Toll-like receptor 4 (TLR4) is the receptor for bacterial lipopolysaccharide (LPS) triggering production of pro-inflammatory cytokines which help eradicate the bacteria but could also be harmful when overproduced. The signaling activity of TLR4 is modulated by cholesterol level in cellular membranes, which in turn is affected by bis(monoacylglycero)phosphate (BMP), a phospholipid enriched in late endosomes. We found that exogenously added BMP isomers become incorporated into the plasma membrane and intracellular vesicles of macrophages and strongly reduced LPS-stimulated production of a chemokine RANTES, which was correlated with inhibition of interferon regulatory factor 3 (IRF3) controlling Rantes expression. To investigate the mechanism underlying the influence of BMP on TLR4 signaling we applied Laurdan and studied the impact of BMP incorporation on lipid packing, a measure for membrane order. Enrichment of model and cellular membranes with BMP significantly reduced their order and the reduction was maintained during stimulation of cells with LPS. This effect of BMP was abolished by enrichment of macrophages with cholesterol. In parallel, the inhibitory effect of BMP exerted on the TLR4-dependent phosphorylation of IRF3 was also reversed. Taken together our results indicate that BMP reduces the order of macrophage membranes which contributes to the inhibition of TLR4-dependent RANTES production.
Keywords: BMP; Cholesterol; Lysobisphosphatidic acid; Membrane order; Toll-like receptors.
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