Bacterial Lipopolysaccharide and IFN-γ Induce Toll-Like Receptor 2 and Toll-Like Receptor 4 Expression in Human Endothelial Cells: Role of NF-κB Activation

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Abstract
Toll-like receptor (TLR) 4 has been identified as the primary receptor for enteric LPS, whereas TLR2 has been implicated as the receptor for Gram-positive and fungal cell wall components and for bacterial, mycobacterial, and spirochetal lipoproteins. Vascular endothelial cell (EC) activation or injury by microbial cell wall components such as LPS is of critical importance in the development of sepsis and septic shock. We have previously shown that EC express predominantly TLR4, and have very little TLR2. These cells respond vigorously to LPS via TLR4, but are unresponsive to lipoproteins and other TLR2 ligands. Here we show that LPS, TNF-α, or IFN-γ induce TLR2 expression in both human dermal microvessel EC and HUVEC. Furthermore, LPS and IFN-γ act synergistically to induce TLR2 expression in EC, and LPS-induced TLR2 expression is NF-κB dependent. LPS and IFN-γ also up-regulate TLR4 mRNA expression in EC. These data indicate that TLR2 and TLR4 expression in ECs is regulated by inflammatory molecules such as LPS, TNF-α, or IFN-γ. TLR2 and TLR4 molecules may render EC responsive to TLR2 ligands and may help to explain the synergy between LPS and lipoproteins, and between LPS and IFN-γ, in inducing shock associated with Gram-negative sepsis.