Src Homology Region 2 Domain-Containing Phosphatase 1 Positively Regulates B Cell Receptor-Induced Apoptosis by Modulating Association of B Cell Linker Protein with Nck and Activation of c-Jun NH2-Terminal Kinase

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Abstract
Src homology region 2 domain-containing phosphatase 1 (SHP-1) is a key mediator in lymphocyte differentiation, proliferation, and activation. We previously showed that B cell linker protein (BLNK) is a physiological substrate of SHP-1 and that B cell receptor (BCR)-induced activation of c-Jun NH2-terminal kinase (JNK) is significantly enhanced in cells expressing a form of SHP-1 lacking phosphatase activity (SHP-1-C/S). In this study, we confirmed that SHP-1 also exerts negative regulatory effects on JNK activation in splenic B cells. To further clarify the role of SHP-1 in B cells, we examined how dephosphorylation of BLNK by SHP-1 affects downstream signaling events. When a BLNK mutant (BLNKΔN) lacking the NH2-terminal region, which contains four tyrosine residues, was introduced in SHP-1-C/S-expressing WEHI-231 cells, the enhanced JNK activation was inhibited. Among candidate proteins likely to regulate JNK activation through BLNK, Nck adaptor protein was found to associate with tyrosine-phosphorylated BLNK and this association was more pronounced in SHP-1-C/S-expressing cells. Furthermore, expression of dominant-negative forms of Nck inhibited BCR-induced JNK activation. Finally, BCR-induced apoptosis was suppressed in SHP-1-C/S-expressing cells and coexpression of Nck SH2 mutants or a dominant-negative form of SEK1 reversed this phenotype. Collectively, these results suggest that SHP-1 acts on BLNK, modulating its association with Nck, which in turn negatively regulates JNK activation but exerts a positive effect on apoptosis.