Isolation and characterization of the mouse cytosolic phosphoenolpyruvate carboxykinase (GTP) gene: evidence for tissue-specific hypersensitive sites

THE EFFECT OF SALBUTAMOL ON PGC-1 α AND GLUT4 mRNA EXPRESSION IN THE LIVER AND MUSCLE OF ELDERLY DIABETIC MICE

Background

Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) plays an important role in the regulation of cellular energy metabolism, and it is involved in obesity and type 2 diabetes mellitus (T2DM). Its expression is elevated in the liver of T2DM mouse models. Literature reports show that chronic β2 stimulation improved insulin sensitivity in T2DM.

Objectives

We aimed to test the hypotheses that chronic β2 stimulation-induced improvement in insulin sensitivity involves changes in the expression of PGC-1α and glucose transporter 4 (GLUT4).

Animals and Methods

We fed a locally inbred, 8 months old mice, a high fat diet (HFD) to induce diabetes. These mice gained weight and became insulin resistant. The β2 agonist salbutamol had a beneficial effect on both glucose tolerance and insulin sensitivity after 4 weeks.

Results

Salbutamol beneficial effect persisted after 4 weeks of its discontinuation. HFD caused an up regulation of the hepatic PGC-1 α expression by 5.23 folds (P< 0.041) and salbutamol reversed this effect and caused a down regulation by 30.3 folds (P< 0.0001). PGC-1 α and GLUT4 expression in the muscle was not affected by salbutamol (P> 0.05).

Conclusion

Down regulation of the liver's PGC-1 α contributes to the beneficial effect of the chronic β2 stimulation on glucose tolerance and insulin sensitivity in T2DM mice.

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

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

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%.

Overexpression of beta2-adrenergic receptors in mouse liver alters the expression of gluconeogenic and glycolytic enzymes

In the livers of humans and many other mammalian species, beta2-adrenergic receptors (beta2-ARs) play an important role in the modulation of glucose production by glycogenolysis and gluconeogenesis. In male mice and rats, however, the expression and physiological role of hepatic beta2-ARs are rapidly lost with development under normal physiological conditions. We previously described a line of transgenic mice, F28 (Andre C, Erraji L, Gaston J, Grimber G, Briand P, and Guillet JG. Eur J Biochem 241: 417-424, 1996), which carry the human beta2-AR gene under the control of its own promoter. In these mice, hepatic beta2-AR levels are shown to increase rapidly after birth and, as in humans, be maintained at an elevated level in adulthood. F28 mice display strongly enhanced adenylyl cyclase responses to beta-AR agonists in their livers and, compared with normal mice, have increased basal hepatic adenylyl cyclase activity. In this report we demonstrate that, under normal physiological conditions, this increased beta2-AR activity affects the expression of the gluconeogenic and glycolytic key enzymes phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and l-pyruvate kinase and considerably decreases hepatic glycogen levels. Furthermore, we show that the effects of beta-adrenergic ligands on liver glycogen observed in humans are reproduced in these mice: liver glycogen levels are strongly decreased by the beta2-AR agonist clenbuterol and increased by the beta-AR antagonist propranolol. These transgenic mice open new perspectives for studying in vivo the hepatic beta2-AR system physiopathology and for testing the effects of beta-AR ligands on liver metabolism.

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.

Effects of src-deficiency on the expression of in vivo toxicity of TCDD in a strain of c-src knockout mice procured through six generations of backcrossings to C57BL/6 mice

The effect of TCDD was studied in c-src-deficient C57BL6-src(tm1sor) (N6 src -/- and -/+) mice, and their wild-type littermate mice (N6 src +/+). The former was created from the original strain of B6, 129-src(tm1sor) mice through six generations of backcrossings with C57BL6 mice. The results of a high dose TCDD toxicity tests in male mice indicated that N6 src-/+ mice were significantly less responsive to the toxic action of TCDD (115 microg/kg single i.p. injection) than N6 src+/+ mice in terms of reduced % body weight gain, the increase in the liver to body weight ratio, and the decrease in the adipose tissue to liver weight ratio and in the weight of pancreas. To understand the cause for these differential effects of TCDD we studied TCDD-induced changes in several biochemical parameters at day 10 and found that most drastically affected ones were glycogen depletion and phosphoenolpyruvate carboxykinase (PEPCK) downregulation. In addition, the degree of triglyceride accumulation in liver was less pronounced in N6-/+ than in N6+/+ mice. These findings suggest that the absence of c-src expression indeed affects the development of selected, TCDD-induced toxic endpoints that are related to wasting syndrome.

Peroxisome proliferator-activated receptor gamma and chicken ovalbumin upstream promoter transcription factor II negatively regulate the phosphoenolpyruvate carboxykinase promoter via a common element

A heterodimer of peroxisome proliferator-activated receptor gamma (PPARgamma) and retinoid X receptor (RXR) is required for adipocyte differentiation. The gene encoding cytosolic phosphoenolpyruvate carboxykinase (PEPCK) is a PPARgamma/RXR target gene in adipose tissue. Of the two PPARgamma response elements, gAF1/PCK1 and PCK2, only PCK2 is required for PEPCK expression and responsiveness to the PPARgamma agonist, rosiglitazone, in adipose tissue even though both elements bind PPARgamma/RXR in vitro. In contrast, gAF1/PCK1 is essential for glucocorticoid inhibition of PPARgamma-induced PEPCK gene expression in adipocytes. We report that chicken ovalbumin upstream promoter transcription factor II (COUP-TFII) is the predominant nuclear receptor bound to gAF1/PCK1 in preadipocytes. COUP-TFII declines during adipogenesis in reciprocal fashion to PPARgamma. In transiently transfected fibroblasts COUP-TFII acts at gAF1/PCK1 to inhibit PPARgamma/RXR activation via PCK2. In contrast COUP-TFs are transcriptional activators of PEPCK in hepatocytes. PPARgamma/RXR occupies gAF1/PCK1 in adipocytes, and mutation of gAF1/PCK1 enhances PEPCK promoter transactivation by PPARgamma/RXR in fibroblasts, suggesting that this element is also a negative PPARgamma response element. These results indicate that gAF1/PCK1 is a pleiotropic element through which COUP-TFII inhibits premature PEPCK expression, and perhaps adipogenesis in general, and PPARgamma/RXR uses this same element in adipocytes to participate in PEPCK modulation by glucocorticoids.

Phosphoenolpyruvate carboxykinase is necessary for the integration of hepatic energy metabolism

We used an allelogenic Cre/loxP gene targeting strategy in mice to determine the role of cytosolic phosphoenolpyruvate carboxykinase (PEPCK) in hepatic energy metabolism. Mice that lack this enzyme die within 3 days of birth, while mice with at least a 90% global reduction of PEPCK, or a liver-specific knockout of PEPCK, are viable. Surprisingly, in both cases these animals remain euglycemic after a 24-h fast. However, mice without hepatic PEPCK develop hepatic steatosis after fasting despite up-regulation of a variety of genes encoding free fatty acid-oxidizing enzymes. Also, marked alterations in the expression of hepatic genes involved in energy metabolism occur in the absence of any changes in plasma hormone concentrations. Given that a ninefold elevation of the hepatic malate concentration occurs in the liver-specific PEPCK knockout mice, we suggest that one or more intermediary metabolites may directly regulate expression of the affected genes. Thus, hepatic PEPCK may function more as an integrator of hepatic energy metabolism than as a determinant of gluconeogenesis.