The dual roles of c-Jun NH2-terminal kinase signaling in Cr(VI)-induced apoptosis in JB6 cells

Hexavalent chromium induces hepatocyte apoptosis via regulation of apoptosis signal-regulating kinase 1/c-Jun amino-terminal kinase signaling

With the spread of hexavalent chromium (Cr(VI)) contamination, Cr(VI)-induced hepatotoxicity has attracted increasing attention in recent years. To date, however, the exact mechanism of Cr(VI) toxicity remains unclear. In this study, we investigated the role of apoptosis signal-regulating kinase 1 (ASK1)/c-Jun amino-terminal kinase (JNK) in Cr(VI)-induced hepatic toxicity and the possible related mechanisms. AML-12 hepatocyte cell-lines were treated with 0, 1, 4, and 16 μmol/Lof Cr(VI) with or without GS-444271 (an ASK1 inhibitor). Adult male mice were administered with 0, 2, 8, and 32 mg/kg body mass (BM)/day of Cr(VI) for 5 days. The level of hepatocyte apoptosis/proliferation, generation of reactive oxygen species (ROS), and expression levels of mRNAs and proteins related to ASK1/JNK and nuclear factor-E2-related factor 2 (Nrf2) signaling were assessed. Results showed that high Cr(VI) exposure induced hepatocyte apoptosis and liver injury by generation of ROS and down-regulation of Nrf2 signaling. In addition, ASK1/JNK signaling activity was upregulated in the Cr(VI)-treated group. Furthermore, GS-444217 treatment significantly rescued Cr(VI)-induced hepatocyte apoptosis and liver dysfunction in vitro and in vivo by down-regulation of ASK1/JNK signaling. Thus, ASK1/JNK signaling appears to play an important role in Cr(VI)-induced hepatocyte apoptosis and liver injury. This study should help improve our understanding of the mechanism of Cr(VI)-induced liver injury and provide support for future investigations on liver disease therapy.

The role of ATF6 in Cr(VI)-induced apoptosis in DF-1 cells

Hexavalent chromium (Cr(VI)) is a common heavy metal pollutant in environment and has been proved possessing the cytotoxicity. In this study, we aimed to investigate the role of activating transcription factor 6 (ATF-6) in apoptosis of chicken embryo fibroblasts cell line (DF-1) induced by Cr(VI). Firstly, DF-1 cells were exposed to Cr(VI) to establish the cytotoxicity model, then the cell apoptosis and ATF-6 protein level were analyzed. By silencing ATF-6 gene, changes of the apoptosis rate and apoptotic proteins were examined. To further explore the regulatory mechanism of ATF-6, endoplasmic reticulum (ER) stress, mitochondrial function, reactive oxygen species (ROS) level, as well as the related pathway were evaluated. Results showed that Cr(VI) can result in DF-1 cell apoptosis, along with mitochondrial membrane potential (MMP) reducing and ER stress. Meanwhile, ATF-6 silencing lowered the apoptosis rate and ER stress level, showing with the decrease of XBP-1, PERK, GRP78, Caspase-12, Cleaved Caspase-3 and the increase of Bcl-2. Further analysis found that ATF-6 silencing down-regulated ROS and caused MMP loss, suggesting that ATF-6 silencing inhibited Cr(VI)-induced mitochondrial damage. In conclusion, this study indicate that ATF-6 plays an important regulatory role in Cr(VI)-induced DF-1 cell apoptosis through the ER stress and mitochondrial pathway.

Accumulation and detoxification dynamics of Chromium and antioxidant responses in juvenile rare minnow, Gobiocypris rarus

Hexavalent chromium (Cr⁶⁺) compounds are hazardous via all exposure routes. To explore the dynamics of Cr accumulation and elimination and to reveal the mechanisms underlying detoxification and antioxidation in juvenile Gobiocypris rarus, one-month old G. rarus larvae were exposed to 0.1mgL^-1 Cr⁶⁺ for four weeks for accumulation and subsequently placed to clean water for another week for depuration. The contents of Cr were measured weekly in the whole body of G. rarus juveniles. The activities of catalase (CAT), glutathione peroxidase (GPx), superoxide dismutase (SOD), glutathione S-transferase (GST) and glutathione reductase (GR), and contents of glutathione (GSH) and malonaldehyde (MDA), and transcripts of cat, Cu/Zn-sod, Mn-sod, gpx1, gstpi, gr, mt1, nrf2 and uba52 were determined. The results indicated that G. rarus juveniles had a strong ability to resist the Cr accumulation by Cr⁶⁺ exposure and to remove Cr from the body in clean water. In addition, GST and MT proteins may be involved in the detoxification of Cr⁶⁺. Moreover, Cr⁶⁺-induced GST detoxification in G. rarus juveniles might be accomplished through the Nrf2-mediated regulation of gene expressions. The antioxidant enzyme systems exhibited a response mechanism of the protective enzymes in organisms when they are subjected to external environmental stress. Two weeks of Cr⁶⁺ treatments could have led to the damage and consecutive degradation of antioxidant enzymes via ubiquitination, and MT proteins could be involved in protecting the activity of these enzymes. The capability of antioxidant enzyme systems to recover from the Cr⁶⁺-induced damage was strong in G. rarus juveniles after Cr⁶⁺ was removed from the water.

Nuclear factor erythroid 2-related factor 2 enhances carcinogenesis by suppressing apoptosis and promoting autophagy in nickel-transformed cells

Nickel-containing compounds are widely used in industry. Nickel is a known human carcinogen that primarily affects the lungs. Proposed mechanisms of nickel-induced carcinogenesis include disruption of cellular iron homeostasis, generation of reactive oxygen species (ROS), and induction of hypoxia signaling. However, the precise molecular mechanisms of nickel-induced malignant transformation and tumor development remain unclear. This study shows that the transcription factor Nrf2 is highly expressed in lung tumor tissue and in nickel-transformed human lung bronchial epithelial BEAS-2B cells (NiT cells). Additionally, constitutively high levels of Nrf2 play a critical role in apoptosis resistance in NiT cells. Basal ROS levels were extremely low in NiT cells and were correlated with elevated expression levels of both antioxidant enzymes (e.g. catalase and superoxide dismutases) and antiapoptotic proteins (e.g. Bcl-2 and Bcl-xL). These processes are tightly controlled by Nrf2. Autophagy inhibition, induced pharmacologically or genetically, enhanced Ni²⁺-induced apoptosis, indicating that the induction of autophagy is the cause of apoptosis resistance in NiT cells. Using similar approaches, we show that in NiT cells the inhibition of apoptosis decreases autophagy. We have shown that Stat3, which is up-regulated by Nrf2, controls autophagy induction in NiT cells. Colony formation and tumor growth were significantly attenuated by knockdown of Nrf2 or Bcl-2. Taken together, this study demonstrates that in NiT cells constitutively high Nrf2 expression inhibits apoptosis by up-regulating antioxidant enzymes and antiapoptotic proteins to increase autophagy via Stat3 signaling. These findings indicate that the Nrf2-mediated suppression of apoptosis and promotion of autophagy contribute to nickel-induced transformation and tumorigenesis.

Cr(VI) induces cytotoxicity in vitro through activation of ROS-mediated endoplasmic reticulum stress and mitochondrial dysfunction via the PI3K/Akt signaling pathway

The occupational and environmental toxicant hexavalent chromium [Cr(VI)] can cause severe damage to the liver; however, the exact mechanisms associated with its toxicity have not been thoroughly demonstrated. In the present study, the underlying mechanisms of Cr(VI)-induced hepatotoxicity were investigated. Our results showed that Cr(VI) inhibited the growth and proliferation of L-02 hepatocytes. Further study revealed that Cr(VI) significantly induced S-phase cell cycle arrest and apoptosis accompanying with the overproduction of reactive oxygen species (ROS). Cr(VI)-induced apoptosis could be prevented by inhibiting ROS with N-acetyl-l-cysteine (NAC). Additionally, our data showed that Cr(VI)-induced endoplasmic reticulum (ER) stress and mitochondrial dysfunction were concentration- and time-dependent. Moreover, inhibition of C/EBA homologous protein (CHOP) expression by siRNA partially prevented Cr(VI)-induced cell apoptosis, mitochondrial dysfunction and ROS generation. We also found that Cr(VI) treatment inhibited the PI3K/Akt pathway in a concentration- and time-dependent manner. After using IGF-1 (50ng/mL), the specific agonist of the PI3K/AKT signaling pathway, the facilitating effects of Cr(VI) were depressed. This finding demonstrated the relationship between the PI3K/Akt pathway, ER stress and mitochondrial dysfunction. Collectively, these findings indicated that Cr(VI) increased ROS production. Increased ROS production may account for inhibition of the PI3K/Akt pathway and lead to ER stress and mitochondrial dysfunction, which consequently induces apoptosis in L-02 hepatocytes. This study provides novel insights into the molecular mechanisms of Cr(VI)-induced cytotoxicity.

JNK1 ablation in mice confers long-term metabolic protection from diet-induced obesity at the cost of moderate skin oxidative damage

Obesity and insulin resistance are associated with oxidative stress, which may be implicated in the progression of obesity-related diseases. The kinase JNK1 has emerged as a promising drug target for the treatment of obesity and type 2 diabetes. JNK1 is also a key mediator of the oxidative stress response, which can promote cell death or survival, depending on the magnitude and context of its activation. In this article, we describe a study in which the long-term effects of JNK1 inactivation on glucose homeostasis and oxidative stress in obese mice were investigated for the first time. Mice lacking JNK1 (JNK1(-/-)) were fed an obesogenic high-fat diet (HFD) for a long period. JNK1(-/-) mice fed an HFD for the long term had reduced expression of antioxidant genes in their skin, more skin oxidative damage, and increased epidermal thickness and inflammation compared with the effects in control wild-type mice. However, we also observed that the protection from obesity, adipose tissue inflammation, steatosis, and insulin resistance, conferred by JNK1 ablation, was sustained over a long period and was paralleled by decreased oxidative damage in fat and liver. We conclude that compounds targeting JNK1 activity in brain and adipose tissue, which do not accumulate in the skin, may be safer and most effective.-Becattini, B., Zani, F., Breasson, L., Sardi, C., D'Agostino, V. G., Choo, M.-K., Provenzani, A., Park, J. M., Solinas, G. JNK1 ablation in mice confers long-term metabolic protection from diet-induced obesity at the cost of moderate skin oxidative damage.

Hexavalent chromium induces apoptosis in male somatic and spermatogonial stem cells via redox imbalance

Hexavalent chromium [Cr(VI)], an environmental toxicant, causes severe male reproductive abnormalities. However, the actual mechanisms of toxicity are not clearly understood and have not been studied in detail. The present in vitro study aimed to investigate the mechanism of reproductive toxicity of Cr(VI) in male somatic cells (mouse TM3 Leydig cells and TM4 Sertoli cells) and spermatogonial stem cells (SSCs) because damage to or dysfunction of these cells can directly affect spermatogenesis, resulting in male infertility. Cr(VI) by inducing oxidative stress was cytotoxic to both male somatic cells and SSCs in a dose-dependent manner, and induced mitochondria-dependent apoptosis. Although the mechanism of Cr(VI)-induced cytotoxicity was similar in both somatic cells, the differences in sensitivity of TM3 and TM4 cells to Cr(VI) could be attributed, at least in part, to cell-specific regulation of P-AKT1, P-ERK1/2, and P-P53 proteins. Cr(VI) affected the differentiation and self-renewal mechanisms of SSCs, disrupted steroidogenesis in TM3 cells, while in TM4 cells, the expression of tight junction signaling and cell receptor molecules was affected as well as the secretory functions were impaired. In conclusion, our results show that Cr(VI) is cytotoxic and impairs the physiological functions of male somatic cells and SSCs.

MiRNA profiling provides insights on adverse effects of Cr(VI) in the midgut tissues of Drosophila melanogaster

Cr(VI), a well-known environmental chemical, is reported to cause various adverse effects on exposed organisms including genomic instability and carcinogenesis. Despite available information on the underlying mechanism of Cr(VI) induced toxicity, studies regarding toxicity modulation by epigenetic mechanisms are limited. It was therefore, hypothesized that the global miRNA profiling in Cr(VI) exposed Drosophila, a genetically tractable model organism, will provide information about mis-regulated miRNAs along with their targeted genes and relevant processes. Third instar larvae of Drosophila melanogaster (Oregon R(+)) were exposed to 5.0-20.0 μg/ml of Cr(VI) for 24 and 48 h. Following miRNA profile analysis on an Agilent platform, 28 of the 36 differentially expressed miRNAs were found to be significantly mis-regulated targeting major biological processes viz., DNA damage repair, oxidation-reduction processes, development and differentiation. Down-regulation of mus309 and mus312 under DNA repair, acon to oxidation-reduction and pyd to stress activated MAPK cascade respectively belonging to these gene ontology classes concurrent with up-regulation of dme-miR-314-3p, dme-miR-79-3p and dme-miR-12-5p confirm their functional involvement against Cr(VI) exposure. These findings assume significance since majority of the target genes in Drosophila have functional homologues in humans. The study further recommends Drosophila as a model to explore the role of miRNAs in xenobiotic induced toxicity.

Nrf2/p62 signaling in apoptosis resistance and its role in cadmium-induced carcinogenesis

The cadmium-transformed human lung bronchial epithelial BEAS-2B cells exhibit a property of apoptosis resistance as compared with normal non-transformed BEAS-2B cells. The level of basal reactive oxygen species (ROS) is extremely low in transformed cells in correlation with elevated expressions of both antioxidant enzymes (catalase, SOD1, and SOD2) and antiapoptotic proteins (Bcl-2/Bcl-xL). Moreover, Nrf2 and p62 are highly expressed in these transformed cells. The knockdown of Nrf2 or p62 by siRNA enhances ROS levels and cadmium-induced apoptosis. The binding activities of Nrf2 on the antioxidant response element promoter regions of p62/Bcl-2/Bcl-xL were dramatically increased in the cadmium-exposed transformed cells. Cadmium exposure increased the formation of LC3-II and the frequency of GFP-LC3 punctal cells in non-transformed BEAS-2B cells, whereas these increases are not shown in transformed cells, an indication of autophagy deficiency of transformed cells. Furthermore, the expression levels of Nrf2 and p62 are dramatically increased during chronic long term exposure to cadmium in the BEAS-2B cells as well as antiapoptotic proteins and antioxidant enzymes. These proteins are overexpressed in the tumor tissues derived from xenograft mouse models. Moreover, the colony growth is significantly attenuated in the transformed cells by siRNA transfection specific for Nrf2 or p62. Taken together, this study demonstrates that cadmium-transformed cells have acquired autophagy deficiency, leading to constitutive p62 and Nrf2 overexpression. These overexpressions up-regulate the antioxidant proteins catalase and SOD and the antiapoptotic proteins Bcl-2 and Bcl-xL. The final consequences are decrease in ROS generation, apoptotic resistance, and increased cell survival, proliferation, and tumorigenesis.

Oxidative stress and metal carcinogenesis

Occupational and environmental exposures to metals are closely associated with an increased risk of various cancers. Although carcinogenesis caused by metals has been intensively investigated, the exact mechanisms of action are still unclear. Accumulating evidence indicates that reactive oxygen species (ROS) generated by metals play important roles in the etiology of degenerative and chronic diseases. This review covers recent advances in (1) metal-induced generation of ROS and the related mechanisms; (2) the relationship between metal-mediated ROS generation and carcinogenesis; and (3) the signaling proteins involved in metal-induced carcinogenesis, especially intracellular reduction-oxidation-sensitive molecules.

Daunorubicin induces cell death via activation of apoptotic signalling pathway and inactivation of survival pathway in muscle-derived stem cells

Daunorubicin (as well as other anthracyclines) is known to be toxic to heart cells and other cells in organism thus limiting its applicability in human cancer therapy. To investigate possible mechanisms of daunorubicin cytotoxicity, we used stem cell lines derived from adult rabbit skeletal muscle. Recently, we have shown that daunorubicin induces apoptotic cell death in our cell model system and distinctly influences the activity of MAP kinases. Here, we demonstrate that two widely accepted antagonistic signalling pathways namely proapoptotic JNK and prosurvival PI3K/AKT participate in apoptosis. Using the Western blot method, we observed the activation of JNK and phosphorylation of its direct target c-Jun along with inactivation of AKT and its direct target GSK in the course of programmed cell death. By means of small-molecule kinase inhibitors and transfection of cells with the genes of the components of these pathways, c-Jun and AKT, we confirm that JNK signalling pathway is proapoptotic, whereas AKT is antiapoptotic in daunorubicin-induced muscle cells. These findings could contribute to new approaches which will result in less toxicity and fewer side effects that are currently associated with the use of daunorubicin in cancer therapies.

Epigallocatechin-3-gallate (EGCG) protects against chromate-induced toxicity in vitro

Hexavalent chromium [Cr(VI)] is a human carcinogen that results in the generation of reactive oxygen species (ROS) and a variety of DNA lesions leading to cell death. Epigallocatechin-3-gallate (EGCG), the major polyphenol present in green tea, possesses potent antioxidative activity capable of protecting normal cells from various stimuli-induced oxidative stress and cell death. Here we demonstrated that co-treatment with EGCG protected human normal bronchial epithelial BEAS-2B cells from Cr(VI)-induced cell death in a dose-dependent manner. Cr(VI) induces apoptosis as the primary mode of cell death. Co-treatment of BEAS-2B cells with EGCG dose-dependently suppressed Cr(VI)-induced apoptosis. Fluorescence microscopic analyses and quantitative measurement revealed that EGCG significantly decreased intracellular levels of ROS induced by Cr(VI) exposure. Using a well-established K(+)/SDS precipitation assay, we further showed that EGCG was able to dose-dependently reduce DNA-protein cross-links (DPC), lesions that could be partially attributed to Cr(VI)-induced oxidative stress. Finally, analyses of Affymetrix microarray containing 28,869 well-annotated genes revealed that, among the 3412 genes changed more than 1.5-fold by Cr(VI) treatment, changes of 2404 genes (70%) were inhibited by pretreatment of EGCG. Real-time PCR confirmed the induction of 3 genes involved in cell death and apoptosis by Cr(VI), which was eliminated by EGCG. In contrast, Cr(VI) reduced the expression of 3 genes related to cellular defense, and this reduction was inhibited by EGCG. Our results indicate that EGCG protects BEAS-2B cells from Cr(VI)-induced cytotoxicity presumably by scavenging ROS and modulating a subset of genes. EGCG, therefore, might serve as a potential chemopreventive agent against Cr(VI) carcinogenesis.

Mycotoxin zearalenone induces AIF- and ROS-mediated cell death through p53- and MAPK-dependent signaling pathways in RAW264.7 macrophages

Zearalenone (ZEN) is commonly found in many food commodities and is known to cause reproductive disorders and genotoxic effects. However, the mode of ZEN-induced cell death of macrophages and the mechanisms by which ZEN causes cytotoxicity remain unclear. The present study shows that ZEN treatment reduces viability of RAW264.7 cells in a dose-dependent manner. ZEN causes predominantly necrotic and late apoptotic cell death. ZEN treatment also results in the loss of mitochondrial membrane potential (MMP), mitochondrial changes in Bcl-2 and Bax proteins, and cytoplasmic release of cytochrome c and apoptosis-inducing factor (AIF). Pre-treatment of the cells with either z-VAD-fmk or z-IETD-fmk does not attenuate ZEN-mediated cell death, whereas catalase suppresses the ZEN-induced decrease in viability in RAW264.7 cells. Treating the cells with c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK), or p53 inhibitor prevented ZEN-mediated changes, such as MMP loss, cellular reactive oxygen species (ROS) increase, and cell death. JNK or p38 MAPK inhibitor inhibited mitochondrial alterations of Bcl-2 and Bax proteins with attendant decreases in cellular ROS levels. Knockdown of AIF via siRNA transfection also diminished ZEN-induced cell death. Further, adenosine triphosphate was markedly depleted in the ZEN-exposed cells. Collectively, these results suggest that ZEN induces cytotoxicity in RAW264.7 cells via AIF- and ROS-mediated signaling, in which the activations of p53 and JNK/p38 play a key role.