alpha(1)-Microglobulin: a yellow-brown lipocalin

The crystal structure of human α1-microglobulin reveals a potential haem-binding site

We describe the 2.3 Å (1 Å=0.1 nm) X-ray structure of α1m (α1-microglobulin), an abundant protein in human blood plasma, which reveals the β-barrel fold typical for lipocalins with a deep pocket lined by four loops at its open rim. Loop #1 harbours the residue Cys34 which is responsible for covalent cross-linking with plasma IgA. A single disulfide bond between Cys72 and Cys169 connects the C-terminal segment to the β-barrel, as in many other lipocalins. The exposed imidazole side chains of His122 and His123 in loop #4 give rise to a double Ni2+-binding site together with a crystallographic neighbour. The closest structural relatives of α1m are the complement protein component C8γ, the L-prostaglandin D synthase and lipocalin 15, three other structurally characterized members of the lipocalin family in humans that have only distant sequence similarity. In contrast with these, α1m is initially expressed as a bifunctional fusion protein with the protease inhibitor bikunin. Neither the electron density nor ESI–MS (electrospray ionization MS) provide evidence for a chromophore bound to the recombinant α1m, also known as ‘yellow/brown lipocalin’. However, the three side chains of Lys92, Lys118 and Lys130 that were reported to be involved in covalent chromophore binding appear to be freely accessible to ligands accommodated in the hydrophobic pocket. A structural feature similar to the well-known Cys–Pro haem-binding motif indicates the presence of a haem-binding site within the loop region of α1m, which explains previous biochemical findings and supports a physiological role in haem scavenging, as well as redox-mediated detoxification.

High-throughput sequence analysis of turbot (Scophthalmus maximus) transcriptome using 454-pyrosequencing for the discovery of antiviral immune genes

Background

Turbot (Scophthalmus maximus L.) is an important aquacultural resource both in Europe and Asia. However, there is little information on gene sequences available in public databases. Currently, one of the main problems affecting the culture of this flatfish is mortality due to several pathogens, especially viral diseases which are not treatable. In order to identify new genes involved in immune defense, we conducted 454-pyrosequencing of the turbot transcriptome after different immune stimulations.

Methodology/Principal Findings

Turbot were injected with viral stimuli to increase the expression level of immune-related genes. High-throughput deep sequencing using 454-pyrosequencing technology yielded 915,256 high-quality reads. These sequences were assembled into 55,404 contigs that were subjected to annotation steps. Intriguingly, 55.16% of the deduced protein was not significantly similar to any sequences in the databases used for the annotation and only 0.85% of the BLASTx top-hits matched S. maximus protein sequences. This relatively low level of annotation is possibly due to the limited information for this specie and other flatfish in the database. These results suggest the identification of a large number of new genes in turbot and in fish in general. A more detailed analysis showed the presence of putative members of several innate and specific immune pathways.

Conclusions/Significance

To our knowledge, this study is the first transcriptome analysis using 454-pyrosequencing for turbot. Previously, there were only 12,471 EST and less of 1,500 nucleotide sequences for S. maximus in NCBI database. Our results provide a rich source of data (55,404 contigs and 181,845 singletons) for discovering and identifying new genes, which will serve as a basis for microarray construction, gene expression characterization and for identification of genetic markers to be used in several applications. Immune stimulation in turbot was very effective, obtaining an enormous variety of sequences belonging to genes involved in the defense mechanisms.

Gene expression profiling in the skin of zebrafish infected with Citrobacter freundii

Skin is considered the largest immunologically active organ, but its molecular mechanism remains unclear in fish. Here, Affymetrix Zebrafish GeneChip was used to assess gene expression in the skin of zebrafish (Danio rerio) infected with the bacterium Citrobacter freundii. The results showed that 229 genes were differentially expressed, of which 196 genes were upregulated and 33 genes were downregulated. Gene Ontology and KEGG pathway analyses indicated 88 genes significantly associated with skin immunity involved in complement activation and acute phase response, defense and immune response, response to stress and stimulus, antigen processing and presentation, cell adhesion and migration, platelet activation and coagulation factors, regulation of autophagy and apoptosis. When compared with transcriptional profiles of previously reported carp (Cyprinus carpio) skin, a similar innate immunity (e.g., interferon, lectin, heat shock proteins, complements), and several different acute phase proteins (transferrin, ceruloplasmin, vitellogenin and alpha-1-microglobulin, etc.) were detected in zebrafish skin. The validity of the microarray results was verified by quantitative real-time PCR analysis of nine representative genes. This is first report that skin play important roles in innate immune responses to bacterial infection, which contribute to understanding the defense mechanisms of the skin in fish.

Plasma proteins present in osteoarthritic synovial fluid can stimulate cytokine production via Toll-like receptor 4

Introduction

Osteoarthritis (OA) is a degenerative disease characterized by cartilage breakdown in the synovial joints. The presence of low-grade inflammation in OA joints is receiving increasing attention, with synovitis shown to be present even in the early stages of the disease. How the synovial inflammation arises is unclear, but proteins in the synovial fluid of affected joints could conceivably contribute. We therefore surveyed the proteins present in OA synovial fluid and assessed their immunostimulatory properties.

Methods

We used mass spectrometry to survey the proteins present in the synovial fluid of patients with knee OA. We used a multiplex bead-based immunoassay to measure levels of inflammatory cytokines in serum and synovial fluid from patients with knee OA and from patients with rheumatoid arthritis (RA), as well as in sera from healthy individuals. Significant differences in cytokine levels between groups were determined by significance analysis of microarrays, and relations were determined by unsupervised hierarchic clustering. To assess the immunostimulatory properties of a subset of the identified proteins, we tested the proteins' ability to induce the production of inflammatory cytokines by macrophages. For proteins found to be stimulatory, the macrophage stimulation assays were repeated by using Toll-like receptor 4 (TLR4)-deficient macrophages.

Results

We identified 108 proteins in OA synovial fluid, including plasma proteins, serine protease inhibitors, proteins indicative of cartilage turnover, and proteins involved in inflammation and immunity. Multiplex cytokine analysis revealed that levels of several inflammatory cytokines were significantly higher in OA sera than in normal sera, and levels of inflammatory cytokines in synovial fluid and serum were, as expected, higher in RA samples than in OA samples. As much as 36% of the proteins identified in OA synovial fluid were plasma proteins. Testing a subset of these plasma proteins in macrophage stimulation assays, we found that Gc-globulin, α₁-microglobulin, and α₂-macroglobulin can signal via TLR4 to induce macrophage production of inflammatory cytokines implicated in OA.

Conclusions

Our findings suggest that plasma proteins present in OA synovial fluid, whether through exudation from plasma or production by synovial tissues, could contribute to low-grade inflammation in OA by functioning as so-called damage-associated molecular patterns in the synovial joint.

Quantitative mass spectrometry analysis using PAcIFIC for the identification of plasma diagnostic biomarkers for abdominal aortic aneurysm

BACKGROUND

Abdominal aortic aneurysm (AAA) is characterized by increased aortic vessel wall diameter (>1.5 times normal) and loss of parallelism. This disease is responsible for 1-4% mortality occurring on rupture in males older than 65 years. Due to its asymptomatic nature, proteomic techniques were used to search for diagnostic biomarkers that might allow surgical intervention under nonlife threatening conditions.

METHODOLOGY/PRINCIPAL FINDINGS

Pooled human plasma samples of 17 AAA and 17 control patients were depleted of the most abundant proteins and compared using a data-independent shotgun proteomic strategy, Precursor Acquisition Independent From Ion Count (PAcIFIC), combined with spectral counting and isobaric tandem mass tags. Both quantitative methods collectively identified 80 proteins as statistically differentially abundant between AAA and control patients. Among differentially abundant proteins, a subgroup of 19 was selected according to Gene Ontology classification and implication in AAA for verification by Western blot (WB) in the same 34 individual plasma samples that comprised the pools. From the 19 proteins, 12 were detected by WB. Five of them were verified to be differentially up-regulated in individual plasma of AAA patients: adiponectin, extracellular superoxide dismutase, protein AMBP, kallistatin and carboxypeptidase B2.

CONCLUSIONS/SIGNIFICANCE

Plasma depletion of high abundance proteins combined with quantitative PAcIFIC analysis offered an efficient and sensitive tool for the screening of new potential biomarkers of AAA. However, WB analysis to verify the 19 PAcIFIC identified proteins of interest proved inconclusive save for five proteins. We discuss these five in terms of their potential relevance as biological markers for use in AAA screening of population at risk.

Chronic kidney disease-related atherosclerosis - proteomic studies of blood plasma

Background

Atherosclerosis is considered the major cause of the dramatic increase in cardiovascular mortality among patients suffering from chronic kidney disease (CKD). Although the close connection between atherosclerosis and kidney dysfunction is undeniable, factors enhancing CKD-mediated plaque formation are still not well recognized.

Results

To increase our knowledge of this process we carried out a comparative proteomic analysis of blood plasma proteins isolated from 75 patients in various stages of renal dysfunction (CKD group), 25 patients with advanced cardiovascular disease (CVD group) and 25 healthy volunteers (HV group). The collected samples were subjected to 2D electrophoresis. Then, individual proteins were identified by mass spectrometry. The comparative analysis involving CKD and HV groups showed a differential accumulation of α-1-microglobulin, apolipoprotein A-IV, γ-fibrinogen and haptoglobin in patients with kidney disease. Exactly the same proteins were identified as differentially expressed when proteomes of CVD patients and HV were compared. However, a direct comparison of CKD and CVD groups revealed significant differences in the accumulation of two proteins: α-1-microglobulin and apolipoprotein A-IV.

Conclusions

The obtained results indicate that at least two processes differentially contribute to the plaque formation in CKD- and CVD-mediated atherosclerosis. It seems that the inflammatory process is more intense in CKD patients. On the other hand, the down- and up-regulation of apolipoprotein A-IV in CVD and CKD groups, respectively, suggests that substantial differences exist in the efficacy of cholesterol transport in both groups of patients.

Blood aging, safety, and transfusion: capturing the "radical" menace

Throughout their life span, circulating red blood cells (RBCs) transport oxygen (O(2)) primarily from the lungs to tissues and return with carbon dioxide (CO(2)) from respiring tissues for final elimination by lungs. This simplistic view of RBCs as O(2) transporter has changed in recent years as other gases, for example, nitric oxide (NO), and small molecules, such as adenosine triphosphate (ATP), have been shown to either be produced and/or carried by RBCs to perform other signaling and O(2) sensing functions. In spite of the numerous biochemical and metabolic changes occurring within RBCs during storage, prior to, and after transfusion, perturbations of RBC membrane are likely to affect blood flow in the microcirculation. Subsequent hemolysis due to storage conditions and/or hemolytic disorders may have some pathophysiological consequences as a result of the release of Hb. In this review, we show that evolution has provided a multitude of protection and intervention strategies against free Hb from "cradle" to "death"; from early biosynthesis to its final degradation and a lot more in between. Furthermore, some of the same naturally occurring protective mechanisms can potentially be employed to oxidatively inactivate this redox active protein and control its damaging side reactions when released outside of the RBC.

Genetic loci that affect aristolochic acid-induced nephrotoxicity in the mouse

Aristolochic acids (AA) are plant-derived nephrotoxins and carcinogens found in traditional medicines and herbal remedies. AA causes aristolochic acid nephropathy (AAN) and is a suspected environmental agent in Balkan endemic nephropathy (BEN) and its associated upper urothelial cancer. Approximately 5-10% of individuals exposed to AA develop renal insufficiency and/or cancer; thus a genetic predisposition to AA sensitivity has been proposed. The mouse is an established animal model of AAN, and inbred murine strains vary in AA sensitivity, confirming the genetic predisposition. We mapped quantitative trait loci (QTL) correlated with proximal tubule dysfunction after exposure to AA in an F2 population of mice, derived from breeding an AA-resistant strain (C57BL/6J) and an AA-sensitive strain (DBA/2J). A single main QTL was identified on chromosome 4 (Aanq1); three other interacting QTLs, (Aanq2-4) also were detected. The Aanq1 region was also detected in untreated mice, raising the possibility that preexisting differences in proximal tubule function may affect the severity of AA-elicited toxicity. This study lays the groundwork for identifying the genetic pathways contributing to AA sensitivity in the mouse and will further our understanding of human susceptibility to AA found widely in traditional medicines.

Proteomics of exhaled breath: methodological nuances and pitfalls

BACKGROUND

The analysis of exhaled breath condensate (EBC) can be an alternative to traditional endoscopic sampling of lower respiratory tract secretions. This is a simple non-invasive method of diagnosing respiratory diseases, in particular, respiratory inflammatory processes.

METHODS

Samples were collected with a special device-condenser (ECoScreen, VIASYS Healthcare, Germany), then treated with trypsin according to the proteomics protocol for standard protein mixtures and analyzed by nanoflow high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) with a 7-Tesla Finnigan LTQ-FT mass spectrometer (Thermo Electron, Germany). Mascot software (Matrixscience) was used for screening the database NCBInr for proteins corresponding to the peptide maps that were obtained.

RESULTS

EBCs from 17 young healthy non-smoking donors were collected. Different methods for concentrating protein were compared in order to optimize EBC preparations for proteomic analysis. The procedure that was chosen allowed identification of proteins exhaled by healthy people. The major proteins in the condensates were cytoskeletal keratins. Another 12 proteins were identified in EBC from healthy non-smokers. Some keratins were found in the ambient air and may be considered exogenous components of exhaled air.

CONCLUSIONS

Knowledge of the normal proteome of exhaled breath allows one to look for biomarkers of different disease states in EBC. Proteins in ambient air can be identified in the respiratory tract and should be excluded from the analysis of the proteome of EBC. The results obtained allowed us to choose the most effective procedure of sample preparation when working with samples containing very low protein concentrations.

Transcriptome analysis of the salivary glands of the female tick Amblyomma americanum (Acari: Ixodidae)

Ticks infest a wide range of hosts while bypassing their immune, inflammatory and haemostatic responses during their extended feeding, which may last for more than two weeks. Here, we present a transcriptome analysis of 3868 expressed sequence tags (ESTs) from three cDNA libraries generated from the salivary glands of adult female Ambyomma americanum ticks at different stages of feeding. We applied a normalization step for one library, significantly decreasing the abundance of mitochondrial sequences amongst the 2292 sequences from the normalized library. Our ESTs include homologues that may modulate haemostatic, immune and inflammatory responses of the hosts. Other ESTs probably represent important components of the highly efficient secretory pathways for salivary proteins and concomitantly transmitted pathogens.

Rational stabilization of the C-LytA affinity tag by protein engineering

The C-LytA protein constitutes the choline-binding module of the LytA amidase from Streptococcus pneumoniae. Owing to its affinity for choline and analogs, it is regularly used as an affinity tag for the purification of proteins in a single chromatographic step. In an attempt to build a robust variant against thermal denaturation, we have engineered several salt bridges on the protein surface. All the stabilizing mutations were pooled in a single variant, C-LytAm7, which contained seven changes: Y25K, F27K, M33E, N51K, S52K, T85K and T108K. The mutant displays a 7 degrees C thermal stabilization compared with the wild-type form, together with a complete reversibility upon heating and a higher kinetic stability. Moreover, the accumulation of intermediates in the unfolding of C-LytA is virtually abolished for C-LytAm7. The differences in stability become more evident when the proteins are bound to a DEAE-cellulose affinity column, as most of wild-type C-LytA is denatured at approximately 65 degrees C, whereas C-LytAm7 may stand temperatures up to 90 degrees C. Finally, the change in the isoelectric point of C-LytAm7 enhances its solubility at acidic pHs. Therefore, C-LytAm7 behaves as an improved affinity tag and supports the engineering of surface salt bridges as an effective approach for protein stabilization.

Absence of Increased α1-Microglobulin in IgA Nephropathy Proteinuria

To search for biomarkers of IgA nephropathy, protein profiles of urine samples from patients with IgA nephropathy and normal volunteers were compared using two-dimensional DIGE. Most of the 172 spots identified in the urine were serum proteins, and their amounts in IgA nephropathy urine were much higher than those in normal urine; this can be explained as proteinuria caused by glomerular dysfunction. However, only alpha(1)-microglobulin, also one of the major serum proteins, in IgA nephropathy urine was not higher in amount than that in normal urine. We confirmed using ELISA analysis that the amounts of transferrin and albumin in IgA nephropathy and diabetic nephropathy urine were much higher than those in normal urine, whereas the amount of alpha(1)-microglobulin in IgA nephropathy urine was not higher than that in normal urine and was much lower than that in diabetic nephropathy urine. Approximately 50% of alpha(1)-microglobulin forms a complex with IgA in serum. These results suggest that alpha(1)-microglobulin in IgA nephropathy urine is a characteristic protein and might be a biomarker for IgA nephropathy and that alpha(1)-microglobulin might have a relationship with IgA nephropathy pathology.

Comparative analysis of serum proteomes to discover biomarkers for ossification of the posterior longitudinal ligament

STUDY DESIGN

Serum proteomes from normal subjects and the patients with ossification of the posterior longitudinal ligament (OPLL) were analyzed by using proteomics.

OBJECTIVES

To identify novel serologic biomarkers for diagnosing OPLL.

SUMMARY OF BACKGROUND DATA

OPLL can compress the spinal cord, and special planning is required for surgeries that are done from the front of the cervical spine. However, the definitive serologic biomarkers for OPLL are still unclear.

METHODS

The 2-dimensional electrophoresis patterns of sera from OPLL patients and normal subjects were compared. The differentially expressed spots were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electrospray ionization quadruple time-of-flight mass spectrometry.

RESULTS

Nine spots that were differentially expressed in the sera of OPLL patients were found and were identified. PRO2675, human serum albumin in a complex with myristic acid and tri-iodobenzoic acid, an unknown protein, chain B of the crystal structure of deoxy-human hemoglobin beta6, pro-apolipoprotein, ALB protein, retinol binding protein and chain A of human serum albumin mutant R218h complexed with thyroxine (3,3',5,5', tetraiodo-L-thyronine), were up-regulated in the sera of OPLL patients, whereas alpha1-microglobulin/bikunin precursor was down-regulated.

CONCLUSIONS

These proteins could be used as diagnostic biomarkers of OPLL.

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.

Up-regulation of alpha1-microglobulin by hemoglobin and reactive oxygen species in hepatoma and blood cell lines

alpha(1)-Microglobulin is a 26-kDa glycoprotein synthesized in the liver, secreted to the blood, and rapidly distributed to the extravascular compartment of all tissues. Recent results show that alpha(1)-microglobulin has heme-binding and heme-degrading properties and it has been suggested that the protein is involved in the defense against oxidation by heme and reactive oxygen species. In the present study the influence of hemoglobin and reactive oxygen species (ROS) on the cellular expression of alpha(1)-microglobulin was investigated. Oxy- and methemoglobin, free heme, and Fenton reaction-induced hydroxyl radicals induced a dose-dependent up-regulation of alpha(1)-microglobulin on both mRNA and protein levels in hepatoma cells and an increased secretion of alpha(1)-microglobulin. The up-regulation was reversed by the addition of catalase and ascorbate, and by reacting hemoglobin with cyanide which prevents redox reactions. Furthermore, the blood cell lines U937 and K562 expressed alpha(1)-microglobulin at low levels, and this expression increased up to 11-fold by the addition of hemoglobin. These results suggest that alpha(1)-microglobulin expression is induced by ROS, arising from redox reactions of hemoglobin or from other sources and are consistent with the hypothesis that alpha(1)-microglobulin participates in the defense against oxidation by hemoglobin, heme, and reactive oxygen species.

Production of recombinant human alpha1-microglobulin and mutant forms involved in chromophore formation

Alpha(1)-Microglobulin, a 26 kDa lipocalin present in plasma and tissues, carries a set of unknown chromophores, bound to C34, K92, K118 and K130, which cause its charge and size heterogeneity. In man, the protein is found in two forms, full length and lacking the C-terminal tetrapeptide LIPR (t-alpha(1)-microglobulin), both which are heme-binding and the latter with heme-degrading properties. We report cloning and overexpression of full length alpha(1)-microglobulin (wt protein), t-alpha(1)-microglobulin (wtdeltaLIPR) and the mutants C34S, K(92,118,130)T and C34S/K(92,118,130)T, the latter subsequently abbreviated as K(3)T and C34S/K(3)T, in Escherichia coli. After purification and refolding from inclusion bodies, all proteins were correctly folded as determined by far-UV circular dichroism and radioimmunoassay. As revealed by gel filtration, recombinant alpha(1)-microglobulins had lower tendencies to form dimers than human plasma or urine analogues. All alpha(1)-microglobulin forms displayed higher amounts of the chromophore than bovine serum albumin but significantly lower than the human urine or plasma counterparts. Differences in the absorbance and fluorescence profiles are consistent with a model where the chromophore is formed by a series of reactions with heme or other chromophore precursors and where C34 is essential for binding of the ligand, K92, K118 and K130 are involved in transformation into the chromophore and LIPR inhibits the latter reaction.

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.

Comparative ligand-binding analysis of ten human lipocalins

At least ten different lipocalins occur in the human body: retinol-binding protein (RBP), alpha1-acid glycoprotein, alpha1-microglobulin, apolipoprotein D, beta-trace protein, complement component 8gamma, glycodelin, neutrophil gelatinase-associated lipocalin, odorant-binding protein, and tear lipocalin. Although many of these lipocalins seem to play an important physiological role, their precise biological function is not always clear. Especially the interpretation of their diverse ligand-binding activities has been hampered by the fact that the natural lipocalins were prepared from different sources and with varying purity. Here we present a generic expression and purification strategy for the recombinant lipocalins, which is based on secretion into the periplasm of E. coli, where disulphide bonds are readily formed, followed by affinity purification via the Strep-tag II and gel filtration. The ten human lipocalins were successfully prepared and their ligand-binding activities were compared via fluorescence titration with a set of typical ligands: retinol, retinoic acid (RA), 11-(5-(dimethylamino)-1-naphthalene-sulfonylamino)undecanoic acid (DAUDA), and 8-anilino-1-naphtalene-sulfonic acid (ANS). As result, merely two lipocalins, RBP and beta-trace, revealed high affinities both for retinol and for RA, which probably reflects a specialized physiological function in retinoid complexation. Surprisingly, the strongest retinol affinity was detected for apolipoprotein D, whereas this lipocalin exhibits much weaker binding activity for retinoic acid. Binding studies with the two spectroscopic probes DAUDA and ANS revealed mixed patterns, which demonstrates that the affinity for lipophilic substances varies considerably among human lipocalins. Notably, RBP with its perfectly moulded retinol-binding site did not show any detectable binding activity for both compounds. Hence, our recombinant expression and purification system should be useful for further structural and functional studies of lipocalins from human origin and beyond.

Lipocalin 2-deficient mice exhibit increased sensitivity to Escherichia coli infection but not to ischemia-reperfusion injury

Diverse functions have been reported for lipocalin 2. To investigate these functions in vivo, we generated gene-targeted lipocalin 2-deficient mice (Lcn2-/- mice). In vitro studies have suggested that lipocalin 2 is important for cellular apoptosis induced by IL-3 withdrawal, and for the induction of kidney differentiation during embryogenesis. Analysis of Lcn2-/- mice showed normal cell death upon IL-3 withdrawal and normal kidney development. However, we found that Lcn2-/- mice exhibited an increased susceptibility to bacterial infections, in keeping with the proposed function of lipocalin 2 in iron sequestration. Neutrophils isolated from Lcn2-/- mice showed significantly less bacteriostatic activity compared with WT controls. The bacteriostatic property of the WT neutrophils was abolished by the addition of exogenous iron, indicating that the main function of lipocalin 2 in the antibacterial innate immune response is to limit this essential element. Another important function ascribed to lipocalin 2 has been its protective role against kidney ischemia-reperfusion injury. We analyzed Lcn2-/- mice using a mouse model for severe renal failure and could not detect any significant differences compared with their WT littermates.

Coelution of other proteins with albumin during size-exclusion HPLC: Implications for analysis of urinary albumin

BACKGROUND

Size-exclusion HPLC has been used as an alternative to immunoassays for quantifying urinary albumin (microalbumin). Systematically higher values for the HPLC method have been proposed to result from nonimmunoreactive albumin.

METHODS

We evaluated separation of purified proteins and urinary components by size-exclusion HPLC using a Zorbax Bio Series GF-250 column eluted with phosphate-buffered saline. Urinary components eluting in the "albumin" peak were analyzed by mass spectrometry and reversed-phase HPLC.

RESULTS

Several proteins, such as transferrin, alpha1-proteinase inhibitor, alpha1-acid glycoprotein, and alpha2-HS glycoprotein, analyzed as purified components, were not resolved from albumin by size-exclusion HPLC. Peaks for other proteins, such as IgG and urinary components identified as dimers of alpha1-microglobulin and immunoglobulin light chains, overlapped with the albumin peak. Profiles of urine specimens showed variable amounts of components overlapping with albumin. Furthermore, the albumin peak obtained by size-exclusion HPLC was found by mass spectrometry and reversed-phase HPLC to contain multiple components in addition to albumin.

CONCLUSIONS

Size-exclusion HPLC does not resolve albumin from several other proteins in urine. The albumin peak resolved by this technique, although predominantly composed of albumin, contains several coeluting globulins that would contribute to overestimation of albumin concentration by size-exclusion HPLC.

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.

Alpha 1-microglobulin: clinical laboratory aspects and applications

BACKGROUND

Urinary microproteins are becoming increasingly important in clinical diagnostics. They can contribute in the non-invasive early detection of renal abnormalities and the differentiation of various nephrological and urological pathologies. Alpha 1-microglobulin (A1M) is an immunomodulatory protein with a broad spectrum of possible clinical applications and seems a promising marker for evaluation of tubular function.

METHOD

We performed a systematic review of the peer-reviewed literature (until end of November 2003) on A1M with emphasis on clinical diagnostic utility and laboratory aspects.

CONCLUSIONS

A1M is a 27-kDa glycoprotein, present in various body fluids, with unknown exact biological function. The protein acts as a mediator of bacterial adhesion to polymer surfaces and is involved in inhibiting renal lithogenesis. Because A1M is not an acute phase protein, is stable in a broad range of physiological conditions and sensitive immunoassays have been developed, its measurement can be used for clinical purposes. Unfortunately, international standardisation is still lacking. Altered plasma/serum levels are usually due to impaired liver or kidney functions but are also observed in clinical conditions such as HIV and mood disorders. Urinary A1M provides a non-invasive, inexpensive diagnostic alternative for the diagnosis and monitoring of urinary tract disorders (early detection of tubular disorders such as heavy metal intoxications, diabetic nephropathy, urinary outflow disorders and pyelonephritis).

Redox properties of the lipocalin alpha1-microglobulin: reduction of cytochrome c, hemoglobin, and free iron

alpha1-Microglobulin (alpha1m) is a 26-kDa plasma and tissue glycoprotein. The protein has a heterogeneous yellow-brown chromophore consisting of small unidentified prosthetic groups localized to a free thiol group (C34) and three lysyl residues (K92, K118, and K130) around the entrance to a hydrophobic pocket. It was recently reported that alpha1m can bind heme and that a C-terminally processed form of alpha1m degrades heme. It is shown here that alpha1m has catalytic reductase and NADH-dehydrogenase-like activities. Cytochrome c, nitroblue tetrazolium (NBT), methemoglobin, and ferricyanide were reduced by alpha1m. Comparison of the reduction rates suggests that methemoglobin is a better substrate than cytochrome c, NBT, and ferricyanide. The reactions with cytochrome c and NBT were mediated by superoxide anions since they were inhibited by superoxide dismutase. The addition of the biological electron donors NADH, NADPH, or ascorbate enhanced the reduction rate of cytochrome c approximately 30-fold. Recombinant alpha1m, which has much less chromophore than plasma and urine alpha1m, was a stronger reductant than the latter alpha1m forms. Site-directed mutagenesis of C34, K92, K118, and K130 and thiol group chemistry showed that the C34 thiol group was involved in the redox reaction but relies upon cooperation with the lysyl residues. The redox properties of alpha1m may provide a physiological protection mechanism against extracellularly exposed heme groups and other oxidants.

Methylprednisolone favourably alters plasma and urinary cytokine homeostasis and subclinical renal injury at cardiac surgery

Whilst elevated urinary transforming growth factor beta-1 (TGFbeta) is associated with chronic renal dysfunction its role in acute peri-operative renal dysfunction is unknown. In contrast, peri-operative increases in urinary IL-1 receptor antagonist (IL-1ra) and TNF soluble receptor-2 (TNFsr-2) mirror pro-inflammatory activity in the nephron and correlate with renal complications. Steroids modulate some plasma cytokines (decreasing TNFalpha, IL-8, IL-6 and increasing IL-10), whereas ability to reduce plasma and urinary TNFsr-2 and IL-1ra and peri-operative renal injury is unknown. Patients undergoing coronary artery bypass grafting with cardiopulmonary bypass (CPB) were randomised to receive methylprednisolone (n = 18) or placebo (n = 17) before induction of anaesthesia. Plasma and urinary pro- and anti-inflammatory cytokine balance was determined along with subclinical proximal tubular injury and dysfunction, measured by urinary N-acetyl-beta-d-glucosaminidase (NAG)/creatinine and alpha-1-microglobulin/creatinine ratios, respectively. In the control group compared with baseline, plasma IL-8, TNFalpha, IL-10, IL-1ra and TNFsr-2 were significantly elevated along with urinary IL-1ra, TNFsr-2 and TGFbeta1. Urinary NAG/creatinine and alpha-1-microglobulin/creatinine ratios rose from completion of revascularisation until 6 h with recovery at 24 h with a further rise in NAG/creatinine ratio at 48 h. Compared to placebo, the methylprednisolone group showed significantly reduced plasma IL-8, TNFalpha, IL-1ra and TNFsr-2 whereas plasma IL-10 increased. Compared to placebo, the methylprednisolone group demonstrated significantly reduced urinary NAG/creatinine ratio, TNFsr-2 and TGFbeta1 at 24 h whereas urinary alpha-1-microglobulin/creatinine ratios increased.

CONCLUSIONS

Methylprednisolone administration during cardiac surgery significantly reduces plasma and urinary TNFsr-2 and IL-1ra, urinary TGFbeta1 and subclinical renal injury but not dysfunction.

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.

Siderocalin (Lcn 2) Also Binds Carboxymycobactins, Potentially Defending against Mycobacterial Infections through Iron Sequestration

Siderocalin, a member of the lipocalin family of binding proteins, is found in neutrophil granules, uterine secretions, and at markedly elevated levels in serum and synovium during bacterial infection; it is also secreted from epithelial cells in response to inflammation or tumorigenesis. Identification of high-affinity ligands, bacterial catecholate-type siderophores (such as enterochelin), suggested a possible function for siderocalin: an antibacterial agent, complementing the general antimicrobial innate immune system iron-depletion strategy, sequestering iron as ferric siderophore complexes. Supporting this hypothesis, siderocalin is a potent bacteriostatic agent in vitro under iron-limiting conditions and, when knocked out, renders mice remarkably susceptible to bacterial infection. Here we show that siderocalin also binds soluble siderophores of mycobacteria, including M. tuberculosis: carboxymycobactins. Siderocalin employs a degenerate recognition mechanism to cross react with these dissimilar types of siderophores, broadening the potential utility of this innate immune defense.

Molecular characterization and developmental expression pattern of the chicken apolipoprotein D gene: Implications for the evolution of vertebrate lipocalins

The insect Lazarillo and the mammalian apolipoprotein D (ApoD) are orthologous members of the lipocalin protein family. We report the cloning and embryonic expression of chicken ApoD, the first molecularly characterized nonmammalian ApoD. We also report the ApoD expression in mouse during postnatal development and some novel aspects of the expression of the paralogous lipocalin prostaglandin D-synthase (PGDS) and discuss these results in view of the lipocalin family evolution in vertebrates. ApoD is expressed in subsets of central nervous system (CNS) neurons and glia during late chicken embryogenesis. Contrary to mouse ApoD, no expression appears in neural crest-derived cephalic mesenchyme and blood vessel pericytes. Also, ApoD is expressed in developing chicken feathers. These expressions are corroborated by quantitative reverse transcriptase-polymerase chain reaction profiles. ApoD is expressed during mouse postnatal development in a subset of CNS neurons, astrocytes and oligodendrocytes, but also in meninges and pericytes. Chicken PGDS is expressed in brain meninges and perivascular cells. Our results suggest that the amniote last common ancestor expressed ApoD and PGDS in the brain during embryogenesis. ApoD appears restricted to ectodermal derivatives, whereas PGDS is expressed by derivatives of the three germ layers.

Human α-1-Microglobulin Is Covalently Bound to Kynurenine-derived Chromophores

Alpha-1-microglobulin carries a set of covalently linked chromophores that give it a peculiar yellow-brown color, fluorescence properties, and both charge and size heterogeneity. In this report it is shown that these features are due to the adducts with the tryptophan metabolite, 3-hydroxykynurenine, and its autoxidation products and that the modification is more pronounced in the protein isolated from urine of hemodialyzed patients. The light yellow amniotic fluid alpha-1-microglobulin acquires the optical properties and charge heterogeneity of the urinary counterpart following incubation with kynurenines. The colored amino acid adducts of urinary and amniotic fluid alpha-1-microglobulins were separated by chromatography after acid hydrolysis and analyzed by mass spectrometry. Human serum albumin samples, native and treated with 3-hydroxykynurenine in the presence of oxygen, were used as a control. The retention times and mass fragmentation products were compared, and a lysyl adduct with hydroxantommathin was identified in the urinary alpha-1-microglobulin and in the modified albumin samples. The more extensive modification of the urinary protein appears to be correlated with uremia, a condition in which the catabolism of tryptophan via the kynurenine pathway is increased, and the consequent rise in the concentration of its derivatives is accompanied by the oxidative processes due to the hemodialysis treatment. The oxidative derivatives of 3-hydroxykynurenine, which are known to act as protein cross-linking agents, are the likely cause of the propensity of urinary alpha-1-microglobulin to form dimers and oligomers. This process, as well as the redox properties of these metabolites, may contribute to the toxic effects of the kynurenine species.

The lipocalin α1-microglobulin binds heme in different species

The lipocalin alpha(1)-microglobulin (alpha(1)m), found in plasma and tissues of various vertebrates, is brown, forms complexes with other proteins and has immunomodulatory effects in vitro, but the physiological function is not yet established. Human alpha(1)m was recently shown to bind heme and, after cleavage of a C-terminal tetrapeptide, initiate heme degradation, thus suggesting a heme-scavenger function. In this work the heme-binding of alpha(1)m was characterized using heme immobilized on agarose beads, spectrophotometry, and electrophoresis. alpha(1)m, both in plasma and in purified form, displayed a concentration-dependent binding to heme-agarose. The apparent dissociation-constant was estimated to be around 2 x 10(-6)M for both free alpha(1)m and the IgA-alpha(1)m complex. Incubation with free heme resulted in two forms of alpha(1)m with different electrophoretic mobility. alpha(1)m, identified on Western blotting, was found in eluates from heme-agarose after incubation with human biological fluids as well as sera from non-human species, indicating evolutionary conservation of the heme-binding property. Heme-binding could be instrumental for isolating new alpha(1)m-homologues.

Study of drug effects of calcium channel blockers on retinal degeneration of rd mouse

In the present study, we studied drug effects of Ca(2+) antagonists on the retinal degeneration of rd mouse to evaluate their efficacy. Several kinds of Ca(2+) antagonists, diltiazem, nicardipine, nilvadipine or nifedipine were administrated intraperitoneally and thereafter retinal morphology and functions were analyzed. In addition, we performed DNA microarray analysis both in nilvadipine treated and control retinas to understand their drug effects at molecular levels. We found that nilvadipine caused significant preservation of retinal thickness in rd mouse during the initial stage of the retinal degeneration, and nicardipine showed also significant but lesser preservation than nilvadipine. However, we recognized no preservation effects of diltiazem and nifedipine. In the total 3774 genes, the expressions of 27 genes were altered upon administration of nilvadipine, including several genes related to the apoptotic pathway, neuro-survival factor, Ca(2+) metabolisms, and other mechanisms. It is suggested that some types of Ca(2+) channel blockers, such as nilvadipine and nicardipine, are able to preserve photoreceptor cells in rd mouse and can potentially be used to treat some RP patients.

Chronicity of pristane-induced arthritis in rats is controlled by genes on chromosome 14

To address the possibility that genes specifically control the chronic phase of arthritis we have isolated a congenic fragment from the resistant E3 rat on the susceptible DA rat background. The isolated fragment covers the Pia6 quantitative trait locus on chromosome 14, which previously has been identified to be linked to chronic pristane induced arthritis (PIA) in gene segregation experiments of an (E3 x DA)F(2)-cross. Heterozygous Pia6 congenic rats (DA.Pia6) were protected from chronic active arthritis specifically, as determined by macroscopic scoring, histopathology and release of cartilage oligomeric matrix protein (-reflecting ongoing cartilage destruction). The difference was seen only during the chronic active phase of arthritis starting approximately 5 weeks after onset of arthritis. Interestingly, the plasma concentration of the lipocalins alpha(1)-acid glycoprotein and alpha(1)-microglobulin was found significantly altered in nai;ve congenic rats compared to the DA rat. The concentration of alpha(1)-microglobulin was found to be significantly higher throughout the disease course, while alpha(1)-acid glycoprotein had a lower concentration in nai;ve rats, which was highly significant in the chronic phase. The altered concentrations of these proteins already before development of chronic arthritis may provide a clue to the pathogenic process controlled by the Pia6 genes. We conclude that the active relapsing chronic arthritis is under a unique genetic control that is different from the control of acute arthritis and postulate that the liver plays an important role in this regulation.

Heme-scavenging role of alpha1-microglobulin in chronic ulcers

Chronic venous ulcers are characterized by chronic inflammation. Heme and iron, originating from blood cell hemolysis as well as extravascular necrosis, have been implicated as important pathogenic factors due to their promotion of oxidative stress. It was recently reported that the plasma and tissue protein alpha1-microglobulin is involved in heme metabolism. The protein binds heme, and a carboxy-terminally processed form, truncated alpha1-microglobulin, also degrades heme. Here, we show the presence of micromolar levels of heme and free iron in chronic leg ulcer fluids. Micromolar amounts of alpha1-microglobulin was also present in the ulcer fluids and bound to added radiolabeled heme. Truncated alpha1-microglobulin was found in the ulcer fluids and exogenously added alpha1-microglobulin was processed into the truncated alpha1-microglobulin form. Histochemical analysis of chronic wound tissue showed the presence of iron deposits, heme/porphyrins in infiltrating cells basement membranes and fibrin cuffs around vessels, and alpha1-microglobulin ubiquitously distributed but especially abundant in basement membranes around vessels and at fibrin cuffs. Our results suggest that alpha1-microglobulin constitutes a previously unknown defense mechanism against high heme and iron levels during skin wound healing. Excessive heme and iron, which are not buffered by alpha1-microglobulin, may underlie the chronic inflammation in chronic ulcers.

Expression of the AMBP gene transcript and its two protein products, alpha(1)-microglobulin and bikunin, in mouse embryogenesis

The expression pattern of the alpha(1)-microglobulin/bikunin precursor (AMBP) gene, and its two protein products were studied in mouse embryos of 8.5-15.5 days of embryonic development by in situ hybridization and immunohistochemistry. AMBP mRNA is strongly transcribed in liver parenchyma, pancreas, and intestine epithelium. Sites of weaker expression are the vessels of the umbilical cord, the developing vertebral bodies, and kidney. The alpha(1)-microglobulin and bikunin proteins are accordingly present in developing hepatocytes, pancreas, kidney, and gut. However, additional sites of protein distribution were found that do not correlate to mRNA localization: alpha(1)-microglobulin was found in myocytes and bikunin in cardiac muscle, nervous system microvasculature, and connective tissue. Both proteins were found in brain mesenchyme and meninges. Thus, a restricted expression of the AMBP mRNA in a few organs contrasts to a widespread and unique distribution of each of the two proteins.

Urinary bikunin determination provides insight into proteinase/proteinase inhibitor imbalance in patients with inflammatory diseases

Bikunin (BK) is a Kunitz-type proteinase inhibitor responsible for most of the antitryptic activity of urine and so is known as the urinary trypsin inhibitor. As its excretion increases in inflammatory conditions, it is often considered to be a positive acute phase protein (APP). However, the gene for BK is downregulated in inflammation. In human plasma the major part of BK is covalently linked through a glycosaminoglycan chain to one or two homologous peptide heavy chains, thus forming high molecular weight proteinase inhibitors called pre-alpha-inhibitor (PalphaI) and inter-alpha-inhibitor (IalphaI), respectively. The C-terminal parts of these heavy chains are very sensitive to proteolysis. Neutrophil proteinases in particular are able to release from IalphaI and PalphaI BK (M, about 25,000) which retains its antitryptic activity and is quickly excreted in urine. It was therefore an early supposition that the higher urinary excretion of BK occurring during inflammatory diseases should be, at least in some respect, related to a partial proteolysis of IalphaI and PalphaI. In this study we observed that BK, determined as antitryptic activity, was clearly increased in urine from 35 patients with inflammatory diseases varying in origin and severity (76.5 +/- 75.5 IU/g vs. reference value <10 IU/g creatinine). This increase seems mainly to be associated with polymorphonuclear leukocyte activation, monitored by human leukocyte elastase (HLE) determination rather than with the acute phase response assessed by C-reactive protein (CRP) measurement. For all the patients we found that the urinary levels of BK and serum concentration of intact IalphaI correlated inversely (r=-0.36; p=0.03), in agreement with the presumed precursor-product relationship linking IalphaI and BK. We also proved that urinary BK was significantly higher, and serum IalphaI was significantly lower, in samples with plasma HLE values above the reference: 90 microg/l. Taken together, our results demonstrate that BK, the urinary excretion of which is increased in inflammatory conditions, originates, at least partly, from IalphaI and PalphaI by proteolytic cleavage. Consequently, urinary BK determination provides information on the severity of systemic proteolysis occurring in inflammation. We also demonstrated that during inflammatory diseases IalphaI and PalphaI concentrations in serum are dependent on their increased utilization as well as on the regulation of their biosynthesis.

Processing of the lipocalin alpha(1)-microglobulin by hemoglobin induces heme-binding and heme-degradation properties

Alpha(1)-microglobulin is a 26-kd protein, widespread in plasma and tissues and well-conserved among vertebrates. Alpha(1)-microglobulin belongs to the lipocalins, a protein superfamily with highly conserved 3-dimensional structures, forming an internal ligand binding pocket. The protein, isolated from urine, has a heterogeneous yellow-brown chromophore bound covalently to amino acid side groups around the entrance of the lipocalin pocket. Alpha(1)-microglobulin is found in blood both in free form and complex-bound to immunoglobulin A (IgA) via a half-cystine residue at position 34. It is shown here that an alpha(1)-microglobulin species, which we name t-alpha(1)-microglobulin (t = truncated), with a free Cys34 thiol group, lacking its C-terminal tetrapeptide, LIPR, and with a more polar environment around the entrance of the lipocalin pocket, is released from IgA-alpha(1)-microglobulin as well as from free alpha(1)-microglobulin when exposed to the cytosolic side of erythrocyte membranes or to purified oxyhemoglobin. The processed t-alpha(1)-microglobulin binds heme and the alpha(1)-microglobulin-heme complex shows a time-dependent spectral rearrangement, suggestive of degradation of heme concomitantly with formation of a heterogeneous chromophore associated with the protein. The processed t-alpha(1)-microglobulin is found in normal and pathologic human urine, indicating that the cleavage process occurs in vivo. The results suggest that alpha(1)-microglobulin is involved in extracellular heme catabolism.

The liver-specific human alpha(1)-microglobulin/bikunin precursor (AMBP) is capable of self-association

alpha-1-Microglobulin (A1M) and bikunin are two plasma glycoproteins encoded by an alpha-1-microglobulin/bikunin precursor (AMBP) gene. Despite their lack of any structural or functional relationship, both A1M and bikunin originate from AMBP cleavage by a furin-like protease that releases the two mature molecules. The AMBP gene maintains a tight control over its expression by a unique enhancer, which is controlled by several hepatocyte-enriched nuclear factors; however, the mechanisms of regulation of the intracellular levels of the AMBP protein are currently unknown. We report the ability of the AMBP protein to self-associate and form a dimer in a yeast environment using the yeast two-hybrid system and an in vitro dimerization assay. We also show that the A1M protein binds to its precursor protein, AMBP, whereas bikunin does not. This observation warrants further investigations for a dimerization-dependent intracellular control that AMBP may be involved in. The relevance of AMBP dimerization and its possible biological significance are postulated.

Inhibition of tumor growth and metastatic spreading by overexpression of inter-alpha-trypsin inhibitor family chains

The inter-alpha trypsin inhibitor (ITI) family is a group of proteins built up from different combinations of I light chain (ITI-L) and 3 highly homologous heavy chains (ITI-HI, -H2 and -H3). To investigate a potential role of the ITI family chains in cancer and metastasis spreading, we engineered human H460M cell lines expressing both the green fluorescent protein (GFP) and one of these chains. These clones were subcutaneously injected in athymic nude mice, and lung metastasis number and primary tumor weight were determined after 28 days. Expression of the ITI-L chain considerably decreased tumor weight and fluorescent lung metastasis number. ITI-HI and ITI-H3 chain expression induced a significant decrease of metastasis number, whereas no decrease of tumor weight could be detected. In vitro, ITI-L expression significantly decreased chemotaxis and ITI-HI and ITI-H3 expression increased cell attachment. These results argue for the antitumoral or antimetastatic properties of ITI-L, -HI and -H3 chains.

Distribution of iodine 125-labeled alpha1-microglobulin in rats after intravenous injection

The 28-kd plasma protein alpha(1)-microglobulin is found in the blood of mammals and fish in a free, monomeric form and as high-molecular-weight complexes with molecular masses above 200 kd. In this study, iodine 125-labeled free and high-molecular weight rat alpha(1)-microglobulin (a mixture of alpha(1)-microglobulin/alpha(1)-inhibitor-3 and alpha(1)-microglobulin/fibronectin complexes) were injected intravenously into rats. The distribution of the proteins was measured by using scintillation camera imaging. Both forms of (125)I-labeled alpha(1)-microglobulin were rapidly cleared from the blood, with a half-life of 2 and 16 minutes for the initial and late phase, respectively, for free alpha(1)-microglobulin; and a half-life of 3 and 130 minutes for the initial and late phase, respectively, for the complexes. After 45 minutes, 6%, 16%, 27%, 13%, and 34% of the free (125)I-labeled alpha(1)-microglobulin and 18%, 21%, 6%, 10%, and 42% of the (125)I-labeled alpha(1)-microglobulin complexes were found in the blood, gastrointestinal tract, kidneys, liver, and the remainder of the body, respectively. The local distribution of injected (125)I-labeled alpha(1)-microglobulin in intestines and kidneys was investigated by microscopy and autoradiography. In the intestine, both forms were distributed in the basal layers, villi, and luminal contents. The results also suggested intracellular labeling of epithelial cells. Well-defined local regions containing higher concentrations of injected protein could be seen in the intestine. In the kidneys, both forms were found mostly in the cortex. Free (125)I-labeled alpha(1)-microglobulin was found predominantly in epithelial cells of a subset of the tubules, whereas the (125)I-labeled complexes were more evenly distributed. Intracellular labeling was indicated for both alpha(1)-microglobulin forms. The results thus indicate a rapid transport of (125)I-labeled alpha(1)-microglobulin from the blood to most tissues.

Chromosomal location, exon/intron organization and evolution of lipocalin genes

Lipocalins exhibit low sequence similarity that contrasts with a tightly conserved folding shared by all members of this superfamily. This conserved folding can be, at least partly, accounted for by a highly conserved gene structure. The array of lipocalin genes that have so far been studied mostly in mammals indicate a large conservation of a typical seven exon/six intron arrangement. Other conserved features include a partly coding exon 1 of variable size, fixed sizes of exons 2-5 that code for an array of lipocalin-specific beta-strands and a tendency of the last exons to either fuse or expand into further exons without major changes in the length of the resulting open reading frame. The conserved exon/intron arrangement as well as a clustering of most lipocalin genes in given chromosomes of human and mouse indicate that the lipocalin genes diverged from a shared ancestor by successive rounds of duplications followed by late changes in exon arrangements.

Beyond the superfamily: the lipocalin receptors

Lipocalins are characterized by multiple molecular recognition properties including the ability to bind to cell surface receptors. Receptors for a number of lipocalins have been identified. These include receptors for alpha-1-microglobulin, insecticyanin, glycodelin, retinol-binding protein, alpha-1-acid glycoprotein, beta-lactoglobulin and odorant-binding protein. The properties of these receptors are summarized and discussed.

Experimentally determined lipocalin structures

Twelve structures of distinct members of the lipocalin protein family have been solved experimentally. These structures have revolutionised our understanding of the properties of the lipocalins. Many more members of the family have been crystallised and now await structure solution. The number of solved lipocalin structures is steadily increasing, and with it increases our knowledge of this enigmatic and challenging protein family.

Immunocalins: a lipocalin subfamily that modulates immune and inflammatory responses

A subset of the lipocalins, notably alpha(1)-acid glycoprotein, alpha(1)-microglobulin, and glycodelin, exert significant immunomodulatory effects in vitro. Interestingly, all three are encoded from the q32-34 region of human chromosome 9, together with at least four other lipocalins (neutrophil gelatinase-associated lipocalin, complement factor gamma-subunit, tear prealbumin, and prostaglandin D synthase) that also may have anti-inflammatory and/or antimicrobial activity. This review addresses important features of this genetically linked subfamily of lipocalins (involvement with the acute phase response, immunomodulatory and anti-inflammatory properties, the tissue localization, complex formation with other proteins and receptors, etc.). It is likely that these proteins have evolved to be an integrated part of the body's defense system as part of the extended cytokine network. Its members exert a regulatory, dampening influence on the inflammatory cascade, thereby protecting against tissue damage from excessive inflammation. That most major mammalian allergens are lipocalins may reflect this connection of lipocalins with the immune system. We propose that this immunologically active lipocalin subset be named the 'immunocalins', signifying not only the structural homology and close genetic linkage of its members, but also their protective involvement with immunological and inflammatory processes. As immune mediators, immunocalins appear to use at least three interactive sites: the lipocalin 'pocket', binding sites for other plasma proteins, and binding sites for cell surface receptors.