Chemistry Reference
In-Depth Information
7.7 Interaction of Cisplatin and Related Platinum Complexes with
Telomeric DNA Duplexes
Due to the presence of triple runs of guanines, a telomeric DNA duplex is a poten-
tial target for cisplatin. Firstly, it has been shown that the platinum content of a
plasmid containing a 800bp (TTAGGG)
n
sequence is higher than in the remaining
plasmid sequence, in proportion to the greater number of guanines.
136
Secondly, it
has been shown that longer telomeres are associated with resistance to cisplatin in
melanoma cells.
137
It is thus conceivable that the telomeric DNA-cisplatin interac-
tion could be relevant for drug sensitivity/resistance status depending on the lengths
of the telomere. However, it is worth noticing that this hypothesis has been invali-
dated in other cell lines.
138
Thirdly, modifi ed guanines are critical for association of
telomere binding proteins since it has been shown that the presence of single or
multiple 8-oxo-G lesions on telomeric sequences interfere with the
in vitro
binding
of TRF1 and TRF2.
139
All these results enable us to hypothesize that the modifi ca-
tion of DNA by cisplatin could occur at telomeres and thus disturb the recognition
of telomeric DNA by TRF1 and TRF2. Based on this hypothesis, we analysed the
in vitro
binding of TRF1 and TRF2 on the telomeric sequences (T
2
AG
3
)
4
/(C
3
TA
2
)
4
platinated at a defi ned GGG site by cisplatin. We showed that the presence of cis-
platin decreases the affi nity of TRF2 more dramatically than that of TRF1 for these
sequences.
140
These results suggest that if there is any platination of telomeres by
cisplatin in cancerous cells, it could have some impact on the binding of TRF2 and
consequently could affect the integrity of the telomeres.
7.8 Interaction of Cisplatin and Related Platinum Complexes
with Telomerase
The RNA component of telomerase, hTR, is necessary for the reverse transcriptase
activity of the enzyme. The template and the conserved structured regions (loops
and pseudo-knot) of hTR are accessible binding sites,
141
consequently hTR is a
potential target for antitelomerase drugs. Antisense oligonucleotides, peptide nucleic
acids (PNAs) directed against the template
142
and small molecules directed against
the tertiary structures
143,144
have been shown to inhibit effi ciently telomerase activity.
The most promising analogue is GRN163L, a lipid conjugate to thiophosphorami-
date
145
that is used in breast cancer therapy.
146
Since electrophilic agents could
interact with the accessible nucleobases of hTR, the effect of cisplatin and some
Pt(II) derivatives has been evaluated
in vitro
on telomerase activity using the TRAP
assay.
147
Particularly, novel platinum complexes bearing aromatic amines as bulky
carrier groups have been prepared for this purpose.
148,149
Inhibition of telomerase,
dependent on Pt(II) complex interactions, has been observed, indicating platination
of hTR; however, we can not exclude that the effect may be due to platination of
essential nucleophilic amino acids of the proteic unit, hTERT. It is know that TRAP
assay occurs with cells extracts during 30 minutes of incubation, followed by PCR
amplifi cation of the telomeric repeats.
150
However, this incubation timescale is
