Hemorrhagic Thrombocytopathy with Platelet Thromboxane A2 Receptor Abnormality: Defective Signal Transduction with Normal Binding Activity

Abstract

SummarySubnormal platelet responses to thromboxane A2 (TXA2) were found in a patient with polycythemia vera, and the mechanism of this dysfunction was analyzed. The patient’s platelets showed defective aggregation and release reaction to arachidonic acid, enzymatically generated TXA2 and synthetic TXA2 mimetics (STA2, U-46619). In contrast, they showed normal responses to thrombin. When the platelet TXA2 receptor was examined with both a 125I-labelled derivative of a TXA2 receptor antagonist ([125I]-PTA-OH) and a 3H-labelled TXA2 agonist ([3H]U-46619), the equilibrium dissociation rate constants (Kd) and the maximal concentrations of binding sites (Bmax) of the patient’s platelets to both ligands were within normal ranges, suggesting that the binding capacity of their TXA2 receptor was normal. STA2 failed to induce normal elevation in the. cytoplasmic free calcium ion concentration, phosphatidic acid formation and 40 kD protein phosphorylation in the patient’s platelets, whereas these responses to thrombin were within normal ranges. 12-O-Tetradecanoyl-phorbol-13-acetate (TPA) also evoked normal response in the 40 kD protein phosphorylation in the patient’s platelets. These results suggested that the patient’s platelets had TXA2 receptor abnormalities which were characterized by defective transduction of the binding signal to postreceptor reactions after normal TXA2 binding.

Arg60 to Leu mutation of the human thromboxane A2 receptor in a dominantly inherited bleeding disorder.

“…We previously described a patient (T.T.) with a mild bleeding disorder characterized by defective platelet response to TXA2 (20,22). The patient's platelets showed defective aggregation response to STA2 (Fig.…”

“…Platelet-rich plasma (PRP) and plateletpoor plasma (PPP) were prepared from the family members and control subjects, as described previously (20). Platelet concentration of PRP was adjusted to 3 x 108 platelets/ml by autologous PPP.…”

“…Platelet concentration of PRP was adjusted to 3 x 108 platelets/ml by autologous PPP. Washed platelet suspensions were prepared as previously described (20). The concentration of the washed platelets was also adjusted to 3 X 10' platelets/ml.…”

“…The physiological importance of the TXA2-mediated signaling pathway has been demonstrated in a group of patients whose hemostatic defects were found to be associated with platelet unresponsiveness to TXA2 (12)(13)(14)(15)(16)(17)(18)(19). Recently, we have reported two unrelated patients with a mild bleeding disorder whose platelets showed impaired aggregation responses to TXA2 and its analogues, despite the normal response to thrombin (20,21). Although the patients' platelets exhibited normal binding activities to TXA2 analogues, they showed decreased GTPase activity and second messenger formation when stimulated by 9,1 1-epithio-I1,12-methano-TXA2 (STA2), a stable TXA2 agonist (20)(21)(22).…”

“…Recently, we have reported two unrelated patients with a mild bleeding disorder whose platelets showed impaired aggregation responses to TXA2 and its analogues, despite the normal response to thrombin (20,21). Although the patients' platelets exhibited normal binding activities to TXA2 analogues, they showed decreased GTPase activity and second messenger formation when stimulated by 9,1 1-epithio-I1,12-methano-TXA2 (STA2), a stable TXA2 agonist (20)(21)(22). These findings led us to speculate that the defect in the patients is due to impaired coupling of the TXA2 receptor to the G protein, which is caused by an abnormality of the TXA2 receptor itself.…”