Regulation of the proapoptotic BH3-only protein BIM by glucocorticoids, survival signals and proteasome in chronic lymphocytic leukemia cells

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Abstract
Glucocorticoids induce apoptosis in chronic lymphocytic leukemia (CLL) cells through a caspase-dependent mechanism. However, their mechanism of action remains unknown. We have studied the regulation of the proapoptotic BH3-only Bcl-2 interacting mediator of cell death (BIM) in CLL cells. We demonstrate that glucocorticoids upregulate BIM at protein and mRNA levels. We have investigated the ability of different survival signals, such as 12-O-tetradecanoylphorbol 13-acetate (TPA), stromal cell-derived factor-1α (SDF-1α), interleukin 4 (IL-4) and B-cell receptor (BCR) activation, to influence the levels of BIM and its induction by glucocorticoids. TPA downregulates BIMEL by extracellular signal-regulated kinase (ERK)-mediated BIM phosphorylation and further proteasome-mediated degradation. However, SDF-1α and BCR activation induce transient BIM phosphorylation, without protein degradation. Proteasome inhibitors do not modify the levels of BIM with respect to untreated cells. However, they induce apoptosis and inhibit TPA-induced BIMEL degradation, leading to its accumulation. In conclusion, the results implicate BIM in glucocorticoid-induced apoptosis in CLL cells. BIMEL phosphorylation through the ERK pathway targets the protein for proteasomal degradation.