Received for publication April 25, 2008.
Revised June 22, 2008.
Accepted for publication July 7, 2008.

Disregulation of Purine Nucleotide Biosynthesis Pathways Modulates Cisplatin Cytotoxicity in Saccharomyces cerevisiae

Abstract

We previously found that inactivation of the FCY2 gene, encoding a purine-cytosine permease, or the HPT1 gene, encoding the hypoxanthine guanine phosphoribosyl transferase, enhances cisplatin resistance in yeast cells. Here, we report that in addition to fcy2 and hpt1 mutants in the salvage pathway of purine nucleotide biosynthesis, mutants in the de novo pathway that disable the feedback inhibition of AMP and GMP biosynthesis also enhanced cisplatin resistance. An activity-enhancing mutant of the ADE4 gene, which constitutively synthesizes AMP and excretes hypoxanthine, and a GMP kinase mutant (guk1), which accumulates GMP and feedback inhibits Hpt1 function, both enhanced resistance to cisplatin. Additionally, over-expression of the ADE4 gene in wild-type cells, which increases de novo synthesis of purine nucleotides, also resulted in elevated cisplatin resistance. Cisplatin cytotoxicity in wild-type cells was abolished by low concentration of extracellular purines (adenine, hypoxanthine, and guanine), but not cytosine. Inhibition of cytotoxicity by exogenous adenine was accompanied by a reduction of DNA-bound cisplatin in wild-type cells. As a membrane permease, Fcy2 may mediate limited cisplatin transport since cisplatin accumulation in whole cells was slightly affected in the fcy2 mutant. However, the fcy2 mutant had a greater effect on the amount of DNA-bound cisplatin which decreased to 50-60% of that in the wild-type cells. Taken together, our results indicate that disregulation of the purine nucleotide biosynthesis pathways as well as addition of exogenous purines can modulate cisplatin cytotoxicity in S. cerevisiae.

Key words: Nucleoside/Nucleotide, Regulation - physiological, Genetics, Resistance