S6 phosphorylation and the p70s6k/p85s6k

Conversion of dendritic cells into tolerogenic or inflammatory cells depends on the activation threshold and kinetics of the mTOR signaling pathway

BACKGROUND

Restoring impaired peripheral immune tolerance is the primary challenge in treating autoimmune diseases. Our previous research demonstrated the effectiveness of small spleen peptides (SSPs), a fraction of low molecular weight proteins, in inhibiting the progression of psoriatic arthritis, even in the presence of high levels of the proinflammatory cytokine TNFα in the bloodstream. When specifically targeting dendritic cells (DCs), SSPs transform them into tolerogenic cells, which efficiently induce the development of regulatory Foxp3+ Treg cells. In this study, we provide further insights into the mechanism of action of SSPs.

RESULTS

We found that SSPs stimulate the activation of the mTOR signaling pathway in dendritic cells, albeit in a different manner than the classical immunogenic stimulus LPS. While LPS-induced activation is rapid, strong, and sustained, the activity induced by SSPs is delayed, less intense, yet still significant. These distinct patterns of activation, as measured by phosphorylation of key components of the pathway are also observed in response to other immunogenic and tolerogenic stimuli such as GM-CSF + IL-4 or IL-10 and TGFβ. The disparity in mTOR activation between immunogenic and tolerogenic stimuli is quantitative rather than qualitative. In both cases, mTOR activation primarily occurs through the PI3K/Akt signaling axis and involves ERK and GSK3β kinases, with minimal involvement of AMPK or NF-kB pathways. Furthermore, in the case of SSPs, mTOR activation seems to involve adenosine receptors. Additionally, we observed that DCs treated with SSPs exhibit an energy metabolism with high plasticity, which is typical of tolerogenic cells rather than immunogenic cells.

CONCLUSION

Hence, the decision whether dendritic cells enter an inflammatory or tolerogenic state seems to rely on varying activation thresholds and kinetics of the mTOR signaling pathway.

A Litopenaeus vannamei p70S6K gene is involved in the antioxidative and apoptosis under low temperature

p70S6K is involved in cellular response, such as tumor metastases, the immune response and tissue repair in vertebrates. The role of p70S6K in these physiological processes in crustaceans remains, however, unknown. In this study, the Lvp70S6K was identified, containing a 5' UTR of 294 bp, an ORF of 1494 bp ad a 3' UTR of 211 bp, encoding 497 amino acids with a theoretical molecular weight of 70 kDa and an estimated isoelectric point of (pI) of 5.16. The multiple alignment found that Lvp70S6K was highly homologous with other invertebrates. Lvp70S6K mRNA was detected in all the tested tissues and the Lvp70S6K expression levels was significantly down-regulated and reached the lowest level (0.44-fold, p < 0.01) at 1.5 h after low temperature stress. The subcellular localization of Lvp70S6K could be detected in cytoplasm. ROS production was significantly up-regulation (1.19-fold, p < 0.01), total hemocyte count (THC) was significantly down-regulation (0.22-fold, p < 0.01), apoptosis rate was markedly increased (1.09-fold, p < 0.01), apoptosis-related genes of LvPDCD4 (1.61-fold, p < 0.01) and LvCyt.C (1.23-fold, p < 0.01) were up-regulated, and anti-apoptotic gene of LvBcl-2 (0.69-fold, p < 0.01), LvIAP1 (0.68-fold, p < 0.01) and LvIAP2 (0.45-fold, p < 0.01) were decreased after low temperature stress in hemolymph of Lvp70S6K-silenced shrimp at 1.5 h. Silencing of LvPTEN significantly increased Lvp70S6K, LvPI3K, LvAKT and LvmTOR expression. In summary, these results indicated that Lvp70S6K play a crucial role in oxidative and apoptosis, which was able to negatively regulate by PTEN.

Thermal Bioprinting Causes Ample Alterations of Expression of LUCAT1, IL6, CCL26, and NRN1L Genes and Massive Phosphorylation of Critical Oncogenic Drug Resistance Pathways in Breast Cancer Cells

Bioprinting technology merges engineering and biological fields and together, they possess a great translational potential, which can tremendously impact the future of regenerative medicine and drug discovery. However, the molecular effects elicited by thermal inkjet bioprinting in breast cancer cells remains elusive. Previous studies have suggested that bioprinting can be used to model tissues for drug discovery and pharmacology. We report viability, apoptosis, phosphorylation, and RNA sequence analysis of bioprinted MCF7 breast cancer cells at separate timepoints post-bioprinting. An Annexin A5-FITC apoptosis stain was used in combination with flow cytometry at 2 and 24 h post-bioprinting. Antibody arrays using a Human phospho-MAPK array kit was performed 24 h post-bioprinting. RNA sequence analysis was conducted in samples collected at 2, 7, and 24 h post-bioprinting. The post-bioprinting cell viability averages were 77 and 76% at 24 h and 48 h, with 31 and 64% apoptotic cells at 2 and 24 h after bioprinting. A total of 21 kinases were phosphorylated in the bioprinted cells and 9 were phosphorylated in the manually seeded controls. The RNA seq analysis in the bioprinted cells identified a total of 12,235 genes, of which 9.7% were significantly differentially expressed. Using a ±2-fold change as the cutoff, 266 upregulated and 206 downregulated genes were observed in the bioprinted cells, with the following 5 genes uniquely expressed NRN1L, LUCAT1, IL6, CCL26, and LOC401585. This suggests that thermal inkjet bioprinting is stimulating large scale gene alterations that could potentially be utilized for drug discovery. Moreover, bioprinting activates key pathways implicated in drug resistance, cell motility, proliferation, survival, and differentiation.

A77 1726 (leflunomide) blocks and reverses cardiac hypertrophy and fibrosis in mice

T-cell infiltration and the subsequent increased intracardial chronic inflammation play crucial roles in the development of cardiac hypertrophy and heart failure (HF). A77 1726, the active metabolite of leflunomide, has been reported to have powerful anti-inflammatory and T cell-inhibiting properties. However, the effect of A77 1726 on cardiac hypertrophy remains completely unknown. Herein, we found that A77 1726 treatment attenuated pressure overload or angiotensin II (Ang II)-induced cardiac hypertrophy in vivo, as well as agonist-induced hypertrophic response of cardiomyocytes in vitro In addition, we showed that A77 1726 administration prevented induction of cardiac fibrosis by inhibiting cardiac fibroblast (CF) transformation into myofibroblast. Surprisingly, we found that the protective effect of A77 1726 was not dependent on its T lymphocyte-inhibiting property. A77 1726 suppressed the activation of protein kinase B (AKT) signaling pathway, and overexpression of constitutively active AKT completely abolished A77 1726-mediated cardioprotective effects in vivo and in vitro Pretreatment with siRNA targetting Fyn (si Fyn) blunted the protective effect elicited by A77 1726 in vitro More importantly, A77 1726 was capable of blocking pre-established cardiac hypertrophy in mice. In conclusion, A77 1726 attenuated cardiac hypertrophy and cardiac fibrosis via inhibiting FYN/AKT signaling pathway.

Gene expression alterations in the medial prefrontal cortex and blood cells in a mouse model of depression during menopause

Aims

The prevalence of major depressive disorder (MDD) is higher in women than in men, and this may be due to the decline in estrogen levels that occurs during the menopausal transition. We studied the biological alterations in the medial prefrontal cortex (mPFC), which is a region that is highly implicated in the neurobiology of MDD, and the blood cells (BCs) of ovariectomized (OVX) mice subjected to chronic mild stress (CMS), which represents a mouse model of depression during menopause.

Main methods

The mPFC and the BCs were obtained from the same individuals. Gene expression levels were analyzed by microarray. The data were used for the Ingenuity Pathway Analysis and the Gene Ontology analysis.

Key findings

The gene expression alterations (GEAs) induced by OVX were mainly associated with ribosomal and mitochondrial functions in both the mPFC and the BCs. Rapamycin-insensitive companion of mTOR (RICTOR) was identified as a possible upstream regulator of the OVX-induced GEAs in both tissues. The CMS-induced GEAs were associated with retinoic acid receptor signaling, inflammatory cytokines and post-synaptic density in the mPFC, but not in the BCs.

Significance

OVX and CMS independently affect biological pathways in the mPFC, which is involved in the development of the depression-like phenotype. Because a subset of the OVX-induced GEAs in the mPFC also occurred in the BCs, the GEAs in the BCs might be a useful probe to predict biological pathways in the corresponding brain tissue under specific conditions such as OVX in females.

Phenotypically Dormant and Immature Leukaemia Cells Display Increased Ribosomal Protein S6 Phosphorylation

Mechanistic/mammalian target of rapamycin (mTOR) activity drives a number of key metabolic processes including growth and protein synthesis. Inhibition of the mTOR pathway promotes cellular dormancy. Since cells from patients with acute myeloid leukaemia (AML) can be phenotypically dormant (quiescent), we examined biomarkers of their mTOR pathway activity concurrently with Ki-67 and CD71 (indicators of cycling cells) by quantitative flow cytometry. Using antibodies to phosphorylated epitopes of mTOR (S2448) and its downstream targets ribosomal protein S6 (rpS6, S235/236) and 4E-BP1 (T36/45), we documented that these phosphorylations were negligible in lymphocytes, but evident in dormant as well as proliferating subsets of both mobilised normal stem cell harvest CD34+ cells and AML blasts. Although mTOR phosphorylation in AML blasts was lower than that of the normal CD34+ cells, p-4E-BP1 was 2.6-fold higher and p-rpS6 was 22-fold higher. Moreover, in contrast to 4E-BP1, rpS6 phosphorylation was higher in dormant than proliferating AML blasts, and was also higher in the immature CD34+CD38- blast subset. Data from the Cancer Genome Atlas show that rpS6 expression is associated with that of respiratory chain enzymes in AML. We conclude that phenotypic quiescence markers do not necessarily predict metabolic dormancy and that elevated rpS6 ser235/236 phosphorylation is characteristic of AML.

p70 Ribosomal protein S6 kinase (Rps6kb1): an update

The Rps6kb1 gene encodes the 70 kDa ribosomal protein S6 kinase (p70S6K), which is a serine/threonine kinase regulated by phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway. p70S6K plays a crucial role in controlling cell cycle, growth and survival. The PI3K/mTOR signalling pathway is one of the major mechanisms for controlling cell survival, proliferation and metabolism and is the central regulator of translation of some components of protein synthesis system. Upon activation, this kinase phosphorylates S6 protein of ribosomal subunit 40S resulting in selective translation of unique family of mRNAs that contain oligopyrimidine tract on 5' transcriptional site (5'TOP). 5'TOP mRNAs are coding the components of translational apparatus including ribosomal proteins and elongation factors. Due to the role of p70S6K in protein synthesis and also its involvement in a variety of human diseases ranging from diabetes and obesity to cancer, p70S6K is now being considered as a new therapeutic target for drug development. Furthermore, p70S6K acts as a biomarker for response to immunosuppressant as well as anticancer effects of inhibitors of the mTOR. Because of the narrow therapeutic index of mTOR inhibitors, drug monitoring is essential, and this is usually done by measuring blood drug levels, therapeutic response and drug-induced adverse effects. Recent studies have suggested that plasma p70S6K is a reliable index for the monitoring of patient response to mTOR inhibitors. Therefore, a better understanding of p70S6K and its role in various pathological conditions could enable the development of strategies to aid diagnosis, prognosis and treatment schedules.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

A new functional role for mechanistic/mammalian target of rapamycin complex 1 (mTORC1) in the circadian regulation of L-type voltage-gated calcium channels in avian cone photoreceptors

In the retina, the L-type voltage-gated calcium channels (L-VGCCs) are responsible for neurotransmitter release from photoreceptors and are under circadian regulation. Both the current densities and protein expression of L-VGCCs are significantly higher at night than during the day. However, the underlying mechanisms of circadian regulation of L-VGCCs in the retina are not completely understood. In this study, we demonstrated that the mechanistic/mammalian target of rapamycin complex (mTORC) signaling pathway participated in the circadian phase-dependent modulation of L-VGCCs. The activities of the mTOR cascade, from mTORC1 to its downstream targets, displayed circadian oscillations throughout the course of a day. Disruption of mTORC1 signaling dampened the L-VGCC current densities, as well as the protein expression of L-VGCCs at night. The decrease of L-VGCCs at night by mTORC1 inhibition was in part due to a reduction of L-VGCCα1 subunit translocation from the cytosol to the plasma membrane. Finally, we showed that mTORC1 was downstream of the phosphatidylionositol 3 kinase-protein kinase B (PI3K-AKT) signaling pathway. Taken together, mTORC1 signaling played a role in the circadian regulation of L-VGCCs, in part through regulation of ion channel trafficking and translocation, which brings to light a new functional role for mTORC1: the modulation of ion channel activities.

Crystal structures of S6K1 provide insights into the regulation mechanism of S6K1 by the hydrophobic motif

The activity of S6K1 (p70 ribosomal protein subunit 6 kinase 1) is stimulated by phosphorylation of Thr389 in the hydrophobic motif by mTORC1 (mammalian target of rapamycin complex 1) and phosphorylation of Thr229 in the activation loop by PDK1 (phosphoinositide-dependent kinase 1); however, the order of the two events is still ambiguous. In the present paper we report six crystal structures of the S6K1 kinase domain alone or plus the hydrophobic motif in various forms, in complexes with a highly specific inhibitor. The structural data, together with the biochemical data, reveal in vivo phosphorylation of Thr389 in the absence of Thr229 phosphorylation and demonstrate the importance of two conserved residues, Gln140 and Arg121, in the establishment of a hydrogen-bonding network between the N-lobe (N-terminal lobe) and the hydrophobic motif. Phosphorylation of Thr389 or introduction of a corresponding negatively charged group leads to reinforcement of the network and stabilization of helix αC. Furthermore, comparisons of S6K1 with other AGC (protein kinase A/protein kinase G/protein kinase C) family kinases suggest that the structural and sequence differences in the hydrophobic motif and helix αC account for their divergence in PDK1 dependency. Taken together, the results of the present study indicate that phosphorylation of the hydrophobic motif in S6K1 is independent of, and probably precedes and promotes, phosphorylation of the activation loop.

Operant behavior to obtain palatable food modifies ERK activity in the brain reward circuit

Food palatability produces behavioral modifications that resemble those induced by drugs of abuse. Palatability-induced behavioral changes require both, the activation of the endogenous cannabinoid system, and changes in structural plasticity in neurons of the brain reward pathway. The ERK intracellular pathway is activated by CB1 receptors (CB1-R) and plays a crucial role in neuroplasticity. We investigated the activation of the ERK signaling cascade in the mesocorticolimbic system induced by operant training to obtain highly palatable isocaloric food and the involvement of the CB1-R in these responses. Using immunofluorescence techniques, we analyzed changes in ERK intracellular pathway activation in the mesocorticolimbic system of wild-type and CB1 knockout mice (CB1-/-) trained on an operant paradigm to obtain standard, highly caloric or highly palatable isocaloric food. Operant training for highly palatable isocaloric food, but not for standard or highly caloric food, produced a robust activation of the ERK signaling cascade in the same brain areas where this training modified structural plasticity. These changes induced by the operant training were absent in CB1-/-. We can conclude that the activation of the ERK pathway is associated to the neuroplasticity induced by operant training for highly palatable isocaloric food and might be involved in CB1-R mediated alterations in behavior and structural plasticity.

Regulation of neuronal mRNA translation by CaM-kinase I phosphorylation of eIF4GII

Ca²⁺/calmodulin-dependent kinases (CaMKs) are essential for neuronal development and plasticity, processes requiring de novo protein synthesis. Roles for CaMKs in modulating gene transcription are well established, but their involvement in mRNA translation is evolving. Here we report that activity-dependent translational initiation in cultured rat hippocampal neurons is enhanced by CaMKI-mediated phosphorylation of Ser1156 in eukaryotic initiation factor eIF4GII (4GII). Treatment with bicuculline or gabazine to enhance neuronal activity promotes recruitment of wild-type 4GII, but not the 4GII S1156A mutant or 4GI, to the heterotrimeric eIF4F (4F) complex that assembles at the 5' cap structure (m⁷GTP) of mRNA to initiate ribosomal scanning. Recruitment of 4GII to 4F is suppressed by pharmacological inhibition (STO-609) of CaM kinase kinase, the upstream activator of CaMKI. Post hoc in vitro CaMKI phosphorylation assays confirm that activity promotes phosphorylation of S1156 in transfected 4GII in neurons. Changes in cap-dependent and cap-independent translation were assessed using a bicistronic luciferase reporter transfected into neurons. Activity upregulates cap-dependent translation, and RNAi knockdown of CaMKIβ and γ isoforms, but not α or δ, led to its attenuation as did blockade of NMDA receptors. Furthermore, RNAi knockdown of 4GII attenuates cap-dependent translation and reduces density of dendritic filopodia and spine formation without effect on dendritic arborization. Together, our results provide a mechanistic link between Ca²⁺ influx due to neuronal activity and regulation of cap-dependent RNA translation via CaMKI activation and selective recruitment of phosphorylated 4GII to the 4F complex, which may function to regulate activity-dependent changes in spine density.

CDNA cloning and characterization of S6 kinase and its effect on yolk protein gene expression in the oriental fruit fly Bactrocera dorsalis (Hendel)

p70 S6 kinase (S6K), a serine/threonine protein kinase, is a downstream target of target of rapamycin (TOR) gene and an important regulator of protein synthesis responsible for cell growth and reproduction. In this study, a S6K gene, named BdS6K (GenBank Accession No. GQ203802), was isolated from the oriental fruit fly Bactrocera dorsalis (Hendel). Quantitative RT-PCR showed that BdS6K mRNA is expressed at a higher level in egg than in other developmental stages, as well as in ovary than in fat body. Downregulation of BdS6K activity by rapamycin treatment in larval stage resulted in the developmental defects of larvae, pupae, and adults, with a reduced yolk protein (YP) expression in the fat body throughout the first reproductive cycle with a substantial reduction in ovary size, and also repressed the egg development in female fruit fly. Knockdown of BdS6K gene by RNA interference in the adult significantly decreased the YP expression. These observations support the involvement of BdS6K signaling in the regulation of the YP synthesis and egg development in B. dorsalis.

Knockdown of insulin receptor substrate 1 reduces proliferation and downregulates Akt/mTOR and MAPK pathways in K562 cells

BCR-ABL kinase activates downstream signaling pathways, including the PI3K-Akt/mTOR and the MAPK pathway. IRS1 has been previously described as constitutively phosphorylated and associated with BCR-ABL in K562 cells, suggesting that IRS1 has role in the BCR-ABL signaling pathways. In this study, we analyzed the effect of IRS1 silencing, by shRNA-lentiviral delivery, in K562 cells, a CML cell line that presents the BCR-ABL. IRS1 silencing decreased cell proliferation and colony formation in K562 cells, which correlates with the delay of these cells at the G0/G1 phase and a decrease in the S phase of the cell cycle. Furthermore, IRS1 silencing in K562 cells resulted in a decrease of Akt, P70S6K and ERK1/2 phosphorylation. Nevertheless, apoptosis was unaffected by IRS1 knockdown and no alterations were found in the phosphorylation of BAD and in the expression of BCL2 and BAX. BCR-ABL and CRKL phosphorylation levels remained unaffected upon IRS1 silencing, and no synergistic effect was observed with imatinib treatment and IRS1 knockdown, indicating that IRS1 is downstream from BCR-ABL. In conclusion, we demonstrated that inhibition of IRS1 is capable of inducing the downregulation of Akt/mTOR and MAPK pathways and further decreasing proliferation, and clonogenicity and induces to cell cycle delay at G0/G1 phase in BCR-ABL cells.

Modulation of oxidative stress and tau phosphorylation by the mTOR activator phosphatidic acid in SH-SY5Y cells

The mammalian target of rapamycin complex 1 (mTORC1) pathway including p70(S6K) (the 70-kDa p70 S6 kinase) and S6, controls protein synthesis, has anti-apoptotic functions and can phosphorylate tau protein. mTORC1 is triggered by nutrients such as phosphatidic acid (PA). Previous experimental studies have shown that oxidative stress may down-regulate this pathway leading to neuronal death. Our results showed that in human neuroblastoma cells, PA exposure can reduce H(2)O(2)-induced apoptosis and can increase tau protein phosphorylation on Ser214 via p70(S6K) activation. These findings reveal that PA, via the mTOR kinase, can trigger tau phosphorylation on a site known to reduce paired helical filament (PHF) formation.

Luteinizing hormone stimulates mammalian target of rapamycin signaling in bovine luteal cells via pathways independent of AKT and mitogen-activated protein kinase: modulation of glycogen synthase kinase 3 and AMP-activated protein kinase

LH stimulates the production of cAMP in luteal cells, which leads to the production of progesterone, a hormone critical for the maintenance of pregnancy. The mammalian target of rapamycin (MTOR) signaling cascade has recently been examined in ovarian follicles where it regulates granulosa cell proliferation and differentiation. This study examined the actions of LH on the regulation and possible role of the MTOR signaling pathway in primary cultures of bovine corpus luteum cells. Herein, we demonstrate that activation of the LH receptor stimulates the phosphorylation of the MTOR substrates ribosomal protein S6 kinase 1 (S6K1) and eukaryotic translation initiation factor 4E binding protein 1. The actions of LH were mimicked by forskolin and 8-bromo-cAMP. LH did not increase AKT or MAPK1/3 phosphorylation. Studies with pathway-specific inhibitors demonstrated that the MAPK kinase 1 (MAP2K1)/MAPK or phosphatidylinositol 3-kinase/AKT signaling pathways were not required for LH-stimulated MTOR/S6K1 activity. However, LH decreased the activity of glycogen synthase kinase 3Beta (GSK3B) and AMP-activated protein kinase (AMPK). The actions of LH on MTOR/S6K1 were mimicked by agents that modulated GSK3B and AMPK activity. The ability of LH to stimulate progesterone secretion was not prevented by rapamycin, a MTOR inhibitor. In contrast, activation of AMPK inhibited LH-stimulated MTOR/S6K1 signaling and progesterone secretion. In summary, the LH receptor stimulates a unique series of intracellular signals to activate MTOR/S6K1 signaling. Furthermore, LH-directed changes in AMPK and GSK3B phosphorylation appear to exert a greater impact on progesterone synthesis in the corpus luteum than rapamycin-sensitive MTOR-mediated events.

Phospho-p70S6K and cdc2/cdk1 as therapeutic targets for diffuse large B-cell lymphoma

p70S6K/p85S6K and cdc2/cdk1 are members of the serine/threonine protein kinase family. p70S6K/p85S6K is one of the downstream effectors of the PI3K/Akt/mTOR signal transduction pathway. It phosphorylates S6 protein of 40S ribosomal subunit and thus functions in protein synthesis and cell growth. cdc2/cdk1 is a cyclin-dependent protein kinase that controls the cell cycle entry from G2 to M phase. Overexpression of phospho-p70S6K and cdc2/cdk1 has recently been identified in the majority of diffuse large B-cell lymphoma (DLBCL) specimens. Combination of small molecules that target phosphorylation of p70S6K and cdc2/cdk1 synergistically induced cell apoptosis and cell cycle G1 and G2 arrest, suggesting that they are potential molecular targets for DLBCL therapy. This review will summarize recent advances in the study of phospho-p70S6K and cdc2/cdk1 as molecular markers and therapeutic targets for DLBCL. We propose that multilevel inhibition of the PI3K/Akt/mTOR pathway and double brake at the G1 and G2 phases of the cell cycle progression are effective strategies in treating DLBCL that overexpress phospho-p70S6K and cdc2/cdk1.

Cannabinoid modulation of hippocampal long-term memory is mediated by mTOR signaling

Cognitive impairment is one of the most important negative consequences associated with cannabis consumption. We found that CB1 cannabinoid receptor (CB1R) activation transiently modulated the mammalian target of rapamycin (mTOR)/p70S6K pathway and the protein synthesis machinery in the mouse hippocampus, which correlated with the amnesic properties of delta9-tetrahydrocannabinol (THC). In addition, non-amnesic doses of either the mTOR blocker rapamycin or the protein synthesis inhibitor anisomycin abrogated the amnesic-like effects of THC, pointing to a mechanism involving new protein synthesis. Moreover, using pharmacological and genetic tools, we found that THC long-term memory deficits were mediated by CB1Rs expressed on GABAergic interneurons through a glutamatergic mechanism, as both the amnesic-like effects and p70S6K phosphorylation were reduced in GABA-CB1R knockout mice and by NMDA blockade.

Enterolactone inhibits insulin-like growth factor-1 receptor signaling in human prostatic carcinoma PC-3 cells

Enterolactone, a major metabolite of plant-based lignans, has been shown to inhibit prostate cancer growth and development, but the mechanistic basis for its anticancer activity remains largely unknown. Activation of insulin-like growth factor-1 (IGF-1) receptor (IGF-1R) signaling is critical for prostate cancer cell growth and progression. This study examined whether the growth inhibitory effect of enterolactone was related to changes in the IGF-1/IGF-1R system in PC-3 prostate cancer cells. At nutritionally relevant concentrations (20-60 micromol/L), enterolactone inhibited IGF-1-induced activation of IGF-1R and its downstream AKT and mitogen-activated protein kinase/extracellular-signal regulated kinase signaling pathways. Inhibition of AKT by enterolactone resulted in decreased phosphorylation of its downstream targets, including p70S6K1 and glycogen synthase kinase-3 beta. Enterolactone also inhibited cyclin D1 expression. As a result, enterolactone inhibited proliferation and migration of PC-3 cells. Knockdown of IGF-1R by plasmids with siRNA (si) against IGF-1R mRNA resulted in inhibition of proliferation of PC-3 cells and cell numbers did not differ when the si-IGF-1R groups (cells transfected with plasmids containing siRNA against IGF-1R mRNA) were treated or untreated with enterolactone. These results suggest that enterolactone suppresses proliferation and migration of prostate cancer cells, at least partially, through inhibition of IGF-1/IGF-1R signaling. The finding of this study provides new insights into the molecular mechanisms that enterolactone exerts against prostate cancer.

Small heat shock proteins Hsp27 or alphaB-crystallin and the protein components of neurofibrillary tangles: tau and neurofilaments

The heat-shock proteins (HSPs) Hsp27 and alphaB-crystallin are up-regulated in Alzheimer's disease (AD), but the extent of this and the consequences are still largely unknown. The HSPs are involved in protein degradation and protection against protein aggregation, and they interact with several cytoskeletal components such as microtubules (MT) and neurofilaments (NF). AD pathology includes aggregated proteins (tau, NF), decreased protein degradation, and cytoskeletal disruption. It is thus of interest to investigate more closely the possible roles of the HSPs in AD pathology. The expressions of Hsp27 and alphaB-crystallin in AD brain samples were significantly increased (by approximately 20% and approximately 30%, respectively) and correlated significantly with phosphorylated tau and NF proteins. To investigate the consequences of increased HSP levels on tau and NF regulation, N2a cells were transfected with Hsp27 or alphaB-crystallin constructs, and overexpression of the HSPs was confirmed in the cells. Increased tau phosphorylation at the Ser262 site in the N2a cells was regulated by Hsp27 overexpression (possibly through p70S6k), whereas the overexpression of alphaB-crystallin resulted in decreased levels of phosphorylated tau, NF, and GSK-3beta. It was also shown that overexpression of HSPs causes an increase in the percentage of cells present in the G(1) phase. The results presented suggest that a cellular defense against dysregulated proteins, in the form of Hsp27 and alphaB-crystallin, might contribute to the cell cycle reentry seen in AD cells. Furthermore, Hsp27 might also be involved in AD pathology by aggravating MT disruption by tau phosphorylation.

Removal of S6K1 and S6K2 leads to divergent alterations in learning, memory, and synaptic plasticity

Protein synthesis is required for the expression of enduring memories and long-lasting synaptic plasticity. During cellular proliferation and growth, S6 kinases (S6Ks) are activated and coordinate the synthesis of de novo proteins. We hypothesized that protein synthesis mediated by S6Ks is critical for the manifestation of learning, memory, and synaptic plasticity. We have tested this hypothesis with genetically engineered mice deficient for either S6K1 or S6K2. We have found that S6K1-deficient mice express an early-onset contextual fear memory deficit within one hour of training, a deficit in conditioned taste aversion (CTA), impaired Morris water maze acquisition, and hypoactive exploratory behavior. In contrast, S6K2-deficient mice exhibit decreased contextual fear memory seven days after training, a reduction in latent inhibition of CTA, and normal spatial learning in the Morris water maze. Surprisingly, neither S6K1- nor S6K2-deficient mice exhibited alterations in protein synthesis-dependent late-phase long-term potentiation (L-LTP). However, removal of S6K1, but not S6K2, compromised early-phase LTP expression. Furthermore, we observed that S6K1-deficient mice have elevated basal levels of Akt phosphorylation, which is further elevated following induction of L-LTP. Taken together, our findings demonstrate that removal of S6K1 leads to a distinct array of behavioral and synaptic plasticity phenotypes that are not mirrored by the removal of S6K2. Our observations suggest that neither gene by itself is required for L-LTP but instead may be required for other types of synaptic plasticity required for cognitive processing.

Octreotide and the mTOR inhibitor RAD001 (everolimus) block proliferation and interact with the Akt-mTOR-p70S6K pathway in a neuro-endocrine tumour cell Line

BACKGROUND/AIM

The mode of action of the somatostatin analog octreotide on neuro-endocrine tumour proliferation is largely unknown. Overexpression of the proto-oncogene Akt/PKB (protein kinase B) has been demonstrated in certain neuro-endocrine tumours: Akt activates downstream proteins including mTOR and p70S6K, which play an important role in cell proliferation. RAD001 (everolimus) is a novel agent that is being trialled in the treatment of neuro-endocrine tumours, and is known to interact with mTOR. We explored the mechanism of action of octreotide, RAD001, and their combination on cell proliferation and kinase activation in a neuro-endocrine tumour cell line (rat insulinoma cell line, INS1).

METHODS

Proliferation assays were used to determine the effects of octreotide, RAD001, and their combination on cell proliferation. Western blotting was used to characterize the expression of phosphorylated Akt, phosphorylated TSC2, phosphorylated mTOR, and phosphorylated 70S6K.

RESULTS

Treatment with octreotide and RAD001 inhibited proliferation and attenuated phosphorylation of all downstream targets of Akt: TSC2, mTOR, and p70S6K.

CONCLUSIONS

In this cell model, octreotide and RAD001 appear to act through a similar pathway and inhibit the Akt-mTOR-p70S6 kinase pathway downstream of Akt. There may be some overlapping effects of the two inhibitors on the mTOR pathway, although it is likely that other additional effects may differentiate the two agents.

Parallel increase in p70 kinase activation and tau phosphorylation (S262) with Abeta overproduction

This study set out to search for a link between overproduction of Abeta and p70S6 kinase (p70S6K) phosphorylation/activation. Results showed that levels of p-p70S6K at T421/S424 and T389 are significantly increased in mouse N2a neuroblastoma cells carrying human APP with Swedish mutation (APPswe), and in transgenic APPswe/PS1 (A246E) mice as compared with respective controls, corresponding to the increase of tau phosphorylation at S262. This parallel increase in p70S6K activation and tau phosphorylation could be demonstrated by treating wild-type N2a cells with Abeta25-35. Our results suggest that the Abeta deposition in senile plaques in Alzheimer disease brains might be a primary event that activates p70S6K and phosphorylates tau at S262, resulting in microtubule disruption.

Chronic alcohol feeding impairs mTOR(Ser 2448) phosphorylation in rat hearts

BACKGROUND

Chronic alcohol administration impairs protein synthesis ultimately causing a loss of proteins in cardiac muscle. Inhibition of protein synthesis resides in the process of mRNA translation. The present set of experiments were designed to examine the potential regulatory effect of chronic alcohol consumption on mammalian target of rapamycin (mTOR), a serine/threonine kinase important in controlling signaling cascades in the mRNA translation initiation pathway in rat hearts.

METHODS

Rats were fed a diet containing ethanol for 20 to 26 weeks. Pair-fed rats served as controls. Rates of protein synthesis were measured following intravenous infusion of [(3)H]-L-phenylalanine (150 mM, 30 microCi/ml; 1 ml/100 g body weight). The phosphorylation state of mTOR, eukaryotic initiation factor 4G (eIF4G), protein kinase B (PKB) and S6K1 in heart were measured using immunoblot techniques with phospho-specific antibodies.

RESULTS

Protein synthesis was reduced by 35% in animals consuming a diet containing ethanol. The fall in protein synthesis was accompanied by diminished S6K1(Thr(389)) and eIF4G (Ser(1108)) phosphorylation, both downstream effectors of mTOR signaling. These changes in phosphorylation of S6K1 and eIF4G were not associated with differences in the distribution of mTOR between TORC1 and TORC2. Instead, phosphorylation of mTOR on Ser(2448) but not on Ser(2481) was significantly reduced following feeding rats an ethanol containing diet. Decreased phosphorylation of mTOR(Ser(2448)) was not associated with a corresponding lessening of tumor suppressor complex 2 phosphorylation or expression of regulated in development and DNA damage 1, both upstream regulators of mTOR. Likewise, phosphorylation of PKB on either Ser(473) or Thr(308) was unaffected by long-term alcohol consumption.

CONCLUSIONS

Chronic ethanol consumption does not alter the distribution of mTOR between TORC1 and TORC2, but instead diminishes mTOR phosphorylation on Ser(2448) independent of changes in tumor suppressor complex 2 and PKB phosphorylation. Furthermore, the data suggest that protein synthesis in rats fed a diet containing ethanol is limited by mTOR-dependent reduction in phosphorylation of S6K1(Thr(389)) and eIF4G(Ser(1108)) secondary to reduced phosphorylation of mTOR(Ser(2448)).

Emerging applications for phospho-proteomics in cancer molecular therapeutics

Protein phosphorylation is a key mechanism of cell regulation in normal and cancer cells. Various new cancer drugs and drug candidates are aimed at protein kinase targets. However, selecting patients likely to respond to these treatments, even among individuals with tumors expressing validated kinase targets remains a major challenge. There exists a need for biomarkers to facilitate the monitoring of modulation of drug-targeted kinase pathways. Phospho-proteomics involves the enrichment of phosphorylated proteins from tissue, and the application of technologies such as mass spectrometry (MS) for the identification and quantification of protein phosphorylation sites. It has potential to provide pharmacodynamic readouts of disease states and cellular drug responses in tumor samples, but technical hurdles and bioinformatics challenges will need to be addressed.

Coordinate activation of intracellular signaling pathways by insulin-like growth factor-1 and platelet-derived growth factor in rat hepatic stellate cells

Proliferation of activated hepatic stellate cells (HSC) is an important event in the development of hepatic fibrosis. Insulin-like growth factor-1 (IGF-1) has been shown to be mitogenic for HSC, but the intracellular signaling pathways involved have not been fully characterized. Thus, the aims of the current study were to examine the roles of the extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3-K) and p70-S6 kinase (p70-S6-K) signaling pathways in IGF-1- and platelet-derived growth factor (PDGF)-induced mitogenic signaling of HSC and to examine the potential crosstalk between these pathways. Both IGF-1 and PDGF increased ERK, PI3-K and p70-S6-K activity. When evaluating potential crosstalk between these signaling pathways, we observed that PI3-K is required for p70-S6-K activation by IGF-1 and PDGF, and is partially responsible for PDGF-induced ERK activation. PDGF and IGF-1 also increased the levels of cyclin D1 and phospho-glycogen synthase kinase-3beta. Coordinate activation of ERK, PI3-K and p70-S6-K is important for perpetuating the activated state of HSC during fibrogenesis.

The phosphoinositide 3-kinase/Akt-signal pathway mediates proliferation and secretory function of hepatic sinusoidal endothelial cells in rats after partial hepatectomy

OBJECTIVE

To investigate the role of AKT signaling pathway in hepatic sinusoidal endothelial cells (SECs) early after partial hepatectomy in rats and the regulatory mechanisms involved.

METHODS

The animal model of 70% hepatectomy was made. Hepatic SECs were isolated and cultured according to Braet et al.'s method with some modifications. The cultured hepatic SECs were divided into two groups: 70% partial hepatectomy groups and LY294002 group (LY). We observed the expressions of AKT and NF-kappaB in cultured hepatic SECs by Western blot, measured the levels of NO, NOs, IL-6, and HGF in the supernatants of hepatic SEC cultures and [3H]thymidine incorporation, and analyzed cell cycle of cultured hepatic SECs by flow cytometer. The relationship of the Akt pathway with secretions and proliferation of hepatic SECs after partial hepatectomy was probed.

RESULTS

The levels of Akt protein expression increased significantly after partial hepatectomy in OG group and with a peak at 24 h post operation. Meanwhile, there was a markedly increase in phosphorylated Akt protein during 2-72 h after operation. But the expression and activity of Akt protein did not change significantly after partial hepatectomy in the LY group. So, partial hepatectomy can marked induce Akt expression and result in rapid and marked phosphorylation of Akt from 2 to 72 h thereafter. The changes of NF-kappaB expression in cultured hepatic SECs were similar to those of Akt expression after operation. The concentrations of HGF and IL-6 in the supernatants of cultured hepatic SECs were relatively low in the LY group, and were markedly increased after partial hepatectomy, with a peak at 24 h in the OG group. There were significant differences between the OG and LY groups at 6 and 24 h (P < 0.05). Both NO and NOS secretion was increased in the OG group compared to the LY group within 24 h after partial hepatectomy. But the secretion of NO and NOS was increased more markedly in the LY group than that in the OG beyond 24 h. These findings suggest that the secretion of the cytokines by hepatic SECs is mediated by Akt signaling. Akt signaling pathway in relationship with proliferation of hepatic SECs and suppression of apoptosis. In OG group, the hepatic SECs in S and G2/M obviously increased. The proliferative index of hepatic SECs in OG group had significant differences with that in LY group at 6, 24, and 72 h, P < 0.05. Meanwhile, the cells of apoptosis in OG group were very low, and the cells in LY group gradually increased.

CONCLUSIONS

These results suggest that AKT signaling pathway plays a crucial role in mediating proliferating and secreted signals in hepatic SECs. AKT has been suggested to play a pivotal role in early liver regeneration involved in the induction of secreted cytokines and proliferation of hepatic SECs.

Meal feeding enhances formation of eIF4F in skeletal muscle: role of increased eIF4E availability and eIF4G phosphorylation

Feeding promotes protein accretion in skeletal muscle through a stimulation of the mRNA translation initiation phase of protein synthesis either secondarily to nutrient-induced rises in insulin or owing to direct effects of nutrients themselves. The present set of experiments establishes the effects of meal feeding on potential signal transduction pathways that may be important in accelerating mRNA translation initiation. Gastrocnemius muscle from male Sprague-Dawley rats trained to consume a meal consisting of rat chow was sampled before, during, and after the meal. Meal feeding enhanced the assembly of the active eIF4G.eIF4E complex, which returned to basal levels within 3 h of removal of food. The increased assembly of the active eIF4G.eIF4E complex was associated with a marked 10-fold rise in phosphorylation of eIF4G(Ser(1108)) and a decreased assembly of inactive 4E-BP1.eIF4E complex. The reduced assembly of 4E-BP1.eIF4E complex was associated with a 75-fold increase in phosphorylation of 4E-BP1 in the gamma-form during feeding. Phosphorylation of S6K1 on Ser(789) was increased by meal feeding, although the extent of phosphorylation was greater at 0.5 h after feeding than after 1 h. Phosphorylation of mammalian target of rapamycin (mTOR) on Ser(2448) or Ser(2481), an upstream kinase responsible for phosphorylating both S6K1 and 4E-BP1, was increased at all times during meal feeding, although the extent of phosphorylation was greater at 0.5 h after feeding than after 1 h. Phosphorylation of PKB, an upstream kinase responsible for phosphorylating mTOR, was elevated only after 0.5 h of meal feeding for Thr(308), whereas phosphorylation Ser(473) was significantly elevated at only 0.5 and 1 h after initiation of feeding. We conclude from these studies that meal feeding stimulates two signal pathways in skeletal muscle that lead to elevated eIF4G.eIF4E complex assembly through increased phosphorylation of eIF4G and decreased association of 4E-BP1 with eIF4E.

A downstream kinase of the mammalian target of rapamycin, p70S6K1, regulates human double minute 2 protein phosphorylation and stability

Human double minute 2 (HDM2) is an oncoprotein overexpressed in many human cancers. HDM2 expression is regulated at multiple levels in cells. Phosphorylation of HDM2 plays an important role in its post-translational regulation. In this study, we have shown that the phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, and the mammalian target of rapamycin (mTOR) inhibitor, rapamycin, have similar effects on the inhibition of HDM2 phosphorylation and protein turnover. Rapamycin inhibited p70S6K1, but not AKT activation, indicating that rapamycin affects HDM2 phosphorylation via an AKT-independent mechanism. Rapamycin also decreased HDM2 protein stability. Knockdown of p70S6K1 by a p70S6K1 siRNA resulted in the inhibition of HDM2 phosphorylation and a decrease in HDM2 protein turnover. Overexpression of p70S6K1 enhanced HDM2 phosphorylation and led to an increase in HDM2 protein turnover. Our results suggest that p70S6K1 regulates turnover of HDM2 protein for cancer development.

P70 S6 kinase mediates tau phosphorylation and synthesis

Currently, we found that the 70-kDa p70 S6 kinase (p70S6K) directly phosphorylates tau at S262, S214, and T212 sites in vitro. By immunoprecipitation, p-p70S6K (T421/S424) showed a close association with p-tau (S262 and S396/404). Zinc-induced p70S6K activation could only upregulate translation of total S6 and tau but not global proteins in SH-SY5Y cells. The requirement of p70S6K activation was confirmed in the SH-SY5Y cells that overexpress wild-type htau40. Level of p-p70S6K (T421/S424) was only significantly correlated with p-tau at S262, S214, and T212, but not T212/S214, in Alzheimer's disease (AD) brains. These suggested that p70S6K might contribute to tau related pathologies in AD brains.

Insulin-like growth factor-I inhibits dexamethasone-induced proteolysis in cultured L6 myotubes through PI3K/Akt/GSK-3beta and PI3K/Akt/mTOR-dependent mechanisms

We and others reported previously that IGF-I inhibits dexamethasone-induced proteolysis in cultured L6 myotubes. Recent evidence suggests that this effect of IGF-I at least in part reflects PI3K/Akt-mediated inhibition of Foxo transcription factors. The potential role of other mechanisms, downstream of PI3K/Akt, is not well understood. Here we tested the hypothesis that PI3K/Akt-mediated inactivation of GSK-3beta and activation of mTOR contribute to the anabolic effects of IGF-I in dexamethasone-treated myotubes. Cultured L6 myotubes were treated with 1 microM dexamethasone in the absence or presence of 0.1 microg/ml of IGF-I and inhibitors of GSK-3beta and mTOR. Protein degradation was measured by determining the release of trichloroacetic acid soluble radioactivity from myotubes that had been prelabeled with (3)H-tyrosine for 48 h. IGF-I reduced basal protein breakdown rates and completely abolished the dexamethasone-induced increase in myotube proteolysis. These effects of IGF-I were associated with increased phosphorylation of Akt, GSK-3beta, and the mTOR downstream targets p70(S6K) and 4E-BP1. The PI3K inhibitor LY294002 and the mTOR inhibitor rapamycin reversed the anabolic effect of IGF-I in dexamethasone-treated myotubes. In addition, the GSK-3beta inhibitors LiCl and TDZD-8 reduced protein degradation in a similar fashion as IGF-I. Our results suggest that PI3K/Akt-mediated inactivation of GSK-3beta and activation of mTOR contribute to the anabolic effects of IGF-I in dexamethasone-treated myotubes.

The PI3' Kinase Pathway in Interferon Signaling

Interferons (IFNs) are pleiotropic cytokines with important immunomodulatory, antitumor, antiviral, and growth inhibitory properties. Consistent with the multiplicity of their effects, the IFNs activate several cellular cascades after they bind to their receptors. Although the Jak-Stat pathway is the most studied pathway, it appears that additional signaling cascades play roles in IFN signaling. The phosphatidylinositol 3'-kinase (PI3'K) pathway has emerged as one of the critical players in IFN signal transduction and is the focus of this review. This pathway is activated distinctively from the Stat pathway but plays important roles in the regulation of IFN-dependent gene transcription and initiation of mRNA translation.

IGF-I stimulates protein synthesis in skeletal muscle through multiple signaling pathways during sepsis

Chronic septic abscess formation causes an inhibition of protein synthesis in gastrocnemius not observed in rats with a sterile abscess. Inhibition is associated with an impaired mRNA translation initiation that can be ameliorated by elevating IGF-I but not insulin. The present study investigated the ability of IGF-I signaling to stimulate protein synthesis in gastrocnemius by accelerating mRNA translation initiation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Protein synthesis in gastrocnemius from septic rats was accelerated twofold by the addition of IGF-I (10 nM) to perfusate. IGF-I increased the phosphorylation of translation repressor 4E-binding protein-1 (4E-BP1). Hyperphosphorylation of 4E-BP1 in response to IGF-I resulted in its dissociation from the inactive eukaryotic initiation factor (eIF) 4E.4E-BP1 complex. Assembly of the active eIF4F complex (as assessed by the association eIF4G with eIF4E) was increased twofold by IGF-I in the perfusate. In addition, phosphorylation of eIF4G and ribosomal protein S6 kinase-1 (S6K1) was also enhanced by IGF-I. Activation of mammalian target of rapamycin, an upstream kinase implicated in phosphorylating both 4E-BP1 and S6K1, was also observed. Thus the ability of IGF-I to accelerate protein synthesis during sepsis may be related to a stimulation of signaling to multiple steps in translation initiation with an ensuing increased phosphorylation of eIF4G, eIF4E availability, and S6K1 phosphorylation.

Mechanisms of type-I interferon signal transduction

Interferons regulate a number of biological functions including control of cell proliferation, generation of antiviral activities and immumodulation in human cells. Studies by several investigators have identified a number of cellular signaling cascades that are activated during engagement of interferon receptors. The activation of multiple signaling cascades by the interferon receptors appears to be critical for the generation of interferon-mediated biological functions and immune surveillance. The present review summarizes the existing knowledge on the multiple signaling cascades activated by Type I interferons. Recent developments in this research area are emphasized and the implications of these new discoveries on our understanding of interferon actions are discussed.

Protein breakdown in muscle from burned rats is blocked by insulin-like growth factor i and glycogen synthase kinase-3beta inhibitors

We reported previously that IGF-I inhibits burn-induced muscle proteolysis. Recent studies suggest that activation of the phosphotidylinositol 3-kinase (PI3K)/Akt signaling pathway with downstream phosphorylation of Forkhead box O transcription factors is an important mechanism of IGF-I-induced anabolic effects in skeletal muscle. The potential roles of other mechanisms in the anabolic effects of IGF-I are less well understood. In this study we tested the roles of mammalian target of rapamycin and glycogen synthase kinase-3beta (GSK-3beta) phosphorylation as well as MAPK- and calcineurin-dependent signaling pathways in the anticatabolic effects of IGF-I by incubating extensor digitorum longus muscles from burned rats in the presence of IGF-I and specific signaling pathway inhibitors. Surprisingly, the PI3K inhibitors LY294002 and wortmannin reduced basal protein breakdown. No additional inhibition by IGF-I was noticed in the presence of LY294002 or wortmannin. Inhibition of proteolysis by IGF-I was associated with phosphorylation (inactivation) of GSK-3beta. In addition, the GSK-3beta inhibitors, lithium chloride and thiadiazolidinone-8, reduced protein breakdown in a similar fashion as IGF-I. Lithium chloride, but not thiadiazolidinone-8, increased the levels of phosphorylated Foxo 1 in incubated muscles from burned rats. Inhibitors of mammalian target of rapamycin, MAPK, and calcineurin did not prevent the IGF-I-induced inhibition of muscle proteolysis. Our results suggest that IGF-I inhibits protein breakdown at least in part through a PI3K/Akt/GSK3beta-dependent mechanism. Additional experiments showed that similar mechanisms were responsible for the effect of IGF-I in muscle from nonburned rats. Taken together with recent reports in the literature, the present results suggest that IGF-I inhibits protein breakdown in skeletal muscle by multiple mechanisms, including PI3K/Akt-mediated inactivation of GSK-3beta and Foxo transcription factors.

IGF-I activates the eIF4F system in cardiac muscle in vivo

IGF-I acutely stimulates protein synthesis in cardiac muscle through acceleration of mRNA translation. In the present study, we examined the regulatory signaling pathways and translation protein factors that potentially contribute to the myocardial responsiveness of protein synthesis to IGF-I in vivo. IGF-I was injected IV into rats and 20 min later the hearts were excised and homogenized for assay of regulatory proteins. IGF-I increased assembly of the translationally active eukaryotic initiation factor (eIF)4G.eIF4E complex. The increased assembly of eIF4G.eIF4E was associated with an enhanced eIF4G phosphorylation and increased availability of eIF4E. Increased availability of eIF4E occurred as a consequence of diminished abundance of the inactive 4E-BP1.eIF4E complex following IGF-I. The assembly of the 4E-BP1.eIF4E complex appeared to be decreased through an IGF-I-induced phosphorylation of 4E-BP1. IGF-I also caused an increase in the phosphorylation of S6K1. Activation of the potential upstream regulators of 4E-BP1 and S6K1 phosphorylation via PKB and mTOR was also observed. In contrast, there was no effect of IGF-I on phosphorylation of elongation factor (eFE)2. The results suggest the major impact of IGF-I in cardiac muscle occurred via stimulation of translation initiation rather than elongation. Furthermore, the results are consistent with a role for assembly of active eIF4G.eIF4E complex and activation of S6K1 in mediating the stimulation of mRNA translation initiation by IGF-I through a PKB/mTOR signaling pathway.

Differential regulation of the p70 S6 kinase pathway by interferon alpha (IFNalpha) and imatinib mesylate (STI571) in chronic myelogenous leukemia cells

The precise mechanisms by which imatinib mesylate (STI571) and interferon alpha (IFNalpha) exhibit antileukemic effects are not known. We examined the effects of IFNs or imatinib mesylate on signaling pathways regulating initiation of mRNA translation in BCR-ABL-expressing cells. Treatment of IFN-sensitive KT-1 cells with IFNalpha resulted in phosphorylation/activation of mammalian target of rapamycin (mTOR) and downstream activation of p70 S6 kinase. The IFN-activated p70 S6 kinase was found to regulate phosphorylation of S6 ribosomal protein, which regulates translation of mRNAs with oligopyrimidine tracts in the 5'-untranslated region. In addition, IFNalpha treatment resulted in an mTOR- and/or phosphatidyl-inositol 3'(PI 3') kinase-dependent phosphorylation of 4E-BP1 repressor of mRNA translation on sites that are required for its deactivation and dissociation from the eukaryotic initiation factor-4E (eIF4E) complex. In contrast to the effects of IFNs, imatinib mesylate suppressed p70 S6 kinase activity, consistent with inhibition of BCR-ABL-mediated activation of the mTOR/p70 S6 kinase pathway. Moreover, the mTOR inhibitor rapamycin enhanced the suppressive effects of imatinib mesylate on primary leukemic granulocyte macrophage-colony-forming unit (CFU-GM) progenitors from patients with chronic myelogenous leukemia (CML). Taken altogether, our data demonstrate that IFNs and imatinib mesylate differentially regulate PI 3' kinase/mTOR-dependent signaling cascades in BCR-ABL-transformed cells, consistent with distinct effects of these agents on pathways regulating mRNA translation. They also support the concept that combined use of imatinib mesylate with mTOR inhibitors may be an appropriate future therapeutic strategy for the treatment of CML.

Acute alcohol intoxication enhances myocardial eIF4G phosphorylation despite reducing mTOR signaling

Acute alcohol intoxication impairs myocardial protein synthesis in rats, secondary to a diminished mRNA translational efficiency. Decreased mRNA translational efficiency occurs through altered regulation of peptide chain initiation. The purpose of the present set of experiments was to determine whether acute alcohol intoxication alters the phosphorylation state of eukaryotic initiation factor (eIF) 4G, eIF4G·eIF4E complex formation, and the mammalian target of rapamycin (mTOR) signaling pathway in the heart. Acute alcohol intoxication was induced by injection of alcohol (75 mmol/kg body wt ip). Control animals received an equal volume of saline. Alcohol administration enhanced phosphorylation of eIF4G (Ser1108) approximately threefold. Alcohol administration lowered formation of the active eIF4G·eIF4E complex by >90%, whereas it increased the abundance of the inactive 4E-binding protein 1 (4E-BP1)·eIF4E complex by ∼160%. Phosphorylation of mTOR on Ser2448and Ser2481was decreased by 50%. Reduced mTOR phosphorylation did not result from decreased phosphorylation of PKB. Phosphorylation of 4E-BP1 and S6 kinase 1 (Thr389), downstream targets of mTOR, were also reduced after acute alcohol administration. These data suggest that acute alcohol-induced impairments in myocardial mRNA translation initiation result, in part, from marked decreases in eIF4G·eIF4E complex formation, which appear to be independent of changes in phosphorylation of eIF4G but dependent on mTOR.

Hydrogen peroxide mediates Rac1 activation of S6K1

We previously reported that hydrogen peroxide (H2O2) mediates mitogen activation of ribosomal protein S6 kinase 1 (S6K1) which plays an important role in cell proliferation and growth. In this study, we investigated a possible role of H2O2 as a molecular linker in Rac1 activation of S6K1. Overexpression of recombinant catalase in NIH-3T3 cells led to the drastic inhibition of H2O2 production by PDGF, which was accompanied by a decrease in S6K1 activity. Similarly, PDGF activation of S6K1 was significantly inhibited by transient transfection or stable transfection of the cells with a dominant-negative Rac1 (Rac1N17), while overexpression of constitutively active Rac1 (Rac1V12) in the cells led to an increase in basal activity of S6K1. In addition, stable transfection of Rat2 cells with Rac1N17 dramatically attenuated the H2O2 production by PDGF as compared with that in the control cells. In contrast, Rat2 cells stably transfected with Rac1V12 produced high level of H2O2 in the absence of PDGF, comparable to that in the control cells stimulated with PDGF. More importantly, elimination of H2O2 produced in Rat2 cells overexpressing Rac1V12 inhibited the Rac1V12 activation of S6K1, indicating the possible role of H2O2 as a mediator in the activation of S6K1 by Rac1. However, H2O2 could be also produced via other pathway, which is independent of Rac1 or PI3K, because in Rat2 cells stably transfected with Rac1N17, H2O2 could be produced by arsenite, which has been shown to be a stimulator of H2O2 production. Taken together, these results suggest that H2O2 plays a pivotal role as a mediator in Rac1 activation of S6K1.

Activation of the p70 S6 kinase by all-trans-retinoic acid in acute promyelocytic leukemia cells

Although the mechanisms by which all-trans-retinoic acid (RA) regulates gene transcription are well understood, very little is known on the signaling events regulating RA-dependent initiation of mRNA translation. We examined whether the mammalian target of rapamycin (mTOR)/p70 S6 kinase pathway is activated by RA. RA treatment of sensitive cell lines resulted in phosphorylation/activation of mTOR and downstream induction of p70 S6 kinase activity. Such phosphorylation/activation of p70 S6 kinase was inducible in primary acute promyelocytic leukemia (APL) blasts and RA-sensitive NB-4 cells, but was defective in an NB-4 variant cell line (NB-4.007/6) that is resistant to the biologic effects of RA. The RA-dependent activation of p70 S6 kinase was also phosphatidylinositol 3' kinase (PI3'K)-dependent, and resulted in downstream phosphorylation of the S6 ribosomal protein on Ser235/236 and Ser240/244, events important for initiation of translation for mRNAs with oligopyrimidine tracts in their 5' untranslated region. RA treatment of leukemia cells also resulted in an mTOR-mediated phosphorylation of the 4E-BP1 repressor of mRNA translation, to induce its deactivation and dissociation from the eukaryotic initiation factor-4E (eIF-4E) complex. Altogether, these findings provide evidence for the existence of a novel RA-activated cellular pathway that regulates cap-dependent translation, and strongly suggest that this cascade plays a role in the induction of retinoid responses in APL cells.

Restoration of protein synthesis in heart and skeletal muscle after withdrawal of alcohol

BACKGROUND

The rate of protein synthesis is diminished after chronic alcohol consumption through changes in both mRNA translation initiation and elongation. It remains unknown how long adverse effects of alcohol on protein synthesis persist after withdrawal from ethanol.

METHODS

We examined the effect of removal of alcohol from the diet of rats for 72 hr after chronic alcohol exposure (16 weeks) on rates of protein synthesis and potential mechanisms for controlling mRNA translation in heart, skeletal muscle, and liver. Rates of protein synthesis were measured after intravenous infusion of [3H]-l-phenylalanine. The formation of active eukaryotic initiation factor (eIF)4E.eIF4G complex, the cellular content of eukaryotic elongation factor (eEF)1A and eEF2, and the phosphorylation state of eEF2 and S6K1 were measured in each tissue.

RESULTS

Withdrawal of alcohol from the diet restored protein synthesis in heart and skeletal muscle to values obtained in pair-fed control rats not exposed to alcohol. However, the organ weight and protein content per muscle was not affected by withdrawal of alcohol from the diet. In both heart and skeletal muscle, the restoration of protein synthesis correlated with reversal of defects in the formation of active eIF4E.eIF4G complex and eEF1A content. Myocardial eEF2 content was also restored to control values after withdrawal of alcohol from the diet. In the gastrocnemius, there was a decrease in the cellular content of eEF2. The lower eIF2 content may have been counterbalanced by an increased activity of eEF2 through a reduction in the phosphorylation state of eEF2 allowing protein synthesis to proceed unimpeded.

CONCLUSIONS

These studies indicate that changes in protein metabolism observed during chronic alcohol intake are reversible and do not, at this stage, represent an irreversible change in cardiac or skeletal muscle.

G1 cell cycle progression and the expression of G1 cyclins are regulated by PI3K/AKT/mTOR/p70S6K1 signaling in human ovarian cancer cells

Ovarian cancer is one of the most common cancers among women. Recent studies demonstrated that the gene encoding the p110alpha catalytic subunit of phosphatidylinositol 3-kinase (PI3K) is frequently amplified in ovarian cancer cells. PI3K is involved in multiple cellular functions, including proliferation, differentiation, antiapoptosis, tumorigenesis, and angiogenesis. In this study, we demonstrate that the inhibition of PI3K activity by LY-294002 inhibited ovarian cancer cell proliferation and induced G(1) cell cycle arrest. This effect was accompanied by the decreased expression of G(1)-associated proteins, including cyclin D1, cyclin-dependent kinase (CDK) 4, CDC25A, and retinoblastoma phosphorylation at Ser(780), Ser(795), and Ser(807/811). Expression of CDK6 and beta-actin was not affected by LY-294002. Expression of the cyclin kinase inhibitor p16(INK4a) was induced by the PI3K inhibitor, whereas steady-state levels of p21(CIP1/WAF1) were decreased in the same experiment. The inhibition of PI3K activity also inhibited the phosphorylation of AKT and p70S6K1, but not extracellular regulated kinase 1/2. The G(1) cell cycle arrest induced by LY-294002 was restored by the expression of active forms of AKT and p70S6K1 in the cells. Our study shows that PI3K transmits a mitogenic signal through AKT and mammalian target of rapamycin (mTOR) to p70S6K1. The mTOR inhibitor rapamycin had similar inhibitory effects on G(1) cell cycle progression and on the expression of cyclin D1, CDK4, CDC25A, and retinoblastoma phosphorylation. These results indicate that PI3K mediates G(1) progression and cyclin expression through activation of an AKT/mTOR/p70S6K1 signaling pathway in the ovarian cancer cells.

Unique, highly proliferative growth phenotype expressed by embryonic and neointimal smooth muscle cells is driven by constitutive Akt, mTOR, and p70S6K signaling and is actively repressed by PTEN

BACKGROUND

At distinct times during embryonic development and after vascular injury, smooth muscle cells (SMCs) exhibit a highly proliferative, serum-independent growth phenotype. The aim of the present study was to evaluate the functional role of S6 ribosomal protein (S6RP) and upstream positive and negative regulators in the control of SMC serum-independent growth.

METHODS AND RESULTS

We previously reported increased expression of S6RP mRNA was associated with this unique growth phenotype. Using immunohistochemistry and Western blot analysis, we report high levels of total and phospho-S6RP and increased levels of Akt and p70S6K phosphorylation, upstream positive regulators of S6RP, in rat embryonic aortas and adult balloon-injured carotid arteries compared with quiescent adult aortas and uninjured carotid arteries. Western blot analysis demonstrated that cultured embryonic and neointimal SMCs that exhibited serum-independent growth capabilities expressed high levels of S6RP and constitutively active Akt, mTOR, and p70S6K. Pharmacological and molecular inhibition of phosphatidylinositol 3-kinase (PI3K) signaling pathways, using PI3K inhibitors, rapamycin, or dominant-negative Akt adenovirus, suppressed embryonic and neointimal SMC serum-independent growth. Finally, decreased activity of PTEN, an endogenous negative regulator of PI3K signaling, was associated with high in vivo SMC growth rates, and morpholino-mediated loss of endogenous PTEN induced a serum-independent growth phenotype in cultured serum-dependent SMCs.

CONCLUSIONS

The possibility exists that cells that exhibit a distinct embryonic-like growth phenotype different from traditional SMCs are major contributors to intimal thickening. Growth of SMCs that exhibit this phenotype is dependent on constitutive Akt and mTOR/p70S6K signaling and is actively inhibited through the timed acquisition of the endogenously produced growth suppressor PTEN.

Gene and protein kinase expression profiling of reactive oxygen species-associated lipotoxicity in the pancreatic beta-cell line MIN6

Oligonucleotide microarrays were used to define oleic acid (OA)-regulated gene expression and proteomic technology to screen protein kinases in MIN6 insulinoma cells. The effects of oxidative stress caused by OA and potential protective effects of N-acetyl-L-cysteine (NAC), a scavenger of reactive oxygen species (ROS), on global gene expression and beta-cell function were investigated. Long-term exposure of MIN6 cells to OA led to a threefold increase in basal insulin secretion, a 50% decrease in insulin content, an inhibition of glucose-stimulated insulin secretion (GSIS), and a twofold increase in the level of ROS. The addition of NAC normalized both the OA-induced insulin content and ROS elevation, but it failed to restore GSIS. Microarray studies and subsequent quantitative PCR analysis showed that OA consistently regulated the expression of 45 genes involved in metabolism, cell growth, signal transduction, transcription, and protein processing. The addition of NAC largely normalized the expression of the OA-regulated genes involved in cell growth and differentiation but not other functions. A protein kinase screen showed that OA regulated the expression and/or phosphorylation levels of kinases involved in stress-response mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and cell cycle control pathways. Importantly, these findings indicate that chronic OA exposure can impair beta-cell function through ROS-dependent and -independent mechanisms.