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.
The phthalocyanine photosensitizer Pc 4 has been shown to bind preferentially to mitochondrial and endoplasmic reticulum membranes. Upon photoirradiation of Pc 4-loaded cells, membrane components, especially Bcl-2, are photodamaged and apoptosis, as indicated by activation of caspase-3 and cleavage of poly(ADP-ribose) polymerase, is triggered. A series of analogs of Pc 4 were synthesized, and the results demonstrate that Pcs with the aminopropylsiloxy ligand of Pc 4 or a similar one on one side of the Pc ring and a second large axial ligand on the other side of the ring have unexpected properties, including enhanced cell uptake, greater monomerization resulting in greater intracellular fluorescence and three-fold higher affinity constants for liposomes. The hydroxyl-bearing axial ligands tend to reduce aggregation of the Pc and direct it to lysosomes, resulting in four to six times more killing of cells, as defined by loss of clonogenicity, than with Pc 4. Whereas Pc 4-PDT photodamages Bcl-2 and Bcl-xL, Pc 181-PDT causes much less photodamage to Bcl-2 over the same dose–response range relative to cell killing, with earlier cleavage of Bid and slower caspase-3-dependent apoptosis. Therefore, within this series of photosensitizers, these hydroxyl-bearing axial ligands are less aggregated than is Pc 4, tend to localize to lysosomes and are more effective in overall cell killing than is Pc 4, but induce apoptosis more slowly and by a modified pathway.
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 numbers and persistence of γ-H2AX foci that were associated with reduced clonogenic survival. DNA damage was associated with a rise in p53R2 but not p21 protein levels 6 h after treatment with radiation and/or cisplatin plus 3-AP. We conclude that blockage of RNR activity by 3-AP impairs DNA damage responses that rely on deoxyribonucleotide production and thereby may substantially increase chemoradiosensitivity of human cervical cancer.
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