A novel protein kinase target for the lipid second messenger phosphatidic acid

Activation of phospholipase D occurs in response to a wide variety of hormo nes, growth factors, and other extracellular signals. The initial product o f phospholipase D, phosphatidic acid (PA), is thought to serve a signaling function, but the intracellular targets for this lipid second messenger are not clearly identified. The production of PA in human neutrophils is close ly correlated with the activation of NADPH oxidase, the enzyme responsible for the respiratory burst. We have developed a cell-free system, in which t he activation of NADPH oxidase is induced by the addition of PA. Characteri zation of this system revealed that a multi-functional cytosolic protein ki nase was a target for PA, and that two NADPH oxidase components were substr ates for the enzyme. Partial purification of the PA-activated protein kinas e separated the enzyme from known protein kinase targets of PA. The partial ly purified enzyme was selectively activated by PA, compared to other phosp holipids, and phosphorylated the oxidase component p47-phox on both serine and tyrosine residues. PA-activated protein kinase activity was present in a variety of hematopoietic cells and cell lines and in rat brain, suggestin g it has widespread distribution. We conclude that this protein kinase may be a novel target for the second messenger function of PA. (C) 1999 Elsevie r Science B.V. All rights reserved.