Molecular mechanisms for the regulation of BCR-mediated signal transduction by SHP-1 and adaptor proteins.

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Based on the previous findings showing that BLNK/SLP-65 is a physiological substrate of SHP-1, a cytosolic protein tyrosine phosphatase, in B cells, we focused on the molecular mechanisms how B cell receptor (BCR)-mediated signaling is regulated by the interaction between SHP-1 and adaptor proteins. The results ate as follows.(1) BCR-induced activation of JNK is significantly enhanced and apoptosis is suppressed in WEHI-231 cells expressing a form of SHP- 1 lacking phosphatase activity (SHP-1-C/S).(2) Among candidate proteins likely to regulate JNK activation through BLNK such as Vav, Nck, TRAF2 and HPK1, Nck adaptor protein was found to associate with tyrosine-phosphorylated BLNK and this association was more pronounced in SHP-1-C/S-expressing cells. Two mutant forms of Nck possessing mutation in SH2 domain failed to bind phosphorylated BLNK, suggesting that this association is mediated via SH2 domain of Nck. Furthermore, expression of SH2 mutants of Nck inhibited the augumentation of BCR-induced JNK activation.(3) Coexpression of Nck SH2 mutants or a dominant negative form of MKK4 with SHP-1-C/S reversed suppression of BCR-induced apoptosis, indicating that JNK activity negatively regulates apoptotic pathway.(4) SLP-76 is an adaptor protein, structurally related to BLNK, has been reported to be expressed preferentially in T cells, NK cells and macrophages so that little is known as for the physiological role in B cells. We found that SLP-76 is expressed in all murine B cell lines tested and splenic B cells and that SHP-1 dephosphorylates SLP-76 in WEHI-231 and murine mature B cell line, BAL-76.