Lysophospholipids increase ICAM-1 expression in HUVEC through a Gi- and NF-κB-dependent mechanism

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Abstract
Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S-1- P ) are both low molecular weight lysophospholipid (LPL) ligands that are recognized by the Edg family of G protein-coupled receptors. In endothelial cells, these two ligands activate Edg receptors, resulting in cell proliferation and cell migration. The intercellular adhesion molecule-1 (ICAM-1, CD54) is one of many cell adhesion molecules belonging to the immunoglobulin superfamily. This study showed that LPA and S-1- P enhance ICAM-1 expression at both the mRNA and protein levels in human umbilical cord vein endothelial cells (HUVECs). This enhanced ICAM-1 expression in HUVECs was first observed at 2 h postligand treatment. Maximal expression appeared at 8 h postligand treatment, as detected by flow cytometry and Western blotting. Furthermore, the effects of S-1- P on ICAM-1 expression were shown to be concentration dependent. Prior treatment of HUVECs with pertussis toxin, a specific inhibitor of Gi, ammonium pyrrolidinedithiocarbamate and BAY 11–7082, inhibitors of the nuclear factor (NF)-κB pathway, or Clostridium difficile toxin B, an inhibitor of Rac, prevented the enhanced effect of LPL-induced ICAM-1 expression. However, pretreatment of HUVECs with exoC3, an inhibitor of Rho, had no effect on S-1- P -enhanced ICAM-1 expression. In a static cell-cell adhesion assay system, pretreatment of LPL enhanced the adhesion between HUVECs and U-937 cells, a human mononucleated cell line. The enhanced adhesion effect could be prevented by preincubation with a functional blocking antibody against human ICAM-1. These results suggest that LPLs released by activated platelets might enhance interactions of leukocytes with the endothelium through a Gi-, NF-κB-, and possibly Rac-dependent mechanism, thus facilitating wound healing and inflammation processes.

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