Insulin inhibits hepatocellular glucose production by utilizing liver-enriched transcriptional inhibitory protein to disrupt the association of CREB-binding protein and RNA polymerase II with the phosphoenolpyruvate carboxykinase gene promoter

PRMT1 promotes hyperglycemia in a FoxO1-dependent manner, affecting glucose metabolism, during hypobaric hypoxia exposure, in rat model

PURPOSE

High-altitude (HA) environment causes changes in cellular metabolism among unacclimatized humans. Previous studies have revealed that insulin-dependent activation of protein kinase B (Akt) regulates metabolic processes via discrete transcriptional effectors. Moreover, protein arginine methyltransferase (PRMT)1-dependent arginine modification of forkhead box other (FoxO)1 protein interferes with Akt-dependent phosphorylation. The present study was undertaken to test the involvement of PRMT1 on FoxO1 activation during hypobaric hypoxia (HH) exposure in rat model.

METHODS

Samples were obtained from normoxia control (NC) and HH-exposed (H) rats, subdivided according to the duration of HH exposure. To explore the specific role played by PRMT1 during HH exposure, samples from 1d pair-fed (PF) NC, 1d acute hypoxia-exposed (AH) placebo-treated, and 1d AH TC-E-5003-treated rats were investigated. Quantitative reverse transcriptase polymerase chain reaction (RT-qPCR) was performed to determine expressions of glycolytic, gluconeogenic enzymes, and insulin response regulating genes. Immuno-blot and enzyme linked immunosorbent assay (ELISA) were used for insulin response regulating proteins. Nuclear translocation of FoxO1 was analyzed using deoxyribonucleic acid (DNA)-binding ELISA kit.

RESULTS

We observed HH-induced increase in glycolytic enzyme expressions in hepatic tissue unlike hypothalamic tissue. PRMT1 expression increased during HH exposure, causing insulin resistance and resulting increase in FoxO1 nuclear translocation, leading to hyperglycemia. Conversely, PRMT1 inhibitor treatment promoted inhibition of FoxO1 activity and increase in glucose uptake during HH exposure leading to reduction in blood-glucose and hepatic glycogen levels.

CONCLUSIONS

PRMT1 might have a potential importance as a therapeutic target for the treatment of HH-induced maladies.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

Insulin, CCAAT/enhancer-binding proteins and lactate regulate the human 11β-hydroxysteroid dehydrogenase type 2 gene expression in colon cancer cell lines

11β-Hydroxysteroid dehydrogenases (11beta-HSD) modulate mineralocorticoid receptor transactivation by glucocorticoids and regulate access to the glucocorticoid receptor. The isozyme 11beta-HSD2 is selectively expressed in mineralocorticoid target tissues and its activity is reduced in various disease states with abnormal sodium retention and hypertension, including the apparent mineralocorticoid excess. As 50% of patients with essential hypertension are insulin resistant and hyperinsulinemic, we hypothesized that insulin downregulates the 11beta-HSD2 activity. In the present study we show that insulin reduced the 11beta-HSD2 activity in cancer colon cell lines (HCT116, SW620 and HT-29) at the transcriptional level, in a time and dose dependent manner. The downregulation was reversible and required new protein synthesis. Pathway analysis using mRNA profiling revealed that insulin treatment modified the expression of the transcription factor family C/EBPs (CCAAT/enhancer-binding proteins) but also of glycolysis related enzymes. Western blot and real time PCR confirmed an upregulation of C/EBP beta isoforms (LAP and LIP) with a more pronounced increase in the inhibitory isoform LIP. EMSA and reporter gene assays demonstrated the role of C/EBP beta isoforms in HSD11B2 gene expression regulation. In addition, secretion of lactate, a byproduct of glycolysis, was shown to mediate insulin-dependent HSD11B2 downregulation. In summary, we demonstrate that insulin downregulates HSD11B2 through increased LIP expression and augmented lactate secretion. Such mechanisms are of interest and potential significance for sodium reabsorption in the colon.

Splicing regulator SLU7 is essential for maintaining liver homeostasis

A precise equilibrium between cellular differentiation and proliferation is fundamental for tissue homeostasis. Maintaining this balance is particularly important for the liver, a highly differentiated organ with systemic metabolic functions that is endowed with unparalleled regenerative potential. Carcinogenesis in the liver develops as the result of hepatocellular de-differentiation and uncontrolled proliferation. Here, we identified SLU7, which encodes a pre-mRNA splicing regulator that is inhibited in hepatocarcinoma, as a pivotal gene for hepatocellular homeostasis. SLU7 knockdown in human liver cells and mouse liver resulted in profound changes in pre-mRNA splicing and gene expression, leading to impaired glucose and lipid metabolism, refractoriness to key metabolic hormones, and reversion to a fetal-like gene expression pattern. Additionally, loss of SLU7 also increased hepatocellular proliferation and induced a switch to a tumor-like glycolytic phenotype. Slu7 governed the splicing and/or expression of multiple genes essential for hepatocellular differentiation, including serine/arginine-rich splicing factor 3 (Srsf3) and hepatocyte nuclear factor 4α (Hnf4α), and was critical for cAMP-regulated gene transcription. Together, out data indicate that SLU7 is central regulator of hepatocyte identity and quiescence.

Stable conditional expression and effect of C/ebpβ-LIP in adipocytes using the pSLIK system

Murine 3T3-L1 adipocytes are widely used as a cellular model of obesity. However, whereas transfection of 3T3-L1 preadipocytes is straightforward, ectopic gene expression in mature 3T3-L1 adipocytes has proved challenging. Here, we used the pSLIK vector system to generate stable doxycycline-inducible expression of the liver-enriched inhibitor protein isoform of CCAAT/enhancer binding protein β (C/ebpβ (Cebpb)) (C/EBPβ-LIP) in fully differentiated 3T3-L1 adipocytes. Because overexpression of C/ebpβ-LIP impairs adipocyte differentiation, the C/ebpβ-LIP construct was first integrated in 3T3-L1 preadipocytes but expression was induced only when adipocytes were fully differentiated. Increased C/EBPβ-LIP in mature adipocytes down-regulated C/ebpβ target genes including 11β-hydroxysteroid dehydrogenase type 1, phosphoenolpyruvate carboxykinase and fatty acid binding protein 4 but had no effect on asparagine synthetase, demonstrating that transcriptional down-regulation by C/ebpβ-LIP in 3T3-L1 adipocytes is not a general effect. Importantly, these genes were modulated in a similar manner in adipose tissue of mice with genetically increased C/ebpβ-LIP levels. The use of the pSLIK system to conditionally express transgenes in 3T3-L1 cells could be a valuable tool to dissect adipocyte physiology.

Cells expressing the C/EBPbeta isoform, LIP, engulf their neighbors

Descriptions of various processes that lead to cell-in-cell structures have been reported for decades. The exact molecular mechanism(s) of their formation and the physiological significance of cell-in-cell structures remain poorly understood. We had previously shown that an isoform of the CCAAT/enhancer-binding protein beta (C/EBPbeta) transcription factor, liver-enriched inhibitory protein (LIP), induces cell death in human breast cancer cells and stimulates autophagy. Here we describe a non-apoptotic cell death process where LIP mediates the engulfment of neighboring cells. We provide evidence of LIP-mediated engulfment via DNA profiling, fluorescent imaging and cell sorting studies, as well as ultrastructure analysis of LIP-expressing MDA-MB-468 breast cancer cells. Our work illustrates that expression of a specific transcription factor, LIP, can mediate cell engulfment.

Regulation of adipocyte 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) by CCAAT/enhancer-binding protein (C/EBP) β isoforms, LIP and LAP

11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) catalyses intracellular regeneration of active glucocorticoids, notably in liver and adipose tissue. 11β-HSD1 is increased selectively in adipose tissue in human obesity, a change implicated in the pathogenesis of metabolic syndrome. With high fat (HF)-feeding, adipose tissue 11β-HSD1 is down-regulated in mice, plausibly to counteract metabolic disease. Transcription of 11β-HSD1 is directly regulated by members of the CCAAT/enhancer binding protein (C/EBP) family. Here we show that while total C/EBPβ in adipose tissue is unaltered by HF diet, the ratio of the C/EBPβ isoforms liver-enriched inhibitor protein (LIP) and liver-enriched activator protein (LAP) (C/EBPβ-LIP:LAP) is increased in subcutaneous adipose. This may cause changes in 11β-HSD1 expression since genetically modified C/EBPβ^(+/L) mice, with increased C/EBPβ-LIP:LAP ratio, have decreased subcutaneous adipose 11β-HSD1 mRNA levels, whereas C/EBPβ^ΔuORF mice, with decreased C/EBPβ-LIP:LAP ratio, show increased subcutaneous adipose 11β-HSD1. C/EBPβ-LIP:LAP ratio is regulated by endoplasmic reticulum (ER) stress and mTOR signalling, both of which are altered in obesity. In 3T3-L1 adipocytes, 11β-HSD1 mRNA levels were down-regulated following induction of ER stress by tunicamycin but were up-regulated following inhibition of mTOR by rapamycin. These data point to a central role for C/EBPβ and its processing to LIP and LAP in transcriptional regulation of 11β-HSD1 in adipose tissue. Down-regulation of 11β-HSD1 by increased C/EBPβ-LIP:LAP in adipocytes may be part of a nutrient-sensing mechanism counteracting nutritional stress generated by HF diet.

Chromatin occupancy of transcription factor 7-like 2 (TCF7L2) and its role in hepatic glucose metabolism

AIMS/HYPOTHESIS

The mechanisms by which transcription factor 7-like 2 (TCF7L2) regulates the pathways that are important in the pathogenesis of type 2 diabetes are unknown. We therefore examined the role of TCF7L2 in hepatic glucose production (HGP) in vitro and characterised the whole-genome chromatin occupancy of TCF7L2 in hepatocytes.

METHODS

We investigated the effect of TCF7L2 silencing and overexpression on HGP from gluconeogenic precursors and used chromatin-immunoprecipitation (ChIP) combined with massively parallel DNA sequencing (ChIP-Seq) to investigate the DNA binding patterns of TCF7L2 across the whole genome.

RESULTS

Silencing of TCF7L2 induced a marked increase in basal HGP, which was accompanied by significant increases in the expression of the gluconeogenic genes Fbp1, Pck1 and G6pc. Overexpression of Tcf7l2 reversed this phenotype and significantly reduced HGP. TCF7L2 silencing did not affect the half-maximal inhibitory concentration of insulin or metformin, but HGP remained elevated in TCF7L2-silenced cells due to the increased baseline HGP. Using ChIP-Seq, we detected 2,119 binding events across the genome. Pathway analysis demonstrated that diabetes genes were significantly over-represented in the dataset. Our results indicate that TCF7L2 binds directly to multiple genes that are important in regulation of glucose metabolism in the liver, including Pck1, Fbp1, Irs1, Irs2, Akt2, Adipor1, Pdk4 and Cpt1a.

CONCLUSIONS/INTERPRETATION

TCF7L2 is an important regulator of HGP in vitro and binds directly to genes that are important in pathways of glucose metabolism in the liver. These data highlight the possibility that TCF7L2 may affect fasting and postprandial hyperglycaemia in carriers of at-risk TCF7L2 genetic polymorphisms.

Regulation of pyruvate dehydrogenase kinase 4 (PDK4) by CCAAT/enhancer-binding protein beta (C/EBPbeta)

The conversion of pyruvate to acetyl-CoA in mitochondria is catalyzed by the pyruvate dehydrogenase complex (PDC). Activity of PDC is inhibited by phosphorylation via the pyruvate dehydrogenase kinases (PDKs). Here, we examined the regulation of Pdk4 gene expression by the CCAAT/enhancer-binding protein β (C/EBPβ). C/EBPβ modulates the expression of multiple hepatic genes including those involved in metabolism, development, and inflammation. We found that C/EBPβ induced Pdk4 gene expression and decreased PDC activity. This transcriptional induction was mediated through two C/EBPβ binding sites in the Pdk4 promoter. C/EBPβ participates in the hormonal regulation of gluconeogenic genes. Previously, we reported that Pdk4 was induced by thyroid hormone (T(3)). Therefore, we investigated the role of C/EBPβ in the T(3) regulation of Pdk4. T(3) increased C/EBPβ abundance in primary rat hepatocytes. Knockdown of C/EBPβ with siRNA diminished the T(3) induction of the Pdk4 and carnitine palmitoyltransferase (Cpt1a) genes. CPT1a is an initiating step in the mitochondrial oxidation of long chain fatty acids. Our results indicate that C/EBPβ stimulates Pdk4 expression and participates in the T(3) induction of the Cpt1a and Pdk4 genes.

Inhibition of glyceroneogenesis by histone deacetylase 3 contributes to lipodystrophy in mice with adipose tissue inflammation

We have reported that the nuclear factor-κB (NF-κB) induces chronic inflammation in the adipose tissue of p65 transgenic (Tg) mice, in which the NF-κB subunit p65 (RelA) is overexpressed from the adipocyte protein 2 (aP2) gene promoter. Tg mice suffer a mild lipodystrophy and exhibit deficiency in adipocyte differentiation. To understand molecular mechanism of the defect in adipocytes, we investigated glyceroneogenesis by examining the activity of cytosolic phosphoenolpyruvate carboxykinase (PEPCK) in adipocytes. In aP2-p65 Tg mice, Pepck expression is inhibited at both the mRNA and protein levels in adipose tissue. The mRNA reduction is a consequence of transcriptional inhibition but not alteration in mRNA stability. The Pepck gene promoter is inhibited by NF-κB, which enhances the corepressor activity through activation of histone deacetylase 3 (HDAC3) in the nucleus. HDAC3 suppresses Pepck transcription by inhibiting the transcriptional activators, peroxisome proliferator-activated receptor-γ, and cAMP response element binding protein. The NF-κB activity is abolished by Hdac3 knockdown or inhibition of HDAC3 catalytic activity. In a chromatin immunoprecipitation assay, HDAC3 interacts with peroxisome proliferator-activated receptor-γ and cAMP response element binding protein in the Pepck promoter when NF-κB is activated by TNF-α. These results suggest that HDAC3 mediates NF-κB activity to repress Pepck transcription. This mechanism is responsible for inhibition of glyceroneogenesis in adipocytes, which contributes to lipodystrophy in the aP2-p65 Tg mice.

Hormonal regulation of gluconeogenic gene transcription in the liver

Glucose homeostasis in mammals is achieved by the actions of counterregulatory hormones, namely insulin, glucagon and glucocorticoids. Glucose levels in the circulation are regulated by the liver, the metabolic centre which produces glucose when it is scarce in the blood. This process is catalysed by two rate-limiting enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) whose gene expression is regulated by hormones. Hormone response units (HRUs) present in the two genes integrate signals from various signalling pathways triggered by hormones. How such domains are arranged in the regulatory region of these two genes, how this complex regulation is accomplished and the latest advancements in the field are discussed in this review.

The C/EBPbeta isoform, liver-inhibitory protein (LIP), induces autophagy in breast cancer cell lines

Autophagy is a process involving the bulk degradation of cellular components in the cytoplasm via the lysosomal degradation pathway. Autophagy manifests a protective role in stressful conditions such as nutrient or growth factor depletion; however, extensive degradation of regulatory molecules or organelles essential for survival can lead to the demise of the cell, or autophagy-mediated cell death. The role of autophagy in cancer is complex with roles in both tumor suppression and tumor promotion proposed. Here we report that an isoform of the C/EBPbeta transcription factor, liver-enriched inhibitory protein (LIP), induces cell death in human breast cancer cells and stimulates autophagy. Overexpression of LIP is incompatible with cell growth and when cell cycle analysis was performed, a DNA profile of cells undergoing apoptosis was not observed. Instead, LIP expressing cells appeared to have large autophagic vesicles when examined via electron microscopy. Autophagy was further assessed in LIP expressing cells by monitoring the development of acidic vesicular organelles and conversion of LC3 from the cytoplasmic form to the membrane-bound form. Our work shows that C/EBPbeta isoform, LIP, is another member of the group of transcription factors, including E2F1 and p53, which are capable of playing a role in autophagy.

Regulation of pyruvate dehydrogenase kinase isoform 4 (PDK4) gene expression by glucocorticoids and insulin

The pyruvate dehydrogenase complex (PDC) catalyzes the conversion of pyruvate to acetyl-CoA in mitochondria and is a key regulatory enzyme in the oxidation of glucose to acetyl-CoA. Phosphorylation of PDC by the pyruvate dehydrogenase kinases (PDK) inhibits its activity. The expression of the pyruvate dehydrogenase kinase 4 (PDK4) gene is increased in fasting and other conditions associated with the switch from the utilization of glucose to fatty acids as an energy source. Transcription of the PDK4 gene is elevated by glucocorticoids and inhibited by insulin. In this study, we have investigated the factors involved in the regulation of the PDK4 gene by these hormones. Glucocorticoids stimulate PDK4 through two glucocorticoid receptor (GR) binding sites located more than 6000 base pairs upstream of the transcriptional start site. Insulin inhibits the glucocorticoid induction in part by causing dissociation of the GR from the promoter. Previously, we found that the estrogen related receptor alpha (ERRalpha) stimulates the expression of PDK4. Here, we determined that one of the ERRalpha binding sites contributes to the insulin inhibition of PDK4. A binding site for the forkhead transcription factor (FoxO1) is adjacent to the ERRalpha binding sites. FoxO1 participates in the glucocorticoid induction of PDK4 and the regulation of this gene by insulin. Our data demonstrate that glucocorticoids and insulin each modulate PDK4 gene expression through complex hormone response units that contain multiple factors.

CCAAT/enhancer-binding protein beta: its role in breast cancer and associations with receptor tyrosine kinases

The CCAAT/enhancer-binding proteins (C/EBPs) are a family of leucine-zipper transcription factors that regulate gene expression to control cellular proliferation, differentiation, inflammation and metabolism. Encoded by an intronless gene, C/EBPbeta is expressed as several distinct protein isoforms (LAP1, LAP2, LIP) whose expression is regulated by the differential use of several in-frame translation start sites. LAP1 and LAP2 are transcriptional activators and are associated with differentiation, whereas LIP is frequently elevated in proliferative tissue and acts as a dominant-negative inhibitor of transcription. However, emerging evidence suggests that LIP can serve as a transcriptional activator in some cellular contexts, and that LAP1 and LAP2 might also have unique actions. The LIP:LAP ratio is crucial for the maintenance of normal growth and development, and increases in this ratio lead to aggressive forms of breast cancer. This review discusses the regulation of C/EBPbeta activity by post-translational modification, the individual actions of LAP1, LAP2 and LIP, and the functions and downstream targets that are unique to each isoform. The role of the C/EBPbeta isoforms in breast cancer is discussed and emphasis is placed on their interactions with receptor tyrosine kinases.

Molecular mechanisms involved in NAFLD progression

Non-alcoholic fatty liver disease (NAFLD) is an emerging metabolic-related disorder characterized by fatty infiltration of the liver in the absence of alcohol consumption. NAFLD ranges from simple steatosis to non-alcoholic steatohepatitis (NASH), which might progress to end-stage liver disease. This progression is related to the insulin resistance, which is strongly linked to the metabolic syndrome consisting of central obesity, diabetes mellitus, and hypertension. Earlier, the increased concentration of intracellular fatty acids within hepatocytes leads to steatosis. Subsequently, multifactorial complex interactions between nutritional factors, lifestyle, and genetic determinants promote necrosis, inflammation, fibrosis, and hepatocellular damage. Up to now, many studies have revealed the mechanism associated with insulin resistance, whereas the mechanisms related to the molecular components have been incompletely characterized. This review aims to assess the potential molecular mediators initiating and supporting the progression of NASH to establish precocious diagnosis and to plan more specific treatment for this disease.

DNA content, chromatin supraorganization, nuclear glycoproteins and RNA amounts in hepatocytes of mice expressing insulin-dependent diabetes

Chromatin supraorganization and extensibility and nuclear glycoprotein content have been reported to change in hepatocytes from mice during development and aging, as well as under starvation and refeeding conditions. In non-obese diabetic (NOD) mice, the expression of insulin-dependent diabetes may be accompanied by metabolic changes in the liver. These changes are likely to be similar to those involved in the aging processes of non-diabetic animals. Therefore, we hypothesized that the chromatin organization, as well as the physical properties and compositions of hepatocyte nuclei would also be affected in NOD mice in the same way as those in aged non-diabetic mice. Nuclear image parameters were evaluated by image analysis of Feulgen-stained preparations. Chromatin extensibility in response to gravity was observed with polarized light after lysis and toluidine blue staining. The Con-A response of nuclear glycoproteins was evaluated with scanning microspectrophotometry. These characteristics were assessed using hepatocyte imprints from female NOD mice after a 28-day period of diabetes expression. Observations and measurements were made in comparison to healthy BALB/c mice. Total RNA amounts were determined for livers of NOD and BALB/c mice. Enhanced polyploidy levels, a decrease in chromatin higher-order packing states, an increased frequency of extended chromatin fiber formation, and deeper Con-A-responsive chromatin areas were observed in the hepatocytes of the NOD mice expressing insulin-dependent diabetes. Reduced amounts of total RNA were also found in the livers of these mice. Our findings for NOD mice expressing insulin-dependent diabetes are consistent with previously reported data for old-aged mice of the inbred strain A/Uni and may reflect changes in transcriptional activities associated with the stressful physiological demands on the liver during the expression of diabetes.

Functional analysis of rat liver citrate carrier promoter: differential responsiveness to polyunsaturated fatty acids

CiC (citrate carrier), a mitochondrial membrane protein, plays an important metabolic role by transporting acetyl-CoA into the cytosol for fatty acid and cholesterol synthesis. Several studies showed that CiC activity and expression is regulated by dietary fatty acids. In the present study we report data on the structural and functional characterization of the 5'-flanking region of the rat Cic gene. By transient transfection assays in H4IIE rat hepatoma cells, a PUFA (polyunsaturated fatty acids) response region has been identified within the CiC promoter. A cluster of putative binding sites for several transcription factors, composed of a NF-Y (nuclear factor-Y) site, an E-box-like site, a SRE1 (sterol regulatory element 1)-like site and four Sp1 (stimulatory protein 1) sites, was localized in the promoter region. Luciferase reporter gene and gel mobility shift assays indicated that a functional E-box-like, essential to the basal CiC promoter activity, confers responsiveness to activation by SREBP (SRE-binding protein)-1c. This study provides evidence for SREBP-1c as a principal target for PUFA regulation of CiC transcription. In H4IIE cells, overexpression of nSREBP (nuclear SREBP)-1c over-rides arachidonic acid (C(20:4, n-6)) suppression, but does not prevent the repression by docosahexaenoic acid (C(22:6, n-3)). ChIP (chromatin immunoprecipitation) assays in H4IIE cells showed that docosahexaenoic acid affects the binding of NF-Y, Sp1 and SREBP-1 to the PUFA response region of CiC promoter, whereas arachidonic acid alters only the binding of SREBP-1. Our data show that PUFA inhibition of hepatic Cic gene transcription is mediated not only by the nuclear level of SREBP-1c, but also might involve a reduction in Sp1 and NF-Y DNA binding, suggesting differential mechanisms in the Cic gene regulation by different PUFA.

COP1 functions as a FoxO1 ubiquitin E3 ligase to regulate FoxO1-mediated gene expression

COP1 is a Ring-Finger E3 ubiquitin ligase that is involved in plant development, mammalian cell survival, growth, and metabolism. Here we report that COP1, whose expression is enhanced by insulin, regulates FoxO1 protein stability. We found that in Fao hepatoma cells, ectopic expression of COP1 decreased, whereas knockdown of COP1 expression increased the level of endogenous FoxO1 protein without impacting other factors such as C/EBPalpha and CREB (cAMP-response element-binding protein). We further showed that COP1 binds FoxO1, enhances its ubiquitination, and promotes its degradation via the ubiquitin-proteasome pathway. To determine the biological significance of COP1-mediated FoxO1 protein degradation, we have examined the impact of COP1 on FoxO1-mediated gene expression and found that COP1 suppressed FoxO1 reporter gene as well as FoxO1 target genes such as glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, two key targets for FoxO1 in the regulation of gluconeogenesis, with corresponding changes of hepatic glucose production in Fao cells. We suggest that by functioning as a FoxO1 E3 ligase, COP1 may play a role in the regulation of hepatic glucose metabolism.

Factors That Control the Tissue-Specific Transcription of the Gene for Phosphoenolpyruvate Carboxykinase-C

Transcription of the gene for PEPCK-C occurs in a number of mammalian tissues, with highest expression occurring in the liver, kidney cortex, and white and brown adipose tissue. Several hormones and other factors, including glucagon, epinephrine, insulin, glucocorticoids and metabolic acidosis, control this process in three responsive tissues, liver, adipose tissue, and kidney cortex. Expression of the gene in these three tissues in regulated in a different manner, responding to the specific physiological role of the tissue. The PEPCK-C gene promoter has been extensively studied and a number of regulatory regions identified that bind key transcription factors and render the gene responsive to hormonal and dietary stimuli. This review will focus on the control of transcription for the gene, with special emphasis on our current understanding of the transcription factors that are involved in the response of PEPCK-C gene in specific tissues. We have also reviewed the biological function of PEPCK-C in each of the tissues discussed in this review, in order to place the control of PEPCK-C gene transcription in the appropriate physiological context. Because of its extraordinary importance in mammalian metabolism and its broad pattern of tissue-specific expression, the PEPCK-C gene has become a model for studying the biological basis of the control of gene transcription.

Increased expression and activity of the transcription factor FOXO1 in nonalcoholic steatohepatitis

OBJECTIVE

Nonalcoholic fatty liver, affecting 34% of the U.S. population, is characterized by hepatic insulin resistance, which is more marked in the presence of steatohepatitis, and frequently precedes hyperglycemia. The molecular mechanisms underlying the relationship between fatty liver and insulin resistance are still undergoing definition and have not been evaluated in humans. Our aim was to evaluate the relationship between insulin resistance and the expression and regulation of forkhead box-containing protein O subfamily-1 (FOXO1), a transcription factor that mediates the effect of insulin on the gluconeogenic genes PEPCK and glucose-6-phosphatase catalytic subunit (G6PC).

RESEARCH DESIGN AND METHODS

FOXO1, PEPCK, and G6PC mRNA levels were evaluated in 84 subjects: 26 with steatohepatitis, 28 with steatosis alone, 14 with normal liver histology without metabolic alterations, and 16 with hepatitis C virus chronic hepatitis, of whom 8 were with and 8 were without steatosis. Protein expression and regulation of FOXO1 and upstream insulin signaling were analyzed in a subset. RESULTS; Expression of PEPCK was higher in steatohepatitis compared with steatosis alone and normal liver, and it was correlated with the homeostasis model assessment of insulin resistance (HOMA-IR) index. FOXO1 mRNA levels were higher in steatohepatitis, correlated with PEPCK and G6PC mRNA and with HOMA-IR. FOXO1 upregulation was confirmed at protein levels in steatohepatitis and, in the presence of oxidative stress, was associated with decreased Ser(256) phosphorylation, decreased Akt1, and increased Jun NH(2)-terminal kinase-1 activity. Consistently, immunohistochemistry showed increased FOXO1 expression and nuclear localization in steatohepatitis. FOXO1 mRNA levels correlated with nonalcoholic steatohepatitis activity score and were modulated by drugs counteracting hepatic lipogenesis.

CONCLUSIONS

FOXO1 expression and activity are increased in patients with steatohepatitis, and mRNA levels are correlated with hepatic insulin resistance.

Deprivation of protein or amino acid induces C/EBPbeta synthesis and binding to amino acid response elements, but its action is not an absolute requirement for enhanced transcription

A nutrient stress signalling pathway is triggered in response to protein or amino acid deprivation, namely the AAR (amino acid response), and previous studies have shown that C/EBPbeta (CCAAT/enhancer-binding protein beta) expression is up-regulated following activation of the AAR. DNA-binding studies, both in vitro and in vivo, have revealed increased C/EBPbeta association with AARE (AAR element) sequences in AAR target genes, but its role is still unresolved. The present results show that in HepG2 human hepatoma cells, the total amount of C/EBPbeta protein, both the activating [LAP* and LAP (liver-enriched activating protein)] and inhibitory [LIP (liver-enriched inhibitory)] isoforms, was increased in histidine-deprived cells. Immunoblotting of subcellular fractions and immunostaining revealed that most of the C/EBPbeta was located in the nucleus. Consistent with these observations, amino acid limitation caused an increase in C/EBPbeta DNA-binding activity in nuclear extracts and chromatin immunoprecipitation revealed an increase in C/EBPbeta binding to the AARE region in vivo, but at a time when transcription from the target gene was declining. A constant fraction of the basal and increased C/EBPbeta protein was phosphorylated on Thr(235) and the phospho-C/EBPbeta did bind to an AARE. Induction of AARE-enhanced transcription was slightly greater in C/EBPbeta-deficient MEFs (mouse embryonic fibroblasts) or C/EBPbeta siRNA (small interfering RNA)-treated HepG2 cells compared with the corresponding control cells. Transient expression of LAP*, LAP or LIP in C/EBPbeta-deficient fibroblasts caused suppression of increased transcription from an AARE-driven reporter gene. Collectively, the results demonstrate that C/EBPbeta is not required for transcriptional activation by the AAR pathway but, when present, acts in concert with ATF3 (activating transcription factor 3) to suppress transcription during the latter stages of the response.

Prevention of HIF-1 activation and iNOS gene targeting by low-dose cadmium results in loss of myocardial hypoxic preconditioning in the rat

This study aimed to underline the interaction between hypoxia-inducible factor-1 (HIF-1) and the inducible nitric oxide synthase (iNOS) gene in vivo and their contribution to the delayed myocardial preconditioning induced by acute intermittent hypoxia (IH) in the rat using chromatin immunoprecipitation and pharmacological inhibition by low-dose cadmium. Langendorff-perfused hearts of Wistar rats exposed to normoxia or IH 24 h earlier were submitted to global ischemia and reperfusion. Effects of iNOS inhibition by aminoguanidine (100 microM) before ischemia or of low-dose injection of cadmium chloride (1 mg/kg) before normoxia or IH were tested. Myocardial HIF-1 and iNOS quantification and in vivo chromatin immunoprecipitation of HIF-1 bound to the iNOS gene promoter were performed. IH-induced delayed cardioprotection resulted in an improvement in coronary flow and functional recovery at reperfusion and a decrease in infarct size. Myocardial HIF-1 activity was increased with resulting targeting of the iNOS gene. Aminoguanidine abolished the cardioprotective effects of IH. Cadmium chloride treatment before IH prevented myocardial HIF-1 activation (72.3 +/- 4.0 vs. 42.1 +/- 9.7 arbitrary units after cadmium chloride; P < 0.05), targeting of the iNOS gene, iNOS expression, and preconditioning (infarct size: 15.9 +/- 5.6 vs. 30.1 +/- 5.4% after cadmium chloride; P < 0.05). This study is the first to demonstrate the interaction of HIF-1 with the myocardial iNOS gene in situ after hypoxic preconditioning. Prevention of HIF-1 activation and iNOS gene targeting by a single low dose of cadmium abolished the delayed cardioprotective effects, bringing insight into the cardiovascular consequences of cadmium exposure.

An intronic locus control region plays an essential role in the establishment of an autonomous hepatic chromatin domain for the human vitamin D-binding protein gene

The human vitamin D-binding protein (hDBP) gene exists in a cluster of four liver-expressed genes. A minimal hDBP transgene, containing a defined set of liver-specific DNase I hypersensitive sites (HSs), is robustly expressed in mouse liver in a copy-number-dependent manner. Here we evaluate these HSs for function. Deletion of HSI, located 5' to the promoter (kb -2.1) had no significant effect on hDBP expression. In contrast, deletion of HSIV and HSV from intron 1 repressed hDBP expression and eliminated copy number dependency without a loss of liver specificity. Chromatin immunoprecipitation analysis revealed peaks of histone H3 and H4 acetylation coincident with HSIV in the intact hDBP locus. This region contains a conserved array of binding sites for the liver-enriched transcription factor C/EBP. In vitro studies revealed selective binding of C/EBPalpha to HSIV. In vivo occupancy of C/EBPalpha at HSIV was demonstrated in hepatic chromatin, and depletion of C/EBPalpha in a hepatic cell line decreased hDBP expression. A nonredundant role for C/EBPalpha was confirmed in vivo by demonstrating a reduction of hDBP expression in C/EBPalpha-null mice. Parallel studies revealed in vivo occupancy of the liver-enriched factor HNF1alpha at HSIII (at kb 0.13) within the hDBP promoter. These data demonstrate a critical role for elements within intron 1 in the establishment of an autonomous and productive hDBP chromatin locus and suggest that this function is dependent upon C/EBPalpha. Cooperative interactions between these intronic complexes and liver-restricted complexes within the target promoter are likely to underlie the consistency and liver specificity of the hDBP activation.

Pax-6 and c-Maf functionally interact with the alpha-cell-specific DNA element G1 in vivo to promote glucagon gene expression

Specific expression of the glucagon gene in the rat pancreas requires the presence of the G1 element localized at -100/-49 base pairs on the promoter. Although it is known that multiple transcription factors such as Pax-6, Cdx-2/3, c-Maf, Maf-B, and Brain-4 can activate the glucagon gene promoter through G1, their relative importance in vivo is unknown. We first studied the expression of Maf-B, c-Maf, and Cdx-2/3 in the developing and adult mouse pancreas. Although Maf-B was detectable in a progressively increasing number of alpha-cells throughout development and in adulthood, c-Maf and Cdx-2/3 were expressed at low and very low levels, respectively. However, c-Maf but not Cdx-2/3 was detectable in adult islets by Western blot analyses. We then demonstrated the in vivo interactions of Pax-6, Cdx-2/3, Maf-B, and c-Maf but not Brain-4 with the glucagon gene promoter in glucagon-producing cells. Although Pax-6, Cdx-2/3, Maf-B, and c-Maf were all able to bind G1 by themselves, we showed that Pax-6 could interact with Maf-B, c-Maf, and Cdx-2/3 and activate transcription of the glucagon gene promoter. Overexpression of dominant negative forms of Cdx-2/3 and Mafs in alpha-cell lines indicated that Cdx-2/3 and the Maf proteins interact on an overlapping site within G1 and that this binding site is critical in the activation of the glucagon gene promoter. Finally, we show that specific inhibition of Pax-6 and c-Maf but not Cdx-2/3 or Maf-B led to decreases in endogenous glucagon gene expression and that c-Maf binds the glucagon gene promoter in mouse islets. We conclude that Pax-6 and c-Maf interact with G1 to activate basal expression of the glucagon gene.

CCAAT/enhancer-binding protein-beta participates in insulin-responsive expression of the factor VII gene

Expression of the human coagulation factor VII (FVII) gene by hepatoma cells was modulated in concert with levels of glucose and insulin in the culture medium. In low glucose medium without insulin, amounts of both FVII mRNA and secreted FVII protein were coordinately increased; in the presence of glucose with insulin, both were decreased. Analysis of the FVII promoter showed that these effects could be reproduced in a reporter-gene system, and a small promoter element immediately upstream of the translation start site of the gene, which mediated these effects, was identified. Mutation of this element largely abrogated the glucose/insulin-responsive change in expression of the reporter gene. Several members of the CCAAT/enhancer-binding protein family were found to be capable of binding the identified sequence element but not the mutated element. The expression of a FVII minigene directed by a segment of the native FVII promoter responded to co-expressed activating and inhibiting forms of CCAAT/enhancer-binding protein beta.

Insulin regulation of phosphoenolpyruvate carboxykinase-c gene transcription: the role of sterol regulatory element-binding protein 1c

The effect of insulin on the regulation of phosphoenolpyruvate carboxykinase C (PEPCK-C) gene transcription, while pivotal for control of carbohydrate metabolism, constitutes only a small part of its overall action in cellular processes. Transcription of the PEPCK-C gene is the target for a number of pathways involved in the signal transduction initiated by insulin, and these processes involve an array of transcription factors and co-regulatory proteins that either alone or in concert bind to a subset of sites in the gene promoter to regulate its expression. This review will focus on a specific transcription factor, sterol regulatory element-binding protein 1c (SREBP-1c), and its role in the control of PEPCK-C gene transcription.

The beta-cell specific transcription factor Nkx6.1 inhibits glucagon gene transcription by interfering with Pax6

The transcription factor Nkx6.1 is required for the establishment of functional insulin-producing beta-cells in the endocrine pancreas. Overexpression of Nkx6.1 has been shown to inhibit glucagon gene expression while favouring insulin gene activation. Down-regulation resulted in the opposite effect, suggesting that absence of Nkx6.1 favours glucagon gene expression. To understand the mechanism by which Nkx6.1 suppresses glucagon gene expression, we studied its effect on the glucagon gene promoter activity in non-islet cells using transient transfections and gel-shift analyses. In glucagonoma cells transfected with an Nkx6.1-encoding vector, the glucagon promoter activity was reduced by 65%. In BHK21 cells, Nkx6.1 inhibited by 93% Pax6-mediated activation of the glucagon promoter, whereas Cdx2/3 and Maf stimulations were unaltered. Although Nkx6.1 could interact with both the G1 and G3 element, only the former displayed specificity for Nkx6.1. Mutagenesis of the three potential AT-rich motifs within the G1 revealed that only the Pax6-binding site preferentially interacted with Nkx6.1. Chromatin immunoprecipitation confirmed interaction of Nkx6.1 with the glucagon promoter and revealed a direct competition for binding between Pax6 and Nkx6.1. A weak physical interaction between Pax6 and Nkx6.1 was detected in vitro and in vivo suggesting that Nkx6.1 predominantly inhibits glucagon gene transcription through G1-binding competition. We suggest that cell-specific expression of the glucagon gene may only proceed when Nkx6.1, in combination with Pdx1 and Pax4, are silenced in early alpha-cell precursors.

CCAAT/enhancer-binding protein beta deletion reduces adiposity, hepatic steatosis, and diabetes in Lepr(db/db) mice

CCAAT/enhancer-binding protein beta (C/EBPbeta) plays a key role in initiation of adipogenesis in adipose tissue and gluconeogenesis in liver; however, the role of C/EBPbeta in hepatic lipogenesis remains undefined. Here we show that C/EBPbeta inactivation in Lepr(db/db) mice attenuates obesity, fatty liver, and diabetes. In addition to impaired adipogenesis, livers from C/EBPbeta(-/-) x Lepr(db/db) mice had dramatically decreased triglyceride content and reduced lipogenic enzyme activity. C/EBPbeta deletion in Lepr(db/db) mice down-regulated peroxisome proliferator-activated receptor gamma2 (PPARgamma2) and stearoyl-CoA desaturase-1 and up-regulated PPARalpha independent of SREBP1c. Conversely, C/EBPbeta overexpression in wild-type mice increased PPARgamma2 and stearoyl-CoA desaturase-1 mRNA and hepatic triglyceride content. In FAO cells, overexpression of the liver inhibiting form of C/EBPbeta or C/EBPbeta RNA interference attenuated palmitate-induced triglyceride accumulation and reduced PPARgamma2 and triglyceride levels in the liver in vivo. Leptin and the anti-diabetic drug metformin acutely down-regulated C/EBPbeta expression in hepatocytes, whereas fatty acids up-regulate C/EBPbeta expression. These data provide novel evidence linking C/EBPbeta expression to lipogenesis and energy balance with important implications for the treatment of obesity and fatty liver disease.

Transcriptional regulation by insulin: from the receptor to the gene

Insulin, after binding to its receptor, regulates many cellular processes and the expression of several genes. For a subset of genes, insulin exerts a negative effect on transcription; for others, the effect is positive. Insulin controls gene transcription by modifying the binding of transcription factors on insulin-response elements or by regulating their transcriptional activities. Different insulin-signaling cascades have been characterized as mediating the insulin effect on gene transcription. In this review, we analyze recent data on the molecular mechanisms, mostly in the liver, through which insulin exerts its effect. We first focus on the key transcription factors (viz. Foxo, sterol-response-element-binding protein family (SREBP), and Sp1) involved in the regulation of gene transcription by insulin. We then present current information on the way insulin downregulates and upregulates gene transcription, using as examples of downregulation phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor binding protein 1 (IGFBP-1) genes and of upregulation the fatty acid synthase and malic enzyme genes. The last part of the paper focuses on the signaling cascades activated by insulin in the liver, leading to the modulation of gene transcription.

Vitamin A regulation of gene expression: molecular mechanism of a prototype gene

Vitamin A regulation of gene expression is a well-characterized example of direct nutrient regulation of gene expression. The downstream metabolites of retinol, all-trans and 9-cis retinoic acids are the bioactive components that bind and activate their cognate nuclear receptors to regulate target genes. There are multiple retinoid receptor subtypes that are encoded by separate genes and each subtype has different isoforms. These receptors are Class II members of the thyroid/retinoid/vitamin D superfamily of nuclear receptors. The characterization of the retinoid receptors and the DNA response elements of target genes that bind these receptors have vastly expanded our knowledge of the mechanism of retinoid regulation of target genes. The basic regulatory mechanism of retinoids interacting with their cognate receptors is further complicated by the interaction of coactivators and corepressors, nuclear proteins that are involved in activation or repression of transcription, respectively. Most of these coregulators are involved in modifying chromatin and nucleosome structure such that chromatin is relaxed or condensed, and in bridging between the upstream enhancer domains and the transcription preinitiation complex. Retinoid regulation of the rate of transcription of target genes and the duration of the retinoid response is further complicated by covalent modification of the retinoid receptors by phosphorylation involved in coactivator association and ubiquitinylation involved in the degradation of retinoid receptors. This review presents a prototype retinoid responsive gene that encodes the phosphoenolpyruate carboxykinase (PEPCK) gene as an example of a specific mechanism of retinoid regulation of a metabolic gene. The retinoid response elements and overall mechanism of retinoid regulation of the PEPCK gene have been well documented by both in vitro and in vivo methods. We provide detailed information on the specific nuclear receptors, coactivators and chromatin modification events that occur when vitamin A is deficient and, therefore, retinoids are not available to activate the nuclear retinoid-signaling cascade.

Increased expression of CCAAT/enhancer binding protein-beta and -delta and monocyte chemoattractant protein-1 genes in aortas from hyperinsulinaemic rats

AIMS/HYPOTHESIS

We evaluated whether hyperinsulinaemia stimulates the expression of transcription factor CCAAT/enhancer binding protein (C/EBP)-beta and C/EBP-delta and leads to the induction of the chemokine (C-C motif) ligand 2 gene (Ccl2, also known as MCP-1) expression in aortas.

METHODS

Hyperinsulinaemia was induced by feeding rats a high-fructose diet. CCL2 production was analysed by ELISA. The expression of Ccl2, Cebpb and Cebpd mRNAs was investigated by quantitative RT-PCR. The binding of C/EBP-beta to Ccl2 was assessed by chromatin immunoprecipitation (ChIP) assay.

RESULTS

Insulin at a concentration of 10 nmol/l significantly stimulated the expression of Cebpb, Cebpd and Ccl2 mRNAs, depending on activation of phosphatidylinositol 3-kinase (PI3K) in cultured vascular smooth muscle cells. The knock-down of C/EBP-beta with siRNA abolished the insulin-induced Ccl2 mRNA expression. In the aortas from fructose-fed rats, the levels of phosphorylation of Akt/protein kinase B, a downstream effector of PI3K, were also increased. The expression of Cebpb, Cebpd and Ccl2 mRNAs in the aortas from fructose-fed rats were significantly elevated, by 330, 300 and 300%, respectively, compared with those of control-fed rats. The induction Ccl2 mRNA expression in the aortas was significantly correlated with the expression of Cebpb and Cebpd mRNAs in the aortas. Furthermore, the ChIP assay showed elevated binding of C/EBP-beta to the 5' upstream region of Ccl2 in the aortas from fructose-fed rats.

CONCLUSIONS/INTERPRETATION

These findings clearly indicate the role of C/EBPs in the mechanism of upregulation of CCL2, an inflammation-related protein, observed in the hyperinsulinaemic state, which may initiate the process of atherosclerosis.

A cellular assay for measuring the modulation of glucose production in H4IIE cells

Type II diabetes and its associated complications are a major health concern of the developed world. One of the hallmarks of diabetes is insulin resistance, where secreted insulin no longer has any effect on its target tissues, namely, liver, muscle, and fat. An important therapeutic strategy is to modulate blood glucose levels using pharmacological agents. Glycogen synthase kinase-3 (GSK3) is a serine-threonine protein kinase that plays important roles in regulating glucose metabolism. It is a key negative regulator of insulin action and is an important contributing factor to insulin resistance in liver, muscle, and adipose tissue. We describe the development of a cell-based assay designed to measure glucose production in rat hepatoma cell line H4IIE liver cells in response to treatment with small molecule inhibitors, including GSK3 inhibitors. The assay is set up in a 96-well format, and glucose production is assessed using a convenient fluorescence-based readout. This disease-relevant cellular assay is a valuable tool for the progression of small molecules that modulate glucose production.

CCAAT/enhancer-binding protein (C/EBP) beta is acetylated at multiple lysines: acetylation of C/EBPbeta at lysine 39 modulates its ability to activate transcription

Transcription factor function can be modulated by post-translational modifications. Because the transcription factor CCAAT/enhancer-binding protein (C/EBP) beta associates with the nuclear coactivator p300, which contains acetyltransferase activity, acetylation of C/EBPbeta was examined to understand its regulation and function. C/EBPbeta is acetylated by acetyltransferases p300 and p300/CREB-binding protein associated factor. Endogenous C/EBPbeta in 3T3-F442A preadipocytes is also recognized by an acetyl-lysine-specific antibody. Analysis of truncations of C/EBPbeta and peptides based on C/EBPbeta sequences identified multiple lysines within C/EBPbeta that can be acetylated. Among these, a novel acetylation site at lysine 39 of C/EBPbeta was identified. Mutation of Lys-39 to arginine or alanine impairs its acetylation and the ability of C/EBPbeta to activate transcription at the promoters for C/EBPalpha and c-fos. Different C/EBPbeta-responsive promoters require different patterns of acetylated lysines in C/EBPbeta for transcription activation. Furthermore, C/EBPbeta acetylation was increased by growth hormone, and mutation of Lys-39 impaired growth hormone-stimulated c-fos promoter activation. These data suggest that acetylation of Lys-39 of C/EBPbeta, alone or in combination with acetylation at other lysines, may play a role in C/EBPbeta-mediated transcriptional activation.

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

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

Hepatocyte nuclear factor-4alpha contributes to carbohydrate-induced transcriptional activation of hepatic fatty acid synthase

Refeeding a carbohydrate-rich meal after a fast produces a co-ordinated induction of key glycolytic and lipogenic genes in the liver. The transcriptional response is mediated by insulin and increased glucose oxidation, and both signals are necessary for optimal induction of FAS (fatty acid synthase). The glucose-regulated component of FAS promoter activation is mediated in part by ChREBP [ChoRE (carbohydrate response element)-binding protein], which binds to a ChoRE between -7300 and -7000 base-pairs in a carbohydrate-dependent manner. Using in vivo footprinting with nuclei from fasted and refed rats, we identify an imperfect DR-1 (direct repeat-1) element between -7110 and -7090 bp that is protected upon carbohydrate refeeding. Electrophoretic mobility-shift assays establish that this DR-1 element binds HNF-4alpha (hepatocyte nuclear factor 4alpha), and chromatin immunoprecipitation establishes that HNF-4alpha binding to this site is increased approx. 3-fold by glucose refeeding. HNF-4alpha transactivates reporter constructs containing the distal FAS promoter in a DR-1-dependent manner, and this DR-1 is required for full glucose induction of the FAS promoter in primary hepatocytes. In addition, a 3-fold knockdown of hepatocyte HNF-4alpha by small interfering RNA produces a corresponding decrease in FAS gene induction by glucose. Co-immunoprecipitation experiments demonstrate a physical interaction between HNF-4alpha and ChREBP in primary hepatocytes, further supporting an important complementary role for HNF-4alpha in glucose-induced activation of FAS transcription. Taken together, these observations establish for the first time that HNF-4alpha functions in vivo through a DR-1 element in the distal FAS promoter to enhance gene transcription following refeeding of glucose to fasted rats. The findings support the broader view that HNF-4alpha is an integral component of the hepatic nutrient sensing system that co-ordinates transcriptional responses to transitions between nutritional states.

Transcriptional Regulation of the Glucose-6-phosphatase Gene by cAMP/Vasoactive Intestinal Peptide in the Intestine

Gluconeogenesis is induced in both the liver and intestine by increased cAMP levels. However, hepatic and intestinal glucose production can have opposite effects on glucose homeostasis. Glucose release into the portal vein by the intestine increases glucose uptake and reduces food intake. In contrast, glucose production by the liver contributes to hyperglycemia in type II diabetes. Glucose-6-phosphatase (Glc6Pase) is the key enzyme of gluconeogenesis in both the liver and intestine. Here we specify the cAMP/protein kinase A regulation of the Glc6Pase gene in the intestine compared with the liver. Similarly to the liver, the molecular mechanism of cAMP/protein kinase A regulation involves cAMP-response element-binding protein, HNF4alpha, CAAT/enhancer-binding protein, and HNF1. In contrast to the situation in the liver, we find that different isoforms of CAAT/enhancer-binding protein and HNF1 contribute to the specific regulation of the Glc6Pase gene in the intestine. Moreover, we show that cAMP-response element binding modulator specifically contributes to the regulation of the Glc6Pase gene in the intestine but not in the liver. These results allow us to identify intestine-specific regulators of the Glc6Pase gene and to improve the understanding of the differences in the regulation of gluconeogenesis in the intestine compared with the liver.

Correlation between FOXO1a (FKHR) and FOXO3a (FKHRL1) binding and the inhibition of basal glucose-6-phosphatase catalytic subunit gene transcription by insulin

Insulin inhibits transcription of the genes encoding the glucose-6-phosphatase catalytic subunit (G6Pase), phosphoenolpyruvate carboxykinase, and IGF binding protein-1 through insulin response sequences (IRSs) that share the same core sequence, T(G/A)TTTT(G/T). The transcription factors FOXO1a and FOXO3a have been shown to bind these elements, but there are conflicting reports as to whether this binding correlates with the action of insulin on gene transcription. Some researchers concluded, from overexpression experiments using FOXO1a, that binding correlated with the insulin response, whereas others concluded, mainly from gel retardation competition experiments using FOXO3a, that it did not. We show here that, although these factors can differentially activate gene transcription in a context-dependent manner, these conflicting data are not explained by a difference in FOXO1a and FOXO3a binding specificity. Instead, we find that gel retardation competition and binding experiments give different results; the latter reveal a correlation between FOXO1a/3a binding and the inhibition of basal G6Pase gene transcription by insulin. In addition, these data show that the binding of FOXO1a/3a to two adjacent IRSs in the G6Pase promoter is cooperative and that promoter context alters the specific IRS base requirements for FOXO1a-stimulated fusion gene expression. Surprisingly, an analysis of insulin action mediated through the G6Pase and IGF binding protein-1 IRSs in the context of a heterologous thymidine kinase promoter reveals that signaling through the latter does not support the accepted model for insulin-stimulated FOXO nuclear exclusion.

Glucocorticoid "Programming" and PTSD Risk

Epidemiological data have linked an adverse fetal environment with increased risks of cardiovascular, metabolic, neuroendocrine, and psychiatric disorders in adulthood. Prenatal stress and/or glucocorticoid excess might underlie this link. In animal models, prenatal stress, glucocorticoid exposure or inhibition/knockout of 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta-HSD-2), the feto-placental barrier to maternal glucocorticoids, reduces birth weight and causes permanent hypertension, hyperglycemia, increased hypothalamic-pituitary-adrenal (HPA) axis activity and behavior resembling of anxiety. In humans, 11 beta-HSD-2 gene mutations cause low birth weight and placental 11 beta-HSD-2 activity correlates directly with birth weight and inversely with infant blood pressure. Low birth weight babies have higher plasma cortisol levels throughout adult life, indicating HPA programming. In human pregnancy, severe maternal stress affects the offspring HPA axis and associates with neuropsychiatric disorders. Posttraumatic stress disorder (PTSD) appears to be a variable in the effects. Intriguingly, some of these effects appear to be 'inherited' into a further generation, itself unexposed to exogenous glucocorticoids at any point in the lifespan from fertilization, implying epigenetic marks persist into subsequent generation(s). Overall, the data suggest that prenatal exposure to excess glucocorticoids programs peripheral and CNS functions in adult life, predisposing to some pathologies, perhaps protecting from others, and these may be transmitted perhaps to one or two subsequent generations.

FoxA2 involvement in suppression of protein C, an outcome predictor in experimental sepsis

Low levels of protein C (PC) predict outcome as early as 10 h after insult in a rat polymicrobial sepsis model and were associated with suppression of PC mRNA, upstream transcription factor FoxA2, and cofactor hepatocyte nuclear factor 6 (HNF6). Small interfering RNA suppression of FoxA2 in isolated hepatocytes demonstrated regulation of both its cofactor HNF6 and PC. Our data suggest that reduced FoxA2 may be important in the suppression of PC and resulting poor outcome in sepsis.

FoxO1 regulates multiple metabolic pathways in the liver: effects on gluconeogenic, glycolytic, and lipogenic gene expression

FoxO transcription factors are important targets of insulin action. To better understand the role of FoxO proteins in the liver, we created transgenic mice expressing constitutively active FoxO1 in the liver using the alpha1-antitrypsin promoter. Fasting glucose levels are increased, and glucose tolerance is impaired in transgenic (TGN) versus wild type (WT) mice. Interestingly, fasting triglyceride and cholesterol levels are reduced despite hyperinsulinemia, and post-prandial changes in triglyceride levels are markedly suppressed in TGN versus WT mice. Activation of pro-lipogenic signaling pathways (atypical protein kinase C and protein kinase B) and the ability to suppress beta-hydroxybutyrate levels are not impaired in TGN. In contrast, de novo lipogenesis measured with (3)H(2)O is suppressed by approximately 70% in the liver of TGN versus WT mice after refeeding. Gene-array studies reveal that the expression of genes involved in gluconeogenesis, glycerol transport, and amino acid catabolism is increased, whereas genes involved in glucose utilization by glycolysis, the pentose phosphate shunt, lipogenesis, and sterol synthesis pathways are suppressed in TGN versus WT. Studies with adenoviral vectors in isolated hepatocytes confirm that FoxO1 stimulates expression of gluconeogenic genes and suppresses expression of genes involved in glycolysis, the shunt pathway, and lipogenesis, including glucokinase and SREBP-1c. Together, these results indicate that FoxO proteins promote hepatic glucose production through multiple mechanisms and contribute to the regulation of other metabolic pathways important in the adaptation to fasting and feeding in the liver, including glycolysis, the pentose phosphate shunt, and lipogenic and sterol synthetic pathways.

Vitamin A status in mice affects the histone code of the phosphoenolpyruvate carboxykinase gene in liver

Vitamin A deficiency decreases hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene expression in mice, and expression is restored with retinoic acid (RA) treatment in vivo. In the studies reported here, we examined changes in histone modification and coregulator association with the regulatory domains of the PEPCK gene in response to alterations in vitamin A status. We identified nuclear receptors that bind to retinoic acid response elements (RAREs) in the PEPCK promoter by electrophoretic mobility shift assay and verified these in vivo using chromatin immunoprecipitation in mouse liver. Hypothetically, nuclear receptors at PEPCK RAREs recruit specific coactivator molecules that contribute to the acetylation of core histones and/or serve as bridging molecules between nuclear receptors and basal transcription factors at the transcription start site. We identified 3 coactivator molecules, cAMP-response element binding protein (CBP), steroid receptor coactivator (SRC)-1, and peroxisome-proliferator activated receptor (PPAR)-gamma-coactivator (PGC)-1alpha, that bound in association with the PEPCK RAREs in vivo. Furthermore, there was differential binding of these coactivators in vitamin A-deficient mice. Related to this, specific lysine residues were acetylated on histones H3 and H4 at the 3 RAREs of the PEPCK promoter, consistent with the action of the above coactivators, and acetylation of certain lysines was significantly decreased with vitamin A deficiency. These results demonstrate the associated changes that occur in nuclear receptor binding, coactivator recruitment, and histone acetylation in response to vitamin A status, identified at specific RAREs in the PEPCK gene in vivo.

Role of p90 Ribosomal S6-Kinase-1 in Oltipraz-Induced Specific Phosphorylation of CCAAT/Enhancer Binding Protein-β for GSTA2 Gene Transactivation

Oltipraz, which has been extensively studied as a cancer chemopreventive agent, promotes phosphatidylinositol 3-kinase-mediated activation of CCAAT/enhancer binding protein-beta (C/EBPbeta). Activated p90 ribosomal S6-kinase-1 (RSK1) phosphorylates major transcription factors, including C/EBPbeta. This study examined whether oltipraz induces phosphorylation of C/EBPbeta at specific residues, and if so, whether RSK1 regulates C/EBPbeta phosphorylation by oltipraz for the GSTA2 gene transactivation. Subcellular fractionation and immunoblot analyses revealed that oltipraz treatment increased the level of C/EBPbeta phosphorylated at Ser(105) in the cytoplasm, which translocated to the nucleus for DNA binding in rat H4IIE cells. Immunoprecipitation-immunoblot, chromatin-immunoprecipitation, and specific mutation analyses revealed that Ser(105)-phosphorylated C/EBPbeta recruited the cAMP response element-binding protein binding protein for histone acetylation and transactivation of the GSTA2 gene. The role of RSK1 in Ser(105)-phosphorylation of C/EBPbeta by oltipraz and its gene transactivation was evidenced by transfection experiments with dominant-negative mutants of RSK1. In mouse Hepa1c1c, human HepG2 cells, and rat primary hepatocytes, oltipraz induced phosphorylation of C/EBPbeta at Thr(217), Thr(266), and Ser(105), respectively, via RSK1. The experiment using small-interference RNA of RSK1 confirmed the essential role of RSK1 in the gene expression. Inhibition of PI3-kinase activity prevented oltipraz-inducible Ser(105)-phosphorylation of rat C/EBPbeta. Oltipraz treatment led to increases in the catalytic activity and nuclear translocation of RSK1, which was abrogated by PI3-kinase inhibition. In summary, oltipraz induces the phosphorylation of rat C/EBPbeta at Ser(105) (functionally analogous Thr(217/266) in mouse and human forms) in hepatocytes, which results in cAMP response element-binding protein-binding protein (CBP) recruitment for the GSTA2 gene transactivation, and the specific C/EBPbeta phosphorylation is mediated by RSK1 downstream of PI3-kinase.

Regulation of elastin gene transcription by proteasome dysfunction

Elastin, a major extracellular matrix protein and the core component of elastic fiber, is essential to maintain lung structural integrity and normal physiological function. We previously found that the downregulation of elastin gene transcription by IL-1β is mediated via activation of NF-κB and CCAAT/enhancer binding protein (C/EBP)β, both targets of the ubiquitin-proteasome pathway. To further investigate the molecular mechanisms that underlie the control of elastin gene expression, we disrupted the ubiquitin-proteasome pathway with specific proteasome inhibitors. We found that specific proteasome inhibitors decreased the steady-state level of elastin mRNA in a dose-responsive manner. Run-on assay and promoter reporter study indicated that the proteasome inhibitor MG-132 repressed the rate of elastin transcription. MG-132 did not affect mRNA levels of NF-κB and C/EBPβ, or the nuclear presence of NF-κB, but markedly increased C/EBPβ isoforms, including liver-enriched transcriptional activating protein and liver-enriched transcriptional inhibitory protein. Addition of cycloheximide blocked these increases and the downregulation of elastin mRNA by MG-132. The MG-132-induced downregulation of elastin transcription was dependent on C/EBPβ expression as assessed with small interfering RNA. These results indicate that the ubiquitin-proteasome pathway plays an essential role in maintaining elastin gene expression in lung fibroblasts. Disruption of this pathway results in the downregulation of tropoelastin transcription via posttranscriptionally induced C/EBPβ isoforms.

Glucocorticoids regulate transcription of the gene for phosphoenolpyruvate carboxykinase in the liver via an extended glucocorticoid regulatory unit

The hepatic transcriptional regulation by glucocorticoids of the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK-C) gene is coordinated by interactions of specific transcription factors at the glucocorticoid regulatory unit (GRU). We propose an extended GRU that consists of four accessory sites, two proximal AF1 and AF2 sites and their distal counterpart dAF1 (-993) and a new site, dAF2 (-1365); together, these four sites form a palindrome. Sequencing and gel shift binding assays of hepatic nuclear proteins interacting with these sites indicated similarity of dAF1 and dAF2 sites to the GRU proximal AF1 and AF2 sites. Chromatin immunoprecipitation assays demonstrated that glucocorticoids enhanced the binding of FOXO1 and peroxisome proliferator-activated receptor-alpha to AF2 and dAF2 sites and not to dAF1 site but enhanced the binding of hepatic nuclear transcription factor-4alpha only to the dAF1 site. Insulin inhibited the binding of these factors to their respective sites but intensified the binding of phosphorylated FOXO1. Transient transfections in HepG2 human hepatoma cells showed that glucocorticoid receptor interacts with several non-steroid nuclear receptors, yielding a synergistic response of the PEPCK-C gene promoter to glucocorticoids. The synergistic stimulation by glucocorticoid receptor together with peroxisome proliferator-activated receptor-alpha or hepatic nuclear transcription factor-4alpha requires all four accessory sites, i.e. a mutation of each of these markedly affects the synergistic response. Mice with a targeted mutation of the dAF1 site confirmed this requirement. This mutation inhibited the full response of hepatic PEPCK-C gene to diabetes by reducing PEPCK-C mRNA level by 3.5-fold and the level of circulating glucose by 25%.

Severe impairment in liver insulin signaling fails to alter hepatic insulin action in conscious mice

Insulin exerts its potent effects on hepatic glucose fluxes via direct and indirect mechanisms. Whereas a liver-specific insulin receptor (IR) knockout (LIRKO) mouse exhibits glucose intolerance as well as insulin resistance, it is unclear whether a more acute decrease in the expression of hepatic IR would be sufficient to induce hepatic insulin resistance. Here we report that the downregulation of hepatic IR expression by up to 95% does not modify hepatic insulin action. The i.p. administration (2 injections over 1 week) of an antisense oligodeoxynucleotide (ASO) directed to reduce insulin expression downregulated hepatic IR expression in C57BL6J mice. A high dose of IR-ASO decreased IR protein approximately 95%, while a control-ASO failed to modify IR expression. At this dose, the IR-ASO also decreased IR expression in adipose tissue but did not significantly decrease IR expression in hypothalamus or skeletal muscle. Insulin action was assessed with insulin clamp studies in conscious mice. The rate of glucose infusion during the clamp studies was comparable in control-ASO- and IR-ASO-treated mice. Importantly, the depletion of liver IR protein markedly impaired downstream insulin signaling in the liver, but it failed to modify the rate of glucose production. Thus, near ablation of liver IR does not alter insulin action on glucose production.

Chronic hyperglycemia enhances PEPCK gene expression and hepatocellular glucose production via elevated liver activating protein/liver inhibitory protein ratio

Acute hyperglycemia normally suppresses hepatic glucose production (HGP) and gluconeogenic gene expression. Conversely, chronic hyperglycemia is accompanied by progressive increases in basal HGP and is a major contributor to hyperglycemia in both type 1 and type 2 diabetes by mechanisms that are poorly understood. The aim of this study was to investigate the molecular mechanisms whereby hyperglycemia contributes to excessive gluconeogenesis in Fao hepatoma cells. Increasing glucose from 5 to 20 mmol/l resulted in loss of glucose inhibition of PEPCK gene expression after 12 h. Furthermore, 24 h of incubation with 20 mmol/l glucose increased cAMP-stimulated PEPCK mRNA by approximately 40% (P < 0.05) and similarly increased glucose production. Although total CCAAT/enhancer-binding protein beta (C/EBPbeta) protein levels were suppressed, 20 mmol/l glucose increased the liver activating protein (LAP; an active isoform of C/EBPbeta)/liver inhibitory protein (LIP; an inhibitory isoform of C/EBPbeta) ratio significantly. Chromatin immunoprecipitation studies of the endogenous PEPCK gene demonstrated an increased association of LAP with the cAMP response element of the promoter. Using transient transfection to manipulate the LAP/LIP ratio, we also demonstrate a direct relationship between this ratio and PEPCK promoter activity. An increased LAP/LIP ratio not only enhanced cAMP- and dexamethasone-induced PEPCK gene expression but also impaired the repressive effect of insulin. These results demonstrate that sustained hyperglycemia diminishes the inhibitory effect of glucose and insulin on PEPCK expression and enhances hormone-stimulated PEPCK gene expression and hepatocellular glucose production. Because prolonged hyperglycemia increases the LAP/LIP ratio and can potentiate hormone induction of PEPCK transcription, our results suggest that a hyperglycemia-driven increased LAP/LIP ratio may be a critical molecular event in the pathogenesis of increased HGP in diabetes.

Glucose-stimulated upregulation of GLUT2 gene is mediated by sterol response element-binding protein-1c in the hepatocytes

GLUT2 is mainly expressed in the liver, beta-cells of the pancreas, and the basolateral membrane of kidney proximal tubules and plays an important role in glucose homeostasis in living organisms. The transcription of the GLUT2 gene is known to be upregulated in the liver during postprandial hyperglycemic states or in type 2 diabetes. However, a molecular mechanism by which glucose activates GLUT2 gene expression is not known. In this study, we report evidence that sterol response element-binding protein (SREBP)-1c plays a key role in glucose-stimulated GLUT2 gene expression. The GLUT2 promoter reporter is activated by SREBP-1c, and the activation is inhibited by a dominant-negative form of SREBP-1c (SREBP-1c DN). Adenoviral expression of SREBP-1c DN suppressed glucose-stimulated GLUT2 mRNA level in primary hepatocytes. An electrophoretic mobility shift assay and mutational analysis of the GLUT2 promoter revealed that SREBP-1c binds to the -84/-76 region of the GLUT2 promoter. Chromatin immunoprecipitation revealed that the binding of SREBP-1c to the -84/-76 region was increased by glucose concentration in a dose-dependent manner. These results indicate that SREBP-1c mediates glucose-stimulated GLUT2 gene expression in hepatocytes.