Purpose: NSC109268 has been described previously as inhibitor of proteasomal degradation and of phosphatase 2Calpha. In a yeast screen, we isolated NSC109268 as an agent altering sensitivity to DNA-damaging agents. We found that NSC109268 and the related compound NSC109272 enhance cellular sensitivity to cis- and transplatin but reduce sensitivity to nitrogen mustard. We explored if similar effects could be found in human cancer cells and if cell cycle analysis could hint at the underlying molecular mechanism.

Methods: Haploid yeast cells were treated in suspension with platinum agents and nitrogen mustard alone or in combination with NSC compounds, and survival was measured by colony-formation assays. Sensitivity of ovarian and prostate cancer cells toward these treatments was evaluated using the MTS assay. Cell cycle progression was determined by flow cytometry.

Results: The enhancement of cisplatin sensitivity by NSC109268 found in yeast was confirmed in cisplatin-sensitive and cisplatin-resistant human ovarian cancer lines and in prostate cancer cells. In yeast and in human carcinoma cells, a correlation of enhanced sensitivity with delaying S-phase progression was revealed.

Conclusion: The known activities of NSC109268 as proteasome or phosphatase inhibitor could explain the phenotype of S-phase delay by assuming a higher initial DNA damage load, inhibition of DNA translesion synthesis or extended checkpoint arrest.

Author: Siede W

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