PML is induced by oncogenicrasand promotes premature senescence

Oncogenicrasprovokes a senescent-like arrest in human diploid fibroblasts involving the Rb and p53 tumor suppressor pathways. To further characterize this response, we compared gene expression patterns betweenras-arrested and quiescent IMR90 fibroblasts. One of the genes up-regulated duringras-induced arrest was promyelocytic leukemia (PML) protein, a potential tumor suppressor that encodes a component of nuclear structures known as promyelocytic oncogenic domains (PODs). PML levels increased during bothras-induced arrest and replicative senescence, leading to a dramatic increase in the size and number of PODs. ForcedPMLexpression was sufficient to promote premature senescence. Like oncogenicras, PMLincreased the levels of p16, hypophosphorylated Rb, phosphoserine-15 p53, and expression of p53 transcriptional targets. The fraction of Rb and p53 that colocalized with PML markedly increased duringras-induced arrest, and expression ofPMLalone forced p53 to the PODs. E1A abolishedPML-induced arrest and prevented PML induction and p53 phosphorylation in response to oncogenicras. These results imply that PML acts with Rb and p53 to promoteras-induced senescence and provide new insights into PML regulation and activity.