OverAKT3: tumor progression and chemoresistance

Akt3 is one of 3 closely related serine/threonine-protein kinases in the Akt family (Akt1, Akt2, and Akt3). Akt is among the most hyperactivated oncogenes in human cancer, regulating key cellular functions such as growth, proliferation, migration, invasion, angiogenesis, metabolism, and survival.^1,2 Studies evaluating the effect of Akt isoform gene knockout on mouse development found that loss of Akt1 increased perinatal mortality and affected overall growth of the mice. Akt2 loss led to a diabetic phenotype but otherwise normal development, whereas loss of Akt3 affected both brain and testes size. Tissue distribution of Akt isoforms also gives an indication of their functional roles under normal circumstances: Akt1 and Akt2 are the most ubiquitously expressed, while Akt3 expression is mostly restricted to the brain and testes.

In recent years, it has become evident that Akt2 and Akt3 are more important than Akt1 in gliomagenesis and progression.³ The first evidence was reported on Akt2, which was found to mediate invasion of rat C6 glioma cells; inhibition of Akt2 prolonged survival in a rat orthotopic glioma model. Subsequent reports have shown that Akt2, and possibly also Akt3, is important for glioma progression and maintenance, whereas Akt1 does not play a major role.³ More specifically, knockdown of Akt2 and EGFR in U87MG-EGFRvIII glioma cells increased apoptosis, reduced tumor growth, and prolonged survival in nude mice. This same result was not observed upon knockdown of Akt1 or Akt3. Another study found that knockdown of Akt2 or Akt3 (but not Akt1) led to decreased Bad phosphorylation and increased caspase-9 and caspase-3 activity, suggesting that these isoforms are important for cell viability through regulation of mitochondrial membrane potential.³ A more recent paper evaluated Akt isoforms by injecting transformed astrocytes into the brains of nude mice to rapidly develop high-grade astrocytoma. The resulting astrocytes had a defined genetic background that included loss of p53 with EGFRvIII expression, with or without loss of PTEN; loss of Akt1 or Akt2 decreased proliferation of PTEN wild-type astrocytes, whereas combined loss of multiple isoforms was needed to inhibit proliferation of PTEN-null astrocytes. In addition, Akt3 displayed a unique phenotype of enabling anchorage-independent growth and invasion.⁴ Our RCAS/Ntv-a glia-specific mouse model revealed that, of the 3 Akt isoforms, Akt3 has the highest capacity to promote high-grade glioma development.⁵

The Akt pathway is among the most hyperactivated signaling pathways in human glioma, and Akt activation status correlates with glioma grade. The Akt kinase is a downstream mediator of the PI3K pathway, resulting in the recruitment of Akt to the plasma membrane via its PH (pleckstrin homology) domain. This anchoring in the proximity of the membrane enables PDK1 (3-phosphoinositide-dependent protein kinase 1) to phosphorylate Akt on Thr-308 and mTORC2 (mammalian target of rapamycin complex 2) to phosphorylate Akt on Ser-473. Once phosphorylated and thus activated, Akt governs the phosphorylation of over 100 substrates.

The three Akt isoforms have shared as well as distinct functions in cancer cells. The function of Akt1 is the most well characterized. Isoform-specific functions are dependent on cellular localization and downstream targets. For example, AEG1-Akt2 interaction prolonged stabilization of Akt2 phosphorylation at S474, regulating downstream signaling cascades that enable glioma cell proliferation and survival.⁶ Phosphorylation of TBX3 by Akt3 promotes TBX3 protein stability, nuclear localization, and transcriptional repression of E-cadherin and contributes to cell migration and invasion. Specific phosphorylation of palladin, an actin bundling protein, by Akt1 led to decreased migration. Conversely, Akt2 has been found to promote breast cancer epithelial-to-mesenchymal transition through mir-200 modulation as well as interaction with Snail1 in the E-cadherin promoter. Akt3 controls mitochondrial biogenesis and autophagy via regulation of the major nuclear export protein CRM-1.⁷

Our current work shows that Akt3 is more sensitive than the other isoforms to growth factor stimulation, including EGF and PDGFB. We further discovered that Akt3 is predominant among the 3 isoforms in modification and localization in the nucleus. We believe that the Akt3 role in glioma progression and therapeutic resistance is dependent on its nuclear function (Fig. 1). Akt3 plays a critical role in DNA double strand break repair and resistance to radiotherapy and chemotherapy. Thus resistance of some glioblastomas to standard treatment approaches may be due to amplification/hyperactivation of Akt3 and the resulting increased DNA repair capacity. Given that Akt3 is a dominant player in glioma progression and aids in tumor resistance to therapy, inhibitors should be developed and tested that specifically block Akt3 function. We also suggest that DNA repair inhibitors should be investigated in combination with standard therapy in tumors with amplified Akt3.

Figure 1.

Akt3 governs glioma progression and therapeutic resistance via regulation of DNA double strand break (DSB) repair. Akt3 is predominant in the nucleus of glioma cells, and the signature genes associated with Akt3 driven tumors are involved in the DNA repair pathway. HR, homologous recombination; NHEJ, non-homologous end joining.

In conclusion, Akt3 plays a critical role in glioma progression and therapeutic resistance. We have observed genomic amplification of Akt3 in a wide range of cancers, including glioblastoma, and the protein is predominantly located in the nucleus of glioma cells. In our glia-specific mouse model, we found that Akt3 promotes glioma progression when combined with PDGFB. The signature genes associated with Akt3-induced glioma are involved in DNA repair pathways. Moreover, Akt3 contributes to therapeutic resistance via regulation of homologous recombination and non-homologous end joining. Targeting Akt3 and/or its downstream DNA repair protein(s) might be an effective therapeutic strategy in glioma.