Abnormal inside-out signal transduction-dependent activation of glycoprotein IIb-IIIa in a patient with impaired pleckstrin phosphorylation

Platelet-agonist interaction results in activation of glycoprotein (GP) IIb-IIIa complex and fibrinogen binding, a prerequisite for platelet aggregation. Fibrinogen binding exposes new antibody binding sites on GPIIb-IIIa (ligand-induced binding sites; LIBS), Signal transduction events, including pleckstrin phosphorylation by protein kinase C (PKC), are considered to regulate GPIIb-IIIa activation. We studied a 16-year-old white male with lifelong mucocutaneous bleeding manifestations and abnormal platelet aggregation and secretion in response to multiple agonists. Pleckstrin phosphorylation was diminished in response to platelet-activating factor (PAF; 4 and 400 nmol/L) and thrombin (0.05 U/ mL). Binding of monoclonal antibodies (MoAbs) 10E5 and A(2)A(9), which bind to both resting and activated GPIIb-IIIa, was normal, Binding of MoAb PAC1, which binds to only activated GPIIb-IIIa, was diminished upon activation with PAF, adenosine diphosphate (ADP), thrombin receptor agonist peptide (SFLLRN), A23187, and 1,2-dioctonylglycerol (DiC(8)). Signal transduction-dependent LIBS expression (studied using MoAb 62) induced by ADP, SFLLRN, and DiC(8) and signal transduction-independent LIBS expression induced by RGDS peptide or disintegrin albolabrin were normal or only minimally decreased, indicating the presence of intact ligand binding sites, We conclude that the patient's platelets have a defect in inside-out signal transduction-dependent GPIIb-IIIa activation due to an upstream defect in the signal transduction mechanisms rather than in the GPIIb-IIIa complex itself, Our findings extend the spectrum of congenital mechanisms leading to impaired aggregation from defects in GPIIb-IIIa per se to aberrations in signaling mechanisms. (C) 1996 by The American Society of Hematology.