Structural specificity of polyamines and polyamine analogues in the protection of DNA from strand breaks induced by reactive oxygen species

Reactive oxygen species are known to induce strand breaks and/or base modifications in DNA. DNA strand breaks are associated with many pathologies and programmed cell death. We have examined the ability of the polyamines and their analogues to protect phi X-174 plasmid DNA from strand breakage induced by a oxygen-radical generating system. Spermine and several unsymmetrically substituted polyamine analogues reduced the amount of strand breakage at a physiologically relevant concentration of 1 mM. However, putrescine, spermidine, N1-acetylspermine, N1-acetylspermidine and symmetrically alkylated polyamine analogues were not able to reduce strand breakage at the same concentration. Thus, the unsymmetrically alkylated polyamine analogues and natural spermine can protect DNA against strand breakage induced by Cu(II)/H2O2 generated ROS similar to other more classical antioxidants.

Inhibition of cell growth in CaCO2 cells by the polyamine analogue N1,N12-bis(ethyl)spermine is preceded by a reduction in MYC oncoprotein levels

The polyamine analogue N1,N12bis(ethyl)spermine (BESpm) is a potent inhibitor of cell proliferation and is representative of a class of agents currently in clinical trials. Previous studies have demonstrated that BESpm treatment can produce a decrease in the mRNA levels of the protooncogene c-myc resulting from decreased transcription. Investigation into the mechanism of the antiproliferative effect of BESpm in the colon cancer cell line CaCO2 indicated that significant reduction in MYC protein, but not c-myc mRNA levels, preceded cytostasis. Specificity of the downregulation of MYC expression by BESpm treatment was demonstrated by comparison to effects on the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase (SSAT) and the polyamine biosynthetic enzyme ornithine decarboxylase (ODC). SSAT activity rapidly increased while levels of ODC activity decreased after BESpm treatment. Measurement of intracellular polyamines demonstrated significant uptake of the analogue after 24 hours, which was concurrent with a reduction of spermine and spermidine levels. Thus, cellular uptake of BESpm mediated a reduction of polyamine levels that was associated with a decrease of MYC protein at the post-transcriptional level.

Translation of ODC mRNA and polyamine transport are suppressed in ras-transformed CREF cells by depleting translation initiation factor 4E

Rapid tumor growth and metastasis require increased polyamine metabolism, which is coordinately regulated by ornithine decarboxylase (ODC) and the polyamine transporter. Both activities are stimulated by ras signalling and are dependent upon protein biosynthesis. T24ras oncogene expression in rat embryo fibroblasts (CREFT24) induces cellular transformation and malignancy, in part, by stimulating the rate-limiting translation initiation factor, eIF-4E. CREFT24 expressing antisense RNA to eIF-4E (AS4E) have markedly decreased tumor growth rates and metastatic capacity, without altered monolayer growth rates. Herein, we demonstrate that in AS4E, ODC is translationally suppressed resulting in decreased ODC activity. Additionally, exogenous polyamine uptake is suppressed in AS4E cells indicating that AS4E can neither generate nor import the polyamines necessary to support rapid tumor growth. These data provide evidence that eIF-4E is the link between ras-induced malignancy and increased polyamine metabolism and support the hypothesis that eIF-4E plays a pivotal role in mediating ras-induced malignancy.

The role of polyamine catabolism in polyamine analogue-induced programmed cell death

N1-ethyl-N11-[(cyclopropyl)methyl]-4,8,-diazaundecane (CPENSpm) is a polyamine analogue that represents a new class of antitumor agents that demonstrate phenotype-specific cytotoxic activity. However, the precise mechanism of its selective cytotoxic activity is not known. CPENSpm treatment results in the superinduction of the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase (SSAT) in sensitive cell types and has been demonstrated to induce programmed cell death (PCD). The catalysis of polyamines by the SSAT/polyamine oxidase (PAO) pathway produces H2O2 as one product, suggesting that PCD produced by CPENSpm may be, in part, due to oxidative stress as a result of H2O2 production. In the sensitive human nonsmall cell line H157, the coaddition of catalase significantly reduces high molecular weight (HMW) DNA (>/=50 kb) and nuclear fragmentation. Important to note, specific inhibition of PAO by N,N'-bis(2, 3-butadienyl)-1,4-butane-diamine results in a significant reduction of the formation of HMW DNA and nuclear fragmentation. In contrast, the coaddition of catalase or PAO inhibitor has no effect on reducing HMW DNA fragmentation induced by N1-ethyl-N11-[(cycloheptyl)methyl]-4,8,-diazaundecane, which does not induce SSAT and does not deplete intracellular polyamines. These results strongly suggest that H2O2 production by PAO has a role in CPENSpm cytotoxicity in sensitive cells via PCD and demonstrate a potential basis for differential sensitivity to this promising new class of antineoplastic agents. Furthermore, the data suggest a general mechanism by which, under certain stimuli, cells can commit suicide through catabolism of the ubiquitous intracellular polyamines.

Ornithine decarboxylase and polyamines in familial adenomatous polyposis

Familial adenomatous polyposis (FAP), due to germ-line mutation of the adenomatous polyposis coli (APC) gene, is characterized by development of colorectal adenomas and ultimately colorectal cancer. The usefulness of ornithine decarboxylase (ODC) activity and polyamine levels in normal-appearing colorectal mucosa to stratify risk for colorectal neoplasia by discriminating presymptomatic individuals with germ-line APC mutation (genotype-positive) from genotype-negative family controls was evaluated in 36 at-risk subjects undergoing endoscopic and genetic screening for FAP. ODC activity and levels of putrescine, spermidine, and spermine were significantly higher in presymptomatic genotype-positive patients compared to genotype-negative persons (P = 0.029, <0.001, 0.002, and <0.001, respectively). Moreover, a putrescine level with a cutoff point of 1.5 nmol/mg protein was the most accurate single discriminator of risk status. ODC activity and polyamine levels are significantly elevated in gene carriers of FAP before the development of polyposis, suggesting a role for these compounds in tumorigenesis of FAP. These assays may be useful in evaluating at-risk members of FAP families in which mutation of the APC gene cannot be found.

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.

Synthesis and evaluation of a polyamine phosphinate and phosphonamidate as transition-state analogue inhibitors of spermidine/spermine-N1-acetyltransferase

Polyamine analogues such as bis(ethyl)norspermine and N1-ethyl-N11-[(cyclopropyl)methyl]-4,8-diazaundecane (CPENSpm) act as inhibitors of the enzyme spermidine/spermine-N1-acetyltransferase (SSAT) in vitro and possess impressive antitumor activity against a number of cell lines. However, the propensity of these compounds to superinduce SSAT in intact cells limits their usefulness in studies aimed at elucidating the role of SSAT in cellular metabolism. The recently synthesized alkylpolyamine analogue N1-ethyl-N11-[(cycloheptyl)methyl]-4,8-diazaundecane (CHENSpm, 3) is also an effective inhibitor of SSAT and has potent antitumor activity, but does not appear to superinduce SSAT. These findings suggest that it is possible to synthesize polyamine analogues that can be used for selective inhibition of the enzyme in cellular metabolic studies. Along these lines, the phosphate-based transition state analogues 4 and 5 were synthesized and evaluated as inhibitors of isolated SSAT. Phosphonamidate 4 was rapidly hydrolyzed under the assay conditions, and thus did not inhibit the enzyme. However, the phosphinate analogue 5 was an effective inhibitor of purified human SSAT, with a Ki value of 250 microM. The inhibitory activity of 5 was also compared with that of CHENSpm (IC50 = 13 microM), as well as a series of bis-substituted alkylpolyamine analogues. The unsymmetrically substituted polyamine analogue CHENSpm (3) and the phosphinate transition state analogue 5 represent the first functional, nonsuperinducing inhibitors of human SSAT.

DPC4 gene in various tumor types

We recently identified a novel tumor-suppressor gene, DPC4, at chromosome 18q21.1 and found that both alleles of DPC4 were inactivated in nearly one-half of the pancreatic carcinomas. Here, we analyzed 338 tumors, originating from 12 distinct anatomic sites, for alterations in the DPC4 gene. Sixty-four specimens were selected for the presence of the allelic loss of 18q and were further analyzed for DPC4 sequence alterations. An alteration of the DPC4 gene sequence was identified in one of eight breast carcinomas and one of eight ovarian carcinomas. These results indicate that whereas DPC4 inactivation is prevalent in pancreatic carcinoma (48%), it is distinctly uncommon (< 10%) in the other tumor types examined. The tissue restriction of alterations in DPC4, as in many other tumor-suppressor genes, emphasizes the complexity of rate-limiting checkpoints in human tumorigenesis.

Polyamine analogue induction of programmed cell death in human lung tumor cells

The naturally occurring polyamines putrescine, spermidine, and spermine are required for cell growth. Based on this requirement, several polyamine analogues that interfere with polyamine function and metabolism have been synthesized as antineoplastic agents. The symmetrically substituted N1,N12-bis(ethyl)spermine (BESpm), and unsymmetrically substituted N1-ethyl-N11-[(cyclopropyl)methyl]-4, 8-diazaundecane (CPENSpm) have previously been shown to cause rapid cytotoxicity of NCI H157 cells, with concurrent high induction of the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase. However, the precise mechanism(s) of the cytotoxic action of the compounds is not known. We now demonstrate that treatment with either BESpm or CPENSpm results in morphological and biochemical changes consistent with the activation of programmed cell death pathways, and that the unsymmetrically substituted CPENSpm more rapidly activates the death program. These studies suggest that the cell type-specific cytotoxicity of these polyamine analogues may be a result of their ability to selectively activate the cell death pathway in sensitive phenotypes and indicate that the relationship between the structure of the polyamine analogues and the ability to induce programmed cell death should be investigated.

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.

Significant induction of spermidine/spermine N1-acetyltransferase without cytotoxicity by the growth-supporting polyamine analogue 1,12-dimethylspermine

The superinduction of the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase (SSAT) has been implicated in the cell type-specific cytotoxic activity of some polyamine analogues. We now report that one polyamine analogue, 1,12-dimethylspermine (DMSpm), produces a large induction of SSAT with no significant effects on growth in the human large cell lung carcinoma line, NCl H157. This cell line has been demonstrated to respond to other analogues with SSAT superinduction and cell death. Treatment of the lung cancer cell line with DMSpm produces a rapid increase in SSAT activity and a near complete depletion of the natural polyamines. Additionally, DMSpm supports cell growth in cells which have been depleted of their natural polyamines by the ornithine decarboxylase inhibitor, 2-difluoromethylornithine. The current results suggest that significant induction of SSAT can occur in the absence of cytotoxicity when the inducing polyamine analogue can support growth and that increased SSAT activity alone is not sufficient for cytotoxicity to occur.

Polyamines and their metabolizing enzymes in human frontal cortex and hippocampus: preliminary measurements in affective disorders

Affective disorders are associated with maladaptive response to stressful life events. Based on the observation that a transient increase in brain polyamine metabolism is a common response to stressful stimuli, our hypothesis is that a maladaptive polyamine stress response may be involved in the pathophysiology of affective disorders. Our current research efforts, therefore, concentrate on the characterization of this PA response, and on its pharmacological regulation. The present preliminary study is the first to measure the polyamines, putrescine, spermidine, and spermine, and their metabolizing enzymes, ornithine decarboxylase, S-adenosylmethionine decarboxylase, and spermidine/spermine N1 acetyltransferase, in brain autopsy samples from people who suffered from depressive disorders or schizophrenia, or from those who committed suicide. The data of affected individuals did not reveal significant differences when compared to those of suicide cases, or to those of people with no known neurologic or psychiatric abnormalities. The following regional differences were observed: spermidine concentrations and ornithine decarboxylase activity were higher, but S-adenosylmethionine decarboxylase activity was lower in the hippocampus as compared to the frontal cortex. Preliminary studies with rat brain indicate that an increase in polyamine metabolizing enzyme activities occurs within several hours after death and persists for at least 48 hours. These observations, in turn, indicate that earlier autopsies are crucial for detection of changes in polyamine metabolism. We conclude that further studies to test the polyamine hypothesis are warranted.

Induction of programmed cell death in human breast cancer cells by an unsymmetrically alkylated polyamine analogue

The need for antineoplastic compounds with novel mechanisms of action is great. One such agent is the recently synthesized polyamine analogue N1-ethyl-N11-((cyclopropyl)methyl)-4,8-diazaundecane (CPENSpm). Exposure of hormone-dependent and -independent human breast cancer cells to 0.1-10 microM CPENSpm led to both growth inhibition and induction of programmed cell death. Fragmentation of DNA to high molecular weight fragments and oligonucleosomal-sized fragments, both characteristic of programmed cell death, was determined to be time and concentration dependent. Depletion of natural polyamine pools and accumulation of the analogue was also demonstrated. These data provide the first evidence that a polyamine analogue induces programmed cell death.

5'-Deoxy-5'-methylthioadenosine phosphorylase and p16INK4 deficiency in multiple tumor cell lines

5'-Deoxy-5'methylthioadenosine phosphorylase (MTA-Pase) gene is localized at the 9p21 region linked to the recently identified putative tumor suppressor gene, p16INK4, which appears implicated in the control of cell division cycle. The phosphorylase is a housekeeping enzyme involved in the purine and amino acid metabolism whose activity is evidentiable in all the normal tissues. Chromosomal deletions encompassing both MTAPase and p16INK4 genes cause the total absence of the enzymatic activity only in malignant cells, thus resulting in defined metabolic differences between malignant and normal cells. MTAPase deficiency was investigated by direct radiochemical assay method and by immunochemical techniques in 35 different human malignant cell lines established from several tumor types. The enzyme-deficient cells derived from breast, lung, ovary and liver cancer, malignant melanomas, malignant gliomas and liposarcomas. Two of the MTAPase-deficient cell preparations (from a liver carcinoma and from a melanoma) are primary cultures thus directly representing the original cancer genotypes. Several of the MTAPase-negative cells were studied for p16INK4 gene deletions and for p16INK4 protein deficiency. In all the examined samples a full correlation exists between the lack of MTAPase and that of p16INK4. A similar result was obtained analysing extracts of Vero cell line, which is a fibroblast MTAPase-negative cell line established from the kidney of a normal adult monkey. Conversely, Cos cells, which also are fibroblasts derived from monkey kidney, show both MTAPase and p16INK4 protein. These results: (i) demonstrate that the phosphorylase deficiency is distributed among almost all the most important human cancers; (ii) confirm and extend the tumor types were p16INK4 gene inactivation is observable and (iii) suggest that deletions at 9p21 (in humans) or at syntenic chromosomes (in other species) might represent a general mechanism of p16INK4 gene loss of function and possibly, in turn, of cancer development and/or progression.

Growth and biochemical effects of unsymmetrically substituted polyamine analogues in human lung tumor cells 1

Three unsymmetrically substituted polyamine analogues demonstrate significant and selective antitumor effects. Each of the analogues N1-ethyl-N11-propargyl-4,8-diazaundecane (PENSpm), N1-ethyl-N11-(cyclobutyl)methyl-4,8-diazaundecane (CBENSpm), and N1-ethyl-N11-(cyclopropyl)methyl-4,8-diazaundecane (CPENSpm) is cytotoxic to a representative non-small-cell lung carcinoma line, NCI H157, while being only growth-inhibitory to a representative small-cell-lung carcinoma line, NCI H82. Cytotoxicity is accompanied by a significant increase in expression of the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase (SSAT) at the levels of activity and steady-state mRNA. These new analogues are significant both for their cell-type-specific activity and as synthetic prototypes for the addition of SSAT-activated functional groups.

Effect of 1,19-bis(ethylamino)-5,10,15-triazanonadecane on human tumor xenografts

The polyamine analogue 1,19-bis(ethylamino)-5,10,15-triazanonadecane (BE-4-4-4-4), 5 mg/kg i.p., was given twice daily on days 0-3 and 7-10 (cycle 1) to nude mice with human malignant gliomas (SF-767 and U-87 MG), lung adenocarcinoma (A549), and colon carcinomas (HCT116 and HT29). A second cycle of drug was given to mice with SF-767 and A549 tumors on days 42-45 and 49-52. The maximum animal weight loss varied between 4 and 12%, which was observed 10-15 days following the initiation of treatment, but no overt toxic reactions were noted. The SF-767 brain tumors were extremely responsive to BE-4-4-4-4 alone (3 of 8 complete regressions after 2 cycles); however, the growth of the U-87 MG brain tumor was only slightly inhibited by BE-4-4-4-4 treatment. There was significant inhibition of tumor growth after treatment with one cycle of BE-4-4-4-4 in animals carrying the A549, HCT116, and HT29 tumors. At day 73, the growth of the A549 tumor was inhibited by 78 and 89% following one or two cycles of BE-4-4-4-4, respectively. The mitotic index of A549 tumors was 18 times greater in control mice than in those treated with BE-4-4-4-4 for one or two cycles 99 days after initiation of treatment. 1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU) was given to mice carrying the U-87 MG or A549 tumors on day 4 (cycle 1) and day 46 (cycle 2) in the maximal tolerated dose of 50 mg/kg for BCNU alone and 40 mg/kg for BCNU plus BE-4-4-4-4. BCNU alone significantly inhibited the growth of U-87 MG tumors but not the growth of A549 tumors. Treatment with the combination of BCNU and BE-4-4-4-4 was significantly better than BCNU alone for A549 tumors and better than BE-4-4-4-4 alone for U87 tumors. However, in both animal groups treated with the combination, there was a significant weight loss, which was not observed for animals treated with either agent alone. These data suggest a role for BE-4-4-4-4 in the treatment of brain, lung, and colon tumors.

Immunohistochemical staining of human spermidine/spermine N1-acetyltransferase superinduced in response to treatment with antitumor polyamine analogues

A superinduction of the polyamine catabolic enzyme, spermidine/spermine N1-acetyltransferase (SSAT) accompanies the phenotype-specific cytotoxic response to a class of antitumor polyamine analogues in several important human solid tumor models. A highly specific antiserum against the human SSAT protein has been developed. Using this antiserum we demonstrate that polyamine analogue treatment in vitro or in vivo results in an induction of SSAT protein which is uniformly distributed throughout the cytoplasm of treated tumor cells. This new biochemical tool will be useful in the examination of the association of superinduced SSAT activity and cell type-specific cytotoxicity. Additionally, since clinical trials have begun on one of the SSAT-inducing polyamine analogues, this antiserum may be useful as a diagnostic tool in differentiating responsive and nonresponsive tumor cell populations in treated patients.

The effect of polyamine depletion on the cytotoxic response to PUVA, gamma rays and UVC in V79 cells in vitro

Difluoromethylornithine (DFMO) depletes cells of polyamines, sensitizing cells against the action of antineoplastic drugs and altering ability to repair radiation-induced DNA strand breaks. Others have hypothesized that the mechanism by which polyamine depletion sensitizes cells is through inhibition of DNA strand break repair or through altering the spectrum of initial DNA damage. We have compared the effect of polyamine depletion on cytotoxic effects in V79 cells for three agents that damage DNA: PUVA (8-methoxypsoralen and ultraviolet light, 365 nm), gamma-rays and UVC (ultraviolet light, 254 nm) in polyamine depleted V79 cells. DFMO pretreatment sensitizes cells to PUVA and gamma-ray toxicity but not to UVC. Unlike UVC photoinduction of DNA lesions, PUVA- and gamma-ray-induced DNA damage is modulated by chromatin structure. Our results suggest that polyamine depletion sensitizes cells to the action of PUVA and gamma-rays by mechanisms disparate from those for UVC, suggesting that the level or type of initial damage, rather than DNA repair, per se, may be the more important determinant of enhanced cytotoxicity.

Lithium exerts a time-dependent and tissue-selective attenuation of the dexamethasone-induced polyamine response in rat brain and liver

It has previously been shown that chronic, but not acute, lithium treatment indirectly prevents the dexamethasone-induced increase in brain polyamine-metabolizing enzymes. In the present study we determined the effects of lithium treatment on changes in cellular polyamines, 6 h after dexamethasone challenge (3 mg/kg intraperitoneally). The findings demonstrate that chronic lithium (daily intraperitoneal 2.5 mmol/kg injections for 2 weeks) treatment completely prevents the accumulation of putrescine, in parallel to its prevention of the dexamethasone-induced increase in ornithine decarboxylase activity. A partial attenuation of this polyamine response was also observed in the liver. Only minor and inconsistent changes were observed in the concentrations of the polyamines, spermidine and spermine. Acute lithium treatment (a single injection at times ranging from 1 to 24 h prior to dexamethasone challenge) did not attenuate the dexamethasone-induced increases in brain putrescine concentration nor in ornithine decarboxylase activity. It is suggested that prevention of the stress-induced polyamine response in the brain may be an important mechanism through which prophylactic lithium may exert its beneficial effect in manic-depressive illness.

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.

Synthesis and evaluation of unsymmetrically substituted polyamine analogues as modulators of human spermidine/spermine-N1-acetyltransferase (SSAT) and as potential antitumor agents

Spermidine/spermine-N1-acetyltransferase (SSAT), the rate-limiting step in polyamine catabolism, is critical for the interconversion and modulation of cellular polyamines. Inhibitor-initiated induction of this enzyme also appears to correlate with the sensitivity of tumor cells to a class of novel polyamine analogues, the bis(ethyl)polyamines. Thus, terminally alkylated polyamines which modulate the cellular level of SSAT could be of great value for understanding the role of this enzyme both in analogue-mediated cytotoxicity and in overall cellular polyamine metabolism. Such analogues could also become important therapeutic agents by disrupting cellular polyamine metabolism. The structure-activity relationships defining the interaction of polyamine analogues with SSAT have not been fully elucidated, and, in particular, unsymmetrically alkylated polyamines have not been synthesized and evaluated as modulators of SSAT. To this end, we now report the synthesis and preliminary biological evaluation of N1-ethyl-N11-propargyl-4,8-diazaundecane and N1-ethyl-N11-((cyclopropyl)methyl)-4,8-diazaundecane via a synthetic pathway which represents an efficient route to a variety of unsymmetrically substituted polyamine analogues. The title compounds act as effective inhibitors of isolated human SSAT and produce a differential superinduction of SSAT in situ which appears to be associated with a cell specific cytotoxic response in two human lung cancer cell lines. In so doing, these analogues exhibit promising antitumor activity against cultured human lung cancer cells.

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

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

Spermidine/spermine N1-acetyltransferase--the turning point in polyamine metabolism

Polyamines are thought to have several vital roles in cell growth and differentiation. The highly regulated polyamine metabolic pathway provides cells with the ability to finely control the intracellular concentration of these ubiquitous polycations. Although earlier studies of regulation of polyamine content were concentrated on the biosynthetic reactions, recently the importance of the catabolic processes, particularly the highly regulated acetylation step in polyamine degradation, has become apparent. This work has led to an understanding of how a cell may, in a tightly controlled manner, facilitate the breakdown, excretion, cycling, and/or intracellular shuttling of the polyamines. This myriad of possibilities appears to be regulated initially at a single rate-limiting enzymatic step, the N1-acetylation of spermidine or spermine, by spermidine/spermine N1-acetyltransferase (SSAT). Recent cloning of the human SSAT gene has facilitated a more detailed study of this enzyme. SSAT appears to have a role in the determination of tumor sensitivity to a new class of antineoplastic agents. The further study of SSAT and the associated polyamine metabolism should provide a better understanding of the regulation and function of these cations.