Liver-specific inactivation of the Nrf1 gene in adult mouse leads to nonalcoholic steatohepatitis and hepatic neoplasia

Deficiency in the nuclear-related factor erythroid 2 transcription factor (Nrf1) leads to genetic instability

Nuclear factor erythroid-derived 2-related factor 1 (Nrf1) regulates cellular stress response genes, and has also been suggested to play a role in other cellular processes. We previously demonstrated that hepatocyte-specific deletion of Nrf1 in mice resulted in spontaneous apoptosis, inflammation, and development of liver tumors. Here, we showed that both fibroblasts derived from Nrf1 null mouse embryos and fibroblasts expressing a conditional Nrf1 allele showed increased micronuclei and formation of abnormal nuclei. Lentiviral shRNA-mediated knockdown of Nrf1 in SAOS-2 cells also resulted in increased micronuclei, abnormal mitosis and multi-nucleated cells. Metaphase analyses showed increased aneuploidy in Nrf1(-/-) embryonic fibroblasts. Nuclear defects in Nrf1-deficient cells were associated with decreased expression of various genes encoding kinetochore and mitotic checkpoint proteins. Our findings suggest that Nrf1 may play a role in maintaining genomic integrity, and that Nrf1 dysregulation may induce tumorigenesis.

Hepatocellular carcinoma in nonalcoholic fatty liver cirrhosis and alcoholic cirrhosis: risk factor analysis in liver transplant recipients

BACKGROUND

Cirrhosis and hepatocellular carcinoma (HCC) evolving from nonalcoholic fatty liver disease (NAFLD) are being increasingly documented. However, clinicopathologic studies to support this are inadequate. Also, the pathogenesis of HCC in alcoholic cirrhosis (ALC) in which the pathologic and clinical features are very similar to those of nonalcoholic fatty liver cirrhosis (NAFLC) is unknown.

METHODS

A clinicomorphologic study on 47 confirmed NAFLC cases, with HCC in eight of them and 75 confirmed ALC cases with HCC in five from among orthotopic liver transplant recipients, was performed.

RESULTS

Patients with NAFLC were older by about 9 years than those with ALC. HCC in NAFLC occurred almost exclusively in men. The presence of NAFLD risk factors, obesity and diabetes both together, was significantly higher in NAFLC than in ALC cases and within the latter, in those with HCC than in those without HCC, whereas in the NAFLC group, this was no different between those with and without the tumor. The steatohepatitic variant of HCC, consistently accompanied by similar histologic changes in the nontumor part of liver, which is a histologic hallmark of association with NAFLC risk factors, was much more frequent in the NAFLC group compared with the ALC group.

CONCLUSION

Hepatic alterations induced by risk factors of NAFLD not only have cirrhogenic but also, very likely, a carcinogenic effect. The incidence of HCC in NAFLC seems higher than in ALC. These findings and their bases need to be established by further studies.

NF-E2-related factor 1 (Nrf1) serves as a novel regulator of hepatic lipid metabolism through regulation of the Lipin1 and PGC-1β genes

Hepatic lipid metabolism is under elaborate regulation, and perturbations in this regulatory process at the transcriptional level lead to pathological conditions. NF-E2-related factor 1 (Nrf1) is a member of the cap'n'collar (CNC) transcription factor family. Hepatocyte-specific Nrf1 gene conditional-knockout mice are known to develop hepatic steatosis, but it remains unclear how Nrf1 contributes to the lipid homeostasis. Therefore, in this study we examined the gene expression profiles of Nrf1-deficient mouse livers. A pathway analysis based on the profiling results revealed that the levels of expression of the genes related to lipid metabolism, amino acid metabolism, and mitochondrial respiratory function were decreased in Nrf1-deficient mouse livers, indicating the profound effects that the Nrf1 deficiency conferred to various metabolic pathways. We discovered that the Nrf1 deficiency leads to the reduced expression of the transcriptional coactivator genes Lipin1 and PGC-1β (for peroxisome proliferator-activated receptor γ coactivator 1β). Chromatin immunoprecipitation analyses showed that Nrf1 binds to the antioxidant response elements (AREs) in regulatory regions of the Lipin1 and PGC-1β genes and the binding of Nrf1 to the AREs activates reporter gene transcription. These results thus identified Nrf1 to be a novel regulator of the Lipin1 and PGC-1β genes, providing new insights into the Nrf1 function in hepatic lipid metabolism.

Nrf1 CNC-bZIP protein promotes cell survival and nucleotide excision repair through maintaining glutathione homeostasis

Skin cancer is the most common cancer in the United States. Its major environmental risk factor is UVB radiation in sunlight. In response to UVB damage, epidermal keratinocytes activate a specific repair pathway, i.e. nucleotide excision repair, to remove UVB-induced DNA lesions. However, the regulation of UVB response is not fully understood. Here we show that the long isoform of the nuclear factor erythroid 2-related factor 1 (Nrf1, also called NFE2L1), a cytoprotective transcription factor critical for the expression of multiple antioxidant response element-dependent genes, plays an important role in the response of keratinocytes to UVB. Nrf1 loss sensitized keratinocytes to UVB-induced apoptosis by up-regulating the expression of the proapoptotic Bcl-2 family member Bik through reducing glutathione levels. Knocking down Bik reduced UVB-induced apoptosis in Nrf1-inhibited cells. In UVB-irradiated surviving cells, however, disruption of Nrf1 impaired nucleotide excision repair through suppressing the transcription of xeroderma pigmentosum C (XPC), a factor essential for initiating the global genome nucleotide excision repair by recognizing the DNA lesion and recruiting downstream factors. Nrf1 enhanced XPC expression by increasing glutathione availability but was independent of the transcription repressor of XPC. Adding XPC or glutathione restored the DNA repair capacity in Nrf1-inhibited cells. Finally, we demonstrate that Nrf1 levels are significantly reduced by UVB radiation in mouse skin and are lower in human skin tumors than in normal skin. These results indicate a novel role of Nrf1 in UVB-induced DNA damage repair and suggest Nrf1 as a tumor suppressor in the skin.

Tumor suppressor functions of FBW7 in cancer development and progression

FBW7 (F-box and WD repeat domain-containing 7) has been characterized as an onco-suppressor protein in human cancers. Recent studies have also shown that FBW7 exerts its anti-tumor function primarily by promoting the degradation of various oncoproteins, through which FBW7 regulates cellular proliferation, differentiation and causes genetic instability. In this review, we will discuss the role of FBW7 downstream substrates and how dysregulation of Fbw7-mediated proteolysis of these substrates contributes to tumorigenesis. Additionally, we will also summarize the currently available various Fbw7-knockout mouse models that support Fbw7 as a tumor suppressor gene in the development and progression of human malignancies.

Hepatocellular carcinoma in non-alcoholic steatohepatitis: Growing evidence of an epidemic?

The incidence of hepatocellular carcinoma in non-viral-related chronic liver disease has gradually increased in Japan. Obesity and diabetes mellitus type 2 have been established as a significant risk factor for hepatocellular carcinoma (HCC) by epidemiologic observations and experimental studies. The risks of these factors for HCC are likely conferred by two factors: the increased risk for development of non-alcoholic steatohepatitis (NASH) and the carcinogenic potential of themselves. Hepatocellular carcinoma in NASH is difficult to evaluate because histological diagnosis is required for diagnosis of NASH, which can lead selection bias. Furthermore, end-stage NASH is in effect "burned-out" NASH, for which the diagnosis of NASH cannot be made any more. At all events, previous studies on the etiology of Japanese HCC showed that non-alcoholic fatty liver disease accounts for 1-5% of all HCC (male predominant, median age 72 years). They have high prevalences of obesity and/or diabetes mellitus type 2 and 10-75% of the HCC arose from non-cirrhotic livers. HCC in NASH may be of multicentric origin, similar to HCC based on viral hepatitis. Regular screening for HCC is extremely important especially in cirrhotic NASH patients and recurrence should be warned. In western and Asian countries, the prevalence of non-alcoholic fatty liver disease in the general population is increasing dramatically. Therefore, there is an urgent need to elucidate pathogenesis and clinical features of HCC in NASH. In this review we summarize current concepts for HCC in NASH.

Redox control of the ubiquitin-proteasome system: from molecular mechanisms to functional significance

In their natural environments, cells are regularly exposed to oxidizing conditions that may lead to protein misfolding. If such misfolded proteins are allowed to linger, they may form insoluble aggregates and pose a serious threat to the cell. Accumulation of misfolded, oxidatively damaged proteins is characteristic of many diseases and during aging. To counter the adverse effects of oxidative stress, cells can initiate an antioxidative response in an attempt to repair the damage, or rapidly channel the damaged proteins for degradation by the ubiquitin-proteasome system (UPS). Recent studies have shown that elements of the oxidative stress response and the UPS are linked on many levels. To manage the extra burden of misfolded proteins, the UPS is induced by oxidative stress, and special proteasome subtypes protect cells against oxidative damage. In addition, the proteasome is directly associated with a thioredoxin and other cofactors that may adjust the particle's response during an oxidative challenge. Here, we give an overview of the UPS and a detailed description of the degradation of oxidized proteins and of the crosstalk between oxidative stress and protein degradation in health and disease.

Transgenic expression of entire hepatitis B virus in mice induces hepatocarcinogenesis independent of chronic liver injury

Hepatocellular carcinoma (HCC), the third leading cause of cancer deaths worldwide, is most commonly caused by chronic hepatitis B virus (HBV) infection. However, whether HBV plays any direct role in carcinogenesis, other than indirectly causing chronic liver injury by inciting the host immune response, remains unclear. We have established two independent transgenic mouse lines expressing the complete genome of a mutant HBV ("preS2 mutant") that is found at much higher frequencies in people with HCC than those without. The transgenic mice show evidence of stress in the endoplasmic reticulum (ER) and overexpression of cyclin D1 in hepatocytes. These mice do not show any evidence of chronic liver injury, but by 2 years of age a majority of the male mice develop hepatocellular neoplasms, including HCC. Unexpectedly, we also found a significant increase in hepatocarcinogenesis independent of necroinflammation in a transgenic line expressing the entire wildtype HBV. As in the mutant HBV mice, HCC was found only in aged--2-year-old--mice of the wildtype HBV line. The karyotype in all the three transgenic lines appears normal and none of the integration sites of the HBV transgene in the mice is near an oncogene or tumor suppressor gene. The significant increase of HCC incidence in all the three transgenic lines--expressing either mutant or wildtype HBV--therefore argues strongly that in absence of chronic necroinflammation, HBV can contribute directly to the development of HCC.

The Fbw7 tumor suppressor regulates nuclear factor E2-related factor 1 transcription factor turnover through proteasome-mediated proteolysis

Nuclear factor E2-related factor 1 (Nrf1) is a basic leucine zipper transcription factor that plays important roles in cellular stress response and development. Currently, the mechanism regulating Nrf1 expression is poorly understood. We report here that Nrf1 is a short-lived protein that is targeted by F-box protein Fbw7, which is the substrate-specifying component of SCF (Skp1-Cul1-Fbox protein-Rbx1)-type ubiquitin ligase for degradation via the ubiquitin-proteasome pathway. We show that Fbw7 directly binds Nrf1 through a Cdc4 phosphodegron and that enforced expression of Fbw7 promotes the ubiquitination and degradation of Nrf1. Conversely, depletion of endogenous Fbw7 leads to decreased Nrf1 ubiquitination and accumulation of Nrf1 protein. Accordingly, expression of Fbw7 leads to down-regulation of antioxidant response element-driven gene activation, whereas disruption of Fbw7-mediated destabilization of Nrf1 leads to increased antioxidant response element-driven gene expression. Together, these data identify Fbw7 as a regulator of Nrf1 expression and reveal a novel function of Fbw7 in cellular stress response.

Dual regulation of the transcriptional activity of Nrf1 by β-TrCP- and Hrd1-dependent degradation mechanisms

A growing body of evidence suggests that Nrf1 is an inducible transcription factor that maintains cellular homeostasis. Under physiological conditions, Nrf1 is targeted to the endoplasmic reticulum (ER), implying that it translocates into the nucleus in response to an activating signal. However, the molecular mechanisms by which the function of Nrf1 is modulated remain poorly understood. Here, we report that two distinct degradation mechanisms regulate Nrf1 activity and the expression of its target genes. In the nucleus, β-TrCP, an adaptor for the SCF (Skp1-Cul1-F-box protein) ubiquitin ligase, promotes the degradation of Nrf1 by catalyzing its polyubiquitination. This activity requires a DSGLS motif on Nrf1, which is similar to the canonical β-TrCP recognition motif. The short interfering RNA (siRNA)-mediated silencing of β-TrCP markedly augments the expression of Nrf1 target genes, such as the proteasome subunit PSMC4, indicating that β-TrCP represses Nrf1 activation. Meanwhile, in the cytoplasm, Nrf1 is degraded and suppressed by the ER-associated degradation (ERAD) ubiquitin ligase Hrd1 and valosin-containing protein (VCP) under normal conditions. We identified a cytoplasmic degradation motif on Nrf1 between the NHB1 and NHB2 domains that exhibited species conservation. Thus, these results clearly suggest that both β-TrCP- and Hrd1-dependent degradation mechanisms regulate the transcriptional activity of Nrf1 to maintain cellular homeostasis.

Molecular characteristics of non-cancerous liver tissue in non-B non-C hepatocellular carcinoma

Although chronic infection with hepatitis B virus (HBV) and/or hepatitis C virus (HCV) are the most important risk factors for the development of hepatocellular carcinoma (HCC) worldwide, the proportion of HCC patients negative for the hepatitis B surface antigen and hepatitis C antibody, so-called "non-B non-C HCC", is rapidly increasing, especially in Japan. The background liver diseases of non-B non-C HCC patients can be multifactorial, including occult HBV infection and non-alcoholic steatohepatitis. It is reasonable to investigate the non-cancerous liver tissues to identify the potential molecular mechanisms responsible for the processes of hepatocarcinogenesis of non-B non-C HCC. However, to date, only a few studies have focused on this research concept based on the idea of "field cancerization". This review highlights the potential importance of the molecular analysis of non-cancerous liver tissues to clarify the molecular characteristics in patients with non-B non-C HCC. A better understanding of the molecular mechanisms underlying the individual predisposition to non-B non-C HCC will lead to improvements in the prevention, early diagnosis and treatment of this neoplastic disease.

NOR1 is an HSF1- and NRF1-regulated putative tumor suppressor inactivated by promoter hypermethylation in nasopharyngeal carcinoma

Promoter hypermethylation-mediated silencing of tumor suppressor genes (TSGs) is a hallmark of oncogenesis. Oxidored-nitro domain-containing protein 1 (NOR1) is a candidate TSG that is downregulated in nasopharyngeal carcinoma (NPC). In the present study, we identified a functional NOR1 promoter that is regulated by heat shock factor 1 and nuclear respiratory factor 1. The promoter is located within a CpG island. Hypermethylation of this CpG island was found in NPC tissue samples and cancer cell lines, whereas no aberrant promoter methylation was detected in non-cancerous nasopharyngeal tissue samples or normal nasopharyngeal epithelial cells. Treatment of NPC 6-10B cells and leukemia HL60 cells with 5'-aza-2'-deoxycytidine increased endogenous levels of NOR1 messenger RNA. Ectopic expression of NOR1 in NPC HNE1 cells inhibited tumor cell colony formation and viability. These findings suggest that promoter hypermethylation may participate in transcriptional inactivation of the NOR1 gene in NPC. Frequent epigenetic inactivation of the NOR1 gene in NPC suggests that it may be a critical tumor suppressor involved in the development of NPC.

Specific SKN-1/Nrf stress responses to perturbations in translation elongation and proteasome activity

SKN-1, the Caenorhabditis elegans Nrf1/2/3 ortholog, promotes both oxidative stress resistance and longevity. SKN-1 responds to oxidative stress by upregulating genes that detoxify and defend against free radicals and other reactive molecules, a SKN-1/Nrf function that is both well-known and conserved. Here we show that SKN-1 has a broader and more complex role in maintaining cellular stress defenses. SKN-1 sustains expression and activity of the ubiquitin-proteasome system (UPS) and coordinates specific protective responses to perturbations in protein synthesis or degradation through the UPS. If translation initiation or elongation is impaired, SKN-1 upregulates overlapping sets of cytoprotective genes and increases stress resistance. When proteasome gene expression and activity are blocked, SKN-1 activates multiple classes of proteasome subunit genes in a compensatory response. SKN-1 thereby maintains UPS activity in the intestine in vivo under normal conditions and promotes survival when the proteasome is inhibited. In contrast, when translation elongation is impaired, SKN-1 does not upregulate proteasome genes, and UPS activity is then reduced. This indicates that UPS activity depends upon presence of an intact translation elongation apparatus; and it supports a model, suggested by genetic and biochemical studies in yeast, that protein synthesis and degradation may be coupled processes. SKN-1 therefore has a critical tissue-specific function in increasing proteasome gene expression and UPS activity under normal conditions, as well as when the UPS system is stressed, but mounts distinct responses when protein synthesis is perturbed. The specificity of these SKN-1–mediated stress responses, along with the apparent coordination between UPS and translation elongation activity, may promote protein homeostasis under stress or disease conditions. The data suggest that SKN-1 may increase longevity, not only through its well-documented role in boosting stress resistance, but also through contributing to protein homeostasis.

The mechanisms through which organisms defend against environmental stresses are critical during diverse disease processes and are likely to be important for longevity. The nematode C. elegans is advantageous for genetic analysis of how stress defenses function and contribute to survival. The evolutionarily conserved C. elegans protein SKN-1 promotes stress resistance and longevity, and it defends against toxic small molecules. We now report that in certain tissues SKN-1 also maintains production of the proteasome, a structure that degrades proteins in a regulated fashion. SKN-1 mounts distinct stress responses to perturbations in protein synthesis and degradation, in which it boosts proteasome levels only in response to proteasome impairment. Remarkably, proteasome activity also depends upon the proper functioning of the protein synthesis apparatus. The specificity of SKN-1 stress responses may be important for protein homeostasis, allowing SKN-1 to maintain levels and activity of the proteasomal degradation apparatus, but not increase degradation when protein synthesis is impaired. This role of SKN-1 in regulating protein turnover may be important for many of its stress defense functions and for protection against disease and aging.

Plasma reactive carbonyl species levels and risk of non-alcoholic fatty liver disease

BACKGROUND AND AIM

Oxidative stress plays a critical role in the pathogenesis of non-alcoholic fatty liver disease (NAFLD). However, there is still no large cohort study to explore the direct risk role of oxidative stress for NAFLD. This study is to test the hypothesis that elevated oxidative stress is a direct risk factor for the pathogenesis of NAFLD under controlling the potential effects of covariates.

METHODS

The levels of serum cholesterol, serum triglyceride, fasting plasma glucose and plasma reactive carbonyl species (RCS) were measured from 1204 Chinese Han adults, and the questionnaire and physical examination were administered to those with known and suspected risk factors for NAFLD.

RESULTS

Statistically significant high levels of blood pressure, fasting plasma glucose, serum cholesterol and triglyceride, body mass index, serum alanine aminotransferase and aspartate aminotransferase, and plasma RCS were observed in NAFLD subjects compared to healthy subjects (P < 0.01). Multivariate-adjusted odds ratio illustrated that, compared with the lowest quartile of plasma RCS levels, the highest quartile subjects had a 132% increase in the risk of developing NAFLD. Further results from multi-interaction analysis demonstrated that the underlying mechanism of the risk of NAFLD by unhealthy physical conditions and lifestyles might be, at least in part, through the oxidative stress.

CONCLUSIONS

Our findings provide credible evidence from a large population that oxidative stress, as indicated by plasma RCS levels, may be a direct risk factor for developing NAFLD.

Central nervous system-specific deletion of transcription factor Nrf1 causes progressive motor neuronal dysfunction

Cap'n'Collar (CNC) proteins heterodimerize with small Maf proteins and regulate the transcription of various genes. Small Maf-deficient mice develop severe neurodegeneration, and it remains unclear whether CNC proteins are involved in this process. In this study, we examined the contribution of Nrf1, one of the CNC proteins, to neuronal homeostasis in vivo. As Nrf1 gene knockout mice are embryonic lethal, we developed a central nervous system (CNS)-specific Nrf1 knockout (CKO) mouse line using mice bearing an Nrf1(flox) allele and Nestin-Cre allele. At birth, the CKO mice appeared indistinguishable from control mice, but thereafter they showed progressive motor ataxia and severe weight loss. All Nrf1 CKO mice died within 3 weeks. These phenotypes are similar to those reported in small Maf-deficient mice, suggesting the presence of collaboration between Nrf1 and small Maf proteins. We also found aberrant accumulation of polyubiquitinated proteins in various CNS regions and apparent neuronal loss in the hippocampus of Nrf1 CKO mice. An oxidative stress marker was accumulated in the spinal cords of the mice, but the expression patterns of oxidative stress response genes regulated by Nrf2 did not change substantially. These results show that Nrf1 sustains the CNS homeostasis through regulating target genes distinct from those regulated by Nrf2.

Loss of nuclear factor E2-related factor 1 in the brain leads to dysregulation of proteasome gene expression and neurodegeneration

The ubiquitin-proteasome pathway plays an important role in the pathogenesis of neurodegeneration, but mechanisms controlling expression of components in this pathway remain poorly understood. Nuclear factor E2-related factor 1 (Nrf1) transcription factor has been shown to regulate expression of antioxidant and cytoprotective genes. To determine the function of Nrf1 in the brain, mice with a late-stage deletion of Nrf1 in neuronal cells were generated. Loss of Nrf1 leads to impaired proteasome function and neurodegeneration. Gene expression profiling and RT-PCR analysis revealed a coordinate down-regulation of various proteasomal genes including PsmB6, which encodes a catalytic subunit of the proteasome. Transcriptional analysis and chromatin immunoprecipitation experiments demonstrated that PsmB6 is an Nrf1 target gene. These findings reveal Nrf1 as a key transcriptional regulator required for the expression of proteasomal genes in neurons and suggest that perturbations of Nrf1 function may contribute to the pathogenesis of neurodegenerative diseases.

Obesity, diabetes mellitus, and liver fibrosis

Obesity is a global epidemic with more than 1 billion overweight adults and at least 300 million obese patients worldwide. Diabetes is characterized by a defect in insulin secretion or a decrease in sensitivity to insulin, which results in elevated fasting blood glucose. Both obesity and elevated fasting glucose are risk factors for nonalcoholic fatty liver disease, a disease spectrum that includes hepatic steatosis (nonalcoholic fatty liver), nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. Increased adiposity and insulin resistance contribute to the progression from NASH to fibrosis through the development of a profibrotic mileau in the liver, including increased hepatocellular death, increased reactive oxygen species generation, and an altered adipokine/cytokine balance. This review will summarize recent advances in our understanding of the pathological interactions among excessive fat accumulation, insulin resistance, and hepatic fibrogenesis and discuss specific molecular pathways that may be of interest in the development of therapeutic interventions to prevent and/or reverse hepatic fibrosis.

Insulin resistance and other metabolic risk factors in the pathogenesis of hepatocellular carcinoma

Coinciding with the increased incidence of hepatocellular carcinoma (HCC), there has been a significant increase in the global incidence of obesity and diabetes mellitus (DM), the two major risk factors for nonalcoholic steatohepatitis (NASH). There are many causes of HCC, and nonalcoholic fatty liver disease/NASH is now emerging as a leading risk factor owing to the epidemic of obesity and type 2 DM. The mechanisms leading to HCC in obesity and type 2 DM likely involve interactions between several signaling pathways, including oxidative stress, inflammation, oncogenes, adiponectins, and insulin resistance associated with visceral adiposity and diabetes.

Overexpression of the IGF2-mRNA binding protein p62 in transgenic mice induces a steatotic phenotype

BACKGROUND & AIMS

The insulin-like growth-factor 2 (IGF2) mRNA binding protein p62 is highly expressed in hepatocellular carcinoma tissue. Still, its potential role in liver disease is largely unknown. In this study, we investigated pathophysiological implications of p62 overexpression in mice.

METHODS

We generated mice overexpressing p62 under a LAP-promotor. mRNA expression levels and stability were examined by real-time RT-PCR. Allele-specific expression of Igf2 and H19 was assessed after crossing mice with SD7 animals. The Igf2 downstream mediators pAKT and PTEN were determined by Western blot.

RESULTS

Hepatic p62 overexpression neither induced inflammatory processes nor liver damage. However, 2.5week old transgenic animals displayed a steatotic phenotype and improved glucose tolerance. p62 overexpression induced the expression of the imprinted genes Igf2 and H19 and their transcriptional regulator Aire (autoimmune regulator). Neither monoallelic expression nor mRNA stability of Igf2 and H19 was affected. Investigating Igf2 downstream signalling pathways showed increased AKT activation and attenuated PTEN expression.

CONCLUSIONS

The induction of a steatotic phenotype implies that p62 plays a role in hepatic pathophysiology.

COP9 signalosome subunit 8 is required for postnatal hepatocyte survival and effective proliferation

Studies using lower organisms and cultured mammalian cells have revealed that the COP9 signalosome (CSN) has important roles in multiple cellular processes. Conditional gene targeting was recently used to study CSN function in murine T-cell development and activation. Using the Cre-loxP system, here we have achieved postnatal hepatocyte-restricted knockout of the csn8 gene (HR-Csn8KO) in mice. The protein abundance of other seven CSN subunits was differentially downregulated by HR-Csn8KO and the deneddylation of all cullins examined was significantly impaired. Moreover, HR-Csn8KO-induced massive hepatocyte apoptosis and evoked extensive reparative responses in the liver, including marked intralobular proliferation of biliary lineage cells and trans-differentiation and proliferation of the oval cells. However, division of pre-existing hepatocytes was significantly diminished in HR-Csn8KO livers. These findings indicate that Csn8 is essential to the ability of mature hepatocytes to proliferate effectively in response to hepatic injury. The histopathological examinations revealed striking hepatocytomegaly in Csn8-deficient livers. The hepatocyte nuclei were dramatically enlarged and pleomorphic with hyperchromasia and prominent nucleoli, consistent with dysplasia or preneoplastic cellular alteration in HR-Csn8KO mice at 6 weeks. Pericellular and perisinusoid fibrosis with distorted architecture was also evident at 6 weeks. It is concluded that CSN8/CSN is essential to postnatal hepatocyte survival and effective proliferation.

Long isoforms of NRF1 contribute to arsenic-induced antioxidant response in human keratinocytes

BACKGROUND

Human exposure to inorganic arsenic (iAs), a potent oxidative stressor, causes various dermal disorders, including hyperkeratosis and skin cancer. Nuclear factor-erythroid 2-related factor 1 (NRF1, also called NFE2L1) plays a critical role in regulating the expression of many antioxidant response element (ARE)-dependent genes.

OBJECTIVES

We investigated the role of NRF1 in arsenic-induced antioxidant response and cytotoxicity in human keratinocytes.

RESULTS

In cultured human keratinocyte HaCaT cells, inorganic arsenite (iAs3+) enhanced the protein accumulation of long isoforms (120-140 kDa) of NRF1 in a dose- and time-dependent fashion. These isoforms accumulated mainly in the nuclei of HaCaT cells. Selective deficiency of NRF1 by lentiviral short-hairpin RNAs in HaCaT cells [NRF1-knockdown (KD)] led to decreased expression of γ-glutamate cysteine ligase catalytic subunit (GCLC) and regulatory subunit (GCLM) and a reduced level of intracellular glutathione. In response to acute iAs3+ exposure, induction of some ARE-dependent genes, including NAD(P)H:quinone oxidoreductase 1 (NQO1), GCLC, and GCLM, was significantly attenuated in NRF1-KD cells. However, the iAs3-induced expression of heme oxygenase 1 (HMOX-1) was unaltered by silencing NRF1, suggesting that HMOX-1 is not regulated by NRF1. In addition, the lack of NRF1 in HaCaT cells did not disturb iAs3+-induced NRF2 accumulation but noticeably decreased Kelch-like ECH-associated protein 1 (KEAP1) levels under basal and iAs3+-exposed conditions, suggesting a potential interaction between NRF1 and KEAP1. Consistent with the critical role of NRF1 in the transcriptional regulation of some ARE-bearing genes, knockdown of NRF1 significantly increased iAs3+-induced cytotoxicity and apoptosis.

CONCLUSIONS

Here, we demonstrate for the first time that long isoforms of NRF1 contribute to arsenic-induced antioxidant response in human keratinocytes and protect the cells from acute arsenic cytotoxicity.

Basic leucine zipper protein Cnc-C is a substrate and transcriptional regulator of the Drosophila 26S proteasome

While the 26S proteasome is a key proteolytic complex, little is known about how proteasome levels are maintained in higher eukaryotic cells. Here we describe an RNA interference (RNAi) screen of Drosophila melanogaster that was used to identify transcription factors that may play a role in maintaining levels of the 26S proteasome. We used an RNAi library against 993 Drosophila transcription factor genes to identify genes whose suppression in Schneider 2 cells stabilized a ubiquitin-green fluorescent protein reporter protein. This screen identified Cnc (cap 'n' collar [CNC]; basic region leucine zipper) as a candidate transcriptional regulator of proteasome component expression. In fact, 20S proteasome activity was reduced in cells depleted of cnc. Immunoblot assays against proteasome components revealed a general decline in both 19S regulatory complex and 20S proteasome subunits after RNAi depletion of this transcription factor. Transcript-specific silencing revealed that the longest of the seven transcripts for the cnc gene, cnc-C, was needed for proteasome and p97 ATPase production. Quantitative reverse transcription-PCR confirmed the role of Cnc-C in activation of transcription of genes encoding proteasome components. Expression of a V5-His-tagged form of Cnc-C revealed that the transcription factor is itself a proteasome substrate that is stabilized when the proteasome is inhibited. We propose that this single cnc gene in Drosophila resembles the ancestral gene family of mammalian nuclear factor erythroid-derived 2-related transcription factors, which are essential in regulating oxidative stress and proteolysis.

Oxidative damage increases and antioxidant gene expression decreases with aging in the mouse ovary

Oxidative stress has been implicated in various aspects of aging, but the role of oxidative stress in ovarian aging remains unclear. Our previous studies have shown that the initiation of apoptotic cell death in ovarian follicles and granulosa cells by various stimuli is initiated by increased reactive oxygen species. Herein, we tested the hypothesis that ovarian antioxidant defenses decrease and oxidative damage increases with age in mice. Healthy, wild-type C57BL/6 female mice aged 2, 6, 9, or 12 mo from the National Institute on Aging Aged Rodent Colony were killed on the morning of metestrus. Quantitative real-time RT-PCR was used to measure ovarian mRNA levels of antioxidant genes. Immunostaining using antibodies directed against 4-hydroxynonenal (4-HNE), nitrotyrosine (NTY), and 8-hydroxy-2'-deoxyguanosine (8-OHdG) was used to localize oxidative lipid, protein, and DNA damage, respectively, within the ovaries. TUNEL was used to localize apoptosis. Ovarian expression of glutathione peroxidase 1 (Gpx1) increased and expression of glutaredoxin 1 (Glrx1), glutathione S-transferase mu 2 (Gstm2), peroxiredoxin 3 (Prdx3), and thioredoxin 2 (Txn2) decreased in a statistically significant manner with age. Statistically significant increases in 4-HNE, NTY, and 8-OHdG immunostaining in ovarian interstitial cells and follicles were observed with increasing age. Our data suggest that the decrease in mRNA expression of mitochondrial antioxidants Prdx3 and Txn2 as well as cytosolic antioxidants Glrx1 and Gstm2 may be involved in age-related ovarian oxidative damage to lipid, protein, DNA, and other cellular components vital for maintaining ovarian function and fertility.

Knockout of the transcription factor NRF2 disrupts spermatogenesis in an age-dependent manner

Oxidative stress occurs when generation of reactive oxygen species (ROS) overwhelms antioxidant defenses. Oxidative stress has been associated with male infertility. The transcription factor nuclear factor-erythroid 2-related factor 2 (NRF2) regulates basal and inducible transcription of genes encoding enzymes important for protection against ROS. We hypothesized that deletion of the Nrf2 gene causes testicular and epididymal oxidative stress, which disrupts spermatogenesis. Our results show that male Nrf2(-/-) mice have decreased fertility compared to wild-type and heterozygous littermates, due to accumulating seminiferous tubule damage with increasing age. Testicular sperm head counts, epididymal sperm counts, and epididymal sperm motility in 2-month-old Nrf2(-/-) males did not differ from those of wild-type littermates; however, by age 6 months, Nrf2(-/-) males had 44% lower testicular sperm head counts, 65% lower epididymal sperm counts, and 66% lower epididymal sperm motility than wild-type males. Two- to 4-month-old Nrf2(-/-) males had elevated levels of testicular and epididymal lipid peroxidation and testicular germ cell apoptosis, and decreased levels of antioxidants, compared to wild-type males. These results provide evidence that oxidative stress has deleterious effects on the testis and epididymis and demonstrate a critical role for the transcription factor NRF2 in preventing oxidative disruption of spermatogenesis.

Identification of topological determinants in the N-terminal domain of transcription factor Nrf1 that control its orientation in the endoplasmic reticulum membrane

Nrf1 [NF-E2 (nuclear factor-erythroid 2)-related factor 1] is a CNC (cap'n'collar) bZIP (basic-region leucine zipper) transcription factor that is tethered to ER (endoplasmic reticulum) and nuclear envelope membranes through its N-terminal signal peptide (residues 1-30). Besides the signal peptide, amino acids 31-90 of Nrf1 also negatively regulate the CNC-bZIP factor. In the present study we have tested the hypothesis that amino acids 31-90 of Nrf1, and the overlapping NHB2 (N-terminal homology box 2; residues 82-106), inhibit Nrf1 because they control its topology within membranes. This region contains three amphipathic alpha-helical regions comprising amino acids 31-50 [called the SAS (signal peptide-associated sequence)], 55-82 [called the CRACs (cholesterol-recognition amino acid consensus sequences)] and 89-106 (part of NHB2). We present experimental data showing that the signal peptide of Nrf1 contains a TM1 (transmembrane 1) region (residues 7-24) that is orientated across the ER membrane in an N(cyt)/C(lum) fashion with its N-terminus facing the cytoplasm and its C-terminus positioned in the lumen of the ER. Once Nrf1 is anchored to the ER membrane through TM1, the remaining portion of the N-terminal domain (NTD, residues 1-124) is transiently translocated into the ER lumen. Thereafter, Nrf1 adopts a topology in which the SAS is inserted into the membrane, the CRACs are probably repartitioned to the cytoplasmic side of the ER membrane, and NHB2 may serve as an anchor switch, either lying on the luminal surface of the ER or traversing the membrane with an N(cyt)/C(lum) orientation. Thus Nrf1 can adopt several topologies within membranes that are determined by its NTD.

Regulation of notch1 signaling by nrf2: implications for tissue regeneration

The Keap1-Nrf2-ARE signaling pathway elicits an adaptive response for cell survival after endogenous and exogenous stresses, such as inflammation and carcinogens, respectively. Keap1 inhibits the transcriptional activation activity of Nrf2 (p45 nuclear factor erythroid-derived 2-related factor 2) in unstressed cells by facilitating its degradation. Through transcriptional analyses in Keap1- or Nrf2-disrupted mice, we identified interactions between the Keap1-Nrf2-ARE and the Notch1 signaling pathways. We found that Nrf2 recognized a functional antioxidant response element (ARE) in the promoter of Notch1. Notch1 regulates processes such as proliferation and cell fate decisions. We report a functional role for this cross talk between the two pathways and show that disruption of Nrf2 impeded liver regeneration after partial hepatectomy and was rescued by reestablishment of Notch1 signaling.

The nrf1 and nrf2 balance in oxidative stress regulation and androgen signaling in prostate cancer cells

Reactive oxygen species (ROS) signaling has recently sparked a surge of interest as being the molecular underpinning for cancer cell survival, but the precise mechanisms involved have not been completely elucidated. This review covers the possible roles of two ROS-induced transcription factors, Nrf1 and Nrf2, and the antioxidant proteins peroxiredoxin-1 (Prx-1) and Thioredoxin-1 (Txn-1) in modulating AR expression and signaling in aggressive prostate cancer (PCa) cells. In androgen independent (AI) C4-2B cells, in comparison to the parental androgen dependent (AD) LNCaP cells, we present evidence of high Nrf1 and Prx-1 expression and low Nrf2 expression in these aggressive PCa cells. Furthermore, in DHT treated C4-2B cells, increased expression of the p65 (active) isoform of Nrf1 correlated with enhanced AR transactivation. Our findings implicate a crucial balance of Nrf1 and Nrf2 signaling in regulating AR activity in AI-PCa cells. Here we will discuss how understanding the mechanisms by which oxidative stress may affect AR signaling may aid in developing novel therapies for AI-PCa.

S-nitrosylation from GSNOR deficiency impairs DNA repair and promotes hepatocarcinogenesis

Human hepatocellular carcinoma (HCC) is associated with elevated expression of inducible nitric oxide synthase (iNOS), but the role of nitric oxide in the pathogenesis of HCC remains unknown. We found that the abundance and activity of S-nitrosoglutathione reductase (GSNOR), a protein critical for control of protein S-nitrosylation, were significantly decreased in approximately 50% of patients with HCC. GSNOR-deficient mice were very susceptible to spontaneous and carcinogen-induced HCC. During inflammatory responses, the livers of GSNOR-deficient mice exhibited substantial S-nitrosylation and proteasomal degradation of the key DNA repair protein O(6)-alkylguanine-DNA alkyltransferase. As a result, repair of carcinogenic O(6)-alkylguanines in GSNOR-deficient mice was significantly impaired. Predisposition to HCC, S-nitrosylation and depletion of alkylguanine-DNA alkyltransferase, and accumulation of O(6)-alkylguanines were all abolished in mice deficient in both GSNOR and iNOS. Thus, our data suggest that GSNOR deficiency, through dysregulated S-nitrosylation, may promote HCC, possibly by inactivating a DNA repair system.

Nonalcoholic fatty liver disease and hepatocellular carcinoma: a weighty connection

Hepatocellular carcinoma (HCC) is a common and deadly malignancy that is increasing in incidence in developed countries. The emergence of hepatitis C virus (HCV) accounts for about half of this increase in HCC, although the etiology of HCC in 15%-50% of new HCC cases remains unclear. The most common form of chronic liver disease in developed countries is nonalcoholic fatty liver disease (NAFLD), which encompasses a broad spectrum of histopathology. The prevalence of NAFLD, including the more aggressive nonalcoholic steatohepatitis (NASH), is increasing with the growing epidemics of diabetes and obesity. NASH can progress to cirrhosis and its related complications. Growing evidence suggests that NASH accounts for a large proportion of idiopathic or cryptogenic cirrhosis, which is associated with the typical risk factors for NASH. HCC is a rare, although important complication of NAFLD. Diabetes and obesity have been established as independent risk factors for the development of HCC. New evidence also suggests that hepatic iron deposition increases the risk of HCC in NASH-derived cirrhosis. Multiple case reports and case reviews of HCC in the setting of NASH support the associations of diabetes and obesity with the risk of HCC, as well as suggest age and advanced fibrosis as significant risks. Insulin resistance and its subsequent inflammatory cascade that is associated with the development of NASH appear to play a significant role in the carcinogenesis of HCC. The complications of NASH, including cirrhosis and HCC, are expected to increase with the growing epidemic of diabetes and obesity.

Nonalcoholic steatohepatitis-associated hepatocellular carcinoma: Our case series and literature review

Recently, nonalcoholic steatohepatitis (NASH) has been considered to be another cause of liver cirrhosis and hepatocellular carcinoma (HCC). The natural history and prognosis of NASH are controversial. Accordingly, we assessed the clinicopathological features of NASH-associated HCC in our experience and reviewed the literature of NASH-associated HCC. We experienced 11 patients with NASH-associated HCC (6 male, 5 female; mean age 73.8 ± 4.9 years) who received curative treatments. Most (91%) patients had been diagnosed with obesity, diabetes, hypertension, or dyslipidemia. Seven patients (64%) also had a non-cirrhotic liver. The recurrence-free survival rates at 1, 3 and 5 years were 72%, 60%, and 60%. We also summarized and reviewed 94 cases of NASH-associated HCC which were reported in the literature (64 male; mean age 66 years). The majority of patients (68%) were obese, 66% of patients had diabetes, and 24% had dyslipidemia. Furthermore, 26% of the HCCs arose from the non-cirrhotic liver. In conclusion, patients with non-cirrhotic NASH may be a high-risk group for HCC, and regular surveillance for HCC is necessary in non-cirrhotic NASH patients as well as cirrhotic patients.

The rise of antioxidant signaling--the evolution and hormetic actions of Nrf2

Organisms have evolved sophisticated and redundant mechanisms to manage oxidative and electrophilic challenges that arise from internal metabolism or xenobiotic challenge for survival. NF-E2-related factor 2 (Nrf2) is a transcription factor that has evolved over millennia from primitive origins, with homologues traceable back to invertebrate Caenorhabditis and Drosophila species. The ancestry of Nrf2 clearly has deep-seated roots in hematopoiesis, yet has diversified into a transcription factor that can mediate a multitude of antioxidant signaling and detoxification genes. In higher organisms, a more sophisticated means of tightly regulating Nrf2 activity was introduced via the cysteine-rich kelch-like ECH-associated protein 1 (Keap1), thus suggesting a need to modulate Nrf2 activity. This is evidenced in Keap1(-/-) mice, which succumb to juvenile mortality due to hyperkeratosis of the gastrointestinal tract. Although Nrf2 activation protects against acute toxicity and prevents or attenuates several disease states, constitutive activation in some tumors leads to poor clinical outcomes, suggesting Nrf2 has evolved in response to a multitude of selective pressures. The purpose of this review is to examine the origins of Nrf2, while highlighting the versatility and protective abilities elicited upon activation. Various model systems in which Nrf2 is normally beneficial but in which exaggerated pharmacology exacerbates a physiological or pathological condition will be addressed. Although Darwinian principles have selected Nrf2 activity for maximal beneficial effect based on environmental and oxidative challenge, both sub- or super-physiological effects have been noted to be detrimental. The functions of Nrf2 thus suggest a hormetic factor that has evolved empirically over time.

Deletion of nuclear factor-E2-related factor-2 leads to rapid onset and progression of nutritional steatohepatitis in mice

Oxidative stress is a critical mediator in liver injury of steatohepatitis. The transcription factor Nrf2 serves as a cellular stress sensor and is a key regulator for induction of hepatic detoxification and antioxidative stress systems. The involvement of Nrf2 in defense against the development of steatohepatitis remains unknown. We aimed to investigate the protective roles of Nrf2 in nutritional steatohepatitis using wild-type (WT) and Nrf2 gene-null (Nrf2-null) mice. WT and Nrf2-null mice were fed a methionine- and choline-deficient (MCD) diet for 3 and 6 wk, and the liver tissues were analyzed for pathology and for expression levels of detoxifying enzymes and antioxidative stress genes via the Nrf2 transcriptional pathway. In WT mice fed an MCD diet, Nrf2 was potently activated in the livers, and steatohepatitis did not develop over the observation periods. However, in Nrf2-null mice fed an MCD diet, the pathological state of the steatohepatitis was aggravated in terms of fatty changes, inflammation, fibrosis, and iron accumulation. In the livers of the Nrf2-null mice, oxidative stress was significantly increased compared with that of WT mice based on the increased levels of 4-hydroxy-2-nonenal and malondialdehyde. This change was associated with the decreased levels of glutathione, detoxifying enzymes, catalase, and superoxide dismutase activity. Correlating well with the liver pathology, the mRNA levels of factors involved in fatty acid metabolism, inflammatory cytokines, and fibrogenesis-related genes were significantly increased in the livers of the Nrf2-null mice. These findings demonstrate that Nrf2 deletion in mice leads to rapid onset and progression of nutritional steatohepatitis induced by an MCD diet. Activation of Nrf2 could be a promising target toward developing new options for prevention and treatment of steatohepatitis.

Identification of two critically deleted regions within chromosome segment 7q35-q36 in EVI1 deregulated myeloid leukemia cell lines

Chromosomal rearrangements involving the EVI1 proto-oncogene are a recurrent finding in myeloid leukemias and are indicative of a poor prognosis. Rearrangements of the EVI1 locus are often associated with monosomy 7 or cytogenetic detectable deletions of part of 7q. As EVI1 overexpression alone is not sufficient to induce leukemia, loss of a 7q tumour suppressor gene might be a required cooperating event. To test this hypothesis, we performed high-resolution array comparative genomic hybridization analysis of twelve EVI1 overexpressing patients and three EVI1 deregulated cell lines to search for 7q submicroscopic deletions. This analysis lead to the delineation of two critical regions, one of 0.39 Mb on 7q35 containing the CNTNAP2 gene and one of 1.33 Mb on chromosome bands 7q35-q36 comprising nine genes in EVI1 deregulated cell lines. These findings open the way to further studies aimed at identifying the culprit EVI1 implicated tumour suppressor genes on 7q.

Targeted disruption of nuclear factor erythroid-derived 2-like 1 in osteoblasts reduces bone size and bone formation in mice

Previous in vitro studies found that nuclear factor erythroid-derived 2-like 1 (NFE2L1) was involved in mediating ascorbic acid-induced osterix expression and osteoblast differentiation via binding to the antioxidant response element of the osterix promoter. To test the role of NFE2L1 in regulating bone formation in vivo, we disrupted NFE2L1 specifically in osteoblasts. Mice expressing Cre under the control of Col1α2 promoter were crossed with NFE2L1 loxP mice to generate Cre+ knockout (KO) and Cre− wild-type (WT) mice. Skeletal measurements by DEXA revealed 8–10% and 9–11% reduction in total body BMC and bone area in the KO mice from 3 to 8 wk of age. Peripheral quantitative computed tomography analyses found both periosteal and endosteal circumferences were reduced by 6% at the middiaphysis of the femurs from 8 wk old KO mice. Histomorphometric analyses revealed reduced bone formation was a cause for reduced bone size in the KO mice. Microcomputed tomography analysis of the metaphysis of the femur revealed that trabecular bone volume/total volume, and trabecular numbers were decreased by 30 and 53% in the NFE2L1 KO mice. Expression of osterix was decreased by 57% in the bones of NFE2L1 KO mice. In vitro nodule assay demonstrated that mineralized nodule area was reduced by 68% in the cultures of bone marrow stromal cells from NFE2L1 KO mice. Treatment of primary osteoblasts with ascorbic acid increased osterix expression by fourfold, whereas loss of NFE2L1 in osteoblasts diminished ascorbic acid stimulation of osterix expression by 50%. Our data provide the first in vivo experimental evidence that NFE2L1 produced by osteoblasts is involved in regulating osterix expression, osteoblast differentiation, and bone formation.

The keap1-nrf2 cellular defense pathway: mechanisms of regulation and role in protection against drug-induced toxicity

Adverse drug reactions pose a significant public health problem. In some cases, the process of drug metabolism can contribute to the onset of toxicity through the bioactivation of a parent molecule to a chemically reactive intermediate. In order to maintain a favorable balance between bioactivation and detoxification, mammalian cells have evolved an inducible cell defense system known as the antioxidant response pathway. The activity of this cytoprotective pathway is largely regulated by the transcription factor Nrf2, which governs the expression of many phase II detoxification and antioxidant enzymes. In turn, the activity of Nrf2 is regulated by the cysteine-rich cytosolic inhibitor Keap1, which acts as a "sensor" for chemical/oxidative stress. This article summarizes our current understanding of the molecular mechanisms that regulate the function of the Keap1-Nrf2 pathway and highlights the importance of Nrf2 in the protection against drug-induced toxicity.

NASH and HCC

Primary liver cancer is the fifth most common malignancy worldwide and the third leading cause of cancer mortality. Non-alcoholic fatty liver disease is the most common cause of chronic liver disease in the United States encompassing a spectrum of entities marked by hepatic steatosis in the absence of significant alcohol consumption. Although simple steatosis follows a generally benign course, the more aggressive form, non-alcoholic steatohepatitis, can progress to cirrhosis and result in complications including hepatocellular carcinoma. A significant number of cases of hepatocellular carcinoma remain cryptogenic without known underlying chronic liver disease. It is increasingly recognized that non-alcoholic steatohepatitis likely accounts for a substantial portion of cryptogenic hepatocellular carcinoma.

Role of Nrf1 in antioxidant response element-mediated gene expression and beyond

Oxidative stress plays an important part in the pathogenesis of a variety of diseases. The ability to mount an efficient response against the continuous threat posed by exogenous and endogenous oxidants is essential for cellular homeostasis and survival. Oxidative stress activates transcription of a variety of antioxidant genes through cis-acting sequence known as antioxidant response element (ARE). Members of the Cap-N-Collar family of transcription factors, including Nrf1 and Nrf2, that bind ARE have been identified. Nrf1 and Nrf2 are expressed in a wide range of tissues and cell types, and both bind the ARE as heterodimers with small Maf proteins. Numerous studies indicate a pivotal role of Nrf2 in ARE function. Herein, we review data derived from cell-based studies and knockout mice in an attempt to define the role and regulation of Nrf1 in oxidative stress response and other functions.

Mechanisms of non-alcoholic steatohepatitis: pathophysiology and molecular biology

Non-alcoholic steatohepatitis (NASH), characterized by liver fatty infiltration, inflammation, hepatocellular injury and fibrosis, may easily develop into liver cirrhosis and hepatocellular carcinoma. The increased flow of FFAs (free fatty acids) to the liver and the de novo lipogenesis in the liver lead to fat overload. Lipotoxicity can induce oxidative stress, inflammatory reaction and apoptosis. Subsequently chronic liver injury activates a fibrogenic response that accelerates the evolution of NASH towards end-stage liver disease. Further research on pathophysiology and molecular biology is beneficial to clinical diagnosis and management of NASH.

Effects and mechanisms of silibinin on human hepatocellular carcinoma xenografts in nude mice

AIM: To investigate the in vivo effects and mechanisms of silibinin on the growth of hepatocellular carcinoma (HCC) xenografts in nude mice.

METHODS: Nude mice bearing HuH7 xenografts were used to assess the anti-HCC effects and mechanisms of silibinin.

RESULTS: Silibinin resulted in a potent dose-dependent reduction of HuH7 xenografts in association with a significant decrease in Ki-67 and α-fetoprotein production, nuclear NF-κB content, polo-like kinase 1, Rb phosphorylation, and E2F1/DP1 complex, but increased p27/CDK4 complex and checkpoint kinase 1 expression, suggesting that the in vivo effects of silibinin are mediated by inhibiting G1-S transition of the cell cycle. Silibinin-induced apoptosis of HuH7 xenografts was associated with inhibited survivin phosphorylation. Silibinin-reduced growth of HuH7 xenografts was associated with decreased p-ERK, increased PTEN expression and the activity of silibinin was correlated with decreased p-Akt production, indicating involvement of PTEN/PI₃K/Akt and ERK pathways in its in vivo anti-HCC effects. Silibinin-reduced growth of HuH7 xenografts was also associated with a significant increase in AC-H3 and AC-H4 expression and the production of superoxide dismutase (SOD)-1.

CONCLUSION: Silibinin reduces HCC xenograft growth through the inhibition of cell proliferation, cell cycle progression and PTEN/P-Akt and ERK signaling, inducing cell apoptosis, and increasing histone acetylation and SOD-1 expression.

Molecular mechanisms involved in NAFLD progression

Non-alcoholic fatty liver disease (NAFLD) is an emerging metabolic-related disorder characterized by fatty infiltration of the liver in the absence of alcohol consumption. NAFLD ranges from simple steatosis to non-alcoholic steatohepatitis (NASH), which might progress to end-stage liver disease. This progression is related to the insulin resistance, which is strongly linked to the metabolic syndrome consisting of central obesity, diabetes mellitus, and hypertension. Earlier, the increased concentration of intracellular fatty acids within hepatocytes leads to steatosis. Subsequently, multifactorial complex interactions between nutritional factors, lifestyle, and genetic determinants promote necrosis, inflammation, fibrosis, and hepatocellular damage. Up to now, many studies have revealed the mechanism associated with insulin resistance, whereas the mechanisms related to the molecular components have been incompletely characterized. This review aims to assess the potential molecular mediators initiating and supporting the progression of NASH to establish precocious diagnosis and to plan more specific treatment for this disease.

The Nrf1 CNC/bZIP protein is a nuclear envelope-bound transcription factor that is activated by t-butyl hydroquinone but not by endoplasmic reticulum stressors

In rat liver RL-34 cells, endogenous Nrf1 (nuclear factor-erythroid 2 p45 subunit-related factor 1) is localized in the ER (endoplasmic reticulum) where it exists as a glycosylated protein. Electron microscopy has demonstrated that ectopic Nrf1 in COS-1 cells is located in the ER and the NE (nuclear envelope). Subcellular fractionation, together with a membrane proteinase protection assay, revealed that Nrf1 is an integral membrane protein with both luminal and cytoplasmic domains. The N-terminal 65 residues of Nrf1 direct its integration into the ER and NE membranes and tether it to a Triton X-100-resistant membrane microdomain that is associated with lipid rafts. The activity of Nrf1 was increased by the electrophile tBHQ (t-butyl hydroquinone) probably through an N-terminal domain-dependent process. We found that the NST (Asn/Ser/Thr-rich) domain, along with AD1 (acidic domain 1), contributes positively to the transactivation activity of full-length Nrf1. Furthermore, the NST domain contains seven putative -Asn-Xaa-Ser/Thr- glycosylation sites and, when glycosylation was prevented by replacing all of the seven asparagine residues with either glutamine (Nrf1(1-7xN/Q)) or aspartic acid (Nrf1(1-7xN/D)), the former multiple point mutant possessed less activity than the wild-type factor, whereas the latter mutant exhibited substantially greater activity. Lastly, the ER stressors tunicamycin, thapsigargin and Brefeldin A were found to inhibit basal Nrf1 activity by approximately 25%, and almost completely prevented induction of Nrf1-mediated transactivation by tBHQ. Collectively, these results suggest that the activity of Nrf1 critically depends on its topology within the ER, and that this is modulated by redox stressors, as well as by its glycosylation status.

Does nonalcoholic fatty liver disease predispose patients to hepatocellular carcinoma in the absence of cirrhosis?

CONTEXT

Hepatocellular carcinoma (HCC) is recognized as a complication of cirrhosis related to nonalcoholic fatty liver disease (NAFLD). Diabetes and the metabolic syndrome are also associated with HCC. However, it is not clear whether NAFLD predisposes patients to HCC in the absence of cirrhosis.

OBJECTIVE

To seek evidence that HCC can develop in NAFLD unaccompanied by cirrhosis.

DESIGN

Retrospective case study was performed on cases from 2004 to 2007 at the University of Illinois at Chicago Medical Center, using the key words hepatocellular carcinoma, liver explant, and liver resection. The diagnosis of HCC was identified and confirmed by hematoxylin-eosin-stained slides in 50 cases. Cause of liver disease was determined by review of liver histology, clinical history, and laboratory data.

RESULTS

Three patients presented with advanced HCC with features of metabolic syndrome, including an elevated body mass index. Each patient had bland steatosis on liver biopsy, without fibrosis or cirrhosis. None of the 3 patients had evidence of any cause for liver disease other than NAFLD.

CONCLUSIONS

The cases presented here suggest that NAFLD may predispose patients to HCC in the absence of cirrhosis. Further studies are needed to confirm this potentially important observation.

Redox mechanisms in hepatic chronic wound healing and fibrogenesis

Reactive oxygen species (ROS) generated within cells or, more generally, in a tissue environment, may easily turn into a source of cell and tissue injury. Aerobic organisms have developed evolutionarily conserved mechanisms and strategies to carefully control the generation of ROS and other oxidative stress-related radical or non-radical reactive intermediates (that is, to maintain redox homeostasis), as well as to 'make use' of these molecules under physiological conditions as tools to modulate signal transduction, gene expression and cellular functional responses (that is, redox signalling). However, a derangement in redox homeostasis, resulting in sustained levels of oxidative stress and related mediators, can play a significant role in the pathogenesis of major human diseases characterized by chronic inflammation, chronic activation of wound healing and tissue fibrogenesis. This review has been designed to first offer a critical introduction to current knowledge in the field of redox research in order to introduce readers to the complexity of redox signalling and redox homeostasis. This will include ready-to-use key information and concepts on ROS, free radicals and oxidative stress-related reactive intermediates and reactions, sources of ROS in mammalian cells and tissues, antioxidant defences, redox sensors and, more generally, the major principles of redox signalling and redox-dependent transcriptional regulation of mammalian cells. This information will serve as a basis of knowledge to introduce the role of ROS and other oxidative stress-related intermediates in contributing to essential events, such as the induction of cell death, the perpetuation of chronic inflammatory responses, fibrogenesis and much more, with a major focus on hepatic chronic wound healing and liver fibrogenesis.

Nrf1 and Nrf2 play distinct roles in activation of antioxidant response element-dependent genes

Nrf1 is a member of the vertebrate Cap'n'Collar (CNC) transcription factor family that commonly contains a unique basic-leucine zipper domain. Among CNC family members, Nrf2 is known to regulate a battery of antioxidant and xenobiotic-metabolizing enzyme genes through the antioxidant response element (ARE). Although Nrf1 has also been shown to bind the ARE, it is unclear whether it plays a distinct role from Nrf2 in regulating genes with this element. To address this issue in vivo, we generated mice bearing a hepatocyte-specific disruption of the Nrf1 gene. AlthoughNrf2 knock-out mice did not exhibit liver damage when they were maintained in an unstressed condition, hepatocyte-specific deletion of Nrf1 caused liver damage resembling the human disease non-alcoholic steatohepatitis. Gene expression analysis revealed that the disruption of Nrf1 causes stress that activates a number of ARE-driven genes in an Nrf2-dependent manner, indicating that Nrf2 cannot compensate completely for loss of Nrf1 function in the liver. In contrast, expression of metallothionein-1 and -2 (MT1 and MT2) genes, each of which harbors at least one ARE in its regulatory region, was decreased in the Nrf1-null mutant mice. Whereas Nrf1 and Nrf2 bound the MT1 ARE with comparable affinity, Nrf1 preferentially activated the reporter gene expression through the MT1 ARE. This study has, thus, identified the first ARE-dependent gene that relies exclusively on Nrf1, suggesting that it plays a distinct functional role in regulating ARE-driven genes.

Time- and dose-based gene expression profiles produced by a bile-duct-damaging chemical, 4,4'-methylene dianiline, in mouse liver in an acute phase

A toxicogenomics study was performed in the mouse liver after treatment of a bile-duct-damaging chemical, 4,4'-methylene dianiline (MDA), across multiple doses and sampling times in an acute phase using the AB Expression Array System. Imprinting control region (ICR) mice were given a single oral administration of a low (10 mg/kg b.w.) or high (100 mg/kg b.w.) dose of MDA. Mice were sacrificed six, twenty-four, and seventy-two hours after treatment for serum chemistry, histopathology, and mRNA preparation from liver samples. Treatment with MDA increased liver-toxicity-related enzymes in blood and induced bile-duct cell injury, followed by regeneration. To explore potential biomarker gene profiles, the altered genes were categorized into four expression patterns depending on dose and time. Numerous functionally defined and unclassified genes in each category were up- or down-regulated throughout the period from cellular injury to the recovery phase, verified by RT-PCR. Many genes associated with liver toxicity and diseases belonged to one of these categories. The chemokine-mediated Th1 pathway was implicated in the inflammatory process. The genes associated with oxidative stress, apoptosis, and cell-cycle regulation were also dynamically responsive to MDA treatment. The Wnt/beta-catenin signaling pathway was likely responsible for the reconstitution process of the MDA-injured liver.

Sirt1 protects against high-fat diet-induced metabolic damage

The identification of new pharmacological approaches to effectively prevent, treat, and cure the metabolic syndrome is of crucial importance. Excessive exposure to dietary lipids causes inflammatory responses, deranges the homeostasis of cellular metabolism, and is believed to constitute a key initiator of the metabolic syndrome. Mammalian Sirt1 is a protein deacetylase that has been involved in resveratrol-mediated protection from high-fat diet-induced metabolic damage, but direct proof for the implication of Sirt1 has remained elusive. Here, we report that mice with moderate overexpression of Sirt1 under the control of its natural promoter exhibit fat mass gain similar to wild-type controls when exposed to a high-fat diet. Higher energy expenditure appears to be compensated by a parallel increase in food intake. Interestingly, transgenic Sirt1 mice under a high-fat diet show lower lipid-induced inflammation along with better glucose tolerance, and are almost entirely protected from hepatic steatosis. We present data indicating that such beneficial effects of Sirt1 are due to at least two mechanisms: induction of antioxidant proteins MnSOD and Nrf1, possibly via stimulation of PGC1alpha, and lower activation of proinflammatory cytokines, such as TNFalpha and IL-6, via down-modulation of NFkappaB activity. Together, these results provide direct proof of the protective potential of Sirt1 against the metabolic consequences of chronic exposure to a high-fat diet.