Characterization of 3H-labelled platelet activating factor receptor complex solubilized from rabbit platelet membranes

Platelet activation

This review article describes the different receptors, second-messengers and mechanisms involved in platelet activation. Several platelet agonists have well-defined receptors at the platelet membrane of which a number are single polypeptides with 7 hydrophobic transmembrane domains. These receptors are connected, via GTP regulatory proteins, with cytoplasmic second-messenger-generating enzymes. Phospholipase C and adenylate cyclase are the two best-known enzymes, generating inositol triphosphate (IP3) and diacyl glycerol from phosphatidylinositol biphosphate and cyclic AMP from ATP respectively. The intraplatelet free calcium level, which is critical for the activation status of the platelet, is increased by IP3 and is lowered in the presence of rising cyclic AMP concentrations. Shape-change occurs with small increases in intraplatelet calcium, while aggregation and secretion of granules take place at higher calcium, levels. The role of myosin and actin filaments and of transmembrane glycoproteins is further discussed.

Platelet-activating factor: receptors and signal transduction

During the past two decades, studies describing the chemistry and biology of PAF have been extensive. This potent phosphoacylglycerol exhibits a wide variety of physiological and pathophysiological effects in various cells and tissues. PAF acts, through specific receptors and a variety of signal transduction systems, to elicit diverse biochemical responses. Several important future directions can be enumerated for the characterization of PAF receptors and their attendant signalling mechanisms. The recent cloning and sequence analysis of the gene for the PAF receptor will allow a number of important experimental approaches for characterizing the structure and analysing the function of the various domains of the receptor. Using molecular genetic and immunological technologies, questions relating to whether there is receptor heterogeneity, the precise mechanism(s) for the regulation of the PAF receptor, and the molecular details of the signalling mechanisms in which the PAF receptor is involved can be explored. Another area of major significance is the examination of the relationship between the signalling response(s) evoked by PAF binding to its receptor and signalling mechanisms activated by a myriad of other mediators, cytokines and growth factors. A very exciting recent development in which PAF receptors undoubtedly play a role is in the regulation of the function of various cellular adhesion molecules. Finally, there remain many incompletely characterized physiological and pathophysiological situations in which PAF and its receptor play a crucial signalling role. Our laboratory has been active in the elucidation of several tissue responses in which PAF exhibits major autocoid signalling responses, e.g. hepatic injury and inflammation, acute and chronic pancreatitis, and cerebral stimulation and/or trauma. As new experimental strategies are developed for characterizing the fine structure of the molecular mechanisms involved in tissue injury and inflammation, the essential role of PAF as a primary signalling molecule will be affirmed. Doubtless the next 20 years of experimental activity will be even more interesting and productive than the past two decades.

Protein kinase C is not involved in the desensitization of platelet activating factor receptor in rabbit platelets

Rabbit platelets pretreated with platelet activating factor (PAF) became refractory to further stimulation by PAF. The effect was specific for PAF. In this study, the alteration in the specific agonist binding to PAF receptor in platelets following desensitization was investigated. As revealed by the Scatchard analysis of radioligand binding data, the affinity for specific PAF binding to desensitized platelet membranes was substantially lowered as compared with that to control platelet membranes. Guanine nucleotide triphosphate, which was shown to decrease the affinity of specific PAF binding to platelet membranes, had less effect on the PAF binding affinity to the desensitized preparation. In platelets pretreated with phorbol 12-myristate-13-acetate, the binding affinity of PAF receptor remained unaltered. Pretreatment of platelets with 1-(5-isoquinolinesulphonyl)-2-methylpiperazine, a protein kinase C inhibitor, or neomycin, an inhibitor of the polyphosphoinositide breakdown, failed to prevent the reduction of specific PAF binding affinity following subsequent exposure to PAF. These results suggest that the agonist-induced desensitization of PAF receptor in rabbit platelets is independent of activation of protein kinase C.

Solubilization of a functionally active platelet-activating factor receptor from rabbit platelets

Binding of platelet-activating factor (PAF) to a specific high-affinity membrane receptor has been demonstrated in numerous cell types, but very little is known about the molecular nature of this receptor. The receptor from rabbit platelets was solubilized using CHAPS, digitonin, octyl glucoside, Nonidet P-40 or sodium cholate, either with pre-bound [3H]PAF or in the absence of ligand. We have been able to demonstrate for the first time that the receptor solubilized with CHAPS, in the absence of ligand, could retain its binding activity. It migrated as a high molecular mass complex (greater than 350 kDa) on a Bio-Gel A-0.5 m gel filtration column. Binding to solubilized receptor rapidly reached an equilibrium at room temperature, but was much slower at 0 degrees C. Scatchard plots were used to calculate the number (approx. 100 per cell) and the affinity (Kd 2.5 +/- 1.4 nM) of the solubilized receptors. These values were comparable with those obtained from whole-cell binding experiments. Competition by PAF antagonists also verified that the assay was measuring PAF receptor binding activity. The presence of a protein in the receptor complex was demonstrated by heat and trypsin inactivation of binding activity. Trypsin had no effect on binding of PAF to whole cells, but was able to decrease binding activity in solubilized receptor preparations. Attempts to demonstrate the involvement of a glycoprotein by use of various lectin columns proved unsuccessful. The latter results are consistent with findings suggesting that the binding site of the PAF receptor may not be exposed at the cell surface.

Photoaffinity labeling of platelet activating factor binding sites in rabbit platelet membranes

A photoreactive, radioiodinated derivative of platelet activating factor (PAF), 1-O-(4-azido-2-hydroxy-3-iodobenzamido)undecyl-2-O-acetyl-sn- glycero-3-phosphocholine ([125I]AAGP), was synthesized and used as a photoaffinity probe to study the PAF binding sites in rabbit platelet membranes. The nonradioactive analog, IAAGP, induced rabbit platelet aggregation with an EC50 value of 3.2 +/- 1.9 nM as compared to 0.40 +/- 0.25 nM for PAF. Specific binding of [125I]AAGP to rabbit platelet membranes was saturable with a dissociation constant (Kd) of 2.4 +/- 0.7 nM and a receptor density (Bmax) of 1.1 +/- 0.2 pmol/mg protein. Photoaffinity labeling of platelet membranes with [125I]AAGP revealed several 125I-labeled components by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A protein species with apparent molecular weight of 52,000 was consistently observed and inhibited significantly by unlabeled PAF at nanomolar concentrations. The labeling was specific since the PAF antagonists, SRI-63,675 and L-652,731, at 1 uM also blocked the appearance of this band; whereas lysoPAF was not effective at the same concentration. These results suggest that the binding sites of PAF receptor in rabbit platelets reside in the polypeptide of Mr = 52,000.

A specific, photolabile and irreversible antagonist (L662,025) of the PAF-receptor

PAF (0.2 microM) induced maximal platelet aggregation in human PRP and [3H]-PAF (1-5 nM) binding to platelet membrane preparations had Kd value of 3.8 nM and Bmax of 200 fmoles/mg of protein. Without UV irradiation, a synthetic azido tetrahydrofuran derivative L662,025 was a reversible and competitive PAF-receptor antagonist with IC50 values of 5.6 +/- 0.3 microM (platelet aggregation) and 1.0 +/- 0.25 microM (receptor binding). Photolysis of L662,025 in the presence of PRP produced an irreversible inhibition of platelet aggregation and specific binding of [3H]-PAF (1 nM). L662,025 did not affect collagen- or ADP-induced human platelet aggregation before or after photolysis. It is a new probe that can be used to identify and characterize the PAF-receptor.