Resistance of human leukemic CEM-C1 cells is overcome by synergism between glucocorticoid and protein kinase A pathways: correlation with c-Myc suppression.

Glucocorticoids (GCs) induce apoptosis in lymphoid cells that contain functional GC receptors (GRs). However, GC resistance often is seen in cells with demonstrable GRs; one such line is CEM-C1. We have tested the hypothesis that positive interactions between GC and cyclic AMP (cAMP) regulate GC actions in CEM clones. Treatment of both GC-resistant CEM-C1 [resistant to 1 microM dexamethasone (Dex)] and the sensitive sister clone, CEM-C7 (approximately 65% cell death with 20 nM Dex, approximately 99% death with 1 microM Dex), with a < or = 20 microM concentration of the protein kinase A activator, forskolin, had no significant effect on cell viability. Cotreatment with Dex and forskolin resulted in a strong synergistic death response, with only approximately 10% CEM-C1 cells surviving treatment with 1 microM Dex and 20 microM forskolin. This death was blocked by the GR antagonist RU 38486. However, the extent of apoptosis did not correlate with the amount of GR protein or binding activity in either C7 or C1 cells. As reported previously, Dex-evoked cell death was associated with suppression of c-Myc in C7 cells. In CEM-C1 cells, Dex alone did not affect c-Myc; however, Dex plus forskolin suppressed c-Myc levels. To evaluate mechanisms of Dex-forskolin synergism, fresh subclones of CEM-C7 (clone 14) and CEM-C1 (clone 15) were isolated, to ensure purity of phenotype. In these, forskolin (with or without Dex) caused a similar increase in cAMP (approximately 300-fold) and phospho-cAMP-responsive element binding protein (approximately 4-5-fold) levels, whereas total cAMP-responsive element binding protein expression was not affected. GR transcription function, as tested from a GR-responsive 330-bp mouse mammary tumor virus promoter-luciferase reporter construct, was induced 8- and 4-fold by 1 microM Dex treatment of CEM-C7-14 and CEM-C1-15 cells, respectively. Forskolin (10 microM) significantly potentiated Dex response in CEM-C1-15 cells (13.5-fold) but had only a modest effect (1.5-fold) in CEM-C7-14 cells. These studies suggest that sensitization of CEM-C1 cells by cross-talk between GR and protein kinase A pathways may occur via cooperative effects on GR-mediated gene transcription.