A hierarchical cascade activated

1Cellular Organization and Signaling, National Centre for Biological Sciences, TIFR, Bangalore 560065, Karnataka, IndiaReceived 3 November 2008; Revised 5 January 2009; Accepted 6 February 2009; Published online 6 March 2009.

Top of pageAbstractThe regulation of cellular metabolism and survival by trophic factors is not completely understood. Here, we describe a signaling cascade activated by the developmental regulator Notch, which inhibits apoptosis triggered by neglect in mammalian cells. In this pathway, the Notch intracellular domain (NIC), which is released after interaction with ligand, converges on the kinase mammalian target of rapamycin (mTOR) and the substrate defining protein rapamycin independent companion of mTOR (Rictor),

cheap ray bans culminating in the activation of the kinase Akt Biochemical and molecular approaches using site directed mutants identified AktS473 as a key downstream target in the antiapoptotic pathway activated by NIC. Despite the

cheap oakleys online demonstrated requirement for Notch processing and its predominant nuclear localization, NIC function was independent of CBF1 an essential DNA binding component required for canonical signaling. In experiments that placed spatial constraints on NIC, enforced nuclear retention abrogated antiapoptotic activity and a membrane anchored form of NIC blocked apoptosis through mTOR, Rictor and Akt dependent signaling. We show that the NIC mTORC2 Akt cascade blocks the
fake ray ban sunglasses apoptotic response triggered by removal of medium or serum deprivation. Consistently, membrane tethered NIC, and AktS473 inhibited apoptosis triggered by cytokine deprivation in activated T cells. Thus, this study identifies a non canonical signaling cascade wherein NIC integrates with multiple pathways to regulate cell survival.

Keywords: apoptosis, Akt, kinase, mammalian cells, mTOR, Notch, neglect, Rictor, signalingCellular dependence on extrinsic factors for survival is developmentally programed and continues through the life of adult metazoans.1 In their absence, cells undergo death by the intrinsic pathway of apoptosis that converges on, and is regulated by mitochondria.2 Although it is increasingly evident that pathways regulating metabolic activity and cell growth also regulate survival, the underlying mechanisms and molecular
cheap michael kors handbags correlates of these interactions are not completely understood. Thus, mechanisms by which growth factors regulate survival and intersect with the nutrient sensing machinery are of interest as core elements of these signaling pathways are conserved across phyla.

Signaling through the Notch family of transmembrane receptors is initiated by interactions with ligands that trigger proteolytic cleavage of the receptor.

We show that Notch signaling converges on the activation of Akt to inhibit apoptosis triggered by neglect, that is, the withdrawal

cheap oakleys of nutritional cues, and describe the molecular intermediates in this cascade. Our experiments reveal integral roles for the nutrient sensor kinase mTOR and its substrate defining protein Rictor, which regulates the activation of Akt for Notch mediated survival. Both biochemical approaches and the analysis of site directed mutants identify AktS473 activity as the downstream target of the NIC mTORC2 signaling cascade.
Cheap Elite Jerseys We present evidence that Notch processing was required for the activation of the signal transduction cascade. However, dominant negative (DN) and RNA interference approaches, as well as the use of
wholesale fake oakleys a deletion mutant, showed that NIC activity did not require CBF1 transcription. Reduced nuclear retention did not inhibit the antiapoptotic activity of NIC. Furthermore, the signaling cascade recapitulated by a modified membrane anchored form of the Notch intracellular domain, supported the cytoplasmic localization of NIC for its antiapoptotic function. This cascade blocked death triggered by the withdrawal of medium or serum in cell lines, and membrane tethered NIC, and AktS473 activity inhibited apoptosis triggered by cytokine deprivation in activated T cells. Thus, the functional interactions with mTOR Rictor and Akt identify a novel Notch mediated signaling cascade that favors cell survival.

Top of pageResultsCharacterization of deprivation (neglect) induced deathIf growth medium is replaced by buffered saline, HeLa cells in culture undergo apoptosis within 6 (neglect induced death). Apoptotic nuclear damage was determined using Hoechst 33 to reveal changes in morphology and the TUNEL assay to detect double stranded breaks in cells (Figure 1a). The TUNEL assay gave comparable
cheap fake oakleys sales results to that obtained with Hoechst 33 (Figure 1b), and in all subsequent analysis, the assessment
wholesale oakleys of nuclear morphology using Hoechst has been presented as the readout of damage. Dying cells were also characterized by the externalization of phosphatidylserine (Figure 1c). Induction of apoptotic damage was detected after 3 of deprivation and increased substantially by 6 (Supplementary Figure 1A). Furthermore, apoptosis was abrogated in cells following the siRNA mediated depletion of the proapoptotic Bcl 2 family protein, Bax (Figure 1d).

Figure 1.

Characterization of the apoptotic response to neglect in HeLa cells. (a) Confocal image of HeLa cells cultured in complete medium (CM; nutrient replete) or buffered saline (neglect). After 6 cells
replica christian louboutin were harvested and assessed for apoptotic changes using Hoechst 33 or by the TUNEL assay; scale bar: 5 (b) Quantitation of apoptotic nuclear damage as assessed by the two readouts, described in panel a, from three separate experiments. (c) Phosphatidylserine exposure assessed using Annexin V FITC in HeLa cells at the onset of the assay (t and after culture in buffered saline for 6 (neglect). (d) HeLa cells were treated with siRNA to Bax, or a scrambled control for 48 Cells were continued in CM or buffered saline, and apoptotic nuclear damage was assessed after 6 The western blot shows the levels of Bax protein in siRNA treated groups. Data from three independent experiments are plotted

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Ligand dependent activation of Notch inhibited neglect induced apoptosisApoptosis induced by the withdrawal of medium was inhibited by the antiapoptotic protein, Bcl 2, or by full length Notch1 FL (Figure 2a), providing a system to explore the signaling cascade downstream of Notch. NFL mediated inhibition was abrogated by the coexpression of soluble Jagged1 (sJag1; Figure 2a), which inhibited Notch processing and release of the intracellular domain (Supplementary Figure 1B). Consistently, apoptosis was prevented by the ectopic expression of the recombinant processed form of the NIC (Figure 2b and c and Supplementary Figure 1C), which protected from
oakleys store apoptosis for an extended period (Supplementary Figure ID). The requirement for ligand dependent processing for Notch activity was verified using mutants disabled for interactions with ligand. The recombinant Notch1LNG, which lacks the epidermal growth factor (EGF) like repeats in its extracellular domain,20 did not block death (Figure 2b), whereas a construct corrected for this defect (Notch1LNG CCSS) imparted protection from death (Figure 2b). As expected, only Notch1LNG CCSS activated the Hes1 promoter (a readout of transcriptional activity) in reporter assays (Figure 2d). Immunofluorescence analysis confirmed that myc tagged Notch1LNG was detected at the membrane as expected (Figure 2d, inset). Furthermore, by immunoblot analysis, both myc tagged constructs were expressed at comparable levels with increased amounts of processed NICD (85 generated in Notch1LNG CCSS transfected cells (Figure 2d inset). Cells undergoing neglect induced apoptosis showed a loss of adherence to substrate, which was apparent in the rounded morphology of cells undergoing neglect and disorganization of mitochondria (Figure 2e, GFP). Both these changes were prevented in cells expressing NIC, but not in cells expressing Notch1LNG (Figure 2e). Thus, the inhibition of nuclear damage by NIC activity correlated with adherence to matrix in cells deprived of nutrients.

Figure 2.

Notch activity inhibits neglect induced death in mammalian cells. (a and b) HeLa cells were transfected with the indicated plasmids and cultured overnight. Culture medium was replaced with complete medium (CM; nutrient replete) or buffered saline (neglect) for each transfected plasmid. Apoptotic nuclear damage was assessed in GFP cells using Hoechst 33 after 6 . The data are corrected to the nutrient replete condition and represent mean from three separate experiments. (c) HeLa cells transfected with RFP or NIC RFP were cultured as described in panel a. After 6 apoptotic damage was analyzed using the TUNEL assay (green), and representative cells from the condition of neglect are shown; scale bar: 5 (d) Hes1 promoter activity assessed in a luciferase assay in HeLa cells transfected with
wholesale cheap jerseys china the constructs shown in the panel; inset: upper panel shows an immunoblot analysis of cells expressing the myc tagged Notch1 constructs. Lower panel shows central plane of a confocal image of HeLa cells expressing Notch1LNG myc stained with an antibody to myc. (e) HeLa cells were transfected with the indicated constructs, cultured in the presence or absence of medium for 6 and stained with Mitotracker Red to mark the mitochondria, and field views imaged at confocal resolution. Images of cultures in the neglect condition are shown. Notch signaling activates Akt in several contexts,7, 8, 9, 10, 21 and we assessed whether Akt was required for Notch activity in the regulation of neglect induced death.

Akt is an intermediate in the antiapoptotic pathway activated by NICThe status of Akt activation in cells experiencing neglect in the presence or absence of Notch FL was determined. Phosphatidylinositol 3,4,5 triphosphates generated through PI3K activity bind the PH domain of Akt enabling its translocation to the plasma membrane and subsequent phosphorylations on T308 and S473.11

replica oakleys
replica ray bans These experiments were initiated because of the observation made in the HEK cell line that ectopically expressed NIC modulated the phosphorylation of Akt, specifically on S473 (Supplementary Figure 2A and B). Neglect triggered a loss of (endogenous) AktS473 phosphorylation, which was prevented in HeLa cells expressing NFL (Figure 3a, upper panel). NFL activity was attenuated by the coexpression of sJag1 (Figure 3a, western blot; compare lanes 2 and 3), consistent with the requirement for ligand dependent interactions for Notch function.

Figure 3.

NIC activates Akt signaling to block neglect induced death. (a) Cells were transfected with the plasmids indicated and cultured overnight. Cells were shifted to buffered saline, and AktS473 phosphorylation was assessed by western blot analysis in cell extracts after 3 incubation in buffered saline. The graph shows data derived by densitometry analysis of the western blots from three separate experiments. The representative western blot shows the phosphorylation of AktS473 in the conditions described in the various lanes. (b) HeLa cells were transfected with GFP or NIC GFP in cells that were pretreated with siRNA to Akt1 or a scrambled control for 48 Cultures were continued with complete medium (CM) or buffered saline as described in Figure 2a. Apoptotic nuclear damage was assessed in GFP cells after 6 inset: levels of Akt1 protein in cells treated with siRNA. (c) HeLa cells transfected with GFP or NIC GFP with or without DN Akt as indicated were cultured in CM or buffered saline. Apoptotic nuclear damage was assessed in GFP cells after 6 data represent mean from a minimum of three separate experiments. In panels b and c, the data are normalized to the CM condition

Full figure and legend (95K)

NIC GFP mediated inhibition of apoptosis was abrogated in cells where Akt1 protein was depleted using siRNA (Figure 3b), or following the cotransfection of a DN (defective ATP binding mutant) construct of Akt (DN
cheap fake ray ban sales Akt; Figure 3c), positioning Akt as an intermediate in the pathway activated by Notch. As Notch lacks intrinsic kinase activity, we speculated that the phosphorylation of Akt in cells expressing Notch was indicative of another signaling intermediate
discount oakleys Sunglasses in this transduction cascade. Recent work has characterized the nutritional sensor mTOR in association with Rictor, mLST8, and SIN1 (mTORC2) as the complex regulating the phosphorylation of Akt on S473.17, 22, 23, 24, 25, 26 Pathways upstream of mTORC2 are not well characterized, which prompted us to test the involvement of mTOR and Rictor in Notch mediated antiapoptotic activity.

Notch activity converges on mTOR and Rictor
cheap ray ban to regulate neglect induced deathThe depletion of mTOR attenuated NIC mediated inhibition of apoptosis (Figure 4a), positioning mTOR as an intermediate in the pathway. The two complexes of mTOR mTORC1 and mTORC2 are distinguished by dependence on the proteins Raptor and Rictor, respectively. To investigate the roles of mTORC1 and mTORC2 in this pathway, the requirement for Raptor or Rictor in the antiapoptotic function of Notch was assessed. Cellular pools of Rictor (mTORC2) or Raptor (mTORC1) were independently depleted using siRNA (Figure 4c). Depletion of Rictor abrogated NIC GFP function (Figure 4b), whereas reducing the levels of Raptor or the scrambled siRNA (included as a control in all experiments) had no effect on NIC mediated inhibition of death (Figure 4b). Although the functional interaction between mTORC2 and Notch signaling revealed by these experiments was consistent with the demonstrated requirement for Akt and its
cheap oakleys outlet phosphorylation on S473, the requirement for mTORC2 for Notch mediated phosphorylation of Akt remained to be established. This was again monitored in conditions where the cellular levels of mTOR, Rictor, or Raptor were reduced by siRNA. Depletion of mTOR or Rictor inhibited the phosphorylation of Akt on S473 in NIC expressing cells (Figure 4d and f), whereas the depletion of Raptor was without effect (Figure 4e). As mTOR and Rictor dependence of a signaling cascade is consistent with the activation of Akt on S473,19 site directed mutants were employed to test whether AktS473 was a functional effector of the NIC signaling cascade in the regulation of cell survival.

Figure 4.

mTOR and Rictor regulate NIC mediated antiapoptotic activity. (a and b) Apoptotic nuclear damage triggered by neglect in HeLa cells expressing GFP or NIC GFP in cells pretreated with siRNA to mTOR (a), Rictor or
nike jerseys Raptor (b) for 48 A scrambled control siRNA was included in all assays. Data are plotted as mean from three independent experiments. (c) Representative immunoblots showing the levels of proteins in cells treated with the indicated siRNA. (d
fake ray ban Immunoblots showing the phosphorylation of AktS473 in HeLa cells transfected with NIC GFP Akt myc in conditions where the cells have been treated with siRNA to mTOR (d), Raptor (e) or Rictor (f)

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