STAT3 contributes to the mitogenic response of hepatocytes during liver regeneration

Class I histone deacetylase activity is required for proliferation of renal epithelial cells

The process of renal regeneration after acute kidney injury is thought to recapitulate renal development, and proliferation of renal proximal tubular cells (RPTCs) is a critical step in the regenerative response. Recent studies indicate that class I histone deacetylases (HDACs) are required for embryonic kidney gene expression, growth, and differentiation. The role and underlying mechanisms of class I HDAC activation in RPTC proliferation, however, remain unclear. In this study, we used cultured RPTCs to examine this issue since four class I HDAC isoforms (1, 2, 3, and 8) are abundantly expressed in this cell type. Blocking class I HDAC activity with a highly selective inhibitor, MS-275, induced global histone H3 hyperacetylation, reduced RPTC proliferation, and diminished expression of cyclin D1 and proliferating cell nuclear antigen. Silencing HDAC1, 3, or 8 with small interfering RNA resulted in similar biological effects. Activation of epidermal growth factor receptor (EGFR) and signal transducers and activators of transcription 3 (STAT3) was required for RPTC proliferation, and STAT3 functioned downstream of EGFR. Treatment with MS-275 or knockdown of HDAC1, 3, or 8 suppressed EGFR expression and phosphorylation, and silencing HDAC1 and 3 also reduced STAT3 phosphorylation. However, HDAC2 downregulation did not affect RPTC proliferation and phosphorylation of EGFR and STAT3. Collectively, these data reveal a critical role of class I HDACs in mediating proliferation of renal epithelial cells through activation of the EGFR/STAT3 signaling pathway.

A20 promotes liver regeneration by decreasing SOCS3 expression to enhance IL-6/STAT3 proliferative signals

Liver regeneration is of major clinical importance in the setting of liver injury, resection, and transplantation. A20, a potent antiinflammatory and nuclear factor kappa B (NF-κB) inhibitory protein, has established pro-proliferative properties in hepatocytes, in part through decreasing expression of the cyclin dependent kinase inhibitor, p21. Both C-terminal (7-zinc fingers; 7Zn) and N-terminal (Nter) domains of A20 were required to decrease p21 and inhibit NF-κB. However, both independently increased hepatocyte proliferation, suggesting that additional mechanisms contributed to the pro-proliferative function of A20 in hepatocytes. We ascribed one of A20's pro-proliferative mechanisms to increased and sustained interleukin (IL)-6-induced signal transducer and activator of transcription 3 (STAT3) phosphorylation, as a result of decreased hepatocyte expression of the negative regulator of IL-6 signaling, suppressor of cytokine signaling 3 (SOCS3). This novel A20 function segregates with its 7Zn not Nter domain. Conversely, total and partial loss of A20 in hepatocytes increased SOCS3 expression, hampering IL-6-induced STAT3 phosphorylation. Following liver resection in mice pro-proliferative targets downstream of IL-6/STAT3 signaling were increased by A20 overexpression and decreased by A20 knockdown. In contrast, IL-6/STAT3 proinflammatory targets were increased in A20-deficient livers, and decreased or unchanged in A20 overexpressing livers. Upstream of SOCS3, levels of its microRNA regulator miR203 were significantly decreased in A20-deficient livers.

CONCLUSION

A20 enhances IL-6/STAT3 pro-proliferative signals in hepatocytes by down-regulating SOCS3, likely through a miR203-dependent manner. This finding together with A20 reducing the levels of the potent cell cycle brake p21 establishes its pro-proliferative properties in hepatocytes and prompts the pursuit of A20-based therapies to promote liver regeneration and repair.

Dynamic recruitment of microRNAs to their mRNA targets in the regenerating liver

Background

Validation of physiologic miRNA targets has been met with significant challenges. We employed HITS-CLIP to identify which miRNAs participate in liver regeneration, and to identify their target mRNAs.

Results

miRNA recruitment to the RISC is highly dynamic, changing more than five-fold for several miRNAs. miRNA recruitment to the RISC did not correlate with changes in overall miRNA expression for these dynamically recruited miRNAs, emphasizing the necessity to determine miRNA recruitment to the RISC in order to fully assess the impact of miRNA regulation. We incorporated RNA-seq quantification of total mRNA to identify expression-weighted Ago footprints, and developed a microRNA regulatory element (MRE) prediction algorithm that represents a greater than 20-fold refinement over computational methods alone. These high confidence MREs were used to generate candidate ‘competing endogenous RNA’ (ceRNA) networks.

Conclusion

HITS-CLIP analysis provide novel insights into global miRNA:mRNA relationships in the regenerating liver.

Increased growth of colorectal liver metastasis following partial hepatectomy

Nearly 50 % of colorectal cancer (CRC) patients develop liver metastases with liver resection being the only option to cure patients. Residual micrometastases or circulating tumor cells are considered a cause of tumor relapse. This work investigates the influence of partial hepatectomy (PH) on the growth and molecular composition of CRC liver metastasis in a syngeneic rat model. One million CC531 colorectal tumor cells were implanted via the portal vein in WAG/Rij rats followed by a 30 % PH a day later. Control groups either received tumor cells followed by a sham-operation or were injected with a buffer solution followed by PH. Animals were examined with magnetic resonance imaging (MRI) and liver tissues were processed for immunolabeling and PCR analysis. One-third PH was associated with an almost threefold increase in relative tumor mass (MRI volumetry: 2.8-fold and transcript levels of CD44: 2.3-fold). Expression of molecular markers for invasiveness and aggressiveness (CD49f, CXCR4, Axin2 and c-met) was increased following PH, however with no significant differences when referring to the relative expression levels (relating to tumor mass). Liver metastases demonstrated a significantly higher proliferation rate (Ki67) 2 weeks following PH and cell divisions also increased in the surrounding liver tissue. Following PH, the stimulated growth of metastases clearly exceeded the compensation in liver volume with long-lasting proliferative effects. However, the distinct tumor composition was not influenced by liver regeneration. Future investigations should focus on the inhibition of cell cycle (i.e. systemic therapy strategies, irradiation) to hinder liver regeneration and therefore restrain tumor growth.

Pancreatic STAT3 protects mice against caerulein-induced pancreatitis via PAP1 induction

The signal transducer and activator of transcription 3 (STAT3) is a transcription factor that controls expressions of several genes involved in cell survival, proliferation and differentiation, and tissue inflammation. However, the significance of pancreatic STAT3 in acute pancreatitis remains unclear. We generated conditional STAT3 knockout (stat3(Δ/Δ)) mice by crossing stat3(flox/flox) mice with Pdx1-promoter Cre transgenic mice. Caerulein administration activated pancreatic STAT3 and induced acute pancreatitis as early as 3 hours in wild-type mice, and full recovery from the induced pancreatic injury was observed within 7 days. The levels of serum amylase and lipase and histologic scores of pancreatic necrosis and inflammatory cell infiltration were significantly higher at 3 hours in stat3(Δ/Δ) mice than in stat3(flox/flox) mice. Pancreatic recovery after pancreatitis was significantly delayed in stat3(Δ/Δ) mice compared with stat3(flox/flox) mice. Although stat3(flox/flox) mice had marked production in the pancreas of pancreatitis-associated protein 1 (PAP1), a serum acute phase protein, this induction was completely abrogated in stat3(Δ/Δ) mice. Enforced production of PAP1 by a hydrodynamic procedure in the liver significantly suppressed pancreatic necrosis and inflammation and also promoted pancreatic regeneration and recovery in stat3(Δ/Δ) mice to levels similar to those observed in stat3(flox/flox) mice. In conclusion, pancreatic STAT3 is indispensable for PAP1 production, and this STAT3/PAP1 pathway plays a protective role in caerulein-induced pancreatitis.

Potential role of signal transducer and activator of transcription (STAT)3 signaling pathway in inflammation, survival, proliferation and invasion of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most lethal malignancies, and is also the fourth most common cancer worldwide with around 700,000 new cases each year. Currently, first line chemotherapeutic drugs used for HCC include fluorouracil, cisplatin, doxorubicin, paclitaxel and mitomycin, but most of these are non-selective cytotoxic molecules with significant side effects. Sorafenib is the only approved targeted therapy by the U.S. Food and Drug Administration for HCC treatment, but patients suffer from various kinds of adverse effects, including hypertension. The signal-transducer-and-activator-of-transcription 3 (STAT3) protein, one of the members of STATs transcription factor family, has been implicated in signal transduction by different cytokines, growth factors and oncogenes. In normal cells, STAT3 activation is tightly controlled to prevent dysregulated gene transcription, whereas constitutively activated STAT3 plays an important role in tumorigenesis through the upregulation of genes involved in anti-apoptosis, proliferation and angiogenesis. Thus, pharmacologically safe and effective agents that can block STAT3 activation have the potential both for the prevention and treatment of HCC. In the present review, we discuss the possible role of STAT3 signaling cascade and its interacting partners in the initiation of HCC and also analyze the role of various STAT3 regulated genes in HCC progression, inflammation, survival, invasion and angiogenesis.

STAT proteins - key regulators of anti-viral responses, inflammation, and tumorigenesis in the liver

Since its discovery in the early 1990s, the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway has been found to play key roles in regulating many key cellular processes such as survival, proliferation, and differentiation. There are seven known mammalian STAT family members: STAT1, 2, 3, 4, 5a, 5b, and 6. In the liver, activation of these STAT proteins is critical for anti-viral defense against hepatitis viral infection and for controlling injury, repair, inflammation, and tumorigenesis. The identification of functions for these STAT proteins has increased our understanding of liver disease pathophysiology and treatments, while also suggesting new therapeutic modalities for managing liver disease.

Omega-3 polyunsaturated fatty acids promote liver regeneration after 90% hepatectomy in rats

AIM: To evaluate the effectiveness of omega-3 polyunsaturated fatty acid (ω-3 PUFA) administration on liver regeneration after 90% partial hepatectomy (PH) in rats.

METHODS: ω-3 PUFAs were intravenously injected in the ω-3 PUFA group before PH surgery. PH, sparing only the caudate lobe, was performed in both the control and the ω-3 PUFA group. Survival rates, liver weight/body weight ratios, liver weights, HE staining, transmission electron microscope imaging, nuclear-associated antigen Ki-67, enzyme-linked immunosorbent assay and signal transduction were evaluated to analyze liver regeneration.

RESULTS: All rats in the control group died within 30 h after hepatectomy. Survival rates in the ω-3 PUFA group were 20/20 at 30 h and 4/20 1 wk after PH. Liver weight/body weight ratios and liver weights increased significantly in the ω-3 PUFA group. The structure of sinusoidal endothelial cells and space of Disse was greatly restored in the ω-3 PUFA group compared to the control group after PH. In the ω-3 PUFA group, interleukin (IL)-4 and IL-10 levels were significantly increased whereas IL-6 and tumor necrosis factor-α levels were dramatically decreased. In addition, activation of protein kinase B (Akt) and of signal transducer and activator of transcription 3 signaling pathway were identified at an earlier time after PH in the ω-3 PUFA group.

CONCLUSION: Omega-3 polyunsaturated fatty acids may prevent acute liver failure and promote liver regeneration after 90% hepatectomy in rats.

The promotional effect of IL-22 on mineralization activity of periodontal ligament cells

OBJECTIVES

Interleukin (IL)-22 acts on non-immune cells to induce anti-microbial responses, protection from tissue damage, and enhance cell regeneration. However, little is known about the involvement of IL-22 in periodontal biology. This study investigated the biological effects of IL-22 on periodontal ligament (PDL) cells as part of studies to assess the involvement of IL-22 in periodontal disease.

MATERIALS AND METHODS

Gene expression levels of IL-22 and its receptors in PDL cells and gingival tissue samples were evaluated by real-time PCR. Proliferative responses and mineralized-matrix forming activities of PDL cells were examined in the presence and absence of IL-22.

RESULTS

In contrast to the expression of IL-22 receptors detected in PDL tissues and their cell lines, gingival tissues showed modest or no gene expressions of IL-22. The production of several cytokines including IL-11, IL-8 and CCL2 was upregulated by IL-22 treatment of PDL cells in a dose-dependent manner. IL-22 treatment had no effect on the proliferative response in PDL cells. Meanwhile, IL-22 precipitated mineralized nodule formation and induced gene expressions of RUNX2, MSX2 and osteocalcin in PDL cells, suggesting that IL-22 enhances the mineralized matrix-forming activities of PDL cells.

CONCLUSION

IL-22 has the potential to promote mineralizing activity in PDL cells and to develop appropriate regenerative therapy.

Ischaemic Preconditioning and Intermittent Clamping Does not Influence Mediators of Liver Regeneration in a Human Liver Sinusoidal Endothelial Cell Model of Ischaemia-Reperfusion Injury

BACKGROUND

The role of surgical technique on liver regeneration following surgery remains inconclusive. The aim of the study was to assess the effect of ischaemic preconditioning (IPC) and intermittent clamping (IC) on mediators of regeneration produced by human liver sinusoidal endothelial cells (SECs), using an in vitro hypoxia-reoxygenation model to mimic ischaemia-reperfusion injury (IRI).

METHODS

Following extraction from samples obtained from liver resection (n = 5), confluent culture flasks of SECs were subjected to IRI (1 hour hypoxia + 1 hour reoxygenation), IPC prior to IRI (10 minutes hypoxia + 10 minutes reoxygenation + 1 hour hypoxia + 1 hour reoxygenation), IC (15 minutes hypoxia + 5 minutes reoxygenation x 3 + 1 hour reoxygenation) and compared to controls. The production of various mediators was determined over 48 hours.

RESULTS

Interleukin (IL)-6, IL-8, granulocyte-colony stimulating factor (G-CSF) and hepatocyte growth factor (HGF) were produced by SECs. Both IPC and IC did not significantly influence the profile of IL-6, IL-8, G-CSF and HGF by SECs compared to IRI over the study period.

CONCLUSION

IPC and IC did not influence the production of pro-regenerative mediators in a SECs model of IRI. The role of surgical technique on liver regeneration remains to be determined.

Proline protects liver from D-galactosamine hepatitis by activating the IL-6/STAT3 survival signaling pathway

The oral administration of proline, one of the non-essential amino acids, has been shown to effectively protect the liver from D-galactosamine (GalN)-induced liver injury and to improve the survival rate. The aim of this study was to investigate the mechanism of this protective action of proline. We paid particular attention to the effect of proline on inflammatory activation, regenerative response, and the associated signal transduction in the liver. Male Fischer rats received intraperitoneal injections of GalN (1.4 g/kg) with or without the oral administration of proline (2 g/kg) 1 h before GalN treatment. Liver pathology, plasma indices of inflammation, and the level of proliferative marker in the liver were monitored. The hepatic activation of interleukin-6 (IL-6)/signal transducer and activator of transcription (STAT)-3 pathway, which is downstream of tumor necrosis factor (TNF)-α/nuclear factor-κB, was also studied. GalN induced massive inflammatory expansion in the liver, leading to a high death rate (60 %) more than 72 h after the treatment. Proline administration significantly suppressed inflammatory infiltration in the live after 48 h, which was accompanied by depletion of plasma TNF-α, glutamic oxaloacetic transaminase, and glutamic pyruvic transaminase. The mRNA expression of histone H3, a marker of proliferation, was significantly upregulated in the liver of proline-treated animals. Furthermore, IL-6/STAT-3 pathway, an anti-inflammatory and regenerative signaling pathway, was strongly activated prior to these observations, with the upregulated expression of downstream genes. These results suggest that the tissue-protective mechanism of proline involves the early activation of IL-6/STAT-3 pathway in the liver, with subsequent activation of the regenerative response and suppression of massive inflammatory activation.

Liver regeneration after liver transplantation

BACKGROUND/PURPOSE

The liver has a remarkable capacity to regenerate after injury or resection. The aim of this review is to outline the mechanisms and factors affecting liver regeneration after liver transplantation.

METHODS

Relevant studies were reviewed using Medline, PubMed and Springer databases.

RESULTS

A variety of cytokines (such as interleukin-6 and tumor necrosis factor-α), growth factors (like hepatocyte growth factor and transforming growth factor-α) and cells are involved in liver regeneration. Several factors affect liver regeneration after transplantation such as ischemic injury, graft size, immunosuppression, steatosis, donor age and viral hepatitis.

CONCLUSION

Liver regeneration has been studied for many years. However, further research is essential to reveal the complex processes affecting liver regeneration, which may provide novel strategies in the management of liver transplantation recipients and donors.

Hepatitis C virus-host interactions: Etiopathogenesis and therapeutic strategies

Hepatitis C virus (HCV) is a significant health problem facing the world. This virus infects more than 170 million people worldwide and is considered the major cause of both acute and chronic hepatitis. Persons become infected mainly through parenteral exposure to infected material by blood transfusions or injections with nonsterile needles. Although the sexual behavior is considered as a high risk factor for HCV infection, the transmission of HCV infection through sexual means, is less frequently. Currently, the available treatment for patients with chronic HCV infection is interferon based therapies alone or in combination with ribavirin and protease inhibitors. Although a sustained virological response of patients to the applied therapy, a great portion of patients did not show any response. HCV infection is mostly associated with progressive liver diseases including fibrosis, cirrhosis and hepatocellular carcinoma. Although the focus of many patients and clinicians is sometimes limited to that problem, the natural history of HCV infection (HCV) is also associated with the development of several extrahepatic manifestations including dermatologic, rheumatologic, neurologic, and nephrologic complications, diabetes, arterial hypertension, autoantibodies and cryglobulins. Despite the notion that HCV-mediated extrahepatic manifestations are credible, the mechanism of their modulation is not fully described in detail. Therefore, the understanding of the molecular mechanisms of HCV-induced alteration of intracellular signal transduction pathways, during the course of HCV infection, may offer novel therapeutic targets for HCV-associated both hepatic and extrahepatic manifestations. This review will elaborate the etiopathogenesis of HCV-host interactions and summarize the current knowledge of HCV-associated diseases and their possible therapeutic strategies.

Remote ischemic preconditioning promotes early liver cell proliferation in a rat model of small-for-size liver transplantation

BACKGROUND

The size of the liver donor graft is a major concern in living donor liver transplantation. Rapid regeneration is essential for the survival of these grafts. The purpose of this study was to investigate the effect of remote ischemic preconditioning (RIPC) on liver regeneration in a rat small-for-size liver transplantation model.

METHODS

We established rat models of small-for-size liver transplantation (30%) in the presence or absence (control) of remote ischemic preconditioning. We observed liver mass regeneration, serum alanine aminotransferase, hepatic pathologic alterations, flow cytometry, and Ki-67 antigen immunohistochemistry. In addition, using Western blotting and reverse-transcriptase-polymerase chain reaction, we assessed the activation of cell cycle progression as well as tumor necrosis factor-α and interleukin-6 expression.

RESULTS

Compared with the control group, serum alanine aminotransferase activity was significantly lower and histopathology changes were significantly attenuated in the RIPC group. Remote ischemic preconditioning induced a high level of interleukin-6 mRNA in small grafts, but suppressed the expression of tumor necrosis factor-α. The proliferation index, indicated by the S-phase and G2/M-phase ratio [(S+G2/M)/(G0/G1+S+G2/M)], was significantly increased in the RIPC group at 24 h (58.25% ± 0.506% versus 53.405% ± 1.25%; P = .007). Meanwhile, cell cycle progression and regeneration (Ki-67) were initiated early in liver grafts treated with RIPC.

CONCLUSIONS

These results suggest that RIPC can protect liver cells against ischemia reperfusion injury in the small grafts and enhance liver regeneration. Interleukin-6 may be a critical mediator in the stimulatory effect on liver cell regeneration, which may make RIPC valuable as a hepatoprotective modality.

Controlling the stem cell compartment and regeneration in vivo: the role of pluripotency pathways

Since the realization that embryonic stem cells are maintained in a pluripotent state through the interplay of a number of key signal transduction pathways, it is becoming increasingly clear that stemness and pluripotency are defined by the complex molecular convergence of these pathways. Perhaps this has most clearly been demonstrated by the capacity to induce pluripotency in differentiated cell types, so termed iPS cells. We are therefore building an understanding of how cells may be maintained in a pluripotent state, and how we may manipulate cells to drive them between committed and pluripotent compartments. However, it is less clear how cells normally pass in and out of the stem cell compartment under normal and diseased physiological states in vivo, and indeed, how important these pathways are in these settings. It is also clear that there is a potential "dark side" to manipulating the stem cell compartment, as deregulation of somatic stem cells is being increasingly implicated in carcinogenesis and the generation of "cancer stem cells." This review explores these relationships, with a particular focus on the role played by key molecular regulators of stemness in tissue repair, and the possibility that a better understanding of this control may open the door to novel repair strategies in vivo. The successful development of such strategies has the potential to replace or augment intervention-based strategies (cell replacement therapies), although it is clear they must be developed with a full understanding of how such approaches might also influence tumorigenesis.

Cytokines in alcoholic liver disease

Alcoholic liver disease (ALD) is associated with a spectrum of liver injury ranging from steatosis and steatohepatitis to fibrosis and cirrhosis. While multifactorial pathogenesis plays a role in the disease progression, enhanced inflammation in the liver during ethanol exposure is a major feature of ALD. Dysregulated cytokine metabolism and activity are crucial to the initiation of alcohol-induced liver injury. The pro-inflammatory cytokine tumor necrosis factor (TNF-α) has been demonstrated to be one of the key factors in the various aspects of pathophysiology of ALD. The immunomodulatory cytokines such as interleukin 10 and interleukin 6 play roles in exerting hepatic protective effects. Adiponectin is an adipose tissue-derived hormone, which displays protective actions on ethanol-induced liver injury. Treatment for mice with adiponectin decreases TNF-α expression, steatosis and prevents alcohol-induced liver injury. Adiponectin exerts its anti-inflammatory effects via suppression of TNF-α expression and induction of anti-inflammatory cytokines such as IL-10. Adiponectin attenuates alcoholic liver injury by the complex network of multiple signaling pathways in the liver, leading to enhanced fatty acid oxidation and reduced steatosis. Interactions between pro- and anti-inflammatory cytokines such as TNFα and adiponectin and other cytokines are likely to play important roles in the development and progression of alcoholic liver disease.

Gene deletions and amplifications in human hepatocellular carcinomas: correlation with hepatocyte growth regulation

Tissues from 98 human hepatocellular carcinomas (HCCs) obtained from hepatic resections were subjected to somatic copy number variation (CNV) analysis. Most of these HCCs were discovered in livers resected for orthotopic transplantation, although in a few cases, the tumors themselves were the reason for the hepatectomies. Genomic analysis revealed deletions and amplifications in several genes, and clustering analysis based on CNV revealed five clusters. The LSP1 gene had the most cases with CNV (46 deletions and 5 amplifications). High frequencies of CNV were also seen in PTPRD (21/98), GNB1L (18/98), KIAA1217 (18/98), RP1-1777G6.2 (17/98), ETS1 (11/98), RSU1 (10/98), TBC1D22A (10/98), BAHCC1 (9/98), MAML2 (9/98), RAB1B (9/98), and YIF1A (9/98). The existing literature regarding hepatocytes or other cell types has connected many of these genes to regulation of cytoskeletal architecture, signaling cascades related to growth regulation, and transcription factors directly interacting with nuclear signaling complexes. Correlations with existing literature indicate that genomic lesions associated with HCC at the level of resolution of CNV occur on many genes associated directly or indirectly with signaling pathways operating in liver regeneration and hepatocyte growth regulation.

Essential role for Stat5a/b in myeloproliferative neoplasms induced by BCR-ABL1 and JAK2(V617F) in mice

STAT5 proteins are constitutively activated in malignant cells from many patients with leukemia, including the myeloproliferative neoplasms (MPNs) chronic myeloid leukemia (CML) and polycythemia vera (PV), but whether STAT5 is essential for the pathogenesis of these diseases is not known. In the present study, we used mice with a conditional null mutation in the Stat5a/b gene locus to determine the requirement for STAT5 in MPNs induced by BCR-ABL1 and JAK2(V617F) in retroviral transplantation models of CML and PV. Loss of one Stat5a/b allele resulted in a decrease in BCR-ABL1-induced CML-like MPN and the appearance of B-cell acute lymphoblastic leukemia, whereas complete deletion of Stat5a/b prevented the development of leukemia in primary recipients. However, BCR-ABL1 was expressed and active in Stat5-null leukemic stem cells, and Stat5 deletion did not prevent progression to lymphoid blast crisis or abolish established B-cell acute lymphoblastic leukemia. JAK2(V617F) failed to induce polycythemia in recipients after deletion of Stat5a/b, although the loss of STAT5 did not prevent the development of myelofibrosis. These results demonstrate that STAT5a/b is essential for the induction of CML-like leukemia by BCR-ABL1 and of polycythemia by JAK2(V617F), and validate STAT5a/b and the genes they regulate as targets for therapy in these MPNs.

Cell signals influencing hepatic fibrosis

Liver fibrosis is the result of the entire organism responding to a chronic injury. Every cell type in the liver contributes to the fibrosis. This paper first discusses key intracellular signaling pathways that are induced during liver fibrosis. The paper then examines the effects of these signaling pathways on the major cell types in the liver. This will provide insights into the molecular pathophysiology of liver fibrosis and should identify therapeutic targets.

Regulation of signal transduction and role of platelets in liver regeneration

Among all organs, the liver has a unique regeneration capability after sustaining injury or the loss of tissue that occurs mainly due to mitosis in the hepatocytes that are quiescent under normal conditions. Liver regeneration is induced through a cascade of various cytokines and growth factors, such as, tumor necrosis factor alpha, interleukin-6, hepatocyte growth factor, and insulin-like growth factor, which activate nuclear factor κB, signal transducer and activator of transcription 3, and phosphatidyl inositol 3-kinase signaling pathways. We previously reported that platelets can play important roles in liver regeneration through a direct effect on hepatocytes and collaborative effects with the nonparenchymal cells of the liver, including Kupffer cells and liver sinusoidal endothelial cells, which participate in liver regeneration through the production of various growth factors and cytokines. In this paper, the roles of platelets and nonparenchymal cells in liver regeneration, including the associated cytokines, growth factors, and signaling pathways, are described.

Liver Regeneration after Partial Hepatectomy Is Not Impaired in Mice with Double Deficiency of Myd88 and IFNAR Genes

Liver regeneration is known to occur in mice lacking one or more Toll-like receptors (TLRs) or the adaptor protein MyD88. Though MyD88 is required for signaling by many TLRs, others signal via MyD88-independent pathways, leading to the induction of type I interferons (IFNs). Here, we assessed liver regeneration after partial hepatectomy (PH) in mice lacking both MyD88 and the type I IFN receptor (Myd88-IFNAR double-KO). Approximately 28% of Myd88-IFNAR double-KO mice had gross liver lesions prior to surgery. In mice without lesions, Myd88-IFNAR deficiency abrogated the increase in circulating IL-6 after PH but did not impair hepatocyte BrdU incorporation, mitotic figure counts, or recovery of liver-to-body weight ratios. These results indicate that type I IFNs are not responsible for the preservation of liver regeneration in Myd88-deficient mice, and they also cast doubt on the idea of microbial products being essential triggers of liver regeneration in mice undergoing PH.

SOCS1 controls liver regeneration by regulating HGF signaling in hepatocytes

BACKGROUND & AIMS

Frequent repression of the Socs1 (suppressor of cytokine signaling 1) gene in hepatocellular carcinoma (HCC) and increased susceptibility of SOCS1-deficient mice to hepatocarcinogens suggest a tumor suppressor role for SOCS1 in the liver, but the underlying mechanisms remain unclear. Here we investigated the role of SOCS1 in regulating hepatocyte proliferation following partial hepatectomy and HGF stimulation.

METHODS

Because Socs1(-/-) mice die prematurely due to deregulated IFNγ signaling, we used Socs1(-/-)Ifng(-/-) mice to study the role of SOCS1 in liver regeneration following partial hepatectomy. We examined the activation of signaling molecules downstream of IL-6 and hepatocyte growth factor (HGF) receptors in the regenerating liver, primary hepatocytes, and in human hepatoma cells. We examined the interaction between SOCS1 and the HGF receptor c-Met by reciprocal immunoprecipitation.

RESULTS

Socs1(-/-)Ifng(-/-) mice displayed accelerated liver regeneration with increased DNA synthesis compared to Ifng(-/-) and wild type mice. The regenerating liver of Socs1(-/-)Ifng(-/-) mice did not show increased IL-6 signaling, but displayed earlier phosphorylation of Gab1, a signaling adaptor downstream of c-Met. Following HGF stimulation, hepatocytes from Socs1(-/-)Ifng(-/-) mice displayed increased phosphorylation of c-Met and Gab1, cell migration and proliferation. Accordingly, SOCS1 overexpression attenuated HGF-induced phosphorylation of c-Met, Gab1, and ERK1/2 in hepatoma cells, and decreased their proliferation and migration. SOCS1 interacted with the Tpr-Met, an oncogenic form of the Met receptor.

CONCLUSIONS

SOCS1 attenuates c-Met signaling and thus negative regulation of HGF signaling could be an important mechanism underlying the anti-tumor role of SOCS1 in the liver.

S6 kinase 1 is required for rapamycin-sensitive liver proliferation after mouse hepatectomy

Rapamycin is an antibiotic inhibiting eukaryotic cell growth and proliferation by acting on target of rapamycin (TOR) kinase. Mammalian TOR (mTOR) is thought to work through 2 independent complexes to regulate cell size and cell replication, and these 2 complexes show differential sensitivity to rapamycin. Here we combine functional genetics and pharmacological treatments to analyze rapamycin-sensitive mTOR substrates that are involved in cell proliferation and tissue regeneration after partial hepatectomy in mice. After hepatectomy, hepatocytes proliferated rapidly, correlating with increased S6 kinase phosphorylation, while treatment with rapamycin derivatives impaired regeneration and blocked S6 kinase activation. In addition, genetic deletion of S6 kinase 1 (S6K1) caused a delay in S phase entry in hepatocytes after hepatectomy. The proliferative defect of S6K1-deficient hepatocytes was cell autonomous, as it was also observed in primary cultures and hepatic overexpression of S6K1-rescued proliferation. We found that S6K1 controlled steady-state levels of cyclin D1 (Ccnd1) mRNA in liver, and cyclin D1 expression was required to promote hepatocyte cell cycle. Notably, in vivo overexpression of cyclin D1 was sufficient to restore the proliferative capacity of S6K-null livers. The identification of an S6K1-dependent mechanism participating in cell proliferation in vivo may be relevant for cancer cells displaying high mTOR complex 1 activity and cyclin D1 accumulation.

Receptor for hyaluronan-mediated motility isoform B promotes liver metastasis in a mouse model of multistep tumorigenesis and a tail vein assay for metastasis

The gene encoding the receptor for hyaluronan-mediated motility (RHAMM) is overexpressed in many human cancers. However, it is unclear whether RHAMM plays a causal role in tumor initiation or progression. Using somatic gene transfer in a mouse model of islet cell tumorigenesis, we demonstrate that RHAMM isoform B (RHAMM(B)) promotes tumor growth and metastases to lymph nodes and the liver. The propensity of RHAMM(B)-expressing cells to metastasize to the liver was confirmed using an experimental metastasis assay in which cells were injected into the tail vein of immunodeficient mice. However, RHAMM(B) did not increase cell migration or proliferation in culture. In initial efforts to identify signaling pathways activated by RHAMM(B), we found that RHAMM(B) induced phosphorylation of epidermal growth factor receptor (EGFR), Erk1/2, and STAT3 and conferred susceptibility to apoptosis after treatment with an EGFR inhibitor, gefitinib. Taken together, the results indicate that RHAMM(B) promotes hepatic metastasis by islet tumor cells, perhaps through growth factor receptor-mediated signaling.

Toll-like receptor 4 and myeloid differentiation factor 88 provide mechanistic insights into the cause and effects of interleukin-6 activation in mouse liver regeneration

Partial hepatectomy (PH) consistently results in an early increase of circulating interleukin-6 (IL-6), which is thought to play a major role in liver regeneration. Activation of this cytokine after PH requires the adaptor protein, MyD88, but the specific MyD88-related receptors involved remain unidentified. It is also unknown whether the magnitude of IL-6 elevation determines the extent of subsequent hepatocyte proliferation. Here, we uncovered artifacts in the assessment of circulating IL-6 levels when using cardiac puncture in mice after PH. By using retro-orbital bleed sampling, we show that the circulating levels of IL-6 after PH were not directly correlated with the extent of hepatocyte DNA synthesis in individual mice. The IL-6 increase after PH was attenuated in all lipopolysaccharide-hyporesponsive mouse strains studied (e.g., C3H/HeJ, Tlr4 null, Cd14 null, Tlr2,4,9 null, and Tlr2,4-Caspase1 null) and was severely abrogated in Myd88 null mice. Despite attenuated IL-6 levels, Tlr4 null mice showed normal signaling downstream of IL-6 and normal hepatocyte proliferation. In contrast, Myd88 null mice showed severe impairments in signal transducer and activator of transcription 3 phosphorylation and Socs3 induction, but had enhanced and prolonged extracellular signal-related kinase 1 and 2 phosphorylation in the first 6 hours after PH. Unexpectedly, these changes were associated with accelerated initiation of hepatocyte proliferation, as assessed by hepatocyte bromodeoxyuridine incorporation, phospho-histone H3 immunostaining, and cyclin E and A protein expression.

CONCLUSION

TLR-4 signaling contributes to IL-6 activation after PH, but the Tlr4-independent component appears sufficient for ensuring intact signaling downstream of IL-6. The lack of correlation between IL-6 levels and hepatocyte proliferation after PH, and the accelerated start of hepatocyte proliferation in Myd88 null mice despite abrogated cytokine activation, may highlight relevant antiproliferative effects of IL-6 signaling, possibly via Socs3, in the regulation of liver regeneration.

Hierarchies of transcriptional regulation during liver regeneration

The remarkable capacity of the liver to regenerate after severe injury or disease has excited interest for centuries. The goal of harnessing this process in treatment of liver disease, and the appreciation of the parallels between regeneration and tumor development in the liver, remain a major driver for research in this area. Studies of liver regeneration as a model system offer a view of intricate and precisely timed regulatory pathways that drive the process toward completion. Successful regeneration of the liver mass demands a hierarchal and well-controlled balance between proliferative and metabolic functions, which is orchestrated by signaling and regulation of transcription factors. Control and regulation of these cascades of transcriptional activities, necessary for induction, renewal, and cessation of liver growth, are the focus of this chapter.

Enhanced liver regeneration in IL-10-deficient mice after partial hepatectomy via stimulating inflammatory response and activating hepatocyte STAT3

Emerging evidence suggests that proinflammatory cytokines, including tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), play a critical role in the initiation and progression of liver regeneration; however, relatively little is known about the role of anti-inflammatory cytokine IL-10 in liver regeneration after partial hepatectomy (PHx). Here, we examined the role of IL-10 in liver regeneration using a model of PHx in several strains of genetically modified mice. After PHx, expression of IL-10 mRNA in the liver and spleen was significantly elevated. Such elevation was diminished in TLR4 mutant mice. Compared with wild-type mice, IL-10(-/-) mice had higher levels of expression of proinflammatory cytokines (IL-6, TNF-α, and IFN-γ) and inflammatory markers (CCR2 and F4/80) in the liver, as well as higher serum levels of proinflammatory cytokines after PHx. The number of neutrophils and macrophages was also higher in the livers of IL-10(-/-) mice than in wild-type mice after PHx. Liver regeneration as determined by BrdU incorporation after PHx was higher in IL-10(-/-) mice than in wild-type mice, which was associated with higher levels of activation of IL-6 downstream signal STAT3 in the liver. An additional deletion of STAT3 in hepatocytes significantly reduced liver regeneration in IL-10(-/-) mice after PHx. Collectively, IL-10 plays an important role in negatively regulating liver regeneration via limiting inflammatory response and subsequently tempering hepatic STAT3 activation.

Molecular mechanisms of liver regeneration and protection for treatment of liver dysfunction and diseases

Liver regeneration is a necessary process that most liver damage depends on for recovery. Regeneration is achieved by a complex interactive network consisting of liver cells (hepatocytes, Kupffer cells, sinusoidal endothelial cells, hepatic stellate cells, and stem cells) and extrahepatic organs (thyroid gland, adrenal gland, pancreas, duodenum, and autonomous nervous system). The restoration of liver volume depends on hepatocyte proliferation, which includes initiation, proliferation, and termination phases. Hepatocytes are "primed" mainly by Kupffer cells via cytokines (IL-6 and TNF-alpha) and then "proliferation" and "cell growth" of hepatocytes are induced by the stimulations of cytokines and growth factors (HGF and TGF-alpha). Liver regeneration is achieved by cell proliferation and cell growth, where the IL-6/STAT3 and PI3-K/PDK1/Akt pathways play pivotal roles, respectively. IL-6/STAT3 pathway regulates hepatocyte proliferation via cyclin D1/p21 and protects against cell death by upregulating FLIP, Bcl-2, Bcl-xL, Ref1, and MnSOD. PI3-K/PDK1/Akt is known to be responsible for regulation of cell size via its downstream molecules such as mTOR in addition to being known for its survival, anti-apoptotic and anti-oxidative properties. Although the molecular mechanisms of liver regeneration have been actively studied, the mechanisms of liver regeneration must be elucidated and leveraged for the sufficient treatment of liver diseases.

Toll-like receptors in acute liver injury and regeneration

Liver is the lymphoid organ with an overwhelming innate immune system, which functions as a filter organ at the first line between the digestive tract and the rest of the body, with receiving 80% of the blood supply through portal vein. TLRs are widely expressed on parenchymal and non-parenchymal cells in the liver, which play critical roles for the liver health. Recent studies indicate that TLR-medicated signals have been involved in almost all liver diseases such as acute and chronic hepatitis, liver fibrosis and cirrhosis, alcoholic and non-alcoholic liver disease, ischemia/reperfusion liver injury, liver regeneration and hepatocellular carcinoma. In this review, the expressions of TLRs in hepatic cell populations including hepatocytes, LSECs, Kupffer cells, lymphocytes, DCs, biliary epithelial cells and HSCs, and TLR ligands and signaling in the liver are summarized. Further, recent advances in the roles of TLRs in acute liver injury and regeneration as mediator and regulator, and their potential therapeutic targets are discussed.

Loss of transforming growth factor β adaptor protein β-2 spectrin leads to delayed liver regeneration in mice

Liver regeneration, following partial hepatectomy (PHx), occurs through precisely controlled and synchronized cell proliferation, in which quiescent hepatocytes undergo one to two rounds of replication, with restoration of liver mass and function. We previously demonstrated that loss of the Smad3/4 adaptor protein β-2 spectrin (β2SP) is associated with faster entry into S phase, and hepatocellular cancer formation. These observations led us to further pursue the role of β2SP in cell cycle progression in vivo. Liver regeneration studies with PHx in β2SP(+/-) mice reveal a surprising and significant decrease in liver/body weight ratio at 48 hours after PHx in β2SP(+/-) mice in comparison to wildtype mice. At 48 hours after PHx we also observe decreased levels of cyclin E (2.4-fold, P < 0.05), Cdk1 (7.2-fold, P < 0.05), cyclin A, pRb (Ser249/Thr252), proliferative cell nuclear antigen (PCNA), cyclin D1 with elevated levels of pCdk1 (Thr14) (3.6-fold, P < 0.05). Strikingly, at 24 hours elevated levels of p53 (4-fold, P < 0.05), phospho-p53 (ser15 and ser20), and p21 (200-fold, P < 0.05) persisting to 48 hours after PHx further correlated with raised expression of the DNA damage markers pChk2 (Thr68) and γH2AX (S139). However, compromised cell cycle progression with loss of β2SP is not rescued by inhibiting p53 function, and that G(2) /M phase arrest observed is independent and upstream of p53.

CONCLUSION

β2SP deficiency results in dysfunctional hepatocyte cell cycle progression and delayed liver regeneration at 48 hours after PHx, which is p53-independent. β2SP loss may increase susceptibility to DNA damage, impair cell cycle progression, and ultimately lead to hepatocellular cancer.

Signal transducer and activator of transcription 3 in liver diseases: a novel therapeutic target

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is activated by many cytokines and growth factors and plays a key role in cell survival, proliferation, and differentiation. STAT3 activation is detected virtually in all rodent models of liver injury and in human liver diseases. In this review, we highlight recent advances of STAT3 signaling in liver injury, steatosis, inflammation, regeneration, fibrosis, and hepatocarcinogenesis. The cytokines and small molecules that activate STAT3 in hepatocytes may have therapeutic benefits to treat acute liver injury, fatty liver disease, and alcoholic hepatitis, while blockage of STAT3 may have a therapeutic potential to prevent and treat liver cancer.

Involvement of STAT3-regulated hepatic soluble factors in attenuation of stellate cell activity and liver fibrogenesis in mice

Glycoprotein 130 (gp130)/signal transducer and activator of transcription 3 (STAT3) signaling in hepatocytes controls a variety of physiological and pathological processes including liver regeneration, apoptosis resistance and metabolism. Recent research has shed light on the importance of acute phase proteins (APPs) regulated by hepatic gp130/STAT3 in host defense through suppression of innate immune responses during systemic inflammation. To examine whether these STAT3-regulated soluble factors directly affect liver fibrogenic responses during liver injury, hepatocyte-specific STAT3 knockout (L-STAT3 KO) mice and control littermates were subjected to bile duct ligation (BDL) and examined 10 days later. In contrast to controls, L-STAT3 KO mice failed to produce APPs, such as serum amyloid A and haptoglobin, after BDL. Whereas L-STAT3 KO mice displayed similar levels of cholestasis, inflammatory cell infiltration and regeneration in the liver, they developed exacerbated liver injury and fibrosis with significant increases in expression of alpha-smooth muscle actin and type I collagen genes. In vitro experiments revealed that attenuated expression of APPs in primary hepatocytes isolated from L-STAT3 KO mice with IL-6 exposure, compared to wild-type hepatocytes. The cultured supernatant from IL-6-treated wild-type hepatocytes inhibited expression of alpha-smooth muscle actin and type I collagen genes in activated hepatic stellate cells (HSCs), whereas this did not occur with the supernatant from IL-6-treated knockout hepatocytes or with control medium. In conclusion, the absence of STAT3 in hepatocytes leads to exacerbation of liver fibrosis during cholestasis. Soluble factors released from hepatocytes, dependent on STAT3, collectively play a protective role in liver fibrogenesis through an inhibitory effect on activated HSCs.

Advances in the regulation of liver regeneration

Liver regeneration is known to be a process involving highly organized and ordered tissue growth triggered by the loss of liver tissue, and remains a fascinating topic. A large number of genes are involved in this process, and there exists a sequence of stages that results in liver regeneration, while at the same time inhibitors control the size of the regenerated liver. The initiation step is characterized by priming of quiescent hepatocytes by factors such as TNF-α, IL-6 and nitric oxide. The proliferation step is the step during which hepatocytes enter into the cell cycle's G1 phase and are stimulated by complete mitogens including HGF, TGF-α and EGF. Hepatic stimulator substance, glucagon, insulin, TNF-α, IL-1 and IL-6 have also been implicated in regulating the regeneration process. Inhibitors and stop signals of hepatic regeneration are not well known and only limited information is available. Furthermore, the effects of other factors such as VEGF, PDGF, hypothyroidism, proliferating cell nuclear antigen, heat shock proteins, ischemic-reperfusion injury, steatosis and granulocyte colony-stimulating factor on liver regeneration are also systematically reviewed in this article. A tissue engineering approach using isolated hepatocytes for in vitro tissue generation and heterotopic transplantation of liver cells has been established. The use of stem cells might also be very attractive to overcome the limitation of donor liver tissue. Liver-specific differentiation of embryonic, fetal or adult stem cells is currently under investigation.

Effect of sorafenib on murine liver regeneration

Hepatocellular carcinoma (HCC) is a common cause of cancer-related death. Sorafenib prolongs survival of patients with advanced disease and is approved for the systemic treatment of unresectable HCC. It possesses antiangiogenic and antiproliferative properties by way of inhibition of the receptor tyrosine kinases vascular endothelial growth factor receptor 2 (VEGFR-2) and platelet-derived growth factor receptor-beta 1/2 (PDGFR-β) and the kinase RAF. Sorafenib represents a candidate compound for adjuvant therapy in HCC patients. The aim of our study was to investigate whether sorafenib affects liver regeneration. C57BL6 mice received sorafenib orally at 30 mg/kg/day or its vehicle either for 14 days until the day before hepatectomy or starting the day after surgery or both. Animals were sacrificed 24, 72, and 120 hours after hepatectomy. Liver regeneration was calculated as a percent of initial liver weight. Bromodeoxyuridine (BrdU) incorporation and phospho-extracellular signal-regulated kinase (pERK1/2) were determined by immunohistochemistry on liver sections. VEGF-A, PDGF-BB, and hepatocyte growth factor (HGF) levels were measured in liver tissue homogenates. Histological analysis of scar tissue was performed. Treatment stopped 1 day before surgery had no impact on liver regeneration. Continuous sorafenib treatment and treatment started 1 day after surgery had statistically significant effects on liver regeneration at 120 hours compared to vehicle-treated control animals (72% ± 12 versus control 88% ± 15 and 70% ± 13 versus control 86% ± 5 at 120 hours, both P ≤ 0.02). BrdU incorporation showed decreased numbers of positive nuclei in both groups receiving sorafenib after surgery. Phospho-ERK levels were reduced in sorafenib-treated animals. An increase of VEGF-A levels was observed in mice receiving sorafenib. Wound-healing complications were observed in animals receiving sorafenib after surgery and confirmed on histological sections.

CONCLUSION

This preclinical study shows that sorafenib did not impact on liver regeneration when ceased before surgery; however, administration after hepatectomy affected late liver regeneration.

Suppression of signal transducer and activator of transcription 3 activation by butein inhibits growth of human hepatocellular carcinoma in vivo

PURPOSE

Hepatocellular carcinoma (HCC) is the fifth most common malignancy worldwide and the third cause of global cancer mortality. Increasing evidence suggest that STAT3 is a critical mediator of oncogenic signaling in HCC and controls the expression of several genes involved in proliferation, survival, metastasis, and angiogenesis. Thus, the novel agents that can suppress STAT3 activation have potential for both prevention and treatment of HCC.

EXPERIMENTAL DESIGN

The effect of butein on STAT3 activation, associated protein kinases, STAT3-regulated gene products, cellular proliferation, and apoptosis was investigated. The in vivo effect of butein on the growth of human HCC xenograft tumors in male athymic nu/nu mice was also examined.

RESULTS

We tested an agent, butein, for its ability to suppress STAT3 activation in HCC cells and nude mice model along with prospectively testing the hypothesis of STAT3 inhibition in a virtual predictive functional proteomics tumor pathway technology platform. We found that butein inhibited both constitutive and inducible STAT3 activation in HCC cells. The suppression was mediated through the inhibition of activation of upstream kinases c-Src and Janus-activated kinase 2. Butein inhibited proliferation and significantly potentiated the apoptotic effects of paclitaxel and doxorubicin in HCC cells. When administered intraperitoneally, butein inhibited the growth of human HCC xenograft tumors in male athymic nu/nu mice.

CONCLUSIONS

Overall, cumulative results from experimental and predictive studies suggest that butein exerts its antiproliferative and proapoptotic effects through suppression of STAT3 signaling in HCC both in vitro and in vivo.

Liver-specific Ldb1 deletion results in enhanced liver cancer development

BACKGROUND & AIMS

LIM-domain-binding (Ldb) proteins have been demonstrated to be essential not only to key embryonic developmental processes but also to carcinogenesis. We have previously demonstrated Ldb1 to be of high biological and developmental relevance, as a targeted deletion of the Ldb1 gene in mice results in an embryonic lethal and pleiotropic phenotype.

METHODS

We have now established a liver-specific Ldb1 knock out to investigate the role of Ldb1 in carcinogenesis, in particular in hepatocellular carcinoma (HCC) development, in vivo.

RESULTS

These mice demonstrated a significantly enhanced growth of liver cancer by means of tumor size and number, advocating for an essential role of Ldb1 in HCC development. In addition, proliferation and resistance against apoptosis were increased. In order to identify the functional disturbances due to a lack of Ldb1, we performed a 15k mouse gene microarray expression analysis. We found the Myc oncogene to be regulated in the microarray analysis and were able to further confirm this regulation by demonstrating an over-expression of its downstream target Cyclin D1. Furthermore, we were able to demonstrate a down-regulation of the tumor suppressor p21. Finally, the liver stem cell marker EpCAM was also identified to be over expressed in Ldb1(-/-) knock out mice.

CONCLUSIONS

We have established a significant role of Ldb1 in cancer development. Furthermore, we provided evidence for a myc/cyclin D1, p21, and EpCAM-dependent signalling to be key downstream regulators of this novel concept in HCC development.

Liver regeneration is impaired in lipodystrophic fatty liver dystrophy mice

We previously reported that mice subjected to partial hepatectomy exhibit rapid development of hypoglycemia followed by transient accumulation of fat in the early regenerating liver. We also showed that disrupting these metabolic alterations results in impaired liver regeneration. The studies reported here were undertaken to further characterize and investigate the functional importance of changes in systemic adipose metabolism during normal liver regeneration. The results showed that a systemic catabolic response is induced in each of two distinct, commonly used experimental models of liver regeneration (partial hepatectomy and carbon tetrachloride treatment), and that this response occurs in proportion to the degree of induced hepatic insufficiency. Together, these observations suggest that catabolism of systemic adipose stores may be essential for normal liver regeneration. To test this possibility, we investigated the hepatic regenerative response in fatty liver dystrophy (fld) mice, which exhibit partial lipodystrophy and have diminished peripheral adipose stores. The results showed that the development of hypoglycemia and hepatic accumulation of fat was attenuated and liver regeneration was impaired following partial hepatectomy in these animals. The fld mice also exhibited increased hepatic p21 expression and diminished plasma levels of the adipose-derived hormones adiponectin and leptin, which have each been implicated as regulators of liver regeneration.

CONCLUSION

These data suggest that the hypoglycemia that develops after partial hepatectomy induces systemic lipolysis followed by accumulation of fat derived from peripheral stores in the early regenerating liver, and that these events may be essential for initiation of normal liver regeneration.

Regulation of liver regeneration by growth factors and cytokines

The capability of the liver to fully regenerate after injury is a unique phenomenon essential for the maintenance of its important functions in the control of metabolism and xenobiotic detoxification. The regeneration process is histologically well described, but the genes that orchestrate liver regeneration have been only partially characterized. Of particular interest are cytokines and growth factors, which control different phases of liver regeneration. Historically, their potential functions in this process were addressed by analyzing their expression in the regenerating liver of rodents. Some of the predicted roles were confirmed using functional studies, including systemic delivery of recombinant growth factors, neutralizing antibodies or siRNAs prior to liver injury or during liver regeneration. In particular, the availability of genetically modified mice and their use in liver regeneration studies has unraveled novel and often unexpected functions of growth factors, cytokines and their downstream signalling targets in liver regeneration. This review summarizes the results obtained by functional studies that have addressed the roles and mechanisms of action of growth factors and cytokines in liver regeneration after acute injury to this organ.

Impact of cell cycle disruption on impaired hepatic regeneration in aged livers with ischemic insult

BACKGROUND

The number of elderly patients with hepatobiliary malignancies has increased with the steady growth of elderly population. However, the safety of major hepatectomy for elderly patients remains controversial. This study investigated the effect of aging on the hepatic regenerative response after partial resection of livers subjected to ischemic insult.

METHODS

Two-thirds hepatectomy following 1-h hepatic ischemia was performed in young (12-wk-old) and old (18-mo-old) rats under portosystemic shunt establishment by subcutaneous transposition of the spleen.

RESULTS

The survival rate 48 h after hepatectomy of the old rats was significantly lower (20%) than that of the young rats (53%), whereas all animals without hepatic ischemia were alive at 48 h. Hepatic necrosis and hepatocyte apoptosis during the early post-hepatectomy phase were more severe in the aged livers, which also showed delayed Akt activation. Liver mass restoration was significantly retarded in the old rats, despite higher plasma IL-6 levels, rapid and prolonged activation of hepatic STAT3, and increased hepatocyte nuclear cyclin D1 levels. In the young livers, cyclin E, which is essential for G1/S transition, and cyclin A, a marker of S phase, were observed in the nucleus from 24 h, reaching peaks 48 h after hepatectomy. In contrast, the old livers showed greatly delayed and decreased nuclear cyclin E and cyclin A levels.

CONCLUSION

Age-related reductions in the regenerative ability of ischemically damaged livers may be caused by cell cycle disruption at either the late G1 phase or the G1/S transition, despite increased cyclin D1 levels and compensatory IL-6/STAT3 activation.

Signal transducer and activator of transcription 3 protects from liver injury and fibrosis in a mouse model of sclerosing cholangitis

BACKGROUND & AIMS

Signal transducer and activator of transcription 3 (Stat3) is the main mediator of interleukin-6-type cytokine signaling required for hepatocyte proliferation and hepatoprotection, but its role in sclerosing cholangitis and other cholestatic liver diseases remains unresolved.

METHODS

We investigated the role of Stat3 in inflammation-induced cholestatic liver injury and used mice lacking the multidrug resistance gene 2 (mdr2(-/-)) as a model for SC.

RESULTS

We show that conditional inactivation of Stat3 in hepatocytes and cholangiocytes (stat3(Deltahc)) of mdr2(-/-) mice strongly aggravated bile acid-induced liver injury and fibrosis. A similar phenotype was observed in mdr2(-/-) mice lacking interleukin-6 production. Biochemical and molecular characterization suggested that Stat3 exerts hepatoprotective functions in both hepatocytes and cholangiocytes. Loss of Stat3 led to increased expression of tumor necrosis factor alpha, which might reduce the barrier function of bile ducts. Moreover, Stat3-deficient hepatocytes displayed up-regulation of bile acid biosynthesis genes and down-regulation of hepatoprotective epidermal growth factor receptor and insulin-like growth factor 1 signaling pathways. Consistently, stat3(Deltahc) mice were more sensitive to cholic acid-induced liver damage than control mice.

CONCLUSIONS

Our data suggest that Stat3 prevents cholestasis and liver damage in sclerosing cholangitis via regulation of pivotal functions in hepatocytes and cholangiocytes.

Down-regulation of the LXR transcriptome provides the requisite cholesterol levels to proliferating hepatocytes

Cholesterol homeostasis is critical for cellular proliferation. Liver X receptor (LXR) alpha and beta are the nuclear receptors responsible for regulation of cholesterol metabolism. In physiological conditions, high intracellular cholesterol levels cause increased synthesis of oxysterols, which activate LXR, thus triggering a transcriptional response for cholesterol secretion and catabolism. Here we employed a mouse model of partial hepatectomy (PH) to dissect the molecular pathways connecting cholesterol homeostasis, cellular proliferation, and LXR. First, we show that hepatic cholesterol content increases after PH, whereas the entire LXR transcriptome is down-regulated. Although LXR messenger RNA (mRNA) levels are unmodified, LXR target genes are significantly down-regulated on day 1 after PH and restored to control levels on day 7, when the liver reaches normal size. The inactivation of LXR following PH is related to the reduced oxysterol availability by way of decreased synthesis, and increased sulfation and secretion. On the contrary, cholesterol synthesis is up-regulated, and extracellular matrix remodeling is enhanced. Second, we show that reactivation of LXR by way of a synthetic ligand determines a negative modulation of hepatocyte proliferation. This effect is sustained by the reactivation of hepatic cholesterol catabolic and secretory pathways, coupled with a significant reduction of cholesterol biosynthesis. Our data unveil a previously unrecognized and apparently paradoxical scenario of LXR modulation. During liver regeneration LXR activity is abated in spite of increasing intracellular cholesterol levels. Turning off LXR-transcriptional pathways is crucial to guaranteeing the requisite intracellular cholesterol levels of regenerating hepatocytes. In line with this hypothesis, pharmacological LXR reactivation during PH significantly reduces liver regeneration capacity.

Interplay of hepatic and myeloid signal transducer and activator of transcription 3 in facilitating liver regeneration via tempering innate immunity

Liver regeneration triggered by two-thirds partial hepatectomy is accompanied by elevated hepatic levels of endotoxin, which contributes to the regenerative process, but liver inflammation and apoptosis remain paradoxically limited. Here, we show that signal transducer and activator of transcription 3 (STAT3), an important anti-inflammatory signal, is activated in myeloid cells after partial hepatectomy and its conditional deletion results in an enhanced inflammatory response. Surprisingly, this is accompanied by an improved rather than impaired regenerative response with increased hepatic STAT3 activation, which may contribute to the enhanced liver regeneration. Indeed, conditional deletion of STAT3 in both hepatocytes and myeloid cells results in elevated activation of STAT1 and apoptosis of hepatocytes, and a dramatic reduction in survival after partial hepatectomy, whereas additional global deletion of STAT1 protects against these effects.

CONCLUSION

An interplay of myeloid and hepatic STAT3 signaling is essential to prevent liver failure during liver regeneration through tempering a strong innate inflammatory response mediated by STAT1 signaling.

Cascades of transcription regulation during liver regeneration

An increasing demand for new strategies in cancer prevention and regenerative medicine requires a better understanding of molecular mechanisms that control cell proliferation in tissue-specific manner. Regenerating liver is a unique model allowing use of biochemical, genetic, and engineering tools to uncover molecular mechanisms and improve treatment of hepatic cancers, liver failure, and fibrotic disease. Molecular mechanisms of liver regeneration involve extra- and intracellular factors to activate transcription of genes normally silenced in quiescent liver. While many upstream signaling pathways of the regenerating liver have been extensively studied, our knowledge of the downstream effectors, transcription factors (TFs), remains limited. This review describes consecutive engagement of pre-existing and de novo synthesized TFs, as cascades that regulate expression of growth-related and metabolic genes during liver regeneration after partial hepatectomy in mice. Several previously recognized regulators of regenerating liver are described in the light of recently identified co-activator and co-repressor complexes that interact with primary DNA-binding TFs. Published results of gene expression and chromatin immunoprecipitation analyses, as well as studies of transgenic mouse models, are used to emphasize new potential regulators of transcription during liver regeneration. Finally, a more detailed description of newly identified transcriptional regulators of liver regeneration illustrates the tightly regulated balance of proliferative and metabolic responses to partial hepatectomy.

Mechanisms of acetaminophen-induced liver necrosis

Although considered safe at therapeutic doses, at higher doses, acetaminophen produces a centrilobular hepatic necrosis that can be fatal. Acetaminophen poisoning accounts for approximately one-half of all cases of acute liver failure in the United States and Great Britain today. The mechanism occurs by a complex sequence of events. These events include: (1) CYP metabolism to a reactive metabolite which depletes glutathione and covalently binds to proteins; (2) loss of glutathione with an increased formation of reactive oxygen and nitrogen species in hepatocytes undergoing necrotic changes; (3) increased oxidative stress, associated with alterations in calcium homeostasis and initiation of signal transduction responses, causing mitochondrial permeability transition; (4) mitochondrial permeability transition occurring with additional oxidative stress, loss of mitochondrial membrane potential, and loss of the ability of the mitochondria to synthesize ATP; and (5) loss of ATP which leads to necrosis. Associated with these essential events there appear to be a number of inflammatory mediators such as certain cytokines and chemokines that can modify the toxicity. Some have been shown to alter oxidative stress, but the relationship of these modulators to other critical mechanistic events has not been well delineated. In addition, existing data support the involvement of cytokines, chemokines, and growth factors in the initiation of regenerative processes leading to the reestablishment of hepatic structure and function.

Lyn-mediated mitochondrial tyrosine phosphorylation is required to preserve mitochondrial integrity in early liver regeneration

Functional alterations in mitochondria such as overproduction of ROS (reactive oxygen species) and overloading of calcium, with subsequent change in the membrane potential, are traditionally regarded as pro-apoptotic conditions. Although such events occur in the early phases of LR (liver regeneration) after two-thirds PH (partial hepatectomy), hepatocytes do not undergo apoptosis but continue to proliferate until the mass of the liver is restored. The aim of the present study was to establish whether tyrosine phosphorylation, an emerging mechanism of regulation of mitochondrial function, participates in the response to liver injury following PH and is involved in contrasting mitochondrial pro-apoptotic signalling. Mitochondrial tyrosine phosphorylation, negligible in the quiescent liver, was detected in the early phases of LR with a trend similar to the events heralding mitochondrial apoptosis and was attributed to the tyrosine kinase Lyn, a member of the Src family. Lyn was shown to accumulate in an active form in the mitochondrial intermembrane space, where it was found to be associated with a multiprotein complex. Our results highlight a role for tyrosine phosphorylation in accompanying, and ultimately counteracting, mitochondrial events otherwise leading to apoptosis, hence conveying information required to preserve the mitochondrial integrity during LR.