Review
This Review is part of a thematic series on New Paradigms of Transcriptional Control of Myocardial and
Vascular Growth, which includes the following articles:
Redox-Dependent Transcriptional Regulation
Excitation-Transcription Coupling
Histone-Modulation as a Regulator of Growth
Gordon F. Tomaselli, Editor
Redox-Dependent Transcriptional Regulation
Hongjun Liu, Renata Colavitti, Ilsa I. Rovira, Toren Finkel
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Abstract—Reactive oxygen species contribute to the pathogenesis of a number of disparate disorders including tissue
inflammation, heart failure, hypertension, and atherosclerosis. In response to oxidative stress, cells activate expression
of a number of genes, including those required for the detoxification of reactive molecules as well as for the repair and
maintenance of cellular homeostasis. In many cases, these induced genes are regulated by transcription factors whose
structure, subcellular localization, or affinity for DNA is directly or indirectly regulated by the level of oxidative stress.
This review summarizes the recent progress on how cellular redox status can regulate transcription-factor activity and
the implications of this regulation for cardiovascular disease. (Circ Res. 2005;97:967-974.)
Key Words: Yap1 䡲 ref-1 䡲 Nrf2 䡲 atherosclerosis 䡲 oxidative stress
O
Numerous studies have implicated oxidative or nitrosative
stress in the progression of atherosclerosis and heart failure as
well as the regulation of angiogenesis and other cardiovascular conditions.2–5 Although the evidence for an oxidative
stress component to disease initiation or progression has been
well documented, the precise and relevant molecular targets
of ROS remain less well understood. One clear mechanism
through which ROS might alter the vessel wall or the
myocardium would be through a redox-dependent change in
transcriptional outputs. Clearly, this is not the only mechanism one could envision because the ROS-mediated oxidation of low-density lipoprotein cholesterol represents a
transcription-independent mechanism through which oxidants could obviously contribute to atherosclerosis. Nonetheless, understanding how changes in intracellular oxidants
might affect transcriptional activity represents an important
avenue in understanding how ROS contribute to numerous
disease states.
In this review, we have decided to present an overview of
a select handful of well-characterized redox-dependent transcriptional regulatory mechanisms. Rather than confine the
xidative stress represents a common threat and danger
for all aerobic organisms. Any enzyme capable of
metabolizing oxygen also carries with it the intentional or
accidental capability of generating reactive oxygen species
(ROS). Once formed, these radical species can damage a
number of essential cellular components including lipid
membranes, DNA, and proteins. In addition, there exists a
growing body of evidence that acting at lower concentrations,
ROS can be purposely made within cells to serve as signaling
molecules.1 The response of a cell or organism to an increase
in superoxide, hydrogen peroxide, or related ROS often
involves the activation of numerous intracellular signaling
pathways. These cytosolic pathways can, in turn, regulate a
host of transcriptional changes that allow the cell to respond
appropriately to the perceived oxidative stress. In addition to
the regulation achieved by classical cytosolic signaling pathways, such as the family of mitogen-activated protein kinases
(eg, mitogen-activated protein kinase, c-Jun N-terminal kinase, and p38 kinase family), evidence suggests that certain
transcription factors can directly or indirectly alter their
activity, depending on cellular redox conditions.
Original received July 20, 2005; revision received September 13, 2005; accepted September 16, 2005.
From the Cardiology Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md.
This manuscript was sent to Eugene Braunwald, Consulting Editor, for review by expert referees, editorial decision, and final disposition.
Correspondence to Toren Finkel, MD, PhD, Chief, Cardiology Branch, National Institutes of Health, Bldg 10 CRC 5-3330, 10 Center Dr, Bethesda,
MD 20892. E-mail [email protected]
© 2005 American Heart Association, Inc.
Circulation Research is available at http://circres.ahajournals.org
DOI: 10.1161/01.RES.0000188210.72062.10
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discussion to those mechanisms already known to operate in
the myocardium or vasculature, we have, instead, purposely
broadened the discussion to include a number of model
systems ranging all the way from bacteria to mammals.
Although some of these examples are known to be directly
applicable to cardiovascular disease, others undoubtedly will
only be relevant with regard to the more general principles
they reveal. It is our belief, however, that the range of
examples provided demonstrate the importance as well as the
complexity of turning oxidant signals into transcriptional
outputs.
Lessons From Simple Organsims: SoxR,
OxyR, and Yap1
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Bacteria respond to either superoxide anions or hydrogen
peroxide with the induction of a discrete set of gene products.6 Interestingly, the bacterial response to elevated but
sublethal concentrations of either superoxide or hydrogen
peroxide significantly differ, suggesting that even these
presumed evolutionary simple organisms have the ability to
sense and distinguish subtly different species of oxidants.
Early work identified 2 separate families of redox-dependent
transcription factors responsible for this dichotomous behavior. The SoxR and SoxS proteins have been demonstrated to
orchestrate the bacterial response to superoxide, whereas the
OxyR protein is involved in responding to hydrogen peroxide. In both cases, the transcriptional machinery directly or
indirectly senses the relevant oxidant species. For the case of
superoxide, the radical species appears to modify the activity
of SoxR, which, in turn, induces expression of SoxS. Increased levels of SoxS result in increased expression of more
than a dozen other genes including enzymes responsible for
DNA repair (eg, nfo, a bacterial endonuclease) as well as
genes involved in detoxification (eg, sodA, a manganesecontaining superoxide dismutase). The capacity for SoxR to
sense superoxide resides in its protein structure and, in
particular, its iron-sulfur (2Fe-2S) clusters. These metalbased clusters can undergo either a 1-electron oxidation or
reduction, switching the protein complex between a [2Fe2S]2⫹ and a [2Fe-2S]3⫹ state. Although both of these oxidation states can bind DNA, only the [2Fe-2S]3⫹ state appears
capable of activating transcription.7
In contrast to the Fe-S– based superoxide sensing system of
SoxR, the sensing of hydrogen peroxide by OxyR revolves
around cysteine chemistry. Once activated, the targets of
OxyR include katG (a hydrogen peroxidase I), gorZ (a
glutathione reductase), and oxyS (a small nontranslated regulatory RNA involved in DNA repair). Two particular cysteine residues (Cys199 and Cys208) appear to play an
essential role in the activation of OxyR. A cysteine-to-serine
substitution of either Cys199 or Cys208 dramatically reduces
the activity of the transcription factor.8 In addition to these 2
critical cysteines, there are 4 additional cysteine residues in
OxyR; however, mutations of these other cysteines do not
appear to have dramatic effects on OxyR function. A considerable amount of effort has gone into understanding how
OxyR function is redox regulated. Storz and colleagues have
suggested an attractive model centered on the formation of a
Figure 1. A proposed mechanism for the regulation of the bacterial transcription factor OxyR. Schematic is based on the
model of Storz and colleagues.8 The inactive form of OxyR contains reduced (SH) thiol groups on the 2 key regulatory cysteines. Increased concentrations of hydrogen peroxide result in
the oxidation of Cys199 to a sulfenic form (S-OH), which then
quickly forms a disulfide bond with Cys208, leading to the
active configuration of OxyR. This active state can then be
reduced back to the original conformation by the combination of
glutaredoxin and glutathione. An alternative model that does not
involve disulfide-bond formation has been proposed by Stamler
and colleagues.12
disulfide bond between Cys199 and Cys208 that forms only
under oxidative-stress conditions. They proposed that the
OxyR transcription factor functions as an “on/off” switch, in
which the active form contains an intramolecular disulfide
bond, and the inactive form contains reduced thiols (see
Figure 1). This disulfide-bond based on/off model of OxyR
was supported by various structural studies of OxyR under
oxidized or reduced conditions.9 –11
In addition to the disulfide bond– based on/off switch
model of OxyR activation, Stamler and colleagues proposed
that the OxyR activity was regulated primarily by the Cys199
residue through various modifications: by oxidative stress to
Cys199 –SOH, by nitrosative stress to Cys199 –SNO, or by
forming a mixed disulfide bond (Cys199 –S-S-G) with glutathione.12 Each modified form of OxyR was proposed to have
different structure, DNA-binding affinity, and promoter activity. This model suggested a graded response on OxyR
transcriptional outputs, depending on the nature of the oxidative stress. Both models appear to agree that the first
structural modification of OxyR by hydrogen peroxide is the
formation of a modified Cys199 to a sulfenic acid (Cys199 –
SOH) form. Whereas Stamler and colleagues suggest that this
form can activate transcription, Storz and colleagues argue
that the sulfenic Cys199 is a chemical intermediate that
quickly reacts with Cys208 to form a stable disulfide bond.
Finally, studies in yeast provide another instructive model
for transcriptional regulation by redox-sensitive factors. In
Liu et al
Redox-Dependent Transcriptional Regulation
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Figure 2. Regulation of the yeast transcription factor Yap1. Nuclear importation of Yap1 is orchestrated by the
nuclear localization signal (NLS), whereas
nuclear export is determined by a Crm1
pathway involving the NES. At any time,
subcellular localization of Yap1 is determined by the balance of nuclear importation and export. Increased concentrations of hydrogen peroxide results in
disulfide-bond formation catalyzed by
glutathione peroxidase 3 (Gpx3), which,
in turn, leads to structural changes in
Yap1. These structural changes involve
formation of 2 disulfide bonds that mask
the NES. The result is that oxidative
stress leads to the nuclear accumulation
of Yap1 and an increase in its biological
activity (eg, induction of antioxidant gene
expression).
Saccharomyces cerevisiae, Yap1 is functionally homologous
to bacterial OxyR.13 In particular, in the face of hydrogen
peroxide exposure, Yap1-deleted strains are unable to induce
antioxidant genes necessary to protect the organism from
oxidative stress. Yap1 is a member of the basic leucine zipper
(bZIP) family of transcription factors and is a distant ortholog
of the mammalian c-Jun family. Yap-1 can exist in the
nucleus, where it is active, as well as in a cytosolic inactive
form (Figure 2). Importation of Yap1 into the nucleus is
regulated by a nuclear-localizing sequence found in the
amino terminus of Yap1, whereas nuclear export is regulated
by a C-terminal nuclear export sequence (NES). Similar to
OxyR, hydrogen peroxide can induce disulfide bonds between particular sets of cysteine residues in Yap1.14 Once
formed, these disulfide bonds produce a conformational
change in Yap1 that masks the NES.15 This oxidant-induced
masking of the NES sequence results in accumulation of the
protein in the nucleus and, hence, a mechanism through
which oxidants regulate Yap1 subcellular localization and, by
extension, regulate Yap1 activity. Genetic and biochemical
evidence suggests that the formation of these Yap1 disulfide
bonds requires an additional protein, in this case a glutathione
peroxidase–related molecule, Gpx3.16 Interestingly, an analogous regulation appears to occur in Schizosaccharomyces
pombe, where the peroxide-sensitive factor is called Pap1.17
Mammalian Redox Control: The Biology
of Ref-1
The redox regulation of mammalian transcription retains
many of the properties already evident in the examples
provided by simpler organisms. In particular, the regulation
of protein function through cysteine-based oxidation and
reduction is a recurrent theme.18 –20 The importance of this
form of regulation is becoming increasingly evident not only
for transcriptional regulation but also for other classes of
molecules such as protein phosphatases.21–23 One important
mammalian redox modulator that has been the focus of
numerous studies is the bifunctional enzyme, Redox factor-1
(Ref-1) (also termed Ape1, HAP1, and APEX1). Two distinct
enzymatic functions have been ascribed to this molecule. The
highly conserved C-terminal region of the protein functions
as a DNA repair enzyme. In particular, this part of the
molecule is involved in the repair of apyrimidinic/apurinic
nucleotides (also known as “AP” sites). Such base-pair
modifications occur on an ongoing basis as cells are exposed
to daily onslaught of ionizing radiation, UV or ROS. The
less-conserved N-terminal region of Ref-1 contains the nuclear localization sequence and the redox regulatory domain,
which is characterized by 2 critical cysteines, Cys65 and
Cys93.24 These cysteines in Ref-1 are believed to be important in the redox-dependent modification of transcription
factors such as activator protein-1 (AP-1) (Fos and Jun),
nuclear factor ␬B (NF-␬B), p53, activating transcription
factor/cAMP-response element– binding protein (ATF/
CREB), hypoxia-inducible factor (HIF)-1␣, and HIF-like
factor.25,26 In general, the oxidized form of these transcription
factors have reduced or absent DNA-binding activity, and
Ref-1 appears as an important factor for the specific reduction
of these transcription factors. Again, the direct oxidation and
reduction of the transcription factors usually involves critical
cysteine residues within the DNA-binding domain of the
protein. Like the cysteine residues in OxyR, the critical
cysteines in both Fos and Jun as well as in other Ref-regulated
transcription factors appear to be surrounded by a stretch of
basic amino acids. This signature is common among “reactive” cysteines. In this case, reactive implies the ability to
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Figure 3. The ubiquitously expressed protein Ref-1 is responsible for the enzymatic reduction of oxidized cysteines within the
DNA-binding domain of a number of transcription factors. The
exact mechanism in which Ref-1 achieves this thiol reduction is
incompletely understood. In addition, the state of oxidation of
the transcription factor in the nucleus is also poorly characterized. Two potential scenarios are shown for 2 known protein
targets, namely the reduction of a sulfenic form of AP-1 or of a
disulfide bond in the tumor suppressor p53.
form a thiolate anion (S-) at physiological pH. Presumably,
the surrounding basic amino acids facilitate this reactivity and
distinguish redox-sensitive cysteine moieties from the majority of nonreactive cysteines found within any given structure
of a protein.
The isolation of Ref-1 resulted from a series of experiments primarily by Curran and colleagues. Similar to
Yap1, the AP-1 transcription factor belongs to the bZIP
family of transcriptional activators. AP-1 results from the
heterodimerization of Fos and Jun proteins, and this
transcriptional complex is important for the proper induction of a number of genes. These AP-1–regulated genes, in
turn, allow the cell to respond appropriately to a host of
environmental stimuli including, but not limited to, oxidative stress. Hints that reduction and oxidation of AP-1 was
important for its biology came from several early observations. Like many other genes involved in growth regulation, c-Jun has been incorporated into an oncogenic
retrovirus. Comparison of the sequence of the viral gene
v-Jun, with its cellular counterpart c-Jun, revealed that 1 of
the amino acid differences in the viral oncogene involved
this conserved cysteine residue found within the DNAbinding domain.27 Interestingly, when the homologous
cysteine residue was purposely mutated in the Fos gene,
the mutant protein appeared to bind to DNA with greater
affinity. In addition, the redox-dependent binding of Fos
was abrogated by this mutation, whereas the ability of the
protein to induce transformation of cells was enhanced.28
These results are consistent with the interpretation that
cysteine oxidation reversibly reduces DNA binding and,
hence, the overall biological activity of AP-1.
The abovementioned studies led to the recognition that
the binding of bZIP family members could be regulated by
the oxidation-reduction of critical cysteine residues within the
DNA-binding domain (see Figure 3). This conjecture was,
however, substantially solidified after the biochemical puri-
fication of Ref-1.29 Subsequent immunodepletion analysis
identified the 36-kDa Ref-1 protein as the major redox
regulator of AP-1 activity in cells.30 Examination of the
structure of human Ref-1 revealed 2 critical cysteine residues
at position 65 (position 64 in mice) and position 93. The
initial data suggested that cysteine 65 was the biologically
important cysteine required for the ultimate reduction of
AP-1.31 This hypothesis has been challenged by recent studies
in which this critical cysteine has been mutated without any
apparent effect on AP-1 activity.32 As such, the exact molecular mechanism underlying the redox activity of Ref-1
remains unclear. Indeed, it is not clear whether Ref-1 acts as
a direct or indirect redox sensor. For instance, previous
studies have indicated that Ref-1 can also bind to the
ubiquitous thiol-containing redox protein thioredoxin.33 Stimuli that activate AP-1 appear to cause the importation of
cytosolic thioredoxin into the nucleus, where it can bind to
Ref-1 and augment AP-1 activity. In many ways, this 2-step
process is similar to the paradigm used by Yap1, where a
glutaredoxin molecule acts as the redox transducer for this
yeast bZIP transcription factor.16 Finally, although Ref-1 was
initially isolated as an AP-1–specific factor, it is now clear
that many transcription factors contain critical single or
multiple cysteine residues within their DNA-binding domain.
Many of these other transcription factors appear to require
Ref-1 for optimal activity.25,26 As such, the redox-dependent
effects of Ref-1 intersect multiple pathways.
The ramification of Ref-1 biology for vascular tissues is
the focus of study for a number of laboratories. For instance,
recent observations have suggested that in endothelial cells,
Ref-1 can inhibit Rac1-dependent activation of the NADPH
oxidase, thereby lowering the production of cytosolic ROS,
decreasing NF-␬B activation, and protecting cells from apoptosis.34 These activities appear separate from the role of the
protein discussed previously. Ref-1 has also been shown to
protect against endothelial cell apoptosis induced by cytokines such as tumor necrosis factor-␣.35 Interestingly, mice
that are heterozygously deleted for Ref-1 (Ref-1⫹/⫺) are
spontaneously hypertensive and exhibit impaired vasodilatation resulting from defective endothelial NO production.36 In
another potentially relevant example, Ref-1 has been shown
to bind and activate the HIF-1␣–inducible transcriptional
complex that regulates the hypoxic induction of vascular
endothelial growth factor.37 Interestingly, this response appears to require hypoxia-induced ROS production. These
oxidants were shown, in turn, to modify the 3⬘ guanine within
the HIF-1 DNA–recognition sequence. The subsequent ROSinduced formation of an apyrimidinic/apurinic site within this
specific region of DNA was shown to facilitate recruitment of
a complex containing both HIF-1 and Ref-1.37 This study,
therefore, provides a potential explanation for how the DNA
repair and redox functional domains of Ref-1 might be
physiologically integrated. It also blurs the line between ROS
acting as random damaging agents and ROS-mediated signaling. Another potentially relevant vascular example of
Ref-1 biology involves platelet-derived growth factor
(PDGF) signaling in smooth muscle cells. In this context, the
PDGF-dependent progression from G0/G1 to S relies on the
reduction of AP-1 by Ref-1. These results hint at a potentially
Liu et al
intriguing connection between dysfunction in Ref-1 signaling
and smooth muscle proliferation and, hence, the pathogenesis
of atherosclerosis.38 Finally, Ref-1 also appears to mediate
the transcriptional inhibition of renin production, a critical
determinant of blood pressure control.39
Oxidative and Xenobiotic Stress: The
Nrf2–Keap1 System
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In mammalian cells, one aspect of the response to oxidants or
xenobiotic stress is the coordinated induction of a host of
detoxifying enzymes known collectively as phase I and II
enzymes.40 Among the gene products that are stimulated are
enzymes such as NAD(P)H quinone oxidoreductase, glutathione S-transferase, cysteine– glutamate exchange transporter, and the multidrug resistance–associated protein. This
coordinated response presumably allows the cell to help
inactivate and expel the offending agent as well as to rapidly
control and repair the damage. Analysis of common cisacting elements in the regulatory regions of these and other
coordinately induced genes has identified a motif termed the
antioxidant responsive element (ARE). Subsequent studies
identified the transcriptional regulator Nrf2 as the factor that
binds to this element.41 Nrf2 is also a bZIP transcription
factor that binds DNA as a heterodimer in conjunction with
members of the Maf family.42 Mice with targeted deletion in
Nrf2 fail to robustly induce phase II enzymes in the face of
xenobiotic exposure, demonstrating the in vivo requirement
of this transcription factor for the overall biological
response.43,44
Similar to previous examples of transcription factors
that coordinate the oxidative stress response, it is not clear
whether Nrf2 directly senses the stress. Two hybrid experiments using Nrf2 as a bait revealed that the transcription
factor could bind the cytosolic protein Keap1.45 Curiously,
Keap1 is a cysteine-rich protein that can bind to the actin
cytoskeleton in addition to binding to Nrf2. Cell culture
experiments demonstrated that stresses that activate Nrf2
induce the dissociation of Nrf2 from Keap1. Once released, Nrf2 efficiently translocates to the nucleus. As
such, these results would suggest that the activity of Nrf2
is negatively regulated by a Keap1-dependent cytosolic
sequestration pathway. Interestingly, whereas Keap1⫺/⫺
mice die shortly after birth, this lethal phenotype can be
reversed by the double knockout of both Keap1⫺/⫺ and
Nrf2⫺/⫺.46 These results again are consistent with the
interpretation of Keap1 as a negative regulator of Nrf2
activity and suggest that either too much or too little Nrf2
activity can result in physiological impairment.
The abovementioned in vitro and in vivo studies suggest
that the Keap1–Nrf2 interaction is an important regulatory
nodal point in the overall response to oxidative or xenobiotic stress. As previously mentioned, Keap1 is a
cysteine-rich protein with nearly 5% of the molecule made
up of this single amino acid. Of the nearly 25 cysteine
residues present, biochemical analysis has identified 5 that
are reactive, namely capable of ionization at physiological
pH. Of these 5 cysteines, 2 appear particularly critical for
Nrf2 regulation.47 An attractive model would be that
Redox-Dependent Transcriptional Regulation
971
modification of these cysteine residues by ROS-induced
stress results in a conformational change in Keap1, allowing for the dissociation of Nrf2. Although such a mechanism is likely, it is important to stress that other regulatory
mechanisms, including phosphorylation and proteasomal
degradation, are also important in regulating Nrf2 activity.48,49 Finally, Nrf2-dependent regulation is beginning to
be explored as an important aspect of the cardiovascular
system. For instance, this transcription factor has already
been implicated in the endothelial cell transcriptional
response to laminar flow,50 the expression of macrophage
scavenger receptors,51 and the induction of heme oxygenase in vascular smooth muscle cells.52
Regulation by Intracellular Redox Buffers
Although the previous discussion has centered on the transcriptional response to direct oxidative challenge, there is a
growing realization that redox regulation extends beyond this
paradigm. Cellular redox status is reflected in the balance
between a number of reduced or oxidized molecular pairs
including GSH/GSSG, NADPH/NADP, and NADH/NAD.
The function of a number of transcription factors, as well as
important DNA modifying enzymes, appears sensitive to
these redox pairs, implying yet another way that cellular
redox status is coupled to transcriptional outputs. Three
important examples will help demonstrate this point. The
transcriptional repressor C-terminal– binding protein (CtBP)
was initially characterized as a cellular binding partner for the
E1A adenoviral gene product. The interaction of CtBP with
E1A resulted in the inhibition of transcription by E1A, a
strong viral oncoprotein. Subsequently, CtBP was shown to
mediate the transcriptional repression of a number of other
transcription partners.53 Curiously, the amino acid structure
of CtBP revealed a high degree of homology to metabolic
enzymes that bind NAD and NADH.54,55 Biochemical analysis has subsequently confirmed that this homology is not
accidental, as the ability of CtBP to function as a transcriptional repressor is dependent on nicotinamide adenine dinucleotide binding.56,57 There is some disagreement as to
whether NAD and NADH differ in their ability to regulate
CtBP-repressor function. One report has suggested that NAD
and NADH are equivalent,57 whereas another has suggested
that NADH is several orders of magnitude more effective
than NAD in regulating CtBP function.56 Although it is
difficult to accurately assess the physiological ratio of free
NAD or NADH in the nucleus, there is some evidence to
suggest that the affinity of CtBP is within the calculated
range.56 In addition, there is some evidence suggesting that
more than simply binding NADH, CtBP may act as a
dehydrogenase.57 At present, the implications of this enzymatic activity are unknown, as is the cellular substrate for
CtBP dehydrogenase activity.
Another example that appears to couple transcriptional
activity to cellular redox status comes from the analysis of
factors that regulate circadian rhythms. A wide range of
organisms regulate the level of certain genes based on the
daily light/dark cycle. In mammals, a family of heterodimeric
transcription factors is essential for maintaining circadian
rhythms. These transcription factors include the Clock gene,
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NPAS2, and BMAL1. Although the details of the circadian
clock and its intricate transcriptional feedback control is
beyond the scope of this review, the observation that the
DNA binding of both Clock–BMAL1 and NPAS2–BMAL1
heterodimers is sensitive to the NAD(P)/NAD(P)H ratios
suggested a mechanism through which environmental inputs
could entrain the circadian clock.58,59 In particular, the suggestion was that neuronal or metabolic activity could alter the
ratio of NAD/NADH or NADP/NADPH and, thereby, provide a mechanism for which environmental cues could
regulate heterodimer binding and, hence, alter circadian
rhythms.
Finally, one additional example is relevant to our
discussion. The mammalian enzyme SIRT1 (silent information regulator) is the closest ortholog of the yeast enzyme
Sir2. Interest in yeast Sir2 increased following the observation from the laboratory of Guarente that overexpression of
Sir2 could significantly extend lifespan in at least 2 model
organisms.60 Initial analysis of yeast Sir2 function suggested
that the protein is essential for transcriptional silencing, a
biochemical property that appears to be maintained by the
mammalian ortholog SIRT1.61,62 In yeast, Sir2 appears to
globally affect transcription by acting as a histone deacetylase. This enzymatic activity has revealed a strict and unique
requirement for NAD, and it is now apparent that mammalian
SIRT1 is also an NAD-dependent deacetylase.60 Some groups
have challenged whether or not subtle shifts in the NADH/
NAD redox ratio represents the predominant physiological
regulator of SIRT1 or Sir2 activity.63 Nonetheless, there are
certain examples already clearly established in which the
ratio of oxidized-to-reduced nicotinamide adenine dinucleotides can regulate the activity of SIRT1.64 These and other
studies suggest the potential for an important link between
redox balance and chromatin dynamics64 and, by extension, a
link between redox status and overall gene expression.
Finally, there is emerging evidence that SIRT1 may also play
an important role in determining apoptotic thresholds in the
myocardium.65
Summary
Redox regulation of transcription is an evolutionary conserved strategy in which alterations in intracellular ROS
are converted into discrete and reproducible alterations in
gene expression (Figure 4). The most well-established
examples have come from analyzing the cellular response
to exogenous oxidative challenge following the addition of
relatively large amounts of superoxide, hydrogen peroxide,
or various electrophiles. The cellular response to these
challenges involves the robust induction of numerous gene
products that primarily function both to inactivate the
threat and to restore homeostasis. Although these examples
are instructive, they presumably represent the extremes of
biological responses. The moderate but sustained rise in
ROS levels seen in diseases such as atherosclerosis or
heart failure presumably results in smaller but equally
regulated alterations in transcription. Nonetheless, it is our
belief that the lessons learned from the examples discussed
will aid in the understanding of these more subtle disease-
Figure 4. ROS levels can regulate transcriptional activity. An
increase in oxidants can trigger alterations in transcription
through a number of distinct mechanisms. Among the possibilities are direct oxidation and reduction of transcription factors
that occurs with OxyR or in the AP-1/Ref-1 system. A rise in
ROS can also result in a change in subcellular localization as
discussed for both Nrf2/Keap1 and Yap1. Finally, an alteration
in ROS levels can be reflected in a change of intracellular redox
buffers that, in turn, modulate the activity of chromatinmodifying enzymes such as SIRT1 or alter the binding of NADHdependent transcription factors such as BMAL.
associated changes. In particular, the use of cysteine-based
sensing as well as the transcriptional modifying effects of
redox buffers such as NAD/NADH represent 2 regulatory
mechanisms that may help to understand how ROS can
initiate, modify, or sustain a wide range of cardiovascular
diseases. Significant progress has been made in the last
decade regarding the physiological role of ROS in signaling and transcriptional regulation. Although oxidants have
been implicated in a wide array of diseases, the discovery
that ROS can covalently modify specific residues within
specific target proteins, be they cytosolic enzymes or
nuclear transcription factors, challenges the preconceived
notion that free radicals contribute to diseases solely as
random damaging agents. These observations may have
important implications for a number of cardiovascular
diseases in which oxidative stress is believed to play a
significant role.
Acknowledgments
This work was supported by intramural NIH funds. We are grateful
to members of our laboratory and the National Heart, Lung, and
Blood Institute Cardiovascular Branch for helpful advice.
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Redox-Dependent Transcriptional Regulation
Hongjun Liu, Renata Colavitti, Ilsa I. Rovira and Toren Finkel
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Circ Res. 2005;97:967-974
doi: 10.1161/01.RES.0000188210.72062.10
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