Lysophosphatidic Acid As a Regulator of Endothelial/leukocyte Interaction

Lysophosphatidic acid (LPA) is produced by a variety of activated cell types and acts as an intercellular mediator of processes associated with inflammation and repair including platelets aggregation, and smooth muscle and fibroblast proliferation. However no previous studies have examined the effects of LPA on endothelial cell leukocyte interactions. We have examined the ability of LPA to activate human aortic endothelial cells (HAEC) to bind monocytes, neutrophils, and HL60 cells (a neutrophil surrogate). Treatment of HAEC for 4 hours with 10 microM LPA caused an increase in the binding of monocytes, neutrophils, and HL60. LPA but not phosphatidic acid dose-dependently increased E-selectin and vascular cell adhesion molecule-1 (VCAM-1) cell surface expression. We performed several studies to characterize the receptor mediating the LPA effect. We demonstrate that at least five potential LPA receptors are expressed by HAEC: Edg-1, -3, -4, and -5 as well as PSP24. Cyclic phosphate-containing phosphatidic acid analogue, an agonist for the type 3 low affinity LPA receptor, was not effective in activating HAEC to bind leukocytes, excluding a role for this receptor. The selective receptor antagonists N-palmitoyl-serine and N-palmitoyl-tyrosine (which inhibits PSP24) completely inhibited LPA-induced VCAM expression; however these antagonists inhibited E-selectin expression by only 30%, suggesting a role for at least one additional LPA receptor mediating E-selectin expression. We propose that Edg-1 might be the second receptor, because this receptor, when expressed in HEK293 cells, similarly to the PSP24 receptor, caused ERK activation to nanomolar concentration of LPA. Exposure of HAEC to sphingosine-1-phosphate, another Edg-1 receptor agonist, increased surface expression of E-selectin and to a much smaller extent VCAM-1. The effects of both LPA and sphingosine-1-phosphate on the induction of both VCAM-1 and E-selectin expression was abolished by pretreatment with pertussis toxin suggesting that both LPA receptors in HAEC couple to a Gi pathway. These findings reveal an important and novel role for LPA and its receptors in inflammatory processes.