TLR4 Activation Induces Nontolerant Inflammatory Response in Endothelial Cells

Abstract

In professional immune cells, Toll-like receptor 4 (TLR4) induces tightly regulated inflammatory response to avoid tissue damage via the induction of "endotoxin tolerance", which is a transient state of cell desensitization in response to lipopolysaccharide (LPS) restimulation after a prior LPS exposure. However, in endothelial cells, the regulation of TLR4-induced inflammation is not fully understood. In this study, we found that the gene transcripts for a lot of Toll-like receptors were expressed in various endothelial cells, including human umbilical vein endothelial cells (HUVEC), human aortic endothelial cell (HAEC), and mouse microvascular endothelial cells (bEND.3). Proteins of TLR4 and its coreceptor CD14 were also detected in HUVEC. LPS treatment significantly upregulated the expression of proinflammation cytokines such as IL-1β, IL-6, and IL-8 only in HUVEC, but not in HAEC and bEND.3, suggesting that vein endothelial cells are important source of proinflammatory cytokines in response to LPS. Unexpectedly, "endotoxin tolerance" was not induced in endothelial cell, but was induced in control glial cells, as LPS pretreatment downregulated the cytokine expression in control glial cells, but did not in endothelial cells, when the cells were restimulated with LPS. The upregulation of cytokine gene expression was dependent on NF-κB signaling, and NF-κB inhibitor repressed the induction of cytokines. Two important signal molecules MyD88 and TRIF, which are TLR4 downstream and NF-κB upstream, were upregulated in vein endothelial cells but were downregulated in control glial cells. These results suggested that vein endothelial cells may play important roles in the pathophysiology of systemic inflammation-associated diseases such as sepsis and septic cardiomyopathy.

Expression and function of toll-like receptors in human circulating endothelial colony forming cells

“…Conversely, ECFCs are a highly differentiated population with a more limited role in immune responses and a less efficient production of IL-6. Nevertheless, ECFCs were still able o produce IL-6, suggesting their involvement in the formation of the cytokine milieu during the innate protective immune responses, as previously demonstrated for HUVECs [29].…”

Systemic Lipopolysaccharide Compromises the Blood-Labyrinth Barrier and Increases Entry of Serum Fluorescein into the Perilymph

“…In studies of the BBB, LPS binds Toll-like receptor 4 (TLR4) on leukocyte and endothelial cell surfaces, which results in the production of inflammatory cytokines, among them, TNF-alpha. Subsequent activation of myeloid differentiation factor 88 (MyD88) plays a critical role in the downstream effects of LPS, with resultant loss of endothelial tight junction integrity within endothelial cells of the brain (McGettrick and O'Neill 2010;Tauseef et al 2012;Wang et al 2011;Zhou et al 2006). These effector molecules activated by inflammation are relatively understudied elements of inner ear dysfunction.…”

Oxidized Low-Density Lipoprotein Induces Inflammatory Responses in Cultured Human Mast Cells Via Toll-Like Receptor 4

“…Toll-like receptor 4 (TLR4), a critical key factor in regulating innate immune response, induces remarkable expression of pro-inflammatory and pro-atherogenic cytokines in macrophages, VSMCs and endothelial cells when activated by binding to its ligand [15,16,17]. While TLR4 was originally described as a pattern receptor that recognizes lipopolysaccharide (LPS), recently, endogenous ligands have been found that are powerful TLR4 activators; examples are ox-LDL, fibronectin and heat shock protein 60 [18].…”