Collagen-like peptide stimulates tyrosine phosphorylation of syk and phospholipase C gamma 2 in platelets independent of the integrin alpha(2)beta(1)

Activation of platelets by collagen is mediated through a tyrosine kinase-dependent pathway that is associated with phosphorylation of the Pc receptor gamma chain, the tyrosine kinase syk, and phospholipase C gamma 2 (PLC gamma 2). We recently described a collagen-related triple-helical peptide (CRP) with the sequence GCP*(GPP*)GCP*G (single letter amino acid code: P* = hydroxyproline; Morton et al, Biochem J 306:337, 1995). The cross-linked peptide is a potent stimulus of platelet activation but, unlike collagen, does not support alpha(2) beta(1)-mediated, Mg2+-dependent adhesion, suggesting that its action is independent of the integrin alpha(2) beta(1). This finding suggests the existence of a platelet receptor other than alpha(2) beta(1) that underlies activation. In the present study, we show that CRP stimulates tyrosine phosphorylation of the same pattern of proteins in platelets as collagen, including syk and PLC gamma 2. Protein tyrosine phosphorylation induced by CRP is not altered in the absence of Mg2+ or the presence of monoclonal antibodies (MoAbs) to the integrin alpha(2) beta(1) (MoAb 6F1 and MoAb 13), conditions that prevent the interaction of collagen with the integrin. In contrast, phosphorylation of syk and PLC gamma 2 by collagen is partially reduced by MoAb 6F1 and MoAb 13 or by removal of Mg2+. This may reflect a direct role of alpha(2) beta(1) in collagen-induced signaling events or an indirect role in which the integrin facilitates the binding of collagen to its signaling receptor. The results show an alpha(2) beta(1)-independent pathway of platelet activation by CRP that involves phosphorylation of syk and PLC gamma 2. This pathway appears to contribute to platelet activation by collagen. (C) 1997 by The American Society of Hematology.