The akt kinase: molecular determinants of oncogenicity

Oncogenic signaling of class I PI3K isoforms

The catalytic subunits of class I PI3Ks comprise four isoforms: p110alpha, p110beta, p110delta and p110gamma. Cancer-specific gain-of-function mutations in p110alpha have been identified in various malignancies. Cancer-specific mutations in the non-alpha isoforms of class I PI3K have not yet been identified, however overexpression of either wild-type p110beta, p110gamma or p110delta is sufficient to induce cellular transformation in chicken embryo fibroblasts. The mechanism whereby these non-alpha isoforms of class I mediate oncogenic signals is unknown. Here we show that potently transforming class I isoforms signal via Akt/mTOR, inhibit GSK3beta and cause degradation of FoxO1. A functional Erk pathway is required for p110gamma and p110beta transformation but not for transformation by p110delta or the H1047R mutant of p110alpha. Transformation and signaling by p110gamma and p110beta are sensitive to loss of interaction with Ras, which acts as a membrane anchor. Mutations in the C2 domain of p110delta reduce transformation, most likely by interfering with membrane association. Several small molecule inhibitors potently and specifically inhibit the oncogenic signaling and transformation of each of the class I PI3K, and, when used in combination with MEK inhibitors, can additively reduce the transformation induced by p110beta and p110gamma.

Live-cell molecular analysis of Akt activation reveals roles for activation loop phosphorylation

Activation of the serine/threonine protein kinase Akt/PKB is a multi-step process involving membrane recruitment, phosphorylation, and membrane detachment. To investigate this process in the cellular context, we employed a live-cell fluorescence imaging approach to examine conformational changes of Akt and its membrane association. A fluorescence resonance energy transfer-based reporter of Akt action (ReAktion) reveals a conformational change that is critically dependent on the existence of a phosphorylatable threonine 308 in the activation loop, because mutations to either aspartate or alanine abolished the change. Furthermore, a mutant carrying a phosphorylation mimic at this position showed diminished membrane association, suggesting that this phosphorylation plays an important role of promoting the dissociation of activated Akt from the membrane. In addition, the membrane-associating pleckstrin homology domain was found to associate with the catalytic domain when Thr308 is phosphorylated, suggesting such an interdomain interaction as a mechanism by which phosphorylation within the catalytic domain can affect membrane association. These studies uncover new regulatory roles of this critical phosphorylation event of Akt for ensuring its proper activation and function.

Phosphorylation by Akt disables the anti-oncogenic activity of YB-1

The Y box-binding protein 1 (YB-1) is a DNA/RNA-binding protein that regulates mRNA transcription and translation. It is a major component of free messenger ribonucleoprotein particles and, at higher concentrations, blocks protein synthesis. In chicken embryo fibroblasts, overexpression of YB-1 confers a specific resistance to oncogenic cellular transformation by phosphoinositide 3-kinase (PI3K) or Akt/PKB. Recent studies have identified YB-1 as a direct substrate of Akt. The functional significance of Akt-mediated phosphorylation remains largely unknown. We generated YB-1 mutants in the Akt phosphorylation consensus sequence to explore the effect of phosphorylated YB-1 in PI3K-induced transformation. In contrast to wild-type YB-1, the phosphomimetic S99E mutant no longer interferes with cellular transformation. This mutant has reduced affinity for the cap of mRNAs and fails to inhibit cap-dependent translation. The data suggest that phosphorylation by Akt disables the inhibitory activity of YB-1 and thereby enhances the translation of transcripts that are necessary for oncogenesis. Overexpression of wild-type YB-1 overrides inactivation by Akt and maintains inhibition of protein synthesis and resistance to transformation.

Coordinate regulation of ribosome biogenesis and function by the ribosomal protein S6 kinase, a key mediator of mTOR function

Current understanding of the mechanisms by which cell growth is regulated lags significantly behind our knowledge of the complex processes controlling cell cycle progression. Recent studies suggest that the mammalian target of rapamycin (mTOR) pathway is a key regulator of cell growth via the regulation of protein synthesis. The key mTOR effectors of cell growth are eukaryotic initiation factor 4E-binding protein 1 (4EBP-1) and the ribosomal protein S6 kinase (S6K). Here we will review the current models for mTOR dependent regulation of ribosome function and biogenesis as well as its role in coordinating growth factor and nutrient signaling to facilitate homeostasis of cell growth and proliferation. We will place particular emphasis on the role of S6K1 signaling and will highlight the points of cross talk with other key growth control pathways. Finally, we will discuss the impact of S6K signaling and the consequent feedback regulation of the PI3K/Akt pathway on disease processes including cancer.

Cholesterol sensitivity of endogenous and myristoylated Akt

The serine-threonine kinase, Akt, has been linked to cholesterol-sensitive signaling mechanisms, suggesting a possible means whereby cholesterol might affect tumor cell growth and survival. However, it has not been shown whether Akt itself, as distinct from upstream components of the pathway (e.g., membrane phosphoinositides), can be directly responsible for cholesterol-mediated effects. Consistent with this possibility, we identified an Akt1 subpopulation in cholesterol-rich lipid raft fractions prepared from LNCaP human prostate cancer cells. Phosphorylation of this Akt subspecies was ablated with methyl-beta-cyclodextrin, a cholesterol-binding compound, under conditions where nonlipid raft-resident Akt was unaffected. A myristoylated Akt1 (MyrAkt1) fusion protein expressed in LNCaP cells was found to be highly enriched in lipid rafts, indicating that oncogenic Akt is overrepresented in cholesterol-rich membranes compared with wild-type Akt. Notably, lipid raft-resident MyrAkt1 exhibited a markedly distinct substrate preference compared with MyrAkt1 immunoprecipitated from cytosol and nonraft membrane fractions, suggesting a redirection of signal transduction when the protein is present in cholesterol-rich membranes. Expression of MyrAkt1 in LNCaP cells overcame their characteristic dependence on constitutive signaling through the phosphoinositide 3'-kinase pathway. This protective effect was substantially diminished with cyclodextrin treatment. Phosphorylation of Akt substrates in lipid raft fractions, but not in cytosol/nonraft membrane fractions, was ablated with cyclodextrin. In addition, in control (LacZ transfected) cells, lipid raft fractions were relatively enriched in phosphorylated Akt substrates. Collectively, these data show that a subpopulation of Akt is cholesterol sensitive and that the oncogenic effects conferred by myristoylation arise, in part, from the tendency of the membrane-targeted form of the protein to reside in cholesterol-rich membrane microdomains.

PTEN, more than the AKT pathway

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN)/phosphatidylinositol 3-kinase (PI3K)/AKT constitute an important pathway regulating the signaling of multiple biological processes such as apoptosis, metabolism, cell proliferation and cell growth. PTEN is a dual protein/lipid phosphatase and its main substrate phosphatidyl-inositol 3,4,5 triphosphate (PIP3) is the product of PI3K. Increase in PIP3 recruits AKT to the membrane where is activated by other kinases also dependent on PIP3. Many components of this pathway have been described as causal forces in cancer. PTEN activity is lost by mutations, deletions or promoter methylation silencing at high frequency in many primary and metastatic human cancers. Germ line mutations of PTEN are found in several familial cancer predisposition syndromes. Recently, many activating mutations in the PI3KCA gene (coding for the p110alpha catalytic subunit of PI3K) have been described in human tumors. Activation of PI3K and AKT are reported to occur in breast, ovarian, pancreatic, esophageal and other cancers. Genetically modified mice confirm these PTEN activities. Tissue-specific deletions of PTEN usually provoke cancer. Moreover, an absence of PTEN cooperates with an absence of p53 to promote cancer. However, we have observed very different results with the expression of activated versions of AKT in several tissues. Activated AKT transgenic lines do not develop tumors in breast or prostate tissues and do not cooperate with an absence of p53. This data suggest that an AKT-independent mechanism contributes to PTEN tumorigenesis. Crosses with transgenic mice expressing possible PTEN targets indicate that neither cyclin D1 nor p53 are these AKT-independent targets. However, AKT is more than a passive bridge toward PTEN tumorigenesis, since its expression not only allows but also enforces and accelerates the tumorigenic process in combination with other oncogenes.

Akt/PKB interacts with the histone H3 methyltransferase SETDB1 and coordinates to silence gene expression

Serine/threonine kinase Akt (also called protein kinase B, PKB) is a downstream effector of phosphoinositide 3-kinase (PI3K) and functions as the focal point for many signal transduction pathways such as glucose metabolism, transcription, cell survival, angiogenesis, and cell motility. Akt is emerging as a central player in tumorigenesis including human ovarian, pancreatic, prostate, breast, and gastric cancers. However, the function and mechanism by which Akt regulate gene transcription in nucleus remains largely unclear. Here we identify histone methyltransferase SETDB1 as a novel nuclear interacting partner of Akt. By yeast two-hybrid screening, we obtained the Akt1-SETDB1 interaction and confirmed the binding both in vitro and in vivo. Both Akt1 and SETDB1 are co-localized in the nucleus in mammalian cells. SETDB1 is not a phosphorylation substrate of Akt kinase. Akt and SETDB1 could coordinate to regulate the activity of certain transcription factors such as forkhead family member. Due to the fact that SETDB1 acts as a specific histone H3, lysine 9-methyltransferase in vivo, we provide the first link between Akt kinase and histone methyltransferase (HMTase). This interaction represents a novel mechanism by which Akt regulates nuclear events including gene transcription.

Cancer-specific mutations in phosphatidylinositol 3-kinase

Cancer-specific mutations in the catalytic subunit of phosphatidylinositol 3-kinase (PI3K) p110 alpha occur in diverse tumors in frequencies that can exceed 30%. The majority of these mutations map to one of three hot spots in the gene, and the rest are distributed over much of the PI3K coding sequence. Most of the cancer-specific mutations induce a gain of function that results in oncogenicity, elevated lipid kinase activity and constitutive signaling through the kinases Akt and TOR. The location of the mutations on a model structure of p110 alpha indicates several distinct mechanisms for the gain of function. The mutated p110 alpha proteins are promising cancer targets. Although identification of mutant-specific small-molecule inhibitors seems technically challenging, the therapeutic benefits from such inhibitors could be extremely important.

Rationale and clinical application of alkylphospholipid analogues in combination with radiotherapy

Concurrent treatment with radiotherapy and chemotherapy has emerged as an effective strategy to improve clinical outcome of cancer. In addition to combining radiation with classical anticancer agents, several new biological response modifiers are under investigation in pre-clinical and clinical studies. Synthetic alkylphospholipids are anticancer agents that in contrast to most anticancer drugs, do not target DNA, but insert in the plasma membrane and subsequently induce a broad range of biological effects, ultimately leading to cell death. Alkylphospholipids kill tumor cells directly by induction of both apoptotic and non-apoptotic cell death, and indirectly by interference with critical signal transduction pathways involved in phospholipid metabolism and survival. Due to their distinct mode of action, these drugs are considered as attractive candidates to combine with radiotherapy. In this review, we will discuss several alkylphospholipids that reached clinical application. These include first-generation alkyl-lysophospholipids edelfosine and ilmofosine, second-generation alkylphosphocholine-prototype miltefosine and more recently developed analogues perifosine and erucylphosphocholine. We focus on mechanisms of action and the rationale to combine these agents with radiotherapy. The preclinical results on molecular targeting underlying this approach will be reviewed, concluded with first clinical data on combined treatment of radiotherapy with perifosine.

Glioma oncogenesis and animal models of glioma formation

Recent advances in animal models have improved our understanding of the pathway abnormalities driving glioma growth. This article reviews key molecular abnormalities that have been modeled in mice, and describes major tumor modeling techniques along with examples of astrocytoma and oligodendroglioma models. Animal models are important not only for the testing of novel therapeutics but also as a means to understand the molecular explanations for treatment success and failure in humans.

Solenopsin, the alkaloidal component of the fire ant (Solenopsis invicta), is a naturally occurring inhibitor of phosphatidylinositol-3-kinase signaling and angiogenesis

Phosphatidylinositol-3-kinase (PI3K), and its downstream effector Akt, or protein kinase Balpha (PKBalpha), play a major regulatory role in control of apoptosis, proliferation, and angiogenesis. PI3K and Akt are amplified or overexpressed in a number of malignancies, including sarcomas, ovarian cancer, multiple myeloma, and melanoma. This pathway regulates production of the potent angiogenic factor vascular endothelial growth factor (VEGF), and protects tumor cells against both chemotherapy and reactive oxygen-induced apoptosis through phosphorylation of substrates such as apoptotic peptidase-activating factor-1 (APAF-1), forkhead proteins, and caspase 9. Given its diverse actions, compounds that suppress the PI3K/Akt pathway have potential pharmacologic utility as angiogenesis inhibitors and antineoplastic agents. Using the SVR angiogenesis assay, a screen of natural products, we isolated the alkaloid solenopsin, and found that it is a potent angiogenesis inhibitor. We also found that solenopsin inhibits the PI3K signaling pathway in cells upstream of PI3K, which may underlie its affects on angiogenesis. Consistent with inhibition of the activation of PI3K, solenopsin prevented the phosphorylation of Akt and the phosphorylation of its substrate forkhead box 01a (FOXO1a), a member of the forkhead family of transcription factors. Interestingly, solenopsin also inhibited Akt-1 activity in an ATP-competitive manner in vitro without affecting 27 of 28 other protein kinases tested.

Akt-PDK1 complex mediates epidermal growth factor-induced membrane protrusion through Ral activation

We studied the spatiotemporal regulation of Akt (also called protein kinase B), phosphatidylinositol-3,4-bisphosphate [PtdIns(3,4)P2], and phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3] by using probes based on the principle of fluorescence resonance energy transfer. On epidermal growth factor (EGF) stimulation, the amount of PtdIns(3,4,5)P3 was increased diffusely in the plasma membrane, whereas that of PtdIns(3,4)P2 was increased more in the nascent lamellipodia than in the plasma membrane of the central region. The distribution and time course of Akt activation were similar to that of increased PtdIns(3,4)P2 levels, which were most prominent in the nascent lamellipodia. Moreover, we found that upon EGF stimulation 3-phosphoinositide-dependent protein kinase-1 (PDK1) was also recruited to nascent lamellipodia in an Akt-dependent manner. Because PDK1 is known to activate Ral GTPase and because Ral is required for EGF-induced lamellipodial protrusion, we speculated that the PDK1-Akt complex may be indispensable for the induction of lamellipodia. In agreement with this idea, EGF-induced lamellipodia formation was promoted by the overexpression of Akt and inhibited by an Akt inhibitor or a Ral-binding domain of Sec5. These results identified the Akt-PDK1 complex as an upstream positive regulator of Ral GTPase in the induction of lamellipodial protrusion.

TGF-beta inhibits Akt-induced transformation in intestinal epithelial cells

BACKGROUND

During the early stages of colorectal carcinogenesis, the phosphatidylinositol 3-kinase (PI3K)/Akt pathway is activated, enabling the transformed cells to survive and grow in the absence of anchorage to extracellular matrix. Transforming growth factor beta (TGF-beta) is an important tumor suppressor in the colon, and it is inactivated during later stages of colorectal carcinogenesis. The purpose of this study was to determine whether TGF-beta inhibits Akt-induced anchorage-independent growth and resistance to anoikis in gut epithelial cells.

METHODS

Rat intestinal epithelial cells (RIE-1) were infected with a retrovirus containing pLXSN-mAkt, and three independent clones were selected. Anchorage-independent growth was examined by colony formation in soft agar and cell counting in ultralow attachment plates. Anoikis was analyzed with the use of Annexin V staining.

RESULTS

All three clones of RIE-1/mAkt formed colonies in soft agar, which were decreased by TGF-beta. TGF-beta induced anoikis and treatment with a general caspase inhibitor, zVAD-fluoromethyl ketone, blocked TGF-beta-mediated decrease in colony formation.

CONCLUSIONS

TGF-beta attenuated Akt-induced anchorage-independent growth in RIE-1 cells in part by enhancing anoikis. Our data demonstrate a novel tumor-suppressor activity of TGF-beta and provide the molecular justification for the required activation of the PI3K/Akt pathway and the subsequent inactivation of TGF-beta signaling during colorectal carcinogenesis.

Akt deficiency impairs normal cell proliferation and suppresses oncogenesis in a p53-independent and mTORC1-dependent manner

Akt contributes to tumorigenesis by inhibiting apoptosis. Here we establish that Akt is required for normal cell proliferation and susceptibility to oncogenesis independently of its antiapoptotic activity. Partial ablation of Akt activity by deleting Akt1 inhibits cell proliferation and oncogenesis. These effects are compounded by deleting both Akt1 and Akt2. In vivo, Akt1 null mice are resistant to MMTV-v-H-Ras-induced tumors and to skin carcinogenesis. Thus, partial ablation of Akt activity is sufficient to suppress tumorigenesis in vitro and in vivo. The effect of Akt deficiency on cell proliferation and oncogenesis is p53 independent but mTORC1 dependent. Surprisingly, upon mTORC1 hyperactivation, the reduction in Akt activity does not impair cell proliferation and susceptibility to oncogenic transformation; thus, Akt may mediate these processes exclusively via mTORC1.

Mitochondrial mutations in cancer

The metabolism of solid tumors is associated with high lactate production while growing in oxygen (aerobic glycolysis) suggesting that tumors may have defects in mitochondrial function. The mitochondria produce cellular energy by oxidative phosphorylation (OXPHOS), generate reactive oxygen species (ROS) as a by-product, and regulate apoptosis via the mitochondrial permeability transition pore (mtPTP). The mitochondria are assembled from both nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) genes. The mtDNA codes for 37 genes essential of OXPHOS, is present in thousands of copies per cell, and has a very high mutations rate. In humans, severe mtDNA mutations result in multisystem disease, while some functional population-specific polymorphisms appear to have permitted humans to adapt to new environments. Mutations in the nDNA-encoded mitochondrial genes for fumarate hydratase and succinate dehydrogenase have been linked to uterine leiomyomas and paragangliomas, and cancer cells have been shown to induce hexokinase II which harnesses OXPHOS adenosine triphosphate (ATP) production to drive glycolysis. Germline mtDNA mutations at nucleotides 10398 and 16189 have been associated with breast cancer and endometrial cancer. Tumor mtDNA somatic mutations range from severe insertion-deletion and chain termination mutations to mild missense mutations. Surprisingly, of the 190 tumor-specific somatic mtDNA mutations reported, 72% are also mtDNA sequence variants found in the general population. These include 52% of the tumor somatic mRNA missense mutations, 83% of the tRNA mutations, 38% of the rRNA mutations, and 85% of the control region mutations. Some associations might reflect mtDNA sequencing errors, but analysis of several of the tumor-specific somatic missense mutations with population counterparts appear legitimate. Therefore, mtDNA mutations in tumors may fall into two main classes: (1) severe mutations that inhibit OXPHOS, increase ROS production and promote tumor cell proliferation and (2) milder mutations that may permit tumors to adapt to new environments. The former may be lost during subsequent tumor oxygenation while the latter may become fixed. Hence, mitochondrial dysfunction does appear to be a factor in cancer etiology, an insight that may suggest new approaches for diagnosis and treatment.

Mitochondria and cancer: Warburg addressed

Otto Warburg recognized that cancer cells generate excessive lactate in the presence of oxygen (aerobic glycolysis). It now appears that this phenomenon is the product of two factors: a return to the more glycolytic metabolism of the embryo and alterations in oxidative phosphorylation (OXPHOS) to increase mitochondrial reactive oxygen species (ROS) production. Alterations in the Ras-PI3K-Akt signal transduction pathway can result in induction of hexokinase II and its attachment to mitochondrial porin redirecting mitochondrial ATP to phosphorylate glucose and drive glycolysis. Furthermore, partial inhibition of OXPHOS by mitochondrial gene mutations (germ-line or somatic) can reduce electron flux through the electron transport chain, increasing mitochondrial ROS production. The increased ROS mutagenizes nuclear proto-oncogenes (initiation) and drives nuclear replication (promotion), resulting in cancer. Therefore, hexokinase II and mitochondrial ROS may be useful alternate targets for cancer therapeutics.

pERK, pAkt and pBad: A Possible Role in Cell Proliferation and Sustained Cellular Survival During Tumorigenesis and Tumor Progression in ENU Induced Transplacental Glioma Rat Model

Gliomas remain to be an unresolved medical problem. Better understanding of complex regulation and key molecules involved in glioma pathology are needed for designing new and effective treatment modalities. Activation of mitogen-activated protein kinase/extracellular signal regulated kinase (ERK) pathway is known to be having a critical role in cell proliferation and differentiation during the invasion and metastasis of the tumor cells. In the present study, N-ethyl N-nitrosourea induced glioma rat model was used to understand the role of ERK1/2 and Akt pathways in the progression of tumor malignancy. Twenty-four glioma rat brains of early (P90) and progressive (P180) stages were used for histological and immunoblot analysis. Results have shown increased levels of activated ERK1/2, activated Akt or protein kinase B, Bcl-2 and pBad in the glioma rats. This study may indicate increased cell proliferation and angiogenesis, mediated through activation of both ERK and Akt pathways along with increased levels of pBad. Further, pAkt and Bcl-2 levels in the progressive stage glioma rats may indicate existence of sustained tumor cell survival signals. Moreover, enhanced pBad levels in tumor may indicate that there are anti-apoptotic mechanisms, further making the malignant cells resistant to apoptosis.

Oncogenic transformation by the jaagsiekte sheep retrovirus envelope protein

Retroviruses have played profound roles in our understanding of the genetic and molecular basis of cancer. Jaagsiekte sheep retrovirus (JSRV) is a simple retrovirus that causes contagious lung tumors in sheep, known as ovine pulmonary adenocarcinoma (OPA). Intriguingly, OPA resembles pulmonary adenocarcinoma in humans, and may provide a model for this frequent human cancer. Distinct from the classical mechanisms of retroviral oncogenesis by insertional activation of or virus capture of host oncogenes, the native envelope (Env) structural protein of JSRV is itself the active oncogene. A major pathway for Env transformation involves interaction of the Env cytoplasmic tail with as yet unidentified cellular adaptor(s), leading to the activation of PI3K/Akt and MAPK signaling cascades. Another potential mechanism involves the cell-entry receptor for JSRV, Hyaluronidase 2 (Hyal2), and the RON receptor tyrosine kinase, but the exact roles of these proteins in JSRV Env transformation remain to be better understood. Recently, a mouse model of lung cancer induced by JSRV Env has been developed, and the tumors in mice resemble those seen in sheep infected with JSRV and in humans. In this review, we summarize recent progress in our understanding the molecular mechanisms of oncogenic transformation by JSRV Env protein, and discuss the relevance to human lung cancer.

Nuclear targeting of Akt antagonizes aspects of cardiomyocyte hypertrophy

The serine/threonine kinase Akt regulates cellular survival, proliferation, gene transcription, protein translation, metabolism, and differentiation. Although Akt substrates are found throughout the cell, activated Akt normally accumulates in the nucleus, suggesting that biologically relevant targets are located there. Consequences of nuclear Akt signaling in cardiomyocytes were explored by using nuclear-targeted Akt (Akt-nuc). Accumulation of Akt-nuc did not provoke hypertrophy, unlike constitutively activated Akt. Instead, Akt-nuc inhibited hypertrophy concurrent with increased atrial natriuretic peptide (ANP) expression that depended upon phosphatidylinositol-3 kinase activity. Akt-nuc antihypertrophic effects were blocked by inhibition of either guanylyl cyclase A receptor or cyclic guanosine monophosphate-dependent protein kinase in cultured cardiomyocytes. Corroborating evidence showed blunted acute hypertrophic remodeling in Akt-nuc transgenic mice after transverse aortic constriction coincident with higher ANP expression and smaller myocyte volume. In addition, Akt-nuc expression improved systolic function and survival in the chronic phase of transverse aortic constriction-induced hypertrophy. Thus, Akt-nuc antagonizes certain aspects of hypertrophy through autocrine/paracrine stimulation of a phosphatidylinositol-3 kinase-dependent signaling cascade that promotes ANP expression, resulting in a unique combination of prosurvival coupled with antihypertrophic signaling.

Oncogenic PI3K and its role in cancer

PURPOSE OF REVIEW

The purpose of this review is to examine the contribution of the PI3K signaling pathway to the development of human tumors and to propose further studies to elucidate how to develop therapeutics for patients with mutations in this pathway.

RECENT FINDINGS

More than 30% of various solid tumor types were recently found to contain mutations in PIK3CA, the catalytic subunit of PI3K. Further analysis of key genes in this pathway identified an additional eight genes altered in tumors. These were generally found to be mutated in a mutually exclusive manner, thus increasing the mutation frequency of the pathway to 40% in colorectal cancers and emphasizing the importance of the PI3K pathway in tumorigenesis. Functional analyses of PIK3CA mutations revealed that they increase its enzymatic activity, stimulate AKT signaling, allow growth factor-independent growth as well as increasing cell invasion and metastasis.

SUMMARY

The PI3K signaling pathway is dysregulated by a variety of mechanisms in a large fraction of human tumors. Both mutational and functional analyses have shown that PIK3CA is an oncogene that plays an important role in tumor progression. Mutant members of the PI3K pathway, including PIK3CA, are good targets for therapeutic intervention because most of them are kinases, making them attractive for drug development. Gaining further insights into PIK3CA oncogenic mechanisms may produce new biomarkers and help the development of targeted therapeutics.

Green tea catechin, epigallocatechin-3-gallate, inhibits vascular endothelial growth factor angiogenic signaling by disrupting the formation of a receptor complex

A potential mechanism by which green tea may prevent cancer development is through the inhibition of angiogenesis. We have shown previously that the green tea catechin, epigallocatechin gallate (EGCG), inhibits endothelial cell tube formation through the inhibition of vascular endothelial growth factor (VEGF)-induced Akt activation and vascular endothelial (VE)-cadherin phosphorylation. Furthermore, EGCG can suppress oxidant-induced production of the proangiogenic cytokine interleukin (IL)-8. To further elucidate the antiangiogenic mechanisms of EGCG, we investigated its regulation of other molecular processes in VEGF-induced signaling in human umbilical vein endothelial cells (HUVECs). We show that EGCG at physiological doses (0.5-10 microM) markedly inhibits the formation of a vascular endothelial growth factor receptor 2 complex formed upon the binding of its ligand VEGF. This disruption results in a significant and dose-dependent decrease in PI3-kinase activity. Electrophoretic mobility shift assay revealed that EGCG decreased the PI3 kinase-dependent activation and DNA-binding ability of NF-kappaB, likely acting through decreasing phosphorylation and degradation of IkappaB. VEGF-induced IL-8 production at the mRNA (real time RT-PCR) and protein levels (ELISA) are also suppressed with EGCG. These results suggest a novel mechanism for green tea's anticancer effects where EGCG can abrogate VEGF signaling by interfering with the formation of a receptor complex, resulting in attenuated mitogenic and angiogenic signaling.

Expression of Akt (protein kinase B) and its isoforms in malignant lymphomas

Akt (protein kinase B) is a serine/threonine kinase involved in the regulation of cell survival signals. Akt is expressed in T- and B-lymphocytes and is activated in response to cytokine and antigen-receptor stimulation. Three isoforms of Akt have been identified, Akt-1, -2 and -3, but the expression pattern and specific functions of each have not yet been determined for many cell types. To determine whether Akt signaling is enhanced in human malignant lymphomas and to analyse the expression pattern of Akt isoforms in these neoplasms, Akt-1, -2 and -3 expression was studied in 38 cell lines derived from hematopoietic neoplasms, by RT-PCR and western blot analysis. The level of phosphorylated (active) Akt was also analysed in cell lines as well as in 72 human malignant non-Hodgkin's lymphoma tissues. The results suggest that there is constitutive activation of Akt in the majority of primary human lymphomas and hematopoietic cell lines and support its proposed key role in lymphoma cell survival.

3-phosphoinositide-dependent protein kinase-1 (PDK1) promotes invasion and activation of matrix metalloproteinases

Background

Metastasis is a major cause of morbidity and mortality in breast cancer with tumor cell invasion playing a crucial role in the metastatic process. PDK1 is a key molecule that couples PI3K to cell proliferation and survival signals in response to growth factor receptor activation, and is oncogenic when expressed in mouse mammary epithelial cells. We now present evidence showing that PDK1-expressing cells exhibit enhanced anchorage-dependent and -independent cell growth and are highly invasive when grown on Matrigel. These properties correlate with induction of MMP-2 activity, increased MT1-MMP expression and a unique gene expression profile.

Methods

Invasion assays in Matrigel, MMP-2 zymogram analysis, gene microarray analysis and mammary isografts were used to characterize the invasive and proliferative function of cells expressing PDK1. Tissue microarray analysis of human breast cancers was used to measure PDK1 expression in invasive tumors by IHC.

Results

Enhanced invasion on Matrigel in PDK1-expressing cells was accompanied by increased MMP-2 activity resulting from stabilization against proteasomal degradation. Increased MMP-2 activity was accompanied by elevated levels of MT1-MMP, which is involved in generating active MMP-2. Gene microarray analysis identified increased expression of the ECM-associated genes decorin and type I procollagen, whose gene products are substrates of MT1-MMP. Mammary fat pad isografts of PDK1-expressing cells produced invasive adenocarcinomas. Tissue microarray analysis of human invasive breast cancer indicated that PDK1pSer241 was strongly expressed in 90% of samples.

Conclusion

These results indicate that PDK1 serves as an important effector of mammary epithelial cell growth and invasion in the transformed phenotype. PDK1 mediates its effect in part by MT1-MMP induction, which in turn activates MMP-2 and modulates the ECM proteins decorin and collagen. The presence of increased PDK1 expression in the majority of invasive breast cancers suggests its importance in the metastatic process.

Phosphoinositide 3-kinase: from viral oncoprotein to drug target

The catalytic subunit p110alpha of the phosphoinositide 3-kinase (PI3K) and the serine-threonine protein kinase Akt have been extensively studied as retroviral oncoproteins. The experimental tools developed with the retroviral vectors are now being applied to PI3K mutations in human cancer. The most frequently occurring mutants of p110alpha are oncogenic in vitro and in vivo, show gain of enzymatic function, activate Akt, and their oncogenic activity is sensitive to rapamycin. The related isoforms p110beta, gamma and delta induce oncogenic transformation as wild-type proteins. Mutated p110alpha proteins are ideal drug targets. Identification of small molecule inhibitors that specifically target these mutant proteins is a realistic and urgent goal.

Oncogenic transformation induced by the p110beta, -gamma, and -delta isoforms of class I phosphoinositide 3-kinase

Class I phosphoinositide 3-kinase contains four isoforms of the catalytic subunit, p110alpha, -beta, -gamma, and -delta. At physiological levels of expression, the wild-type p110alpha isoform lacks oncogenic potential, but gain-of-function mutations and overexpression of p110alpha are correlated with oncogenicity. The p110beta, -gamma, and -delta isoforms induce transformation of cultured cells as wild-type proteins. This oncogenic potential requires kinase activity and can be suppressed by the target of rapamycin inhibitor rapamycin. The p110delta isoform constitutively activates the Akt signaling pathway; p110gamma activates Akt only in the presence of serum. The isoforms differ in their requirements for upstream signaling. The transforming activity of the p110gamma isoform depends on rat sarcoma viral oncogene homolog (Ras) binding; preliminary data suggest the same for p110beta and indicate Ras-independent oncogenic potential of p110delta. The surprising oncogenic potential of the wild-type non-alpha isoforms of class I phosphoinositide 3-kinase may explain the dearth of cancer-specific mutations in these proteins, because these non-alpha isoforms could contribute to the oncogenic phenotype of the cell by differential expression.

Phosphatidylinositide 3-Kinase Is Important in Late-Stage Fibroblast Growth Factor-1-Mediated Angiogenesis in vivo

We previously reported that overexpression of a secreted version of fibroblast growth factor-1 (sp-FGF-1) has the ability to induce angiogenesis in the chicken chorioallantoic membrane (CAM). In our current study, we examine the effects of sp-FGF-1 through a time course analysis of angiogenesis in the chicken CAM on days 3, 4, and 5 after gene transfection. Significant angiogenesis was observed on days 4 and 5 after gene transfection in the CAM assay. To evaluate the role of phosphatidylinositide 3-kinase (PI3K) signaling in sp-FGF-1-induced angiogenesis, we analyzed mRNA expression levels of PI3K and protein activity through its immediate downstream target, AKT-1. We found upregulation of both PI3K and AKT mRNA expression levels in day 5 sp-FGF-1 versus day 5 vector control-transfected CAMs. Furthermore, by blocking PI3K phosphorylation using the specific inhibitor, LY294002, we found that downstream phosphorylation of AKT-1 was inhibited. More importantly, the blockade of the PI3K pathway via LY294002 in sp-FGF-1-transfected CAMs significantly inhibited angiogenesis. These results further elucidate the molecular mechanisms of the sp-FGF-1 signaling pathway and it underscores the importance of PI3K signaling in FGF-1-stimulated angiogenesis in vivo. It also provides a basis for the role of sp-FGF-1 in the development of therapeutic treatments to combat vascular insufficiencies and angiogenesis-dependent cancers.

Malignant transformation of an epithelial cell by v-Src via tv-a-mediated retroviral infection: A new cell model for studying carcinogenesis

Most human cancers are of epithelial origin, but many cell culture models for the study of cancer-causing genes use fibroblasts. In addition, efficient delivery and stable expression of foreign genes into non-transformed cell lines are often difficult. To address both questions, we here established a non-transformed rat kidney epithelial RK3E cell line that constitutively expresses tv-a (receptor for subgroup A avian leukosis virus, ALV) for delivery of foreign genes via avian retroviral infection. This cell line (RK3E/tv-a) allows efficient and stable expression of either single or multiple foreign genes. Furthermore, tv-a-mediated delivery of various oncogenes (v-src, H-ras, myc or akt) leads to malignant transformation. v-src-transformed cells exhibited classical cancerous phenotypes in vitro, and induced tumor formation and lung metastasis upon injecting into immunodeficient mice. Expression profiles of downstream molecular effectors (E-cadherin, beta-catenin, cyclin D1, Myc, VEGF, MMP-2, and MMP-9) in these cells correlate with characteristics of cancerous phenotypes. This new cell model serves as a useful tool to study cancer-causing genes in epithelial cell type.

Oncogenic PI3K deregulates transcription and translation

There have long been indications of a role for PI3K (phosphatidylinositol 3-kinase) in cancer pathogenesis. Experimental data document a requirement for deregulation of both transcription and translation in PI3K-mediated oncogenic transformation. The recent discoveries of cancer-specific mutations in PIK3CA, the gene that encodes the catalytic subunit p110alpha of PI3K, have heightened the interest in the oncogenic potential of this lipid kinase and have made p110alpha an ideal drug target.

mTOR controls FLIPS translation and TRAIL sensitivity in glioblastoma multiforme cells

The tumor-selective, proapoptotic, death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a mediator of antitumor drug activity and in itself is a promising agent for the treatment of human malignancies. Like many tumors, however, glioblastoma multiforme (GBM), the most fatal form of glioma, exhibits a range of TRAIL sensitivity, and only a small percentage of GBM tumors undergo TRAIL-induced apoptosis. We here show that TRAIL resistance in GBM is a consequence of overexpression of the short isoform of the caspase-8 inhibitor, c-FLICE inhibitory protein (FLIP(S)), and that FLIP(S) expression is in turn translationally enhanced by activation of the Akt-mammalian target of rapamycin (mTOR)-p70 S6 kinase 1 (S6K1) pathway. Conversely, pharmacologic or genetic inhibition of mTOR, or the mTOR target S6K1, suppresses polyribosomal accumulation of FLIP(S) mRNA, FLIP(S) protein expression, and TRAIL resistance. In archived material from 12 human GBM tumors, PTEN status was a predictor of activation of the Akt-mTOR-S6K1 pathway and of FLIP(S) levels, while in xenografted human GBM, activation status of the PTEN-Akt-mTOR pathway distinguished the tumors inherently sensitive to TRAIL from those which could be sensitized by the mTOR inhibitor rapamycin. These results define the mTOR pathway as a key limiter of tumor elimination by TRAIL-mediated mechanisms, provide a means by which the TRAIL-sensitive subset of GBM can be identified, and provide rationale for the combined use of TRAIL with mTOR inhibitors in the treatment of human cancers.

Functional and therapeutic significance of Akt deregulation in malignant melanoma

Identification of specific genes or signaling pathways involved in development of melanoma could lead to new therapies that target and correct these defects. Recent studies have revealed deregulation of the Akt signaling pathway occurring in 43-67% of melanomas. Akt kinase family members, Akt1/PKBalpha, Akt2/PKBbeta and Akt3/PKBgamma, share extensive structural similarity and perform common as well as unique functions within cells. The Akt signaling cascade initiates at the cell surface when growth factors or other extracellular stimuli activate phosphoinositide 3-kinase (PI3K). Activated PI3K generates a lipid second messenger, phosphatidylinositol-3,4,5-trisphosphate (PIP3), causing translocation of Akt to the plasma membrane where it becomes phosphorylated and activated. The balance of cellular PIP3 is regulated primarily by a phosphatase called PTEN that reduces PIP3 levels thereby lowering Akt activity. In melanomas, decreased PTEN activity elevates PIP3 levels resulting in Akt activation. Active Akt then phosphorylates downstream cellular proteins that promote melanoma cell proliferation and survival. Recently, Akt3 was discovered to be the predominant isoform activated in sporadic melanomas. Levels of activity increased during melanoma progression with metastatic melanomas having the highest activity. Although mechanisms of Akt3 activation remain to be fully characterized, overexpression of Akt3 and decreased PTEN activity play important roles in this process. Targeted reduction of Akt3 activity decreased survival of melanoma tumor cells leading to inhibition of tumor development, which may be therapeutically effective for shrinking tumors in melanoma patients. This review surveys recent developments in Akt deregulation in melanoma and its potential as a selective therapeutic target in patients in the advanced stages of this disease.

Mechanism of von Hippel-Lindau protein-mediated suppression of nuclear factor kappa B activity

Biallelic inactivating mutations of the von Hippel-Lindau tumor suppressor gene (VHL) are a hallmark of clear cell renal cell carcinoma (CCRCC), the most common histologic subtype of RCC. Biallelic VHL loss results in accumulation of hypoxia-inducible factor alpha (HIFalpha). Restoring expression of the wild-type protein encoded by VHL (pVHL) in tumors with biallelic VHL inactivation (VHL(-)(/)(-)) suppresses tumorigenesis, and pVHL-mediated degradation of HIFalpha is necessary and sufficient for VHL-mediated tumor suppression. The downstream targets of HIFalpha that promote renal carcinogenesis have not been completely elucidated. Recently, VHL loss was shown to activate nuclear factor kappa B (NF-kappaB), a family of transcription factors that promotes tumor growth. Here we show that VHL loss drives NF-kappaB activation by resulting in HIFalpha accumulation, which induces expression of transforming growth factor alpha, with consequent activation of an epidermal growth factor receptor/phosphatidylinositol-3-OH kinase/protein kinase B (AKT)/IkappaB-kinase alpha/NF-kappaB signaling cascade. We also show that components of this signaling pathway promote the growth of VHL(-)(/)(-) tumor cells. Members of this pathway represent viable drug targets in VHL(-)(/)(-) tumors, such as those associated with CCRCC.

Akt1 contains a functional leucine-rich nuclear export sequence

Nuclear Akt1 expression and Akt activation are common in cancer invasion. However, the mechanisms for this association and its causal role in invasion are uncertain. In an effort to identify potential mechanisms for regulating Akt subcellular localization, we analyzed the Akt gene sequences and identified a highly conserved leucine-rich potential nuclear export sequence (NES). Initial experiments demonstrated that leptomycin B induced nuclear Akt1 localization. Transient expression experiments demonstrated that, in comparison to wild-type Akt1, NES-mutated (AKT/NES) Akt1 has reduced interactions with CRM-1 and persistent nuclear localization. Subsequent stable transfection experiments in Akt1-/- fibroblasts confirmed that expression of AKT/NES resulted in persistent nuclear localization and activation1. Finally, stable expression of AKT/NES in Akt1-/- fibroblasts was sufficient to enhance cell migration in vitro. Thus, Akt1 contains a functional NES and mutation of the NES results in nuclear-predominant Akt1 activation that is sufficient to induce migration.

Functional analysis of PIK3CA gene mutations in human colorectal cancer

Mutations in the PIK3CA gene, which encodes the p110alpha catalytic subunit of phosphatidylinositol 3-kinase (PI3K), have been reported in human cancers, including colorectal cancer. Most of the mutations cluster at hotspots within the helical and kinase domains. Whereas H1047R, one of the hotspot mutants, is reported to have elevated lipid kinase activity, the functional consequences of other mutations have not been examined. In this study, we examined the effects of colon cancer-associated PIK3CA mutations on the lipid kinase activity in vitro, activation of the downstream targets Akt and p70S6K in vivo and NIH 3T3-transforming ability. Of eight mutations examined, all showed increased lipid kinase activity compared with wild-type p110alpha. All the mutants strongly activated Akt and p70S6K compared with wild-type p110alpha as determined by immunoblotting using phospho-specific antibodies. These mutants also induced morphologic changes, loss of contact inhibition, and anchorage-independent growth of NIH 3T3 cells. The hotspot mutations examined in this study, E542K, E545K, and H1047R, all had high enzymatic and transforming activities. These results show that almost all the colon cancer-associated PIK3CA mutations are functionally active so that they are likely to be involved in carcinogenesis.

Inhibition of protein synthesis by Y box-binding protein 1 blocks oncogenic cell transformation

The multifunctional Y box-binding protein 1 (YB-1) is transcriptionally repressed by the oncogenic phosphoinositide 3-kinase (PI3K) pathway (with P3K as an oncogenic homolog of the catalytic subunit) and, when reexpressed with the retroviral vector RCAS, interferes with P3K- and Akt-induced transformation of chicken embryo fibroblasts. Retrovirally expressed YB-1 binds to the cap of mRNAs and inhibits cap-dependent and cap-independent translation. To determine the requirements for the inhibitory role of YB-1 in P3K-induced transformation, we conducted a mutational analysis, measuring YB-1-induced interference with transformation, subcellular localization, cap binding, mRNA binding, homodimerization, and inhibition of translation. The results show that (i) interference with transformation requires RNA binding and a C-terminal domain that is distinct from the cytoplasmic retention domain, (ii) interference with transformation is tightly correlated with inhibition of translation, and (iii) masking of mRNAs by YB-1 is not sufficient to block transformation or to inhibit translation. We identified a noncanonical nuclear localization signal (NLS) in the C-terminal half of YB-1. A mutant lacking the NLS retains its ability to interfere with transformation, indicating that a nuclear function is not required. These results suggest that YB-1 interferes with P3K-induced transformation by a specific inhibition of translation through its RNA-binding domain and a region in the C-terminal domain. Potential functions of the C-terminal region are discussed.

Prognostic significance of activated Akt expression in melanoma: a clinicopathologic study of 292 cases

PURPOSE

Akt is a serine/threonine kinase that leads to stimulation of cell cycle progression, cell proliferation, and inhibition of apoptosis. To investigate the role of Akt in melanoma pathogenesis, we examined the expression of phospho-Akt (p-Akt; Ser-473) in melanocytic lesions at different stages and analyzed the correlations between the p-Akt expression level and clinicopathologic factors and patient survival.

PATIENTS AND METHODS

We evaluated the p-Akt expression in 12 cases of normal nevi, 58 cases of dysplastic nevi, 170 cases of primary melanomas, and 52 cases of melanoma metastases using tissue microarray and immunohistochemistry.

RESULTS

Strong p-Akt expression was observed in 17%, 43%, 49%, and 77% of the biopsies in normal nevi, dysplastic nevi, primary melanoma, and melanoma metastases, respectively. Significant differences for p-Akt staining pattern were observed between normal nevi and primary melanomas (P < .05), and between primary melanomas and melanoma metastases (P < .001). Furthermore, our Kaplan-Meier survival curves showed that strong p-Akt expression is inversely correlated with both overall and disease-specific 5-year survival of patients with primary melanoma (P < .05 for both). Strikingly, our multivariate Cox regression analysis revealed that p-Akt is an independent prognostic factor in low-risk melanomas (thickness < or = 1.5 mm; relative risk, 6.44; 95% CI, 1.28 to 32.55; P = .018).

CONCLUSION

The expression of p-Akt increases dramatically with melanoma invasion and progression and is inversely correlated with patient survival. In addition, p-Akt may serve as an independent prognostic marker and help to identify those patients with low-risk melanomas who are at increased risk of death.

Sonic hedgehog and insulin-like growth factor signaling synergize to induce medulloblastoma formation from nestin-expressing neural progenitors in mice

Medulloblastoma (MB) is a malignant brain tumor that arises in the cerebellum of children. Activation of the Sonic hedgehog/Patched (Shh/Ptc) signaling pathway in neural progenitor cells of the cerebellum induces MBs in mice. The incomplete penetrance of tumor formation in mice, coupled with the low frequency of mutations in Shh/Ptc pathway genes in human tumors, suggests that other signaling molecules cooperate with Shh to enhance MB formation. We modeled the ability of insulin-like growth factor (IGF) signaling to induce MB using the RCAS/tv-a system, which allows postnatal gene transfer and expression in a cell-type-specific manner. We used RCAS retroviral vectors to target expression of Shh, IGF2, and activated Akt to nestin-expressing neural progenitors in the cerebella of newborn mice. The incidence of Shh-induced tumor formation (15%) was enhanced by coexpression with IGF2 (39%) and Akt (48%). Neither IGF2 nor Akt caused tumors when expressed independently. The induced tumors showed upregulated expression of insulin receptor substrate 1 and phosphorylated forms of IGF1 receptor and Akt, mimicking activated IGF signaling found in human MBs. These results indicate that combined activation of the Shh/Ptc and IGF signaling pathways is an important mechanism in MB pathogenesis.

Genetic and hypoxic regulation of angiogenesis in gliomas

Infiltrative astrocytic neoplasms are by far the most common malignant brain tumors in adults. Clinically, they are highly problematic due to their widely invasive nature which makes a complete resection almost impossible. Biologic progression of these tumors is inevitable and adjuvant therapies are only moderately effective in prolonging survival. Glioblastoma multiforme (GBM; WHO grade IV), the most malignant form of infiltrating astrocytoma, can evolve from a lower grade precursor tumor (secondary GBM) or can present as high grade lesion from the outset, so-called de novo GBM. Molecular genetic investigations suggest that GBMs are comprised of multiple molecular genetic subsets. Notwithstanding the diversity of genetic alterations leading to the GBM phenotype, the vascular changes that evolve in this disease, presumably favoring further growth, are remarkably similar. Underlying genetic alterations in GBM may tilt the balance in favor of an angiogenic phenotype by upregulation of pro-angiogenic factors and down-regulation of angiogenesis inhibitors. Increased vascularity and endothelial cell proliferation in GBMs are also driven by hypoxia-induced expression of pro-angiogenic cytokines, such vascular endothelial growth factor (VEGF). Understanding the contribution of genetic alterations and hypoxia in angiogenic dysregulation in astrocytic neoplasms will lead to the development of better anti-angiogenic therapies for this disease. This review will summarize the properties of angiogenic dysregulation that lead to the highly vascularized nature of these tumors.

Akt phosphorylation is a risk factor for early disease recurrence and poor prognosis in hepatocellular carcinoma

BACKGROUND

Patients with hepatocellular carcinoma (HCC) who showed early massive disease recurrence due to hematogenous intrahepatic metastasis after curative resection had a poor prognosis. The authors previously reported that Akt phosphorylation was correlated with hematogenous intrahepatic metastasis, using HCC cell lines.

METHODS

The authors analyzed clinicopathologic features and the status of selected biologic markers, including phosphorylated Akt, to identify risk factors for early disease recurrence and poor prognosis in HCC. In the current series, 49 postoperative patients developed intrahepatic disease recurrence within 6 months (Group 1) and 86 patients remained disease recurrence free > 3 years after resection (Group 2). Group 1 was further divided into 2 subgroups: 19 patients who died of disease recurrence within a year after resection (Group 1A) and 27 patients who survived > 1 year (Group 1B).

RESULTS

Using univariate analysis, the risk factors for early disease recurrence were tumor size, macroscopic classification, tumor differentiation, microscopic capsule infiltration, microscopic portal vein (MPV) invasion, microscopic intrahepatic metastasis (MIM), and positive immunostaining for phosphorylated Akt, Ki-67, and p53 (P < 0.05). The risk factors for poor prognosis were the number of intrahepatic metastases, tumor differentiation, and positive immunostaining for phosphorylated Akt and Ki-67 (P < 0.05). Multivariate analysis revealed that the risk factors for early disease recurrence were MPV invasion, MIM, and positive immunostaining for phosphorylated Akt, and that the risk factors for poor prognosis were positive immunostaining for phosphorylated Akt and Ki-67 (P < 0.05).

CONCLUSIONS

The current clinical study showed the critical involvement of Akt phosphorylation in the aggressiveness of HCC. The potential benefits of surgery should be assessed carefully in patients with any of these risk factors.

Restoration of SHIP activity in a human leukemia cell line downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle

The inositol 5-phosphatase SHIP (SHIP-1) is a negative regulator of signal transduction in hematopoietic cells and targeted disruption of SHIP in mice leads to a myeloproliferative disorder. We analyzed the effects of SHIP on the human leukemia cell line Jurkat in which expression of endogenous SHIP protein is not detectable. Restoration of SHIP expression in Jurkat cells with an inducible expression system caused a 69% reduction of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) and a 65% reduction of Akt kinase activity, which was associated with reduced phosphorylation of glycogen synthase kinase 3beta (GSK-3beta) (Ser-9) without changing the phosphorylation of Bad (Ser-136), FKHR (Ser-256) or MAPK (Thr-202/Tyr-204). SHIP-expressing Jurkat cells showed an increased transit time through the G1 phase of the cell cycle, but SHIP did not cause a complete cell cycle arrest or apoptosis. Extension of the G1 phase was associated with an increased stability of the cell cycle inhibitor p27(Kip1) and reduced phosphorylation of the retinoblastoma protein Rb at serine residue 780. Our data indicate that restoration of SHIP activity in a human leukemia cell line, which has lost expression of endogenous SHIP, downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle.

The mood stabilizer valproic acid stimulates GABA neurogenesis from rat forebrain stem cells

Valproate, an anticonvulsant drug used to treat bipolar disorder, was studied for its ability to promote neurogenesis from embryonic rat cortical or striatal primordial stem cells. Six days of valproate exposure increased by up to fivefold the number and percentage of tubulin beta III-immunopositive neurons, increased neurite outgrowth, and decreased by fivefold the number of astrocytes without changing the number of cells. Valproate also promoted neuronal differentiation in human fetal forebrain stem cell cultures. The neurogenic effects of valproate on rat stem cells exceeded those obtained with the neurotrophins brain-derived growth factor (BDNF) or NT-3, and slightly exceeded the effects obtained with another mood stabilizer, lithium. No effect was observed with carbamazepine. Most of the newly formed neurons were GABAergic, as shown by 10-fold increases in neurons that immunostained for GABA and the GABA-synthesizing enzyme GAD65/67. Double immunostaining for bromodeoxyuridine and tubulin beta III showed that valproate increased by four- to fivefold the proliferation of neuronal progenitors derived from rat stem cells and increased cyclin D2 expression. Valproate also regulated the expression of survival genes, Bad and Bcl-2, at different times of treatment. The expression of prostaglandin E synthase, analyzed by quantitative RT-PCR, was increased by ninefold as early as 6 h into treatment by valproate. The enhancement of GABAergic neuron numbers, neurite outgrowth, and phenotypic expression via increases in the neuronal differentiation of neural stem cell may contribute to the therapeutic effects of valproate in the treatment of bipolar disorder.

Activation of aPKC is required for vanadate-induced phosphorylation of protein kinase B (Akt), but not p70S6k in mouse epidermal JB6 cells

Vanadium is a metal widely distributed in the environment. Although vanadate-containing compounds exert potent toxic effects on a wide variety of biological systems, the mechanisms by which vanadate mediates adverse effects are not well understood. The present study investigated the vanadate-induced phosphorylation of Akt and p70S6K, two kinases known to be vital for cell survival, growth, transformation, and transition of the cell cycle in mammals. Exposure of mouse epidermal JB6 cells to vanadium led to phosphorylation of Akt and p70S6K in a time- and dose-dependent manner. Vanadium exposure also caused translocation of atypical isoforms of PKC (lambda, zeta) from the cytosol to the membrane, but had no effect on PKCalpha translocation, suggesting that the atypical PKCs (aPKC) were specifically involved in vanadium-induced cellular response. Importantly, overexpression of a dominant negative mutant PKClambda blocked Akt phosphorylation at Ser473 and Thr308, whereas it did not inhibit p70S6k phosphorylation at Thr389 and Thr421/Ser424, suggesting that aPKC activation is specifically involved in vanadium-induced activation of Akt, but not in activation of p70S6k. Furthermore, vanadium-induced p70S6k phosphorylation at Thr389 and Thr421/Ser424 and Akt phosphorylation at Thr308 occurred through a PI-3K-dependent pathway because a PI-3K dominant negative mutant inhibited induction as compared with vector control cells. These results indicate that there was a differential role of aPKC in vanadate-induced phosphorylation of Akt and p70S6k, suggesting that signal transduction pathways leading to the activation of Akt and p70S6k were different.

Proteasomal degradation of the FoxO1 transcriptional regulator in cells transformed by the P3k and Akt oncoproteins

The P3k oncoprotein [homolog of the catalytic subunit p110alpha of class 1A phosphoinositide 3-kinase (PI3K)] and its downstream effector Akt induce oncogenic transformation in cultures of chicken embryo fibroblasts (CEF). The winged helix transcription factor FoxO1 is a growth-attenuating and proapoptotic protein and serves as a substrate of Akt. Here we show that FoxO1 expression is constitutively suppressed in CEF transformed by P3k or Akt. The FoxO1 protein level is high in serum-starved normal CEF, but platelet-derived growth factor treatment induces rapid phosphorylation and disappearance of FoxO1. PI3K inhibitors or the proteasome inhibitor lactacystin interfere with this process. These data suggest that phosphorylation-dependent degradation of FoxO1 by means of proteasomes plays a role in oncogenic transformation by P3k and Akt. A dominant negative mutant of FoxO1 containing the repressor domain of the Drosophila Engrailed protein induces partial oncogenic transformation of CEF and interferes with FoxO1-dependent transcriptional activation. The FoxG1 oncoprotein also inhibits transcriptional activation by FoxO1. Inhibition of FoxO1, albeit by different mechanisms, appears to be a common denominator of the PI3K and FoxG1 oncogenic pathways.

A proline-rich motif in the C terminus of Akt contributes to its localization in the immunological synapse

The serine/threonine kinases of the Akt/protein kinase B family are regulated in part by recruitment to the plasma membrane, which is accomplished by the binding of an N-terminal PH domain to the phosphatidylinositol 3-kinase products phosphoinositol 3,4,5-trisphosphate and phosphoinositol 3,4-bisphosphate. We have examined Akt localization in a murine T cell clone (D10) before and after stimulation by APC/Ag, and we found that whereas the pleckstrin homology domain is required for plasma membrane recruitment of Akt upon T cell activation, the C terminus of the kinase restricts its cellular localization to the immunologic synapse formed at the site of T cell/APC contact. A recently described proline-rich motif in this region appears to be important for proper localization of full-length Akt. Moreover, a form of Akt in which this motif was mutated acts as a potent dominant negative construct to block T cell activation. Therefore, multiple mechanisms are involved in the proper targeting of Akt during the early events of T cell activation.

AKT status controls susceptibility of malignant keratinocytes to the early-activated and UVB-induced apoptotic pathway

In previous work, we have described an early-activated and ultraviolet B (UVB)-induced apoptotic pathway in human keratinocytes, which can be completely inhibited by AKT activation. We now compared this response of primary human keratinocytes with the response of two p53-mutated squamous cell carcinoma (SCC)-derived cell lines (A431 and A253) to an apoptotic UVB dose. In these cell lines, both the basal AKT phosphorylation status and the apoptotic response to UVB diverged strongly from the response of healthy primary keratinocytes. Even more, a remarkable correlation was found between the two. Although a constitutive dual phosphorylation of AKT rendered the A253 SCC cell line completely resistant to the early-activated and UVB-induced apoptotic pathway, deficient T308 phosphorylation of AKT in the SCC cell line A431 led to a greatly augmented sensitivity to the early-activated, UVB-induced apoptotic pathway. These results indicate that the preservation of a healthy AKT pathway is essential for a wild-type UVB-induced apoptotic response in skin, and suggest that AKT-mediated dysregulation of the early-activated apoptotic response to UVB is an important event in the oncogenic transformation of keratinocytes.

Transformation potency of ErbB heterodimer signaling is determined by B-Raf kinase

Cellular transformation occurs only in cells that express both ErbB1 and ErbB4 receptors, but not in cells expressing only one or the other of these receptors. However, when both receptors are coexpressed and ligand-stimulated, they interact with virtually the same adaptor/effector proteins as when expressed singly. To reveal the underlying regulatory mechanism of the kinase/phosphatase network in ErbB homo- and heterodimer receptor signaling, extracellular signal-regulated kinase (ERK) and Akt activities were evaluated in the presence of several enzyme inhibitors in ligand-induced cells expressing ErbB1 (E1), ErbB4 (E4), and ErbB1/ErbB4 (E1/4) receptor. The PP2A inhibitor okadaic acid showed receptor-specific inhibitory profiles for ERK and Akt activities. Moreover, B-Raf isolated only from E1/4 cells could induce in vitro phosphorylation for MEK; this B-Raf kinase activity was abolished by pretreatment of the cells with okadaic acid. Our study further showed that the E1/4 cell-specific B-Raf activity was stimulated by PLC gamma and subsequent Rap1 activation. The present study suggests that B-Raf kinase, which was specifically activated in the cells coexpressing ErbB1 and ErbB4 receptors, elevates total ERK activity within the cell and, therefore, can induce cellular transformation.

Akt activation and localisation correlate with tumour invasion and oncogene expression in thyroid cancer

INTRODUCTION

Akt activation is involved in the pathogenesis of inherited thyroid cancer in Cowden's syndrome and in sporadic thyroid cancers. In cell culture, Akt regulates thyroid cell growth and survival; but recent data suggest that Akt also regulates cell motility in non-thyroid cell lines. We therefore sought to evaluate the role of Akt in thyroid cancer progression.

METHODS

We evaluated 46 thyroid cancer, 20 thyroid follicular adenoma, and adjacent normal tissues samples by immunohistochemistry for activated Akt (pAkt), Akt 1, 2, and 3, and p27 expression. Immunoblots were performed in 14 samples.

RESULTS

Akt activation was identified in 10/10 follicular cancers, 26/26 papillary cancers, and 2/10 follicular variant of papillary cancers, but in only 4/66 normal tissue samples and 2/10 typical benign follicular adenomas. Immunoactive pAkt was greatest in regions of capsular invasion; and was localised to the nucleus in follicular cancers and the cytoplasm in papillary cancers, except for invasive regions of papillary cancers where it localised to both compartments. Immunoactive Akt 1, but not Akt 2 or Akt 3, correlated with pAkt localisation, and nuclear pAkt was associated with cytoplasmic expression of p27. In vitro studies using human thyroid cancer cells demonstrated that nuclear translocation of Akt 1 and pAkt were associated with cytoplasmic p27 and cell invasion and migration. Cell migration and the localisation of Akt 1, pAkt, and p27 were inhibited by PI3 kinase, but not MEK inhibition.

DISCUSSION

These data suggest an important role for nuclear activation of Akt 1 in thyroid cancer progression.

A role for MKP3 in axial patterning of the zebrafish embryo

Fibroblast growth factors (FGFs) are secreted molecules that can activate the RAS/mitogen-activated protein kinase (MAPK) pathway to serve crucial functions during embryogenesis. Through an in situ hybridization screen for genes with restricted expression patterns during early zebrafish development,we identified a group of genes that exhibit similar expression patterns to FGF genes. We report the characterization of zebrafish MAP kinase phosphatase 3(MKP3; DUSP6 - Zebrafish Information Network), a member of the FGF synexpression group, showing that it has a crucial role in the specification of axial polarity in the early zebrafish embryo. MKP3 dephosphorylates the activated form of MAPK, inhibiting the RAS/MAPK arm of the FGF signaling pathway. Gain- and loss-of-function studies reveal that MKP3 is required to limit the extent of FGF/RAS/MAPK signaling in the early embryo, and that disturbing this inhibitory pathway disrupts dorsoventral patterning at the onset of gastrulation. The earliest mkp3 expression is restricted to the future dorsal region of the embryo where it is initiated by a maternalβ-catenin signal, but soon after its initiation, mkp3 expression comes under the control of FGF signaling. Thus, mkp3 encodes a feedback attenuator of the FGF pathway, the expression of which is initiated at an early stage so as to ensure correct FGF signaling levels at the time of axial patterning.

Overexpression of c-MYC promotes an undifferentiated phenotype in cultured astrocytes and allows elevated Ras and Akt signaling to induce gliomas from GFAP-expressing cells in mice

The c-MYC protooncogene is overexpressed in the most malignant primary brain tumor, glioblastoma multiforme (GBM), and has been correlated with the undifferentiated character of several cell types. However, the role of Myc activity in the generation of GBMs is not known. In this report, we show that gene transfer of c-MYC to GFAP-expressing astrocytes in vitro promotes the outgrowth of GFAP-negative, nestin-expressing cells with progenitor-like morphology, growth characteristics and gene-expression pattern. In addition, gene transfer of c-MYC to GFAP-expressing astrocytes in vivo induces GBMs when co-expressed with activated Ras and Akt. Without c-MYC, Ras+Akt induces GBMs from nestin-expressing CNS progenitors but is insufficient in GFAP-expressing differentiated astrocytes. The ability of Myc activity to enhance the oncogenic effects of Ras+Akt appears to be limited to GFAP-expressing astrocytes because nestin-expressing progenitors show no increase in GBM formation with the addition of MYC to Ras+Akt. These studies indicate that one role of MYC activity in the formation of gliomas might be to either promote or reinforce an undifferentiated phenotype required for glioma cells to respond to the oncogenic effects of elevated Ras and Akt activity.

An essential role for protein synthesis in oncogenic cellular transformation

The induction and maintenance of oncogenic transformation requires interference with the controls that regulate translation and transcription. The PI 3-kinase pathway, which shows gain of function in numerous and diverse human cancers, generates signals that have a positive effect on the initiation of protein synthesis. Here we review the components of the PI 3-kinase signaling pathway and the mRNA-binding protein YB-1, exploring their roles in protein synthesis and oncogenic cell transformation.

Nuclear targeting of Akt enhances kinase activity and survival of cardiomyocytes

Heart failure is associated with death of cardiomyocytes leading to loss of contractility. Previous studies using membrane-targeted Akt (myristolated-Akt), an enzyme involved in antiapoptotic signaling, showed inhibition of cell death and prevention of pathogenesis induced by cardiomyopathic stimuli. However, recent studies by our group have found accumulation of activated Akt in the nucleus, suggesting that biologically relevant target(s) of Akt activity may be located there. To test this hypothesis, a targeted Akt construct was created to determine the antiapoptotic action of nuclear Akt accumulation. Nuclear localization of the adenovirally encoded Akt construct was confirmed by confocal microscopy. Cardiomyocytes expressing nuclear-targeted Akt showed no evidence of morphological remodeling such as altered myofibril density or hypertrophy. Nuclear-targeted Akt significantly elevated levels of phospho-Akt and kinase activity and inhibited apoptosis as effectively as myristolated-Akt in hypoxia-induced cell death. Transgenic overexpression of nuclear-targeted Akt did not result in hypertrophic remodeling, altered cardiomyocyte DNA content or nucleation, or enhanced phosphorylation of typical cytoplasmic Akt substrates, yet transgenic hearts were protected from ischemia-reperfusion injury. Gene array analyses demonstrated changes in the transcriptional profile of Akt/nuc hearts compared with nontransgenic controls distinct from prior characterizations of Akt expression in transgenic hearts. Collectively, these experiments show that targeting of Akt to the nucleus mediates inhibition of apoptosis without hypertrophic remodeling, opening new possibilities for therapeutic applications of nuclear-targeted Akt to inhibit cell death associated with heart disease.