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 G(1 )/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 respon se of endoansactivation function to DNA damage in a normal cell, we ir radiated early-passage rat embryo fibroblasts with 10 or 50 J/m(2) 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, north ern analysis revealed differential regulation of the p53 targets p21(W AF1/CIP1), Mdm2, Ccng (also known as cyclin G) and Bar in accordance w ith their proposed functions in the cell. The growth suppressor p21(WA F1/CIP1) was activated initially (within 6 h) after exposure to 10 J/m (2), but not after 50 J/m(2), in a p53-dependent manner. Both Ccng and Mdm2 were activated later than p21 (12-24 h) after exposure to 10 J/m (2). Expression of Bar was increased after exposure to both 10 J/m(2) (24 h after UV exposure) and 50 J/m(2) (6 h after UV exposure), which correlated well with the apoptosis seen in cells exposed to either dos e. These fibroblasts also exhibited a temporary cell cycle arrest (<8 h) at 10 J/m(2). 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 radi ation, indicating that the amount of DNA damage is not the sole determ inant of the cellular response. (C) 1997 by Radiation Research Society .