Liver progenitor cells (LPCs) represent the cell compartment facilitating hepatic regeneration during chronic injury while hepatocyte-mediated repair mechanisms are compromised. LPC proliferation is frequently observed in human chronic liver diseases such as hereditary hemochromatosis, fatty liver disease, and chronic hepatitis. In vivo studies have suggested that a tumor necrosis factor family member, tumor necrosis factor–like weak inducer of apoptosis (TWEAK), is promitotic for LPCs; whether it acts directly is not known. In our murine choline-deficient, ethionine-supplemented (CDE) model of chronic liver injury, TWEAK receptor [fibroblast growth factor-inducible 14 (Fn14)] expression in the whole liver is massively upregulated. We therefore set out to investigate whether TWEAK/Fn14 signaling promotes the regenerative response in CDE-induced chronic liver injury by mitotic stimulation of LPCs. Fn14 knockout (KO) mice showed significantly reduced LPC numbers and attenuated inflammation and cytokine production after 2 weeks of CDE feeding. The close association between LPC proliferation and activation of hepatic stellate cells in chronic liver injury prompted us to investigate whether fibrogenesis was also modulated in Fn14 KO animals. Collagen deposition and expression of key fibrogenesis mediators were reduced after 2 weeks of injury, and this correlated with LPC numbers. Furthermore, the injection of 2-week-CDE-treated wildtype animals with TWEAK led to increased proliferation of nonparenchymal pan cytokeratin–positive cells. Stimulation of an Fn14-positive LPC line with TWEAK led to nuclear factor kappa light chain enhancer of activated B cells (NFκB) activation and dose-dependent proliferation, which was diminished after targeting of the p50 NFκB subunit by RNA interference. Conclusion: TWEAK acts directly and stimulates LPC mitosis in an Fn14-dependent and NFκB-dependent fashion, and signaling via this pathway mediates the LPC response to CDE-induced injury and regeneration. (Hepatology 2010)

Lymphotoxin-beta (LTβ) is a proinflammatory cytokine and a member of the tumor necrosis factor (TNF) superfamily known for its role in mediating lymph node development and homeostasis. Our recent studies suggest a role for LTβ in mediating the pathogenesis of human chronic liver disease. We hypothesize that LTβ co-ordinates the wound healing response in liver injury via direct effects on hepatic stellate cells. This study used the choline-deficient, ethionine-supplemented (CDE) dietary model of chronic liver injury, which induces inflammation, liver progenitor cell proliferation, and portal fibrosis, to assess (1) the cellular expression of LTβ, and (2) the role of LTβ receptor (LTβR) in mediating wound healing, in LTβR^−/− versus wild-type mice. In addition, primary isolates of hepatic stellate cells were treated with LTβR-ligands LTβ and LTβ-related inducible ligand competing for glycoprotein D binding to herpesvirus entry mediator on T cells (LIGHT), and mediators of hepatic stellate cell function and fibrogenesis were assessed. LTβ was localized to progenitor cells immediately adjacent to activated hepatic stellate cells in the periportal region of the liver in wild-type mice fed the CDE diet. LTβR^−/− mice fed the CDE diet showed significantly reduced fibrosis and a dysregulated immune response. LTβR was demonstrated on isolated hepatic stellate cells, which when stimulated by LTβ and LIGHT, activated the nuclear factor kappa B (NF-κB) signaling pathway. Neither LTβ nor LIGHT had any effect on alpha-smooth muscle actin, tissue inhibitor of metalloproteinase 1, transforming growth factor beta, or procollagen α₁(I) expression; however, leukocyte recruitment-associated factors intercellular adhesion molecule 1 and regulated upon activation T cells expressed and secreted (RANTES) were markedly up-regulated. RANTES caused the chemotaxis of a liver progenitor cell line expressing CCR5. Conclusion: This study suggests that LTβR on hepatic stellate cells may be involved in paracrine signaling with nearby LTβ-expressing liver progenitor cells mediating recruitment of progenitor cells, hepatic stellate cells, and leukocytes required for wound healing and regeneration during chronic liver injury. (HEPATOLOGY 2009;49:227–239.)