The Phosphoinositol Phosphatase Activity of PTEN Mediates a Serum-sensitive Gl Growth Arrest in Glioma Cells

The PTEN gene (also called MMACl and TEPl) at chromosome 10q23 is mutated in a variety of predominantly late-stage tumors and has been shown to suppress glioma cell growth in vitro and in vivo. Here we sought tu determine the mechanism by which PTEN mediates growth inhibition. Using the mutant PTEN glioma cell line, U87MG, as a transfection recip ient for a series of PTEN alÃ-eles, we provide direct evidence that this capacity requires phosphatase activity. Mutations mapping upstream, within, and downstream of the catalytic domain ablated activity toward a 3' phosphorylated phosphoinositide substrate of PTEN, whereas alÃ-eles with mutations flanking the catalytic domain retained activity toward the acidic protein polymer substrate, Glu4Tyr,. Thus, catalytic activity to ward phosphoinositide substrates was required for growth suppression, whereas activity toward the protein substrate was dispensable for growth suppression. Finally, we used apoptotic and cell proliferation analyses to show that /'// Y mriliatril growth inhibition under reduced serum con ditions was due to a (., cell cycle block rather than to an induction of apoptosis.