PTEN gene targeting reveals a radiation-induced size checkpoint in human cancer cells

Following DNA damage, human cells arrest primarily in the G(1) and G(2) phases of the cell cycle. Here, we show that after irradiation, human cancer cells with targeted deletion of PTEN or naturally occurring PTEN mutations can exert G(1) and G(2) arrests but are unable to arrest in size. Pharmacological inhibition of phosphoinositol-3-kinase, or mTOR in PTEN-/- cells restored the size arrest, whereas siRNA-mediated depletion of TSC2 in PTEN+/+ cells attenuated the size arrest. Radiation treatment potentiated Akt activation in PTEN-/- but not PTEN+/+ cells. Finally, abrogation of the size arrest via PTEN deletion conferred radiosensitivity both in vitro and in vivo. These results identify a new tumor suppressor gene-regulated, DNA damage-inducible arrest that occurs simultaneously with the G(1) and G(2) arrests but is genetically separable from them. We suggest that aberrant regulation of cell size during cell cycle arrest may be important in human cancer pathogenesis.