Chronic Lymphocytic Leukemia Cells Have Increased Expression of Cyclic Nucleotide Phosphodiesterase 7B, Which May Serve as Disease-Specific Therapeutic Target.

Non-specific inhibition of cyclic nucleotide phosphodiesterase (PDE) in chronic lymphocytic leukemia (CLL) cells leads to intracellular accumulation of cyclic nucleotides, which in turn can sensitize the CLL cells to spontaneous and/or drug-induced apoptosis. Recent studies have identified at least 11 isoforms of PDE, each of which catalyzes the hydrolysis of cAMP and/or cGMP and regulates the intracellular levels of cyclic nucleotides. The finding that various tissues differentially express selected PDE isoforms has prompted development of isoform-specific inhibitors that can selectively increase intracellular levels of cyclic nucleotides in target cells that over-express the inhibited PDE isoform. Accordingly, we examined for differential expression of PDE isoforms by CLL B cells by assessing the levels of mRNA encoding each of the 11 different PDE isoforms in CLL cells and normal blood lymphocytes using quantitative real-time RT-PCR. CLL cell samples (n = 24) and lymphocytes of healthy adults (n = 16) each expressed detectable levels of PDE isoforms 1A, 1B, 2A, 3A, 3B, 4A, 4B, 4C, 4D, 5A, 7A, 7B, 8A, 8B, and 9A. However, we discovered that CLL cells of each patient had significantly higher levels of PDE7B mRNA (2.8-fold to 368-fold) and significantly lower levels of PDE3B mRNA (5-fold to 138 fold) than did lymphocytes from healthy donors (n = 16). As such, the ratios of PDE7B/PDE3B in CLL cell samples were >3 (ranging from 3 to 1019), whereas normal lymphocytes had ratios of < 0.3 (ranging from 0.006 to 0.23). The mean PDE7B/PDE3B ratio for CLL cells (123.6 ± 45.0, S.D., n=24) was significantly higher than that for B-lymphocytes of normal donors (3.8 ± 1.1, n=10) (P<0.0001). Immunoblot analyses demonstrated that CLL cells uniformly expressed high levels of PDE7B and low levels of PDE3B relative to those of normal lymphocytes. Moreover, we found that PDE7B contributed predominantly to the total PDE activity in CLL cells but not in normal lymphocytes. We thus studied effect of a selective PDE7 inhibitor (BRL-50481) on CLL cells and normal lymphocytes in vitro and found that BRL-50481 dose-dependently promoted apoptosis of CLL cells, but not normal lymphocytes. Collectively these findings indicate that CLL B cells selectively over-express PDE7B and under-express PDE3B relative to normal lymphocytes or isolated blood B cells and suggest that selective inhibitors of PDE7B may be effective in the treatment of this disease.