Regulation of G1 progression by the PTEN tumor suppressor protein is linked to inhibition of the phosphatidylinositol 3-kinase/Akt pathway

Levels of PTEN protein modulate Akt phosphorylation on serine 473, but not on threonine 308, in IGF-II-overexpressing rhabdomyosarcomas cells

Constitutive activation of Akt has been found in many types of human cancer, and is believed to promote proliferation and increased cell survival thereby contributing to cancer progression. In this study, we examined Akt phosphorylation on Ser473 and Thr308 in seven IGF-II-overexpressing rhabdomyosarcomas (RMS) cells. All the RMS cell lines tested had high levels of Akt phosphorylation on Thr308, whereas three cell lines (Rh5, Rh18, and CTR) had a much lower level of Akt phosphorylation on Ser473. To determine whether the difference in Akt phosphorylation on Ser473, but not on Thr308, observed among cell lines is a cell-specific phenomenon or due to other factors, which possibly downregulate Akt phosphorylation, we examined expression of PTEN protein, which acts as a negative regulator of the PI3K/Akt signaling pathway through its ability to dephosphorylate phosphatidylinositol 3,4,5-triphosphate (PIP3). The levels of PTEN expression inversely correlate with Akt phosphorylation on Ser473, but not on Thr308. Consistent with this finding, transfection of wild-type PTEN into RMS and mouse myoblast C2C12 cells resulted in reduced Akt phosphorylation on Ser473, but not on Thr308. Our data suggest that Ser473 may be a key target residue for PTEN to modulate the effects of IGF-II on activating the PI3K/Akt pathway in RMS cells. A better understanding of the pathway in RMS will likely contribute to insights into the biology of the RMS tumorigenesis and hopefully lead to novel therapeutic options.

A chemical genetic screen identifies inhibitors of regulated nuclear export of a Forkhead transcription factor in PTEN-deficient tumor cells

The PI3K/PTEN/Akt signal transduction pathway plays a key role in many tumors. Downstream targets of this pathway include the Forkhead family of transcription factors (FOXO1a, FOXO3a, FOXO4). In PTEN null cells, FOXO1a is inactivated by PI3K-dependent phosphorylation and mislocalization to the cytoplasm, yet still undergoes nucleocytoplasmic shuttling. Since forcible localization of FOXO1a to the nucleus can reverse tumorigenicity of PTEN null cells, a high-content, chemical genetic screen for inhibitors of FOXO1a nuclear export was performed. The compounds detected in the primary screen were retested in secondary assays, and structure-function relationships were identified. Novel general export inhibitors were found that react with CRM1 as well as a number of compounds that inhibit PI3K/Akt signaling, among which are included multiple antagonists of calmodulin signaling.

Inhibition of phosphatidylinositol 3-kinase dephosphorylates BAD and promotes apoptosis in myeloid leukemias

The phosphatidylinositol 3-kinase (PI3K)/AKT protein kinase pathway is involved in cell growth, proliferation, and apoptosis. The functional activation of PI3K/AKT provides survival signals and blockade of this pathway may facilitate cell death. Downstream targets of PI3K-AKT include the proapoptotic protein BAD, caspase-9, NF-kappaB, and Forkhead. We have previously reported that BAD is constitutively phosphorylated in primary acute myeloid leukemia (AML) cells, a post-transcriptional modification, which inactivates its proapoptotic function. In this study, we tested the hypothesis that the inhibition of PI3K by LY294002 results in the dephosphorylation of AKT and BAD, and thus promote leukemia cell apoptosis. We investigated the effects of LY294002 in megakaryocytic leukemia-derived MO7E cells, primary AML and normal bone marrow progenitor cells. In MO7E cells, LY294002 reduced AKT kinase activity, induced dephosphorylation of AKT and BAD, and increased apoptosis. Concomitant inhibition of mitogen-activated protein kinase signaling or combination with all-trans retinoic acid further enhanced apoptosis of leukemic cells. In primary AML samples, clonogenic cell growth was significantly reduced. Normal hematopoietic progenitors were less affected, suggesting preferential targeting of leukemia cells. In conclusion, the data suggest that the inhibition of the PI3K/AKT signaling pathway restores apoptosis in AML and may be explored as a novel target for molecular therapeutics in AML.

A new human cell line, PDSS-26, from poorly differentiated synovial sarcoma, with unique chromosomal anomalies

Permanent synovial sarcoma cell lines are invaluable tools for understanding of the biology of this tumor. The present study reports the establishment of a new human cell line, PDSS-26, derived from a surgical specimen of a poorly differentiated synovial sarcoma. PDSS-26 has a doubling time of a 72 hours and grows as a monolayer of spindle cells that retain immunoreactivity for bcl-2 and vimentin. Karyotypic analysis revealed a rearrangement involving chromosomes 17 and 18, at the breakpoints q11.2 and q11.2, respectively, as the only structural aberrations. Analysis by reverse transcriptase polymerase chain reaction showed the presence of the SYT-SSX1 fusion transcript in both the primary tumor and the cell line. Cytoplasmic PTEN staining was detected by immunohistochemistry in both the PDSS-26 cell line and in original tumor, whereas no mutation was identified by automatic sequencing. Thus, PDSS-26 cells could be useful for future functional studies.

Gene therapy for ovarian cancer: progress and potential

Gene therapy remains a promising therapeutic modality for ovarian cancer. Yet much work remains to be done to see gene therapy realize its full potential in elucidating the complex genetic interactions of delivered genes within target cancer cells and in the development of improved vector systems. Because most neoplasms involve multiple mutations, the targeting of a single mutation is unlikely to achieve total tumor control: gene therapy strategies that target multiple cellular processes or invoke various antitumor approaches need to be investigated. Additionally, current vector systems do not transduce ovarian cancer cells efficiently and are hampered by immune responses that further limit their efficacy. Additionally, limitations in vector specificity lead to transduction of normal cells and subsequent toxicity. Investigators are developing refinements to current gene therapy approaches that would address these limitations and that are soon to be incorporated into clinical trials. It is hoped that these advances will lead to improvements in the therapeutic index for ovarian cancer gene therapy and provide another effective therapeutic tool for this deadly disease.

Inactivation of NF-kappaB by genistein is mediated via Akt signaling pathway in breast cancer cells

Genistein, a natural isoflavonoid found in soybean products, has been proposed to be associated with a lower rate of breast cancer in Asian women. Studies from our laboratory and others have shown that genistein can induce apoptosis by regulating the expression of apoptosis-related genes in breast cancer cells. However, the precise molecular mechanism(s) by which genistein induces apoptotic cell death is not clear. In order to investigate such mechanism, we tested the role of Akt and NF-kappaB in genistein-treated MDA-MB-231 breast cancer cells. We found that inhibition of cell growth and induction of apoptosis by genistein are partly mediated through the downregulation of Akt and NF-kappaB pathways. Gel shift assay showed that NF-kappaB DNA-binding activity in MDA-MB-231 cells transfected with Akt cDNA was induced, suggesting that there is a cross-talk between NF-kappaB and Akt signaling pathway. Moreover, we found that genistein could abrogate EGF and Akt induced NF-kappaB activation. From these results, we conclude that the inactivation of NF-kappaB by genistein in MDA-MB-231 breast cancer cells is partly mediated via Akt pathway, which could be useful for rational design of strategies for the prevention and/or treatment of breast cancer.

Roles of the PI-3K and MEK pathways in Ras-mediated chemoresistance in breast cancer cells

Activated Ras utilises several downstream pathways, including the mitogen-activated protein kinase (MAPK) kinase (MEK)/MAPK pathway and the phosphoinositide 3-kinase (PI-3k)/Akt pathway, to promote cell proliferation and to inhibit apoptosis. To investigate which pathway plays a major role in Ras-induced drug resistance to chemotherapeutic agents in breast cancer cells, we transfected MCF7 breast cancer cells with a constitutively active H-RasG12V and examined the toxicities of three commonly used breast cancer chemotherapeutic agents, paclitaxel, doxorubicin, and 5-fluorouracil in these cells under the conditions that PI-3K or MEK were selectively inhibited by their respective specific inhibitors or dominant negative expression vectors. We found that Ras-mediated drug resistance is well correlated with resistance to apoptosis induced by anticancer agents in MCF7 breast cancer cells. Although inhibition of MEK/MAPK or PI-3K/Akt can each enhance the cytotoxicity of paclitaxel, doxorubicin, or 5-fluorouracil, inhibition of the PI-3K/Akt pathway seems to have a greater effect than inhibition of the MEK/MAPK pathway in reversing Ras-mediated drug resistance. Our results indicate that the PI-3K pathway may play a more important role in receptor tyrosine kinase-mediated resistance to chemotherapy and suggest that PI-3K/Akt might be a critical target molecule for anticancer intervention in breast cancer.

Prostate intraepithelial neoplasia induced by prostate restricted Akt activation: the MPAKT model

To determine whether Akt activation was sufficient for the transformation of normal prostate epithelial cells, murine prostate restricted Akt kinase activity was generated in transgenic mice (MPAKT mice). Akt expression led to p70S6K activation, prostatic intraepithelial neoplasia (PIN), and bladder obstruction. mRNA expression profiles from MPAKT ventral prostate revealed similarities to human cancer and an angiogenic signature that included three angiogenin family members, one of which was found elevated in the plasma of men with prostate cancer. Thus, the MPAKT model may be useful in studying the role of Akt in prostate epithelial cell transformation and in the discovery of molecular markers relevant to human disease.

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.

TRB3: a tribbles homolog that inhibits Akt/PKB activation by insulin in liver

Insulin resistance is a major hallmark in the development of type II diabetes, which is characterized by the failure of insulin to promote glucose uptake in muscle and to suppress glucose production in liver. The serine-threonine kinase Akt (PKB) is a principal target of insulin signaling that inhibits hepatic glucose output when glucose is available from food. Here we show that TRB3, a mammalian homolog of Drosophila tribbles, functions as a negative modulator of Akt. TRB3 expression is induced in liver under fasting conditions, and TRB3 disrupts insulin signaling by binding directly to Akt and blocking activation of the kinase. Amounts of TRB3 RNA and protein were increased in livers of db/db diabetic mice compared with those in wild-type mice. Hepatic overexpression of TRB3 in amounts comparable to those in db/db mice promoted hyperglycemia and glucose intolerance. Our results suggest that, by interfering with Akt activation, TRB3 contributes to insulin resistance in individuals with susceptibility to type II diabetes.

Biphasic estradiol-induced AKT phosphorylation is modulated by PTEN via MAP kinase in HepG2 cells

We reported previously in HepG2 cells that estradiol induces cell cycle progression throughout the G1-S transition by the parallel stimulation of both PKC-alpha and ERK signaling molecules. The analysis of the cyclin D1 gene expression showed that only the MAP kinase pathway was involved. Here, the presence of rapid/nongenomic, estradiol-regulated, PI3K/AKT signal transduction pathway, its modulation by the levels of the tumor suppressor PTEN, its cross-talk with the ERK pathway, and its involvement in DNA synthesis and cyclin D1 gene promoter activity have all been studied in HepG2 cells. 17beta-Estradiol induced the rapid and biphasic phosphorylation of AKT. These phosphorylations were independent of each other, being the first wave of activation independent of the estrogen receptor (ER), whereas the second was dependent on ER. Both activations were dependent on PI3K activity; furthermore, the ERK pathway modulated AKT phosphorylation by acting on the PTEN levels. The results showed that the PI3K pathway, as well as ER, were strongly involved in both G1-S progression and cyclin D1 promoter activity by acting on its proximal region (-254 base pairs). These data indicate that in HepG2 cells, different rapid/nongenomic estradiol-induced signal transduction pathways modulate the multiple steps of G1-S phase transition.

HER2/PI-3K/Akt activation leads to a multidrug resistance in human breast adenocarcinoma cells

Growth factor receptor-mediated signal transduction has been implicated in conferring resistance to conventional chemotherapy on cancer cells. In this study, we delineated a pathway that involves HER2/PI-3K/Akt in mediating multidrug resistance in human breast cancer cells. We found that the cell lines that express both HER2 and HER3 appear to have a higher phosphorylation level of Akt (activated Akt). Transfection of HER2 in MCF7 breast cancer cells that express HER3 caused a phosphoinoside-3 kinase (PI-3K)-dependent activation of Akt, and was associated with an increased resistance of the cells to multiple chemotherapeutic agents (paclitaxel, doxorubicin, 5-fluorouracil, etoposide, and camptothecin). Selective inhibition of PI-3K or Akt activity with their respective dominant-negative expression vectors sensitized the cells to the induction of apoptosis by the chemotherapeutic agents. We further demonstrated that MCF7 cells expressing a constitutively active Akt, in which the phospholipid-interactive PH domain of Akt was replaced by a farnesylation sequence for constitutive membrane anchorage (DeltaPH-Akt1-farn), showed a similar increased resistance to the chemotherapeutic agents. Our results suggest that activation of Akt1 by HER2/PI-3K plays an important role in conferring a broad-spectrum chemoresistance on breast cancer cells and that Akt may therefore be a novel molecular target for therapies that would improve the outcome of patients with breast cancer.

PTEN signaling pathways in melanoma

Phosphatase and tensin homolog deleted in from chromosome ten (PTEN), initially also known as mutated in multiple advanced cancers or TGF-beta-regulated and epithelia cell-enriched phosphatase, is a tumor suppressor gene that is mutated in a large fraction of human melanomas. A broad variety of human cancers carry PTEN alterations, including glioblastomas, endometrial, breast, thyroid and prostate cancers. The PTEN protein has at least two biochemical functions: it has both lipid phosphatase and protein phosphatase activity. The lipid phosphatase activity of PTEN decreases intracellular PtdIns(3,4,5)P(3) level and downstream Akt activity. Cell-cycle progression is arrested at G1/S, mediated at least partially through the upregulation of the cyclin-dependent kinase inhibitor p27. In addition, agonist-induced apoptosis is mediated by PTEN, through the upregulation of proapoptotic machinery involving caspases and BID, and the downregulation of antiapoptotic proteins such as Bcl2. The protein phosphatase activity of PTEN is apparently less central to its involvement in tumorigenesis. It is involved in the inhibition of focal adhesion formation, cell spreading and migration, as well as the inhibition of growth factor-stimulated MAPK signaling. Therefore, the combined effects of the loss of PTEN lipid and protein phosphatase activity may result in aberrant cell growth and escape from apoptosis, as well as abnormal cell spreading and migration. In melanoma, PTEN loss has been mostly observed as a late event, although a dose-dependent loss of PTEN protein and function has been implicated in early stages of tumorigenesis as well. In addition, loss of PTEN and oncogenic activation of RAS seem to occur in a reciprocal fashion, both of which could cooperate with CDKN2A loss in contribution to melanoma tumorigenesis.

Alterations of cell cycle-regulatory genes in prostate cancer

Deregulated proliferation is one of the main events in neoplastic transformation, and this has prompted increased attention being given to the understanding of the mechanisms involved in cell cycle regulation and its alterations. The 'retinoblastoma pathway', a key effector controlling G1-S phase transition, includes several oncogenes and tumour suppressor genes which display a wide range of abnormalities with potential usefulness as markers of evolution or treatment response in prostate cancer. Among these, the existence of p53 mutations seems to predict resistance to radiotherapy or systemic treatment, and p16 overexpression or p27 downregulation seems to serve as markers of poor evolution. The well-established existence of a critical hormonal role in prostate carcinogenesis coupled with the relationship of androgenic activity and regulation of several cell cycle modulators forces cell cycle control in the prostate to be envisioned as a highly complex steroid-influenced system, which will undoubtedly have critical implications in the future management of prostate cancer patients.

RRM1-induced metastasis suppression through PTEN-regulated pathways

Lung cancer is the leading cause of cancer-related mortality in the United States. Only 15% of patients with this disease survive 5 years or longer. Early metastatic spread is the single most important reason for this poor outcome. The survival of patients with pathological stage I disease, that is, no evidence for metastatic spread, and molecular aberrations on chromosome 11p15.5 is equal to that of patients with stage II disease, that is, metastatic spread to hilar lymph nodes. RRM1 is a gene in this region, and it is haploinsufficient in at least 34% stage I patients. Here, we show that overexpression of RRM1 in human and mouse lung cancer cell lines induced PTEN expression, reduced phosphorylation of focal adhesion kinase (FAK), suppressed migration, invasion, and metastasis formation, and increased survival in an animal model. Increased PTEN expression was required for the RRM1-induced suppression of cell motility and FAK phosphorylation. We conclude that RRM1 functions as a metastasis suppressor gene through induction of PTEN expression.

Frequent monoallelic deletion of PTEN and its reciprocal associatioin with PIK3CA amplification in gastric carcinoma

Mutational alterations of PTEN and PIK3CA, which negatively and positively regulate PI3-kinase activity, respectively, have been observed in many types of human cancer. To explore the implication of PTEN and PIK3CA mutations in gastric tumorigenesis, we characterized the expression and mutation status of the genes in 126 gastric tissues and 15 cell lines. Expression of PTEN transcript was abnormally low in 5 of 15 (33%) cell lines and 20 of 55 (36%) primary carcinomas, whereas 0 of 71 noncancerous tissues including 16 benign tumors showed altered expression. Allelotyping analysis using an intragenic polymorphism (IVS4+109) revealed that 14 of 30 (47%) informative cases carried LOH of the gene, which is closely linked to low expression. The LOH rate was significantly higher in advanced tumors [12 of 19 (63%)] compared to early-stage tumors [2 of 11 (18%)] and more frequent in poorly differentiated tumors [9 of 13 (69%)] than well- or moderately differentiated tumors [5 of 17 (29%)]. Interestingly, however, none of the LOH tumors carried mutational disruption of the remaining allele, suggesting haploinsufficiency of PTEN in gastric tumorigenesis. Methylation studies revealed that PTEN pseudogene, but not PTEN, is methylated in cell lines and primary tumors, indicating that PTEN is not a target of epigenetic silencing in gastric cancers and that the pseudogene should be considered more carefully in methylation analysis of the PTEN promoter. Genomic amplification of PIK3CA was found in 9 of 15 (60%) cell lines and 20 of 55 (36.4%) primary tumors but in no noncancerous tissues. Furthermore, PIK3CA amplification was predominantly detected in tumors with no PTEN alterations, suggesting that mutations of PTEN and PIK3CA are mutually exclusive events in gastric tumorigenesis. Amplification of PIK3CA was strongly associated with increased expression of PIK3CA transcript and elevated levels of phospho-AKT. Collectively, our data reveal that 13 of 15 (87%) gastric cell lines and 31 of 55 (56%) primary carcinomas harbored either amplification of PIK3CA or abnormal reduction of PTEN. Mutually exclusive alterations of PTEN and PIK3CA also suggest that mutations of either gene could activate the PI3-kinase/AKT signaling pathway, which is directly linked to the malignant progression of gastric tumor cells.

Conditional loss of PTEN leads to testicular teratoma and enhances embryonic germ cell production

The tumor suppressor gene PTEN, which is frequently mutated in human cancers, encodes a lipid phosphatase for phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3] and antagonizes phosphatidylinositol 3 kinase. Primordial germ cells (PGCs), which are the embryonic precursors of gametes, are the source of testicular teratoma. To elucidate the intracellular signaling mechanisms that underlie germ cell differentiation and proliferation, we have generated mice with a PGC-specific deletion of the Pten gene. Male mice that lacked PTEN exhibited bilateral testicular teratoma, which resulted from impaired mitotic arrest and outgrowth of cells with immature characters. Experiments with PTEN-null PGCs in culture revealed that these cells had greater proliferative capacity and enhanced pluripotent embryonic germ (EG) cell colony formation. PTEN appears to be essential for germ cell differentiation and an important factor in testicular germ cell tumor formation.

Lipid phosphatases in the regulation of T cell activation: living up to their PTEN-tial

The initiating events associated with T activation in response to stimulation of the T cell antigen receptor (TCR) and costimulatory receptors, such as CD28, are intimately associated with the enzymatically catalyzed addition of phosphate not only to key tyrosine, threonine and serine residues in proteins but also to the D3 position of the myo-inositol ring of phosphatidylinositol (PtdIns). This latter event is catalyzed by the lipid kinase phosphoinositide 3-kinase (PI3K). The consequent production of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 serves both to recruit signaling proteins to the plasma membrane and to induce activating conformational changes in proteins that contain specialized domains for the binding of these phospholipids. The TCR signaling proteins that are subject to regulation by PI3K include Akt, phospholipase Cgamma1 (PLCgamma1), protein kinase C zeta (PKC-zeta), Itk, Tec and Vav, all of which play critical roles in T cell activation. As is the case for phosphorylation of protein substrates, the phosphorylation of PtdIns is under dynamic regulation, with the D3 phosphate being subject to hydrolysis by the 3-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10), thereby placing PTEN in direct opposition to PI3K. In this review we consider recent data concerning how PTEN may act in regulating the process of T cell activation.

Ras modulates Myc activity to repress thrombospondin-1 expression and increase tumor angiogenesis

Tumor angiogenesis is postulated to be regulated by the balance between pro- and anti-angiogenic factors. We demonstrate that the critical step in establishing the angiogenic capability of human cells is the repression of the critical anti-angiogenic factor, thrombospondin-1 (Tsp-1). This repression is essential for tumor formation by mammary epithelial cells and kidney cells engineered to express SV40 early region proteins, hTERT, and H-RasV12. We have uncovered the signaling pathway leading from Ras to Tsp-1 repression. Ras induces the sequential activation of PI3 kinase, Rho, and ROCK, leading to activation of Myc through phosphorylation; phosphorylation of Myc via this mechanism enables it to repress Tsp-1 expression. We thus describe a novel mechanism by which the cooperative activity of the oncogenes, ras and myc, leads directly to angiogenesis and tumor formation.

IKKalpha regulates mitogenic signaling through transcriptional induction of cyclin D1 via Tcf

The Wnt/beta-catenin/Tcf and IkappaB/NF-kappaB cascades are independent pathways involved in cell cycle control, cellular differentiation, and inflammation. Constitutive Wnt/beta-catenin signaling occurs in certain cancers from mutation of components of the pathway and from activating growth factor receptors, including RON and MET. The resulting accumulation of cytoplasmic and nuclear beta-catenin interacts with the Tcf/LEF transcription factors to induce target genes. The IkappaB kinase complex (IKK) that phosphorylates IkappaB contains IKKalpha, IKKbeta, and IKKgamma. Here we show that the cyclin D1 gene functions as a point of convergence between the Wnt/beta-catenin and IkappaB pathways in mitogenic signaling. Mitogenic induction of G(1)-S phase progression and cyclin D1 expression was PI3K dependent, and cyclin D1(-/-) cells showed reduced PI3K-dependent S-phase entry. PI3K-dependent induction of cyclin D1 was blocked by inhibitors of PI3K/Akt/IkappaB/IKKalpha or beta-catenin signaling. A single Tcf site in the cyclin D1 promoter was required for induction by PI3K or IKKalpha. In IKKalpha(-/-) cells, mitogen-induced DNA synthesis, and expression of Tcf-responsive genes was reduced. Reintroduction of IKKalpha restored normal mitogen induction of cyclin D1 through a Tcf site. In IKKalpha(-/-) cells, beta-catenin phosphorylation was decreased and purified IKKalpha was sufficient for phosphorylation of beta-catenin through its N-terminus in vitro. Because IKKalpha but not IKKbeta induced cyclin D1 expression through Tcf activity, these studies indicate that the relative levels of IKKalpha and IKKbeta may alter their substrate and signaling specificities to regulate mitogen-induced DNA synthesis through distinct mechanisms.

AKT-independent protection of prostate cancer cells from apoptosis mediated through complex formation between the androgen receptor and FKHR

Recent studies suggested that the protection of cell apoptosis by AKT involves phosphorylation and inhibition of FKHR and related FOXO forkhead transcription factors and that androgens provide an AKT-independent cell survival signal in prostate cancer cells. Here, we report receptor-dependent repression of FKHR function by androgens in prostate cancer cells. Transcriptional analysis demonstrated that activation of the androgen receptor caused an inhibition of both wild-type FKHR and a mutant in which all three known AKT sites were mutated to alanines, showing that the repression is AKT independent. In vivo and in vitro coprecipitation studies demonstrated that the repression is mediated through protein-protein interaction between FKHR and the androgen receptor. Mapping analysis localized the interacting domains to the carboxyl terminus between amino acids 350 and 655 of FKHR and to the amino-terminal A/B region and the ligand binding domain of the receptor. Further analysis demonstrated that the activated androgen receptor blocked FKHR's DNA binding activity and impaired its ability to induce Fas ligand expression and prostate cancer cell apoptosis and cell cycle arrest. These studies identify a new mechanism for androgen-mediated prostate cancer cell survival that appears to be independent of the activity of the receptor on androgen response element-mediated transcription and establish FKHR and related FOXO forkhead proteins as important nuclear targets for both AKT-dependent and -independent survival signals in prostate cancer cells.

Transduction of growth or mitogenic signals into translational activation of TOP mRNAs is fully reliant on the phosphatidylinositol 3-kinase-mediated pathway but requires neither S6K1 nor rpS6 phosphorylation

Translation of terminal oligopyrimidine tract (TOP) mRNAs, which encode multiple components of the protein synthesis machinery, is known to be controlled by mitogenic stimuli. We now show that the ability of cells to progress through the cell cycle is not a prerequisite for this mode of regulation. TOP mRNAs can be translationally activated when PC12 or embryonic stem (ES) cells are induced to grow (increase their size) by nerve growth factor and retinoic acid, respectively, while remaining mitotically arrested. However, both growth and mitogenic signals converge via the phosphatidylinositol 3-kinase (PI3-kinase)-mediated pathway and are transduced to efficiently translate TOP mRNAs. Translational activation of TOP mRNAs can be abolished by LY294002, a PI3-kinase inhibitor, or by overexpression of PTEN as well as by dominant-negative mutants of PI3-kinase or its effectors, PDK1 and protein kinase Balpha (PKBalpha). Likewise, overexpression of constitutively active PI3-kinase or PKBalpha can relieve the translational repression of TOP mRNAs in quiescent cells. Both mitogenic and growth signals lead to phosphorylation of ribosomal protein S6 (rpS6), which precedes the translational activation of TOP mRNAs. Nevertheless, neither rpS6 phosphorylation nor its kinase, S6K1, is essential for the translational response of these mRNAs. Thus, TOP mRNAs can be translationally activated by growth or mitogenic stimuli of ES cells, whose rpS6 is constitutively unphosphorylated due to the disruption of both alleles of S6K1. Similarly, complete inhibition of mammalian target of rapamycin (mTOR) and its effector S6K by rapamycin in various cell lines has only a mild repressive effect on the translation of TOP mRNAs. It therefore appears that translation of TOP mRNAs is primarily regulated by growth and mitogenic cues through the PI3-kinase pathway, with a minor role, if any, for the mTOR pathway.

Phosphatidylinositol-3 kinase p85 enhances expression from the myelin basic protein promoter in oligodendrocytes

Phosphatidylinositol-3 kinase (PI3K) is a family of enzymes that phosphorylates the D3 position of phosphoinositides in membranes which can then act as a second messenger and affect many essential cellular processes such as survival, proliferation and differentiation. Class IA PI3K is composed of two subunits: a regulatory subunit, p85, and a catalytic subunit, p110. The p85 subunit is composed of several adapter domains which, upon interaction with the appropriate molecules, transmit the signal to activate p110. We have used the spontaneously immortalized oligodendrocyte cell line, CG4, to examine the role of PI3K in maturation of the oligodendrocyte. We show that overexpression of the p85 subunit enhances expression of myelin basic protein (MBP) upon differentiation of CG4 cells and primary oligodendrocytes. In experiments in CG4 cells, neither cotransfection with the tumor suppressor PTEN, which dephosphorylates the D3 position of phosphoinositides, nor inhibition of PI3K activity with wortmannin mimics this effect. Further, we have shown that this effect is dependent on the coexpression of the two SH2 domains within p85. Thus, the p85-mediated enhancement of MBP promoter activity in oligodendrocytes appears to be independent of PI3K activity and dependent on the adapter functions of the p85 subunit's SH2 domains.

Phosphatidylinositol 3-kinase/Akt stimulates androgen pathway through GSK3beta inhibition and nuclear beta-catenin accumulation

PI3K/Akt plays a critical role in prostate cancer cell growth and survival. Recent studies have shown that the effect of PI3K/Akt in prostate cells is mediated through androgen signaling. The PI3K inhibitor, LY294002, and a tumor suppressor, PTEN, negatively regulate the PI3K/Akt pathway and repress AR activity. However, the molecular mechanisms whereby PI3K/Akt and PTEN regulate the androgen pathway are currently unclear. Here, we demonstrate that blocking the PI3K/Akt pathway reduces the expression of an endogenous AR target gene. Moreover, we show that the repression of AR activity by LY294002 is mediated through phosphorylation and inactivation of GSK3beta, a downstream substrate of PI3K/Akt, which results in the nuclear accumulation of beta-catenin. Given the recent evidence that beta-catenin acts as a coactivator of AR, our findings suggest a novel mechanism by which PI3K/Akt modulates androgen signaling. In a PTEN-null prostate cancer cell line, we show that PTEN expression reduces beta-catenin-mediated augmentation of AR transactivation. Using the mutants of beta-catenin, we further demonstrate that the repressive effect of PTEN is mediated by a GSK3beta-regulated degradation of beta-catenin. Our results delineate a novel link among the PI3K, wnt, and androgen pathways and provide fresh insights into the mechanisms of prostate tumor development and progression.

Functional involvement of Akt signaling downstream of Jak1 in v-Abl-induced activation of hematopoietic cells

Activation of intracellular signaling pathways is important for cellular transformation and tumorigenesis. The nonreceptor tyrosine kinases Jak1 and Jak3, which bind to the v-Abl oncoprotein, are constitutively activated in cells transformed with the Abelson murine leukemia virus. A mutant of p160 v-Abl lacking the Jak1-binding region (v-Abl Delta858-1080) has a significant defect in Jak/STAT (signal transducers and activators of transcription) activation, cytokine-independent cell growth/survival, and tumorigenesis. To identify the pathways downstream of Jak kinases in v-Abl-mediated signaling, we examined the activation of several signaling molecules by p160 v-Abl or the v-Abl Delta858-1080 mutant. We demonstrate that, in addition to the decreased Ras activation, signaling through phosphatidylinositol-3 kinase and Akt are impaired in cells expressing mutant v-Abl. The proliferative defect of v-Abl Delta858-1080 was rescued by activated v-Akt and was also moderately rescued by activated v-H-Ras. However, constitutive active phosphatidylinositol-3 kinase (p110CAAX) did not complement this effect. Cells expressing v-Abl Delta858-1080 demonstrated reduced tumor formation in nude mice. In contrast, cells coexpressing v-Akt with v-Abl Delta858-1080 demonstrated reduced latency and increased frequency of tumor formation in nude nice compared with cells expressing v-Abl Delta858-1080 alone, whereas v-H-Ras or p110CAAX had minimum effects on tumor formation. These results suggest that Jak1-dependent Akt activation is important in v-Abl-mediated transformation.

Immunohistochemical evidence of PTEN in oral squamous cell carcinoma and its correlation with the histological malignancy grading system

PTEN is a tumor suppressor gene that encodes a dual phosphatase protein capable of modulating membrane receptors and interaction of the cell and extracellular stimuli. PTEN regulates cell physiology such as division, differentiation/apoptosis and also migration and adhesion. The expression of PTEN was evaluated by immunohistochemistry in OSCC and compared to a well-established histological malignancy grading system. The well-differentiated OSCC were 59.1% and poorly differentiated were 40.9%. According to PTEN expression, the cases were 45.5% positive (the entire tumor showed stained), 22.7% mixed (both negative and positive cells were present) and 31.8% negative (no staining was seen in the tumor cells). PTEN expression in OSCC was related to the malignancy grade (P < 0.0005). Aggressive tumors with a high score of malignancy did not express PTEN, and clearly, the PTEN expression was present in the epithelium adjacent to the tumor. Negative cells were in the invasion border of the tumor. This result suggests that PTEN is related to histologic pattern and biological behavior of OSCC and may be a used as a prognostic marker in the future. The role of PTEN during carcinogenesis and as a biomarker should be further investigated.

Loss of cellular adhesion to matrix induces p53-independent expression of PTEN tumor suppressor

BACKGROUND

The 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, exogenous PTEN was able to suppress their ability to grow anchorage-independently, and thus reverted one of the typical characteristics of tumor cells. As these findings indicated that PTEN might be involved in the regulation of anchorage-dependent cell growth, we analyzed this aspect of PTEN function in non-tumor cells with an anchorage-dependent phenotype.

RESULTS

We found that in response to the disruption of cell-matrix interactions, expression of endogenous PTEN was transcriptionally activated, and elevated levels of PTEN protein and activity were present in the cells. These events correlated with decreased phosphorylation of focal adhesion kinase, and occurred even in the absence of p53, a tumor suppressor protein and recently established stimulator of PTEN transcription.

CONCLUSIONS

In view of PTEN's potent growth-inhibitory capacity, we conclude that its induction after cell-matrix disruptions contributes to the maintenance of the anchorage-dependent phenotype of normal cells.

Identification of the tuberous sclerosis complex-2 tumor suppressor gene product tuberin as a target of the phosphoinositide 3-kinase/akt pathway

The S/T-protein kinases activated by phosphoinositide 3-kinase (PI3K) regulate a myriad of cellular processes. Here, we show that an approach using a combination of biochemistry and bioinformatics can identify substrates of these kinases. This approach identifies the tuberous sclerosis complex-2 gene product, tuberin, as a potential target of Akt/PKB. We demonstrate that, upon activation of PI3K, tuberin is phosphorylated on consensus recognition sites for PI3K-dependent S/T kinases. Moreover, Akt/PKB can phosphorylate tuberin in vitro and in vivo. We also show that S939 and T1462 of tuberin are PI3K-regulated phosphorylation sites and that T1462 is constitutively phosphorylated in PTEN(-/-) tumor-derived cell lines. Finally, we find that a tuberin mutant lacking the major PI3K-dependent phosphorylation sites can block the activation of S6K1, suggesting a means by which the PI3K-Akt pathway regulates S6K1 activity.

Essential role of AKT-1/protein kinase B alpha in PTEN-controlled tumorigenesis

PTEN is mutated at high frequency in many primary human cancers and several familial cancer predisposition disorders. Activation of AKT is a common event in tumors in which the PTEN gene has been inactivated. We previously showed that deletion of the murine Pten gene in embryonic stem (ES) cells led to increased phosphatidylinositol triphosphate (PIP(3)) accumulation, enhanced entry into S phase, and better cell survival. Since PIP(3) controls multiple signaling molecules, it was not clear to what degree the observed phenotypes were due to deregulated AKT activity. In this study, we mutated Akt-1 in Pten(-/-) ES cells to directly assess the role of AKT-1 in PTEN-controlled cellular processes, such as cell proliferation, cell survival, and tumorigenesis in nude mice. We showed that AKT-1 is one of the major downstream effectors of PTEN in ES cells and that activation of AKT-1 is required for both the cell survival and cell proliferation phenotypes observed in Pten(-/-) ES cells. Deletion of Akt-1 partially reverses the aggressive growth of Pten(-/-) ES cells in vivo, suggesting that AKT-1 plays an essential role in PTEN-controlled tumorigenesis.

Retroviral expression of a kinase-defective IGF-I receptor suppresses growth and causes apoptosis of CHO and U87 cells in-vivo

BACKGROUND

Phosphatidylinositol-3,4,5-triphosphate (PtdInsP3) signaling is elevated in many tumors due to loss of the tumor suppressor PTEN, and leads to constitutive activation of Akt, a kinase involved in cell survival. Reintroduction of PTEN in cells suppresses transformation and tumorigenicity. While this approach works in-vitro, it may prove difficult to achieve in-vivo. In this study, we investigated whether inhibition of growth factor signaling would have the same effect as re-expression of PTEN.

METHODS

Dominant negative IGF-I receptors were expressed in CHO and U87 cells by retroviral infection. Cell proliferation, transformation and tumor formation in athymic nude mice were assessed.

RESULTS

Inhibition of IGF-IR signaling in a CHO cell model system by expression of a kinase-defective IGF-IR impairs proliferation, transformation and tumor growth. Reduction in tumor growth is associated with an increase in apoptosis in-vivo. The dominant-negative IGF-IRs also prevented growth of U87 PTEN-negative glioblastoma cells when injected into nude mice. Injection of an IGF-IR blocking antibody alphaIR3 into mice harboring parental U87 tumors inhibits tumor growth and increases apoptosis.

CONCLUSION

Inhibition of an upstream growth factor signal prevents tumor growth of the U87 PTEN-deficient glioma to the same extent as re-introduction of PTEN. This result suggests that growth factor receptor inhibition may be an effective alternative therapy for PTEN-deficient tumors.

Infrequent genetic alterations of the tumor suppressor gene PTEN/MMAC1 in squamous cell carcinoma of the oral cavity

BACKGROUND

Fifty tumor specimens from primarily untreated patients were analyzed to elucidate the involvement of the tumor suppressor gene PTEN/MMAC1 in the development of oral squamous cell cancer.

METHODS

Eight microsatellite markers, spanning 10 cM of genomic DNA located centromeric, telomeric or intragenic of PTEN/MMAC1 were used for loss of heterozygosity (LOH) and breakpoint analysis. The microsatellite panel within or in close proximity (1 cM) to the 10q23.3 locus showed a LOH rate of 12%. Complete sequence analysis of the genes coding region was performed in all 10 cases that exhibited LOH in one of the eight microsatellite markers within or around the PTEN/MMAC1 gene. Comparative multiplex PCR reactions served to screen for homozygous deletions.

RESULTS

There was no association between allelic loss of the gene, overall patient survival and recurrence-free survival. Sequencing did not reveal any mutation in the coding region of PTEN/MMAC1. Differential PCR analysis failed to detect any homozygous deletion.

CONCLUSIONS

We conclude that PTEN/MMAC1 gene alterations do not play a key role in tumorigenesis of oral squamous cell cancers.

p53 regulates cell survival by inhibiting PIK3CA in squamous cell carcinomas

Interactions between the p53 and PI3K/AKT pathways play a significant role in the determination of cell death/survival. In benign cells these pathways are interrelated through the transcriptional regulation of PTEN by p53, which is required for p53-mediated apoptosis. PTEN exerts its effects by decreasing the phosphorylated AKT fraction, thereby diminishing prosurvival activities. However, the link between these pathways in cancer is not known. In this study, PIK3CA, encoding the p110alpha catalytic subunit of PI3K, is identified as an oncogene involved in upper aerodigestive tract (UADT) carcinomas. Simultaneous abnormalities in both pathways are rare in primary tumors, suggesting that amplification of PIK3CA and mutation of p53 are mutually exclusive events and either event is able to promote a malignant phenotype. Moreover, the negative effect of p53 induction on cell survival involves the transcriptional inhibition of PIK3CA that is independent of PTEN activity, as PTEN is not expressed in the primary tumors. Conversely, constitutive activation of PIK3CA results in resistance to p53-related apoptosis in PTEN deficient cells. Thus, p53 regulates cell survival by inhibiting the PI3K/AKT prosurvival signal independent of PTEN in epithelial tumors. This inhibition is required for p53-mediated apoptosis in malignant cells.

The phosphatidylinositol phosphatase PTEN is under control of costimulation and regulates proliferation in human T cells

The phosphatidylinositol phosphatase gene PTEN is a dual specific phosphatase acting on phospho amino acids but also on three phosphorylated inositol phospholipids. Present results demonstrate that PTEN is inducible by costimulatory signals in human CD4(+) T cells. PTEN expression was up-regulated on RNA and protein level in freshly isolated human CD4(+) T cells following stimulation with CD28 or CD2. In contrast, PTEN expression was high but remained CD28 and CD2 unresponsive in lymphoma cells. Intracellular staining revealed PTEN expression in CD4(+) T cell populations stimulated with anti-CD28 or anti-CD28 / anti-CD3. Stimulation with anti-CD3 alone did not induce PTEN expression. Inhibition of PTEN expression by antisense oligonucleotides in CD4(+) T cells stimulated with non-mitogenic anti-CD28 resulted in massively increased proliferation, which was sensitive to the phosphatidylinositol 3-kinase (PI3 K) inhibitor wortmannin. Although CD28 and CD2 induce PI3 K signal transduction, wortmannin did not block PTEN up-regulation by CD28 or CD2 indicating that PTEN gene expression is PI3 K independent. These results demonstrate that PTEN negatively controls costimulatory signals by antagonizing PI3 K activity in the absence of TCR engagement.

Dissection of angiogenic signaling in zebrafish using a chemical genetic approach

Striking homology between signaling molecules in zebrafish and humans suggests that compounds known to inhibit human kinases may enable a chemical genetic approach to dissect signaling pathways in the zebrafish embryo. We tested this hypothesis using a vascular endothelial growth factor receptor inhibitor, PTK787/ZK222584. Zebrafish embryos treated with this compound lacked all major blood vessels. Overexpression of AKT/PKB, a putative effector of vascular endothelial growth factor signaling, allowed blood vessels to form in the presence of drug. Endothelial cell apoptosis induced by the drug is prevented by increasing AKT/PKB activity, thus establishing the physiological relevance of AKT/PKB in the angiogenic process. This approach allowed us to examine the effects of blood flow and the role of endothelial signals in organogenesis.

PTEN: The down side of PI 3-kinase signalling

The PTEN tumour suppressor protein is a phosphoinositide 3-phosphatase that, by metabolising phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)), acts in direct antagonism to growth factor stimulated PI 3-kinases. A wealth of data has now illuminated pathways that can be controlled by PTEN through PtdIns(3,4,5)P(3), some of which, when deregulated, give a selective advantage to tumour cells. Early studies of PTEN showed that its activity was able to promote cell cycle arrest and apoptosis and inhibit cell motility, but more recent data have identified other functional consequences of PTEN action, such as effects on the regulation of angiogenesis. The structure of PTEN includes several features not seen in related protein phosphatases, which adapt the enzyme to act efficiently as a lipid phosphatase, including a C2 domain tightly associated with the phosphatase domain, and a broader and deeper active site pocket. Several pieces of data indicate that PTEN is a principal regulator of the cellular levels of PtdIns(3,4,5)P(3), but work is only just beginning to uncover mechanisms by which the cellular activity of PTEN can be controlled. There also remains the vexing question of whether any of PTEN's cellular functions reflect its evolutionary roots as a member of the protein tyrosine phosphatase superfamily.

Expression and prognostic significance of PTEN product protein in patients with esophageal squamous cell carcinoma

BACKGROUND

PTEN is a candidate tumor-suppressor gene in a variety of malignant tumors. The prognostic importance of PTEN product protein (PTEN) and its correlation with clinicopathologic characteristics have yet to be delineated in patients with esophageal carcinoma.

METHODS

Specimens from 97 patients with esophageal squamous cell carcinoma were used for the immunohistochemical evaluation of PTEN expression.

RESULTS

PTEN expression was detected in the nucleus in 48 specimens (49.5%). There were statistically significant correlations between nuclear PTEN expression and macroscopic tumor classification, T stage, and American Joint Committee on Cancer (AJCC) stage (P < 0.01), indicating that PTEN expression was down-regulated by advancement of the disease process. There were no statistically significant correlations between nuclear PTEN expression and the intensity and extent of cytoplasmic PTEN expression. The 10-year overall survival rate was significantly better in patients with positive nuclear PTEN expression (n = 48 patients) compared with the rate in patients with negative nuclear PTEN expression (n = 49 patients; P < 0.01). The results of a multivariate analysis of factors that were prognostic for survival showed that AJCC stage (P < 0.05; relative risk, 2.038) and negative nuclear PTEN expression (P < 0.05; relative risk, 1.825) were significant factors indicative of poor survival.

CONCLUSIONS

Nuclear PTEN expression may be a favorable biologic marker and a useful prognostic indicator in patients with esophageal squamous cell carcinoma.

Control of cell cycle exit and entry by protein kinase B-regulated forkhead transcription factors

AFX-like Forkhead transcription factors, which are controlled by phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signaling, are involved in regulating cell cycle progression and cell death. Both cell cycle arrest and induction of apoptosis are mediated in part by transcriptional regulation of p27(kip1). Here we show that the Forkheads AFX (FOXO4) and FKHR-L1 (FOXO3a) also directly control transcription of the retinoblastoma-like p130 protein and cause upregulation of p130 protein expression. Detailed analysis of p130 regulation demonstrates that following Forkhead-induced cell cycle arrest, cells enter G(0) and become quiescent. This is shown by a change in phosphorylation of p130 to G(0)-specific forms and increased p130/E2F-4 complex formation. Most importantly, long-term Forkhead activation causes a sustained but reversible inhibition of proliferation without a marked increase in apoptosis. As for the activity of the Forkheads, we also show that protein levels of p130 are controlled by endogenous PI3K/PKB signaling upon cell cycle reentry. Surprisingly, not only nontransformed cells, but also cancer cells such as human colon carcinoma cells, are forced into quiescence by Forkhead activation. We therefore propose that Forkhead inactivation by PKB signaling in quiescent cells is a crucial step in cell cycle reentry and contributes to the processes of transformation and regeneration.

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.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Phosphoinositide-specific inositol polyphosphate 5-phosphatase IV inhibits Akt/protein kinase B phosphorylation and leads to apoptotic cell death

Phosphoinositide-specific inositol polyphosphate 5- phosphatase IV has the affinity for PI(3,4,5)P(3) (K(m) = 0.65 microM) that is approximately 10-fold greater than the other inositol polyphosphate 5-phosphatases, which use this substrate including SHIP, OCRL, and 5ptase II, suggesting that it may be important in controlling intracellular levels of this metabolite. We created cell lines stably expressing the enzyme to study its effect on cell function. We found that overexpression of 5ptase IV in 293 cells caused the rapid depletion of both PI(4,5)P(2) and PI(3,4,5)P(3) in cells with corresponding increases in the products, PI(4)P and PI(3,4)P(2), changing the balance of two phosphoinositol products of phosphoinositide 3-kinase, PI(3,4)P(2) and PI(3,4,5)P(3), in the cell. One of the targets of these phosphoinositides is the serine/threonine kinase Akt, which plays an important role in the control of apoptosis. We were able to address the relative roles of PI(3,4)P(2) and PI(3,4,5)P(3) in the activation of Akt by selective depletion of these phosphoinositides in cells stably transfected with 5ptase IV and inositol polyphosphate 4-phosphatase (4ptase I). In cells transfected with 4ptase I, the level of PI(3,4)P(2) was reduced, and PI(3,4,5)P(3) was increased. Expression of the two enzymes had the opposite effect on the phosphorylation of Akt in response to stimulation with growth factors or heat shock. Akt phosphorylation was inhibited in cells expressing 5ptase IV but increased in 4ptase I cells and correlated with the intracellular level of PI(3,4,5)P(3) and not that of PI(3,4)P(2). The inhibition of Akt phosphorylation in cells expressing 5ptase IV makes them highly susceptible to FAS-induced apoptosis, whereas overexpressing of the 4ptase I protects cells from apoptosis. Our results place 5ptase IV as a relevant biological regulator of PI3K/Akt pathway in cells.

Emerging targets in the AKT pathway for treatment of androgen-independent prostatic adenocarcinoma

Prostatic adenocarcinoma (CaP) is the most common, non-cutaneous malignancy and the second-leading cause of cancer death in men. The disease has two distinct phases: the androgen-dependent phase, which can be treated effectively with androgen ablation therapies, and the androgen-independent phase, for which there is no effective life-prolonging therapy. An estimated 32,000 men will die this year from androgen-independent, metastatic CaP. Efforts to understand the metastatic progression of CaP and the emergence of androgen-independent disease have begun to illuminate the molecular events involved. Recent work suggests that CaP progression to androgen-independent, metastatic disease involves a dampened apoptotic response, a release from the cell cycle block that initially follows androgen withdrawal and a shift from dependence on paracrine-derived growth and survival factors to autonomous production of these key proteins. Functional loss of the tumour suppressor phosphatase and tensin homologue deleted on chromosome ten (PTEN) and subsequent activation of the AKT pathway, have been prominently implicated in the progression of CaP to androgen-independence. Activation of the AKT pathway can suppress the apoptotic response, undermine cell cycle control and selectively enhance the production of key growth and survival factors. Though many proteins and intracellular signalling pathways can influence these biological processes, activation of the AKT pathway may be a particularly potent signal involved in CaP progression to androgen-independence and therefore presents a series of potential targets for therapy of advanced androgen-independent CaP.

Signaling pathways of D3-phosphoinositide-binding kinases in T cells and their regulation by PTEN

Phosphoinositide 3-kinases (PI3Ks) phosphorylate the D3 position of the myo -inositol ring of inositol phospholipids, producing, amongst others, phosphatidylinositol-(3,4,5)-trisphosphate. This activity is opposed by the lipid phosphatase PTEN, which catalyzes the removal of this phosphate. Stimulation of PI3Ks is elicited by engagement of receptors for antigen, cytokines and chemokines, and by co-stimulatory molecules. Kinases and other enzymes containing pleckstrin homology domains are activated by binding to these phospholipids, affecting a variety of cellular processes that control lymphocyte function, including cell survival, proliferation, chemotaxis and cytoskeletal reorganization. This review highlights the signaling pathways of these kinases and other enzymes in T cells, their biological effects, and their regulation by PTEN.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

Phosphorylation of the PTEN tail acts as an inhibitory switch by preventing its recruitment into a protein complex

PTEN is a tumor suppressor protein that functions, in large part, by dephosphorylating the lipid second messenger phosphatidylinositol 3,4,5-trisphosphate and by doing so antagonizing the action of phosphoinositide 3-kinase. PTEN structural domains include an N-terminal phosphatase domain, a lipid-binding C2 domain, and a 50-amino acid C-terminal tail that contains a PDZ binding sequence. We showed previously that phosphorylation of the PTEN tail negatively regulates PTEN activity. We now show that phosphorylated PTEN exists in a monomeric "closed" conformation and has low affinity for PDZ domain-containing proteins. Conversely, when unphosphorylated, PTEN is in an "open" conformation, is recruited into a high molecular weight complex (PTEN-associated complex), and strongly interacts with PDZ-containing proteins such as MAGI-2. As a consequence, when compared with wild-type PTEN, the phosphorylation-deficient mutant form of PTEN strongly cooperates with MAGI-2 to block Akt activation. These results indicate that phosphorylation of the PTEN tail causes a conformational change that results in the masking of the PDZ binding domain. Consequently, the ability of PTEN to bind to PDZ domain-containing proteins is reduced dramatically. These data suggest that phosphorylation of the PTEN tail suppresses the activity of PTEN by controlling the recruitment of PTEN into the PTEN-associated complex.

Phosphatidylinositol 3-kinase-dependent pathways oppose Fas-induced apoptosis and limit chloride secretion in human intestinal epithelial cells. Implications for inflammatory diarrheal states

The epithelial lining of the intestine serves as a barrier to lumenal bacteria and can be compromised by pathologic Fas-mediated epithelial apoptosis. Phosphatidylinositol (PI)3-kinase signaling has been described to limit apoptosis in other systems. We hypothesized that PI3-kinase-dependent pathways regulate Fas-mediated apoptosis and barrier function in intestiynal epithelial cells (IEC). IEC lines (HT-29 and T84) were exposed to agonist anti-Fas antibody in the presence or absence of chemical inhibitors of PI3-kinase (LY294002 and wortmannin). Apoptosis, barrier function, changes in short circuit current (DeltaI(sc)), and expression of adhesion molecules were assessed. Inhibition of PI3-kinase strongly sensitized IEC to Fas-mediated apoptosis. Expression of constitutively active Akt, a principal downstream effector of the PI3-kinase pathway, protected against Fas-mediated apoptosis to an extent that was comparable with expression of a genetic caspase inhibitor, p35. PI3-kinase inhibition sensitized to apoptosis by increasing and accelerating Fas-mediated caspase activation. Inhibition of PI3-kinase combined with cross-linking Fas was associated with increased permeability to molecules that were <400 Da but not those that were >3,000 Da. Inhibition of PI3-kinase resulted in chloride secretion that was augmented by cross-linking Fas. Confocal analyses revealed polymerization of actin and maintenance of epithelial cell adhesion molecule-mediated interactions in monolayers exposed to anti-Fas antibody in the context of PI3-kinase inhibition. PI3-kinase-dependent pathways, especially Akt, protect IEC against Fas-mediated apoptosis. Inhibition of PI3-kinase in the context of Fas signaling results in increased chloride secretion and barrier dysfunction. These findings suggest that agonists of PI3-kinase such as growth factors may have a dual effect on intestinal inflammation by protecting epithelial cells against immune-mediated apoptosis and limiting chloride secretory diarrhea.

Deletion of Pten in mouse brain causes seizures, ataxia and defects in soma size resembling Lhermitte-Duclos disease

Initially identified in high-grade gliomas, mutations in the PTEN tumor-suppressor are also found in many sporadic cancers and a few related autosomal dominant hamartoma syndromes. PTEN is a 3'-specific phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P3) phosphatase and functions as a negative regulator of PI3K signaling. We generated a tissue-specific deletion of the mouse homolog Pten to address its role in brain function. Mice homozygous for this deletion (PtenloxP/loxP;Gfap-cre), developed seizures and ataxia by 9 wk and died by 29 wk. Histological analysis showed brain enlargement in PtenloxP/loxP;Gfap-cre mice as a consequence of primary granule-cell dysplasia in the cerebellum and dentate gyrus. Pten mutant cells showed a cell-autonomous increase in soma size and elevated phosphorylation of Akt. These data represent the first evidence for the role of Pten and Akt in cell size regulation in mammals and provide an animal model for a human phakomatosis condition, Lhermitte-Duclos disease (LDD).