The roles Of αIIbβ3-mediated outside-in signal transduction, thromboxane A2, and adenosine diphosphate in collagen-induced platelet aggregation

Collagen-induced activation of platelets in suspension leads to alpha(IIb)beta(3)-mediated outside-in signaling, granule release, thromboxane A2 (TxA2) production, and aggregation. Although much is known about collagen-induced platelet signaling, the roles of TxA2 production, adenosine diphosphate (ADP) and dense-granule secretion, and alpha(IIb)beta(3)-mediated outside-in signaling in this process are unclear. Here, we demonstrate that TxA2 and ADP are required for collagen-induced platelet activation in response to a low, but not a high, level of collagen and that alpha(IIb)beta(3)-mediated outside-in signaling is required, at least in part, for this TxA2 production and ADP secretion. A high level of collagen can activate platelets deficient in PLCgamma2, Galphaq, or TxA2 receptors, as well as platelets treated with a protein kinase C inhibitor, Ro31-8220. Thus, activation of alpha(IIb)beta 3 in response to a high level of collagen does not require these signaling proteins. Furthermore, a high level of collagen can cause weak TxA2 and ADP-independent aggregation, but maximal aggregation induced by a high level of collagen requires TxA2 or secretion.