No abstract

Toxicity testing and regulation of advanced materials at the nanoscale, i.e. nanosafety, is challenged by the growing number of nanomaterials and their property variants requiring assessment for potential human health impacts. The existing animal-reliant toxicity testing tools are onerous in terms of time and resources and are less and less in line with the international effort to reduce animal experiments. Thus, there is a need for faster, cheaper, sensitive and effective animal alternatives that are supported by mechanistic evidence. More importantly, there is an urgency for developing alternative testing strategies that help justify the strategic prioritization of testing or targeting the most apparent adverse outcomes, selection of specific endpoints and assays and identifying nanomaterials of high concern. The Adverse Outcome Pathway (AOP) framework is a systematic process that uses the available mechanistic information concerning a toxicological response and describes causal or mechanistic linkages between a molecular initiating event, a series of intermediate key events and the adverse outcome. The AOP framework provides pragmatic insights to promote the development of alternative testing strategies. This review will detail a brief overview of the AOP framework and its application to nanotoxicology, tools for developing AOPs and the role of toxicogenomics, and summarize various AOPs of relevance to inhalation toxicity of nanomaterials that are currently under various stages of development. The review also presents a network of AOPs derived from connecting all AOPs, which shows that several adverse outcomes induced by nanomaterials originate from a molecular initiating event that describes the interaction of nanomaterials with lung cells and involve similar intermediate key events. Finally, using the example of an established AOP for lung fibrosis, the review will discuss various in vitro tests available for assessing lung fibrosis and how the information can be used to support a tiered testing strategy for lung fibrosis. The AOPs and AOP network enable deeper understanding of mechanisms involved in inhalation toxicity of nanomaterials and provide a strategy for the development of alternative test

It has been shown that glutathione S-transferase (GST) interacts with and suppresses the activity of c-Jun NH 2 -terminal kinase (JNK). GST-deficient mice (GST ؊/؊ ) have higher levels of circulating white blood cells, with similar proportions of lymphocytes, monocytes, and granulocytes. Interestingly, a selective expansion of splenic B lymphocytes was observed in GST ؊/؊ animals but no change in T lymphocytes or natural killer cells. A peptidomimetic inhibitor of GST that disrupts the interaction between GST and JNK mimics in wild type mice the increased myeloproliferation observed in GST ؊/؊ animals. Until now, the molecular basis for this effect has not been defined. In an in vitro hematopoiesis assay, interleukin-3, granulocyte colonystimulating factor, and granulocyte/macrophage colonystimulating factor were more effective at stimulating proliferation of hematopoietic cells in GST ؊/؊ mice than in wild type. The JNK inhibitor SP600125 which caused little inhibition of cytokine-induced myeloproliferation in wild type mice, decreased the number of colonies in GST ؊/؊ animals. A more sustained phosphorylation of the STAT family of proteins was also observed in GST ؊/؊ bone marrow-derived mast cells exposed to interleukin-3. This was associated with an increased proliferation and a down-regulation of expression of negative regulators of the Janus kinase-STAT pathway SHP, Src homology 2 domain-containing tyrosine phosphatase-1 and -2. The increased activation of JNK and STATs in GST-deficient mice provides a viable mechanism for the increased myeloproliferation in these animals. These data also confirm the important role that GST plays in the regulation of cell signaling pathways in a myeloproliferative setting.