Toll–Like Receptor 4 Mediates Inflammatory Signaling by Bacterial Lipopolysaccharide in Human Hepatic Stellate Cells

Abstract

Bacterial lipopolysaccharide (LPS) stimulates Kupffer cells and participates in the pathogenesis of alcohol–induced liver injury. However, it is unknown whether LPS directly affects hepatic stellate cells (HSCs), the main fibrogenic cell type in the injured liver. This study characterizes LPS–induced signal transduction and proinflammatory gene expression in activated human HSCs. Culture–activated HSCs and HSCs isolated from patients with hepatitis C virus–induced cirrhosis express LPS–associated signaling molecules, including CD14, toll–like receptor (TLR) 4, and MD2. Stimulation of culture–activated HSCs with LPS results in a rapid and marked activation of NF–κB, as assessed by in vitro kinase assays for IκB kinase (IKK), IκBα steady–state levels, p65 nuclear translocation, NF–κB–dependent luciferase reporter gene assays, and electrophoretic mobility shift assays. Lipid A induces NF–κB activation in a similar manner. Both LPS– and lipid A–induced NF–κB activation is blocked by preincubation with either anti–TLR4 blocking antibody (HTA125) or Polymyxin B. Lipid A induces NF–κB activation in HSCs from TLR4–sufficient (C3H/OuJ) mice but not from TLR4–deficient (C3H/HeJ) mice. LPS also activates c–Jun N–terminal kinase (JNK), as assessed by in vitro kinase assays. LPS up–regulates IL–8 and MCP–1 gene expression and secretion. LPS–induced IL–8 secretion is completely inhibited by the IκB super repressor (Ad5IκB) and partially inhibited by a specific JNK inhibitor, SP600125. LPS also up–regulates cell surface expression of ICAM–1 and VCAM–1. In conclusion, human activated HSCs utilize components of TLR4 signal transduction cascade to stimulate NF–κB and JNK and up–regulate chemokines and adhesion molecules. Thus, HSCs are a potential mediator of LPS–induced liver injury.