Oncogene (2008) 27, 6473–6488 & 2008 Macmillan Publishers Limited All rights reserved 0950-9232/08 $32.00 www.nature.com/onc REVIEW PI3K/Akt: getting it right matters

TF Franke

Departments of Psychiatry and Pharmacology, New York University School of Medicine, New York, NY, USA

The Akt serine/threonine kinase (also called protein induction and maintenance of its kinase activity (for kinase B) has emerged as a critical signaling molecule review, see Chan et al., 1999). Mammalian cells express within eukaryotic cells. Significant progress has been three closely related Akt isoforms: Akt1 (PKBa), Akt2 made in clarifying its regulation by upstream kinases (PKBb) and Akt3 (PKBg), all encoded by different and identifying downstream mechanisms that mediate its genes. Akt1 is ubiquitously expressed at high levels effects in cells and contribute to signaling specificity. (Coffer and Woodgett, 1991; Jones et al., 1991b; Here, we provide an overview of present advances in the Bellacosa et al., 1993). In contrast, Akt2 is highly field regarding the function of Akt in physiological and expressed in insulin-sensitive tissues including the liver, pathological cell function within a more generalized skeletal muscle and adipose tissue (Jones et al., 1991a; framework of Akt signal transduction. An emphasis is Konishi et al., 1994). The expression of Akt2 is placed on the involvement of Akt in human diseases drastically increased during the differentiation of ranging from cancer to metabolic dysfunction and mental adipose tissue and skeletal muscle (Hill et al., 1999; disease. Vandromme et al., 2001). Akt3 is expressed most highly Oncogene (2008) 27, 6473–6488; doi:10.1038/onc.2008.313 in the brain and testis and exhibits lower levels of expression in intestinal organs and muscle tissue Keywords: Akt/PKB; PI3K/PTEN; PDK2; mTORC; (Nakatani et al., 1999a). Akt-interacting proteins; human disease The structural homology between different Akt iso- forms extends to their activation mechanism. All Akt isoforms contain PH domains with similar specificities for the D3-phosphorylated phosphoinositide products of phosphatidylinositol 3-kinase (PI3K) (Franke et al., Akt kinases: a family of homologous kinases 1997). Another similarity is that they are regulated by phosphoinositide-dependent kinase-1 (PDK1), a PH The Akt gene is the cellular homolog of the v-akt domain-containing kinase downstream of PI3K, which oncogene transduced by AKT8, an acute transforming phosphorylates Akt isoforms and other AGC kinase retrovirus in mice that was originally described in 1977 family members on a critical threonine residue in the (Staal et al., 1977). In 1991, three independent research activation loop (for review, see Toker and Newton, groups cloned and characterized Akt kinases. Philip 2000b). Tsichlis’ group identified v-akt as the gene transduced by Several findings point to functional differences the rodent retrovirus AKT8 (Bellacosa et al., 1991), and between Akt isoforms. These include the observation that overexpression of Akt2, but not Akt1, is sufficient subsequently showed that its cellular homolog, then À/À named c-akt, encoded the cytoplasmic serine-threonine to restore insulin-mediated glucose uptake in Akt2 protein kinase Akt in mice (Bellacosa et al., 1993). adipocytes (Bae et al., 2003); that glucose uptake in Also in 1991, two research groups in England and wild-type adipocytes is potently inhibited by siRNA Switzerland identified Akt when searching for novel knockdown of Akt2 but not Akt1 (Jiang et al., 2003); kinases related to protein kinases A and C (PKA/PKC) and that only the expression of human Akt2 and Akt3 is (Coffer and Woodgett, 1991; Jones et al., 1991b). pathologically increased in human cancer, suggesting Thus, Akt is also frequently referred to as protein the involvement of specific Akt isoforms in the onset or kinase B (PKB). propagation of cancer (Cheng et al., 1996; Nakatani All Akt isoforms share similar structures, namely an et al., 1999b). Furthermore, although increasing Akt2 N-terminal regulatory domain including a pleckstrin activity increases cell motility, invasion and metastatic homology (PH) domain; a hinge region connecting the potential, cells expressing activated Akt1 fail to exhibit PH domain to a kinase domain with serine/threonine this cancer-related phenotype (Arboleda et al., 2003; specificity; and a C-terminal region necessary for the Hutchinson et al., 2004). Marked differences regarding the involvement of specific Akt isoforms have also been reported for other human diseases: inherited muta- Correspondence: Dr TF Franke, New York University School of tions in AKT2 have been described in familial Medicine, 550 First Avenue, Milhauser Laboratories TCH-550, New York, NY 10016, USA. diabetes (George et al., 2004); in contrast, AKT1 E-mail: [email protected] coding variations have been associated with familial Akt signaling in animal physiology and human disease TF Franke 6474 schizophrenia (Emamian et al., 2004). Taken together, In addition to lipid kinase activity, Class I PI3Ks also these findings of varying and isoform-specific involve- exert serine/threonine protein kinase activity against ment in human disease suggest that different Akt protein substrates. However, the physiological relevance isoforms are associated with distinct biological pro- of their protein kinase activity differs significantly cesses. between the different members of Class IPI3Ks.More specifically, although only a few examples of Class IA-dependent protein phosphorylation such as the Phosphatidylinositol 3-kinases: signaling networks phosphorylation of insulin receptor substrate (IRS)-1 upstream of Akt are known (Lam et al., 1994), the protein kinase activity of Class IB PI3Ks has significant implications for the Critical to the understanding of Akt regulation in cells downstream activation of the mitogen-activated protein was the finding that Akt kinase activity is induced kinase cascade (Lopez-Ilasaca et al., 1997). Other following activation of PI3K in growth factor receptor- members of the ‘extended’ family of PI3Ks include class mediated signaling cascades (Burgering and Coffer, II-type kinases that consist of a single p110-like 1995; Franke et al., 1995; Kohn et al., 1995). At the subunit and prefer PIand PI-4-P as substrates over plasma membrane, PI3K phosphorylates phosphoinosi- PI-4,5-P2 (Virbasius et al., 1996) and class III PI3Ks tides on the 30-OH (D3) position of the inositol ring to such as VPS34 that are involved in intracellular vesicle generate the second messengers phosphatidylinositol-3, transport and sorting (Schu et al., 1993). In addition to 4-bisphosphate (PI-3,4-P2) and phosphatidylinositol-3, trafficking vesicles, VPS34 is involved in the regulation 4,5-trisphosphate (PIP3), using phosphatidyinositol-4- of mammalian target of rapamycin (mTOR) activity phosphate (PI-4-P) and phosphatidylinositol-4,5- in response to amino-acid availability (for review, bisphosphate (PI-4,5-P2) as substrates, respectively (for see Nobukuni et al., 2007), which may be relevant review, see Engelman et al., 2006; Hawkins et al., 2006). for the autophagic response to nutrient starvation Although it had already been demonstrated that the (Kihara et al., 2001; Wurmser and Emr, 2002; Juhasz levels of D3-phosphorylated phosphatidylinositols et al., 2008). change following growth factor stimulation of cells (Auger et al., 1989), it was not until 1992 that a cDNA for a kinase capable of phosphorylating phosphatidyli- Oncogenic Aktivation nositol (PI) at the D3 position was identified (Hiles et al., 1992). Subsequently, PI3K was shown to consist The original discovery of Akt as the protein encoded by of 85 kDa regulatory and 110 kDa catalytic subunits. To the cellular homolog of the viral oncogene v-akt already date, four catalytic subunits that can phosphorylate PI, implied an important function in cell growth and PI-4-P and PI-4,5-P2 in vitro have been cloned (Stephens survival. Numerous studies have since examined onco- et al., 1991; Carter et al., 1994). Class IA PI3Ks (p110a, genic amplifications of different Akt genes in primary p110b and p110d) associate with various regulatory human tumors and cancer cells (for review, see Brazil subunits to form active heterodimers with distinct and Hemmings, 2001; Scheid and Woodgett, 2001). expression and activity. These regulatory subunits Findings of altered AKT2 and AKT3 expression in include: p85a, p55a and p50a, which are all encoded human cancer support a critical function of the kinases by the pik3r1 gene and have identical C-terminal in the disease. The importance of Akt signaling in cancer regions; p85b encoded by pik3r2; and p55g encoded by is further underscored by the discovery of mutations in pik3r3 (for review, see Engelman et al., 2006; Hawkins upstream regulatory molecules including activating et al., 2006). somatic mutations in PIK3CA (Bachman et al., 2004; For Class IA PI3Ks, a simple model for activation by Campbell et al., 2004; Samuels et al., 2004). The gene for growth factors has been proposed. Upon receptor the tumor suppressor phosphatase and tensin homolog tyrosine kinase (RTK)-dependent activation, PI3K (PTEN), a phosphatase that inactivates products of complexes are recruited to the plasma membrane PI3K and inhibits Akt activation (Maehama and Dixon, through the phosphorylation-dependent interaction 1998), is often mutated in human cancer. Silencing of between Src homology 2 (SH2) domains in their PTEN is frequently observed in primary tumors and regulatory subunit and the phospho-tyrosine residues cancer cell lines (for review, see Carnero et al., 2008; on the receptor. As a result, the catalytic subunits are Maehama, 2007; Salmena et al., 2008). Still, these are stabilized and brought into proximity of phosphoinosi- not the only oncogenic changes leading to increased Akt tides at the plasma membrane, thus resulting in the activity. Alterations of growth factor receptors and Ras subsequent generation of D3-phosphorylated lipid also result in Akt hyperactivity in tumor cells (for products (for review, see Engelman et al., 2006; review, see Shaw and Cantley, 2006) and protect cancer Hawkins et al., 2006). In the case of Class IB PI3K, cells against apoptotic insults. Akt hyperactivation is only one regulatory subunit (p101) has been shown found more frequently in poorly differentiated tumors to interact with the p110g catalytic subunit (Stephens that are more invasive, grow faster and respond less to et al., 1997). Class IB PI3K is primarily regulated treatment. Thus, the pathological finding of hyperacti- by small G-proteins such as Ras and by the b/g subunits vated Akt in primary tumors is considered to be a of G protein-coupled receptors (Lopez-Ilasaca et al., negative prognostic marker for disease outcome (for 1997). review, see LoPiccolo et al., 2008). The relevance of

Oncogene Akt signaling in animal physiology and human disease TF Franke 6475 altered Akt signaling in primary tumors is further being Akt kinase kinases: PDK1 and PDK2 supported by findings in transgenic mouse models in which expression of activated Akt in the mammary or In addition to phosphoinositide binding, phosphoryla- prostate gland induces pathological changes closely tion events are required to fully activate Akt. Thus, resembling those in human cancers (Hutchinson et al., phosphorylation of threonine residue-308 in its kinase 2001; White et al., 2001; Majumder et al., 2003). domain (T-loop) and of serine residue-473 within the It was nearly two decades since the initial description hydrophobic motif (HM) in the C-terminal tail (accord- of AKT by Staal (Staal et al., 1977) before polymorph- ing to the mouse Akt1 sequence) potently activate the isms and somatic mutations were identified that directly enzyme and lock it in an active conformation (Alessi link mutations in Akt isoforms to human cancer. Large- et al., 1996a). The structural analysis of the Akt kinase scale sequencing efforts have identified germline single- domain has unraveled the precise nature of the nucleotide polymorphisms and somatic mutations in phosphorylation-dependent mechanisms of Akt activa- AKT2 and AKT3 (Sjoblom et al., 2006; Greenman et al., tion (Yang et al., 2002a, 2002b). These studies also 2007). Carpten et al. (2007) have described an activating, provide an explanation for the requirement of HM although rare, mutation in the pleckstrin homology phosphorylation to induce full Akt activity, as phos- domain of AKT1 in breast, colorectal and ovarial phorylation at the HM site is required to facilitate a clinical cancer specimens. Others have since confirmed disorder-to-order transition within the structure of the these findings in various cancers (Bleeker et al., 2008; Akt kinase domain and enable the proper alignment of Kim et al., 2008; Malanga et al., 2008). Specifically, substrates within its active site. In addition to separately a G>A mutation at nucleotide 49 results in a lysine solving the structures of both the PH and kinase domain substitution for glutamic acid at amino acid 17 of Akt, advances have been made in understanding the [AKT1(E17K)] that results in a mutant Akt1 protein dynamics of the entire polypeptide. Calleja et al. (2007) with increased plasma membrane recruitment. In have used a fluorescent in vivo probe in combination culture, cells carrying the mutant allele exhibit a with molecular modeling to examine domain inter- transformed phenotype. Retroviral transduction into actions during recruitment and activation of Akt at the Em-Myc transgenic mice induces leukemia similar to plasma membrane. Using this approach, the authors myristoylated Akt1 that recapitulates the GAG-Akt find that Akt is maintained in an inactive state by fusion protein transduced by AKT8 (Carpten et al., the interaction of its PH and kinase domains. This 2007). Activation of a cytoplasmic kinase by means of conformation is relaxed upon binding of phosphoinosi- a PH domain mutation has previously been reported for tides to the PH domain, thus allowing phosphorylation Bruton’s tyrosine kinase (Btk) (Li et al., 1995). Still, the of Akt by PDK1 and subsequent activation. identification of such a mutation in Akt is an important Although other kinases, including calmodulin-depen- confirmation for a direct involvement of Akt in cancer dent kinase kinase (CaM-KK), have been proposed to which was previously lacking supporting genetic evi- phosphorylate the critical T-loop residues in Akt (Yano dence. Thus, both direct changes in Akt and also et al., 1998), clear biochemical and genetic evidence has indirect changes in modifiers of Akt activity exist that confirmed that in most biological paradigms this activity increase pathway strength and drive inherited and is conferred by PDK1 (Alessi et al., 1997; Stephens et al., somatic tumors in humans (for review, see Brugge 1998). PDK1 also contains a PH domain and a serine/ et al., 2007). threonine kinase domain. The PH domain in PDK1 is These studies strongly validate the search for new located at its C-terminus and binds to PIP3 at a higher treatment approaches that target the Akt pathway. affinity than the PH domain of Akt (Franke et al., Compounds that inhibit Akt kinase activity are under 1997). Under physiological conditions, PDK1 is present development; other drugs have been designed that target in the plasma membrane of non-stimulated cells, where the regulatory domain of Akt (for review, see LoPiccolo it resides as a constitutively activated enzyme. Akt et al., 2008). Being a protein kinase adds to the value of shuttles from the cytoplasm to the plasma membrane Akt as a potential drug target, as, historically speaking, only after the levels of D3-phosphoinositides are loss of function has been easier to achieve through increased following stimulation of PI3K (Stephens pharmacological targeting than restoration of function. et al., 1998). Of note, other members of the AGC One caveat for targeting Akt, however, is that genetic ‘superfamily’ of kinases are also phosphorylated by changes in upstream signaling molecules (for example, PDK1 on corresponding T-loop motifs, although they PIK3CA) will not only affect Akt signaling, but also may use different mechanisms for colocalization with other parallel pathways that are regulated by these PDK1 (for review, see Toker and Newton, 2000b). upstream activators. As such, the observation that the The molecular mechanisms governing HM phosphor- E17K mutation, although not changing the sensitivity of ylation in Akt have remained elusive for a long period. the mutant kinase to ATP-competitive inhibitors, alters Preliminary experimental data supported both mechan- its sensitivity to an allosteric kinase inhibitor is very isms involving Akt autophosphorylation (Toker and promising (Carpten et al., 2007). The identification Newton, 2000a) and phosphorylation by other serine of this mutation provides a defined target for kinases including integrin-linked kinase-1 (ILK1) (Persad pharmacological therapy with the intrinsic promise of et al., 2000), PDK1 (Balendran et al., 1999; Biondi et al., minimizing side effects on normal Akt function in 2000) and DNA-dependent protein kinase (DNA-PK) cellular homeostasis. (Feng et al., 2004; Dragoi et al., 2005; Lu et al., 2006),

Oncogene Akt signaling in animal physiology and human disease TF Franke 6476 among others (for review, see Dong and Liu, and invertebrate species examined to date and 2005). Biochemically distinct properties argue against include Akt-related kinases in Drosophila melanogaster, PDK1 and ILK1 in favor of a separate molecular Caenorhabditis elegans and Dictyostelium discoides identity of HM kinase activity (Hill et al., 2002; (Franke et al., 1994; Paradis and Ruvkun, 1998; Meili Hodgkinson et al., 2002). In an elegant study from the et al., 1999). Genetic findings in D. melanogaster suggest Sabatini laboratory, mTOR complex-2 (mTORC2) was a dual function for Akt depending on the developmental ultimately identified as this physiological PDK2 kinase stage. In early embryos, loss of Akt activity after activity (Sarbassov et al., 2005), a finding which has overexpression of dominant-negative PI3K or PTEN since been repeated in other systems and is generally results in ectopic apoptosis (Scanga et al., 2000), and accepted in the field (for review, see Guertin and experiments using genetic mutants suggest a conserved Sabatini, 2007; Reiling and Sabatini, 2006). mTORC2 function for Akt in apoptosis regulation and tumori- directly phosphorylates the HM site in vitro and genesis (Staveley et al., 1998). At later developmental enhances subsequent phosphorylation of Akt by stages, the pathway primarily regulates cell size and PDK1 (Sarbassov et al., 2005). In addition, mTORC2 does not affect apoptosis (Staveley et al., 1998; is sensitive to PI3K inhibitors but insensitive to Verdu et al., 1999). staurosporine, pharmacological properties that have In C. elegans, genetic complementation groups have been the gold standard for candidate PDK2 activities significantly contributed to the analysis and under- (for review, see Dong and Liu, 2005). In further studies, standing of the Akt signal transduction pathway. A siRNA knockdown of rictor, mTOR, mammalian comparison of the Akt signaling pathways in C. elegans LST8/G-protein b-subunit like protein (mLST8/GbL) and mammals indicates strong evolutionary conserva- and mammalian stress-activated protein kinase inter- tion of the pathway (Paradis and Ruvkun, 1998). acting protein 1 (mSIN1) as primary components of Upstream regulators of akt-1 and akt-2 in C. elegans mTORC2 confirms its molecular identity: siRNA are close homologs of their mammalian counterparts. knockdown of any mTORC2 component inhibits They include genes such as: DAF-18, a gene encoding a phosphorylation of the HM site both after stimulation structural homolog of PTEN; age-1, a gene encoding a with insulin and in genetically modified mice (Frias PI3K homolog; pdk-1, a gene encoding a PDK1 et al., 2006; Guertin et al., 2006). homolog; and other upstream components encoded by All of these results provide strong support for the insulin-like (ins-1) and insulin/IGF1 receptor (daf-2) mTORC2 to be PDK2. Still, they do not preclude that genes (Ogg and Ruvkun, 1998; Paradis et al., 1999; other candidate kinases for PDK2 (or yet to be Pierce et al., 2001). These findings, by complementing uncovered kinases) could have functions in Akt activa- experimental evidence in mammalian cells, underscore a tion under certain conditions. Bozulic et al. (2008) have pivotal and conserved function of Akt. provided evidence that DNA-PK mediates Akt phos- phorylation at the HM site after induction of DNA double-strand break ends. Similarly, the ability of ILK1 Mouse models of altered Akt function to function as a protein kinase is still controversial, as it has been noted that ILK1 is missing critical residues that Selective disruption of Akt genes in the mouse germ line are found to be essential for catalysing phosphotransfer results in isoform-deficient mice with specific pheno- reactions in other kinases (Lynch et al., 1999). However, types: Akt1(À/À) mice show placental hypotrophy, a possible answer to the conundrum surrounding the accompanied by retardation of growth and reduction function of ILK1 in Akt regulation may be found in of body weight (Chen et al., 2001; Cho et al., 2001b); data from the Dedhar laboratory in which ILK1 was Akt2(À/À) mice become hyperinsulemic and hyper- shown to interact with mTORC2 (McDonald et al., glycemic (Cho et al., 2001a); and Akt3(À/À) mice exhibit 2008). Using pharmacological inhibitors and siRNA- reduced brain sizes (Easton et al., 2005; Tschopp et al., based genetic manipulations, the authors of this study 2005). The absence of full functional compensation, show that interaction between rictor and ILK1 is according to which not all Akt isoforms perform required for promoting Akt phosphorylation at the equally, is also suggested by studies in mouse embryonic HM site. fibroblasts from different strains of Akt-deficient mice, The molecular mechanisms of Akt regulation are in which the lack of specific Akt isoforms exerts distinct summarized in Figure 1. cellular phenotypes (Yun et al., 2008). Still, the fact that all singular isoform knockout mutants are viable with only subtle differences in phenotype indicates the Genetic models of Akt signaling capacity of the three Akt isoforms to compensate for each other. In support of this idea, the phenotype of In addition to genetic implications in human disease, the Akt1/Akt2 double-null mice shows severe dwarfism, second argument in strong support of the biological atrophy of multiple organ systems including skin and relevance of the Akt signaling cascade arises from skeletal muscle, failed adipogenesis and results in early findings of its evolutionary conservation that extend to neonatal lethality (Peng et al., 2003). In spite of all the downstream components and their interdependence enumerated defects, Akt1/Akt2 double-null mice devel- within the signaling pathway. Structural homologs of op normally during embryogenesis and no significant Akt kinases have been identified in all vertebrate apoptosis is observed. Akt2/Akt3 double-null mice are

Oncogene Akt signaling in animal physiology and human disease TF Franke 6477

Figure 1 Canonical schematic depicting the present state of our understanding of Akt activation and regulation of downstream biological responses. Autophosphorylation of RTKs induces the recruitment of p85 regulatory subunits leading to PI3K activation. Once activated, p110 catalytic subunits phosphorylate plasma membrane-bound phosphoinositides (PI-4-P and PI-4,5-P2) on the D3- position of their inositol rings. The second messengers resulting from this PI3K-dependent reaction are PI-3,4-P2 and PI-3,4,5-P3 (also called PIP3). PIP3, in turn, is the substrate for the phosphoinositide 3-phosphatase PTEN, an endogenous inhibitor of PI3K signaling in cells. The phosphoinositide products of PI3K form high-affinity binding sites for the PH domains of intracellular molecules. PDK1 and Akt are two of the many targets of PI3K products in cells. Following binding of the Akt PH domain to PI3K products, Akt is phosphorylated by PDK1 on a critical threonine residue in its kinase domain. mTORC2 is the main kinase activity that through phosphorylation of a C-terminal HM serine residue locks Akt enzyme into an active conformation. Other kinases such as DNA-PK and ILK1 are also capable of phosphorylating Akt at the HM site but may do so in a cell- or context-dependent manner. Akt activation is blunted by phosphatases including PP2A and PHLPP that inhibit Akt activity by dephosphorylation. Studies examining Akt-interacting proteins such as CTMP or second messengers such as Ins(1,3,4,5)P4 suggest that this common pathway of Akt regulation may be further specified within certain functional contexts or during development. Once activated, Akt activation is channeled into a plethora of downstream biological responses reaching from angiogenesis, cell survival, proliferation, translation to metabolism. viable but display impaired glucose homeostasis and tance, tumor formation and increased autoimmunity in growth deficiencies (Dummler et al., 2006). Future aged transgenic animals (Jones et al., 2000; Malstrom studies will determine which mechanisms compensate et al., 2001). Studies using overexpression of activated for the loss of Akt function. Candidate kinases to Akt1 in mouse mammary glands or neural progenitor substitute for Akt function exist: kinases regulate similar cells have observed increased tumor formation, but only substrates [such as serum- and glucocorticoid-induced when Akt1-transgenic animals were crossed with ani- protein kinase (SGK)] (Brunet et al., 2001); others are mals carrying activated alleles of v-Ki-ras2 Kirsten rat even activated by similar factors [such as cytokine- sarcoma viral oncogene homolog (Kras) or mutated independent survival kinase (CISK)/SGK3] (Liu et al., middle T oncoprotein (Holland et al., 2000; Hutchinson 2000). et al., 2001). In the mammary model, apoptosis during Transgenic studies have expressed mutant Akt from the involution of the mammary gland after weaning of tissue-specific promoter systems. Studies using activated pups was suppressed (Hutchinson et al., 2001). Other Akt1 in T-cells have found increased apoptosis resis- groups have studied the importance of Akt in ovarian

Oncogene Akt signaling in animal physiology and human disease TF Franke 6478 tumor models (Orsulic et al., 2002). Prostate-restricted partners such as keratin-10 (Paramio et al., 2001), the activation of Akt in mice is sufficient to induce cell binding sites on Akt have yet to be characterized. proliferation (prostatic intraepithelial neoplasia) and Thus far, the importance of Akt binding proteins results in phenotypic changes (bladder obstruction) was mainly defined under pathological conditions. For (Majumder et al., 2003). Taken together, data from example, overexpression of Tcl-1 in human T-cell genetic models suggest that Akt1 expression by itself prolymphocytic leukemia contributes to pathogenesis does not promote oncogenic transformation, even in by facilitating Akt activation and altering its intracel- systems where PI3K overexpression causes transforma- lular localization (for review, see Noguchi et al., 2007). tion (Aoki et al., 1998; Zhao et al., 2003). Instead, cell However, it is an attractive idea that interactors of Akt size is found to be frequently increased. indeed may guide Akt activity and specificity under physiological conditions. In support of the physiological importance of endogenous competitors for PI3K second Interacting proteins that regulate Akt activity and messenger binding to the Akt PH domain, Jia et al. substrate specificity (2007) have demonstrated that inositol 1,3,4,5-tetraki- sphosphate [Ins(1,3,4,5)P4] is a specific functional The findings of nonredundancy in Akt isoforms suggest inhibitor of PI3K-dependent signaling in neutrophils. additional mechanisms that fine-tune the participation In this paradigm, Ins(1,3,4,5)P4 is generated as a result of Akt in its diverse biological functions. Mainly by of Ins(1,4,5)P3 kinase activation after chemoattractant using yeast two-hybrid screens, numerous proteins have stimulation and competes for binding to PI3K products, been identified that interact with one or more domains thus blocking Akt activation. In addition, a study from of Akt and one or more of its isoforms (for review, see the Molling laboratory has shown that expression of the Brazil et al., 2002; Du and Tsichlis, 2005; Franke, 2008). Akt-interacting protein WD-repeat propeller-FYVE Akt-interaction proteins include several enzymes: PDK1 (ProF) is upregulated during adipocyte differentiation, (Balendran et al., 1999), inosine-50 monophosphate at which point it increases interaction of Akt with the dehydrogenase (Ingley and Hemmings, 2000), histone forkhead-related transcription factor FoxO1 (Fritzius H3 methyltransferase SETDB1 (Gao et al., 2007), and Moelling, 2008). Similarly, Hao et al. (2008) have Src (Jiang and Qiu, 2003) and both known isoforms provided evidence that JIP1 can act as a functional of the PH domain leucine-rich repeat protein phospha- bridge between Akt and the guanine nucleotide ex- tase family (PHLPP) (Gao et al., 2005; Brognard change factor RalGDS, which in turn may determine the et al., 2007). Other Akt-interacting proteins lack signal strength of PDK1 toward Akt downstream of discernible intrinsic enzymatic activity. This shared Ras. And finally, at least in zebrafish, members of the property suggests that their interaction with Akt iso- APPL protein family modulate signaling specificity of forms affects enzymatic activity, access to upstream the signal network downstream of Akt during develop- regulatory components, or intracellular localization and ment (Schenck et al., 2008). In these experiments, distribution. Thus, consequences of binding for Akt depletion of Appl1 by morpholino knockdown in function are diverse and may differ depending on cell zebrafish selectively decreases the phosphorylation of types and experimental conditions (for review, see glycogen synthase kinase 3 (GSK3)-b but not tuberous Franke, 2007). sclerosis protein-2 (TSC2). Akt-interacting proteins bind to different functional domains. PH domain-interacting proteins include the Tcl1 oncogene and its related family members (for Substrate phosphorylation by Akt review, see Noguchi et al., 2007), Ras GTPase-activating protein (RasGAP) (Yue et al., 2004), c-Jun N-terminal Consequences of Akt activation include diverse biolo- kinase (JNK) interacting protein 1 (JIP1) (Kim et al., gical responses, ranging from primarily metabolic 2002, 2003), growth factor receptor-binding protein-10 functions such as glucose transport, glycolysis, glycogen (Grb10) (Jahn et al., 2002), casein kinase 2-interacting synthesis and the suppression of gluconeogenesis to protein-1 (Tokuda et al., 2007) and Ca2 þ /calmodulin protein synthesis, increased cell size, cell-cycle progres- (Dong et al., 2007). Heat shock protein 90 (HSP90) sion and apoptosis suppression. (Basso et al., 2002), tribbles homolog 3 (TRB3) Insights into the molecular consequences of increased (Du et al., 2003), adaptor protein containing PH Akt activation were derived from seminal studies that domain, PTB domain and leucine zipper motif (APPL) ultimately identified the ‘orphan’ proto-oncogene as an (Mitsuuchi et al., 1999) and the BTB/POZ domain- obligate intermediate downstream of PI3K in the containing protein BTBD10 (Nawa et al., 2008) interact insulin-dependent metabolic control of glycogen synth- with its kinase domain. Examples of proteins interacting esis. When searching for kinases that could regulate with its C-terminus include C-terminal modulator GSK3, the groups of Brian Hemmings and Phil Cohen protein (CTMP) (Maira et al., 2001), Akt phosphoryla- realized that Akt inhibited GSK3 activity in an insulin- tion enhancer (APE) (Anai et al., 2005), Arg-binding stimulated and PI3K-dependent manner by direct protein 2 (ArgBP2)-g (Yuan et al., 2005) and prohibitin phosphorylation of an N-terminal regulatory serine 2 (Sun et al., 2004). The JNK adaptor Plenty of SH3s residue (Cross et al., 1995). By systematically permutat- (POSH) binds to both the N- and C-terminus of Akt ing the amino-acid sequence surrounding the Akt (Figueroa et al., 2003). Finally, for other Akt-interacting phosphorylation site in GSK3, Alessi et al. (1996b)

Oncogene Akt signaling in animal physiology and human disease TF Franke 6479 derived an optimal peptide sequence for Akt phosphor- surprisingly, other components of the post-mitochon- ylation (R-X-R-X-X-S/T; where R is an arginine drial machinery such as the X-linked inhibitor of residue, S is serine, T is threonine and X is any amino apoptotic proteins (XIAP) have been suggested as acid). This Akt consensus motif is a common feature of potential Akt substrates (Dan et al., 2004). known substrates of Akt, and its presence predicts reasonably well whether a given protein may be phosphorylated by Akt enzyme in vitro (for review, see Akt-dependent inhibition of stress-regulated kinase Manning and Cantley, 2007). Experiments using rando- cascades mized permutations on the basis of the motif to optimize substrate peptides have defined the requirement for Another important class of Akt targets are proteins optimal phosphorylation by Akt further (Obata et al., involved in the stress-activated/mitogen activated pro- 2000). The preferred phosphoacceptor for Akt-depen- tein kinase (SAPK/MAPK) cascades. Growing experi- dent phosphorylation is a serine residue, but a synthetic mental evidence points to a close functional relationship substrate peptide with a threonine residue as the between the Akt survival pathway and SAPK/MAPK phosphoacceptor instead (R-P-R-A-A-T-F; P ¼ proline, cascades that are activated by various cellular stresses A ¼ alanine, F ¼ phenylalanine) is also easily phos- and are linked to apoptosis. Increased Akt activity has phorylated. For achieving optimal phosphorylation been shown to suppress the JNK and p38 pathways efficiency, the phosphoacceptor is best followed by a (Berra et al., 1998; Cerezo et al., 1998; Okubo et al., hydrophobic residue with a large side-chain in the p þ 1 1998). It has been shown that apoptosis signal-regulat- position, and preceded by a serine or threonine at the ing kinase 1 (ASK1) is regulated by Akt and contains an pÀ2 position. Akt-specific phosphorylation site (Kim et al., 2001). These findings have been confirmed independently by other groups (Yuan et al., 2003; Mabuchi et al., 2004). Targets of Akt with implications in apoptosis suppression Thus, ASK1 is likely to be one of the points of convergence between PI3K/Akt signaling and stress- One of the first targets of Akt to be identified that has activated kinase cascades, although probably not the direct implications for regulating cell survival is the pro- only one. Akt also phosphorylates the small G protein apoptotic BCL2-antagonist of death (BAD) protein. Rac1 (Kwon et al., 2000), the MAP2K stress-activated BAD regulation by Akt has exemplified the molecular protein kinase kinase-1 (SEK1; also known as JNKK1 pathways linking survival factor signaling to apoptosis or MKK4) (Park et al., 2002) and the MAP3K mixed suppression (for review, see Franke and Cantley, 1997). lineage kinase 3 (MLK3) (Barthwal et al., 2003; When BAD is not phosphorylated, it will inhibit Bcl-xL Figueroa et al., 2003). Using yeast-2-hybrid screens to and other anti-apoptotic Bcl-2 family members by direct identify interacting partners for Akt kinases, Figueroa binding of its Bcl-2 homology domain to their hydro- et al. (2003) found binding of Akt2 to the JNK adaptor phobic grooves (Gajewski and Thompson, 1996). Once POSH. These authors showed that the binding of Akt2 phosphorylated, these phospho-serine residues of BAD to POSH results in an inhibition of JNK activity, and form high-affinity binding sites for cytoplasmic 14-3-3 that this inhibition is mediated by phosphorylation of molecules. As a result, phosphorylated BAD is retained the upstream kinase MLK3 and leads to the disassembly in the cytosol where its pro-apoptotic activity is of the JNK signaling complex. In turn, POSH is also an effectively neutralized (Zha et al., 1996). The importance Akt substrate (Lyons et al., 2007). Taken together, these of BAD as an integration point of survival signaling is findings point to an intriguing model for the regulation underscored by the fact that it is a substrate for multiple of the JNK pathway by Akt, in which the Akt- independent kinase pathways in cells, not all of which dependent phosphorylation of specific components can phosphorylate BAD at the same site(s) as Akt (Datta block signal transduction through the stress-regulated et al., 2000). The mechanisms of 14-3-3-dependent kinase cascade. In spite of this, it has been reported that regulation of BAD function hereby resemble the Akt- Akt also blocks the pro-apoptotic activity of other dependent inhibition of FoxO transcription factors MAP3Ks such as MLK1 and MLK2 that act in parallel that regulate the transcription of pro-apoptotic genes to MLK3 but do not contain a typical Akt consensus (Brunet et al., 1999). phosphorylation motif (Xu et al., 2001). Thus, phos- The function of Akt extends beyond maintaining phorylation-based mechanisms may be limited in mitochondrial integrity to keep cytochrome c and other explaining the role of Akt in blocking JNK signaling. apoptogenic factors in the mitochondria (Kennedy et al., 1999). Akt activity also mitigates the response of cells to the release of cytochrome c into the cytoplasm. Akt substrates with metabolic functions Although caspase-9 is an Akt substrate in human cells, where it may explain cytochrome c resistance (Cardone Several Akt-dependent mechanisms mediate the regula- et al., 1998), it may not be the only, or even the most tion of insulin-dependent glucose homeostasis down- important, target because Akt-dependent cytochrome c stream of PI3K. These include the insulin-mediated resistance can be observed in animal species where increase in glycogen synthesis and suppression of caspase-9 lacks a potential Akt phosphorylation glycogenolysis through the Akt-dependent phosphoryla- site (Fujita et al., 1999; Zhou et al., 2000). Not tion and inactivation of GSK3. Nonphosphorylated

Oncogene Akt signaling in animal physiology and human disease TF Franke 6480 GSK3 is catalytically active and inhibits the activity of tenance of the cellular machinery, the production and glycogen synthase, which is the rate-limiting enzyme in availability of nutrients, and also other building blocks glycogen synthesis through a phosphorylation-dependent needed for basic cellular functions (for review, see Plas mechanism (Cross et al., 1995). In the liver, suppression et al., 2002). of hepatic glucose production and glycogenolysis is further enhanced by the insulin-mimicking peripheral effects of leptin. Leptin induces Akt activity in vitro and Mechanisms of Akt-dependent cell-cycle regulation in vivo in pathways parallel to those utilized in insulin signaling (Kim et al., 2000; Szanto and Kahn, 2000). It The regulation of the cell cycle by Akt is well antagonizes the increase of cAMP levels by glucagon, established. Akt activity affects at least two key cell- presumably by inducing the Akt-dependent phosphoryla- cycle regulator proteins: p21Cip1/WAF1 and p27Kip1. tion of phosphodiesterase 3B (Zhao et al., 2000). p21Cip1/WAF1 is stabilized after activation of the Akt Other metabolic effects of Akt activation include the pathway by direct phosphorylation and also after Akt- phosphorylation and activation of 6-phosphofructo-2- dependent inhibition of GSK3 (Rossig et al., 2001, 2002; kinase (Deprez et al., 1997). In addition, Akt activation Li et al., 2002). Furthermore, Zhou et al. (2001) showed has been shown to cause the translocation of hexokinase that the phosphorylation of p21Cip1/WAF1 by Akt inhibits to the mitochondria for the purpose of eliciting the its interaction with proliferating cell nuclear antigen capture of intracellular glucose by phosphorylation (PCNA) and promotes the assembly of the cyclin (Gottlob et al., 2001). Furthermore, Akt has been D1:cyclin-dependent kinase 4 (CDK4) complex. Akt implicated in increased lipogenesis through the phos- also phosphorylates p27Kip1, which in turn is a target phorylation of ATP-citrate lyase (Berwick et al., 2002). gene of FoxO transcription factors. Phosphorylation of Finally, in addition to modifying glucose metabolism, p27Kip1 induces cytoplasmic localization and inhibition, Akt activity increases the transcription of the glucose similar to the regulation of p21Cip1/WAF1 (Liang et al., transporters GLUT1 (Barthel et al., 1999) and the 2002; Shin et al., 2002). translocation of GLUT4 to the plasma membrane in Additional evidence for the involvement of Akt in the muscle and fat cells (Kohn et al., 1996), thus increasing regulation of cell proliferation is found at the interface the uptake of glucose into cells. Other substrates of Akt between p53 tumor suppressor signaling and Akt with a possible involvement in metabolic regulation activity. Akt phosphorylates the ubiquitin ligase mouse include Akt substrate of 160 kDa (AS160), which may double minute homolog 2 (Mdm2) and its human mediate the regulation of GLUT4 translocation by Akt homolog HDM2 that are induced by p53 and block (Sano et al., 2003), phosphoinositide kinase for five p53 function. Two proposed mechanisms of Akt- position containing a Fyve finger (PIKfyve) (Berwick dependent regulation of Mdm2 include the Akt-depen- et al., 2004) and syntaxin binding protein 4 (STXBP4/ dent increase of Mdm2’s nuclear import (Mayo and synip) (Yamada et al., 2005). Donner, 2001) and its intrinsic ubiquitin ligase activity Although many of its substrates are involved in clearly (Zhou et al., 2001; Ashcroft et al., 2002; Ogawara et al., defined biological functions within a circumscribed 2002). There is also cross talk between PTEN and p53. context such as cell proliferation, a more thorough PTEN not only regulates p53 protein levels through Akt analysis of Akt signaling has suggested that the bound- and Mdm2, but also directly interacts with p53 and aries between metabolic processes and apoptosis suppres- enhances its transcriptional activity (Freeman et al., sion may be artificial. For example, the Akt target GSK3 2003). Conversely, on the basis of the finding of a p53 has been implicated both in the regulation of glucose binding site within the PTEN promoter, Stambolic et al. metabolism and cell survival (Pap and Cooper, 1998). (2001) have presented evidence for the regulation of These findings suggest that the distinctions between cell PTEN transcription by p53. growth, survival, metabolism and apoptosis regulation do not properly reflect functional interactions between concurrent biological processes in cells. This shift in mTOR and Akt: increasing cell size matters perception has been fueled by studies from the Korsmeyer laboratory that have demonstrated a canonical function Although Akt promotes cell proliferation by regulating for the pro-apoptotic Bcl-2 family member BAD in the specific targets related to the cell cycle machinery and to regulation of glucokinase activity (Danial et al., 2003). It DNA damage-dependent cell responses, a conserved is conceivable that findings of PKA-dependent regulation function of PI3K/Akt signal transduction is found in of BAD in glucose metabolism can be extrapolated to regulating cell volume (for review, see Franke et al., BAD inhibition by Akt. Still, a formal confirmation for 2003). For example, expression of constitutively active a role of Akt in this process has yet to be presented mutants or dominant-negative mutants specifically in (for review, see Downward, 2003). cardiomyocytes of transgenic mice results in enlarged or Overall, the efforts in attributing metabolic functions reduced heart sizes, respectively (Shioi et al., 2000, with implications for cell viability to Akt have been 2002). Conversely, cell volumes are decreased in Akt1 limited, as many of the subtle effects of Akt on and Akt3 knockout mice (Cho et al., 2001b; Easton cellular homeostasis may be difficult to detect (Gottlob et al., 2005). et al., 2001; Plas et al., 2001). Still, these functions of The main effector of Akt in the context of cell growth Akt may be of fundamental relevance for the main- pathway is the product of the tuberous sclerosis (TSC)

Oncogene Akt signaling in animal physiology and human disease TF Franke 6481 gene TSC2. Reduced TSC2 (also called tuberin) together related proteins huntingtin (Humbert et al., 2002) and with loss of TSC1 (also known as hamartin) is ataxin (Chen et al., 2003). frequently found in TSC, an autosomal dominant tumor Other studies suggest that the involvement of Akt in disease affecting 1 in 6000 individuals. TSC2 phosphor- brain function extends beyond the protection of ylation by Akt induces the activity of mTORC1 by neuronal cells against apoptotic insults. Indeed, patho- relieving Ras enriched in brain (Rheb) GTPase from the logical changes in Akt signal transduction have been inhibition of the TSC1/TSC2 complex (Tee et al., 2003). described that are associated with mental diseases. A second binding partner of mTOR and also a substrate Significantly decreased Akt1 expression has been re- for Akt is proline-rich Akt substrate 40 kDa (PRAS40). ported in patients suffering from familial schizophrenia Underphosphorylated PRAS40 binds raptor and (Emamian et al., 2004). Decreased Akt1 levels are inhibits mTOR kinase activity (Sancak et al., 2007; correlated with increased GSK3 activity, presumably Wang et al., 2007). As a result, ribosomal protein kinase because of the lack of the Akt-dependent inhibitory p70S6k and eukaryotic translation initiation factor 4E input on GSK3. In support of AKT1 being a suscept- binding protein 1 (EIF4EBP1) are phosphorylated by ibility gene for schizophrenia, Akt1(À/À) mice exhibit mTOR (Inoki et al., 2002; Tee et al., 2002), thus leading increased sensitivity to the sensorimotor disruptive to increased translation of existing mRNA transcripts effect of amphetamine, which is partly reversed by the (for review, see Mamane et al., 2006). treatment of mutant mice with the antipsychotic drug The dual involvement of mTOR in mTORC1 as an haloperidol (Emamian et al., 2004). Additional support effector downstream of Akt and also as an upstream for a contribution of impaired Akt signaling in the regulatory kinase when in mTORC2 suggests the pathogenesis of schizophrenia derives from the finding evolution of a tightly regulated feedback loop to of mutant PI3K signaling in schizophrenia (for review, coordinate growth and nutrient utilization. In addition see Arnold et al., 2005). A direct involvement of Akt in to regulating Akt phosphorylation, mTOR attenuates dopaminergic action is indicated by the observation that PI3K activity by phosphorylation of the insulin-receptor Akt1(À/À) mutant mice exhibit a behavioral phenotype substrate IRS1 (for review, see Harrington et al., 2005; resembling enhanced dopaminergic transmitter function Manning, 2004). This may explain in part why TSC is (Emamian et al., 2004). By interacting with the GSK3 characterized by an increase in cell size but rarely leads pathway, Akt modulates the suppression of dopamine to the kind of highly malignant tumors that are observed (DA)-associated behaviors after treatment with the after PTEN loss of function (for review, see Cheadle mood stabilizer lithium (Beaulieu et al., 2004). et al., 2000). Additional data showing that chronic Furthermore, a b-arrestin 2-mediated kinase/phospha- treatment with rapamycin reduces Akt phosphorylation, tase scaffold of Akt and protein phosphatase A (PP2A) presumably by soaking up mTOR in mTORC2, offer is required for the regulation of Akt downstream of DA promising routes for therapeutic intervention in cancers receptors (Beaulieu et al., 2005). Still, the role of Akt in caused by hyperactivation of Akt (Sarbassov et al., dopaminergic responses by far exceeds actions down- 2006; Zeng et al., 2007). In spite of this, the observation stream of DA receptors: the insulin-dependent regula- that certain tumors respond to rapamycin with increased tion of DA transporter also depends on Akt activity Akt signaling and a more aggressive phenotype (for (Garcia et al., 2005). review, see Easton et al., 2006) indicates the need to Since the field of Akt signaling in psychiatric disorders develop mTOR inhibitors that act on sites other than is still emerging, it may be too early to speculate about the rapamycin binding site (Sarbassov et al., 2006). the molecular involvement of Akt in regulating higher brain function. Possible functional outlets of Akt include some of the substrates mentioned above, Beyond cancer: PI3K/Akt signaling in higher brain including mTORC1 and GSK3. In addition, substrates function of Akt related to synaptic plasticity and transmission have been described. One such novel substrate of Akt The critical importance of Akt signaling for neuronal related to neuronal excitability is the b2-subunit of the function is implied from several lines of in vitro evidence type A g-aminobutyric acid receptor (GABAA-R) (Lin using neuronal cell lines and dispersed primary neuronal et al., 2001). In support of a direct involvement of Akt in cultures that have demonstrated a requirement for Akt synaptic function, studies directed at working memory in the protection against trophic factor deprivation, performance performed in Akt1(À/À) mice (Lai et al., oxidative stress and ischemic injury (Dudek et al., 1997; 2006) and in healthy individuals carrying the AKT1 Salinas et al., 2001; Noshita et al., 2002). Dysregulation coding variation observed in familial schizophrenia (Tan of Akt activity is observed in neurodegenerative diseases et al., 2008) find a strong correlation with cognitive including Alzheimer’s disease (Rickle et al., 2004; Ryder performance. Additional roles for Akt in higher brain et al., 2004), Parkinson’s disease (Hashimoto et al., function are suggested by studies from the Nestler 2004) and Huntington’s disease (Humbert et al., 2002), laboratory that have explored the IRS2-Akt pathway and it is also associated with the pathobiological during the development of tolerance to opiate reward mechanisms underlying spinocerebellar ataxia (Chen (Russo et al., 2007). By using viral-mediated gene et al., 2003). A mechanistic involvement of impaired Akt transfer to express mutant Akt in midbrain neurons, signaling in neurodegeneration is further supported by these authors demonstrate that downregulation of the Akt-dependent phosphorylation of the disease- the IRS2-Akt pathway mediates morphine-induced

Oncogene Akt signaling in animal physiology and human disease TF Franke 6482

Figure 2 Akt kinase regulates diverse aspects of neuronal cell function. Akt activation in neuronal cells follows similar mechanisms to those outlined in Figure 1, including activation of PI3K by RTKs including IGF-1/insulin and nerve growth factor (BDNF/NT-3) receptors. Other mechanisms governing Akt activity in neuronal cells include G-coupled receptors for the monoamine neurotransmitters serotonin (5-HT) (for review, see Raymond et al., 2001) and dopamine (DA) (for review, see Beaulieu et al., 2007). Depending on receptor type (D2-DA and 5-HT1A receptors vs D1-DA receptors), binding of 5-HT or DA decreases or increases the activity of adenylyl cyclase (AC), respectively. Changes in cAMP second messenger levels, in turn, alter PKA and PP2A activity. PP2A is inhibited by increased PKA activity, thus maintaining Akt in an activated state after 5-HT1A receptor simulation (Hsiung et al., 2008). After binding of DA to D2-DA receptor, following initial inhibition of AC, a secondary internalization complex is formed between b-arrestin 2, PP2A and Akt leading to the inhibition of Akt. In neuronal cells, activated Akt regulates diverse targets that have been implicated in the regulation of protein translation and cell size (mTORC1), axonal outgrowth (GSK3), apoptosis suppression (BAD) and synaptic plasticity (GABAA-R). Details regarding functional consequences of Akt regulation for higher brain function are discussed in the text.

decreases in cell size of DA neurons in brain regions that The emerging involvement of Akt in higher brain are critically involved in the reward circuitry and function is summarized in Figure 2. affected in individuals addicted to drugs of abuse. Finally, TSC patients show an increased incidence of autism spectrum disorders (ASD) ranging from 25 to Conclusions 50% (for review, see Wiznitzer, 2004). Individuals with macrocephaly due to Lhermitte–Duclos disease When considering the present understanding of all the are prone to ASD and show a pronounced incidence signals leading to and from Akt, we face a growing of mutations in the PTEN tumor suppressor gene complexity that is in part compounded by the intersec- (Butler et al., 2005). Additional experimental support tion of multiple signaling cascades. Many substrates of for a possible involvement of PTEN/Akt in ASD is Akt are shared with other kinases that have similar provided by data from the Parada laboratory examining specificities. Moreover, signals originating from acti- the morphology and behavior of mutant mice with vated Akt do not simply lead to changes in the neuron-specific knockout of PTEN (Kwon et al., biological activity of specific downstream substrates, 2006). Future studies will be required to clarify the but affect entire signaling networks. In spite of this, function of Akt in cognition and characterize the there is hope that there is order to the far-reaching underlying molecular mechanisms. In spite of this, physiological involvement of Akt. One possibility is these initial studies suggest a complex function of that differential regulation of the binding partners of Akt in conditions affecting brain function and Akt may determine cell- and context-specific signaling mental health. by Akt. Studies are now needed to elucidate the

Oncogene Akt signaling in animal physiology and human disease TF Franke 6483 physiological functions of the binding partners of Akt in Akt, exchange their findings and ideas. Today, tens of mammalian physiology. thousands of papers on Akt have been published, with A second challenge that the field is facing arises from the rate of publications still increasing. To organize this the involvement of Akt in multiple areas of physiology. information in a comprehensive manner and expand the These now exceed cancer and diabetes and, as briefly field beyond the confines of specific research fields has outlined above, include higher brain functions related to become a difficult Akt to follow. cognition. This far-reaching involvement poses impor- tant questions to researchers trying to find inhibitors of Akt signaling in cancer therapy. Indeed, psychiatrists Acknowledgements may want to boost Akt signaling in patients suffering from schizophrenia. Most likely, tissue-specific solutions Work on the Akt signaling cascade in Dr Franke’s laboratory can be applied. However, the plethora of Akt-dependent is supported by the National Science Foundation (NSF award IOS-0757780), the National Institutes of Health (grant biological responses may also require new approaches in RO1MH079439), Susan G Komen for the Cure and the G the scientific community when examining Akt. Thus, Harold & Leila Y Mathers Foundation. Ithank Alexander rather than examining the Akt pathway within a specific Broad and Andrew Sideris for assistance with the Akt field, it may be necessary that researchers from different signaling schematics, and Terry McIntosh for diligent subject areas, but all interested in questions related to production of this manuscript.

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