Characterization of the Platelet Prostaglandin D₂ Receptor: LOSS OF PROSTAGLANDIN D₂ RECEPTORS IN PLATELETS OF PATIENTS WITH MYELOPROLIFERATIVE DISORDERS

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Abstract

Prostaglandin (PG) D2 is synthesized in platelets at concentrations which could inhibit aggregation via activation of adenylate cyclase. To more directly define platelet-PG interactions, a binding assay has been developed for platelet PG receptors with [3H]PGD2 as ligand. [3H]PGD2 binding to intact platelets was saturable and rapid with the ligand bound by 3 min at 20°C. PG competed with the [3H]PGD2 binding site with a potency series: PGD2 (IC50 = 0.08 μM) ≫ PGI2 (IC50 = 2 μM) > PGE1 (IC50 = 6 μM) > PGF2α (IC50 = 8 μM). Scatchard analysis of binding data from six normal subjects showed a single class of binding sites with a dissociation constant (Kd) of 53 nM and 210 binding sites per platelet. This PGD2 receptor assay was then used to study platelets from five patients with myeloproliferative disorders (polycythemia vera, essential thrombocythemia, and chronic myelogenous leukemia), as over 90% of these patients have platelets resistant to the effects of PGD2 on aggregation and adenylate cyclase activity (1978. Blood.52: 618-626.). In the presence of 50 nM [3H]PGD2, the patients' platelets bound 7.1±2.9 fmol ligand/108 platelets compared with 15.1±1 fmol/108 platelets in normals, a decrease of 53% (P < 0.01). Scatchard analysis showed that the Kd of [3H]PGD2 binding (33 nM) was comparable to normal platelets, which indicates that the decreased PGD2 binding in these platelets represented fewer receptors rather than altered affinity of the ligand for the binding site. The 53% decrease in [3H]PGD2 binding correlated with a 48% decrease in PGD2-activated platelet adenylate cyclase. The characterization of the platelet PGD2 binding site provides further direct evidence that there are at least two PG receptors on platelets, one for PGE1 and PGI2, and a separate receptor for PGD2. Direct binding analysis will be a useful tool for studying the role of PG in regulating platelet function, as demonstrated by the selective loss of PGD2 binding sites in patients with myeloproliferative disorders.

Authors

Barry Cooper, David Ahern