The C-terminus of Pten contributes to tumor suppression in mice

PTEN is a tumor suppressor that functions to antagonize the AKT-mediated growth-signaling pathway. Several studies have suggested the C-terminus of PTEN is essential for the inhibition of cell spreading and focal adhesion. Removal of the C-terminus permits anchorage-independent growth and Pten mutants lacking the lipid phosphatase domain, although enzymatically inactive, still inhibit cell migration, spreading and focal adhesion. These results suggest that Pten's ability to regulate cell spreading and motility is dependent on its lipid phosphatase activity. Similarly, truncated versions of Pten have been found in some malignant tumors implying that loss of the C-terminus of Pten may promote cell invasion by destroying intracellular matrix contacts allowing for increased cell motility.;In order to elucidate the function of the C-terminus of Pten, we have generated a Pten knockout mouse containing a C-terminal deletion of the last 46 amino acids of the protein (Pten Delta46). Homozygous PtenDelta46/Delta46 mice are embryonic-lethal and die at embryonic day E8.5. However, heterozygous mice develop hyperplasia and tumors in multiple organs, similar to the phenotype of lipid phosphatase domain deletion knockout mice that have already been described. These results suggest that the C-terminus of Pten plays a crucial role in its activity and loss of this domain impairs Pten function.;Because the phenotype of our heterozygous mice (PtenDelta46/+ ) is similar to other Pten lipid phosphatase deficient heterozygous mice, we wanted to determine if the loss of Pten's C-terminus generates an unstable protein. Expression of a Pten Delta46 cDNA was seen in transiently transfected cells however, Pten Delta46 protein could not be detected in either heterozygous PtenDelta46/+ MEF cells or in homozygous PtenDelta46/Delta46 ES cells. RT-PCR revealed that the Pten Delta46 message was made in vivo suggesting that degradation occurs at the protein level. Preliminary studies have shown that treatment of ES cells with lactacystin, a proteasome inhibitor, stabilizes Pten Delta46 protein suggesting this protein may be regulated through ubiqutin/proteasome-mediated degradation. This rapid degradation may explain tumor formation in the Pten Delta46 heterozygous mice in which loss of Pten through increased degradation leads to increased PI3K signaling and enhanced growth and survival. We conclude that deletion of the C-terminus of Pten results in an unstable protein leading to loss of tumor suppression in mice.