Binding and internalization of platelet-activating factor 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine in washed rabbit platelets

The binding profile of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (AGEPC, platelet-activating factor) to washed rabbit platelets was investigated through the use of structural analogs of AGEPC, e.g. U66985, which specifically suppressed AGEPC biological activities on rabbit platelets. This interaction of AGEPC with platelets could be divided into three different components termed A, B, and C. Component A was considered as one of high affinity (Kd = 0.5 X 10(-9) M) and with a low capacity (about 400 sites/platelet). The binding of AGEPC to component A was reversible and was blocked by the inhibitory analogs of AGEPC. This was considered to be the AGEPC receptor site(s). Component B was irreversible in nature and was presumed to be associated with internalization of AGEPC. The latter process was sensitive to the structural inhibitors. Component C was not affected by the inhibitors and probably represented a nonspecific binding to the lipid layer of the membrane. The binding profile of 1-O-alkyl-2-(lyso)-sn-glycero-3-phosphocholine, a biologically inactive and noninhibitory analog of AGEPC, was observed to consist of a single component and was (also) unaffected by the inhibitors. Internalization of AGEPC into rabbit platelets was further examined by the bovine serum albumin extraction method, which was originally developed by Mohandas et al. (Mohandas, N., Wyatt, J., Mel, S. F., Rossi, M. E., and Shohet, S. B. (1982) J. Biol. Chem. 257, 6537-6543). AGEPC was instantly taken up by the cell and internalization into its membrane, where it remained and was not released into cytosol. The internalization of AGEPC was suppressed by pretreating the cells with AGEPC analogs. In platelets desensitized to AGEPC, no down-regulation of the receptor site(s) was observed. The internalization of AGEPC in the desensitized cells was clearly enhanced and this was obvious even in the presence of the AGEPC inhibitor(s). Even in the presence of the inhibitors, effective internalization of AGEPC was also evident in thrombin-treated cells. These results suggested that the internalization of AGEPC was irreversibly enhanced in the platelets which were activated by AGEPC itself as well as by thrombin.     

Some unique features of the metabolic conversion of platelet-activating factor (AGEPC) to alkyl acyl PC by washed rabbit platelets

Recently several synthetic analogs of 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine (AGEPC; platelet-activating factor) were characterized as selective inhibitors of this agonist's effects on rabbit platelets (Tokumura, A., Homma, H., and Hanahan, D. J. (1985) J. Biol. Chem. 260, 12710-12714). In this current investigation, these studies have been extended to include a further inquiry into the biochemical nature of the metabolic inactivation of AGEPC in rabbit platelets, and the effect of these analogs on this process. Two of the latter components (U66985 and CV3988), which blocked AGEPC biological activity on rabbit platelets, also blocked the metabolism of this agonist. The metabolic conversion of AGEPC to alkyl acyl PC was inhibited nearly sevenfold by the most potent analog, U66985. Those analogs with low (U68043) or no biological inhibitory activity (lysoGEPC) had marginal effects on the metabolism of AGEPC. The effects of these compounds on the metabolism of AGEPC was not simply due to competitive inhibition. In platelets which had been pretreated with AGEPC in absence of extracellular Ca2+ (desensitized) and washed, the metabolic conversion of AGEPC to alkyl acyl PC was actually enhanced. This enhanced metabolic inactivation of AGEPC was also observed upon the treatment of the cells with thrombin, collagen, or ionophore A23187, indicating that the metabolism of AGEPC in platelets was enhanced not only by AGEPC itself but by other agonists as well. Nearly 85% of the fatty acyl residues was arachidonate in the alkyl acyl PC derived from AGEPC. This specific acylation with arachidonate was observed in the presence and absence of the inhibitor and in desensitized cells, indicating that selectivity for arachidonate is not dependent on the enhancement of the metabolism of AGEPC. The alkyl acyl PC found in the cells treated with thrombin, collagen, or A23187 was also predominantly alkyl arachidonoyl PC. Thus it has been shown that the inactivation of AGEPC by its conversion to alkyl acyl PC by rabbit platelets is enhanced by this agonist itself and that excess amounts of AGEPC could be further inactivated by the enhanced capacity of the metabolism process.     

Metabolism of 1-O-alkyl-2-acetyl-sn-glycerol by washed rabbit platelets: formation of platelet activating factor

A new type of neutral lipid, 1-O-alkyl-2-acetyl-sn-glycerol (AAG), induced a delayed aggregation pattern on interaction with washed rabbit platelets. Although far less potent on a molar basis than platelet activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, AGEPC, nevertheless this compound caused an aggregation, albeit delayed in time, remarkably similar to that exhibited by AGEPC. In view of the possible formation of AGEPC in this reaction, AAG was incubated with washed rabbit platelets, and a lipid corresponding in chromatographic behavior to AGEPC was isolated and identified as such by a combined gas-liquid chromatography/mass spectrometry technique coupled with selected ion monitoring.     

Specific binding of phospholipid platelet-activating factor by human platelets

The binding of the phospholipid platelet-activating factor 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphorylcholine (AGEPC) to washed human platelets was more than 80% complete within 2 min, which coincided with the time of initiation of platelet aggregation by AGEPC. Scatchard plot analysis of the binding of [3H]AGEPC to platelets without and with an excess of unlabeled AGEPC revealed two distinct types of binding sites. One platelet site for AGEPC exhibited a high affinity (KD = 37 +/- 13 nM, mean +/- SD), was saturable, and had a low maximal capacity of 1399 +/- 498 (mean +/- SD) molecules of AGEPC/platelet. The other platelet site demonstrated a nearly infinite binding capacity, consistent with nonreceptor uptake of AGEPC into cellular structures. The specificity of the high-affinity binding site for AGEPC was assessed by comparing the capacity of several analogues of AGEPC to inhibit the binding of [3H]AGEPC to platelets and to induce platelet aggregation. An ether linkage in position 1, a short-chain fatty acid in position 2, and a choline moiety in the polar head group proved to be critical both for the binding of [3H]AGEPC to platelets and for the initiation of platelet aggregation. Exposure of platelets to AGEPC for 5 min at 37 degrees C functionally deactivated the exposed platelets to subsequent stimulation by AGEPC, as assessed by diminished aggregation, and concomitantly reduced the specific binding of [3H]AGEPC. Evaluation of the time course of the events of deactivation revealed the loss of an aggregation response to AGEPC after 90 sec at 37 degrees C, despite the retention of up to 50% of the specific binding sites for AGEPC.     

Production of platelet-activating factor by chick retina

In the present study it is demonstrated that platelet-activating factor (PAF) was produced by chick retinas, upon stimulation with neurotransmitters such as acetylcholine (ACh), dopamine, or with calcium ionophore A23187, but not upon stimulation with gamma-amino-n-butyric acid, L-glycine, L-glutamate, epinephrine, or histamine. PAF produced in response to ACh, dopamine, or A23187 was not released into supernatants but was extractable from retinas. The amounts of extractable PAF increased after sonication of stimulated retinas. While no PAF activity could be recovered from unstimulated retinas, small amounts of this lipid can be detected following sonication of the tissue. The amount of extractable PAF from ACh-, dopamine-, or A23187-stimulated retinas was dependent upon the incubation time and concentration of the agonists. PAF was identified on the basis of chemical and lipase treatments, biological activity with washed rabbit platelets, behavior on thin layer chromatography, and high pressure liquid chromatography. Control cell preparations (leukocytes, erythrocytes, and embryogenic fibroblasts) did not produce PAF upon neurotransmitter stimulation. ACh and dopamine promoted PAF production by increasing dithiothreitol-insensitive cholinephosphotransferase activity, without affecting the acetyltransferase activity. In contrast, the A23187 ionophore stimulated the acetyltransferase activity but did not affect the dithiothreitol-insensitive cholinephosphotransferase.     

Binding of coagulation factor XI to washed human platelets

The binding of human coagulation factor XI to washed human platelets was studied in the presence of zinc ions, calcium ions, and high molecular weight kininogen. Significant factor XI binding occurred at physiological levels of these metal ions when high molecular weight kininogen was present. Binding required platelet stimulation and was specific, reversible, and saturable. Scatchard analysis of the binding yielded approximately 1500 binding sites per platelet with an apparent dissociation constant of approximately 10 nM. Since the concentration of factor XI in plasma is about 25 nM, this suggests that in plasma factor XI binding sites on stimulated platelets might be saturated. Calcium ions and high molecular weight kininogen acted synergistically to enhance the ability of low concentrations of zinc ions to promote factor XI binding. The similarity between the concentrations of metal ions optimal for factor XI binding and those optimal for high molecular weight kininogen binding, as well as the ability of high molecular weight kininogen to modulate these metal ion effects, implies that factor XI and high molecular weight kininogen may form a complex on the platelet surface as they do in solution and on artificial negatively charged surfaces.     

Characterization of platelet-activating factor receptors in porcine platelets

Despite a large number of studies describing the properties and effects of platelet-activating factor (PAF), little is known about its receptor structure. The characterization of the PAF receptor from additional cell types and species is important for the design of strategies to purify and characterize the receptor molecule. Porcine platelets were shown to bind PAF with characteristics similar to several other species, based on receptor number, affinity, and the activity of PAF antagonists. We found that the affinity for binding was higher in porcine than in rabbit platelets (Kd = 0.68 +/- 0.13 nM for rabbit and 0.29 +/- 0.10 nM for porcine). Porcine platelets have approximately 281 +/- 158 receptors per cell compared with 689 +/- 229 receptors in rabbit platelets. Rabbit platelets respond to concentrations of PAF that are approximately 10(5)-fold lower than those required for aggregation of porcine platelets, but this difference is probably not due to the differences in receptor number alone. When binding was compared between purified membranes from these two cell types, porcine platelets had 20-fold fewer receptors per milligram of membrane protein, but this difference may have been due to an artifact of the membrane preparation procedure. Binding of PAF was severely hindered at cold temperatures. It was undetectable in whole cells on ice and greatly reduced with purified membranes. This study is the first to characterize PAF receptors in porcine platelets, which represent a potentially useful source of receptor for further biochemical characterization.     

Occurrence of platelet-activating factor in rabbit spermatozoa

Spermatozoa obtained from rabbit ejaculate were analyzed for the presence of platelet-activating factor [PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (AGEPC)] by using standard HPLC and TLC procedures. Fractions corresponding to synthetic PAF (AGEPC) revealed PAF-like activity amounting to 0.35 +/- 0.06 pmol/10(8) cells (mean +/- SE) as determined by bioassays based on the release of [3H]serotonin from washed rabbit platelets. This activity was lost upon base-catalyzed methanolysis, but was restored to the original level after reacetylation. Analysis of the phosphatidylcholine (PC) fraction by GC-MS subsequent to base-catalyzed methanolysis showed that 1-O-alkyl-2-acylphosphocholine comprises about 12% of the PC fraction with alkyl chain lengths of 16:0 (88%) and 18:0 (12%).     

Localization of fibrinogen during ADP- or thrombin-induced aggregation of washed rabbit platelets

In contrast to human platelets, which aggregate poorly in response to ADP unless fibrinogen is present in the external medium, washed rabbit platelets form large aggregates in response to ADP without fibrinogen in the suspending medium. Addition of fibrinogen to the suspending medium of rabbit platelets frequently has little or no effect on the extent of ADP-induced platelet aggregation. We examined washed rabbit platelets by immunocytochemistry during ADP-induced aggregation and deaggregation and during thrombin-induced aggregation when the external medium did not contain added fibrinogen to determine if (a) fibrinogen was expressed on the surface of rabbit platelets that could support aggregation when the platelets were stimulated, or (b) fibrinogen secreted from the alpha granules supports platelet aggregation. Glutaraldehyde-fixed samples were prepared at different times after addition of ADP or thrombin, embedded in Lowicryl K4M, sectioned, incubated with sheep anti-rabbit fibrinogen, washed, reacted with gold-labeled anti-sheep IgG, and prepared for electron microscopy. The alpha granules of rabbit platelets were heavily labeled with immunogold; the platelet membrane was not labeled. During platelet aggregation and deaggregation in response to ADP, fibrinogen was not detectable on the platelet surface. In response to thrombin, large aggregates formed before fibrinogen was secreted from the alpha granules; fibrinogen was detectable focally at sites of granule discharge by 30-60 sec and fibrin formed by 3 min. Therefore, stimulated washed rabbit platelets can adhere to each other without large amounts of fibrinogen taking part in the close platelet-to-platelet contact, since aggregation occurs before detectable secretion, and large areas where the platelets are in contact are devoid of fibrinogen between the adherent membranes. Adhesion mechanisms not involving fibrinogen may support the aggregation of washed rabbit platelets.     

The release of a platelet-activating factor by stimulated rabbit neutrophils.

Normal rabbit peripheral blood neutrophils released a platelet-activating factor upon stimulation by opsonized zymosan. The liberation was Ca++ dependent and the time course of release was closely associated with phagocytosis. The material extracted into chloroform and exhibited an identical mobility by thin layer chromatography to basophil-derived, IgE-stimulated, platelet-activating factor (PAFb). It was similar to PAFb in its effect on platelets in both aggregation and release but was distinguished from ADP, thrombin, arachidonic acid, and thromboxanes. This factor appears to be responsible for some previously reported neutrophil-platelet interactions.     

Production of platelet-activating factor by the pre-implantation sheep embryo.

Embryos were collected from superovulated ewes on Day 2 (2-8 cell), Day 4 (8-16 cell) and Day 6 (morula/early blastocyst). Two embryos were cultured in 1 ml of one of four media: (i) Ham's F10 + 4 mg bovine serum albumin (BSA)/ml, (ii) synthetic oviduct fluid medium + 20% human serum, (iii) Quinn's human tubal fluid medium (HTF) + 3 mg BSA/ml or (iv) HTF + 10% acid-treated fetal calf serum for 24 h. They were transferred to fresh media of the same type and their further development was monitored. A quantitative bioassay and radioimmunoassay was used to measure the concentration of platelet-activating factor (PAF, 1-o-alkyl-2-acetyl-sn-glyceryl-3-phosphocholine) produced. Following extraction and partial purification, 21/95 (22.1%) of the embryo-conditioned media samples had PAF concentrations greater than that measured in corresponding control media. This was designated as embryo-derived PAF and the corresponding cultures were termed 'PAF-positive'. PAF was produced by embryos at all three developmental stages examined and in each of the four media used, and the average amount of PAF produced was 60.9 +/- 9.8 pmol/embryo/24 h. However, neither the developmental stage of the embryo, nor the type of media affected the proportion of PAF-positive cultures nor the amount of PAF produced during culture. Thus, it is demonstrated for the first time that early ovine embryos can secrete PAF in vitro, and that there is considerable variability in their capacity for PAF secretion.     

Production of platelet-activating factor in slugs.

The land slug, Incilaria bilineata, was shown to contain a large amount of 1-O-alkyl-2-acyl (long chain)-sn-glycero-3-phosphocholine, which accounts for as much as 47% of the choline glycerophospholipid fraction. Since this unique either phospholipid has been regarded as a stored precursor form of platelet-activating factor (PAF) in mammalian inflammatory cells, we examined the possibility of the presence of PAF in this animal. We obtained the evidence for the occurrence of significant amounts of PAF in two species of slugs, Incilaria bilineta and Incilaria fruhstorferi. Gas chromatography-mass spectrometry analysis revealed that the alkyl fatty chain of PAF principally consists of 16:0. We confirmed the presence of both enzyme activities catalyzing the formation of PAF, one for the remodeling pathway and the other for the de novo pathway. We also found the occurrence of other enzyme activities involved in PAF metabolism: acetylhydrolase activity which inactivates PAF and phospholipase A2 activity toward alkylacylglycerophosphocholine. However, we failed to detect cofactor-independent transacylation activity in this animal. The amounts of PAF in slugs were markedly increased when slugs were administered several treatments which are considered to induce shock, such as the injection of dimethyl sulfoxide or injuries. These results suggest that PAF is produced and may have certain physiological and pathological roles in the land slug, as in the case of mammals.     

Release of acetylhydrolase from platelets on aggregation with platelet-activating factor

Acetylhydrolase, the enzyme which inactivates platelet-activating factor (PAF, 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine), was selectively released from bovine platelets by aggregation with physiological concentrations (0.1-10 nM) of PAF with no cell lysis. The release of the acetylhydrolase paralleled that of serotonin. The acetylhydrolase released was active over a broad pH range (pH 5.4-8.6) and was not affected by Ca2+ (1-4 mM) or EDTA (1-8 mM). The Km value of the enzyme was 4.6 microM. Net specific acetylhydrolase activity recovered in the 130,000 x g supernatant after stimulation with PAF could be determined in the presence of EDTA without the activity of Ca2+-dependent phospholipase A2 which was also released from the cells at the same concentration of PAF. The acetylhydrolase was inhibited competitively by specific PAF antagonists, rac-3-(N-n-octadecylcarbamoyloxy)-2-methyoxypropyl-2-thiazolioe thyl phosphate (CV-3988) and (2RS)-1-O-hexadecyl-2-O-ethyl-3-O-(7-thiazolinoheptyl)-glycerol methanesulfonate (ONO-6040). Their Ki values for the enzyme were 1.17 microM and 0.84 microM, respectively. The release of the enzyme could also be detected when the platelets were aggregated with ADP (2.3 microM) or thrombin (0.5 unit). These results suggest that the enzyme released from the aggregated platelets to the blood plasma may also have a physiological function cooperating with the plasma acetylhydrolase.     

Radioligand binding of antagonists of platelet-activating factor to intact human platelets

Two new antagonists of platelet-activating factor (PAF), the pyrrolothiazole derivative 52770 RP and the triazolodiazepine WEB 2086, have been studied as radioligands in intact human platelets. [3H]52770 RP and [3H]WEB 2086 bound specifically to high-affinity sites with dissociation constants (Kd) of 14.8 and 6.1 nM, respectively. The maximal number of sites for [3H]52770 RP binding was approx. 15-fold higher than for [3H]PAF and [3H]WEB 2086. In addition, C16-PAF, lyso-PAF, WEB 2086 and 52770 RP had Ki values which were nearly identical for both [3H]PAF and [3H]WEB 2086, whereas only 52770 RP competed for [3H]52770 RP-binding sites. These results demonstrate that in human platelets the sites of [3H]WEB 2086 binding are identical to [3H]PAF-binding sites, whereas those of [3H]52770 RP are not. [3H]WEB 2086 appears, therefore, to be a suitable antagonist radioligand for labelling PAF receptors.     

Activation of washed chicken thrombocytes by 1-O-hexadecyl/octadecyl-2-acetyl-sn-glycero-3-phosphorylcholine (platelet-activating factor)

Washed, [3H]serotonin-labeled chicken thrombocytes aggregated and secreted [3H]serotonin when stimulated in vitro with platelet-activating factor (PAF), collagen and calcium ionophore A23187. The effective dose causing a 25% secretion of [3H]serotonin (ED25) from washed chicken thrombocytes was 10(-8) M for PAF, 5 X 10(-8) M for collagen and 3 X 10(-7) M for A23187. Chicken thrombocyte activation by PAF required Ca2+ and appeared to be mediated through a specific receptor for PAF.     

Effect of prostacyclin on platelet-activating factor induced rabbit platelet aggregation

In this paper, the effect of prostacyclin (PGI₂) on the aggregation induced by Platelet-activating factor (PAF), a phospholipid mediator of anaphylaxis, was studied. Synthetic PGI₂ and PGI₂-like activity generated from rabbit aorta were demonstrated to be effective inhibitors of PAF-induced rabbit platelet aggregation and release of ³H-serotonin (³H-5HT).