Deletion of IKK2 in hepatocytes does not sensitize these cells to TNF-induced apoptosis but protects from ischemia/reperfusion injury

Contradictory functions of NF-kappaB in liver physiology and cancer

Rudolf Virchow (1821-1902), one of the founding fathers of modern pathology, hypothesized that cancer and inflammatory processes are linked, due to the presence of leukocytes in the tumor tissue. Today, chronic inflammation is believed to be one of the major causes for cancer development, accounting for nearly 20% of cancer cases worldwide. Hepatocellular carcinoma (HCC) is the third leading cause of cancer mortality throughout the world, and its incidence is increasing in the United States. HCC is widely accepted to be the outcome of continuous injury and chronic inflammation, and thus provides a good model to gain insight into inflammatory related cancer processes. Nuclear Factor- kappa B (NF-kappaB) was first identified as an enhancer protein of the kappa light-chain gene in B lymphocytes. Later it was realized that there are five NF-kappaB transcription factors with important roles in inflammation, innate immunity, cancer and apoptosis aborting. Consequently, NF-kappaB was shown to link inflammation and cancer, but recent reports have revealed it to play a much more complex role, where in some disease processes it promotes cancer and in others it impedes carcinogenesis. In this review, we will focus on the seemingly contradictory role of NF-kappaB in liver homeostasis, as well as in liver cancer.

Emerging drugs for complications of end-stage liver disease

BACKGROUND

The prevalence of end-stage liver disease is rising rapidly and constitutes a major healthcare burden currently. Many cases are diagnosed at a later stage when liver transplantation is the only effective treatment option. There is thus an urgent need for novel treatments to reverse the earlier stages of cirrhosis as well as to treat the many associated life-threatening complications.

OBJECTIVES

To review the current drugs available for treating the complications of advanced liver disease. To address novel treatment strategies that are in development, with particular reference to the rapidly developing area of antifibrotic therapy. To assess how the obstacles that have so far impeded the development of effective new drugs for end-stage liver disease may be overcome in the future.

METHODS

The literature was reviewed to define current therapies and therapies in clinical trials. We used the current models of the molecular basis of liver fibrogenesis to determine potential new therapeutic targets for antifibrotic therapy.

CONCLUSIONS

Insights into the pathogenesis of liver injury and fibrosis have opened up new avenues for therapy and there are now candidates and targets with real potential for the development of a new generation of antifibrotic therapies.

Generation and activation of multiple dimeric transcription factors within the NF-kappaB signaling system

The NF-kappaB signaling pathway regulates the activity of multiple dimeric transcription factors that are generated from five distinct monomers. The availabilities of specific dimers are regulated during cell differentiation and organ development and determine the cell's responsiveness to inflammatory or developmental signals. An altered dimer distribution is a hallmark of many chronic diseases. Here, we reveal that the cellular processes that generate different NF-kappaB dimers are highly connected through multiple cross-regulatory mechanisms. First, we find that steady-state expression of RelB is regulated by the canonical pathway and constitutive RelA activity. Indeed, synthesis control of RelB is the major determinant of noncanonical NF-kappaB dimer activation. Second, processing, not synthesis, of p100 and p105 is mechanistically linked via competitive dimerization with a limited pool of RelA and RelB. This homeostatic cross-regulatory mechanism determines the availability of the p50- and p52-containing dimers and also of the noncanonical IkappaB p100. Our results inform a wiring diagram to delineate NF-kappaB dimer formation that emphasizes that inflammatory and developmental signaling cannot be considered separately but are highly interconnected.

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.

Targeting IKK2 by pharmacological inhibitor AS602868 prevents excitotoxic injury to neurons and oligodendrocytes

Among the diverse mechanisms involved in the pathophysiology of post-ischemic and post-traumatic injuries, excitotoxicity and nuclear factor-kappaB (NF-kappaB) activation through induction of IkappaB kinase (IKK) complex have a primary role. We investigated the effects of the selective inhibitor of the IKK2 subunit, the anilinopyrimidine derivative AS602868, on excitotoxic injury produced in rat organotypic hippocampal slices and cerebellar primary neurons. Brief exposure to N-methyl-D-aspartate (NMDA) induces astrocyte reactivity, neuron cell death and oligodendrocyte degeneration in hippocampal slices. Application of AS602868 elicited a long-lasting protection of both neurons and oligodendrocytes. Maximal effect was observed with prolonged application of the compound after NMDA exposure. Neuroprotection was also evident in primary cultures of cerebellar granule cells starting from 20 nM concentration. AS602868-elicited neuroprotection correlated with inhibition of NF-kappaB activity. Our results suggest that AS602868 may prove to be a valuable approach in treating neurodegeneration and demyelination associated with cerebral trauma and ischemia.

Recombinant human erythropoietin attenuates hepatic injury induced by ischemia/reperfusion in an isolated mouse liver model

INTRODUCTION

Apoptosis is a central mechanism of cell death following reperfusion of the ischemic liver. Recombinant human erythropoietin (rhEPO) have an important role in the treatment of myocardial ischemia/reperfusion (I/R) injury, by preventing apoptosis. The aim of the study was to investigate the effect of different regimens of rhEPO in preventing apoptosis following I/R-induced hepatic injury.

MATERIAL AND METHODS

Isolated mouse livers were randomly divided into five groups: (1) control group, perfused for the whole study period (105 min); (2) 30-min perfusion followed by 90 min of ischemia and 15 min of reperfusion; (3), (4) and (5) like group 2, but with administration of rhEPO 5,000 units/kg i.p. at 30 min, 24 h, or both 30 min and 24 h respectively, before induction of ischemia. Perfusate liver enzyme levels and intrahepatic caspase-3 activity were measured, and apoptotic cells were identified by morphological criteria, TUNEL assay, and immunohistochemistry for caspase-3. Using immunoblot the expression of the proapoptotic JNK and inhibitor of NFkappaB (IkappaBalpha) were also evaluated. von Willebrand factor (vWF) immunohistochemistry was used as a marker of endothelial cells.

RESULTS

Compared to the I/R livers, all 3 rhEPO pretreated groups showed: a significant reduction in liver enzyme levels (P < 0.05) and intrahepatic caspase-3 activity (P < 0.05), fewer apoptotic hepatocytes (P < 0.05) and positive vWF staining in numerous endothelial cells lining the sinusoids. EPO decreased JNK phosphorylation and the degradation of the inhibitor of NFkappaB (IkappaBalpha) during I/R. There was no added benefit of the multiple- over the single-dose rhEPO regimen.

CONCLUSION

Pretreatment with one dose of rhEPO can attenuate post-I/R hepatocyte apoptotic liver damage. NFkappaB and JNK activation is likely to play a pivotal role in the pathophysiology of I/R hepatic injury and might have a key role in EPO-mediated protective effects. This effect is associated with the increase in sinusoidal vWF immunostaining suggests an additional effect of rhEPO in liver angiogenesis recovery. These findings have important implications for the potential use of rhEPO in I/R injury during liver transplantation.

A mechanism for the anti-fibrogenic effects of the pregnane X receptor (PXR) in the liver: inhibition of NF-kappaB?

The liver is susceptible to chronic damage through exposure to a variety of toxins (e.g. alcohol) and viruses (e.g. hepatitis C). Obesity, autoimmune diseases (e.g. autoimmune hepatitis) and a variety of genetic diseases (e.g. Wilson's disease) also lead to chronic liver damage. This damage results in scarring fibrogenesis, structural disruption and functional impairment of the organ. Recent work suggests that there is cross-talk between the PXR and NF-kappaB pathways. This cross-talk may explain the observation that PXR activators inhibit liver fibrosis in in vitro and in vivo animal models of the disease. This reveiw will focus on the two transcription factors and their potential interaction.

Nuclear factor-kappaB and the hepatic inflammation-fibrosis-cancer axis

Nuclear factor-kappaB (NF-kappaB) is a transcriptional regulator of genes involved in immunity, inflammatory response, cell fate, and function. Recent attention has focused on the pathophysiological role of NF-kappaB in the diseased liver. In vivo studies using rodent models of liver disease and cell-targeted perturbation of NF-kappaB activity have revealed complex and multicellular functions in hepatic inflammation, fibrosis, and the development of hepatocellular carcinoma - a process we have termed the "inflammation-fibrosis-cancer axis". This review summarizes the current state of knowledge and provides insight into the vast complexity of the hepatic NF-kappaB signaling system, which should provide a rich source of new therapeutic targets.

Vascular endothelial cell-specific NF-kappaB suppression attenuates hypertension-induced renal damage

Nuclear factor kappa B (NF-kappaB) participates in hypertension-induced vascular and target-organ damage. We tested whether or not endothelial cell-specific NF-kappaB suppression would be ameliorative. We generated Cre/lox transgenic mice with endothelial cell-restricted NF-kappaB super-repressor IkappaBalphaDeltaN (Tie-1-DeltaN mice) overexpression. We confirmed cell-specific IkappaBalphaDeltaN expression and reduced NF-kappaB activity after TNF-alpha stimulation in primary endothelial cell culture. To induce hypertension with target-organ damage, we fed mice a high-salt diet and N(omega)-nitro-l-arginine-methyl-ester (L-NAME) and infused angiotensin (Ang) II. This treatment caused a 40-mm Hg blood pressure increase in both Tie-1-DeltaN and control mice. In contrast to control mice, Tie-1-DeltaN mice developed a milder renal injury, reduced inflammation, and less albuminuria. RT-PCR showed significantly reduced expression of the NF-kappaB targets VCAM-1 and ICAM-1, compared with control mice. Thus, the data demonstrate a causal link between endothelial NF-kappaB activation and hypertension-induced renal damage. We conclude that in vivo NF-kappaB suppression in endothelial cells stops a signaling cascade leading to reduced hypertension-induced renal damage despite high blood pressure.

Hepatocyte-specific IKK gamma/NEMO expression determines the degree of liver injury

BACKGROUND & AIMS

NEMO is the regulatory subunit of the I kappa B kinase (IKK) complex and is involved in controlling nuclear factor kappaB (NF-kappaB) activation. NEMO knockout mice die during embryogenesis due to massive hepatocyte apoptosis. Here we investigated the role of NEMO-dependent signaling in hepatocytes during acute liver injury.

METHODS

We generated conditional hepatocyte-specific NEMO knockout mice using the loxP system with the Cre recombinase under the control of the albumin promoter (NEMODeltaLPC). In these mice, we studied mechanisms of tumor necrosis factor (TNF)- and ischemia/reperfusion-dependent liver cell damage.

RESULTS

In adult NEMODeltaLPC animals, NEMO is specifically deleted in hepatocytes and no differences in survival, growth, and fertility were found when compared with wild-type (NEMO(f/f)) mice. TNF stimulation of NEMODeltaLPC mice resulted in high serum transaminase levels and massive hepatocyte apoptosis, which were associated with lack of I kappa B alpha degradation, inhibition of NF-kappaB activation, and target gene transcription. Additionally, ischemia/reperfusion resulted in higher nonparenchymal cell-dependent induction of oxidative stress and stronger inflammation in NEMODeltaLPC mice. This led to massive hepatocyte apoptosis and death of the animals, while NEMO(f/f) mice survived with significantly lesser liver damage, showing mainly necrotic cell death. Thus, complete inhibition of NF-kappaB activation in hepatocytes, in contrast to attenuation in hepatocyte-specific IKK2(-/-) mice, determines the type of liver cell damage during ischemia/reperfusion injury and is associated with a poor prognosis.

CONCLUSIONS

Our results show that understanding of the fine tuning of NF-kappaB modulation during liver injury is essential to develop new therapeutic strategies.

Hepatic tumor necrosis factor signaling and nuclear factor-kappaB: effects on liver homeostasis and beyond

The proinflammatory cytokine TNF has a pivotal role in liver pathophysiology because it holds the capacity to induce both hepatocyte cell death and hepatocyte proliferation. This dual effect of TNF on hepatocytes reflects its ability to induce both nuclear factor kappaB (NF-kappaB)-dependent gene expression and cell death. Multiple studies have demonstrated the crucial role of the transcription factor NF-kappaB in the decision between life and death of a hepatocyte. Massive hepatocyte apoptosis preceding embryonic lethality in NF-kappaB-deficient mice constituted the first indication of an essential antiapoptotic function of NF-kappaB in the liver. Although many studies confirmed this crucial cytoprotective role of NF-kappaB in adult liver, a number of genetic studies recently obtained conflicting results on the exact role of NF-kappaB in different mouse models of TNF hepatotoxicity, demonstrating that caution should be taken when interpreting studies using different NF-kappaB-deficient mice in distinct models of liver injury. Recent reports showing a role for hepatic NF-kappaB activation in the proliferation of malignant cells during hepatocarcinogenesis, and in the progression of fatty liver diseases to insulin resistance and type 2 diabetes mellitus demonstrate that NF-kappaB can also have more detrimental effects in the liver. Moreover, its role in the development of the metabolic syndrome emphasizes that hepatic NF-kappaB activation might also have adverse effects on the endocrine system. Therefore, understanding the regulation of hepatic TNF signaling and NF-kappaB activation is of critical therapeutic importance. In this review, we summarize how studies on the role of NF-kappaB in different mouse models of liver pathologies have contributed to this understanding.

Caspase inhibition sensitizes inhibitor of NF-kappaB kinase beta-deficient fibroblasts to caspase-independent cell death via the generation of reactive oxygen species

Cells lacking functional NF-kappaB die after ligation of some tumor necrosis factor (TNF) receptor family members through failure to express NF-kappaB-dependent anti-apoptotic genes. NF-kappaB activation requires the IkappaB kinase (IKK) complex containing two catalytic subunits named IKKalpha and IKKbeta that regulate distinct NF-kappaB pathways. IKKbeta is critical for classical signaling that induces pro-inflammatory and anti-apoptotic gene profiles, whereas IKKalpha regulates the non-canonical pathway involved in lymphoid organogenesis and B-cell development. To determine whether IKKalpha and IKKbeta differentially function in rescuing cells from death induced by activators of the classical and non-canonical pathways, we analyzed death after ligation of the TNF and lymphotoxin-beta receptors, respectively. Using murine embryonic fibroblasts (MEFs) lacking each of the IKKs, the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, and dominant negative Fas-associated death domain protein, we found that deletion of these kinases sensitized MEFs to distinct cell death pathways. MEFs lacking IKKalpha were sensitized to death in response to both cytokines that was entirely caspase-dependent, demonstrating that IKKalpha functions in this process. Surprisingly, death of IKKbeta-/- MEFs was not blocked by caspase inhibition, demonstrating that IKKbeta negatively regulates caspase-independent cell death (CICD). CICD was strongly activated by both TNF and lymphotoxin-beta receptor ligation in IKKbeta-/- MEFs and was accompanied by loss of mitochondrial membrane potential and the generation of reactive oxygen species. CICD was inhibited by the anti-oxidant butylated hydroxyanosole and overexpression of Bcl-2, neither of which blocked caspase-dependent apoptosis. Our findings, therefore, demonstrate that both IKKalpha and IKKbeta regulate cytokine-induced apoptosis, and IKKbeta additionally represses reactive oxygen species- and mitochondrial-dependent CICD.

Genetic inactivation of RelA/p65 sensitizes adult mouse hepatocytes to TNF-induced apoptosis in vivo and in vitro

BACKGROUND & AIMS

The transcription factor nuclear factor (NF)-kappaB plays a critical role in mediating survival of hepatocytes in response to tumor necrosis factor (TNF)-alpha during development because mice deficient for the NF-kappaB subunit RelA/p65 die in utero because of TNF-induced liver apoptosis. For the adult liver, conflicting concepts exist as to whether soluble TNF can trigger apoptosis when NF-kappaB activation is impaired. By creating a mouse model in which the transactivating NF-kappaB subunit RelA/p65 can be genetically inactivated in hepatocytes using the Cre/lox system, we sought to clarify the role of NF-kappaB in TNF-mediated hepatocyte apoptosis.

METHODS

Deletion of RelA/p65 in the liver was achieved using an inducible conditional knockout system (rela(F/F)MxCre mice) or, hepatocyte-specifically, using a developmental conditional knockout system (rela(F/F)AlbCre mice).

RESULTS

Disruption of RelA/p65 rendered mice sensitive to lethal liver injury upon TNF administration. Primary RelA/p65-deficient hepatocytes showed no NF-kappaB activation and undergo rapid apoptosis after TNF treatment. In contrast, hepatocytes deficient for I kappa B-kinase beta (IKK beta), displayed residual NF-kappaB activity and consecutively only mild apoptosis in response to TNF. TNF-induced apoptosis in RelA/p65-deficient hepatocytes was accompanied by prolonged activation of c-jun activating kinase (JNK) and rapid, largely proteasome-independent elimination of the long splice form of the antiapoptotic cellular FLICE inhibitor protein (c-FLIP(L)). Gene silencing of caspase-8, caspase-inhibitors, inhibition of JNK, or administration of antioxidants inhibited apoptosis and elimination of c-FLIP(L).

CONCLUSIONS

RelA/p65 is essential for TNF-induced NF-kappaB activation in adult hepatocytes. Genetic deletion of a functional RelA/p65 sensitizes these cells to apoptosis in response to soluble TNF in vivo and in vitro.

Sodium nitroprusside regulates mRNA expressions of LTC4 synthesis enzymes in hepatic ischemia/reperfusion injury rats via NF-kappaB signaling pathway

Leukotriene (LT) C4 (LTC4) synthesis enzymes including LTC4 synthase (LTC4S), microsomal glutathione S-transferase (MGST) 2 and MGST3 can all conjugate LTA4 and reduced glutathione (GSH) to form LTC4, which is related to hepatic ischemia/reperfusion (I/R) injury. The relationship between nitric oxide (NO) and cysteinyl LTs has been shown in previous studies. However, the mechanisms of NO action on gene expression of LTC4 synthesis enzymes are still largely unclear during hepatic I/R. Adult male Sprague-Dawley rats were divided into 5 groups: a sham group (control), an I/R group, and sodium nitroprusside (SNP, 2.5, 5 and 10 microg/kg/min)+I/R groups. Livers were subjected to 60 min of partial hepatic ischemia followed by 5 h of reperfusion, saline or SNP (2.5, 5 and 10 microg/kg/min) administered intravenously. The mRNA levels of LTC4 synthesis enzymes, inducible NO synthase (iNOS) and endothelial No synthase (eNOS) in rat liver tissue were examined by RT-PCR; the protein expressions of NF-kappaB p65, p50 and IkappaBalpha in liver cell lysates and nuclear extracts were detected by Western blot analysis, and serum NO2. levels were also evaluated. Serum NO2. levels, the protein expressions of NF-kappaB p65 and p50 in the nucleus extract, and hepatic mRNA expressions of LTC4S and iNOS were decreased while hepatic mRNA of eNOS was increased in the SNP (5 and 10 microg/kg/min)+I/R groups when compared with those in the I/R group. SNP (2.5 microg/kg/min) promoted the mRNA expressions of both MGST2 and MGST3, whereas SNP (10 microg/kg/min) increased MGST2 mRNA but decreased MGST3 mRNA compared to those in I/R group. Compared with control, the mRNA expression of MGST2 and MGST3 were elevated in SNP (2.5 microg/kg/min)+I/R group, MGST3 mRNA was significantly declined in the SNP (5 and 10 microg/kg/min)+I/R groups. Immunohistochemistry staining revealed that I/R liver exhibited strong cytoplasmic and nuclear staining for NF-kappaB p65, but the livers of the SNP (2.5 microg/kg/min)+I/R group presented slight cytoplasmic and nuclear staining. But IkappaBalpha protein in all groups remains unchanged. It was concluded that SNP downregulated LTC4S mRNA expression by inhibiting NF-kappaB activation independent of IkappaBalpha, but appeared to have a dual influence on the mRNA expressions of MGST2 and MGST3 by other signaling pathways during hepatic I/R injury.

Role of heat shock protein 70 in hepatic ischemia-reperfusion injury in mice

It is well established that liver ischemia-reperfusion induces the expression of heat shock protein (HSP) 70. However, the biological function of HSP70 in this injury is unclear. In this study, we sought to determine the role of HSP70 in hepatic ischemia-reperfusion injury in mice. Male mice were subjected to 90 min of partial hepatic ischemia followed by up to 8 h of reperfusion. HSP70 was rapidly upregulated after reperfusion. To explore the function of HSP70, sodium arsenite (8 mg/kg iv) was injected before surgery. We found that this dose induced HSP70 expression within 6 h of treatment. Induction of HSP70 with arsenite resulted in a >50% reduction in liver injury as determined by serum transaminases and histology. In addition, arsenite similarly reduced liver neutrophil recruitment and liver nuclear factor-kappaB activation, and attenuated serum levels of tumor necrosis factor-alpha and macrophage inflammatory protein-2, but increased levels of interleukin (IL)-6. In HSP70 knockout mice, arsenite did not protect against liver injury but did reduce liver neutrophil accumulation. Arsenite-induced reductions in neutrophil accumulation in HSP70 knockout mice were found to be mediated by IL-6. To determine whether extracellular HSP70 contributed to the injury, recombinant HSP70 was injected before surgery. Intravenous injection of 10 microg of recombinant HSP70 had no effect on liver injury after ischemia-reperfusion. The data suggest that intracellular HSP70 is directly hepatoprotective during ischemia-reperfusion injury and that extracellular HSP70 is not a significant contributor to the injury response in this model. Targeted induction of HSP70 may represent a potential therapeutic option for postischemic liver injury.

Deletion of NEMO/IKKgamma in liver parenchymal cells causes steatohepatitis and hepatocellular carcinoma

The IkappaB kinase (IKK) subunit NEMO/IKKgamma is essential for activation of the transcription factor NF-kappaB, which regulates cellular responses to inflammation. The function of NEMO in the adult liver remains elusive. Here we show that ablation of NEMO in liver parenchymal cells caused the spontaneous development of hepatocellular carcinoma in mice. Tumor development was preceded by chronic liver disease resembling human nonalcoholic steatohepatitis (NASH). Antioxidant treatment and genetic ablation of FADD demonstrated that death receptor-mediated and oxidative stress-dependent death of NEMO-deficient hepatocytes triggered disease pathogenesis in this model. These results reveal that NEMO-mediated NF-kappaB activation in hepatocytes has an essential physiological function to prevent the spontaneous development of steatohepatitis and hepatocellular carcinoma, identifying NEMO as a tumor suppressor in the liver.

Hepatocyte NF-kappaB activation is hepatoprotective during ischemia-reperfusion injury and is augmented by ischemic hypothermia

The present study examined the role of hepatocyte NF-kappaB activation during ischemia-reperfusion injury. Second, we evaluated the effects of ischemic hypothermia on NF-kappaB activation and liver injury. C57BL/6 mice underwent 90 min of partial hepatic ischemia and up to 8 h of reperfusion. Body temperature was regulated during the ischemic period between 35 and 37 degrees C, 33 and 35 degrees C, 29 and 33 degrees C or unregulated, where temperature fell to <29 degrees C. Liver injury, as measured by serum alanine aminotransferase as well as liver histopathology, was inversely proportional to regulated body temperature, with the unregulated group (<29 degrees C) being highly protected and the normothermic group (35-37 degrees C) displaying the greatest injury. Inflammation, as measured by production of TNF-alpha and liver recruitment of neutrophils, was greatest in the normothermic groups and lowest in the ischemic hypothermia groups. Interestingly, hepatocyte NF-kappaB activation was highest in the hypothermic group and least in the normothermic group. Paradoxically, degradation of IkappaB proteins, IkappaB-alpha and IkappaB-beta, was greatest in the normothermic group, suggesting an alternate NF-kappaB regulatory mechanism during ischemia-reperfusion injury. Subsequently, we found that NF-kappaB p65 protein was increasingly degraded in normothermic versus hypothermic groups, and this degradation was specific for hepatocytes and was associated with decreased expression of the peptidyl-prolyl isomerase Pin1. The data suggest that NF-kappaB activation in hepatocytes is a protective response during ischemia-reperfusion and can be augmented by ischemic hypothermia. Furthermore, it appears that Pin1 promotes NF-kappaB p65 protein stability such that decreased expression of Pin1 during ischemia-reperfusion results in p65 degradation, reduced nuclear translocation of NF-kappaB, and enhanced hepatocellular injury.

Hepatic fibrosis 2006: report of the Third AASLD Single Topic Conference

The Third American Associated for the Study of Liver Diseases (AASLD)-sponsored Single Topic Conference on hepatic fibrosis was held in June 2006. The conference was both international, with 6 countries represented, and cross-disciplinary, linking the basic molecular and cellular biology of fibrogenic cells to clinical trial design for emerging antifibrotic therapies. The specific goals of the conference were: (1) to consolidate knowledge about the natural history of fibrosis; (2) to clarify potential endpoints and markers; (3) to emphasize new antifibrotic targets developed on the basis of advances in basic science; and (4) to understand current critical issues pertaining to clinical trial design. Given the tremendous growth of the field and the constraints of a 2-day format, the selection of speakers was a challenge. A number of topics not included in the oral presentations were featured at poster sessions, lending breadth and depth to the meeting as a whole. Surprising new themes emerged about molecular, clinical, and regulatory aspects of the field, and a consensus emerged that hepatic fibrosis has matured into an integrated discipline that promises to significantly improve the prognosis of patients with fibrosing liver disease.

STAT3 is required for IL-6-gp130-dependent activation of hepcidin in vivo

BACKGROUND & AIMS

Hepcidin is a peptide hormone that is central to the regulation of iron homeostasis. In response to interleukin 6 (IL-6), hepatocytes produce hepcidin that decreases iron release/transfer from enterocytes and macrophages and causes hypoferremia. To clarify the molecular pathways involved in hepcidin activation by IL-6, we used different mice strains in which the main IL-6/gp130 signaling pathways have been genetically disrupted.

METHODS

We generated mice with hepatocyte-specific deletion of the IL-6 signal-transducing gp130 receptor (alfpgp130 (LoxP/LoxP)), with a gp130 receptor lacking the essential region for STAT1 and -3 activation (alfpCre gp130(DeltaSTAT/LoxP)) or mice expressing a gp130 allele lacking the essential tyrosine for RAS-MAPK activation (alfpCregp130(Y757F/LoxP)). We studied gp130-dependent pathways and hepcidin mRNA expression by Western blot, reverse-transcription polymerase chain reaction, and Northern blot in vivo and ex vivo.

RESULTS

IL-6 stimulated phospho STAT3, serum amyloid A (SAA), and suppressor of cytokine signaling 3 (SOCS3) expression in livers of wild-type and alfpCregp130(Y757F/LoxP) mice, whereas this response was blocked in alfpCre gp130(LoxP/LoxP) and alfpCre gp130(DeltaSTAT/LoxP) mice. In wild-type and alfpCregp130(Y757F/LoxP) animals, significantly higher hepcidin mRNA expression was found 3 to 6 hours after IL-6 stimulation. In contrast, no IL-6-dependent regulation of hepcidin mRNA expression was found in alfpgp130 (DeltaSTAT/LoxP) and AlfpCre gp130 (LoxP/LoxP) animals. In primary hepatocytes, higher hepcidin mRNA expression after IL-6 stimulation was only observed when gp130-STAT3-dependent signaling was intact.

CONCLUSIONS

We have demonstrated that both in vivo and in vitro STAT3 is the key transcription factor responsible for IL-6 activation of hepcidin gene expression in the liver.

Cholestasis protects the liver from ischaemic injury and post-ischaemic inflammation in the mouse

BACKGROUND AND HYPOTHESIS

Cholestasis is associated with high morbidity and mortality in patients undergoing major liver surgery, but the mechanisms responsible remain elusive. Increased ischaemic liver injury and inflammation may contribute to the poor outcome.

METHODS

Common bile duct ligation (biliary obstruction with hyperbilirubinaemia) or selective ligation of the left hepatic duct (biliary obstruction without hyperbilirubinaemia) was performed in C57BL/6 mice before 1 h of hepatic ischaemia and 1, 4 or 24 h of reperfusion. Infection with the intracellular hepatic pathogen Listeria monocytogenes for 12 and 48 h was used to study ischaemia-independent hepatic inflammation.

RESULTS

Cholestatic mice showed considerable protection from ischaemic liver injury as determined by transaminase release, histological liver injury and neutrophil infiltration. In cholestatic mice, reduced injury correlated with a failure to activate nuclear factor kappaB (NFkappaB) and tumour necrosis factor alpha (TNFalpha) mRNA synthesis, two key mediators of post-ischaemic liver inflammation. After selective bile duct ligation, both the ligated and the non-ligated lobes showed blocked activation of NFkappaB as well as reduced induction of TNFalpha mRNA synthesis and neutrophil infiltration. By contrast, infection with L monocytogenes showed comparable activation of NFkappaB and hepatic recruitment of neutrophils 12 h after infection.

CONCLUSION

Cholestasis does not increase but rather dramatically protects the liver from ischaemic injury and inflammation. This effect is mediated by a systemic factor, but not bilirubin, and is associated with a preserved capacity to trigger an inflammatory response to other stimuli such as a bacterial pathogen.

Targeted ablation of IKK2 improves skeletal muscle strength, maintains mass, and promotes regeneration

NF-kappaB is a major pleiotropic transcription factor modulating immune, inflammatory, cell survival, and proliferative responses, yet the relevance of NF-kappaB signaling in muscle physiology and disease is less well documented. Here we show that muscle-restricted NF-kappaB inhibition in mice, through targeted deletion of the activating kinase inhibitor of NF-kappaB kinase 2 (IKK2), shifted muscle fiber distribution and improved muscle force. In response to denervation, IKK2 depletion protected against atrophy, maintaining fiber type, size, and strength, increasing protein synthesis, and decreasing protein degradation. IKK2-depleted mice with a muscle-specific transgene expressing a local Igf-1 isoform (mIgf-1) showed enhanced protection against muscle atrophy. In response to muscle damage, IKK2 depletion facilitated skeletal muscle regeneration through enhanced satellite cell activation and reduced fibrosis. Our results establish IKK2/NF-kappaB signaling as an important modulator of muscle homeostasis and suggest a combined role for IKK inhibitors and growth factors in the therapy of muscle diseases.

Unravelling the complexities of the NF-κB signalling pathway using mouse knockout and transgenic models

The nuclear factor-kappaB (NF-kappaB) signalling pathway serves a crucial role in regulating the transcriptional responses of physiological processes that include cell division, cell survival, differentiation, immunity and inflammation. Here we outline studies using mouse models in which the core components of the NF-kappaB pathway, namely the IkappaB kinase subunits (IKKalpha, IKKbeta and NEMO), the IkappaB proteins (IkappaBalpha, IkappaBbeta, IkappaBvarepsilon and Bcl-3) and the five NF-kappaB transcription factors (NF-kappaB1, NF-kappaB2, c-Rel, RelA and RelB), have been genetically manipulated using transgenic and knockout technology.

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.

Mechanisms of crosstalk between TNF-induced NF-kappaB and JNK activation in hepatocytes

Hepatocyte cell death is a universal feature of inflammatory liver diseases. The observation that mice deficient in the activation of nuclear factor-kappaB (NF-kappaB) are not viable because of excessive hepatocyte apoptosis induced by tumor necrosis factor (TNF) made it crystal-clear that NF-kappaB plays a central role in protecting hepatocytes against TNF-induced cell death. Also during TNF-mediated liver injury, NF-kappaB was shown to have an essential anti-apoptotic effect, underscoring the therapeutic importance of understanding its underlying molecular mechanisms. For a long time, the ability of NF-kappaB to induce the expression of a variety of anti-apoptotic proteins was thought to be solely responsible for its cytoprotective effects. However, during the past few years it has become clear that NF-kappaB-mediated inhibition of cell death also involves attenuating TNF-induced activation of c-Jun activating kinase (JNK). Whereas transient activation of JNK upon TNF treatment is associated with cellular survival, prolonged JNK activation contributes to cell death. Several studies have shown that NF-kappaB activation inhibits the sustained phase of TNF-induced JNK activation and thus protects cells against TNF cytotoxicity. In this review, we will discuss the various mechanisms by which NF-kappaB activation blunts TNF-induced JNK activation, including the induction of JNK inhibitory proteins and controlling the levels of reactive oxygen species (ROS). Moreover, because the cytoprotective effects of NF-kappaB activation are particularly important in liver physiology, we will put each of these JNK-inhibitory mechanisms into a 'hepatic perspective' by discussing their role in various mouse models of TNF-mediated liver injury.

PKCzeta at the crossroad of NF-kappaB and Jak1/Stat6 signaling pathways

The atypical protein kinase C (PKC) isoforms (aPKC) have been implicated in the regulation of a number of essential signaling events. Early studies using dominant-negative mutants suggested that they are important intermediaries in the activation of the canonical nuclear factor (NF)-kappaB pathway. More recent data using knockout mice genetically demonstrate that in fact the PKCzeta isoform is essential for the adequate activation of this cascade both upstream and downstream the IkappaB kinase complex. In this review, we summarize the mechanistic details whereby the aPKC pathway regulates important cellular functions and how this is achieved by the ability of these kinases to interact with different protein regulators and adapters, as well as to impinge in NF-kappaB-independent signaling cascades such as the Janus kinase-1/signal transducer and activator of transcription 6 system, which plays a critical role in T-cell-mediated hepatitis and asthma.

Dissection of the NF-kappaB signalling cascade in transgenic and knockout mice

Studies in transgenic and knockout mice have made a major contribution to our current understanding of the physiological functions of the NF-kappaB signalling cascade. The generation and analysis of mice with targeted modifications of individual components of the NF-kappaB pathway tremendously advanced our knowledge of the roles of the NF-kappaB proteins themselves, and also of the many activators and negative regulators of NF-kappaB. These studies have highlighted the complexity of the NF-kappaB system, by revealing the multiple interactions, redundancies, but also diverse functions, performed by the different molecules participating in the regulation of NF-kappaB signalling. Furthermore, inhibition or enforced activation of NF-kappaB in transgenic mice has uncovered the critical roles that NF-kappaB plays in the pathogenesis of various diseases such as liver failure, diabetes and cancer.

Good cop, bad cop: the different faces of NF-kappaB

Complexes formed from the nuclear factor kappaB (NF-kappaB) family of transcription factors are ubiquitously expressed and are induced by a diverse array of stimuli. This results in their becoming activated in a wide variety of different settings. While the functions of NF-kappaB in many of these contexts have been the subject of intense research and are now well established, it is also clear that there is great diversity in the effects and consequences of NF-kappaB activation. NF-kappaB subunits do not necessarily regulate the same genes, in an identical manner, in all of the different circumstances in which they are induced. This review will discuss the different functions of NF-kappaB, the pathways that modulate NF-kappaB subunit activity and, in contrast to its more commonly thought of role as a promoter of cancer cell growth and survival, the ability of NF-kappaB, under some circumstances, to behave as a tumor suppressor.

IκB kinase 2 inhibition corrects defective nitrergic erectile mechanisms in diabetic mouse corpus cavernosum

OBJECTIVES

Oxidative or glyco-oxidative stress-induced activation of the transcription factor, nuclear factor (NF)-kappaB, is associated with the neurovascular complications of diabetes mellitus. Antioxidant treatment has beneficial effects in diabetic patients; however, delineating a possible role for NF-kappaB deactivation against direct antioxidant effects has been difficult. NF-kappaB is negatively regulated by the inhibitor of kappaB (IkappaB) complex that, in turn, is activated by specific kinases. Thus, the aim was to investigate the effects of the IkappaB kinase 2 inhibitor, AS602868, on corpus cavernosum function in diabetic mice.

METHODS

Diabetes was induced by streptozotocin; the duration was 6 weeks. Intervention AS602868 treatment (100 mg/kg/day) was given for 2 weeks after 4 weeks of untreated diabetes. Corpora cavernosum were isolated in organ baths for measurement of agonist-evoked or electrical stimulation-evoked smooth muscle tensions.

RESULTS

The maximal nitrergic nerve-mediated relaxation of phenylephrine-precontracted cavernosum was reduced approximately 30% by diabetes (P <0.001). AS602868 treatment completely reversed the deficit (P <0.001). Maximal nitric oxide-mediated endothelium-dependent relaxation to acetylcholine was attenuated approximately 32% by diabetes (P <0.05). This was completely restored by IkappaB kinase 2 inhibition (P <0.01). Furthermore, AS602868 treatment also completely corrected (P <0.01) an approximate 20% diabetic deficit (P <0.001) in maximal endothelium-independent relaxation to the nitric oxide donor, sodium nitroprusside.

CONCLUSIONS

Inhibition of IkappaB kinase 2 can correct nitric oxide-dependent indexes of diabetic erectile dysfunction. This suggests that NF-kappaB activation is important in the development of diabetic cavernosum nitrergic neuropathy and vasculopathy.

Modulation of Hsp90 function in neurodegenerative disorders: a molecular-targeted therapy against disease-causing protein

Abnormal accumulation of disease-causing protein is a commonly observed characteristic in chronic neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and polyglutamine (polyQ) diseases. A therapeutic approach that could selectively eliminate would be a promising remedy for neurodegenerative disorders. Spinal and bulbar muscular atrophy (SBMA), one of the polyQ diseases, is a late-onset motor neuron disease characterized by proximal muscle atrophy, weakness, contraction fasciculations, and bulbar involvement. The pathogenic gene product is polyQ-expanded androgen receptor (AR), which belongs to the heat shock protein (Hsp) 90 client protein family. 17-Allylamino-17-demethoxygeldanamycin (17-AAG), a novel Hsp90 inhibitor, is a new derivative of geldanamycin that shares its important biological activities but shows less toxicity. 17-AAG is now in phase II clinical trials as a potential anti-cancer agent because of its ability to selectively degrade several oncoproteins. We have recently demonstrated the efficacy and safety of 17-AAG in a mouse model of SBMA. The administration of 17-AAG significantly ameliorated polyQ-mediated motor neuron degeneration by reducing the total amount of mutant AR. 17-AAG accomplished the preferential reduction of mutant AR mainly through Hsp90 chaperone complex formation and subsequent proteasome-dependent degradation. 17-AAG induced Hsp70 and Hsp40 in vivo as previously reported; however, its ability to induce HSPs was limited, suggesting that the HSP induction might support the degradation of mutant protein. The ability of 17-AAG to preferentially degrade mutant protein would be directly applicable to SBMA and other neurodegenerative diseases in which the disease-causing proteins also belong to the Hsp90 client protein family. Our proposed therapeutic approach, modulation of Hsp90 function by 17-AAG treatment, has emerged as a candidate for molecular-targeted therapies for neurodegenerative diseases. This review will consider our research findings and discuss the possibility of a clinical application of 17-AAG to SBMA and other neurodegenerative diseases.

Hepatic ischemia-reperfusion injury: roles of Ca2+ and other intracellular mediators of impaired bile flow and hepatocyte damage

Liver resection and liver transplantation have been successful in the treatment of liver tumors and end-stage liver disease. This success has led to an expansion in the pool of patients potentially treatable by liver surgery and, in the case of transplantation, to a shortage of liver donors. At present, there are significant numbers of potential candidates for liver resection and liver donation who have fatty livers, are aged, or have livers damaged by chemotherapy. All of these are at high risk for ischemic reperfusion (IR) injury. The aims of this review are to assess current knowledge of the clinical effectiveness of ischemic preconditioning and intermittent ischemia in reducing IR damage in liver surgery; to evaluate the use of bile flow as a sensitive indicator of IR liver damage; and to analyze the molecular mechanisms, especially intracellular Ca2+, involved in IR injury and ischemic preconditioning. It is concluded that bile flow is a sensitive indicator of IR injury. Together with reactive oxygen species (ROS) and other extracellular and intracellular signaling molecules, intracellular Ca2+ in hepatocytes plays a key role in the normal regulation of bile flow and in IR-induced injury and cell death. Ischemic preconditioning is an effective strategy to reduce IR injury but there is considerable scope for improvement, especially in patients with fatty and aged livers. The development of effective new strategies to reduce IR injury will depend on improved understanding of the molecular mechanisms involved, especially by gaining a better perspective of the relative importance of the various intrahepatocyte signaling pathways involved.

Mechanisms of Liver Injury. I. TNF-alpha-induced liver injury: role of IKK, JNK, and ROS pathways

TNF-alpha activates several intracellular pathways to regulate inflammation, cell death, and proliferation. In the liver, TNF-alpha is not only a mediator of hepatotoxicity but also contributes to the restoration of functional liver mass by driving hepatocyte proliferation and liver regeneration. This review summarizes recent advances in TNF-alpha signaling mechanisms that demonstrate how the IKK, ROS, and JNK pathways interact with each other to regulate hepatocyte apoptosis and proliferation. Activation of these pathways is causatively linked to liver injury induced by concanavalin A, TNF-alpha, and ischemia-reperfusion and to liver regeneration and hepatocarcinogenesis. In light of recent findings, pharmacological inhibitors of JNK and IKK and antioxidants may be promising new tools for the treatment of hepatitis, ischemia-reperfusion injury, and hepatocellular carcinoma.

Pro-apoptotic role of NF-kappaB: implications for cancer therapy

Nuclear factor-kappaB (NF-kappaB) is generally viewed as anti-apoptotic and oncogenic, leading to a quest for its inhibitors. However, recent evidence suggests that in some situations NF-kappaB may promote apoptosis. Depending on the specific cell type and the stimulus involved, NF-kappaB activation may lead to either anti- or pro-apoptotic response. Both these effects can be mediated by NF-kappaB in a context-dependent manner by selectively regulating its target genes. In this review, we discuss the evidence for NF-kappaB's pro-apoptotic role and explore the possible mechanisms behind it. We emphasize that rather than trying to inhibit NF-kappaB in cancer therapy, agents should be developed to unleash its pro-apoptotic ability.

Kinase inhibitors and airway inflammation

Kinases are believed to play a crucial role in the expression and activation of inflammatory mediators in the airway, in T-cell function and airway remodelling. Important kinases such as Inhibitor of kappaB kinase (IKK)2, mitogen activated protein (MAP) kinases and phsopho-inositol (PI)3 kinase regulate inflammation either through activation of pro-inflammatory transcription factors such as activating protein-1 (AP-1) and nuclear factor kappaB (NF-kappaB), which are activated in airway disease, or through regulation of mRNA half-life. Selective kinase inhibitors have been developed which reduce inflammation and some characteristics of disease in animal models. Targeting specific kinases that are overexpressed or over active in disease should allow for selective treatment of respiratory diseases. Interest in this area has intensified due to the success of the specific Abelson murine leukaemia viral oncogene (Abl) kinase inhibitor imatinib mesylate (Gleevec) in the treatment of chronic myelogenous leukaemia. Encouraging data from animal models and primary cells and early Phase I and II studies in other diseases suggest that inhibitors of p38 MAP kinase and IKK2 may prove to be useful novel therapies in the treatment of severe asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis and other inflammatory airway diseases.

IKK mediates ischemia-induced neuronal death

The IkappaB kinase complex IKK is a central component of the signaling cascade that controls NF-kappaB-dependent gene transcription. So far, its function in the brain is largely unknown. Here, we show that IKK is activated in a mouse model of stroke. To investigate the function of IKK in brain ischemia we generated mice that contain a targeted deletion of Ikbkb (which encodes IKK2) in mouse neurons and mice that express a dominant inhibitor of IKK in neurons. In both lines, inhibition of IKK activity markedly reduced infarct size. In contrast, constitutive activation of IKK2 enlarged the infarct size. A selective small-molecule inhibitor of IKK mimicked the effect of genetic IKK inhibition in neurons, reducing the infarct volume and cell death in a therapeutic time window of 4.5 h. These data indicate a key function of IKK in ischemic brain damage and suggest a potential role for IKK inhibitors in stroke therapy.

Nuclear factor kappa B signaling in macrophage function and atherogenesis

PURPOSE OF REVIEW

Atherosclerosis is a chronic inflammatory disease of the medium and large-sized arteries. Nuclear factor kappaB transcription factors are major regulators of inflammatory responses, and aberrant nuclear factor kappaB regulation is linked to a large number of diseases. Focusing on macrophages, this review will discuss recent literature on the role of nuclear factor kappaB and the signaling pathways regulating its activity in atherosclerosis.

RECENT FINDINGS

After the initial identification of activated nuclear factor kappaB in human atherosclerotic lesions, the involvement of this family of transcription factors in atherogenesis has gained growing attention. It is now clear that signaling pathways activating nuclear factor kappaB, and nuclear factor kappaB action, constitute major players at all stages of the atherosclerotic process. Long considered a pro-atherogenic factor, recent studies indicate that the actual role of nuclear factor kappaB might prove to be far more complex. Apart from activating many pro-inflammatory genes linked to atherogenesis, nuclear factor kappaB regulates cellular processes such as cell survival and proliferation. In addition, its important role in inflammatory resolution and anti-inflammatory gene transcription suggests that its activation at different cell types or different stages of the atherosclerotic process might have distinct and opposing results.

SUMMARY

The numerous diseases in which aberrant nuclear factor kappaB action is found to play a crucial role makes it an intensively studied target for drug interventions. However, given its pleiotropic functions in inflammation and immunity, a more targeted modulation of its activity, at a cell type-specific or disease stage-specific level, could provide safer therapeutic solutions.

Activation and function of hepatocyte NF-kappaB in postischemic liver injury

The inhibitor of NF-kappaB (I-kappaB) kinase (IKK) complex consists of 3 subunits, IKK1, IKK2, and NF-kappaB essential modulator (NEMO), and is involved in the activation of NF-kappaB by various stimuli. IKK2 or NEMO constitutive knockout mice die during embryogenesis as a result of massive hepatic apoptosis. Therefore, we examined the role of IKK2 in TNF-induced apoptosis and ischemia/reperfusion (I/R) injury in the liver by using conditional knockout mice. Hepatocyte-specific ablation of IKK2 did not lead to impaired activation of NF-kappaB or increased apoptosis after TNF-alpha stimulation whereas conditional NEMO knockout resulted in complete block of NF-kappaB activation and massive hepatocyte apoptosis. In a model of partial hepatic I/R injury, mice lacking IKK2 in hepatocytes displayed significantly reduced liver necrosis and inflammation than wild-type mice. AS602868, a novel chemical inhibitor of IKK2, protected mice from liver injury due to I/R without sensitizing them toward TNF-induced apoptosis and could therefore emerge as a new pharmacological therapy for liver resection, hemorrhagic shock, or transplantation surgery.