Glucocorticoid-mediated co-regulation of RCAN1-1, E4BP4 and BIM in human leukemia cells susceptible to apoptosis.

Glucocorticoids (GCs) are known to induce apoptosis of leukemia cells via gene regulatory changes affecting key pro-and anti-apoptotic genes. Three genes previously implicated in GC-evoked apoptosis in the CEM human T-cell leukemia model, RCAN1, E4BP4 and BIM, were studied in a panel of human lymphoid and myeloid leukemia cell lines. Of the two RCAN1 transcripts, the synthetic GC Dexamethasone (Dex) selectively upregulates RCAN1-1, but not RCAN1-4, in GC-susceptible Sup-B15, RS4;11, Kasumi-1 cells but not in GC-resistant Sup T1 and Loucy cells. E4BP4 and BIM regulation correlated with that of RCAN1-1. A putative GRE and four EBPREs were identified within 1500bp upstream from the transcription start site of RCAN1-1. GC-refractory CEM C1-15 cells sensitized to GC-evoked apoptosis by ectopic E4BP4 expression, CEM C1-15mE#3, showed restored RCAN1-1 upregulation, suggesting that RCAN1-1 is a downstream target of E4BP4. A model for coordinated regulation of RCAN1-1, E4BP4 and BIM, and their role in GC-evoked apoptosis is proposed.