Beyond the superfamily: the lipocalin receptors

Lipocalin signaling controls unicellular tube development in the Caenorhabditis elegans excretory system

Unicellular tubes or capillaries composed of individual cells with a hollow lumen perform important physiological functions including fluid or gas transport and exchange. These tubes are thought to build intracellular lumina by polarized trafficking of apical membrane components, but the molecular signals that promote luminal growth and luminal connectivity between cells are poorly understood. Here we show that the lipocalin LPR-1 is required for luminal connectivity between two unicellular tubes in the Caenorhabditis elegans excretory (renal) system, the excretory duct cell and pore cell. Lipocalins are a large family of secreted proteins that transport lipophilic cargos and participate in intercellular signaling. lpr-1 is required at a time of rapid luminal growth, it is expressed by the duct, pore and surrounding cells, and it can function cell non-autonomously. These results reveal a novel signaling mechanism that controls unicellular tube formation, and provide a genetic model system for dissecting lipocalin signaling pathways.

Ethanol effect on the structure of beta-lactoglobulin b and its ligand binding

The changes of structure and ligand binding properties of beta-LG B have been studied by fluorescence and circular dichroism spectroscopy in ethanolic solutions. Fluorescence measurements of retinol/beta-LG interactions at 480 nm in various ethanol concentrations show that the maximal fluorescence intensity induced by this interaction between retinol and beta-LG is observed around 20% v/v of ethanol. It is reduced to zero at 40% and 50% of ethanol. These results suggest that there are two distinct structural changes in beta-LG occurring between 20% and 30% and around 40% of ethanol. The first transition, which increases affinity and the apparent number of binding sites for retinol, may be related or similar to the Tanford transition. The strong quenching of retinol emission at 480 nm in 40% of ethanol indicates the radical transformation of beta-LG tertiary structure and the release of retinol. CD spectra at the aromatic region show that secondary and tertiary structures of beta-LG are not significantly affected between 0% and 20% of ethanol. In 30% of ethanol, beta-sheet percentage of beta-LG decreases with respect to native beta-LG (from 55% to 46%). beta-Sheet percentage in beta-LG increases in 40% and 50% alcohol (51% and 53%) relative to 30% of ethanol, which also indicates the strong rearrangement of the secondary structure of beta-LG, while its tertiary structure and beta-LG interactions are radically changed.

On a chip demonstration of a functional role for Odorant Binding Protein in the preservation of olfactory receptor activity at high odorant concentration

The molecular mechanisms underlying odorant detection have been investigated using the chip based SPR technique by focusing on the dynamic interactions between transmembrane Olfactory Receptor OR1740, odorant ligands and soluble Odorant-Binding Protein (OBP-1F). The OR1740 present in the lipid bilayer of nanosomes derived from transformed yeasts specifically bound OBP-1F. The receptor preferential odorant ligand helional released bound OBP-1F from the OR-OBP complex, while unrelated odorants failed to do so. OBP-1F modified the functional OR1740 dose-response to helional, from a bell-shaped to a saturation curve, thus preserving OR activity at high ligand concentration. This unravels an active role for OBPs in olfaction, in addition to passive transport or a scavenger role. This sensorchip technology was applied to assessing native OBP-1F in a biological sample: rat olfactory mucus also displayed significant binding to OR1740 nanosomes, and the addition of helional yielded the dissociation of mucus OBP from the receptor.

Resolution of liver cirrhosis using vitamin A-coupled liposomes to deliver siRNA against a collagen-specific chaperone

There are currently no approved antifibrotic therapies for liver cirrhosis. We used vitamin A-coupled liposomes to deliver small interfering RNA (siRNA) against gp46, the rat homolog of human heat shock protein 47, to hepatic stellate cells. Our approach exploits the key roles of these cells in both fibrogenesis as well as uptake and storage of vitamin A. Five treatments with the siRNA-bearing vitamin A-coupled liposomes almost completely resolved liver fibrosis and prolonged survival in rats with otherwise lethal dimethylnitrosamine-induced liver cirrhosis in a dose- and duration-dependent manner. Rescue was not related to off-target effects or associated with recruitment of innate immunity. Receptor-specific siRNA delivery was similarly effective in suppressing collagen secretion and treating fibrosis induced by CCl(4) or bile duct ligation. The efficacy of the approach using both acute and chronic models of liver fibrosis suggests its therapeutic potential for reversing human liver cirrhosis.

The role of lipocalin 2 in the regulation of inflammation in adipocytes and macrophages

Adipose tissue-derived cytokines (adipokines) are associated with the development of inflammation and insulin resistance. However, which adipokine(s) mediate this linkage and the mechanisms involved during obesity is poorly understood. Through proteomics and microarray screening, we recently identified lipocalin 2 (LCN 2) as an adipokine that potentially connects obesity and its related adipose inflammation. Herein we show that the levels of LCN2 mRNA are dramatically increased in adipose tissue and liver of ob/ob mice and primary adipose cells isolated from Zucker obese rats, and thiazolidinedione administration reduces LCN2 expression. Interestingly, addition of LCN2 induces mRNA levels of peroxisome proliferator-activated receptor-gamma (PPARgamma) and adiponectin. Reducing LCN2 gene expression causes decreased expression of PPARgamma and adiponectin, slightly reducing insulin-stimulated Akt2 phosphorylation at Serine 473 in 3T3-L1 adipocytes. LCN2 administration to 3T3-L1 cells attenuated TNFalpha-effect on glucose uptake, expression of PPARgamma, insulin receptor substrate-1, and glucose transporter 4, and secretion of adiponectin and leptin. When added to macrophages, LCN2 suppressed lipopolysaccharide-induced cytokine production. Our data suggest that LCN2, as a novel autocrine and paracrine adipokine, acts as an antagonist to the effect of inflammatory molecules on inflammation and secretion of adipokines.

Structural Insight into the Dual Ligand Specificity and Mode of High Density Lipoprotein Association of Apolipoprotein D

Human apolipoprotein D (ApoD) occurs in plasma associated with high density lipoprotein. Apart from the involvement in lipid metabolism, its binding activity for progesterone and arachidonic acid plays a role in cancer development and neurological diseases. The crystal structures of free ApoD and its complex with progesterone were determined at 1.8A resolution and reveal a lipocalin fold. The narrow, mainly uncharged pocket within the typical beta-barrel accommodates progesterone with its acetyl side chain oriented toward the bottom. The cavity adopts essentially the same shape in the absence of progesterone and allows complexation of arachidonic acid as another cognate ligand. Three of the four extended loops at the open end of the beta-barrel expose hydrophobic side chains, which is an unusual feature for lipocalins and probably effects association with the high density lipoprotein particle by mediating insertion into the lipid phase. This mechanism is in line with an unpaired Cys residue in the same surface region that can form a disulfide cross-link with apolipoprotein A-II.

Age-related change of endocytic receptors megalin and cubilin in the kidney in rats

Megalin and cubilin are the major endocytic receptors responsible for resorption of glomerular filtrate proteins, particularly albumin, in the renal proximal tubule. In order to better understand the mechanism of the development of albuminuria with age in rats, we investigated age-related change of the amount and cellular localization of both receptors in the kidney. Immunoblot analysis of the kidney extracts showed that the amount of megalin significantly decreased with age. Although there was no age-related change in the amount of intact cubilin, the amount of cubilin fragments increased with age. Immunohistochemical study revealed that megalin and cubilin were predominantly localized in brush border membrane of proximal tubular cells in young rats, but the receptors tended to diffuse into the cytoplasm in the old rats. Interestingly, low but significant amounts of megalin and cubilin were present in the glomerular cells in addition to the proximal tubular cells. The quantity of receptors progressively increased in the glomerulus with age. This age-related increase might be to compensate for the age-related defect of the uptake of albumin by the proximal tubules. Thus, although it is unclear whether megalin and cubilin in the glomerulus contribute to the uptake of albumin in primary urine, the age-related increase in the amount of albumin in urine might at least partly be due to quantitative and qualitative alterations of both receptors in the proximal tubule.

A novel alternative spliced variant of neutrophil gelatinase-associated lipocalin receptor in oesophageal carcinoma cells

Recent studies suggest that NGAL (neutrophil gelatinase-associated lipocalin) is a novel iron transporter with functions distinct from that of transferrin and mediates a new iron-delivery pathway. To get a better understanding of NGAL function in oesophageal carcinoma, we analysed the expression of NGAL receptors in oesophageal carcinoma cells and identified a novel spliced variant designated NgalR-3. When expressed in a heterologous system, the protein produced from this novel spliced variant exhibits the biochemical characteristics of interaction and co-localization with NGAL protein in vivo. This new finding suggests that NgalR-3 may act as a potential NGAL receptor and play a role in NGAL-mediated iron transport in oesophageal carcinoma.

Acute endotoxemia is associated with upregulation of lipocalin 24p3/Lcn2 in lung and liver

Acute endotoxemia is associated with production of acute phase proteins which regulate inflammatory responses to tissue injury. Consistent with DNA microarray experiments, we found that acute endotoxemia, induced by administration of lipopolysaccharide (LPS) to mice (1 mg/kg) or rats (5 mg/kg), resulted in increased expression of the hepatic acute phase protein, lipocalin 24p3, which was evident within 4 h and persisted for 24-48 h. Increases in 24p3 expression were also observed in the lung after LPS administration, as well as in isolated liver and lung macrophages, and Type II alveolar epithelial cells. The actions of LPS are dependent, in part, on Toll-like receptor (TLR) proteins. Macrophages from C3H/HeJ mice, which possess a nonfunctional TLR-4, expressed low levels of 24p3 mRNA when compared to cells from control C3H/OuJ mice. Whereas LPS administration increased 24p3 expression in lung and liver macrophages from control C3H/OuJ mice, minimal effects were observed in TLR-4 mutant mice demonstrating that TLR-4 is important in regulating 24p3 expression during acute endotoxemia. Promoters for genes encoding lipocalin proteins including 24p3 contain consensus sequences for transcription factors including NF-kappaB, and C/EBP. Acute endotoxemia resulted in NF-kappaB nuclear binding activity in both alveolar macrophages and Type II cells. In contrast, C/EBP activation was evident only in Type II cells, suggesting differential effects of LPS on these cell types. These data suggest that the acute phase response to acute endotoxemia involves induction of 24p3 in both the lung and liver. This protein may be important in restoring tissue homeostasis following LPS-induced injury.

Structural features of the ligand binding site on human complement protein C8gamma: a member of the lipocalin family

Human C8 is one of five components of the cytolytic membrane attack complex of complement. It contains three subunits (C8alpha, C8beta, C8gamma) arranged as a disulfide-linked C8alpha-gamma heterodimer that is noncovalently associated with C8beta. C8gamma has the distinction of being the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small hydrophobic ligand. A natural ligand for C8gamma has not been identified; however previous structural studies indicate C8gamma has a typical lipocalin fold that is suggestive of a ligand-binding capability. A distinctive feature of C8gamma is the division of its putative ligand binding pocket into a hydrophilic upper portion and a large hydrophobic lower cavity. Access to the latter is restricted by the close proximity of two tyrosine side chains (Y83 and Y131). In the present study, binding experiments were performed using lauric acid as a pseudoligand to investigate the potential accessibility of the lower cavity. The crystal structure of a C8gamma.laurate complex revealed that Y83 and Y131 can move to allow penetration of the hydrocarbon chain of laurate into the lower cavity. Introducing a Y83W mutation blocked access but had no effect on the ability of C8gamma to enhance C8 cytolytic activity. Together, these results indicate that the lower cavity in C8gamma could accommodate a ligand if such a ligand has a narrow hydrophobic moiety at one end. Entry of that moiety into the lower cavity would require movement of Y83 and Y131, which act as a gate at the cavity entrance.

Vitamins A and E: metabolism, roles and transfer to offspring

Vitamins A and E are essential, naturally occurring, fat-soluble nutrients that are involved in several important biological processes such as immunity, protection against tissue damage, reproduction, growth and development. They are extremely important during the early stages of life and must be transferred adequately to the young during gestation and lactation. The present article presents an overview of their biological functions, metabolism and dynamics of transfer to offspring in mammals. Among other topics, the review focuses on the biochemical aspects of their intestinal absorption, blood transport, tissue uptake, storage and catabolism. It also describes their different roles as well as their use as preventive and therapeutic agents. Finally, the mechanisms involved in their transfer during gestation and lactation are discussed.

Mouse 24p3 protein has an effect on L929 cell viability

It is well known that mouse uterine 24p3 protein, is an acute phase protein, secreted from the L929 cell line, and that it will be induced by the dexamethasone stimulation of the cell. We investigated the possible effects of 24p3 protein on the L929 cell line, by observing its morphological change, ROS increase and viability decrease, by the process of culturing in a 24p3 protein-supplemented medium. Following the L929 cells′ exposure to the 24p3 protein supplement for a period of 72 hours, S-phase cells accumulated to a significant degree, suggesting that the entry into the G2/M phase from the S phase, in the cell cycle progression, was blocked. There was a significant decrease in cell numbers and increased DNA damage within the cells in the presence of 24p3 protein within the medium for 96 hours, implying that they have undergone pathway of cell death. After 96h incubation in low concentration of 24p3 protein, the result of PI/annexin V double staining showed cell death obviously. These results suggest that 24p3 protein-induced S phase arrest in the cell cycle, would cause DNA damage, followed by cell death in the L929 cells.

Gene expression and biomarkers in renal transplant ischemia reperfusion injury

The incidence of postischemic acute renal allograft failure (ARF) occurs in roughly 25% of cadaveric donor kidney recipients. This high rate remained virtually unchanged over the last decades despite modification in recipient management and modern immunosuppressive strategies. It has recently been shown that among other reasons, the systemic inflammation in the brain death cadaveric organ donor contributes to subsequent ARF in the recipient. This review focuses on the consequences of ischemia and reperfusion on the cellular level and offers potential solutions for the reduction of ARF. Genome-wide gene expression analysis together with sophisticated biostatistical analysis made it possible to identify several candidate gene products and proteins that may act as specific and sensitive biomarker for renal inflammation and ischemia. These markers may be very helpful in the clinical management of patients with a high a priori risk of subsequent ARF such as recipients of marginal donor kidneys. Ongoing clinical trials will evaluate whether immunosuppression of the cadaveric organ donor before organ harvest will have the potential to reduce inflammation in the transplant kidney and subsequently lead to a reduction in the rate of ARF.

Cumulus oophorus-associated glycodelin-C displaces sperm-bound glycodelin-A and -F and stimulates spermatozoa-zona pellucida binding

Spermatozoa have to swim through the oviduct and the cumulus oophorus before fertilization in vivo. In the oviduct, spermatozoa are exposed to glycodelin-A and -F that inhibit spermatozoa-zona pellucida binding. In this study, we determined whether these glycodelins would inhibit fertilization. The data showed that the spermatozoa without previous exposure to glycodelin-A and -F acquired glycodelin immunoreactivity during their passage through the cumulus oophorus. On the other hand, when glycodelin-A or -F-pretreated spermatozoa were exposed to the cumulus oophorus, the zona pellucida binding inhibitory activity of glycodelin-A and -F was not only removed, but the spermatozoa acquired enhanced zona pellucida binding ability. These actions of the cumulus oophorus were due to the presence of a cumulus isoform of glycodelin, designated as glycodelin-C. The cumulus cells could convert exogenous glycodelin-A and -F to glycodelin-C, which was then released into the surrounding medium. The protein core of glycodelin-C was identical to that in other glycodelin isoforms, as demonstrated by mass spectrum, peptide mapping, and affinity to anti-glycodelin antibody recognizing the protein core of glycodelin. In addition to having a smaller size and a higher isoelectric point, glycodelin-C also had lectin binding properties different from other isoforms. Glycodelin-C stimulated spermatozoazona pellucida binding in a dose-dependent manner, and it effectively displaced sperm-bound glycodelin-A and -F. In conclusion, the cumulus cells transform glycodelin-A and -F to glycodelin-C, which in turn removes the spermatozoazona binding inhibitory glycodelin isoforms and enhances the zona binding capacity of spermatozoa passing through the cumulus oophorus.

Protein biomarkers associated with acute renal failure and chronic kidney disease

Acute renal failure (ARF) as well as chronic kidney disease (CKD) are currently categorized according to serum creatinine concentrations. Serum creatinine, however, has shortcomings because of its low predictive values. The need for novel markers for the early diagnosis and prognosis of renal diseases is imminent, particularly for markers reflecting intrinsic organ injury in stages when glomerular filtration is not impaired. This review summarizes protein markers discussed in the context of ARF as well as CKD, and provides an overview on currently available discovery results following 'omics' techniques. The identified set of candidate marker proteins is discussed in their cellular and functional context. The systematic review of proteomics and genomics studies revealed 56 genes to be associated with acute or chronic kidney disease. Context analysis, i.e. correlation of biological processes and molecular functions of reported kidney markers, revealed that 15 genes on the candidate list were assigned to the most significant ontology groups: immunity and defence. Other significantly enriched groups were cell communication (14 genes), signal transduction (22 genes) and apoptosis (seven genes). Among 24 candidate protein markers, nine proteins were also identified by gene expression studies. Next generation candidate marker proteins with improved diagnostic and prognostic values for kidney diseases will be derived from whole genome scans and protemics approaches. Prospective validation still remains elusive for all proposed candidates.

Binding of the lipocalin C8γ to human complement protein C8α is mediated by loops located at the entrance to the C8γ ligand binding site

Human C8 is one of five complement components (C5b, C6, C7, C8 and C9) that interact to form the membrane attack complex (MAC). C8 is composed of a disulfide-linked C8alpha-gamma heterodimer and a noncovalently associated C8beta chain. C8alpha and C8beta are homologous to C6, C7 and C9, whereas C8gamma is the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small molecule. In C8gamma, the calyx opening is surrounded by four loops that connect beta-strands. Loop 1 is the largest and contains Cys40 that links to Cys164 in C8alpha. To determine if these loops mediate binding of C8alpha prior to interchain disulfide bond formation in C8alpha-gamma, the loops were substituted separately and in combination for the corresponding loops in siderocalin (NGAL, Lcn2), a lipocalin that is structurally similar to C8gamma. The siderocalin-C8gamma chimeric constructs were expressed in E. coli, purified, and assayed for their ability to bind C8alpha. Results indicate at least three of the four loops surrounding the entrance to the C8gamma calyx are involved in binding C8alpha. Binding near the calyx entrance suggests C8alpha may restrict and possibly regulate access to the C8gamma ligand binding site.

Interaction of uteroglobin with lipocalin-1 receptor suppresses cancer cell motility and invasion

Cellular migration and invasion are critical for important biological processes including cancer metastasis. We previously reported that uteroglobin (UG), a multifunctional secreted protein, binds to several cell types inhibiting migration and invasion [G.C. Kundu, A.K. Z. Zhang Mandal, G. Mantile-Selvaggi, A.B. Mukherjee (1998) Uteroglobin (UG) suppresses extracellular matrix invasion by normal and cancer cells that express the high affinity UG-binding proteins. J Biol Chem. 273: 22819-22824]. More recently, we reported that HTB-81 adenocarcinoma cells, which do not bind UG, are refractory to UG-mediated inhibition of migration and invasion [Z. Zhang, G.C. Kundu, D. Panda, A.K. Mandal et al. (1999) Loss of transformed phenotype in cancer cells by overexpression of the uteroglobin gene. Proc Natl Acad Sci U S A. 96, 3963-3968]. Since UG shares several biological properties with lipocalin-1 that mediates some of its biological effects via its receptor (Lip-1R), we sought to determine whether UG might interact with Lip-1R and inhibit migration and invasion of HTB-81 cells. To address this question, we first transfected COS-1 cells, which do not bind UG, with a Lip-1R-cDNA construct and performed binding assays using 125I-human UG (hUG). The results show that hUG binds Lip-1R on these cells with high specificity. Further, transfection of HTB-81 cells with the same construct yielded 125I-hUG binding with high affinity (Kd=18 nM) and specificity. The hUG-Lip-1R interaction was further confirmed by transfecting HTB-81, HTB-30 and HTB-174 cells, which are refractory to UG-binding, with a green fluorescent protein (GFP)-Lip-1R-cDNA construct and testing for Lip-1R-hUG colocalization by fluorescence microscopy. Finally, we demonstrate that Lip-1R-hUG interaction on Lip-1R transfected HTB-81 cells renders them fully responsive to hUG-mediated inhibition of migration and invasion. Taken together, these results suggest that Lip-1R is at least one of the UG-binding proteins through which UG exerts anti-motility and anti-invasive effects.

Identification, expression, and evolutionary analyses of plant lipocalins

Lipocalins are a group of proteins that have been characterized in bacteria, invertebrate, and vertebrate animals. However, very little is known about plant lipocalins. We have previously reported the cloning of the first true plant lipocalins. Here we report the identification and characterization of plant lipocalins and lipocalin-like proteins using an integrated approach of data mining, expression studies, cellular localization, and phylogenetic analyses. Plant lipocalins can be classified into two groups, temperature-induced lipocalins (TILs) and chloroplastic lipocalins (CHLs). In addition, violaxanthin de-epoxidases (VDEs) and zeaxanthin epoxidases (ZEPs) can be classified as lipocalin-like proteins. CHLs, VDEs, and ZEPs possess transit peptides that target them to the chloroplast. On the other hand, TILs do not show any targeting peptide, but localization studies revealed that the proteins are found at the plasma membrane. Expression analyses by quantitative real-time PCR showed that expression of the wheat (Triticum aestivum) lipocalins and lipocalin-like proteins is associated with abiotic stress response and is correlated with the plant's capacity to develop freezing tolerance. In support of this correlation, data mining revealed that lipocalins are present in the desiccation-tolerant red algae Porphyra yezoensis and the cryotolerant marine yeast Debaryomyces hansenii, suggesting a possible association with stress-tolerant organisms. Considering the plant lipocalin properties, tissue specificity, response to temperature stress, and their association with chloroplasts and plasma membranes of green leaves, we hypothesize a protective function of the photosynthetic system against temperature stress. Phylogenetic analyses suggest that TIL lipocalin members in higher plants were probably inherited from a bacterial gene present in a primitive unicellular eukaryote. On the other hand, CHLs, VDEs, and ZEPs may have evolved from a cyanobacterial ancestral gene after the formation of the cyanobacterial endosymbiont from which the chloroplast originated.

Anticalins as an alternative to antibody technology

Anticalins are a class of engineered ligand-binding proteins that are based on the lipocalin scaffold. The lipocalin protein architecture is characterised by a compact, rigid beta-barrel that supports four structurally hypervariable loops. These loops form a pocket for the specific complexation of differing target molecules. Natural lipocalins occur in human plasma and body fluids, where they usually function in the transport of vitamins, steroids or metabolic compounds. Using targeted mutagenesis of the loop region and biochemical selection techniques, variants with novel ligand specificities, both for low-molecular weight substances and for macromolecular protein targets, can be generated. Due to their small size, typically between 160 and 180 residues, robust tertiary structure and composition of a single polypeptide chain, such 'anticalins' provide several advantages over antibodies concerning economy of production, stability during storage, faster pharmacokinetics and better tissue penetration. At present, anticalins offer three major mechanisms for therapeutic application: (i) as antidotes, by quickly removing toxic or otherwise irritating compounds from the human body; (ii) as antagonists, for example, by binding to cellular receptors and blocking them from interaction with their natural signalling molecules; (iii) as tissue-targeting vehicles, by addressing toxic molecules or enzymes to disease-related cell surface proteins.

Studies on retinol-binding protein during vitellogenesis in the Rainbow Trout (Oncorhynchus mykiss)

Retinoids are important regulatory signaling molecules during embryonic development and therefore, should be present in the eggs of oviparous animals that develop independently of the maternal organism. Studies were initiated in Rainbow Trout to elucidate the role of retinol-binding protein (RBP), the specific retinol carrier protein in vertebrate plasma, during vitellogenesis. Plasma levels of RBP in pre-vitellogenic, vitellogenic, and post-vitellogenic females were compared to plasma of male trout, using a rabbit polyclonal antiserum that was generated to His-tagged RBP recombinant protein. Western-blot analyses showed that there were no differences in the relative plasma levels of RBP between pre-vitellogenic, vitellogenic or post-vitellogenic females and similar levels were also found in males. In contrast, strong elevation of vitellogenin (VTG) was observed in the plasma from vitellogenic females. Northern-blot analysis of hepatic mRNA revealed that there were no dramatic changes in the abundance of RBP transcripts in the liver of females during vitellogenesis, but showed a significant increase in the expression of VTG in the livers of vitellogenic females. These results indicate differences in the regulation of RBP and VTG during vitellogenesis, suggesting that RBP may not be the main transporter protein for retinoids to fish egg. Recent publications on the association of retinal with VTG in fish and the occurrence of RBP transcripts in ovarian tissues raise the need for reevaluation of the role of RBP during vitellogenesis in oviparous non-mammalian vertebrate species.

Lipocalin 2 diminishes invasiveness and metastasis of Ras-transformed cells

Lipocalin 2, an iron-siderophore-binding protein, converts embryonic kidney mesenchyme to epithelia. We found that lipocalin 2 could also convert 4T1-Ras-transformed mesenchymal tumor cells to an epithelial phenotype, increase E-cadherin expression, and suppress cell invasiveness in vitro and tumor growth and lung metastases in vivo. The Ras-MAPK pathway mediated the epithelial to mesenchymal transition in part by increasing E-cadherin phosphorylation and degradation. Lipocalin 2 antagonized these effects at a point upstream of Raf activation. Lipocalin 2 action was enhanced by iron-siderophore. These data characterize lipocalin 2 as an epithelial inducer in Ras malignancy and a suppressor of metastasis.

Expression of the carrier protein apolipoprotein D in the mouse inner ear

The cochlear portion of the inner ear converts movements produced by sound waves into electrical impulses. Transcripts enriched in the cochlea are likely to have an important role in hearing. In this paper, we report that microarray analyses of the Soares NMIE inner ear library revealed cochlear enriched expression of apolipoprotein D (apoD), a glycoprotein and member of the lipocalin family that transport small hydrophobic ligands. The cochlear enriched expression of Apod was validated by quantitative real time PCR analysis. To investigate the function of apoD in the inner ear the transcript and protein were localised in the cochlea. Apod messenger RNA (mRNA) expression was localised to the spiral ligament and spiral limbus, particularly in the suprastrial and supralimbral regions. The apoD protein was detected in the spiral ligament, spiral limbus and also in the outer hair cells of the organ of Corti. Investigation of cell lines exhibiting characteristics of hair and supporting cells revealed no Apod mRNA expression in these cells. This suggests transport of the protein within the cochlea, followed by internalisation into outer hair cells. The spiral limbus and ligament contain subpopulations of fibrocytes that are intimately involved in regulation of ion balance in the cochlear fluids and type I, II and III fibrocytes of the spiral ligament were all shown to be positive for apoD protein. On the basis of these results it was hypothesised that apoD could be involved in maintaining cochlear fluid homeostasis. To determine whether the apoD gene product was important for normal auditory function the hearing ability of an apoD knockout mouse was tested. The mouse was found to have a hearing threshold that was not significantly different to the control strain.

Anticalins in Drug Development

Anticalins are a novel class of engineered ligand-binding proteins that are prepared from lipocalins--conventional plasma proteins in humans--via targeted random mutagenesis and selection against prescribed haptens or antigens. The first anticalins were selected to bind to small ligands, such as the cardioactive drug digoxin. Recently, libraries that also permit the generation of anticalins with high affinities and specificities for protein targets, especially disease-related cell-surface receptors, have been constructed. Anticalins are much smaller than antibodies or their antigen-binding fragments, lack glycosylation as well as immunological effector functions, and consist of a single, stably folded polypeptide chain. Thus, they offer benefits as biopharmaceuticals in several areas of medical therapy, for example as receptor antagonists or as effective antidotes against toxic compounds.

The endocytic receptor megalin binds the iron transporting neutrophil-gelatinase-associated lipocalin with high affinity and mediates its cellular uptake

Neutrophil-gelatinase-associated lipocalin (NGAL) is a prominent protein of specific granules of human neutrophils also synthesized by epithelial cells during inflammation. NGAL binds bacterial siderophores preventing bacteria from retrieving iron from this source. Also, NGAL may be important in delivering iron to cells during formation of the tubular epithelial cells of the primordial kidney. No cellular receptor for NGAL has been described. We show here that megalin, a member of the low-density lipoprotein receptor family expressed in polarized epithelia, binds NGAL with high affinity, as shown by surface plasmon resonance analysis. Furthermore, a rat yolk sac cell line known to express high levels of megalin, endocytosed NGAL by a mechanism completely blocked by an antibody against megalin.

Protein sequence randomization: efficient estimation of protein stability using knowledge-based potentials

Modifications of the amino acid sequence generally affect protein stability. Here, we use knowledge-based potentials to estimate the stability of protein structures under sequence variation. Calculations on a variety of protein scaffolds result in a clear distinction of known mutable regions from arbitrarily chosen control patches. For example, randomly changing the sequence of an antibody paratope yields a significantly lower number of destabilized mutants as compared to the randomization of comparable regions on the protein surface. The technique is computationally efficient and can be used to screen protein structures for regions that are amenable to molecular tinkering by preserving the stability of the mutated proteins.

Glycodelin A and differentiation of first trimester trophoblast cells in vitro

AIM

The glycoprotein, glycodelin A (GdA) is a main product of the maternal decidua in the first trimester of pregnancy and is secreted into the amniotic fluid. The purpose of this study was to investigate the effect of GdA on secretion and surface markers of isolated first trimester trophoblasts in vitro.

METHODS

Cytotrophoblasts were prepared from human first trimester placentae and incubated with varying concentrations of GdA or transfected separately with the expression plasmid of GdA. Supernatants were assayed for human chorionic gonadotropin (hCG) protein concentrations. Expression of human placental lactogen (hPL), mucin 1 (MUC1) and the Thomsen-Friedenreich (TF) epitope was analysed in stimulated trophoblast cells and in unstimulated controls by immunocytochemistry.

RESULTS

Glycodelin A induced a reduced expression of hPL compared with unstimulated controls. Expression of MUC1 was not affected by GdA. Freshly isolated trophoblast cells showed no TF expression but became positive for this antigen after 96 h of cultivation. GdA-stimulated trophoblast cells inhibited TF expression after 96 h of cultivation. GdA plasmids induced a significantly higher hCG production in transfected cells than in cells transfected with the empty plasmid.

CONCLUSIONS

The results obtained in this study suggest that GdA is involved in the differentiation of trophoblast cells. The treatment of GdA plasmid transfected trophoblast cells stimulated hCG production in isolated trophoblast cells and inhibited hPL and TF expression, suggesting a functional link between hCG and GdA.

Aphrodisin, an aphrodisiac lipocalin secreted in hamster vaginal secretions

Vertebrates communicate through pheromones, which favor biological regulations within each species. Aphrodisin, a protein belonging to the lipocalin superfamily, found in hamster vaginal secretions, is detected by the male accessory olfactory system and induces or facilitates its copulatory behavior. Although much is known about aphrodisin structure, the question of whether aphrodisin bears itself the pheromonal function or is simply a carrier for hydrophobic small pheromones has not been definitely solved. Arguments based on use of recombinant aphrodisin deprived of any natural ligand and its capability to convey hamster pheromonal compounds will be discussed, together with progresses concerning putative natural ligand(s).

Delivery of ferric ion to mouse spermatozoa is mediated by lipocalin internalization

The aim of this study was to illustrate the further process of 24p3 protein after association with epididymal spermatozoa. We have previously identified a caput-initiated 24p3 protein, which interacts with the spermatozoa surface in vitro. In the present study, we investigate another role of the 24p3 protein with spermatozoa. Mouse epididymal spermatozoa exhibit the ability to bind spontaneously with exogenous 24p3 protein, a part of which is further internalized into the spermatozoa in epididymal caput. We have now focused on this issue using freshly prepared spermatozoa from caudal region of epididymis. First, the cytosolic fractionation of spermatozoa has revealed that biotinylated 24p3 protein signal could be detected by supplying biotinylated protein under 37 degrees C incubation after 30 min at this experiment. Further, flow cytometric analysis of FITC-protein containing spermatozoa has revealed two distinct types of fluorescent spermatozoa, and microscopical experimentation with fluorescent FITC-24p3 protein has shown that the 24p3 protein did accumulate in the cytosolic portion of spermatozoa. All of these events, which showed protein uptake into the cell, demonstrated time- and temperature-dependence of endocytotic characteristics, these constituting the critical points in the process of endocytosis for spermatozoa as for other cells. Using a fluorometric method, the binding affinities of ferrous ion and ferric ion to 24p3 protein were shown to be (1.5+/-0.2)x10(6) and (3.0+/-0.4)x10(7)M(-1), respectively. We have also determined the internalization of this protein in the transition of iron into spermatozoa. We report here that spermatozoa, from the caudal epididymis, demonstrate the ability to bind with 24p3 protein and further internalize it and deliver the ferric ion to the spermatozoa via protein internalization. We suggest that the 24p3 protein plays a physiological role in spermatozoa in the context of protein-ligand complex internalization.

Prostaglandin synthases: recent developments and a novel hypothesis

Cells are continuously exposed to cues, which signal cell survival or death. Fine-tuning of these conflicting signals is essential for tissue development and homeostasis, and defective pathways are linked to many disease processes, especially cancer. It is well established that prostaglandins (PGs), as signalling molecules, are important regulators of cell proliferation, differentiation and apoptosis. PG production has been a focus of many researchers interested in the mechanisms of parturition. Previously, investigators have focussed on the committed step of PG biosynthesis, the conversion by prostaglandin H synthase (PGHS; also termed cyclo-oxygenase, COX) of arachidonic acid (AA) (substrate) to PGH2, the common precursor for biosynthesis of the various prostanoids. However, recently the genes encoding the terminal synthase enzymes involved in converting PGH2 to each of the bioactive PGs, including the major uterotonic PGs, PGE2 (PGE synthase) and PGF2alpha (PGF synthase), have been cloned and characterized. This review highlights how the regulation of the expression and balance of key enzymes can produce, from a single precursor, prostanoids with varied and often opposing effects.

RPE65 of Retinal Pigment Epithelium, A Putative Receptor Molecule for Plasma Retinol-Binding Protein, is Expressed in Human Keratinocytes

Retinoids are important modulators for cell growth and differentiation of normal skin. In plasma, retinol is transported coupled to plasma retinol-binding protein. In this study, we investigated gene and protein expression of RPE65, a putative receptor for plasma retinol-binding protein in human epidermal keratinocytes. We performed real-time PCR analysis to evaluate expression of RPE65 mRNA in proliferating and differentiating keratinocytes. Immunoblotting with anti-RPE65 antibody shows distinct reactivity to a 61-kDa protein. Indirect immunofluorescence on normal human epidermis reveals cell surface labeling of keratinocytes. Laser scan microscopy exhibits colocalization of plasma retinol-binding protein and RPE65 on cultured keratinocytes. Internalization experiments with [3H]retinoic acid-retinol-binding protein complex in the presence and absence of excess of retinol-binding protein indicates receptor-dependent uptake of retinoids. We further show isolation of RPE65 protein by affinity chromatography from lysates of keratinocytes using a retinol-binding protein-matrix gel column. In summary, we demonstrate mRNA and protein expression of RPE65 in epidermal keratinocytes. Colocalization of plasma retinol-binding protein with RPE65 and affinity binding suggest a direct interaction of RPE65 with plasma retinol-binding protein in cultured human keratinocytes that might be involved in retinoid uptake of keratinocytes.

LCN6, a novel human epididymal lipocalin

BACKGROUND

The lipocalin (LCN) family of structurally conserved hydrophobic ligand binding proteins is represented in all major taxonomic groups from prokaryotes to primates. The importance of lipocalins in reproduction and the similarity to known epididymal lipocalins prompted us to characterize the novel human epididymal LCN6.

METHODS AND RESULTS

LCN6 cDNA was identified by database analysis in a comprehensive human library sequencing program. Macaca mulatta (rhesus monkey) cDNA was obtained from an epididymis cDNA library and is 93% homologous to the human. The gene is located on chromosome 9q34 adjacent LCN8 and LCN5. LCN6 amino acid sequence is most closely related to LCN5, but the LCN6 beta-barrel structure is best modeled on mouse major urinary protein 1, a pheromone binding protein. Northern blot analysis of RNAs isolated from 25 human tissues revealed predominant expression of a 1.0 kb mRNA in the epididymis. No other transcript was detected except for weak expression of a larger hybridizing mRNA in urinary bladder. Northern hybridization analysis of LCN6 mRNA expression in sham-operated, castrated and testosterone replaced rhesus monkeys suggests mRNA levels are little affected 6 days after castration. Immunohistochemical staining revealed that LCN6 protein is abundant in the caput epithelium and lumen. Immunofluorescent staining of human spermatozoa shows LCN6 located on the head and tail of spermatozoa with the highest concentration of LCN6 on the post-acrosomal region of the head, where it appeared aggregated into large patches.

CONCLUSIONS

LCN6 is a novel lipocalin closely related to Lcn5 and Lcn8 and these three genes are likely products of gene duplication events that predate rodent-primate divergence. Predominant expression in the epididymis and location on sperm surface are consistent with a role for LCN6 in male fertility.

Steroid hormone modulation of olfactory processing in the context of socio-sexual behaviors in rodents and humans

Primer pheromones and other chemosensory cues are important factors governing social interactions and reproductive physiology in many species of mammals. Responses to these chemosignals can vary substantially within and between individuals. This variability can stem, at least in part, from the modulating effects steroid and non-steroid hormones exert on olfactory processing. Such modulation frequently augments or facilitates the effects that prevailing social and environmental conditions have on the reproductive axis. The mechanisms underlying the hormonal regulation of responses to chemosensory cues are diverse. They are in part behavioral, achieved through the modulation of chemoinvestigative behaviors, and in part a product of the modulation of the intrinsic responsiveness of the main and accessory olfactory systems to conspecific, as well as other classes, of chemosignals. The behavioral and non-behavioral effects complement one another to ensure that mating and other reproductive processes are confined to reproductively favorable conditions.

Antisense down-regulation of lipocalin-interacting membrane receptor expression inhibits cellular internalization of lipocalin-1 in human NT2 cells

There is increasing experimental evidence demonstrating that many lipocalins bind to specific cell surface receptors. However, whereas the binding of lipocalins to their lipophilic ligands has now been characterized in much detail, there is a lack of knowledge about the nature of lipocalin receptors, the physiological role of receptor binding, and the molecular mechanism of ligand delivery. We previously identified a novel human membrane protein (lipocalin-1-interacting membrane receptor (LIMR)), which interacts with lipocalin-1 (Wojnar, P., Lechner, M., Merschak, P., and Redl, B. (2001) J. Biol. Chem. 276, 20206-20212). In the present study, we investigated the physiological role of LIMR and found this protein to be essential for mediating internalization of lipocalin-1 (Lcn-1) in NT2 cells, leading to its degradation. Whereas control NT2 cells rapidly internalized (125)I-Lcn-1 or fluorescein isothiocyanate-labeled Lcn-1, NT2 cells that were made LIMR deficient by cDNA antisense expression greatly accumulated Lcn-1 in the culture medium but did not internalize it. Because sequence and structure analysis indicated that proteins similar to LIMR are present in several organisms and at least two closely related orthologues are found in human and mouse, we suggest LIMR to be the prototype of a new family of endocytic receptors, which are topographically characterized by nine putative transmembrane domains and a characteristic large central cytoplasmic loop.

The neutrophil lipocalin NGAL is a bacteriostatic agent that interferes with siderophore-mediated iron acquisition

First identified as a neutrophil granule component, neutrophil gelatinase-associated lipocalin (NGAL; also called human neutrophil lipocalin, 24p3, uterocalin, or neu-related lipocalin) is a member of the lipocalin family of binding proteins. Putative NGAL ligands, including neutrophil chemotactic agents such as N-formylated tripeptides, have all been refuted by recent biochemical and structural results. NGAL has subsequently been implicated in diverse cellular processes, but without a characterized ligand, the molecular basis of these functions remained mysterious. Here we report that NGAL tightly binds bacterial catecholate-type ferric siderophores through a cyclically permuted, hybrid electrostatic/cation-pi interaction and is a potent bacteriostatic agent in iron-limiting conditions. We therefore propose that NGAL participates in the antibacterial iron depletion strategy of the innate immune system.

Cloning and sequence analysis of Galleria mellonella juvenile hormone binding protein--a search for ancestors and relatives

Juvenile hormone binding proteins (JHBPs) serve as specific carriers of juvenile hormone (JH) in insect hemolymph. As shown in this report, Galleria mellonella JHBP is encoded by a cDNA of 1063 nucleotides. The pre-protein consists of 245 amino acids with a 20 amino acid leader sequence. The concentration of the JHBP mRNA reaches a maximum on the third day of the last larval instar, and decreases five-fold towards pupation. Comparison of amino acid sequences of JHBPs from Bombyx mori, Heliothis virescens, Manduca sexta and G. mellonella shows that 57 positions out of 226 are occupied by identical amino acids. A phylogeny tree was constructed from 32 proteins, which function could be associated to JH. It has three major branches: (i) ligand binding domains of nuclear receptors, (ii) JHBPs and JH esterases (JHEs), and (iii) hypothetical proteins found in Drosophila melanogaster genome. Despite the close positioning of JHEs and JHBPs on the tree, which probably arises from the presence of a common JH binding motif, these proteins are unlikely to belong to the same family. Detailed analysis of the secondary structure modeling shows that JHBPs may contain a beta-barrel motif flanked by alpha-helices and thus be evolutionary related to the same superfamily as calycins.

Molecular characterization of a new adult male putative calycin specific to tergal aphrodisiac secretion in the cockroach Leucophaea maderae

Lma-p18 is an epicuticular surface protein specific to the tergal gland aphrodisiac secretion of Leucophaea maderae adult males. Native Lma-p18 was purified and the complete cDNA sequence was determined by RT-PCR using primers based on Edman degradation fragments. Northern blot and in situ hybridization analyses showed that Lma-p18 is expressed exclusively in the anterior part of male tergal gland, which is exposed only during sexual behavior. Sequence analysis indicated that Lma-p18 belongs to the calycin superfamily and is very similar to Lma-p22, the first known male-specific tergal protein in L. maderae. Lma-p18 and Lma-p22 were proposed to bind different sexually attractive compounds as other calycins.

Prostaglandin D2 synthase induces apoptosis in PC12 neuronal cells

Apoptosis of neuronal cells is a proposed cause of certain neurological disorders. Here, we report on a 5- to 6-fold increase in apoptosis by exposure to prostaglandin D2 synthase (PGD2S) in PC12 neuronal cells. Apoptosis was detected by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling (TUNEL) assay, and appears to be mediated via caspase-3 activation. Neutralization with anti-PGD2S antibody or pre-treatment with selenium, which inhibits PGD2S enzymatic activity, both significantly inhibited the PGD2S-induced apoptosis, however, neither had any effect on the apoptosis induced by the known neuronal apoptotic inducer, glutamate. In addition, prostaglandins E1, E2, and F2alpha all inhibited the PGD2S-induced apoptosis while prostaglandin H2 had no significant effect. Furthermore, PGD2S isolated from human serum was more effective at inducing apoptosis then recombinantly expressed protein, presumably due to glycosylation. This novel role of PGD2S, as an inducer of apoptosis, may have implications in PC12 differentiation and possibly some neurological disorders.

Molecular cloning of a novel lipocalin-1 interacting human cell membrane receptor using phage display

Human lipocalin-1 (Lcn-1, also called tear lipocalin), a member of the lipocalin structural superfamily, is produced by a number of glands and tissues and is known to bind an unusually large array of hydrophobic ligands. Apart from its specific function in stabilizing the lipid film of human tear fluid, it is suggested to act as a physiological scavenger of potentially harmful lipophilic compounds, in general. To characterize proteins involved in the reception, detoxification, or degradation of these ligands, a cDNA phage-display library from human pituitary gland was constructed and screened for proteins interacting with Lcn-1. Using this method an Lcn-1 interacting phage was isolated that expressed a novel human protein. Molecular cloning and analysis of the entire cDNA indicated that it encodes a 55-kDa protein, lipocalin-1 interacting membrane receptor (LIMR), with nine putative transmembrane domains. The cell membrane location of this protein was confirmed by immunocytochemistry and Western blot analysis of membrane fractions of human NT2 cells. Independent biochemical investigations using a recombinant N-terminal fragment of LIMR also demonstrated a specific interaction with Lcn-1 in vitro. Based on these data, we suggest LIMR to be a receptor of Lcn-1 ligands. These findings constitute the first report of cloning of a lipocalin interacting, plasma membrane-located receptor, in general. In addition, a sequence comparison supports the biological relevance of this novel membrane protein, because genes with significant nucleotide sequence similarity are present in Takifugu rubripes, Drosophila melanogaster, Caenorhabditis elegans, Mus musculus, Bos taurus, and Sus scrofa. According to data derived from the human genome sequencing project, the LIMR-encoding gene has to be mapped on human chromosome 12, and its intron/exon organization could be established. The entire LIMR-encoding gene consists of about 13.7 kilobases in length and contains 16 introns with a length between 91 and 3438 base pairs.