Relationships between phosphatidic acid and cyclic nucleotide phosphodiesterases in activated human blood mononuclear cells

We have previously shown that mitogenic activation of human PBMC rapidly in creases both the intracellular phosphatidic acid (PA) level and cyclic nucl eotide phosphodiesterase (PDE) activity, with time-course. responses, sugge sting a causative relationship between the two events. PA also directly sti mulated cAMP-PDE activity in acellular systems. Thus the mitogenic properti es of PA might be due to its ability to lower the level of cAMP, a negative effector of lymphocyte activation, through PDE activation. In this study, human PBMC were stimulated either with the mitogenic lectin ConA, the anti- CDS mAb OKT3, or the Phorbol ester TPA. All three agonists increased the ra diolabeled PA level and the PA mass in treated cells and simultaneously inc reased cytosolic and particulate cAMP- and cGMP-PDE activities, with signif icant positive correlations between PA accumulation and PDE activities. Fur thermore, the ConA-induced PDE activation was dose-dependently reduced by t reatment of PBMC with the diacylglycerol-kinase inhibitor R59022. This comp ound also dose-dependently lowered the PA level and inhibited the prolifera tive response to ConA. In addition, TPA-induced PDE activation was totally abolished by ethanol, which strongly reduced PA accumulation in response to the phorbol ester. These data suggest that PA increase may be linked to mi togen-induced PDE activation. Experiments performed in the presence of roli pram indicated that ConA and TPA stimulated both the rolipram-sensitive PDE 4 and the rolipram-insensitive PDE activities, OKT3 being more active on PD E4. All three agonists stimulated the cGMP-specific PDE5. These results sug gest that PA is an important component of the mechanisms that maintain a lo w level of cyclic nucleotides, which is a prerequisite for an optimal lymph oproliferative response.