WIP1 dephosphorylation of p27Kip1 Serine 140 destabilizes p27Kip1 and reverses anti-proliferative effects of ATM phosphorylation

The phosphoinositide-3-kinase like kinases (PIKK) such as ATM and ATR play a key role in initiating the cellular DNA damage response (DDR). One key ATM target is the cyclin-dependent kinase inhibitor p27(Kip1) that promotes G1 arrest. ATM activates p27(Kip1)-induced arrest in part through phosphorylation of p27(Kip1) at Serine 140. Here we show that this site is dephosphorylated by the type 2C serine/threonine phosphatase, WIP1 (Wildtype p53-Induced Phosphatase-1), encoded by the PPM1D gene. WIP1 has been shown to dephosphorylate numerous ATM target sites in DDR proteins, and its overexpression and/or mutation has often been associated with oncogenesis. We demonstrate that wildtype, but not phosphatase-dead WIP1, efficiently dephosphorylates p27(Kip1) Ser140 both in vitro and in cells and that this dephosphorylation is sensitive to the WIP1-specific inhibitor GSK 2830371. Increased expression of wildtype WIP1 reduces stability of p27(Kip1) while increased expression of similar amounts of phosphatase-dead WIP1 has no effect on p27(Kip1) protein stability. Overexpression of wildtype p27(Kip1) reduces cell proliferation and colony forming capability relative to the S140A (constitutively non-phosphorylated) form of p27. Thus, WIP1 plays a significant role in homeostatic modulation of p27(Kip1) activity following activation by ATM.