| Cisplatin is a widely used chemotherapeutic drug for several tumors, such as those of the ovary, testes, cervix, head and neck, lung, pancreas and breast. A significant problem associated with cisplatin therapy is cisplatin resistance in patients administered with cisplatin. In such cisplatin resistant cancers, dose escalation broadens the side effect profile of cisplatin limiting usefulness of cisplatin. This project deals with a compound NSC109268 from the National Cancer Institute's Diversity Set of compounds and the molecular mechanism underlying its application as a cellular sensitizer to cisplatin. It was found that NSC109268, a 20S proteasome inhibitor and a PP2C and PP2A type phosphatase inhibitor, synergistically sensitized Wild Type yeast, 2008 and 2008/C13, ovarian carcinoma cisplatin sensitive and resistant cell lines respectively, to cisplatin. This sensitization effect correlated with a slower S phase traversal in both yeast and the cisplatin resistant cell line model. To probe for the target pathway involved in cisplatin sensitization, it was reasoned that if the targeted pathway of NSC109268 is already inactive, the cell will be sensitive to cisplatin and no additional sensitization by combination treatment will be achieved. A survey of a large number of yeast mutants showed this phenotype exclusively for mutants defective in damage tolerance by template switch pathway (e.g. rad5, ubc13) which includes a subset of recombination functions (e.g. rad51). Template switching mediates lesion bypass by temporarily replacing the lesion-containing DNA template with an undamaged template, the newly synthesized daughter strand of the sister duplex. Cell cycle delays following cisplatin treatment were not synergistically influenced in a rad5Delta mutant confirmed damage tolerance by template switch as the target pathway of NSC109268 in mediating cisplatin sensitization. Furthermore, preliminary studies hinted at the involvement of both the PP2A and the PP2C phosphatases type inhibitory activities of NSC109268 as critical in mediating cellular sensitization to cisplatin. NSC109268 may be useful as a pharmacological adjuvant to cisplatin for cisplatin based chemotherapy by overcoming cisplatin resistance and increasing cisplatin sensitivity. |
