Anna Moles

Mechanism of Mitochondrial Glutathione-Dependent Hepatocellular Susceptibility to TNF Despite NF-κB Activation

BACKGROUND & AIMS Nuclear factor kappaB (NF-kappaB) is the master regulator of tumor necrosis factor (TNF) susceptibility. Although mitochondrial glutathione (mGSH) depletion was shown to sensitize hepatocytes to TNF despite NF-kappaB activation, the mechanisms involved, particularly the role of Bax oligomerization and mitochondrial outer membrane (MOM) permeabilization, 2 critical steps in cell death, remained unexplored. METHODS TNF signaling at the premitochondrial and mitochondrial levels was analyzed in primary mouse hepatocytes with or without mGSH depletion. RESULTS Unexpectedly, we observed that TNF activates caspase-8 independently of NF-kappaB inactivation, causing Bid cleavage and mitochondrial Bax oligomerization. However, their predicted consequences on MOM permeabilization, cytochrome c release, caspase-3 activation, and hepatocellular death occurred only on mGSH depletion. These events were preceded by stimulated mitochondrial reactive oxygen species that predominantly oxidized cardiolipin, changes not observed in acidic sphingomyelinase (ASMase)(-/-) hepatocytes. Oxidized cardiolipin potentiated oligomerized Bax-induced MOM-like liposome permeabilization by restructuring the lipid bilayer, without effect on membrane Bax insertion or oligomerization. ASMase(-/-) mice with mGSH depletion by cholesterol loading were resistant to TNF-induced liver injury in vivo. CONCLUSIONS Thus, MOM-localized oligomeric Bax is not sufficient for TNF-induced MOM permeabilization and cell death requiring mGSH-controlled ASMase-mediated mitochondrial membrane remodeling by oxidized cardiolipin generation.

Critical role of tumor necrosis factor receptor 1, but not 2, in hepatic stellate cell proliferation, extracellular matrix remodeling, and liver fibrogenesis†‡§

Tumor necrosis factor (TNF) has been implicated in the progression of many chronic liver diseases leading to fibrosis; however, the role of TNF in fibrogenesis is controversial and the specific contribution of TNF receptors to hepatic stellate cell (HSC) activation remains to be established. Using HSCs from wild‐type, TNF‐receptor‐1 (TNFR1) knockout, TNF‐receptor‐2 (TNFR2) knockout, or TNFR1/R2 double‐knockout (TNFR‐DKO) mice, we show that loss of both TNF receptors reduced procollagen‐α1(I) expression, slowed down HSC proliferation, and impaired platelet‐derived growth factor (PDGF)‐induced promitogenic signaling in HSCs. TNFR‐DKO HSCs exhibited decreased AKT phosphorylation and in vitro proliferation in response to PDGF. These effects were reproduced in TNFR1 knockout, but not TNFR2 knockout, HSCs. In addition, matrix metalloproteinase 9 (MMP‐9) expression was dependent on TNF binding to TNFR1 in primary mouse HSCs. These results were validated in the human HSC cell line, LX2, using neutralizing antibodies against TNFR1 and TNFR2. Moreover, in vivo liver damage and fibrogenesis after bile‐duct ligation were reduced in TNFR‐DKO and TNFR1 knockout mice, compared to wild‐type or TNFR2 knockout mice. Conclusion: TNF regulates HSC biology through its binding to TNFR1, which is required for HSC proliferation and MMP‐9 expression. These data indicate a regulatory role for TNF in extracellular matrix remodeling and liver fibrosis, suggesting that targeting TNFR1 may be of benefit to attenuate liver fibrogenesis. (HEPATOLOGY 2011;)

Acidic sphingomyelinase controls hepatic stellate cell activation and in vivo liver fibrogenesis

The mechanisms linking hepatocellular death, hepatic stellate cell (HSC) activation, and liver fibrosis are largely unknown. Here, we investigate whether acidic sphingomyelinase (ASMase), a known regulator of death receptor and stress-induced hepatocyte apoptosis, plays a role in liver fibrogenesis. We show that selective stimulation of ASMase (up to sixfold), but not neutral sphingomyelinase, occurs during the transdifferentiation/activation of primary mouse HSCs into myofibroblast-like cells, coinciding with cathepsin B (CtsB) and D (CtsD) processing. ASMase inhibition or genetic down-regulation by small interfering RNA blunted CtsB/D processing, preventing the activation and proliferation of mouse and human HSCs (LX2 cells). In accordance, HSCs from heterozygous ASMase mice exhibited decreased CtsB/D processing, as well as lower levels of alpha-smooth muscle actin expression and proliferation. Moreover, pharmacological CtsB inhibition reproduced the antagonism of ASMase in preventing the fibrogenic properties of HSCs, without affecting ASMase activity. Interestingly, liver fibrosis induced by bile duct ligation or carbon tetrachloride administration was reduced in heterozygous ASMase mice compared with that in wild-type animals, regardless of their sensitivity to liver injury in either model. To provide further evidence for the ASMase-CtsB pathway in hepatic fibrosis, liver samples from patients with nonalcoholic steatohepatitis were studied. CtsB and ASMase mRNA levels increased eight- and threefold, respectively, in patients compared with healthy controls. These findings illustrate a novel role of ASMase in HSC biology and liver fibrogenesis by regulating its downstream effectors CtsB/D.

Cathepsins B and D drive hepatic stellate cell proliferation and promote their fibrogenic potential.

Cathepsins have been best characterized in tumorigenesis and cell death and implicated in liver fibrosis; however, whether cathepsins directly regulate hepatic stellate cell (HSC) activation and proliferation, hence modulating their fibrogenic potential, is largely unknown. Here, we show that expression of cathepsin B (CtsB) and cathepsin D (CtsD) is negligible in quiescent HSCs but parallels the increase of α‐smooth muscle actin and transforming growth factor‐β during in vitro mouse HSC activation. Both cathepsins are necessary for HSC transdifferentiation into myofibroblasts, because their silencing or inhibition decreased HSC proliferation and the expression of phenotypic markers of HSC activation, with similar results observed with the human HSC cell line LX2. CtsB inhibition blunted AKT phosphorylation in activated HSCs in response to platelet‐derived growth factor. Moreover, during in vivo liver fibrogenesis caused by CCl4 administration, CtsB expression increased in HSCs but not in hepatocytes, and its inactivation mitigated CCl4‐induced inflammation, HSC activation, and collagen deposition. Conclusion: These findings support a critical role for cathepsins in HSC activation, suggesting that the antagonism of cathepsins in HSCs may be of relevance for the treatment of liver fibrosis. (HEPATOLOGY 2009.)

Cathepsin B Overexpression Due to Acid Sphingomyelinase Ablation Promotes Liver Fibrosis in Niemann-Pick Disease

Background: The mechanism of liver fibrosis in Niemann-Pick disease (NPD) is unknown. Results: The loss of ASMase stimulates cathepsin B (CtsB) activation promoting liver fibrosis. Conclusion: CtsB contributes to the hepatic phenotype of NPD. Significance: CtsB may be a novel therapeutic target to treat liver disease in NPD. Niemann-Pick disease (NPD) is a lysosomal storage disease caused by the loss of acid sphingomyelinase (ASMase) that features neurodegeneration and liver disease. Because ASMase-knock-out mice models NPD and our previous findings revealed that ASMase activates cathepsins B/D (CtsB/D), our aim was to investigate the expression and processing of CtsB/D in hepatic stellate cells (HSCs) from ASMase-null mice and their role in liver fibrosis. Surprisingly, HSCs from ASMase-knock-out mice exhibit increased basal level and activity of CtsB as well as its in vitro processing in culture, paralleling the enhanced expression of fibrogenic markers α-smooth muscle actin (α-SMA), TGF-β, and pro-collagen-α1(I) (Col1A1). Moreover, pharmacological inhibition of CtsB blunted the expression of α-SMA and Col1A1 and proliferation of HSCs from ASMase-knock-out mice. Consistent with the enhanced activation of CtsB in HSCs from ASMase-null mice, the in vivo liver fibrosis induced by chronic treatment with CCl4 increased in ASMase-null compared with wild-type mice, an effect that was reduced upon CtsB inhibition. In addition to liver, the enhanced proteolytic processing of CtsB was also observed in brain and lung of ASMase-knock-out mice, suggesting that the overexpression of CtsB may underlie the phenotype of NPD. Thus, these findings reveal a functional relationship between ASMase and CtsB and that the ablation of ASMase leads to the enhanced processing and activation of CtsB. Therefore, targeting CtsB may be of relevance in the treatment of liver fibrosis in patients with NPD.

Alcohol, signaling, and ECM turnover.

Alcohol is recognized as a direct hepatotoxin, but the precise molecular pathways that are important for the initiation and progression of alcohol-induced tissue injury are not completely understood. The current understanding of alcohol toxicity to organs suggests that alcohol initiates injury by generation of oxidative and nonoxidative ethanol metabolites and via translocation of gut-derived endotoxin. These processes lead to cellular injury and stimulation of the inflammatory responses mediated through a variety of molecules. With continuing alcohol abuse, the injury progresses through impairment of tissue regeneration and extracellular matrix (ECM) turnover, leading to fibrogenesis and cirrhosis. Several cell types are involved in this process, the predominant being stellate cells, macrophages, and parenchymal cells. In response to alcohol, growth factors and cytokines activate many signaling cascades that regulate fibrogenesis. This mini-review brings together research focusing on the underlying mechanisms of alcohol-mediated injury in a number of organs. It highlights the various processes and molecules that are likely involved in inflammation, immune modulation, susceptibility to infection, ECM turnover and fibrogenesis in the liver, pancreas, and lung triggered by alcohol abuse.

1059 CRITICAL ROLE OF TNF-RECEPTOR 1 BUT NOT 2 IN HEPATIC STELLATE CELL PROLIFERATION, EXTRACELLULAR MATRLX REMODELING AND LIVER FIBROGENESIS

Trabajo presentado en el 46th Annual Meeting of the European Association for the Study of the Liver (EASL), celebrado en Berlin, Alemania, del 30 de marzo al 3 de abril de 2011

Mitochondrial–Lysosomal Axis in Acetaminophen Hepatotoxicity

Acetaminophen (APAP) toxicity is the most common cause of acute liver failure and a major indication for liver transplantion in the United States and Europe. Although significant progress has been made in understanding the molecular mechanisms underlying APAP hepatotoxicity, there is still an urgent need to find novel and effective therapies against APAP-induced acute liver failure. Hepatic APAP metabolism results in the production of the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI), which under physiological conditions is cleared by its conjugation with glutathione (GSH) to prevent its targeting to mitochondria. APAP overdose or GSH limitation leads to mitochondrial NAPQI-protein adducts formation, resulting in oxidative stress, mitochondrial dysfunction, and necrotic cell death. As mitochondria are a major target of APAP hepatotoxicity, mitochondrial quality control and clearance of dysfunctional mitochondria through mitophagy, emerges as an important strategy to limit oxidative stress and the engagement of molecular events leading to cell death. Recent evidence has indicated a lysosomal–mitochondrial cross-talk that regulates APAP hepatotoxicity. Moreover, as lysosomal function is essential for mitophagy, impairment in the fusion of lysosomes with autophagosomes-containing mitochondria may compromise the clearance of dysfunctional mitochondria, resulting in exacerbated APAP hepatotoxicity. This review centers on the role of mitochondria in APAP hepatotoxicity and how the mitochondrial/lysosomal axis can influence APAP-induced liver failure.

Hepatic stellate cell proliferation. A potential role of protein kinase R: Reply

We read with great interest the article by Martinot-Peignoux et al. In this report from France, undetectable serum hepatitis C virus (HCV) RNA at 12 weeks (Wþ12) (409 patients) post-treatment follow-up was as relevant as undetectable serum HCV RNA at 24 weeks (Wþ24) (sustained virological response [SVR]; 408 patients) after the end of treatment. Current standard therapy is based on a combination of pegylated interferon (PEG-IFN) and ribavirin, but it leads to only 50% SVR in patients with HCV genotype 1 and high viral loads. IFN reduced the risk for HCC, especially among patients with SVR. Then, we need to accurately judge whether the patient is SVR or non-SVR, applying the present standard for the judgment of SVR with the undetectability of serum HCV RNA at post-treatment Wþ24. We investigated 102 patients with chronic hepatitis C genotype 1 treated with PEG-IFN-alfa 2a plus ribavirin for 48 weeks. Some of these patients had already been included in previous reports. Serum HCV RNA was measured using the COBAS TaqMan HCV test with a detection limit of 1.2 logIU/mL. At the Wþ24 posttreatment follow-up, 40 (39.2%) patients had SVR, and 31 (48.4%) and 9 (23.6%) were treatment naı̈ve and previously treated patients, respectively. At Wþ12, serum HCV RNA was undetectable in 42 patients, and 40 patients were SVR (PPV, 95.2%). We found two relapsers at Wþ24 (undetectable at Wþ12). In the case of using direct-acting antivirals, earlier knowledge of treatment outcome would be useful for retreatment for the same patient. Taken together, our findings show that Wþ12 undetectable serum HCV RNA is not suitable for predicting persistent virological response. Further understanding of the mechanism of relapse could be useful in reducing the posttreatment follow-up period from the current standard of 24 weeks.