Abstract 4421: PTEN-deficient tumors depend on Akt2 for maintenance
Authors: Chin Rebecca , Balk Steve P. , Toker Alex 
Published: May 2015

Abstract
PTEN deficiency is frequently found in solid tumors. Since PTEN is yet to be effectively targeted with drugs, its downstream target Akt may provide more viable therapeutic strategies. The essential role of Akt in PTEN-null tumor initiation has been demonstrated in a number of studies. However, the role of Akt in the maintenance of established PTEN-deficient tumors is not known. Moreover, whether PTEN-null tumors depend on a specific Akt isoform for survival signaling remains elusive.
In this study, we have developed a tet-on inducible system for expressing Akt1/2/3 shRNAs to determine the role of Akt isoforms in PTEN-deficient cancer progression. Three-dimensional (3D) matrigel cultures are employed to better recapitulate tumors growing in vivo. Administration of doxycycline (dox) in Akt1 or Akt2 shRNA PTEN-null prostate tumor cells at the start of culture resulted in the inhibition of spheroid formation, suggesting an important role of both Akt1 and Akt2 in the initial growth of spheroids. Conversely, when spheroids were allowed to form for 7 days, followed by dox for further 10 days, no difference was observed between Akt1-depleted and control spheroids. In contrast, silencing of Akt2 resulted in complete disintegration of spheroids. Confocal microscopy studies show a significant induction of active caspase-3 and fragmented nuclei in Akt2-silenced cells. Moreover, whereas Akt1 is uniformly expressed, Akt2 is localized in the cytoplasm and/or on the plasma membrane. Interestingly, the differences of effect of Akt isoforms on cell survival in conventional 2D culture are considerably more modest. The exclusive requirement of Akt2 for survival is also observed in other PTEN-deficient solid tumors, including breast cancer and glioblastoma. Importantly, shRNA silencing of Akt2 but not Akt1 promotes regression of prostate cancer xenografts. Mechanistically, we show that Akt2 silencing up-regulates p21 and the pro-apoptotic protein Bax and downregulates the insulin-like growth factor receptor-1. We also show that p21 is an effector of Akt2 in mediating prostate tumor cell survival.
These results indicate that Akt2, but not Akt1, plays a critical role in PTEN-deficient tumor maintenance, and provide a rationale for developing therapeutics targeting Akt2. Current efforts are aimed at identifying novel Akt isoform-specific substrates that contribute to cancer progression using a genome-wide proteomic screen. The results of these studies could point to new targets for anti-cancer therapeutics.
This work was supported by grants from NIH (A.T., CA092644; R.C., K99CA157945, T32 CA081156-09; S.B., P01 CA163227), the Department of Defense (S.B., W81XWH-09-1-0435) and a sponsored research grant from ImClone/Eli Lilly Inc. (A.T and R.C.).
Citation Format: Rebecca Chin, Steve P. Balk, Alex Toker. PTEN-deficient tumors depend on Akt2 for maintenance. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4421. doi:10.1158/1538-7445.AM2014-4421