Evaluation of pathways involved in pentachlorophenol-induced apoptosis in rat neurons

Pentachlorophenol exposure induced neurotoxicity by disrupting citrulline metabolism in larvae and adult zebrafish

Pentachlorophenol (PCP) is a ubiquitous environmental toxicant with various adverse effects. Although its neurotoxicity has been reported, the underlying mechanism and subsequent detoxification remain unclear. In this study, embryos and adult zebrafish were exposed to PCP to determine its potential neurotoxic mechanism and protective indicators. The survival rate, heart rate, mobility time, active status and moving distance were significantly decreased in larvae after 30 μg/L PCP exposure. Likewise, the mobile time, latency to the first movement, velocity and moving distance of adult zebrafish were significantly reduced by PCP exposure. Untargeted metabolomics analysis of larvae revealed that arginine and proline metabolism was the primary pathway affected by PCP exposure, reflected by increased proline and decreased citrulline (CIT) contents, which were confirmed by quantitative data. PCP exposure suppressed the conversion from arginine to CIT in larvae by downregulating the expression of nos1 and nos2a. Ornithine content was increased in the brains and intestines of adult zebrafish after PCP exposure, which inhibited ornithine catabolism to CIT by downregulating otc, resulting in reduced CIT. Intriguingly, CIT supplementation significantly restored the neurobehavioral defects induced by PCP in larvae and adult zebrafish. CIT supplementation upregulated the expression of ef1α and tuba1 in larvae and inhibited the downregulation of ef1α in the brains of adult zebrafish. Taken together, these results indicated that CIT supplementation could protect against PCP-induced neurotoxicity by upregulating the expression of genes involved in neuronal development and function.

microRNA-106b-containing extracellular vesicles affect autophagy of neurons by regulating CDKN2B in Parkinson's disease

Parkinson's disease (PD) is the second most frequent neurodegenerative disorder, and autophagy dysfunction is involved in the pathogenesis of PD. Mesenchymal stem cells (MSC)-derived extracellular vesicles (EVs) have been established as an attractive therapeutic tool, since they can serve as biological nanoparticles with beneficial effects in PD. Herein, the study aimed to investigate the effects of EVs derived microRNA (miR)-106b on autophagy of neurons in PD. Following the development of a mouse model of PD, we conducted behavior test, TUNEL assay and HE staining to verify the success of modeling. Afterward, MSC-derived EVs were extracted and identified. In hippocampal tissues and neurons of PD mice, miR-106b was poorly expressed, while CDKN2B was highly expressed. miR-106b shuttled by MSC-derived EVs increased neuronal survival, autophagy, LC3II/LC3I ratio and Bcl-2 protein expression, while inhibited neuronal apoptosis and Bax expression in PD mice. It was also confirmed that CDKN2B is a downstream target of miR-106b. Overexpression of CDKN2B reversed the protective effects of miR-106b-containing EVs on neurons in mice with PD. Collectively, miR-106b-containing EVs alleviate neuronal apoptosis and enhance neuronal autophagy in PD by downregulating CDKN2B.

Phenol and chlorinated phenols exhibit different apoptotic potential in human red blood cells (in vitro study)

Phenol and chlorinated phenols are widely spread in the environment and human surrounding, which leads to a common environmental and occupational exposure of humans to these substances. The aim of this study was to assess eryptotic changes in human red blood cells treated with phenol, 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP). The erythrocytes were incubated with phenols studied in the concentrations ranging from 1 to 100 μg/mL for 24 h or 48 h. The results of the study revealed that all compounds studied caused phosphatidylserine translocation and increased cytosolic calcium ions level in human erythrocytes. It was also noticed that phenol and chlorophenols caused an increase in caspase-3 and calpain activation, which confirmed that they were capable of inducing suicidal death of erythrocytes. The results also revealed that PCP most strongly altered the parameters studied, while phenol exhibited the weakest eryptotic potential in the incubated cells.

Quantitative optical measurement of mitochondrial superoxide dynamics in pulmonary artery endothelial cells

Reactive oxygen species (ROS) play a vital role in cell signaling and redox regulation, but when present in excess, lead to numerous pathologies. Detailed quantitative characterization of mitochondrial superoxide anion ( O2•- ) production in fetal pulmonary artery endothelia cells (PAECs) has never been reported. The aim of this study is to assess mitochondrial O2•- production in cultured PAECs over time using a novel quantitative optical approach. The rate, the sources, and the dynamics of O2•- production were assessed using targeted metabolic modulators of the mitochondrial electron transport chain (ETC) complexes, specifically an uncoupler and inhibitors of the various ETC complexes, and inhibitors of extra-mitochondrial sources of O2•- . After stabilization, the cells were loaded with nanomolar mitochondrial-targeted hydroethidine (Mito-HE, MitoSOX) online during the experiment without washout of the residual dye. Time-lapse fluorescence microscopy was used to monitor the dynamic changes in O2•- fluorescence intensity over time in PAECs. The transient behaviors of the fluorescence time course showed exponential increases in the rate of O2•- production in the presence of the ETC uncoupler or inhibitors. The most dramatic and the fastest increase in O2•- production was observed when the cells were treated with the uncoupling agent, PCP. We also showed that only the complex IV inhibitor, KCN, attenuated the marked surge in O2•- production induced by PCP. The results showed that mitochondrial respiratory complexes I, III and IV are sources of O2•- production in PAECs, and a new observation that ROS production during uncoupling of mitochondrial respiration is mediated in part via complex IV. This novel method can be applied in other studies that examine ROS production under stress condition and during ROS-mediated injuries in vitro.

Early developmental exposure to pentachlorophenol causes alterations on mRNA expressions of caspase protease family in zebrafish embryos

Caspase proteases play an essential role in cell apoptosis and inflammation, thus matter greatly in animal development and other biological processes. As a ubiquitous environmental pollutant, pentachlorophenol (PCP) is considered to have adverse effects on animal apoptosis during embryonic development, yet the evidence that PCP interfere with caspase genes was seldom reported. To uncover the effects of PCP on caspases expression in early embryos of zebrafish, two concentrations of PCP (5 μg/L and 200 μg/L) were chosen and 14 types of caspase genes at two different developmental stages, 8 h post-fertilization (hpf) and 24 hpf were analyzed. Lower survival and hatching rates, distinct developmental delay and morphological deformities of head and tail were observed. PCP, especially in the high concentration, significantly altered the expressions of most caspase genes. At 8 hpf, PCP had the most significant inductive effects on gene casp8l2 with fold changes (FCs) of 6.87 at 5 μg/L and 4.48 at 200 μg/L, and casp6l1 (with FCs of 3.15/3.69), and inhibitory effects on caspa (with FCs of 0.93/0.53) and caspb (with FCs of 0.99/0.57). At 24 hpf, PCP had the most significant effects on casp6l2, casp9, and caspc. PCP exposure possibly disrupted intrinsic apoptosis pathway considering its effects on casp9 expression. In addition, most caspase genes exhibited higher levels at 24 hpf than 8 hpf except caspc. Our results suggested that PCP had different effects on varied caspase genes, which probably resulting in a profound impact on caspase proteins and apoptosis processes and, ultimately, developmental abnormality.

Activating Transcription Factor 4 (ATF4)-ATF3-C/EBP Homologous Protein (CHOP) Cascade Shows an Essential Role in the ER Stress-Induced Sensitization of Tetrachlorobenzoquinone-Challenged PC12 Cells to ROS-Mediated Apoptosis via Death Receptor 5 (DR5) Signaling

Tetrachlorobenzoquinone (TCBQ) is a downstream metabolite of pentachlorophenol (PCP). Previously, we demonstrated that TCBQ caused cytotoxicity due to mitochondrial-related apoptosis. Here, we confirmed the upregulation of death receptor 5 (DR5) followed by the construction of the death-inducing signaling complex (DISC). We also detected the activation of the caspase cascade, which was correlated with TCBQ-induced apoptotic cell death in PC12 cells. The upregulation of DR5 included transcriptional activation and de novo protein synthesis in response to TCBQ. We also identified the endoplasmic reticulum (ER) as a new target for the TCBQ challenge in PC12 cells. The protein kinase R-like ER kinase/eukaryotic translation initiation factor 2α (PERK/eIF2α)-mediated activating transcription factor 4 (ATF4)-ATF3-C/EBP homologous protein (CHOP) signaling pathway contributed to the process of TCBQ-induced ER stress. Blocking ATF4, ATF3, or CHOP signaling by gene silencing technology resulted in decreased cell apoptosis after exposure to TCBQ. Finally, NAC ameliorated TCBQ-induced apoptosis and ER stress, which illustrated that TCBQ-induced apoptosis is somehow ROS-dependent. In summary, this study provided important mechanistic insight into how TCBQ utilizes ER stress-related signaling to exhibit pro-apoptotic activity in PC12 cells.

2,4-dichlorophenol induces ER stress-mediated apoptosis via eIF2α dephosphorylation in vitro

2,4-Dichlorophenol (2,4-DCP) has been widely used to produce herbicides and pharmaceutical intermediates, which exhibits various toxic effects including apoptosis. However, the mechanisms underlying 2,4-DCP-induced apoptosis, especially mediated by endoplasmic reticulum (ER) stress, are still unknown. In the present study, the mouse embryonic fibroblasts (MEFs) were used as an in vitro model system to figure out whether 2,4-DCP could induce ER stress, and further to elucidate the role of ER stress in 2,4-DCP-induced apoptosis. The results showed that 2,4-DCP dramatically caused the decrease of cell viability, the increase of apoptotic cells, the collapse of mitochondrial membrane potential (MMP) and the activation of caspase-3, suggesting that 2,4-DCP did induce apoptosis. Meanwhile, 2,4-DCP acted similarly as ER stress agonist tunicamycin (Tu) to activate all three branches (IRE1α, ATF6 and eIF2α) of ER stress. Furthermore, repression of ER stress or inhibition of eIF2α dephosphorylation significantly alleviated 2,4-DCP-induced apoptosis. Taking these results together, the present study firstly showed that 2,4-DCP induced ER stress-mediated apoptosis via eIF2α dephosphorylation in mammalian cells. These findings will provide new insights into the mechanisms underlying apoptosis after chlorophenols exposure.

Tetrachlorobenzoquinone exhibits neurotoxicity by inducing inflammatory responses through ROS-mediated IKK/IκB/NF-κB signaling

Tetrachlorobenzoquinone (TCBQ) is a joint metabolite of persistent organic pollutants (POPs), hexachlorobenzene (HCB) and pentachlorophenol (PCP). Previous studies have been reported that TCBQ contributes to acute hepatic damage due to its pro-oxidative nature. In the current study, TCBQ showed the highest capacity on the cytotoxicity, ROS formation and inflammatory cytokines release among four compounds, i.e., HCB, PCP, tetrachlorohydroquinone (TCHQ, reduced form of TCBQ) and TCBQ, in PC 12 cells. Further mechanistic study illustrated TCBQ activates nuclear factor-kappa B (NF-κB) signaling. The activation of NF-κB was identified by measuring the protein expressions of inhibitor of nuclear factor kappa-B kinase (IKK) α/β, p-IKKα/β, an inhibitor of NF-κB (IκB) α, p-IκBα, NF-κB (p65) and p-p65. The translocation of NF-κB was assessed by Western blotting of p65 in nuclear/cytosolic fractions, electrophoretic mobility shift assay (EMSA) and luciferase reporter gene assay. In addition, TCBQ significantly induced protein and mRNA expressions of inflammatory cytokines and mediators, such as interleukin-1 beta (IL-1β), IL-6, tumor necrosis factor-alpha (TNF-α), inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2) and the production of nitric oxide (NO) and prostaglandin E2 (PGE2). Pyrrolidine dithiocarbamate (PDTC), a specific NF-κB inhibitor inhibited these effects efficiently, further suggested TCBQ-induced inflammatory responses involve NF-κB signaling. Moreover, antioxidants, i.e., N-acetyl-l-cysteine (NAC), Vitamin E and curcumin, ameliorated TCBQ-induced ROS generation as well as the activation of NF-κB, which implied that ROS serve as the upstream molecule of NF-κB signaling. In summary, TCBQ exhibits a neurotoxic effect by inducing oxidative stress-mediated inflammatory responses via the activation of IKK/IκB/NF-κB pathway in PC12 cells.

Transcriptional profiling of the soil invertebrate Folsomia candida in pentachlorophenol-contaminated soil

Pentachlorophenol (PCP), a widely used pesticide, is considered to be an endocrine disruptor. The molecular effects of chemicals with endocrine-disrupting potential on soil invertebrates are largely unknown. In the present study, the authors explored the transcriptional expression changes of collembola (Folsomia candida) in response to PCP contamination. A total of 92 genes were significantly differentially expressed at all exposure times, and the majority of them were found to be downregulated. In addition to the transcripts encoding cytochrome P450s and transferase enzymes, chitin-binding protein was also identified in the list of common differentially expressed genes. Analyses of gene ontology annotation and enrichment revealed that cell cycle-related transcripts were significantly induced by PCP, indicating that PCP can stimulate cell proliferation in springtail, as has been reported in human breast cancer cells. Enrichment of functional terms related to steroid receptors was observed, particularly in 20 significant differentially expressed genes involved in chitin metabolism in response to PCP exposure. Combined with confirmation by quantitative polymerase chain reaction, the results indicate that the adverse effects on reproduction of springtails after exposure to PCP can be attributed to a chemical-induced delay in the molting cycle and that molting-associated genes may serve as possible biomarkers for assessing toxicological effects.

2,4,6-Trichlorophenol Cytotoxicity Involves Oxidative Stress, Endoplasmic Reticulum Stress, and Apoptosis

This study aims to evaluate the cytotoxicity and potential mechanisms of 2,4,6-trichlorophenol (2,4,6-TCP) in mouse embryonic fibroblasts. Our results show that 2,4,6-TCP causes morphological changes and reduces cell viability. The overproduction of reactive oxygen species, the upregulation of nuclear factor-E2-related factor 2 (Nrf2) and heme oxygenase 1 (HMOX1) messenger RNA (mRNA) expressions, and the nuclear translocation of Nrf2 protein demonstrate that 2,4,6-TCP induces oxidative stress, and the Nrf2/HMOX1 pathway might be involved in 2,4,6-TCP-induced antioxidative response. Simultaneously, our data also demonstrate that 2,4,6-TCP upregulates the expressions of binding immunoglobulin protein, inositol-requiring enzyme/endonuclease 1α, and C/EBP homologous protein; stimulates α subunit of eukaryotic translation initiation factor 2 phosphorylation; and induces the splicing of Xbp1 mRNA, suggesting that endoplasmic reticulum (ER) stress is triggered. Moreover, 2,4,6-TCP alters the mitochondrial membrane potential and increases the apoptosis rate, the caspase 3 activity, and the Bax/Bcl-2 ratio, demonstrating that the mitochondrial pathway is involved in the 2,4,6-TCP-induced apoptosis. Thus, these results show that 2,4,6-TCP induces oxidative stress, ER stress, and apoptosis, which together contribute to its cytotoxicity in vitro.

Cytotoxic effects exerted by pentachlorophenol by targeting nodal pro-survival signaling pathways in human pancreatic cancer cells

Pancreatic adenocarcinoma is one of the deadliest human solid tumors in the developed countries characterized by high resistance toward chemotherapeutic treatment. We have previously shown that silencing of the pro-survival protein kinase CK2 by RNA interference contributes to enhance the cytotoxicity of the chemotherapeutic agent 2′,2′-difluoro 2′-deoxycytidine (gemcitabine). Initial experiments showed that pentachlorophenol (PCP) inhibits CK2 and induces cell death in human pancreatic cancer cell lines. We report here evidence that exposure of this type of cells to PCP induces caspase-mediated apoptosis, inhibition of the lysosome cysteine protease cathepsin B and mitochondrial membrane depolarization. Beside cellular inhibition of CK2, the analysis of signaling pathways deregulated in pancreatic cancer cells revealed that PCP causes decreased phosphorylation levels of NF-κB/p65, suppresses its nuclear translocation and leads to activation of JNK-mediated stress response. Surprisingly, exposure to PCP results in increased phosphorylation levels of AKT at the canonical S473 and T308 activation sites supporting previous data showing that AKT phosphorylation is not predictive of tumor cell response to treatment. Taken together, our study provides novel insights into the effects induced by the exposure of pancreatic cancer cells to chlorinated aromatic compounds posing the basis for more advanced studies in vivo.

2,4-dichlorophenol induces apoptosis in primary hepatocytes of grass carp (Ctenopharyngodon idella) through mitochondrial pathway

2,4-Dichlorophenol (2,4-DCP), a major type of chlorophenols, has been widely used to produce some herbicides and pharmaceuticals, yet due to its incomplete degradation and bioaccumulation characteristics, it is toxic to aquatic organisms. Apoptosis is one of the most severe outcomes of cell poisoning and injury. So far, the potential molecular mechanism of 2,4-DCP-induced apoptosis has not been reported. This study showed that 2,4-DCP significantly induced apoptosis in primary hepatocytes of grass carp (Ctenopharyngodon idella). 2,4-DCP exposure upregulated mRNA of caspase-3, reduced the mitochondrial membrane potential (Δψm), increased intracellular reactive oxygen species (ROS) and the Bax/Bcl-2 ratio, while protection of mitochondria with acetyl-l-carnitine hydrochloride (ALC) rescued 2,4-DCP-induced apoptosis, restored the Δψm and reduced the Bax/Bcl-2 ratio. Taken together, this is the first study that has identified that 2,4-DCP exposure induced apoptosis through the mitochondria-dependent pathway in primary hepatocytes of grass carp.

The relationships between pesticide metabolites and neurobehavioral test performance in the third National Health and Nutrition Examination Survey

Regression analysis was used to estimate and test for relationships between urinary pesticide metabolites and neurobehavioral test performance in adults, 20 to 59 years old, participating in the third National Health and Nutrition Examination Survey. The 12 pesticide metabolites included 2 naphthols, 8 phenols, a phenoxyacetic acid, and a pyridinol. The 3 neurobehavioral tests included in the survey were simple reaction time, symbol-digit substitution, and serial digit learning. As the 2,4-dichlorophenol, 2,5-dichlorophenol, and the pentachlorophenol concentrations increased, performance on the serial digit learning test improved. As the 2,5-dichlorophenol concentration increased, performance on the symbol-digit substitution test improved. At low concentrations, the parent compounds of these metabolites may act at acetylcholine and γ-aminobutyric acid synapses in the central nervous system to improve neurobehavioral test performance.

Identification of tetrabromobisphenol A diallyl ether as an emerging neurotoxicant in environmental samples by bioassay-directed fractionation and HPLC-APCI-MS/MS

Brominated flame retardants (BFRs) have been widely used as additives in products to reduce their flammability. Recent findings suggested that some BFRs exhibit neurotoxicity and thus might pose a threat to human health. In this work, a neurotoxicity assay-directed analysis was developed, combining sample cleanup, fractionation, chemical identification, and bioassay. Viability of primary cultured cerebellar granule neurons (CGNs) was used to evaluate the neurotoxicity of extracts or separated fractions from environmental samples. Tetrabromobisphenol A diallyl ether (TBBPA DAE) was identified as the causative toxicant in sediment samples collected from a river near a brominated flame retardant (BFR) manufacturing plant in South China. Liquid chromatography atmospheric pressure chemical ionization tandem mass spectrometry (LC-APCI-MS/MS) was optimized to determine TBBPA DAE levels in the potent fractions and to confirm TBBPA DAE as the key neurotoxicant. On the basis of comparison with the structure of other TBBPA derivatives, the 1-propenyl group in TBBPA DAE appears to be the cause for the neurotoxic potency. The levels of TBBPA DAE in samples along the river were found at up to 49 ng/L for river water, 10,183 ng/g dry weight (dw) in surface sediments, and 42 ng/g dw in soils. According to the distribution of TBBPA DAE in the environmental samples, the manufacturing plant was identified as the release source of TBBPA DAE. To our knowledge, this study is the first to demonstrate potential neurotoxicity induced by TBBPA DAE in real environmental samples.

Identification of differential hepatic proteins in rare minnow (Gobiocypris rarus) exposed to pentachlorophenol (PCP) by proteomic analysis

Pentachlorophenol (PCP) is a ubiquitous contaminant that has been shown to lead to hepatoxicity and is implicated in the incidence of liver tumors in human. A number of previous studies have described the toxic effects of PCP based on conventional toxicological indices. However, little evidence on protein levels is available at present. For further understanding of mechanisms of action and identifying the potential protein biomarkers for PCP exposure, two-dimensional electrophoresis coupled with mass spectrometry has been used to identify proteins differentially expressed in the livers of rare minnow (Gobiocypris rarus) following PCP exposure of 0.5, 5, 50 μg/L. After comparison of the protein profiles from treated and control groups, 39 protein spots were found altered in abundance (>2-fold) from male and female PCP-treated groups. Matrix-assisted laser desorption/ionization (MALDI) tandem time-of-flight mass spectrometry (TOF/MS) analysis allowed the unambiguous identification, and 18 protein spots were identified successfully, 12 proteins in females and 6 proteins in males, respectively. These proteins were involved in transport, metabolism, response to oxidative stress and other biological processes. Of these proteins, four differentially expressed mRNA encoding proteins underwent quantitative analysis by quantitative real-time PCR (QRT-PCR). The consistent and discrepant results between mRNA and protein levels suggested that complicated regulatory mechanisms of gene expression were implicated in the response to PCP exposure. In addition, marked gender differences in response to PCP have been described from the comparison of the male and female liver protein profiles.