A preferential inhibition by Zn2+ on platelet activating factor- and thrombin-induced serotonin secretion from washed rabbit platelets

Structurally diverse metal coordination compounds, bearing imidodiphosphinate and diphosphinoamine ligands, as potential inhibitors of the platelet activating factor

Metal complexes bearing dichalcogenated imidodiphosphinate [R₂P(E)NP(E)R₂′]⁻ ligands (E = O, S, Se, Te), which act as (E,E) chelates, exhibit a remarkable variety of three-dimensional structures. A series of such complexes, namely, square-planar [Cu{(OPPh₂)(OPPh₂)N-O, O}₂], tetrahedral [Zn{(EPPh₂)(EPPh₂)N-E,E}₂], E = O, S, and octahedral [Ga{(OPPh₂)(OPPh₂)N-O,O}₃], were tested as potential inhibitors of either the platelet activating factor (PAF)- or thrombin-induced aggregation in both washed rabbit platelets and rabbit platelet rich plasma. For comparison, square-planar [Ni{(Ph₂P)₂N-S-CHMePh-P, P}X₂], X = Cl, Br, the corresponding metal salts of all complexes and the (OPPh₂)(OPPh₂)NH ligand were also investigated. Ga(O,O)₃ showed the highest anti-PAF activity but did not inhibit the thrombin-related pathway, whereas Zn(S,S)₂, with also a significant PAF inhibitory effect, exhibited the highest thrombin-related inhibition. Zn(O,O)₂ and Cu(O,O)₂ inhibited moderately both PAF and thrombin, being more effective towards PAF. This work shows that the PAF-inhibitory action depends on the structure of the complexes studied, with the bulkier Ga(O,O)₃ being the most efficient and selective inhibitor.

Alanine exchanges of polar amino acids in the transmembrane domains of a platelet-activating factor receptor generate both constitutively active and inactive mutants

To determine ligand-binding sites of a platelet-activating factor (PAF) receptor, alanine-scanning mutagenesis was carried out. All 23 polar amino acids in the putative 7-transmembrane (TM) domains of a guinea pig PAF receptor were individually replaced with alanine. The ligand-binding properties of mutant receptors were determined after transient expression in COS-7 cells. Mutants in TM II (N58A, D63A), TM III (N100A, T101A, S104A) and TM VII (D289A) displayed higher PAF-binding affinities than seen with the wild-type receptor. In contrast, mutants in TM V (H188A), TM VI (H248A, H249A, Q252A), and TM VII (Q276A, T278A) showed lower affinities. Representative mutants were then stably expressed in Chinese hamster ovary cells to observe PAF-induced cellular signals (arachidonate release, phosphatidylinositol hydrolysis, adenylyl cyclase inhibition). An N100A mutant with the highest affinity was constitutively active and was responsive to lyso-PAF, an inactive derivative of PAF. One nanomolar PAF induced no signals in low affinity mutants, an EC50 value for the wild-type receptor. Three histidines (His-188, His-248, His-249) might form a binding pocket for the phosphate group of PAF, since zinc effectively inhibited ligand binding. Based on these results, a three-dimensional molecular model of PAF and its receptor was generated using bacteriorhodopsin as a reference protein.

Inhibition of PAF-acether effects on isolated guinea pig hearts by zinc ions (Zn2+)

PAF-acether is a phospholipid synthesized by most animal tissues and exerting a strong decrease on the heart's contractile force and coronary flow. PAF-acether (10(-9) and 10(-10)M) was administered to isolated guinea pig hearts perfused via the Langendorff apparatus with Chenoweth solution. Zinc (1.5 microM) is known to benefit heart function thus, Zn2+ (1.5, 7.5, and 30 microM) was added in the perfusing solution before or after PAF-acether administration. Contractile force, coronary flow, and heart rate were recorded by means of a Narco MK-IV Physiograph throughout all modes of perfusion. Calcium inhibitor (Verapamil 10(-10)M) and Pb+2 Co2+ (1.5 x 10(-6)M) were used subsequently in the perfusing solutions in order to elucidate some of the Zn and PAF interactions observed. All hearts were analyzed for their Zn and Ca content by means of an Atomic Absorption Spectrophotometry (AAS). Our data suggest that low concentrations of zinc (1.5 microM) can strongly inhibit PAF-induced decrease of contractile force and coronary flow. Zinc-inhibiting effects on PAF's negative inotropic action (myocytic level) is not exerted through Zn-Ca antagonism. Nevertheless, a Zn-Ca antagonism in the arteriolar level cannot be excluded. Zinc inhibits PAF selectively only if it is administered before PAF injection and this strongly suggests a receptor interaction between the metal and the phospholipid at the heart level.

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.

Synthesis of a PAF immunogen and production of PAF-specific antibodies

Platelet activating factor (PAF), a naturally occurring phospholipid with many potent physiological and pharmacological activities, is implicated as a mediator of many diseases. An immunoassay for PAF would greatly improve quantitation, and hence PAF-specific antibodies were required. Chemically-reactive analogs of PAF, containing an aldehyde group at the end of the 1-O-alkyl chain (hexyl or dodecyl), were synthesized from readily available materials. During the multi-step synthetic procedure, the aldehyde group was protected as an acetal, which was converted by mild acidic hydrolysis to the aldehyde immediately prior to protein coupling. These analogs were coupled to methylated bovine serum albumin and the resultant conjugates were injected into rabbits. Antibodies to PAF were detected using a solid phase radioimmunoassay based on Protein A-Sepharose. The dodecyl PAF conjugate proved to be the more immunogenic conjugate with more than half of the rabbits producing significant levels of antibodies (at least a 10-fold increase in radioactive uptake over pre-immune levels). Results from solid phase immunoassays employing nitrocellulose discs impregnated with PAF, lysoPAF, lecithin, lysolecithin and 2-O-methyl-lysoPAF indicated that the antibodies recognized only PAF. PAF-specific antibodies were isolated by affinity chromatography using a column of PAF-poly(lysine) conjugated to carboxy-activated polyacrylamide. The antibodies may be employed in a sensitive and specific immunoassay for PAF and for many other studies involving PAF.

Quantitation by radioimmunoassay of PAF in human saliva

Approximately 75% of the PAF present in saliva is recovered on extraction of whole saliva (0.8 vol) with chloroform/methanol/water (2:2:1, v/v/v). PAF levels, determined by our recently developed radioimmunoassay, in saliva extracts ranged from 0.5-21 ng/mL with 59% between 2-6 ng/mL. These figures, for apparently healthy subjects, are higher than previously reported levels obtained by platelet assays. The validity of our radioimmunoassay results was checked by isolating and quantitating the PAF fraction from whole saliva. In addition, when we examined our saliva samples by platelet aggregation, low levels of PAF, comparable with the values found in the literature, were detected. Investigations revealed the presence of a substance(s) which inhibited PAF-induced platelet aggregation but which did not affect the radioimmunoassay.

Inhibitor(s) of platelet-activating factor (PAF) in human saliva

Quantitation of platelet-activating factor (PAF) in human saliva samples by radioimmunoassay indicated there was, at times, sufficient PAF present to aggregate platelets. However, in certain samples, we observed little or no aggregation, and furthermore, these samples were found to inhibit aggregation induced by PAF (200 pg). Chromatographic fractionation of pooled saliva increased the PAF activity 4-fold, and the observed inhibitory activity was found to co-migrate with the fatty acids. The inhibitory fraction was found to be active against platelet aggregation induced by arachidonic acid (3.4 nmole) as well as PAF (25 pg), but not thrombin (20 mU). These results indicate the existence of a PAF inhibitor in saliva, which may explain why potentially toxic levels of PAF can occur in the saliva of normal, healthy individuals. These findings also highlight an important advantage of the radioimmunoassay over platelet aggregation for the quantitation of PAF in, at least, some biological fluids.

Zn(2+)-dependent tyrosine phosphorylation of a 68-kDa protein and its differentiation from Mg(2+)-dependent tyrosine phosphorylation in sheep platelets

A 68-kDa protein that was tyrosine phosphorylated in the presence of Zn2+ and two proteins of 52 and 46 kDa that were tyrosine phosphorylated in the presence of Mg2+ were separated by column chromatography of a sheep platelet high speed supernatant on poly(Glu, Tyr)4:1 copolymer-Sepharose or tyrosine-Sepharose. Phosphorylation of the 68-kDa protein occurred maximally in the presence of Zn2+ while Mg2+ was ineffective. The kinases responsible for the Zn(2+)- and Mg(2+)-dependent tyrosine phosphorylation could also tyrosine phosphorylate poly(Glu, Tyr)4:1, histone, and angiotensin II with the same metal ion specificity. The two tyrosine kinase activities could be also distinguished by their differential response to polyamines and quercetin. Zn2+ stimulation did not appear to be due to the inhibition of a protein tyrosine phosphatase. Sephadex G-100 gel filtration of the fraction showing Zn(2+)-dependent tyrosine phosphorylation of the 68-kDa protein showed that the tyrosine kinase activity corresponded to a molecular mass of 68,000 and it showed a protein band of 68 kDa as detected by silver staining on sodium dodecyl sulfate-polyacrylamide gel.

Platelet-activating factor and related acetylated lipids as potent biologically active cellular mediators

Platelet-activating factor (PAF or 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is the most potent lipid mediator yet discovered. It is known to stimulate a wide span of biological responses ranging from aggregation and degranulation of platelets and neutrophils to a variety of cellular effects involving the stimulation of chemotaxis; chemokinesis; superoxide formation; protein phosphorylation; activation of protein kinase C, arachidonic acid, and phosphoinositide metabolites; glycogenolysis; and tumor necrosis factor production. Obviously, with such a diversity of biological activities, it is not surprising that PAF has been considered to be a key component in numerous diseases related to hypersensitivity and inflammatory responses. Evidence has also been presented for the role of PAF in physiological processes, particularly those involving reproduction and fetal development. Furthermore, because of its potent hypotensive action, PAF has been implicated as a contributing factor in blood pressure regulation. PAF is produced by two independent enzymatic pathways. The remodeling route involves the structural modification of a membrane lipid (1-alkyl-2-acyl-sn-glycero-3-phosphocholine) by replacement of the acyl moiety with an acetate group. An alternate route is the de novo synthesis of PAF from an O-alkyl analogue of a lysophosphatidic acid that requires a reaction sequence of acetylation, dephosphorylation, and phosphocholine addition steps. Hypersensitivity and other pathophysiological reactions are thought to be caused by activation of the remodeling pathway, whereas the de novo route is believed to be the source of endogenous levels of PAF required for physiological functions. Inactivation of PAF occurs when the acetate group is hydrolyzed by an acetylhydrolase that is present in both extra- and intracellular compartments, although the catalytic activity of the two forms of acetylhydrolase are identical, some of their properties differ. The control of PAF metabolism is very complex, but acetylhydrolase, Ca2+, phosphorylation/dephosphorylation of enzymes, and fatty acids (especially polyunsaturates) appear to be important regulatory factors. Specific PAF receptors have clearly been demonstrated on several different types of cells, and although the mechanism of PAF actions is poorly understood, it appears that the PAF/receptor-induced responses are closely associated with the signal transduction process; both G proteins and adenyl cyclase appear to be involved. Because significant quantities of PAF are often retained within certain cells, the possibility of PAF serving as an intracellular mediator has also been proposed.

Zinc monoglycerolate: a slow-release source of zinc with anti-arthritic activity in rats

Zinc repletion by parental administration of zinc monoglycerolate (ZMG) or certain other lipophilic zinc complexes, suppressed the development of adjuvant-induced polyarthritis in rats. While ZMG was effective when given parenterally over various limited time schedules (immunosuppressant, therapeutic, singledose), it was not effective given orally. The complex showed no acute anti-inflammatory activity in the carrageenan paw oedema assay and little gastric-intestinal or other organ toxicity. When injected s.c. it caused very much less local irritation than most zinc salts. Being lubrous, ZMG could be applied as the dry powder for rubbing into the skin and by this route was found to have anti-arthritic activity. 65Zn was shown to be absorbed and excreted in the faeces (biliary excretion) after applying 65Zn-ZMG dermally to shaved dorsal skin of rats. ZMG showed consistent anti-arthritic activity in rats under conditions in which 2 gold drugs (aurothiomalate, Auranofin) exhibited variable effects, depending on the strain of rat. The role of zinc and its availability in chronic inflammation are discussed on the basis of these studies.

Inhibition of [3H]platelet activating factor (PAF) binding by Zn2+: a possible explanation for its specific PAF antiaggregating effects in human platelets

Zinc ions in the micromolar range exhibited a strong inhibitory activity toward platelet activating factor (PAF)-induced human washed platelet activation, if added prior to this lipid chemical mediator. The concentration of Zn2+ required for 50% inhibition of aggregation (IC50) was inversely proportional to the concentration of PAF present. The IC50 values (in microM) for Zn2+ were 8.8 +/- 3.9, 27 +/- 5.8, and 34 +/- 1.7 against 2, 5, and 10 nM PAF, respectively (n = 3-6). Zn2+ exhibited comparable inhibitory effects on [3H]serotonin secretion and the IC50 values (in microM) were 10 +/- 1.2, 18 +/- 3.5, and 35 +/- 0.0 against 2, 5, and 10 nM PAF, respectively (n = 3). Under the same experimental conditions, aggregation and serotonin secretion induced by ADP (5 microM), arachidonic acid (3.3 microM), or thrombin (0.05 U/ml) were not inhibited. Introduction of Zn2+ within 0-2 min after PAF addition not only blocked further platelet aggregation and [3H]serotonin secretion but also caused reversal of aggregation. Analysis of [3H]PAF binding to platelets showed that Zn2+ as well as unlabeled PAF prevented the specific binding of [3H]PAF. The inhibition of [3H]PAF specific binding was proportional to the concentration of Zn2+ and the IC50 value was 18 +/- 2 microM against 1 nM [3H]PAF (n = 3). Other cations, such as Cd2+, Cu2+, and La3+, were ineffective as inhibitors of PAF at concentrations where Zn2+ showed its maximal effects. However, Cd2+ and Cu2+ at high concentrations exhibited a significant inhibition of the aggregation induced by 10 nM PAF with IC50 values being five- and sevenfold higher, respectively, than the IC50 for Zn2+, and with the IC50 values for inhibition of binding of 1 nM [3H]PAF being 5 and 19 times higher, respectively, than the IC50 for Zn2+. The specific inhibition of PAF-induced platelet activation and PAF binding to platelets suggested strongly that Zn2+ interacted with the functional receptor site of PAF or at a contiguous site.