Showing papers on "PI3K/AKT/mTOR pathway published in 2001"
TL;DR: It is concluded that the activation of the Akt/mTOR pathway and its downstream targets, p70S6K and PHAS-1/4E-BP1, is requisitely involved in regulating skeletal muscle fibre size, and that activation of this pathway can oppose muscle atrophy induced by disuse.
Abstract: Skeletal muscles adapt to changes in their workload by regulating fibre size by unknown mechanisms. The roles of two signalling pathways implicated in muscle hypertrophy on the basis of findings in vitro, Akt/mTOR (mammalian target of rapamycin) and calcineurin/NFAT (nuclear factor of activated T cells), were investigated in several models of skeletal muscle hypertrophy and atrophy in vivo. The Akt/mTOR pathway was upregulated during hypertrophy and downregulated during muscle atrophy. Furthermore, rapamycin, a selective blocker of mTOR, blocked hypertrophy in all models tested, without causing atrophy in control muscles. In contrast, the calcineurin pathway was not activated during hypertrophy in vivo, and inhibitors of calcineurin, cyclosporin A and FK506 did not blunt hypertrophy. Finally, genetic activation of the Akt/mTOR pathway was sufficient to cause hypertrophy and prevent atrophy in vivo, whereas genetic blockade of this pathway blocked hypertrophy in vivo. We conclude that the activation of the Akt/mTOR pathway and its downstream targets, p70S6K and PHAS-1/4E-BP1, is requisitely involved in regulating skeletal muscle fibre size, and that activation of the Akt/mTOR pathway can oppose muscle atrophy induced by disuse.
2,439 citations
TL;DR: It is shown that Akt promotes hypertrophy by activating downstream signalling pathways previously implicated in activating protein synthesis: the pathways downstream of mammalian target of rapamycin (mTOR) and the pathway activated by phosphorylating and thereby inhibiting glycogen synthase kinase 3 (GSK3).
Abstract: Skeletal muscle is composed of multinucleated fibres, formed after the differentiation and fusion of myoblast precursors. Skeletal muscle atrophy and hypertrophy refer to changes in the diameter of these pre-existing muscle fibres. The prevention of atrophy would provide an obvious clinical benefit; insulin-like growth factor 1 (IGF-1) is a promising anti-atrophy agent because of its ability to promote hypertrophy. However, the signalling pathways by which IGF-1 promotes hypertrophy remain unclear, with roles suggested for both the calcineurin/NFAT (nuclear factor of activated T cells) pathway and the PtdIns-3-OH kinase (PI(3)K)/Akt pathway. Here we employ a battery of approaches to examine these pathways during the hypertrophic response of cultured myotubes to IGF-1. We report that Akt promotes hypertrophy by activating downstream signalling pathways previously implicated in activating protein synthesis: the pathways downstream of mammalian target of rapamycin (mTOR) and the pathway activated by phosphorylating and thereby inhibiting glycogen synthase kinase 3 (GSK3). In contrast, in addition to demonstrating that calcineurin does not mediate IGF-1-induced hypertrophy, we show that IGF-1 unexpectedly acts via Akt to antagonize calcineurin signalling during myotube hypertrophy.
1,491 citations
TL;DR: The demonstration that PI3-kinase/Akt signaling affects Mdm2 localization provides insight into how this pathway, which is inappropriately activated in many malignancies, affects the function of p53.
Abstract: The Mdm2 oncoprotein promotes cell survival and cell cycle progression by inhibiting the p53 tumor suppressor protein. To regulate p53, Mdm2 must gain nuclear entry, and the mechanism that induces this is now identified. Mitogen-induced activation of phosphatidylinositol 3-kinase (PI3-kinase) and its downstream target, the Akt/PKB serine-threonine kinase, results in phosphorylation of Mdm2 on serine 166 and serine 186. Phosphorylation on these sites is necessary for translocation of Mdm2 from the cytoplasm into the nucleus. Pharmacological blockade of PI3-kinase/Akt signaling or expression of dominant-negative PI3-kinase or Akt inhibits nuclear entry of Mdm2, increases cellular levels of p53, and augments p53 transcriptional activity. Expression of constitutively active Akt promotes nuclear entry of Mdm2, diminishes cellular levels of p53, and decreases p53 transcriptional activity. Mutation of the Akt phosphorylation sites in Mdm2 produces a mutant protein that is unable to enter the nucleus and increases p53 activity. The demonstration that PI3-kinase/Akt signaling affects Mdm2 localization provides insight into how this pathway, which is inappropriately activated in many malignancies, affects the function of p53.
1,209 citations
TL;DR: It is demonstrated that statins potently augment endothelial progenitor cell differentiation in mononuclear cells and CD34-positive hematopoietic stem cells isolated from peripheral blood.
Abstract: HMG-CoA reductase inhibitors (statins) have been developed as lipid-lowering drugs and are well established to reduce morbidity and mortality from coronary artery disease. Here we demonstrate that statins potently augment endothelial progenitor cell differentiation in mononuclear cells and CD34-positive hematopoietic stem cells isolated from peripheral blood. Moreover, treatment of mice with statins increased c-kit(+)/Sca-1(+)--positive hematopoietic stem cells in the bone marrow and further elevated the number of differentiated endothelial progenitor cells (EPCs). Statins induce EPC differentiation via the PI 3-kinase/Akt (PI3K/Akt) pathway as demonstrated by the inhibitory effect of pharmacological PI3K blockers or overexpression of a dominant negative Akt construct. Similarly, the potent angiogenic growth factor VEGF requires Akt to augment EPC numbers, suggesting an essential role for Akt in regulating hematopoietic progenitor cell differentiation. Given that statins are at least as potent as VEGF in increasing EPC differentiation, augmentation of circulating EPC might importantly contribute to the well-established beneficial effects of statins in patients with coronary artery disease.
1,207 citations
TL;DR: This review uses transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated as a framework to build a profile of PI3k function within both the cell and the organism and focuses, in particular, on the role ofPI3K in cell regulation, immunity, and development.
Abstract: The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.
1,119 citations
TL;DR: Three-dimensional structures of the Trk receptors, in one instance in association with a neurotrophin, have revealed the structural bases underlying specificity in neurotrophIn signaling, where key intermediates are localized to different membrane compartments.
1,103 citations
TL;DR: It is shown that HER-2/neu-mediated cell growth requires the activation of Akt, which associates with p 21Cip1/WAF1 and phosphorylates it at threonine 145, resulting in cytoplasmic localization of p21Cip 1/Waf1.
Abstract: Amplification or overexpression of HER-2/neu in cancer cells confers resistance to apoptosis and promotes cell growth. The cellular localization of p21Cip1/WAF1 has been proposed to be critical either in promoting cell survival or in inhibiting cell growth. Here we show that HER-2/neu-mediated cell growth requires the activation of Akt, which associates with p21Cip1/WAF1 and phosphorylates it at threonine 145, resulting in cytoplasmic localization of p21Cip1/WAF1. Furthermore, blocking the Akt pathway with a dominant-negative Akt mutant restores the nuclear localization and cell-growth-inhibiting activity of p21Cip1/WAF1. Our results indicate that HER-2/neu induces cytoplasmic localization of p21Cip1/WAF1 through activation of Akt to promote cell growth, which may have implications for the oncogenic activity of HER-2/neu and Akt.
1,093 citations
TL;DR: The involvement of PA in mTOR signaling reveals an important function of this lipid in signal transduction and protein synthesis, as well as a direct link between mTOR and mitogens, and suggests a potential mechanism for the in vivo actions of the immunosuppressant rapamycin.
Abstract: The mammalian target of rapamycin (mTOR) governs cell growth and proliferation by mediating the mitogen- and nutrient-dependent signal transduction that regulates messenger RNA translation. We identified phosphatidic acid (PA) as a critical component of mTOR signaling. In our study, mitogenic stimulation of mammalian cells led to a phospholipase D-dependent accumulation of cellular PA, which was required for activation of mTOR downstream effectors. PA directly interacted with the domain in mTOR that is targeted by rapamycin, and this interaction was positively correlated with mTOR's ability to activate downstream effectors. The involvement of PA in mTOR signaling reveals an important function of this lipid in signal transduction and protein synthesis, as well as a direct link between mTOR and mitogens. Furthermore, these studies suggest a potential mechanism for the in vivo actions of the immunosuppressant rapamycin.
1,027 citations
TL;DR: It is shown that the D AF-2 pathway prevents DAF-16 accumulation in nuclei, and it is found that both sensory neurons and germline activity regulate DAF/IGF-1 signaling, but the nuclear localization patterns are different, which reveal unexpected complexity in the Daf-16-dependent pathways that regulate aging.
Abstract: The lifespan of Caenorhabditis elegans is regulated by the insulin/insulin-like growth factor (IGF)-1 receptor homolog DAF-2, which signals through a conserved phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathway. Mutants in this pathway remain youthful and active much longer than normal animals and can live more than twice as long. This lifespan extension requires DAF-16, a forkhead/winged-helix transcription factor. DAF-16 is thought to be the main target of the DAF-2 pathway. Insulin/IGF-1 signaling is thought to lead to phosphorylation of DAF-16 by AKT activity, which in turn shortens lifespan. Here, we show that the DAF-2 pathway prevents DAF-16 accumulation in nuclei. Disrupting Akt-consensus phosphorylation sites in DAF-16 causes nuclear accumulation in wild-type animals, but, surprisingly, has little effect on lifespan. Thus the DAF-2 pathway must have additional outputs. Lifespan in C. elegans can be extended by perturbing sensory neurons or germ cells. In both cases, lifespan extension requires DAF-16. We find that both sensory neurons and germline activity regulate DAF-16 accumulation in nuclei, but the nuclear localization patterns are different. Together these findings reveal unexpected complexity in the DAF-16-dependent pathways that regulate aging.
1,020 citations
TL;DR: In vitro and in vivo studies of isogenic PTEN+/+ and PTEN−/− mouse cells as well as human cancer cells with defined PTEN status showed that the growth of PTEN null cells was blocked preferentially by pharmacologic FRAP/mTOR inhibition, indicating that FRAP-mTOR functions downstream of Akt in tumorigenesis.
Abstract: Recent evidence places the FRAP/mTOR kinase downstream of the phosphatidyl inositol 3-kinase/Akt-signaling pathway, which is up-regulated in multiple cancers because of loss of the PTEN tumor suppressor gene. We performed biological and biochemical studies to determine whether PTEN-deficient cancer cells are sensitive to pharmacologic inhibition of FRAP/mTOR by using the rapamycin derivative CCI-779. In vitro and in vivo studies of isogenic PTEN+/+ and PTEN−/− mouse cells as well as human cancer cells with defined PTEN status showed that the growth of PTEN null cells was blocked preferentially by pharmacologic FRAP/mTOR inhibition. Enhanced tumor growth caused by constitutive activation of Akt in PTEN+/+ cells also was reversed by CCI-779 treatment, indicating that FRAP/mTOR functions downstream of Akt in tumorigenesis. Loss of PTEN correlated with increased S6 kinase activity and phosphorylation of ribosomal S6 protein, providing evidence for activation of the FRAP/mTOR pathway in these cells. Differential sensitivity to CCI-779 was not explained by differences in biochemical blockade of the FRAP/mTOR pathway, because S6 phosphorylation was inhibited in sensitive and resistant cell lines. These results provide rationale for testing FRAP/mTOR inhibitors in PTEN null human cancers.
988 citations
TL;DR: A molecular link between activation of the PI 3-kinase/AKT survival pathways, hormone-independent activation of ERα, and inhibition of tamoxifen-induced apoptotic regression is defined.
TL;DR: This study indicates that blocking the Akt pathway mediated by HER-2/neu would increase the cytotoxic effect of DNA-damaging drugs in tumour cells with wild-type p53.
Abstract: HER-2/neu amplification or overexpression can make cancer cells resistant to apoptosis and promotes their growth. p53 is crucial in regulating cell growth and apoptosis, and is often mutated or deleted in many types of tumour. Moreover, many tumours with a wild-type gene for p53 do not have normal p53 function, suggesting that some oncogenic signals suppress the function of p53. In this study, we show that HER-2/neu-mediated resistance to DNA-damaging agents requires the activation of Akt, which enhances MDM2-mediated ubiquitination and degradation of p53. Akt physically associates with MDM2 and phosphorylates it at Ser166 and Ser186. Phosphorylation of MDM2 enhances its nuclear localization and its interaction with p300, and inhibits its interaction with p19ARF, thus increasing p53 degradation. Our study indicates that blocking the Akt pathway mediated by HER-2/neu would increase the cytotoxic effect of DNA-damaging drugs in tumour cells with wild-type p53.
TL;DR: It is demonstrated that the mTOR pathway is influenced by the intracellular concentration of ATP, independent of the abundance of amino acids, and that mTOR itself is an ATP sensor.
Abstract: The bacterial macrolide rapamycin is an efficacious anticancer agent against solid tumors. In a hypoxic environment, the increase in mass of solid tumors is dependent on the recruitment of mitogens and nutrients. When nutrient concentrations change, particularly those of essential amino acids, the mammalian Target of Rapamycin (mTOR) functions in regulatory pathways that control ribosome biogenesis and cell growth. In bacteria, ribosome biogenesis is independently regulated by amino acids and adenosine triphosphate (ATP). Here we demonstrate that the mTOR pathway is influenced by the intracellular concentration of ATP, independent of the abundance of amino acids, and that mTOR itself is an ATP sensor.
TL;DR: SGK1, like Akt, promotes cell survival and that it does so in part by phosphorylating and inactivating FKHRL1, however, SGK and Akt display differences with respect to the efficacy with which they phosphorylate the three regulatory sites on FKhRL1.
Abstract: Serum- and glucocorticoid-inducible kinases (SGKs) form a novel family of serine/threonine kinases that are activated in response to a variety of extracellular stimuli. SGKs are related to Akt (also called PKB), a serine/threonine kinase that plays a crucial role in promoting cell survival. Like Akt, SGKs are activated by the phosphoinositide-3 kinase (PI3K) and translocate to the nucleus upon growth factor stimulation. However the physiological substrates and cellular functions of SGKs remained to be identified. We hypothesized that SGKs regulate cellular functions in concert with Akt by phosphorylating common targets within the nucleus. The best-characterized nuclear substrates of Akt are transcription factors of the Forkhead family. Akt phosphorylates Forkhead transcription factors such as FKHRL1, leading to FKHRL1's exit from the nucleus and the consequent shutoff of FKHRL1 target genes. We show here that SGK1, like Akt, promotes cell survival and that it does so in part by phosphorylating and inactivating FKHRL1. However, SGK and Akt display differences with respect to the efficacy with which they phosphorylate the three regulatory sites on FKHRL1. While both kinases can phosphorylate Thr-32, SGK displays a marked preference for Ser-315 whereas Akt favors Ser-253. These findings suggest that SGK and Akt may coordinately regulate the function of FKHRL1 by phosphorylating this transcription factor at distinct sites. The efficient phosphorylation of these three sites on FKHRL1 by SGK and Akt appears to be critical to the ability of growth factors to suppress FKHRL1-dependent transcription, thereby preventing FKHRL1 from inducing cell cycle arrest and apoptosis. These findings indicate that SGK acts in concert with Akt to propagate the effects of PI3K activation within the nucleus and to mediate the biological outputs of PI3K signaling, including cell survival and cell cycle progression.
TL;DR: Results provide the first direct link between Akt and the family of stress-activated kinases and reduce apoptosis induced by ASK1 in a manner dependent on phosphorylation of serine 83 ofASK1.
Abstract: The Akt family of serine/threonine-directed kinases promotes cellular survival in part by phosphorylating and inhibiting death-inducing proteins. Here we describe a novel functional interaction between Akt and apoptosis signal-regulating kinase 1 (ASK1), a mitogen-activated protein kinase kinase kinase. Akt decreased ASK1 kinase activity stimulated by both oxidative stress and overexpression in 293 cells by phosphorylating a consensus Akt site at serine 83 of ASK1. Activation of the phosphoinositide 3-kinase (PI3-K)/Akt pathway also inhibited the serum deprivation-induced activity of endogenous ASK1 in L929 cells. An association between Akt and ASK1 was detected in cells by coimmunoprecipitation. Phosphorylation by Akt inhibited ASK1-mediated c-Jun N-terminal kinase and activating transcription factor 2 activities in intact cells. Finally, activation of the PI3-K/Akt pathway reduced apoptosis induced by ASK1 in a manner dependent on phosphorylation of serine 83 of ASK1. These results provide the first direct link between Akt and the family of stress-activated kinases.
TL;DR: The identification of PTEN, its mutational status in cancer, its role as a regulator of PI3K, and its domain structure are discussed.
TL;DR: It is shown that transformed cells of PTEN+/− mice have elevated levels of phosphorylated Akt and activated p70/S6 kinase associated with an increase in proliferation and that inhibition of these proteins may be therapeutic for cancer patients with deranged PI3K signaling.
Abstract: PTEN phosphatase acts as a tumor suppressor by negatively regulating the phosphoinositide 3-kinase (PI3K) signaling pathway. It is unclear which downstream components of this pathway are necessary for oncogenic transformation. In this report we show that transformed cells of PTEN+/− mice have elevated levels of phosphorylated Akt and activated p70/S6 kinase associated with an increase in proliferation. Pharmacological inactivation of mTOR/RAFT/FRAP reduced neoplastic proliferation, tumor size, and p70/S6 kinase activity, but did not affect the status of Akt. These data suggest that p70/S6K and possibly other targets of mTOR contribute significantly to tumor development and that inhibition of these proteins may be therapeutic for cancer patients with deranged PI3K signaling.
Journal Article•
TL;DR: It is shown that insulin and epidermal growth factor or an inactivating mutation in the tumor suppressor PTEN specifically increase the protein levels of hypoxia-inducible factor (HIF) 1alpha but not of HIF-1beta in human cancer cell lines.
Abstract: The phosphatidylinositol 3-kinase (PI3K) signaling pathway has inherent oncogenic potential. It is upregulated in diverse human cancers by either a gain of function in PI3K itself or in its downstream target Akt or by a loss of function in the negative regulator PTEN. However, the complete consequences of this upregulation are not known. Here we show that insulin and epidermal growth factor or an inactivating mutation in the tumor suppressor PTEN specifically increase the protein levels of hypoxia-inducible factor (HIF) 1a but not of HIF-1b in human cancer cell lines. This specific elevation of HIF-1a protein expression requires PI3K signaling. In the prostate carcinomaderived cell lines PC-3 and DU145, insulin- and epidermal growth factor-induced expression of HIF-1a was inhibited by the PI3K-specific inhibitors LY294002 and wortmannin in a dose-dependent manner. HIF-1b expression was not affected by these inhibitors.
TL;DR: The mTOR/p70 S6 kinase signaling pathway is identified as a novel modulator of insulin-stimulated glucose transport in skeletal muscle cells.
TL;DR: It is shown that PI3-K/Akt signaling mediates growth, survival, and cell cycle regulatory effects of IL-6 in MM, and LY294002 completely abrogates this signaling cascade.
Abstract: Previous studies demonstrate that interleukin-6 (IL-6) mediates growth and survival in human multiple myeloma (MM) cells via the MEK/MAPK and JAK/STAT signaling pathways, respectively. IL-6 also confers protection against Dexamethasone (Dex)-induced apoptosis via activation of protein tyrosine phosphatase (SHP2). In the current study, we characterized IL-6 triggered phophatidylinositol-3 kinase/Akt kinase (PI3-K/Akt) signaling in MM cells. IL-6 induces Akt/PKB phosphorylation in a time and dose dependent manner in MM.1S MM cells. IL-6 also induced phosphorylation of downstream targets of Akt, including Bad, GSK-3beta, and FKHR, confirming Akt activation. Inhibition of Akt activation by the PI3-K inhibitor LY294002 partially blocked IL-6 triggered MEK/MAPK activation and proliferation in MM.1S cells, suggesting cross-talk between PI3-K and MEK signaling. We demonstrate that Dex-induced apoptosis in MM.1S cells is mediated by downstream activation of caspase-9, with resultant caspase-3 cleavage; and conversely, that IL-6 triggers activation of PI3-K and its association with SHP2, inactivates caspase-9, and protects against Dex-induced apoptosis. LY294002 completely abrogates this signaling cascade, further confirming the importance of PI3-K/Akt signaling in conferring the protective effect of IL-6 against Dex-induced apoptosis. Finally, we show that IL-6 triggered PI3-K/Akt signaling in MM.1S cells inactivates forkhead transcriptional factor (FKHR), with related G1/S phase transition, whereas LY294002 blocks this signaling, resulting in upregulation of p27(KIP1) and G1 growth arrest. Our data therefore suggest that PI3-K/Akt signaling mediates growth, survival, and cell cycle regulatory effects of IL-6 in MM.
TL;DR: In this article, the authors investigate the role of β-adrenergic receptor (AR) and β-AR subtypes in regulating cardiomyocyte survival and apoptosis and explore underlying mechanisms.
Abstract: The goal of this study was to determine whether beta(1)-adrenergic receptor (AR) and beta(2)-AR differ in regulating cardiomyocyte survival and apoptosis and, if so, to explore underlying mechanisms. One potential mechanism is that cardiac beta(2)-AR can activate both G(s) and G(i) proteins, whereas cardiac beta(1)-AR couples only to G(s). To avoid complicated crosstalk between beta-AR subtypes, we expressed beta(1)-AR or beta(2)-AR individually in adult beta(1)/beta(2)-AR double knockout mouse cardiac myocytes by using adenoviral gene transfer. Stimulation of beta(1)-AR, but not beta(2)-AR, markedly induced myocyte apoptosis, as indicated by increased terminal deoxynucleotidyltransferase-mediated UTP end labeling or Hoechst staining positive cells and DNA fragmentation. In contrast, beta(2)-AR (but not beta(1)-AR) stimulation elevated the activity of Akt, a powerful survival signal; this effect was fully abolished by inhibiting G(i), G(beta gamma), or phosphoinositide 3 kinase (PI3K) with pertussis toxin, beta ARK-ct (a peptide inhibitor of G(beta gamma)), or LY294002, respectively. This indicates that beta(2)-AR activates Akt via a G(i)-G(beta gamma)-PI3K pathway. More importantly, inhibition of the G(i)-G(beta gamma)-PI3K-Akt pathway converts beta(2)-AR signaling from survival to apoptotic. Thus, stimulation of a single class of receptors, beta(2)-ARs, elicits concurrent apoptotic and survival signals in cardiac myocytes. The survival effect appears to predominate and is mediated by the G(i)-G(beta gamma)-PI3K-Akt signaling pathway.
TL;DR: Hormone stimulation of MCF‐7 cells rapidly triggers association of ERα with Src and p85, which indicates a novel reciprocal cross‐talk between PI3‐kinase and Src, which converge on cell cycle progression.
Abstract: The p85-associated phosphatidylinositol (PI) 3-kinase/Akt pathway mediates the oestradiol-induced S-phase entry and cyclin D1 promoter activity in MCF-7 cells. Experiments with Src, p85α and Akt dominant-negative forms indicate that in oestradiol-treated cells these signalling effectors target the cyclin D1 promoter. Oestradiol acutely increases PI3-kinase and Akt activities in MCF-7 cells. In NIH 3T3 cells expressing ERα, a dominant-negative p85 suppresses hormone stimulation of Akt. The Src inhibitor, PP1, prevents hormone stimulation of Akt and PI3-kinase activities in MCF-7 cells. In turn, stimulation of Src activity is abolished in ERα-expressing NIH 3T3 fibroblasts by co-transfection of the dominant-negative p85α and in MCF-7 cells by the PI3-kinase inhibitor, LY294002. These findings indicate a novel reciprocal cross-talk between PI3-kinase and Src. Hormone stimulation of MCF-7 cells rapidly triggers association of ERα with Src and p85. In vitro these proteins are assembled in a ternary complex with a stronger association than that of the binary complexes composed by the same partners. The ternary complex probably favours hormone activation of Src- and PI3-kinase-dependent pathways, which converge on cell cycle progression.
TL;DR: Although these different thyrocyte systems constitute interesting models of the wide diversity of possible mechanisms of cAMP-dependent proliferation in various cell types, extrapolation of in vitro mechanistic data to TSH-dependent goitrogenesis in man can only be accepted in the cases where independent validation is provided.
Abstract: TSH via cAMP, and various growth factors, in cooperation with insulin or IGF-I stimulate cell cycle progression and proliferation in various thyrocyte culture systems, including rat thyroid cell lines (FRTL-5, WRT, PC Cl3) and primary cultures of rat, dog, sheep and human thyroid. The available data on cell signaling cascades, cell cycle kinetics, and cell cycle-regulatory proteins are thoroughly and critically reviewed in these experimental systems. In most FRTL-5 cells, TSH (cAMP) merely acts as a priming/competence factor amplifying PI3K and MAPK pathway activation and DNA synthesis elicited by insulin/IGF-I. In WRT cells, TSH and insulin/IGF-I can independently activate Ras and PI3K pathways and DNA synthesis. In dog thyroid primary cultures, TSH (cAMP) does not activate Ras and PI3K, and cAMP must be continuously elevated by TSH to directly control the progression through G(1) phase. This effect is exerted, at least in part, via the cAMP-dependent activation of the required cyclin D3, itself synthesized in response to insulin/IGF-I. This and other discrepancies show that the mechanistic logics of cell cycle stimulation by cAMP profoundly diverge in these different in vitro models of the same cell. Therefore, although these different thyrocyte systems constitute interesting models of the wide diversity of possible mechanisms of cAMP-dependent proliferation in various cell types, extrapolation of in vitro mechanistic data to TSH-dependent goitrogenesis in man can only be accepted in the cases where independent validation is provided.
TL;DR: The data suggest that mTOR may be a good target for breast cancer therapy, especially in tumors with Akt activation resulting from either growth factor dependency or loss of PTEN function.
Abstract: The mammalian target of rapamycin (mTOR) is a central regulator of G1 cell cycle protein synthesis that precedes commitment to normal cellular replication. We have studied the effect of cell cycle inhibitor-779 (CCI-779), a rapamycin ester that inhibits mTOR function, on the proliferation of a panel of breast cancer cell lines. Six of eight lines studied were sensitive (IC(50) 1.0 microM) to CCI-779. Sensitive lines were estrogen dependent (MCF-7, BT-474, T-47D), or lacked expression of the tumor suppressor PTEN (MDA-MB-468, BT-549), and/or overexpressed the Her-2/neu oncogene (SKBR-3, BT-474). Resistant lines (MDA-MB-435, MDA-MB-231) shared none of these properties. CCI-779 (50 nM) inhibited mTOR function in both a sensitive and a resistant line. In nu/nu mouse xenografts, CCI-779 inhibited growth of MDA-MB-468 (sensitive) but not MDA-MB-435 resistant tumors. Treatment of sensitive lines with CCI-779 resulted in a decrease in D-type cyclin and c-myc levels and an increase in p27(kip-1) levels. There was good correlation between activation of the Akt pathway and sensitivity to CCI-779. Amplification of mTOR-regulated p70S6 kinase, which is downstream of Akt, may also have conferred CCI-779 sensitivity to MCF-7 cells. Taken together, the data suggest that mTOR may be a good target for breast cancer therapy, especially in tumors with Akt activation resulting from either growth factor dependency or loss of PTEN function.
TL;DR: It is demonstrated that Akt phosphorylates the androgen receptor (AR) at Ser-210 and Ser-790, providing a molecular basis for cross-talk between two signaling pathways at the level of Akt and AR–AR coregulators that may help to better understand the roles of AkT in the androgens/AR-mediated apoptosis.
Abstract: Whereas several apoptosis-related proteins have been linked to the antiapoptotic effects of Akt serine–threonine kinase, the search continues to explain the Akt signaling role in promoting cell survival via antiapoptotic effects. Here, we demonstrate that Akt phosphorylates the androgen receptor (AR) at Ser-210 and Ser-790. A mutation at AR Ser-210 results in the reversal of Akt-mediated suppression of AR transactivation. Activation of the phosphatidylinositol-3-OH kinase/Akt pathway results in the suppression of AR target genes, such as p21, and the decrease of androgen/AR-mediated apoptosis, which may involve the inhibition of interaction between AR and AR coregulators. Together, these findings provide a molecular basis for cross-talk between two signaling pathways at the level of Akt and AR–AR coregulators that may help us to better understand the roles of Akt in the androgen/AR-mediated apoptosis.
TL;DR: The results suggest that TNF impairs insulin signaling through IRS-1 by activation of a PI 3-kinase/Akt/mTOR pathway, which is antagonized by PTEN.
Abstract: Tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) by the insulin receptor permits this docking protein to interact with signaling proteins that promote insulin action. Serine phosphorylation uncouples IRS-1 from the insulin receptor, thereby inhibiting its tyrosine phosphorylation and insulin signaling. For this reason, there is great interest in identifying serine/threonine kinases for which IRS-1 is a substrate. Tumor necrosis factor (TNF) inhibited insulin-promoted tyrosine phosphorylation of IRS-1 and activated the Akt/protein kinase B serine-threonine kinase, a downstream target for phosphatidylinositol 3-kinase (PI 3-kinase). The effect of TNF on insulin-promoted tyrosine phosphorylation of IRS-1 was blocked by inhibition of PI 3-kinase and the PTEN tumor suppessor, which dephosphorylates the lipids that mediate PI 3-kinase functions, whereas constitutively active Akt impaired insulin-promoted IRS-1 tyrosine phosphorylation. Conversely, TNF inhibition of IRS-1 tyrosine phosphorylation was blocked by kinase dead Akt. Inhibition of IRS-1 tyrosine phosphorylation by TNF was blocked by rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), a downstream target of Akt. mTOR induced the serine phosphorylation of IRS-1 (Ser-636/639), and such phosphorylation was inhibited by rapamycin. These results suggest that TNF impairs insulin signaling through IRS-1 by activation of a PI 3-kinase/Akt/mTOR pathway, which is antagonized by PTEN.
TL;DR: The findings suggest that 17β-E2 induced eNOS activation through an Akt-dependent mechanism, which is mediated by ERα via a nongenomic mechanism.
TL;DR: In Wnt-overexpressing PC12 cells, dominant-negative Akt decreased free β-catenin and derepressed nerve growth factor-induced differentiation, and acts in association with Dvl as an important regulator of the Wnt signaling pathway.
TL;DR: The results identify β1 integrin signaling as an important survival pathway in drug-induced apoptosis in breast cancer cells and suggest that activation of this pathway may contribute to the generation of drug resistance.
Abstract: Inherent or acquired drug resistance is one of the major problems in chemotherapy. The mechanisms by which cancer cells survive and escape the cytotoxic effects of chemotherapeutic agents are essentially unknown. In the present study, we demonstrate that in the MDA-MB-231 and MDA-MB-435 breast cancer cells, ligation of β1 integrins by their extracellular matrix ligands inhibits significantly apoptosis induced by paclitaxel and vincristine, two microtubule-directed chemotherapeutic agents that are widely used in the therapy of breast cancer. We show that β1 integrin signaling inhibits drug-induced apoptosis by inhibiting the release of cytochrome c from the mitochondria in response to drug treatment. Further, integrin-mediated protection from drug-induced apoptosis and inhibition of cytochrome c release are dependent on the activation of the PI 3-kinase/Akt pathway. Our results identify β1 integrin signaling as an important survival pathway in drug-induced apoptosis in breast cancer cells and suggest that activation of this pathway may contribute to the generation of drug resistance.
TL;DR: The macrolide antibiotic rapamycin effectively blocks oncogenic transformation induced by either P3k or Akt but does not reduce the transforming activity of 11 other oncoproteins, suggesting a correlation between the oncogenicity of Akt and phosphorylation of S6K and 4E-BP1.
Abstract: The oncoproteins P3k (homolog of the catalytic subunit of class IA phosphoinositide 3-kinase) and Akt (protein kinase B) induce oncogenic transformation of chicken embryo fibroblasts. The transformed cells show constitutive phosphorylation of the positive regulator of translation p70S6 kinase (S6K) and of the eukaryotic initiation factor 4E-BP1 binding protein (4E-BP1), a negative regulator of translation. Phosphorylation activates S6K and inactivates 4E-BP1. A mutant of Akt that retains kinase activity but does not induce phosphorylation of S6K or of 4E-BP1 fails to transform chicken embryo fibroblasts, suggesting a correlation between the oncogenicity of Akt and phosphorylation of S6K and 4E-BP1. The macrolide antibiotic rapamycin effectively blocks oncogenic transformation induced by either P3k or Akt but does not reduce the transforming activity of 11 other oncoproteins. Rapamycin inhibits the kinase mTOR, an important regulator of translation, and this inhibition requires binding of the antibiotic to the immunophilin FKBP12. Displacement of rapamycin from FKBP12 relieves the inhibition of mTOR and also restores P3k-induced transformation. These data are in accord with the hypothesis that transformation by P3k or Akt involves intervention in translational controls.