PTEN-LONG, a translational variant of the canonical tumor suppressor PTEN

Phosphatase and Tensin Homologue on chromosome Ten (PTEN) is a tumor suppressor and an antagonist of the phosphotidylinositol-3 kinase (PI3K) pathway. We identified a 576-amino acid translational variant of PTEN, termed PTEN-Long, that arises from an alternative translation start site 519 base-pairs upstream of the ATG initiation sequence, adding 173 N-terminal amino acids to the normal PTEN open reading frame. We demonstrate that PTEN-Long is able to act as a lipid phosphatase and that it is able to down regulate PI3K signaling when transiently transfected into cells. We observe that PTEN-Long is down regulated in the tumor cells of breast cancer cases, and that in some cases it is highly expressed by immune cells in the tumor microenvironment indicating that PTEN-Long may play a role in the endogenous response to tumor formation. We demonstrate that PTEN-Long is secreted from cells and can enter other cells. As an exogenous agent, PTEN-Long antagonized PI3K signaling and induced tumor cell death in vitro and in vivo. Furthermore we were able to use the PTEN-Long alternately translated region (ATR) to shuttle fused RFP and p53 into cells, thus opening the possibility that ATR-fusion proteins may be capable of restoring proteins such as lost tumor suppressors back into cells. Using pancreatic ductal adenocarcinoma (PDAC) as a case study in order to expand upon our initial observations, we observed loss/down-regulation of PTEN-Long in human PDAC tumor samples as well as in tumor derived cell lines. We demonstrate that this feature of the human disease is recapitulated in a Kras G12D p53 H172R Pdx1-Cre (KPC) mouse model. We examined the efficacy of PTEN-Long treatment alone or in combination with gemcitabine in the KPC model. To our surprise PTEN-Long preformed equivalently to gemcitabine as a monotherapy, and the animals had a significantly increased survival when treated with the PTEN-Long/gemcitabine combination. Further studies are needed to understand the mechanism by which PTEN- Long is cooperating with gemcitabine in this model. The present data strongly supports further exploration of PTEN-Long's utility and potential as a therapeutic agent.