Biology Reference
In-Depth Information
side effects such as alopecia and mucositis can occur.
While the intensity of these side effects depends heavily
upon the dose and duration of exposure to the chemo-
therapeutic agent, these conditions are not considered
limitations in therapy.
a chemopreventive strategy to suppress the generation
of disease-causing mutations.
In vitro
applications of
this strategy have been demonstrated using DSBs as
a model form of DNA damage. For example, selective
inhibitors of TdT have been evaluated as anticancer
agents against TdT-positive forms of leukemia. One
nucleoside analog is cordycepin (3'-deoxyadenosine)
that displays cytotoxicity alone and in combination
with the adenosine deaminase inhibitor, pentosta-
tin.
274,275
The cytotoxic activity of cordycepin correlates
with the ability of the triphosphate form to inhibit
single-stranded DNA synthesis by TdT
in vitro
.
276
Since
cordycepin lacks a 3'-OH moiety, it terminates primer
extension after its incorporation to presumably generate
abortive
FUTURE DIRECTIONS IN THE
DEVELOPMENT OF POLYMERASE
INHIBI
TORS AS ANTICANCER A
GENTS
It is clear that nucleoside analogs are the current drug
of choice for inhibiting the activity of DNA polymerases.
Unfortunately, since these analogs use Watson
Crick
base pairing selection rules, they lack the ability to selec-
tively inhibit a specific DNA polymerase or even class of
DNA polymerase, i.e., those involved in DNA repair
versus chromosomal DNA synthesis. Indeed, it would
be advantageous to inhibit certain repair polymerases,
especially those involved in BER as this pathway plays
a critical role in the repair of bases damaged by alkylat-
ing agents commonly used in chemotherapy. To test if
inhibiting BER would produce an apoptotic response,
Stachelek
et al.
used the bile acid, lithocholic acid, as
an inhibitor of pol
b
to disrupt BER.
272
In vitro
studies
demonstrated that lithocholic acid suppresses the poly-
merization and 5'-deoxyribose phosphate lyase activi-
ties of pol
b
. The inhibitory effects of lithocholic acid
result from the ability of the bile acid to prevent the
binding of pol
b
to DNA. Colony-forming assays
demonstrated that a synergistic cytotoxic effect of
combining lithocholic acid with temozolomide. Cells
treated with the combination of lithocholic acid and
temozolomide show an increase in
g
-H2AX immunoflu-
orescence. This suggests that the potentiation of temozo-
lomide cytotoxicity by lithocholic acid is caused by the
conversion of SSBs generated by the inhibition of pol
b
into DSBs double-strand breaks during DNA replica-
tion. The ability of lithocholic acid to inhibit BER
provides a mechanism explaining the carcinogenic
effects of this bile acid in animal studies.
273
While it is
unlikely that lithocholic acid could be employed as
a therapeutic agent, the results of this study prove that
inhibiting polymerases involved in BER provide new
ways to increase the efficacy of conventional anticancer
agents.
Alternative approaches to potentiate the effects of
DNA-damaging agents include the development of
nucleoside analogs that are selectively incorporated
opposite lesions formed by DNA damaging agents and
that would subsequently block their replication. This
approach could be used as an adjunctive therapy to
sensitize cancer cells to the effects of other chemothera-
peutic agents that damage DNA. In addition, the ability
to inhibit translesion DNA synthesis could be used as
e
intermediates
along
the
recombination
pathway to induce apoptosis.
Other preclinical studies have targeted other non-
instructional DNA lesions such as apurinic/apyrimi-
dinic sites that arise from the enzymatic processing of
various DNA lesions. Despite the non-templating nature
of this non-instructional lesion, most DNA polymerases
preferentially incorporate dATP opposite the lesion.
277
In vitro
studies have been performed demonstrating
the ability of various 5-substituted indolyl-2'-deoxyribo-
side triphosphates that mimic the core structure of dATP
to inhibit the misreplication of apurinic/apyrimidinic
sites.
278
e
280
Kinetic analyses reveal that several of these
non-natural are incorporated opposite non-instructional
DNA lesions ~1000 fold more efficiently than the
preferred natural substrate, dATP. In addition, these
analogs are selective as they are poorly incorporated
opposite undamaged nucleobases. Finally, these analogs
are refractory to both elongation and exonuclease proof-
reading activity. Thus, they act as selective chain termi-
nators of translesion DNA synthesis. It will be of
significant interest to evaluate the application of these
non-natural nucleosides as adjunctive chemothera-
peutic agents.
References
1. Garg P, Burgers PM. DNA polymerases that propagate the
eukaryotic DNA replication fork.
Crit Rev Biochem Mol Biol
2005;40:115
28.
2. Burgers PM. Polymerase dynamics at
e
the eukaryotic DNA
5.
3. Kunkel TA, Burgers PM. Dividing the workload at a eukaryotic
replication fork.
Trends Cell Biol
2008;18:521
replication fork.
J Biol Chem
2009;284:4041
e
7.
4. Harmalkar MN, Shirsat NV. Staurosporine-induced growth
inhibition of glioma cells is accompanied by altered expression of
cyclins, CDKs and CDK inhibitors.
Neurochem Res
2006;31:
685
e
92.
5. Koh J, Kubota T, Migita T, Abe S, Hashimoto M, Hosoda Y,
Kitajima M. UCN-01 (7-hydroxystaurosporine) inhibits the
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e
e
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