Constitutive Expression of Ectopic C-Myc Delays Glucocorticoid-evoked Apoptosis of Human Leukemic CEM-C7 Cells

Sensitivity to glucocorticoid (GC)-evoked apoptosis in lymphoid cell lines correlates closely with GC-mediated suppression of c-Myc expression. To establish a functional role for c-Myc in GC-mediated apoptosis, we have stably expressed MycER(TM), the human c-Myc protein fused to the modified ligand-binding domain of the murine estrogen receptor alpha, in GC-sensitive CEM-C7-14 cells. In CEM-C7-14 cells, MycER(TM) constitutively imparts c-Myc functions. Cells expressing MycER(TM) (C7-MycER(TM)) exhibited a marked reduction in cell death after 72 h in 100 nM dexamethasone (Dex), with 10-20-fold more viable cells when compared to the parental CEM-C7-14 clone. General GC responsiveness was not compromised, as evidenced by Dex-mediated suppression of endogenous c-Myc and cyclin D3, and induction of c-Jun and the glucocorticoid receptor. MycER(TM) also blunted Dex-mediated upregulation of p27(kipI) and suppression of the Myc target p53. In comparison to parental CEM-C7-14 cells, Dex-evoked DNA strand breaks were negligible and caspase activation was delayed, but the extent of G1 cell cycle arrest was similar in C7-MycER(TM) cells. Myc-ER(TM) did not result in permanent, complete resistance to GC however, and the GC-treated cells eventually died, indicative of redundant or interactive mechanisms in the GC-evoked lytic response of lymphoid cells. Our results emphasize the importance of c-Myc suppression in GC-evoked apoptosis of CEM-C7-14 cells.