The P53-dependent Gamma-ray Response of GADD45

Activation of the human GADD45 gene by ionizing radiation (IR) has previously been shown to be dependent on the tumor suppressor and transcription factor p53 (M. B. Kastan, et al., Cell 71: 587-597, 1992). Unlike GADD45, the response of other DNA damage-inducible genes to IR is not dependent on p53 based on the observation that induction in a panel of cell lines did not correlate with a normal p53 status; this included human GADD153, another member of the gadd (growth arrest and DNA damage inducible) group; MyD118, a gene related to GADD45; and the protooncogenes c-jun and c-fos. This p53-dependent response of GADD45 was further investigated in human cells with halogenated pyrimidines, which act as radiosensitizers when incorporated into cellular DNA. When cellular DNA contained halogenated pyrimidines such as iododeoxyuridine (IdUrd), GADD45 gamma-ray induction, as measured by increased mRNA, was enhanced. Rapid induction could be seen with doses as low as 0.5 Gy, and substitution with IdUrd resulted in an approximately 2-fold increase in induction over a wide dose range. This level of IdUrd substitution produced a similar fold increase in cellular radiosensitivity and has been shown previously (T. M. Kinsella et al., Int. J. Radiation Oncology Biol. Phys. 13: 733-739, 1987) to produce a similar fold increase in DNA strand breaks after IR. Considering that substitution with halogenated pyrimidines would be expected to have little effect on other cellular targets after IR, these experiments indicate that actual damage to DNA, primarily strand breaks, is a major signal for the activation of this p53-dependent pathway that is required for GADD45 induction and for activation of the G1 "checkpoint" cell cycle delay.