Figure 3.

BCR signalling in untreated (a) and ES-62-exposed (b) B cells. Following ligation of the B-cell antigen receptor (BCR) of untreated B cells (a) the kinase, Lyn, tyrosine phosphorylates the immunoreceptor tyrosine activation motifs (ITAMs) on the accessory transducing molecules Ig-α and Ig-β resulting in the recruitment and activation of the PI-3-K and PLC-γ-signalling pathways. Whilst PI-3-K activation results in the activation of atypical PKC isoforms (aPKC), PLC-γ activation induces inositol trisphosphate (IP₃) and diacylglycerol (DAG) generation, ultimately resulting in activation of classical (cPKC) and novel (nPKC) PKC isoforms. Binding of the adaptor proteins Shc and BLNK to the phosphorylated ITAMs leads to the recruitment of the Grb2Sos complexes (Grb2 is an adaptor protein which binds Sos, a guanine nucleotide exchange factor) required for activation of the GTPase, Ras. Active Ras initiates the Erk MAP kinase cascade by binding and activating the ser/thr kinase, Raf leading to stimulation of the thr/tyr kinase MEK and consequent activation and nuclear translocation of the ser/thr kinase Erk. ES-62/PC signalling (b) disrupts BCR coupling to the PI-3-K cascade as well as targeting major negative regulatory sites in the control of the Erk, p38 and JNK MAP kinase cascades. First, ES-62 signalling promotes the BCR-activation of SHP-1 tyrosine phosphatase to prevent initiation of BCR signalling by maintaining the ITAMs in a resting, dephosphorylated state and hence prevents recruitment of the ShcGrb2Sos complexes required to activate the Ras- and Rac-MAP kinase cascades. Second, ES-62 signalling promotes the BCR-mediated recruitment of RasGAP to terminate ongoing Ras signals. In addition, ES-62 is also likely to target MAP kinase activation by down-regulating PKC isoform expression. Finally, ES-62-signalling promotes the BCR-driven association of the nuclear MAP kinase dual (thr/tyr) phosphatase, Pac-1 with Erk to terminate any ongoing Erk signals. This multipronged mechanism results in a rapid and profound desensitization of BCR coupling to the MAP kinase cascades.