Lipocalin Superfamily Research Articles

Ligand Recognition of Lipocalin-Type Prostaglandin D Synthase

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is a multi functional protein that functions as a PGD2 synthesizing enzyme, a scavenger of various lipophilic ligands, and an amyloid β chaperone in the brain. L-PGDS is a member of the lipocalin superfamily and has the ability to bind various lipophilic molecules, such as prostanoid, retinoid, bile pigment, and amyloid β peptide. However, the molecular mechanism for this wide variety of ligand binding is not fully understood. In this study, we determined the NMR structures of recombinant mouse L-PGDS and a L-PGDS/PGH2 analog. L-PGDS has a typical lipocalin fold, consisting of an eight-stranded β-barrel and a single α-helix. The interior of the barrel forms a hydrophobic cavity and the upper end of the barrel is open. The size of the barrel was found to be larger than those of other lipocalins and the cavity contained two pockets. The results of NMR titration, kinetic and molecular docking experiments revealed that PGH2 and retinoic acid occupied the hydrophilic pockets 1 and 2, respectively. A structural comparison of the L-PGDS/PGH2 analog complex with apo-L-PGDS showed that the H2-helix, CD-loop, and EF-loop located at the upper end of the β-barrel undergo a conformational change and cover the entrance of the cavity upon U-46619 binding. These results indicate that the two binding sites in the large cavity are responsible for the broad ligand specificity of L-PGDS in its induced-fit mechanism.

Drug delivery system for poorly water-soluble compounds using lipocalin-type prostaglandin D synthase

Lipocalin-type prostaglandin D synthase (L-PGDS) is a member of the lipocalin superfamily and a secretory lipid-transporter protein, which binds a wide variety of hydrophobic small molecules. Here we show the feasibility of a novel drug delivery system (DDS), utilizing L-PGDS, for poorly water-soluble compounds such as diazepam (DZP), a major benzodiazepine anxiolytic drug, and 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione (NBQX), an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist and anticonvulsant. Calorimetric experiments revealed for both compounds that each L-PGDS held three molecules with high binding affinities. By mass spectrometry, the 1:3 complex of L-PGDS and NBQX was observed. L-PGDS of 500μM increased the solubility of DZP and NBQX 7- and 2-fold, respectively, compared to PBS alone. To validate the potential of L-PGDS as a drug delivery vehicle in vivo, we have proved the prospective effects of these compounds via two separate delivery strategies. First, the oral administration of a DZP/L-PGDS complex in mice revealed an increased duration of pentobarbital-induced loss of righting reflex. Second, the intravenous treatment of ischemic gerbils with NBQX/L-PGDS complex showed a protective effect on delayed neuronal cell death at the hippocampal CA1 region. We propose that our novel DDS could facilitate pharmaceutical development and clinical usage of various water-insoluble compounds.

NGAL: a biomarker of acute and chronic renal dysfunction

Neutrophil gelatinase-associated lipocalin (NGAL) is a glycoprotein of 25 kDa belonging to the superfamily of lipocalins, which counts low molecular mass proteins having the capacity to fix the iron. The NGAL presents bacteriostatic properties and is a factor of growth and differentiation, especially in response to renal tissue damage and during the nephrogenesis. Since the past 10 years, numerous clinical studies suggest that urinary and/or blood levels of NGAL could be a relevant biomarker of acute or chronic renal failure, in particular in the context of the diabetic nephropathy. NGAL could be a more sensitive and more specific marker than the albuminuria and might detect the early appearance of the renal lesions, and thus could be useful to prevent or reduce severity of renal function alterations.

Ligand Recognition Mechanism of Lipocalin-type Prostaglandin D Synthase

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is a multi functional protein acting as a PGD(2) synthesizing enzyme, a transporter or scavenger of various lipophilic ligands, and an amyloid β chaperon in the brain. L-PGDS is a member of the lipocalin superfamily and has the ability to bind various lipophilic molecules such as prostanoid, retinoid, bile pigment, and amyloid β peptide. However, the molecular mechanism for a wide variety of ligand binding has not been well understood. In this study, we determined by NMR the structure of recombinant mouse L-PGDS and L-PGDS/PGH(2) analog complex. L-PGDS has the typical lipocalin fold, consisting of an eight-stranded β-barrel and a long α-helix. The interior of the barrel formed a hydrophobic cavity opening to the upper end of the barrel, the size of which was larger than those of other lipocalins and the cavity contained two pockets. Kinetic studies and molecular docking studies based on the result of NMR titration experiments provide the direct evidence for two binding sites for PGH(2) and retinoic acid in the large cavity of L-PGDS. Structural comparison of L-PGDS/U-46619 complex with apo-L-PGDS showed that the H2-helix, CD-loop, and EF-loop located at the upper end of the β-barrel change the conformation to cover the entry of the cavity upon U-46619 binding. These results indicated that the two binding sites in the large cavity and induced fit mechanism were responsible for the broad ligand specificity of L-PGDS.

Importance of pH and disulfide bridges on the structural and binding properties of human α 1-acid glycoprotein

Human α 1-acid glycoprotein (AGP) is an acute phase plasma glycoprotein containing two disulfide bridges. As a member of the lipocalin superfamily, it binds and transports several basic and neutral ligands, but a number of other activities have also been described. Thanks to its binding properties, AGP is also a good candidate for the development of biosensors and affinity chromatography media, and in this context detailed structural information is needed. The structural properties of AGP at different p 2Hs and under reducing conditions were analysed by FT-IR spectroscopy. The obtained data indicate that AGP, when denatured, does not aggregate at neutral or basic p 2Hs whilst it does at acidic p 2Hs. Under reducing conditions the protein is remarkably less thermostable than its oxidized counterpart and presents an enhanced tendency to aggregate, even at neutral p 2H. A heat-induced molten globule-like state (MG) was detected at 55 °C at p 2H 7.4 and 5.5. At p 2H 4.5 the MG occurred at 45 °C with an onset of formation at 40 °C. The MG was not observed under reducing conditions. A lower affinity of chlorpromazine and progesterone for the MG formed at p 2H 4.5 and 40 °C was observed, suggesting that ligand(s) may be released near the negative surfaces of biological membranes. Furthermore, the reduced AGP displays an enhanced affinity for progesterone, indicating the importance of disulfide bonds for the binding capacity of AGP.

The conserved disulfide bond of human tear lipocalin modulates conformation and lipid binding in a ligand selective manner

The primary aim of this study is the elucidation of the mechanism of disulfide induced alteration of ligand binding in human tear lipocalin (TL). Disulfide bonds may act as dynamic scaffolds to regulate conformational changes that alter protein function including receptor–ligand interactions. A single disulfide bond, (Cys61―Cys153), exists in TL that is highly conserved in the lipocalin superfamily. Circular dichroism and fluorescence spectroscopies were applied to investigate the mechanism by which disulfide bond removal effects protein stability, dynamics and ligand binding properties. Although the secondary structure is not altered by disulfide elimination, TL shows decreased stability against urea denaturation. Free energy change (ΔG0) decreases from 4.9±0.2 to 2.1±0.3kcal/mol with removal of the disulfide bond. Furthermore, ligand binding properties of TL without the disulfide vary according to the type of ligand. The binding of a bulky ligand, NBD-cholesterol, has a decreased time constant (from 11.8±0.2 to 3.3s). In contrast, the NBD-labeled phospholipid shows a moderate decrease in the time constant for binding, from 33.2±0.2 to 22.2±0.4s. FRET experiments indicate that the hairpin CD is directly involved in modulation of both ligand binding and flexibility of TL. In TL complexed with palmitic acid (PA–TL), the distance between the residues 62 of strand D and 81 of loop EF is decreased by disulfide bond reduction. Consequently, removal of the disulfide bond boosts flexibility of the protein to reach a CD-EF loop distance (24.3Å, between residues 62 and 81), which is not accessible for the protein with an intact disulfide bond (26.2Å). The results suggest that enhanced flexibility of the protein promotes a faster accommodation of the ligand inside the cavity and an energetically favorable ligand-protein complex.

Molecular Cloning, Sequencing, and Gene Expression Analysis of Tributyltin-Binding Protein Type 1 in Japanese Medaka Fish,Oryzias latipes

The full-length cDNA sequence of tributyltin-binding protein type 1 in Japanese medaka (Oryzias latipes) (Olat.TBT-bp1) was determined by means of rapid amplification of cDNA ends (RACE) of liver tissue. Analysis of the structure of the gene encoding Olat.TBT-bp1 revealed that the exonintron organization of this gene corresponds to that of the genes encoding lipocalin superfamily proteins, suggesting that Olat.TBT-bp1 can be categorized as a member of the lipocalin superfamily, which may play an important role in transportation, detoxification, and excretion of xenobiotic compounds. Reverse transcription - PCR revealed that Olat.TBT-bp1 was expressed mainly in the liver, and upregulation of its expression was detected 1, 2, and 4 weeks post hatching. Relative expression of the Olat.TBT-bp1 gene was significantly downregulated, compared with that in the solvent control, by exposure to tributyltin at 0.01 mg/l or triclosan at 1.7 mg/l. Further studies on Olat.TBT-bp1 expression in conjunction with other biochemical and physiological toxicities in response to chemical exposures are needed to increase our understanding and information of TBT-bps mechanisms and as molecular biomarkers of chemical exposures. The role of Olat.TBT-bp1 in xenobiotic detoxification and/or excretion needs more investigations.

Induction of lipocalin-2 expression in acute and chronic experimental liver injury moderated by pro-inflammatory cytokines interleukin-1β through nuclear factor-κB activation

Lipocalin-2 (LCN2) belongs to the lipocalin superfamily, sharing a barrel-shaped tertiary structure with a hydrophobic pocket and an ability to bind lipophilic molecules. LCN2 has recently emerged as an important modulator of cellular homeostasis in several organs, i.e. heart, lung and kidney, but little is known about the expression of LCN2 in acute and chronic liver injury. In this study, we wanted to analyse the expression and regulation of LCN2 in models of acute and chronic experimental liver injury. We analysed LCN2 expression in livers of rats subjected to bile duct ligation or repeated doses of carbon tetrachloride and tested the impact of various pro-inflammatory cytokines in cultured primary liver cells. By using primary cultures of hepatic stellate cells and hepatocytes isolated from normal and injured rat livers, we found a significant LCN2 expression in early hepatic stellate cell cultures, a lower expression in fully transdifferentiated myofibroblasts and no expression in freshly isolated hepatocytes. However, LCN2 expression and secretion in hepatocytes increased dramatically during culturing. In addition, chronic in vivo liver injury resulting from both bile duct ligation and repeated application of carbon tetrachloride resulted in rapid and well-sustained induction of LCN2 expression. Immunohistochemistry and primary liver cell isolation identified injured hepatocytes as the main source of LCN2 production. LCN2 is strongly induced in both primary hepatocytes and immortalized hepatocellular carcinoma cell line HepG2 by the pro-inflammatory cytokine interleukin-1β via nuclear factor-κB activation, but not by the profibrotic cytokines platelet-derived growth factor and transforming growth factor-β. LCN2 expression shows clear correlation to liver damage and resulting inflammatory responses, rather than to the degree of liver fibrosis, which in fact may imply a distinct diagnostic value as an early biomarker of liver inflammation.

NGAL is a Precocious Marker of Therapeutic Response

NGAL (Neutrophil Gelatinase-Associated Lipocalin) is a small 25-kD peptide belonging to the lipocalin superfamily. Several studies highlight its role as an organ injury and disease activity biomarker. In the present review, instead, we wanted to study NGAL as a precocious marker of therapeutic response in renal and non-renal diseases (glomerulonephritis, vasculitis, LES, Crohn's disease and other chronic inflammatory pathologies). The obtained outcomes support the hypothesis that NGAL could be employed as a biomarker of response to different therapeutic schemes, because its levels sensibly and precociously change compared to other haematologic and biochemical parameters.

Lipocalin-1: a potential marker for noninvasive aneuploidy screening

This study has identified the first protein, lipocalin-1, in the secretome of human blastocysts that is associated with chromosome aneuploidy. The development of a noninvasive technique to determine an embryo's developmental competence, including chromosomal constitution, by analysis of spent IVF culture medium will be a powerful tool for embryo selection in assisted reproductive technologies.

Lipocalin-13 Regulates Glucose Metabolism by both Insulin-Dependent and Insulin-Independent Mechanisms

Insulin sensitivity is impaired in obesity, and insulin resistance is the primary risk factor for type 2 diabetes. Here we show that lipocalin-13 (LCN13), a lipocalin superfamily member, is a novel insulin sensitizer. LCN13 was secreted by multiple cell types. Circulating LCN13 was markedly reduced in mice with obesity and type 2 diabetes. Three distinct approaches were used to increase LCN13 levels: LCN13 transgenic mice, LCN13 adenoviral infection, and recombinant LCN13 administration. Restoration of LCN13 significantly ameliorated hyperglycemia, insulin resistance, and glucose intolerance in mice with obesity. LCN13 enhanced insulin signaling not only in animals but also in cultured adipocytes. Recombinant LCN13 increased the ability of insulin to stimulate glucose uptake in adipocytes and to suppress hepatic glucose production (HGP) in primary hepatocyte cultures. Additionally, LCN13 alone was able to suppress HGP, whereas neutralization of LCN13 increased HGP in primary hepatocyte cultures. These data suggest that LCN13 regulates glucose metabolism by both insulin-dependent and insulin-independent mechanisms. LCN13 and LCN13-related molecules may be used to treat insulin resistance and type 2 diabetes.

Dipetalodipin, a Novel Multifunctional Salivary Lipocalin That Inhibits Platelet Aggregation, Vasoconstriction, and Angiogenesis through Unique Binding Specificity for TXA2, PGF2α, and 15(S)-HETE

Dipetalodipin (DPTL) is an 18 kDa protein cloned from salivary glands of the triatomine Dipetalogaster maxima. DPTL belongs to the lipocalin superfamily and has strong sequence similarity to pallidipin, a salivary inhibitor of collagen-induced platelet aggregation. DPTL expressed in Escherichia coli was found to inhibit platelet aggregation by collagen, U-46619, or arachidonic acid without affecting aggregation induced by ADP, convulxin, PMA, and ristocetin. An assay based on incubation of DPTL with small molecules (e.g. prostanoids, leukotrienes, lipids, biogenic amines) followed by chromatography, mass spectrometry, and isothermal titration calorimetry showed that DPTL binds with high affinity to carbocyclic TXA(2), TXA(2) mimetic (U-46619), TXB(2), PGH(2) mimetic (U-51605), PGD(2,) PGJ(2), and PGF(2α). It also interacts with 15(S)-HETE, being the first lipocalin described to date to bind to a derivative of 15-lipoxygenase. Binding was not observed to other prostaglandins (e.g. PGE(1), PGE(2), 8-iso-PGF(2α), prostacyclin), leukotrienes (e.g. LTB(4), LTC(4), LTD(4), LTE(4)), HETEs (e.g. 5(S)-HETE, 12(S)-HETE, 20-HETE), lipids (e.g. arachidonic acid, PAF), and biogenic amines (e.g. ADP, serotonin, epinephrine, norepinephrine, histamine). Consistent with its binding specificity, DPTL prevents contraction of rat uterus stimulated by PGF(2α) and induces relaxation of aorta previously contracted with U-46619. Moreover, it inhibits angiogenesis mediated by 15(S)-HETE and did not enhance inhibition of collagen-induced platelet aggregation by SQ29548 (TXA(2) antagonist) and indomethacin. A 3-D model for DPTL and pallidipin is presented that indicates the presence of a conserved Arg(39) and Gln(135) in the binding pocket of both lipocalins. Results suggest that DPTL blocks platelet aggregation, vasoconstriction, and angiogenesis through binding to distinct eicosanoids involved in inflammation.

Purification and characterization of tributyltin-binding protein of tiger puffer, Takifugu rubripes

We successfully purified Trub.TBT-bpα, a tributyltin (TBT) binding protein (bp) of the tiger puffer, Takifugu rubripes. Tiger puffer was injected intraperitoneally with TBT (1.0 mg/kg body weight) and Trub.TBT-bpα was purified from serum by ammonium sulfate fractionation, gel filtration chromatography and polyacrylamide gel electrophoresis. Gel electrophoresis revealed that the Trub.TBT-bpα has a molecular mass of approximately 48.5 kDa and contains at least 40% N-glycan. The deduced 212 amino acid sequence of the protein showed the highest identity (41%, 212 amino acid overlap and E-value: 9e−42) with TBT-binding protein type 1 (TBT-bp1) of Paralichthys olivaceus (Japanese flounder). Analysis of the gene structure of Trub.TBT-bpα suggests that this protein belongs to the lipocalin superfamily, which may be important in the accumulation and elimination of TBT. Phylogenetic analysis suggests that functionalization of TBT-bps has occurred during evolution, and that the functions of this group of proteins might be important for fish survival.

Roborovskin, a Lipocalin in the Urine of the Roborovski Hamster, Phodopus roborovskii

Many rodents are now known to exhibit an obligate proteinuria that delivers urine-mediated chemosignals. In this paper, we explore the urinary proteins of the Roborovski hamster (Phodopus roborovskii). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of urine from individual male and female Roborovski hamsters revealed 2 proteins, with approximate masses of 6 and 17 kDa, the expression pattern of which showed little variation between individuals or between sexes. Peptide mass fingerprints obtained from these 2 proteins revealed a number of features: 1) the proteins of a given mass were the same in all individuals regardless of sex, 2) the 6 kDa protein was not a fragment of the 21 kDa protein, and 3) neither protein was a fragment of a larger, conserved protein such as serum albumin. Electrospray mass spectrometry of purified protein preparations established the mass of the larger protein as invariant, at 17144 ± 2 Da in all samples. This protein has been termed roborovskin. The primary structure of roborovskin was determined by tandem mass spectrometry of peptides derived from independent and overlapping digestion with 3 proteases, supported by Edman degradation of the protein N-terminus. Roborovskin shared significant homology with olfactory-binding proteins from Myodes glareolus (bank vole) and with aphrodisin and submandibular protein from the golden hamster Mesocricetus auratus, all of which belong to the lipocalin superfamily. Lower levels of homology were also indicated between a variety of other lipocalins including the major urinary proteins from house mice and Norway rats. A model of the tertiary structure of roborovskin was constructed from the primary sequence by homology modeling. This model structure resembled other 8-stranded beta barrel lipocalins. Thus, the Roborovski hamster may demonstrate another variant of urinary lipocalin expression, as for the animals studied here, there appears to be no polymorphism in expression either between sexes or individuals.

Cloning, overexpression, purification, crystallization and preliminary X-ray analysis of a female-specific lipocalin (FLP) expressed in the lacrimal glands of Syrian hamsters.

Proteins belonging to the lipocalin superfamily are usually secretory proteins of molecular mass approximately 20 kDa with a hydrophobic pocket for the binding and transport of diverse small ligands. Various lipocalins have been associated with many biological processes, e.g. immunomodulation, odorant transport, pheromonal activity, retinoid transport, cancer-cell interactions etc. However, the exact functions of many lipocalins and the ligands bound by them are unclear. Previously, the cDNA of a 20 kDa lipocalin (FLP) which is female-specifically expressed in the lacrimal glands of Syrian (golden) hamsters and secreted in the tears of females has been identified and cloned. His-tagged recombinant FLP (rFLP) has now been cloned, overexpressed in Escherichia coli as a soluble protein and purified to homogeneity using Ni-affinity followed by size-exclusion chromatography. Purified rFLP was crystallized using the sitting-drop vapour-diffusion method. The crystals tested belonged to space group P2(1)2(1)2(1) and diffracted to beyond 1.86 A resolution. Solvent-content analysis indicated the presence of one monomer in the asymmetric unit.

Discrepancy between mRNA and Protein Expression of Neutrophil Gelatinase-Associated Lipocalin in Bronchial Epithelium Induced by Sulfur Mustard

Sulfur mustard (SM) is a potent vesicant that has been employed as a chemical weapon in various conflicts during the 20th century. More recently, mustard was used in the Iraq conflict against Iranian troops and civilians. At the present time there are more than 40.000 people suffering from pulmonary lesions special bronchiolitis obliterans (BOs) due to mustard gas. SM increases the endogenous production of reactive oxygen species (ROS). Neutrophil Gelatinase-associated Lipocalin 2 (Lcn2, NGAL) is a member of the lipocalin superfamily for which a variety of functions such as cellular protection against oxidative stress have been reported. Ten normal and Twenty SM-induced COPD patient individuals were studied. Assessment of NGAL expressions in healthy and the patients endobrinchial biopsies were performed by semiquantitative RT-PCR, real-time RT-PCR, and Immunohistochemistry analysis. While Normal control samples expressed same level of mRNA NGAL, expression level of mRNA-NGAL was upregulated about 1.4- to 9.8-folds compared to normal samples. No significant immunoreactivity was revealed in both samples. As we are aware this is the first report of induction of NGAL in patients exposed to SM. NGAL may play an important role in cellular protection against oxidative stress toxicity induced by mustard gas in airway wall of patients.

Neutrophil Gelatinase-Associated Lipocalin induces the expression of heme oxygenase-1 and superoxide dismutase 1, 2

Lipocalin-2 (Lcn2, NGAL) is a member of the lipocalin super family with diverse function such as the induction of apoptosis, the suppression of bacterial growth, and modulation of inflammatory response. Much interest has recently been focused on the physiological/pathological role of the lipocalin-2 that is considered to be a novel protective factor against oxidative stress. However, its precise biological roles in this protection are not fully understood. In this report we intended to test the effect of lipocalin-2 on the expression of heme oxygenase ((1, 2)) and superoxide dismutase ((1, 2)) which are two strong antioxidants. NGAL was cloned to pcDNA3.1 plasmid by using genetic engineering method. The recombinant vector was transfected to CHO and HEK293T to establish stable cell expressing NGAL and the expression of HO-1, 2 and SOD(1, 2) were compared with appropriate controls by RT-PCR and western blot. On the other hand, expression of NGAL was suppressed by siRNA transfection in order to study the effect of lipocalin-2 on mentioned genes/proteins. The results showed that the expression of HO-1 and SOD(1, 2) enzymes were higher in cells expressing recombinant lipocalin-2 compared with the control cells. Although the expression of HO-1 was lower in NGAL silencing cells, the expression of SOD(1) and SOD(2) were higher. Our data suggest that NGAL is a potent inducer of HO-1 and somewhat SOD(1) and SOD(2) and it appears that part of antioxidant property of NGAL could be attributed to the induction of HO-1 and SOD(1, 2).

Adipocytokines and Insulin Resistance

It is well known that adipocytes and resident macrophages that have migrated to adipose tissue produce and secrete a variety of biologically active mediators (adipocytokines), which are thought to contribute to the development of insulin resistance, type 2 diabetes, and cardiovascular disease (1). The abnormal function of adipocytes may play an important role in the development of a chronic low-grade proinflammatory state associated with obesity (2). For example, adipocyte hypertrophy appears to lead to an imbalance between pro- and anti-inflammatory adipokines. The secretion of interleukin (IL)-6, IL-8, monocyte chemoattractant protein-1, and granulocyte colony–stimulating factor have been positively correlated with adipocyte size. Adipose tissue is an important inflammatory source in obesity and type 2 diabetes, not only because of cytokines produced from the adipocyte itself, but also because of infiltration by proinflammatory macrophages (3). Not only do adipocytes, but also adipose tissue macrophage numbers, increase with obesity and participate in inflammatory pathways of obese individuals. Macrophages from adipose tissue are responsible for almost all adipose tissue tumor necrosis factor (TNF)-α and significant amounts of IL-6 production. Macrophages migrating to adipose tissue in response to high-fat feeding overexpress proinflammatory cytokines. Different cytokines synthesized by adipocytes or by macrophages from adipose tissue may induce insulin resistance, such as IL-6, TNF-α, leptin, resistin, adiponectin, retinol binding protein-4 (RBP4), or lipocalin-2 (LCN2). This review focuses on the latter adipocytokines, hinting at their role in obesity-associated insulin resistance. LCN2 (or neutrophil gelatinase-associated lipocalin) is a recently identified adipokine that belongs to the superfamily of lipocalins (such as RBP4), which seems to affect glucose metabolism and insulin sensitivity (4). LCN2 protein has been implicated in diverse actions, such as apoptosis and innate immunity, and is expressed in several tissues, including neutrophils, liver, kidney, adipocytes, and macrophages (5). Lipocalins comprise a class of proteins that are …

NGAL: a biomarker of acute kidney injury and other systemic conditions

Neutrophil gelatinase-associated lipocalin (NGAL) is a 25 kDa protein belonging to the lipocalin superfamily. It was initially found in activated neutrophils, however, many other cells, like kidney tubular cells, may produce NGAL in response to various insults. Recently, it has been found to have a role in iron metabolism by virtue of its binding with siderophores. It has also been found to have a role in kidney development and tubular regeneration after injury. In experimental studies, it was found to be highly expressed in response to tubular injury. In subsequent clinical studies, urine NGAL has been found to be an early predictor for acute kidney injury (AKI). Newer devices for early bedside detection of NGAL are now available. Since serum creatinine is known to be an inadequate and late marker of AKI, NGAL might soon emerge as a troponin-like early marker for AKI. Recent evidence also suggests its role as a biomarker in a variety of other renal and non-renal conditions.

Major Urinary Protein-1 Increases Energy Expenditure and Improves Glucose Intolerance through Enhancing Mitochondrial Function in Skeletal Muscle of Diabetic Mice

Major urinary protein-1 (MUP-1) is a low molecular weight secreted protein produced predominantly from the liver. Structurally it belongs to the lipocalin family, which carries small hydrophobic ligands such as pheromones. However, the physiological functions of MUP-1 remain poorly understood. Here we provide evidence demonstrating that MUP-1 is an important player in regulating energy expenditure and metabolism in mice. Both microarray and real-time PCR analysis demonstrated that the MUP-1 mRNA abundance in the liver of db/db obese mice was reduced by approximately 30-fold compared with their lean littermates, whereas this change was partially reversed by treatment with the insulin-sensitizing drug rosiglitazone. In both dietary and genetic obese mice, the circulating concentrations of MUP-1 were markedly decreased compared with the lean controls. Chronic elevation of circulating MUP-1 in db/db mice, using an osmotic pump-based protein delivery system, increased energy expenditure and locomotor activity, raised core body temperature, and decreased glucose intolerance as well as insulin resistance. At the molecular level, MUP-1-mediated improvement in metabolic profiles was accompanied by increased expression of genes involved in mitochondrial biogenesis, elevated mitochondrial oxidative capacity, decreased triglyceride accumulation, and enhanced insulin-evoked Akt signaling in skeletal muscle but not in liver. Altogether, these findings raise the possibility that MUP-1 deficiency might contribute to the metabolic dysregulation in obese/diabetic mice, and suggest that the beneficial metabolic effects of MUP-1 are attributed in part to its ability in increasing mitochondrial function in skeletal muscle.