Inhibition of the phosphoinositide 3-kinase pathway induces a senescence-like arrest mediated by p27Kip1

Abstract

A senescence-like growth arrest is induced in mouse primary embryo fibroblasts by inhibitors of phosphoinositide 3-kinase (PI3K). We observed that senescence-like growth arrest is correlated with an increase in p27Kip1 but that down-regulation of other cyclin-dependent kinase (CDK) inhibitors, including p15INK4b, p16INK4a, p19 INK4d, and p21Cip1 as well as other negative cell cycle regulators such as p53 and p19ARF, implies that this senescence-related growth arrest is independent of the activity of p53, p19ARF, p16INK4a, and p21Cip1, which are associated with replicative senescence. The p27Kip1 binds to the cyclin/CDK2 complexes and causes a decrease in CDK2 kinase activity. We demonstrated that ectopic expression of p27Kip1 can induce permanent cell cycle arrest and a senescence-like phenotype in wild-type mouse embryo fibroblasts. We also obtained results suggesting that the kinase inhibitors LY294002 and Wortmannin arrest cell growth and induce a senescence-like phenotype, at least partially, through inhibition of PI3K and protein kinase B/Akt, activation of the forkhead protein AFX, and up-regulation of p27Kip1expression. In summary, these observations taken together suggest that p27Kip1 is an important mediator of the permanent cell cycle arrest induced by PI3K inhibitors. Our data suggest that repression of CDK2 activity by p27Kip1 is required for the PI3K-induced senescence, yet mouse embryo fibroblasts derived from p27Kip1/ mice entered cell cycle arrest after treatment with LY294002. We show that this is due to a compensatory mechanism by which p130 functionally substitutes for the loss of p27Kip1. This is the first description that p130 may have a role in inhibiting CDK activity during senescence.