A Sequence within the Cytoplasmic Tail of GpIIb Independently Activates Platelet Aggregation and Thromboxane Synthesis*

Abstract

All integrin α subunits contain a highly conserved KXGFFKR motif in their cytoplasmic domains that plays a crucial role in the regulation of integrin affinity for their ligands. We show that a lipid-modified peptide corresponding to the cytoplasmic region, 989–995, of the platelet integrin subunit glycoprotein GpIIb (αIIb), palmitoyl-KVGFFKR (Ppep; 10 μm), but not a similarly modified scrambled peptide (palmitoyl-FKFVRGK), can specifically induce platelet activation and aggregation equivalent to that of strong agonists such as thrombin. Ppep-induced aggregation is also associated with indices of platelet activation including thromboxane A₂ (TXA₂) synthesis (EC₅₀ = 45 ± 5 μm), secretion of α-granules detected as enhanced surface expression of P-selectin (EC₅₀ = 52 ± 8 μm), and conformational changes in GpIIb/IIIa measured by the monoclonal antibody, PAC-1 (EC₅₀ = 3.7 ± 1 μm). The TXA₂ receptor antagonist, SQ29548, PGE₁, and the ADP scavenger, apyrase, differentially inhibit the aggregation response and TXA₂ synthesis in response to Ppep. Similarly, GpIIb/IIIa antagonists (RO-449883 and integrelin), which inhibit aggregation by greater than 90%, have little effect on peptide-induced TXA₂ synthesis, suggesting that this event is independent of fibrinogen binding to GpIIb/IIIa. Alanine-stepping of the Ppep sequence identifies GFFK(991–994) as the critical residues in all peptide-mediated events. We conclude that this peptide can imitate the cytoplasmic domain of GpIIb and initiate parallel but independent signaling pathways, one leading to ligand binding and platelet aggregation and the other to intracellular signaling events such as TXA₂ synthesis and secretion.