Neutralization and Transfer of Lipopolysaccharide by Phospholipid Transfer Protein
Journal of Biological Chemistry
Phospholipid transfer protein (PLTP) and lipopolysaccharide-binding protein (LBP) are lipid transfer proteins found in human plasma. PLTP shares 24% sequence similarity with LBP. PLTP mediates the transfer and exchange of phospholipids between lipoprotein particles, whereas LBP transfers bacterial lipopolysaccharide (LPS) either to lipoprotein particles or to CD14, a soluble and cell-surface receptor for LPS. We asked whether PLTP could interact with LPS and mediate the transfer of LPS to
... fer of LPS to lipoproteins or to CD14. PLTP was able to bind and neutralize LPS: incubation of LPS with purified recombinant PLTP (rPLTP) resulted in the inhibition of the ability of LPS to stimulate adhesive responses of neutrophils, and addition of rPLTP to blood inhibited cytokine production in response to LPS. Transfer of LPS by rPLTP was examined using fluorescence dequenching experiments and native gel electrophoresis. The results suggested that rPLTP was able to mediate the exchange of LPS between micelles and the transfer of LPS to reconstituted HDL particles, but it did not transfer LPS to CD14. Consonant with these findings, rPLTP did not mediate CD14-dependent adhesive responses of neutrophils to LPS. These results suggest that while PLTP and LBP both bind and transfer LPS, PLTP is unable to transfer LPS to CD14 and thus does not mediate responses of cells to LPS. Lipopolysaccharide (LPS 1 ; endotoxin) is a membrane lipid of Gram-negative bacteria that acts as a potent inflammatory stimulus in humans and other mammals (1). Recent work has suggested that a plasma protein called LPS-binding protein (LBP) is important in trafficking LPS in blood. LBP can transfer LPS to lipoprotein particles, resulting in the functional neutralization of LPS (2). LBP also mediates functional responses of cells to LPS (3) by facilitating the transfer of LPS to CD14 (4), a glycoprotein found both as a soluble monomer in the blood (soluble CD14, sCD14) and as a glycosylphosphatidylinositol-linked membrane protein (mCD14) on monocytes, macrophages, and neutrophils. LBP has sequence similarity to two other plasma lipid transfer proteins, phospholipid transfer protein (PLTP, 24% amino acid identity) (5) and cholesteryl ester transfer protein (CETP, 23% amino acid identity) (3). LBP, PLTP, and CETP are found in plasma associated with HDL particles (2, 6, 7). CETP facilitates the transfer of cholesteryl esters, triglycerides, and phospholipids between lipoproteins (7) . PLTP mediates the exchange and transfer of phospholipids between lipoprotein particles (6). PLTP also mediates high density lipoprotein (HDL) conversion, the transformation of HDL into smaller and larger particles (8 -10). Through these activities, PLTP may regulate HDL level and composition and thereby affect cholesterol metabolism (6). Because our previous studies indicated that the transfer of LPS may be important in modulating inflammatory responses to LPS, and because LBP and PLTP share sequence similarity as well as related functions, we investigated the ability of PLTP to interact with LPS and transfer LPS to R-HDL or to CD14.