Dose- and Time-Dependent Effects of Luteolin on Liver Metallothioneins and Metals in Carbon Tetrachloride-Induced Hepatotoxicity in Mice

The aim of this study was to investigate the protective effect of luteolin on liver Ca, Mg, Zn, Cu, Fe, and Mn content in mice with carbon tetrachloride (CCl4)-induced hepatotoxicity. Additionally, liver metallothionein (MT) expression was studied. Luteolin was administered intraperitoneally (i.p.) as a single 5- or 50-mg/kg dose or once daily for two consecutive days, respectively. Two hours after the last injection, the mice were treated with CCl4 (20 mg/kg, i.p.). CCl4 injection reduced hepatic level of all metals except Ca, with an intense cytoplasmic staining pattern in hepatocytes located in periportal areas, indicating induction of MTs. Pretreatment with 50 mg/kg of luteolin for 2 days remarkably elevated metal content to control values (Mg and Cu) or even above them (Zn and Fe). Luteolin pretreatment increased pericentral MTs immunopositivity and histological architecture improvement in a time- and dose-dependent manner, being the most prominent in mice pretreated with 50 mg/kg for 2 days. The liver in this group showed pronounced MT expression in almost all hepatocytes throughout the liver parenchyma. In conclusion, these results suggest the protective effect of luteolin on CCl4-induced hepatotoxicity and an enhancement of hepatocyte proliferative capabilities.     

Sphingosine 1-phosphate regulates regeneration and fibrosis after liver injury via sphingosine 1-phosphate receptor 2

Sphingosine 1-phosphate (S1P), a bioactive lipid mediator, stimulates proliferation and contractility in hepatic stellate cells, the principal matrix-producing cells in the liver, and inhibits proliferation via S1P receptor 2 (S1P(2)) in hepatocytes in rats in vitro. A potential role of S1P and S1P(2) in liver regeneration and fibrosis was examined in S1P(2)-deficient mice. Nuclear 5-bromo-2'-deoxy-uridine labeling, proliferating cell nuclear antigen (PCNA) staining in hepatocytes, and the ratio of liver weight to body weight were enhanced at 48 h in S1P(2)-deficient mice after a single carbon tetrachloride (CCl(4)) injection. After dimethylnitrosamine (DMN) administration with a lethal dose, PCNA staining in hepatocytes was enhanced at 48 h and survival rate was higher in S1P(2)-deficient mice. Serum aminotransferase level was unaltered in those mice compared with wild-type mice in both CCl(4)- and DMN-induced liver injury, suggesting that S1P(2) inactivation accelerated regeneration not as a response to enhanced liver damage. After chronic CCl(4) administration, fibrosis was less apparent, with reduced expression of smooth-muscle alpha-actin-positive cells in the livers of S1P(2)-deficient mice, suggesting that S1P(2) inactivation ameliorated CCl(4)-induced fibrosis due to the decreased accumulation of hepatic stellate cells. Thus, S1P plays a significant role in regeneration and fibrosis after liver injury via S1P(2).     

Early growth response (EGR)-1 is required for timely cell-cycle entry and progression in hepatocytes after acute carbon tetrachloride exposure in mice

Cell-cycle induction in hepatocytes protects from prolonged tissue damage after toxic liver injury. Early growth response (Egr)-1(-/-) mice exhibit increased liver injury after carbon tetrachloride (CCl(4)) exposure and reduced TNF-α production. Because TNF-α is required for prompt cell-cycle induction after liver injury, here, we tested the hypothesis that Egr-1 is required for timely hepatocyte entry into the cell cycle after CCl(4)-induced liver injury. Acute liver injury was induced by a single injection of CCl(4). Assays were employed to assess indices of the cell cycle in liver after CCl(4) exposure. Bromodeoxyuridine incorporation peaked in wild-type mice at 48 h after CCl(4) but was reduced by 80% in Egr-1(-/-) mice. Proliferating-cell nuclear-antigen immunohistochemistry revealed blocks in cell-cycle entry and progression to DNA synthesis in Egr-1-deficient mice 48 h after CCl(4). Cyclin D, important for G0/G1 progression, was reduced at baseline and 36 h after CCl(4). Cyclin E1, required for G1/S-phase transition, was reduced in Egr-1(-/-) mice 24 and 48 h after CCl(4) exposure and was associated with reduced phosphorylation of the retinoblastoma protein. Proliferation in Egr-1(-/-) mice was delayed, rather than blocked, because indices of cell-cycle progression were restored 72 h after CCl(4) exposure. We concluded that Egr-1 was required for prompt cell-cycle entry (G0- to G1-phase) and G1/S-phase transition after toxic liver injury. These data support the hypothesis that Egr-1 provides hepatoprotection in the CCl(4)-injured liver, attributable, in part, to timely cell-cycle induction and progression.     

C3a and C3b activation products of the third component of complement (C3) are critical for normal liver recovery after toxic injury

Although the complement system has been implicated in liver regeneration after toxic injury and partial hepatectomy, the mechanism or mechanisms through which it participates in these processes remains ill-defined. In this study, we demonstrate that complement activation products (C3a, C3b/iC3b) are generated in the serum of experimental mice after CCl(4) injection and that complement activation is required for normal liver regeneration. Decomplementation by cobra venom factor resulted in impaired entry of hepatocytes into S phase of the cell cycle. In addition, livers from C3-deficient (C3(-/-)) mice showed similarly impaired proliferation of hepatocytes, along with delayed kinetics of both hepatocyte hyperplasia and removal of injured liver parenchyma. Restoration of hepatocyte proliferative capabilities of C3(-/-) mice through C3a reconstitution, as well as the impaired regeneration of C3a receptor-deficient mice, demonstrated that C3a promotes liver cell proliferation via the C3a receptor. These findings, together with data showing two waves of complement activation, indicate that C3 activation is a pivotal mechanism for liver regeneration after CCl(4) injury, which fulfills multiple roles; C3a generated early after toxin injection is relevant during the priming of hepatocytes, whereas C3 activation at later times after CCl(4) treatment contributes to the clearance of injured tissue.     

Suppression of acute hepatic injury by a synthetic prostacyclin agonist through hepatocyte growth factor expression

Previous studies have demonstrated that mice disrupted with the cyclooxygenase-2 gene showed much more severe liver damage compared with wild-type mice after liver injury, and prostaglandins (PGs) such as PGE(1/2) and PGI(2) have decreased hepatic injury, but the mechanisms by which prostaglandins exhibit protective action on the liver have yet to be addressed. In the present study, we investigated the mechanism of the protective action of PGI(2) using the synthetic IP receptor agonist ONO-1301. In primary cultures of hepatocytes and nonparenchymal liver cells, ONO-1301 did not show protective action directly on hepatocytes, whereas it stimulated expression of hepatocyte growth factor (HGF) in nonparenchymal liver cells. In mice, peroral administration of ONO-1301 increased hepatic gene expression and protein levels of HGF. Injections of CCl4 induced acute liver injury in mice, but the onset of acute liver injury was strongly suppressed by administration of ONO-1301. The increases in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) by CCl4 were suppressed by 10 mg/kg ONO-1301 to 39.4 and 33.6%, respectively. When neutralizing antibody against HGF was administered with ONO-1301 and CCl4, the decreases by ONO-1301 in serum ALT and AST, apoptotic liver cells, and expansion of necrotic areas in liver tissue were strongly reversed by neutralization of endogenous HGF. These results indicate that ONO-1301 increases expression of HGF and that hepatoprotective action of ONO-1301 in CCl4-induced liver injury may be attributable to its activity to induce expression of HGF, at least in part. The potential for involvement of HGF-Met-mediated signaling in the hepatotrophic action of endogenous prostaglandins generated by injury-dependent cyclooxygenase-2 induction is considerable.     

Protective role of estrogen-induced miRNA-29 expression in carbon tetrachloride-induced mouse liver injury

Previous studies have indicated that female animals are more resistant to carbon tetrachloride (CCl(4))-induced liver fibrosis than male animals, and that estradiol (E(2)) treatment can inhibit CCl(4)-induced animal hepatic fibrosis. The underlying mechanism governing these phenomena, however, has not been fully elucidated. Here we reported the role of estrogen-induced miRNA-29 (miR-29) expression in CCl(4)-induced mouse liver injury. Hepatic miR-29 levels were differentially regulated in female and male mice during CCl(4) treatment. Specifically, the levels of miR-29a and miR-29b expression were significantly decreased in the livers of male, but not female, mice following 4 weeks of CCl(4) treatment. The down-regulation of miR-29a and miR-29b in male mouse livers correlated with the early development of liver fibrosis, as indicated by increased expressions of fibrotic markers in male mice relative to female mice. In addition, E(2) was maintained at a higher level in female mice than in male mice. In contrast to TGF-β1 that decreased miR-29a/b expression in murine hepatoma IAR20 cells and normal hepatocytes, E(2) enhanced the expression of miR-29a/b through suppression of the nuclear factor-κB (NF-κB) signal pathway, which negatively regulates miR-29 expression. Furthermore, both E(2) treatment and intravenous injection of the recombinant adenovirus expressing miR-29a/b markedly increased the miR-29a/b level and attenuated the expression of fibrotic markers in mouse livers during CCl(4) treatment, supporting the protective role of E(2)-induced miR-29 in CCl(4)-induced hepatic injury. In conclusion, our results collectively demonstrate that estrogen can inhibit CCl(4)-induced hepatic injury through the induction of hepatic miR-29.     

A novel role of complement: mice deficient in the fifth component of complement (C5) exhibit impaired liver regeneration

Components of innate immunity have recently been implicated in the regulation of developmental processes. Most strikingly, complement factors appear to be involved in limb regeneration in certain urodele species. Prompted by these observations and anticipating a conserved role of complement in mammalian regeneration, we have now investigated the involvement of complement component C5 in liver regeneration, using a murine model of CCl(4)-induced liver toxicity and mice genetically deficient in C5. C5-deficient mice showed severely defective liver regeneration and persistent parenchymal necrosis after exposure to CCl(4.) In addition, these mice showed a marked delay in the re-entry of hepatocytes into the cell cycle (S phase) and diminished mitotic activity, as demonstrated, respectively, by the absence of 5-bromo-2'-deoxyuridine incorporation in hepatocytes, and the rare occurrence of mitoses in the liver parenchyma. Reconstitution of C5-deficient mice with murine C5 or C5a significantly restored hepatocyte regeneration after toxic injury. Furthermore, blockade of the C5a receptor (C5aR) abrogated the ability of hepatocytes to proliferate in response to liver injury, providing a mechanism by which C5 exerts its function, and establishing a critical role for C5aR signaling in the early events leading to hepatocyte proliferation. These results support a novel role for C5 in liver regeneration and strongly implicate the complement system as an important immunoregulatory component of hepatic homeostasis.     

Protective effect of L-theanine on carbon tetrachloride-induced acute liver injury in mice

We studied effects of L-theanine, a unique amino acid in tea, on carbon tetrachloride (CCl(4))-induced liver injury in mice. The mice were pre-treated orally with L-theanine (50, 100 or 200 mg/kg) once daily for seven days before CCl(4) (10 ml/kg of 0.2% CCl(4) solution in olive oil) injection. L-theanine dose-dependently suppressed the increase of serum activity of ALT and AST and bilirubin level as well as liver histopathological changes induced by CCl(4) in mice. L-theanine significantly prevented CCl(4)-induced production of lipid peroxidation and decrease of hepatic GSH content and antioxidant enzymes activities. Our further studies demonstrated that L-theanine inhibited metabolic activation of CCl(4) through down-regulating cytochrome P450 2E1 (CYP2E1). As a consequence, L-theanine inhibited oxidative stress-mediated inflammatory response which included the increase of TNF-α and IL-1β in sera, and expression of cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) in livers. CCl(4)-induced activation of apoptotic related proteins including caspase-3 and PARP in mouse livers was also prevented by L-theanine treatment. In summary, L-theanine protects mice against CCl(4)-induced acute liver injury through inhibiting metabolic activation of CCl(4) and preventing CCl(4)-induced reduction of anti-oxidant capacity in mouse livers to relieve inflammatory response and hepatocyte apoptosis.     

Genetic abolishment of hepatocyte proliferation activates hepatic stem cells

Quiescent hepatic stem cells (HSCs) can be activated when hepatocyte proliferation is compromised. Chemical injury rodent models have been widely used to study the localization, biomarkers, and signaling pathways in HSCs, but these models usually exhibit severe promiscuous toxicity and fail to distinguish damaged and non-damaged cells. Our goal is to establish new animal models to overcome these limitations, thereby providing new insights into HSC biology and application. We generated mutant mice with constitutive or inducible deletion of Damaged DNA Binding protein 1 (DDB1), an E3 ubiquitin ligase, in hepatocytes. We characterized the molecular mechanism underlying the compensatory activation and the properties of oval cells (OCs) by methods of mouse genetics, immuno-staining, cell transplantation and gene expression profiling. We show that deletion of DDB1 abolishes self-renewal capacity of mouse hepatocytes in vivo, leading to compensatory activation and proliferation of DDB1-expressing OCs. Partially restoring proliferation of DDB1-deficient hepatocytes by ablation of p21, a substrate of DDB1 E3 ligase, alleviates OC proliferation. Purified OCs express both hepatocyte and cholangiocyte markers, form colonies in vitro, and differentiate to hepatocytes after transplantation. Importantly, the DDB1 mutant mice exhibit very minor liver damage, compared to a chemical injury model. Microarray analysis reveals several previously unrecognized markers, including Reelin, enriched in oval cells. Here we report a genetic model in which irreversible inhibition of hepatocyte duplication results in HSC-driven liver regeneration. The DDB1 mutant mice can be broadly applied to studies of HSC differentiation, HSC niche and HSCs as origin of liver cancer.     

Cytokine-responsive gene-2/IFN-inducible protein-10 expression in multiple models of liver and bile duct injury suggests a role in tissue regeneration.

IFN-inducible protein-10 (IP-10/CXCL10) is a CXC chemokine that targets both T cells and NK cells. Elevation of IP-10 expression has been demonstrated in a number of human diseases, including chronic cirrhosis and biliary atresia. Cytokine-responsive gene-2 (Crg-2), the murine ortholog of IP-10, was induced following CCl(4) treatment of the hepatocyte-like cell line AML-12. Crg-2 expression was noted in vivo in multiple models of hepatic and bile duct injury, including bile duct ligation and CCl(4), D-galactosamine, and methylene dianiline toxic liver injuries. Induction of Crg-2 was also examined following two-thirds hepatectomy, a model that minimally injures the remaining liver, but that requires a large hepatic regenerative response. Crg-2 was induced in a biphasic fashion after two-thirds hepatectomy, preceding each known peak of hepatocyte DNA synthesis. Induction of Crg-2 was also observed in the kidney, gut, thymus, and spleen within 1 h of two-thirds hepatectomy. Characteristic of an immediate early gene, pretreatment of mice with the protein synthesis inhibitor cycloheximide before either two-thirds hepatectomy or CCl(4) injection led to Crg-2 superinduction. rIP-10 was demonstrated to have hepatocyte growth factor-inducing activity in vitro, but alone had no direct mitogenic effect on hepatocytes. Our data demonstrate that induction of Crg-2 occurs in several distinct models of liver injury and regeneration, and suggest a role for CRG-2/IP-10 in these processes.     

IL-6 modulates hepatocyte proliferation via induction of HGF/p21cip1: regulation by SOCS3

The precise role of IL-6 in liver regeneration and hepatocyte proliferation is controversial and the role of SOCS3 in liver regeneration remains unknown. Here we show that in vitro treatment with IL-6 inhibited primary mouse hepatocyte proliferation. IL-6 induced p21cip1 protein expression in primary mouse hepatocytes. Disruption of the p21cip1 gene abolished the inhibitory effect of IL-6 on cell proliferation. Co-culture with nonparenchymal liver cells diminished IL-6 inhibition of hepatocyte proliferation, which was likely due to IL-6 stimulation of nonparenchymal cells to produce HGF. Finally, IL-6 induced higher levels of p21cip1 protein expression and a slightly stronger inhibition of cell proliferation in SOCS3+/- mouse hepatocytes compared to wild-type hepatocytes, while liver regeneration was enhanced and prolonged in SOCS3+/- mice. Our findings suggest that IL-6 directly inhibits hepatocyte proliferation via a p21cip1-dependent mechanism and indirectly enhances hepatocyte proliferation via stimulating nonparenchymal cells to produce HGF. SOCS3 negatively regulates liver regeneration.     

Different sensitivity of inbred mice to hepatocarcinogen ortho-aminoazotoluene may be due to differences in the negative control mechanisms of hepatocyte proliferation

A most convenient model to study mechanisms of live organism response to chemical carcinogens is tumor induction in murine liver by aminoazodyes, in particular by ortho-aminoazotoluene (OAT). We studied both early and late stages of hepatocarcinogenesis on several lines of inbred mice differing in sensibility to OAT. By means of autoradiography, we examined proliferative activity of hepatocytes obtained from the liver of sensitive (A/He, DD, SWR) and resistant to OAT AKR, CC57Br, BALB/c lines of mice, which were injected carcinogen. The level of p53, p21Cip1, bax, mdm2, cyclin G, gadd45 genes expression in the liver of mice of different lines given OAT injection was studied by multiplex PCR method. Carcinogen caused a decrease of hepatocyte proliferative activity induced by partial hypatectomy (PHE), and an increase in p53, p21Cip, bax, mdm2, and cyclin G genes within mice of A/He, DD and SWR lines. Cell fusion experiments on hepatocytes obtained from regenerating murine liver sensitive to A/He line carcinogen and given long-time OAT administrations with resting and proliferating fibroblasts of NIH 3T3 mice revealed no obvious suppression of DNA synthesis in heterokaryons. Unlike, in fusion experiments on serum-stimulated fibroblasts with hepatocytes obtained from the liver of BALB/c line mice also given OAT suppression of DNA synthesis in stimulated fibroblasts in heterokaryons was observed 15 days following PHE. These results enable us to conclude that OAT administrations break negative endogenous mechanisms of hepatocyte proliferation control in the liver of mice sensitive to carcinogenes.     

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.     

Hepatocyte-specific ablation of spermine/spermidine-N1-acetyltransferase gene reduces the severity of CCl4-induced acute liver injury

Activation of spermine/spermidine-N(1)-acetyltransferase (SSAT) leads to DNA damage and growth arrest in mammalian cells, and its ablation reduces the severity of ischemic and endotoxic injuries. Here we have examined the role of SSAT in the pathogenesis of toxic liver injury caused by carbon tetrachloride (CCl(4)). The expression and activity of SSAT increase in the liver subsequent to CCl(4) administration. Furthermore, the early liver injury after CCl(4) treatment was significantly attenuated in hepatocyte-specific SSAT knockout mice (Hep-SSAT-Cko) compared with wild-type (WT) mice as determined by the reduced serum alanine aminotransferase levels, decreased hepatic lipid peroxidation, and less severe liver damage. Cytochrome P450 2e1 levels remained comparable in both genotypes, suggesting that SSAT deficiency does not affect the metabolism of CCl(4). Hepatocyte-specific deficiency of SSAT also modulated the induction of cytokines involved in inflammation and repair as well as leukocyte infiltration. In addition, Noxa and activated caspase 3 levels were elevated in the livers of WT compared with Hep-SSAT-Cko mice. Interestingly, the onset of cell proliferation was significantly more robust in the WT compared with Hep-SSAT Cko mice. The inhibition of polyamine oxidases protected the animals against CCl(4)-induced liver injury. Our studies suggest that while the abrogation of polyamine back conversion or inhibition of polyamine oxidation attenuate the early injury, they may delay the onset of hepatic regeneration.     

Targeted Deletion of Fibrinogen Like Protein 1 Reveals a Novel Role in Energy Substrate Utilization

Fibrinogen like protein 1(Fgl1) is a secreted protein with mitogenic activity on primary hepatocytes. Fgl1 is expressed in the liver and its expression is enhanced following acute liver injury. In animals with acute liver failure, administration of recombinant Fgl1 results in decreased mortality supporting the notion that Fgl1 stimulates hepatocyte proliferation and/or protects hepatocytes from injury. However, because Fgl1 is secreted and detected in the plasma, it is possible that the role of Fgl1 extends far beyond its effect on hepatocytes. In this study, we show that Fgl1 is additionally expressed in brown adipose tissue. We find that signals elaborated following liver injury also enhance the expression of Fgl1 in brown adipose tissue suggesting that there is a cross talk between the injured liver and adipose tissues. To identify extra hepatic effects, we generated Fgl1 deficient mice. These mice exhibit a phenotype suggestive of a global metabolic defect: Fgl1 null mice are heavier than wild type mates, have abnormal plasma lipid profiles, fasting hyperglycemia with enhanced gluconeogenesis and exhibit differences in white and brown adipose tissue morphology when compared to wild types. Because Fgl1 shares structural similarity to Angiopoietin like factors 2, 3, 4 and 6 which regulate lipid metabolism and energy utilization, we postulate that Fgl1 is a member of an emerging group of proteins with key roles in metabolism and liver regeneration.     

Obstructive cholestasis induces TNF-alpha- and IL-1 -mediated periportal downregulation of Bsep and zonal regulation of Ntcp, Oatp1a4, and Oatp1b2

Inverse acinar regulation of Mrp2 and 3 represents an adaptive response to hepatocellular cholestatic injury. We studied whether obstructive cholestasis (bile duct ligation) and LPS treatment affect the zonal expression of Bsep (Abcb11), Mrp4 (Abcc4), Ntcp (Slc10a1), and Oatp isoforms (Slco1a1, Slco1a4, and slco1b2) in rat liver, as analyzed by semiquantitative immunofluorescence. Contribution of TNF-alpha and IL-1beta to transporter zonation in obstructive cholestasis was studied by cytokine inactivation. In normal liver Bsep, Mrp4, Ntcp, and Oatp1a1 were homogeneously distributed in the acinus, whereas Oatp1a4 and Oatp1b2 expression increased from zone 1 to 3. Glutamine synthetase-positive pericentral hepatocytes exhibited markedly lower Oatp1a4 expression than the remaining zone 3 hepatocytes. In cholestatic liver Bsep and Ntcp immunofluorescence in periportal hepatocytes significantly decreased to 66 +/- 4% (P < 0.01) and 67 +/- 7% (P < 0.05), whereas it was not altered in pericentral hepatocytes. Oatp1a4 was significantly induced in hepatocytes with a primarily low expression, i.e., in periportal hepatocytes and in glutamine synthetase-positive pericentral hepatocytes. Likewise, Oatp1b2 was upregulated in periportal hepatocytes. Mrp4 zonal induction was homogeneous. Inactivation of TNF-alpha and IL-1beta prevented periportal downregulation of Bsep. Recruitment of neutrophils and polymorphonuclear cells mainly occurred in the periportal zone. Likewise, IL-1beta induction was largely found periportally. No significant transporter zonation was seen following LPS treatment. In conclusion, zonal downregulation of Bsep in obstructive cholestasis is associated with portal inflammation and is mediated by TNF-alpha and IL-1beta. Periportal downregulation of Ntcp and induction of Oatp1a4 and Oatp1b2 may represent adaptive mechanisms to reduce cholestatic injury in hepatocytes with profound downregulation of Bsep and Mrp2.