p38-MAP Kinase Activation Followed by BIM Induction Is Critical for Glucocorticoid-Induced Apoptosis in Lymphoblastic Leukemia Cells.

Glucocorticoids (GC) are common components in chemotherapeutic protocols for lymphoid malignancies. GC-induced apoptosis requires the BCL-2 family-regulated pathway, but it remains largely undefined how the signaling pathways and their downstream target molecules, including the BCL-2 family members, participate in GC-induced cell death. BIM (BCL-2-interacting mediator of cell death) identified as a BH3-only protein induces apoptosis and its activity can be modulated at the transcriptional and post-translational levels. BIM is required for apoptosis induced by Taxol, dexamethasone, Gleevec, and HDAC inhibitors, indicating that this protein has a role in anticancer therapy against human malignancies. However, the molecular mechanisms how BIM is regulated and how BIM regulates downstream cell death machinery are not entirely clear.

In this study, we used a T cell acute lymphoblastic leukemia (T-ALL) cell line, CCRF-CEM to examine the molecular mechanisms of Dexamethasone (Dex)-induced cell death. Dex treatment induces the mRNA and protein of BIM in CCRF-CEM cells before the onset of apoptosis. Bim mRNA induction is completely inhibited by a transcription inhibitor, actinomycin D. However, a protein synthesis inhibitor, cycloheximide treatment augments Dex-induced Bim mRNA accumulation (super-induction), suggesting that de novo protein synthesis is not required for the induction of Bim mRNA. BIM protein induction is inhibited by actinomycin D or cycloheximide, thus new BIM protein synthesis is required for its accumulation. These data indicate that Dex treatment increases BIM levels mainly by new mRNA synthesis (transcription). In contrast, the overall expression of the multi-domain pro-apoptotic effectors, BAX and BAK, are constant. The anti-apoptotic members, BCL-2, MCL-1 and BCL-XL, also show constant expression. Down-regulation of BIM by shRNA strongly inhibits Dex-induced apoptosis. Thus, BIM is an essential molecule in Dex-induced apoptosis in CCRF-CEM cells. We then investigated the upstream signaling pathways to induce BIM. p38-mitogen activated protein kinase (p38-MAPK) is activated by Dex treatment prior to BIM induction. Treatment with a p38-MAPK inhibitor, SB203580, dramatically reduces Dex-induced apoptosis as well as BIM induction.

These findings indicate that BIM induction through p38-MAPK activation is a critical pathway in GC-induced cell death.