EXOGENOUS PHOSPHOLIPASE-D GENERATES LYSOPHOSPHATIDIC ACID AND ACTIVATES RAS, RHO AND CA2+ SIGNALING PATHWAYS

Background: Phospholipase D (PLD) hydrolyzes phospholipids to generate phosphatidic acid (PA) and a free headgroup. PLDs occur as both intra cellular and secreted forms; the latter can act as potent virulence fa ctors. Exogenous PLD has growth-factor-like properties, in that it ind uces proto-oncogene transcription, mitogenesis and cytoskeletal change s in target cells. The underlying mechanism is unknown, although it-is generally assumed that PLD action is mediated by PA serving asa putat ive second messenger. Results: In quiescent fibroblasts, exogenous PLD (from Streptomyces chromofuscus) stimulated accumulation of the GTP-b ound form of Ras, activation of mitogen-activated protein (MAP) kinase and DNA synthesis, through the pertussis-toxin-sensitive inhibitory G protein G(i). Furthermore, PLD mimicked bioactive lysophospholipids ( but not PA) in inducing Ca2+ mobilization, membrane depolarization and Rho-mediated neurite retraction. PLD action was mediated by lysophosp hatidic acid (LPA) derived from lysophosphatidylcholine acting on cogn ate G-protein-coupled LPA receptor(s). There was no evidence for the i nvolvement of PA in mediating the effects of exogenous PLD. Conclusion s: Our results provide a molecular explanation for the multiple cellul ar responses to exogenous PLDs. These PLDs generate bioactive LPA from pre-existing lysophosphatidylcholine in the outer membrane leaflet, r esulting in activation of G-protein-coupled LPA receptors and conseque nt activation of Ras, Rho and Ca2+ signaling pathways. Unscheduled act ivation of LPA receptors may underlie, at least in part, the known pat hogenic effects of exogenous PLDs.