Specific binding of the transglutaminase, platelet factor XIII, to HSP27

Let's cross-link: diverse functions of the promiscuous cellular transglutaminase factor XIII-A

Essentials Plasma Factor XIII, a heterodimer of A and B subunits FXIIIA2 B2 , is a transglutaminase enzyme with a well-established role in haemostasis. Cells of bone marrow and mesenchymal lineage express the FXIII-A gene (F13A1) that encodes the cellular form of the transglutaminase, a homodimer of the A subunits, FXIII-A. FXIII-A was presumed to function intracellularly, however, several lines of evidence now indicate that FXIII-A is externalised by an as yet unknown mechanism This review describes the mounting evidence that FXIII-A is a diverse transglutaminase with many intracellular and extracellular substrates that can participate in an array of biological processes SUMMARY: Factor XIII is a tranglutaminase enzyme that catalyzes the formation of ε-(γ-glutamyl)lysyl isopeptide bonds in protein substrates. The plasma form, FXIII-A2 B2 , has an established function in hemostasis, where its primary substrate is fibrin. A deficiency in FXIII manifests as a severe bleeding diathesis, underscoring its importance in this pathway. The cellular form of the enzyme, a homodimer of the A-subunits, denoted FXIII-A, has not been studied in as extensive detail. FXIII-A was generally perceived to remain intracellular, owing to the lack of a classical signal peptide for its release. In the last decade, emerging evidence has revealed that this diverse transglutaminase can be externalized from cells, by an as yet unknown mechanism, and can cross-link extracellular substrates and participate in a number of diverse pathways. The FXIII-A gene (F13A1) is expressed in cells of bone marrow and mesenchymal lineage, notably megakaryocytes, monocytes/macrophages, dendritic cells, chrondrocytes, osteoblasts, and preadipocytes. The biological processes that FXIII-A is coupled with, such as wound healing, phagocytosis, and bone and matrix remodeling, reflect its expression in these cell types. This review describes the mounting evidence that this cellular transglutaminase can be externalized, usually in response to stimuli, and participate in extracellular cross-linking reactions. A corollary of being involved in these biological pathways is the participation of FXIII-A in pathological processes. In conclusion, the functions of this transglutaminase extend far beyond its role in hemostasis, and our understanding of this enzyme in terms of its secretion, regulation and substrates is in its infancy.

A review of platelet secretion assays for the diagnosis of inherited platelet secretion disorders

Measurement of platelet granule release to detect inherited platelet secretion disorders (IPSDs) is essential for the evaluation of patients with abnormal bleeding and is necessary to distinguish which granule sub-types are affected and whether there is abnormal granule bio-synthesis or secretion. The radioactive serotonin incorporation and release assay, described before 1970, is still considered the “gold standard” test to assess platelet δ-granule release, although is unsuitable for clinical diagnostic laboratories. Luciferin-based assays, such as lumiaggregometry, are the most widely performed alternatives, although these methods do not distinguish defects in δ-granule biosyn-thesis from defects in secretion. Platelet α-granule release is commonly evaluated using flow cytometry by measuring surface exposure of P-selectin after platelet activation. However, this assay has poor sensitivity for some α-granule disorders. Only few studies have been published with more recently developed assays and no critical reviews on these methods are available. In this review, we describe the rationale for developing robust and accurate laboratory tests of platelet granule release and describe the characteristics of the currently available tests. We identify an unmet need for further systematic evaluation of new assays and for standardisation of methodologies for clinical diagnostic laboratories.

Interactive protein network of FXIII-A1 in lipid rafts of activated and non-activated platelets

Lipid-rafts are defined as membrane microdomains enriched in cholesterol and glycosphingolipids within platelet plasma membrane. Lipid raft-mediated clot retraction requires factor XIII and other interacting proteins. The aim of this study was to investigate the proteins that interact with factor XIII in raft and non-raft domains of activated and non-activated platelet plasma membrane. By lipidomics analysis, we identified cholesterol- and sphingomyelin-enriched areas as lipid rafts. Platelets were activated by thrombin. Proteomics analysis provided an overview of the pathways in which proteins of rafts and non-rafts participated in the interaction network of FXIII-A1, a catalytic subunit of FXIII. "Platelet activation" was the principal pathway among KEGG pathways for proteins of rafts, both before and after activation. Network analysis showed four types of interactions (activation, binding, reaction, and catalysis) in raft and non-raft domains in interactive network of FXIII-A1. FXIII-A1 interactions with other proteins in raft domains and their role in homeostasis highlight the specialization of the raft domain in clot retraction via the Factor XIII protein network.

Functional factor XIII-A is exposed on the stimulated platelet surface

Factor XIII (FXIII) stabilizes thrombi against fibrinolysis by cross-linking α2-antiplasmin (α2AP) to fibrin. Cellular FXIII (FXIII-A) is abundant in platelets, but the extracellular functions of this pool are unclear because it is not released by classical secretion mechanisms. We examined the function of platelet FXIII-A using Chandler model thrombi formed from FXIII-depleted plasma. Platelets stabilized FXIII-depleted thrombi in a transglutaminase-dependent manner. FXIII-A activity on activated platelets was unstable and was rapidly lost over 1 hour. Inhibiting platelet activation abrogated the ability of platelets to stabilize thrombi. Incorporating a neutralizing antibody to α2AP into FXIII-depleted thrombi revealed that the stabilizing effect of platelet FXIII-A on lysis was α2AP dependent. Platelet FXIII-A activity and antigen were associated with the cytoplasm and membrane fraction of unstimulated platelets, and these fractions were functional in stabilizing FXIII-depleted thrombi against lysis. Fluorescence confocal microscopy and flow cytometry revealed exposure of FXIII-A on activated membranes, with maximal signal detected with thrombin and collagen stimulation. FXIII-A was evident in protruding caps on the surface of phosphatidylserine-positive platelets. Our data show a functional role for platelet FXIII-A through exposure on the activated platelet membrane where it exerts antifibrinolytic function by cross-linking α2AP to fibrin.

Heat shock proteins and heat shock factor 1 in carcinogenesis and tumor development: an update

Heat shock proteins (HSP) are a subset of the molecular chaperones, best known for their rapid and abundant induction by stress. HSP genes are activated at the transcriptional level by heat shock transcription factor 1 (HSF1). During the progression of many types of cancer, this heat shock transcriptional regulon becomes co-opted by mechanisms that are currently unclear, although evidently triggered in the emerging tumor cell. Concerted activation of HSF1 and the accumulation of HSPs then participate in many of the traits that permit the malignant phenotype. Thus, cancers of many histologies exhibit activated HSF1 and increased HSP levels that may help to deter tumor suppression and evade therapy in the clinic. We review here the extensive work that has been carried out and is still in progress aimed at (1) understanding the oncogenic mechanisms by which HSP genes are switched on, (2) determining the roles of HSF1/HSP in malignant transformation and (3) discovering approaches to therapy based on disrupting the influence of the HSF1-controlled transcriptome in cancer.

Transglutaminase 2 as a biomarker of osteoarthritis: an update

Osteoarthritis is a progressive joint disease characterized by cartilage degradation and bone remodelling. Under physiologic conditions, articular cartilage displays a stable chondrocyte phenotype, whereas in osteoarthritis a chondrocyte hypertrophy develops near the sites of cartilage surface damage and associates to the pathologic expression of type X collagen. Transglutaminases (TGs) include a family of Ca(2+)-dependent enzymes that catalyze the formation of γ-glutamyl cross-links. Their substrates include a variety of intracellular and extracellular macromolecular components. TGs are ubiquitously and abundantly expressed and implicated in a variety of physiopathological processes. TGs activity is modulated by inflammatory cytokines. TG2 (also known as tissue transglutaminase) mediates the hypertrophic differentiation of joint chondrocytes and interleukin-1-induced calcification. Histomorphometrical and biomolecular investigations document increased TG2 expression in human and experimental osteoarthritis. Consequently, the level of TG2 expression may represent an adjuvant additional marker to monitor tissue remodelling occurring in osteoarthritic joint tissue. Experimental induction of osteoarthritis in TG2 knockout mice is followed from reduced cartilage destruction and increased osteophyte formation compared to wild-type mice, suggesting a different influence on joint bone and cartilage remodelling. The capacity of transamidation by TG2 to regulate activation of latent TGF-β seems to have a potential impact on the regulation of inflammatory response in osteoarthritic tissues. Additional studies are needed to define TG2-regulated pathways that are differently modulated in osteoblasts and chondrocytes during osteoarthritis.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Differential expression of a novel gene in response to hsp27 and cell differentiation in human keratinocytes

The 27 kDa heat shock protein (hsp27) is expressed in keratinocytes in a differentiation-related pattern. Keratinocyte differentiation involves a coordinated program of expression and interaction of specific differentiation-related genes and proteins. To investigate the functional role of hsp27 in these processes we used a differential display approach to identify genes that might be regulated by the expression of hsp27 in human keratinocytes. mRNA was extracted from the human squamous carcinoma cell line A431 and a subclone stably transfected with human hsp27. Reverse transcriptase differential display polymerase chain reaction was performed using one base anchored oligo-dT and arbitrary primers. Differentially expressed genes were confirmed by northern blot analysis and further characterized by sequencing. Their expression in human skin and other tissues was investigated by northern blot and in situ hybridization. Out of five fragments detected with the initial reverse transcriptase differential display polymerase chain reaction screen one could be confirmed by northern blot to be downregulated in hsp27-overexpressing A431. This mRNA (G24) is not only downregulated by overexpression of hsp27 in A431 but also during differentiation in normal human keratinocytes in culture and in situ, situations where hsp27 is known to be induced. According to sequence analysis G24 represents a novel gene that does not code for a protein and thus might belong to the growing family of noncoding RNAs. These results not only demonstrate for the first time that overexpression of hsp27 by gene transfer is associated with regulation of gene expression but also reveal a novel differentiation-associated gene in human keratinocytes.

Association of HSPB2, a member of the small heat shock protein family, with mitochondria

We previously identified HSPB2, a new member of the small heat shock protein family, expressed in heart and skeletal muscles. In this study, we used a polyclonal anti-HSPB2 antibody and examined the subcellular localization of HSPB2 in differentiated C2C12 cells, KNS-81 cells, and NIH3T3 transfectants expressing human HSPB2. Double staining with anti-HSPB2 and various markers for cytoplasmic structures showed that HSPB2 was present in the cytosol as granules, some of which colocalized with mitochondria. This colocalization was not altered by a colchicine treatment, indicating that it is independent of microtubules. The subcellular fractionation of differentiated C2C12 cells revealed that HSPB2 was mainly detected in the postmitochondrial supernatant, but mild heat treatment enriched the amount of HSPB2 in the mitochondrial fraction. The expression of HSPB2 protected the cells from heat-induced cell death. In addition, Northern blot analysis revealed that expression of HSPB2 mRNA is higher in slow-twitch muscle than in fast-twitch muscle, which correlates with the amounts of mitochondria present in these two types of tissue. Taken together, these results suggest that HSPB2 may not localize in the matrix, but rather associates with the outer membrane components of the mitochondria and thus plays a role in the stress response.

Identification and characterization of hic-5/ARA55 as an hsp27 binding protein

hsp27 has been reported to participate in a wide variety of activities, including resistance to thermal and metabolic stress, regulation of growth and differentiation, and acting as a molecular chaperone or a regulator of actin polymerization. We hypothesized that these diverse functions are regulated in a cell- or tissue-specific manner via interaction with various binding proteins. To investigate this hypothesis, we used hsp27 as a "bait" to screen a yeast two-hybrid cDNA library from rat kidney glomeruli and identified a novel hsp27 binding protein, hic-5 (also known as ARA55), a focal adhesion protein and steroid receptor co-activator. Biochemical interaction between hsp27 and hic-5 was confirmed by co-immunoprecipitation, and critical protein.protein interaction regions were mapped to the hic-5 LIM domains and the hsp27 C-terminal domain. Initial analysis of the functional role of hsp27.hic-5 interaction revealed that hic-5 significantly inhibited the protection against heat-induced cell death conferred by hsp27 overexpression in co-transfected 293T cells. In contrast, when a non-hsp27-interacting hic-5 truncation mutant (hic-5/DeltaLIM4) was co-expressed with hsp27, the hic-5 inhibition of hsp27 protection was absent. We conclude that hic-5 is a true hsp27 binding protein and inhibits the ability of hsp27 to provide protection against heat shock in an interaction-dependent manner.

HSP22, a new member of the small heat shock protein superfamily, interacts with mimic of phosphorylated HSP27 ((3D)HSP27)

Most of the members of the superfamily of mammalian small heat shock or stress proteins are abundant in muscles where they play a role in muscle function and maintenance of muscle integrity. One member of this protein superfamily, human HSP27, is rapidly phosphorylated on three serine residues (Ser(15), Ser(78), and Ser(82)) during cellular response to a number of extracellular factors. To understand better the role of HSP27, we performed a yeast two-hybrid screen of a human heart cDNA library for HSP27-interacting proteins. By using the triple aspartate mutant, a mimic of phosphorylated HSP27, as "bait" construct, a protein with a molecular mass of 21.6 kDa was identified as an HSP27-binding protein. Sequence analysis revealed that this new protein shares an overall sequence identity of 33% with human HSP27. This protein also contains the alpha-crystallin domain in its C-terminal half, a hallmark of the superfamily of small stress proteins. Thus, the new protein itself is a member of this protein superfamily, and consequently we designated it HSP22. According to the two-hybrid data, HSP22 interacts preferentially with the triple aspartate form of HSP27 as compared with wild-type HSP27. HSP22 is expressed predominantly in muscles. In vitro, HSP22 is phosphorylated by protein kinase C (at residues Ser(14) and Thr(63)) and by p44 mitogen-activated protein kinase (at residues Ser(27) and Thr(87)) but not by MAPKAPK-2.

Heat shock protein 27 is a substrate of cGMP-dependent protein kinase in intact human platelets: phosphorylation-induced actin polymerization caused by HSP27 mutants

Phosphorylation of heat shock protein 27 (Hsp27) in human platelets by mitogen-activated protein kinase-activated protein kinase (MAPKAP) 2 is associated with signaling events involved in platelet aggregation and regulation of microfilament organization. We now show that Hsp27 is also phosphorylated by cGMP-dependent protein kinase (cGK), a signaling system important for the inhibition of platelet aggregation. Stimulation of washed platelets with 8-para-chlorophenylthio-cGMP, a cGK specific activator, resulted in a time-dependent phosphorylation of Hsp27. This is supported by the ability of cGK to phosphorylate Hsp27 in vitro to an extent comparable with the cGK-mediated phosphorylation of its established substrate vasodilator-stimulated phosphoprotein. Studies with Hsp27 mutants identified threonine 143 as a yet uncharacterized phosphorylation site in Hsp27 specifically targeted by cGK. To test the hypothesis that cGK could inhibit platelet aggregation by phosphorylating Hsp27 and interfering with the MAPKAP kinase phosphorylation of Hsp27, the known MAPKAP kinase 2-phosphorylation sites (Ser15, Ser78, and Ser82) as well as Thr143 were replaced by negatively charged amino acids, which are considered to mimic phosphate groups, and tested in actin polymerization experiments. Mimicry at the MAPKAP kinase 2 phosphorylation sites led to mutants with a stimulating effect on actin polymerization. Mutation of the cGK-specific site Thr143 alone had no effect on actin polymerization, but in the MAPKAP kinase 2 phosphorylation-mimicking mutant, this mutation reduced the stimulation of actin polymerization significantly. These data suggest that phosphorylation of Hsp27 and Hsp27-dependent regulation of actin microfilaments contribute to the inhibitory effects of cGK on platelet function.

Identification and characterization of a novel protein from Sertoli cells, PASS1, that associates with mammalian small stress protein hsp27

hsp27 is involved in development of tolerance to stress, possibly by its involvement in molecular chaperoning, maintenance of glutathione status, and/or modulation of microfilament structure and function. We hypothesize that hsp27 function depends on specific association with other proteins. To discover proteins that associate with hsp27, we made a differentiated rat Sertoli cell cDNA expression library and screened it using the yeast two-hybrid system. We obtained a cDNA coding for a novel protein of 428 amino acids that we have named PASS1 (protein associated with small stress proteins 1). BLAST searches did not reveal major similarity of PASS1 to any known protein, but the cDNA sequence matched several mouse EST clones and shares 34% homology with a Caenorhabditis elegans genomic sequence. In vitro, bacterially expressed glutathione S-transferase-PASS1 fusion protein bound to hsp27, and hsp27 was co-immunoprecipitated with c-Myc-tagged PASS1 overexpressed in several cell lines. The region of PASS1 responsible for association with hsp27 was identified as existing predominantly between amino acids 108 and 208 of PASS1. Northern hybridization and Western blot analysis demonstrated that PASS1 is expressed in several tissues, with the highest expression occurring in testis, primarily in Sertoli cells. The presence of a 1.4-kilobase PASS1 mRNA in kidney as well as the 1. 8-kilobase mRNA seen in other tissues suggests that alternate splicing may occur in this organ. Ectopic expression of PASS1 in two cultured cell lines was observed to inhibit the ability of hsp27 to protect cells against heat shock, indicating that PASS1 does interact with hsp27 in the live cell.

Factor XIII modulates intestinal epithelial wound healing in vitro

BACKGROUND

An acquired deficiency of blood coagulation factor XIII has been proposed to cause an impairment of intestinal wound healing and hemostasis in patients with inflammatory bowel diseases. Substitution of factor XIII seems to result in a rapid improvement of intestinal wound healing. Our aim was therefore to characterize the role of factor XIII in the modulation of intestinal wound healing in vitro.

METHODS

Factor XIII was added to subconfluent cultures of two non-transformed small-intestinal epithelial cell lines (IEC-6, IEC-18) and three human colon cancer-derived epithelial cell lines (T84, CaCo-2, HT-29) with subsequent assessment of cell proliferation with a colorimetric 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenylformazan (MTT) assay. The effects on epithelial cell migration in vitro were assessed with an in vitro wounding model of confluent IEC-6 cell monolayers.

RESULTS

Factor XIII caused a modest inhibition of proliferation of IEC-6 and IEC-18 cells. However, factor XIII significantly stimulated proliferation of T84, CaCo-2. and HT-29 cell lines. In addition, thrombin-activated factor Xill promoted intestinal epithelial cell restitution in vitro on average 2.5-fold. The modulatory effects of factor XIII could not be significantly blocked by anti-transforming growth factor beta (TGFbeta).

CONCLUSIONS

Factor XIII may promote intestinal epithelial wound healing by enhancement of epithelial cell restitution through a TGFbeta-independent pathway. This may explain previously described beneficial effects of factor XIII in the treatment of active ulcerative colitis.

Thiol/disulfide exchange between small heat shock protein 25 and glutathione

Murine small heat shock protein 25 (Hsp25) carries a single Cys-residue at position 141 of its amino acid sequence. In glutathione redox buffers, Hsp25 equilibrates between reduced protein (PSH), mixed disulfide (PSSG) and protein dimer (PSSP) forms. At highly oxidative conditions, native Hsp25 predominantly forms PSSP while denatured Hsp25 forms PSSG. Conversion of PSSP to PSSG correlates with urea and temperature denaturation of tertiary and/or quaternary structure of Hsp25. At pH 7.5, 25 degreesC, the second-order rate constant for the formation of PSSP in the reaction of native PSH with GSSG is 20.1+/-1.4 M-1 min-1. This is approximately 3-fold lower than the reaction velocity of GSSG with a typical, unhindered thiol of pKa 8.6. At redox equilibrium, the fractions of PSSP, PSSG, and PSH depend on the concentration of GSH and less on the ratio [GSH]/[GSSG] (R). At a constant R, the fractions of PSSG and PSH species depend similarly on GSH concentration, being approximately equal in glutathione redox buffers with low R. It is concluded that in oligomeric complexes, Hsp25 subunits in vitro form stable dimers, in which the reacting -SH groups are in a proximity to form intersubunit disulfide bonds. Within a reaction of one of these -SH groups with GSSG, steric hindrances and electrostatic repulsion complicate penetration of another reduced or oxidized glutathione molecule to the reaction site.

Plasma transglutaminase in hypertrophic chondrocytes: expression and cell-specific intracellular activation produce cell death and externalization

We previously used subtractive hybridization to isolate cDNAs for genes upregulated in chick hypertrophic chondrocytes (Nurminskaya, M. , and T.F. Linsenmayer. 1996. Dev. Dyn. 206:260-271). Certain of these showed homology with the "A" subunit of human plasma transglutaminase (factor XIIIA), a member of a family of enzymes that cross-link a variety of intracellular and matrix molecules. We now have isolated a full-length cDNA for this molecule, and confirmed that it is avian factor XIIIA. Northern and enzymatic analyses confirm that the molecule is upregulated in hypertrophic chondrocytes (as much as eightfold). The enzymatic analyses also show that appreciable transglutaminase activity in the hypertrophic zone becomes externalized into the extracellular matrix. This externalization most likely is effected by cell death and subsequent lysis-effected by the transglutaminase itself. When hypertrophic chondrocytes are transfected with a cDNA construct encoding the zymogen of factor XIIIA, the cells convert the translated protein to a lower molecular weight form, and they initiate cell death, become permeable to macromolecules and eventually undergo lysis. Non-hypertrophic cells transfected with the same construct do not show these degenerative changes. These results suggest that hypertrophic chondrocytes have a novel, tissue-specific cascade of mechanisms that upregulate the synthesis of plasma transglutaminase and activate its zymogen. This produces autocatalytic cell death, externalization of the enzyme, and presumably cross-linking of components within the hypertrophic matrix. These changes may in turn regulate the removal and/or calcification of this hypertrophic matrix, which are its ultimate fates.

Type X collagen and other up-regulated components of the avian hypertrophic cartilage program

Elucidating the cellular and molecular processes involved in growth and remodeling of skeletal elements is important for our understanding of congenital limb deformities. These processes can be advantageously studied in the epiphyseal growth zone, the region in which all of the increase in length of a developing long bone is achieved. Here, young chondrocytes divide, mature, become hypertrophic, and ultimately are removed. During cartilage hypertrophy, a number of changes occur, including the acquisition of synthesis of new components, the most studied being type X collagen. In this review, which is based largely on our own work, we will first examine the structure and properties of the type X collagen molecule. We then will describe the supramolecular forms into which the molecule becomes assembled within tissues, and how this changes its physical properties, such as thermal stability. Certain of these studies involve a novel, immunohistochemical approach that utilizes an antitype X collagen monoclonal antibody that detects the native conformation of the molecule. We describe the developmental acquisition of the molecule, and its transcriptional regulation as deduced by in vivo footprinting, transient transfection, and gel-shift assays. We provide evidence that the molecule has unique diffusion and regulatory properties that combine to alter the hypertrophic cartilage matrix. These conclusions are derived from an in vitro system in which exogenously added type X collagen moves rapidly through the cartilage matrix and subsequently produces certain changes mimicking ones that have been shown normally to occur in vivo. These include altering the cartilage collagen fibrils and effecting changes in proteoglycans. Last, we describe the subtractive hybridization, isolation, and characterization of other genes up-regulated during cartilage hypertrophy, with specific emphasis on one of these--transglutaminase.

The effect of the intersubunit disulfide bond on the structural and functional properties of the small heat shock protein Hsp25

The murine small heat shock protein Hsp25 carries a single cysteine residue in position 141 of its amino acid sequence. Interestingly, Hsp25 can exist within the cell as covalently bound dimer which is linked by an intermolecular disulfide bond between two monomers. Oxidative stress caused by treatment of the cells with diamide, arsenite, or hydrogen peroxide leads to an increase in Hsp25-dimerisation which can be blocked by simultaneous treatment with reducing agents. Recombinant Hsp25 was prepared in an oxidized dimeric (oxHsp25) and reduced monomeric (redHsp25) from. The two species were compared with regard to secondary structure, stability, oligomerization properties and their chaperone activity. It is demonstrated by CD measurements in the far UV region that there are no significant differences in the secondary structure and temperature- or pH-stability of oxHsp25 and redHsp25. However, according to CD measurements in the near UV region an increase in the asymmetry of the microenvironment of aromatic residues in oxHsp25 is observed. Furthermore, an increase in stability of the hydrophobic environment of the tryptophan residues mainly located in the N-terminal domain of the protein against urea denaturation is detected in oxHsp25. Both reduced and oxidized Hsp25 from oligomeric complexes of similar size and stability against detergents and both species prevent thermal aggregation of citrate synthase and assist significantly in oxaloacetic acid-induced refolding of the enzyme. Hence, the overall secondary structure, the degree of oligomerization and the chaperone activity of Hsp25 seem independent of the formation of the intermolecular disulfide bond and only the stability of the hydrophobic N-terminal part of the molecule is influenced by formation of this bound. The obtained data do not exclude the possible involvement of dimerization of this protein in other cellular functions, e.g. in intracellular sulfhydryl-buffering or in the protection of actin filaments from fragmentation upon oxidative stress.

Mechanism of platelet inhibition by nitric oxide: in vivo phosphorylation of thromboxane receptor by cyclic GMP-dependent protein kinase

Nitric oxide (NO) is a potent vasodilator and inhibitor of platelet activation. NO stimulates production of cGMP and activates cGMP-dependent protein kinase (G kinase), which by an unknown mechanism leads to inhibition of Galphaq-phospholipase C-inositol 1, 4,5-triphosphate signaling and intracellular calcium mobilization for several important agonists, including thromboxane A2 (TXA2). To explore the mechanism of platelet inhibition by NO, activation of platelet TXA2 receptors in the presence of cGMP was studied. The nonhydrolyzable analog 8-bromo-cyclic GMP (8-Br-cGMP) potently inhibited activation of the TXA2-specific GTPase in platelet membranes in a concentration-dependent fashion, suggesting that G kinase catalyzes the phosphorylation of some proximal component of the receptor-G protein signaling pathway. Nanomolar concentrations of G kinase were found to catalyze the phosphorylation of platelet TXA2 receptors in vitro, but not Galphaq copurifying with the TXA2 receptors in these experiments. Using immunoaffinity methods, in vivo phosphorylation of TXA2 receptors by cyclic GMP was demonstrated from 32P-labeled cells treated with 8-Br-cGMP. Peptide mapping studies of in vivo phosphorylated TXA2 receptors demonstrated cGMP mediates phosphorylation of the carboxyl terminus of the TXA2 receptor. G kinase also catalyzed the phosphorylation of peptides corresponding to the cytoplasmic tails of both alpha and beta forms of the receptor but not control peptide or a peptide corresponding to the third intracytoplasmic loop of the TXA2 receptor. These data identify TXA2 receptors as cGMP-dependent protein kinase substrates and support a novel mechanism for the inhibition of cell function by NO in which activation of G kinase inhibits signaling by G protein-coupled receptors by catalyzing their phosphorylation.

Novel aspects of blood coagulation factor XIII. I. Structure, distribution, activation, and function

Blood coagulation factor XIII (FXIII) is a protransglutaminase that becomes activated by the concerted action of thrombin and Ca2+ in the final stage of the clotting cascade. In addition to plasma, FXIII also occurs in platelets, monocytes, and monocyte-derived macrophages. While the plasma factor is a heterotetramer consisting of paired A and B subunits (A2B2), its cellular counterpart lacks the B subunits and is a homodimer of potentially active A subunits (A2). The gene coding for the A and B subunits has been localized to chromosomes 6p24-25 and 1q31-32.1, respectively. The genomic as well as the primary protein structure of both subunits has been established, and most recently the three-dimensional structure of recombinant cellular FXIII has also been revealed. Monocytes/macrophages synthesize their own FXIII, and very likely FXIII in platelets is synthesized by the megakaryocytes. Cells of bone marrow origin seem to be the primary site for the synthesis of subunit A in plasma FXIII, but hepatocytes might also contribute. The B subunit of plasma FXIII is synthesized in the liver. Plasma FXIII circulates in association with its substrate precursor, fibrinogen. Fibrin(ogen) has an important regulatory role in the activation of plasma FXIII. The most important steps of the activation of plasma FXIII are the proteolytic removal of activation peptide by thrombin, the dissociation of subunits A and B, and the exposure of the originally buried active site on the free A subunits. The end result of this process is the formation of an active transglutaminase, which cross-links peptide chains through epsilon(gamma-glutamyl)lysyl isopeptide bonds. Cellular FXIII in platelets becomes activated through a nonproteolytic process. When intracytoplasmic Ca2+ is raised during platelet activation, the zymogen--in the absence of subunit B--assumes an active configuration. The protein substrates of activated FXIII include components of the clotting-fibrinolytic system, adhesive and contractile proteins. The main physiological function of plasma FXIII is to cross-link fibrin and protect it from the fibrinolytic plasmin. The latter effect is achieved mainly by covalently linking alpha 2 antiplasmin, the most potent physiological inhibitor of plasmin, to fibrin. Plasma FXIII seems to be involved in wound healing and tissue repair, and it is essential to maintaining pregnancy. Cellular FXIII, if exposed to the surface of the cells, might support or perhaps take over the hemostatic functions of plasma FXIII; however, its intracellular role has remained mostly unexplored.

Biosynthesis of plasma factor XIII: evidence for transcription and translation in hepatoma cells

Factor XIIIa belongs to a family of ubiquitous transglutaminases, which catalyze formation of covalent bonds between the epsilon-amino group of specific lysines and the gamma-carboxyl group of glutamines. Factor XIII is synthesized as a zymogen and after activation, it participates in both the coagulation and fibrinolytic mechanisms. Most transglutaminases are intracellular, but factor XIII is both intracellular and extracellular. the biosynthesis of extracellular (plasma) factor XIII, with the structure of a noncovalent heterotetramer, A2B2, is complex. Here, evidence is presented from PCR analysis and Northern blotting that mRNAs for both A and B subunits are present in the liver. The distribution of mRNA, specific for factor XIII subunits, in various human tissues was also analyzed. Among the tissues examined, the only signal for B subunit was found in the liver. For subunit A, the signal was observed in placenta, liver, kidney, lung, skeletal muscle and heart with varying intensities; in brain or pancreas there was no signal. With an immunoperoxidase method, factor XIII A subunit was identified in the PLC/PRF/5 cell line. By ELISA and reverse immunoblotting, with antibodies specific for the A-B complex, it was also shown that these cells produce and secrete factor XIII. From all of these results, we conclude that the liver is a source of plasma factor XIII, and that the complex A2B2 is secreted from these cells.