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-ye ar-old white male with lifelong mucocutaneous bleeding manifestations and abnormal platelet aggregation and secretion in response to multipl e agonists. Pleckstrin phosphorylation was diminished in response to p latelet-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), whi ch 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 recep tor 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-indepen dent LIBS expression induced by RGDS peptide or disintegrin albolabrin were normal or only minimally decreased, indicating the presence of i ntact ligand binding sites, We conclude that the patient's platelets h ave a defect in inside-out signal transduction-dependent GPIIb-IIIa ac tivation due to an upstream defect in the signal transduction mechanis ms rather than in the GPIIb-IIIa complex itself, Our findings extend t he spectrum of congenital mechanisms leading to impaired aggregation f rom defects in GPIIb-IIIa per se to aberrations in signaling mechanism s. (C) 1996 by The American Society of Hematology.