Induction of c-fos gene by mercury chloride in LLC-PK1 cells

Neurobehavioral effects, c-Fos/Jun expression and tissue distribution in rat offspring prenatally co-exposed to MeHg and PFOA: PFOA impairs Hg retention

Exposure to methylmercury (MeHg) and perfluorooctanoic acid (PFOA) can occur simultaneously as both contaminants are found in the same food sources, especially fish, seafood, marine mammals and milk. The aim of this study was to assess the effects of exposure to MeHg (10 μg mL(-1) in drinking water) and PFOA (10 μg mL(-1) in drinking water) from gestational day 1 to postnatal day (PND) 21, alone and in combination, on neurobehavioral development and the expression of c-Fos/Jun in different brain regions in the offspring. Our findings showed that exposure to MeHg alone, and exposure to MeHg combined with PFOA significantly induced cliff avoidance reflexes and negative geotaxis reflexes. And these effects appeared to be greater following exposure to MeHg alone. MeHg and/or PFOA exposure did not significantly impair motor coordination functions, or cause significant changes in c-Fos expression in the hippocampus and cerebellum, and spatial learning tests were similar to those in the controls, thus it was impossible to determine whether combined exposure to MeHg and PFOA had any additional effects on both hippocampus and cerebellum regions. However, a significant increase in the frequency of line crossing was observed in rats treated with MeHg or PFOA alone, and there were no significant differences between the MeHg+PFOA-treated group and the controls, suggesting that PFOA was antagonistic to MeHg toxicity in the locomotor activity test. Co-exposure to MeHg and PFOA decreased all tissue Hg concentrations in pups compared to the group exposed to MeHg only, suggesting that PFOA impaired Hg retention in different tissues.

Effects of heavy metals on mitogen-activated protein kinase pathways

The signaling pathways leading to cellular protection or cell death following exposure to heavy metals have not been fully clarified. Mitogen-activated protein kinases (MAPKs), i.e., extracellular signal-regulated protein kinase (ERK), c-Jun NH(2)-terminal kinase (JNK) and p38 MAPK transmit extracellular signals into the nucleus, and have been shown to participate in a diverse array of cellular functions such as cell growth, differentiation and apoptosis. Treatment with cadmium, inorganic mercury or tributyltin can activate ERK, JNK and p38 MAPK, and induces the expression of c-fos and c-jun genes prior to the development of apoptosis. However, the members of the MAPK family appear to be differentially activated depending on the heavy metal and the cell type exposed. Consequently, various cellular responses may be caused by the distinct pattern of MAPKs activation. MAPKs may be one of the important cellular signal transduction pathways affected by various environmental pollutants, including heavy metals.

Assessing noxious effects of dietary exposure to methylmercury, PCBs and Se coexisting in environmentally contaminated rice in male mice

Polychlorinated biphenyls and methylmercury are two of the most ubiquitous environmental contaminants in Guizhou province. Rice is eaten with almost every meal and provides more calories than any single food in Guizhou province. The estimated tolerable daily intake of total mercury, MeHg, Se and PCBs from Guizhou contaminated rice by Chinese people showed that MeHg and/or PCBs exceeded the corresponding limits. The aim of this study was to characterize the effects of exposure to environmental contaminated rice on neurobehavioral development and neurobiological disruptions in mice. Animals were treated from postnatal day (PND) 22 to 91. At PND 26-91 days of age, mice were tested for neurobehavioural development and neurochemical level changes. We showed that dietary exposure to environmentally contaminated rice gave rise to different changes in antioxidants. Reduced superoxide dismutase (SOD) activity and excess increased nitric oxide (NO) indicated aggravation of oxidative status after long-term dietary intake of Hg and PCBs. Neurobehavioral derangement in the central nervous system and significant delay in the Morris water maze test response on PND 91 are correlated with the increased of c-fos/c-jun expression levels in the cerebral cortex. These results suggest that MeHg neurotoxicity might be a greater hazard than that associated with PCB, but PCB may augment the neurobehavioral deficits caused by increased levels of mercury exposure. The simultaneous intake of selenium might have a protective effect on Hg accumulation in the body, and vitamin C might protect mice against the toxic effects of PCBs. However, the protective role of Se and vitamin C is very limited for multiple-agent pollution. Immediately early genes in the brain response to contaminated rice might be dependent on interaction among NO, NO synthase (NOS), SOD and reduced glutathione (GSH). We should be alert to mental health problems in human beings when any kind of Hg- and PCB-polluted food is consumed.

Involvement of the lymphocytic muscarinic acetylcholine receptor in methylmercury-induced c-Fos expression and apoptosis in human leukemic T cells

Methylmercury (MeHg) is an environmental toxicant that is known to induce lymphocyte apoptosis; however, little is known about the molecular mechanism involved. Data showed that MOLT-3 cells were more sensitive to MeHg-induced cytotoxic effects than Jurkat clone E6-1 cells, suggesting that the lymphocytic muscarinic cholinergic system may be involved since the expressions of five subtypes (M1-M5) of muscarinic acetylcholine receptor (mAChR) in MOLT-3 cells are higher than in Jurkat cells. The role of mAChR-linked pathways in MeHg-induced apoptosis in human leukemic T cells was examined in this study. Treatment of the MOLT-3 cells with 1 microM MeHg produced induction of c-Fos expression, apoptotic cell death, and downregulation of mAChR. MeHg-induced c-Fos expression was significantly reduced by pretreatment with atropine (a nonselective mAChR antagonist), or 4-DAMP (a selective M1/M3 mAChR antagonist), whereas pirenzipine (a selective M1 mAChR antagonist) or himbazine (a selective M2/M4 mAChR antagonist) did not reduce this induction, suggesting that MeHg-induced c-Fos expression through the activation of the mAChR, at least M3 subtype, is involved. Pretreatment with 4-DAMP or SB 203580 (a specific p38 inhibitor) resulted in decreases in the level of phosphorylated p38, c-Fos expression, and apoptotic cell death induced by MeHg. Taken together, these data suggest that the mAChR-p38-dependent pathway participates in the increase of c-Fos expression, which is involved in MeHg-induced lymphocyte apoptosis. In addition, a noncytotoxic concentration of MeHg (0.1 microM) inhibited PHA/PMA-stimulated interleukin (IL)-2 production, and this inhibition was reversed by pretreatment with atropine or 4-DAMP. Overall, this study provides initial evidence that MeHg may alter the immune system by targeting the lymphocytic mAChR.

Comparative cytotoxicity of cadmium and mercury in a human bronchial epithelial cell line (BEAS-2B) and its role in oxidative stress and induction of heat shock protein 70

A number of toxic heavy metals, such as cadmium (Cd) and mercury (Hg), are widely used in occupational settings, and exposure to these metals is associated with the development of pulmonary diseases. Cytotoxicity, apoptosis, and reactive oxygen species (ROS) generation were tested to compare the biological reactivity of these two heavy metals using a human bronchial epithelial cell line, BEAS-2B. Further, heat-shock protein 70 (Hsp70) expression was observed as a sensitive indicator of cellular stress. Exposure to metals (0-50 microM) for 72 h showed more significant cytotoxicity in Cd-treated than Hg-treated cells. Apoptosis was significantly increased in the cells exposed to 50 microM of Cd (3.5-fold) and Hg (3.6-fold). Cd and Hg produced an induction of Hsp70 protein as assayed by Western blotting and enzyme-linked immunosorbent assay (ELISA). Induction of Hsp70 protein by these metals was inhibited by addition of N-acetylcysteine. However, addition of catalase blocked the synthesis of Hsp70 only in Hg-treated cells. Hsp70B and Hsp70C mRNA expression was induced by both metals, while Hsp70A mRNA expression showed no change. Electron spin resonance (ESR) tests showed that hydroxyl radical generation was greater in the reaction of cells with Hg compared to Cd. Intracellular generation of ROS was detected in the cells exposed to both Cd and Hg. These results suggest that both cytotoxicity and apoptosis were significantly elevated with all metals tested; however, Cd was relatively more toxic. Hsp70 protein and mRNA were sensitive to exposure to these metals. Depletion of sulfhydryl groups of cellular proteins and generation of ROS may be involved in metal-induced lung cell damage.

Mercury pollution in two typical areas in Guizhou province, China and its neurotoxic effects in the brains of rats fed with local polluted rice

Guizhou province, which located in southwestern of China, is an important mercury (Hg) production center. This study was to investigate the environmental levels and ecological effects of mercury in two typical Hg polluted areas in Guizhou province. In addition, to improve the understanding of the neurotoxic effects of Hg, a rats based laboratory study was also carried out in this study. Samples of water, soil, plants, crops and animals collected from Wanshan mercury mine area, Guzhou province, were analyzed by mercury analyzer. The effects of Hg contaminated rice on the expression of c-jun mRNA in rat's brain and the expression of c-JUN protein in cortex, hippocampus were observed using reverse transcription polymerase chain reaction (RT-PCR) and immunocytochemical methods. The results showed that the mercury contents in most environmental samples of aquatics, soil, atmosphere and the biomass of corn, plant and animals, were higher than the national standard and the corresponding data from unpolluted area. It was found mercury pollutions were significant in soil and air. In the laboratory study, the expression of c-jun mRNA and its protein was significantly induced by Hg polluted rice collected from local area. Selenium could reduce the Hg accumulation in the body and had antagonist effect on Hg in terms of the expression of c-jun mRNA and c-JUN protein. The environmental data and Hg levels in different creatures collected in this study will facilitate the environmental and ecological risk assessment of Hg in the polluted areas. It was urged to be alert of mental health problem in human beings when any kind of Hg-polluted food was taken. More efforts should be performed to protect the local ecosystem and human health in the mercury polluted area of Wanshan, Guizhou province of China.

Involvement of c-Fos in fenretinide-induced apoptosis in human ovarian carcinoma cells

Fenretinide (HPR), a synthetic retinoid that exhibits lower toxicity than other retinoids, has shown preventive and therapeutic activity against ovarian tumors. Although the growth inhibitory effects of HPR have been ascribed to its ability to induce apoptosis, little is known about the molecular mechanisms involved. Since the proto-oncogene c-Fos has been implicated in apoptosis induction, we analyzed its role in mediating HPR response in a human ovarian carcinoma cell line (A2780) sensitive to HPR apoptotic effect. In these cells, HPR treatment caused induction of c-Fos expression, whereas such an effect was not observed in cells made resistant to HPR-induced apoptosis (A2780/HPR). Moreover, in a panel of other human ovarian carcinoma cell lines, c-Fos inducibility and HPR sensitivity were closely associated. Ceramide, which is involved in HPR-induced apoptosis, was also involved in c-Fos induction because its upregulation by HPR was reduced by fumonisin B(1), a ceramide synthase inhibitor. The causal relationship between c-Fos induction and apoptosis was established by the finding of an increased apoptotic rate in cells overexpressing c-Fos. Similarly to that observed for c-Fos expression, HPR treatment increased c-Jun expression in HPR-sensitive but not in HPR-resistant cells, suggesting the involvement of the transcription factor activating protein 1 (AP-1) in HPR-induced apoptosis. In gene reporter experiments, HPR stimulated AP-1 transcriptional activity and potentiated the AP-1 activity induced by 12-tetradecanoylphorbol 13-acetate. Furthermore, inhibition of AP-1 DNA binding, by transfecting A2780 cells with a dominant-negative Fos gene, caused decreased sensitivity to HPR apoptotic effects. Overall, the results indicate that c-Fos plays a role in mediating HPR-induced growth inhibition and apoptosis in ovarian cancer cells and suggest that c-Fos regulates these processes as a member of the AP-1 transcription factor.

Apoptotic response to growth factor deprivation involves cooperative interactions between c-Fos and p300

Two preneoplastic cell lines have been utilized to study changes in the regulation of apoptosis during neoplastic progression [sup+I (stage I) and sup-II (stage II)]. Sup+I cells are prone to undergo apoptosis, while sup-II cells are relatively resistant. We report that induction of apoptosis in sup+I cells is tightly correlated with the formation of c-Fos/p300 complexes, which were not present in the non-apoptotic sup-II cells under the same conditions. When apoptosis was induced in the sup-II cells by over-expression of c-Fos, concomitant c-Fos:p300 complexes were detected. Over-expression of p300 resulted in apoptosis in sup-II cells and also in p53wt human tumor cells, but not in p53mutant human tumor cells. Over-expression of the C-terminal fragment of p300, which contains the c-Fos binding site, enhanced apoptosis, suggesting that the c-Fos:p300 complex is actively involved in apoptosis. We propose that p300 could function as a general mediator of transcription factor-induced apoptosis.

Selective activation in the MAPK pathway by Hg(II) in precision-cut rabbit renal cortical slices

The kidneys are the primary organ for the accumulation and toxicity of inorganic mercury. In these studies the molecular response of precision-cut rabbit renal cortical slices to low levels of inorganic mercury was examined. Cortical slices (275 microm) were obtained from 1.0 kg NZW rabbits and exposed to mercuric chloride [Hg(II)] at concentrations of 0.01-10 microM for 2-8 h. Overt cytotoxicity, as assessed by intracellular K(+) levels, was not observed following exposure to these concentrations of Hg(II). However, an induction of heme-oxygenase-1 (Hsp32) was seen following a 2-h challenge to Hg(II). A dose-dependent induction of the DNA binding activity of the AP-1 transcription factor after 4 h of Hg(II) exposure correlated with a dose-dependent enhancement of c-jun gene expression following 2 h of Hg(II) exposure. Additionally, an increase in phosphorylated c-Jun NH(2)-terminal protein kinase (JNK) was observed following 2 h of Hg(II) exposure. These results suggest activation of the mitogen-activated protein (MAP) signal transduction pathway, specifically the c-Jun NH(2)-terminal protein kinase (JNK) pathway. No changes were observed, however, in the DNA binding activity of ATF2 and Elk-1, transcription factors involved in both the JNK and p38 pathways of MAP signal transduction, nor in the gene expression of c-myc. This selectivity of alterations in molecular signaling suggests an acute response in signal transduction, specifically activation of the JNK pathway in renal tissue following exposure to nanomolar concentrations of Hg(II).