Biology Reference
In-Depth Information
cloned in 1991 by Demple and colleagues,
63
who also
characterized its enzymatic activity on AP site-contain-
ing DNA. One year later, Xanthoudakis and Curran
described a protein which stimulated AP-1 DNA-
binding activity through the conserved Cys residues in
Fos and Jun and named this protein Ref-1, acronym of
Redox Effector Factor-1. They also suggested that the
newly identified protein may represent a novel redox
component of the signal transduction processes that
regulated eukaryotic gene expression.
134
It turned out
that the two groups independently identified the same
protein and described its endonuclease and redox func-
tions which are exerted by two functionally independent
domains.
production can lead to alterations in cellular state, repre-
senting a signal of changes in cell functions. Based
on their ability to be reversibly oxidized, thiol groups
may act as redox sensitive switches, thereby providing a
common trigger for a variety of ROS-mediated signaling
events.
139
The N-terminal redox portion of APE1 is necessary
for its redox-coactivator activity on oxidized transcrip-
tional factors such as p53,
140
AP-1,
134
Myb,
141
NF-
k
B,
142
Egr-1,
143
and Pax proteins.
144
Structural data
indicate that this region forms an extended loop which
lies across the
b
-strands
b
13 and
b
14 making a number
of hydrogen bond and salt bridge interactions with
the globular core of the molecule. Both Cys65 and
Cys93 are considered critical for redox activity.
145
e
146
However, these residues are inaccessible to solvent and
would therefore be unable to directly interact with resi-
dues from other proteins, unless APE1 undergoes
a dramatic conformational change resulting in the expo-
sure of Cys65 and the creation of a binding site that will
accommodate the different transcriptional factors.
147
A redox-independent mode of action by APE1 on tran-
scription factors activities has also been hypothesized
for p53
148
and AP-1.
149
Recently, a novel APE1 activity, termed “redox chap-
erone activity,” was discovered, by which APE1 may
regulate the DNA binding activity of various transcrip-
tion factors by promoting reduction of their critical
Cys residues by means of a third party of reducing mole-
cules, such as GSH and thioredoxin.
150
The redox-
chaperone activity seems to be mediated by direct inter-
actions between APE1 and the target transcription
factor, and does not require as high concentrations of
APE1 as does its redox activity. It is therefore plausible
that APE1 may regulate redox-sensitive transcription
factors essentially through this redox-chaperone activity
in living cells. In conclusion, through its redox and the
redox-chaperone activities, APE1 stimulates the tran-
scriptional activity of several transcriptional factors
involved in cell cycle control, apoptosis, cellular growth
and differentiation and may therefore contribute to
preserve the cell from the genotoxic insults due to
increased ROS concentration.
151
APE1 Gene and Protein
The gene encoding APE1 protein maps to chromo-
some 14 bands q11.2
12 in the human genome.
135
It
consists of four introns and five exons, spanning 2.64
kilobases, the first of which is non-coding. The 5' flanking
region (-890 bp) is located in a CpG island and lacks
typical TATA and CAAT boxes, but contains TATA-
and CAAT-like sequences and putative transcriptional
factor recognition sites for Sp1, AP-1, ATF, CREB, and
USF.
136
Mature transcripts codify for a 37 kDa globular
protein composed by 318 amino acid residues, acts as
a monomer, and is organized into three functional inde-
pendent subdomains. The first 33
e
35 N-terminal resi-
dues contain the nuclear localization signal and
constitute an unstructured domain essentially involved
in protein
e
protein interactions. This portion of the
protein is also involved in the RNA-binding activity of
APE1
76
and the modulation of its catalytic activity on
AP site-containing DNA.
137
e
The portion comprising
amino acids 35
127 constitutes the redox portion of
APE1, while the DNA repair section spans the C-
terminal part from about residue 161 onwards. APE1 is
recognized as a multifunctional protein because, beyond
its endonuclease activity discussed above, it also exhibits
several other functions within the cell (Figure 3.3).
e
Redox Regulation of Transcriptional
Factors Activity
As previously mentioned, normal cellular metabo-
lism as well as treatment with chemotherapeutic agents
can lead to ROS generation. Cellular redox balance is
maintained by an array of enzymatic (such as catalase,
peroxidase, superoxide dismutase) and non-enzymatic
(such as glutathione,
a
-tocopherol, L-ascorbic acid)
systems.
138
However, growing interest is focused on
the formation of sub-toxic levels of ROS and their poten-
tial biological role as signaling molecules. Sub-toxic ROS
Transcriptional Repressor Activity
APE1 transcriptional repressor activity was first
discovered on the human parathyroid hormone (PTH)
gene through its binding on negative calcium respon-
sive elements (nCaRE) present in the promoter
region.
152
Increased hematic levels of PTH lead to
Ca
2
þ
release from the storage tissues,
153
starting a nega-
tive feedback mechanism that involves the binding of
trans-acting complex to nCaRE sequences and, as final
consequence, gene transcriptional repression.
152
Izumi