Action of rolipram on specific PDE4 cAMP phosphodiesterase isoforms and onthe phosphorylation of cAMP-response-element-binding protein (CREB) and p38 mitogen-activated protein (MAP) kinase in U937 monocytic cells

U937 monocytic cells are shown here to express a range of PDE4, cAMP-specif ic phosphodiesterase (PDE) isoenzymes: the long isoenzymes, PDE4A4, PDE4D5 and PDE4D3, plus the short isoenzyme, PDE4B2. These isoenzymes provide arou nd 76% of the total cAMP PDE activity of U937 cells. The specific activitie s of the total PDE4A, PDE4B and PDE4D activities were 0.63+/-0.09, 8.8+/-0. 2 and 34.4+/-2.9 pmol/min per mg of protein respectively. The PDE4 selectiv e inhibitor, rolipram, inhibited immunopurified PDE4B and PDE4D activities similarly, with IC50 values of approx. 130 nM and 240 nM respectively. In c ontrast, rolipram inhibited immunopurified PDE4A activity with a dramatical ly lower IC50 value of around 3 nM. Rolipram increased phosphorylation of c AMP-response-element-binding protein (CREB) in U937 cells in a dose-depende nt fashion, which implied the presence of both high affinity (IC50 value ap prox. 1 nM) and low affinity (IC50 value approx. 120 nM) components. Rolipr am dose-dependently inhibited the interferon-gamma (IFN-gamma)-stimulated p hosphorylation of p38 mitogen-activated protein (MAP) kinase in a simple mo notonic fashion with an IC50 value of approx. 290 nM. On this basis, it is suggested that rolipram inhibition of PDE4A4 is involved in regulating CREB phosphorylation but not IFN-gamma-stimulated p38 MAP kinase phosphorylatio n. PDE4A4 was also selectively activated by challenge of U937 cells with ei ther bacterial lipopolysaccharide (LPS) or IFN-gamma through a process whic h was attenuated by both wortmannin and rapamycin. It is proposed that the PDE4A4 isoform is involved in compartmentalized cAMP signalling responses i n U937 monocytes.