POSTTRANSCRIPTIONAL REGULATION OF THE P53 TUMOR-SUPPRESSOR GENE DURING GROWTH-INDUCTION OF HUMAN PERIPHERAL-BLOOD MONONUCLEAR-CELLS

Regulation of p53 gene expression at the posttranscriptional level was investigated during growth induction of human peripheral blood mononu clear cells (PBMCs). Freshly isolated PBMCs, which are in the G(0) pha se of the cell cycle, were shown to express low levels of p53 mRNA tha t was rapidly degraded with a half life of Ih. The rapid decay of p53 mRNA in quiescent PBMCs was dependent on global protein synthesis as t reatment with cycloheximide resulted in stabilization of the p53 messa ge. PBMCs were stimulated to enter the cell cycle by treatment with a combination of the mitogenic lectin phytohemagglutinin (PHA) and phorb ol ester (TPA). Progressive stabilization of the p53 message occurred in PBMCs during growth induction. By 24 h of incubation in the presenc e of PHA and TPA, the half life of p53 mRNA was 6 h and p53 mRNA stead y state levels were increased 4.5 to 5.0-fold. p53 protein was not det ected in quiescent PBMCs, but was readily detected in PBMCs stimulated for 24 h with PHA and TPA. Stabilization of p53 mRNA was observed in PBMCs treated with either PHA or TPA, but to a lesser degree than when PKA and TPA were used as co-stimulants. These results indicate that d ifferential degradation of p53 messenger RNA occurs in quiescent vs mi togen stimulated PBMCs and suggest that post-transcriptional regulatio n importantly contributes to increased p53 mRNA steady state levels as PBMCs enter the cell cycle.