Biology Reference
In-Depth Information
therapeutic agents.
145
e
148
Through the use of anti-sense
oligonucleotides or siRNA to reduce the levels of APE1,
cell growth is blocked and sensitivity to DNA damaging
agents increases
148
e
150
establishing APE1 as an impor-
tant protein in cancer. Although these findings suggest
an important role for APE1 in cancer, they do not estab-
lish which of APE1's functions may be most important.
There has been considerable debate in the literature
regarding the wisdom of inhibiting essential DNA
repair enzymes.
120,121,151
e
153
However, targeting of
APE1's redox function is a novel approach in the devel-
opment of a cancer therapeutic agent with potentially
fewer concerns regarding toxicity in normal vs. cancer
cells. Both natural products and novel compounds that
directly or indirectly target the redox function of APE1
are discussed here.
Compounds reported to inhibit the redox activity of
APE1 in cells either directly or indirectly include
a number of natural products and have been recently
reviewed.
46,120,154,155
These compounds include soy iso-
flavones, a component in soybeans, that are thought to
have potential as chemopreventative agents in prostate
cancer.
156
Treatment of PC-3 prostate cells and xenograft
mice with soy isoflavones following radiation treatment
resulted in increased cell killing, reduced NF-
k
B binding
to DNA and reduced APE1 levels. The authors
concluded that the soy isoflavones reduced APE1 levels
and subsequently resulted in a reduction of the ability of
APE1 to reduce NF-
k
B resulting in cells' inability to
respond to the stress.
157
However, at this point the
data is merely correlative.
Another natural product reported to affect the redox
activity of APE1 is resveratrol, a component of red
wine and grapes.
158
In this study, resveratrol inhibited
both the endonuclease activity of APE1 and the DNA
binding activities of AP-1 in cellular extracts, presum-
ably through inhibition of APE1 redox function.
However, this has not been corroborated by others, nor
has it been shown to be effective at levels that are phys-
iologically relevant.
Further characterization of E3330 as a redox inhibitor
of APE1 includes the demonstration that E3330 blocks
the redox function of APE1 with AP-1 as the down-
stream target in vitro as well as after the treatment of
ovarian cancer cells with E3330.
46
Additionally, E3330
blocks APE1 redox activity with HIF-1
a
discussed above, then it follows that Ref-1 (APE1) plays
an important role not only in the redox modulation of
p53 but also in the regulation of DNA repair.
The activator protein-1 (AP-1) family of proteins
regulate expression of a number of DNA repair genes
involved in homologous recombination, NER global
genome repair, mismatch repair, and base excision
repair.
9
In one example of regulation of DNA repair
proteins by AP-1 composed of c-Jun and ATF2, 23
DNA repair or repair-associated genes whose promoters
are bound upon phosphorylation of ATF2 and c-Jun
were identified following cisplatin treatment. Chro-
matin immunoprecipitation (ChIP) with antibodies
against ATF2 and c-Jun followed by hybridization to
promoter arrays was used to identify relevant genes
including ERCC1, ERCC3, XPA, MSH2, MSH6, RAD50,
RAD23B, MLH1, PMS2, UNG2, and ATM.
142
Expression
of some genes including ERCC1, ERCC3, XPA, RAD23B,
and MSH2 and some genes that have been specifically
implicated in the repair of DNA-cisplatin adducts such
as RAD23B, XPA, ERCC3, XPF-ERCC1, MSH2, and
PMS2, were confirmed in additional studies. DNA
adducts induced by cisplatin are repaired primarily
through the NER pathway. Important proteins including
XPA, RAD23B, ERCC1, and ERCC3 in NER were also
observed on the promoter array. The MMR complex
members MSH2, MSH6, MLH1, and PMS2, which are
included in a large complex involved in the recognition
of DNA-cisplatin adducts, were bound strongly by acti-
vated c-Jun or ATF2 or both. AP-1 was also found to
upregulate MSH2 expression in the myeloid leukemia
U937 cell line treated with a phorbol ester (TPA).
143
In
addition, in response to TPA treatment in human
ovarian cancer cells, ERCC1 was transcriptionally upre-
gulated by AP-1.
144
These studies support the role of AP-1's involvement
in regulating a significant number of DNA repair
proteins involved primarily in the NER and MMR path-
ways (
Figure 11.8
). As the DNA-binding activity of AP-1
is redox regulated, redox control of this protein has
significant implications not only for response to oxida-
tive stress as discussed above but also in regulating
the expression of DNA repair proteins. APE1 is impli-
cated as a potential point of control for regulating the
DNA-binding activity of AP-1 and p53 and thereby
modulating the expression of DNA repair proteins.
159
and other
downstream transcription factors. This demonstrates
that the redox inhibition is not specific for the down-
stream target. Although E3330 blocked the redox func-
tion of APE1, it had no effect on APE1 repair
endonuclease activity. Inhibition of APE1 redox function
significantly attenuates retinal vascular endothelial cells
(RVEC) proliferation and capillary formation in vitro, but
does not cause cell death. Furthermore, the capillary
formation of RVECs appears much more sensitive to
APE1 REDOX INHIBITORS
AND CLINICAL IMPLICATIONS
(E3330 AND ANALOGS)
The levels of APE1 are elevated in a variety of cancers
and are typically associated with a poor prognosis due
to aggressive proliferation and increased resistance to
