Polyamine modulon in yeast—Stimulation of COX4 synthesis by spermidine at the level of translation

Unveiling the novel role of spermidine in leaf senescence: A study of eukaryotic translation factor 5A-independent and dependent mechanisms

Senescence is a crucial and highly active process in plants, optimising resource allocation and promoting phenotypic plasticity under restricted conditions. It involves global metabolic reprogramming for the organised disintegration and remobilization of resources. Polyamines (PAs) are polycationic biogenic amines prevalent in all eukaryotes and are necessary for cell survival. The commonly used PAs in plants include putrescine, spermidine, and spermine. Notably, the leaf's expression of S-adenosylmethionine decarboxylase and spermidine synthase gene family transcripts significantly changes during senescence. This suggests these genes are critical in spermidine metabolism and may condition metabolic reprogramming. One key role of spermidine in eukaryotes is to provide the 4-aminobutyl group for the posttranslational modification of lysine in eukaryotic translation factor 5A (eIF5A). This modification is catalysed by two sequential enzymatic steps leading to the activation of eIF5A by converting lysine to the unusual amino acid hypusine. Although eIF5A is well characterised to be involved in the translation of proline-rich repeat proteins and other hard-to-read motifs, the biological role of eIF5A has recently been clarified only in mammals. It could be better described at the plant functional level. The expression patterns of eIF5A isoforms and genes encoding machinery responsible for hypusination, differ between induced and developmental leaf senescence. In this paper, we summarise the existing knowledge on spermidine-dependent senescence control mechanisms in plants, raising the possibility that spermidine could be an element of a biological switch controlling the onset of a different type of senescence in an eIF5A-independent and dependent manner.

Ovarian aging: energy metabolism of oocytes

In women who are getting older, the quantity and quality of their follicles or oocytes and decline. This is characterized by decreased ovarian reserve function (DOR), fewer remaining oocytes, and lower quality oocytes. As more women choose to delay childbirth, the decline in fertility associated with age has become a significant concern for modern women. The decline in oocyte quality is a key indicator of ovarian aging. Many studies suggest that age-related changes in oocyte energy metabolism may impact oocyte quality. Changes in oocyte energy metabolism affect adenosine 5'-triphosphate (ATP) production, but how related products and proteins influence oocyte quality remains largely unknown. This review focuses on oocyte metabolism in age-related ovarian aging and its potential impact on oocyte quality, as well as therapeutic strategies that may partially influence oocyte metabolism. This research aims to enhance our understanding of age-related changes in oocyte energy metabolism, and the identification of biomarkers and treatment methods.

Evidence for adduction of biologic amines with reactive metabolite of 8-epidiosbulbin E acetate in vitro and in vivo

Dioscorea bulbifera L. (DBL) is one of traditional Chinese medicines and has been used for the treatment of goiter, tumor and carbuncles. However, clinic application of the herbal medicine has been limited, due to reported severe hepatotoxicity. 8-Epidiosbulbin E acetate (EEA), one of the major components of DBL, can cause severe liver damage. The furan ring of EEA is metabolized by CYP3A4 to a cis-enedial reactive intermediate prone to react amino and/or thiol groups of amino acid residues. In this study, we investigated the interaction of the reactive intermediate with biologic amines. EEA-derived biologic amine adducts, including spermidine, spermine, putrescine, ornithine, lysine and glutamine were detected in cultured mouse primary hepatocytes treated with EEA. Only spermidine adduct was observed in bile of mice given EEA. The detection of the adducts was established by labeling with bromobenzyl mercaptan and LC-MS/MS analysis. Exposure of EEA resulted in concentration dependent cytotoxicity in hepatocytes. Pretreatment with spermidine attenuated the susceptibility of cells to the cytotoxicity of EEA, because of the compensation of the depleted spermidine.

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.

Functional roles of polyamines and their metabolite acrolein in eukaryotic cells

Among low molecular weight substances, polyamines (spermidine, spermine and their precursor putrescine) are present in eukaryotic cells at the mM level together with ATP and glutathione. It is expected therefore that polyamines play important roles in cell proliferation and viability. Polyamines mainly exist as a polyamine-RNA complex and regulate protein synthesis. It was found that polyamines enhance translation from inefficient mRNAs. The detailed mechanisms of polyamine stimulation of specific kinds of protein syntheses and the physiological functions of these proteins are described in this review. Spermine is metabolized into acrolein (CH2 = CH-CHO) and hydrogen peroxide (H2O2) by spermine oxidase. Although it is thought that cell damage is mainly caused by reactive oxygen species (O2-, H2O2, and •OH), it was found that acrolein is much more toxic than H2O2. Accordingly, the level of acrolein produced becomes a useful biomarker for several tissue-damage diseases like brain stroke. Thus, the mechanisms of cell toxicity caused by acrolein are described in this review.

Translational Regulation of Clock Genes BMAL1 and REV-ERBα by Polyamines

Polyamines stimulate the synthesis of specific proteins at the level of translation, and the genes encoding these proteins are termed as the "polyamine modulon". The circadian clock generates daily rhythms in mammalian physiology and behavior. We investigated the role of polyamines in the circadian rhythm using control and polyamine-reduced NIH3T3 cells. The intracellular polyamines exhibited a rhythm with a period of about 24 h. In the polyamine-reduced NIH3T3 cells, the circadian period of circadian clock genes was lengthened and the synthesis of BMAL1 and REV-ERBα was significantly reduced at the translation level. Thus, the mechanism of polyamine stimulation of these protein syntheses was analyzed using NIH3T3 cells transiently transfected with genes encoding enhanced green fluorescent protein (EGFP) fusion mRNA with normal or mutated 5'-untranslated region (5'-UTR) of Bmal1 or Rev-erbα mRNA. It was found that polyamines stimulated BMAL1 and REV-ERBα synthesis through the enhancement of ribosomal shunting during the ribosome shunting within the 5'-UTR of mRNAs. Accordingly, the genes encoding Bmal1 and Rev-erbα were identified as the members of "polyamine modulon", and these two proteins are significantly involved in the circadian rhythm control.

Correlation of polyamines, acrolein-conjugated lysine and polyamine metabolic enzyme levels with age in human liver

The polyamines spermidine, spermine and putrescine are essential for normal cellular functions. The contents of polyamines in tissue decreased in aged mice compared to young mice. In this study, the polyamine contents and their metabolic byproduct acrolein-conjugated lysine (N^ε-(3-formyl-3,4-dehydropiperidino)-lysine, FDP-Lys) in human liver tissue were measured and analyzed the correlation with age of the subjects. The putrescine and FDP-Lys levels were significantly increased with age. On the other hand, spermine level was decreased with age. Spermidine did not significantly correlate with age. The relative amount of spermine oxidase (SMOX) significantly correlated with the age of subjects whereas ornithine decarboxylase (ODC) and adenosylmethionine decarboxylase (AMD1) significantly reduced by the age. Our results suggested that an increase in oxidation and reduction in polyamine synthesis may cause the change of polyamine profile in the elderly.

5' untranslated regions: the next regulatory sequence in yeast synthetic biology

When developing industrial biotechnology processes, Saccharomyces cerevisiae (baker's yeast or brewer's yeast) is a popular choice as a microbial host. Many tools have been developed in the fields of synthetic biology and metabolic engineering to introduce heterologous pathways and tune their expression in yeast. Such tools mainly focus on controlling transcription, whereas post-transcriptional regulation is often overlooked. Herein we discuss regulatory elements found in the 5' untranslated region (UTR) and their influence on protein synthesis. We provide not only an overall picture, but also a set of design rules on how to engineer a 5' UTR. The reader is also referred to currently available models that allow gene expression to be tuned predictably using different 5' UTRs.

GSTΠ stimulates caveolin-1-regulated polyamine uptake via actin remodeling

Polyamines spermidine and spermine, and their diamine precursor putrescine, are essential for normal cellular functions in both pro- and eukaryotes. Cellular polyamine levels are regulated by biosynthesis, degradation and transport. Transport of dietary and luminal bacterial polyamines in gastrointestinal (GI) tissues plays a significant role in tissue polyamine homeostasis. We have reported that caveolin-1 play an inhibitory role in polyamine uptake in GI tissues. We investigated the mechanism of caveolin-1-regulated polyamine transport. We found that glutathione S-transferase Π(GSTΠ) was secreted from caveolin-1 knockdown cells and stimulated spermidine transport in human colon-derived HCT116 cells. GSTΠ secreted in the medium increased S-glutathionylated protein level in the plasma membrane fraction. Proteomic analysis revealed that actin was S-glutathionylated by GSTΠ. Immunofluorescence microscopy demonstrated that actin filaments around plasma membrane were S-glutathionylated in caveolin-1 knockdown cells. Inhibition of actin remodeling by jasplakinolide caused a decrease in polyamine uptake activity. These data support a model in which caveolin-1 negatively regulates polyamine uptake by inhibiting GSTΠ secretion, which stimulates actin remodeling and endocytosis.

The functional role of polyamines in eukaryotic cells

Polyamines, consisting of putrescine, spermidine and spermine are essential for normal cell growth and viability in eukaryotic cells. Since polyamines are cations, they interact with DNA, ATP, phospholipids, specific kinds of proteins, and especially with RNA. Consequently, the functions of these acidic compounds and some proteins are modified by polyamines. In this review, the functional modifications of these molecules by polyamines are presented. Structural change of specific mRNAs by polyamines causes the stimulation of the synthesis of several different proteins, which are important for cell growth and viability. eIF5 A, the only known protein containing a spermidine derivative, i.e. hypusine, also functions at the level of translation. Experimental results thus far obtained strongly suggest that the most important function of polyamines is at the level of translation.

The roles of polyamines in microorganisms

Polyamines are small polycations that are well conserved in all the living organisms except Archae, Methanobacteriales and Halobacteriales. The most common polyamines are putrescine, spermidine and spermine, which exist in varying concentrations in different organisms. They are involved in a variety of cellular processes such as gene expression, cell growth, survival, stress response and proliferation. Therefore, diverse regulatory pathways are evolved to ensure strict regulation of polyamine concentration in the cells. Polyamine levels are kept under strict control by biosynthetic pathways as well as cellular uptake driven by specific transporters. Reverse genetic studies in microorganisms showed that deletion of the genes in polyamine metabolic pathways or depletion of polyamines have negative effects on cell survival and proliferation. The protein products of these genes are also used as drug targets against pathogenic protozoa. These altogether confirm the significant roles of polyamines in the cells. This mini-review focuses on the differential concentrations of polyamines and their cellular functions in different microorganisms. This will provide an insight about the diverse evolution of polyamine metabolism and function based on the physiology and the ecological context of the microorganisms.

Two stems with different characteristics and an internal loop in an RNA aptamer contribute to spermine-binding

Though polyamines (putrescine, spermidine, and spermine) bind to the specific position in RNA molecules, interaction mechanisms are poorly understood. SELEX procedure has been used to isolate high-affinity oligoribonucleotides (aptamers) from randomized RNA libraries. Selected aptamers are useful in exploring sequences and/or structures in RNAs for binding molecules. In this study, to analyze the interaction mechanism of polyamine to RNA, we selected RNA aptamers targeted for spermine. Two spermine-binding aptamers (#5 and #24) were obtained and both of them had two stem-loop structures. The 3′ stem-loop of #5 (SL_2) bound to spermine more effectively than the 5′ stem-loop of #5 did. A thermodynamic analysis by an isothermal titration calorimetry revealed that the dissociation constant of SL_2 for spermine was 27.2 μM and binding ratio was nearly 1:1. Binding assay with base-pair replaced variants showed that two stem regions and an internal loop in SL_2 were important for their spermine-binding activities. NMR analyses proposed that a terminal-side and a loop-side stem in SL_2 take a loose and a stable structure, respectively and a conformational change of SL_2 is induced by spermine. It is conclusive that two stems with different characteristics and an internal loop in SL_2 contribute to the specific spermine-binding.

Modulation of protein synthesis by polyamines

Polyamines are ubiquitous small basic molecules that play important roles in cell growth and viability. Since polyamines mainly exist as a polyamine-RNA complex, we looked for proteins whose synthesis is preferentially stimulated by polyamines at the level of translation, and thus far identified 17 proteins in Escherichia coli and 6 proteins in eukaryotes. The mechanisms of polyamine stimulation of synthesis of these proteins were investigated. In addition, the role of eIF5A, containing hypusine formed from spermidine, on protein synthesis is described. These results clearly indicate that polyamines and eIF5A contribute to cell growth and viability through modulation of protein synthesis.

Polyamine stimulation of eEF1A synthesis based on the unusual position of a complementary sequence to 18S rRNA in eEF1A mRNA

It is thought that Shine-Dalgarno-like sequences, which exhibit complementarity to the nucleotide sequences at the 3'-end of 18S rRNA, are not present in eukaryotic mRNAs. However, complementary sequences consisting of more than 5 nucleotides to the 3'-end of 18S rRNA, i.e., a CR sequence, are present at -17 to -32 upstream from the initiation codon AUG in 18 mRNAs involved in protein synthesis except eEF1A mRNA. Thus, effects of the CR sequence in mRNAs and polyamines on protein synthesis were examined using control and polyamine-reduced FM3A and NIH3T3 cells. Polyamines did not stimulate protein synthesis encoded by 18 mRNAs possessing a normal CR sequence. When the CR sequence was deleted, protein synthetic activities decreased to less than 70% of intact mRNAs. In eEF1A mRNA, the CR sequence was located at -33 to -39 upstream from the initiation codon AUG, and polyamines stimulated eEF1A synthesis about threefold. When the CR sequence was shifted to -22 to -28 upstream from the AUG, eEF1A synthesis increased in polyamine-reduced cells and the degree of polyamine stimulation decreased greatly. The results indicate that the CR sequence exists in many eukaryotic mRNAs, and the location of a CR sequence in mRNAs influences polyamine stimulation of protein synthesis.

Endogenous polyamine function--the RNA perspective

Recent progress with techniques for monitoring RNA structure in cells such as ‘DMS-Seq’ and ‘Structure-Seq’ suggests that a new era of RNA structure-function exploration is on the horizon. This will also include systematic investigation of the factors required for the structural integrity of RNA. In this context, much evidence accumulated over 50 years suggests that polyamines play important roles as modulators of RNA structure. Here, we summarize and discuss recent literature relating to the roles of these small endogenous molecules in RNA function. We have included studies directed at understanding the binding interactions of polyamines with polynucleotides, tRNA, rRNA, mRNA and ribozymes using chemical, biochemical and spectroscopic tools. In brief, polyamines bind RNA in a sequence-selective fashion and induce changes in RNA structure in context-dependent manners. In some cases the functional consequences of these interactions have been observed in cells. Most notably, polyamine-mediated effects on RNA are frequently distinct from those of divalent cations (i.e. Mg²⁺) confirming their roles as independent molecular entities which help drive RNA-mediated processes.

The roles of polyamines during the lifespan of plants: from development to stress

Compelling evidence indicates that free polyamines (PAs) (mainly putrescine, spermidine, spermine, and its isomer thermospermine), some PA conjugates to hydroxycinnamic acids, and the products of PA oxidation (hydrogen peroxide and γ-aminobutyric acid) are required for different processes in plant development and participate in abiotic and biotic stress responses. A tight regulation of PA homeostasis is required, since depletion or overaccumulation of PAs can be detrimental for cell viability in many organisms. In plants, homeostasis is achieved by modulation of PA biosynthesis, conjugation, catabolism, and transport. However, recent data indicate that such mechanisms are not mere modulators of PA pools but actively participate in PA functions. Examples are found in the spermidine-dependent eiF5A hypusination required for cell division, PA hydroxycinnamic acid conjugates required for pollen development, and the involvement of thermospermine in cell specification. Recent advances also point to implications of PA transport in stress tolerance, PA-dependent transcriptional and translational modulation of genes and transcripts, and posttranslational modifications of proteins. Overall, the molecular mechanisms identified suggest that PAs are intricately coordinated and/or mediate different stress and developmental pathways during the lifespan of plants.

Toxicity of polyamines and their metabolic products

Polyamines are ubiquitous and essential components of mammalian cells. They have multiple functions including critical roles in nucleic acid and protein synthesis, gene expression, protein function, protection from oxidative damage, the regulation of ion channels, and maintenance of the structure of cellular macromolecules. It is essential to maintain a correct level of polyamines, and this amount is tightly regulated at the levels of transport, synthesis, and degradation. Catabolic pathways generate reactive aldehydes including acrolein and hydrogen peroxide via a number of oxidases. These metabolites, particularly those from spermine, can cause significant toxicity with damage to proteins, DNA, and other cellular components. Their production can be increased as a result of infection or cell damage that releases free polyamines and activates the oxidative catabolic pathways. Since polyamines also have an important physiological role in protection from oxidative damage, the reduction in polyamine content may exacerbate the toxic potential of these agents. Increases in polyamine catabolism have been implicated in the development of diseases including stroke, other neurological diseases, renal failure, liver disease, and cancer. These results provide new opportunities for the early diagnosis, prevention, and treatment of disease.

Acetaldehyde-induced cytotoxicity involves induction of spermine oxidase at the transcriptional level

Ethanol consumption causes serious liver injury including cirrhosis and hepatocellular carcinoma. Ethanol is metabolized mainly in the liver to acetic acid through acetaldehyde. We investigated the effect of ethanol and acetaldehyde on polyamine metabolism since polyamines are essential factors for normal cellular functions. We found that acetaldehyde induced spermine oxidase (SMO) at the transcriptional level in HepG2 cells. The levels and activities of ornithine decarboxylase (ODC) and spermidine/spermine acetyltransferase (SSAT) were not affected by acetaldehyde. Spermidine content was increased and spermine content was decreased by acetaldehyde treatment. Knockdown of SMO expression using siRNA reduced acetaldehyde toxicity. Acetaldehyde exposure increased free acrolein levels. An increase of acrolein by acetaldehyde was SMO dependent. Our results indicate that cytotoxicity of acetaldehyde involves, at least in part, oxidation of spermine to spermidine by SMO, which is induced by acetaldehyde.

Increase in cell viability by polyamines through stimulation of the synthesis of ppGpp regulatory protein and ω protein of RNA polymerase in Escherichia coli

It is known that polyamines increase cell growth through stimulation of the synthesis of several kinds of proteins encoded by the so-called "polyamine modulon". We recently reported that polyamines also increase cell viability at the stationary phase of cell growth through stimulation of the synthesis of ribosome modulation factor, a component of the polyamine modulon. Accordingly, we looked for other proteins involved in cell viability whose synthesis is stimulated by polyamines. It was found that the synthesis of ppGpp regulatory protein (SpoT) and ω protein of RNA polymerase (RpoZ) was stimulated by polyamines at the level of translation. Stimulation of the synthesis of SpoT and RpoZ by polyamines was due to an inefficient initiation codon UUG in spoT mRNA and an unusual location of a Shine-Dalgarno (SD) sequence in rpoZ mRNA. Accordingly, the spoT and rpoZ genes are components of the polyamine modulon involved in cell viability. Reduced cell viability caused by polyamine deficiency was prevented by modified spoT and rpoZ genes whose synthesis was not influenced by polyamines. Under these conditions, the level of ppGpp increased in parallel with increase of SpoT protein. The results indicate that polyamine stimulation of synthesis of SpoT and RpoZ plays important roles for cell viability through stimulation of ppGpp synthesis by SpoT and modulation of RNA synthesis by ppGpp-RpoZ complex.

Interactions between polyamines and abiotic stress pathway responses unraveled by transcriptome analysis of polyamine overproducers

Plant development and productivity are negatively regulated by adverse environmental conditions. The identification of stress-regulatory genes, networks, and signaling molecules should allow the development of novel strategies to obtain tolerant plants. Polyamines (PAs) are polycationic compounds with a recognized role in plant growth and development, as well as in abiotic and biotic stress responses. During the last years, knowledge on PA functions has been achieved using genetically modified plants with altered PA levels. In this review, we combine the information obtained from global transcriptome analyses in transgenic Arabidopsis plants with altered putrescine or spermine levels. Comparison of common and specific gene networks affected by elevation of endogenous PAs, support the view that these compounds actively participate in stress signaling through intricate crosstalks with abscisic acid (ABA), Ca(2+) signaling and other hormonal pathways in plant defense and development.

Current status of the polyamine research field

This chapter provides an overview of the polyamine field and introduces the 32 other chapters that make up this volume. These chapters provide a wide range of methods, advice, and background relevant to studies of the function of polyamines, the regulation of their content, their role in disease, and the therapeutic potential of drugs targeting polyamine content and function. The methodology provided in this new volume will enable laboratories already working in this area to expand their experimental techniques and facilitate the entry of additional workers into this rapidly expanding field.