Post-transcriptional regulation of the content of spermidine/spermine N1-acetyltransferase by N1N12-bis(ethyl)spermine

N1-acetylspermidine is a determinant of hair follicle stem cell fate

Stem cell differentiation is accompanied by increased mRNA translation. The rate of protein biosynthesis is influenced by the polyamines putrescine, spermidine and spermine, which are essential for cell growth and stem cell maintenance. However, the role of polyamines as endogenous effectors of stem cell fate and whether they act through translational control remains obscure. Here, we investigate the function of polyamines in stem cell fate decisions using hair follicle stem cell (HFSC) organoids. Compared to progenitor cells, HFSCs showed lower translation rates, correlating with reduced polyamine levels. Surprisingly, overall polyamine depletion decreased translation but did not affect cell fate. In contrast, specific depletion of natural polyamines mediated by spermidine/spermine N1-acetyltransferase (SSAT; also known as SAT1) activation did not reduce translation but enhanced stemness. These results suggest a translation-independent role of polyamines in cell fate regulation. Indeed, we identified N1-acetylspermidine as a determinant of cell fate that acted through increasing self-renewal, and observed elevated N1-acetylspermidine levels upon depilation-mediated HFSC proliferation and differentiation in vivo. Overall, this study delineates the diverse routes of polyamine metabolism-mediated regulation of stem cell fate decisions.

This article has an associated First Person interview with the first author of the paper.

Summary: Reduced protein synthesis is required for stem cell functions. Here, we delineate a complex interplay of polyamines and mRNA translation that determines hair follicle stem cell fate decisions.

Roles of polyamines in translation

Polyamines are organic polycations that bind to a variety of cellular molecules, including nucleic acids. Within cells, polyamines contribute to both the efficiency and fidelity of protein synthesis. In addition to directly acting on the translation apparatus to stimulate protein synthesis, the polyamine spermidine serves as a precursor for the essential post-translational modification of the eukaryotic translation factor 5A (eIF5A), which is required for synthesis of proteins containing problematic amino acid sequence motifs, including polyproline tracts, and for termination of translation. The impact of polyamines on translation is highlighted by autoregulation of the translation of mRNAs encoding key metabolic and regulatory proteins in the polyamine biosynthesis pathway, including S-adenosylmethionine decarboxylase (AdoMetDC), antizyme (OAZ), and antizyme inhibitor 1 (AZIN1). Here, we highlight the roles of polyamines in general translation and also in the translational regulation of polyamine biosynthesis.

Downregulation of the polyamine regulator spermidine/spermine N(1)-acetyltransferase by Epstein-Barr virus in a Burkitt's lymphoma cell line

Transition of Akata Burkitt's lymphoma (BL) from a malignant to nonmalignant phenotype upon loss of Epstein-Barr virus (EBV) is evidence for a viral contribution to tumorigenesis despite the tight restriction of EBV gene expression in BL. Examination of global cellular gene expression in Akata subclones that retained or lost EBV identified spermidine/spermine N(1)-acetyltransferase (SAT1), an inducible enzyme whose catabolism of polyamines affects both apoptosis and cell growth, as one of a limited number of cellular genes downregulated by EBV. Re-infection of the EBV-negative Akata clone reduced SAT1 mRNA to a level comparable with the parental EBV-positive Akata. EBV-positive Akata cells demonstrated decreased SAT1 enzyme activity concomitant with altered intracellular polyamine constituents. Reduction of SAT1 in EBV-positive BL was a transcriptional effect. Forced expression of the viral BCL2 homologue, BHRF1, in an EBV-negative Akata clone reduced SAT1 mRNA. Thus, EBV repression of polyamine catabolism becomes a complementary alteration to dysregulated c-myc enhancement of polyamine synthesis in BL and favorable to BL lymphomagenesis.

A novel assay platform for the detection of translation modulators of spermidine/spermine acetyltransferase

Spermidine/spermine-N1-acetyltransferase (SSAT) is a mitochondrial-localized enzyme that is highly inducible and tightly controlled and is the rate-limiting enzyme in polyamine catabolism. It is known that SSAT is induced when polyamine level increases. Although multiple mechanisms have been implicated, translational control is thought to be paramount. Previous studies with transgenic and knockout mice suggested that for certain human conditions, the modulation of SSAT levels could offer therapeutic benefits. Besides polyamines and their analogs, certain stimuli can increase SSAT levels, suggesting that the development of reporters for high throughput screening can lead to the identification of novel pharmacophores that can modulate SSAT translation. Here we report the development and validation of a luciferase-based biosensor system for the identification of compounds that are able to either promote or prevent the translation of SSAT. The system uses HEK293T cells transfected with a construct composed of SSAT mRNA modified to lack upstream open reading frame (uORF) function, is mutated to reduce translational repression and is linked with luciferase. As a proof of principle of the utility of the SSAT translation sensor, we screened the Prestwick drug library (1,200 FDA Approved compounds). The library contained 15 compounds that activated SSAT translation by at least 40% more than the basal expression, but none exceeded the positive control N1, N11-diethylnorspermine. On the other hand, 38 compounds were found to strongly inhibit SSAT translation. We conclude that this biosensor can lead to the identification of novel pharmacophores that are able to modulate the translation of SSAT.

Polyamine-regulated translation of spermidine/spermine-N1-acetyltransferase

Rapid synthesis of the polyamine catabolic enzyme spermidine/spermine-N(1)-acetyltransferase (SSAT) in response to increased polyamines is an important polyamine homeostatic mechanism. Indirect evidence has suggested that there is an important control mechanism involving the release of a translational repressor protein that allows the immediate initiation of SSAT protein synthesis without RNA transcription, maturation, or translocation. To identify a repressor protein, we used a mass spectroscopy-based RNA-protein interaction system and found six proteins that bind to the coding region of SSAT mRNA. Individual small interfering RNA (siRNA) experiments showed that nucleolin knockdown enhances SSAT translation. Nucleolin exists in several isoforms, and we report that the isoform that binds to SSAT mRNA undergoes autocatalysis in the presence of polyamines, a result suggesting that there is a negative feedback system that helps control the cellular content of polyamines. Preliminary molecular interaction data show that a nucleolin isoform binds to a 5' stem-loop of the coding region of SSAT mRNA. The glycine/arginine-rich C terminus of nucleolin is required for binding, and the four RNA recognition motif domains are included in the isoform that blocks SSAT translation. Understanding SSAT translational control mechanisms has the potential for the development of therapeutic strategies against cancer and obesity.

A combined model of hepatic polyamine and sulfur amino acid metabolism to analyze S-adenosyl methionine availability

Many molecular details remain to be uncovered concerning the regulation of polyamine metabolism. A previous model of mammalian polyamine metabolism showed that S-adenosyl methionine availability could play a key role in polyamine homeostasis. To get a deeper insight in this prediction, we have built a combined model by integration of the previously published polyamine model and one-carbon and glutathione metabolism model, published by different research groups. The combined model is robust and it is able to achieve physiological steady-state values, as well as to reproduce the predictions of the individual models. Furthermore, a transition between two versions of our model with new regulatory factors added properly simulates the switch in methionine adenosyl transferase isozymes occurring when the liver enters in proliferative conditions. The combined model is useful to support the previous prediction on the role of S-adenosyl methionine availability in polyamine homeostasis. Furthermore, it could be easily adapted to get deeper insights on the connections of polyamines with energy metabolism.

Novel anti-apoptotic effect of the retinoblastoma protein: implications for polyamine analogue toxicity

The retinoblastoma protein (pRb) pathway is frequently altered in breast cancer cells. pRb is involved in the regulation of cell proliferation and cell death. The breast cancer cell line L56Br-C1 does not express pRb and is extremely sensitive to treatment with the polyamine analogue N(1),N(11)-diethylnorspermine (DENSPM) which causes apoptosis. Polyamines are essential for the regulation of cell proliferation, cell differentiation and cell death. DENSPM depletes cells of polyamines, e.g., by inducing the activity of the polyamine catabolic enzyme spermidine/spermine N(1)-acetyltransferase (SSAT). In this study, L56Br-C1 cells were transfected with human pRb-cDNA. Overexpression of pRb inhibited DENSPM-induced cell death and DENSPM-induced SSAT activity. This suggests that the pRb protein level is a promising marker for polyamine depletion sensitivity and that there is a connection between pRb and the regulation of SSAT activity. We also show that SSAT protein levels and SSAT activity do not always correlate, suggesting that there is an unknown regulation of SSAT.

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.

Polyamine catabolism in colorectal cancer cells following treatment with oxaliplatin, 5-fluorouracil and N1, N11 diethylnorspermine

PURPOSE

Our previous studies showed that combined treatment of oxaliplatin and N(1), N(11) diethyl-norspermine (DENSPM) results in massive induction of spermidine/spermine N(1)-acetyltransferase (SSAT) mRNA and activity. Since oxaliplatin and 5-fluorouracil (5FU) are used clinically in treatment of colorectal cancers, this study examines the effect of adding DENSPM to oxaliplatin/5FU combination on SSAT and spermine oxidase (SMO) in HCT-116 cells.

METHODS

HCT-116 cells were treated with clinically relevant concentrations of drugs for 20 h followed by 24 h in drug free medium. SSAT and SMO mRNA and protein were assayed by QRT-PCR and Westerns respectively; polyamine pools were measured by HPLC. SSAT and SMO mRNA in tumor biopsies from patients with rectal cancer receiving oxaliplatin, capecitabine and radiation were measured by QRT-PCR.

RESULTS

Oxaliplatin + 5FU + DENSPM produced significantly higher levels of SSAT and SMO mRNA, protein and activity than those seen with oxaliplatin+5FU with a significant depletion of cellular spermine and spermidine pools. Oxaliplatin/DENSPM was superior to 5FU/DENSPM in SSAT induction but similar for SMO. Oxaliplatin + DENSPM revealed synergistic growth inhibition at >IC(50) concentrations and antagonism at

CONCLUSIONS

These studies demonstrated that combining DENSPM with oxaliplatin + 5FU provides an added benefit by aiming at the clinically relevant therapeutic target, the polyamine catabolism. Further, we show for the first time, that SMO and SSAT induction could be measured in tumor biopsies in patients receiving chemo-radiation. Optimization of treatment conditions in vivo should facilitate a clinical evaluation of the three drug combination.

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Key Words Collapse

MESH Headings

• Acetyltransferases/genetics
• Acetyltransferases/metabolism
• Antineoplastic Combined Chemotherapy Protocols/pharmacology
• Antineoplastic Combined Chemotherapy Protocols/therapeutic use
• Blotting, Western
• Cell Line, Tumor
• Colonic Neoplasms/drug therapy
• Colonic Neoplasms/metabolism
• Colonic Neoplasms/pathology
• Drug Synergism
• Fluorouracil/pharmacology
• Fluorouracil/therapeutic use
• Gene Expression/drug effects
• Humans
• Organoplatinum Compounds/pharmacology
• Organoplatinum Compounds/therapeutic use
• Oxaliplatin
• Oxidoreductases Acting on CH-NH Group Donors/genetics
• Oxidoreductases Acting on CH-NH Group Donors/metabolism
• Polyamines/metabolism
• Rectal Neoplasms/drug therapy
• Rectal Neoplasms/metabolism
• Rectal Neoplasms/pathology
• Spermine/analogs & derivatives
• Spermine/pharmacology
• Spermine/therapeutic use
• Polyamine Oxidase

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Grants

• CA 10656 NCI NIH HHS
• CA 109619 NCI NIH HHS

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Affiliation(s)

Suzanne Hector Department of Medicine, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NY 14263, USA
Ramakumar Tummala
Nicholas D Kisiel
Paula Diegelman
Slavoljub Vujcic
Kimberly Clark
Marwan Fakih
Debora L Kramer
Carl W Porter
Lakshmi Pendyala

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Spermidine/spermine-N(1)-acetyltransferase: a key metabolic regulator

Spermidine/spermine-N(1)-acetyltransferase (SSAT) regulates cellular polyamine content. Its acetylated products are either excreted from the cell or oxidized by acetylpolyamine oxidase. Since polyamines play critical roles in normal and neoplastic growth and in ion channel regulation, SSAT is a key enzyme in these processes. SSAT is very highly regulated. Its content is adjusted in response to alterations in polyamine content to maintain polyamine homeostasis. Certain polyamine analogs can mimic the induction of SSAT and cause a loss of normal polyamines. This may have utility in cancer chemotherapy. SSAT activity is also induced via a variety of other stimuli, including toxins, hormones, cytokines, nonsteroidal anti-inflammatory agents, natural products, and stress pathways, and by ischemia-reperfusion injury. These increases are initiated by alterations in Sat1 gene transcription reinforced by alterations at the other regulatory steps, including protein turnover, mRNA processing, and translation. Transgenic manipulation of SSAT activity has revealed that SSAT activity links polyamine metabolism to lipid and carbohydrate metabolism by means of alterations in the content of acetyl-CoA and ATP. A high level of SSAT stimulates flux through the polyamine biosynthetic pathway, since biosynthetic enzymes are induced in response to the fall in polyamines. This sets up a futile cycle in which ATP is used to generate S-adenosylmethionine for polyamine biosynthesis and acetyl-CoA is consumed in the acetylation reaction. A variety of other effects of increased SSAT activity include death of pancreatic cells, blockage of regenerative tissue growth, behavioral changes, keratosis follicularis spinulosa decalvans, and hair loss. These are very likely due to changes in polyamine and putrescine levels, although increased oxidative stress via the oxidation of acetylated polyamines may also contribute. Recently, it was found that the SSAT protein and/or a related protein, thialysine acetyltransferase, interacts with a number of other important proteins, including the hypoxia-inducible factor-1 alpha-subunit, the p65 subunit of NF-kappaB, and alpha9beta1-integrin, altering the function of these proteins. It is not yet clear whether this functional alteration involves protein acetylation, local polyamine concentration changes, or other effects. It has been suggested that SSAT may also be a useful target in diseases other than cancer, but the wide-ranging physiological and pathophysiological effects of altered SSAT expression will require very careful limitation of such strategies to the relevant cells to avoid toxic effects.

Polyamine-dependent Regulation of Spermidine-Spermine N1-Acetyltransferase mRNA Translation

Spermidine-spermine N(1)-acetyltransferase (SSAT) is induced in response to an elevation in intracellular polyamine pools. The increased enzyme activity is the result of an increase in gene transcription, mRNA translation, and protein stability. Induction of SSAT by polyamine analogues can lead to intracellular polyamine depletion and apoptosis. The mechanism by which polyamines alter the translational efficiency of SSAT mRNA is not well understood. In this study, we investigated the regulation of SSAT translation by the polyamine analogue N(1),N(11)-diethylnorspermine (DENSPM). DENSPM induced expression of both FLAG-tagged SSAT and SSAT fused to Renilla luciferase in a time- and concentration-dependent manner. This effect was not inhibited by actinomycin D indicating that changes in gene transcription did not explain the enhanced expression in the presence of DENSPM. Furthermore, because FLAG-SSAT did not contain the 5'- or 3'-untranslated regions of SSAT, translational regulation involved the coding sequence only. By contrast, cycloheximide completely inhibited induction by DENSPM, indicating a requirement for new protein synthesis. Deletion constructs identified two regions of the SSAT protein-coding RNA sequence that conferred polyamine responsiveness. Using these regions as probes in RNA electrophoretic mobility shift assays, we observed specific binding of a cytoplasmic protein. In addition, we found that the interaction between the RNA probes and the binding protein could be inhibited by DENSPM in a concentration-dependent manner. These results suggest that polyamines regulate SSAT mRNA translational efficiency by inhibiting a repressor protein from binding to regions of the coding sequence of the SSAT transcript.

Targeting polyamine metabolism and function in cancer and other hyperproliferative diseases

The polyamines spermidine and spermine and their diamine precursor putrescine are naturally occurring, polycationic alkylamines that are essential for eukaryotic cell growth. The requirement for and the metabolism of polyamines are frequently dysregulated in cancer and other hyperproliferative diseases, thus making polyamine function and metabolism attractive targets for therapeutic intervention. Recent advances in our understanding of polyamine function, metabolic regulation, and differences between normal cells and tumour cells with respect to polyamine biology, have reinforced the interest in this target-rich pathway for drug development.

Mammalian polyamine catabolism: a therapeutic target, a pathological problem, or both?

With the recent discovery of the polyamine catabolic enzyme spermine oxidase (SMO/PAOh1), the apparent complexity of the polyamine metabolic pathway has increased considerably. Alone or in combination with the two other known members of human polyamine catabolism, spermidine/spermine N(1)-acetyltransferase, and N(1)-acetylpolyamine oxidase (PAO), SMO/PAOh1 expression has the potential to alter polyamine homeostasis in response to normal cellular signals, drug treatment and environmental and/or cellular stressors. The activity of the oxidases producing toxic aldehydes and the reactive oxygen species (ROS) H(2)O(2), suggest a mechanism by which these oxidases can be exploited as an antineoplastic drug target. However, inappropriate activation of the pathways may also lead to pathological outcomes, including DNA damage that can lead to cellular transformation. The most recent data suggest that the two polyamine catabolic pathways exhibit distinct properties and understanding these properties should aid in their exploitation for therapeutic and/or chemopreventive strategies.

Polyamine-regulated unproductive splicing and translation of spermidine/spermine N1-acetyltransferase

Spermidine/spermine N1-acetyltransferase (SSAT), the rate-controlling enzyme in the interconversion of spermidine and spermine, is regulated by polyamines and their analogs at many levels of gene expression. Recently, SSAT pre-mRNA has been shown to undergo alternative splicing by inclusion of an exon that contains premature termination codons. In the present study, we show that alterations in the intracellular polyamine level resulted in a change in the relative abundance of SSAT transcripts. Addition of polyamines or their N-diethylated analogs reduced the amount of the variant transcript, whereas polyamine depletion by 2-difluoromethylornithine or MG-132 enhanced the exon inclusion. Experiments performed with protein synthesis inhibitors and siRNA-mediated down-regulation of Upf1 protein verified that the variant transcript was degraded by nonsense-mediated mRNA decay (NMD). Interestingly, several proteins have been shown to regulate their expression by alternative splicing-coupled NMD, termed regulated unproductive splicing and translation (RUST). Our present results suggest that in the case of SSAT, RUST is mediated by polyamines, and this system functions to fine-tune the polyamine metabolism.

Mathematical modeling of polyamine metabolism in mammals

Polyamines are considered as essential compounds in living cells, since they are involved in cell proliferation, transcription, and translation processes. Furthermore, polyamine homeostasis is necessary to cell survival, and its deregulation is involved in relevant processes, such as cancer and neurodegenerative disorders. Great efforts have been made to elucidate the nature of polyamine homeostasis, giving rise to relevant information concerning the behavior of the different components of polyamine metabolism, and a great amount of information has been generated. However, a complex regulation at transcriptional, translational, and metabolic levels as well as the strong relationship between polyamines and essential cell processes make it difficult to discriminate the role of polyamine regulation itself from the whole cell response when an experimental approach is given in vivo. To overcome this limitation, a bottom-up approach to model mathematically metabolic pathways could allow us to elucidate the systemic behavior from individual kinetic and molecular properties. In this paper, we propose a mathematical model of polyamine metabolism from kinetic constants and both metabolite and enzyme levels extracted from bibliographic sources. This model captures the tendencies observed in transgenic mice for the so-called key enzymes of polyamine metabolism, ornithine decarboxylase, S-adenosylmethionine decarboxylase and spermine spermidine N-acetyl transferase. Furthermore, the model shows a relevant role of S-adenosylmethionine and acetyl-CoA availability in polyamine homeostasis, which are not usually considered in systemic experimental studies.

Pharmacogenomic identification of novel determinants of response to chemotherapy in colon cancer

DNA microarray analysis was used to analyze the transcriptional profile of HCT116 colorectal cancer cells that were treated with 5-fluorouracil (5-FU) or oxaliplatin and selected for resistance to these agents. Bioinformatic analyses identified sets of genes that were constitutively dysregulated in drug-resistant cells and transiently altered following acute exposure of parental cells to drug. We propose that these genes may represent molecular signatures of sensitivity to 5-FU and oxaliplatin. Using real-time reverse transcription-PCR (RT-PCR), the robustness of our microarray data was shown with a strong overall concordance of expression trends for > or =82% (oxaliplatin) and > or =85% (5-FU) of a representative subset of genes. Furthermore, strong correlations between the microarray and real-time RT-PCR measurements of average fold changes in gene expression were observed for both the 5-FU (R(2) > or = 0.73) and oxaliplatin gene sets (R(2) > or = 0.63). Functional analysis of three genes identified in the microarray study [prostate-derived factor (PDF), calretinin, and spermidine/spermine N(1)-acetyl transferase (SSAT)] revealed their importance as novel regulators of cytotoxic drug response. These data show the power of this novel microarray-based approach to identify genes which may be important markers of response to treatment and/or targets for therapeutic intervention.

Effects ofN1,N13-Diethylnorspermine (DENSPM) and X-Radiation Treatment on Human Colorectal Tumor Clones with Varying X-Radiation and Drug Responses

This study was designed to examine the effects of treatment with N1, N13-diethylnorspermine (DENSPM), a spermine analog, and X radiation on survival and on the polyamine and spermidine/spermine N1-acetyltransferase (SSAT) levels in closely related human colorectal tumor (HCT116) clones exhibiting a wide range of X-radiation and drug responses. After treatment with DENSPM and X radiation, clonogenic cell survival was measured. SSAT protein levels were measured by Western blot analysis and SSAT enzymatic activities by the conversion of [1-14C]acetyl-CoA into [1-14C]acetylspermidine. Polyamine [i.e. putrescine (PUT), spermine (SPM) and spermidine (SPD)] levels were measured with high-performance liquid chromatography. DENSPM enhanced the efficacy of radiation treatment in HCT116, HCT116-Clone2 (a radiation-resistant clone) and HCT116-Clone10 (a clone with similar X-radiation response as the parental HCT116 cells) but not in HCT116-CloneK (an X-radiation-sensitive but relatively drug-resistant clone). Treatment with DENSPM without X radiation caused the most significant increase in SSAT activity (approximately 22-fold) and an almost complete depletion of SPD levels in HCT116-CloneK. Our results suggest that (a) the lack of sensitization of X-radiation treatment by DENSPM in HCT116-CloneK was likely due to the prior depletion of SPD levels by DENSPM alone, (b) natural polyamine contents and/or inducibility of SSAT may be important factors influencing cellular response to combined X-radiation and DENSPM treatments, and (c) more importantly, there may be a potentially novel role for combining polyamine analogs such as DENSPM with X rays.

Polyamine transport in parasites: a potential target for new antiparasitic drug development

The metabolism of the naturally occurring polyamines-putrescine, spermidine and spermine-is a highly integrated system involving biosynthesis, uptake, degradation and interconversion. Metabolic differences in polyamine metabolism have long been considered to be a potential target to arrest proliferative processes ranging from cancer to microbial and parasitic diseases. Despite the early success of polyamine inhibitors such as alpha-difluoromethylornithine (DFMO) in treating the latter stages of African sleeping sickness, in which the central nervous system is affected, they proved to be ineffective in checking other major diseases caused by parasitic protozoa, such as Chagas' disease, leishmaniasis or malaria. In the use and design of new polyamine-based inhibitors, account must be taken of the presence of up-regulated polyamine transporters in the plasma membrane of the infectious agent that are able to circumvent the effect of the drug by providing the parasite with polyamines from the host. This review contains information on the polyamine requirements and molecular, biochemical and genetic characterization of different transport mechanisms in the parasitic agents responsible for a number of the deadly diseases that afflict underdeveloped and developing countries.

The ubiquitin-proteasome system is responsible for cysteine-responsive regulation of cysteine dioxygenase concentration in liver

Hepatic cysteine dioxygenase (CDO) activity is a critical regulator of cellular cysteine concentration and availability of cysteine for anabolic processes and is markedly higher in animals fed diets containing excess sulfur amino acids compared with those fed levels at or below the requirement. Rat hepatocytes responded to a deficiency or excess of cysteine in the culture medium with a decrease or increase in CDO level but no change in CDO mRNA level. The cysteine analog, cysteamine, but not cysteine metabolites or thiol reagents, was also effective in increasing CDO. Inhibitors of the 26S proteasome blocked CDO degradation in cysteine-deficient cells but had little or no effect on CDO concentration in hepatocytes cultured with excess cysteine. High-molecular-mass CDO-ubiquitin conjugates were observed in cells cultured in cysteine-deficient medium, whether or not proteasome inhibitor was present, but these CDO-ubiquitin conjugates were not observed in cells cultured in cysteine-supplemented medium with or without proteasome inhibitor. Similar results were observed for degradation of recombinant CDO expressed in human heptocarcinoma cells cultured in cysteine-deficient or cysteine-supplemented medium. CDO is an example of a mammalian enzyme that is robustly regulated via its substrate, with the presence of substrate blocking the ubiquitination of CDO and, hence, the targeting of CDO for proteasomal degradation. This regulation occurs in primary hepatocytes in a manner that corresponds with changes observed in intact animals.

Genetic approaches to the cellular functions of polyamines in mammals

The polyamines putrescine, spermidine and spermine are organic cations shown to participate in a bewildering number of cellular reactions, yet their exact functions in intermediary metabolism and specific interactions with cellular components remain largely elusive. Pharmacological interventions have demonstrated convincingly that a steady supply of these compounds is a prerequisite for cell proliferation to occur. The last decade has witnessed the appearance of a substantial number of studies, in which genetic engineering of polyamine metabolism in transgenic rodents has been employed to unravel their cellular functions. Transgenic activation of polyamine biosynthesis through an overexpression of their biosynthetic enzymes has assigned specific roles for these compounds in spermatogenesis, skin physiology, promotion of tumorigenesis and organ hypertrophy as well as neuronal protection. Transgenic activation of polyamine catabolism not only profoundly disturbs polyamine homeostasis in most tissues, but also creates a complex phenotype affecting skin, female fertility, fat depots, pancreatic integrity and regenerative growth. Transgenic expression of ornithine decarboxylase antizyme has suggested that this unique protein may act as a general tumor suppressor. Homozygous deficiency of the key biosynthetic enzymes of the polyamines, ornithine and S-adenosylmethionine decarboxylase, as achieved through targeted disruption of their genes, is not compatible with murine embryogenesis. Finally, the first reports of human diseases apparently caused by mutations or rearrangements of the genes involved in polyamine metabolism have appeared.

Spermidine/spermine N1-acetyltransferase (SSAT) activity in human small-cell lung carcinoma cells following transfection with a genomic SSAT construct

Spermidine/spermine N (1)-acetyltransferase (SSAT) activity is typically highly inducible in non-small-cell lung carcinomas in response to treatment with anti-tumour polyamine analogues, and this induction is associated with subsequent cell death. In contrast, cells of the small-cell lung carcinoma (SCLC) phenotype generally do not respond to these compounds with an increase in SSAT activity, and usually are only moderately affected with respect to growth. The goal of the present study was to produce an SSAT-overexpressing SCLC cell line to further investigate the role of SSAT in response to these anti-tumour analogues. To accomplish this, NCI-H82 SCLC cells were stably transfected with plasmids containing either the SSAT genomic sequence or the corresponding cDNA sequence. Individual clones were selected based on their ability to show induced SSAT activity in response to exposure to a polyamine analogue, and an increase in the steady-state SSAT mRNA level. Cells transfected with the genomic sequence exhibited a significant increase in basal SSAT mRNA expression, as well as enhanced SSAT activity, intracellular polyamine pool depletion and growth inhibition following treatment with the analogue N (1), N (11)-bis(ethyl)norspermine. Cells containing the transfected cDNA also exhibited an increase in the basal SSAT mRNA level, but remained phenotypically similar to vector control cells with respect to their response to analogue exposure. These studies indicate that both the genomic SSAT sequence and polyamine analogue exposure play a role in the transcriptional and post-transcriptional regulation and subsequent induction of SSAT activity in these cells. Furthermore, this is the first production of a cell line capable of SSAT protein induction from a generally unresponsive parent line.

Agmatine modulates the in vivo biosynthesis and interconversion of polyamines and cell proliferation

Agmatine has recently gained wide interest as a bioactive arginine metabolite with a multitude of physiological functions. This study evaluates the in vivo role of agmatine in the modulation of metabolism and intracellular level of polyamines. Here, we report that agmatine, administered to mice, differentially affects the renal and liver activity of the two key enzymes regulating polyamine biosynthesis and interconversion/degradation. Thus, agmatine exerts a negative regulation of ODC activity and protein content, and positive regulation of SSAT activity, having no effect on ODC and SSAT transcript level. Agmatine modulation of ODC and SSAT activities is noticeably augmented by the inhibitor of its catabolism, aminoguanidine. Antizyme and eIF4E protein content appears to be affected by agmatine only insignificantly and apparently do not contribute to agmatine-induced down-regulation of ODC content. The homeostasis of spermidine and spermine is preserved after agmatine injection, while the putrescine level decreases. Furthermore, when tested in a mouse kidney injury model, agmatine, partially but significantly, reduces [3H] thymidine incorporation into DNA. This is consistent with suppressed renal tubule epithelial cell proliferation. The findings provide in vivo evidence of a substantial role of agmatine as a modulator of polyamine biosynthesis and degradation and suggest its suppressive effect on cell proliferation.

Polyamine analogues inhibit the ubiquitination of spermidine/spermine N1-acetyltransferase and prevent its targeting to the proteasome for degradation

Spermidine/spermine N(1)-acetyltransferase (SSAT), a key enzyme in mammalian polyamine catabolism, undergoes rapid turnover (half-life approx. 30 min) and is highly inducible in response to polyamine analogues such as bis(ethyl)spermine (BE-3-4-3), which greatly stabilize the enzyme. Rapid degradation of SSAT in reticulocyte lysates was preceded by formation of a ladder of ubiquitinated forms, and required the production of high-molecular-mass complexes with ubiquitin (HMM-SSAT-Ubs). Mutation of all 11 lysines in SSAT separately to arginine demonstrated that no single lysine residue is critical for its degradation in vitro, but mutant K87R had a significantly longer half-life, suggesting that lysine-87 may be the preferred site for ubiquitination. Mutations at the C-terminus of SSAT, such as E171Q, resulted in marked stabilization of the protein, due to the lack of formation of the HMM-SSAT-Ubs. Addition of BE-3-4-3 prevented the accumulation of ubiquitin conjugates and the proteasomal degradation of wild-type SSAT. These results indicate that conformational changes brought about by the binding of polyamine analogues prevent the efficient polyubiquitination of SSAT, leading to a major increase in the amount of SSAT protein, and that alteration of the C-terminal end of the protein has a similar effect in preventing the productive interaction with an E2 or E3 component of the ubiquitin pathway.

Histamine prevents polyamine accumulation in mouse C57.1 mast cell cultures

The effects of histamine on polyamine uptake and metabolism was studied in a mouse mast cell line (C57.1), as a cell model in which both biogenic amines are important for maintaining cell function and viability. Results obtained after incubations with exogenous histamine indicated that histamine prevents polyamine accumulation by affecting polyamine uptake. A plasma membrane transport system for polyamines has been also studied in mast cells. It seems to be a Na(+)-dependent uptake with high affinity for both spermine and spermidine and lower affinity for putrescine and agmatine. Polyamine uptake was reduced in both cells treated with exogenous histamine and histamine-preloaded cells. However, ornithine decarboxylase activity and cell proliferation were not affected by histamine. Incubation with histamine enhanced the spermidine/spermine acetyl transferase induction caused by N(1)-ethyl-N(11)-[(cyclopropyl)methyl]-4,8-diazaundecane, suggesting that polyamine acetylation could be another mechanism by which histamine prevents polyamine accumulation in C57.1 mast cells.

Differential induction of spermidine/spermine N1-acetyltransferase activity in cisplatin-sensitive and -resistant ovarian cancer cells in response to N1,N12-bis(ethyl)spermine involves transcriptional and post-transcriptional regulation

The growth inhibition that occurs in cisplatin-sensitive 2008 human ovarian cancer cells in response to the spermine analogue, N1,N12-bis(ethyl)spermine (BESpm), is associated with a potent induction of spermidine/spermine N1-acetyltransferase (SSAT), the rate-limiting enzyme in polyamine catabolism. Conversely, in cisplatin-resistant C13* cells, which are less responsive to BESpm, enzyme induction does not occur at comparable levels after exposure to the bis(ethyl)-derivative. In this study, we investigated the molecular mechanisms underlying the differential induction of SSAT activity in cisplatin-sensitive and -resistant cells. Northern blot analysis revealed a difference in the level of SSAT mRNA expression in the two cell lines; in particular, 2008 cells treated with 10 microM BESpm for progressively increasing periods of time accumulated more heteronuclear (3.5 kb) and mature (1.3/1.5 kb) SSAT mRNAs than its resistant variant. SSAT mRNA accumulation paralleled enzyme activity and both were almost completely prevented in the two lines by co-treatment with 5 microg/ml actinomycin-D (Act-D), suggesting that transcription plays a major role in the analogue-mediated induction of SSAT. Moreover, when Act-D was added 48 h after BESpm exposure, SSAT mRNA and enzyme activity were stabilised in both cell lines. Therefore, the marked difference in the induction of SSAT activity seems to be related to increased enzyme synthesis, particularly in sensitive cells, whose SSAT protein turnover was also greatly reduced (half-life >12 h in 2008 cells versus 5 h in C13* cells) in the presence of BESpm. These findings suggest that cisplatin-resistance modulates the SSAT response to BESpm at transcriptional and post-transcriptional levels.

Effects of conditional overexpression of spermidine/spermine N1-acetyltransferase on polyamine pool dynamics, cell growth, and sensitivity to polyamine analogs

Acetylation of polyamines by spermidine/spermine N(1)-acetyltransferase (SSAT) has been implicated in their degradation and/or export out of the cell. The relationship of SSAT to polyamine pool dynamics and cell growth is not yet clearly understood. MCF-7 human breast carcinoma cells were transfected with tetracycline-regulated (Tet-off) SSAT human cDNA or murine gene. Doxycycline removal for >2 days caused a approximately 20-fold increase in SSAT RNA and a approximately 10-fold increase in enzyme activity. After 4 days, intracellular putrescine and spermidine pools were markedly lowered, and cell growth was inhibited. Growth inhibition could not be prevented with exogenous polyamines due to a previously unrecognized ability of SSAT to rapidly acetylate influxing polyamines and thereby prevent restoration of the endogenous pools. Instead, cells accumulated high levels of N(1)-acetylspermidine, N(1)-acetylspermine, and N(1), N(12)-diacetylspermine, a metabolite not previously reported in mammalian cells. Doxycycline deprivation before treatment with N(1), N(11)-diethylnorspermine markedly increased analog induction of SSAT mRNA and activity and enhanced growth sensitivity to the analog by approximately 100-fold. Overall, the findings demonstrate that conditional overexpression of SSAT lowers polyamine pools, inhibits cell growth, and markedly enhances growth sensitivity to certain analogs. The enzyme also plays a remarkably efficient role in maintaining polyamine pool homeostasis during challenges with exogenous polyamines.

Effects of methionine on endogenous antioxidants in the heart

The deficiency of methionine, an essential amino acid, is associated with cardiovascular lesions. Because different types of cardiac pathologies are caused by a decrease in antioxidants, we examined the effects of methionine on myocardial antioxidant enzymes in hemodynamically assessed rats that were treated with methionine (10 mg/ml) in drinking water for 12, 24, and 48 h. Glutathione peroxidase (GSHPx) activity was significantly increased to 150.5 +/- 12.2 and 191.7 +/- 13.7% of the control value at 12 and 24 h, respectively, followed by a decline to 120 +/- 24.6% at 48 h. The mRNA levels of GSHPx at these time points were 151.2 +/- 12.0, 218.7 +/- 35.3, and 173.5 +/- 25.2%, respectively. Superoxide dismutase (SOD) activity was 144.3 +/- 3.7, 114.3 +/- 10.1, and 143.1 +/- 11. 2% at 12, 24, and 48 h, respectively. Catalase (Cat) activity was 272.4 +/- 5.4, 237.8 +/- 16.6, and 224.1 +/- 17.3% of the control value. The expression of Cat and SOD mRNA was unchanged at 12, 24, and 48 h. The lipid peroxidation was decreased by 24.4 +/- 11.2, 54. 9 +/- 0.1, and 6.4 +/- 2.1% at 12, 24, and 48 h, respectively. Methionine had no effect on the ventricular or aortic pressures, heart rate, and myocardial glutathione levels at any of the time points. The study shows that methionine has a significant effect on the myocardial antioxidant enzyme activities, and only changes in GSHPx enzyme activity correlated with the mRNA changes. These antioxidant changes may have a role in the beneficial effects of methionine in pathological rather than physiological conditions.

Properties and regulation of human spermidine/spermine N1-acetyltransferase stably expressed in Chinese hamster ovary cells

Spermidine/spermine N1-acetyltransferase (SSAT) appears to be the rate-limiting enzyme of polyamine catabolism, yet studies of its regulation have been limited by the low amounts of SSAT in uninduced cells. A system for studying SSAT was established by stably transfecting Chinese hamster ovary cells with a construct where SSAT cDNA was under control of the cytomegalovirus promoter. Thirteen of 44 clones expressed significantly increased SSAT activity (650-1900 compared with 24 pmol/min/mg protein in control cells). SSAT activity was directly proportional to SSAT protein, which turned over very rapidly (t(1)/(2) of 29 min) and was degraded through the ubiquitin/proteasomal pathway. The increased SSAT activity caused perturbations in polyamine homeostasis and led to a reduction in the rate of growth under clonal conditions. N1,N12-bis(ethyl)spermine greatly increased SSAT activity in controls and SSAT transfected clones (to about 10 and 60 nmol/min/mg protein, respectively). N1, N12-Bis(ethyl)spermine caused an increase in the SSAT half-life and a slight increase in SSAT mRNA, but these changes were insufficient to account for the increase in SSAT protein suggesting that translational regulation of SSAT must also occur.

Overexpression of spermidine/spermine N1-acetyltransferase under the control of mouse metallothionein I promoter in transgenic mice: evidence for a striking post-transcriptional regulation of transgene expression by a polyamine analogue

We recently generated a transgenic mouse line overexpressing spermidine/spermine N1-acetyltransferase (SSAT) gene under its own promoter. The tissue polyamine pools of these animals were profoundly affected and the mice were hairless from early age. We have now generated another transgenic-mouse line overexpressing the SSAT gene under the control of a heavy-metal-inducible mouse metallothionein I (MT) promoter. Even in the absence of heavy metals, changes in the tissue polyamine pools indicated that a marked activation of polyamine catabolism had occurred in the transgenic animals. As with the SSAT transgenic mice generated previously, the mice of the new line (MT-SSAT) suffered permanent hair loss, but this occurred considerably later than in the previous SSAT transgenic animals. Liver was the most affected tissue in the MT-SSAT transgenic animals, revealed by putrescine overaccumulation, significant decrease in spermidine concentration and >90% reduction in the spermine pool. Even though hepatic SSAT mRNA accumulated to massive levels in non-induced transgenic animals, SSAT activity was only moderately elevated. Administration of ZnSO4 further elevated the level of hepatic SSAT message and induced enzyme activity, but not more than 2- to 3-fold. Treatment of the transgenic animals with the polyamine analogue N1,N11-diethylnorspermine (DENSPM) resulted in an immense induction, more than 40000-fold, of enzyme activity in the liver of transgenic animals, and minor changes in the SSAT mRNA level. Liver spermidine and spermine pools were virtually depleted within 1-2 days in response to the treatment with the analogue. The treatment also resulted in a marked mortality (up to 60%) among the transgenic animals which showed ultrastructural changes in the liver, most notably mitochondrial swelling, one of the earliest signs of cell injury. These results indicated that, even without its own promoter, SSAT is powerfully induced by the polyamine analogue through a mechanism that appears to involve a direct translational and/or heterogenous nuclear RNA processing control. It is likewise significant that overexpression of SSAT renders the animals extremely sensitive to polyamine analogues.

The identification of a cis-element and a trans-acting factor involved in the response to polyamines and polyamine analogues in the regulation of the human spermidine/spermine N1-acetyltransferase gene transcription

The superinduction of spermidine/spermine N1-acetyltransferase (SSAT) gene has been associated with a cytotoxic response to a new class of antineoplastic polyamine analogues. The initial mechanism of SSAT superinduction is an increase in transcription in response to analogue exposure. This increased transcription appears to be modulated through the association between a nuclear protein factor and a cis-element described here as the polyamine-responsive element (PRE). The PRE was identified as a 9-base pair sequence, 5'-TATGACTAA-3', in the context of a 31-base pair stretch from -1522 to -1492 base pairs with respect to the SSAT transcriptional start site. This element binds a nuclear factor from polyamine analogue-responsive cells, but not from polyamine analogue-insensitive cells. The labeled PRE was used to clone and identify the transcription factor, Nrf-2, that binds constitutively to the PRE sequence. Although the PRE sequence shares homology to the originally identified Nrf-2 recognition sequence, the two sequences are not identical. The Nrf-2 transcription factor appears only to be present in cell types that are capable of expressing high amounts of SSAT. The results of these studies suggest that Nrf-2, bound to the PRE, plays an important regulatory role of expression of the human SSAT gene.

Polyamine metabolism of rat gastric mucosa after oral administration of hypertonic sodium chloride solution

Oral administration of 1 ml of 3.42 M NaCl solution to rats induced spermidine/spermine N1-acetyltransferase (SSAT) activity in gastric mucosa as well as ornithine decarboxylase (ODC) activity. SSAT activity increased and peaked at 5 h and again at 7 h, whereas ODC activity peaked at 6 h. SSAT mRNA also increased after 3.42 M NaCl administration to an extent similar to the increase in SSAT activity at 5 h. Intracellular putrescine level and DNA synthesis were increased by NaCl administration. A polyamine oxidase inhibitor, N,N'-bis(2,3-butadienyl)-1,4-butanediamine (MDL-72527), but not an ODC inhibitor, alpha-difluoromethylornithine (DFMO), inhibited the increases in putrescine level and DNA synthesis at 5 h. The inhibition of DNA synthesis by MDL-72527 was reversed by putrescine administration. In contrast, both MDL-72527 and DFMO inhibited the increase in putrescine level and DNA synthesis at 16.5 h. These findings suggest that putrescine produced from preexistent spermidine by SSAT is responsible for the initial DNA synthesis after mucosal injury induced by NaCl and that both SSAT and ODC are involved in formation of putrescine, which is required for subsequent DNA synthesis.

Correlation of polyamine and growth responses to N1,N11-diethylnorspermine in primary fetal fibroblasts derived from transgenic mice overexpressing spermidine/spermine N1-acetyltransferase

A recently generated transgenic mouse line having activated polyamine catabolism due to systemic overexpression of spermidine/spermine N1-acetyltransferase (SSAT) was used to isolate primary fetal fibroblasts as a means to further elucidate the cellular consequences of activated polyamine catabolism. Basal levels of SSAT activity and steady-state mRNA in the transgenic fibroblasts were about approximately 20- and approximately 40-fold higher than in non-transgenic fibroblasts. Consistent with activated polyamine catabolism, there was an overaccumulation of putrescine and N1-acetylspermidine and a decrease in spermidine and spermine pools. Treatment with the polyamine analogue N1,N11-diethylnorspermine (DENSPM) increased SSAT activity in the transgenic fibroblasts approximately 380-fold, whereas mRNA increased only approximately 3-fold, indicating post-mRNA regulation. SSAT activity in the nontransgenic fibroblasts increased approximately 200-fold. By Western blot, enzyme protein was found to increase approximately 46 times higher in the treated transgenic fibroblasts than non-transgenic fibroblasts: a value comparable to 36-fold differential in enzyme activity. With DENSPM treatment, spermidine pools were more rapidly depleted in the transgenic fibroblasts than in nontransgenic fibroblasts. Similarly, transgenic fibroblasts were much more sensitive to DENSPM-induced growth inhibition. This was not diminished by co-treatment with an inhibitor of polyamine oxidase, suggesting that growth inhibition was due to polyamine depletion per se as opposed to oxidative stress. Since the two fibroblasts were genetically identical except for the transgene, the various metabolic and growth response differences are directly attributable to overexpression of SSAT.

Activation of polyamine catabolism profoundly alters tissue polyamine pools and affects hair growth and female fertility in transgenic mice overexpressing spermidine/spermine N1-acetyltransferase

We have generated a transgenic mouse line that overexpresses the rate-controlling enzyme of polyamine catabolism, spermidine/spermine N1-acetyltransferase. Tissues of these mice showed markedly distorted polyamine pools, which in most cases were characterized by the appearance of N1-acetylspermidine, not normally found in mouse tissues, a massive accumulation of putrescine, and decreases in spermidine and/or spermine pools. The most striking phenotypic change was permanent hair loss at the age of 3 to 4 weeks which was typified histologically by the appearance of extensive follicular cysts in the dermis. The effect seemed attributable to putrescine interference with hair development, possibly with differentiation/proliferation of epidermal cells located in hair follicles. Female members of the transgenic line were found to be infertile apparently due to ovarian hypofunction and hypoplastic uteri. The findings demonstrate the utility of spermidine/spermine N1-acetyltransferase overexpression as an effective means for genetically modulating total tissue polyamine pools in transgenic animals and examining the developmental and oncogenic consequences.

Proteasomal degradation of spermidine/spermine N1-acetyltransferase requires the carboxyl-terminal glutamic acid residues

The rapid turnover of spermidine/spermine N1-acetyltransferase (SSAT), a key enzyme in the regulation of polyamine levels, was found to be mediated via ubiquitination and the proteasomal system. SSAT degradation was blocked by the binding of polyamines or of the polyamine analog, N1,N12-bis(ethyl)spermine (BE-3-4-3), to the protein, providing a mechanism for the increase of SSAT activity in response to these agents. Site-directed mutagenesis indicated that a number of residues including arginine 19, cysteine 122, histidine 126, glutamic acid 152, arginine 155, and methionine 167 were needed for protection of SSAT by BE-3-4-3. These residues have previously been shown to reduce the affinity for the binding of polyamines to the SSAT protein, and these results indicate that the change in protein configuration brought about by this binding renders the protein resistant to proteasomal degradation. Mutations to alanines of residues arginine 7, cysteine 14, and lysine 141 also prevented the protection by BE-3-4-3, and these residues may be required for the formation of the protected conformation. The rapid degradation of SSAT required the carboxyl-terminal region of the protein, and the two terminal glutamic acid residues at positions 170 and 171 were found to be of critical importance. Truncation of the protein to remove these residues or the mutation of either of these acidic residues to glutamine completely abolished the rapid degradation of SSAT. The addition of two extra lysine residues at the carboxyl terminus or the conversion of the glutamic acids at positions 170 and 171 to lysines also prevented SSAT degradation by the proteasome. These results show the key role of the acidic residues at the carboxyl terminus of the protein in reacting with the proteasome. In contrast, mutation of lysine 166 to alanine, which extends the length of the acidic region in the carboxyl-terminal fragment of SSAT, actually increased the rate of degradation of SSAT without affecting its stabilization by BE-3-4-3. The binding of BE-3-4-3 or polyamines is therefore likely to change the configuration of the SSAT protein in a way that prevents the exposure of the carboxyl-terminal region of the ubiquitinated protein to the proteasome.

Effects of polyamines, polyamine analogs, and inhibitors of protein synthesis on spermidine-spermine N1-acetyltransferase gene expression

The key polyamine catabolizing enzyme spermidine-spermine N1-acetyltransferase (SSAT) is among the few genes known to be inducible by the natural polyamines. Certain polyamine analogs markedly exaggerate this response and thus provide useful tools for studying the underlying regulatory mechanisms. As shown here, the analog which most potently induces SSAT activity, N1, N11-diethylnorspermine (DENSPM), increases SSAT mRNA in MALME-3M human melanoma cells to a maximum of > 20-fold and immunodetectable SSAT protein to > 300-fold. By comparison, the natural polyamine spermine is far less effective, increasing SSAT mRNA by approximately 3-fold and protein by approximately 7-fold. In particular, the difference in mRNA accumulation by spermine and the analog was shown to be due to differential effects on both gene transcription and mRNA stabilization. Although the analog DENSPM has been regarded as the most potent inducer of SSAT activity and mRNA, we now report that inhibitors of protein synthesis are capable of increasing SSAT mRNA to nearly comparable levels. Inhibitor-induced accumulation in SSAT mRNA was shown to involve increased gene transcription and mRNA stabilization. This suggests that, under basal conditions, SSAT gene expression is suppressed by a labile protein (or proteins). While induction of SSAT mRNA by inhibitors of protein synthesis only occurred at concentrations which blocked protein synthesis, that by DENSPM took place at concentrations which did not. The combination of either protein inhibitor with DENSPM or spermine produced an additive increase in SSAT mRNA. Taken together, these findings suggest the involvement of two separate but possibly converging pathways in the regulation of SSAT mRNA, one mediated by polyamines and their analogs and the other mediated by a labile repressor of SSAT gene transcription and/or mRNA stabilization. In addition to its apparent regulatory importance, induction of SSAT mRNA by inhibitors of protein synthesis represents a potentially useful system for studying the posttranscriptional regulation of this interesting gene.

Differential post-transcriptional control of ornithine decarboxylase and spermidine-spermine N1-acetyltransferase by polyamines

Ornithine decarboxylase (ODC) and spermidine/ spermine N1-acetyltransferase (SSAT) are short-lived polyamine enzymes with rate-limiting roles in controlling polyamine biosynthesis and catabolism, respectively. We have found that treatment of MALME-3M human melanoma cells for 6 h with 10 micrograms/ml cycloheximide (CHX) increases ODC and SSAT mRNA 6-9-fold. When cells containing CHX-induced SSAT mRNA were washed and post-incubated for an additional 6 h in drug free media, enzyme activity increased only 2-fold above that in untreated cells despite the > 6-fold increase in accumulated mRNA. Inclusion of 10 microM spermine or spermidine in the post-incubation medium increased SSAT activity approximately 7-fold without further elevating SSAT mRNA levels. This indicates post-transcriptional regulation which, due to the similarity between polyamine-mediated increases in SSAT activity and available mRNA, probably occurs at the level of mRNA translation. In contrast to the SSAT response, polyamines markedly reduced ODC activity (but not mRNA) to one sixth that in cells not exposed to polyamines. The findings illustrate how via post-transcriptional mechanisms, shifts in intracellular polyamine pools can simultaneously and differentially regulate polyamine biosynthesis and catabolism. It is hypothesized that these post-transcriptional responses enable cells to rapidly and sensitively control intracellular spermidine and spermine pools.

Structure and critical residues at the active site of spermidine/spermine-N1-acetyltransferase

Spermidine/spermine-N1-acetyltransferase (SSAT) is a key enzyme in the degradation of polyamines. Alanine-scanning mutagenesis of all eight arginine residues was used to investigate the arginine residues involved in acetyl-CoA binding. The results indicate that Arg101, Arg142 and Arg143 are important for such binding. The apparent Km values for acetyl-CoA were significantly increased when any one of these residues was replaced by an alanine residue. These mutations also abolished the ability of acetyl-CoA to protect the protein from digestion by trypsin. Co-expression of the inactive R101A (Arg101 --> Ala) mutant and an E152K (Glu152 --> Lys) mutant, previously known to inactivate SSAT, led to restoration of activity, showing that the active enzyme is a dimer with residues contributed by both subunits. The double mutant R101A/E152K acted as a dominant negative when co-expressed with the wild-type SSAT. Transfection of COS-7 cells with a plasmid producing this mutant greatly attenuated the increase in SSAT activity brought about by N1, N12-bis(ethyl)spermine. These results indicate that the double mutant R101A/E152K-SSAT protein can be used to evaluate the importance of SSAT activity in response to exogenous polyamines or polyamine analogues.

Effect of polyamine analogues and inhibition of polyamine oxidase on spermidine/spermine N1-acetyltransferase activity and cell proliferation

Several bis(ethyl)polyamine analogues are currently undergoing trials as antitumor agents. The ability of some of these analogues to induce spermidine/spermine N1-acetyltransferase and to inhibit cell proliferation was examined in a number of different cell lines. Although N1,N11 bis(ethyl)norspermine was a potent inducer of the acetylase in all cell lines tested, there was a striking difference in the acetylase induction in response to N,N'-bis(ethylamino)propyl]-1,7-heptanediamine. This was a very strong inducer in CHO cells but had no effect in HT29 cells and very little effect in COS-7 or L1210 cells. There was no correlation between the induction of the acetylase and the ability of these analogues to inhibit cell proliferation since N1,N11-bis(ethylamino)-propyl]-1,7-heptanediamine was as at least as strongly antiproliferative as N1,N11-bis(ethyl)-norspermine or N1,N12-bis(ethyl)spermine. Acetylase induction and the intracellular level of the analogues were increased in CHO cells by treatment with a polyamine oxidase inhibitor suggesting that they are degraded by polyamine oxidase. The absence of polyamine oxidase in some tumors may therefore contribute to their sensitivity to these analogues.