Modulating Radiation Resistance by Inhibiting Ribonucleotide Reductase in Cancers with Virally or Mutationally Silenced p53 Protein

Abstract

Therapeutic ionizing radiation damages DNA, increasing p53-regulated ribonucleotide reductase (RNR) activity required for de novo synthesis of the deoxyribonucleotide triphosphates used during DNA repair. This study investigated the pharmacological inhibition of RNR in cells of virally or mutationally silenced p53 cancer cell lines using 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine ® NSC #663249), a chemotherapeutic radiosensitizer that equally inhibits RNR M2 and p53R2 small subunits. The effects of 3-AP on RNR inhibition and resulting radiosensitization were evaluated in cervical (CaSki, HeLa and C33-a) and colon (RKO, RKO-E6) cancer cells. 3-AP treatment significantly enhanced radiation-related cytotoxicity in cervical and colon cancer cells. 3-AP treatment significantly decreased RNR activity, caused prolonged radiationinduced DNA damage, and resulted in an extended G 1 /S-phase cell cycle arrest in all cell lines. Similar effects were observed in both RKO and RKO-E6 cells, suggesting a p53-independent mechanism of radiosensitization. We conclude that inhibition of ribonucleotide reductase by 3-AP enhances radiation-mediated cytotoxicity independent of p53 regulation by impairing repair processes that rely on deoxyribonucleotide production, thereby substantially increasing the radiation sensitivity of human cancers.

Chemoradiotherapy for Cervical Cancer in 2010

“…A phase 1 study including ten patients treated with 3-AP, weekly cisplatin, and radiation therapy for cervical cancer reported minimal toxicity with electrolyte abnormalities and no recurrences at a median follow-up of 18 months [37].…”

A Phase I-II Evaluation of Veliparib (NSC #737664), Topotecan, and Filgrastim or Pegfilgrastim in the Treatment of Persistent or Recurrent Carcinoma of the Uterine Cervix

“…19Y21 The ribonucleotide reductase M2b subunit persists throughout the cell cycle, 30 is regulated in the cytosol by a protein-protein bond with p53, 31 and once freed from p53 by p53 phosphorylation may localize to the nucleus for immediate supply of deoxyribonucleotides when nuclear DNA is damaged. 20,21 To evaluate whether ribonucleotide reductase M2b expression correlated with treatment outcome, immunohistochemical analyses were conducted (Fig. 1).…”

Ribonucleotide Reductase Expression in Cervical Cancer

“…Viral or mutational silencing of p53 in cervical cancers eliminates the G1/S cell cycle checkpoint, resulting in a compensatory rise in RNR M2. 1 For the purposes of this immunohistochemical analysis and based on these preclinical in vitro cervical cancer cell observations, RNR M2 3+ immunoreactivity was arbitrarily deemed as the clinically meaningful level of overexpression. A high 3+ level of RNR M2 expression seen in the cytosol occurred in most (41/51 [80%]) cervical cancers (Fig.…”