Nitric oxide donors selectively potentiate thrombin-stimulated p70(S6k) activity and morphological changes in Swiss 3T3 cells

L-Arginine Enhances Protein Synthesis by Phosphorylating mTOR (Thr 2446) in a Nitric Oxide-Dependent Manner in C2C12 Cells

Muscle atrophy may arise from many factors such as inactivity, malnutrition, and inflammation. In the present study, we investigated the stimulatory effect of nitric oxide (NO) on muscle protein synthesis. Primarily, C2C12 cells were supplied with extra L-arginine (L-Arg) in the culture media. L-Arg supplementation increased the activity of inducible nitric oxide synthase (iNOS), the rate of protein synthesis, and the phosphorylation of mTOR (Thr 2446) and p70S6K (Thr 389). L-NAME, an NOS inhibitor, decreased NO concentrations within cells and abolished the stimulatory effect of L-Arg on protein synthesis and the phosphorylation of mTOR and p70S6K. In contrast, SNP (sodium nitroprusside), an NO donor, increased NO concentrations, enhanced protein synthesis, and upregulated mTOR and p70S6K phosphorylation, regardless of L-NAME treatment. Blocking mTOR with rapamycin abolished the stimulatory effect of both L-Arg and SNP on protein synthesis and p70S6K phosphorylation. These results indicate that L-Arg stimulates protein synthesis via the activation of the mTOR (Thr 2446)/p70S6K signaling pathway in an NO-dependent manner.

Intestinal ribosomal p70(S6K) signaling is increased in piglet rotavirus enteritis

Recent identification of the mammalian target of rapamycin (mTOR) pathway as an amino acid-sensing mechanism that regulates protein synthesis led us to investigate its role in rotavirus diarrhea. We hypothesized that malnutrition would reduce the jejunal protein synthetic rate and mTOR signaling via its target, ribosomal p70 S6 kinase (p70(S6K)). Newborn piglets were artificially fed from birth and infected with porcine rotavirus on day 5 of life. Study groups included infected (fully fed and 50% protein calorie malnourished) and noninfected fully fed controls. Initially, in "worst-case scenario studies," malnourished infected piglets were killed on days 1, 3, 5, and 11 postinoculation, and jejunal samples were compared with controls to determine the time course of injury and p70(S6K) activation. Using a 2 x 2 factorial design, we subsequently determined if infection and/or malnutrition affected mTOR activation on day 3. Western blot analysis and immunohistochemistry were used to measure total and phosphorylated p70(S6K); [(3)H]phenylalanine incorporation was used to measure protein synthesis; and lactase specific activity and villus-crypt dimensions were used to quantify injury. At the peak of diarrhea, the in vitro jejunal protein synthetic rate increased twofold (compared with the rate in the uninfected pig jejunum), concomitant with increased jejunal p70(S6K) phosphorylation (4-fold) and an increased p70(S6K) level (3-fold, P < 0.05). Malnutrition did not alter the magnitude of p70(S6K) activation. Immunolocalization revealed that infection produced a major induction of cytoplasmic p70(S6K) and nuclear phospho-p70(S6K), mainly in the crypt. A downregulation of semitendinosus muscle p70(S6K) phosphorylation was seen at days 1-3 postinoculation. In conclusion, intestinal activation of p70(S6K) was not inhibited by malnutrition but was strongly activated during an active state of mucosal regeneration.

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

We have examined the role of endogenous 70-kDa S6 kinase (p70(S6K)) in actin cytoskeletal organization and cell migration in Swiss 3T3 fibroblasts. Association of p70(S6K) with the actin cytoskeleton was demonstrated by cosedimentation of p70(S6K) with F-actin and by subcellular fractionation in which p70(S6K) activity was measured in the F-actin cytoskeletal fraction. Immunocytochemical studies showed that p70(S6K), Akt1, PDK1, and p85 phosphoinositide 3-kinase (PI 3-kinase) were localized to the actin arc, a caveolin-enriched cytoskeletal structure located at the leading edge of migrating cells. Using a phospho-specific antibody to mammalian target of rapamycin (mTOR), we find that activated mTOR is enriched at the actin arc, suggesting that activation of the p70(S6K) signaling pathway is important to cell migration. Using the actin arc to assess migration, epidermal growth factor (EGF) stimulation was found to induce actin arc formation, an effect that was blocked by rapamycin treatment. We show further that actin stress fibers may function to down-regulate p70(S6K). Fibronectin stimulated stress fiber formation in the absence of growth factors and caused an inactivation of p70(S6K). Conversely, cytochalasin D and the Rho kinase inhibitor Y-27632, both of which cause stress fiber disruption, increased p70(S6K) activity. These studies provide evidence that the p70(S6K) pathway is important for signaling at two F-actin microdomains in cells and regulates cell migration.

Nitric oxide induces polarization of actin in encephalitogenic T cells and inhibits their in vitro trans-endothelial migration in a p70S6 kinase-independent manner

Nitric oxide (NO) inhibits both actively induced and transferred autoimmune encephalomyelitis. To explore potential mechanisms, we examined the ability of NO to inhibit migration of T lymphoblasts through both collagen matrices and monolayers of rat brain endothelial cells. The NO donor 1-hydroxy-2-oxo-3, 3-bis (2-aminoethyl)-1-triazene (HOBAT) inhibited migration in a concentration-dependent manner. NO pretreatment of T cells inhibited migration through untreated endothelial cells, but NO pretreatment of endothelial cells had no inhibitory effect on untreated T cells. Therefore NO's migration inhibitory action was mediated through its effect on T cells and not endothelial cells. HOBAT did not inhibit migration by inducing T-cell death but rather by polarizing the T cells, resulting in a morphology suggestive of migrating cells. P70S6 kinase, shown to have a role in NO-induced migration inhibition in fibroblasts, had no role in the inhibitory effect of NO on T-cell migration. Thus, HOBAT did not alter p70S6K activity nor did rapamycin, a specific inhibitor of p70S6K, inhibit HOBAT-induced T-cell morphological changes or T-cell migration. We suggest that NO-induced morphological changes result in T cells with predefined migratory directionality, thus limiting the ability of these cells to respond to other migratory signals.

Molecular crosstalk between p70S6k and MAPK cell signaling pathways

We report here for the first time that the specific MAPK kinase (MEK) inhibitor, PD-98059, completely knocked out granulocyte-macrophage colony-stimulating factor (GM-CSF)-stimulated MAPK activity but also partially inactivated the ribosomal kinase p70S6K. Since a connection between the two major signaling pathways, Ras/MEK/MAPK and PI3-K/p70S6K was suspected, experiments were designed to prove a molecular crosstalk between those. First, p70S6K protein could be co-immunoprecipitated with anti-MAPK antibodies, MAPK protein was similarly present in anti-p70S6K immunoprecipitates, indicating close spatial proximity of both signaling molecules. Second, p70S6K enzymatic activity was found in anti-MAPK immunoprecipitates and MAPK in anti-p70S6K immunoprecipitates, being the latter activity higher in samples derived from GM-CSF-treated cells. Since an upstream activator of p70S6K, phosphatidylinositol (PI)3-kinase, has been associated to cell movement in phagocytic cells, we studied a possible participation of p70S6K in chemotaxis and whether MAPK had an input. Our data show that functional chemotaxis was inhibited by rapamycin, a specific p70S6K inhibitor, as well as by PD-98059. Thus, a connection between these two kinases extends from the molecular level to cell migration, a key functionality in non-proliferative, mature phagocytes such as neutrophils.

The phosphoinositide 3-kinase and p70 S6 kinase regulate long-term potentiation in hippocampal neurons

The mechanisms by which long-term changes in synaptic efficacy (e.g., long-term potentiation) are maintained are not well understood. There is evidence that reorganization of the neuronal actin cytoskeleton is important for consolidation of long-term potentiation. In non-neuronal cells, phosphoinositide 3-kinase and p70 S6 kinase have been shown to regulate actin polymerization. We have investigated the subcellular localization of these enzymes in cultured hippocampal pyramidal neurons and their possible role in hippocampal long-term potentiation. Immunohistochemical analysis revealed enrichment of both enzymes in the growth cones and filopodia of extending neurites, whereas p70 S6 kinase was also present at the soma. Antibodies to the phosphorylated form of p70 S6 kinase confirmed its activity in these locations. Interestingly, both enzymes displayed strong colocalization with F-actin in discrete regions of developing neurites. In hippocampal slices, the maintenance of long-term potentiation was attenuated by either rapamycin or 2-(4-morpholinyl)-8-phenyl-1(4H)-1-benzopyran-4-one, inhibitors of p70 S6 kinase and phosphoinositide 3-kinase, respectively. Our findings provide evidence for a novel biochemical pathway involving phosphoinositide 3-kinase and p70 S6 kinase that is important for the maintenance of hippocampal long-term potentiation, possibly via regulation of actin dynamics.

Lysophosphatidic acid activates the 70-kDa S6 kinase via the lipoxygenase pathway

Many hormones are known to activate the 70-kDa S6 kinase (p70(S6K)). The signalling pathways mediating p70(S6K) activation are only partially characterized. We investigate, in this report, the mechanisms by which lysophosphatidic acid (LPA) activates p70(S6K). We observed that p70(S6K) activation was conventional, in that it was sensitive to both rapamycin and PI3 kinase inhibition. p70(S6K) activation appeared to be caused by the activation of several phospholipase pathways. LPA was an effective stimulus of phospholipase C induced intracellular calcium mobilization, which appeared to participate in p70(S6K) activation. Similarly, the effect of LPA on p70(S6K) activity was antagonized by butan-1-ol but not butan-2-ol suggesting the involvement of agonist stimulated phospholipase D activity. Further, antagonism of the phospholipase A(2) and lipoxygenase pathways attenuated p70(S6K) activation indicating a novel mechanism of p70(S6K) regulation. We conclude that in Swiss 3T3 cells LPA coordinates activation of several phospholipases to regulate p70(S6K).

The flightless I protein colocalizes with actin- and microtubule-based structures in motile Swiss 3T3 fibroblasts: evidence for the involvement of PI 3-kinase and Ras-related small GTPases

The flightless I protein contains an actin-binding domain with homology to the gelsolin family and is likely to be involved in actin cytoskeletal rearrangements. It has been suggested that this protein is involved in linking the cytoskeletal network with signal transduction pathways. We have developed antibodies directed toward the leucine rich repeat and gelsolin-like domains of the human and mouse homologues of flightless I that specifically recognize expressed and endogenous forms of the protein. We have also constructed a flightless I-enhanced green fluorescent fusion vector and used this to examine the localization of the expressed protein in Swiss 3T3 fibroblasts. The flightless I protein localizes predominantly to the nucleus and translocates to the cytoplasm following serum stimulation. In cells stimulated to migrate, the flightless I protein colocalizes with beta-tubulin- and actin-based structures. Members of the small GTPase family, also implicated in cytoskeletal control, were found to colocalize with flightless I in migrating Swiss 3T3 fibroblasts. LY294002, a specific inhibitor of PI 3-kinase, inhibits the translocation of flightless I to actin-based structures. Our results suggest that PI 3-kinase and the small GTPases, Ras, RhoA and Cdc42 may be part of a common functional pathway involved in Fliih-mediated cytoskeletal regulation. Functionally, we suggest that flightless I may act to prepare actin filaments or provide factors required for cytoskeletal rearrangements necessary for cell migration and/or adhesion.

Activation of endogenous thrombin receptors causes clustering and sensitization of epidermal growth factor receptors of swiss 3T3 cells without transactivation

The G protein-coupled thrombin receptor can induce cellular responses in some systems by transactivating the epidermal growth factor (EGF) receptor. This is in part due to the stimulation of ectoproteases that generate EGF receptor ligands. We show here that this cannot account for the stimulation of proliferation or migration by thrombin of Swiss 3T3 cells. Thrombin has no direct effect on the activation state of the EGF receptor or of its downstream effectors. However, thrombin induces the subcellular clustering of the EGF receptor at filamentous actin-containing structures at the leading edge and actin arcs of migrating cells in association with other signaling molecules, including Shc and phospholipase Cgamma1. In these thrombin-primed cells, the subsequent migratory response to EGF is potentiated. Thrombin did not potentiate the EGF-stimulated EGF receptor phosphorylation. Thus, in Swiss 3T3 cells the G protein-coupled thrombin receptor can potentiate the EGF tyrosine kinase receptor response when activated by EGF, and this appears to be due to the subcellular concentration of the receptor with downstream effectors and not to the overall ability of EGF to induce receptor transphosphorylation. Thus, the EGF receptor subcellular localization which is altered by thrombin appears to be an important determinant of the efficacy of downstream EGF receptor signaling in cell migration.

Molecular pathways of cyclic nucleotide-induced inhibition of arterial smooth muscle cell proliferation

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a wide variety of extracellular stimuli. These signals regulate many biological processes including cell proliferation, differentiation, migration, and apoptosis. Recently, significant progress has been achieved in the molecular basis underlying cyclic nucleotide regulation of cell proliferation. This review summarizes our knowledge of the signaling pathways regulated by cyclic nucleotides in arterial smooth muscle cells.

An automated fluorescence based assay of neurite formation

The assessment of neurite-promoting activity of growth factors and neurotrophins in vitro is usually done by the laborious process of counting neuronal processes manually under a standard microscope. This often requires fixation of the cells so that samples can be counted over a period of days when large numbers of cells or samples are assessed. This, therefore, limits the investigator to a single time point for that plate of cells. In an effort to provide an assay that could be used to screen samples quickly to grade overall neurotrophic activity, and to study time-dependent responses easily, an assay is described that quantitates neurite formation based on automated fluorescence detection. Dissociated embryonic mouse dorsal root ganglion neurons were placed in the upper chamber of 1 microm pore size Fluoroblok (Falcon) transwell chambers. The porous membrane of these transwells is optically opaque. NGF placed in the lower chamber of the transwell induced the production of neurites that preferentially passed from the neuronal cell body in the upper chamber through the pores into the lower NGF-containing chamber. The degree of neurite production was assessed both in live cells and in fixed cells by fluorescent labeling of the neurons and neuronal processes. Quantitation of neurite formation was done with a multiwell fluorescence plate reader and verified by confocal microscopy. This method allows the screening and quantitation of neurotrophic activity in both primary neurons and neuronal cell lines. In addition, this also provides experimental access to neuronal processes that are optically separated from the cell body. The latter aspect may also be of great use where fluorescence measurements within neurites and growth cones, such as for Ca2+ release studies, need to be isolated from contaminating fluorescence or synaptic influences of cell bodies.

Cellular function of p70S6K: a role in regulating cell motility

The 70 kDa ribosomal S6 kinase (p70S6K) is activated by numerous mitogens, growth factors and hormones. Activation of p70S6K occurs through phosphorylation at a number of sites and the primary target of the activated kinase is the 40S ribosomal protein S6, a major component of the machinery involved in protein synthesis in mammalian cells. In addition to its involvement in regulating translation, p70S6K activation has been implicated in cell cycle control and neuronal cell differentiation. Recent data obtained in this laboratory suggests that p70S6K may also function in regulating cell motility, a cellular response that is important in tumour metastases, the immune response and tissue repair. The present paper reviews the regulation and cellular function of p70S6K and proposes a novel function of p70S6K in regulating cell motility.

Nuclear and cytoskeletal translocation and localization of heterotrimeric G-proteins

Heterotrimeric GTP-binding proteins (G-proteins) are involved in a diverse array of signalling pathways. They are generally thought to be membrane-bound proteins, which disassociate on receptor activation and binding of GTP. A model to explain this has been proposed, which is often described as 'the G-protein cycle'. The 'G-protein cycle' is discussed in the present paper in relation to evidence that now exists regarding the non- membranous localization of G-proteins. Specifically, the experimental evidence demonstrating association of G-proteins with the cytoskeleton and the nucleus, and the mechanisms by which G-proteins translocate to these sites are reviewed. Furthermore, the possible effector pathways and the physiological function of G-proteins at these sites are discussed.

Insulin induces epidermal growth factor (EGF) receptor clustering and potentiates EGF-stimulated DNA synthesis in swiss 3T3 cells: a mechanism for costimulation in mitogenic synergy

In many cellular systems, activation with more than one ligand can produce a cellular response that is greater than the sum of the individual responses to the ligands. This synergy is sometimes referred to as coactivation. In Swiss 3T3 fibroblasts, activation of the epidermal growth factor (EGF) receptor produces a weak induction of DNA synthesis. Insulin has no stimulatory effect on this response. However, in combination, EGF and insulin synergize to cause a large induction of S phase. The underlying cellular biochemistry of this effect has been examined. The data indicate that phospholipase C activation is a major component of agonist-induced DNA synthesis. In contrast, activation of p70 S6 kinase by single agonists was inversely related to their ability to stimulate DNA synthesis. Therefore, it was examined whether stimulation of Swiss 3T3 cells with insulin causes changes in the subcellular distribution of EGF receptors and phospholipase Cgamma1 that could potentially explain the observed synergy or costimulation. It was found that insulin effectively induced the accumulation of EGF receptors on the actin arc of cells without activation of the EGF receptor. In contrast, EGF, when added for several hours, did not cause accumulation of the EGF receptor at this site. However, both EGF and insulin stimulated the accumulation of phospholipase Cgamma1 at the actin arc, which was coincident with the EGF receptor in the case of insulin- stimulated cells. Therefore, it is suggested that the insulin-induced coclustering of the EGF receptor with phospholipase Cgamma1 at the actin arc may allow for greater efficiency of signal transduction, resulting in the synergy observed for these two hormones in stimulation of DNA synthesis.