Regulation of the phosphorylation and integrity of protein synthesis initiation factor eIF4GI and the translational repressor 4E-BP1 by p53

Interruption of p53-MDM2 Interaction by Nutlin-3a in Human Lymphoma Cell Models Initiates a Cell-Dependent Global Effect on Transcriptome and Proteome Level

In most lymphomas, p53 signaling pathway is inactivated by various mechanisms independent to p53 gene mutations or deletions. In many cases, p53 function is largely regulated by alterations in the protein abundance levels by the action of E3 ubiquitin-protein ligase MDM2, targeting p53 to proteasome-mediated degradation. In the present study, an integrating transcriptomics and proteomics analysis was employed to investigate the effect of p53 activation by a small-molecule MDM2-antagonist, nutlin-3a, on three lymphoma cell models following p53 activation. Our analysis revealed a system-wide nutlin-3a-associated effect in all examined lymphoma types, identifying in total of 4037 differentially affected proteins involved in a plethora of pathways, with significant heterogeneity among lymphomas. Our findings include known p53-targets and novel p53 activation effects, involving transcription, translation, or degradation of protein components of pathways, such as a decrease in key members of PI3K/mTOR pathway, heat-shock response, and glycolysis, and an increase in key members of oxidative phoshosphorylation, autophagy and mitochondrial translation. Combined inhibition of HSP90 or PI3K/mTOR pathway with nutlin-3a-mediated p53-activation enhanced the apoptotic effects suggesting a promising strategy against human lymphomas. Integrated omic profiling after p53 activation offered novel insights on the regulatory role specific proteins and pathways may have in lymphomagenesis.

Metabolism responses in the intestine of Oreochromis mossambicus exposed to salinity, alkalinity and salt-alkalinity stress using LC-MS/MS-based metabolomics

Multiple abiotic stresses are imposed on fish as a result of unprecedented changes in temperature and precipitation patterns in recent decades. It is unclear how teleosts respond to severe ambient salinity, alkalinity, and saline-alkalinity in terms of their metabolic and molecular osmoregulation processes. The metabolic reactions in the intestine of Oreochromis mossambicus under salinity (25 g/L, S_C), alkalinity (4 g/L, A_C), and saline-alkalinity (salinity: 25 g/L & alkalinity: 4 g/L, SA_C) stresses were examined in this research utilizing LC-MS/MS-based metabolomics. The findings demonstrated that the three osmotic-stressed groups' metabolic profiles were considerably different from those of the control group. Osmolytes, energy sources, free amino acids, and several intermediate metabolites were all synthetically adjusted as part of the osmoregulation associated with the salinity, alkalinity, and saline-alkalinity stress. Following osmotic stress, osmoregulation-related pathways, including the mTOR signaling pathway, TCA cycle, glycolysis/gluconeogenesis, etc., were also discovered in the intestine of O. mossambicus. Overall, our findings can assist in better comprehending the molecular regulatory mechanism in euryhaline fish under various osmotic pressures and can offer a preliminary profile of osmotic regulation.

Mechanisms of miR-3189-3p-mediated inhibition of c-MYC translation in triple negative breast cancer

BACKGROUND

Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by the lack of estrogen receptor, progesterone receptor, and HER2. Our lab previously characterized miR-3189-3p as a microRNA with potent anti-cancer activity against glioblastoma. Here, we hypothesized a similar activity in TNBC cells. As miR-3189-3p is predicted to target a variety of RNA binding proteins, we further hypothesized an inhibitory effect of this miRNA on protein synthesis.

METHODS

MDA-MB-231 and MDA-MB-468 cells were used to investigate the effect of miR-3189-3p on cell proliferation, migration, and invasion. TGCA database was used to analyze the expression of miR-3189-3p, c-MYC, 4EPB1, and eIF4E in breast cancer. Western blotting and RT-qPCR assays were used to assess the expression of selected proteins and RNAs after transfections.

RESULTS

Although c-MYC is not a predicted gene target for miR-3189-3p, we discovered that c-MYC protein is downregulated in miRNA-treated TNBC cells. We found that the downregulation of c-MYC by miR-3189-3p occurs in both normal growth conditions and in the absence of serum. The mechanism involved the direct inhibition of eIF4EBP1 by miR-3189-3p. Additionally, we found that miR-3189-3p could negatively affect cap-independent translation mediated by internal ribosome entry sites (IRES) or by m6A. Finally, miR-3189-3p sensitized TNBC cells to doxorubicin.

CONCLUSION

Overall, results indicated that miR-3189-3p exerts its anti-tumor activity through targeting translational regulatory proteins leading to an impairment in c-MYC translation, and possibly other oncogenic factors, suggesting that miR-3189-3p, alone or in combination, could be a valuable therapeutic approach against a malignancy with few treatment options.

Suppression of the doxorubicin response by hypoxia-inducible factor-1α is strictly dependent on oxygen concentrations under hypoxic conditions

Hypoxia-inducible factor-1α (HIF-1α) and p53 are involved in anticancer drug resistance under hypoxic conditions. Here, we found that the cytotoxicity of anticancer drugs (doxorubicin, gemcitabine, and cisplatin) was lower at 1% O₂ than at 5% O₂. We examined the effects of these drugs on HIF-1α and p53 expression under different hypoxic oxygen concentrations. At 5% O₂, the drugs decreased HIF-1α expression and increased p53 levels. At 1% O₂, the drugs increased HIF-1α expression but did not alter p53 levels. When the HIF-1α protein was stabilized by DMOG under normoxic conditions, doxorubicin did not increase the level of p53 expression. These results show that the maintenance of HIF-1α expression blocked doxorubicin-dependent increases in p53 expression. We hypothesized the mechanism of HIF-1α protein translation might be different between at 5% and at 1% O₂, because many reports indicate that the same mechanism of HIF-1α protein stabilization occurs under hypoxic conditions, such as 5% and 1% O₂. The level of phosphorylated-4E-BP1, which causes translation of HIF-1α, was higher at 1% O₂ than at 5% O₂. Our results suggest that the sensitivity of tumor cells to anticancer drugs is dependent oxygen concentrations under hypoxic conditions, and involves 4E-BP1-dependent stabilization of the HIF-1α protein.

Target-binding behavior of IDPs via pre-structured motifs

Pre-Structured Motifs (PreSMos) are transient secondary structures observed in many intrinsically disordered proteins (IDPs) and serve as protein target-binding hot spots. The prefix "pre" highlights that PreSMos exist a priori in the target-unbound state of IDPs as the active pockets of globular proteins pre-exist before target binding. Therefore, a PreSMo is an "active site" of an IDP; it is not a spatial pocket, but rather a secondary structural motif. The classical and perhaps the most effective approach to understand the function of a protein has been to determine and investigate its structure. Ironically or by definition IDPs do not possess structure (here structure refers to tertiary structure only). Are IDPs then entirely structureless? The PreSMos provide us with an atomic-resolution answer to this question. For target binding, IDPs do not rely on the spatial pockets afforded by tertiary or higher structures. Instead, they utilize the PreSMos possessing particular conformations that highly presage the target-bound conformations. PreSMos are recognized or captured by targets via conformational selection (CS) before their conformations eventually become stabilized via structural induction into more ordered bound structures. Using PreSMos, a number of, if not all, IDPs can bind targets following a sequential pathway of CS followed by an induced fit (IF). This chapter presents several important PreSMos implicated in cancers, neurodegenerative diseases, and other diseases along with discussions on their conformational details that mediate target binding, a structural rationale for unstructured proteins.

Aberrant regulation of the Akt signaling network by human cytomegalovirus allows for targeting of infected monocytes

Primary peripheral blood monocytes are responsible for the hematogenous dissemination of human cytomegalovirus (HCMV) following a primary infection. In order to facilitate viral spread, HCMV extends the naturally short 48-h lifespan of monocytes by stimulating a non-canonical activation of Akt during viral entry, which leads to the increased expression of a specific subset of antiapoptotic proteins. In this study, global analysis of the Akt signaling network showed HCMV induced a more robust activation of the entire network when compared to normal myeloid growth factors. Furthermore, we found a unique interplay between HCMV-activated Akt and the stress response transcription heat shock factor 1 (HSF1) that allowed for the synthesis of both cap- and internal ribosome entry site (IRES)-containing antiapoptotic mRNAs such as myeloid cell leukemia-1 (Mcl-1) and X-linked inhibitor of apoptosis (XIAP), respectively. As generally a switch from cap-dependent to IRES-mediated translation occurs during cellular stress, the ability of HCMV to concurrently drive both types of translation produces a distinct milieu of prosurvival proteins needed for the viability of infected monocytes. Indeed, we found inhibition of XIAP led to death of ∼99% of HCMV-infected monocytes while having minimal effect on the viability of uninfected cells. Taken together, these data indicate that the aberrant activation of the Akt network by HCMV induces the upregulation of a unique subset of antiapoptotic proteins specifically required for the survival of infected monocytes. Consequently, our study highlights the possibility of exploiting these virus-induced changes to prevent viral spread in immunocompromised patients at high-risk for HCMV exposure.

Translation Control by p53

The translation of mRNAs plays a critical role in the regulation of gene expression and therefore, in the regulation of cell proliferation, differentiation and apoptosis. Unrestricted initiation of translation causes malignant transformation and plays a key role in the maintenance and progression of cancers. Translation initiation is regulated by the ternary complex and the eukaryotic initiation factor 4F (eIF4F) complex. The p53 tumor suppressor protein is the most well studied mammalian transcription factor that mediates a variety of anti-proliferative processes. Post-transcriptional mechanisms of gene expression in general and those of translation in particular play a major role in shaping the protein composition of the cell. The p53 protein regulates transcription and controls eIF4F, the ternary complex and the synthesis of ribosomal components, including the down-regulation of rRNA genes. In summary, the induction of p53 regulates protein synthesis and translational control to inhibit cell growth.

Enantiomerically pure β-dipeptide derivative induces anticancer activity against human hormone-refractory prostate cancer through both PI3K/Akt-dependent and -independent pathways

The use of peptides that target cancer cells and induce anticancer activities through various mechanisms is developing as a potential anticancer strategy. KUD983, an enantiomerically pure β-dipeptide derivative, displays potent activity against hormone-refractory prostate cancer (HRPC) PC-3 and DU145 cells with submicromolar IC₅₀. KUD983 induced G1 arrest of the cell cycle and subsequent apoptosis associated with down-regulation of several related proteins including cyclin D1, cyclin E and Cdk4, and the de-phosphorylation of RB. The levels of nuclear and total c-Myc protein, which could increase the expression of both cyclin D1 and cyclin E, were profoundly inhibited by KUD983. Furthermore, it inhibited PI3K/Akt and mTOR/p70S6K/4E-BP1 pathways, the key signaling in multiple cellular functions. The transient transfection of constitutively active myristylated Akt (myr-Akt) cDNA significantly rescued KUD983-induced caspase activation but did not blunt the inhibition of mTOR/p70S6K/4E-BP1 signaling cascade suggesting the presence of both Akt-dependent and -independent pathways. Moreover, KUD983-induced effect was enhanced with the down-regulation of anti-apoptotic Bcl-2 members (e.g., Bcl-2, and Mcl-1) and IAP family members (e.g., survivin). Notably, KUD983 induced autophagic cell death using confocal microscopic examination, tracking the level of conversion of LC3-I to LC3-II and flow cytometric detection of acidic vesicular organelles-positive cells. In conclusion, the data suggest that KUD983 is an anticancer β-dipeptide against HRPCs through the inhibition of cell proliferation and induction of apoptotic and autophagic cell death. The suppression of signaling pathways regulated by c-Myc, PI3K/Akt and mTOR/p70S6K/4E-BP1 and the collaboration with down-regulation of Mcl-1 and survivin may explain KUD983-induced anti-HRPC mechanism.

PC-1/PrLZ confers resistance to rapamycin in prostate cancer cells through increased 4E-BP1 stability

An important strategy for improving advanced PCa treatment is targeted therapies combined with chemotherapy. PC-1, a prostate Leucine Zipper gene (PrLZ), is specifically expressed in prostate tissue as an androgen-induced gene and is up-regulated in advanced PCa. Recent work confirmed that PC-1 expression promotes PCa growth and androgen-independent progression. However, how this occurs and whether this can be used as a biomarker is uncertain. Here, we report that PC-1 overexpression confers PCa cells resistance to rapamycin treatment by antagonizing rapamycin-induced cytostasis and autophagy (rapamycin-sensitivity was observed in PC-1-deficient (shPC-1) C4-2 cells). Analysis of the mTOR pathway in PCa cells with PC-1 overexpressed and depressed revealed that eukaryotic initiation factor 4E-binding protein 1(4E-BP1) was highly regulated by PC-1. Immunohistochemistry assays indicated that 4E-BP1 up-regulation correlates with increased PC-1 expression in human prostate tumors and in PCa cells. Furthermore, PC-1 interacts directly with 4E-BP1 and stabilizes 4E-BP1 protein via inhibition of its ubiquitination and proteasomal degradation. Thus, PC-1 is a novel regulator of 4E-BP1 and our work suggests a potential mechanism through which PC-1 enhances PCa cell survival and malignant progression and increases chemoresistance. Thus, the PC-1-4E-BP1 interaction may represent a therapeutic target for treating advanced PCa.

PAT4 levels control amino-acid sensitivity of rapamycin-resistant mTORC1 from the Golgi and affect clinical outcome in colorectal cancer

Tumour cells can use strategies that make them resistant to nutrient deprivation to outcompete their neighbours. A key integrator of the cell's responses to starvation and other stresses is amino-acid-dependent mechanistic target of rapamycin complex 1 (mTORC1). Activation of mTORC1 on late endosomes and lysosomes is facilitated by amino-acid transporters within the solute-linked carrier 36 (SLC36) and SLC38 families. Here, we analyse the functions of SLC36 family member, SLC36A4, otherwise known as proton-assisted amino-acid transporter 4 (PAT4), in colorectal cancer. We show that independent of other major pathological factors, high PAT4 expression is associated with reduced relapse-free survival after colorectal cancer surgery. Consistent with this, PAT4 promotes HCT116 human colorectal cancer cell proliferation in culture and tumour growth in xenograft models. Inducible knockdown in HCT116 cells reveals that PAT4 regulates a form of mTORC1 with two distinct properties: first, it preferentially targets eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), and second, it is resistant to rapamycin treatment. Furthermore, in HCT116 cells two non-essential amino acids, glutamine and serine, which are often rapidly metabolised by tumour cells, regulate rapamycin-resistant mTORC1 in a PAT4-dependent manner. Overexpressed PAT4 is also able to promote rapamycin resistance in human embryonic kidney-293 cells. PAT4 is predominantly associated with the Golgi apparatus in a range of cell types, and in situ proximity ligation analysis shows that PAT4 interacts with both mTORC1 and its regulator Rab1A on the Golgi. These findings, together with other studies, suggest that differentially localised intracellular amino-acid transporters contribute to the activation of alternate forms of mTORC1. Furthermore, our data predict that colorectal cancer cells with high PAT4 expression will be more resistant to depletion of serine and glutamine, allowing them to survive and outgrow neighbouring normal and tumorigenic cells, and potentially providing a new route for pharmacological intervention.

The estrogen metabolite 2-methoxyestradiol regulates eukaryotic initiation factor 4E (eIF4E) and inhibits protein synthesis in MG63 osteosarcoma cells

Osteosarcoma is a primary bone tumor that affects children and young adults. The estrogen metabolite 2-methoxyestradiol (2-ME) induces cell death in osteosarcoma cells. To determine whether 2-ME actions involve the control of protein synthesis, we studied the effect of 2-ME on eukaryotic initiation factor 4E (eIF4E) and eIF4E-binding protein 1 (4E-BP1) in MG63 osteosarcoma cells. Our results show that 2-ME treatment increases the association of eIF4E with 4E-BP1 in osteosarcoma cells. Also, 2-ME decreases the binding of eIF4E protein to 7-methyl-guanosine cap structure, indicating that 2-ME treatment results in the inhibition of translational initiation. These findings are further supported by the inhibition of protein synthesis in 2-ME-treated osteosarcoma cells. Taken together, our studies show that 2-ME-mediated antitumor effects in osteosarcoma cells involve the regulation of protein synthesis, and translational machinery could serve as a target in the treatment of osteosarcoma.

mTOR Signaling in Protein Translation Regulation: Implications in Cancer Genesis and Therapeutic Interventions

mTOR is a central nutrient sensor that signals a cell to grow and proliferate. Through distinct protein complexes it regulates different levels of available cellular energy substrates required for cell growth. One of the important functions of the complex is to maintain available amino acid pool by regulating protein translation. Dysregulation of mTOR pathway leads to aberrant protein translation which manifests into various pathological states. Our review focuses on the role mTOR signaling plays in protein translation and its physiological role. It also throws some light on available data that show translation dysregulation as a cause of pathological complexities like cancer and the available drugs that target the pathway for cancer treatment.

Modulation of the sensitivity of Jurkat T-cells to inhibition of protein synthesis by tumor necrosis factor α-related apoptosis-inducing ligand

Tumor necrosis factor α-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in Jurkat T lymphoma cells. One of the characteristics of the phase preceding overt apoptosis is the marked downregulation of protein synthesis. We have investigated factors that can influence this response and have explored some of the signaling pathways involved. We show that interferon-α (IFNα) pretreatment desensitizes Jurkat cells to TRAIL-induced inhibition of protein synthesis, such that the concentration of TRAIL required for 50% inhibition is increased by 10-fold. The inhibition of translation is characterized by dephosphorylation of the eIF4E-binding protein 4E-BP1 and IFNα desensitizes Jurkat cells to this effect. IFNα also inhibits TRAIL-mediated dephosphorylation of the growth-promoting protein kinase B (Akt). Since Jurkat cells are defective for phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and therefore have constitutive phosphoinositide 3-kinase (PI3K) activity, we investigated the consequences for protein synthesis of inhibiting PI3K using LY294002. Inhibition of PI3K partially inhibits translation, but also enhances the effect of a suboptimal concentration of TRAIL. However, LY294002 does not block the ability of IFNα to protect protein synthesis from TRAIL-induced inhibition. Data are presented suggesting that IFNα impairs the process of activation of caspase-8 within the TRAIL death-inducing signaling complex.

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.

Hyperglycemia-induced O-GlcNAcylation and truncation of 4E-BP1 protein in liver of a mouse model of type 1 diabetes

4E-BP1 is a protein that, in its hypophosphorylated state, binds the mRNA cap-binding protein eIF4E and represses cap-dependent mRNA translation. By doing so, it plays a major role in the regulation of gene expression by controlling the overall rate of mRNA translation as well as the selection of mRNAs for translation. Phosphorylation of 4E-BP1 causes it to release eIF4E to function in mRNA translation. 4E-BP1 is also subject to covalent addition of N-acetylglucosamine to Ser or Thr residues (O-GlcNAcylation) as well as to truncation. In the truncated form, it is both resistant to phosphorylation and able to bind eIF4E with high affinity. In the present study, Ins2(Akita/+) diabetic mice were used to test the hypothesis that hyperglycemia and elevated flux of glucose through the hexosamine biosynthetic pathway lead to increased O-GlcNAcylation and truncation of 4E-BP1 and consequently decreased eIF4E function in the liver. The amounts of both full-length and truncated 4E-BP1 bound to eIF4E were significantly elevated in the liver of diabetic as compared with non-diabetic mice. In addition, O-GlcNAcylation of both the full-length and truncated proteins was elevated by 2.5- and 5-fold, respectively. Phlorizin treatment of diabetic mice lowered blood glucose concentrations and reduced the expression and O-GlcNAcylation of 4E-BP1. Additionally, when livers were perfused in the absence of insulin, 4E-BP1 phosphorylation in the livers of diabetic mice was normalized to the control value, yet O-GlcNAcylation and the association of 4E-BP1 with eIF4E remained elevated in the liver of diabetic mice. These findings provide insight into the pathogenesis of metabolic abnormalities associated with diabetes.

Insights into 4E-BP1 and p53 mediated regulation of accelerated cell senescence

Senescence is a valid tumor suppressive mechanism in cancer. Accelerated cell senescence describes the growth arrested state of cells that have been treated with anti-tumor drugs, such as doxorubicin that induce a DNA damage response. Discodermolide, a microtubule-stabilizing agent, is a potent inducer of accelerated cell senescence. Resistance to discodermolide is mediated via resistance to accelerated cell senescence, and is associated with reduced expression of the mTORC1 substrate, 4E-BP1 and increased expression of p53 [1]. Although the association of p53 with senescence induction is well-characterized, senescence reversion in the presence of high expression of p53 has not been well-documented. Furthermore, studies addressing the role of mTOR signaling in regulating senescence have been limited and recent data implicate a novel, senescence-associated role for 4E-BP1 in crosstalk with the transcription factor p53. This research perspective will address these somewhat contradictory findings and summarize recent research regarding senescence and mTORC1 signaling.

DNA damaging agents and p53 do not cause senescence in quiescent cells, while consecutive re-activation of mTOR is associated with conversion to senescence

When the cell cycle is arrested, growth-promoting pathways such as mTOR (Target of Rapamycin) drive cellular senescence, characterized by cellular hyper-activation, hypertrophy and permanent loss of the proliferative potential. While arresting cell cycle, p53 (under certain conditions) can inhibit the mTOR pathway. Senescence occurs when p53 fails to inhibit mTOR. Low concentrations of DNA-damaging drugs induce p53 at levels that do not inhibit mTOR, thus causing senescence. In quiescence caused by serum starvation, mTOR is deactivated. This predicts that induction of p53 will not cause senescence in such quiescent cells. Here we tested this prediction. In proliferating normal cells, etoposide caused senescence (cells could not resume proliferation after removal of etoposide). Serum starvation prevented induction of senescence, but not of p53, by etoposide. When etoposide was removed, such cells resumed proliferation upon addition of serum. Also, doxorubicin did not cause senescent morphology in the absence of serum. Re-addition of serum caused mTOR-dependent senescence in the presence of etoposide or doxorubicin. Also, serum-starvation prevented senescent morphology caused by nutlin-3a in MCF-7 and Mel-10 cells. We conclude that induction of p53 does not activate the senescence program in quiescent cells. In cells with induced p53, re-activation of mTOR by serum stimulation causes senescence, as an equivalent of cellular growth.

Paradoxical suppression of cellular senescence by p53

The tumor suppressor p53 is a canonical inducer of cellular senescence (irreversible loss of proliferative potential and senescent morphology). p53 can also cause reversible arrest without senescent morphology, which has usually been interpreted as failure of p53 to induce senescence. Here we demonstrate that p53-induced quiescence actually results from suppression of senescence by p53. In previous studies, suppression of senescence by p53 was masked by p53-induced cell cycle arrest. Here, we separated these two activities by inducing senescence through overexpression of p21 and then testing the effect of p53 on senescence. We found that in p21-arrested cells, p53 converted senescence into quiescence. Suppression of senescence by p53 required its transactivation function. Like rapamycin, which is known to suppress senescence, p53 inhibited the mTOR pathway. We suggest that, while inducing cell cycle arrest, p53 may simultaneously suppress the senescence program, thus causing quiescence and that suppression of senescence and induction of cell cycle arrest are distinct functions of p53. Thus, in spite of its ability to induce cell cycle arrest, p53 can act as a suppressor of cellular senescence.

p53-dependent translational control of senescence and transformation via 4E-BPs

eIF4E, the mRNA 5' cap-binding translation initiation factor, is overexpressed in numerous cancers and is implicated in mechanisms underlying oncogenesis and senescence. 4E-BPs (eIF4E-binding proteins) inhibit eIF4E activity, and thereby act as suppressors of eIF4E-dependent pathways. Here, we show that tumorigenesis is increased in p53 knockout mice that lack 4E-BP1 and 4E-BP2. However, primary fibroblasts lacking 4E-BPs, but expressing p53, undergo premature senescence and resist oncogene-driven transformation. Thus, the p53 status governs 4E-BP-dependent senescence and transformation. Intriguingly, the 4E-BPs engage in senescence via translational control of the p53-stabilizing protein, Gas2. Our data demonstrate a role for 4E-BPs in senescence and tumorigenesis and highlight a p53-mediated mechanism of senescence through a 4E-BP-dependent pathway.

Regulation of protein synthesis by ionizing radiation

Ionizing radiation (IR) is a physiologically important stress to which cells respond by the activation of multiple signaling pathways. Using a panel of immortalized and transformed breast epithelial cell lines, we demonstrate that IR regulation of protein synthesis occurs in nontransformed cells and is lost with transformation. In nontransformed cells, IR rapidly activates the MAP kinases ERK1/2, resulting in an early transient increase in cap-dependent mRNA translation that involves mTOR and is radioprotective, enhancing the translation of a subset of mRNAs encoding proteins involved in DNA repair and cell survival. Following a transient increase in translation, IR-sensitive (nontransformed) cells inhibit cap-dependent protein synthesis through a mechanism that involves activation of p53, induction of Sestrin 1 and 2 genes, and stimulation of AMP kinase, inhibiting mTOR and hypophosphorylating 4E-BP1. IR is shown to block proteasome-mediated decay of 4E-BP1, increasing its abundance and the sequestration of eIF4E. The IR signal that impairs mTOR-dependent protein synthesis at late times is assembly of the DNA damage response machinery, consisting of Mre11, Rad50, and NBS1 (MRN); activation of the MRN complex kinase ATM; and p53. These results link genotoxic signaling from the DNA damage response complex to the control of protein synthesis.

Cell signaling in endometrial carcinoma: phosphorylated 4E-binding protein-1 expression in endometrial cancer correlates with aggressive tumors and prognosis

In the oncogenic process, cell growth control plays a crucial role, and growth factor receptors and their signaling pathways are known to be altered in endometrial cancer, mostly in type I carcinomas. Two main pathways are involved in transmitting the proliferative signal from the membrane receptors to the nucleus: phosphatydil-inositol-3-kinase-protein kinase B-mammalian target of rapamycin and RAS-RAF-ERK pathways. A final effector of these signaling cascades is the cap-dependent mRNA translation initiation complex, which is negatively regulated by 4E-BP1. The aim of our work was to study the relative importance of the factors involved in these pathways and to see their correlation with the clinicopathologic features of the tumors and their prognosis. We studied 120 endometrial carcinomas, including 93 type I and 27 type II carcinomas, and 18 control cases. Tissue microarrays were constructed and immunohistochemistry was performed for HER2, p53, and the phosphorylated forms of protein kinase B, extracellular signal-regulated kinase, and 4E-BP1. HER2 was overexpressed in 11% of carcinomas but not in control cases, and 30% of carcinomas showed activation of protein kinase B and extracellular signal-regulated kinase, mostly in type II carcinomas. The phosphorylated form of 4E-BP1 was found to be cytoplasmic in 31% of cases, and in 63% of cases it showed nuclear expression; the latter was only found in carcinomas. p53 positivity was found in type II and in grade 3 type I carcinomas. This nuclear expression of phospho-4E-BP1 and HER2 overexpression were the only characteristics with prognostic significance. The activation of the signaling pathways that control cell growth is a common event in endometrial carcinomas. 4E-BP1 is a downstream effector of these pathways whose activation status correlates with aggressive phenotypes and prognosis. This factor can reflect the activity of these pathways, regardless of the upstream molecular alterations, and, therefore, it can be a hallmark of the transmission of the oncogenic signal to the nucleus.

Acute doxorubicin cardiotoxicity is associated with p53-induced inhibition of the mammalian target of rapamycin pathway

BACKGROUND

Doxorubicin is used to treat childhood and adult cancer. Doxorubicin treatment is associated with both acute and chronic cardiotoxicity. The cardiotoxic effects of doxorubicin are cumulative, which limits its chemotherapeutic dose. Free radical generation and p53-dependent apoptosis are thought to contribute to doxorubicin-induced cardiotoxicity.

METHODS AND RESULTS

Adult transgenic (MHC-CB7) mice expressing cardiomyocyte-restricted dominant-interfering p53 and their nontransgenic littermates were treated with doxorubicin (20 mg/kg cumulative dose). Nontransgenic mice exhibited reduced left ventricular systolic function (predoxorubicin fractional shortening [FS] 61+/-2%, postdoxorubicin FS 45+/-2%, mean+/-SEM, P<0.008), reduced cardiac mass, and high levels of cardiomyocyte apoptosis 7 days after the initiation of doxorubicin treatment. In contrast, doxorubicin-treated MHC-CB7 mice exhibited normal left ventricular systolic function (predoxorubicin FS 63+/-2%, postdoxorubicin FS 60+/-2%, P>0.008), normal cardiac mass, and low levels of cardiomyocyte apoptosis. Western blot analyses indicated that mTOR (mammalian target of rapamycin) signaling was inhibited in doxorubicin-treated nontransgenic mice but not in doxorubicin-treated MHC-CB7 mice. Accordingly, transgenic mice with cardiomyocyte-restricted, constitutively active mTOR expression (MHC-mTORca) were studied. Left ventricular systolic function (predoxorubicin FS 64+/-2%, postdoxorubicin FS 60+/-3%, P>0.008) and cardiac mass were normal in doxorubicin-treated MHC-mTORca mice, despite levels of cardiomyocyte apoptosis similar to those seen in doxorubicin-treated nontransgenic mice.

CONCLUSIONS

These data suggest that doxorubicin treatment induces acute cardiac dysfunction and reduces cardiac mass via p53-dependent inhibition of mTOR signaling and that loss of myocardial mass, and not cardiomyocyte apoptosis, is the major contributor to acute doxorubicin cardiotoxicity.

Activation of p53 stimulates proteasome-dependent truncation of eIF4E-binding protein 1 (4E-BP1)

BACKGROUND INFORMATION

The translational inhibitor protein 4E-BP1 [eIF4E (eukaryotic initiation factor 4E)-binding protein 1] regulates the availability of polypeptide chain initiation factor eIF4E for protein synthesis. Initiation factor eIF4E binds the 5' cap structure present on all cellular mRNAs. Its ability to associate with initiation factors eIF4G and eIF4A, forming the eIF4F complex, brings the mRNA to the 43S complex during the initiation of translation. Binding of eIF4E to eIF4G is inhibited in a competitive manner by 4E-BP1. Phosphorylation of 4E-BP1 decreases the affinity of this protein for eIF4E, thus favouring the binding of eIF4G and enhancing translation. We have previously shown that induction or activation of the tumour suppressor protein p53 rapidly leads to 4E-BP1 dephosphorylation, resulting in sequestration of eIF4E, decreased formation of the eIF4F complex and inhibition of protein synthesis.

RESULTS

We now report that activation of p53 also results in modification of 4E-BP1 to a truncated form. Unlike full-length 4E-BP1, which is reversibly phosphorylated at multiple sites, the truncated protein is almost completely unphosphorylated. Moreover, the latter interacts with eIF4E in preference to full-length 4E-BP1. Inhibitor studies indicate that the p53-induced cleavage of 4E-BP1 is mediated by the proteasome and is blocked by conditions that inhibit the dephosphorylation of full-length 4E-BP1. Measurements of the turnover of 4E-BP1 indicate that the truncated form is much more stable than the full-length protein.

CONCLUSIONS

The results suggest a model in which proteasome activity gives rise to a stable, hypophosphorylated and truncated form of 4E-BP1, which may exert a long-term inhibitory effect on the availability of eIF4E, thus contributing to the inhibition of protein synthesis and the growth-inhibitory and pro-apoptotic effects of p53.

mTOR signalling in human cancer

Inhibitors of mTOR, the mammalian target of rapamycin, have been extensively studied in clinical trials for cancer treatment. Results have been promising, mostly in certain lymphomas, but in solid tumours the results have been generally less encouraging. However, recent results, particularly in renal cell carcinoma, have provided renewed interest in the role of mTOR inhibitors in solid tumours. A rational, and potentially more successful, development of these agents (i.e., RAD001, temsirolimus and AP23573) likely relies in a deeper knowledge of mTOR signalling in cancer, both at the preclinical and clinical levels. These would allow a better selection of patients more likely to respond to the use of biologically active doses of the agents and the development of mechanistically based combinations with other agents. The goal of this review is to provide an update on the complex signalling of mTOR in cancer and on the biological effects of mTOR inhibitors in cancer cells.

Effects of protein phosphorylation on ubiquitination and stability of the translational inhibitor protein 4E-BP1

The availability of the eukaryotic polypeptide chain initiation factor 4E (eIF4E) for protein synthesis is regulated by the 4E-binding proteins (4E-BPs), which act as inhibitors of cap-dependent mRNA translation. The ability of the 4E-BPs to sequester eIF4E is regulated by reversible phosphorylation at multiple sites. We show here that, in addition, 4E-BP1 is a substrate for polyubiquitination and that some forms of 4E-BP1 are simultaneously polyubiquitinated and phosphorylated. In Jurkat cells inhibition of proteasomal activity by MG132 enhances the level of hypophosphorylated, unmodified 4E-BP1 but only modestly increases the accumulation of high-molecular-weight, phosphorylated forms of 4E-BP1. In contrast, inhibition of protein phosphatase activity with calyculin A reduces the level of unmodified 4E-BP1 but strongly enhances the amount of phosphorylated, high-molecular-weight 4E-BP1. Turnover measurements in the presence of cycloheximide show that, whereas 4E-BP1 is normally a very stable protein, calyculin A decreases the apparent half-life of the normal-sized protein. Affinity chromatography on m(7)GTP-Sepharose indicates that the larger forms of 4E-BP1 bind very poorly to eIF4E. We suggest that the phosphorylation of 4E-BP1 may play a dual role in the regulation of protein synthesis, both reducing the affinity of 4E-BP1 for eIF4E and promoting the conversion of 4E-BP1 to alternative, polyubiquitinated forms.

4E-binding protein 1, a cell signaling hallmark in breast cancer that correlates with pathologic grade and prognosis

PURPOSE

Cell signaling pathways include a complex myriad of interconnected factors from the membrane to the nucleus, such as erbB family receptors and the phosphoinositide-3-kinase/Akt/mTOR and Ras-Raf-ERK cascades, which drive proliferative signals, promote survival, and regulate protein synthesis.

EXPERIMENTAL DESIGN

To find pivotal factors in these pathways, which provide prognostic information in malignancies, we studied 103 human breast tumors with an immunohistochemical profile, including total and phosphorylated (p) proteins: human epidermal growth factor receptor 2 (HER2), epidermal growth factor receptor, extracellular signal-regulated kinase 1/2, Akt, 4E-binding protein 1 (4EBP1), eukaryotic initiation factor 4E, phosphorylated ribosomal protein S6 kinase 1, phosphorylated ribosomal protein S6, and Ki67. Western blot and reverse lysate protein arrays were also done in a subset of tumors.

RESULTS

Significantly, activation of the phosphoinositide-3-kinase/Akt/mTOR cascade was detected in a high proportion of tumors (41.9%). Tumors with HER2 overexpression showed higher p-Akt as compared with negative tumors (P < 0.001). Levels of p-Akt correlated with the downstream molecules, p-4EBP1 (P = 0.001) and p-p70S6K (P = 0.05). Although 81.5% of tumors expressed p-4EBP1, in 16.3% of these tumors, concomitant activation of the upstream factors was not detected. Interestingly, p-4EBP1 was mainly expressed in poorly differentiated tumors (P < 0.001) and correlated with tumor size (P < 0.001), presence of lymph node metastasis (P = 0.002), and locoregional recurrences (P = 0.002). Coexpression of p-4EBP1 and p-eIF4G correlated with a high tumor proliferation rate (P = 0.012).

CONCLUSION

In this study, p-4EBP1 was the main factor in signaling pathways that associate with prognosis and grade of malignancy in breast tumors. Moreover, p-4EBP1 was detected in both HER2-positive and HER2-negative tumors. This factor seems to be a channeling point at which different upstream oncogenic alterations converge and transmit their proliferative signal, modulating protein translation.

Phosphorylated 4E binding protein 1: a hallmark of cell signaling that correlates with survival in ovarian cancer

BACKGROUND

Growth factor receptors and cell signaling factors play a crucial role in human carcinomas and have been studied in ovarian tumors with varying results. Cell signaling involves multiple pathways and a myriad of factors that can be mutated or amplified. Cell signaling is driven through the mammalian target of rapamycin (mTOR) and extracellular regulated kinase (ERK) pathways and by some downstream molecules, such as 4E binding protein 1 (4EBP1), eukaryotic initiation factor 4E, and p70 ribosomal protein S6 kinase (p70S6K). The objectives of this study were to analyze the real role that these pathways play in ovarian cancer, to correlate them with clinicopathologic characteristics, and to identify the factors that transmit individual proliferation signals and are associated with pathologic grade and prognosis, regardless specific oncogenic alterations upstream.

METHODS

One hundred twenty-nine ovarian epithelial tumors were studied, including 20 serous cystadenomas, 7 mucinous cystadenomas, 11 serous borderline tumors, 16 mucinous borderline tumors, 29 serous carcinomas, 16 endometrioid carcinomas, 15 clear cell carcinomas, and 15 mucinous carcinomas. Tissue microarrays were constructed, and immunohistochemistry for the receptors epidermal growth factor receptor (EGFR) and c-erb-B2 was performed and with phosphorylated antibodies for protein kinase B (AKT), 4EBP1, p70S6K, S6, and ERK.

RESULTS

Among 129 ovarian neoplasms, 17.8% were positive for c-erb-B2, 9.3% were positive for EGFR, 47.3% were positive for phosphorylated AKT (p-AKT), 58.9% were positive for p-ERK, 41.1% were positive for p-4EBP1, 26.4% were positive for p70S6K, and 15.5% were positive for p-S6. Although EGFR, p-AKT, and p-ERK expression did not differ between benign, borderline, or malignant tumors, c-erb-B2, p-4EBP1, p-p70S6K, and p-S6 were expressed significantly more often in malignant tumors. Only p-4EBP1 expression demonstrated prognostic significance (P = .005), and only surgical stage and p-4EBP1 expression had statistical significance in the multivariate analysis.

CONCLUSIONS

In patients with ovarian carcinoma, significant expression of p-4EBP1 was associated with high-grade tumors and a poor prognosis, regardless other oncogenic alterations upstream. This finding supports the study of this factor as a hallmark or pivotal factor in cell signaling in ovarian carcinoma that may crucial in the transmission of the proliferation cell signal and may reflect the real oncogenic role of this pathway in ovarian tumors.

When translation meets transformation: the mTOR story

There is currently a high level of interest in signalling through the mammalian target of rapamycin (mTOR). This reflects both its key role in many cell functions and its involvement in disease states such as cancers. The best understood targets for mTOR signalling are proteins involved in controlling the translational machinery, including the ribosomal protein S6 kinases and proteins that regulate the initiation and elongation phases of translation. Indeed, there is compelling evidence that at least one of these targets of mTOR (eukaryotic initiation factor eIF4E) plays a key role in tumorigenesis. It is regulated through the mTOR-dependent phosphorylation of inhibitory proteins such as eIF4E-binding protein 1. Thus, targeting mTOR signalling may be an effective anticancer strategy, in at least a significant subset of tumours. Not all effects of mTOR are sensitive to the classical anti-mTOR drug rapamycin, and this compound also interferes with other processes besides eIF4E function. Developing new approaches to targeting mTOR for cancer therapy requires more detailed knowledge of signalling downstream of mTOR. Such advances are likely to come from further work to understand the regulation of mTOR targets such as components of the translational apparatus.

Upstream of the mammalian target of rapamycin: do all roads pass through mTOR?

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that controls many aspects of cellular physiology, including transcription, translation, cell size, cytoskeletal organization and autophagy. Recent advances in the mTOR signaling field have found that mTOR exists in two heteromeric complexes, mTORC1 and mTORC2. The activity of mTORC1 is regulated by the integration of many signals, including growth factors, insulin, nutrients, energy availability and cellular stressors such as hypoxia, osmotic stress, reactive oxygen species and viral infection. In this review we highlight recent advances in the mTOR signaling field that relate to how the two mTOR complexes are regulated, and we discuss stress conditions linked to the mTOR signaling network that have not been extensively covered in other reviews. Given the diversity of signals that have been shown to impinge on mTOR, we also speculate on other signal-transduction pathways that may be linked to mTOR in the future.

Stress and mTORture signaling

The TOR (target of rapamycin) pathway is an evolutionarily conserved signaling module regulating cell growth (accumulation of mass) in response to a variety of environmental cues such as nutrient availability, hypoxia, DNA damage and osmotic stress. Its pivotal role in cellular and organismal homeostasis is reflected in the fact that unrestrained signaling activity in mammals is associated with the occurrence of disease states including inflammation, cancer and diabetes. The existence of TOR homologs in unicellular organisms whose growth is affected by environmental factors, such as temperature, nutrients and osmolarity, suggests an ancient role for the TOR signaling network in the surveillance of stress conditions. Here, we will summarize recent advances in the TOR signaling field with special emphasis on how stress conditions impinge on insulin/insulin-like growth factor signaling/TOR signaling.

Regulation of translation factors eIF4GI and 4E-BP1 during recovery of protein synthesis from inhibition by p53

Activation of the tumour suppressor protein p53 rapidly inhibits protein synthesis. This is associated with dephosphorylation and cleavage of initiation factor eIF4GI and the eIF4E-binding protein 4E-BP1. When the activation of p53 is reversed within 16 h 4E-BP1 becomes rephosphorylated, the level of intact eIF4GI slowly increases and protein synthesis gradually recovers. The recovery of protein synthesis is partially blocked by rapamycin and wortmannin but not by the protein kinase inhibitors PD98059 and CGP74514A. Both rapamycin and wortmannin, but not PD98059 or CGP74514A, delay the reappearance of eIF4GI. In contrast, full-length 4E-BP1 rapidly becomes rephosphorylated and this process is partially inhibited by rapamycin, PD98059 and CGP74514A. Thus, activation of p53 results in the inhibition of distinct rapamycin- and wortmannin-sensitive pathways that target eIF4GI, and rapamycin-sensitive and -insensitive pathways that target 4E-BP1. Following inactivation of p53 the gradual recovery is determined largely by the kinetics of restoration of eIF4GI rather than by the rephosphorylation of full-length 4E-BP1. These findings suggest that the ability of cells to rephosphorylate 4E-BP1, resynthesise eIF4GI and restore the rate of protein synthesis after inactivation of p53 is an important aspect of recovery following the relief of physiological stress.

Pifithrin-α Enhances Chemosensitivity by a p38 Mitogen-Activated Protein Kinase-Dependent Modulation of the Eukaryotic Initiation Factor 4E in Malignant Cholangiocytes

Pifithrin-alpha is the lead compound for a novel group of small molecules that are being developed for use as anticancer agents. The eukaryotic initiation factor 4E (eIF-4E) is overexpressed in many cancers, it can mediate sensitivity to therapy, and it may be regulated by p53. We examined the utility of pifithrin-alpha as an adjunct to therapy for the treatment of human cholangiocarcinoma, a tumor that is highly refractory to therapy, and we assessed the involvement of p53-dependent eIF-4E regulation in cellular responses to pifithrin-alpha. The expression of eIF-4E was increased in human cholangiocarcinomas compared with normal liver. Modulation of eIF-4E expression by RNA interference enhanced the efficacy of gemcitabine in KMCH cholangiocarcinoma cells. Preincubation of KMCH cells with pifithrin-alpha enhanced gemcitabine-induced cytotoxicity in an eIF-4E-dependent manner. Furthermore, pifithrin-alpha increased eIF-4E phosphorylation at serine 209 via activation of p38 mitogen-activated protein kinase (MAPK). Pifithrin-alpha was shown to activate aryl hydrocarbon receptor (AhR) signaling and p38 MAPK activation. Sequencing analysis indicated the presence of a functionally inactivating p53 mutation in KMCH cells, and small interfering RNA to p53 did not modulate chemosensitization by pifithrin-alpha. Pifithrin-alpha enhanced chemosensitivity by a mechanism independent of p53 and involving AhR and p38 MAPK deregulation of eIF-4E phosphorylation. Thus, pifithrin-alpha may prove useful for enhancing chemosensitivity in tumors with mutated p53. Moreover, modulation of eIF-4E is an attractive therapeutic target for intervention in cancer treatment.

Regulation of protein synthesis by inducible wild-type p53 in human lung carcinoma cells

Activation of an over-expressed mutant form of the tumour suppressor protein p53 has been shown to inhibit protein synthesis. To determine whether this effect is due only to high level expression or the mutant nature of the protein, we have used a doxycycline-inducible lung carcinoma cell line capable of expressing wild-type p53. We now show that levels of wild-type p53 similar to those expressed endogenously also inhibit protein synthesis. The mechanism involves dephosphorylation and accumulation of the translational inhibitor 4E-BP1, and increased association of 4E-BP1 with initiation factor eIF4E. The inhibition of translation is not a consequence of p53-mediated apoptosis.

Immunohistochemical mapping of total and phosphorylated eukaryotic initiation factor 4G in rat hippocampus following global brain ischemia and reperfusion

Partial proteolysis and phosphorylation of the translation initiation factor eukaryotic initiation factor 4G (eIF4G) occur in reperfused brain, but the contribution of eIF4G alterations to brain injury has not been established. A component of the complex delivering mRNA to the small ribosomal subunit, eIF4G is also found in stress granules. Stress granules sequester inactive 48S preinitiation complexes during stress-induced translation arrest. We performed double-labeling immunofluorescence histochemistry for total or ser 1108 phosphorylated eIF4G and the stress granule component T-cell internal antigen following normothermic, 10 min cardiac arrest-induced global brain ischemia and up to 4 h reperfusion in the rat. In cornu ammonis (Ammon's horn; CA) 1 at 90 min and 4 h reperfusion, eIF4G staining transformed from a homogeneous to an aggregated distribution. The number of eIF4G-containing stress granules differed between CA1 and CA3 during reperfusion. In hippocampal pyramidal neurons, phosphorylated eIF4G appeared exclusively in stress granules. Supragranular interneurons of the dentate gyrus showed a large increase in cytoplasmic eIF4G(P) following reperfusion. Immunoblot analysis with antisera against different portions of eIF4G showed a large increase in phosphorylated C-terminal eIF4G fragments, suggesting these accumulate in the cytoplasm of dentate gyrus interneurons. Thus, altered eIF4G subcellular compartmentalization may contribute to prolonged translation arrest in CA1 pyramidal neurons. Accumulation of phosphorylated eIF4G fragments may contribute to the vulnerability of dentate interneurons. Ischemia and reperfusion invoke different translational control responses in distinct hippocampal neuron populations, which may contribute to the differential ischemic vulnerabilities of these cells.