Identification of type-2 phosphatidic acid phosphohydrolase (PAPH-2) in neutrophil plasma membranes

Characterization and purification of neutrophil ecto-phosphatidic acid phosphohydrolase

Phosphatidic acid and its derivatives play potentially important roles as extracellular messengers in biological systems. An ecto-phosphatidic acid phosphohydrolase (ecto-PAPase) has been identified which effectively regulates neutrophil responses to exogenous phosphatidic acid by converting the substrate to diacylglycerol. The present study was undertaken to characterize this ecto-enzyme on intact cells and to isolate the enzyme from solubilized neutrophil extracts. In the absence of detergent, short chain phosphatidic acids were hydrolysed most effectively by neutrophil plasma membrane ecto-PAPase; both saturated and unsaturated long chain phosphatidic acids were relatively resistant to hydrolysis. Both long (C18:1) and short (C8) chain lyso-phosphatidic acids were hydrolysed at rates comparable with those observed for short chain (diC8) phosphatidic acid. Activity of the ecto-enzyme accounted for essentially all of the N-ethylmaleimide-insensitive, Mg2+-independent PAPase activity recovered from disrupted neutrophils. At 37 degrees C and pH7.2, the apparent Km for dioctanoyl phosphatidic acid (diC8PA) was 1. 4x10(-3) M. Other phosphatidic acids and lysophosphatidic acids inhibited hydrolysis of [32P]diC8PA in a rank order that correlated with competitor solubility, lysophosphatidic acids and unsaturated phosphatidic acids being much more effective inhibitors than long chain saturated phosphatidic acids. Dioleoyl (C18:1) phosphatidic acid was an unexpectedly strong inhibitor of activity, in comparison with its ability to act as a direct substrate in the absence of detergent. Other inhibitors of neutrophil ecto-PAPase included sphingosine, dimethyl- and dihydro-sphingosine, propranolol, NaF and MgCl2. Of several leucocyte populations isolated from human blood by FACS, including T cells, B cells, NK lymphocytes and monocytes, ecto-PAPase was most prevalent on neutrophils; erythrocytes were essentially devoid of activity. A non-hydrolysable, phosphonate analogue of phosphatidic acid, phosphonate 1, efficiently solubilized catalytic activity from intact neutrophils without causing cell disruption or increasing permeability. Enzyme activity in solubilized extracts was purified in the absence of detergent by successive heparin-Sepharose, gel filtration and anion exchange chromatography. By assaying activity in renatured SDS/polyacrylamide gel slices, the molecular mass of neutrophil ecto-PAPase was estimated to be between 45 and 52 kDa, similar to the molecular mass of previously purified plasma membrane PAPases. Since a large portion of neutrophil plasma membrane PAPase is available for hydrolysis of exogenous substrates, ecto-PAPase may play an important role in regulating inflammatory cell responses to extracellular phosphatidic acid in biological systems.

Phosphatidic acid: a lipid messenger involved in intracellular and extracellular signalling

Generated during the initial phases of cell signalling, phosphatidic acid has been implicated as a messenger involved in the activation of cellular kinases and phospholipases as well as certain proto-oncogene products and low-molecular-weight G-proteins. Although many of the reported effects of phosphatidic acid can be attributed to metabolites generated by cellular hydrolases, the parent compound clearly possesses important biological activities. However, instead of acting as a ubiquitous second messenger mediating signalling events shared by a wide variety of cells, in many systems the phospholipid seems to function specifically, regulating unique functions confined to specialized groupings of cells. One such function is neutrophil superoxide generation, which is induced when phosphatidic acid, generated by activated phospholipase D (PLD), facilitates the interaction of a cytoplasmic low-molecular-weight G-protein with dormant, membrane-bound reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Positioned on the outer surface of the plasma membrane of triggering cells, phosphatidic acid potentially mediates the "juxtacrine" stimulation of cells in direct contact. This review critically evaluates the known biological effects of phosphatidic acid as opposed to functions induced by its metabolites and addresses the mechanisms by which these effects are specifically induced by this phospholipid.

Messenger functions of phosphatidic acid

Under physiological conditions, phosphatidic acid (PA) is an anionic phospholipid with moderate biological reactivity. Some of its biological effects can be attributed to lyso-PA and diacylglycerol generated by the action of cellular hydrolases. However, it is clear that the parent compound exhibits biological activities of its own. Early studies implicated PA in the transport of Ca++ across plasma membranes as well as in the mobilization of intracellular stored calcium. Both responses may be induced as a consequence of other cellular processes activated by PA, as opposed to being directly mediated by the lipid. PA may be involved in the activation of certain functions confined to specialized groupings of cells, such as the neutrophil superoxide-generating enzyme or actin polymerization. Recent studies implicate PA as an activator of intracellular protein kinases, and a PA-dependent superfamily of kinases involved in cellular signalling has been hypothesized. Deployed on the outer surface of the plasma membrane, PA potentially provides a method of communication between cells in direct contact. This review will explore the known functions of PA as an intracellular mediator and extracellular messenger of biological activities and address ways in which these functions are potentially regulated by cellular enzymes which hydrolyse the phospholipid.