Regulation of phospholipase D-induced hydrolysis of choline-containing phosphoglycerides by cyclic AMP in human neutrophils.
In human neutrophils, the chemotactic tripeptide FMLP and the protein kinase C activator PMA stimulate the breakdown of 1-O-[3H]alkyl-2-acyl-sn-glycero-3-phosphocholine ([3H]EAPC) and the formation of 1-O-[3H]alkyl-2-acyl-phosphatidic acid ([3H]-EAPA) and 1-O-[3H]alkyl-2-acylglycerol ([3H]EAG) via mechanism(s) that may involve the activation of phospholipase D. We have investigated the regulation of phospholipase D by determining the effects of elevation of intracellular levels of cAMP on receptor-mediated and PMA-induced breakdown of [3H]-EAPC and formation of products. Pretreatment of neutrophils with either 10(-3) M dibutyryl-cAMP or 10(-5) M PGE2, in the presence of 4 x 10(-4) M isobutylmethylxanthine (IBMX), inhibited FMLP- and leukotriene B4-induced breakdown of [3H]EAPC and formation of [3H]EAPA and [3H]EAG. Inhibition was apparent at all time points of stimulation examined (15-120 s). In addition, the mass of diradyl-phosphatidic acid was decreased in FMLP-stimulated neutrophils pretreated with PGE2 and IBMX. In contrast, pretreatment of cells with PGE2 and IBMX did not inhibit PMA-induced breakdown of [3H]EAPC and the formation of products at 3 and 10 min after stimulation. Furthermore, formation of 1-O-[3H]alkyl-2-acyl-phosphatidylethanol, produced by phospholipase D in the presence of ethanol by a transphosphatidylation reaction, was significantly inhibited by pretreatment of cells with PGE2 and IBMX in FMLP- but not PMA-stimulated neutrophils. This differential modulation by cAMP of receptor-mediated and PMA-induced activation of phospholipase D suggests agonist-dependent activation of separate pathways and/or activation of separate phospholipase D enzymes. Thus, cAMP elevation may exert inhibitory effects directly on the phospholipase D activated by FMLP and/or on sites proximal to the enzyme that are involved in signal transmission.