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 (c AMP) regulate GC actions in CEM clones. Treatment of both GC-resistant CEM-CI [resistant to 1 mu M dexamethasone (Dex)] and the sensitive si ster clone, CEM-C7 (similar to 65% cell death with 20 nhl Dex, similar to 99% death with 1 mu M Dex), with a less than or equal to 20 mu M c oncentration of the protein kinase A activator, forskolin, had no sign ificant effect on cell viability. Cotreatment with Dex and forskolin r esulted in a strong synergistic death response, with only similar to 1 0% CEM-C1 cells surviving treatment with 1 phl Dex and 20 mu M forskol in, This death was blocked by the GR antagonist RU 38486. However, the extent of apoptosis did not correlate with the amount of GR protein o r binding activity in either C7 or C1 cells. As reported previously, D ex-evoked cell death was associated with suppression of c-Myc in C7 ce lls. In CEM-C1 cells, Dex alone did not affect c-Myc; however, Dex plu s forskolin suppressed c-Myc levels, To evaluate mechanisms of Dex-for skolin synergism, fresh subclones of CEM-C7 (clone 14) and CEM-CI (clo ne 15) were isolated, to ensure purity of phenotype. In these, forskol in (with or without Dex) caused a similar increase in cAMP (similar to 300-fold) and phospho-cAMP-responsive element binding protein (simila r to 4-5-fold) levels, whereas total cAMP-responsive element binding p rotein expression was not affected. GR transcription function, as test ed from a GR-responsive 330-bp mouse mammary tumor virus promoter-luci ferase reporter construct, was induced 8- and 4-fold by 1 mu M Dex tre atment of CEM-C7 14 and CEM-C1-15 cells, respectively. Forskolin (10 m u M) significantly potentiated Dex response in CEM-C1-15 cells (13.5-f old) but had only a modest effect (1.5-fold) in CEM-C7-14 cells. These studies suggest that sensitization of CEM-CI cells by cross-talk betw een GR and protein kinase A pathways may occur via cooperative effects on GR-mediated gene transcription.