In medicine, glycoprotein IIb/IIIa (GPIIb/IIIa, also known as integrin αIIbβ3) is an integrin complex found on platelets. It is a receptor for fibrinogen and von Willebrand factor and aids in platelet activation. The complex is formed via calcium-dependent association of gpIIb and gpIIIa, a required step in normal platelet aggregation and endothelial adherence. Platelet activation by ADP (blocked by clopidogrel) leads to the aforementioned conformational change in platelet gpIIb/IIIa receptors that induces binding to fibrinogen. The gpIIb/IIIa receptor is a target of several drugs including abciximab, eptifibatide, tirofiban.
gpIIb/IIIa Complex Formation
Once platelets are activated, they exocytose granule contents, including both ADP and TXA2. These then bind their respective receptors on platelet surfaces, in both an autocrine and paracrine fashion (binds both itself and other platelets). The binding of these receptors result in a cascade of events resulting in an increase in intracellular calcium (e.g. via Gq receptor activation leading to Ca2+ release from platelet endoplasmic reticulum Ca2+ stores, who may activate PKC). Hence, this calcium increase triggers the calcium-dependent association of gpIIb and gpIIIa to form the activated membrane receptor complex gpIIb/IIIa, which is capable of binding fibrinogen (factor I), resulting in many platelets "sticking together" as they may connect to the same strands of fibrinogen, resulting in clot. The coagulation cascade then follows to stabilize the clot, as thrombin (factor IIa) converts the soluble fibrinogen into insoluble fibrin strands. These strands are then cross-linked by factor XIII to form a stabilized blood clot.
Defects in glycoprotein IIb/IIIa cause Glanzmann's thrombasthenia.
Autoantibodies against IIb/IIIa can be produced in immune thrombocytopenic purpura.
Glycoprotein IIb/IIIa inhibitors can be used to prevent blood clots in an effort to decrease the risk of heart attack or stroke.
- ^ a b Vickers JD (July 1999). "Binding of polymerizing fibrin to integrin alpha(IIb)beta(3) on chymotrypsin-treated rabbit platelets decreases phosphatidylinositol 4,5-bisphosphate and increases cytoskeletal actin". Platelets 10 (4): 228–37. PMID 16801097. doi:10.1080/09537109976077.
- ^ Calvete JJ (1995). "On the structure and function of platelet integrin alpha IIb beta 3, the fibrinogen receptor". Proc. Soc. Exp. Biol. Med. 208 (4): 346–60. PMID 7535429.
- ^ Shattil SJ (1999). "Signaling through platelet integrin alpha IIb beta 3: inside-out, outside-in, and sideways". Thromb. Haemost. 82 (2): 318–25. PMID 10605720.
- ^ Bellucci S, Caen J (2002). "Molecular basis of Glanzmann's Thrombasthenia and current strategies in treatment". Blood Rev. 16 (3): 193–202. PMID 12163005. doi:10.1016/S0268-960X(02)00030-9.
- ^ McMillan R (October 2007). "The pathogenesis of chronic immune thrombocytopenic purpura". Semin. Hematol. 44 (4 Suppl 5): S3–S11. PMID 18096470. doi:10.1053/j.seminhematol.2007.11.002.