3 O - Reentry into the cell cycle of live-irradiated quiescent normal human diploid fibroblasts: its relationships with p53 induction or inhibition

Abstract

In order to assess the possible role of p53 in response to DNA damage occuring in normal quiescent cells, human diploid fibroblasts in confluence for more than two weeks were exposed to ‘UVc irradiation (20 J/m²), with and without p53 antisens oligonucleotides, and time-course cell cycle analysis was performed by flow cytometry for the next 72 hrs. The accumulation of p53 in these irradiated cells at 24^th and the inhibition of this induction by p53 antisens oligonuclcotides were, verified by ELISA. After reseeding of control unirradiated fibroblasts, reentry into the cell cycle occured at the 48^th hr. In UVc-irradiated cells, absolutly no reentry into the cell cycle was observed. In quiescent irradiated fibroblasts in which p53 accumulation was inhibited by p53 antisens, a reentry into the cell cycle appeared as soon as the 24^th, i.e even before unirradiated control cells. These results show that in confluenl normal human diploid fibroblasts, the induction of p53 by a DNA damaging agent creates a complete block and inhibits reentry into the cell cycle. The inhibition of p53 induction not only overcomes this block, regardless of DNA damages, but also permits an earlier reentry into the cell cycle In conclusion, we demonstrate that p53 protein inhibits the reentry into the cell cycle of quiescent normal human fibroblasts exposed to DNA damaging agents. This suggests that p53 could induce a G0/G1block, in addition of a G1/S block which is moreover controversed. To clearly evaluate if this block is a G0/G1 or a G1/S block some more experiments are going on, including p27^kipl, p21^wafl/cipl and cyclin D expression studies