Evidence That Glucocorticoid- and Cyclic AMP-Induced Apoptotic Pathways in Lymphocytes Share Distal Events

Abstract

WEHI7.2 murine lymphocytes undergo apoptotic death when exposed to glucocorticoids or elevated levels of intracellular cyclic AMP (cAMP), and these pathways are initiated by the glucocorticoid receptor (GR) and protein kinase A, respectively. We report the isolation and characterization of a novel WEHI7.2 variant cell line, WR256, which was selected in a single step for growth in the presence of dexamethasone and arose at a frequency of ~10^-10. The defect was not GR-related, as WR256 expressed functional GR and underwent GR-dependent events associated with apoptosis, such as hormone-dependent gene transcription and inhibition of cell proliferation. Moreover, the glucocorticoid-resistant phenotype was stable in culture and did not revert after treatment with 5-azacytidine or upon stable expression of GR cDNA. In addition, WR256 did not exhibit the diminished mitochondrial activity commonly associated with apoptosis. Interestingly, WR256 was also found to be resistant to 8-bromo-cAMP and forskolin despite having normal levels of protein kinase A activity and the ability to induce cAMP-dependent transcription. We examined the steady-state transcript levels of bcl-2, a gene whose protein product acts dominantly to inhibit thymocyte apoptosis, to determine whether elevated bcl-2 expression could account for the resistant phenotype. Our data showed that bcl-2 RNA levels were similar in the two cell lines and not altered by either dexamethasone or 8-bromo-cAMP treatment. These results suggest that WR256 exhibits a “deathless” phenotype and has a unique defect in a step of the apoptotic cascade that may be common to the glucocorticoid- and cAMP-mediated cell death pathways.