Direct identification of PTEN phosphorylation sites

Reconstitution of PTEN activity by CK2 inhibitors and interference with the PI3-K/Akt cascade counteract the antiapoptotic effect of human stromal cells in chronic lymphocytic leukemia

Evidence suggests that tumor microenvironment is critically involved in supporting survival of chronic lymphocytic leukemia (CLL) cells. However, the molecular mechanisms of this effect and the clinical significance are not fully understood. We applied a microenvironment model to explore the interaction between CLL cells and stromal cells and to elucidate the role of phosphatidylinositol 3 kinase (PI3-K)/Akt/phosphatase and tensin homolog detected on chromosome 10 (PTEN) cascade in this process and its in vivo relevance. Primary human stromal cells from bone marrow, lymph nodes, and spleen significantly inhibited spontaneous apoptosis of CLL cells. Pan-PI3-K inhibitors (LY294002, wortmannin, PI-103), isotype-specific inhibitors of p110α, p110β, p110γ, and small interfering RNA against PI3-K and Akt1 counteracted the antiapoptotic effect of the stromal cells. Induction of apoptosis was associated with a decrease in phosphatidylinositol-3,4,5-triphosphate, PI3-K-p85, and dephosphorylation of phosphatidylinositol-dependent kinase-1 (PDK-1), Akt1, and PTEN. Freshly isolated peripheral blood mononuclear cells from patients with CLL (n = 44) showed significantly higher levels of phosphorylated Akt1, PDK-1, PTEN, and CK2 than healthy persons (n = 8). CK2 inhibitors (4,5,6,7-tetrabromo-1H-benzotriazole, apigenin, and 5,6-dichloro-1-β-D-ribofuranosylbenzimidazol) decreased phosphorylation of PTEN and Akt, induced apoptosis in CLL cells, and enhanced the response to fludarabine. In conclusion, bone marrow microenvironment modulates the PI3-K/Akt/PTEN cascade and prevents apoptosis of CLL cells. Combined inhibition of PI3-K/Akt and recovery of PTEN activity may represent a novel therapeutic concept for CLL.

Casein Kinase II: an attractive target for anti-cancer drug design

Casein Kinase II (CK2) is a ubiquitous serine/threonine kinase that is highly conserved in eukaryotic cells. CK2 has been shown to impact cell growth and proliferation, as numerous growth-related proteins are substrates of CK2. More importantly, experimental evidence linking increased expression and activity of CK2 to human cancers underscores the relevance of CK2 biology to cellular transformation and carcinogenesis. Due to the critical regulatory role CK2 plays in cell fate determination in cancer cells, there is a tremendous interest in the development of CK2-specific therapies. Supporting this, recent reports have demonstrated that genetic manipulation of CK2 expression as well as pharmacological inhibition of its enzymatic activity sensitizes cancers to apoptotic stimuli. Here we provide a succinct account of the biology of CK2, its cellular substrates, its pro-survival and pro-proliferation activity, and highlight evidence for its involvement in human cancer.

Network of Cancer Genes: a web resource to analyze duplicability, orthology and network properties of cancer genes

The Network of Cancer Genes (NCG) collects and integrates data on 736 human genes that are mutated in various types of cancer. For each gene, NCG provides information on duplicability, orthology, evolutionary appearance and topological properties of the encoded protein in a comprehensive version of the human protein-protein interaction network. NCG also stores information on all primary interactors of cancer proteins, thus providing a complete overview of 5357 proteins that constitute direct and indirect determinants of human cancer. With the constant delivery of results from the mutational screenings of cancer genomes, NCG represents a versatile resource for retrieving detailed information on particular cancer genes, as well as for identifying common properties of precompiled lists of cancer genes. NCG is freely available at: http://bio.ifom-ieo-campus.it/ncg.

Dephosphorylation and inactivation of Akt/PKB is counteracted by protein kinase CK2 in HEK 293T cells

Akt (PKB) is a critical kinase in cell-survival pathways. Its activity depends on the phosphorylation of Thr308 and Ser473, by PDK1 and mTORC2, respectively. We found that Akt can be further stimulated through phosphorylation of Ser129 by another kinase, CK2. Here we show that phosphorylation of Akt at Ser129 also facilitates its association with Hsp90 chaperone, thus preventing Thr308 dephosphorylation. This is supported by the following observations: (1) phospho-Thr308 decreases when Ser129 is mutated to alanine, (2) this decrease is abolished by cell treatment with okadaic acid (to inactivate PP2A) or geldanamycin (to inactivate Hsp90), (3) phosphorylation of Ser129 neither enhances the activity of PDK1 nor hampers the in vitro activity of PP2A on Thr308, but increases the Hsp90 association to Akt. These data support the view that the antiapoptotic potential of CK2 is at least in part mediated by its ability to maintain Akt in its active form.

Analysis of PTEN complex assembly and identification of heterogeneous nuclear ribonucleoprotein C as a component of the PTEN-associated complex

PTEN (phosphatase and tensin homolog deleted on chromosome 10) is well characterized for its role in antagonizing the phosphoinositide 3-kinase pathway. Previous studies using size-exclusion chromatography demonstrated PTEN recruitment into high molecular mass complexes and hypothesized that PTEN phosphorylation status and PDZ binding domain may be required for such complex formation. In this study, we set out to test the structural requirements for PTEN complex assembly and identify the component(s) of the PTEN complex(es). Our results demonstrated that the PTEN catalytic function and PDZ binding domain are not absolutely required for its complex formation. On the other hand, PTEN phosphorylation status has a significant impact on its complex assembly. Our results further demonstrate enrichment of the PTEN complex in nuclear lysates, suggesting a mechanism through which PTEN phosphorylation may regulate its complex assembly. These results prompted further characterization of other protein components within the PTEN complex(es). Using size-exclusion chromatography and two-dimensional difference gel electrophoresis followed by mass spectrometry analysis, we identified heterogeneous nuclear ribonucleoprotein C (hnRNP C) as a novel protein recruited to higher molecular mass fractions in the presence of PTEN. Further analysis indicates that endogenous hnRNP C and PTEN interact and co-localize within the nucleus, suggesting a potential role for PTEN, alongside hnRNP C, in RNA regulation.

Altering PI3K-Akt signalling in zebrafish embryos affects PTEN phosphorylation and gastrulation

BACKGROUND INFORMATION

PTEN (phosphatase and tensin homologue deleted on chromosome 10) is a negative regulator of the PI3K (phosphoinositide 3-kinase)-Akt (also called protein kinase B) signalling pathway and is essential for embryogenesis, but its function in early vertebrate embryos is unclear.

RESULTS

To address how PTEN functions in early embryos, we overexpressed one of the four zebrafish PTEN isoforms at the 1-2-cell stage. Overexpression of Ptena454 alters phospho-Akt levels and impairs cell movements associated with gastrulation. Heat shocking embryos increases phospho-Akt levels and lowers phospho-Ptena454 levels. Inhibiting CK2 (protein kinase CK2) activity reduces phospho-Pten levels and augments the effects due to Ptena454 overexpression. Low phospho-Akt and corresponding low phospho-GSK-3 (glycogen synthase kinase-3) and high phospho-Pten levels accompany wortmannin or LY294002 treatment, which inhibit PI3K activity.

CONCLUSIONS

These results suggest that Ptena454 regulation is correlated to changes in phospho-Akt levels. We propose a model in which homoeostasis in rapidly dividing and migrating embryonic cells depends on a counterbalance between pro-survival signalling employing CK2 and GSK-3 and the pro-apoptotic activity of Ptena454.

The PTEN-AKT3 signaling cascade as a therapeutic target in melanoma

Melanocytes undergo extensive genetic changes during transformation into aggressive melanomas. These changes deregulate genes whose aberrant activity promotes the development of this disease. The phosphoinositide-3-kinase (PI3K) and mitogen-activated protein (MAP) kinase pathways are two key signaling cascades that have been found to play prominent roles in melanoma development. These pathways relay extra-cellular signals via an ordered series of consecutive phosphorylation events from cell surface throughout the cytoplasm and nucleus regulating diverse cellular processes including proliferation, survival, invasion and angiogenesis. It is generally accepted that therapeutic agents would need to target these two pathways to be an effective therapy for the long-term treatment of advanced-stage melanoma patients. This review provides an overview of the PI3 kinase pathway focusing specifically on two members of the pathway, called PTEN and Akt3, which play important roles in melanoma development. Mechanisms leading to deregulation of these two proteins and therapeutic implications of targeting this signaling cascade to treat melanoma are detailed in this review.

Protein kinase CK2 in health and disease: CK2: a key player in cancer biology

Elevated levels of protein kinase CK2 (formerly casein kinase 2 or II) have long been associated with increased cell growth and proliferation both in normal and cancer cells. The ability of CK2 to also act as a potent suppressor of apoptosis offers an important link to its involvement in cancer since deregulation of both cell proliferation and apoptosis are among the key features of cancer cell biology. Dysregulated CK2 may impact both of these processes in cancer cells. All cancers that have been examined show increased CK2 expression, which may also relate to prognosis. The extensive involvement of CK2 in cancer derives from its impact on diverse molecular pathways controlling cell proliferation and cell death. Downregulation of CK2 by various approaches results in induction of apoptosis in cultured cell and xenograft cancer models suggesting its potential as a therapeutic target.

Leptin-dependent phosphorylation of PTEN mediates actin restructuring and activation of ATP-sensitive K+ channels

Leptin activates multiple signaling pathways in cells, including the phosphatidylinositol 3-kinase pathway, indicating a degree of cross-talk with insulin signaling. The exact mechanisms by which leptin alters this signaling pathway and how it relates to functional outputs are unclear at present. A previous study has established that leptin inhibits the activity of the phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10), an important tumor suppressor and modifier of phosphoinositide signaling. In this study we demonstrate that leptin phosphorylates multiple sites on the C-terminal tail of PTEN in hypothalamic and pancreatic beta-cells, an action not replicated by insulin. Inhibitors of the protein kinases CK2 and glycogen synthase kinase 3 (GSK3) block leptin-mediated PTEN phosphorylation. PTEN phosphorylation mutants reveal the critical role these sites play in transmission of the leptin signal to F-actin depolymerization. CK2 and GSK3 inhibitors also prevent leptin-mediated F-actin depolymerization and consequent ATP-sensitive K(+) channel opening. GSK3 kinase activity is inhibited by insulin but not leptin in hypothalamic cells. Both hormones increase N-terminal GSK3 serine phosphorylation, but in hypothalamic cells this action of leptin is transient. Leptin, not insulin, increases GSK3 tyrosine phosphorylation in both cell types. These results demonstrate a significant role for PTEN in leptin signal transmission and identify GSK3 as a potential important signaling node contributing to divergent outputs for these hormones.

A phosphorylation-dependent intramolecular interaction regulates the membrane association and activity of the tumor suppressor PTEN

The PI 3-phosphatase PTEN (phosphatase and tensin homologue deleted on chromosome 10), one of the most important tumor suppressors, must associate with the plasma membrane to maintain appropriate steady-state levels of phosphatidylinositol 3,4,5-triphosphate. Yet the mechanism of membrane binding has received little attention and the key determinants that regulate localization, a phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding motif and a cluster of phosphorylated C-terminal residues, were not included in the crystal structure. We report that membrane binding requires PIP(2) and show that phosphorylation regulates an intramolecular interaction. A truncated version of the enzyme, PTEN(1-351), bound strongly to the membrane, an effect that was reversed by co-expression of the remainder of the molecule, PTEN(352-403). The separate fragments associated in vitro, an interaction dependent on phosphorylation of the C-terminal cluster, a portion of the PIP(2) binding motif, integrity of the phosphatase domain, and the CBR3 loop. Our investigation provides direct evidence for a model in which PTEN switches between open and closed states and phosphorylation favors the closed conformation, thereby regulating localization and function. Small molecules targeting these interactions could potentially serve as therapeutic agents in antagonizing Ras or PI3K-driven tumors. The study also stresses the importance of determining the structure of the native enzyme.

Understanding PTEN regulation: PIP2, polarity and protein stability

The PTEN tumour suppressor is a lipid and protein phosphatase that inhibits phosphoinositide 3-kinase (PI3K)-dependent signalling by dephosphorylating phosphatidylinositol 3,4,5-trisphosphate (PtdInsP(3)). Here, we discuss the concept of PTEN as an 'interfacial enzyme', which exists in a high activity state when bound transiently at membrane surfaces containing its substrate and other acidic lipids, such as PtdIns(4,5)P(2) and phosphatidylserine (PtdSer). This mechanism ensures that PTEN functions in a spatially restricted manner, and may explain its involvement in forming the gradients of PtdInsP(3), which are necessary for generating and/or sustaining cell polarity during motility, in developing neurons and in epithelial tissues. Coordinating PTEN activity with alternative mechanisms of PtdInsP(3) metabolism, by the tightly regulated SHIP 5-phoshatases, synthesizing the independent second messenger PtdIns(3,4)P(2), may also be important for cellular polarization in some cell types. Superimposed on this interfacial mechanism are additional post-translational regulatory processes, which generally act to reduce PTEN activity. Oxidation of the active site cysteine residue by reactive oxygen species and phosphorylation of serine/threonine residues at sites in the C-terminus of the protein inhibit PTEN. These phosphorylation sites also appear to play a role in regulating both stability and localization of PTEN, as does ubiquitination of PTEN. Because genetic studies in mice show that the level of expression of PTEN in an organism profoundly influences tumour susceptibility, factors that regulate PTEN, localization, activity and turnover should be important in understanding its biological functions as a tumour suppressor.

PTEN posttranslational inactivation and hyperactivation of the PI3K/Akt pathway sustain primary T cell leukemia viability

Mutations in the phosphatase and tensin homolog (PTEN) gene leading to PTEN protein deletion and subsequent activation of the PI3K/Akt signaling pathway are common in cancer. Here we show that PTEN inactivation in human T cell acute lymphoblastic leukemia (T-ALL) cells is not always synonymous with PTEN gene lesions and diminished protein expression. Samples taken from patients with T-ALL at the time of diagnosis very frequently showed constitutive hyperactivation of the PI3K/Akt pathway. In contrast to immortalized cell lines, most primary T-ALL cells did not harbor PTEN gene alterations, displayed normal PTEN mRNA levels, and expressed higher PTEN protein levels than normal T cell precursors. However, PTEN overexpression was associated with decreased PTEN lipid phosphatase activity, resulting from casein kinase 2 (CK2) overexpression and hyperactivation. In addition, T-ALL cells had constitutively high levels of ROS, which can also downmodulate PTEN activity. Accordingly, both CK2 inhibitors and ROS scavengers restored PTEN activity and impaired PI3K/Akt signaling in T-ALL cells. Strikingly, inhibition of PI3K and/or CK2 promoted T-ALL cell death without affecting normal T cell precursors. Overall, our data indicate that T-ALL cells inactivate PTEN mostly in a nondeletional, posttranslational manner. Pharmacological manipulation of these mechanisms may open new avenues for T-ALL treatment.

Resistance to TRAIL-induced apoptosis caused by constitutional phosphorylation of Akt and PTEN in acute lymphoblastic leukemia cells

OBJECTIVE

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor superfamily, which induces apoptosis in cancer cells but not in normal cells. Akt/protein kinase B, when phosphorylated to its active form, promotes cell survival and blocks apoptosis. The aim of this study was to investigate the role of Akt pathway in acquired TRAIL resistance of acute lymphoblastic leukemia cells.

MATERIALS AND METHODS

MB-IT and NALM-24 cells that developed resistance to TRAIL, i.e., TRAIL-resistant cells (MB-IT R and NALM-24 R) were established from TRAIL-sensitive acute lymphoblastic leukemia cell lines (MB-IT S and NALM-24 S), respectively, through application of TRAIL and repetitive limiting dilution. Apoptosis was measured by flow cytometry using propidium iodide/Annexin-V fluorescein isothiocyanate staining. TRAIL receptor cell surface expression of MB-IT and NALM-24 were analyzed by flow cytometry. Protein levels were analyzed by Western blot analysis.

RESULTS

The obtained resistant cell lines presented the same pattern of receptor expression as sensitive parent cells, and the internalization of DR5 after TRAIL treatment was similar. Caspase-8/3, FLIP, BID, XIAP were cleaved/downregulated in sensitive cells after treatment with TRAIL, but not in the resistant cells. We also observed that phosphoinositide-3-kinase (PI3K)/Akt pathway was constitutively active in resistant clones, and was not downregulated upon TRAIL treatment. Phosphate and tensin homologue deleted on chromosome 10 (PTEN) level was the same in both sensitive cells and resistant cells, but was quickly downregulated in sensitive cells after TRAIL treatment. Also, resistant cells expressed a high level of phosphorylated inactive form of PTEN than the sensitive cells. Expression levels of PH domain leucine-rich repeat protein phosphatase were slightly higher in sensitive than resistant cells. When resistant cells were treated with LY 294002 (a PI3K inhibitor), the expression level of phosphorylated Akt was distinctly downregulated, and there was induction of apoptosis when these cells were treated with a combination of TRAIL and LY 294002. When MB-IT-sensitive cells were treated with okadaic acid, a phosphatase inhibitor, TRAIL-induced apoptosis was significantly reduced.

CONCLUSION

These results suggest that cellular resistance to TRAIL could be developed through phosphorylation (activation) of Akt and phosphorylation (inactivation) of PTEN.

PTEN in brain tumors

The tumor suppressor PTEN dephosphorylates phospholipids generated through the activity of PI3K. PTEN thus antagonizes PI3K activity and regulates a multitude of cellular processes such as angiogenesis, motility, invasiveness, survival and proliferation, all of which can initiate and sustain the malignant phenotype. Although PTEN's lipid phosphatase activity is key to its tumor suppressive functions, it also dephosphorylates protein substrates and interacts with other key regulatory molecules, salient among them the tumor suppressor p53. Given the critical roles of PTEN in cellular homeostasis, it is not surprising that both PTEN expression levels and PTEN protein activities are tightly controlled by a complex conglomeration of molecules that regulate post-translational modifications, subcellular localization, transcriptional activation and transcriptional repression. As one of the most commonly altered molecules in human disease, PTEN plays an important role in a myriad of signaling cascades, and plays a central role in normal brain development and brain tumor pathogenesis. As such it influences prognosis of human cancer, predicts response to therapy, constitutes the lynchpin of genetic syndromes, and may underlie neurocognitive abnormalities such as autism spectrum disorders and Alzheimer's disease. Thus, targeting PTEN and its signaling affiliates sows the seeds for combating not only cancer but also neurocognitive disorders.

Tenets of PTEN tumor suppression

Since its discovery as the elusive tumor suppressor gene at the frequently mutated 10q23 locus, PTEN has been identified as lost or mutated in several sporadic and heritable tumor types. A decade of work has established that PTEN is a nonredundant phosphatase that is essential for regulating the highly oncogenic prosurvival PI3K/AKT signaling pathway. This review discusses emerging modes of PTEN function and regulation, and speculates about how manipulation of PTEN function could be used for cancer therapy.

Phosphatase PTEN in neuronal injury and brain disorders

The phosphatase and tensin homologue PTEN was originally identified as a tumor suppressor. In the CNS, mutation or inactivation of PTEN is best known for playing a tumorigenic role in the molecular pathogenesis of glioblastoma. However, recent studies show that PTEN is associated with several brain diseases other than cancer, suggesting a broader role of PTEN in CNS pathophysiology. Here, we review the evidence for the crucial involvement of PTEN in neuronal injury as well as in neurological and psychiatric disorders, and discuss the potential of PTEN as a molecular target for the development of a novel CNS therapeutic strategy.

PTEN: its deregulation and tumorigenesis

The tumor suppressor phosphatase and tensin homolog (PTEN) functions as a phosphoinositide 3-phosphatase, that antagonizes phosphatidylinositol 3-kinase action, and negatively regulates cell proliferation and survival signals. Inactivation of PTEN by loss-of-function mutations gives rise to deregulated hyperproliferation of cells, leading to oncogenic transformation. Recent studies have identified a number of upstream regulatory factors for PTEN and unveiled that the impairment in the PTEN regulatory system potentially becomes a causal factor for oncogenic transformation of cells. This article will review the PTEN inactivation mechanism which is linked to human tumorigenesis, particularly focusing on recent research progress in PTEN regulators.

PTEN is destabilized by phosphorylation on Thr366

Although PTEN (phosphatase and tensin homologue deleted on chromosome 10) is one of the most commonly mutated tumour suppressors in human cancers, loss of PTEN expression in the absence of mutation appears to occur in an even greater number of tumours. PTEN is phosphorylated in vitro on Thr366 and Ser370 by GSK3 (glycogen synthase kinase 3) and CK2 (casein kinase 2) respectively, and specific inhibitors of these kinases block these phosphorylation events in cultured cells. Although mutation of these phosphorylation sites did not alter the phosphatase activity of PTEN in vitro or in cells, blocking phosphorylation of Thr366 by either mutation or GSK3 inhibition in glioblastoma cell lines led to a stabilization of the PTEN protein. Our data support a model in which the phosphorylation of Thr366 plays a role in destabilizing the PTEN protein.

Regulation of PTEN Activity by Its Carboxyl-terminal Autoinhibitory Domain

The regulation of PTEN intrinsic biochemical properties has not been fully elucidated. In this report, we investigated the role of the PTEN carboxyl-terminal tail domain in regulating its membrane targeting and catalytic functions. Characterization of a panel of PTEN phosphorylation site mutants revealed that mutating Ser-385 to alanine (S385A) promoted membrane localization in vivo and phosphatase activity in vitro. Furthermore, S385A mutation was associated with a substantial reduction in the phosphorylation of the Ser-380/Thr-382/Thr-383 cluster. Therefore, Ser-385 could prime additional dephosphorylation events to regulate PTEN catalytic activity. Moreover, substituting Ser-380/Thr-382/Thr-383 to phosphomimic residues reversed the phosphatase activity of the S385A mutation. Next, we further defined the underlying mechanisms responsible for the COOH-terminal tail region in modulating PTEN biological activity. We have identified an interaction between the 71-amino acid carboxyl-terminal tail region and the CBRIII motif of the C2 domain, which has been implicated in membrane binding. In addition, a synthetic phosphomimic peptide encompassing the phosphorylation site cluster between amino acids 368 and 390 within the tail region mediated the suppression of PTEN catalytic activity in vitro. This same peptide when expressed in cultured cells also impeded PTEN membrane localization and enhanced phospho-Akt levels. Thus, our data suggest that the COOH-terminal tail can act as an autoinhibitory domain to control both PTEN membrane recruitment and phosphatase activity.

The p110delta isoform of PI 3-kinase negatively controls RhoA and PTEN

Inactivation of PI 3-kinase (PI3K) signalling is critical for tumour suppression by PTEN. This is thought to be a unidirectional relationship in which PTEN degrades the lipids produced by PI3K, thus controlling cell proliferation, survival and migration. We now show that this relationship is in fact bidirectional, whereby PI3K reciprocally controls PTEN. We report that the p110delta PI3K negatively regulates PTEN, through a pathway involving inhibition of RhoA. Inactivation of p110delta in macrophages led to reduced Akt and Rac1 activation, but paradoxically to increased RhoA and PTEN activity. Partial inactivation of p190RhoGAP and a reduced binding of cytoplasmic RhoA to the cyclin-dependent kinase inhibitor p27 both contributed to the increased RhoA-GTP levels upon p110delta inactivation. Pharmacological inhibition of ROCK, a downstream effector kinase of RhoA, restored all signalling and functional defects of p110delta inactivation, including Akt phosphorylation, chemotaxis and proliferation. This work identifies the RhoA/ROCK pathway as a major target of p110delta-mediated PI3K signalling, and establishes for the first time that PI3K controls itself, via a feedback loop involving PTEN.

Expression, generation, and purification of unphosphorylated and phospho-Ser-380/Thr-382/Thr-383 form of recombinant PTEN phosphatase

The dual specificity phosphatase PTEN exerts its tumour suppressor and cell-migration regulatory functions by dephosphorylating the phospholipid substrate, phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P(3)), and phosphotyrosine protein substrates. PTEN functions are regulated by phospholipid binding, interactions with other cellular proteins and phosphorylation at multiple sites. Precisely, how the phosphorylation and binding events modulate PTEN activity and structure remains mostly unclear. Detailed studies of this issue require the availability of significant quantity of both the unphosphorylated and phosphorylated forms of purified recombinant PTEN. Here, we describe the successful expression and purification of recombinant rat PTEN using a baculovirus-infected Spodoptera frugiperda (Sf9) cell expression system. The recombinant PTEN was purified to near homogeneity using four sequential column chromatographic steps. The specific enzymatic activity of the purified preparation in dephosphorylating PI(3,4,5,)P(3) and the artificial phosphotyrosine substrate poly(Glu/Tyr) are 6.7 nmol/min/microg and 0.006 pmol/min/microg, respectively. Intriguingly, similar to PTEN expressed in mammalian cells, the recombinant PTEN was phosphorylated in the infected insect cells at Ser-380, Thr-382, and Thr-383 at the C-terminal tail. Treatment with alkaline phosphatase fully dephosphorylated these sites. After the treatment, the unphosphorylated PTEN and alkaline phosphatase could be separated by ion exchange column chromatography. The availability of the phosphorylated and unphosphorylated forms of recombinant PTEN permits future investigations into the three-dimensional structures of the phosphorylated and unphosphorylated forms of PTEN, and the role of phosphorylation in regulating PTEN activity, phospholipid- and protein-binding affinities.

Protein kinase CK2alpha as an unfavorable prognostic marker and novel therapeutic target in acute myeloid leukemia

INTRODUCTION

Protein kinase CK2 is implicated in cellular proliferation and transformation. However, the clinical and biological significances of CK2 have not been elucidated in acute myeloid leukemia (AML).

EXPERIMENTAL DESIGN

We evaluated the biological significances of catalytic subunit of CK2 (CK2alpha) expression in leukemia cell lines and primary leukemic blasts obtained from AML patients.

RESULTS

In this study, the expression of CK2alpha was elevated in a substantial proportion of AML. In AML patients with normal karyotype, the disease-free survival and overall survival rates were significantly lower in the CK2alpha-high compared with the CK2alpha-low AML cases (P=0.0252 and P=0.0392, respectively). An induced overexpression of CK2alpha increased the levels of Ser473 phosphorylated (p)-Akt/protein kinase B (PKB), p-PDK1, pFKHR, p-BAD, Bcl-2, Bcl-xL, Mcl-1, and XIAP. Treatment of U937 cell line and primary AML blasts with selective CK2 inhibitor, tetrabromobenzotriazole or apigenin, reduced the levels of these molecules in a dose-dependent manner. CK2alpha small interfering RNA treatment also resulted in a down-regulation of p-Akt/PKB and Bcl-2 in U937 cells. Apigenin-induced cell death was preferentially observed in the CK2alpha-high leukemia cell lines, HL-60 and NB4, which was accompanied by cytoplasmic release of SMAC/DIABLO and proteolytic cleavage of procaspase-9, procaspase-3, procaspase-8, and poly(ADP)ribose polymerase. An induced overexpression of CK2alpha potentially enhanced the sensitivity of U937 cells to the apigenin-induced cell death. Apigenin-induced cell death was significantly higher in CK2alpha-high AML compared with CK2alpha-low AML (P<0.0001) or normal bone marrow samples (P<0.0001).

CONCLUSION

These findings strongly suggest protein kinase CK2alpha as an unfavorable prognostic marker and novel therapeutic target in AML.

Ultraviolet B-induced matrix metalloproteinase-1 and -3 secretions are mediated via PTEN/Akt pathway in human dermal fibroblasts

Matrix Metalloproteinases (MMPs) are crucial enzymes for ultraviolet irradiation-induced photoaging in human skin. Ultraviolet B (UVB) stimulates dermal fibroblasts to increase MMP-1 and -3 expression and extracellular matrix (ECM) degradation in photoaging. We investigated whether phosphatase and tensin homolog (PTEN)/Akt pathway is involved in secretions of MMP-1 and -3 in human dermal fibroblasts. The increase in MMP-1 and -3 expression and secretion occurred along with the increase in PTEN and Akt phosphorylation by UVB irradiation in a dose- and time-dependent manner. However, treatment with a casein kinase 2 inhibitor, 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole, inhibited their phosphorylations and MMP-1 and -3 secretions. Transfection of wild-type PTEN (Wt-PTEN) decreased basal and UVB-induced MMP-1 and -3 secretions, as well as activator protein-1 (AP-1) activity, while transfection of small interference RNA of PTEN (siRNA-PTEN), phosphatase-inactive PTEN (C124S-PTEN), or lipid phosphatase-inactive PTEN (G129E-PTEN) increased basal or UVB-induced MMP-1 and -3 secretions and AP-1 activity. Transfection of constitutively active Akt (Myr-Akt) also increased basal or UVB-induced MMP-1 and -3 secretions, as well as AP-1 activity. However, transfection of kinase-inactive Akt (K179M-Akt) decreased their secretions, but showed no significant change of AP-1 activity without UVB irradiation, and a significant increase of AP-1 activity with UVB irradiation. Treatment with the phosphatidylinositol 3-kinase inhibitors, LY294002 or wortmannin, downregulated basal and UVB-induced MMP-1 and -3 secretions. In conclusion, UVB irradiation increases PTEN and Akt phosphorylation in human dermal fibroblasts, and these inhibition of PTEN and activation of Akt by phosphorylation are involved in UVB-induced MMP-1 and -3 secretions partly through upregulation of AP-1 activity.

Phosphatase PTEN is inactivated in bovine aortic endothelial cells exposed to cyclic strain

Hemodynamic forces, including cyclic strain (CS) and shear stress (SS), have been recognized as important modulators of vascular cell morphology and function. PTEN (also known as MMAC1/TEP1) is a lipid phosphatase that leads to a decrease in intracellular phosphatidylinositol 3,4,5 trisphosphate (PIP3) and therefore can modulate the stimulating effect of phosphatidylinositol 3-kinase (PI3K). In this study, we focused on the upstream regulators of the PI3K-Akt pathway by assessing Akt, PTEN, casein kinase 2 (CK2) (a kinase that catalyzes phosphorylation of PTEN), and PI3K activity in endothelial cells (EC) exposed to CS. The activity of phospho-PTEN (n = 4) and phospho-CK2 (n = 4) increased in a time-dependent fashion, reaching maximal activity by 10 min of CS stimulation. The peak of phospho-Akt activity (n = 4) occurred later, at 60 min. Akt activity was altered by transfection of EC with dominant negative PTEN plasmids. Furthermore, CS increased PIP3 immunoreactivity in a time-dependent manner, reaching maximal activity after 60 min of CS stimulation, and these effects were affected by transfection of EC with dominant negative PTEN plasmids. Inhibition of PTEN activity had no effect on CS-mediated cell proliferation but inhibited CS-mediated suppression of apoptosis.

Hodgkin's lymphoma: molecular targets and novel treatment strategies

The WHO classification of Hodgkin's lymphoma (HL) distinguishes between two major subtypes, classical and nodular lymphocyte predominant HL. Approximately 95% of patients with HL will have the classical HL histology, which is characterized by the presence of rare malignant Hodgkin's and Reed-Sternberg cells among an overwhelming number of benign reactive cells. In recent years, new studies have shed more light on the biological and molecular features of Hodgkin's and Reed-Sternberg cells, providing hope that new targeted therapy may be developed to enhance the cure rate and to reduce treatment-related toxicity. In this review, the current understanding of the pathology and biology of HL will be discussed, as well as the current treatment approaches for patients with classical HL. Future treatment strategies will also be discussed based on our understanding of HL biology.

Critical role of PICT-1, a tumor suppressor candidate, in phosphatidylinositol 3,4,5-trisphosphate signals and tumorigenic transformation

The tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) regulates diverse cellular functions by dephosphorylating the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP(3)). Recent study revealed that PICT-1/GLTSCR2 bound to and stabilized PTEN protein in cells, implicating its roles in PTEN-governed PIP(3) signals. In this study, we demonstrate that RNA interference-mediated knockdown of PICT-1 in HeLa cells down-regulated endogenous PTEN and resulted in the activation of PIP(3) downstream effectors, such as protein kinase B/Akt. Furthermore, the PICT-1 knockdown promoted HeLa cell proliferation; however the proliferation of PTEN-null cells was not altered by the PICT-1 knockdown, suggesting its dependency on PTEN status. In addition, apoptosis of HeLa cells induced by staurosporine or serum-depletion was alleviated by the PICT-1 knockdown in the similar PTEN-dependent manner. Most strikingly, the PICT-1 knockdown in HeLa and NIH3T3 cells promoted anchorage-independent growth, a hallmark of tumorigenic transformation. Furthermore, PICT-1 was aberrantly expressed in 18 (41%) of 44 human neuroblastoma specimens, and the PICT-1 loss was associated with reduced PTEN protein expression in spite of the existence of PTEN mRNA. Collectively, these results suggest that PICT-1 plays a role in PIP(3) signals through controlling PTEN protein stability and the impairment in the PICT-1-PTEN regulatory unit may become a causative factor in human tumor(s).

Emodin negatively affects the phosphoinositide 3-kinase/AKT signalling pathway: a study on its mechanism of action

The development of selective cell-permeable inhibitors of protein kinases whose aberrant activation contributes to cell transformation is a promising approach in cancer treatment. Emodin is a natural anthraquinone derivative that exhibits anti-proliferative effects in various cancer cell lines by efficient induction of apoptosis. The phosphoinositide 3-kinase (PI3K)/AKT pathway has been shown to be central in the promotion of cell survival since the alteration of this signalling cascade is a frequent event in human malignancies. Previous published results indicated that treatment of cells with inhibitors of protein kinase CK2, such as emodin, induces apoptosis and that the anti-apoptotic effect of CK2 is partially mediated by target phosphorylation and up-regulation of AKT by CK2. In the present study, a screening with selected CK2 inhibitors induced a variable response with respect to AKT down-regulation, emodin being the most effective, suggesting that other mechanisms other than the inhibition of CK2 were responsible for the emodin-mediated modulation of AKT. We found that emodin does not directly affect AKT kinase. Furthermore, we show that the down-regulation of AKT is due to the emodin-mediated target inhibition of components of the PI3K pathway, which directly or indirectly affect AKT activity, i.e. the mammalian target of rapamycin and the phosphatase and tensin homolog deleted on chromosome 10, but not the phosphoinositide-dependent kinase 1. Taken together, our results highlight a new mechanism by which emodin exerts anti-cancer activity and suggest the further investigation of plant polyphenols, such as emodin, as therapeutic and preventive agents for cancer therapy.

Phosphoinositide 3-kinase regulatory subunit p85alpha suppresses insulin action via positive regulation of PTEN

The phosphoinositide 3-kinase (PI3K) pathway is central to the metabolic actions of insulin on liver. Here, we show that mice with a liver-specific deletion of the p85alpha regulatory subunit of PI3K (L-Pik3r1KO) exhibit a paradoxical improvement of hepatic and peripheral insulin sensitivity. Although PI3K enzymatic activity is diminished in L-Pik3r1KO livers because of a reduced level of regulatory and catalytic subunits of PI3K, insulin-stimulated Akt activity is actually increased. This increased Akt activity correlates with increased phosphatidylinositol (3,4,5)-trisphosphate levels which are due, at least in part, to diminished activity of the (3,4,5)-trisphosphate phosphatase PTEN. Thus, the regulatory subunit p85alpha is a critical modulator of insulin sensitivity in vivo not only because of its effects on PI3K activation, but also as a regulator of PTEN activity.

Troglitazone inhibits endothelial cell proliferation through suppression of casein kinase 2 activity

Troglitazone, an agonist of peroxisome proliferator activated receptor gamma (PPARgamma), has been reported to inhibit endothelial cell proliferation by suppressing Akt activation. Recently, it has been also proposed that phosphatase and tensin homolog deleted from chromosome 10 (PTEN) plays an important role in such effect of troglitazone. However, the mechanism of how troglitazone regulates PTEN remains to be elucidated. We therefore investigated the effects of troglitazone on casein kinase 2 (CK2), which is known to negatively regulate PTEN activity. Troglitazone significantly inhibited serum-induced proliferation of HUVEC in a concentration dependent manner. Serum-induced Akt and its downstream signaling pathway activation was attenuated by troglitazone (10 microM) pretreatment. The phosphorylation of PTEN, which was directly related to Akt activation, was decreased with troglitazone pretreatment and was inversely proportional to CK2 activity. DRB, a CK2 inhibitor, also showed effects similar to that of troglitazone on Akt and its downstream signaling molecules. In conclusion, our results suggest that troglitazone inhibits proliferation of HUVECs through suppression of CK2 activity rendering PTEN to remain activated, and this effect of troglitazone in HUVECs seems to be PPARgamma independent.

Nuclear localization of PTEN by a Ran-dependent mechanism enhances apoptosis: Involvement of an N-terminal nuclear localization domain and multiple nuclear exclusion motifs

The targeting of the tumor suppressor PTEN protein to distinct subcellular compartments is a major regulatory mechanism of PTEN function, by controlling its access to substrates and effector proteins. Here, we investigated the molecular basis and functional consequences of PTEN nuclear/cytoplasmic distribution. PTEN accumulated in the nucleus of cells treated with apoptotic stimuli. Nuclear accumulation of PTEN was enhanced by mutations targeting motifs in distinct PTEN domains, and it was dependent on an N-terminal nuclear localization domain. Coexpression of a dominant negative Ran GTPase protein blocked PTEN accumulation in the nucleus, which was also affected by coexpression of importin alpha proteins. The lipid- and protein-phosphatase activity of PTEN differentially modulated PTEN nuclear accumulation. Furthermore, catalytically active nuclear PTEN enhanced cell apoptotic responses. Our findings indicate that multiple nuclear exclusion motifs and a nuclear localization domain control PTEN nuclear localization by a Ran-dependent mechanism and suggest a proapoptotic role for PTEN in the cell nucleus.

Nuclear Ptdlns(3,4,5)P3 signaling: an ongoing story

Phosphatidylinositol 3,4,5-trisphosphate (Ptdlns(3,4,5)P(3)) is linked to a variety of cellular functions, such as growth, cell survival, and differentiation. Ptdlns(3,4,5)P(3) is primarily synthesized by class I phosphoinositide 3-kinases and its hydrolysis by two 3-phosphoinositide 3-phosphatases, PTEN and SHIP proteins, leads to the production of two other second messengers, Ptdlns(4,5)P(2) and Ptdlns(3,4)P(2), respectively. Evidence accumulated over the last years strongly suggest that Ptdlns(3,4,5)P(3) is an important component of signaling pathway operating within the nucleus. Moreover, recent advances indicated that nuclear translocation of cell surface receptors could activate nuclear phosphoinositide 3-kinase suggesting a new mode of signal transduction. The aim of this review is intended to summarize the state of our knowledge on nuclear Ptdlns(3,4,5)P(3) and its metabolizing enzymes, and to highlight the emerging roles for intranuclear Ptdlns(3,4,5)P(3).

Regulation of the PTEN phosphatase

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) is a phosphatidylinositol phosphate phosphatase and is frequently inactivated in human cancers. The balance between phosphoinositide 3-kinase (PI3K) and PTEN determines PI(3,4,5)P3 levels. PI3K is regulated by a variety of intracellular and extracellular signals, but little is known about the regulation of PTEN. In this article, we review control of PTEN function by phosphorylation as well as by binding of lipid and protein partners.

A novel leptin signalling pathway via PTEN inhibition in hypothalamic cell lines and pancreatic beta-cells

In obesity and diabetes, the ability of hypothalamic neurons to sense and transduce changes in leptin and insulin levels is compromised. The effects of both hormones require intracellular signalling via the PI3-kinase pathway, which is inhibited by the phosphatase PTEN. We show that leptin-stimulated F-actin depolymerization in mouse hypothalamic cells is inhibited by PTEN, a process involving independent effects of both its lipid and protein phosphatase activities. Potentially mediating this F-actin depolymerization, leptin, but not insulin, stimulated the phosphorylation of PTEN in a CK2 dependent manner, and inhibited its phosphatase activity. Similarly, hyperpolarization of mouse pancreatic beta-cells by leptin also requires coincident PtdIns(3,4,5)P3 generation and actin depolymerization, and could be inhibited by mechanisms requiring both the lipid and protein phosphatase activities of PTEN. These results demonstrate a critical role for PTEN in leptin signalling and indicate a mechanism by which leptin and insulin can produce PI3K dependent differential cellular outputs.

Inhibition of the phosphatidylinositol-3 kinase/Akt promotes G1 cell cycle arrest and apoptosis in Hodgkin lymphoma

Activation of the phosphatidylinositol 3-kinase (PI(3)K) pathway has been linked with tumour cell growth, survival and resistance to therapy in several cancer types. The active, phosphorylated form of Akt (pAkt) was found to be aberrantly expressed in Hodgkin lymphoma (HL)-derived cell lines and in Hodgkin-Reed-Sternberg (HRS) cells in 27 of 42 (64.3%) of primary lymph node sections of HL, indicative of PI(3)K activity. Akt phosphorylation was not associated with loss of phosphatase and tensin homologue deleted on chromosome 10 (PTEN) expression, but with its phosphorylation in HL-cell lines, suggesting that its biological function is impaired. Akt phosphorylation was further induced by CD30 ligand (CD30L), CD40L and receptor activator of nuclear factor kappa B (RANK) ligand. The PI(3)K inhibitor LY294002 demonstrated antiproliferative effects in a dose- and time-dependent manner, which was associated with Akt dephosphorylation on Thr308 and Ser473 sites and dephosphorylation of the downstream ribosomal protein S6. LY209002 induced cell cycle arrest in the G0/G1 phase and apoptosis, which were associated with upregulation of MDM2, downregulation of cyclin D1, activation of caspase 9 and poly-ADP-ribose polymerase cleavage. The Akt inhibitor QLT394 also demonstrated antiproliferative effects in a dose- and time-dependent manner, dephosphorylated ribosomal S6 and cleaved caspase 9. Collectively, these data suggest that the aberrant activation of the PI(3)K/Akt survival pathway in HRS cells is not because of loss of PTEN expression. Our data suggest that PTEN phosphorylation and activation of CD30, CD40 and RANK may play a role in activating Akt in HRS cells.

PTEN: A crucial mediator of mitochondria-dependent apoptosis

The highly frequent mutation of phosphatase and tensin homologue deleted on chromosome 10 (PTEN) in various cancers has attracted much attention to study its role in tumorigenesis. As an important tumor suppressor, the pro-apoptotic function of PTEN has been linked to its capacity antagonizing the PI3K/Akt signaling pathway. However, less data are available concerning its role in neurodegeneration in which apoptotic processes are also involved. In the present study, we attempted to study the role and the underlying mechanism of PTEN in neuronal apoptosis. Using primary rat hippocampal cultures, staurosporine (STS, 100 nM) induced a time-dependent apoptosis, accompanied by a marked production of reactive oxygen species (ROS), release of cytochrome c and activation of caspase 9 and 3. However, the expression of PTEN, and the levels of phospho-PTEN and phospho-Akt were not changed at all time points tested (0.5-24 h) after STS stimulation, suggesting that the protein level as well as the phosphorylation status of PTEN were not related to the procession of apoptosis. Interestingly, immunostaining revealed a punctate intracellular distribution of PTEN from 2 to 8 h after adding STS. Double labeling and Western blotting of mitochondrial fraction demonstrated a mitochondrial location and accumulation of PTEN, respectively, after challenging with STS. Furthermore, we provide evidence for the first time that PTEN was associated with Bax in the absence and the presence of STS. Of note, the STS-induced marked increase in the cellular ROS level, release of cytochrome c and activation of caspase 3 were inhibited in cultured hippocampal cells when PTEN was knocked down by a specific antisense. Moreover, knockdown of PTEN significantly protected hippocampal cells from apoptotic damage. These findings demonstrated that PTEN is a crucial mediator of mitochondria-dependent apoptosis, and thus could become a molecular target for interfering with neurodegenerative diseases.

The tumor suppressor PTEN is necessary for human Sprouty 2-mediated inhibition of cell proliferation

Sprouty family proteins are novel regulators of growth factor actions. Human Sprouty 2 (hSPRY2) inhibits the proliferation of a number of different cell types. However, the mechanisms involved in the anti-proliferative actions of hSPRY2 remain to be elucidated. Here we have demonstrated that hSPRY2 increases the amount of the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and decreases its phosphorylation. The resultant increase in PTEN activity is reflected in decreased activation of Akt by epidermal growth factor and serum. Consistent with increased PTEN activity, in hSPRY2-expressing cells, the progression of cells from the G1 to S phase is decreased. By using PTEN null primary mouse embryonic fibroblasts and their isogenic controls as well as small interfering RNA against PTEN, we demonstrated that PTEN is necessary for hSPRY2 to inhibit Akt activation by epidermal growth factor as well as cell proliferation. Overall, we concluded that hSPRY2 mediates its anti-proliferative actions by altering PTEN content and activity.

Constitutively activated CK2 potentially plays a pivotal role in Theileria-induced lymphocyte transformation

Activation of casein kinase II (CK2) was one of the first observations made on how Theileria parasites manipulate host cell signal transduction pathways and we argue that CK2 induction may in fact contribute to many of the different activation events that have been described since 1993 for Theileria-infected lymphocytes such as sustained activation of transcription factors c-Myc and NF-κB. CK2 also contributes to infected lymphocyte survival by inhibiting caspase activation and is probably behind constitutive PI3-K activation by phosphorylating PTEN. Finally, we also discuss how CK2A may act not only as a kinase, but also as a stimulatory subunit for the protein phosphatase PP2A, so dampening down the MEK/ERK and Akt/PKB pathways and for all these reasons we propose CK2 as a central player in Theileria-induced lymphocyte transformation.

Casein kinase Iepsilon down-regulates phospho-Akt via PTEN, following genotoxic stress-induced apoptosis in hematopoietic cells

Here, we show a functional role of casein kinase I (CKI) epsilon in hematopoietic cell survival through the modification of phosphatidylinositol 3-kinase (PI3K)/Akt signaling. Introduction of wild-type (WT)-CKIepsilon into interleukin-3 (IL-3)-dependent 32D cells increased the sensitivity to genotoxic stresses, such as gamma-irradiation, etoposide, and IL-3 deprivation, whereas kinase-negative (KN)-CKIepsilon suppressed it. Contrary to KN-CKIepsilon, WT-CKIepsilon attenuated the IL-3-induced activation of Akt with the increase of PTEN activity. Similarly, the increase of Akt activation, as well as PTEN inactivation, was accompanied both by a decrease of CKIepsilon expression induced by all-trans retinoic acid and by the addition of a specific inhibitor for CKIepsilon in HL-60 cells. CKIepsilon seems to activate PTEN by physical interaction. These results suggest that the CKIepsilon-induced down-regulation of PI3K/Akt signaling through PTEN lead to amplified sensitivity to apoptosis. Thus, the suppression of CKIepsilon in many human leukemia cell lines may play a role in the cell immortalization.

Protein kinase CK2 phosphorylates and upregulates Akt/PKB

Treatment of Jurkat cells with specific inhibitors of protein kinase CK2 induces apoptosis. Here we provide evidence that the anti-apoptotic effect of CK2 can be at least partially mediated by upregulation of the Akt/PKB pathway. Such a conclusion is based on the following observations: (1) inhibition of CK2 by cell treatment with two structurally unrelated CK2 inhibitors induces downregulation of Akt/PKB, as judged from decreased phosphorylation of its physiological targets, and immunoprecipitate kinase assay; (2) similar results are observed upon reduction of CK2 catalytic subunit by the RNA-interference technique; (3) Akt/PKB Ser129 is phosphorylated by CK2 in vitro and in vivo; (4) such a phosphorylation of activated Akt/PKB correlates with a further increase in catalytic activity. These data disclose an unanticipated mechanism by which constitutive phosphorylation by CK2 may be required for maximal activation of Akt/PKB.

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.

Cooperative phosphorylation of the tumor suppressor phosphatase and tensin homologue (PTEN) by casein kinases and glycogen synthase kinase 3beta

The phosphatase and tensin homologue (PTEN) tumor suppressor is a phosphatidylinositol D3-phosphatase that counteracts the effects of phosphatidylinositol 3-kinase and negatively regulates cell growth and survival. PTEN is itself regulated by phosphorylation on multiple serine and threonine residues in its C terminus. Previous work has implicated casein kinase 2 (CK2) as the kinase responsible for this phosphorylation. Here we showed that CK2 does not phosphorylate all sites in PTEN and that glycogen synthase kinase 3beta (GSK3beta) also participates in PTEN phosphorylation. Although CK2 mainly phosphorylated PTEN at Ser-370 and Ser-385, GSK3beta phosphorylated Ser-362 and Thr-366. More importantly, prior phosphorylation of PTEN at Ser-370 by CK2 strongly increased its phosphorylation at Thr-366 by GSK3beta, suggesting that the two may synergize. Using RNA interference, we showed that GSK3 phosphorylates PTEN in intact cells. Finally, PTEN phosphorylation was affected by insulin-like growth factor in intact cells. We concluded that multiple kinases, including CK2 and GSK3beta, participate in PTEN phosphorylation and that GSK3beta may provide feedback regulation of PTEN.

Constitutive activation of P I3 K-Akt and NF-kappaB during prostate cancer progression in autochthonous transgenic mouse model

BACKGROUND

Cancer progression is usually facilitated by independent growth signals that may lead to increased cell survival and evasion of apoptosis. Phosphatidylinositol 3'-OH kinase (P I3 K)-Akt and transcription factor NF-kappaB are important signaling molecules and key survival factors involved in the control of cell proliferation, apoptosis, and oncogenesis. Although P I3 K-Akt and NF-kappaB have been implicated in the development and progression of prostate cancer, expression of these molecules during progression of autochthonous disease has not been elucidated.

METHODS

Prostate cancer growth and progression in autochthonous transgenic adenocarcinoma of the mouse prostate (TRAMP) mice and male non-transgenic littermates were observed by magnetic resonance imaging (MRI). Expression patterns of P I3 K-Akt, NF-kappaB, I kappaB, and associated signaling molecules during different stages of cancer progression in these mice were examined by Western blot analysis, electrophoretic mobility shift assay (EMSA), enzyme-linked immunoabsorbent assay (ELISA), kinase assay, and immunohistochemistry.

RESULTS

Sequential MRI and gross analysis of prostate gland exhibited increasing prostate volume associated with the development and progression of prostatic adenocarcinoma in TRAMP mice, compared to male non-transgenic littermates. Differential protein expression of P I3 K, phosphorylated-Akt (Ser 473), I kappa Balpha and its phosphorylation, IKK kinase activity, NF-kappaB/p65, p50, DNA binding, and transcriptional-regulated genes, viz., Bc l2, cyclin D1, MMP-9, and VEGF were observed during prostate cancer progression in TRAMP mice, compared to male non-transgenic littermates. Expressions of these molecules were significantly increased during cancer progression observed at 24 and 32 weeks of age.

CONCLUSIONS

Differential expression pattern of P I3 K-Akt, NF-kappaB and I kappaB during prostate cancer progression in TRAMP mice suggest that these molecules represent potential molecular targets for prevention and/or therapeutic intervention.

LKB1 interacts with and phosphorylates PTEN: a functional link between two proteins involved in cancer predisposing syndromes

Germline mutations of the LKB1 (STK11) tumor suppressor gene lead to Peutz-Jeghers syndrome (PJS) and predisposition to cancer. LKB1 encodes a serine/threonine kinase generally inactivated in PJS patients. We identified the dual phosphatase and tumor suppressor protein PTEN as an LKB1-interacting protein. Several LKB1 point mutations associated with PJS disrupt the interaction with PTEN suggesting that the loss of this interaction might contribute to PJS. Although PTEN and LKB1 are predominantly cytoplasmic and nuclear, respectively, their interaction leads to a cytoplasmic relocalization of LKB1. In addition, we show that PTEN is a substrate of the kinase LKB1 in vitro. As PTEN is a dual phosphatase mutated in autosomal inherited disorders with phenotypes similar to those of PJS (Bannayan-Riley-Ruvalcaba syndrome and Cowden disease), our study suggests a functional link between the proteins involved in different hamartomatous polyposis syndromes and emphasizes the central role played by LKB1 as a tumor suppressor in the small intestine.

Redox regulation of PTEN by S-nitrosothiols

PTEN (phosphatase with sequence homology to tensin) is a phosphatidylinositol 3,4,5-trisphosphate phosphatase that regulates many cellular processes. Activity of the enzyme is dependent on the redox state of the active site cysteine such that oxidation by H2O2 leads to inhibition. Because S-nitrosothiols are known to modify enzymes containing reactive cysteines, we hypothesized that S-nitrosothiols would oxidize PTEN and inhibit its phosphatase activity. In the present study, we show that S-nitrosocysteine (CSNO), S-nitrosoglutathione (GSNO), and S-nitroso-N-acetylpenicillamine (SNAP) reversibly oxidized recombinant PTEN. In addition, CSNO led to concentration- and time-dependent oxidation of endogenous cellular PTEN. However, in contrast, GSNO and SNAP were effective only when coincubated with cysteine, suggesting that these nitrosothiols must react with cysteine to form CSNO, which can be transferred across cell membranes. Oxidation of cellular PTEN resulted from thiol modification and led to reversible inhibition of phosphatase activity. Although oxidation of PTEN by H2O2 led to formation of an intramolecular disulfide, oxidation of PTEN by CSNO seemed to lead to formation of a mixed disulfide. Glutathionylation of cellular proteins by incubating cells with diamide or incubating cellular extracts with GSSG oxidized PTEN in a manner similar to that of CSNO. Overall, these data demonstrate for the first time that S-nitrosothiols oxidatively modify PTEN, leading to reversible inhibition of its phosphatase activity, and suggest that the oxidized species is a mixed disulfide.

PTEN function: how normal cells control it and tumour cells lose it

The PTEN (phosphatase and tensin homologue deleted on chromosome 10) tumour suppressor is a PI (phosphoinositide) 3-phosphatase that can inhibit cellular proliferation, survival and growth by inactivating PI 3-kinase-dependent signalling. It also suppresses cellular motility through mechanisms that may be partially independent of phosphatase activity. PTEN is one of the most commonly lost tumour suppressors in human cancer, and its deregulation is also implicated in several other diseases. Here we discuss recent developments in our understanding of how the cellular activity of PTEN is regulated, and the closely related question of how this activity is lost in tumours. Cellular PTEN function appears to be regulated by controlling both the expression of the enzyme and also its activity through mechanisms including oxidation and phosphorylation-based control of non-substrate membrane binding. Therefore mutation of PTEN in tumours disrupts not only the catalytic function of PTEN, but also its regulatory aspects. However, although mutation of PTEN is uncommon in many human tumour types, loss of PTEN expression seems to be more frequent. It is currently unclear how these tumours lose PTEN expression in the absence of mutation, and while some data implicate other potential tumour suppressors and oncogenes in this process, this area seems likely to be a key focus of future research.

Mitochondrial H2O2 regulates the angiogenic phenotype via PTEN oxidation

Recent studies have demonstrated that the tumor suppressor PTEN (phosphatase and tensin homolog deleted from chromosome 10), the antagonist of the phosphosphoinositol-3-kinase (PI3K) signaling cascade, is susceptible to H2O2-dependent oxidative inactivation. This study describes the use of redox-engineered cell lines to identify PTEN as sensitive to oxidative inactivation by mitochondrial H2O2. Increases in the steady state production of mitochondrial derived H2O2, as a result of manganese superoxide dismutase (Sod2) overexpression, led to PTEN oxidation that was reversed by the coexpression of the H2O2-detoxifying enzyme catalase. The accumulation of an oxidized inactive fraction of PTEN favored the formation of phosphatidylinositol 3,4,5-triphosphate at the plasma membrane, resulting in increased activation of Akt and modulation of its downstream targets. PTEN oxidation in response to mitochondrial H2O2 enhanced PI3K signaling, leading to increased expression of the key regulator of angiogenesis, vascular endothelial growth factor. Overexpression of PTEN prevented the H2O2-dependent increase in vascular endothelial growth factor promoter activity and immunoreactive protein, whereas a mutant PTEN (G129R), lacking phosphatase activity, did not. Furthermore, mitochondrial generation of H2O2 by Sod2 promoted endothelial cell sprouting in a three-dimensional in vitro angiogenesis assay that was attenuated by catalase coexpression or the PI3K inhibitor LY2949002. Moreover, Sod2 overexpression resulted in increased in vivo blood vessel formation that was H2O2-dependent as assessed by the chicken chorioallantoic membrane assay. Our findings provide the first evidence for the involvement of mitochondrial H2O2 in regulating PTEN function and the angiogenic switch, indicating that Sod2 can serve as an alternative physiological source of the potent signaling molecule, H2O2.