Induction of apoptosis in human T-cells by organomercuric compounds: a flow cytometric analysis

EtHg is more toxic than MeHg to human peripheral blood mononuclear cells: Involvement of apoptotic, mitochondrial, oxidative and proliferative parameters

BACKGROUND

Methylmercury (MeHg) and ethylmercury (EtHg) are potent toxicants affecting the environment and human healthy. In this way, the present study aimed to investigate and compare the effects of MeHg and EtHg exposure on human peripheral blood mononuclear cells (PBMCs), which are critical components of the mammalian immune system.

METHODS

PBMCs were exposed to 2.5 μM MeHg or 2.5 μM EtHg. The number of cells and incubation times varied according to each assay. After exposures, the PBMCs were subjected to different evaluations, including cell viability, morphological aspects, cell cycle phases, indices of apoptosis and necrosis, reactive species (RS) production, and mitochondrial functionality.

RESULTS

PBMCs exposed to EtHg were characterized by decreased viability and size, increased granularity, RS production, and apoptotic indexes accompanied by an intensification of Sub-G1 and reduction in G0-G1 cell cycle phases. Preceding these effects, we found mitochondrial dysfunctions, namely a reduction in the electron transport system related to mitochondrial complex I. In contrast, PBMCs exposed to MeHg showed only reduced viability. By ICP-MS, we found that PBMCs treated with EtHg accumulated Hg + levels ∼1.8-fold greater than MeHg-exposed cells.

CONCLUSIONS AND SIGNIFICANCE

Taken together, our findings provide important insights about mercury immunotoxicity, showing that EtHg is more immunotoxic to human PBMCs than MeHg.

Dietary exposure to mercury and its relation to cytogenetic instability in populations from "La Mojana" region, northern Colombia

Fish consumption and chronic exposure to low doses of mercury (Hg) seems to activate several molecular mechanisms leading to carcinogenic and/or teratogenic processes. However, Hg genotoxic effects on humans are not completely described. In the present study, we assessed cytogenetic damage in isolated human peripheral lymphocytes using the cytokinesis-block micronucleus cytome assay (CBMN-Cyt), micronucleus formation with anti-kinetochore antibody (CREST staining), levels of total Hg in hair (T-Hg), fish consumption, and estimated Hg dose. The study comprised 39 non-exposed, and 73 residents from La Mojana region, an area with a well-documented Hg contamination. Data showed a significant increase in micronuclei (MNBN), nucleoplasmic bridges (NPB), and necrotic and apoptotic cell frequencies in residents of "La Mojana." The overall mean T-Hg level in hair for exposed residents was 1.12 ± 0.94 mg kg^-1 and 0.15 ± 0.05 in individuals from the reference area. Approximately 40% of analyzed individuals showed T-Hg levels that exceeded US Environmental Protection Agency (USEPA) reference dose. Increased T-Hg levels in hair were related to increased MNBN frequencies and high fish consumption. Other cellular markers, such as necrotic and apoptotic cell frequencies, were also correlated with high fish intake and T-Hg contents. Results of the CREST staining demonstrated that in vivo exposure to Hg induces genetic instability by chromosome fragment loss (clastogenic). Additionally, a high average intake of some fish species, particularly with carnivorous habits like Caquetaia kraussii, Hoplias malabaricus, and Sorubin cuspicaudus, seems to increase MNBN frequencies significantly.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Study of the Cytotoxic and Bactericidal Effects of Sila-substituted Thioalkyne and Mercapto-thione Compounds based on 1,2,3-Triazole Scaffold

A series of sila-organosulphur compounds containing 1,2,3-triazole cores were screened for their cytotoxic activity on human breast cancer cell line MCF-7. Most of the tested compounds exhibited moderate-to-good activity against the cancer cells. Especially, the compound 4-((2-(trimethylsilyl)ethynylthio)methyl)-1-benzyl-1H-1,2,3-triazole (3a) from series of sila-substituted thioalkyne 1,2,3-triazoles (STATs) and the compounds 3-(1-benzyl-1H-1,2,3-triazol-4-yl)-1-mercapto-1,1-bis(trimethylsilyl)propane-2-thione (4a) and 1-mercapto-1,1-bis(trimethylsilyl)-3-(1-phenethyl-1H-1,2,3-triazol-4-yl)propane-2-thione (4e) from series of sila-substituted mercapto-thione 1,2,3-triazoles (SMTTs) exhibited promising cytotoxicity against MCF-7 with IC₅₀ values of 35.17, 32.63 and 30.3 μg/mL, respectively. In addition, the possible mechanisms for inhibition of cell growth and induction of apoptotic cell death were explored by DAPI staining, cell cycle analysis and qRT-PCR. The synthetic compounds were evaluated for their in vitro antibacterial activities, and as a result, the most prominent effects were observed for 3e and 4e. Especially, 3e was found to be quite active against all the tested strains with the MIC values ranging from 15 to 62 μg/mL, except P. aeruginosa. The results of the time-kill assay suggested that the compound of 3e completely inhibited the growth of both gram-negative bacteria, A. baumannii, and gram-positive bacteria, S. aureus. In addition, SEM analysis confirmed morphostructural damage of the bacteria. Our findings could be applicable for developing dual-targeting anticancer/antibacterial therapeutics.

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.

Comparative analysis of hemocyte properties from Mytilus edulis desolationis and Aulacomya ater in the Kerguelen Islands

The capability of bivalve molluscs to respond to environmental stresses largely depends upon their cellular immunity. Accordingly, shift in habitat conditions following thermal stress or exposure to pollutants may harm sensitive species differently, thereby modulating the biodiversity of a given ecosystem by favoring stress-tolerant species. Here, we have compared the sensitivity of hemocytes from Mytilus edulis desolationis (M. edulis desolationis) and Aulacomya ater (A. ater) to acute thermal stress and exposure to cadmium. The two subantarctic species are commonly found in the same habitat in the isolated Kerguelen archipelago. Our results showed that the phagocytic activity and viability of hemocytes from both species were equally sensitive to increasing concentrations of cadmium. However, although in vitro exposure to cadmium induced apoptosis in hemocytes of M. edulis desolationis and A. ater, flow cytometric analyses showed that the apoptotic profile of both species differed greatly when using Annexin V and YO-PRO-1 as apoptotic markers. We also found that the total hemocyte counts decreased strongly in A. ater but not in M. edulis desolationis following an acute thermal stress. Taken together, these results showed that stress responses differed significantly in hemocytes from both species. This suggests that the co-existence of both species may be at risk following exposure to pollutants and/or changes in temperature.

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.

Apoptotic effect of eugenol in human colon cancer cell lines

Eugenol, a natural compound available in honey and various plants extracts including cloves and Magnoliae flos, is exploited for various medicinal applications. Since most of the drugs used in the cancer are apoptotic inducers, the apoptotic effect and anticancer mechanism of eugenol were investigated against colon cancer cells. Antiproliferative effect was estimated using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay]. Earlier events like MMP (mitochondrial membrane potential), thiol depletion and lipid layer break were measured by using flow cytometry. Apoptosis was evaluated using PI (propidium iodide) staining, TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling) assay and DNA fragmentation assay. MTT assay signified the antiproliferative nature of eugenol against the tested colon cancer cells. PI staining indicated increasing accumulation of cells at sub-G1-phase. Eugenol treatment resulted in reduction of intracellular non-protein thiols and increase in the earlier lipid layer break. Further events like dissipation of MMP and generation of ROS (reactive oxygen species) were accompanied in the eugenol-induced apoptosis. Augmented ROS generation resulted in the DNA fragmentation of treated cells as shown by DNA fragmentation and TUNEL assay. Further activation of PARP (polyadenosine diphosphate-ribose polymerase), p53 and caspase-3 were observed in Western blot analyses. Our results demonstrated molecular mechanism of eugenol-induced apoptosis in human colon cancer cells. This research will further enhance eugenol as a potential chemopreventive agent against colon cancer.

Molecular mechanisms of methylmercury-induced cell death in human HepG2 cells

Methylmercury (MeHg) has been suggested to exert cytotoxicity through multiple mechanisms, but the precise biochemical machinery has not been fully defined. This study was aimed at investigating the time-course (0-24h) effect of 2mg/L MeHg on cell death in human HepG2 cells. MeHg decreased cell viability in a time-dependent manner, which was concomitant with increased LDH leakage, reduced GSH levels, CAT activity and altered activity of the antioxidant enzymes GPx and GR at the longest times of incubation (16 and 24h). Activity of the detoxifying enzyme GST was also early enhanced (2h). Caspase-3 activity reached a maximum value at 8h and continued increased up to 24h. This feature was preceded by an enhancement in the caspase-9 activity (2h), whereas caspase-8 activity remained unchanged. MeHg early diminished Bcl-x(L)/Bcl-x(S) ratio and increased levels of the pro-apoptotic Bax and Bad. Moreover, MeHg-induced cytotoxicity was completely inhibited by the antioxidants (GSH and NAC) and notably by the mitochondrial complex I inhibitor rotenone, but not by the NADH oxidase inhibitor DPI. In summary, MeHg induced an oxidative stress responsible for apoptosis in HepG2 cells through direct activation of the caspase cascade and altered the cellular antioxidant and detoxificant enzymatic system to later provoke necrosis at later stages.

Involvement of non-protein thiols, mitochondrial dysfunction, reactive oxygen species and p53 in honey-induced apoptosis

Honey is a complex mixture of different biologically active constituents. Honey possesses anti-inflammatory, antioxidant and antitumor properties. Our chief investigation was to assess the honey induced apoptosis and its molecular mechanism in colon cancer cell growth inhibition. Honey exerted antiproliferative potential against the HCT-15 and HT-29 colon cancer cells as assessed by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT) assay. Flow cytometric analysis showed the increasing accumulation of hypodiploid nuclei in the sub-G(1) phase of cell cycle indicating apoptosis. Honey transduced the apoptotic signal via initial depletion of intracellular non protein thiols, consequently reducing the mitochondrial membrane potential (MMP) and increasing the reactive oxygen species (ROS) generation. An increasing earlier lipid layer break was observed in the treated cells compared to the control. Honey induced apoptosis was accompanied by up-regulating the p53 and modulating the expression of pro and anti-apoptotic proteins. Further apoptosis induction was substantiated using DNA fragmentation assay and YO-PRO-1 staining. Results showed honey as a plausible candidate for induction of apoptosis through ROS and mitochondria-dependent mechanisms in colon cancer cells. This will promote honey as a potential chemotherapeutic agent against colon cancer.

Immune function effects of dental amalgam in children: a randomized clinical trial

BACKGROUND

Dental amalgam is a widely used restorative material containing 50 percent elemental mercury that emits mercury vapor. No randomized clinical trials have determined whether there are adverse immunological effects associated with this low-level mercury exposure in children. The objective of this study was to evaluate a subpopulation of the participants in the New England Children's Amalgam Trial for in vitro manifestations of immunotoxic effects of dental amalgam.

METHODS

The authors conducted a randomized clinical trial in which children requiring dental restorative treatment were randomly assigned to receive either amalgam for posterior restorations or resin-based composite restorations. They assessed 66 children, aged 6 to 10 years, for total white blood cell counts, specific lymphocyte (T-cell and B-cell) counts and lymphocyte, neutrophil and monocyte responsiveness across a five-year period. Because of the small number of participants, the authors acknowledge that the study is exploratory in nature and has limited statistical power.

RESULTS

The mean number of tooth surfaces restored during the five-year period was 7.8 for the amalgam group and 10.1 for the composite group. In the amalgam group, there was a slight, but not statistically significant, decline in responsiveness of T cells and monocytes at five to seven days after treatment; the authors consistently observed no differences at six, 12 or 60 months.

CONCLUSIONS

The findings of this study confirm that treatment of children with amalgam restorations leads to increased, albeit low-level, exposure to mercury. In this exploratory analysis of immune function, amalgam exposure did not cause overt immune deficits, although small transient effects were observed five to seven days after restoration placement.

CLINICAL IMPLICATIONS

These findings suggest that immunotoxic effects of amalgam restorations are minimal and transient in children and most likely do not need to be of concern to practitioners considering the use of this restorative dental material.

Methylmercury Induces Alveolar Macrophages Apoptosis

Through the use of a scanning electronic microscope, it was found that alveolar macrophages treated with 10 μM of methylmercury for 24 h showed a decrease of surface microvilli, and those treated with 15 μM of methylmercury underwent deformity and subsequent cell death. To investigate their death patterns, DNA was aspirated from alveolar macrophages and analyzed by electrophoresis. It was discovered that the DNA ladder phenomenon became more obvious as the methylmercury increased in concentration. When 5 mM EGTA was used to eliminate calcium ions, a decrease of the ladder phenomenon was observed. Zinc at 1 mM had a similar inhibitory effect. Moreover, an apoptosis peak was observed on flow cytometry analysis of DNA stained with propidium iodide. Alveolar macrophages stained with Hoechst 33342 demonstrated apoptotic bodies induced by methylmercury. The above data indicate that methylmercury can induce a typical apoptosis in alveolar macrophages. Continuing onto the study of the mechanism of apoptosis as induced by methylmercury in alveolar macrophages, it was discovered that methylmercury could increase the intracellular calcium ion concentration and decrease the pH in alveolar macrophages. To find out which endonuclease was responsible for the methylmercury-induced DNA fragmentation of alveolar macrophages, the nuclear proteins of alveolar macrophages was aspirated and tested under different pH values and in conditions with or without calcium ions, and it was discovered that the endonuclease was calcium dependent without relations to pH values.

Acute depletion and recovery of peritoneal B-1 lymphocytes in BALB/c mice after a single injection of mercury chloride

The acute toxicity of mercury (Hg) to B cells was studied in the peritoneal cavity of BALB/c mice, a coelomic space where both B-1 and B-2 subsets of B lymphocytes are present. Up to 24 hr after a single in situ Hg injection, the peritoneal cavity became virtually devoid of lymphocytes, particularly of the B-1 subset. Lymphocyte depletion was more severe for B than T cells. This depletion was associated with partial lymphocyte activation (CD69(+)) at 6 hr of treatment and it was due to apoptosis rather than to necrosis. Partial recovery of both B and T cells was observed in the peritoneal cavity 48 hr after the Hg injection. The phenomenon was followed by a second decrease in peritoneal lymphocytes 72 hr after Hg. Neutrophils that entered the peritoneal cavity because of the Hg injection were resistant to apoptosis. No significant changes in lymphocyte number or subpopulation were found in the spleen and thymus of the mice up to 72 hr after the Hg treatment. We concluded that B lymphocytes were severely affected by the toxic effects of Hg. Our data suggest that Hg-induced unbalance in the repertoire of B cells, of the B-1 subset in particular, may result later in the secretion of the high titres of pathogenic autoantibodies that are found in the Hg-induced lupus disorder of BALB/c mice.

Cell death and cytotoxic effects in YAC-1 lymphoma cells following exposure to various forms of mercury

The effects of 1 min-4 h exposures to four Hg compounds (mercuric chloride [HgCl2], methyl mercuric chloride [CH3HgCl], p-chloromercuribenzoate [p-CMB] and thimerosal [TMS; ethylmercurithiosalicylate]) on cell death, microtubules, actin, CD3 receptor expression, protein tyrosine phosphorylation (PTyr-P) and intracellular calcium ([Ca2+]i) levels were investigated in YAC-1 lymphoma cells using flow cytometry. YOPRO-1 (YP) and propidium iodide (PI) dye uptake indicated all forms of Hg tested were toxic at concentrations ranging from 25.8-48.4 microM, with two distinct patterns of effects. Early apoptosis was prolonged for CH3HgCl- and TMS-treated cells, with more than 50% remaining YP+/PI- after 4h. Both CH3HgCl and TMS induced complete loss of beta-tubulin fluorescence, indicative of microtubule depolymerization and inhibition of tubulin synthesis and/or beta-tubulin degradation, while F-actin fluorescence diminished to a lesser degree and only after loss beta-tubulin. CH3HgCl and TMS induced an almost immediate two-fold increase in CD3 fluorescence, with levels returning to baseline within minutes. With continued exposure, CD3 fluorescence was reduced to approximately 50% of baseline values. Both compounds also increased PTyr-P two- to three-fold immediately, with levels returning to baseline at 4h. Similarly, two- to three-fold increases in [Ca2+]i were noted after 1 min exposure. [Ca2+]i increased progressively, reaching levels five- to eight-fold greater than control values. In contrast, dye uptake was delayed with HgCl2 and p-CMB, although cell death proceeded rapidly, with almost all non-viable cells being late apoptotic (YP+/PI+) by 4h. p-CMB produced early reductions in F-actin, and after 4h, complete loss of F-actin with only partial reduction of total beta-tubulin was seen with both p-CMB and HgCl2. HgCl2 reduced CD3 expression and PTyr-P slightly within minutes, while p-CMB produced similar effects on CD3 only at 4h, at which time PTyr-P was increased two- to three-fold. Both compounds increased [Ca2+]i within minutes, though levels remained under twice the baseline concentration after 15 min exposure. With continued exposure, [Ca2+]i increased to levels two- to five-fold greater than control values. These findings indicate the two groups of Hg compounds may induce cell death by distinct pathways, reflecting interactions with different cellular targets leading to cell death.

Stimulation of erythrocyte phosphatidylserine exposure by mercury ions

The sequelae of mercury intoxication include induction of apoptosis. In nucleated cells, Hg2+-induced apoptosis involves mitochondrial damage. The present study has been performed to elucidate effects of Hg2+ in erythrocytes which lack mitochondria but are able to undergo apoptosis-like alterations of the cell membrane. Previous studies have documented that activation of a Ca2+-sensitive erythrocyte scramblase leads to exposure of phosphatidylserine at the erythrocyte surface, a typical feature of apoptotic cells. The erythrocyte scramblase is activated by osmotic shock, oxidative stress and/or energy depletion which increase cytosolic Ca2+ activity and/or activate a sphingomyelinase leading to formation of ceramide. Ceramide sensitizes the scramblase to Ca2+. The present experiments explored the effect of Hg2+ ions on erythrocytes. Phosphatidylserine exposure after mercury treatment was estimated from annexin binding as determined in FACS analysis. Exposure to Hg2+ (1 microM) indeed significantly increased annexin binding from 2.3+/-0.5% (control condition) to 23+/-6% (n=6). This effect was paralleled by activation of a clotrimazole-sensitive K+-selective conductance as measured by patch-clamp recordings and by transient cell shrinkage. Further experiments revealed also an increase of ceramide formation by approximately 66% (n=7) after challenge with mercury (1 microM). In conclusion, mercury ions activate a clotrimazole-sensitive K+-selective conductance leading to transient cell shrinkage. Moreover, Hg2+ increases ceramide formation. The observed mechanisms could similarly participate in the triggering of apoptosis in nucleated cells by Hg2+.

Mercury impairment of mouse thymocyte survival in vitro: involvement of cellular thiols

Heavy metals are well known to be able to induce immunotoxicity, but comparative metal studies related to apoptosis have not been conducted. In the present study, the effects of arsenic, cadmium, gold, lead, manganese, and mercury on thymocytes from BALB/c mice were analyzed. Thymic cells were cultured for 3-24 h in vitro in the absence or presence of metal, and markers of apoptosis or cell death, including annexin V binding, DNA loss/oligonucleosomal fragmentation, 7-amino-actinomycin D uptake (loss of impermeance), changes of the mitochondrial membrane potential (JC-1 fluorescence), and Western analysis of cellular thiols, were assayed. Mercury (Hg) was the only metal shown to be consistently toxic with the dose and times utilized. Cadmium (Cd) was the only other metal tested that also produced some significant level of DNA loss; however, the induction of apoptosis by Cd was not as consistent as that observed with Hg. When Hg was added with 2-mercaptoethanol (2-ME), Hg produced greater toxicity. Endogenous DNA synthesis by thymocytes was immediately inhibited by Hg and Hg + 2-ME. The Hg + 2-ME-induced apoptosis appeared to be associated with altered levels of cellular thiols, in that glutathione (GSH) depletion was significant in comparison to the non-metal control and Hg alone. The increased Hg-induced toxicity in the presence of 2-ME likely was due to the ability of 2-ME to enhance (10- to 20-fold) the cellular uptake of Hg. Western analysis with biotin maleimide demonstrated that Hg + 2-ME and to a lesser extent the positive control dexamethasone eliminated many reactive thiols; the major thiol-reactive protein still reactive with the maleimide probe had an approximate Molecular Mass of 45 kD. Surprisingly, Hg alone enhanced the expression of this thiol-expressing protein, which by Mass Spectrometry (MS)/MS analysis was shown to be beta-actin. Hg also produced the appearance of yet to be identified new proteins. Based on the results with Hg + 2-ME, it is suggested that numerous protein thiols participate in maintenance of cell survival and their loss is associated with apoptosis. The increased expression of new thiol-reactive proteins or thiol-reactive proteins with altered electrophoretic profiles needs to be further investigated. However, the enhanced toxicity attributed to Hg + 2-ME suggests that increased intracellular oxidative stress, observed as increased depletion of GSH, is responsible for the accelerated cell death.

Comparison of cytopathological changes induced by mercury chloride exposure in renal cell lines (VERO and BGM)

The response to mercury chloride was assessed in two cell lines of renal origin, determining the range of toxic concentrations by Neutral Red assay after 24-h of exposure. Morphological changes in the Buffalo Green Monkey (BGM) and VERO cell lines after exposure to subcytotoxic doses (0.045 and 0.038mM, respectively) equivalent to EC10 (effective concentrations 10%) of mercury chloride were evaluated at the structural and ultrastructural level by optic, transmission and scanning microscopy. Using transmission electron microscopy, the most notable findings in treated cells were the presence of intracytoplasmic inclusion bodies and apoptotic bodies. Scanning microscopy pointed to a cell with a disrupted perinuclear region and a decreased number of surface microvilli. Similar alterations in both in vivo and in vitro experiments have been described by other authors. We conclude that BGM and VERO renal cell lines can be considered as useful tools for toxicological studies involving mercury chloride.

Plasma membrane aquaporin activity can affect the rate of apoptosis but is inhibited after apoptotic volume decrease

Apoptosis is characterized by a conserved series of morphological events beginning with the apoptotic volume decrease (AVD). This study investigated a role for aquaporins (AQPs) during the AVD. Inhibition of AQPs blocked the AVD in ovarian granulosa cells undergoing growth factor withdrawal and blocked downstream apoptotic events such as cell shrinkage, changes in the mitochondrial membrane potential, DNA degradation, and caspase-3 activation. The effects of AQP inhibition on the AVD and DNA degradation were consistent in thymocytes and with two additional apoptotic signals, thapsigargin and C6-ceramide. Overexpression of AQP-1 in Chinese hamster ovary (CHO-AQP-1) cells enhanced their rate of apoptosis. The AVD is driven by loss of K+from the cell, and we hypothesize that after the AVD, AQPs become inactive, which halts further water loss and allows K+concentrations to decrease to levels necessary for apoptotic enzyme activation. Swelling assays on granulosa cells, thymocytes, and CHO-AQP-1 cells revealed that indeed, the shrunken (apoptotic) subpopulation has very low water permeability compared with the normal-sized (nonapoptotic) subpopulation. In thymocytes, AQP-1 is present and was shown to colocalize with the plasma membrane receptor tumor necrosis factor receptor-1 (TNF-R1) both before and after the AVD, which suggests that this protein is not proteolytically cleaved and remains on the cell membrane. Overall, these data indicate that AQP-mediated water loss is important for the AVD and downstream apoptotic events, that the water permeability of the plasma membrane can control the rate of apoptosis, and that inactivation after the AVD may help create the low K+concentration that is essential in apoptotic cells. Furthermore, inactivation of AQPs after the AVD does not appear to be through degradation or removal from the cell membrane.

Effect of vitamin B6 on oxygen radicals, mitochondrial membrane potential, and lipid peroxidation in H2O2-treated U937 monocytes

Vitamin B6 (Vit.B6) supplementation has been shown to be beneficial in reducing diabetic complications, cognitive aging, and in the prevention of coronary heart disease. It was hypothesized that Vit.B6 compounds may function as antioxidants and thus offer protection against oxidative stress under various pathophysiological and or experimental conditions. To test this hypothesis, U937 monocytes were cultured with pyridoxine (P), pyridoxal phosphate (PP) and pyridoxamine (PM) and H2O2, either alone or together for 2 h. Oxidative stress was determined by measuring superoxide radical production, lipid peroxidation, and mitochondrial transmembrane potential. Results demonstrate that Vit.B6 compounds can prevent the oxygen radical generation and lipid peroxidation caused by hydrogen peroxide in U937 monocytes, and that some of the protective effect of Vit.B6 may occur via modification of mitochondrial function.

Contrasting changes of sensitivity by lymphocytes and neutrophils to mercury in developing grey seals

Mercury is the principal metal contaminant in the St Lawrence Estuary. It impairs humoral, cellular and non-specific immune responses in many species. Since the immune system of juvenile seals is immature, it should react differently to the effects of contamination compared to that of mature animals. Phagocytosis and lymphoblastic transformation responses have been evaluated in the peripheral blood leukocytes of eight juvenile grey seals at different intervals of time over 11 weeks. Dose-response curves of 10(-9)-10(-3)M of methylmercury chloride have also been performed in vitro for evaluation of these two immune functions. The immune response of grey seals differs during their development. The phagocytosis response increased from the 2nd to 5th week post-weaning and then reached a plateau. As for the lymphoblastic transformation response, it was stable from the 2nd to the 3rd week post-weaning, increased significantly at week 4 post-weaning and then reached a plateau. These data suggest that these animals should be particularly vulnerable to infections, diseases and parasites before the 5th week post-weaning. Furthermore, mercury decreased the immune response, and age of seals had an effect on cell sensitivity to mercury. Concentrations of 10(-5)M of methylmercury chloride decreased phagocytosis and lymphoblastic transformation responses. Phagocytosis is more affected by MeHgCl contamination before this immune function reaches complete development which occurs at week 5 post-weaning. On the other hand, lymphoblastic transformation is more affected by this contaminant after its complete development which occurs at week 4 post-weaning.

Morphological characterisation of BGM (Buffalo Green Monkey) cell line exposed to low doses of cadmium chloride

Morphological changes in the Buffalo Green Monkey (BGM) cell line after exposure to a subcytotoxic dose (0.062 mM, equivalent to EC(10)-effective concentration 10%) of cadmium chloride have been evaluated. Cells were exposed for 24 h and the effects observed at the ultrastructural level by transmission and scanning microscopy. Using transmission electron microscopy, the most notable findings in treated cells were the presence of intranuclear inclusion bodies and thin intracytoplasmic granules associated to myelin figures and the presence of apoptotic bodies. Other morphological alterations included cell vacuolisation and a reduced cytoplasm volume, condensation of the mitochondria and a decreased number of cytoplasmic organelles, except lysosomes and autophagic vacuoles, which increased in number. Scanning electron microscopy pointed to a cell with a disrupted perinuclear region and a decrease in the number of surface microvilli. We conclude that the BGM cell line may be considered an useful tool for toxicological studies involving cadmium.

Oxygen radical generation and endosulfan toxicity in Jurkat T-cells

The mechanism by which endosulfan exposure causes cellular dysfunction in experimental animals and humans is not clear. In the present study, we provide experimental evidence in support of the role of oxidative stress in endosulfan toxicity. Using both cell free system and Jurkat cells as in vitro models, we demonstrate that endosulfan can generate oxygen radicals that is inhibitable by superoxide dismutase (SOD) and glutathione (GSH). In the cell culture model, oxygen radical generation in response to endosulfan was dose- (0-100 microM) and time-dependent (0-12 h). Two-color flowcytometric analysis showed that endosulfan mediated changes in delta psi(m) and generation of superoxide radicals do occur simultaneously in the affected cell population. It was hypothesized that endosulfan exerts a severe oxidative stress in Jurkat cells and this could be prevented or minimized by an antioxidant. To test this hypothesis, GSH was added exogenously and endosulfan toxicity was evaluated by alamarblue assay. In conclusion, our results demonstrate a role for reactive oxygen species (ROS) in endosulfan toxicity and that supplementation of antioxidants such as GSH may be useful in individuals who are at risk to endosulfan toxicity.

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.

Mercuric chloride induces apoptosis via a mitochondrial-dependent pathway in human leukemia cells

Mercurial compounds modulate immunologic functions by inducing cytotoxicity. Although mercury chloride (HgCl(2)) is known to induce apoptosis in various immune system cells, the mechanism of the induction of apoptosis is poorly understood. In this study, we examined the activation of caspase-3, an important cysteine aspartic protease, during HgCl(2)-induced apoptosis in a human leukemia cell line (HL-60 cells). Both DNA fragmentation, a characteristic of apoptotic cells, and proteolysis of poly(ADP-ribose) polymerase (PARP), a substrate of caspase-3, occurred at 6 h after HgCl(2) treatment in HL-60 cells. These results suggest that the activation of caspase-3 was involved in HgCl(2)-induced apoptosis. The release of cytochrome c (Cyt c) from mitochondria into the cytosol, which is an initiator of the activation of caspase cascades, was also observed in HgCl(2)-treated HL-60 cells. Moreover, the release of Cyt c from mitochondria was observed in HgCl(2)-treated mitochondria isolated from mice liver, and this was followed by mitochondrial permeability transition (PT). The PT was inhibited by cyclosporin A (CsA), a potent inhibitor of PT. CsA also suppressed the occurrence of DNA fragmentation induced by HgCl(2) treatment in HL-60 cells. Taken together, these findings indicate that HgCl(2) is a potent inducer of apoptosis via Cyt c release from the mitochondria in HL-60 cells.

Ixodes ricinus tick salivary gland extract inhibits IL-10 secretion and CD69 expression by mitogen-stimulated murine splenocytes and induces hyporesponsiveness in B lymphocytes

Tick saliva contains immunosuppressive factors allowing this blood-feeding ectoparasite to remain on hosts and enhancing pathogen transmission. In this study, we examined the modulation of mitogen-induced activation of naive murine splenocytes by the saliva and salivary gland extract (SGE) of I. ricinus ticks. We found that saliva-specific factors reduced IL-10 production by both concanavalin A (ConA) and lipopolysaccharide (LPS) stimulated splenocytes. The LPS-induced IL-10 production is 10 times more sensitive to SGE than the ConA-induced IL-10 production. Flow cytometric analysis determined that SGE particularly inhibited B (B220+) cell IL-10 production in mitogen-stimulated splenocyte preparations. Moreover, SGE reduced the early activation marker CD69 expression on ConA-activated T cells and also on B cells in presence of ConA or LPS. Annexin V and Via-probe staining demonstrated that SGE did not increase cell death in activated splenocytes and slightly decreased apoptosis in B lymphocytes. By employing assays with isolated B cells, we further showed that SGE had a direct effect on B cells and inhibited LPS-induced B cell proliferation. Taken together, our results indicate that salivary immunomodulators induce hyporesponsiveness to mitogen in both T and B cells, and that a direct B-cell inhibitory activity is present in tick saliva.

Cellular specificity related to monoglyceride-induced cell death

We have recently observed that monoglycerides (MGs), a family of lipids consisting of a single fatty acid moiety attached to a glycerol backbone, induce rapid dose-dependent apoptosis in murine thymocytes. In this work, we evaluated the sensitivity of various normal and malignant immune and non-immune cells to MGs. We demonstrate that the propensity to MG-induced death displayed by both T and B lymphocytes is clearly modulated according to their differentiation and activation status. For instance, the earliest T and B cell precursors are refractory to MG-mediated cell death. In the T-cell lineage, immature thymocytes are the most susceptible to MG treatment, while B cells from peripheral lymphoid organs appear more sensitive than B-cell subsets from the bone marrow. On the other hand, both activated T and B cells are more resistant to MG exposure than their non-activated counterparts. In addition, other hematopoietic lineages such as natural killer cells, macrophages, and erythroid cells are quite resistant to MG-induced death. Furthermore, using various immortalized cell lines from different tissues, we found that lymphomas and thymomas are the most sensitive among all lineages tested, while epithelial cells and fibroblasts are unaffected by MG treatment. Finally, MG-induced death was shown to be independent of Fas/Fas ligand (FasL) interactions. Altogether, our findings indicate that there is a cellular specificity related to MG-mediated cell death biased towards T and B lymphocytes. This suggests that MGs could potentially be used in the treatment of specific lymphoid disorders by bypassing the requirement for the Fas/FasL system.

Effect of arsenic, cadmium and lead on the induction of apoptosis of normal human mononuclear cells

The aim of this work was to investigate the effect of cadmium, lead and arsenic on the apoptosis of human immune cells. Peripheral blood mononuclear cells (MNC) were incubated with increasing concentrations of these metals and then cellular apoptosis was determined by flow cytometry and by DNA electrophoresis. We found that arsenic induced a significant level of apoptosis at 15 microM after 48h of incubation. Cadmium had a similar effect, but at higher concentrations (65 microM). In addition, cadmium exerted a cytotoxic effect on MNC that seemed to be independent of the induction of apoptosis. In contrast, concentrations of lead as high as 500 microM were nontoxic and did not induce a significant degree of apoptosis. Additional experiments showed that arsenic at concentrations as low as 1.0 microM had a significant pro-apoptotic effect when cells were cultured in the presence of this pollutant for more than 72. Non-T cells were more susceptible than T lymphocytes to the effect of arsenic and cadmium. Interestingly, MNC from children chronically exposed to arsenic showed a high basal rate of apoptosis and a diminished in vitro sensibility to this metalloid. Our results indicate that both arsenic and cadmium are able to induce apoptosis of lymphoid cells, and suggest that this phenomenon may contribute to their immunotoxic effect in vivo.

Mercury-induced apoptosis in human lymphocytes: caspase activation is linked to redox status

There is growing evidence that heavy metals, in general, and mercurial compounds, in particular, are toxic to the human immune system. We have previously shown that methyl mercuric chloride (MeHgCl) is a potent human T-cell apoptogen; moreover, mitochondria appear to be a target organelle for the induction of cell death. The objective of this study was to determine the impact of MeHgCl on mitochondrial function in lymphocytes in terms of modulating reactive oxygen species (ROS) generation, thiol status, and caspase activation. Using the fluorescent probe, 3,3'-dihexyloxacarbocyanine, we demonstrated that exposure to MeHgCl for 1 h resulted in a profound decrease in the mitochondrial transmembrane potential. We next observed the release of cytochrome c from mitochondria into the cytosol; significant translocation was noted between 4 and 8 h following treatment with mercury. ROS generation was monitored by following the conversion of dihydroethidium to the fluorescent product, ethidium. Kinetic analysis indicated that ROS generation was maximal after 16 h of exposure to MeHgCl. The toxicant also depleted the thiol reserves of the cell; glutathione levels were depleted in a dose-dependent fashion reaching minimal levels at 16 h. Real-time RT-PCR analysis demonstrated a significant reduction in both glutathione S-transferase and glutathione peroxidase gene expression in mercury-treated cells. Finally, after 16 h of treatment with MeHgCl, we observed activation of caspase-8, -9, and -3 along with increased expression of caspase-8 and -9. We propose that the target organelle for MeHgCl is the mitochondrion and that induction of oxidative stress is critical to activation of death-signaling pathways. Additonally, mercury acts as a genotoxin significantly altering the expression of genes that affect cell survival and apoptosis.

Prolongation of human neutrophil survival by low-level mercury via inhibition of spontaneous apoptosis

Low levels of organic and inorganic mercury compounds have been reported previously to induce cell death by apoptosis in human peripheral blood mononuclear cells (MNC). but little is known about their potential effects on the viability and death of polymorphonuclear neutrophils (PMN). In contrast to MNC, PMN are known to undergo readily spontaneous apoptosis both in vivo and in vitro. Therefore, it was hypothesized that PMN may differ from MNC in their reactions to low mercury levels. The effects of methylmercuric chloride (MeHgCl) and mercuric chloride (HgCl2) were evaluated in concentration-response and time-course studies on human PMN viability and on their modes of cell death after in vitro incubation at 37 degrees C. Cell death by apoptosis or necrosis was assessed by annexin V-fluorescein isothiocyanate binding to externalized phosphatidylserine in conjunction with propidium iodide, and flow cytometry analysis. Morphologic counting of pyknotic nuclei and the fluorescence properties of the DNA-binding dye Hoechst 33342 in combination with propidium iodide were used to further confirm apoptotic cell death and to characterize the sequence of Hg-induced cell death. Results show that low concentrations of MeHgCl (1-7.5 microM) that were cytotoxic to MNC actually inhibited PMN spontaneous apoptosis. Low-level HgCl, reproduced the anti-apoptotic effects of MeHgCl on PMN, but to a lower extent. Higher concentrations of MeHgCl and HgCl2 were necrogenic to PMN, but MeHgCl was about an order of magnitude more toxic, and discrete differences were observed in the modalities of cell death induced by both species. These data reveal for the first time that (1) low levels of organic and inorganic mercury species protect human PMN from cell death via inhibition of spontaneous apoptosis, and (2) PMN are more resistant than MNC to mercury-induced cytotoxicity. Since delayed apoptosis and increased resistance to toxicant-induced cell death may lead to excessive accumulation of senescent PMN, evidence indicates that findings of this study may have implications for mercury-induced autoimmunity and inflammation.

p21(WAF1/CIP1) inhibits cell cycle progression but not G2/M-phase transition following methylmercury exposure

Methylmercury (MeHg) is an environmentally prevalent organometal that is particularly toxic to the developing central nervous system (CNS). Prenatal MeHg exposure is associated with reduced brain size and weight and a reduced number of neurons, which have been associated with impaired cell proliferation. We evaluate the role of p21, a cell cycle protein involved in the G1- and G2-phase checkpoint control, in the cell cycle inhibition induced by MeHg. Primary mouse embryonic fibroblasts (MEFs) of different p21 genotypes (wild-type, heterozygous, and null) were isolated at day 14 of gestation and treated at passages 4-6 with either 0, 2, 4, or 6 microM MeHg or 50 nM colchicine for 24 h. Changes in cell cycle distribution after continuous toxicant treatment were analyzed by DNA content-based flow cytometry using DAPI. MeHg induced an increase in the proportion of cells in G2/M at 2 and 4 microM MeHg (p < or = 0.05) irrespective of p21 genotype. Effects of MeHg on cell cycle progression were subsequently evaluated using BrdU-Hoechst flow cytometric analysis. Inhibition of cell cycle progression was observed in all p21 genotypes after continuous exposure to MeHg for 24 and 48 h. p21 null (-/-) cells reached the second-round G1 at a higher fraction compared to the wild type (+/+) and heterozygous (+/-) cells (p < or = 0.05). These data support previous observations that MeHg inhibits cell cycle progression through delayed G2/M transition. Whereas the G2/M accumulation induced by MeHg was independent of p21 status, a greater proportion of p21(-/-) cells were able to complete one round of cell division in the presence of MeHg compared to p21(+/-) or p21(+/+) cells. These data suggest a role for p21 in retarding cell cycle progression, but not mitotic inhibition, following exposure to MeHg.

Induction of apoptosis in human T cells by Actinobacillus actinomycetemcomitans cytolethal distending toxin is a consequence of G2 arrest of the cell cycle

We have previously shown that Actinobacillus actinomycetemcomitans produces an immunosuppressive factor that is encoded by the cdtB gene, which is homologous to a family of cytolethal distending toxins (Cdt) expressed by several Gram-negative bacteria. Moreover, we have shown that CdtB impairs lymphocyte function by inducing G(2) arrest of the cell cycle. We now report that both CdtB as well as an extract prepared from an Escherichia coli strain that expresses all three of the A. actinomycetemcomitans cdt genes (rCdtABC) induce apoptosis. Pretreatment of lymphocytes with either CdtB or rCdtABC leads to DNA fragmentation in activated lymphocytes at 72 and 96 h. No DNA fragmentation was induced in nonactivated cells. Flow cytometric analysis of the Cdt-treated lymphocytes demonstrates a reduction in cell size and an increase in nuclear condensation. Mitochondrial function was also perturbed in cells pretreated with either CdtB or rCdtABC. An increase in the expression of the mitochondria Ag, Apo 2.7, was observed along with evidence of the development of a mitochondrial permeability transition state; this includes a decrease in the transmembrane potential and elevated generation of reactive oxygen species. Activation of the caspase cascade, which is an important biochemical feature of the apoptotic process, was also observed in Cdt-treated lymphocytes. Overexpression of the bcl-2 gene in the human B lymphoblastoid cell line, JY, led to a decrease in Cdt-induced apoptosis. Interestingly, Bcl-2 overexpression did not block Cdt-induced G(2) arrest. The implications of our results with respect to the immunosuppressive functions of Cdt proteins are discussed.

Mercury-induced apoptosis in human lymphoid cells: evidence that the apoptotic pathway is mercurial species dependent

There is growing evidence that heavy metals, in general, and mercurial compounds, in particular, are toxic to the human immune system. In this regard, we have previously shown that both inorganic and organic mercurials are potent human T-cell apoptogens; moreover, mitochondria appear to be a target organelle for the induction of cell death. To ascertain whether both mercury species utilize the same molecular pathway to trigger the apoptotic cascade, cells were treated with MeHgCl or HgCl2 and mitochondrial activity was examined. We show that both mercury species affect mitochondrial activity by inducing the development of a membrane permeability transition. This state is characterized by a decline in both the transmembrane potential and the intracellular pH, as well as the generation of reactive oxygen species. We also determined that mercury exposure results in a decline in the T-cell GSH content. Since mitochondrial dysfunction and the development of a permeability transition may result in the release of cytochrome c, a factor that promotes apoptosis, we assessed the abilities of both species of mercury to induce the translocation of cytochrome c from mitochondria to the cytosol. We noted that MeHgCl caused a significant increase in cytosolic cytochrome c. Surprisingly, however, HgCl2 did not alter the level of cytosolic cytochrome c. We next determined whether the mercurials could alter the level of the anti-apoptotic protein Bcl-2. Our results demonstrate that HgCl2 induces a significant elevation in the Bcl-2 content of T-cells; in contrast, T-cells treated with MeHgCl did not exhibit altered levels of this anti-apoptotic protein. Regardless of whether cytochrome c is released from the mitochondria, both mercurial species were capable of activating the caspase cascade, as evident by cleavage of poly (ADP-ribose) polymerase. Thus, our study shows that, whereas each of the mercury species shares common features in the apoptotic process, profound differences exist in a number of key steps in the pathway. The significance of these differences is discussed.

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

Methylmercury chloride (MeHgCl) is known to induce cellular and humoral immunodeficiencies in mice. In this study, the involvement of lymphoid subset disorders due to low concentrations of methylmercury (0.001-1.0 microM) has been examined. Cytofluorometric analysis of splenic cells exposed in vitro to low concentrations of MeHgCl for 48 h revealed two distinct populations: the first expressed a typical lymphoid forward light scatter (FSC)/side light scatter (SSC) pattern (R1 region), and the second was characterized by a lower FSC and a higher SSC (R2 region). A dose-dependent shift of cells from R1 region toward R2 region was observed in splenic cells treated with MeHgCl. The apoptotic state of cells in the R2 region was confirmed by the TdT-mediated dUTP nick end labeling (TUNEL) assay. Analysis of DNA content in splenic lymphoid cells showed that low concentrations of MeHgCl increased both hypoploid cells and cells in G0-G1/S phase, both in the R1 and R2 regions. However, the numbers of cells in G0-G1/S and G2/M phases were decreased, but hypoploid cells increased in both regions due to exposure to 1 microM MeHgCl. MeHgCl-induced apoptosis disappeared when splenic cells were pretreated with anti-Fas antibodies, indicating that Fas expressing cells were the target cells for MeHgCl-mediated apoptosis. Furthermore, T cells from the V beta 8 family were found to be more sensitive to apoptosis induced by low concentrations of MeHgCl. Taken together, these results suggest that MeHgCl at low concentrations mediates the development of apoptosis in peripheral T cell via the Fas/FasL pathway.

Evidence for the induction of apoptosis by endosulfan in a human T-cell leukemic line

Several organochlorinated pesticides including DDT, PCBs and dieldrin have been reported to cause immune suppression and increase susceptibility to infection in animals. Often this manifestation is accompanied by atrophy of major lymphoid organs. It has been suggested that increased apoptotic cell death leading to altered T-B cell ratios, and loss of regulatory cells in critical numbers leads to perturbations in immune function. The major objective of our study was to define the mechanism by which endosulfan, an organochlorinated pesticide, induces human T-cell death using Jurkat, a human T-cell leukemic cell line, as an in vitro model. We exposed Jurkat cells to varying concentrations of endosulfan for 0-48 h and analyzed biochemical and molecular features characteristic of T-cell apoptosis. Endosulfan lowered cell viability and inhibited cell growth in a dose- and time-dependent manner. DAPI staining was used to enumerate apoptotic cells and we observed that endosulfan at 10-200 microM induced a significant percentage of cells to undergo apoptotic cell death. At 48 h, more than 90% cells were apoptotic with 50 microM of endosulfan. We confirmed these observations using both DNA fragmentation and annexin-V binding assays. It is now widely being accepted that mitochondria undergo major changes early during the apoptotic process. We examined mitochondrial transmembrane potential (deltapsim) in endosulfan treated cells to understand the role of the mitochondria in T-cell apoptosis. Within 30 min of chemical exposure, a significant percentage of cells exhibited a decreased incorporation of DiOC6(3), a cationic lipophilic dye into mitochondria indicating the disruption of deltapsim. This drop in deltapsim was both dose- and time-dependent and correlated well with other parameters of apoptosis. We also examined whether this occurred by the down regulation of bcl-2 protein expression that is likely to increase the susceptibility of Jurkat cells to endosulfan toxicity. Paradoxically, the intracellular expression of bcl-2 protein was elevated in a dose dependent manner suggesting endosulfan-induced apoptosis occurred by a non-bcl-2 pathway. Based on these data, as well as those reported elsewhere, we propose the following sequence of events to account for T-cell apoptosis induced by endosulfan: uncoupling of oxidative phosphorylation --> excess ROS production --> GSH depletion --> oxidative stress --> disruption of deltapsim --> release of cytochrome C and other apoptosis related proteins to cytosol --> apoptosis. This study reports for the first time that endosulfan can induce apoptosis in a human T-cell leukemic cell line which may have direct relevance to loss of T cells and thymocytes in vivo. Furthermore, our data strongly support a role of mitochondrial dysfunction and oxidative stress in endosulfan toxicity.

Reactive oxygen species are involved in the apoptosis induced by nonsteroidal anti-inflammatory drugs in cultured gastric cells

We previously reported the induction of apoptotic DNA fragmentation by nonsteroidal anti-inflammatory drugs (NSAIDs) in cultured rat gastric cells, and indicated that prostaglandin-synthesis is only marginally involved in the apoptotic process. In the present study, we examined whether the generation of reactive oxygen species is critically involved in NSAID-induced apoptosis. Indomethacin, sodium diclofenac, flurbiprofen, zaltoprofen, etodolac, but not mofezolac, enhanced apoptotic DNA fragmentation and mRNA expression for cyclooxygenase-2 in AGS cells, a cell line derived from human gastric epithelium. The apoptotic effect of indomethacin was then confirmed by fluorescent staining of the cells with annexin V. Apoptotic DNA fragmentation induced by indomethacin and flurbiprofen was suppressed by incubation of the cells with the anti-oxidants pyrrolidine dithiocarbamate, diphenyleneiodonium chloride, and N-acetyl-L-cysteine. These two NSAIDs also enhanced release from the cells of 8-isoprostane, a nonenzymatic product by free-radical-mediated peroxidation of arachidonic acid. Further, lucigenin chemiluminescence showed that the intracellular production of reactive oxygen species increased in cells treated with indomethacin. The present data thus indicate a crucial association between the generation of reactive oxygen species and NSAID-induced apoptosis in gastric epithelial cells.

The involvement of microtubular disruption in methylmercury-induced apoptosis in neuronal and nonneuronal cell lines

Methylmercury (MeHg) is known to interfere with cell cycle progression by disruption of microtubules. The relationship between the changes in cell cycle and the induction of apoptosis caused by MeHg was investigated in cultured mammalian cells. MeHg caused nuclear fragmentation and DNA ladder formation in rat pheochromocytoma (PC12) and mouse neuroblastoma cells exposed to MeHg. Flow cytometric analysis revealed that the occurrence of apoptosis was preceded by the accumulation of cells in G2/M after MeHg treatment. Exposure to colchicine, a well-characterized mitotic inhibitor, also caused G2/M-phase arrest followed by the appearance of apoptotic cells. These results suggest that G2/M-phase arrest through the disruption of microtubules is an important event in the development of apoptosis by MeHg. MeHg treatment led to G2/M-phase arrest followed by apoptosis in nonneuronal HeLa cells also. Bcl-2 was phosphorylated by MeHg treatment in HeLa cells but not in PC12 cells; however, p53 expression was not changed in either cell line. Thus, MeHg induces apoptosis via a p53-independent pathway in both cell lines, however, different pathways may be activated after the disruption of microtubules in PC12 and HeLa cells.

Protection Against CD95-Mediated Apoptosis by Inorganic Mercury in Jurkat T Cells

Dysregulation of CD95/Fas-mediated apoptosis has been implicated as a contributing factor in autoimmune disorders. Animal studies clearly have established a connection between mercury exposure and autoimmune disease in rodents, while case reports have suggested a link between accidental mercury contamination and autoimmune disease in humans. The mechanism(s) for these associations are poorly understood. Using the Jurkat cell model, we have found that low levels (≤10 μM) of inorganic mercury (i.e., HgCl2) attenuated anti-CD95-mediated growth arrest and markedly enhanced cell survival. Several biochemical assays for apoptosis, including DNA degradation, poly(ADP-ribose) polymerase degradation, and phosphatidylserine externalization, directly verified that HgCl2 attenuated anti-CD95-mediated apoptosis. In an attempt to further characterize the effect of mercury on CD95-mediated apoptosis, several signaling components of the CD95 death pathway were analyzed to determine whether HgCl2 could modulate them. HgCl2 did not modulate CD95 expression; however, it did block CD95-induced caspase-3 activation. HgCl2 was not able to attenuate TNF-α-mediated apoptosis in U-937 cells, or ceramide-C6-mediated apoptosis in Jurkat cells, suggesting that mercury acts upstream of, or does not involve, these signals. Thus, inorganic mercury specifically attenuates CD95-mediated apoptosis likely by targeting a signaling component that is upstream of caspase-3 activation and downstream of CD95.

Induction of apoptosis in human T-cells by methyl mercury: temporal relationship between mitochondrial dysfunction and loss of reductive reserve

The objective of our study was to define the mechanism by which MeHgCl induces human T-cell apoptosis. We asked the question: does mercury disrupt the Deltapsim and induce a mitochondrial permeability transition state? Using two fluorescent reagents, JC-1 and DiOC6(3), we demonstrated that MeHgCl exposure resulted in a decrease in the Deltapsim. Since a decline in Deltapsim can disturb the pHi, we employed SNARF-1 to assess pHi; results indicate that mercury treatment reduced the pHi from 7.0 to 6.5. Consistent with these observations, we noted that uncoupled electron transfer reactions generated ROS, while cardiolipin, a mitochondrial phospholipid, was oxidized. In concert with the biochemical changes, there was a decrease in overall dimension of the mitochondria of mercury-treated cells and a loss in cristae architecture. The toxicant also depleted the thiol reserves of the cell and promoted translocation of cytochrome c from the mitochondria to the cytosol. Furthermore, when T cells were thiol-depleted, there was increased susceptibility to MeHgCl-induced apoptosis. Finally, we established a temporal relationship between the decline in Deltapsim, generation of ROS, and depletion of thiol reserves. The earliest detectable event was at the level of the mitochondrion; in the presence of MeHgCl there was a profound reduction in mitochondrial Deltapsim and a decline in GSH levels within 1 h. Subsequently, a further decrease in thiol reserves was linked to the generation of ROS. We propose that the target organelle for MeHgCl is the mitochondrion and that induction of oxidative stress leads to activation of death-signaling pathways.

Mercuric chloride-induced apoptosis is dependent on protein synthesis

Apoptosis is a mode of cell death with morphologic and biochemical features that distinguish it from necrosis. Recent studies demonstrating that mercury compounds initiate apoptosis in cultured cells did not elucidate if the biochemical mechanism of apoptosis involved a dependence on macromolecular synthesis post-insult, i.e. programmed cell death. The objectives of this in vitro study were (1) to determine if HgCl2 cytotoxicity includes an apoptotic component, and (2) to determine if apoptosis is dependent on protein synthesis, i.e. proceeds by an inducible mechanism. Suspensions of mouse lymphoma (L5178Y-R) cells were exposed to 0, 1, 5, or 10 microM HgCl2 and apoptosis was evaluated utilizing qualitative and quantitative methods. At 24 h after exposure, transmission electron microscopy revealed a concentration-related increase in morphologic changes typical of apoptosis: margination of condensed chromatin to the nuclear membrane, dilation of the rough endoplasmic reticulum, cytoplasmic condensation and vacuolation, nuclear dissolution, and plasma membrane blebbing. An increase in Hg-induced DNA fragmentation (DNA 'ladder') was observed using agarose gel electrophoresis. Time- and concentration-dependent increases in the percent of apoptotic cells were observed at 1, 6, 12, and 24 h after HgCl2 exposure using a flow cytometric method that discriminates between cells according to size and granularity. Pretreatment of cells with cycloheximide (CHX), an inhibitor of translation, prior to HgCl2 exposure resulted in a 25-50% reduction in apoptotic cells 24 h after exposure to 10 and 20 microM HgCl2, and concomitantly reduced the overall cytotoxicity compared to HgCl2 alone. These results, although limited to a single cell line, support the hypothesis that HgCl2 induces apoptosis that is dependent, at least in part, upon protein synthesis.

Apoptotic human lymphocytes have diminished CD4 and CD8 receptor expression

We used quantitative multiparameter flow cytometric assays to simultaneously detect viable, apoptotic, and necrotic human peripheral blood mononuclear cells (PBMC) and immunophenotyped lymphocyte subsets within the PBMC. Apoptosis was induced by a spectrum of treatments, including camptothecin, cisplatin, dexamethasone, hyperthermia, staurosporine, and etoposide in anti-CD3 mAb-stimulated cells and by cyclohexamide in both quiescent and stimulated cells; apoptosis in the latter was augmented by anti-fas mAb. We found that CD4(+) and CD8(+) cells were significantly underrepresented in the apoptotic PBMC and that the percentage of CD4(+) and CD8(+) PBMC each markedly decreased as apoptosis increased. This suggested that surface expression of these receptors was lessened on apoptotic CD4(+) and CD8(+) cells. This was directly confirmed by observation of sorted CD4(+) PBMC. This analysis of a wide variety of apoptotic stimuli demonstrates that diminished CD4 and CD8 surface receptor expression is a common feature of human T lymphocyte apoptosis.

Mercuric chloride induces apoptosis in human T lymphocytes: evidence of mitochondrial dysfunction

The major objective of our study was to define the mechanism by which mercuric chloride (HgCl2) induces human T-cell death. Human peripheral blood T-cells were exposed to 0-40 microm HgCl2 and then analyzed for biochemical and molecular features of T-cell apoptosis. HgCl2-treated cells exhibited increased Hoechst 33258 fluorescence while maintaining their ability to exclude the vital stain 7-aminoactinomycin D. To further evaluate cell death and distinguish between apoptosis and necrosis, translocation of phosphatidylserine to the outer layer of the plasma membrane (annexin V binding), DNA fragmentation (TUNEL assay), and cleavage of poly (ADP-ribose) polymerase (PARP) were assessed. In the presence of 20-40 microm HgCl2, T-cells exhibited increased annexin V binding (28%) and DNA fragmentation (31%). HgCl2-dependent PARP cleavage was also observed by Western blot analysis. Because degradative changes associated with apoptosis are often preceded by disruption of mitochondrial function, HgCl2-treated cells were assessed for disruption of the mitochondrial transmembrane potential (DeltaPsim) and development of the mitochondrial permeability transition state. Using DiOC6(3), we demonstrated that HgCl2 exposure resulted in a decrease in the DeltaPsim. Because a decline in DeltaPsim can disturb the intracellular pH (pHi), we used the fluorescent probe, SNARF-1, to assess intracellular acidification. Treatment of T-cells with HgCl2 resulted in reduced pHi from 7.0 to 6.7. Concomitant with these observations, the fluorescent probe, hydroethidine, was utilized to demonstrate that uncoupled mitochondrial electron transport resulted in increased reactive oxygen species (ROS) generation. Interestingly, in spite of these alterations to mitochondrial function, translocation of cytochrome c to the cytosol was not detected; this correlated with enhanced bcl-2 levels in HgCl2-treated cells. In conclusion, HgCl2 exposure results in oxidative stress and activation of death signaling pathways leading to apoptosis. Collectively, our studies indicate that individual mercurial species are capable of inducing T-cell death by activating specific apoptotic cascades.

Molecular and biochemical mechanisms of Pasteurella haemolytica leukotoxin-induced cell death

Pasteurella haemolytica leukotoxin (LKT) is a member of the RTX family of pore-forming toxins that kill bovine immune cells. Several studies have suggested that RTX toxins kill target cells by the induction of apoptosis. In the present study, BL3 bovine leukaemia cells were exposed to LKT and assessed by molecular and flow cytometric techniques that measure different aspects of apoptotic cell death. The intoxicated cells demonstrated morphological, light scatter and Hoechst 33258 staining characteristics consistent with cells undergoing apoptosis. The cells also exhibited internucleosomal DNA fragmentation and poly (ADP-ribose) polymerase (PARP) cleavage, both indicators of apoptosis. LKT-treated cells bound annexin-V-FITC indicating that phosphatidylserine groups were translocated from the inner to the outer leaflet of the cell membrane. The effect of LKT on cells was dose dependent and inhibitable by incubation with anti-LKT monoclonal antibody. Finally, an early step for induction of apoptosis appears to be the binding of LKT to a beta2 integrin since pre-incubating cells with anti-beta2 integrin antibodies inhibited LKT-induced apoptosis. This study provides new insights into understanding the pathogenesis of bovine pasteurellosis and could lead to the development of both preventative and therapeutic strategies for disease management.

Inhibition of poly(ADP-ribose) polymerase rescues human T lymphocytes from methylmercury-induced apoptosis

The objective of this investigation was to determine the role of poly(ADP-ribose) polymerase (PARP) in methylmercuric chloride (MeHgCl)-induced T-cell apoptosis. Following exposure of human T-cells to 2.5 microM MeHgCl, we observed PARP activation within 45 min. Maximal activation was observed at 90 min after MeHgCl treatment; thereafter, PARP activity declined. The loss in enzyme activity was coincidental with the cleavage of 116-kDa intact PARP protein to an 85-kDa fragment. To address the relationship between PARP activation and induction of apoptosis, we first examined the redox status of T cells treated with MeHgCl. We found that exposure of T cells to low concentrations of this toxicant resulted in decreased levels of reduced pyridine nucleotides and an increase in the relative amounts of oxidized flavoproteins. Thus, the possibility exists that activation of PARP leads to NAD+ depletion and thereby alters mitochondrial redox status. To determine if PARP activation is indeed part of the proapoptotic (destructive) response or a component of the antiapoptotic (protective) response, we employed two inhibitors: 3-aminobenzamide and nicotinamide. Pretreatment of T cells with these inhibitors protected cells from MeHgCl-induced apoptosis; this was seen as a reduction in the uptake of Hoechst 33258 and DNA fragmentation. Moreover, these inhibitors blocked MeHgCl-induced oxidative stress as evidenced by a reduction in reactive oxygen species (ROS) generation. These agents, however, failed to block MeHgCl-dependent decline in mitochondrial transmembrane potential (delta psi m). We conclude that PARP activation leads to proapoptotic events that contribute to MeHgCl-induced cell death.

Actinobacillus actinomycetemcomitans leukotoxin induces apoptosis in HL-60 cells

Actinobacillus actinomycetemcomitans leukotoxin (Ltx) is a member of the repeats-in-toxin (RTX) family of pore-forming toxins and kills human immune cells. Currently, it remains unclear whether toxin-mediated killing of target cells involves the induction of necrosis or apoptosis. Therefore, the goal of this investigation was to determine whether Ltx is capable of causing apoptotic cell death in toxin-sensitive promyelocytic HL-60 cells. Multiparameter flow cytometric analysis of toxin-treated cells stained with Hoechst 33258 (or 33342) and 7-aminoactinomycin D allowed us to identify four populations: viable cells, early apoptotic cells, late apoptotic and/or secondarily necrotic cells, and a final population that was composed of cellular debris. Compared with control cells, HL-60 cells treated with Ltx exhibited a gradual decrease in forward light scatter with a coincident increase in side light scatter, indicative of a decrease in cell size and organelle condensation, respectively. Additional experiments demonstrated that Ltx-treated cells showed evidence of internucleosomal DNA fragmentation and phosphatidylserine translocation. The results of our studies clearly demonstrate that Ltx can kill HL-60 cells by inducing apoptosis. We hypothesize that elimination of acute inflammatory cells via this mechanism plays a critical role in the pathogenesis of diseases caused by A. actinomycetemeomitans.