E4BP4 facilitates glucocorticoid-evoked apoptosis of human leukemic CEM cells via upregulation of Bim

Abstract

BackgroundSynthetic GCs serve as therapeutic agents for some lymphoid leukemias because of their ability to induce transcriptional changes via the GC receptor (GR) and trigger apoptosis. Upregulation of the BH3-only member of Bcl-2 family proteins, Bim, has been shown to be essential for GC-evoked apoptosis of leukemic lymphoblasts. Using human T cell leukemic sister clones CEM-C7-14 and CEM-C1-15, we have previously shown that the bZIP transcriptional repressor, E4BP4, is preferentially upregulated by GCs in CEM-C7-14 cells that are susceptible to GC-evoked apoptosis, but not in refractory CEM-C1-15 cells. E4BP4 is an evolutionarily conserved member of the PAR family of bZIP transcription factors related to the C. elegans death specification gene ces2.ResultsMouse E4BP4 was ectopically expressed in CEM-C1-15 cells, resulting in sensitization to GC-evoked apoptosis in correlation with restoration of E4BP4 and Bim upregulation. shRNA mediated modest knockdown of E4BP4 in CEM-C7-14 cells resulted in concomitant reduction in Bim expression, although GC-evoked fold-induction and sensitivity to apoptosis was similar to parental cells.ConclusionData presented here suggest that GC-mediated upregulation of E4BP4 facilitates Bim upregulation and apoptosis of CEM cells. Since the Bim promoter does not contain any consensus GRE or EBPRE sequences, induction of Bim may be a secondary response.

Correlation of glucocorticoid-mediated E4BP4 upregulation with altered expression of pro- and anti-apoptotic genes in CEM human lymphoblastic leukemia cells

“…We have previously demonstrated that E4BP4-dependent upregulation of Bim contributes to GC-evoked apoptosis of human leukemic CEM cells, using a cell line derived from the GC-resistant CEM C1-15 cells that ectopically expresses mouse E4BP4 (CEM C1-15mE#3) [15]. Here, we have extended our studies to evaluate the role of E4BP4 in GC-dependent regulation of genes whose protein products are known to crosstalk with E4BP4, and genes implicated in apoptosis.…”

“…CEM C1-15 cells express functional GR, which has been shown to be transcriptionally active with a blunted response to GC stimulus, hence modest regulation of GC-dependent genes is to be expected (4). Bim is a well-known mediator of GC-evoked apoptosis of lymphoid leukemic cells [14], and we have shown that its expression correlates with E4BP4 expression [15] (Figure 2A). BIRC3 or IAP-1 (Inhibitor of Apoptosis-1) is known to bind to TRAFs ( T umor necrosis factor R eceptor A ssociated F actors) and inhibit apoptosis by interfering with activation of caspases.…”

“…The CEM C1-15m#3 subclone, stably transfected with mouse E4BP4, has been characterized previously and is sensitive to GC-evoked apoptosis [15]. Cells were maintained in log phase at 37°C in a 5% CO 2 incubator in RPMI 1640 containing L-Glutamine (Catalog #50-020-PB) from Cellgro (Manassas, VA,) supplemented with 5% heat-inactivated fetal bovine serum (FBS, Cat #S11150) from Atlanta Biologicals (Lawrenceville, GA).…”

“…BIM is upregulated in GC sensitive human leukemic CEM C7-14 cells in conjunction with GC-evoked apoptosis. We have demonstrated that ectopic E4BP4 expression restores sensitivity to GC-evoked apoptosis in a refractory CEM sister cell line, CEM C1-15, via restoration of BIM induction [15]. …”

“…To investigate the role of E4BP4 in GC-evoked apoptosis of human leukemic cells, we have previously created a cell line, CEM C1-15mE#3, that ectopically expresses mouse E4BP4 in the GC-resistant CEM C1-15 cells [15]. These cells are sensitized to GC-evoked apoptosis and BIM upregulation, analogous to the GC-sensitive sister cell line, CEM-C7-14.…”