The effect of platelet releasate on wound healing in animal models

The alpha granules of platelets contain growth factors that are important in wound healing. We found that a major effect of thrombin-induced human platelet releasates in animal models of wound healing is to enhance the development of granulation tissue and new connective tissue matrix. These studies provide further evidence that platelet-derived protein factors may be useful in treating full-thickness dermal wounds by increasing the rate of granulation tissue formation.

Two burgeoning families of platelet factor 4-related proteins: mediators of the inflammatory response

The inflammatory response involves the recruitment and activation of various types of cells from the systemic circulation and from local tissues. One important component of the inflammatory response is the activation of platelets at sites of tissue injury and inflammation. In particular, activated platelets release large amounts of two proteins, platelet factor 4 (PF4) and beta-thromboglobulin (beta TG), which mediate several inflammatory processes. Recently, many novel proteins that are structurally related to PF4 and beta TG have been identified. The PF4-related proteins are secreted by white blood cells, endothelial cells, and fibroblasts in response to various inflammatory and mitogenic stimuli. Like PF4, these proteins appear to be inflammatory response mediators; several of them are potent chemoattractants, activating agents, or mitogens for specific cell types that are involved in the inflammatory response. The study of PF4-related proteins provides new insight into the mechanisms of the immune response, and may result in the development of new therapeutic agents.

Biologic and biochemic properties of recombinant platelet factor 4 demonstrate identity with the native protein

Platelet factor 4 (PF4) is a 70 amino acid protein released from the alpha-granules of platelets after activation. The exact biologic function of this protein is unknown. We have constructed an expression vector for recombinant PF4 (rPF4) in the T7-based promoter vector pT7-7 to better study the relationship between PF4 structure and function. The protein was expressed in Escherichia coli and purified to homogeneity by heparin-agarose affinity chromatography and reverse-phase high-performance liquid chromatography. Purity of protein was confirmed by immunoblot analysis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, which resulted in a single component with a molecular weight of 8,000 daltons. The amino acid composition and sequence of the N-terminal 20 residues showed that rPF4 is identical to PF4 prepared from human platelets (hPF4), except for an N-terminal initiating methionine residue. Immunoblots revealed that rPF4 and hPF4 bound polyclonal anti-hPF4 equally well, while chemotaxis experiments demonstrated similar potencies as neutrophil attractants. Dose-dependent neutrophil chemotactic responses and competitive studies with polyclonal anti-hPF4 antiserum further demonstrate similar chemotactic properties of the two PF4 species. In conclusion, our data show that this recombinant protein and the native protein appears to have similar immunologic, heparin-binding, and chemotactic properties. The chemotactic properties of hPF4 appear to be entirely intrinsic to the protein and not due, in part, to any contaminating protein. Furthermore, our expression vector should prove useful for the construction of recombinant forms of PF4 to investigate structure/function relationships of this biologically important protein.

Inhibition of angiogenesis by recombinant human platelet factor-4 and related peptides

Recombinant human platelet factor-4 (rhPF4), purified from Escherichia coli, inhibited blood vessel proliferation in the chicken chorioallantoic membrane in a dose-dependent manner. Treatment of several cell types with rhPF4 in vitro suggested that the angiostatic effect was due to specific inhibition of growth factor-stimulated endothelial cell proliferation. The inhibitory activities were associated with the carboxyl-terminal, heparin-binding region of the molecule and could be abrogated by including heparin in the test samples, an indication that sulfated polysaccharides might modulate the angiostatic activity of platelet factor-4 in vivo. Understanding of the mechanisms of control of angiogenesis by endogenous proteins should facilitate the development of effective treatments for diseases of pathogenic neovascularization such as Kaposi's sarcoma, diabetic retinopathy, and malignant tumor growth.

Identification by sequence analysis of chemotactic factors for monocytes produced by normal and transformed cells stimulated with virus, double-stranded RNA or cytokine

A monocyte chemotactic activity was found to be released by various types of cultured human cells after appropriate stimulation: normal diploid fibroblasts, peripheral blood mononuclear cells or monocytes isolated therefrom, and a number of tumor cell lines, including osteosarcoma (MG-63) and hepatoma (Malavu) but not melanoma (Bowes) cells. Cultures of diploid human fibroblasts and these tumor cells stimulated with interleukin (IL) 1 or double-stranded RNA [poly(rI).poly(rC)], or infected with viruses (measles or rubella viruses) were found to produce chemotactic activity for both monocytes and granulocytes. Media collected from fibroblasts treated with E. coli or IL 6 did not contain such activity. Granulocyte and monocyte chemotactic activities were serologically distinct, and could be separated by successive chromatographical procedures. While the granulocyte chemotactic activity of both fibroblasts and MG-63 cells had previously been identified as granulocyte chemotactic protein/IL 8, the monocyte chemotactic activity from MG-63 cells was identified by amino acid sequence analysis as a different protein recently described to be released by human glioma and myelomonocytic cell lines. In view of the similarity in their chromatographical behavior, monocyte chemotactic activities from fibroblasts, MG-63 cells and fresh monocytes can probably be assigned to identical molecules. Cultures of unfractionated peripheral blood cells, however, were found to release an additional monocyte chemotactic protein, identifiable by amino acid sequence analysis as platelet factor 4.

Molecular mechanisms of avian neural crest cell migration on fibronectin and laminin

We have examined the molecular interactions of avian neural crest cells with fibronectin and laminin in vitro during their initial migration from the neural tube. A 105-kDa proteolytic fragment of fibronectin encompassing the defined cell-binding domain (65 kDa) promoted migration of neural crest cells to the same extent as the intact molecule. Neural crest cell migration on both intact fibronectin and the 105-kDa fragment was reversibly inhibited by RGD-containing peptides. The 11.5-kDa fragment containing the RGDS cell attachment site was also able to support migration, whereas a 50-kDa fragment corresponding to the adjacent N-terminal portion of the defined cell-binding domain was unfavorable for neural crest cell movement. In addition to the putative "cell-binding domain," neural crest cells were able to migrate on a 31-kDa fragment corresponding to the C-terminal heparin-binding (II) region of fibronectin, and were inhibited in their migration by exogenous heparin, but not by RGDS peptides. Heparin potentiated the inhibitory effect of RGDS peptides on intact fibronectin, but not on the 105-kDa fragment. On substrates of purified laminin, the extent of avian neural crest cell migration was maximal at relatively low substrate concentrations and was reduced at higher concentrations. The efficiency of laminin as a migratory substrate was enhanced when the glycoprotein occurred complexed with nidogen. Moreover, coupling of the laminin-nidogen complex to collagen type IV or the low density heparan sulfate proteoglycan further increased cell dispersion, whereas isolated nidogen or the proteoglycan alone were unable to stimulate migration and collagen type IV was a significantly less efficient migratory substrate than laminin-nidogen. Neural crest cell migration on laminin-nidogen was not affected by RGDS nor by YIGSR-containing peptides, but was reduced by 35% after addition of heparin. The predominant motility-promoting activity of laminin was localized to the E8 domain, possessing heparin-binding activity distinct from that of the N-terminal E3 domain. Migration on the E8 fragment was reduced by greater than 70% after addition of heparin. The E1' fragment supported a minimal degree of migration that was RGD-sensitive and heparin-insensitive, whereas the primary heparin-binding E3 fragment and the cell-adhesive P1 fragment were entirely nonpermissive for cell movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of platelet factor 4 on the neutralization of heparin by protamine

In vitro, PF4 is comparable to protamine sulfate in the neutralization of heparin, but the complexes formed with heparin are different. Even with an excess of PF4, no large PF4-heparin complexes are formed and none of the complexes are able to activate ATIII, nor do these complexes dissociate on incubation in plasma at 37 degrees C. The action of PF4 and protamine is complementary. However, excess protamine displaces PF4 or prevents its complexing with heparin. When excess protamine is used to neutralize heparin in the presence of PF4, large heparin-protamine complexes are formed incorporating PF4. In contrast to the heparin-protamine complexes formed without PF4, these do not activate ATIII nor do they dissociate on incubation. Since PF4 is liberated during ECB procedures, its contribution to the stability of heparin-protamine complexes in vivo may influence the amount of protamine needed to neutralize heparin as well as affect the reactions which have been reported on injection of protamine after ECB.

The blood platelet as an inflammatory cell

Platelets seem to be involved in inflammatory responses. They release substances which can cause vasodilation or vasoconstriction. Platelet Derived Growth Factor (PDGF) induces monocyte accumulation and activation and there is evidence that some mediators released from these cells can affect neutrophil migration as well as function. Furthermore, platelet derived cytokines participate in the processes of tissue repair, particularly fibroblast proliferation and matrix remodelling. We review some of the possible mechanisms which have been proposed to explain this role of platelets in the appearance and persistence of inflammatory reactions at the tissue sites.

Inhibition of human megakaryocytopoiesis in vitro by platelet factor 4 (PF4) and a synthetic COOH-terminal PF4 peptide

We report that highly purified human platelet factor 4 (PF4) inhibits human megakaryocytopoiesis in vitro. At greater than or equal to 25 micrograms/ml, PF4 inhibited megakaryocyte colony formation approximately 80% in unstimulated cultures, and approximately 58% in cultures containing recombinant human IL 3 and granulocyte-macrophage colony-stimulating factor. Because PF4 (25 micrograms/ml) had no effect on either myeloid or erythroid colony formation lineage specificity of this effect was suggested. A synthetic COOH-terminal PF4 peptide of 24, but not 13 residues, also inhibited megakaryocyte colony formation, whereas a synthetic 18-residue beta-thromboglobulin (beta-TG) peptide and native beta-TG had no such effect when assayed at similar concentrations. The mechanism of PF4-mediated inhibition was investigated. First, we enumerated total cell number, and examined cell maturation in control colonies (n = 200) and colonies (n = 100) that arose in PF4-containing cultures. Total cells per colony did not differ dramatically in the two groups (6.1 +/- 3.0 vs. 4.2 +/- 1.6, respectively), but the numbers of mature large cells per colony was significantly decreased in the presence of PF4 when compared with controls (1.6 +/- 1.5 vs. 3.9 +/- 2.3; P less than 0.001). Second, by using the human leukemia cell line HEL as a model for primitive megakaryocytic cells, we studied the effect of PF4 on cell doubling time, on the expression of both growth-regulated (H3, p53, c-myc,and c-myb), and non-growth-regulated (beta 2-microglobulin) genes. At high concentrations of native PF4 (50 micrograms/ml), no effect on cell doubling time, or H3 or p53 expression was discerned. In contrast, c-myc and c-myb were both upregulated. These results suggested the PF4 inhibited colony formation by impeding cell maturation, as opposed to cell proliferation, perhaps by inducing expression of c-myc and c-myb. The ability of PF4 to inhibit a normal cell maturation function was then tested. Megakaryocytes were incubated in synthetic PF4, or beta-TG peptides for 18 h and effect on Factor V steady-state mRNA levels was determined in 600 individual cells by in situ hybridization. beta-TG peptide had no effect on FV mRNA levels, whereas a approximately 60% decrease in expression of Factor V mRNA was found in megakaryocytes exposed to greater than or equal 100 ng/ml synthetic COOH-terminal PF4 peptide. Accordingly, PF4 modulates megakaryocyte maturation in vitro, and may function as a negative autocrine regulator of human megakaryocytopoiesis.

Interaction of platelet factor four with cultured vascular endothelial cells

Platelets secrete a low-molecular-weight protein, platelet factor four (PF-4), which binds to and neutralizes heparin and related sulfated glycosaminoglycans (GAGs). To examine the interactions of PF-4 with the GAGs present on endothelial cell surfaces, we incubated 125I-PF-4 with cell suspensions derived from confluent monolayers of cultured bovine aortic endothelium. Binding of 125I-PF-4 was inhibited by a 100-fold excess of nonradioactive PF-4 and varied with duration and temperature of incubation. At 4 degrees C, binding reached equilibrium at 20 minutes with kd = 2.87 mumol/L and Bmax of 63.83 pmol/10(5) cells. Binding capacity was reduced 83.4% by brief incubation of endothelial cells with trypsin and 46.67% by incubation with Flavobacterium heparinase, but was unchanged by chondroitin-ABCase treatment. At 37 degrees C, PF-4 was internalized by confluent monolayer of bovine aortic endothelial cells primarily through low-affinity adsorptive endocytosis. The internalized PF-4 was degraded to amino acids and small peptides with 50% conversion after 18-hour incubation. These studies demonstrate that a secreted platelet protein can bind to and enter endothelial cells. Binding may explain the rapid clearance of released PF-4 from plasma and could have important local effects on endothelial structure and function.

No evidence of platelet activation during atrial pacing in subjects with stable angina

Platelet behaviour (activation) in ischemic heart disease (stable angina) during pacing-induced tachycardia was studied. ECG was recorded during the trial. Ischemic heart disease (IHD) subjects had 75% or more narrowing of the luminal diameter of a coronary artery, demonstrated by coronary angiography. Eight subjects needing cardiac catheterism because of supraventricular rhythm disturbances with no evidence of IHD were studied as controls. Beta-thromboglobulin (beta-tg) and platelet factor 4 (PF4) were studied as platelet activation markers; beta-tg and PF4 were evaluated before atrial pacing in peripheral venous blood and, by catheterism, before and at maximum pacing rate in coronary venous sinus (CVS) and in ascending aorta (AA). Catheterism and blood withdrawals were performed in order to reduce platelet activation in vivo. No significant difference in platelet activation between IHD patients and control group in peripheral venous blood were found. No trans-myocardial gradient neither in IHD subjects nor in controls were observed. In conclusion, no platelet activation in IHD patients during pacing-induced tachycardia could be observed.

Platelet specific proteins (beta-thromboglobulin and platelet factor 4) in normal pregnancy and in pregnancy complicated by preeclampsia

Pregnancy is accompanied by significant alterations of platelet function. Platelet activity can be determined by measurement of plasma levels of secreted platelet proteins. In this study we determined plasma levels of beta-Thromboglobulin (beta-TG) and Platelet Factor 4 (PF4) simultaneously in 35 women with uncomplicated pregnancy and in 15 patients with preeclampsia in third trimester of gestation. Additionally, PF4 plasma levels were measured using a commercially available Radio Immunoassay (RIA) and an Enzyme Immunoassay (EIA) simultaneously and values obtained were compared. Platelet count and creatinine were in the normal range in both groups; however, significantly higher levels of beta-TG (P less than 0.0005) and PF4 (P less than 0.0001) were found in case of preeclampsia. High levels of platelet proteins emphasize the active role of the platelets in the alterations of hemostasis in cases of preeclampsia.

Human platelet interactions with surfaces of type I collagen, chondroitin-4-sulphate, and chondroitin-6-sulphate in vitro

In vitro human platelet interactions with surfaces of type I collagen, chondroitin-6-sulphate (CH-6-S), chondroitin-4-sulphate (CH-4-S), a CH-6-S/collagen layer, and a collagen-CH-6-S complex were investigated. Polystyrene and silanized glass served as controls. Platelet counts, platelet factor 4 released, and platelet aggregating ability for the different surfaces were compared with controls. Platelet count and platelet factor 4 release data showed that there were no differences between surfaces of CH-6-S, CH-4-S and the controls. However, significant differences in platelet counts and platelet factor 4 released were found when collagen, the CH-6-S/collagen layer, and the collagen-CH-6-S complex were compared with controls. The pure type I collagen surface had the greatest influence on platelet activation. The collagen-CH-6-S complex had a greater effect that the CH-6-S/collagen layer on platelet activation. It appears that chondroitin-6-sulphate can modify the platelet activity of type I collagen.

Failure of platelet markers to evaluate membrane biocompatibility during a combined hemodialysis-hemoperfusion treatment

Plasma levels of the platelet markers beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4) are among the most sophisticated indexes of biocompatibility available to evaluate new members for hemodialysis. This investigation was designed to determine the extent of platelet activation by measuring the alpha-granule release products, beta-TG and PF4; anticoagulation and thrombogenesis by monitoring plasma heparin; and fibrinopeptide A (FPA) and thromboxane B2 (TX B2) levels during treatment with a combined hemodialysis-hemoperfusion system. Both in vivo and in vitro results showed that the platelet markers had a pattern different from that generally observed during treatment with hemodialysis alone. This is due to the avidity of charcoal for the markers studied, which therefore cannot be used to evaluate the biocompatibility of the system.

Platelet function and prostaglandins in patients with peripheral vascular disease treated with calcium dobesilate

In a double-blind placebo controlled study, 25 male patients (age range: 48-67 years) suffering from peripheral vascular disease were treated daily for 4 weeks with either 2 g of calcium dobesilate (n = 13) or placebo (n = 12). Different platelet and prostaglandin parameters were examined before and at the end of therapy. The number of circulating endothelial cells decreased significantly (4.9 +/- 2.9 to 2.0 +/- 1.9; p less than 0.0004). In addition, a decrease in serum TXB2 was noted (p less than 0.0001), but no significant change in plasma TXB2. The fibrinogen half-life was shortened (p less than 0.0001) and the platelet half-life was prolonged (p less than 0.04). The level of beta TG was decreased (p less than 0.006), but PF4 was unchanged. No alteration in the conversion of exogenous 14C-arachidonic acid by platelets to eicosanoids was observed. These observations indicate that calcium dobesilate is able to exert a favourable effect on some parameters of platelet function and prostaglandin synthesis which are important in the regulation of the hemostatic process. It is suggested that these pharmacological actions might explain, at least in part, the beneficial clinical effect of the drug.

An unclassified platelet function disorder associated with bleeding tendency

A 7-year-old boy with a history of bleeding tendency showed a prolongation of bleeding time and a decrease in platelet adhesiveness. The platelets of the patient, however, had a normal reaction to ADP, collagen, epinephrine, arachidonic acid, bovine fibrinogen, ristocetin, A-23187 and thrombin-induced aggregation, and their shape was determined by electron microscopy to be normal. Therefore, this disorder could not be thought to belong to any known platelet dysfunction. On the other hand, an increase in clot retraction, a reversal of ATP/ADP, a decrease in beta-thromboglobulin and platelet factor 4 in the platelets, and an elevation of plasma levels of released beta-thromboglobulin from the platelets were observed in the patient. We don't know any cases with such an association of hypo- and hyperfunction of platelets.

Platelet factor 4 enhances fibrin fiber polymerization

In order to further examine the role of platelet secretory products on fibrin assembly, we have examined the effects of purified platelet factor 4 (PF4) on fibrin formation in vitro. The kinetics of fibrin formation and the structural properties of the resulting fibrin gel are strongly affected by PF4. In the presence of 7.6 ng/mL PF4, the fibrin fiber mass-length ratio increases from 3.5 X 10(12) daltons/cm. These results emphasize the importance of the local microenvironment in clot structure and its subsequent fate at the site of vascular injury.

Perisinusoidal fibrosis of the liver in patients with thrombocytopenic purpura

10 patients with thrombocytopenic purpura (TP) underwent splenectomy. Eight of these patients had idiopathic TP (certain or probable). All had normal liver function tests. Liver histology of the surgical biopsy was normal with the exception of a non specific mild portal infiltration in 6 cases. On Sirius red staining the perisinusoidal network was normal in 3 cases, mildly or moderately increased in 5 cases and often associated with perivenular fibrosis. Collagen types I, III, IV, laminin and fibronectin were increased in the 8 biopsies tested. On semi-thin sections, numerous Kupffer cells were observed. Under the electron microscope, sinusoidal abnormalities were very similar in all 7 patients studied: numerous Kupffer cells containing abundant lysosomes, numerous collagen bundles in the Disse space, active endothelial cells, transformation of some perisinusoidal cells into cells with some of the characteristics of fibroblasts (increased RER) and myofibroblasts (peripheral condensations of the filamentous network), increased fragments of basement membrane-like material. In two cases there was an increase in the number of perisinusoidal cells loaded with lipids. The similarity of the lesions and the absence of other fibrogenic causes (except in 2 cases) suggest that TP may represent another group of diseases with perisinusoidal fibrosis. The aetiology of fibrosis remains unknown but platelet derived growth factor and activated macrophages may play a major role.

Platelet and vessel wall interaction and the genesis of atherosclerosis

On the basis of in vitro studies, biochemistry and cell biology the interactions between platelets, other circulating blood cells and the vessel wall are complex and wide-ranging. In essence, the effects of platelets adhering to injured luminal surface might include: Aggregation and adherence of further platelets; Activation of coagulation and fibrin formation; Displacement of AT-III from endothelial surface; Recruitment and activation of potentially injurious polymorphonuclear leukocytes and monocytes; Recruitment and proliferation of smooth muscles cells from the vessel media to the intima; Vasoconstriction; Initiation of feedback mechanisms such as PGI2 release to limit the process. These effects are capable of contributing to the initial lesions and the progression of the atherosclerosis. As the lesions become more complex, and particularly as the normal endothelium is lost, there is morphological evidence that there is deposition of thrombus (for discussion see Chapter 4) and 111In-labelled platelet imaging experiments in patients document platelet uptake onto atherosclerotic plaques (Davis et al, 1978).

Protease-induced immunoregulatory activity of platelet factor 4

Intravenous injection of human or mouse serum or platelet material secreted from appropriately stimulated platelets ("releasate") together with antigen alleviates the immunosuppression in SJL/J mice induced by injection of irradiated lymphoma cells or in (CB6)F1 mice induced by injection of concanavalin A. We now report that injection of releasate from 10(6) human platelets restores plaque-forming cells to the unsuppressed number; greater amounts increase responses further. Immunoregulatory activity is released from platelets exposed to thrombin in parallel with other alpha-granule components. Heparin-agarose absorbs activity. Purified platelet factor 4 (PF4) has activity; beta-thromboglobulin and platelet-derived growth factor have little or none. Activity in serum is neutralized by goat anti-human PF4. An enzymatic step is necessary for production of immunoregulatory activity. Releasates boiled immediately after platelet aggregation with 250 nM A23187 or those produced by adding A23187 in the presence of 100 microM serine protease inhibitor (p-amidinophenyl)methanesulfonyl fluoride (APMSF) are ineffective, whereas releasates boiled or mixed with APMSF after incubation for 60 min are active. Activity is generated by incubating a mixture of heparin-absorbed releasate (as enzyme source) and heparin-agarose eluate of releasate made in the presence of APMSF (as substrate source). The enzymatic step does not alter the heparin-neutralizing activity of PF4. Apparently a secreted platelet protease converts PF4 to a form with immunoregulatory activity.