Assembly and routing of von Willebrand factor variants: the requirements for disulfide-linked dimerization reside within the carboxy-terminal 151 amino acids.

The precursor protein of von Willebrand factor (pro-vWF) consists of four different repeated domains, denoted D1-D2-D'-D3-A1-A2-A3-D4-B1-B2-B3-C1-C2, followed by a carboxy-terminal region of 151 amino acids without obvious internal homology. Previously, we have shown the requirement of the domains D1, D2, D', and D3 of pro-vWF in the assembly of pro-vWF dimers into multimers. Here, we define the domains of vWF involved in dimerization, using deletion mutants of full-length vWF cDNA transiently expressed in monkey kidney COS-1 cells. It is shown that only the carboxy-terminal 151 amino acid residues of vWF are required for dimerization. In addition, by analyzing a construct, encoding only the carboxy-terminal 151 amino acids of vWF, we find that the formation of dimers is an event independent of other domains present on pro-vWF, such as the domains C1 and C2 previously suggested to be involved in dimerization. Furthermore, it is shown that a deletion mutant of vWF, lacking the carboxy-terminal 151 amino acid residues and thus unable to dimerize, is proteolytically degraded in the ER. In contrast, a mutant protein, composed only of the carboxy-terminal 151 amino acids of vWF, and able to dimerize, is transported from the ER in a similar fashion as wild-type vWF. The role of the ER in the assembly of vWF is discussed with regard to the data presented in this paper on the intracellular fate of several vWF mutant proteins.

Thrombin neutralizes plasminogen activator inhibitor 1 (PAI-1) that is complexed with vitronectin in the endothelial cell matrix.

Vitronectin endows plasminogen activator inhibitor 1 (PAI-1), the fast-acting inhibitor of both tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), with additional thrombin inhibitory properties. In view of the apparent association between PAI-1 and vitronectin in the endothelial cell matrix (ECM), we analyzed the interaction between PAI-1 and thrombin in this environment. Upon incubating 125I-labeled alpha-thrombin with endothelial cell matrix (ECM), the protease formed SDS-stable complexes exclusively with PAI-1, with subsequent release of these complexes into the supernatant. Vitronectin was required as a cofactor for the association between PAI-1 and thrombin in ECM. Metabolic labeling of endothelial cell proteins, followed by incubation of ECM with t-PA, u-PA, or thrombin, indicated that all three proteases depleted PAI-1 from ECM by complex formation and proteolytic cleavage. Proteolytically inactive thrombin as well as anticoagulant thrombin, i.e., thrombin in complex with its endothelial cell surface receptor thrombomodulin, did not neutralize PAI-1, emphasizing that the procoagulant moiety of thrombin is required for a functional interaction with PAI-1. A physiological implication of our findings may be related to the mutual neutralization of both PAI-1 and thrombin, providing a new link between plasminogen activation and the coagulation system. Evidence is provided that in ECM, procoagulant thrombin may promote plasminogen activator activity by inactivating PAI-1.

Interaction between plasminogen activator inhibitor type 1 (PAI-1) bound to fibrin and either tissue-type plasminogen activator (t-PA) or urokinase-type plasminogen activator (u-PA). Binding of t-PA/PAI-1 complexes to fibrin mediated by both the finger and the kringle-2 domain of t-PA.

Plasminogen activation is catalyzed both by tissue-type-(t-PA) and by urokinase-type plasminogen activator (u-PA). This reaction is controlled by plasminogen activator inhibitor type 1 (PAI-1) that is either present in plasma or bound to fibrin, present in a thrombus. We studied the mechanism of in vitro inhibition of both t-PA and u-PA activity by PAI-1 bound to fibrin. It is shown that activation of latent PAI-1 unmasks a specific fibrin-binding site that is distinct from its reactive site. This reactive site of activated PAI-1 bound to fibrin is fully exposed to form complexes with t-PA and u-PA, that are unable to activate plasminogen. Upon complex formation with either one of the plasminogen activators, PAI-1 apparently undergoes a conformational change and loses its affinity for fibrin. Consequently, complexes of u-PA and PAI-1 dissociate from the fibrin matrix and are encountered in the fluid phase. In contrast, t-PA/PAI-1 complexes remain bound to fibrin. By employing recombinant t-PA deletion-mutant proteins, that precisely lack domains involved in fibrin binding, we demonstrate that binding of t-PA/PAI-1 complexes is mediated by both the "finger" (F) and the "kringle-2" (K2) domain of t-PA. A model is proposed that explains inhibition of the fibrinolytic process, at the level of plasminogen activation by t-PA, directed by PAI-1 bound to fibrin. An implication of the proposed model is that t-PA/PAI-1 complexes and free t-PA compete for the same binding sites on fibrin.

Genetic linkage of two intragenic restriction fragment length polymorphisms with von Willebrand's disease type IIA. Evidence for a defect in the von Willebrand factor gene.

Restriction fragment length polymorphisms (RFLPs), using the enzymes Bgl II and Xba I in conjunction with human von Willebrand factor (vWF) cDNA probes, have been described previously. In the present study we demonstrate the localization of both genetic markers within the vWF gene. The RFLPs were used to study the segregation of alleles associated with von Willebrand's disease (vWD) type IIA in a comprehensive, affected family. Individuals of this family were tested for their bleeding time and their plasma was analyzed for vWF antigen concentration and vWF ristocetin-cofactor activity. Based on these data, the affected members were diagnosed as vWD type-IIA patients; this conclusion was confirmed by the analysis of the multimeric vWF pattern of some of the patients. It was demonstrated that both RFLPs are completely linked with the vWD type-IIA trait. From this finding, we conclude that the defect that causes the vWD type IIA is most likely due to a mutation in the vWF gene and not to a mutation in a gene involved in posttranslational processing of the vWF protein.

The human kinesin-like protein RB6K is under tight cell cycle control and is essential for cytokinesis.

Several members of the kinesin superfamily are known to play a prominent role in the motor-driven transport processes that occur in mitotic cells. Here we describe a new mitotic human kinesin-like protein, RB6K (Rabkinesin 6), distantly related to MKLP-1. Expression of RB6K is regulated during the cell cycle at both the mRNA and protein level and, similar to cyclin B, shows a maximum during M phase. Isolation of the RB6K promoter allowed identification of a CDE-CHR element and promoter activity was shown to be maximal during M phase. Immunofluorescence microscopy using antibodies raised against RB6K showed a weak signal in interphase Golgi but a 10-fold higher signal in prophase nuclei. During M phase, the newly synthesized RB6K does not colocalise with Rab6. In later stages of mitosis RB6K localized to the spindle midzone and appeared on the midbodies during cytokinesis. The functional significance of this localization during M phase was revealed by antibody microinjection studies which resulted exclusively in binucleate cells, showing a complete failure of cytokinesis. These results substantiate a crucial role for RB6K in late anaphase B and/or cytokinesis, clearly distinct from the role of MKLP-1.

Human activin-A is expressed in the atherosclerotic lesion and promotes the contractile phenotype of smooth muscle cells.

Activin is a member of the transforming growth factor-beta superfamily, and it modulates the proliferation and differentiation of various target cells. In this study, we investigated the role of activin in the initiation and progression of human atherosclerosis. The expression of activin, its physiological inhibitor follistatin, and activin receptors were assayed in human vascular tissue specimens that represented various stages of atherogenesis. In situ hybridization experiments revealed activin mRNA in endothelial cells and macrophages and a strong induction of activin expression in neointimal smooth muscle cells from the early onset of atherogenesis. We developed an "in situ free-activin binding assay" by using biotinylated follistatin, which allowed us to detect bioactive activin at specific sites in atherosclerotic lesions. The mRNAs encoding the activin receptors are expressed similarly in normal and atherosclerotic tissue, which indicates that activin-A signaling in atherogenesis is most likely dependent on changes in growth factor concentrations rather than on receptor levels. In vitro, activin induces the contractile, nonproliferative phenotype in cultured smooth muscle cells, as is reflected by increased expression of smooth muscle-specific markers (SMalpha-actin and SM22alpha). Our data provide evidence that activin induces redifferentiation of neointimal smooth muscle cells, and we hypothesize that activin is involved in plaque stabilization.

High-density mutagenesis by combined DNA shuffling and phage display to assign essential amino acid residues in protein-protein interactions: application to study structure-function of plasminogen activation inhibitor 1 (PAI-I).

The identification of specific amino acid residues involved in protein-protein interaction is fundamental to understanding structure-function relationships. Supported by mathematical calculations, we designed a high-density mutagenesis procedure for the generation of a mutant library of which a limited number of random clones would suffice to exactly localize amino acid residues essential for a particular protein-protein interaction. This goal was achieved experimentally by consecutive cycles of DNA shuffling, under error prone conditions, each followed by exposure of the target protein on the surface of phages to screen and select for correctly folded, functional mutants. To validate the procedure, human plasminogen activator inhibitor 1 (PAI-1) was chosen, because its 3D structure is known, many experimental tools are available and it may serve as a model protein for structure-function studies of serine proteinases and their inhibitors (serpins). After five cycles of DNA shuffling and selection for t-PA binding, analysis of 27 randomly picked clones revealed that PAI-1 mutants contained an average of 9.1 amino acid substitutions distributed over 114 different positions, which were preferentially located at the surface of the protein. This limited collection of mutant PAI-1 preparations contained multiple mutants defective in binding to three out of four tested anti-PAI-1 monoclonal antibodies. Alignment of the nucleotide sequence of defective clones permitted assignment of single dominant amino acid residues for binding to each monoclonal antibody. The importance of these residues was confirmed by testing the properties of single point mutants. From the position of these amino acid residues in the 3D structure of PAI-1 and the effects of the corresponding monoclonal antibodies on t-PA-PAI-1 interaction, conclusions can be drawn with respect to this serpin-serine proteinase interaction.

Different structural requirements for plasminogen activator inhibitor 1 (PAI-1) during latency transition and proteinase inhibition as evidenced by phage-displayed hypermutated PAI-1 libraries.

Plasminogen activator inhibitor type 1 (PAI-1) is a member of the serine protease inhibitor (serpin) superfamily. Its highly mobile reactive-center loop (RCL) is thought to account for both the rapid inhibition of tissue-type plasminogen activator (t-PA), and the rapid and spontaneous transition of the unstable, active form of PAI-1 into a stable, inactive (latent) conformation (t(1/2) at 37 degrees C, 2.2 hours). We determined the amino acid residues responsible for the inherent instability of PAI-1, to assess whether these properties are independent and, consequently, whether the structural basis for inhibition and latency transition is different. For that purpose, a hypermutated PAI-1 library that is displayed on phage was pre-incubated for increasing periods (20 to 72 hours) at 37 degrees C, prior to a stringent selection for rapid t-PA binding. Accordingly, four rounds of phage-display selection resulted in the isolation of a stable PAI-1 variant (st-44: t(1/2) 450 hours) with 11 amino acid mutations. Backcrossing by DNA shuffling of this stable mutant with wt PAI-1 was performed to eliminate non-contributing mutations. It was shown that the combination of mutations at positions 50, 56, 61, 70, 94, 150, 222, 223, 264 and 331 increases the half-life of PAI-1 245-fold. Furthermore, within the limits of detection the stable mutants isolated are functionally indistinguishable from wild-type PAI-1 with respect to the rate of inhibition of t-PA, cleavage by t-PA, and binding to vitronectin. These stabilizing mutations constitute largely reversions to the stable "serpin consensus sequence" and are located in areas implicated in PAI-1 stability (e.g. the vitronectin-binding domain and the proximal hinge). Collectively, our data provide evidence that the structural requirements for PAI-1 loop insertion during latency transition and target proteinase inhibition can be separated.

The variable region-1 from tissue-type plasminogen activator confers specificity for plasminogen activator inhibitor-1 to thrombin by facilitating catalysis: release of a kinetic block by a heterologous protein surface loop.

Substitution of the native variable region-1 (VR1/37-loop) of thrombin by the corresponding VR1 of tissue-type plasminogen activator (thrombin-VR1(tPA)) increases the rate of inhibition by plasminogen activator inhibitor type 1 (PAI-1) by three orders of magnitude, and is thus sufficient to confer PAI-1 specificity to a heterologous serine protease. A structural and kinetical approach to establish the function of the VR1 loop of t-PA in the context of the thrombin-VR1(tPA) variant is described. The crystal structure of thrombin-VR1(tPA) was resolved and showed a conserved overall alpha-thrombin structure, but a partially disordered VR1 loop as also reported for t-PA. The contribution of a prominent charge substitution close to the active site was studied using charge neutralization variants thrombin-E39Q(c39) and thrombin-VR1(tPA)-R304Q(c39), resulting in only fourfold changes in the PAI-1 inhibition rate. Surface plasmon resonance revealed that the affinity of initial reversible complex formation between PAI-1 and catalytically inactive Ser195-->Ala variants of thrombin and thrombin-VR1(tPA) is only increased fivefold, i.e. KD is 652 and 128 nM for thrombin-S195A and thrombin-S195A-VR1(tPA), respectively. We established that the partition ratio of the suicide substrate reaction between the proteases and PAI-1 was largely unaffected in any variant studied. Hirugen allosterically decreases the rate of thrombin inhibition by PAI-1 2.5-fold and of thrombin-VR1(tPA) 20-fold, by interfering with a unimolecular step in the reaction, not by decreasing initial complex formation or by altering the stoichiometry. Finally, kinetic modeling demonstrated that acylation is the rate-limiting step in thrombin inhibition by PAI-1 (k approximately 10(-3) s(-1)) and this kinetic block is alleviated by the introduction of the tPA-VR1 into thrombin (k>1 s(-1)). We propose that the length, flexibility and different charge architecture of the VR1 loop of t-PA invoke an induced fit of the reactive center loop of PAI-1, thereby enhancing the rate of acylation in the Michaelis complex between thrombin-VR1(t-PA) and PAI-1 by more than two orders of magnitude.

Molecular advances in plasminogen activator inhibitor 1 interaction with thrombin and tissue-type plasminogen activator.

Plasminogen activator inhibitor 1 (PAI-1) is a glycoprotein that controls the activity of the key enzymes of the fibrinolytic system, the serine proteases tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). Inhibition is accomplished by rapid formation of inactive, equimolar PAI-1/PA complexes. The physiological importance of PAI-1 for the fibrinolytic system has been underscored by the observation that in humans, a homozygous defect results in hemorrhagic episodes. In addition to its function in surveillance of the integrity of clots, PAI-1 efficiently inhibits the serine protease thrombin in vitro, provided that either the high molecular weight glycosaminoglycan heparin or the glycoprotein vitronectin is present. These cofactors accelerate the rate of thrombin inhibition by PAI-1 by more than two orders of magnitude. Inhibition of thrombin by PAI-1 proceeds according to a "suicide substrate mechanism," typified by a branched reaction pathway, leading either to stable PAI-1/thrombin complexes or to degradation of the inhibitor and recycling of enzyme. The cofactors heparin and vitronectin, although increasing inhibition through different mechanisms, essentially promote PAI-1 degradation by thrombin. In view of the multitude of functions attributed to thrombin, the authors propose that the relevance of thrombin inhibition by PAI-1 is to restrict its mitogenic activity, rather than to affect its coagulation function in plasma. (Trends Cardiovasc Med 1997;7:47-51). © 1997, Elsevier Science Inc.

Vascular calcification: expression patterns of the osteoblast-specific gene core binding factor alpha-1 and the protective factor matrix gla protein in human atherogenesis.

OBJECTIVE: Increasing evidence suggests that vascular calcification is a regulated process. We studied the vascular expression pattern of a key factor in mineralization and a counteracting, protective factor. Based on the phenotype of null mice, Core binding factor alpha-1 (Cbfa-1) plays a pivotal role in bone formation, whereas Matrix Gla Protein (MGP) is a potent inhibitor of vascular calcification. METHODS: We investigated the expression of MGP and Cbfa-1 in cultured, human monocytic cells, endothelial cells and smooth muscle cells (SMC), as well as in normal and atherosclerotic vessel specimens. RESULTS: In cultured cells MGP is expressed in endothelial cells and SMC, whereas Cbfa-1 mRNA is predominantly present in macrophages and to a lesser extent in SMC. In the normal vessel wall MGP expression is high at the luminal side and declines toward the center of the media, whereas Cbfa-1 is absent. Moderate, diffuse calcification of the aorta media was observed only in those regions where MGP is low or absent. In atherosclerotic lesions MGP is expressed in endothelial cells and SMC that form fibrous caps, but is never present in macrophages. Cbfa-1 is synthesized in regions without MGP, it is associated with calcified areas and Cbfa-1 may be considered a marker for osteoprogenitor-like cells in the vessel wall. CONCLUSIONS: Our observations on MGP expression confirm and extend published data and are consistent with a protective function of MGP. Cbfa-1 expression is absent in normal medial SMC and co-localizes with neointimal macrophages and focal calcifications.

Relief of polarity caused by transposon Tn5: application in mapping a cloned region of the Escherichia coli uvrB locus essential for UV resistance.

The structure and function of recombinant plasmid pNP5, which consists of vector pMB9 and a 2.5 kb EcoRI fragment harbouring the Escherichia coli uvrB gene, has been investigated. Insertional inactivation with the transposons Tn1 (Apr) or Tn5 (Kmr) has been used to determine the region on pNP5 DNA that is essential for UV resistance in uvrB deletion strains. This region spans approx. 1.8 kb and is separated by at least 280 bp from the pMB9 promoter to which it has been fused. Furthermore, a procedure is described to eliminate the polarity exerted by the transposon Tn5. A combination of in vitro digestion of pNP5::Tn5 DNA with restriction endonuclease XHoI, followed by ligation and subsequent in vivo propagation of the resulting plasmid DNA yields predominantly pNP5 molecules with a site-specific nonpolar mutation. The method allows an investigation of cloned complex genetic units, such as operons.

Functional analysis of the human tissue-type plasminogen activator protein: the light chain.

A pBR322::Rous sarcoma virus(RSV)-based shuttle vector was used to insert fused genes, composed of the amino-terminal portion of the bacterial chloramphenicol-acetyltransferase gene (cat) and the entire coding region for the C-terminally derived light (L) chain of human tissue-type plasminogen activator (t-PA) cDNA. Cotransfection of rat 3Y1 cells with pRSVneo DNA and pRSVcat/t-PA DNA yielded stably integrated G418-resistant transfectants which contain unrearranged copies of pRSVcat/t-PA DNA. These transfectants synthesize cat/t-PA L-chain mRNA, apparently correctly initiated and terminated. With the help of an enzyme-linked immunosorbent assay (ELISA), it is demonstrated that these cells produce human t-PA antigen. Furthermore, pRSVcat/t-PA L-chain cDNA-containing rat 3Y1 cells synthesize a plasminogen-dependent amidolytic activity which is suppressed by specific anti-human t-PA antibodies. This activity cannot be stimulated by fibrin, a property displayed by native t-PA. It is concluded that the t-PA L-chain cDNA contains the complete genetic information for the plasminogen activator activity.

Identification of functional interaction sites on proteins using bacteriophage-displayed random epitope libraries.

We describe a phage-display-based method to identify epitopes or interaction sites on proteins. DNA encoding the protein of interest is partially degraded with DNase I to generate random fragments of 50-200 bp. These fragments are then cloned into a phagemid vector that has been modified to allow the expression of the random fragments and the construction of a (bacterio)phage-displayed random epitope library. Phages displaying functional epitopes can be selected from these libraries by affinity selection or panning. To test this method we have constructed a random-epitope library for human plasminogen-activator inhibitor 1 and used this library to map the epitope of a monoclonal antibody (mAb) directed against this protein. By alignment of the selected overlapping epitope-containing fragments, we were able to locate the epitope of the mAb on a stretch of 39 amino acids spanning from E128 to V166. The approach may also be applied to more complex systems than single-protein genes, such as viral genomes or complete cDNA libraries.

Functional display of human plasminogen-activator inhibitor 1 (PAI-1) on phages: novel perspectives for structure-function analysis by error-prone DNA synthesis.

The synthesis of the human plasminogen-activator inhibitor 1 (PAI-1) protein in the cytoplasm of transformed Escherichia coli cells results in inactive protein preparations that can be activated by denaturation and renaturation. We have used the phagemid pComb3, designed for combinatorial immunoglobulin repertoire cloning, for routing of PAI-1 to the periplasm and subsequent exposure on the surface of filamentous phages. Phage-displayed PAI-1 specifically binds to immobilized polyclonal and monoclonal anti-human PAI-1 antibodies. In addition, PAI-1 retains its capacity to form equimolar complexes with its target serine protease tissue-type plasminogen activator (t-PA), as well as its ability to inhibit t-PA activity. Finally, we have explored and manipulated the error-prone property of TaqI DNA polymerase during PCR amplification of the full-length PAI-1 cDNA to generate a large library of predominantly single, random PAI-1 mutants. In addition, a computer simulation program has been devised that converts the number of mutations per codogenic region (in this case PAI-1) into actual mutant proteins. The PAI-1-phage mutant library is composed of 46% single and 34% double mutants and 20% wild-type PAI-1 and can be employed to isolate mutants defective in interactions of PAI-1 with other components. The method described here is applicable to other studies on the structure-function analysis of eukaryotic proteins.

Serial analysis of gene expression to assess the endothelial cell response to an atherogenic stimulus.

Activation of human, arterial endothelial cells (ECs) is an early event in the pathogenesis of atherosclerosis. To identify the repertoire of genes that are differentially expressed after activation, we used serial analysis of gene expression (SAGE) to compare the mRNA spectrum of quiescent ECs with that of ECs activated for 6h with a strong atherogenic stimulus. SAGE methodology generates concatenated 'tags' of 10bp that are derived from a specific mRNA. About 5% of over 12000 tags analyzed is derived from genes that are differentially expressed (at least 5-fold up- or downregulated). These transcript tags are derived from only 56 genes, close to 1% of the total number of analyzed genes. Among these 56 differentially expressed genes are 42 known genes, including the hallmark endothelial cell activation markers interleukin 8 (IL-8), monocyte chemoattractant protein 1 (MCP-1), vascular cell adhesion molecule 1 (VCAM-1), plasminogen activator inhibitor 1 (PAI-1), Gro-alpha, Gro-beta and E-selectin. Differential transcription of a selection of the upregulated genes was confirmed by Northern blot analysis. A novel observation is the upregulation of activin betaA mRNA, a member of the transforming growth factor beta family. Apparent discrepancies between this novel technology and conventional methods are discussed. In conclusion, we demonstrate that for the application of SAGE, a moderate number of analyzed transcript tags suffices to reveal the significant alterations of EC transcription that results from a strong atherogenic stimulus.

Determination of the vitronectin binding site on plasminogen activator inhibitor 1 (PAI-1).

Vitronectin is the carrier protein of plasminogen activator inhibitor 1 (PAI-1). We used a well-characterized panel of anti-human PAI-1 monoclonal antibodies (MoAbs) to localize the vitronectin-binding site on PAI-1. By employing a direct vitronectin/PAI-1 binding assay and two vitronectin-dependent inhibition assays, we demonstrate that the anti-PAI-1 MoAbs CLB-5, CLB-10, CLB-2C8 and I1, directed against different epitopes in the region between amino acids 110 and 145, prevent the interaction of PAI-1 with vitronectin. We conclude that the region between amino acids 110 and 145 of PAI-1 harbours an important determinant for the interaction with vitronectin.

Lack of gradual regulation of tetracycline-controlled gene expression by the tetracyclin-repressor/VP16 transactivator (tTA) in cultured cells.

Von Willebrand factor (vWF) is an essential multimeric protein for adhesion of platelets to an injured vessel wall. Endothelial cells secrete vWF by either a constitutive or a regulated pathway. It is unknown whether the secretory partitioning of vWF is dependent on the level of vWF synthesis. We employed the widely applied tetracycline-controlled transactivator system (tTA) to study the regulation of vWF mRNA synthesis in stably transfected Madin Darby kidney (MDCK-II) cells in a quantitative manner. Immunofluorescence staining with anti-vWF antibodies revealed that increasing the concentration of tetracycline resulted in a decreased number of MDCK-II cells that synthesize vWF. Apparently, tTA-regulated gene expression in an individual cell functions as an 'on/off' system rather than regulating the level of gene expression in a dose-response manner, as reported previously.

Selection of peptides that bind to plasminogen activator inhibitor 1 (PAI-1) using random peptide phage-display libraries.

Large random hexa- and decapenta-peptide libraries were constructed and displayed on the surface of the filamentous phagemid pComb8. Panning of the hexa-peptide library on immobilized plasminogen activator inhibitor 1 (PAI-1) specifically selected a minor fraction of concatemers, indicating that binding to PAI-1 requires an extended amino acid sequence. Accordingly, the decapenta-peptide library exclusively yielded PAI-1 binding peptides of 15 amino acid residues. None of these phage-bound peptides prevented the interaction between PAI-1 and its target serine protease urokinase (u-PA). To isolate peptides that block the interaction between PAI-1 and u-PA, phages bound to immobilized PAI-1 were eluted by incubation with u-PA. Remarkably, this procedure resulted in elution of a unique phage type that harbors a concatemer of decapentamers, consisting of 49 amino acid residues with no obvious similarity to the primary sequence of PAI-1 or u-PA.

Gene expression in atherogenesis.

It is conceivable that the extent and spatio-temperal expression of dozens or even a few hundred genes are significantly altered during the development and progression of atherosclerosis as compared to normal circumstances. Differential gene expression in vascular cells and in blood cells, due to gene-gene and gene-environment interactions can be considered the molecular basis for this disease. To comprehend the coherence of the complex genetic response to systemic and local atherosclerotic challenges, one needs accessible high through-put technologies to analyze a panel of differentially expressed genes and to describe the interactions between and among their gene products. Fortunately, new technologies have been developed which allow a complete inventory of differential gene expression, i.e. DD/RT-PCR, SAGE and DNA micro-array. The initial data on the application of these technologies in cardiovascular research are now being reported. This review summarizes a number of key observations. Special attention is paid to a few central transcription factors which are differentially expressed in endothelial cells, smooth muscle cells or monocytes/ macrophages. Recent data on the role of nuclear factor-kappaB (NF-kappaB) and peroxisome proliferation-activating receptors (PPARs) are discussed. Like the PPARs, the NGFI-B subfamily of orphan receptors (TR3, MINOR and NOT) also belongs to the steroid/thryroid hormone receptor superfamily of transcription factors. We report that this subfamily is specifically induced in a sub-population of neointimal smooth muscle cells. Furthermore, intriguing new data implicating the Sp/XKLF family of transcription factors in cell-cell communication and maintenance of the atherogenic phenotype are mentioned. A member of the Sp/XKLF family, the shear stress-regulated lung Krüppel-like factor (LKLF) is speculated to be instrumental for the communication between endothelial cells and smooth muscle cells. Taken together, the expectation is that the fundamental knowledge obtained on atherogenesis and the data that will be acquired during the coming decade with the new, powerful high through-put methodologies will lead to novel modalities to treat patients suffering from cardiovascular disease. In view of the phenotypic changes of vascular and blood-borne cells during atherogenesis, therapeutic interventions likely will focus on reversal of an acquired phenotype by gene therapy approach or by using specific drugs which interfere with aberrant gene expression.