Acute and selective inhibition of adipocyte glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase by interferon gamma

Interferon gamma (IFN-gamma) was previously shown to promote fatty acid (FA) release from adipose tissue (AT). Net lipolysis is an equilibrium between triglyceride breakdown and FA re-esterification. The latter requires activated glyceroneogenesis for glycerol-3-phosphate synthesis and increased cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), the key enzyme in this pathway. We wondered whether glyceroneogenesis and PEPCK-C would be IFN-gamma targets. We injected mice with IFN-gamma, and exposed either AT explants and isolated adipocytes from humans and mice or 3T3-F442A adipocytes to IFN-gamma before monitoring expression of genes involved in lipid metabolism and the metabolic consequences. We show that IFN-gamma induces a large increase in FA release without affecting glycerol output and decreases [1-(14)C]-pyruvate incorporation into lipids, thus demonstrating that FA re-esterification is reduced due to diminished glyceroneogenesis. A series of mRNA encoding proteins involved in FA metabolism remained unaffected by IFN-gamma, while that of PEPCK-C was rapidly and drastically lowered. IFN-gamma effect opposed that of the beta-agonist isoproterenol and of 8-Br-cAMP. In IFN-gamma-treated mice, PEPCK-C gene expression was decreased in AT, but not in liver or kidney. Thus, IFN-gamma exerts a tissue-specific action in rodents and humans, having glyceroneogenesis and the PEPCK-C gene as selective targets to intensify FA release from adipocytes.

Candidate gene association study of insulin signaling genes and Alzheimer's disease: evidence for SOS2, PCK1, and PPARgamma as susceptibility loci

Epidemiological evidence supports the existence of a possible link between type II diabetes mellitus (T2DM) and late-onset Alzheimer's disease (LOAD). Polymorphisms from candidate genes for T2DM were genotyped in a two-stage approach to identify novel risk factors for LOAD. One hundred fifty-two polymorphisms were initially genotyped in a case:control cohort: nine SNPs showed individual association with disease status under at least one genetic model, while an additional two SNPs showed a haplotype association. In a replication study, we confirmed significant association of SNPs within three genes--PPARgamma, SOS2, and PCK1--with Alzheimer's disease. In particular, our data suggest that the effect of variants within these genes might be influenced by gender.

Hepatocyte differentiation of mesenchymal stem cells from rat peritoneal adipose tissue in vitro and in vivo

Mesenchymal tissues harbour stromal cells capable of multilineage differentiation. Here, we demonstrate the isolation of mesenchymal stem cells (MSC) from rat peritoneal adipose tissue capable of osteogenic and adipogenic differentiation. Under in vitro conditions favouring hepatocyte differentiation, these MSC gained characteristic functions of hepatocytes such as the capacity to synthesize urea or store glycogen. Hepatocyte-specific transcripts of dipeptidylpeptidase type IV (CD26), albumin, cytochrome P450 type 1A1 (CYP1A1) and connexin CX32 (CX32) were detected only in differentiated but not undifferentiated cells. Transient transgenic expression of luciferase could be stimulated by cAMP when driven by the hepatocyte-specific promoter of the cytosolic phosphoenolpyruvate carboxykinase (PCK1) gene. Finally, stem cell-derived hepatocytes from wild type (CD26+/+) rats were transplanted into the livers of CD26-deficient animals after lentiviral transduction with the GFP gene under the control of the ubiquitin promoter. GFP-positive cells engrafted in the host liver predominantly in the periportal region of the liver lobule. They continued to express CD26, a prominent feature of differentiated hepatocytes, indicating their topologically and functionally proper integration into the host liver parenchyma. Thus, MSCs from rat peritoneal adipose tissue exhibit the potential to differentiate into hepatocyte-like cells in vitro and in vivo.

Depot-specific modulation of rat intraabdominal adipose tissue lipid metabolism by pharmacological inhibition of 11beta-hydroxysteroid dehydrogenase type 1

The metabolic consequences of visceral obesity have been associated with amplification of glucocorticoid action by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) in adipose tissue. This study aimed to assess in a rat model of diet-induced obesity the effects of pharmacological 11beta-HSD1 inhibition on the morphology and expression of key genes of lipid metabolism in intraabdominal adipose depots. Rats fed a high-sucrose, high-fat diet were treated or not with a specific 11beta-HSD1 inhibitor (compound A, 3 mg/kg.d) for 3 wk. Compound A did not alter food intake or body weight gain but specifically reduced mesenteric adipose weight (-18%) and adipocyte size, without significantly affecting those of epididymal or retroperitoneal depots. In mesenteric fat, the inhibitor decreased (to 25-50% of control) mRNA levels of genes involved in lipid synthesis (FAS, SCD1, DGAT1) and fatty acid cycling (lipolysis/reesterification, ATGL and PEPCK) and increased (30%) the activity of the fatty acid oxidation-promoting enzyme carnitine palmitoyltransferase 1. In striking contrast, in the epididymal depot, 11beta-HSD1 inhibition increased (1.5-5-fold) mRNA levels of those genes related to lipid synthesis/cycling and slightly decreased carnitine palmitoyltransferase 1 activity, whereas gene expression remained unaffected in the retroperitoneal depot. Compound A robustly reduced liver triacylglycerol content and plasma lipids. The study demonstrates that pharmacological inhibition of 11beta-HSD1, at a dose that does not alter food intake, reduces fat accretion specifically in the mesenterical adipose depot, exerts divergent intraabdominal depot-specific effects on genes of lipid metabolism, and reduces steatosis and lipemia.

Nuclear corepressor is required for inhibition of phosphoenolpyruvate carboxykinase expression by tumor necrosis factor-alpha

Inhibition of phosphoenolpyruvate carboxykinase (PEPCK) by TNF-alpha contributes to the pathogenesis of hypoglycemia in endotoxin shock. In this study, the molecular mechanism underlying the inhibition was investigated in hepatoma cells (rat H4IIE and human HepG2). PEPCK expression was induced by cAMP, and the induction was reduced by TNF-alpha at protein and mRNA levels in H4IIE cells. The inhibition was observed in the PEPCK gene promoter in a PEPCK-luciferase reporter. Activation of nuclear factor kappaB (NF-kappaB) pathway was required for the transcriptional inhibition of PEPCK gene. Degradation of NF-kappaB inhibitor (IkappaB) and p65 nuclear translocation were involved in the inhibition. An interaction of histone deacetylase 3 (HDAC3) and silencing mediator for retinoic acid receptor and thyroid hormone receptor (SMRT) with the PEPCK gene promoter was induced by TNF-alpha and observed in a chromatin immunoprecipitation assay. The TNF-induced inhibition was blocked by HDAC inhibitor or HDAC3 knockdown. The blocking effect was also observed in knockdown of corepressor SMRT. Point mutation suggests that cAMP response element (CRE) is required for TNF-induced inhibition of the PEPCK gene promoter. Phosphorylation of cAMP response element-binding protein at Ser133 and expression of peroxisome proliferator-activated receptor-gamma coactivator 1alpha were not changed by TNF-alpha in H4IIE cells. The transcriptional activity of CRE-binding protein was inhibited by TNF-alpha in a CRE-luciferase reporter. The data suggests that the nuclear corepressor proteins of HDAC3 and SMRT mediate TNF inhibition of PEPCK transcription. The inhibition mechanism is related to activation of NF-kappaB and inhibition of CRE-binding protein activity by the corepressor. These data suggest a novel activity of nuclear corepressor in the regulation of PEPCK expression by TNF-alpha.

Cytosolic phosphoenolpyruvate carboxykinase does not solely control the rate of hepatic gluconeogenesis in the intact mouse liver

When dietary carbohydrate is unavailable, glucose required to support metabolism in vital tissues is generated via gluconeogenesis in the liver. Expression of phosphoenolpyruvate carboxykinase (PEPCK), commonly considered the control point for liver gluconeogenesis, is normally regulated by circulating hormones to match systemic glucose demand. However, this regulation fails in diabetes. Because other molecular and metabolic factors can also influence gluconeogenesis, the explicit role of PEPCK protein content in the control of gluconeogenesis was unclear. In this study, metabolic control of liver gluconeogenesis was quantified in groups of mice with varying PEPCK protein content. Surprisingly, livers with a 90% reduction in PEPCK content showed only a approximately 40% reduction in gluconeogenic flux, indicating a lower than expected capacity for PEPCK protein content to control gluconeogenesis. However, PEPCK flux correlated tightly with TCA cycle activity, suggesting that under some conditions in mice, PEPCK expression must coordinate with hepatic energy metabolism to control gluconeogenesis.

Mouse embryo culture induces changes in postnatal phenotype including raised systolic blood pressure

A key factor in the use of assisted reproductive technologies (ART) for diverse species is the safety of procedures for long-term health. By using a mouse model, we have investigated the effect of in vitro culture and embryo transfer (ET) of superovulated embryos on postnatal growth and physiological activity compared with that of embryos developing in vivo. Embryo culture from two-cell to blastocyst stages in T6 medium either with or without a protein source reduced blastocyst trophectoderm and inner cell mass cell number compared with that of embryos developing in vivo. Embryo culture and ET had minimal effects on postnatal growth when compared with in vivo development with an equivalent litter size. However, embryo culture, and to a lesser extent ET, led to an enhanced systolic blood pressure at 21 weeks compared with in vivo development independent of litter size, maternal origin, or body weight. Moreover, activity of enzymatic regulators of cardiovascular and metabolic physiology, namely, serum angiotensin-converting enzyme and the gluconeogenesis controller, hepatic phosphoenolpyruvate carboxykinase, were significantly elevated in response to embryo culture and/or ET in female offspring at 27 weeks, independent of maternal factors and postnatal growth. These animal data indicate that postnatal physiological criteria important in cardiovascular and metabolic health may be more sensitive to routine ART procedures than growth.

Acute and selective regulation of glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase in adipose tissue by thiazolidinediones in type 2 diabetes

AIMS/HYPOTHESIS

Regulation of glyceroneogenesis and its key enzyme cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) plays a major role in the control of fatty acid release from adipose tissue. Here we investigate the effect of rosiglitazone on the expression of genes involved in fatty acid metabolism and the resulting metabolic consequences.

MATERIALS AND METHODS

Rosiglitazone was administered to Zucker fa/fa rats for 4 days and to 24 diabetic patients for 12 weeks, then mRNA expression for the genes encoding PEPCK-C, mitochondrial PEPCK, adipocyte lipid-binding protein, glycerol kinase, lipoprotein lipase and glycerol-3-phosphate dehydrogenase was examined in s.c. adipose tissue by real-time RT-PCR. Glyceroneogenesis was determined using [1-(14)C]pyruvate incorporation into lipids. Cultured adipose tissue explants from overweight women undergoing plastic surgery were incubated with rosiglitazone for various times before mRNA determination and analysis of PEPCK-C protein, activity and glyceroneogenesis.

RESULTS

Rosiglitazone administration to rats induced the expression of the gene encoding PEPCK-C mRNA (PCK1) and PEPCK-C activity in adipose tissue with a resulting 2.5-fold increase in glyceroneogenesis. This was accompanied by an improvement in dyslipidaemia as demonstrated by the decrease in plasma NEFAs and triacylglycerol. In rosiglitazone-treated diabetic patients, PCK1 mRNA was raised 2.5-fold in s.c. adipose tissue. Rosiglitazone treatment of adipose tissue explants from overweight women caused a selective augmentation in PCK1 mRNA which reached a maximum of 9-fold at 14 h, while mRNA for other genes remained unaffected. Experiments with inhibitors showed a direct and transcription-only effect, which was followed by an increase in PEPCK-C protein, enzyme activity and glyceroneogenesis.

CONCLUSIONS/INTERPRETATION

These results favour adipocyte glyceroneogenesis as the initial thiazolidinedione-responsive pathway leading to improvement in dyslipidaemia.

Effect of Biotin on Activity and Gene Expression of Biotin-Dependent Carboxylases in the Liver of Dairy Cows

Biotin is a cofactor of the gluconeogenic enzymes pyruvate carboxylase (PC) and propionyl-coenzyme A carboxylase (PCC). We hypothesized that biotin supplementation increases the activity and gene expression of PC and PCC and the gene expression of phosphoenol-pyruvate carboxykinase (PEPCK) in the liver of lactating dairy cows. Eight multiparous Holstein cows (40 +/- 2 kg/d of milk yield and 162 +/- 35 d in milk) were randomly assigned to 1 of 2 diet sequences in a crossover design with two 22-d periods. Treatments consisted of a basal diet (60% concentrate) containing 0 or 0.96 mg/kg of supplemental biotin. On d 21 of each period, liver tissue was collected by percutaneous liver biopsy. Activities of PC and PCC were determined by measuring the fixation of [14C]O2 in liver homogenates. Abundance of mRNA for PCC, PC, and PEPCK was determined by quantitative reverse-transcription PCR. Biotin supplementation did not affect milk production or composition. Biotin supplementation increased the activity of PC but had no effect on PCC activity. Biotin supplementation did not affect the gene expression of PC, PCC, and PEPCK. The increased activity of PC without changes in mRNA abundance may have been caused by increased activation of the apoenzymes by holocarboxylase synthetase. In conclusion, biotin supplementation affected the activity of PC in the liver of lactating dairy cows, but whether biotin supplementation increases glucose production in the liver remains to be determined.

Immunocytochemical localization of glucose 6-phosphatase and cytosolic phosphoenolpyruvate carboxykinase in gluconeogenic tissues reveals unsuspected metabolic zonation

Immunohistochemical analysis was used to define the precise cell-specific localization of Glucose-6-phosphatase (Glc6Pase) and cytosolic form of the phosphoenolpyruvate carboxykinase (PEPCK-C) in the digestive system (liver, small intestine and pancreas) and the kidney. Co-expression of Glc6Pase and PEPCK-C was shown to take place in hepatocytes, in proximal tubules of the cortex kidney and at the top of the villi of the small intestine suggesting that these tissues are all able to perform complete gluconeogenesis. On the other hand, intrahepatic bile ducts, collecting tubes of the nephron and the urinary epithelium in the calices of the kidney, as well as the crypts of the small intestine, express Glc6Pase without significant levels of PEPCK-C. In such cases, the function of Glc6Pase could be related to the transepithelial transport of glucose characteristic of these tissues, rather than to the neoformation of glucose. Lastly, PEPCK-C expression in the absence of Glc6Pase was noted in both the exocrine pancreas and the endocrine islets of Langerhans. Possible roles of PEPCK-C in exocrine pancreas might be the provision of gluconeogenic intermediates for further conversion into glucose in the liver, whereas PEPCK-C would be instrumental in pyruvate cycling, which has been suggested to play a regulatory role in insulin secretion by the beta-cells of the islets.

Liver glyconeogenesis: a pathway to cope with postprandial amino acid excess in high-protein fed rats?

This paper provides molecular evidence for a liver glyconeogenic pathway, that is, a concomitant activation of hepatic gluconeogenesis and glycogenesis, which could participate in the mechanisms that cope with amino acid excess in high-protein (HP) fed rats. This evidence is based on the concomitant upregulation of phosphoenolpyruvate carboxykinase (PEPCK) gene expression, downregulation of glucose 6-phosphatase catalytic subunit (G6PC1) gene expression, an absence of glucose release from isolated hepatocytes and restored hepatic glycogen stores in the fed state in HP fed rats. These effects are mainly due to the ability of high physiological concentrations of portal blood amino acids to counteract glucagon-induced liver G6PC1 but not PEPCK gene expression. These results agree with the idea that the metabolic pathway involved in glycogen synthesis is dependent upon the pattern of nutrient availability. This nonoxidative glyconeogenic disposal pathway of gluconeogenic substrates copes with amino excess and participates in adjusting both amino acid and glucose homeostasis. In addition, the pattern of PEPCK and G6PC1 gene expression provides evidence that neither the kidney nor the small intestine participated in gluconeogenic glucose production under our experimental conditions. Moreover, the main glucose-6-phosphatase (G6Pase) isoform expressed in the small intestine is the ubiquitous isoform of G6Pase (G6PC3) rather than the G6PC1 isoform expressed in gluconeogenic organs.

Growth hormone corrects proliferation and transcription of phosphoenolpyruvate carboxykinase in livers of old mice via elimination of CCAAT/enhancer-binding protein alpha-Brm complex

Growth hormone (GH), which is reduced with age, corrects the impaired proliferative capacity of livers of old animals. In this paper, we present a mechanism by which GH eliminates age-dependent negative control of proliferation and increases transcription of liver-specific genes in livers of old mice. The reduced proliferative capacities of the liver of old animals are associated with the CCAAT/enhancer-binding protein alpha (C/EBPalpha)-Brm complex, which inhibits E2F-dependent promoters. We found that a sequestration of C/EBPalpha into complexes with Brm leads to a weak interaction of C/EBPalpha with promoters of liver-specific genes, expression of which is reduced in old animals. Injection of either GH or the regulator of the amplitude of endogenous GH release, ghrelin, reduces the C/EBPalpha-Brm complex in livers of old mice, leading to a derepression of E2F targets, to increased interactions of C/EBPalpha with promoters of liver-specific genes, and to correction of their expression. GH-dependent elimination of the complex is mediated by the inhibition of cyclin D3-CDK4 activity and by elevation of a phosphatase, protein phosphatase 2A, which dephosphorylates C/EBPalpha and dissociates the complex.

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

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

Epigallocatechin gallate supplementation alleviates diabetes in rodents

As the prevalence of type 2 diabetes mellitus is increasing at an alarming rate, effective nutritional and exercise strategies for the prevention of this disease are required. Specific dietary components with antidiabetic efficacy could be one aspect of these strategies. This study investigated the antidiabetic effects of the most abundant green tea catechin, epigallocatechin gallate (EGCG, TEAVIGO), in rodent models of type 2 diabetes mellitus and H4IIE rat hepatoma cells. We assessed glucose and insulin tolerance in db/db mice and ZDF rats after they ingested EGCG. Using gene microarray and real-time quantitative RT-PCR we investigated the effect of EGCG on gene expression in H4IIE rat hepatoma cells as well as in liver and adipose tissue of db/db mice. EGCG improved oral glucose tolerance and blood glucose in food-deprived rats in a dose-dependent manner. Plasma concentrations of triacylglycerol were reduced and glucose-stimulated insulin secretion was enhanced. In H4IIE cells, EGCG downregulated genes involved in gluconeogenesis and the synthesis of fatty acids, triacylgycerol, and cholesterol. EGCG decreased the mRNA expression of phosphoenolpyruvate carboxykinase in H4IIE cells as well as in liver and adipose tissue of db/db mice. Glucokinase mRNA expression was upregulated in the liver of db/db mice in a dose-dependent manner. This study shows that EGCG beneficially modifies glucose and lipid metabolism in H4IIE cells and markedly enhances glucose tolerance in diabetic rodents. Dietary supplementation with EGCG could potentially contribute to nutritional strategies for the prevention and treatment of type 2 diabetes mellitus.

Perturbation of glucose flux in the liver by decreasing F26P2 levels causes hepatic insulin resistance and hyperglycemia

Hepatic insulin resistance is one of the characteristics of type 2 diabetes and contributes to the development of hyperglycemia. How changes in hepatic glucose flux lead to insulin resistance is not clearly defined. We determined the effects of decreasing the levels of hepatic fructose 2,6-bisphosphate (F26P(2)), a key regulator of glucose metabolism, on hepatic glucose flux in the normal 129J mice. Upon adenoviral overexpression of a kinase activity-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, the enzyme that determines F26P(2) level, hepatic F26P(2) levels were decreased twofold compared with those of control virus-treated mice in basal state. In addition, under hyperinsulinemic conditions, hepatic F26P(2) levels were much lower than those of the control. The decrease in F26P(2) leads to the elevation of basal and insulin-suppressed hepatic glucose production. Also, the efficiency of insulin to suppress hepatic glucose production was decreased (63.3 vs. 95.5% suppression of the control). At the molecular level, a decrease in insulin-stimulated Akt phosphorylation was consistent with hepatic insulin resistance. In the low hepatic F26P(2) states, increases in both gluconeogenesis and glycogenolysis in the liver are responsible for elevations of hepatic glucose production and thereby contribute to the development of hyperglycemia. Additionally, the increased hepatic gluconeogenesis was associated with the elevated mRNA levels of peroxisome proliferator-activated receptor-gamma coactivator-1alpha and phosphoenolpyruvate carboxykinase. This study provides the first in vivo demonstration showing that decreasing hepatic F26P(2) levels leads to increased gluconeogenesis in the liver. Taken together, the present study demonstrates that perturbation of glucose flux in the liver plays a predominant role in the development of a diabetic phenotype, as characterized by hepatic insulin resistance.

Renal bone morphogenetic protein-7 protects against diabetic nephropathy

Longstanding diabetes causes renal injury with early dropout of podocytes, albuminuria, glomerular and tubulointerstitial fibrosis, and progressive renal failure. The renal pathology seems to be driven, in part, by TGF-beta and is associated with a loss of renal bone morphogenic protein-7 (BMP-7) expression. Here, the hypothesis that maintenance of renal (especially podocyte) BMP-7 by transgenic expression reduces diabetic renal injury was tested. Diabetic mice that expressed the phosphoenolpyruvate carboxykinase promoter-driven BMP-7 transgene and nondiabetic, transgenic mice as well as diabetic and nondiabetic wild-type controls were studied for up to 1 yr. Transgenic expression of BMP-7 in glomerular podocytes and proximal tubules prevents podocyte dropout and reductions in nephrin levels in diabetic mice. Maintenance of BMP-7 also reduces glomerular fibrosis and interstitial collagen accumulation as well as collagen I and fibronectin expression. Diabetic wild-type mice develop progressive albuminuria, which is substantially reduced in transgenic mice. These effects of the BMP-7 transgene occur without changing renal TGF-beta levels. It is concluded that maintenance of renal BMP-7 during the evolution of diabetic nephropathy reduces diabetic renal injury, especially podocyte dropout. The findings also establish a role for endogenous glomerular BMP-7 as an autocrine regulator of podocyte integrity in vivo.

Recombinant human insulin-like growth factor-I treatment inhibits gluconeogenesis in a transgenic mouse model of type 2 diabetes mellitus

IGF-I and insulin are structurally related polypeptides that mediate a similar pattern of biological effects via receptors that display considerably homology. Administration of recombinant human IGF-I (rhIGF-I) has been proven to improve glucose control and liver and muscle insulin sensitivity in patients with type 2 diabetes mellitus (DM). The effect of rhIGF-I treatment was evaluated in a mouse model of type 2 DM (MKR mouse), which expresses a dominant-negative form of the human IGF-I receptor under the control of the muscle creatine kinase promoter specifically in skeletal muscle. MKR mice have impaired IGF-I and insulin signaling in skeletal muscle, leading to severe insulin resistance in muscle, liver, and fat, developing type 2 DM at 5 wk of age. Six-week-old MKR mice were treated with either saline or rhIGF-I for 3 wk. Blood glucose levels were decreased in response to rhIGF-I treatment in MKR mice. rhIGF-I treatment also increased body weight in MKR with concomitant changes in body composition such as a decrease in fat mass and an increase in lean body mass. Insulin, fatty acid, and triglyceride levels were not affected by rhIGF-I, nor were insulin or glucose tolerance in MKR mice. Hyperinsulinemic-euglycemic clamp analysis demonstrated no improvement in overall insulin sensitivity. Pyruvate and glutamine tolerance tests proved that there was a decrease in the rate of glucose appearance in MKR mice treated with rhIGF-I, suggesting a reduction in the gluconeogenic capacity of liver, kidney, and small intestine. Taken together these results demonstrate that the improvement of the hyperglycemia was achieved by inhibition of gluconeogenesis rather than an improvement in insulin sensitivity. Also, these results suggest that a functional IGF-I receptor in skeletal muscle is required for IGF-I to improve insulin sensitivity in this mouse model of type 2 DM.

Merit of physical exercise to reverse the higher gene expression of hepatic phosphoenolpyruvate carboxykinase in obese Zucker rats

Effects of endurance training on the phosphoenolpyruvate carboxykinase (PEPCK), a rate-limiting enzyme of gluconeogenesis, were studied in the obese Zucker rats. We used a moderate exercise program consisting of treadmill running at 20 m/min and 0-degree gradient for 1 h/day, 7 days/week, for 8 weeks. At the end of the experimental period, insulin action on glucose disposal rate was measured using the glucose-insulin index, the product of the areas under the curve of glucose and insulin during the intraperitoneal glucose tolerance test. Furthermore, changes of hepatic PEPCK gene expression were detected using reverse transcriptase polymerase chain reaction to assay the mRNA level and Western blot analysis to detect the protein level. Different to sedentary obese rats, an elevation in the value of glucose-insulin index from the exercised obese rats declined, indicating the marked effect of regular moderate exercise on the improvement of insulin sensitivity in this insulin resistant animal model. Moreover, the diabetes-related elevation in mRNA level and protein content of hepatic PEPCK were observed in non-exercise obese groups but they were markedly reduced by exercise training. In addition, chronic exercise training enhanced the insulin sensitivity of lean Zucker rats, since the value of glucose-insulin index was lower than that of untrained lean groups. Also, the hepatic PEPCK gene expressions both the mRNA and protein levels were reduced in exercised lean Zucker rats as compared with their sedentary littermates. These results suggest that modulation of hepatic PEPCK gene expression by chronic exercise training might be related to the enhancement of insulin sensitivity. Thus, endurance exercise training could aid in the prevention and/or treatment of individuals with insulin resistance.

Expression of human fructose-1,6-bisphosphatase in the liver of transgenic mice results in increased glycerol gluconeogenesis

In type 2 diabetes, increased endogenous glucose production (EGP) as a result of elevated gluconeogenesis contributes to hyperglycemia. An increase in glycerol gluconeogenesis has led to the suggestion that, in obese human subjects with type 2 diabetes, there may be an increase in the activity of the gluconeogenic enzyme fructose-1,6-bisphosphatase (FBPase). The aim of this study was to generate transgenic mice that overexpress human liver FBPase in the liver and assess the consequences to whole-body and hepatic glucose metabolism. FBPase transgenic mice had significantly higher levels of transgene expression in the liver and, as a result, had increased FBPase protein and enzyme activity levels in the liver. This resulted in an increase in the rate of glycerol conversion to glucose but not in EGP. The increased expression of FBPase in the liver did not result in any significant differences compared with littermate control mice in insulin or glucose tolerance. Therefore, it appears that, on its own, an increase in FBPase does not lead to impaired regulation of EGP and hence does not affect whole-body glucose metabolism. This suggests that, for EGP to be increased, other factors associated with obesity are also required.

knocking down liver ccaat/enhancer-binding protein alpha by adenovirus-transduced silent interfering ribonucleic acid improves hepatic gluconeogenesis and lipid homeostasis in db/db mice

CCAAT/enhancer-binding protein-alpha (C/EBPalpha) is a member of the basic leucine zipper transcription factor family and regulates expression of several enzymes in the liver that control glucose and lipid metabolism. Using adenovirus-transduced silent interfering (si)RNA against C/EBPalpha, endogenous liver C/EBPalpha protein was knocked down by 70-80% in 8-wk-old wild-type (WT) and db/db mice. In WT mice, fasting blood glucose concentrations were reduced approximately 24% without changes in plasma free fatty acid and triglycerides, when compared with LacZ adenovirus-treated control mice. Ad-C/EBPalpha siRNA treatment nearly normalized fasting glucose and significantly reduced plasma insulin and free fatty acid content, even though there was no elevation of C/EBPalpha protein in the livers of db/db mice. In parallel with the changes in glucose levels, hepatic glucose production was significantly reduced in C/EBPalpha siRNA-treated WT and db/db mice. mRNA levels of phyosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and liver glycogen synthase were decreased in the C/EBPalpha siRNA-treated WT and db/db mice. Interestingly, the magnitude of reduction in these enzymes was more profound in db/db mice. C/EBPalpha siRNA also decreased mRNA levels of proliferator activator protein-gamma coactivator-1alpha in both the WT and db/db mice but reduced cAMP response element-binding protein only in WT and did not alter hepatic nuclear factor-4alpha and CBP/p300 expression. Expression of genes involved in lipogenesis, such as fatty acid synthase, acetyl-CoA carboxylase, and sterol regulatory element-binding protein-1c was robustly suppressed in the C/EBPalpha siRNA-treated db/db mice. Taken together, these results indicate that C/EBPalpha plays an important role in maintaining glucose and lipid homeostasis in the liver.

Identification and characterization of a small molecule AMPK activator that treats key components of type 2 diabetes and the metabolic syndrome

AMP-activated protein kinase (AMPK) is a key sensor and regulator of intracellular and whole-body energy metabolism. We have identified a thienopyridone family of AMPK activators. A-769662 directly stimulated partially purified rat liver AMPK (EC50 = 0.8 microM) and inhibited fatty acid synthesis in primary rat hepatocytes (IC50 = 3.2 microM). Short-term treatment of normal Sprague Dawley rats with A-769662 decreased liver malonyl CoA levels and the respiratory exchange ratio, VCO2/VO2, indicating an increased rate of whole-body fatty acid oxidation. Treatment of ob/ob mice with 30 mg/kg b.i.d. A-769662 decreased hepatic expression of PEPCK, G6Pase, and FAS, lowered plasma glucose by 40%, reduced body weight gain and significantly decreased both plasma and liver triglyceride levels. These results demonstrate that small molecule-mediated activation of AMPK in vivo is feasible and represents a promising approach for the treatment of type 2 diabetes and the metabolic syndrome.

Nuclear receptor binding to the retinoic acid response elements of the phosphoenolpyruvate carboxykinase gene in vivo: effects of vitamin A deficiency

Vitamin A deficiency decreases hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene expression in mice and expression is restored with retinoic acid treatment in vivo. This report examines further the mechanism of retinoid regulation of the PEPCK gene in vivo. We have identified nuclear receptors that bind to retinoic acid response elements (RAREs) in the PEPCK promoter by electrophoretic mobility shift assay and have verified these in vivo using chromatin immunoprecipitation (ChIP) in mouse liver. Based on the results of our ChIP assay, hepatic nuclear factor (HNF)-4alpha, retinoid X receptor (RXR) alpha, retinoic acid receptor (RAR) alpha, peroxisome proliferator-activated receptor (PPAR) alpha and chicken ovalbumin upstream promoter transcription factor (COUP-TF) II bind to the downstream retinoic acid response unit RARE1/RARE2, and PPARalpha and RXRalpha bind to the upstream RARE3 of the PEPCK gene. HNF-4alpha, RXRalpha, RARalpha, PPARalpha and COUP-TFII bind PEPCK RAREs in a specific pattern that, with the exception of PPARalpha, does not change significantly with vitamin A deficiency. PPARalpha binding to the upstream retinoic acid response element is decreased in the vitamin A-deficient liver, when compared to the vitamin A-sufficient state. These results provide the first in vivo measures of nuclear receptor binding to the upstream and downstream RAREs of the PEPCK gene under conditions where the nucleosomal structure of the chromatin is maintained and the nuclear receptors are physically cross-linked in situ to the PEPCK DNA in intact liver.

Novel mechanism for plasma glucose-lowering action of metformin in streptozotocin-induced diabetic rats

To better understand the insulin-independent plasma glucose-lowering action of metformin, we used streptozotocin (STZ)-induced diabetic rats to investigate the possible mechanisms. Oral intake of metformin decreased the plasma glucose of STZ-induced diabetic rats with a parallel increase of plasma beta-endorphin-like immunoreactivity (BER). Mediation of opioid mu-receptors in the action of metformin was identified by the blockade of receptors with antagonist in STZ-induced diabetic rats and the failure of action in opioid mu-receptor knockout diabetic mice. Release of BER from adrenal glands by metformin was characterized, using bilateral adrenalectomy and the release of BER from isolated adrenal medulla of STZ-induced diabetic rats. Repeated treatment with metformin in STZ-induced diabetic rats increased the mRNA and protein levels of GLUT-4 in soleus muscle that was blocked by naloxonazine. Reduction of the mRNA or protein levels of hepatic PEPCK was also impeded in the same group of STZ-induced diabetic rats. In conclusion, our results provide novel mechanisms for the plasma glucose-lowering action of metformin, via an increase of beta-endorphin secretion from adrenal glands to stimulate opioid mu-receptor linkage, leading to an increase of GLUT-4 gene expression and an attenuation of hepatic PEPCK gene expression in STZ-induced diabetic rats.

Salicornia herbacea prevents high fat diet-induced hyperglycemia and hyperlipidemia in ICR mice

Salicornia herbacea L. (Chenopodiaceae) has been used as a seasoned vegetable by living in coastal areas. S. herbacea (SH) has been demonstrated to stimulate cytokine production, nitric oxide release, and to show anti-oxidative effect. In a series of investigations to develop potential anti-diabetic and/or anti-hyperlipidemic agents from Korean indigenous plants, 50% ethanol extract of Salicornia herbacea was found to prevent the onset of the hyperglycemia and hyperlipidemia induced by high fat diet in ICR mice. At 6 week old, the ICR mice were randomly divided into five groups; two control and three treatment groups. The control mice were to receive either a regular diet (RD) or high-fat diet (HFD), and the treatment groups were fed a high fat diet with either 350 mg/kg, 700 mg/kg of SH (SH350 and SH700) or 250 mg/kg of metformin (MT250) for a 10-week period. SH not only reduced body weight but also corrected associated hyperglycemia and hyperlipidemia in a dose dependent manner. SH exerted beneficial effects on the plasma glucose and lipid homeostasis possibly ascribed to its specific effects on lipogenesis related genes (SREBP1a, FAS, GAPT), and PEPCK, glucose 6-phosphatase gene expressions in liver. Ethanol extract of S. herbacea has potential as a preventive agent for type 2 diabetes (and possibly hyperlipidemia) and deserves future clinical trial.