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)

In this study we have investigated the effects of insulin, chemical and hyperthermic stresses upon the activity of the System A amino acid transporter in L6 rat muscle cells. Uptake of alpha-methyl-aminoisobutyric acid (Me-AIB), a non-metabolisable System A substrate, was increased by between 50% and 80% when muscle cells were exposed to a maximally effective concentration of insulin (100 nM), sodium arsenite (ARS, 0.5 mM) or a 42 degrees C heat shock (HS). The observed activation in System A in response to all three stimuli was maximal within 20 min and in the case of insulin and ARS primarily involved an increase in the Vmax of System A transport. In contrast, HS induced significant increases in both Vmax and Km of System A transport suggesting that hyperthermic stress may activate System A by a mechanism distinct from that mediating the effects of insulin and ARS. The hormonal stimulation of System A was blocked by the phosphoinositide 3-kinase (PI3k) inhibitor, wortmannin, but not by rapamycin or PD 98059 which respectively inhibit the mTOR and classical MAP kinase pathways. Exposure of L6 cells to ARS, but not HS, caused a 4.7-fold stimulation in MAPKAP-K2 activity that was blocked by SB 203580, a specific inhibitor of the stress activated protein kinase SAPK2/p38. However, neither SB 203580, rapamycin nor wortmannin were able to suppress the ARS- or HS-induced stimulation in System A transport. In summary, our results demonstrate that activity of the System A transporter can be rapidly upregulated in response to hormonal and stress stimuli through changes in the transport kinetics of the System A carrier. Our data show that whilst the hormonal response is PI3k dependent, the signalling mechanisms which instigate changes in System A activity in response to chemical or hyperthermic stress do not appear to involve PI3k or components of the mTOR, p42/p44 MAP kinase or SAPK2/p38 signalling pathways.
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PMID:Regulation of System A amino acid transport in L6 rat skeletal muscle cells by insulin, chemical and hyperthermic stress. 987 56

In mouse embryo NIH 3T3 fibroblasts, ethanol (60-80 mM) was found to enhance the stimulatory effects of sphingosine 1-phosphate (S1P) on both DNA synthesis and cell proliferation. Well-detectable potentiating effects of ethanol on S1P-induced mitogenesis required the presence of calcium (>1 mM) and zinc (20-40 microM) in the incubation medium. The amphibian tetrapeptide bombesin, which is known to mobilize intracellular calcium in fibroblasts, had no effect alone, but it approximately doubled the combined stimulatory effects of ethanol and S1P on DNA synthesis. The synergistic mitogenic effects of ethanol and S1P were also slightly enhanced, rather than inhibited, by the alcohol dehydrogenase inhibitor 4-methylpyrazole (5 mM). Of the various growth regulatory enzymes examined, ethanol detectably enhanced the stimulatory effects of S1P on the phosphosphorylation (activation) of p42/p44 mitogen-activated protein (MAP) kinases, but not of p38 MAP kinase. Cotreatment of fibroblasts with ethanol for 10 min also enhanced the stimulatory effects of S1P on the activities of c-Raf-1 kinase and p70 S6 kinase, but neither S1P nor ethanol had effects on phosphatidylinositol 3'-kinase and Akt/PKB kinase activities. Ethanol-plus-S1P-induced DNA synthesis was partially inhibited by both PD 98059 (50 microM) and rapamycin (10 nM), inhibitors of p42/p44 MAP kinase kinase and mTOR/p70 S6 kinases, respectively. The results indicate that in NIH 3T3 fibroblasts, ethanol can enhance the mitogenic effects of S1P by a zinc- and calcium-dependent mechanism involving both the rapamycin-sensitive p70 S6 kinase-dependent and the c-Raf-1/MAP kinase-dependent growth regulatory pathways.
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PMID:Ethanol potentiates the mitogenic effects of sphingosine 1-phosphate by a zinc- and calcium-dependent mechanism in fibroblasts. 1033 73

Endothelin-1 (ET-1), a potent endothelium-derived vasoconstrictor peptide, exerts a growth-promoting effect on vascular smooth muscle cells, implicating its pathogenic role in vascular remodeling. To gain insight into the cellular and molecular mechanism whereby ET-1 induces vascular growth, we studied whether transactivation of receptor tyrosine kinases, such as epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor, are required for activation of p42/p44 mitogen-activated protein (MAP) kinase and p70 S6 kinase (p70S6K), and subsequent growth-promotion by ET-1 in cultured rat vascular smooth muscle cells. Immunoblotting with antiphosphotyrosine antibody revealed that ET-1 rapidly (within 2 min) and transiently induced tyrosine phosphorylation of several proteins, among which 180-kDa protein was shown to be EGFR. ET-1 rapidly increased association of EGFR and Shc with glutathione-S-transferase-Grb2 fusion protein. The ET-1-induced activation of MAP kinase was reduced by an EGFR kinase inhibitor (AG1478) but not by a platelet-derived growth factor receptor kinase inhibitor (AG1296). AG1478 dose-dependently decreased ET-1-stimulated MAP kinase activity as well as [3H]leucine and [3H]thymidine uptake. The ET-1-induced tyrosine phosphorylation of EGFR, as well as MAP kinase activation, was inhibited by an ETA receptor antagonist and intracellular Ca2+ antagonists but not by an ETB receptor antagonist, pertussis toxin, or protein kinase C inhibitors. In addition, dominant negative mutant of H-Ras and a MAP kinase kinase (MEK-1) inhibitor (PD98059) completely blocked ET-1-induced MAP kinase activation as well as [3H]leucine and [3H]thymidine uptake. Both AG1478 and PD98059 inhibited ET-1-induced phosphorylation and activation of p70S6K. Furthermore, rapamycin, a selective inhibitor of mammalian target of rapamycin, completely blocked ET-1-stimulated [3H]leucine and [3H]thymidine uptake. These results suggest that ETA receptor-mediated vascular growth by ET-1 requires both MAP kinase and p70S6K cascades mediated partly via Ca2+-dependent EGFR transactivation.
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PMID:Endothelin-mediated vascular growth requires p42/p44 mitogen-activated protein kinase and p70 S6 kinase cascades via transactivation of epidermal growth factor receptor. 1049 23

Amino acid availability is known to regulate diverse cell processes including the activation of p70 S6 kinase, initiation factors involved in mRNA translation, gene expression and cellular amino acid uptake. Essential amino acids, in particular the branched-chain amino acids (e.g. leucine), have been shown to be the dominant players in mediating these effects, although the precise nature by which they regulate these processes remain poorly understood. In this study we have investigated the mechanisms involved in the leucine-induced modulation of p70 S6 kinase and addressed whether this kinase participates in the up-regulation of the System A amino acid transporter in L6 muscle cells. Incubation of muscle cells that had been amino acid-deprived for 1 h with L-leucine (2 mM) led to a rapid (>2-fold) activation of p70 S6 kinase, which was suppressed by both wortmannin and rapamycin. Consistent with this finding, addition of leucine caused a rapid ( approximately 5-fold) but transient stimulation of phosphatidylinositol 3-kinase (PI3K). PI3K activation was inhibited by wortmannin and was not dependent upon insulin receptor substrate-1 activation. Unlike stimulation by insulin, activation of neither protein kinase B nor p42/p44 mitogen-activated protein kinase accompanied the leucine-induced stimulation of PI3K. However, the leucine-induced activation of PI3K and p70 S6 kinase did result in the concomitant inactivation of glycogen synthase kinase-3 (GSK-3). Leucine enhanced System A transport by approximately 50%. We have shown previously that this stimulation is protein-synthesis-dependent and in the current study we show that it was blocked by both wortmannin and rapamycin. Our findings indicate that PI3K and the mammalian target of rapamycin are components of a nutrient signalling pathway that regulates the activation of p70 S6 kinase and induction of System A in L6 cells. The activation of this pathway by leucine is also responsible for the inactivation of GSK-3, and this is likely to have important regulatory implications for translation initiation.
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PMID:L-leucine availability regulates phosphatidylinositol 3-kinase, p70 S6 kinase and glycogen synthase kinase-3 activity in L6 muscle cells: evidence for the involvement of the mammalian target of rapamycin (mTOR) pathway in the L-leucine-induced up-regulation of system A amino acid transport. 1094 49

The potent vasoconstrictor arginine vasopressin (AVP) is also a mitogen for mesangial cells. Treatment with AVP decreased transit time through the cell cycle. AVP-stimulated mesangial cell growth by activating both the Ras mitogen-activated protein kinase (MAPK) and the phosphatidylinositol 3-kinase (PI3K) cell signaling pathways. Both the selective PI3K inhibitor LY-294002 and the MAPK kinase (MEK) inhibitor PD-98059 inhibited AVP-stimulated mesangial cell proliferation. However, LY-294002 was more potent, indicating an important role for PI3K activation in AVP-stimulated mesangial cell proliferation. AVP appeared to exert its effect on MAPK and PI3K activation, as well as on cell proliferation, by activating the epidermal growth factor receptor (EGF-R). Pretreatment with the tyrphostin-derived EGF-R antagonist AG-1478 inhibited mesangial cell proliferation as well as the activation of extracellular signal-regulated kinase 1/2 (ERK1/2 or p42/p44(MAPK)), and p70S6 kinase, a downstream effector of PI3K, providing evidence that MAPK and PI3K activation, respectively, occurred downstream of EGF-R activation. Treatment with rapamycin, an inhibitor of the p70S6 kinase activator mTOR, also resulted in growth inhibition, further suggesting the importance of the PI3K signaling pathway in AVP-induced proliferation. AVP treatment appeared to transactivate EGF-R by inducing tyrosine phosphorylation of the Ca(2+)/protein kinase C (PKC)-dependent nonreceptor tyrosine kinase, Pyk2, leading to Pyk2/c-Src association and c-Src activation. This was followed by association of c-Src with EGF-R and EGF-R activation. These data suggested that AVP-stimulated Pyk2 tyrosine phosphorylation to activate c-Src, thereby leading to EGF-R transactivation.
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PMID:Arginine vasopressin stimulates mesangial cell proliferation by activating the epidermal growth factor receptor. 1135 36

Hepatic expression of insulin-like growth factor-binding protein-1 (IGFBP-1) is rapidly and completely inhibited by insulin. The signalling pathway that mediates this effect of insulin requires the activation of phosphoinositide 3-kinase (PI 3-kinase). Many of the cellular actions of insulin, including activation of PI 3-kinase, can be 'mimicked' by oxidative stresses, such as H(2)O(2). In the present study, we demonstrate that H(2)O(2) does not 'mimic' but rather antagonizes insulin repression of IGFBP-1 gene expression in H4IIE cells. This effect is accompanied by a decrease in the insulin-induced activation of mammalian target of rapamycin (mTOR)-dependent signalling. However, insulin-induced phosphorylation and regulation of protein kinase B, glycogen synthase kinase-3 and FKHR (forkhead in rhabdomyosarcoma) are not affected by H(2)O(2) in the same cells. In addition, H(2)O(2) strongly activates the p42/p44 mitogen-activated protein kinases, but the presence of PD184352 (an inhibitor of this pathway) does not block the effect of H(2)O(2) on IGFBP-1 gene expression. Our results support the view that the insulin-mediated repression of IGFBP-1 gene expression is partly mTOR-dependent, and demonstrate that H(2)O(2) selectively antagonizes mTOR-dependent insulin action. The implications for the use of H(2)O(2)-generating agents as therapeutics for the treatment of insulin resistance, as well as the role of oxidative stress in the development of insulin resistance, are discussed.
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PMID:Insulin regulation of hepatic insulin-like growth factor-binding protein-1 (IGFBP-1) gene expression and mammalian target of rapamycin (mTOR) signalling is impaired by the presence of hydrogen peroxide. 1194 57

Impaired glucose tolerance precedes type 2 diabetes and is characterized by hyperinsulinemia, which develops to balance peripheral insulin resistance. To gain insight into the deleterious effects of hyperinsulinemia on skeletal muscle, we studied the consequences of prolonged insulin treatment of L6 myoblasts on insulin-dependent signaling pathways. A 24-h long insulin treatment desensitized the phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB) and p42/p44 MAPK pathways toward a second stimulation with insulin or insulin-like growth factor-1 and led to decreased insulin-induced glucose uptake. Desensitization was correlated to a reduction in insulin receptor substrate (IRS)-1 and IRS-2 protein levels, which was reversed by the PI3K inhibitor LY294002. Co-treatment of cells with insulin and LY294002, while reducing total IRS-1 phosphorylation, increased its phosphotyrosine content, enhancing IRS-1/PI3K association. PDK1, mTOR, and MAPK inhibitors did not block insulin-induced reduction of IRS-1, suggesting that the PI3K serine-kinase activity causes IRS-1 serine phosphorylation and its commitment to proteasomal degradation. Contrarily, insulin-induced IRS-2 down-regulation occurred via a PI3K/mTOR pathway. Suppression of IRS-1/2 down-regulation by LY294002 rescued the responsiveness of PKB and MAPK toward acute insulin stimulation. Conversely, adenoviral-driven expression of constitutively active PI3K induced an insulin-independent reduction in IRS-1/2 protein levels. IRS-2 appears to be the chief molecule responsible for MAPK and PKB activation by insulin, as knockdown of IRS-2 (but not IRS-1) by RNA interference severely impaired activation of both kinases. In summary, (i) PI3K mediates insulin-induced reduction of IRS-1 by phosphorylating it while a PI3K/mTOR pathway controls insulin-induced reduction of IRS-2, (ii) in L6 cells, IRS-2 is the major adapter molecule linking the insulin receptor to activation of PKB and MAPK, (iii) the mechanism of IRS-1/2 down-regulation is different in L6 cells compared with 3T3-L1 adipocytes. In conclusion, the reduction in IRS proteins via different PI3K-mediated mechanisms contributes to the development of an insulin-resistant state in L6 myoblasts.
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PMID:Phosphoinositide 3-kinase-mediated reduction of insulin receptor substrate-1/2 protein expression via different mechanisms contributes to the insulin-induced desensitization of its signaling pathways in L6 muscle cells. 1259 28

We previously reported that p70 S6 kinase takes part in bone morphogenetic protein-4 (BMP-4)-stimulated vascular endothelial growth factor (VEGF) synthesis in osteoblast-like MC3T3-E1 cells. Recently, we showed that BMP-4-induced osteocalcin synthesis is regulated by p44/p42 MAP kinase and p38 MAP kinase in these cells. In the present study, we investigated whether the MAP kinases are involved in the BMP-4-stimulated synthesis of VEGF in MC3T3-E1 cells. PD-98059 and U-0126, inhibitors of the upstream kinase of p44/p42 MAP kinase, failed to affect BMP-4-stimulated VEGF synthesis. SB-203580 and PD-169316, inhibitors of p38 MAP kinase, significantly reduced VEGF synthesis, whereas SB-202474, a negative control for p38 MAP kinase inhibitor, had little effect on VEGF synthesis. The BMP-4-stimulated phosphorylation of p38 MAP kinase was not affected by rapamycin, an inhibitor of p70 S6 kinase. On the contrary, SB-203580 and PD-169316 reduced the BMP-4-stimulated phosphorylation of p70 S6 kinase. In addition, anisomycin, an activator of p38 MAP kinase, phosphorylates p70 S6 kinase, and the phosphorylation was suppressed by SB-203580. LY-294002, an inhibitor of phosphatidylinositol 3-kinase, failed to suppress the phosphorylation of p38 MAP kinase induced by BMP-4. Not BMP-4 but anisomycin weakly induced the phosphorylation of phosphoinositide-dependent kinase-1. However, anisomycin had little effect on phosphorylation of either Akt or the mammalian target of rapamycin. Taken together, our results suggest that p38 MAP kinase functions in BMP-4-stimulated VEGF synthesis as a positive regulator at a point upstream from p70 S6 kinase in osteoblasts.
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PMID:p38 MAP kinase regulates BMP-4-stimulated VEGF synthesis via p70 S6 kinase in osteoblasts. 1263 56

Cdk5 regulates myogenesis but the signaling cascade through which Cdk5 modulates this process remains to be characterized. Here, we investigated whether PI3K, Akt, p70S6K, p38 MAPK, p44/42 MAPK, and Egr-1 serve as upstream regulators of Cdk5 during L6 myoblast differentiation. Upon serum reduction, we found that besides elevated expression of Cdk5 and its activator, p35, and increased Cdk5/p35 activity, Egr-1, Akt, p70S6K, and p38 MAPK activity were upregulated in differentiating L6 cells. However, p44/42 MAPK was downregulated and SAPK/JNK was unaffected. LY294002, a PI3K inhibitor, blocked the activation of Akt and p70S6K, indicating that Akt and p70S6K activation is linked to PI3K activation. The lack of LY294002 effect on p38 MAPK suggests that p38 MAPK activation is not associated with PI3K activation. Rapamycin, a specific inhibitor of FRAP/mTOR (the upstream kinase of p70S6K), also blocked p70S6K activation, indicating the involvement of FRAP/mTOR activation. LY294002 and rapamycin also blocked the enhancement of Egr-1 level, Cdk5 activity, and myogenin expression, suggesting that upregulation of these factors is coupled to PI3K-p70S6K activation. Overexpression of dominant-negative-Akt also reduced Cdk5/p35 activity and myogenin expression, indicating that the PI3K-p70S6K-Egr-1-Cdk5 signaling cascade is linked to Akt activation. SB2023580, a p38 MAPK inhibitor, had no effect on p70S6K, Egr-1, or Cdk5 activity, suggesting that p38 MAPK activation lies in a pathway distinct from the PI3K-Akt-p70S6K-Egr-1 pathway that we identify as the upstream modulator of Cdk5 activity during L6 myoblast differentiation.
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PMID:L6 myoblast differentiation is modulated by Cdk5 via the PI3K-AKT-p70S6K signaling pathway. 1520 59

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a transmissible lung cancer of sheep. The virus can induce tumors rapidly, and we previously found that the JSRV envelope protein (Env) functions as an oncogene, because it can transform mammalian and avian fibroblast cell lines. (N. Maeda, Proc. Natl. Acad. Sci. USA 98:4449-4454, 2001). The molecular mechanisms of JSRV Env transformation are of considerable interest. Several reports suggested that the phosphatidylinositol 3-kinase/Akt pathway is important for transformation of mammalian fibroblasts but not for chicken fibroblasts. In this study, we found that Akt/mTOR is involved in JSRV transformation of mouse NIH 3T3 fibroblasts, because treatment with the mTOR inhibitor rapamycin reduced transformation. We also found that H/N-Ras inhibitor FTI-277 and MEK1/2 inhibitors PD98059 and U0126 strongly inhibited JSRV transformation of NIH 3T3 fibroblasts, suggesting that the H/N-Ras-MEK-mitogen-activated protein kinase (MAPK) p44/42 pathway is necessary for the transformation. In RK3E epithelial cells, the MEK1/2 inhibitors also eliminated transformation, but FTI-277 only partially inhibited transformation. It was noteworthy that p38 MAPK inhibitors enhanced JSRV transformation in both fibroblasts and epithelial cells. Treatment of transformed cells with p38 inhibitors both increased levels of phospho-MEK1/2 and phospho-p44/42 and induced rapid enhancement of the transformed phenotype. Immunohistochemical staining of tumor tissues from naturally and experimentally induced OPA and naturally occurring enzootic nasal adenocarcinoma revealed strong activation of MAPK p44/42 in all cases examined. However, p38 activation was not generally observed. These results indicate that signaling through two pathways (in particular, H/N-Ras-MEK-MAPK and, to a lesser extent, Akt-mTOR) is important for JSRV-induced transformation and that p38 MAPK has a negative regulatory effect on transformation, perhaps via MEK1/2 and p44/42.
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PMID:Roles of the Ras-MEK-mitogen-activated protein kinase and phosphatidylinositol 3-kinase-Akt-mTOR pathways in Jaagsiekte sheep retrovirus-induced transformation of rodent fibroblast and epithelial cell lines. 1576 44


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