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

Glucocorticoids (GC) are common components in chemotherapeutic protocols for lymphoid malignancies. GC-induced apoptosis requires the intrinsic, BCL-2 family-regulated pathway. Treatment of CCRF-CEM (T cell acute lymphoblastic leukemia) cells with the GC, dexamethasone (Dex), activates p38-mitogen activated protein kinase (p38-MAPK) and then induces mRNA transcription and synthesis levels of BIM, a BH3-only pro-apoptotic BCL-2 family member. Dex-induced apoptosis is dramatically inhibited by downregulation of BIM by shRNA or by pretreatment with a p38-MAPK inhibitor, SB203580, which also reduces BIM induction. These findings indicate that BIM induction through p38-MAPK activation is a critical pathway in GC-induced cell death.