The polyamine-derived amino acid hypusine: its post-translational formation in eIF-5A and its role in cell proliferation

SUMOylation modulates eIF5A activities in both yeast and pancreatic ductal adenocarcinoma cells

BACKGROUND

The eukaryotic translation initiation protein eIF5A is a highly conserved and essential factor that plays a critical role in different physiological and pathological processes including stress response and cancer. Different proteomic studies suggest that eIF5A may be a small ubiquitin-like modifier (SUMO) substrate, but whether eIF5A is indeed SUMOylated and how relevant is this modification for eIF5A activities are still unknown.

METHODS

SUMOylation was evaluated using in vitro SUMOylation assays, Histidine-tagged proteins purification from His6-SUMO2 transfected cells, and isolation of endogenously SUMOylated proteins using SUMO-binding entities (SUBES). Mutants were engineered by site-directed mutagenesis. Protein stability was measured by a cycloheximide chase assay. Protein localization was determined using immunofluorescence and cellular fractionation assays. The ability of eIF5A1 constructs to complement the growth of Saccharomyces cerevisiae strains harboring thermosensitive mutants of a yeast EIF5A homolog gene (HYP2) was analyzed. The polysome profile and the formation of stress granules in cells expressing Pab1-GFP (a stress granule marker) by immunofluorescence were determined in yeast cells subjected to heat shock. Cell growth and migration of pancreatic ductal adenocarcinoma PANC-1 cells overexpressing different eIF5A1 constructs were evaluated using crystal violet staining and transwell inserts, respectively. Statistical analysis was performed with GraphPad Software, using unpaired Student's t-test, or one-way or two-way analysis of variance (ANOVA).

RESULTS

We found that eIF5A is modified by SUMO2 in vitro, in transfected cells and under endogenous conditions, revealing its physiological relevance. We identified several SUMO sites in eIF5A and found that SUMOylation modulates both the stability and the localization of eIF5A in mammalian cells. Interestingly, the SUMOylation of eIF5A responds to specific stresses, indicating that it is a regulated process. SUMOylation of eIF5A is conserved in yeast, the eIF5A SUMOylation mutants are unable to completely suppress the defects of HYP2 mutants, and SUMOylation of eIF5A is important for both stress granules formation and disassembly of polysomes induced by heat-shock. Moreover, mutation of the SUMOylation sites in eIF5A abolishes its promigratory and proproliferative activities in PANC-1 cells.

CONCLUSIONS

SUMO2 conjugation to eIF5A is a stress-induced response implicated in the adaptation of yeast cells to heat-shock stress and required to promote the growth and migration of pancreatic ductal adenocarcinoma cells.

Good cop, bad cop: Polyamines play both sides in host immunity and viral replication

Viruses rely on host cells for energy and synthesis machinery required for genome replication and particle assembly. Due to the dependence of viruses on host cells, viruses have evolved multiple mechanisms by which they can induce metabolic changes in the host cell to suit their specific requirements. The host immune response also involves metabolic changes to be able to react to viral insult. Polyamines are small ubiquitously expressed polycations, and their metabolism is critical for viral replication and an adequate host immune response. This is due to the variety of functions that polyamines have, ranging from condensing DNA to enhancing the translation of polyproline-containing proteins through the hypusination of eIF5A. Here, we review the diverse mechanisms by which viruses exploit polyamines, as well as the mechanisms by which immune cells utilize polyamines for their functions. Furthermore, we highlight potential avenues for further study of the host-virus interface.

Role of eIF5A in Mitochondrial Function

The eukaryotic translation initiation factor 5A (eIF5A) is an evolutionarily conserved protein that binds ribosomes to facilitate the translation of peptide motifs with consecutive prolines or combinations of prolines with glycine and charged amino acids. It has also been linked to other molecular functions and cellular processes, such as nuclear mRNA export and mRNA decay, proliferation, differentiation, autophagy, and apoptosis. The growing interest in eIF5A relates to its association with the pathogenesis of several diseases, including cancer, viral infection, and diabetes. It has also been proposed as an anti-aging factor: its levels decay in aged cells, whereas increasing levels of active eIF5A result in the rejuvenation of the immune and vascular systems and improved brain cognition. Recent data have linked the role of eIF5A in some pathologies with its function in maintaining healthy mitochondria. The eukaryotic translation initiation factor 5A is upregulated under respiratory metabolism and its deficiency reduces oxygen consumption, ATP production, and the levels of several mitochondrial metabolic enzymes, as well as altering mitochondria dynamics. However, although all the accumulated data strongly link eIF5A to mitochondrial function, the precise molecular role and mechanisms involved are still unknown. In this review, we discuss the findings linking eIF5A and mitochondria, speculate about its role in regulating mitochondrial homeostasis, and highlight its potential as a target in diseases related to energy metabolism.

Identification of differentially expressed genes associated with precocious puberty by suppression subtractive hybridization in goat pituitary tissues

The aim of this study was to identify genes related to precocious puberty expressed in the pituitary of goats at different growth stages by suppression subtractive hybridization (SSH). The pituitary glands from Jining Gray (JG) goats (early puberty) and Liaoning Cashmere (LC) goats (late puberty) at 30, 90, and 180 days were used in this study. To identify differentially expressed genes (DEGs) in the pituitary glands, mRNA was extracted from these tissues, and SSH libraries were constructed and divided into the following groups: juvenile group (30-JG vs. 30-LC, API), puberty group (90-JG vs. 180-LC, BPI), and control group (90-JG vs. 90-LC, EPI). A total of 60, 49, and 58 DEGs were annotated by 222 Gene Ontology (GO) terms and 75 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. Most of the DEGs were significantly enriched in GO terms related to 'structural constituent of ribosome', 'translation' and 'GTP binding', and numerous DEGs were also significantly enriched in KEGG terms related to the Jak-STAT signaling and oocyte meiosis pathways. Candidate genes associated with precocious puberty and sexual development were screened from the SSH libraries. These genes were analyzed to determine if they were expressed in the pituitary tissues of the goats at different growth stages and to identify genes that may influence the hypothalamic-pituitary-gonadal (HPG) axis. In this study, we found precocious puberty-related genes (such as PRLP0, EIF5A, and YWHAH) that may be interesting from an evolutionary perspective and that could be investigated for use in future goat breeding programs. Our results provide a valuable dataset that will facilitate further research into the reproductive biology of goats.

LHCGR Expression During Follicle Stimulating Hormone-Induced Follicle Growth Is Negatively Regulated by Eukaryotic Initiation Factor 5A

We have shown that the transient changes in the expression of luteinizing hormone/choriogonadotropin receptor (LHCGR) messenger RNA (mRNA) during the ovarian cycle occurs, at least in part, through a posttranscriptional mechanism involving an LHCGR mRNA-binding protein (LRBP). Eukaryotic initiation factor 5A (eIF5A), an LRBP-interacting protein, participates in this process. eIF5A undergoes hypusination, a unique posttranslational modification that is necessary for its functions. This study examined the role of eIF5A in follicle-stimulating hormone (FSH)-induced LHCGR expression during follicular growth. Treatment of primary cultures of rat granulosa cells with FSH and 17β-estradiol (E2) showed a time-dependent increase in LHCGR mRNA expression. Conversely, inhibition of endogenous hypusination of eIF5A using N1-guanyl-1,7-diaminoheptane (GC7), a hypusination inhibitor, showed a greater increase in LHCGR mRNA expression over that produced by FSH and E2 alone. Further studies were carried out to determine the mechanism by which inhibition of hypusination of eIF5A causes an increase in LHCGR mRNA expression. Because LHCGR expression is negatively regulated by LRBP, the effect of inhibiting hypusination of eIF5A on LRBP expression was examined. The results showed a decrease in the expression of LRBP mRNA and protein when hypusination of eIF5A was inhibited by GC7. Because LRBP promotes LHCGR mRNA degradation, the results of this study support the notion that by inhibiting eIF5A hypusination, FSH reduces the expression of LRBP. This increases LHCGR mRNA expression by abrogating the inhibitory action of LRBP.

Polyamines and Hypusination Are Required for Ebolavirus Gene Expression and Replication

Ebolavirus (EBOV) is an RNA virus that is known to cause severe hemorrhagic fever in humans and other primates : EBOV successfully enters and replicates in many cell types. This replication is dependent on the virus successfully coopting a number of cellular factors. Many of these factors are currently unidentified but represent potential targets for antiviral therapeutics. Here we show that cellular polyamines are critical for EBOV replication. We found that small-molecule inhibitors of polyamine synthesis block gene expression driven by the viral RNA-dependent RNA polymerase. Short hairpin RNA (shRNA) knockdown of the polyamine pathway enzyme spermidine synthase also resulted in reduced EBOV replication. These findings led us to further investigate spermidine, a polyamine that is essential for the hypusination of eukaryotic initiation factor 5A (eIF5A). Blocking the hypusination of eIF5A (and thereby inhibiting its function) inhibited both EBOV gene expression and viral replication. The mechanism appears to be due to the importance of hypusinated eIF5A for the accumulation of VP30, an essential component of the viral polymerase. The same reduction in hypusinated eIF5A did not alter the accumulation of other viral polymerase components. This action makes eIF5A function an important gate for proper EBOV polymerase assembly and function through the control of a single virus protein.

IMPORTANCE

Ebolavirus (EBOV) is one of the most lethal human pathogens known. EBOV requires host factors for replication due to its small RNA genome. Here we show that the host protein eIF5A in its activated form is necessary for EBOV replication. We further show that the mechanism is through the accumulation of a single EBOV protein, VP30. To date, no other host proteins have been shown to interfere with the translation or stability of an EBOV protein. Activated eIF5A is the only protein in the cell known to contain the specific modification of hypusine; therefore, this pathway is a target for drug development. Further investigation into the mechanism of eIF5A interaction with VP30 could provide insight into therapeutics to combat EBOV.

Hypusination of eukaryotic initiation factor 5A via cAMP-PKA-ERK1/2 pathway is required for ligand-induced downregulation of LH receptor mRNA expression in the ovary

Luteinizing hormone receptor (LHR) mRNA expression in the ovary is regulated post-transcriptionally by an LH receptor mRNA binding protein (LRBP). Eukaryotic initiation factor 5A (EIF5A), identified as an LRBP-interacting protein plays a crucial role in LHR mRNA expression. In this study, we have demonstrated that during hCG-induced LHR downregulation, a significant upregulation of eIF5A mRNA expression and hypusination of eIF5A protein occurs in a time dependent manner. Pretreatment with H89, a specific inhibitor of PKA, and U0126, a specific inhibitor of ERK1/2 significantly inhibited both hCG-induced eIF5A mRNA expression and hypusination of eIF5A protein. Pretreatment with GC7, a specific inhibitor of eIF5A hypusination significantly abolished hCG-induced LRBP mRNA and protein expression. Furthermore, GC7 pretreatment significantly inhibited hCG-induced interaction of LRBP with LHR mRNA as assessed by RNA electrophoretic mobility gel shift assay (REMSA). GC7 treatment also reversed LHR mRNA downregulation. Taken together, these results suggest that hCG-induced LHR mRNA downregulation is mediated by cAMP-PKA-ERK1/2 signaling leading to activation of eIF5A hypusination.

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.

Regulation of polyamine metabolism by translational control

Polyamines are low molecular weight, positively charged compounds that are ubiquitous in all living cells. They play a crucial role in many biochemical processes including regulation of transcription and translation, modulation of enzyme activities, regulation of ion channels and apoptosis. A strict balance between synthesis, catabolism and excretion tightly controls the cellular concentration of polyamines. The concentrations of rate-limiting enzymes in the polyamine synthesis and degradation pathways are regulated at different levels, including transcription, translation and degradation. Polyamines can modulate the translation of most of the enzymes required for their synthesis and catabolism through feedback mechanisms that are unique for each enzyme. Translational control is associated with cis-acting and trans-acting factors that can be influenced by the concentration of polyamines through mechanisms that are not completely understood. In this review, we present an overview of the translational control mechanisms of the proteins in the polyamine pathway, including ornithine decarboxylase (ODC), ODC antizyme, S-adenosylmethionine decarboxylase and spermidine/spermine N(1) acetyltransferase, highlighting the areas where more research is needed. A better understanding of the translational control of these enzymes would offer the possibility of a novel pharmacological intervention against cancer and other diseases.

Revival of 2-(difluoromethyl)ornithine (DFMO), an inhibitor of polyamine biosynthesis, as a cancer chemopreventive agent

ODC (ornithine decarboxylase), a key enzyme of polyamine biosynthesis, is an inducible enzyme exhibiting high activity in tumour cells, suggesting ODC as a target for antineoplastic therapy. Among the inhibitors of polyamine-related enzymes, the ODC inactivator DFMO [2-(difluoromethyl)ornithine] became the most well-known. The drug is usually cytostatic and its effects on growth are reversed by micromolar concentrations of polyamines in the cellular environment. ODC inactivation is associated with decreased transcription of the growth-related c-myc and c-fos genes. DFMO used as a single drug has only minor effects on tumour growth. The low efficacy of the drug is due to the use of exogenous (gastrointestinal) polyamines by the mammalian organism. Although it was disappointing in most therapeutic attempts, DFMO showed potential in cancer chemoprevention based on its ability to lower polyamine levels in colorectal mucosa at low dosages with no demonstrable toxicity over long periods of use. DFMO in combination with other drugs prevents and inhibits the development of a variety of chemically induced cancers in animals with doses far lower than those administered for therapy. Low doses of several NSAIDs (non-steroidal anti-inflammatory drugs) and DFMO administered in combination have been shown to be more effective in inhibiting chemically induced colon tumours in rats than are high doses of these agents given individually. This combination has gained further interest after findings suggesting that ODC polymorphism is a genetic marker for colon cancer risk and supporting the use of DFMO and aspirin or other NSAIDs in combination as a strategy for colon cancer prevention.

Posttranslational synthesis of hypusine: evolutionary progression and specificity of the hypusine modification

A naturally occurring unusual amino acid, hypusine [N (epsilon)-(4-amino-2-hydroxybutyl)-lysine] is a component of a single cellular protein, eukaryotic translation initiation factor 5A (eIF5A). It is a modified lysine with structural contribution from the polyamine spermidine. Hypusine is formed in a novel posttranslational modification that involves two enzymes, deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). eIF5A and deoxyhypusine/hypusine modification are essential for growth of eukaryotic cells. The hypusine synthetic pathway has evolved in eukaryotes and eIF5A, DHS and DOHH are highly conserved, suggesting maintenance of a fundamental cellular function of eIF5A through evolution. The unique feature of the hypusine modification is the strict specificity of the enzymes toward its substrate protein, eIF5A. Moreover, DHS exhibits a narrow specificity toward spermidine. In view of the extraordinary specificity and the requirement for hypusine-containing eIF5A for mammalian cell proliferation, eIF5A and the hypusine biosynthetic enzymes present new potential targets for intervention in aberrant cell proliferation.

Homospermidine synthase, the first pathway-specific enzyme of pyrrolizidine alkaloid biosynthesis, evolved from deoxyhypusine synthase

Pyrrolizidine alkaloids are preformed plant defense compounds with sporadic phylogenetic distribution. They are thought to have evolved in response to the selective pressure of herbivory. The first pathway-specific intermediate of these alkaloids is the rare polyamine homospermidine, which is synthesized by homospermidine synthase (HSS). The HSS gene from Senecio vernalis was cloned and shown to be derived from the deoxyhypusine synthase (DHS) gene, which is highly conserved among all eukaryotes and archaebacteria. DHS catalyzes the first step in the activation of translation initiation factor 5A (eIF5A), which is essential for eukaryotic cell proliferation and which acts as a cofactor of the HIV-1 Rev regulatory protein. Sequence comparison provides direct evidence for the evolutionary recruitment of an essential gene of primary metabolism (DHS) for the origin of the committing step (HSS) in the biosynthesis of pyrrolizidine alkaloids.

In vitro manipulation of L1210 cell cycle kinetics with 4-amidinoindan-1-one 2'-amidinohydrazone, alpha-difluoromethylornithine and N1-acetylspermine

We investigated whether in vitro L1210 growth inhibition by alpha-difluoromethylornithine (DFMO; 740 microM) and 4-amidinoindan-1-one 2'-amidinohydrazone (CGP 48664A; 1.7 microM) is reversible with N1-acetylspermine (N1-acSp). Influences of N1-acSp dose (1-100 microM), time (0-12 h at 100 microM), aminoguanidine (AG, 1 mM) and cell numbers (at 1 microM N1-acSp) on percentage S-phase, polyamine contents and viability were determined. DFMO/CGP 48664A decreased percentage S-phase from 58 to 26%, decreased spermidine (Sd) and spermine (Sp) contents 3-fold, but did not affect viability. With increasing N1-acSp dose, S-phase percentage and Sd contents increased concomitantly, reaching plateau values that were comparable with those of untreated controls. S-phase and Sd content increased from 4-6 h after N1-acSp administration, reaching plateau values from 11 and 6 h, respectively. N1-acSp content was dose dependent and increased linearly to reach plateau values from 8 h. AG did not affect any of these parameters. Addition of 1 microM N1-acSp to decreasing numbers of DFMO/CGP 48664A-treated cells caused increasing S-phase percentage, Sd and N1-acSp contents. We conclude that cell cycle kinetics of cultured L1210 cells can be manipulated by the induction of growth inhibition with DFMO/CGP 48664A and its subsequent abolishment with N1-acSp. N1-acSp accumulation rate and its subsequent conversion to Sd is relatively slow compared with intracellular Sd needs. The data support the notion that Sd is the most important polyamine for growth.

Identification of the eukaryotic initiation factor 5A as a retinoic acid-stimulated cellular binding partner for tissue transglutaminase II

GTP-binding protein/transglutaminases (tissue transglutaminases or TGases) have been implicated in a variety of cellular processes including retinoic acid (RA)-induced apoptosis. Recently, we have shown that RA activates TGases as reflected by stimulated GTP binding, increased membrane association, and stimulated phosphoinositide lipid turnover. This prompted us to search for cellular proteins that bind TGases in a RA-stimulated manner. In this report, we show that the eukaryotic initiation factor (eIF-5A), a protein that is essential for cell viability, perhaps through effects on protein synthesis and/or RNA export, associates with the TGase in vivo. The interaction between eIF-5A and TGase is specific for the GDP-bound form of the TGase and is not detected when the TGase is pre-loaded with GTP gamma S. The TGase-eIF-5A interaction also is promoted by Ca2+, Mg2+, and RA treatment of HeLa cells. In the presence of retinoic acid, millimolar levels of Ca2+ are no longer required for the TGase-eIF-5A interaction. Nocodazole treatment, which blocks the cell cycle at mitosis (M phase), strongly inhibits the interaction between eIF-5A and cytosolic TGase. The interaction between TGase and eIF-5A and its sensitivity to the nucleotide-occupied state of the TGase provides a potentially interesting connection between RA signaling and protein synthesis and/or RNA trafficking activities.

Enzyme-substrate intermediate formation at lysine 329 of human deoxyhypusine synthase

Deoxyhypusine (Nepsilon-(4-aminobutyl)lysine) is the key intermediate in the posttranslational synthesis of the unique amino acid, hypusine (Nepsilon-(4-amino-2-hydroxybutyl)lysine). Deoxyhypusine synthase catalyzes the formation of deoxyhypusine by conjugation of the butylamine moiety of spermidine to the epsilon-amino group of one specific lysine residue of the eukaryotic translation initiation factor 5A (eIF-5A) precursor protein. However, in the absence of the eIF-5A precursor, catalysis involves only the NAD-dependent cleavage of spermidine to generate 1,3-diaminopropane and a putative 4-carbon amine intermediate that gives rise to Delta1-pyrroline. We have obtained evidence for a covalent enzyme-substrate intermediate that accumulates in the absence of the eIF-5A precursor. Incubation of human recombinant enzyme with [1, 8-3H]spermidine and NAD, followed by reduction with NaBH3CN, resulted in specific radiolabeling of the enzyme. The radioactive component in the reduced enzyme intermediate was identified as deoxyhypusine and was shown to occur at a single locus. The fact that labeled deoxyhypusine was found after treatment with a reducing agent suggests an intermediate with the butylamine moiety derived from spermidine attached through an imine linkage to the epsilon-amino group of a specific lysine residue of the enzyme. This residue has been identified as lysine 329. Separate experiments showing efficient transfer of labeled butylamine moiety from enzyme intermediate to eIF-5A precursor strongly support a reaction mechanism involving an imine intermediate.