The ShcA adaptor activates AKT signaling to potentiate breast tumor angiogenesis by stimulating VEGF mRNA translation in a 4E-BP-dependent manner.

The ShcA adaptor protein is engaged by numerous receptor tyrosine kinases (RTKs) in breast cancer cells. Once activated, RTKs phosphorylate three key tyrosine phosphorylation sites (Y239, Y240 and Y317) within ShcA that creates a docking site for Grb2/SOS and Grb2/Gab-containing complexes to activate the MAPK and AKT signaling pathways, respectively. We previously demonstrated that a tyrosine to phenylalanine substitution of the ShcA tyrosine phosphorylation sites (Shc3F-Y239/240/313F) significantly impairs breast tumor growth and angiogenesis in transgenic mouse models, in part, through the regulation of vascular endothelial growth factor (VEGF) production. Despite this fact, the underlying molecular mechanisms by which ShcA transduces pro-tumorigenic signals in breast cancer cells remain poorly defined. In this study, we demonstrate that ShcA-dependent activation of AKT, but not the RAS/MAPK pathway, induces VEGF production by bolstering VEGF mRNA translation. Accordingly, ShcA drives breast tumor growth and angiogenesis in vivo in a 4E-BP-dependent manner. These findings establish ShcA as a biological bridge that links AKT activation downstream of RTKs to cap-dependent VEGF mRNA translation in order to promote mammary tumorigenesis.

Phosphorylation of eIF4E promotes EMT and metastasis via translational control of SNAIL and MMP-3.

The progression of cancers from primary tumors to invasive and metastatic stages accounts for the overwhelming majority of cancer deaths. Understanding the molecular events which promote metastasis is thus critical in the clinic. Translational control is emerging as an important factor in tumorigenesis. The messenger RNA (mRNA) cap-binding protein eIF4E is an oncoprotein that has an important role in cancer initiation and progression. eIF4E must be phosphorylated to promote tumor development. However, the role of eIF4E phosphorylation in metastasis is not known. Here, we show that mice in which eukaryotic translation initiation factor 4E (eIF4E) cannot be phosphorylated are resistant to lung metastases in a mammary tumor model, and that cells isolated from these mice exhibit impaired invasion. We also demonstrate that transforming growth factor-beta (TGFß) induces eIF4E phosphorylation to promote the translation of Snail and Mmp-3 mRNAs, and the induction of epithelial-to-mesenchymal transition (EMT). Furthermore, we describe a new model wherein EMT induced by TGFß requires translational activation via the non-canonical TGFß signaling branch acting through eIF4E phosphorylation.

The intra-tumor heterogeneity of cell signaling factors in breast cancer: p4E-BP1 and peIF4E are diffusely expressed and are real potential targets.

Breast cancers and most malignant tumors are composed of heterogeneous tumor cells both at genetic and morphological levels; intra-tumor heterogeneity can be one underlying cause of therapeutic resistance. Classical studies have focused on analyses of the relationship between primary tumors and metastatic dissemination, and on subclone evolution. However, it should be noted that tumor heterogeneity at the level of protein expression (proteomics) has not been yet studied in depth. The differences in protein expression also can play an important role in elucidating the relationship between intra-tumor heterogeneity and resistance to systemic therapy. In fact, in human tumors there is not always a homogeneous expression of many of the crucial factors involved in cell signaling, such as pMAPK, pAKt, pMTOR, even with constitutive oncogenic alterations upstream, such as HER2, PI3 K. Conversely, two of these factors, peIF4E and p4E-BP1, which are downstream, and control protein translation, show a diffuse and strong protein expression. In summary, most of cell signaling factors show a heterogeneous expression, regardless of oncogenic alterations. Tissue heterogeneity could be driven by local factors, including hypoxia. The fact that the phosphorylation of crucial proteins such as 4E-BP1 and eIF4E is observed homogeneously throughout most tumors and are druggable opens the chance to get real potential targets in cancer therapy.

mRNA translation and energy metabolism in cancer: the role of the MAPK and mTORC1 pathways.

mRNA translation is the most energy-consuming process in the cell and strongly correlates with cellular metabolic activity. Translation and energy metabolism play important roles in homeostatic cell growth and proliferation, and when dysregulated lead to cancer. eIF4E is a key regulator of translation, which promotes oncogenesis by selectively enhancing translation of a subset of tumor-promoting mRNAs (e.g., cyclins and c-myc). PI3K/AKT and mitogen-activated protein kinase (MAPK) pathways, which are strongly implicated in cancer etiology, exert a number of their biological effects by modulating translation. The PI3K/AKT pathway regulates eIF4E function by inactivating the inhibitory 4E-BPs via mTORC1, whereas MAPKs activate MAP kinase signal-integrating kinases 1 and 2, which phosphorylate eIF4E. In addition, AMP-activated protein kinase, which is a central sensor of the cellular energy balance, impairs translation by inhibiting mTORC1. Thus, eIF4E plays a major role in mediating the effects of PI3K/AKT, MAPK, and cellular energetics on mRNA translation.

Nuclear assortment of eIF4E coincides with shut-off of host protein synthesis upon poliovirus infection.

Eukaryotic initiation factor (eIF) 4E is a subunit of the cap-binding protein complex, eIF4F, which recognizes the cap structure of cellular mRNAs to facilitate translation initiation. eIF4E is assembled into the eIF4F complex via its interaction with eIF4G, an event that is under Akt/mTOR regulation. The eIF4E-eIF4G interaction is regulated by the eIF4E binding partners, eIF4E-binding proteins and eIF4E-transporter. Cleavage of eIF4G occurs upon poliovirus infection and is responsible for the shut-off of host-cell protein synthesis observed early in infection. Here, we document that relocalization of eIF4E to the nucleus occurs concomitantly with cleavage of eIF4G upon poliovirus infection. This event is not dependent upon virus replication, but is dependent on eIF4G cleavage. We postulate that eIF4E nuclear relocalization may contribute to the shut-off of host protein synthesis that is a hallmark of poliovirus infection by perturbing the circular status of actively translating mRNAs.

The eIF4E-binding proteins are modifiers of cytoplasmic eIF4E relocalization during the heat shock response.

Stress granules (SGs) arise as a consequence of cellular stress, contain stalled translation preinitiation complexes, and are associated with cell survival during environmental insults. SGs are dynamic entities with proteins relocating into and out of them during stress. Among the repertoire of proteins present in SGs is eukaryotic initiation factor 4E (eIF4E), a translation factor required for cap-dependent translation and that regulates a rate-limiting step for protein synthesis. Herein, we demonstrate that localization of eIF4E to SGs is dependent on the presence of a family of repressor proteins, eIF4E-binding proteins (4E-BPs). Our results demonstrate that 4E-BPs regulate the SG localization of eIF4E.

Targeting mTOR-dependent tumours with specific inhibitors: a model for personalized medicine based on molecular diagnoses.

Cancer cells are characterized by aberrant growth arising from deregulated signalling pathways. The mammalian target of rapamycin (mTOR) pathway integrates multiple growth signals coming from both intracellular and extracellular cues. In this short review, we summarize what is known about the efficacy of targeting the mTOR pathway to treat cancer patients, and we explain the rationale behind promising new inhibitors that could show more potent tumour growth inhibition than did the first generation of these drugs.

Akt phosphorylation of La regulates specific mRNA translation in glial progenitors.

The Akt signaling pathway activity increases as normal tissue progresses to malignant transformation, and regulates the translation of specific messenger RNAs (mRNAs) through multiple mechanisms. We have identified one such mechanism of Akt-dependent translation control as involving the lupus autoantigen La. La is an RNA-associated protein that contains multiple trafficking elements to support the interaction with RNAs in different subcellular locations. We show here that the La protein is a direct target of the serine/threonine protein kinase Akt on threonine 301, and La nuclear export in mouse glial progenitors, as well as its association with polysomes is modulated by Akt activity. Using a functional approach to determine the network of genes affected by La in the cytoplasm by microarray analysis of polysome-bound mRNAs, we found that La binds 34% of the polysome bound mRNAs and regulates the expression of a specific pool of mRNAs under KRas/Akt activation. Therefore, La appears to be an important contributor to Akt-mediated translational regulation of these transcripts in murine glial cells.

mTOR signaling: implications for cancer and anticancer therapy.

Mounting evidence links deregulated protein synthesis to tumorigenesis via the translation initiation factor complex eIF4F. Components of this complex are often overexpressed in a large number of cancers and promote malignant transformation in experimental systems. mTOR affects the activity of the eIF4F complex by phosphorylating repressors of the eIF4F complex, the eIF4E binding proteins. The immunosuppressant rapamycin specifically inhibits mTOR activity and retards cancer growth. Importantly, mutations in upstream negative regulators of mTOR cause hamartomas, haemangiomas, and cancers that are sensitive to rapamycin treatment. Such mutations lead to increased eIF4F formation and consequently to enhanced translation initiation and cell growth. Thus, inhibition of translation initiation through targeting the mTOR-signalling pathway is emerging as a promising therapeutic option.

mTOR, translation initiation and cancer.

Control of mRNA translation plays a fundamental role in many aspects of cell metabolism. It constitutes a critical step in the control of gene expression, and consequently cell growth, proliferation and differentiation. Translation is regulated in response to nutrient availability, hormones, mitogenic and growth factor stimulation and is coupled with cell cycle progression and cell growth. Signaling by the PI3K/Akt/mTOR pathway profoundly affects mRNA translation through phosphorylation of downstream targets such as 4E-BP and S6K. Inhibitors of this pathway and thus cap-dependent translation are emerging as promising therapeutic options for the treatment of cancer.

mTOR signaling: implications for cancer and anticancer therapy.

Mounting evidence links deregulated protein synthesis to tumorigenesis via the translation initiation factor complex eIF4F. Components of this complex are often overexpressed in a large number of cancers and promote malignant transformation in experimental systems. mTOR affects the activity of the eIF4F complex by phosphorylating repressors of the eIF4F complex, the eIF4E binding proteins. The immunosuppressant rapamycin specifically inhibits mTOR activity and retards cancer growth. Importantly, mutations in upstream negative regulators of mTOR cause hamartomas, haemangiomas, and cancers that are sensitive to rapamycin treatment. Such mutations lead to increased eIF4F formation and consequently to enhanced translation initiation and cell growth. Thus, inhibition of translation initiation through targeting the mTOR-signalling pathway is emerging as a promising therapeutic option.

Regulation of poly(A)-binding protein through PABP-interacting proteins.

Translation initiation requires the participation of eukaryotic translation initiation factors (eIFs). The poly(A)-binding protein (PABP) is thought to stimulate translation by promoting mRNA circularization through simultaneous interactions with eIF4G and the 3' poly(A) tail. PABP activity is regulated by the PABP-interacting proteins (Paips), a family of proteins consisting of Paip1, a translational stimulator, and Paip2A and Paip2B, two translational inhibitors. Paip2A controls PABP homeostasis via ubiquitination. When the cellular concentration of PABP is reduced, Paip2A becomes ubiquitinated and degraded, resulting in the relief of PABP repression. Paip1 interacts with eIF4A and eIF3, which promotes translation. The regulation of PABP activity by Paips represents the first known mechanism for controlling PABP, adding a new layer to the existing knowledge of PABP function.

Contrasting mechanisms of regulating translation of specific Drosophila germline mRNAs at the level of 5'-cap structure binding.

Translational control is a key genetic regulatory mechanism underlying the initial establishment of the major spatial axes of the Drosophila embryo. Many translational control mechanisms target eIF4E (eukaryotic initiation factor 4E), an initiation factor that recognizes the 5'-cap structure of the mRNA. Cap recognition by eIF4E, in complex with eIF4G, is essential for recruitment of the mRNA to the small ribosomal subunit. One established mechanism for repressing translation involves eIF4E-binding proteins, which competitively inhibit the eIF4E-eIF4G interaction. Our group has uncovered a novel mechanism for repression in which an eIF4E cognate protein called d4EHP, which cannot bind eIF4G, binds to the 5'-cap structure of cad mRNA thus rendering it translationally inactive. These two related, but distinct, mechanisms are discussed and contrasted in this review.

Alternative splicing facilitates internal ribosome entry on the ornithine decarboxylase mRNA.

Ornithine decarboxylase (ODC) is the ratelimiting enzyme in the biosynthesis of polyamines, which are required for optimal cell growth and proliferation. ODC is overexpressed in many tumors and, conversely, its overexpression induces transformation. We have previously reported that ODC mRNA alternative splicing relieves the translation repression normally imposed by a long and structured 5' untranslated region (UTR), and that the ODC 5' UTR contains an internal ribosome entry site (IRES). Here we show that ODC IRES activity is enhanced following inclusion of alternative sequences generated by splicing at cryptic acceptor sites. Furthermore, the alternative ODC IRES is more sensitive to cell-cycledependent changes in the rate of translation. These findings uncover a new biological property of differentially spliced transcripts. This is the first example of alternative splicing that modulates mRNA translation through the cell cycle in a cap-independent manner.

mTOR signaling to translation.

Over the past few years, the target of rapamycin (TOR) pathway has been implicated in the control of translation, both in yeast and in higher eukaryotes. In this review, we provide an overview of translation in eukaryotes, and discuss the mechanisms and advantages of the regulation of translation. We then describe how the TOR pathway can modulate translation in yeast and in mammals, through the modulation of the phosphorylation of key translation components, and the regulation of the abundance of ribosomes and translation factors.

Hierarchical phosphorylation of the translation inhibitor 4E-BP1.

In most instances, translation is regulated at the initiation phase, when a ribosome is recruited to the 5' end of an mRNA. The eIF4E-binding proteins (4E-BPs) interdict translation initiation by binding to the translation factor eIF4E, and preventing recruitment of the translation machinery to mRNA. The 4E-BPs inhibit translation in a reversible manner. Hypophosphorylated 4E-BPs interact avidly with eIF4E, whereas 4E-BP hyperphosphorylation, elicited by stimulation of cells with hormones, cytokines, or growth factors, results in an abrogation of eIF4E-binding activity. We reported previously that phosphorylation of 4E-BP1 on Thr 37 and Thr 46 is relatively insensitive to serum deprivation and rapamycin treatment, and that phosphorylation of these residues is required for the subsequent phosphorylation of a set of unidentified serum-responsive sites. Here, using mass spectrometry, we identify the serum-responsive, rapamycin-sensitive sites as Ser 65 and Thr 70. Utilizing a novel combination of two-dimensional isoelectric focusing/SDS-PAGE and Western blotting with phosphospecific antibodies, we also establish the order of 4E-BP1 phosphorylation in vivo; phosphorylation of Thr 37/Thr 46 is followed by Thr 70 phosphorylation, and Ser 65 is phosphorylated last. Finally, we show that phosphorylation of Ser 65 and Thr 70 alone is insufficient to block binding to eIF4E, indicating that a combination of phosphorylation events is necessary to dissociate 4E-BP1 from eIF4E.

FRAP/mTOR is required for proliferation and patterning during embryonic development in the mouse.

The FKBP-12-rapamycin associated protein (FRAP, also known as mTOR and RAFT-1) is a member of the phosphoinositide kinase related kinase family. FRAP has serine/threonine kinase activity and mediates the cellular response to mitogens through signaling to p70s6 kinase (p70(s6k)) and 4E-BP1, resulting in an increase in translation of subsets of cellular mRNAs. Translational up-regulation is blocked by inactivation of FRAP signaling by rapamycin, resulting in G(1) cell cycle arrest. Rapamycin is used as an immunosuppressant for kidney transplants and is currently under investigation as an antiproliferative agent in tumors because of its ability to block FRAP activity. Although the role of FRAP has been extensively studied in vitro, characterization of mammalian FRAP function in vivo has been limited to the immune system and tumor models. Here we report the identification of a loss-of-function mutation in the mouse FRAP gene, which illustrates a requirement for FRAP activity in embryonic development. Our studies also determined that rapamycin treatment of the early embryo results in a phenotype indistinguishable from the FRAP mutant, demonstrating that rapamycin has teratogenic activity.

Regulation of translation via TOR signaling: insights from Drosophila melanogaster.

The target of rapamycin (TOR) proteins are large protein kinases evolutionarily conserved from yeast to human. A large body of evidence demonstrates that TOR proteins function in a nutrient-sensing checkpoint whose role is to restrict growth under conditions of low nutrient availability. Under such conditions, TOR blocks the transmission of growth-promoting signals from extracellular stimuli. Recent data obtained by genetic studies in the fruit fly Drosophila melanogaster demonstrate the importance of both insulin-like signaling and TOR signaling in promoting growth. Importantly, these studies identified a major downstream target of TOR and insulin-like signaling as the translational machinery.

Adipose tissue reduction in mice lacking the translational inhibitor 4E-BP1.

All nuclear-encoded mRNAs contain a 5' cap structure (m7GpppN, where N is any nucleotide), which is recognized by the eukaryotic translation initiation factor 4E (eIF4E) subunit of the eIF4F complex. The eIF4E-binding proteins constitute a family of three polypeptides that reversibly repress cap-dependent translation by binding to eIF4E, thus preventing the formation of the eIF4F complex. We investigated the biological function of 4E-BP1 by disrupting its gene (Eif4ebp1) in the mouse. Eif4ebp1-/- mice manifest markedly smaller white fat pads than wild-type animals, and knockout males display an increase in metabolic rate. The males' white adipose tissue contains cells that exhibit the distinctive multilocular appearance of brown adipocytes, and expresses the uncoupling protein 1 (UCP1), a specific marker of brown fat. Consistent with these observations, translation of the peroxisome proliferator-activated receptor-gamma co-activator 1 (PGC1), a transcriptional co-activator implicated in mitochondrial biogenesis and adaptive thermogenesis, is increased in white adipose tissue of Eif4ebp1-/- mice. These findings demonstrate that 4E-BP1 is a novel regulator of adipogenesis and metabolism in mammals.

Translational silencing of ceruloplasmin requires the essential elements of mRNA circularization: poly(A) tail, poly(A)-binding protein, and eukaryotic translation initiation factor 4G.

Ceruloplasmin (Cp) is a glycoprotein secreted by the liver and monocytic cells and probably plays roles in inflammation and iron metabolism. We showed previously that gamma interferon (IFN-gamma) induced Cp synthesis by human U937 monocytic cells but that the synthesis was subsequently halted by a transcript-specific translational silencing mechanism involving the binding of a cytosolic factor(s) to the Cp mRNA 3' untranslated region (UTR). To investigate how protein interactions at the Cp 3'-UTR inhibit translation initiation at the distant 5' end, we considered the "closed-loop" model of mRNA translation. In this model, the transcript termini are brought together by interactions of poly(A)-binding protein (PABP) with both the poly(A) tail and initiation factor eIF4G. The effect of these elements on Cp translational control was tested using chimeric reporter transcripts in rabbit reticulocyte lysates. The requirement for poly(A) was shown since the cytosolic inhibitor from IFN-gamma-treated cells minimally inhibited the translation of a luciferase reporter upstream of the Cp 3'-UTR but almost completely blocked the translation of a transcript containing a poly(A) tail. Likewise, a requirement for poly(A) was shown for silencing of endogenous Cp mRNA. We considered the possibility that the cytosolic inhibitor blocked the interaction of PABP with the poly(A) tail or with eIF4G. We found that neither of these interactions were inhibited, as shown by immunoprecipitation of PABP followed by quantitation of the poly(A) tail by reverse transcription-PCR and of eIF4G by immunoblot analysis. We considered the alternate possibility that these interactions were required for translational silencing. When PABP was depleted from the reticulocyte lysate with anti-human PABP antibody, the cytosolic factor did not inhibit translation of the chimeric reporter, thus showing the requirement for PABP. Similarly, in lysates treated with anti-human eIF4G antibody, the cytosolic extract did not inhibit the translation of the chimeric reporter, thereby showing a requirement for eIF4G. These data show that translational silencing of Cp requires interactions of three essential elements of mRNA circularization, poly(A), PABP, and eIF4G. We suggest that Cp mRNA circularization brings the cytosolic Cp 3'-UTR-binding factor into the proximity of the translation initiation site, where it silences translation by an undetermined mechanism. These results suggest that in addition to its important function in increasing the efficiency of translation, transcript circularization may serve as an essential structural determinant for transcript-specific translational control.