doi: 10.1161/hh1201.093162
© 2001 American Heart Association, Inc.
Editorial |
Ref-1 and Transcriptional Control of Endothelial Apoptosis
From the Division of Biomedical Sciences, University of California, Riverside, Calif.
Correspondence to Michael B. Stemerman, MD, Division of Biomedical Sciences, University of California Riverside, Riverside, CA 92521. E-mail michael.stemerman{at}ucr.edu
Key Words: endotheliumapoptosis • transcription factor • oxidative stress
Vascular
endothelium, when unperturbed, provides a surface to
the blood vessel, which is passive to the development of thrombosis,
and potentially adherent blood cells. This characteristic is the
quintessence of vascular
homeostasis.1 However,
endothelial cells (ECs) can undergo apoptosis
in vitro in response to a variety of
pathophysiological conditions including
hypoxia, proinflammatory cytokines, bacterial
endotoxins, and atherogenic risk factors such as homocysteine and
lipoproteins (reviewed in
Stefanec2 and Dimmeler and
Zeiher3 ). These cellular
perturbations have in common the generation of intracellular reactive
oxygen intermediates, referred to as oxidative stress. ECs respond to
these adverse conditions by altering their intracellular
reduction/oxidization (redox) state and making their ultimate decision
between adaptation (survival) and apoptosis (see
Figure
).
Understanding the precise mechanisms controlling such a process is an
important component to our knowledge of cardiovascular
diseases. In this issue of Circulation
Research, Hall et
al4 provide novel evidence
for a critical role of Ref-1, a redox-sensitive regulator, in affecting
EC apoptosis.
|
Ref-1 was cloned as Redox factor, also known as apurinic
(apyrimidinic) endonuclease
(APE).5 As a ubiquitously
expressed multifunctional 36-kDa protein, Ref-1 is involved in the
repair of DNA damage as well as in the transcriptional regulation of
genes. Its 5'AP-endonuclease functions in base excision repair, and its
3'-diesterase activity removes phosphoglycolate residues from DNA
damaged by genotoxic stresses. In addition, Ref-1 is also important for
the activation of transcription factors, such as activator
protein-1
(AP-1),6 7 nuclear
factor-
B (NF-B),8
p53,9 10 and
hypoxia-inducible factor-1
(HIF-1).11 Activation of
transcription factors, which occurs via a redox-based mechanism,
pertains to its 6-kDa N-terminal domain. Following its discovery,
Xanthoudakis and
Curran12 identified Ref-1 as
a reductive activator of c-Fos and c-Jun (two major
components of AP-1) via a reduction of the conserved cysteine residues
in their DNA binding domains. Interestingly, Ref-1 also acts as a
transcriptional repressor of its own gene and other genes such as that
coding for the parathyroid hormone. Although it has been observed that
a decrease in Ref-1 protein level precedes apoptotic changes in
rodent models for ischemic or traumatic brain
injury,13 a role of Ref-1 in
EC apoptosis has not been investigated previously. In the
present study, hypoxia resulted in decrease in Ref-1
protein expression in both human umbilical vein ECs and bovine
pulmonary artery ECs. Moreover, overexpression of Ref-1 rescued
both hypoxia-and tumor necrosis factor (TNF-)–induced
apoptosis. This demonstrates that the decline in Ref-1 is a
cause of, but not a response to, hypoxia-induced
apoptosis.4 Further,
Ref-1 appears to be an antiapoptotic factor in ECs. This agrees
with a recent report showing a protective effect for Ref-1 in
dopamine-induced neuron
apoptosis.14
Numerous agents are categorized as having pro- or anti-EC
apoptotic properties. What remains as inconclusive and
perhaps controversial are the roles of specific transcription factors
controlling EC response to these various perturbations. Transcription
factor NF-B has, for years, been recognized as a central mediator of
gene expression induced by proinflammatory cytokines and
pathogens. It is thought to play a pivotal role in
cardiovascular diseases including
atherosclerosis (see review by Collins and
Cybulsky15 ). Activation of
NF-B has been linked to apoptosis, with the factor playing
either an antiapoptotic or proapoptotic role, depending
on the cell type. Activation of NF-B is essential to protect
TNF-–induced
apoptosis,16 which
appears to be a common mechanism in many cell types. Although how
NF-B protects against apoptosis is far from established, it
is believed that a major mechanism by which the transcription factor
inhibits cell death is to induce the expression of
antiapoptotic genes whose products, in turn, provide
protection to the cells under adverse conditions. A number of such
protective genes that are induced by NF-B have been identified,
including inhibitors of apoptosis (IAPs),
TNF-receptor–associated factor-1 and –2 (TRAF-1 and TRAF-2),
Bcl-2–like factors and A20, a zinc-finger protein that was originally
identified as a TNF-inducible gene in
ECs.17 Although NF-B
protects ECs from TNF-–induced apoptosis, this survival
pathway seems to provide little protection against some other
apoptotic stimuli such as lipopolysaccharide (LPS),
interleukin-1ß,18 and
hypoxia, despite the fact that NF-B is also
activated in these
scenarios.18 19
In addition, certain endothelial survival factors such
as Bcl-2,
Bcl-XL,20 and
A20, which although suppressing NF-B, can override cellular
apoptotic signaling and make NF-B dispensable in EC
protection.21 Because
NF-B is a key transcription factor governing a variety of
proinflammatory genes including chemokines and adhesion molecules, an
NF-B–independent antiapoptotic pathway can protect
endothelial integrity without converting the
endothelium to a proinflammatory state. Such a
mechanism is desirable for therapeutic intervention for many clinical
conditions such as reperfusion injury and
xenotransplantation.17 In
the present study, Ref-1 rescues ECs from apoptosis via
both an NF-B–dependent and –independent mechanism, depending on
whether it is triggered by TNF- or by
hypoxia.4 This result
reinforces the concept that transcriptional mechanisms regulating EC
responses are context-specific.
AP-1 is also a transcription factor regulated by Ref-1 during redox change. AP-1 complexes are composed of various dimers between Jun (c-Jun, JunB, and JunD) and Fos (c-Fos, Fra-1, and Fra-2).22 Mounting evidence has implied that activation of AP-1 is also associated with an apoptotic response in ECs. Many EC-perturbing agents, such as inflammatory cytokines, LPS, reactive oxygen, and oxidized LDL (oxLDL) induce EC apoptosis as well as c-jun expression.23 Sustained activation of c-Jun N-terminal kinase (JNK), an immediate c-Jun–activating molecule, induces EC apoptosis.24 More direct evidence pointing to a proapoptotic role for c-Jun originates from the observation that overexpression of c-Jun in ECs triggers marked apoptosis. However, an N-terminal truncate of c-Jun, missing the transactivation domain, not only loses its proapoptotic property but also provides protection against hydrogen peroxide–induced EC death.23 A recent report demonstrated the involvement of JNK–c-Jun pathway in oxLDL-induced apoptosis in human coronary artery ECs.25
What appears to be more paradoxical is that Ref-1 is also
known as a potent activator for the tumor suppressor p53,
which, when activated in cells, can induce either cell cycle
arrest or apoptosis. The p53 is activated in response
to genotoxic stresses and is associated with hypoxia-induced EC
death.26 Gaiddon et
al9 recently showed that
Ref-1 enhances the proapoptotic functions of p53 in a
transformed cell line. It thus seems to be in apparent contradiction
that in the present study ECs were prevented from undergoing
apoptosis by overexpression of Ref-1, which, on the other hand,
may activate p53. However, it should be pointed out that, in
the study of Gaiddon et al, Ref-1 increased the ability of p53 to
induce apoptosis only when exogenous p53 and Ref-1 were both
overexpressed by cotransfection. Thus, it is unclear whether Ref-1
activation of endogenous p53 is to induce apoptosis
or, alternatively, to arrest cell cycle. In fact, laminar shear stress,
known to promote EC survival, can cause sustained activation of p53 and
endothelial growth
arrest.27 Given that Ref-1
possesses dual functions as transcriptional regulator and DNA repair
enzyme, it is rational to speculate that these two domains of Ref-1,
although they can function independently, may act in concert to protect
cells from oxidative damage: one activates p53 to ensure
efficient cell-cycle arrest for the other to fix the DNA damage.
Nevertheless, precise interactions between Ref-1 and certain
transcription factors as well as their functional readouts under
specific endothelial conditions would be of
considerable importance in understanding the transcriptional regulation
of EC apoptosis. The need to understand this effect of Ref-1 is
underscored by the increasing number of transcription factors that have
been found to interact with Ref-1. On this expanding list are HIF-1,
HIF-like factor (HLF), activating transcription factor (ATF), cAMP
response element–binding protein (CREB), the oncogene Myb, nuclear
factor-Y (NF-Y), and early growth response-1 gene (Egr-1), Pax-5, and
Pax-8.28 Although the
consequences of activating these transcription factors remain poorly
understood, it can be hypothesized that Ref-1 may play a pivotal role
in integrating the transcriptional response and, thus, control EC fate
under specific oxidative conditions. However, an important caveat must
be considered regarding the pathogenetic importance of EC
apoptosis. Most reports studying EC programmed cell death have
examined the process in cell culture. Although a few studies have shown
in situ detection of EC apoptosis in
microvessels29 and
transplant coronary artery
disease,30 it is yet
uncertain as to its importance in major circulatory disorders such as
atherosclerosis. Until such in vivo studies are carried
out, the role for EC apoptosis in vascular diseases remains
speculative.
Finally, the finding that Ref-1 increases EC survival under
conditions of hypoxia and TNF- stimulation has potential
clinical relevance to vascular diseases. For example, upregulation of
Ref-1 by either gene transfer or pharmacological agonists can be
expected to promote angiogenesis that is therapeutically desirable for
ischemic diseases and wound healing. On the other hand,
antagonizing Ref-1 may exert an angiostatic effect and, in turn,
inhibit tumor growth.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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