The molecular physiology of hepatic nuclear factor 3 in the regulation of gluconeogenesis

A high concentrate diet inhibits forkhead box protein A2 expression, and induces oxidative stress, mitochondrial dysfunction and mitochondrial unfolded protein response in the liver of dairy cows

High-concentrate diet induce subacute ruminal acidosis (SARA) and cause liver damage in ruminants. It has been reported that forkhead box protein A2 (FOXA2) can enhance mitochondrial membrane potential but its function in mitochondrial dysfunction induced by high concentrate diets is still unknown. Therefore, the aim of this study was to elucidate the effect of high-concentrate (HC) diet on hepatic FOXA2 expression, mitochondrial unfolded protein response (UPRmt), mitochondrial dysfunction and oxidative stress. A total of 12 healthy mid-lactation Holstein cows were selected and randomized into 2 groups: the low concentrate (LC) diet group (concentrate:forage = 4:6) and HC diet group (concentrate:forage = 6:4). The trial lasted 21 d. The rumen fluid, blood and liver tissue were collected at the end of the experiment. The results showed that the rumen fluid pH level was reduced in the HC group and the pH was lower than 5.6 for more than 4 h/d, indicating that feeding HC diets successfully induced SARA in dairy cows. Both FOXA2 mRNA and protein abundance were significantly reduced in the liver of the HC group compared with the LC group. The activity of antioxidant enzymes (CAT, G6PDH, T-SOD, Cu/Zn SOD, Mn SOD) and mtDNA copy number in the liver tissue of the HC group decreased, while the level of H2O2 significantly increased, this increase was accompanied by a decrease in oxidative phosphorylation (OXPHOS). The balance of mitochondrial division and fusion was disrupted in the HC group, as evidenced by the decreased mRNA level of OPA1, MFN1, and MFN2 and increased mRNA level of Drp1, Fis1, and MFF. At the same time, HC diet downregulated the expression level of SIRT1, SIRT3, PGC-1α, TFAM, and Nrf 1 to inhibit mitochondrial biogenesis. The HC group induced UPRmt in liver tissue by upregulating the mRNA and protein levels of CLPP, LONP1, CHOP, Hsp10, and Hsp60. In addition, HC diet could increase the protein abundance of Bax, CytoC, Caspase 3 and Cleaved-Caspase 3, while decrease the protein abundance of Bcl-2 and the Bcl-2/Bax ratio. Overall, our study suggests that the decreased expression of FOXA2 may be related to UPRmt, mitochondrial dysfunction, oxidative stress, and apoptosis in the liver of dairy cows fed a high concentrate diet.

AMP activated kinase negatively regulates hepatic Fetuin-A via p38 MAPK-C/EBPβ/E3 Ubiquitin Ligase Signaling pathway

Fetuin-A (Fet-A) is a liver-secreted phosphorylated protein, known to impair insulin signaling, which has been shown to be associated with obesity, insulin resistance, and incident diabetes. Fet-A interacts with the insulin-stimulated insulin receptor (IR) and inhibits IR tyrosine kinase activity and glucose uptake. It has been shown that high glucose increases Fet-A expression through the ERK1/2 signaling pathway. However, factors that downregulate Fet-A expression and their potential mechanisms are unclear. We examined the effect of AMP-activated protein kinase (AMPK) on high-glucose induced Fet-A expression in HepG2 cells, Hep3B cells and primary rat hepatocytes. High glucose increased Fet-A and phosphorylated (Ser312) fetuin-A (pFet-A) expression, which are known to impair insulin signaling. AICAR-induced AMPK activation significantly down-regulated high glucose-induced Fet-A expression and secretion of pFet-A while treatment with Compound C (AMPK inhibitor), SB202190 (p38 MAPK inhibitor) or p38 MAPK siRNA transfection prevented AICAR-induced downregulation of Fet-A expression. In addition, activation of p38 MAPK, by anisomycin, decreased the hepatic expression of Fet-A. Further, we our studies have shown that short-term effect of AICAR-treatment on Fet-A expression was mediated by proteosomal degradation, and long-term treatment of AICAR was associated with decrease in hepatic expression of C/EBP beta, an important transcription factor involved in the regulation of Fet-A. Taken together, our studies implicate a critical role for AMPK-p38 MAPK-C/EBPb-ubiquitin-proteosomal axis in the regulation of the expression of hepatic Fet-A.

Intricate Regulation of Phosphoenolpyruvate Carboxykinase (PEPCK) Isoforms in Normal Physiology and Disease

BACKGROUND

The phosphoenolpyruvate carboxykinase (PEPCK) isoforms are considered as rate-limiting enzymes for gluconeogenesis and glyceroneogenesis pathways. PEPCK exhibits several interesting features such as a) organelle-specific isoforms (cytosolic and a mitochondrial) in vertebrate clade, b) tissue-specific expression of isoforms and c) organism-specific requirement of ATP or GTP as a cofactor. In higher organisms, PEPCK isoforms are intricately regulated and activated through several physiological and pathological stimuli such as corticoids, hormones, nutrient starvation and hypoxia. Isoform-specific transcriptional/translational regulation and their interplay in maintaining glucose homeostasis remain to be fully understood. Mounting evidence indicates the significant involvement of PEPCK isoforms in physiological processes (development and longevity) and in the progression of a variety of diseases (metabolic disorders, cancer, Smith-Magenis syndrome).

OBJECTIVE

The present systematic review aimed to assimilate existing knowledge of transcriptional and translational regulation of PEPCK isoforms derived from cell, animal and clinical models.

CONCLUSION

Based on current knowledge and extensive bioinformatics analysis, in this review we have provided a comparative (epi)genetic understanding of PCK1 and PCK2 genes encompassing regulatory elements, disease-associated polymorphisms, copy number variations, regulatory miRNAs and CpG densities. We have also discussed various exogenous and endogenous modulators of PEPCK isoforms and their signaling mechanisms. A comprehensive review of existing knowledge of PEPCK regulation and function may enable identification of the underlying gaps to design new pharmacological strategies and interventions for the diseases associated with gluconeogenesis.

AMPK activation by pterostilbene contributes to suppression of hepatic gluconeogenic gene expression and glucose production in H4IIE cells

Pterostilbene, a bioactive component of blueberries and grapes, shows structural similarity to resveratrol, and exhibits antioxidant, anti-inflammatory, anti-cancer, hypoglycemic, and cholesterol lowering effects. Recent evidence indicates that pterostilbene is an agonist of the nuclear receptor, peroxisome proliferator-activated receptor-alpha (PPAR-α). Since PPAR-α agonists induce peroxisomal proliferation and fatty acid oxidation, we examined gene expression of acyl CoA oxidase (ACO) and carnitine palmitoyl transferase-1 (CPT-1). Pterostilbene treatment, at concentrations that demonstrated over 75% cell viability (20 μM, 50 μM), significantly increased gene expression of ACO, CPT-1, and PPAR-α. Pterostilbene treatment (50 μM) also demonstrated potent activation of AMP-activated kinase (AMPK), compared to AICAR (0.5 mM) or metformin (2 mM), consistent with upregulation in fatty acid oxidation gene expression. Since AMPK activators mimic the actions of insulin by repressing hepatic gluconeogenesis, we examined pterostilbene's effects on hepatic gluconeogenic gene expression. Pterostilbene treatment significantly repressed dexamethasone-induced phosphoenol pyruvate carboxykinase (PEPCK) and glucose6-phosphatase (G6Pase) gene expression, and decreased glucose production in H4IIE cells. Taken together, our studies demonstrate that pterostilbene, a natural compound and PPAR-α agonist, modulate several AMPK-dependent metabolic functions. The results of the present study suggest that pterostilbene may have beneficial effects in the prevention and management of type 2 diabetes and related disorders. In this study, we found that pterostilbene activated AMP-activated kinase (AMPK) and increased the expression of fatty acid oxidation genes, including acyl CoA oxidase and carnitine palmitoyl transferase-1.

Minireview: new molecular mediators of glucocorticoid receptor activity in metabolic tissues

The glucocorticoid receptor (GR) was one of the first nuclear hormone receptors cloned and represents one of the most effective drug targets available today for the treatment of severe inflammation. The physiologic consequences of endogenous or exogenous glucocorticoid excess are well established and include hyperglycemia, insulin resistance, fatty liver, obesity, and muscle wasting. However, at the molecular and tissue-specific level, there are still many unknown protein mediators of glucocorticoid response and thus, much remains to be uncovered that will help determine whether activation of the GR can be tailored to improve therapeutic efficacy while minimizing unwanted side effects. This review summarizes recent discoveries of tissue-selective modulators of glucocorticoid signaling that are important in mediating the unwanted side effects of therapeutic glucocorticoid use, emphasizing the downstream molecular effects of GR activation in the liver, adipose tissue, muscle, and pancreas.

Functional analyses of the mutation nt-128 T→G in the hepatocyte nuclear factor-1α promoter region in Chinese diabetes pedigrees

AIMS

Hepatocyte nuclear factor-1α (HNF-1α) regulates the expression of genes encoding proteins involved in glucose metabolism and insulin secretion. Mutations in the HNF-1α gene cause maturity-onset diabetes of the young Type 3. However, the mechanism leading to this disease has not been completely ascertained. Previously, we found a novel mutation in the regulatory element of the human HNF-1α gene in two Chinese diabetes pedigrees. The nucleotide at position -128 T was substituted by G (nt-128 T→G). In this study, we analysed the functional defect of nt-128 T→G in HNF-1α transcription activity.

METHODS

Luciferase reporter gene assays were carried out to examine the functional characteristics of this mutant. Electrophoretic mobility shift assays and chromatin immunoprecipitation were performed to confirm the binding of nuclear proteins to oligonucleotides.

RESULTS

The variant construct (nt-128 T→G) had a 1.65-fold increase in promoter activity compared with that of the wild-type construct in HepG2 cells and a 1.33-fold increase in MIN6 cells, respectively. The variant resided at a FOXA/HNF-3 binding site identified by a series of competitive electrophoretic mobility shift assays and antibody supershift analyses. The assays showed a differential binding affinity in the wild-type and the nt-128 T→G mutant fragments by FOXA/HNF-3. Chromatin immunoprecipitation indicated that FOXA/HNF-3 bound to this region in vivo. One nucleotide substitution in the FOXA/HNF-3 site in the human HNF-1α regulatory element caused an increase of HNF-1α transcriptional activity.

CONCLUSIONS

Our data suggested that this substitution in the promoter region affects DNA-protein interaction and HNF-1α gene transcription. The mutant may contribute to the development of diabetes in these two nt-128 T→G pedigrees of Chinese.

Mechanistic evaluation of the insulin response in H4IIE hepatoma cells: new endpoints for toxicity testing?

This study was designed to evaluate if the rat H4IIE hepatoma cell line is a physiologically relevant model to study hepatic insulin responses to hint at its prospective application in pollutant-related insulin resistance research. DNA microarray analysis, real-time PCR and flow cytometric cell cycle analysis were used to assess the relevance of the insulin response in H4IIE cells. Insulin dose dependently stimulated H4IIE growth and time dependently altered the expression of the known insulin responsive genes: Fasn, Pck1 and Irs2. Microarray analysis performed on cells exposed to insulin (100nM) for 6h and 24h showed that genes related to carbohydrate and lipid metabolism were most profoundly afflicted, in accordance with in vivo hepatic insulin action. Since changes in carbohydrate and lipid metabolism are pivotal in the pathogenesis of insulin resistance, the presence of a physiological relevant insulin response in H4IIE cells pleads for further testing of its potential use in research on pollutant-driven insulin resistance.

Docosahexaenoic acid suppresses apolipoprotein A-I gene expression through hepatocyte nuclear factor-3β

BACKGROUND

Dietary fish-oil supplementation has been shown in human kinetic studies to lower the production rate of apolipoprotein (apo) A-I, the major protein component of HDL. The underlying mechanism responsible for this effect is not fully understood.

OBJECTIVE

We investigated the effect and the mechanism of action of the very-long-chain n-3 (omega-3) polyunsaturated fatty acid docosahexaenoic acid (DHA), relative to the saturated fatty acid palmitic acid (PA), on the hepatic expression of apo A-I in HepG2 cells.

DESIGN

HepG2 cells were treated with different doses of DHA and PA (0-200 μmol/L). mRNA expression levels of apo A-I were assessed by real-time polymerase chain reaction, and apo A-I protein concentrations were measured by immunoassay. DHA dose-dependently suppressed apo A-I mRNA levels and also lowered apo A-I protein concentrations in the media, with maximum effects at 200 μmol/L. This concentration of fatty acids was used in all subsequent experiments.

RESULTS

To elucidate the mechanism mediating the reduction in apo A-I expression by DHA, transfection experiments were conducted with plasmid constructs containing serial deletions of the apo A-I promoter. The DHA-responsive region was mapped to the -185 to -148 nucleotide region of the apo A-I promoter, which binds the hepatocyte nuclear factor (HNF)-3β. Nuclear extracts from cells treated with DHA or PA had a similar nuclear abundance of HNF-3β. However, electrophoresis mobility shift assays showed less binding of HNF-3β to the -180 to -140 sequence of the apo A-I promoter than did PA-treated cells. As shown by chromatin immunoprecipitation analysis, less HNF-3β was recruited to the apo A-I promoter in DHA-treated cells than in PA-treated cells, which supports the concept of an interference of DHA with the binding of HNF-3β to the apo A-I promoter.

CONCLUSION

These findings suggest that, in human hepatoma HepG2 cells, DHA inhibits the binding of HNF-3β to the apo A-I promoter, resulting in the repression of apo A-I promoter transactivity and thus a reduction in apo A-I expression.

Hepatocyte metabolic signalling pathways and regulation of hepatitis B virus expression

Hepatitis B virus (HBV) is a small DNA virus responsible for significant morbidity and mortality worldwide. The liver, which is the main target organ for HBV infection, provides the virus with the machinery necessary for persistent infection and propagation, a process that might ultimately lead to severe liver pathologies such as chronic hepatitis, cirrhosis and liver cancer. HBV gene expression is regulated mainly at the transcriptional level by recruitment of a whole set of cellular transcription factors (TFs) and co-activators to support transcription. Over the years, many of these TFs were identified and interestingly enough most are associated with the body's nutritional state. These include the hepatocyte nuclear factors, forkhead Box O1, Farnesoid X receptor, cyclic-AMP response element-binding (CREB), CCAAT/enhancer-binding protein (C/EBP) and glucocorticoid receptor TFs and the transcription coactivator PPARγ coactivator-1α. Consequently, HBV gene expression is linked to hepatic metabolic processes such as glucose and fat production and utilization as well as bile acids' production and secretion. Furthermore, recent evidence indicates that HBV actively interferes with some of these hepatic metabolic processes by manipulating key TFs, such as CREB and C/EBP, to meet its requirements. The discovery of the mechanisms by which HBV is controlled by the hepatic metabolic milieu may broaden our understanding of the unique regulation of HBV expression and may also explain the mechanisms by which HBV induces liver pathologies. The emerging principle of the intimate link between HBV and liver metabolism can be further exploited for host-targeted therapeutic strategies.

FoxOs function synergistically to promote glucose production

Hepatic glucose production (HGP) plays a vital role in maintaining the supply of glucose to the body, and transcription factor FoxO1 is known to confer hormone responsiveness onto HGP. Mice with a liver-specific FoxO1 deletion (L-FoxO1) show reduced HGP and reduced expression of glucose production genes. To determine the contribution of additional transcription factors to HGP, we created double and triple liver-specific knock-outs lacking FoxO1, FoxO3, and FoxO4 or the related protein FoxA2. We show that, when compared with single knock-out of FoxO1, triple ablation of FoxO genes causes more pronounced fasting hypoglycemia, increased glucose tolerance, and enhanced insulin sensitivity, with decreased plasma insulin levels. In contrast, combined ablation of FoxO1 and FoxA2 phenocopied the single knock-out of FoxO1. These data indicate that FoxOs work in concert to regulate multiple aspects of hepatic glucose metabolism.

Roles of hepatocyte nuclear factors (HNF) in the regulation of reproduction in teleosts

Hepatocyte nuclear factor (HNF) families are composed of liver-enriched transcription factors and upstream regulators of many liver-specific genes. HNF are involved in liver-specific gene expression, metabolism, development, cell growth and many cellular functions in the body. HNF genes can be activated or influenced by several hormones and insulin-like growth factors (IGF), and different combinations of the four HNF factors form a network in controlling the expression of liver-specific or liver-enriched genes. The functions of these factors and their interactions within the gonads of bony fishes, however, are not well understood, and the related literature is scant. Recently, several members of the HNF families have been detected in teleost gonads together with their downstream genes (IGF-I and IGF-II), suggesting that these HNF could be upregulated in vitro by steroid hormones. Thus, the hormone-HNF-IGF-gonad interaction may be an alternative axis in the reproductive mechanism that acts in concert with the conventional hypothalamus-pituitary-gonad pathway. This may help the early development and maturation of the gonad or gamete, sexual maturity or reversion and spawning-regulating mechanisms among fishes to be understood.

Tumour necrosis factor-alpha attenuates insulin action on phosphoenolpyruvate carboxykinase gene expression and gluconeogenesis by altering the cellular localization of Foxa2 in HepG2 cells

Circulating tumour necrosis factor-alpha (TNFalpha) levels, which are elevated in obesity-associated insulin resistance and diabetes, inhibit insulin signalling at several points in the signalling cascade. The liver is critical in maintaining circulating glucose levels and, in a preliminary investigation using the human hepatoma (HepG2) cell line in this study, we demonstrated the role of TNFalpha in the regulation of this phenomenon and determined the underlying molecular mechanisms. As the transcription factor Foxa2 has been implicated, in part, in the regulation of gluconeogenic genes, we studied the effects of TNFalpha and/or insulin on its cellular status in hepatocytes, followed by an assessment of its occupancy on the phosphoenolpyruvate carboxykinase (PEPCK) promoter. Preincubation of cells with TNFalpha, followed by insulin, significantly prevented insulin-mediated nuclear exclusion of Foxa2 and substantially increased its nuclear concentration. Foxa2 was subsequently found to occupy its binding element on the PEPCK promoter. TNFalpha alone, however, did not alter the status of cellular Foxa2 or its occupancy on the PEPCK promoter. TNFalpha preincubation also significantly attenuated insulin-induced inhibition of the expression of gluconeogenic enzymes and hepatic glucose production. Insulin inhibition of PEPCK expression and the preventive effect of TNFalpha could be partially but significantly restored in the presence of Foxa2 siRNA. Several other well-known mediators of insulin action in the liver in general and of gluconeogenic genes in particular include Foxo1, PGC-1 and SREBP-1c. Our results indicate that another transcription factor, Foxa2, is at least partly responsible for the attenuating effect of TNFalpha on insulin action on PEPCK expression and glucose production in HepG2 cells.

Inhibition of forkhead box O1 protects pancreatic beta-cells against dexamethasone-induced dysfunction

Forkhead Box O1 (FoxO1) is a key transcription regulator of insulin/IGF-I signaling pathway, and its activity can be increased by dexamethasone (DEX) in several cell types. However, the role of FoxO1 in DEX-induced pancreatic beta-cell dysfunction has not been fully understood. Therefore, in this study, we investigated whether FoxO1 could mediate DEX-induced beta-cell dysfunction and the possible underlying mechanisms in pancreatic beta-cell line RINm5F cells and primary rat islet. We found that DEX markedly increased FoxO1 mRNA and protein expression and decreased FoxO1 phosphorylation through the Akt pathway, which resulted in an increase in active FoxO1 in RINm5F cells and isolated rat islets. Activated FoxO1 subsequently inhibited pancreatic duodenal homeobox-1 expression and induced nuclear exclusion of pancreatic duodenal homeobox-1. Knockdown of FoxO1 by RNA interference restored the expression of pancreatic duodenal homeobox-1 and prevented DEX-induced dysfunction of glucose-stimulated insulin secretion in rat islets. Together, the results of present study demonstrate that FoxO1 is integrally involved in DEX-induced inhibition of pancreatic duodenal homeobox-1 and glucose-stimulated insulin secretion dysfunction in pancreatic islet beta-cells. Inhibition of FoxO1 can effectively protect beta-cells against DEX-induced dysfunction.

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.

Prediction of CYP3A4 enzyme activity using haplotype tag SNPs in African Americans

The CYP3A locus encodes hepatic enzymes that metabolize many clinically used drugs. However, there is marked interindividual variability in enzyme expression and clearance of drugs metabolized by these enzymes. We utilized comparative genomics and computational prediction of transcriptional factor binding sites to evaluate regions within CYP3A that were most likely to contribute to this variation. We then used a haplotype tagging single-nucleotide polymorphisms (htSNPs) approach to evaluate the entire locus with the fewest number of maximally informative SNPs. We investigated the association between these htSNPs and in vivo CYP3A enzyme activity using a single-point IV midazolam clearance assay. We found associations between the midazolam phenotype and age, diagnosis of hypertension and one htSNP (141689) located upstream of CYP3A4. 141689 lies near the xenobiotic responsive enhancer module (XREM) regulatory region of CYP3A4. Cell-based studies show increased transcriptional activation with the minor allele at 141689, in agreement with the in vivo association study findings. This study marks the first systematic evaluation of coding and noncoding variation that may contribute to CYP3A phenotypic variability.

Pharmacological investigations of the anti-diabetic effect of Cortex Moutan and its active component paeonol

Cortex Moutan (CM, root bark of Paeonia suffruticosa Andr.) is one of the common herbs found in anti-diabetic traditional Chinese medicine formulae. To study the potential anti-diabetic mechanisms of CM, four in vitro models (intestinal brush border membrane vesicles (BBMV), rat hepatoma cell line H4IIE, human skin fibroblasts cell line Hs68 and mouse adipocytes 3T3-L1) were used. CM showed significant in vitro anti-diabetic effects by inhibiting glucose uptake of BBMV and enhancing glucose uptake into Hs68 and 3T3-L1 cells. Using bioassay-guided fractionation, paeonol was confirmed to be one of the active constituents for inhibiting BBMV glucose uptake. With neonatal-streptozotocin diabetic rats, paeonol (200 and 400mg/kgbody wt.) was found to improve oral glucose tolerance in vivo. To the best of our knowledge, this is the first report on the anti-diabetic effect of paeonol.

Steroid hormones (17β-estradiol and hydrocortisone) upregulate hepatocyte nuclear factor (HNF)-3β and insulin-like growth factors I and II expression in the gonads of tilapia (Oreochromis mossambicus) in vitro

Hepatocyte nuclear factors (HNF-1alpha, -1beta and -3beta) and insulin-like growth factors (IGF-I and -II), which are involved in liver-specific gene expression, metabolism, development and cell growth, have been found in the gonads of tilapia (Oreochromis mossambicus). However, the functions of these factors and how they interact within the gonads of bony fish are not understood. In the present study, we provided experimental evidence that the expression of HNF-3beta in the gonads of tilapia, but not HNF-1alpha and -1beta, was affected in vitro by 17beta-estradiol and hydrocortisone. Immunohistochemical staining confirmed that tilapia HNF-3beta was mainly found in the nuclei of hepatocytes, the follicular granulosa cells of the ovaries, and the interstitial cells of the testes of adult tilapia. Further data were gathered at various steroid concentrations (0.1, 1, 10, 100, and 1000 nM) over various culture intervals (6, 12, 18, 24, 30, and 36 h) and subjected to semi-quantitative RT-PCR analysis. The expression of downstream genes (IGF-I and -II) followed the same temporal patterns as HNF-3beta, albeit at decreased levels for 30 and 36 h culture intervals. Both hormones upregulated HNF-3beta mRNA expression at concentrations of 0.1-10 nM, and reached optimal physiological concentrations for induction of IGFs at 1-10 nM. The identity of the PCR fragments was concurrently verified by sequencing and PCR-Southern hybridization. We inferred that HNF-3beta and IGFs may play a regulatory role in tilapia gonads during oocyte maturation and spermatogenesis.

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.

Protein-tyrosine phosphatase 1B deficiency reduces insulin resistance and the diabetic phenotype in mice with polygenic insulin resistance

Mice heterozygous for insulin receptor (IR) and IR substrate (IRS)-1 deficiency provide a model of polygenic type 2 diabetes in which early-onset, genetically programmed insulin resistance leads to diabetes. Protein-tyrosine phosphatase 1B (PTP1B) dephosphorylates tyrosine residues in IR and possibly IRS proteins, thereby inhibiting insulin signaling. Mice lacking PTP1B are lean and have increased insulin sensitivity. To determine whether PTP1B can modify polygenic insulin resistance, we crossed PTP1B-/- mice with mice with a double heterozygous deficiency of IR and IRS-1 alleles (DHet). DHet mice weighed slightly less than wild-type mice and exhibited severe insulin resistance and hyperglycemia, with approximately 35% of DHet males developing diabetes by 9-10 weeks of age. Body weight in DHet mice with PTP1B deficiency was similar to that in DHet mice. However, absence of PTP1B in DHet mice markedly improved glucose tolerance and insulin sensitivity at 10-11 weeks of age and reduced the incidence of diabetes and hyperplastic pancreatic islets at 6 months of age. Insulin-stimulated phosphorylation of IR, IRS proteins, Akt/protein kinase B, glycogen synthase kinase 3beta, and p70(S6K) was impaired in DHet mouse muscle and liver and was differentially improved by PTP1B deficiency. In addition, increased phosphoenolpyruvate carboxykinase expression in DHet mouse liver was reversed by PTP1B deficiency. In summary, PTP1B deficiency reduces insulin resistance and hyperglycemia without altering body weight in a model of polygenic type 2 diabetes. Thus, even in the setting of high genetic risk for diabetes, reducing PTP1B is partially protective, further demonstrating its attractiveness as a target for prevention and treatment of type 2 diabetes.

Influence of an anti-diabetic foot ulcer formula and its component herbs on tissue and systemic glucose homeostasis

Complications of diabetes impose major public health burdens worldwide. The positive effect of a Radix Astragali-based herbal preparation on healing diabetic foot ulcers in patients has been reported. Formula 1 is also referred as the 'Herbal drink to strengthen muscle and control swelling'. This formula contains six Chinese medical herbs, including Radix Astragali, Radix Rehmanniae, Rhizoma Smilacis Chinensis, Rhizoma Atractylodis Macrocephalae, Radix Polygoni Multiflori Preparata, and Radix Stephania Tetrandrae. Three of these herbs (Radix Astragali, Radix Rehmanniae, Rhizoma Atractylodis Macrocephalae) are commonly used in different anti-diabetic formulae of Chinese medicine. The objective of the current study is to use an interdisciplinary approach to test the hypothesis that Formula 1 and its components influence tissue and systemic glucose homeostasis. In vitro and in vivo models have been established including: (1) glucose absorption into intestinal brush border membrane vesicles (BBMV); (2) gluconeogenesis by H4IIE hepatoma cells; (3) glucose uptake by 3T3-L1 adipocytes and Hs68 skin fibroblasts; (4) normalization of glycaemic control in a diabetic rat model. The results of in vitro studies indicated that all herbal extracts can modify cellular glucose homeostasis. Since Formula 1 and Rhizoma Smilacis Chinensis extracts demonstrated potent effects on modifying glucose homeostasis in multiple tissues in vitro, they were further studied for their anti-diabetic activities in vivo using a streptozotocin (STZ)-induced diabetic rat model. The results showed that Formula 1 and Rhizoma Smilacis Chinensis extracts did not significantly improve oral glucose tolerance or basal glycaemia in diabetic rats. In conclusion, the anti-diabetic foot ulcer Formula 1 contains ingredients active in modifying tissue glucose homeostasis in vitro but these biological activities could not be associated with improved glycaemic control of diabetes in vivo.

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.

Characterization of a novel Foxa (hepatocyte nuclear factor-3) site in the glucagon promoter that is conserved between rodents and humans

The pancreatic islet hormone glucagon stimulates hepatic glucose production and thus maintains blood glucose levels in the fasting state. Transcription factors of the Foxa [Fox (forkhead box) subclass A; also known as HNF-3 (hepatocyte nuclear factor-3)] family are required for cell-specific activation of the glucagon gene in pancreatic islet alpha-cells. However, their action on the glucagon gene is poorly understood. In the present study, comparative sequence analysis and molecular characterization using protein-DNA binding and transient transfection assays revealed that the well-characterized Foxa-binding site in the G2 enhancer element of the rat glucagon gene is not conserved in humans and that the human G2 sequence lacks basal enhancer activity. A novel Foxa site was identified that is conserved in rats, mice and humans. It mediates activation of the glucagon gene by Foxa proteins and confers cell-specific promoter activity in glucagon-producing pancreatic islet alpha-cell lines. In contrast with previously identified Foxa-binding sites in the glucagon promoter, which bind nuclear Foxa2, the novel Foxa site was found to bind preferentially Foxa1 in nuclear extracts of a glucagon-producing pancreatic islet alpha-cell line, offering a mechanism that explains the decrease in glucagon gene expression in Foxa1-deficient mice. This site is located just upstream of the TATA box (between -30 and -50), suggesting a role for Foxa proteins in addition to direct transcriptional activation, such as a role in opening the chromatin at the start site of transcription of the glucagon gene.

Distinct Mechanisms of Glucose Lowering by Specific Agonists for Peroxisomal Proliferator Activated Receptor γ and Retinoic Acid X Receptors

Agonists for the nuclear receptor peroxisomal proliferator-activated receptor-gamma (PPARgamma) and its heterodimeric partner, retinoid X receptor (RXR), are effective agents for the treatment of type 2 diabetes. To gain insight into the antidiabetic action of these compounds, we treated female Zucker diabetic rats (ZFF) with AGN194204, which we show to be a homodimer-specific RXR agonist, or the PPARgamma agonist, troglitazone. Hyperinsulinemic-euglycemic clamps in ZFF showed that troglitazone and AGN194204 reduced basal endogenous glucose production (EGP) approximately 30% and doubled the insulin suppression of EGP. AGN194204 had no effect on peripheral glucose utilization, whereas troglitazone increased insulin-stimulated glucose utilization by 50%, glucose uptake into skeletal muscle by 85%, and de novo skeletal muscle glycogen synthesis by 300%. Troglitazone increased skeletal muscle Irs-1 and phospho-Akt levels following in vivo insulin treatment, whereas AGN194204 increased hepatic Irs-2 and insulin stimulated phospho-Akt in liver. Gene profiles of AGN194204-treated mouse liver analyzed by Ingenuity Pathway Analysis identified increases in fatty acid synthetic genes, including Srebp-1 and fatty acid synthase, a pathway previously shown to be induced by RXR agonists. A network of down-regulated genes containing Foxa2, Foxa3, and G-protein subunits was identified, and decreases in these mRNA levels were confirmed by quantitative reverse transcription-PCR. Treatment of HepG2 cells with AGN194204 resulted in inhibition of glucagon-stimulated cAMP accumulation suggesting the G-protein down-regulation may provide an additional mechanism for hepatic insulin sensitization by RXR. These studies demonstrate distinct molecular events lead to insulin sensitization by high affinity RXR and PPARgamma agonists.

Adiponectin represses gluconeogenesis independent of insulin in hepatocytes

Adiponectin plays important roles in regulating insulin sensitivity and atherogenesis. Adiponectin has been shown to suppress hepatic glucose production in rodents. It has not been reported whether ectopically expressed adiponectin could regulate glucose metabolism in cultured hepatocyte-like cells. In the current study, the effect of adiponectin on glucose production was analyzed by ectopically expressing the protein in hepatoma H4IIE cells using an adenovirus delivery system to generate both human full-length and the globular domain of the protein. Expression of adiponectin in hepatoma H4IIE cells, in the absence of insulin, suppressed expression of the genes encoding glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, rate-limiting enzymes in the gluconeogenic pathway. Furthermore, expression of adiponectin in H4IIE cells suppressed glucose production from lactate and pyruvate. Purified recombinant human adiponectin also reduced glucose production in H4IIE cells and in rat primary hepatocytes in the absence of insulin. These data suggest that adiponectin protein could exert its function independent of the presence of insulin in these culture systems.

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%.

Serum retinol binding protein 4 contributes to insulin resistance in obesity and type 2 diabetes

In obesity and type 2 diabetes, expression of the GLUT4 glucose transporter is decreased selectively in adipocytes. Adipose-specific Glut4 (also known as Slc2a4) knockout (adipose-Glut4(-/-)) mice show insulin resistance secondarily in muscle and liver. Here we show, using DNA arrays, that expression of retinol binding protein-4 (RBP4) is elevated in adipose tissue of adipose-Glut4(-/-) mice. We show that serum RBP4 levels are elevated in insulin-resistant mice and humans with obesity and type 2 diabetes. RBP4 levels are normalized by rosiglitazone, an insulin-sensitizing drug. Transgenic overexpression of human RBP4 or injection of recombinant RBP4 in normal mice causes insulin resistance. Conversely, genetic deletion of Rbp4 enhances insulin sensitivity. Fenretinide, a synthetic retinoid that increases urinary excretion of RBP4, normalizes serum RBP4 levels and improves insulin resistance and glucose intolerance in mice with obesity induced by a high-fat diet. Increasing serum RBP4 induces hepatic expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) and impairs insulin signalling in muscle. Thus, RBP4 is an adipocyte-derived 'signal' that may contribute to the pathogenesis of type 2 diabetes. Lowering RBP4 could be a new strategy for treating type 2 diabetes.

Foxa2 integrates the transcriptional response of the hepatocyte to fasting

Survival during prolonged food deprivation depends on the activation of hepatic gluconeogenesis. Inappropriate regulation of this process is a hallmark of diabetes and other metabolic diseases. Activation of the genes encoding gluconeogenic enzymes is mediated by hormone-responsive transcription factors such as the cyclic AMP response element binding protein (CREB) and the glucocorticoid receptor (GR). Here we show using cell-type-specific gene ablation that the winged helix transcription factor Foxa2 is required for activation of the hepatic gluconeogenic program during fasting. Specifically, Foxa2 promotes gene activation both by cyclic AMP, the second messenger for glucagon, and glucocorticoids. Foxa2 mediates these effects by enabling recruitment of CREB and GR to their respective target sites in chromatin. We conclude that Foxa2 is required for execution of the hepatic gluconeogenic program by integrating the transcriptional response of the hepatocyte to hormonal stimulation.

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.

Compensatory roles of Foxa1 and Foxa2 during lung morphogenesis

Foxa1 and Foxa2 are closely related family members of the Foxa group of transcription factors that are coexpressed in subsets of respiratory epithelial cells throughout lung morphogenesis. Shared patterns of expression, conservation of DNA binding, and transcriptional activation domains indicate that they may serve complementary functions in the regulation of gene expression during lung morphogenesis. Whereas branching morphogenesis of the fetal lung occurs normally in the Foxa2Delta/Delta and Foxa1-/- mice, deletion of both Foxa1 and Foxa2 (in Foxa2Delta/Delta, Foxa1-/- mice) inhibited cell proliferation, epithelial cell differentiation, and branching. Dilation of terminal lung tubules and decreased branching were observed as early as embryonic day 12.5. Foxa1 and Foxa2 regulated Shh (sonic hedgehog) and Shh-dependent genes in the respiratory epithelial cells that influenced the expression of genes in the pulmonary mesenchyme that are required for branching morphogenesis. Epithelial cell differentiation, as indicated by lack of expression of surfactant protein B, surfactant protein C, the Clara cell secretory protein, and Foxj1, was inhibited. Foxa family members regulate signaling and transcriptional programs required for morphogenesis and cell differentiation during formation of the lung.

HIV-1 accessory protein Vpr inhibits the effect of insulin on the Foxo subfamily of forkhead transcription factors by interfering with their binding to 14-3-3 proteins: potential clinical implications regarding the insulin resistance of HIV-1-infected patients

HIV-1 accessory protein Vpr arrests host cells at the G2/M phase of the cell cycle by interacting with members of the protein family 14-3-3, which regulate the activities of "partner" molecules by binding to their phosphorylated serine or threonine residues and changing their intracellular localization and/or stability. Vpr does this by facilitating the association of 14-3-3 to its partner protein Cdc25C, independent of the latter's phosphorylation status. Here we report that the same viral protein interfered with and altered the activity of another 14-3-3 partner molecule, Foxo3a, a subtype of the forkhead transcription factors, by inhibiting its association with 14-3-3. Foxo3a's transcriptional activity is normally suppressed by insulin-induced translocation of this protein from the nucleus into the cytoplasm. Vpr inhibited the ability of insulin or its downstream protein kinase Akt to change the intracellular localization of Foxo3a preferentially to the cytoplasm. This HIV-1 protein also interfered with insulin-induced coprecipitation of 14-3-3 and Foxo3a in vivo and antagonized the negative effect of insulin on Foxo3a-induced transactivation of a FOXO-responsive promoter. Moreover, Vpr antagonized insulin-induced suppression of the mRNA expression of the glucose 6-phosphatase, manganese superoxide dismutase, and sterol carrier protein 2 genes, which are known targets of insulin and FOXO, in HepG2 cells. These findings indicate that Vpr interferes with the suppressive effects of insulin on FOXO-mediated transcription of target genes via 14-3-3. Vpr thus may contribute to the tissue-selective insulin resistance often observed in HIV-1-infected individuals.

Development of a long-acting insulin analog using albumin fusion technology

The primary therapeutic goal for the treatment of diabetes is maintenance of a long-term, near-normoglycemic condition and prevention of the onset or progression of the complications associated with the disease. Although several analogs of human insulin have been developed, the currently prescribed long-acting insulin analogs do not provide a stable basal glycemia for more than a few hours. Here, we report the development of Albulin, a long-acting insulin analog obtained by direct gene fusion of a single-chain human insulin to human serum albumin. Albulin showed an elimination t(1/2) of approximately 7 h in normoglycemic mice. In vitro pharmacodynamic profiles for Albulin characterized by receptor binding, inhibition of gluconeogenesis, induction of glucose uptake, and global regulation of gene expression in relevant cell types showed that Albulin produced similar activity profiles compared with that of recombinant human insulin. A single Albulin administration in vivo normalized blood glucose level in diabetic mice in a relatively peakless and sustained (24-h) fashion. A further reduction in glucose levels was achieved by administering a recombinant human insulin a few hours after Albulin injection in mice, indicating the potential for Albulin therapy in combination with available fast-acting insulin derivatives. In summary, Albulin displays characteristics of a potent long-acting insulin analog that can be evaluated for use as a novel insulin therapy for patients with insulin-dependent diabetes.

Allelic expression imbalance of the human CYP3A4 gene and individual phenotypic status

The human cytochrome P450 3A4 (CYP3A4) plays a dominant role in the metabolism of numerous clinically useful drugs. Alterations in the activity or expression of this enzyme may account for a major part of the variation in drug responsiveness and toxicity. However, it is generally accepted that most of the known single nucleotide polymorphisms in the coding and 5'-flanking regions are not the main determinants for the large inter-individual variability of CYP3A4 expression and activity. We show that the allelic variation is critically involved in determining the individual total hepatic CYP3A4 mRNA level and metabolic capability. There exists a definite correlation between the total CYP3A4 mRNA level and allelic expression ratio, the relative transcript level ratio derived from the two alleles. Individuals with a low expression ratio, exhibiting a large difference of transcript level between the two alleles, revealed extremely low levels of total hepatic CYP3A4 mRNA, and thus low metabolic capability as assessed by testosterone 6beta-hydroxylation. These results present a new insight into the individualized CYP3A4-dependent pharmacotherapy and the importance of expression imbalance to human phenotypic diversity.

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.

Human immunodeficiency virus type-1 accessory protein Vpr: a causative agent of the AIDS-related insulin resistance/lipodystrophy syndrome?

Recent advances in the development of three different types of antiviral drugs, the nucleotide and non-nucleotide analogues acting as reverse transcriptase inhibitors (NRTIs) and the nonpeptidic viral protease inhibitors (PI), and their introduction in the management of patients with AIDS, either alone or in combination, have dramatically improved the clinical course of the disease and prolonged life expectancy in patients with AIDS. The increase in life expectancy in association with the long-term use of the above antiviral agents, however, have generated novel morbidities and complications. Central among them is the quite common AIDS-related insulin resistance and lipodystrophy syndrome, which is characterized by a striking phenotype and marked metabolic disturbances. To look for the pathologic causes of this particular syndrome, we focused on one of the HIV-1 accessory proteins, Vpr, which has multiple functions, such as virion incorporation, nuclear translocation of the HIV-1 preintegration complex, nucleo-cytoplasmic shuttling, transcriptional activation, and induction of apoptosis. Vpr may also act like a hormone, which is secreted into the extracellular space and affects the function of distant organs. Vpr functions as a coactivator of the glucocorticoid receptor and potentiates the action of glucocorticoid hormones, thereby inducing tissue glucocorticoid hypersensitivity. Vpr also arrests host cells at the G2/M phase of the cell cycle by interacting with novel 14-3-3 proteins. Vpr facilitates the interaction of 14-3-3 and its partner protein Cdc25C, which is critical for the transition of G2/M checkpoint in the cell cycle, and suppresses its activity by segregating it into the cytoplasm. The same Vpr protein also suppresses the association of 14-3-3 with other partner molecules, the Foxo transcription factors. Since the Foxo proteins function as negative transcription factors for insulin, Vpr may cause resistance of tissues to insulin. Through these two newly identified functions of Vpr, namely, coactivation of glucocorticoid receptor activity and inhibition of insulin effects on Foxo proteins, Vpr may participate in the development of AIDS-related insulin resistance/lipodystrophy syndrome.

Hepatic nuclear factor 3 and nuclear factor 1 regulate 5-aminolevulinate synthase gene expression and are involved in insulin repression

Although the negative regulation of gene expression by insulin has been widely studied, the transcription factors responsible for the insulin effect are still unknown. The purpose of this work was to explore the molecular mechanisms involved in the insulin repression of the 5-aminolevulinate synthase (ALAS) gene. Deletion analysis of the 5'-regulatory region allowed us to identify an insulin-responsive region located at -459 to -354 bp. This fragment contains a highly homologous insulin-responsive (IRE) sequence. By transient transfection assays, we determined that hepatic nuclear factor 3 (HNF3) and nuclear factor 1 (NF1) are necessary for an appropriate expression of the ALAS gene. Insulin overrides the HNF3beta or HNF3beta plus NF1-mediated stimulation of ALAS transcriptional activity. Electrophoretic mobility shift assay and Southwestern blotting indicate that HNF3 binds to the ALAS promoter. Mutational analysis of this region revealed that IRE disruption abrogates insulin action, whereas mutation of the HNF3 element maintains hormone responsiveness. This dissociation between HNF3 binding and insulin action suggests that HNF3beta is not the sole physiologic mediator of insulin-induced transcriptional repression. Furthermore, Southwestern blotting assay shows that at least two polypeptides other than HNF3beta can bind to ALAS promoter and that this binding is dependent on the integrity of the IRE. We propose a model in which insulin exerts its negative effect through the disturbance of HNF3beta binding or transactivation potential, probably due to specific phosphorylation of this transcription factor by Akt. In this regard, results obtained from transfection experiments using kinase inhibitors support this hypothesis. Due to this event, NF1 would lose accessibility to the promoter. The posttranslational modification of HNF3 would allow the binding of a protein complex that recognizes the core IRE. These results provide a potential mechanism for the insulin-mediated repression of IRE-containing promoters.

Novel concepts in insulin regulation of hepatic gluconeogenesis

The regulation of hepatic gluconeogenesis is an important process in the adjustment of the blood glucose level, and pathological changes in the glucose production of the liver are a central characteristic in type 2 diabetes. The pharmacological intervention in signaling events that regulate the expression of the key gluconeogenic enzymes phosphoenolpyruvate carboxykinase (PEPCK) and the catalytic subunit glucose-6-phosphatase (G-6-Pase) is regarded as a potential strategy for the treatment of metabolic aberrations associated with this disease. However, such intervention requires a detailed understanding of the molecular mechanisms involved in the regulation of this process. Glucagon and glucocorticoids are known to increase hepatic gluconeogenesis by inducing the expression of PEPCK and G-6-Pase. The coactivator protein PGC-1 has been identified as an important mediator of this regulation. In contrast, insulin is known to suppress both PEPCK and G-6-Pase gene expression by the activation of PI 3-kinase. However, PI 3-kinase-independent pathways can also lead to the inhibition of gluconeogenic enzymes. This review focuses on signaling mechanisms and nuclear events that transduce the regulation of gluconeogenic enzymes.

Inhibition of Foxo1 function is associated with improved fasting glycemia in diabetic mice

Excessive hepatic glucose production is a contributing factor to fasting hyperglycemia in diabetes. Insulin suppresses hepatic glucose production by inhibiting the expression of two gluconeogenic enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G-6-Pase). The forkhead transcription factor Foxo1 has been implicated as a mediator of insulin action in regulating hepatic gluconeogenesis, and a Foxo1 mutant (Foxo1-Delta256), devoid of its carboxyl domain, has been shown to interfere with Foxo1 function and inhibit gluconeogenic gene expression in cultured cells. To study the effect of Foxo1-Delta256 on glucose metabolism in animals, the Foxo1-Delta256 cDNA was delivered to the livers of mice by adenovirus-mediated gene transfer. Hepatic Foxo1-Delta256 production resulted in inhibition of gluconeogenic activity, as evidenced by reduced PEPCK and G-6-Pase expression in the liver. Mice treated with the Foxo1-Delta256 vector exhibited significantly reduced blood glucose levels. In contrast, blood glucose levels in control vector-treated animals remained unchanged, which coincided with the lack of alterations in the expression levels of PEPCK and G-6-Pase. When tested in diabetic db/db mice, hepatic production of Foxo1-Delta256 was shown to reduce fasting hyperglycemia. Furthermore, we showed that hepatic Foxo1 expression was deregulated as a result of insulin resistance in diabetic mice and that Foxo1-Delta256 interfered with Foxo1 function via competitive binding to target promoters. These results demonstrated that functional inhibition of Foxo1, caused by hepatic expression of its mutant, is associated with reduced hepatic gluconeogenic activity and improved fasting glycemia in diabetic mice.

Transcriptional regulation of human CYP3A4 basal expression by CCAAT enhancer-binding protein alpha and hepatocyte nuclear factor-3 gamma

Cytochrome P450 3A4 (CYP3A4) is involved in the metabolism of more than 50% of currently used therapeutic drugs, yet the mechanisms that control CYP3A4 basal expression in liver are poorly understood. Several putative binding sites for CCAAT/enhancer-binding protein (C/EBP) and hepatic nuclear factor 3 (HNF-3) were found by computer analysis in CYP3A4 promoter. The use of reporter gene assays, electrophoretic mobility shift assays, and site-directed mutagenesis revealed that one proximal and two distal C/EBP alpha binding sites are essential sites for the trans-activation of CYP3A4 promoter. No trans-activation was found in similar reporter gene experiments with a HNF-3 gamma expression vector. The relevance of these findings was further explored in the more complex DNA/chromatin structure within endogenous CYP3A4 gene. Using appropriate adenoviral expression vectors, we found that both hepatic and nonhepatic cells overexpressing C/EBP alpha had increased CYP3A4 mRNA levels, but no effect was observed when HNF-3 gamma was overexpressed. In contrast, overexpression of HNF-3 gamma simultaneously with C/EBP alpha resulted in a greater activation of the CYP3A4 gene. This cooperative effect was hepatic-specific and also occurred in CYP3A5 and CYP3A7 genes. To investigate the mechanism for HNF-3 gamma action, we studied its binding to CYP3A4 promoter and the effect of the deacetylase inhibitor trichostatin A. HNF-3 gamma was able to bind CYP3A4 promoter at a distal position, near the most distal C/EBP alpha binding site. Trichostatin A increased C/EBP alpha effect but abolished HNF-3 gamma cooperative action. These findings revealed that C/EBP alpha and HNF-3 gamma cooperatively regulate CYP3A4 expression in hepatic cells by a mechanism that probably involves chromatin remodeling.

Glucocorticoids repress transcription of phosphoenolpyruvate carboxykinase (GTP) gene in adipocytes by inhibiting its C/EBP-mediated activation

The cytosolic form of the phosphoenolpyruvate carboxykinase (PEPCK-C) gene is selectively expressed in several tissues, primarily in the liver, kidney, and adipose tissue. The transcription of the gene is reciprocally regulated by glucocorticoids in these tissues. It is induced in the liver and kidney but repressed in the white adipose tissue. To elucidate which adipocyte-specific transcription factors participate in the repression of the gene, DNase I footprinting analyses of nuclear proteins from 3T3-F442A adipocytes and transient transfection experiments in NIH3T3 cells were utilized. Glucocorticoid treatment slightly reduced the nuclear C/EBP alpha concentration but prominently diminished the binding of adipocyte-derived nuclear proteins to CCAAT/enhancer-binding protein (C/EBP) recognition sites, without affecting the binding to nuclear receptor sites in the PEPCK-C gene promoter. Of members of the C/EBP family of transcription factors, C/EBP alpha was the strongest trans-activator of the PEPCK-C gene promoter in the NIH3T3 cell line. The glucocorticoid receptor (GR), in the presence of its hormone ligand, inhibited the activation of the PEPCK-C gene promoter by C/EBP alpha or C/EBP beta but not by the adipocyte-specific peroxisome proliferator-activated receptor gamma 2. This inhibition effect was similar using the wild type or mutant GR and did not depend on GR binding to the DNA. The glucocorticoid response unit (GRU) in the PEPCK-C gene promoter (-2000 to +73) restrained C/EBP alpha-mediated trans-activation, because mutation of each single GRU element increased this activation by 3-4-fold. This series of GRU mutations were repressed by wild type GR to the same percent as was the nonmutated PEPCK-C gene promoter. In contrast, the repression by mutant GR depended on the intact AF1 site in the gene promoter, whereby mutation of the AF1 element abolished the repression.

An integrated functional genomics screening program reveals a role for BMP-9 in glucose homeostasis

A coordinated functional genomics program was implemented to identify secreted polypeptides with therapeutic applications in the treatment of diabetes. Secreted factors were predicted from a diverse expressed-sequence tags (EST) database, representing >1,000 cDNA libraries, using a combination of bioinformatic algorithms. Subsequently, approximately 8,000 human proteins were screened in high-throughput cell-based assays designed to monitor key physiological transitions known to be centrally involved in the physiology of type 2 diabetes. Bone morphogenetic protein-9 (BMP-9) gave a positive response in two independent assays: reducing phosphoenolpyruvate carboxykinase (PEPCK) expression in hepatocytes and activating Akt kinase in differentiated myotubes. Purified recombinant BMP-9 potently inhibited hepatic glucose production and activated expression of key enzymes of lipid metabolism. In freely fed diabetic mice, a single subcutaneous injection of BMP-9 reduced glycemia to near-normal levels, with maximal reduction observed 30 hours after treatment. BMP-9 represents the first hepatic factor shown to regulate blood glucose concentration. Using a combination of bioinformatic and high-throughput functional analyses, we have identified a factor that may be exploited for the treatment of diabetes.

c-Myc is required for the glucose-mediated induction of metabolic enzyme genes

Glucose exerts powerful effects on hepatocyte gene transcription by mechanisms that are incompletely understood. c-Myc regulates hepatic glucose metabolism by increasing glycolytic enzyme gene transcription while concomitantly decreasing gluconeogenic and ketogenic enzyme gene expression. However, the molecular mechanisms by which c-Myc exerts these effects is not known. In this study, the glucose-mediated induction of L-type pyruvate kinase and glucose-6-phosphatase mRNA levels was diminished by maneuvers involving recombinant adenoviral vectors that interfere with (i) c-Myc protein levels by antisense expression or (ii) c-Myc function through a dominant-negative Max protein. These results were obtained using both HL1C rat hepatoma cells and primary rat hepatocytes. Furthermore, a decrease in c-Myc abundance reduced glucose production in HL1C cells, presumably by decreasing glucose-6-phosphatase activity. The repression of hormone-activated phosphoenolpyruvate carboxykinase gene transcription by glucose was not affected by a reduction in c-Myc levels. The basal mRNA levels for L-pyruvate kinase and glucose-6-phosphatase were not altered to any significant degree by adenoviral treatment. Furthermore, adenoviral overexpression of the c-Myc protein induced glucose-6-phosphatase mRNA in the absence of glucose stimulation. We conclude that multiple mechanisms exist to communicate the glucose-derived signal and that c-Myc has a key role in the hepatic glucose signaling pathway.

Epigallocatechin gallate, a constituent of green tea, represses hepatic glucose production

Herbs have been used for medicinal purposes, including the treatment of diabetes, for centuries. Plants containing flavonoids are used to treat diabetes in Indian medicine and the green tea flavonoid, epigallocatechin gallate (EGCG), is reported to have glucose-lowering effects in animals. We show here that the regulation of hepatic glucose production is decreased by EGCG. Furthermore, like insulin, EGCG increases tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 (IRS-1), and it reduces phosphoenolpyruvate carboxykinase gene expression in a phosphoinositide 3-kinase-dependent manner. EGCG also mimics insulin by increasing phosphoinositide 3-kinase, mitogen-activated protein kinase, and p70(s6k) activity. EGCG differs from insulin, however, in that it affects several insulin-activated kinases with slower kinetics. Furthermore, EGCG regulates genes that encode gluconeogenic enzymes and protein-tyrosine phosphorylation by modulating the redox state of the cell. These results demonstrate that changes in the redox state may have beneficial effects for the treatment of diabetes and suggest a potential role for EGCG, or derivatives, as an antidiabetic agent.

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

Hormones regulate glucose homeostasis, in part, by controlling the expression of gluconeogenic enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK). Insulin and glucocorticoids reciprocally regulate PEPCK expression primarily at the level of gene transcription. We demonstrate here that glucocorticoids promote, whereas insulin disrupts, the association of CREB-binding protein (CBP) and RNA polymerase II with the hepatic PEPCK gene promoter in vivo. We also show that accessory factors, such as CCAAT/enhancer-binding protein beta (C/EBP beta), can recruit CBP to drive transcription. Insulin increases protein levels of liver-enriched transcriptional inhibitory protein (LIP), an inhibitory form of C/EBP beta, in a phosphatidylinositol 3-kinase-dependent manner. LIP concomitantly replaces liver-enriched transcriptional activator protein on the PEPCK gene promoter, which can abrogate the recruitment of CBP and polymerase II, culminating in the repression of PEPCK expression and the attenuation of hepatocellular glucose production.

Foxa2 (HNF3beta ) controls multiple genes implicated in metabolism-secretion coupling of glucose-induced insulin release

The transcription factor Foxa2 is implicated in blood glucose homeostasis. Conditional expression of Foxa2 or its dominant-negative mutant DN-Foxa2 in INS-1 cells reveals that Foxa2 regulates the expression of genes important for glucose sensing in pancreatic beta-cells. Overexpression of Foxa2 results in blunted glucose-stimulated insulin secretion, whereas induction of DN-Foxa2 causes a left shift of glucose-induced insulin release. The mRNA levels of GLUT2 and glucokinase are drastically decreased after induction of Foxa2. In contrast, loss of Foxa2 function leads to up-regulation of hexokinase (HK) I and II and glucokinase (HK-IV) mRNA expression. The glucokinase and the low K(m) hexokinase activities as well as glycolysis are increased proportionally. In addition, induction of DN-Foxa2 also reduces the expression of beta-cell K(ATP) channel subunits Sur1 and Kir6.2 by 70%. Furthermore, in contrast to previous reports, induction of Foxa2 causes pronounced decreases in the HNF4alpha and HNF1alpha mRNA levels. Foxa2 fails to regulate the expression of Pdx1 transcripts. The expression of insulin and islet amyloid polypeptide is markedly suppressed after induction of Foxa2, while the glucagon mRNA levels are significantly increased. Conversely, Foxa2 is required for glucagon expression in these INS-1-derived cells. These results suggest that Foxa2 is a vital transcription factor evolved to control the expression of genes essential for maintaining beta-cell glucose sensing and glucose homeostasis.

Adenovirus-mediated increase in HNF-3beta or HNF-3alpha shows differences in levels of liver glycogen and gene expression

We previously generated a transgenic mouse line (T-77) in which increased hepatic expression of the hepatocyte nuclear factor-3beta (HNF-3beta) protein was used to assess its role in hepatocyte-specific gene transcription. The T-77 transgenic mice displayed elevated serum bile acid and bilirubin levels and a complete absence of hepatic glycogen storage. These postnatal liver defects were associated with diminished expression of hepatocyte genes involved in gluconeogenesis and bile acid transport as well as reduced levels of hepatocyte transcription factors. In this study, we show that mouse tail vein injections of adenovirus expressing the rat HNF-3beta (AdHNF3beta) cDNA efficiently increased its levels throughout the liver lobule and recapitulated the T-77 transgenic liver phenotype within several days postinfection. Likewise, the AdHNF3beta-infected liver phenotype was associated with reduced hepatic expression of genes involved in glucose homeostasis, bile acid transport, and bilirubin conjugation, which were not found with control adenovirus infections. These studies show that adenovirus-mediated gene transfer is an effective method for rapid hepatic increases in transcription factor levels to determine in vivo target genes. In contrast, AdHNF3alpha-infected liver displayed only a transient reduction in hepatic glycogen levels and was associated with less severe decreases in hepatic expression of gluconeogenic and bilirubin metabolism genes. Consistent with these findings, only T-77 transgenic and AdHNF3beta-infected liver exhibited diminished hepatic expression of the HNF-6 transcription factor, suggesting that reduced HNF-6 levels contribute to diminished HNF-3beta-specific transcriptional activity.

Foxa3 (hepatocyte nuclear factor 3gamma ) is required for the regulation of hepatic GLUT2 expression and the maintenance of glucose homeostasis during a prolonged fast

The winged helix transcription factors, hepatocyte nuclear factors 3alpha, -beta, and -gamma (HNF-3, encoded by the Foxa1, -a2, and -a3 genes, respectively), are expressed early in embryonic endoderm and play important roles in the regulation of gene expression in liver and pancreas. Foxa1 has been shown to be required for glucagon secretion in the pancreas, whereas Foxa2 is critical for the regulation of insulin secretion in pancreatic beta-cells. Here we address the role of Foxa3 in the maintenance of glucose homeostasis. Mice homozygous for a null mutation in Foxa3 appear normal under fed conditions. However, when fasted, Foxa3(-/-) mice have a significantly lower blood glucose compared with control mice. The fasting hypoglycemia in Foxa3(-/-) mice could not be attributed to defects in pancreatic hormone secretion, ketone production, or hepatic glycogen breakdown. Surprisingly, mRNA levels for several gluconeogenic enzymes were up-regulated appropriately in fasted Foxa3(-/-) mice, despite the fact that the corresponding genes had been shown to be activated by FOXA proteins in vitro. However, the mRNA for the plasma membrane glucose transporter GLUT2 was decreased by 64% in the fasted and 93% in the fed state, suggesting that efflux of newly synthesized glucose is limiting in Foxa3(-/-) hepatocytes. Thus, Foxa3 is the dominating transcriptional regulator of GLUT2 expression in hepatocytes in vivo. In addition, we investigated the hepatic transcription factor network in Foxa3(-/-) mice and found that the normal activation of HNF-4alpha, HNF-1alpha, and PGC-1 induced by fasting is attenuated in mice lacking Foxa3.