IKK1 and IKK2 cooperate to maintain bile duct integrity in the liver

Inflammatory destruction of intrahepatic bile ducts is a common cause of vanishing bile duct syndrome and cholestasis, often progressing to biliary cirrhosis and liver failure. However, the molecular mechanisms underlying the pathogenesis of inflammatory biliary disease are poorly understood. Here, we show that the two IkappaB kinases, IKK1/IKKalpha and IKK2/IKKbeta, display distinct collaborative and specific functions that are essential to protect the liver from cytokine toxicity and bile duct disease. Combined conditional ablation of IKK1 and IKK2, but not of each kinase alone, sensitized the liver to in vivo LPS challenge, uncovering a redundant function of the two IkappaB kinases in mediating canonical NF-kappaB signaling in hepatocytes and protecting the liver from TNF-induced failure. Unexpectedly, mice with combined ablation of IKK1 and IKK2 or IKK1 and NEMO spontaneously developed severe jaundice and fatal cholangitis characterized by inflammatory destruction of small portal bile ducts. This bile duct disease was caused by the combined impairment of canonical NF-kappaB signaling together with inhibition of IKK1-specific functions affecting the bile-blood barrier. These results reveal a novel function of the two IkappaB kinases in cooperatively regulating liver immune homeostasis and bile duct integrity and suggest that IKK signaling may be implicated in human biliary diseases.     

TAK1: Another mesh in the NF-κB - JNK controlled network causing hepatocellular carcinoma

The MAP3-kinase TGF-beta-activated kinase 1 (TAK1) critically modulates innate and adaptive immune responses and connects cytokine stimulation with activation of inflammatory signaling pathways. Here, we report that conditional ablation of TAK1 in liver parenchymal cells (hepatocytes and cholangiocytes) causes hepatocyte dysplasia and early-onset of hepatocarcinogenesis, coinciding with biliary ductopenia and cholestasis. TAK1-mediated cancer suppression is exerted through activating NF-kappaB in response to tumor necrosis factor (TNF) and through preventing Caspase-3-dependent hepatocyte and cholangiocyte apoptosis. Moreover, TAK1 suppresses a procarcinogenic and pronecrotic pathway, which depends on NF-kappaB-independent functions of the I kappaB-kinase (IKK)-subunit NF-kappaB essential modulator (NEMO). Therefore, TAK1 serves as a gatekeeper for a protumorigenic, NF-kappaB-independent function of NEMO in parenchymal liver cells.     

Hepatic NF-kappa B essential modulator deficiency prevents obesity-induced insulin resistance but synergizes with high-fat feeding in tumorigenesis

Development of obesity-associated insulin resistance and diabetes mellitus type 2 has been linked to activation of proinflammatory pathways in the liver, leading to impaired insulin signal transduction. To further define the role of hepatic NF-kappaB activation in this process, we have analyzed glucose metabolism in mice with liver-specific inactivation of the NF-kappaB essential modulator gene (NEMO(L-KO) mice) exposed to a high-fat diet (HFD). These animals are protected from the development of obesity-associated insulin resistance, highlighting the importance of hepatic NF-kappaB activation in this context. However, hepatic NEMO deficiency synergizes with HFD in the development of liver steatosis as a consequence of decreased peroxisome proliferator-activated receptor (PPAR-alpha) and increased PPAR-gamma expression. Steatosis interacts with increased inflammation, causing elevated apoptosis in the livers of these mice under HFD. These changes result in liver tumorigenesis of NEMO(L-KO) mice under normal diet, a process that is largely aggravated when these mice are exposed to HFD. These data directly demonstrate the interaction of hepatic inflammation, dietary composition, and metabolism in the development of liver tumorigenesis.     

Critical role of acidic sphingomyelinase in murine hepatic ischemia-reperfusion injury

The molecular mechanisms of hepatic ischemia/reperfusion (I/R) damage are incompletely understood. We investigated the role of ceramide in a murine model of warm hepatic I/R injury. This sphingolipid induces cell death and participates in tumor necrosis factor (TNF) signaling. Hepatic ceramide levels transiently increased after the reperfusion phase of the ischemic liver in mice, because of an early activation of acidic sphingomyelinase (ASMase) followed by acid ceramidase stimulation. In vivo administration of an ASMase inhibitor, imipramine, or ASMase knockdown by siRNA decreased ceramide generation during I/R, and attenuated serum ALT levels, hepatocellular necrosis, cytochrome c release, and caspase-3 activation. ASMase-induced ceramide generation activated JNK resulting in BimL phosphorylation and translocation to mitochondria, as the inhibition of ASMase by imipramine prevented these events. In contrast, blockade of ceramide catabolism by N-oleyolethanolamine (NOE), a ceramidase inhibitor, enhanced ceramide levels and potentiated I/R injury compared with vehicle-treated mice. Pentoxifylline treatment prevented TNF upregulation and ASMase activation. Furthermore, 9 of 11 mice treated with imipramine survived 7 days after total liver ischemia, compared with 4 of 12 vehicle-treated mice, whereas 8 of 8 NOE-treated mice died within 2 days of total liver ischemia. In conclusion, ceramide generated from ASMase plays a key role in I/R-induced liver damage, and its modulation may be of therapeutic relevance.     

Delayed-onset caspase-dependent massive hepatocyte apoptosis upon Fas activation in Bak/Bax-deficient mice

The proapoptotic Bcl-2 family proteins Bak and Bax serve as an essential gateway to the mitochondrial pathway of apoptosis. When activated by BH3-only proteins, Bak/Bax triggers mitochondrial outer membrane permeabilization leading to release of cytochrome c followed by activation of initiator and then effector caspases to dismantle the cells. Hepatocytes are generally considered to be type II cells because, upon Fas stimulation, they are reported to require the BH3-only protein Bid to undergo apoptosis. However, the significance of Bak and Bax in the liver is unclear. To address this issue, we generated hepatocyte-specific Bak/Bax double knockout mice and administered Jo2 agonistic anti-Fas antibody or recombinant Fas ligand to them. Fas-induced rapid fulminant hepatocyte apoptosis was partially ameliorated in Bak knockout mice but not in Bax knockout mice, and was completely abolished in double knockout mice 3 hours after Jo2 injection. Importantly, at 6 hours, double knockout mice displayed severe liver injury associated with repression of XIAP, activation of caspase-3/7 and oligonucleosomal DNA breaks in the liver, without evidence of mitochondrial disruption or cytochrome c-dependent caspase-9 activation. This liver injury was not ameliorated in a cyclophilin D knockout background nor by administration of necrostatin-1, but was completely inhibited by administration of a caspase inhibitor after Bid cleavage. CONCLUSION: Whereas either Bak or Bax is critically required for rapid execution of Fas-mediated massive apoptosis in the liver, delayed onset of mitochondria-independent, caspase-dependent apoptosis develops even in the absence of both. The present study unveils an extrinsic pathway of apoptosis, like that in type I cells, which serves as a backup system even in type II cells.     

Roles of AKT and sphingosine kinase in the antiapoptotic effects of bile duct ligation in mouse liver

Tumor necrosis factor (TNF) receptor- and Fas-mediated apoptosis are major death processes of hepatocytes in liver disease. Although antiapoptotic effects in the injured liver promote chronic hepatitis and carcinogenesis, scant information is known about these mechanisms. To explore this issue, we compared acute liver injury after TNF-alpha or anti-Fas antibody (Jo2) between livers from sham-operated mice and chronic injured liver via bile duct ligation (BDL). BDL inhibited hepatocyte apoptosis induced by TNF-alpha but not by Jo2. On the other hand, BDL inhibited the massive hemorrhage seen in livers treated with either TNF-alpha or Jo2. Inactivation of AKT blocked the antiapoptotic effect of BDL. Sphingosine kinase knockout mice also lost the antihemorrhagic effect of BDL and attenuated the antiapoptotic effects of BDL. In bile duct-ligated livers, hepatic stellate cells (HSCs) were activated and produced tissue inhibitor of metalloproteinase 1 in a sphingosine kinase (SphK)-1-dependent mechanism. In conclusion, BDL exerts antiapoptotic effects that appear to require activation of AKT in hepatocytes and SphK in HSCs.     

Protective effects of apocynin and allopurinol on ischemia/reperfusion-induced liver injury in mice

AIM： To determine the effects of allopurinol, an inhibitor of xanthine oxidase, and apocynin, an inhibitor of NADPH oxidase, on oxidant stress and liver injury caused by hepatic ischemia/reperfusion （I/R） procedure in mice.
METHODS： Mice were pretreated with a xanthine oxidase inhibitor, allopurinol, or NADPH oxidase （NOX） inhibitor, apocynin before the hepatic I/R procedure. Then treated or untreated mice underwent the hepatic I/R procedure. The effects on hepatic injury and superoxide anions were determined after starting reperfusion.
RESULTS： A standard warm hepatic I/R procedure led to a marked increase in superoxide anion production as indicated by a superoxide anion tracer, MCLA. At the same time, the procedure caused profound acute liver injury, as indicated by elevated serum alanine aminotransferase and tumor necrosis factor-α levels, reduced liver glutathione levels and elevated malondialdehyde contents, as well as a high apoptotic cell count. All these changes were reversed by the use of apocynin or allopurinol prior to the hepatic I/R procedure.
CONCLUSION： Allopurinol and apocynin exerted protective effects on hepatic ischemia/reperfusion injury. The protection is associated with blocking the generationof superoxide anions during the hepatic I/R procedure by inhibiting xanthine oxidase and NADPH oxidase activity.     

NEMO recognition of ubiquitinated Bcl10 is required for T cell receptor-mediated NF-kappaB activation

The mechanism by which the Carma1-Bcl10-MALT1 (CBM) complex couples T cell antigen receptor (TCR) signaling to IkappaB kinase (IKK) and NF-kappaB activation is not known. Here, we show that Bcl10 undergoes K63-linked polyubiquitination in response to T cell activation and subsequently binds NEMO, the regulatory subunit of IKK. This interaction requires the ubiquitin-binding activity of NEMO. The sites of Bcl10 ubiquitination were mapped to K31 and K63. Mutation of these residues did not affect TCR signaling-induced CBM complex assembly but prevented Bcl10 ubiquitination, NEMO binding, and NF-kappaB activation. Therefore, the regulated ubiquitination of Bcl10 and its recognition by NEMO are a critical link between the CBM complex, IKK recruitment, and NF-kappaB activation.     

The role of JNK2 in toxic liver injury

Tumor necrosis factor-induced toxic liver injury results from JNK2-dependent activation of caspase-8 and the mitochondrial death pathway. Wang Y, Singh R, Lefkowitch JH, Rigoli RM, Czaja MJ. In vitro studies of hepatocytes have implicated over-activation of c-Jun N-terminal kinase (JNK) signaling as a mechanism of tumor necrosis factor-alpha (TNF)-induced apoptosis. However, the functional significance of JNK activation and the role of specific JNK isoforms in TNF-induced hepatic apoptosis in vivo remain unclear. JNK1 and JNK2 function was, therefore, investigated in the TNF-dependent, galactosamine/lipopolysaccharide (GalN/LPS) model of liver injury. The toxin GalN converted LPS-induced JNK signaling from a transient to prolonged activation. Liver injury and mortality from GalN/LPS was equivalent in wild-type and jnk1-/- mice but markedly decreased in jnk2-/- mice. This effect was not secondary to down-regulation of TNF receptor 1 expression or TNF production. In the absence of jnk2, the caspase-dependent, TNF death pathway was blocked, as reflected by the failure of caspase-3 and -7 and poly(ADP-ribose) polymerase cleavage to occur. JNK2 was critical for activation of the mitochondrial death pathway, as in jnk2-/- mice Bid cleavage and mitochondrial translocation and cytochrome c release were markedly decreased. This effect was secondary to the failure of jnk2-/- mice to activate caspase-8. Liver injury and caspase activation were similarly decreased in jnk2 null mice after GalN/TNF treatment. Ablation of jnk2 did not inhibit GalN/LPS-induced c-Jun kinase activity, although activity was completely blocked in jnk1-/- mice. Toxic liver injury is, therefore, associated with JNK over-activation and mediated by JNK2 promotion of caspase-8 activation and the TNF mitochondrial death pathway through a mechanism independent of c-Jun kinase activity. [Abstract reproduced by permission of J Biol Chem 2006;281:15258-67].     

Toll-like receptor 4 mediates inflammatory signaling by bacterial lipopolysaccharide in human hepatic stellate cells

Bacterial lipopolysaccharide (LPS) stimulates Kupffer cells and participates in the pathogenesis of alcohol-induced liver injury. However, it is unknown whether LPS directly affects hepatic stellate cells (HSCs), the main fibrogenic cell type in the injured liver. This study characterizes LPS-induced signal transduction and proinflammatory gene expression in activated human HSCs. Culture-activated HSCs and HSCs isolated from patients with hepatitis C virus-induced cirrhosis express LPS-associated signaling molecules, including CD14, toll-like receptor (TLR) 4, and MD2. Stimulation of culture-activated HSCs with LPS results in a rapid and marked activation of NF-kappaB, as assessed by in vitro kinase assays for IkappaB kinase (IKK), IkappaBalpha steady-state levels, p65 nuclear translocation, NF-kappaB-dependent luciferase reporter gene assays, and electrophoretic mobility shift assays. Lipid A induces NF-kappaB activation in a similar manner. Both LPS- and lipid A-induced NF-kappaB activation is blocked by preincubation with either anti-TLR4 blocking antibody (HTA125) or Polymyxin B. Lipid A induces NF-kappaB activation in HSCs from TLR4-sufficient (C3H/OuJ) mice but not from TLR4-deficient (C3H/HeJ) mice. LPS also activates c-Jun N-terminal kinase (JNK), as assessed by in vitro kinase assays. LPS up-regulates IL-8 and MCP-1 gene expression and secretion. LPS-induced IL-8 secretion is completely inhibited by the IkappaB super repressor (Ad5IkappaB) and partially inhibited by a specific JNK inhibitor, SP600125. LPS also up-regulates cell surface expression of ICAM-1 and VCAM-1. In conclusion, human activated HSCs utilize components of TLR4 signal transduction cascade to stimulate NF-kappaB and JNK and up-regulate chemokines and adhesion molecules. Thus, HSCs are a potential mediator of LPS-induced liver injury.     

Nuclear factor-kappaB decoy oligodeoxynucleotides attenuates ischemia/reperfusion injury in rat liver graft

AIM: To evaluate the protective effect of NF-kappaB decoy oligodeoxynucleotides (ODNs) on ischemia/reperfusion (I/R) injury in rat liver graft. METHODS: Orthotopic syngeneic rat liver transplantation was performed with 3 h of cold preservation of liver graft in University of Wisconsin solution containing phosphorothioated double-stranded NF-kappaB decoy ODNs or scrambled ODNs. NF-kappaB decoy ODNs or scrambled ODNs were injected intravenously into donor and recipient rats 6 and 1 h before operation, respectively. Recipients were killed 0 to 16 h after liver graft reperfusion. NF-kappaB activity in the liver graft was analyzed by electrophoretic mobility shift assay (EMSA). Hepatic mRNA expression of TNF-alpha, IFN-gamma and intercellular adhesion molecule-1 (ICAM-1) were determined by semiquantitative RT-PCR. Serum levels of TNF-alpha and IFN-gamma were measured by enzyme-linked immunosorbent assays (ELISA). Serum level of alanine transaminase (ALT) was measured using a diagnostic kit. Liver graft myeloperoxidase (MPO) content was assessed. RESULTS: NF-kappaB activation in liver graft was induced in a time-dependent manner, and NF-kappaB remained activated for 16 h after graft reperfusion. NF-kappaB activation in liver graft was significant at 2 to 8 h and slightly decreased at 16 h after graft reperfusion. Administration of NF-kappaB decoy ODNs significantly suppressed NF-kappaB activation as well as mRNA expression of TNF-alpha, IFN-gamma and ICAM-1 in the liver graft. The hepatic NF-kappaB DNA binding activity [presented as integral optical density (IOD) value] in the NF-kappaB decoy ODNs treatment group rat was significantly lower than that of the I/R group rat (2.16+/-0.78 vs 36.78+/-6.35 and 3.06+/-0.84 vs 47.62+/- 8.71 for IOD value after 4 and 8 h of reperfusion, respectively, P<0.001). The hepatic mRNA expression level of TNF-alpha, IFN-gamma and ICAM-1 [presented as percent of beta-actin mRNA (%)] in the NF-kappaB decoy ODNs treatment group rat was significantly lower than that of the I/R group rat (8.31+/-3.48 vs 46.37+/-10.65 and 7.46+/- 3.72 vs 74.82+/-12.25 for hepatic TNF-alpha mRNA, 5.58+/-2.16 vs 50.46+/-9.35 and 6.47+/-2.53 vs 69.72+/-13.41 for hepatic IFN-gamma mRNA, 6.79+/-2.83 vs 46.23+/-8.74 and 5.28+/-2.46 vs 67.44+/-10.12 for hepatic ICAM-1 mRNA expression after 4 and 8 h of reperfusion, respectively, P<0.001). Administration of NF-kappaB decoy ODNs almost completely abolished the increase of serum level of TNF-alpha and IFN-gamma induced by hepatic ischemia/reperfusion, the serum level (pg/mL) of TNF-alpha and IFN-gamma in the NF-kappaB decoy ODNs treatment group rat was significantly lower than that of the I/R group rat (42.7+/-13.6 vs 176.7+/-15.8 and 48.4+/-15.1 vs 216.8+/-17.6 for TNF-alpha level, 31.5+/-12.1 vs 102.1+/-14.5 and 40.2+/-13.5 vs 118.6+/-16.7 for IFN-gamma level after 4 and 8 h of reperfusion, respectively, P<0.001). Liver graft neutrophil recruitment indicated by MPO content and hepatocellular injury indicated by serum ALT level were significantly reduced by NF-kappaB decoy ODNs, the hepatic MPO content (A655) and serum ALT level (IU/L) in the NF-kappaB decoy ODNs treatment group rat was significantly lower than that of the I/R group rat (0.17+/-0.07 vs 1.12+/-0.25 and 0.46+/-0.17 vs 1.46+/-0.32 for hepatic MPO content, 71.7+/-33.2 vs 286.1+/-49.6 and 84.3+/-39.7 vs 467.8+/-62.3 for ALT level after 4 and 8 h of reperfusion, respectively, P<0.001). CONCLUSION: The data suggest that NF-kappaB decoy ODNs protects against I/R injury in liver graft by suppressing NF-kappaB activation and subsequent expression of proinflammatory mediators.     

Roles of Akt/PKB and IKK complex in constitutive induction of NF-kappaB in hepatocellular carcinomas of transforming growth factor alpha/c-myc transgenic mice.

NF-kappaB regulates liver cell death during development, regeneration, and neoplastic transformation. For example, we showed that oncogenic Ras- or Raf-mediated transformation of rat liver epithelial cells (RLEs) led to altered NF-kappaB regulation through IKK complex activation, which rendered these cells more resistant to TGF-beta1-induced apoptosis. Thus, based on these findings, we sought to determine whether NF-kappaB could also be involved in tumor growth of liver cells in vivo. Hepatocellular carcinomas (HCCs) derived from bitransgenic mice harboring TGF-alpha and c-myc transgenes targeted specifically to the liver were compared with HCCs from c-myc single transgenic mice. Tumors from bitransgenic mice are characterized by a higher frequency of appearance, lower apoptotic index, and a higher rate of cell proliferation. Here we show that NF-kappaB is activated in HCCs of double TGF-alpha/c-myc transgenic mice, but not of c-myc single transgenic mice, suggesting that TGF-alpha mediates induction of NF-kappaB. Activation of the IKK complex was observed in the HCCs of double TGF-alpha/c-myc transgenic mice, implicating this pathway in NF-kappaB induction. Lastly, activation of the Akt/protein kinase B (PKB), which has recently been implicated in NF-kappaB activation by PDGF, TNF-alpha, and Ras, was also observed. Importantly, human HCC cell lines similarly displayed NF-kappaB activation. Thus, these studies elucidate an anti-apoptotic mechanism by a TGF-alpha-Akt/PKB-IKK pathway, which likely contributes to survival and proliferation, thereby accelerating c-myc-induced liver neoplastic development in vivo.