Molecular Insight into Iron Homeostasis of Acute Myeloid Leukemia Blasts.

Acute myeloid leukemia (AML) remains a disease of gloomy prognosis despite intense efforts to understand its molecular foundations and to find efficient treatments. In search of new characteristic features of AML blasts, we first examined experimental conditions supporting the amplification of hematological CD34+ progenitors ex vivo. Both AML blasts and healthy progenitors heavily depended on iron availability. However, even if known features, such as easier engagement in the cell cycle and amplification factor by healthy progenitors, were observed, multiplying progenitors in a fully defined medium is not readily obtained without modifying their cellular characteristics. As such, we measured selected molecular data including mRNA, proteins, and activities right after isolation. Leukemic blasts showed clear signs of metabolic and signaling shifts as already known, and we provide unprecedented data emphasizing disturbed cellular iron homeostasis in these blasts. The combined quantitative data relative to the latter pathway allowed us to stratify the studied patients in two sets with different iron status. This categorization is likely to impact the efficiency of several therapeutic strategies targeting cellular iron handling that may be applied to eradicate AML blasts.

Time-dependent effects of hypoxia on cell metabolism, signaling, and iron homeostasis in the hematopoietic progenitor model KG1a.

Lowered availability of oxygen in the micro-environment of cells perturbs metabolic and signaling pathways. It affects proliferation, tissue morphology, and differentiation. Leukemia impairs maturation of hematopoietic progenitors: the immune system, healing, and erythropoiesis are weakened, thereby perturbing iron homeostasis and further lowering oxygen provision to tissues. Here, the time-dependent molecular consequences of sudden hypoxia were studied in the KG1a model of immature hematopoietic progenitors. The oxygen tension of KG1a cells was abruptly lowered from the experimentally usual ca. 20 to 1%. Growth and key hubs of signaling, metabolism, and iron homeostasis were monitored by a combination of immunological methods and functional assays. The collapse of oxygen availability stopped proliferation after one generation. The number of cells then remained approximately constant over several days, including after anaerobic changes in the culture medium. Lowered oxygen resulted in transient increase of the hypoxia-inducible factor 1α and of its REDD1 target, inhibition of mechanistic (or mammalian) target of rapamycin, decreased autophagy, altered cap-dependent translation, and minimal repression of the already weak oxidative phosphorylation. These adjustments did not trigger important cellular iron fluxes since the cells relied on their internal iron stores to survive. In conclusion, the response of the KG1a cells to stringent hypoxia is varied, with some established hypoxia-sensitive pathways exhibiting activation whereas others were unaffected. The results draw attention to the flexibility of the environmental adaptation of cancer cells. They suggest that thorough characterization of early leukemic blasts is warranted to propose informed treatments to patients.

Low-level cadmium doses do not jeopardize the insulin secretion pathway of ß-cell models until the onset of cell death.

BACKGROUND: Cadmium is an inescapable environmental pollutant that permeates the food chain and has been debatably associated with diabetes in humans. PURPOSE AND PROCEDURES: To probe the specific impact of low-level cadmium exposure on insulin production, largely sub-cytotoxic (50-500 nM) concentrations of cadmium chloride challenged the INS-1 and MIN6 rodent models of pancreatic ß-cells for the longest possible time, up to 4 days, before sub-culturing. MAIN FINDINGS: The concentration of detectable oxidants, the pattern of the actin cytoskeleton, the translocation of ß-catenin, the activity of protein phosphatases, calcium traffic, and the phosphorylation status of several key signaling nodes, such as AMP kinase and mitogen activated kinases including nuclear translocation of Extracellular signal-Regulated Kinase, were all insensitive to the applied very low cadmium doses. Accordingly, low-level cadmium exposure did not alter the insulin secretion ability, the functional hallmark of ß-cells, before the onset of cell death. CONCLUSIONS: These data define an operational toxicological threshold for these cellular models of ß-cells that should be useful to address insulin secretion and the diabetogenic effects of chronic low-level cadmium exposure in animal models and in humans.

Cellular Dynamics of Transition Metal Exchange on Proteins: A Challenge but a Bonanza for Coordination Chemistry.

Transition metals interact with a large proportion of the proteome in all forms of life, and they play mandatory and irreplaceable roles. The dynamics of ligand binding to ions of transition metals falls within the realm of Coordination Chemistry, and it provides the basic principles controlling traffic, regulation, and use of metals in cells. Yet, the cellular environment stands out against the conditions prevailing in the test tube when studying metal ions and their interactions with various ligands. Indeed, the complex and often changing cellular environment stimulates fast metal-ligand exchange that mostly escapes presently available probing methods. Reducing the complexity of the problem with purified proteins or in model organisms, although useful, is not free from pitfalls and misleading results. These problems arise mainly from the absence of the biosynthetic machinery and accessory proteins or chaperones dealing with metal / metal groups in cells. Even cells struggle with metal selectivity, as they do not have a metal-directed quality control system for metalloproteins, and serendipitous metal binding is probably not exceptional. The issue of metal exchange in biology is reviewed with particular reference to iron and illustrating examples in patho-physiology, regulation, nutrition, and toxicity.

Emerging Links between Cadmium Exposure and Insulin Resistance: Human, Animal, and Cell Study Data.

Recent research has helped clarify the role of cadmium (Cd) in various pathological states. We have demonstrated Cd involvement in pancreatic cancer, as well as the bioaccumulation of Cd in the pancreas. Bioaccumulation and increased toxicity suggest that Cd may also be involved in other pancreas-mediated diseases, like diabetes. Cd falls into the category of "hyperglycemic" metals, i.e., metals that increase blood glucose levels, which could be due to increased gluconeogenesis, damage to ß-cells leading to reduced insulin production, or insulin resistance at target tissue resulting in a lack of glucose uptake. This review addresses the current evidence for the role of Cd, leading to insulin resistance from human, animal, and in vitro studies. Available data have shown that Cd may affect normal insulin function through multiple pathways. There is evidence that Cd exposure results in the perturbation of the enzymes and modulatory proteins involved in insulin signal transduction at the target tissue and mutations of the insulin receptor. Cd, through well-described mechanisms of oxidative stress, inflammation, and mitochondrial damage, may also alter insulin production in ß-cells. More work is necessary to elucidate the mechanisms associated with Cd-mediated insulin resistance.

Theoretical Modeling of Oral Glucose Tolerance Tests Guides the Interpretation of the Impact of Perinatal Cadmium Exposure on the Offspring's Glucose Homeostasis.

Oral glucose tolerance tests, in which the concentration of glucose is monitored in the circulation over 2 h after ingesting a bolus, probe diabetic or pre-diabetic conditions. The resulting glucose curves inform about glucose turnover, insulin production and sensitivity, and other parameters. However, extracting the relevant parameters from a single complex curve is not straightforward. We propose a simple modeling method recapitulating the most salient features of the role of insulin-secreting pancreatic ß -cells and insulin sensitive tissues. This method implements four ordinary differential equations with ten parameters describing the time-dependence of glucose concentration, its removal rate, and the circulating and stored insulin concentrations. From the initial parameter set adjusted to a reference condition, fitting is done by minimizing a weighted least-square residual. In doing so, the sensitivity of ß -cells to glucose was identified as the most likely impacted function at weaning for the progeny of rats that were lightly exposed to cadmium in the perigestational period. Later in life, after young rats received non-contaminated carbohydrate enriched food, differences are more subtle, but modeling agrees with long-lasting perturbation of glucose homeostasis.

Human mesenchymal stem cells improve rat islet functionality under cytokine stress with combined upregulation of heme oxygenase-1 and ferritin.

BACKGROUND: Islets of Langerhans transplantation is a promising therapy for type 1 diabetes mellitus, but this technique is compromised by transplantation stresses including inflammation. In other tissues, co-transplantation with mesenchymal stem cells has been shown to reduce damage by improving anti-inflammatory and anti-oxidant defences. Therefore, we probed the protection afforded by bone marrow mesenchymal stem cells to islets under pro-inflammatory cytokine stress. METHODS: In order to evaluate the cytoprotective potential of mesenchymal stem cells on rat islets, co-cultures were exposed to the interleukin-1, tumour necrosis factor α and interferon γ cocktail for 24 h. Islet viability and functionality tests were performed. Reactive oxygen species and malondialdehyde were measured. Expression of stress-inducible genes acting as anti-oxidants and detoxifiers, such as superoxide dismutases 1 and 2, NAD(P)H quinone oxidoreductase 1, heme oxygenase-1 and ferritin H, was compared to non-stressed cells, and the corresponding proteins were measured. Data were analysed by a two-way ANOVA followed by a Holm-Sidak post hoc analysis. RESULTS: Exposure of rat islets to cytokines induces a reduction in islet viability and functionality concomitant with an oxidative status shift with an increase of cytosolic ROS production. Mesenchymal stem cells did not significantly increase rat islet viability under exposure to cytokines but protected islets from the loss of insulin secretion. A drastic reduction of the antioxidant factors heme oxygenase-1 and ferritin H protein levels was observed in islets exposed to the cytokine cocktail with a prevention of this effect by the presence of mesenchymal stem cells. CONCLUSIONS: Our data evidenced that MSCs are able to preserve islet insulin secretion through a modulation of the oxidative imbalance mediated by heme and iron via heme oxygenase-1 and ferritin in a context of cytokine exposure.

Impact of maternal low-level cadmium exposure on glucose and lipid metabolism of the litter at different ages after weaning.

Cadmium (Cd) is a metal which may participate in the development of type II diabetes even if Cd exposure levels are mild. However, experimental studies focusing on daily environmentally relevant doses are scarce, particularly for glucose metabolism of the offspring of chronically exposed mothers. The aim is to measure the impact of maternal low level Cd exposure on glucose and lipid metabolism of offspring. Female rats were exposed to 0, 50 or 500 µg.kg-1.d-1 of CdCl2, 21 days before mating and during 21 days of gestation and 21 days of lactation. Pups exposure was organized in 3 groups (control, Cd1, Cd2) according to renal dams' Cd burden. Parameters of glucose and lipid metabolisms were measured for the pups on post-natal day 21, 26 and 60. Maternal Cd exposure led to significant amounts of Cd in the liver and kidney of pups. At weaning, insulin secretion upon glucose stimulation was unchanged, but the removal of circulating glucose was slower for pups born from the lowest impregnated dams (Cd1). Five days after, glucose tolerance of all groups was identical. Thus, this loss of insulin sensitivity was reversed, in part by increased adiponectin secretion for the Cd1 group. Furthermore, pups from dams accumulating the highest levels of Cd (Cd2) exhibited a compensatory increased insulin pancreatic secretion, together with increased circulating non-esterified fatty acids, indicating the establishment of insulin resistance, 2 months after birth. This study has demonstrated the influence of maternal exposure to low levels of Cd on glucose homeostasis in the offspring that might increase the risk of developing type II diabetes later in life.

Impact of chronic and low cadmium exposure of rats: sex specific disruption of glucose metabolism.

BACKGROUND: Several epidemiological and animal studies suggest a positive association between cadmium (Cd) exposure and incidence of type 2 diabetes, but the association remains controversial. Besides, the experimental data have mainly been obtained with relatively high levels of Cd, over various periods of time, and with artificial routes of administration. OBJECTIVES: Do environmental exposures to Cd induce significant disruption of glucose metabolism? METHODS: Adults Wistar rats were exposed for three months to 0, 5, 50 or 500 µg.kg-1.d-1 of CdCl2 in drinking water. Relevant parameters of glucose homeostasis were measured. RESULTS: Cd accumulated in plasma, kidney and liver of rats exposed to 50 and 500 µg.kg-1.d-1, without inducing signs of organ failure. In rats drinking 5 µg.kg-1.d-1 for 3 months, Cd exposure did not lead to any significant increase of Cd in these organs. At 50 and 500 µg.kg-1.d-1 of Cd, glucose and insulin tolerance were unchanged in both sexes. However, females exhibited a significant increase of both fasting and glucose-stimulated plasma insulin that was assigned to impaired hepatic insulin extraction as indicated by unaltered fasting C-peptide plasma levels. CONCLUSIONS: Glucose homeostasis is sensitive to chronic Cd exposure in a gender-specific way. Moreover, this study proves that an environmental pollutant such as Cd can have, at low concentrations, an impact on the glucose homeostatic system and it highlights the importance of a closer scrutiny of the underlying environmental causes to understand the increased incidence of type 2 diabetes.

The iron regulatory proteins are defective in repressing translation via exogenous 5' iron responsive elements despite their relative abundance in leukemic cellular models.

In animal cells the specific translational control of proteins contributing to iron homeostasis is mediated by the interaction between the Iron Regulatory Proteins (IRP1 and IRP2) and the Iron Responsive Elements (IRE) located in the untranslated regions (UTR) of regulated messengers, such as those encoding ferritin or the transferrin receptor. The absolute concentrations of the components of this regulatory system in hematopoietic cells and the ability of the endogenous IRP to regulate exogenous IRE have been measured. The IRP concentration is in the low µM (10-6 M) range, whereas the most abundant IRE-containing messenger RNA (mRNA), i.e. those of the ferritin subunits, do not exceed 100 nM (10-7 M). Most other IRP mRNA targets are around or below 1 nM. The distribution of the mRNA belonging to the cellular iron network is similar in human leukemic cell lines and in normal cord blood progenitors, with differences among the cellular models only associated with their different propensities to synthesize hemoglobin. Thus, the IRP regulator is in large excess over its presently identified regulated mRNA targets. Yet, despite this excess, endogenous IRP poorly represses translation of transfected luciferase cDNA engineered with a series of IRE sequences in the 5' UTR. The cellular concentrations of the central hubs of the mammalian translational iron network will have to be included in the description of the proliferative phenotype of leukemic cells and in assessing any therapeutic action targeting iron provision.

Mitochondrial Morphology and Function of the Pancreatic ß-Cells INS-1 Model upon Chronic Exposure to Sub-Lethal Cadmium Doses.

The impact of chronic cadmium exposure and slow accumulation on the occurrence and development of diabetes is controversial for human populations. Islets of Langerhans play a prominent role in the etiology of the disease, including by their ability to secrete insulin. Conversion of glucose increase into insulin secretion involves mitochondria. A rat model of pancreatic ß-cells was exposed to largely sub-lethal levels of cadmium cations applied for the longest possible time. Cadmium entered cells at concentrations far below those inducing cell death and accumulated by factors reaching several hundred folds the basal level. The mitochondria reorganized in response to the challenge by favoring fission as measured by increased circularity at cadmium levels already ten-fold below the median lethal dose. However, the energy charge and respiratory flux devoted to adenosine triphosphate synthesis were only affected at the onset of cellular death. The present data indicate that mitochondria participate in the adaptation of ß-cells to even a moderate cadmium burden without losing functionality, but their impairment in the long run may contribute to cellular dysfunction, when viability and ß-cells mass are affected as observed in diabetes.

Reactive oxygen species levels control NF-κB activation by low dose deferasirox in erythroid progenitors of low risk myelodysplastic syndromes.

Anemia is a frequent cytopenia in myelodysplastic syndromes (MDS) and most patients require red blood cell transfusion resulting in iron overload (IO). Deferasirox (DFX) has become the standard treatment of IO in MDS and it displays positive effects on erythropoiesis. In low risk MDS samples, mechanisms improving erythropoiesis after DFX treatment remain unclear. Herein, we addressed this question by using liquid cultures with iron overload of erythroid precursors treated with low dose of DFX (3µM), which corresponds to DFX 5 mg/kg/day, an unusual dose used for iron chelation. We highlight a decreased apoptosis rate and an increased proportion of cycling cells, both leading to higher proliferation rates. The iron chelation properties of low dose DFX failed to activate the Iron Regulatory Proteins and to support iron depletion, but low dose DFX dampers intracellular reactive oxygen species. Furthermore low concentrations of DFX activate the NF-κB pathway in erythroid precursors triggering anti-apoptotic and anti-inflammatory signals. Establishing stable gene silencing of the Thioredoxin (TRX) 1 genes, a NF-κB modulator, showed that fine-tuning of reactive oxygen species (ROS) levels regulates NF-κB. These results justify a clinical trial proposing low dose DFX in MDS patients refractory to erythropoiesis stimulating agents.

Association between iron level, glucose impairment and increased DNA damage during pregnancy.

Elevated circulating ferritin has been reported to increase the risk of gestational diabetes mellitus (GDM). When high ferritin translates into high iron stores, iron excess is also a condition leading to free radical damage. We aimed to evaluate the relationship between oxidative stress (OS) induced by iron status and GDM risk in non iron-supplemented pregnant women. METHODS: This was a pilot observational study conducted on 93 non-anemic pregnant women. Iron status was assessed at the first trimester of gestation. Blood sampling was done at 24-28 weeks' gestation for oral glucose tolerance test (OGTT), insulin and biological markers of oxidative damage tests. RESULTS: A significant increase in DNA damage was found in patients who developed GDM. Women with elevated DNA damage had a six-fold increased risk of developing GDM (Exp (B)=6.851, P=0.038; 95% CI [1.108-42.375]). The serum ferritin levels at first trimester were significantly correlated to lipid peroxidation (rho=0.24, p=0.012). The stratified analysis suggests that ferritin is a modifying factor for the correlation of oxidative stress (OS) and glucose intolerance. CONCLUSION: Moderate ferritin levels due to iron intake without iron-supplement, at early pregnancy is a modifying factor for the correlation of oxidative damage and glucose intolerance in pregnant women. Larger studies to evaluate the risk of food iron intake induced increased oxidative damage in offspring are warranted to propose nutrition advice regarding iron intake in women with a high risk of GDM.

Chronic Exposure to Low-Level Cadmium in Diabetes: Role of Oxidative Stress and Comparison with Polychlorinated Biphenyls.

Among the most important physiological functions, maintenance of the oxidation reduction equilibrium in cells stands out as a major homeostatic event. Many environmental contaminants efficiently trap cellular reducing compounds, but the actual importance of this mode of toxicity is far from being precisely known. This statement applies to cases of slowly developing chronic diseases, such as neurodegenerations, diabetes, and many others. The involvement of oxidative challenge in diabetes is considered in connection with chronic dietary exposure to low-level concentrations of cadmium. Comparison is made with polychlorobiphenyl molecules (PCB): they are structurally unrelated to cadmium, they preferentially distribute into different organs than cadmium, and they follow different metabolic pathways. Yet, they have also pro-oxidative properties, and they are associated with diabetes. Since neither cadmium nor PCB is a direct oxidant, they both follow indirect pathways to shift the redox equilibrium. Thus, a difference must be made between the adaptable response of the organism, i.e. the anti-oxidant response, and the irreversible damage generated by oxidizing species, i.e. oxidative damage, when exposure occurs at low concentrations. The approximate border between high and low levels of exposure is estimated in this review from the available relevant data, and the strengths and weaknesses of experimental models are delineated. Eventually, chronic low level exposure to these contaminants sparks cellular responses setting ground for dysfunction and disease, such as diabetes: oxidative damage is an accompanying phenomenon and not necessarily an early mechanism of toxicity.

Iron for proliferation of cell lines and hematopoietic progenitors: Nailing down the intracellular functional iron concentration.

Iron is an essential nutrient which must be provided in sufficient amounts to support growth of eukaryotic cells. All organisms devote specialized pathways to ensure proper delivery. Yet, a quantitative assessment of the intra-cellular iron concentration needed to allow the cell cycle to proceed in mammalian cells is missing. Starting from iron-depleted cell lines or primary hematopoietic progenitors prepared with clinically implemented iron chelators, replenishment via transferrin and other iron sources has been quantitatively monitored through the main endogenous markers of the cellular iron status, namely proteins involved in the uptake (transferrin receptor), the storage (ferritin), and the sensing (Iron Regulatory Proteins) of iron. When correlated with measurements of iron concentrations and indicators of growth, this minimally intrusive approach provided an unprecedented estimate of the intracellular iron concentration acting upon iron-centered regulatory pathways. The data were analyzed with the help of a previously developed theoretical treatment of cellular iron regulation. The minimal cellular iron concentration required for cell division was named functional iron concentration (FIC) to distinguish it from previous estimates of the cellular labile iron. The FIC falls in the low nanomolar range for all studied cells, including hematopoietic progenitors. These data shed new light on basic aspects of cellular iron homeostasis by demonstrating that sensing and regulation of iron occur well below the concentrations requiring storage or becoming noxious in pathological conditions. The quantitative assessment provided here is relevant for monitoring treatments of conditions in which iron provision must be controlled to avoid unwanted cellular proliferation.

The bioinorganic chemistry of cadmium in the context of its toxicity.

Cadmium is known for its toxicity in animals and man as it is not used in these species. Its only role in biology is as a zinc replacement at the catalytic site of a particular class of carbonic anhydrases in some marine diatoms. The toxicity of cadmium continues to be a significant public health concern as cadmium enters the food chain and it is taken up by tobacco smokers. The biochemical basis for its toxicity has been the objective of research for over 50 years. Cadmium damages the kidneys, the lungs upon inhalation, and interferes with bone metabolism. Evidence is accumulating that it affects the cardiovascular system. Cadmium is classified as a human carcinogen. It generates oxidative stress. This chapter discusses the chemistry and biochemistry of cadmium(II) ions, the only important state of cadmium in biology. This background is needed to interpret the countless effects of cadmium in laboratory experiments with cultured cells or with animals with regard to their significance for human health. Evaluation of the risks of cadmium exposure and the risk factors that affect cadmium's biological effects in tissues is an on-going process. It appears that the more we learn about the biochemistry of cadmium and the more sensitive assays we develop for determining exposure, the lower we need to set the upper limits for exposure to protect those at risk. But proper control of cadmium's presence and interactions with living species and the environment still needs to be based on improved knowledge about the mechanisms of cadmium toxicity; the gaps in our knowledge in this area are discussed herein.

Prion protein expression and functional importance in developmental angiogenesis: role in oxidative stress and copper homeostasis.

AIM: It has been convincingly shown that oxidative stress and toxicity by deregulated metals, such as copper (Cu), are tightly linked to the development of pre-eclampsia and intrauterine growth retardation (IUGR), the most threatening pathologies of human pregnancy. However, mechanisms implemented to control these effects are far from being understood. Among proteins that bind Cu and insure cellular protection against oxidative stress is the cellular prion protein (PrP(C)), a glycosyl phosphatidyl inositol-anchored glycoprotein, which we reported to be highly expressed in human placenta. Herein, we investigated the pathophysiological role of PrP(C) in Cu and oxidative stress homeostasis in vitro using human placenta and trophoblast cells, and in vivo using three strains of mice (C57Bl6, PrP(C) knockout mice [PrP(-/-)], and PrP(C) overexpressing mice [Tga20]). RESULTS: At the cellular level, PrP(C) protection against oxidative stress was established in multiple angiogenic processes: proliferation, migration, and tube-like organization. For the animal models, lack (PrP(-/-)) or overexpression (Tga20) of PrP(C) in gravid mice caused severe IUGR that was correlated with a decrease in litter size, changes in Cu homeostasis, increase in oxidative stress response, development of hypoxic environment, failure in placental function, and maintenance of growth defects of the offspring even 7.5 months after delivery. INNOVATION: PrP(C) could serve as a marker for the idiopathic IUGR disease. CONCLUSION: These findings demonstrate the stress-protective role of PrP(C) during development, and propose PrP(C) dysregulation as a novel causative element of IUGR.

A bacteria-specific 2[4Fe-4S] ferredoxin is essential in Pseudomonas aeruginosa.

BACKGROUND: Ferredoxins are small iron-sulfur proteins belonging to all domains of life. A sub-group binds two [4Fe-4S] clusters with unequal and extremely low values of the reduction potentials. These unusual properties are associated with two specific fragments of sequence. The functional importance of the very low potential ferredoxins is unknown. RESULTS: A bioinformatic screening of the sequence features defining very low potential 2[4Fe-4S] ferredoxins has revealed the almost exclusive presence of the corresponding fdx gene in the Proteobacteria phylum, without occurrence in Archaea and Eukaryota. The transcript was found to be monocistronic in Pseudomonas aeruginosa, and not part of an operon in most bacteria. Only fdx genes of bacteria which anaerobically degrade aromatic compounds belong to operons. As this pathway is not present in all bacteria having very low potential 2[4Fe-4S] ferredoxins, these proteins cannot exclusively be reductants of benzoyl CoA reductases. Expression of the ferredoxin gene did not change in response to varying growth conditions, including upon macrophage infection or aerobic growth with 4-hydroxy benzoate as carbon source. However, it increased along the growth curve in Pseudomonas aeruginosa and in Escherichia coli. The sequence immediately 5' upstream of the coding sequence contributed to the promotor activity. Deleting the fdx gene in Pseudomonas aeruginosa abolished growth, unless a plasmid copy of the gene was provided to the deleted strain. CONCLUSIONS: The gene of the very low potential 2[4Fe-4S] ferredoxin displays characteristics of a housekeeping gene, and it belongs to the minority of genes that are essential in Pseudomonas aeruginosa. These data identify a new potential antimicrobial target in this and other pathogenic Proteobacteria.

Cellular mechanisms of cadmium toxicity related to the homeostasis of essential metals.

The widespread occurrence of cadmium in the environment continues to pose a threat to human health despite attempts at limiting its technological uses. The biologically significant ionic form of cadmium, Cd(2+), binds to many bio-molecules and these interactions underlie the toxicity mechanisms of cadmium. Some of the molecules specialized in the handling of alkaline earth (Mg(2+), Ca(2+)) and transition metal ions (e.g. Zn(2+), Cu(2+/+), Fe(3+/2+)) should be particularly sensitive to the presence of Cd(2+), because they enclose cationic sites to which the toxic metal can bind. The possible molecular targets of this kind for cadmium are considered herein. Whereas in vitro evidence for native cation replacement by Cd(2+) in bio-molecules has been largely provided, the demonstration of such occurrences in vivo is scarce, with the notable exception of metallothionein. One reason might be that realistic low-level Cd(2+) contaminations involve cellular concentrations far smaller than those of endogenous cations that usually saturate their binding sites. It is very likely that cadmium toxicity is most often mediated by biological systems amplifying the signals triggered by the presence of Cd(2+). The interference of Cd(2+) with redox sensitive systems acting at the transcriptional and post-transcriptional levels is instrumental in such processes. A better understanding of cadmium toxicity to tackle the environmental challenges lying ahead thus requires properly designed studies implementing biologically relevant cadmium concentrations on different cell types, improved knowledge of the homeostasis of essential metals, and use of these data in a theoretical framework integrating all cellular aspects of cadmium effects.