Biology Reference
In-Depth Information
activity.
249
However there are conflicting reports
regarding this compound's efficacy for inhibition of
APE1,
195
and the computer docking of the compound
on APE1 did not coincide with the portion of the mole-
cule in which the DNA repair activity resides. Studies
performed in melanoma cells demonstrated that resver-
atrol can block the DNA binding activity of AP-1 using
an EMSA assay and can enhance the sensitivity of
dacarbazine.
249
activities is still not fully known, but the critical cysteine
residues are cysteine 65 and 93.
145,147,165,206
The most
selective and well-characterized APE1 redox inhibitor
is the napthoquinone compound, E3330 (2E-3-[5-(2, 3
dimethoxy-6-methyl-1, 4-benzoquinolyl)]-2-nonyl-2-
propenoic acid). Previous studies demonstrated that
the quinone derivative E3330 selectively inhibited
NF
k
B-mediated gene expression without affecting any
other regulatory steps such as phosphorylation or trans-
location to the nucleus.
243
Biochemical studies using
radiolabeled E3330 and proteins renatured on
membrane blots demonstrated that
14
C-labeled E3330
selectively bound to both recombinant APE1 and puri-
fied APE1 from cell nuclear extracts.
243
e
244
While
E3330 blocks APE1's redox function, it has no effect on
APE1 endonuclease activity or BER activity of an AP
site.
208
A recent study by Su
et al.
utilized the APE1
redox inhibitor E3330 to probe into how APE1 reduces
cysteines in transcription factors.
245
They found that
E3330 recognizes an alternate, redox active conforma-
tion of APE1, and potentially inhibits its redox activity
by inducing disulfide bond formation within
APE1.
245
e
246
Kelley's group has extensively character-
ized the effect of E3330 on APE1 redox activity in cancer
cell lines and endothelial cells
208
as well as developing
more potent analogs (RN8-51).
246
In addition, soy iso-
flavones
247
e
248
and resveratrol
249
e
250
have reported
activity against the redox activity of APE1 (
Figure 3.3
).
Soy isoflavones, potential chemopreventive agent
and component of soybeans, may also function as poten-
tial modulators of signaling through APE1.
248,251
Using
prostate cancer cells as a model system, treatment with
soy isoflavones in combination with radiation resulted
in a decrease in APE1 levels and a concomitant decrease
in NF
k
B and HIF-1
a
activity.
247,248
The prostate cells did
not respond to radiation as robustly through Src/STAT3
pathways when APE1 levels were decreased. The
increase in sensitivity to radiation is a favorable
response clinically and these studies suggest it is medi-
ated through APE1. However, there are multiple compo-
nents in soy isoflavone including genistein which has
been shown to modulate several pathways and be rather
non-specific in its effects, therefore some of the effects
attributed to APE1 inhibition may be indirect and not
causal.
252
Regardless of the exact mechanism by which
soy isoflavones sensitize prostate cancer cells to radia-
tion, decreases in APE1, NF
k
B, and HIF-1
a
coincide
with dramatically increased prostate cancer cells' sensi-
tivity to radiation.
Lastly, Resveratrol (3,4',5-trihydroxystilbene), a com-
ponent of red wine and grapes, is a polyphenolic
compound with generally accepted health benefits in
humans.
253
The most accepted mechanism of action is
believed to be as an antioxidant. Resveratrol has been
reported to inhibit APE1's DNA repair and redox
PARP1 INHIBITORS
There are some inherent differences between normal
and cancer cells. As our understanding of the differences
grows, scientists and clinicians hope to take advantage
of these differences and selectively target the cancer cells
therapeutically. Cancer cells are characterized by uncon-
trolled cell division, genetic instability, ability to recruit
new blood vessel growth termed angiogenesis, ability
to invade and metastasize, and the insensitivity to
signals of programmed cell death or apoptosis.
254
A component in the resistance and evasion of apoptosis
by tumor cells is believed to be through inappropriate
signaling in response to DNA damage.
207,255
e
256
The
lack of DNA repair protein expression contributes to
the genomic instability of tumor cells, e.g. mismatch
repair proteins.
257
Conversely, the expression of DNA
repair proteins such as PARP,
O
6
-methylguanine
DNA-methyltransferase (MGMT) or APE1, can confer
resistance to therapy and offer a survival advan-
tage.
209,258
e
259
The success of PARP inhibitors in breast
cancer is believed to be due to the tumor's reduced
capacity for DNA repair through loss or mutation of
the BRCA proteins. This has been referred to as
“synthetic lethality,” which describes the observation
that a deficit in one critical gene is not sufficient to kill
the tumor cell but when a specific second hit is acquired,
this event is lethal to the cancer cell. Inhibition of PARP
in these cancer cells removes another pathway for
dealing with DNA strand breaks and renders the tumor
cells sensitive (
Figure 3.5
). Yet, the normal cells with
either no mutation or loss of BRCA have sufficient
DNA repair capacity and are not overly sensitive to
PARP inhibition.
260
Many of the PARP inhibitors that
are being tested preclinically and clinically were devel-
oped using a pharmacophore and structure-activity rela-
tionship (SAR) approach.
261
e
263
The inhibitors can bind
to PARP in a similar orientation as NAD
þ
and thereby
block the interaction of NAD
þ
with PARP. Upon activa-
tion of PARP by DSBs, the C-terminal end of PARP
synthesizes, via the catalysis of NAD
þ
, chains of poly
(ADP-ribose)- or PAR- branched chains.
264
With
a blockade of PARP catalysis of NAD
þ
, downstream
signaling through PARP is blocked. PARP inhibitors
