PDE4 Inhibitors Augment Glucocorticoid Receptor Levels in B-CLL Cells but Not in T Cells, B Cells, Monocytes or Neutrophils.

cAMP analogues have long been known to induce apoptosis in specific lymphoid subsets and to augment the apoptotic effect of glucocorticoids. However, a clinically feasible means by which to take advantage of these effects for the treatment of lymphoid malignancies has not previously been identified. Inhibition of type 4 cAMP phosphodiesterases (PDE4) activates cAMP-mediated signaling in many cell types and several PDE4 inhibitors are currently in clinical trials for inflammatory illnesses such as asthma, COPD and psoriasis. We have previously reported that PDE4 inhibitors induce apoptosis in B-CLL and augment the apoptotic effects of glucocorticoids such as dexamethasone. However, the mechanism by which PDE4 inhibitors increase glucocorticoid-induced apoptosis has remained unclear. In gene chip studies, we find that the PDE4 inhibitor rolipram (10 uM) augments transcript levels of glucocorticoid receptor alpha (GRa). A rolipram-induced dose and time-dependent increase in GRa levels was confirmed both by real-time PCR and by Western analysis. In leukemic cells derived from eight B-CLL patients, GRa transcript levels rose over the first four hours to a mean of 4.8 +/− 0.2-fold above baseline and maintained such a fourfold increase for at least 24 hours. An analysis of GRa transcript half-life following actinomycin D treatment demonstrates that rolipram augments transcript levels in B-CLL cells by a transcriptional mechanism. Consistent with this, rolipram treatment augments GRa transcripts derived from promoters 1A and 1C to a greater degree than 1B. Remarkably, rolipram does not augment GRa transcript levels in normal T cells, B cells, monocytes or neutrophils, even in the presence of the adenylate cyclase activator forskolin. Similarly, no increase in GRa transcript levels was observed in leukemic T-CLL cells. As previously reported in B cell lines, treatment of B-CLL cells with dexamethasone reduced GRa transcript levels in a dose-dependent manner. Co-treatment with PDE4 inhibitors maintained GRa transcript levels above basal levels even at the highest dexamethasone concentration tested (1 uM). Finally, we have confirmed the ability of PDE4 inhibitors to augment dexamethasone-induced apoptosis using several PDE4 inhibitors currently in clinical trials for other indications. Our results suggest that co-treatment with PDE4 inhibitors and high-dose dexamethasone may increase leukemic cell GRa levels and augment the apoptotic efficacy of glucocorticoid therapy in B-CLL patients.