AKT plays a central role in tumorigenesis.

A is emerging as a central player in tumorigenesis. In this issue of PNAS, Mayo and Donner (1) report on yet another function of AKT, involving regulation of the Mdm2yp53 pathway. The first evidence pointing to a role of AKT in oncogenesis was provided by early studies of transforming viruses. A novel retrovirus, isolated from an AKR mouse T cell lymphoma (2), harbored transduced sequences of cellular origin (3). In 1991, our collaborative studies with Philip Tsichlis and Stephen Staal resulted in the cloning of the viral oncogene v-akt (4). The predicted oncoprotein contained viral Gag sequences fused to a kinase related to protein kinase C. The oncogenic potential of v-akt arises from the creation of a myristylation site at the amino terminus and consequent constitutive kinase activity (5). By different approaches, aimed at identifying novel protein kinases, two other groups independently cloned the identical cellular sequence at about the same time (6, 7). AKT is now known to define a family of closely related, highly conserved cellular homologues (reviewed in ref. 8). In human, these are designated AKT1, AKT2, and AKT3, located at chromosomes 14q32, 19q13, and 1q44, respectively (reviewed in ref. 9). The encoded proteins are serineythreonine kinases belonging to the protein kinase B (PKB) family, and the AKT1, AKT2, and AKT3 proteins are also known as PKBa, PKBb, and PKBg, respectively. Each AKT family member contains an amino-terminal pleckstrin homology (PH) domain, a short a-helical linker, and a carboxyl-terminal kinase domain (8). PH domains exist in diverse signaling molecules and permit anchorage of proteins to the cell membrane via phospholipid interactions (10). The degree of functional redundancy between AKT1, AKT2, and AKT3 is currently unclear. Although each kinase responds similarly to various stimuli, their different tissue-specific expression patterns suggest distinct roles, e.g., compared to Akt1, Akt2 transcripts are especially abundant in highly insulin-responsive tissues such as brown fat (11). Moreover, Akt2 knockout mice exhibit impaired ability of insulin to lower blood glucose as a result of defects in the action of the hormone on liver and skeletal muscle (12). Expression of Akt1 and Akt3 does not compensate for loss of Akt2, thus establishing Akt2 as an essential gene for the maintenance of normal glucose homeostasis. Mounting evidence suggests that AKT perturbations play an important role in human malignancy. In 1992, we reported the first recurrent involvement of an AKT gene in a human cancer, demonstrating amplification and overexpression of AKT2 in ovarian tumors and cell lines (13). Subsequent studies documented AKT2 amplification andyor mRNA overexpression in 10–20% of human ovarian and pancreatic cancers (14, 15) and activation of the AKT2 kinase in '40% of ovarian cancers (16). Overexpression of AKT2 can transform NIH 3T3 cells (17), and AKT2 antisense RNA inhibits the tumorigenic phenotype of cancer cells exhibiting amplified AKT2 (15). Amplification of AKT1 was observed in a human gastric cancer (3), and AKT1 kinase activity is often increased in prostate and breast cancers and is associated with a poor prognosis (18). To date, amplification of AKT3 has not been described. However, AKT3 mRNA is up-regulated in estrogen receptor-negative breast tumors, and increased AKT3 enzymatic activity was found in estrogen receptor-deficient breast cancer and androgen-insensitive prostate cancer cell lines (19), suggesting that AKT3 may contribute to the aggressiveness of steroid hormone-insensitive cancers. There has been enormous interest in the mechanisms and cellular consequences of signal propagation from receptor tyrosine kinases to AKT (reviewed in refs. 8 and 20–28). The AKT kinases are major downstream targets of growth factor receptor tyrosine kinases that signal via phosphatidylinositol 3-kinase (PI3K). AKT activation is a multistep process involving both membrane translocation and phosphorylation (29). The pleckstrin homology domain of AKT kinases has affinity for the 39-phosphorylated phosphoinositides 3,4,5-trisphosphate (PI3,4,5-P3) and PI-3,4,-P2 produced by PI3K, and they are activated specifically by the latter lipid. Phospholipid binding triggers the translocation of AKT kinases to the plasma membrane. Upon membrane localization, AKT molecules are phosphorylated at Thr-308y309 in the kinase activation loop and Ser-473y474 in the carboxyl-terminal tail. Thr-308y309 phosphorylation is necessary for AKT activation, and Ser-473y474 phosphorylation is only required for maximal activity. Phosphorylation on these residues is induced by growth factor stimulation and inhibited by the PI3K inhibitor, LY294002. Indeed, the kinase responsible for Thr-308y309 phosphorylation, PDK1 (for 3-phosphoinositide-dependent kinase) is activated by the PI3K lipid products PI-3,4,5-P3 and PI-3,4-P2. More controversial is the identity of PDK2, the kinase(s) responsible for Ser-473y474 phosphorylation (30). Interestingly, avian sarcoma virus 16 contains a potent transforming sequence derived from the cellular gene for the catalytic subunit of PI3K (31), and its human homologue, PIK3CA, was implicated as an oncogene in human ovarian cancer (32). Furthermore, the negative regulator of this pathway, the tumor suppressor PTEN, inhibits AKT activation by dephosphorylating PI-3,4,-P2yPI-3,4,5-P3 (reviewed in refs. 33 and 34). Recent studies have revealed a burgeoning list of AKT substrates implicated in oncogenesis (reviewed in ref. 26). Among its pleiotropic effects, activated AKT is a well-established survival factor, exerting anti-apoptotic activity by preventing release of cytochrome c from mitochondria and inactivating forkhead transcription factors known to induce expression of pro-apoptotic factors such as Fas ligand. AKT phosphorylates and inactivates the pro-apoptotic factors BAD and pro-caspase-9. Moreover, AKT activates IkB kinase, a positive regulator of NF-kB, which results in transcription of anti-apoptotic genes. AKT kinases also phosphorylate and inactivate glycogen synthase kinase 3, thereby stimulating glycogen synthesis (35). AKT activation affects cell cycle progression, through regulation of cyclin D stability (36) and inhibition of p27Kip1 protein levels (37), and mRNA translation, via phosphorylation of 4E-BP1 and its dissociation from the mRNA cap binding protein elF4E