Activation of Akt/protein kinase B overcomes a G(2)/m cell cycle checkpoint induced by DNA damage

Activation of Akt, or protein kinase B, is frequently observed in human cancers. Here we report that Akt activation via overexpression of a constitutively active form or via the loss of PTEN can overcome a G(2)/M cell cycle checkpoint that is induced by DNA damage. Activated Akt also alleviates the reduction in CDC2 activity and mitotic index upon exposure to DNA damage. In addition, we found that PTEN null embryonic stem (ES) cells transit faster from the G(2)/M to the G(1) phase of the cell cycle when compared to wild-type ES cells and that inhibition of phosphoinositol-3-kinase (PI3K) in HEK293 cells elicits G(2) arrest that is alleviated by activated Akt. Furthermore, the transition from the G(2)/M to the G(1) phase of the cell cycle in Akt1 null mouse embryo fibroblasts (MEFs) is attenuated when compared to that of wild-type MEFs. These results indicate that the PI3K/PTEN/Akt pathway plays a role in the regulation of G(2)/M transition. Thus, cells expressing activated Akt continue to divide, without being eliminated by apoptosis, in the presence of continuous exposure to mutagen and accumulate mutations, as measured by inactivation of an exogenously expressed herpes simplex virus thymidine kinase (HSV-tk) gene. This phenotype is independent of p53 status and cannot be reproduced by overexpression of Bcl-2 or Myc and Bcl-2 but seems to counteract a cell cycle checkpoint mediated by DNA mismatch repair (MMR). Accordingly, restoration of the G(2)/M cell cycle checkpoint and apoptosis in MMR-deficient cells, through reintroduction of the missing component of MMR, is alleviated by activated Akt. We suggest that this new activity of Akt in conjunction with its antiapoptotic activity may contribute to genetic instability and could explain its frequent activation in human cancers.     

The inducible expression of the tumor suppressor gene PTEN promotes apoptosis and decreases cell size by inhibiting the PI3K/Akt pathway in Jurkat T cells.

In this study, we characterize the function of the tumor suppressor gene PTEN in Jurkat T cells. We established stable clones of Jurkat T cells that inducibly express either wild-type or phosphatase-inactive PTEN. We show here that PTEN potently inhibited the growth and reduced the size of Jurkat cells. The growth-suppressive effect of PTEN was associated with its ability to induce apoptotic cell death with little or no effect on cell cycle. PTEN also rendered Jurkat cells more susceptible to apoptosis induced by various stimuli. Furthermore, PTEN expression led to a reduction in the level of 3'-phosphorylated phospholipids and thus altered the activity and localization of Akt. Finally, coexpression of constitutively active Akt reversed the effects caused by PTEN. In summary, our results suggest that PTEN suppresses cell growth, promotes apoptosis, and decreases cell size by negatively regulating the phosphoinositide 3-kinase/Akt pathway in Jurkat T cells.     

PTEN/MMAC1 overexpression decreases insulin-like growth factor-I-mediated protection from apoptosis in neuroblastoma cells.

Insulin-like growth factor I (IGF-I) protects cells from apoptosis primarily through the action of phosphatidylinositol-3 kinase and the downstream serine/threonine kinase Akt. The PTEN gene product, a protein which dephosphorylates phosphatidylinositol lipids, prevents activation of Akt and regulates several cellular functions, including cell cycle progression, cell migration, and survival from apoptosis. In this study, PTEN overexpression decreases IGF-I-induced Akt activity, enhances serum withdrawal-induced apoptosis, and decreases IGF-I protection and cell growth in SHEP cells. The PTEN lipid phosphatase mutant G129E fails to inhibit IGF-I-stimulated Akt activity and protection from apoptosis. The C124S mutation, which abolishes both lipid and protein phosphatase activity, fails to inhibit Akt activity and IGF-I protection against hyperosmotic-induced apoptosis but still inhibits growth and IGF-I protection against serum withdrawal-induced apoptosis. These data suggest a role for PTEN in modulating the effect of IGF-I on Akt activity, neuroblastoma cell growth, and protection against apoptotic stimuli.     

PTEN regulates RANKL- and osteopontin-stimulated signal transduction during osteoclast differentiation and cell motility

PTEN (also known as MMAC-1 or TEP-1) is a frequently mutated tumor suppressor gene in human cancer. PTEN functions have been identified in the regulation of cell survival, growth, adhesion, migration, and invasiveness. Here, we characterize the diverse signaling networks modulated by PTEN in osteoclast precursors stimulated by RANKL and osteopontin (OPN). RANKL dose-dependently stimulated transient activation of Akt before activation of PTEN, consistent with a role for PTEN in decreasing Akt activity. PTEN overexpression blocked RANKL-activated Akt stimulated survival and osteopontin-stimulated cell migration while a dominant-negative PTEN increased the actions of RANKL and OPN. PTEN overexpression suppressed RANKL-mediated osteoclast differentiation and OPN-stimulated cell migration. The PTEN dominant-negative constitutively induced osteoclast differentiation and cell migration. Our data demonstrate multiple roles for PTEN in RANKL-induced osteoclast differentiation and OPN-stimulated cell migration in RAW 264.7 osteoclast precursors.     

The Effect of a DNA Damaging Agent on Embryonic Cell Cycles of the Cnidarian Hydractinia echinata

The onset of gastrulation at the Mid-Blastula Transition can accompany profound changes in embryonic cell cycles including the introduction of gap phases and the transition from maternal to zygotic control. Studies in Xenopus and Drosophila embryos have also found that cell cycles respond to DNA damage differently before and after MBT (or its equivalent, MZT, in Drosophila). DNA checkpoints are absent in Xenopus cleavage cycles but are acquired during MBT. Drosophila cleavage nuclei enter an abortive mitosis in the presence of DNA damage whereas post-MZT cells delay the entry into mitosis. Despite attributes that render them workhorses of embryonic cell cycle studies, Xenopus and Drosophila are hardly representative of diverse animal forms that exist. To investigate developmental changes in DNA damage responses in a distant phylum, I studied the effect of an alkylating agent, Methyl Methanesulfonate (MMS), on embryos of Hydractinia echinata. Hydractinia embryos are found to differ from Xenopus embryos in the ability to respond to a DNA damaging agent in early cleavage but are similar to Xenopus and Drosophila embryos in acquiring stronger DNA damage responses and greater resistance to killing by MMS after the onset of gastrulation. This represents the first study of DNA damage responses in the phylum Cnidaria.     

PTEN induces cell cycle arrest by decreasing the level and nuclear localization of cyclin D1

PTEN is a tumor suppressor frequently inactivated in brain, prostate, and uterine cancers that acts as a phosphatase on phosphatidylinositol-3,4,5-trisphosphate, antagonizing the activity of the phosphatidylinositol 3'-OH kinase. PTEN manifests its tumor suppressor function in most tumor cells by inducing G(1)-phase cell cycle arrest. To study the mechanism of cell cycle arrest, we established a tetracycline-inducible expression system for PTEN in cell lines lacking this gene. Expression of wild-type PTEN but not of mutant forms unable to dephosphorylate phosphoinositides reduced the expression of cyclin D1. Cyclin D1 reduction was accompanied by a marked decrease in endogenous retinoblastoma (Rb) protein phosphorylation on cyclin D/CDK4-specific sites, showing an early negative effect of PTEN on Rb inactivation. PTEN expression also prevented cyclin D1 from localizing to the nucleus during the G(1)- to S-phase cell cycle transition. The PTEN-induced localization defect and the cell growth arrest could be rescued by the expression of a nucleus-persistent mutant form of cyclin D1, indicating that an important effect of PTEN is at the level of nuclear availability of cyclin D1. Constitutively active Akt/PKB kinase counteracted the effect of PTEN on cyclin D1 translocation. The data are consistent with an oncogenesis model in which a lack of PTEN fuels the cell cycle by increasing the nuclear availability of cyclin D1 through the Akt/PKB pathway.     

Overexpression of PTEN suppresses lipopolysaccharide-induced lung fibroblast proliferation, differentiation and collagen secretion through inhibition of the PI3-K-Akt-GSK3beta pathway

Background

Abnormal and uncontrolled proliferation of lung fibroblasts may contribute to pulmonary fibrosis. Lipopolysaccharide (LPS) can induce fibroblast proliferation and differentiation through activation of phosphoinositide3-Kinase (PI3-K) pathway. However, the detail mechanism by which LPS contributes to the development of lung fibrosis is not clearly understood. To investigate the role of phosphatase and tensin homolog (PTEN), a PI3-K pathway suppressor, on LPS-induced lung fibroblast proliferation, differentiation, collagen secretion and activation of PI3-K, we transfected PTEN overexpression lentivirus into cultured mouse lung fibroblasts with or without LPS treatment to evaluate proliferation by MTT and Flow cytometry assays. Expression of PTEN, alpha-smooth muscle actin (alpha-SMA), glycogen synthase kinase 3 beta (GSK3beta) and phosphorylation of Akt were determined by Western-blot or real-time RT-PCR assays. The PTEN phosphorylation activity was measured by a malachite green-based assay. The content of C-terminal propeptide of type I procollagen (PICP) in cell culture supernatants was examined by ELISA.

Results

We found that overexpression of PTEN effectively increased expression and phosphatase activity of PTEN, and concomitantly inhibited LPS-induced fibroblast proliferation, differentiation and collagen secretion. Phosphorylation of Akt and GSK3beta protein expression levels in the LPS-induced PTEN overexpression transfected cells were significantly lower than those in the LPS-induced non-transfected cells, which can be reversed by the PTEN inhibitor, bpV(phen).

Conclusions

Collectively, our results show that overexpression and induced phosphatase activity of PTEN inhibits LPS-induced lung fibroblast proliferation, differentiation and collagen secretion through inactivation of PI3-K-Akt-GSK3beta signaling pathways, which can be abrogated by a selective PTEN inhibitor. Thus, expression and phosphatase activity of PTEN could be a potential therapeutic target for LPS-induced pulmonary fibrosis. Compared with PTEN expression level, phosphatase activity of PTEN is more crucial in affecting lung fibroblast proliferation, differentiation and collagen secretion.     

Dissection of the XChk1 signaling pathway in Xenopus laevis embryos

Checkpoint pathways inhibit cyclin-dependent kinases (Cdks) to arrest cell cycles when DNA is damaged or unreplicated. Early embryonic cell cycles of Xenopus laevis lack these checkpoints. Completion of 12 divisions marks the midblastula transition (MBT), when the cell cycle lengthens, acquiring gap phases and checkpoints of a somatic cell cycle. Although Xenopus embryos lack checkpoints prior to the MBT, checkpoints are observed in cell-free egg extracts supplemented with sperm nuclei. These checkpoints depend upon the Xenopus Chk1 (XChk1)-signaling pathway. To understand why Xenopus embryos lack checkpoints, xchk1 was cloned, and its expression was examined and manipulated in Xenopus embryos. Although XChk1 mRNA is degraded at the MBT, XChk1 protein persists throughout development, including pre-MBT cell cycles that lack checkpoints. However, when DNA replication is blocked, XChk1 is activated only after stage 7, two cell cycles prior to the MBT. Likewise, DNA damage activates XChk1 only after the MBT. Furthermore, overexpression of XChk1 in Xenopus embryos creates a checkpoint in which cell division arrests, and both Cdc2 and Cdk2 are phosphorylated on tyrosine 15 and inhibited in catalytic activity. These data indicate that XChk1 signaling is intact but blocked upstream of XChk1 until the MBT.     

Insulin-like growth factor-II regulates PTEN expression in the mammary gland

The tumor suppressor PTEN is altered in many cancers, including breast cancer, but only a handful of factors are known to control its expression. PTEN plays a vital role in cell survival and proliferation by regulating Akt phosphorylation, a key component of the phosphatidylinositol 3 kinase (PI3K) pathway. Here we show that insulin-like growth factor-II (IGF-II), which signals through PI3K, regulates PTEN expression in the mammary gland. IGF-II injection into mouse mammary gland significantly increased PTEN expression. Transgenic IGF-II expression also increased mammary PTEN protein, leading to reductions in Akt phosphorylation, epithelial proliferation, and mammary morphogenesis. IGF-II induced PTEN promoter activity and protein levels and this involved the immediate early gene egr-1. Thus, we have identified a novel negative feedback loop within the PI3K pathway where IGF-II induces PTEN expression to modulate its physiologic effects.     

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

Background information. PTEN (phosphatase and tensin homologue deleted on chromosome 10) is a negative regulator of the PI3K (phosphoinositide 3‐kinase)–Akt (also called protein kinase B) signalling pathway and is essential for embryogenesis, but its function in early vertebrate embryos is unclear. Results. To address how PTEN functions in early embryos, we overexpressed one of the four zebrafish PTEN isoforms at the 1–2‐cell stage. Overexpression of Ptena454 alters phospho‐Akt levels and impairs cell movements associated with gastrulation. Heat shocking embryos increases phospho‐Akt levels and lowers phospho‐Ptena454 levels. Inhibiting CK2 (protein kinase CK2) activity reduces phospho‐Pten levels and augments the effects due to Ptena454 overexpression. Low phospho‐Akt and corresponding low phospho‐GSK‐3 (glycogen synthase kinase‐3) and high phospho‐Pten levels accompany wortmannin or LY294002 treatment, which inhibit PI3K activity. Conclusions. These results suggest that Ptena454 regulation is correlated to changes in phospho‐Akt levels. We propose a model in which homoeostasis in rapidly dividing and migrating embryonic cells depends on a counterbalance between pro‐survival signalling employing CK2 and GSK‐3 and the pro‐apoptotic activity of Ptena454.     

Akt isoform-specific inhibition of MDA-MB-231 cell proliferation

To dissect the isoform-specific roles of Akt in breast cancer cells, constitutively active Akt isoforms were introduced into MDA-MB-231 cells. Both Akt1 and Akt2 efficiently inhibited the growth of MDA-MB-231 cells. Overexpression of Akt1 down-regulated ERK activity inhibiting Ser 259 phosphorylation of c-Raf and subsequent downstream signaling. Akt2 overexpression up-regulated the cell cycle inhibitor p27. Cycloheximide decay assays showed that Akt2 increased the stability and nuclear localization of p27, thus inhibiting the cyclin E/CDK2 complex. These results suggest that the inhibition of cell proliferation by Akt1 and Akt2 is mediated by isoform-specific mechanisms.     

Cyclin A1/Cdk2 is Sufficient but not Required for the Induction of Apoptosis in Early Xenopus laevis Embryos

The role of cyclin-dependent kinases in cell proliferation is well characterized, whereas their somewhat paradoxical role in catalyzing apoptosis is less understood. One Cdk complex implicated in both cell proliferation and cell death is cyclin A/Cdk2. During early embryonic development of Xenopus laevis, distinct isoforms of cyclin A are expressed at different times. From fertilization through gastrulation, cyclin A1 is the predominant isoform. Cyclin A1 dimerizes with Cdk2 but not Cdk1. In contrast, cyclin A2 is expressed at a low level until gastrulation, when it becomes the major A-type cyclin and associates with both Cdk1 and Cdk2. When Xenopus embryos are treated with ionizing radiation (IR) prior to the midblastula transition (MBT), cyclin A1 protein persists beyond the MBT and forms an active complex with Cdk2. During this window of cyclin A1/Cdk2 activity, the embryo undergoes apoptosis. To test the hypothesis that cyclin A1-associated activity is a mediator of apoptosis, cyclin A1 protein level and associated kinase activity were measured in embryos treated with aphidicolin to induce apoptosis. Both cyclin A1 content and associated kinase activity were sustained after the MBT as embryos underwent apoptosis. To determine whether cyclin A1/Cdk2 was sufficient to induce apoptosis, recombinant cyclin A1/Cdk2 complex was injected into single-celled embryos, which induced apoptosis after the MBT. However, morpholinos targeting translation of cyclins A1 and A2 did not block apoptosis in embryos treated with X-rays or aphidicolin. These data indicate that cyclin A1/Cdk2 is sufficient, but not required for apoptosis during early development.     

Cell cycle arrest and apoptosis by expression of a novel TPIP (TPIP-C2) cDNA encoding a C2-domain in HEK-293 cells

The human TPIP (TPTE and PTEN homologous Inositol lipid Phosphatase) belongs to the PTEN (Phosphatase and TENsin homologue deleted on chromosome 10) family of dual-specific phosphatases and is expressed from the human chromosome 13 as multiple splice-variants, e.g., TPIPα, β, γ mRNAs. PTEN is a well characterized tumor suppressor, which controls survival, adhesion, motility and migration of mammalian cells, its C2-domain plays crucial role in controlling these functions. However, role of isolated C2-domain protein in regulation of cell proliferation and apoptosis is not reported. We report sequence analysis and function of a novel human TPIP (TPIP-C2) cDNA encoding a 193 amino acid C2-domain in cell proliferation and apoptosis regulation. In silico analysis and homology modelling revealed that the C2-domain of TPIP-C2 is similar to that of PTEN but with short disorder sequences overlapping or adjacent to the post-translational modification sites. Overexpression of TPIP-C2 cDNA in human embryonic kidney (HEK-293) cells caused cell cycle arrest, inhibition of cell proliferation and induced apoptosis in an activated caspase 3 and PARP-dependent manner in comparison to overexpression of the full length human PTEN cDNA. TPIP-C2 overexpressed cells also showed S-phase cell cycle arrest. We suggest that C2-domain of TPIP-C2 may act as a dominant negative effector, which may bind to and arrest the cell proliferation signalling complex and isolated TPIP-C2-domain-like proteins expressed in mammalian cells/tissues may play important role in regulation of cell proliferation and apoptosis. The TPIP-C2 cDNA may be exploited for inducing cell cycle-inhibition and apoptosis in human cancer cells and tissues.     

Phosphatase and Tensin Homolog Deleted on Chromosome 10 (PTEN) and PH Domain and Leucine-rich Repeat Phosphatase Cross-talk (PHLPP) in Cancer Cells and in Transforming Growth Factor β-Activated Stem Cells

Akt kinase controls cell survival, proliferation, and invasive growth and is a critical factor for cancer development. Here we describe a cross-talk between phosphatases that may preserve levels of activated/phosphorylated Akt and confer aggressive growth of cancer cells. In prostatic cancer cells, but not in non-transformed cells or in prostate stem cells, we found that the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) overexpression down-regulated PH domain and leucine-rich repeat phosphatase (PHLPP) and that PHLPP overexpression down-regulated PTEN. We also show that silencing PTEN by siRNA increased the levels of PHLPPs. This cross-talk facilitated invasive migration and was mediated by epigenetic alterations, including activation of miR-190, miR-214, polycomb group of proteins, as well as DNA methylation. A role for the purinergic receptor P2X4, previously associated with wound healing, was indicated. We also show that TGF-β1 induced cross-talk concomitant with epithelial-mesenchymal transition in stem cells. The cross-talk emerged as an integrated part of epithelial-mesenchymal transition. We conclude that cross-talk between PTEN and PHLPPs is silenced in normal prostate cells but activated in TGF-β1 transformed prostate stem and cancer cells and facilitates invasive growth.     

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

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

Dominant negative E2F inhibits progression of the cell cycle after the midblastula transition in Xenopus

The cleavage cycle, which is initiated by fertilization, consists of only S and M phases, and the gap phases (G1 and G2) appear after the midblastula transition (MBT) in the African clawed frog, Xenopus laevis. During early development in Xenopus, we examined the E2F activity, which controls transition from the G1 to S phase in the somatic cell cycle. Gel retardation and transactivation assays revealed that, although the E2F protein was constantly present throughout early development, the E2F transactivation activity was induced in a stage-specific manner, that is, low before MBT and rapidly increased after MBT. Introduction of the recombinant dominant negative E2F (dnE2F), but not the control, protein into the 2-cell stage embryos specifically suppressed E2F activation after MBT. Cells in dnE2F-injected embryos appeared normal before MBT, but ceased to proliferate and eventually died at the gastrula. These cells contained decreased cdk activity with enhanced inhibitory phosphorylation of Cdc2 at Tyr15. Thus, E2F activity is required for cell cycle progression and cell viability after MBT, but not essential for MBT transition and developmental progression during the cleavage stage.