Role of the p70(S6K) pathway in regulating the actin cytoskeleton and cell migration

Fibronectin increases matrix metalloproteinase 9 expression through activation of c-Fos via extracellular-regulated kinase and phosphatidylinositol 3-kinase pathways in human lung carcinoma cells

Enhanced expression of matrix metalloproteinase-9 (MMP-9) is associated with human lung tumor invasion and/or metastasis. We have demonstrated that fibronectin (FN), a matrix glycoprotein, stimulates human non-small cell lung carcinoma (NSCLC) cell proliferation. The current study examines the effect of FN on MMP-9 expression in NSCLC cells. We show that FN increases MMP-9 protein, mRNA expression, and gelatinolytic activity in NSCLC cells. The integrin alpha5beta1 mediated the effects of FN because alpha5 small interfering RNA blocked FN-stimulated MMP-9 protein expression, and also abrogated FN-induced phosphorylation of ERK and phosphatidylinositol 3-kinase (PI3K) signals. The inhibitor of ERK, PD98095, and of PI3K, wortmannin, but not that of protein kinase A, H89, of Rho kinase, Y-27632, of mTOR, rapamycin, or of JNK, SP600125, prevented FN-induced MMP-9 gelatinolytic activity and gene expression. FN enhanced MMP-9 gene promoter activity; however, there was no response to FN in DNA constructs with an AP-1 site mutation. FN increased AP-1 DNA binding activity, and this was abrogated by cyclic AMP response element decoy oligonucleotides, which also diminished FN-induced MMP-9 promoter activity. FN increased the expression of the AP-1 subunit c-Fos protein, but not in the presence of PD98095 and wortmannin. The AP-1 inhibitor, nordihydroguaiaretic acid, and a c-Fos small interfering RNA eliminated the effect of FN on MMP-9 expression. This study indicates that FN, by binding to the integrin alpha5beta1 receptor, stimulates the expression of MMP-9 through increased AP-1/DNA binding and c-Fos protein expression via ERK and PI3K signaling pathways. The data unveils a novel mechanism by which FN could promote NSCLC cell invasion and metastasis.

Rapamycin inhibits cell motility by suppression of mTOR-mediated S6K1 and 4E-BP1 pathways

Rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), inhibits tumor cell motility. However, the underlying mechanism is poorly understood. Here, we show that rapamycin inhibited type I insulin-like growth factor (IGF-I)-stimulated motility of a panel of cell lines. Expression of a rapamycin-resistant mutant of mTOR (mTORrr) prevented rapamycin inhibition of cell motility. However, cells expressing a kinase-dead mTORrr remained sensitive to rapamycin. Downregulation of raptor or rictor by RNA interference (RNAi) decreased cell motility. However, only downregulation of raptor mimicked the effect of rapamycin, inhibiting phosphorylation of S6 kinase 1 (S6K1) and 4E-BP1. Cells infected with an adenovirus expressing constitutively active and rapamycin-resistant mutant of p70 S6K1, but not with an adenovirus expressing wild-type S6K1, or a control virus, conferred to resistance to rapamycin. Further, IGF-I failed to stimulate motility of the cells, in which S6K1 was downregulated by RNAi. Moreover, downregulation of eukaryotic initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) by RNAi-attenuated rapamycin inhibition of cell motility. In contrast, expression of constitutively active 4E-BP1 dramatically inhibited IGF-I-stimulated cell motility. The results indicate that both S6K1 and 4E-BP1 pathways, regulated by TORC1, are required for cell motility. Rapamycin inhibits IGF-I-stimulated cell motility, through suppression of both S6K1 and 4E-BP1/eIF4E-signaling pathways, as a consequence of inhibition of mTOR kinase activity.

Mammalian target of rapamycin (mTOR) regulates both proliferation of megakaryocyte progenitors and late stages of megakaryocyte differentiation

A major determinant in platelet production is the megakaryocyte (MK) size that is regulated both by ploidization and the increase in cytoplasmic volume at the end of maturation. Here we investigated the involvement of the mammalian target of rapamycin (mTOR) pathway in the regulation of megakaryopoiesis. We show that phosphorylation of mTOR, p70S6K1, and 4E-BP1 was diminished in thrombopoietin-cultured human MKs after rapamycin treatment. Rapamycin induced an inhibition in the G1/S transition and a decrease in the mean MK ploidy via a diminution of p21 and cyclin D3 occurring at a transcriptional level. Both cycling (2N/4N) and polyploid (8N/16N) MKs were reduced in size, with a size reduction slightly more pronounced in mature polyploid MKs than in immature ones. Rapamycin also induced a delay in the expression of MK markers and prevented the generation of proplatelet MKs. Additional experiments performed in vitro with MKs from mutant mice showed that the decrease in mean ploidy level and the delay in MK differentiation in the presence of rapamycin were less pronounced in CdknIa (p21)–/– MKs than in CdknIa (p21)+/+ MKs. These findings indicate that the mTOR pathway plays an important role during megakaryopoiesis by regulating ploidy, cell size, and maturation, in part by regulating p21 and cyclin D3.

Oncogenic Ras in tumour progression and metastasis

The ras genes give rise to a family of related GTP-binding proteins that exhibit potent transforming potential. Mutational activation of Ras proteins promotes oncogenesis by disturbing a multitude of cellular processes, such as gene expression, cell cycle progression and cell proliferation, as well as cell survival, and cell migration. Ras signalling pathways are well known for their involvement in tumour initiation, but less is known about their contribution to invasion and metastasis. This review summarises the role and mechanisms of Ras signalling, especially the role of the Ras effector cascade Raf/MEK/ERK, as well as the phosphatidylinositol 3-kinase/Akt pathway in Ras-mediated transformation and tumour progression. In addition, it discusses the impact of Rho GTPases on Ras-mediated transformation and metastasis.

Identification and characterization of a constitutively T-loop phosphorylated and active recombinant S6K1: expression, purification, and enzymatic studies in a high capacity non-radioactive TR-FRET Lance assay

The p70 S6 ribosomal protein kinase 1 (S6K) is a substrate and effector of the mammalian target of rapamycin (mTOR). The mTOR/S6K pathway is implicated in cancer and metabolic disorders. To study the molecular regulation of S6K and identify specific inhibitors, availability of active recombinant S6K and robust enzyme assays are critically needed. To date, however, expression of active recombinant S6K has not been feasible as S6K activation requires a cascade of phosphorylation events. We have compared several engineered S6K enzymes. Expression of the Flag-S6KDeltaCT(T389E) in HEK293 cells resulted in a highly active S6K that was constitutively phosphorylated on T229 in the activation-loop (T-loop). The active enzyme was readily purified in large scale by anti-Flag affinity chromatography achieving a high purity. We developed a high capacity homogeneous time-resolved fluorescence resonance energy transfer. Lance assay for measurement of substrate phosphorylation and analysis of kinetic parameters. The Michaelis constant (Km) values of S6K for ATP and the Biotin-S6 substrate peptide were determined to be 21.4+/-0.29 and 0.9+/-0.48 microM, respectively. The Lance assay was further validated with a diverse panel of literature inhibitors, in which the PKC inhibitors staurosporine, Ro-318220, and the PKA inhibitor Balanol potently inhibited S6K. Dose-response and inhibition mechanism by these inhibitors were also studied. Our data provide a new simplified strategy to achieve rapid production of active S6K and demonstrate utility of the Lance assay for S6K enzyme screen in searching for specific inhibitors.

Regulation of mTOR by amino acids and resistance exercise in skeletal muscle

Resistance exercise disturbs skeletal muscle homeostasis leading to activation of catabolic and anabolic processes within the muscle cell. A current challenge of exercise biology is to describe the molecular mechanisms of regulation by which contractile activity stimulates net protein breakdown during exercise and net protein synthesis during recovery. Muscle growth is optimized by combining exercise and appropriate nutritional strategies, such as amino acid (AA) and carbohydrate ingestion. The effects are integrated at the level of one central regulatory protein, mTOR (mammalian target of rapamycin). mTOR is a complex protein integrating signals of the energetic status of the cell and environmental stimuli to control protein synthesis, protein breakdown and therefore cell growth. mTOR is known to be activated by insulin, and the mechanisms involved are well documented. The ways by which exercise and AA lead to mTOR activation remain partially unclear. Exercise and AA use different signalling pathways upstream of mTOR. Exercise seems to recruit partially the same pathway as insulin, whereas AA could act more directly on mTOR. During resistance exercise, the activity of mTOR could be acutely blunted by AMP-activated protein kinase (AMPK), thus inhibiting protein synthesis and enhancing AA availability for energy metabolism. During recovery, the inhibition of mTOR by AMPK is suppressed, and its activation is maximized by the presence of AA. There appears to be a requirement for a minimal concentration of plasma insulin to stimulate muscle protein synthesis in response to resistance exercise and AA ingestion.

Phosphorylated osteopontin promotes migration of human choriocarcinoma cells via a p70 S6 kinase-dependent pathway

This study examined the role of osteopontin (OPN), a phosphorylated secreted glycoprotein, in the promotion of trophoblastic cell migration, an early event in the embryo implantation process. Three human choriocarcinoma cell lines, namely JAR, BeWo, and JEG-3, were treated with variants of OPN differing in the extent of phosphorylation following sequential dephosphorylation with tartrate-resistant acid phosphatase (TRAP), and their migratory response was measured. The highly phosphorylated human milk form of OPN (OPN-1) strongly triggered migration in all three cell lines, whereas the less phosphorylated variants, OPN-2a and OPN-2b, failed to stimulate migration. JAR cell migration in response to OPN-1 was accompanied by a rapid rearrangement of actin filaments to the cellular membrane. Using broad spectrum protein kinase profiling, we identified p70 S6 kinase as a major signal transduction pathway activated by OPN-1 during the migratory response in JAR cells. Activation was blocked completely by rapamycin and LY294002, thus demonstrating that OPN-1-stimulated migration occurs through mTOR and PI3K pathways, respectively. Conversely, PD98059 did not affect the activation of p70 S6 kinase by OPN-1, therefore, this response does not involve the Ras/ MAPK signaling cascade. Together, these data show that the highly phosphorylated human OPN-1 can stimulate trophoblastic cell migration and provides evidence for the involvement of the PI3K/mTOR/p70 S6 kinase pathway in the JAR cells response. Because both OPN and TRAP are expressed in the uterus during early pregnancy, it is conceivable that extracellular phosphatases such as TRAP may modify OPN charge state and thus modulate cell migration.

Antibody-based profiling of the phosphoinositide 3-kinase pathway in clinical prostate cancer

PURPOSE

As kinase inhibitors transition from the laboratory to patients, it is imperative to develop biomarkers that can be used in the clinic. The primary objectives are to identify patients most likely to benefit from molecularly targeted therapies and to document modulation of the drug target. Constitutive activation of the phosphoinositide 3-kinase (PI3K) pathway and its downstream effectors, as a result of PTEN loss or by other mechanisms, occurs in a high proportion of prostate cancers, making it an ideal template for the design of clinical trials involving PI3K pathway inhibitors. Prostate cancers also present unique organ-specific challenges, in that tumors are heterogeneous and diagnostic tissue is extremely limited.

EXPERIMENTAL DESIGN

Working within these limitations, we have developed a set of immunohistochemical assays that define activation of the PI3K pathway in clinical samples.

RESULTS AND CONCLUSIONS

Using both univariate and multivariate analyses, we show that loss of PTEN is highly correlated with the activation of AKT, and this, in turn, is associated with the phosphorylation of S6, one of its main effectors. These three antibodies are potentially able to define a molecular signature of PTEN loss and/or AKT pathway activation in prostate cancer.

The Serine/Threonine Kinase Akt Promotes Fcγ Receptor-mediated Phagocytosis in Murine Macrophages through the Activation of p70S6 Kinase

Fc gamma receptor (Fc gamma R) clustering by immune complexes activates multiple signaling pathways leading to phagocytosis. We and others have previously reported that Akt is phosphorylated in response to Fc gamma R clustering. However, the functional consequence of Akt activation by Fc gamma R is not known. Using Raw 264.7 macrophage cells transfected to overexpress either constitutively active myristoylated (Myr)-Akt or a dominant-negative CAAX-Akt and bone marrow macrophages (BMMs) from wild-type and transgenic mice expressing macrophage-specific Myr-Akt, we analyzed the function of Akt in phagocytosis. We report that overexpression of Myr-Akt resulted in significant increase in phagocytic efficiency, whereas CAAX-Akt down-regulated phagocytosis in Raw 264.7 cells. Likewise BMMs expressing Myr-Akt displayed enhanced phagocytic ability. Analyzing the downstream effectors of Akt, we demonstrate that p70S6 kinase is constitutively phosphorylated in Myr-Akt-expressing BMMs. p70S6 kinase is reported to influence actin cytoskeleton and cell migration, suggesting that Akt may influence phagocytosis through the activation of p70S6 kinase. Consistent with this, overexpression of either wild-type or constitutively active but not a kinase-inactive p70S6 kinase in Raw 264.7 cells significantly enhanced phagocytosis. Likewise suppression of p70S6 kinase with rapamycin down-regulated phagocytic efficiency conferred by the expression of constitutively active Akt. These findings demonstrate a novel role for Akt in phagocytosis through the activation of p70S6 kinase.