Release of platelet factor 4 by adenosine diphosphate and other platelet-aggregating agents

Recent Advances in the Discovery and Function of Antimicrobial Molecules in Platelets

The conventional function described for platelets is maintaining vascular integrity. Nevertheless, increasing evidence reveals that platelets can additionally play a crucial role in responding against microorganisms. Activated platelets release molecules with antimicrobial activity. This ability was first demonstrated in rabbit serum after coagulation and later in rabbit platelets stimulated with thrombin. Currently, multiple discoveries have allowed the identification and characterization of PMPs (platelet microbicidal proteins) and opened the way to identify kinocidins and CHDPs (cationic host defense peptides) in human platelets. These molecules are endowed with microbicidal activity through different mechanisms that broaden the platelet participation in normal and pathologic conditions. Therefore, this review aims to integrate the currently described platelet molecules with antimicrobial properties by summarizing the pathways towards their identification, characterization, and functional evaluation that have promoted new avenues for studying platelets based on kinocidins and CHDPs secretion.

Hepatic arterial infusion of recombinant platelet factor-4 suppresses metastases to the lungs from tumors implanted into the livers of rabbits

BACKGROUND

This study evaluated the toxicity (Part I) and antitumor effects (Part II) associated with hepatic arterial infusion of recombinant platelet factor-4 (rPF4), an antiangiogenic protein.

METHODS

Healthy rabbits (Part I) and rabbits with tumors implanted in their livers (Part II) received saline or rPF4 via hepatic arterial infusion. Three saline-receiving and four rPF4-receiving animals died 2-3 days postinfusion from gastroduodenal thromboembolism. The remaining animals were necropsied 3, 7, 10, or 14 days postinfusion. Blood analyses and hepatic angiography were performed before infusion and at the time of sacrifice.

RESULTS

In Part I, focal coagulation necrosis of the hepatic parenchyma was observed in 1 of 11 rabbits that received saline and in 6 of 10 that received rPF4. In Part II, hepatic arterial infusion of rPF4 had no effect on growth of the implanted liver tumors. However, the protein significantly reduced the incidence of lung metastasis.

CONCLUSIONS

Intraarterial infusion of rPF4 significantly reduced the incidence of lung metastasis. Nonheparin systemic anticoagulation may be needed during catheterization and infusion procedures to prevent thromboemboli.

Induction of local inflammation by recombinant human platelet factor 4 in the mouse

Platelet factor 4 (PF-4) has been shown to be chemotactic for neutrophils and monocytes in vitro. To assess whether these observations have in vivo relevance, we tested the ability of recombinant human PF-4 (rPF-4) to induce acute and chronic dermal inflammation in the mouse. When injected as a single dose intradermally, rPF-4 induced an acute inflammatory response that peaked at 6 to 12 hr and which resolved by 36 hr. Injection of an equivalent amount of cytochrome c, buffer alone, or an amino-terminal PF-4 peptide failed to elicit a significant inflammatory response; however, the carboxy-terminal PF-4 peptide retained proinflammatory properties. The inflammatory infiltrate induced by a single injection of either rPF-4 or the 41 amino acid carboxy-terminal peptide was composed of neutrophils and smaller numbers of mononuclear cells. Repeated injection of rPF-4 resulted in nearly equal numbers of neutrophils and mononuclear cells. Moreover, marked dermal fibrosis developed after only 5 days of daily injection of rPF-4. Although relatively high concentrations of rPF-4 were required to elicit an inflammatory response, these concentrations may be locally attainable during platelet aggregation. Our findings thus support the hypothesis that PF-4 may contribute to the development of inflammatory responses at sites of platelet aggregation.

Radioimmunoassay of platelet factor 4 and beta-thromboglobulin: development and application to studies of platelet release in relation to fibrinopeptide A generation

Platelet and fibrinogen survival and turnover studies have shown that platelet activation and fibrin formation may occur to different degrees in different thrombotic disorders. More direct evidence of differential involvement of platelet activation and fibrin formation should be provided by specifically measuring the products of these reactions, i.e. released platelet proteins and fibrinopeptide A. Two platelet proteins, platelet factor 4 (PF4) and beta-thromboglobulin (betaTG), were isolated and characterized, and sensitive and specific radioimmunoassays were developed to measure them. These assays were employed, along with the radioimmunoassay for fibrinopeptide A (FPA), to study the release of PF4 and betaTG in relation to FPA cleavage. PF4 and betaTG were released by ADP and collagen with time course and concentration dependence similar to that of [14C]serotonin release. FPA was not cleaved from fibrinogen during ADP or collagen-induced platelet release. Thrombin caused release of PF4 and betaTG as well as cleavage of FPA. Cleavage of FPA occurred with concentrations of thrombin about 100 times less than did release of PF4 and betaTG, and release of [14C]serotinin required still higher thrombin concentrations. Release of [14C]serotonin and platelet proteins was similar as a function of time. Sodium citrate was found to inhibit platelet release induced by thrombin.

Platelet factor 4: an inhibitor of collagenase

Human platelet factor 4 (PF4) is known to bind to heparin and inhibit its anticoagulant effect. This factor also inhibits the enzyme collagenase derived from cultured human skin and collagenase extracted from human granulocytes. The addition of heparin to the PF4-collagenase assay system has no effect on the observed inhibition of collagenase. Thus PF4 inhibits collagenase, in addition to neutralizing heparin.

Proteins secreted by platelets: significance in detecting thrombosis

Because platelet survival measurements are time-consuming and may not completely reflect platelet involvement in hemostasis and thrombosis, other tests have been sought. Measurement of two proteins released by platelets, platelet factor 4 (PF4) and beta-thromboglobulin (betaTG), may provide simpler, more direct means of quantitating platelet involvement. The radioimmunoassays for these proteins reviewed in this paper are sensitive and specific. Although there are technical problems still to be resolved in their clinical application, clinical studies to date suggest that such assays will be useful in studying the pathogenesis and course of thromboembolic disorders. PF4 and betaTG levels apparently do reflect in vivo platelet release. Because release of PF4 and betaTG parallels release of platelet-derived growth factor, plasma PF4 and betaTG levels should also reflect release of that protein. The PF4 and beta TG assays along with an assay for fibrinopeptide A in clinical samples should help elucidate the relative importance of platelet release and fibrin formation in thromboembolic disorders.

Plasma heparin concentrations during subcutaneous heparin therapy

Plasma heparin concentrations were measured at two-hourly intervals following the subcutaneous injection of varying the doses of heparin. With doses of 5000 and 10,000 units, peak plasma concentrations were seen most commonly four hours after the injection. There was a wide range of peak values at both dose levels with considerable overlap and no correlation could be shown between plasma heparin concentration and the age, sex or weight of the individual. Traces only of heparin could be detected in the blood after a 1000 unit injection. Comparable results were obtained up to six hours after the injection with either the calcium or sodium salts of heparin.

Platelet consumption and plasma concentration of platelet factor 4 (PF-J)

In 10 cancer patients platelet concentration, platelet consumption, platelet half-life time and plasms concentrations of platelet factor 4 were determined. A high freguency of thrombocytosis and increased platelet consumption were observed, and the plasms concentrations of platelet factor 4 was increased. However, no significant correlation was found between platelet consumption and the result of a single determination of platelet factor 4 concentrations in plasma.

Subcellular localization of the heparin-neutralizing factor in blood platelets

1. The distribution of the heparin-neutralizing factor (platelet factor 4, PF4) in subcellular organelles of blood platelets of rabbits and man was investigated. 2. In both species the organelles storing 5-hydroxytryptamine (5-HT storage organelles) contained only trivial amounts of PF4. 3. In contrast, the content of PF4 was highest in the subcellular fractions rich in alpha-granules. 4. In conclusion, PF4 is probably localized in the alpha-granules and therefore the platelets contain at least two types of organelles (5-HT organelles and alpha-granules) capable of releasing their contents in response to the same stimuli, such as exposure to collagen, thrombin, etc.

Platelets, thrombosis and drugs

The development of thrombosis involves 4 main factors: the vessel wall, the formed elements of the blood, blood coagulation, and blood flow. In venous thrombosis, however, the major part in both the initiation and growth of thrombi is played by the platelets. In selecting drugs which inhibit platelet function it is helful to know which of the platelet reactions that contribute to thrombus formation can be inhibited by various agents. Platelets adhere to the damaged vessel wall, collagen being probably the most important constituent involved. They are then stimulated to release the contents of their storage granules. Release-inducing agents promote the discharge of adenosine diphosphate (ADP) which causes platelets in the vicinity to swell to a more spherical shape, extend pseudopods and adhere to each other. Platelet aggregation is reversible, and a number of drugs have been shown to be capable of inhibiting platelet function at various stages, both in vitro and in vivo. Adrenaline, noradrenaline, oestrogens and nicotine enhance aggregation. Drugs which inhibit platelet function include the non-steroidal anti-inflammatory drugs, the pyrimido-pyrimidines (e.g. dipyridamole), hydroxychloroquine, clofibrate, and dextran. In this review the effects of drugs which inhibit platelet function are outlined and the extent to which they can be used to influence the course of thromboembolic disease in man is discussed. It is suggested that combination of anti-platelet drugs with anticoagulants could prove clinically useful.

Lipoprotein-lipase responses to heparin

Recent evidence suggests an interrelationship between platelets, heparin, and the lipolytic enzyme, lipoprotein-lipase. It seems likely that heparin might influence platelet adherence through its activation of lipoprotein-lipase. The mode of activation of the enzyme by heparin may, therefore, be important clinically.

In these experiments the enzyme responses to increasing doses of heparin have been measured in dogs. The mean enzyme responses were found to be directly related to the logarithm of the heparin dose.

Inhibition by aspirin of release of antiheparin activity from human platelets

Both in vitro and in vivo, aspirin inhibited the adenosine diphosphate and collagen-induced release of platelet factor 4 (antiheparin factor). The release induced by adrenaline and thrombin was not affected. The in-vivo effect in normal persons lasted for at least three days. Platelet uptake of acetyl-(14)C-aspirin was significantly greater than that of carboxyl-(14)C-aspirin.

Heterogeneity of human platelets. II. Functional evidence suggestive of young and old platelets

In the previous communication, suggestive evidence was presented for large-heavy platelets being "young" platelets and light-small platelets being "old" platelets. Large-heavy, light-small, and total human platelet populations were compared with respect to their platelet function. After addition of adenosine diphosphate (ADP), thrombin, or epinephrine, platelet aggregation time was 3.0-, 4.5-, and 3.3-fold shorter with large-heavy platelets compared with light-small platelets, and large-heavy platelets released 3.7-, 7.6-, and 8.1-fold greater adenosine triphosphate (ATP) into the medium, respectively, than did light-small platelets. After platelet aggregation by thrombin or epinephrine, large-heavy platelets released 6.0- and 3.8-fold more ADP into the medium than did light-small platelets. After platelet aggregation by ADP, light-small platelets consumed 5.9-fold greater added extracellular ADP than did large-heavy platelets.Large-heavy platelets aggregated by ADP, thrombin, or epinephrine released 9.1-, 8.5-, and 12.7-fold greater platelet factor 4 than light-small platelets similarly treated.