| Lysophosphatidate (LPA) is among the most investigated signaling lipid, and is linked to roles that promote wound healing, development, and angiogenesis. LPA signaling is also linked to disease states such as tumor growth, metastasis, atherosclerosis, and inflammatory diseases. Therefore, regulation of the metabolism of LPA is critical in controlling its accumulation and signaling effects. Lipid phosphate phosphatases (LPP) are transmembrane enzymes that can dephosphorylate various signaling lipid phosphates including LPA, and phosphatidate (PA). There are three isoforms of LPP, and each has specific functions in vivo. This thesis will focus on LPP1 that acts partly in the plasma membrane as an ecto-enzyme that can degrade external LPA. LPA stimulates pertussis toxin-sensitive migration of fibroblasts through cell surface receptors. We found that LPP1 over-expression can attenuate LPA-, but not platelet-derived growth factor (PDGF)- or endothelin-induced fibroblast migration. Conversely, knockdown of endogenous LPP1 activity increased LPA-, but not PDGF-induced migration. LPP1 also attenuated migration induced by a non-hydrolysable LPA receptor agonist. This demonstrates that inhibition of LPA-induced migration was not dependent on external LPA dephosphorylation, and that LPP1 controls signaling downstream of LPA receptors. This is consistent with work that shows LPP1 can regulate intracellular accumulation of lipid phosphates. Indeed we found that LPP1 can decrease PA accumulation by attenuating LPA-induced phospholipase D (PLD) activation, and that PLD2 activity was necessary for LPA-, but not PDGF-induced fibroblast migration. Increased LPP1 activity also decreased LPA-, but not PDGF-stimulated ERK and Rho activities, and the basal activities of Rac and Cdc42. These signaling proteins are important in fibroblast migration. LPP1 affected the phosphorylation of cellular proteins stimulated with various signaling agonists, such as PDGF, which also had its PLD activation inhibited by LPP1. In addition, over-expression of LPP1 decreased LPA-induced migration of neighboring wild-type cells, as did the conditioned media from the former cells. The affect by the conditioned media could be due to the alteration of secretion of extracellular matrix remodeling proteins, matrix metalloproteinases and their regulators. This thesis increases our knowledge of how LPP1 regulates intracellular signaling by bioactive lipids. |
