EXPRESSION/REGULATION OF PHOSPHODIESTERASE 3 ISOFORMS

通过催化水解cAMP和cGMP，环

By catalyzing hydrolysis of cAMP and cGMP, cyclic nucleotide phosphodiesterases (PDEs) are critical regulators of intracellular concentrations of, and biological responses mediated by, cyclic nucleotides, including immune/inflammatory responses. Understanding cellular regulation of PDE isoforms [which belong to nine gene families (PDE1-9)] will be of increasing importance for targeting specific PDEs in treating pulmonary disorders. Although individual cells usually contain representatives of several PDE gene families, little is known of signalling pathways involved in cytokine and growth factor regulation of different PDEs in a single cell. In rat adipocytes insulin phosphorylates (serine 302) and activates PDE3B via phosphatidylinositol 3-kinase (PI3-K) and perhaps, PKB-dependent signals. In cytosolic extracts from adipocytes incubated with insulin, activated PKB co-purified with the kinase that phosphorylated PDE3B. In FDCP2 hematopoietic cells, IL-4, IGF-1 and insulin activate PDE3B and PDE4, whereas IL-3, GMCSF and PMA (phorbol ester) only activate PDE4. IL-4, IGF-1 and IL-3 activate PDE3B and 4 by PI3-K-dependent signals; PMA activates PDE4 via PKC. Downstream of PI3-K and PKC, IL-4, IGF-1, IL3 and PMA activate PDE4 by MAP kinase signals; IL-4 and IGF-1 activate PDE3B by MAP kinase-independent signals. To elucidate the role of PKB in activation of PDE3B in intact cells, FDCP2 cells were transfected with wild-type (F/B), constituitively active (F/B*) and kinase-inactive (F/B-)PKB and control vector (F/V). In F/V cells, IGF-1 increased PKB, PDE3B, and PDE4 activities approx. 2-fold. In F/B cells, however, IGF-1, in a wortmannin-sensitive manner, increased PKB activity approx. 10-fold and PDE3B phosphorylation and activity (approx. 4-fold), but increased PDE4 to the same extent as in F/V cells. In F/B* cells, in the absence of IGF-1, PKB activity was markedly increased (approx. 10-fold) and PDE3B was phosphorylated and activated (3-4 fold); these effects were inhibited by wortmannin. In F/B* cells, IGF-1 had little or no further effect on PKB and activation/phosphorylation of PDE3B. Rapamycin (p70S6 kinase inhibitor) and PD98059 [Mitogen-activated kinase kinase-1 (MEK-1) inhibitor] did not affect PKB or PDE3B activities in F/B* cells. Thymidine incorporation was greater in F/B* cells than in F/V cells and was inhibited to a much greater extent by PDE3 inhibitors than by rolipram, a PDE4 inhibitor. These results suggest that PDE3B, not PDE4, is a downstream target, if not substrate, for PKB and that activated PDE3B may regulate cAMP pools that modulate, at least in part, effects of PKB on proliferation/survival of FDCP2 hematopoietic cells. Although PDE4 is thought to be the predominant PDE isoform in most inflammatory/immune cells, PDE3 and PDE4, which exhibit a high affinity for cAMP, are both present in lymphocytes and macrophages. RT-PCR amplification of lymphocyte and macrophage RNA demonstrated the presence of PDE3B, not PDE3A, mRNA. After 5-7 days in culture elutriated peripheral human monocytes adhere to plastic surfaces and increase dramatically in size. The morphologic change is accompanied by a relative increase in PDE3B mRNA and activity, Glut 5 transporter and PDE1C mRNAs and a relative decrease in PDE4 activity and PDE4 mRNA. Incubation of elutriated monocytes with phorbol ester for four hours was associated with their strong adherence and an increase in PDE3B activity and mRNA (RT-PCR) relative to that of PDE4, without a marked change in mononuclear cell morphology. It will be important to define specific PDE isoforms present in alveolar macrophages and to determine if cellular inflammatory responses can be altered by PDE inhibitors.