Nuclear localization of liver FBPase isoenzyme in kidney and liver

Regulation of gene expression by glycolytic and gluconeogenic enzymes

Gene transcription and cell metabolism are two fundamental biological processes that mutually regulate each other. Upregulated or altered expression of glucose metabolic genes in glycolysis and gluconeogenesis is a major driving force of enhanced aerobic glycolysis in tumor cells. Importantly, glycolytic and gluconeogenic enzymes in tumor cells acquire moonlighting functions and directly regulate gene expression by modulating chromatin or transcriptional complexes. The mutual regulation between cellular metabolism and gene expression in a feedback mechanism constitutes a unique feature of tumor cells and provides specific molecular and functional targets for cancer treatment.

Fructose-1,6-bisphosphatase: From a glucose metabolism enzyme to multifaceted regulator of a cell fate

Fructose-1,6-bisphosphatase (FBPase) is one of the ancient, evolutionarily conserved enzymes of carbohydrate metabolism. It has been described for a first time in 1943, however, for the next half a century mostly kinetic and structural parameters of animal FBPases have been studied. Discovery of ubiquitous expression of the muscle isozyme of FBPase, thus far considered to merely regulate glycogen synthesis from carbohydrate precursors, and its nuclear localisation in several cell types has risen new interest in the protein, resulting in numerous publications revealing complex functions/properties of FBPase. This review summarises the current knowledge of FBPase in animal cells providing evidence that the enzyme merits the name of moonlighting protein.

A new level of regulation in gluconeogenesis: metabolic state modulates the intracellular localization of aldolase B and its interaction with liver fructose-1,6-bisphosphatase

Understanding how glucose metabolism is finely regulated at molecular and cellular levels in the liver is critical for knowing its relationship to related pathologies, such as diabetes. In order to gain insight into the regulation of glucose metabolism, we studied the liver-expressed isoforms aldolase B and fructose-1,6-bisphosphatase-1 (FBPase-1), key enzymes in gluconeogenesis, analysing their cellular localization in hepatocytes under different metabolic conditions and their protein-protein interaction in vitro and in vivo. We observed that glucose, insulin, glucagon and adrenaline differentially modulate the intracellular distribution of aldolase B and FBPase-1. Interestingly, the in vitro protein-protein interaction analysis between aldolase B and FBPase-1 showed a specific and regulable interaction between them, whereas aldolase A (muscle isozyme) and FBPase-1 showed no interaction. The affinity of the aldolase B and FBPase-1 complex was modulated by intermediate metabolites, but only in the presence of K(+). We observed a decreased association constant in the presence of adenosine monophosphate, fructose-2,6-bisphosphate, fructose-6-phosphate and inhibitory concentrations of fructose-1,6-bisphosphate. Conversely, the association constant of the complex increased in the presence of dihydroxyacetone phosphate (DHAP) and non-inhibitory concentrations of fructose-1,6-bisphosphate. Notably, in vivo FRET studies confirmed the interaction between aldolase B and FBPase-1. Also, the co-expression of aldolase B and FBPase-1 in cultured cells suggested that FBPase-1 guides the cellular localization of aldolase B. Our results provide further evidence that metabolic conditions modulate aldolase B and FBPase-1 activity at the cellular level through the regulation of their interaction, suggesting that their association confers a catalytic advantage for both enzymes.

Preclinical and Clinical Studies for Sodium Tungstate: Application in Humans

Diabetes is a complex metabolic disorder triggered by the deficient secretion of insulin by the pancreatic β-cell or the resistance of peripheral tissues to the action of the hormone. Chronic hyperglycemia is the major consequence of this failure, and also the main cause of diabetic problems. Indeed, several clinical trials have agreed in that tight glycemic control is the best way to stop progression of the disease. Many anti-diabetic drugs for treatment of type 2 diabetes are commercially available, but no ideal normoglycemic agent has been developed yet. Moreover, weight gain is the most common side effect of many oral anti-diabetic agents and insulin, and increased weight has been shown to worsen glycemic control and increase the risk of diabetes progression. In this sense, the inorganic salt sodium tungstate (NaW) has been studied in different animal models of metabolic syndrome and diabetes, proving to have a potent effect on normalizing blood glucose levels and reducing body weight, without any hypoglycemic action. Although the liver has been studied as the main site of NaW action, positive effects have been also addressed in muscle, pancreas, brain, adipose tissue and intestine, explaining the effective anti-diabetic action of this salt. Here, we review NaW research to date in these different target organs. We believe that NaW deserves more attention, since all available anti-diabetic treatments remain suboptimal and new therapeutics are urgently needed.

Nuclear accumulation of fructose 1,6-bisphosphatase is impaired in diabetic rat liver

Using a streptozotocin-induced type 1 diabetic rat model, we analyzed and separated the effects of hyperglycemia and hyperinsulinemia over the in vivo expression and subcellular localization of hepatic fructose 1,6-bisphosphatase (FBPase) in the multicellular context of the liver. Our data showed that FBPase subcellular localization was modulated by the nutritional state in normal but not in diabetic rats. By contrast, the liver zonation was not affected in any condition. In healthy starved rats, FBPase was localized in the cytoplasm of hepatocytes, whereas in healthy re-fed rats it was concentrated in the nucleus and the cell periphery. Interestingly, despite the hyperglycemia, FBPase was unable to accumulate in the nucleus in hepatocytes from streptozotocin-induced diabetic rats, suggesting that insulin is a critical in vivo modulator. This idea was confirmed by exogenous insulin supplementation to diabetic rats, where insulin was able to induce the rapid accumulation of FBPase within the hepatocyte nucleus. Besides, hepatic FBPase was found phosphorylated only in the cytoplasm, suggesting that the phosphorylation state is involved in the nuclear translocation. In conclusion, insulin and not hyperglycemia plays a crucial role in the nuclear accumulation of FBPase in vivo and may be an important regulatory mechanism that could account for the increased endogenous glucose production of liver of diabetic rodents.

Proteomic profiling of the effect of metabolic acidosis on the apical membrane of the proximal convoluted tubule

The physiological response to the onset of metabolic acidosis requires pronounced changes in renal gene expression. Adaptations within the proximal convoluted tubule support the increased extraction of plasma glutamine and the increased synthesis and transport of glucose and of NH(4)(+) and HCO(3)(-) ions. Many of these adaptations involve proteins associated with the apical membrane. To quantify the temporal changes in these proteins, proteomic profiling was performed using brush-border membrane vesicles isolated from proximal convoluted tubules (BBMV(PCT)) that were purified from normal and acidotic rats. This preparation is essentially free of contaminating apical membranes from other renal cortical cells. The analysis identified 298 proteins, 26% of which contained one or more transmembrane domains. Spectral counts were used to assess changes in protein abundance. The onset of acidosis produced a twofold, but transient, increase in the Na(+)-dependent glucose transporter and a more gradual, but sustained, increase (3-fold) in the Na(+)-dependent lactate transporter. These changes were associated with the loss of glycolytic and gluconeogenic enzymes that are contained in the BBMV(PCT) isolated from normal rats. In addition, the levels of γ-glutamyltranspeptidase increased twofold, while transporters that participate in the uptake of neutral amino acids, including glutamine, were decreased. These changes could facilitate the deamidation of glutamine within the tubular lumen. Finally, pronounced increases were also observed in the levels of DAB2 (3-fold) and myosin 9 (7-fold), proteins that may participate in endocytosis of apical membrane proteins. Western blot analysis and accurate mass and time analyses were used to validate the spectral counting.

Proteomic analysis of brush-border membrane vesicles isolated from purified proximal convoluted tubules

The renal proximal convoluted tubule is the primary site of water, electrolyte and nutrient reabsorption and of active secretion of selected molecules. Proteins in the apical brush-border membrane facilitate these functions and initiate some of the cellular responses to altered renal physiology. The current study uses two-dimensional liquid chromatography/mass spectrometry to compare brush border membrane vesicles isolated from rat renal cortex (BBMV(CTX)) and from purified proximal convoluted tubules (BBMV(PCT)). Both proteomic data and Western blot analysis indicate that the BBMV(CTX) contain apical membrane proteins from cortical cells other than the proximal tubule. This heterogeneity was greatly reduced in the BBMV(PCT). Proteomic analysis identified 193 proteins common to both samples, 21 proteins unique to BBMV(CTX), and 57 proteins unique to BBMV(PCT). Spectral counts were used to quantify relative differences in protein abundance. This analysis identified 42 and 50 proteins that are significantly enriched (p values <or=0.001) in the BBMV(CTX) and BBMV(PCT), respectively. These data were validated by measurement of gamma-glutamyltranspeptidase activity and by Western blot analysis. The combined results establish that BBMV(PCT) are primarily derived from the proximal convoluted tubule (S1 and S2 segments), whereas BBMV(CTX) include proteins from the proximal straight tubule (S3 segment). Analysis of functional annotations indicated that BBMV(PCT) are enriched in mitochondrial proteins and enzymes involved in glucose and organic acid metabolism. Thus the current study reports a detailed proteomic analysis of the brush-border membrane of the rat renal proximal convoluted tubule and provides a database for future hypothesis-driven research.

Effect of leaf position on levels of the chloroplast and cytosolic fructose bisphosphatase isozymes in the pea leaf nucleus

Immunocytolocalization experiments indicate that nuclear levels of the pea leaf cytosolic fructose bisphosphatase are higher in leaves located near the base of the plant and lower in expanded leaves at the apex. It seems possible that the cytosolic isozyme in the nucleus has a role in tissue aging. In contrast, there is no indication that leaf position or tissue age affects levels of the chloroplastic enzyme in the nucleus. The density of the chloroplast fructose bisphosphatase is higher in the nucleolus than in the nucleoplasm. Conversely, the density of the cytosolic isozyme is slightly higher in the nucleoplasm. Analysis of double immunolabeling experiments indicates that both isozymes are distributed nonrandomly with respect to DNA, and therefore colocalized with DNA, in the nucleus, and that the chloroplast isozyme is also distributed nonrandomly with respect to DNA in the chloroplast.

Expression of key substrate cycle enzymes in rat spermatogenic cells: Fructose 1,6 bisphosphatase and 6 phosphofructose 1-kinase

A substrate cycle composed of phosphofructo 1-kinase I (PFK) and fructose 1,6 bisphosphatase I (FBPase) has been proposed in rat spermatids. This substrate cycle can explain the ability of glucose to induce a decrease in intracellular ATP, a phenomenon that was related to regulation of [Ca(2+)]i in these cells. In spite of the importance of this metabolic cycle, the expression and activities of the enzymes that compose such cycle have not been systematically studied in spermatogenic cells. Here, we show that PFK and FBPase activities were present in pachytene spermatocytes and round spermatids extracts. Expression of PFK at the mRNA and protein levels showed a relatively similar expression in spermatogenic cells, but a stronger expression in Sertoli cells. Instead, expression of FBPase at the mRNA and protein levels was stronger in round and elongating spermatids as compared to other spermatogenic cells. A similar pattern was observed when evidencing FBPase activity by a NADPH-nitroblue tetrazolium-linked cytochemical assay in isolated pachytene spermatocytes and round spermatids. Rat spermatids also showed the ability to convert lactate to fructose- and glucose-6-P, indicating that both glycolytic and gluconeogenic fluxes are present in these cells. Our results indicate that a coordinated expression of key substrate cycle enzymes, at the level of PFK/FBPase, appear in the last stages of spermatogenic cell differentiation, suggesting that the co-regulation of these enzymes are required for the ability of these cells to respond to glucose and induce metabolic and Ca(2+) signals that can be important for sperm development and function.

Subcellular localization of muscle FBPase in carp (Cyprinus carpio) tissues

Subcellular localization of muscle FBPase-a regulatory enzyme of glyconeogenesis-was investigated in carp using immunohistochemistry and protein exchange method. Results of the experiments revealed that, in striated muscles, FBPase associates with alpha-actinin of the Z-line and co-localizes with aldolase. Additionally, in cardiac and smooth muscle cells FBPase is present inside the nuclei. In the light of findings on mammalian muscle FBPase, the data presented here indicates that interaction of the enzyme with specific cellular partners and nuclear presence of FBPase is a general phenomenon in contemporary vertebrates.

Nuclear Localization of Fructose 1,6-bisphosphatase in Smooth Muscle Cells

Fructose 1,6-bisphosphatase (FBPase)--a key enzyme of gluconeogenesis--for a long time was regarded to be soluble, and freely diffused in the cytoplasm. Our recent investigation revealed however, that in skeletal muscles of mammals, FBPase is located on both sides of the Z-line and, in cardiomyocytes, it is also present inside the cells' nuclei. In the current paper we demonstrate that, in smooth muscle cells, FBPase is located in the cytoplasm and the nucleus, and that the presence of the enzyme in the nucleus is almost completely restricted to the heterochromatin area. In search for additional evidence for the nuclear localization of FBPase and for a possible explanation of its role in the nucleus, we have analyzed the primary structures of muscle FBPases, finding on their molecular surface a number of domains specific for proteins transported into the nucleus.

Different involvement for aldolase isoenzymes in kidney glucose metabolism: aldolase B but not aldolase A colocalizes and forms a complex with FBPase

The expression of aldolase A and B isoenzyme transcripts was confirmed by RT-PCR in rat kidney and their cell distribution was compared with characteristic enzymes of the gluconeogenic and glycolytic metabolic pathway: fructose-1,6-bisphosphatase (FBPase), phosphoenol pyruvate carboxykinase (PEPCK), and pyruvate kinase (PK). We detected aldolase A isoenzyme in the thin limb and collecting ducts of the medulla and in the distal tubules and glomerula of the cortex. The same pattern of distribution was found for PK, but not for aldolase B, PEPCK, and FBPase. In addition, co-localization studies confirmed that aldolase B, FBPase, and PEPCK are expressed in the same proximal cells. This segregated cell distribution of aldolase A and B with key glycolytic and gluconeogenic enzymes, respectively, suggests that these aldolase isoenzymes participate in different metabolic pathways. In order to test if FBPase interacts with aldolase B, FBPase was immobilized on agarose and subjected to binding experiments. The results show that only aldolase B is specifically bound to FBPase and that this interaction was specifically disrupted by 60 microM Fru-1,6-P2. These data indicate the presence of a modulated enzyme-enzyme interaction between FBPase and isoenzyme B. They affirm that in kidney, aldolase B specifically participates, along the gluconeogenic pathway and aldolase A in glycolysis.

Novel expression of liver FBPase in Langerhans islets of human and rat pancreas

Several reports have indicated the absence of gluconeogenic enzymes in pancreatic islet cells. In contrast, here we demonstrate that liver fructose-1,6-bisphosphatase (FBPase) is highly expressed both in human and rat pancreas. Interestingly, pancreatic FBPase is active and functional, and is inhibited by AMP and fructose-2,6-bisphosphate (Fru-2,6-P2). These results suggest that FBPase may participate as a component of a metabolic sensing mechanism present in the pancreas. Immunolocalization analysis showed that FBPase is expressed both in human and rat Langerhans islets, specifically in beta cells. In humans, FBPase was also located in the canaliculus and acinar cells. These results indicate that FBPase coupled with phosphofructokinase (PFK) plays a crucial role in the metabolism of pancreatic islet cells. The demonstration of gluconeogenic recycling of trioses as a new metabolic signaling pathway may contribute to our understanding of the differences between the insulin secretagogues trioses, fructose, and glucose in pancreas.

Subcellular localization of liver FBPase is modulated by metabolic conditions

In primary cultured hepatocytes, fructose-1,6-bisphosphatase (FBPase) localization is modulated by glucose, dihydroxyacetone (DHA) and insulin. In the absence of these substrates, FBPase was present in the cytoplasm, but the addition of glucose or DHA induced its translocation to the nucleus. As expected, we observed the opposite effect of glucose on glucokinase localization. The addition of insulin in the absence of glucose largely increased the amount of nuclear FBPase. Moreover, at high concentrations of glucose or DHA, FBPase shifted from the cytosol to the cell periphery and co-localized with GS. Interestingly, the synthesis of Glu-6-P and glycogen induced by DHA was not inhibited by insulin. These results indicate that FBPase is involved in glycogen synthesis from gluconeogenic precursors. Overall, these findings show that translocation may be a new integrative mechanism for gluconeogenesis and glyconeogenesis.

Methyl jasmonate-induced ethylene production is responsible for conifer phloem defense responses and reprogramming of stem cambial zone for traumatic resin duct formation

Conifer stem pest resistance includes constitutive defenses that discourage invasion and inducible defenses, including phenolic and terpenoid resin synthesis. Recently, methyl jasmonate (MJ) was shown to induce conifer resin and phenolic defenses; however, it is not known if MJ is the direct effector or if there is a downstream signal. Exogenous applications of MJ, methyl salicylate, and ethylene were used to assess inducible defense signaling mechanisms in conifer stems. MJ and ethylene but not methyl salicylate caused enhanced phenolic synthesis in polyphenolic parenchyma cells, early sclereid lignification, and reprogramming of the cambial zone to form traumatic resin ducts in Pseudotsuga menziesii and Sequoiadendron giganteum. Similar responses in internodes above and below treated internodes indicate transport of a signal giving a systemic response. Studies focusing on P. menziesii showed MJ induced ethylene production earlier and 77-fold higher than wounding. Ethylene production was also induced in internodes above the MJ-treated internode. Pretreatment of P. menziesii stems with the ethylene response inhibitor 1-methylcyclopropene inhibited MJ and wound responses. Wounding increased 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase protein, but MJ treatment produced a higher and more rapid ACC oxidase increase. ACC oxidase was most abundant in ray parenchyma cells, followed by cambial zone cells and resin duct epithelia. The data show these MJ-induced defense responses are mediated by ethylene. The cambial zone xylem mother cells are reprogrammed to differentiate into resin-secreting epithelial cells by an MJ-induced ethylene burst, whereas polyphenolic parenchyma cells are activated to increase polyphenol production. The results also indicate a central role of ray parenchyma in ethylene-induced defense.