Human platelet factor 4: subcellular localization and characteristics of release from intact platelets

Platelet release reaction during EDTA-induced platelet agglutinations and inhibition of EDTA-induced platelet agglutination by anti-glycoprotein II b/III a complex monoclonal antibody

To characterize the nature of EDTA-induced platelet agglutination, the spontaneous release of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4) was examined during EDTA-induced platelet agglutinations. A slight release of beta-TG and PF4 was observed when EDTA-anticoagulated whole blood from cases with EDTA-induced platelet agglutination was kept for 60 minutes, whereas a high spontaneous release of these proteins was found from normal blood anticoagulated with EDTA. These findings imply that EDTA-dependent platelet agglutinin may stabilize the platelet membrane surfaces. Secondly, we found that pretreatment of fresh blood with anti-glycoprotein (GP) II b/III a complex monoclonal antibody dramatically reduced EDTA-induced platelet agglutinations. This study indicated that the binding sites of EDTA-dependent antibody might be GP II b/III a complex. The use of an anti-GP II b/III a complex monoclonal antibody may be useful in avoiding analytical errors in some cases with EDTA-induced pseudothrombocytopenia.

Human platelet dense granules: improved isolation and preliminary characterization of [3H]-serotonin uptake and tetrabenazine-displaceable [3H]-ketanserin binding

An improved method for the isolation of human platelet dense granules was developed. A good yield (45%) of highly enriched (69-fold, based on serotonin content) dense granules was obtained after mild sonication and Percoll gradient centrifugation. The method has facilitated characterization of the granule, permitting the first report of Km and Vmax values for [3H]-serotonin uptake, as well as the first determination of Kd and Bmax values for tetrabenazine-displaceable [3H]-ketanserin binding, in the human platelet dense granule. The rates and affinities (Vmax 1.45 nmol/mg/min, Km 0.93 uM) of [3H]-serotonin uptake were similar to those previously reported for porcine dense granules. Tetrabenazine-displaceable [3H]-ketanserin binding was observed with a Kd (9.4 nM) similar to, and a Bmax (5.4 pmol/mg) approximately 10-fold lower than, that previously seen in bovine chromaffin granules.

Relationships between chemical structure and inhibition of ADP-stimulated human thrombocyte release of serotonin and platelet factor 4

The inhibitory potencies of carbamoylpiperidinoalkane and N-alkylnipecotoylpiperazine derivatives on ADP-stimulated human blood platelet aggregation, serotonin (5-HT) release and platelet factor 4 (PF-4) release were evaluated. The procedure was designed to allow concurrent determination of all three sets of values. Most compounds were more than twice as potent in blocking PF-4 (X = 91 +/- 1 (S.E., n = 7)%) compared to their inhibition of 5-HT (X = 38 +/- 1(S.E., n = 6)%) release; the one compound which failed to meet these criteria was still decidedly more powerful in impeding PF-4 than 5-HT release. Since the compounds' platelet aggregation-inhibitory values were within the same range as their 5-HT release-blocking potencies, but had a strikingly greater impact in arresting PF-4 release, it is suggested that the platelet plasma membrane and the lining enveloping the dense bodies may share certain commonalities, while the sheathing encasing the alpha-granules may differ from both in a tangible manner.

Study of in vivo platelet activation in uncomplicated essential hypertension

Beta-thromboglobulin (BTG) and platelet factor 4 (PF4), platelet alpha-granule specific proteins, and serotonin (5-HT) which is stored in dense granules are released when platelets are activated. To investigate in vivo platelet activation in uncomplicated essential hypertension, platelet 5-HT and PF4 contents, plasma BTG and PF4 concentrations, as well as urinary BTG levels were assessed in normotensive and hypertensive subjects. Plasma BTG and PF4 concentrations and urinary BTG levels were comparable in both groups. Mean platelet 5-HT content was significantly decreased in hypertensive subjects without modification of the intraplatelet PF4 content. These data suggest first of all that the decrease in platelet 5-HT content is due mainly to the inhibition of platelet 5-HT uptake previously described, and second of all that no significant in vivo platelet activation occurs in essential hypertensive subjects devoid of cardiovascular complications.

Expression of coagulant activity in human platelets: release of membranous vesicles providing platelet factor 1 and platelet factor 3

The relationship between the appearance of membrane-associated factor V-like activity (platelet factor 1, PF1) and phospholipid-like catalytic activity (platelet factor 3, PF3) has been examined, in vitro, in collagen-stimulated, human platelets. Both activities increased 7 fold upon collagen treatment relative to stirred controls. After sedimentation of stimulated platelets, 31% of total PF1 and 41% of PF3 remained in the supernatant fraction. PF1 eluted from a Sepharose CL-4B column in the same void volume fractions as PF3, phospholipid, and vesicular particles. These fractions had roughly 100 fold (lipid basis) or 1000 fold (protein basis) enhanced specific activity when compared to the stimulated platelet suspension. Freeze-fracture electron microscopy demonstrated that these void volume fractions contain two populations of membranous vesicles (80-200 nm and 400-600 nm in diameter). Upon centrifugation of the void volume fractions, PF1 and PF3 activities, phosphate-containing material, and ultraviolet-absorbing material all sedimented at the same rate, indicating that PF1 and PF3 are activities associated with one or both of the platelet-derived vesicle populations. Finally, we examined the effects of inhibitors on the appearance of PF1, PF3, platelet factor 4, total intrinsic factor V activity, and serotonin as well as on platelet aggregation. These studies suggest that the collagen-stimulated release of PF1 and PF3 is not coupled to either platelet aggregation or PF4 release but is probably a separate phase of the release reaction.

Platelet-derived collagenase inhibitor: characterization and subcellular localization

Purified human platelets were found to contain a collagenase inhibitor that is immunologically, functionally, and chromatographically identical to that produced by human skin fibroblasts. None of the other formed elements of the blood (erythrocytes, granulocytes, mononuclear cells) possessed detectable quantities of this protein. Virtually all the collagenase inhibitor contained within platelets was released following platelet activation with thrombin. Similarly, platelet activation accompanying blood clotting also resulted in the release of this protein, the ratio of plasma to serum inhibitor levels being approximately equal to 0.5. When platelets were subjected to subcellular fractionation, essentially all of the platelet-associated collagenase inhibitor was found to be located in the alpha-granule. Studies with radiolabeled inhibitor failed to detect uptake of inhibitor by platelets. Furthermore, immunologically reactive protein of similar quantity to that found in platelets was identified in human megakaryocyte lysates. Thus, the data suggest that the collagenase inhibitor is endogenously produced and stored within platelet alpha-granules. The platelet-derived collagenase inhibitor was antigenically identical to the collagenase inhibitor from human skin fibroblasts in double immunodiffusion and, like its fibroblast counterpart, inhibited collagenase on a 1:1 stoichiometric basis. When subjected to several of the chromatographic procedures utilized to purify the fibroblast protein, the platelet inhibitor behaved in an indistinguishable manner. Platelet factor 4, previously reported to be a collagenase inhibitor, was found to be immunologically unrelated to the platelet-derived collagenase inhibitor. Furthermore, platelet factor 4 displayed no collagenase inhibitory activity. Although the function of platelet-derived collagenase inhibitor is unknown, such a protein released by activated platelets may serve to regulate collagen turnover during the early stages of the inflammatory process.

Optimal techniques for the immunocytochemical demonstration of beta-thromboglobulin, platelet factor 4, and fibrinogen in the alpha granules of unstimulated platelets

The distribution of beta-thromboglobulin, platelet factor 4, and fibrinogen in unstimulated platelets was investigated by several immunocytochemical techniques. All three substances were found to be localized in the majority of platelet alpha granules either by immunoperoxidase methods on saponin-treated platelets or by colloidal gold immunoconjugates on frozen thin sections. The optimal conditions for preparing and fixing platelets for immunocytochemistry were also determined. Platelets obtained from blood dripped directly into fixative or anticoagulated blood were compared systematically with respect to shape. Temperature was found to be the most important variable. Immediately fixed platelets were generally disc-shaped, regardless of the temperature of the fixative. Reducing the temperature of blood (stored with anticoagulant) before fixation resulted in more swollen and fewer disc-shaped platelets. However, if the blood was mixed with an anticoagulant and maintained at 37 degrees C for 1 h before fixation, the same number of disc-shaped platelets were present as in samples from blood fixed immediately. The intracellular localization of beta-thromboglobulin, platelet factor 4, and fibrinogen was consistent regardless of platelet preparatory procedure, but several technical problems were encountered with respect to plasma membrane labelling when control experiments were analysed. Immediately fixed, non-permeabilized platelet plasma membranes were always labelled, no matter which control substances or immunoperoxidase markers were used. However, when platelets were washed by centrifugation, the plasma membranes were negative. Exposure to saponin markedly diminished labelling of the plasma membranes. Optimal techniques for the immunocytochemical demonstration of these alpha granule proteins in platelets are presented in this report.

Stimulation of histamine release from human basophils by human platelet factor 4

Human basophils were stimulated to release histamine noncytotoxically by purified human platelet factor 4 (PF4) and the synthetic substituent peptide PF4(59-70). Histamine release was augmented significantly by 10(-7) M PF4 and 10(-5) M PF4(59-70), increased in a concentration-dependent manner, and attained a maximum at 3 X 10(-5) M PF4 and 3 X 10(-4) M PF4(59-70) similar to that achieved by goat anti-human myeloma IgE. PF4 (1-60) failed to initiate the release of histamine, which confirmed that the critical determinant of activity is in the carboxy-terminal sequence. Histamine release from basophils by optimally effective concentrations of PF4 and PF4(59-70) reached a plateau by 1-3 min, as contrasted with 10 min or longer for anti-IgE. The elimination of calcium and magnesium from the buffer suppressed the release of histamine by anti-IgE by 79-83%, but had no effect on that elicited by PF4(59-70). The rate of uptake of [125I]PF4 by purified basophils was similar on a molar basis to the rate of release of histamine by the same concentrations of PF4. The noncytotoxic release of histamine from human basophils by PF4 thus is temporally and biochemically distinct from that mediated by IgE and may be similar to that evoked by other polycationic stimuli.

Human platelet-derived growth factor: radioimmunoassay and discovery of a specific plasma-binding protein

The platelet-derived growth factor (PDGF) is the principal mitogen in serum for cultured cells of mesenchymal origin. PDGF also is a potent chemotactic protein for inflammatory cells and for cells required for wound repair. Because activity levels of PDGF in biological fluids are difficult to measure, we attempted to develop a radioimmunoassay for PDGF. Rabbits were immunized with purified PDGF; the antiserum obtained was monospecific for PDGF in immunodiffusion analysis against concentrated platelet lysates, serum, and plasma. A radioimmunoassay for PDGF was developed with a sensitivity of congruent to 0.2 ng/ml. Levels of PDGF in plasma/serum were measured and compared with PDGF levels determined by a receptor-competition assay and by a standard biological assay measuring incorporation of [3H]thymidine into 3T3 cells. Radioimmunoassay showed apparent PDGF levels of 50 ng/ml in human plasma and 103 ng/ml in serum. The 50 ng/ml PDGF in plasma was unexpected because the plasma samples contained little or no platelet release products as determined by very low levels of platelet factor 4. We therefore sought an immunologically reactive PDGF molecule in human plasma. No immunologically reactive protein was detected by immunodiffusion analysis or when plasma was treated with an immunoaffinity gel. Subsequently, a 125I-PDGF-binding protein was identified; the 125I-PDGF-plasma-binding protein complex was not reactive with anti-PDGF immunoglobulin. Correction for 125I-PDGF bound by the plasma-binding protein established serum levels of PDGF of congruent to 50 ng/ml; congruent to 50 ng/ml PDGF was found in serum by radioreceptor-competition assays and by mitogenic assays as well. The plasma-binding protein may serve to clear PDGF released in the circulation, thereby limiting PDGF activity to its local interactions at the site of blood-vessel injury.

In vivo platelet activation with in vitro hyperaggregability to arachidonic acid in renal allograft recipients

Renal allograft recipients were investigated to determine the extent and possible nature of in vivo platelet activation. In 92 allografted patients stable for more than 4 months' duration, intraplatelet serotonin in circulating platelets was depleted significantly. In a further 16 patients studied serially for 12 to 16 weeks following transplantation, intraplatelet serotonin fell abruptly within 4 days from transplantation to very low levels, and remained thus for 10 weeks, rising toward normal at about 12 weeks. Although some patients showed abrupt falls in intraplatelet serotonin coincident with acute rejection episodes, there was no difference in intraplatelet serotonin in seven patients whose grafts functioned well immediately and remained stable, and seven in whom repeated rejection led to graft loss within 3 months. Thus, these tests of platelet function do not permit diagnosis of rejection or prediction of graft outcome. Plasma platelet factor 4 (PF4) concentrations, in contrast, were normal in most patients during the first 6 weeks after grafting, then rose and remained abnormal up to 13 years following the allograft in the long-term stable graft recipients. This discrepancy suggests a different mode of platelet activation in the first few weeks after grafting from subsequent months. Despite universal depletion of intraplatelet amines and alpha-granule contents only four out of 14 early allograft recipients had an abnormal bleeding time, and platelet aggregation thresholds with adenosine-5'-diphosphate and collagen were not different from controls. However, thresholds for platelet aggregation with arachidonic acid were reduced significantly (P less than 0.01) and thromboxane B2 generation was increased in vitro. There was no correlation between depletion of intraplatelet serotonin and circulating platelet-agglutinating material, but nine of 17 biopsy specimens from rejecting allografts taken during the first 3 months showed extensive glomerular localization of platelet membrane antigens and PF4.

Evidence for serotonin binding in vitro by platelet factor 4 and beta-thromboglobulin

Evidence is presented for in vitro high affinity binding of serotonin (5-HT) by beta-thromboglobulin (beta TG) and platelet factor 4 (PF4) from human blood. Results include: 1) identification by radioimmunoassay of PF4 in specifically bound material obtained by 5-HT affinity chromatography of human platelet extracts; 2) binding of 72% and 6% of radiolabelled PF4 on 5-HT and control affinity columns, respectively; and 3) binding of approximately 8 moles of 5-HT per mole of purified beta TG in the presence of ferrous ion and heparin in ultrafiltration studies, with Scatchard analysis indicating a dissociation constant of about 4 X 10(-8) M.

New synthesis of a platelet-specific protein: platelet factor 4 synthesis in a megakaryocyte-enriched rabbit bone marrow culture system

The site of synthesis of platelet-specific proteins remains to be established. With the use of short-term megakaryocyte-enriched cultures, direct evidence was obtained to show that megakaryocytes synthesize the platelet-specific protein, platelet factor 4. A megakaryocyte-enriched fraction of rabbit bone marrow for culture was obtained by centrifugal elutriation and cultured with [3H]leucine. Newly synthesized 3H-platelet factor 4 was sought by copurification with added carrier rabbit platelet factor 4, using heparin agarose affinity chromatography and immunoprecipitation with specific goat anti-rabbit platelet factor 4 antisera. SDS PAGE of the washed immunoprecipitates demonstrated a [3H]leucine-containing peak which migrated identically with purified homogeneous rabbit platelet factor 4. A second, slightly larger molecular-weight protein was identified in the gels also, suggesting that rabbit platelet factor 4 may be synthesized as a larger molecular-weight precursor in rabbit megakaryocytes. These results provide direct evidence that the platelet-specific protein, platelet factor 4, is synthesized in rabbit megakaryocytes before it is packaged into alpha-granules for release in circulating platelets.

Kinetic analysis of alpha-granule secretion by platelets. A methodological report

A method is described for the kinetic measurement of alpha-granule secretion by platelets. The method uses formaldehyde as a secretion-blocking reagent. This treatment alters the antigenecity of beta-thromboglobulin but not of Platelet Factor 4, both measured with commercially available reagents. Evidence is shown that this formaldehyde effect does not alter the secretion kinetics when the data are expressed as a percentage of a similarly treated reference sample. The method shows that following stimulation with thrombin or A 23187 alpha-granule secretion is much slower than dense granule secretion.

Myelofibrosis and acute megakaryoblastic leukemia in a child: topographic relationship between fibroblasts and megakaryocytes with an alpha-granule defect

In a child with acute megakaryoblastic leukemia--severe thrombocytopenia and myelofibrosis, EM studies on bone marrow showed a strict topographic relationship between the presence of clusters of abnormal megakaryocytes and the increased number of fibroblasts and extracellular fibers. Megakaryocytes and platelets lacked alpha-granules while the plasma thromboglobulin level was three times the normal level. This suggested that the alpha-granular proteins were synthesized but not retained in alpha-granules. If this occurs, the increased marrow levels of platelet-derived growth factor and factor 4 would favor the proliferation of fibroblasts and the synthesis of collagen, and thereby promote myelofibrosis. After therapy-induced remission, the number of marrow megakaryocytes decreased, the alpha-granules were normally produced, the plasma beta-thromboglobulin level was normal and the myelofibrosis disappeared. These observations suggest that during acute megakaryoblastic leukemia, an acquired gray-platelet syndrome occurs and that the local excretion of alpha-granule proteins triggers the myelofibrosis.

Platelet factor 4 is chemotactic for neutrophils and monocytes

Platelet factor 4 is shown to be a chemotactic protein for human polymorphonuclear leukocytes and monocytes at concentrations found in human serum and reached locally in injured tissue. The maximum chemotactic response to platelet factor 4 nearly equals that achieved with saturating concentrations of the chemotactic activity derived from the fifth component of human complement, C5. Cells desensitized to C5 chemotactic activity retain chemotactic responsiveness to platelet factor 4. Serum contains inhibitory capacity against the chemotactic activity associated with platelet factor 4. Our results suggest that the local release of platelet factor 4 may be an important stimulus attracting inflammatory cells to sites of blood vessel injury.