Dose-dependent apoptosis induced by low concentrations of methylmercury in murine splenic Fas+ T cell subsets

Shedding New Light on Methylmercury-induced Neurotoxicity Through the Crosstalk Between Autophagy and Apoptosis

Methylmercury (MeHg) is a bio-accumulative global environmental contaminant present in fish and seafood. MeHg accumulates in the aquatic environment and eventually reaches the human system via the food chain by bio-magnification. The central nervous system is the primary target of toxicity and is particularly vulnerable during development. It is well documented that developmental MeHg exposure can lead to neurological alterations, including cognitive and motor dysfunction. Apoptosis is a primary characteristic of MeHg-induced neurotoxicity, and may be regulated by autophagic activity. However, mechanisms mediating the interaction between apoptosis and autophagy remains to be explored. Autophagy is an adaptive response under stressful conditions, and the basal level of autophagy ensures the physiological turnover of old and damaged organelles. Autophagy can regulate cell fate through different crosstalk signaling pathways. A complex interplay between autophagy and apoptosis determines the degree of apoptosis and the progression of MeHg-induced neurotoxicity as demonstrated by pre-clinical models and clinical trials. This review summarizes recent advances in the roles of autophagy and apoptosis in MeHg neurotoxicity and thoroughly explores the relationship between them. The autophagic pathway may be a potential therapeutic target in MeHg neurotoxicity through modulation of apoptosis.

Effects of Methylmercury on Harbour Seal Peripheral Blood Leucocytes In Vitro Studied by Electron Microscopy

Methylmercury (MeHg) is highly immunotoxic and can alter the health status of the harbour seal, Phoca vitulina, from the North Sea. To investigate the mechanism of MeHg-induced toxicity in harbour seal lymphocytes, Concanavalin A (ConA)-stimulated peripheral blood leucocytes were exposed in vitro to sublethal concentrations of MeHgCl (0.2, 1, and 2 µM) for 72 h and then analysed for their viability and ultrastructure. After 72 h of incubation, cells were counted with a propidium iodide staining technique, a metabolic MTS assay was performed, and cells exposed to 1 µM of MeHgCl were observed by transmission electron microscopy (TEM). Alive cell numbers decreased with increased MeHgCl concentrations. In presence of ConA and 1 µM of MeHgCl, TEM images revealed a higher frequency of apoptotic cells. Exposed cells displayed condensation of the chromatin at the nuclear membrane and mitochondrial damages. The results suggest that in vitro MeHgCl-induced apoptosis in harbour seal lymphocytes through a mitochondrial pathway.

Decreased immune response in zebra finches exposed to sublethal doses of mercury

Mercury (Hg) is a ubiquitous contaminant with deleterious effects on many wildlife species. Most studies to date have focused on fish-eating birds and mammals because much historical Hg pollution is aquatic. Recently, however, comparable blood-Hg levels have been found in terrestrial insectivorous songbirds. As a result, research is needed to clarify the effects of Hg exposure on songbirds. One fundamental end point that is still poorly understood is the effect of Hg on the songbird immune system. If Hg affects the functioning of the immune system, exposed songbirds may be less able to mount an appropriate immune response against invading pathogens. To gain insight into how Hg affects songbird immune function on a cellular level, a flow cytometric assay was developed to measure lipopolysaccharide-induced B-lymphocyte proliferation in zebra finches (Taeniopygia guttata). This is the first experimental (dosing) study of the potential effect of Hg on songbird immune system functioning. Decreased B cell proliferation was observed after lipopolysaccharide exposure in individuals with greater concentrations of Hg in their blood and tissues. In addition, these individuals had decreased ratios of proliferating-to-resting B cells. This decrease in lymphocyte proliferation in response to an effective mitogen suggests that environmental exposure to sublethal levels of Hg may inhibit or delay B cell proliferation in songbirds, potentially increasing susceptibility to disease and decreasing survivorship.

Caspase-10 is the key initiator caspase involved in tributyltin-mediated apoptosis in human immune cells

Tributyltin (TBT) is one of the most toxic compounds produced by man and distributed in the environment. A multitude of toxic activities have been described, for example, immunotoxic, neurotoxic, and endocrine disruptive effects. Moreover, it has been shown for many cell types that they undergo apoptosis after treatment with TBT and the cell death of immune cells could be the molecular background of its immunotoxic effect. As low as 200 nM up to 1 μM of TBT induces all signs of apoptosis in Jurkat T cells within 1 to 24 hrs of treatment. When compared to Fas-ligand control stimulation, the same sequence of events occurs: membrane blebbing, phosphatidylserine externalisation, the activation of the “death-inducing signalling complex,” and the following sequence of cleavage processes. In genetically modified caspase-8-deficient Jurkat cells, the apoptotic effects are only slightly reduced, whereas, in FADD-negative Jurkat cells, the TBT effect is significantly diminished. We could show that caspase-10 is recruited by the TRAIL-R2 receptor and apoptosis is totally prevented when caspase-10 is specifically inhibited in all three cell lines.

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.

Threshold Level of p53 Required for the Induction of Apoptosis in X-Irradiated MOLT-4 Cells

PURPOSE

To determine the threshold level for the initiation of apoptosis by studying the quantitative aspect of p53 response to DNA damage in individual cells, to better understand the process in X-ray-induced p53-dependent apoptosis.

METHODS AND MATERIALS

Time-sequential changes in p53 protein level were obtained for X-irradiated MOLT-4 cells using flow cytometry and analyzed.

RESULTS

The changes in the cellular frequency distribution pattern of p53 content could be divided into two parts at a certain p53 level. The p53 vs. side-scatter in flow cytometry showed the sequential changes of p53 increase followed by an increase in cell death. On the basis of these results we determined a threshold level of p53 for the initiation of apoptosis. The level was estimated to be (1.08 +/- 0.05) x 10(5) molecules per cell, which was approximately threefold higher than the mean content of control cells. The minimum times for p53 level to reach this threshold level were independent of X-ray dose and 1.4-1.6 h. The times for the signal transduction from the p53 accumulation to disruption of the mitochondrial membrane potential, caspase-3 activation, and cell death were 1.6, 2.1, and 2.8 h, respectively.

CONCLUSIONS

The threshold level of p53 for the initiation of apoptosis and the time sequence in the course of apoptotic events were determined in X-irradiated MOLT-4 cells.

Sensitivity to methylmercury-induced autoimmune disease in mice correlates with resistance to apoptosis of activated CD4+ lymphocytes

The sensitivity of splenic lymphoid cells to apoptosis induced by low concentrations of methylmercury (MeHgCl) has been examined in C57BL/6 and SJL mice, which are, respectively, resistant and sensitive to a genetically determined autoimmune disease induced by subtoxic doses of MeHgCl. To determine the implications of subtoxic doses of MeHgCl in the susceptibility of SJL mice to autoimmune disease, Concanavalin A (ConA) stimulated spleen cells from both mouse strains were treated in vitro with MeHgCl concentrations varying between 0.001 and 1.0 microM for 48h. Results have shown that ConA-activated splenic lymphoid cells from SJL mice increased in the presence of low concentrations of MeHgCl while the number of lymphoid cells from C57BL/6 mice rather decreased. Flow cytometric analysis of the cells showing a typical lymphoid forward scatter (FSC)/side scatter (SSC) pattern (region R1), and those characterized by a lower FSC and a higher SSC parameters (region R2), morphologically corresponding to apoptotic cells, revealed that lymphoid cells from C57BL/6 mice suffered a dose-dependent shift from region R1 toward region R2 when treated with concentrations ranging between 0.01 and 1 microM of MeHgCl. However, SJL splenic lymphoid cells cultured in the presence of low concentrations of MeHgCl proved more resistant to apoptosis. The level of apoptosis induced by MeHgCl in both regions was verified by AnnexinV-propidium iodide (PI) and TdT-mediated dUTP nick end labeling (TUNEL) immunolabelings. Phenotyping of lymphoid cells from both mouse strains cultured in the presence of low concentrations of MeHgCl and stimulated with ConA, indicated that CD4+ T cells from SJL mice increased while the corresponding cell subset from C57BL/6 mice became apoptotic. The resistance to apoptosis of ConA-activated lymphoid cells from SJL mice seemed related to an increase of CD4+ cells induced by the lower concentrations of MeHgCl (0.001 and 0.01 microM). However, these SJL cells were sensitive to anti-Fas-mediated apoptosis while residual anti-Fas-resistant cells from C57BL/6 mice were, themselves, sensitive to MeHgCl-induced apoptosis. The in vivo significance of these results has been confirmed by an observed increase in splenic cellularity and in the percentage of activated CD4+ cells from SJL mice. These increases were not observed in C57BL/6 mice.