The effect of ionizing radiation on the expression of two DNA-damage-inducible genes, designated gadd45 and gaddl53, was examined in cultured human cells. These genes have previously been shown to be strongly and coordinately induced by UV radiation and alkylating agents in human and hamster cells. We found that the gadd45 but not the gaddl53 gene is strongly induced by X rays in human cells. The level of gadd45 mRNA increased rapidly after X rays at doses as low as 2 Gy. After 20 Gy of X rays, gadd45 induction, as measured by increased amounts of mRNA, was similar to that produced by the most effective dose of the alkylating agent methyl methanesulfonate. No induction was seen after treatment of either human or hamster cells with 12-O-tetradecanoylphorbol-13-acetate, a known activator of protein kinase C (PKC). Therefore, gadd45 represents the only known mammalian X-ray-responsive gene whose induction is not mediated by PKC. However, induction was blocked by the protein kinase inhibitor H7, indicating that induction is mediated by some other kinase(s). Sequence analysis of human and hamster cDNA clones demonstrated that this gene has been highly conserved and encodes a novel 165-amino-acid polypeptide which is 96% identical in the two species. This gene was localized to the short arm of human chromosome 1 between p12 and p34. When induction in lymphoblast lines from four normal individuals was compared with that in lines from four patients with ataxia telangiectasia, induction by X rays ofgadd45 mRNA was less in the cell lines from this cancer-prone radiosensitive disorder. Our results provide evidence for the existence of an X-ray stress response in human cells which is independent of PKC and which is abnormal in ataxia telangiectasia.There is increasing evidence that ionizing radiation can induce specific genes in mammalian and other eucaryotic cells. Such genes may have functions like those in procaryotes, in which genes encoding protective functions, such as DNA repair processes, can be induced (39). Prior treatment with a low dose of X rays induces a transient (usually small) protection, manifested as an increased survival or as a reduction in chromosomal damage, against a second, higher dose in human lymphocytes (25), Chinese hamster cells (12), and insect cells (15). These protective effects are blocked by inhibitors of protein (12) and RNA synthesis (15), indicating that such processes may be mediated by the induction of particular genes. The finding that certain proteins increase in abundance after X irradiation of human cells also provides evidence for X-ray-inducible genes (2). Recently, tumor necrosis factor a (TNF) mRNA has been reported to increase in human sarcoma cells after ionizing radiation (9). TNF has a role in a variety of cellular processes, including the acute-phase response and inflammation (1), and thus its induction probably represents a general response to cell injury rather than a specific response to X rays or other DNA-damaging agents. Transcription from the long terminal repeat o...

The effect of ionizing radiation on the expression of two DNA-damage-inducible genes, designated gadd45 and gadd153, was examined in cultured human cells. These genes have previously been shown to be strongly and coordinately induced by UV radiation and alkylating agents in human and hamster cells. We found that the gadd45 but not the gadd153 gene is strongly induced by X rays in human cells. The level of gadd45 mRNA increased rapidly after X rays at doses as low as 2 Gy. After 20 Gy of X rays, gadd45 induction, as measured by increased amounts of mRNA, was similar to that produced by the most effective dose of the alkylating agent methyl methanesulfonate. No induction was seen after treatment of either human or hamster cells with 12-O-tetradecanoylphorbol-13-acetate, a known activator of protein kinase C (PKC). Therefore, gadd45 represents the only known mammalian X-ray-responsive gene whose induction is not mediated by PKC. However, induction was blocked by the protein kinase inhibitor H7, indicating that induction is mediated by some other kinase(s). Sequence analysis of human and hamster cDNA clones demonstrated that this gene has been highly conserved and encodes a novel 165-amino-acid polypeptide which is 96% identical in the two species. This gene was localized to the short arm of human chromosome 1 between p12 and p34. When induction in lymphoblast lines from four normal individuals was compared with that in lines from four patients with ataxia telangiectasia, induction by X rays of gadd45 mRNA was less in the cell lines from this cancer-prone radiosensitive disorder. Our results provide evidence for the existence of an X-ray stress response in human cells which is independent of PKC and which is abnormal in taxia telangiectasia.