The mechanisms controlling neural stem cell proliferation are poorly understood. Here we demonstrate that the PTEN tumor suppressor plays an important role in regulating neural stem/progenitor cells in vivo and in vitro. Mice lacking PTEN exhibited enlarged, histoarchitecturally abnormal brains, which resulted from increased cell proliferation, decreased cell death, and enlarged cell size. Neurosphere cultures revealed a greater proliferation capacity for tripotent Pten-/- central nervous system stem/progenitor cells, which can be attributed, at least in part, to a shortened cell cycle. However, cell fate commitments of the progenitors were largely undisturbed. Our results suggest that PTEN negatively regulates neural stem cell proliferation.

To investigate the molecular basis of PTENmediated tumor suppression, we introduced a null mutation into the mouse Pten gene by homologous recombination in embryonic stem (ES) cells. Pten ؊/؊ ES cells exhibited an increased growth rate and proliferated even in the absence of serum. ES cells lacking PTEN function also displayed advanced entry into S phase. This accelerated G 1 ͞S transition was accompanied by down-regulation of p27 KIP1 , a major inhibitor for G 1 cyclindependent kinases. Inactivation of PTEN in ES cells and in embryonic fibroblasts resulted in elevated levels of phosphatidylinositol 3,4,5,-trisphosphate, a product of phosphatidylinositol 3 kinase. Consequently, PTEN deficiency led to dosagedependent increases in phosphorylation and activation of Akt͞ protein kinase B, a well-characterized target of the phosphatidylinositol 3 kinase signaling pathway. Akt activation increased Bad phosphorylation and promoted Pten ؊/؊ cell survival. Our studies suggest that PTEN regulates the phosphatidylinositol 3,4,5,-trisphosphate and Akt signaling pathway and consequently modulates two critical cellular processes: cell cycle progression and cell survival.The tumor susceptibility gene encoding PTEN͞MMAC1͞TEP1 (1-3) is mutated at high frequency in many primary human cancers and several familial cancer predisposition disorders (4). PTEN contains the sequence motif that is highly conserved in the members of the protein tyrosine phosphatase family. PTEN has been shown in vitro to possess phosphatase activity on phosphotyrosyl, phosphothreonyl-containing substrates (3, 5) and more recently, on phosphatidylinositol 3,4,5-trisphosphate (PIP3), a product of phosphatidylinositol 3 (PI3) kinase (6). Many cancerrelated mutations have been mapped within the conserved catalytic domain of PTEN, suggesting that the phosphatase activity of PTEN is required for tumor suppressor function. In addition, wild-type PTEN, but not mutant derivatives lacking phosphatase activity, suppresses the growth of glioblastoma cells and their tumorigenecity in nude mice (7-9), confirming the functional relevance of the PTEN phosphatase domain for tumor suppression. Very recently, inactivation of PTEN in a mouse model has confirmed the role of PTEN as a bona fide tumor suppressor (10). However, the exact function of PTEN in regulation of cell growth and tumorigenesis remains unclear.In this study, we have investigated the molecular basis underlying the tumor suppression function of PTEN by using a combination of molecular genetic, cell biological, and biochemical approaches. We have identified PIP3, a product of PI3 kinase, as an intracellular target of PTEN. Our studies suggest that PTEN acts as a negative regulator for the PI3-kinase͞Akt signaling pathway, which controls and coordinates two major cellular processes: cell cycle progression and cell death.

In the liver, insulin controls both lipid and glucose metabolism through its cell surface receptor and intracellular mediators such as phosphatidylinositol 3-kinase and serine-threonine kinase AKT. The insulin signaling pathway is further modulated by protein tyrosine phosphatase or lipid phosphatase. Here, we investigated the function of phosphatase and tension homologue deleted on chromosome 10 (PTEN), a negative regulator of the phosphatidylinositol 3-kinase/AKT pathway, by targeted deletion of Pten in murine liver. Deletion of Pten in the liver resulted in increased fatty acid synthesis, accompanied by hepatomegaly and fatty liver phenotype. Interestingly, Pten liver-specific deletion causes enhanced liver insulin action with improved systemic glucose tolerance. Thus, deletion of Pten in the liver may provide a valuable model that permits the study of the metabolic actions of insulin signaling in the liver, and PTEN may be a promising target for therapeutic intervention for type 2 diabetes

We show in this study that PTEN regulates p53 protein levels and transcriptional activity through both phosphatase-dependent and -independent mechanisms. The onset of tumor development in p53(+/-);Pten(+/-) mice is similar to p53(-/-) animals, and p53 protein levels are dramatically reduced in Pten(-/-) cells and tissues. Reintroducing wild-type or phosphatase-dead PTEN mutants leads to a significant increase in p53 stability. PTEN also physically associates with endogenous p53. Finally, PTEN regulates the transcriptional activity of p53 by modulating its DNA binding activity. This study provides a novel mechanism by which the loss of PTEN can functionally control "two" hits in the course of tumor development by concurrently modulating p53 activity.