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

Phosphatidylinositol (PI) 3-kinase is required for G1 to S phase cell cycle progression stimulated by a variety of growth factors and is implicated in the activation of several downstream effectors, including p70(S6K). However, the molecular mechanisms by which PI 3-kinase is engaged in activation of the cell cycle machinery are not well understood. Here we report that the expression of a dominant negative (DN) form of either the p110alpha catalytic or the p85 regulatory subunit of heterodimeric PI 3-kinase strongly inhibited epidermal growth factor (EGF)-induced upregulation of cyclin D1 protein in NIH 3T3(M17) fibroblasts. The PI 3-kinase inhibitors LY294002 and wortmannin completely abrogated increases in both mRNA and protein levels of cyclin D1 and phosphorylation of pRb, inducing G1 arrest in EGF-stimulated cells. By contrast, rapamycin, which potently suppressed p70(S6K) activity throughout the G1 phase, had little inhibitory effect, if any, on either of these events. PI 3-kinase, but not rapamycin-sensitive pathways, was also indispensable for upregulation of cyclin D1 mRNA and protein by other mitogens in NIH 3T3 (M17) cells and in wild-type NIH 3T3 cells as well. We also found that an enforced expression of wild-type p110 was sufficient to induce cyclin D1 protein expression in growth factor-deprived NIH 3T3(M17) cells. The p110 induction of cyclin D1 in quiescent cells was strongly inhibited by coexpression of either of the PI 3-kinase DN forms, and by LY294002, but was independent of the Ras-MEK-ERK pathway. Unlike mitogen stimulation, the p110 induction of cyclin D1 was sensitive to rapamycin. These results indicate that the catalytic activity of PI 3-kinase is necessary, and could also be sufficient, for upregulation of cyclin D1, with mTOR signaling being differentially required depending upon cellular conditions.
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PMID:Cyclin D1 expression mediated by phosphatidylinositol 3-kinase through mTOR-p70(S6K)-independent signaling in growth factor-stimulated NIH 3T3 fibroblasts. 989 Oct 68

A key regulatory step in translation is initiation, or the recruitment of the translational machinery to the 5' end of mRNA. The 5' terminus of most mRNAs is demarcated by a m7GpppN cap (where m is a methyl group, and N is any nucleotide). The m7 cap is essential for the translation of most mRNAs, as it directs the translational machinery to the 5' end of the mRNA via its interaction with the cap binding protein, the eukaryotic translation initiation factor 4E (eIF4E). eIF4E is the limiting initiation factor in most cells. Thus, eIF4E activity plays a principal role in determining global translation rates. Consistent with this role, eIF4E is required for cell cycle progression, exhibits anti-apoptotic activity, and, when overexpressed, transforms cells. This review focuses upon the various mechanisms utilized in the regulation of eIF4E activity. (1) eIF4E is regulated transcriptionally; it is one of the few identified transcriptional targets of c-myc. (2) eIF4E is phosphorylated following activation of the MNK1 kinase, a substrate of the ERK and p38 MAPKs. The recent determination of the three-dimensional structure of eIF4E bound to a m7 cap analog has provided insight into the mechanisms involved in the regulation of the eIF4E-cap and eIF4E-mRNA interactions. As suggested by the crystal structure, phosphorylation of eIF4E may enhance its affinity for mRNA. (3) eIF4E is also regulated through binding to a family of translational repressor proteins. Interaction with the 4E-BPs prevents the incorporation of eIF4E into an active translation initiation complex, and thus, inhibits cap-dependent translation. This inhibitory interaction is relieved following phosphorylation of the 4E-BPs by a PI3K-dependent pathway, involving signalling by the anti-apoptotic kinase Akt/PKB, as well as FRAP/mTOR.
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PMID:eIF4E activity is regulated at multiple levels. 1021 43

Adipocyte number, a determinant of adipose tissue mass, reflects the balance between the rates of proliferation/differentiation vs. apoptosis of preadipocytes. The percentage of 3T3-L1 preadipocytes undergoing cell death following serum deprivation was reduced by 10 nM insulin-like growth factor (IGF)-1 (from 50.0 +/- 0.7% for control starved cells to 27.5 +/- 3.1%). TUNEL staining confirmed the apoptotic nature of the cell death. The protective effect of IGF-1 was blocked by phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin, and LY294002, but was unaffected by rapamycin, PD98059, or SB203580, which inhibit mammalian target of rapamycin (mTOR), ERK kinase (MEK1), and p38 MAPK respectively. Exogenous PI(3,4,5)P3 (10 microM), the principal product of IGF-1-stimulated PI3K in 3T3-L1 preadipocytes, had a modest survival effect on its own, reducing cell death from 47.9 +/- 3.4% to 35.6 +/- 3.5%. When added to the combination of IGF-1 and LY294002, PI(3,4,5)P3 reversed most of the inhibitory effect of LY294002 on IGF-1-dependent cell survival, protein kinase B/Akt phosphorylation, and caspase-3 activity. Taken together, these results implicate PI(3,4,5)P3 as a necessary signal for the anti-apoptotic action of IGF-1 on 3T3-L1 preadipocytes.
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PMID:Phosphatidylinositol-3,4,5-trisphosphate is required for insulin-like growth factor 1-mediated survival of 3T3-L1 preadipocytes. 1114 83

The alpha(1)-adrenergic agonist phenylephrine (PE) and insulin each stimulate protein synthesis in cardiomyocytes. Activation of protein synthesis by PE is involved in the development of cardiac hypertrophy. One component involved here is p70 S6 kinase 1 (S6K1), which lies downstream of mammalian target of rapamycin, whose regulation is thought to involve phosphatidylinositol 3-kinase and protein kinase B (PKB). S6K2 is a recently identified homolog of S6K1 whose regulation is poorly understood. Here we demonstrate that in adult rat ventricular cardiomyocytes, PE and insulin each activate S6K2, activation being 3.5- and 5-fold above basal, respectively. Rapamycin completely blocked S6K2 activation by either PE or insulin. Three different inhibitors of MEK1/2 abolished PE-induced activation of S6K2 whereas expression of constitutively active MEK1 activated S6K2, without affecting the p38 mitogen-activated protein kinase and JNK pathways, indicating that MEK/ERK signaling plays a key role in regulation of S6K2 by PE. PE did not activate PKB, and expression of dominant negative PKB failed to block activation of S6K2 by PE, indicating PE-induced S6K2 activation is independent of PKB. However, this PKB mutant did partially block S6K2 activation by insulin, indicating PKB is required here. Another hypertrophic agent, endothelin 1, also activated S6K2 in a MEK-dependent manner. Our findings provide strong evidence for novel signaling connections between MEK/ERK and S6K2.
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PMID:Cross-talk between the ERK and p70 S6 kinase (S6K) signaling pathways. MEK-dependent activation of S6K2 in cardiomyocytes. 1143 69

Insulin regulates the expression of several hepatic genes. Although the general definition of insulin signaling has progressed dramatically, the elucidation of the complete signaling pathway from insulin receptor to transcription factors involved in the regulation of a specific gene remains to be established. In fact, recent works suggest that multiple divergent insulin signaling pathways regulate the expression of distinct genes. 5-Aminolevulinate synthase (ALAS) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of heme biosynthesis. It has been reported that insulin caused the rapid inhibition of housekeeping ALAS transcription, but the mechanism involved in this repression has not been explored. The present study investigates the role of phosphatidylinositol 3-kinase (PI3-kinase) and mitogen-activated protein kinase pathways in insulin signaling relevant to ALAS inhibition. To explore this, we combined the transient overexpression of regulatory proteins involved in these pathways and the use of small cell permeant inhibitors in rat hepatocytes and HepG2 cells. Wortmannin and LY294002, PI3-kinase inhibitors, as well as lovastatin and PD152440, Ras farnesylation inhibitors, and MEK inhibitor PD98059 abolished the insulin repression of ALAS transcription. The inhibitor of mTOR/p70(S6K) rapamycin had no effect whatsoever upon hormone action. The overexpression of vectors encoding constitutively active Ras, MEK, or p90(RSK) mimicked the inhibitory action of insulin. Conversely, negative mutants of PKB, Ras, or MEK impaired insulin inhibition of ALAS promoter activity. Furthermore, inhibition of one of the pathways blocks the inhibitory effect produced by the activation of the other. Our findings suggest that factors involved in two signaling pathways that are often considered to be functionally separate during insulin action, the Ras/ERK/p90(RSK) pathway and the PI3K/PKB pathway, are jointly required for insulin-mediated inhibition of ALAS gene expression in rat hepatocytes and human hepatoma cells.
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PMID:Phosphatidylinositol 3-kinase and Ras/mitogen-activated protein kinase signaling pathways are required for the regulation of 5-aminolevulinate synthase gene expression by insulin. 1171 32

Interleukin-6 (IL-6) is a prominent tumor growth factor for malignant multiple myeloma cells. In addition to its known activation of the Janus tyrosine kinase-STAT and RAS-MEK-ERK pathways, recent work suggests that IL-6 can also activate the phosphatidylinositol 3-kinase (PI3-K)/AKT kinase pathway in myeloma cells. Because activation of the PI3-K/AKT as well as RAS-MEK-ERK pathways may result in downstream stimulation of the p70(S6K) (p70) and phosphorylation of the 4E-BP1 translational repressor, we assessed these potential molecular targets in IL-6-treated myeloma cells. IL-6 rapidly activated p70 kinase activity and p70 phosphorylation. Activation was inhibited by wortmannin, rapamycin, and the ERK inhibitors PD98059 and UO126, as well as by a dominant negative mutant of AKT. The concurrent requirements for both ERK and PI3-K/AKT appeared to be a result of their ability to phosphorylate p70 on different residues. In contrast, IL-6-induced phosphorylation of 4E-BP1 was inhibited by rapamycin, wortmannin, and dominant negative AKT but ERK inhibitors had no effect, indicating ERK function was dispensable. In keeping with these data, a dominant active AKT mutant was sufficient to induce 4E-BP1 phosphorylation but could not by itself activate p70 kinase activity. Prevention of IL-6-induced p70 activation and 4E-BP1 phosphorylation by the mammalian target of rapamycin inhibitors rapamycin and CCI-779 resulted in inhibition of IL-6-induced myeloma cell growth. These results indicate that both ERK and PI3-K/AKT pathways are required for optimal IL-6-induced p70 activity, but PI3-K/AKT is sufficient for 4E-BP1 phosphorylation. Both effects are mediated via mammalian target of rapamycin function, and, furthermore, these effects are critical for IL-6-induced tumor cell growth.
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PMID:Signal pathways involved in activation of p70S6K and phosphorylation of 4E-BP1 following exposure of multiple myeloma tumor cells to interleukin-6. 1187 47

p70S6 kinase (S6K1) plays a pivotal role in hypertrophic cardiac growth via ribosomal biogenesis. In pressure-overloaded myocardium, we show S6K1 activation accompanied by activation of protein kinase C (PKC), c-Raf, and mitogen-activated protein kinases (MAPKs). To explore the importance of the c-Raf/MAPK kinase (MEK)/MAPK pathway, we stimulated adult feline cardiomyocytes with 12-O-tetradecanoylphorbol-13-acetate (TPA), insulin, or forskolin to activate PKC, phosphatidylinositol-3-OH kinase, or protein kinase A (PKA), respectively. These treatments resulted in S6K1 activation with Thr-389 phosphorylation as well as mammalian target of rapamycin (mTOR) and S6 protein phosphorylation. Thr-421/Ser-424 phosphorylation of S6K1 was observed predominantly in TPA-treated cells. Dominant negative c-Raf expression or a MEK1/2 inhibitor (U0126) treatment showed a profound blocking effect only on the TPA-stimulated phosphorylation of S6K1 and mTOR. Whereas p38 MAPK inhibitors exhibited only partial effect, MAPK-phosphatase-3 expression significantly blocked the TPA-stimulated S6K1 and mTOR phosphorylation. Inhibition of mTOR with rapamycin blocked the Thr-389 but not the Thr-421/Ser-424 phosphorylation of S6K1. Therefore, during PKC activation, the c-Raf/MEK/extracellular signal-regulated kinase-1/2 (ERK1/2) pathway mediates both the Thr-421/Ser-424 and the Thr-389 phosphorylation in an mTOR-independent and -dependent manner, respectively. Together, our in vivo and in vitro studies indicate that the PKC/c-Raf/MEK/ERK pathway plays a major role in the S6K1 activation in hypertrophic cardiac growth.
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PMID:c-Raf/MEK/ERK pathway controls protein kinase C-mediated p70S6K activation in adult cardiac muscle cells. 1194 May 78

Overexpression of the growth factor receptor subunit c-erbB2, leading to its ligand-independent homodimerization and activation, has been implicated in the pathogenesis of mammary carcinoma. Here, we have examined the effects of c-erbB2 on the adhesive properties of a mammary epithelial cell line, HB2/tnz34, in which c-erbB2 homodimerization can be induced by means of a transfected hybrid "trk-neu" construct. trk-neu consists of the extracellular domain of the trkA nerve growth factor (NGF) receptor fused to the transmembrane and cytoplasmic domains of c-erbB2, allowing NGF-induced c-erbB2 homodimer signaling. Both spreading and adhesion on collagen surfaces were impaired on c-erbB2 activation in HB2/tnz34 cells. Antibody-mediated stimulation of alpha(2)beta(1) integrin function restored adhesion, suggesting a direct role for c-erbB2 in integrin inactivation. Using pharmacological inhibitors and transient transfections, we identified signaling pathways required for suppression of integrin function by c-erbB2. Among these was the MEK-ERK pathway, previously implicated in integrin inactivation. However, we could also show that downstream of phosphoinositide-3-kinase (PI3K), protein kinase B (PKB) acted as a previously unknown, potent inhibitor of integrin function and mediator of the disruptive effects of c-erbB2 on adhesion and morphogenesis. The integrin-linked kinase, previously identified as a PKB coactivator, was also found to be required for integrin inactivation by c-erbB2. In addition, the PI3K-dependent mTOR/S6 kinase pathway was shown to mediate c-erbB2-induced inhibition of adhesion (but not spreading) independently of PKB. Overexpression of MEK1 or PKB suppressed adhesion without requirement for c-erbB2 activation, suggesting that these two pathways partake in integrin inhibition by targeting common downstream effectors. These results demonstrate a major novel role for PI3K and PKB in regulation of integrin function.
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PMID:c-erbB2-induced disruption of matrix adhesion and morphogenesis reveals a novel role for protein kinase B as a negative regulator of alpha(2)beta(1) integrin function. 1218 54

Anaplastic thyroid carcinomas (ATCs) are highly aggressive, extremely lethal human cancers with poor therapeutic response. Chemokines are a superfamily of small cytokine-like proteins that induce, through their interaction with G protein-coupled receptors, cytoskeletal rearrangement, firm adhesion to endothelial cells, and directional migration. In this study, we characterized the expression of CXC chemokine receptor 4 (CXCR4) and analyzed its functions in ARO cells, a human ATC cell. The normal primary cultured thyroid cells and ATC cell lines expressed CXCR4 and stromal cell-derived factor (SDF)-1 alpha transcripts, detected by RT-PCR. Fluorescence activated cell sorting analysis of CXCR4 expression in normal and ATC cells showed that ARO cells expressed significant levels of CXCR4. FRO, NPA, and normal thyroid cells did not express membrane CXCR4, as determined by fluorescence activated cell sorting analysis. To identify the functional role of CXCR4 in ARO cells, we treated ARO cells with SDF-1 alpha and analyzed the signaling pathways, cellular migration, and proliferation. SDF-1alpha enhanced the migration but did not affect the proliferation of ARO cells or activate the Janus kinase/signal transducer and activator of transcription signaling pathways. However, SDF-1 alpha/CXCR4 activation resulted in phosphorylation of the p70S6 kinase and its target protein, ribosomal S6 protein, and also activation of the ERK1/ERK2 signaling pathways. Furthermore, SDF-1 alpha/CXCR4- mediated activation of the p70S6 kinase and phosphorylation of the S6 protein were inhibited by treatment with an mTOR/FRAP inhibitor. The specificity of the CXCR4-mediated migration of ARO cells was demonstrated by the dose-dependent inhibition of migration by neutralizing anti-CXCR4. The ATC cells, FRO and NPA, which do not express CXCR4, did not demonstrate significant SDF-1 alpha-mediated migration in vitro. In addition, the CXCR4-mediated migration of ARO cells was inhibited by treatment with pertussis toxin (a Gi-protein inhibitor) and PD 98059 (a mitogen-activated ERK kinase inhibitor) but not by LY294002 and wortmanin, phosphatidylinositol 3-kinase inhibitors. These findings suggest that a subset of ATC cells expresses functional CXCR4, which may be important in tumor cell migration and local tumor invasion.
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PMID:CXC chemokine receptor 4 expression and function in human anaplastic thyroid cancer cells. 1251 84

Cholecystokinin (CCK) acting through its G protein-coupled receptor is now known to activate a variety of intracellular signaling mechanisms and thereby regulate a complex array of cellular functions in pancreatic acinar cells. The best studied mechanism is the coupling through heterotrimeric G proteins of the Gq family to activate a phospholipase C leading to an increase in inositol trisphosphate and release of intracellular Ca2+. This pathway along with protein kinase C activation in response to the increase in diacylglycerol stimulates the secretion of digestive enzymes by the process of exocytosis. CCK also activates signaling pathways in acini more related to other processes. The three mitogen activated protein kinase cascades leading to ERKs, JNKs and p38 MAPK are all activated by CCK. CCK activates the ERK cascade by PKC activation of Raf which in turn activates MEK and ERKs. JNKs are activated by a distinct mechanism which requires higher concentrations of CCK. Both ERKs and JNKs are presumed to regulate gene expression. CCK activation of p38 MAPK also plays a role in regulating the actin cytoskeleton through phosphorylation of the small heat shock protein HSP27. The PI3K-PKB-mTOR pathway is activated by CCK and plays a major role in regulating protein synthesis at the translational level. This includes both activation of p70 S6K leading to phosphorylation of ribosomal protein S6 and the phosphorylation of the binding protein for initiation factor 4E leading to formation of the mRNA cap binding complex. Other signaling pathways activated by CCK receptors include NF-kappaB and a variety of tyrosine kinases. Further work is needed to understand how CCK receptors activate most of the above pathways and to better understand the biological events regulated by these diverse signaling pathways.
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PMID:Cholecystokinin activates a variety of intracellular signal transduction mechanisms in rodent pancreatic acinar cells. 1268 72


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