Fibronectin controls cap-dependent translation through beta1 integrin and eukaryotic initiation factors 4 and 2 coordinated pathways

The Paradox of Ribosomal Insufficiency Coupled with Increased Cancer: Shifting the Perspective from the Cancer Cell to the Microenvironment

Ribosomopathies are defined as inherited diseases in which ribosomal factors are mutated. In general, they present multiorgan symptoms. In spite of the fact that in cellular models, ribosomal insufficiency leads to a reduced rate of oncogenic transformation, patients affected by ribosomopathies present a paradoxical increase in cancer incidence. Several hypotheses that explain this paradox have been formulated, mostly on the assumption that altered ribosomes in a stem cell induce compensatory changes that lead to a cancer cell. For instance, the lack of a specific ribosomal protein can lead to the generation of an abnormal ribosome, an oncoribosome, that itself leads to altered translation and increased tumorigenesis. Alternatively, the presence of ribosomal stress may induce compensatory proliferation that in turns selects the loss of tumor suppressors such as p53. However, modern views on cancer have shifted the focus from the cancer cell to the tumor microenvironment. In particular, it is evident that human lymphocytes are able to eliminate mutant cells and contribute to the maintenance of cancer-free tissues. Indeed, many tumors develop in conditions of reduced immune surveillance. In this review, we summarize the current evidence and attempt to explain cancer and ribosomopathies from the perspective of the microenvironment.

The Impact of 3D Nichoids and Matrix Stiffness on Primary Malignant Mesothelioma Cells

Malignant mesothelioma is a type of cancer that affects the mesothelium. It is an aggressive and deadly form of cancer that is often caused by exposure to asbestos. At the molecular level, it is characterized by a low number of genetic mutations and high heterogeneity among patients. In this work, we analyzed the plasticity of gene expression of primary mesothelial cancer cells by comparing their properties on 2D versus 3D surfaces. First, we derived from primary human samples four independent primary cancer cells. Then, we used Nichoids, which are micro-engineered 3D substrates, as three-dimensional structures. Nichoids limit the dimension of adhering cells during expansion by counteracting cell migration between adjacent units of a substrate with their microarchitecture. Tumor cells grow effectively on Nichoids, where they show enhanced proliferation. We performed RNAseq analyses on all the samples and compared the gene expression pattern of Nichoid-grown tumor cells to that of cells grown in a 2D culture. The PCA analysis showed that 3D samples were more transcriptionally similar compared to the 2D ones. The 3D Nichoids induced a transcriptional remodeling that affected mainly genes involved in extracellular matrix assembly. Among these genes responsible for collagen formation, COL1A1 and COL5A1 exhibited elevated expression, suggesting changes in matrix stiffness. Overall, our data show that primary mesothelioma cells can be effectively expanded in Nichoids and that 3D growth affects the cells' tensegrity or the mechanical stability of their structure.

Fibronectin, DHPS and SLC3A2 Signaling Cooperate to Control Tumor Spheroid Growth, Subcellular eIF5A1/2 Distribution and CDK4/6 Inhibitor Resistance

Extracellular matrix (ECM) protein expression/deposition within and stiffening of the breast cancer microenvironment facilitates disease progression and correlates with poor patient survival. However, the mechanisms by which ECM components control tumorigenic behaviors and responses to therapeutic intervention remain poorly understood. Fibronectin (FN) is a major ECM protein controlling multiple processes. In this regard, we previously reported that DHPS-dependent hypusination of eIF5A1/2 is necessary for fibronectin-mediated breast cancer metastasis and epithelial to mesenchymal transition (EMT). Here, we explored the clinical significance of an interactome generated using hypusination pathway components and markers of intratumoral heterogeneity. Solute carrier 3A2 (SLC3A2 or CD98hc) stood out as an indicator of poor overall survival among patients with basal-like breast cancers that express elevated levels of DHPS. We subsequently discovered that blockade of DHPS or SLC3A2 reduced triple negative breast cancer (TNBC) spheroid growth. Interestingly, spheroids stimulated with exogenous fibronectin were less sensitive to inhibition of either DHPS or SLC3A2 - an effect that could be abrogated by dual DHPS/SLC3A2 blockade. We further discovered that a subset of TNBC cells responded to fibronectin by increasing cytoplasmic localization of eIF5A1/2. Notably, these fibronectin-induced subcellular localization phenotypes correlated with a G0/G1 cell cycle arrest. Fibronectin-treated TNBC cells responded to dual DHPS/SLC3A2 blockade by shifting eIF5A1/2 localization back to a nucleus-dominant state, suppressing proliferation and further arresting cells in the G2/M phase of the cell cycle. Finally, we observed that dual DHPS/SLC3A2 inhibition increased the sensitivity of both Rb-negative and -positive TNBC cells to the CDK4/6 inhibitor palbociclib. Taken together, these data identify a previously unrecognized mechanism through which extracellular fibronectin controls cancer cell tumorigenicity by modulating subcellular eIF5A1/2 localization and provides prognostic/therapeutic utility for targeting the cooperative DHPS/SLC3A2 signaling axis to improve breast cancer treatment responses.

Cap-dependent translation initiation monitored in living cells

mRNA translation is tightly regulated to preserve cellular homeostasis. Despite extensive biochemical, genetic, and structural studies, a detailed understanding of mRNA translation regulation is lacking. Imaging methodologies able to resolve the binding dynamics of translation factors at single-cell and single-mRNA resolution were necessary to fully elucidate regulation of this paramount process. Here live-cell spectroscopy and single-particle tracking were combined to interrogate the binding dynamics of endogenous initiation factors to the 5'cap. The diffusion of initiation factors (IFs) changed markedly upon their association with mRNA. Quantifying their diffusion characteristics revealed the sequence of IFs assembly and disassembly in cell lines and the clustering of translation in neurons. This approach revealed translation regulation at high spatial and temporal resolution that can be applied to the formation of any endogenous complex that results in a measurable shift in diffusion.

Phosphorylation of Arl4A/D promotes their binding by the HYPK chaperone for their stable recruitment to the plasma membrane

The Arl4 small GTPases participate in a variety of cellular events, including cytoskeleton remodeling, vesicle trafficking, cell migration, and neuronal development. Whereas small GTPases are typically regulated by their GTPase cycle, Arl4 proteins have been found to act independent of this canonical regulatory mechanism. Here, we show that Arl4A and Arl4D (Arl4A/D) are unstable due to proteasomal degradation, but stimulation of cells by fibronectin (FN) inhibits this degradation to promote Arl4A/D stability. Proteomic analysis reveals that FN stimulation induces phosphorylation at S143 of Arl4A and at S144 of Arl4D. We identify Pak1 as the responsible kinase for these phosphorylations. Moreover, these phosphorylations promote the chaperone protein HYPK to bind Arl4A/D, which stabilizes their recruitment to the plasma membrane to promote cell migration. These findings not only advance a major mechanistic understanding of how Arl4 proteins act in cell migration but also achieve a fundamental understanding of how these small GTPases are modulated by revealing that protein stability, rather than the GTPase cycle, acts as a key regulatory mechanism.

Intracellular mRNA transport and localized translation

Fine-tuning cellular physiology in response to intracellular and environmental cues requires precise temporal and spatial control of gene expression. High-resolution imaging technologies to detect mRNAs and their translation state have revealed that all living organisms localize mRNAs in subcellular compartments and create translation hotspots, enabling cells to tune gene expression locally. Therefore, mRNA localization is a conserved and integral part of gene expression regulation from prokaryotic to eukaryotic cells. In this Review, we discuss the mechanisms of mRNA transport and local mRNA translation across the kingdoms of life and at organellar, subcellular and multicellular resolution. We also discuss the properties of messenger ribonucleoprotein and higher order RNA granules and how they may influence mRNA transport and local protein synthesis. Finally, we summarize the technological developments that allow us to study mRNA localization and local translation through the simultaneous detection of mRNAs and proteins in single cells, mRNA and nascent protein single-molecule imaging, and bulk RNA and protein detection methods.

Ribosome profiling unveils translational regulation of metabolic enzymes in primary CD4+ Th1 cells

The transition from a naïve to an effector T cell is an essential event that requires metabolic reprogramming. We have recently demonstrated that the rapid metabolic changes that occur following stimulation of naïve T cells require the translation of preexisting mRNAs. Here, we provide evidence that translation regulates the metabolic asset of effector T cells. By performing ribosome profiling in human CD4⁺ Th1 cells, we show that the metabolism of glucose, fatty acids and pentose phosphates is regulated at the translational level. In Th1 cells, each pathway has at least one enzyme regulated at the translational level and selected enzymes have high translational efficiencies. mRNA expression does not predict protein expression. For instance, PKM2 mRNA is equally present in naïve T and Th1 cells, but the protein is abundant only in Th1. 5'-untranslated regions (UTRs) may partly account for this regulation. Overall we suggest that immunometabolism is controlled by translation.

Translational control and the cancer cell response to stress

The evidence for the importance of aberrant translation in cancer cells is overwhelming. Reflecting the wealth of data, there are excellent reviews delineating how ribosomes and initiation factors are linked to cancer [1-3], and the therapeutic strategies being devised to target them [4]. Changes in translational efficiency can engender a malignant phenotype without the need for chromatin reorganization, transcription, splicing and mRNA export [5,6]. Thus, cancer-related modulations of the translational machinery are ideally suited to allow cancer cells to respond to the various stresses encountered along the path of tumorigenesis and organism-wide dissemination [7^•,8,9,10^•]. Emerging findings supporting this notion are the focus of this review.

Structural analysis of ribosomal RACK1 and its role in translational control

Receptor for Activated C-Kinase 1 (RACK1) belongs to the WD40 family of proteins, known to act as scaffolding proteins in interaction networks. Accordingly, RACK1 is found to have numerous interacting partners ranging from kinases and signaling proteins to membrane bound receptors and ion channels. Interestingly, RACK1 has also been identified as a ribosomal protein present in all eukaryotic ribosomes. Structures of eukaryotic ribosomes have shown RACK1 to be located at the back of the head of the small ribosomal subunit. This suggests that RACK1 could act as a ribosomal scaffolding protein recruiting regulators of translation to the ribosome, and several studies have in fact found RACK1 to play a role in regulation of translation. To fully understand the role of RACK1 we need to understand whether the many reported interaction partners of RACK1 bind to free or ribosomal RACK1. In this review we provide a structural analysis of ribosome-bound RACK1 to provide a basis for answering this fundamental question. Our analysis shows that RACK1 is tightly bound to the ribosome through highly conserved and specific interactions confirming RACK1 as an integral ribosomal protein. Furthermore, we have analyzed whether reported binding sites for RACK1 interacting partners with a proposed role in translational control are accessible on ribosomal RACK1. Our analysis shows that most of the interaction partners with putative regulatory functions have binding sites that are available on ribosomal RACK1, supporting the role of RACK1 as a ribosomal signaling hub. We also discuss the possible role for RACK1 in recruitment of ribosomes to focal adhesion sites and regulation of local translation during cell spreading and migration.

Integrin β6 can be translationally regulated by eukaryotic initiation factor 4E: Contributing to colonic tumor malignancy

It is well known that both eukaryotic initiation factor 4E (eIF4E) and integrin αvβ6 can contribute to malignant behavior of colon cancer. We have found that integrin αvβ6 and eIF4E were co-expressed and positively correlated in colon cancer tissues. Recently, deregulation of the protein synthesis apparatus has begun to gain attention as a major participant in cancer development and progression. However, the regulation of integrin β6 expression at translational level has never been investigated before. In present study, gene-silencing technique for eIF4E by small interfering RNA (siRNA) was used in all the subsequent experiments, in order to investigate whether eIF4E could translationally regulate expression of integrin β6 in colon cancer SW480 and HT-29 cell lines. Additionally, the subsequent effects of eIF4E knockdown on cellular malignant behavior were observed. siRNA in SW480 and HT-29 transfectants. Subsequently, protein expression of β6 was markedly suppressed, while mRNA expression of β6 showed no significant variation before and after eIF4E RNA interfering. Therefore, it could be seen that eIF4E could upregulate the expression of β6, without effect on β6 mRNA expression. More importantly, after treated with eIF4E siRNA, cellular migratory capacity on fibronectin of HT-29 and β6-transfected SW480 as well as their survival to 5-FU was decreased distinctly. Expression of integrin β6 could be translationally regulated by eIF4E, which subsequently contributed to tumor malignancy through enhancing cellular migration, survival, anti-apoptosis, and chemoresistance of colon cancer in vitro. Thus, targeting eIF4E in integrin αvβ6 expressing tumors can be a potential therapeutic strategy for patients with colon cancer.

Radiation Enhancement of Head and Neck Squamous Cell Carcinoma by the Dual PI3K/mTOR Inhibitor PF-05212384

PURPOSE

Radiation remains a mainstay for the treatment of nonmetastatic head and neck squamous cell carcinoma (HNSCC), a malignancy characterized by a high rate of PI3K/mTOR signaling axis activation. We investigated the ATP-competitive dual PI3K/mTOR inhibitor, PF-05212384, as a radiosensitizer in preclinical HNSCC models.

EXPERIMENTAL DESIGN

Extent of radiation enhancement of two HNSCC cell lines (UMSCC1-wtP53 and UMSCC46-mtP53) and normal human fibroblast (1522) was assessed by in vitro clonogenic assay with appropriate target inhibition verified by immunoblotting. Radiation-induced DNA damage repair was evaluated by γH2AX Western blots with the mechanism of DNA double-strand break repair abrogation investigated by cell cycle analysis, immunoblotting, and RT-PCR. PF-05212384 efficacy in vivo was assessed by UMSCC1 xenograft tumor regrowth delay, xenograft lysate immunoblotting, and tissue section immunohistochemistry.

RESULTS

PF-05212384 effectively inhibited PI3K and mTOR, resulting in significant radiosensitization of exponentially growing and plateau-phase cells with 24-hour treatment following irradiation, and variable radiation enhancement with 24-hour treatment before irradiation. Tumor cells radiosensitized to a greater extent than normal human fibroblasts. Postirradiation PF-05212384 treatment delays γH2AX foci resolution. PF-05212384 24-hour exposure resulted in an evident G1-S phase block in p53-competent cells. Fractionated radiation plus i.v. PF-05212384 synergistically delayed nude mice bearing UMSCC1 xenograft regrowth, with potential drug efficacy biomarkers identified, including pS6, pAkt, p4EBP1, and Ki67.

CONCLUSIONS

Taken together, our results of significant radiosensitization both in vitro and in vivo validate the PI3K/mTOR axis as a radiation modification target and PF-05212384 as a potential clinical radiation modifier of nonmetastatic HNSCC.

Translation factors and ribosomal proteins control tumor onset and progression: how?

Gene expression is shaped by translational control. The modalities and the extent by which translation factors modify gene expression have revealed therapeutic scenarios. For instance, eukaryotic initiation factor (eIF)4E activity is controlled by the signaling cascade of growth factors, and drives tumorigenesis by favoring the translation of specific mRNAs. Highly specific drugs target the activity of eIF4E. Indeed, the antitumor action of mTOR complex 1 (mTORc1) blockers like rapamycin relies on their capability to inhibit eIF4E assembly into functional eIF4F complexes. eIF4E biology, from its inception to recent pharmacological targeting, is proof-of-principle that translational control is druggable. The case for eIF4E is not isolated. The translational machinery is involved in the biology of cancer through many other mechanisms. First, untranslated sequences on mRNAs as well as noncoding RNAs regulate the translational efficiency of mRNAs that are central for tumor progression. Second, other initiation factors like eIF6 show a tumorigenic potential by acting downstream of oncogenic pathways. Third, genetic alterations in components of the translational apparatus underlie an entire class of inherited syndromes known as 'ribosomopathies' that are associated with increased cancer risk. Taken together, data suggest that in spite of their evolutionary conservation and ubiquitous nature, variations in the activity and levels of ribosomal proteins and translation factors generate highly specific effects. Beside, as the structures and biochemical activities of several noncoding RNAs and initiation factors are known, these factors may be amenable to rational pharmacological targeting. The future is to design highly specific drugs targeting the translational apparatus.

Stress signaling and the shaping of the mammary tissue in development and cancer

The postnatal mammary gland develops extensively through cycles of proliferation, branching, involution and remodeling. We review recent advances made in the field of stress signaling pathways and its roles in mammary gland organogenesis, how they contribute to normal organ specification and homeostasis and how its subversion by oncogenes leads to cancer. We analyze stress signaling in mammary gland biology taking into account the interrelationship with the extracellular matrix and adhesion signaling during morphogenesis. By integrating the information gathered from in vivo and three dimensional in vitro organogenesis studies, we review the novel contribution of p38(SAPK), c-Jun NH2-terminal kinase and PKR-like endoplasmic reticulum kinase (PERK) signaling pathways to the timely activation of cell death, correct establishment of polarity and growth arrest and autophagy, respectively. We also review the evidence supporting that the activation of the aforementioned stress kinases maintain breast acinar structures as part of a tumor suppressive program and that its deregulation is commonplace during breast cancer initiation.

Translational control of the fibroblast-extracellular matrix association: An application to pulmonary fibrosis

Pulmonary fibrosis is a severe lung disease characterized by sustained propagation of lung fibroblasts and relentless accumulation of extracellular matrix (ECM). Idiopathic pulmonary fibrosis (IPF) is the most severe chronic form of pulmonary fibrosis and results both in the gradual exchange of normal lung parenchyma with fibrotic tissue and in the irreversible impairment of gas exchange in the lung. Despite the urgency for novel therapies in IPF treatment, there is no effective and proven medical therapy available. Molecular mechanisms underlying IPF pathogenesis include aberrant ECM signaling through the canonical integrin/PI3K/Akt/mTORC1 signal transduction pathway. One important and well-characterized downstream effector of this pathway is the cellular protein synthesis machinery. Here we will review the recent advances in our understanding of the function of ECM and integrin receptor signaling in development of IPF and will present evidence indicating that the dysregulation of the eIF4F-mediated translational apparatus is an important factor in the development and progression of IPF and other fibrotic disorders. We further discuss the perspectives and challenges to curbing this deadly disease by targeting aberrant translation.

The intracellular interactome of tetraspanin-enriched microdomains reveals their function as sorting machineries toward exosomes

Extracellular vesicles are emerging as a potent mechanism of intercellular communication because they can systemically exchange genetic and protein material between cells. Tetraspanin molecules are commonly used as protein markers of extracellular vesicles, although their role in the unexplored mechanisms of cargo selection into exosomes has not been addressed. For that purpose, we have characterized the intracellular tetraspanin-enriched microdomain (TEM) interactome by high throughput mass spectrometry, in both human lymphoblasts and their derived exosomes, revealing a clear pattern of interaction networks. Proteins interacting with TEM receptors cytoplasmic regions presented a considerable degree of overlap, although some highly specific CD81 tetraspanin ligands, such as Rac GTPase, were detected. Quantitative proteomics showed that TEM ligands account for a great proportion of the exosome proteome and that a selective repertoire of CD81-associated molecules, including Rac, is not correctly routed to exosomes in cells from CD81-deficient animals. Our data provide evidence that insertion into TEM may be necessary for protein inclusion into the exosome structure.

Signalling pathways passing Src in pancreatic endocrine tumours: relevance for possible combined targeted therapies

The most frequent molecular abnormalities in pancreatic endocrine tumours (PETs) are mutations of the MEN1 gene, deregulation of the PI3K/AKT/mTOR signalling pathway and overactivation of growth factors and their receptors, such as the VEGF. On this basis, everolimus (Afinitor®; Novartis) and sunitinib (Sutent®; Pfizer) have both been approved by the FDA for the treatment of progressive, unresectable, locally advanced or metastatic PETs. However, molecular or surrogate markers able to predict the response of PET patients to treatment with these drugs are not available, and cancer cells treated with targeted therapies might develop escape pathways that evoke pro-survival feedback responses. The existence of cross-talk between different molecular pathways in PETs has been poorly investigated. In the present review, we present data supporting an important role for Src family kinases (SFKs) in PETs, together with the recent observation of a novel role for SFK in modulating the mTOR pathway activity. Of note, while treatment with everolimus triggered the activation of a survival response dependent on PI3K/AKT signalling in vitro, the simultaneous inhibition of SFKs blocked the activation of this unwanted escape signal. These studies might set the ground for the investigation of combined treatment of PETs with SFK and mTOR inhibitors.

Mena binds α5 integrin directly and modulates α5β1 function

Mena binds to the cytoplasmic tail of α5 integrin and modulates key α5β1 integrin functions in adhesion, motility, and fibrillogenesis.

Mena is an Ena/VASP family actin regulator with roles in cell migration, chemotaxis, cell–cell adhesion, tumor cell invasion, and metastasis. Although enriched in focal adhesions, Mena has no established function within these structures. We find that Mena forms an adhesion-regulated complex with α5β1 integrin, a fibronectin receptor involved in cell adhesion, motility, fibronectin fibrillogenesis, signaling, and growth factor receptor trafficking. Mena bound directly to the carboxy-terminal portion of the α5 cytoplasmic tail via a 91-residue region containing 13 five-residue “LERER” repeats. In fibroblasts, the Mena–α5 complex was required for “outside-in” α5β1 functions, including normal phosphorylation of FAK and paxillin and formation of fibrillar adhesions. It also supported fibrillogenesis and cell spreading and controlled cell migration speed. Thus, fibroblasts require Mena for multiple α5β1-dependent processes involving bidirectional interactions between the extracellular matrix and cytoplasmic focal adhesion proteins.

Sensitivity of global translation to mTOR inhibition in REN cells depends on the equilibrium between eIF4E and 4E-BP1

Initiation is the rate-limiting phase of protein synthesis, controlled by signaling pathways regulating the phosphorylation of translation factors. Initiation has three steps, 43S, 48S and 80S formation. 43S formation is repressed by eIF2α phosphorylation. The subsequent steps, 48S and 80S formation are enabled by growth factors. 48S relies on eIF4E-mediated assembly of eIF4F complex; 4E-BPs competitively displace eIF4E from eIF4F. Two pathways control eIF4F: 1) mTORc1 phosphorylates and inactivates 4E-BPs, leading to eIF4F formation; 2) the Ras-Mnk cascade phosphorylates eIF4E. We show that REN and NCI-H28 mesothelioma cells have constitutive activation of both pathways and maximal translation rate, in the absence of exogenous growth factors. Translation is rapidly abrogated by phosphorylation of eIF2α. Surprisingly, pharmacological inhibition of mTORc1 leads to the complete dephosphorylation of downstream targets, without changes in methionine incorporation. In addition, the combined administration of mTORc1 and MAPK/Mnk inhibitors has no additive effect. The inhibition of both mTORc1 and mTORc2 does not affect the metabolic rate. In spite of this, mTORc1 inhibition reduces eIF4F complex formation, and depresses translocation of TOP mRNAs on polysomes. Downregulation of eIF4E and overexpression of 4E-BP1 induce rapamycin sensitivity, suggesting that disruption of eIF4F complex, due to eIF4E modulation, competes with its recycling to ribosomes. These data suggest the existence of a dynamic equilibrium in which eIF4F is not essential for all mRNAs and is not displaced from translated mRNAs, before recycling to the next.

Src kinase activity coordinates cell adhesion and spreading with activation of mammalian target of rapamycin in pancreatic endocrine tumour cells

Pancreatic endocrine tumours (PETs) are rare and heterogeneous neoplasms, often diagnosed at metastatic stage, for which no cure is currently available. Recently, activation of two pathways that support proliferation and invasiveness of cancer cells, the Src family kinase (SFK) and mammalian target of rapamycin (mTOR) pathways, was demonstrated in PETs. Since both pathways represent suitable targets for therapeutic intervention, we investigated their possible interaction in PETs. Western blot and immunofluorescence analyses indicated that SFK and mTOR activity correlate in PET cell lines. We also found that SFKs coordinate cell adhesion and spreading with activation of the mTOR pathway in PET cells. Live cell metabolic labelling and biochemical studies demonstrated that SFK activity enhance mTOR-dependent translation initiation. Furthermore, microarray analysis of the mRNAs associated with polyribosomes revealed that SFKs regulate mTOR-dependent translation of specific transcripts, with an enrichment in mRNAs encoding cell cycle proteins. Importantly, a synergic inhibition of proliferation was observed in PET cells concomitantly treated with SFK and mTOR inhibitors, without activation of the phosphatidylinositol 3-kinase/AKT pro-survival pathway. Tissue microarray analysis revealed activation of Src and mTOR in some PET samples, and identified phosphorylation of 4E-BP1 as an independent marker of poor prognosis in PETs. Thus, our work highlights a novel link between the SFK and mTOR pathways, which regulate the translation of mRNAs for cell cycle regulators, and suggest that crosstalk between these pathways promotes PET cell proliferation.

Impairment of cytoplasmic eIF6 activity restricts lymphomagenesis and tumor progression without affecting normal growth

Eukaryotic Initiation Factor 6 (eIF6) controls translation by regulating 80S subunit formation. eIF6 is overexpressed in tumors. Here, we demonstrate that eIF6 inactivation delays tumorigenesis and reduces tumor growth in vivo. eIF6(+/-) mice resist to Myc-induced lymphomagenesis and have prolonged tumor-free survival and reduced tumor growth. eIF6(+/-) mice are also protected by p53 loss. Myc-driven lymphomas contain PKCβII and phosphorylated eIF6; eIF6 is phosphorylated by tumor-derived PKCβII, but not by the eIF4F activator mTORC1. Mutation of PKCβII phosphosite of eIF6 reduces tumor growth. Thus, eIF6 is a rate-limiting controller of initiation of translation, able to affect tumorigenesis and tumor growth. Modulation of eIF6 activity, independent from eIF4F complex, may lead to a therapeutical avenue in tumor therapy.

JNK inhibition arrests cotranslational degradation

Adhesion to fibronectin stimulates protein synthesis (translation) of fibroblasts. Protein synthesis stimulation is dependent from the activation of beta(1)-integrin. beta(1)-Integrin elicits a PI3K cascade that modulates eIF4F (eukaryotic initiation factor 4F) complex formation. In the attempt to further dissect elements of the PI3K cascade that might be responsible for fibronectin-stimulated translation, we used pharmacological inhibitors of known kinases. We found that JNK inhibition, by SP600125 treatment, increased (35)S-methionine incorporation. Paradoxically, the increase in methionine incorporation was associated to a reduction of initiation of translation. These data imply that, during the adhesion of fibroblasts to fibronectin, conspicuous protein degradation occurs. Indeed, we found that inhibition of the proteasome by MG132 also increased methionine incorporation. Cotranslational degradation depended on PI3K activation. In spite of this, serum promoted translation, but not cotranslational degradation. The crosstalk between translation and degradation was further analyzed by studying the phosphorylation of initiation factors. Briefly, inhibition of JNK leads to eIF2alpha phosphorylation, which may account for the decrease in initiation of translation. In conclusion, beta(1)-integrin-activated translation causes the synthesis of short-lived proteins, whose degradation is controlled by the JNK pathway. We hypothesize that JNK is a general regulator of cotranslational degradation.

Localization of ribosomes and translation initiation factors to talin/beta3-integrin-enriched adhesion complexes in spreading and migrating mammalian cells

BACKGROUND INFORMATION

The spatial localization of translation can facilitate the enrichment of proteins at their sites of function while also ensuring that proteins are expressed in the proximity of their cognate binding partners.

RESULTS

Using human embryonic lung fibroblasts and employing confocal imaging and biochemical fractionation techniques, we show that ribosomes, translation initiation factors and specific RNA-binding proteins localize to nascent focal complexes along the distal edge of migrating lamellipodia. 40S ribosomal subunits appear to associate preferentially with beta3 integrin in focal adhesions at the leading edges of spreading cells, with this association strongly augmented by a synergistic effect of cell engagement with a mixture of extracellular matrix proteins. However, both ribosome and initiation factor localizations do not require de novo protein synthesis.

CONCLUSIONS

Taken together, these findings demonstrate that repression, complex post-transcriptional regulation and modulation of mRNA stability could potentially be taking place along the distal edge of migrating lamellipodia.

Spreading of mesothelioma cells is rapamycin-sensitive and requires continuing translation

The interaction of cancer cells with extracellular matrix (ECM) is important in metastasization. Here we identified the molecules of the ECM expressed by sarcomatous malignant mesothelioma, and their effect on adhesion and spreading. In addition, by analyzing the relationship between translation and attachment to matrix, we found that mesothelioma cells rely on continuing translation to efficiently attach to matrix, and rapamycin inhibition affects spreading and migration of cancer cells. Specifically, we found that sarcomatous cells produce high amounts of fibronectin, able to support the spreading of mesothelioma cells. Spreading of cancer cells on fibronectin does not require de novo transcription but is sensitive to cycloheximide, an inhibitor of protein synthesis. Next, we analyzed the involvement of the mammalian target of rapamycin (mTOR) pathway, a major pathway controlling translation. Cancer cells have a constitutively active mTOR pathway; surprisingly, inhibition of mTOR complex 1 (mTORC1) by rapamycin barely affects the global rate of translation and of initiation of translation, but deeply inhibits mesothelioma spreading on ECM. The effects of rapamycin and cycloheximide on spreading were observed in several mesothelioma cell lines, although with different magnitude. Overall, data suggest that adhesion and spreading of mesothelioma cells on ECM require the translation of pre-synthesized mRNAs, and mTORC1 activity. We speculate that mTORC1 activity is required either for the translation of specific mRNAs or for the direct modulation of cytoskeletal remodeling.

Proteomic analysis of integrin-associated complexes identifies RCC2 as a dual regulator of Rac1 and Arf6

The binding of integrin adhesion receptors to their extracellular matrix ligands controls cell morphology, movement, survival, and differentiation in various developmental, homeostatic, and disease processes. Here, we report a methodology to isolate complexes associated with integrin adhesion receptors, which, like other receptor-associated signaling complexes, have been refractory to proteomic analysis. Quantitative, comparative analyses of the proteomes of two receptor-ligand pairs, alpha(4)beta(1)-vascular cell adhesion molecule-1 and alpha(5)beta(1)-fibronectin, defined both core and receptor-specific components. Regulator of chromosome condensation-2 (RCC2) was detected in the alpha(5)beta(1)-fibronectin signaling network at an intersection between the Rac1 and adenosine 5'-diphosphate ribosylation factor 6 (Arf6) subnetworks. RCC2 knockdown enhanced fibronectin-induced activation of both Rac1 and Arf6 and accelerated cell spreading, suggesting that RCC2 limits the signaling required for membrane protrusion and delivery. Dysregulation of Rac1 and Arf6 function by RCC2 knockdown also abolished persistent migration along fibronectin fibers, indicating a functional role for RCC2 in directional cell movement. This proteomics workflow now opens the way to further dissection and systems-level analyses of adhesion signaling.

Loss of the tumor suppressor gene NF2, encoding merlin, constitutively activates integrin-dependent mTORC1 signaling

Integrin signaling promotes, through p21-activated kinase, phosphorylation and inactivation of the tumor suppressor merlin, thus removing a block to mitogenesis in normal cells. However, the biochemical function of merlin and the effector pathways critical for the pathogenesis of malignant mesothelioma and other NF2-related malignancies are not known. We report that integrin-specific signaling promotes activation of mTORC1 and cap-dependent mRNA translation. Depletion of merlin rescues mTORC1 signaling in cells deprived of anchorage to a permissive extracellular matrix, suggesting that integrin signaling controls mTORC1 through inactivation of merlin. This signaling pathway controls translation of the cyclin D1 mRNA and, thereby, cell cycle progression. In addition, it promotes cell survival. Analysis of a panel of malignant mesothelioma cell lines reveals a strong correlation between loss of merlin and activation of mTORC1. Merlin-negative lines are sensitive to the growth-inhibitory effect of rapamycin, and the expression of recombinant merlin renders them partially resistant to rapamycin. Conversely, depletion of merlin restores rapamycin sensitivity in merlin-positive lines. These results indicate that integrin-mediated adhesion promotes mTORC1 signaling through the inactivation of merlin. Furthermore, they reveal that merlin-negative mesotheliomas display unregulated mTORC1 signaling and are sensitive to rapamycin, thus providing a preclinical rationale for prospective, biomarker-driven clinical studies of mTORC1 inhibitors in these tumors.

PKCbetaII modulates translation independently from mTOR and through RACK1

RACK1 (receptor for activated C kinase 1) is an abundant scaffolding protein, which binds active PKCbetaII (protein kinase C betaII) increasing its activity in vitro. RACK1 has also been described as a component of the small ribosomal subunit, in proximity to the mRNA exit channel. In the present study we tested the hypothesis that PKCbetaII plays a specific role in translational control and verified whether it may associate with the ribosomal machinery. We find that specific inhibition of PKCbetaI/II reduces translation as well as global PKC inhibition, but without affecting phosphorylation of mTOR (mammalian target of rapamycin) targets. These results suggest that PKCbetaII acts as a specific PKC isoform affecting translation in an mTOR-independent fashion, possibly close to the ribosomal machinery. Using far-Western analysis, we found that PKCbetaII binds ribosomes in vitro. Co-immunoprecipitation studies indicate that a small but reproducible pool of PKCbetaII is associated with membranes containing ribosomes, suggesting that in vivo PKCbetaII may also physically interact with the ribosomal machinery. Polysomal profiles show that stimulation of PKC results in an increased polysomes/80S ratio, associated with a shift of PKCbetaII to the heavier part of the gradient. A RACK1-derived peptide that inhibits the binding of active PKCbetaII to RACK1 reduces the polysomes/80S ratio and methionine incorporation, suggesting that binding of PKCbetaII to RACK1 is important for PKC-mediated translational control. Finally, down-regulation of RACK1 by siRNA (small interfering RNA) impairs the PKC-mediated increase of translation. Taken together the results of the present study show that PKCbetaII can act as a specific PKC isoform regulating translation, in an mTOR-independent fashion, possibly close to the ribosomal machinery.

Eukaryotic initiation factor 6 is rate-limiting in translation, growth and transformation

Cell growth and proliferation require coordinated ribosomal biogenesis and translation. Eukaryotic initiation factors (eIFs) control translation at the rate-limiting step of initiation. So far, only two eIFs connect extracellular stimuli to global translation rates: eIF4E acts in the eIF4F complex and regulates binding of capped messenger RNA to 40S subunits, downstream of growth factors, and eIF2 controls loading of the ternary complex on the 40S subunit and is inhibited on stress stimuli. No eIFs have been found to link extracellular stimuli to the activity of the large 60S ribosomal subunit. eIF6 binds 60S ribosomes precluding ribosome joining in vitro. However, studies in yeasts showed that eIF6 is required for ribosome biogenesis rather than translation. Here we show that mammalian eIF6 is required for efficient initiation of translation, in vivo. eIF6 null embryos are lethal at preimplantation. Heterozygous mice have 50% reduction of eIF6 levels in all tissues, and show reduced mass of hepatic and adipose tissues due to a lower number of cells and to impaired G1/S cell cycle progression. eIF6(+/-) cells retain sufficient nucleolar eIF6 and normal ribosome biogenesis. The liver of eIF6(+/-) mice displays an increase of 80S in polysomal profiles, indicating a defect in initiation of translation. Consistently, isolated hepatocytes have impaired insulin-stimulated translation. Heterozygous mouse embryonic fibroblasts recapitulate the organism phenotype and have normal ribosome biogenesis, reduced insulin-stimulated translation, and delayed G1/S phase progression. Furthermore, eIF6(+/-) cells are resistant to oncogene-induced transformation. Thus, eIF6 is the first eIF associated with the large 60S subunit that regulates translation in response to extracellular signals.

Integrin-dependent translational control: Implication in cancer progression

The importance of translational control in cancer progression has been underscored by a number of recent studies. However, little is known how cancer cells maintain their high efficiency of translation. Here, we summarize studies that support the role of integrins in translational control, especially at the initiation step, and discuss the various mechanisms by which integrins regulate the recruitment of translational machinery. This review also examines the hypothesis that integrins contribute to various aspects of cancer progression such as proliferation, survival, angiogenesis, and invasion through translational control.

Semaphorin controls epidermal morphogenesis by stimulating mRNA translation via eIF2alpha in Caenorhabditis elegans

Conserved semaphorin-plexin signaling systems govern various aspects of animal development, including axonal guidance in vertebrates and epidermal morphogenesis in Caenorhabditis elegans. Here we provide in vivo evidence that stimulation of mRNA translation via eukaryotic initiation factor 2alpha (eIF2alpha) is an essential downstream event of semaphorin signaling in C. elegans. In semaphorin/plexin mutants, a marked elevation in the phosphorylation of eIF2alpha is observed, which causes translation repression and is causally related to the morphological epidermal phenotype in the mutants. Conversely, removal of constraints on translation by genetically reducing the eIF2alpha phosphorylation largely bypasses requirement for the semaphorin signal in epidermal morphogenesis. We also identify an actin-depolymerizing factor/cofilin, whose expression in the mutants is predominantly repressed, as a major translational target of semaphorin signaling. Thus, our results reveal a physiological significance for translation of mRNAs for cytoskeletal regulators, linking environmental cues to cytoskeletal rearrangement during cellular morphogenesis in vivo.

Extracellular matrix regulation of autophagy

Integrin-mediated attachment of epithelial cells to extracellular matrix (ECM) is crucial for proper growth and survival. Although detachment leads to apoptosis, termed anoikis, recent work demonstrates that ECM detachment also robustly induces autophagy, a tightly regulated lysosomal self-digestion process that actually promotes survival. Autophagy presumably protects epithelial cells from the stresses of ECM detachment, allowing them to survive provided that they reattach in a timely manner. Currently, the intracellular signals linking integrin engagement to autophagy remain unclear, but certain growth factor, energy-sensing, and stress-response pathways represent attractive candidates. Moreover, autophagy may be a previously unrecognized mechanism utilized by detached cancer cells to survive anoikis, which may facilitate tumor cell dormancy, dissemination, and metastasis.

Inhibition of proliferation by PERK regulates mammary acinar morphogenesis and tumor formation

Endoplasmic reticulum (ER) stress signaling can be mediated by the ER kinase PERK, which phosphorylates its substrate eIF2α. This in turn, results in translational repression and the activation of downstream programs that can limit cell growth through cell cycle arrest and/or apoptosis. These responses can also be initiated by perturbations in cell adhesion. Thus, we hypothesized that adhesion-dependent regulation of PERK signaling might determine cell fate. We tested this hypothesis in a model of mammary acini development, a morphogenetic process regulated in part by adhesion signaling. Here we report a novel role for PERK in limiting MCF10A mammary epithelial cell proliferation during acinar morphogenesis in 3D Matrigel culture as well as in preventing mammary tumor formation in vivo. We show that loss of adhesion to a suitable substratum induces PERK-dependent phosphorylation of eIF2α and selective upregulation of ATF4 and GADD153. Further, inhibition of endogenous PERK signaling during acinar morphogenesis, using two dominant-negative PERK mutants (PERK-ΔC or PERK-K618A), does not affect apoptosis but results instead in hyper-proliferative and enlarged lumen-filled acini, devoid of proper architecture. This phenotype correlated with an adhesion-dependent increase in translation initiation, Ki67 staining and upregulation of Laminin-5, ErbB1 and ErbB2 expression. More importantly, the MCF10A cells expressing PERKΔC, but not a vector control, were tumorigenic in vivo upon orthotopic implantation in denuded mouse mammary fat pads. Our results reveal that the PERK pathway is responsive to adhesion-regulated signals and that it is essential for proper acinar morphogenesis and in preventing mammary tumor formation. The possibility that deficiencies in PERK signaling could lead to hyperproliferation of the mammary epithelium and increase the likelihood of tumor formation, is of significance to the understanding of breast cancer.