The Effects of G2-Phase Enrichment and Checkpoint Abrogation on Low-Dose Hyper-Radiosensitivity

Abstract

Purpose An association between low-dose hyper-radiosensitivity (HRS) and the “early” G2/M checkpoint has been established. An improved molecular understanding of the temporal dynamics of this relationship is needed before clinical translation can be considered. This study was conducted to characterize the dose response of the early G2/M checkpoint and then determine whether low-dose radiation sensitivity could be increased by synchronization or chemical inhibition of the cell cycle. Methods and Materials Two related cell lines with disparate HRS status were used (MR4 and 3.7 cells). A double–thymidine block technique was developed to enrich the G2-phase population. Clonogenic cell survival, radiation-induced G2-phase cell cycle arrest, and deoxyribonucleic acid double-strand break repair were measured in the presence and absence of inhibitors to G2-phase checkpoint proteins. Results For MR4 cells, the dose required to overcome the HRS response (approximately 0.2 Gy) corresponded with that needed for the activation of the early G2/M checkpoint. As hypothesized, enriching the number of G2-phase cells in the population resulted in an enhanced HRS response, because a greater proportion of radiation-damaged cells evaded the early G2/M checkpoint and entered mitosis with unrepaired deoxyribonucleic acid double-strand breaks. Likewise, abrogation of the checkpoint by inhibition of Chk1 and Chk2 also increased low-dose radiosensitivity. These effects were not evident in 3.7 cells. Conclusions The data confirm that HRS is linked to the early G2/M checkpoint through the damage response of G2-phase cells. Low-dose radiosensitivity could be increased by manipulating the transition of radiation-damaged G2-phase cells into mitosis. This provides a rationale for combining low-dose radiation therapy with chemical synchronization techniques to improve increased radiosensitivity.

Radiobiological Response of Cervical Cancer Cell Line in Low Dose Region: Evidence of Low Dose Hypersensitivity (HRS) and Induced Radioresistance (IRR)

“…The DNA is the principal cellular target of ionizing radiation and the molecular mechanisms involved in the effect of radiation on the cell is complex. Series of molecular events take place after irradiation to the cell [13][14][15]. As a response of radiation induced damage, Ionizing Radiation Induced Factor (IRIF), a histone (H2AX) based protein complex is formed at the damaged site of the DNA.…”

Low-dose radiation hyper-radiosensitivity in multicellular tumour spheroids

“…It is important to note that LDHRS has not been observed for high-linearenergy-transfer radiations (such as neutrons) [2,16], and this indicates that reparation plays an important role. It has also been suggested that the repair induction is a consequence of the fact that the cell cycle is stopped in the G2 phase, because this permits the cell to repair the lesions produced before it divides, and this only occurs for a given threshold dose [18][19][20]. According to several authors [21][22][23], this links LDHRS with the so-called dose rate inverse effect [24,25].…”

[32P]ATP inhibits the growth of xenografted tumors in nude mice

“…This inhibition of xenografted tumor may result from direct cytotoxic ability of the radioisotope, significant injury to B-cell lymphocytes and natural killer cells or, possibly, the welldocumented phenomenon of low-dose radiation hypersensitivity. 10 Hematoxylin and eosin staining of sections of the xenografted tumors showed no immune cells present in tumors from mice that received the highest doses of [ 32 P]ATP, while the small, disrupted tumors examined from mice receiving optimal [ 32 P]ATP doses did contain infiltrating immune cells (data not shown).…”