From the Laboratory of Pathology and ¶ Pediatric
Branch, NCI, National Institutes of Health,
Bethesda, Maryland 20892
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ABSTRACT |
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Host infection by the pathogenic fungus Candida albicans is initiated by adhesion and mediated by binding to several host extracellular matrix proteins. Previously, we demonstrated that hemoglobin supplemented into a chemically defined medium significantly and specifically induced fibronectin binding to C. albicans. We now report that hemoglobin also induces binding of laminin, fibrinogen, and type IV collagen but not of thrombospondin-1 or type I collagen. The binding of each protein was inhibited by the respective unlabeled ligand in a concentration-dependent manner. Fibrinogen inhibited the binding of radiolabeled fibronectin, laminin, and fibrinogen with similar IC50 values, suggesting that a single promiscuous receptor recognizes these three proteins. Competitive binding studies indicated that a second class of receptor binds specifically to laminin. Growth of C. albicans in the presence of hemoglobin also increased cell adhesion to immobilized fibronectin, laminin, fibrinogen, and type IV collagen but not to thrombospondin-1 or type I collagen. Exposure to hemoglobin induced increased or de novo expression of several surface proteins on C. albicans. One of these proteins with a molecular weight of 55,000 recognized fibronectin, based on ligand protection and affinity chromatography on immobilized fibronectin. Thus, hemoglobin induces both promiscuous and specific receptors for extracellular matrix proteins and, therefore, may regulate matrix adhesion during dissemination of C. albicans infections.
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INTRODUCTION |
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Candida albicans is an important opportunistic pathogen for humans, causing both superficial and disseminated infections (reviewed in Refs. 1 and 2) with significant morbidity and mortality among immunocompromised patients (3). Adhesion of the organism to mucosal epithelium is a prerequisite for colonization and, therefore, is regarded as an initial step in the process leading to infections (3, 4). Additional adhesion events to endothelium and extracellular matrix (ECM)1 components are required for dissemination of C. albicans. A number of ECM proteins bind to C. albicans, including fibronectin (5-8), laminin (9), vitronectin (10, 11), complement (12), fibrinogen (13), gelatin (7), and types I and IV collagen (14).
Interaction with individual ECM proteins is mediated by binding to respective receptors on the surface of Candida cells (5-7, 9-11). Some of these receptors are probably homologs of mammalian integrins (15). However, fibronectin binds to C. albicans both through the cell binding domain recognized by mammalian integrins (8) and through the collagen binding domain (6), and laminin and fibrinogen bind to distinct classes of receptors on C. albicans that are probably not integrins (9, 13, 16). Thus, both integrin and non-integrin receptors may mediate adhesion of C. albicans.
Previously, we reported that hemoglobin induces a marked enhancement of fibronectin binding activity in C. albicans (17). This induction is reversible, requires cell growth in the presence of hemoglobin, and is not due to a bridge effect of hemoglobin between a receptor on the organism and fibronectin. In addition, adhesion of C. albicans to corneal endothelial cells was significantly increased when grown in hemoglobin-containing defined medium compared with those grown in defined medium alone. Although the ability to acquire iron has long been considered one of the most important adaptive responses for microbial pathogenesis (18, 19), the enhancement of fibronectin binding by hemoglobin was not simply due to iron acquisition from the hemoglobin, because other ferroproteins, ferrous ions, or iron-containing porphyrins were inactive (17). Thus, hemoglobin itself may act as a potent regulator for the fibronectin receptor in C. albicans, although its precise mechanism remains unclear.
We have now examined whether this specific induction by hemoglobin influences binding of C. albicans to other ECM proteins. We demonstrate that binding of C. albicans to laminin, fibrinogen, and type IV collagen, are increased to various degrees, but binding to thrombospondin-1 and type I collagen are not. In addition to inducing specific receptors for some ECM ligands on C. albicans, growth in the presence of hemoglobin induces a promiscuous receptor for several ECM proteins including fibronectin (FN), fibrinogen, and laminin. Adhesion to these proteins is also coordinately increased, and expression of specific cell surface proteins that bind FN is increased. Among these, a 55-kDa protein was identified as a hemoglobin-induced fibronectin receptor. Based on these findings, hemoglobin seems to be a potent regulator of the adhesive phenotype of C. albicans.
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EXPERIMENTAL PROCEDURES |
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Strains and Growth Conditions--
ATCC strain 44807 of C. albicans was purchased from the American Type Culture Collection
and used throughout this study. The organism was initially grown in
Sabouraud medium (6) to early stationary phase at 26 °C, and
blastoconidia were stored at 70 °C with 20% sterile glycerol
until used. On a biweekly basis, a fresh culture was made as a stock
for daily use. For each experiment, the organism was inoculated into
the 4 × of yeast nitrogen base (YNB) broth with or without
hemoglobin at a final concentration of 0.1% and incubated at 26 °C
for 20-48 h (17). YNB is a chemically defined medium, and under this
growth condition no germination was found upon microscopic examination
of cultures in the presence or absence of hemoglobin.
Extracellular Matrix Proteins-- FN was purified from frozen human plasma as described previously (6). Thrombospondin-1 was isolated from human platelets (20). Mouse laminin and gelatin were purchased from Life Technologies, Inc., bovine fibrinogen was from Calbiochem, and collagens type I and IV were from Collaborative Research Incorporated (Bedford, MA). Iodination of individual ECM proteins was accomplished using Iodogen (Pierce), and unbound iodine was removed by gel filtration through a PD-10 column (6).
Cell Binding Assay-- In a typical binding assay, 2 × 106 C. albicans were exposed to each 125I matrix protein at a final concentration of 0.5 µg/ml in a total volume of 200 µl of Dulbecco's phosphate-buffered saline (PBS) without CaCl2 and MgCl2, 0.1% BSA, pH 6.0, in 12 × 75-mm polypropylene tubes (PGC, Gaithersburg, MD) and incubated for 3 h at room temperature with shaking at 160 rpm. The cell suspensions were then transferred to microfuge tubes, and the blastoconidia were separated from unbound radioactive ligand by centrifugation through 100 µl of an oil mixture of dibutyl phthalate/dioctyl phthalate (2:1). Radioactive ECM protein bound to the cell pellet was quantified in a gamma counter (Packard Instrument Company, Downers Grove, IL). For inhibition assays, the binding of radiolabeled protein was determined in the presence of various concentrations of unlabeled homologous or heterologous ECM ligands.
Adhesion to Immobilized Matrix Proteins-- Extracellular matrix proteins were coated onto glass chamber slides (Nalge Nunc International, Naperville, IL) wells by adding 300 µl/well of FN solution at 1 or 10 µg/ml into each well and incubating at 4 °C overnight. Adhesion was measured as described previously (17).
Biotinylation and Extraction of Surface Proteins of C. albicans--
Biotinylation of Candida surface proteins was
achieved using sulfohydroxysuccinimidyl-6-(biotinamido) hexanoate
(Pierce). Briefly, 20 ml of Candida cultures grown in 4 × YNB with or without hemoglobin at 26 °C for 48 h were
harvested by centrifugation, and the cells were washed twice with DPBS,
pH 7.4, without Ca2+ and Mg2+ (Life
Technologies, Inc.) to remove trapped or precipitated hemoglobin. The
pellets were suspended in 1 ml of 50 mM sodium bicarbonate buffer, pH 8.5, in microfuge tubes, and 74 µl of 1 mg/ml
sulfohydroxysuccinimidyl-6-(biotinamido) hexanoate were added. The cell
suspension was incubated at room temperature with shaking for 1 h,
and excess biotinylation reagent was removed by washing the cells with
DPBS. After biotinylation, cell surface proteins were extracted.
Candida cells were suspended either in lyticase (Sigma) at a
final concentration of 2 mg/ml, 8 mM dithiothreitol, or a
combination of both and incubated at 37 °C for 1 h with
vigorous shaking. Cell pellets were removed by centrifugation, and
extracted cell surface proteins in the supernatant fluids were either
separated by 10% SDS-polyacrylamide gel electrophoresis containing 8 M urea or stored at 70 °C for future use. After
electrophoresis, proteins were transferred onto a nitrocellulose
membrane, blocked with 3% BSA in 50 mM Tris, pH 7.5, 150 mM NaCl, and incubated with streptavidin-horseradish peroxidase (Amersham Life Science, Inc.). Biotinylated cell surface proteins were visualized using an ECL chemiluminescent detection kit
(Amersham).
Purification of a FN-binding Protein Using Affinity Chromatography-- For preparation of the affinity gel, FN was conjugated to Reacti-Gel (Pierce) at a concentration of 1 mg/ml at 4 °C overnight, and excess FN was removed by washing. Unbound reactive groups were blocked by incubating with 100 mM Tris buffer, pH 7.5, at room temperature for 1 h. Extracted Candida biotinylated surface proteins were incubated with a suspension of the affinity gel at room temperature for 4 h and transferred to a glass column. The column was washed with 10 column volumes of DPBS, pH 7.4, containing 300 mM NaCl. Bound proteins were eluted stepwise with DPBS buffer, pH 7.4, containing 650 mM NaCl followed by 0.2 M sodium acetate, pH 4.0. Eluted fractions were dialyzed, freeze-dried, and analyzed by SDS-polyacrylamide gel electrophoresis. FN-binding cell surface proteins were visualized by ECL detection.
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RESULTS |
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Induction of ECM Protein Binding by Hemoglobin Is Not Limited to Fibronectin-- After growth in YNB supplemented with 0.1% hemoglobin for 48 h, binding of Candida cells to several radiolabeled ECM proteins was examined. Among the ECM proteins tested, binding of fibronectin, laminin, fibrinogen, and type IV collagen increased more than 10-fold relative to their binding to cells grown in the absence of hemoglobin (Fig. 1). In contrast, hemoglobin failed to enhance the binding of type I collagen or thrombospondin-1 to C. albicans. Binding of FN, laminin, fibrinogen, and type IV collagen were consistently and markedly enhanced in all experiments. Some enhancement of gelatin binding was observed but varied from 2 to 16-fold in three separate experiments (results not shown).
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Hemoglobin Enhances Adhesion of Candida albicans to Several Immobilized Ligands-- Although increased binding of soluble matrix proteins could have biological significance, the ability of immobilized ECM proteins to promote C. albicans adhesion to a surface is the parameter that is required for dissemination of the pathogen. We previously reported that hemoglobin induced an increased adhesion of C. albicans to immobilized FN coated onto glass Chamber slides (17). Using the same technique, adhesion of C. albicans grown in hemoglobin-containing medium was examined on slides coated with various ECM proteins (Fig. 2). Compared with the cells grown in the absence of hemoglobin, hemoglobin-induced C. albicans demonstrated a significantly greater adhesion (p < 0.05 by a two-tailed t test) to immobilized FN, laminin, fibrinogen, or type IV collagen at a concentration of 10 µg/ml, respectively. Although adhesion to type I collagen increased slightly for Candida cells prepared from cultures grown in the presence of hemoglobin, the difference was not statistically significant (p > 0.1). Growth in medium containing hemoglobin did not alter adhesion of C. albicans to immobilized thrombospondin-1 or gelatin (p > 0.2).
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Hemoglobin Induces a Shared Receptor for Fibronectin, Laminin, and Fibrinogen-- Binding of fibronectin to C. albicans induced by hemoglobin was previously shown to be saturable and described by a single class of binding sites (17). Similarly, induced binding of fibrinogen and laminin were also saturable (Fig. 3), but cross-competition experiments suggested that these three proteins partially share a common binding site. Fibrinogen was equally active as an inhibitor of iodinated laminin, fibrinogen, or FN binding, and the binding of all three labeled proteins was completely inhibited by excess fibrinogen (Fig. 3A). This result demonstrates that fibrinogen binds to a site recognized by all three proteins. In contrast, laminin was a better inhibitor of iodinated laminin binding than of fibronectin binding, suggesting that more than one class of receptor(s) interacts with these two proteins (Fig. 3B).
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Expression of Novel Surface Proteins Induced by Hemoglobin-- Since hemoglobin dramatically induced binding of several ECM proteins involving both distinct receptors and a class of promiscuous receptor, we expected that hemoglobin may induce increased or de novo expression of proteins on the surface of Candida cells. To detect these changes in cell surface protein expression, Candida cells grown in medium with or without hemoglobin supplement were labeled with biotin. Labeled surface components were extracted with either dithiothreitol alone or together with lyticase. Equal amounts of protein for each extract were loaded onto SDS gels and detected using peroxidase-streptavidin (Fig. 5). Biotinylated cell surface proteins ranging from 14 to >100 kDa were observed in hemoglobin-induced and noninduced C. albicans. The proteins detected by streptavidin binding should represent exclusively cell wall or cell membrane proteins, since the sulfonated biotin derivative used is not membrane-permeable. This was verified by comparing electrophoretic profiles of surface-labeled cells to those of cells broken using glass beads, which showed few proteins with similar molecular weights (results not shown). Furthermore, the plasma membrane of the cells remained intact throughout the labeling and lyticase/dithiothreitol extraction. Viability of the cells decreased only 1.8% following labeling and extraction of the surface proteins, based on trypan blue exclusion.
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Identification of a FN-binding Protein from the Cell Surface-- Several approaches were used to identify which of these induced surface proteins bind to the ECM proteins. Ligand blots of lyticase extracts with 125I-fibronectin or fibrinogen identified a 70-kDa protein, but this protein was identified as an impurity in the lyticase.2 In preliminary experiments, we observed that modification of exposed amino groups on C. albicans inhibited binding of 125I-fibronectin (Fig. 6). This implied that amino groups on a surface protein are required for fibronectin binding to its receptor. We therefore used ligand protection from chemical modification to identify a fibronectin-binding protein. C. albicans cells were incubated with 0, 10, or 100 µg/ml FN for 2 h to allow FN to bind to its receptors on the cell surface. Excess FN was removed, and exposed amino groups on surface proteins were modified using sulfo-SHPP in the presence of the bound FN. After removal of unreacted sulfo-SHPP and dissociation of the bound FN by thorough washing, the cells were biotinylated. By this approach, biotinylation was limited to those amino groups that were protected by the bound FN. Surface proteins were released by dithiothreitol/lyticase digestion, and biotin-labeled proteins were identified by blotting with streptavidin-peroxidase after separation by SDS gel electrophoresis (Fig. 7). Protection by 10 or 100 µg/ml FN resulted in prominent labeling of a 55-kDa protein (Fig. 7, lanes b and c) that was absent in the cells without FN protection (Fig. 7, lane a). At the higher FN concentration, additional bands were revealed at molecular masses of 45 and 66 kDa (Fig. 7, lane c). In contrast to specific protection of the 55-kDa protein, biotin labeling of the 30- and 70-kDa hemoglobin-induced proteins identified in lane d of Fig. 5 were not altered by preincubation with FN (Fig. 7, lanes a-c).
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DISCUSSION |
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Both solution phase cell binding assays and adhesion to immobilized extracellular matrix proteins demonstrate that growth of C. albicans in a defined medium containing hemoglobin coordinately up-regulates interactions with laminin, fibronectin, fibrinogen, and type IV collagen. This up-regulation is specific in that interactions with at least two other extracellular matrix proteins, type I collagen and thrombospondin-1 are unaffected. Both binding to the proteins in solution and adhesion to the immobilized proteins are increased by growth in the presence of hemoglobin. A larger increase was observed in binding to the soluble proteins than in adhesion to immobilized proteins. This was noted previously using fibronectin (17) and probably reflects a greater sensibility of soluble ligand binding assays to changes in receptor number compared with the multivalent avidity measured in adhesion assays. Growth in the presence of hemoglobin induced increased expression of several proteins on the surface of C. albicans. FN binding to C. albicans protected amino groups on a 55-kDa protein from chemical modification, identifying this surface protein as a candidate for the hemoglobin-induced ECM receptor. A 55-kDa protein was also identified as a potential FN receptor by purification using affinity chromatography on immobilized FN.
To address whether a single promiscuous receptor or a family of receptors with varying specificities are responsible for the enhancement of ECM protein binding by hemoglobin, we performed quantitative analysis of heterologous displacement experiments. By comparing the Ki values for a single protein to inhibit binding of the three labeled proteins, fibronectin, laminin, and fibrinogen, we demonstrated that fibronectin and fibrinogen bind to a common class of sites that overlap only partially with those sites recognizing laminin. Therefore, growth in the presence of hemoglobin induces expression on C. albicans of a class of promiscuous receptors that bind fibronectin, fibrinogen, laminin, and type IV collagen and specific receptors for some ECM proteins, such as laminin. Binding of both fibronectin and fibrinogen to the promiscuous site produces a linear Scatchard plot, indicating that a homogeneous class of receptors accounts for binding of each of these ligands. Because some of these ECM proteins bind to each other, we could not examine heterologous displacement between all pairs of proteins. Using fibrinogen as a tracer for binding to the promiscuous receptor, however, we can show that native type I collagen does not bind to this receptor. Although C. albicans apparently has receptors that mediate adhesion to type I collagen and thrombospondin-1, these receptors are probably distinct from the promiscuous receptor because their binding and ability to promote adhesion are not affected by growth with hemoglobin.
Studies in other organisms provide precedent for both specific and
shared promiscuous receptors for ECM proteins. Promiscuous integrins
have been identified in mammalian cells. The best defined of these are
the platelet integrin IIbIIIa (IIb/
3), which binds to fibrinogen,
thrombospondin, fibronectin, and von Willebrand factor (25), and the
leukocyte
2 integrin CR3 (CD11b/CD18, reviewed in Ref. 26).
Mammalian cells also express several families of scavenger receptors
that bind multiple ligands (27). Among these, the low density
lipoprotein receptor-related protein and CD36 have been demonstrated to
bind to several ECM proteins (28-30). Microbial interactions with
multiple ECM proteins are also frequently observed.
Staphylococcus aureus interacts with fibronectin, laminin, collagens, thrombospondin-1, and elastin (31) reviewed in (32, 33).
Some of these interactions are mediated by distinct receptors, but a
S. aureus protein that binds several ECM proteins has also been reported (34). Blood stages of the protozoan pathogen responsible for malaria, Plasmodium falciparum, recognize the host
proteins thrombospondin-1, CD36, VCAM1, E-selectin, and ICAM1 via a
family of related cell surface receptors (35). The promiscuous receptor on C. albicans resembles a mammalian cell scavenger receptor
(27) in that the proteins bound are apparently unrelated in sequence, but only a subset of proteins are recognized by the receptor.
Previous studies of extracellular matrix interactions with C. albicans have identified candidate receptors for fibronectin (5,
8), laminin (16), fibrinogen (36), and entactin (37) and an analog of
mammalian integrin subunits that may mediate adhesion to epithelial
cells (reviewed in Ref. 15). Limited evidence has been obtained for
partial competition between laminin, fibronectin, and entactin for
binding to cell wall extracts of C. albicans (37). However,
no competition was observed between fibrinogen and the complement
receptor (36). Heparin inhibited binding of C. albicans to
several ECM proteins, including fibronectin, laminin, and types I and
IV collagen. This inhibition does not reflect binding of the
glycosaminoglycans to a C. albicans binding site shared with
these proteins but probably resulted from sequestration of ligands
after binding of heparin to the proteins (38).
Up-regulated expression of potential receptors for selected ECM proteins was observed at the functional level by increased binding activity and adhesion to the immobilized proteins as well as at the protein level as increased expression of several surface proteins on C. albicans, including a 55-kDa protein. Binding of FN to the cells differentially protected the protein from chemical modification (Fig. 7), suggesting that it may directly mediate hemoglobin-induced interactions with fibronectin. The same 55-kDa protein was also identified as a major protein bound to a FN affinity column and eluted with acetic acid. Proteins of 68-72 kDa were identified previously as putative receptors for laminin, fibrinogen, and C3d (9) and are reviewed in (3). Proteins with molecular masses of 60 and 105 kDa were identified as potential FN receptors in uninduced C. albicans by affinity chromatography on immobilized FN (5, 14). Because lyticase was used to release the surface proteins, protease contamination in the lyticase could result in some degradation of the receptor. Thus, the native molecular mass of the receptor identified here may be larger than 55 kDa.
The finding that hemoglobin is a potent regulator of the binding of C. albicans to several ECM proteins may contribute to understanding the pathogenesis of systemic candidasis. Hemoglobin is an abundant circulating protein in the body, and it is often present in sites of tissue injury. C. albicans may readily encounter hemoglobin at sites of tissue injury, which may in turn change its binding to ECM proteins in the tissue. Furthermore, virulent strains of C. albicans express a hemolytic activity that could release hemoglobin from erythrocytes exposed to C. albicans (39). Recently, hyphal cells of C. albicans were reported to bind to human hemoglobin, suggesting that this organism expresses hemoglobin receptors (40). Induction of ECM binding to C. albicans by hemoglobin would facilitate colonization of this organism after it enters the vascular compartment. Therefore, defining of the mechanism by which hemoglobin regulates adherence in this pathogen could identify new targets to prevent or treat C. albicans infections.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ Present address: Dept. of Medicine, University of Rochester Medical School, Rochester, NY 14642.
To whom correspondence and reprint requests should be
addressed: Bldg. 10, Rm 2A33, 10 Center Dr. MSC 1500, NIH, Bethesda, MD
20892-1500. Tel.: 301-496-6264; Fax: 301-402-0043.
1 The abbreviations used are: ECM, extracellular matrix; BSA, bovine serum albumin; FN, fibronectin; DPBS, Dulbecco's phosphate-buffered saline; sulfo-SHPP: sulfosuccinimidyl-3-(4-hydroxyphenyl) propionate; YNB, yeast nitrogen base.
2 S. Yan, H. Krutzsch, and D. D. Roberts, unpublished results.
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REFERENCES |
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