Cellular characterization of a new irreversible inhibitor of S-adenosylmethionine decarboxylase and its use in determining the relative abilities of individual polyamines to sustain growth and viability of L1210 cells

Hydroxylamine derivatives for regulation of spermine and spermidine metabolism

The biogenic polyamines spermine, spermidine, and their precursor putrescine are present in micro-to-millimolar concentrations in all cell types and are vitally important for their normal growth. High intracellular content of spermine and spermidine determines the multiplicity of the cellular functions of the polyamines. Many of these functions are not well characterized at the molecular level, ensuring the ongoing development of this field of biochemistry. Tumor cells have elevated polyamine level if compared with normal cells, and this greatly stimulates the search for new opportunities to deplete the intracellular pool of spermine and spermidine resulting in decrease in cell growth and even cell death. O-Substituted hydroxylamines occupy their own place among chemical regulators of the activity of the enzymes of polyamine metabolism. Varying the structure of the alkyl substituent made it possible to obtain within one class of chemical compounds highly effective inhibitors and regulators of the activity of all the enzymes of putrescine, spermine and spermidine metabolism (with the exception of FAD-dependent spermine oxidase and acetylpolyamine oxidase), effectors of the polyamine transport system, and even actively transported in cells "proinhibitor" of ornithine decarboxylase. Some principles for the design of specific inhibitors of these enzymes as well as the peculiarities of cellular effects of corresponding O-substituted hydroxylamines are discussed.

Polyamine acetylation modulates polyamine metabolic flux, a prelude to broader metabolic consequences

Recent studies suggest that overexpression of the polyamine-acetylating enzyme spermidine/spermine N(1)-acetyltransferase (SSAT) significantly increases metabolic flux through the polyamine pathway. The concept derives from the observation that SSAT-induced acetylation of polyamines gives rise to a compensatory increase in biosynthesis and presumably to increased flow through the pathway. Despite the strength of this deduction, the existence of heightened polyamine flux has not yet been experimentally demonstrated. Here, we use the artificial polyamine precursor 4-fluoro-ornithine to measure polyamine flux by tracking fluorine unit permeation of polyamine pools in human prostate carcinoma LNCaP cells. Conditional overexpression of SSAT was accompanied by a massive increase in intracellular and extracellular acetylated spermidine and by a 6-20-fold increase in biosynthetic enzyme activities. In the presence of 300 microM 4-fluoro-ornithine, SSAT overexpression led to the sequential appearance of fluorinated putrescine, spermidine, acetylated spermidine, and spermine. As fluorinated polyamines increased, endogenous polyamines decreased, so that the total polyamine pool size remained relatively constant. At 24 h, 56% of the spermine pool in the induced SSAT cells was fluorine-labeled compared with only 12% in uninduced cells. Thus, SSAT induction increased metabolic flux by approximately 5-fold. Flux could be interrupted by inhibition of polyamine biosynthesis but not by inhibition of polyamine oxidation. Overall, the findings are consistent with a paradigm whereby flux is initiated by SSAT acetylation of spermine and particularly spermidine followed by a marked increase in key biosynthetic enzymes. The latter sustains the flux cycle by providing a constant supply of polyamines for subsequent acetylation by SSAT. The broader metabolic implications of this futile metabolic cycling are discussed in detail.

Key role for p27Kip1, retinoblastoma protein Rb, and MYCN in polyamine inhibitor-induced G1 cell cycle arrest in MYCN-amplified human neuroblastoma cells

Alpha-difluoromethylornithine (DFMO) inhibits the proto-oncogene ornithine decarboxylase (ODC) and is known to induce cell cycle arrest. However, the effect of DFMO on human neuroblastoma (NB) cells and the exact mechanism of DFMO-induced cell death are largely unknown. Treatment with DFMO in combination with SAM486A, an S-adenosylmethionine decarboxylase (AdoMetDC) inhibitor, has been shown to enhance polyamine pool depletion. Therefore, we analysed the mechanism of action of DFMO and/or SAM486A in two established MYCN-amplified human NB cell lines. DFMO and SAM486A caused rapid cell growth inhibition, polyamine depletion, and G1 cell cycle arrest without apoptosis in cell lines LAN-1 and NMB-7. These effects were enhanced with combined inhibitors and largely prevented by cotreatment with exogenous polyamines. The G1 cell cycle arrest was concomitant with an increase in cyclin-dependent kinase inhibitor p27Kip1. In a similar fashion, DFMO and DFMO/SAM486A inhibited the phosphorylation of the G1/S transition-regulating retinoblastoma protein Rb at residues Ser795 and Ser807/811. Moreover, we observed a dramatic decrease in MYCN protein levels. Overexpression of MYCN induces an aggressive NB phenotype with malignant behavior. We show for the first time that DFMO and SAM486A induce G1 cell cycle arrest in NB cells through p27Kip1 and Rb hypophosphorylation.

Metabolic and antiproliferative consequences of activated polyamine catabolism in LNCaP prostate carcinoma cells

Depletion of intracellular polyamine pools invariably inhibits cell growth. Although this is usually accomplished by inhibiting polyamine biosynthesis, we reasoned that this might be more effectively achieved by activation of polyamine catabolism at the level of spermidine/spermine N(1)-acetyltransferase (SSAT); a strategy first validated in MCF-7 breast carcinoma cells. We now examine the possibility that, due to unique aspects of polyamine homeostasis in the prostate gland, tumor cells derived from it may be particularly sensitive to activated polyamine catabolism. Thus, SSAT was conditionally overexpressed in LNCaP prostate carcinoma cells via a tetracycline-regulatable (Tet-off) system. Tetracycline removal resulted in a rapid approximately 10-fold increase in SSAT mRNA and an increase of approximately 20-fold in enzyme activity. SSAT products N(1)-acetylspermidine, N(1)-acetylspermine, and N(1),N(12)-diacetylspermine accumulated intracellularly and extracellularly. SSAT induction also led to a growth inhibition that was not accompanied by polyamine pool depletion as it was in MCF-7 cells. Rather, intracellular spermidine and spermine pools were maintained at or above control levels by a robust compensatory increase in ornithine decarboxylase and S-adenosylmethionine decarboxylase activities. This, in turn, gave rise to a high rate of metabolic flux through both the biosynthetic and catabolic arms of polyamine metabolism. Treatment with the biosynthesis inhibitor alpha-difluoromethylornithine during tetracycline removal interrupted flux and prevented growth inhibition. Thus, flux-induced growth inhibition appears to derive from overaccumulation of metabolic products and/or from depletion of metabolic precursors. Metabolic effects that were not excluded as possible contributing factors include high levels of putrescine and acetylated polyamines, a 50% reduction in S-adenosylmethionine, and a 45% decline in the SSAT cofactor acetyl-CoA. Overall, the study demonstrates that activation of polyamine catabolism in LNCaP cells elicits a compensatory increase in polyamine biosynthesis and downstream metabolic events that culminate in growth inhibition.

A perspective of polyamine metabolism

Polyamines are essential for the growth and function of normal cells. They interact with various macromolecules, both electrostatically and covalently and, as a consequence, have a variety of cellular effects. The complexity of polyamine metabolism and the multitude of compensatory mechanisms that are invoked to maintain polyamine homoeostasis argue that these amines are critical to cell survival. The regulation of polyamine content within cells occurs at several levels, including transcription and translation. In addition, novel features such as the +1 frameshift required for antizyme production and the rapid turnover of several of the enzymes involved in the pathway make the regulation of polyamine metabolism a fascinating subject. The link between polyamine content and human disease is unequivocal, and significant success has been obtained in the treatment of a number of parasitic infections. Targeting the polyamine pathway as a means of treating cancer has met with limited success, although the development of drugs such as DFMO (alpha-difluoromethylornithine), a rationally designed anticancer agent, has revolutionized our understanding of polyamine function in cell growth and provided 'proof of concept' that influencing polyamine metabolism and content within tumour cells will prevent tumour growth. The more recent development of the polyamine analogues has been pivotal in advancing our understanding of the necessity to deplete all three polyamines to induce apoptosis in tumour cells. The current thinking is that the polyamine inhibitors/analogues may also be useful agents in the chemoprevention of cancer and, in this area, we may yet see a revival of DFMO. The future will be in adopting a functional genomics approach to identifying polyamine-regulated genes linked to either carcinogenesis or apoptosis.

Hydroxylamine-containing inhibitors of polyamine biosynthesis and impairment of colon cancer cell growth

Polyamine synthesis (by the action of ornithine decarboxylase [ODC] and S-adenosylmethionine decarboxylase [SAMDC]) and polyamine content are high in colon cancer. In addition, colonic lumen is rich in polyamines synthesised by colonic microflora; for this reason, polyamine depletion in colon cancer may be a logical approach to impair growth of colon cancer cells. We evaluated highly specific and reportedly non-toxic hydroxylamine-containing inhibitors of ODC (1-aminooxy-3-aminopropane, APA) and SAMDC (S-(5'-deoxy-5'-adenosyl)-methylthioethyl-hydroxylamine, AMA) in human colon cancer cells (Caco-2 and HT-29) in culture. APA depleted ODC activity within 24 hr, more rapidly than did difluoromethylornithine. APA and AMA in combination (100 microM each) reduced ODC and SAMDC activities to undetectable levels within 24 hr and intracellular polyamines to 8-23% of control. The resulting growth arrest could be reversed only by twice as much spermidine as is physiologically present in the colonic lumen. In concentrations sufficient to deplete growth, APA and AMA were not toxic. Simultaneous treatment with APA, AMA, and 5-fluorouracil reduced colon cancer cell survival more potently than treatment with 5-fluorouracil alone. The hydroxylamine-containing ODC and SAMDC inhibitors APA and AMA are potent inhibitors of colon cancer cell proliferation and might be therapeutically promising in colon cancer.

Polyamine regulation of ornithine decarboxylase and its antizyme in intestinal epithelial cells

Ornithine decarboxylase (ODC) is feedback regulated by polyamines. ODC antizyme mediates this process by forming a complex with ODC and enhancing its degradation. It has been reported that polyamines induce ODC antizyme and inhibit ODC activity. Since exogenous polyamines can be converted to each other after they are taken up into cells, we used an inhibitor of S-adenosylmethionine decarboxylase, diethylglyoxal bis(guanylhydrazone) (DEGBG), to block the synthesis of spermidine and spermine from putrescine and investigated the specific roles of individual polyamines in the regulation of ODC in intestinal epithelial crypt (IEC-6) cells. We found that putrescine, spermidine, and spermine inhibited ODC activity stimulated by serum to 85, 46, and 0% of control, respectively, in the presence of DEGBG. ODC activity increased in DEGBG-treated cells, despite high intracellular putrescine levels. Although exogenous spermidine and spermine reduced ODC activity of DEGBG-treated cells close to control levels, spermine was more effective than spermidine. Exogenous putrescine was much less effective in inducing antizyme than spermidine or spermine. High putrescine levels in DEGBG-treated cells did not induce ODC antizyme when intracellular spermidine and spermine levels were low. The decay of ODC activity and reduction of ODC protein levels were not accompanied by induction of antizyme in the presence of DEGBG. Our results indicate that spermine is the most, and putrescine the least, effective polyamine in regulating ODC activity, and upregulation of antizyme is not required for the degradation of ODC protein.

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

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

Putrescine does not support the migration and growth of IEC-6 cells

The migration of IEC-6 cells is inhibited when the cells are depleted of polyamines by inhibiting ornithine decarboxylase with alpha-difluoromethylornithine (DFMO). Exogenous putrescine, spermidine, and spermine completely restore cell migration inhibited by DFMO. Because polyamines are interconverted during their synthesis and catabolism, the specific role of individual polyamines in intestinal cell migration, as well as growth, remains unclear. In this study, we used an inhibitor of S-adenosylmethionine decarboxylase, diethylglyoxal bis(guanylhydrazone)(DEGBG), to block the synthesis of spermidine and spermine from putrescine. We found that exogenous putrescine does not restore migration and growth of IEC-6 cells treated with DFMO plus DEGBG, whereas exogenous spermine does. In addition, the normal distribution of actin filaments required for migration, which is disrupted in polyamine-deficient cells, could be achieved by adding spermine but not putrescine along with DFMO and DEGBG. These results indicate that putrescine, by itself, is not essential for migration and growth, but that it is effective because it is converted into spermidine and/or spermine.

In vitro manipulation of L1210 cell cycle kinetics with 4-amidinoindan-1-one 2'-amidinohydrazone, alpha-difluoromethylornithine and N1-acetylspermine

We investigated whether in vitro L1210 growth inhibition by alpha-difluoromethylornithine (DFMO; 740 microM) and 4-amidinoindan-1-one 2'-amidinohydrazone (CGP 48664A; 1.7 microM) is reversible with N1-acetylspermine (N1-acSp). Influences of N1-acSp dose (1-100 microM), time (0-12 h at 100 microM), aminoguanidine (AG, 1 mM) and cell numbers (at 1 microM N1-acSp) on percentage S-phase, polyamine contents and viability were determined. DFMO/CGP 48664A decreased percentage S-phase from 58 to 26%, decreased spermidine (Sd) and spermine (Sp) contents 3-fold, but did not affect viability. With increasing N1-acSp dose, S-phase percentage and Sd contents increased concomitantly, reaching plateau values that were comparable with those of untreated controls. S-phase and Sd content increased from 4-6 h after N1-acSp administration, reaching plateau values from 11 and 6 h, respectively. N1-acSp content was dose dependent and increased linearly to reach plateau values from 8 h. AG did not affect any of these parameters. Addition of 1 microM N1-acSp to decreasing numbers of DFMO/CGP 48664A-treated cells caused increasing S-phase percentage, Sd and N1-acSp contents. We conclude that cell cycle kinetics of cultured L1210 cells can be manipulated by the induction of growth inhibition with DFMO/CGP 48664A and its subsequent abolishment with N1-acSp. N1-acSp accumulation rate and its subsequent conversion to Sd is relatively slow compared with intracellular Sd needs. The data support the notion that Sd is the most important polyamine for growth.

4-Amidinoindan-1-one 2'-amidinohydrazone (CGP 48664A) exerts in vitro growth inhibitory effects that are not only related to S-adenosylmethionine decarboxylase (SAMdc) inhibition

The competitive S-adenosylmethionine decarboxylase (SAMdc; EC 4.1.1.50) inhibitor 4-amidinoindan-1-one 2'-amidinohydrazone (CGP 48664A) inhibits growth more effectively than the irreversible SAMdc inhibitor 5'-[[(Z)-4-amino-2-butenyl]methylamino]-5'-deoxyadenosine (AbeAdo), while having similar effects on polyamine contents. We hypothesized that growth inhibition by CGP 48664A is not merely accomplished by SAMdc inhibition. Concentration-related growth inhibitory effects of AbeAdo, CGP 48664A and methylglyoxal bis(guanylhydrazone) (MGBG) were investigated in L1210 cells that were additionally exposed to 10 microM AbeAdo. This concentration causes maximal growth inhibition, profound SAMdc inhibition and plateau polyamine contents. Almost complete inhibition of functional SAMdc activity by 10 microM AbeAdo was confirmed by demonstration of poor conversion of tetradeuterated spermidine to tetradeuterated spermine by gas chromatography-mass spectrometry. Increasing AbeAdo did not affect L1210 cell numbers, viability, nor polyamine contents. MGBG proved highly toxic. CGP 48664A did not affect L1210 polyamine contents, but cell numbers and viability decreased dose-dependently to 50% and 70% of control, respectively. We conclude that CGP 48664A inhibits L1210 growth not only through SAMdc inhibition, but also by an as yet poorly understood second effect with higher IC50. The alleged second effect of CGP 48664A appears important for its potent antitumor effect.

CGP 48664, a potent and specific S-adenosylmethionine decarboxylase inhibitor: effects on regulation and stability of the enzyme

Mammalian S-adenosylmethionine decarboxylase (AdoMetDC) catalyses a regulatory important step in the biosynthesis of polyamines and is a potential target for therapeutic agents against various parasitic diseases and proliferative disorders. In the present study we examined the effects of a newly synthesized AdoMetDC inhibitor. 4-amidinoindan-1-one 2'-amidinohydrazone (CGP 48664), on polyamine metabolism in the mouse leukaemia cell line L1210. Treatment of the cells with 2 microM CGP 48664 led to a depletion of cellular spermidine and spermine. The putrescine content, in contrast, was markedly increased. Cells seeded in the presence of the inhibitor showed a significant decrease in growth rate, which was fully reversed by the addition of 2 microM spermidine or 1 microM spermine. The syntheses of ornithine decarboxylase and AdoMetDC were greatly increased in cells treated with CGP 48664. These increases were not correlated with similar changes in the mRNA levels, indicating the involvement of a translational mechanism. CGP 48664 was demonstrated to be a very poor competitor of spermidine uptake in the L1210 cells. L1210 cells deficient in polyamine transport were as sensitive to the antiproliferative effect of the inhibitor as were the parental cells, indicating that CGP 48664 did not enter the cells by the polyamine transport system. In addition to inhibiting AdoMetDC, CGP 48664 stabilized the enzyme against degradation. In the present study we also demonstrated that aminoguanidine (AMG), which is frequently used in cellular systems to inhibit any action of serum polyamine oxidase, apparently inhibits AdoMetDC by an irreversible mechanism that markedly stabilizes the enzyme against proteolytic degradation. CGP 48664 and the parental compound methylglyoxal bis(guanylhydrazone), which is also a potent inhibitor of AdoMetDC, contain one or two AMG-like moieties; the importance of these residues in the inhibition of AdoMetDC is discussed.

Inhibition of spermidine synthase gene expression by transforming growth factor-beta 1 in hepatoma cells

We screened genes responsive to transforming growth factor-beta (TGF-beta 1) protein in a human hepatoma cell line (Hep3B) using a PCR-mediated differential display technique, in order to investigate the mechanisms involved in TGF-beta-induced growth suppression. We found a gene that was down-regulated by TGF-beta 1 to be completely identical in an approx. 620 bp segment to the gene for the enzyme spermidine synthase, which mediates the conversion of putrescine into spermidine. Both spermidine synthase mRNA expression and its enzyme activity were decreased after TGF-beta 1 treatment of Hep3B cells. The inhibition of spermidine synthase gene expression by TGF-beta 1 protein was also observed in other hepatoma cell lines. The expression of genes for other biosynthetic enzymes in polyamine metabolism (ornithine decarboxylase and S-adenosylmethionine decarboxylase) was also inhibited to the same extent as for spermidine synthase, while the gene expression of spermidine/spermine N1-acetyltransferase, a catabolic enzyme, was relatively resistant to TGF-beta 1. Spermine levels in Hep3B cells were decreased by TGF-beta 1 treatment, although the levels of spermidine and putrescine were unchanged, probably due to compensation by remaining spermidine/spermine N1-acetyltransferase activity. Exogenously added spermidine or spermine, but not putrescine, partially antagonized the growth-inhibitor effects of TGF-beta 1 on Hep3B cells. Our data suggest that down-regulation of gene expression of the enzymes involved in polyamine metabolism, including spermidine synthase, may be associated with the mechanism of TGF-beta-induced growth suppression.

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.

Use of 4-fluoro-L-ornithine to monitor metabolic flux through the polyamine biosynthetic pathway

The mechanistic effectiveness of various polyamine analogs and enzyme inhibitors is typically determined by their ability to deplete intracellular polyamine pools. In this study, we describe an assay that may prove useful in augmenting this relatively static assessment of drug action. The assay relies upon the substitution of 4-fluoro-L-ornithine (Fl-Orn) for ornithine as a polyamine precursor to provide a means to measure metabolic flux through polyamine pools. At concentrations up to 500 microM, the analog did not inhibit the growth of L1210 murine leukemia cells during incubations of up to 72 hr. Using HPLC, the analog was processed metabolically over time to what was deduced to be 2-fluoroputrescine, 6-fluorospermidine and 6-fluorospermine. The relative proportion of fluorinated polyamine analog to the natural polyamine increased with time and Fl-Orn concentration. The sum of the two was found to be nearly identical to the respective polyamine pool of control cells exposed instead to 500 microM ornithine. This indicates that Fl-Orn was recognized and utilized as a precursor at a rate very similar to that of ornithine itself. Using L1210 cells at different stages of cell growth, it was determined that the metabolic flux through the pools, as indicated by the rate of appearance of individual fluorinated polyamine species, reflected the proliferation status of the cells--non-growing cells failed to incorporate the analog. Likewise, in cell types with varying polyamine pool profiles, such as polyamine enzyme overproducers or those with constitutively different spermidine of spermine ratios, the incorporation of the fluorinated analogs into pools was found to be proportional to the size to the natural polyamine pool. In cells treated with inhibitors of S-adenosylmethionine decarboxylase, Fl-Orn incorporation indicated a total blockade of polyamine synthesis at that enzyme site. Overall, the Fl-Orn assay has demonstrated that polyamine pool profiles generally reflect the rate of flux through the pathway in proliferating cells, suggesting that most intracellular polyamines are freely exchangeable with those undergoing metabolic flux.

In vivo effects of 4-amidinoindan-1-one 2'-amidinohydrazone (CGP 48664A) and alpha-difluoromethylornithine (DFMO) on L1210 growth, cell-cycle phase distribution and polyamine contents

We studied the in vivo effects of 4-amidinoindan-1-one 2'-amidinohydrazone (CGP 48664A), alpha-difluoromethylornithine (DFMO) and a combination of CGP 48664A-DFMO on tumor growth, cell-cycle phase distribution and polyamine contents. DBA-2 mice were inoculated i.p. with 10(5) L1210 cells on day 0, treated i.p. on days 1-4 and killed on day 5. As compared to controls, CGP 48664A, DFMO and the CGP 48664A-DFMO combination reduced L1210 cell numbers by 33, 43 and 85%, respectively. CGP 48664A did not affect cell-cycle phase distribution. DFMO and the CGP 48664A-DFMO combination caused a moderate and a heavy accumulation in G0/G1- and G2/M-phases, respectively. Compared with controls, the CGP 48664A-DFMO combination reduced putrescine, spermidine and total polyamines, but did not affect spermine. Compared with CGP 48664A, the CGP 48664A-DFMO combination caused lower putrescine and total polyamines, higher spermine, but no change in spermidine. Compared with DFMO, the CGP 48664A-DFMO combination caused higher putrescine and spermidine, lower spermine, but no change in total polyamine levels. We conclude that CGP 48664A potentiates the cystostatic effect of DFMO in vivo. The resulting growth inhibition is accompanied by an accumulation in G0/G1- and G2/M-phases and a reduction of putrescine and spermidine. The data suggest that perturbed polyamine composition rather than reduced spermidine or total polyamine pool size causes a profound growth inhibition.

Irreversible inhibition of human S-adenosylmethionine decarboxylase by the pure diastereomeric forms of S-(5'-deoxy-5'-adenosyl)-1-ammonio-4- methylsulfonio-2-cyclopentene (AdoMac)

The S-adenosylmethionine (AdoMet) analogue AdoMac [S-(5'-deoxy-5'-adenosyl)-1-ammonio-4-methylsulfonio-2-cyclopen ten e], an enzyme-activated, irreversible inhibitor of the Escherchia coli form of S-adenosylmethionine decarboxylase (AdoMet-DC), also acts as a potent inhibitor of the human form of the enzyme. This analogue has been resolved recently into its four possible diastereomeric forms, and each pure diastereomer has now been evaluated as an inhibitor of human AdoMet-DC. As was the case for the bacterial enzyme, kinetic analysis revealed that the four pure diastereomeric forms of AdoMac differentially inhibit human S-adenosylmethionine decarboxylase (K(i) values ranging between 11 and 63 microM). Although the human and bacterial forms of the enzyme each discriminated between the four diastereomers of AdoMac, each form appeared to bind optimally to a distinctly different diastereomer of the inhibitor. These data suggest that the active sites of human and bacterial AdoMet-DC are distinctly different, and that it may be possible to design inhibitors that are specific for a given form of the enzyme.

Stable amplification of the S-adenosylmethionine decarboxylase gene in Chinese hamster ovary cells

A Chinese hamster ovary cell subline (CHO/664) > 1000-fold resistant to the S-adenosylmethionine decarboxylase (AdoMetDC) inhibitor, CGP-48664 (4-(aminoiminomethyl)-2,3-dihydro-1H-inden-1-one diaminomethylenehydrazone), has been developed and characterized. The cells were also cross-resistant to the highly specific nucleoside analog inhibitor of AdoMetDC, MDL-73811. These unique cells stably overexpress AdoMetDC due to a 10-16-fold amplification of the AdoMetDC gene, which resulted in a similar increase in AdoMetDC transcript levels. In the presence of 100 microM CGP-48664, the CHO/664 cells displayed AdoMetDC activities similar to the parental line. Following removal of the inhibitor, AdoMetDC activity increased steadily over 20 days to 10-12 times that found in parental CHO cells. Decarboxylated (dc) AdoMet pools accumulated rapidly from < 5 pmol/10(6) cells to approximately 1000-1500 pmol/10(6) cells at 3 days due to diffusion away of intracellular inhibitor and to the depletion of putrescine and spermidine as aminopropyl acceptors in dcAdoMet-mediated synthase reactions. Polyamine pools shifted as putrescine, and spermidine pools were processed forward to spermine. During the period from 3 days to 20 days, dcAdoMet pools fell steadily and eventually stabilized at 100-200 pmol/10(6) cells. Providing excess putrescine at this time as an aminopropyl acceptor rapidly lowered dcAdoMet pools and led to a near normalization of polyamine pools, indicating that both dcAdoMet and putrescine are essential in maintaining steady-state polyamine pool profiles. As with cell line variants that overproduce ornithine decarboxylase, polyamine transport was found to be increased in CHO/664 cells due to an apparent inability of the system to down-regulate polyamine transport in response to polyamine excess. Given the unique metabolic disturbances seen in these cells, we anticipate that in addition to providing a useful system for evaluating the specificity of newly developed AdoMetDC inhibitors, they will undoubtedly prove valuable for investigating the various regulatory interrelationships involved in polyamine homeostasis and possibly other aspects of purine metabolism.

Effects of the S-adenosylmethionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, on cell growth and polyamine metabolism and transport in Chinese hamster ovary cell cultures

The regulation of polyamine transport and the roles of polyamine transport and synthesis in cell growth were investigated using cultured Chinese hamster ovary (CHO) cells and CHOMG cells which are mutants lacking polyamine-transport activity. Metabolically stable methylated polyamine analogues were used to measure polyamine accumulation, and the irreversible S-adenosyl-L-methionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (AbeAdo), was used to inhibit synthesis. Exposure to AbeAdo lead to a dose-dependent decrease in growth for both cell lines, although CHOMG cells were more sensitive. Intracellular putrescine levels were greatly increased in AbeAdo-treated CHO cells and to a lesser extent in CHOMG cells, whereas intracellular spermidine and spermine levels were substantially reduced in both. Treatment with AbeAdo increased putrescine content in the culture medium to a much greater extent in CHOMG cultures indicating that a portion of the excess putrescine synthesized in response to AbeAdo treatment is excreted, but that CHO cells salvage this putrescine whereas it is lost to CHOMG cells which cannot take up polyamines. AbeAdo treatment increased polyamine transport into CHO cells despite high intracellular putrescine, suggesting that spermidine and/or spermine, and not putrescine, are the major factors regulating transport activity. The accumulation of either 1-methylspermidine or 1,12-dimethylspermine was significantly increased by AbeAdo treatment. Accumulation was increased even further when protein synthesis was blocked by cycloheximide, indicating that a short-lived protein is involved in the regulation of polyamine uptake. In the presence of cycloheximide and AbeAdo or alpha-difluoromethylornithine, methylated polyamine derivatives accumulated to very high levels leading to cell death. These results show that the polyamine-transport system plays an important role in retaining intracellular polyamines and that down-regulation of the transport system in response to increased intracellular polyamine content is necessary to prevent accumulation of toxic levels of polyamines.

Consequences of aberrant ornithine decarboxylase regulation in rat hepatoma cells

DH23A cells, an alpha-difluoromethylornithine (DFMO)-resistant variant of rat hepatoma tissue culture cells (HTC), contain high levels of very stable ornithine decarboxylase (ODC). In the absence of DFMO, the high ODC activity results in a large accumulation of endogenous putrescine. Concomitant with the putrescine increase is a period of cytostasis and a subsequent loss of viable cells. In contrast, HTC cells with a moderate polyamine content can be maintained in exponential growth. This suggests that a moderate polyamine concentration is necessary for both optimal cell growth and survival. The cytotoxicity observed in the DH23A cells is apparently not due to byproducts of polyamine oxidation or alterations in steady state intracellular pH or free [Ca2+]. It is possible to mimic the effects of high levels of stable ODC by treatment of cells with exogenous putrescine in the presence of DFMO. This suggests that overaccumulation of putrescine is the causative agent in the observed cytotoxicity, although the mechanism is unclear. These data support the hypothesis that downregulation of ODC may be necessary to prevent accumulation of cytotoxic concentrations of the polyamines.

Polyamines in testosterone-induced hypertrophic and antifolate-induced hyperplastic mouse kidney. Differential effect of alpha-difluoromethylornithine

In the testosterone-induced hypertrophic and antifolate (N10-propargyl,5,6-dideazafolic acid, CB 3717)-induced hyperplastic mouse kidney models, a marked increase of two diamine levels--putrescine and cadaverine--occurred which paralleled induced ornithine decarboxylase (ODC) activity. Under these conditions the augmentation of spermidine levels was much smaller, while spermine levels were affected differentially--increased by testosterone and decreased by CB 3717; this resulted in an increase of spermidine/spermine ratio in hyperplastic, but not hypertrophic kidney. alpha-Difluoromethylornithine (DFMO) prevented testosterone- or CB 3717-induced increment of both diamine levels. Spermidine and spermine depletion in response to DFMO was significant in hyperplastic kidney only. DFMO also significantly affected the other biochemical markers of hyperplasia, namely lowered CB 3717-induced cell proliferation rate and increased S-adenosylmethionine decarboxylase (AdoMetDC) activity. In contrast, testosterone-induced hypertrophy was not influenced by DFMO, as judged by the lack of its effect on S-adenosylmethionine synthetase and cystathionine and synthase activity. These results indicate that the increase of putrescine levels does not mediate testosterone-induced renal hypertrophy and possibly also antifolate-induced hyperplasia. The involvement of spermidine in mediation of renal hyperplasia is highly possible, while that of spermine is excluded.

Regulation of polyamine transport by polyamines and polyamine analogs

Regulation of polyamine transport in murine L1210 leukemia cells was characterized in order to better understand its relationship to specific intracellular polyamines and their analogs and to quantitate the sensitivity by which it is controlled. Up-regulation of polyamine uptake was evaluated following a 48-hr treatment with a combination of biosynthetic enzyme inhibitors to deplete intracellular polyamine pools. The latter declined gradually over 48 hr and was accompanied by a steady increase in spermidine (SPD) and spermine (SPM) transport as indicated by rises in Vmax to levels approximately 4.5 times higher than control values. Restoration of individual polyamine pools during a 6-hr period following inhibitor treatment revealed that SPD and SPM uptake could not be selectively affected by specific pool changes. The effectiveness of individual polyamines in reversing inhibitor-induced stimulation of uptake was as follows: putrescine < SPD < SPM = the SPM analog, N1, N12-bis(ethyl)spermine (BESPM). In contrast to stimulation of transport, down-regulation by exogenous polyamines or analogs occurred rapidly and in response to subtle increases in intracellular pools. Following a 1-hr exposure to 10 microM BESPM, Vmax values for SPD and SPM fell by 70%, whereas the analog pool increased to only 400-500 pmol/10(6) cells--about 15-20% of the total polyamine pool (approximately 2.8 nmol/10(6) cells). SPM produced nearly identical regulatory effects on transport kinetics. Both BESPM and SPM were even more effective at down-regulating transport that had been previously stimulated four to fivefold by polyamine depletion achieved with enzyme inhibitors. A dose response with BESPM at 48 hr revealed a biphasic effect on uptake whereby concentrations of analog < 3 microM produced an increase in SPD and SPM Vmax values, whereas concentrations 3 microM and higher produced a marked suppression of these values. Cells treated with 3 microM BESPM for 2 hr and placed in analog-free medium recovered transport capability in only 3 hr. Thus, whereas stimulation of polyamine transport is a relatively insensitive and slowly responsive process that tends to parallel polyamine depletion, down-regulation of polyamine transport by exogenous polyamines and analogs and its reversal are rapidly responsive events that correlate with relatively small (i.e., 15-20%) changes in intracellular polyamine pools.

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

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

Effects of chronic 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxy- adenosine (AbeAdo) treatment on polyamine and eIF-5A metabolism in AbeAdo-sensitive and -resistant L1210 murine leukaemia cells

We have previously reported that prolonged chronic exposure to the S-adenosyl-L-methionine decarboxylase (AdoMetDC) inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxy-adenosine (MDL 73811, AbeAdo), leads to cytostasis of L1210 cells [Byers, Ganem and Pegg (1992) Biochem. J. 287, 717-724]. Further studies to investigate the mechanism by which these effects are brought about were carried out by comparing an L1210-derived cell line (R20) that is resistant to AbeAdo with the parent cells. The R20 cells were derived by two rounds of AbeAdo-induced cytostasis followed by rescue with exogenous polyamines. Cytostasis was induced in L1210 cells treated for 12 days with 10 microM AbeAdo; however, exposure to up to 40 microM AbeAdo did not induce cytostasis in R20 cells. Putrescine levels were elevated and spermine levels were depleted in both treated L1210 and treated R20 cells. Spermidine was depleted in treated L1210 cells but was only partly reduced in treated R20 cells. AdoMetDC activity was below the limit of detection in treated L1210 cells but, although greatly reduced, could be measured in the treated R20 cells. The resistance of the R20 cells to the effects of AbeAdo on cell growth and spermidine depletion correlated with reduced AbeAdo accumulation by R20 cells. In the absence of spermidine synthesis, unhypusinated eukaryotic translation initiation factor 5A (eIF-5A) accumulated in AbeAdo-treated L1210 cells. There was no detectable accumulation of unhypusinated eIF-5A in R20 cells. Unhypusinated eIF-5A accumulated during AbeAdo treatment was depleted in L1210 cells rescued by exogenous spermidine. These findings are consistent with the hypothesis that AbeAdo-induced cytostasis is due to the loss of hypusinated eIF-5A. However, spermine was able to rescue AbeAdo-treated L1210 cells without significantly reducing the unhypusinated eIF-5A accumulated during AbeAdo treatment, suggesting that only a small amount of the unmodified protein must be hypusinated to restore cell growth.

Regulation of S-adenosylmethionine decarboxylase activity by alterations in the intracellular polyamine content

The effects of addition of exogenous spermidine and spermine and of two inhibitors of polyamine biosynthesis, alpha-difluoromethylornithine (DFMO), which decreases spermidine concentrations, and n-butyl-1,3-diaminopropane, which depletes spermine, on the expression of S-adenosylmethionine decarboxylase (AdoMetDC) activity were studied in mammalian cell lines (HT29, CHO and COS-7). AdoMetDC levels were inversely related to the polyamine content, and spermine was the more potent repressor of AdoMetDC activity, but only spermidine affected the amount of AdoMetDC mRNA. Transfection of COS-7 cells or CHO cells with plasmid constructs containing a chloramphenicol acetyltransferase (CAT) reporter gene driven by portions of the AdoMetDC promoter region indicated that CAT expression was altered by spermidine, but not by spermine, suggesting that there is a spermidine-responsive element in this promoter. Transient transfection of COS-7 cells with pSAMh1, a plasmid containing the AdoMetDC cDNA in a vector with the SV40 promoter and origin of replication, led to a large increase in AdoMetDC expression. Although treatment of COS-7 cells with n-butyl-1,3-diaminopropane greatly increased endogenous AdoMetDC activity, the spermine depletion brought about by this inhibitor did not stimulate AdoMetDC expression from pSAMh1. The pSAMh1 cDNA is missing 72 nucleotides from the 5' end of the AdoMetDC mRNA, and it is possible that translational regulation by spermine involves this region. The expression of AdoMetDC from pSAMh1 in COS-7 cells was greatly inhibited by DFMO treatment, although endogenous AdoMetDC activity was increased. The expression of other plasmids containing the SV40 origin of replication was also inhibited by DFMO in COS-7 cells, but not in CHO cells. DFMO treatment did not interfere with the expression of plasmids driven by the RSV promoter. These results suggest that low spermidine levels interfere with the replication of plasmids containing the SV40 origin of replication.

S-adenosylmethionine decarboxylase as an enzyme target for therapy

The polyamine biosynthetic pathway has attracted much interest as a therapeutic target. Many studies have shown the potential value of inhibitors of the first enzyme in the biosynthetic pathway, ornithine decarboxylase, which forms putrescine. In order to convert putrescine into the polyamines, spermidine and spermine, the aminopropyl donor, decarboxylated S-adenosylmethionine, is needed. Therefore, S-adenosylmethionine decarboxylase (AdoMetDC, EC 4.1.1.50) is essential for polyamine synthesis. Early studies of the inhibition of this enzyme were carried out with compounds such as methylglyoxal bis(guanylhydrazone) that lack specificity and also lack potency since they are competitive inhibitors whose effects are overcome by a compensatory increase in the amount of the target enzyme. Recently, powerful irreversible inhibitors of AdoMetDC have become available including 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, an enzyme activated inhibitor and 5'-deoxy-5'-[(3-hydrazinopropyl)methylamino]adenosine which binds to the active site and forms a covalent bond with the pyruvate prosthetic group. This review describes the current state of knowledge of the structure and properties of AdoMetDC, the available inhibitors of this enzyme, their mechanism of action and their effects on polyamines and on the growth of tumors and protozoan parasites. These effects indicate that AdoMetDC inhibitors may be of therapeutic value either alone or in combination with ornithine decarboxylase inhibitors and that further trials of these compounds should be considered.

Cytostasis induced in L1210 murine leukaemia cells by the S-adenosyl-L-methionine decarboxylase inhibitor 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine may be due to hypusine depletion

The effects of inhibition of the capacity to form spermidine and spermine on cell growth were investigated using murine leukaemia L1210 cells and 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811, AbeAdo), an enzyme-activated irreversible inhibitor of S-adenosyl-L-methionine decarboxylase. Putrescine levels were increased 80-fold, and spermidine and spermine levels were greatly reduced after a 3-day exposure to a maximally inhibitory dose of 10 microM-AbeAdo. Addition of AbeAdo to the culture medium inhibited the growth of L1210 cells measured 3 days later in a dose-dependent manner, but, even at a dose of 10 microM, which was maximally effective, exposure to AbeAdo was not immediately cytostatic. However, the growth rate of L1210 cells chronically exposed to 10 microM-AbeAdo declined steadily until day 12, when the cells stopped growing. L1210 cells exposed to AbeAdo for 12 days could not be rescued from cytostasis by removal of the drug from the culture, but could be rescued by exposure to exogenous spermidine or spermine, indicating that the growth-inhibitory effects of AbeAdo were a result of spermidine and/or spermine depletion. It is suggested that elevated intracellular putrescine in AbeAdo-treated cells sustained limited growth in the absence of physiological levels of spermidine and spermine until certain critical and specific physiological role(s) fulfilled by spermidine (and/or spermine) became deficient resulting in cytostasis. N-(3-Aminopropyl)-1,4-diamino-cis-but-2-ene, a spermidine analogue that is a substrate for deoxyhypusine synthase, was able to mimic the effects of spermidine in reversing AbeAdo-induced cytostasis. Spermidine analogues such as 5,5-dimethylspermidine, which are not substrates for deoxyhypusine synthase, were not active in this way. These results provide evidence that the formation of hypusine in the protein-synthesis initiation factor eIF-5A may be a critical role of spermidine essential for cell growth.

Irreversible inhibition of S-adenosylmethionine decarboxylase of Trypanosoma brucei brucei by S-adenosylmethionine analogues

S-Adenosylmethionine analogues designed as active-site directed inhibitors were tested in vitro for their effects on S-adenosylmethionine decarboxylase (AdoMetDC) of Trypanosoma brucei brucei. These analogues contained a tertiary nitrogen atom in place of the sulfonium and had a side chain of variable length ending in a reactive group (hydrazino-, aminooxy-, hydrazido- or a methylnitrosourea). The hydrazino- derivatives were the most potent inhibitors with IC50 values in the range of 40-100 nM. The most active compound (IC50 of 0.04 microM) was 5'-deoxy-5'-[(2-hydrazinoethyl)-methylamino]adenosine (MHZEA). Addition of MHZEA produced a time-dependent inactivation with an apparent Ki of 0.4 microM, and the enzyme half-life at a saturating concentration of MHZEA was 0.4 min. Increasing the length of the side chain or changing the methyl group attached to the nitrogen to an ethyl group reduced the potency. Replacement of the hydrazino moiety with an aminooxy group resulted in about a 30- to 35-fold decrease in inhibition potency. However, the relative order of activities of these aminooxy analogues was similar to that found in the hydrazino series with 5'-deoxy-5'-[(2-aminooxyethyl)methylamino]adenosine (MAOEA), which had an IC50 of 1.3 microM, being the most active. The hydrazido analogs were even less effective with 5'-deoxy-5'-[(3-hydrazino-3-oxopropyl)-methylamino]adenosine, the best inhibitor, having an IC50 value of 8.7 microM. The methylnitrosourea derivatives were inactive. The inactivation of trypanosomal AdoMetDC with MHZEA or MAOEA was irreversible and was greatly stimulated by putrescine, a known activator of the enzyme, indicating that the compounds bind to the active site and form a covalent bond with the enzyme. These inhibitors may have considerable potential as chemotherapeutic agents against trypanosomiasis and other protozoal infections and may also be useful in studying the role of AdoMetDC in the regulation of polyamine levels in these organisms.

Purification of human S-adenosylmethionine decarboxylase expressed in Escherichia coli and use of this protein to investigate the mechanism of inhibition by the irreversible inhibitors, 5'-deoxy-5'-[(3-hydrazinopropyl)methylamino]adenosine and 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine

Human S-adenosylmethionine decarboxylase (AdoMetDC) was expressed in high yield in Escherichia coli using the pIN-III(lppP-5) expression vector and purified to apparent homogeneity using affinity chromatography on methylglyoxal bis(guanylhydrazone)-Sepharose. The inactivation of the purified enzyme by 5'-deoxy-5'-[(3-hydrazinopropyl)methylamino]adenosine (MHZPA) was accompanied by an increase in absorbance at 260 nm of the large subunit. This increase was equivalent to the addition of 1 molecule of MHZPA. After digestion with the protease Lys-C, a peptide that contained the bound MHZPA was isolated and found to have the amino acid composition consistent with that expected from the amino terminus of the large subunit. These results indicate that MHZPA inactivates AdoMetDC by forming a hydrazone derivative at the pyruvate prosthetic group. Inactivation of AdoMetDC by 5'-([(Z)-4-amino-2-butenyl]methylamino]-5'-deoxyadenosine (AbeAdo) led to the appearance of a new peptide peak in the Lys-C protease digest. This peptide had the sequence ASMFVSK. This agrees with the expected sequence from the amino terminus, which is pyruvoyl-SMFVSK, with the exception that the pyruvate has been converted to alanine. Direct gas-phase sequencing of the large subunit of the enzyme also indicated the presence of alanine at the amino terminus after inactivation with AbeAdo. These results indicate that this inhibitor leads to transamination of the pyruvate prosthetic group. Since the pyruvate is covalently linked to the protein, its replacement by alanine leads to an irreversible inactivation of AdoMetDC.

Sequestered end products and enzyme regulation: the case of ornithine decarboxylase

The polyamines (putrescine, spermidine, and spermine) are synthesized by almost all organisms and are universally required for normal growth. Ornithine decarboxylase (ODC), an initial enzyme of polyamine synthesis, is one of the most highly regulated enzymes of eucaryotic organisms. Unusual mechanisms have evolved to control ODC, including rapid, polyamine-mediated turnover of the enzyme and control of the synthetic rate of the protein without change of its mRNA level. The high amplitude of regulation and the rapid variation in the level of the protein led biochemists to infer that polyamines had special cellular roles and that cells maintained polyamine concentrations within narrow limits. This view was sustained in part because of our continuing uncertainty about the actual biochemical roles of polyamines. In this article, we challenge the view that ODC regulation is related to precise adjustment of polyamine levels. In no organism does ODC display allosteric feedback inhibition, and in three types of organism, bacteria, fungi, and mammals, the size of polyamine pools may vary radically without having a profound effect on growth. We suggest that the apparent stability of polyamine pools in unstressed cells is due to their being largely bound to cellular polyanions. We further speculate that allosteric feedback inhibition, if it existed, would be inappropriately responsive to changes in the small, freely diffusible polyamine pool. Instead, mechanisms that control the amount of the ODC protein have appeared in most organisms, and even these are triggered inappropriately by variation of the binding of polyamines to ionic binding sites. In fact, feedback inhibition of ODC might be maladaptive during hypoosmotic stress or at the onset of growth, when organisms appear to require rapid increases in the size of their cellular polyamine pools.

Regulation of ornithine decarboxylase gene expression, polyamine levels, and DNA synthetic rates by all-trans-retinoic acid in cultured human keratinocytes

Regulation of ornithine decarboxylase (ODC) gene expression and cell growth by all-trans-retinoic acid in the presence and absence of exogenous putrescine were examined in normal keratinocyte cultures maintained in serum-free medium containing 0.15 mM Ca++. Putrescine and the higher polyamines are negative feedback regulators of ODC synthesis and are essential for cell growth. Human keratinocytes were incubated with and without 1 microM putrescine and the effects of 5 x 10(-7) M retinoic acid on ODC mRNA levels, ODC activity, polyamine levels, and DNA synthetic rates were determined. Northern blot analysis of total RNA isolated from breast reduction keratinocytes treated with retinoic acid up to 24 h showed a time-dependent suppression of ODC mRNA levels that was unaffected by putrescine. ODC activity was suppressed more rapidly in keratinocytes grown in the absence of putrescine; however, at 24 h, ODC activity was suppressed to an equal extent under both culture conditions. The effect of retinoic acid on polyamine levels was determined in the absence of exogenous putrescine. Retinoic acid treatment markedly suppressed putrescine and N1-acetylspermidine levels, whereas spermidine and spermine levels were relatively unaffected. The effect of retinoic acid on DNA synthetic rates, as measured by 3H-thymidine incorporation, was variable. Retinoic acid either stimulated or had little effect on keratinocyte DNA synthetic rates in cells derived from breast reductions and cultured in the absence of putrescine; these effects were not opposed by the presence of exogenous putrescine. In contrast, DNA synthesis in keratinocytes derived from neonatal foreskins was consistently suppressed by retinoic acid, independent of the polyamine status. Our data, therefore, suggest that the effect of retinoic acid on cell growth, as indicated by DNA synthetic rates, does not necessarily parallel its effect on ODC activity and mRNA levels.

Polyamines: from molecular biology to clinical applications

The polyamines putrescine, spermidine and spermine represent a group of naturally occurring compounds exerting a bewildering number of biological effects, yet despite several decades of intensive research work, their exact physiological function remains obscure. Chemically these compounds are organic aliphatic cations with two (putrescine), three (spermidine) or four (spermine) amino or amino groups that are fully protonated at physiological pH values. Early studies showed that the polyamines are closely connected to the proliferation of animal cells. Their biosynthesis is accomplished by a concerted action of four different enzymes: ornithine decarboxylase, adenosylmethionine decarboxylase, spermidine synthase and spermine synthase. Out of these four enzyme, the two decarboxylases represent unique mammalian enzymes with an extremely short half life and dramatic inducibility in response to growth promoting stimuli. The regulation of ornithine decarboxylase, and to some extent also that of adenosylmethionine decarboxylase, is complex, showing features that do not always fit into the generally accepted rules of molecular biology. The development and introduction of specific inhibitors to the biosynthetic enzymes of the polyamines have revealed that an undisturbed synthesis of the polyamines is a prerequisite for animal cell proliferation to occur. The biosynthesis of the polyamines thus offers a meaningful target for the treatment of certain hyperproliferative diseases, most notably cancer. Although most experimental cancer models responds strikingly to treatment with polyamine antimetabolites--namely, inhibitors of various polyamine synthesizing enzymes--a real breakthrough in the treatment of human cancer has not yet occurred. It is, however, highly likely that the concept is viable. An especially interesting approach is the chemoprevention of cancer with polyamine antimetabolites, a process that appears to work in many experimental animal models. Meanwhile, the inhibition of polyamine accumulation has shown great promise in the treatment of human parasitic diseases, such as African trypanosomiasis.

Effect of magnesium ions on the inhibition of S-adenosylmethionine decarboxylase from Escherichia coli by [2-(amino-oxy)ethyl](5'-deoxyadenosin-5'-yl)(methyl)sulphonium

[2-(Amino-oxy)ethyl](5'-deoxyadenosin-5'-yl)(methyl)sulphonium+ ++, the amino-oxy analogue of decarboxylated S-adenosylmethionine, is a potent irreversible inhibitor of Escherichia coli S-adenosylmethionine decarboxylase [Khomutov, Zavalova, Syrku, Artamonova & Khomutov (1983) Bioorg. Khim. 9, 130-131; Artamonova, Zavalova, Khomutov & Khomutov (1986) Bioorg. Khim. 12, 206-212]. We have shown that Mg2+ ions are required for the irreversible inhibition of the decarboxylase, and that S-adenosylmethionine protects against this inhibition.

Regulation of S-adenosylmethionine decarboxylase in L1210 leukemia cells. Studies using an irreversible inhibitor of the enzyme

A potent irreversible inhibitor of S-adenosylmethionine (AdoMet) decarboxylase, S-(5'-adenosyl)-methylthio-2-aminooxyethane (AdoMeSaoe), was used to study the regulatory control of this key enzyme in the polyamine biosynthetic pathway. Treatment of L1210 cells with the inhibitor completely eradicated the growth-induced rise in AdoMet decarboxylase activity, resulting in a marked decrease in cellular content of spermidine and spermine. The putrescine content, on the other hand, was greatly elevated. Although no detectable AdoMet decarboxylase activity was found in the L1210 cells after treatment with AdoMeSaoe, the cells contained 50-fold higher amounts of AdoMet decarboxylase protein, compared to untreated cells during exponential growth. Part of this increase was shown to be due to elevated synthesis of the enzyme. This stimulation appeared to be related to the decrease in cellular spermidine and spermine content, since addition of either one of the polyamines counteracted the rise in AdoMet decarboxylase synthesis. The synthesis rate was determined by immunoprecipitation of labeled enzyme after a short pulse with [35S]methionine. In addition to a protein that co-migrated with pure rat AdoMet decarboxylase (Mr approximately 32,000), the antibody precipitated a somewhat larger labeled protein (Mr approximately 37,000) that most likely represents the proenzyme form. Treatment of the L1210 cells with AdoMetSaoe also gave rise to a marked stabilization of the decarboxylase which contributed to the increase in its cellular protein content. Addition of spermidine did not significantly affect this stabilization, whereas the addition of spermine reduced the half-life of the enzyme to almost that of the control cells.

The genetics of polyamine synthesis in Neurospora crassa

New mutations of the polyamine pathway of Neurospora crassa fell into three categories. The majority affected ornithine decarboxylase and lay at the previously defined spe-1 locus. One mutation, JP100, defining the new spe-2 locus, eliminated S-adenosyl-methionine decarboxylase and led to putrescine accumulation. Revertants of this mutation suggested that the locus encodes the enzyme. Two other mutations, LV105 and JP120, defined a third locus, spe-3. Strains with these mutations also accumulated putrescine and were presumed to lack spermidine synthase activity, which catalyzes the formation of spermidine from putrescine and decarboxylated S-adenosylmethionine. The three spe loci lay within about 20 map units of one another on the right arm of Linkage Group V in the order: centromere-spe-2-spe-1-spe-3. The requirement for spermidine for growth was much less in spe-2 and spe-3 mutants than in spe-1 mutants, which do not accumulate putrescine. This suggested that putrescine fulfills many, but not all, of the functions of spermidine, or that high levels of putrescine render spermidine more effective in its essential roles.