Genomically amplified Akt3 activates DNA repair pathway and promotes glioma progression

Akt is a robust oncogene that plays key roles in the development and progression of many cancers, including glioma. We evaluated the differential propensities of the Akt isoforms toward progression in the well-characterized RCAS/Ntv-a mouse model of PDGFB-driven low grade glioma. A constitutively active myristoylated form of Akt1 did not induce high-grade glioma (HGG). In stark contrast, Akt2 and Akt3 showed strong progression potential with 78% and 97% of tumors diagnosed as HGG, respectively. We further revealed that significant variations in polarity and hydropathy values among the Akt isoforms in both the pleckstrin homology domain (P domain) and regulatory domain (R domain) were critical in mediating glioma progression. Gene expression profiles from representative Akt-derived tumors indicated dominant and distinct roles for Akt3, consisting primarily of DNA repair pathways. TCGA data from human GBM closely reflected the DNA repair function, as Akt3 was significantly correlated with a 76-gene signature DNA repair panel. Consistently, compared with Akt1 and Akt2 overexpression models, Akt3-expressing human GBM cells had enhanced activation of DNA repair proteins, leading to increased DNA repair and subsequent resistance to radiation and temozolomide. Given the wide range of Akt3-amplified cancers, Akt3 may represent a key resistance factor.Akt is among the most hyperactivated signaling pathways in human cancer and is an important kinase that regulates key cellular functions including cell growth, proliferation, angiogenesis, glucose metabolism, invasion, and survival, among others (1–3). Analysis of human glioblastomas (GBMs) from The Cancer Genome Atlas (TCGA) has revealed that the Akt signaling pathway is one of the top altered pathways in GBM (4, 5). Additionally, researchers have observed that the Akt activation status correlates with glioma grade (6). However, much less is known regarding the precise roles of the Akt isoforms (Akt1, Akt2, Akt3) in this disease.Akt1, Akt2, and Akt3 share roughly 80% overall sequence identity and each contain three similar domains: a pleckstrin homology, kinase, and regulatory domain (7). Although they appear to be very similar proteins, developmental studies as well as assessment of the isoforms in cancer models have revealed that they possess distinct functions (1, 8). The most extensive work on isoform differences has been performed in breast cancer. Akt2 is reported to promote migration, invasion, and metastasis, whereas Akt1 is inhibitory (8, 9). Specific phosphorylation of palladin, an actin bundling protein, by Akt1 led to decreased migration (10). Conversely, Akt2 has been found to promote breast cancer epithelial to mesenchymal transition (EMT) through miR-200 modulation (11) as well as an interaction with Snail1 on the E-cadherin promoter (12). Functional differences in glioma have been slightly variable by using knockdown systems in transformed cells. The first studies were reported on Akt2 only, which was found to mediate invasion in rat C6 glioma cells (13), and inhibition of Akt2 prolonged survival in an orthotopic rat model (14). Subsequent reports have found that Akt2, and possibly Akt3, are important for disease progression and maintenance, whereas Akt1 does not play a major role (15, 16). Another study in transformed mouse astrocytes indicate that Akt isoform function is potentially modulated by the genetic landscape of the tumor (17). The underlying basis behind these differences and the major pathways that the isoforms use remains largely unknown. To our knowledge, our study is the first to use a glial-specific transgenic mouse model to demonstrate Akt3 as a powerful inducer of glioma progression and to further reveal the previously unidentified DNA repair function of Akt3.