We are interested in understanding the
mechanisms by which cancer cells exhibit resistance to cisplatin, an
anticancer drug that exerts its cytotoxic effect by forming
intrastrand crosslinks on DNA. We carried out a selection for mutants
that are able to grow in the presence of cisplatin. One of the mutants
we identified was defective in the gene MAC1, which encodes a
transcription factor that regulates transcription of genes involved in
copper and iron uptake, and catalases. We deleted each of the Mac1p
target genes and observed that cells deleted for the CTR1 gene
are as resistant to cisplatin as mac1 mutant. We found that the
level of cisplatin-DNA adducts is reduced to half in ctr1
compared to WT. This decrease was independent of RAD2, a gene
involved in repairing the adducts, suggesting that the decrease in the
adduct level is not due to enhanced repair. We also found that the
amount of cisplatin in whole cells is decreased to half in
ctr1. Since Ctr1p is a copper transporter, we studied the
effect of copper in cisplatin resistance. Addition of copper results
in better survival of WT cells and less cisplatin accumulation inside
the cell, but not in ctr1. As cells were treated with copper,
the level of Ctr1p decreased. Cells overexpressing CTR1 accumulated
more cisplatin inside the cell. The correlation between Ctr1p level
and cellular cisplatin level suggests that Ctr1p plays an important
role in facilitating cisplatin uptake.
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