The extracellular signal-regulated kinase pathway regulates the phosphorylation of 4E-BP1 at multiple sites

CDK1-loaded extracellular vesicles promote cell cycle to reverse impaired wound healing in diabetic obese mice

Small extracellular vesicles (sEVs) mediate intercellular signaling to coordinate the proliferation of cell types that promote re-epithelialization of skin following injury. Cyclin-dependent kinase 1 (CDK1) drives cell division and is a key regulator of entry to the cell cycle. To understand the potential of sEV-mediated delivery of CDK1 to reverse impaired wound healing, we generated CDK1-loaded sEVs (CDK1-sEVs) and evaluated their ability to mediate cell proliferation, re-epithelialization, and downstream signaling responses in the wound bed. We found that treatment of human keratinocytes with CDK1-sEVs increased phosphorylation of the CDK1 target, eukaryotic translation inhibition factor 4E-binding protein 1 (4E-BP1), and histone H3 within 24 h via AKT and ERK phosphorylation, driving increased proliferation and cell migration. Treatment of the wound bed of diabetic obese mice, a model of delayed wound healing, with a single dose of CDK1-sEVs accelerated wound closure, increased re-epithelialization, and promoted the proliferation of keratinocytes. These studies show that delivery of CDK1 by sEVs can stimulate selective and transient proliferation of cell types that increase re-epithelialization and promote proliferation of keratinocytes to accelerate wound healing.

Increased PHLPP1 expression through ERK-4E-BP1 signaling axis drives nicotine induced oxidative stress related damage of cardiomyocytes

Nicotine, a key constituent of tobacco/electronic cigarettes causes cardiovascular injury and mortality. Nicotine is known to induce oxidative stress and mitochondrial dysfunction in cardiomyocytes leading to cell death. However, the underlying mechanisms remain unclear. Pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP) is a member of metal-dependent protein phosphatase (PPM) family and is known to dephosphorylate several AGC family kinases and thereby regulate a diverse set of cellular functions including cell growth, survival, and death. Our lab has previously demonstrated that PHLPP1 removal reduced cardiomyocyte death and cardiac dysfunction following injury. Here, we present a novel finding that nicotine exposure significantly increased PHLPP1 protein expression in the adolescent rodent heart. Building upon our in vivo finding, we determined the mechanism of PHLPP1 expression in cardiomyocytes. Nicotine significantly increased PHLPP1 protein expression without altering PHLPP2 in cardiomyocytes. In cardiomyocytes, nicotine significantly increased NADPH oxidase 4 (NOX4), which coincided with increased reactive oxygen species (ROS) and increased cardiomyocyte apoptosis which were dependent on PHLPP1 expression. PHLPP1 expression was both necessary and sufficient for nicotine induced mitochondrial dysfunction. Mechanistically, nicotine activated extracellular signal-regulated protein kinases (ERK1/2) and subsequent eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) to increase PHLPP1 protein expression. Inhibition of protein synthesis with cycloheximide (CHX) and 4EGI-1 abolished nicotine induced PHLPP1 protein expression. Moreover, inhibition of ERK1/2 activity by U0126 significantly blocked nicotine induced PHLPP1 expression. Overall, this study reveals a novel mechanism by which nicotine regulates PHLPP1 expression through ERK-4E-BP1 signaling axis to drive cardiomyocyte injury.

Multiple Roles of PLK1 in Mitosis and Meiosis

Cells are equipped with a diverse network of signaling and regulatory proteins that function as cell cycle regulators and checkpoint proteins to ensure the proper progression of cell division. A key regulator of cell division is polo-like kinase 1 (PLK1), a member of the serine/threonine kinase family that plays an important role in regulating the mitotic and meiotic cell cycle. The phosphorylation of specific substrates mediated by PLK1 controls nuclear envelope breakdown (NEBD), centrosome maturation, proper spindle assembly, chromosome segregation, and cytokinesis. In mammalian oogenesis, PLK1 is essential for resuming meiosis before ovulation and for establishing the meiotic spindle. Among other potential roles, PLK1 regulates the localized translation of spindle-enriched mRNAs by phosphorylating and thereby inhibiting the translational repressor 4E-BP1, a downstream target of the mTOR (mammalian target of rapamycin) pathway. In this review, we summarize the functions of PLK1 in mitosis, meiosis, and cytokinesis and focus on the role of PLK1 in regulating mRNA translation. However, knowledge of the role of PLK1 in the regulation of meiosis remains limited.

Hippocampal mTOR Dysregulation and Morphological Changes in Male Rats after Fetal Growth Restriction

Fetal growth restriction (FGR) has been linked to long-term neurocognitive impairment, especially in males. To determine possible underlying mechanisms, we examined hippocampal cellular composition and mTOR signaling of male rat FGR offspring during main brain growth and development (postnatal days (PND) 1 and 12). FGR was either induced by a low-protein diet throughout pregnancy, experimental placental insufficiency by bilateral uterine vessel ligation or intrauterine stress by “sham” operation. Offspring after unimpaired gestation served as common controls. Low-protein diet led to a reduced cell density in the molecular dentate gyrus subregion, while intrauterine surgical stress was associated with increased cell density in the cellular CA2 subregion. Experimental placental insufficiency caused increased mTOR activation on PND 1, whereas intrauterine stress led to mTOR activation on PND 1 and 12. To determine long-term effects, we additionally examined mTOR signaling and Tau phosphorylation, which is altered in neurodegenerative diseases, on PND 180, but did not find any changes among the experimental groups. Our findings suggest that hippocampal cellular proliferation and mTOR signaling are dysregulated in different ways depending on the cause of FGR. While a low-protein diet induced a decreased cell density, prenatal surgical stress caused hyperproliferation, possibly via increased mTOR signaling.

Activation of ERK and p38 Reduces AZD8055-Mediated Inhibition of Protein Synthesis in Hepatocellular Carcinoma HepG2 Cell Line

Protein synthesis is important for maintaining cellular homeostasis under various stress responses. In this study, we screened an anticancer drug library to select compounds with translational repression functions. AZD8055, an ATP-competitive mechanistic target of rapamycin complex 1/2 (mTORC1/2) inhibitor, was selected as a translational suppressor. AZD8055 inhibited protein synthesis in mouse embryonic fibroblasts and hepatocellular carcinoma HepG2 cells. Extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) were activated during the early phase of mTORC1/2 inhibition by AZD8055 treatment. Combined treatment of AZD8055 with the MAPK kinase1/2 (MEK1/2) inhibitor refametinib or the p38 inhibitor SB203580 markedly decreased translation in HepG2 cells. Thus, the inhibition of ERK1/2 or p38 may enhance the efficacy of AZD8055-mediated inhibition of protein synthesis. In addition, AZD8055 down-regulated the phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), and AZD8055-induced phosphorylation of ERK1/2 and p38 had no effect on phosphorylation status of 4E-BP1. Interestingly, AZD8055 modulated the 4E-BP1 mRNA pool by up-regulating ERK1/2 and p38 pathways. Together, these results suggest that AZD8055-induced activation of MAPKs interferes with inhibition of protein synthesis at an early stage of mTORC1/2 inhibition, and that it may contribute to the development of resistance to mTORC1/2 inhibitors.

mTOR-dependent translation amplifies microglia priming in aging mice

Microglia maintain homeostasis in the brain. However, with age, they become primed and respond more strongly to inflammatory stimuli. We show here that microglia from aged mice had upregulated mTOR complex 1 signaling controlling translation, as well as protein levels of inflammatory mediators. Genetic ablation of mTOR signaling showed a dual yet contrasting effect on microglia priming: it caused an NF-κB-dependent upregulation of priming genes at the mRNA level; however, mice displayed reduced cytokine protein levels, diminished microglia activation, and milder sickness behavior. The effect on translation was dependent on reduced phosphorylation of 4EBP1, resulting in decreased binding of eIF4E to eIF4G. Similar changes were present in aged human microglia and in damage-associated microglia, indicating that upregulation of mTOR-dependent translation is an essential aspect of microglia priming in aging and neurodegeneration.

Inhibitors of the PI3K/Akt/mTOR Pathway in Prostate Cancer Chemoprevention and Intervention

The phosphatidylinositol 3-kinase (PI3K)/serine-threonine kinase (Akt)/mammalian target of the rapamycin (mTOR)-signaling pathway has been suggested to have connections with the malignant transformation, growth, proliferation, and metastasis of various cancers and solid tumors. Relevant connections between the PI3K/Akt/mTOR pathway, cell survival, and prostate cancer (PC) provide a great therapeutic target for PC prevention or treatment. Recent studies have focused on small-molecule mTOR inhibitors or their usage in coordination with other therapeutics for PC treatment that are currently undergoing clinical testing. In this study, the function of the PI3K/Akt/mTOR pathway, the consequence of its dysregulation, and the development of mTOR inhibitors, either as an individual substance or in combination with other agents, and their clinical implications are discussed. The rationale for targeting the PI3K/Akt/mTOR pathway, and specifically the application and potential utility of natural agents involved in PC treatment is described. In addition to the small-molecule mTOR inhibitors, there are evidence that several natural agents are able to target the PI3K/Akt/mTOR pathway in prostatic neoplasms. These natural mTOR inhibitors can interfere with the PI3K/Akt/mTOR pathway through multiple mechanisms; however, inhibition of Akt and suppression of mTOR 1 activity are two major therapeutic approaches. Combination therapy improves the efficacy of these inhibitors to either suppress the PC progression or circumvent the resistance by cancer cells.

The Role of Processed Aloe vera Gel in Intestinal Tight Junction: An In Vivo and In Vitro Study

Leaky gut is a condition of increased paracellular permeability of the intestine due to compromised tight junction barriers. In recent years, this affliction has drawn the attention of scientists from different fields, as a myriad of studies prosecuted it to be the silent culprit of various immune diseases. Due to various controversies surrounding its culpability in the clinic, approaches to leaky gut are restricted in maintaining a healthy lifestyle, avoiding irritating factors, and practicing alternative medicine, including the consumption of supplements. In the current study, we investigate the tight junction-modulating effects of processed Aloe vera gel (PAG), comprising 5–400-kD polysaccharides as the main components. Our results show that oral treatment of 143 mg/kg PAG daily for 10 days improves the age-related leaky gut condition in old mice, by reducing their individual urinal lactulose/mannitol (L/M) ratio. In concordance with in vivo experiments, PAG treatment at dose 400 μg/mL accelerated the polarization process of Caco-2 monolayers. The underlying mechanism was attributed to enhancement in the expression of intestinal tight junction-associated scaffold protein zonula occludens (ZO)-1 at the translation level. This was induced by activation of the MAPK/ERK signaling pathway, which inhibits the translation repressor 4E-BP1. In conclusion, we propose that consuming PAG as a complementary food has the potential to benefit high-risk patients.

mTOR pathway and DNA damage response: A therapeutic strategy in cancer therapy

The mammalian target of rapamycin (mTOR) is a conserved serine/threonine-protein kinase, comprising two subunit protein complexes: mTORC1 and mTORC2. In response to insult and cancer, the mTOR pathway plays a crucial role in regulating growth, metabolism, cell survival, and protein synthesis. Key subunits of mTORC1/2 catalyze the phosphorylation of various molecules, including eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1), ribosomal protein S6 kinase β-1 (S6K1). The DNA damage response (DDR) maintains genomic stability and provides an opportunity for treating tumors with defects caused by DNA damaging agents. Many mTOR inhibitors are utilized for the treatment of cancers. However, several clinical trials are still assessing the efficacy of mTOR inhibitors. This paper discusses the role of the mTOR signaling pathway and its regulators in developing cancer. In the following, we will review the interaction between DDR and mTOR signaling and the innovative therapies applied in preclinical and clinical trials for treating cancers.

RIOK2 phosphorylation by RSK promotes synthesis of the human small ribosomal subunit

Ribosome biogenesis lies at the nexus of various signaling pathways coordinating protein synthesis with cell growth and proliferation. This process is regulated by well-described transcriptional mechanisms, but a growing body of evidence indicates that other levels of regulation exist. Here we show that the Ras/mitogen-activated protein kinase (MAPK) pathway stimulates post-transcriptional stages of human ribosome synthesis. We identify RIOK2, a pre-40S particle assembly factor, as a new target of the MAPK-activated kinase RSK. RIOK2 phosphorylation by RSK stimulates cytoplasmic maturation of late pre-40S particles, which is required for optimal protein synthesis and cell proliferation. RIOK2 phosphorylation facilitates its release from pre-40S particles and its nuclear re-import, prior to completion of small ribosomal subunits. Our results bring a detailed mechanistic link between the Ras/MAPK pathway and the maturation of human pre-40S particles, which opens a hitherto poorly explored area of ribosome biogenesis.

mTORC1 promotes TOP mRNA translation through site-specific phosphorylation of LARP1

LARP1 is a key repressor of TOP mRNA translation. It binds the m7Gppp cap moiety and the adjacent 5'TOP motif of TOP mRNAs, thus impeding the assembly of the eIF4F complex on these transcripts. mTORC1 controls TOP mRNA translation via LARP1, but the details of the mechanism are unclear. Herein we elucidate the mechanism by which mTORC1 controls LARP1's translation repression activity. We demonstrate that mTORC1 phosphorylates LARP1 in vitro and in vivo, activities that are efficiently inhibited by rapamycin and torin1. We uncover 26 rapamycin-sensitive phospho-serine and -threonine residues on LARP1 that are distributed in 7 clusters. Our data show that phosphorylation of a cluster of residues located proximally to the m7Gppp cap-binding DM15 region is particularly sensitive to rapamycin and regulates both the RNA-binding and the translation inhibitory activities of LARP1. Our results unravel a new model of translation control in which the La module (LaMod) and DM15 region of LARP1, both of which can directly interact with TOP mRNA, are differentially regulated: the LaMod remains constitutively bound to PABP (irrespective of the activation status of mTORC1), while the C-terminal DM15 'pendular hook' engages the TOP mRNA 5'-end to repress translation, but only in conditions of mTORC1 inhibition.

LFA-1 cluster formation in T-cells depends on L-plastin phosphorylation regulated by P90RSK and PP2A

The integrin LFA-1 is crucial for T-cell/ APC interactions and sensitive recognition of antigens. Precise nanoscale organization and valency regulation of LFA-1 are mandatory for an appropriate function of the immune system. While the inside-out signals regulating the LFA-1 affinity are well described, the molecular mechanisms controlling LFA-1 avidity are still not fully understood. Here, we show that activation of the actin-bundling protein L-plastin (LPL) through phosphorylation at serine-5 enables the formation of clusters containing LFA-1 in high-affinity conformation. Phosphorylation of LPL is induced by an nPKC-MEK-p90RSK pathway and counter-regulated by the serine-threonine phosphatase PP2A. Interestingly, recruitment of LFA-1 into the T-cell/APC contact zone is not affected by LPL phosphorylation. Instead, for this process, activation of the actin-remodeling protein cofilin through dephosphorylation is essential. Together, this study reveals a dichotomic spatial regulation of LFA-1 clustering and microscale movement in T-cells by two different actin-binding proteins, LPL and cofilin.

Differential responses to kinase inhibition in FGFR2-addicted triple negative breast cancer cells: a quantitative phosphoproteomics study

Fibroblast Growth Factor (FGF) dependent signalling is frequently activated in cancer by a variety of different mechanisms. However, the downstream signal transduction pathways involved are poorly characterised. Here a quantitative differential phosphoproteomics approach, SILAC, is applied to identify FGF-regulated phosphorylation events in two triple- negative breast tumour cell lines, MFM223 and SUM52, that exhibit amplified expression of FGF receptor 2 (FGFR2) and are dependent on continued FGFR2 signalling for cell viability. Comparative Gene Ontology proteome analysis revealed that SUM52 cells were enriched in proteins associated with cell metabolism and MFM223 cells enriched in proteins associated with cell adhesion and migration. FGFR2 inhibition by SU5402 impacts a significant fraction of the observed phosphoproteome of these cells. This study expands the known landscape of FGF signalling and identifies many new targets for functional investigation. FGF signalling pathways are found to be flexible in architecture as both shared, and divergent, responses to inhibition of FGFR2 kinase activity in the canonical RAF/MAPK/ERK/RSK and PI3K/AKT/PDK/mTOR/S6K pathways are identified. Inhibition of phosphorylation-dependent negative-feedback pathways is observed, defining mechanisms of intrinsic resistance to FGFR2 inhibition. These findings have implications for the therapeutic application of FGFR inhibitors as they identify both common and divergent responses in cells harbouring the same genetic lesion and pathways of drug resistance.

CDK Phosphorylation of Translation Initiation Factors Couples Protein Translation with Cell-Cycle Transition

Protein translation in eukaryotes is cell-cycle dependent, with translation rates more robust in G1 phase of the cell cycle than in mitosis. However, whether the fundamental cell-cycle control machinery directly activates protein translation during the G1/S cell-cycle transition remains unknown. Using the early divergent eukaryote Trypanosoma brucei as a model organism, we report that the G1 cyclin-dependent kinase CRK1 phosphorylates two translation initiation factors, eIF4E4 and PABP1, to promote the G1/S cell-cycle transition and global protein translation. Phosphorylation of eIF4E4 by CRK1 enhances binding to the m⁷G cap structure and interaction with eIF4E4 and eIF4G3, and phosphorylation of PABP1 by CRK1 promotes association with the poly(A) sequence, self-interaction, and interaction with eIF4E4. These findings demonstrate that cyclin-dependent kinase-mediated regulation of translation initiation factors couples global protein translation with the G1/S cell-cycle transition.

Protein translation is cell-cycle dependent, with more robust translation rates in the G1 phase of the cell cycle than in mitosis. An et al. show that the G1 cyclin-dependent kinase CRK1 phosphorylates translation initiation factors eIF4E4 and PABP1 to couple protein translation initiation with the G1/S cell-cycle transition.

S 47445 counteracts the behavioral manifestations and hippocampal neuroplasticity changes in bulbectomized mice

S 47445 is a positive allosteric modulator of glutamate AMPA-type receptors that possesses procognitive, neurotrophic and enhancing synaptic plasticity properties. Its chronic administration promotes antidepressant- and anxiolytic-like effects in different rodent models of depression. We have evaluated the behavioral effects of S 47445 in the bilateral olfactory bulbectomy mice model (OB) and the adaptive changes in those proteins associated to brain neuroplasticity (BDNF and mTOR pathway). Following OB surgery, adult C57BL/6J male mice were chronically administered S 47445 (1, 3 and 10 mg/kg/day; i.p.) and fluoxetine (18 mg/kg/day; i.p.), and then behaviorally tested in the open field test. Afterwards, the expression levels of BDNF, mTOR, phospho-mTOR, 4EBP1 and phospho-4EBP1 were evaluated in hippocampus and prefrontal cortex. Both drugs reduced the OB-induced locomotor activity, a predictive outcome of antidepressant efficacy, with a similar temporal pattern of action. S 47445, but not fluoxetine, showed an anxiolytic effect as reflected by an increased central activity. Chronic administration of S 47445 reversed OB-induced changes in BDNF and phopho-mTOR expression in hippocampus but not in prefrontal cortex. The chronic administration of S 47445 induced antidepressant- and anxiolytic-like effects at low-medium doses (1 and 3 mg/kg/day, i.p.) associated with the reversal of OB-induced changes in hippocampal BDNF and mTOR signaling pathways.

The Secret Life of Translation Initiation in Prostate Cancer

Prostate cancer (PCa) is the second most prevalent cancer in men worldwide. Despite the advances understanding the molecular processes driving the onset and progression of this disease, as well as the continued implementation of screening programs, PCa still remains a significant cause of morbidity and mortality, in particular in low-income countries. It is only recently that defects of the translation process, i.e., the synthesis of proteins by the ribosome using a messenger (m)RNA as a template, have begun to gain attention as an important cause of cancer development in different human tissues, including prostate. In particular, the initiation step of translation has been established to play a key role in tumorigenesis. In this review, we discuss the state-of-the-art of three key aspects of protein synthesis in PCa, namely, misexpression of translation initiation factors, dysregulation of the major signaling cascades regulating translation, and the therapeutic strategies based on pharmacological compounds targeting translation as a novel alternative to those based on hormones controlling the androgen receptor pathway.

Translation regulation in skin cancer from a tRNA point of view

Protein synthesis is a central and dynamic process, frequently deregulated in cancer through aberrant activation or expression of translation initiation factors and tRNAs. The discovery of tRNA-derived fragments, a new class of abundant and, in some cases stress-induced, small Noncoding RNAs has perplexed the epigenomics landscape and highlights the emerging regulatory role of tRNAs in translation and beyond. Skin is the biggest organ in human body, which maintains homeostasis of its multilayers through regulatory networks that induce translational reprogramming, and modulate tRNA transcription, modification and fragmentation, in response to various stress signals, like UV irradiation. In this review, we summarize recent knowledge on the role of translation regulation and tRNA biology in the alarming prevalence of skin cancer.

A rapamycin derivative, biolimus, preferentially activates autophagy in vascular smooth muscle cells

Although rapamycin is a well-known conformational inhibitor of mTORC1, it is now widely used for treating arterial restenosis. Various rapamycin analogues (rapalogue) have been made for applying to drug-eluting stents. Here we show that two major rapalogues, everolimus and biolimus, exert a differential effect on the mTORC1-mediated signaling pathways in vascular smooth muscle cells. In balloon-injured carotid arteries, both rapalogues strongly inhibit neointimal hyperplasia. Signaling pathway analyses reveal that everolimus exert cytotoxicity by increasing cellular reactive oxygen species and consequently reduce energy metabolism. By contrast, biolimus confers a preferential induction of autophagy by more strongly activating major autophagy regulator, ULK1, in vascular smooth muscle cells than everolimus does. As a consequence, the implantation of biolimus-eluting stent reduces endothelial loss, which in turn reduces inflammation, in porcine coronary arteries. Thus, this study reveals that a chemical derivatization can cause a change among mTORC1-dependent signaling pathways in vascular smooth muscle cells, thereby enabling to elicit a differential efficacy on arterial restenosis.

The squirrel with the lagging eIF2: Global suppression of protein synthesis during torpor

Hibernating mammals use strong metabolic rate depression and a reduction in body temperature to near-ambient to survive the cold winter months. During torpor, protein synthesis is suppressed but can resume during interbout arousals. The current study aimed to identify molecular targets responsible for the global suppression of protein synthesis during torpor as well as possible mechanisms that could allow for selective protein translation to continue over this time. Relative changes in protein expression and/or phosphorylation levels of key translation factors (ribosomal protein S6, eIF4E, eIF2α, eEF2) and their upstream regulators (mTOR, TSC2, p70 S6K, 4EBP) were analyzed in liver and kidney of 13-lined ground squirrels (Ictidomys tridecemlineatus) sampled from six points over the torpor-arousal cycle. The results indicate that both organs reduce protein synthesis during torpor by decreasing mTOR and TSC2 phosphorylation between 30 and 70% of control levels. Translation resumes during interbout arousal when p-p70 S6K, p-rpS6, and p-4EBP levels returned to control values or above. Only liver translation factors were activated or disinhibited during periods of torpor itself, with >3-fold increases in total eIF2α and eEF2 protein levels, and a decrease in p-eEF2 (T56) to as low as 16% of the euthermic control value. These data shed light on a possible molecular mechanism involving eIF2α that could enable the translation of key transcripts during times of cell stress.

LARP1 on TOP of ribosome production

The ribosome is an essential unit of all living organisms that commands protein synthesis, ultimately fuelling cell growth (accumulation of cell mass) and cell proliferation (increase in cell number). The eukaryotic ribosome consists of 4 ribosomal RNAs (rRNAs) and 80 ribosomal proteins (RPs). Despite its fundamental role in every living organism, our present understanding of how higher eukaryotes produce the various ribosome components is incomplete. Uncovering the mechanisms utilized by human cells to generate functional ribosomes will likely have far-reaching implications in human disease. Recent biochemical and structural studies revealed La-related protein 1 (LARP1) as a key new player in RP production. LARP1 is an RNA-binding protein that belongs to the LARP superfamily; it controls the translation and stability of the mRNAs that encode RPs and translation factors, which are characterized by a 5' terminal oligopyrimidine (5'TOP) motif and are thus known as TOP mRNAs. The activity of LARP1 is regulated by the mammalian target of rapamycin complex 1 (mTORC1): a eukaryotic protein kinase complex that integrates nutrient sensing with mRNA translation, particularly that of TOP mRNAs. In this review, we provide an overview of the role of LARP1 in the control of ribosome production in multicellular eukaryotes. This article is categorized under: Translation > Translation Regulation RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Processing > Capping and 5' End Modifications.

Recombinant human erythropoietin stimulates melanoma tumor growth through activation of initiation factor eIF4E

Recombinant human erythropoietin (EPO) is standard treatment for anemia in cancer patients. Recent clinical trials suggest that EPO may accelerate tumor progression and increase mortality. However, the evidence supporting a growth-promoting effect of EPO has remained controversial. Employing an in vivo model of B16 murine melanoma, we observed that administration of EPO to tumor bearing C57BL/6 mice resulted in pronounced acceleration of melanoma growth. Our in vitro studies demonstrate that B16 murine melanoma cells express EPOR, both at the protein and mRNA levels. Interestingly, expression of EPOR was retained in the established tumors. EPO stimulation of B16 cells enhanced proliferation and protein synthesis rates, and correlated with activation of the receptor associated Janus kinase 2 (Jak2) as well as phosphorylation of extracellular signal–regulated kinase (Erk) 1/2 and Akt kinases. Treatment with EPO and Jak-2 antagonists significantly inhibited EPO-mediated B16 cell proliferation. Moreover, EPO dose-dependently induced the phosphorylation and activation of the translation initiation factor eIF4E as well as the phosphorylation of its repressor, the eIF4E binding protein 4E-BP1. Finally, using eIF4E small interfering RNA (siRNA), we observed that EPO-mediated stimulation of B16 cell proliferation is eIF4E-dependent. Our results indicate that EPO exerts a powerful stimulatory effect on cell proliferation and de novo protein synthesis in melanoma cells through activation of the initiation factor eIF4E.

Protein Translation in the Nucleus Accumbens Is Dysregulated during Cocaine Withdrawal and Required for Expression of Incubation of Cocaine Craving

Exposure to drug-associated cues can induce drug craving and relapse in abstinent addicts. Cue-induced craving that progressively intensifies ("incubates") during withdrawal from cocaine has been observed in both rats and humans. Building on recent evidence that aberrant protein translation underlies incubation-related adaptations in the NAc, we used male rats to test the hypothesis that translation is dysregulated during cocaine withdrawal and/or when rats express incubated cocaine craving. We found that intra-NAc infusion of anisomycin, a general protein translation inhibitor, or rapamycin, an inhibitor of mammalian target of rapamycin, reduced the expression of incubated cocaine craving, consistent with previous results showing that inhibition of translation in slices normalized the adaptations that maintain incubation. We then examined signaling pathways involved in protein translation using NAc synaptoneurosomes prepared after >47 d of withdrawal from cocaine or saline self-administration, or after withdrawal plus a cue-induced seeking test. The most robust changes were observed following seeking tests. Most notably, we found that eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 2α (eIF2α) are dephosphorylated when cocaine rats undergo a cue-induced seeking test; both effects are consistent with increased translation during the test. Blocking eIF2α dephosphorylation and thereby restoring its inhibitory influence on translation, via intra-NAc injection of Sal003 just before the test, substantially reduced cocaine seeking. These results are consistent with dysregulation of protein translation in the NAc during cocaine withdrawal, enabling cocaine cues to elicit an aberrant increase in translation that is required for the expression of incubated cocaine craving.SIGNIFICANCE STATEMENT Cue-induced cocaine craving progressively intensifies (incubates) during withdrawal in both humans and rats. This may contribute to persistent vulnerability to relapse. We previously demonstrated a role for protein translation in synaptic adaptations in the NAc closely linked to incubation. Here, we tested the hypothesis that translation is dysregulated during cocaine withdrawal, and this contributes to incubated craving. Analysis of signaling pathways regulating translation suggested that translation is enhanced when "incubated" rats undergo a cue-induced seeking test. Furthermore, intra-NAc infusions of drugs that inhibit protein translation through different mechanisms reduced expression of incubated cue-induced cocaine seeking. These results demonstrate that the expression of incubation depends on an acute increase in translation that may result from dysregulation of several pathways.

Distinct PKC-mediated posttranscriptional events set cytokine production kinetics in CD8+ T cells

Effective T cell responses against invading pathogens require the concerted production of three key cytokines: TNF-α, IFN-γ, and IL-2. The cytokines functionally synergize, but their production kinetics widely differ. How the differential timing of expression is regulated remains, however, poorly understood. We compared the relative contribution of transcription, mRNA stability, and translation efficiency on cytokine production in murine effector and memory CD8⁺ T cells. We show that the immediate and ample production of TNF-α is primarily mediated by translation of preformed mRNA through protein kinase C (PKC)-induced recruitment of mRNA to polyribosomes. Also, the initial production of IFN-γ uses translation of preformed mRNA. However, the magnitude and subsequent expression of IFN-γ, and of IL-2, depends on calcium-induced de novo transcription and PKC-dependent mRNA stabilization. In conclusion, PKC signaling modulates translation efficiency and mRNA stability in a transcript-specific manner. These cytokine-specific regulatory mechanisms guarantee that T cells produce ample amounts of cytokines shortly upon activation and for a limited time.

Dendritic overgrowth and elevated ERK signaling during neonatal development in a mouse model of autism

Autism spectrum disorder (hereafter referred to as “ASD”) is a heterogeneous neurodevelopmental condition characterized by impaired social communication and interactions, and restricted, repetitive activities or interests. Alterations in network connectivity and memory function are frequently observed in autism patients, often involving the hippocampus. However, specific changes during early brain development leading to disrupted functioning remain largely unclear. Here, we investigated the development of dendritic arbor of hippocampal CA1 pyramidal neurons in the BTBR T+tf/J (BTBR) mouse model of autism. BTBR mice display the defining behavioural features of autism, and also exhibit impaired learning and memory. We found that compared to control C57BL/6J (B6) animals, the lengths of both apical and basal dendrites were significantly greater in neonatal BTBR animals. Further, basal dendrites in the BTBR mice had higher branching complexity. In contrast, cross-sectional area of the soma was unchanged. In addition, we observed a similar density of CA1 pyramidal neurons and thickness of the neuronal layer between the two strains. Thus, there was a specific, compartmentalized overgrowth of dendrites during early development in the BTBR animals. Biochemical analysis further showed that the extracellular signal-regulated kinases (ERK) pathway was up-regulated in the hippocampus of neonatal BTBR animals. Since dendritic structure is critical for information integration and relay, our data suggest that altered development of dendrites could potentially contribute to impaired hippocampal function and behavior observed in the BTBR model, and that this might be related to increased activation of the ERK pathway.

A researcher's guide to the galaxy of IRESs

The idea of internal initiation is frequently exploited to explain the peculiar translation properties or unusual features of some eukaryotic mRNAs. In this review, we summarize the methods and arguments most commonly used to address cases of translation governed by internal ribosome entry sites (IRESs). Frequent mistakes are revealed. We explain why "cap-independent" does not readily mean "IRES-dependent" and why the presence of a long and highly structured 5' untranslated region (5'UTR) or translation under stress conditions cannot be regarded as an argument for appealing to internal initiation. We carefully describe the known pitfalls and limitations of the bicistronic assay and artefacts of some commercially available in vitro translation systems. We explain why plasmid DNA transfection should not be used in IRES studies and which control experiments are unavoidable if someone decides to use it anyway. Finally, we propose a workflow for the validation of IRES activity, including fast and simple experiments based on a single genetic construct with a sequence of interest.

Monosodium urate crystal-induced pro-interleukin-1β production is post-transcriptionally regulated via the p38 signaling pathway in human monocytes

IL-1β is a key mediator of sterile inflammation in response to endogenous particulates, a type of damage-associated molecular pattern (DAMPs) molecule derived from damaged cells. Despite the well-known role of sterile particulates such as monosodium urate (MSU) crystals as inflammasome inducers in monocytes/macrophages, little is known regarding how pro-IL-1β synthesis is induced under sterile inflammatory conditions. We provide evidence that MSU crystals post-transcriptionally induce the rapid production of pro-IL-1β in human primary monocytes. Metabolic labeling and pull-down assays for newly-synthesized proteins clearly showed that MSU crystals rapidly, within 30 min, induce the synthesis of pro-IL-1β as well as global proteins. Notably, MSU crystal-induced pro-IL-1β synthesis is selectively dependent on the p38 MAPK pathway, whereas global protein synthesis is mediated via the mTOR, ERK1/2, and p38 pathways. Furthermore, inhibition of Mnk1, a substrate of p38, blocked MSU crystal-induced pro-IL-1β synthesis downstream of eIF4E phosphorylation. In addition, the p38 MAPK pathway leading to phosphorylation of MK2 was also critical for stabilization of pro-IL-1β mRNA following MSU stimulation. Our findings demonstrate that post-transcriptional regulation via p38 MAPK plays a central role in the rapid synthesis of pro-IL-1β in response to MSU crystals, which is an essential step for IL-1β production in human monocytes.

Twist1-mediated 4E-BP1 regulation through mTOR in non-small cell lung cancer

Twist1 overexpression corresponds with poor survival in non-small cell lung cancer (NSCLC), but the underlining mechanism is not clear. The objective of the present study was to investigate the tumorigenic role of Twist1 and its related molecular mechanisms in NSCLC. Twist1 was overexpressed in 34.7% of NSCLC patients. The survival rate was significantly lower in patients with high Twist1 expression than low expression (P < 0.05). Twist1 expression levels were higher in H1650 cells, but relatively lower in H1975 cells. H1650 with stable Twist1 knockdown, H1650shTw, demonstrated a significantly slower rate of wound closure; however, H1975 with stable Twist1 overexpression, H1975Over, had an increased motility velocity. A significant decrease in colony number and size was observed in H1650shTw, but a significant increase in colony number was found in H1975Over (P < 0.05). Tumor growth significantly decreased in mice implanted with H1650shTw compared to H1650 (P < 0.05). 4E-BP1 and p53 gene expressions were increased, but p-4E-BP1 and p-mTOR protein expressions were decreased in H1650shTw. However, 4E-BP1 gene expression was decreased, while p-4E-BP1 and p-mTOR protein expressions were increased in H1975Over. p-4E-BP1 was overexpressed in 24.0% of NSCLC patients. Survival rate was significantly lower in patients with high p-4E-BP1 expression than low p-4E-BP1 (P < 0.01). A significant correlation was found between Twist1 and p-4E-BP1 (P < 0.01). A total of 13 genes in RT-PCR array showed significant changes in H1650shTw. Altogether, Twist1 is correlated with p-4E-BP1 in predicting the prognostic outcome of NSCLC. Inhibition of Twist1 decreases p-4E-BP1 expression possibly through downregulating p-mTOR and increasing p53 expression in NSCLC.

Mitotic protein kinase CDK1 phosphorylation of mRNA translation regulator 4E-BP1 Ser83 may contribute to cell transformation

Mammalian target of rapamycin (mTOR)-directed eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) phosphorylation promotes cap-dependent translation and tumorigenesis. During mitosis, cyclin-dependent kinase 1 (CDK1) substitutes for mTOR and fully phosphorylates 4E-BP1 at canonical sites (T37, T46, S65, and T70) and the noncanonical S83 site, resulting in a mitosis-specific hyperphosphorylated δ isoform. Colocalization studies with a phospho-S83 specific antibody indicate that 4E-BP1 S83 phosphorylation accumulates at centrosomes during prophase, peaks at metaphase, and decreases through telophase. Although S83 phosphorylation of 4E-BP1 does not affect general cap-dependent translation, expression of an alanine substitution mutant 4E-BP1.S83A partially reverses rodent cell transformation induced by Merkel cell polyomavirus small T antigen viral oncoprotein. In contrast to inhibitory mTOR 4E-BP1 phosphorylation, these findings suggest that mitotic CDK1-directed phosphorylation of δ-4E-BP1 may yield a gain of function, distinct from translation regulation, that may be important in tumorigenesis and mitotic centrosome function.

Selective Disruption of Metabotropic Glutamate Receptor 5-Homer Interactions Mimics Phenotypes of Fragile X Syndrome in Mice

Altered function of the Gq-coupled, Group 1 metabotropic glutamate receptors, specifically mGlu5, is implicated in multiple mouse models of autism and intellectual disability. mGlu5 dysfunction has been most well characterized in the fragile X syndrome mouse model, the Fmr1 knock-out (KO) mouse, where pharmacological and genetic reduction of mGlu5 reverses many phenotypes. mGlu5 is less associated with its scaffolding protein Homer in Fmr1 KO mice, and restoration of mGlu5-Homer interactions by genetic deletion of a short, dominant negative of Homer, H1a, rescues many phenotypes of Fmr1 KO mice. These results suggested that disruption of mGlu5-Homer leads to phenotypes of FXS. To test this idea, we examined mice with a knockin mutation of mGlu5 (F1128R; mGlu5(R/R)) that abrogates binding to Homer. Although FMRP levels were normal, mGlu5(R/R) mice mimicked multiple phenotypes of Fmr1 KO mice, including reduced mGlu5 association with the postsynaptic density, enhanced constitutive mGlu5 signaling to protein synthesis, deficits in agonist-induced translational control, protein synthesis-independent LTD, neocortical hyperexcitability, audiogenic seizures, and altered behaviors, including anxiety and sensorimotor gating. These results reveal new roles for the Homer scaffolds in regulation of mGlu5 function and implicate a specific molecular mechanism in a complex brain disease.

SIGNIFICANCE STATEMENT

Abnormal function of the metabotropic, or Gq-coupled, glutamate receptor 5 (mGlu5) has been implicated in neurodevelopmental disorders, including a genetic cause of intellectual disability and autism called fragile X syndrome. In brains of a mouse model of fragile X, mGlu5 is less associated with its binding partner Homer, a scaffolding protein that regulates mGlu5 localization to synapses and its ability to activate biochemical signaling pathways. Here we show that a mouse expressing a mutant mGlu5 that cannot bind to Homer is sufficient to mimic many of the biochemical, neurophysiological, and behavioral symptoms observed in the fragile X mouse. This work provides strong evidence that Homer-mGlu5 binding contributes to symptoms associated with neurodevelopmental disorders.

Prevention of irradiation-induced salivary hypofunction by rapamycin in swine parotid glands

Radiotherapy is commonly used in patients with oral cavity and pharyngeal cancers, usually resulting in irreversible salivary hypofunction. Currently management of radiation damage to salivary glands still remains a great challenge. Recent studies show that activation of mammalian target of rapamycin (mTOR) occurs in salivary gland lesions, making it possible to apply mTOR inhibitor for treatment. Our results indicate inhibition of mTOR by rapamycin significantly alleviated irradiation-induced salivary hypofunction by restoring 46% salivary flow rate and protecting histological structures in swine. Furthermore, rapamycin protected human submandibular gland cell line (HSG) from irradiation-induced cell depletion and loss of cell proliferation capacity. These findings lay the foundation for a new clinical application of rapamycin to prevent irradiation-induced salivary hypofunction.

Protein Kinase C-Independent Inhibition of Organic Cation Transporter 1 Activity by the Bisindolylmaleimide Ro 31-8220

Ro 31–8220 is a potent protein kinase C (PKC) inhibitor belonging to the chemical class of bisindolylmaleimides (BIMs). Various PKC-independent effects of Ro 31–8220 have however been demonstrated, including inhibition of the ATP-binding cassette drug transporter breast cancer resistance protein. In the present study, we reported that the BIM also blocks activity of the solute carrier organic cation transporter (OCT) 1, involved in uptake of marketed drugs in the liver, in a PKC-independent manner. Ro 31–8220, in contrast to other pan-PKC inhibitors such as staurosporine and chelerythrine, was thus shown to cis-inhibit uptake of the reference OCT1 substrate tetraethylammonium in OCT1-transfected HEK293 cells in a concentration-dependent manner (IC₅₀ = 0.18 μM) and without altering membrane expression of OCT1. This blockage of OCT1 was also observed in human hepatic HepaRG cells that constitutionally express OCT1. It likely occurred through a mixed mechanism of inhibition. Ro 31–8220 additionally trans-inhibited TEA uptake in OCT1-transfected HEK293 cells, which likely discards a transport of Ro 31–8220 by OCT1. Besides Ro 31–8220, 7 additional BIMs, including the PKC inhibitor LY 333531, inhibited OCT1 activity, whereas 4 other BIMs were without effect. In silico analysis of structure-activity relationships next revealed that various molecular descriptors, especially 3D-WHIM descriptors related to total size, correspond to key physico-chemical parameters for inhibition of OCT1 activity by BIMs. In addition to activity of OCT1, Ro 31–8220 inhibited those of other organic cation transporters such as multidrug and toxin extrusion protein (MATE) 1 and MATE2-K, whereas, by contrast, it stimulated that of OCT2. Taken together, these data extend the nature of cellular off-targets of the BIM Ro 31–8220 to OCT1 and other organic cation transporters, which has likely to be kept in mind when using Ro 31–8220 and other BIMs as PKC inhibitors in experimental or clinical studies.

Working hard at the nexus between cell signaling and the ribosomal machinery: An insight into the roles of RACK1 in translational regulation

RACK1 is a ribosome-associated protein which functions as a receptor for activated PKCs. It also acts as a scaffold for many other proteins involved in diverse signaling pathways, e.g. Src, JNK, PDE4D and FAK signaling. With such a broad interactome, RACK1 has been suggested to function as a linker between cell signaling and the translation machinery. Accordingly, RACK1 modulates translation at different levels in several model organisms. For instance, it regulates ribosome stalling and mRNA quality control in yeasts and promotes translation efficiency downstream of specific cellular stimuli in mammals. However, the molecular mechanism by which RACK1 exerts these roles is widely uncharacterized. Moreover, the full list of ribosome-recruited RACK1 interactors still needs characterization. Here we discuss in vivo and in vitro findings to better delineate the roles of RACK1 in regulating ribosome function and translation.

Tumor promoter-induced cellular senescence: cell cycle arrest followed by geroconversion

Phorbol ester (PMA or TPA), a tumor promoter, can cause either proliferation or cell cycle arrest, depending on cellular context. For example, in SKBr3 breast cancer cells, PMA hyper-activates the MEK/MAPK pathway, thus inducing p21 and cell cycle arrest. Here we showed that PMA-induced arrest was followed by conversion to cellular senescence (geroconversion). Geroconversion was associated with active mTOR and S6 kinase (S6K). Rapamycin suppressed geroconversion, maintaining quiescence instead. In this model, PMA induced arrest (step one of a senescence program), whereas constitutively active mTOR drove geroconversion (step two). Without affecting Akt phosphorylation, PMA increased phosphorylation of S6K (T389) and S6 (S240/244), and that was completely prevented by rapamycin. Yet, T421/S424 and S235/236 (p-S6K and p-S6, respectively) phosphorylation became rapamycin-insensitive in the presence of PMA. Either MEK or mTOR was sufficient to phosphorylate these PMA-induced rapamycin-resistant sites because co-treatment with U0126 and rapamycin was required to abrogate them. We next tested whether activation of rapamycin-insensitive pathways would shift quiescence towards senescence. In HT-p21 cells, cell cycle arrest was caused by IPTG-inducible p21 and was spontaneously followed by mTOR-dependent geroconversion. Rapamycin suppressed geroconversion, whereas PMA partially counteracted the effect of rapamycin, revealing the involvement of rapamycin-insensitive gerogenic pathways. In normal RPE cells arrested by serum withdrawal, the mTOR/pS6 pathway was inhibited and cells remained quiescent. PMA transiently activated mTOR, enabling partial geroconversion. We conclude that PMA can initiate a senescent program by either inducing arrest or fostering geroconversion or both. Rapamycin can decrease gero-conversion by PMA, without preventing PMA-induced arrest. The tumor promoter PMA is a gero-promoter, which may be useful to study aging in mammals.

Exploring the phosphoproteome profiles during Xenopus egg activation by calcium stimulation using a fully automated phosphopeptide purification system

To explore the phosphoproteome profiles duringXenopusegg activation by Ca(2+)-stimulation, an automated phosphopeptide purification system involving a titania column was improved by introducing 4-step elution with phosphate buffers. The number of detected phosphopeptides in the tryptic digest of aXenopusegg cytosol fraction on mass spectrometry (MS) was increased 1.5-fold and the percentage of multiply phosphorylated peptides increased from 17 to 24% with introduction of the 4-step elution method. Phosphopeptides were purified by the improved method from tryptic digests of cytosol fractions ofXenopuseggs without and with a Ca(2+)-stimulus, and then, analysed by MS. One thousand three hundred and seventy-five and 994 phosphopeptides were reproducibly detected on duplicate MS, respectively. They included 818 and 437 phosphopeptides specific to each digest, respectively. A method involving isobaric tags for relative and absolute quantitation (iTRAQ) was also applied to compare the phosphorylation levels inXenopuseggs without and with a Ca(2+)-stimulus, the ratios for 112 phosphopeptides in tryptic digests of these egg cytosol fractions being obtained. It was suggested from all the results that the phosphorylation sites and levels change duringXenopusegg activation for many known and unknown sites on structural proteins, signalling related proteins, cell cycle-related proteins and others.

p53 Deletion or Hotspot Mutations Enhance mTORC1 Activity by Altering Lysosomal Dynamics of TSC2 and Rheb

The activity of mammalian target of rapamycin complex 1 (mTORC1) is frequently enhanced in carcinomas, an effect thought to contribute to the malignant phenotype. Here, it is demonstrated that either deletion or mutation of TP53 in colon or lung carcinoma cells substantially enhances mTORC1 kinase activity by an effect downstream of and independent of AMPK. Mechanistically, it was determined that loss or mutation of p53 decreased expression of TSC2 and Sestrin2 (SESN2). Complementation of p53 null cells with TSC2 or Sestrin2 reduced mTORC1 activity to levels found in p53 wild-type (wt) cells, whereas their genetic depletion enhanced mTORC1 activity in p53 wt cells. However, the primary causal event in enhanced mTORC1 activity upon loss of p53 appeared to be a diminished distribution of TSC2 to lysosomal membranes containing mTOR. Subsequently, there was increased Rheb in the lysosomal compartment, and a higher mTOR association with Raptor. Transfection of TSC2 into p53 null cells replaced TSC2 and diminished Rheb at the lysosome, recapitulating cells with wt p53. In contrast, transfection of Sestrin2 decreased mTOR in lysosomes, but the lower levels of Sestrin2 in p53 null cells did not change lysosomal mTOR. In summary, loss of the transcriptional activity of p53, either by deletion or by key mutations in the DNA-binding domain, diminishes expression of TSC2 and Sestrin2, thus, shifting membrane-bound TSC2 out of lysosomal membranes, increasing lysosomal Rheb and increasing the kinase activity of mTORC1.

IMPLICATIONS

This study establishes that loss of p53 function decreases lysosomal TSC2 and increases lysosomal Rheb resulting in hyperactive mTORC1, findings that are consistent with a more malignant phenotype.

CDK1 substitutes for mTOR kinase to activate mitotic cap-dependent protein translation

Mitosis is commonly thought to be associated with reduced cap-dependent protein translation. Here we show an alternative control mechanism for maintaining cap-dependent translation during mitosis revealed by a viral oncoprotein, Merkel cell polyomavirus small T (MCV sT). We find MCV sT to be a promiscuous E3 ligase inhibitor targeting the anaphase-promoting complex, which increases cell mitogenesis. MCV sT binds through its Large T stabilization domain region to cell division cycle protein 20 (Cdc20) and, possibly, cdc20 homolog 1 (Cdh1) E3 ligase adapters. This activates cyclin-dependent kinase 1/cyclin B1 (CDK1/CYCB1) to directly hyperphosphorylate eukaryotic initiation factor 4E (eIF4E)-binding protein (4E-BP1) at authentic sites, generating a mitosis-specific, mechanistic target of rapamycin (mTOR) inhibitor-resistant δ phospho-isoform not present in G1-arrested cells. Recombinant 4E-BP1 inhibits capped mRNA reticulocyte translation, which is partially reversed by CDK1/CYCB1 phosphorylation of 4E-BP1. eIF4G binding to the eIF4E-m(7)GTP cap complex is resistant to mTOR inhibition during mitosis but sensitive during interphase. Flow cytometry, with and without sT, reveals an orthogonal pH3(S10+) mitotic cell population having higher inactive p4E-BP1(T37/T46+) saturation levels than pH3(S10-) interphase cells. Using a Click-iT flow cytometric assay to directly measure mitotic protein synthesis, we find that most new protein synthesis during mitosis is cap-dependent, a result confirmed using the eIF4E/4G inhibitor drug 4E1RCat. For most cell lines tested, cap-dependent translation levels were generally similar between mitotic and interphase cells, and the majority of new mitotic protein synthesis was cap-dependent. These findings suggest that mitotic cap-dependent translation is generally sustained during mitosis by CDK1 phosphorylation of 4E-BP1 even under conditions of reduced mTOR signaling.

Amyloid precursor protein (APP) affects global protein synthesis in dividing human cells

Hypoxic non‐small cell lung cancer (NSCLC) is dependent on Notch‐1 signaling for survival. Targeting Notch‐1 by means of γ‐secretase inhibitors (GSI) proved effective in killing hypoxic NSCLC. Post‐mortem analysis of GSI‐treated, NSCLC‐burdened mice suggested enhanced phosphorylation of 4E‐BP1 at threonines 37/46 in hypoxic tumor tissues. In vitro dissection of this phenomenon revealed that Amyloid Precursor Protein (APP) inhibition was responsible for a non‐canonical 4E‐BP1 phosphorylation pattern rearrangement—a process, in part, mediated by APP regulation of the pseudophosphatase Styx. Upon APP depletion we observed modifications of eIF‐4F composition indicating increased recruitment of eIF‐4A to the mRNA cap. This phenomenon was supported by the observation that cells with depleted APP were partially resistant to silvestrol, an antibiotic that interferes with eIF‐4A assembly into eIF‐4F complexes. APP downregulation in dividing human cells increased the rate of global protein synthesis, both cap‐ and IRES‐dependent. Such an increase seemed independent of mTOR inhibition. After administration of Torin‐1, APP downregulation and Mechanistic Target of Rapamycin Complex 1 (mTORC‐1) inhibition affected 4E‐BP1 phosphorylation and global protein synthesis in opposite fashions. Additional investigations indicated that APP operates independently of mTORC‐1. Key phenomena described in this study were reversed by overexpression of the APP C‐terminal domain. The presented data suggest that APP may be a novel regulator of protein synthesis in dividing human cells, both cancerous and primary. Furthermore, APP appears to affect translation initiation using mechanisms seemingly dissimilar to mTORC‐1 regulation of cap‐dependent protein synthesis. J. Cell. Physiol. 230: 1064–1074, 2015. © 2014 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc.

The ever-evolving role of mTOR in translation

Control of translation allows for the production of stoichiometric levels of each protein in the cell. Attaining such a level of fine-tuned regulation of protein production requires the coordinated temporal and spatial control of numerous cellular signalling cascades impinging on the various components of the translational machinery. Foremost among these is the mTOR signalling pathway. The mTOR pathway regulates both the initiation and elongation steps of protein synthesis through the phosphorylation of numerous translation factors, while simultaneously ensuring adequate folding of nascent polypeptides through co-translational degradation of misfolded proteins. Perhaps most remarkably, mTOR is also a key regulator of the synthesis of ribosomal proteins and translation factors themselves. Two seminal studies have recently shown in translatome analysis that the mTOR pathway preferentially regulates the translation of mRNAs encoding ribosomal proteins and translation factors. Therefore, the role of the mTOR pathway in the control of protein synthesis extends far beyond immediate translational control. By controlling ribosome production (and ultimately ribosome availability), mTOR is a master long-term controller of protein synthesis. Herein, we review the literature spanning the early discoveries of mTOR on translation to the latest advances in our understanding of how the mTOR pathway controls the synthesis of ribosomal proteins.

Rapamycin delays salivary gland atrophy following ductal ligation

Salivary gland atrophy is a frequent consequence of head and neck cancer irradiation therapy but can potentially be regulated through the mammalian target of rapamycin (mTOR). Excretory duct ligation of the mouse submandibular gland provokes severe glandular atrophy causing activation of mTOR. This study aims to discover the effects of blocking mTOR signaling in ligation-induced atrophic salivary glands. Following 1 week of unilateral submandibular excretory duct ligation: gland weights were significantly reduced, 4E-BP1 and S6rp were activated, and tissue morphology revealed typical signs of atrophy. However, 3 days following ligation with rapamycin treatment, a selective mTOR inhibitor, gland weights were maintained, 4E-BP1 and S6rp phosphorylation was inhibited, and there were morphological signs of recovery from atrophy. However, following 5 and 7 days of ligation and rapamycin treatment, glands expressed active mTOR and showed signs of considerable atrophy. This evidence suggests that inhibition of mTOR by rapamycin delays ligation-induced atrophy of salivary glands.

Temporal changes in ERK phosphorylation are harmonious with 4E-BP1, but not p70S6K, during clenbuterol-induced hypertrophy in the rat gastrocnemius

Extracellular signal-regulated kinase (ERK) is required for clenbuterol (CB)-dependent fast-type myofibril enlargement; however, its contribution to translation control is unclear. ERK mediates translational regulation through mammalian target of rapamycin complex 1 (mTORC1) activation and (or) mTORC1-independent pathways. In this study, we aimed to investigate the role of ERK in translational control during CB-induced muscular hypertrophy by measuring time-dependent changes in the phosphorylation statuses of ERK, p70 ribosomal S6 kinase (p70S6K; an indicator of mTORC1 activity), 4E-binding protein 1 (4E-BP1), eukaryotic elongation factor 2 (eEF2), and other related signaling molecules in rat gastrocnemius muscles. Five-day administration of CB induced phenotypes associated with muscular hypertrophy (significant increases in wet weight and isometric ankle flexion torque in the gastrocnemius muscle), but was not accompanied by elevated ERK or p70S6K phosphorylation. One-day administration of CB caused significant increases in the phosphorylation of ERK, p70S6K, and 4E-BP1. In contrast, 3-day administration of CB caused significant increases in the phosphorylation of ERK and 4E-BP1, but not p70S6K. In addition, positive correlations were observed between ERK and 4E-BP1 on days 1 and 3, whereas a correlation between ERK and p70S6K was only observed on day 1. eEF2 phosphorylation was unchanged on both days 1 and 3. These findings suggest that ERK accelerates the initiation of translation, but does not support the involvement of ERK in translational elongation. Furthermore, ERK may play a major role in promoting translational initiation by mediating the phosphorylation of 4E-BP1, and may contribute to the initial activation of mTORC1 during CB administration.

Morphoproteomics: exposing protein circuitries in tumors to identify potential therapeutic targets in cancer patients

Morphoproteomics combines the disciplines of histopathology, molecular biology and protein chemistry to paint a portrait of the protein circuitry in diseased cells for the purpose of uncovering molecular targets amenable to specific intervention, thereby customizing therapy for individual patients. This review considers the clinical application of morphoproteomics in malignant cells in the context of currently available pharmaceutical agents and discusses opportunities for combinatorial approaches that involve one or more small molecule inhibitors and single-agent chemotherapy with relatively low toxicity profiles. Future directions that involve focusing on points of convergence in signal transduction pathways and which integrate morphoproteomic with genomic and pharmacoproteomic and protein-function microarray data are offered.

The Akt/mTor signaling cascade is modified during placentation in the porcine uterine tissue

Recently we showed that essential components for the initiation of protein synthesis, namely the eukaryotic initiation factor 4E (eIF4E, mRNA-cap-binding protein) and its repressors 4E-BP1 as well as 4E-BP2, are proteolytically processed in the porcine endometrium during implantation. Here, the situation during placentation was compared with ovariectomized (OVX) animals and animals on pregnancy day 1 (PD1). Furthermore, the research was extended to factors which phosphorylate eIF4E and 4E-BPs and regulate their activities. These are the protein kinase B/mammalian target of rapamycin kinase (Akt/mTor) with the regulators Raptor and Rictor as well as the mitogen activated protein kinases (MAPKs): extra cellular-signal regulated kinase 1 and 2 (ERK1 and ERK2). Striking differences in the placentation site (PS) and the areas aside from PS (peri-PS) were observed. EIF4E and 4E-BP2 truncation as well as 4E-BP1 degradation took place in the endometrium of the peri-PS on PD24. Accompanied by a fragmentation of Akt/mTor, no expression of Rictor was observed, whereas the abundance of Raptor was not altered. On the contrary, MAPKs expression and phosphorylation remained almost stable in the peri-PS. In conclusion, the results indicated that on PD24 the translational regulation was shifted to 4E-BP2 control. Furthermore, the Akt/mTor signaling cascade seemed to be down regulated which suggest reduced phosphorylation of 4E-BP2. Whereas Akt was proteolyzed, the observed mTor fragments represented most likely splicing variants. The results indicate that translational control of gene expression is an important feature in the porcine endometrium during early pregnancy.

Evidence that TMEM67 causes polycystic kidney disease through activation of JNK/ERK-dependent pathways

TMEM67 mutations are associated with severe autosomal recessive polycystic kidney disease (ARPKD) in both humans and animals. However, the molecular mechanisms underlying the pathogenesis of PKD caused by TMEM67 mutations remain to be determined. We have investigated the possible signalling pathways involved in the pathogenesis of PKD. Overexpression of TMEM67 in human embryonic kidney (HEK293) cells triggered the activation of overall tyrosine phosphorylated proteins, extracellular signal-regulated kinase (ERK) and c-jun N-terminal KINASE (JNK). Activation was suppressed by pharmacological inhibitors of ERK or JNK. Activation of the mammalian target of rapamycin (mTOR) or p70s kinase (S6K) did not occur, although elevated phosphorylation of eIF4E-binding protein 1 (4E-BP1), a target of S6K, was seen. In animal studies, activation of a variety of signalling molecules was linked to ERK, JNK and 4E-BP1. Significant induction of phosphorylation of tyrosine phosphorylated proteins, ERK and 4E-BP1, at different postnatal ages was detected in mutant kidneys of B6C3Fe a/a-bpck mice, a cystic renal disease mouse model caused by TMEM67 loss of function mutation. Based on these in vitro and in vivo observations, we propose that TMEM67 mutations cause PKD through ERK- and JNK-dependent signalling pathways, which may provide novel insight into the therapy of polycystic kidney diseases.

The permissive effects of glucose on receptor-operated potentiation of insulin secretion from mouse islets: a role for ERK1/2 activation and cytoskeletal remodelling

AIMS/HYPOTHESIS

Glucose plays two distinct roles in regulating insulin secretion from beta cells--an initiatory role, and a permissive role enabling receptor-operated secretagogues to potentiate glucose-induced insulin secretion. The molecular mechanisms underlying the permissive effects of glucose on receptor-operated insulin secretion remain uncertain. We have investigated the role of extracellular signal-regulated kinase 1/2 (ERK1/2) activation and consequent cytoskeletal remodelling in this process.

METHODS

Insulin release was measured from groups of isolated mouse islets using static incubation experiments and subsequent radioimmunoassay of samples. ERK1/2 activation was measured by western blotting of islet protein samples for both phosphorylated and total ERK1/2. Rhodamine-phalloidin staining was used to measure filamentous actin in dispersed primary beta cells.

RESULTS

Inhibition of ERK1/2 blocked potentiation of glucose-induced insulin release by the receptor-operated secretagogues kisspeptin, A568, exendin-4 and JWH015, although the agonists alone had minimal effects on ERK1/2 activation, suggesting a permissive rather than causal role for ERK1/2 activation in receptor-operated insulin release. Following pharmacological activation of ERK1/2 all agonists caused a significant increase in insulin release from islets incubated with sub-stimulatory levels of glucose. ERK1/2 inhibition significantly reduced the glucose-dependent decreases in filamentous actin observed in primary beta cells, while pharmacological dissociation of actin filaments enabled all receptor-operated secretagogues tested to significantly stimulate insulin release from islets at a sub-stimulatory glucose concentration.

CONCLUSIONS/INTERPRETATION

Glucose-induced ERK1/2 activation in beta cells mediates the permissive effects of stimulatory glucose concentrations on receptor-operated insulin secretagogues, at least in part through effects on actin depolymerisation and cytoskeletal remodelling.

4E-BP1 participates in maintaining spindle integrity and genomic stability via interacting with PLK1

The essential function of eIF4E-binding protein 1 (4E-BP1) in translation initiation has been well established; however, the role of 4E-BP1 in normal cell cycle progression is coming to attention. Here, we revealed the role of 4E-BP1 on mitotic regulation and chromosomal DNA dynamics during mitosis. First, we have observed the co-localization of the phosphorylated 4E-BP1 at T37/46 with Polo-like kinase 1 (PLK1) at the centrosomes during. Depression of 4E-BP1 by small interfering RNA in HepG2 or HeLa cells resulted in an increased outcome of polyploidy and aberrant mitosis, including chromosomal DNA misaligned and multi-polar spindles or multiple centrosomes. We observed that 4E-BP1 interacted with PLK1 directly in vitro and in vivo in mitotic cells, and the C-terminal aa 77-118 of 4E-BP1 mediates its interaction with PLK1. PLK1 can phosphorylate 4E-BP1 in vitro. Furthermore, the depletion of 4E-BP1 sensitized HepG2 and HeLa cells to the microtubule disruption agent paclitaxel. These results demonstrate that 4E-BP1, beyond its role in translation regulation, can function as a regulator of mitosis via interacting with PLK1, and possibly plays a role in genomic stability maintaining.

Transcription and translation are primary targets of Pim kinase inhibitor SGI-1776 in mantle cell lymphoma

Proviral integration site for Moloney murine leukemia virus (Pim) kinases are serine/threonine/tyrosine kinases and oncoproteins that promote tumor progression. Three isoforms of Pim kinases have been identified and are known to phosphorylate numerous substrates, with regulatory functions in transcription, translation, cell cycle, and survival pathways. These kinases are involved in production, proliferation, and survival of normal B cells and are overexpressed in B-cell malignancies such as mantle cell lymphoma (MCL). SGI-1776 is a small molecule and Pim kinase inhibitor with selectivity for Pim-1. We hypothesize that Pim kinase function can be inhibited by SGI-1776 in MCL and that inhibition of phosphorylation of downstream substrates will disrupt transcriptional, translational, and cell cycle processes and promote cell death. SGI-1776 treatment in 4 MCL cell lines resulted in apoptosis induction. Phosphorylation of transcription (c-Myc) and translation targets (4E-BP1), tested in Jeko-1 and Mino, was declined. Consistent with these data, Mcl-1 and cyclin D1 protein levels were decreased. Importantly, similar to cell line data, MCL primary cells but not normal cells showed similar inhibition of substrate phosphorylation and cytotoxicity from SGI-1776 treatment. Genetic knockdown of Pim-1/Pim-2 affected similar proteins in MCL cell lines. Collectively these data demonstrate Pim kinases as therapeutic targets in MCL.

Rapamycin-insensitive mTORC1 activity controls eIF4E:4E-BP1 binding

The recent development of mammalian target of rapamycin (mTOR) kinase domain inhibitors and genetic dissection of rapamycin-sensitive and -insensitive mTOR protein complexes (mTORC1 and mTORC2) have revealed that phosphorylation of the mTOR substrate 4E-BP1 on amino acids Thr37 and/or Thr46 represents a rapamycin-insensitive activity of mTORC1. Despite numerous previous reports utilizing serine (Ser)-to-alanine (Ala) and threonine (Thr)-to-Ala phosphorylation site mutants of 4E-BP1 to assess which post-translational modification(s) directly regulate binding to eIF4E, an ambiguous understanding persists. This manuscript demonstrates that the initial, rapamycin-insensitive phosphorylation event at Thr46 is sufficient to prevent eIF4E:4E-BP1 binding. This finding is relevant, particularly as mTOR kinase domain inhibitors continue to be assessed for clinical efficacy, since it clarifies a difference between the action of these second-generation mTOR inhibitors and those of rapamycin analogues.