Institute of Fundamental Technological Research

Lipopolysaccharide (LPS) activates macrophages by binding to the TLR4/MD-2 complex and triggers two pro-inflammatory signaling pathways: one relies on MyD88 at the plasma membrane, and the other one depends on TRIF in endosomes. When present in high doses, LPS is internalized and undergoes detoxification. We found that the uptake of a high concentration of LPS (1000 ng/ml) in macrophage-like J774 cells was upregulated upon inhibition of clathrin- and dynamin-mediated endocytosis which, on the other hand, strongly reduced the production of pro-inflammatory mediators TNF-α and RANTES. The binding and internalization of high amounts of LPS was mediated by scavenger receptor A (SR-A) with participation of CD14 without an engagement of TLR4. Occupation of SR-A by dextran sulfate or anti-SR-A antibodies enhanced LPS-induced production of TNF-α and RANTES by about 70%, with CD14 as a limiting factor. Dextran sulfate also elevated the cell surface levels of TLR4 and CD14, which could have contributed to the upregulation of the pro-inflammatory responses. Silencing of SR-A expression inhibited the LPS-triggered TNF-α production whereas RANTES release was unchanged. These data indicate that SR-A is required for maximal production of TNF-α in cells stimulated with LPS, possibly by modulating the cell surface levels of TLR4 and CD14.

CD14, CTX-FITC, Endocytosis, HEPES-buffered saline, LBP, LPS conjugated with Alexa Fluor 488, LPS-AF488, LPS-binding protein, Lipopolysaccharide, PD buffer, SR-A, Scavenger receptor, TLR, poly(I:C), polyinosinic–polycytidylic acid, scavenger receptor A, subunit B of cholera toxin conjugated with FITC, transferrin conjugated with Alexa Fluor 647, transferrin-AF647

Upon bacterial infection lipopolysaccharide (LPS) induces migration of monocytes/macrophages to the invaded region and production of pro-inflammatory mediators. We examined mechanisms of LPS-stimulated motility and found that LPS at 100 ng/ml induced rapid elongation and ruffling of macrophage-like J774 cells. A wound-healing assay revealed that LPS also activated directed cell movement that was followed by TNF-α production. The CD14 and TLR4 receptors of LPS translocated to the leading lamella of polarized cells, where they transiently colocalized triggering local accumulation of actin filaments and phosphatidylinositol 4,5-bisphosphate. Fractionation of Triton X-100 cell lysates revealed that LPS induced polymerization of cytoskeletal actin filaments by 50%, which coincided with the peak of cell motility. This microfilament population appeared at the expense of short filaments composing the plasma membrane skeleton of unstimulated cells and actin monomers consisting prior to the LPS stimulation about 60% of cellular actin. Simultaneously with actin polymerization, LPS stimulated phosphorylation of two actin-regulatory proteins, paxillin on tyrosine 118 by 80% and N-WASP on serine 484/485 by 20%, and these events preceded activation of NF-κB. LPS-induced protein phosphorylation and reorganization of the actin cytoskeleton were inhibited by PP2, a drug affecting activity of tyrosine kinases of the Src family. The data indicate that paxillin and N-WASP are involved in the reorganization of actin cytoskeleton driving motility of LPS-stimulated cells. Disturbances of actin organization induced by cytochalasin D did not inhibit TNF-α production suggesting that LPS-induced cell motility is not required for TNF-α release.

ACTIN CYTOSKELETON, CELL MOTILITY, LPS, N-WASP, PAXILLIN, SRC KINASES