Lipopolysaccharide and sphingosine-1-phosphate cooperate to induce inflammatory molecules and leukocyte adhesion in endothelial cells

Abstract

Given that TLRs and sphingosine-1-phosphate (S1P) are key players in inflammation, we explored the potential interplay between TLRs and S1P in the adhesion/inflammatory pathways in primary human endothelial cells. As determined by Western blot and flow cytometry, cells treated with LPS (a TLR4 ligand) and S1P showed significantly enhanced expression of adhesion molecules such as ICAM-1 and E-selectin compared with the effect of either ligand alone. Cell-type differences on E-selectin upregulation were observed. In contrast, no cooperation effect on ICAM-1 or E-selectin was observed with a TLR2/TLR1 ligand. Consistent with an increase in adhesion molecule expression, endothelial cell treatment with LPS plus S1P significantly enhanced adhesion of PBMCs under shear stress conditions compared with the effect of either ligand alone and exhibited comparable levels of cell adhesion strength as those after TNF-α treatment. Moreover, LPS and S1P cooperated to increase the expression of proinflammatory molecules such as IL-6, cyclooxygenase-2, and prostacyclin, as determined by ELISA and Western blot. The analysis of signaling pathways revealed the synergistic phosphorylation of ERK upon LPS plus S1P treatment of HUVEC and human aortic endothelial cells and cell-type differences on p38 and NF-κB activation. Moreover, pharmacological and small interfering RNA experiments disclosed the involvement of S1P(1/3) and NF-κB in the cooperation effect and that cell origin determines the S1P receptors and signaling routes involved. Sphingosine kinase activity induction upon LPS plus S1P treatment suggests S1P- Sphingosine kinase axis involvement. In summary, LPS and S1P cooperate to increase proinflammatory molecules in endothelial cells and, in turn, to augment leukocyte adhesion, thus exacerbating S1P-mediated proadhesive/proinflammatory properties.