Mapping the hyaluronan-binding site on the link module from human tumor necrosis factor-stimulated gene-6 by site-directed mutagenesis

Link modules are hyaluronan-binding domains found in extracellular proteins involved in matrix assembly, development, and immune cell migration. Previously we have expressed the Link module from the inflammation-associated protein tumor necrosis factor-stimulated gene-6 (TSG-6) and determined its tertiary structure in solution. Here we generated 21 Link module mutants, and these were analyzed by nuclear magnetic resonance spectroscopy and a hyaluronan-binding assay. The individual mutation of five amino acids, which form a cluster on one face of the Link module, caused large reductions in functional activity but did not affect the Link module fold. This ligand-binding site in TSG-6 is similar to that determined previously for the hyaluronan receptor, CD44, suggesting that the location of the interaction surfaces may also be conserved in other Link module-containing proteins. Analysis of the sequences of TSG-6 and CD44 indicates that the molecular details of their association with hyaluronan are likely to be significantly different. This comparison identifies key sequence positions that may be important in mediating hyaluronan binding, across the Link module superfamily. The use of multiple sequence alignment and molecular modeling allowed the prediction of functional residues in link protein, and this approach can be extended to all members of the superfamily.     

Specificity of the tumor necrosis factor-induced protein 6-mediated heavy chain transfer from inter-alpha-trypsin inhibitor to hyaluronan: implications for the assembly of the cumulus extracellular matrix

The formation of the hyaluronan-rich cumulus extracellular matrix is crucial for female fertility and accompanied by a transesterification reaction in which the heavy chains (HCs) of inter-alpha-trypsin inhibitor (IalphaI)-related proteins are covalently transferred to hyaluronan. Tumor necrosis factor-induced protein-6 (TNFIP6) is essential for this transfer reaction. Female mice deficient in TNFIP6 are infertile due to the lack of a correctly formed cumulus matrix. In this report, we characterize the specificity of TNFIP6-mediated HC transfer from IalphaI to hyaluronan. Hyaluronan oligosaccharides with eight or more monosaccharide units are potent acceptors in the HC transfer, with longer oligosaccharides being somewhat more efficient. Epimerization of the N-acetyl-glucosamine residues to N-acetyl-galactosamines (i.e. in chondroitin) still allows the HC transfer although at a significantly lower efficiency. Sulfation of the N-acetyl-galactosamines in dermatan-4-sulfate or chondroitin-6-sulfate prevents the HC transfer. Hyaluronan oligosaccharides disperse cumulus cells from expanding cumulus cell-oocyte complexes with the same size specificity as their HC acceptor specificity. This process is accompanied by the loss of hyaluronan-linked HCs from the cumulus matrix and the appearance of oligosaccharide-linked HCs in the culture medium. Chondroitin interferes with the expansion of cumulus cell-oocyte complexes only when added with exogenous TNFIP6 before endogenous hyaluronan synthesis starts, supporting that chondroitin is a weaker HC acceptor than hyaluronan. Our data indicate that TNFIP6-mediated HC transfer to hyaluronan is a prerequisite for the correct cumulus matrix assembly and hyaluronan oligosaccharides and chondroitin interfere with this assembly by capturing the HCs of the IalphaI-related proteins.     

Characterization of the interaction between chymotrypsin and heparin.

Heparin forms a complex with chymotrypsin which is active towards glutaryl-L-phenylalanine-p-nitroanilide (GPANA) and glutaryl-L-phenylalanine-beta-naphthylamide (GPNA) at pH 7.6. The activity of chymotrypsin towards GPANA at pH 7.6 is enhanced in the presence of heparin. Heparin does not bind at the active site of the enzyme since proflavin is not displaced from the active site of chymotrypsin upon complex formation. The heparin-chymotrypsin complex migrates under basic polyacrylamide disc gel electrophoresis conditions to a position intermediate between heparin and free chymotrypsin. The complex is dissociable under acidic polyacrylamide gel electrophoresis conditions. It is estimated that one to three molecules of heparin can bind to each chymotrypsin molecule on the basis of electrophoretic and enzymic activity data.     

Activation of the coagulation mechanism on tumor necrosis factor-stimulated cultured endothelial cells and their extracellular matrix. The role of flow and factor IX/IXa

Infusion of tumor necrosis factor (TNF) into tumor-bearing mice led to intravascular clot formation with fibrin deposition in microvessels in the tumor bed in close association with the vessel wall, which could be prevented by active site-blocked factor IXa (IXai). This observation prompted us to examine the role of the intrinsic system in activation of the coagulation mechanism on TNF-stimulated human endothelial cell monolayers and endothelial-derived matrix during exposure to purified coagulation factors or flowing blood. Treatment of endothelial cells in intact monolayers with TNF induced expression of the procoagulant cofactor tissue factor (TF) in a dose-dependent manner, and after removal of the cells, TF was present in the matrix. TNF-treated endothelial cell monolayers exposed to blood anticoagulated with low molecular weight heparin induced activation of coagulation. Addition of IXai blocked the procoagulant response on TNF-treated endothelial cells, and consistent with this, the presence of factor IX/VIIIa enhanced endothelial TF/factor VII(a) factor X activation over a wide range of cytokine concentrations (0-600 pM). When TF-dependent factor X activation on endothelial cells was compared with preparations of subendothelium, the extracellular matrix was 10-20 times more effective. IXai blocked TF/factor VII(a) mediated activated coagulation on matrix, but only at lower concentration of TNF (less than 50 pM). Similarly, enhancement of factor Xa formation on matrix by factors IX/VIIIa was most evident at lower TNF concentrations. When anticoagulated whole blood flowing with a shear of 300 s-1 was exposed to matrices from TNF-treated endothelial cells, but not matrices from control cells, fibrinopeptide A (FPA) generation, fibrin deposition, and platelet aggregate formation were observed. FPA generation could be prevented by a blocking antibody to TF and by active site-blocked factor Xa (Xai) over a wide range of TNF concentrations (0-600 pM), whereas IXai only blocked FPA generation at lower TNF concentrations (less than 50 pM). Activation of coagulation on matrix from TNF-stimulated endothelial cells was dependent on the presence of platelets, indicating the important role of platelets in propagating the reactions leading to fibrin formation. These observations demonstrate the potential of cytokine-stimulated endothelium and their matrix to activate coagulation and suggest the importance of the intrinsic system in factor Xa formation on cellular surfaces.     

Mechanochemical switching between growth and differentiation during fibroblast growth factor-stimulated angiogenesis in vitro: role of extracellular matrix

The angiogenic factor, basic fibroblast growth factor (FGF), either stimulates endothelial cell growth or promotes capillary differentiation depending upon the microenvironment in which it acts. Analysis of various in vitro models of spontaneous angiogenesis, in combination with time-lapse cinematography, demonstrated that capillary tube formation was greatly facilitated by promoting multicellular retraction and cell elevation above the surface of the rigid culture dish or by culturing endothelial cells on malleable extracellular matrix (ECM) substrata. These observations suggested to us that mechanical (i.e., tension-dependent) interactions between endothelial cells and ECM may serve to regulate capillary development. To test this hypothesis, FGF-stimulated endothelial cells were grown in chemically defined medium on bacteriological (nonadhesive) dishes that were precoated with different densities of fibronectin. Extensive cell spreading and growth were promoted by fibronectin coating densities that were highly adhesive (greater than 500 ng/cm2), whereas cell rounding, detachment, and loss of viability were observed on dishes coated with low fibronectin concentrations (less than 100 ng/cm2). Intermediate fibronectin coating densities (100-500 ng/cm2) promoted cell extension, but they could not completely resist cell tractional forces. Partial retraction of multicellular aggregates resulted in cell shortening, cessation of growth, and formation of branching tubular networks within 24-48 h. Multicellular retraction and subsequent tube formation also could be elicited on highly adhesive dishes by overcoming the mechanical resistance of the substratum using higher cell plating numbers. Dishes coated with varying concentrations of type IV collagen or gelatin produced similar results. These results suggest that ECM components may act locally to regulate the growth and pattern-regulating actions of soluble FGF based upon their ability to resist cell-generated mechanical loads. Thus, we propose that FGF-stimulated endothelial cells may be "switched" between growth, differentiation, and involution modes during angiogenesis by altering the adhesivity or mechanical integrity of their ECM.     

Characterization of a recombinant extracellular domain of the type 1 tumor necrosis factor receptor: evidence for tumor necrosis factor-alpha induced receptor aggregation.

An expression plasmid encoding the extracellular portion of the human tumor necrosis factor (TNF) type 1 receptor (TNF-R1) was constructed and used to generate a stable cell line secreting soluble TNF-R1 (sTNF-R1). The sTNF-R1 was purified, and its biochemical properties and its interactions with human TNF-alpha were examined. SDS-PAGE resolved the purified sTNF-R1 into three bands of approximate Mr 24,200, 28,200, and 32,800. Sedimentation equilibrium analysis gave a molecular weight of 25,000 for sTNF-R1 whereas the molecular weight obtained by gel filtration chromatography was approximately 55,000-60,000. Scatchard analysis of [125I]TNF-alpha binding to sTNF-R1 revealed high-affinity binding (Kd = 93 pM), comparable to that observed for the intact receptor on whole cells. Competitive binding experiments showed that sTNF-R1 has a 50-60-fold higher affinity for TNF-alpha than for TNF-beta, in contrast to the equal affinities of TNF-alpha and TNF-beta for the full-length TNF-R1 transiently expressed in mammalian cells. The sTNF-R1 was found to block the cytotoxicity of TNF-alpha and TNF-beta on a murine L-M cell assay. The sizes of the sTNF-R1.TNF-alpha complex determined by gel filtration chromatography and sedimentation equilibrium were approximately 141 and 115 kDa, respectively. The stoichiometry of the complex was examined by Scatchard analysis, size-exclusion chromatography, HPLC separation, amino acid composition, sequence analysis, and sedimentation equilibrium. The data from these studies suggest that at least two molecules of sTNF-R1 can bind to a single TNF-alpha trimer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of the interaction between Robo1 and heparin and other glycosaminoglycans

Roundabout 1 (Robo1) is the cognate receptor for secreted axon guidance molecule, Slits, which function to direct cellular migration during neuronal development and angiogenesis. The Slit2–Robo1 signaling is modulated by heparan sulfate, a sulfated linear polysaccharide that is abundantly expressed on the cell surface and in the extracellular matrix. Biochemical studies have further shown that heparan sulfate binds to both Slit2 and Robo1 facilitating the ligand–receptor interaction. The structural requirements for heparan sulfate interaction with Robo1 remain unknown. In this report, surface plasmon resonance (SPR) spectroscopy was used to examine the interaction between Robo1 and heparin and other GAGs and determined that heparin binds to Robo1 with an affinity of ∼650 nM. SPR solution competition studies with chemically modified heparins further determined that although all sulfo groups on heparin are important for the Robo1–heparin interaction, the N-sulfo and 6-O-sulfo groups are essential for the Robo1–heparin binding. Examination of differently sized heparin oligosaccharides and different GAGs also demonstrated that Robo1 prefers to bind full-length heparin chains and that GAGs with higher sulfation levels show increased Robo1 binding affinities.     

Characterization of the interaction between murine tumour necrosis factor and monoclonal antibodies

We characterized the interaction between murine TNF (mTNF) and the neutralizing monoclonal antibodies TN3 and 1F3F3. The epitopes were localized by comparing the detection efficiency for a panel of TNF chimaeric proteins and site-specific muteins in ELISA. Mutation of mTNF amino acid Q131 inhibited the interaction with 1F3F3, whereas mutation of D71/Y72 inhibited the binding to TN3. As D71/Y72 are located in an exposed loop near the TNF intersubunit groove, binding of TN3 promoted the dissociation and/or interfered with the reassociation of subunits into mTNF trimers. 1F3F3, on the other hand, prevented the spontaneous dissociation of bound (hetero)trimeric mTNF.     

Remodeling and homeostasis of the extracellular matrix: implications for fibrotic diseases and cancer

Dynamic remodeling of the extracellular matrix (ECM) is essential for development, wound healing and normal organ homeostasis. Life-threatening pathological conditions arise when ECM remodeling becomes excessive or uncontrolled. In this Perspective, we focus on how ECM remodeling contributes to fibrotic diseases and cancer, which both present challenging obstacles with respect to clinical treatment, to illustrate the importance and complexity of cell-ECM interactions in the pathogenesis of these conditions. Fibrotic diseases, which include pulmonary fibrosis, systemic sclerosis, liver cirrhosis and cardiovascular disease, account for over 45% of deaths in the developed world. ECM remodeling is also crucial for tumor malignancy and metastatic progression, which ultimately cause over 90% of deaths from cancer. Here, we discuss current methodologies and models for understanding and quantifying the impact of environmental cues provided by the ECM on disease progression, and how improving our understanding of ECM remodeling in these pathological conditions is crucial for uncovering novel therapeutic targets and treatment strategies. This can only be achieved through the use of appropriate in vitro and in vivo models to mimic disease, and with technologies that enable accurate monitoring, imaging and quantification of the ECM.     

Relaxin and the extracellular matrix: molecular mechanisms of action and implications for cardiovascular disease

Myocardial fibrosis is a common endpoint in a variety of cardiac pathologies. It results from excessive accumulation of collagen and other materials that together comprise the extracellular matrix (ECM). In the past decade, the peptide hormone relaxin has emerged as an important regulator of the ECM within several organs, including the heart, and has been suggested as a novel therapeutic agent for the treatment of fibrotic disorders. This review summarises research on the anti-fibrotic actions of relaxin, outlines the potential mechanisms by which relaxin regulates the ECM in cardiovascular tissues and examines the implications of this research for the management of heart disease. Some of the contradictions in the literature are also addressed in order to clarify the role of relaxin as an anti-fibrotic factor in vivo.     

The role of the cytoskeleton in cellular adhesion molecule expression in tumor necrosis factor-stimulated endothelial cells

Leukocyte infiltration is a hallmark of the atherosclerotic lesion. These cells are captured by cellular adhesion molecules (CAMs), including vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), platelet-endothelial cell adhesion molecule (PECAM), and E-selectin, on endothelial cells (EC). We examined the role of the actin cytoskeleton in tumor necrosis factor-alpha (TNF-alpha)-induced translocation of CAMs to the cell surface. Human aortic EC were grown on 96-well plates and an ELISA was used to assess surface expression of the CAMs. TNF-alpha increased VCAM-1, ICAM-1, and E-selectin by 4 h but had no affect on the expression of PECAM. A functioning actin cytoskeleton was important for VCAM-1 and ICAM-1 expression as both cytochalasin D, an actin filament disruptor, and jasplakinolide, an actin filament stabilizer, attenuated the expression of these CAMs. These compounds were ineffective in altering E-selectin surface expression. Myosin light chains are phosphorylated in response to TNF-alpha and this appears to be regulated by Rho kinase instead of myosin light chain kinase. However, the Rho kinase inhibitor, Y27632, had no affect on TNF-alpha-induced CAM expression. ML-7, a myosin light chain kinase inhibitor, had a modest inhibitory effect on the translocation of VCAM-1 but not on ICAM-1 or E-selectin. These data suggest that the surface expression of VCAM-1 and ICAM-1 is dependent on cycling of the actin cytoskeleton. Nevertheless, modulation of actin filaments via myosin light chain phosphorylation is not necessary. The regulation of E-selectin surface expression differs from that of the other CAMs.     

Characterization and distribution of extracellular matrix components and receptors in GH3B6 prolactin cells

GH3B6 cells, a rat pituitary tumor cell line, synthesize and secrete large amounts of prolactin (PRL) in vitro. In the present work, we evaluated the capacity of these cells to express extracellular matrix (ECM) components and receptors in vitro. The expression of laminin (LN), fibronectin (FN) and type IV collagen (CIV) was investigated by immunofluorescence assays. In comparison to PRL distribution, where around 50-70% of the cells contained PRL concentrated in the Golgi region, a variable immunolabeling for the three ECM components could be observed in the majority of GH3B6 cells. Importantly, this pattern was not modified when cells were cultured in the presence of 30 nM thyroliberin (TRH). The expression of the ECM receptors: alpha5beta1 (FN receptor), alpha6beta1 (LN receptor) and CD44 (hyaluronic acid receptor) could be demonstrated by cytofluorometric analysis. Using biochemical procedures, we analyzed the synthesis and secretion of glycosaminoglycans (GAGs). The cells synthesized and secreted mainly heparan sulfate (75%) with a minor amount of chondroitin sulfate/dermatan sulfate. In an attempt to evaluate the individual contribution of the ECM components to influence cell morphology and PRL distribution in vitro, GH3B6 cells were cultivated separately on LN, FN and CIV substrates. Under all conditions, it was possible to observe an increase of cell adherence to the substrate, accompanied with changes of cellular morphology, characterized by the appearance of cytoplasmatic processes. However, no changes on PRL distribution could be observed. Our results suggest that endocrine tumor cell lines are involved in synthesis of ECM components and receptors.     

Features of the interaction between alpha2-antiplasmin and plasminogen/plasmin

The kinetic of plasmin, Va1442-plasmin, Lys530-plasmin inhibition reaction by alpha 2-antiplasmin as well as interaction of the inhibitor with different derivatives of the plasminogen and its fragments were studied. It was shown that plasmin, mini- and micro-plasmin activity decreased by 97, 88 and 85%, respectively, for equimolar ratio 1:1 of the inhibitor. The value of the inhibition reached its maximum in 1-2, 5-10 and 10-15 min, respectively. The constants of the complex formation rate were 1.4 x 10(6); 1.7 x 10(5) and 6.2 x 10(4) M-1s-1 for the plasmin, mini- and micro-plasmin with alpha 2-antiplasmin, respectively. Both 10(-2) M 6-aminohexanoic acid and 10(-1) M arginine reduced the complex formation rate between plasmin, mini-plasmin and alpha 2-antiplasmin to the value of the rate reaction between micro-plasmin and inhibitor. alpha 2-Antiplasmin bound with all investigated derivatives and fragments of plasminogen. The amount of inhibitor decreased in the series: plasmin, kringle 1-3, kringle 4, mini-plasminogen, micro-plasminogen. The kringle 1-4 and kringle 5 were determined to control the rate of reaction between enzyme and inhibitor, being not necessary for the inhibition. The comparison of the inhibitor interaction with DPP-plasmin, mini-plasminogen and micro-plasminogen displayed the possibility of the additional region existence in catalytic domain. This region participated in the complex with alpha 2-antiplasmin formation. It is supposed that the multisite interaction between plasmin and alpha 2-antiplasmin provides for the specificity and efficiency the inhibitor action.     

Interaction between the extracellular matrix and lymphatics: Consequences for lymphangiogenesis and lymphatic function

The lymphatic system is important for body fluid balance as well as immunological surveillance. Due to the identification of new molecular markers during the last decade, there has been a recent dramatic increase in our knowledge on the molecular mechanisms involved in lymphatic vessel growth (lymphangiogenesis) and lymphatic function. Here we review data showing that although it is often overlooked, the extracellular matrix plays an important role in the generation of new lymphatic vessels as a response to physiological and pathological stimuli. Extracellular matrix-lymphatic interactions as well as biophysical characteristics of the stroma have consequences for tumor formation, growth and metastasis. During the recent years, anti-lymphangiogenesis has emerged as an additional therapeutic modality to the clinically applied anti-angiogenesis strategy. Oppositely, enhancement of lymphangiogenesis in situations of lymph accumulation is seen as a promising strategy to a set of conditions where few therapeutic avenues are available. Knowledge on the interaction between the extracellular matrix and the lymphatics may enhance our understanding of the underlying mechanisms and may ultimately lead to better therapies for conditions where reduced or increased lymphatic function is the therapeutic target.     

Physiological roles of matrix metalloproteinases: implications for tumor growth and metastasis.

Physiological processes involving remodelling of the extracellular matrix, such as wound healing, embryogenesis, angiogenesis, and the female reproductive cycle, require the activity of matrix metalloproteinases (MMPs). This group of proteases degrades basal membranes and connective tissues and plays an essential role in the homeostasis of the extracellular matrix. An imbalance in the expression or activity of MMPs can have important consequences in diseases such as multiple sclerosis, Alzheimer's disease, or the development of cancers. Because of the pathophysiological importance of MMPs, their activity is highly controlled in order to confine them to specific areas. An activation cascade, initiated by the proteolysis of plasminogen, cleaves proMMPs, and every step is controlled by specific activators or inhibitors. MMPs destabilize the organization of the extracellular matrix and influence the development of cancer by contributing to cell migration, tumor cell proliferation, and angiogenesis. Accordingly, these proteases possess an important role in cell-matrix interactions by affecting fundamental processes such as cell differentiation and proliferation. Therefore, the characterization of MMPs involved in specific types and stages of tumors will significantly improve the diagnosis and treatment of these cancers in humans.     

Characterization of the InsP6-dependent interaction between CK2 and Nopp140

Nopp140, a highly phosphorylated nucleolar protein, negatively regulates CK2, a kinase essential for cell proliferation. We quantitatively analyzed the interaction between two subunits of CK2 and Nopp140 and characterized the mechanism by which InsP₆ inhibits the interaction. Nopp140 specifically binds to the catalytic subunit of CK2 (CK2α) with a dissociation constant of (K_d) of 4 nM, which interferes with the catalytic activity of CK2. The C-terminal region of Nopp140 is determined as CK2α-binding region by a yeast two-hybrid method as well as a direct measurement of the interaction between CK2α and deletion mutants of Nopp140. InsP₆ specifically binds to CK2α and disrupts the interaction between CK2α and Nopp140 with an IC₅₀ value of 25 μM, thereby attenuating the Nopp140-mediated repression of CK2 activity.     

Focal contacts: transmembrane links between the extracellular matrix and the cytoskeleton

The sites of tightest adhesion that form between cells and substrate surfaces in tissue culture are termed focal contacts. The external faces of focal contacts include specific receptors, belonging to the integrin family of proteins, for fibronectin and vitronectin, two common components of extracellular matrices. On the internal (cytoplasmic) side of focal contacts, several proteins, including talin and vinculin, mediate interactions with the actin filament bundles of the cytoskeleton. The changes that occur in focal contacts as a result of viral transformation are discussed.