Ribonucleotide Reductase Inhibition Enhances Chemoradiosensitivity of Human Cervical Cancers

Abstract For repair of damaged DNA, cells increase de novo synthesis of deoxyribonucleotide triphosphates through the rate-limiting, p53-regulated ribonucleotide reductase (RNR) enzyme. In this study we investigated whether pharmacological inhibition of RNR by 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC #663249) enhanced chemoradiation sensitivity through a mechanism involving sustained DNA damage. RNR inactivation by 3-AP and resulting chemoradiosensitization were evaluated in human cervical (CaSki, C33-a) cancer cells through study of DNA damage (γ-H2AX signal) by flow cytometry, RNR subunit p53R2 and p21 protein steady-state levels by Western blot analysis and laser scanning imaging cytometry, and cell survival by colony formation assays. 3-AP treatment led to sustained radiation- and cisplatin-induced DNA damage (i.e. increased γ-H2AX signal) in both cell lines through a mechanism of inhibited RNR activity. Radiation, cisplatin and 3-AP exposure resulted in significantly elevated num...