Abstract
Fractionated radiation (FR) is used in radiotherapy to allow the recovery of normal tissues from sublethal damage. To elucidate the biological effect of FR exposure, we studied the DNA damage response of FR in human tumor cell lines, HepG2 and HeLa. We found that cells with long-term FR exposure to 0.5 Gy X-rays twice per day for more than 31 days (31FR cells) overexpressed cyclinD1. Cyclin D1 overexpression was also observed in 31FR-31NR cells, 31FR cells cultured without FR for further 31 days. Interestingly, 31FR-31NR cells exhibited gamma-H2AX foci (phosphorylated H2AX), the marker of DNA double strand brakes (DSBs), without FR. Thus, these cells induced DSBs for a long time after FR exposure. Gamma-H2AX positive cells were also positive for PCNA, the marker of S-phase of the cell cycle, indicating that cells with DSBs were in S phase.
In this study we analyzed the correlation between cyclin D1 overexpression and DSBs formation. Gamma-H2AX was decreased in 31FR cells by down-regulation of cyclinD1 expression using siRNA targeting cyclinD1. This study is first to demonstrate that cyclin D1 overexpression is associated with induction of DSBs.
The level of cyclinD1 is tightly controlled for the normal progression of the cell cycle and its deregulation is linked to the development of various types of cancer. CyclinD1 overexpression caused by long-term FR exposure may lead to genomic instability due to the induction of DSBs.
