Page 225 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

210 Principles and Methods

represent another material type that has undergone some toxicity test-
ing [11]. Water-soluble, monodisperse or colloidal fullerene aggregates
induce O -anions, lipid peroxidation in cells and tissue as well as the
2
ability to affect GSH depletion and cytotoxicity [23]. In vitro incubation
of keratinocytes and bronchial epithelial cells with relatively high doses
of single-wall carbon nanotubes (SWNT) results in ROS generation,
lipid peroxidation, oxidative stress, mitochondrial dysfunction, and
changes in cell morphology [24,25]. MWNT also elicits pro-inflammatory
effects in keratinocytes [26].

Oxidative stress elicits quantifiable
cellular responses that can be used
to study NM toxicity
Cells respond to oxidative stress by mounting a number of protective and
injurious responses that, depending on the stress level, can lead to a pro-
tected or an injurious outcome [7, 11]. The protective responses are
elicited by even minor changes in the cellular redox equilibrium. This
sensitivity is rooted in the behavior of the cap ’n’ collar transcription
factor, Nrf-2, that operates on the antioxidant response element (ARE)
in the promoter of > 200 phase II enzymes [27]. These phase II enzymes
exert antioxidant, detoxification and anti-inflammatory effects that are
responsible for preventing or slowing the effects of oxidant injury [27].
Examples of phase II enzymes include HO-1, glutathione-S-transferase,
NADPH quinone oxidoreductase, catalase, superoxide dismutase, glu-
tathione peroxidase, and UDP-glucoronosyltransferase [27, 28]. A
reduced or compromised phase II response promotes susceptibility to oxi-
dant injury. In studies conducted in epithelial cells and macrophages,
phase II enzyme expression is the most sensitive oxidative response
parameter and has also been referred to as Tier 1 of the hierarchical
oxidative stress response [29–31]. A higher level of oxidative stress can
lead to a Tier 2 response, which is characterized by pro-inflammatory
effects that follow the activation of intracellular signaling cascades,
including the MAP kinase and NF- B signaling cascades [30–32]. These
signaling cascades are responsible for the activation of a number of
cytokines, chemokines, and adhesion molecules that play a role in local
and systemic inflammatory responses [32–34]. The dynamic equilib-
rium between the protective (Tier 1) and pro-inflammatory responses
(Tier 2) determines the outcome of the oxidative stress response and the
likelihood that this will lead to injury.
2+
Tier 3 responses involve mitochondrial perturbation, Ca flux, and
activation of apoptosis pathways [21, 30, 32, 35]. Although the in vivo
significance of the mitochondrial pathway is still under investigation,
it has been demonstrated in tissue culture cells that ambient PM inter-
feres in mitochondrial electron transfer, thereby leading to dissipation