Activation of PIP3 signaling by inactivating mutations in the PTEN tumor suppressor or activating mutations in the PIK3CA oncogene is found in a wide range of common human cancers. Though the biochemical details of PIP3 signal transduction are rapidly emerging, the phenotypic consequences of PIP3 pathway activation in human cancer cells remain less well defined. As such, the long-term goal of this project is to define the effects of PIP3 activation in human cancer cells. In our initial preliminary studies, we employed human somatic cell gene targeting to create isogenic sets of PTEN+/+ and PTEN-/- human cancer cell lines. Using these and related model systems, we found that PTEN controls a novel radiation-induced size checkpoint in human cells that is distinct and genetically separable from the radiation-induced, p53-dependent G1 and G2 checkpoints. To our knowledge, the existence of such a DNA damage-inducible size arrest has neither been postulated or demonstrated in any organism. Since the initial submission of this application we have employed human somatic cell gene targeting to create an isogenic set of PIK3CA gene targeted human cancer cells, and also created NIH3T3 cell lines that express wild-type, oncogenic, or inactive forms of PIK3CA. Using these systems we have obtained preliminary data suggesting that, like PTEN, the PIK3CA oncogene may be able to regulate size checkpoint function as well. In this application we propose three related specific aims to further pursue and extend these observations. Aim #1 - Identify additional inducers of the PTEN-dependent size checkpoint. Aim #2 - Demonstrate the presence of the PTEN-dependent size checkpoint in untransformed human cells. Aim #3 - Characterize crosstalk between the radiation-induced, p53-dependent G1/G2 checkpoints and the radiation-induced, PTEN-dependent cell size checkpoint. Pursuit of these aims will enable us to further characterize the properties and mechanism(s) of a tumor suppressor gene-regulated, radiation-induced cell size checkpoint.