Glyceroneogenesis, the pathway that almost wasn't

An Organism-Level Quantitative Flux Model of Mammalian Energy Metabolism

Mammalian tissues feed on nutrients in the blood circulation. At the organism-level, mammalian energy metabolism comprises of oxidation, interconverting, storing and releasing of circulating nutrients. Though much is known about the individual processes and nutrients, a holistic and quantitative model describing these processes for all major circulating nutrients is lacking. Here, by integrating isotope tracer infusion, mass spectrometry, and isotope gas analyzer measurement, we developed a framework to systematically quantify fluxes through these processes for 10 major circulating energy nutrients in mice, resulting in an organism-level quantitative flux model of energy metabolism. This model revealed in wildtype mice that circulating nutrients' metabolic cycling fluxes are more dominant than their oxidation fluxes, with distinct partition between cycling and oxidation flux for individual circulating nutrients. Applications of this framework in obese mouse models showed on a per animal basis extensive elevation of metabolic cycling fluxes in ob/ob mice, but not in diet-induced obese mice. Thus, our framework describes quantitatively the functioning of energy metabolism at the organism-level, valuable for revealing new features of energy metabolism in physiological and disease conditions.

Phosphoenolpyruvate carboxykinase in cell metabolism: Roles and mechanisms beyond gluconeogenesis

BACKGROUND

Phosphoenolpyruvate carboxykinase (PCK) has been almost exclusively recognized as a critical enzyme in gluconeogenesis, especially in the liver and kidney. Accumulating evidence has shown that the enhanced activity of PCK leads to increased glucose output and exacerbation of diabetes, whereas the defects of PCK result in lethal hypoglycemia. Genetic mutations or polymorphisms are reported to be related to the onset and progression of diabetes in humans.

SCOPE OF REVIEW

Recent studies revealed that the PCK pathway is more complex than just gluconeogenesis, depending on the health or disease condition. Dysregulation of PCK may contribute to the development of obesity, cardiac hypertrophy, stroke, and cancer. Moreover, a regulatory network with multiple layers, from epigenetic regulation, transcription regulation, to posttranscription regulation, precisely tunes the expression of PCK. Deciphering the molecular basis that regulates PCK may pave the way for developing practical strategies to treat metabolic dysfunction.

MAJOR CONCLUSIONS

In this review, we summarize the metabolic and non-metabolic roles of the PCK enzyme in cells, especially beyond gluconeogenesis. We highlight the distinct functions of PCK isoforms (PCK1 and PCK2), depict a detailed network regulating PCK's expression, and discuss its clinical relevance. We also discuss the therapeutic potential targeting PCK and the future direction that is highly in need to better understand PCK-mediated signaling under diverse conditions.

Transcriptome signature for dietary fructose-specific changes in rat renal cortex: A quantitative approach to physiological relevance

Fructose consumption causes metabolic diseases and renal injury primarily in the renal cortex where fructose is metabolized. Analyzing gene differential expression induced by dietary manipulation is challenging. The effects may depend on the base diet and primary changes likely induce secondary or higher order changes that are difficult to capture by conventional univariate transcriptome analyses. We hypothesized that dietary fructose induces a genetic program in the kidney cortex that favors lipogenesis and gluconeogenesis. To test this, we analyzed renal cortical transcriptomes of rats on normal- and high-salt base diets supplemented with fructose. Both sets of data were analyzed using the Characteristic Direction method to yield fructose-induced gene vectors of associated differential expression values. A fructose-specific "signature" of 139 genes differentially expressed was extracted from the 2 diet vectors by a new algorithm that takes into account a gene's rank and standard deviation of its differential expression value. Of these genes, 97 were annotated and the top 34 accounted for 80% of the signal in the annotated signature. The genes were predominantly proximal tubule-specific, coding for metabolic enzymes or transporters. Cosine similarity of signature genes in the two fructose-induced vectors was >0.78. These 139 genes of the fructose signature contributed 27% and 38% of total differential expression on normal- and high- salt diet, respectively. Principal Component Analysis showed that the individual animals could be grouped according to diet. The fructose signature contained a greater enrichment of Gene Ontology processes related to nutrition and metabolism of fructose than two univariate analysis methods. The major feature of the fructose signature is a change in metabolic programs of the renal proximal tubule consistent with gluconeogenesis and de-novo lipogenesis. This new "signature" constitutes a new metric to bridge the gap between physiological phenomena and differential expression profile.

Glycerol-3-phosphatase and not lipid recycling is the primary pathway in the accumulation of high concentrations of glycerol in rainbow smelt (Osmerus mordax)

Rainbow smelt is a small fish that accumulates glycerol in winter as a cryoprotectant when the animal is in seawater. Glycerol is synthesized in liver from different substrates that all lead to the formation of glycerol-3-phosphate (G3P). This study assesses whether glycerol is produced by a direct dephosphorylation of G3P by a phosphatase (G3Pase) or by a cycling through the glycerolipid pool followed by lipolysis. Foremost, concentrations of on-board glycerolipids and activity of G3Pase and of enzymes involved in lipid metabolism were measured in smelt liver over the glycerol cycle. Concentrations of on-board glycerolipids did not change over the cycle and were too low to significantly contribute directly to glycerol production but activities of enzymes involved in both potential pathways were up-regulated at the onset of glycerol accumulation. A second experiment conducted with isolated hepatic cells producing glycerol showed 1) that on-board glycerolipids were not sufficient to produce the glycerol released even though phospholipids could account for up to 17% of it, 2) that carbon cycling through the glycerolipid pool was not involved as glycerol was produced at similar rates following inhibition of this pathway, and 3) that G3Pase activity measured was sufficient to allow the synthesis of glycerol at the rate observed. These results are the first to clearly support G3Pase as the metabolic step leading to glycerol production in rainbow smelt and the first to provide strong support for a G3Pase in any animal species.

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

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