Modulation of activities and RNA level of the components of the plasminogen activation system during fusion of human myogenic satellite cells in vitro

Role of plasminogen activator inhibitor-1 in glucocorticoid-induced muscle change in mice

We recently revealed that plasminogen activator inhibitor-1 (PAI-1), a serine protease inhibitor, is involved in diabetes, osteoporosis and muscle wasting induced by glucocorticoid (GC) treatment in mice. In the present study, we investigated the detailed mechanisms by which GC induces muscle wasting through PAI-1 in vivo and in vitro. PAI-1 deficiency suppressed the mRNA levels of atrogin1 and muscle RING-Finger Protein 1 (MuRF1), ubiquitin ligases leading to muscle degradation, elevated by GC treatment in the gastrocnemius muscle of mice. In vitro study revealed that active PAI-1 treatment augmented the increase in atrogin1 mRNA levels enhanced by dexamethasone (Dex) in mouse myoblastic C2C12 cells. Moreover, a reduction in endogenous PAI-1 level by siRNA suppressed the mRNA levels of atrogin1 and MuRF1 enhanced by Dex in C2C12 cells. In contrast, a reduction in endogenous PAI-1 levels and active PAI-1 did not affect the phosphorylations of Akt and p70S6 kinase nor myogenic differentiation with or without Dex in C2C12 cells. In addition, PAI-1 deficiency blunted IGF-1 mRNA levels decreased by GC treatment in the gastrocnemius muscle of mice, although neither active PAI-1 nor a reduction in endogenous PAI-1 levels affected the levels of IGF-1 mRNA in C2C12 cells in the presence of Dex. In conclusion, our data suggest that paracrine PAI-1 is involved in GC-induced muscle wasting through the enhancement of muscle degradation in mice.

Improved autograft survival of mesenchymal stromal cells by plasminogen activator inhibitor 1 inhibition

Mesenchymal stromal cells (MSCs) display robust reparative properties through their ability to limit apoptosis, enhance angiogenesis, and direct positive tissue remodeling. However, low in vivo survival of transplanted cells limits their overall effectiveness and significantly affects their clinical usage. Consequently, identifying strategies to improve cell survival in vivo are a priority. One explanation for their low survival is that MSCs are often transplanted into ischemic tissue, such as infarcted myocardium, where there is poor blood supply and low oxygen tension. Therefore, we examined how MSCs respond to a hypoxic, nutrient-poor stress environment to identify trophic factors that could be manipulated in advance of MSC transplantation. Combining microarray and proteomic screens we identified plasminogen activator inhibitor 1 (PAI-1) as one factor consistently upregulated in our in vitro ischemia-mimicking conditions. Subsequent genetic and chemical manipulation studies define PAI-1 as a negative regulator of MSC survival in vivo. Mechanistically, MSC-derived PAI-1 does not alter MSC survival through a plasmin-dependent mechanism but rather directly impacts on the adhesiveness of MSCs to their surrounding matrices. Thus we can conclude that post-transplantation, PAI-1 negatively impacts MSC survival by promoting anoikis via matrix detachment.

Genetically determined proteolytic cleavage modulates alpha7beta1 integrin function

The dystrophin-glycoprotein complex and the alpha7beta1 integrin are trans-sarcolemmal linkage systems that connect and transduce contractile forces between muscle fibers and the extracellular matrix. alpha7beta1 is the major laminin binding integrin in skeletal muscle. Different functional variants of this integrin are generated by alternative splicing and post-translational modifications such as glycosylation and ADP-ribosylation. Here we report a species-specific difference in alpha7 chains that results from an intra-peptide proteolytic cleavage, by a serine protease, at the 603RRQ605 site. Site-directed mutagenesis of RRQ to GRQ prevents this cleavage. This RRQ sequence in the alpha7 integrin chain is highly conserved among vertebrates but it is absent in mice. Protein structure modeling indicates this cleavage site is located in an open region between the beta-propeller and thigh domains of the alpha7 chain. Compared with the non-cleavable alpha7 chain, the cleaved form enhances cell adhesion and spreading on laminin. Cleavage of the alpha7 chain is elevated upon myogenic differentiation, and this cleavage may be mediated by urokinase-type plasminogen activator. These results suggest proteolytic cleavage is a novel mechanism that regulates alpha7 integrin functions in skeletal muscle, and that the generation of such cleavage sites is another evolutionary mechanism for expanding and modifying protein functions.

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

Interleukin-4 improves the migration of human myogenic precursor cells in vitro and in vivo

Different molecules are available to recruit new neighboring myogenic cells to the site of regeneration. Formerly called B cell stimulatory factor-1, IL-4 can now be included in the list of motogenic factors. The present report demonstrates that human IL-4 is not required for fusion between mononucleated myoblasts but is required for myotube maturation. In identifying IL-4 as a pro-migratory agent for myogenic cells, these results provide a mechanism which partly explains IL-4 demonstrated activity during differentiation. Among the different mechanisms by which IL-4 might enhance myoblast migration processes, our results indicate that there are implications of some integrins and of three major components of the fibrinolytic system. Indeed, increases in the amount of active urokinase plasminogen activator and its receptor were observed following an IL-4 treatment, while the plasminogen activator inhibitor-1 decreased. Finally, IL-4 did not modify the amount of cell surface alpha5 integrin but increased the presence of beta3 and beta1 integrins. This integrin modulation might favor myogenic cell migration and its interaction with newly formed myotubes. Therefore, IL-4 co-injection with transplanted myoblasts might be an approach to enhance the migration of transplanted cells for the treatment of a damaged myocardium or of a Duchenne Muscular Dystrophy patient.

Age-related changes in the mitotic and metabolic characteristics of muscle-derived cells

Age-related sarcopenia could partly result from cumulative repeated episodes of incomplete repair and regeneration. We hypothesized that mitotic and metabolic events associated with satellite cell activation and proliferation could be altered with aging. Muscle-derived cells (mdc) were isolated from gastrocnemius and quadriceps muscles of young (3 wk old), adult (9 mo old), and old (24 mo old) Sprague-Dawley male rats (n = 10/group). The mdc from young growing rats started to proliferate earlier compared with adult and old animals. Cell cycle duration was significantly reduced with aging from 36.5 +/- 3.2 to 28.0 +/- 2.2 h. However, the proportion of noncycling (G0 phase) and cycling (G1 + S + G2 + M phases) cultured mdc was statistically unchanged among the three age groups. Significantly lower increase in c-met and proliferating cell nuclear antigen expression were observed in cultured mdc of old rats upon serum stimulation. Major changes in the expression of citrate synthase, lactate dehydrogenase, proteasome, caspase 3, plasminogen activators (PAs), and matrix metalloproteinase 2-9 (MMP2-9) were observed upon serum stimulation, but no age-related difference was noted. However, when measured on crushed muscle extracts, PAs and MMP2-9 enzyme activities were significantly decreased with aging. Our results show that cellular and biochemical events associated with the control of mdc activation and proliferation occur with aging. These alterations may participate in the accumulation of repeated episodes of incomplete repair and regeneration throughout the life span, thus contributing to the loss of skeletal muscle mass and function with aging.

Satellite cells attract monocytes and use macrophages as a support to escape apoptosis and enhance muscle growth

Once escaped from the quiescence niche, precursor cells interact with stromal components that support their survival, proliferation, and differentiation. We examined interplays between human myogenic precursor cells (mpc) and monocyte/macrophages (MP), the main stromal cell type observed at site of muscle regeneration. mpc selectively and specifically attracted monocytes in vitro after their release from quiescence, chemotaxis declining with differentiation. A DNA macroarray–based strategy identified five chemotactic factors accounting for 77% of chemotaxis: MP-derived chemokine, monocyte chemoattractant protein-1, fractalkine, VEGF, and the urokinase system. MP showed lower constitutive chemotactic activity than mpc, but attracted monocytes much strongly than mpc upon cross-stimulation, suggesting mpc-induced and predominantly MP-supported amplification of monocyte recruitment. Determination of [³H]thymidine incorporation, oligosomal DNA levels and annexin-V binding showed that MP stimulate mpc proliferation by soluble factors, and rescue mpc from apoptosis by direct contacts. We conclude that once activated, mpc, which are located close by capillaries, initiate monocyte recruitment and interplay with MP to amplify chemotaxis and enhance muscle growth.

Mitotic activity of rat muscle satellite cells in response to serum stimulation: relation with cellular metabolism

Cellular and molecular adaptations of satellite cells isolated from rat hindlimb muscles (n = 10) were investigated in response to serum stimulation. Flow cytometry analysis of the amounts of DNA and RNA indicated that 97.7 +/- 0.7% of satellite cells were in G0 at the end of the isolation procedure, whereas 93.2 +/- 2.0% of cells were cycling after serum exposure. The length of cell division was 34.0 +/- 2.8 h. Myoblast proliferation was asynchronous, suggesting the existence of heterogeneous cell populations in skeletal muscle. Myoblast proliferation was also accompanied by a significant increase in c-met expression, and major adaptations of energetic and proteolytic metabolisms, including an increase in the relative contribution of glycolytic metabolism for energy production, an increase in proteasome and matrix metalloproteinases 2 and 9 activities, and a decrease in plasminogen activator activities. Our data suggest that, along with molecular adaptations leading to cell cycle activation itself, adaptations in energetic and proteolytic metabolisms are crucial events involved in satellite cell activation and myoblast proliferation.

Plasmin activity is required for myogenesis in vitro and skeletal muscle regeneration in vivo

Plasmin, the primary fibrinolytic enzyme, has a broad substrate spectrum and is implicated in biologic processes dependent upon proteolytic activity, such as tissue remodeling and cell migration. Active plasmin is generated from proteolytic cleavage of the zymogen plasminogen (Plg) by urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA). Here, we have investigated the role of plasmin in C2C12 myoblast fusion and differentiation in vitro, as well as in skeletal muscle regeneration in vivo, in wild-type and Plg-deficient mice. Wild-type mice completely repaired experimentally damaged skeletal muscle. In contrast, Plg(-/-) mice presented a severe regeneration defect with decreased recruitment of blood-derived monocytes and lymphocytes to the site of injury and persistent myotube degeneration. In addition, Plg-deficient mice accumulated fibrin in the degenerating muscle fibers; however, fibrinogen depletion of Plg-deficient mice resulted in a correction of the muscular regeneration defect. Because we found that uPA, but not tPA, was induced in skeletal muscle regeneration, and persistent fibrin deposition was also reproducible in uPA-deficient mice following injury, we propose that fibrinolysis by uPA-dependent plasmin activity plays a fundamental role in skeletal muscle regeneration. In summary, we identify plasmin as a critical component of the mammalian skeletal muscle regeneration process, possibly by preventing intramuscular fibrin accumulation and by contributing to the adequate inflammatory response after injury. Finally, we found that inhibition of plasmin activity with alpha2-antiplasmin resulted in decreased myoblast fusion and differentiation in vitro. Altogether, these studies demonstrate the requirement of plasmin during myogenesis in vitro and muscle regeneration in vivo.

Urokinase-dependent plasminogen activation is required for efficient skeletal muscle regeneration in vivo

Plasminogen activators urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA) are extracellular proteases involved in various tissue remodeling processes. A requirement for uPA activity in skeletal myogenesis was recently demonstrated in vitro. The role of plasminogen activators in skeletal muscle regeneration in vivo in wild-type, uPA-deficient, and tPA-deficient mice is investigated here. Wild-type and tPA-/- mice completely repaired experimentally damaged skeletal muscle. In contrast, uPA-/- mice had a severe regeneration defect, with decreased recruitment of blood-derived monocytes to the site of injury and with persistent myotube degeneration. In addition, uPA-deficient mice accumulated fibrin in the degenerating muscle fibers; however, the defibrinogenation of uPA-deficient mice resulted in a correction of the muscle regeneration defect. A similar severe regeneration deficit with persistent fibrin deposition was also reproducible in plasminogen-deficient mice after injury, suggesting that fibrinolysis by uPA-mediated plasminogen activation plays a fundamental role in skeletal muscle regeneration. In conclusion, the uPA-plasmin system is identified as a critical component of the mammalian skeletal muscle regeneration process, possibly because it prevents intramuscular fibrin accumulation and contributes to the adequate inflammatory response after injury. These studies demonstrate the requirement of an extracellular proteolytic cascade during muscle regeneration in vivo.

Thrombin receptor induction by injury-related factors in human skeletal muscle cells

Thrombin is involved in tissue repair through its proteolytic activation of a specific thrombin receptor (PAR-1). Previous studies have shown that serine proteases and their inhibitors are involved in neuromuscular junction plasticity. We hypothesized that thrombin could also be involved during skeletal muscle inflammation. Thus we investigated the expression of PAR-1 in human myoblasts and myotubes in vitro and its regulation by injury-related factors. The functionality of this receptor was tested by measuring thrombin's ability to elicit Ca2+ signals. Western blot analysis and immunocytochemistry demonstrated the presence of PAR-1 in myoblasts but not in myotubes unless they were treated by tumor necrosis factor-alpha (10 ng/ml), interleukin-1beta (5 ng/ml), or transforming growth factor-beta(1) (10 ng/ml). The addition of 10 nM alpha-thrombin evoked a strong Ca2+ signal in myoblasts while a limited response in myotubes was observed. However, in the additional presence of injury-related factors, the amplitude of the Ca2+ response was significantly enhanced, representing 88, 65, 48% of their respective basal level, compared to 27% of that obtained in controls. Moreover, immunochemical studies on human skeletal muscle biopsies of patients suffering from inflammatory myopathies showed an overexpression of PAR-1. These results suggest that PAR-1 synthesis may be induced in response to muscle injury, thereby implicating thrombin signaling in certain muscle inflammatory diseases.

Cell invasion is affected by differential expression of the urokinase plasminogen activator/urokinase plasminogen activator receptor system in muscle satellite cells from normal and dystrophic patients

The aim of this study was to evaluate the differential expression and the function in cell movement and proliferation of the urokinase plasminogen activator (u-PA) system in muscle satellite cells (MSC) of normal individuals and patients with Duchenne muscular dystrophy (DMD). By immunoenzymatic, zymographic, and radioligand binding methods and by quantitative polymerase chain reaction of the specific mRNA we have shown that both normal and DMD MSC produce u-PA and the plasminogen activator inhibitor-1 and express u-PA receptors (u-PAR). During the proliferation phase of their growth-differentiation program, MSC from DMD patients show more u-PAR than their normal counterpart, produce more plasminogen activator inhibitor-1, and release low amounts of u-PA into the culture medium. By Boyden chamber Matrigel invasion assays we have shown that normal MSC are more prone than DMD cells to spontaneous invasion but, when subjected to a chemotactic gradient of u-PA, DMD MSC sense the ligand much better and to a greater extent than normal MSC. u-PA also stimulates proliferation of MSC, but no difference is observable between normal and DMD patients. Antagonization of u-PA/u-PAR interaction with specific anti-u-PA and anti-u-PAR monoclonal antibodies and with antisense oligonucleotides inhibiting u-PAR expression indicates that u-PA/u-PAR interaction is required in spontaneous and u-PA-induced invasion, as well as in u-PA-induced proliferation.

Involvement of the [uPAR:uPA:PAI-1:LRP] complex in human myogenic cell motility

The urokinase-type plasminogen activator system is a proteolytic system involved in tissue remodeling and cell migration. At the cell surface, receptor (uPAR)-bound urokinase (uPA) binds its inhibitor PAI-1, localized in the matrix, and the complex is internalized by endocytic receptors, such as the low-density lipoprotein receptor-related protein (LRP). We previously proposed a nonproteolytic role for the uPA system in human myogenic cell differentiation in vitro, i.e., cell fusion, and showed that myogenic cells can use PAI-1 as an adhesion matrix molecule. The aim of this study was to define the role of the uPA system in myogenic cell migration that is necessary for fusion. Using a two-dimensional motility assay and microcinematography, we showed that any interference with the [uPAR:uPA:PAI-1] complex formation, and interference with LRP binding to this complex, markedly decreased myogenic cell motility. This phenomenon was reversible and independent of plasmin activity. Inhibition of cell motility was associated with suppression of both filopodia and membrane ruffling activity. [uPAR:uPA:PAI-1:LRP] complex formation involves high-affinity molecular interactions and results in quick internalization of the complex. It is likely that this complex supports the membrane ruffling activity involved in the guidance of the migrating cell toward appropriate sites for attachment.

TGF-beta autocrine loop regulates cell growth and myogenic differentiation in human rhabdomyosarcoma cells

Transforming growth factor beta (TGF) is a well-known inhibitor of myogenic differentiation as well as an autocrine product of rhabdomyosarcoma cells. We studied the role of the TGF-beta autocrine loop in regulating growth and myogenic differentiation in the human rhabdomyosarcoma cell line, RD. We previously reported that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induces growth arrest and myogenic differentiation in these cells, which constitutively express muscle regulatory factors. We show that TPA inhibits the activation of secreted latent TGF-beta, thus decreasing the concentration of active TGF-beta to which the cells are exposed. This event is mediated by the TPA-induced alteration of the uPA/PAI serine-protease system. Complete removal of TGF-beta, mediated by the ectopic expression of a soluble type II TGF-beta receptor dominant negative cDNA, induces growth arrest, but does not trigger differentiation. In contrast, a reduction in the TGF-beta concentration, to a range of 0.14-0.20 x 10(-2) ng/ml (which is similar to that measured in TPA-treated cells), mimics TPA-induced differentiation. Taken together, these data demonstrate that cell growth and suppression of differentiation in rhabdomyosarcoma cells require overproduction of active TGF-beta; furthermore, they show that a 'critical' concentration of TGF-beta is necessary for myogenic differentiation to occur, whereas myogenesis is abolished below and above this concentration. By impairing the TGF-beta autocrine loop, TPA stabilizes the factor concentration within the range compatible for differentiation to occur. In contrast, in human primary muscle cells a much higher concentration of exogenous TGF-beta is required for the differentiation inhibitory effect and TPA inhibits differentiation in these cells probably through a TGF-beta independent mechanism. These data thus clarify the mechanism underlying the multiple roles of TGF-beta in the regulation of both the transformed and differentiated phenotype.

Exposure to tissue culture conditions can adversely affect myoblast behavior in vivo in whole muscle grafts: implications for myoblast transfer therapy

The effects of tissue culture conditions on the viability of myoblasts in whole muscles transplanted in vivo were investigated. Whole male (SJL/J) donor muscles were exposed to various tissue culture reagents and proteolytic enzymes, and allografted into female (SJL/J) host mice. Desmin immunohistochemistry was used to assess the numbers of myogenic cells (as an index of myoblast viability and the extent of regeneration) in tissue sections of whole-muscle grafts sampled on days 7 and 14. DNA quantitation with a Y-chromosome-specific probe was used to determine the total Y-1 sequence DNA (as an index of myoblast survival and proliferation) in whole-muscle grafts sampled on days 1, 3, and 7. In grafts exposed to serum-free medium, there was a delay in myoblast fusion at 7 days that was recovered by 14 days, but exposure to serum (10% or 20%) had a prolonged adverse effect on myotube formation at 14 days. DNA quantitation demonstrated that either serum-free culture medium or 10% serum enhanced the number of male cells within whole-muscle grafts at 7 days. Proteolytic digestion (even for 5 min) of whole muscles prior to grafting was extremely detrimental to myoblast survival and viability at 7 and 14 days. The unexpected finding of adverse effects of tissue culture conditions on the regeneration of whole-muscle grafts in vivo appears to parallel the major problem of the rapid death of isolated cultured donor myoblasts after injection in myoblast transfer therapy. The use of whole-muscle grafts provides an alternative and sensitive model to analyze the crucial effects of various tissue culture components on the subsequent survival and proliferation of myogenic cells in vivo.

Protease nexin I expression is up-regulated in human skeletal muscle by injury-related factors

Protease nexin I is a 43-50 kDa glycoprotein capable of inhibiting a number of serine proteases. In cultured differentiated human skeletal muscle (myotubes), we previously found that protease nexin I was localized in patches at their surface where it was active and able to inhibit thrombin. To understand the role of skeletal muscle protease nexin I after injury or in inflammatory conditions where thrombin might be extravasated by blood vessels, we examined the role of inflammatory factors on protease nexin I synthesis and secretion by myotubes in culture. By enzyme-linked immunosorbent assay (ELISA) and Western blotting, we found that this serine protease inhibitor is secreted by cultured human myotubes. Protease nexin I secretion is stimulated by tumor necrosis factor-alpha, transforming growth factor-beta and interleukin-1. Complex formation experiments with labeled thrombin reveal active protease nexin I bound to the surface of the treated cells. Secreted protease nexin I-thrombin complex was enhanced in the presence of transforming growth factor-beta and tumor necrosis factor-alpha. Protease nexin I mRNA was detected by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis. Whatever the conditions, no significantly different levels were observed, indicating that the changes in cell and media protease nexin I concentration are elicited at the translational/posttranslational levels. Immunocytochemical studies on human skeletal muscle biopsies of patients suffering from inflammatory myopathies showed an overexpression of protease nexin I together with the above inflammatory factors. These findings suggest that skeletal muscle protease nexin I might play a role after injury or inflammatory pathologies.

The transcription factor EGR-1 suppresses transformation of human fibrosarcoma HT1080 cells by coordinated induction of transforming growth factor-beta1, fibronectin, and plasminogen activator inhibitor-1

Re-expression of EGR-1 in fibrosarcoma HT1080 suppresses transformation including tumorigenicity (Huang, R.-P., Liu, C., Fan, Y., Mercola, D., and Adamson, E. (1995) Cancer Res. 55, 5054-5062) owing in part to up-regulation of the transforming growth factor (TGF)-beta1 promoter by EGR-1 which suppresses growth by an autocrine mechanism (Liu, C., Adamson, E., and Mercola, D. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 11831-11836). Here we show that enhanced cell attachment contributes to the suppression via increased secretion of fibronectin (FN) and also of plasminogen activator inhibitor-1 (PAI-1). The secretion of FN and PAI-1 is strongly correlated with EGR-1 expression (RPEARSON = 0.971 and 0. 985, respectively). Addition of authentic TGF-beta1 to parental cells greatly stimulated secretion of PAI-1 but not FN, whereas addition of TGF-beta antibody or lipofection with specific antisense TGF-beta1 oligonucleotides to EGR-1-regulated cells completely inhibits the secretion of PAI-1 but not FN. However, in gel mobility shift assays pure EGR-1 or nuclear extracts of EGR-1-regulated cells specifically bind to two GC-rich elements of the human FN promoter at positions -75/-52 and -4/+18, indicating that the increased secretion of FN is likely due to direct up-regulation by EGR-1. Moreover, adhesion was greatly enhanced in EGR-1-regulated cells and was reversed by treatment with Arg-Gly-Asp (RGD) or PAI-1 antibody indicating that the secreted proteins are functional. We conclude that EGR-1 regulates the coordinated expression of gene products important for cell attachment ("oikis" factor) and normal growth control.

Biochemical characterization of the mouse muscle-specific enolase: developmental changes in electrophoretic variants and selective binding to other proteins

The glycolytic enzyme enolase (EC 4.2.1.11) is active as dimers formed from three subunits encoded by different genes. The embryonic alphaalpha isoform remains distributed in many adult cell types, whereas a transition towards betabeta and gammagamma isoforms occurs in striated muscle cells and neurons respectively. It is not understood why enolase exhibits tissue-specific isoforms with very close functional properties. We approached this problem by the purification of native betabeta-enolase from mouse hindlimb muscles and by raising specific antibodies of high titre against this protein. These reagents have been useful in revealing a heterogeneity of the beta-enolase subunit that changes with in vivo and in vitro maturation. A basic carboxypeptidase appears to be involved in generating an acidic beta-enolase variant, and may regulate plasminogen binding by this subunit. We show for the first time that pure betabeta-enolase binds with high affinity the adjacent enzymes in the glycolytic pathway (pyruvate kinase and phosphoglycerate mutase), favouring the hypothesis that these three enzymes form a functional glycolytic segment. betabeta-Enolase binds with high affinity sarcomeric troponin but not actin and tropomyosin. Some of these binding properties are shared by the alphaalpha-isoenolase, which is also expressed in striated muscle, but not by the neuron-specific gammagamma-enolase. These results support the idea that specific interactions with macromolecules will address muscle enolase isoforms at the subcellular site where ATP, produced through glycolysis, is most needed for contraction. Such a specific targeting could be modulated by post-translational modifications.

Evidence of a non-conventional role for the urokinase tripartite complex (uPAR/uPA/PAI-1) in myogenic cell fusion

Urokinase can form a tripartite complex binding urokinase receptor (uPAR) and plasminogen activator inhibitor type-1 (PAI-1), a component of the extracellular matrix (ECM). The components of the tripartite complex are modulated throughout the in vitro myogenic differentiation process. A series of experiments aimed at elucidating the role of the urokinase tripartite complex in the fusion of human myogenic cells were performed in vitro. Myogenic cell fusion was associated with increased cell-associated urokinase-type plasminogen activator (uPA) activity, cell-associated uPAR, and uPAR occupancy. Incubation of cultures with either uPA anticatalytic antibodies, or the amino-terminal fragment of uPA (ATF), which inhibits competitively uPA binding to its receptor, or anti-PAI-1 antibodies, which inhibit uPA binding to PAI-1, resulted in a 30 to 47% decrease in fusion. Incubation of cultures with the plasmin inhibitor aprotinin did not affect fusion. Decreased fusion rates induced by interfering with uPAR/uPA/PAI-1 interactions were not associated with significant changes in mRNA levels of both the myogenic regulatory factor myogenin and its inhibitor of DNA binding, Id. Incubation of cultures with purified uPA resulted in a decrease in fusion, likely due to a competitive inhibition of PAI-1 binding of endogenous uPA. We conclude that muscle cell fusion largely depends on interactions between the members of the urokinase complex (uPAR/uPA/PAI-1), but does not require proteolytic activation of plasmin. Since the intrinsic muscle cell differentiation program appears poorly affected by the state of integrity of the urokinase complex, and since cell migration is a prerequisite for muscle cell fusion in vitro, it is likely that the urokinase system is instrumental in fusion through its connection with the cell migration process. Our results suggest that the urokinase tripartite complex may be involved in cell migration in a non conventional way, playing the role of an adhesion system bridging cell membrane to ECM.

Binding of urokinase to plasminogen activator inhibitor type-1 mediates cell adhesion and spreading

Urokinase plasminogen activator and its receptor are both found at the surface of the cell membrane in many cell types. The plasminogen activator inhibitor type-1 (PAI-1) is often associated with the extracellular matrix. The spatial localization of these three molecules could account for their involvement in cell adhesion and/or migration. We have shown previously that the urokinase receptor mediates mechanical force transmission across the cell surface to the cytoskeleton. Here we investigated whether immobilized plasminogen activator inhibitor type 1 (PAI-1) could regulate cell spreading and cytoskeleton reorganization. Serum deprived human myogenic cells were plated in serum free medium onto bacteriologic dishes precoated with different extracellular matrix ligands (fibronectin, vitronectin, or type 1 collagen) or PAI-1 at increasing concentrations. The number of adherent cells and their projected area were quantitated after 3 hours of plating. PAI-1 promoted cell adhesion and spreading in a dose dependent manner. Addition of antibodies to PAI-1 inhibited the adhesion on PAI-1 coated dishes in a dose dependent way. The PAI-1 mediated cell adhesion required the presence of urokinase at the cell surface. Removal of the glycosylphosphatidylinositol (GPI)-linked proteins abolished cell adhesion on PAI-1 dish, suggesting its dependence on the presence of the urokinase receptor, a GPI-linked receptor. Furthermore, addition of antibodies against alpha v beta3 integrin completely inhibited cell adhesion on PAI-1, suggesting that alpha v beta3 might be the transmembrane molecule that physically connects the complex of PAI-1, urokinase, and urokinase receptor to the cytoskeleton. Visualization of spread cells stained for filamentous actin with confocal microscopy showed a dose-dependent increase of filopodia on PAI-1 coated dishes and cytoskeletal reorganization, suggesting a migratory profile. These data indicate that PAI-1 plays a direct role in dynamic cell adhesion particularly at the leading edge, where increased levels of urokinase plasminogen activator (uPA) and its receptor (uPAR) are localized in migrating cells. Immobilized PAI-1 could therefore serve to bridge the cell surface with the extracellular matrix via the formation of a multimolecular complex that includes alpha v beta3 integrins in myogenic cells.

Regulated localization confers multiple functions on the protease urokinase plasminogen activator

We have investigated the role of the plasminogen activation cascade in skeletal muscle differentiation. Migrating, undifferentiated myoblasts express urokinase plasminogen activator (uPA) and its cell surface receptor (uPAR). Consequently, uPA is localized predominantly to the cell surface. Preventing uPA from associating with its receptor with a noncatalytic form of uPA (NC-uPA) hinders migration of myoblasts and inhibits differentiation. When myoblasts reach confluence, cease migrating, and start to differentiate, uPAR gets downregulated, and uPA becomes redistributed from the cell surface to the extracellular space. The function of uPA at this stage was tested using the protease inhibitors aprotinin, alpha2-antiplasmin, or plasminogen activator inhibitor-1 (PAI-1). Contrary to the role of cell-associated uPA, inhibition of soluble uPA/plasmin stimulates differentiation of myoblasts. Aprotinin can inhibit activation of latent TGFbeta and stimulates differentiation, suggesting PAI-1 and alpha2-antiplasmin also may stimulate differentiation via this mechanism. These data suggest that regulation of uPA localization allows a dual function for this protease in regulating cell migration and controlling cell differentiation.

Serine proteinase inhibitors in human skeletal muscle: expression of beta-amyloid protein precursor and alpha 1-antichymotrypsin in vivo and during myogenesis in vitro

The balance of serine proteases and inhibitors in nerve and muscle is altered during programmed- and injury-induced remodeling. A serpin, alpha 1-antichymotrypsin (alpha 1-ACT), and Kunitz-inhibitor containing forms of the beta-amyloid precursor protein (beta APP) may be important components of this balance. In the present study, we analyzed their expression in primary cultures of human myogenic (satellite) cells that mimic myogenic differentiation using Western blotting and immunocytochemistry. In vitro results were compared to in vivo results from normal adult human skeletal muscle biopsies. Using an anti-alpha 1-ACT polyclonal antibody, we detected a 62 kDa immunoreactive band both in cultured human myogenic cells (mononucleated myoblasts as well as multi-nucleated myotubes) and in extracts of human muscle biopsies. With a polyclonal anti-beta APP antibody we found two bands (105 and 120 kDa) in myoblasts and myotubes in culture. However, the same antibody recognized only a single band at 92 kDa in biopsies. By immunocytochemistry, both alpha 1-ACT and beta APP were indistinctly present on localized to the surface of myoblasts in culture. In contrast, these inhibitors were dense on myotube surfaces, where they often formed distinct aggregates and frequently co-localized. In permeabilized muscle cells, alpha 1-ACT and beta APP appeared to be localized to the perikarya of both myoblasts and myotubes. Confirming previous results, both alpha 1-ACT and beta APP were present at the neuromuscular junction in human muscle sections. These developmental changes found during in vitro myogenesis for alpha 1-ACT and beta APP, both serine protease inhibitors, reinforce the hypothesis that regulation of the serine proteases and serine protease inhibitors plays an important role in neuromuscular differentiation.

The urokinase/urokinase-receptor system and cancer invasion

u-PA binds with high affinity to its specific GPI-anchored receptor on the cell surface. The binding has at least two important consequences: (1) it enhances the rate of plasminogen activation on the cell surface; and (2) it focuses the u-PA proteolytic activity at the leading front of migrating cells. Several recent findings suggest that surface-bound u-PA is essential for the invasive ability of tumour cells, even if a picture is emerging indicating a concerted action with other proteases, like collagenases and cathepsin B (Kobayashi et al, 1992; Ossowski, 1992; Schmitt et al, 1992; (Danø et al, 1994). In some tumours, e.g. colon, breast and skin cancer, in situ hybridization studies have given an insight into the u-PA/u-PAR tumour biology showing a complex interplay between stromal and cancer cells Danø et al, 1994). u-PA, u-PAR, and PAI-1 tumour content are now well established prognostic factor in breast cancer. This body of knowledge could be used for theurapeutic purposes. For example, a large study with 671 patients has allowed the identification of node-negative patients which, according to their u-PA levels, would need adjuvant therapy (Foekens et al, 1992). Many other tumours, especially colorectal cancer, expect a direct clinical evaluation of u-PA, u-PAR and serpins as prognostic factors.

Urokinase receptor mediates mechanical force transfer across the cell surface

The tripartite complex formed by the urokinase receptor, urokinase, and its inhibitor is an enzymatic system that controls plasmin formation involved in degradation of extracellular matrix proteins. With the use of magnetic twisting cytometry with urokinase-coated ferromagnetic beads, we applied mechanical stress directly to the urokinase receptor on the surface of human myogenic cells in culture. The stiffness and the stiffening response measured through the urokinase receptor resembled those of integrins, which are linked mechanically to the cytoskeleton. Furthermore, stiffness decreased with disruption of actin microfilaments. These results demonstrate that the urokinase receptor is coupled mechanically to the cytoskeleton. Inhibition of the tripartite complex formation with antibodies led to a twofold increase in cytoskeletal stiffness. A stiffened cytoskeleton might impede cytoskeletal remodeling and reorganization and thus impede cell motility. Our results demonstrate that the urokinase receptor mediates mechanical force transfer across the cell surface. As such, it is a novel pathway to regulate cytoskeletal stiffness and, thereby, possibly to modulate motility of normal and abnormal adherent cells.

Plasminogen activation by human keratinocytes: molecular pathways and cell-biological consequences

Keratinocytes are the major cellular constituent of stratified epithelia. Defects in these epithelia are re-epithelialized by keratinocytes migrating from the edge of the defect into the wound. The cells form a monolayer with subsequent differentiation into a multilayered epithelium. It is thought that plasminogen activation by migrating keratinocytes is an important event during re-epithelialization. In the present report we summarize the studies on plasminogen activation by human keratinocytes in vitro and in vivo. Under the aspect of pericellular proteolysis the discussion is focused on the molecular mechanisms of plasminogen activation at the keratinocyte surface and on the cell-biological consequences of pericellular plasmin formation. We describe a cell surface-associated pathway of plasminogen activation which crucially depends on cell surface receptors for (pro)-uPA and plasmin(ogen). uPA bound to its receptor converts cell-bound plasminogen into the active protease plasmin. Compared to plasminogen activation in solution, activation at the keratinocyte cell surface is accelerated by a factor of approx. 7-10, and the plasmin generated and bound at the cell surface is protected against its specific inhibitor alpha 2-antiplasmin. Plasmin thus provided in the pericellular space leads to detachment of cultured keratinocytes from the growth substratum. Plasmin interferes with the adhesion of keratinocytes to fibrin, but not with the adhesion to collagen type I. By demonstrating that keratinocytes of the epithelial outgrowth in healing skin wounds express uPA and the uPA-R and that plasmin(ogen) is colocalized with uPA and/or uPA-R, indirect evidence is provided that this pathway may be operative in vivo. In view of previous findings that plasminogen activation is also observed under certain pathologic conditions in the epidermis, we conclude that plasminogen activation by keratinocytes is rather related to tissue damage and subsequent repair mechanisms than to a specific pathologic situation.

Synthesis and secretion of matrix-degrading metalloproteases by human skeletal muscle satellite cells

The expression of matrix-degrading metalloproteases (MMPs) by human skeletal muscle satellite cells was investigated by zymography of cell culture media and by Northern blot analysis of mRNA prepared from satellite cells. Zymography in gelatin substrate gels revealed that satellite cells constitutively synthesize and secrete 72 kDa gelatinase (MMP-2). In addition, treatment of satellite cell cultures with phorbol ester resulted in an induction of 92 kDa gelatinase (MMP-9) activity. On casein substrate gels, little or no proteolytic activity was detectable in control or phorbol ester treated satellite cell cultures, suggesting that compared to fibroblasts, satellite cells secrete little or no interstitial collagenase (MMP-1) or stromelysin (MMP-3) activity. Northern blotting, however, revealed that there is detectable expression of mRNA transcripts encoding MMP-1 in satellite cell cultures, and that increased accumulation of MMP-1 mRNA transcripts occurs upon treatment of these cells with phorbol ester. In contrast, no constitutive, or induced expression of transcripts encoding MMP-3 was detectable in satellite cells. These findings show that satellite cells can synthesize and secrete selected members of the MMP family and suggest that skeletal muscle cells may participate directly in remodelling of the extracellular matrix during myogenesis and the regeneration of skeletal muscle.

Heparan sulfate proteoglycans as transducers of FGF-2 signalling

The fibroblast growth factor-2 (FGF-2) low-affinity binding sites, heparan sulfate proteoglycans (HSPGs), function as modulators of FGF-2 activity. It is noteworthy that HSPG binding protects FGF-2 from denaturation and proteolytic degradation, provides a matrix-bound or cell-surface reservoir of this factor for the cells and is required for the activation of FGF high-affinity receptors. In our study we investigated the biological meaning of FGF-2 internalization mediated through its low-affinity binding sites, HSPGs. Using as model system L6 myoblasts lacking endogenous FGF receptors (FGFRs), we demonstrated that these cells internalize FGF-2 efficiently through an HSPG-mediated pathway. FGF-2 internalization occurring through HSPGs was paralleled by an increase in the activity of urokinase plasminogen activator (u-PA). The u-PA-inducing activity of FGF-2 was strictly correlated to its internalization, as chlorate treatment, which causes a strong inhibition of FGF-2 internalization, abrogated the u-PA-inducing activity of FGF-2. In addition, expression of functional FGF high-affinity receptors (FGFR-1) did not enhance u-PA in L6 myoblasts upon FGF-2 stimulation. According to our results we propose that FGF-2 internalization mediated through HSPGs may transduce FGF-2 signalling such as u-PA-activity stimulation. Thus, HSPGs may act as direct transducers of FGF signalling and indeed, different FGF-signalling pathways must exist.

Skeletal muscle satellite cells

Evidence now suggests that satellite cells constitute a class of myogenic cells that differ distinctly from other embryonic myoblasts. Satellite cells arise from somites and first appear as a distinct myoblast type well before birth. Satellite cells from different muscles cannot be functionally distinguished from one another and are able to provide nuclei to all fibers without regard to phenotype. Thus, it is difficult to ascribe any significant function to establishing or stabilizing fiber type, even during regeneration. Within a muscle, satellite cells exhibit marked heterogeneity with respect to their proliferative behavior. The satellite cell population on a fiber can be partitioned into those that function as stem cells and those which are readily available for fusion. Recent studies have shown that the cells are not simply spindle shaped, but are very diverse in their morphology and have multiple branches emanating from the poles of the cells. This finding is consistent with other studies indicating that the cells have the capacity for extensive migration within, and perhaps between, muscles. Complexity of cell shape usually reflects increased cytoplasmic volume and organelles including a well developed Golgi, and is usually associated with growing postnatal muscle or muscles undergoing some form of induced adaptive change or repair. The appearance of activated satellite cells suggests some function of the cells in the adaptive process through elaboration and secretion of a product. Significant advances have been made in determining the potential secretion products that satellite cells make. The manner in which satellite cell proliferative and fusion behavior is controlled has also been studied. There seems to be little doubt that cellcell coupling is not how satellite cells and myofibers communicate. Rather satellite cell regulation is through a number of potential growth factors that arise from a number of sources. Critical to the understanding of this form of control is to determine which of the many growth factors that can alter satellite cell behavior in vitro are at work in vivo. Little work has been done to determine what controls are at work after a regeneration response has been initiated. It seems likely that, after injury, growth factors are liberated through proteolytic activity and initiate an activation process whereby cells enter into a proliferative phase. After myofibers are formed, it also seems likely that satellite cell behavior is regulated through diffusible factors arising from the fibers rather than continuous control by circulating factors.(ABSTRACT TRUNCATED AT 400 WORDS)

Urokinase and urokinase receptor: a paracrine/autocrine system regulating cell migration and invasiveness

Urokinase and its receptor are essential components of the cell migration machinery, providing an inducible, transient and localized cell surface proteolytic activity. This activity has been shown to be required in normal and pathological forms of cellular invasiveness (i.e. in several embryonic developmental processes, during inflammatory responses and cancer metastasis and spreading). It represents one of the best known of the proteolytic systems which are currently under investigation in this field. The urokinase receptor allows a continuous regulation of the proteolytic activity at cell contacts, utilizing the different localization of urokinase and its inhibitors. The receptor, in fact, in addition to focusing the enzymatic activity at focal and cell-cell contacts, also regulates it by internalizing and degrading only the inhibited form of urokinase. Internalized receptor releases the ligands to the lysosomes and recycles back to surface. In this way, the proteolytically active areas of the cell surface can be continuously monitored for their activity and their location modified. The cell can thus coordinate its migration efforts with a step-wise modification of the proteolytic activity-map of the cell surface. The urokinase cycle can be supported by one individual cell (autocrine) or by two or more cells. In the latter case, complementation and synergism of urokinase and its receptor are found.