Protease-activated receptors (PARs) in cancer: Novel biased signaling and targets for therapy

Despite the fact that G protein-coupled receptors (GPCRs) mediate numerous physiological processes and represent targets for therapeutics for a vast array of diseases, their role in tumor biology is under appreciated. Protease-activated receptors (PARs) form a family which belongs to GPCR class A. PAR1&2 emerge with a central role in epithelial malignancies. Although the part of PAR1&2 in cancer is on the rise, their underlying signaling events are poorly understood. We review hereby past, present, and future cancer-associated PAR biology. Mainly, their role in physiological (placenta-cytotophobalst) and patho-physiological invasion processes. The identification and characterization of signal pleckstrin homology (PH)-domain-binding motifs established critical sites for breast cancer growth in PAR1&2. Among the proteins found to harbor important PH-domains and are involved in PAR biology are Akt/PKB as also Etk/Bmx and Vav3. A point mutation in PAR2, H349A, but not R352A, abrogated PH-protein association and is sufficient to markedly reduce PAR2-instigated breast tumor growth in vivo as also placental extravillous trophoblast (EVT) invasion in vitro is markedly reduced. Similarly, the PAR1 mutant hPar1-7A, which is unable to bind PH-domain, inhibits mammary tumors and EVT invasion, endowing these motifs with physiological significance and underscoring the importance of these previously unknown PAR1 and PAR2 PH-domain-binding motifs in both pathological and physiological invasion processes.

Tumor cell invasion is promoted by activation of protease activated receptor-1 in cooperation with the alpha vbeta 5 integrin

The first prototype of the protease activated receptor (PAR) family, the thrombin receptor PAR1, plays a central role both in the malignant invasion process of breast carcinoma metastasis and in the physiological process of placental implantation. The molecular mechanism underlying PAR1 involvement in tumor invasion and metastasis, however, is poorly defined. Here we show that PAR1 increases the invasive properties of tumor cells primarily by increased adhesion to extracellular matrix components. This preferential adhesion is accompanied by the cytoskeletal reorganization of F-actin toward migration-favoring morphology as detected by phalloidin staining. Activation of PAR1 increased the phosphorylation of focal adhesion kinase and paxillin, and the induced formation of focal contact complexes. PAR1 activation affected integrin cell-surface distribution without altering their level of expression. The specific recruitment of alpha(v)beta(5) to focal contact sites, but not of alpha(v)beta(3) or alpha(5)beta(1), was observed by immunofluorescent microscopy. PAR1 overexpressing cells showed selective reciprocal co-precipitation with alpha(v)beta(5) and paxillin but not with alpha(v)beta(3) that remained evenly distributed under these conditions. This co-immunoprecipitation failed to occur in cells containing the truncated form of PAR1 that lacked the entire cytoplasmic portion of the receptor. Thus, the PAR1 cytoplasmic tail is essential for conveying the cross-talk and recruiting the alpha(v)beta(5) integrin. While PAR1 overexpressing cells were invasive in vitro, as reflected by their migration through a Matrigel barrier, invasion was further enhanced by ligand activation of PAR1. Moreover, the application of anti-alpha(v)beta(5) antibodies specifically attenuated this PAR1 induced invasion. We propose that the activation of PAR1 may lead to a novel cooperation with the alpha(v)beta(5) integrin that supports tumor cell invasion.

Protection of thrombin receptor expression under hypoxia

Thrombin receptor (ThR) plays a significant role in myocyte contractility and hypertrophy. Heart myocyte ischemic damage, caused by insufficient blood supply, is the leading cause of heart infarction. Here we demonstrate that when primary myocyte cultures are subjected to hypoxic stress, ThR mRNA levels are reduced markedly. This takes place also in vivo in a model of ischemic pig heart, exhibiting reduced levels of ThR compared with normal heart sections. Prior activation of ThR however, by either thrombin receptor-activating peptide (TRAP) or by alpha-thrombin resulted in full protection of ThR mRNA levels under hypoxia. The effect appeared specific to ThR because the addition of TRAP did not affect the hypoxic damage as shown by the levels of lactic dehydrogenase release and up-regulated GLUT-1, a glucose transporter gene. This protection effect took place not only in primary myocytes but also in NIH3T3 fibroblasts. ThR protection occurs via specific cell signaling events because activation of the receptor by TRAP, following interruption of the signaling cascade by calphostin C, a protein kinase C inhibitor, resulted in loss of ThR mRNA protection. Because Ras and Src are part of the ThR signaling cascade, the introduction of either dominant ras or src oncogenes to NIH3T3 murine fibroblasts gave rise to similar protection of ThR mRNA levels under hypoxic conditions without the exogenous addition of TRAP. Likewise, ThR mRNA protection was obtained after transfection with proto-oncogene vav. The 95-kDa protein Vav undergoes tyrosine phosphorylation after ThR activation, serving thus as part of the receptor machinery cascade. We therefore conclude that the initiation of the signaling cascades either exogenously by TRAP or within the cell via src or ras, as well as via vav oncogene interconnecting G-binding protein to the tyrosine kinase pathway, ultimately results in ThR protection under hypoxia. We present hereby, a novel concept of activated receptors, which under minimal oxygen tension protect their otherwise decaying mRNA. Maintaining the level of ThR that plays an active role in normal myocyte function may provide a significant repair mechanism in ischemic tissue, assisting in the regaining of normal myocyte functions.

Thrombin receptor overexpression in malignant and physiological invasion processes

Although the involvement of soluble and matrix-immobilized proteases in tumor cell invasion and metastasis is well recognized, the role of proteolytically activated cell surface receptors has not been elucidated. We report here that thrombin receptor, a member of the protease-activated receptor family, is preferentially expressed in highly metastatic human breast carcinoma cell lines and breast carcinoma biopsy specimens. Introduction of thrombin receptor antisense cDNA considerably inhibited the invasion of metastatic breast carcinoma cells in culture through a reconstituted basement membrane. During placental implantation of the human embryo, thrombin receptor is transiently expressed in the invading cytotrophoblasts. These results emphasize the involvement of thrombin receptor in cell invasion associated with tumor progression and normal embryonic development.

Specific involvement of glypican in thrombin adhesive properties

We have previously demonstrated that thrombin possesses an active yet cryptic Arg-Gly-Asp (RGD) site which upon exposure induces endothelial cell (EC) adhesion via alpha nu beta 3 integrin [Bar-Shavit et al. (1991): J Cell Biol 112:335]. This was achieved in the presence of cell surface-associated heparan sulfate proteoglycans (HSPG) and exceedingly low concentrations of plasmin [Bar-Shavit et al. (1993): J Cell Biol 123:1279]. A portion of the cell surface-associated HSPG (glypican) is anchored via a covalently linked glycosyl-phosphatidylinositol (PI) residue, which can be released by treatment with glycosyl-PI-specific phospholipase C (PI-PLC). We report here that exposure of either bovine aortic EC, smooth muscle cells (SMC), or wild-type CHO cells to PI-PLC released HSPG involved in the conversion of thrombin to an adhesive molecule. The adhesion-promoting activity of the released HSPG was abolished following treatment with heparinase but not chondroitinase ABC. Incubation of thrombin with heparan sulfate-deficient CHO cells or cells that were pretreated with PI-PLC failed to induce its conversion to an adhesive molecule, indicating that glypican was playing a major role in this conversion. Moreover, affinity-purified glypican, but not syndecan or fibroglycan, elicited efficient conversion of plasmin-treated thrombin into an adhesive molecule. Antibodies raised against the RGD site in thrombin failed to interact with native thrombin, prothrombin, or the RGD site in other adhesive proteins such as vitronectin, fibrinogen, or fibronectin. Anti-thrombin-RGD antibodies which blocked the adhesion-promoting activity of thrombin were also capable of recognizing thrombin that was first incubated with a suboptimal concentration of plasm in in the presence of PI-PLC-released HSPG. Heparin, heparan sulfate, and PI-PLC-released HSPG had no effect on other cellular properties of thrombin such as receptor binding and growth-promoting activity. Altogether we have demonstrated that the heparin binding domain in thrombin plays a specific role in promoting thrombin adhesive properties and that membrane-associated glypican is likely to be the major physiological inducer of this property.

Structural requirements for inhibition of melanoma lung colonization by heparanase inhibiting species of heparin

Heparanase activity correlates with metastatic potentials of lymphoma, melanoma and mammary adenocarcinoma cell lines. We investigated the ability of various modified species of heparin and size homogeneous oligosaccharides derived from depolymerized heparin to inhibit: a) heparanase-mediated degradation of heparan sulfate (HS) in the extracellular matrix (ECM) deposited by cultured endothelial cells, and b) lung colonization of B16-BL6 melanoma cells in C57BL mice. For this purpose, melanoma cells or conditioned medium were incubated with metabolically sulfate-labeled subendothelial ECM in the absence and presence of heparin, heparin fragment or nonanticoagulant species of heparin. Labeled HS degradation fragments released into the incubation medium were analyzed by gel filtration over Sepharose 6B. The B16-BL6 melanoma cells were also tested for lung colonization following their intravenous administration to C57BL mice, in the absence and presence of the various species of heparin. Inhibition of both heparanase and melanoma lung colonization depended on the size and degree of sulfation of the heparin molecule, the position of sulfate groups, and the occupancy of the N position of the hexosamines. Inhibition of heparanase was best achieved by heparin species containing 16 sugar units or more and having sulfate groups at both the N and O positions. Low sulfate oligosaccharides were less effective heparanase inhibitors than medium and high sulfate fractions of the same size saccharide. While O-desulfation abolished the heparanase inhibiting effect of heparin. O-sulfated, N-substituted (e.g., N-acetyl or N-hexanoyl) species of heparin retained a high inhibitory activity, provided that the N-substituted molecules had a molecular size of about > or = 4,000 daltons. Potent inhibitors of heparanase activity were also efficient inhibitors of tumor invasion and lung colonization. The antimetastatic and anticoagulant activities of heparin were unrelated, as indicated by using heparin fractions with high and low affinity for antithrombin III. These heparins differ about 200-fold in their anticoagulant activity, but expressed similar high antiheparanase and antimetastatic activities. It appears that heparanase-inhibiting species of heparin interfere with the passing of tumor cells across the capillary wall, as they significantly inhibited metastasis even when injected up to 3 h after lodgment. Structural requirements for inhibition of heparanase activity and lung colonization of melanoma cells by species of heparin were different from those identified for a) release of ECM-bound basic fibroblast growth factor (bFGF), and b) stimulation of bFGF receptor binding and mitogenic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

The involvement of thrombin RGD in metastasis: characterization of a cryptic adhesive site

Adhesive interactions between cells and the subendothelial extracellular matrix take place at several stages during tumor progression and metastasis. We have previously demonstrated that thrombin possesses an active yet cryptic Arg-Gly-Asp (RGD) site which can be exposed in the presence of low concentrations of plasmin and cell-associated heparan sulfate proteoglycan. Thus, thrombin may act as a matrix-adhesive molecule via activation of the alpha v beta 3 integrin. We have now identified a 31 amino acid fragment as the minimal thrombin-generated cleavage product, which contains an active RGD site, following gel filtration analysis on FPLC Superdex 75 column. The role of membrane-associated heparan sulfate in thrombin conversion to an adhesive protein was demonstrated by using CHO cell mutants defective in various aspects of glycosaminoglycan synthesis. Incubation of both thrombin and a low concentration of plasmin on the surface of wild type CHO cells resulted in a typical digestion cleavage profile upon gel filtration. No cleavage products were observed when thrombin and a suboptimal plasmin concentration were incubated on monolayers of CHO cell mutants lacking heparan sulfate. Next, we examined the possible role of the thrombin RGD site during the progression of tumor development and metastasis. Toward this, we tested murine melanoma cells expressing low (B16-F1 cells) and high (B16-BL6 cells) lung colonization potentials in cell adhesion assays in vitro. Differential adherence capability of the cells was observed: while high attachment levels of B16-BL6 cells were obtained, the low metastatic B16-F1 cells did not adhere to thrombin RGD. Antibodies raised against the RGD site in thrombin specifically recognized thrombin digested with plasmin, but were unable to interact with native thrombin or prothrombin and inhibited potently B16-BL6 melanoma cell adhesion. Furthermore, the antibodies failed to recognize RGD in other adhesive plasma proteins such as vitronectin, fibrinogen, or fibronectin. Provided that the RGD-containing fragments of thrombin are widely distributed throughout the vascular system, they may have a significant role during tumor progression and dislodgement of metastatic cells. The development of RGD mimetics and/or specific antibodies might thus be applied to inhibit a critical step in metastatic spread.

Antiproliferative activity to vascular smooth muscle cells and receptor binding of heparin-mimicking polyaromatic anionic compounds

Proliferation of bovine aortic smooth muscle cells (SMCs) induced by thrombin, basic fibroblast growth factor, or serum is inhibited by anionic, nonsulfated aromatic compounds that mimic many of the effects of heparin. Among these compounds are aurintricarboxylic acid (ATA) and a newly synthesized polymer of 4-hydroxyphenoxy acetic acid (compound RG-13577). Iodinated- or 14C-labeled compound RG-13577 binds to cultured SMCs in a highly specific and saturable manner. Scatchard analysis of the binding data revealed the presence of an estimated 1 x 10(7) binding sites per cell with an apparent dissociation constant of 3 x 10(-6) mol/L. Binding of radiolabeled RG-13577 was efficiently competed for by related aromatic anionic compounds and by apolipoprotein E, but not by heparin, heparan sulfate, suramin, or various purified growth factors and extracellular matrix proteins. Receptor cross-linking of SMC-bound 125I-RG-13577 revealed a single species of high M(r) (approximately 280 kD) cell surface receptors detected in the absence but not the presence of excess unlabeled compound RG-13577. Binding was susceptible to downregulation and restoration of receptor levels in a manner similar to that of hormone and growth factor receptors. We suggest that the antiproliferative activity of compound RG-13577 and related compounds is initiated by binding to specific growth-inhibiting cell surface receptors. Heparin-mimicking compounds may be applied to inhibit SMC proliferation associated with atherosclerosis and restenosis.

Thrombin adhesive properties: induction by plasmin and heparan sulfate

We have previously demonstrated that chemically modified thrombin preparations induce endothelial cell (EC) adhesion, spreading and cytoskeletal reorganization via an Arg-Gly-Asp (RGD) sequence and the alpha v beta 3 integrin. Native thrombin, however, did not exhibit adhesive properties, consistent with crystal structure analysis, showing that Gly-Asp residues of the RGD epitope are buried within the molecule. We have now identified a possible physiological mean of converting thrombin to an adhesive protein. Plasmin, the major end product of the fibrinolytic system, converted thrombin to an adhesive protein for EC in a time and dose-dependent manner. EC adhesion and spreading was also induced by a low molecular weight (approximately 3,000 D) cleavage fragment generated upon incubation of thrombin with plasmin. Cell adhesion mediated by this fragment was completely inhibited by the synthetic peptide GRGDSP. Conversion of thrombin to an adhesive molecule was significantly enhanced in the presence of heparin or heparan sulfate, while other glycosaminoglycans (GAGs) (e.g., dermatan sulfate, keratan sulfate, chondroitin sulfate) had no effect. The role of cell surface heparan sulfate in thrombin conversion to EC adhesive protein was investigated using CHO cell mutants defective in various aspects of GAG synthesis. Incubation of both thrombin and a suboptimal amount of plasmin on the surface of formaldehyde fixed wild-type CHO-KI cells resulted in an efficient conversion of thrombin to an adhesive molecule, as indicated by subsequent induction of EC attachment. In contrast, there was no effect to incubation of thrombin and plasmin with fixed CHO mutant cells lacking both heparan sulfate and chondroitin sulfate, or with cells expressing no heparan sulfate and a three-fold increase in chondroitin sulfate. A similar gain of adhesive properties was obtained upon incubation of thrombin and plasmin in contact with native, but not heparinase-treated extracellular matrix (ECM) produced by cultured ECs. It appears that cell surface and ECM-associated heparan sulfate modulate thrombin adhesive properties through its heparin binding site in a manner that enables suboptimal amounts of plasmin to expose the RGD domain. Our results demonstrate, for the first time, a significant modulation of thrombin molecule by heparin, resulting in its conversion to a potent adhesive protein for ECs. This conversion is most effective in contact with cell surfaces, basement membranes and ECM.

Thrombin-induced release of active basic fibroblast growth factor-heparan sulfate complexes from subendothelial extracellular matrix

The angiogenic factor, basic fibroblast growth factor (bFGF), is sequestered and protected by binding to heparan sulfate proteoglycans (HSPG) in the subendothelial extracellular matrix (ECM). Release of ECM-bound bFGF provides a novel mechanism for regulation of cell proliferation and neovascularization in normal and pathologic situations. Exposure of ECM to thrombin, the final activation product of the clotting cascade, resulted in release of high molecular weight HSPG-bFGF complex, as indicated by its immunoprecipitation with anti-bFGF antibodies, susceptibility to degradation by bacterial heparinase, and inhibition of its mitogenic activity in the presence of neutralizing anti-bFGF antibodies. The ECM-resident bFGF-HSPG complex was not released by thrombin in the presence of hirudin or antithrombin III, or by catalytically blocked thrombin preparations. A threefold to fivefold higher mitogenic activity was released by thrombin from ECM that was preheated (1 hour, 80 degrees C), as compared with native ECM. This difference is attributed to heat stable bFGF-HSPG complexes that are more readily released after heat treatment of the ECM and to activation and release of ECM-resident transforming growth factor-beta (TGF-beta) activity. Our results indicate that the large reservoir of proteolytic activity present in plasma in the form of prothrombin may participate in release from the subendothelial ECM of biologically active bFGF and TGF-beta, depending on the accessibility of thrombin. Thrombin may gain access to the subendothelium on clot formation after tissue injury and as a result of the conversion of prothrombin to thrombin induced by the ECM itself.

Charge-dependent binding of granzyme A (MTSP-1) to basement membranes

The murine T cell-associated serine proteinase granzyme A [also termed Murine T cell-associated serine proteinase-1 (MTSP-1), or SE-1] expresses optimal enzymatic activity under extracellular milieu conditions. It degrades a variety of proteins that are constituents of basement membranes. Granzyme A is harbored within intracellular storage granules from which its release can be induced by appropriate ligand binding to extracellular matrix receptors of T cells. Secreted granzyme A has, therefore, been implicated in the degradation of extracellular matrix barriers during T cell migration. Here we show that granzyme A binds to natural basement membranes in a charge-dependent manner. Binding of granzyme A to charged surfaces protects if from inhibition by natural high molecular weight inhibitors. The interaction of granzyme A with in vitro-produced extracellular matrices liberates basic fibroblast growth factor, which is bound to negatively charged heparan sulfate glycosaminoglycans of the extracellular matrix. We propose that the charge-dependent interaction of granzyme A with basement membranes has multiple, biologically relevant consequences.

Reversal of basic fibroblast growth factor-mediated autocrine cell transformation by aromatic anionic compounds

NIH-3T3 cells transfected with basic fibroblast growth factor (bFGF) fused to a signal peptide sequence (spbFGF cells) are transformed in vitro and tumorigenic in vivo. Treatment of spbFGF cells with low and nontoxic concentrations (0.5-2.5 micrograms/ml) of negatively charged, nonsulfated aromatic compounds (e.g., aurin tricarboxylic acid, 4-hydroxyphenoxyacetic acid) resulted in restoration of their normal proliferative rate, morphological appearance, and adhesion properties. Binding and cross-linking experiments using 125I-labeled bFGF revealed that these alterations were associated with an up-regulation of high affinity receptors bFGF receptors was induced by these compounds in spbFGF cells that were seeded on fibronectin to enforce a firm cell attachment and flattening. Thus, induction of spbFGF cell adhesion and spreading may not be related to restoration of normal bFGF-receptor interactions. Although the negatively charged aromatic compounds mimic many of the effects of heparin in other systems (e.g., release of heparin- and heparan sulfate-bound proteins, inhibition of heparanase), heparin, heparan sulfate, and dextran sulfate were not effective at the low concentrations of the anionic compounds used in the present study. Likewise, suramin, a sulfated aromatic molecule, was effective at toxic concentrations, 400-600-fold higher than the nonsulfated aromatic compounds. The development of defined, nontoxic anionic compounds may provide a new strategy to interfere with the autonomous and anchorage independent mode of cell growth involved in autocrine cell transformation and cancer.

Thrombin enhances degradation of heparan sulfate in the extracellular matrix by tumor cell heparanase

The ability of normal and malignant blood-borne cells to extravasate correlates with the activity of an endo-beta-D-glucuronidase (heparanase) which degrades heparan sulfate (HS) in the subendothelial extracellular matrix (ECM). The association of malignancy with different types of coagulopathies prompted us to study the effect of thrombin (EC 3.4.21.5), a serine protease elaborated during activation of the clotting cascade, on the ability of heparanase to degrade the ECM-HS. The circulating zymogen form of thrombin, prothrombin, was converted to proteolytically active thrombin during incubation with ECM. Thrombin generation by the ECM was time and dose dependent, reaching maximal conversion by 6 h incubation at 3 U/ml of prothrombin. Heparanase-mediated release of low Mr HS cleavage products from sulfate-labeled ECM was stimulated four- to sixfold in the presence of alpha-thrombin, but there was no effect on degradation of soluble HS. Similar results were obtained with heparanase preparations derived from mouse lymphoma and human hepatoma cell lines and from human placenta. Incubation of ECM with alpha-thrombin alone resulted in release of nearly intact high-Mr labeled proteoglycans. Thrombin stimulation of heparanase action was dose and time dependent, reaching a maximal value at 24 h incubation with 1 microM alpha-thrombin. The effect of modified thrombin preparations correlated with their proteolytic activity. Catalytically blocked preparations of thrombin (e.g., DIP-alpha-thrombin, MeSO2-alpha-thrombin) failed to facilitate heparanase action, while catalytically modified preparations (e.g., gamma-thrombin, NO2-alpha-thrombin) exerted only a slight enhancement. Antithrombin III (ATIII) and hirudin both inhibited thrombin-stimulated heparanase degradation of ECM-bound HS. Heparanase action was also facilitated by ECM-immobilized thrombin to an extent which was similar to that induced by soluble thrombin. This result implies that thrombin sequestered by the subendothelial ECM and protected from interaction with its natural inhibitor ATIII (Bar-Shavit et al., 1989, J. Clin. Invest. 84, 1096-1104) may participate locally in cellular invasion during tumor metastasis, inflammation, and autoimmunity.

Thrombin as a multifunctional protein: induction of cell adhesion and proliferation

The serine protease thrombin (E.C.3.4.21.5) is well recognized for its central role in hemostasis. In addition, thrombin is unique among the enzymes participating in the clotting cascade, by virtue of its cell activation effects induced via the enzymatic pocket or via functional domains located throughout the molecule. In this review, we elaborate on "nonhemostatic" activities of thrombin among which are interactions with vessel wall components. These activities include promotion of cellular adhesion and induction of smooth muscle cell proliferation. Thrombin can exert these effects when it is in a fluid phase and when it is immobilized to extracellular matrix.

Coordinate secretion and functional synergism of T cell-associated serine proteinase-1 (MTSP-1) and endoglycosidase(s) of activated T cells

Cell lysates and exocytosed soluble mediator(s) (ESM) released from CD8+ T cell lines (TCL) by receptor-triggered secretory exocytosis were tested for degradation of proteoglycans associated with in vitro produced subendothelial extracellular matrix (ECM). ESM was found to release low-molecular weight (kav 0.5-0.6) fragments from the sulfated proteoglycans in ECM. In the presence of heparin, an inhibitor for endoglycosidase activity, only high-molecular-weight products (kav 0.2) were formed. Preincubation of ESM with HD-prolylphenylalanyl-arginyl-chloromethylketone (PFR-CK) an inhibitor for the T cell-associated serine proteinase-1 (MTSP-1) totally prevented release of high- and low-molecular weight proteoglycan fragments. Furthermore, it was shown that purified MTSP-1 is able to release from ECM high-molecular weight proteoglycans and that this process is inhibitable by PFR-CK but not by heparin. Further treatment of these soluble high-molecular weight sulfated proteoglycans with ESM from TCL 1.D9 led to appearance of low-molecular weight split products (kav 0.5-0.6). This conversion was inhibitable by heparin but not by PFR-CK. These findings indicate that activated T cells contain two enzymatic activities, i.e. MTSP-1 and at least one endoglycosidase, which after receptor-triggered secretion can synergize in the degradation of sulfated proteoglycans in subendothelial ECM.

Extracellular sequestration and release of fibroblast growth factor: a regulatory mechanism?

Basic fibroblast growth factor, (bFGF), promotes the formation of new blood capillaries and is sequestered and protected by binding to heparan sulfate (HS), both on the cell surface and in the extracellular matrix. Release of HS-bound bFGF by heparin-like molecules and HS-degrading enzymes (i.e., heparanase) provides a novel mechanism for regulation of the growth of capillary blood vessels in normal and pathological situations. The extracellular matrix also serves as a storage depot for other growth factors and enzymes.

Extracellular matrix-resident basic fibroblast growth factor: implication for the control of angiogenesis

Despite the ubiquitous presence of basic fibroblast growth factor (bFGF) in normal tissues, endothelial cell proliferation in these tissues is usually very low, suggesting that bFGF is somehow sequestered from its site of action. Immunohistochemical staining revealed the localization of bFGF in basement membranes of diverse tissues, suggesting that the extracellular matrix (ECM) may serve as a reservoir for bFGF. Moreover, functional studies indicated that bFGF is an ECM component required for supporting endothelial cell proliferation and neuronal differentiation. We have found that bFGF is bound to heparan sulfate (HS) in the ECM and is released in an active form when the ECM-HS is degraded by heparanase expressed by normal and malignant cells (i.e. platelets, neutrophils, lymphoma cells). It is proposed that restriction of bFGF bioavailability by binding to ECM and local regulation of its release provide a novel mechanism for neovascularization in normal and pathological situations. The subendothelial ECM contains also tissue type- and urokinase type-plasminogen activators which participate in cell invasion and tissue remodeling. These results and studies on the properties of other ECM-immobilized enzymes (i.e. thrombin, plasmin, lipoprotein lipase) and growth factors (GM-CSF, IL-3, osteogenin), suggest that the ECM provides a storage depot for biologically active molecules which are thereby stabilized and protected. This may allow a more localized and persistent mode of action, as compared to the same molecules in a fluid phase.

An Arg-Gly-Asp sequence within thrombin promotes endothelial cell adhesion

Thrombin, in addition to its central role in hemostasis, possesses diverse cellular bioregulatory functions implicated in wound healing, inflammation, and atherosclerosis. In the present study we demonstrate that thrombin molecules modified either at the procoagulant or catalytic sites induce endothelial cell (EC) adhesion, spreading, and cytoskeletal reorganization. The most potent adhesive thrombin analogue (NO2-alpha-thrombin) was obtained by nitration of tyrosine residues. The cell adhesion promoting activity of NO2-alpha-thrombin was blocked upon the formation of thrombin-antithrombin III (ATIII) complexes and by antiprothrombin antibodies, but was unaffected by hirudin. Arg-Gly-Asp-containing peptides, fully inhibited EC adhesion to NO2-alpha-thrombin, while synthetic peptides corresponding to thrombin "Loop B" mitogenic site and the thrombin-derived chemotactic fragment "CB67-129", were uneffective. Immunofluorescence studies indicated that EC adhesion to NO2-alpha-thrombin was followed by cell spreading, actin microfilament assembly, and formation of focal contacts. By the use of specific antibodies, the vitronectin (vn) receptor (alpha v beta 3) was found to be localized in clusters upon cell adhesion to NO2-alpha-thrombin. An anti alpha v beta 3 antibody blocked EC adhesion and spreading while antifibronectin (fn) receptor (alpha 5 beta 1) antibodies were uneffective. While native thrombin exhibited a very low cell attachment activity, thrombin that was incubated at 37 degrees C before coating of plastic surfaces induced EC attachment and spreading. We propose that under certain conditions the naturally hindered RGD domain within thrombin is exposed for interaction with alpha v beta 3 on EC. This in turn promotes cell adhesion, spreading, and reorganization of cytoskeletal elements, which may altogether contribute to repair mechanisms in the disturbed vessel wall. This study defines a new biological role of thrombin and characterizes a new recognition mechanism on EC for this molecule.

Extracellular matrix-resident growth factors and enzymes: possible involvement in tumor metastasis and angiogenesis

Neoplastic cells require an appropriate pericellular environment and new formation of stroma and blood vessels in order to constitute a solid tumor. Tumor progression also involves degradation of various extracellular matrix (ECM) constituents. In this review we have focused on the possible involvement of ECM-resident growth factors and enzymes in neovascularization and cell invasion. We demonstrate that the pluripotent angiogenic factor, basic fibroblast growth factor (bFGF) is an ECM component required for supporting cell proliferation and differentiation. Basic FGF has been identified in the subendothelial ECM produced in vitro and in basement membranes of the cornea and blood vessels in vivo. Despite the ubiquitous presence of bFGF in normal tissues, endothelial cell (EC) proliferation in these tissues is usually very low, suggesting that bFGF is somehow sequestered from its site of action. Our results indicate that bFGF is bound to heparan sulfate (HS) in the ECM and is released in an active form when the ECM-HS is degraded by cellular heparanase. We propose that restriction of bFGF bioavailability by binding to ECM and local regulation of its release, provides a novel mechanism for regulation of capillary blood vessel growth in normal and pathological situations. Heparanase activity correlates with the metastatic potential of various tumor cells and heparanase inhibiting molecules markedly reduce the incidence of lung metastasis in experimental animals. Heparanase may therefore participate in both tumor cell invasion and angiogenesis through degradation of the ECM-HS and mobilization of ECM-resident EC growth factors. The subendothelial ECM contains also tissue type- and urokinase type- plasminogen activators (PA), as well as PA inhibitor which may regulate cell invasion and tissue remodeling. Heparanase and the ECM-resident PA participate synergistically in sequential degradation of HS-proteoglycans in the ECM. These results together with similar observations on the properties of other ECM-immobilized enzymes and growth factors, suggest that the ECM provides a storage depot for biologically active molecules which are thereby stabilized and protected. This may allow a more localized, regulated and persistent mode of action, as compared to the same molecules in a fluid phase.

Thrombin immobilized to extracellular matrix is a potent mitogen for vascular smooth muscle cells: nonenzymatic mode of action

Esterolytically inactive diisopropyl fluorophosphate-conjugated thrombin (DIP-alpha-thrombin) stimulated 3H-thymidine incorporation and proliferation of growth-arrested vascular smooth muscle cells (SMCs), similar to native alpha-thrombin. Half-maximal mitogenic response of SMCs was obtained at 1 nM thrombin and was specifically blocked by the leech-derived, high-affinity thrombin inhibitor, hirudin. Native thrombin and a variety of thrombin species that were chemically modified to alter thrombin procoagulant or esterolytic functions were found to induce 3H-thymidine incorporation to a similar extent. Exposure of SMCs to DIP-alpha-thrombin caused a rapid and transient expression of the c-fos protooncogene, determined by Northern blot analysis. These results indicate that thrombin is a potent mitogen for SMCs through a distinct non-enzymatic domain. Binding of 125I-alpha-thrombin to SMC cultures revealed an apparent dissociation constant of 6 nM and an estimated 5.4 x 10(5) binding sites per cell. This binding was inhibited to the same extent by native thrombin and by its nonenzymatic form, DIP-alpha-thrombin. Moreover, the chemotactic fragment of thrombin (CB67-129), which failed to elicit a mitogenic response, competed for 125I-alpha-thrombin binding to SMCs. Cross-linking analysis of 125I-alpha-thrombin to SMCs revealed a specific cell-surface binding site 55 kDa in size. Finally, thrombin immobilized to a naturally produced extracellular matrix retained potent mitogenic activity toward SMCs. These observations lend support to the possibility that in vivo, subendothelial basement membranes sequester thrombin (as well as other bioactive molecules), which may stimulate localized and persistent growth of arterial SMCs. Thrombin may thus be involved directly in progression of atherosclerotic plaque formation.

Interaction of lipoprotein lipase with subendothelial extracellular matrix

We have analyzed the binding of lipoprotein lipase (LPL) to the subendothelial extracellular matrix produced by cultured endothelial cells. Binding was linear up to a concentration of 0.5 microgram/ml (10 nM) enzyme used in this study, and equilibrium was achieved after 2 h of incubation with bovine 125I-LPL at 4 degrees C. Heparin and heparan sulfate effectively inhibited the binding of LPL to extracellular-matrix-coated plates; chondroitin sulfate had no effect, while high concentrations of dermatan sulfate or keratan sulfate inhibited binding of LPL to extracellular matrix by only 40%. Basic fibroblast growth factor (bFGF) did not affect LPL binding, while antithrombin-III (AT-III) caused up to a 50% inhibition of enzyme binding to extracellular matrix. alpha-Thrombin. 5.10(-6) M, and its esterolytically inactive derivative, DIP-alpha-thrombin, effectively inhibited binding of LPL to extracellular-matrix-coated plates. alpha-Thrombin was also able to release the extracellular-matrix-bound LPL in an active form. Extracellular-matrix-bound LPL detached into medium containing triolein emulsion and/or serum, and was catalytically active after being released. Extracellular-matrix-bound LPL lost 30% of its activity following incubation at 37 degrees C for 4 h. in contrast to soluble LPL which lost 75% of its activity. It is plausible to conclude from these data that in vivo the subendothelial basement membrane, similarly to extracellular matrix, sequesters and stabilizers LPL secreted into the subendothelial space by non-endothelial cells, and thus may play an important role in determining the route of LPL from its site of synthesis to its site of action.

Binding of thrombin to subendothelial extracellular matrix. Protection and expression of functional properties

We have analyzed the binding of thrombin, a serine protease with central roles in hemostasis, to the subendothelial extracellular matrix (ECM) produced by cultured endothelial cells. This substrate provides a thrombogenic surface where hemostasis is initiated. Binding was saturable and equilibrium was achieved after 3 h incubation with 125I-alpha-thrombin. Scatchard analysis of thrombin binding revealed the presence of 5.1 X 10(9) binding sites per squared millimeter ECM, with an apparent Kd of 13 nM. The catalytically blocked enzyme, diisofluorophosphate (DIP)-alpha-thrombin competed efficiently with 125I-alpha-thrombin, indicating that the binding was independent of its catalytic site. Moreover, high concentrations of the synthetic tetradecapeptide, representing residues 367-380 of thrombin B chain (the macrophage mitogenic domain of thrombin), competed with thrombin binding to ECM, indicating that the binding site may reside in the vicinity of "loop B" region. Thrombin binds to dermatan sulfate in the ECM, as demonstrated by the inhibition of 125I-alpha-thrombin binding to ECM pretreated with chondroitinase ABC, but not with heparitinase or chondroitinase AC. This stands in contrast to 125I-FGF (fibroblast growth factor) binding to ECM, which was inhibited by heparitinase but not by chondroitinase ABC, ECM-bound thrombin exhibits an exposed proteolytic site as monitored by the Chromozyme TH assay and by its ability to convert fibrinogen to a fibrin clot and to induce platelet activation as indicated by 14C-serotonin release. ECM-bound thrombin failed to form a complex with its major circulating inhibitor-antithrombin III (AT III), compared with rapid complex formation with soluble thrombin. We propose that thrombin binds to subendothelial ECM where it remains functionally active, localized, and protected from inactivation by circulating inhibitors.

Striated muscle fibers differentiate in primary cultures of adult anterior pituitary cells

Anterior pituitary cells from adult male rats were cultured on a natural extracellular matrix (ECM). From the 5th day, spindle-shaped cells, fusing to form elongated fibers, were observed among the epithelial cells. These fibers later increased in size and number and were identified as striated muscle fibers, based on their multi-nucleation, cross-striation and rhythmic contraction. Striated muscle development was further validated by the appearance of cholinergic-nicotinic receptors, as demonstrated by the binding of 125I-alpha bungarotoxin (a-Btx). The effect of various supplements added to the cultures on a-Btx binding was used as a measure of muscle cell differentiation. ECM and serum were essential for myotube formation. Insulin (or IGF-I) and cortisol synergistically caused a 20-fold increase in myogenesis. FGF and EGF were effective only in the presence of insulin and cortisol. The data suggest that: (a) cells, derived from adult pituitary, differentiate, in primary culture, into striated muscle fibers; (b) the induction of this differentiation is multi-factorial; ECM, serum, insulin and glucocorticoids facilitate this myogenesis.

Thrombin chemotactic stimulation of HL-60 cells: studies on thrombin responsiveness as a function of differentiation

Thrombin, a major procoagulant enzyme and growth factor, is also selectively chemotactic for monocytes and macrophages but not for neutrophils. This effect stands in contrast to other well-known chemotactic agents such as fMet-Leu-Phe, C5a fragments, and LTB4, which stimulate directed cell movement in both cell types, and have important physiological implications. The human leukemic cell line HL-60, which is capable of differentiating either along granulocytic or monocytic lineages, was therefore used to explore the development of this selective monocyte/macrophage chemotactic response to thrombin. Esterolytically inactive DIP-alpha-thrombin, as well as the thrombin-derived chemotactic peptide CB67-129, elicits a dose-dependent chemotactic response in HL-60 cells differentiated to monocytelike cells by treatment with 1,25(OH)2D3 (HL-60/mono), whereas no such response is evident in either undifferentiated HL-60 cells or in cells differentiated into granulocytes by treatment with DMSO (HL-60/gran). Similarly, early events which characterize stimulation of inflammatory cells by chemotactic agents are also evident, but only in monocyte-differentiated cells. In HL-60/mono, thrombin selectively stimulates rapid cytosolic Ca2+ elevation as well as rapid cytoskeletal association of cytosolic actin. Following thrombin stimulation, maximal actin association in these cells occurs within 30 sec (declining to basal levels at the end of 5 min), and maximal Ca2+ elevations are also evident within 15-20 sec, suggesting a temporal relationship between these two events. Thus, the events accompanying stimulation of HL-60/mono by thrombin are characteristic of those seen following stimulation of inflammatory cells by chemotaxins, with a major difference being the selectivity of thrombin as a chemotaxin for cells of macrophage/monocytic lineage. The selective chemotactic responsiveness of HL-60/mono to thrombin appears to relate to the development of specific receptors on these cells as part of monocytic differentiation: HL-60/mono (but HL-60/gran nor undifferentiated HL-60) are capable of significant specific 125-I-labeled alpha-thrombin-binding (ka approximately 20 nM), and possess an estimated 400,000 thrombin-binding sites per cell. Our findings further suggest that the thrombin response of HL-60 and particularly the expression of thrombin receptors on these cells may serve as a useful model system for exploring the biology of monocyte/macrophage differentiation.

Hormone-like activity of human thrombin

Recently, we have shown that thrombin is a chemotaxin and growth-promoting agent for cells of the mononuclear phagocytic lineage. These activities are independent of thrombin's enzymatic activity. Unlike other chemotactic factors, thrombin is specific for monocytes and does not attract granulocytes. To further explore the cellular specificity we have used a human leukemia cell line HL-60 that is capable of in vitro differentiation toward either monocytes (HL-60/mono) following incubation with 1,25(OH)2D3, or granulocytes (HL-60/gran) following incubation with DMSO. In contrast to undifferentiated HL-60 cells or HL-60/gran, we find that HL-60/mono respond chemotactically to intact human alpha-thrombin, esterolytically inactive iPR2P-alpha-thrombin, and the thrombin-derived peptide CB67-129, previously shown to contain the thrombin chemotactic exosite. In addition, thrombin induces in HL-60/mono association of actin with the cytoskeleton and causes an increase in levels of free cytosolic Ca2+. These phenomena are well characterized as early events occurring concomitant with directed cell movement associated with exposure to chemotactic agents such as FMLP. Furthermore, in contrast to fibroblasts, both iPR2P-alpha-thrombin and the thrombin chemotactic peptide CB67-129 evoke dose-dependent [3H]TdR incorporation, protein synthesis, and cell replication in growth-arrested J-744 cells, a murine macrophage-like cell line. Limited tryptic digests of CB67-129 lose chemotactic activity but retain full mitogenic activity, demonstrating that as with PDGF, the sites on CB67-129 required for chemotaxis and mitogenesis are clearly dissociable. The mitogenic effects of the CB67-129 digest can be mimicked by a synthetic tetradecapeptide analogue of CB67-129 (residues 367-380) that includes the loop B insertion sequence, previously shown to be critical for thrombin's chemotactic effects. From these data, it is apparent that the loop B insertion is critical for thrombin's nonenzymic biological effects on cells, but additional sites are required for stimulation of cell movement.

Biologic activities of nonenzymatic thrombin: elucidation of a macrophage interactive domain

Our studies to date have clearly demonstrated the existence of a unique cell interactive exosite region in thrombin, which mediates specific biologic effects on cells of monocytic-macrophage lineage. These findings are of potential physiologic significance, since even proteolytically degraded forms of thrombin (beta- and gamma-thrombin) or fragments of thrombin arising due to breakdown of thrombin-thrombomodulin complexes are potentially biologically active and are not subject to inhibition by inhibitors such as antithrombin III. Such degraded thrombin forms and proteolytic fragments are likely to be present at sites of inflammation and wound repair where they may be important modulators of macrophage-monocyte responsiveness. Importantly, as emphasized in this communication, the chemotactic and growth-promoting properties of this site, although overlapping, are clearly dissociable. It is apparent that the chemotactic effects require the presence of most, if not all, of the 338-400 sequence, whereas the mitogenic effects are mediated exclusively through the so-called loop B insertion sequence that is a unique feature of thrombin. Although there are ample precedents for differing functional domains within discrete proteins and the presence of hormonelike regions in proteins (such as fibronectin and immunoglobulin G) has been documented, it is fascinating to speculate how functionally unique regions such as this can arise. Data from Craik and associates suggest that unique amino acid sequences in proteins originate from gene insertions at intron-exon junctions and are characteristically surface expressed. These insertion sequences give rise to unique structural and functional features that characterize the particular protein, and also serve to differentiate it from other related members within its family.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a thrombin sequence with growth factor activity on macrophages

In contrast to fibroblasts, the exposure of G0/G1-arrested J774 cells, a murine macrophage-like tumor cell line, with either active or esterolytically inactive diisopropyl phosphorofluoridate-conjugated alpha-thrombin (the enzymatically active form of thrombin, EC 3.4.21.5) results in a mitogenic response as measured by increased [3H]thymidine incorporation. This response to thrombin is optimal at 10 nM and is specifically blocked by hirudin, a high-affinity thrombin inhibitor. When prethrombin 1 [a single-chain prothrombin derivative lacking fragment 1, resulting from the action of thrombin on prothrombin] is cleaved with cyanogen bromide, a fragment (peptide CB67-129) is produced that, like the parent thrombin molecule, is mitogenic for J774 cells but not for fibroblasts. Limited tryptic digests of this fragment retain the ability to stimulate macrophages--a function that can be mimicked by a synthetic tetradecapeptide homologue of CB67-129 (representing residues 367-380 of the human thrombin B chain sequence) but not by any of a series of well-known growth promoters, including platelet-derived growth factor, epidermal growth factor, nerve growth factor, and fibroblast epidermal growth factor, nerve growth factor, and fibroblast growth factor. The mitogenic effects of this peptide are not limited to J774 cells but can be expressed in other macrophage-like tumor cell lines, including P388D1, RAW, and PU5. In addition to increased [3H]thymidine incorporation, the synthetic B chain peptide stimulates cell proliferation as evidenced by a dose-dependent increase in total protein per culture well and cell number. We conclude that the thrombin molecule contains a macrophage growth factor domain that is separate and distinct from its active center. Thus, thrombin, in addition to its major role in hemostasis and thrombosis, may also have important functions in such basic processes as the inflammatory response and monocytopoiesis.

Growth-promoting effects of esterolytically inactive thrombin on macrophages

It has been recognized for many years that alpha-thrombin, like other better known mitogens (eg, PDGF, EGF, etc) is capable of initiating proliferation in quiescent cells belonging to the fibroblast family. However, unlike these other peptides, thrombin is a serine protease whose function as a growth stimulator for fibroblasts is intimately linked to its esterolytic activity. Thus, while native alpha-thrombin is capable of evoking DNA synthesis in G0/G1-arrested cells, neither enzymatically inactive thrombin (eg, iPR2P-alpha-thrombin) nor partially degraded thrombin (eg, gamma-thrombin) shares in this capability. Data from our laboratory have shown that thrombin is chemotactic for peripheral blood monocytes and for cells belonging to the monocyte/macrophage family and that this activity is not dependent upon thrombin's enzymatic properties. Our recent findings demonstrate that thrombin also serves as a growth factor for these cells, and this mitogenic capability is independent of esterolytic function and resides in the same region of the molecule as that responsible for chemotaxis. Additionally, by means of techniques such as computer modeling and peptide synthesis, we have now been able to delineate a distinct mitogenic subsite within this chemotactic thrombin sequence. Thus, the sequence in the thrombin B chain that mediates chemotaxis represents a true cell interactive exosite additionally capable of stimulating growth and possibly other biological functions in cells of macrophage/monocyte lineage.

Inhibition of acetylcholine receptor synthesis by thyroid hormones

Studies were made on the effect of thyroid hormones on the level of acetylcholine receptors (AChR) in cultured rat skeletal muscle. Treatment of differentiated myotubes in vitro with thyroxine (T4; 2 X 10(-7) mol/l) for 2-3 days caused a marked decrease in the amount of AChR (P less than 0.05) and an increase in activity of Na+-K+-ATPase (P less than 0.05). There was no significant effect of hormone treatment on other muscle proteins, such as creatine kinase and acetylcholinesterase. Measurements of the turnover rate of AChR in T4-treated myotubes showed only a very slight effect of T4 on the rate of AChR degradation. To study the mechanism by which the hormone exerts its effect, muscle cells were labelled with radioactive amino acid and the rate of its incorporation into AChR protein was measured. The AChR was then isolated using anti-AChR antibodies. The specific activity of labelled AChR was lower in hormone-treated cells. These experiments suggest that the decreased level of AChR in response to thyroid hormone treatment is due to a partial suppression of receptor synthesis.

Localization of a chemotactic domain in human thrombin

The cyanogen bromide fragment CB67-129 of human prethrombin 1, corresponding to residues 54-116 of the thrombin B chain, is a potent chemotaxin for human peripheral blood monocytes and the murine macrophage like cell line, J774. Both of these cell types have been shown to respond chemotactically to alpha-thrombin and iPr2P-alpha-thrombin. Effective concentrations for stimulating directed cell movement with the fragment vary from 10(-11) to 10(-7) M. Moreover, CB67-129 and its parent protein compete for the same chemotactic receptor site. Fragment CB67-129, representing residues 54-116 of the human thrombin B chain sequence, contains a nine-residue insertion ("loop B") that is absent in homologous sequences derived from the closely related proteases chymotrypsin and trypsin. Unlike iPr2P-alpha-thrombin, iPr2P derivatives of these latter enzymes possess little or no chemotactic activity, suggesting a relationship between the insertion sequence and thrombin chemotactic activity. The loop B sequence is unique insofar as it contains all of the carbohydrate moieties known to reside in alpha-thrombin. However, chemotactic activity is only minimally reduced subsequent to hydrolysis by both neuraminidase and beta-galactosidase, indicating that receptor recognition and stimulated cell movement are mainly a function of structure of the cyanogen bromide derived fragment rather than of asparagine-linked carbohydrates.

Receptor-mediated chemotactic response of a macrophage cell line (J774) to thrombin

Human alpha-thrombin, the procoagulant activation product of prothrombin, elicits chemotaxis in several macrophage-like continuous cell lines, most notably J774. Effective alpha-thrombin concentrations eliciting cell movement range from 10(-10) to 10(-6) M, with the optimal response occurring at about 10(-8) M. At the latter concentration, the response is equivalent, on a molar basis, to that observed with f-Met-Leu-Phe-OH, the positive control used in these experiments. Blockage of alpha-thrombin's active center with diisopropylfluorophosphate or by tryptic proteolysis of the procoagulant exosite (i.e., gamma-thrombin) does not decrease chemotactic activity. However, formation of enzymatically inactive complexes with AT3 or hirudin eliminates chemotactic potency. Competition chemotaxis assays, carried out by comparing the abilities of varying concentrations of test substances placed in the upper compartment of Boyden chambers to inhibit gradient-oriented movement of cells toward a fixed concentration of thrombin in the lower compartment, demonstrate that, although the modified forms of thrombin cross-compete, formylated peptide will not inhibit thrombin's chemotactic effects. Binding studies carried out using 125I-alpha-thrombin on paraformaldehyde-fixed J774 cells show a Kd of approximately 7.5 nM with an estimated 14,100-binding sites/cell. This Kd is in agreement with the optimal chemotactic thrombin dose (approximately 10 nM). On the basis of the competition chemotaxis assays and the radiolabeled thrombin-binding data, it is proposed that unique thrombin-specific chemotactic receptors exist on J774 cell membranes, and that these receptors are distinct from those mediating chemotaxis by agents such as formylated peptides.

Monocyte chemotaxis: stimulation by specific exosite region in thrombin

Human alpha-thrombin is a potent chemoattractant for human monocytes, with optimum activity occurring at about 10 nanomoles per liter. A variety of thrombins that were chemically modified to alter procoagulant or esterolytic functions showed a similar optimum activity, but complexes of prothrombin or alpha-thrombin with either antithrombin III or hirudin did not. These findings indicate that the regions in thrombin responsible for monocyte chemotaxis are proximate to those involved in certain protein recognition interactions of alpha-thrombin (for example, hirudin binding) but are distinct from the catalytic site and from certain exosites required for clotting.

Chemotactic response of monocytes to thrombin

Human alpha-thrombin, the procoagulant activation product of prothrombin, elicits chemotaxis in human peripheral blood monocytes and several macrophagelike continuous cell lines, most notably J-774.2, but not in human peripheral blood granulocytes. alpha-Thrombin is effective in stimulating cell movement at concentrations ranging from 10(-10) to 10(-6) M but is optimally active at 10(-8) M. At the latter concentration, the degree of response is equivalent, on a molar basis, to that observed with the peptide formylmethionylleucylphenylalanine, (FMP). In contrast to thrombin, prothrombin produces a minimal chemotactic response in monocytes and J-774.2. Blockade of alpha-thrombin's active center with diisopropylfluorophosphate (DIP-F) or tryptic proteolysis of the procoagulant exosite (i.e., gamma-thrombin) fails to alter chemotactic activity. On the other hand, addition of equimolar amounts of antithrombin III (AT3) to alpha-thrombin reduces thrombin-mediated chemotaxis by 60%, and increased ratios of AT3 to enzyme completely suppress chemotaxis. We conclude that thrombin is a potent monocyte chemotaxin and that the domains in thrombin involved in stimulating cell movement are distinct from the catalytic site and the fibrin recognition exosite. These chemotactic domains appear to be sequestered in prothrombin and in the thrombin-AT3 complex and, as such, are unavailable to the chemotactic receptor on the monocyte cell membrane.

Okazaki pieces grow opposite to the replication fork direction during simian virus 40 DNA replication

Simian virus 40 replicating DNA was pulse labeled with alpha-32P-dATP using an acellular DNA replication system. Nascent DNA chains of less than 200 nucleotides (Okazaki pieces) were then isolated from the denatured replicating DNA by electrosieving through a polyacrylamide gel column. The purified Okazaki pieces were hybridized to separated strands of Bg1(1)+Hpa1 simian virus 40 DNA restriction fragments immobilized on nitrocellulose filters. Only strands with polarity of the DNA replication fork direction hybridized with Okazaki pieces. Hence, Okazaki pieces in simian virus 40 are synthesized against the DNA replication fork direction.

The frequencies of transcription from the E- and L-strands of polyoma DNA

Exhaustion type hybridization was used to measure the amount of nuclear virus RNA complementary to the early (E) and late (L) polyoma virus DNA strands. At 36 h after infection between 2.5 and 7.3% of the newly synthesized virus RNA was complementary to the E-strand (-strand) and between 92.7 and 97.5% was complementary to the L-strand (+strand). This proportion was independent of the labelling time, indicating similar accumulation of the E- and L-RNA transcripts in the nucleus. The nuclear E- and L-RNA transcripts sedimented in a similar manner through sucrose gradients.