SHPS-1 negatively regulates integrin αIIbβ3 function through CD47 without disturbing FAK phosphorylation

Authors


Yoshiaki Tomiyama, Department of Internal Medicine and Molecular Science, Graduate School of Medicine B5, Osaka University, 2–2 Yamadaoka, Suita Osaka 565–0871, Japan.
Tel.: + 81 6 6879 3732; Fax: + 81 6 6879 3739; e-mail: yoshi@hp-blood.med.osaka-u.ac.jp

Abstract

Summary.  CD47 (integrin-associated protein) serves as a receptor for thrombospondin-1 (TSP-1) and Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1), and the TSP-1/CD47 interaction has been believed to augment integrin-mediated platelet function. Here, employing SHPS-1-immunoglobulin (Ig) as a ligand, we have newly demonstrated that CD47 acts as an inhibitory receptor for platelet function. The binding of SHPS-1-Ig was solely mediated by CD47, because CD47-deficient platelets failed to bind murine SHPS-1-Ig. The human SHPS-1/CD47 interaction inhibited the platelet aggregation induced by several kinds of agonists at a low concentration. Moreover, human SHPS-1 expressed on the cell surface as well as soluble SHPS-1-Ig markedly inhibited the platelet spreading on, but not initial adhesion to, immobilized fibrinogen. Again, neither murine SHPS-1 expressed on the cell surface nor murine SHPS-1-Ig inhibited the spreading of CD47-deficient platelets. We further investigated the tyrosine phosphorylation of signaling proteins during platelet spreading on immobilized fibrinogen. Unexpectedly, SHPS-1 inhibited αIIbβ3-mediated platelet spreading without disturbing focal adhesion kinase (FAK) tyrosine phosphorylation. Further examination revealed that SHPS-1 inhibited the tyrosine phosphorylation of α-actinin, a downstream effector of FAK, but not of cortactin. Thus, it is likely that the SHPS-1/CD47 interaction inhibits αIIbβ3-mediated outside-in signaling by interfering with the downstream pathway of FAK. Taken together, our data suggest that SHPS-1 negatively regulates platelet function via CD47, especially αIIbβ3-mediated outside-in signaling.

Ancillary