The adaptor protein HSH2 attenuates apoptosis in response to ligation of the B cell antigen receptor complex on the B lymphoma cell line, WEHI-231

Signals transduced by the B cell antigen receptor (BCR) play a central role in regulating the functional response of the cell to antigen. Depending on the nature of the antigenic signal and the developmental or differentiation state of the B cell, antigen receptor signaling can promote either apoptosis or survival and activation. Understanding the molecular mechanisms underlying BCR-mediated apoptosis constitutes an important area of research because aberrations in programmed cell death can result in the development of autoimmunity or cancer. Expression of the adaptor protein hematopoietic Src homology 2 (HSH2) was found to significantly decrease BCR-mediated apoptosis in the murine WEHI-231 cell line. Analysis of signal transduction pathways activated in response to BCR ligation revealed that HSH2 does not significantly alter total protein tyrosine phosphorylation or Ca2+ mobilization. HSH2 does not potentiate the activation-dependent phosphorylation of AKT either. With respect to MAPK activation, HSH2 was not observed to alter the activation of ERK or p38 in response to BCR ligation, but it does significantly potentiate JNK activation. Analysis of processes directly associated with apoptosis revealed that HSH2 inhibits mitochondrial depolarization to a significant degree, whereas it has only a slight effect on caspase activation and poly ADP-ribose polymerase cleavage. BCR-induced apoptosis of WEHI-231 cells is associated with the loss of endogenous HSH2 expression within 12 h, whereas inhibition of apoptosis in response to CD40-mediated signaling leads to stabilization of HSH2 expression. Thus, endogenous HSH2 expression correlates directly with survival of WEHI-231 cells, which supports the hypothesis that HSH2 modulates the apoptotic response through its ability to directly or indirectly promote mitochondrial stability.