Structure of the human spermidine/spermine N1-acetyltransferase gene (exon/intron gene organization and localization to Xp22.1)

Salivary RNA signatures in oral cancer detection

Oral squamous cell carcinomas (OSCC) are common malignancies that affect almost a million people every year. The key issue in reducing mortality and morbidity associated with OSCC is to develop novel strategies to identify OSCC at an early stage. One such strategy is the identification of biomarkers. So far, more than 100 biomarkers are recognized in the detection of oral cancer and they range from proteins to nucleic acids (DNAs, RNAs). Detection of ribose nucleic acids in saliva is a recent trend in diagnosing oral cancer. Studies have shown statistically significant changes in the levels of salivary transcriptomes in patients with oral squamous cell carcinomas. These biomarkers have displayed high sensitivity and specificity. Also, new point-of-care platforms such as oral fluid nanosensor test are now available that will soon emerge as chair-side tools for early detection of oral cancer. The aim of this review is to highlight the importance of salivary transcriptomes in oral cancer detection.

Lack of evidence for the association of ornithine decarboxylase (+316 G>A), spermidine/spermine acetyl transferase (-1415 T>C) gene polymorphisms with calcium oxalate stone disease

Urolithiasis is a complex and multifactorial disorder characterized by the presence of stones in the urinary tract. Urea cycle is an important process involved in disease progression. L-ornithine is a key amino acid in the urea cycle and is converted to putrescine by ornithine decarboxylase (ODC). Putrescine, spermidine and spermine are natural polyamines that are catabolized by a specific enzyme, spermidine/spermine acetyltransferase (SSAT). The single-nucleotide polymorphisms (SNPs) in the intron region of ODC (+316 G>A) and promoter region of SSAT (-1415 T>C) genes have been found to be associated with the polyamines expression levels. The aim of this study was to examine whether the ODC (+316 G>A) intron 1 region gene polymorphism and SAT-1 promoter region (-1415 T>C) gene polymorphisms are potential genetic markers for susceptibility to urolithiasis. A control group of 104 healthy subjects and a group of 65 patients with recurrent idiopathic calcium oxalate stone disease were enrolled into this study. Polymerase chain reaction (PCR)-based restriction analysis was performed for the ODC intron 1 (+316 G>A) region and SAT-1 (-1415 T>C) promoter gene polymorphisms by PstI and MspI restriction enzyme digestion, respectively. The genotype distribution of polymorphisms studied in the ODC intron 1 region (+316 G>A) and SAT-1 -1415 T>C promoter region did not reveal a significant difference between urolithiasis and the control groups (P=0.713 and 0.853), respectively. Furthermore, no significant difference was observed between the control and patient groups for ODC +316 G>A and SAT-1 -1415 T>C allele frequencies (P=0.877 and 0.644), respectively. In conclusion, results of the present study suggest that ODC + 316 G>A and SAT-1 -1415 T>C gene polymorphisms might not be a risk factor for urolithiasis.

Epigenetic regulation of spermidine/spermine N1-acetyltransferase (SAT1) in suicide

We have recently shown that the expression of spermidine/spermine N1-acetyltransferase (SAT1) is downregulated across the brains of suicide completers, and that its expression is influenced by genetic variations in the promoter. Several promoter polymorphisms in SAT1, including rs6526342, have been associated with suicide and other psychiatric disorders, and display haplotype-specific effects on expression. However, these effects cannot explain total variability in SAT1 expression, and other regulatory mechanisms, such as epigenetic factors, may also be at play. In this study, we assessed the involvement of epigenetic factors in controlling SAT1 expression in the prefrontal cortex of suicide completers by mapping CpG methylation across a 1880-bp region of the SAT1 promoter, and measuring levels of tri-methylated histone-3-lysine 27 (H3K27me3) at the promoter in suicide completers and controls. Our results demonstrated that CpG methylation was significantly negatively correlated with SAT1 expression. Although overall or site-specific CpG methylation was not associated with suicide or SAT1 expression, we observed high levels of methylation at the polymorphic CpG site created by rs6526342, indicating a relationship between promoter haplotypes and methylation. There was no association between H3K27me3 and suicide, nor was this modification associated with SAT1 expression. Overall, our results indicate that epigenetic factors in the promoter region of SAT1 influence gene expression levels, and may provide a mechanism for both our previous findings of haplotype-specific effects of promoter variations on SAT1 expression, as well as the widespread downregulation of SAT1 expression observed in the brains of suicide completers.

Pretreatment with anti-oxidants sensitizes oxidatively stressed human cancer cells to growth inhibitory effect of suberoylanilide hydroxamic acid (SAHA)

PURPOSE

Most prostate, colon and breast cancer cells are resistant to growth inhibitory effects of suberoylanilide hydroxamic acid (SAHA). We have examined whether the high oxidative stress in these cells causes a loss of SAHA activity and if so, whether pretreatment with an anti-oxidant can sensitize these cells to SAHA.

METHODS

A DNA-Hoechst dye fluorescence measured cell growth and dichlorfluorescein-diacetate (DCF-DA) dye fluorescence measured reactive oxygen species (ROS). Growth inhibitory and ROS-generating activities of SAHA in androgen-treated or untreated LNCaP cells and PC-3 prostate cancer cells, HT-29 and HCT-115 colon cancer cells, MDA-MB231 breast cancer cells and A549 and NCI-H460 lung cancer cells with or without pretreatment with an anti-oxidant Vitamin E was determined. SAHA activity against LNCaP cells treated with another anti-oxidant N-acetyl cysteine (NAC) was also determined. Liquid chromatography-mass spectrometry (LC-MS) was used to determine intracellular SAHA level.

RESULTS

SAHA treatment markedly inhibits LNCaP cell growth, when the cells are at a low ROS level. SAHA is, however, inactive against the same cell line, when the cells are at a high ROS level. A significant decrease in SAHA level was observed in LNCaP cells with high ROS after 24- and 72-h treatment when compared to cells with low ROS. Vitamin E pretreatment that reduces cellular ROS, synergistically sensitizes oxidatively stressed LNCaP, PC-3, HT-29, HCT-115 and MDA-MB231 cells, but not the A-549 and NCI-H460 cells with low ROS to SAHA. NAC treatment also sensitized androgen-treated LNCaP cells to the growth inhibitory effects of SAHA.

CONCLUSION

Response to SAHA could be improved by combining anti-oxidants such as Vitamin E with SAHA for the treatment of oxidatively stressed human malignancies that are otherwise resistant to SAHA.

Association of the SAT1 in/del polymorphism with suicide completion

Several studies have observed decreased expression of spermidine/spermine N1-acetyltransferase (SAT1) in the brains of suicide completers, and we previously identified a single-nucleotide polymorphism in the promoter region of SAT1 which was associated with suicide completion and SAT1 expression in the brain. We recently characterized the haplotype structure of the SAT1 promoter region and identified an insertion/deletion (in/del) of 15 adenine residues. This variant appears to be a predictor of SAT1 expression, and we were thus interested in determining if the lower expressing deletion allele was found more frequently among suicide completers. To this end, we genotyped the in/del in a sample of 771 French-Canadian males, comprising 326 suicide completers and 445 non-suicide controls. We found no significant difference in the frequencies of the two alleles between suicide completers and controls in the entire sample. However, we observed a significantly higher frequency of the deletion in the depressed suicide completers compared to the depressed non-suicides. These results add support for a role of SAT1 in conferring a risk for suicide completion, in particular in the context of depressive disorders.

A small molecule polyamine oxidase inhibitor blocks androgen-induced oxidative stress and delays prostate cancer progression in the transgenic adenocarcinoma of the mouse prostate model

High levels of reactive oxygen species (ROS) present in human prostate epithelia are an important etiologic factor in prostate cancer (CaP) occurrence, recurrence, and progression. Androgen induces ROS production in the prostate by a yet unknown mechanism. Here, to the best of our knowledge, we report for the first time that androgen induces an overexpression of spermidine/spermine N1-acetyltransferase, the rate-limiting enzyme in the polyamine oxidation pathway. As prostatic epithelia produce a large excess of polyamines, the androgen-induced polyamine oxidation that produces H2O2 could be a major reason for the high ROS levels in the prostate epithelia. A small molecule polyamine oxidase inhibitor N,N'-butanedienyl butanediamine (MDL 72,527 or CPC-200) effectively blocks androgen-induced ROS production in human CaP cells, as well as significantly delays CaP progression and death in animals developing spontaneous CaP. These data show that polyamine oxidation is not only a major pathway for ROS production in prostate, but inhibiting this pathway also successfully delays CaP progression.

Identification and characterization of spermidine/spermine N1-acetyltransferase promoter variants in suicide completers

BACKGROUND

We have previously shown that the expression of spermidine/spermine N1-acetyltransferase (SAT1) is decreased in the brain Brodmann areas (BA)4, BA8/9, and BA11 of suicide completers and found an association between rs6526342, a SAT1 promoter single nucleotide polymorphism (SNP), with suicide completion (1).

METHODS

We genotyped 18 promoter polymorphisms in SAT1 in a French-Canadian population. The relationship between haplotypes and gene expression was assessed with microarray analysis of three brain regions as well as reporter gene assays in three cell lines. Site-directed mutagenesis was used to examine the role of individual polymorphisms in reporter gene expression.

RESULTS

We identified two major and several minor haplotypes in the promoter region of SAT1. Subjects who possessed the haplotype containing the risk allele for rs6526342 demonstrated decreased SAT1 expression in BA4, BA8/9, and BA11. This haplotype was also associated with decreased expression in reporter gene assays. Site-directed mutagenesis identified three polymorphisms-an insertion/deletion (rs6151267), and two SNPs (rs6526342 and rs928931)-that were involved in determining reporter gene expression. These polymorphisms do not seem to exert their effects through the polyamine responsive element, because all constructs were induced to a similar extent in the presence of spermine.

CONCLUSIONS

Our results indicate that genetic variations in the promoter region of SAT1 are involved in determining levels of gene expression and might provide a mechanism for the decreased SAT1 expression observed in suicide completers.

Positive association between SAT-1 -1415T/C polymorphism and anxiety

Limbic glutamatergic neurotransmission plays a pivotal role in the pathogenesis of anxiety disorders. Polyamines modulate the activity of several ionotropic glutamate receptors and have been involved in the regulation of fear-conditioning response. Spermidine/spermine N1-acetyltransferase (SSAT-1) is the main enzyme regulating polyamine catabolism. The aim of the present study was to examine the association between anxiety disorders and the -1415T/C (rs1960264) single nucleotide polymorphism (SNP) of the gene (SAT1) coding for SSAT-1. A case-control design was used in order to compare the genotypes for the -1415T/C (rs1960264) SNP between anxiety patients (n = 218), other non-anxiety psychiatric patients (n = 362), and healthy controls (n = 251). DSM-IV diagnoses were provided using MINI 4.4. Genomic DNA was extracted from peripheral blood samples collected from participants. In males, there was a significant difference in the distribution of the two genotypes (T and C) for the SAT-1 -1415T/C SNP between anxiety patients, non-anxiety psychiatric controls, and healthy controls. The T genotype was significantly more frequent in males suffering from anxiety disorders than in male psychiatric controls and healthy controls. This is the first study linking polymorphic variants of genes involved in polyamine metabolism with anxiety disorders.

SAT-1 -1415T/C polymorphism and susceptibility to schizophrenia

Patients suffering from psychosis show increased blood and fibroblast total polyamine levels. Spermidine/spermine N1-acetyltransferase (SSAT-1) and its coding gene (SAT-1) are the main factors regulating polyamine catabolism. The aim of the present study was to examine the association between the SAT-1 -1415T/C single nucleotide polymorphism (SNP) and schizophrenia. A case-control design was used in order to compare the genotypes for the SNP between schizophrenia patients (n=180, 83 females and 97 males), other non-psychotic psychiatric patients (n=413, 256 females and 157 males), and healthy controls (n=251, 101 females and 150 males). No significant differences in the distribution of the genotypes of the SAT-1 -1415T/C SNP were found groups among groups. We failed to demonstrate a significant association between the SAT-1 -1415T/C SNP and schizophrenia, but a mild association between allele C and psychopathology was found in the female group.

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.

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.

Tumor Necrosis Factor α Induces Spermidine/Spermine N1-Acetyltransferase through Nuclear Factor κBin Non-small Cell Lung Cancer Cells

Tumor necrosis factor alpha (TNFalpha) is a potent pleiotropic cytokine produced by many cells in response to inflammatory stress. The molecular mechanisms responsible for the multiple biological activities of TNFalpha are due to its ability to activate multiple signal transduction pathways, including nuclear factor kappaB (NFkappaB), which plays critical roles in cell proliferation and survival. TNFalpha displays both apoptotic and antiapoptotic properties, depending on the nature of the stimulus and the activation status of certain signaling pathways. Here we show that TNFalpha can lead to the induction of NFkappaB signaling with a concomitant increase in spermidine/spermine N(1)-acetyltransferase (SSAT) expression in A549 and H157 non-small cell lung cancer cells. Induction of SSAT, a stress-inducible gene that encodes a rate-limiting polyamine catabolic enzyme, leads to lower intracellular polyamine contents and has been associated with decreased cell growth and increased apoptosis. Stable overexpression of a mutant, dominant negative IkappaBalpha protein led to the suppression of SSAT induction by TNFalpha in these cells, thereby substantiating a role of NFkappaB in the induction of SSAT by TNFalpha. SSAT promoter deletion constructs led to the identification of three potential NFkappaB response elements in the SSAT gene. Electromobility shift assays, chromatin immunoprecipitation experiments and mutational studies confirmed that two of the three NFkappaB response elements play an important role in the regulation of SSAT in response to TNFalpha. The results of these studies indicate that a common mediator of inflammation can lead to the induction of SSAT expression by activating the NFkappaB signaling pathway in non-small cell lung cancer cells.

Induction of spermidine/spermine N1-acetyltransferase (SSAT) by aspirin in Caco-2 colon cancer cells

Epidemiological, experimental and clinical results suggest that aspirin and other NSAIDs (non-steroidal anti-inflammatory drugs) inhibit the development of colon cancer. It has been shown that the NSAID sulindac induces apoptosis and suppresses carcinogenesis, in part, by a mechanism leading to the transcriptional activation of the gene encoding SSAT (spermidine/spermine N1-acetyltransferase), a rate-limiting enzyme in polyamine catabolism. In the present study, we show that a variety of NSAIDs, including aspirin, sulindac, ibuprofen and indomethacin, can induce SSAT gene expression in Caco-2 cells. Aspirin, at physiological concentrations, can induce SSAT mRNA via transcriptional initiation mechanisms. This induction leads to increased SSAT protein levels and enzyme activity. Promoter deletion analysis of the 5' SSAT promoter-flanking region led to the identification of two NF-kappaB (nuclear factor kappaB) response elements. Electrophoretic mobility-shift assays showed binding of NF-kappaB complexes at these sequences after aspirin treatment. Aspirin treatment led to the activation of NF-kappaB signalling and increased binding at these NF-kappaB sites in the SSAT promoter, hence providing a potential mechanism for the induction of SSAT by aspirin in these cells. Aspirin-induced SSAT ultimately leads to a decrease in cellular polyamine content, which has been associated with decreased carcinogenesis. These results suggest that activation of SSAT by aspirin and different NSAIDs may be a common property of NSAIDs that plays an important role in their chemopreventive actions in colorectal cancer.

Heat shock protein 10 and signal transduction: a "capsula eburnea" of carcinogenesis?

To date, little is known either about the physical interactions of heat shock protein 10 (Hsp10) with other proteins within the cell or its involvement in signal transduction pathways. Hsp10 has been considered mainly as a partner of Hsp60 in the Hsp60/10 protein folding machine. Only recently, Hsp10 was reported to interact with proteins involved in deoxyribonucleic acid checkpoint inactivation, termination of M-phase, messenger ribonucleic acid export, import of nuclear proteins, nucleocytoplasmic transport, and pheromone signaling pathways. At the same time, Hsp10 expression can be up-regulated in cancer cells, because it accumulates as the cell transformation progresses. Recent data suggest that Hsp10 may be not only a component of the folding machine but also an active player of the cell signaling network, influencing cell cycle, nucleocytoplasmic transport, and metabolism, with putative roles in the lack of cell differentiation and in the inhibition of apoptosis. In this review, we revise the involvement of Hsp10 in signal transduction pathways and its possible role in cancer etiology.

Spermidine/spermine-N1-acetyltransferase-2 (SSAT2) acetylates thialysine and is not involved in polyamine metabolism

Spermidine/spermine-N1-acetyltransferase (SSAT1) is a short-lived polyamine catabolic enzyme inducible by polyamines and polyamine analogues. Induction of SSAT1 plays an important role in polyamine homoeostasis, since the N1-acetylated polyamines can be excreted or oxidized by acetylpolyamine oxidase. We have purified a recombinant human acetyltransferase (SSAT2) that shares 45% identity and 61% homology with human SSAT1, but is only distally related to other known members of the GNAT (GCN5-related N-acetyltransferase) family. Like SSAT1, SSAT2 is widely expressed, but did not turn over rapidly, and levels were unaffected by treatments with polyamine analogues. Despite similarity in sequence to SSAT1, polyamines were found to be poor substrates of purified SSAT2, having K(m) values in the low millimolar range and kcat values of <0.01 s(-1). The kcat/K(m) values for spermine and spermidine for SSAT2 were <0.0003% those of SSAT1. Expression of SSAT2 in NIH-3T3 cells was not detrimental to growth, and did not reduce polyamine content or increase acetylpolyamines. These results indicate that SSAT2 is not a polyamine catabolic enzyme, and that polyamines are unlikely to be its natural intracellular substrates. A promising candidate for the physiological substrate of SSAT2 is thialysine [S-(2-aminoethyl)-L-cysteine], which is acetylated predominantly at the epsilon-amino group with K(m) and kcat values of 290 muM and 5.2 s(-1). Thialysine is a naturally occurring modified amino acid that can undergo metabolism to form cyclic ketimine derivatives found in the brain and as urinary metabolites, which can undergo further reaction to form antioxidants. SSAT2 should be renamed 'thialysine N(epsilon)-acetyltransferase', and may regulate this pathway.

Induction of a SSAT isoform in response to hypoxia or iron deficiency and its protective effects on cell death

Spermidine/spermine N(1)-acetyltransferase (SSAT) is the key enzyme with regard to the maintenance of intracellular polyamine levels. It is an inducible enzyme, which may participate in adaptive responses to environmental stress. However, little is known regarding its responses to oxygen or nutrient deficiencies. Using microarray assays, we discovered that SSAT was enhanced under both oxygen- and iron-deficient conditions. However, RT-PCR revealed that the SSAT mRNA was not induced; rather, an mRNA variant was newly expressed. In this variant, the splicing-in of 110 bases induces early termination, generating a truncated isoform which lacks catalytic motifs. The variant expression occurs in other cancer cells and was irrelevant to both hypoxia-inducible factor 1 and to the redox state. We attempted to determine its role, using stable cell-lines. The expressed isoform was found to promote cell survival under iron-deficient conditions and blocked the cleavage of poly(ADP-ribose) polymerase. This isoform may contribute to the progression of tumors of a more malignant phenotype under poor conditions and may constitute a potential target for anticancer therapy.

Polyamine catabolism in platinum drug action: Interactions between oxaliplatin and the polyamine analogue N1,N11-diethylnorspermine at the level of spermidine/spermine N1-acetyltransferase

A great deal of experimental evidence connects induction of polyamine catabolism via spermidine/spermine N1-acetyltransferase (SSAT) to antiproliferative activity and apoptosis. Following our initial observation from gene expression profiling that platinum drugs induce SSAT, we undertook this present study to characterize platinum drug induction of SSAT and other polyamine catabolic enzymes and to examine how these responses might be enhanced with the well-known inducer of SSAT and clinically relevant polyamine analogue, N1,N11-diethylnorspermine (DENSPM). The results obtained in A2780 ovarian cancer cells by real-time quantitative RT-PCR and Northern blot analysis show that a 2-hour exposure of A2780 cells to platinum drugs induces expression of SSAT, a second SSAT (SSAT-2), spermine oxidase, and polyamine oxidase in a dose-dependent manner. At equitoxic doses, oxaliplatin is more effective than cisplatin in SSAT induction. The most affected enzyme, SSAT, increased 15-fold in mRNA expression and 2-fold in enzyme activity. When combined with DENSPM to further induce SSAT and to enhance conversion of mRNA to activity, oxaliplatin increased SSAT mRNA 50-fold and activity, 210-fold. Polyamine pools declined in rough proportion to levels of SSAT induction. At pharmacologically relevant oxaliplatin exposure times (20 hours) and drug concentrations (5 to 15 μmol/L), these responses were increased even further. Combining low-dose DENSPM with oxaliplatin produced a greater than additive inhibition of cell growth based on the sulforhodamine-B assay. Taken together, the findings confirm potent induction of polyamine catabolic enzymes, such as SSAT by platinum drugs, and demonstrate that these biochemical responses as well as growth inhibition can be potentiated by co-treatment with the polyamine analogue DENSPM. With appropriate in vitro and in vivo optimization, these findings could lead to clinically relevant therapeutic strategies.

Suppression of polyamine catabolism by activated Ki-ras in human colon cancer cells

An activated Ki-ras was expressed in the human colon adenocarcinoma cell line Caco-2 to study the effects of Ki-ras oncogene on polyamine metabolism during gastrointestinal tumorigenesis. Multiple clones selected for expression of the mutant Ki-ras transgene displayed a suppression of transcription of a key catabolic enzyme in polyamine catabolism spermidine/spermine N1-acetyltransferase (SSAT). Gene expression analysis, with cDNA microarrays, showed that Ki-ras transfected clones had decreased levels of expression, compared to mock transfected cells, of peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the nuclear hormone receptor family and an important regulator of cell proliferation and differentiation. The activated Ki-ras suppressed SSAT expression by a mechanism involving the PPARgamma response element (PPRE) located at +48 bp relative to the transcription start site of the SSAT gene. Transient expression of the PPARgamma protein in Ki-ras expressing Caco-2 clones, or treatment with the PPARgamma ligand ciglitazone, led to an increase in the SSAT promoter activity. A MEK1/2 inhibitor PD98059 induced transcription of both PPARgamma and SSAT genes in the activated Ki-ras clones, suggesting that the mitogen-activated protein kinases (MAPKs) were involved in the regulation of SSAT expression by PPARgamma. We concluded that mutated Ki-ras suppressed SSAT via a transcriptional mechanism involving the PPARgamma signaling pathway.

Novel perspective: focusing on the X chromosome in reproductive cancers

In an XX female, one of the two X chromosomes has been inactivated during early embryonic life to achieve a compensation of X-linked gene products between males and females, leaving only one allele of X-linked genes functional. There are some X-linked genes escaping the X-inactivation, i.e., being expressed from both alleles. Escape from X-inactivation varies at different levels; some genes have both alleles active in some women but only one allele active in others, whereas some other genes have both alleles active in neoplastic tissue but only one allele active normally. The X-inactivation may be considered functionally equivalent to a loss of heterozygosity (LOH) for some genes, whereas escape from X-inactivation may be equivalent to functional gene amplification for others. The physiological LOH may make X-linked tumor suppressor genes lose their function more easily, compared with autosomal tumor suppressor genes, thus predisposing women to cancer formation more easily. Moreover, the human X chromosome contains many genes related to cancer or to sex and reproduction. All these properties of the X chromosome suggest that it may play more important roles than any autosomal chromosome in the development and progression of reproductive and urologic cancers.

Cyclooxygenase-independent induction of apoptosis by sulindac sulfone is mediated by polyamines in colon cancer

Sulindac, a non-steroidal anti-inflammatory prodrug, is metabolized into pharmacologically active sulfide and sulfone derivatives. Sulindac sulfide, but not sulindac sulfone, inhibits cyclooxygenase (COX) enzyme activities, yet both derivatives have growth inhibitory effects on colon cancer cells. Microarray analysis was used to detect COX-independent effects of sulindac on gene expression in human colorectal cells. Spermidine/sperm-ine N1-acetyltransferase (SSAT) gene, which encodes a polyamine catabolic enzyme, was induced by clinically relevant sulindac sulfone concentrations. Northern blots confirmed increased SSAT RNA levels in these colon cancer cells. Deletion analysis and mutational studies were done to map the sulindac sulfone-dependent response sequences in the SSAT 5'-flanking sequences. This led us to the identification of two peroxisome proliferator-activated receptor (PPAR) response elements (PPREs) in the SSAT gene. PPRE-2, at +48 bases relative to the transcription start site, is required for the induction of SSAT by sulindac sulfone and is specifically bound by PPAR gamma in the Caco-2 cells as shown by transfection and gel shift experiments. PPRE-1, at-323 bases relative to the start site, is not required for the induction of SSAT by sulindac sulfone but can be bound by both PPAR delta and PPAR gamma. Sulindac sulfone reduced cellular polyamine contents in the absence but not in the presence of verapamil, an inhibitor of the export of monoacetyl diamines, inhibited cell proliferation and induced apoptosis. The induced apoptosis could be partially rescued by exogenous putrescine. These data suggest that apoptosis induced by sulindac sulfone is mediated, in part, by the COX-independent, PPAR-dependent transcriptional activation of SSAT, leading to reduced tissue polyamine contents in human colon cancer cells.

Expression of spermidine/spermine N1-acetyltransferase in HeLa cells is regulated by amino acid sufficiency

The effect of amino acids on the regulation of the expression of spermidine/spermine N(1)-acetyltransferase (SSAT), the key enzyme of polyamine catabolism, was studied in HeLa cells. When compared with similar exposure to complete medium, deprivation of arginine, methionine or leucine gave rise to a time-dependent, slowly reversible increase in the cellular level of SSAT mRNA that started to be significant after 8, 12 or 16h and reached four-, five- and two-fold after 16h, respectively. Experiments utilizing (i) constructs containing fragments of the SSAT promoter linked to a luciferase reporter gene or (ii) actinomycin D (Act-D)-treated cells indicated that the increase in the SSAT mRNA level was due to an augmentation in gene transcription and message stability after omission of one of the polyamine precursor amino acids. By contrast, SSAT mRNA stabilisation was only observed when leucine was the omitted amino acid. Amino acid deprivation was also found to cause increased intracellular activity of SSAT concurrent with changes in the cell polyamine content, namely increased putrescine but decreased spermine levels. Furthermore, stable expression of a dominant negative mutant of stress-activated protein kinase/extracellular signal-regulated protein kinase (SAPK/ERK) kinase 1 in HeLa cells was found to inhibit the increase in SSAT mRNA by amino acid deprivation. The data suggest that c-Jun N-terminal kinase/SAPK (JNK/SAPK) may be involved in the amino acid-dependent regulation of SSAT expression.

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.

Functional characterization of the human spermidine/spermine N(1)-acetyltransferase gene promoter

Spermidine/spermine N(1)-acetyltransferase (SSAT), the key enzyme of polyamine catabolism, is induced by antiproliferative stresses. We analyzed the 5' flanking region of the human SSAT gene, and clarified that the binding of Sp1 to the GC-box located 42 to 51 bp upstream from the transcription start site is essential for transcription in HeLa S3 cells. A polyamine-responsive element (PRE) seemed to be responsible for the elevated transcription after X-ray irradiation.

Cloning and characterization of human polyamine-modulated factor-1, a transcriptional cofactor that regulates the transcription of the spermidine/spermine N(1)-acetyltransferase gene

The increased transcription and ultimate superinduction of the spermidine/spermine N(1)-acetyltransferase (SSAT) gene has been associated with the antineoplastic activity of several new antitumor polyamine analogues. In sensitive tumor cell types, the transcriptional induction appears to be regulated by the constitutive association of the transcription factor Nrf-2 with the recently discovered polyamine-responsive element. Using the yeast two-hybrid system, a new transcriptional cofactor, polyamine-modulated factor-1 (PMF-1), has been identified as a partner protein of Nrf-2 that, in combination with Nrf-2, regulates the polyamine analogue-induced transcription of SSAT. The human PMF-1 gene, located on chromosome 1 near the 1q12/1q21 border, yields an mRNA transcript of approximately 1.2 kilobases that codes for a 165-amino acid protein with a predicted molecular mass of approximately 20 kDa. The PMF-1 mRNA appears to increase in response to analogue exposure only in analogue-responsive cells. In addition to the transcriptional regulation of SSAT, PMF-1 or similar factors should be considered in the regulation of other polyamine-dependent genes.

Analysis of the spermine synthase gene region in Fugu rubripes, Tetraodon fluviatilis, and Danio rerio

A prerequisite to understanding the evolution of the human X chromosome is the analysis of synteny of X-linked genes in different species. We have focused on the spermine synthase gene in human Xp22. 1. We show that whereas the human gene spans a genomic region of 54 kb, the Fugu rubripes gene is encompassed in a 4.7-kb region. However, we could not find conserved synteny between this region of human Xp22 and the equivalent F. rubripes region. A cosmid clone containing the F. rubripes gene does not contain other X-linked genes. Instead we identified homologs of human genes that are autosomally localized: the ryanodine receptor type I (RYRI), which is implicated in malignant hyperthermia and central core disease, and the HE6 gene. Comparison of the F. rubripes, Tetraodon fluviatilis, mouse, human, and Danio rerio 5'UTRs of spermine synthase highlights conserved sequences potentially involved in regulation. Interestingly, pseudogenes of this gene that are present in the human and mouse genomes seem to be absent in the compact F. rubripes genome. Analysis of a D. rerio PAC clone containing spermine synthase shows an intermediate genomic size in this fish. Sequence analysis of this PAC clone did not reveal other known genes: neither the RYRI gene, nor the HE6 gene, nor other human Xp22 genes were identified.

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.

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.

Differential transcription of the human spermidine/spermine N1-acetyltransferase (SSAT) gene in human lung carcinoma cells

The expression of spermidine/spermine N1-acetyltransferase (SSAT), the rate-limiting enzyme in the catabolism of polyamines, is highly regulated by a number of factors including the natural polyamines and their analogues. The phenotype-specific cytotoxicity that occurs in response to a class of polyamine analogues, the diethylpolyamines, is associated with a phenotype-specific superinduction of SSAT in human non-small-cell lung carcinomas, whereas in non-responding cell types, including the small-cell lung carcinomas, the superinduction of SSAT does not occur. In this study, we have investigated the molecular basis of this phenotype-specific SSAT induction in human lung carcinoma cells in response to N1,N12-diethylspermine (BESpm). To facilitate the study of transcriptional regulation, we have cloned and characterized 11 kb of the human SSAT locus, including 3500 bp of the 5' promoter region. Nuclear run-on transcription studies suggest that the initial induction of SSAT results from an increase in the rate of gene transcription. Results from Northern blot analysis and ribonuclease protection assays indicate a differential expression of SSAT mRNA between the analogue-responsive H157 and non-responsive H82 cells. There is no detectable SSAT mRNA in H82 cells, even after a 24-h analogue treatment, whereas SSAT mRNA in H157 cells was detectable by Northern blot analysis and increased more than 100-fold following drug exposure. Furthermore, nuclear run-on transcription assays do not detect any active transcription of SSAT gene in either treated or untreated H82 cells. These results indicate that at least one component of the phenotype-specific induction of SSAT appears to be due to differences in transcriptional regulation of the gene. In addition, mapping of DNase I-hypersensitive sites of the SSAT gene suggest that the cell type-specific promoter/enhancer utilization may control the expression of the SSAT gene in differentially sensitive cell types in vivo.

Isolation of a polyamine transport deficient cell line from the human non-small cell lung carcinoma line NCI H157

In an effort to study the mechanism underlying the observed phenotype-specific response of human lung cancer cell lines to a polyamine analogue, N1,N12-bis(ethyl)spermine(BESpm), we have isolated a BESpm resistant cell line from the BESpm-sensitive large cell lung carcinoma line NCI H157. The mutant line exhibits identical growth rates in the presence or absence of the analogue. However, the overall growth of mutant cells reaches stationary phase earlier than that of the parental cells. In contrast to the parental cells, where a superinduction of spermidine/spermine N1-acetyltransferase (SSAT) is associated with BESpm toxicity, treatment of this resistant line with BESpm did not induce SSAT mRNA or enzyme activity. BESpm treatment was not effective in depleting the intracellular polyamine pools and very low intracellular BESpm levels were detected. This BESpm resistance is not mediated by multidrug resistance (MDR) protein, since these cells maintain their sensitivity to the antineoplastic agent adriamycin. Treatment of these cells with methylglyoxal bis(guanylhydrazone) (MGBG), an AdoMetDC inhibitor which enters cell using polyamine transport system, shows no inhibition of cell growth. Our data suggest that these mutant cells are deficient in polyamine transport. Consistent with this hypothesis, exogenous polyamines did not prevent difluoromethylornithine (DFMO) induced growth inhibition in the mutant cells.

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.