Characterization of Cd-induced molecular events prior to cellular damage in primary rat hepatocytes in culture: activation of the stress activated signal protein JNK and transcription factor AP-1

Genomic Redistribution of Metal-Response Transcription Factor-1 (MTF-1) in Cadmium Resistant Cells

(1) Background: Metal homeostasis is an important part of cellular programs and is disrupted when cells are exposed to carcinogenic heavy metals. Metal response is mediated by the metal response element transcription factor MTF-1. However, where MTF-1 binds and how that binding changes in response to heavy metals, such as cadmium, remains unknown. (2) Methods: To investigate the effects of prolonged cadmium exposure on the genomic distribution of MTF-1, we performed MTF-1 CUT&RUN, RNA-seq and ATAC-seq on control and cadmium-resistant cells. (3) Results: Changes in MTF-1 binding primarily occur distal to the transcription start sight. Newly occupied MTF-1 sites are enriched for FOS/JUN DNA binding motifs, while regions that lose MTF-1 binding in cadmium are enriched for the FOX transcription factor family member DNA binding sites. (4) Conclusions: Relocalization of MTF-1 to new genomic loci does not alter the accessibility of these locations. Our results support a model whereby MTF-1 is relocalized to accessible FOS/JUN-bound genomic locations in response to cadmium.

Growth performance, biochemical, cytological and molecular aspects of rabbits exposed to lead toxicity

Heavy metals have enormous variety of deleterious effects on many organs in the body. This study demonstrated the toxic influences of lead on the growth, biochemical, cellular and molecular aspects of developing rabbits. Seventy-five rabbits (New Zealand NZW) were divided into five equal groups as follows; control (C) and four treatment groups (T1-4) orally administered lead acetate solution as follow T1: 20, T2: 30, T3: 50 and T4: 70 mg/kg body weight. Lead resulted in a significant decrease in live body weight, daily body weight gain and feed intake in T3 and T4 compared to those in other groups. Blood haematology measurements such as red blood cells, haematocrit (HCT), mean corpuscular volume, platelet, white blood cells and lymphocytes were significantly influenced by the high level of lead. Oral administration of lead significantly reduced total proteins in the serum. It was observed that the high lead level led to significantly (p < 0.05) increased levels of aspartate aminotransferase, alanine aminotransferase enzymes, urea and creatinine. Four random amplified polymorphic DNA primers polymorphism were detected among the treatment groups. Total number of induced bands (loss or appearance) compared with control group were 4, 10, 10 and 14 bands using primers P1, P2, P3 and P4 respectively. Number of micronuclei showed a dose-response increase and the difference was highly significant especially between control compared with T3 and T4 groups. From our results, we can conclude that exposure of rabbits to lead acetate resulted in negative effects on the growth performance and altered the haematological and biochemical parameters, in addition to its adverse impact on cytological and molecular characterization of animals.

Engineered nickel oxide nanoparticles affect genome stability in Allium cepa (L.)

Indiscriminate uses of engineered nickel oxide nanoparticles (NiO-NPs) in heavy industries have ushered their introduction into the natural environment, ensuing novel interactions with biotic components of the ecosystem. Though much is known about the toxicity of NiO-NPs on animals, their phytotoxic potential is not well elucidated. NiO-NP hinders intra-cellular homeostasis by producing ROS in excess, having profound effect on the antioxidant profile of exposed animal and plant tissues. In the present study, bulbs of the model plant Allium cepa were treated with varying concentrations of NiO-NP (10 mg L^-1 - 500 mg L^-1) to study changes in ROS production and potential genotoxic effect. The data generated proved a concomitant upsurge in intracellular ROS accumulation with NiO-NP dosage that could be correlated with increased genotoxicity in A. cepa. Augmented in situ ROS production was revealed through DCFH-DA assay, with highest increase in fluorescence (70% over control) in bulbs exposed to 125 mg L^-1 NiO-NP. Effect of NiO-NP on genomic DNA was studied through detailed analyses of RAPD profiles which allows detection of even slightest changes in DNA sequence of treated plants. Significant differences in band intensity, loss and appearance of bands as well as genomic template stability and band sharing indices of treated plants revealed increased vulnerability of genomic DNA to NiO-NP, at even lowest concentration (10 mg L^-1). This is the first report of NiO-NP induced genotoxicity on A. cepa, which confirms the nanoparticle as a potent environmental hazard.

Cadmium and cellular signaling cascades: interactions between cell death and survival pathways

Cellular stress elicited by the toxic metal Cd(2+) does not coerce the cell into committing to die from the onset. Rather, detoxification and adaptive processes are triggered concurrently, allowing survival until normal function is restored. With high Cd(2+), death pathways predominate. However, if sublethal stress levels affect cells for prolonged periods, as in chronic low Cd(2+) exposure, adaptive and survival mechanisms may deregulate, such that tumorigenesis ensues. Hence, death and malignancy are the two ends of a continuum of cellular responses to Cd(2+), determined by magnitude and duration of Cd(2+) stress. Signaling cascades are the key factors affecting cellular reactions to Cd(2+). This review critically surveys recent literature to outline major features of death and survival signaling pathways as well as their activation, interactions and cross talk in cells exposed to Cd(2+). Under physiological conditions, receptor activation generates 2nd messengers, which are short-lived and act specifically on effectors through their spatial and temporal dynamics to transiently alter effector activity. Cd(2+) recruits physiological 2nd messenger systems, in particular Ca(2+) and reactive oxygen species (ROS), which control key Ca(2+)- and redox-sensitive molecular switches dictating cell function and fate. Severe ROS/Ca(2+) signals activate cell death effectors (ceramides, ASK1-JNK/p38, calpains, caspases) and/or cause irreversible damage to vital organelles, such as mitochondria and endoplasmic reticulum (ER), whereas low localized ROS/Ca(2+) levels act as 2nd messengers promoting cellular adaptation and survival through signal transduction (ERK1/2, PI3K/Akt-PKB) and transcriptional regulators (Ref1-Nrf2, NF-κB, Wnt, AP-1, bestrophin-3). Other cellular proteins and processes targeted by ROS/Ca(2+) (metallothioneins, Bcl-2 proteins, ubiquitin-proteasome system, ER stress-associated unfolded protein response, autophagy, cell cycle) can evoke death or survival. Hence, temporary or permanent disruptions of ROS/Ca(2+) induced by Cd(2+) play a crucial role in eliciting, modulating and linking downstream cell death and adaptive and survival signaling cascades.

Cadmium induces transcription independently of intracellular calcium mobilization

BACKGROUND

Exposure to cadmium is associated with human pathologies and altered gene expression. The molecular mechanisms by which cadmium affects transcription remain unclear. It has been proposed that cadmium activates transcription by altering intracellular calcium concentration ([Ca(2+)](i)) and disrupting calcium-mediated intracellular signaling processes. This hypothesis is based on several studies that may be technically problematic; including the use of BAPTA chelators, BAPTA-based fluorescent sensors, and cytotoxic concentrations of metal.

METHODOLOGY/PRINCIPAL FINDING

In the present report, the effects of cadmium on [Ca(2+)](i) under non-cytotoxic and cytotoxic conditions was monitored using the protein-based calcium sensor yellow cameleon (YC3.60), which was stably expressed in HEK293 cells. In HEK293 constitutively expressing YC3.60, this calcium sensor was found to be insensitive to cadmium. Exposing HEK293::YC3.60 cells to non-cytotoxic cadmium concentrations was sufficient to induce transcription of cadmium-responsive genes but did not affect [Ca(2+)](i) mobilization or increase steady-state mRNA levels of calcium-responsive genes. In contrast, exposure to cytotoxic concentrations of cadmium significantly reduced intracellular calcium stores and altered calcium-responsive gene expression.

CONCLUSIONS/SIGNIFICANCE

These data indicate that at low levels, cadmium induces transcription independently of intracellular calcium mobilization. The results also support a model whereby cytotoxic levels of cadmium activate calcium-responsive transcription as a general response to metal-induced intracellular damage and not via a specific mechanism. Thus, the modulation of intracellular calcium may not be a primary mechanism by which cadmium regulates transcription.

Estrogenic response in male bullfrog (Rana catesbeiana) hepatocytes after single or combined exposure to cadmium (Cd) and 17beta-estradiol (E2)

Contamination by heavy metals and sex hormones in a water environment is an important health issue. In this study, we investigated the estrogenic effects of cadmium (Cd) administration alone and in combination with 17beta-estradiol (E2) on the hepatocytes of male Bullfrog (Rana catesbeiana). Their vitellogenin (VTG) expression and reactive oxygen species (ROS) were analyzed upon exposure to Cd alone or to both Cd and E2. Our results suggest that the VTG levels induced by the co-treatment of 100 nM E2 and 100 nM CdCl(2) were significantly higher than those induced by 100 nM E2 alone (p < 0.05), and were comparable to vitellogenin induction observed with 1 μM E2. A similar result was observed by western blot analysis in the culture medium of hepatocytes. Meanwhile, Cd (but not E2) increased the ROS levels. These results suggest that Cd has a cooperative effect with E2 in the induction of VTG, thus acting as an estrogenic disruptor. Cd also causes oxidative stress that occurs with the enhanced vitellogenesis.

Cadmium promotes breast cancer cell proliferation by potentiating the interaction between ERalpha and c-Jun

Cadmium is an environmental contaminant that enters the body through diet or cigarette smoke. It affects multiple cellular processes, including cell proliferation, differentiation, and apoptosis. Recently, cadmium has been shown to function as an endocrine disruptor, to stimulate estrogen receptor alpha (ERalpha) activity and promote uterine and mammary gland growth in mice. Although cadmium exposure has been associated with the development of breast cancer, the mechanism of action of cadmium remains unclear. To address this deficit, we examined the effects of cadmium treatment on breast cancer cells. We found that ERalpha is required for both cadmium-induced cell growth and modulation of gene expression. We also determined that ERalpha translocates to the nucleus in response to cadmium exposure. Additionally, we provide evidence that cadmium potentiates the interaction between ERalpha and c-Jun and enhances recruitment of this transcription factor complex to the proximal promoters of cyclin D1 and c-myc, thus increasing their expression. This study provides a mechanistic link between cadmium exposure and ERalpha and demonstrates that cadmium plays an important role in the promotion of breast cancer.

MAPK activation is involved in cadmium-induced Hsp70 expression in HepG2 cells

Cadmium is one of the most toxic elements to which man can be exposed at work or in the environment. By far, the most salient toxicological property of Cd is its exceptionally long half-life in the human body. Once absorbed, Cd accumulates in the human body, particularly in the liver and other vital organs. The cellular actions of Cd are extensively documented, but the molecular mechanisms underlying these actions are still not resolved. It is known that Cd activates the activator protein-1 (AP-1), but no data about the pathway involved are reported for liver. The objective was to provide a greater insight into the effect of cadmium on mitogen-activated protein kinases (MAPK's) involved in signal transduction, its relationship with AP-1 activation, and heat shock protein (Hsp) 70 expression, in HepG2 cells. AP-1 activation as a result of 5 microM CdCl(2) exposure was increased 24.5-fold over control cells after 4 h treatment. To investigate the role of the extracellular signal-regulated protein kinases (ERK's), c-Jun N-terminal kinases (JNK's) and p38 kinases in cadmium-induced AP-1 activation, specific MAPKs inhibitors were used. AP-1 activation decreased by 74% with ERK inhibition, by 83% with p38 inhibition, while inhibition of JNK decreased by 70%. Only ERK and JNK participated in Hsp70 production, conferring cell protection against cadmium damage.

Heavy metal and growth hormone pathways in metallothionein regulation in fish RTH-149 cell line

Interference between heavy metals and growth hormone (GH) on cell signaling has been previously demonstrated in fish cells. This study was aimed at assessing their effects on expression of the metallothionein isoforms MT-A and MT-B. The results indicate that all heavy metals induce MT-A more markedly than MT-B, but differences appeared when metals were combined with GH. For MT-B induction, a positive interference between metals and GH was observed for Zn(2+)/GH and Cd(2+)/GH, a negative interference for Hg(2+)/GH. With regards to MT-A, no interference was observed for Zn(2+)/GH and Hg(2+)/GH, while a negative interference occurred with Cu(2+)/GH and a positive interference with Cd(2+)/GH. The possible mechanisms underlying the differential regulation of metallothioneins include different signaling pathways. The results show that STAT5 and ERKs responded differently to different combinations, and Zn(2+)/GH and Cd(2+)/GH exerted a slight positive interference on ERK activation. On the other hand, a synergic rise in [Ca(2+)](i) occurred for all combinations except for Cu(2+)/GH. Our data suggest that the cross-talk between heavy metals and GH resulting in MT transcription modulation does not strictly depend on Ca(2+) signalling; (ii)ERK activation may represent the point of cross-talk between Zn(2+) or Cd(2+) and GH, converging on MT-B transcription, probably through a differential recruitment of transcription factors.

Common and distinct mechanisms of different redox-active carcinogens involved in the transformation of mouse JB6P+ cells

We transformed JB6P+ cells with prolonged intermittent low-dose UVB radiation or prolonged exposure to low-dose H(2)O(2) or CdCl(2). Stable transformation was confirmed by an anchorage-independence assay. The JB6P+ transformants formed more colonies (approximately six folds) in soft agar as compared to their JB6P+ parent cells and were associated with increased intracellular reactive oxygen species (ROS) levels. Activating protein-1 (AP-1) is a family of transcription factors that are rapidly activated by elevated intracellular ROS levels, and their composition is important in the process of cellular transformation and/or tumor progression. To investigate if carcinogenesis induced by distinct carcinogens was via similar molecular mechanisms in these transformants, gel mobility shift and immunoblot analyses were utilized to determine the distinct AP-1 compositions. Compared to parent JB6P+ cells, the gain of JunB and Fra-1 in AP-1 DNA binding complexes was markedly increased in all transformed cells, which might contribute to a more proliferative phenotype, while loss of Fra-2 occurred in JB6P+/H(2)O(2) and JB6P+/Cd cells. Differential AP-1 components in the transformants suggested that their transformations might be mediated by distinct transcription signalings with distinct AP-1 dimer compositions. However, all three transformants exhibited increased activation of pathways involved in cell proliferation (ERK/Fra-1/AP-1 and JNK/c-jun/AP-1) and anti-apoptosis (Bcl-xl). The development of the JB6P+ transformants (JB6P+/UVB; JB6P+/H(2)O(2); JB6P+/Cd) provides a unique tool to study the mechanisms that contribute to different redox-active carcinogens in a single model.

Impact assessment of cadmium contamination on rice (Oryza sativa L.) seedlings at molecular and population levels using multiple biomarkers

Assessment of environmental contamination on ecology (plant) at molecular and population levels is important in risk quantification and remediation study. Random amplified polymorphic DNA (RAPD) assay and related other fingerprinting techniques have been employed to detect the genotoxin-induced DNA damage and mutations. This research compared the effects occurring at molecular and population levels in rice seedlings exposed to cadmium (Cd) concentrations of 15-60 mg l(-1) for 8 days with quartz sand culture. Inhibition of root growth and increase of total soluble protein content in root tips of rice seedlings were observed with the increase of Cd concentration. For the RAPD analyses, 12 RAPD primers of 50-70% GC content were found to produce unique polymorphic band patterns and subsequently were used to produce a total of 180 bands of 179-3056 bp in molecular size in the control root tips of rice seedlings. Results produced by these RAPD primers indicate that changes in RAPD profiles of root tips after Cd treatment include modifications in band intensity and gain or loss of bands by comparison with control. The effect of changes was dose-dependent. Genomic template stability compares favourably with the traditional indices such as root growth and soluble protein content. The DNA polymorphisms detected by RAPD analysis can be applied as a suitable biomarker assay for the detection of genotoxic effects of Cd contamination on plants.

Role of oxidative stress and apoptosis in cadmium induced thymic atrophy and splenomegaly in mice

Cadmium immunotoxicity in rodents is primarily characterized by marked thymic damage and splenomegaly. To understand the toxicity of Cd on lymphoid cells in vivo, a single dose of Cd as CdCl2 (1.8 mg/kg, i.p.) was administered to male BALB/c mice and cytotoxicity (MTT assay), oxidative stress indicators (glutathione, reactive oxygen species) and apoptotic markers (mitochondrial membrane potential, caspase-3 activity, phosphatidylserine externalization, apoptotic DNA, intranucleosomal DNA fragmentation) were assessed in thymic and splenic single cell suspensions, at various time intervals. Lowering of body weight gain and cellularity and a loss in cell viability was seen in the Cd treated mice. The earliest significant increase in ROS at 18 h, followed by mitochondrial membrane depolarization, caspase-3 activation and GSH depletion at 24h in spleen and later at 48 h in thymus, strongly implicate the possible involvement of ROS. A pronounced inhibition of cell proliferative response at 48 h and 72 h may also be linked to Cd induced apoptosis. The morphological alterations including thymic cortical cell depletion and an increase in red pulp with diminished white pulp in spleen were observed at 48 h and beyond. The splenic cells appeared more susceptible than thymus cells to the adverse effects of Cd. The present study, therefore, demonstrates potentiation of oxidative stress followed by mitochondrial-caspase dependent apoptotic pathway. This may, in part, be responsible for causing suppression of cell proliferative response, thymic atrophy and splenomegaly.

Attenuation of cadmium chloride induced cytotoxicity in murine hepatocytes by a protein isolated from the leaves of the herb Cajanus indicus L

Cadmium has been recognized as a strong environmental pollutant. Exposure to this heavy metal occurs through the intake of foodstuffs, drinking water and also via the inhalation of air. Present study was conducted to evaluate the protective effect of a 43 kDa protein, isolated from the leaves of the herb Cajanus indicus, against cadmium-induced cytotoxicity in hepatocytes. For this study, cadmium chloride (CdCl(2)) has been used as the source of cadmium. Treatment of hepatocytes with 800 microM CdCl(2) for 3 h caused significant reduction in cell viability in association with the increased levels of glutamate pyruvate transaminase (GPT) and alkaline phosphatase (ALP) leakage. The activities of the antioxidant enzymes, superoxide dismutase, catalase (CAT), glutathione-S-transferase and glutathione reductase, and the levels of cellular metabolites, reduced glutathione (GSH) as well as total thiols have also been decreased under the same treatment. In addition, the toxin enhanced the levels of the lipid peroxidation end products and oxidized glutathione (GSSG). Incubation of hepatocytes with the protein at a dose of 0.1 mg/ml for 3 h prior to the toxin treatment (at a dose of 800 microM for 3 h) restored the activities of all the antioxidant enzymes, the levels of GSH, total thiols, cell viability and also attenuated the increased levels of GPT, ALP, lipid peroxidation and GSSG. In addition, the protein resisted CdCl(2) induced alterations of all the parameters when applied in combination with CdCl(2). Effects of a known antioxidant, vitamin E, and a non-relevant protein, bovine serum albumin against CdCl(2) induced cytotoxicity have also been included in the study. Combining all, we would like to say that the protein possessed protective activity against CdCl(2) induced cytotoxicity in mouse hepatocytes probably via its antioxidant property.

Efects of growth hormone and cadmium on the transcription regulation of two metallothionein isoforms

The effect of growth hormone (GH) and cadmium (Cd) on metallothionein (MT) expression was investigated in hepatoma cells. In fish the constitutive isoform MT-B and the metal-responsive MT-A are expressed. Real-time RT-PCR revealed that: Cd up-regulates mostly MT-A, GH slightly induces MT-B and the GH/Cd combination induces synergistically both MTs. Perturbations in Ca2+ levels suppressed or reduced the Cd-induction of MTs and abolished the GH/Cd synergy. Similar results were obtained by inhibition of tyrosine kinases. Also the signaling molecules recruited by the GH receptor responded differently to GH and Cd, with ERKs showing a synergistic activation upon GH/Cd. The following conclusions can be drawn: (1) cytosolic Ca2+ is mainly involved in MT-A regulation; (2) both Ca2+ and tyrosine phosphorylation are essential for Cd-induction and GH/Cd synergy on MTs. The synergy could depend on interactions in different signaling pathways, leading to a differential recruitment of MTF-1 and AP-1 transcription factors.

Melatonin inhibits both ER alpha activation and breast cancer cell proliferation induced by a metalloestrogen, cadmium

Cadmium (Cd) is a heavy metal affecting human health both through environmental and occupational exposure. There is evidence that Cd accumulates in several organs and is carcinogenic to humans. In vivo, Cd mimics the effect of estrogens in the uterus and mammary gland. In estrogen-responsive breast cancer cell lines, Cd stimulates proliferation and can also activate the estrogen receptor independent of estradiol. The ability of this metalloestrogen to increase gene expression in MCF7 cells is blocked by anti-estrogens suggesting that the activity of these compounds is mediated by ER alpha. The aims of this work were to test whether melatonin inhibits Cd-induced proliferation in MCF7 cells, and also to study whether melatonin specifically inhibits Cd-induced ER alpha transactivation. We show that melatonin prevents the Cd-induced growth of synchronized MCF7 breast cancer cells. In transient transfection experiments, we prove that both ER alpha- and ER beta-mediated transcription are stimulated by Cd. Melatonin is a specific inhibitor of Cd-induced ER alpha-mediated transcription in both estrogen response elements (ERE)- and AP1-containing promoters, whereas ER beta-mediated transcription is not inhibited by the pineal indole. Moreover, the mutant ER alpha-(K302G, K303G), unable to bind calmodulin, is activated by Cd but becomes insensitive to melatonin treatment. These results proved that melatonin inhibits MCF7 cell growth induced by Cd and abolishes the stimulatory effect of the heavy metal in cells expressing ER alpha at both ERE-luc and AP1-luc sites. We can infer from these experiments that melatonin regulates Cd-induced transcription in both ERE- and AP1 pathways. These results also reinforce the hypothesis of the anti-estrogenic properties of melatonin as a valuable tool in breast cancer therapies.

DNA changes in barley (Hordeum vulgare) seedlings induced by cadmium pollution using RAPD analysis

In recent years, several plant species have been used as bioindicators, and several tests have been developed to evaluate the toxicity of environmental contaminants on vegetal organisms. In this study, barley (Hordeum vulgare L) seedling was used as bioindicator of cadmium (Cd) pollution in the range of 30-120 mgl(-1). Inhibition of root growth and reduction of total soluble protein content in root tips of barley seedlings were observed with the increase of Cd concentrations. The changes occurring in random amplified polymorphic DNA (RAPD) profiles of root tips following Cd treatment included variation in band intensity, loss of normal bands and appearance of new bands compared with the normal seedlings. Additionally, we found that the effect of changes was dose-dependent. These results indicated that genomic template stability (a qualitative measure reflecting changes in RAPD profiles) was significantly affected at the above Cd concentration. Thus, DNA polymorphisms detected by RAPD analysis could be used as an investigation tool for environmental toxicology and as a useful biomarker assay for the detection of genotoxic effects of Cd pollution on plants.

Kinetics of the early subcellular distribution of cadmium in rat hepatocytes

The kinetics of the early subcellular distribution of cadmium (Cd) was characterized in primary cultures of rat hepatocytes exposed to 10, 50 and 100 microM Cd in a serum-free WME medium for 10, 30 or 60 min. Our results demonstrate a time- and concentration-dependent increase in Cd content with the highest metal concentration measured in the cytosol, whereas the lowest was observed in the mitochondria. With the exception of early localization in the plasma membrane, Cd concentrations in fractions were characterized by the following decreasing order of magnitude: cytosol > low density molecules approximately nuclei > lysosomes approximately mitochondria. We also found evidence for: (i) a two-step process for Cd distribution in the nuclei and mitochondria; and (ii) a time-dependent 'slow' process of transfer from the plasma membrane to the cytosol. Saturation in Cd uptake was observed at 50 microM in most cell fractions at 10 and 30 min, except for the plasma membrane. The lack of apparent saturation for Cd accumulation at 60 min was not related to an increase in metallothionein synthesis. Altogether, our data provide insights into the dynamics of transfer between intracellular compartments, and allow a better identification of the organelles that are the most subjected to Cd toxicity for early exposure conditions.