PDE4B expression and activity limit the growth-inhibitory effects of cAMP. (A) Growth inhibition following forskolin treatment. DHL6, DHL7, OCI-Ly3, and OCI-Ly10 cells were incubated with 40 μM forskolin for 24 hours. (B) Growth inhibition following treatment with PDE4 inhibitors. DHL6, DHL7, OCI-Ly3, and OCI-Ly10 cells were incubated with 40 μM forskolin for 24 hours in the presence or absence of rolipram (10 μM), PLX513 (10 μM [DHL6] or 20 μM [DHL7, OCI-Ly3, and OCI-Ly10]), or with these PDE4 inhibitors alone. A 20-μM dose of PLX513 was cytotoxic for DHL6 cells regardless of forskolin treatment. Data in panels A-B represent the ratio of proliferation of treated cells to cells cultured with vehicle alone for the same time periods. Cell proliferation was measured by MTS assay. (C) cAMP induction following forskolin treatment. DHL6, DHL7, OCI-Ly3, and OCI-Ly10 cells were incubated with 40 μM forskolin for 10 to 60 minutes and intracellular cAMP levels determined by ELISA. The cAMP induction in DHL7 and OCI-Ly10 cells was minimal (5 and 3 pmol/million cells, respectively), and it is not apparent in this display. (D) cAMP levels following inhibition of PDE4. DHL6, DHL7, OCI-Ly3, and OCI-Ly10 cells were incubated with 40 μM forskolin for 10 to 60 minutes following preincubation with PLX513 for 30 minutes. Results are displayed as the percentage of increase in cAMP levels in cells treated with forskolin alone versus those treated with forskolin and PLX513. All proliferation and cAMP data are the mean and SD of 3 independent experiments. All individual experiments were performed in triplicate.