Hyaluronan-binding properties of human serum hemopexin

Investigating the Synergistic Interactions of Surface Immobilized and Free Natural Ocular Lubricants for Contact Lens Applications: A Comparative Study between Hyaluronic Acid and Proteoglycan 4 (Lubricin)

The main reasons for the discontinuation of contact lens wear are ocular dryness and discomfort. Proteoglycan 4 (PRG4), a mucinous glycoprotein, and hyaluronic acid (HA), a nonsulfated linear glycosaminoglycan, are naturally present in the eye and contribute to ocular hydration and lubrication. This study aimed to investigate the impact of the structure of the recombinant human PRG4 (rhPRG4)/HA complex on contact lens properties, when one agent is grafted and the counterpart is physisorbed on the surface of model conventional or silicone contact lens materials. Investigation of the wettability, water retention, antifouling, and boundary lubricant properties of the prepared hydrogels showed that the rhPRG4/HA interactions varied with the rhPRG/HA configuration on the hydrogel surface as well as the composition of the underlying substrate used. The rhPRG4-physisorbed/HA-grafted sample was characterized by better antifouling and boundary lubricant properties on the model conventional hydrogels, while the HA-physisorbed/rhPRG4-grafted sample exhibited improved surface wettability, antifouling, and water-retentive properties on the model silicone hydrogels. The results of this study contribute to the design of biomimetic contact lens surfaces that work synergistically with ocular fluid-phase biological agents to enhance compatibility between the contact lens and the ocular environment, alleviating dry eye symptoms and improving comfort.

Pathogenesis of proteinuria in idiopathic minimal change disease: molecular mechanisms

Minimal change disease (MCD) is the most common type of nephrotic syndrome in children and adolescents. The pathogenesis of proteinuria in this condition is currently being reassessed. Following the Shalhoub hypothesis, most efforts have been placed on identifying the putative circulating factor, but recent advancement in podocyte biology has focused attention on the molecular changes at the glomerular capillary wall, which could explain the mechanism of proteinuria in MCD. This report critically reviews current knowledge on the different postulated mechanisms at the glomerular capillary wall level for increased permeability to plasma proteins in MCD. The report helps describe the rationale behind novel therapies and suggests future targeted therapies for MCD.

An alternative view of the proposed alternative activities of hemopexin

Hemopexin is a plasma protein that plays a well-established biological role in sequestering heme that is released into the plasma from hemoglobin and myoglobin as the result of intravascular or extravascular hemolysis as well as from skeletal muscle trauma or neuromuscular disease. In recent years, a variety of additional biological activities have been attributed to hemopexin, for example, hyaluronidase activity, serine protease activity, pro-inflammatory and anti-inflammatory activity as well as suppression of lymphocyte necrosis, inhibition of cellular adhesion, and binding of divalent metal ions. This review examines the challenges involved in the purification of hemopexin from plasma and in the recombinant expression of hemopexin and evaluates the questions that these challenges and the characteristics of hemopexin raise concerning the validity of many of the new activities proposed for this protein. As well, an homology model of the three-dimensional structure of human hemopexin is used to reveal that the protein lacks the catalytic triad that is characteristic of many serine proteases but that hemopexin possesses two highly exposed Arg-Gly-Glu sequences that may promote interaction with cell surfaces.

Lubricin: a novel potential biotherapeutic approaches for the treatment of osteoarthritis

Osteoarthritis (OA) is a multi-factor disorder of sinovial joints, which characterized by escalated degeneration and loss of articular cartilage. Treatment of OA is a critical unmet need in medicine for regeneration of damaged articular cartilage in elderly. On the other hand, lubricin, a glycoprotein specifically synthesized by chondrocytes located at the surface of articular cartilage, has been shown to provide boundary lubrication of congruent articular surfaces under conditions of high contact pressure and near zero sliding speed. Lubrication of these surfaces is critical to normal joint function, while different gene expressions of lubricin had been found in the synovium of rheumatoid arthritis (RA) and OA. Moreover, mutations or lacking of lubricin gene have been shown to link to the joint disease such as camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP), synovial hyperplasia and failure of joint function, suggesting an important role of lubricin in the pathogenesis of these joint disease. Recent studies demonstrate that administration with recombinant lubricin in the joint cavity would be effective in the prevention of cartilage degeneration in animal OA models. Therefore, a treatment with lubricin which would protect cartilage in vivo would be desirable. This article reviews recent findings with regard to the possible role of lubricin in the progression of OA, and further discusses lubricin as a novel potential biotherapeutic approaches for the treatment of OA.

Hemopexin induces nephrin-dependent reorganization of the actin cytoskeleton in podocytes

Hemopexin is an abundant plasma protein that effectively scavenges heme. When infused into rats, hemopexin induces reversible proteinuria, and activated hemopexin is increased in children with minimal change nephrotic syndrome. These observations suggest a role for hemopexin in glomerular disease; in this study, the effects of active hemopexin on human podocytes and glomerular endothelial cells, the two cell types that compose the glomerular filtration barrier, were investigated. Within 30 min of treatment with hemopexin, actin reorganized from stress fibers to cytoplasmic aggregates and membrane ruffles in wild-type podocytes. This did not occur in nephrin-deficient podocytes unless they were transfected with nephrin-expressing plasmids. Furthermore, hemopexin did not affect actin organization in cells that do not express nephrin, specifically human glomerular endothelial cells, fibroblasts, and HEK293 cells. The effects of hemopexin on wild-type podocytes reversed within 4 h and were inhibited by preincubation with human plasma. Treatment with hemopexin activated protein kinase B in both wild-type and nephrin-deficient podocytes but activated RhoA only in wild-type cells. In addition, hemopexin led to a selective increase in the passage of albumin across monolayers of glomerular endothelial cells and to a reduction in glycocalyx. In summary, active hemopexin causes nephrin-dependent remodeling of podocytes and affects permeability of the glomerular filtration barrier by degrading the glycocalyx.

Chromatographically distinguishable heme insertion isoforms of human hemopexin

Two spectroscopically distinct, non-interconverting forms of human hemopexin have been isolated by immobilized metal ion affinity chromatography and characterized spectroscopically. Form alpha (characterized by a bisignate Soret CD spectrum) and form beta (Soret CD characterized by a positive Cotton effect) exhibit different spectroscopic responses to addition of Zn2+ or Cu2+, yet both forms exhibit the same metal ion-induced decrease in Tm for the thermally induced release of the heme prosthetic group. Far UV-CD spectra indicate that the two isoforms possess essentially identical secondary structures, but their differential retention during metal ion affinity chromatography indicates slight differences in exposure of His residues on the protein surface. We propose that these observations result from the binding of heme in form beta with an orientation that differs from the crystallographically observed binding orientation for rabbit hemopexin by rotation of the heme prosthetic group by 180 degrees about the alpha-gamma meso-carbon axis and from interaction of metal ions at two separate binding sites.

PRG4 exchange between the articular cartilage surface and synovial fluid

The boundary lubrication function of articular cartilage is mediated in part by proteoglycan 4 (PRG4) molecules, found both in synovial fluid (SF) and bound to the articular cartilage surface. Currently the mechanism by which PRG4 binds to the articular surface is not well understood. The objectives of this study were to determine (1) the effect of bathing fluid contents on PRG4 concentration at the articular surface ([PRG4](cart)), and (2) whether native PRG4 can be removed from the surface and subsequently repleted with PRG4 from synovial fluid. In one experiment, cylindrical cartilage disks were stored in solutions of various PRG4 concentrations, either in phosphate-buffered saline (PBS) or SF as the carrier fluid. In a separate experiment, cartilage disks were stored in solutions expected to remove native PRG4 from the articular surface and allow subsequent repletion with PRG4 from SF. [PRG4](cart) was independent of PRG4 concentration of the bathing fluid, and was similar for both carrier fluids. PRG4 was removed from cartilage by treatment with hyaluronidase, reduction/alkylation, and sodium dodecyl sulphate, and was repleted fully by subsequent bathing in SF. These results suggest that the articular surface is normally saturated with tightly bound PRG4, but this PRG4 can exchange with the PRG4 in SF under certain conditions. This finding suggests that all tissues surrounding the joint cavity that secrete PRG4 into the SF may help to maintain lubrication function at the articular surface.

Protease activity of plasma hemopexin

BACKGROUND

Previous studies into the relevance of a putative circulating factor in the pathogenesis of minimal change nephrotic syndrome have opened the possibility that plasma hemopexin might be an important effector molecule in this disorder. Thus, intra renal infusion of isolated plasma hemopexin into rats induced minimal change like glomerular lesions and proteinuria. Both, in vivo and in vitro effects of the active isoform of hemopexin could be attributed to protease activity of this molecule. However, the question remained whether hemopexin per se rather than some contaminating plasma factor is responsible for the potential enzymatic activity of this molecule.

METHODS

Recombinant hemopexin was prepared according to standard methods in Pichia pastoris and compared for its identity and protease activity with plasma hemopexin using Western blotting and various in vitro assays. Unilateral renal perfusion in anesthetized rats was used to test the proteinuria inducing capacity of recombinant hemopexin versus heat-inactivated recombinant hemopexin.

RESULTS

The blotting results show identical 85 kD bands in both native as well as recombinant hemopexin. Incubation of kidney tissue with recombinant hemopexin resulted in loss of of glomerular ectoapyrase and sialoglycoproteins, as shown by immunohistochemistry, which effect can be inhibited with the serine protease inhibitor phenylmethanesulfonyl fluoride. Artificial substrates for serine proteases, like kallikrein or thrombin, are hydrolysed by recombinant hemopexin in vitro, and not by heat-inactivated recombinant hemopexin or saline. Unilateral kidney perfusion of recombinant hemopexin, in contrast to control Pichia transfection products or heat-inactivated recombinant hemopexin, followed by a protein marker showed significantly enhanced urinary protein leakage 5.0, 10.0, and 15.0 minutes after perfusion.

CONCLUSION

It is concluded that the hemopexin molecule as such can potentially act as a toxic protease, leading in the rat to proteinuria and glomerular alterations characteristic for minimal change nephrotic syndrome.

Identification of hemopexin as a GH-regulated gene

A cDNA library from the liver of a growth hormone (GH)-treated hypophysectomized rat was constructed and screened for GH-inducible genes (GIGs). Three cDNAs specific for putative GIG mRNAs (GIG-3, -7 and -12) were isolated and, when sequenced, were found to be homologous to portions of rat hemopexin, a Class 2 acute-phase gene. Hemopexin is an essential heme scavenger produced primarily in the liver, which upon binding to free heme, transports it to the liver where the heme iron is re-utilized. Hemopexin has not been previously described as being GH-responsive. GIG-3 and GIG-12 encode overlapping portions of the entire coding sequence starting within a few hundred base pairs from the 5' end of the hemopexin mRNA, and GIG-7 encodes the 3'-most end of the hemopexin mRNA. Northern analysis and ribonuclease protection assays of RNA from livers of control rats using the cDNA probes demonstrated a major transcript of approximately 2.0 kb. The hemopexin mRNA was low or undetectable in livers of hypophysectomized rats. Daily treatment with bovine growth hormone (bGH) for 10 days restored hemopexin mRNA to levels comparable or greater than that of intact rats. GH-dependence in cultured rat H4IIE hepatoma cells was then examined. Using hemopexin cDNA probes (GIG-3, -7, and -12) we identified a mRNA on Northern blots, which increased in concentration following bGH, compared with untreated cells. When measured by ribonuclease protection assay, a maximal increase in hemopexin mRNA concentration was obtained following 4-6 h of bGH administration. We conclude that hemopexin is a GH-inducible gene in rat liver in vivo and in cultured rat hepatoma cells.

Identification and characterization of a Streptococcus pyogenes operon involved in binding of hemoproteins and acquisition of iron

The hemolytic Streptococcus pyogenes can use a variety of heme compounds as an iron source. In this study, we investigate hemoprotein utilization by S. pyogenes. We demonstrate that surface proteins contribute to the binding of hemoproteins to S. pyogenes. We identify an ABC transporter from the iron complex family named sia for streptococcal iron acquisition, which consists of a lipoprotein (siaA), membrane permease (siaB), and ATPase (siaC). The sia transporter is part of a highly conserved, iron regulated, 10-gene operon. SiaA, which was localized to the cell membrane, could specifically bind hemoglobin. The operon's first gene encodes a novel bacterial protein that bound hemoglobin, myoglobin, heme-albumin, and hemoglobin-haptoglobin (but not apo-haptoglobin) and therefore was named Shr, for streptococcal hemoprotein receptor. PhoZ fusion and Western blot analysis showed that Shr has a leader peptide and is found in both membrane-bound and soluble forms. An M1 SF370 strain with a polar mutation in shr was more resistant to streptonigrin and hydrogen peroxide, suggesting decreased iron uptake. The addition of hemoglobin to the culture medium increased cell resistance to hydrogen peroxide in SF370 but not in the mutant, implying the sia operon may be involved in hemoglobin-dependent resistance to oxidative stress. The shr mutant demonstrated reduced hemoglobin binding, though cell growth in iron-depleted medium supplemented with hemoglobin, whole blood, or ferric citrate was not affected, suggesting additional systems are involved in hemoglobin utilization. SiaA and Shr are the first hemoprotein receptors identified in S. pyogenes; their possible role in iron capture is discussed.

Inhibition of Mg2+-dependent Adhesion of Polymorphonuclear Leukocytes by Serum Hemopexin: Differences in Divalent-Cation Dependency of Cell Adhesion in the Presence and Absence of Serum

Circulating and nonadherent polymorphonuclear leukocytes (PMNs) become activated to attain adhesive state in an integrin-dependent manner by various stimuli, and perform a variety of microbicidal functions such as phagocytosis and superoxide production. We found that, in the absence of serum, a physiological concentration of hemopexin has a strong inhibitory action on Mg(2+)-dependent adhesion of PMA-activated PMNs to fibrinogen- and serum-coated surfaces. Under these conditions, Ca(2+) had no effect on Mg(2+)-dependent adhesion or the adhesion-inhibitory activity of hemopexin. In contrast, PMNs suspended in serum containing sufficient amounts of hemopexin to inhibit adhesion showed marked adherence, which was inhibited by EGTA. Next, we prepared a small-molecule fraction of serum by ultrafiltration followed by boiling. PMA-activated PMNs was found to adhere in the presence of both hemopexin and the small-molecule fraction, and the adhesion was enhanced by exogenous Ca(2+). EGTA abolished the effect of the small molecule fraction. The data suggest that serum contains adhesion-promoting factor(s) which allows PMNs to adhere despite the presence of hemopexin and that Ca(2+) is required for adhesion-promoting activity. Further study of hemopexin may provide clues for new therapeutic strategies aimed at interfering with PMN adhesion to control inflammation and tissue injury.

Hemopexin: a review of biological aspects and the role in laboratory medicine

BACKGROUND

Hemopexin is a heme-binding plasma glycoprotein which, after haptoglobin, forms the second line of defense against hemoglobin-mediated oxidative damage during intravascular hemolysis. A decrease in plasma hemopexin concentration reflects a recent release of heme compounds in the extracellular compartment. Heme-hemopexin complexes are delivered to hepatocytes by receptor-mediated endocytosis after which hemopexin is recycled to the circulation.

METHODS OF ANALYSIS

Immunonephelometric and -turbidimetric hemopexin assays are available as more precise and rapid alternatives to the radial immunodiffusion technique.

INTERPRETATIONS

Hemopexin determinations are not subject to interference by in vitro hemolysis. Altered serum or plasma concentrations of hemopexin are found not only in hemolytic anemias but also in other conditions such as chronic neuromuscular diseases and acute intermittent porphyria. In laboratory medicine, while hemopexin determination in tandem with haptoglobin has potential applications in the assessment of intravascular hemolysis and allows for the monitoring of the severity of hemolysis after depletion of haptoglobin, its diagnostic utility is less clear in other pathological conditions. Further studies are necessary to fully establish the clinical significance of hemopexin determination.

Homology of lubricin and superficial zone protein (SZP): products of megakaryocyte stimulating factor (MSF) gene expression by human synovial fibroblasts and articular chondrocytes localized to chromosome 1q25

We have previously identified megakaryocyte stimulating factor (MSF) gene expression by synovial fibroblasts as the origin of lubricin in the synovial cavity. Lubricin is a mucinous glycoprotein responsible for the boundary lubrication of articular cartilage. MSF has a significant homology to vitronectin and is composed of 12 exons. RNA was purified from human synovial fibroblasts and articular chondrocytes grown in vitro from tissue explants obtained from subjects without degenerative joint disease. RT-PCR was used with multiple complimentary primer pairs spanning the central mucin expressing exon 6 of the MSF gene and individual exons on both the N- and C-terminal sides of exon 6. Exons 2, 4 and 5 appear to be variably expressed by synovial fibroblasts and articular chondrocytes. Lubricating mucin, in the form of MSF, is expressed by both chondrocytes and synovial fibroblasts in vitro. Both lubricin and superficial zone protein (SZP), a related proteoglycan, share a similar primary structure but could differ in post-translational modifications with O-linked oligosaccharides which are predominant in lubricin and with limited amounts chondroitin and keratan sulfate found in SZP. Since most of the MSF exons are involved in the expression of lubricating mucin, a strong homology to vitronectin persists. It is therefore appropriate to consider that both SZP and lubricin occupy a new class of biomolecules termed tribonectins. Screening of a human genome bacterial artificial chromsome (BAC) library with a cDNA primer pair complimentary for exon 6 identified two clones. Both clones were complimentary for chromosome 1q25 by in situ hybridization. This same locus was previously implicated in camptodactyl-arthropathy-pericarditis syndrome (CAP) by genetic mapping. It is hypothesized that CAP, a large joint arthropathy, may be associated with ineffective boundary lubrication provided by synovial fluid.

Stromal and epithelial expression of tumor markers hyaluronic acid and HYAL1 hyaluronidase in prostate cancer

Hyaluronic acid (HA), a glycosaminoglycan, regulates cell adhesion and migration. Hyaluronidase (HAase), an endoglycosidase, degrades HA into small angiogenic fragments. Using an enzyme-linked immunosorbent assay-like assay, we found increased HA levels (3-8-fold) in prostate cancer (CaP) tissues when compared with normal (NAP) and benign (BPH) tissues. The majority ( approximately 75-80%) of HA in prostate tissues was found to exist in the free form. Primary CaP fibroblast and epithelial cells secreted 3-8-fold more HA than respective NAP and BPH cultures. Only CaP epithelial cells and established CaP lines secreted HAase and the secretion increased with tumor grade and metastasis. The pH activity profile and optimum (4.2; range 4.0-4.3) of CaP HAase was identical to the HYAL1-type HAase present in human serum and urine. Full-length HYAL1 transcript and splice variants were detected in CaP cells by reverse transcriptase-polymerase chain reaction, cloning, and sequencing. Immunoblotting confirmed secretion of a approximately 60-kDa HYAL1-related protein by CaP cells. Immunohistochemistry showed minimal HA and HYAL1 staining in NAP and BPH tissues. However, a stromal and epithelial pattern of HA and HYAL1 expression was observed in CaP tissues. While high HA staining was observed in tumor-associated stroma, HYAL1 staining in tumor cells increased with tumor grade and metastasis. The gel-filtration column profiles of HA species in NAP, BPH, and CaP tissues were different. While the higher molecular mass and intermediate size HA was found in all tissues, the HA fragments were found only in CaP tissues. In particular, the high-grade CaP tissues, which showed both elevated HA and HYAL1 levels, contained angiogenic HA fragments. The stromal-epithelial HA and HYAL1 expression may promote angiogenesis in CaP and may serve as prognostic markers for CaP.

Respective roles of inflammation and axonal breakdown in the regulation of peripheral nerve hemopexin: an analysis in rats and in C57BL/Wlds mice

We have previously demonstrated that one of the peripheral nerve responses to injury is the overexpression of hemopexin (HPX). Here, we demonstrate that Wallerian degeneration is required for this response, since HPX does not increase in C57BL/Wlds mice, which display a severely impaired Wallerian degeneration. We also show that HPX synthesis is dramatically increased in macrophages during their activation or after IL-6 stimulation. However, IL-6-driven HPX overexpression occurs in vivo and in vitro in the absence of substantial macrophage invasion. We conclude that, after nerve injury, HPX overexpression occurs first in Schwann cells as a result of axotomy and is subsequently regulated by inflammation. Furthermore, our results and those already described suggest that IL-6, synthesized by the various cell types producing HPX, control nerve HPX expression via paracrine and autocrine mechanisms.

Is 100KF an isoform of hemopexin? Immunochemical characterization of the vasoactive plasma factor 100KF

The human vasoactive plasma factor 100KF has been proposed to play a role in minimal change disease in relapse. Since preliminary data suggested similarity between 100KF and the human plasma glycoprotein hemopexin (Hx), this study was conducted to compare 100KF with purified Hx for sequence homology, immunostaining properties in Western and dot-blot assays, ability to affect glomerular ecto-ATPase and glomerular polyanions in vitro, as well as their glomerular permeability increasing effect following alternate perfusion into the rat kidney ex vivo. 100KF was purified from normal pooled plasma according to standard chromatographic techniques, and from the same batch Hx was prepared using affinity chromatography. A second batch of Hx was prepared directly from human serum according to a standard protocol. (For comparison, additional Hx samples obtained from other centers were also included in the study.) The results show: (1) 100% homology of 100KF with plasma Hx after internal sequence analysis; (2) positive staining of the eluate with both monoclonal and polyclonal anti-Hx IgG as well as anti-100KF IgG in dot-blot assays, and similar bands on Western blotting using the same antibodies; (3) affection of glomerular polyanions and glomerular ecto-ATPase after incubation of kidney tissue with either 100KF or Hx (1.5 respectively 1.0 mg/ml; 1.0 h, 37 degrees C), as detected by computerized histochemical quantification; and (4) significant enhancement of urinary protein leakage after Hx perfusion followed by diluted rat serum into the rat kidney ex vivo (Hx: 210.65+/-49.79 microg protein leakage per min versus heat-inactivated Hx control: 112.2+/-49.18 microg per min [both n = 6]). From these data and from the observation that both Hx and 100KF activity can be inhibited by serine protease inhibitors but not by broad spectrum collagenase inhibitors, it is concluded that Hx may be closely related or identical to the active moiety of 100KF.

Regulation of hemopexin synthesis in degenerating and regenerating rat sciatic nerve

In injured peripheral nerves, hemopexin mRNA is expressed by fibroblasts, Schwann cells, and invading blood macrophages, and the protein accumulates in the extracellular matrix. This and its absence of regulation in injured central optic nerve suggest that hemopexin could play a positive role in peripheral nerve repair. Here, we studied the regulation of hemopexin expression in degenerating and regenerating nerves. After a sciatic nerve injury, both the synthesis of hemopexin and the level of its mRNA increase sharply during the first 2 days, leading to an accumulation of hemopexin in the nerve. Afterward, hemopexin expression decreases progressively in regenerating nerves. In permanently degenerated nerves, it is again transiently increased and then strongly decreased, whereas hemopexin from blood origin is accumulating. As part of the elucidation of the complex regulation of hemopexin expression in injured nerves, we demonstrate that interleukin-6 increases hemopexin synthesis in intact nerves, whereas adult rat serum, but not purified hemopexin, inhibits it in degenerated nerves. Hemopexin, known as acute-phase protein, is therefore one of the molecules rapidly and specifically up-regulated in injured peripheral nerves. More generally, our findings suggest that the acute phase could be not only a systemic liver-specific response but also a reaction of injured tissues themselves.

Changes in expression and localization of hemopexin and its transcripts in injured nervous system: a comparison of central and peripheral tissues

The recent demonstration of hemopexin synthesis in the adult rat sciatic nerve and its accumulation after injury has raised the question of the possible role of this acute phase protein during the process of nerve repair. To gain insight into its function, we have compared the distribution of both hemopexin and its messenger RNA in the peripheral and the central nervous systems. We find that hemopexin is present in all types of peripheral nerves and ganglia, confined to the extracellular matrix and basement membranes of the endoneurium, blood vessels and connective tissues. After injury, hemopexin messenger RNA is overexpressed by Schwann cells, fibroblasts and invading macrophages. The content in hemopexin protein increases in all nerves studied, without changes in localization. Therefore, hemopexin does not appear to be associated with the fate of myelin or with the regeneration of a particular type of nerve fibre. In the central nervous system, hemopexin messenger RNA cannot be detected and the protein is only found in basement membranes of the vascular system (capillaries, meninges and choroid plexus). Furthermore, hemopexin and its messenger RNA remain absent from the distal part of the injured optic nerves. Our results further support the idea that hemopexin plays specific roles during nerve repair, and that it may be associated with the endoneurial extracellular matrix.