Aspects of the control of phosphoenolpyruvate carboxykinase gene transcription

Regulation of glycolysis and gluconeogenesis by acetylation of PKM and PEPCK

Glycolysis is a catabolic process of glucose hydrolysis needed for energy and biosynthetic intermediates, whereas gluconeogenesis is a glucose production process important for maintaining blood glucose levels during starvation. Although they share many enzymes, these two processes are not simply the reverse of each other and are instead reciprocally regulated. Two key enzymes that regulate irreversible steps in these two processes are pyruvate kinase (PK) and phosphoenolpyruvate carboxy kinase (PEPCK), which catalyze the last and first step of glycolysis and gluconeogenesis, respectively, and are both regulated by lysine acetylation. Acetylation at Lys305 of the PKM (muscle form of PK) decreases its activity and also targets it for chaperone-mediated autophagy and subsequent lysosome degradation. Acetylation of PEPCK, on the other hand, targets it for ubiquitylation by the HECT E3 ligase, UBR5/EDD1, and subsequent proteasomal degradation. These studies established a model in which acetylation regulates metabolic enzymes via different mechanisms and also revealed cross talk between acetylation and ubiquitination. Given that most metabolic enzymes are acetylated, we propose that acetylation is a major posttranslational modifier that regulates cellular metabolism.

Dietary protein restriction during F0 pregnancy in rats induces transgenerational changes in the hepatic transcriptome in female offspring

There is considerable evidence for non-genomic transmission between generations of phenotypes induced by environmental exposures during development, although the mechanism is poorly understood. We investigated whether alterations in expression of the liver transcriptome induced in F1 offspring by feeding F0 dams a protein-restricted (PR) diet during pregnancy were passed with or without further change to two subsequent generations. The number of genes that differed between adult female offspring of F0 protein-restricted (PR) and protein-sufficient (PS) dams was F1 1,684 genes, F2 1,680 and F3 2,062. 63/113 genes that were altered in all three generations showed directionally opposite differences between generations. There was a trend toward increased proportions of up-regulated genes in F3 compared to F1. KEGG analysis showed that only the Adherens Junctions pathway was altered in all three generations. PR offspring showed altered fasting glucose homeostasis and changes in phosphoenolpyruvate carboxykinase promoter methylation and expression in all three generations. These findings show that dietary challenge during F0 pregnancy induced altered gene expression in all three generations, but relatively few genes showed transmission of altered expression between generations. For the majority of altered genes, these changes were not found in all generations, including some genes that were changed in F3 but not F1, or the direction and magnitude of difference between PR and PS differed between generations. Such variation may reflect differences between generations in the signals received by the fetus from the mother as a consequence of changes in the interaction between her phenotype and the environment.

Functional characterization of the proximal promoter of the murine pyruvate carboxylase gene in hepatocytes: role of multiple gc boxes

Pyruvate carboxylase (PC) catalyzes the first committed step in gluconeogenesis in the liver. The murine PC gene possesses two promoters, the proximal (P1) and the distal (P2) which mediate production of distinct tissue-specific mRNA isoforms. By comparing the luciferase activities of 5'-nested deletions of the P1-promoter in the AML12 mouse hepatocyte cell line, the critical cis-acting elements required for maintaining basal transcription were located within the 166 nucleotides proximal to the transcription start site. Three GC boxes were identified within this region and shown by gel shift and ChIP assays to bind Sp1/Sp3. Over-expression of Sp1/Sp3 in AML12 and NIH3T3 cells increased P1-promoter activity, with Sp1 being a stronger activator than Sp3. Mutation of any one of the three GC boxes dramatically reduced basal promoter activity by 60-80% suggesting that all three boxes are equally strong regulatory elements. In AML12 cells, over-expression of Sp1/Sp3 restored the transcriptional activity of GC1 and GC2 but not GC3 mutants to levels similar to that of the WT construct, suggesting that GC3 is particularly critical for Sp1/Sp3-mediated induction. In NIH3T3 cells, however, the three boxes were equally important, indicating that the GC boxes differentially contribute to transcriptional regulation of the P1-promoter in the two cell lines. Mutants harboring two disrupted GC boxes showed a further decrease in promoter activity similar to the triple GC box mutant. Neither Sp1 nor Sp3 was able to fully restore the promoter activities of these mutants to that the WT level.

The intrauterine growth restriction phenotype: fetal adaptations and potential implications for later life insulin resistance and diabetes

The intrauterine growth restricted (IUGR) fetus develops unique metabolic adaptations in response to exposure to reduced nutrient supply. These adaptations provide survival value for the fetus by enhancing the capacity of the fetus to take up and use nutrients, thereby reducing the need for nutrient supply. Each organ and tissue in the fetus adapts differently, with the brain showing the greatest capacity for maintaining nutrient supply and growth. Such adaptations, if persistent, also have the potential in later life to promote nutrient uptake and storage, which directly lead to complications of obesity, insulin resistance, reduced insulin production, and type 2 diabetes.

Effects of crustacean hyperglycemic hormone (CHH) on the transcript expression of carbohydrate metabolism-related enzyme genes in the kuruma prawn, Marsupenaeus japonicus

Crustacean hyperglycemic hormone (CHH), a member of a neuropeptide family present only in arthropods, plays a pivotal role in the modulation of hemolymph glucose levels, molting, reproduction, and the stress response. Although it has been determined that hepatopancreas and muscle are the major tissues in which CHH regulates hyperglycemic activity, the molecular mechanism by which CHH regulates carbohydrate metabolism remains unclear. In this study, we analyzed the mRNA expression levels of enzymes involved in glycogen metabolism and gluconeogenesis in order to determine how CHH regulates hemolymph glucose levels. We first cloned cDNAs encoding four carbohydrate metabolism-related enzymes from the kuruma prawn, Marsupenaeus japonicus, glycogen phosphorylase (MjGP), glycogen synthase (MjGS), fructose 1,6-bisphosphatase (MjFBPase), and phosphoenolpyruvate carboxykinase (MjPEPCK). RT-PCR analysis showed that eyestalk ablation remarkably decreased MjGP and increased MjGS transcript levels in the hepatopancreas, but not in muscle. Considering the fact that various eyestalk factors, including MIH, are removed by eyestalk ablation, these results indicate that after eyestalk ablation the metabolic state proceeds towards glycogen accumulation in the specific tissues related to molting. In contrast, MjFBPase and MjPEPCK transcript levels were not significantly changed by eyestalk ablation, indicating that CHH and other eyestalk-derived factors might not induce gluconeogenesis. Quantitative real-time PCR analysis showed that exposure of hepatopancreas to recombinant CHH significantly changed the expression levels of MjGP and MjGS, but not MjFBPase and MjPEPCK. Collectively, these results indicate that CHH is involved in glycogen metabolism in hepatopancreas.

Inhibition of glyceroneogenesis by histone deacetylase 3 contributes to lipodystrophy in mice with adipose tissue inflammation

We have reported that the nuclear factor-κB (NF-κB) induces chronic inflammation in the adipose tissue of p65 transgenic (Tg) mice, in which the NF-κB subunit p65 (RelA) is overexpressed from the adipocyte protein 2 (aP2) gene promoter. Tg mice suffer a mild lipodystrophy and exhibit deficiency in adipocyte differentiation. To understand molecular mechanism of the defect in adipocytes, we investigated glyceroneogenesis by examining the activity of cytosolic phosphoenolpyruvate carboxykinase (PEPCK) in adipocytes. In aP2-p65 Tg mice, Pepck expression is inhibited at both the mRNA and protein levels in adipose tissue. The mRNA reduction is a consequence of transcriptional inhibition but not alteration in mRNA stability. The Pepck gene promoter is inhibited by NF-κB, which enhances the corepressor activity through activation of histone deacetylase 3 (HDAC3) in the nucleus. HDAC3 suppresses Pepck transcription by inhibiting the transcriptional activators, peroxisome proliferator-activated receptor-γ, and cAMP response element binding protein. The NF-κB activity is abolished by Hdac3 knockdown or inhibition of HDAC3 catalytic activity. In a chromatin immunoprecipitation assay, HDAC3 interacts with peroxisome proliferator-activated receptor-γ and cAMP response element binding protein in the Pepck promoter when NF-κB is activated by TNF-α. These results suggest that HDAC3 mediates NF-κB activity to repress Pepck transcription. This mechanism is responsible for inhibition of glyceroneogenesis in adipocytes, which contributes to lipodystrophy in the aP2-p65 Tg mice.

Gestational high fat diet programs hepatic phosphoenolpyruvate carboxykinase gene expression and histone modification in neonatal offspring rats

In insulin resistance and type II diabetes, there is an elevation of hepatic gluconeogenesis, which contributes to hyperglycaemia. Studies in experimental animals have provided evidence that consumption of high fat (HF) diets by female rats programs the progeny for glucose intolerance in adulthood, but the mechanisms behind the in utero programming remain poorly understood. The present study analysed the effect of a maternal HF diet on fetal gluconeogenic gene expression and potential regulation mechanism related to histone modifications. Dams were fed either a Control (C, 16% kcal fat) or a high-fat (HF, 45% kcal fat) diet throughout gestation. Livers of the offspring were collected on gestational day 21 and analysed to determine the consequences of a maternal HF diet on molecular markers of fetal liver gluconeogenesis. We demonstrated that offspring of HF-fed dams were significantly heavier and had significantly higher blood glucose levels at the time of delivery than offspring of dams fed the C diet. While maternal gluconeogenesis and plasma glucose were not affected by the HF diet, offspring of HF-fed dams had significantly higher mRNA contents of gluconeogenic genes in addition to the elevated plasma glucose. In addition to increased transcription rate, a gestational HF diet resulted in modifications of the Pck1 histone code in livers of offspring. Our results demonstrate that in utero exposure to HF diet has the potential to program the gluconeogenic capacity of offspring through epigenetic modifications, which could potentially lead to excessive glucose production and altered insulin sensitivity in adulthood.

Hormonal regulation of gluconeogenic gene transcription in the liver

Glucose homeostasis in mammals is achieved by the actions of counterregulatory hormones, namely insulin, glucagon and glucocorticoids. Glucose levels in the circulation are regulated by the liver, the metabolic centre which produces glucose when it is scarce in the blood. This process is catalysed by two rate-limiting enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) whose gene expression is regulated by hormones. Hormone response units (HRUs) present in the two genes integrate signals from various signalling pathways triggered by hormones. How such domains are arranged in the regulatory region of these two genes, how this complex regulation is accomplished and the latest advancements in the field are discussed in this review.

A retrospective review of the roles of multifunctional glucose-6-phosphatase in blood glucose homeostasis: Genesis of the tuning/retuning hypothesis

In a scientific career spanning from 1955 to 2000, my research focused on phosphoenolpyruvate carboxykinase and glucose-6-phosphatase. Grounded in basic enzymology, and initially pursuing the steady-state rate behavior of isolated preparations of these critically important gluconeogenic enzymes, our key findings were confirmed and extended by in situ enzyme rate experiments exploiting isolated liver perfusions. These efforts culminated in the discovery of the liver cytosolic isozyme of carboxykinase, known today as (GTP)PEPCK-C (EC4.1.1.32) and also revealed a biosynthetic function and multicomponent nature of glucose-6-phosphatase (EC3.1.3.9). Discovery that glucose-6-phosphatase possessed an intrinsically biosynthetic activity, now known as carbamyl-P:glucose phosphotransferase - along with a deeper consideration of the enzyme's hydrolytic activity as well as the action of liver glucokinase resulted in the evolution of Tuning/Retuning Hypothesis for blood glucose homeostasis in health and disease. This THEN & NOW review shares with the reader the joy and exhilaration of major scientific discovery and also contrasts the methodologies and approaches on which I relied with those currently in use.

Electrostatic interactions play a significant role in the affinity of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase for Mn2+

Phosphoenolpyruvate (PEP) carboxykinases catalyse the reversible formation of oxaloacetate (OAA) and ATP (or GTP) from PEP, ADP (or GDP) and CO(2). They are activated by Mn(2+), a metal ion that coordinates to the protein through the epsilon-amino group of a lysine residue, the N(epsilon-2)-imidazole of a histidine residue, and the carboxylate from an aspartic acid residue. Neutrality in the epsilon-amino group of Lys213 of Saccharomyces cerevisiae PEP carboxykinase is expected to be favoured by the vicinity of ionised Lys212. Glu272 and Glu284, located close to Lys212, should, in turn, electrostatically stabilise its positive charge and hence assist in keeping the epsilon-amino group of Lys213 in a neutral state. The mutations Glu272Gln, Glu284Gln, and Lys212Met increased the activation constant for Mn(2+) in the main reaction of the enzyme up to seven-fold. The control mutation Lys213Gln increased this constant by ten-fold, as opposed to control mutation Lys212Arg, which did not affect the Mn(2+) affinity of the enzyme. These observations indicate a role for Glu272, Glu284, and Lys212 in assisting Lys213 to properly bind Mn(2+). In an unexpected result, the mutations Glu284Gln, Lys212Met and Lys213Gln changed the nucleotide-independent OAA decarboxylase activity of S. cerevisiae PEP carboxykinase into an ADP-requiring activity, implying an effect on the OAA binding characteristics of PEP carboxykinase.