Differential Effects of Endotoxin Lipopolysaccharide on Stellate Cells and Portal Fibroblasts: Implications in Fibrosis Originating in Central Versus Portal Injury

Abstract

Background and Aims Activated hepatic stellate cells (aHSCs) are considered to be the major cells to cause liver fibrosis of various etiologies, but portal fibroblasts (Pfbs) that are quickly activated in biliary injury are also highly fibrogenic. A critical role for the gut-derived inflammatory endotoxin lipopolysaccharide (LPS) has been implicated in liver fibrosis. However, LPS down-modulates pro-fibrogenic factors in primary HSCs. We examined mechanisms of LPS actions in early and late fibrosis due to carbon tetrachloride (CCl4) and in bile duct ligation (BDL)-induced fibrosis. Methods C57BL/6(B6) mice were administered oil or CCl4 every 3 days, or underwent sham surgery or BDL. After 4 or 16 weeks (CCl4) or on day 14 (BDL), mice were challenged with 5 mg/kg LPS for 24h (CCl4) or 6h (BDL). Liver tissue was processed for immunohistochemical and molecular analyses. HSCs isolated from mouse or human liver were treated with 10 or 100 ng/ml LPS on day 3 or day 10 of culture or in passage (P) 1 through 5. Cells were then used for immunohistochemical and molecular analyses. Results H/E staining showed injury and inflammation at 4 and 16 weeks in CCl4-treated mice, which increased further after LPS treatment. Smooth muscle alpha (αSMA)+ aHSCs accumulated in the fibrotic (4 weeks) and cirrhotic (16 weeks) regions, and LPS reduced their number at 4 but not 16 weeks. However, mRNA expression of Acta2 (encodes αSMA) and Col1a1 (major extracellular matrix component) was reduced at both time points. Interestingly, mitogenic PDGFβR (expressed by aHSCs) expression decreased at 4 but not 16 weeks. LPS-induced altered expression of TGFβ1 (most potent fibrogenic cytokine), TGFβR1 and associated signaling molecules (SMAD7, C/EBPα, C/EBPβ and C/EBPδ) indicated down-modulation of TGFβ1-induced fibrogenic activity at 4 but not 16 weeks. In vitro, LPS-induced modulation of the expression of these factors demonstrated down-regulation of fibrogenic activity in cultured mouse and human HSCs till P1 or P2, but the cells became resistant to these LPS effects at P3-P5. Interestingly, increasing number of cells from P1 onward started expressing Thy1 (a marker of Pfbs) + αSMA with progressive reduction in αSMA alone expressing cells. LPS caused an increase in Thy1+αSMA+ cells. However, no Thy1-expressing cells were observed in fibrotic regions of CCl4-treated mice. In BDL mice, the majority of cells in fibrotic regions were Thy1+αSMA+ Pfbs. LPS caused strong necrotic damage and increased fibrosis with profound accumulation of αSMA+Thy1- aHSCs, and increased expression of pro-fibrogenic markers. Conclusion LPS exerts antifibrotic effects at early stages of HSC activation and fibrosis (in vivo and in vitro). At late stages of activation, HSCs become resistant to these actions. Pfbs are the primary fibrogenic cells in biliary injury, and are impervious to antifibrotic effects of LPS. LPS-stimulated Pfbs may provide signals for recruitment and activation of HSCs during progression of biliary fibrosis.