Biosafety of Mesoporous Silica Nanoparticles.

Careful analysis of any new nanomedicine device or disposal should be undertaken to comprehensively characterize the new product before application, so that any unintended side effect is minimized. Because of the increasing number of nanotechnology-based drugs, we can anticipate that regulatory authorities might adapt the approval process for nanomedicine products due to safety concerns, e.g., request a more rigorous testing of the potential toxicity of nanoparticles (NPs). Currently, the use of mesoporous silica nanoparticles (MSN) as drug delivery systems is challenged by a lack of data on the toxicological profile of coated or non-coated MSN. In this context, we have carried out an extensive study documenting the influence of different functionalized MSN on the cellular internalization and in vivo behaviour. In this article, a synthesis of these works is reviewed and the perspectives are drawn. The use of magnetic MSN (Fe3O4@MSN) allows an efficient separation of coated NPs from cell cultures with a simple magnet, leading to results regarding corona formation without experimental bias. Our interest is focused on the mechanism of interaction with model membranes, the adsorption of proteins in biological fluids, the quantification of uptake, and the effect of such NPs on the transcriptomic profile of hepatic cells that are known to be readily concerned by NPs' uptake in vivo, especially in the case of an intravenous injection.

Biocompatibility assessment of functionalized magnetic mesoporous silica nanoparticles in human HepaRG cells.

Magnetic mesoporous silica nanoparticles (M-MSNs) are a promising class of nanoparticles for drug delivery. However, a deep understanding of the toxicological mechanisms of action of these nanocarriers is essential, especially in the liver. The potential toxicity on HepaRG cells of pristine, pegylated (PEG), and lipid (DMPC) M-MSNs were compared. Based on MTT assay and real-time cell impedance, none of these NPs presented an extensive toxicity on hepatic cells. However, we observed by transmission electron microscopy (TEM) that the DMPC and pristine M-MSNs were greatly internalized. In comparison, PEG M-MSNs showed a slower cellular uptake. Whole gene expression profiling revealed the M-MSNs molecular modes of action in a time- and dose-dependent manner. The lowest dose tested (1.6 µg/cm2) induced no molecular effect and was defined as 'No Observed Transcriptional Effect level.' The dose 16 µg/cm2 revealed nascent but transient effects. At the highest dose (80 µg/cm2), adverse effects have clearly arisen and increased over time. The limit of biocompatibility for HepaRG cells could be set at 16 µg/cm2 for these NPs. Thanks to a comparative pathway-driven analysis, we highlighted the sequence of events that leads to the disruption of hepatobiliary system, elicited by the three types of M-MSNs, at the highest dose. The Adverse Outcome Pathway of hepatic cholestasis was implicated. Toxicogenomics applied to cell cultures is an effective tool to characterize and compare the modes of action of many substances. We propose this strategy as an asset for upstream selection of the safest nanocarriers in the framework of regulation for nanobiosafety.

The species origin of the serum in the culture medium influences the in vitro toxicity of silica nanoparticles to HepG2 cells.

The formation of a protein corona around nanoparticles can influence their toxicity, triggering cellular responses that may be totally different from those elicited by pristine nanoparticles. The main objective of this study was to investigate whether the species origin of the serum proteins forming the corona influences the in vitro toxicity assessment of silica nanoparticles. Coronas were preformed around nanoparticles before cell exposures by incubation in fetal bovine (FBS) or human (HS) serum. The compositions of these protein coronas were assessed by nano-LC MS/MS. The effects of these protein-coated nanoparticles on HepG2 cells were monitored using real-time cell impedance technology. The nanoparticle coronas formed in human or fetal bovine serum comprised many homologous proteins. Using human compared with fetal bovine serum, nanoparticle toxicity in HepG2 cells decreased by 4-fold and 1.5-fold, when used at 50 and 10µg/mL, respectively. It is likely that "markers of self" are present in the serum and are recognized by human cell receptors. Preforming a corona with human serum seems to be more appropriate for in vitro toxicity testing of potential nanocarriers using human cells. In vitro cytotoxicity assays must reflect in vivo conditions as closely as possible to provide solid and useful results.

The timeline of corona formation around silica nanocarriers highlights the role of the protein interactome.

Magnetic mesoporous silica nanoparticles (M-MSNs) represent promising targeting tools for theranostics. Engineering the interaction of nanoparticles (NPs) with biological systems requires an understanding of protein corona formation around the nanoparticles as this drives the biological fate of nanocarriers. We investigated the behavior of proteins in contact with M-MSNs by high-throughput comparative proteomics, using human and bovine sera as biological fluids, in order to assess the adsorption dynamics of proteins in these media. Using system biology tools, and especially protein-protein interaction databases, we demonstrated how the protein network builds up within the corona over the course of the experiment. Based on these results, we introduce and discuss the role of the "corona interactome" as an important factor influencing protein corona evolution. The concept of the "corona interactome" is an original methodology which could be generalized to all NP candidates. Based on this, pre-coating nanocarriers with specific proteins presenting minimal interactions with opsonins might provide them with properties such as stealth.

Ex vivo assessment of testicular toxicity induced by carbendazim and iprodione, alone or in a mixture.

To measure the testicular toxicity of two fungicides (carbendazim and iprodione), alone or in a mixture, we used a rat ex vivo model of seminiferous tubules, greatly reducing the number of rodents used, in accordance with the 3R rule (Replacement, Reduction, and Refinement). This model allows the representation of puberty, a critical life period with regard to endocrine disruptors. The cellular modifications were followed for three weeks through transcriptomic and proteomic profiling analysis. A quantitative and comparative method was developed to estimate how known pathways were disturbed by each substance. This pathway-driven analysis revealed a strong alteration of steroidogenesis and an impairment of meiosis in all cases, albeit the initial molecular events were different for both substances. The ex vivo cytogenetic analysis confirmed that both fungicides alter the course of the first meiotic prophase. In addition, the mixture of both substances triggered effects greater than the sum of their cumulative effects and compromised future sperm motility after a shorter time of exposure compared with the fungicides tested separately. The alliance of an ex vivo culture with "omics" strategies complemented with a physiological examination is a powerful combination of tools for testing substances, separately or in a mixture, for their testicular toxicity. In particular, proteomics allowed the identification of systematically differentially expressed proteins in the secretomes of exposed cultures, such as FUCO and PEBP1, two proteins linked with the motility and fertilizing ability of spermatozoa, respectively. These proteins may be potential biomarkers of testicular dysfunction and infertility.

High-throughput, quantitative assessment of the effects of low-dose silica nanoparticles on lung cells: grasping complex toxicity with a great depth of field.

BACKGROUND: The toxicity of manufactured fumed silica nanoparticles (NPs) remains poorly investigated compared to that of crystalline silica NPs, which have been associated with lung diseases after inhalation. Amorphous silica NPs are a raw material for manufactured nanocomposites, such as cosmetics, foods, and drugs, raising concerns about their potential toxicity. RESULTS: The size of the NPs was determined by dynamic light scattering and their shape was visualized by atomic force microscopy (10 ± 4 nm). The pertinent toxicological concentration and dynamic ranges were determined using viability tests and cellular impedance. We combined transcriptomics and proteomics to assess the cellular and molecular effects of fumed silica in A549 human alveolar epithelial cells. The "no observed transcriptomic adverse effect level" (NOTEL) was set to 1.0 µg/cm(2), and the "lowest observed adverse transcriptional effect level" (LOTEL) was set at 1.5 µg/cm(2). We carried out genome-wide expression profiles with microarrays and identified, by shotgun proteomics, the exoproteome changes in lung cells after exposure to NP doses (0.1, 1.0, 1.5, 3.0, and 6.0 µg/cm(2)) at two time points (24 h and 72 h). The data revealed a hierarchical, dose-dependent cellular response to silica NPs. At 1.5 µg/cm(2), the Rho signaling cascade, actin cytoskeleton remodeling, and clathrin-mediated endocytosis were induced. At 3.0 µg/cm(2), many inflammatory mediators were upregulated and the coagulation system pathway was triggered. Lastly, at 6.0 µg/cm(2), oxidative stress was initiated. The proteins identified in the extracellular compartment were consistent with these findings. CONCLUSIONS: The alliance of two high-throughput technologies allowed the quantitative assessment of the cellular effects and molecular consequences of exposure of lung cells to low doses of NPs. These results were obtained using a pathway-driven analysis instead of isolated genes. As in photography, toxicogenomics allows, at the same time, the visualization of a wide spectrum of biological responses and a "zoom in" to the details with a great depth of field. This study illustrates how such an approach based on human cell culture models is a valuable predictive screening tool to evaluate the toxicity of many potentially harmful emerging substances, alone or in mixtures, in the framework of future regulatory reinforcements.

Uranium in drinking-water: a unique case of guideline value increases and discrepancies between chemical and radiochemical guidelines.

BACKGROUND: Uranium represents a unique case for an element naturally present in the environment, as its chemical guideline value in drinking water significantly increased from 2 µg/L in 1998 up to 15 µg/L in 2004 and then to 30 µg/L in 2011, to date corresponding to a multiplication factor of 15 within a period of just 13 years. OBJECTIVES: In this commentary we summarize the evolution of uranium guideline values in drinking-water based on both radiological and chemical aspects, emphasizing the benefit of human studies and their contribution to recent recommendations. We also propose a simpler and better consistency between radiological and chemical values. DISCUSSION: The current chemical guideline value of 30 µg/L is still designated as provisional because of scientific uncertainties regarding uranium toxicity. During the same period, the radiological guideline for (238)U increased from 4 Bq/L to 10 Bq/L while that for (234)U decreased from 4 Bq/L to 1 Bq/L. These discrepancies are discussed here, and a value of 1 Bq/L for all uranium isotopes is proposed to be more consistent with the current chemical value of 30 µg/L. CONCLUSION: Continuous progress in the domains of toxicology and speciation should enable a better interpretation of the biological effects of uranium in correlation with epidemiological human studies. This will certainly aid future proposals for uranium guideline values.

Exposure to low-dose bisphenol A impairs meiosis in the rat seminiferous tubule culture model: a physiotoxicogenomic approach.

BACKGROUND: Bisphenol A (BPA) is one of the most widespread chemicals in the world and is suspected of being responsible for male reproductive impairments. Nevertheless, its molecular mode of action on spermatogenesis is unclear. This work combines physiology and toxicogenomics to identify mechanisms by which BPA affects the timing of meiosis and induces germ-cell abnormalities. METHODS: We used a rat seminiferous tubule culture model mimicking the in vivo adult rat situation. BPA (1 nM and 10 nM) was added to the culture medium. Transcriptomic and meiotic studies were performed on the same cultures at the same exposure times (days 8, 14, and 21). Transcriptomics was performed using pangenomic rat microarrays. Immunocytochemistry was conducted with an anti-SCP3 antibody. RESULTS: The gene expression analysis showed that the total number of differentially expressed transcripts was time but not dose dependent. We focused on 120 genes directly involved in the first meiotic prophase, sustaining immunocytochemistry. Sixty-two genes were directly involved in pairing and recombination, some of them with high fold changes. Immunocytochemistry indicated alteration of meiotic progression in the presence of BPA, with increased leptotene and decreased diplotene spermatocyte percentages and partial meiotic arrest at the pachytene checkpoint. Morphological abnormalities were observed at all stages of the meiotic prophase. The prevalent abnormalities were total asynapsis and apoptosis. Transcriptomic analysis sustained immunocytological observations. CONCLUSION: We showed that low doses of BPA alter numerous genes expression, especially those involved in the reproductive system, and severely impair crucial events of the meiotic prophase leading to partial arrest of meiosis in rat seminiferous tubule cultures.

Toxicity evaluation of manufactured CeO2 nanoparticles before and after alteration: combined physicochemical and whole-genome expression analysis in Caco-2 cells.

BACKGROUND: Engineered nanomaterials may release nanosized residues, by degradation, throughout their life cycle. These residues may be a threat for living organisms. They may be ingested by humans through food and water. Although the toxicity of pristine CeO2 nanoparticles (NPs) has been documented, there is a lack of studies on manufactured nanoparticles, which are often surface modified. Here, we investigated the potential adverse effects of CeO2 Nanobyk 3810™ NPs, used in wood care, and their residues, altered by light or acid. RESULTS: Human intestinal Caco-2 cells were exposed to residues degraded by daylight or in a medium simulating gastric acidity. Size and zeta potential were determined by dynamic light scattering. The surface structure and redox state of cerium were analyzed by transmission electronic microscopy (TEM) and X-ray absorption spectroscopy, respectively. Viability tests were performed in Caco-2 cells exposed to NPs. Cell morphology was imaged with scanning electronic microscopy. Gene expression profiles obtained from cells exposed to NPs before and after their alteration were compared, to highlight differences in cellular functions.No change in the cerium redox state was observed for altered NPs. All CeO2 NPs suspended in the culture medium became microsized. Cytotoxicity tests showed no toxicity after Caco-2 cell exposure to these various NPs up to 170 µg/mL (24 h and 72 h). Nevertheless, a more-sensitive whole-gene-expression study, based on a pathway-driven analysis, highlighted a modification of metabolic activity, especially mitochondrial function, by altered Nanobyk 3810™. The down-regulation of key genes of this pathway was validated by qRT-PCR. Conversely, Nanobyk 3810™ coated with ammonium citrate did not display any adverse effect at the same concentration. CONCLUSION: The degraded nanoparticles were more toxic than their coated counterparts. Desorption of the outside layer was the most likely cause of this discrepancy in toxicity. It can be assumed that the safe design of engineered nanoparticles could include robust protective layers conferring on them greater resistance to alteration during their life cycle.

Reply to comment on Fisichella et al. (2012), "Intestinal toxicity evaluation of TiO2 degraded surface-treated nanoparticles: a combined physico-chemical and toxicogenomics approach in Caco-2 cells" by Faust et al.

In this response, we discuss the major differences that clearly distinguish our results from those mentioned by Faust et al. In particular, the experiments have been conducted on nanoparticles of different nature, what mainly explains the observed discrepancies.

Intestinal toxicity evaluation of TiO2 degraded surface-treated nanoparticles: a combined physico-chemical and toxicogenomics approach in caco-2 cells.

BACKGROUND: Titanium dioxide (TiO2) nanoparticles (NPs) are widely used due to their specific properties, like UV filters in sunscreen. In that particular case TiO2 NPs are surface modified to avoid photocatalytic effects. These surface-treated nanoparticles (STNPs) spread in the environment and might release NPs as degradation residues. Indeed, degradation by the environment (exposure to UV, water and air contact …) will occur and could profoundly alter the physicochemical properties of STNPs such as chemistry, size, shape, surface structure and dispersion that are important parameters for toxicity. Although the toxicity of surface unmodified TiO2 NPs has been documented, nothing was done about degraded TiO2 STNPs which are the most likely to be encountered in environment. The superoxide production by aged STNPs suspensions was tested and compared to surface unmodified TiO2 NPs. We investigated the possible toxicity of commercialized STNPs, degraded by environmental conditions, on human intestinal epithelial cells. STNPs sizes and shape were characterized and viability tests were performed on Caco-2 cells exposed to STNPs. The exposed cells were imaged with SEM and STNPs internalization was researched by TEM. Gene expression microarray analyses were performed to look for potential changes in cellular functions. RESULTS: The production of reactive oxygen species was detected with surface unmodified TiO2 NPs but not with STNPs or their residues. Through three different toxicity assays, the STNPs tested, which have a strong tendency to aggregate in complex media, showed no toxic effect in Caco-2 cells after exposures to STNPs up to 100 µg/mL over 4 h, 24 h and 72 h. The cell morphology remained intact, attested by SEM, and internalization of STNPs was not seen by TEM. Moreover gene expression analysis using pangenomic oligomicroarrays (4x 44000 genes) did not show any change versus unexposed cells after exposure to 10 µg/ mL, which is much higher than potential environmental concentrations. CONCLUSIONS: TiO2 STNPs, degraded or not, are not harmful to Caco-2 cells and are unlikely to penetrate the body via oral route. It is likely that the strong persistence of the aluminium hydroxide layer surrounding these nanoparticles protects the cells from a direct contact with the potentially phototoxic TiO2 core.

From cell to man: evaluation of osteopontin as a possible biomarker of uranium exposure.

BACKGROUND: Ore workers are conventionally monitored for exposure by measuring the uranium in their urine, but specific biomarkers of kidney damage still remain to be discovered. A recent toxicogenomics study allowed us to focus on osteopontin (OSTP) normally excreted in human urine and linked to mineral metabolism. OBJECTIVES: We examined the association between osteopontin and uranium exposure both in vitro, in a human kidney cell model, and in the urine of exposed individuals. METHODS: OSTP was measured in supernatants of uranium-exposed HK2 cells to establish a dose-response curve and a time course experiment. Its role was studied through a gene extinction experiment. Uranium and OSTP were then monitored in the urine of exposed nuclear fuel industry workers and a chronically exposed population. These levels were compared with those found in a non-exposed population. RESULTS: The study of HK2 cells indicated that OSTP secretion decreased after uranium exposure in a concentration and time dependent manner, but its suppression does not affect cell sensitivity to uranium. In spite of wide inter-individual variability, this parameter decreases also in human urine when urinary uranium exceeds 30 µg/L after an acute exposure, a value considered to be critical for kidney damage. CONCLUSION: This study reports how toxicogenomics can highlight putative toxicity biomarkers in an easy to access biological fluid. The decrease of urinary osteopontin in response to uranium exposure suggests kidney damage and would thus be complementary to current markers.

Alterations in gene expression in cultured human cells after acute exposure to uranium salt: Involvement of a mineralization regulator.

The risk of exposure of workers or populations to materials, such as uranium, of nuclear fuel process origins is a major concern worldwide. Our goal is to improve the knowledge of mechanisms ruling its chemical toxicity, and to search for proteins as potential indicator of effect. Such a marker of internal damage remains to be discovered in the case of uranium. This study, based on DNA microarrays, reports a comparative gene expression analysis following acute uranium exposure of several human cell lines taken from kidneys or lungs as representative targets. Among uranium altered genes, no common gene was found between cells originating from lungs and kidney. In contrast, a set of 24 altered genes was common to two kidney cell lines. Transcriptional levels of a subset of renal genes were assessed with qRT-PCR. Furthermore, we highlighted a gene (SPP1) coding for a secreted protein (osteopontin) linked to ectopic mineralization. Immunoblotting assays showed that uranyl ions affect the excretion of osteopontin in a time- and dose-dependent manner. We consider that osteopontin, described as associated with bone resorbtion and kidney mineral stones, is a worthwhile candidate to be tested in vivo as a potential indicator of uranyl mineralization effects.

Uranium speciation in drinking water from drilled wells in southern Finland and its potential links to health effects.

Exceptionally high concentrations of natural uranium have been found in drinking water originating from drilled wells in Southern Finland. However, no clear clinical symptoms have been observed among the exposed population. Hence a question arose as to whether uranium speciation could be one reason for the lack of significant adverse health effects. Uranium species were determined using time-resolved laser-induced fluorescence spectroscopy. We performed multi-element chemical analyses in these water samples, and predictive calculations were carried out using up-to-date thermodynamic data. The results indicated good agreement between measurements and modeling. The low toxicity of Finnish bedrockwater may be due to the predominance of two calcium-dependent species, Ca2UO2(CO3)3(aq) and CaUO2(CO3)3(2-), whose nontoxicity for cells has been described previously. This interdisciplinary study describes chemical speciation of drinking water with elevated uranium concentrations and the potential consequence on health. From these results, it appears that modeling could be used for a better understanding of uranium toxicity of drinking water in the event of contamination.

Identification of uranyl binding proteins from human kidney-2 cell extracts by immobilized uranyl affinity chromatography and mass spectrometry.

To improve our knowledge on protein targets of uranyl ion (UO(2)(2+)), we set up a proteomic strategy based on immobilized metal-affinity chromatography (IMAC). The successful enrichment of UO(2)(2+)-interacting proteins from human kidney-2 (HK-2) soluble cell extracts was obtained using an ion-exchange chromatography followed by a dedicated IMAC process previously described and designed for the uranyl ion. By mass spectrometry analysis we identified 64 proteins displaying varied functions. The use of a computational screening algorithm along with the particular ligand-based properties of the UO(2)(2+) ion allowed the analysis and categorization of the protein collection. This profitable approach demonstrated that most of these proteins fulfill criteria which could rationalize their binding to the UO(2)(2+)-loaded phase. The obtained results enable us to focus on some targets for more in-depth studies and open new insights on its toxicity mechanisms at molecular level.

Global gene expression profiling in human lung cells exposed to cobalt.

BACKGROUND: It has been estimated that more than 1 million workers in the United States are exposed to cobalt. Occupational exposure to 59 Co occurs mainly via inhalation and leads to various lung diseases. Cobalt is classified by the IARC as a possible human carcinogen (group 2B). Although there is evidence for in vivo and in vitro toxicity, the mechanisms of cobalt-induced lung toxicity are not fully known. The purpose of this work was to identify potential signatures of acute cobalt exposure using a toxicogenomic approach. Data analysis focused on some cellular processes and protein targets that are thought to be relevant for carcinogenesis, transport and biomarker research. RESULTS: A time course transcriptome analysis was performed on A549 human pulmonary cells, leading to the identification of 85 genes which are repressed or induced in response to soluble 59 Co. A group of 29 of these genes, representing the main biological functions, was assessed by quantitative RT-PCR. The expression profiles of six of them were then tested by quantitative RT-PCR in a time-dependent manner and three modulations were confirmed by Western blotting. The 85 modulated genes include potential cobalt carriers (FBXL2, ZNT1, SLC12A5), tumor suppressors or transcription factors (MAZ, DLG1, MYC, AXL) and genes linked to the stress response (UBC, HSPCB, BNIP3L). We also identified nine genes coding for secreted proteins as candidates for biomarker research. Of those, TIMP2 was found to be down-regulated and this modulation was confirmed, in a dose-dependent manner, at protein level in the supernatant of exposed cells. CONCLUSION: Most of these genes have never been described as related to cobalt stress and provide original hypotheses for further study of the effects of this metal ion on human lung epithelial cells. A putative biomarker of cobalt toxicity was identified.

Actinide speciation in relation to biological processes.

In case of accidental release of radionuclides into the environment, actinides represent a severe health risk to human beings following internal contamination (inhalation, ingestion or wound). For a better understanding of the actinide behaviour in man (in term of metabolism, retention, excretion) and in specific biological systems (organs, cells or biochemical pathways), it is of prime importance to have a good knowledge of the relevant actinide solution chemistry and biochemistry, in particular of the thermodynamic constants needed for computing actinide speciation. To a large extent, speciation governs bioavailability and toxicity of elements and has a significant impact on the mechanisms by which toxics accumulate in cell compartments and organs and by which elements are transferred and transported from cell to cell. From another viewpoint, speciation is the prerequisite for the design and success of potential decorporation therapies. The purpose of this review is to present the state of the art of actinide knowledge within biological media. It is also to discuss how actinide speciation can be determined or predicted and to highlight the areas where information is lacking with the aim to encourage new research efforts.

Proteomic analysis of the response of human lung cells to uranium.

The industrial use of uranium and particularly of depleted uranium, has pinpointed the need to review its chemical impact on human health. A proteomic approach was used to evaluate the response of a human lung cell line (A549) to uranium. We established the first 2-D reference map of the A549 cell line, identifying 87 spots corresponding to 81 major proteins. Uranium treatment triggered differential expression of 18 spots, of which 14 corresponded to fragments of cytokeratin 8 (CK8) and cytokeratin (CK18) and 1 to peroxiredoxin 1. We probed several hypotheses regarding CK cleavage, and observed that it did not result from caspase or calpain activity. Furthermore, we showed that the fragments are recognised by an anti-ubiquitin antibody (KM691). These results suggest a regulatory pathway involving CK ubiquitinylation or dysfunction in the proteasome-ubiquitin system in response to uranium exposure in human lung cells.

Transcriptomic and proteomic responses of human renal HEK293 cells to uranium toxicity.

The industrial use of uranium, in particular depleted uranium, has pin-pointed the need to review its chemical impact on human health. Global methodologies, applied to the field of toxicology, have demonstrated their applicability to investigation of fine molecular mechanisms. This report illustrate the power of toxicogenomics to evaluate the involvement of certain genes or proteins in response to uranium. We particularly show that 25% of modulated genes concern signal transduction and trafficking, that the calcium pathway is heavily disturbed and that nephroblastomas-related genes are involved (WIT-1, STMN1, and STMN2). A set of 18 genes was deregulated whatever the concentration of toxicant, which could constitute a signature of uranium exposure. Moreover, a group of downregulated genes, with corresponding disappearing proteins (HSP90, 14-3-3 protein, HMGB1) in two-dimensional polyacrylamide gel electrophoresis (2-D PAGE), are good candidates for use as biomarkers of uranium effects. These results reveal a cross-checking between transcriptomic and proteomic technologies. Moreover, our temporal gene expression profiles suggest the existence of a concentration threshold between adaptive response and severe cell deregulation. Our results confirm the involvement of genes already described and also provide new highlights on cellular response to uranium.