Neuritogenesis and the nerve growth factor-induced differentiation of PC-12 cells requires annexin II-mediated plasmin generation

One of the key morphological changes associated with the nerve growth factor (NGF)-induced differentiation of rat adrenal pheochromocytoma (PC-12) cells is the growth of axon-like processes called neurites. A growing body of evidence suggests that this process may be dependent upon plasmin, a serine protease generated from plasminogen (Plg) by either urokinase Plg activator (u-PA) or tissue Plg activator (t-PA). Prior work in our laboratory has identified annexin II (Ann-II) as a co-receptor for Plg and t-PA that promotes and localizes plasmin generation near the cell surface. In the present study, we report a 3-9-fold increase in Ann-II protein and message levels in NGF-treated PC-12 cells. Message stability and nuclear run-on assays suggest that this induction occurs at the level of gene transcription. Neurite outgrowth assays on and within a three-dimensional matrix demonstrate the inhibition of NGF-induced PC-12 cell differentiation by polyclonal and monoclonal antibodies directed against Ann-II as well as by the overexpression of antisense Ann-II mRNA. Neuritogenesis is also impaired by alpha(2)-plasmin inhibitor, antibodies directed against t-PA and u-PA, and epsilon-aminocaproic acid, a lysine analog that inhibits Plg activation and the binding of Plg to Ann-II. Plasmin generation assays reveal a 2-fold increase in plasmin production on NGF-treated PC-12 cells, which can be blocked by a polyclonal antibody directed against the tail region of Ann-II. From these data, we conclude that Ann-II is transcriptionally up-regulated by NGF and that Ann-II-mediated plasmin generation may play an important role during neurite development in the differentiating PC-12 cell.

Recombinant annexin II modulates impaired fibrinolytic activity in vitro and in rat carotid artery

Fibrinolytic activity has been reported to be decreased in atherosclerosis. Recently, annexin II was identified as a coreceptor on endothelial cells for plasminogen and tissue plasminogen activator. In this study, we examined whether recombinant annexin II (rAN II) protein can modulate fibrinolytic activity on vascular endothelium in vitro and in vivo. The effect of rAN II on human umbilical vein endothelial cells (HUVECs) was measured. Addition of a fluorescent plasmin substrate revealed that HUVECs treated with rAN II exhibited significantly more plasmin generation than those treated with BSA. Moreover, rAN II treatment of HUVECs restored plasmin generation impaired by plasminogen activator inhibitor-1 or homocysteine pretreatment. In a rat carotid artery thrombus model, the patency of thrombosed carotid arteries was significantly enhanced by rAN II injection, in contrast to BSA injection, without systemic blood coagulation dysregulation. We found that rAN II enhanced plasmin generation on vascular endothelium in vitro and reduced thrombus formation in vivo, and concluded that enhancement of endothelial fibrinolytic activity by annexin II could modulate the hypercoagulable state of atherosclerosis. Further study of rAN II in vitro and in vivo may lead to the establishment of novel therapeutic approaches to thrombogenic vascular disease.

A mediator of cell surface-specific plasmin generation

It has become increasingly evident that the generation of cell surface proteases including plasmin is fundamental to a wide variety of in vivo biological processes. Cell surface receptors allow for specific controlled proteolysis, provide protection from inhibitors, and enhance catalytic efficiency. Here we describe one such receptor, annexin II, which serves as a coreceptor for tissue plasminogen activator and plasminogen and is found on a wide variety of cell types including endothelial cells, some tumor cells, monocytes and macrophages, and neuronal cells. Evidence indicates that annexin II may be crucial to the efficient generation of cell surface plasmin, endothelial cell formation of new blood vessels, and maintenance of vascular patency. Additionally, it has been shown that annexin II expression in acute promyelocytic leukemia contributes to the bleeding diathesis seen in this disease and that inhibition of annexin II may be an important mechanism in the formation of atherosclerotic plaque. Furthermore, emerging evidence reveals the importance of annexin II on the surface of monocytes and macrophages, where it may contribute to the cells' ability to degrade extracellular matrix proteins and migrate to sites of injury or inflammation.

Potential role of recombinant annexin II in diabetic vascular injury

Hyperinsulinemia and hyperglycemia have been associated with vascular injury such as atherosclerosis in diabetes mellitus. Recently, annexin II, a member of annexin family proteins, has been found to work as co-receptor on endothelial cells for plasminogen and tissue plasminogen activator, facilitating plasmin generation on the surface of vascular endothelium. In this review, we overviewed the effect of glucose and insulin on plasmin generation in endothelial cells and its potential modulation by recombinant annexin II (rAN II) based on our data.

Impaired recruitment of bone-marrow-derived endothelial and hematopoietic precursor cells blocks tumor angiogenesis and growth

The role of bone marrow (BM)-derived precursor cells in tumor angiogenesis is not known. We demonstrate here that tumor angiogenesis is associated with recruitment of hematopoietic and circulating endothelial precursor cells (CEPs). We used the angiogenic defective, tumor resistant Id-mutant mice to show that transplantation of wild-type BM or vascular endothelial growth factor (VEGF)-mobilized stem cells restore tumor angiogenesis and growth. We detected donor-derived CEPs throughout the neovessels of tumors and Matrigel-plugs in an Id1+/-Id3-/- host, which were associated with VEGF-receptor-1-positive (VEGFR1+) myeloid cells. The angiogenic defect in Id-mutant mice was due to impaired VEGF-driven mobilization of VEGFR2+ CEPs and impaired proliferation and incorporation of VEGFR1+ cells. Although targeting of either VEGFR1 or VEGFR2 alone partially blocks the growth of tumors, inhibition of both VEGFR1 and VEGFR2 was necessary to completely ablate tumor growth. These data demonstrate that recruitment of VEGF-responsive BM-derived precursors is necessary and sufficient for tumor angiogenesis and suggest new clinical strategies to block tumor growth.

Recombinant angiostatin prevents retinal neovascularization in a murine proliferative retinopathy model

Retinal neovascularization is central to the pathogenesis of proliferative diabetic retinopathy, the leading cause of blindness among the middle-aged population. Angiostatin, a proteolytic fragment of plasminogen is one of the most promising inhibitors of angiogenesis currently in clinical trials. Here we show that recombinant angiostatin can inhibit retinal neovascularization in a mouse model of proliferative retinopathy. Because proliferative diabetic retinopathy is a recurrent disease, effective therapy will need to be sustained. Recombinant adeno-associated viruses permit long-term expression of transfected genes; however, they can only accommodate a small insert sequence. Thus, we engineered and tested a shortened recombinant angiostatin derivative containing a signal sequence to permit secretion. Recombinant protein was purified from the medium of transfected HEK293 cells and injected subcutaneously into treated animals. The retinal vasculature was analyzed in retinal flat mounts and using immunohistochemically stained sections. Both methods demonstrate that this short, secreted form of angiostatin is effective in reducing the development of blood vessels in a nontumor environment and has therapeutic potential for neovascular retinopathies such as diabetic retinopathy, retinopathy of prematurity, retinal vein occlusion and, possibly, age-related macular degeneration.

Plasminogen-mediated matrix invasion and degradation by macrophages is dependent on surface expression of annexin II

Genetic evidence demonstrates the importance of plasminogen activation in the migration of macrophages to sites of injury and inflammation, their removal of necrotic debris, and their clearance of fibrin. These studies identified the plasminogen binding protein annexin II on the surface of macrophages and determined its role in their ability to degrade and migrate through extracellular matrices. Calcium-dependent binding of annexin II to RAW264.7 macrophages was shown using flow cytometry and Western blot analysis of EGTA eluates. Ligand blots demonstrated that annexin II comigrates with one of several proteins in lysates and membranes derived from RAW264.7 macrophages that bind plasminogen. Preincubation of RAW264.7 macrophages with monoclonal anti-annexin II IgG inhibited (35%) their binding of 125I-Lys-plasminogen. Likewise, plasmin binding to human monocyte-derived macrophages and THP-1 monocytes was inhibited (50% and 35%, respectively) when cells were preincubated with anti-annexin II IgG. Inhibition of plasminogen binding to annexin II on RAW264.7 macrophages significantly impaired their ability to activate plasminogen and degrade [3H]-glucosamine-labeled extracellular matrices. The migration of THP-1 monocytes through a porous membrane, in response to monocyte chemotactic protein-1, was blocked when the membranes were coated with extracellular matrix. The addition of plasminogen to the monocytes restored their ability to migrate through the matrix-coated membrane. Preincubation of THP-1 monocytes with anti-annexin II IgG inhibited (60%) their plasminogen-dependent chemotaxis through the extracellular matrix. These studies identify annexin II as a plasminogen binding site on macrophages and indicate an important role for annexin II in their invasive and degradative phenotype.

New concepts in fibrinolysis and angiogenesis

Angiogenesis, the process by which new blood vessels form from preexisting vasculature, underlies a number of biologic processes including embryologic development, inflammation, wound healing, hypoxic retinal vascular proliferation, tumor growth, and atherosclerosis. The fibrinolytic system represents a cascade of serine protease activation events that culminate in the generation of plasmin. Although in-vitro studies suggest several possible roles that plasmin might play in angiogenesis, angiogenesis and fibrinolytic activity do not always correlate in in-vivo systems. During cutaneous and corneal wound healing, for example, angiogenesis proceeds normally in plasminogen-deficient animals. Similarly, the growth of most neoplasms is unimpaired in the absence of plasminogen. On the other hand, hypoxia-driven vascular proliferation may require plasmin-like activity, and angiogenesis within the atherosclerotic plaque seems to be associated with increased expression of fibrinolytic proteins. Recently, several nonplasmin fibrinolysins that may support the invasive phenotype of endothelial cells under specific circumstances have been identified. Thus, the contribution of individual fibrinolysins appears to be context-specific, just as the profile of endothelial cell gene expression depends upon the surrounding tissue milieu.

High affinity binding of beta 2-glycoprotein I to human endothelial cells is mediated by annexin II

Beta(2)-glycoprotein I (beta(2)GPI) is an abundant plasma phospholipid-binding protein and an autoantigen in the antiphospholipid antibody syndrome. Binding of beta(2)GPI to endothelial cells targets them for activation by anti-beta(2)GPI antibodies, which circulate and are associated with thrombosis in patients with the antiphospholipid antibody syndrome. However, the binding of beta(2)GPI to endothelial cells has not been characterized and is assumed to result from association of beta(2)GPI with membrane phospholipid. Here, we characterize the binding of beta(2)GPI to endothelial cells and identify the beta(2)GPI binding site. (125)I-beta(2)GPI bound with high affinity (K(d) approximately 18 nm) to human umbilical vein endothelial cells (HUVECs). Using affinity purification, we isolated beta(2)GPI-binding proteins of approximately 78 and approximately 36 kDa from HUVECs and EAHY.926 cells. Amino acid sequences of tryptic peptides from each of these were identical to sequences within annexin II. A role for annexin II in binding of beta(2)GPI to cells was confirmed by the observations that annexin II-transfected HEK 293 cells bound approximately 10-fold more (125)I-beta(2)GPI than control cells and that anti-annexin II antibodies inhibited the binding of (125)I-beta(2)GPI to HUVECs by approximately 90%. Finally, surface plasmon resonance studies revealed high affinity binding between annexin II and beta(2)GPI. These results demonstrate that annexin II mediates the binding of beta(2)GPI to endothelial cells.

Construction of recombinant adenoviral vector of annexin II

Annexin II is a member of the annexin family of calcium-dependent phospholipid binding proteins expressed in vascular endothelium. Recently this molecule was reported to play a role in control of fibrinolysis on the endothelial surface. To examine the role of annexin II in vascular endothelium critically, we developed a recombinant adenoviral vector containing the annexin II cDNA. A full-length annexin II cDNA was inserted into a shuttle vector, pAdRSV4, and co-transfected into 293 cells with a replication-deficient type 5 adenovirus, pJM17. Resulting plaques were isolated and checked for protein expression. The verified clone (AdRSV-ANII) was further analyzed. Characterization of this vector will facilitate the investigation of the mechanism of fibrinolysis on vascular endothelium.

Annexin II and regulation of cell surface fibrinolysis

The regulated function of the fibrinolytic system is fundamental to the solubilization of fibrin-containing thrombi and to a number of other biologic processes. In recent years, several receptors, which serve to localize proteolytic activity on the cell surface, have been identified on endothelial cells, blood cells, neuronal cells, and tumor cells. One such receptor is annexin II, a calcium- and phospholipid-binding protein that serves as a profibrinolytic co-receptor for tissue plasminogen activator and plasminogen on endothelial cells. Accumulating evidence suggests that impaired cell surface fibrinolytic assembly could lead to progressive atherothrombotic disease. In addition, dysregulation of annexin II expression in acute promyelocytic leukemia is an important mechanism for the bleeding diathesis associated with this malignancy.

Inhibition of endothelial cell thromboresistance by homocysteine

Homocysteine (HC) is a highly reactive thiol intermediate in amino acid metabolism, which can modify the function of endothelial cells in a myriad of ways. In vitro, homocysteine can inhibit the thromboresistance properties of the endothelial cell by induction of procoagulant factors, inactivation of natural anticoagulant systems, and suppression of vasodilatory and platelet-modulating factors. HC also inhibits the fibrinolytic system by impairing the ability of the endothelial cell to bind tissue plasminogen activator (t-PA), by interacting directly with the t-PA binding "tail" domain of its endothelial cell receptor, annexin II. Moreover, HC influences endothelial cell gene expression as exemplified by induction of the elongation factor-1 family of polypeptides, which promote polypeptide chain elongation during mRNA translation. Induction of EF-1 subunits alpha, beta, gamma and delta by homocysteine is associated with increased turnover of at least one free thiol-containing protein, suggesting that up-regulation of these subunits may represent a mechanism for replacement of damaged or modified proteins. A more complete understanding of the diverse effects of homocysteine on endothelial cell function may provide important clues to the precise role homocysteine may play in the initiation and progression of vascular disease.

Simplified production of a recombinant human angiostatin derivative that suppresses intracerebral glial tumor growth

Angiostatin is an endogenous inhibitor of tumor neovascularization that inhibits the proliferation of endothelial cells. Production of sufficient quantities of biologically active angiostatin by the enzymatic cleavage of plasminogen has proven difficult in that it has delayed clinical testing. We have cloned, expressed, and purified a recombinant human angiostatin derivative (K1-3) using a mammalian expression system. Through the addition of a secretory signal and polyhistidine sequence tag, K1-3 can be purified from post-culture medium by simple column chromatography. Purified K1-3 protein is apparently folded in an active conformation, as evidenced by its ability to bind to lysine-Sepharose. In vitro, recombinant K1-3 significantly suppressed endothelial cell proliferation in a dose-dependent manner with an IC50 of 50 nM. Using an animal model of intracranial brain tumors in immune-competent rats, systemic administration of purified recombinant K1-3 resulted in up to 85% suppression of tumor growth (P = 0.011). Growth suppression was accompanied by a 32% decrease (P = 0.01) in tumor neovascularization. This study demonstrates a simple method to produce a biologically active recombinant angiostatin derivative. The ability to suppress intracerebral tumor growth after systemic administration suggests that K1-3 is likely to have therapeutic value in the treatment of malignant glial tumors.

Annexin II: a mediator of the plasmin/plasminogen activator system

The annexins constitute a family of calcium-dependent membrane binding proteins. Recently, annexin II has been shown to accelerate the activation of the clot-dissolving protease plasmin by complexing with the plasmin precursor plasminogen and with tissue plasminogen activator. Binding of plasminogen to annexin II is inhibited by the atherogenic lipoprotein, lipoprotein(a), while binding of tissue plasminogen activator to annexin II is blocked by the thiol amino acid homocysteine. Formation of the plasminogen/tissue plasminogen activator/annexin II complex may represent a key regulatory mechanism in fibrinolytic surveillance.

Annexin II and bleeding in acute promyelocytic leukemia

BACKGROUND

Acute promyelocytic leukemia (APL) is associated with a hemorrhagic disorder of unknown cause that responds to treatment with all-trans-retinoic acid.

METHODS

We studied a newly described receptor for fibrinolytic proteins, annexin II, in cells from patients with APL or other leukemias. We examined initial rates of in vitro generation of plasmin by tissue plasminogen activator (t-PA) in the presence of APL cells that did or did not have the characteristic translocation of APL, t(15;17). We also determined the effect of all-trans-retinoic acid on the expression of annexin II and the generation of cell-surface plasmin.

RESULTS

The expression of annexin II, as detected by a fluorescein-tagged antibody, was greater on leukemic cells from patients with APL than on other types of leukemic cells (mean fluorescence intensity, 6.9 and 2.9, respectively; P<0.01). The t(15;17)-positive APL cells stimulated the generation of cell-surface, t-PA-dependent plasmin twice as efficiently as the t(15;17)-negative cells. This increase in plasmin was blocked by an anti-annexin II antibody and was induced by transfection of t(15;17)-negative cells with annexin II complementary DNA. The t(15;17)-positive APL cells contained abundant messenger RNA for annexin II, which disappeared through a transcriptional mechanism after treatment with all-trans-retinoic acid.

CONCLUSIONS

Abnormally high levels of expression of annexin II on APL cells increase the production of plasmin, a fibrinolytic protein. Overexpression of annexin II may be a mechanism for the hemorrhagic complications of APL.

Modulation of annexin II by homocysteine: implications for atherothrombosis

Recent evidence indicates a potential role for the plasmin/plasminogen activator system in the prevention of atherosclerotic vascular disease. Fibrin deposition is a common histologic feature of the tissues of mice that are genetically deficient in one or more key components of the fibrinolytic system. Cell surface receptors may support fibrinolytic surveillance in both intravascular and extravascular locations by stimulating the efficiency plasmin generation and by protecting plasmin from its inhibitors. In vitro studies suggest that the endothelial cell receptor, annexin II, which independently binds both plasminogen and t-PA, could play a key role in the process. Binding of plasminogen to annexin II is specifically inhibited in the presence of excess concentrations of the atherogenic LDL-like particle Lp(a). Similarly, t-PA binding to annexin II is blocked by homocysteine, a sulfhydryl-containing amino acid that is associated with atherogenesis and that directly derivatizes the t-PA binding domain of annexin II. Elucidation of the precise role of annexin II in fibrinolytic surveillance, however, will await in vivo study.

Induction of acute translational response genes by homocysteine. Elongation factors-1alpha, -beta, and -delta

The thiol amino acid homocysteine (HC) accumulates in homocystinuria and homocyst(e)inemia, and is associated with a wide variety of clinical manifestations. To determine whether HC influences the cell's program of gene expression, vascular endothelial cells were treated with HC for 6-42 h and analyzed by differential display. We found a 3-7-fold, time-dependent induction of a 220-base pair fragment, which demonstrated complete sequence identity with elongation factor-1delta (EF-1delta), a member of the multimeric complex regulating mRNA translation. Fibroblasts from cystathionine beta-synthase -/- individuals also showed up to 3.0-fold increased levels of mRNA for EF-1alpha, -beta, and -delta when compared with normal cells, and treatment of normal cells with the HC precursor, methionine, induced a 1.5-2.0-fold increase in EF-1alpha, -beta, and -delta mRNA. This induction was completely inhibited by cycloheximide and reflected a doubling in the rate of gene transcription in nuclear run-on analyses. In HC-treated endothelial cells, pulse-chase studies revealed a doubling in the rate of synthesis of the thiol-containing protein, annexin II, but no change in synthesis of the cysteineless protein, plasminogen activator inhibitor-1. Thus, HC induces expression of a family of acute translational response genes through a protein synthesis-dependent transcriptional mechanism. This process may mediate accelerated synthesis of free thiol-containing proteins in response to HC-induced oxidative stress.

Tissue plasminogen activator binding to the annexin II tail domain. Direct modulation by homocysteine

Tissue plasminogen activator binds to endothelial cells via the calcium-regulated phospholipid-binding protein annexin II, an interaction that is inhibited by the prothrombotic amino acid homocysteine. We sought to identify the tissue plasminogen activator binding domain of annexin II and to determine the mechanism of its modulation by homocysteine. Tissue plasminogen activator binding to immobilized annexin II was inhibited by intact fluid phase annexin II but not by its "core" fragment (residues 25-339). Two overlapping "tail" peptides specifically blocked 65-75% of binding. Localization of the tissue plasminogen activator binding domain was confirmed upon specific inhibition by the hexapeptide LCKLSL (residues 7-12). Expressed C9G annexin II protein failed to support tissue plasminogen activator binding, while binding to C133G, C262G, and C335G was equivalent to that of wild type annexin II. Upon exposure to homocysteine, annexin II underwent a 135 +/- 4-Da increase in mass localizing specifically to Cys9 and a 60-66% loss in tissue plasminogen activator-binding capacity (I50 = 11 microM). Upon treatment of cultured endothelial cells with [35S]homocysteine, the dithiothreitol-sensitive label was recovered by immunoprecipitation with anti-annexin II IgG. These data provide a potential mechanism for the prothrombotic effect of homocysteine by demonstrating direct blockade of the tissue plasminogen activator binding domain of annexin II.

Profibrinolytic properties characterize a stably transformed human endothelial cell line

A stable immortalized venous endothelial cell (IVEC) line, obtained by transfection of human umbilical vein endothelial cells (HUVEC), retains many normal differentiated endothelial characteristics. We compared the fibrinolytic activities of IVEC and HUVEC, and observed that IVEC express a more profibrinolytic phenotype than HUVEC, since they bind and activate plasminogen more efficiently, produce more tissue plasminogen activator and urokinase-type plasminogen activator antigens, and secrete less plasminogen activator inhibitor-1 antigen both under basal conditions and after stimulation with lipopolysaccharide, phorbol ester and tumor necrosis factor. Moreover, immunostaining and Western blotting of IVEC for the plasminogen/tissue plasminogen activator receptor annexin II, as well as Northern blotting of annexin II mRNA, revealed similar patterns of surface expression in IVEC and HUVEC. Plasminogen activator inhibitor-2 is expressed similarly in both cell types. IVEC may be a useful human model for functional and pharmacological explorations and modulations of fibrinolytic system components.

The endothelial cell ecto-ADPase responsible for inhibition of platelet function is CD39

We previously demonstrated that when platelets are in motion and in proximity to endothelial cells, they become unresponsive to agonists (Marcus, A.J., L.B. Safier, K.A. Hajjar, H.L. Ullman, N. Islam, M.J. Broekman, and A.M. Eiroa. 1991. J. Clin. Invest. 88:1690-1696). This inhibition is due to an ecto-ADPase on the surface of endothelial cells which metabolizes ADP released from activated platelets, resulting in blockade of the aggregation response. Human umbilical vein endothelial cells (HUVEC) ADPase was biochemically classified as an E-type ATP-diphosphohydrolase. The endothelial ecto-ADPase is herein identified as CD39, a molecule originally characterized as a lymphoid surface antigen. All HUVEC ecto-ADPase activity was immunoprecipitated by monoclonal antibodies to CD39. Surface localization of HUVEC CD39 was established by confocal microscopy and flow cytometric analyses. Transfection of COS cells with human CD39 resulted in both ecto-ADPase activity as well as surface expression of CD39. PCR analyses of cDNA obtained from HUVEC mRNA and recombinant human CD39 revealed products of the same size, and of identical sequence. Northern blot analyses demonstrated that HUVEC express the same sized transcripts for CD39 as MP-1 cells (from which CD39 was originally cloned). We established the role of CD39 as a prime endothelial thromboregulator by demonstrating that CD39-transfected COS cells acquired the ability to inhibit ADP-induced aggregation in platelet-rich plasma. The identification of HUVEC ADPase/CD39 as a constitutively expressed potent inhibitor of platelet reactivity offers new prospects for antithrombotic therapeusis.

Thrombotic thrombocytopenic purpura and sporadic hemolytic-uremic syndrome plasmas induce apoptosis in restricted lineages of human microvascular endothelial cells

Thrombotic thrombocytopenic purpura (TTP) and sporadic hemolytic-uremic syndrome (HUS) are thrombotic microangiopathies that occur in the absence of an inflammatory response. Ultrastructural features of tissues involved in TTP/sporadic HUS suggest an apoptotic process. Consistent with these findings, we observed that TTP plasmas induce apoptosis in primary human endothelial cells (EC) of dermal microvascular but not umbilical vein origin (Laurence et al, Blood 87:3245, 1996). We now document the ability of plasmas from both TTP and sporadic HUS patients, but not from a patient with childhood/diarrhea-associated HUS, to induce apoptosis and expression of the apoptosis-associated molecule Fas (CD95) in restricted lineages of microvascular EC. EC of small vessel dermal, renal, and cerebral origin were susceptible to induction of Fas and an apoptotic cell death. In contrast, microvascular EC of pulmonary and hepatic origin, as well as EC of a large vessel, coronary artery, were resistant to both processes. This dichotomy parallels the in vivo pathology of TTP/sporadic HUS, with notable sparing of the pulmonary and hepatic microvasculature. Apoptotic EC also had some features of a procoagulant phenotype, including depressed production of prostaglandin I2 (prostacyclin). These phenomena support the pathophysiologic significance of microvascular EC apoptosis in TTP, extend it to a related disorder (sporadic HUS), and suggest consideration of apoptosis inhibitors in the experimental therapeutics of these syndromes.

Interaction of the fibrinolytic receptor, annexin II, with the endothelial cell surface. Essential role of endonexin repeat 2

Endothelial cells express a cell surface co-receptor for plasminogen and tissue plasminogen activator (t-PA) which we recently identified as annexin II (Hajjar, K. A., Jacovina, A. T., and Chacko, J. (1994) J. Biol. Chem. 269, 21191-21197). This protein enhances the catalytic efficiency of t-PA-dependent plasmin generation by 60-fold (Cesarman, G. M., Guevara, C. A., and Hajjar, K. A. (1994) J. Biol. Chem. 269, 21198-21203). Here, we demonstrate that annexin II is constitutively translocated to the endothelial cell surface within 16 h of biosynthesis, and that cell surface annexin II comprises 4.3 +/- 1.0% of the total cellular pool. Exogenous 125I-annexin II bound to EGTA-washed endothelial cells with high affinity (Kd 49 nM) and in a calcium-dependent (I50 = 3 microM), phospholipid-sensitive manner. Peptides KASMKGLGTDED and YDSMKGKGTRDK, mimicking the calcium-binding "endonexin" motif (KGXGT) of annexin II, blocked its interaction with endothelial cells. Recombinant annexin II, bearing the calcium-binding site substitution D161A of core repeat 2, failed to compete with binding of the wild type protein to the cell surface, while E246A and D321A mutants, corresponding to core repeats 3 and 4, behaved as effective competitors. These data suggest that translocated annexin II interacts with cell surface phospholipid via a high affinity calcium-dependent binding site that includes residues 118-122 (KGLGT) and the coordinating Asp161 of core repeat 2. Thus, calcium-regulated expression of annexin II on the endothelial cell surface may play a central role in control of plasmin-mediated processes.

The role of lipoprotein(a) in atherogenesis and thrombosis

Lipoprotein(a) [Lp(a)] represents an important independent risk factor for atherosclerotic cardiovascular disease. Lp(a) constitutes a class of low-density lipoprotein-like particles that are structurally heterogeneous due to variability within the distinguishing apoprotein, apolipoprotein(a) [Apo(a)]. Apo(a) bears a high degree of homology to the fibrinolytic zymogen, plasminogen, the parent molecule of the serine protease plasmin. Apo(a) contains a variable number of tandemly repeated triple-loop units called kringles, which appear to mediate Lp(a)'s interactions with fibrin and cell surface receptors. Although the mechanism of its atherogenicity is unknown, Lp(a) has been implicated in the delivery of cholesterol to the injured blood vessel, in blockade of plasmin generation on fibrin and cell surfaces, and as a stimulus for smooth muscle cell proliferation. In addition, new members of the plasminogen/Apo(a) gene family have been defined, creating a potential link between Lp(a) and the control of angiogenesis in both health and disease. Pharmacologic therapy of elevated Lp(a) levels has been only modestly successful; apheresis remains the most effective therapeutic modality.

Changing concepts in fibrinolysis

New advances in molecular biology have prompted reevaluation of traditional concepts in fibrinolysis. Identification of novel cell surface activation receptors as well as a series of plasminogen/apolipoprotein(a) homologues suggests new potential mechanisms for controlled generation of the multifunctional protease plasmin.

Cellular receptors in the regulation of plasmin generation

Cell surface receptors may play a significant role in the regulation of plasmin generation. Although structurally diverse, these receptors can be classified on a functional basis into two groups. Activation receptors for plasminogen and plasminogen activators serve to localize, and in some cases, potentiate plasminogen activation, and are expressed on endothelial cells, blood cells, neuronal cells, and tumor cells. Clearance receptors, on the other hand, serve to eliminate plasmin and plasminogen activators from blood or focal micro-environments. They are found primarily on parenchymal hepatocytes and tissue macrophages. It is likely that integrated actions of both classes of receptors are essential to the homeostatic control of plasmin activity. Knowledge of how these receptors themselves are regulated may provide an important key to understanding a host of biologic processes.

Thrombosis and inflammation as multicellular processes: significance of cell-cell interactions

Platelet activation as a result of vascular injury provokes endothelial cells to respond in a manner which limits or reverses the occlusive consequences of platelet accumulation. If the agonistic forces are strong, platelet accumulation is irreversible. In vitro data from our laboratory have repeatedly demonstrated that platelets become unresponsive to all agonists when in proximity to endothelial cells. This unresponsiveness is due to at least three separate endothelial "thromboregulatory" systems: eicosanoids, endothelium-derived relaxing factor (EDRF/NO), and most importantly an endothelial cell ecto-nucleotidase which metabolizes released platelet adenosine diphosphate (ADP) with consequent restoration of platelets to the resting state. This nucleotidase is operative in the complete absence of EDRF/NO and eicosanoids, indicating that the latter two are dispensable thromboregulators. We have solubilized the human endothelial cell ectoADPase, as well as that from placental tissue. Candidate proteins from a purified ADPase fraction are now being studied in further detail. An understanding of the molecular biology of the ADPase gene may lead to development of therapeutic agents such as soluble forms of the enzyme as well as approaches toward up-regulation of ectoADPase activity. This could result in "early thromboregulation", i.e. prevention and/or reversal of platelet accumulation at sites of vascular damage via immediate metabolic removal of the prime platelet agonist-ADP.

An endothelial cell receptor for plasminogen/tissue plasminogen activator (t-PA). II. Annexin II-mediated enhancement of t-PA-dependent plasminogen activation

In the preceding paper (Hajjar, K. A., Jacovina, A. T., and Chacko, J. (1994) J. Biol. Chem. 269, 21191-21197), we identified a M(r) = 40,000 endothelial cell receptor for tissue plasminogen activator (t-PA) and plasminogen (PLG) as the calcium- and phospholipid-binding protein, annexin II (Ann-II). Here, we examined the effect of Ann-II on t-PA-dependent plasminogen activation in a purified system. Purified native Ann-II bound t-PA, plasminogen, and plasmin with high affinity (Kd = 25 nM, 161 nM, and 75 nM, respectively). At fixed plasminogen concentrations, preincubation with purified native Ann-II was associated with an approximately 21-fold increase in the rate of Glu-PLG activation and an approximately 14-fold increase in activation of Lys-PLG. Three irrelevant proteins had no effect on plasmin formation, while fibrinogen increased the rate of Glu-PLG activation by approximately 4-fold. Annexin-II-mediated enhancement of t-PA-dependent plasminogen activation was 90-95% inhibited by epsilon-aminocaproic acid or by pretreatment of Ann-II with carboxypeptidase B, indicating a carboxyl-terminal lysine-dependent interaction. Kinetic analyses revealed that Ann-II conferred an approximately 60-fold increase in catalytic efficiency upon t-PA-dependent activation of either Glu-PLG or Lys-PLG. Thus, Ann-II-mediated assembly of plasminogen and t-PA may promote and localize constitutive plasmin generation on the surface of the blood vessel wall.

An endothelial cell receptor for plasminogen/tissue plasminogen activator. I. Identity with annexin II

Sequencing of two internal peptides from the putative human endothelial cell tissue plasminogen activator (t-PA) receptor identified an analog of the calcium- and phospholipid-binding protein, annexin II (Ann-II). The polymerase chain reaction-derived, full-length cDNA revealed complete sequence identity with the heavy chain of Ann-II, and ligand-precipitated receptor protein immunoreacted specifically with a monoclonal antibody to Ann-II. Transfected 293 cells bound plasminogen (Kd = 114 nM; Bmax = 347,000) as well as t-PA (Kd = 48 nM; Bmax = 380,000). Antisense oligonucleotides directed against endothelial cell Ann-II mRNA inhibited binding of both t-PA and plasminogen by 49% and 38%, respectively. The K307T mutant of Ann-II expressed on 293 cells failed to bind plasminogen, while the K328I mutant bound this ligand in a manner equivalent to the wild-type. Binding of plasminogen to both the wild-type and the K328I mutant was blocked by pretreatment of 293 cells with carboxypeptidase B. These data suggest a novel mechanism whereby a plasmin-like serine protease may cleave Ann-II at Lys307-Arg308, exposing a new carboxyl-terminal lysine residue (Lys307) for binding and efficient activation of plasminogen.

alpha-Fucose-mediated binding and degradation of tissue-type plasminogen activator by HepG2 cells

The glycoprotein tissue-type plasminogen activator (t-PA) is subject to hepatic clearance in humans. Here, the interaction of t-PA with a well-differentiated hepatoma cell line (HepG2) was examined. Suspended HepG2 cells bound 125I-t-PA in a specific, saturable, and reversible fashion through a Ca(2+)-dependent, active site-independent mechanism. Binding isotherms indicated a high affinity system with a single class of saturable binding sites (Kd 39 nM; maximum binding capacity 493,000 sites per cell). Bound t-PA was rapidly degraded at 37 degrees C in a manner inhibited by lysosomotropic agents or metabolic inhibitors. Pretreatment of t-PA with monoclonal antibodies against the EGF/fibronectin finger domain, but not kringle 2 or kringle 1, reduced total binding by 86%. Binding of 125I-t-PA to HepG2 cells was inhibited by monosaccharides fucose and galactose and by the neoglycoprotein fucosyl-albumin. Enzymatic removal of alpha-fucose residues, but not alpha-galactose, high mannose, or complex oligosaccharide from 125I-t-PA, reduced specific binding by 60 +/- 5%. Binding was also inhibited by high, but not low, molecular weight urokinase, which contains an EGF-based threonine-linked alpha-fucose homologous to that of t-PA. These data suggest that EGF-associated O-linked alpha-fucose may mediate t-PA binding and degradation by HepG2 cells. This mechanism may be relevant to other proteins with analogous structures.

Homocysteine-induced modulation of tissue plasminogen activator binding to its endothelial cell membrane receptor

Endothelial cells impart thromboresistance to the blood vessel wall. As modulators of fibrinolytic activity, these cells synthesize and secrete tissue plasminogen activator (t-PA) as well as its physiologic inhibitor, plasminogen activator inhibitor-1. In addition, endothelial cells support membrane-associated assembly of plasminogen and tissue plasminogen activator. Recently, an M(r) approximately 40,000 protein expressed on endothelial cells has been shown to interact noncompetitively through disparate mechanisms with both t-PA and plasminogen, suggesting trimolecular assembly of enzyme, substrate, and receptor (Hajjar, K. A. 1991. J. Biol. Chem. 266:21962-21970). In the present study, treatment of cultured endothelial cells with DL-homocysteine was specifically associated with a selective reduction in cellular binding sites for t-PA. This 65% decrease in binding was associated with a 60% decrease in cell-associated t-PA activity. No change in affinity for t-PA or plasminogen or in the maximal number of binding sites for plasminogen was observed. Matrix-associated t-PA binding sites were not affected. These data suggest a new mechanism whereby homocysteine may perturb endothelial cell function, thus promoting a prothrombotic state at the surface of the blood vessel wall.

Plasminogen and plasminogen activator assembly on the human endothelial cell

Through assembly of plasminogen and its activators, the endothelial cell surface may provide a favorable environment for constitutive generation of plasmin. This system may be regulated at multiple levels. Abundant expression of a 40-kDa protein with dual ligand-binding capacity may promote cell surface plasmin formation by colocalizing t-PA and plasminogen in a catalytically favorable configuration. Conversion of Glu-PLG to the preactivated form Lys-PLG, in the vicinity of the cell surface, may also precede plasmin formation. Physiologic concentrations of Lp(a), furthermore, may serve to modulate plasminogen activation at the cell surface by competing for binding sites, whereas elevated levels of Lp(a) might suppress this mechanism and lead to a subclinical prothrombotic state. Finally, cell surface binding sites for both plasmin and t-PA appear to protect these molecules from their physiologic antagonists, alpha 2-plasmin inhibitor and plasminogen activator inhibitor, type-1, respectively. Plasmin formation may contribute to the nonthrombogenicity of the blood vessel wall.

Assembly of plasmin-generating proteins on the surface of human endothelial cells

Traditionally, plasmin generation has been conceptualized as a process oriented on the surface of a fibrin-containing thrombus. Recent work, however, indicated that plasminogen and its activators, tissue plasminogen activator (t-PA) and urokinase, can assemble on the surface of cultured human umbilical vein endothelial cells (HUVECs). On binding to HUVECs, plasminogen is activated by t-PA approximately 12-fold more efficiently than fluid-phase plasminogen, and is converted to a plasmin-modified form, possibly unique to cell surfaces. In addition, t-PA interacts with HUVECs at two sites. The major binding site preserves its activity and represents a true (relative molecular weight 40,000) membrane-associated exoreceptor. The low-density lipoprotein (LDL)-like lipoprotein, lipoprotein(a), is highly associated with atherosclerosis, bears striking sequence homology to plasminogen, and competes with plasminogen for cell surface binding. In summary, functional assembly of plasminogen and t-PA may represent an important thromboregulatory system.

The endothelial cell tissue plasminogen activator receptor. Specific interaction with plasminogen

Human endothelial cells (EC) assemble plasmin-generating proteins on their surface. We have previously identified an EC membrane protein (Mr approximately 40,000) which specifically binds tissue plasminogen activator (t-PA) but not urokinase (Hajjar, K.A., and Hamel, N. M. (1990) J. Biol. Chem. 265, 2908-2916). In the present study, t-PA receptor protein (t-PA-R) was purified to apparent homogeneity from a detergent extract of human placental tissue by diisopropyl fluorophosphate-t-PA affinity chromatography and preparative gel electrophoresis. In a solid phase binding assay wells coated with t-PA-R bound both 125I-t-PA and 125I-Lys-plasminogen (PLG), but not 125I-urokinase in a specific, reversible, and noncompetitive fashion. Binding of 125I-Lys-PLG, but not 125I-t-PA, to t-PA-R was 80% inhibited by a 20-100-fold molar excess of the PLG-like lipoprotein(a), or by the lysine analog, epsilon-aminocaproic acid (50 mM). A polyclonal anti-t-PA-R antibody inhibited 66 and 79% of the specific 125I-t-PA and 125I-Lys-PLG binding, respectively, to EC monolayers. Biosynthetically labeled 40-kDa protein coprecipitated with t-PA- or Lys-PLG-Sepharose beads, but not with unconjugated Sepharose. In a functional assay, t-PA associated with immobilized t-PA-R generated 6.4 times more plasmin than an equivalent amount of t-PA in the fluid phase. These results suggest that t-PA-R can bind both t-PA and Lys-PLG in a manner that mimics the EC surface. This protein may play a role in modulating plasmin generation on cell surfaces.

Inhibition of platelet function by an aspirin-insensitive endothelial cell ADPase. Thromboregulation by endothelial cells

We previously reported that platelets become unresponsive to agonists when stimulated in combined suspension with aspirin-treated human umbilical vein endothelial cells. Inhibition occurred concomitant with metabolism of platelet-derived endoperoxides to prostacyclin by endothelial cells. We now demonstrate that if aspirin-treated platelets which fully respond to appropriate doses of agonists are exposed to aspirin-treated endothelial cells, they remain unresponsive despite absence of prostacyclin. Platelet inhibition is due in large part to ecto-ADPase activity on the endothelial cells. This was established by incubating aspirin-treated endothelial cells with 14C-ADP. Radio-thin layer chromatography and aggregometry demonstrated that 14C-ADP and induction of platelet activation decreased rapidly and concurrently. AMP accumulated transiently, was further metabolized to adenosine, and deaminated to inosine. The apparent Km of the endothelial cell ADPase was 33-42 microM and the Vmax 17-43 nmol/min per 10(6) cells, values in the range of antithrombotic potential. Thus, at least three complementary systems in human endothelial cells control platelet responsiveness: a cell-associated, aspirin-insensitive ADPase which functions in parallel with fluid phase autacoids such as the aspirin-inhibitable eicosanoids, and the aspirin-insensitive endothelium-derived relaxing factor.

Lipoprotein (a) regulates plasminogen activator inhibitor-1 expression in endothelial cells. A potential mechanism in thrombogenesis

Lipoprotein (a) (Lp(a)) is a low density lipoprotein-like particle which contains the plasminogen-like apolipoprotein a. Lp(a) levels are elevated in patients with atherosclerotic coronary artery disease. Recent studies suggest that Lp(a) competitively inhibits plasminogen binding to the endothelial cell and interferes with surface-associated plasmin generation. In this study, we present evidence for the presence of Lp(a) in the microvasculature of inflamed tissue. In addition, we demonstrate that Lp(a) regulates endothelial cell synthesis of a major fibrinolytic protein, plasminogen activator inhibitor-1 (PAI-1). In cultured human endothelial cells, Lp(a) enhanced PAI-1 antigen, activity, and steady-state mRNA levels without altering tissue plasminogen activator activity or mRNA transcript levels. This effect was cell-specific. Although other lipoproteins did not coordinately raise PAI-1 mRNA levels in endothelial cells, low density lipoprotein treatment selectively raised the level of the 3.4-kilobase mRNA species of PAI-1 without a concomitant increase in PAI-1 activity or antigen. Endothelial cell exposure to Lp(a) did not cause generalized endothelial cell activation since the functional activity and mRNA levels for tissue factor, platelet-derived growth factor and interleukin-6 were not elevated following Lp(a) exposure. These data suggest a molecular mechanism whereby Lp(a) may support a specific prothrombotic endothelial cell phenotype, namely by increasing PAI-1 expression.

Endothelial cell fibrinolytic assembly

Endothelial cells play a critical role in thromboregulation by controlling the assembly of fibrinolytic constituents on the membrane. The assembly system illustrated in FIGURE 6 is characterized by the binding of circulating glu-plasminogen to a membrane receptor (Pathway 1). A membrane-associated protease (possibly plasmin) converts the inactive zymogen into a catalytically more efficient zymogen lys-plasminogen (Pathway 2). T-PA binds to a specific receptor, retains its catalytic activity, and is protected from its natural inhibitor PAI-1. The membrane provides a favorable environment for plasmin generation (Pathway 3) at the vessel surface and contributes to the maintenance of a physiological nonthrombogenic state. The immobilization and surface activation of plasminogen provides an important mechanism for localizing proteolytic activity at the surface of other cells such as macrophages and tumor cells. Lp(a), a plasminogen-like lipoprotein, by competing at the endothelial surface for plasminogen binding down-regulates endothelial cell plasmin generation and may thus promote localized thrombogenesis that over a period of time contributes to progressive atherosclerosis.

Identification and characterization of human endothelial cell membrane binding sites for tissue plasminogen activator and urokinase

Cultured human endothelial cells synthesize and secrete two types of plasminogen activator, tissue plasminogen activator (t-PA) and urokinase (u-PA). Previous work from this laboratory (Hajjar, K.A., Hamel, N. M., Harpel, P. C., and Nachman, R. L. (1987) J. Clin. Invest. 80, 1712-1719) has demonstrated dose-dependent, saturable, and high affinity binding of t-PA to two sites associated with cultural endothelial cell monolayers. We now report that an isolated plasma membrane-enriched endothelial cell fraction specifically binds 125I-t-PA at a single saturable site (Kd 9.1 nM; Bmax 3.1 pmol/mg membrane protein). Ligand blotting experiments demonstrated that both single and double-chain t-PA specifically bound to a Mr 40,000 membrane protein present in detergent extracts of isolated membranes, while high molecular weight, low molecular weight, and single-chain u-PA associated with a Mr 48,000 protein. Both binding interactions were reversible and cell-specific and were inhibitable by pretreatment of intact cells with nanomolar concentrations of trypsin. The relevant binding proteins were not found in subendothelial cell matrix, failed to react with antibodies to plasminogen activator inhibitor type 1 and interacted with their respective ligands in an active site-independent manner. The isolated t-PA binding site was resistant to reduction and preserved the capacity for plasmin generation. In contrast, the isolated u-PA binding protein was sensitive to reduction, and did not maintain the catalytic activity of the ligand on the blot. The results suggest that in addition to sharing a matrix-associated binding site (plasminogen activator inhibitor type 1), both t-PA and u-PA have unique membrane binding sites which may regulate their function. The results also provide further support for the hypothesis that plasminogen and t-PA can assemble on the endothelial cell surface in a manner which enhances cell surface generation of plasmin.

Lipoprotein(a) modulation of endothelial cell surface fibrinolysis and its potential role in atherosclerosis

Endothelial cells play a critical role in thromboregulation by virtue of a surface-connected fibrinolytic system. Cultured endothelial cells synthesize and secrete tissue-type plasminogen activator (t-PA) which can bind to at least two discrete sites on the cell surface. These binding sites preserve the catalytic activity of t-PA and protect it from its physiological inhibitor (PAI-1). N-terminal glutamic acid plasminogen (Glu-PLG), the main circulating fibrinolytic zymogen, also interacts specifically with the endothelial cell surface. Binding is associated with a 12-fold increase in catalytic efficiency of plasmin generation by t-PA which may reflect conversion of Glu-PLG to its plasmin-modified form, N-terminal lysine plasminogen (Lys-PLG). Lipoprotein(a) is an atherogenic lipoprotein particle which contains the plasminogen-like apolipoprotein(a) bound to low density lipoprotein. We report here that lipoprotein(a) interferes with endothelial cell fibrinolysis by inhibiting plasminogen binding and hence plasmin generation. In addition, we demonstrate lipoprotein(a) accumulation in atherosclerotic lesions. These findings may provide a link between impaired cell surface fibrinolysis and progressive atherosclerosis.

Decreased messenger RNA translation in herpesvirus-infected arterial cells: effects on cholesteryl ester hydrolase

Herpes simplex viruses (HSVs) contain a function that can cause the degradation of host mRNA and mediate the shutoff of host protein synthesis. Previously, we observed that HSV infection causes a 40-fold increase in cholesteryl ester (CE) accretion in arterial smooth muscle cells due, in part, to a substantial decrease in CE hydrolysis. In studies reported herein, we found that HSV infection leads to reduced immunoprecipitable lysosomal (acid) CE hydrolase (ACEH) and beta-galactosidase, another lysosomal enzyme in vascular smooth muscle cells. The HSV-induced reduction was greater with respect to ACEH than beta-galactosidase. To determine whether degradation of host cellular mRNA or inhibition of cellular translation was responsible for decreased CE hydrolysis in HSV-infected smooth muscle cells, we utilized an in vitro translation system that permitted us to compensate for any mRNA degradation during viral infection. Reduced ACEH activity was observed in the total cellular RNA translation products of HSV-infected smooth muscle cells compared to uninfected cells owing to posttranscriptional modification. We conclude that the decrease in CE hydrolysis in HSV-infected smooth muscle cells is caused primarily by decreased ACEH synthesis and activity, which can contribute to CE accretion in these vascular cells.

Endothelial cell-mediated conversion of Glu-plasminogen to Lys-plasminogen. Further evidence for assembly of the fibrinolytic system on the endothelial cell surface

Lysine-plasminogen (Lys-PLG), the plasmin-modified form of native glutamic acid-plasminogen (Glu-PLG), displays enhanced binding affinity for fibrin and also enhanced activation by urokinase and tissue plasminogen activator. We previously demonstrated high-affinity, specific, and functional binding of Glu-PLG as well as tissue plasminogen activator to cultured human umbilical vein endothelial cells (HUVEC). In the present study, we demonstrate binding of Lys-PLG to HUVEC, as well as conversion of Glu-PLG to Lys-PLG at the cell surface. Binding of Lys-PLG to HUVEC was saturable, reversible, epsilon-aminocaproic acid-sensitive, and involved two saturable sites with Kd's of 142 pM and 120 nM, respectively. Upon incubation with Glu-PLG, HUVEC, as well as endothelium in situ, partially converted the ligand to a Lys-PLG-like species. Conversion by HUVEC was blocked by diisopropyl-fluorophosphate, but not by other serine protease inhibitors, including alpha 2-plasmin inhibitor. Eluates of intact umbilical cord vessels contained Lys-PLG by immunoblot analysis. Lys-PLG was also identified immunohistochemically on the endothelial surface of vessels from a variety of normal and inflamed tissues. Thus, endothelial cells appear to actively modify circulating Glu-PLG, bind Lys-PLG to their surface, and thus enhance the fibrinolytic potential of the blood vessel wall.

Interactions of arterial cells: III. Stathmokinetic analyses of smooth muscle cells cocultured with endothelial cells

Arterial endothelial cells (EC) or their conditioned medium (ECCM) can alter the proliferation of cocultured arterial smooth muscle cells (SMC). Previously, we have shown, as have others, that EC regulate the growth of cocultured SMC depending on the density of both cell types. To ascertain the rate of cell-cycle traverse in preconfluent arterial SMC cocultured with arterial EC or ECCM (derived from preconfluent EC), we have conducted a series of stathmokinetic experiments using flow cytometry to determine where specific changes may occur in the cell cycle. Results of our experiments indicate for the first time that ECCM stimulates the proliferation of preconfluent SMC by significantly shortening the residence times in the G1 and S phases of the cell cycle. The predominant relative effect occurs within the early G1 (G1A) compartment where pretreatment with ECCM shortens the residence time by approximately 55%. Furthermore, we have observed that preincubation of serum-free ECCM with antiplatelet-derived growth factor (PDGF) antibody abolishes any mitogenic effect on SMC. This suggests that EC secrete PDGF-like molecules which enhance the proliferation rate of preconfluent, cocultured SMC. These findings support the hypothesis that arterial EC may secrete mitogens which stimulate arterial SMC proliferation in the vascular wall.

Binding of tissue plasminogen activator to cultured human endothelial cells

Tissue plasminogen activator (t-PA) and urokinase (u-PA), the major activators of plasminogen, are synthesized and released from endothelial cells. We previously demonstrated specific and functional binding of plasminogen to cultured human umbilical vein endothelial cells (HUVEC). In the present study we found that t-PA could bind to HUVEC. Binding of t-PA to HUVEC was specific, saturable, plasminogen-independent, and did not require lysine binding sites. The t-PA bound in a rapid and reversible manner, involving binding sites of both high (Kd, 28.7 +/- 10.8 pM; Bmax, 3,700 +/- 300) and low (Kd, 18.1 +/- 3.8 nM; Bmax 815,000 +/- 146,000) affinity. t-PA binding was 70% inhibited by a 100-fold molar excess of u-PA. When t-PA was bound to HUVEC, its apparent catalytic efficiency increased by three- or fourfold as measured by plasminogen activation. HUVEC-bound t-PA was active site-protected from its rapidly acting inhibitor: plasminogen activator inhibitor. These results demonstrate that t-PA specifically binds to HUVEC and that such binding preserves catalytic efficiency with respect to plasminogen activation. Therefore, endothelial cells can modulate hemostatic and thrombotic events at the cell surface by providing specific binding sites for activation of plasminogen.