T-cell receptor–induced phosphorylation of the ζ chain is efficiently promoted by ZAP-70 but not Syk

Abstract

Engagement of the T-cell receptor (TCR)results IntroductionT-cell immune responses are coupled to activation of the T-cell receptor (TCR). Stimulation of this receptor results in the activation of signal transduction pathways, culminating in expression of cytokines and cellular proliferation. 1 Current models of antigen/ major histocompatibility complex (MHC)-induced T-cell activation are presented as ordered events with a sequential interaction of Src and ZAP-70/Syk protein tyrosine kinases (PTKs) with the TCR/CD3/ complex. Specifically, TCR engagement activates the Src family PTKs Lck/Fyn, which phosphorylate the tyrosines present in the immunoreceptor tyrosine activation motif (ITAM) [2][3][4] conserved in the CD3 and subunits of the TCR complex. 5,6 The ZAP-70/Syk PTKs then bind to the phosphorylated ITAMs via their respective SH2 domains, allowing their activation. 2,7,8 Once activated, ZAP-70/Syk kinases phosphorylate downstream, signaling intermediates such as Vav, Lat, and SLP-76 that are required for appropriate recruitment of downstream signaling cascades.ZAP-70 and Syk are structurally homologous; both proteins are composed of 2 tandemly arranged Src homology 2 (SH2) domains and a carboxy-terminal kinase domain. 7,9 Overall, these 2 kinases share more than 50% sequence identity with conserved tyrosine and serine phosphorylation sites. Although these 2 PTKs have some overlapping functions, 1 it has been hypothesized that in vivo, they are nonredundant because of their distinct expression profiles. ZAP-70 was initially reported to be expressed exclusively in thymocytes, T cells, and natural killer (NK) cells, whereas Syk was described as being expressed in a wide variety of hematopoietic cells but in only low levels in peripheral T cells. 10 Nevertheless, it has since been shown that Syk is expressed at high levels in human CD4 ϩ effector T cells and a subpopulation of TCR-stimulated ␣␤ T cells. 1,11 Moreover, the expression of Syk is not limited to hematopoietic cells, as it has been detected in vascular endothelial cells, 12 in epithelial cells, and in breast tissue. 13 With regard to ZAP-70, this PTK now has been shown to be expressed throughout normal B-cell development, at least in mice. 14 In humans, ZAP-70 has recently been detected in a subset of chronic lymphocytic leukemia (CLL) B cells, and importantly, its expression has been correlated with a poor prognosis. [15][16][17] The increased phosphorylation of Syk in ZAP-70-expressing CLL cells was the first indication that ZAP-70 may modulate the activity of Syk 18,19 rather than vice versa. Indeed, until these recent data were reported, Syk was assumed to be "superior" to ZAP-70, as its kinase activity is 100-fold higher than that of 20 it can be activated in an Lck-independent fashion, 21-25 and TCR-induced calcium flux is higher in the presence of Syk than 27 It has previously been shown that both Syk and ZAP-70 bind to Lck [28][29][30] and that in vitro, the 2 kinases bind to chain ITAMs with similar affinities. 31-33 Indeed, it has been hypo...

A New Approach for Quantitative Phosphoproteomic Dissection of Signaling Pathways Applied to T Cell Receptor Activation

“…5). These seemingly upstream perturbations in phosphorylation are, nevertheless, consistent with previous reports for the P116 Jurkat clone (39,40). Although Lck-mediated phosphorylation of CD3 ITAMs is widely believed to precede Zap-70 recruitment and activation (41), previous studies have revealed a synergistic role of Zap-70-mediated recruitment as well as stabilization of the interaction between Lck and CD3 at the ITAM regions independent of Zap-70 kinase activity (Fig.…”

ZAP-70 enhances IgM signaling independent of its kinase activity in chronic lymphocytic leukemia

“…In this regard, it is noteworthy that ZAP-70 is more effective than p72 Syk in promoting phosphorylation of TCR-z by src kinases following TCR ligation. 28 The association of ZAP-70 with Ig receptor signaling has been challenged in studies by Deglesne and colleagues, who used immobilized anti-to stimulate CLL cells in vitro and then monitored for CLL cell survival 72 hours later. They segregated patients into 2 groups, one of which had enhanced CLL cell survival with anti-(group A), and the other, which had CLL cell survival that apparently was unaffected by treatment with anti-(group B).…”

“…Indeed, prior studies found that ZAP-70 could promote phosphorylation of TCR-z ITAMs in thymocytes independent of its kinase activity, serving instead as an adapter protein that could facilitate recruitment of lck to the TCR complex. 27,28 We investigated whether the kinase activity of ZAP-70 or its ability to interact with c-Cbl was required for it to enhance Ig signaling in CLL cells. For this, we transfected ZAP-70 Ϫ CLL cells with adenovirus vectors encoding ZAP-70 or well-defined mutant forms of ZAP-70 (eg, ZAP-70-KA 369 or ZAP-70-YF 292 ).…”

Redundant role for Zap70 in B cell development and activation

“…While this appears to be true for Zap70, Syk can be activated by TCR signaling even in the absence of Src family kinase activation, probably because, unlike Zap70, it can phosphorylate the ITAM directly [35][36][37]. Despite this, Zap70 has been reported to be much better than Syk at promoting phosphorylation of the TCRf chain by recruiting Lck to the signaling complex [38]. Finally, detailed examination of the roles of Syk and Zap70 in thymic development has concluded that they play distinct roles [39].…”