Mechanisms of translational regulation by a human eIF5-mimic protein

USF2-Mediated Transcription of BZW2 Contributes to CRC Malignant Progression by Affecting LAMP3

BACKGROUND

Colorectal cancer (CRC) is one of the most frequent causes of cancer death in China, and its occurrence, development, and prognosis are closely related to the living state of patients. Basic leucine zipper and W2 domains 2 (BZW2), also known as eIF5-mimin protein 1 (5MP1), is a translational regulatory protein and highly expressed in CRC and promotes malignant progression of CRC, but the specific mechanism has not been clarified.

METHODS

The databases were used to mine related genes. The expression levels of genes were detected by quantitative real-time PCR (qRT-PCR) and western blot. 3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay, 5-ethynyl-2'-deoxyuridine (EdU) staining, flow cytometry, transwell assay, and sphere formation assay were employed to examine the effects of BZW2 on the phenotypes in CRC cells in vitro. The mechanism of BZW2 in CRC progression was determined by chromatin immunoprecipitation (CHIP) and dual luciferase reporter assay. In vivo, xenograft animal model was performed to verify the results.

RESULTS

BZW2 was elevated in CRC tissues and cells and was associated with poor prognosis of patients. Functionally, BZW2 enhanced CRC cell proliferation, invasion, and sphere formation but restrained apoptosis. CHIP and dual luciferase reporter assay confirmed that upstream transcription factor 2 (USF2) regulated BZW2 transcription. Also, BZW2 could attenuate the effects of USF2 defection in CRC progression. Meanwhile, lysosomal associated membrane protein 3 (LAMP3) acted as the target of BZW2 and restored the action of BZW2 knockdown. Similarly, BZW2 was involved in tumorigenesis in vivo by the same mechanism in vitro.

CONCLUSION

These findings revealed a molecular basis for BZW2's promotion of CRC malignant progression and highlighted the role of BZW2 in promoting cancer stemness.

BZW2 is a potential regulator of non-small cell lung cancer progression

BACKGROUND

Personalized targeted therapy has become an important strategy for cancer treatment owing to its remarkable therapeutic efficacy and safety. However, drug resistance remains the primary cause of treatment failure. Basic leucine zipper and W2 domain 2 (BZW2), which is aberrantly expressed in cancer, has been implicated in tumor progression and may serve as a new therapeutic target. Therefore, the role of BZW2 in non-small cell lung cancer (NSCLC) requires further investigation.

METHODS

The expression and genetic alterations of BZW2 in pan-cancers were explored using The Cancer Genome Atlas (TCGA) PanCancer databases. The mRNA and protein levels of BZW2 in patients with NSCLC were verified in our cohort. Functional experiments including CCK8, colony formation, and transwell assays were performed to evaluate the impact of BZW2 on the proliferative, migratory, and invasive capacities of SK-MES-1 cells. Gene Set Enrichment Analysis was used to identify underlying biological processes and pathways. Single-cell RNA (scRNA) sequencing data were employed to investigate the tumor microenvironment of NSCLC and the co-expression of BZW2 and stemness-related genes.

RESULTS

Dysregulated BZW2 expression was observed in various malignant tumors. BZW2 expression was found to be significantly elevated in NSCLC. BZW2 depletion inhibited the growth, mobility, and invasive abilities of lung squamous cell carcinoma SK-MES-1 cells. BZW2 may be related to signaling pathways such as nucleotide excision repair, ubiquitin-mediated proteolysis, and the P53 signaling pathway. Biological processes, including translational initiation, tRNA processing, and RNA methylation, were observed to be enriched in the high-BZW2 group. Furthermore, there was a positive correlation between BZW2 and the m6A- and m5C-related genes. scRNA analysis revealed a co-expression relationship between BZW2 and stemness-related genes such as CD44, SOX9, and CD133.

CONCLUSIONS

Elevated BZW2 expression is associated with the proliferation, migration, and invasion of NSCLC, and BZW2 may be a potential therapeutic target for NSCLC.

Critical cis-parameters influence STructure assisted RNA translation (START) initiation on non-AUG codons in eukaryotes

In eukaryotes, translation initiation is a highly regulated process, which combines cis-regulatory sequences located on the messenger RNA along with trans-acting factors like eukaryotic initiation factors (eIF). One critical step of translation initiation is the start codon recognition by the scanning 43S particle, which leads to ribosome assembly and protein synthesis. In this study, we investigated the involvement of secondary structures downstream the initiation codon in the so-called START (STructure-Assisted RNA translation) mechanism on AUG and non-AUG translation initiation. The results demonstrate that downstream secondary structures can efficiently promote non-AUG translation initiation if they are sufficiently stable to stall a scanning 43S particle and if they are located at an optimal distance from non-AUG codons to stabilize the codon-anticodon base pairing in the P site. The required stability of the downstream structure for efficient translation initiation varies in distinct cell types. We extended this study to genome-wide analysis of functionally characterized alternative translation initiation sites in Homo sapiens. This analysis revealed that about 25% of these sites have an optimally located downstream secondary structure of adequate stability which could elicit START, regardless of the start codon. We validated the impact of these structures on translation initiation for several selected uORFs.

Identification of genes that promote PI3K pathway activation and prostate tumour formation

We have performed a functional in vivo mutagenesis screen to identify genes that, when altered, cooperate with a heterozygous Pten mutation to promote prostate tumour formation. Two genes, Bzw2 and Eif5a2, which have been implicated in the process of protein translation, were selected for further validation. Using prostate organoid models, we show that either Bzw2 downregulation or EIF5A2 overexpression leads to increased organoid size and in vivo prostate growth. We show that both genes impact the PI3K pathway and drive a sustained increase in phospho-AKT expression, with PTEN protein levels reduced in both models. Mechanistic studies reveal that EIF5A2 is directly implicated in PTEN protein translation. Analysis of patient datasets identified EIF5A2 amplifications in many types of human cancer, including the prostate. Human prostate cancer samples in two independent cohorts showed a correlation between increased levels of EIF5A2 and upregulation of a PI3K pathway gene signature. Consistent with this, organoids with high levels of EIF5A2 were sensitive to AKT inhibitors. Our study identified novel genes that promote prostate cancer formation through upregulation of the PI3K pathway, predicting a strategy to treat patients with genetic aberrations in these genes particularly relevant for EIF5A2 amplified tumours.

Variant to gene mapping for carpal tunnel syndrome risk loci implicates skeletal muscle regulatory elements

BACKGROUND

Carpal tunnel syndrome (CTS) is a common disorder caused by compression of the median nerve in the wrist, resulting in pain and numbness throughout the hand and forearm. While multiple behavioural and physiological factors influence CTS risk, a growing body of evidence supports a strong genetic contribution. Recent genome-wide association study (GWAS) efforts have reported 53 independent signals associated with CTS. While GWAS can identify genetic loci conferring risk, it does not determine which cell types drive the genetic aetiology of the trait, which variants are "causal" at a given signal, and which effector genes correspond to these non-coding variants. These obstacles limit interpretation of potential disease mechanisms.

METHODS

We analysed CTS GWAS findings in the context of chromatin conformation between gene promoters and accessible chromatin regions across cellular models of bone, skeletal muscle, adipocytes and neurons. We identified proxy variants in high LD with the lead CTS sentinel SNPs residing in promoter connected open chromatin in the skeletal muscle and bone contexts.

FINDINGS

We detected significant enrichment for heritability in skeletal muscle myotubes, as well as a weaker correlation in human mesenchymal stem cell-derived osteoblasts. In myotubes, our approach implicated 117 genes contacting 60 proxy variants corresponding to 20 of the 53 GWAS signals. In the osteoblast context we implicated 30 genes contacting 24 proxy variants coinciding with 12 signals, of which 19 genes shared. We subsequently prioritized BZW2 as a candidate effector gene in CTS and implicated it as novel gene that perturbs myocyte differentiation in vitro.

INTERPRETATION

Taken together our results suggest that the CTS genetic component influences the size, integrity, and organization of multiple tissues surrounding the carpal tunnel, in particular muscle and bone, to predispose the nerve to being compressed in this disease setting.

FUNDING

This work was supported by NIH Grant UM1 DK126194 (SFAG and WY), R01AG072705 (SFAG & KDH) and the Center for Spatial and Functional Genomics at CHOP (SFAG & ADW). SFAG is supported by the Daniel B. Burke Endowed Chair for Diabetes Research. WY is supported by the Perelman School of Medicine of the University of Pennsylvania.

BZW2 promotes malignant progression in lung adenocarcinoma through enhancing the ubiquitination and degradation of GSK3β

The role of Basic leucine zipper and W2 domains 2 (BZW2) in the advancement of different types of tumors is noteworthy, but its involvement and molecular mechanisms in lung adenocarcinoma (LUAD) remain uncertain. Through this investigation, it was found that the upregulation of BZW2 was observed in LUAD tissues, which was associated with an unfavorable prognosis for individuals diagnosed with LUAD, as indicated by data from Gene Expression Omnibus and The Cancer Genome Atlas databases. Based on the clinicopathologic characteristics of LUAD patients from the tissue microarray, both univariate and multivariate analyses indicated that BZW2 functioned as an independent prognostic factor for LUAD. In terms of mechanism, BZW2 interacted with glycogen synthase kinase-3 beta (GSK3β) and enhanced the ubiquitination-mediated degradation of GSK3β through slowing down of the dissociation of the ubiquitin ligase complex, which consists of GSK3β and TNF receptor-associated factor 6. Moreover, BZW2 stimulated Wnt/β-catenin signaling pathway through GSK3β, thereby facilitating the advancement of LUAD. In conclusion, BZW2 was a significant promoter of LUAD. The research we conducted identified a promising diagnostic and therapeutic target for LUAD.

The interplay between cis- and trans-acting factors drives selective mRNA translation initiation in eukaryotes

Translation initiation consists in the assembly of the small and large ribosomal subunits on the start codon. This important step directly modulates the general proteome in living cells. Recently, genome wide studies revealed unexpected translation initiation events from unsuspected novel open reading frames resulting in the synthesis of a so-called 'dark proteome'. Indeed, the identification of the start codon by the translation machinery is a critical step that defines the translational landscape of the cell. Therefore, translation initiation is a highly regulated process in all organisms. In this review, we focus on the various cis- and trans-acting factors that rule the regulation of translation initiation in eukaryotes. Recent discoveries have shown that the guidance of the translation machinery for the choice of the start codon require sophisticated molecular mechanisms. In particular, the 5'UTR and the coding sequences contain cis-acting elements that trigger the use of AUG codons but also non-AUG codons to initiate protein synthesis. The use of these alternative start codons is also largely influenced by numerous trans-acting elements that drive selective mRNA translation in response to environmental changes.

Interactome Analysis Identifies the Role of BZW2 in Promoting Endoplasmic Reticulum-Mitochondria Contact and Mitochondrial Metabolism

Understanding the molecular functions of less-studied proteins is an important task of life science research. Despite reports of basic leucine zipper and W2 domain-containing protein 2 (BZW2) promoting cancer progression first emerging in 2017, little is known about its molecular function. Using a quantitative proteomic approach to identify its interacting proteins, we found that BZW2 interacts with both endoplasmic reticulum (ER) and mitochondrial proteins. We thus hypothesized that BZW2 localizes to and promotes the formation of ER-mitochondria contact sites and that such localization would promote calcium transport from ER to the mitochondria and promote ATP production. Indeed, we found that BZW2 localized to ER-mitochondria contact sites and that BZW2 knockdown decreased ER-mitochondria contact, mitochondrial calcium levels, and ATP production. These findings provide key insights into molecular functions of BZW2, the potential role of BZW2 in cancer progression, and highlight the utility of interactome data in understanding the function of less-studied proteins.

BZW2 Inhibition Reduces Colorectal Cancer Growth and Metastasis

Because survival of patients with metastatic colorectal cancer remain poor, there is an urgent need to identify potential novel druggable targets that are associated with colorectal cancer progression. One such target, basic leucine zipper and W2 domains 2 (BZW2), is involved in regulation of protein translation, and its overexpression is associated with human malignancy. Thus, we investigated the expression and regulation of BZW2, assessed its role in activation of WNT/β-catenin signaling, identified its downstream molecules, and demonstrated its involvement in metastasis of colorectal cancer. In human colorectal cancers, high mRNA and protein expression levels of BZW2 were associated with tumor progression. BZW2-knockdown reduced malignant phenotypes, including cell proliferation, invasion, and spheroid and colony formation. BZW2-knockdown also reduced tumor growth and metastasis; conversely, transfection of BZW2 into BZW2 low-expressing colorectal cancer cells promoted malignant features, including tumor growth and metastasis. BZW2 expression was coordinately regulated by microRNA-98, c-Myc, and histone methyltransferase enhancer of zeste homolog 2 (EZH2). RNA sequencing analyses of colorectal cancer cells modulated for BZW2 identified P4HA1 and the long noncoding RNAs, MALAT1 and NEAT1, as its downstream targets. Further, BZW2 activated the Wnt/β-catenin signaling pathway in colorectal cancers expressing wild-type β-catenin. In sum, our study suggests the possibility of targeting BZW2 expression by inhibiting EZH2 and/or c-Myc.

IMPLICATIONS

FDA-approved small-molecule inhibitors of EZH2 can indirectly target BZW2 and because BZW2 functions as an oncogene, these inhibitors could serve as therapeutic agents for colorectal cancer.

Between Order and Chaos: Understanding the Mechanism and Pathology of RAN Translation

Repeat-associated non-AUG (RAN) translation is a pathogenic mechanism in which repetitive sequences are translated into aggregation-prone proteins from multiple reading frames, even without a canonical AUG start codon. Since its discovery in spinocerebellar ataxia type 8 (SCA8) and myotonic dystrophy type 1 (DM1), RAN translation is now known to occur in the context of 12 disease-linked repeat expansions. This review discusses recent advances in understanding the regulatory mechanisms controlling RAN translation and its contribution to the pathophysiology of repeat expansion diseases. We discuss the key findings in the context of Fragile X Tremor Ataxia Syndrome (FXTAS), a neurodegenerative disorder caused by a CGG repeat expansion in the 5' untranslated region of FMR1.

Mass spectrometry analysis of affinity-purified cytoplasmic translation initiation complexes from human and fly cells

eIF5-mimic protein (5MP) controls translation through binding to the ribosomal pre-initiation complex (PIC) and alters non-AUG translation rates for cancer oncogenes and repeat-expansions in neurodegenerative diseases. Here, we describe a semi-quantitative protocol for detecting 5MP-associated proteins in cultured human and fly cells. We detail one-step anti-FLAG affinity purification and whole-lane mass spectrometry analysis of samples resolved by SDS-PAGE. This protocol allows for quantitative evaluation of the effect of 5MP mutations on its molecular interactions, to elucidate translational control by 5MP. For complete details on the use and execution of this protocol, please refer to Singh et al. (2021).

Translation regulatory factor BZW1 regulates preimplantation embryo development and compaction by restricting global non-AUG Initiation

Protein synthesis is an essential step in gene expression during the development of mammalian preimplantation embryos. This is a complex and highly regulated process. The accuracy of the translation initiation codon is important in various gene expression programs. However, the mechanisms that regulate AUG and non-AUG codon initiation in early embryos remain poorly understood. BZW1 is a key factor in determining the mRNA translation start codon. Here, we show that BZW1 is essential for early embryonic development in mice. Bzw1-knockdown embryos fail to undergo compaction, and show decreased blastocyst formation rates. We also observe defects in the differentiation capacity and implantation potential after Bzw1 interference. Further investigation revealed that Bzw1 knockdown causes the levels of translation initiation with CUG as the start codon to increase. The decline in BZW1 levels result in a decrease in protein synthesis in preimplantation embryos, whereas the total mRNA levels are not altered. Therefore, we concluded that BZW1 contributes to protein synthesis during early embryonic development by restricting non-AUG translational initiation.

Aberrant protein synthesis and cancer development: The role of canonical eukaryotic initiation, elongation and termination factors in tumorigenesis

In tumourigenesis, oncogenes or dysregulated tumour suppressor genes alter the canonical translation machinery leading to a reprogramming of the translatome that, in turn, promotes the translation of selected mRNAs encoding proteins involved in proliferation and metastasis. It is therefore unsurprising that abnormal expression levels and activities of eukaryotic initiation factors (eIFs), elongation factors (eEFs) or termination factors (eRFs) are associated with poor outcome for patients with a wide range of cancers. In this review we discuss how RNA binding proteins (RBPs) within the canonical translation factor machinery are dysregulated in cancers and how targeting such proteins is leading to new therapeutic avenues.

Expression, oncological and immunological characterizations of BZW1/2 in pancreatic adenocarcinoma

Background: Despite the progress in early diagnosis and treatment, prognosis of pancreatic adenocarcinoma (PAAD) is still poor. Basic leucine zipper and W2 domain-containing protein 1 (BZW1) and protein 2 (BZW2) are attached to the basic leucine zipper (bZIP) superfamily. Recently, BZW1 was identified as an important role in glycolysis of PAAD. However, the comprehensive reports about BZW1/2 in PAAD are not sufficient.

Methods: RNA-seq data in the Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases were retrospectively analyzed. We explored the expression of BZW1/2 in PAAD tissues and the associations between BZW1/2 and prognosis. In addition, the potential roles of BZW1/2 in tumor microenvironment (TME) of PAAD were analyzed. Finally, clinicopathological data of 49 patients with PAAD in our institution were collected. Immunohistochemistry was used to determine the expression of BZW1/2 in PAAD samples.

Results: BZW1 and BZW2 were upregulated in PAAD tissues compared to normal tissues (p < 0.05). The expression of BZW1/2 were not significantly correlated with gender, grade and stage of PAAD (p > 0.05). High expression of BZW2 was an independent predictor for poor prognosis of PAAD (HR 1.834, 95%CI 1.303–2.581, p = 0.001). And a nomogram to predict overall survival (OS) of PAAD was established with a C-index of 0.685. BZW1 and BZW2 expression were positively associated with T cell mediated immune response to tumor cell and Th2 cells in xCell database. Tumor Immune Single-Cell Hub (TISCH) analyses indicated that BZW1 and BZW2 were mainly expressed in B cells and malignant cells. External cohort furtherly validated that high expression of BZW1 and BZW2 were predictors for poor prognosis of PAAD.

Conclusion: We found that BZW1 and BZW2 are highly expressed in malignant cells and B cells in the TME of PAAD. BZW2 is an independent predictor for OS of PAAD. BZW1 and BZW2 expression are positively associated with T cell mediated immune response to tumor cell and Th2 cells in PAAD.

Label-free protocol to quantify protein affinity using isothermal titration calorimetry and bio-layer interferometry of a human eIF5-mimic protein

eIF5-mimic protein (5MP) controls translation through its interaction with eukaryotic translation initiation factor (eIF) 2 and eIF3 and alters non-AUG translation rates for oncogenes in cancer and repeat expansions in neurodegenerative disease. To precisely evaluate the effect of 5MP mutations on binding affinity against eIFs, here we describe two label-free protocols of affinity measurement for 5MP binding to eIF2 or eIF3 protein segments, termed isothermal titration calorimetry (ITC) and bio-layer interferometry (BLI), starting with how to purify proteins used. For complete details on the use and execution of this protocol, please refer to Singh et al. (2021).

The PCI domains are “winged” HEAT domains

The HEAT domains are a family of helical hairpin repeat domains, composed of four or more hairpins. HEAT is derived from the names of four family members: huntingtin, eukaryotic translation elongation factor 3 (eEF3), protein phosphatase 2 regulatory A subunit (PP2A), and mechanistic target of rapamycin (mTOR). HEAT domain-containing proteins play roles in a wide range of cellular processes, such as protein synthesis, nuclear transport and metabolism, and cell signaling. The PCI domains are a related group of helical hairpin domains, with a “winged-helix” (WH) subdomain at their C-terminus, which is responsible for multi-subunit complex formation with other PCI domains. The name is derived from the complexes, where these domains are found: the 26S Proteasome “lid” regulatory subcomplex, the COP9 signalosome (CSN), and eukaryotic translation initiation factor 3 (eIF3). We noted that in structure similarity searches using HEAT domains, sometimes PCI domains appeared in the search results ahead of other HEAT domains, which indicated that the PCI domains could be members of the HEAT domain family, and not a related but separate group, as currently thought. Here, we report extensive structure similarity analysis of HEAT and PCI domains, both within and between the two groups of proteins. We present evidence that the PCI domains as a group have greater structural similarity with individual groups of HEAT domains than some of the HEAT domain groups have among each other. Therefore, our results indicate that the PCI domains have evolved from a HEAT domain that acquired a WH subdomain. The WH subdomain in turn mediated self-association into a multi-subunit complex, which eventually evolved into the common ancestor of the Proteasome lid/CSN/eIF3.

The metaphorical swiss army knife: The multitude and diverse roles of HEAT domains in eukaryotic translation initiation

Biomolecular associations forged by specific interaction among structural scaffolds are fundamental to the control and regulation of cell processes. One such structural architecture, characterized by HEAT repeats, is involved in a multitude of cellular processes, including intracellular transport, signaling, and protein synthesis. Here, we review the multitude and versatility of HEAT domains in the regulation of mRNA translation initiation. Structural and cellular biology approaches, as well as several biophysical studies, have revealed that a number of HEAT domain-mediated interactions with a host of protein factors and RNAs coordinate translation initiation. We describe the basic structural architecture of HEAT domains and briefly introduce examples of the cellular processes they dictate, including nuclear transport by importin and RNA degradation. We then focus on proteins in the translation initiation system featuring HEAT domains, specifically the HEAT domains of eIF4G, DAP5, eIF5, and eIF2Bϵ. Comparative analysis of their remarkably versatile interactions, including protein-protein and protein-RNA recognition, reveal the functional importance of flexible regions within these HEAT domains. Here we outline how HEAT domains orchestrate fundamental aspects of translation initiation and highlight open mechanistic questions in the area.

Non-AUG translation initiation in mammals

Recent proteogenomic studies revealed extensive translation outside of annotated protein coding regions, such as non-coding RNAs and untranslated regions of mRNAs. This non-canonical translation is largely due to start codon plurality within the same RNA. This plurality is often due to the failure of some scanning ribosomes to recognize potential start codons leading to initiation downstream—a process termed leaky scanning. Codons other than AUG (non-AUG) are particularly leaky due to their inefficiency. Here we discuss our current understanding of non-AUG initiation. We argue for a near-ubiquitous role of non-AUG initiation in shaping the dynamic composition of mammalian proteomes.

Assessing eukaryotic initiation factor 4F subunit essentiality by CRISPR-induced gene ablation in the mouse

Eukaryotic initiation factor (eIF) 4F plays a central role in the ribosome recruitment phase of cap-dependent translation. This heterotrimeric complex consists of a cap binding subunit (eIF4E), a DEAD-box RNA helicase (eIF4A), and a large bridging protein (eIF4G). In mammalian cells, there are two genes encoding eIF4A (eIF4A1 and eIF4A2) and eIF4G (eIF4G1 and eIF4G3) paralogs that can assemble into eIF4F complexes. To query the essential nature of the eIF4F subunits in normal development, we used CRISPR/Cas9 to generate mouse strains with targeted ablation of each gene encoding the different eIF4F subunits. We find that Eif4e, Eif4g1, and Eif4a1 are essential for viability in the mouse, whereas Eif4g3 and Eif4a2 are not. However, Eif4g3 and Eif4a2 do play essential roles in spermatogenesis. Crossing of these strains to the lymphoma-prone Eμ-Myc mouse model revealed that heterozygosity at the Eif4e or Eif4a1 loci significantly delayed tumor onset. Lastly, tumors derived from Eif4e∆38 fs/+/Eμ-Myc or Eif4a1∆5 fs/+/Eμ-Myc mice show increased sensitivity to the chemotherapeutic agent doxorubicin, in vivo. Our study reveals that eIF4A2 and eIF4G3 play non-essential roles in gene expression regulation during embryogenesis; whereas reductions in eIF4E or eIF4A1 levels are protective against tumor development in a murine Myc-driven lymphoma setting.

Human oncoprotein 5MP suppresses general and repeat-associated non-AUG translation via eIF3 by a common mechanism

eIF5-mimic protein (5MP) is a translational regulatory protein that binds the small ribosomal subunit and modulates its activity. 5MP is proposed to reprogram non-AUG translation rates for oncogenes in cancer, but its role in controlling non-AUG initiated synthesis of deleterious repeat-peptide products, such as FMRpolyG observed in fragile-X-associated tremor ataxia syndrome (FXTAS), is unknown. Here, we show that 5MP can suppress both general and repeat-associated non-AUG (RAN) translation by a common mechanism in a manner dependent on its interaction with eIF3. Essentially, 5MP displaces eIF5 through the eIF3c subunit within the preinitiation complex (PIC), thereby increasing the accuracy of initiation. In Drosophila, 5MP/Kra represses neuronal toxicity and enhances the lifespan in an FXTAS disease model. These results implicate 5MP in protecting cells from unwanted byproducts of non-AUG translation in neurodegeneration.

BZW2/5MP1 acts as a promising target in hepatocellular carcinoma

Basic leucine zipper and W2 domain 2 (BZW2), also known as 5MP1, is a protein related to translation regulation. Evidence from previous studies indicates that BZW2 is involved in tumorigenesis in several cancers. However, little is known about the role of BZW2 in hepatocellular carcinoma (HCC). In this study, we first analyzed the gene expression profile of BZW2 in multiple HCC datasets. Next, we explored the biological effects of BZW2 in HCC cell lines. BZW2 was overexpressed in different HCC cohorts. Multivariate analysis confirmed that increased BZW2 expression is an independent prognostic indicator of shorter overall survival. BZW2 coexpressed genes were mainly enriched in the biological processes of ribonucleoprotein complex biogenesis, rRNA metabolism, translational initiation, and negative regulation of metabolic processes. BZW2 depletion reduced proliferation, clonality, and invasion and increased apoptosis in MHCC97-H cells. Furthermore, BZW2 overexpression or knockdown enhanced or impaired c-Myc expression, respectively. Overall, these findings identified BZW2 as a biomarker of HCC and provided novel insight that the effect of BZW2 on the translatome is a potential mechanism that promotes HCC progression via the c-Myc pathway.

Origin of translational control by eIF2α phosphorylation: insights from genome-wide translational profiling studies in fission yeast

During amino acid limitation, the protein kinase Gcn2 phosphorylates the α subunit of eIF2, thereby regulating mRNA translation. In yeast Saccharomyces cerevisiae and mammals, eIF2α phosphorylation regulates translation of related transcription factors Gcn4 and Atf4 through upstream open reading frames (uORFs) to activate transcription genome wide. However, mammals encode three more eIF2α kinases activated by distinct stimuli. Did the translational control system involving eIF2α phosphorylation evolve from so simple (as found in yeast S. cerevisiae) to complex (as found in humans)? Recent genome-wide translational profiling studies of amino acid starvation response in the fission yeast Schizosaccharomyces pombe provide an unexpected answer to this question.

Gcn2 eIF2α kinase mediates combinatorial translational regulation through nucleotide motifs and uORFs in target mRNAs

The protein kinase Gcn2 is a central transducer of nutritional stress signaling important for stress adaptation by normal cells and the survival of cancer cells. In response to nutrient deprivation, Gcn2 phosphorylates eIF2α, thereby repressing general translation while enhancing translation of specific mRNAs with upstream ORFs (uORFs) situated in their 5'-leader regions. Here we performed genome-wide measurements of mRNA translation during histidine starvation in fission yeast Schizosaccharomyces pombe. Polysome analyses were combined with microarray measurements to identify gene transcripts whose translation was up-regulated in response to the stress in a Gcn2-dependent manner. We determined that translation is reprogrammed to enhance RNA metabolism and chromatin regulation and repress ribosome synthesis. Interestingly, translation of intron-containing mRNAs was up-regulated. The products of the regulated genes include additional eIF2α kinase Hri2 amplifying the stress signaling and Gcn5 histone acetyl transferase and transcription factors, together altering genome-wide transcription. Unique dipeptide-coding uORFs and nucleotide motifs, such as '5'-UGA(C/G)GG-3', are found in 5' leader regions of regulated genes and shown to be responsible for translational control.

BZW2 promotes the malignant progression of colorectal cancer via activating the ERK/MAPK pathway

Colorectal cancer (CRC) is one of the most prevalent malignant solid cancers worldwide involving the dysregulation of multiple signaling molecules. However, the role and corresponding mechanism of basic leucine zipper and W2 domains 2 (BZW2) in CRC development, to our knowledge, has not been reported. We found BZW2 was overexpressed in human CRC tissues compared with that in paired adjacent colorectal samples. BZW2 overexpression was closely associated with tumor T stage (p = .030), metastatic lymph nodes (p = .037), TNM stage (p = .018) and the worse prognosis of CRC patients (p = .009). Moreover, BZW2 was an independent disadvantage prognostic factor (p = .031). BZW2 also showed an increased expression in different invasive CRC cell lines. Its silencing and overexpression diminished and increased cell proliferation, invasion, and migration in Colo205 and HCT116 cells via specifically activating of extracellular-signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) signaling. Moreover, ERK/MAPK inhibitor PD98059 reverse the enhancement of cell proliferation, invasion, and migration in BZW2 overexpressing HCT116 cells. BZW2 silencing also inhibited subcutaneous tumors growth and p-ERK expression in vivo. BZW2 promotes the malignant progression of CRC via activating ERK/MAPK signaling, which provided a promising gene target therapy for CRC.

Translational Control in Cancer

The translation of messenger RNAs (mRNAs) into proteins is a key event in the regulation of gene expression. This is especially true in the cancer setting, as many oncogenes and transforming events are regulated at this level. Cancer-promoting factors that are translationally regulated include cyclins, antiapoptotic factors, proangiogenic factors, regulators of cell metabolism, prometastatic factors, immune modulators, and proteins involved in DNA repair. This review discusses the diverse means by which cancer cells deregulate and reprogram translation, and the resulting oncogenic impacts, providing insights into the complexity of translational control in cancer and its targeting for cancer therapy.

Novel oncogene 5MP1 reprograms c-Myc translation initiation to drive malignant phenotypes in colorectal cancer

BACKGROUND

Translational reprogramming through controlled initiation from non-AUG start codons is considered a crucial driving force in tumorigenesis and tumor progression. However, its clinical impact and underlying mechanism are not fully understood.

METHODS

Using a bioinformatics approach, we identified translation initiation regulator 5MP1/BZW2 on chromosome 7p as a potential oncogenic driver gene in colorectal cancer (CRC), and explored the biological effect of 5MP1 in CRC in vitro or in vivo. Pathway analysis was performed to identify the downstream target of 5MP1, which was verified with transcriptomic and biochemical analyses. Finally, we assessed the clinical significance of 5MP1 expression in CRC patients.

FINDINGS

5MP1 was ubiquitously amplified and overexpressed in CRC. 5MP1 promoted tumor growth and induced cell cycle progression of CRC. c-Myc was identified as its potential downstream effector. c-Myc has two in-frame start codons, AUG and CUG (non-AUG) located upstream of the AUG. 5MP1 expression increased the AUG-initiated c-Myc isoform relative to the CUG-initiated isoform. The AUG-initiated c-Myc isoform displayed higher protein stability and a stronger transactivation activity for oncogenic pathways than the CUG-initiated isoform, accounting for 5MP1-driven cell cycle progression and tumor growth. Clinically, high 5MP1 expression predicts poor survival of CRC patients.

INTERPRETATION

5MP1 is a novel oncogene that reprograms c-Myc translation in CRC. 5MP1 could be a potential therapeutic target to overcome therapeutic resistance conferred by tumor heterogeneity of CRC. FUND: Japan Society for the Promotion of Science; Priority Issue on Post-K computer; National Institutes of Health; National Science Foundation; KSU Johnson Cancer Center.

Cellular eIF2B subunit localization: implications for the integrated stress response and its control by small molecule drugs

Eukaryotic initiation factor 2 (eIF2) is a G protein critical for translation. It is tightly regulated in the integrated stress response (ISR) via phosphorylation of eIF2α and the subsequent control of eukaryotic initiation factor 2B (eIF2B), a multisubunit guanine nucleotide exchange factor. Through studying the localization of eIF2B subunits, we identified cytoplasmic eIF2B bodies in mammalian cells. We highlight a relationship between body size and the eIF2B subunits localizing to them; larger bodies contain all subunits and smaller bodies contain predominantly catalytic subunits. eIF2 localizes to eIF2B bodies and shuttles within these bodies in a manner that correlates with eIF2B activity. On stress, eIF2α-P localizes predominately to larger bodies and results in a decreased shuttling of eIF2. Interestingly, drugs that inhibit the ISR can rescue eIF2 shuttling in a manner correlating to levels of eIF2α-P. In contrast, smaller bodies show increased eIF2 shuttling in response to stress, which is accompanied by the localization of eIF2Bδ to these bodies, suggesting the formation of a novel trimeric complex of eIF2B. This response is mimicked by ISR-inhibiting drugs, providing insight into their potential mechanism of action. This study provides evidence that the composition and function of mammalian eIF2B bodies are regulated by the ISR and the drugs that control it.

Analysis of eIF2B bodies and their relationships with stress granules and P-bodies

Eukaryotic cells respond to stress and changes in the environment in part by repressing translation and forming cytoplasmic assemblies called stress granules and P-bodies, which harbor non-translating mRNAs and proteins. A third, but poorly understood, assembly called the eIF2B body can form and contains the eIF2B complex, an essential guanine exchange factor for the translation initiation factor eIF2. Hypomorphic EIF2B alleles can lead to Vanishing White Matter Disease (VWMD), a leukodystrophy that causes progressive white matter loss. An unexplored question is how eIF2B body formation is controlled and whether VWMD alleles in EIF2B alter the formation of eIF2B bodies, stress granules, or P-bodies. To examine these issues, we assessed eIF2B body, stress granule, and P-body induction in wild-type yeast cells and cells carrying VWMD alleles in the EIF2B2 (GCD7) and EIF2B5 (GCD6) subunits of eIF2B. We demonstrate eIF2B bodies are rapidly and reversibly formed independently of stress granules during acute glucose deprivation. VWMD mutations had diverse effects on stress-induced assemblies with some alleles altering eIF2B bodies, and others leading to increased P-body formation. Moreover, some VWMD-causing mutations in GCD7 caused hyper-sensitivity to chronic GCN2 activation, consistent with VWMD mutations causing hyper-sensitivity to eIF2α phosphorylation and thereby impacting VWMD pathogenesis.

Regulation of translation initiation factor eIF2B at the hub of the integrated stress response

Phosphorylation of the translation initiation factor eIF2 is one of the most widely used and well-studied mechanisms cells use to respond to diverse cellular stresses. Known as the integrated stress response (ISR), the control pathway uses modulation of protein synthesis to reprogram gene expression and restore homeostasis. Here the current knowledge of the molecular mechanisms of eIF2 activation and its control by phosphorylation at a single-conserved phosphorylation site, serine 51 are discussed with a major focus on the regulatory roles of eIF2B and eIF5 where a current molecular view of ISR control of eIF2B activity is presented. How genetic disorders affect eIF2 or eIF2B is discussed, as are syndromes where excess signaling through the ISR is a component. Finally, studies into the action of recently identified compounds that modulate the ISR in experimental systems are discussed; these suggest that eIF2B is a potential therapeutic target for a wide range of conditions. This article is categorized under: Translation > Translation Regulation.

Translational autoregulation of BZW1 and BZW2 expression by modulating the stringency of start codon selection

The efficiency of start codon selection during ribosomal scanning in eukaryotic translation initiation is influenced by the context or flanking nucleotides surrounding the AUG codon. The levels of eukaryotic translation initiation factors 1 (eIF1) and 5 (eIF5) play critical roles in controlling the stringency of translation start site selection. The basic leucine zipper and W2 domain-containing proteins 1 and 2 (BZW1 and BZW2), also known as eIF5-mimic proteins, are paralogous human proteins containing C-terminal HEAT domains that resemble the HEAT domain of eIF5. We show that translation of mRNAs encoding BZW1 and BZW2 homologs in fungi, plants and metazoans is initiated by AUG codons in conserved unfavorable initiation contexts. This conservation is reminiscent of the conserved unfavorable initiation context that enables autoregulation of EIF1. We show that overexpression of BZW1 and BZW2 proteins enhances the stringency of start site selection, and that their poor initiation codons confer autoregulation on BZW1 and BZW2 mRNA translation. We also show that overexpression of these two proteins significantly diminishes the effect of overexpressing eIF5 on stringency of start codon selection, suggesting they antagonize this function of eIF5. These results reveal a surprising role for BZW1 and BZW2 in maintaining homeostatic stringency of start codon selection, and taking into account recent biochemical, genetic and structural insights into eukaryotic initiation, suggest a model for BZW1 and BZW2 function.

Translation control of mRNAs encoding mammalian translation initiation factors

Eukaryotic cells evolved highly complex and accurate protein synthesis machinery that is finely tuned by various signaling pathways. Dysregulation of translation is a hallmark of many diseases, including cancer, and thus pharmacological approaches to modulate translation become very promising. While there has been much progress in our understanding of mammalian mRNA-specific translation control, surprisingly, relatively little is known about whether and how the protein components of the translation machinery shape translation of their own mRNAs. Here we analyze mammalian mRNAs encoding components of the translation initiation machinery for potential regulatory features such as 5'TOP motifs, TISU motifs, poor start codon nucleotide context and upstream open reading frames.

Competition between translation initiation factor eIF5 and its mimic protein 5MP determines non-AUG initiation rate genome-wide

In the human genome, translation initiation from non-AUG codons plays an important role in various gene regulation programs. However, mechanisms regulating the non-AUG initiation rate remain poorly understood. Here, we show that the non-AUG initiation rate is nearly consistent under a fixed nucleotide context in various human and insect cells. Yet, it ranges from <1% to nearly 100% compared to AUG translation, depending on surrounding sequences, including Kozak, and possibly additional nucleotide contexts. Mechanistically, this range of non-AUG initiation is controlled in part, by the eIF5-mimic protein (5MP). 5MP represses non-AUG translation by competing with eIF5 for the Met-tRNAi-binding factor eIF2. Consistently, eIF5 increases, whereas 5MP decreases translation of NAT1/EIF4G2/DAP5, whose sole start codon is GUG. By modulating eIF5 and 5MP1 expression in combination with ribosome profiling we identified a handful of previously unknown non-AUG initiation sites, some of which serve as the exclusive start codons. If the initiation rate for these codons is low, then an AUG-initiated downstream ORF prevents the generation of shorter, AUG-initiated isoforms. We propose that the homeostasis of the non-AUG translatome is maintained through balanced expression of eIF5 and 5MP.

Downregulation of BZW2 inhibits osteosarcoma cell growth by inactivating the Akt/mTOR signaling pathway

Osteosarcoma is the most common malignant bone tumor in adolescents. The function of basic leucine zipper and W2 domains 2 (BZW2) in tumor progression has been reported. However, the role and mechanisms of BZW2 in osteosarcoma remain to be determined. The aim of the present study was to reveal the expression and biological functions of BZW2 in osteosarcoma and to elucidate the proximal mechanisms underlying these functions. The expression of BZW2 in osteosarcoma tissues and cell lines was assessed by qRT-PCR, western blotting and immunohistochemistry. BZW2 overexpression was detected in osteosarcoma cell lines. Clinically, BZW2 expression was higher in osteosarcoma tissues than in corresponding non-tumor tissues and was associated with advanced Enneking stage and tumor recurrence. The knockdown of BZW2 using siRNA inhibited osteosarcoma cell proliferation, colony-forming ability, and the cell cycle at the G2/M phase in vitro. Host signaling pathways affected by BZW2 were detected using a PathScan Intracellular Signaling Antibody Array kit. These data demonstrated that the knockdown of BZW2 suppresses protein phosphorylation in the Akt/mTOR signaling pathway. These observations suggest that BZW2 is upregulated and has a pro-tumor effect in osteosarcoma via activation of the Akt/mTOR signaling pathway and thus is a potential target for gene therapy.

Fail-safe control of translation initiation by dissociation of eIF2α phosphorylated ternary complexes

Phosphorylation of eIF2α controls translation initiation by restricting the levels of active eIF2-GTP/Met-tRNAi ternary complexes (TC). This modulates the expression of all eukaryotic mRNAs and contributes to the cellular integrated stress response. Key to controlling the activity of eIF2 are translation factors eIF2B and eIF5, thought to primarily function with eIF2-GDP and TC respectively. Using a steady-state kinetics approach with purified proteins we demonstrate that eIF2B binds to eIF2 with equal affinity irrespective of the presence or absence of competing guanine nucleotides. We show that eIF2B can compete with Met-tRNAi for eIF2-GTP and can destabilize TC. When TC is formed with unphosphorylated eIF2, eIF5 can out-compete eIF2B to stabilize TC/eIF5 complexes. However when TC/eIF5 is formed with phosphorylated eIF2, eIF2B outcompetes eIF5 and destabilizes TC. These data uncover competition between eIF2B and eIF5 for TC and identify that phosphorylated eIF2-GTP translation initiation intermediate complexes can be inhibited by eIF2B.

Role of Eukaryotic Initiation Factors during Cellular Stress and Cancer Progression

Protein synthesis can be segmented into distinct phases comprising mRNA translation initiation, elongation, and termination. Translation initiation is a highly regulated and rate-limiting step of protein synthesis that requires more than 12 eukaryotic initiation factors (eIFs). Extensive evidence shows that the transcriptome and corresponding proteome do not invariably correlate with each other in a variety of contexts. In particular, translation of mRNAs specific to angiogenesis, tumor development, and apoptosis is altered during physiological and pathophysiological stress conditions. In cancer cells, the expression and functions of eIFs are hampered, resulting in the inhibition of global translation and enhancement of translation of subsets of mRNAs by alternative mechanisms. A precise understanding of mechanisms involving eukaryotic initiation factors leading to differential protein expression can help us to design better strategies to diagnose and treat cancer. The high spatial and temporal resolution of translation control can have an immediate effect on the microenvironment of the cell in comparison with changes in transcription. The dysregulation of mRNA translation mechanisms is increasingly being exploited as a target to treat cancer. In this review, we will focus on this context by describing both canonical and noncanonical roles of eIFs, which alter mRNA translation.

Overexpression of eIF5 or its protein mimic 5MP perturbs eIF2 function and induces ATF4 translation through delayed re-initiation

ATF4 is a pro-oncogenic transcription factor whose translation is activated by eIF2 phosphorylation through delayed re-initiation involving two uORFs in the mRNA leader. However, in yeast, the effect of eIF2 phosphorylation can be mimicked by eIF5 overexpression, which turns eIF5 into translational inhibitor, thereby promoting translation of GCN4, the yeast ATF4 equivalent. Furthermore, regulatory protein termed eIF5-mimic protein (5MP) can bind eIF2 and inhibit general translation. Here, we show that 5MP1 overexpression in human cells leads to strong formation of 5MP1:eIF2 complex, nearly comparable to that of eIF5:eIF2 complex produced by eIF5 overexpression. Overexpression of eIF5, 5MP1 and 5MP2, the second human paralog, promotes ATF4 expression in certain types of human cells including fibrosarcoma. 5MP overexpression also induces ATF4 expression in Drosophila The knockdown of 5MP1 in fibrosarcoma attenuates ATF4 expression and its tumor formation on nude mice. Since 5MP2 is overproduced in salivary mucoepidermoid carcinoma, we propose that overexpression of eIF5 and 5MP induces translation of ATF4 and potentially other genes with uORFs in their mRNA leaders through delayed re-initiation, thereby enhancing the survival of normal and cancer cells under stress conditions.

eIF2β is critical for eIF5-mediated GDP-dissociation inhibitor activity and translational control

In protein synthesis translation factor eIF2 binds initiator tRNA to ribosomes and facilitates start codon selection. eIF2 GDP/GTP status is regulated by eIF5 (GAP and GDI functions) and eIF2B (GEF and GDF activities), while eIF2α phosphorylation in response to diverse signals is a major point of translational control. Here we characterize a growth suppressor mutation in eIF2β that prevents eIF5 GDI and alters cellular responses to reduced eIF2B activity, including control of GCN4 translation. By monitoring the binding of fluorescent nucleotides and initiator tRNA to purified eIF2 we show that the eIF2β mutation does not affect intrinsic eIF2 affinities for these ligands, neither does it interfere with eIF2 binding to 43S pre-initiation complex components. Instead we show that the eIF2β mutation prevents eIF5 GDI stabilizing nucleotide binding to eIF2, thereby altering the off-rate of GDP from eIF2•GDP/eIF5 complexes. This enables cells to grow with reduced eIF2B GEF activity but impairs activation of GCN4 targets in response to amino acid starvation. These findings provide support for the importance of eIF5 GDI activity in vivo and demonstrate that eIF2β acts in concert with eIF5 to prevent premature release of GDP from eIF2γ and thereby ensure tight control of protein synthesis initiation.

Protein expression changes induced in a malignant melanoma cell line by the curcumin analogue compound D6

Background

We have previously demonstrated that the hydroxylated biphenyl compound D6 (3E,3′E)-4,4′-(5,5′,6,6′-tetramethoxy-[1,1′-biphenyl]-3,3′-diyl)bis(but-3-en-2-one), a structural analogue of curcumin, exerts a strong antitumor activity on melanoma cells both in vitro and in vivo. Although the mechanism of action of D6 is yet to be clarified, this compound is thought to inhibit cancer cell growth by arresting the cell cycle in G2/M phase, and to induce apoptosis through the mitochondrial intrinsic pathway. To investigate the changes in protein expression induced by exposure of melanoma cells to D6, a differential proteomic study was carried out on D6-treated and untreated primary melanoma LB24Dagi cells.

Methods

Proteins were fractionated by SDS-PAGE and subjected to in gel digestion. The peptide mixtures were analyzed by liquid chromatography coupled with tandem mass spectrometry. Proteins were identified and quantified using database search and spectral counting. Proteomic data were finally uploaded into the Ingenuity Pathway Analysis software to find significantly modulated networks and pathways.

Results

Analysis of the differentially expressed protein profiles revealed the activation of a strong cellular stress response, with overexpression of several HSPs and stimulation of ubiquitin-proteasome pathways. These were accompanied by a decrease of protein synthesis, evidenced by downregulation of proteins involved in mRNA processing and translation. These findings are consistent with our previous results on gene expression profiling in melanoma cells treated with D6.

Conclusions

Our findings confirm that the curcumin analogue D6 triggers a strong stress response in melanoma cells, turning down majority of cell functions and finally driving cells to apoptosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2362-6) contains supplementary material, which is available to authorized users.

Sliding of a 43S ribosomal complex from the recognized AUG codon triggered by a delay in eIF2-bound GTP hydrolysis

During eukaryotic translation initiation, 43S ribosomal complex scans mRNA leader unless an AUG codon in an appropriate context is found. Establishing the stable codon-anticodon base-pairing traps the ribosome on the initiator codon and triggers structural rearrangements, which lead to Pi release from the eIF2-bound GTP. It is generally accepted that AUG recognition by the scanning 43S complex sets the final point in the process of start codon selection, while latter stages do not contribute to this process. Here we use translation reconstitution approach and kinetic toe-printing assay to show that after the 48S complex is formed on an AUG codon, in case GTP hydrolysis is impaired, the ribosomal subunit is capable to resume scanning and slides downstream to the next AUG. In contrast to leaky scanning, this sliding is not limited to AUGs in poor nucleotide contexts and occurs after a relatively long pause at the recognized AUG. Thus, recognition of an AUG per se does not inevitably lead to this codon being selected for initiation of protein synthesis. Instead, it is eIF5-induced GTP hydrolysis and Pi release that irreversibly trap the 48S complex, and this complex is further stabilized by eIF5B and 60S joining.

eIF5A1/RhoGDIα pathway: a novel therapeutic target for treatment of spinal cord injury identified by a proteomics approach

Spinal cord injury (SCI) is frequently accompanied by a degree of spontaneous functional recovery. The underlying mechanisms through which such recovery is generated remain elusive. In this study, we observed a significant spontaneous motor function recovery 14 to 28 days after spinal cord transection (SCT) in rats. Using a comparative proteomics approach, caudal to the injury, we detected difference in 20 proteins. Two of these proteins, are eukaryotic translation initiation factor 5A1 (eIF5A1) that is involved in cell survival and proliferation, and Rho GDP dissociation inhibitor alpha (RhoGDIα), a member of Rho GDI family that is involved in cytoskeletal reorganization. After confirming the changes in expression levels of these two proteins following SCT, we showed that in vivo eIF5A1 up-regulation and down-regulation significantly increased and decreased, respectively, motor function recovery. In vitro, eIF5A1 overexpression in primary neurons increased cell survival and elongated neurite length while eIF5A1 knockdown reversed these results. We found that RhoGDIα up-regulation and down-regulation rescues the effect of eIF5A1 down-regulation and up-regulation both in vivo and in vitro. Therefore, we have identified eIF5A1/RhoGDIα pathway as a new therapeutic target for treatment of spinal cord injured patients.

Essential role of eIF5-mimic protein in animal development is linked to control of ATF4 expression

Translational control of transcription factor ATF4 through paired upstream ORFs (uORFs) plays an important role in eukaryotic gene regulation. While it is typically induced by phosphorylation of eIF2α, ATF4 translation can be also induced by expression of a translational inhibitor protein, eIF5-mimic protein 1 (5MP1, also known as BZW2) in mammals. Here we show that the 5MP gene is maintained in eukaryotes under strong purifying selection, but is uniquely missing in two major phyla, nematoda and ascomycota. The common function of 5MP from protozoa, plants, fungi and insects is to control translation by inhibiting eIF2. The affinity of human 5MP1 to eIF2β was measured as being equivalent to the published value of human eIF5 to eIF2β, in agreement with effective competition of 5MP with eIF5 for the main substrate, eIF2. In the red flour beetle, Tribolium castaneum, RNA interference studies indicate that 5MP facilitates expression of GADD34, a downstream target of ATF4. Furthermore, both 5MP and ATF4 are essential for larval development. Finally, 5MP and the paired uORFs allowing ATF4 control are conserved in the entire metazoa except nematoda. Based on these findings, we discuss the phylogenetic and functional linkage between ATF4 regulation and 5MP expression in this group of eukaryotes.

Why is start codon selection so precise in eukaryotes?

Translation generally initiates with the AUG codon. While initiation at GUG and UUG is permitted in prokaryotes (Archaea and Bacteria), cases of CUG initiation were recently reported in human cells. The varying stringency in translation initiation between eukaryotic and prokaryotic domains largely stems from a fundamental problem for the ribosome in recognizing a codon at the peptidyl-tRNA binding site. Initiation factors specific to each domain of life evolved to confer stringent initiation by the ribosome. The mechanistic basis for high accuracy in eukaryotic initiation is described based on recent findings concerning the role of the multifactor complex (MFC) in this process. Also discussed are whether non-AUG initiation plays any role in translational control and whether start codon accuracy is regulated in eukaryotes.

Upregulation of eIF-5A1 in the paralyzed muscle after spinal cord transection associates with spontaneous hindlimb locomotor recovery in rats by upregulation of the ErbB, MAPK and neurotrophin signal pathways

It is well known that trauma is frequently accompanied by spontaneous functional recovery after spinal cord injury (SCI), but the underlying mechanisms remain elusive. In this study, BBB scores showed a gradual return of locomotor functions after SCT. Proteomics analysis revealed 16 differential protein spots in the gastrocnemius muscle between SCT and normal rats. Of these differential proteins, eukaryotic translation initiation factor 5A1 (elf-5A1), a highly conserved molecule throughout eukaryotes, exhibited marked upregulation in the gastrocnemius muscle after SCT. To study the role of eIF-5A1 in the restoration of hindlimb locomotor functions following SCT, we used siRNA to downregulate the mRNA level of eIF-5A1. Compared with untreated SCT control rats, those subjected to eIF-5A1 knockdown exhibited impaired functional recovery. Moreover, gene expression microarrays and bioinformatic analysis showed high correlation between three main signal pathways (ErbB, MAPK and neurotrophin signal pathways) and eIF-5A1. These signal pathways regulate cell proliferation, differentiation and neurocyte growth. Consequently, eIF-5A1 played a pivotal role via these signal pathways in hindlimb locomotor functional recovery after SCT, which could pave the way for the development of a new strategy for the treatment of spinal cord injury in clinical trials.