would contribute to the zonated expression of GcgR mRNA.