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Reactive Oxygen Species and Oxidative Stress 287

Overview of ROS-Mediated Modulation of Gene Expression in Eukaryotes
Although an understanding of ROS-driven gene regulation in prokaryotes can be useful in developing
hypotheses regarding responses to ROS in eukaryotic organisms, significant differences exist between
the prokaryotes and eukaryotes thus far studied (Crawford, 1999; McCord, 2000). One major difference
is that transcription factors that are responsive exclusively to ROS have not been identified in eukaryotes;
rather, signaling molecules also involved in responses to other stimuli are used. Another major difference
is that a different array of proteins is induced in eukaryotes, including many that are not thought of as
classical antioxidants and sometimes not including proteins that typically are thought of as classical
antioxidants. A third major difference is the presence in some mammalian species of an enhancer element
known as the antioxidant response element (ARE), which regulates the expression of certain antioxidants
and other proteins. Some of these differences may be attributable to the large differences that exist
between single-cell and multicellular organisms in terms of their dependence on oxygen and the desir-
ability of maximizing rates of cell division (McCord, 2000). These differences are discussed at greater
length below.
The expression of many genes is affected by ROS; however, it is important to bear in mind that
not all such changes necessarily represent adaptive, antioxidant responses. Some changes in gene
expression simply result from oxidative damage; for example, low levels of damage may alter the
activity of transcription factors (Gutteridge and Halliwell, 1999). Additionally, ROS are generated
•–
purposefully not only as physiological effector molecules (such as O generated by phagocytes,
2
discussed previously) but also as signaling molecules in pathways not necessarily related to oxidative
stress per se (Finkel, 2000; Morel and Barouki, 1999; Palmer and Paulson, 1997; Sauer et al., 2001).
Thus, ROS may be generated by enzymes such as NADPH oxidase or in the mitochondria as part of
normally functioning signaling pathways (Palmer and Paulson, 1997; Sauer et al., 2001). It may be
useful to conceptualize the effect of the signaling elicited by ROS as varying qualitatively according
to the amount and duration of the associated oxidative stress, with low levels in some cases producing
a cellular response of proliferation at least in vitro, intermediate levels often producing temporary
growth arrest and an adaptive response, higher levels leading to apoptosis, and very high doses causing
necrosis (Chandra et al., 2000; Davies, 1999; Gutteridge and Halliwell, 1999; Martindale and Hol-
brook, 2002). Thus, ROS cannot be thought of as simply and exclusively damage-causing agents;
rather, it is the production of inappropriate amounts of ROS or production in the wrong place or at
the wrong time that is problematic. Furthermore, the ability of ROS-generating chemicals to cause
damage is not limited to their ability to generate highly reactive toxic intermediates but is also related
to their ability to alter the functioning of normal signaling pathways that utilize ROS as messenger
molecules. Finally, signaling by reactive nitrogen, copper, iron, and other redox-active metal species,
in addition to reactive oxygen species, likely plays an important role in modulating gene expression
(Gutteridge and Halliwell, 1999).

Effects of ROS on Transcription Factors
The activity of many transcription factors is dependent upon the reduced or oxidized state of redox-
responsive moieties such as thiols or Fe–S clusters; as a result, oxidative stress can alter the activity of
those transcription factors by altering the redox status of the cell (Arrigo, 1999; Primiano et al., 1997;
Schafer and Butner, 2001; Sen, 2000). Transcription factors in eukaryotes reported to be affected in this
fashion include AP-1, Maf, Nrl, NF-IL6, Sp-1 family members, protein kinase C, glucocorticoid recep-
tors, estrogen receptors, aryl hydrocarbon receptor, NF-κB, p53, and others (Arrigo, 1999; Crawford,
1999; Dalton et al., 1999; Michiels et al., 2002; Primiano et al., 1997; Sen, 2000). ROS also increase
the rate of transcription of many transcription factors, including members of the AP-1, NF-κB, and AP-2
families (Dalton et al., 1999). The upstream events leading to activation of transcription of these factors
are complex and include alterations in phosphorylation of signaling molecules, release of arachidonic
acid from cell membranes and subsequent metabolism of the arachidonic acid, and mobilization of Ca 2+
(for reviews, see Dalton et al., 1999; Martindale and Holbrook, 2002; Sauer et al., 2001).