Regulation of the pp72syk protein tyrosine kinase by platelet integrin alpha IIbbeta 3

Abstract

pp72syk is essential for development and function of several hematopoietic cells, and it becomes activated through tandem SH2 interaction with ITAM motifs in immune response receptors. Since Syk is also activated through integrins, which do not contain ITAMs, a CHO cell model system was used to study Syk activation by the platelet integrin, alpha IIb beta 3. As in platelets, Syk underwent tyrosine phosphorylation and activation during CHO cell adhesion to alpha IIb beta 3 ligands, including fibrinogen. This involved Syk autophosphorylation and the tyrosine kinase activity of Src, and it exhibited two novel features. Firstly, unlike alpha IIb beta 3-mediated activation of pp125FAK, Syk activation could be triggered by the binding of soluble fibrinogen and abolished by truncation of the alpha IIb or beta 3 cytoplasmic tail, and it was resistant to inhibition by cytochalasin D. Secondly, it did not require phosphorylated ITAMs since it was unaffected by disruption of an ITAM-interaction motif in the SH2(C) domain of Syk or by simultaneous overexpression of the tandem SH2 domains. These studies demonstrate that Syk is a proximal component in alpha IIb beta 3 signaling and is regulated as a consequence of intimate functional relationships with the alpha IIb beta 3 cytoplasmic tails and with Src or a closely related kinase. Furthermore, there are fundamental differences in the activation of Syk by alpha IIb beta 3 and immune response receptors, suggesting a unique role for integrins in Syk function.

Ligation of HLA-DR Molecules on B Cells Induces Enhanced Expression of IgM Heavy Chain Genes in Association with Syk Activation

“…In this respect, although DR-mediated signals activate Syk, cytoplasmic domains of DR␣ and DR␤ chains are devoid of ITAM motif. Likewise, the IL-2 receptor (30), CD29 (59), and platelet integrin ␣ IIb ␤ 3 (60) are also capable of activating Syk, although these receptors carry no ITAM motif in their cytoplasmic domains. Because Syk was not co-precipitated with DR molecules in our study (data not shown), certain molecules associated with DR may play a role in activating Syk, which are currently under investigation, using mass mapping techniques.…”

Src kinase activation by direct interaction with the integrin β cytoplasmic domain

“…Chinese hamster ovary (CHO) cell lines that stably express human wild-type or mutant ␣IIb␤3 have been described (11)(12)(13). The amount of ␤3 integrin expressed on the surface of these cell lines was equivalent, as assessed by flow cytometry by using a ␤3-specific antibody.…”

Aggretin, a Heterodimeric C-type Lectin from Calloselasma rhodostoma (Malayan Pit Viper), Stimulates Platelets by Binding to α2β1 Integrin and Glycoprotein Ib, Activating Syk and Phospholipase Cγ2, but Does Not Involve the Glycoprotein VI/Fc Receptor γ Chain Collagen Receptor

“…However, platelets can adhere to and be activated by a fibrinogen-coated surface (42), so there must be a recognition potential even in the "nonactivated" conformation. Clustering of GPIIb-IIIa leads to activation of p72 SYK without obvious Fc␥ involvement; however, the mechanism for this is not yet known (43). Aggretin clustering of ␣ 2 ␤ 1 and GPIb appears to induce a similar signaling pathway that may be common to several integrins.…”