Characterisation of the conformational and quaternary structure-dependent heparin-binding region of bovine seminal plasma protein PDC-109.
(9/6624)
PDC-109, the major heparin-binding protein of bull seminal plasma, binds to sperm choline lipids at ejaculation and modulates capacitation mediated by heparin. Affinity chromatography on heparin-Sepharose showed that polydisperse, but not monomeric, PDC-109 displayed heparin-binding capability. We sought to characterise the surface topology of the quaternary structure-dependent heparin-binding region of PDC-109 by comparing the arginine- and lysine-selective chemical modification patterns of the free and the heparin-bound protein. A combination of reversed-phase peptide mapping of endoproteinase Lys-C-digested PDC-109 derivatives and mass spectrometry was employed to identify modified and heparin-protected residues. PDC-109 contains two tandemly arranged fibronectin type II domains (a, Cys24-Cys61; b, Cys69-Cys109). The results show that six basic residues (Lys34, Arg57, Lys59, Arg64, Lys68, and Arg104) were shielded from reaction with acetic anhydride and 1,2-cyclohexanedione in heparin-bound PDC-109 oligomers. In the 1H-NMR solution structures of single fibronectin type II domains, residues topologically equivalent to PDC-109 Arg57 (Arg104) and Lys59 lay around beta-strand D on the same face of the domain. In full-length PDC-109, Arg64 and Lys68 are both located in the intervening polypeptide between domains a and b. Our data suggest possible quaternary structure arrangements of PDC-109 molecules to form a heparin-binding oligomer. (+info)
Adhesion to fibronectin stimulates proliferation of wild-type and bcr/abl-transfected murine hematopoietic cells.
(10/6624)
Cells of most tissues require adhesion to a surface to grow. However, for hematopoietic cells, both stimulation and inhibition of proliferation by adhesion to extracellular matrix components have been described. Furthermore, it has been suggested that progenitor cells from chronic myelogenous leukemia show decreased beta1 integrin-mediated adhesion to fibronectin, resulting in increased proliferation and abnormal trafficking. However, we show here that the chronic myelogenous leukemia-specific fusion protein p210bcr/abl stimulates the expression of alpha5beta1 integrins and induces adhesion to fibronectin when expressed in the myeloid cell line 32D. Moreover, proliferation of both p210bcr/abl-transfected 32D (32Dp210) cells and untransfected 32D cells is stimulated by immobilized fibronectin. Cell cycle analysis revealed that nonadherent 32D and 32Dp210 cells are arrested in late G1 or early S phase, whereas the adherent fractions continue cycling. Although both adherent and nonadherent p210bcr/abl-transfected and parental 32D cells express equal amounts of cyclin A, a protein necessary for cell cycle progression at the G1/S boundary, cyclin A complexes immunoprecipitated from 32D cells cultured on immobilized fibronectin were found to be catalytically inactive in nonadherent but not in adherent cells. In addition, as compared with untransfected 32D cells, cyclin A immunoprecipitates from 32Dp210 cells exhibited a greatly elevated kinase activity and remained partially active irrespective of the adhesion status. The lack of cyclin A/cyclin-dependent kinase (CDK) 2 activity in nonadherent 32D cells appeared to result from increased expression and cyclin A complex formation of the CDK inhibitor p27(Kip1). Taken together, our results indicate that adhesion stimulates cell cycle progression of hematopoietic cells by down-regulation of p27(Kip1), resulting in activation of cyclin A/CDK2 complexes and subsequent transition through the G1/S adhesion checkpoint. (+info)
Dynamics and elasticity of the fibronectin matrix in living cell culture visualized by fibronectin-green fluorescent protein.
(11/6624)
Fibronectin (FN) forms the primitive fibrillar matrix in both embryos and healing wounds. To study the matrix in living cell cultures, we have constructed a cell line that secretes FN molecules chimeric with green fluorescent protein. These FN-green fluorescent protein molecules were assembled into a typical matrix that was easily visualized by fluorescence over periods of several hours. FN fibrils remained mostly straight, and they were seen to extend and contract to accommodate movements of the cells, indicating that they are elastic. When fibrils were broken or detached from cells, they contracted to less than one-fourth of their extended length, demonstrating that they are highly stretched in the living culture. Previous work from other laboratories has suggested that cryptic sites for FN assembly may be exposed by tension on FN. Our results show directly that FN matrix fibrils are not only under tension but are also highly stretched. This stretched state of FN is an obvious candidate for exposing the cryptic assembly sites. (+info)
TGF-beta induces fibronectin synthesis through a c-Jun N-terminal kinase-dependent, Smad4-independent pathway.
(12/6624)
Transforming growth factor-beta (TGF-beta) exerts its effects on cell proliferation, differentiation and migration in part through its modulation of extracellular matrix components, such as fibronectin and plasminogen activator inhibitor-1 (PAI-1). Although the SMAD family of proteins recently has been shown to be a key participant in TGF-beta signaling, other signaling pathways have also been shown to be activated by TGF-beta. We report here that c-Jun N-terminal kinase (JNK), a member of the MAP kinase family, is activated in response to TGF-beta in the human fibrosarcoma HT1080-derived cell line BAHgpt. Stable expression of dominant-negative forms of JNK1 and MKK4, an upstream activator of JNK, results in loss of TGF-beta-stimulated fibronectin mRNA and protein induction, while having little effect on TGF-beta-induced levels of PAI-1. The human fibronectin promoter contains three CRE elements, one of which has been shown to bind a c-Jun-ATF-2 heterodimer. Utilizing a GAL4 fusion trans-reporting system, we demonstrate a decrease in transactivating potential of GAL4-c-Jun and GAL4-ATF-2 in dominant-negative JNK1- and MKK4-expressing cells. Finally, we show that TGF-beta-induced fibronectin synthesis is independent of Smad4. These results demonstrate that TGF-beta-mediated fibronectin induction requires activation of JNK which in turn modulates the activity of c-Jun and ATF-2 in a Smad4independent manner. (+info)
Identification of protein-disulfide isomerase activity in fibronectin.
(13/6624)
Assembly and degradation of fibronectin-containing extracellular matrices are dynamic processes that are up-regulated during wound healing, embryogenesis, and metastasis. Although several of the early steps leading to fibronectin deposition have been identified, the mechanisms leading to the accumulation of fibronectin in disulfide-stabilized multimers are largely unknown. Disulfide-stabilized fibronectin multimers are thought to arise through intra- or intermolecular disulfide exchange. Several proteins involved in disulfide exchange reactions contain the sequence Cys-X-X-Cys in their active sites, including thioredoxin and protein-disulfide isomerase. The twelfth type I module of fibronectin (I12) contains a Cys-X-X-Cys motif, suggesting that fibronectin may have the intrinsic ability to catalyze disulfide bond rearrangement. Using an established protein refolding assay, we demonstrate here that fibronectin has protein-disulfide isomerase activity and that this activity is localized to the carboxyl-terminal type I module I12. I12 was as active on an equal molar basis as intact fibronectin, indicating that most of the protein-disulfide isomerase activity of fibronectin is localized to I12. Moreover, the protein-disulfide isomerase activity of fibronectin appears to be partially cryptic since limited proteolysis of I10-I12 increased its isomerase activity and dramatically enhanced the rate of RNase refolding. This is the first demonstration that fibronectin contains protein-disulfide isomerase activity and suggests that cross-linking of fibronectin in the extracellular matrix may be catalyzed by a disulfide isomerase activity contained within the fibronectin molecule. (+info)
Modulation of cell proliferation and differentiation through substrate-dependent changes in fibronectin conformation.
(14/6624)
Integrin-mediated cell adhesion to extracellular matrices provides signals essential for cell cycle progression and differentiation. We demonstrate that substrate-dependent changes in the conformation of adsorbed fibronectin (Fn) modulated integrin binding and controlled switching between proliferation and differentiation. Adsorption of Fn onto bacterial polystyrene (B), tissue culture polystyrene (T), and collagen (C) resulted in differences in Fn conformation as indicated by antibody binding. Using a biochemical method to quantify bound integrins in cultured cells, we found that differences in Fn conformation altered the quantity of bound alpha5 and beta1 integrin subunits but not alphav or beta3. C2C12 myoblasts grown on these Fn-coated substrates proliferated to different levels (B > T > C). Immunostaining for muscle-specific myosin revealed minimal differentiation on B, significant levels on T, and extensive differentiation on C. Differentiation required binding to the RGD cell binding site in Fn and was blocked by antibodies specific for this site. Switching between proliferation and differentiation was controlled by the levels of alpha5beta1 integrin bound to Fn, and differentiation was inhibited by anti-alpha5, but not anti-alphav, antibodies, suggesting distinct integrin-mediated signaling pathways. Control of cell proliferation and differentiation through conformational changes in extracellular matrix proteins represents a versatile mechanism to elicit specific cellular responses for biological and biotechnological applications. (+info)
Neutrophils sense flow-generated stress and direct their migration through alphaVbeta3-integrin.
(15/6624)
During inflammation neutrophils are recruited from the blood onto the surface of microvascular endothelial cells. In this milieu the presence of soluble chemotactic gradients is disallowed by blood flow. However, directional cues are still required for neutrophils to migrate to the junctions of endothelial cells where extravasation occurs. Shear forces generated by flowing blood provide a potential alternative guide. In our flow-based adhesion assay neutrophils preferentially migrated in the direction of flow when activated after attachment to platelet monolayers. Neutralizing alphaVbeta3-integrin with monoclonal antibodies or turning the flow off randomized the direction of migration without affecting migration velocity. Purified, immobilized alphaVbeta3-integrin ligands, CD31 and fibronectin, could both support flow-directed neutrophil migration in a concentration-dependent manner. Migration could be randomized by neutralizing alphaVbeta3-integrin interactions with the substrate using antibodies or Arg-Gly-Asp-containing peptide. These results exemplify mechanical signal transduction through integrin-ligand interactions and reveal a guidance system that was hitherto unknown in neutrophils. In more general terms, it demonstrates that cells can use integrin molecules to "sample" their physical microenvironment through adhesion and use this information to modulate their behavior. (+info)
Affinity modulation of very late antigen-5 through phosphatidylinositol 3-kinase in mast cells.
(16/6624)
Adhesiveness of integrins is up-regulated rapidly by a number of molecules, including growth factors, cytokines, chemokines, and other cell surface receptors, through a mechanism termed inside-out signaling. The inside-out signaling pathways are thought to alter integrin affinity for ligand, or cell surface distribution of integrin by diffusion/clustering. However, it remains to be clarified whether any physiologically relevant agonists induce a rapid change in the affinity of beta1 integrins and how ligand-binding affinity is modulated upon stimulation. In this study, we reported that affinity of beta1 integrin very late Ag-5 (VLA-5) for fibronectin was rapidly increased in bone marrow-derived mast cells by Ag cross-linking of FcepsilonRI. Ligand-binding affinity of VLA-5 was also augmented by receptor tyrosine kinases when the phospholipase Cgamma-1/protein kinase C pathway was inhibited. Wortmannin suppressed induction of the high affinity state VLA-5 in either case. Conversely, introduction of a constitutively active p110 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) increased the binding affinity for fibronectin. Failure of a constitutively active Akt to stimulate adhesion suggested that the affinity modulation mechanisms mediated by PI 3-kinase are distinct from the mechanisms to control growth and apoptosis by PI 3-kinase. Taken together, our findings demonstrated that the increase of affinity of VLA-5 was induced by physiologically relevant stimuli and PI 3-kinase was a critical affinity modulator of VLA-5. (+info)