Involvement of the electrophile responsive element and p53 in the activation of hepatic stellate cells as a response to electrophile menadione

Loss of Tid1/DNAJA3 Co-Chaperone Promotes Progression and Recurrence of Hepatocellular Carcinoma after Surgical Resection: A Novel Model to Stratify Risk of Recurrence

Simple Summary

Tid1 acts as a tumor suppressor in various cancer types, however, its role in hepatocellular carcinoma (HCC) remains unclear. Here, we observed a low protein level of Tid1 in poorly differentiated HCC cell lines. The expression of Tid1 affected the malignancy in human HCC cell lines; meanwhile the protein level of Nrf2 was negatively regulated by Tid1. In multivariate analysis, using immunohistochemical (IHC) assay in 210 HCC cases, we found the tumor size > 5 cm, multiple tumors, presence of vascular invasion, low Tid1 expression in the non-tumor part, and high Nrf2 expression in the non-tumor part, were independently associated with worse recurrence-free survival (RFS). A scoring system by integrating the five clinical and pathological factors predicts the RFS among HCC patients after surgical resection. In summary, Tid1 plays a prognostic role for surgically resected HCC.

Abstract

Tid1, a mitochondrial co-chaperone protein, acts as a tumor suppressor in various cancer types. However, the role of Tid1 in hepatocellular carcinoma (HCC) remains unclear. First, we found that a low endogenous Tid1 protein level was observed in poorly differentiated HCC cell lines. Further, upregulation/downregulation of Tid1 abrogated/promoted the malignancy of human HCC cell lines, respectively. Interestingly, Tid1 negatively modulated the protein level of Nrf2. Tissue assays from 210 surgically resected HCC patients were examined by immunohistochemistry (IHC) analyses. The protein levels of Tid1 in the normal and tumor part of liver tissues were correlated with the clinical outcome of the 210 HCC cases. In multivariate analysis, we discovered that tumor size > 5 cm, multiple tumors, presence of vascular invasion, low Tid1 expression in the non-tumor part, and high Nrf2 expression in the non-tumor part were significant factors associated with worse recurrence-free survival (RFS). A scoring system by integrating the five clinical and pathological factors predicts the RFS among HCC patients after surgical resection. Together, Tid1, serving as a tumor suppressor, has a prognostic role for surgically resected HCC to predict RFS.

Molecular and cellular basis for the unique functioning of Nrf1, an indispensable transcription factor for maintaining cell homoeostasis and organ integrity

The consensuscis-regulatory AP-1 (activator protein-1)-like AREs (antioxidant-response elements) and/or EpREs (electrophile-response elements) allow for differential recruitment of Nrf1 [NF-E2 (nuclear factor-erythroid 2)-related factor 1], Nrf2 and Nrf3, together with each of their heterodimeric partners (e.g. sMaf, c-Jun, JunD or c-Fos), to regulate different sets of cognate genes. Among them, NF-E2 p45 and Nrf3 are subject to tissue-specific expression in haemopoietic and placental cell lineages respectively. By contrast, Nrf1 and Nrf2 are two important transcription factors expressed ubiquitously in various vertebrate tissues and hence may elicit putative combinational or competitive functions. Nevertheless, they have de facto distinct biological activities because knockout of their genes in mice leads to distinguishable phenotypes. Of note, Nrf2 is dispensable during development and growth, albeit it is accepted as a master regulator of antioxidant, detoxification and cytoprotective genes against cellular stress. Relative to the water-soluble Nrf2, less attention has hitherto been drawn to the membrane-bound Nrf1, even though it has been shown to be indispensable for embryonic development and organ integrity. The biological discrepancy between Nrf1 and Nrf2 is determined by differences in both their primary structures and topovectorial subcellular locations, in which they are subjected to distinct post-translational processing so as to mediate differential expression of ARE-driven cytoprotective genes. In the present review, we focus on the molecular and cellular basis for Nrf1 and its isoforms, which together exert its essential functions for maintaining cellular homoeostasis, normal organ development and growth during life processes. Conversely, dysfunction of Nrf1 results in spontaneous development of non-alcoholic steatohepatitis, hepatoma, diabetes and neurodegenerative diseases in animal models.

Nrf2 is a potential prognostic marker and promotes proliferation and invasion in human hepatocellular carcinoma

Background

Nuclear factor E2-related factor 2 (Nrf2 or NFE2L2) is abundantly expressed in cancer cells and relates to proliferation, invasion, and chemoresistance. Our early observations also found that expression of Nrf2 was up-regulated in kinds of cancer including human hepatocellular carcinoma (HCC) cells. But there are limited reports about the expression, significance, function of Nrf2 in HCC.

Methods

First, Nrf2 expression was analyzed in HCC cell lines and tumor samples. Then, the relationship of Nrf2 with clinicopathological factors and survival were analyzed. Further, the effect of Nrf2 on cell proliferation, apoptosis, and metastasis was examined in vitro by modulating expression of Nrf2 through specific shRNA or expression plasmid. Last, the potential mechanisms were also investigated.

Results

Nrf2 was up-regulated in HCC, and expression of Nrf2 was correlated with tumor differentiation, metastasis, and tumor size. Univariate and multivariate analyses indicated that high Nrf2 expression might be a poor prognostic factor. Further studies demonstrated that inhibition of Nrf2 expression inhibited proliferation by inducing apoptosis and repressed invasion, and up-regulation of Nrf2 expression resulted in opposite phenotypes. Moreover, there are positive correlation between Nrf2 expression and that of Bcl-xL and MMP-9.

Conclusion

Nrf2 is a potential prognostic marker and promotes proliferation and invasion in human hepatocellular carcinoma partly through regulating expression of Bcl-xL and MMP-9.

Identification and quantification of the basal and inducible Nrf2-dependent proteomes in mouse liver: biochemical, pharmacological and toxicological implications

The transcription factor Nrf2 is a master regulator of cellular defence: Nrf2 null mice (Nrf2^(−/−)) are highly susceptible to chemically induced toxicities. We report a comparative iTRAQ-based study in Nrf2^(−/−) mice treated with a potent inducer, methyl-2-cyano-3,12-dioxooleana-1,9(11)dien-28-oate (CDDO-me; bardoxolone -methyl), to define both the Nrf2-dependent basal and inducible hepatoproteomes. One thousand five hundred twenty-one proteins were fully quantified (FDR < 1%). One hundred sixty-one were significantly different (P < 0.05) between WT and Nrf2^(−/−) mice, confirming extensive constitutive regulation by Nrf2. Treatment with CDDO-me (3 mg/kg; i.p.) resulted in significantly altered expression of 43 proteins at 24 h in WT animals. Six proteins were regulated at both basal and inducible levels exhibiting the largest dynamic range of Nrf2 regulation: cytochrome P4502A5 (CYP2A5; 17.2-fold), glutathione-S-transferase-Mu 3 (GSTM3; 6.4-fold), glutathione-S-transferase Mu 1 (GSTM1; 5.9-fold), ectonucleoside-triphosphate diphosphohydrolase (ENTPD5; 4.6-fold), UDP-glucose-6-dehydrogenase (UDPGDH; 4.1-fold) and epoxide hydrolase (EPHX1; 3.0-fold). These proteins, or their products, thus provide a potential source of biomarkers for Nrf2 activity. ENTPD5 is of interest due to its emerging role in AKT signalling and, to our knowledge, this protein has not been previously shown to be Nrf2-dependent. Only two proteins altered by CDDO-me in WT animals were similarly affected in Nrf2^(−/−) mice, demonstrating the high degree of selectivity of CDDO-me for the Nrf2:Keap1 signalling pathway.

Biological significance

The Nrf2:Keap1 signalling pathway is attracting considerable interest as a therapeutic target for different disease conditions. For example, CDDO-me (bardoxolone methyl) was investigated in clinical trials for the treatment of acute kidney disease, and dimethyl fumarate, recently approved for reducing relapse rate in multiple sclerosis, is a potent Nrf2 inducer. Such compounds have been suggested to act through multiple mechanisms; therefore, it is important to define the selectivity of Nrf2 inducers to assess the potential for off-target effects that may lead to adverse drug reactions, and to provide biomarkers with which to assess therapeutic efficacy. Whilst there is considerable information on the global action of such inducers at the mRNA level, this is the first study to catalogue the hepatic protein expression profile following acute exposure to CDDO-me in mice. At a dose shown to evoke maximal Nrf2 induction in the liver, CDDO-me appeared highly selective for known Nrf2-regulated proteins. Using the transgenic Nrf2^(−/−) mouse model, it could be shown that 97% of proteins induced in wild type mice were associated with a functioning Nrf2 signalling pathway. This analysis allowed us to identify a panel of proteins that were regulated both basally and following Nrf2 induction. Identification of these proteins, which display a large magnitude of variation in their expression, provides a rich source of potential biomarkers for Nrf2 activity for use in experimental animals, and which may be translatable to man to define individual susceptibility to chemical stress, including that associated with drugs, and also to monitor the pharmacological response to Nrf2 inducers.

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Liver proteomes from WT, Nrf2-null and Nrf2-induced mice were compared by iTRAQ

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Of 1521 proteins quantified, 161 were regulated basally and 43 following induction

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Six proteins were both basally and inducibly regulated, with high dynamic ranges

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In order of fold change, these proteins were CYP2A5, GSTM3, GSTM1, ENTPD5, G6PD, EPHX1

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These proteins may yield translatable biomarkers for clinical development

SK-N-MC cell death occurs by distinct molecular mechanisms in response to hydrogen peroxide and superoxide anions: involvements of JAK2-STAT3, JNK, and p38 MAP kinases pathways

Oxidative stress plays a vital role in the pathogenesis of neurodegenerative diseases. Nerve cells are incessantly exposed to environmental stresses leading to overproduction of some harmful species like reactive oxygen species (ROS). ROS including hydrogen peroxide and superoxide anion are potent inducers of various signaling pathways encompassing MAPKs and JAK-STAT pathways. In the current study, we scrutinized the effects of hydrogen peroxide and/or menadione (superoxide anion generator) on JNK/p38-MAPKs and JAK2-STAT3 pathways to elucidate the mechanism(s) by which each oxidant modulated the above-mentioned pathways leading to SK-N-MC cell death. Our results delineated that hydrogen peroxide and superoxide anion radical induced distinct responses as we showed that STAT3 and p38 were activated in response to hydrogen peroxide, but not superoxide anion radicals indicating the specificity in ROS-induced signaling pathways activations and behaviors. We also observed that menadione induced JNK-dependent p53 expression and apoptotic death in SK-N-MC cells while H2O2-induced JNK activation was p53 independent. Thus, we declare that ROS type has a key role in selective instigation of JNK/p38-MAPKs and JAK2-STAT3 pathways in SK-N-MC cells. Identifying these differential behaviors and mechanisms of hydrogen peroxide and superoxide anion functions illuminates the possible therapeutic targets in the prevention or treatment of ROS-induced neurodegenerative diseases such as Alzheimer's disease.

Endoplasmic reticulum stress induces fibrogenic activity in hepatic stellate cells through autophagy

BACKGROUND & AIMS

Metabolic stress during liver injury enhances autophagy and provokes stellate cell activation, with secretion of scar matrix. Conditions that augment protein synthesis increase demands on the endoplasmic reticulum (ER) folding capacity and trigger the unfolded protein response (UPR) to cope with resulting ER stress. Generation of reactive oxygen species (ROS) is a common feature of hepatic fibrogenesis, and crosstalk between oxidant stress and ER stress has been proposed. The aim of our study was to determine the impact of oxidant and ER stress on stellate cell activation.

METHODS

Oxidant stress was induced in hepatic stellate cells using H2O2 in culture or by ethanol feeding in vivo, and the UPR was analyzed. Because the branch of the UPR mainly affected was IREα, we blocked this pathway in stellate cells and analyzed the fibrogenic response, together with autophagy and downstream MAPK signaling. The Nrf2 antioxidant response was also evaluated in stellate cells under oxidant stress conditions.

RESULTS

H2O2 treatment in culture or ethanol feeding in vivo increased the UPR based on splicing of XBP1 mRNA, which triggered autophagy. The Nrf2-mediated antioxidant response, as measured by qRT-PCR of its target genes was also induced under ER stress conditions. Conversely, blockade of the IRE1α pathway in stellate cells significantly decreased both their activation and autophagic activity in a p38 MAPK-dependent manner, leading to a reduced fibrogenic response.

CONCLUSIONS

These data implicate mechanisms underlying protein folding quality control in regulating the fibrogenic response in hepatic stellate cells.

Role of the Nrf2-ARE pathway in liver diseases

The liver is a central organ that performs a wide range of functions such as detoxification and metabolic homeostasis. Since it is a metabolically active organ, liver is particularly susceptible to oxidative stress. It is well documented that liver diseases including hepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma are highly associated with antioxidant capacity. NF-E2-related factor-2 (Nrf2) is an essential transcription factor that regulates an array of detoxifying and antioxidant defense genes expression in the liver. It is activated in response to electrophiles and induces its target genes by binding to the antioxidant response element (ARE). Therefore, the roles of the Nrf2-ARE pathway in liver diseases have been extensively investigated. Studies from several animal models suggest that the Nrf2-ARE pathway collectively exhibits diverse biological functions against viral hepatitis, alcoholic and nonalcoholic liver disease, fibrosis, and cancer via target gene expression. In this review, we will discuss the role of the Nrf2-ARE pathway in liver pathophysiology and the potential application of Nrf2 as a therapeutic target to prevent and treat liver diseases.

Nuclear translocation of Nrf2 in hepatocytes of mice with hepatic fibrosis

AIM: To investigate the nuclear translocation of NF-E2-related factor 2 (Nrf2) in hepatocytes of mice with hepatic fibrosis.

METHODS: Mice were randomly divided into two groups: normal control group and model group. For the model group, carbon tetrachloride (CCl₄) dissolved in mineral oil was injected intraperitoneally for 10 weeks, while the normal control group was injected with the same volume of mineral oil. At the end of the 10th week, specimens were collected to assess the degrees of hepatic fibrosis and inflammation by haematoxylin-eosin staining and Masson staining. Western blot was used to detect the protein expression of Nrf2 and NAD(P)H quinine oxidoreductase 1 (Nqo1) and nuclear translocation of Nrf2.

RESULTS: Compared to the normal control group, the degrees of hepatic inflammation and fibrosis in the model group were significantly increased. Western blot analysis showed that, compared to the normal control group (0.490 ± 0.088, 0.430 ± 0.057, 0.370 ± 0.022), the expression levels of Nrf2 and Nqo1 proteins, as well as nuclear translocation of Nrf2 were increased significantly in the model group (0.490 ± 0.088 vs 0.790 ± 0.045, 0.430 ± 0.057 vs 0.720 ± 0.040, 0.370 ± 0.022 vs 0.670 ± 0.044; F = 2.027, 0.772, 1.552, all P < 0.05).

CONCLUSION: In mice with hepatic fibrosis, the nuclear translocation of Nrf2 in hepatocytes is increased to up-regulate its target protein Nqo1.

Superoxide anions and hydrogen peroxide inhibit proliferation of activated rat stellate cells and induce different modes of cell death

BACKGROUND

In chronic liver injury, hepatic stellate cells (HSCs) proliferate and produce excessive amounts of connective tissue causing liver fibrosis and cirrhosis. Oxidative stress has been implicated as a driving force of HSC activation and proliferation, although contradictory results have been described.

AIM

To determine the effects of oxidative stress on activated HSC proliferation, survival and signalling pathways.

METHODS

Serum-starved culture-activated rat HSCs were exposed to the superoxide anion donor menadione (5-25 micromol/L) or hydrogen peroxide (0.2-5 mmol/L). Haem oxygenase-1 mRNA expression, glutathione status, cell death, phosphorylation of mitogen-activated protein (MAP) kinases and proliferation were investigated.

RESULTS

Menadione induced apoptosis in a dose- and time-dependent, but caspase-independent manner. Hydrogen peroxide induced necrosis only at extremely high concentrations. Both menadione and hydrogen peroxide activated Jun N-terminal kinase (JNK) and p38. Hydrogen peroxide also activated extracellular signal-regulated protein. Menadione, but not hydrogen peroxide, reduced cellular glutathione levels. Inhibition of JNK or supplementation of glutathione reduced menadione-induced apoptosis. Non-toxic concentrations of menadione or hydrogen peroxide inhibited platelet-derived growth factor- or/and serum-induced proliferation.

CONCLUSION

Reactive oxygen species (ROS) inhibit HSC proliferation and promote HSC cell death in vitro. Different ROS induce different modes of cell death. Superoxide anion-induced HSC apoptosis is dependent on JNK activation and glutathione status.

Oxidative stress and antioxidants in the pathogenesis of pulmonary fibrosis: a potential role for Nrf2

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive disorder in which excessive deposition of extracellular matrix leads to irreversible scarring of interstitial lung tissue. The etiology of IPF remains unknown, but growing evidence suggests that disequilibrium in oxidant/antioxidant balance contributes significantly. IPF is currently regarded as a fibroproliferative disorder triggered by repeated alveolar epithelial cell injury. Oxidative stress plays a role in many processes involved in alveolar epithelial cell injury and fibrogenesis. Here we review the role of oxidative stress in IPF, and other forms of pulmonary fibrosis, with particular attention to antioxidant defenses regulated by the redox-sensitive transcription factor nuclear factor, erythroid derived 2, like (Nrf2). Nrf2 binds specific antioxidant response elements (AREs) in the promoter of antioxidant enzyme and defense protein genes and regulates their expression in many tissue types. Nrf2 protects from several phenotypes in which enhanced oxidative burden contributes to disease pathogenesis, including cancer, acute lung injury, and pulmonary fibrosis. We suggest that promoter polymorphisms in human NRF2 may contribute to IPF susceptibility, although this hypothesis has not been tested. Pulmonary fibrosis is a highly complex disease and involves multiple genes and processes, and new therapies for cellular and molecular targets involved in pathogenic mechanisms are needed.

Emerging role of Nrf2 in protecting against hepatic and gastrointestinal disease

Transcription factor NF-E2-related factor 2 (Nrf2) belongs to the basic region-leucine zipper family and is activated in response to electrophiles and reactive oxygen species. Nrf2 coordinately regulates the constitutive and inducible transcription of a wide array of genes involved in drug metabolism, detoxification, and antioxidant defenses. During periods of oxidative stress, Nrf2 is released from sequestration in the cytoplasm and translocates to the nucleus. Nrf2 binds antioxidant response elements (AREs) in the regulatory regions of target genes and activates transcription. Genetically modified mice lacking Nrf2 serve as a useful tool for identifying new ARE-regulated genes and assessing the ability of Nrf2 to confer protection against a variety of pathologies in numerous organs including the liver, intestine, lung, skin, and nervous system. With regards to the liver and gastrointestinal tract, Nrf2 knockout mice are more susceptible to acetaminophen-induced hepatocellular injury, benzo[a]pyrene-induced tumor formation and Fas- and TNFalpha -mediated hepatocellular apoptosis. The higher sensitivity of Nrf2 knockout mice to chemical toxicity is due in part to reduced basal and inducible expression of detoxification enzymes. Nrf2 may also be important in protecting against liver fibrosis, gallstone development, and formation of aberrant crypt foci. Research of Nrf2 has opened up new opportunities in understanding how antioxidant defense pathways are regulated, how oxidative stress contributes to disease progression and may serve as a novel target for designing therapies to prevent and treat diseases in which oxidative stress is implicated.

Bifunctional Alkylating Agent-Induced p53 and Nonclassical Nuclear Factor κB Responses and Cell Death Are Altered by Caffeic Acid Phenethyl Ester: A Potential Role for Antioxidant/Electrophilic Response-Element Signaling

Bifunctional alkylating agents (BFA) such as mechlorethamine (nitrogen mustard) and bis-(2-chloroethyl) sulfide (sulfur mustard; SM) covalently modify DNA and protein. The roles of nuclear factor kappaB (NF-kappaB) and p53, transcription factors involved in inflammatory and cell death signaling, were examined in normal human epidermal keratinocytes (NHEK) and immortalized HaCaT keratinocytes, a p53-mutated cell line, to delineate molecular mechanisms of action of BFA. NHEK and HaCaT cells exhibited classical NF-kappaB signaling as degradation of inhibitor protein of NF-kappaBalpha (IkappaBalpha) occurred within 5 min after exposure to tumor necrosis factor-alpha. However, exposure to BFA induced nonclassical NF-kappaB signaling as loss of IkappaBalpha was not observed until 2 or 6 h in NHEK or HaCaT cells, respectively. Exposure of an NF-kappaB reporter gene-expressing HaCaT cell line to 12.5, 50, or 100 muM SM activated the reporter gene within 9 h. Pretreatment with caffeic acid phenethyl ester (CAPE), a known inhibitor of NF-kappaB signaling, significantly decreased BFA-induced reporter gene activity. A 1.5-h pretreatment or 30-min postexposure treatment with CAPE prevented BFA-induced loss of membrane integrity by 24 h in HaCaT cells but not in NHEK. CAPE disrupted BFA-induced phosphorylation of p53 and p90 ribosomal S6 kinase (p90RSK) in both cell lines. CAPE also increased nuclear factor E2-related factor 2 and decreased aryl hydrocarbon receptor protein expression, both of which are involved in antioxidant/electrophilic response element (ARE/EpRE) signaling. Thus, disruption of p53/p90RSK-mediated NF-kappaB signaling and activation of ARE/EpRE pathways may be effective strategies to delineate mechanisms of action of BFA-induced inflammation and cell death signaling in immortalized versus normal skin systems.

Leptin represses matrix metalloproteinase-1 gene expression in LX2 human hepatic stellate cells

BACKGROUND/AIMS

Collagen accumulation in liver fibrosis is due in part to decreased expression of matrix metalloproteinase (MMP)-1 relative to TIMP-1. LX-2 hepatic stellate cells produce increased amounts of collagen and tissue inhibitor of metalloproteinase (TIMP)-1 in response to leptin. The effect of leptin on MMP-1 production has not been reported.

METHODS

LX-2 cells were treated with leptin with or without inhibitors. We determined: phosphorylation of Janus kinase (JAK) 1 and -2, signal transducer and activator of transcription (STAT)3 and -5, extracellular signal-regulated kinase (ERK)1/2 and p38 by Western blot; H2O2 concentration by a colorimetric method; MMP-1 mRNA levels and stability by Northern blot; MMP-1 promoter activity as well as pro-MMP-1 by ELISA; and active MMP-1 by fluorescence.

RESULTS

LX-2 cells constitutively expressed the MMP-1 gene and leptin repressed the basal level of MMP-1 mRNA and its promoter activity. The repression was mediated by JAK/STAT pathway in synergism with JAK-mediated H2O2-dependent ERK1/2 and p38 pathways. ERK1/2 inhibited MMP-1 promoter activity, whereas p38 decreased the message stability, contributing to mRNA down-regulation. Inhibition of MMP-1 gene diminished secreted pro-MMP-1 and active MMP-1.

CONCLUSIONS

Leptin represses MMP-1 gene expression via the synergistic actions of the JAK/STAT pathway and JAK-mediated H2O2-dependent ERK1/2 and p38 pathways.

DLPC and SAMe prevent alpha1(I) collagen mRNA up-regulation in human hepatic stellate cells, whether caused by leptin or menadione

We previously reported that the combination of dilinoleoylphosphatidylcholine (DLPC) and S-adenosylmethionine (SAMe), which have antioxidant properties and antifibrogenic actions, prevented leptin-stimulated tissue inhibitor of metalloproteinase (TIMP)-1 production in hepatic stellate cells (HSCs) by inhibiting H2O2-mediated signal transduction. We now show that DLPC and SAMe inhibit alpha1(I) collagen mRNA expression induced by leptin or menadione in LX-2 human HSCs. We found that DLPC and SAMe prevent H2O2 generation and restore reduced glutathione (GSH) depletion whether caused by leptin or menadione. Blocking H2O2 signaling through ERK1/2 and p38 pathways resulted in a complete inhibition of leptin or menadione-induced alpha1(I) collagen mRNA. The inhibition of collagen mRNA by DLPC and SAMe combined is at least two times more effective than that by DLPC or SAMe alone. In conjunction with the prevention of TIMP-1 production, the ability of DLPC and SAMe to inhibit alpha1(I) collagen mRNA expression provides a mechanistic basis for these innocuous compounds in the prevention of hepatic fibrosis, because enhanced TIMP-1 and collagen productions are associated with hepatic fibrogenesis and their attenuation may diminish fibrosis.

Human aldehyde dehydrogenase 3A1 inhibits proliferation and promotes survival of human corneal epithelial cells

Aldehyde dehydrogenase 3A1 (ALDH3A1) is a NAD(P)+-dependent enzyme that is highly expressed in mammalian corneal epithelial cells and has been shown to protect against UV- and 4-hydroxynonenal-induced cellular damage, mainly by metabolizing toxic lipid peroxidation aldehydes. Here we report a novel function of ALDH3A1 as a negative cell cycle regulator. We noticed a reduction in ALDH3A1 gene expression in actively proliferating primary human corneal epithelium explant cultures, indicating that ALDH3A1 expression is inversely correlated with replication. To examine this further, a human corneal epithelial cell line (HCE) lacking endogenous ALDH3A1 was stably transfected to express ALDH3A1 at levels similar to those found in vivo. ALDH3A1-transfected cells exhibited an elongated cell cycle, decreased plating efficiency, and reduced DNA synthesis compared with the mock-transfected cells. These effects were associated with reduced cyclin A- and cyclin B-dependent kinase activities and reduced phosphorylation of the retinoblastoma protein (pRb) as well as decreased protein levels of cyclins A, B, and E, the transcription factor E2F1, and the cyclin-dependent kinase inhibitor p21. These observations were further expanded and confirmed on human keratinocyte cells (NCTC-2544) overexpressing ALDH3A1. Consistent with a protective role of an elongated cell cycle, ALDH3A1-transfected cells exhibited increased resistance to the cytotoxic effects of the DNA-damaging agents mitomycin C and Vp-16. Immunohistochemistry and biochemical fractionation revealed that ALDH3A1 is localized both in the nucleus and cytosol of ALDH3A1-transfected cells, implying a possible association between the nuclear localization of the enzyme and its proliferation-suppressive functions. In conclusion, these results suggest that ALDH3A1 may protect corneal epithelial cells against oxidative damage not only through its metabolic function but also by prolonging the cell cycle.