TGFbeta1-induced suppression of glutathione antioxidant defenses in hepatocytes: caspase-dependent post-translational and caspase-independent transcriptional regulatory mechanisms

Glutathione depletion and disruption of intracellular ionic homeostasis regulate lymphoid cell apoptosis

Intracellular glutathione (GSH) depletion is an important hallmark of apoptosis. We have recently shown that GSH depletion by its extrusion regulates apoptosis independently of excessive reactive oxygen species accumulation. However, the mechanisms by which GSH depletion regulates apoptosis are still unclear. Because disruption of intracellular ionic homeostasis, associated with apoptotic volume decrease (AVD), is necessary for the progression of apoptotic cell death, we sought to evaluate the relationship between GSH transport and ionic homeostasis during Fas ligand (FasL)-induced apoptosis in Jurkat cells. GSH depletion in response to FasL was paralleled by distinct degrees of AVD identified by differences in cellular forward scatter and electronic impedance analysis. Inhibition of GSH efflux prevented AVD, K+ loss, and the activation of two distinct ionic conductances, mediated by Kv1.3 and outward rectifying Cl- channels. Reciprocally, stimulation of GSH loss accelerated the loss of K+, AVD, and consequently the progression of the execution phase of apoptosis. Although high extracellular K+ inhibited FasL-induced apoptosis, GSH depletion was largely independent of K+ loss. These results suggest that deregulation of GSH and ionic homeostasis converge in the regulation of apoptosis in lymphoid cells.

Upregulation of the NADPH oxidase NOX4 by TGF-beta in hepatocytes is required for its pro-apoptotic activity

BACKGROUND/AIMS

The transforming growth factor-beta (TGF-beta) induces apoptosis in hepatocytes through an oxidative stress process. Here, we have analyzed the role of different NADPH oxidase isoforms in the intracellular signalling induced by TGF-beta in hepatocytes, to later explore whether this mechanism is altered in liver tumor cells.

METHODS

Primary cultures of rat and human hepatocytes, HepG2 and Hep3B cells were used in in vitro studies to analyze the TGF-beta response.

RESULTS

TGF-beta-induced apoptosis in rat hepatocytes does not require Rac-dependent NADPH oxidases. TGF-beta upregulates the Rac-independent Nox4, which correlates with its pro-apoptotic activity. Regulation of Nox4 occurs at the transcriptional level and is counteracted by intracellular survival signals. siRNA targeted knock-down of Nox4 attenuates NADPH oxidase activity, caspase activation and cell death in rat hepatocytes. NOX4 upregulation by TGF-beta is also observed in human hepatocytes, coincident with apoptosis. In human hepatocellular carcinoma (HCC) cell lines, NOX4 upregulation by TGF-beta is only observed in cells that are sensitive to its cytotoxic effect, such as Hep3B cells. siRNA targeted knock-down of NOX4 in these cells impairs TGF-beta-induced apoptosis.

CONCLUSIONS

Upregulation of NOX4 by TGF-beta is required for its pro-apoptotic activity in hepatocytes. Impairment of this TGF-beta-induced response might confer apoptosis resistance in HCC cells.

Nutritional lipid-induced oxidative stress leads to mitochondrial dysfunction followed by necrotic death in FaO hepatocytes

OBJECTIVE

Mitochondrial dysfunction and hepatocyte cell death have been reported in fatty liver and non-alcoholic steatohepatitis. Our aim in this study was to evaluate whether direct exposure of hepatocytes to extracellular fat could facilitate such deleterious effects.

METHODS

FaO hepatic cells treated with fat was used as an in vitro model for steatosis. FaO hepatocytes were exposed to 0.1% triacylglycerols using commercially available lipid emulsion (LE) for various periods and studied for production of reactive oxygen species (ROS), mitochondrial function, and cell death parameters. To study the type of cell death, high-mobility group box chromosomal protein 1cellular levels, DNA fragmentation, and caspase activity were evaluated.

RESULTS

Cells incubated with LE for 6 h exhibited a marked increase in the production of intracellular ROS. Using treatments with peroxisome proliferator-activated receptor activators, mitochondrial electron-transfer chain inhibitor, and different sources of LE that did or did not contain medium-chain triacylglycerols, the mitochondria were found to be the source of ROS. LE treatment resulted in phosphorylation of adenosine monophosphate-activated protein kinase, accompanied by a decrease in adenosine triphosphate levels. Changes in intracellular ROS and energy levels were followed by cell death. FaO hepatocytes showed a significant reduction in high-mobility group box chromosomal protein-1 and little DNA fragmentation. Incubation with LE for 24 h did not change caspase-3 activity, indicating that hepatocyte death was necrotic. The antioxidant N-acetylcysteine was able to attenuate the changes in intracellular energy levels and ROS levels and to prevent cell death after exposure to LE.

CONCLUSION

These results suggest that exposure of FaO cells to LE leads to an increase in mitochondrial ROS production and a decrease in cellular energy levels followed by necrotic cell death.

The inhibition of the epidermal growth factor (EGF) pathway enhances TGF-beta-induced apoptosis in rat hepatoma cells through inducing oxidative stress coincident with a change in the expression pattern of the NADPH oxidases (NOX) isoforms

Transforming growth factor-beta (TGF-beta) induces apoptosis in hepatocytes, through a mechanism mediated by reactive oxygen species (ROS) production. Numerous tumoral cells develop mechanisms to escape from the TGF-beta-induced tumor suppressor effects. In this work we show that in FaO rat hepatoma cells inhibition of the epidermal growth factor receptor (EGFR) with the tyrphostin AG1478 enhances TGF-beta-induced cell death, coincident with an elevated increase in ROS production and GSH depletion. These events correlate with down-regulation of genes involved in the maintenance of redox homeostasis, such as gamma-GCS and MnSOD, and elevated mitochondrial ROS. Nonetheless, not all the ROS proceed from the mitochondria. Emerging evidences indicate that ROS production by TGF-beta is also mediated by the NADPH oxidase (NOX) system. TGF-beta-treated FaO cells induce nox1 expression. However, the treatment with TGF-beta and AG1478 greatly enhanced the expression of another family member: nox4. NOX1 and NOX4 targeted knock-down by siRNA experiments suggest that they play opposite roles, because NOX1 knockdown increases caspase-3 activity and cell death, whilst NOX4 knock-down attenuates the apoptotic process. This attenuation correlates with maintenance of GSH and antioxidant enzymes levels. In summary, EGFR inhibition enhances apoptosis induced by TGF-beta in FaO rat hepatoma cells through an increased oxidative stress coincident with a change in the expression pattern of NOX enzymes.

Concurrent administration of ascorbic acid enhances liver tumor-promoting activity of kojic acid in rats

We previously found that administration of ascorbic acid (AA) enhances the liver tumor-promoting activity of kojic acid (KA) in mice. To examine the reproducibility of these results in rats and the underlying mechanism of this effect, we employed a two-stage liver carcinogenesis model using male F344 rats. Two weeks after initiation with diethylnitrosamine (DEN), the animals received a diet containing 2% KA and drinking water with or without 5,000 ppm AA for a period of 7 weeks. A DEN-alone group was also established as a control. One week after the commencement of the administration, the animals were subjected to two-thirds partial hepatectomy. At the end of the experiment, the livers were analyzed immunohistochemically, and the mRNA expression level and extent of lipid peroxidation were measured. AA treatment enhanced the KA-induced tumor-promoting activity in terms of the number and area of liver cell foci that were positive for glutathione-S-transferase placental form. AA coadministration increased the number of hepatocytes positive for proliferating cell nuclear antigen and inversely decreased the number of TUNEL-positive cells. However, the increased level of thiobarbituric acid reactive substances resulting from KA treatment was suppressed by coadministration of AA. Gene expression analyses using low-density microarrays and real-time RT-PCR showed that coadministration of AA resulted in upregulation of genes related to cell proliferation and downregulation of those involved in apoptosis and/or cell cycle arrest. These results indicate that the concerted effects of AA on cell proliferation and apoptosis/cell cycle arrest probably through its antioxidant activity are involved in this enhancement.

Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase

Glutathione (GSH) is a tripeptide composed of glutamate, cysteine, and glycine. The first and rate-limiting step in GSH synthesis is catalyzed by glutamate cysteine ligase (GCL, previously known as gamma-glutamylcysteine synthetase). GCL is a heterodimeric protein composed of catalytic (GCLC) and modifier (GCLM) subunits that are expressed from different genes. GCLC catalyzes a unique gamma-carboxyl linkage from glutamate to cysteine and requires ATP and Mg(++) as cofactors in this reaction. GCLM increases the V(max) and K(cat) of GCLC, decreases the K(m) for glutamate and ATP, and increases the K(i) for GSH-mediated feedback inhibition of GCL. While post-translational modifications of GCLC (e.g. phosphorylation, myristoylation, caspase-mediated cleavage) have modest effects on GCL activity, oxidative stress dramatically affects GCL holoenzyme formation and activity. Pyridine nucleotides can also modulate GCL activity in some species. Variability in GCL expression is associated with several disease phenotypes and transgenic mouse and rat models promise to be highly useful for investigating the relationships between GCL activity, GSH synthesis, and disease in humans.

Regulation of glutathione synthesis

Glutathione (GSH) is a ubiquitous intracellular peptide with diverse functions that include detoxification, antioxidant defense, maintenance of thiol status, and modulation of cell proliferation. GSH is synthesized in the cytosol of all mammalian cells in a tightly regulated manner. The major determinants of GSH synthesis are the availability of cysteine, the sulfur amino acid precursor, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL). GCL is composed for a catalytic (GCLC) and modifier (GCLM) subunit and they are regulated at multiple levels and at times differentially. The second enzyme of GSH synthesis, GSH synthase (GS) is also regulated in a coordinated manner as GCL subunits and its up-regulation can further enhance the capacity of the cell to synthesize GSH. Oxidative stress is well known to induce the expression of GSH synthetic enzymes. Key transcription factors identified thus far include Nrf2/Nrf1 via the antioxidant response element (ARE), activator protein-1 (AP-1) and nuclear factor kappa B (NFkappaB). Dysregulation of GSH synthesis is increasingly being recognized as contributing to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary fibrosis, cholestatic liver injury, endotoxemia and drug-resistant tumor cells. Manipulation of the GSH synthetic capacity is an important target in the treatment of many of these disorders.

Oxidative stress, plasminogen activator inhibitor 1, and lung fibrosis

Fibrosis is characterized by excessive accumulation of extracellular matrix (ECM) in basement membranes and interstitial tissues, resulting from increased synthesis or decreased degradation of ECM or both. The plasminogen activator/plasmin system plays an important role in ECM degradation, whereas the plasminogen activator inhibitor 1 (PAI-1) is a physiologic inhibitor of plasminogen activators. PAI-1 expression is increased in the lung fibrotic diseases and in experimental fibrosis models. The deletion of the PAI-1 gene reduces, whereas the overexpression of PAI-1 enhances, the susceptibility of animals to lung fibrosis induced by different stimuli, indicating an important role of PAI-1 in the development of lung fibrosis. Many growth factors, including transforming growth factor beta (TGF-beta) and tumor necrosis factor alpha (TNF-alpha), as well as other chemicals/agents, induce PAI-1 expression in cultured cells and in vivo. Reactive oxygen and nitrogen species (ROS/RNS) have been shown to mediate the induction of PAI-1 by many of these stimuli. This review summarizes some recent findings that help us to understand the role of PAI-1 in the development of lung fibrosis and ROS/RNS in the regulation of PAI-1 expression during fibrogenesis.

Epigallocatechin-3-gallate inhibits growth of activated hepatic stellate cells by enhancing the capacity of glutathione synthesis

Activation of hepatic stellate cells (HSC), the key effectors in hepatic fibrogenesis, is characterized by enhanced cell proliferation and overproduction of extracellular matrix. Oxidative stress promotes HSC activation. Glutathione (GSH) is the most important intracellular antioxidant, whose synthesis is mainly regulated by glutamate-cysteine ligase (GCL). We reported previously that (-)-epigallocatechin-3-gallate (EGCG), the major and most active component in green tea extracts, inhibited HSC activation. The aim of this study is to elucidate the underlying mechanisms. We hypothesize that this inhibitory effect of EGCG might mainly result from its antioxidant capability by increasing de novo synthesis of GSH. In this report, we observe that EGCG enhances the levels of cytoplasmic and mitochondrial GSH and increases GCL activity by inducing gene expression of the catalytic subunit GCLc, leading to de novo synthesis of GSH. Real-time polymerase chain reaction and Western blotting analyses show that de novo synthesis of GSH is required for EGCG to regulate the expression of genes relevant to apoptosis and to cell proliferation. Additional experiments demonstrate that exogenous transforming growth factor (TGF)-beta1 suppresses GCLc gene expression and reduces the level of GSH in cultured HSC. Transient transfection assays and Western blotting analyses further display that EGCG interrupts TGF-beta signaling by reducing gene expression of TGF-beta receptors and Smad4, leading to increased expression of GCLc. These results support our hypothesis and collectively demonstrate that EGCG increases the level of cellular GSH in HSC by stimulating gene expression of GCLc, leading to the inhibition of cell proliferation of activated HSC in vitro.

Effect of ethanol on pro-apoptotic mechanisms in polarized hepatic cells

Chronic ethanol consumption is associated with serious and potentially fatal alcohol-related liver injuries such as hepatomegaly, alcoholic hepatitis and cirrhosis. Moreover, it has been documented that the clinical progression of alcohol-induced liver damage may be associated with an increase in hepatocellular death that involves apoptotic mechanisms. Although much information has been learned about the clinical manifestations associated with alcohol-related diseases, the search continues for a better understanding of the molecular and/or cellular mechanisms by which ethanol exerts its deleterious effects such as the induction of pro-apoptotic mechanisms and related cell damaging events. As part of the effort to enhance our understanding of those particular cellular pathways and mechanisms associated with ethanol toxicity, researchers over the years have utilized a variety of model systems. Recently, work has come forth demonstrating the utility of a hybrid cell line (WIF-B) as a cell culture model system for the study of alcohol-associated alterations in hepatocellular mechanisms. Success with such emerging model systems could aid in the development of potential therapeutic treatments for the prevention of alcohol-induced apoptotic cell death that may ultimately serve as a significant target in delaying the onset and/or progression of clinical symptoms of alcohol-mediated liver disease. This review article summarizes the current understanding of ethanol-mediated modifications in cell survival and thus the promotion of pro-apoptotic events with emphasis on analyses made in various experimental model systems, particularly the more recently characterized WIF-B cell system.

Activation of the Nrf2 pathway, but decreased gamma-glutamylcysteine synthetase heavy subunit chain levels and caspase-3-dependent apoptosis during exposure of primary mouse hepatocytes to diphenylarsinic acid

Diphenylarsinic acid (DPAsV) is a degradation product of chemical warfare agents, over which there has been a public outcry in the Kamisu Area of Ibaraki Prefecture in Japan. In this study, we investigated the cytotoxicity of and cellular response to DPAsV in primary mouse hepatocytes. Exposure of the hepatocytes to DPAsV resulted in cell damage accompanied by cellular accumulation of DPAsV in a time-dependent manner. The cell death caused by DPAsV was attributable to apoptosis. DPAsV activated a basic leucine-zipper transcription factor Nrf2 as determined by the nuclear translocation of Nrf2, anti-oxidant response element (ARE)-dependent luciferase activity, and upregulation of downstream gene products. However, gamma-glutamylcysteine synthetase heavy subunit chain (gamma-GCS(H)), which is regulated by Nrf2, underwent cleavage by activated caspase-3 to a 17 kDa fragment, leading to a minimal level of constitutive gamma-GCS(H) expression 72 h following the exposure (25 microM). Experiments with cycloheximide revealed that the DPAsV-mediated reduction in gamma-GCS(H) was due to a post-translational modification. The results suggest that DPAsV causes caspase-3-dependent cleavage of gamma-GCS(H) regardless of Nrf2 activation in primary mouse hepatocytes.

Cigarette smoke extract-induced suppression of caspase-3-like activity impairs human neutrophil phagocytosis

Neutrophils are the primary inflammatory cell in smokers’ lungs, but little is known about the ability of cigarette smoke to modulate neutrophil function. Neutrophils undergo caspase-3-dependent spontaneous, as well as phagocytosis-induced, apoptosis. This study investigated the ability of cigarette smoke extract (CSE) to alter neutrophil caspase-3 activity, apoptosis, and phagocytosis. CSE treatment resulted in a dramatic suppression of neutrophil caspase-3-like activity, which correlated with reduced cleavage of glutamate-l-cysteine ligase catalytic subunit, a known target of active caspase-3. CSE did not affect procaspase-3 processing to its active fragment, suggesting a direct effect of CSE on active caspase-3. Consistent with this, CSE inhibited active recombinant caspase-3 activity, which was abolished by dithiothreitol, suggesting a redox-sensitive mechanism. CSE-induced suppression of caspase-3 activity did not alter spontaneous apoptosis but did impair phagocytic activity. Since CSE treatment resulted in profound suppression of caspase-3 activity but did not alter apoptosis, the possibility of a threshold level of caspase-3 activity was investigated. CSE reduced caspase-3 activity in a concentration-dependent manner. Despite near complete suppression of caspase-3 activity, spontaneous apoptosis was not altered. Conversely, treatment with the pan-caspase inhibitor, Z-Val-Ala-Asp-fluoromethylketone, reduced spontaneous apoptosis. These data demonstrate that CSE does not suppress caspase-3 activity below a threshold level to prevent spontaneous apoptosis, but the level of inhibition is sufficient to impair neutrophil phagocytic activity. These divergent functions of caspase-3 may contribute to the persistence of neutrophils in the lungs of smokers, as well as be a factor in their higher incidence of community-acquired pneumonia.

Involvement of reactive oxygen species on the apoptotic mechanism induced by IFN-alpha2b in rat preneoplastic liver

Interferon-alpha2b (IFN-alpha2b) is an important component in the preventive treatment of patients who have severe hepatic illness such as hepatitis B or C and hepatocarcinomas. In a previous work, using a rat liver preneoplastic model, we have demonstrated that IFN-alpha2b reduces the number and volume of altered hepatic foci (AHF) inducing apoptosis through a mechanism mediated by TGF-beta(1). In this study, the implication of hepatocytes redox status of IFN-alpha2b-treated preneoplastic liver in the TGF-beta(1)-induced apoptotic death was analyzed. Results indicate that IFN-alpha2b induces hepatocytic TGF-beta(1) production and secretion by induction of reactive oxygen species (ROS) formation through the activation of a membrane bound NADPH oxidase complex. TGF-beta(1), in turn, reduces hepatocytes antioxidant defenses and induces programmed cell death. On the other hand, it was also demonstrated that treatment of rats with IFN-alpha2b plus a ROS scavenger such as ascorbic acid, abolishes the apoptotic effect of IFN-alpha2b in rat preneoplastic livers, leading to an increase of the foci volume. In conclusion, these findings strongly suggest that ROS have a fundamental role as signaling and/or regulator molecules in the IFN-alpha2b-induced apoptosis in hepatic preneoplastic cells.

Basal reactive oxygen species determine the susceptibility to apoptosis in cirrhotic hepatocytes

Hepatocytes from cirrhotic murine livers exhibit increased basal ROS activity and resistance to TGFbeta-induced apoptosis, yet when ROS levels are decreased by antioxidant pretreatment, these cells recover susceptibility to apoptotic stimuli. To further study these redox events, hepatocytes from cirrhotic murine livers were pretreated with various antioxidants prior to TGFbeta treatment and the ROS activity, apoptotic response, and mitochondrial ROS generation were assessed. In addition, normal hepatocytes were treated with low-dose H(2)O(2) and ROS and apoptotic responses determined. Treatment of cirrhotic hepatocytes with various antioxidants decreased basal ROS and rendered them susceptible to apoptosis. Examination of normal hepatocytes by confocal microscopy demonstrated colocalization of ROS activity and respiring mitochondria. Basal assessment of cirrhotic hepatocytes showed nonfocal ROS activity that was abolished by antioxidants. After pretreatment with an adenovirus expressing MnSOD, basal cirrhotic hepatocyte ROS were decreased and TGFbeta-induced colocalization of ROS and mitochondrial respiration was present. Treatment of normal hepatocytes with H(2)O(2) resulted in a sustained increase in ROS and resistance to TGFbeta apoptosis that was reversed when these cells were pretreated with an antioxidant. In conclusion, cirrhotic hepatocytes have a nonfocal distribution of ROS. However, normal and cirrhotic hepatocytes exhibit mitochondrial localization of ROS that is necessary for apoptosis.

Transforming growth factor-beta 1 stimulates angiotensinogen gene expression in kidney proximal tubular cells

The present study investigated whether transforming growth factor-beta 1 (TGF-beta1) exerts an autocrine positive effect on angiotensinogen (ANG) gene expression in rat kidney proximal tubular cells, and delineates its underlying mechanism(s) of action. Rat immortalized renal proximal tubular cells (IRPTCs) and freshly isolated mouse renal proximal tubules were incubated in the absence or presence of active human TGF-beta1. IRPTCs were also stably transfected with rat TGF-beta1 or p53 tumor suppressor protein (p53) cDNA in sense (S) and antisense (AS) orientations. ANG mRNA and p53 protein expression were assessed by reverse transcription-polymerase chain reaction and Western blotting, respectively. Reactive oxygen species (ROS) generation was quantified by lucigenin assay. Active TGF-beta1 evoked ROS generation and stimulated ANG mRNA and p53 protein expression, whereas a superoxide scavenger and inhibitors of nicotinamide adenine dinucleotide oxidase and p38 mitogen-activated protein kinase (p38 MAPK) abolished the TGF-beta1 effect. Stable transfer of p53 cDNA (S) enhanced and p53 cDNA (AS) abolished the stimulatory effect of TGF-beta1 on ANG mRNA expression in IRPTCs. Our results demonstrate that TGF-beta1 stimulates ANG gene expression and its action is mediated, at least in part, via ROS generation, p38 MAPK activation, and p53 expression, suggesting that angiotensin II and TGF-beta1 may form a positive feedback loop to enhance their respective gene expression, leading to renal injury.

Increased toxicity by transforming growth factor-beta 1 in liver cells overexpressing CYP2E1

Ethanol treatment causes an increase in expression of TGF-beta1 and CYP2E1 in the centrilobular area. Alcoholic liver disease is usually initiated in the centrilobular region of the liver. We hypothesized that the combination of TGF-beta1 and CYP2E1 produces increased oxidative stress and liver cell toxicity. To test this possibility, we studied the effects of TGF-beta1 on the viability of HepG2 E47 cells that express human CYP2E1, and C34 HepG2 cells, which do not express CYP2E1. E47 cells underwent greater growth inhibition and enhanced apoptosis after TGF-beta1 treatment, as compared to the C34 cells. There was an enhanced production of reactive oxygen species (ROS) and a decline in reduced glutathione (GSH) levels in the TGF-beta1-treated E47 cells and the enhanced cell death could be prevented by antioxidants. The CYP2E1 inhibitor diallyl sulfide prevented the potentiated cell death in E47 cells validating the role of CYP2E1. Mitochondrial membrane potential declined in the TGF-beta1-treated E47 cells, prior to developing toxicity, and cell death could be prevented by trifluoperazine, an inhibitor of the mitochondrial membrane permeability transition. TGF-beta1 also produced a loss of cell viability in hepatocytes from pyrazole-treated rats with elevated levels of CYP2E1, compared to control hepatocytes. In conclusion, increased toxic interactions by TGF-beta1 plus CYP2E1 can occur by a mechanism involving increased production of intracellular ROS and depletion of GSH, resulting in mitochondrial membrane damage and loss of membrane potential, followed by apoptosis. Potentiation of TGF-beta1-induced cell death by CYP2E1 may contribute to mechanisms of alcohol-induced liver disease.

Acetaminophen-induced liver injury is attenuated in male glutamate-cysteine ligase transgenic mice

Acetaminophen overdose is a leading cause of drug-related acute liver failure in the United States. Glutathione, a tripeptide antioxidant protects cells against oxidative damage from reactive oxygen species and plays a crucial role in the detoxification of xenobiotics, including acetaminophen. Glutathione is synthesized in a two-step enzymatic reaction. Glutamate-cysteine ligase carries out the rate-limiting and first step in glutathione synthesis. We have generated C57Bl/6 mice that conditionally overexpress glutamate-cysteine ligase, and report here their resistance to acetaminophen-induced liver injury. Indices of liver injury included histopathology and serum alanine aminotransferase activity. Male transgenic mice induced to overexpress glutamate-cysteine ligase exhibited resistance to acetaminophen-induced liver injury when compared with acetaminophen-treated male mice carrying, but not expressing glutamate-cysteine ligase transgenes, or to female glutamate-cysteine ligase transgenic mice. We conclude that glutamate-cysteine ligase activity is an important factor in determining acetaminophen-induced liver injury in C57Bl/6 male mice. Because people are known to vary in their glutamate-cysteine ligase activity, this enzyme may also be an important determinant of sensitivity to acetaminophen-induced liver injury in humans.

EGF blocks NADPH oxidase activation by TGF-beta in fetal rat hepatocytes, impairing oxidative stress, and cell death

Epidermal growth factor (EGF) is a survival signal for transforming growth factor-beta (TGF-beta)-induced apoptosis in hepatocytes, phosphatidylinositol 3-kinase (PI 3-K) being involved in this effect. Here, we analyze the possible cross talks between EGF and TGF-beta signals to understand how EGF impairs the early pro-apoptotic events induced by TGF-beta. Data have indicated that neither SMAD nor c-Jun NH2 Terminal Kinase (JNK) activations are altered by EGF, which clearly interferes with events directly related to the radical oxygen species (ROS) production, impairing oxidative stress, p38 MAP kinase activation, and cell death. Activation of a NADPH-oxidase-like system, which is responsible for the early ROS production by TGF-beta, is completely inhibited by EGF, through a PI 3-K-dependent mechanism. Activity of RAC1 increases by TGF-beta, but also by EGF, and both act synergistically to get maximum effects. Fetal rat hepatocytes express nox4, in addition to nox1 and nox2, and TGF-beta clearly upregulates nox4. EGF blocks up-regulation of nox4 by TGF-beta. Interestingly, in the presence of PI 3-K inhibitors, EGF is not able to counteract the nox4 upregulation by TGF-beta. Taking together these results indicate that impairment of TGF-beta-induced NADPH oxidase activation by EGF is a RAC1-independent process and correlates with an inhibition of the mechanisms that address the increase of nox4 mRNA levels by TGF-beta.

Resistance of Leishmania donovani to sodium stibogluconate is related to the expression of host and parasite gamma-glutamylcysteine synthetase

Sequencing studies showed that the gamma-glutamylcysteine synthetase (gamma-GCS) heavy chain genes from sodium stibogluconate (SSG)-resistant (SSG-R) and SSG-susceptible (SSG-S) Leishmania donovani strains were identical, indicating that SSG resistance was related to quantitative differences in gamma-GCS expression rather than gene interstrain polymorphisms. In vitro infection of murine macrophages with the SSG-R strain, but not the SSG-S strain, down regulated expression of host gamma-GCS, which would result in a reduction in intramacrophage glutathione (GSH) levels and promote an oxidative intramacrophage environment. This would inhibit, or minimize, the reduction of SSG pentavalent antimony to its more toxic trivalent form. Macrophage studies showed that the SSG-R strain expressed higher levels of gamma-GCS compared to the SSG-S strain, which would result in higher GSH levels, giving increased protection against oxidative stress and facilitating SSG efflux. However a similar differential effect on host and parasite gamma-GCS expression was not obtained when using tissues from infected mice. In this case gamma-GCS expression was organ and strain dependent for both the host and the parasite, indicating that environmental conditions have a profound effect on gamma-GCS expression. Consistent with the proposed mechanism from in vitro studies, increasing tissue GSH levels in the presence of SSG by cotreatment of L. donovani-infected mice with SSG solution and GSH incorporated into nonionic surfactant vesicles was more effective in reducing liver, spleen, and bone marrow parasite burdens than monotherapy with SSG. Together, these results indicate that SSG resistance is associated with manipulation of both host and parasite GSH levels by L. donovani.

Focus on antioxidant enzymes and antioxidant strategies in smoking related airway diseases

Cigarette smoke causes significant oxidant stress which is further enhanced by recruitment and activation of inflammatory cells to the lung. Polymorphisms in some detoxification enzymes are thought to increase the risk of developing chronic obstructive pulmonary disease (COPD), but the ultimate role of genetic variability in antioxidant and/or detoxification enzymes in COPD remains obscure. Some antioxidant enzymes are inducted, but the extent of induction is insufficient to protect the lung/alveolar epithelium against cigarette smoke. Exogenous antioxidants such as vitamins do not seem to protect against cigarette smoke related lung injury. Glutathione related synthetic drugs such as N-acetylcysteine have shown some benefits, but they may have pro-oxidant side effects. Synthetic compounds with superoxide dismutase and catalase activities have shown promising results in animal models against a variety of oxidant exposures including cigarette smoke in the lung. These results are in agreement with studies highlighting the importance of alveolar antioxidant protection mechanisms in oxidant stress and their inducibility. These new drugs need to be tested in cigarette smoking related lung injury/inflammation since inflammation/oxidant stress can continue after discontinuation of smoking.

Hepatic progenitor cell resistance to TGF-beta1's proliferative and apoptotic effects

The success of hepatocellular therapies using stem or progenitor cell populations is dependent upon multiple factors including the donor cell, microenvironment, and etiology of the liver injury. The following experiments investigated the impact of TGF-beta1 on a previously described population of hepatic progenitor cells (HPC). The majority of the hepatic progenitor cells were resistant to endogenously produced TGF-beta1's proapoptotic and anti-proliferative effects unlike more well-differentiated cellular populations (e.g., mature hepatocytes). Surprisingly, in vitro TGF-beta1 supplementation significantly inhibited de novo hepatic progenitor cell colony formation possibly via an indirect mechanism(s). Therefore despite the HPC's direct resistance to supplemental TGF-beta1, this cytokine's inhibitory effect on colony formation could have a potential negative impact on the use of these cells as a therapy for patients with liver disease.

Differential regulation of rodent hepatocyte and oval cell proliferation by interferon gamma

Hepatocytes and intrahepatic progenitor cells (oval cells) have similar responses to most growth factors but rarely proliferate together. Oval cells constitute a reserve compartment that is activated when hepatocyte proliferation is inhibited. Interferon gamma (IFN-gamma) increases in liver injury that involves oval cell responses, but it is not upregulated during liver regeneration after partial hepatectomy. Based on these observations, we used well-characterized lines of hepatocytes (AML-12 cells) and oval cells (LE-6 cells) to investigate the potential mechanisms that regulate differential growth responses in hepatocytes and oval cells. We show that IFN-gamma blocks hepatocyte proliferation in vivo, and that in combination with either tumor necrosis factor (TNF) or lipopolysaccharide (LPS), it causes cell cycle arrest in hepatocytes but stimulates oval cell proliferation in cultured cells. The hepatocyte cell cycle arrest is reversible, is p53-independent, and is not associated with apoptosis. Treatment of AML-12 hepatocytes with IFN-gamma/LPS or IFN-gamma/TNF, but not with individual cytokines, induced NO synthase and generated NO, while similarly treated oval cells produced little if any NO. Generation of NO by an NO donor reproduced the inhibitory effect of the cytokine combinations on AML-12 cell replication, while NO inhibitors abolish the replication deficiency. In conclusion, we propose that IFN-gamma, in conjunction with TNF or LPS, can both inhibit hepatocyte proliferation through the generation of NO and stimulate oval cell replication. The response of hepatocytes and oval cells to cytokine combinations may contribute to the differential proliferation of these cells in hepatic growth processes.

Smad3-ATF3 signaling mediates TGF-beta suppression of genes encoding Phase II detoxifying proteins

This study provides evidence that in mammary epithelial cells the pluripotent cytokine TGF-beta1 repressed expression of multiple genes involved in Phase II detoxification. GCLC, the gene that encodes the catalytic subunit of the enzyme glutamate cysteine ligase, the rate-limiting enzyme in the biosynthesis of glutathione, was used as a molecular surrogate for investigating the mechanisms by which TGF-beta suppressed Phase II gene expression. TGF-beta was found to suppress luciferase reporter activity mediated by the human GCLC proximal promoter, as well as reporter activity mediated by the GCLC antioxidant response element, ARE4. TGF-beta downregulated expression of endogenous GCLC mRNA and GCLC protein. TGF-beta suppression of the Phase II genes correlated with a decrease in cellular glutathione and an increase in cellular reactive oxygen species. Ectopic expression of constitutively active Smad3E was sufficient to inhibit both reporters in the absence of TGF-beta, whereas dominant negative Smad3A blocked TGF-beta suppression. Smad3E suppressed Nrf2-mediated activation of the GCLC reporter. We demonstrate that TGF-beta increased ATF3 protein levels, as did transient overexpression of Smad3E. Ectopic expression of ATF3 was sufficient to suppress the GCLC reporter activity, as well as endogenous GCLC expression. These results demonstrate that Smad3-ATF3 signaling mediates TGF-beta repression of ARE-dependent Phase II gene expression and potentially provide critical insight into mechanisms underlying TGF-beta1 function in carcinogenesis, tissue repair, and fibrosis.

4-hydroxynonenal and TGF-beta1 concur in inducing antiproliferative effects on the CaCo-2 human colon adenocarcinoma cell line

4-Hydroxynonenal (HNE) has been demonstrated to exert its antiproliferative effect by up-regulating the c-Jun-N-terminal kinase (JNK), a member of the mitogen-activated protein kinase family (MAPKs). Transforming growth factor-beta1 (TGF-beta1) is the major negative regulatory factor in controlling cell proliferation, and Smads are its intracellular transducers. Recent data on human colon adenocarcinoma has shown a low HNE content paralleled by a marked alteration of TGF-beta1 levels within the tumor mass. The two events appear related because of the demonstrated marked ability of HNE to up-regulate expression and synthesis of TGF-beta1; the combined decreases of HNE and TGF-beta1 found in cancer cells provide a favorable condition for neoplastic progression. Furthermore, HNE is likely able to interact with the cytokine to enhance apoptosis and increase intracellular reactive oxygen species (ROS) formation in the CaCo-2 colon carcinoma cell line. The probable mechanism whereby HNE and TGF-beta1 interact to induce apoptosis is through cross-talk between the main signaling pathways of the two molecules (JNK and Smads), and the observed ROS production might only contribute to amplifying the apoptotic pathways. The network between the two signaling pathways here involved is now under investigation.

Resistance to TGF-beta-induced apoptosis in regenerating hepatocytes

Treatment with transforming growth factor beta (TGF-beta) of hepatocytes from two different proliferative conditions, such as fetal development and adult liver regeneration, shows that regenerating cells respond to this cytokine in terms of growth inhibition, but are less sensitive than the fetal ones to the apoptosis induced by this factor. Regenerating TGF-beta treated cells show higher cell viability and lower percentage of apoptotic cells than the fetal treated ones. Furthermore, TGF-beta treated regenerating hepatocytes maintain a well-preserved parenchyma-like organization. Treatment with TGF-beta induces the loss of mitochondrial transmembrane potential in fetal but not in regenerating hepatocytes and activation of caspase-3 is lower in regenerating than in fetal cells. Regenerating hepatocytes show higher intracellular levels of some antiapoptotic proteins, such as Bcl-x(L) and c-IAP-1 and, interestingly, they present higher intracellular glutathione levels, which might provide of mechanisms to avoid potential dangerous effects of the oxidative stress-mediated apoptosis induced by TGF-beta. In fact, treatment with BSO (a glutathione synthesis inhibitor) restores the response of regenerating hepatocytes to TGF-beta in terms of cell death. In conclusion, increased levels of Bcl-x(L) and cIAP-1 and higher intracellular glutathione levels could confer resistance to the apoptosis induced by TGF-beta during liver regeneration.

Glutamate-cysteine ligase attenuates TNF-induced mitochondrial injury and apoptosis

Glutathione (GSH) is important in free radical scavenging, maintaining cellular redox status, and regulating cell survival in response to a wide variety of toxicants. The rate-limiting enzyme in GSH synthesis is glutamate-cysteine ligase (GCL), which is composed of catalytic (GCLC) and modifier (GCLM) subunits. To determine whether increased GSH biosynthetic capacity enhances cellular resistance to tumor necrosis factor-alpha- (TNF-alpha-) induced apoptotic cell death, we have established several mouse liver hepatoma (Hepa-1) cell lines overexpressing GCLC and/or GCLM. Cells overexpressing GCLC alone exhibit modest increases in GCL activity, while cells overexpressing both subunits have large increases in GCL activity. Importantly, cells overexpressing both GCL subunits exhibit increased resistance to TNF-induced apoptosis as judged by a loss of redox potential; mitochondrial membrane potential; translocation of cytochrome c to the cytoplasm; and activation of caspase-3, caspase-8, and caspase-9. Analysis of the effects of TNF on these parameters indicates that maintaining mitochondrial integrity mediates this protective effect in GCL-overexpressing cells.

Source of early reactive oxygen species in the apoptosis induced by transforming growth factor-beta in fetal rat hepatocytes

Transforming growth factor-beta (TGF-beta) induces an oxidative stress process in hepatocytes that mediates its apoptotic activity. To determine the cellular source of the early reactive oxygen species (ROS) generated by fetal rat hepatocytes in response to TGF-beta, we used inhibitors that block different ROS-producing systems. Diphenyleneiodonium, which inhibits NADPH oxidase and other flavoproteins, completely blocked the increase in ROS induced by TGF-beta, coincidently with an impairment of caspase-3 activation and cell death. Rotenone, an inhibitor of the NADH dehydrogenase in mitochondrial complex I, attenuated, but did not completely inhibit, ROS-production, caspase activation, and cell death mediated by TGF-beta. No significant protection was observed with inhibitors of other ROS-producing systems, such as cytochrome P450 (metyrapone), cyclooxygenase (indomethacin), and xanthine oxidase (allopurinol). Additional experiments have indicated that two different mechanisms could be involved in the early ROS production by TGF-beta. First, an inducible (cycloheximide-inhibited) NADPH oxidase-like system could account for the extramitochondrial production of ROS. Second, TGF-beta could increase ROS by a rapid downregulation of antioxidant genes. In particular, intramitochondrial ROS would increase by depletion of MnSOD. Finally, glutathione depletion is a late event and it would be more the consequence than the cause of the increase in ROS induced by TGF-beta.