p53 Response to Senescence Figure 4. Enforced growth arrest does not induce senescence. P53ts or p53ts-Ras cells were grown at 39uC or incubated at 32uC for different times as indicated. Cells were fixed and stained with DAPI to identify the nuclei, or with antibodies against 53BP1 or phosphorylated gH2AH. A) Representative picture. B) Foci of .60 nuclei of each condition were counted and data represented as the average of the number of foci per nuclei. Bars = StDev. doi:10.1371/journal.pone.0003230.g004 process occurs later, acting on pRb to stabilize its active unpho- sphorylated form, independent of p53. Unphosphorylated pRb will bind and inactivate E2F factors blocking cell cycle progression and altering local chromatin [27]. PPP1CA activation will take part in this second process, contributing to irreversible proliferative arrest by enforcing pRb dephosphorylation. PLoS ONE | www.plosone.org 8 September 2008 | Volume 3 | Issue 9 | e3230