Molecular cloning of rat liver glucocorticoid-receptor translocation promoter

Multiplex Fluorescent Bead-Based Immunoassay for the Detection of Cytokines, Chemokines, and Growth Factors

The comprehensive analysis of serum cytokine levels can be challenging due to low sample volumes and time consuming when using single-target methods like enzyme-linked immunosorbent assay (ELISA). Bead-based detection systems allow the simultaneous detection of multiple analytes using minimal sample volumes. Here we describe the use of a multiplex cytokine, chemokine, and growth factor assay for mouse cytokines in a 96-well format. This assay is based on antibody-coupled fluorescent magnetic beads combined with biotinylated secondary detection antibody followed by fluorescent-tagged streptavidin in a sandwich-like composition. Final assay readout provides the concentrations of 23 different cytokines, chemokines, and growth factors in up to 76 samples.

The ATP-stimulated translocation promoter (ASTP) activity of glycerol kinase plays central role in adipogenesis

Glycerol kinase (GK) is a multifunctional enzyme located at the interface of carbohydrate and fat metabolism. It contributes to both central carbon metabolism and adipogenesis; specifically, through its role as the ATP-stimulated translocation promoter (ASTP). GK overexpression leads to increased ASTP activity and increased fat storage in H4IIE cells. We performed metabolic flux analysis in human GK-overexpressing H4IIE cells and found that overexpressing cells had significantly altered fluxes through central carbon and lipid metabolism including increased flux through the pentose phosphate pathway and increased production of lipids. We also observed an equal contribution of glycerol to carbohydrate metabolism in all cell lines, suggesting that GK's alternate functions rather than its enzymatic function are important for these processes. To further elucidate the contributions of the enzymatic (phosphorylation) and alternative (ASTP) functions of GK in adipogenesis, we performed experiments on mammalian GK and E. coli GK. We determined that the ASTP function of GK (which is absent in E. coli GK) plays a greater role than the enzymatic activity in these processes. These studies further emphasize GK's diverse functionality and provides fundamental insights into the multiple protein functions of glycerol kinase.

Disruption of glycerol metabolism by RNAi targeting of genes encoding glycerol kinase results in a range of phenotype severity in Drosophila

In Drosophila, RNAi targeting of either dGyk or dGK can result in two alternative phenotypes: adult glycerol hypersensitivity or larval lethality. Here we compare these two phenotypes at the level of glycerol kinase (GK) phosphorylation activity, dGyk and dGK-RNA expression, and glycerol levels. We found both phenotypes exhibit reduced but similar levels of GK phosphorylation activity. Reduced RNA expression levels of dGyk and dGK corresponded with RNAi progeny that developed into glycerol hypersensitive adult flies. However, quantification of dGyk/dGK expression levels for the larval lethality phenotype revealed unexpected levels possibly due to a compensatory mechanism between dGyk and dGK or RNAi inhibition. The enzymatic role of glycerol kinase converts glycerol to glycerol 3-phosphate. As expected, elevated glycerol levels were observed in larvae that went on to develop into glycerol hypersensitive adults. Interestingly, larvae that died before eclosion revealed extremely low glycerol levels. Further characterization identified a wing phenotype that is enhanced by a dGpdh null mutation, indicating disrupted glycerol metabolism underlies the wing phenotype. In humans, glycerol kinase deficiency (GKD) exhibits a wide range of phenotypic variation with no obvious genotype-phenotype correlations. Additionally, disease severity often does not correlate with GK phosphorylation activity. It is intriguing that both human GKD patients and our GKD Drosophila model show a range of phenotype severity. Additionally, the lack of correlation between GK phosphorylation and dGyk/dGK-RNA expression with phenotypic severity suggests further study including understanding the alternative functions of the GK protein, could provide insights into the complex pathogenic mechanism observed in human GKD patients.

Glycerol hypersensitivity in a Drosophila model for glycerol kinase deficiency is affected by mutations in eye pigmentation genes

Glycerol kinase plays a critical role in metabolism by converting glycerol to glycerol 3-phosphate in an ATP dependent reaction. In humans, glycerol kinase deficiency results in a wide range of phenotypic variability; patients can have severe metabolic and CNS abnormalities, while others possess hyperglycerolemia and glyceroluria with no other apparent phenotype. In an effort to help understand the pathogenic mechanisms underlying the phenotypic variation, we have created a Drosophila model for glycerol kinase deficiency by RNAi targeting of dGyk (CG18374) and dGK (CG7995). As expected, RNAi flies have reduced glycerol kinase RNA expression, reduced phosphorylation activity and elevated glycerol levels. Further investigation revealed these flies to be hypersensitive to fly food supplemented with glycerol. Due to the hygroscopic nature of glycerol, we predict glycerol hypersensitivity is a result of greater susceptibility to desiccation, suggesting glycerol kinase to play an important role in desiccation resistance in insects. To evaluate a role for genetic modifier loci in determining severity of the glycerol hypersensitivity observed in knockdown flies, we performed a preliminary screen of lethal transposon insertion mutant flies using a glycerol hypersensitive survivorship assay. We demonstrate that this type of screen can identify both enhancer and suppressor genetic loci of glycerol hypersensitivity. Furthermore, we found that the glycerol hypersensitivity phenotype can be enhanced or suppressed by null mutations in eye pigmentation genes. Taken together, our data suggest proteins encoded by eye pigmentation genes play an important role in desiccation resistance and that eye pigmentation genes are strong modifiers of the glycerol hypersensitive phenotype identified in our Drosophila model for glycerol kinase deficiency.

Moonlighting function of glycerol kinase causes systems-level changes in rat hepatoma cells

Glycerol kinase (GK) is an enzyme with diverse (moonlighting) cellular functions. GK overexpression affects central metabolic fluxes substantially; therefore, to elucidate the mechanism underlying these changes, we employed a systems-level evaluation of GK overexpression in H4IIE rat hepatoma cells. Microarray analysis revealed altered expression of genes in metabolism (central carbon and lipid), which correlated with previous flux analysis, and of genes regulated by the glucocorticoid receptor (GR). Oil Red O staining showed that GK overexpression leads to increased fat storage in H4IIE cells. Network component analysis revealed that activities of peroxisome proliferator-activated receptor alpha, GR, and seven other transcription factors were altered by GK overexpression. The increased activity of GR was experimentally verified by quantitative RT-PCR of GR-responsive genes in the presence and absence of the glucocorticoid agonist, dexamethasone. This systems biology approach further emphasizes GK's essential role in central and lipid metabolism and experimentally verifies GK's alternative (moonlighting) function of affecting GR transcription factor activity.

Conserved family of glycerol kinase loci in Drosophila melanogaster

Glycerol kinase (GK) is an enzyme that catalyzes the formation of glycerol 3-phosphate from ATP and glycerol, the rate-limiting step in glycerol utilization. We analyzed the genome of the model organism Drosophila melanogaster and identified five GK orthologs, including two loci with sequence homology to the mammalian Xp21 GK protein. Using a combination of sequence analysis and evolutionary comparisons of orthologs between species, we characterized functional domains in the protein required for GK activity. Our findings include additional conserved domains that suggest novel nuclear and mitochondrial functions for glycerol kinase in apoptosis and transcriptional regulation. Investigation of GK function in Drosophila will inform us about the role of this enzyme in development and will provide us with a tool to examine genetic modifiers of human metabolic disorders.

Cell type-specific activation of metabolism reveals that beta-cell secretion suppresses glucagon release from alpha-cells in rat pancreatic islets

Abnormal glucagon secretion is often associated with diabetes mellitus. However, the mechanisms by which nutrients modulate glucagon secretion remain poorly understood. Paracrine modulation by beta- or delta-cells is among the postulated mechanisms. Herein we present further evidence of the paracrine mechanism. First, to activate cellular metabolism and thus hormone secretion in response to specific secretagogues, we engineered insulinoma INS-1E cells using an adenovirus-mediated expression system. Expression of the Na+-dependent dicarboxylate transporter (NaDC)-1 resulted in 2.5- to 4.6-fold (P < 0.01) increases in insulin secretion in response to various tricarboxylic acid cycle intermediates. Similarly, expression of glycerol kinase (GlyK) increased insulin secretion 3.8- or 4.2-fold (P < 0.01) in response to glycerol or dihydroxyacetone, respectively. This cell engineering method was then modified, using the Cre-loxP switching system, to activate beta-cells and non-beta-cells separately in rat islets. NaDC-1 expression only in non-beta-cells, among which alpha-cells are predominant, caused an increase (by 1.8-fold, P < 0.05) in glucagon secretion in response to malate or succinate. However, the increase in glucagon release was prevented when NaDC-1 was expressed in whole islets, i.e., both beta-cells and non-beta-cells. Similarly, an increase in glucagon release with glycerol was observed when GlyK was expressed only in non-beta-cells but not when it was expressed in whole islets. Furthermore, dicarboxylates suppressed basal glucagon secretion by 30% (P < 0.05) when NaDC-1 was expressed only in beta-cells. These data demonstrate that glucagon secretion from rat alpha-cells depends on beta-cell activation and provide insights into the coordinated mechanisms underlying hormone secretion from pancreatic islets.

Human and murine glycerol kinase: influence of exon 18 alternative splicing on function

Glycerol kinase (GK) is a key enzyme in glycerol metabolism with two alternatively spliced forms-one with an 87bp insertion corresponding to exon 18 (GK+EX18), and one lacking exon 18 (GK-EX18). We report the expression of GK+/-EX18 in various tissues and cell lines, as well as their enzymatic characteristics and subcellular localization. RT-PCR revealed differential expression in tissues and cell lines. Northern blot analysis revealed that both forms of the murine ortholog, Gyk, were highly expressed in murine heart and increased during embryonic development. K(m) values for glycerol for GK+/-EX18 were not significantly different, although GK-EX18 had a higher V(max) for glycerol. GK-EX18 had a lower K(m) and V(max) for ATP than GK+EX18. Immunofluorescence experiments showed that GK+EX18 co-localized to the mitochondria and the perinuclear region while GK-EX18 had a diffuse expression pattern. These data suggest specific and divergent roles for GK+EX18 and GK-EX18 in cellular metabolism and development.

Single-gene disorders: what role could moonlighting enzymes play?

Single-gene disorders with "simple" Mendelian inheritance do not always imply that there will be an easy prediction of the phenotype from the genotype, which has been shown for a number of metabolic disorders. We propose that moonlighting enzymes (i.e., metabolic enzymes with additional functional activities) could contribute to the complexity of such disorders. The lack of knowledge about the additional functional activities of proteins could result in a lack of correlation between genotype and phenotype. In this review, we highlight some notable and recent examples of moonlighting enzymes and their possible contributions to human disease. Because knowledge and cataloging of the moonlighting activities of proteins are essential for the study of cellular function and human physiology, we also review recently reported and recommended methods for the discovery of moonlighting activities.

Five cases of isolated glycerol kinase deficiency, including two families: failure to find genotype:phenotype correlation

Little is understood of the genotype/phenotype correlations in X linked glycerol kinase deficiency (GKD) where most cases are caused by extensive deletions of Xp21, which often include genes flanking the GK locus. Few cases of isolated GKD have been investigated where the phenotype is not influenced by neighbouring genes. In this paper, we present the mutation data from four confirmed and one suspected case of non-deletion, isolated, X linked GKD and therefore extend the base of patients that can allow an assessment of genotype/phenotype correlations for this disease. The mutations found were two terminations leading to premature truncation of the GK polypeptide chain, one insertion, and an amino acid substitution. Phenotypic variation was observed in two families, where there was more than one affected subject carrying the same mutation, confirming previous studies that suggest there is no correlation between disease severity and genotype. Furthermore, the nature of the mutation in different families does not appear to influence the spectrum of phenotypic variation. In addition, one coding polymorphism in exon 3 has been found. The characterisation of the gene structure has been completed and shows that instead of 19 there are 21 exons.

X-linked glycerol kinase deficiency in the mouse leads to growth retardation, altered fat metabolism, autonomous glucocorticoid secretion and neonatal death

Glycerol kinase is an X chromosome-encoded enzyme involved in the metabolism of endogenous and dietary glycerolipids. The physiological significance of its activity in mammals is not well understood. Glycerol kinase deficiency in humans occurs as an isolated enzyme deficiency or as part of a contiguous gene deletion syndrome in variable association with Duchenne muscular dystrophy and adrenal hypoplasia congenita. Isolated glycerol kinase deficiency has an inconstant phenotype, ranging from asymptomatic hyperglycerolemia to a severe metabolic disorder with growth and psychomotor retardation. Although intragenic mutations were reported recently, the pathophysiological basis for the phenotypic variability remains unknown. To understand better the physiological significance of glycerol kinase and the pathophysiology of its deficiency, we generated glycerol kinase-deficient mice by gene targeting. Mutant male mice appear normal at birth, but exhibit postnatal growth retardation, altered fat metabolism with profound hyperglycerolemia and elevated free fatty acids, autonomous glucocorticoid synthesis and death by 3-4 days of age. Heterozygous females are healthy and biochemically normal. The biochemical features observed in glycerol kinase-deficient mice provide the basis for further investigations into the pathogenesis of the human disorder.

Identification of new members of a carbohydrate kinase-encoding gene family

In a sequence database search using the human glycerol kinase-encoding sequence (HUMGLYKINB) as a query, we identified six previously unidentified carbohydrate kinase sequences. Five of the six newly identified sequences appear to be known types of carbohydrate kinases, four are glycerol kinases and one is a gluconokinase. The sixth newly identified sequence, the Caenorhabditis elegans gene, CER08D7.7-CEF59B2.1, shows similarity to the family of carbohydrate kinases including other glycerol kinases, xylulokinases, gluconokinases, ribulokinases, rhamnulokinases, and fucokinases. A phylogenetic comparison of this newly identified Caenorhabditis elegans gene with the other members of the carbohydrate kinase family demonstrated that this sequence cannot be assigned to one of the known classes of carbohydrate kinases.

Nitric oxide- and hydrogen peroxide-mediated gene expression by glucocorticoids and FK506 in histamine paw edema of mice

An immunosuppressant FK506 binds with a component (hsp 56) of glucocorticoid receptor (GR) complex. Dexamethasone (Dex) never suppressed histamine paw edema of mice before 1 hr after its dosing as new protein(s) synthesis is required. However, FK506 (0.01-10 mg/kg, oral) 1.5 hr before 0.1 mg/kg Dex (s.c.), suppressed edema at 30 min. This suppression and that at 3 hr, were abolished by nitric oxide (NO) synthesis inhibitors (1-300 mg/kg). Nitroprusside (NO donor), catalase and molybdate (GR complex stabilizing protease inhibitor) enhanced the suppression. FK506, not cyclosporin A, was demonstrated for the first time in vivo to enhance GR and a hypothesis is proposed that FK506 might enhance GR and AP-1 signalings in a system reciprocally controlled by NO and H2O2.

Purification and characterization of a macromolecular-translocation inhibitor III of activated glucocorticoid-receptor-complex binding to nuclei from rat liver

Macromolecular-translocation inhibitors (MTI) of binding of the activated glucocorticoid-receptor complex (GRC) to nuclei from rat liver are separated into at least three components (MTI-I-III) by DEAE-cellulose column chromatography [Okamoto, K., Isohashi, F., Horiuchi, M. & Sakamoto, Y. (1982) Biochem. Biophys. Res. Commun. 108, 1655-1660]. In this study, we have purified MTI-III from the livers of adrenalectomized rats to apparent homogeneity, as determined by SDS/PAGE. The purification procedure consisted of DEAE-cellulose chromatography, acid treatment and sequential chromatographies using gel filtration, S-Sepharose and Mono S columns. The purified protein had a molecular mass of approximately 69 kDa, as estimated by SDS/PAGE, and the molecular mass of the inhibitor was approximately 68 kDa, as estimated by gel filtration. Thus, MTI-III exerts its inhibitory activity as a monomer. The sedimentation coefficient of MTI-III was approximately 3.7 S. Purified MTI-III was fairly stable at 4 degrees C but at higher temperatures, especially above 25 degrees C, it was rapidly inactivated. Under low-salt conditions, MTI-III was associated with activated GRC (4.2 S) and the resulting complex was detected on sucrose density gradients as a larger species (6.8 S). Initial treatment of nuclei or DNA-cellulose with MTI-III did not alter their abilities to bind activated GRC. These results indicate that MTI-III acts through an interaction with GRC.