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

Polyamine Protection Against Chemically Induced Hepatotoxicity

The protective effect of putrescine (a polyamine) on chemically induced hepatotoxicity in male Sprague-Dawley rats was assessed by mortality, clinical pathological changes (specifically alanine aminotransferase and aspartate aminotransferase activities), and liver histopathological changes. A reduction in hepatotoxicant-induced mortality by 20% to 25% was observed when putrescine (100 mg/kg/day) was administered intraperitoneally for 3 days prior to hepatotoxicant administration (either carbon tetrachloride or allyl alcohol at dose levels approximating the LD50). Putrescine significantly reduced the hepatoxicant-induced increases in serum alanine aminotransferase and aspartate aminotransferase activities. Histological assessment revealed that putrescine pretreatment also reduced the severity and frequency of hepatotoxicant-induced liver necrosis. Administration of putrescine at 0.5 and 3 hours following hepatotoxicant treatment decreased both hepatoxicant-induced mortality and hepatoxicant-induced increases in serum alanine aminotransferase and aspartate aminotransferase activities, with the 0.5 hour postdose treatment being more effective than the 3 hours postdose treatment. Early intervention reduced the mortality rate in the allyl alcohol plus putrescine group by 20% and by 10% in the carbon tetrachloride as well as the carbon tetrachloride plus putrescine groups. However, the effectiveness of postdose putrescine treatment was less than when putrescine was administered prior to the hepatotoxicant.

Specific phosphorylation of histone demethylase KDM3A determines target gene expression in response to heat shock

Phosphorylation of histone demethylase KDM3A in response to thermal stress enables its specific recruitment to target genes by Stat1.

Histone lysine (K) residues, which are modified by methyl- and acetyl-transferases, diversely regulate RNA synthesis. Unlike the ubiquitously activating effect of histone K acetylation, the effects of histone K methylation vary with the number of methyl groups added and with the position of these groups in the histone tails. Histone K demethylases (KDMs) counteract the activity of methyl-transferases and remove methyl group(s) from specific K residues in histones. KDM3A (also known as JHDM2A or JMJD1A) is an H3K9me2/1 demethylase. KDM3A performs diverse functions via the regulation of its associated genes, which are involved in spermatogenesis, metabolism, and cell differentiation. However, the mechanism by which the activity of KDM3A is regulated is largely unknown. Here, we demonstrated that mitogen- and stress-activated protein kinase 1 (MSK1) specifically phosphorylates KDM3A at Ser264 (p-KDM3A), which is enriched in the regulatory regions of gene loci in the human genome. p-KDM3A directly interacts with and is recruited by the transcription factor Stat1 to activate p-KDM3A target genes under heat shock conditions. The demethylation of H3K9me2 at the Stat1 binding site specifically depends on the co-expression of p-KDM3A in the heat-shocked cells. In contrast to heat shock, IFN-γ treatment does not phosphorylate KDM3A via MSK1, thereby abrogating its downstream effects. To our knowledge, this is the first evidence that a KDM can be modified via phosphorylation to determine its specific binding to target genes in response to thermal stress.

Histone methylation regulates gene expression and can have drastic consequences for health if the process is defective. Histone lysine demethylases (KDMs) counteract the activity of methyl-transferases and remove methyl group(s) from histones. KDM3A is a H3K9me2/1 demethylase that performs diverse functions via the regulation of its target genes, which are involved in spermatogenesis, metabolism, and cell differentiation. However, the mechanisms underlying KDM3A regulation of specific genes at specific times are largely unknown. Here we found that a physiological stress—elevated temperature—induces KDM3A phosphorylation in human cells via the MSK1 kinase. This phosphorylated form of KDM3A directly interacts with the transcription factor Stat1, which enables Stat1 to recruit KDM3A to Stat1-binding sequences at the promoters of specific target genes. KDM3A then acts to demethylate H3K9me2/1 at these targets, thereby causing specific gene expression in response to the thermal stress. We conclude that heat shock can affect the expression of many genes in human cells via a novel activation mechanism that is centered around the phosphorylation of KDM3A.

Histone deacetylase inhibition overcomes drug resistance through a miRNA-dependent mechanism

The treatment of specific tumor cell lines with poly- and oligoamine analogs results in a superinduction of polyamine catabolism that is associated with cytotoxicity; however, other tumor cells show resistance to analog treatment. Recent data indicate that some of these analogs also have direct epigenetic effects. We, therefore, sought to determine the effects of combining specific analogs with an epigenetic targeting agent in phenotypically resistant human lung cancer cell lines. We show that the histone deacetylase inhibitor MS-275, when combined with (N(1), N(11))-bisethylnorspermine (BENSpm) or (N(1), N(12))-bis(ethyl)-cis-6,7-dehydrospermine tetrahydrochloride (PG-11047), synergistically induces the polyamine catabolic enzyme spermidine/spermine N(1)-acetyltransferase (SSAT), a major determinant of sensitivity to the antitumor analogs. Evidence indicates that the mechanism of this synergy includes reactivation of miR-200a, which targets and destabilizes kelch-like ECH-associated protein 1 (KEAP1) mRNA, resulting in the translocation and binding of nuclear factor (erythroid-derived 2)-like 2 (NRF2) to the polyamine-responsive element of the SSAT promoter. This transcriptional stimulation, combined with positive regulation of SSAT mRNA and protein by the analogs, results in decreased intracellular concentrations of natural polyamines and growth inhibition. The finding that an epigenetic targeting agent is capable of inducing a rate-limiting step in polyamine catabolism to overcome resistance to the antitumor analogs represents a completely novel chemotherapeutic approach. In addition, this is the first demonstration of miRNA-mediated regulation of the polyamine catabolic pathway. Furthermore, the individual agents used in this study have been investigated clinically; therefore, translation of these combinations into the clinical setting holds promise.

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.

Chemically induced oxidative stress increases polyamine levels by activating the transcription of ornithine decarboxylase and spermidine/spermine-N1-acetyltransferase in human hepatoma HUH7 cells

Biogenic polyamines spermine and spermidine participate in numerous cellular processes including transcription, RNA processing and translation. Specifically, they counteract oxidative stress, an alteration of cell redox balance involved in generation and progression of various pathological states including cancer. Here, we investigated how chemically induced oxidative stress affects polyamine metabolism, specifically the expression and activities of enzymes catalyzing polyamine synthesis (ornithine decarboxylase; ODC) and degradation (spermidine/spermine-N(1)-acetyltransferase; SSAT), in human hepatoma cells. Oxidative stress induced the up-regulation of ODC and SSAT gene transcription mediated by Nrf2, and in case of SSAT, also by NF-κB transcription factors. Activation of transcription led to the elevated intracellular activities of both enzymes. The balance in antagonistic activities of ODC and SSAT in the stressed hepatoma cells was shifted towards polyamine biosynthesis, which resulted in increased intracellular levels of putrescine, spermidine, and spermine. Accumulation of putrescine is indicating for accelerated degradation of polyamines by SSAT - acetylpolyamine oxidase (APAO) pathway generating toxic products that promote carcinogenesis, whereas accelerated polyamine synthesis via activation of ODC is favorable for proliferation of cells including those sub-lethally damaged by oxidative stress.

Human PIH1 associates with histone H4 to mediate the glucose-dependent enhancement of pre-rRNA synthesis

Ribosome biogenesis is critical in the growth of eukaryotic cells, in which the synthesis of precursor ribosomal RNA is the first and rate-limiting step. Here, we show that human PIH1 domain-containing protein 1 (PIH1) interacts directly with histone H4 and recruits the Brg1-SWI/SNF complex via SNF5 to human rRNA genes. This process is likely involved in PIH1-dependent DNase I-hypersensitive chromatin remodeling at the core promoter of the rRNA genes. PIH1 mediates the occupancy of not only the Brg1 complex but also the Pol I complex at the core promoter and enhances transcription initiation of rRNA genes. Additionally, the interaction between PIH1 and H4K16 expels TIP5, a component of the silencing nucleolar remodeling complex (NoRC), from the core region, suggesting that PIH1 is involved in the derepression of NoRC-silenced rRNA genes. These data indicate that PIH1 is a positive regulator of human rRNA genes and is of great importance for the recovery of human cells from nutrient starvation and the transition to glucose-induced exponential growth in vivo.

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.

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.

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.

Concentration-dependent effects ofN1,N11-diethylnorspermine on melanoma cell proliferation

N1, N11-diethylnorspermine (DENSPM) is a polyamine analog that is currently under investigation as a novel anticancer drug. Although it has shown promising preclinical activity, there has been large variation in responsiveness reported between different human cancers. During our studies into the causes of this variation, we observed a consistent increase in cell proliferation at low drug concentrations (<10 microM) in human melanoma cells resistant to the drug. At higher concentrations, growth inhibition was seen in all cell lines, with IC50 values ranging 2-180 microM. We hypothesized that DENSPM may mimic endogenous polyamines at low concentrations, supporting cell growth in resistant lines. We also observed that DENSPM downregulated polyamine transport in a manner similar to that for spermidine, a finding that confirms previous reports. Finally, DENSPM could rescue cells from growth arrest by the ornithine decarboxylase inhibitor difluoromethylornithine, which depletes intracellular polyamines. Taken together, these results suggest that DENSPM, at clinically relevant concentrations, can mimic endogenous polyamines and induce proliferation in resistant human melanoma cells.

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.

cDNA microarray analysis of isogenic paclitaxel- and doxorubicin-resistant breast tumor cell lines reveals distinct drug-specific genetic signatures of resistance

cDNA microarray analysis is a highly useful tool for the classification of tumors and for prediction of patient prognosis to specific cancers based on this classification. However, to date, there is little evidence that microarray approaches can be used to reliably predict patient response to specific chemotherapy drugs or regimens. This is likely due to an inability to differentiate between genes affecting patient prognosis and genes that play a role in response to specific drugs. Thus, it would be highly useful to identify genes whose expression correlates with tumor cell sensitivity to specific chemotherapy agents in a drug-specific manner. Using cDNA microarray analysis of wildtype MCF-7 breast tumor cells and isogenic paclitaxel-resistant (MCF-7(TAX)) or doxorubicin-resistant (MCF-7(DOX)) derivative cell lines, we have uncovered drug-specific changes in gene expression that accompany the establishment of paclitaxel or doxorubicin resistance. These changes in gene expression were confirmed by quantitative reverse transcription polymerase chain reaction and immunoblotting experiments, with a confirmation rate of approximately 91-95%. The genes identified may prove highly useful for prediction of response to paclitaxel or doxorubicin in patients with breast cancer. To our knowledge this is the first report of drug-specific genetic signatures of resistance to paclitaxel or doxorubicin, based on a comparison of gene expression between isogenic wildtype and drug-resistant tumor cell lines. Moreover, this study provides significant insight into the wide variety of mechanisms through which resistance to these agents may be acquired in breast cancer.

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.

Molecular mechanisms of polyamine analogs in cancer cells

The natural polyamines are aliphatic cations with multiple functions and are essential for cell growth. Soon after the critical requirement of polyamines for cell proliferation was recognized, the metabolism of polyamines was pursued as a target for antineoplastic therapy. Initially, much attention was focused on the development of inhibitors of polyamine biosynthesis as a means to inhibit tumor growth. The best-characterized inhibitor is alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase. While compensatory mechanisms in polyamine metabolism reduce the effectiveness of DFMO as a single chemotherapeutic agent, it is currently undergoing extensive testing and clinical trials for chemoprevention and other diseases. There has been increasing interest over the last two decades in the cytotoxic response to agents that target the regulation of polyamine metabolism rather than directly inhibiting the metabolic enzymes in tumor cells. This interest resulted in the development of a number of polyamine analogs that exhibit effective cytotoxicity against tumor growth in preclinical models. The analogs enter cells through a selective polyamine transport system and can be either polyamine antimetabolites that deplete the intracellular polyamines or polyamine mimetics that displace the natural polyamines from binding sites, but do not substitute in terms of growth-promoting function. Synthesis of the first generation of symmetrically substituted bis(alkyl)polyamine analogs in the mid-1980s was based on the theory that polyamines may utilize feedback mechanisms to auto-regulate their synthesis. In the 1990s, unsymmetrically substituted bis(alkyl) polyamine analogs were developed. These compounds display structure-dependent and cell type-specific cellular effects and regulation on polyamine metabolism. More recently, a novel class of analogs has been synthesized, which include conformationally restricted, cyclic and long-chain oligoamine analogs. The development and use of these analogs have provided valuable information for understanding the molecular mechanisms of targeting the polyamine pathway as a means of cancer therapy.

Induction of human spermine oxidase SMO(PAOh1) is regulated at the levels of new mRNA synthesis, mRNA stabilization and newly synthesized protein

The oxidation of polyamines induced by antitumour polyamine analogues has been associated with tumour response to specific agents. The human spermine oxidase, SMO(PAOh1), is one enzyme that may play a direct role in the cellular response to the antitumour polyamine analogues. In the present study, the induction of SMO(PAOh1) enzyme activity by CPENSpm [N1-ethyl-N11-(cyclopropyl)methyl-4,8,diazaundecane] is demonstrated to be a result of newly synthesized mRNA and protein. Inhibition of new RNA synthesis by actinomycin D inhibits both the appearance of SMO(PAOh1) mRNA and enzyme activity. Similarly, inhibition of newly synthesized protein with cycloheximide prevents analogue-induced enzyme activity. Half-life determinations indicate that stabilization of SMO(PAOh1) protein does not play a significant role in analogue-induced activity. However, half-life experiments using actinomycin D indicate that CPENSpm treatment not only increases mRNA expression, but also leads to a significant increase in mRNA half-life (17.1 and 8.8 h for CPENSpm-treated cells and control respectively). Using reporter constructs encompassing the SMO(PAOh1) promoter region, a 30-90% increase in transcription is observed after exposure to CPENSpm. The present results are consistent with the hypothesis that analogue-induced expression of SMO(PAOh1) is a result of increased transcription and stabilization of SMO(PAOh1) mRNA, leading to increased protein production and enzyme activity. These data indicate that the major level of control of SMO(PAOh1) expression in response to polyamine analogues exposure is at the level of mRNA.

Induction of spermidine/spermine N 1-acetyltransferase in breast cancer tissues treated with the polyamine analogue N 1,N 11-diethylnorspermine

PURPOSE

The polyamine analogue, N1, N11-diethylnorspermine (DENSpm), is currently being evaluated in clinical trials for the treatment of solid tumors. The response of solid tumors to this drug has been associated with superinduction of the polyamine catabolic enzyme, spermine/spermidine N1-acetyltransferase (SSAT). Therefore, to estimate the response of breast cancers to DENSpm, we measured induction of SSAT in breast cancer explants treated in vitro with this polyamine analogue.

EXPERIMENTAL DESIGN

Expression of SSAT protein was evaluated by immunohistochemistry in tissue explants from 38 invasive breast cancer tumors incubated in vitro in the presence (or absence) of DENSpm. In addition, SSAT enzymatic activity was measured in tissue explants from four tumors with high cellularity.

RESULTS

SSAT expression was significantly increased in 30 of 38 tumor samples treated with DENSpm compared to untreated controls. This induction of SSAT protein expression was found specifically in neoplastic cells of the treated samples, and was seen in all histologic patterns (ductal, lobular, and mucinous) of breast cancer examined. In tumor samples evaluated for changes in SSAT enzymatic activity, these changes correlated closely with changes in protein expression.

CONCLUSIONS

Immunohistochemical staining for induction of SSAT correlates with measures of enzymatic activity in a small sample where measurements were possible and suggests that immunohistochemistry may be used for predicting response of breast cancers to DENSpm. A high proportion of breast cancers induced SSAT in response to DENSpm, supporting the continued consideration of this class of agents for treatment of breast cancer.

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.

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.

Differential hypersensitivity to DNase I in the regulatory region of human hsp90 beta gene in heat shock and constitutive expression

DNase I hypersensitivity analysis is a useful tool to investigate impact of structure changes in chromatin on the expression of a gene. In order to unravel chromatin regulation on human hsp90 beta gene, differential sensitivity to DNase I in non-treated and heat-shocked Jurkat cells are examined. Four major hypersensitive sites at -120/-20 bp (HS1), +360 bp (HS2), +630/+780 bp (HS4) and around +1020 bp (HS5) with respect to the transcription start site of hsp90 beta gene have been identified. The HS1 is shared by both constitutive and heat shock, while the intronic sites of HS4 and HS5 are elicited by heat shock and HS2 is illustrated only in constitutive expression. In addition, distal HSs at around 8.7 kb upstream and 6.8 kb downstream are found in both constitutive and heat shock expression, which indicate that the boundaries of the hsp90 beta gene extend for some 16 kb. The HS patterns confirm chromatin regulation in the expression of hsp90 beta gene under various treatments.

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.

Protein kinase C-epsilon promotes survival of lung cancer cells by suppressing apoptosis through dysregulation of the mitochondrial caspase pathway

The serine/threonine protein kinase C (PKC) has been implicated in the regulation of drug resistance and cell survival in many types of cancer cells. However, the one or more precise mechanisms remain elusive. In this study, we have identified and determined the mechanism by which PKC-epsilon, a novel PKC isoform, modulates drug resistance in lung cancer cells. Western blot analysis demonstrates that expression of PKC-epsilon, but not other PKC isoforms, is associated with the chemo-resistant phenotype of non-small cell lung cancer (NSCLC) cell lines. Northern blotting and nuclear run-on transcription analysis further reveals that the failure of expression of PKC-epsilon in the chemo-sensitive phenotype of small cell lung cancer (SCLC) cells results from transcriptional inactivation of the gene. Importantly, forced expression of PKC-epsilon in NCI-H82 human SCLC cells confers a significant resistance to the chemotherapeutic drugs, etoposide and doxorubicin. Resistance is characterized by a significant reduction in apoptosis in PKC-epsilon-expressing cells. Treatment of NCI-H82 cells with etoposide induces a series of time-dependent events, including the release of cytochrome c from the mitochondria to the cytosol, activation of caspase-9 and caspase-3, and cleavage of poly(ADP-ribose) polymerase (PARP). All of these events are blocked by PKC-epsilon expression. Furthermore, caspase-specific inhibitors, z-VAD-fmk and z-DEVD-fmk, significantly attenuate the accumulation of sub-G(1) population and block the PARP cleavage in response to etoposide. These results suggest that PKC-epsilon prevents cells from undergoing apoptosis through inhibition of the mitochondrial-dependent caspase activation, thereby leading to cell survival. Finally, down-regulation of PKC-epsilon expression by the antisense cDNA in NSCLC cells results in increased sensitivity to etoposide. Taken together, our findings suggest an important role for PKC-epsilon in regulating survival of lung cancer cells.

Induction of alternatively spliced spermidine/spermine N1-acetyltransferase mRNA in the human kidney cells infected by venezuelan equine encephalitis and tick-borne encephalitis viruses

293 and RH cells derived from human embryo kidney were infected by Venezuelan equine encephalitis and tick-borne encephalitis viruses and cDNA libraries representing cellular mRNAs induced or suppressed due to the infection were prepared using suppressive subtractive hybridization. Among the up-regulated clones the RT-PCR and Northern analyses revealed an unusual transcript of the spermidine/spermine N1-acetyltransferase (SSAT) gene that was shown to be an alternatively spliced form containing an additional 110-bp exon. The alternatively spliced transcript is polyadenylated and can be expected to yield only a truncated 71 amino acid polypeptide. This first evidence of the host gene alternatively spliced mRNA induction by RNA viruses raises the questions of its biological role, regulation mechanisms of alternative splicing, and significance for the virus life cycle.

Cloning and characterization of the mouse polyamine-modulated factor-1 (mPMF-1) gene: an alternatively spliced homologue of the human transcription factor

The natural polyamines and their analogues have been implicated in transcriptional regulation of specific genes. Human polyamine-modulated factor-1 (hPMF-1) was the first polyamine-responsive transcription factor identified. Here the mouse homologue of the hPMF-1 gene is described. Interestingly, the mouse gene (mPMF-1) codes for two alternatively spliced mRNAs. Both of the mouse splice variants, mPMF-1S and mPMF-1L, possess C-terminal coiled-coil domains nearly identical to that found in hPMF-1 and are highly homologous with the human protein. The C-terminal coiled-coil structure is necessary for transcriptional activation. However, the shorter protein, mPMF-1S, does not contain an N-terminal coiled-coil region as do both hPMF-1 and the longer mPMF-1L. mPMF-1L mRNA codes for a protein of 202 amino acids, 37 amino acids longer than the human protein. By contrast, mPMF-1S codes for only 133 amino acids, as a result of two exons being omitted compared with mPMF-1L. Both mouse transcription factors can interact with Nrf-2 (nuclear factor-E2-related factor 2), the normal partner of hPMF-1, substantiating the importance of the C-terminal coiled-coil region responsible for this interaction. Finally, the expression of mPMF-1 is induced when mouse M1 myeloid leukaemia cells are exposed to polyamine analogues, suggesting control similar to that observed for the hPMF-1.

Polyamine- and insulin-like growth factor-I-mediated proliferation of porcine uterine endometrial cells: a potential role for spermidine/spermine N(1)-acetyltransferase during peri-implantation

Insulin-like growth factor-I (IGF-I) and the polyamine catabolic enzyme spermidine/spermine N(1)-acetyltransferase (SSAT) are progesterone-regulated genes with maximal expression at peri-implantation in the porcine uterine endometrium. However, while IGF-I stimulates cell proliferation, SSAT, by acetylating the naturally occurring polyamines (PA) spermine (SPM) and spermidine (SPD), typically functions as a cell growth inhibitor. The present study examined the functional relationships of IGF-I, SSAT, and PA in the control of endometrial cell proliferation. Northern blot analysis indicated that SSAT mRNA levels change with distinct pregnancy stages, in contrast to those for the PA biosynthetic enzyme ornithine decarboxylase (ODC). Primary cultures of luminal and glandular epithelial (LE, GE) and stromal (ST) cells isolated from Day 12 pregnant pig endometrium had IGF-I mRNA levels for ST > LE > GE cells. The mRNA levels for SSAT and ODC were transiently diminished by IGF-I treatment, but only in GE cells. By contrast, SPM and SPD increased SSAT mRNA levels in GE and ST cells, but increased ODC mRNA levels only in GE cells. IGF-I, putrescine (PUT), and SPM individually increased cellular DNA synthesis as measured by tritiated thymidine incorporation in GE and ST cells, while SPD had an effect only in ST cells. IGF-I enhanced the proliferative effect of each PA in GE cells, but only of SPD in ST cells. The mitogen-activated protein kinase inhibitor, PD98059, inhibited the induction by SPM of GE cell DNA synthesis but not that of IGF-I. Wortmannin, a phosphatidylinositol-3-kinase inhibitor had no effect on either IGF-I or SPM induction of GE cell DNA synthesis. The relative concentrations of SPM, SPD, and PUT in uterine luminal fluids differed, with the levels for each PA higher at pregnancy Day 12 than at 11.5. These results suggest that IGF-I and PA act through distinct signaling pathways to mediate cell-type-specific growth of early pregnancy pig uterine endometrium. Further, SSAT, through its control of intracellular PA levels, likely plays a modulatory role in the establishment of an optimal uterine environment for successful embryo attachment.

Epstein-Barr virus SM protein interacts with mRNA in vivo and mediates a gene-specific increase in cytoplasmic mRNA

SM is an Epstein-Barr virus (EBV) gene expressed during early lytic replication of EBV. SM encodes a nuclear phosphoprotein that functions as a posttranscriptional regulator of gene expression. SM has been implicated in several aspects of gene regulation, including nuclear mRNA stabilization, posttranscriptional processing, and nuclear mRNA export. Activation by SM is promoter independent but gene specific. The mechanism by which SM selectively activates some EBV target genes or heterologous reporter genes remains to be determined. SM binds RNA in vitro, suggesting that sequence- or structure-specific mRNA interactions might mediate SM specificity. We have further analyzed RNA binding by SM and demonstrated that proteolytic cleavage of SM and consequent exposure of an arginine-rich region are necessary to allow RNA binding in vitro. However, SM mutants with deletions of this arginine-rich region localized normally in the nucleus and were fully functional in gene activation. We therefore developed an assay to study in vivo interactions of SM with target mRNAs based on immunoprecipitation of SM from cell lysates followed by RNase protection analysis. Using this assay, we demonstrated that SM forms complexes with specific mRNAs in vivo. SM binds mRNAs from both SM-responsive as well as nonresponsive intronless genes and increases the nuclear accumulation of both types of mRNAs. In addition, SM preferentially associates with newly transcribed mRNAs. These data indicate that SM forms complexes with mRNAs in the nucleus and enhances their nuclear accumulation. However, SM does not enhance cytoplasmic accumulation of all transcripts that it binds to the same degree, suggesting that additional mRNA-specific characteristics, such as nuclear retention motifs or binding sites for cellular proteins, also determine responsiveness to SM.

Characterization of the interaction between the transcription factors human polyamine modulated factor (PMF-1) and NF-E2-related factor 2 (Nrf-2) in the transcriptional regulation of the spermidine/spermine N1-acetyltransferase (SSAT) gene

Polyamines and polyamine analogues have been demonstrated to modulate the transcription of various genes. Spermidine/spermine N(1)-acetyltransferase (SSAT) is transcriptionally regulated through the interaction of at least two trans-acting transcription factors, NF-E2-related factor 2 (Nrf-2) and PMF-1 (polyamine modulated factor-1). Nrf-2 has previously been shown to regulate transcription of other genes through interactions between its C-terminal leucine zipper and the leucine-zipper region of other members of the small Maf protein family (the term "Maf" is derived from MusculoAponeurotic-Fibrosarcoma virus). Here it is demonstrated that the interaction between Nrf-2 and PMF-1 is mediated through the binding of the leucine-zipper region of Nrf-2 and a C-terminal coiled-coil region of PMF-1 that does not contain a leucine zipper. Mutations that interrupt either the leucine zipper of Nrf-2 or the coiled-coil region of PMF-1 are demonstrated to alter the ability of these factors to interact, thus their ability to regulate the transcription of the SSAT gene is lost.

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.

Differential expression of multiple unexpected genes during U937 cell and macrophage differentiation detected by suppressive subtractive hybridization

OBJECTIVE

The objective of this study was to identify new markers of myelomonocytic differentiation using a sensitive technique that permits detection of rare differential gene expression.

MATERIALS AND METHODS

[corrected] Suppressive subtractive hybridization (SSH) was performed between the human myelomonocytic U937 cell line and 1 alpha, 25-dihydroxyvitamin D3 and transforming growth factor beta 1 differentiated U937 cells. cDNA clones with significant increased expression in differentiated U937 cells over nondifferentiated U937 cells were characterized by sequencing. [corrected] The pattern of differential gene expression obtained by SSH was confirmed by cDNA Southern and Northern blots on the undifferentiated vs. differentiated U937 cells, and by reverse transcriptase polymerase chain reaction on undifferentiated human CD34(+) stem cells isolated from bone marrow vs. peripheral blood CD14(+) mature monocytes.

RESULTS

Seven cDNAs never associated with in vitro U937 cell myelomonocytic differentiation (prolactin, 11-beta hydroxysteroid dehydrogenase [11 beta-HSD)] haptoglobin alpha (2FS)-beta precursor, GLIPR, RTVP, the RNA helicase P68, and spermidine-spermine N1-acetyltransferase) were identified. The first five of these genes previously were associated with immune function and the last two are important for intermediary metabolism. Differential expression was confirmed in CD34(+)/CD14(+) monocyte differentiation for all genes but 11 beta-HSD.

CONCLUSIONS

We identified six new markers of U937 cell differentiation, which also are differentially expressed during normal human myelomonocytic differentiation.

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.

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.

Saccharomyces cerevisiae GNA1, an essential gene encoding a novel acetyltransferase involved in UDP-N-acetylglucosamine synthesis

The Saccharomyces cerevisiae gene, YFL017C, for a putative acetyltransferase was characterized. Disruption of YFL017C was lethal, leading to a morphology similar to those caused by the depletion of AGM1 or UAP1, the genes encoding phospho-N-acetylglucosamine mutase and UDP-N-acetylglucosamine pyrophosphorylase, respectively. This implies the involvement of YFL017C in UDP-N-acetylglucosamine synthesis. The recombinant protein for YFL017C displayed phosphoglucosamine acetyltransferase activities in vitro and utilized glucosamine 6-phosphate as the substrate. When incubated with Agm1p and Uap1p, the Yfl017c protein produced UDP-N-acetylglucosamine from glucosamine 6-phosphate. These results indicate that YFL017C specifies glucosamine-6-phosphate acetyltransferase; therefore, the gene was designated GNA1 (glucosamine-6-phosphate acetyltransferase). In addition, whereas bacterial phosphoglucosamine acetyltransferase and UDP-N-acetylglucosamine pyrophosphorylase activities are intrinsic in a single polypeptide, they are encoded by distinct essential genes in yeast. When the sequence of ScGna1p was compared with those of other acetyltransferases, Ile97, Glu98, Val102, Gly112, Leu115, Ile116, Phe142, Tyr143, and Gly147 were found to be highly conserved. When alanine was substituted for these amino acids, the enzyme activity for the substituted Phe142 or Tyr143 enzymes was severely diminished. Although the activity of Y143A was too low to perform kinetics, F142A displayed a significantly increased Km value for acetyl-CoA, suggesting that the Phe142 and Tyr143 residues are essential for the catalysis.

Changes in polyamine catabolism in HL-60 human promyelogenous leukaemic cells in response to etoposide-induced apoptosis

The topoisomerase II inhibitor etoposide induced apoptosis in HL-60 cells within 4 h of exposure to the drug, as measured by changes in morphology, DNA fragmentation and cytotoxicity assays. Etoposide-induced apoptosis was accompanied by an increase in polyamine efflux from the cells and a decrease in total polyamine content during the first 24 h of exposure to the drug. Although both enzyme activities increased slightly, there were no significant changes in spermidine/spermine N1-acetyltransferase activity or polyamine oxidase activity. After longer exposures (48-72 h), significant induction of spermidine/spermine N1-acetyltransferase activity and loss of polyamine content occurred. These results suggest that polyamine oxidation and the resultant hydrogen peroxide produced may be associated with the initiation of apoptosis, while induction of the acetyltransferase and overall loss of intracellular polyamines may be involved in the final, possibly necrotic, stages of cell death.

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.

Significant increases in the steady states of putrescine and spermidine/spermine N1-acetyltransferase mRNA in HeLa cells accompanied by growth arrest

Polyamines are intrinsic polycations which play critical roles in cell proliferation. Ornithine decarbolylase (ODC) catalyzes the first step of polyamine biosynthesis converting ornithine to putrescine. In addition to polyamine degradation, spermidine/spermine acetyltransferase (SSAT) regulates interconversion pathway of spermine and spermidine to putrescine. We quantified the polyamines and mRNAs of ODC and SSAT in HeLa S3 cells at various stages during exponential and plateau phases of culturing. Unexpectedly, putrescine and SSAT mRNA levels increased remarkably at the plateau phase, in contrast to the decrease of ODC mRNA level. It will be suggested that the putrescine has a novel function linked to the arrest of cell growth in which the SSAT-mediated pathway producing putrescine takes part.

Regulation of spermidine/spermine N1-acetyltransferase expression by cytokines and polyamines in human hepatocarcinoma cells (HepG2)

Spermidine/spermine N1-acetyltransferase (cSAT), a key enzyme in polyamine degradation, is induced by various hepatotoxins and liver tumor promoters. In this paper we demonstrate that physiological factors, such as cytokines, control cSAT expression in HepG2 human hepatocarcinoma cells. Hepatocyte growth factor (HGF) induced the cSAT mRNA precursor (3.5 kb) at 4 h. The mature form of mRNA (1.3 kb) increased 6-8-fold between 8 and 10 h, and remained elevated until 18 h. An increase in cSAT activity (2-fold) and high levels of N1-acetylspermidine were observed concomitantly. Interleukin-1 beta (IL-1 beta) enhanced cSAT expression (both mRNA and enzyme activity) similar to HGF, while tumor necrosis factor-alpha (TNF-alpha) was less effective. This system also provides a useful means for examining the involvement of negative and positive changes of polyamines in the induction of cSAT and c-jun, a gene that participates in the control of cSAT expression. alpha-Difluoromethylornithine (DFMO) pretreatment, by lowering putrescine and spermidine in HGF- or IL-1 beta-treated cells, prevented the induction of cSAT. This effect was reversed by exogenous putrescine or spermidine. IL-1 beta induced c-jun mRNA more than HGF. DFMO prevented almost completely the enhancement of c-jun mRNA expression by IL-1 beta, and this effect was reversed by exogenous putrescine or spermidine. Therefore, we suggest that cSAT and c-jun expression is specifically regulated by polyamine-mediated mechanisms in IL-1 beta treated HepG2 cells. Since cSAT is inducibile by cytokines that control tumor metabolism and growth as well as tumor-host interaction, we hypothesize an involvement of cSAT in hepatoma growth.

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.

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.