Differential regulation of glyceroneogenesis by glucocorticoids in epididymal and retroperitoneal white adipose tissue from rats

PURPOSE

Investigate the glycerol-3-phosphate generation pathways in epididymal (EPI) and retroperitoneal (RETRO) adipose tissues from dexamethasone-treated rats.

METHODS

Rats were treated with dexamethasone for 7 days. Glycerol-3-phosphate generation pathways via glycolysis, glyceroneogenesis and direct phosphorylation of glycerol were evaluated, respectively, by 2-deoxyglucose uptake, phosphoenolpyruvate carboxykinase (PEPCK-C) activity and pyruvate incorporation into triacylglycerol (TAG)-glycerol, and glycerokinase activity and glycerol incorporation into TAG-glycerol.

RESULTS

Dexamethasone treatment markedly decreased the body weight, but increased the weight and lipid content of EPI and RETRO and plasma insulin, glucose, non-esterified fatty acid and TAG levels. EPI and RETRO from dexamethasone-treated rats showed increased rates of de novo fatty acid synthesis (80 and 100%) and basal lipolysis (20%). In EPI, dexamethasone decreased the 2-deoxyglucose uptake (50%), as well as glyceroneogenesis, evidenced by a decrease of PEPCK-C activity (39%) and TAG-glycerol synthesis from pyruvate (66%), but increased the glycerokinase activity (50%) and TAG-glycerol synthesis from glycerol (72%) in this tissue. In spite of a similar reduction in 2-deoxyglucose uptake in RETRO, dexamethasone treatment increased glyceroneogenesis, evidenced by PEPCK activity (96%), and TAG-glycerol synthesis from pyruvate (110%), accompanied by a decrease in glycerokinase activity (50%) and TAG-glycerol synthesis from glycerol (50%). Dexamethasone effects on RETRO were accompanied by a decrease in p-Akt content and by lower insulin effects on the rates of glycerol release in the presence of isoproterenol and on the rates of glucose uptake in isolated adipocytes.

CONCLUSION

Our data demonstrated differential regulation of glyceroneogenesis and direct phosphorylation of glycerol by glucocorticoids in EPI and RETRO from rats.

GCN2 regulates the CCAAT enhancer binding protein beta and hepatic gluconeogenesis

Mice deficient for general control nondepressible-2 (Gcn2) either globally or specifically in the liver display reduced capacity to maintain glucose homeostasis during fasting, suggesting the hypothesis that GCN2 may regulate gluconeogenesis (GNG), which normally plays a key role maintaining peripheral glucose homeostasis. Gcn2-deficient mice exhibit normal insulin sensitivity and plasma insulin but show reduced GNG when administered pyruvate, a gluconeogenic substrate. The basal expression of phosphoenolpyruvate carboxykinase, a rate-limiting enzyme in GNG, is abnormally elevated in Gcn2 knockout (KO) mice in the fed state but fails to be further induced during fasting. The level of tricarboxylic acid cycle intermediates, including malate and oxaloacetate, and the NADH-to-NAD(+) ratio are perturbed in the liver of Gcn2 KO mice either in the fed or fasted state, which may directly impinge upon GNG. Additionally, the expression of the CCAAT enhancer-binding protein-β (C/EBPβ) in the liver fails to be induced in Gcn2 KO mice after 24 h fasting, and the liver-specific Cebpβ KO mice show reduced fasting GNG similar to that seen in Gcn2-deficient mice. Our study demonstrates that GCN2 is important in maintaining GNG in the liver, which is likely to be mediated through regulation of C/EBPβ.

Decreased fasting blood glucose is associated with impaired hepatic glucose production in thyroid-stimulating hormone receptor knockout mice

Our previous study reported that thyroid-stimulating hormone (TSH) promotes cholesterol synthesis via the cyclic adenosine monophosphate/protein kinase A/cAMP regulatory element-binding protein (cAMP/PKA/CREB) pathway after binding to TSH receptors (TSHR) in the liver. The hepatic cAMP/PKA/CREB pathway also plays an important role in maintaining fasting glucose homeostasis. These findings implied a possible role for TSH in hepatic glucose metabolism. In this study, we used TSH receptor knockout mice (Tshr-ko mice) to clarify the effect of Tshr deletion on hepatic glucose metabolism, and investigated whether the effects of TSH directly regulate hepatic gluconeogenesis in HepG2 cells. Tshr-ko mice exhibited decreased fasting blood glucose levels, increased insulin sensitivity but normal level of fasting plasma insulin. Tshr deletion impaired hepatic glucose production by down-regulating the expression of glucose-6-phosphatase (G6P) and phosphoenolpyruvate pyruvate carboxylase (PEPCK) mRNA, two rate-limiting enzymes in hepatic gluconeogenesis, and enhancing the abundance of hepatic glucokinase (GK), the first enzyme regulating glycogen synthesis. Moreover, Tshr deletion inhibited the protein expression of hepatic phospho-CREB and increased the protein expression of hepatic phospho-AMP-activated protein kinase (p-AMPK), two up-stream regulators of PEPCK and G6P mRNA. In HepG2 cells, TSH increased the expression of G6P and PEPCK at mRNA level. These results indicated the simulative effects of TSH on hepatic glucose production in vivo and in vitro, suggesting a novel role for TSH in hepatic glucose metabolism.

Rosiglitazone increases the expression of peroxisome proliferator-activated receptor-gamma target genes in adipose tissue, liver, and skeletal muscle in the sheep fetus in late gestation

Exposure to maternal overnutrition increases the expression of peroxisome proliferator-activated receptor-gamma (PPARgamma) in adipose tissue before birth, and it has been proposed that the precocial activation of PPARgamma target genes may lead to increased fat deposition in postnatal life. In this study, we determined the effect of intrafetal administration of a PPARgamma agonist, rosiglitazone, on PPARgamma target gene expression in fetal adipose tissue as well indirect actions of rosiglitazone on fetal liver and skeletal muscle. Osmotic pumps containing rosiglitazone (n = 7) or vehicle (15% ethanol, n = 7) were implanted into fetuses at 123-126 d gestation (term = 150 +/- 3 d gestation). At 137-141 d gestation, tissues were collected and mRNA expression of PPARgamma, lipoprotein lipase (LPL), adiponectin, and glycerol-3-phosphate dehydrogenase (G3PDH) in adipose tissue, PPARalpha and PPARgamma-coactivator 1alpha (PGC1alpha) in liver and muscle and phosphoenolpyruvate carboxykinase (PEPCK) in liver determined by quantitative real-time RT-PCR. Plasma insulin concentrations were lower in rosiglitazone-treated fetuses (P < 0.02). Rosiglitazone treatment resulted in increased expression of LPL and adiponectin mRNA (P < 0.01) in fetal adipose tissue. The expression of PPARalpha mRNA in liver (P < 0.05) and PGC1alpha mRNA (P < 0.02) in skeletal muscle were also increased by rosiglitazone treatment. Rosiglitazone treatment increased expression of PPARgamma target genes within fetal adipose tissue and also had direct or indirect actions on the fetal liver and muscle. The effects of activating PPARgamma in fetal adipose tissue mimic those induced by prenatal overnutrition, and it is therefore possible that activation of PPARgamma may be the initiating mechanism in the pathway from prenatal overnutrition to postnatal obesity.

Feed restriction induces pyruvate carboxylase but not phosphoenolpyruvate carboxykinase in dairy cows

The ability of dairy cattle to adapt to changes in nutrient intake requires appropriately responsive expression of several key genes in liver. Holstein cows were used in 2 experiments to determine the effect of short-term feed restriction on expression of mRNA for gluconeogenic and ureagenic enzymes in liver. In experiment 1, cows were fed a total mixed diet for ad libitum intake for a 5-d period followed by 5 d of 50% of their previous 5-d ad libitum intake followed by 10 d of ad libitum feeding. Liver biopsies and blood samples were obtained on d 5, 10, and 20 of the experiment, the last day of each feeding period. Pyruvate carboxylase (PC) mRNA increased with feed restriction, but phosphoenolpyruvate carboxykinase (PEPCK) was unchanged. Expression of carbamoyl phosphate synthetase (CPS-I), argininosuccinate synthetase, and ornithine transcarbamylase mRNA were not altered by feed restriction; however, CPS-I mRNA expression tended to increase during realimentation. In experiment 2, cows were fed for ad libitum intake for 5 d and then fed 50% of previous intake for 5 d. Liver biopsy samples collected on d 5 and 10 were used for PC mRNA, PEPCK mRNA, and in vitro measure of gluconeogenesis from radiolabelled propionate and lactate. The data indicate expression of genes for key metabolic processes in liver of lactating cows is responsive to feeding level. Expression of PC mRNA is part of the adaptive response to feed intake restriction and is matched by increased capacity for gluconeogenesis from lactate.

Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein

Protein feeding is known to decrease hunger and subsequent food intake in animals and humans. It has also been suggested that glucose appearance into portal vein, as occurring during meal assimilation, may induce comparable effects. Here, we connect these previous observations by reporting that intestinal gluconeogenesis (i.e., de novo synthesis of glucose) is induced during the postabsorptive time (following food digestion) in rats specifically fed on protein-enriched diet. This results in glucose release into portal blood, counterbalancing the lowering of glycemia resulting from intestinal glucose utilization. Comparable infusions into the portal vein of control postabsorptive rats (fed on starch-enriched diet) decrease food consumption and activate the hypothalamic nuclei regulating food intake. Similar hypothalamic activation occurs on protein feeding. All these effects are absent after denervation of the portal vein. Thus, portal sensing of intestinal gluconeogenesis may be a novel mechanism connecting the macronutrient composition of diet to food intake.

Proposed involvement of adipocyte glyceroneogenesis and phosphoenolpyruvate carboxykinase in the metabolic syndrome

Elevated concentration of plasma non-esterified fatty acids (NEFA) is now recognized as a key factor in the onset of insulin-resistance and type 2 diabetes mellitus. During fasting, circulating NEFAs arise from white adipose tissue (WAT) as a consequence of lipolysis from stored triacylglycerols. However, a significant part of these FAs (30-70%) is re-esterified within the adipocyte, so that a recycling occurs and net FA output is much less than << true >> lipolysis. Indeed, a balance between two antagonistic processes, lipolysis and FA re-esterification, controls the rate of net FA release from WAT. During fasting, re-esterification requires glyceroneogenesis defined as the de novo synthesis of glycerol-3-P from pyruvate, lactate or certain amino acids. The key enzyme in this process is the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C; EC 4.1.1.32). Recent advance has stressed the role of glyceroneogenesis and of PEPCK-C in FA release from WAT. Results indicate that glyceroneogenesis is indeed important to lipid homeostasis and that a disregulation in this pathway may have profound pathophysiological effects. The present review focuses on the regulation of glyceroneogenesis and of PEPCK-C gene expression and activity by FAs, retinoic acids, glucocorticoids and the hypolipidemic class of drugs, thiazolidinediones.

An extract of Syzygium aromaticum represses genes encoding hepatic gluconeogenic enzymes

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

Peroxisome proliferator-activated receptor gamma coactivator-1alpha, as a transcription amplifier, is not essential for basal and hormone-induced phosphoenolpyruvate carboxykinase gene expression

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the initial step in hepatic gluconeogenesis. In the fasted state, PEPCK gene expression is activated by glucagon (via cAMP) and glucocorticoids. Peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) plays an important role in energy homeostasis and is considered to be a key regulator of hepatic gluconeogenesis in response to fasting. It is not clear whether PGC-1alpha is obligatory for the activation of the transcription program of gluconeogenic genes, or whether it amplifies an existing process. H4IIE hepatoma cells were used to address this key point. These cells respond appropriately to all of the hormones involved in the regulation of gluconeogenic genes, yet they are devoid of PGC-1alpha. Also, these hormone responses occur in the absence of ongoing protein synthesis, so the necessary complement of transcription factors exists in untreated cells. However, exogenous expression of PGC-1alpha in these cells does enhance basal and hormone-induced expression of the PEPCK and glucose-6-phosphatase genes. Mutational analyses of the PEPCK gene promoter reveal that one element in the PEPCK gene promoter, glucocorticoid accessory factor 3, which binds chicken ovalbumin upstream promoter-transcription factor, is of particular importance. Taken together, these data suggest that, under chronic fasting conditions, i.e. when high levels of cAMP and glucocorticoids induce PGC-1alpha expression, this coactivator markedly amplifies PEPCK gene expression and gluconeogenesis.

A modest glucokinase overexpression in the liver promotes fed expression levels of glycolytic and lipogenic enzyme genes in the fasted state without altering SREBP-1c expression

Hepatic genes crucial for carbohydrate and lipid homeostasis are regulated by insulin and glucose metabolism. However, the relative contributions of insulin and glucose to the regulation of metabolic gene expression are poorly defined in vivo. To address this issue, adenovirus-mediated hepatic overexpression of glucokinase was used to determine the effects of increased hepatic glucose metabolism on gene expression in fasted or ad libitum fed rats. In the fasted state, a 3 fold glucokinase overexpression was sufficient to mimic feeding-induced increases in pyruvate kinase and acetyl CoA carboxylase mRNA levels, demonstrating a primary role for glucose metabolism in the regulation of these genes in vivo. Conversely, glucokinase overexpression was unable to mimic feeding-induced alterations of fatty acid synthase, glucose-6-phosphate dehydrogenase, carnitine palmitoyl transferase I or PEPCK mRNAs, indicating insulin as the primary regulator of these genes. Interestingly, glucose-6-phosphatase mRNA was increased by glucokinase overexpression in both the fasted and fed states, providing evidence, under these conditions, for the dominance of glucose over insulin signaling for this gene in vivo. Importantly, glucokinase overexpression did not alter sterol regulatory element binding protein 1-c mRNA levels in vivo and glucose signaling did not alter the expression of this gene in primary hepatocytes. We conclude that a modest hepatic overexpression of glucokinase is sufficient to alter expression of metabolic genes without changing the expression of SREBP-1c.

Effect of hyperosmotic shock on phosphoenolpyruvate carboxykinase gene expression and gluconeogenic activity in the crab muscle

Chasmagnathus granulata phosphoenolpyruvate carboxykinase (PEPCK) cDNA from jaw muscle was cloned and sequenced, showing a specific domain to bind phosphoenolpyruvate in addition to the kinase-1 and kinase-2 motifs to bind guanosine triphosphate (GTP) and Mg(2+), respectively, specific for all PEPCKs. In the kinase-1 motifs the GK was changed to RK. The first 19 amino acids of the putative enzyme contain hydrophobic amino acids and hydroxylated residues specific to a mitochondrial type signal. The PEPCK is expressed in hepatopancreas, muscles, nervous system, heart, and gills. Hyperosmotic stress for 24 h increased the PEPCK mRNA level, gluconeogenic and PEPCK activities in muscle.

The glucocorticoid receptor and the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor II interact with and mutually affect each other's transcriptional activities: implications for intermediary metabolism

Glucocorticoids exert their metabolic effect via their intracellular receptor, the glucocorticoid receptor (GR). In a yeast two-hybrid screening, we found the chicken ovalbumin upstream promoter transcription factor II (COUP-TFII), an orphan nuclear receptor that plays important roles in glucose, cholesterol, and xenobiotic metabolism, as a partner of GR. In an in vitro glutathione-S-transferase pull-down assay, COUP-TFII interacted via its DNA-binding domain with the hinge regions of both GRalpha and its splicing variant GRbeta, whereas COUP-TFII formed a complex with GRalpha, but not with GRbeta, in an in vivo chromatin immunoprecipitation and a regular immunoprecipitation assay. Accordingly, GRalpha, but not GRbeta, enhanced COUP-TFII-induced transactivation of the simple COUP-TFII-responsive 7alpha-hydroxylase promoter through the transcriptional activity of its activation function-1 domain, whereas COUP-TFII repressed GRalpha-induced transactivation of the glucocorticoid-responsive promoter by attracting the silencing mediator for retinoid and thyroid hormone receptors. Importantly, mutual protein-protein interaction of GRalpha and COUP-TFII was necessary for glucocorticoid-induced enhancement of the promoter activity and the endogenous mRNA expression of the COUP-TFII-responsive phosphoenolpyruvate carboxykinase, the rate-limiting enzyme of hepatic gluconeogenesis. We suggest that COUP-TFII may participate in some of the metabolic effects of glucocorticoids through direct interactions with GRalpha. These interactions influence the transcription of both COUP-TFII- and GRalpha-responsive target genes, seem to be promoter specific, and can be in either a positive or negative direction.

SREBP-1 interacts with hepatocyte nuclear factor-4 alpha and interferes with PGC-1 recruitment to suppress hepatic gluconeogenic genes

The hepatocyte nuclear factor-4alpha (HNF-4alpha)/PGC-1 pathway plays a crucial role in the transcriptional regulation of hepatic gluconeogenic enzymes such as phosphoenolpyruvate carboxykinase (PEPCK) and Glc-6-Pase, genes that are activated at fasting and suppressed in a fed state. SREBP-1c dominates the nutritional regulation of lipogenic genes inverse to gluconeogenesis. Here we show the mechanism by which SREBP-1 suppresses expression of gluconeogenic genes. A series of luciferase reporter assays demonstrated that SREBP-1a and -1c effectively inhibited the PEPCK promoter activity that was induced by HNF-4alpha. The HNF-4alpha-binding site in the glucocorticoid-response unit was responsible for the SREBP-1 inhibition, although SREBP-1 did not bind to the PEPCK promoter as demonstrated by electrophoretic mobility shift assays. The inhibitory effect was more potent in the isoform of SREBP-1a than SREBP-1c and was eliminated by deletion of the amino-terminal transactivation domain of SREBP-1. Coimmunoprecipitation experiments demonstrated that these two transcription factors directly interact through the transactivation domain of SREBP-1 and the ligand binding/AF2 domains of HNF-4alpha. Estimation of coactivator recruitment using HNF-4alpha-Gal4DBD fusion assay showed that SREBP-1 competitively inhibited PGC-1 recruitment, a requirement for HNF-4alpha activation. Consistent with these results, hepatic PEPCK and Glc-6-Pase mRNA levels are suppressed by overexpression of SREBP-1a and -1c in the transgenic mice. Our data indicate that SREBP-1 has a novel role as negative regulator of gluconeogenic genes through a cross-talk with HNF-4alpha interference with PGC-1 recruitment.

Severe diabetes, age-dependent loss of adipose tissue, and mild growth deficiency in mice lacking Akt2/PKB beta

The serine/threonine kinase Akt/PKB plays key roles in the regulation of cell growth, survival, and metabolism. It remains unclear, however, whether the functions of individual Akt/PKB isoforms are distinct. To investigate the function of Akt2/PKBbeta, mice lacking this isoform were generated. Both male and female Akt2/PKBbeta-null mice exhibit mild growth deficiency and an age-dependent loss of adipose tissue or lipoatrophy, with all observed adipose depots dramatically reduced by 22 weeks of age. Akt2/PKBbeta-deficient mice are insulin resistant with elevated plasma triglycerides. In addition, Akt2/PKBbeta-deficient mice exhibit fed and fasting hyperglycemia, hyperinsulinemia, glucose intolerance, and impaired muscle glucose uptake. In males, insulin resistance progresses to a severe form of diabetes accompanied by pancreatic beta cell failure. In contrast, female Akt2/PKBbeta-deficient mice remain mildly hyperglycemic and hyperinsulinemic until at least one year of age. Thus, Akt2/PKBbeta-deficient mice exhibit growth deficiency similar to that reported previously for mice lacking Akt1/PKBalpha, indicating that both Akt2/PKBbeta and Akt1/PKBalpha participate in the regulation of growth. The marked hyperglycemia and loss of pancreatic beta cells and adipose tissue in Akt2/PKBbeta-deficient mice suggest that Akt2/PKBbeta plays critical roles in glucose metabolism and the development or maintenance of proper adipose tissue and islet mass for which other Akt/PKB isoforms are unable to fully compensate.

Thiazolidinediones block fatty acid release by inducing glyceroneogenesis in fat cells

Thiazolidinediones are used to treat type 2 diabetes mellitus because they decrease plasma glucose, insulin, triglyceride, and fatty acid levels. Thiazolidinediones are agonists for peroxisome proliferator-activated receptor gamma, a nuclear receptor that is highly expressed in fat tissue. We identify glyceroneogenesis as a target of thiazolidinediones in cultured adipocytes and fat tissues of Wistar rats. The activation of glyceroneogenesis by thiazolidinediones occurs mainly in visceral fat, the same fat depot that is specifically implicated in the progression of obesity to type 2 diabetes. The increase in glyceroneogenesis is a result of the induction of its key enzyme, phosphoenolpyruvate carboxykinase, whose gene expression is peroxisome proliferator-activated receptor gamma-dependent in adipocytes. The main role of this metabolic pathway is to allow the re-esterification of fatty acids via a futile cycle in adipocytes, thus lowering fatty acid release into the plasma. The importance of such a fatty acid re-esterification process in the control of lipid homeostasis is highlighted by the existence of a second thiazolidinedione-induced pathway involving glycerol kinase. We show that glyceroneogenesis accounts for at least 75% of the whole thiazolidinedione effect. Because elevated plasma fatty acids promote insulin resistance, these results suggest that the glyceroneogenesis-dependent fatty acid-lowering effect of thiazolidinediones could be an essential aspect of the antidiabetic action of these drugs.

Gluconeogenic enzyme gene expression is decreased by dietary carbohydrates in common carp (Cyprinus carpio) and gilthead seabream (Sparus aurata)

Our objective is to understand the low metabolic utilization of dietary carbohydrates in fish. We compared the regulation of gluconeogenic enzymes at a molecular level in two fish species, the common carp (Cyprinus carpio) and gilthead seabream (Sparus aurata), known to be relatively tolerant to dietary carbohydrates. After cloning of partial cDNA sequences for three key gluconeogenic enzymes (glucose-6-phosphatase (G6Pase), fructose biphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK) in the two species, we analyzed gene expressions of these enzymes 6 and 24 h after feeding with (20%) or without carbohydrates. Our data show that there is at least one gluconeogenic enzyme strongly regulated (decreased expression after feeding) in the two fish species, i.e. the PEPCK for common carp and G6Pase/FBPase for gilthead seabream. In these fish species, the regulation seems to be similar to the mammals at least at the molecular level.

Phosphoenolpyruvate carboxykinase is induced in growth-arrested hepatoma cells

Phosphoenolpyruvate carboxykinase (PEPCK) mRNA is elevated in H4IIEC3 rat hepatoma cells cultured at high density, suggesting that PEPCK expression and growth arrest may be coordinately regulated. Induction of growth arrest either by contact inhibition (high culture density) or by serum deprivation correlated with significant increases in PEPCK protein and its mRNA. The observation that PEPCK mRNA was induced by contact inhibition in the presence of serum indicates that the effect of high density is independent of insulin or any other serum component. The magnitudes of the changes in PEPCK expression during growth arrest were greatly enhanced in KRC-7 cells, an H4IIEC3 subclone that is much more sensitive to growth arrest than its parental cell line. Restimulation of proliferation in growth-arrested KRC-7 cells, either by addition of serum or insulin to serum-deprived cells or by replating contact-inhibited cells at low density, caused a rapid decrease in PEPCK expression. However, PEPCK mRNA is not always reduced in proliferating cells since treatment of serum-starved cells with epidermal growth factor stimulated entry into the cell cycle but did not affect PEPCK mRNA levels. Finally, dexamethasone induction of PEPCK mRNA was blunted in cells cultured at high density but was unaffected by the presence or absence of serum. Collectively, these data suggest the possibility of cross-talk between the control of PEPCK expression and growth arrest in KRC-7 cells.

Archaebacterial relationships of the phosphoenolpyruvate carboxykinase gene reveal mosaicism of Giardia intestinalis core metabolism

A gene encoding a putative GTP-specific phosphoenolpyruvate carboxykinase has been cloned and sequenced from the type I amitochondriate protist Giardia intestinalis. The deduced amino acid sequence is related most closely to homologs from hyperthermophilic archaebacteria and only more distantly to homologs from Eubacteria and Metazoa. Most enzymes of Giardia core metabolism, however, are related more closely to eubacterial and metazoan homologs. An archaebacterial relationship has been noted previously for the unusual acetyl-CoA synthetase (ADP-forming) of this organism. The results suggest that phosphoenolpyruvate carboxykinase and acetyl-CoA synthetase have been acquired from different sources than most enzymes of Giardia core metabolism.

Calorie restriction enhances the expression of key metabolic enzymes associated with protein renewal during aging

Our studies show that dietary caloric restriction (CR) alters the expression of key metabolic enzymes in a manner consistent with an increased rate of extrahepatic protein turnover and renewal during aging. Of the key hepatic gluconeogenic enzyme genes affected by CR, glucose 6-phosphatase mRNA increased 1.7- and 2.3-fold in young and old CR mice. Phosphoenolpyruvate carboxykinase mRNA increased 2-fold in young mice, and its mRNA and activity increased 2.5- and 1.7-fold in old mice. These changes indicate that CR enhances the enzymatic capacity for gluconeogenesis. The carbon required for gluconeogenesis appears to be generated from peripheral protein turnover. Muscle glutamine synthetase mRNA increased 1.3- and 2.1-fold in young and old CR mice, suggesting increased disposal of nitrogen and carbon derived from protein catabolism for energy. mRNA for the key liver nitrogen disposal enzymes glutaminase, carbamyl phosphate synthase I, and tyrosine aminotransferase were increased by 2.4-, 1.8-, and 1.8-fold in CR mice. Consistent with increased hepatic nitrogen disposal, hepatic glutamine synthetase mRNA and activity were each decreased about 40% in CR mice. Together, these and our other published data suggest that CR enhances and maintains protein turnover, and thus protein renewal, into old age. These effects are likely to resist the well-documented decline in whole body protein renewal with age. Enhanced renewal may reduce the level of damaged and toxic proteins that accumulate during aging, contributing to the extension of life span by CR.

AMP-activated protein kinase counteracted the inhibitory effect of glucose on the phosphoenolpyruvate carboxykinase gene expression in rat hepatocytes

The effect of AMP-activated protein kinase (AMPK) in the regulation of the phosphoenolpyruvate carboxykinase (PEPCK) gene expression was studied in isolated rat hepatocytes. Activation of AMPK by AICAR counteracted the inhibitory effect of glucose on the PEPCK gene expression, both at the mRNA and the transcriptional levels. It is proposed that a target for AMPK is involved in the inhibitory effect of glucose on PEPCK gene transcription.

Antihyperglycemic action of isoferulic acid in streptozotocin-induced diabetic rats

Wistar rats with streptozotocin-induced diabetes (STZ-diabetic rats), which is similar to human insulin-dependent diabetic mellitus (IDDM), were employed to investigate the antihyperglycemic action of isoferulic acid. A single intravenous injection of isoferulic acid decreased the plasma glucose in a dose-dependent manner in the STZ-diabetic rats. Repeated intravenous administration of STZ-diabetic rats with isoferulic acid (5.0 mg kg(-1)) also resulted in the lowering of plasma glucose after one day. Stimulatory effects of isoferulic acid on the glucose uptake and glycogen synthesis in soleus muscles isolated from STZ-diabetic rats were also obtained indicating an increase of glucose utilization following isoferulic acid treatment which was not dependent on insulin. The mRNA level of glucose transporter subtype 4 form (GLUT4) in soleus muscle was raised by isoferulic acid after repeated treatment for 1 day in STZ-diabetic rats. Similar repeated treatment with isoferulic acid reversed the elevated mRNA level of phosphoenolpyruvate carboxykinase (PEPCK) in liver of STZ-diabetic rats to the normal level. However, expression of GLUT4 and PEPCK genes in nondiabetic rats were not influenced by similar treatment with isoferulic acid. These results suggest that isoferulic acid can inhibit hepatic gluconeogenesis and/or increase the glucose utilization in peripheral tissue to lower plasma glucose in diabetic rats lacking insulin.

The inhibitory effect of glucose on phosphoenolpyruvate carboxykinase gene expression in cultured hepatocytes is transcriptional and requires glucose metabolism

Phosphoenolpyruvate carboxykinase (PEPCK) is the rate-limiting enzyme of gluconeogenesis in the liver. PEPCK gene expression is controlled at the transcriptional level and is mainly regulated by hormones that are involved in glucose homeostasis. In this study, we have investigated the role of glucose on PEPCK gene expression in cultured hepatocytes. We demonstrate that glucose counteracts the stimulatory effect of glucocorticoids and cAMP on PEPCK expression. Glucose must be metabolized through glucokinase to have its inhibitory effect. The effect of glucose is mainly transcriptional and the region responsible for glucose inhibition is localized in the first 490 bp of the promoter.

Insulin replacement therapy in diabetic rats using an osmotic pump normalizes expression of enzymes key to hepatic carbohydrate metabolism

Intensively treating type I diabetics with continuous subcutaneous insulin infusions or multiple daily insulin injections to normalize mean blood glucose concentrations significantly reduces the onset of secondary diabetic complications when compared to conventionally treated diabetics. Our studies focused on characterizing hepatic enzyme expression in intensively and conventionally treated diabetic rats. Alloxan-induced diabetic rats were conventionally treated with insulin injected twice daily or intensively treated with similar daily dosages of insulin administered via a surgically implanted osmotic pump. Our results demonstrate a significant difference in hepatic enzyme expression when these treatment regimes are compared. In conventionally treated diabetic rats, phosphoenolpyruvate carboxykinase (PEPCK) protein and mRNA levels remained slightly elevated when compared to normal animals, glycogen phosphorylase (GP) protein levels were still slightly decreased, and glycogen synthase (GS) protein and mRNA levels remained at the elevated levels observed in untreated diabetics. In contrast, the protein and mRNA levels of all three enzymes were normalized in the insulin pump-treated animals. These results suggest that intensive insulin therapy improves glycemia directly by normalizing hepatic gene expression while conventional insulin therapy normalizes plasma glucose concentrations indirectly.

Dominant negative forms of Akt (protein kinase B) and atypical protein kinase Clambda do not prevent insulin inhibition of phosphoenolpyruvate carboxykinase gene transcription

Transcriptional regulation of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting enzyme in hepatic gluconeogenesis, by insulin was investigated with the use of adenovirus vectors encoding various mutant signaling proteins. Insulin inhibited transcription induced by dexamethasone and cAMP of a chloramphenicol acetyltransferase (CAT) reporter gene fused with the PEPCK promoter sequence in HL1C cells stably transfected with this construct. A dominant negative mutant of phosphoinositide (PI) 3-kinase blocked insulin inhibition of transcription of the PEPCK-CAT fusion gene, whereas a constitutively active mutant of PI 3-kinase mimicked the effect of insulin. Although a constitutively active mutant of Akt (protein kinase B) inhibited PEPCK-CAT gene transcription induced by dexamethasone and cAMP, a mutant Akt (Akt-AA) in which the phosphorylation sites targeted by insulin are replaced by alanine did not affect the ability of insulin to inhibit transcription of the fusion gene. Akt-AA almost completely inhibited insulin-induced activation of both endogenous and recombinant Akt in HL1C cells. Furthermore, neither a kinase-defective mutant protein kinase Clambda (PKClambda), which blocked insulin-induced activation of endogenous PKClambda, nor a dominant negative mutant of the small GTPase Rac prevented inhibition of PEPCK-CAT gene transcription by insulin. These data suggest that phosphoinositide 3-kinase is important for insulin-induced inhibition of PEPCK gene transcription and that a downstream effector of phosphoinositide 3-kinase distinct from Akt, PKClambda, and Rac may exist for mediating the effect of insulin.

Characterization of a kidney-specific pattern of chromatin structure in the rat phosphoenolpyruvate carboxykinase gene

The kidney-specific chromatin structure of the phosphoenolpyruvate carboxykinase (PEPCK) gene was examined and compared to that of the liver. Kidney nuclear extracts were found to lack a liver-enriched factor, pepA, that binds to HSS A, a distal enhancer of the PEPCK gene that may be involved in opening the chromatin domain of the PEPCK gene in the liver. To begin the characterization of the kidney-specific chromatin structure of the PEPCK gene, nuclease hypersensitive sites (HSS) were mapped by indirect end-labeling analysis in proximal tubules from control rats, proximal tubules from acidotic rats which express induced levels of PEPCK, and NRK52E cells, a rat kidney epithelial cell line which does not express the PEPCK gene. A subset of HSS, at -400/+1 over the proximal promoter and at +1900 within the coding region, correlate with kidney-specific PEPCK expression. Two other HSS, at -3.1 kb and +6.2 kb, are detected in kidney cells regardless of PEPCK expression. The HSS at -4800, -1240, and +4650, previously identified in PEPCK-expressing liver cells, were not observed in the kidney. As in the liver, the pattern of hypersensitivity in the kidney does not change by altering the rate of transcription.

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

A 72 kilobase pair DNA fragment that contains the mouse phosphoenolpyruvate carboxykinase (PEPCK) gene locus, pck1, was isolated from a genomic bacterial artificial chromosome library. The region from approximately -5.5 to +6.6 kilobase pairs relative to the pck1 transcription start site was sequenced and exhibits a high degree of homology to the rat and human genes. Additionally, the chromatin structure of the PEPCK gene in mouse liver resembles that seen in rat. Backcross panel analysis of a microsatellite sequence confirms that the gene is located on chromosome 2. Hypersensitive site analysis was performed on nuclei isolated from the adipocyte cell line 3T3-F442A in the preadipose and adipose states. Several hypersensitive sites are present in the undifferentiated 3T3-F442A cells, before PEPCK mRNA is detected. The same sites are present after differentiation, however, the sensitivity of mHS 3 increases relative to the others. We conclude that the chromatin is open in 3T3-F442A cells and that factors are able to bind in the undifferentiated state but that something else is required for transcription.

The repression of hormone-activated PEPCK gene expression by glucose is insulin-independent but requires glucose metabolism

Phosphoenolpyruvate carboxykinase (PEPCK) is a rate-controlling enzyme in hepatic gluconeogenesis, and it therefore plays a central role in glucose homeostasis. The rate of transcription of the PEPCK gene is increased by glucagon (via cAMP) and glucocorticoids and is inhibited by insulin. Under certain circumstances glucose also decreases PEPCK gene expression, but the mechanism of this effect is poorly understood. The glucose-mediated stimulation of a number of glycolytic and lipogenic genes requires the expression of glucokinase (GK) and increased glucose metabolism. HL1C rat hepatoma cells are a stably transfected line of H4IIE rat hepatoma cells that express a PEPCK promoter-chloramphenicol acetyltransferase fusion gene that is regulated in the same manner as the endogenous PEPCK gene. These cells do not express GK and do not normally exhibit a response of either the endogenous PEPCK gene, or of the trans-gene, to glucose. A recombinant adenovirus that directs the expression of glucokinase (AdCMV-GK) was used to increase glucose metabolism in HL1C cells to test whether increased glucose flux is also required for the repression of PEPCK gene expression. In AdCMV-GK-treated cells glucose strongly inhibits hormone-activated transcription of the endogenous PEPCK gene and of the expressed fusion gene. The glucose effect on PEPCK gene promoter activity is blocked by 5 mM mannoheptulose, a specific inhibitor of GK activity. The glucose analog, 2-deoxyglucose mimics the glucose response, but this effect does not require GK expression. 3-O-methylglucose is ineffective. Glucose exerts its effect on the PEPCK gene within 4 h, at physiologic concentrations, and with an EC50 of 6.5 mM, which approximates the Km of glucokinase. The effects of glucose and insulin on PEPCK gene expression are additive, but only at suboptimal concentrations of both agents. The results of these studies demonstrate that, by inhibiting PEPCK gene transcription, glucose participates in a feedback control loop that governs its production from gluconeogenesis.

Effect of starvation on gene expression of regulatory enzymes of glycolysis/gluconeogenesis in genetically obese (fa/fa) Zucker rats

OBJECTIVE

To study the mechanism that controls fructose-2,6-bisphosphate (Fru-2,6-P2) accumulation, as well as the mRNAs levels of the glycolytic/gluconeogenic regulatory enzymes in the livers of fed and starved lean (fa/-) and obese (fa/fa) Zucker rats.

DESIGN

Rats were fed a standard chow or deprived of food for 24 h.

SUBJECTS

Male lean (fa/-) and genetically obese (fa/fa) rats (nine weeks old).

MEASUREMENTS

Fru-2,6-P2 concentration, 6-phosphofructo-2-kinase (PFK-2), glucokinase (GK), pyruvate kinase (PK) activities and the mRNA levels of GK, PFK-2, L-type pyruvate kinase, fructose-1,6-bisphosphatase (FBPase-1) and phosphoenolpyruvate carboxykinase (PEPCK) were analyzed.

RESULTS

PFK-2/FBPase-2 mRNA decreased during starvation in both fa/- and fa/fa animals. Although PFK-2/FBPase-2 mRNA levels were similar in fed lean and obese rats, PFK-2 concentration and activity were higher in fed obese than in fed lean animals, which might explain the high concentration of Fru-2,6-P2 observed in obese animals. During starvation, PFK-2 protein concentration decreased, correlating with the enzymatic activity and Fru-2,6-P2 levels. The activities of GK and L-pyruvate kinase (L-PK) also increased in fed obese (fa/fa) rats compared with fed lean (fa/-) animals, but decreased during starvation. The mRNA levels of glycolytic enzymes in fed obese rats were similar (PFK-2) or higher than (GK, L-PK) in fed lean animals. During starvation, they decreased in lean and obese rats with one important exception, GK mRNA remained high in obese animals. The mRNA of gluconeogenic enzymes remained constant (FBPase-1) or increased (PEPCK) during fasting.

CONCLUSION

The changes observed might be explained by the hyperinsulinaemia observed in the liver of obese rats, which might lead to the stimulation of glycolysis and lipogenesis.

Ontogeny and cellular localization of the pyruvate recycling system in rat brain

The ontogeny of the cerebral pyruvate recycling pathway and the cellular localization of associated enzymes, malic enzyme (ME) and phosphoenolpyruvate carboxykinase (PEPCK), have been investigated using a combination of 13C NMR spectroscopy, enzymatic analysis, and molecular biology approaches. Activity of the pathway, using [1,2-(13)C2]acetate as a substrate, was detected by 13C NMR in brain extracts 3 weeks after birth, increasing progressively up to the third month of age. In whole-brain homogenates, ME activity increased to adult levels with the same time course as the recycling pathway. PEPCK activity was low during the first 2 weeks of life and decreased further toward adulthood. ME and PEPCK activity were found in primary cultures of astrocytes and in synaptosomal fractions of adult brain. Primary cultures of cortical neurons showed PEPCK activity but no detectable ME activity. The cytosolic ME gene was expressed in primary cultures of neurons and in astrocytes as well as in the neonatal and adult brain. The PEPCK gene was expressed both in primary cultures of cortical neurons and in astrocytes, but the level of its expression in the neonatal and adult brain was undetectable.

Repression of phosphoenolpyruvate carboxykinase gene activity by insulin is blocked by 3-aminobenzamide but not by PD128763, a selective inhibitor of poly(ADP-ribose) polymerase

Expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene is induced by 3-aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase. Synthesis of PEPCK mRNA is repressed by insulin, but remains detectable in H4IIE hepatoma cells exposed simultaneously to both 3-aminobenzamide and insulin. This capability of 3-aminobenzamide to block the inhibitory actions of insulin suggests that ADP-ribosylation is required for the regulation of PEPCK gene expression by insulin. Furthermore, neither changes in chromatin condensation nor cell growth status were linked to these events. The inability of 3,4-dihydro-5-methylisoquinolinone (PD128763), a selective inhibitor of poly(ADP-ribose) polymerase, to impede insulin-dependent repression of PEPCK gene expression, however, indicated that 3-aminobenzamide does not operate by inhibiting poly(ADP-ribosyl)ation. The potential involvement of mono(ADP-ribosyl)ation, a process that is also inhibited by 3-aminobenzamide, in the regulation of PEPCK gene activity was then evaluated. Analysis of poly(ADP-ribose) polymerase activity and poly(ADP-ribosyl)ation confirmed that there were no significant changes in response to insulin, while microsomal mono(ADP-ribosyl)transferase activity was elevated approximately fourfold. An increase in protein hydroxylamine-sensitive mono(ADP-ribosyl)ation was observed following insulin treatment. The sensitivity of the mono(ADP-ribosyl)transferase activity to 3-aminobenzamide but not PD128763 makes it plausible that mono(ADP-ribosyl)ation rather than poly(ADP-ribosyl)ation contributes to the regulation of PEPCK gene expression.

Cell volume regulates liver phosphoenolpyruvate carboxykinase and fructose-1,6-bisphosphatase genes

Hypertonic-induced cell shrinkage increases glucose release in H-4-II-E rat hepatoma cells. This is paralleled by a concomitant increase in the mRNA levels of the rate-limiting enzymes of the pathway of gluconeogenesis, phosphoenolpyruvate carboxykinase (PCK) and fructose-1,6-bisphosphatase (FBP), of seven- and fivefold, respectively. In contrast, hypotonic-induced swelling of the cells results in a transient decrease in PCK and FBP mRNAs to 15% and 39% of control levels. The antagonistic effects of hyper- and hypotonicity mimic the counteracting effects of adenosine 3',5'-cyclic monophosphate (cAMP) and insulin on PCK and FBP mRNA levels. The hypertonic-induced increase in mRNA levels is due to an enhanced transcriptional rate, whereas the decrease in mRNAs caused by hypotonicity results from a decrease in transcription as well as mRNA stability. The inductive effect of hypertonicity does not require ongoing protein synthesis and acts independently of the cAMP-dependent protein kinase and protein kinase C pathways. These results suggest that cell volume changes in liver cells may play an important role in regulating hepatic glucose metabolism by altered gene expression.

Protein synthesis is involved in the modulation of the level of the phosphoenolpyruvate carboxykinase mRNA by changes in cell volume in isolated rat hepatocytes

The mechanism of action of hydration state was studied on phosphoenolpyruvate carboxykinase (PCK) gene expression in isolated rat hepatocytes. Hypoosmolarity decreased the level of the PCK mRNA after a lag period of about 60 min. The decreasing effect of hypoosmolarity was totally blocked by inhibitors of both protein synthesis and gene transcription. Moreover, hypoosmolarity specifically increased the synthesis of a 45000 Mr protein, which decreased in the presence of inhibitors of transcription. A close relationship between the synthesis of the 45000 Mr protein and the decrease in the PCK mRNA level was observed, suggesting that this protein might potentially be involved in the regulation of the level of the PCK mRNA by cell swelling.

Hepatic fibrosis, glomerulosclerosis, and a lipodystrophy-like syndrome in PEPCK-TGF-beta1 transgenic mice

Transgenic mice overexpressing a constitutively active human TGF-beta1 under control of the rat phosphoenolpyruvate carboxykinase regulatory sequences developed fibrosis of the liver, kidney, and adipose tissue, and exhibited a severe reduction in body fat. Expression of the transgene in hepatocytes resulted in increased collagen deposition, altered lobular organization, increased hepatocyte turnover, and in extreme cases, hemorrhage and thrombosis. Renal expression of the transgene was localized to the proximal tubule epithelium, and was associated with tubulointerstitial fibrosis, characterized by excessive collagen deposition and increased fibronectin and plasminogen activator inhibitor-1 immunoreactivity. Pronounced glomerulosclerosis was evident, and hydronephrosis developed with low penetrance. Expression of TGF-beta1 in white and brown adipose tissue resulted in a lipodystrophy-like syndrome. All white fat depots and brown fat pads were severely reduced in size, and exhibited prominent fibroplasia. This reduction in WAT was due to impaired adipose accretion. Introduction of the transgene into the ob/ob background suppressed the obesity characteristic of this mutation; however, transgenic mutant mice developed severe hepato- and splenomegaly. These studies strengthen the link between TGF-beta1 expression and fibrotic disease, and demonstrate the potency of TGF-beta1 in modulating mesenchymal cell differentiation in vivo.

Expression of PEP carboxylase from Escherichia coli complements the phenotypic effects of pyruvate carboxylase mutations in Saccharomyces cerevisiae

We investigated the effects of the expression of the Escherichia coli ppc gene encoding PEP carboxylase in Saccharomyces cerevisiae mutants devoid of pyruvate carboxylase. Functional expression of the ppc gene restored the ability of the yeast mutants to grow in glucose-ammonium medium. Growth yield in this medium was the same in the transformed yeast than in the wild type although the growth rate of the transformed yeast was slower. Growth in pyruvate was slowed down in the transformed strain, likely due to a futile cycle produced by the simultaneous action of PEP carboxykinase and PEP carboxylase.

Early effects of streptozotocin-induced diabetes on insulin-like growth factor-I in the kidneys of growth hormone-transgenic and growth hormone-deficient dwarf mice

BACKGROUND

The renal growth and hyperfiltration observed in humans and animals with early diabetes might be dependent on growth hormone (GH) and insulin-like growth factor (IGF)-I. The aim of this study was to investigate the early changes in kidney IGF-I in experimental diabetes in mice transgenic for bovine GH and in genetically GH-deficient Ames dwarf mice.

METHODS

At 2, 4 and 8 days after a single intraperitoneal injection with streptozotocin, animals were weighted, bled and killed; plasma was analyzed for glucose and IGF-I. IGF-I levels were determined in tissue from snap-frozen kidney and liver.

RESULTS

Body weight decreased significantly after the induction of diabetes. Kidney weight increased significantly in GH-transgenic, but not in normal or dwarf mice. Plasma IGF-I was significantly decreased at day 2 in GH-transgenic and normal mice, while liver IGF-I was increased at day 4 in all mice. Kidney IGF-I increased significantly in normal and GH-transgenic mice and was increased more than 3-fold at day 4 in GH-transgenic mice. In dwarf mice, no kidney IGF-I was detectable.

CONCLUSION

The diabetes-induced increase in renal IGF-I is dependent on the presence of GH. GH deficiency may protect diabetic animals from early changes in the kidney.

Vitamin A regulates genes involved in hepatic gluconeogenesis in mice: phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

We examined the effects of vitamin A deficiency and all-trans retinoic acid (RA) supplementation on regulation of three important genes in hepatic gluconeogenesis: the genes for phosphoenolpyruvate carboxykinase (PEPCK), fructose-1,6-bisphosphatase (Fru-1,6-P2ase) and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6-PF-2-K/Fru-2,6-P2ase). Mice were made vitamin A deficient in the second generation by initiating a vitamin A-deficient diet on d 10 of gestation. At 7 wk of age, vitamin A-deficient mice were treated with all-trans RA or vehicle alone and killed for RNA analysis. In liver, vitamin A deficiency resulted in PEPCK mRNA levels that were 74% lower and 6-PF-2-K/Fru-2,6-P2ase mRNA levels that were 42% lower than the respective mRNA measured in control mice. The Fru-1,6-P2ase mRNA abundance was not affected by vitamin A deficiency. The decrease in hepatic PEPCK and 6-PF-2-K/Fru-2,6-P2ase mRNA levels was reversed by treatment with all-trans RA within 3 h of administration. In mice fed the control diet, food deprivation for 15 h resulted in PEPCK mRNA levels that were 3.5-fold higher, Fru-1,6-P2ase mRNA levels that were 2-fold higher, and 6-PF-2-K/Fru-2,6-P2ase mRNA levels that were 3.4-fold higher than in fed mice. Vitamin A-deficient mice did not respond to food deprivation with induced PEPCK mRNA levels, whereas 6-PF-2-K/Fru-2,6-P2ase and Fru-1,6-P2ase mRNA levels were induced. The pattern of 6-PF-2-K/Fru-2,6-P2ase mRNA abundance with vitamin A deficiency and food deprivation was complex and different from that for either PEPCK or Fru-1,6-P2ase transcripts. The cAMP-responsiveness of the PEPCK gene in vitamin A-deficient mice was tested. Vitamin A deficiency caused a significant reduction in cAMP stimulation of PEPCK mRNA levels in liver. These results in the whole animal indicate that vitamin A regulation of the hepatic PEPCK gene is physiologically important; without adequate vitamin A nutriture, stimulation of the PEPCK gene by food deprivation or cAMP treatment is inhibited in the liver.

Hepatic fatty acid synthase gene transcription is induced by a dietary copper deficiency

A dietary copper (Cu) deficiency is associated with a twofold increase in hepatic fatty acid biosynthesis. We hypothesized that the induction of hepatic lipogenesis associated with a dietary Cu deficiency reflected an enhanced expression of genes encoding lipogenic enzymes, i.e., fatty acid synthase (FAS). Male weanling rats were pair-meal fed for 42 days a high-sucrose diet that was Cu deficient (CuD; 0.7 microgram Cu/g) or Cu adequate (CuA; 5.0 micrograms Cu/g). The CuD diet increased FAS enzymatic activity twofold (P < 0.05). This rise in enzymatic activity was accompanied by a threefold increase in FAS mRNA and a 2.5-fold increase in FAS gene transcription (P < 0.05). Neither the mRNA abundance nor the rate of gene transcription for phosphoenolpyruvate carboxykinase or beta-actin was affected by the CuD diet. The induction of FAS gene transcription was associated with a 65-85% increase in hepatic reduced glutathione (GSH; P < 0.05). When hepatic GSH synthesis was suppressed by treating CuD rats with L-buthionine sulfoximine, the induction of FAS expression was completely prevented. Similarly, feeding N-acetylcysteine to CuA rats increased hepatic GSH levels 2.5-fold, and this was accompanied by a significant induction in FAS expression. These data indicate that the increase in hepatic lipogenesis associated with a Cu deficiency reflects an induction in hepatic lipogenic gene transcription (i.e., FAS) and that the rate of gene transcription may be dependent on hepatic thiol redox.

Messenger RNA deadenylylation precedes decapping in mammalian cells

In yeast, the major mRNA degradation pathway is initiated by poly(A) tail shortening that triggers mRNA decapping. The mRNA is then degraded by 5'-to-3' exonucleolysis. In mammalian cells, even though poly(A) tail shortening also precedes mRNA degradation, the degradation pathway has not been elucidated. We have used a reverse transcription-PCR approach that relies on mRNA circularization to measure the poly(A) tail length of four mammalian mRNAs. This approach allows for the simultaneous analysis of the 5' and 3' ends of the same mRNA molecule. For all four mRNAs analyzed, this strategy permitted us to demonstrate the existence of small amounts of decapped mRNA species which have a shorter poly(A) tail than their capped counterparts. Kinetic analysis of one of these mRNAs indicates that the decapped species with a short poly(A) tail are mRNA degradation products. Therefore, our results indicate that decapping is preceded by a shortening of the poly(A) tail in mammalian cells, as it is in yeast, suggesting that this mRNA degradation pathway is conserved throughout eukaryotic evolution.

A multicomponent insulin response sequence mediates a strong repression of mouse glucose-6-phosphatase gene transcription by insulin

Glucose-6-phosphatase (G6Pase) catalyzes the final step in the gluconeogenic and glycogenolytic pathways. The transcription of the gene encoding the catalytic subunit of G6Pase is stimulated by glucocorticoids, whereas insulin strongly inhibits both basal G6Pase gene transcription and the stimulatory effect of glucocorticoids. To identify the insulin response sequence (IRS) in the G6Pase promoter through which insulin mediates its action, we have analyzed the effect of insulin on the basal expression of mouse G6Pase-chloramphenicol acetyltransferase (CAT) fusion genes transiently expressed in hepatoma cells. Deletion of the G6Pase promoter sequence between -271 and -199 partially reduces the inhibitory effect of insulin, whereas deletion of additional sequence between -198 and -159 completely abolishes the insulin response. The presence of this multicomponent IRS may explain why insulin potently inhibits basal G6Pase-CAT expression. The G6Pase promoter region between -198 and -159 contains an IRS, since it can confer an inhibitory effect of insulin on the expression of a heterologous fusion gene. This region contains three copies of the T(G/A)TTTTG sequence, which is the core motif of the phosphoenolpyruvate carboxykinase (PEPCK) gene IRS. This suggests that a coordinate increase in both G6Pase and PEPCK gene transcription is likely to contribute to the increased hepatic glucose production characteristic of patients with non-insulin-dependent diabetes mellitus.

Oxidative and chemical stress mimic insulin by selectively inhibiting the expression of phosphoenolpyruvate carboxykinase in hepatoma cells

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the rate-limiting step in hepatic gluconeogenesis. Glucagon (via the second messenger cAMP), retinoic acid, and glucocorticoids stimulate transcription of the PEPCK gene, whereas insulin and phorbol esters have a dominant inhibitory effect. We now show that oxidative and chemical stress (hydrogen peroxide and sodium meta-arsenite, respectively) also produce a dominant inhibitory effect, both on the endogenous PEPCK gene and on a stably transfected PEPCK-chloramphenicol acetyl transferase (CAT) fusion gene. Wortmannin, an inhibitor of 1-phosphatidylinositol 3-kinase (PI 3-kinase), blocks the inhibition of glucocorticoid and cAMP-induced PEPCK gene transcription by insulin; however, it has no effect on the inhibition elicited by oxidative or chemical stress. Thus, the mechanism(s) used by hydrogen peroxide and sodium meta-arsenite to regulate PEPCK gene expression are PI 3-kinase independent. This suggests that these agents operate by a pathway distinct from that used by insulin or that the pathways converge at a point downstream of PI 3-kinase. The reactivating kinase (RK, also known as p38 mitogen activated protein kinase) is induced by insulin, hydrogen peroxide, or sodium meta-arsenite in hepatoma cells, and these effects are blocked by SB203580, a selective inhibitor of RK. However, SB203580 has no effect on the ability of any of these agents to regulate PEPCK-CAT fusion gene expression. Thus, although RK has a role in the regulation of lymphokine gene expression in monocytes, it is not required for the regulation of PEPCK expression by either insulin or oxidative and chemical stress in hepatoma cells.

Insulin-like growth factor I and insulin induce adipogenic-related gene expression in fetal brown adipocyte primary cultures

Fetal rat brown adipocytes show high-affinity binding sites for both insulin-like growth factor I (IGF-I) and insulin. Cell culture for 24 h in the presence of IGF-I or insulin, independently, up-regulated the mRNA expression of adipogenic-related genes, such as fatty acid synthase (FAS), glycerol-3-phosphate de-hydrogenase and insulin-regulated glucose transporter Glut4, and down-regulated the expression of phosphoenolpyruvate carboxykinase mRNA in a dose-dependent manner. Moreover, both IGF-I and insulin increased the FAS gene transcription rate at 2 h, producing a time-dependent accumulation of FAS mRNA. Furthermore IGF-I or insulin increased glucose uptake and lipid content throughout the 24 h culture period. Our results suggest that both IGF-I and insulin are major signals involved in initiating and/or maintaining the expression of adipogenic-related genes in fetal rat brown adipocytes.

Correction of diabetic alterations by glucokinase

Hyperglycemia is a common feature of diabetes mellitus. It results from a decrease in glucose utilization by the liver and peripheral tissues and an increase in hepatic glucose production. Glucose phosphorylation by glucokinase is an initial event in glucose metabolism by the liver. However, glucokinase gene expression is very low in diabetic animals. Transgenic mice expressing the P-enolpyruvate carboxykinase/glucokinase chimeric gene were generated to study whether the return of the expression of glucokinase in the liver of diabetic mice might prevent metabolic alterations. In contrast to nontransgenic mice treated with streptozotocin, mice with the transgene previously treated with streptozotocin showed high levels of both glucokinase mRNA and its enzyme activity in the liver, which were associated with an increase in intracellular levels of glucose 6-phosphate and glycogen. The liver of these mice also showed an increase in pyruvate kinase activity and lactate production. Furthermore, normalization of both the expression of genes involved in gluconeogenesis and ketogenesis in the liver and the production of glucose and ketone body by hepatocytes in primary culture were observed in streptozotocin-treated transgenic mice. Thus, glycolysis was induced while gluconeogenesis and ketogenesis were blocked in the liver of diabetic mice expressing glucokinase. This was associated with normalization of blood glucose, ketone bodies, triglycerides, and free fatty acids even in the absence of insulin. These results suggest that the expression of glucokinase during diabetes might be a new approach to the normalization of hyperglycemia.

Efficacy of anti-tumor necrosis factor polyclonal antibody on phosphoenolpyruvate carboxykinase expression in septic and endotoxemic rats

We studied the protective effect of anti-tumor necrosis factor-alpha (anti-TNF) polyclonal antibody on phosphoenolpyruvate carboxykinase (PEPCK) expression in lipopolysaccharide-induced endotoxemia and peritonitis sepsis induced by cecal incision. At 3 h after intraperitoneal lipopolysaccharide injection, levels of serum glucose, liver glycogen, and PEPCK expression were decreased and serum TNF was elevated. In contrast, 3 h after cecal incision, levels of serum glucose, serum TNF, and PEPCK expression were elevated. At 6 h after cecal incision (terminal sepsis), serum TNF remained elevated and levels of serum glucose, liver glycogen, and PEPCK expression were decreased. Circulating TNF was not detected in septic and endotoxemic rats pretreated with anti-TNF. Passive immunization with rat anti-TNF antibody restored PEPCK expression in early endotoxemia and sepsis (3 h), but not in terminal sepsis (6 h). Anti-TNF failed to reverse sepsis-induced hypoglycemia and hyperglycemia, suggesting that besides TNF, some other mediators are involved in glucose dyshomeostasis.

Developmental changes in the phospho(enol)pyruvate carboxykinase gene expression in small intestine and liver of suckling rats

The cytosolic enzyme phospho(enol)pyruvate carboxykinase (PEPCK) is markedly expressed in the intestinal mucosa of suckling rats. The expression is located in the small intestine, but there is no expression in stomach, colon, or cecum. The expression changes with age. The mRNA levels at birth are very low, increase after the first lactation, reach maximum levels between 3 and 9 days after birth, and then decrease smoothly. At weaning, when animals begin to feed on a solid chow diet, the expression falls to adult levels, which are hardly detectable. Mother's milk may influence the intestinal expression, since in rats weaned at Day 18, 3 days before normal weaning, the mRNA levels decreased dramatically. mRNA levels for PEPCK in liver present a rather different developmental pattern from that of intestine, remaining high at weaning and in adult rats. On the ninth day after birth, the mRNA levels are the same in intestine and liver.

Multiple signalling pathways trigger the exquisite sensitivity of yeast gluconeogenic mRNAs to glucose

The transcription of the yeast FBP1 and PCK1 genes, which encode the gluconeogenic enzymes fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase, is repressed by glucose. Here, we show that this repression is both very strong and exceptionally sensitive to glucose, being triggered by glucose at concentrations less than 0.005% (0.27 mM). This repression remains operative in yeast mutants carrying any one of the three hexose kinases, but is lost in a triple hxk1, hxk2, glk1 mutant. In addition, 2-deoxyglucose can trigger the repression, but 6-deoxyglucose cannot, suggesting that internalization and phosphorylation of the glucose is essential for repression to occur. While gluconeogenic gene transcription is subject to the Mig 1p-dependent pathway of glucose repression, the exquisite response to glucose is maintained in hxk2 and mig1 mutants, suggesting that this pathway is not essential for the response. The response can also be triggered by the addition of exogenous cAMP, suggesting that the Ras/cAMP pathway can mediate repression of the FPB1 and PCK1 mRNAs. However, the response is not dependent upon this pathway because it remains intact in Ras, adenyl cyclase and protein kinase A mutants. The data show that yeast cells can detect very low glucose concentrations in the environment, and suggest that several distinct signalling pathways operate to repress FPB1 and PCK1 transcription in the presence of glucose.

Effects of short-chain acyl-CoA dehydrogenase deficiency on development expression of metabolic enzyme genes in the mouse

Patients with an acyl-CoA dehydrogenase deficiency share the disease features of hypoglycemia, hyperammonemia, tissue fatty change, hypoketonemia, carnitine deficiency, and organic acidemia due to apparent disruption of normal fatty acid, glucose, and urea metabolism. Most of the acute clinical episodes occur in young children. These episodes are precipitated by fasting and are often fatal, with the in vivo mechanisms essentially unknown. Since the genes of the rate controlling enzymes of these pathways are tissue and developmentally regulated at the transcriptional level, we measured, throughout neonatal development, the steady-state mRNA levels of long-chain, medium-chain, and short-chain (SCAD) acyl-CoA dehydrogenases, pyruvate carboxylase (PC), phosphoenolpyruvate carboxykinase (PEPCK), carbamyl phosphate synthetase I (CPS), ornithine transcarbamylase (OTC), and argininosuccinate synthetase (AS) in fed or fasted SCAD-deficient BALB/ByJ mice compared to BALB/cBy controls. Overall, our results showed no major effects on expression of acyl-CoA dehydrogenases due to SCAD deficiency, regardless of age or fasting. In SCAD-deficient mice we found depressed mRNA expression and enzyme activity for the urea cycle enzymes CPS and AS at 6 days of age, and found no apparent effects on expression of gluconeogenic enzymes PC or PEPCK. There was a period of overall lower gene expression for most genes at 6 and 15 days, which appears to be in parallel with the developmental period when children with these diseases are most severely affected.

Anisoosmotic regulation of hepatic gene expression

The effect of anisoosmolarity on the abundance of various mRNA species was examined in perfused rat liver and H4IIE rat hepatoma cells. Hyperosmotic exposure (385 mosmol/l) of isolated rat livers increased mRNA levels for tyrosine aminotransferase (TAT) by 246% and those for phosphoenolpyruvate carboxykinase (PEPCK) by 186%, whereas hypoosmotic exposure (225 mosmol/l) decreased their levels to 43% and 42%, respectively. mRNA levels for fructose-1,6-bisphosphatase (FBP), argininosuccinate lyase (ASL), argininosuccinate synthetase (ASS), glutamine synthetase (GS), glutaminase (GA) and glucokinase (GK) were largely unaffected. In H4IIE cells the modulation of TAT and PEPCK mRNA levels by anisoosmotic exposure was similar to that found in perfused rat liver. ASL and glutaminase mRNA levels were influenced in an opposite manner. The effects of anisoosmolarity on PEPCK mRNA levels in H4IIE cells were largely abolished in the presence of the protein kinase inhibitors H-7, H-89 and HA-1004. Other protein kinase inhibitors such as Go-6850, KN-62, Rp-8-CPT-cAMPS, rapamycin, wortmannin, genistein or herbimycin did not prevent the osmosensitivity of PEPCK mRNA levels. Also pertussis and cholera toxin, vanadate and colchicine did not affect the osmosensitivity of PEPCK mRNA levels. The data suggest that anisoosmotic exposure acts on the levels of some but not all mRNA species and that this action may involve changes in protein phosphorylation. They further indicate that the recently identified osmosensitive signal transduction pathway which involves a G-protein and tyrosine kinase dependent activation of mitogen-activated protein kinases is apparently not involved in the osmoregulation of PEPCK mRNA levels.

Oral selenate improves glucose homeostasis and partly reverses abnormal expression of liver glycolytic and gluconeogenic enzymes in diabetic rats

Selenium is a trace element that exerts certain insulin-like actions in vitro. In this study, we evaluated its in vivo effects on the glucose homeostasis of rats made diabetic and insulin-deficient by streptozotocin. Na2SeO4 was administered ad libitum in drinking water and/or food for 10 weeks. The elevated plasma glucose levels (approximately 25 mmol/l) and glucosuria (approximately 85 mmol/day) of untreated rats were decreased by 50 and 80%, respectively, by selenate treatment. The beneficial effect of selenate was also evident during oral and intravenous glucose tolerance tests: the integrated glucose responses were decreased by 40-50% as compared to those in untreated rats. These effects were not due to an increase in plasma insulin levels. Compared to non-diabetic rats, pancreatic insulin reserves were reduced by more than 90% in treated and untreated diabetic rats. The hepatic activities and mRNA levels of two key glycolytic enzymes, glucokinase and L-type pyruvate kinase were blunted in diabetic rats. They increased approximately two- to threefold after selenate treatment, to reach 40-75% of the values in non-diabetic rats. In contrast, elevated activity and mRNA levels of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase, were reduced by 40-65% after selenate administration. Since selenate induced a moderate decrease in body weight due to an anorexigenic effect, we checked that there was no improvement of glucose homeostasis or hepatic glucose metabolism in an additional group of calorie-restricted diabetic rats, which was weight-matched with the selenate group. In addition, no obvious toxic side-effects on the kidney or liver were observed in the rats receiving selenate. In conclusion, selenate induces a sustained improvement of glucose homeostasis in streptozotocin-diabetic rats by an insulin-like action, which involves partial correction of altered pretranslational regulatory mechanisms in liver metabolism.

Skeletal growth of transgenic mice with elevated levels of circulating insulin-like growth factor-II

Insulin-like growth factor-II (IGF-II) is a major factor produced by skeletal tissues. To evaluate endocrine effects of IGF-II on bone growth, we measured skeletal dimensions of 12-week-old transgenic mice harbouring fusion genes where a human IGF-II cDNA is transcriptionally controlled by rat phospheonolpyruvate carboxykinase (PEPCK) promoter sequences. Transgene expression in liver, kidney and intestine resulted in circulating IGF-II levels in transgenic mice which were 2-3-fold higher than in controls. Serum IGF-I concentrations of transgenic mice were lower than in controls. Body weight was not influenced by the expression of the IGF-II transgene. Only 1 out of 5 measurements taken from the radius was significantly affected by the presence of the transgene, while in 60 measurements taken from eight other bones there was no difference between transgenic mice and controls. Furthermore, serum levels of calcium and phosphate as well as alkaline phosphatase activity were not significantly altered in PEPCK-IGF-II transgenic mice. Our findings demonstrate that moderately increased levels of circulating IGF-II do not cause major changes in skeletal growth and turnover in mice. This may be due to a lack of activity of circulating IGF-II on bone growth or to physiological consequences of elevated IGF-II, like a reduction of circulating IGF-I or an increase in IGF binding proteins.

The role of the TATA box in the hormonal regulation of phosphoenolpyruvate carboxykinase gene expression

In this study, we analyzed the role of the TATA box in the regulation of the phosphoenolpyruvate carboxykinase (PEPCK) gene expression by dexamethasone (DEX), retinoic acid (RA), glucagon (via cAMP) and insulin (INS). The PEPCK TATA box (TATTTAAA) was absolutely required for both basal promoter activity and hormone-mediated transactivation. However, the relative induction of PEPCK gene expression by DEX, RA and cAMP, and its repression by INS, remained unaltered despite the substitution of the PEPCK TATA box with TATA elements from the herpes simplex virus-thymidine kinase gene, gene 33 or a consensus TATA box sequence, TATAAA. The results indicate that the TATA box serves a permissive, but not defining, function in the response of the PEPCK gene to hormones, and that this function can be equally facilitated by heterologous TATA box elements.