Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:P42345 (mTOR)
 26,049 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Recent studies indicate that dysregulation of the Akt/PKB family of serine/threonine kinases is a prominent feature of many human cancers. The Akt/PKB family is composed of three members termed Akt1/PKBalpha, Akt2/PKBbeta, and Akt3/PKBgamma. It is currently not known to what extent there is functional overlap between these family members. We have recently identified small molecule inhibitors of Akt. These compounds have pleckstrin homology domain-dependent, isozyme-specific activity. In this report, we present data showing the relative contribution that inhibition of the different isozymes has on the apoptotic response of tumor cells to a variety of chemotherapies. In multiple cell backgrounds, maximal induction of caspase-3 activity is achieved when both Akt1 and Akt2 are inhibited. This induction is not reversed by overexpression of functionally active Akt3. The level of caspase-3 activation achieved under these conditions is equivalent to that observed with the phosphatidylinositol-3-kinase inhibitor LY294002. We also show that in different tumor cell backgrounds inhibition of mammalian target of rapamycin, a downstream substrate of Akt, is less effective in inducing caspase-3 activity than inhibition of Akt1 and Akt2. This shows that the survival phenotype conferred by Akt can be mediated by signaling pathways independent of mammalian target of rapamycin in some tumor cell backgrounds. Finally, we show that inhibition of both Akt1 and Akt2 selectively sensitizes tumor cells, but not normal cells, to apoptotic stimuli.
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PMID:Tumor cell sensitization to apoptotic stimuli by selective inhibition of specific Akt/PKB family members. 1571 98

Hemodynamic forces, including shear stress and cyclic strain, have been recognised as important modulators of vascular cell morphology and function. However, the mechanism by which vascular cells sense and transduce the extracellular mechanical signals into the cell nucleus has not yet been clarified. The purpose of our study was to assess the involvement of the signal transducer and activator of transcription-3 (STAT-3) in the signaling pathway mediating the response of vascular smooth muscle cells (SMC) to cyclic strain. Embryonic A7r5 SMC derived from thoracic aortas of DB1X rats were seeded on flexible collagen I-coated plates. Cells were subjected to 10% average strain at 60 cycles/min for various time periods. Activation of STAT-3, p38, extracellular signal-regulated kinase (ERK) 1/2 and Src was assessed by immunoblotting using phosphospecific antibodies. The interactions between STAT-3 phosphorylation and p38, ERK1/2, phosphatidylinositol-3 (PI3K), mammalian target of rapamycin (mTOR), Janus kinase (JAK) 2 and Src were evaluated by pretreating the cells with specific inhibitors including SB202190, PD98059, LY294002, wortmannin, rapamycin, AG490 and PP1. Serine phosphorylation of STAT-3 was increased by 2-fold after 15 min of cyclic strain, while tyrosine phosphorylation was increased by 2.3-fold after 60 min. Inhibition of ERK1/2 by PD98059 prevented serine phosphorylation of STAT-3, whereas inhibition of Src by PP1 prevented STAT-3 tyrosine phosphorylation. Pretreating the cells with SB202190, a specific inhibitor of p38, resulted in an increase in basal phosphorylation of ERK1/2 and a subsequent increase in basal serine phosphorylation of STAT-3. In conclusion, both serine and tyrosine phosphorylation of STAT-3 are involved in the signaling pathway mediating the effects of cyclic strain on vascular SMC. Serine phosphorylation of STAT-3 is mediated by ERK1/2, while tyrosine phosphorylation is mediated by Src. A negative feedback loop was also found between p38 and ERK1/2.
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PMID:The role of STAT-3 in the mediation of smooth muscle cell response to cyclic strain. 1583 72

Replicative cell division is an energetically demanding process that can be executed only if cells have sufficient metabolic resources to support a doubling of cell mass. Here we show that proliferating mammalian cells have a cell-cycle checkpoint that responds to glucose availability. The glucose-dependent checkpoint occurs at the G(1)/S boundary and is regulated by AMP-activated protein kinase (AMPK). This cell-cycle arrest occurs despite continued amino acid availability and active mTOR. AMPK activation induces phosphorylation of p53 on serine 15, and this phosphorylation is required to initiate AMPK-dependent cell-cycle arrest. AMPK-induced p53 activation promotes cellular survival in response to glucose deprivation, and cells that have undergone a p53-dependent metabolic arrest can rapidly reenter the cell cycle upon glucose restoration. However, persistent activation of AMPK leads to accelerated p53-dependent cellular senescence. Thus, AMPK is a cell-intrinsic regulator of the cell cycle that coordinates cellular proliferation with carbon source availability.
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PMID:AMP-activated protein kinase induces a p53-dependent metabolic checkpoint. 1605 73

The mammalian target of rapamycin (mTOR/TOR) is implicated in cancer and other human disorders and thus an important target for therapeutic intervention. To study human TOR in vitro, we have produced in large scale both the full-length TOR (289 kDa) and a truncated TOR (132 kDa) from HEK293 cells. Both enzymes demonstrated a robust and specific catalytic activity towards the physiological substrate proteins, p70 S6 ribosomal protein kinase 1 (p70S6K1) and eIF4E binding protein 1 (4EBP1), as measured by phosphor-specific antibodies in Western blotting. We developed a high capacity dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA) for analysis of kinetic parameters. The Michaelis constant (Km) values of TOR for ATP and the His6-S6K substrate were shown to be 50 and 0.8 microM, respectively. Dose-response and inhibition mechanisms of several known inhibitors, the rapamycin-FKBP12 complex, wortmannin and LY294002, were also studied in DELFIA. Our data indicate that TOR exhibits kinetic features of those shared by traditional serine/threonine kinases and demonstrate the feasibility for TOR enzyme screen in searching for new inhibitors.
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PMID:Characterization of the cloned full-length and a truncated human target of rapamycin: activity, specificity, and enzyme inhibition as studied by a high capacity assay. 1589 31

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.
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PMID:Identification of S6 kinase 1 as a novel mammalian target of rapamycin (mTOR)-phosphorylating kinase. 1590 73

Insulin receptor substrate-1 (IRS-1) plays a pivotal role in insulin signal transduction. It has been shown that the amino acids modulate insulin signaling at the level of IRS-1. Here we show that an amino acid unbalanced diet causes a reduction in serine phosphorylation as well as an elevation in insulin-induced tyrosine phosphorylation of IRS-1 in rat muscle. In fibroblasts and myotube cells, the effect of amino acid deprivation on IRS-1 phosphorylation was evident only when cells were pretreated with reagents causing hyperphosphorylation of serines of IRS-1. But, the target kinases of these reagents were not inactivated by amino acid deprivation, suggesting that amino acid deprivation activates serine/threonine phosphatase(s) of IRS-1. The phosphatases regulated by mammalian target of rapamycin do not appear to participate in the dephosphorylation either. These results suggest that amino acid deprivation dephosphorylates IRS-1 through unidentified serine/threonine phosphatases and thereby potentiates insulin signaling.
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PMID:Disruption of the availability of amino acids induces a rapid reduction of serine phosphorylation of insulin receptor substrate-1 in vivo and in vitro. 1591 20

Protein kinase C (PKC) is a family of serine/threonine kinases whose activity is controlled, in part, by phosphorylation on three conserved residues that are located on the catalytic domain of the enzyme, known as the activation-loop, the turn-motif, and the C-terminal hydrophobic-motif sites. Using a panel of phospho-specific antibodies, we have determined that PKC beta(I) and delta are constitutively phosphorylated on all three sites in unstimulated and activated T cells. Although PKC theta is constitutively phosphorylated at the activation-loop and turn-motif sites in T cells, PMA or anti-CD3/CD28 stimulation results in an increase in phosphorylation at the hydrophobic-motif (Ser695), an event that coincides with translocation of the enzyme from the cytosol/cytoskeleton to the membrane. Studies on the stimulus-induced phosphorylation of PKC theta demonstrate that an upstream kinase activity involving a conventional PKC isoform(s) and the PI3-kinase pathway, rather than autophosphorylation or the rapamycin-sensitive mTOR pathway, regulates this site in T lymphocytes. However, hydrophobic-motif phosphorylation does not appear to control membrane translocation, suggesting that this site may control other aspects of PKC theta signalling.
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PMID:Stimulus-induced phosphorylation of PKC theta at the C-terminal hydrophobic-motif in human T lymphocytes. 1600 40

Activation of members of the protein kinase AGC (cAMP dependent, cGMP dependent, and protein kinase C) family is regulated primarily by phosphorylation at two sites: a conserved threonine residue in the activation loop and a serine/threonine residue in a hydrophobic motif (HM) near the COOH terminus. Although phosphorylation of these kinases in the activation loop has been found to be mediated by phosphoinositide-dependent protein kinase-1 (PDK1), the kinase(s) that catalyzes AGC kinase phosphorylation in the HM remains uncharacterized. So far, at least 10 kinases have been suggested to function as an HM kinase or the so-called "PDK2," including mitogen-activated protein (MAP) kinase-activated protein kinase-2 (MK2), integrin-linked kinase (ILK), p38 MAP kinase, protein kinase Calpha (PKCalpha), PKCbeta, the NIMA-related kinase-6 (NEK6), the mammalian target of rapamycin (mTOR), the double-stranded DNA-dependent protein kinase (DNK-PK), and the ataxia telangiectasia mutated (ATM) gene product. However, whether any or all of these kinases act as a physiological HM kinase remains to be established. Nonetheless, available data suggest that multiple systems may be used in cells to regulate the activation of the AGC family kinases. It is possible that, unlike activation loop phosphorylation, phosphorylation of the HM site in the different AGC family kinases is mediated by distinct kinases. In addition, phosphorylation of the AGC family kinase at the HM site could be cell type, signaling pathway, and substrate specific. Identification and characterization of the bonafide HM kinase(s) will be essential to verify these hypotheses.
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PMID:PDK2: the missing piece in the receptor tyrosine kinase signaling pathway puzzle. 1601 56

The function of insulin receptor substrate-1 (IRS-1), a key molecule of insulin signaling, is modulated by phosphorylation at multiple serine/threonine residues. Phorbol ester stimulation of cells induces phosphorylation of two inhibitory serine residues in IRS-1, i.e. Ser-307 and Ser-318, suggesting that both sites may be targets of protein kinase C (PKC) isoforms. However, in an in vitro system using a broad spectrum of PKC isoforms (alpha, beta1, beta2, delta, epsilon, eta, mu), we detected only Ser-318, but not Ser-307 phosphorylation, suggesting that phorbol ester-induced phosphorylation of this site in intact cells requires additional signaling elements and serine kinases that link PKC activation to Ser-307 phosphorylation. As we have observed recently that the tyrosine phosphatase Shp2, a negative regulator of insulin signaling, is a substrate of PKC, we studied the role of Shp2 in this context. We found that phorbol ester-induced Ser-307 phosphorylation is reduced markedly in Shp2-deficient mouse embryonic fibroblasts (Shp2-/-) whereas Ser-318 phosphorylation is unaltered. The Ser-307 phosphorylation was rescued by transfection of mouse embryonic fibroblasts with wild-type Shp2 or with a phosphatase-inactive Shp2 mutant, respectively. In this cell model, tumor necrosis factor-alpha-induced Ser-307 phosphorylation as well depended on the presence of Shp2. Furthermore, Shp2-dependent phorbol ester effects on Ser-307 were blocked by wortmannin, rapamycin, and the c-Jun NH2-terminal kinase (JNK) inhibitor SP600125. This suggests an involvement of the phosphatidylinositol 3-kinase/mammalian target of rapamycin cascade and of JNK in this signaling pathway resulting in IRS-1 Ser-307 phosphorylation. Because the activation of these kinases does not depend on Shp2, it is concluded that the function of Shp2 is to direct these activated kinases to IRS-1.
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PMID:Shp2 is required for protein kinase C-dependent phosphorylation of serine 307 in insulin receptor substrate-1. 1605 40

In 3T3-L1 adipocytes, insulin or anisomycin stimulated phosphorylation of IRS-1 at Ser(307) and Ser(636/639), both of which were partially reduced by the mTOR inhibitor, rapamycin, or the JNK inhibitor, SP600125, and were further inhibited by a combination of them. Interestingly, anisomycin-induced p70(S6K) phosphorylation was reduced by SP600125, while insulin-induced p70(S6K) phosphorylation was not. Furthermore, unlike insulin, anisomycin failed to elicit translocation or degradation of IRS-1. These results indicate that mTOR and JNK play roles in phosphorylating IRS-1 serine residues, and that insulin and anisomycin are different in terms of the relationship of activation between mTOR and JNK, and the effects on IRS-1 localization and stability.
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PMID:Roles of mTOR and JNK in serine phosphorylation, translocation, and degradation of IRS-1. 1609 28


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