Caspase activation during hepatocyte apoptosis induced by tumor necrosis factor-alpha in galactosamine-sensitized mice

Bromelain ameliorates D-galactosamine-induced acute liver injury: role of SIRT1/LKB1/AMPK, GSK3β/Nrf2 and NF-κB p65/TNF-α/caspase-8, -9 signalling pathways

OBJECTIVES

The present research focused on estimating, for the first time, the potential protective effects of bromelain against D-galactosamine-induced acute liver injury in rats as well as identifying the possible underlying mechanisms.

METHODS

Silymarin (100 mg/kg/day, p.o.) as a reference drug or bromelain (20 and 40 mg/kg/day, p.o.) were administered for 10 days, and on the 8th day of the experiment, a single dose of galactosamine (400 mg/kg/i.p.) induced acute liver injury.

KEY FINDINGS

Pretreatment with bromelain improved liver functions and histopathological alterations induced by galactosamine. Bromelain ameliorated oxidative stress by inducing SIRT1 protein expression and increasing LKB1 content. This resulted in phosphorylating the AMPK/GSK3β axis, which stimulated Nrf2 activation in hepatic cells and thus increased the activity of its downstream antioxidant enzymes [HO-1 and NQO1]. Besides, bromelain exerted significant anti-apoptotic and anti-inflammatory effects by suppressing hepatic contents of TNF-α, NF-κB p65, as well as caspase-8 and caspase-9. The protective effects of bromelain40 were proved to be better than silymarin and bromelain20 in most of the assessed parameters.

CONCLUSIONS

Our results highlight the significant hepatoprotective effects of bromelain against acute liver injury through modulation of SIRT1/LKB1/AMPK, GSK3β/Nrf2 signalling in addition to NF-κB p65/TNF-α/ caspase-8 and -9 pathway.

Does high mobility group 1 protein function as a late mediator for LPS- or TNF-induced shock in galactosamine-sensitized mice?

The role of high mobility group-1 protein (HMG-1) in LPS- and TNF-a-induced lethal shock in galactosamine (GalN)-sensitized mice was investigated. No detectable HMG-1 levels were observed by immunoblotting analysis in plasma from untreated or GalN-sensitized BALB/c mice 5 h after LPS injection, although significant levels of HMG-1 were detected in plasma 6 h after the challenge. All GalN-sensitized BALB/c but not BALB/lpsd mice succumbed by 6 h after LPS injection. When GalN-sensitized mice were injected with TNF-α, the presence of HMG-1 was seen at 5.5 h in plasma of BALB/c mice and at 6 h in BALB/lpsd mice, although almost all GalN-sensitized BALB/c mice died by 6 h after challenge. The time-dependent phenomenon correlated with elevated serum aspartate aminotransferase (AST) levels and the appearance of apoptotic cells in livers. Administration of pooled plasma, equivalent to approximately 200 μg recombinant murine HMG-1, taken from mice on the verge of near death, did not result in induction of lethal shock in GalN-sensitized mice. Taken together with the late appearance of HMG-1 in moribund mice, these data suggest that HMG-1 does not decisively contribute to lethality in the GalN sensitization model.

Tumor necrosis factor-α promotes cholestasis-induced liver fibrosis in the mouse through tissue inhibitor of metalloproteinase-1 production in hepatic stellate cells

Tumor necrosis factor (TNF)-α, which is a mediator of hepatotoxicity, has been implicated in liver fibrosis. However, the roles of TNF-α on hepatic stellate cell (HSC) activation and liver fibrosis are complicated and remain controversial. To explore this issue, the role of TNF-α in cholestasis-induced liver fibrosis was examined by comparing between TNF-α(-/-) mice and TNF-α(+/+) mice after bile duct ligation (BDL). Serum TNF-α levels in mice were increased by common BDL combined with cystic duct ligation (CBDL+CDL). TNF-α deficiency reduced liver fibrosis without affecting liver injury, inflammatory cell infiltration, and liver regeneration after CBDL+CDL. Increased expression levels of collagen α1(I) mRNA, transforming growth factor (TGF)-β mRNA, and α-smooth muscle actin (αSMA) protein by CBDL+CDL in the livers of TNF-α(-/-) mice were comparable to those in TNF-α(+/+) mice. Exogenous administration of TNF-α decreased collagen α1(I) mRNA expression in isolated rat HSCs. These results suggest that the reduced fibrosis in TNF-α(-/-) mice is regulated in post-transcriptional level. Tissue inhibitor of metalloproteinase (TIMP)-1 plays a crucial role in the pathogenesis of liver fibrosis. TIMP-1 expression in HSCs in the liver was increased by CBDL+CDL, and the induction was lower in TNF-α(-/-) mice than in TNF-α(+/+) mice. Fibrosis in the lobe of TIMP-1(-/-) mice with partial BDL was also reduced. These findings indicate that TNF-α produced by cholestasis can promote liver fibrosis via TIMP-1 production from HSCs. Thus, targeting TNF-α and TIMP-1 may become a new therapeutic strategy for treating liver fibrosis in cholestatic liver injury.

Camel's milk alleviates alcohol-induced liver injury in rats

Alcoholic liver disease (ALD) represents a spectrum of clinical illness and morphological changes that range from fatty liver, hepatic inflammation and necrosis to progressive fibrosis. For the etiology of ALD, oxidative stress, increased expression of proinflammatory cytokines and apoptosis have been described. The present study aimed to investigate the effectiveness of camel's milk (CM) in alleviating alcohol-induced hepatotoxicity as a model of clinical liver illness. Male rats were grouped into four groups from which one group received normal saline and served as control. Groups from 2 to 4 received a daily oral dose of 56% ethanol for 4 weeks. Group 2 served as untreated control while groups 3 and 4 were respectively treated with CM either in a prophylactic or a curative approach. Alanine transaminase, aspartate transaminase, alkaline phosphatase, triglycerides, as well as cholesterol levels were estimated in the serum. Malondialdehyde, total antioxidant capacity, and tumor necrosis factor-alpha levels along with caspase-3 activity were determined in liver tissue homogenate. A histopathological analysis of liver tissue was also achieved. Results showed amelioration of all tested parameters following administration of CM. Conclusively, treatment with camel's milk alleviates alcohol-associated hazards and protects hepatic tissue from alcohol-induced toxicity.

Disruption of TAK1 in hepatocytes causes hepatic injury, inflammation, fibrosis, and carcinogenesis

TGF-beta-activated kinase 1 (TAK1) is a MAP3K family member that activates NF-kappaB and JNK via Toll-like receptors and the receptors for IL-1, TNF-alpha, and TGF-beta. Because the TAK1 downstream molecules NF-kappaB and JNK have opposite effects on cell death and carcinogenesis, the role of TAK1 in the liver is unpredictable. To address this issue, we generated hepatocyte-specific Tak1-deficient (Tak1DeltaHEP) mice. The Tak1DeltaHEP mice displayed spontaneous hepatocyte death, compensatory proliferation, inflammatory cell infiltration, and perisinusoidal fibrosis at age 1 month. Older Tak1DeltaHEP mice developed multiple cancer nodules characterized by increased expression of fetal liver genes including alpha-fetoprotein. Cultures of primary hepatocytes deficient in Tak1 exhibited spontaneous cell death that was further increased in response to TNF-alpha. TNF-alpha increased caspase-3 activity but activated neither NF-kappaB nor JNK in Tak1-deficient hepatocytes. Genetic abrogation of TNF receptor type I (TNFRI) in Tak1DeltaHEP mice reduced liver damage, inflammation, and fibrosis compared with unmodified Tak1DeltaHEP mice. In conclusion, hepatocyte-specific deletion of TAK1 in mice resulted in spontaneous hepatocyte death, inflammation, fibrosis, and carcinogenesis that was partially mediated by TNFR signaling, indicating that TAK1 is an essential component for cellular homeostasis in the liver.

Bezafibrate decreases growth stimulatory action of the sphingomyelin signaling pathway in regenerating rat liver

Liver regeneration after partial hepatectomy (PH) is achieved through proliferation of hepatocytes and non-parenchymal cells. The nuclear peroxisome proliferator-activated receptor alpha (PPARalpha) is involved in regulation of lipid metabolism and proliferation of hepatic cells. The sphingomyelin signal transduction pathway is involved in the regulation of the cell cycle in eukaryotic organisms. Sphingosine-1-phosphate (S1P) and ceramide (CER)-- the intermediates of the pathway--are known to stimulate and to inhibit cellular proliferation. The aim of the present study was to investigate the effect of PPARalpha activation by bezafibrate on the sphingomyelin signaling pathway during the first 24h of liver regeneration after PH in the rat. The content of sphingomyelin, ceramide, sphingosine, sphinganine, sphingosine-1-phosphate and the activity of sphingomyelinases and ceramidases were determined at various time points after PH. It has been found that the activity of neutral Mg(2+)-dependent sphingomyelinase (nSMase) increased, whereas the activity of acidic sphingomyelinase (aSMase) decreased in the regenerating liver. Activation of PPARalpha by bezafibrate lower the activity of nSMase and increased the activity of aSMase in the regenerating rat liver. The content of ceramide was higher in bezafibrate-treated rats, whereas the content of sphingosine-1-phosphate was markedly lower as compared to the untreated rats. Therefore, it is concluded that activation of PPARalpha by bezafibrate decreases the growth-stimulatory activity of the sphingomyelin pathway in regenerating rat liver.

Chylomicron-bound LPS selectively inhibits the hepatocellular response to proinflammatory cytokines

BACKGROUND/AIMS

Pretreatment of rodent hepatocytes with chylomicron-bound lipopolysaccharide (CM-LPS) renders these cells unresponsive to subsequent stimulation by proinflammatory cytokines. We sought to test the selectivity of this response.

METHODS

Cellular responses to hypoxia, oxidative stress, apoptosis, and heat-shock response, and thermotolerance induced in CM-LPS pretreated hepatocytes were compared with responses in non-pretreated cells.

RESULTS

CM-LPS inhibited the hepatocellular response to proinflammatory cytokines without affecting the response to the other cellular stressors. It did not affect the response to oxidative stress, as measured by mitochondrial activity after hydrogen peroxide was added, or protein induction before or after stimulation with cobalt chloride. Also, induction of heat shock proteins did not differ between the CM-LPS pretreated cells and non-pretreated cells. CM-LPS did not interfere with the adoption of the thermotolerant phenotype, as shown by similar mitochondrial activity between pretreated and non-pretreated cells. Although stimulation with tumor necrosis factor-alpha and actinomycin D increased activity of the apoptotic enzymes, there were no differences between cells pretreated with CM-LPS and non-pretreated hepatocytes.

CONCLUSION

When the response to proinflammatory cytokines is inhibited, hepatocellular responses to hypoxia, oxidative stress, heat shock, and apoptosis remain intact after pretreatment with CM-LPS. CM-LPS may have a specific anti-inflammatory effect on hepatocytes.

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.

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.

Systemic mediators induce fibrogenic effects in normal liver after partial bile duct ligation

BACKGROUND/AIMS

Collagen production by activated hepatic stellate cells (HSCs) is a key event in liver fibrosis, and a number of factors have been characterized that trigger HSC activation and collagen production. However, it remains unclear if these factors act locally at the site of injury or also affect HSCs distant to the site of injury.

METHODS

A model of partial bile duct ligation (PBDL) in which fibrogenesis can be compared between the injured ligated lobe and the non-ligated lobe.

RESULTS

After PBDL, HSCs showed an increased expression of procollagen type I alpha1 mRNA and collagen-reporter gene activity not only in the ligated lobe, but also in the non-ligated lobe, albeit at a lower level. In contrast, an increase in the number of desmin- and alpha-smooth muscle actin positive HSCs, and accumulation of inflammatory cells were observed only in the ligated lobe. Although transforming growth factor-beta (TGF-beta) mRNA was increased only in the ligated lobe, Smad2/3 were activated in the ligated and the non-ligated lobe. These data suggest that the systemic increase in profibrogenic mediators including TGF-beta induces collagen transcription in the uninjured liver.

CONCLUSION

Systemic profibrogenic mediators from the injury site affect the residual non-injured liver.

CpG DNA-mediated Induction of Acute Liver Injury in d-Galactosamine-sensitized Mice

Unmethylated CpG motifs present in bacterial DNA (CpG DNA) induce innate inflammatory responses, including rapid induction of proinflammatory cytokines. Although innate inflammatory responses induced by CpG DNA and other pathogen-associated molecular patterns are essential for the eradication of infectious microorganisms, excessive activation of innate immunity is detrimental to the host. In this study, we demonstrate that CpG DNA, but not control non-CpG DNA, induces a fulminant liver failure with subsequent shock-mediated death by promoting massive apoptotic death of hepatocytes in D-galactosamine (D-GalN)-sensitized mice. Inhibition of mitochondrial membrane permeability transition pore opening or caspase 9 activity in vivo protects D-GalN-sensitized mice from the CpG DNA-mediated liver injury and death. CpG DNA enhanced production of proinflammatory cytokines in D-GalN-sensitized mice via a TLR9/MyD88-dependent pathway. In addition, CpG DNA failed to induce massive hepatocyte apoptosis and subsequent fulminant liver failure and death in D-GalN-sensitized mice that lack TLR9, MyD88, tumor necrosis factor (TNF)-alpha, or TNF receptor I but not interleukin-6 or -12p40. Taken together, our results provide direct evidence that CpG DNA induces a severe acute liver injury and shock-mediated death through the mitochondrial apoptotic pathway-dependent death of hepatocytes caused by an enhanced production of TNF-alpha through a TLR9/MyD88 signaling pathway in D-GalN-sensitized mice.

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.

Roles for C16-ceramide and sphingosine 1-phosphate in regulating hepatocyte apoptosis in response to tumor necrosis factor-alpha

Tumor necrosis factor (TNF)-alpha signals cell death and simultaneously induces the generation of ceramide, which is metabolized to sphingosine and sphingosine 1-phosphate (S1P) by ceramidase (CDase) and sphingosine kinase. Because the dynamic balance between the intracellular levels of ceramide and S1P (the "ceramide/S1P rheostat") may determine cell survival, we investigated these sphingolipid signaling pathways in TNF-alpha-induced apoptosis of primary hepatocytes. Endogenous C16-ceramide was elevated during TNF-alpha-induced apoptosis in both rat and mouse primary hepatocytes. The putative acid sphingomyelinase (ASMase) inhibitor imipramine inhibited TNF-alpha-induced apoptosis and C16-ceramide increase as did the knock out of ASMase. Overexpression of neutral CDase (NCDase) inhibited the TNF-alpha-induced increase of C16-ceramide and apoptosis in rat primary hepatocytes. Moreover, NCDase inhibited liver injury and hepatocyte apoptosis in mice treated with D-galactosamine plus TNF-alpha. This protective effect was abrogated by the sphingosine kinase inhibitor N,N-demethylsphingosine, suggesting that the survival effect of NCDase is due to not only C16-ceramide reduction but also S1P formation. Administration of S1P or overexpression of NCDase activated the pro-survival kinase AKT, and overexpression of dominant negative AKT blocked the survival effect of NCDase. In conclusion, activation of ASMase and generation of C16-ceramide contributed to TNF-alpha-induced hepatocyte apoptosis. NCDase prevented apoptosis both by reducing C16-ceramide and by activation of AKT through S1P formation. Therefore, the cross-talk between sphingolipids and AKT pathway may determine hepatocyte apoptosis by TNF-alpha.

PGE1 abolishes the mitochondrial-independent cell death pathway induced by D-galactosamine in primary culture of rat hepatocytes

BACKGROUND AND AIM

PGE1 reduces in vivo and in vitro D-galactosamine (D-GalN)-induced cell death in hepatocytes. The present study was undertaken to elucidate the intracellular pathway by which D-GalN induces cell death in cultured hepatocytes. In addition, we evaluated if PGE1 was able to modulate different parameters related to D-GalN-induced apoptosis in cultured rat hepatocytes.

METHODS

Hepatocytes were isolated from male Wistar rats (225-275 g) by the classical collagenase procedure. PGE1 (1 microM) was administered 2 h before D-GalN (5 mM) in primary culture of rat hepatocytes. Apoptosis was determined by DNA fragmentation and caspase-3, -6, -8 and -9 activation in hepatocytes. Caspase activation was evaluated by the detection of the related cleaved product and its associated activity. Cell necrosis was determined by the measurement of lactate dehydrogenase (LDH) activity in culture medium. To elucidate the role of mitochondria, we measured neutral (nSMase) and acid (aSMase) sphingomyelinase, as well as the expression of cytochrome c in mitochondria and cytoplasm fractions from D-GalN treated hepatocytes.

RESULTS

D-GalN induced caspase-3 activation and DNA fragmentation in hepatocytes. This apoptotic response was not associated with the activation of caspase-6, -8 or -9. The use of specific inhibitors confirmed that only caspase-3 was involved in D-GalN-induced apoptosis. D-GalN did not modify nSMase and aSMase activities, nor mitochondrial cytochrome c release in hepatocytes.

CONCLUSIONS

D-GalN induced apoptosis through caspase-3 activation but without modification of the activity of caspase-6, -8, -9, SMases or cytochrome c release. PGE1 appears to prevent D-GalN-induced apoptosis by a mitochondria-independent mechanism.

Role of tumor necrosis factor-alpha in the development of spontaneous hepatic toxicity in Long-Evans Cinnamon rats

The objective of this study was to evaluate the potential role of TNF-alpha in the onset of acute hepatitis in the Long-Evans Cinnamon (LEC) rat, an animal model for inherited copper (Cu) toxicosis. In LEC rats, Cu is accumulated in the liver with age, and clinical signs of acute hepatitis were observed as, icterus, reduced body weight, nasal bleeding, dehydration, and reduced food intake at 12 weeks of age. Cellular changes such as apoptosis in the liver were evident in these rats with increasing age. Positive TNF-alpha and TNFR1 immunostainings were observed in hepatocytes and Kupffer cells in LEC rats. Hepatic levels of caspase-3 activity, TNF-alpha mRNA, and protein were also increased in LEC rats from 6 to 12 weeks of age as compared with control Long-Evans (LE) rats. The neutralization of TNF-alpha by passive immunization or the inhibition of caspase activity can block the apoptotic process initiated by TNF-alpha. In this study, we evaluated the effects of passive immunization of LEC rats with weekly administration of anti-rat TNF-alpha on Cu-induced acute hepatitis. This treatment resulted in a reduction of the percentage of apoptotic cells in the liver, decreased activity of caspase-3, and also in down-regulation of the TNF-alpha gene expression. Thus, these results suggest a major role for TNF-alpha on the pathogenesis of Cu-induced acute hepatitis in LEC rats.

Nuclear import of proinflammatory transcription factors is required for massive liver apoptosis induced by bacterial lipopolysaccharide

Stimulation of macrophages with lipopolysaccharide (LPS) leads to the production of cytokines that elicit massive liver apoptosis. We investigated the in vivo role of stress-responsive transcription factors (SRTFs) in this process focusing on the precipitating events that are sensitive to a cell-permeant peptide inhibitor of SRTF nuclear import (cSN50). In the absence of cSN50, mice challenged with LPS displayed very early bursts of inflammatory cytokines/chemokines, tumor necrosis factor alpha (1 h), interleukin 6 (2 h), interleukin 1 beta (2 h), and monocyte chemoattractant protein 1 (2 h). Activation of both initiator caspases 8 and 9 and effector caspase 3 was noted 4 h later when full-blown DNA fragmentation and chromatin condensation were first observed (6 h). At this time an increase of pro-apoptotic Bax gene expression was observed. It was preceded by a decrease of anti-apoptotic Bcl2 and BclX(L) gene transcripts. Massive apoptosis was accompanied by microvascular injury manifested by hemorrhagic necrosis and a precipitous drop in blood platelets observed at 6 h. An increase in fibrinogen/fibrin degradation products and a rise in plasminogen activator inhibitor 1 occurred between 4 and 6 h. Inhibition of SRTFs nuclear import with the cSN50 peptide abrogated all these changes and increased survival from 7 to 71%. Thus, the nuclear import of SRTFs induced by LPS is a prerequisite for activation of the genetic program that governs cytokines/chemokines production, liver apoptosis, microvascular injury, and death. These results should facilitate the rational design of drugs that protect the liver from inflammation-driven apoptosis.

Role of FAN in tumor necrosis factor-alpha and lipopolysaccharide-induced interleukin-6 secretion and lethality in D-galactosamine-sensitized mice

Tumor necrosis factor (TNF) alpha-induced neutral sphingomyelinase-mediated generation of ceramide, a bioactive lipid molecule, is transduced by the adaptor protein FAN, which binds to the intracellular region of the CD120a TNFalpha receptor. FAN-deficient mice do not exhibit any gross abnormality. To further explore the functions of FAN in vivo and because CD120a-deficient mice are resistant to endotoxin-induced liver failure and lethality, we investigated the susceptibility of FAN-deficient animals to lipopolysaccharide (LPS). We show that after d-galactosamine sensitization, FAN-deficient mice were partially resistant to LPS- and TNFalpha-induced lethality. Although LPS challenge resulted in a hepatic ceramide content lower in mutant mice than in control animals, it triggered similar histological alterations, caspase activation, and DNA fragmentation in the liver. Interestingly, LPS-induced elevation of IL-6 (but not TNFalpha) serum concentrations was attenuated in FAN-deficient mice. A less pronounced secretion of IL-6 was also observed after LPS or TNFalpha treatment of cultured peritoneal macrophages and embryonic fibroblasts isolated from FAN-deficient mice, as well as in human fibroblasts expressing a mutated FAN. Finally, we show that d-galactosamine-sensitized IL-6-deficient mice were partially resistant to endotoxin-induced liver apoptosis and lethality. These findings highlight the role of FAN and IL-6 in the inflammatory response initiated by endotoxin, implicating TNFalpha.

Inhibition of nuclear factor kappaB and phosphatidylinositol 3-kinase/Akt is essential for massive hepatocyte apoptosis induced by tumor necrosis factor alpha in mice

BACKGROUND/AIMS

Tumor necrosis factor (TNF)-alpha itself does not induce liver injury in normal mice or hepatocytes. Rather, this event, especially in vitro, is explained by the fact that the TNF-alpha/TNF receptor system not only triggers downstream signals leading to apoptosis but also induces an antiapoptotic pathway through the activation of nuclear factor (NF)-kappaB. The aim of this study was to determine whether inhibition of antiapoptotic pathways influences the susceptibility of mice to TNF-alpha. Here, we focused on the roles of NF-kappaB and phosphatidylinositol 3-kinase (PI3K)-regulated serine/threonine kinase Akt.

METHODS

TNF-alpha was administered to BALB/c mice after treatment with an adenovirus expressing a mutant form IkappaBalpha (Ad5IkappaB), the PI3K inhibitor wortmannin, or both. Liver injury was assessed biochemically and histologically. The expression of Bcl-2 family members and caspase activity were examined.

RESULTS

In the mice livers, treatment with Ad5IkappaB or the wortmannin suppressed the activation of NF-kappaB or Akt, respectively. Suppression of either NF-kappaB or Akt showed a slight increase in transaminase levels and focal liver cell death after TNF-alpha administration. However, in mice treated with both Ad5IkappaB and wortmannin, TNF-alpha administration resulted in massive hepatocyte apoptosis and hemorrhagic liver destruction in mice. The combination of Ad5IkappaB, wortmannin, and TNF-alpha markedly increased the activation of caspase-3 and -9, and activated caspase-8 to a lesser degree, suggesting that TNF-alpha-induced hepatocyte apoptosis is dependent on type II cell death signaling pathway, probably through the mitochondria. Inhibition of the NF-kappaB and PI3K/Akt pathways had no effect on expression of Bcl-2 families.

CONCLUSION

The inducible activation of NF-kappaB and constitutive activation of Akt regulate hepatocyte survival against TNF-alpha, which occurs independent of Bcl-2 families.

Anoikis: roadblock to cell transplantation?

Cell therapy, in particular liver cell transplantation, holds great therapeutic potential and is partially hindered by the high rate of apoptosis during cell isolation, cryopreservation, and engraftment. Apoptosis occurring due to cell detachment from the extracellular matrix is a phenomenon termed "anoikis." The purpose of this review is to describe signaling mechanisms pertinent to anoikis in both immortalized cell lines, but particularly in primary normal epithelial cells. The mechanisms described include integrin signaling and survival molecules, caspase activation, and the role of mitochondrial proteins in anoikis. Strategies to prevent anoikis during isolation and cryopreservation of hepatocytes are discussed.

Expression of the NF-kappa B target gene X-ray-inducible immediate early response factor-1 short enhances TNF-alpha-induced hepatocyte apoptosis by inhibiting Akt activation

Using a cDNA microarray analysis, we identified x-ray-inducible immediate early response factor-1 (IEX-1) as a proapoptotic gene which was induced by TNF-alpha and also depend on NF-kappaB activation in Hc human hepatocytes. In these cells only the original form of IEX-1, termed IEX-1S, but not its longer transcript IEX-1L, was expressed. Overexpression of IEX-1S resulted in promotion of TNF-alpha-induced apoptosis in Hc cells expressing a mutant form of IkappaB. This proapoptotic action can be explained by its inhibitory findings on survival signals; inhibition of TNF-alpha-induced activation and expression of phosphatidylinositol 3-kinase (PI3K)/Akt, and also blockage of expression of Mcl-1, an antiapoptotic Bcl-2 family member which is located downstream of Akt, was inhibited by IEX-1S. LY 294002, an inhibitor of PI3K, increased IEX-1S expression induced by TNF-alpha and accelerated TNF-alpha-induced apoptosis in IkappaB-treated Hc cells. Overexpression of the dominant-negative Akt enhanced, but the constitutively active Akt suppressed, TNF-alpha-induced IEX-1S expression, suggesting that PI3K/Akt negatively regulated IEX-1S expression. These results demonstrate that NF-kappaB-dependent recruitment of IEX-1S may play a proapoptotic role in TNF-alpha-stimulated hepatocytes through blockage of the PI3K/Akt pathway. Moreover, the reciprocal cross-talk between IEX-1S and PI3K/Akt may closely be involved in the regulation of TNF-alpha-induced hepatocyte apoptosis.

Tumor necrosis factor alpha-induced interleukin-8 production via NF-kappaB and phosphatidylinositol 3-kinase/Akt pathways inhibits cell apoptosis in human hepatocytes

Tumor necrosis factor alpha (TNF-alpha) not only induces apoptotic signals but also causes antiapoptotic and regenerative responses in the liver. However, the molecular mechanism(s) of the latter events remains unclear. In the present study, we examined TNF-alpha-induced genes in Hc human normal (unsensitized) hepatocytes by cDNA microarray analysis. Interleukin-8 (IL-8) induction was the most pronounced of the upregulated genes. The IL-8 protein level was also increased. IL-8 belongs to the ELR-CXC chemokine family and appears to exert mitogenic and antiapoptotic functions in other cell systems. IL-8 expression by TNF-alpha was inhibited when two survival signals, nuclear factor kappaB (NF-kappaB) and phosphatidylinositol 3-kinase (PI3K)/Akt, were inhibited by a mutant form of inhibitor of NF-kappaB (IkappaB); by dominant negative (kinase-dead) Akt; or by treatment with LY 294002, an inhibitor of PI3K. TNF-alpha induced apoptosis in Hc cells that were sensitized by inhibition of NF-kappaB and PI3K activation. IL-8 administration protected mice against concanavalin A-induced hepatitis in vivo. IL-8 also rescued the sensitized Hc cells, at least in part, from TNF-alpha-induced apoptosis in vitro. TNF-alpha inhibited DNA synthesis in unsensitized Hc cells in the absence of serum. Exogenous IL-8 reversed, though anti-IL-8 neutralization antibody enhanced, growth inhibition by TNF-alpha. These results indicate that IL-8, the production of which is stimulated by TNF-alpha, inhibits apoptosis of sensitized hepatocytes and releases normal (unsensitized) hepatocytes from growth inhibition induced by TNF-alpha.