| Summary: | Platinum-based anticancer drugs such as cisplatin, carboplatin and nedaplatin have
been widely used in the chemotherapy of a variety of solid tumours for several
decades. However, the development of both inherent and acquired resistance has
greatly limited the efficacy of all of these drugs. Several mechanisms were
proposed to explain the cellular resistance to these platinum drugs, including
decreased drug accumulation. Previously, it was suggested that cisplatin enters
cells via passive diffusion, followed by intracellular hydrolysis and activation
prior to targeting DNA. However, recent in vivo and in vitro studies confirmed
that transporters and carriers involved in copper homeostasis play important roles
on the transport as well as cellular resistance to the platinum drugs. CTR1, a major
plasma-membrane transporter involved in intracellular copper(I) homeostasis, was
found to facilitate the uptake of several platinum drugs although the molecular
mechanism remains unclear. The extracellular N-terminal domain of human CTR1
(hCTR1) with two methionine(Met)-rich and two histidine(His)-rich motifs has
been proved to be essential for the uptake of both copper and platinum drugs by
the transporter.
In this thesis, the extracellular domain of hCTR1 (hCTR1_N, residues 1-55) was
overexpressed and the role of the Met- and His-rich motifs on cisplatin binding
was examined by either mutagenesis or chemical modification. Cisplatin was
found to directly and rapidly bind to the Met residues of hCTR1_N by the
formation of monofunctional cisplatin-thioether adducts. The kinetics of the
binding process was found to correlate with the number of Met residues,
indicating that all Met residues are exposed to solvents and capable for cisplatin
binding. Such a non-sequence-specific binding may increase the likelihood of
capturing the anticancer drug in extracellular fluid by the N-terminus of hCTR1.
The effect of hCTR_N on the binding and activation of second-generation
platinum anticancer drugs, e.g. carboplatin and nedaplatin, were subsequently
investigated. hCTR1_N was found to significantly facilitate the activation of these
platinum drugs by the formation of ring-opened monofunctional Pt-thioether
species through Met residues. Although the activities of platinum drugs against
hCTR1_N are significantly different, their monofunctional protein-bound species
demonstrated great similarity in both structure and kinetic aspects, suggesting the
uptake of these platinum drugs by hCTR1 might follow the same mechanism. The
formation of active ring-opened species of carboplatin and nedaplatin by
chloride/bicarbonate was observed, indicating these nucleophiles may play a
critical role in the pre-activation of the drugs prior to their reaching cellular targets.
Pt-thioether species were proposed as intermediates for the platination of other
biomolecules. The monofunctional cisplatin adduct of hCTR1_N was proved to
further transfer its active platinum species to either cysteine- or guaninecontaining
biomolecules which mimic the C-ternimus of hCTR1 and DNA.
Methionine residues of hCTR1 may therefore serve as key residues for the
activation and transport of platinum anticancer drugs in the form of
monofunctional Pt-thioether species through the pole of trimeric hCTR1 and
eventually to their final target – DNA. === published_or_final_version === Chemistry === Doctoral === Doctor of Philosophy
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