The akt kinase: molecular determinants of oncogenicity

PDK2: a complex tail in one Akt

The kinase Akt contains two phosphatidylinositol-3 kinase (PI3K)-dependent phosphorylation sites, one in the activation loop (Thr(308)) and one in the carboxyl-terminal tail (Ser(473)), both of which are conserved among the members of the AGC kinase family. Under physiological conditions, the phosphorylation of Thr(308) appears to be coordinately regulated with the phosphorylation of Ser(473). Under experimental conditions, however, the two sites can be uncoupled, suggesting that their phosphorylation is controlled by different kinases and phosphatases. Phosphoinositide-dependent kinase 1 (PDK1), the kinase that phosphorylates the activation loop site, has been unambiguously identified. However, PDK2, a kinase that is hypothesized to phosphorylate the hydrophobic carboxyl-terminal site, remains elusive. This Perspective examines the regulation and biological significance of Akt phosphorylation at Ser(473). The authors propose that Ser(473) undergoes both autophosphorylation and phosphorylation by other kinases. Both events may be promoted by interactions between PDK1 and phosphorylated or phosphomimetically altered hydrophobic phosphorylation motifs in kinases associated with Akt. These interactions may induce conformational changes in Akt that make Ser(473) accessible to phosphorylation.

A role of the kinase mTOR in cellular transformation induced by the oncoproteins P3k and Akt

The oncoproteins P3k (homolog of the catalytic subunit of class IA phosphoinositide 3-kinase) and Akt (protein kinase B) induce oncogenic transformation of chicken embryo fibroblasts. The transformed cells show constitutive phosphorylation of the positive regulator of translation p70S6 kinase (S6K) and of the eukaryotic initiation factor 4E-BP1 binding protein (4E-BP1), a negative regulator of translation. Phosphorylation activates S6K and inactivates 4E-BP1. A mutant of Akt that retains kinase activity but does not induce phosphorylation of S6K or of 4E-BP1 fails to transform chicken embryo fibroblasts, suggesting a correlation between the oncogenicity of Akt and phosphorylation of S6K and 4E-BP1. The macrolide antibiotic rapamycin effectively blocks oncogenic transformation induced by either P3k or Akt but does not reduce the transforming activity of 11 other oncoproteins. Rapamycin inhibits the kinase mTOR, an important regulator of translation, and this inhibition requires binding of the antibiotic to the immunophilin FKBP12. Displacement of rapamycin from FKBP12 relieves the inhibition of mTOR and also restores P3k-induced transformation. These data are in accord with the hypothesis that transformation by P3k or Akt involves intervention in translational controls.

A role of the kinase mTOR in cellular transformation induced by the oncoproteins P3k and Akt

The oncoproteins P3k (homolog of the catalytic subunit of class IA phosphoinositide 3-kinase) and Akt (protein kinase B) induce oncogenic transformation of chicken embryo fibroblasts. The transformed cells show constitutive phosphorylation of the positive regulator of translation p70S6 kinase (S6K) and of the eukaryotic initiation factor 4E-BP1 binding protein (4E-BP1), a negative regulator of translation. Phosphorylation activates S6K and inactivates 4E-BP1. A mutant of Akt that retains kinase activity but does not induce phosphorylation of S6K or of 4E-BP1 fails to transform chicken embryo fibroblasts, suggesting a correlation between the oncogenicity of Akt and phosphorylation of S6K and 4E-BP1. The macrolide antibiotic rapamycin effectively blocks oncogenic transformation induced by either P3k or Akt but does not reduce the transforming activity of 11 other oncoproteins. Rapamycin inhibits the kinase mTOR, an important regulator of translation, and this inhibition requires binding of the antibiotic to the immunophilin FKBP12. Displacement of rapamycin from FKBP12 relieves the inhibition of mTOR and also restores P3k-induced transformation. These data are in accord with the hypothesis that transformation by P3k or Akt involves intervention in translational controls.

Forkhead transcription factors are critical effectors of cell death and cell cycle arrest downstream of PTEN

PTEN acts as a tumor suppressor, at least in part, by antagonizing phosphoinositide 3-kinase (PI3K)/Akt signaling. Here we show that Forkhead transcription factors FKHRL1 and FKHR, substrates of the Akt kinase, are aberrantly localized to the cytoplasm and cannot activate transcription in PTEN-deficient cells. Restoration of PTEN function restores FKHR to the nucleus and restores transcriptional activation. Expression of a constitutively active form of FKHR that cannot be phosphorylated by Akt produces the same effect as reconstitution of PTEN on PTEN-deficient tumor cells. Specifically, activated FKHR induces apoptosis in cells that undergo PTEN-mediated cell death and induces G(1) arrest in cells that undergo PTEN-mediated cell cycle arrest. Furthermore, both PTEN and constitutively active FKHR induce p27(KIP1) protein but not p21. These data suggest that Forkhead transcription factors are critical effectors of PTEN-mediated tumor suppression.

Distinct roles for PI3K in proliferation and survival of oligodendrocyte progenitor cells

Phosphoinositol 3-kinase (PI3K) is a downstream effector for multiple ligand-activated receptors and modulates cell responses through activation of its target protein kinase B (Akt). We examined the roles of PI3K-Akt signaling in a primary glial (oligodendrocyte) progenitor cell culture system that is ligand-dependent for cell proliferation, survival, and prevention of differentiation. We demonstrate that PI3K and Akt (Ser-473 phosphorylation) are activated in response to platelet-derived growth factor but not basic fibroblast growth factor-2 (FGF2) and that distinct forms of PI3K are activated in early progenitors and later-maturation pro-oligodendroblasts as identified by their sensitivity to wortmannin. By establishing conditions to examine effects on cell proliferation and survival independently, we demonstrate that PI3K is necessary for a full mitogenic response and that PI3K is also necessary for early progenitor survival. Our results therefore demonstrate that PI3K-Akt signaling independently regulates proliferation and survival, that the form of PI3K is distinct in early progenitors and pro-oligodendroblasts, and that FGF2 does not activate this pathway in either primary glial cell population.

ets-2 is a target for an akt (Protein kinase B)/jun N-terminal kinase signaling pathway in macrophages of motheaten-viable mutant mice

The transcription factor ets-2 was phosphorylated at residue threonine 72 in a colony-stimulating factor 1 (CSF-1)- and mitogen-activated protein kinase-independent manner in macrophages isolated from motheaten-viable (me-v) mice. The CSF-1 and ets-2 target genes coding for Bcl-x, urokinase plasminogen activator, and scavenger receptor were also expressed at high levels independent of CSF-1 addition to me-v cells. Akt (protein kinase B) was constitutively active in me-v macrophages, and an Akt immunoprecipitate catalyzed phosphorylation of ets-2 at threonine 72. The p54 isoform of c-jun N-terminal kinase-stress-activated kinase (JNK- SAPK) coimmunoprecipitated with Akt from me-v macrophages, and treatment of me-v cells with the specific phosphatidylinositol 3-kinase inhibitor LY294002 decreased cell survival, Akt and JNK kinase activities, ets-2 phosphorylation, and Bcl-x mRNA expression. Therefore, ets-2 is a target for phosphatidylinositol 3-kinase-Akt-JNK action, and the JNK p54 isoform is an ets-2 kinase in macrophages. Constitutive ets-2 activity may contribute to the pathology of me-v mice by increasing expression of genes like the Bcl-x gene that promote macrophage survival.

Oncogenic transformation by the FOX protein Qin requires DNA binding

Some functions of the Qin oncoprotein are not dependent on DNA binding. In order to test the requirement for DNA binding in Qin-induced oncogenic transformation, site directed mutations were introduced in the winged helix (WH) DNA binding domain of the Qin protein. In cellular Qin (c-Qin), the glycine at position 233 was either deleted or substituted with the amino acids aspartic acid, alanine, glutamic acid, asparagine, proline or lysine. The same position carries aspartic acid in the viral Qin protein (v-Qin). The adjacent residues, threonine 232 and lysine 234, were separately mutated to proline. Several additional amino acid substitutions believed to be involved in DNA contacts were introduced at the following c-Qin positions: asparagine 189, histidine 193, serine 196 or arginine 236. Most of the substitutions reduced DNA binding of Qin, one mutation, H193A, completely abolished DNA binding, and another mutation, T232P, increased DNA binding affinity. Mutant H193A failed to transform chicken embryo fibroblasts (CEF), all other mutants, even those showing minimal DNA binding, retained oncogenicity for CEF. The efficiencies of focus formation induced by these mutant proteins in cell culture were not significantly different from that of wild type. However, the rate of focus development and the size of foci induced by the Qin mutants were greater with strong DNA binders than with weak DNA binders. Transdominant negative constructs consisting of the winged helix domain of cQin or v-Qin interfered with focus formation induced by full length Qin proteins. These results suggest that DNA binding is a prerequisite for transformation by Qin, and strong DNA binding is related to accelerated transformation in CEF.

Restricted 12p amplification and RAS mutation in human germ cell tumors of the adult testis

Human testicular germ-cell tumors of young adults (TGCTs), both seminomas and nonseminomas, are characterized by 12p overrepresentation, mostly as isochromosomes, of which the biological and clinical significance is still unclear. A limited number of TGCTs has been identified with an additional high-level amplification of a restricted region of 12p including the K-RAS proto-oncogene. Here we show that the incidence of these restricted 12p amplifications is approximately 8% in primary TGCTs. Within a single cell formation of i(12p) and restricted 12p amplification is mutually exclusive. The borders of the amplicons cluster in short regions, and the amplicon was never found in the adjacent carcinoma in situ cells. Seminomas with the restricted 12p amplification virtually lacked apoptosis and the tumor cells showed prolonged in vitro survival like seminoma cells with a mutated RAS gene. However, no differences in proliferation index between these different groups of seminomas were found. Although patients with a seminoma containing a homogeneous restricted 12p amplification presented at a significantly younger age than those lacking it, the presence of a restricted 12p amplification/RAS mutation did not predict the stage of the disease at clinical presentation and the treatment response of primary seminomas. In 55 primary and metastatic tumors from 44 different patients who failed cisplatinum-based chemotherapy, the restricted 12p amplification and RAS mutations had the same incidence as in the consecutive series of responding patients. These data support the model that gain of 12p in TGCTs is related to invasive growth. It allows tumor cells, in particular those showing characteristics of early germ cells (ie, the seminoma cells), to survive outside their specific microenvironment. Overexpression of certain genes on 12p probably inhibits apoptosis in these tumor cells. However, the copy numbers of the restricted amplification of 12p and K-RAS mutations do not predict response to therapy and survival of the patients.

Induction of postmitotic neuroretina cell proliferation by distinct Ras downstream signaling pathways

Ras-induced cell transformation is mediated through distinct downstream signaling pathways, including Raf, Ral-GEFs-, and phosphatidylinositol 3-kinase (PI 3-kinase)-dependent pathways. In some cell types, strong activation of the Ras-Raf-MEK-extracellular signal-regulated kinase (ERK) cascade leads to cell cycle arrest rather than cell division. We previously reported that constitutive activation of this pathway induces sustained proliferation of primary cultures of postmitotic chicken neuroretina (NR) cells. We used this model system to investigate the respective contributions of Ras downstream signaling pathways in Ras-induced cell proliferation. Three RasV12 mutants (S35, G37, and C40) which differ by their ability to bind to Ras effectors (Raf, Ral-GEFs, and the p110 subunit of PI 3-kinase, respectively) were able to induce sustained NR cell proliferation, although none of these mutants was reported to transform NIH 3T3 cells. Furthermore, they all repressed the promoter of QR1, a neuroretina growth arrest-specific gene. Overexpression of B-Raf or activated versions of Ras effectors Rlf-CAAX and p110-CAAX also induced NR cell division. The mitogenic effect of the RasC40-PI 3-kinase pathway appears to involve Rac and RhoA GTPases but not the antiapoptotic Akt (protein kinase B) signaling. Division induced by RasG37-Rlf appears to be independent of Ral GTPase activation and presumably requires an unidentified mechanism. Activation of either Ras downstream pathway resulted in ERK activation, and coexpression of a dominant negative MEK mutant or mKsr-1 kinase domain strongly inhibited proliferation induced by the three Ras mutants or by their effectors. Similar effects were observed with dominant negative mutants of Rac and Rho. Thus, both the Raf-MEK-ERK and Rac-Rho pathways are absolutely required for Ras-induced NR cell division. Activation of these two pathways by the three distinct Ras downstream effectors possibly relies on an autocrine or paracrine loop, implicating endogenous Ras, since the mitogenic effect of each Ras effector mutant was inhibited by RasN17.

The new serine-threonine kinase, Qik, is a target of the Qin oncogene

The winged helix transcription factor Qin is the avian homolog of the mammalian brain factor 1 (BF-1) and has the potential to act as an oncogenic protein. We used representational difference analysis to identify genes that are differentially expressed in chicken embryo fibroblasts (CEF) transformed by Qin. One of the up-regulated Qin targets identified in this analysis is a serine-threonine kinase termed Qik (Qin-induced kinase). Qik belongs to the AMPK/SNF1 kinase family. It is a ubiquitously expressed protein and is upregulated rapidly after a hormone-regulated form of Qin is activated. In vitro kinase tests demonstrate that Qik is capable of autophosphorylation. Elevated levels of Qik transcripts are also observed in Src-transformed cells, suggesting that Src and Qin share some targets.

SHIP inhibits Akt activation in B cells through regulation of Akt membrane localization

Activation of the serine/threonine kinase Akt and the regulation of its activation are recognized as critical in controlling proliferative/survival signals via many hematopoietic receptors. In B lymphocytes, the B-cell receptor (BCR)-mediated activation of Akt is attenuated by co–cross-linking of BCR with the inhibitory receptor FcγRIIB1, and the binding of the SH2 domain-containing inositol phosphatase, SHIP, to FcγRIIB1. Because SHIP dephosphorylates phosphatidylinositol 3,4,5-trisphosphate (PIP3) and activation of Akt requires PIP3, the destruction of this phospholipid has been proposed as the mechanism for Akt inhibition. However, upstream kinases that activate Akt, such as PDK1, also require PIP3 for activation. In this report, we addressed whether SHIP inhibits Akt directly at the level of Akt recruitment to the membrane, indirectly through PDK recruitment/phosphorylation of Akt, or both. We generated stable B-cell lines expressing a regulatable, but constitutively membrane-bound Akt that still required PDK-dependent phosphorylation for activation. Several lines of evidence suggested that activation of this membrane-targeted Akt is not inhibited by FcγRIIB1/SHIP and that PDK is not a target for SHIP-mediated inhibition. These data demonstrate that SHIP inhibits Akt primarily through regulation of Akt membrane localization. We also observed during these studies that FcγRIIB1/SHIP does not inhibit p70S6k activation, even though several other PIP3-dependent events were down-regulated. Because the enhanced activation of Akt in the absence of SHIP correlates with hyperproliferation in the myeloid lineage, our data have implications for SHIP and Akt-dependent regulation of proliferation in the hematopoietic lineage.

SHIP inhibits Akt activation in B cells through regulation of Akt membrane localization

Activation of the serine/threonine kinase Akt and the regulation of its activation are recognized as critical in controlling proliferative/survival signals via many hematopoietic receptors. In B lymphocytes, the B-cell receptor (BCR)-mediated activation of Akt is attenuated by co–cross-linking of BCR with the inhibitory receptor FcγRIIB1, and the binding of the SH2 domain-containing inositol phosphatase, SHIP, to FcγRIIB1. Because SHIP dephosphorylates phosphatidylinositol 3,4,5-trisphosphate (PIP3) and activation of Akt requires PIP3, the destruction of this phospholipid has been proposed as the mechanism for Akt inhibition. However, upstream kinases that activate Akt, such as PDK1, also require PIP3 for activation. In this report, we addressed whether SHIP inhibits Akt directly at the level of Akt recruitment to the membrane, indirectly through PDK recruitment/phosphorylation of Akt, or both. We generated stable B-cell lines expressing a regulatable, but constitutively membrane-bound Akt that still required PDK-dependent phosphorylation for activation. Several lines of evidence suggested that activation of this membrane-targeted Akt is not inhibited by FcγRIIB1/SHIP and that PDK is not a target for SHIP-mediated inhibition. These data demonstrate that SHIP inhibits Akt primarily through regulation of Akt membrane localization. We also observed during these studies that FcγRIIB1/SHIP does not inhibit p70S6k activation, even though several other PIP3-dependent events were down-regulated. Because the enhanced activation of Akt in the absence of SHIP correlates with hyperproliferation in the myeloid lineage, our data have implications for SHIP and Akt-dependent regulation of proliferation in the hematopoietic lineage.

B cell antigen receptor-induced activation of Akt promotes B cell survival and is dependent on Syk kinase

Signaling through the B cell Ag receptor (BCR) is a key determinant in the regulation of B cell physiology. Depending on additional factors, such as microenvironment and developmental stage, ligation of the BCR can trigger B lymphocyte activation, proliferation, or apoptosis. The regulatory mechanisms determining B cell apoptosis and survival are not known. Using the chicken B lymphoma cell line DT40 as a model system, we investigated the role of the serine/threonine kinase Akt in B cell activation. While parental DT40 cells undergo apoptosis in response to BCR cross-linking, cells overexpressing Akt show a greatly diminished apoptotic response. By contrast, limiting the activation of Akt, either by inhibiting phosphatidylinositol 3-kinase or by ectopic expression of the phospholipid phosphatase MMAC1, results in a significant increase in the percentage of apoptotic cells after BCR cross-linking. Using various DT40 knockout cell lines, we further demonstrate that the tyrosine kinase Syk is required for Akt activation and that Lyn tyrosine kinase inhibits Akt activation. Taken together, the data demonstrate that Akt plays an important role in B cell survival and that Akt is activated in a Syk-dependent pathway.

AKT/PKB and other D3 phosphoinositide-regulated kinases: kinase activation by phosphoinositide-dependent phosphorylation

The protein kinase Akt/PKB is activated via a multistep process by a variety of signals. In the early steps of this process, PI-3 kinase-generated D3-phosphorylated phosphoinositides bind the Akt PH domain and induce the translocation of the kinase to the plasma membrane where it co-localizes with phosphoinositide-dependent kinase-1. By binding to the PH domains of both Akt and phosphoinositide-dependent kinase-1, D3-phosphorylated phosphoinositides appear to also induce conformational changes that permit phosphoinositide-dependent kinase-1 to phosphorylate the activation loop of Akt. The paradigm of Akt activation via phosphoinositide-dependent phosphorylation provided a framework for research into the mechanism of activation of other members of the AGC kinase group (p70S6K, PKC, and PKA) and members of the Tec tyrosine kinase family (TecI, TecII, Btk/Atk, Itk/Tsk/Emt, Txk/Rlk, and Bm/Etk). The result was the discovery that these kinases and Akt are activated by overlapping pathways. In this review, we present our current understanding of the regulation and function of the Akt kinase and we discuss the common and unique features of the activation processes of Akt and the AGC and Tec kinase families. In addition, we present an overview of the biosynthesis of phosphoinositides that contribute to the regulation of these kinases.

v-Crk activates the phosphoinositide 3-kinase/AKT pathway in transformation

v-Crk induces cellular tyrosine phosphorylation and transformation of chicken embryo fibroblasts (CEF). We studied the molecular mechanism of the v-Crk-induced transformation. Experiments with Src homology (SH)2 and SH3 domain mutants revealed that the induction of tyrosine phosphorylation of cellular proteins requires only the SH2 domain, but both the SH2 and SH3 domains are required for complete transformation. Analysis of three well defined signaling pathways, the mitogen-activated protein kinase (MAPK) pathway, the Jun N-terminal kinase (JNK) pathway, and the phosphoinositide 3-kinase (PI3K)/AKT pathway, demonstrated that only the PI3K/AKT pathway is constitutively activated in v-Crk-transformed CEF. Both the SH2 and SH3 domains are required for this activation of the PI3K/AKT pathway in CEF. We also found that the colony formation of CEF is strongly induced by a constitutively active PI3K mutant, and that a PI3K inhibitor, LY294002, suppresses the v-Crk-induced transformation. These results strongly suggest that constitutive activation of the PI3K/AKT pathway plays an essential role in v-Crk-induced transformation of CEF.

The PI3K-PDK1 connection: more than just a road to PKB

Phosphoinositide 3-kinases (PI3Ks) generate specific inositol lipids that have been implicated in the regulation of cell growth, proliferation, survival, differentiation and cytoskeletal changes. One of the best characterized targets of PI3K lipid products is the protein kinase Akt or protein kinase B (PKB). In quiescent cells, PKB resides in the cytosol in a low-activity conformation. Upon cellular stimulation, PKB is activated through recruitment to cellular membranes by PI3K lipid products and phosphorylation by 3'-phosphoinositide-dependent kinase-1 (PDK1). Here we review the mechanism by which PKB is activated and the downstream actions of this multifunctional kinase. We also discuss the evidence that PDK1 may be involved in the activation of protein kinases other than PKB, the mechanisms by which this activity of PDK1 could be regulated and the possibility that some of the currently postulated PKB substrates targets might in fact be phosphorylated by PDK1-regulated kinases other than PKB.

Combined activation of Ras and Akt in neural progenitors induces glioblastoma formation in mice

Gliomas are the most common primary malignant brain tumours and are classified into four clinical grades, with the most aggressive tumours being grade 4 astrocytomas (also known as glioblastoma multiforme; GBM). Frequent genetic alterations in GBMs (refs 2-5) result in stimulation of common signal transduction pathways involving Ras, Akt and other proteins. It is not known which of these pathways, if any, are sufficient to induce GBM formation. Here we transfer, in a tissue-specific manner, genes encoding activated forms of Ras and Akt to astrocytes and neural progenitors in mice. We found that although neither activated Ras nor Akt alone is sufficient to induce GBM formation, the combination of activated Ras and Akt induces high-grade gliomas with the histological features of human GBMs. These tumours appear to arise after gene transfer to neural progenitors, but not after transfer to differentiated astrocytes. Increased activity of RAS is found in many human GBMs (ref. 11), and we show here that Akt activity is increased in most of these tumours, implying that combined activation of these two pathways accurately models the biology of this disease.

Signalling through the insulin receptor

The insulin receptor substrates function at the heart of the insulin signalling network. It has recently become apparent that the intracellular localisation of these molecules is regulated in a precise manner that is critical for both the generation and the termination of the insulin signal. Some insulin receptor substrate isoforms appear to be associated with an insoluble matrix that resembles the cytoskeleton. When inappropriately dissociated from this matrix the signalling network collapses concomitant with loss of insulin sensitivity.

The catalytic subunit of phosphoinositide 3-kinase: requirements for oncogenicity

The retroviral oncogene p3k (v-p3k) of avian sarcoma virus 16 (ASV16) codes for the catalytic subunit of phosphoinositide (PI) 3-kinase, p110alpha. The v-P3k protein is oncogenic in vivo and in vitro; its cellular counterpart, c-P3k, lacks oncogenicity. Fusion of viral Gag sequences to the amino terminus of c-P3k activates the transforming potential. Activation can also be achieved by the addition of a myristylation signal to the amino terminus or of a farnesylation signal to the carboxyl terminus of c-P3k. A mutated myristylation signal was equally effective; it also caused a strong increase in the kinase activity of P3k. Mutations that inactivate lipid kinase activity abolish oncogenicity. The transforming activity of P3k is correlated with the ability to induce activating phosphorylation in Akt. Point mutations and amino-terminal deletions recorded in v-P3k were shown to be irrelevant to the activation of oncogenic potential. Interactions of P3k with the regulatory subunit of PI 3-kinase, p85, or with Ras are not required for transformation. These results support the conclusion that the oncogenicity of P3k depends on constitutive lipid kinase activity. Akt is an important and probably essential downstream component of the oncogenic signal from P3k.

Phosphatidylinositol 3-kinase signaling mediates angiogenesis and expression of vascular endothelial growth factor in endothelial cells

Phosphatidylinositol 3-kinase (PI 3-kinase) is a signaling molecule that controls numerous cellular properties and activities. The oncogene v-p3k is a homolog of the gene coding for the catalytic subunit of PI 3-kinase, p110alpha. P3k induces transformation of cells in culture, formation of hemangiosarcomas in young chickens, and myogenic differentiation in myoblasts. Here, we describe a role of PI 3-kinase in angiogenesis. Overexpression of the v-P3k protein or of cellular PI 3-kinase equipped with a myristylation signal, Myr-P3k, can induce angiogenesis in the chorioallantoic membrane (CAM) of the chicken embryo. This process is characterized by extensive sprouting of new blood vessels and enlargement of preexisting vessels. Overexpression of the myristylated form of the PI 3-kinase target Akt, Myr-Akt, also induces angiogenesis. Overexpression of the tumor suppressor PTEN or of dominant-negative constructs of PI 3-kinase inhibits angiogenesis in the yolk sac of chicken embryos, suggesting that PI 3-kinase and Akt signaling is required for normal embryonal angiogenesis. The levels of mRNA for vascular endothelial growth factor (VEGF) are elevated in cells expressing activated PI 3-kinase or Myr-Akt. VEGF mRNA levels are also increased by insulin treatment through the PI 3-kinase-dependent pathway. VEGF mRNA levels are decreased in cells treated with the PI 3-kinase inhibitor LY294002 and restored by overexpression of v-P3k or Myr-Akt. Overexpression of VEGF by the RCAS vector induces angiogenesis in chicken embryos. These results suggest that PI 3-kinase plays an important role in angiogenesis and regulates VEGF expression.

Loss of PTEN facilitates HIF-1-mediated gene expression

In glioblastoma-derived cell lines, PTEN does not significantly alter apoptotic sensitivity or cause complete inhibition of DNA synthesis. However, in these cell lines PTEN regulates hypoxia- and IGF-1-induced angiogenic gene expression by regulating Akt activation of HIF-1 activity. Restoration of wild-type PTEN to glioblastoma cell lines lacking functional PTEN ablates hypoxia and IGF-1 induction of HIF-1-regulated genes. In addition, Akt activation leads to HIF-1α stabilization, whereas PTEN attenuates hypoxia-mediated HIF-1α stabilization. We propose that loss ofPTEN during malignant progression contributes to tumor expansion through the deregulation of Akt activity and HIF-1-regulated gene expression.

Loss of PTEN facilitates HIF-1-mediated gene expression

In glioblastoma-derived cell lines, PTEN does not significantly alter apoptotic sensitivity or cause complete inhibition of DNA synthesis. However, in these cell lines PTEN regulates hypoxia- and IGF-1-induced angiogenic gene expression by regulating Akt activation of HIF-1 activity. Restoration of wild-type PTEN to glioblastoma cell lines lacking functional PTEN ablates hypoxia and IGF-1 induction of HIF-1-regulated genes. In addition, Akt activation leads to HIF-1alpha stabilization, whereas PTEN attenuates hypoxia-mediated HIF-1alpha stabilization. We propose that loss of PTEN during malignant progression contributes to tumor expansion through the deregulation of Akt activity and HIF-1-regulated gene expression.

The MMAC1 tumor suppressor phosphatase inhibits phospholipase C and integrin-linked kinase activity

Loss of the tumor suppressor MMAC1 has been shown to be involved in breast, prostate and brain cancer. Consistent with its identification as a tumor suppressor, expression of MMAC1 has been demonstrated to reduce cell proliferation, tumorigenicity, and motility as well as affect cell-cell and cell-matrix interactions of malignant human glioma cells. Subsequently, MMAC1 was shown to have lipid phosphatase activity towards PIP3 and protein phosphatase activity against focal adhesion kinase (FAK). The lipid phosphatase activity of MMAC1 results in decreased activation of the PIP3-dependent, anti-apoptotic kinase, AKT. It is thought that this inhibition of AKT culminates with reduced glioma cell proliferation. In contrast, MMAC1's effects on cell motility, cell - cell and cell - matrix interactions are thought to be due to its protein phosphatase activity towards FAK. However, recent studies suggest that the lipid phosphatase activity of MMAC1 correlates with its ability to be a tumor suppressor. The high rate of mutation of MMAC1 in late stage metastatic tumors suggests that effects of MMAC1 on motility, cell - cell and cell - matrix interactions are due to its tumor suppressor activity. Therefore the lipid phosphatase activity of MMAC1 may affect PIP3 dependent signaling pathways and result in reduced motility and altered cell - cell and cell - matrix interactions. We demonstrate here that expression of MMAC1 in human glioma cells reduced intracellular levels of inositol trisphosphate and inhibited extracellular Ca2+ influx, suggesting that MMAC1 affects the phospholipase C signaling pathway. In addition, we show that MMAC1 expression inhibits integrin-linked kinase activity. Furthermore, we show that these effects require the catalytic activity of MMAC1. Our data thus provide a link of MMAC1 to PIP3 dependent signaling pathways that regulate cell - matrix and cell - cell interactions as well as motility. Lastly, we demonstrate that AKT3, an isoform of AKT highly expressed in the brain, is also a target for MMAC1 repression. These data suggest an important role for AKT3 in glioblastoma multiforme. We therefore propose that repression of multiple PIP3 dependent signaling pathways may be required for MMAC1 to act as a tumor suppressor.

The regulation and activities of the multifunctional serine/threonine kinase Akt/PKB

The serine/threonine kinase Akt, or protein kinase B (PKB), has recently been a focus of intense research. It appears that Akt/PKB lies in the crossroads of multiple cellular signaling pathways and acts as a transducer of many functions initiated by growth factor receptors that activate phosphatidylinositol 3-kinase (PI 3-kinase). Akt/PKB is particularly important in mediating several metabolic actions of insulin. Another major activity of Akt/PKB is to mediate cell survival. In addition, the recent discovery of the tumor suppressor PTEN as an antagonist of PI 3-kinase and Akt/PKB kinase activity suggests that Akt/PKB is a critical factor in the genesis of cancer. Thus, elucidation of the mechanisms of Akt/PKB regulation and its physiological functions should be important for the understanding of cellular metabolism, apoptosis, and cancer.

Regulation of PTEN expression in neuronal apoptosis

PTEN phosphatase is a tumor suppressor gene that dephosphorylates phosphatidylinositol phosphates. PTEN restrains the function of a major antiapoptotic and survival pathway involving phosphoinositide 3-kinase and Akt kinase. Our purpose was to find out whether apoptotic inducers affect the expression of PTEN in cerebellar granule neurons and neuroblastoma 2a cells (Neuro-2a). PTEN mRNA expression showed a major 5.5-kb and a lower abundance 2.5-kb transcripts. In Neuro-2a cells, serum withdrawal induced a prominent, continuous decrease both in 5.5- and 2.5-kb transcripts of PTEN mRNA. Simultaneously, the expression level of 56-kDa PTEN protein decreased in Neuro-2a cells. The decrease in PTEN expression precedes apoptotic changes observed after serum withdrawal. On the contrary, okadaic acid and etoposide only slightly affected the expression of PTEN although they induce a prominent apoptosis in Neuro-2a cells. In cerebellar granule neurons, okadaic acid treatment induced a prominent increase in PTEN mRNA expression after 6-h treatment, both at the 5.5- and 2.5-kb transcripts. The early response in PTEN mRNA expression disappeared in 5.5-kb transcripts already at 12 h and in the case of 2.5-kb transcripts it lasted up to 24 h. Potassium deprivation, known to induce apoptosis in cerebellar granule cells, did not affect PTEN mRNA expression but together with serum deprivation induced a clear decrease in the 5. 5-kb PTEN transcripts. It seems that the changes in PTEN expression level and neuronal apoptosis are not related to each other in general but the expression of PTEN phosphatase seems to regulate certain apoptotic signals affecting phosphoinositide 3-kinase function.

Heregulin regulation of Akt/protein kinase B in breast cancer cells

In the present studies, we demonstrate that heregulin is a potent and rapid activator of the serine/threonine kinase called Akt in the MCF-7 breast cancer cell line but not in 3 other breast cancer cell lines (T47D, HBL-100, and MDA-231). The extent of activation of Akt in the 4 cell lines correlated with the ability of heregulin to activate phosphatidylinositol 3-kinase and inhibition of the kinase blocked Akt activation. A monoclonal antibody to HER2 inhibited the ability of heregulin to activate Akt in the MCF-7 cells. BT474, a breast cancer cell line which overexpresses HER2, had high basal Akt enzymatic activity. This high basal activity was lowered when cells were pre-incubated with an anti-HER2 monoclonal antibody which is used to treat breast cancer patients. Our results indicate that heregulin is a potent activator of Akt and that overexpression of HER2 in breast cancers could also lead to activation of Akt.

Up-regulation of Akt3 in estrogen receptor-deficient breast cancers and androgen-independent prostate cancer lines

We measured the insulin-stimulated amount of Akt1, Akt2, and Akt3 enzymatic activities in four breast cancer cell lines and three prostate cancer cell lines. In the estrogen receptor-deficient breast cancer cells and the androgen-insensitive prostate cells, the amount of Akt3 enzymatic activity was approximately 20-60-fold higher than in the cells that were estrogen- or androgen-responsive. In contrast, the levels of Akt1 and -2 were not increased in these cells. The increase in Akt3 enzyme activity correlated with an increase in both Akt3 mRNA and protein. In a prostate cancer cell line lacking the tumor suppressor PTEN (a lipid and protein phosphatase), the basal enzymatic activity of Akt3 was constitutively elevated and represented the major active Akt in these cells. Finally, reverse transcription-PCR was used to examine the Akt3 expression in 27 primary breast carcinomas. The expression levels of Akt3 were significantly higher in the estrogen receptor-negative tumors in comparison to the estrogen receptor-positive tumors. To see if the increase in Akt3 could be due to chromosomal abnormalities, the Akt3 gene was assigned to human chromosome 1q44 by fluorescence in situ hybridization and radiation hybrid cell panel analyses. These results indicate that Akt3 may contribute to the more aggressive clinical phenotype of the estrogen receptor-negative breast cancers and androgen-insensitive prostate carcinomas.

Protein kinase B localization and activation differentially affect S6 kinase 1 activity and eukaryotic translation initiation factor 4E-binding protein 1 phosphorylation

Recent studies indicate that phosphatidylinositide-3OH kinase (PI3K)-induced S6 kinase (S6K1) activation is mediated by protein kinase B (PKB). Support for this hypothesis has largely relied on results obtained with highly active, constitutively membrane-localized alleles of wild-type PKB, whose activity is independent of PI3K. Here we set out to examine the importance of PKB signaling in S6K1 activation. In parallel, glycogen synthase kinase 3beta (GSK-3beta) inactivation and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) phosphorylation were monitored as markers of the rapamycin-insensitive and -sensitive branches of the PI3K signaling pathway, respectively. The results demonstrate that two activated PKBalpha mutants, whose basal activity is equivalent to that of insulin-induced wild-type PKB, inhibit GSK-3beta to the same extent as a highly active, constitutively membrane-targeted wild-type PKB allele. However, of these two mutants, only the constitutively membrane-targeted allele of PKB induces S6K1 activation. Furthermore, an interfering mutant of PKB, which blocks insulin-induced PKB activation and GSK-3beta inactivation, has no effect on S6K1 activation. Surprisingly, all the activated PKB mutants, regardless of constitutive membrane localization, induce 4E-BP1 phosphorylation and the interfering PKB mutant blocks insulin-induced 4E-BP1 phosphorylation. The results demonstrate that PKB mediates S6K1 activation only as a function of constitutive membrane localization, whereas the activation of PKB appears both necessary and sufficient to induce 4E-BP1 phosphorylation independently of its intracellular location.

Myogenic signaling of phosphatidylinositol 3-kinase requires the serine-threonine kinase Akt/protein kinase B

The oncogene p3k, coding for a constitutively active form of phosphatidylinositol 3-kinase (PI 3-kinase), strongly activates myogenic differentiation. Inhibition of endogenous PI 3-kinase activity with the specific inhibitor LY294002, or with dominant-negative mutants of PI 3-kinase, interferes with myotube formation and with the expression of muscle-specific proteins. Here we demonstrate that a downstream target of PI 3-kinase, serine-threonine kinase Akt, plays an important role in myogenic differentiation. Expression of constitutively active forms of Akt dramatically enhances myotube formation and expression of the muscle-specific proteins MyoD, creatine kinase, myosin heavy chain, and desmin. Transdominant negative forms of Akt inhibit myotube formation and the expression of muscle-specific proteins. The inhibition of myotube formation and the reduced expression of muscle-specific proteins caused by the PI 3-kinase inhibitor LY294002 are completely reversed by constitutively active forms of Akt. Wild-type cellular Akt effects a partial reversal of LY294002-induced inhibition of myogenic differentiation. This result suggests that Akt can substitute for PI 3-kinase in the stimulation of myogenesis; Akt may be an essential downstream component of PI 3-kinase-induced muscle differentiation.

Nuclear endpoint of Wnt signaling: neoplastic transformation induced by transactivating lymphoid-enhancing factor 1

The interaction between beta-catenin and LEF-1/TCF transcription factors plays a pivotal role in the Wnt-1 signaling pathway. The level of beta-catenin is regulated by partner proteins, including glycogen synthase kinase-3beta (GSK-3beta) and the adenomatous polyposis coli (APC) tumor suppressor protein. Genetic defects in APC are responsible for a heritable predisposition to colon cancer. APC protein and GSK-3beta bind beta-catenin, retain it in the cytoplasm, and facilitate the proteolytic degradation of beta-catenin. Abrogation of this negative regulation allows beta-catenin to translocate to the nucleus and to form a transcriptional activator complex with the DNA-binding protein lymphoid-enhancing factor 1 (LEF-1). This complex is thought to be involved in tumorigenesis. Here we show that covalent linkage of LEF-1 to beta-catenin and to transcriptional activation domains derived from the estrogen receptor or the herpes simplex virus protein VP16 generates transcriptional regulators that induce oncogenic transformation of chicken embryo fibroblasts. The chimeras between LEF-1 and beta-catenin or VP16 are constitutively active, whereas fusions of LEF-1 to the estrogen receptor are regulatable by estrogen. These experiments document the oncogenicity of transactivating LEF-1 and show that the transactivation domain normally provided by beta-catenin can be replaced by heterologous activation domains. These results suggest that the transactivating function of the LEF-1/beta-catenin complex is critical for tumorigenesis and that this complex transforms cells by activating specific LEF-1 target genes.