Chemotactic response of monocytes to thrombin

Inhibitory effect of heparin cofactor II on thrombin-stimulated prostacyclin production by cultured vascular smooth muscle cells

We investigated the effect of heparin cofactor II (HCII) on thrombin-induced prostacyclin (PGI2) production by A10 cells, an established cell line of vascular smooth muscle cells from murine aorta. Confluent monolayers of A10 cells were incubated with 0.1 NIH U/ml of thrombin for 30 min in the presence of antithrombin III (ATIII) or HCII, and PGI2 production by the cells was measured by radioimmunoassay as 6-keto-prostaglandin F1a, the stable metabolite of PGI2. ATIII at 40 mInh.U/ml and more significantly inhibited thrombin-induced PGI2 production by A10 cells, but HCII at the same doses did only slightly. However, when A10 cells were preincubated with HCII for 30 min before exposure to thrombin, the PGI2 production was markedly inhibited. The medium conditioned by A10 cells for 30 min did not enhance the inhibitory effect of HCII on thrombin-induced PGI2 production by the cells. On the other hand, A10 cells synthesized both dermatan sulfate and heparan sulfate which are capable of activating HCII. From these results, it was suggested that HCII would be activated by glycosaminoglycans (GAGs) such as dermatan sulfate of the cell layers and could inhibit thrombin-induced PGI2 production. HCII may be a modulator of thrombin on the physiological functions of vascular smooth muscle cells, reacting to the cell surface GAGs.

Thrombin signalling in U937 human monocytic cells is coupled to inositol phosphate formation but not to thromboxane B2 synthesis nor to inhibition of adenylate cyclase: distinct differences in thrombin signalling between U937 cells and platelets

The blood coagulation factor, human thrombin has been shown to have chemotactic and mitogenic effects on mononuclear phagocytic inflammatory cells. In the present study, we have used the U937 human monocytic cell line to explore the signal transduction mechanisms utilised by thrombin in these cells. In U937 cells differentiated into a macrophage-like phenotype, thrombin stimulated the formation of inositol trisphosphate (IP3) and the mobilisation of intracellular Ca2+ [( Ca2+]i) via a mechanism which was partially sensitive to pertussis toxin. Thrombin failed, however, to evoke thromboxane (Tx) B2 synthesis in the differentiated cells. In contrast, the chemotactic peptide N-formyl-L-methionylleucyl-L-phenylalanine (FMLP) stimulated TxB2 synthesis under conditions where it evoked increases in IP3 formation and [Ca2+]i mobilisation, via a pertussis toxin-sensitive mechanism, comparable in extent to those mediated by thrombin. Thrombin also failed to cause inhibitory guanine nucleotide binding protein (Gi)-mediated inhibition of adenylate cyclase activity in U937 cell membranes. These results indicate that U937 cells express receptors for thrombin which are in part coupled via a pertussis toxin-sensitive guanine nucleotide binding protein to phospholipase C activation, the formation of IP3 and the mobilisation of [Ca2+]i. However, the failure of thrombin to stimulate TxB2 synthesis or cause Gi-mediated inhibition of adenylate cyclase in U937 cells contrasts with its effects in human platelets and other thrombin-responsive cells. These results suggest that the thrombin receptor or receptor-effector coupling mechanism(s) in mononuclear cells is functionally distinct from the thrombin receptor or receptor-effector coupling mechanism(s) present in other thrombin-responsive cells.

The reaction of thrombin with platelet-derived nexin requires a secondary recognition site in addition to the catalytic site

A protease nexin released by activated platelets forms stable complexes with alpha-thrombin. Active-site-blocked thrombin does not form the stable complex, but it inhibits formation of the stable complex by active alpha-thrombin. gamma-Thrombin, which has a damaged substrate recognition site (the anion-binding exosite), did not form the complex and did not inhibit formation of the stable complex by alpha-thrombin. Complex formation was inhibited by the C-terminal dodecapeptide of hirudin, which has been shown to bind to the anion-binding exosite. A monoclonal antibody that blocks reactions of thrombin that involve the anion-binding exosite also inhibited formation of a stable complex of alpha-thrombin and the platelet-derived protease nexin. It is concluded that the anion-binding exosite of thrombin, a site that confers a high degree of specificity for substrates with a complementary site, binds to the platelet nexin prior to reaction of the catalytic site with the serpin.

Changes of DNA-acridine orange binding in monocytes and endothelial cells of hypertensive arteries

The interactions of monocytes and endothelial cells were examined employing acridine orange (AO) binding to chromatin as an electron microscopic probe for studying changes in nuclear activity. The number of AO labeled nuclei were determined in adherent and sub-endothelial monocytes, and endothelial cells either associated with or devoid of monocytes in mesenteric arteries of renal hypertensive and normotensive rats. No AO-positive cell nuceli were found in the few adherent monocytes in the normotensive rats while in hypertensive rats 63% of the adherent monocytes and 86% of the subendothelial monocytes were labeled by AO. The number of AO-positive cell nuclei increased in the endothelium from 2% in normotensive rats to 17% in hypertensive animals. Fifty-seven percent of these endothelial cells were associated with monocytes adhering to their surfaces. Thirty-nine (52%) of AO-positive and 24 (52%) of AO-negative cell pairs represented 84% of cell pairs with identical nuclear activity. These findings indicate a sequential interaction of monocytes and adjacent endothelial cells recognized at the nuclear level. According to other experimental work the binding capacity of chromatin for AO increases as cell nuclei are reactivated by stimulation of cell proliferation. In addition, AO labeling of chromatin visualized electron microscopically is confined to three of the four stages of the cell cycle. Therefore AO labeling in the present experiments might indicate the reentrance of monocytes and endothelial cells into the cell cycle.

Localization of the structural domain responsible for the chemotactic properties of thrombin on polymorphonuclear leukocytes

Human alpha thrombin at 1.1.10(-5) M is chemotactic for human polymorphonuclear leukocytes. This thrombin property disappears when the alpha thrombin (1.1.10(-5) M) hirudin (1.32.10(-5) M) mixture is realized. The same result is obtained when the thrombin at 1.1. 10(-5) M is inhibited by antithrombin III in a ratio of 1 mol of thrombin for 4.5 mol of antithrombin III. The hirudin and the antithrombin III appear therefore to mask, by their binding the structural domain responsible for the chemotactic properties of thrombin on polymorphonuclear leukocytes.

Coagulation impact on chemotactic activity generation for polymorphonuclear leukocytes

Chemotactic technique in agarose gel has exposed the attractive properties of human alpha thrombin with respect to human polymorphonuclear leukocytes. The observed chemotaxis is maximal between 1.4.10-5 M and 1.6.10-5 M but extends from 2.8.10-6 M to 2.2.10-5 M. Human prothrombin, in an identical concentration zone as that studied for thrombin shows no chemotactic activity on the polymorphonuclear leukocytes. During coagulation the formed alpha thrombin attracts the polymorphonuclear leukocytes to it's formation site.

Viral activation of the coagulation cascade: molecular interactions at the surface of infected endothelial cells

Herpesviral infection of endothelial cells (ECs) induces arterial injury. We now demonstrate that such infection promoted enhanced monocyte-endothelial adhesion. Enhanced adhesion was blocked by monoclonal antibodies to the viral-encoded cell surface glycoprotein gC but not by antibodies to gD or gE. Adhesion was also blocked by treating ECs with specific thrombin inhibitors or by growing cells in prothrombin-depleted serum. We found that gC bound and promoted activation of factor X on infected ECs, thereby contributing to thrombin generation. Factor X also bound to transfected L cells that were induced to express gC. Cross-linking and immunoprecipitation studies demonstrated factor X-gC complex formation on the surface of these cells. We suggest that gC-dependent thrombin generation by herpes-infected endothelium may be an important mediator of vascular pathology during viral infection.

A selective defect in tissue factor mRNA expression in monocytes from AIDS patients

Expression of tumor necrosis factor (TNF alpha), tissue factor (TF), and interleukin 1-beta (IL-1 beta) mRNA was evaluated in monocytes isolated from patients infected with human immunodeficiency virus (HIV). There was a significant depression (66%) of the induced level of TF mRNA expression in response to lipopolysaccharide. Conversely, the response of TNF alpha and IL-1 beta, following LPS induction, was "normal." TF mRNA reduction was also observed to a lesser degree in AIDS-related complex patients (20%) but not in asymptomatic seropositives. TF is necessary for initiation of the coagulation protease cascade, leading to thrombin production and fibrin deposition, which play a role in inflammatory responses. Its selective reduction may be a factor in the diminished resistance to secondary infections observed in AIDS. Further, since the TF defect increases as patients progress toward AIDS, it may serve as a marker for disease progression.

Interaction of thrombin with heparin cofactor II and antithrombin III on prostacyclin production by cultured endothelial cells

To investigate the physiologic function of heparin cofactor II (HCII), endothelial cells from human umbilical vein were incubated in vitro for 20 min with 0.5 NIH U/ml thrombin in the presence of HCII or antithrombin III (ATIII), and prostacyclin production determined by radioimmunoassay for 6-keto-prostaglandin F1 alpha, the stable metabolite of prostacyclin. Although ATIII at 20 mInh.U/ml slightly but significantly inhibited thrombin-induced prostacyclin production, neither unfractionated heparin (UFH) nor low molecular weight heparin (LMWH) at 1 U/ml accelerated the inhibitory effect of ATIII. HCII at 10 and 20 mInh.U/ml did not decrease thrombin stimulation of prostacyclin production in the presence or absence of UFH or LMWH. However, HCII caused a marked decrease in the thrombin-stimulated prostacyclin prostacyclin production in the presence of 2 mg/ml dermatan sulfate (DS). The significant inhibition by HCII occurred when the DS concentrations were 0.2 microgram/ml and higher. From these results we suggest that HCII may prevent a prostacyclin-induced inhibition of platelet aggregation for hemostasis when plasma is exposed to vascular smooth muscle cells or fibroblasts which synthesize a significant amount of DS.

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.

Thrombin and bradykinin enhance prostaglandin production in human peripheral blood monocytes

Thrombin and bradykinin stimulate production of prostaglandin E2 (PGE2), and 6-keto-prostaglandin F1 alpha (the stable breakdown product of prostacyclin) in isolated human peripheral blood monocytes in a dose- and time-dependent manner. Since PGE2 and prostacyclin can affect the activity of immunocompetent cells and bone resorbing osteoclasts, our finding indicates that thrombin and bradykinin, both of which are formed in inflammatory processes as a consequence of activation of the Hageman factor (coagulation factor XII), may have important roles in the modulation of the inflammatory response and the loss of alveolar bone in periodontitis.

Formation of factor Va by atherosclerotic rabbit aorta mediates factor Xa-catalyzed prothrombin activation

Vascular cell procoagulant activity may be important in the pathogenesis of atherosclerosis. In previous studies, we described the ability of the atherogenic metabolite homocysteine to activate endothelial cell Factor V, a key coagulation cofactor for thrombin generation. The present study was designed to investigate Factor V activity and Factor Xa-catalyzed prothrombin activation by control and atherosclerotic aorta from normal and hypercholesterolemic rabbits. Factor Xa generated ninefold more thrombin on atherosclerotic aortic segments than on control segments. Atherosclerotic segments activated 125I-prothrombin with Factor Xa in the presence of the thrombin inhibitor dansyl arginine-4-ethylpiperidine amide and cleaved 125I-Factor V. This suggests that increases in vessel-wall Factor V activity and Factor Xa-catalyzed prothrombin activation result from activation of vessel-wall Factor V. 125I-Factor Va peptides generated by atherosclerotic aorta were very similar in molecular weight to those generated by homocysteine-treated cells. When vascular endothelium was mechanically removed by brushing, atherosclerotic vessels still generated four- to fivefold more thrombin than control vessels. These data and results from immunocytochemical studies suggest that Factor V in atherosclerotic vessels is associated with both endothelium and other cells of the lesion. In contrast, Factor V in control vessels is associated primarily with endothelium. The increases in Factor V activity and thrombin formation in the blood vessel wall of hypercholesterolemic rabbits may contribute to the development of atherosclerosis and its complications.

Visualization of monocyte recruitment into atherosclerotic arteries using fluorescent labelling

It has been shown that monocytes are present in early atherosclerotic lesions and in mechanically-injured arterial intima, but direct morphological tracing of specific leukocyte populations into such areas has been lacking. A method for FITC-labelling of leukocytes was therefore evaluated for monocyte studies. Monocyte (95% pure) populations were isolated from blood by counterflow centrifugation and labelled by incubation with free fluorescein isothiocyanate 1-hydrochloride (FITC) in Hank's balanced salt solution. FITC-labelled monocytes showed glass adherence, spreading and migration, as well as acid phosphatase positivity and phagocytosis for up to 20 days in tissue culture. For in vivo experiments, hypercholesterolemic (H) and normal (N) swine were bled repeatedly, and monocyte populations were isolated, labelled and reinjected. Labelled cells were found in blood samples. Animals were killed after 9 days, and formaldehyde-fixed and frozen samples of aortae were studied en face and/or sectioned and examined microscopically under fluorescence. FITC-labelled leukocytes could be found adherent to sites of thickened intima but not to normal areas. Labelled cells were also detected within atherosclerotic lesions. These results show the feasibility of the labelling technique and provide direct visualization of monocyte recruitment from the blood into atherosclerotic and lesion-prone areas.

Inhibition of bone resorption in vitro by serine-esterase inhibitors

The effect of two synthetic serine esterase inhibitors, N-alpha-dansyl(p-guanidino)phenylalaninepiperidine hydrochloride (I 2581) and D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone (D-Phe-Pro-Arg-CH2Cl), on bone resorption in organ cultured mouse calvaria from neonatal mice has been examined. Mineral mobilization was assessed by analyzing the release of 45Ca, stable calcium (Ca2+) and inorganic phosphate (Pi). Organic matrix degradation was studied by analyzing the release of 3H from [3H]proline-labelled bones, and by quantifying the amounts of hydroxyproline in bone after culture. It was found that I 2581, at and above 30 mumol/l, dose-dependently inhibited 45Ca release induced by thrombin, parathyroid hormone (PTH), prostaglandin E2 and 1-alpha-hydroxyvitamin D-3. I 2581 (50 mumol/l) inhibited PTH-stimulated release of 3H from [3H]proline-labelled bones, and this effect was reversible after withdrawal of I 2581. I 2581 (50 mumol/l) inhibited the release of Ca2+, Pi, beta-glucuronidase and beta-N-acetylglucosaminidase in bones stimulated by PTH and 1-alpha-hydroxyvitamin D-3, without affecting the release of lactate dehydrogenase. In parallel, I 2581 decreased PTH and 1-alpha-hydroxyvitamin D-3 induced reduction of hydroxyproline levels in bones after culture. I 2581 (50 mumol/l) did not affect the basal release of 45Ca, Ca2+, beta-glucuronidase and beta-N-acetylglucosaminidase, nor the basal amounts of hydroxyproline in bones after culture. D-Phe-Pro-Arg-CH2Cl (100 mumol/l) significantly inhibited PTH- and PGE2-induced release of 45Ca without affecting basal release of radioactive calcium. These data indicate that activation of serine proteinase(s) may be a necessary step in the mechanism of action of several stimulators of bone resorption.

Monoclonal antibody to the thrombin receptor stimulates DNA synthesis in combination with gamma-thrombin or phorbol myristate acetate

Studies with various thrombin derivatives have shown that initiation of cell proliferation by thrombin requires two separate types of signals: one, generated by high affinity interaction of thrombin or DIP-thrombin (alpha-thrombin inactivated at ser 205 of the B chain by diisopropylphosphofluoridate) with receptors and the other, by thrombin's enzymic activity. To further study the role of high affinity thrombin receptors in initiation, we immunized mice with whole human fibroblasts and selected antibodies that blocked the binding of 125I-thrombin to high affinity receptors on hamster fibroblasts. One of these antibodies, TR-9, inhibits from 80 to 100% of 125I-thrombin binding, exhibits an immunofluorescent pattern indistinguishable from that of thrombin bound to receptors on these cells, and selectively binds solubilized thrombin receptors. By itself, TR-9 did not initiate DNA synthesis nor did it block thrombin initiation, but TR-9 addition to cells in the presence of alpha-thrombin, gamma-thrombin (0.5 microgram/ml), or PMA stimulated thymidine incorporation up to threefold over controls. In all cases, maximal stimulation was observed at concentrations of TR-9, ranging from 1 to 4 nM corresponding to concentrations required to inhibit from 30 to 100% of 125I-thrombin binding. These results demonstrate that the binding of the monoclonal antibody to the alpha-thrombin receptor can mimic the effects of thrombin's high affinity interaction with this receptor in stimulating cell proliferation.

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.

Coagulation and the expression of cell-mediated immunity

Induction of monocyte/macrophage procoagulants may occur as the result of activation of the cell-mediated immune (CMI) response. Macrophage procoagulant inducing factor (MPIF), a soluble product of stimulated TDTH lymphocytes, may act together with two monokines, interleukin 1 and tumour necrosis factor alpha, which induce thromboplastin on endothelial cells, to initiate the fibrin deposition which is a common feature of many diseases in which CMI plays a role. Murine MPIF is chemically distinct from a number of other well characterized lymphokines in that the two major activities, MPIF alpha and MPIF beta are heparin-binding proteins with high isoelectric points. Fractions highly enriched for MPIF induced interstitial fibrin deposition when injected intradermally. In addition, intense infiltration of polymorphonuclear leucocytes (PWN) and mononuclear cells was seen 4-24 h following intradermal injection. In vitro experiments have confirmed that this lymphokine is a potent chemotactic agent for these cells. These results suggest that MPIF plays a central role in the expression of histopathological features of delayed-type hypersensitivity reactions. Monocyte and macrophage procoagulants induced by MPIF would contribute significantly to the activation of coagulation which not only results in fibrin deposition but also in the production of activated serine proteases and fibrinopeptides which may potentiate an inflammatory response.

Monocyte chemoattractants in pigeon aortic atherosclerosis

Atherosclerosis occurs in the aorta of White Carneau pigeons proximal to the celiac bifurcation, where monocyte adhesion and migration into lesions have been demonstrated. This study documents chemoattractants that might be responsible for monocyte adherence and migration. Ten-week-old pigeons were fed either a cholesterol-free (normal) diet or a 0.4% cholesterol diet for 12 or 24 weeks. Birds with a normal diet did not have lesions in the lesion-prone area of the aorta, whereas birds fed a cholesterol-containing diet had simple intimal foam-cell lesions (12 weeks) or foam-cell lesions complicated with extracellular lipid and fibrillar matrix material (24 weeks). Plasma cholesterol levels in birds on the cholesterol-containing diet were 780-1080 mg/dl versus 140-240 mg/dl in the normal diet control group(s) at necropsy. To assay for chemoattractants, tissue was collected from lesion-prone and nonsusceptible (nonlesion) areas of the aortas. Samples from the two types of regions were separately pooled, then homogenized and tested for chemoattractant activity for pigeon peripheral blood monocytes. Monocyte chemoattractants were demonstrated in lesion area homogenates from pigeons fed cholesterol for 12 or 24 weeks and also in analogous homogenates from pigeons fed a normal diet. Monocyte migration to lesion-prone homogenates was significantly greater than that to nonlesion area homogenates. The chemoattractants in homogenates were monocyte-specific. The chemoattractant activity in the birds fed cholesterol for 12 weeks was confined to the aqueous phase of lipid extracts. This activity was abolished by pronase but unaffected by heat (100 C, 30 minutes), which indicated that the chemoattractant(s) in these homogenates was heat-stable protein(s). Activity in lipid extracts of lesion area homogenates from birds fed a cholesterol-containing diet for 24 weeks was found in both the aqueous and organic phases, suggesting that these samples contained lipid as well as proteinaceous chemoattractants.

Failure of guinea pig plasma kallikrein to cause neutrophil migration

Guinea pig plasma kallikrein was apparently chemotactic for guinea pig neutrophils when assayed in Boyden's chamber. However, it was concluded that this phenomenon was artificial due to the following reasons: Kallikrein was only chemotactic in the presence of bovine serum albumin (BSA) in the chamber but not when BSA was substituted by guinea pig serum albumin or egg albumin. Kallikrein injection failed to cause dermal tissue leukocytosis. Kallikrein did not polarize neutrophils. A chemoattractant(s) seemed to be generated from BSA preparation time dependently. Although this generation required the enzymatic activity of kallikrein, the chemotactic activity once generated was resistant to a kallikrein inhibitor (diisopropylfluorophosphate) and antiprekallikrein antibody.

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.

Thrombin stimulates neutrophil adherence by an endothelial cell-dependent mechanism: characterization of the response and relationship to platelet-activating factor synthesis

Thrombin, a serine coagulation protease that is generated at sites of tissue injury and inflammation, stimulates the adherence of PMNs and neutrophils to EC. We found that thrombin enhanced the adhesion of neutrophils to primary monolayers of human umbilical vein EC when assayed by the binding of 111Indium-labeled PMNs to the EC, the recovery of unlabeled PMNs after incubation with thrombin-treated EC, and by phase contrast and scanning electron microscopy (SEM). SEM demonstrated that thrombin caused PMNs to intimately adhere to the EC plasma membrane and under some conditions to become polarized. The thrombin-stimulated adherence was a rapid, time-dependent response with an onset within 1 minute of addition of thrombin, a peak at 5-10 minutes, and a decline thereafter. The response was concentration-dependent over the range 0.01-2 U/ml thrombin, and required active thrombin. Prothrombin, factor Xa, and fibrinogen were not effective. Thrombin-stimulated PMN adherence was dependent on the EC, because thrombin did not significantly stimulate neutrophils to adhere to albumin-coated petri dishes, subendothelial matrices, or primary cultures of smooth muscle cells. Human EC, when treated with thrombin, also produce platelet-activating factor (PAF; 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine). The concentration-response relationships and time courses for thrombin-stimulated PMN adherence and PAF production were tightly correlated. Furthermore, PAF itself stimulated the adherence of PMNs to EC, and pretreatment of PMNs with PAF selectively inhibited their adherence response to thrombin. These findings demonstrate two novel biologic activities of thrombin, the stimulation of EC-dependent adherence of PMNs and the production of PAF by EC, and suggest that they are functionally related. In addition, they suggest that thrombin may act as a plasma-derived humoral mediator of inflammation under some conditions.

Thrombin-induced chemotaxis and aggregation of neutrophils

Thrombin-induced neutrophil chemotaxis and aggregation were studied using cells isolated from either human or sheep blood. Sheep neutrophils (10(8) cells/ml) exhibited maximum chemotactic migration towards 10(-8)M human alpha-thrombin, 10(-8)M gamma-thrombin (which lacks the fibrinogen site), and 10(-12)MD-Phe-Pro-Arg-CH2-alpha-thrombin (catalytically inactive thrombin). Chemotactic responses of the same magnitude were obtained with human neutrophils (10(8) cells/ml). The chemotactic responses to thrombin were comparable to those obtained with diluted (1:200 v/v) zymosan activated serum (ZAS) and 10(-11)M FMLP. Premixing of the thrombin forms with hirudin in 1:1 stoichiometric amounts abolished the chemotaxis but not chemokinesis Aggregatory responses of human and sheep neutrophils were comparable for ZAS, alpha-thrombin, and gamma-thrombin. The responses of both human and sheep neutrophils to D-Phe-Pro-Arg-CH2-alpha-thrombin were attenuated, indicating that the proteolytic site may be involved in the aggregatory response. The results suggest that thrombin-induced neutrophil chemotaxis and aggregation are mediated by different mechanisms, since chemotaxis is a catalytically independent response whereas aggregation is an active site independent response.

Effects of fibrinogen derivatives upon the inflammatory response. Studies with human fibrinopeptide B

Fibrin formation and turnover are intimately associated with inflammation and wound healing. To explore whether fibrin(ogen)-derived peptides exert direct effects upon cells involved in inflammation and tissue repair we examined the capacity of human fibrinopeptide B (hFpB), a thrombin-derived proteolytic cleavage product of the fibrinogen B beta-chain, to stimulate neutrophils (PMN), monocytes, and fibroblasts. hFpB caused directed cell migration of PMN and fibroblasts that was optimal at approximately 10(-8) M. This chemotactic activity was blocked by preincubating hFpB with antiserum to hFpB. hFpB was not chemotactic for monocytes. The chemotactic potency of hFpB for PMN was equivalent to that of anaphylatoxin from the fifth component of human complement (C5a), leukotriene B4 (LTB4), and formyl-methionyl-leucyl-phenylalanine (fMLP), and for fibroblasts its chemotactic activity was comparable to that of platelet-derived growth factor. hFpB did not interact with PMN receptors for C5a, LTB4, or fMLP as (a) desensitization with 10(-7) M hFpB abolished chemotaxis to hFpB but had no effect upon chemotaxis to C5a, LTB4, or fMLP and (b) induction of chemotactic responses to fMLP and LTB4 in neutrophilic leukemic cells (HL-60 cells) by incubation with dimethylsulfoxide did not extend to hFpB. Like fMLP, hFpB caused a rapid, dose-dependent increase in PMN cytoskeletal associated actin, but unlike fMLP, hFpB did not cause PMN aggregation, release of lysosomal enzymes (lysozyme and beta-glucuronidase), or the production of superoxide anion. These results suggest that hFpB may have a role in recruiting PMN and fibroblasts at sites of fibrin deposition and turnover. The capacity of hFpB to cause PMN chemotaxis without causing concurrent release of lysosomal enzymes or the production of superoxide anion is further evidence for the complexity of PMN responses to chemotactic agents.

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.

Specific association of thrombin-antithrombin complexes with a human hepatoma cell line

The clearance of thrombin-antithrombin (TAT) complexes from blood by the liver is through a receptor mediated pathway. We have used the established human hepatoma cell line, Hep G2, to determine if these hepatocytes have the capacity to bind this enzyme-inhibitor complex. The TAT complex was bound to the cells in a time and temperature dependent manner, reaching an apparent steady state at 90 minutes at both 4 and 37 degrees C. Binding at 4 degrees C was 5-7-fold less extensive than at 37 degrees C. The bound TAT was structurally similar to the added ligand. This interaction was specific, as it was inhibited by nonlabeled TAT but not by 50-fold molar excesses of unrelated proteins or by the individual constituents, thrombin or antithrombin. Factor Xa- antithrombin complex inhibited the binding reactions slightly. Specific binding isotherms at 37 degrees C were subjected to Scatchard plots. The apparent dissociation constant was 247 +/- 74 nM, and the number of TAT molecules bound per cell was 5.19 +/- 0.89 X 10(5). Bound TAT complexes did not undergo degradation at 4 or 37 degrees C for up to 2.5 hr, as greater than 85% of the bound ligand was acid precipitable during the time course of binding. Internalization of the TAT complex was compared with transferrin, a molecule known to be internalized by Hep G2 cells, by resistance of the cell-bound ligands to degradation by trypsin or pronase. In contrast to transferrin, most of the TAT complexes remained cell-surface associated for at least 2 hr at both 4 degrees and 37 degrees C, indicating that TAT was not substantially internalized by the Hep G2 cells.

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.

Thrombin stimulates the adherence of neutrophils to human endothelial cells in vitro

Highly purified human thrombin stimulates the adherence of polymorphonuclear leukocytes (PMNs) to vascular endothelial cells (EC). When Indium-labeled PMNs were incubated with primary monolayers of cultured human umbilical vein EC, the basal adherence was 10 +/- 1% of the PMNs at 5 min. Addition of thrombin (2 U/ml) increased the mean adherence to 42 +/- 15%. Enhanced neutrophil adherence in response to thrombin was confirmed by experiments with unlabeled leukocytes, examined by phase contrast and scanning electron microscopy. The action of thrombin was on the EC, since it did not directly stimulate PMN adhesiveness when measured by aggregation or by adherence to nylon fiber columns. Furthermore, enhanced neutrophil adherence occurred when endothelial monolayers were treated with thrombin and washed before adding 111Indium (111In)-labeled PMNs. Thrombin that had been inactivated with antithrombin III and heparin did not enhance neutrophil adherence. Prothrombin, Factor Xa, and fibrinogen were also ineffective. The stimulated adherence of PMNs was maximal 5 min after incubation of the EC with thrombin, and decreased thereafter. The response was dose-dependent, with half-maximal stimulation at 0.2-0.25 U thrombin/ml. The enhanced PMN adherence caused by thrombin may result in part from the production of platelet-activating factor (PAF) by the stimulated EC since thrombin-stimulated EC synthesize PAF with a time course and concentration dependence that are similar to the time and concentration relationships for thrombin-stimulated PMN adherence, PAF itself promoted neutrophil adherence to the EC monolayers, and pretreatment of PMNs with PAF decreased the adherence stimulated by thrombin and PAF, but not adherence stimulated by N-formylmethionyl-leucyl-phenylalanine and C5a fragments, which indicates specific desensitization of PAF-mediated adherence. These studies demonstrate the endothelial cell-dependent stimulation of PMN adherence by thrombin, a novel mechanism of enhanced leukocyte adherence that may be important in interactions between the coagulation and inflammatory systems.

Cutaneous tissue repair: basic biologic considerations. I

Wound repair of the integument is reviewed in the context of new developments in cell biology and biochemistry. Injury of the skin and concomitant blood vessel disruption lead to extravasation of blood constituents, followed by platelet aggregation and blood clotting. These events initiate inflammation and set the stage for repair processes. The macrophage plays a pivotal role in the transition between wound inflammation and repair (granulation tissue formation), since this cell both scavenges tissue debris and releases a plethora of biologically active substances that include growth factors. Although concrete evidence is lacking, growth factors are probably at least partially responsible for the angiogenesis and fibroplasia (granulation tissue) that gradually fill the wound void. If the epidermal barrier is disrupted during injury, reepithelialization begins within 24 hours and proceeds first over the margin of residual dermis and subsequently over granulation tissue. The signals for angiogenesis, fibroplasia, neomatrix formation, and reepithelialization in wound repair are not known, but a number of possibilities are discussed. Matrix remodeling is the last stage of wound repair and gradually increases the scar tensile strength to 70% to 80% of normal skin.

Endothelial cell-derived chemotactic activity for mouse peritoneal macrophages and the effects of modified forms of low density lipoprotein

Cultured rabbit and bovine aortic endothelial cells generated chemotactic activity for mouse resident peritoneal macrophages, demonstrable in the conditioned medium. This chemotactic activity was heat stable and was not extracted into chloroform/methanol. It was inhibited by addition of endothelial cell-modified low density lipoprotein (EC-modified LDL), a form of LDL shown previously to contain peroxidized lipids, increased lysophosphatidylcholine, and partially degraded apoprotein B. The chemotactic activity was also inhibited by LDL previously oxidized in the absence of cells with 5 microM Cu2+. Inhibitory activity was present in the lipid extract of EC-modified LDL but not in that of native LDL, presumably representing peroxidized lipid components. EC-modified LDL also inhibited the chemotactic activity of zymosan-activated serum. Because EC-modified LDL is taken up in part by way of the acetyl-LDL receptor, the effects of acetyl-LDL were tested. Rather than inhibiting chemotaxis, acetyl LDL showed intrinsic positive chemotactic activity as did also fucoidin and polyinosinic acid, both of which also interact with the acetyl-LDL receptor. These studies suggest mechanisms by which macrophages may be recruited into the subendothelial space by endothelial cell-derived chemotactic factors or by natural polyanions structurally related to fucoidin or polyinosinic acid and then become "trapped" there because of the inhibitory effects of peroxidized lipid components in modified forms of LDL.

Fibrin: mediator of in vivo and in vitro injury and inflammation

We examined the role that fibrin deposition and fibrin-associated factors (FAF) play in acute anterior segment inflammatory responses in the rabbit eye. It was demonstrated by immunofluorescence that fibrin represented a major component of the exudative meshwork deposited within the anterior chamber and on leukocyte surfaces therein. Using our in vivo model of endocular inflammation we next demonstrated that fibrin and fibrinogen-derived peptides, but not thrombin, induced inflammatory responses characterized by both leukocyte influx and endothelial cell injury. Fibrin formation within the anterior chamber induced a leukocyte influx consisting primarily of PMN's. Fibrinogen-derived peptides induced primarily a monocyte influx. This dichotomy suggests that multiple inflammatory mediators are elaborated or released during endocular fibrinogenesis and fibrinolysis. To investigate direct effects of fibrin deposition on the corneal endothelial cells (CEC) an in vitro "corneal cup" organ culture model was next developed. Studies comparing various types of mediators demonstrated that only fibrin- derived preparations directly induced CEC injury. Fibrin deposition may thus play multiple roles in endocular inflammation, including the modulation of leukocyte influx, and the direct mediation of corneal endothelial cell injury.

Nerve growth factor. A structural relationship between its proteolytic and leukocyte-chemotactic active sites

High molecular weight mouse nerve growth factor (HMW-NGF), in addition to its effects on certain neural elements, is also chemotactic for human polymorphonuclear leukocytes. One of the subunits of HMW-NGF is a protease of the serine family and its active site contains a serine residue and a closely-neighboring histidine residue that are both essential for proteolysis. Elimination of enzyme activity by irreversibly blocking the single serine has no effect on leukotaxis, but blocking the histidine abolishes leukotaxis. These results suggest the possibility that part of the proteolytic active site of this enzyme may have evolved to perform more than one, completely different, biologic function-proteolysis as well as nonproteolytically mediated chemotaxis.

Locomotion and chemotaxis of mononuclear phagocytes

The capacity for locomotion and for chemotaxis is probably very different in monocytes and macrophages from different sources. Numerous techniques have been established for studying the locomotion of these cells. Many of the factors are sparsely documented and the reports are scattered among various cell types. Heterogeneity of locomotion and chemotactic responsiveness is evident when established macrophage lines and mouse peritoneal macrophage are studied. The effects of mononuclear phagocytes and their released products on the locomotion of other cell types are reviewed.

Clotting factors secreted by monocytes and macrophages: analytical considerations

The secretion of clotting factors by rat spleen macrophages and human peripheral blood monocytes has been studied. The results show that the amount of clotting factors measured depends critically upon the characteristics of the assay system used. The presence of warfarin, salicylic acid or thrombin in the culture medium is shown to decrease the vitamin K dependent clotting factor activity in the supernatant after in vitro culture of rat spleen macrophages and human peripheral blood monocytes.

A cellular binding site for the Mr 55,000 form of the human plasminogen activator, urokinase

The secretion of plasminogen activators has been implicated in the controlled extracellular proteolysis that accompanies cell migration and tissue remodeling. We found that the human plasminogen activator urokinase (Uk) (Mr 55,000 form) binds rapidly, specifically, and with high affinity to fresh human blood monocytes and to cells of the monocyte line U937. Upon binding Mr 55,000 Uk was observed to confer high plasminogen activator activity to the cells. Binding of the enzyme did not require a functional catalytic site (located on the B chain of the protein) but did require the noncatalytic A chain of Mr 55,000 Uk, since Mr 33,000 Uk did not bind. These results demonstrate the presence of a membrane receptor for Uk on monocytes and show a hitherto unknown function for the A chain of Uk: binding of secreted enzyme to its receptor results in Uk acting as a membrane protease. This localizes plasminogen activation near the cell surface, an optimal site to facilitate cell migration.

Initial characterization of a peripheral blood mononuclear cell chemoattractant derived from cultured arterial smooth muscle cells

A mononuclear cell chemoattractant of high specific activity produced by baboon (Papio cynocephalus) aortic medial smooth-muscle cells (SMCs) in culture has been partially characterized. Smooth-muscle cells, between the third and eighth passage, were grown to confluence in Medium 199 containing 10% fetal calf serum and then incubated for 24 hours in either serumless medium (Neuman and Tytell) or Medium 199 containing 0.2% bovine serum albumin. The 24-hour SMC-conditioned medium was fractionated on Sephadex G100-Superfine and potent chemoattractant activity (SMC-CF) eluted in the 10,000-12,000 dalton region. SMC-CF displayed chemotactic and chemokinetic activity for peripheral blood mononuclear cells but not for polymorphonuclear leukocytes. Production of SMC-CF by the cells was significantly inhibited in the presence of cycloheximide, and its activity was abolished after incubation with the bacterial protease subtilisin. Chromatofocusing experiments indicate that SMC-CF is a cationic protein with a pI of greater than 10.5. The role of SMC-CF may play as an inflammatory mediator in monocyte recruitment to the arterial intima in atherogenesis is discussed.

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.

Structure-function relationships in human alpha- and gamma-thrombins

Human pro-coagulant alpha-thrombin may be proteolyzed under controlled conditions to the non-coagulant beta- and gamma-thrombin forms. These derivative forms nonetheless retain esterase and amidase activities with small substrates as well as several other thrombin functions. Structurally, human gamma-thrombin consists of three non-covalently associated fragments which retain structural integrity as measured by several spectroscopic criteria as well as enzymatic function. The protein folding characteristics of three-chain gamma-thrombin indicate that each fragment (domain) contains sufficient information to result in a correct renaturation of protein conformation. Those subtle structural differences which distinguish gamma- from alpha-thrombin are most likely the obstructions to fibrinogen binding which account for the loss of clotting activity.