Endotoxin Stimulates Cytokine and Nitric Oxide Production in Rat Hepatic Stellate Cells (HSCs) via p38 MAPK and Reactive Oxygen Species

Abstract

Endotoxin-stimulated HSCs may contribute to liver failure by hepatocyte growth inhibition and apoptosis. Two major effects of endotoxin on HSCs are increased synthesis of nitric oxide (NO) and potent inflammatory cytokines TNF-á and IL-6. These effects of endotoxin are novel as they do not require LBP, CD14/TLR-4. Since the intracellular signaling mechanisms of these effects are not known, we investigated if they are mediated by oxidative stress and signaling via NF-êB and mitogen-activated protein kinases (MAPKs) in rat HSCs. HSCs isolated from rat liver were activated in culture for 8-10 days and challenged with endotoxin. Activation of MAPKs and NF-êB, production of reactive oxygen species, and expression of iNOS, IL-6 and TNF-á were determined. Endotoxin induced time- and dose-dependent increases in the mRNA and protein expression of iNOS, NO synthesis as well as mRNA expression and release of IL-6 and TNF-á. Endotoxin also stimulated activation NF-êB, ERK1/2, p38 and JNK, and generation of H2O2. Exogenous H2O2 stimulated IL-6 and TNF-á but not NO synthesis, and did not activate NF-êB or MAPKs. Inhibition of p38 MAPK by SB203580 and that of NF-êB activation by pyrrolidine dithiocarbamate blocked endotoxin-induced H2O2, NO and cytokine synthesis. Inhibition of ERK1/2 and JNK phosphorylation did not alter the effects of endotoxin on NO or cytokine synthesis. While SB203580 inhibited LPS-induced NF-êB activation, PDTC did not affect p38 phosphorylation in LPS-stimulated cells. The results indicate that endotoxin-induced NO and cytokine synthesis in HSCs is initiated by p38 MAPK and mediated by NF-êB, with involvement of H2O2 in the cytokine production. Supported by a grant (RO1-DK 54411) from the NIH.