Differential activation of p53 targets in cells treated with ultraviolet radiation that undergo both apoptosis and growth arrest.
The ability of p53 to act as a tumor suppressor is tightly correlated with its ability to function as a transcriptional activator at the G1/S-phase cell cycle checkpoint. Previous overexpression studies have indicated simultaneous induction of p53 target genes, despite opposing cellular functions of their protein products. To delineate the response of endoansactivation function to DNA damage in a normal cell, we irradiated early-passage rat embryo fibroblasts with 10 or 50 J/m2 of ultraviolet light (mostly UV-C). We investigated the induction of p53 targets and the response of the cells over 48 h. In this system, northern analysis revealed differential regulation of the p53 targets p21WAFI/CIPI, Mdm2, Ccng (also known as cyclin G) and Bax in accordance with their proposed functions in the cell. The growth suppressor p21WAFI/CIPI was activated initially (within 6 h) after exposure to 10 J/m2, but not after 50 J/m2, in a p53-dependent manner. Both Ccng and Mdm2 were activated later than p21 (12-24 h) after exposure to 10 J/m2. Expression of Bax was increased after exposure to both 10 J/m2 (24 h after UV exposure) and 50 J/m2 (6 h after UV exposure), which correlated well with the apoptosis seen in cells exposed to either dose. These fibroblasts also exhibited a temporary cell cycle arrest (< 8 h) at 10 J/m2. Thus we have investigated the physiological response of the p53 pathway in normal cells and identified a temporal order for induction of p53 targets. We demonstrate that both apoptosis and cell cycle arrest occur simultaneously when cells are treated with UV radiation, indicating that the amount of DNA damage is not the sole determinant of the cellular response.
Reinke
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Department of Molecular Genetics, University of Texas, M. D. Anderson Cancer Center, Houston 77030, USA.
Lozano
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D000818:Animals; D017209:Apoptosis / Q000528:radiation effects*; D002453:Cell Cycle / Q000528:radiation effects; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000096:biosynthesis / Q000235:genetics; D005260:Female; D009687:Nuclear Proteins; D011401:Promoter Regions, Genetic; D011518:Proto-Oncogene Proteins / Q000235:genetics; D051736:Proto-Oncogene Proteins c-mdm2; D051381:Rats; D016159:Tumor Suppressor Protein p53 / Q000528:radiation effects*; D014466:Ultraviolet Rays
C493958:Cdkn1a protein, rat; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D016159:Tumor Suppressor Protein p53; C497208:Mdm2 protein, rat; D051736:Proto-Oncogene Proteins c-mdm2