αIIbβ3-mediated outside-in signaling induced by the agonist peptide LSARLAF utilizes ADP and thromboxane A2 receptors to cause α-granule secretion by platelets

Summary.  The peptide LSARLAF (LSA) causes αIIbβ3-dependent platelet activation that results in α-granule secretion and aggregation. LSARLAF-induced, αIIbβ3-mediated outside-in signaling causing α-granule secretion and platelet aggregation was studied using washed mouse platelets. ADP receptor antagonists, enzyme inhibitors, normal platelets and platelets from mice that lack either Gαq or thromboxane (Tx) A2 receptors were used for this investigation. The results demonstrate that LSA-induced αIIbβ3-mediated signaling producing aggregation of washed platelets is mediated through the release of ADP and thromboxane, which cause α-granule release by mediating their effects though Gαq and/or Gi depending on the level of LSA used to activate the platelets. Specifically, αIIbβ3 elicited aggregation of washed platelets in response to a low level of LSA requires signaling through the ADP receptor P2Y1 and Gαq, and the ADP receptor P2Y12 and Gi as well as TxA2 receptors. However, this aggregation is independent of Gαq and TxA2 signaling in response to high LSA concentrations, but is dependent on ADP signaling through its receptor P2Y12, and therefore presumably Gi, regardless of the level of LSA used to activate the platelets. PKC function is required for ADP secretion and the subsequent signaling through P2Y12 regardless of the level of LSA used to activate the platelets. The end point of the LSA-induced αIIbβ3-mediated signaling characterized in this study is α-granule secretion, which provides the fibrinogen required for aggregation of washed platelets.