| The PTEN tumor suppressor gene is mutationally inactivated in a wide range of common human cancers, most notably glioblastomas, endometrial carcinomas, melanomas, and advanced prostate adenocarcinomas. Germline mutations of PTEN are similarly associated with a group of phenotypically similar cancer predisposition syndromes. The encoded protein is a lipid phosphatase that negatively regulates signaling pathways mediated by phosphoinositide 3-kinase (P13K) and its downstream effector, Akt.;Despite the importance of PTEN inactivation in the pathogenesis of human cancers, its function in human cells is relatively unknown. As such, we employed gene targeting to create isogenic sets of human cancer cells that differ only in the presence or absence of their endogenous, wild-type PTEN genes. Phenotypic analyses of these cells and several genetically unmodified, PTEN-mutant cell lines revealed that PTEN controls a novel, radiation-induced cell size checkpoint that is distinct and genetically separable from the radiation-induced, p53-regulated, G1 and G2 checkpoints. Following irradiation, PTEN-proficient cells enlarge slightly and then growth-arrest, while PTEN-deficient cells enlarge continuously and are unable to enforce an arrest in cell size. Radiation potentiates Akt activation in cells with targeted deletion of PTEN, and the Akt-regulated genes, TSC2 and mTOR, modulate the integrity of the size checkpoint. Abrogation of the PTEN-dependent size checkpoint also results in radiosensitization in vitro and in vivo. To our knowledge, this represents the first report of a genetic mechanism that enforces an arrest in cell size during cell cycle arrest.;Comparative microarray-based expression analyses of isogenic PTEN +/+ and PTEN-/- cells revealed the identity of many genes that are modulated by PTEN. Notably, PTEN deletion resulted in upregulation of several known p53-induced genes. We show that loss of PTEN function stabilizes p53 and leads to a concomitant increase in p21 and PUMA protein levels. These results reveal a novel relationship between PTEN and p53. |
