Manufactured nanomaterials: categorization and approaches to hazard assessment.

Nanotechnology offers enormous potential for technological progress. Fortunately, early and intensive efforts have been invested in investigating toxicology and safety aspects of this new technology. However, despite there being more than 6,000 publications on nanotoxicology, some key questions still have to be answered and paradigms need to be challenged. Here, we present a view on the field of nanotoxicology to stimulate the discussion on major knowledge gaps and the critical appraisal of concepts or dogma. First, in the ongoing debate as to whether nanoparticles may harbour a specific toxicity due to their size, we support the view that there is at present no evidence of 'nanospecific' mechanisms of action; no step-change in hazard was observed so far for particles below 100 nm in one dimension. Therefore, it seems unjustified to consider all consumer products containing nanoparticles a priori as hazardous. Second, there is no evidence so far that fundamentally different biokinetics of nanoparticles would trigger toxicity. However, data are sparse whether nanoparticles may accumulate to an extent high enough to cause chronic adverse effects. To facilitate hazard assessment, we propose to group nanomaterials into three categories according to the route of exposure and mode of action, respectively: Category 1 comprises nanomaterials for which toxicity is mediated by the specific chemical properties of its components, such as released ions or functional groups on the surface. Nanomaterials belonging to this category have to be evaluated on a case-by-case basis, depending on their chemical identity. Category 2 focuses on rigid biopersistent respirable fibrous nanomaterials with a specific geometry and high aspect ratio (so-called WHO fibres). For these fibres, hazard assessment can be based on the experiences with asbestos. Category 3 focuses on respirable granular biodurable particles (GBP) which, after inhalation, may cause inflammation and secondary mutagenicity that may finally lead to lung cancer. After intravenous, oral or dermal exposure, nanoscaled GBPs investigated apparently did not show 'nanospecific' effects so far. Hazard assessment of GBPs may be based on the knowledge available for granular particles. In conclusion, we believe the proposed categorization system will facilitate future hazard assessments.

Replacement of in vivo acute oral toxicity studies by in vitro cytotoxicity methods: opportunities, limits and regulatory status.

The development of a new medicinal product is a long and costly process in particular due to the regulatory requirements for quality, safety and efficacy. There is a common interest to increase the efficiency of drug development and to provide new, better quality medicinal products much faster to the public. One possible way to economize time and costs, as well as to consider animal protection issues, is to introduce new alternative methods into non-clinical toxicity testing. Currently, animal tests are mandatory for the evaluation of acute toxicity of chemicals and new drugs. The replacement of the in vivo tests by alternative in vitro assays would offer the opportunity to screen and assess numerous compounds at the same time, to predict acute oral toxicity and thus accelerate drug development. Moreover, the substitution of in vivo tests by in vitro methods shows a proactive pursuit of ethical and animal welfare issues. Importantly, the implementation of in vitro assays for acute oral toxicity would require the establishment of common test guidelines across the EU, USA and Japan, i.e., the regions of ICH (International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use). Presently, alternative in vitro tests are being investigated internationally. Yet, in order to achieve regulatory acceptance and implementation of in vitro assays, convincing results from validation studies are required. In this review, we discuss the current regulatory status of acute oral toxicity testing and point out achievements of alternative methods. We describe the application of in vitro tests, correlating in vitro with in vivo data. The use of in vitro data to predict in vivo acute oral toxicity is analyzed using the Registry of Cytotoxicity, an official independent database. We have then analyzed opportunities and drawbacks for future implementation of in vitro test methods, with particular focus on industrial use.

Prevalidation of potential protein biomarkers in toxicology using iTRAQ reagent technology.

Today, toxicoproteomics still relies mainly on 2-DE followed by MS for detection and identification of proteins, which might characterize a certain state of disease, indicate toxicity or even predict carcinogenicity. We utilized the classical 2-DE/MS approach for the evaluation of early protein biomarkers which are predictive for chemically induced hepatocarcinogenesis in rats. We were able to identify statistically significantly deregulated proteins in N-nitrosomorpholine exposed rat liver tissue. Based on literature data, biological relevance in the early molecular process of hepatocarcinogenicity could be suggested for most of these potential biomarkers. However, in order to ensure reliable results and to create the prerequisites necessary for integration in routine toxicology studies in the future, these protein expression patterns need to be prevalidated using independent technology platforms. In the current study, we evaluated the usefulness of iTRAQ reagent technology (Applied Biosystems, Framingham, USA), a recently introduced MS-based protein quantitation method, for verification of the 2-DE/MS biomarkers. In summary, the regulation of 26 2-DE/MS derived protein biomarkers could be verified. Proteins like HSP 90-beta, annexin A5, ketohexokinase, N-hydroxyarylamine sulfotransferase, ornithine aminotransferase, and adenosine kinase showed highly comparable fold changes using both proteomic quantitation strategies. In addition, iTRAQ analysis delivered further potential biomarkers with biological relevance to the processes of hepatocarcinogenicity: e.g. placental form of glutathione S-transferase (GST-P), carbonic anhydrase, and aflatoxin B1 aldehyde reductase. Our results show both the usefulness of iTRAQ reagent technology for biomarker prevalidation as well as for identification of further potential marker proteins, which are indicative for liver hepatocarcinogenicity.

Use of two-dimensional gel electrophoresis in predictive toxicology: identification of potential early protein biomarkers in chemically induced hepatocarcinogenesis.

Our current approach focused on the identification of potential early protein biomarker signatures which are indicative of the carcinogenic processes in rats exposed to 20 mg/kg of the liver carcinogen N-nitrosomorpholine (NNM). Treated liver was investigated at different timepoints. Therefore, proteins were separated by two-dimensional gel electrophoresis as a first step prior to identification of differentially expressed proteins by mass spectrometry. Proteomic analysis of liver samples after one day of exposure revealed significant upregulation of proteins involved in response to cellular stress induced by NNM (superoxide dismutase, heat shock protein 60, peroxiredoxin). Eighteen weeks after withdrawal of NNM, we were able to identify cancer-related proteins in rat liver bearing malignant, transformed cells (caspase-8 precursor, vimentin, Rho GDP dissociation inhibitor). Some of these proteins were already deregulated after three weeks of exposure indicating their potential usefulness as early predictive biomarkers for liver carcinogenicity (annexin A5, fructose-1,6-bisphosphatase). As regulatory toxicology approaches usually include the investigation of carcinogenicity in two-years studies in rodents, especially the detection of early protein biomarker signatures which precede the appearance of neoplasia, demonstrates the high potential of proteomics approaches to substantially reduce the time and costs of carcinogenicity testing.

[Toxicoproteomics: first experiences in a BMBF-study]. / Toxikoproteomics: Erste Erfahrungen in einer BMBF-Studie.

The rapid development of molecular toxicology is providing innovative approaches to an improved investigation and recognition of toxic substances. Proteome analysis offers, with 2DE/MS (two-dimensional gel electrophoresis and mass spectrometry) and SELDI (surface enhanced laser desorption/ionisation), a promising discipline to classify molecular changes caused by toxic exposure. The Rat Liver Foci Bioassay (RLFB) is a detailed, well-described model for the investigation of liver carcinogenesis induced by chemical substances. Based on this model, we examined whether proteomic methods of molecular toxicology can be used for the early recognition of toxic and/or carcinogenic characteristics of toxic substances. In addition, identification and subsequent prevalidation of new hepatocellular biomarkers was performed, enabling better prediction of toxic and/or carcinogenic effects. This could lead to a more meaningful RLFB and thus to an improved risk assessment of chemicals. 2DE analysis in this study showed that deregulated proteins are assigned to mainly anabolic and catabolic metabolism pathways in the cell. Beyond this, individual proteins were identified which play a key role in the carcinogenic process. A comparison of the differentially expressed proteins in tissue from tumour-bearing animals and tissue derived from the start of the study revealed that protein expression changes (biomarkers) were already detectable shortly after exposure. In addition, analysis by SELDI clearly showed several differentially expressed proteins and/or derived masses. The spectra represented specific differences in tissues, which could be assigned to the same histopathological endpoints. With bioinformatics analysis it was possible to identify individual discriminating mass peaks, which were indicative of tumour formation. Group specific changes can be illustrated and/or represented in more detail with further cluster analysis methods. These results give hope for an improved prediction of hepatotoxicity and carcinogenicity by means of protein markers, which could in the future lead to a shortening of carcinogenicity studies and to a reduction in the use of experimental animals.

Activation of estrogen receptor alpha and ERbeta by 4-methylbenzylidene-camphor in human and rat cells: comparison with phyto- and xenoestrogens.

4-Methylbenzylidene-camphor (4-MBC) is an organic sunscreen that protects against UV radiation and may therefore help in the prevention of skin cancer. Recent results on the estrogenicity of 4-MBC have raised concerns about a potential of 4-MBC to act as an endocrine disruptor. Here, we investigated the direct interaction of 4-MBC with estrogen receptor (ER) alpha and ERbeta in a series of studies including receptor binding, ER transactivation and functional tests in human and rat cells. 4-MBC induced alkaline phosphatase activity, a surrogate marker for estrogenic activity, in human endometrial Ishikawa cells. Interestingly, 4-MBC induced weakly ERalpha and with a higher potency ERbeta mediated transactivation in Ishikawa cells at doses more than 1 microM, but showed no distinct binding affinity to ERalpha or ERbeta. In addition, 4-MBC was an effective antagonist for ERalpha and ERbeta. In an attempt to put 4-MBC's estrogenic activity into perspective we compared binding affinity and potency to activate ER with phyto- and xenoestrogens. 4-MBC showed lower estrogenic potency than genistein, coumestrol, resveratrol, bisphenol A and also camphor. Analysis of a potential metabolic activation of 4-MBC that could account for 4-MBC's more distinct estrogenic effects observed in vivo revealed that no estrogenic metabolites of 4-MBC are formed in primary rat or human hepatocytes. In conclusion, we were able to show that 4-MBC is able to induce ERalpha and ERbeta activity. However, for a hazard assessment of 4-MBC's estrogenic effects, the very high doses of 4-MBC required to elicit the reported effects, its anti-estrogenic properties as well as its low estrogenic potency compared to phytoestrogens and camphor has to be taken into account.