SELECTIVE ACTIVATION OF ROLIPRAM-SENSITIVE, CAMP-SPECIFIC PHOSPHODIESTERASE ISOFORMS BY PHOSPHATIDIC-ACID

In rat thymic lymphocytes, accumulation of phosphatidic acid (PA) occu rs at the same time as decrease in cAMP levels and activation of a cAM P-specific phosphodiesterase (PDE) [type 4, EC 3.1.4.17 (PDE4)]. We in vestigated the nature of the PDE activated by PA and the mechanism of activation by using recombinant cAMP-specific PDE4 isoforms derived fr om three different genes (PDE4A, PDE4B, and PDE4D). The ''long'' varia nts expressed from each gene (PDE4A5, PDE4B1, and PDE4D3) were activat ed by PA, whereas the ''short'' variants (PDE4A1, PDE4B2, PDE4D1, and PDE4D2) were not. Phosphatidylserine was an activator that was as effe ctive as PA, whereas phosphatidylcholine was ineffective, indicating t hat activation was restricted to anionic phospholipids. PA caused an i ncrease in the V-max value of PDE4D3 without affecting the K-m value o f the enzyme for the cAMP substrate. PA also caused a change in the Mg 2+ requirement for hydrolysis. Half-maximal stimulation of the PDE was obtained with similar to 10 mu g/ml PA. Although protein kinase A-med iated phosphorylation of PDE4D3 produces effects similar to those elic ited by PA, the mechanism of PA-induced activation was not found to in volve a phosphorylation. Instead, several observations suggest that PA may directly interact with the enzyme. The stimulation of cAMP PDEs b y PA and other acidic phospholipids may be a mechanism by which growth factors and hormones modulate the cAMP-dependent signal transduction pathway during cell stimulation.