Pleiotropic functions of TNF-alpha determine distinct IKKbeta-dependent hepatocellular fates in response to LPS

TNF-alpha influences morbidity and mortality during the course of endotoxemia. However, the complex pleiotropic functions of TNF-alpha remain poorly understood. We evaluated how hepatic induction of NF-kappaB and TNF-alpha influence survival and hepatocellular death in a lethal murine model of endotoxic shock. Using dominant-negative viral vectors to inhibit the IKK complex, we demonstrate through this study that the liver is a major source of TNF-alpha during the course of lethal endotoxemia and that IKKbeta (but not IKKalpha) is predominantly responsible for activating NF-kappaB and TNF-alpha in the liver after LPS administration. Using TNF-alpha knockout mice and hepatic-specific inhibition of IKKbeta, we demonstrate that the status of TNF-alpha and NF-kappaB balances necrotic and apoptotic fates of hepatocytes in the setting of endotoxemia. In the presence of TNF-alpha, inhibiting hepatic IKKbeta resulted in increased survival, reduced serum proinflammatory cytokines, and reduced hepatocyte necrosis in response to a lethal dose of endotoxin. In contrast, inhibiting hepatic IKKbeta in TNF-alpha knockout mice resulted in decreased survival and increased caspase 3-mediated hepatocyte apoptosis after endotoxin challenge, despite a reduced proinflammatory cytokine response. In the presence of TNF-alpha, NF-kappaB-dependent hepatocellular necrosis predominated, while in the absence of TNF-alpha, NF-kappaB primarily influenced apoptotic fate of hepatocytes. Changes in JNK phosphorylation after LPS challenge were also dynamically affected by both IKKbeta and TNF-alpha; however, this pathway could not solely explain the differential outcomes in hepatocellular fates. In conclusion, our studies demonstrate that induction of NF-kappaB and TNF-alpha balances protective (antiapoptotic) and detrimental (proinflammatory) pathways to determine hepatocellular fates during endotoxemia.     

Serine peptidase inhibitor Kazal type I (SPINK1) promotes BRL‐3A cell proliferation via p38, ERK,and JNK pathways

Serine peptidase inhibitor Kazal type I (SPINK1) has the similar spatial structure as epidermal growth factor (EGF); EGF can interact with epidermal growth factor receptor (EGFR) to promote proliferation in different cell types. However, whether SPINK1 can interact with EGFR and further regulate the proliferation of hepatocytes in liver regeneration remains largely unknown. In this study, we investigated the role of SPINK1 in a rat liver hepatocyte line of BRL‐3A in vitro. The results showed the upregulation of endogenous Spink1 (gene addition) significantly increased not only the cell viability, cell numbers in S and G₂/M phase, but also upregulated the genes/proteins expression related to cell proliferation and anti‐apoptosis in BRL‐3A. In contrast, the cell number in G₁ phase and the expression of pro‐apoptosis‐related genes/proteins were significantly decreased. The similar results were observed when the cells were treated with exogenous rat recombinant SPINK1. Immunoblotting suggested SPINK1 can interact with EGFR. By Ingenuity Pathway Analysis software, the SPINK1 signalling pathway was built; the predicted read outs were validated by qRT‐PCR and western blot; and the results showed that p38, ERK, and JNK pathways‐related genes/proteins were involved in the cell proliferation upon the treatment of endogenous Spink1 and exogenous SPINK1. Collectively, SPINK1 can associate with EGFR to promote the expression of cell proliferation‐related and anti‐apoptosis‐related genes/proteins; inhibit the expression of pro‐apoptosis‐related genes/proteins via p38, ERK, and JNK pathways; and consequently promote the proliferation of BRL‐3A cells. For the first time, we demonstrated that SPINK1 can associate with EGFR to promote the proliferation of BRL‐3A cells via p38, ERK, and JNK pathways. This work has direct implications on the underlying mechanism of SPINK1 in regulating hepatocytes proliferation in vivo and liver regeneration after partial hepatectomy.     

IL-22-mediated liver cell regeneration is abrogated by SOCS-1/3 overexpression in vitro

The IL-10-like cytokine IL-22 is produced by activated T cells. In this study, we analyzed the role of this cytokine system in hepatic cells. Expression studies were performed by RT-PCR and quantitative PCR. Signal transduction was analyzed by Western blot experiments and ELISA. Cell proliferation was measured by MTS and [(3)H]thymidine incorporation assays. Hepatocyte regeneration was studied in in vitro restitution assays. Binding of IL-22 to its receptor complex expressed on human hepatic cells and primary human hepatocytes resulted in the activation of MAPKs, Akt, and STAT proteins. IL-22 stimulated cell proliferation and migration, which were both significantly inhibited by the phosphatidylinositol 3-kinase inhibitor wortmannin. IL-22 increased the mRNA expression of suppressor of cytokine signaling (SOCS)-3 and the proinflammatory cytokines IL-6, IL-8, and TNF-alpha. SOCS-1/3 overexpression abrogated IL-22-induced STAT activation and decreased IL-22-mediated liver cell regeneration. Hepatic IL-22 mRNA expression was detectable in different forms of human hepatitis, and hepatic IL-22 mRNA levels were increased in murine T cell-mediated hepatitis in vivo following cytomegalovirus infection, whereas no significant differences were seen in an in vivo model of ischemia-reperfusion injury. In conclusion, IL-22 promotes liver cell regeneration by increasing hepatic cell proliferation and hepatocyte migration through the activation of Akt and STAT signaling, which is abrogated by SOCS-1/3 overexpression.     

High levels of Myc expression are required for the robust proliferation of hepatocytes,but not for the sustained weak proliferation

In contrast to the robust proliferation exhibited following acute liver injury, hepatocytes exhibit long-lasting proliferative activity in chronic liver injury. The mechanistic differences between these distinct modes of proliferation are unclear. Hepatocytes exhibited robust proliferation that peaked at 2 days following partial hepatectomy in mice, but this proliferation was completely inhibited by hepatocyte-specific expression of MadMyc, a Myc-suppressing chimeric protein. However, Myc suppression induced weak but continuous hepatocyte proliferation, thereby resulting in full restoration of liver mass despite an initial delay. Late-occurring proliferation was accompanied by prolonged suppression of proline dehydrogenase (PRODH) expression, and forced PRODH overexpression inhibited hepatocyte proliferation. In hepatocytes in chronic liver injury, Myc was not activated but PRODH expression was suppressed in regenerating hepatocytes. In liver tumors, PRODH expression was often suppressed, especially in the highly proliferative tumors with distinct Myc expression. Our results indicate that the robust proliferation of hepatocytes following acute liver injury requires high levels Myc expression and that there is a compensatory Myc-independent mode of hepatocyte proliferation with the regulation of proline metabolism, which might be relevant to liver regeneration in chronic injury.     

CXC Chemokines Function as a Rheostat for Hepatocyte Proliferation and Liver Regeneration

Background

Our previous in vitro studies have demonstrated dose-dependent effects of CXCR2 ligands on hepatocyte cell death and proliferation. In the current study, we sought to determine if CXCR2 ligand concentration is responsible for the divergent effects of these mediators on liver regeneration after ischemia/reperfusion injury and partial hepatectomy.

Methods

Murine models of partial ischemia/reperfusion injury and hepatectomy were used to study the effect of CXCR2 ligands on liver regeneration.

Results

We found that hepatic expression of the CXCR2 ligands, macrophage inflammatory protein-2 (MIP-2) and keratinocyte-derived chemokine (KC), was significantly increased after both I/R injury and partial hepatectomy. However, expression of these ligands after I/R injury was 30-100-fold greater than after hepatectomy. Interestingly, the same pattern of expression was found in ischemic versus non-ischemic liver lobes following I/R injury with expression significantly greater in the ischemic liver lobes. In both systems, lower ligand expression was associated with increased hepatocyte proliferation and liver regeneration in a CXCR2-dependent fashion. To confirm that these effects were related to ligand concentration, we administered exogenous MIP-2 and KC to mice undergoing partial hepatectomy. Mice received a “high” dose that replicated serum levels found after I/R injury and a “low” dose that was similar to that found after hepatectomy. Mice receiving the “high” dose had reduced levels of hepatocyte proliferation and regeneration whereas the “low” dose promoted hepatocyte proliferation and regeneration.

Conclusions

Together, these data demonstrate that concentrations of CXC chemokines regulate the hepatic proliferative response and subsequent liver regeneration.     

The effect of Z-FA.FMK on D-galactosamine/TNF-alpha-induced liver injury in mice

Cathepsin B is a cysteine proteinase, considered to have an important role in apoptosis, which is activated by D-galactosamine and tumor necrosis factor-alpha (D-GalN/TNF-alpha). Benzyloxycarbonyl-L-phenylalanine fluoromethyl ketone (Z-FA.FMK) is a cathepsin B inhibitor used in research on apoptotic pathways. The aim of this study was to investigate the role of Z-FA.FMK on apoptotic cell death, cell proliferation and liver damage induced by a D-GalN/TNF-alpha combination in mice. In the study, 1 h after administration of 8 mg/kg Z-FA.FMK by intravenous injection, D-GalN (700 mg/kg) and TNF-alpha (15 microg/kg) were administered by a single intraperitoneal injection. In the group given D-GalN/TNF-alpha, the following results were found: Degenerative changes in the liver tissue, significant increase in the number of both TUNEL and activated caspase-3-positive hepatocytes, a decrease in the number of PCNA-positive hepatocytes, an increase in lipid peroxidation (LPO) levels and a decrease in glutathione (GSH) and DNA levels in the liver tissue. In contrast, in the group given D-GalN/TNF-alpha and Z-FA.FMK, a decrease in the damage of the liver tissue, a significant decrease in TUNEL and activated caspase-3-positive hepatocytes, a significant increase in the number of PCNA-positive hepatocytes, a decrease in the LPO levels, an increase in GSH and DNA levels in the liver tissue were found. As a result, microscopic and biochemical evaluations indicate that Z-FA.FMK plays a protective role against liver injury induced by D-GalN/TNF-alpha and it has an inverse effect on hepatocyte apoptosis and proliferation in BALB/c mice.     

The role of hepatocytes and oval cells in liver regeneration and repopulation

The liver has the unique capacity to regulate its growth and mass. In rodents and humans, it grows rapidly after resection of more than 50% of its mass. This growth process, as well as that following acute chemical injury is known as liver regeneration, although growth takes place by compensatory hyperplasia rather than true regeneration. In addition to hepatocytes and non-parenchymal cells, the liver contains intra-hepatic "stem" cells which can generate a transit compartment of precursors named oval cells. Liver regeneration after partial hepatectomy does not involve intra or extra-hepatic (hemopoietic) stem cells but depends on the proliferation of hepatocytes. Transplantation and repopulation experiments have demonstrated that hepatocytes, which are highly differentiated and long-lived cells, have a remarkable capacity for multiple rounds of replication. In this article, we review some aspects of the regulation of hepatocyte proliferation as well as the interrelationships between hepatocytes and oval cells in different liver growth processes. We conclude that in the liver, normally quiescent differentiated cells replicate rapidly after tissue resection, while intra-hepatic precursor cells (oval cells) proliferate and generate lineage only in situations in which hepatocyte proliferation is blocked or delayed. Although bone marrow stem cells can generate oval cells and hepatocytes, transdifferentiation is very rare and inefficient.     

MiR-429 regulates rat liver regeneration and hepatocyte proliferation by targeting JUN/MYC/BCL2/CCND1 signaling pathway

Increasing evidence indicates that miR-429 is involved in tumor suppression in various human cancers. However, its role in liver regeneration remains unexplored. Liver regeneration is a highly orchestrated process that can be regulated by microRNAs (miRNAs), although the mechanisms are largely unclear. In this study, we aimed to identify the role of miR-429 in hepatocyte proliferation during liver regeneration. First, we performed microarray analysis and qRT-PCR. Results indicated that miR-429 level in rat liver markedly decreased 30 h after partial hepatectomy, and miR-429 overexpression disrupted BRL-3A proliferation and the transition of G1 to S phase in rat hepatocyte and promoted hepatocyte apoptosis. By contrast, miR-429 down-regulation had inverse effects. MiR-429 negatively regulated JUN expression in vitro and in vivo. After using JUN siRNA, we found that JUN inhibition mediates the effect of miR-429 in hepatocyte proliferation and growth and miR-429 negatively regulates JUN/MYC/BCL2/CCND1 signaling pathways. Our results also indicated that miR-429 inhibits hepatocyte proliferation and liver regeneration by targeting JUN/MYC/BCL2/CCND1.     

Byakangelicin protects against carbon tetrachloride–induced liver injury and fibrosis in mice

Liver fibrosis is a disease caused by long‐term damage that is related to a number of factors. The current research on the treatment of liver fibrosis mainly focuses on the activation of hepatic stellate cell, in addition to protecting liver cells. byakangelicin has certain anti‐inflammatory ability, but its effect on liver fibrosis is unclear. This study aims to explore whether byakangelicin plays a role in the development of liver fibrosis and to explore its mechanism. We determined that byakangelicin has a certain ability to resist fibrosis and reduce liver cell damage in a model of carbon tetrachloride–induced liver fibrosis in mice. Thereafter, we performed further verification in vitro. The signalling pathways of two important pro‐fibrotic cytokines, transforming growth factor‐β and platelet‐derived growth factor, were studied. Results showed that byakangelicin can inhibit related pathways. According to the hepatoprotective effect of byakangelicin observed in animal experiments, we studied the effect of byakangelicin on 4‐HNE–induced hepatocyte (HepG2) apoptosis and explored its related pathways. The results showed that byakangelicin could attenuate 4‐HNE–induced hepatocyte apoptosis via inhibiting ASK‐1/JNK signalling. In conclusion, byakangelicin could improve carbon tetrachloride–induced liver fibrosis and liver injury by inhibiting hepatic stellate cell proliferation and activation and suppressing hepatocyte apoptosis.     

Exploring the priming mechanism of liver regeneration: proteins and protein complexes

The liver has the ability to restore its functional capacity following injury or resection and the priming of liver regeneration is a complex process that has not been completely elucidated. In the current research, to further reveal the priming mechanism of liver regeneration, hepatocyte total protein and hepatocyte cytosol of the rats at 4 h after 2/3 partial hepatectomy (PHx) were studied, respectively, by 2‐DE and 2‐D blue native gel electrophoresis. Seventeen unique differential proteins were identified in hepatocyte total protein samples. Nine differential protein complexes containing 41 protein components were identified in hepatocyte cytosol samples. For the first time, at the priming stage of liver regeneration, the variations of serine protease inhibitor 2c, sulfite oxidase and valosin‐containing protein (VCP) were presented and validated by Western blotting, and the VCP complex was further validated by antibody super‐shift experiments. The current results suggested that at 4 h after PHx, VCP complex was down‐regulated in hepatocyte cytosol, apoptosis pathways were inhibited, nuclear factor‐κB and interleukin 6 pathways worked together and triggered the liver regeneration.     

Involvement of IQGAP3, a regulator of Ras/ERK‐related cascade,in hepatocyte proliferation in mouse liver regeneration and development

The spatio‐temporal regulation of hepatocyte proliferation is a critical issue in liver regeneration. Here, in normal and regenerating liver as well as in developing liver, we examined its expression/localization of IQGAP3, which was most recently reported as a Ras/Rac/Cdc42‐binding proliferation factor associated with cell–cell contacts in epithelial‐type cells. In parallel, the expression/localization of Rac/Cdc42‐binding IQGAP1/2 was examined. IQGAP3 showed a specific expression in proliferating hepatocytes positive for the proliferating marker Ki‐67, the levels of expressions of mRNAs and proteins were significantly increased in hepatocytes in liver regeneration and development. In immunofluorescence, IQGAP3 was highly enriched at cell–cell contacts of hepatocytes. IQGAP1 and IQGAP2 were exclusively expressed in Kupffer and sinusoidal endothelial cells, respectively, in normal, regenerating, and developing liver. The expression of IQGAP1, but not of IQGAP2, was increased in CCl₄‐induced (but not in partial hepatectomy‐induced) liver regeneration. Exclusive expression/localization of IQGAP3 to hepatocytes in the liver likely reflects the specific involvement of the IQGAP3/Ras/ERK signaling cascade in hepatocyte proliferation in addition to the previously identified signaling pathways, possibly by integrating cell–cell contact‐related proliferating signaling events. On the other hand, the Rac/Cdc42‐binding properties of IQGAP1/2/3 may be related to the distinct modes of remodeling due to the different strategies which induced proliferation of liver cells; partial hepatectomy, CCl₄ injury, or embryonic development. Thus, the functional orchestration of Ras and the Ras homologous (Rho) family proteins Rac/Cdc42 likely plays a critical role in liver regeneration and development. J. Cell. Physiol. 220: 621–631, 2009. © 2009 Wiley‐Liss, Inc.     

Effects of sizofiran on endotoxin-enhanced cold ischemia-reperfusion injury of the rat liver

Kupffer cells (KC), resident macrophages of the liver, have been strongly implicated in lipopolysaccharide (LPS)-induced liver graft injury. However, our recent study showed that sizofiran (schizophyllan glucan) (SPG), which activates KC, did not influence cold ischemia-reperfusion liver injury of LPS-exposed rats. Here we investigated some mechanisms by which SPG does not aggravate LPS-enhanced cold ischemia-reperfusion rat liver injury. Control and SPG-treated rats were exposed to LPS for 2 h prior to hepatectomy. The livers were cold-preserved in University of Wisconsin solution followed by reperfusion with Krebs-Henseleit buffer. We found that SPG dramatically inhibited LPS-induced increases of tumor necrosis factor-alpha (TNF-alpha) in the plasma and bile in vivo. Moreover, LPS-induced TNF- release into the washout solution after cold ischemia was also abrogated by SPG pretreatment. However, SPG increased TNF- release into the perfusate after reperfusion. On the other hand, SPG completely abolished expression of c-myc protooncogene, which is known to sensitize cells to TNF-alpha cytotoxicity. In conclusion, inhibition of both TNF- release after LPS challenge and c-myc expression may explain why activation of KC with SPG does not aggravate endotoxin-enhanced cold ischemia-reperfusion liver injury.     

SOCS2 Balances Metabolic and Restorative Requirements during Liver Regeneration

After significant injury, the liver must maintain homeostasis during the regenerative process. We hypothesized the existence of mechanisms to limit hepatocyte proliferation after injury to maintain metabolic and synthetic function. A screen for candidates revealed suppressor of cytokine signaling 2 (SOCS2), an inhibitor of growth hormone (GH) signaling, was strongly induced after partial hepatectomy. Using genetic deletion and administration of various factors we investigated the role of SOCS2 during liver regeneration. SOCS2 preserves liver function by restraining the first round of hepatocyte proliferation after partial hepatectomy by preventing increases in growth hormone receptor (GHR) via ubiquitination, suppressing GH pathway activity. At later times, SOCS2 enhances hepatocyte proliferation by modulating a decrease in serum insulin-like growth factor 1 (IGF-1) that allows GH release from the pituitary. SOCS2, therefore, plays a dual role in modulating the rate of hepatocyte proliferation. In particular, this is the first demonstration of an endogenous mechanism to limit hepatocyte proliferation after injury.     

Skeletal muscle cell activation by low-energy laser irradiation: a role for the MAPK/ERK pathway

Low-energy laser irradiation (LELI) has been shown to promote skeletal muscle regeneration in vivo and to activate skeletal muscle satellite cells, enhance their proliferation and inhibit differentiation in vitro. In the present study, LELI, as well as the addition of serum to serum-starved myoblasts, restored their proliferation, whereas myogenic differentiation remained low. LELI induced mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) phosphorylation with no effect on its expression in serum-starved myoblasts. Moreover, a specific MAPK kinase inhibitor (PD098059) inhibited the LELI- and 10% serummediated ERK1/2 activation. However, LELI did not affect Jun N-terminal kinase (JNK) or p38 MAPK phosphorylation or protein expression. Whereas a 3-sec irradiation induced ERK1/2 phosphorylation, a 12-sec irradiation reduced it, again with no effect on JNK or p38. Moreover, LELI had distinct effects on receptor phosphorylation: it caused phosphorylation of the hepatocyte growth factor (HGF) receptor, previously shown to activate the MAPK/ERK pathway, whereas no effect was observed on tumor suppressor necrosis alpha (TNF-alpha) receptor which activates the p38 and JNK pathways. Therefore, by specifically activating MAPK/ERK, but not JNK and p38 MAPK enzymes, probably by specific receptor phosphorylation, LELI induces the activation and proliferation of quiescent satellite cells and delays their differentiation.     

Molecular dissection of gp130-dependent pathways in hepatocytes during liver regeneration

Interleukin-6 (IL-6) via its signal transducer gp130 is an important mediator of liver regeneration involved in protecting from lipopolysaccharide (LPS)-induced liver injury after partial hepatectomy (PH). Here we generated mice either defective (Delta) in hepatocyte-specific gp130-dependent Ras or STAT activation to define their role during liver regeneration. Deletion of gp130-dependent signaling had major impact on acute phase gene (APG) regulation after PH. APG expression was blocked in gp130-DeltaSTAT animals, whereas gp130-DeltaRas mice showed an enhanced APG response and stronger SOCS3 regulation correlating with delayed hepatocyte proliferation. To define the role of SOCS3 during hepatocyte proliferation, primary hepatocytes were co-stimulated with IL-6 and hepatocyte growth factor. Higher SOCS3 expression in gp130-DeltaRas hepatocytes correlated with delayed hepatocyte proliferation. Next, we tested the impact of LPS, mimicking bacterial infection, on liver regeneration. LPS and PH induced SOCS3 and APG in all animal strains and delayed cell cycle progression. Additionally, IL-6/gp130-dependent STAT3 activation in hepatocytes was essential in mediating protection and thus required for maximal proliferation. Unexpectedly, oncostatin M was most strongly induced in gp130-DeltaSTAT animals after PH/LPS-induced stress and was associated with hepatocyte proliferation in this strain. In summary, gp130-dependent STAT3 activation and concomitant SOCS3 during liver regeneration is involved in timing of DNA synthesis and protects hepatocyte proliferation during stress conditions.     

PTEN-mediated AKT/β-catenin signaling enhances the proliferation and expansion of Lgr5+ hepatocytes

Rationale: Compelling evidence suggests that Lgr5+ hepatocytes repair liver damage by promoting the regeneration of hepatocytes and ductal cells in the case of liver injury. The PTEN-mediated AKT/β-catenin signaling plays a key role in the regulation of innate immune regulation in the liver. However, the signaling pathways that control Lgr5+ hepatocyte proliferation in the liver remain unclear.Methods: In order to assess the involvement of PTEN-mediated AKT/β-catenin signaling in the expansion of Lgr5+ hepatocytes upon liver injuries, the Lgr5-CreER; Rosa-mTmG lineage tracing system was used to target Lgr5+ hepatocytes.Results: The tracing of Lgr5+ hepatocytes showed that PTEN deletion and β-catenin activation significantly promoted the proliferation of Lgr5+ hepatocytes. In converse, the simultaneous inhibition of PTEN and β-catenin limited Lgr5+ hepatocyte proliferation in the liver. Our findings provide an insight into understanding how PTEN-mediated AKT/β-catenin signaling regulates the proliferation of Lgr5+ hepatocytes.Conclusion: The outcomes can improve the application potential of Lgr5+ hepatocytes in the treatment of liver injury diseases and provide a new treatment option for liver cancer.     

Simulated microgravity promoted differentiation of bipotential murine oval liver stem cells by modulating BMP4/Notch1 signaling

Faster growth and differentiation of liver stem cells to hepatocyte is one of the key factors during liver regeneration. In recent years, simulated microgravity, a physical force has shown to differentially regulate the differentiation and proliferation of stem cells. In the present work, we studied the effect of simulated microgravity on differentiation and proliferation of liver stem cells. The cells were subjected to microgravity, which was simulated using indigenously fabricated 3D clinostat. Proliferation, apoptosis, immunofluorescence assays and Western blot analysis were carried out to study the effects of simulated microgravity on liver stem cells. Microgravity treatment for 2 h enhanced proliferation of stem cells by twofold without inducing apoptosis and compromising cell viability. Analysis of hepatocyte nuclear factor 4‐α (HNF4‐α) expression after 2 h of microgravity treatment revealed that microgravity alone can induce the differentiation of stem cells within 2–3 days. Probing bone morphogenic protein 4 (BMP4) and Notch1 in microgravity treated stem cells elaborated downregulation of Notch1 and upregulation of BMP4 after 2 days of incubation. Further, blocking BMP4 using dorsomorphin and chordin conditioned media from chordin plasmid transfected cells attenuated microgravity mediated differentiation of liver stem cells. In conclusion, microgravity interplays with BMP4/Notch1 signaling in stem cells thus inducing differentiation of stem cells to hepatocytes. Present findings can be implicated in clinical studies where microgravity activated stem cells can regenerate the liver efficiently after liver injury. J. Cell. Biochem. 112: 1898–1908, 2011. © 2011 Wiley‐Liss, Inc.