Ionizing radiation has many medical, industrial and military uses. Although ionizing radiation can be used in the therapy of diseases such as cancer, exposure to biologically significant levels of such radiation can also cause genotoxic stress. Similarly, many industrial processes (such as the production of nuclear power) and military uses (such as nuclear weapons) can expose individuals to hazardous levels of ionizing radiation. Such radiation can elicit a variety of cellular responses, ranging from cell-cycle arrest to mutation, malignant transformation, or cell death. Many of the responses (such as genotoxicity) are often subtle, and exposed persons may be unaware or unsure if they have been exposed. Moreover, it may require years to evince an untoward effect (such as the development of a malignancy) caused by the exposure.
Many of the assumptions about low dose effects have been based on extrapolations from effects measured at high doses. Transcriptional responses to doses of ionizing radiation with relatively little effect on cell survival have not been as well investigated, although small variations in expression levels of several isolated genes have been detected. A dose of 50 cGy reportedly reduced expression of β- and γ-actin (Woloschakand Chang-Liu, Int. J. Radiat. Biol. 59:1173-83, 1991) and induced RB-1 and H4 histone (Woloschak and Chang-Liu, Cancer Lett. 97:169-75, 1995) in Syrian Hamster Embryo cells, while a decrease in c-myc and increase in c-jun was detected in these cells following a dose as low as 6 cGy (Woloschak and Chang-Liu, Cancer Lett. 97:169-75, 1995). In a transformed human lymphoblast cell line, activation of NF-κB has been reported with as little as 10 cGy of radiation (Prasad et al., Radiat. Res. 138:367-72, 1994), along with induction of c-FOS, c-JUN, c-MYC and c-Ha-RAS in the 25-200 cGy range (Prasad et al., Radiat. Res. 143:263-72, 1995). The induction by 25 cGy of PBP74, a member of the heat shock 70 gene family, has also been reported in two human cancer cell lines (Sadekova et al., Int. J. Radiat. Biol. 72:653-60, 1997).