PDE4B controls the cAMP-mediated SYK inhibition in DLBCL. (A,B) Stable expression of PDE4B-WT but not a PI mutant blocked cAMP-mediated inhibition of SYK phosphorylation in the DLBCL cell lines DHL6 and DHL10. Immunoblotting with total anti-SYK antibody confirmed equal loading, and anti-FLAG demonstrate the ectopic expression of PDE4B in these cell lines. (C) Quantitative real-time RT-PCR shows an approximate 65% decrease in PDE4B expression in the OCI-Ly3 cell line, 72 hours after 200 nM siRNA oligonucleotide transfection. Results are displayed as expression relative to siRNA-control transfected cells, determined using the ΔΔcycle threshold (CT) method and reported as 2^−ΔΔCT. (D) siRNA-based knockdown of PDE4B expression with 2 independent sequences in the DLCBL cell line OCI-Ly3 led to a marked cAMP-mediated decrease in SYK Tyr525/526 phosphorylation. Immunoblotting with total SYK confirms equal loading. In these experiments, induction of cAMP was performed by incubating the cell lines with 40 μM Forskolin for 1 hour.