Hepatocyte nuclear factor-4 alpha mediates the stimulatory effect of peroxisome proliferator-activated receptor gamma co-activator-1 alpha (PGC-1 alpha) on glucose-6-phosphatase catalytic subunit gene transcription in H4IIE cells

Licochalcone A alleviates abnormal glucolipid metabolism and restores energy homeostasis in diet-induced diabetic mice

Licochalcone A (LCA) is a bioactive chalcone compound identified in licorice. This study aimed to investigate the effects of LCA on glucolipid metabolism and energy homeostasis, as well as the underlying mechanisms. Blood glucose levels, oral glucose tolerance, serum parameters, and histopathology were examined in high-fat-high-glucose diet (HFD)-induced diabetic mice, with metformin as a positive control. Additionally, changes in key markers related to glucolipid metabolism and mitochondrial function were analyzed to comprehensively assess LCA's effects on metabolism. The results showed that LCA alleviated metabolic abnormalities in HFD-induced diabetic mice, which were manifested by suppression of lipogenesis, promotion of lipolysis, reduction of hepatic steatosis, increase in hepatic glycogenesis, and decrease in gluconeogenesis. In addition, LCA restored energy homeostasis by promoting mitochondrial biogenesis, enhancing mitophagy, and reducing adenosine triphosphate production. Mechanistically, the metabolic benefits of LCA were associated with the downregulation of mammalian target of rapamycin complex 1 and activation of adenosine monophosphate-activated protein kinase, the two central regulators of metabolism. This study demonstrates that LCA can alleviate abnormal glucolipid metabolism and restore energy balance in diet-induced diabetic mice, highlighting its therapeutical potential for the treatment of diabetes.

PPARα activation partially drives NAFLD development in liver-specific Hnf4a-null mice

HNF4α regulates various genes to maintain liver function. There have been reports linking HNF4α expression to the development of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis. In this study, liver-specific Hnf4a-deficient mice (Hnf4aΔHep mice) developed hepatosteatosis and liver fibrosis, and they were found to have difficulty utilizing glucose. In Hnf4aΔHep mice, the expression of fatty acid oxidation-related genes, which are PPARα target genes, was increased in contrast to the decreased expression of PPARα, suggesting that Hnf4aΔHep mice take up more lipids in the liver instead of glucose. Furthermore, Hnf4aΔHep/Ppara-/- mice, which are simultaneously deficient in HNF4α and PPARα, showed improved hepatosteatosis and fibrosis. Increased C18:1 and C18:1/C18:0 ratio was observed in the livers of Hnf4aΔHep mice, and the transactivation of PPARα target gene was induced by C18:1. When the C18:1/C18:0 ratio was close to that of Hnf4aΔHep mouse liver, a significant increase in transactivation was observed. In addition, the expression of Pgc1a, a coactivator of PPARs, was increased, suggesting that elevated C18:1 and Pgc1a expression could contribute to PPARα activation in Hnf4aΔHep mice. These insights may contribute to the development of new diagnostic and therapeutic approaches for NAFLD by focusing on the HNF4α and PPARα signaling cascade.

Changes in PGC-1α-Dependent Mitochondrial Biogenesis Are Associated with Inflexible Hepatic Energy Metabolism in the Offspring Born to Dexamethasone-Treated Mothers

In the present study we investigated the participation of hepatic peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) in the metabolic programming of newborn rats exposed in utero to dexamethasone (DEX). On the 21st day of life, fasted offspring born to DEX-treated mothers displayed increased conversion of pyruvate into glucose with simultaneous upregulation of PEPCK (phosphoenolpyruvate carboxykinase) and G6Pase (glucose-6-phosphatase). Increased oxidative phosphorylation, higher ATP/ADP ratio and mitochondrial biogenesis and lower pyruvate levels were also found in the progeny of DEX-treated mothers. On the other hand, the 21-day-old progeny of DEX-treated mothers had increased hepatic triglycerides (TAG) and lower CPT-1 activity when subjected to short-term fasting. At the mechanistic level, rats exposed in utero to DEX exhibited increased hepatic PGC-1α protein content with lower miR-29a-c expression. Increased PGC-1α content was concurrent with increased association to HNF-4α and NRF1 and reduced PPARα expression. The data presented herein reveal that changes in the transcription machinery in neonatal liver of rats born to DEX-treated mothers leads to an inflexible metabolic response to fasting. Such programming is hallmarked by increased oxidative phosphorylation of pyruvate with impaired FFA oxidation and hepatic TAG accumulation.

18β-Glycyrrhetinic acid acts through hepatocyte nuclear factor 4 alpha to modulate lipid and carbohydrate metabolism

Hepatocyte nuclear factor 4 alpha (HNF4α) regulates the expression of essential genes involved in very-low-density lipoprotein (VLDL) homeostasis and gluconeogenesis.　18β-glycyrrhetinic acid (GA) is an active ingredient of Glycyrrhiza uralensis an herbal medicine used for treating liver aliments. In this study, we established that GA functions as a partial antagonist of HNF4α through HNF4α-driven reporter luciferase assay and co-immunoprecipitation experiments with co-activator PGC1α. By virtual docking and site-directed mutagenesis analysis, we confirmed that serine 190 and arginine 235 of HNF4α are both essential for GA to exert its antagonistic action on HNF4α. Importantly, GA suppressed the expression of HNF4α target genes such as apolipoprotein B (ApoB), microsomal triglyceride transfer protein (MTP) and phospholipase A₂ G12B (PLA2G12B) modulating hepatic VLDL secretion in mice fed on a high fat diet. In addition, GA also suppressed gluconeogenesis and ameliorated glucose intolerance via down-regulating the expression of HNF4α target genes glucose-6-phosphatase (G6pc) and phosphoenolpyruvate carboxykinase (Pepck). Furthermore, GA significantly lowered blood glucose and improved insulin resistance in db/db mice. In all, we established that GA acts as a partial HNF4α antagonist modulating lipid and carbohydrate metabolism.

Transcriptional coactivator NT-PGC-1α promotes gluconeogenic gene expression and enhances hepatic gluconeogenesis

The transcriptional coactivator PGC‐1α plays a central role in hepatic gluconeogenesis. We previously reported that alternative splicing of the PGC‐1α gene produces an additional transcript encoding the truncated protein NT‐PGC‐1α. NT‐PGC‐1α is co‐expressed with PGC‐1α and highly induced by fasting in the liver. NT‐PGC‐1α regulates tissue‐specific metabolism, but its role in the liver has not been investigated. Thus, the objective of this study was to determine the role of hepatic NT‐PGC‐1α in the regulation of gluconeogenesis. Adenovirus‐mediated expression of NT‐PGC‐1α in primary hepatocytes strongly stimulated the expression of key gluconeogenic enzyme genes (PEPCK and G6Pase), leading to increased glucose production. To further understand NT‐PGC‐1α function in hepatic gluconeogenesis in vivo, we took advantage of a previously reported FL‐PGC‐1α^−/− mouse line that lacks full‐length PGC‐1α (FL‐PGC‐1α) but retains a slightly shorter and functionally equivalent form of NT‐PGC‐1α (NT‐PGC‐1α²⁵⁴). In FL‐PGC‐1α^−/− mice, NT‐PGC‐1α²⁵⁴ was induced by fasting in the liver and recruited to the promoters of PEPCK and G6Pase genes. The enrichment of NT‐PGC‐1α²⁵⁴ at the promoters was closely associated with fasting‐induced increase in PEPCK and G6Pase gene expression and efficient production of glucose from pyruvate during a pyruvate tolerance test in FL‐PGC‐1α^−/− mice. Moreover, FL‐PGC‐1α^−/− primary hepatocytes showed a significant increase in gluconeogenic gene expression and glucose production after treatment with dexamethasone and forskolin, suggesting that NT‐PGC‐1α²⁵⁴ is sufficient to stimulate the gluconeogenic program in the absence of FL‐PGC‐1α. Collectively, our findings highlight the role of hepatic NT‐PGC‐1α in stimulating gluconeogenic gene expression and glucose production.

Importance of Hepatocyte Nuclear Factor 4α in Glycerol-induced Glucose-6-phosphatase Expression in Liver

Glucose-6-phosphatase (G6Pase) is a key regulator of gluconeogenesis. We previously found that administration of glycerol, a substrate for gluconeogenesis, transactivates G6Pase in the mouse liver. To clarify its cell-autonomous transcriptional activation in hepatocytes, we examined the mechanism of expression of the gene G6pc, which encodes G6Pase, in rat hepatoma cell line FAO cells. Endogenous G6pc expression in FAO cells was increased by glycerol administration as well as by the fatty acid oleate. Luciferase reporter assay revealed that the ~2.0 kb mouse G6pc promoter contains the element(s) responsible for glycerol-stimulated G6pc transactivation. Using several deletion- or chimeric-constructs of G6pc promoter, we found that the DNA response element for hepatocyte nuclear factor 4α (HNF4α) (-77/-65) in the G6pc promoter is essential for transactivation by glycerol. Similarly to glycerol, oleate also increased G6pc expression through its action on the HNF4α element (-77/-65). Furthermore, the reporter activities were higher in the cells co-treated with glycerol plus oleate than in those singly treated with glycerol or oleate. In addition, the temporal profiles of G6pc expression differed between glycerol and oleate administration. Our present results suggest that glycerol and oleate induce G6pc expression both via the HNF4αelement (-77/-65) and also through other regulatory mechanisms.

PGC-1α, glucose metabolism and type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a chronic disease characterized by glucose metabolic disturbance. A number of transcription factors and coactivators are involved in this process. Peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) is an important transcription coactivator regulating cellular energy metabolism. Accumulating evidence has indicated that PGC-1α is involved in the regulation of T2DM. Therefore, a better understanding of the roles of PGC-1α may shed light on more efficient therapeutic strategies. Here, we review the most recent progress on PGC-1α and discuss its regulatory network in major glucose metabolic tissues such as the liver, skeletal muscle, pancreas and kidney. The significant associations between PGC-1α polymorphisms and T2DM are also discussed in this review.

The cAMP-dependent protein kinase downregulates glucose-6-phosphatase expression through RORα and SRC-2 coactivator transcriptional activity

Fasting hormones activate the cAMP/PKA signaling pathway and stimulate expression of hepatic gluconeogenic enzymes including glucose-6-phosphatase (G6Pase). Previously it was shown that steroid receptor coactivator 2 (SRC-2) knock-out mice exhibit fasting hypoglycemia and that SRC-2 coactivates RAR-related orphan receptor alpha (RORα) at the proximal G6Pase promoter. We have investigated the upstream regulation and functional implications of this RORα/SRC-2 complex on G6Pase expression. In HepG2 cells, overexpression of the catalytic PKA subunit (PKA-Cα) reduced the SRC-2 protein level, recruitment to the G6Pase promoter, and its ability to coactivate RORα. Knock-down and transactivation experiments employing G6Pase promoter constructs demonstrated that RORα and SRC-2 are required for PGC-1α to stimulate G6Pase expression. These results suggest that PKA inhibits SRC-2 coactivation of RORα and in turn reduces PGC-1α dependent regulation of G6Pase. This indirect feedback mechanism may underlie the suppression of gluconeogenesis throughout long-term starvation.

Regulation of Glucose Homeostasis by Glucocorticoids

Glucocorticoids are steroid hormones that regulate multiple aspects of glucose homeostasis. Glucocorticoids promote gluconeogenesis in liver, whereas in skeletal muscle and white adipose tissue they decrease glucose uptake and utilization by antagonizing insulin response. Therefore, excess glucocorticoid exposure causes hyperglycemia and insulin resistance. Glucocorticoids also regulate glycogen metabolism. In liver, glucocorticoids increase glycogen storage, whereas in skeletal muscle they play a permissive role for catecholamine-induced glycogenolysis and/or inhibit insulin-stimulated glycogen synthesis. Moreover, glucocorticoids modulate the function of pancreatic α and β cells to regulate the secretion of glucagon and insulin, two hormones that play a pivotal role in the regulation of blood glucose levels. Overall, the major glucocorticoid effect on glucose homeostasis is to preserve plasma glucose for brain during stress, as transiently raising blood glucose is important to promote maximal brain function. In this chapter we will discuss the current understanding of the mechanisms underlying different aspects of glucocorticoid-regulated mammalian glucose homeostasis.

Metabolic control through glucocorticoid hormones: an update

In the past decades, glucocorticoid (GC) hormones and their cognate, intracellular receptor, the glucocorticoid receptor (GR), have been well established as critical checkpoints in mammalian energy homeostasis. Whereas many aspects in healthy nutrient metabolism require physiological levels and/or action of GC, aberrant GC/GR signalling has been linked to severe metabolic dysfunction, including obesity, insulin resistance and type 2 diabetes. Consequently, studies of the molecular mechanisms within the GC signalling axis have become a major focus in biomedical research, up-to-date particularly focusing on systemic glucose and lipid handling. However, with the availability of novel high throughput technologies and more sophisticated metabolic phenotyping capabilities, as-yet non-appreciated, metabolic functions of GC have been recently discovered, including regulatory roles of the GC/GR axis in protein and bile acid homeostasis as well as metabolic inter-organ communication. Therefore, this review summarises recent advances in GC/GR biology, and summarises findings relevant for basic and translational metabolic research.

MicroRNA-29a-c decrease fasting blood glucose levels by negatively regulating hepatic gluconeogenesis

BACKGROUND & AIMS

The expression levels of microRNA-29 (miR-29) family members (miR-29a, miR-29b, miR-29c, here denoted collectively as miR-29a-c) are increased in livers of Goto-Kakizaki diabetic rats and db/db diabetic mice. However, the functional consequences of miR-29a-c upregulation in diabetic livers are not explored. The objective of this study was to evaluate the roles of miR-29a-c in the regulation of hepatic glucose production and blood glucose levels using different mouse models.

METHODS

db/m, db/db diabetic and diet-induced obese (DIO) mice were injected with adenovirus expressing miR-29a-c through the tail vein. Blood glucose levels were measured and glucose-tolerance tests and pyruvate-tolerance tests were performed. To explore the molecular mechanism by which miR-29a-c regulate hepatic glucose metabolism, gain or loss of miR-29a-c function studies were performed in primary mouse hepatocytes and the direct effectors of miR-29-mediated effects on glucose metabolism were identified.

RESULTS

Adenovirus-mediated overexpression of miR-29a-c in the livers of db/m, db/db, and DIO mice decreased fasting blood glucose levels and improved glucose tolerance. Overexpression of miR-29a-c in primary hepatocytes and mouse livers decreased the protein levels of PGC-1α and G6Pase, the direct targets of miR-29a-c, thereby reducing cellular, and hepatic glucose production. In contrast, loss of miR-29a-c function in primary hepatocytes increased the protein levels of PGC-1α and G6Pase and increased cellular glucose production. Finally, enforced expression of PGC-1α increased miR-29a-c expression levels in primary hepatocytes, thus forming a negative feedback regulation loop.

CONCLUSIONS

miR-29a-c can regulate hepatic glucose production and glucose tolerance in mice.

Construction and characterization of peroxisome proliferator-activated receptor-gamma co-activator 1 alpha (PGC-1α over-expressing cell line derived from human hepatocyte carcinoma HepG2 cells)

AIMS

The aim was develop stable human cell line stable over-expressing transcription co-activator peroxisome proliferator-activated receptor gamma co-activator 1α (PGC-1α) with restored hepatospecific functions and increased expression of major xenobiotic metabolizing enzymes.

METHODS

Six clones of HepG2-PGC-1α and one control clone HepG2-pcDNA3 were isolated and analyzed for secretion of hepatospecific markers, fibrinogen, albumin and alpha1-antitrypsin. Expression levels of protein and mRNA of hepatocyte nuclear factor (HNF4α), pregnane X receptor (PXR) and aryl hydrocarbon receptor (AhR) were determined. We measured basal and ligand inducible expression of CYP1A1 and CYP3A4.

RESULTS

Stably transfected cell line HepG2-PGC-1α derived from HepG2 cells over-expressing PGC-1α displayed increased secretion of fibrinogen, but not albumin or alpha1-antitrypsin compared to parent HepG2 cells. We found increased levels of HNF4α, PXR and AhR proteins but not their mRNAs in HepG2-PGC1 cells. Basal expression of CYP3A4 protein in HepG2-PGC-1α cells was increased but rifampicin-inducible expression of CYP3A4 protein was lowered in comparison with parent HepG2 cells. Induction of CYP3A4 mRNA varied between 1.3 - 1.9 fold in individual clones. Expression of TCDD-inducible CYP1A1 protein was lower in HepG2-PGC-1α cells than in parent HepG2 cells. Induction of CYP1A1 mRNA by TCDD in HepG2-PGC-1α cells was comparable with that in parent HepG2 cells and ranged between 103 - 198 fold.

CONCLUSION

Stable expression of PGC-1α in HepG2 cells restores several hepatospecific functions, such as secretion of fibrinogen, expression of HNF4α1 and xenoreceptors PXR and AhR. However, the expression and induction of key drug-metabolizing enzymes (CYP1A1 and CYP3A4) were not improved.

Antidiabetic Potentials of a Novel Polyherbal Preparation Formulated According to Principles of Siddha System of Medicine

According to the principles of Siddha system of medicine, the following polyherbal preparation consisting of 5 plant parts in equal ratio namely, Asparagus racemosus, Emblica officinalis, Salacia oblonga, Syzygium aromaticum, and Tinospora cordifolia was formulated to treat experimental type 2 diabetic rats. So, using plants having aphrodisiac property in the formulation is a rational approach and first of its kind, as there have been no reports so far. Phenolics and other bioactive compounds present in polyherbal preparation may be responsible for lipid-lowering effects and strong antioxidant activity. Polyherbal preparation treatment reverted the activities of glycolytic and gluconeogenic enzymes that are disturbed in diabetic rats. It is concluded that polyherbal preparation treatment improves deranged lipid profile, antioxidant status, glycogen content, and decreases lipid peroxidation, which provides stability to membrane integrity and thus favors insulin receptor to achieve better glucose tolerance through a holistic approach.

Molecular Mechanism Underlying the Antidiabetic Effects of a Siddha Polyherbal Preparation in the Liver of Type 2 Diabetic Adult Male Rats

A siddha polyherbal preparation consisting of 5 medicinal plants, namely, Asparagus racemosus, Emblica officinalis, Salacia oblonga, Syzygium aromaticum, and Tinospora cordifolia, in equal ratio, was formulated to examine the molecular mechanism by which it exhibits antidiabetic effects in the liver of high-fat and fructose-induced type 2 diabetic rats. The polyherbal preparation treated type 2 diabetic rats showed an increase in insulin receptor, Akt, and glucose transporter2 mRNA levels compared with diabetic rats. Insulin receptor, insulin receptor substrate-2, Akt, phosphorylated Akt substrate of 160kDaThreonine642, α-Actinin-4, β-arrestin-2, and glucose transporter2 proteins were also markedly decreased in diabetic rats, whereas the polyherbal preparation treatment significantly improved the expression of these proteins more than that of metformin-treated diabetic rats. The expression pattern of insulin signaling molecules analyzed in the present study signifies the therapeutic efficacy of the siddha polyherbal preparation.

Inhibitory Effects of Chung Hun Wha Dam Tang (CHWDT) on High-Fat Diet-Induced Obesity via AMP-Activated Protein Kinase Activation

The Chung Hun Wha Dam Tang (CHWDT) herbal combination was reported to cease dizziness and phlegm. However, the effect of CHWDT in obesity has not yet been known mechanically. Therefore, we investigated whether this CHWDT could protect the cells from lipogenesis, gluconeogenesis, and inflammation in both in vivo and in vitro. CHWDT significantly decreased body weight, epididymal and perirenal fat content without affecting feed intake in high-fat diet-induced obese mice model. Additionally, CHWDT inhibited obesity-induced SREBP1, FAS, PGC1α, G6Pase, PEPCK and increased CPT1, ACO, and LCAD genes expression in vivo and in vitro. Proinflammatory cytokines like TNF-α and iNOS expression were reduced by CHWDT in both Raw264.7 macrophages and HepG2 cells. In addition, NO production was also significantly decreased by CHWDT in LPS-stimulated macrophages. Furthermore, AMPKα activation by CHWDT was involved in inhibition of obesity by reducing triglycerides production and increasing CPT1 expression. Based on all of the results, we suggest that CHWDT has inhibitory effects on obesity-induced lipogenesis, gluconeogenesis, and inflammation via AMPKα activation.

PARIS (ZNF746) repression of PGC-1α contributes to neurodegeneration in Parkinson's disease

A hallmark of Parkinson's disease (PD) is the preferential loss of substantia nigra dopamine neurons. Here, we identify a new parkin interacting substrate, PARIS (ZNF746), whose levels are regulated by the ubiquitin proteasome system via binding to and ubiquitination by the E3 ubiquitin ligase, parkin. PARIS is a KRAB and zinc finger protein that accumulates in models of parkin inactivation and in human PD brain. PARIS represses the expression of the transcriptional coactivator, PGC-1α and the PGC-1α target gene, NRF-1 by binding to insulin response sequences in the PGC-1α promoter. Conditional knockout of parkin in adult animals leads to progressive loss of dopamine (DA) neurons in a PARIS-dependent manner. Moreover, overexpression of PARIS leads to the selective loss of DA neurons in the substantia nigra, and this is reversed by either parkin or PGC-1α coexpression. The identification of PARIS provides a molecular mechanism for neurodegeneration due to parkin inactivation.

Peroxisome proliferator-activated receptor {alpha} is responsible for the up-regulation of hepatic glucose-6-phosphatase gene expression in fasting and db/db Mice

Glucose-6-phosphatase (G6Pase) is a key enzyme that is responsible for the production of glucose in the liver during fasting or in type 2 diabetes mellitus (T2DM). During fasting or in T2DM, peroxisome proliferator-activated receptor α (PPARα) is activated, which may contribute to increased hepatic glucose output. However, the mechanism by which PPARα up-regulates hepatic G6Pase gene expression in these states is not well understood. We evaluated the mechanism by which PPARα up-regulates hepatic G6Pase gene expression in fasting and T2DM states. In PPARα-null mice, both hepatic G6Pase and phosphoenolpyruvate carboxykinase levels were not increased in the fasting state. Moreover, treatment of primary cultured hepatocytes with Wy14,643 or fenofibrate increased the G6Pase mRNA level. In addition, we have localized and characterized a PPAR-responsive element in the promoter region of the G6Pase gene. Chromatin immunoprecipitation (ChIP) assay revealed that PPARα binding to the putative PPAR-responsive element of the G6Pase promoter was increased in fasted wild-type mice and db/db mice. These results indicate that PPARα is responsible for glucose production through the up-regulation of hepatic G6Pase gene expression during fasting or T2DM animal models.

cAMP-mediated regulation of HNF-4alpha depends on the level of coactivator PGC-1alpha

Hepatocyte nuclear factor-4 alpha (HNF-4alpha) is a member of the nuclear receptor superfamily with important roles in hepatic metabolism. Fasting induces the cAMP/protein kinase A (PKA)-signaling pathway. The mechanisms whereby cAMP regulates HNF-4alpha transcriptional activity are incompletely understood. We have therefore investigated the role of cAMP/PKA in regulation of HNF-4alpha in COS-1 cells and the hepatoma HepG2 cell line. cAMP/PKA inhibited the transcriptional activity of HNF-4alpha in COS-1 cells, whereas a stimulatory effect was observed in HepG2 cells. The cAMP-induced inhibition of HNF-4alpha in COS-1 cells was counteracted by overexpression of the nuclear receptor coactivator PGC-1alpha, and cAMP/PKA-dependent induction of the PGC1A gene in HepG2 cells seems to explain the cell specific differences. This was further supported by knock-down of PGC-1alpha in HepG2 cells, which abolished the stimulatory effect of PKA on HNF-4alpha transcriptional activity. Similar to the cAMP/PKA-mediated regulation of HNF-4alpha, overexpression of the cAMP-response element binding protein (CREB) inhibited the transcriptional activity of HNF-4alpha in COS-1 cells, regardless of cAMP/PKA activation and CREB phosphorylation. Moreover, activation of CREB by cAMP/PKA further stimulated HNF-4alpha transactivation in HepG2 cells. cAMP induced the expression of the HNF-4alpha target genes PCK1 and G6Pase in these cells. In conclusion, our results suggest that the level of PGC-1alpha determines whether the cAMP/PKA-pathway overall stimulates or inhibits HNF-4alpha transcriptional activation.

Acute exercise reduces hepatic glucose production through inhibition of the Foxo1/HNF-4alpha pathway in insulin resistant mice

Protein hepatocyte nuclear factor 4alpha (HNF-4alpha) is atypically activated in the liver of diabetic rodents and contributes to hepatic glucose production. HNF-4alpha and Foxo1 can physically interact with each other and represent an important signal transduction pathway that regulates the synthesis of glucose in the liver. Foxo1 and HNF-4alpha interact with their own binding sites in the phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) promoters, and this binding is required for their effects on those promoters. However, the effect of physical activity on the HNF-4alpha/Foxo1 pathway is currently unknown. Here, we investigate the protein levels of HNF-4alpha and the HNF-4alpha/Foxo1 pathway in the liver of leptin-deficient (ob/ob) and diet-induced obese Swiss (DIO) mice after acute exercise. The ob/ob and DIO mice swam for four 30 min periods, with 5 min rest intervals for a total swimming time of 2h. Eight hours after the acute exercise protocol, the mice were submitted to an insulin tolerance test (ITT) and determination of biochemical and molecular parameters. Acute exercise improved insulin signalling, increasing insulin-stimulated Akt and Foxo1 phosphorylation and decreasing HNF-4alpha protein levels in the liver of DIO and ob/ob mice under fasting conditions. These phenomena were accompanied by a reduction in the expression of gluconeogenesis genes, such as PEPCK and G6Pase. Importantly, the PI3K inhibitor LY292004 reversed the acute effect of exercise on fasting hyperglycaemia, confirming the involvement of the PI3K pathway. The present study shows that exercise acutely improves the action of insulin in the liver of animal models of obesity and diabetes, resulting in increased phosphorylation and nuclear exclusion of Foxo1, and a reduction in the Foxo1/HNF-4alpha pathway. Since nuclear localization and the association of these proteins is involved in the activation of PEPCK and G6Pase, we believe that the regulation of Foxo1 and HNF-4alpha activities are important mechanisms involved in exercise-induced improvement of glucose homeostasis in insulin resistant states.

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.

Insulin and epidermal growth factor suppress basal glucose-6-phosphatase catalytic subunit gene transcription through overlapping but distinct mechanisms

The G6Pase (glucose-6-phosphatase catalytic subunit) catalyses the final step in the gluconeogenic and glycogenolytic pathways, the hydrolysis of glucose-6-phosphate to glucose. We show here that, in HepG2 hepatoma cells, EGF (epidermal growth factor) inhibits basal mouse G6Pase fusion gene transcription. Several studies have shown that insulin represses basal mouse G6Pase fusion gene transcription through FOXO1 (forkhead box O1), but Stoffel and colleagues have recently suggested that insulin can also regulate gene transcription through FOXA2 (forkhead box A2) [Wolfrum, Asilmaz, Luca, Friedman and Stoffel (2003) Proc. Natl. Acad. Sci. 100, 11624-11629]. A combined GR (glucocorticoid receptor)-FOXA2 binding site is located between -185 and -174 in the mouse G6Pase promoter overlapping two FOXO1 binding sites located between (-188 and -182) and (-174 and -168). Selective mutation of the FOXO1 binding sites reduced the effect of insulin, whereas mutation of the GR/FOXA2 binding site had no effect on the insulin response. In contrast, selective mutation of the FOXO1 and GR/FOXA2 binding sites both reduced the effect of EGF. The effect of these mutations was additive, since the combined mutation of both FOXO1 and GR/FOXA2 binding sites reduced the effect of EGF to a greater extent than the individual mutations. These results suggest that, in HepG2 cells, GR and/or FOXA2 are required for the inhibition of basal G6Pase gene transcription by EGF but not insulin. EGF also inhibits hepatic G6Pase gene expression in vivo, but in cultured hepatocytes EGF has the opposite effect of stimulating expression, an observation that may be explained by a switch in ErbB receptor sub-type expression following hepatocyte isolation.

Transcriptional control of energy homeostasis by the estrogen-related receptors

Transcriptional control of cellular energy metabolic pathways is achieved by the coordinated action of numerous transcription factors and associated coregulators. Several members of the nuclear receptor superfamily have been shown to play important roles in this process because they can translate hormonal, nutrient, and metabolite signals into specific gene expression networks to satisfy energy demands in response to distinct physiological cues. Estrogen-related receptor (ERR) alpha, ERRbeta, and ERRgamma are nuclear receptors that have yet to be associated with a natural ligand and are thus considered as orphan receptors. However, the transcriptional activity of the ERRs is exquisitely sensitive to the presence of coregulatory proteins known to be essential for the control of energy homeostasis, and for all intents and purposes, these coregulators function as protein ligands for the ERRs. In particular, functional genomics and biochemical studies have shown that ERRalpha and ERRgamma operate as the primary conduits for the activity of members of the family of PGC-1 coactivators. As transcription factors, the ERRs control vast gene networks involved in all aspects of energy homeostasis, including fat and glucose metabolism as well as mitochondrial biogenesis and function. Phenotypic analyses of knockout mouse models have shown that all three ERRs are indispensable for proper development and/or survival of the organism when subjected to a variety of physiological challenges. The focus of this review is on the recent and rapid advances in understanding the functions of the ERRs in regulating bioenergetic pathways, with an emphasis on their roles in the specification of energetic properties required for cell- and tissue-specific functions.

Sequence variation between the mouse and human glucose-6-phosphatase catalytic subunit gene promoters results in differential activation by peroxisome proliferator activated receptor gamma coactivator-1alpha

AIMS/HYPOTHESIS

The glucose-6-phosphatase catalytic subunit (G6PC) plays a key role in hepatic glucose production by catalysing the final step in gluconeogenesis and glycogenolysis. Peroxisome proliferator activated receptor gamma coactivator-1alpha (PGC-1alpha) stimulates mouse G6pc-luciferase fusion gene expression through hepatocyte nuclear factor-4alpha (HNF-4alpha), which binds an element located between -76 and -64 in the promoter. The aim of this study was to compare the regulation of mouse G6pc and human G6PC gene expression by PGC-1alpha.

METHODS

PGC-1alpha action was analysed by transient transfection and gel retardation assays.

RESULTS

In H4IIE cells, PGC-1alpha alone failed to stimulate human G6PC-luciferase fusion gene expression even though the sequence of the -76 to -64 HNF-4alpha binding site is perfectly conserved in the human promoter. This difference could be explained, in part, by a 3 bp sequence variation between the mouse and human promoters. Introducing the human sequence into the mouse G6pc promoter reduced PGC-1alpha-stimulated fusion gene expression, whereas the inverse experiment, in which the mouse sequence was introduced into the human G6PC promoter, resulted in the generation of a G6PC-luciferase fusion gene that was now induced by PGC-1alpha. This critical 3 bp region is located immediately adjacent to a consensus nuclear hormone receptor half-site that is perfectly conserved between the mouse G6pc and human G6PC promoters. Gel retardation experiments revealed that this 3 bp region influences the affinity of HNF-4alpha binding to the half-site.

CONCLUSIONS/INTERPRETATION

These observations suggest that PGC-1alpha may be more important in the control of mouse G6pc than human G6PC gene expression.

Coactivator PGC-1alpha regulates the fasting inducible xenobiotic-metabolizing enzyme CYP2A5 in mouse primary hepatocytes

The nutritional state of organisms and energy balance related diseases such as diabetes regulate the metabolism of xenobiotics such as drugs, toxins and carcinogens. However, the mechanisms behind this regulation are mostly unknown. The xenobiotic-metabolizing cytochrome P450 (CYP) 2A5 enzyme has been shown to be induced by fasting and by glucagon and cyclic AMP (cAMP), which mediate numerous fasting responses. Peroxisome proliferator-activated receptor gamma coactivator (PGC)-1alpha triggers many of the important hepatic fasting effects in response to elevated cAMP levels. In the present study, we were able to show that cAMP causes a coordinated induction of PGC-1alpha and CYP2A5 mRNAs in murine primary hepatocytes. Furthermore, the elevation of the PGC-1alpha expression level by adenovirus mediated gene transfer increased CYP2A5 transcription. Co-transfection of Cyp2a5 5' promoter constructs with the PGC-1alpha expression vector demonstrated that PGC-1alpha is able to activate Cyp2a5 transcription through the hepatocyte nuclear factor (HNF)-4alpha response element in the proximal promoter of the Cyp2a5 gene. Chromatin immunoprecipitation assays showed that PGC-1alpha binds, together with HNF-4alpha, to the same region at the Cyp2a5 proximal promoter. In conclusion, PGC-1alpha mediates the expression of Cyp2a5 induced by cAMP in mouse hepatocytes through coactivation of transcription factor HNF-4alpha. This strongly suggests that PGC-1alpha is the major factor mediating the fasting response of CYP2A5.

Chronic late-gestation hypoglycemia upregulates hepatic PEPCK associated with increased PGC1alpha mRNA and phosphorylated CREB in fetal sheep

Hepatic glucose production is normally activated at birth but has been observed in response to experimental hypoglycemia in fetal sheep. The cellular basis for this process remains unknown. We determined the impact of 2 wk of fetal hypoglycemia during late gestation on enzymes responsible for hepatic gluconeogenesis, focusing on the insulin-signaling pathway, transcription factors, and coactivators that regulate gluconeogenesis. Hepatic phosphoenolpyruvate carboxykinase and glucose-6-phosphatase mRNA increased 12-fold and 7-fold, respectively, following chronic hypoglycemia with no change in hepatic glycogen. Chronic hypoglycemia decreased fetal plasma insulin with no change in glucagon but increased plasma cortisol 3.5-fold. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha mRNA and phosphorylation of cAMP response element binding protein at Ser(133) were both increased, with no change in Akt, forkhead transcription factor FoxO1, hepatocyte nuclear factor-4alpha, or CCAAT enhancer binding protein-beta. These results demonstrate that chronic fetal hypoglycemia triggers signals that can activate gluconeogenesis in the fetal liver.

Human nuclear pregnane X receptor cross-talk with CREB to repress cAMP activation of the glucose-6-phosphatase gene

The nuclear PXR (pregnane X receptor) was originally characterized as a key transcription factor that activated hepatic genes encoding drug-metabolizing enzymes. We have now demonstrated that PXR also represses glucagon-activated transcription of the G6Pase (glucose-6-phosphatase) gene by directly binding to CREB [CRE (cAMP-response element)-binding protein]. Adenoviral-mediated expression of human PXR (hPXR) and its activation by rifampicin strongly repressed cAMP-dependent induction of the endogenous G6Pase gene in Huh7 cells. Using the -259 bp G6Pase promoter construct in cell-based transcription assays, repression by hPXR of PKA (cAMP-dependent protein kinase)-mediated promoter activation was delineated to CRE sites. GST (glutathione transferase) pull-down and immunoprecipitation assays were employed to show that PXR binds directly to CREB, while gel-shift assays were used to demonstrate that this binding prevents CREB interaction with the CRE. These results are consistent with the hypothesis that PXR represses the transcription of the G6Pase gene by inhibiting the DNA-binding ability of CREB. In support of this hypothesis, treatment with the mouse PXR activator PCN (pregnenolone 16alpha-carbonitrile) repressed cAMP-dependent induction of the G6Pase gene in primary hepatocytes prepared from wild-type, but not from PXR-knockout, mice, and also in the liver of fasting wild-type, but not PXR-knockout, mice. Moreover, ChIP (chromatin immunoprecipitation) assays were performed to show a decreased CREB binding to the G6Pase promoter in fasting wild-type mice after PCN treatment. Thus drug activation of PXR can repress the transcriptional activity of CREB, down-regulating gluconeogenesis.

Glucocorticoids, metabolism and metabolic diseases

Since the discovery of the beneficial effects of adrenocortical extracts for treating adrenal insufficiency more than 80 years ago, glucocorticoids (GC) and their cognate, intracellular receptor, the glucocorticoid receptor (GR) have been characterized as critical components of the delicate hormonal control system that determines energy homeostasis in mammals. Whereas physiological levels of GCs are required for proper metabolic control, excessive GC action has been tied to a variety of pandemic metabolic diseases, such as type II diabetes and obesity. Highlighted by its importance for human health, the investigation of molecular mechanisms of GC/GR action has become a major focus in biomedical research. In particular, the understanding of tissue-specific functions of the GC-GR pathway has been proven to be of substantial value for the identification of novel therapeutic options in the treatment of severe metabolic disorders. Therefore, this review focuses on the role of the GC-GR axis for metabolic homeostasis and dysregulation, emphasizing tissue-specific functions of GCs in the control of energy metabolism.

The nuclear receptor cofactor, receptor-interacting protein 140, is required for the regulation of hepatic lipid and glucose metabolism by liver X receptor

The liver X receptors (LXRs) are nuclear receptors that play important roles in the regulation of lipid metabolism. In this study, we demonstrate that receptor-interacting protein 140 (RIP140) is a cofactor for LXR in liver. Analysis of RIP140 null mice and hepatocytes depleted of RIP140 indicate that the cofactor is essential for the ability of LXR to activate the expression of a set of genes required for lipogenesis. Furthermore we demonstrate that RIP140 is required for the ability of LXR to repress the expression of the phosphoenolpyruvate carboxykinase gene in Fao cells and mice. Thus, we conclude that the function of RIP140 as a cofactor for LXR in liver varies according to the target genes and metabolic process, serving as a coactivator in lipogenesis but as a corepressor in gluconeogenesis.

Several transcription factors are recruited to the glucose-6-phosphatase gene promoter in response to palmitate in rat hepatocytes and H4IIE cells

Fatty acids and glucose are strong modulators of the expression of glucose-6-phosphatase (Glc-6-Pase), an enzyme that plays a key role in glucose homeostasis. PUFA inhibit, whereas SFA and monounsaturated fatty acids induce the expression of the Glc-6-Pase gene. Palmitate and oleate are the most abundant fatty acid species in circulation during food deprivation in mammals. Although dietary fats have been shown to modulate the expression of genes involved in both lipid and carbohydrate metabolism in liver, little is known regarding the molecular mechanism of transcriptional response of the Glc-6-Pase gene to long-chain fatty acids. Using H4IIE hepatoma cells and hepatocytes from adult rats, we investigated the mechanism of the induction of this gene by palmitate and oleate. Both of these fatty acids stimulated Glc-6-Pase gene transcription but did not affect the stability of its mRNA. In transient transfection assays, transcription from the Glc-6-Pase gene promoter was markedly enhanced by both palmitate and oleate but not by arachidonate. Chromatin immunoprecipitation analysis was used to show that palmitate induced the recruitment of an array of transcription factors viz hepatic nuclear factor(NF)-4alpha, CAAT/enhancer binding proteinbeta, PPARalpha, chicken ovalbumin upstream promoter transcription factor (COUP-TF), cAMP regulatory element binding protein, and NF-kappaB to this gene promoter. Although it is presently unclear how these various transcription factors interact at this promoter, the data are consistent with the view that multiple regulatory elements in the Glc-6-Pase gene promoter are responsible for the modulation of gene transcription by fatty acids.

Gluconeogenesis: re-evaluating the FOXO1-PGC-1alpha connection

Increased expression of the gene encoding the enzyme glucose-6-phosphatase (G6Pase) contributes to the increased production of glucose by the liver that occurs in individuals with diabetes. Puigserver et al. show that the transcription factor FOXO1 and the transcriptional co-activator PGC-1alpha act synergistically to stimulate the expression of genes in the gluconeogenesis pathway and propose that PGC-1alpha acts, in part, directly through FOXO1. Here we show that FOXO1 is neither required nor sufficient for the stimulation of G6Pase-luciferase fusion gene expression by PGC-1alpha. Our results indicate that the transcriptional interaction between FOXO1 and PGC-1alpha is indirect.

Threshold levels of hepatocyte nuclear factor 6 (HNF-6) acting in synergy with HNF-4 and PGC-1alpha are required for time-specific gene expression during liver development

During liver development, hepatocytes undergo a maturation process that leads to the fully differentiated state. This relies at least in part on the coordinated action of liver-enriched transcription factors (LETFs), but little is known about the dynamics of this coordination. In this context we investigate here the role of the LETF hepatocyte nuclear factor 6 (HNF-6; also called Onecut-1) during hepatocyte differentiation. We show that HNF-6 knockout mouse fetuses have delayed expression of glucose-6-phosphatase (g6pc), which catalyzes the final step of gluconeogenesis and is a late marker of hepatocyte maturation. Using a combination of in vivo and in vitro gain- and loss-of-function approaches, we demonstrate that HNF-6 stimulates endogenous g6pc gene expression directly via a synergistic and interdependent action with HNF-4 and that it involves coordinate recruitment of the coactivator PGC-1alpha. The expression of HNF-6, HNF-4, and PGC-1alpha rises steadily during liver development and precedes that of g6pc. We provide evidence that threshold levels of HNF-6 are required to allow synergism between HNF-6, HNF-4, and PGC-1alpha to induce time-specific expression of g6pc. Our observations on the regulation of g6pc by HNF-6 provide a model whereby synergism, interdependency, and threshold concentrations of LETFs and coactivators determine time-specific expression of genes during liver development.

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.

Estrogen-related receptor alpha is a repressor of phosphoenolpyruvate carboxykinase gene transcription

The orphan nuclear receptor estrogen-related receptor (ERR) alpha is a downstream effector of the transcriptional coactivator PGC-1alpha in the regulation of genes important for mitochondrial oxidative capacity. PGC-1alpha is also a potent activator of the transcriptional program required for hepatic gluconeogenesis, and in particular of the key gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK). We report here that the regulatory sequences of the PEPCK gene harbor a functional ERRalpha binding site. However, in contrast to the co-stimulating effects of ERRalpha and PGC-1alpha on mitochondrial gene expression, ERRalpha acts as a transcriptional repressor of the PEPCK gene. Suppression of ERRalpha expression by small interfering RNA leads to reduced binding of ERRalpha to the endogenous PEPCK gene, and an increase in promoter occupancy by PGC-1alpha, suggesting that part of the ERRalpha function at this gene is to antagonize the action of PGC-1alpha. In agreement with the in vitro studies, animals that lack ERRalpha show increased expression of gluconeogenic genes, including PEPCK and glycerol kinase, but decreased expression of mitochondrial genes, such as ATP synthase subunit beta and cytochrome c-1. Our findings suggest that ERRalpha has opposing effects on genes important for mitochondrial oxidative capacity and gluconeogenesis. The different functions of ERRalpha in the regulation of these pathways suggest that enhancing ERRalpha activity could have beneficial effects on glucose metabolism in diabetic subjects by two distinct mechanisms: increasing mitochondrial oxidative capacity in peripheral tissues and liver, and suppressing hepatic glucose production.

Cloning of the rat pyruvate dehydrogenase kinase 4 gene promoter: activation of pyruvate dehydrogenase kinase 4 by the peroxisome proliferator-activated receptor gamma coactivator

The pyruvate dehydrogenase complex catalyzes the conversion of pyruvate to acetyl-CoA in mitochondria and is a key regulatory enzyme in the metabolism of glucose to acetyl-CoA. Phosphorylation of pyruvate dehydrogenase by the pyruvate dehydrogenase kinases (PDK) inhibits pyruvate dehydrogenase complex activity. There are four PDK isoforms, and expression of PDK4 and PDK2 genes is elevated in starvation and diabetes, allowing glucose to be conserved while fatty acid oxidation is increased. In these studies we have investigated the transcriptional mechanisms by which the expression of the PDK4 gene is increased. The peroxisome proliferator-activated receptor gamma coactivator (PGC-1alpha) stimulates the expression of genes involved in hepatic gluconeogenesis and mitochondrial fatty acid oxidation. We have found that PGC-1alpha will induce the expression of both the PDK2 and PDK4 genes in primary rat hepatocytes and ventricular myocytes. We cloned the promoter for the rat PDK4 gene. Hepatic nuclear factor 4 (HNF4), which activates many genes in the liver, will induce PDK4 expression. Although HNF4 and PGC-1alpha interact to stimulate several genes encoding gluconeogenic enzymes, the induction of PDK4 does not involve interactions of PGC-1alpha with HNF4. Using the chromatin immunoprecipitation assay, we have demonstrated that HNF4 and PGC-1alpha are associated with the PDK4 gene in vivo. Our data suggest that by inducing PDK genes PGC-1alpha will direct pyruvate away from metabolism into acetyl-CoA and toward the formation of oxaloacetate and into the gluconeogenic pathway.

Tumour necrosis factor alpha decreases glucose-6-phosphatase gene expression by activation of nuclear factor kappaB

The key insulin-regulated gluconeogenic enzyme G6Pase (glucose-6-phosphatase) has an important function in the control of hepatic glucose production. Here we examined the inhibition of G6Pase gene transcription by TNF (tumour necrosis factor) in H4IIE hepatoma cells. TNF decreased dexamethasone/dibtuyryl cAMP-induced G6Pase mRNA levels. TNFalpha, but not insulin, led to rapid activation of NFkappaB (nuclear factor kappaB). The adenoviral overexpression of a dominant negative mutant of IkappaBalpha (inhibitor of NFkappaB alpha) prevented the suppression of G6Pase expression by TNFalpha, but did not affect that by insulin. The regulation of G6Pase by TNF was not mediated by activation of the phosphoinositide 3-kinase/protein kinase B pathway, extracellular-signal-regulated protein kinase or p38 mitogen-activated protein kinase. Reporter gene assays demonstrated a concentration-dependent down-regulation of G6Pase promoter activity by the transient overexpression of NFkappaB. Although two binding sites for NFkappaB were identified within the G6Pase promoter, neither of these sites, nor the insulin response unit or binding sites for Sp proteins, was necessary for the regulation of G6Pase promoter activity by TNFalpha. In conclusion, the data indicate that the activation of NFkappaB is sufficient to suppress G6Pase gene expression, and is required for the regulation by TNFalpha, but not by insulin. We propose that NFkappaB does not act by binding directly to the G6Pase promoter.

Studies of the Gly482Ser polymorphism of the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) gene in Danish subjects with the metabolic syndrome

The peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1alpha) is a novel transcriptional co-activator that holds an important role in lipid and glucose metabolism. PGC-1alpha is a candidate gene for the metabolic syndrome (MS) as well as type 2 diabetes. Recent studies suggested linkage between the chromosomal region of PGC-1alpha and fasting serum insulin levels, and associates a Gly482Ser polymorphism of the gene with type 2 diabetes and hypertension. In this study, we investigated whether the Gly482Ser variant is associated with the MS per se or other phenotypic traits related to this syndrome. The variant was examined, using PCR-RFLP, in the DanMONICA cohort comprising a population-based sample of 2349 subjects. MS was defined using the National Cholesterol Education Program -- Adult Treatment Panel III (NCEP-ATPIII) criteria. The allelic frequency of the Ser482 allele was 35.8% in the MS group and 35.6% in the non-MS group (P = 0.74). There were no significant differences across the three groups of genotypes with respect to any of the examined variables, including BMI, waist, fasting serum lipids, plasma glucose, serum insulin, HOMA estimates of insulin resistance and insulin secretion, 24-ambulatory blood pressure or left ventricular mass index. In conclusion, the Gly482Ser polymorphism of the PGC-1alpha gene is not associated with the metabolic syndrome, related quantitative traits or cardiac hypertrophy among Danish Caucasian subjects.

An extract of Syzygium aromaticum represses genes encoding hepatic gluconeogenic enzymes

Insulin action is impaired in diabetic patients, which leads to increased hepatic glucose production. Plants and herbs have been used for medicinal purposes, including the treatment of diabetes, for centuries. Since dietary management is a starting point for the treatment of diabetes, it is important to recognize the effect of plant-based compounds on tissues that regulate glucose metabolism, such as the liver. In a recent study, several herbs and spices were found to increase glucose uptake into adipocytes, an insulin-like effect. Our data reveal that Syzygium aromaticum (L.) Merrill and Perry (Myrtaceae) (commonly referred to as clove) extract acts like insulin in hepatocytes and hepatoma cells by reducing phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase) gene expression. Much like insulin, clove-mediated repression is reversed by PI3K inhibitors and N-acetylcysteine (NAC). A more global analysis of gene expression by DNA microarray analysis reveals that clove and insulin regulate the expression of many of the same genes in a similar manner. These results demonstrate that consumption of certain plant-based diets may have beneficial effects for the treatment of diabetes and indicate a potential role for compounds derived from clove as insulin-mimetic agents.

A Distal Region Involving Hepatocyte Nuclear Factor 4α and CAAT/Enhancer Binding Protein Markedly Potentiates the Protein Kinase A Stimulation of the Glucose-6-Phosphatase Promoter

Glucose-6-phosphatase (Glc6Pase) is the last enzyme of gluconeogenesis and is only expressed in the liver, kidney, and small intestine. In these tissues, the mRNA and its activity are increased when cAMP levels increased (e.g. in fasting or diabetes). We first report that a proximal region (within −200 bp relative to the transcription start site) and a distal region (−694/−500 bp) are both required for a potent cAMP and a protein kinase A (PKA) responsiveness of the Glc6Pase promoter. Using different molecular approaches, we demonstrate that hepatocyte nuclear factor (HNF4α), CAAT/ enhancer-binding protein-α (C/EBPα), C/EBPβ, and cAMP response element-binding protein (CREB) are involved in the potentiated PKA responsiveness: in the distal region, via one HNF4α- and one C/EBP-binding sites, and in the proximal region, via two HNF4α and two CREB-binding sites. We also show that HNF4α, C/EBPα, and C/EBPβ are constitutively bound to the endogenous Glc6Pase gene, whereas CREB and CREB-binding protein (CBP) will be bound to the gene upon stimulation by cAMP. These data strongly suggest that the cAMP responsiveness of the Glc6Pase promoter requires a tight cooperation between a proximal and a distal region, which depends on the presence of several HNF4α-, C/EBP-, and CREB-binding sites, therefore involving an intricate association of hepatic and ubiquitous transcription factors.

Peroxisomal proliferator activated receptor gamma coactivator (PGC-1alpha) stimulates carnitine palmitoyltransferase I (CPT-Ialpha) through the first intron

Peroxisomal proliferator activated receptor gamma coactivator-1 (PGC-1alpha) is a transcriptional coactivator that promotes mitochondrial biogenesis and energy metabolism in brown fat, skeletal muscle and heart. Previous studies demonstrated that PGC-1alpha is present at low levels in the liver but that the hepatic abundance of PGC-1alpha is elevated in diabetic and fasted animals. Elevated PGC-1alpha expression is associated with increased fatty acid oxidation and hepatic glucose production. Carnitine palmitoyltransferase-I (CPT-I) is a rate controlling step in the mitochondrial oxidation of long chain fatty acids. CPT-I transfers the acyl moiety from fatty acyl-CoA to carnitine for the translocation of long chain fatty acids across the mitochondrial membrane. There are two isoforms of CPT-I including a liver isoform CPT-Ialpha and a muscle isoform CPT-Ibeta. Here, we characterized the regulation of CPT-Ialpha isoform by PGC-1alpha. PGC-1alpha stimulates CPT-Ialpha primarily through multiple sites in the first intron. We found that PGC-1alpha can induce CPT-Ialpha gene expression in cardiac myocytes and primary hepatocytes. Our results indicate that PGC-1alpha elevates the expression of CPT-Ialpha via a unique mechanism that utilizes elements within the intron.

IMPACT OF TRANSCRIPTION FACTOR PROFILE AND CHROMATIN CONFORMATION ON HUMAN HEPATOCYTE CYP3A GENE EXPRESSION

Recent data have made it increasingly clear that the gene expression profile of a cell system, and its alteration in response to external stimuli, is highly dependent on both the higher order chromatin structure of the genome and the interaction of gene products in interpreting stimuli. To further explore this phenomenon, we have examined the role of both of these factors in controlling xenobiotic-mediated gene expression changes in primary and transformed human hepatocytes (HuH7). Using quantitative polymerase chain reaction, expression levels of several transcription factors implicated in the liver-specific regulation of the CYP3A gene family were examined in human adult and fetal liver RNA samples. These expression profiles were then compared with those obtained from both primary and transformed human hepatocytes, showing that, in general, cultured cells exhibit a distinct profile compared with either the fetal or adult samples. Transcriptome profiles before and after exposure to the CYP3A transcriptional activators rifampicin, dexamethasone, pregnane-16alpha-carbonitrile, and phenobarbital were subsequently examined. Whereas exposure to these compounds elicited a dose-dependent increase in CYP3A transcription in primary hepatocytes, no alteration in expression levels was observed for the hepatoma cell line HuH7. Alteration in the expression levels of pregnane X receptor and chicken ovalbumin upstream promoter transcription factor I, and the disruption of higher order chromatin within HuH7 cells altered CYP3A expression and/or activation by xenobiotics toward that observed in primary hepatocytes. These data provide potential roles for these two processes in regulating CYP3A expression in vivo.

Peroxisomal proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) enhances the thyroid hormone induction of carnitine palmitoyltransferase I (CPT-I alpha)

Carnitine palmitoyltransferase I (CPT-I) catalyzes the rate-controlling step in the pathway of mitochondrial fatty acid oxidation. Thyroid hormone will stimulate the expression of the liver isoform of CPT-I (CPT-I alpha). This induction of CPT-I alpha gene expression requires the thyroid hormone response element in the promoter and sequences within the first intron. The peroxisomal proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) is a coactivator that promotes mitochondrial biogenesis, mitochondrial fatty acid oxidation, and hepatic gluconeogenesis. In addition, PGC-1 alpha will stimulate the expression of CPT-I alpha in primary rat hepatocytes. Here we report that thyroid hormone will increase PGC-1 alpha mRNA and protein levels in rat hepatocytes. In addition, overexpression of PGC-1 alpha will enhance the thyroid hormone induction of CPT-I alpha indicating that PGC-1 alpha is a coactivator for thyroid hormone. By using chromatin immunoprecipitation assays, we show that PGC-1 alpha is associated with both the thyroid hormone response element in the CPT-I alpha gene promoter and the first intron of the CPT-I alpha gene. Our data demonstrate that PGC-1 alpha participates in the stimulation of CPT-I alpha gene expression by thyroid hormone and suggest that PGC-1 alpha is a coactivator for thyroid hormone.

Orphan nuclear receptor small heterodimer partner represses hepatocyte nuclear factor 3/Foxa transactivation via inhibition of its DNA binding

Small heterodimer partner (SHP; NR0B2) is an atypical orphan nuclear receptor and acts as a coregulator of various nuclear receptors. Herein, we examined a novel cross talk between SHP and a forkhead transcription factor HNF3 (hepatocyte nuclear factor 3/Foxa. Transient transfection assay demonstrated that SHP inhibited the transcriptional activity of all three isoforms of HNF3, HNF3alpha, beta, and gamma. In vivo and in vitro protein interaction studies showed that SHP physically interacted with HNF3. Adenovirus-mediated overexpression of SHP significantly decreased the mRNA levels of glucose-6-phosphase (G6Pase), cholesterol 7-alpha-hydroxylase (CYP7A1), and phosphoenolpyruvate carboxykinase (PEPCK) in HepG2 cells and rat primary hepatocytes. Moreover, the mRNA level of G6Pase was notably increased by down-regulation of SHP with small interfering RNA. Interestingly, HNF3 transactivity was still repressed by SHPDelta128-139 that fails to repress nuclear receptors. Mapping of interaction domain revealed that SHP interacted with forkhead DNA binding domain of HNF3alpha. Gel mobility shift and chromatin immunoprecipitation assays demonstrated that SHP inhibits DNA binding of HNF3. These results suggest that SHP is involved in the regulation of G6Pase, CYP7A1, and PEPCK gene expression via novel mechanism of inhibition of HNF3 activity and expand the role of SHP as a coregulator of other family of transcription factors in addition to nuclear receptors.

Inverse gene expression patterns for macrophage activating hepatotoxicants and peroxisome proliferators in rat liver

Macrophage activation contributes to adverse effects produced by a number of hepatotoxic compounds. Transcriptional profiles elicited by two macrophage activators, LPS and zymosan A, were compared to those produced by 100 paradigm compounds (mostly hepatotoxicants) using cDNA microarrays. Several hepatotoxicants previously reported to activate liver macrophages produced transcriptional responses similar to LPS and zymosan, and these were used to construct a gene signature profile for macrophage activators in the liver. Measurement of cytokine mRNAs in the same liver samples by RT-PCR independently confirmed that these compounds are associated with macrophage activation. In addition to expected effects on acute phase proteins and metabolic pathways that are regulated by LPS and inflammation, a strong induction was observed for many endoplasmic reticulum-associated stress/chaperone proteins. Additionally, many genes in our macrophage activator signature profile were well-characterized PPARalpha-induced genes which were repressed by macrophage activators. A shared gene signature profile for peroxisome proliferators was determined using a training set of clofibrate, WY 14643, diethylhexylphthalate, diisononylphthalate, perfluorodecanoic acid, perfluoroheptanoic acid, and perfluorooctanoic acid. The signature profile included macrophage activator-induced genes that were repressed by peroxisome proliferators. NSAIDs comprised an interesting pharmacological class in that some compounds, notably diflunisal, co-clustered with peroxisome proliferators whereas several others co-clustered with macrophage activators, possibly due to endotoxin exposure secondary to their adverse effects on the gastrointestinal system. While much of these data confirmed findings from the literature, the transcriptional patterns detected using this toxicogenomics approach showed relationships between genes and biological pathways requiring complex analysis to be discerned.

Hepatic de novo synthesis of glucose 6-phosphate is not affected in peroxisome proliferator-activated receptor alpha-deficient mice but is preferentially directed toward hepatic glycogen stores after a short term fast

Apart from impaired beta-oxidation, Pparalpha-deficient (Pparalpha(-/-)) mice suffer from hypoglycemia during prolonged fasting, suggesting alterations in hepatic glucose metabolism. We compared hepatic glucose metabolism in vivo in wild type (WT) and Pparalpha(-/-) mice after a short term fast, applying novel isotopic methods. After a 9-h fast, mice were infused with [U-(13)C]glucose, [2-(13)C]glycerol, [1-(2)H]galactose, and paracetamol for 6 h, and blood and urine was collected in timed intervals. Plasma glucose concentrations remained constant and were not different between the groups. Hepatic glycogen content was 69 +/- 11 and 90 +/- 31 microM/g liver after 15 h of fasting in WT and Pparalpha(-/-) mice, respectively. The gluconeogenic flux toward glucose 6-phosphate was not different between the groups (i.e. 157 +/- 9 and 153 +/- 9 microM/kg/min in WT and Pparalpha(-/-) mice, respectively). The gluconeogenic flux toward plasma glucose, however, was decreased in PPARalpha(-/-) mice (i.e. 142 +/- 9 versus 124 +/- 13 microM/kg/min) (p < 0.05), accounting for the observed decrease (-15%) in hepatic glucose production in Pparalpha(-/-) mice. Expression of the gene encoding glucose-6-phosphate hydrolase (G6ph) was lower in the PPARalpha(-/-) mice compared with WT mice. In conclusion, Pparalpha(-/-) mice were able to maintain a normal total gluconeogenic flux to glucose 6-phosphate during moderate fasting, despite their inability to up-regulate beta-oxidation. However, this gluconeogenic flux was directed more toward glycogen, leading to a decreased hepatic glucose output. This was associated with a down-regulation of the expression of G6ph in PPARalpha-deficient mice.

Hepatocyte nuclear factor 4alpha controls the development of a hepatic epithelium and liver morphogenesis

Although advances have been made in understanding cell differentiation, only rudimentary knowledge exists concerning how differentiated cells form tissues and organs. We studied liver organogenesis because the cell and tissue architecture of this organ is well defined. Approximately 60% of the adult liver consists of hepatocytes that are arranged as single-cell anastomosing plates extending from the portal region of the liver lobule toward the central vein. The basal surface of the hepatocytes is separated from adjacent sinusoidal endothelial cells by the space of Disse, where the exchange of substances between serum and hepatocytes takes place. The hepatocyte's apical surface forms bile canaliculi that transport bile to the hepatic ducts. Proper liver architecture is crucial for hepatic function and is commonly disrupted in disease states, including cirrhosis and hepatitis. Here we report that hepatocyte nuclear factor 4alpha (Hnf4alpha) is essential for morphological and functional differentiation of hepatocytes, accumulation of hepatic glycogen stores and generation of a hepatic epithelium. We show that Hnf4alpha is a dominant regulator of the epithelial phenotype because its ectopic expression in fibroblasts induces a mesenchymal-to-epithelial transition. Most importantly, the morphogenetic parameters controlled by Hnf4alpha in hepatocytes are essential for normal liver architecture, including the organization of the sinusoidal endothelium.