The “BH3-only” proapoptotic BCL-2 family members initiate the intrinsic apoptotic pathway. A small interfering RNA knockdown of BIM confirms this BH3-only member is important for the cytokine-mediated homeostasis of hematopoietic cells. We show here that the phosphorylation status of BIM controls its proapoptotic activity. IL-3, a hematopoietic survival factor, induces extracellular signal-regulated kinase/mitogen-activated protein kinase-mediated phosphorylation of BIM on three serine sites (S55, S65, and S100). After IL-3 withdrawal, only nonphosphorylated BIM interacts with the multidomain proapoptotic effector BAX. Phosphorylation of BIM on exposure of cells to IL-3 dramatically reduces the BIM/BAX interaction. A nonphosphorylatable BIM molecule (S55A, S65A, and S100A) demonstrates enhanced interaction with BAX and enhanced proapoptotic activity. Thus, ERK/mitogen-activated protein kinase-dependent phosphorylation of BIM in response to survival factor regulates BIM/BAX interaction and the pro-death activity of BIM

In the absence of costimulating signals, B cell receptor (BCR) crosslinking on immature B cells triggers the apoptotic cell death program. In the WEHI-231 B cell lymphoma model, anti-IgM crosslinking triggers activation of caspase-7 independently of caspase-8, followed by apoptosis. Two main mechanisms for caspase-7 activation have been proposed: (i) caspase-8 recruitment to death receptors (Fas or tumour necrosis factor); and (ii) changes in mitochondrial membrane permeability and cytochrome c release, which activate caspase-9. Here we report that caspase-7 activation induced by BCR crosslinking is independent of caspase-8 and cytochrome c translocation from mitochondria to the cytosol, as well as of mitochondrial depolarization. In addition, in a cell-free system, the S-100 fraction of anti-IgM-treated WEHI-231 cells induces a caspase activation pattern different from that activated by cytochrome c and dATP. We demonstrate that calpain specifically triggers activation and processing of caspase-7 both in vitro and in vivo, and that both processes are inhibited by calpain inhibitors. Furthermore, calpain activation is associated with decreased expression levels of calpastatin, which is upregulated by CD40 ligation. These data confirm a role for calpain during BCR crosslinking, which may be critical for cell deletion by apoptosis during B cell development and activation.