Transition in Survival From Low-Dose Hyper-Radiosensitivity to Increased Radioresistance Is Independent of Activation of ATM SER1981 Activity

• Published in: International journal of radiation oncology, biology, physics Vol. 69; no. 4; pp. 1262 - 1271
• Main Authors: Krueger, Sarah A., Ph.D, Collis, Spencer J., Ph.D, Joiner, Michael C., Ph.D, Wilson, George D., Ph.D, Marples, Brian, Ph.D
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15.11.2007
• Subjects: Animals; Ataxia telangiectasia mutated; Ataxia Telangiectasia Mutated Proteins; ATM; CAFFEINE; CELL CYCLE; Cell Cycle Proteins - antagonists & inhibitors; Cell Cycle Proteins - physiology; Cell Line; Cell Survival - physiology; Cell Survival - radiation effects; Cricetinae; Cricetulus; DNA-Binding Proteins - antagonists & inhibitors; DNA-Binding Proteins - physiology; G 2-checkpoint arrest; G2 Phase - physiology; G2 Phase - radiation effects; Hematology, Oncology and Palliative Medicine; Humans; INHIBITION; Ionizing radiation; IONIZING RADIATIONS; IRRADIATION; Low dose; MITOSIS; PHOSPHORYLATION; Protein-Serine-Threonine Kinases - antagonists & inhibitors; Protein-Serine-Threonine Kinases - physiology; Radiation Dosage; RADIATION DOSES; Radiation Tolerance - physiology; Radiology; RADIOLOGY AND NUCLEAR MEDICINE; RADIOSENSITIVITY; RADIOTHERAPY; Rats; RNA, Small Interfering; Tumor Suppressor Proteins - antagonists & inhibitors; Tumor Suppressor Proteins - physiology; G 2-checkpoint arrest; Ionizing radiation; ATM; Low dose; Cell cycle; Ataxia telangiectasia mutated
• ISSN: 0360-3016; 1879-355X
• DOI: 10.1016/j.ijrobp.2007.08.012

Purpose The molecular basis of low-dose hyper-radiosensitivity (HRS) is only partially understood. The aim of this study was to define the roles of ataxia telangiectasia mutated (ATM) activity and the downstream ATM-dependent G2 -phase cell cycle checkpoint in overcoming HRS and triggering radiation resistance. Methods and Materials Survival was measured using a high-resolution clonogenic assay. ATM Ser1981 activation was measured by Western blotting. The role of ATM was determined in survival experiments after molecular (siRNA) and chemical (0.4 mM caffeine) inhibition and chemical (20 μg/mL chloroquine, 15 μM genistein) activation 4–6 h before irradiation. Checkpoint responsiveness was assessed in eight cell lines of differing HRS status using flow cytometry to quantify the progression of irradiated (0–2 Gy) G2 -phase cells entering mitosis, using histone H3 phosphorylation analysis. Results The dose–response pattern of ATM activation was concordant with the transition from HRS to radioresistance. However, ATM activation did not play a primary role in initiating increased radioresistance. Rather, a relationship was discovered between the function of the downstream ATM-dependent early G2 -phase checkpoint and the prevalence and overcoming of HRS. Four cell lines that exhibited HRS failed to show low-dose (<0.3-Gy) checkpoint function. In contrast, four HRS-negative cell lines exhibited immediate cell cycle arrest for the entire 0–2-Gy dose range. Conclusion Overcoming HRS is reliant on the function of the early G2 -phase checkpoint. These data suggest that clinical exploitation of HRS could be achieved by combining radiotherapy with chemotherapeutic agents that modulate this cell cycle checkpoint.