Glioma is formed by active Akt1 alone and promoted by active Rac1 in transgenic zebrafish

Primary brain tumors account for 1.4% of all cancers and 2.4% of all cancer-related deaths in the United States, and malignant gliomas account for ∼70% of malignant primary brain tumors.1 Glioma can be classified histologically from grade I to grade IV, according to World Health Organization (WHO) criteria, with glioblastoma (grade IV) accounting for 70% of malignant gliomas.2 Primary glioblastoma is predominant, with 10% being secondary glioblastoma formed by progression of pre-existing low-grade glioma. Glioblastomas are formed by the sequential accumulation of genetic alterations, including the loss of the tumor-suppressor function, deregulation, and activation of growth factor signaling and activation of the survival pathway.1,3 Although molecular signatures vary between primary and secondary glioblastomas, p16, p53, retinoblastoma, and PTEN are among the genes most frequently lost in glioblastoma. Deregulation of growth factor signaling includes overexpression or activating mutations of platelet-derived growth factor (PDGF), PDGF receptor, epidermal growth factor receptor (EGFR), mouse double minute-2 (mdm2), CDK4, and PI3K.4,5 These alterations most often result in the activation of the Akt pathway. Akt (also known as PKB), a serine-threonine protein kinase, is a key mediator of the phosphatidylinositol 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signal pathway, which is activated in up to 90% of all glioblastomas.6 The activation of Akt signaling can be initiated by PTEN inactivation, PI3K activation, or upregulation of growth factors.7,8 Studies of Akt have showed its involvement in such diverse physiological actions as nutrient metabolism, protein synthesis, cell survival, transcriptional regulation, the cell cycle, cell apoptosis, and proliferation. The anti-apoptotic signal is largely mediated by phosphorylation resulting in the inhibition of Bad, caspase-9, and forkhead transcriptional factor and the activation of I-κB kinase and Mdm2.9,10 Akt has also been identified as a positive regulator of the survivin gene,11 which is a member of the inhibitors of apoptosis (IAP) family expressed during embryonic development but not in terminally differentiated adult tissues.12–14 Subsequent studies have revealed that survivin is re-expressed in transformed cell lines and in a variety of human tumors and is essential for the anti-apoptotic function.15,16 mTOR plays a key role in mediating Akt-induced cell proliferation. Activated mTOR complex activates ribosomal protein S6 kinase (RS6K) and inactivates the eukaryotic translation initiation factor 4E binding protein 1 (4EBP1), which inhibits eukaryotic translation initiation factor 1 (Eif1a). Akt also inhibits GSK3β, which acts as a negative regulator of cell cycle progression through inhibition of cyclin D1.17,18 Rho GTPase family proteins mediate distinct cytoskeletal rearrangements in response to receptor stimulations and have been implicated in the establishment and maintenance of cadherin-based cell-cell adhesions.19,20 Rac is a member of this family and counteracts Rho activity. The reciprocal balance between Rho and Rac activity is a major determinant of cellular morphology and motility.21 Active Rac1 signaling is associated with acquiring the mesenchymal phenotype of cancer cells and with enhanced motility and invasion.22,23 In vitro studies have shown that suppression of Rac1 activity is associated with the immobilization and induction of apoptosis of glioma cells.24,25 In addition, Rac1 controls the nuclear localization of β-catenin by phosphorylation, which induces cell proliferation.26 Although evidence has been reported on the critical role of Akt signaling in gliomagenesis, the expression of Akt alone was insufficient to induce glioma in mouse models3,27; coactivation of Kras signaling was needed to induce glioblastoma. In the current study, we established transgenic zebrafish expressing dominant-active (DA) DAAkt1 or DARac1 at the ptf1a domain. We found that DAAkt1 alone induced glioma, and this process was accelerated by coexpression of DARac1; the relevant mechanism in which Akt1 and Rac1 are involved for glioma formation and progression was also investigated.