University of California Los Angeles
Development of Methods and Models for Nanoparticle Toxicity Screening: Applicatio
Andre Nel, Ph.D.
Data for performing a preliminary risk assessment of manufactured nanomaterials are just beginning to emerge. However, early studies of nanomaterial toxicity in aqueous media have tended to be more observational than mechanistic, and have often focused on a single, advanced stage of toxicity that could yield contradictory results. Moreover, the ability to generalize findings to other nanomaterials is limited by the lack of a rational basis for categorizing nanomaterials. Elucidating the mechanisms of toxicity for a given nanomaterial will provide a basis for classifying materials for regulatory purposes, postulating dose-response curves, screening potential risks, and prescribing strategies for risk management. The primary objective of this work is to elucidate the mechanism(s) by which manufactured nanoparticles may induce toxicity in vitro and in vivo.
Specifically, this study will consider fullerene-based materials, comparing them with,
- Reference standards (TiO2 and carbon black)
- Ultrafine particles obtained from an urban airshed (well characterized by in vitro toxicology studies)
We will explore a methodology for rapidly screening potentially toxic nanoparticles based on their propensity to generate ROS. The principal hypothesis is that certain classes of nanoparticles such as fullerenes induce ROS production, cellular oxidative stress and cytotoxicity. Fullerenes are selected based on the relatively novel properties (e.g. strength.arid electron affinity) that make them attractive for commercialization. The investigators propose that oxidative stress induced by fullerene derivatives occurs in several stages (tiers), beginning with the induction of phase II antioxidant defenses at the lowest tier of oxidative stress (tier 1), followed by pro-inflammatory (tier 2) and mitochondrion-mediated cytotoxic effects (tier 3) as the level of oxidative stress increases. Particle size, shape, surface area, charge, and chemical composition are important physical variables that could determine their ROS-generating or scavenging properties. Rapid physicochemical determination of ROS production might provide a paradigm to assess the possible toxicity of nanomaterials that act via these mechanisms. Specific Aim 1 will characterize commercial nanoparticles and their derivatives in terms of particle size, shape, surface area, charge, aqueous solubility, propensity to aggregate, and their ability to catalyze or quench ROS production in vitro. Materials will also be characterized in model solutions containing naturally occurring organic matter, proteins and ions at levels similar to those present in natural waters. Aim 2 will determine whether various fullerenes can generate a hierarchical oxidative stress response in macrophages, bronchial epithelial cells, endothelial cells, neural cells and hepatocytes.
- This will be accomplished by comparing the effects of fullerenes and reference nanoparticles on,
Phase II enzyme expression and activation of the heme oxygenase 1 (HO-1) promoter (tier 1)
- Ytokine and chernokine expression as well as assays for MAP kinase activation (tier 2)
- Mitochondria! perturbation and induction of cellular apoptosis (tier 3)
These biological responses will be compared to the physicochemieal properties of nanomaterials elucidated in Aim 1. Aim 3 will perform in vivo imaging of the oxidative stress-sensitive HO-1 promoter linked to a luciferase reporter in transgenic mice. Organs and tissues showing increased luciferase activity will be investigated for histological evidence of inflammation and cytotoxicity. Aim 4 will compare the biologic responses elicited by each of the nano-scale particles with their ability to generate ROS abiotically, and test the hypothesis that ROS generation can be used to screen toxicity. By focusing on mechanisms of toxicity rather than outcomes alone, this work will provide the basis for classifying nanomaterials for regulatory purposes. Based on preliminary results presented in this proposal, we anticipate that ROS generation in solution and under UV radiation will be good predictors of nanoparticle toxicity and that ROS measurements can be adapted to screen nanomaterials. A broader assessment of nanomaterial toxicity in the context of the hierarchical oxidative stress response is likely to yield a more sensitive paradigm for toxicity testing, perhaps resolving inconsistencies reported in the literature.