Nucleotide sequence of hamster spermidine/spermine-N1-acetyltransferase cDNA

Characterization of the SSAT1 gene and its expression profiling in various tissues and follicles in geese

Spermidine/spermine N1-acetyltransferase (SSAT ) is a catabolic regulator of polyamines, ubiquitous molecules essential for cell proliferation and differentiation. In this study, the molecular characterization of the SSAT1 gene of the Sichuan white goose was analyzed, as well as its expression profiles in various follicles and tissues. The open reading frame of the SSAT1 cDNA (GenBank No. KM925008) is 516 bp in length and encodes a 171-amino acid protein with a putative molecular weight of 20 kDa. The predicted SSAT1 protein is highly conserved with those of other species, especially Gallus gallus. SSAT1 mRNA was ubiquitously expressed in all the examined tissues. The highest level of SSAT1 mRNA expression was found in the pineal gland (P<0.05), and was 12-fold greater than in the heart. The level of SSAT1 mRNA expression was relatively lower in preovulatory follicles, while it was higher in postovulatory follicles (POFs), particularly in POF1. Furthermore, as postovulatory follicles degenerated, SSAT1 expression gradually decreased. Our findings suggest that SSAT1 might play important roles in mediating the physiological function of the pineal gland and regulating the regression of POFs.

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.

Properties of recombinant human N1-acetylpolyamine oxidase (hPAO): potential role in determining drug sensitivity

The recent cloning of the mammalian gene coding for N(1)-acetylpolyamine oxidase (PAO) provides the opportunity to directly examine the role of human PAO (hPAO) in polyamine homeostasis as well as its potential role in determining cellular response to antitumor polyamine analogues. To facilitate the study of this enzyme, the production, purification, and characterization of the recombinant hPAO is reported. hPAO oxidizes N(1)-acetylspermidine (K(m)=2.1 microM, K(cat)=15.0 s(-1)) and has very high affinity for N(1)-acetylspermine (K(m)=0.85 microM, K(cat)=31.7 s(-1)). The recombinant hPAO does not efficiently oxidize spermine, thereby demonstrating a significant difference in substrate specificity from the previously described human spermine oxidase PAOh1/SMO. Importantly, hPAO demonstrates the ability to oxidize a subset of antitumor polyamine analogues, suggesting that this oxidase activity could have a significant effect on determining tumor sensitivity to these or similar agents. Transfection of A549 human lung cancer cells with an hPAO-expressing plasmid leads to a profound decrease in sensitivity to those analogues which act as substrates, confirming its potential to alter drug response. One similarity that hPAO shares with human PAOh1/SMO, is that certain oligoamine analogues are potent inhibitors of its oxidase activity. The results of these studies demonstrate how changes in polyamine catabolism may affect drug response.

Altered spermidine/spermine N1-acetyltransferase activity as a mechanism of cellular resistance to bis(ethyl)polyamine analogues

To develop a model system to investigate mechanisms of antiproliferative action of bis(ethyl)polyamine analogues, intermittent analogue treatments followed by recovery periods in drug-free medium were used to select an N(1), N(12)-bis(ethyl)spermine-resistant derivative of the Chinese hamster ovary cell line C55.7. The resulting C55.7Res line was at least 10-fold resistant to N(1),N(12)-bis(ethyl)spermine and N(1), N(11)-bis(ethyl)norspermine. The stability of the resistance in the absence of selection pressure was >/=9 months, indicating that a heritable genotypic change was responsible for the resistance phenotype. Polyamine transport alterations and multi-drug resistance were eliminated as causes of the resistance. Spermidine/spermine N(1)-acetyltransferase (SSAT) activity and regulation were altered in C55.7Res cells as basal activity was decreased, and no activity induction resulted from exposure to analogue concentrations, which caused 300-fold enzyme induction in parental cells. SSAT mRNA levels in the absence and presence of analogue were unchanged, but no SSAT protein was detected in C55.7Res cells. A point mutation, which results in the change leucine156 (a fully conserved residue) to phenylalanine, was identified in the C55.7Res SSAT cDNA. Expression of wtSSAT activity in C55.7Res cells restored sensitivity to bis(ethyl)polyamines. These results provided definitive evidence that SSAT activity is a critical target of the cytotoxic action of these analogues.

Polyamines. An overview

The polyamines spermine, spermidine, and putrescine are small organic molecules one or more of which are present in all living organisms. Many natural products contain polyamine residues. Polyamines are synthesized by a highly regulated pathway from arginine or ornithine and also can be transported in and out of cells. Polyamines are degraded to a variety of compounds the functions of which are largely unknown. Polyamines influence the transcriptional and translational stages of protein synthesis, stabilize membranes, and, in mammalian systems, modulate neurophysiological functions and may act as intracellular messengers. However, at the molecular level the mode of action of the polyamines is largely unknown.

Gossypol induces spermidine/spermine N1-acetyltransferase in canine prostate epithelial cells

Gossypol is an antisteroidogenic compound naturally found in cottonseed. Gossypol has been shown to inhibit steroidogenesis in the canine prostate and may inhibit canine prostate growth. Its mechanism of action, however, is largely unknown. Our laboratory has previously demonstrated that in vivo administration of gossypol to male dogs can reduce circulating levels of testosterone and estradiol. Gossypol also showed an ability to reduce prostate weights. To search for genes regulated by gossypol in the canine prostate, differential display RT-PCR was performed on total RNAs isolated from control and gossypol-treated male dogs. Gossypol was demonstrated to induce expression of spermidine/spermine-N1-acetyl-transferase (SSAT), the major catabolic enzyme for polyamines. This induction was confirmed by Northern hybridization analysis of total RNA isolated from prostates of mature dogs treated with gossypol for 2 months. Gossypol was also shown to inhibit the progression of cells into the S phase mediated by spermidine. Our findings support the notion that gossypol can inhibit prostate cell proliferation and may be a potential therapeutic agent for use in controlling overgrowth of the prostate.

RGFGIGS is an amino acid sequence required for acetyl coenzyme A binding and activity of human spermidine/spermine N1acetyltransferase

Polyamine catabolism is rate limited by spermidine/spermine N1-acetyltransferase (SSAT). Although the amino acid sequence of SSAT is known, the substrate binding and catalytic sites are not. The goal of this study was to define the region responsible for acetyl coenzyme A binding. Human SSAT contains a region of 20 amino acids homologous to several microbial antibiotic N-acetyltransferases. The highest homology is represented in the Campylobacter coli streptothricin acetyltransferase sat4 gene, where 16 identical or highly conserved amino acids exist in a 20-residue stretch. The most conserved residues within this region are RGFGIGS beginning at Arg-101 in the human SSAT. Site-directed mutations to Arg-101, Gly-104, and Gly-106 resulted in proteins with no measurable activity. The G102D mutation produced a partially active protein with a decreased affinity for acetyl coenzyme A and with a Km >10-fold that of the wild-type protein. Analysis using the PredictProtein program suggests a common structure among the microbial and eukaryotic N-acetyltransferases in the region corresponding to the RGFGIGS of human SSAT consisting of an alpha-helix usually preceded by a glycine loop. Our data are consistent with the hypothesis that Arg-101 and the proximal glycine loop are necessary for the activity of human SSAT.

Spermidine/spermine N1-acetyltransferase mRNA levels show marked and region-specific changes in the early phase after transient forebrain ischemia

Considerable evidence points to an involvement of natural polyamines (putrescine, spermidine and spermine) in trophic regulation of brain tissue. Spermidine/spermine N1-acetyltransferase is the key enzyme in the interconversion pathway which leads to the formation of spermidine and putrescine from spermine and spermidine, respectively. In the present paper we have studied using in situ hybridization histochemistry the levels of spermidine/spermine N1-acetyltransferase mRNA in the rat central nervous system after transient forebrain ischemia. In the first hours after the insult, a modest increase in spermidine/spermine N1-acetyltransferase mRNA levels was observed in ependymal cells and other non-neuronal cells of all telencephalic and diencephalic regions. In addition, major increases in spermidine/spermine N1-acetyltransferase mRNA levels were observed in regions selectively vulnerable to the ischemic insult, such as striatum, hippocampus and cerebral cortex, during the first day post-reperfusion. The time course and extent of labelling increase were subregion- and cell-specific. At the cellular level, the labelling appeared markedly increased in neurons (8-10 fold in ventromedial striatum and CA1 region) and, to a lesser extent, in non-neuronal cells. The increase in SSAT mRNA levels was not directly related to cell degeneration, as it was detected in both some vulnerable and some resistant cell populations. However, the peak increase of SSAT labelling was precocious in resistant neurons (such as those of ventromedial striatum and dentate gyrus granular layer) and delayed or very limited in vulnerable neurons (such as those of CA1 pyramidal layer and dorsolateral striatum). The increase in spermidine/spermine N1-acetyltransferase may contribute to the increase in putrescine and decrease in spermidine levels observed after ischemia and gives further support to the notion that polyamine metabolism in the early phase after lesion is oriented towards putrescine production. This phenomenon could be relevant in determining the prevalence of neurotrophic vs. neurotoxic effects of polyamines.

Regional and cellular distribution of spermidine/spermine N1-acetyltransferase (SSAT) mRNA in the rat central nervous system

Spermidine/spermine N1-acetyltransferase (SSAT) is the key enzyme responsible for polyamine interconversion. SSAT mRNA (visualized by in situ hybridization histochemistry) was shown to have a wide but heterogeneous distribution in the central nervous system (CNS) at both regional and cellular levels. The highest labelling was observed in hippocampus (pyramidal and polymorph neurons) and olfactory bulb. Present data suggest that polyamine metabolism in the CNS is not homogeneous but rather that the preferential production of a polyamine species is region- and cell type-specific.

Different localization of spermidine/spermine N1-acetyltransferase and ornithine decarboxylase transcripts in the rat kidney

In situ hybridization histochemistry of transverse sections from male rat kidney showed that the mRNA of the regulatory enzyme of polyamine degradation, spermidine/spermine N1-acetyltransferase, has a spotty distribution in the cortex, is low and diffused in the outer stripe and high and diffused in the inner stripe of the outer medulla. At the cellular level, this mRNA is solely expressed by the epithelium of the distal straight and convoluted nephron tubules. Since biosynthetic ornithine decarboxylase mRNA is solely found in the proximal straight tubules, it is proposed that polyamine biosynthesis and degradation occur at separate sites along the nephron.

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

Spermidine/spermine N1-acetyltransferase (SSAT) is the rate-limiting enzyme for the degradation and excretion of polyamines in mammalian cells, and its activity is known to be increased enormously on exposure to polyamines and polyamine analogues. The mechanism by which such an analogue, BESM [N1N12-bis(ethyl)spermine], increases the content of SSAT was investigated by transfecting COS-7 cells with plasmids containing SSAT cDNA in the pEUK expression vector. Despite a large increase in mRNA production, there was only a very small increase in SSAT activity in the transfected cells. When BESM was added at 36 h after transfection, there was a large and very rapid increase in SSAT protein amounting to 380-fold in 12 h without any increase in the mRNA. SSAT protein turned over very rapidly, with a half-life of about 20 min. In the presence of BESM, this turnover was greatly reduced, and the half-life increased to more than 13 h. However, this increase was not sufficient to account for all of the increase in SSAT protein, suggesting that there is also regulation of the translation of the mRNA by BESM. Further evidence for such translation regulation was obtained by studying the polysomal distribution of the SSAT mRNA. In the absence of BESM, most of the mRNA was present in fractions which sedimented more slowly than the monoribosome peak. In BESM-treated cells, a significant proportion of the SSAT mRNA was moved into the small-polysome region of the gradient. The expression of SSAT and the effects of BESM on the polysomal distribution of SSAT mRNA were not affected by the 5'- or 3'-untranslated regions of the mRNA, since constructs which lacked all of these regions gave similar results to constructs containing the entire mRNA sequence. These results show that the increased transcription of the SSAT gene that occurs in the presence of polyamine analogues such as BESM is not sufficient for SSAT expression and that post-transcriptional regulation is critical for the control of SSAT content.

Cloning and sequence analysis of the gene and cDNA encoding mouse spermidine/spermine N1-acetyltransferase--a gene uniquely regulated by polyamines and their analogs

The polyamine catabolizing enzyme, spermidine/spermine N1-acetyltransferase (SSAT), has been implicated as a critical determinant of polyamine pool maintenance. SSAT has recently been shown to be positively regulated in human cell lines by polyamines and their analogs at the level of mRNA accumulation. Mouse LA-4 lung adenoma cells treated with either spermine or the spermine analog, N1,N12-bis(ethyl)spermine, produced a 2.3 and 6.5-fold increase, respectively, in SSAT mRNA. Prior evidence for transcriptional control of the enzyme prompted investigation of SSAT gene structure and its regulatory elements. The mouse SSAT gene was isolated as a 3650 bp EcoRI fragment from a lambda-J1 Mus saxicola genomic library by hybridization with human SSAT cDNA. An additional 431 bp downstream from the 3' EcoRI site were sequenced from a BamHI fragment (total gene sequence, 4066 bp). The gene contains six exons and five introns. Sequence analysis of the 774 bp of the 5' non-coding region revealed the absence of TATAA or CCAAT sequence motifs and the presence of a number of binding motifs in the 5' region of the gene with consensus binding sequences for transcription factors SP1, AP1, E2F, AP2, PEA-3 and others. The deduced amino acid sequence of the coding region differs from that of the human SSAT cDNA by five amino acids. The 527 bp of the 3' non-coding region contains four possible polyadenylation signal sites of which only one displays a typical consensus sequence. A 940 bp SSAT cDNA was isolated from Mus domesticus (BALB-C) liver lambda gt11 cDNA library. It contains a 5' untranslated region 89 bp in length and a 3' untranslated region 376 bp in length. The amino acid sequence deduced from Mus domesticus differs from that of Mus saxicola by one amino acid, from the hamster cDNA, by four amino acids and from the human cDNA by six amino acids. Further elucidation of the structural features of the SSAT gene may reveal how it is positively regulated by polyamines and their analogs.

Stress induction of the spermidine/spermine N1-acetyltransferase by a post-transcriptional mechanism in mammalian cells

Heat shock and diethyldithiocarbamate stimulate polyamine catabolism in animal cells by a mechanism involving the induction of spermidine/spermine N1-acetyltransferase (N1-SSAT) activity. Steady-state levels of RNA encoding this enzyme remain essentially unchanged during periods after these stresses when N1-SSAT activity is increased by 3.5-10-fold or more in three different cell lines of hamster and human origin. Depletion of intracellular spermidine pools by alpha-difluoromethylornithine (DFMO) inhibits stress induction of N1-SSAT activity. Exogenous spermidine can restore stress inducibility of N1-SSAT to DFMO-treated cells, and induce this enzyme activity in non-heat-shocked but polyamine-depleted cells. Acetylation at N1 suppresses the ability of spermidine to induce N1-SSAT activity, relative to this same modification at N8. Fluorinated spermidine analogues, which decrease the pKa values of the amine groups at positions 4 and 8, neither induce nor inhibit N1-SSAT activity in DFMO-treated cells. These data demonstrate that certain stresses induce N1-SSAT by a spermidine-dependent post-transcriptional mechanism. The mode of induction is affected by both the propyl and butyl moieties of spermidine.

Regulation of spermidine/spermine N1-acetyltransferase by intracellular polyamine pools. Evidence for a functional role in polyamine homeostasis

Through its role in polyamine acetylation and the back-conversion pathway, spermidine/spermine N1-acetyltransferase (SSAT) has the potential to control intracellular polyamine pools by facilitating their catabolism and/or excretion. The possibility that the enzyme is subject to regulation by intracellular polyamine pools was investigated in MALME-3 human melanoma cells. Increases in intracellular polyamine pools by treatment with 3 microM exogenous spermidine or spermine for 48 h caused SSAT activity to increase 111% and 226%, respectively, and SSAT-specific mRNA to rise 19% and 66%, respectively. Decreases in polyamine pools by treatment with inhibitors of polyamine biosynthesis caused SSAT activity to decrease by 46% and mRNA to fall by 89%. Both SSAT activity and mRNA were more sensitive to changes in spermine than spermidine. The identification of a positive regulatory relationship between SSAT and intracellular polyamine pools further implicates this enzyme in a proposed model for polyamine pool homeostasis.