Plasminogen activator inhibitor-1 is a critical downstream target of p53 in the induction of replicative senescence

genesPAI-1PI(3)KPKBGSK3cyclin D1p19ARFp21CIP1uPAp16INK4Acyclin EPCNAp53 limits the proliferation of primary diploid fibroblasts by inducing a state of growth arrest named replicative senescence — a process which protects against oncogenic transformation and requires integrity of the p53 tumour suppressor pathway1, 2, 3. However, little is known about the downstream target genes of p53 in this growth-limiting response. Here, we report that suppression of the p53 target gene encoding plasminogen activator inhibitor-1 (PAI-1) by RNA interference (RNAi) leads to escape from replicative senescence both in primary mouse embryo fibroblasts and primary human BJ fibroblasts. PAI-1 knockdown results in sustained activation of the PI(3)K–PKB–GSK3 pathway and nuclear retention of cyclin D1, consistent with a role for PAI-1 in regulating growth factor signalling. In agreement with this, we find that the PI(3)K–PKB–GSK3–cyclin D1 pathway is also causally involved in cellular senescence. Conversely, ectopic expression of PAI-1 in proliferating p53-deficient murine or human fibroblasts induces a phenotype displaying all the hallmarks of replicative senescence. Our data indicate that PAI-1 is not merely a marker of senescence, but is both necessary and sufficient for the induction of replicative senescence downstream of p53