Abstract
Centrosomes are important cytoplasmic organelles which are involved in proper chromosome segregation. They are maintained through the activity of several factors, including DNA repair proteins such as BRCA1 and NBS1. It is shown here that ionizing radiation (IR) induces an excess number of centrosomes in a dose-dependent manner, and that the number of excess centrosomes significantly decreases after protracted exposures vs acute exposures; frequencies of the aberrant centrosomes, after an exposure to radiation at a dose rate of 0.5 mGy/min, were reduced to 70-80% of the extent observed after an acute exposure with a dose rate of 1 Gy/min. The excess number of centrosomes could be due to the overduplication of centrosomes, since the centrioles were also duplicated. Interestingly, when the IR-induced frequency of centrosome overduplication was plotted against radiation induced cell killing, similar almost coincident curves were seen for both types of exposures, in which the frequencies were significantly increased at low cell killing levels but reached a saturated level at higher cell killing levels. Moreover, these curves from Ku70- and DNA-PKcs-deficient cells overlapped with those of wild type cells, while cell killing was considerably enhanced by deficiencies in these proteins. These results suggest that IR-induced centrosome overduplication is caused by a common mechanism, which is involved in either WT cells or NHEJ-deficient cells, regardless of the radiation dose-rate. Conversely, the absence of either BRCA1 or NBS1 enhanced IR-induced overduplication frequencies by 2-4-fold and they were significantly increased even at higher cell killing levels.Thus, the present results indicate that protracted exposure to IR significantly alleviates the frequency of centrosome overduplication, although this type of centrosome aberration was pronounced in BRCA1- and NBS1-deficient cells even after irradiation at low dose-rates.
