Characterization of the platelet prostaglandin D2 receptor. Loss of prostaglandin D2 receptors in platelets of patients with myeloproliferative disorders

Development of a radioimmunoassay for prostaglandin D2 using an antiserum against 11-methoxime prostaglandin D2

A sensitive and specific radioimmunoassay for prostaglandin D2 has been developed using its stabilized 11-methoxime derivative, which was obtained after treatment of prostaglandin D2 with methoxamine-HCl. The antiserum was obtained after injection of prostaglandin D2-methoxamine coupled to bovine serum albumin. A (125I)-Histamide prostaglandin D2-methoxamine tracer was prepared by iodination of the corresponding histamide, followed by thin layer chromatography purification. The sensitivity of the assay was 280 femtomoles per ml at 50% displacement. The cross reactivities were 15% with prostaglandin D1-methoxamine and less than 0.20% with other prostaglandins. Determination of the half-life of prostaglandin D2 in a solution containing albumin was also carried out, since it has been shown to catalyze prostaglandin D2 destruction. The unstability of this prostaglandin is due to the presence of a beta-hydroxy ketone group, and all prostaglandins possessing this labile moiety could be stabilized by such a derivatization before developing a radioimmunoassay.

Heparin does not interfere with prostacyclin and prostaglandin D2 binding to platelets

Heparin has been reported to antagonize the platelet antiaggregating activity of prostacyclin (PGI2) and prostaglandin D2 (PGD2). To investigate whether heparin interferes with PGI2 and/or PGD2 binding to the specific membrane platelet receptors, the number and the affinity of binding sites for tritiated PGI2 and PGD2 were determined in 8 healthy subjects (aged 25-32; 4 for PGI2 and 4 for PGD2 studies) in the absence and in the presence of heparin. Preliminary aggregation tests indicated that the heparin concentration tested (2 I.U./ml) reduced the antiaggregating activity of PGI2 and PGD2. Heparin, when preincubated with platelet suspension, did not induce any significant change in the number/platelet (bs/plt) and affinity (Kd) of PGI2 and PGD2 binding sites (bs) in comparison to control values (PGI2 high affinity bs: bs/plt = 106 +/- 12 vs 107 +/- 17 - Kd = 9.7 +/- 3.1 vs 10.0 +/- 2.3 nM; PGI2 low affinity bs: bs/plt = 3551 +/- 233 vs 3670 +/- 465 - Kd = 877 +/- 125 +/- 125 vs 833 +/- 104 nM; PGD2 bs: bs/plt = 228 +/- 14 vs 234 +/- 24 - Kd = 68 +/- 10 vs 73 +/- 7 nM; p greater than 0.4 for all the differences). No significant binding modification was also observed when heparin was preincubated with 3H-PGI2 and 3H-PGD2. The present demonstration that heparin does not interfere with PGI2 and PGD2 binding to platelets is consistent with the hypothesis that heparin reduces the antiaggregating effects of PGI2 and PGD2 by directly potentiating platelet aggregation.

Thromboxane synthase inhibition causes re-direction of prostaglandin endoperoxides to prostaglandin D2 during collagen stimulated aggregation of human platelet rich plasma

Prostanoid synthesis and release during collagen-induced aggregation of human platelet rich plasma (PRP) was studied using a novel gas chromatography/mass spectrometry assay technique. Aggregation was associated with the production of mainly thromboxane A2 (TXA2), measured as TXB2, and smaller amounts of the prostaglandins (PGs) D2, E2 and F2 alpha. UK 37,248 inhibited TXB2 formation by greater than 95% and increased the production of PGD2, PGE2 and PGF2 alpha twenty-fold. The relative amounts of these three prostanoids were not changed by UK 37,248. Even though high concentrations of PGD2 were formed, aggregation was not inhibited. In contrast, flurbiprofen inhibited aggregation, demonstrating that platelet aggregation produced by this concentration of collagen is cyclooxygenase dependent. These results support the proposal that the prostaglandin endoperoxides can induce aggregation alone, irrespective of the amount of PGD2 that is produced.

Age related changes of platelet prostacyclin receptors in humans

Platelet receptors for PGI2 were investigated in eighteen healthy volunteers divided in two classes of age: nine were 18-40 years old and nine were 41-65 years old. PGI2 platelet receptors were found to decrease significantly with age; the young subjects had 106 +/- 16 (mean +/- SD) high affinity receptors/platelet and 3509 +/- 529 low affinity receptors/platelet whereas the old subjects had 86 +/- 14 high affinity receptors/platelet (P less than 0.02) and 2471 +/- 640 low affinity receptors/platelet (P less than 0.005). The cumulated data from all the subjects yielded a significant negative correlation with age (r = -0.71, P less than 0.001 for the high affinity class and r = -0.65, P less than 0.01 for the low affinity class). The affinity of receptor sites for the ligand did not statistically change with aging (P less than 0.30 and P less than 0.90 respectively for high and low affinity receptors).

A novel approach for the study of thromboxane A2 and prostaglandin H2 receptors using an 125I-labeled ligand

Previous studies of eicosanoid receptors have utilized 3H-labeled ligands. However, 125I has a higher theoretical specific activity (approximately 2000 Ci/mmole) than 3H (29 Ci/mmole), which provides a potential advantage for 125I ligand binding studies when the receptor density is low. Since eicosanoids do not possess an easily iodinatable structure (e.g. a phenol or imidazole ring), it is not feasible to directly incorporate 125I into the molecule. The thromboxane A2/prostaglandin H2 receptor antagonist, cis-7-(2-p-hydroxyphenylethanolaminocyclopentyl)-heptanoic acid (cis-APO), was synthesized to test the concept that it could be labeled with 125I and used as a ligand for binding studies. cis-APO is a structural analog of 13-azaprostanoic acid, a TXA2/PGH2 antagonist [G. C. Le Breton, D. L. Venton, S. E. Enke and P. V. Halushka, Proc. natn. Acad. Sci. U.S.A. 76, 4097 (1979)], in which the omega aliphatic chain was substituted with 2-p-hydroxyphenylethanol, which contains a phenolic group. [127I]cis-APO was synthesized by insertion of 127I (stable isotope) into the phenolic portion of the molecule under alkaline conditions. [125I]-cis-APO was synthesized via insertion of 125I (unstable isotope) into the molecule in the presence of chloramine T. cis-APO inhibited human platelet aggregation induced by the thromboxane mimetic U46619 [C. Malmsten, Life Sci. 18, 169 (1976)]. The IC50 for cis-APO was 6.4 +/- 0.7 microM and for [127I]-cis-APO was 9.8 +/- 1.3 microM (P less than 0.001). [125I]-cis-APO binding to a human platelet membrane preparation at 4 degrees was time and protein concentration dependent, saturable, and reduced or abolished by trypsin or boiling respectively. The Kd for iodo-cis-APO determined at equilibrium using a Scatchard analysis was 1.48 microM and the maximum binding capacity was 18.7 pmoles/mg protein. The forward rate constant (k+1) was 2.3 X 10(3) M-1 s-1 and the dissociation constant (k-1) was 2.12 X 10(-3) s-1. The Kd determined from k-1/k+1 was 0.92 microM. These observations show that the omega side chain of eicosanoid analogs can be substituted with a phenolic group, iodinated, and retain biological activity. These molecules may then be utilized to study thromboxane A2 or prostaglandin H2 receptors.

Decreased number of PGD2 binding sites on platelets from patients with type IIa hyperlipoproteinemia

Platelets from patients with familial hypercholesterolemia (type IIa hyperlipoproteinemia), a condition associated with a high prevalence of atherosclerosis and its ischemic complications, are claimed to be hyperresponsive to aggregating stimuli. We investigated the platelet responsiveness to and the binding of PGD2, a potent endogenous inhibitor of platelet aggregation via stimulation of adenylate cyclase, in a group of 7 patients affected by IIa hyperlipoproteinemia (IIa HLP) and in a control group of 10 healthy subjects. Inhibition by PGD2 of ADP-induced platelet aggregation was significantly lower in IIa HLP patients than in controls. The number of binding sites for PGD2 of platelets from IIa HLP patients was significantly reduced in comparison with that from controls (93 +/- 19 and 232 +/- 23 receptors/platelet, respectively), whereas the affinity for PGD2 was comparable to that of controls (Kd = 68.8 +/- 19.8 nM in patients and 66.1 +/- 15.9 nM in controls). The reduced number of platelet PGD2 binding sites in IIa HLP patients may account for the impaired sensitivity to PGD2 shown in vitro by platelets and may contribute to the increased tendency to thrombotic manifestations observed in IIa HLP.

Reduction in prostacyclin platelet receptors in active spontaneous angina

The number and affinity of binding sites for tritiated prostacyclin (PGI2) in platelets were investigated in twenty-eight patients with spontaneous angina (fourteen in the active and fourteen in the inactive phase) and in nine healthy controls of similar age. Active-phase patients had significantly lower numbers of both classes of platelet PGI2 receptors (high affinity and low affinity) than controls or inactive-phase patients. In contrast, the affinity for PGI2 was not significantly different in the three groups. In six active-phase patients restudied in the inactive phase the previously low number of PGI2 receptors had returned to normal. These results suggest that the instability of angina is associated with functional changes not confined to the coronary vasculature.

Synthesis of [17,18-3H] trans-13-azaprostanoic acid. A labeled probe for the PGH2/TXA2 receptor

Because of its highly unstable nature, TXA2, produced by platelet metabolism of arachidonic acid, does not lend itself to use as a receptor probe for its own receptor. As such, the stable TXA2/PGH2 antagonist, trans-13-azaprostanoic acid (trans-13-APA, 12b), was prepared as the [17, 18 3H] derivative [( 3H] trans-13-APA, 12c) to study this receptor and to better evaluate the mechanism of action of these azaprostanoids. Tritiated trans-13-APA, 12c, was prepared in nearly theoretical specific activity (57 Ci/mmole) from (17Z)-trans-13-azaprost-17-enoic acid (11b) by catalytic tritiation. The unsaturated 11b was prepared by condensation of cis-7-amino-3-heptene (8) with 2-(6-carboxyhexyl) cyclopentanone (9), NaBH4 reduction, chromatography, and hydrolysis of the trans isomer so isolated. The olefins 11a and b were also of biochemical interest because of the unsaturation in the lower side chain. The presence of similar unsaturation in PGH3(4) and TXA3 (3) renders these prostaglandins inactive as proaggregatory agents. Evaluation of the antiaggregatory activity of 11a and b indicated it to be about the same potency in inhibiting human platelet aggregation as the parent cis and trans-13-APAs, suggesting that introduction of a double bond at the 17 position in platelet prostaglandin antagonists is unlikely to result in enhanced antiplatelet activity.

On the multiplicity of platelet prostaglandin receptors. II. The use of N-0164 for distinguishing the loci of action for PGI2, PGD2, PGE2 and hydantoin analogs

The omega-chain variant analogs of prostacyclin (PGI2) and PGD2 in which n-amyl side-chain has been replaced by a cyclohexyl group have been prepared and their cardiovascular activities have been compared to those of BW-245C(Fig. 1) a potent anti-aggregatory vasodilator bearing a cyclohexyl-terminated side-chain on a hydantoin skeleton. The cyclohexyl group has little effect on PGI2, but converts PGD2 to a long lasting hypotensive agent and increases the platelet anti-aggregatory potency of PGD2 by a factor of 8. The prostaglandin antagonist N-0164 selectively blocks the anti-aggregatory actions of PGD2, cyclohexyl-PGD2, and BW-245C; with essentially no effect on PGI2, cyclohexyl-PGI2 and PGE2 at comparably effective doses. The latter observation is contrary to an earlier report by MacIntyre, but supports the view that the anti-aggregatory effect of high doses of PGE2 (EC50=50 microM) is mediated by the PGI2 receptor. The hydantoin acts at the platelet PGD2 receptor.

Autoradiographic localization of a binding protein(s) specific for prostaglandin D2 in rat brain

The specific [3H]prostaglandin (PG) D2 binding was detected by using the slide-mounted sections of rat brain fixed by perfusion with 2% paraformaldehyde. The binding was reversible, saturable, high affinity, Na+ dependent, and highly specific for PGD2. These binding characteristics are essentially similar to those observed with the synaptic membrane of rat brain as previously reported. Using autoradiographic image analyses by computerized densitometry and color coding, we visualized the localization of [3H]PGD2 binding in rat brain. A high density of the binding sites was observed in the cerebral cortex, preoptic area, amygdala, hypothalamic nuclei (arcuate nucleus, ventromedial nucleus, and posterior hypothalamic nucleus), thalamic nuclei (reuniens nucleus and rhomboid nucleus), hippocampus, pineal body, and cerebellar cortex. The binding was not significantly observed in the striatum and also was negative in the white matter, arachnoid membranes, and vasculatures.

Direct evidence for the presence of selective binding sites for (3H) prostaglandin E2 on rat peritoneal macrophages

A method is presented which provides for a simple and rapid determination of PGE2 receptors on viable peritoneal macrophages. Incubation of the harvested cells with (3H)PGE2 revealed specific binding of (3H)PGE2 by use of the Millipore filter assay system. Maximum binding was attained in the presence of 1 mM EDTA. Specific binding was saturable at 65 fmol/mg protein with an equilibrium dissociation constant (Kd) of 3.2 X 10(-8)M. Inhibition of (3H)PGE2 binding with unlabelled prostaglandins revealed a potency series of PGE2 greater than PGE2 greater than PGI2. The PGE2 concentration which displaced 50% of the labelled ligand was 10(-7)M. Comparable kinetic data were obtained for adenylate cyclase stimulation, since the concentration which showed a halfmaximal stimulation of cAMP production was 2 X 10(-7)M of PGE2. Since PGE1 and PGI2 compete with (3H)PGE2 binding in a non-parallel manner compared to PGE2 itself, it is proposed that macrophages possess different types of PG receptors.

Specific binding of the thromboxane A2 antagonist 13-azaprostanoic acid to human platelet membranes

In the present study we characterized the interaction between the thromboxane A2/prostaglandin H2 antagonist, trans-13-azaprostanoic acid (13-APA), and isolated human platelet membranes. In these studies, we developed a binding assay using trans [3H] 13-APA as the ligand. It was found that trans [3H] 13-APA specific binding was rapid, reversible, saturable and temperature dependent. Scatchard analysis of the binding data yielded a curvilinear plot which indicated the existence of two classes of binding sites: a high-affinity binding site with an estimated dissociation constant (Kd) of 100 nM; and a low-affinity binding site with an estimated Kd of 3.5 microM. At saturation, approximately 1 pmol/mg protein of [3H] 13-APA was bound to the high affinity site. In order to further characterize the nature of the [3H] 13-APA binding site, we evaluated competitive binding by cis 13-APA, cis 15-APA, prostaglandin F2 alpha, U46619, 6-ketoprostaglandin F1 alpha and thromboxane B2. It was found that the [3H] 13-APA binding site was stereospecific and structurally specific. Thus, the cis isomer of 13-APA exhibited substantially reduced affinity for binding. Furthermore, the prostaglandin derivatives, thromboxane B2 and 6-ketoprostaglandin F1 alpha, which do not possess biological activity, also did not compete for [3H] 13-APA binding. On the other hand, U46619 which acts as a thromboxane A2/prostaglandin H2 mimetic, and prostaglandin F2 alpha which acts as a thromboxane A2/prostaglandin H2 antagonist, both effectively competed for [3H] 13-APA binding. These findings indicate that trans 13-APA binds to a specific site on the platelet membrane which presumably represents the thromboxane A2/prostaglandin H2 receptor.

Technical considerations for platelet aggregation and related problems

Platelet aggregation generally is ordered by the physician to evaluate platelet function in hemorrhagic or thrombotic disorders. Malfunction of the platelet may be the result of an intrinsic congenital defect or an acquired problem induced by drugs or certain circulating plasma factors. It is necessary to obtain information from the patient with respect to family history, drug ingestion, physical or mental stress. In addition, other laboratory studies should be obtained to rule out general coagulation disorders affecting the plasma factors. A bleeding time will be helpful in establishing the severity of any platelet dysfunction. Technical considerations with regard to the preparation of the samples are of primary importance in determining platelet aggregation. Aggregating studies require the use of a variety of binding agents. (Studies on shape change, adhesion of platelets, release of platelet granule substance, and or lysis with extrusion of cytoplasmic constituents may be helpful in certain cases.) Instrumentation for platelet aggregation presently is available in many hospitals. The technical factors to be considered for routine aggregation studies include the type and strength of anticoagulant, centrifugation technique used in preparing the platelet-rich and platelet-poor plasma, platelet concentration, time of storage of the sample after venipuncture and after centrifugation, temperature, and the mixing of the sample. In general, critical concentrations of each reagent should be employed to improve the discrimination capability of the assay. Small differences in response may be obliterated by using excessive concentrations of a given reagent. Comparison in response to the test platelets with control platelets is best done at the same time by performing the aggregation in a dual instrument so that handling procedures will be identical and artifactual differences eliminated.

Interrelationships among prostaglandins, vasopressin and cAMP in renal papillary collecting tubule cells in culture

To determine the influence of prostaglandins on cAMP metabolism in renal papillary collecting tubule (RPCT) cells, intracellular cAMP levels were measured after incubating cells with prostaglandins (PGs) alone or in combination with arginine vasopressin (AVP). PGE1, PGE2 and PGI2, but not PGD2 or PGF2 alpha, increased intracellular cAMP concentrations. At maximal concentrations (10(-5) M) the effects of PGE2 plus PGI2 (or PGE1), but not of PGI2 plus PGE1, were additive suggesting that at least two different PG receptors may be present in RPCT cell populations. Bradykinin treatment of RPCT cells caused an accumulation of intracellular cAMP which was blocked by aspirin and was quantitatively similar to that observed with 10(-5) M PGE2. PGs, when tested at concentrations (e.g. 10(-9) M) which had no independent effect on intracellular cAMP levels, did not inhibit the AVP-induced accumulation of intracellular cAMP in RPCT cells. These results indicate that PGs do not block AVP-induced accumulation of intracellular cAMP in RPCT cells at concentrations of PGs which have been shown to inhibit the hydroosmotic effect of AVP on perfused collecting tubule segments. However, at higher concentrations of PGs (e.g. 10(-5) M), the effects of AVP plus PGE1, PGE2, PGI2 or bradykinin on intracellular cAMP levels were not additive. Thus, under certain conditions, there is an interaction between PGs and AVP at the level of cAMP metabolism in RPCT cells.

Defective platelet aggregation and increased platelet turnover in patients with myelofibrosis and other myeloproliferative diseases

In 9 patients with myeloproliferative diseases (MPD) (6 with myelofibrosis, MF, 1 with Ph1 positive chronic granulocytic leukaemia, CGL, 1 with primary eosinophilia, PE, 1 with pre-leukaemia syndrome, preL) collagen, epinephrine, and ADP-induced aggregation, N-ethylmaleimide-induced malondialdehyde (MDA) production, beta-thromboglobulin (beta-TG) plasma levels, and platelet turnover were studied. Collagen-induced aggregation was found to be normal in 7 patients, absent in 1, and reduced in 1. In all but 3 patients, aggregation with ADP was markedly reduced. Epinephrine-induced aggregation was decreased in 7 patients. No difference was found between mean MDA production in MPD (3.21 +/- 0.50 nmol/10(9) PLTs) and in control group of 21 normal subjects (3.04 +/- 0.26 nmol/10(9) PLTs). Mean beta-TG levels were significantly higher (P less than 0.01) in MPD patients (165.00 +/- 28.29 ng/ml) than in healthy controls (81.76 +/- 14.63 ng/ml). Mean platelet production half-time was significantly shorter in MPD (2.48 +/- 0.24 d) than in the control group (3.43 +/- 0.17 d), after adjustment for age by covariance analysis (P less than 0.005). Our data do not indicate an abnormal prostaglandin synthesis and are consistent with the hypothesis that a disseminated intravascular platelet aggregation might take place in MPD patients.

Platelet Fc receptor. Increased expression in myeloproliferative disease

The platelet Fc receptor, a membrane receptor for immune complexes or aggregated immunoglobulin (Ig)G, was compared in normal and myeloproliferative platelets. Washed platelets from 11 normal donors and 27 patients were incubated with fluorescein-conjugated ovalbumin-anti-ovalbumin complexes and examined by phase and fluorescence microscopy. Only 3.2+/-1% of the normal platelets stained, whereas 76+/-16% of the myeloproliferative platelets stained with the immune complex. The fluorescent staining was mediated by a platelet Fc receptor, as shown by the absence of platelet staining with immune complex containing antibody preincubated with Staphylococcal protein A to block the Fc region. In addition, no staining occurred with antigen or antibody alone or after preincubation of platelets with aggregated IgG. Platelets from normal or myeloproliferative donors did not stain with the immune complexes when the incubation was performed in plasma. The increased expression of Fc receptors on myeloproliferative platelets was corroborated by studies of [(14)C]serotonin release by immune complexes or aggregated IgG in 8 patients and 17 normal donors. Serotonin uptake was similar in both groups. Myeloproliferative platelets released significantly more serotonin than normal platelets at each concentration of immune complex or aggregated IgG; in addition, myeloproliferative platelets released serotonin in response to much smaller concentrations of immune complex or aggregated IgG. [(14)C]Serotonin release by myeloproliferative platelets was not increased above that of normal platelets when thrombin was used as the stimulus. The results were independent of patient age, sex, therapy, hematocrit, or platelet size. Interaction of circulating immune complexes with platelets bearing increased Fc receptors may contribute to the abnormal hemostasis associated with the myeloproliferative syndromes.