Phenotypic analysis of human glioma cells expressing the MMAC1 tumor suppressor phosphatase

Involvement of Akt/NF-κB pathway in antitumor effects of parthenolide on glioblastoma cells in vitro and in vivo

BACKGROUND

Glioblastoma is the most common and most aggressive form of malignant glioma and is very difficult to treat. Controlling tumour cell invasion and angiogenesis is essential to improve the prognosis of glioblastoma patients. Since constitutive activation of nuclear factor-κB (NF-κB) is necessary for tumour progression, NF-κB may be an important pharmacological target for this disease. Our study aimed to evaluate the antitumour effects of parthenolide, a NF-κB inhibitor, in two human glioblastoma cell lines (U87MG and U373) and in glioblastoma xenografts. Furthermore, we aimed to investigate the molecular mechanisms underlying these effects.

METHODS

The anti-invasive and anti-angiogenic effects of parthenolide were analysed using in vitro invasion and angiogenesis assays. Parthenolide-induced growth inhibition of glioblastoma cells in vitro was determined using the MTT (methyl thiazolyl tetrazolium) assay. In addition, the effect of parthenolide on orthotropic implantation in vivo was evaluated using an intracerebral human glioblastoma xenograft model.

RESULTS

We found that parthenolide suppresses proliferation, invasion, and tumour- induced angiogenesis of glioblastoma cells. Molecular studies demonstrated that parthenolide suppresses gene and protein expression of angiogenic factors. Furthermore, parthenolide reduced Akt phosphorylation and activated mitochondrial signalling, suggesting that the antitumour function of parthenolide may be mediated not only by the inhibition of NF-κB but also by the inhibition of Akt signalling and the activation of apoptotic proteins. Parthenolide suppressed neovascularity and tumour growth in glioblastoma xenografts.

CONCLUSION

The present study identified parthenolide as a new therapeutic agent for glioblastomas.

Phosphatase and tensin homolog on chromosome 10 is phosphorylated in primary effusion lymphoma and Kaposi's sarcoma

Primary effusion lymphoma (PEL) is a non-Hodgkin's B-cell lymphoma driven by Kaposi's sarcoma-associated herpesvirus. It is uniquely sensitive to mTOR, PI3K, and Akt inhibitors; however, the basis of this requirement for the mTOR pathway remains to be elucidated. The phosphatase and tensin homolog gene (PTEN) on chromosome 10 controls the first step in the phosphatidylinositol 3 kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway and is genetically inactivated in many solid tumors. We find an absence of PTEN deletions, mutations, or protein mislocalization in PEL. However, we find consistent hyperphosphorylation at serine 380 of PTEN, which is an inactivating modification, in PEL cell lines and in tumor xenografts. We also evaluated a large tissue microarray of Kaposi's sarcoma biopsies and observed concordant high levels of phospho-PTEN, phospho-Akt, and phospho-S6 ribosomal protein. Reintroduction of PTEN into PEL inhibited colony formation in soft agar, verifying the functional dependence of PEL on PI3K signaling. This was also true for PEL cell lines that carried mutant p53 and for KS-like cell lines. Activating PTEN in these cancers may yield a new treatment strategy for PEL, KS, and similar PTEN wild-type lymphomas.

Expression of Livin, Smac/DIABLO and PTEN and their relationships with human gastric adenocarcinoma

AIM: To investigate the regulating role and significance of protein Livin, mitochondrial apoptosis-promoting protein Smac/DIABLO and PTEN in carcinogenesis and progression mechanism.

METHODS: Real-time RT PCR was used to examine the expression of Livin mRNA and Smac/DIABLO mRNA in 75 gastric carcinoma specimens, 20 normal gastric tissues and 20 adjacent tissues. The expression and location of Livin, Smac/DIABLO and PTEN were detected using Western blot combined with immunohistochemistry (SP).

RESULTS: The expression of Livin mRNA was significantly up-regulated in gastric carcinoma specimens (6.374 ± 4.759), however, no expression was found in normal or adjacent tissues. There was a significance in expression between low differentiated carcinoma group and lymph node metastases group (χ² = 9.60, 5.51, P < 0.01 or 0.05). The expression of Livin mRNA had no correlation with tumor size, invasion of nerve or TNM stage. The expression of Smac/DIABLO mRNA was lower in gastric cancer tissues than in normal gastric tissues and adjacent tissues, but there was no significant difference (0.731 ± 0.420 vs 1.104 ± 0.276, 1.061 ± 0.737, all P > 0.05). The expression of Smac/DIABLO mRNA in gastric cancer tissues had no correlation with clinical pathological factors of gastric carcinoma. The expression of Smac/DIABLO held significant difference from intestinal-type gastric carcinoma to diffuse-type gastric carcinoma (χ² = 5.06, P < 0.05). The expression of PTEN was not determined in gastric carcinoma tissues and normal tissues.

CONCLUSION: There is a diversity of expression of Livin, Smac/DIABLO and PTEN in different stages and pathological types of gastric carcinoma. Real-time RT PCR and the expression of Livin and Smac/DIABLO could be beneficial to diagnosis of gastric carcinogenesis, severity of differentiation, and chemotherapy sensitivity.

Silencing mammalian target of rapamycin signaling by small interfering RNA enhances rapamycin-induced autophagy in malignant glioma cells

The mammalian target of rapamycin (mTOR) plays a central role in regulating the proliferation of malignant glioma cells, and mTOR-specific inhibitors such as rapamycin analogs are considered as promising therapy for malignant gliomas. However, the efficacy of mTOR inhibitors alone in the treatment of patients with malignant gliomas is only modest, potentially because these agents rather than acting as mTOR kinase inhibitors instead interfere with the function of only mTOR/raptor (regulatory-associated protein of mTOR) complex and thus do not perturb all mTOR functions. The purpose of this study was to determine whether global inhibition of the mTOR molecule enhances the antitumor effect of rapamycin on malignant glioma cells. We showed that rapamycin induced autophagy and that inhibition of autophagy by small interfering RNA (siRNA) directed against autophagy-related gene Beclin 1 attenuated the cytotoxicity of rapamycin in rapamycin-sensitive tumor cells, indicating that the autophagy was a primary mediator of rapamycin's antitumor effect rather than a protective response. Exogenous expression of an mTOR mutant interfering with its kinase activity markedly enhanced the incidence of rapamycin-induced autophagy. Moreover, silencing of mTOR with siRNA augmented the inhibitory effect of rapamycin on tumor cell viability by stimulating autophagy. Importantly, not only rapamycin-sensitive malignant glioma cells with PTEN mutations but also rapamycin-resistant malignant glioma cells with wild-type PTEN were sensitized to rapamycin by mTOR siRNA. These results indicate that rapamycin-induced autophagy is one of the agent's antitumor effects and that silencing or inhibiting mTOR kinase activity could enhance the effectiveness of rapamycin.

A novel function of OLIG2 to suppress human glial tumor cell growth via p27Kip1 transactivation

The basic helix-loop-helix transcription factor OLIG2 is specifically expressed in cells of the oligodendrocyte lineage. It is also expressed in various tumors originating from glial cells; however, the expression of OLIG2 is rare or weak in glioblastomas, the most malignant gliomas. The role of OLIG2 in glioma remains unclear. To investigate the function of OLIG2 in glial tumor cells, we have established a glioblastoma cell line, U12-1, in which the expression of OLIG2 is induced by the Tet-off system. Induction of OLIG2 resulted in suppression of both the proliferation and anchorage-independent growth of U12-1. It also resulted in an increase in the expression of p27Kip1. A luciferase assay revealed that the CTF site of the p27Kip1 gene promoter was essential for OLIG2-dependent activation of p27Kip1 gene transcription. Electrophoretic mobility shift assays confirmed that a nuclear extract of OLIG2-expressing U12-1 cells contained a protein complex that binds to the CTF site of the p27Kip1 gene promoter. Furthermore, siRNA against p27Kip1 rescued the OLIG2-mediated growth and DNA synthesis inhibition of U12-1 cells. These results indicate that OLIG2 suppresses the proliferation of U12-1 and that this effect is mediated by transactivation of the p27Kip1 gene, and low expression of OLIG2 may be related to the malignant behavior of human glioblastoma.

Synergistic Augmentation of Rapamycin-Induced Autophagy in Malignant Glioma Cells by Phosphatidylinositol 3-Kinase/Protein Kinase B Inhibitors

The mammalian target of rapamycin (mTOR) is a downstream effector of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway and a central modulator of cell proliferation in malignant gliomas. Therefore, the targeting of mTOR signaling is considered a promising therapy for malignant gliomas. However, the mechanisms underlying the cytotoxic effects of a selective mTOR inhibitor, rapamycin, on malignant glioma cells are poorly understood. The purpose of this study was thus to elucidate how rapamycin exerts its cytotoxic effects on malignant glioma cells. We showed that rapamycin induced autophagy but not apoptosis in rapamycin-sensitive malignant glioma U87-MG and T98G cells by inhibiting the function of mTOR. In contrast, in rapamycin-resistant U373-MG cells, the inhibitory effect of rapamycin was minor, although the phosphorylation of p70S6 kinase, a molecule downstream of mTOR, was remarkably inhibited. Interestingly, a PI3K inhibitor, LY294002, and an Akt inhibitor, UCN-01 (7-hydroxystaurosporine), both synergistically sensitized U87-MG and T98G cells as well as U373-MG cells to rapamycin by stimulating the induction of autophagy. Enforced expression of active Akt in tumor cells suppressed the combined effects of LY294002 or UCN-01, whereas dominant-negative Akt expression was sufficient to increase the sensitivity of tumor cells to rapamycin. These results indicate that rapamycin exerts its antitumor effect on malignant glioma cells by inducing autophagy and suggest that in malignant glioma cells a disruption of the PI3K/Akt signaling pathway could greatly enhance the effectiveness of mTOR inhibitors.

The human tumour suppressor PTEN regulates longevity and dauer formation in Caenorhabditis elegans

The PTEN tumour suppressor is a phosphatase that dephosphorylates phosphatidylinositol 3, 4, 5 triphosphate (PIP3) and protein substrates. PTEN function is modulated by its carboxy-terminal region, which contains several clustered phosphorylation sites and a PDZ-binding motif (PDZbm). Although PTEN growth suppression effect is well demonstrated, its additional biological roles are less well understood. DAF-18, a Caenorhabditis elegans homologue PTEN, is a component of the insulin/IGF-I signalling pathway that controls entry to the dauer larval stage and adult longevity. To further explore the role of PTEN in the insulin signalling cascade and its possible involvement in the mechanisms of ageing, we undertook a study of PTEN function in C. elegans. We now report that human PTEN can substitute for DAF-18 and restores the dauer and longevity phenotypes in worms devoid of DAF-18. Furthermore, we provide genetic and biochemical evidence that dauer and lifespan control depends on PTEN-mediated regulation of PIP3 levels. Finally, we established that phosphorylation sites in the C-terminus of PTEN and its PDZbm are necessary for PTEN control of the insulin/IGF-I pathway. These results demonstrate that PTEN negatively regulates the insulin/IGF pathway in a whole organism and raise the hypothesis that PTEN may be involved in mammalian ageing.

Zinc-induced PTEN protein degradation through the proteasome pathway in human airway epithelial cells

The tumor suppressor PTEN is a putative negative regulator of the phosphatidylinositol 3-kinase/Akt pathway. Exposure to Zn2+ ions induces Akt activation, suggesting that PTEN may be modulated in this process. Therefore, the effects of Zn2+ on PTEN were studied in human airway epithelial cells and rat lungs. Treatment with Zn2+ resulted in a significant reduction in levels of PTEN protein in a dose- and time-dependent fashion in a human airway epithelial cell line. This effect of Zn2+was also observed in normal human airway epithelial cells in primary culture and in rat airway epithelium in vivo. Concomitantly, levels of PTEN mRNA were also significantly reduced by Zn2+ exposure. PTEN phosphatase activity evaluated by measuring Akt phosphorylation decreased after Zn2+ treatment. Pretreatment of the cells with a proteasome inhibitor significantly blocked zinc-induced reduction of PTEN protein as well as the increase in Akt phosphorylation, implicating the involvement of proteasome-mediated PTEN degradation. Further study revealed that Zn2+-induced ubiquitination of PTEN protein may mediate this process. A phosphatidylinositol 3-kinase inhibitor blocked PTEN degradation induced by Zn2+, suggesting that phosphatidylinositol 3-kinase may participate in the regulation of PTEN. However, both the proteasome inhibitor and phosphatidylinositol 3-kinase inhibitor failed to prevent significant down-regulation of PTEN mRNA expression in response to Zn2+. In summary, exposure to Zn2+ ions causes PTEN degradation and loss of function, which is mediated by an ubiquitin-associated proteolytic process in the airway epithelium.

Protein tyrosine phosphatase RQ is a phosphatidylinositol phosphatase that can regulate cell survival and proliferation

Protein tyrosine phosphatase RQ (PTPRQ) was initially identified as a protein tyrosine phosphatase (PTPase)-like protein that is upregulated in a model of renal injury. Here we present evidence that, like PTEN, the biologically important enzymatic activity of PTPRQ is as a phosphatidylinositol phosphatase (PIPase). The PIPase specificity of PTPRQ is broader than that of PTEN and depends on different amino acid residues in the catalytic domain. In vitro, the recombinant catalytic domain of PTPRQ has low PTPase activity against tyrosine-phosphorylated peptide and protein substrates but can dephosphorylate a broad range of phosphatidylinositol phosphates, including phosphatidylinositol 3,4,5-trisphosphate and most phosphatidylinositol monophosphates and diphosphates. Phosphate can be hydrolyzed from the D3 and D5 positions in the inositol ring. PTPRQ does not have either of the basic amino acids in the catalytic domain that are important for the PIPase activity of PTEN or the sequence motifs that are characteristic of type II phosphatidylinositol 5-phosphatases. Instead, the PIPase activity depends on the WPE sequence present in the catalytic cleft of PTPRQ, and in the "inactive" D2 domains of many dual-domain PTPases, in place of the WPD motif present in standard active PTPases. Overexpression of PTPRQ in cultured cells inhibits proliferation and induces apoptosis. An E2171D mutation that retains or increases PTPase activity but eliminates PIPase activity, eliminates the inhibitory effects on proliferation and apoptosis. These results indicate that PTPRQ represents a subtype of the PTPases whose biological activities result from its PIPase activity rather than its PTPase activity.

PTEN/MMAC1 expression in melanoma resection specimens

PTEN/MMAC1, a tumour suppressor gene located on chromosome 10q23.3, has been found to be deleted in several types of human malignancies. As the chromosomal region 10q22-qter commonly is affected by losses in melanomas, we addressed this gene as tumour suppressor candidate in melanomas. Investigating PTEN/MMAC1 expression at mRNA level by semi-quantitative reverse transcription-polymerase chain reaction, we did not find a statistically significant down-regulation in melanoma resection specimens in comparison to acquired melanocytic nevi from which melanomas quite often are known to arise. Upon immunohistochemistry, PTEN/MMAC1 protein expression in melanomas was not lost. Sequencing the PTEN/MMAC1 cDNAs in 26 melanoma resection specimens (21 primary melanomas, five metastases), we detected three point mutations and two nucleotide deletions which did not represent genetic polymorphisms. With respect to the predicted protein sequences, all three point mutations were silent whereas the two frame shifts at the extreme C-terminus resulted in a loss of the putative PDZ-targeting consensus sequence. As loss of this motif possibly impairs localization and function of PTEN/MMAC1 in the two corresponding primary tumours, alterations of this tumour suppressor protein may participate in some melanomas.

PTEN associates with the vault particles in HeLa cells

PTEN is a tumor suppressor that primarily dephosphorylates phosphatidylinositol 3,4,5-trisphosphate to down-regulate the phosphoinositide 3-kinase/Akt signaling pathway. Although the cellular functions of PTEN as a tumor suppressor have been well characterized, the mechanism by which PTEN activity is modulated by other signal molecules in vivo remains poorly understood. In searching for potential PTEN modulators through protein-protein interaction, we identified the major vault protein (MVP) as a dominant PTEN-binding protein in a yeast two-hybrid screen. MVP is the major structural component of vault, the largest intracellular ribonucleoprotein particle. Co-immunoprecipitation confirmed the interaction between PTEN and MVP in transfected mammalian cells. More importantly, we found that a significant portion of endogenous PTEN associates with vault particles in human HeLa cells. Deletion mutation analysis demonstrated that MVP binds to the C2 domain of PTEN and that PTEN interacts with MVP through its EF hand-like motif. Furthermore, the in vitro binding experiments revealed that the interaction of PTEN with MVP is Ca(2+)-dependent.

Tumor suppressor PTEN is mutated in canine osteosarcoma cell lines and tumors

Canine osteosarcoma (OS) cell lines contain mutations that directly or indirectly inactivate the tumor suppressor genes p53 and retinoblastoma. Another important tumor suppressor, PTEN, is mutated in many human cancers. To determine whether inactivation of PTEN plays a role in the pathogenesis of canine OS, we studied its expression in canine OS cell lines and tumors. Four of five canine OS cell lines (CO2, C03, CO5, and CO7) constitutively express high levels of the phosphorylated form of Akt, an indirect indicator of aberrant PTEN expression. PTEN protein is essentially absent from three of these cell lines (CO2, CO5, and CO7), whereas C03 contains a potentially inactivating amino acid substitution in PTEN at codon 340. Genomic hybridization experiments indicate that CO2, CO5, and CO7 contain large deletions within the PTEN gene. Ten of 15 OS tumors exhibit variable or negative PTEN staining. Evaluation of a PTEN-negative staining tumor by Southern blotting indicates that the PTEN gene is deleted in this tumor. These results indicate that PTEN is mutated or downregulated in a high percentage of canine OS cell lines and tumors and likely plays an important role in the pathogenesis of the disease.

Motif analysis of the tumor suppressor gene MMAC/PTEN identifies tyrosines critical for tumor suppression and lipid phosphatase activity

The tumor suppressor gene, MMAC/PTEN, has phosphatase, C2, and PDZ-binding domains as well as potential sites of regulation by phosphorylation, including tyrosine phosphorylation, which may contribute to its ability to modulate cell growth and viability. Several obvious and significant motifs were found in MMAC/PTEN, including most notably, a catalytic domain of tyrosine phosphatase (IHCxxGxxRS/T) and several potential tyrosine phosphorylation sites. To examine the functional significance of tyrosine phosphorylation of MMAC/PTEN, retroviral constructs were generated with mutations at two putative tyrosine phosphorylation sites (Y240A/Y240F and Y315A/Y315F). Stable expression of wild-type MMAC/PTEN in U251 human glioma cells (which do not normally produce a functional MMAC/PTEN gene product) resulted in a significant reduction of tumor growth in nude mice, decreased growth rate, saturation density, and colony formation in vitro, as well as dephosphorylation of D3-phosphorylated phosphatidylinositols (PtdIns) in vitro. Mutation of Y240 or Y315 to either alanine or phenylalanine abrogated the ability of MMAC/PTEN to alter growth rate, saturation density, and colony formation in vitro. The ability of MMAC/PTEN to limit tumor growth in nude mice was markedly decreased but not abrogated by mutation of Y240 or Y315 to alanine. Thus, Y240 and Y315 are required for MMAC/PTEN to decrease tumor growth in vitro and in vivo. In contrast to wild-type MMAC/PTEN, mutant MMAC/PTEN containing Y240A or Y315A was unable to dephosphorylate D3-phosphorylated PtdIns in vitro. Thus, Y240A and Y315A are involved in the ability of MMAC/PTEN to dephosphorylate PtdIns and regulate tumor cell growth in vitro and in vivo.

PTEN and myotubularin: novel phosphoinositide phosphatases

Protein tyrosine phosphatases (PTPs) are a diverse group of enzymes that contain a highly conserved active site motif, Cys-x5-Arg (Cx5R). The PTP superfamily enzymes, which include tyrosine-specific, dual specificity, low-molecular-weight, and Cdc25 phosphatases, are key mediators of a wide variety of cellular processes, including growth, metabolism, differentiation, motility, and programmed cell death. The PTEN/MMAC1/TEP1 gene was originally identified as a candidate tumor suppressor gene located on human chromosome 10q23; it encodes a protein with sequence similarity to PTPs and tensin. Recent studies have demonstrated that PTEN plays an essential role in regulating signaling pathways involved in cell growth and apoptosis, and mutations in the PTEN gene are now known to cause tumorigenesis in a number of human tissues. In addition, germ line mutations in the PTEN gene also play a major role in the development of Cowden and Bannayan-Zonana syndromes, in which patients often suffer from increased risk of breast and thyroid cancers. Biochemical studies of the PTEN phosphatase have revealed a molecular mechanism by which tumorigenesis may be caused in individuals with PTEN mutations. Unlike most members of the PTP superfamily, PTEN utilizes the phosphoinositide second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP3), as its physiologic substrate. This inositol lipid is an important regulator of cell growth and survival signaling through the Ser/Thr protein kinases PDK1 and Akt. By specifically dephosphorylating the D3 position of PIP3, the PTEN tumor suppressor functions as a negative regulator of signaling processes downstream of this lipid second messenger. Mutations that impair PTEN function result in a marked increase in cellular levels of PIP3 and constitutive activation of Akt survival signaling pathways, leading to inhibition of apoptosis, hyperplasia, and tumor formation. Certain structural features of PTEN contribute to its specificity for PIP3, as well as its role(s) in regulating cellular proliferation and apoptosis. Recently, myotubularin, a second PTP superfamily enzyme associated with human disease, has also been shown to utilize a phosphoinositide as its physiologic substrate.

Suppression of matrix metalloproteinase-2 gene expression and invasion in human glioma cells by MMAC/PTEN

Human gliomas are highly invasive, and remain to be a major obstacle for any effective therapeutic remedy. Among many other factors, gliomas express elevated levels of matrix metalloproteinases (MMPs), which have been implicated to play an important role in tumor invasion as well as neovascularization. The tumor suppressor gene mutated in multiple advanced cancers/phosphatase and tensin homologue (MMAC/PTEN) has been shown to inhibit cell migration, spreading, and focal adhesion. In this study, we determined whether MMAC/PTEN inhibits tumor invasion by modulating MMP-2 activity. Our results showed that reintroduction of the MMAC/PTEN gene into human glioma U251 and U87 cells modified their phenotype and growth characteristics. The ability of MMAC/PTEN to induce anoikis in U251 cells was accompanied by a significant inhibition of in vitro invasion (70%). Expression of MMAC/PTEN in U251 and U87 cells inhibited MMP-2 enzymatic activity as determined by zymography. Furthermore, MMAC/PTEN expression strongly decreased MMP-2 mRNA levels, which correlated well with the inhibition of invasion capacity in these cells. Concomitant with MMP-2 expression and activity, MMP-2 promoter activity was also reduced in MMAC/PTEN expressing cells. Our observations suggest that MMAC/PTEN inhibits tumor cell invasion in part by regulating MMP-2 gene transcription and thereby its enzymatic activity. Further characterization of this regulation will facilitate the development of MMAC/PTEN based gene therapy for gliomas.

Role of serine/threonine protein phosphatase 2A in cancer

The serine/threonine protein phosphatase 2A (PP2A) appears to be critically involved in cellular growth control and potentially in the development of cancer. A few studies indicated that this enzyme might actually exert tumor suppressive function. However, other findings demonstrated the requirement for PP2A in cell growth and survival, which is not a characteristic of a typical tumor suppressor. This apparent discrepancy might be due to the fact that PP2A is a multitask enzyme system, rather than a single enzyme. Its individual subunits are encoded by a heterogeneous group of genes which give rise to a multitude of different PP2A holoenzyme complexes. Thus, the puzzling observation that PP2A exerts inhibitory, as well as stimulatory, effects on cell growth could be due to the activity of different PP2A complexes with distinct subcellular location and divers substrate specificity. At the same time, this abundance of PP2A components provides a large target for mutations that might derail proper enzyme function and could contribute to the process of tumorigenesis. So far, however, it has not been unequivocally established whether such mutations, examples of which have indeed been found in human cancer cells, result in the activation of an oncogenic function or rather in the inactivation of the presumed tumor suppressive role of PP2A. Therefore, the general opinion of PP2A as being a tumor suppressor needs to be viewed with caution.

The tumor suppressor PTEN is phosphorylated by the protein kinase CK2 at its C terminus. Implications for PTEN stability to proteasome-mediated degradation

The tumor suppressor phosphatase PTEN regulates cell migration, growth, and survival by dephosphorylating phosphatidylinositol second messengers and signaling phosphoproteins. PTEN possesses a C-terminal noncatalytic regulatory domain that contains multiple putative phosphorylation sites, which could play an important role in the control of its biological activity. The protein kinase CK2 phosphorylated, in a constitutive manner, a cluster of Ser/Thr residues located at the PTEN C terminus. PTEN-phosphorylated defective mutants showed decreased stability in comparison with wild type PTEN and were more rapidly degraded by the proteasome. Inhibition of PTEN phosphorylation by the CK2 inhibitor 5,6-dichloro-1-beta-d-ribofuranosyl-benzimidazole also diminished the PTEN protein content. Our results support the notion that proper phosphorylation of PTEN by CK2 is important for PTEN protein stability to proteasome-mediated degradation.

Regulation of brain fatty acid-binding protein expression by differential phosphorylation of nuclear factor I in malignant glioma cell lines

Brain fatty acid-binding protein (B-FABP) is expressed in the radial glial cells of the developing central nervous system as well as in a subset of human malignant glioma cell lines. Most of the malignant glioma lines that express B-FABP also express GFAP, an intermediate filament protein found in mature astrocytes. We are studying the regulation of the B-FABP gene to determine the basis for its differential expression in malignant glioma lines. By DNase I footprinting, we have identified five DNA-binding sites located within 400 base pairs (bp) of the B-FABP transcription start site, including two nuclear factor I (NFI)-binding sites at -35 to -58 bp (footprint 1, fp1) and -237 to -260 bp (fp3), respectively. Competition experiments, supershift experiments with anti-NFI antibody, and methylation interference experiments all indicate that the factor binding to fp1 and fp3 is NFI. By site-directed mutagenesis of both NFI-binding sites, we show that the most proximal NFI site is essential for B-FABP promoter activity in transiently transfected malignant glioma cells. Different band shift patterns are observed with nuclear extracts from B-FABP(+) and B-FABP(-) malignant glioma lines, with the latter generating complexes that migrate more slowly than those obtained with B-FABP(+) extracts. All bands are converted to a faster migrating form with potato acid phosphatase treatment, indicating that NFI is differentially phosphorylated in B-FABP(+) and B-FABP(-) lines. Our results suggest that B-FABP expression in malignant glioma lines is determined by the extent of NFI phosphorylation which, in turn, is controlled by a phosphatase activity specific to B-FABP(+) lines.

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

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

Analysis of the cellular functions of PTEN using catalytic domain and C-terminal mutations: differential effects of C-terminal deletion on signalling pathways downstream of phosphoinositide 3-kinase

The tumour suppressor protein, PTEN (phosphatase and tensin homolog deleted on chromosome 10), is a phosphatase that can dephosphorylate tyrosine-containing peptides, Shc, focal adhesion kinase and phosphoinositide substrates. In cellular assays, PTEN has been shown to antagonize the PI-3K-dependent activation of protein kinase B (PKB) and to inhibit cell spreading and motility. It is currently unclear, however, whether PTEN accomplishes these effects through its lipid- or protein-phosphatase activity, although strong evidence has demonstrated the importance of the latter for tumour suppression by PTEN. By using a PTEN G129E (Gly(129)-->Glu) mutant that has lost its lipid phosphatase activity, while retaining protein phosphatase activity, we demonstrated a requirement for the lipid phosphatase activity of PTEN in the regulation of PKB activity, cell viability and membrane ruffling. We also made a small C-terminal deletion of PTEN, removing a putative PDZ (PSD95, Dlg and ZO1)-binding motif, with no detectable effect on the phosphatase activity of the protein expressed in HEK293 cells (human embryonic kidney 293 cells) assayed in vitro. Surprisingly, expression of this mutant revealed differential requirements for the C-terminus in the different functional assays. Wild-type and C-terminally deleted PTEN appeared to be equally active in down-regulating PKB activity, but this mutant enzyme had no effect on platelet-derived growth factor (PDGF)-induced membrane ruffling and was only partially active in a cell viability assay. These results stress the importance of the lipid phosphatase activity of PTEN in the regulation of several signalling pathways. They also identify a mutation, similar to mutations that occur in some human tumours, which removes the effect of PTEN on membrane ruffling but not that on PKB.

Analysis of the cellular functions of PTEN using catalytic domain and C-terminal mutations: differential effects of C-terminal deletion on signalling pathways downstream of phosphoinositide 3-kinase

The tumour suppressor protein, PTEN (phosphatase and tensin homolog deleted on chromosome 10), is a phosphatase that can dephosphorylate tyrosine-containing peptides, Shc, focal adhesion kinase and phosphoinositide substrates. In cellular assays, PTEN has been shown to antagonize the PI-3K-dependent activation of protein kinase B (PKB) and to inhibit cell spreading and motility. It is currently unclear, however, whether PTEN accomplishes these effects through its lipid- or protein-phosphatase activity, although strong evidence has demonstrated the importance of the latter for tumour suppression by PTEN. By using a PTEN G129E (Gly(129)-->Glu) mutant that has lost its lipid phosphatase activity, while retaining protein phosphatase activity, we demonstrated a requirement for the lipid phosphatase activity of PTEN in the regulation of PKB activity, cell viability and membrane ruffling. We also made a small C-terminal deletion of PTEN, removing a putative PDZ (PSD95, Dlg and ZO1)-binding motif, with no detectable effect on the phosphatase activity of the protein expressed in HEK293 cells (human embryonic kidney 293 cells) assayed in vitro. Surprisingly, expression of this mutant revealed differential requirements for the C-terminus in the different functional assays. Wild-type and C-terminally deleted PTEN appeared to be equally active in down-regulating PKB activity, but this mutant enzyme had no effect on platelet-derived growth factor (PDGF)-induced membrane ruffling and was only partially active in a cell viability assay. These results stress the importance of the lipid phosphatase activity of PTEN in the regulation of several signalling pathways. They also identify a mutation, similar to mutations that occur in some human tumours, which removes the effect of PTEN on membrane ruffling but not that on PKB.

PTEN mutation analysis in two genetic subtypes of high-grade oligodendroglial tumors. PTEN is only occasionally mutated in one of the two genetic subtypes

We recently identified two genetic subtypes of high-grade oligodendroglial tumors (HG-OT): 1p-/19q- HG-OT are characterized by a loss of chromosome 1p32-36 (del(1)(p32-p36) and/or a del(19)(q13. 3); whereas +7/-10 HG-OT harbor a gain of chromosome 7 (+7) and/or a -10 without a loss of 1p32-36 and 19q13.3. Because a -10 and a +7 are most frequently detected in glioblastomas (GBM), the genotype of +7/-10 HG-OT suggests that these tumors are GBM with a prominent oligodendroglial phenotype rather than anaplastic oligodendrogliomas. PTEN is a tumor suppressor gene, located at 10q23.3, which is involved in tumor progression of GBM and other neoplasms. In this study, we screened for PTEN mutations in six low-grade oligodendroglial tumors (LG-OT), five 1p-/19q- HG-OT, seven +7/-10 HG-OT, and nine xenografted GBM. PTEN mutations were detected in none of the LG-OT and 1p-/19q- HG-OT, once in +7/-10 HG-OT, and frequently in GBM. As one of the +7/-10 HG-OT harbored a PTEN mutation, this demonstrates that PTEN can be involved in the oncogenesis of this genetic subtype of HG-OT. The lower frequency of PTEN mutations in +7/-10 HG-OT compared to GBM suggests that these tumors are of a distinct tumor type rather than GBM. Published by Elsevier Science Inc.

Cell cycle arrest by the PTEN tumor suppressor is target cell specific and may require protein phosphatase activity

PTEN, a tumor suppressor commonly targeted in human cancer, possesses phosphatase activities toward both protein and lipid substrates. While PTEN suppresses gliomas through cell cycle inhibition which requires its lipid phosphatase activity, PTEN's effects on other tumor types and the role of its protein phosphatase activity are controversial or unknown. Here we show that exogenous wild-type PTEN arrests some, but not all human breast cancer cell lines in G1, in a manner independent of endogenous PTEN. Unexpectedly, the G129E mutant of PTEN selectively deficient in the lipid phosphatase activity still blocked the cell cycle of MCF-7 cells, while the G129R and H123Y mutants lacking both phosphatase activities were ineffective. These results suggest that PTEN's protein phosphatase activity likely contributes to its tumor suppressor function in subsets of tumors and that elucidation of downstream targets which dictate cellular responses to PTEN may have important implications for future cancer treatment strategies.

Evidence for regulation of the PTEN tumor suppressor by a membrane-localized multi-PDZ domain containing scaffold protein MAGI-2

PTEN is a tumor suppressor gene mutated in human cancers. Although many mutations target the phosphatase domain, others create a truncated protein lacking the C-terminal PDZ-binding motif or a protein that extends beyond the PDZ-binding motif. Using the yeast two-hybrid system, we isolated a membrane-associated guanylate kinase family protein with multiple PDZ domains [AIP-1 (atrophin interacting protein 1), renamed MAGI-2 (membrane associated guanylate kinase inverted-2)]. MAGI-2 contains eight potential protein-protein interaction domains and is localized to tight junctions in the membrane of epithelial cells. PTEN binds to MAGI-2 through an interaction between the PDZ-binding motif of PTEN and the second PDZ domain of MAGI-2. MAGI-2 enhances the ability of PTEN to suppress Akt activation. Furthermore, certain PTEN mutants have reduced stability, which is restored by adding the minimal PDZ-binding motif back to the truncated protein. We propose that MAGI-2 improves the efficiency of PTEN signaling through assembly of a multiprotein complex at the cell membrane.

Inhibition of H-Ras transformation by the PTEN/MMAC1/TEP1 tumor suppressor gene

The human PTEN/MMAC1/TEP1 (PTEN) tumor suppressor gene encodes a phosphatase with specificity towards the D3 phosphate of phosphatidylinositides. PTEN mutations have been reported in the endometrioid type of uterine tumors which are associated with frequent activations of the Ras oncogenes. In this study, we report the ability of PTEN to potently inhibit H-Ras induced morphological transformation and anchorage-independent growth in NIH3T3 cells. This novel activity of PTEN was correlated more with its ability to suppress the phosphatidylinositol 3-kinase (PI3-K)-dependent signaling cascade, but not the mitogen-activated protein kinase (MAPK) pathway. To define the minimal region in PTEN protein that is responsible for this anti-oncogenic activity, a panel of carboxyl-terminal truncation mutants was generated. While deletions of 4 and 33 amino acids do not have marked effects, removal of up to 68 amino acids drastically reduced the ability of PTEN to inhibit Ras transformation. The propensity of these mutants to suppress Ras transformation is correlated with their relative ability to dephosphorylate inositol (1,3,4,5)-tetrakisphosphate in vitro, and to suppress Akt kinase activity in cultured cells. In addition, we have evidence to suggest that the C-terminal region of PTEN contributes to the stability of the encoded gene product.

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

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

Transcriptional activation of p21WAF1 by PTEN/MMAC1 tumor suppressor

The recently discovered tumor suppressor gene PTEN has been found mutated in many types of advanced tumors. When introduced into tumor cells that lack the wild-type allele of the gene, PTEN was able to suppress the growth of these cells. Here, we have analyzed how PTEN might alter cell cycle-regulatory controls to achieve this growth-inhibitory effect. We found that overexpression of PTEN stimulates the synthesis of three inhibitors of cyclin-dependent kinases, p21WAF1, p27KIP1, and p57KIP2. This effect is very specific, as the expression of other components of the cell cycle engine, various cyclins and cyclin-dependent kinases, is not affected. For p21WAF1 we show that this induction is due to the p53-independent transcriptional activation of its promoter. In addition, increased expression of PTEN rendered the cells more sensitive to apoptotic cell death. Therefore, our data suggest a two-fold mechanism of growth inhibition by PTEN: one that acts via the increased expression of CKIs such as p21WAF1, and another that augments the cellular propensity for apoptotic cell death.

PTEN/MMAC1/TEP1 in signal transduction and tumorigenesis

The level of phosphorylation within cells is tightly regulated by the concerted action of protein kinases and protein phosphatases [Hunter, T. (1995) Cell 80, 225-236]. Disregulation in the activity of either of these players can lead to cellular transformation. Many protein tyrosine kinases are proto-oncogenes and it has been postulated that some protein phosphatases may act as tumor suppressors. Herein we will review the recent findings addressing the roles the candidate tumor suppressor PTEN/MMAC1/TEP1 (PTEN, phosphatase and tensin homologue deleted from chromosome 10; MMAC 1, mutated in multiple advanced cancers 1; TEP1, TGF beta regulated and epithelial cell enriched phosphatase 1) plays in signal transduction and tumorigenesis. PTEN is a dual specificity protein phosphatase (towards phospho-Ser/Thr and phospho-Tyr) and, unexpectedly, also has a phosphoinositide 3-phosphatase activity. PTEN plays an important role in the modulation of the 1-phosphatidylinositol 3-kinase (PtdIns 3-kinase) pathway, by catalyzing the degradation of the PtdIns(3,4,5)P3 generated by PtdIns 3-kinase; this inhibits the downstream functions mediated by the PtdIns 3-kinase pathway, such as activation of protein kinase B (PKB, also known as Akt), cell survival and cell proliferation. Furthermore, PTEN modulates cell migration and invasion by negatively regulating the signals generated at the focal adhesions, through the direct dephosphorylation and inhibition of focal adhesion kinase (FAK). Growth factor receptor signaling is also negatively regulated by PTEN, through the inhibition of the adaptor protein Shc. While some of the functions of PTEN have been elucidated, it is clear that there is much more to discover about the roles of this unique protein.