p70 S6 kinase is regulated by protein kinase Czeta and participates in a phosphoinositide 3-kinase-regulated signalling complex

Protein Kinase-zeta inhibits collagen I-dependent and anchorage-independent growth and enhances apoptosis of human Caco-2 cells

Colonic carcinogenesis is accompanied by abnormalities in multiple signal transduction components, including alterations in protein kinase C (PKC). The expression level of PKC-zeta, an atypical PKC isoform, increases from the crypt base to the luminal surface and parallels crypt cell differentiation in normal colon. In prior studies in the azoxymethane model of colon cancer, we showed that PKC-zeta was down-regulated in rat colonic tumors. In this study, we showed that PKC-zeta is expressed predominantly in colonic epithelial and not stromal cells, and loss of PKC-zeta occurs as early as the adenoma stage in human colonic carcinogenesis. To assess the regulation of growth and differentiation by PKC-zeta, we altered this isoform in human Caco-2 colon cancer cells using stable constitutive or inducible expression vectors, specific peptide inhibitors or small interfering RNA. In ecdysone-regulated transfectants grown on collagen I, ponasterone A significantly induced PKC-zeta expression to 135% of empty vector cells, but did not alter nontargeted PKC isoforms. This up-regulation was accompanied by a 2-fold increase in basal and 4-fold increase in insulin-stimulated PKC-zeta biochemical activity. Furthermore, PKC-zeta up-regulation caused >50% inhibition of cell proliferation on collagen I (P < 0.05). Increased PKC-zeta also significantly enhanced Caco-2 cell differentiation, nearly doubling alkaline phosphatase activity, while inducing a 3-fold increase in the rate of apoptosis (P < 0.05). In contrast, knockdown of this isoform by small interfering RNA or kinase inhibition by myristoylated pseudosubstrate significantly and dose-dependently increased Caco-2 cell growth on collagen I. In transformation assays, constitutively up-regulated wild-type PKC-zeta significantly inhibited Caco-2 cell growth in soft agar, whereas a kinase-dead mutant caused a 3-fold increase in soft agar growth (P < 0.05). Taken together, these studies indicate that PKC-zeta inhibits colon cancer cell growth and enhances differentiation and apoptosis, while inhibiting the transformed phenotype of these cells. The observed down-regulation of this growth-suppressing PKC isoform in colonic carcinogenesis would be predicted to contribute to tumorigenesis.

Requirement of androgen-dependent activation of protein kinase Czeta for androgen-dependent cell proliferation in LNCaP Cells and its roles in transition to androgen-independent cells

A cell line that we designed, AILNCaP, proliferated in androgen-depleted medium after emerging from long-term androgen-depleted cultures of an androgen-sensitive prostate cancer cell line, LNCaP. Using this cell line as a model of progression to androgen independence, we demonstrated that the activity of the mammalian target of rapamycin/p70 S6 kinase transduction pathway is down-regulated after androgen depletion in LNCaP, whereas its activation is related to transition of this cell line to androgen-independent proliferation. Kinase activity of protein kinase Czeta is regulated by androgen stimulation in LNCaP cells, whereas it is activated constitutively in AILNCaP cells under androgen-depleted conditions. Treatment with a protein kinase Czeta pseudosubstrate inhibitor reduced p70 S6 kinase activity and cell proliferation in both cell lines. We identified that both protein kinase Czeta and p70 S6 kinase were associated in LNCaP cells and this association was enhanced by the androgen stimulation. We examined the expression of phospho-protein kinase Czeta and phospho-p70 S6 kinase in hormone-naive prostate cancer specimens and found that the expression of both kinases was correlated with each other in those specimens. Significant correlation was observed between the expression of both kinases and Ki67 expression. Most of the prostate cancer cells that survived after prior hormonal treatment also expressed both kinases. This is the first report that shows the significance of this pathway for both androgen-dependent and -independent cell proliferation in prostate cancer. Our data suggest that protein kinase Czeta/mammalian target of rapamycin/S6 kinase pathway plays an important role for the transition of androgen-dependent to androgen-independent prostate cancer cells.

Ganglioside GM3 promotes carcinoma cell proliferation via urokinase plasminogen activator-induced extracellular signal-regulated kinase-independent p70S6 kinase signaling

Overexpression of NeuAcalpha2-3Galbeta1-4Glcbeta1-Cer (GM3), a major ganglioside of cutaneous tumor cell membranes, inhibits ligand-dependent and ligand-independent activation of the epidermal growth factor (EGF) receptor in normal and neoplastic epithelial cells. This leads to the suppression of Ras/extracellular signal-regulated kinase (ERK) activation and, in the presence of EGF or fibronectin, inhibits cell proliferation. However, some tumor cells show increased levels of GM3, and vaccines that target GM3 can inhibit the growth of neoplastic cells in vivo, especially melanomas. We report that in the presence of urokinase plasminogen activator (uPA), overexpression of GM3 paradoxically increases the proliferation of carcinoma cells by augmenting ERK-independent p70S6 kinase activation. Functional blockade of uPA receptor (uPAR) or inhibition of p70S6 kinase, but not inhibition of Ras/ERK signaling, suppresses this GM3-induced stimulation of cell proliferation. The ERK-independent activation of p70S6 kinase involves phosphorylation at threonine-389, threonine-421/serine-424, and serine-411 sites with intermediate phosphatidylinositol 3 kinase and protein kinase C-zeta activation. These studies implicate gangliosides as enhancers of uPAR-related signaling and suggest that the response to GM3 depends on the local concentration of uPA. Therapeutic modalities that target or supplement gangliosides may require concomitant treatment that suppresses EGFR or uPAR signaling, respectively, to control neoplastic cell proliferation.

AKT-independent phosphorylation of TSC2 and activation of mTOR and ribosomal protein S6 kinase signaling by prostaglandin F2alpha

Prostaglandin F2alpha (PGF2alpha) is an important mediator of corpus luteum (CL) regression, although the cellular signaling events that mediate this process have not been clearly identified. It is established that PGF2alpha binds to a G-proteincoupled receptor (GPCR) to stimulate protein kinase C (PKC) and Raf-MEK-Erk signaling in luteal cells. The present experiments were performed to determine whether PGF2alpha stimulates the mammalian target of rapamycin (mTOR)/ribosomal protein S6 kinase 1 (S6K1) signaling pathway in steroidogenic luteal cells. We demonstrate that PGF2alpha treatment results in a timeand concentration-dependent stimulation of the phosphorylation and activation of S6K1. The stimulation of S6K1 in response to PGF2alpha treatment was abolished by the mTOR inhibitor rapamycin. Treatment with PGF2alpha did not increase AKT phosphorylation but increased the phosphorylation of Erk and the tumor suppressor protein tuberous sclerosis complex 2 (TSC2), an upstream regulator of mTOR. The effects of PGF2alpha were mimicked by the PKC activator PMA and inhibited by U0126, a MEK1 inhibitor. The activation of mTOR/S6K1 and putative down stream processes involving the translational apparatus (i.e. 4EBP1 phosphorylation, release of 4EBP1 binding in m(7)G cap binding assays, and the phosphorylation and synthesis of S6) were completely sensitive to treatment with rapamycin, implicating mTOR in the actions of PGF2alpha. Taken together, our data suggest that GPCR activation in response to PGF2alpha stimulates the mTOR pathway which increases the translational machinery in luteal cells. The translation of proteins under the control of mTOR may have implications for luteal development and regression and offer new strategies for therapeutic intervention in PGF2alpha-target tissues.

Muscarinic receptor signaling in the pathophysiology of asthma and COPD

Anticholinergics are widely used for the treatment of COPD, and to a lesser extent for asthma. Primarily used as bronchodilators, they reverse the action of vagally derived acetylcholine on airway smooth muscle contraction. Recent novel studies suggest that the effects of anticholinergics likely extend far beyond inducing bronchodilation, as the novel anticholinergic drug tiotropium bromide can effectively inhibit accelerated decline of lung function in COPD patients. Vagal tone is increased in airway inflammation associated with asthma and COPD; this results from exaggerated acetylcholine release and enhanced expression of downstream signaling components in airway smooth muscle. Vagally derived acetylcholine also regulates mucus production in the airways. A number of recent research papers also indicate that acetylcholine, acting through muscarinic receptors, may in part regulate pathological changes associated with airway remodeling. Muscarinic receptor signalling regulates airway smooth muscle thickening and differentiation, both in vitro and in vivo. Furthermore, acetylcholine and its synthesizing enzyme, choline acetyl transferase (ChAT), are ubiquitously expressed throughout the airways. Most notably epithelial cells and inflammatory cells generate acetylcholine, and express functional muscarinic receptors. Interestingly, recent work indicates the expression and function of muscarinic receptors on neutrophils is increased in COPD. Considering the potential broad role for endogenous acetylcholine in airway biology, this review summarizes established and novel aspects of muscarinic receptor signaling in relation to the pathophysiology and treatment of asthma and COPD.

Expression of the SNT-1/FRS2 phosphotyrosine binding domain inhibits activation of MAP kinase and PI3-kinase pathways and antiestrogen resistant growth induced by FGF-1 in human breast carcinoma cells

Fibroblast growth factor (FGF) signaling can bypass the requirement for estrogen receptor (ER) activation in the growth of ER-positive (ER+) breast cancer cells. Fibroblast growth factor-1 stimulation leads to phosphorylation of the adaptor protein Suc1-associated neurotrophic factor-induced tyrosine-phosphorylated target (SNT-1) on C-terminal tyrosine residues, whereas it is constitutively bound through its N-terminal phosphotyrosine-binding domain (PTB) to FGF receptors (FGFRs). By expressing the PTB domain of SNT-1 (SNT-1 PTB) in an inducible manner in an ER+ breast carcinoma line, ML20, we asked whether we could uncouple FGFR activation from its downstream signaling components and abrogate FGF-1-induced antiestrogen-resistant growth. Induction of SNT-1 PTB resulted in a significant decrease of FGF-1-dependent tyrosine phosphorylation of endogenous SNT-1, strong inhibition of complex formation between SNT-1, Gab-1 and Sos-1, and reduced activation of Ras, mitogen-activated protein kinase (MAP kinase), and Akt. SNT-1 PTB also inhibited the phosphorylation of p70S6K on Thr421/Ser424 and Ser411, which may result from the abrogation of MAP kinase activity. Moreover, we also observed a decreased phosphorylation of the MAP kinase-independent site Thr389. This may reflect both inhibition of PI-3 kinase pathways and mammalian target of rapamycin (mTOR)-dependent signaling, as the phosphorylation of Thr389 site was sensitive to treatment with the PI3-K and mTOR inhibitors, LY294002 and rapamycin, respectively. Collectively these results suggest that SNT-1 plays a pivotal role in FGF-dependent activation of the Ras-MAP kinase, PI-3 kinase, and mTOR pathways in these cells. Fibroblast growth factor-1 dependent colony formation of ML20 cells in media containing the pure antiestrogen ICI 182,780 was also markedly inhibited upon induction of SNT-1 PTB, suggesting that blockade of FGFR-SNT-1 interactions might abrogate FGF-mediated antiestrogen resistance in breast cancers.

Bradykinin augments EGF-induced airway smooth muscle proliferation by activation of conventional protein kinase C isoenzymes

This study aims to investigate the effects of bradykinin, alone and in combination with growth factors on proliferation of cultured bovine tracheal smooth muscle cells. Bradykinin did not induce mitogenic responses by itself, but concentration-dependently augmented growth factor-induced [3H]thymidine incorporation and cell proliferation. The bradykinin effect was mediated by bradykinin B2 receptors, and not dependent on cyclo-oxygenase. Bradykinin-induced synergism with epidermal growth factor (EGF) could be suppressed by the protein kinase C (PKC) inhibitors GF 109203X (Bisindolylmaleimide I; specific for conventional and novel PKCs) and Gö 6976 (12-(2-Cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole; specific for conventional PKCs). In addition, sole activation of PKC using Phorbol 12-myristate 13-acetate (PMA) was sufficient for a synergistic interaction with EGF. In contrast to bradykinin however, PMA was mitogenic by itself which was not at all affected by Gö 6976, but abolished by GF 109203X. Bradykinin transiently activated the p42/p44 MAP kinase pathway, whereas PMA-induced activation of p42/p44 mitogen activated protein (MAP) kinase was sustained. Neither the combination of bradykinin and EGF nor that of PMA and EGF induced synergistic activation of p42/p44 MAP kinase, however. These results show that bradykinin B2 receptor-stimulation augments growth factor-induced mitogenic responses of airway smooth muscle cells through activation of conventional PKC isozymes. In addition, the results show that PKC isozyme-specificity underlies stimulus-specific differences in mitogenic capacity for bradykinin and PMA.

mTOR and S6K1 mediate assembly of the translation preinitiation complex through dynamic protein interchange and ordered phosphorylation events

In response to nutrients, energy sufficiency, hormones, and mitogenic agents, S6K1 phosphorylates several targets linked to translation. However, the molecular mechanisms whereby S6K1 is activated, encounters substrate, and contributes to translation initiation are poorly understood. We show that mTOR and S6K1 maneuver on and off the eukaryotic initiation factor 3 (eIF3) translation initiation complex in a signal-dependent, choreographed fashion. When inactive, S6K1 associates with the eIF3 complex, while the S6K1 activator mTOR/raptor does not. Cell stimulation promotes mTOR/raptor binding to the eIF3 complex and phosphorylation of S6K1 at its hydrophobic motif. Phosphorylation results in S6K1 dissociation, activation, and subsequent phosphorylation of its translational targets, including eIF4B, which is then recruited into the complex in a phosphorylation-dependent manner. Thus, the eIF3 preinitiation complex acts as a scaffold to coordinate a dynamic sequence of events in response to stimuli that promote efficient protein synthesis.

A model of the roles of essential kinases in the induction and expression of late long-term potentiation

The induction of late long-term potentiation (L-LTP) involves complex interactions among second-messenger cascades. To gain insights into these interactions, a mathematical model was developed for L-LTP induction in the CA1 region of the hippocampus. The differential equation-based model represents actions of protein kinase A (PKA), MAP kinase (MAPK), and CaM kinase II (CAMKII) in the vicinity of the synapse, and activation of transcription by CaM kinase IV (CAMKIV) and MAPK. L-LTP is represented by increases in a synaptic weight. Simulations suggest that steep, supralinear stimulus-response relationships between stimuli (e.g., elevations in [Ca(2+)]) and kinase activation are essential for translating brief stimuli into long-lasting gene activation and synaptic weight increases. Convergence of multiple kinase activities to induce L-LTP helps to generate a threshold whereby the amount of L-LTP varies steeply with the number of brief (tetanic) electrical stimuli. The model simulates tetanic, -burst, pairing-induced, and chemical L-LTP, as well as L-LTP due to synaptic tagging. The model also simulates inhibition of L-LTP by inhibition of MAPK, CAMKII, PKA, or CAMKIV. The model predicts results of experiments to delineate mechanisms underlying L-LTP induction and expression. For example, the cAMP antagonist RpcAMPs, which inhibits L-LTP induction, is predicted to inhibit ERK activation. The model also appears useful to clarify similarities and differences between hippocampal L-LTP and long-term synaptic strengthening in other systems.

Thrombopoietin (TPO) induces c-myc expression through a PI3K- and MAPK-dependent pathway that is not mediated by Akt, PKCzeta or mTOR in TPO-dependent cell lines and primary megakaryocytes

Thrombopoietin (TPO) and its receptor (c-Mpl) are the major regulators of megakaryocyte and platelet production and serve a critical and non-redundant role in hematopoietic stem cell (HSC) biology. TPO signals through the Jak-STAT, Ras-Raf-MAPK, and PI3K pathways, and promotes survival, proliferation, and polyploidization in megakaryocytes. The proto-oncogene c-myc also plays an important role in many of these same processes. In this work we studied the regulated expression of c-myc in megakaryocytic cell lines and primary cells by quantitative real-time RT-PCR. We found that TPO induced expression of c-myc in 1 h in both hematopoietic cell lines (UT-7 and BaF3/Mpl) and mature murine megakaryocytes. The TPO-induced expression of c-myc was blocked by a phosphatidylinositol 3-kinase (PI3K) inhibitor, suggesting that TPO stimulated c-myc expression through a PI3K-dependent pathway. Of interest, our study showed that overexpression of active Akt did not rescue the effect of PI3K blockade on c-myc expression, rather, enhanced it. In addition, inhibitors of protein kinase C (PKC)zeta and the target of rapamycin (mTOR) also failed to affect c-myc mRNA expression, while c-myc mRNA expression was reduced by inhibition of the mitogen activated protein kinase (MAPK) pathway. Therefore, we conclude that TPO stimulates c-myc expression in primary megakaryocytes through a PI3K- and MAPK-dependent pathway that is not mediated by Akt, PKCzeta or mTOR.

Insulin signalling downstream of protein kinase B is potentiated by 5'AMP-activated protein kinase in rat hearts in vivo

AIMS/HYPOTHESIS

5'AMP-activated protein kinase (AMPK) and insulin stimulate glucose transport in heart and muscle. AMPK acts in an additive manner with insulin to increase glucose uptake, thereby suggesting that AMPK activation may be a useful strategy for ameliorating glucose uptake, especially in cases of insulin resistance. In order to characterise interactions between the insulin- and AMPK-signalling pathways, we investigated the effects of AMPK activation on insulin signalling in the rat heart in vivo.

METHODS

Male rats (350-400 g) were injected with 1 g/kg 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) or 250 mg/kg metformin in order to activate AMPK. Rats were administered insulin 30 min later and after another 30 min their hearts were removed. The activities and phosphorylation levels of components of the insulin-signalling pathway were subsequently analysed in individual rat hearts.

RESULTS

AICAR and metformin administration activated AMPK and enhanced insulin signalling downstream of protein kinase B in rat hearts in vivo. Insulin-induced phosphorylation of glycogen synthase kinase 3 (GSK3) beta, p70 S6 kinase (p70S6K)(Thr389) and IRS1(Ser636/639) were significantly increased following AMPK activation. To the best of our knowledge, this is the first report of heightened insulin responses of GSK3beta and p70S6K following AMPK activation. In addition, we found that AMPK inhibits insulin stimulation of IRS1-associated phosphatidylinositol 3-kinase activity, and that AMPK activates atypical protein kinase C and extracellular signal-regulated kinase in the heart.

CONCLUSIONS/INTERPRETATIONS

Our data are indicative of differential effects of AMPK on the activation of components in the cardiac insulin-signalling pathway. These intriguing observations are critical for characterisation of the crosstalk between AMPK and insulin signalling.

Formononetin, a phyto-oestrogen, and its metabolites up-regulate interleukin-4 production in activated T cells via increased AP-1 DNA binding activity

Phyto-oestrogens are polyphenolic non-steroidal plant compounds with oestrogen-like biological activity. Phyto-oestrogens have many biological effects including oestrogen agonist/antagonist properties. However, the effect of phyto-oestrogens on allergic responses remains unclear. In this study we investigated whether formononetin, a phyto-oestrogen, and its metabolites, daidzein and equol, affect production of interleukin-4 (IL-4), a pro-inflammatory cytokine closely associated with allergic immune response, in primary CD4+ T cells and EL4 T lymphoma cells. Formononetin, daidzein and equol significantly enhanced IL-4 production from both CD4+ T cells and EL4 cells in a dose-dependent manner. Formononetin, daidzein and equol also enhanced IL-4 gene promoter activity in EL4 cells transiently transfected with IL-4 gene promoter constructs, but this effect was impaired in EL4 cells transfected with an IL-4 promoter construct deleted of P4 site carrying nuclear factor of activated T cells (NF-AT) and activator protein-1 (AP-1) binding sites. In addition, formononetin, daidzein and equol increased AP-1 DNA binding activities while did not affect NF-AT DNA binding activities. The enhancing effects on IL-4 production and AP-1 DNA binding activities were abrogated by specific inhibitors for phosphatidylinositol-3-kinase (PI3K), protein kinase C (PKC) and p38 mitogen-activated protein kinase (MAPK), indicating that formononetin, daidzein and equol might enhance IL-4 production by increased activation of AP-1 through the PI3-K/PKC/p38 MAPK signalling pathway. These results suggest that phyto-oestrogens and some of their metabolites may increase allergic responses via the enhancement of IL-4 production in T cells.

Novel functions of the phospholipase D2-Phox homology domain in protein kinase Czeta activation

It has been established that protein kinase Czeta (PKCzeta) participates in diverse signaling pathways and cellular functions in a wide variety of cells, exhibiting properties relevant to cellular survival and proliferation. Currently, however, the regulation mechanism of PKCzeta remains elusive. Here, for the first time, we determine that phospholipase D2 (PLD2) enhances PKCzeta activity through direct interaction in a lipase activity-independent manner. This interaction of the PLD2-Phox homology (PX) domain with the PKCzeta-kinase domain also induces the activation loop phosphorylation of PKCzeta and downstream signal stimulation, as measured by p70 S6 kinase phosphorylation. Furthermore, only the PLD2-PX domain directly stimulates PKCzeta activity in vitro, and it is necessary for the formation of the ternary complex with phosphoinositide-dependent kinase 1 and PKCzeta. The mutant that substitutes the triple lysine residues (Lys101, Lys102, and Lys103) within the PLD2-PX domain with alanine abolishes interaction with the PKCzeta-kinase domain and activation of PKCzeta. Moreover, breast cancer cell viability is significantly affected by PLD2 silencing. Taken together, these results suggest that the PLD2-mediated PKCzeta activation is induced by its PX domain performing both direct activation of PKCzeta and assistance of activation loop phosphorylation. Furthermore, we find it is an important factor in the survival of breast cancer cells.

Identification of S6 kinase 1 as a novel mammalian target of rapamycin (mTOR)-phosphorylating kinase

Here we demonstrate that mammalian target of rapamycin (mTOR) is phosphorylated in a rapamycin-sensitive manner. We show that S6 kinase 1 (S6K1), but not Akt, directly phosphorylates mTOR in cell-free in vitro system and in cells. Expression of a constitutively active, rapamycin- and wortmannin-resistant S6K1 leads to constitutive phosphorylation of mTOR, whereas knock-down of S6K1 using small inhibitory RNA greatly reduces mTOR phosphorylation despite elevated Akt activity. Importantly, phosphorylation of mTOR by S6K1 occurs at threonine 2446/serine 2448. This region has been shown previously to be part of a regulatory repressor domain. These sites are also constitutively phosphorylated in the breast cancer cell line MCF7 carrying an amplification of the S6K1 gene, but not in a less tumorigenic cell line, MCF10a. Many models for Akt signaling to mTOR have been presented, suggesting direct phosphorylation by Akt. These models must be reconsidered in light of the present findings.

Differential regulation of the p70 S6 kinase pathway by interferon alpha (IFNalpha) and imatinib mesylate (STI571) in chronic myelogenous leukemia cells

The precise mechanisms by which imatinib mesylate (STI571) and interferon alpha (IFNalpha) exhibit antileukemic effects are not known. We examined the effects of IFNs or imatinib mesylate on signaling pathways regulating initiation of mRNA translation in BCR-ABL-expressing cells. Treatment of IFN-sensitive KT-1 cells with IFNalpha resulted in phosphorylation/activation of mammalian target of rapamycin (mTOR) and downstream activation of p70 S6 kinase. The IFN-activated p70 S6 kinase was found to regulate phosphorylation of S6 ribosomal protein, which regulates translation of mRNAs with oligopyrimidine tracts in the 5'-untranslated region. In addition, IFNalpha treatment resulted in an mTOR- and/or phosphatidyl-inositol 3'(PI 3') kinase-dependent phosphorylation of 4E-BP1 repressor of mRNA translation on sites that are required for its deactivation and dissociation from the eukaryotic initiation factor-4E (eIF4E) complex. In contrast to the effects of IFNs, imatinib mesylate suppressed p70 S6 kinase activity, consistent with inhibition of BCR-ABL-mediated activation of the mTOR/p70 S6 kinase pathway. Moreover, the mTOR inhibitor rapamycin enhanced the suppressive effects of imatinib mesylate on primary leukemic granulocyte macrophage-colony-forming unit (CFU-GM) progenitors from patients with chronic myelogenous leukemia (CML). Taken altogether, our data demonstrate that IFNs and imatinib mesylate differentially regulate PI 3' kinase/mTOR-dependent signaling cascades in BCR-ABL-transformed cells, consistent with distinct effects of these agents on pathways regulating mRNA translation. They also support the concept that combined use of imatinib mesylate with mTOR inhibitors may be an appropriate future therapeutic strategy for the treatment of CML.

Lithium Can Relieve Translational Repression of TOP mRNAs Elicited by Various Blocks along the Cell Cycle in a Glycogen Synthase Kinase-3- and S6-Kinase-independent Manner

TOP mRNAs are translationally controlled by mitogenic, growth, and nutritional stimuli through a 5'-terminal oligopyrimidine tract. Here we show that LiCl can alleviate the translational repression of these mRNAs when progression through the cell cycle is blocked at G(0), G(1)/S, or G(2)/M phases in different cell lines and by various physiological and chemical means. This derepressive effect of LiCl does not involve resumption of cell division. Unlike its efficient derepressive effect in mitotically arrested cells, LiCl alleviates inefficiently the repression of TOP mRNAs in amino acid-deprived cells and has no effect in lymphoblastoids whose TOP mRNAs are constitutively repressed even when they are proliferating. LiCl is widely used as a relatively selective inhibitor of glycogen synthase kinase-3. However, inhibition per se of this enzyme by more specific drugs failed to derepress the translation of TOP mRNAs, implying that relief of the translational repression of TOP mRNAs by LiCl is carried out in a glycogen synthase kinase-3-independent manner. Moreover, this effect is apparent, at least in some cell lines, in the absence of S6-kinase 1 activation and ribosomal protein S6 phosphorylation, thus further supporting the notion that translational control of TOP mRNAs does not rely on either of these variables.

SKAR is a specific target of S6 kinase 1 in cell growth control

BACKGROUND

The mammalian target of rapamycin (mTOR) and phosphatidylinositol 3-kinase (PI3K) signaling pathways promote cell growth and cell cycle progression in response to nutritional, energy, and mitogenic cues. In mammalian cells, the ribosomal protein S6 kinases, S6K1 and S6K2, lie downstream of mTOR and PI3K, suggesting that translational control through the phosphorylation of S6 regulates cell growth. Interestingly, genetic experiments predict that a substrate that is specific to S6K1 but not S6K2 regulates cell growth.

RESULTS

Here we identify SKAR as a novel and specific binding partner and substrate of S6K1 but not S6K2. We find that serines 383 and 385 of human SKAR are insulin-stimulated and rapamycin-sensitive S6K1 phosphorylation sites. Quantitative mass spectrometry reveals that serine 383/385 phosphorylation is sensitive to RNA interference (RNAi)-mediated S6K1 reduction, but not S6K2 reduction. Furthermore, RNAi-mediated reduction of SKAR decreases cell size. SKAR is nuclear protein with homology to the Aly/REF family of RNA binding proteins, which has been proposed to couple transcription with pre-mRNA splicing and mRNA export.

CONCLUSIONS

We have identified a novel and specific target of S6K1, SKAR, which regulates cell growth. The homology of SKAR to the Aly/REF family links S6K1 with mRNA biogenesis in the control of cell growth.

Postsynaptic signaling networks: cellular cogwheels underlying long-term plasticity

Learning depends on positive or negative changes in synaptic transmission that are synapse-specific and sustained. Synaptic signals can be directly measured and respond to certain kinds of stimulation by becoming persistently enhanced (long-term potentiation, LTP) or decreased (long-term depression, LTD). Studying LTP and LTD opens a window on to the molecular mechanisms of memory. Although changes in both pre- and postsynaptic strength have been implicated in LTP and LTD, most attention has been focused on changes in postsynaptic glutamate receptor density. This is controlled by intracellular Ca(2+) ions via a network of signaling molecules. Changes in postsynaptic Ca(2+) concentration depend on the coincidence of appropriate synaptic signals, as is found in learning situations. The long-term persistence of LTP and LTD requires gene transcription and translation. It is posited that local translation at the synapse, in a self-sustaining manner, mediates the persistence of long-term changes despite constant turnover of the synaptic components.

Extracellular ATP-induced proliferation of adventitial fibroblasts requires phosphoinositide 3-kinase, Akt, mammalian target of rapamycin, and p70 S6 kinase signaling pathways

Extracellular nucleotides are increasingly recognized as important regulators of growth in a variety of cell types. Recent studies have demonstrated that extracellular ATP is a potent inducer of fibroblast growth acting, at least in part, through an ERK1/2-dependent signaling pathway. However, the contributions of additional signaling pathways to extracellular ATP-mediated cell proliferation have not been defined. By using both pharmacologic and genetic approaches, we found that in addition to ERK1/2, phosphatidylinositol 3-kinase (PI3K), Akt, mammalian target of rapamycin (mTOR), and p70 S6K-dependent signaling pathways are required for ATP-induced proliferation of adventitial fibroblasts. We found that extracellular ATP acting in part through G(i) proteins increased PI3K activity in a time-dependent manner and transient phosphorylation of Akt. This PI3K pathway is not involved in ATP-induced activation of ERK1/2, implying activation of independent parallel signaling pathways by ATP. Extracellular ATP induced dramatic increases in mTOR and p70 S6K phosphorylation. This activation of the mTOR/p70 S6 kinase (p70 S6K) pathway in response to ATP is because of independent contributions of PI3K/Akt and ERK1/2 pathways, which converge on the level of p70 S6K. ATP-dependent activation of mTOR and p70 S6K also requires additional signaling inputs perhaps from pathways operating through Galpha or Gbetagamma subunits. Collectively, our data demonstrate that ATP-induced adventitial fibroblast proliferation requires activation and interaction of multiple signaling pathways such as PI3K, Akt, mTOR, p70 S6K, and ERK1/2 and provide evidence for purinergic regulation of the protein translational pathways related to cell proliferation.

Continuous ethanol exposure inhibits agonist-stimulated phosphorylation of p70S6 kinase and ribosomal S6 protein in cultured rat astrocytes

The effects of ethanol exposure on agonist-stimulated phosphorylation of p70S6 kinase and ribosomal S6 protein were determined in confluent astrocyte monolayers. Basal phosphorylation of p70S6 kinase and S6 protein was either unaffected or reduced, respectively, after exposure to 50 mM ethanol for 4 days. The abilities of norepinephrine, carbachol and epidermal growth factor to phosphorylate these proteins were significantly decreased after ethanol exposure. In contrast, ethanol exposure had no effect on the protein expression of either p70S6 kinase or S6 protein. Our data suggest that continuous ethanol exposure results in a generalized decrease in agonist-activation of the p70S6 pathway.

PI3-kinase and TOR: PIKTORing cell growth

Regulation of growth and proliferation in higher eukaryotic cells results from an integration of nutritional, energy, and mitogenic signals. Biochemical processes underlying cell growth and proliferation are governed by the phosphatidylinositol 3-kinase (PI3K) and target of rapamycin (TOR) signaling pathways. The importance of the interplay between these two pathways is underscored by the discovery that the TOR inhibitor rapamycin is effective against tumors caused by misregulation of the PI3K pathway. We review here recent data concerning the convergence of the PI3K and TOR pathways, the role of these pathways in cell growth and proliferation, and the regulation of growth by downstream TOR targets.

Role of atypical protein kinase C in estradiol-triggered G1/S progression of MCF-7 cells

Expression of a dominant negative atypical protein kinase C (aPKC), PKCzeta, prevents nuclear translocation of extracellular regulated kinase 2 (ERK-2), p27 nuclear reduction, and DNA synthesis induced by estradiol in human mammary cancer-derived MCF-7 cells. aPKC action upstream of these events has been analyzed. In hormone-stimulated NIH 3T3 and Cos cells ectopically expressing human estrogen receptor alpha (hERalpha), aPKC is activated by phosphatidylinositol 3-kinase (PI 3-kinase) and, in turn, controls the Ras/MEK-1/ERK cascade. In MCF-7 and Cos cells stimulated by hormone, PI 3-kinase activates PKCzeta by Thr410 phosphorylation. Serine phosphorylation of PKCzeta is simultaneously induced. PKCzeta activation leads to recruitment of Ras to a multimolecular complex that also includes hERalpha, Src, PI 3-kinase, and aPKC. We propose that PKCzeta pushes Ras and the signaling complex close together in such a way that it facilitates the Src-dependent Ras activation. This activation is crucial for the interplay between estradiol-triggered signaling and cell cycle machinery.

Apelin (65‐77) activates p70 S6 kinase and is mitogenic for umbilical endothelial cells

We report here that apelin (65-77) activates p70 S6 kinase (p70S6K), not only in CHO cells that have been stably transfected with the apelin receptor, but also in umbilical endothelial cells (HUVEC), which express it endogenously. Apelin (65-77) induces a time-dependent phosphorylation of p70S6K at residues T421/S424 and T389. This dual phosphorylation is associated with two transduction cascades, involving a PI3K pathway and an ERK pathway, respectively. The PI3K pathway, which can be blocked by wortmannin, leads to phosphorylation of Akt at residues T308 or S473, which then promotes the phosphorylation of p70S6K at T421/S424 and T389. The ERK pathway is blocked by PD 098059, a MEK inhibitor, and results in the phosphorylation of p70S6K at T421/S424. Phosphorylation both of Akt and p70S6K is abrogated by pretreatment with pertussis toxin (PTX) and an inhibitor of atypical PKCs. In addition, we demonstrate that apelin (65-77) also increases the enzymatic activity of p70S6K and that the effects of the previously mentioned inhibitors on the level of T389 phosphorylation correlate with their action on enzyme activity. Interestingly, the main findings were reproduced in umbilical endothelial cells and apelin (65-77) promoted thymidine incorporation into DNA of these cells, revealing that apelin is a new mitogenic peptide for the endothelial cell.

Activation of aPKC is required for vanadate-induced phosphorylation of protein kinase B (Akt), but not p70S6k in mouse epidermal JB6 cells

Vanadium is a metal widely distributed in the environment. Although vanadate-containing compounds exert potent toxic effects on a wide variety of biological systems, the mechanisms by which vanadate mediates adverse effects are not well understood. The present study investigated the vanadate-induced phosphorylation of Akt and p70S6K, two kinases known to be vital for cell survival, growth, transformation, and transition of the cell cycle in mammals. Exposure of mouse epidermal JB6 cells to vanadium led to phosphorylation of Akt and p70S6K in a time- and dose-dependent manner. Vanadium exposure also caused translocation of atypical isoforms of PKC (lambda, zeta) from the cytosol to the membrane, but had no effect on PKCalpha translocation, suggesting that the atypical PKCs (aPKC) were specifically involved in vanadium-induced cellular response. Importantly, overexpression of a dominant negative mutant PKClambda blocked Akt phosphorylation at Ser473 and Thr308, whereas it did not inhibit p70S6k phosphorylation at Thr389 and Thr421/Ser424, suggesting that aPKC activation is specifically involved in vanadium-induced activation of Akt, but not in activation of p70S6k. Furthermore, vanadium-induced p70S6k phosphorylation at Thr389 and Thr421/Ser424 and Akt phosphorylation at Thr308 occurred through a PI-3K-dependent pathway because a PI-3K dominant negative mutant inhibited induction as compared with vector control cells. These results indicate that there was a differential role of aPKC in vanadate-induced phosphorylation of Akt and p70S6k, suggesting that signal transduction pathways leading to the activation of Akt and p70S6k were different.

Tumor-promoting phorbol esters and activated Ras inactivate the tuberous sclerosis tumor suppressor complex via p90 ribosomal S6 kinase

Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in either of the two tumor suppressor genes TSC1 or TSC2, which encode hamartin and tuberin, respectively. Tuberin and hamartin form a complex that inhibits signaling by the mammalian target of rapamycin (mTOR), a critical nutrient sensor and regulator of cell growth and proliferation. Phosphatidylinositol 3-kinase (PI3K) inactivates the tumor suppressor complex and enhances mTOR signaling by means of phosphorylation of tuberin by Akt. Importantly, cellular transformation mediated by phorbol esters and Ras isoforms that poorly activate PI3K promote tumorigenesis in the absence of Akt activation. In this study, we show that phorbol esters and activated Ras also induce the phosphorylation of tuberin and collaborates with the nutrient-sensing pathway to regulate mTOR effectors, such as p70 ribosomal S6 kinase 1 (S6K1). The mitogen-activated protein kinase (MAPK)-activated kinase, p90 ribosomal S6 kinase (RSK) 1, was found to interact with and phosphorylate tuberin at a regulatory site, Ser-1798, located at the evolutionarily conserved C terminus of tuberin. RSK1 phosphorylation of Ser-1798 inhibits the tumor suppressor function of the tuberin/hamartin complex, resulting in increased mTOR signaling to S6K1. Together, our data unveil a regulatory mechanism by which the Ras/MAPK and PI3K pathways converge on the tumor suppressor tuberin to inhibit its function.

Target of rapamycin (TOR): an integrator of nutrient and growth factor signals and coordinator of cell growth and cell cycle progression

Cell growth (an increase in cell mass and size through macromolecular biosynthesis) and cell cycle progression are generally tightly coupled, allowing cells to proliferate continuously while maintaining their size. The target of rapamycin (TOR) is an evolutionarily conserved kinase that integrates signals from nutrients (amino acids and energy) and growth factors (in higher eukaryotes) to regulate cell growth and cell cycle progression coordinately. In mammals, TOR is best known to regulate translation through the ribosomal protein S6 kinases (S6Ks) and the eukaryotic translation initiation factor 4E-binding proteins. Consistent with the contribution of translation to growth, TOR regulates cell, organ, and organismal size. The identification of the tumor suppressor proteins tuberous sclerosis1 and 2 (TSC1 and 2) and Ras-homolog enriched in brain (Rheb) has biochemically linked the TOR and phosphatidylinositol 3-kinase (PI3K) pathways, providing a mechanism for the crosstalk that occurs between these pathways. TOR is emerging as a novel antitumor target, since the TOR inhibitor rapamycin appears to be effective against tumors resulting from aberrantly high PI3K signaling. Not only may inhibition of TOR be effective in cancer treatment, but rapamycin is an FDA-approved immunosuppressive and cardiology drug. We review here what is known (and not known) about the function of TOR in cellular and animal physiology.

Molecular regulation of angiogenesis and tumorigenesis by signal transduction pathways: evidence of predictable and reproducible patterns of synergy in diverse neoplasms

A large number of oncogenes, tumor suppressor genes, and signal transduction pathways have been described. Currently, a framework that allows prediction of tumor behavior based upon oncogenes, tumor suppressors, and signal transduction pathways is lacking. In 1869, Mendeleev published a periodic table of elements which allowed prediction of properties of elements based upon atomic weights that allowed prediction of chemical and physical properties of elements yet to be discovered. In this paper, I will discuss recurrent patterns of synergy found in the literature and our laboratory between tumor suppressor genes, oncogenes, and signaling pathways that allows one to predict the signaling pathway in a given tumor based upon the inactivation of a tumor suppressor gene. These patterns can be found in multiple different human neoplasms. Conversely, one can predict the inactivation of a tumor suppressor based upon the activation status of a signaling pathway. This knowledge can be used by a clinician or pathologist with access to immunohistochemistry to make predictions based upon simple technologies and determine the signaling pathways involved in a patient's tumor. These strategies may be useful in the design of prevention and treatment strategies for cancer.

Critical role of T-loop and H-motif phosphorylation in the regulation of S6 kinase 1 by the tuberous sclerosis complex

The tuberous sclerosis gene products Tsc1 and Tsc2 behave as tumor suppressors by restricting cell growth, a function conserved among metazoans. Recent evidence has indicated that hyperactivation of S6 kinase 1 (S6K1) may represent an important biochemical change in the development of tuberous sclerosis-associated lesions. We show here that deletion of either Tsc1 or Tsc2 or expression of the Rheb (Ras homolog enriched in brain) GTPase leads to hyperphosphorylation of S6K1 at a subset of regulatory sites, particularly those of two essential residues functionally conserved among AGC superfamily serine/threonine kinases, i.e. the activation loop (T-loop; Thr-229) and the hydrophobic motif (H-motif; Thr-389). These sites are reciprocally and dose-dependently regulated when S6K1 is coexpressed with the Tsc1-Tsc2 complex. Mutations that render S6K1 mTOR (mammalian target of rapamycin)-resistant also protect S6K1 activity and phosphorylation from down-regulation by Tsc1/2. We demonstrate that two disease-associated mutations in Tsc2 fail to negatively regulate S6K1 activity concomitant with a failure to modify T-loop and H-motif phosphorylation. Finally, we identify one pathological Tsc2 mutation that retains its ability to negatively regulate S6K1, suggesting that, in some cases, tuberous sclerosis may develop independently of S6K1 hyperactivation. These results also highlight the importance of dual control of T-loop and H-motif phosphorylation of S6K1 by the Tsc1-Tsc2 complex.

Insulin Promotes Rat Retinal Neuronal Cell Survival in a p70S6K-dependent Manner

The purpose of this study was to examine the role of the ribosomal protein S6 protein kinase (p70S6K), a protein synthesis regulator, in promoting retinal neuronal cell survival. Differentiated R28 rat retinal neuronal cells were used as an experimental model. Cells were maintained in Dulbecco's modified Eagle's medium supplemented with 10% newborn calf serum, and during the period of experimentation were exposed either to the absence or presence of 10 nm insulin. Insulin treatment induced p70S6K, mTOR, and Akt phosphorylation, effects that were completely prevented by the PI3K inhibitor, LY294002. Insulin-induced phosphorylation of p70S6K and mTOR was prevented by the mTOR inhibitor, rapamycin. Apoptosis, induced by serum deprivation and evaluated by Hoechst staining, was inhibited by insulin treatment in R28 cells, but not in L6 muscle cells. This effect of insulin was also largely prevented by rapamycin. Inhibition of p70S6K activity by exogenous expression of a dominant negative mutant of p70S6K prevented insulin-induced cell survival, whereas, overexpression of wild type p70S6K or expression of a rapamycin resistant form of the kinase enhanced the effect of insulin on survival. Enhanced cell survival under the latter condition was accompanied by increased p70S6K activity and phosphorylation. Rapamycin did not inhibit insulin induced p70S6K phosphorylation and activity in cells transfected with the rapamycin-resistant mutant. Together, these results suggest that p70S6K plays a key role in insulin stimulated retinal neuronal cell survival.

Role of protein kinase C in arginine vasopressin-stimulated ERK and p70S6 kinase phosphorylation

We previously showed in rat renal glomerular mesangial cells, that arginine vasopressin (AVP)-stimulated cell proliferation was mediated by epidermal growth factor receptor (EGF-R) transactivation, and activation (phosphorylation) of ERK1/2 and p70S6 kinase (Ghosh et al. [2001]: Am J Physiol Renal Physiol 280:F972-F979]. In this paper, we extend these observations and show that different protein kinase C (PKC) isoforms play different roles in mediating AVP-stimulated ERK1/2 and p70S6 kinase phosphorylation and cell proliferation. AVP treatment for 0-60 min stimulated the serine/threonine phosphorylation of PKC isoforms alpha, delta, epsilon, and zeta. The activation of PKC was dependent on EGF-R and phosphatidylinositol 3-kinase (PI3K) activation. In addition, inhibition of conventional and novel PKC isoforms by chronic (24 h) exposure to phorbol 12-myristate 13-acetate (PMA) inhibited AVP-induced activation of ERK and p70S6 kinase as well as EGF-R phosphorylation. Rottlerin, a specific inhibitor of PKCdelta, inhibited both ERK and p70S6 kinase phosphorylation and cell proliferation. In contrast, a PKCepsilon translocation inhibitor decreased ERK1/2 activation without affecting p70S6 kinase or cell proliferation, while a dominant negative PKCzeta (K281W) cDNA delayed p70S6 kinase activation without affecting ERK1/2. On the other hand, Gö6976, an inhibitor of conventional PKC isoforms, did not affect p70S6 kinase, but stimulated ERK1/2 phosphorylation without affecting cell proliferation. Our results indicate that PKCdelta plays an important role in AVP-stimulated ERK and p70S6 kinase activation and cell proliferation.

Insulin-dependent signaling regulates azurophil granule-selective macroautophagy in human myeloblastic cells

We show that insulin-dependent signals regulate azurophil granule-selective macroautophagy in human myeloid cells. Depletion of insulin from an insulin-transferrin-supplemented serum-free medium caused growth retardation of myeloblastic HL-60 cells, in which sequestration of electronic-dense cytoplasmic materials by autophagosomes was observed. Positive immunoreactivity with anti-CD68, anti-cathepsin D, and anti-myeloperoxidase antibodies indicated that the sequestrated materials were azurophil granules, the granulocyte/macrophage lineage-specific lysosome-like particles. By contrast, other organelles, including the mitochondria, endoplasmic reticulum, and Golgi apparatus remained intact, indicating that the macroautophagy selectively targeted azurophil granules. The addition of insulin induced rapid activations of p70S6K and Akt, and the cells were rescued from macroautophagy. Rapamycin, an inhibitor of mammalian target of rapamycin, did not block the insulin-mediated rescue from macroautophagy, although it nullified the activation of p70S6K and cell growth. Low doses of LY294002, a phosphatidyl-inositol-3-kinase inhibitor, which abolished cell growth and p70S6K activity but did not influence Akt activity, did not block the insulin-mediated rescue either. By contrast, low doses of Akt-specific inhibitors, which inhibited neither cell growth nor p70S6K activity, completely blocked the insulin-mediated rescue from macroautophagy. Thus, insulin-dependent signals are responsible for the control of azurophil granule-selective macroautophagy via Akt-dependent pathways, while p70S6K-dependent pathways promote cell growth.

Activation of the p70 S6 kinase and phosphorylation of the 4E-BP1 repressor of mRNA translation by type I interferons

The Type I IFN receptor-generated signals required for initiation of mRNA translation and, ultimately, induction of protein products that mediate IFN responses, remain unknown. We have previously shown that IFNalpha and IFNbeta induce phosphorylation of insulin receptor substrate proteins and downstream engagement of the phosphatidylinositol (PI) 3'-kinase pathway. In the present study we provide evidence for the existence of a Type I IFN-dependent signaling cascade activated downstream of PI 3'-kinase, involving p70 S6 kinase. Our data demonstrate that p70 S6K is rapidly phosphorylated on threonine 421 and serine 424 and is activated during treatment of cells with IFNalpha or IFNbeta. Such activation of p70 S6K is blocked by pharmacological inhibitors of the PI 3'-kinase or the FKBP 12-rapamycin-associated protein/mammalian target of rapamycin (FRAP/mTOR). Consistent with this, the Type I IFN-dependent phosphorylation/activation of p70 S6K is defective in embryonic fibroblasts from mice with targeted disruption of the p85alpha and p85beta subunits of the PI 3'-kinase (p85alpha-/-beta-/-). Treatment of sensitive cell lines with IFNalpha or IFNbeta also results in phosphorylation/inactivation of the 4E-BP-1 repressor of mRNA translation. Such 4E-BP1 phosphorylation is also PI3'-kinase-dependent and rapamycin-sensitive, indicating that the Type I IFN-inducible activation of PI3'-kinase and FRAP/mTOR results in dissociation of 4E-BP1 from the eukaryotic initiation factor-4E (eIF4E) complex. Altogether, our data establish that the Type I IFN receptor-activated PI 3'-kinase pathway mediates activation of the p70 S6 kinase and inactivation of 4E-BP1, to regulate mRNA translation and induction of Type I IFN responses.

Ca(2+)-independent protein kinase C activity is required for alpha1-adrenergic-receptor-mediated regulation of ribosomal protein S6 kinases in adult cardiomyocytes

The alpha(1)-adrenergic agonist, phenylephrine (PE), exerts hypertrophic effects in the myocardium and activates protein synthesis. Both Ca(2+)-dependent protein kinase C (PKC, PKCalpha) and Ca(2+)-independent PKC isoforms (PKCdelta and epsilon ) are detectably expressed in adult rat cardiomyocytes. Stimulation of the alpha(1)-adrenergic receptor by PE results in activation of Ca(2+)-independent PKCs, as demonstrated by translocation of the delta and epsilon isoenzymes from cytosol to membrane fractions. PE also induces activation of p70 ribosomal protein S6 kinases (S6K1 and 2) in adult cardiomyocytes. We have studied the role of Ca(2+)-independent PKCs in the regulation of S6K activity by PE. Activation of S6K1/2 by PE was blocked by the broad-spectrum PKC inhibitor bisindolylmaleimide (BIM) I, whereas Gö6976, a compound that only inhibits Ca(2+)-dependent PKCs, did not inhibit S6K activation. Rottlerin, which selectively inhibits PKCdelta, also prevented PE-induced S6K activation. The isoform-specific PKC inhibitors had similar effects on the phosphorylation of eukaryotic initiation factor 4E (eIF4E)-binding protein 1, a translation repressor that, like the S6Ks, lies downstream of the mammalian target of rapamycin (mTOR). Infection of cells with adenoviruses encoding dominant-negative PKCdelta or epsilon inhibited the activation of extracellular-signal-regulated kinase (ERK) by PE, and also inhibited the activation and/or phosphorylation of S6Ks 1 and 2. The PE-induced activation of protein synthesis was abolished by BIM I and markedly attenuated by rottlerin. Our data thus suggest that Ca(2+)-independent PKC isoforms play an important role in coupling the alpha(1)-adrenergic receptor to mTOR signalling and protein synthesis in adult cardiomyocytes.

Signalling specificity of Ser/Thr protein kinases through docking-site-mediated interactions

Signal transduction pathways use protein kinases for the modification of protein function by phosphorylation. A major question in the field is how protein kinases achieve the specificity required to regulate multiple cellular functions. Here we review recent studies that illuminate the mechanisms used by three families of Ser/Thr protein kinases to achieve substrate specificity. These kinases rely on direct docking interactions with substrates, using sites distinct from the phospho-acceptor sequences. Docking interactions also contribute to the specificity and regulation of protein kinase activities. Mitogen-activated protein kinase (MAPK) family members can associate with and phosphorylate specific substrates by virtue of minor variations in their docking sequences. Interestingly, the same MAPK docking pocket that binds substrates also binds docking sequences of positive and negative MAPK regulators. In the case of glycogen synthase kinase 3 (GSK3), the presence of a phosphate-binding site allows docking of previously phosphorylated (primed) substrates; this docking site is also required for the mechanism of GSK3 inhibition by phosphorylation. In contrast, non-primed substrates interact with a different region of GSK3. Phosphoinositide-dependent protein kinase-1 (PDK1) contains a hydrophobic pocket that interacts with a hydrophobic motif present in all known substrates, enabling their efficient phosphorylation. Binding of the substrate hydrophobic motifs to the pocket in the kinase domain activates PDK1 and other members of the AGC family of protein kinases. Finally, the analysis of protein kinase structures indicates that the sites used for docking substrates can also bind N- and C-terminal extensions to the kinase catalytic core and participate in the regulation of its activity.

Coordinate regulation of translation by the PI 3-kinase and mTOR pathways

Control of translation initiation is an important means by which cells tightly regulate the critical processes of growth and proliferation. Multiple effector proteins contribute to translation initiation of specially modified mRNAs that modulate these processes. Coordinated regulation of these translational effectors by multiple signaling pathways allows the integration of information regarding mitogenic signals, energy levels, and nutrient sufficiency. The mTOR protein, in particular, serves as a sensor of all of these signals and is thought to thus serve as a crucial checkpoint control protein. Signals from the mTOR pathway converge with mitogenic inputs from the phosphoinositide (PI) 3-kinase pathway on translational effector proteins to coordinately control cellular growth, size, and cell proliferation. The translational effectors regulated by the PI 3-kinase and mTOR pathways and their roles in regulation of cellular growth will be the primary focus of this review.

Activation of S6K1 (p70 ribosomal protein S6 kinase 1) requires an initial calcium-dependent priming event involving formation of a high-molecular-mass signalling complex

The mitogen-stimulated protein kinase p70 ribosomal protein S6 kinase 1 (S6K1) is a key enzyme in the regulation of cell growth and proliferation. Activation of S6K1 requires a complex, ordered series of conformational changes and phosphorylation reactions. While the role of sequential, multi-site phosphorylation has been extensively detailed, characterization of the priming step required to initiate this cascade has remained elusive. In the present study we show for the first time that this priming process is dependent on calcium. Calcium-dependent regulation of S6K1 did not specifically target Thr-229 and Thr-389, the key regulatory phosphorylation sites; rather, calcium chelation resulted in a global inhibition of S6K1 phosphorylation. Mutation of individual phosphorylation sites in the auto-inhibitory and hydrophobic domains to acidic residues (to mimic phosphorylation) yields a kinase that remains sensitive to calcium chelation, while the combined mutations alleviate the requirement for calcium. Furthermore, deletion of the C-terminal residues (398-502) also renders the kinase insensitive to calcium. We hypothesize that the initial calcium-dependent process is required to release an inhibitory interaction between the C- and N-termini of S6K1, thus allowing phosphorylation of these key domains. The requirement for this priming step can only be overcome by mutations mimicking the phosphorylation of both the auto-inhibitory and hydrophobic domains. We further propose that the priming event involves formation of a calcium-dependent protein complex that releases the interaction between the N- and C-termini. S6K1 is then accessible for activation by the kinases that target the known regulatory phosphorylation sites. Consistent with this hypothesis, serum stimulation of S6K1 activity is associated with its incorporation into a calcium-dependent high-molecular-mass complex.

Mechanism of Ca2+-mediated regulation of NDR protein kinase through autophosphorylation and phosphorylation by an upstream kinase

NDR1 (nuclear Dbf2-related) is a serine/threonine protein kinase belonging to subfamily of kinases implicated in the regulation of cell division and morphology. Previously, we demonstrated that the activity of NDR1 is controlled by phosphorylation of two regulatory residues, Ser-281 and Thr-444. Moreover, we found that NDR1 becomes activated through a direct interaction with EF-hand Ca(2+)-binding proteins of the S100 family. In this work, we characterize this regulatory mechanism in detail. We found that NDR1 autophosphorylates in vitro predominantly on Ser-281 and to a lesser extent on Thr-74 and Thr-444. All of these residues proved to be crucial also for NDR1 activity in vivo; however, in contrast to Ser-281 and Thr-444, Thr-74 seems to be involved only in binding to S100B rather than directly regulating NDR1 activity per se. When we added Ca(2+)/S100B, we observed an increased autophosphorylation on Ser-281 and Thr-444, resulting in stimulation of NDR1 activity in vitro. Using phosphospecific antibodies, we found that Ser-281 also becomes autophosphorylated in vivo, whereas Thr-444 is targeted predominantly by an as yet unidentified upstream kinase. Significantly, the Ca(2+)-chelating agent BAPTA-AM suppressed the activity and phosphorylation of NDR1 on both Ser-281 and Thr-444, and specifically, these effects were reversed when we added the sarcoplasmic-endoplasmic reticulum Ca(2+) ATPase pump inhibitor thapsigargin.

Protein kinase C phosphorylates ribosomal protein S6 kinase betaII and regulates its subcellular localization

The ribosomal protein S6 kinase (S6K) belongs to the AGC family of Ser/Thr kinases and is known to be involved in the regulation of protein synthesis and the G(1)/S transition of the cell cycle. There are two forms of S6K, termed S6Kalpha and S6Kbeta, which have cytoplasmic and nuclear splice variants. Nucleocytoplasmic shuttling has been recently proposed for S6Kalpha, based on the use of the nuclear export inhibitor, leptomycin B. However, the molecular mechanisms regulating subcellular localization of S6Ks in response to mitogenic stimuli remain to be elucidated. Here we present data on the in vitro and in vivo phosphorylation of S6Kbeta, but not S6Kalpha, by protein kinase C (PKC). The site of phosphorylation was identified as S486, which is located within the C-terminal nuclear localization signal. Mutational analysis and the use of phosphospecific antibodies provided evidence that PKC-mediated phosphorylation at S486 does not affect S6K activity but eliminates the function of its nuclear localization signal and causes retention of an activated form of the kinase in the cytoplasm. Taken together, this study uncovers a novel mechanism for the regulation of nucleocytoplasmic shuttling of S6KbetaII by PKC-mediated phosphorylation.

Paclitaxel induces inactivation of p70 S6 kinase and phosphorylation of Thr421 and Ser424 via multiple signaling pathways in mitosis

The 70 kDa ribosomal S6 kinase (p70S6K) is important for cell growth and survival. Activation of p70S6K requires sequential phosphorylation of multiple serine and threonine sites often triggered by growth factors and hormones. Here, we report that paclitaxel, a microtubule-damaging agent, induces phosphorylation of p70S6K at threonine 421 and serine 424 (T421/S424) in a concentration- and time-dependent manner in multiple breast and ovarian cancer cell lines demonstrated by a T421/S424 phospho-p70S6K antibody. Phosphoamino-acid analysis and Western blot analysis by serine-/threonine-specific antibodies further confirms that both serine and threonine residues are phosphorylated in p70S6K following treatment with paclitaxel. Paclitaxel-induced p70S6K(T421/S424) phosphorylation requires both de novo RNA and protein synthesis via multiple signaling pathways including ERK1/2 MAP kinase, JNK, PKC, Ca(++), PI3K, and mammalian target of rapamycin (mTOR). Despite phosphorylation of p70S6K(T421/S424), paclitaxel inactivates this kinase in a concentration- and time-dependent manner as illustrated by in vitro kinase assay. Inhibitors of mTOR, PI3K, and Ca(++) impair p70S6K activity, whereas inhibitors of JNK and PKC stimulate p70S6K activity. Inhibition of PKC and JNK prevents paclitaxel-induced p70S6K inactivation. Moreover, the paclitaxel-induced phosphorylation and low activity of p70S6K mainly occurs during mitosis. In summary, paclitaxel is able to induce p70S6K(T421/S424) phosphorylation and decrease its activity in mitotic cells via multiple signaling pathways. Our data suggest that paclitaxel-induced p70S6K(T421/S424) phosphorylation and kinase inactivation are differentially regulated. Our data also indicate that paclitaxel may exert its antitumor effect, at least in part, via inhibition of p70S6K.

Inhibition of muscarinic receptor-induced proliferation of astroglial cells by ethanol: mechanisms and implications for the fetal alcohol syndrome

In utero exposure to ethanol is deleterious to fetal brain development. Children born with the fetal alcohol syndrome (FAS) display a number of abnormalities, the most significant of which are central nervous system (CNS) dysfunctions, such as microencephaly and mental retardation. An interaction of ethanol with glial cells, particularly astrocytes, has been suggested to contribute to the developmental neurotoxicity of this alcohol. At low concentrations (10-100 mM) ethanol inhibits the proliferation of astroglial cells in vitro, particularly when stimulated by acetycholine through muscarinic M3 receptors. Of the several signal transduction pathways activated by these receptors in astrocytes or astrocytoma cells, which are involved in mitogenic signaling, only some (e.g. protein kinase C (PKC) zeta, p70S6 kinase) appear to be targeted by ethanol at the same low concentrations which effectively inhibit proliferation. Inhibition of astroglial proliferation by ethanol may contribute to the microencephaly seen in FAS.

The Rb-family protein p107 inhibits translation by a PDK1-dependent mechanism

The Rb family of proteins, which consists of Rb, p107 and p130, are critical regulators of cell proliferation. In addition to their inhibitory effects on cell cycle progression, Rb-family proteins repress transcription by RNA polymerases I and III, and may therefore restrain cell growth. However, it is not known if Rb, p107 or p130 have direct effects on protein synthesis. Here we report that ectopic expression of p107 in rat fibroblasts markedly attenuates the stimulation of mRNA translation and global protein synthesis by serum growth factors. This effect is associated with a reduction in the phosphorylation and activation of the serine-threonine kinases Akt1 and p70 S6 kinase (S6K1), two downstream targets of phosphoinositide-dependent kinase 1 (PDK1). We show that overexpression of p107 interferes with the recruitment of PDK1 to the plasma membrane in response to growth factors. Overexpression of PDK1 restores the defect in translation elicited by p107. These results suggest that p107 restricts cell growth by interfering with the phosphoinositide 3-kinase (PI3K) signaling pathway.

Characterization of phosphatidylinositol 3-kinase-dependent phosphorylation of the hydrophobic motif site Thr(389) in p70 S6 kinase 1

Phosphorylation of the highly conserved hydrophobic motif site in AGC kinases is necessary for phosphotransferase activity. Phosphorylation of this motif (FLGFT389Y) in p70 S6 kinase (S6K1) is both rapamycin- and wortmannin-sensitive, suggesting a role for both mammalian target of rapamycin- and phosphatidylinositol 3-kinase-dependent pathways. We report here that co-expression of phosphoinositide-dependent kinase-1 (PDK1) and the phosphatidylinositol 3-kinase-regulated atypical protein kinase Czeta cooperate to increase both phosphorylation of the hydrophobic motif site Thr(389), as well as the activation loop site Thr(229). Interestingly, although PDK1 alone can promote an increase in Thr(389) phosphorylation in both wild type S6K1 and a kinase-inactive mutant of S6K1, the cooperative effect between PDK1 and protein kinase Czeta required S6K1 activity. Furthermore, Akt, another phosphatidylinositol 3-kinase effector and regulator of S6K1, also increased Thr(389) phosphorylation in a S6K1 activity-dependent manner. Consistent with this, epidermal growth factor-induced Thr(389) phosphorylation in wild type S6K1 persisted for up to 120 min, whereas kinase-inactive mutants of S6K1 displayed only a reduced and transient increase in Thr(389) phosphorylation. We conclude that S6K1 activity is required for maximal Thr(389) phosphorylation by mitogens and by multiple phosphatidylinositol 3-kinase-dependent inputs including PDK1, PKCzeta, and Akt, and we propose that autophosphorylation is an important regulatory mechanism for phosphorylation of the hydrophobic motif Thr(389) site in S6K1.

Naringin-sensitive phosphorylation of plectin, a cytoskeletal cross-linking protein, in isolated rat hepatocytes

To identify phosphoproteins that might play a role in naringin-sensitive hepatocellular cytoskeletal disruption and apoptosis induced by algal toxins, hepatocyte extracts were separated by gel electrophoresis and immunostained with a phosphothreonine-directed antibody. Use of dilute (5%) polyacrylamide gels containing 6 m urea allowed the resolution of one very large (approximately 500-kDa) okadaic acid- and naringin-sensitive phosphoprotein, identified by tryptic fingerprinting, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and immunostaining as the cytolinker protein, plectin. The naringin-sensitive phosphorylation induced by okadaic acid and microcystin-LR probably reflected inhibition of a type 2A protein phosphatase, whereas the naringin-resistant phosphorylation induced by calyculin A, tautomycin, and cantharidin probably involved a type 1 phosphatase. Okadaic acid caused a collapse of the plectin-immunostaining bile canalicular sheaths and the general cytoskeletal plectin network into numerous medium-sized plectin aggregates. Inhibitors of protein kinase C, cAMP-dependent protein kinase, or Ca(2+)/calmodulin-dependent kinase II had moderate or no protective effects on plectin network disruption, whereas naringin offered 86% protection. Okadaic acid induced a naringin-sensitive phosphorylation of AMP-activated protein kinase (AMPK), the stress-activated protein kinases SEK1 and JNK, and S6 kinase. The AMPK-activating kinase (AMPKK) is likely to be the target of inhibition by naringin, the other kinases serving as downstream components of an AMPKK-initiated signaling pathway.

Role of the p70 S6 kinase cascade in neutrophilic differentiation and proliferation of HL-60 cells-a study of transferrin receptor-positive and -negative cells obtained from dimethyl sulfoxide- or retinoic acid-treated HL-60 cells

Previously, we suggested that p70 S6 kinase (p70 S6K) plays an important role in the regulation of neutrophilic differentiation of HL-60 cells; this conclusion was based on our analysis of transferrin receptor (Trf-R) positive (Trf-R(+)) and negative (Trf-R(-)) cells that appeared after treatment with dimethyl sulfoxide (Me(2)SO). In this study, we analyzed the upstream of p70 S6K in relation to the differentiation and proliferation of both cell types. The granulocyte colony-stimulating factor (G-CSF)-induced enhancement of phosphatidylinositol 3-kinase (PI3K) activity in Trf-R(+) cells was markedly higher than that in Trf-R(-) cells. Wortmannin, a specific inhibitor of PI3K, partially inhibited G-CSF-induced p70 S6K activity and G-CSF-dependent proliferation, whereas rapamycin, an inhibitor of p70 S6K, completely inhibited these activities. The wortmannin-dependent enhancement of neutrophilic differentiation was similar to that induced by rapamycin. From these results, we conclude that the PI3K/p70 S6K cascade may play an important role in negative regulation of neutrophilic differentiation in HL-60 cells. For the G-CSF-dependent proliferation, however, p70 S6K appears to be a highly important pathway through not only a PI3K-dependent but also possibly an independent cascade.

Tuberin regulates p70 S6 kinase activation and ribosomal protein S6 phosphorylation. A role for the TSC2 tumor suppressor gene in pulmonary lymphangioleiomyomatosis (LAM)

Although the cellular functions of TSC2 and its protein product, tuberin, are not known, somatic mutations in the TSC2 tumor suppressor gene are associated with tumor development in lymphangioleiomyomatosis (LAM). We found that ribosomal protein S6 (S6), which exerts translational control of protein synthesis and is required for cell growth, is hyperphosphorylated in the smooth muscle-like cell lesions of LAM patients compared with smooth muscle cells from normal human blood vessels and trachea. Smooth muscle (SM) cells derived from these lesions (LAMD-SM) also exhibited S6 hyperphosphorylation, constitutive activation of p70 S6 kinase (p70S6K), and increased basal DNA synthesis. In parallel, TSC2-/- smooth muscle cells (ELT3) and TSC2-/- epithelial cells (ERC15) also exhibited hyperphosphorylation of S6, constitutive activation of p70S6K, and increased basal DNA synthesis. Re-introduction of wild type tuberin into LAMD-SM, ELT3, and ERC15 cells abolished phosphorylation of S6 and significantly inhibited p70S6K activity and DNA synthesis. Rapamycin, an immunosuppressant, inhibited hyperphosphorylation of S6, p70S6K activation, and DNA synthesis in LAMD-SM cells. Interestingly, the basal levels of phosphatidylinositol 3-kinase, Akt/protein kinase B, and p42/p44 MAPK activation were unchanged in LAMD-SM and ELT3 cells relative to levels in normal human tracheal and vascular SM. These data demonstrate that tuberin negatively regulates the activity of S6 and p70S6K specifically, and suggest a potential mechanism for abnormal cell growth in LAM.

The Src-family tyrosine kinase inhibitor PP1 interferes with the activation of ribosomal protein S6 kinases

Considerable biochemical and pharmacological evidence suggests that the activation of ribosomal protein S6 kinases (S6Ks) by activated receptor tyrosine kinases involves multiple co-ordinated input signals. However, the identities of many of these inputs remain poorly described, and their precise involvement in S6K activation has been the subject of great investigative effort. In the present study, we have shown that 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1), a selective inhibitor of the Src family of non-receptor tyrosine kinases, interferes with the activation of 70 and 85 kDa S6K gene products (p70S6K1 and p85S6K1) by insulin, insulin-like growth factor 1, sodium orthovanadate and activated alleles of phosphoinositide 3-kinase and H-Ras. PP1 also impedes the activation of AKT/protein kinase B and the extracellular signal-regulated protein kinases 1 and 2 by these various stimuli. Insulin-like growth factor 1 was observed to induce a sustained increase in c-Src autophosphorylation as revealed using anti-phospho-Y416 antisera, but this effect was absent from the cells treated with PP1. To conclude, an activated allele of p70S6K1 is compared with the wild-type allele, resistant to inhibition by PP1 when co-expressed with phosphoinositide-dependent kinase 1 (PDK1), suggesting that PP1 affects p70S6K1 via a PDK1-independent pathway. Thus activation of Src may supply a necessary signal for the activation of p70S6K1 and possibly other S6Ks.

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.

Effect of ethanol on protein kinase Czeta and p70S6 kinase activation by carbachol: a possible mechanism for ethanol-induced inhibition of glial cell proliferation

The signal transduction pathways that mediate the mitogenic response of muscarinic acetylcholine receptors in astroglial cells have not been fully elucidated. In this study we investigated the activation of p70S6 kinase (p70S6K) by carbachol in 1321 N1 astroctyoma cells. Carbachol induced a dose- and time-dependent activation of p70S6K, as evidenced by increased phosphorylation at Thr-389, Thr-421 and Ser-424, by increased p70S6K activity, and by a shift in its molecular weight. Activation of p70S6K was mediated by M3 muscarinic acetylcholine receptors (mAChRs) and was inhibited by two phosphatidylinositol-3-kinase (PI3-K) inhibitors, by a pseudosubstrate to protein kinase C (PKC) zeta, and by the p70S6K inhibitor rapamycin. Carbachol-induced DNA synthesis was strongly inhibited by rapamycin, suggesting that p70S6K activation plays an important role in carbachol-induced cell proliferation. Ethanol (25-100 mm) has been shown to inhibit carbachol-induced proliferation of astroglial cells. In the same range of concentrations, ethanol also inhibits carbachol-induced activation of PKCzeta and of p70S6K. On the other hand, inhibition of PI3-kinase was only observed at higher ethanol concentrations. These results indicate that activation of the PKCzeta--> p70S6K pathway by M3 mAChRs may play a role in the increased DNA synthesis and may represent a target for ethanol-induced inhibition of astroglial cell proliferation.

Role of PDK1 in insulin-signaling pathway for glucose metabolism in 3T3-L1 adipocytes

To investigate the role of 3-phosphoinositide-dependent protein kinase 1 (PDK1) in the insulin-signaling pathway for glucose metabolism, wild-type (wt), the kinase-dead (kd), or the plecstrin homology (PH) domain deletion (DeltaPH) mutant of PDK1 was expressed using an adenovirus gene transduction system in 3T3-L1 adipocytes. wt-PDK1 and kd-PDK1 were found in both membrane and cytosol fractions, whereas DeltaPH-PDK1, which exhibited PDK1 activity similar to that of wt-PDK1, was detected exclusively in the cytosol fraction. Insulin dose dependently activated protein kinase B (PKB) but did not change atypical protein kinase C (aPKC) activity in control cells. aPKC activity was not affected by expression of wt-, kd-, or DeltaPH-PDK1 in either the presence or the absence of insulin. Overexpression of wt-PDK1 enhanced insulin-induced activation of PKB as well as insulin-induced phosphorylation of glycogen synthase kinase (GSK)3alpha/beta, a direct downstream target of PKB, although insulin-induced glycogen synthesis was not significantly enhanced by wt-PDK1 expression. Neither DeltaPH-PDK1 nor kd-PDK1 expression affected PKB activity, GSK3 phosphorylation, or glycogen synthesis. Thus membrane localization of PDK1 via its PH domain is essential for insulin signaling through the PDK1-PKB-GSK3alpha/beta pathway. Glucose transport activity was unaffected by expression of wt-PDK1, kd-PDK1, or DeltaPH-PDK1 in either the presence or the absence of insulin. These findings suggest the presence of a signaling pathway for insulin-stimulated glucose transport in which PDK1 to PKB or aPKC is not involved.