Inhibition of platelet integrin alpha(IIb)beta(3) by peptides that interfere with protein kinases and the beta(3) tail

a-Thrombin stimulation of human platelets initiates inside-out signaling to integrin alpha(IIb)beta(3) (glycoprotein IIb/IIIa), resulting in the expos ure of ligand binding sites. In the present study, the regulation of alpha( IIb)beta(3) via protein kinases was investigated in platelets permeabilized with streptolysin O by introducing peptides that interfere with these enzy mes and with possible regulatory domains in the cytosolic tail of the beta( 3) subunit. Compared with intact platelets, the permeabilized platelets pre served >80% of the aggregation, secretion, and alpha(IIb)beta(3) ligand bin ding capacity. The peptide YIYGSFK, a substrate for Src kinases, inhibited alpha-thrombin-induced ligand binding to alpha(IIb)beta(3) but a reversed p eptide with Y-->F substitutions (KFSGFIF) had no effect. Ligand binding to alpha(IIb)beta(3) was also inhibited by the peptide RKRCLRRL, which binds i rreversibly to the catalytic domain of protein kinase C. Peptides correspon ding to parts of the protein C inhibitor and beta(2)-glycoprotein I were us ed as negative controls and failed to interfere with ligand binding. Possib le target domains for protein kinases are present in the cytoplasmic tail o f the beta(3) subunit. The LLITIHDR peptide, matching the membrane-proximal domain of beta(3) (residues 717 to 724), had no effect, but NNPLYKEA (resi dues 743 to 750), EATSTFTN (residues 749 to 756), and TNITYRGT (residues 75 5 to 762), which mimicked overlapping domains of the carboxy-terminal part of beta(3), reduced alpha-thrombin-induced ligand binding by 60+/-4%, 97+/- 1%, and 97+/-2% (n=3) at 500 mu mol/L peptide, respectively. These observat ions indicate that Src kinases and protein kinase C take part in inside-out signaling to integrin alpha(IIb)beta(3) and identify target domains in bet a(3) that contribute to the regulation of this integrin.