Inorganics and hormesis

Investigation of ZnO nanoparticles' ecotoxicological effects towards different soil organisms

INTRODUCTION

Nanomaterials have widespread applications in several industrial sectors. ZnO nanoparticles (NPs) are among the most commonly used metal oxide NPs in personal care products, coating and paints. However, their potential toxicological impact on the environment is largely unexplored.

MATERIALS AND METHODS

The aim of this work was to evaluate whether ZnO nanoparticles exert toxic and genotoxic effects upon terrestrial organisms: plants (Lepidium sativum, Vicia faba), crustaceans (Heterocyipris incongruens), insects (Folsomia candida). To achieve this purpose, organisms pertaining to different trophic levels of the soil ecosystem have been exposed to ZnO NPs. In parallel, the selected soil organisms have been exposed to the same amount of Zn in its ionic form (Zn(2+)) and the effects have been compared.

RESULTS

The most conspicuous effect, among the test battery organisms, was obtained with the ostracod H. incongruens, which was observed to be the most sensitive organism to ZnO NPs. The root elongation of L. sativum was also mainly affected by exposure to ZnO NPs with respect to ZnCl(2), while collembolan reproduction test produced similar results for both Zn compounds. Slight genotoxic effects with V. faba micronucleus test were observed with both soils.

CONCLUSION

Nanostructured ZnO seems to exert a higher toxic effect in insoluble form towards different terrestrial organisms with respect to similar amounts of zinc in ionic form.

Interactions between iron and titanium metabolism in spinach: a chlorophyll fluorescence study in hydropony

One of the elements showing strong beneficial effect on plants at low concentrations and toxic effects at higher concentrations is titanium (Ti). We investigated the interconnection between the Fe uptake and the Ti intoxication in model experiment on Fe-deficient spinach (Spinacia oleracea) plants to help to elucidate the mechanism of the biological activity of titanium in plants. The two different Ti (0 and 20 mg L⁻¹) and two different Fe (0 and 1.35 mg L⁻¹) concentrations in hydroponic medium were used in all four possible combinations. We compared chemical analysis of Ti and Fe in roots and shoots with the changes of the in vivo chlorophyll fluorescence. Although Fe and Ti concentration found in shoots of Ti-non-treated Fe-deficient plants was comparable with that in Ti-treated Fe-deficient plants, the soluble form of Ti present in the growth media had a negative effect on photosynthetic activity monitored by chlorophyll fluorescence measurements. The presence of Fe in growth medium significantly decreased the Ti concentration in shoots and increased the photosynthetic activity. Here, we propose that Ti affect components of electron transport chain containing Fe in their structure (particularly photosystem I) and decrease the photosystem II efficiency.

Cellular stress responses, the hormesis paradigm, and vitagenes: novel targets for therapeutic intervention in neurodegenerative disorders

Despite the capacity of chaperones and other homeostatic components to restore folding equilibrium, cells appear poorly adapted for chronic oxidative stress that increases in cancer and in metabolic and neurodegenerative diseases. Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This article introduces the concept of hormesis and its applications to the field of neuroprotection. It is argued that the hormetic dose response provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose-response relationships, their mechanistic foundations, and their relationship to the concept of biological plasticity, as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This article describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways, including sirtuin and Nrf2 and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. Particular attention is given to the emerging role of nitric oxide, carbon monoxide, and hydrogen sulfide gases in hormetic-based neuroprotection and their relationship to membrane radical dynamics and mitochondrial redox signaling.

Hormesis is central to toxicology, pharmacology and risk assessment

This paper summarizes numerous conceptual and experimental advances over the past two decades in the study of hormesis. Hormesis is now generally accepted as a real and reproducible biological phenomenon, being highly generalized and independent of biological model, endpoint measured and chemical class/physical stressor. The quantitative features of the hormetic dose response are generally highly consistent, regardless of the model and mechanism, and represent a quantitative index of biological plasticity at multiple levels of biological organization. The hormetic dose-response model has been demonstrated to make far more accurate predictions of responses in low dose zones than either the threshold or linear at low dose models. Numerous therapeutic agents widely used by humans are based on the hormetic dose response and its low dose stimulatory characteristics. It is expected that as low dose responses come to dominate toxicological research that risk assessment practices will incorporate hormetic concepts in the standard setting process.

Environmental monitoring of Domingo Rubio stream (Huelva Estuary, SW Spain) by combining conventional biomarkers and proteomic analysis in Carcinus maenas

Element load, conventional biomarkers and altered protein expression profiles were studied in Carcinus maenas crabs, to assess contamination of "Domingo Rubio" stream, an aquatic ecosystem that receives pyritic metals, industrial contaminants, and pesticides. Lower antioxidative activities - glucose-6-phosphate and 6-phosphogluconate dehydrogenases, catalase - were found in parallel to higher levels of damaged biomolecules - malondialdehyde, oxidized glutathione -, due to oxidative lesions promoted by contaminants, as the increased levels of essential - Zn, Cu, Co - and nonessential - Cr, Ni, Cd - elements. Utility of Proteomics to assess environmental quality was confirmed, especially after considering the six proteins identified by de novo sequencing through capLC-muESI-ITMS/MS and homology search on databases. They include tripartite motif-containing protein 11 and ATF7 transcription factor (upregulated), plus CBR-NHR-218 nuclear hormone receptor, two components of the ABC transporters and aldehyde dehydrogenase (downregulated). These proteins could be used as novel potential biomarkers of the deleterious effects of pollutants present in the area.

Comparative population growth of Ceriodaphnia dubia and Daphnia pulex (Cladocera) exposed to zinc toxicity

Population growth of two cladocerans (Ceriodaphnia dubia and Daphnia pulex) exposed to 4 different concentrations of ZnCl(2) (0.125, 0.25, 0.5 and 1.0 mg L(-1), plus controls) at one algal food (Scenedesmus acutus) density (0.5 X 10(6) cells mL(-1)) was quantified for 30 days. Population densities of C. dubia and D. pulex decreased with increasing concentration of Zn in the medium. At a concentration of 1 mg L(-1) of ZnCl(2), both C. dubia and D. pulex did not reproduce and died within a week. The peak population densities of C. dubia ranged from 0.2 to 6.0 ind. mL(-1), depending on the Zn level in the medium, whereas this range was lower for D. pulex (0.2 to 4.1 ind. mL(-1)). The peak population density was inversely related to the Zn concentration. The rate of population increase (r) varied from -0.12 to +0.14 and -0.02 to +0.23 per day for C. dubia and D. pulex, respectively, depending on the Zn level in the medium. Statistically, both the peak population density and the r were significantly affected by the heavy metal concentration in the medium. Multiple comparison tests showed that the rate of population increase (r) of D. pulex in the lowest ZnCl(2) level (0.125 mg L(-1)) was significantly higher than controls. However, under similar conditions, the r of C. dubia was significantly lower than controls. With a further increase in Zn level, the growth rates of both the cladoceran species were significantly reduced as compared to controls. The results are discussed in relation to published data on the toxicity of zinc to freshwater zooplankton.

Population level indicators of stress: Effects of two heavy metals (copper and mercury) on the growth of Lecane quadridentata (Ehrenberg, 1830) (Rotifera: Lecanidae)

We quantified the population level responses of Lecane quadridentata exposed separately to five different concentrations of Cu (0.3125, 0.625, 1.25, 2.5 and 5.0 micro g/L as CuSO(4)) and Hg (0.0844, 0.1688, 0.3375, 0.675 and 1.35 micro g/L as HgCl(2)) at 23 degrees C and under an algal food density of 1 x 10(6) cells/mL of Chlorella vulgaris for 24 days. We derived peak population densities and the rate of population increase (r) per day in both controls and in heavy metal treatments. In general, the population growth of L. quadridentata decreased with increasing concentration of Cu or Hg in the medium. The growth was similar in controls and in the lowest tested concentration of Cu (0.3125 micro g/L), whereas in Hg treatments, even at the lowest tested concentration (0.0844 micro g/L), there was a drastic decline of rotifer densities. The peak population densities in controls were 63+/- 5 ind./mL, whereas in heavy metal treatments, they ranged from 1 to 58 ind./mL depending on the concentration. The rate of population increase also decreased with increasing concentration of Cu or Hg. In general, the r varied from +0.23+/- 0.01 to -0.12+/- 0.02, depending on the metal type and concentration. The differences in the population level responses of L. quadridentata to Cu and Hg were interpreted in terms of relative toxicity of the chosen heavy metals. Since our study showed that peak population density was also sensitive to metal stress, we recommend the use of this variable together with the rate of population increase in ecotoxicological evaluations.

In vivo screening to determine hazards of nanoparticles: nanosized TiO2

A single-species laboratory test with terrestrial invertebrates was used to identify the hazard of nanosized TiO(2). Feeding parameters, weight change, mortality, and the activities of catalase and glutathione-S-transferase were evaluated after 3 or 14 days of dietary exposure. The effects of nano-TiO(2) were dependent on exposure concentration and duration, total consumed quantity, size and pre-treatment of particles. The intensity of a response was ruled by duration of exposure and not by consumed quantity of nano-TiO(2) or exposure concentration as expected. The response to nano-TiO(2) is described as threshold-like. The exposure concentrations 10-1000 microg TiO(2)/g dry food (1.35-1025 microg of total consumed quantity of TiO(2)/g animal wet wt.) were identified as safe for tested species after tested exposure period. We conclude that the response to nanoparticles is different from that of soluble chemicals therefore these two types of data should be interpreted and processed differently.

Developmental mechanisms of arsenite toxicity in zebrafish (Danio rerio) embryos

Arsenic usually accumulates in soil, water and airborne particles, from which it is taken up by various organisms. Exposure to arsenic through food and drinking water is a major public health problem affecting some countries. At present there are limited laboratory data on the effects of arsenic exposure on early embryonic development and the mechanisms behind its toxicity. In this study, we used zebrafish as a model system to investigate the effects of arsenite on early development. Zebrafish embryos were exposed to a range of sodium arsenite concentrations (0-10.0mM) between 4 and 120h post-fertilization (hpf). Survival and early development of the embryos were not obviously influenced by arsenite concentrations below 0.5mM. However, embryos exposed to higher concentrations (0.5-10.0mM) displayed reduced survival and abnormal development including delayed hatching, retarded growth and changed morphology. Alterations in neural development included weak tactile responses to light (2.0-5.0mM, 30hpf), malformation of the spinal cord and disordered motor axon projections (2.0mM, 48hpf). Abnormal cardiac function was observed as bradycardia (0.5-2.0mM, 60hpf) and altered ventricular shape (2.0mM, 48hpf). Furthermore, altered cell proliferation (2.0mM, 24hpf) and apoptosis status (2.0mM, 24 and 48hpf), as well as abnormal genomic DNA methylation patterning (2.0mM, 24 and 48hpf) were detected in the arsenite-treated embryos. All of these indicate a possible relationship between arsenic exposure and developmental failure in early embryogenesis. Our studies suggest that the negative effects of arsenic on vertebrate embryogenesis are substantial.

A perspective on the scientific, philosophical, and policy dimensions of hormesis

The hormesis concept has broad implications for biology and the biomedical sciences. This perspective on hormesis concentrates on toxicology and toxicological risk assessment and secondarily explores observations from other fields. It considers the varied manifestations of hormesis in the context of a broad family of biological stress responses. Evidence for hormesis is reviewed, and the hormesis model is contrasted with more widely accepted dose-response models in toxicology: a linear nonthreshold (LNT) model for mutagenesis and carcinogenesis, and a threshold model for most other toxicologic effects. Scientific, philosophical, and political objections to the hormesis concept are explored, and complications in the hormesis concept are analyzed. The review concludes with a perspective on the current state of hormesis and challenges that the hormesis model poses for risk assessment.

Induction of cytoplasmic accumulation of p53: a mechanism for low levels of arsenic exposure to predispose cells for malignant transformation

Although epidemiologic studies have linked arsenic exposure to the development of human cancer, the mechanisms underlying the tumorigenic role of arsenic remain largely undefined. We report here that treatment of cells with sodium arsenite at the concentrations close to environmental exposure is associated with the up-regulation of Hdm2 and the accumulation of p53 in the cytoplasm. Through the mitogen-activated protein kinase pathway, arsenite stimulates the P2 promoter-mediated expression of Hdm2, which then promotes p53 nuclear export. As a consequence, the p53 response to genotoxic stress is compromised, as evidenced by the impaired p53 activation and apoptosis in response to UV irradiation or 5FU treatment. The ability of arsenite to impede p53 activation is further demonstrated by a significantly blunted p53-dependent tissue response to 5FU treatment when mice were fed with arsenite-containing water. Together, our data suggests that arsenic compounds predispose cells to malignant transformation by up-regulation of Hdm2 and subsequent p53 inactivation.

Sodium arsenite induces ROS generation, DNA oxidative damage, HO-1 and c-Myc proteins, NF-kappaB activation and cell proliferation in human breast cancer MCF-7 cells

Epidemiological evidence has associated exposure to arsenic (As) in drinking water with an increased incidence of human cancers in the skin, bladder, liver, kidney and lung. Sodium arsenite mimics the effects of estradiol and induces cell proliferation in the estrogen responsive breast cancer cell line MCF-7. Therefore, our aim was to further explore the ability of sodium arsenite to induce MCF-7 epithelial breast cell proliferation and some of its underlying mechanisms by studying ROS production, c-Myc and HO-1 protein levels, 8-OHdG formation and NF-kappaB activation. Low arsenite concentrations (0.5-5 microM) induced ROS production and ROS-related depolarization of the mitochondrial membrane suggesting that mitochondria played an important role in the oxidative effects of As. ROS-mediated DNA damage as measured by the presence of 8-OHdG DNA-adducts in their nuclei, IkappaB phosphorylation, NF-kappaB activation and increases in c-Myc and HO-1 protein levels were also observed, suggesting that these factors play a relevant role in the arsenite induced MCF-7 cell recruitment into the S-phase of the cell cycle and cell proliferation observed. In conclusion, arsenite activates several pathways involved in MCF-7 cell proliferation suggesting that arsenite exposure may pose a risk for breast cancer in human exposed populations notwithstanding that most studies to date have not yet implicated this metalloid as a cofactor in the etiology of this disease.

Antinociceptive, hypoglycemic and spasmolytic effects of Brickellia veronicifolia

INTRODUCTION

Brickellia veronicifolia (Kunth) Gray (Asteraceae) (BV) is broadly commercialized for treating gastrointestinal diseases (stomach aches, biliary colics and dyspepsia), arthritis, diabetes and painful inflammatory complaints.

AIMS OF THE STUDY

In order to complete the preclinical pharmacological profile of BV, first the antinociceptive effect of an organic extract (BVE) and isolated metabolites on the hot plate and writhing tests was assessed.

EXPERIMENTAL

Then, their potential hypoglycemic effects were analyzed in normoglycemic and diabetic rats; in addition, an oral glucose tolerance test (OGTT) was performed. Finally, the spasmolytic activity of BVE was assessed in vivo using the gastrointestinal motility test (GMT) in mice.

RESULTS

The results revealed that BVE (100-600 mg/kg), 6-methoxysalicylic acid (1), 2-methoxybenzoic acid (2), benzyl-2,6-dimethoxybenzoate (3), and taraxasteryl acetate (4) showed significant analgesic effects. Compounds 2 and 3 were the most active (1-100mg/kg) in the hot plate and writhing tests, respectively. In the antidiabetic assays, BVE (100mg/kg) showed an important hypoglycemic action. Furthermore, at the same dose, it provoked a significant postprandial decrease of blood glucose level after 30 min of a glucose challenge. Finally, the GMT in mice revealed the spasmolytic activity in vivo of BVE (31.6 mg/kg).

CONCLUSION

The overall information tends to support the vernacular uses of the plant.

Body size and population growth of Brachionus patulus (Rotifera) in relation to heavy metal (copper and mercury) concentrations

The effect of different sublethal concentrations of Cu(+ 2) and Hg(+ 2) was evaluated on the population growth and morphometry (body width and postero-lateral spines) of Brachionus patulus. When exposed to Cu(+ 2) at lower concentrations (i.e., 2.5% and 5.0% of LC(50)), the population growth of B. patulus did not differ significantly from controls, while at the corresponding conditions, Hg(+ 2) had adverse effects. The rate of population increase (r) varied from 0.13 to 0.28 per day, depending on the metal type and its concentration in the medium. In general, increase in metal concentration decreased the rate of population increase. When the daily growth rate was plotted as function of population density over the entire growth phase, a significantly inverse relation was obtained for controls and in treatments containing lower concentrations (2.5 and 5% of LC(50)) of heavy metals. However, under higher metal concentrations, the relation between population density and the daily growth rate became non-significant. Morphometric data showed that Cu caused larger bodied individuals, while the opposite effect was resulted due to Hg(+ 2). Postero-lateral spines of B. patulus were also significantly influenced by the concentration of Cu(+ 2) or Hg(+ 2). Thus, compared to controls, there was reduction in the spine length, at low concentrations of Cu(+ 2), but at all the four tested levels of Hg(+ 2). Our data suggested that in addition to the population growth rate, morphometric measurements need to be considered for understanding the impact of heavy metals to brachionid rotifers.

Aqueous crude extract of Rhoeo discolor, a Mexican medicinal plant, decreases the formation of liver preneoplastic foci in rats

There are many plants in Mexico with medicinal properties, some of them used in alternative medicine to treat cancer, such is the case of Rhoeo discolor L. Hér Hance (Commelinaceae family); however, there are not scientific reports that validate their antitumoral property. The present study shows the protective effects of the Rhoeo discolor aqueous crude extract (ACE) against rat liver cancer using the resistant-hepatocyte model. The carcinogenesis protocol consisted on the initiation with N-diethylnitrosamine, followed by the promotion with 2-acetylaminofluorene and a partial hepatectomy. After 24 days, the gamma-glutamyl transpeptidase positive, corresponding to altered hepatocytes foci (AHF), were quantified. Additionally to discard a possible carcinogenic effect of ACE, it was first tested as promoting agent instead 2-acetylaminofluorene, and second, ACE was administered as initiator and promoter instead of the whole carcinogenic treatment. In summary, firstly, ACE administration reduced the number and area of preneoplastic lesions with dose below 20mg/kg body weight and secondly, ACE administration neither presented a promoting or initiator effects nor induced the development of AHF. Results of this investigation justify continuing with further studies of Rhoeo discolor components to develop chemoprevention strategies as an option in the treatment of cancer.

Biochemical biomarkers in chronically metal-stressed daphnids

Biochemical biomarkers are a popular measure of toxic effects on organisms due to their assumed fast response, and are usually assessed after acute exposure of the organism to the stressor. However, increasing interest in the use of biochemical biomarkers in environmental pollution monitoring calls for more laboratory long-term studies of contaminants' effects on biochemical endpoints. In this study, four biochemical biomarkers (protein content, activity of cholinesterase (ChE), catalase (CAT) and glutathione S-transferase (GST), were correlated with standardised reproductive and survival endpoints of water fleas (Daphnia magna) after chronic exposure to Cr (VI) and Cd. No effect on the reproduction and survival was noticed up to the highest tested concentration of Cr (VI) (52.5 microg/L), while the protein content, and the ChE and CAT activity decreased, and GST activity increased. Cd affected reproduction of daphnids above 0.656 microg/L, but the protein content and ChE activity were changed at 0.328 microg/L and 0.082 microg/L of Cd, respectively. Biochemical biomarkers in some cases proved to be equally or more sensitive than reproduction and mortality. We recommend more frequent use of a battery of biochemical biomarkers in combination with other higher-level biomarkers also in chronic studies and not only in the acute ones.

Hormesis and precaution: the twain shall meet

Regulatory focus on quantifying risk of disease or death from exposure to hazardous substances via monotonic dose-response models has downplayed or even rejected potential benefits to human health from exposures to low (sub-threshold) doses, and thus represented by either U-shaped or J-shaped models. On the other hand, most environmental health policy hypothesizes, without firm evidence, that cancer risk is proportional to exposure at low doses of current ambient exposures. An acceptable exposure is determined by either setting a somewhat arbitrary ;acceptable' level of risk, such as one in a million excess individual lifetime cancer risk or, in the case of several types of animal toxicological test results, applying multiplicative safety factors to a specific concentration, generally derived from a benchmark dose or NOAEL. This seemingly precautionary approach is questionable in light of much experimental evidence indicating protective effects of exposure at low doses - U-shaped or J-shaped models. We demonstrate that incorporating the possibility of hormesis into regulatory decision-making is precautionary, while use of default results in policy conflicts with precaution.

Prey (Brachionus calyciflorus and Brachionus havanaensis) exposed to heavy metals (Cu and Cd) for different durations and concentrations affect predator's (Asplanchna brightwellii) population growth

In this work, we evaluated the combined effects of heavy metal concentration (CuSO(4) and CdCl(2) separately, at 0.1, 0.2 and 0.4 mg/L) and exposure time (6 and 24 h) on the population growth of the predatory rotifer Asplanchna brightwellii (predator) via its prey (Brachionus calyciflorus or Brachionus havanaensis). At low Cu concentration, B. calyciflorus stimulated the population growth of its predator as compared to controls. However, under higher Cd concentration and longer exposure time, B. calyciflorus caused lower growth of A. brightwellii. B. havanaensis previously exposed to Cu led to reduced growth of its predator, especially under high metal concentration. Regardless of the treatments, the population growth rates (r) of A. brightwellii varied from 0.11 +/- 0.02 to 0.45 +/- 0.04 d(- 1). The exposure time had significant influence on r of A. brightwellii via only the Cd-exposed prey B. havanaensis. However, regardless of the exposure time, B. calyciflorus exposed to Cu and B. havanaensis exposed to Cd stimulated the growth rate of the predator. In our study the predator was not directly exposed to heavy metals. Therefore, the toxicants acted in an indirect way on A. brightwellii, i. e., the predator's growth was affected through the ingestion of metal-contaminated prey.

The combined effects of heavy metals (copper and zinc), temperature and food (Chlorella vulgaris) level on the demographic characters of Moina macrocopa (Crustacea: Cladocera)

In this work we evaluated the effect of temperature (22 degrees and 27 degrees C), algal density (Chlorella vulgaris 0.5 x 10(6) and 2.0 x 10(6) cells/mL) and different combinations of 2 heavy metals (Zn at 25.25-101.0 microg/L and Cu at 17.75-71.0 microg/L) on the population level variables of Moina macrocopa. Median lethal concentration (24 h bioassay at 1 x 10(6) cells/mL of algal diet) of Zn and Cu for M. macrocopa were 1010 microg/L and 710 microg/L, respectively. In the survivorship curves at 27 degrees C there was a reduction in the survival of cladocerans exposed to Cu compared to controls or Zn. The fecundity curves (m(x)) indicated a steady reproductive output throughout the life span of M. macrocopa, but the negative impact of copper was more than that of zinc. Reproductive phase of M. macrocopa was longer at 22 degrees C than at 27 degrees C. The average lifespan was higher at 22 degrees C and at the higher food level. It was significantly affected by temperature, food level and toxicant concentration, as well as their interaction. The net reproductive rate was also influenced by food and temperature but not by the toxicant level. The generation time ranged between 4-8 days and was lower at 27 degrees C. The population growth rate (r) derived from life table experiments varied from 0.6 to 0.9 per day, depending on the treatment. Regardless of the toxicant level, at 22 degrees C, the population growth was higher at the higher food level. In treatments containing only Cu, the population growth of M. macrocopa was lower than when present together with Zn. Peak population densities of around 30 ind./mL were reached under high food conditions. Higher temperature and lower food level had an adverse effect on M. macrocopa in treatments containing only Cu. In the presence of higher food density, the adverse impact of copper was not evident. The r derived from growth study ranged from 0.25 to 0.64 per day depending on the test conditions. Data were interpreted in terms of sensitivity M. macrocopa for its possible use as a complement, but not as an alternative to Daphnia magna for evaluating the toxicity of heavy metals.

The role of oxidative stress in hormesis induced by sodium arsenite in human embryo lung fibroblast (HELF) cellular proliferation model

Hormetic dose-response relationships induced by environmental agents are often characterized by a low-dose stimulation and a high-dose inhibition. The mechanisms underlying hormesis induced by environmental agents still remain an enigma; however, hormetic consequences may have significant implications for health risk assessments. To investigate the role of oxidative stress in hormetic phenomena associated with cell proliferation induced by sodium arsenite, the levels of reactive oxygen species (ROS), lipid peroxidation (LPO), and heat-shock proteins (HSP) and the activities of glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) were measured in human embryo lung fibroblast (HELF) cells after treatment with sodium arsenite at various concentrations for differing times. Results showed that sodium arsenite induced significant cell proliferation at low concentrations (0.5 microM for 12, 24, and 48 h), but inhibited cell growth at high amounts (5 and 10 microM for 24 and 48 h), reflected as a beta concentration-response curve. Data indicated that the relationship between ROS levels and sodium arsenite exposure concentration displayed a positive correlation. It was found out that sodium arsenite at high concentrations induced LPO damage. The activities of SOD were enhanced at low metal concentrations but inhibited with high amounts in a concentration-dependent manner. Similarly, heat-shock protein 27 (HSP27) levels were increased by sodium arsenite of low concentrations with early exposure time (3, 6, and 12 h), but decreased with high metal concentrations with greater exposure time (24 and 48 h). Sodium arsenite decreased HSP70 expression at lower concentrations, but increased HSP70 expression at higher concentration. The results indicated that this cellular hormetic model of cell proliferation induced by sodium arsenite occurred in HELF cells, which may explain contradictory effects seen with this metal. Sodium arsenite at low concentrations induced enhanced ROS generation without cytotoxicity and a cellular protective effect. In contrast, sodium arsenite at high concentrations produced marked ROS formation, marked oxidative stress, and cellular damage, as evidenced by LPO.

Light quanta modulated characteristics of Ni uptake by Brassica juncea seedlings: the interdependence of plant metal concentration and biomass

The relationships between the concentration of metal in the growth medium, Cs, the concentration of metal absorbed by the plant, Cp, and the total biomass achieved, M, all of which are factors relevant to the efficiency of metal uptake and tolerance by the plant, have been investigated via the physiological response of Brassica juncea seedlings to Ni stress. The factorial growth experiments treated the Ni concentration in agar medium and the diurnal light quanta as independently variable parameters. Observations included the evidence of light enhancement of Ni toxicity in the root, as well as at the whole-plant level. The shoot mass index possibly is an indicator of the amount of shoot metal sequestration in B. juncea, as are the logarithmic variation of Cp with Cs and the power-law dependence of M on Cp. The sum total of these observations indicates that, for the Ni accumulating plant B. juncea, the overall metabolic allocation to either growth or metal tolerance of the plant is important. Neither a rapid biomass increase nor a high metal absorbed concentration favored the removal of high metal mass from the medium. Rather, the plants with a moderate rate of biomass growth and a moderate absorbed metal concentration demonstrated the ability to remove the maximum mass of metal from the medium. The implication of these results as related to the extant model of phyoextraction efficiency is discussed.

Hormesis and its place in nonmonotonic dose-response relationships: some scientific reality checks

OBJECTIVE

This analysis is a critical assessment of current hormesis literature. I discuss definitions, characterization, generalizability, mechanisms, absence of empirical data specific for hormesis hypothesis testing, and arguments that hormesis be the "default assumption" in risk assessment.

DATA SOURCES

Hormesis, a biological phenomenon typically described as low-dose stimulation from substances producing higher-dose inhibition, has recently garnered interest in several quarters. The principal sources of published materials for this analysis are the writings of certain proponents of hormesis. Surprisingly few systematic critiques of current hormesis literature exist. Limits to the phenomenon's appropriate role in risk assessment and health policy have been published.

DATA SYNTHESIS

Serious gaps in scientific understanding remain: a stable definition; generalizability, especially for humans; a clear mechanistic basis; limitations in the presence of multiple toxic end points, target organs, and mechanisms. Absence of both arms-length, consensus-driven, scientific evaluations and empirical data from studies specifically designed for hormesis testing have limited its acceptance.

CONCLUSIONS

Definition, characterization, occurrence, and mechanistic rationale for hormesis will remain speculative, absent rigorous studies done specifically for hormesis testing. Any role for hormesis in current risk assessment and regulatory policies for toxics remains to be determined.

Potential of Chilopsis linearis for gold phytomining: using XAS to determine gold reduction and nanoparticle formation within plant tissues

This study reports on the capability of the desert plant Chilopsis linearis (Cav.) Sweet (desert willow) to uptake gold (Au) from gold-enriched media at different plant-growth stages. Plants were exposed to 20, 40, 80, 160, and 320 mg Au L(-1) in agar-based growing media for 13, 18, 23, and 35 d. The Au content and oxidation state of Au in the plants were determined using an inductively coupled plasma/optical emission spectrometer (ICP/OES) and X-ray absorption spectroscopy (XAS), respectively. Gold concentrations ranging from 20 to 80 mg Au L(-1) did not significantly affect Chilopsis linearis plant growth. The concentration of gold in the plants increased as the age of the plant increased. The Au concentrations in leaves for the 20, 40, 80, and 160 mg Au L(-1) treatments were 32, 60, 62, and 179 mg Au kg(-1) dry weight mass, respectively, demonstrating the gold uptake capability of desert willow. The XAS data indicated that desert willow produced gold nanoparticles within plant tissues. Plants exposed to 160 mg Au L(-1) formed nanoparticles that averaged approximately 8, 35, and 18 A in root, stem, and leaves, respectively. It was observed that the average size of the Au nanoparticles formed by the plants is related to the total Au concentration in tissues and their location in the plant

The applicability of acetylcholinesterase and glutathione S-transferase in Daphnia magna toxicity test

The most commonly used toxicity test worldwide is the acute Daphnia magna test. The relevance of acetylcholinesterase (AChE) and glutathione S-transferase (GST) activity in D. magna exposed to chromium, cadmium, and diazinon was evaluated in connection with this standard test. We found no link between enzyme activities and immobility. Concentrations of Cr(6+) up to 280 microg/L had no effect on AChE and GST activities, while 20% immobility was observed. At concentrations of 20-25 microg/L of Cd(2+) AChE activity was increased by about 50%. The effect of diazinon on both enzymes was insignificant up to concentrations that caused 27% immobility. Consequently, while the use of AChE and GST activities is recommended when the mode of action of chemicals is studied, the value of these biomarkers in routine acute toxicity tests is limited because the relationship between enzyme activities and immobility of D. magna exposed to different chemicals is unclear.

Turbidity mitigates lead toxicity to cladocerans (Cladocera)

To test the hypothesis that sediment would have a synergistic effect on the toxicity of lead to cladocerans, we performed life table demography experiments with two pelagic (Diaphanosoma birgei and Moina micrura) and one littoral (Alona rectangula) cladoceran species. Life table demography experiments were conducted at three levels of turbidity (0, 17 and 170 NTU) and six concentrations of lead (as PbCl2) from 0 to 0.71 mg l(-1). Median lethal concentrations (LC50) forA. rectangula, D. birgei and M. micrura were 7.06 +/- 0.39, 3.16 +/- 0.25 and 3.24 +/- 0.69 mg l(-1) of Pb. Life table study showed that in general, the presence of sediments in test jars allowed an overall increase of 20-75% in both survivorship and reproduction of the cladoceran species exposed to different concentrations of Pb. At 0.04 mg l(-1) of Pb, the population growth rates were 0.127 forA. rectangula, 0.037 for D. birgei and 0.471 d(-1) for M. micrura in the absence of sediments but were elevated in their presence (0.309, 0.141 and 0.722 d(-1), respectively). The data have been discussed in relation to their importance in shallow, turbid Mexican waterbodies.

The thyroid endocrine disruptor perchlorate affects reproduction, growth, and survival of mosquitofish

The perchlorate anion--an oxidizer found in rockets, missiles, some ammunition, flares, airbags, and fireworks--occurs as a contaminant in ground and surface water in many parts of the United States. Its toxic effects include inhibition of thyroid hormone synthesis. To investigate its chronic toxicity, mosquitofish (Gambusia holbrooki) adults and fry were exposed to aqueous sodium perchlorate at 1, 10, and 100mg/L, and growth and reproductive performance (fecundity, eggs/embryos mass, and gonadosomatic index [GSI]) were determined. Five-day acute toxicity tests were also performed. Perchlorate had a stimulatory effect on fecundity, GSI, and egg/embryo mass, at least for some treatments. The LC50 of sodium perchlorate was 404 mg/L. Growth was enhanced at 1mg/L but inhibited at 10mg/L. These results suggest that, at environmentally relevant concentrations, perchlorate does not induce acutely toxic effects but may have mild stimulatory or hormetic effects on fitness parameters in this species.

Review of recent ecotoxicological studies on cladocerans

Cladocerans have been widely used as the bioassay organisms in evaluating the impact of different toxic substances. Literature survey during the last 10 years revealed that cladoceran ecotoxicological research has been in an exponential phase constituting nearly 10% of publications on this group. Many studies have considered typically planktonic taxa such as Daphnia magna, D. pulex, Moina macrocopa, M. micrura and Ceriodaphania dubia. Experimental data on toxicity tests, to a lesser extent, are also available for littoral-benthic genera such as Simocephalus, Macrothrix and Alona. Most toxicity tests are limited to the derivation of median lethal concentrations of various durations but mostly at 24 or 48 h. Studies related to the evaluation of changes in the life history variables of cladocerans as a result of sublethal exposure to toxic substances are not many, but gaining importance. The common toxic substances used in the cladoceran toxicity tests appear to be heavy metals, pesticides and a few natural toxins such as cyanotoxins. We review here the effect of different toxic substances on cladocerans based on both the field and the laboratory studies from an ecotoxicology point of view. Suggestions for the future cladoceran ecotoxicology are also commented on.

Paradigm lost, paradigm found: the re-emergence of hormesis as a fundamental dose response model in the toxicological sciences

This paper provides an assessment of the toxicological basis of the hormetic dose-response relationship including issues relating to its reproducibility, frequency, and generalizability across biological models, endpoints measured and chemical class/physical stressors and implications for risk assessment. The quantitative features of the hormetic dose response are described and placed within toxicological context that considers study design, temporal assessment, mechanism, and experimental model/population heterogeneity. Particular emphasis is placed on an historical evaluation of why the field of toxicology rejected hormesis in favor of dose response models such as the threshold model for assessing non-carcinogens and linear no threshold (LNT) models for assessing carcinogens. The paper argues that such decisions were principally based on complex historical factors that emerged from the intense and protracted conflict between what is now called traditional medicine and homeopathy and the overly dominating influence of regulatory agencies on the toxicological intellectual agenda. Such regulatory agency influence emphasized hazard/risk assessment goals such as the derivation of no observed adverse effect levels (NOAELs) and the lowest observed adverse effect levels (LOAELs) which were derived principally from high dose studies using few doses, a feature which restricted perceptions and distorted judgments of several generations of toxicologists concerning the nature of the dose-response continuum. Such historical and technical blind spots lead the field of toxicology to not only reject an established dose-response model (hormesis), but also the model that was more common and fundamental than those that the field accepted.

Effects of low doses of dietary lead on puberty onset in female mice

Female Swiss mice typically display signs of puberty at about 33-37 days of age. In the present investigation (96 female mice tested in 8 Pb exposure levels, n=12 per exposure level), the time to puberty onset was markedly influenced by exposure to dietary lead. While modest increases in blood lead concentrations from a normal background of 2-3 to 13.2 microg/dl delayed the onset of puberty by 15-20% to about 40-43 days, reducing blood lead from 2-3 to 0.7 microg/dl was associated with an acceleration of puberty to 21 days, an enhancement by over 30%. This dose-response relationship represents novel findings of possible ecological as well as public health significance and indicates that lead is able to induce biologically significant changes at blood lead levels previously thought to be without effect.

Differential uptake and transport of trivalent and hexavalent chromium by tumbleweed (Salsola kali)

Experiments were conducted to determine the differential absorption of Cr species by tumbleweed (Salsola kali) as well as the effect of this heavy metal on plant growth and nutrient uptake. Tumbleweed seeds were grown in an agar-based media containing different concentrations of either Cr(III) or Cr(VI). The results demonstrated that the uptake of Cr was influenced by the Cr concentration in the growth medium and the speciation of this heavy metal. When supplied in the hexavalent form, the concentration of Cr in the different plant parts (2900, 790, and 600 mg kg(-1) for roots, stems, and leaves, respectively) was between 10 and 20 times higher than the amounts found when Cr was supplied in the trivalent form. In addition, it was found that in most of the experiments, Cr(III) exhibited more toxic effects on tumbleweed plants than Cr(VI). The size of roots of plants grown in 20 mg L(-1) Cr(III) were significantly smaller (p < 0.05) than those grown in 20 mg L(-1) Cr(VI). Plants exposed to 20 mg L(-1) Cr(III) produced shoots significantly shorter (p < 0.05) compared with the size of control plants and with those grown in 20 mg L(-1) Cr(VI). In addition, the absorption of macronutrients and microelements was in general lower when the plants were grown in the medium containing Cr(III). The amounts of Cr concentrated in the aerial plant parts under experimental conditions may indicate tumbleweed as a new option for the phytoremediation of Cr-contaminated soil.