Regulation of S-adenosylmethionine decarboxylase activity in rat liver and prostate

The regulation of polyamine pathway proteins in models of skeletal muscle hypertrophy and atrophy: a potential role for mTORC1

Polyamines have been shown to be absolutely required for protein synthesis and cell growth. The serine/threonine kinase, the mechanistic target of rapamycin complex 1 (mTORC1), also plays a fundamental role in the regulation of protein turnover and cell size, including in skeletal muscle, where mTORC1 is sufficient to increase protein synthesis and muscle fiber size, and is necessary for mechanical overload-induced muscle hypertrophy. Recent evidence suggests that mTORC1 may regulate the polyamine metabolic pathway, however, there is currently no evidence in skeletal muscle. This study examined changes in polyamine pathway proteins during muscle hypertrophy induced by mechanical overload (7 days), with and without the mTORC1 inhibitor, rapamycin, and during muscle atrophy induced by food deprivation (48 h) and denervation (7 days) in mice. Mechanical overload induced an increase in mTORC1 signaling, protein synthesis and muscle mass, and these were associated with rapamycin-sensitive increases in adenosylmethione decarboxylase 1 (Amd1), spermidine synthase (SpdSyn), and c-Myc. Food deprivation decreased mTORC1 signaling, protein synthesis, and muscle mass, accompanied by a decrease in spermidine/spermine acetyltransferase 1 (Sat1). Denervation, resulted increased mTORC1 signaling and protein synthesis, and decreased muscle mass, which was associated with an increase in SpdSyn, spermine synthase (SpmSyn), and c-Myc. Combined, these data show that polyamine pathway enzymes are differentially regulated in models of altered mechanical and metabolic stress, and that Amd1 and SpdSyn are, in part, regulated in a mTORC1-dependent manner. Furthermore, these data suggest that polyamines may play a role in the adaptive response to stressors in skeletal muscle.

BSCTV C2 attenuates the degradation of SAMDC1 to suppress DNA methylation-mediated gene silencing in Arabidopsis

Plant viruses are excellent tools for studying microbial-plant interactions as well as the complexities of host activities. Our study focuses on the role of C2 encoded by Beet severe curly top virus (BSCTV) in the virus-plant interaction. Using BSCTV C2 as bait in a yeast two-hybrid screen, a C2-interacting protein, S-adenosyl-methionine decarboxylase 1 (SAMDC1), was identified from an Arabidopsis thaliana cDNA library. The interaction was confirmed by an in vitro pull-down assay and a firefly luciferase complemention imaging assay in planta. Biochemical analysis further showed that the degradation of the SAMDC1 protein was inhibited by MG132, a 26S proteasome inhibitor, and that C2 could attenuate the degradation of the SAMDC1 protein. Genetic analysis showed that loss of function of SAMDC1 resulted in reduced susceptibility to BSCTV infection and reduced viral DNA accumulation, similar to the effect of BSCTV C2 deficiency. Bisulfite sequencing analysis further showed that C2 deficiency caused enhanced DNA methylation of the viral genome in infected plants. We also showed that C2 can suppress de novo methylation in the FWA transgenic assay in the C2 transgene background. Overexpression of SAMDC1 can mimic the suppressive activity of C2 against green fluorescent protein-directed silencing. These results suggest that C2 interferes with the host defense mechanism of DNA methylation-mediated gene silencing by attenuating the 26S proteasome-mediated degradation of SAMDC1.

Spermine synthase activity affects the content of decarboxylated S-adenosylmethionine

dcAdoMet (decarboxylated S-adenosylmethionine) is an essential intermediate in the synthesis of polyamines. Its content is normally very low, amounting to less than 5% of that of S-adenosylmethionine itself. It was found that in mice lacking spermine synthase there was a large increase in dcAdoMet and that overexpression of spermine synthase reduced the amount of this nucleoside. There was also an increase in dcAdoMet in cells derived from patients with Snyder–Robinson syndrome, a rare X-linked recessive human disease caused by SMS gene mutations that greatly reduce the content of spermine synthase. These results suggest that there is an inverse relationship between the amount of spermine synthase protein and the content of dcAdoMet and raise the possibility that some of the abnormalities seen in mammals deficient in spermine synthase might be due to changes in dcAdoMet pools.

ODC1 is a critical determinant of MYCN oncogenesis and a therapeutic target in neuroblastoma

Neuroblastoma is a frequently lethal childhood tumor in which MYC gene deregulation, commonly as MYCN amplification, portends poor outcome. Identifying the requisite biopathways downstream of MYC may provide therapeutic opportunities. We used transcriptome analyses to show that MYCN-amplified neuroblastomas have coordinately deregulated myriad polyamine enzymes (including ODC1, SRM, SMS, AMD1, OAZ2, and SMOX) to enhance polyamine biosynthesis. High-risk tumors without MYCN amplification also overexpress ODC1, the rate-limiting enzyme in polyamine biosynthesis, when compared with lower-risk tumors, suggesting that this pathway may be pivotal. Indeed, elevated ODC1 (independent of MYCN amplification) was associated with reduced survival in a large independent neuroblastoma cohort. As polyamines are essential for cell survival and linked to cancer progression, we studied polyamine antagonism to test for metabolic dependence on this pathway in neuroblastoma. The Odc inhibitor alpha-difluoromethylornithine (DFMO) inhibited neuroblast proliferation in vitro and suppressed oncogenesis in vivo. DFMO treatment of neuroblastoma-prone genetically engineered mice (TH-MYCN) extended tumor latency and survival in homozygous mice and prevented oncogenesis in hemizygous mice. In the latter, transient Odc ablation permanently prevented tumor onset consistent with a time-limited window for embryonal tumor initiation. Importantly, we show that DFMO augments antitumor efficacy of conventional cytotoxics in vivo. This work implicates polyamine biosynthesis as an arbiter of MYCN oncogenesis and shows initial efficacy for polyamine depletion strategies in neuroblastoma, a strategy that may have utility for this and other MYC-driven embryonal tumors.

Targeting polyamine metabolism and function in cancer and other hyperproliferative diseases

The polyamines spermidine and spermine and their diamine precursor putrescine are naturally occurring, polycationic alkylamines that are essential for eukaryotic cell growth. The requirement for and the metabolism of polyamines are frequently dysregulated in cancer and other hyperproliferative diseases, thus making polyamine function and metabolism attractive targets for therapeutic intervention. Recent advances in our understanding of polyamine function, metabolic regulation, and differences between normal cells and tumour cells with respect to polyamine biology, have reinforced the interest in this target-rich pathway for drug development.

Spermine causes loss of innate immune response to Helicobacter pylori by inhibition of inducible nitric-oxide synthase translation

Helicobacter pylori infection of the stomach elicits a vigorous but ineffective host immune and inflammatory response, resulting in persistence of the bacterium for the life of the host. We have reported that in macrophages, H. pylori up-regulates inducible NO synthase (iNOS) and antimicrobial NO production, but in parallel there is induction of arginase II, generating ornithine, and of ornithine decarboxylase (ODC), generating polyamines. Spermine, in particular, has been shown to restrain immune response in activated macrophages by inhibiting proinflammatory gene expression. We hypothesized that spermine could prevent the antimicrobial effects of NO by inhibiting iNOS in macrophages activated by H. pylori. Spermine did not affect the up-regulation of iNOS mRNA levels but in a concentration-dependent manner significantly attenuated iNOS protein levels and NO production. Reduction in iNOS protein was due to inhibition of iNOS translation and not due to iNOS degradation. ODC knockdown with small interfering (si) RNA resulted in increased H. pylori-stimulated iNOS protein expression and NO production without altering iNOS mRNA levels. When macrophages were cocultured with H. pylori, killing of bacteria was enhanced by transfection of ODC siRNA and prevented by addition of spermine. These results identify a mechanism of immune dysregulation induced by H. pylori in which stimulated spermine synthesis by the arginase-ODC pathway inhibits iNOS translation and NO production, leading to persistence of the bacterium and risk for peptic ulcer disease and gastric cancer.

S-adenosylmethionine decarboxylase degradation by the 26 S proteasome is accelerated by substrate-mediated transamination

The short-lived enzyme S-adenosylmethionine decarboxylase uses a covalently bound pyruvoyl cofactor to catalyze the formation of decarboxylated S-adenosylmethionine, which then donates an aminopropyl group for polyamine biosynthesis. Here we demonstrate that S-adenosylmethionine decarboxylase is ubiquitinated and degraded by the 26 S proteasome in vivo, a process that is accelerated by inactivation of S-adenosylmethionine decarboxylase by substrate-mediated transamination of its pyruvoyl cofactor. Proteasome inhibition in COS-7 cells prevents the degradation of S-adenosylmethionine decarboxylase antigen; however, even brief inhibition of the 26 S proteasome caused substantial losses of S-adenosylmethionine decarboxylase activity despite accumulation of S-adenosylmethionine decarboxylase antigen. Levels of the enzyme's substrate (S-adenosylmethionine) increased rapidly after 26 S proteasome inhibition, and this increase in substrate level is consistent with the observed loss of activity arising from an increased rate of inactivation by substrate-mediated transamination. Evidence is also presented that this substrate-mediated transamination accelerates normal degradation of S-adenosylmethionine decarboxylase, as the rate of degradation of the enzyme was increased in the presence of AbeAdo (5'-([(Z)-4-amino-2-butenyl]methylamino]-5'-deoxyadenosine) (a substrate analogue that transaminates the enzyme); conversely, when the intracellular substrate level was reduced by methionine deprivation, the rate of degradation of the enzyme was decreased. Ubiquitination of S-adenosylmethionine decarboxylase is demonstrated by isolation of His-tagged AdoMetDC (S-adenosylmethionine decarboxylase) from COS-7 cells co-transfected with hemagglutinin-tagged ubiquitin and showing bands that were immunoreactive to both anti-AdoMetDC antibody and anti-hemagglutinin antibody. This is the first study to demonstrate that AdoMetDC is ubiquitinated and degraded by the 26 S proteasome, and substrate-mediated acceleration of degradation is a unique finding.

Extremely rapid turnover of S-adenosylmethionine decarboxylase inCrithidia fasciculata

The activity of S-adenosylmethionine decarboxylase (AdoMetDC) in Crithidia fasciculata was shown to be correlated to the growth of the parasite. An increase in activity was observed during exponential growth. Inhibition of protein synthesis induced an extremely rapid decay of AdoMetDC activity. The half-life of the enzyme was estimated to be about 3 min, which is the shortest half-life ever recorded for an eukaryotic AdoMetDC. The reduction in AdoMetDC activity was correlated with a decrease in AdoMetDC protein content, demonstrating a rapid turnover of the enzyme. No polyamine-mediated feedback regulation of AdoMetDC was observed in the parasite.

In vitro translation of the upstream open reading frame in the mammalian mRNA encoding S-adenosylmethionine decarboxylase

The upstream open reading frame (uORF) in the mRNA encoding S-adenosylmethionine decarboxylase is a polyamine-responsive element that suppresses translation of the associated downstream cistron in vivo. In this paper, we provide the first direct evidence of peptide synthesis from the S-adenosylmethionine decarboxylase uORF using an in vitro translation system. We examine both the influence of cation concentration on peptide synthesis and the effect of altering the uORF sequence on peptide synthesis. Synthesis of wild type and altered peptides was similar at all concentrations of magnesium tested. In contrast, synthesis of the wild type peptide was more sensitive than that of altered peptides to elevated concentrations of the naturally occurring polyamines, spermidine and spermine, as well as several polyamine analogs. The sensitivity of in vitro synthesis to spermidine was influenced by both the amino acid sequence and the length of the peptide product of the uORF. Findings from the present study correlate with the effects of the uORF and polyamines on translation of a downstream cistron in vivo and support the hypothesis that polyamines and the structure of the nascent peptide create a rate-limiting step in uORF translation, perhaps through a ribosome stalling mechanism.

Mechanistic studies of the processing of human S-adenosylmethionine decarboxylase proenzyme. Isolation of an ester intermediate

Human S-adenosylmethionine decarboxylase is synthesized as a proenzyme that undergoes an autocatalytic cleavage reaction generating the alpha and beta subunits and forming the pyruvate prosthetic group, which is derived from an internal Ser residue (Ser-68). The mechanism of this processing reaction was studied using site-directed mutagenesis of conserved residues (His-243 and Ser-229) located close to the cleavage site. Mutant S229A failed to process, and mutant S229C cleaved very slowly, whereas mutant S229T processed normally, suggesting that the hydroxyl group of residue 229 is required for the processing reaction where Ser-229 may act as a proton acceptor. Mutant His-243A cleaved very slowly, forming a small amount of the correctly processed pyruvoyl enzyme but a much larger proportion of the alpha subunit with an amino-terminal Ser. The cleavage to form the latter was greatly enhanced by hydroxylamine. This result suggests that the N-O acyl shift needed for ester formation occurs normally in this mutant but that the next step, which is a beta-elimination reaction leading to the two subunits, does not occur. His-243 may therefore act as the basic residue that extracts the hydrogen of the alpha-carbon of Ser-68 in the ester in order to facilitate this reaction. The availability of the recombinant H243A S-adenosylmethionine decarboxylase proenzyme provides a useful model system to examine the processing reaction in vitro and test the design of specific inactivators aimed at blocking the production of the pyruvoyl prosthetic group.

Role of cysteine-82 in the catalytic mechanism of human S-adenosylmethionine decarboxylase

S-Adenosylmethionine decarboxylase is a pyruvate-dependent enzyme. The enzyme forms a Schiff base with substrate, S-adenosylmethionine, through the pyruvoyl moiety. This facilitates the release of CO2 from the substrate, which must then be protonated on the alpha carbon in order to permit hydrolysis of the Schiff base to release the product. The catalytic mechanism of human S-adenosylmethionine decarboxylase was investigated via mutagenic and kinetic approaches. The results of enzyme kinetic studies indicated that Cys-82 is a crucial residue for activity and this residue has a basic pKa. Iodoacetic acid inhibited wild-type enzyme activity in a time- and pH-dependent manner but did not affect the already reduced activity of mutant C82A. Reaction of this mutant with iodoacetic acid led to approximately one less mole of reagent being incorporated per mole of enzyme alphabeta dimer than with wild-type S-adenosylmethionine decarboxylase. Both wild-type and C82A mutant S-adenosylmethionine decarboxylases were inactivated by substrate-mediated transamination, but this reaction occurred much more frequently with C82A than with wild-type enzyme. A major proportion of the recombinant C82A mutant protein was in the transaminated form in which the pyruvoyl cofactor is converted into alanine. This suggests that incorrect protonation of the pyruvate, rather than the substrate, occurs much more readily when Cys-82 is altered. On the basis of these results, it was postulated that residue Cys-82 may be the proton donor of the decarboxylation reaction catalyzed by S-adenosylmethionine decarboxylase.

Processing of mammalian and plant S-adenosylmethionine decarboxylase proenzymes

S-Adenosylmethionine decarboxylase (AdoMetDC) is a pyruvoyl enzyme, and the pyruvate is formed in an intramolecular reaction that cleaves a proenzyme precursor and converts a serine residue into pyruvate. The wild type potato AdoMetDC proenzyme processed much faster than the human proenzyme and did not require putrescine for an optimal rate of processing despite the presence of three acidic residues (equivalent to Glu11, Glu178, and Glu256) that were demonstrated in previous studies to be required for the putrescine activation of human AdoMetDC proenzyme processing (Stanley, B. A., Shantz, L. M., and Pegg, A. E. (1994) J. Biol. Chem. 269, 7901-7907). A fourth residue that is also needed for the putrescine stimulation of human AdoMetDC proenzyme processing was identified in the present studies, and this residue (Asp174) is not present in the potato sequence. The site of potato AdoMetDC proenzyme processing was found to be Ser73 in the conserved sequence, YVLSESS, which is the equivalent of Ser68 in the human sequence. Replacement of the serine precursor with threonine or cysteine by site-directed mutagenesis in either the potato or the human AdoMetDC proenzyme did not prevent processing but caused a significant reduction in the rate. Although the COOH-terminal regions of the known eukaryotic AdoMetDCs are not conserved, only relatively small truncations of 8 residues from the human protein and 25 residues from the potato proenzyme were compatible with processing. The maximally truncated proteins show no similarity in COOH-terminal amino acid sequence but each contained 46 amino acid residues after the last conserved sequence, suggesting that the length of this section of the protein is essential for maintaining the proenzyme conformation needed for autocatalytic processing.

The upstream open reading frame of the mRNA encoding S-adenosylmethionine decarboxylase is a polyamine-responsive translational control element

S-Adenosylmethionine decarboxylase (AdoMetDC) is a key enzyme in the pathway of polyamine biosynthesis. The cellular levels of the polyamines specifically regulate AdoMetDC translation through the 5'-leader of the mRNA, which contains a small upstream open reading frame (uORF) 14 nucleotides from the cap. Mutating the initiation codon of the uORF, which encodes a peptide product with the sequence MAGDIS, abolished regulation. In addition, the uORF is sufficient, by itself, to provide polyamine regulation when inserted into the 5'-leader of the human growth hormone mRNA. Changing the amino acid sequence at the carboxyl terminus of the peptide product of the uORF abolished polyamine regulation. In contrast, altering the nucleotide sequence of the uORF at degenerate positions, without changing the amino acid sequence of the peptide, did not affect regulation. Extending the distance between cap and uORF, thereby changing the rate of initiation at the initiator AUG of the uORF, did not alter polyamine regulation. When the uORF was extended so as to overlap, out of frame, the downstream major cistron, polyamine regulation was abolished. We propose that polyamines do not modulate the rate of recognition of the uORF but rather regulate interaction of the peptide product of the uORF with its target.

Regulation of mammalian S-adenosylmethionine decarboxylase as studied in a transient expression system

Mammalian S-adenosylmethionine decarboxylase (AdoMetDC), which catalyzes a key step in the biosynthesis of polyamines, is regulated by a multitude of mechanisms. The polyamines exert a strong feedback control of the enzyme. In the present study we have used a transient expression system to study the regulation of mammalian AdoMetDC. COS cells were transfected with a SV 40-based expression vector containing a 5'- and 3'-truncated human AdoMetDC cDNA (pSDC:16). The cells were shown to contain high levels of AdoMetDC activity 2 days after expression. This was partly due to an increase in the synthesis of the enzyme. However a marked stabilization of the enzyme against degradation did also contribute to the high AdoMetDC activity seen in the COS cells after the transfection with pSDC:16. The high expression of AdoMetDC was reflected in a marked change in intracellular polyamine levels. The cells were almost depleted of their putrescine, and their spermidine content was decreased to about 35% of that found in the mock-transfected cells. The spermine content, on the other hand, was increased. This change in polyamine levels was most likely attributable to the pSDC:16-induced increase in decarboxylated S-adenosylmethinine, which favors the accumulation of spermine at the expenses of putrescine and spermidine. The effects on the expression of AdoMetDC of polyamine synthesis inhibitors varied dependent on whether the COS cells were transfected with control vector or pSDC:16, in spite of similar effects on cellular polyamine levels, indicating a difference in feedback regulation of 'native' and recombinant AdoMetDC. The construct used in the present study gave rise to an AdoMetDC mRNA without a 5' and devoid of most of the 3' untranslated regions. However, whether these parts of the mRNA are involved in the polyamine-mediated translational control of the enzyme remains to be confirmed.

Overproduction of S-adenosylmethionine decarboxylase in ethylglyoxal-bis(guanylhydrazone)-resistant mouse FM3A cells

A variant cell line, termed SAM-1, which overproduced S-adenosylmethionine decarboxylase (AdoMetDC), was isolated by treatment of mouse FM3A cells with N-methyl-N'-nitro-N-nitrosoguanidine and subsequent incubation with ethylglyoxal bis(guanylhydrazone), an inhibitor of the enzyme. The cells were resistant to ethylglyoxal bis(guanylhydrazone), and showed AdoMetDC activity approximately five-times higher than control cells. The rate of AdoMetDC synthesis and the amount of AdoMetDC existing in SAM-1 cells were about five-times those in control cells. The amount of AdoMetDC mRNA existing in SAM-1 cells was five-times more than that in control cells. The amount of 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, an irreversible inhibitor of AdoMetDC, necessary to inhibit cell growth was also five-times more in SAM-1 cells than in control cells. However, the following were the same in both SAM-1 and control cells; the amount of genomic DNA for AdoMetDC, the size and nucleotide sequence of 5' untranslated region of AdoMetDC mRNA, the deduced amino acid sequence (334 residues) from the nucleotide sequence of AdoMetDC cDNA and the degradation rate (t1/2 = about 4 h) of AdoMetDC. In addition, AdoMetDC mRNA in control cells was slightly more stable than that in SAM-1 cells. The results indicate that the overproduction of AdoMetDC in SAM-1 cells was caused by the increase of AdoMetDC mRNA. The variant cell line is convenient for studying the regulation of AdoMetDC and the physiological function of polyamines.

Polyamine-mediated regulation of S-adenosylmethionine decarboxylase expression in mammalian cells. Studies using 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, a suicide inhibitor of the enzyme

Cell proliferation is dependent on an adequate supply of the polyamines putrescine, spermidine and spermine. One of the key steps in the polyamine biosynthetic pathway is catalyzed by S-adenosylmethionine decarboxylase (AdoMetDC). In the present study we have used a newly synthesized enzyme-activated irreversible AdoMetDC inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine [(Z)-AbeAdo], to investigate the regulation of this enzyme. Treatment of mouse L1210 leukemia cells with (Z)-AbeAdo resulted in a total inhibition of their AdoMetDC activity followed by depletion of the spermidine and spermine content. The putrescine content, however, was dramatically increased after treatment with (Z)-AbeAdo. In spite of the cellular depletion of spermidine and spermine, only a minor inhibitory effect was obtained on cell growth, indicating that putrescine at a high concentration might partly replace spermidine and spermine in their growth-promoting functions. Cells grown in the presence of (Z)-AbeAdo exhibited an increased synthesis of AdoMetDC, which was counteracted by the addition of either spermidine or spermine. The change in AdoMetDC synthesis could not be fully explained by a change in the level of AdoMetDC mRNA, indicating also a translational control. Mammalian AdoMetDC is synthesized as a larger proenzyme, which is then cleaved into two subunits of different sizes. The conversion of the proenzyme into the subunits is a very rapid process, which is stimulated greatly by putrescine in vitro. However, the processing of the proenzyme in the (Z)-AbeAdo-treated L1210 cells was not affected by their very high putrescine content, indicating that the conversion might be saturated at low levels of putrescine, or that most of the putrescine in the (Z)-AbeAdo-treated L1210 cells might be bound to sites normally occupied by spermidine and spermine.

Inhibition of ornithine decarboxylase and S-adenosylmethionine decarboxylase synthesis by antisense oligodeoxynucleotides

Oligodeoxynucleotides 18 nucleotides in length having sequences complementary to regions spanning the initiation codon regions of ornithine decarboxylase or S-adenosylmethionine decarboxylase mRNAs were tested for their ability to inhibit translation of these mRNAs. In reticulocyte lysates, a strong and dose dependent reduction of ornithine decarboxylase synthesis in response to mRNA from D-R L1210 cells was brought about by 5'-AAAGCTGCTCATGGTTCT-3' which is complementary to the sequence from -6 to +12 of the mRNA sequence but there was no inhibition by 5'-TGCAGCTTCCATCACCGT-3'. Conversely, the latter oligodeoxynucleotide which is complementary to the sequence from -6 to +12 of the mRNA of S-adenosyl methionine decarboxylase was a strong inhibitor of the synthesis of this enzyme in response to rat prostate mRNA and the antisense sequence from ornithine decarboxylase had no effect. The translation of ornithine decarboxylase mRNA in a wheat germ system was inhibited by the antisense oligodeoxynucleotide at much lower concentration than those needed in the reticulocyte lysate suggesting that degradation of the hybrid by ribonuclease H may be an important factor in this inhibition. These results indicate that such oligonucleotides may be useful to regulate cellular polyamine levels and as probes to study control of mRNA translation.

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.

Ornithine decarboxylase activity in mouse tumour tissue in response to refeeding and diet components

Ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase activity (SAMD) were measured in tumour tissue in mice during periods of starvation (24 h) and refeeding. Starvation led to a 60% reduction in tumour ODC activity. Refeeding normalised the activity within 4 h. Restitution in ODC activity, representing de novo enzyme synthesis, preceded DNA resynthesis. SAMD activity continued to fall along the increase in ODC activity during refeeding, while difluoro-methyl-ornithine (DFMO) caused a compensatory increase in SAMD activity as expected. A fall and regain in ODC activity was associated with inhibition and regrowth of the tumour. Starvation-refeeding was not related to any decrease in tumour polyamine concentrations, while systemic DFMO blockade was. Glucose stimulated ODC when refed orally, but not when given systemically. Tumour ODC activity was not decreased in refed mice by anti-insulin, a procedure that antagonised insulin's bioactivity. Exogenous insulin did not stimulate tumour ODC activity. Our results suggest that gastrointestinal metabolism of carbohydrates stimulates the release of a factor, which initiates both ODC activity and DNA synthesis in tumour cells. This factor was not insulin.

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.

Regulation of the Na(+)-dependent and the Na(+)-independent polyamine transporters in renal epithelial cells (LLC-PK1)

We have studied the regulation of the Na(+)-dependent and Na(+)-independent polyamine transport pathways in the renal LLC-PK1 cell line. Most of the experiments were performed in the presence of 5 mM DL-2-difluoromethylornithine (DFMO) in order to inhibit the cellular synthesis of polyamines. The activity of both transporters as measured by putrescine uptake was increased by growth-promoting stimuli and decreased by exogenous polyamines. The time course of the increase in uptake activity induced by fetal calf serum could be fitted by a single exponential, and the process was three times faster for the Na(+)-dependent than for the Na(+)-independent transporter. Maximum activity was reached after more than 24 h. This increase could be inhibited by actinomycin D and by cycloheximide. Other growth-promoting stimuli, such as subconfluent cell density, as well as growth factors also induced an increase in the transport activity. Particularly, there was a marked stimulation of the Na(+)-dependent pathway by epidermal growth factor in combination with insulin. On the other hand, the transport activity decayed very rapidly upon addition of exogenous polyamines (t1/2 less than 60 min). The diamine putrescine was much less effective in this respect than the polyamines spermidine and spermine. The non-metabolizable substrate methylglyoxal bis(guanylhydrazone) did not induce a decay of the transport activity, but it protected the Na(+)-dependent pathway against the polyamine-induced decay. Inhibition of the protein synthesis by cycloheximide did not induce a rapid decrease of the transport activity; neither did it affect the polyamine-induced decay. These observations suggest that this polyamine-induced decay is not owing to an inhibitory effect on the rate of synthesis of the transporters, but rather to a degradation or an inactivation of the transporters. The polyamine-induced decay slowed down at lower cell density. This effect was particularly pronounced for the Na(+)-dependent transporter. Since the uptake of polyamines was increased at low cell density, the decreased rate of decay in this condition pleads against a simple mechanism of transinhibition by the substrate. In conclusion, both transport pathways were similarly affected by the regulatory parameters, but the Na(+)-dependent transporter was more rapidly and more effectively regulated. The numerous interacting regulatory steps furthermore suggest a physiological role for these transporters, such as an involvement in urinary polyamine disposal.

Androgen regulation of polyamine synthesis in seminal vesicle and in different lobes of the rat prostate

Endogenous levels of the polyamines putrescine, spermidine, and spermine have been examined in the 10(6) m/s2 supernatant of different lobes of the rat prostate and in the seminal vesicles of castrates, androgen-stimulated castrates, and intact controls. Content of the polyamines varied between the lobes, with spermidine highest in intact animals. After castration, the content of polyamines fell significantly in all lobes but the coagulating gland (CG). Spermidine levels were highest, except in the lateral prostate (LP) and CG, where the content of putrescine was highest. In castrated animals treated with testosterone propionate for 72 h, the amount of the three polyamines examined increased dramatically in the ventral prostate (VP) and moderately in the CG and seminal vesicle (SV). Concerning individual polyamines, spermidine increased significantly in all lobes, while putrescine increased significantly only in the two saccular parts of rat prostate, i.e., CG and SV. Spermidine content decreased significantly in the DP. Major differences in the content of the three polyamines--putrescine, spermidine, and spermine--in the various tissues studied have been found. Moreover, distinct differences among intact, castrated, and testosterone-treated castrated animals have been revealed.

Polyamine-mediated control of mammalian S-adenosyl-L-methionine decarboxylase expression: effects on the content and translational efficiency of the mRNA

The expression of mammalian AdoMet decarboxylase, a key enzyme in polyamine synthesis, was shown to be regulated by polyamines at two different levels. Polyamine depletion of Ehrlich ascites tumor cells induced a marked compensatory increase in the synthesis of the enzyme, as measured by 35S-methionine pulse-labeling and immuno-precipitation. This increase in synthesis rate was counteracted by provision of spermidine, which reduced the synthesis of AdoMet decarboxylase to an undetectable level. Northern analysis revealed a nearly 2-fold increase in the amount of AdoMet decarboxylase mRNA when the putrescine and spermidine content was depleted. This increase in AdoMet decarboxylase mRNA content cannot account for the more than 5-fold increase in synthesis rate, indicating a feedback regulation also at the level of mRNA translation.

Polyamine-directed preferential nutritional repletion of normal tissues in tumor-bearing hosts

Our previous study demonstrated that total parenteral nutrition (TPN) increases erythrocyte (RBC) polyamines selectively in cancer patients but not in non-cancer patients, suggesting that these changes may relate to tumor presence. We therefore studied the possible relationship between RBC polyamine levels and potential tumor growth induced by TPN. In addition, we attempted to control tumor growth during TPN with alpha-difluoromethylornithine (DFMO), a suicide inhibitor of ornithine decarboxylase (ODC), in a fibrosarcoma-bearing rat model. Rats in a control group received chow ad libitum for 3 weeks and their tumors grew exponentially. A restricted intake (RI) tumor-bearing group received 8 g chow per day per rat; this produced malnourished rats in which tumor growth significantly slowed. The third group (RI----TPN) received the RI diet for 2 weeks and TPN for 1 week, which resulted in an increase in tumor growth. The last group (RI----TPN + DFMO) received a 2-week RI regimen and 1-week TPN plus DFMO regimen, which inhibited tumor growth to the level of the RI group due to the addition of DFMO (1 g/kg/day). Although the addition of TPN to the RI regimen markedly increased tumor ODC activity, as well as tumor putrescine and RBC putrescine levels, the addition of DFMO to the RI----TPN regimen entirely abolished these increases. Despite DFMO treatment, liver ODC activity in RI----TPN + DFMO group was unaffected. The plasma albumin level, an indicator of host nutrition, of the RI----TPN + DFMO group was significantly higher than that of the RI group. Liver, spleen, kidney and total body weights of the RI----TPN or RI----TPN + DFMO group were significantly higher than those of the RI group. These results indicate that TPN can enhance tumor growth in malnourished hosts and that the addition of DFMO to TPN can effectively control this TPN-enhanced tumor growth, leading to preferential nutritional repletion of the host.

Detection of proenzyme form of S-adenosylmethionine decarboxylase in extracts from rat prostate

Previous work in which the synthesis of S-adenosylmethionine decarboxylase was studied by translation of its mRNA indicated that it was formed as a proenzyme having a M.W. of about 37,000 that was cleaved to form the enzyme sub-unit of M.W. 32,000 in a putrescine-stimulated reaction. The extent to which the proenzyme accumulates in vivo and is affected by the putrescine concentration was studied by subjecting prostate extracts to Western immunoblotting procedures. The proenzyme form was readily detectable in control prostates (about 4% of the total) and this proportion was increased to 25% when the rats were pretreated for 3 days with the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine. Conversely, it was decreased to almost undetectable levels after treatment with methylglyoxal bis(guanylhydrazone). These results indicate that the processing of the proenzyme form of S-adenosylmethionine decarboxylase is regulated by the cellular putrescine concentration. This conversion provides another step at which polyamine biosynthesis may be controlled.

Regulation of mammalian S-adenosylmethionine decarboxylase

S-Adenosylmethionine decarboxylase is a key enzyme in the biosynthesis of polyamines that is the rate limiting step in the formation of spermidine and spermine. The activity of S-adenosylmethionine decarboxylase is known to be regulated negatively by these polyamines and positively by their precursor, putrescine. A specific antiserum to S-adenosylmethionine decarboxylase was raised by immunizing rabbits with the homogeneous enzyme purified from rat prostate and a specific radioimmunoassay for the protein was set up. Using this radioimmunoassay it was found that a number of inhibitors of other steps in the polyamine biosynthetic pathway lead to increases in the amount of S-adenosylmethionine decarboxylase protein. These changes were caused by both a decreased rate of degradation and an increased rate of synthesis of the protein. The increased synthesis was due to two factors; a rise in the amount of translatable mRNA and an enhanced translation efficiency. The mRNA content of the prostate was substantially increased by treatment for 3 days with alpha-difluoromethylornithine (2% in drinking water). The translation of mRNA for S-adenosylmethionine decarboxylase was studied using a polyamine-depleted reticulocyte lysate supplemented with mRNA from rat prostate and the antiserum to precipitate the proteins corresponding to S-adenosylmethionine decarboxylase. These studies indicated that the enzyme was synthesized as an inactive precursor of Mr 37,000 which was converted to the enzyme sub-unit of Mr 32,000. The conversion of the precursor to the active sub-unit in vitro was increased by putrescine. The precursor could also be detected by immunoblotting of extracts from prostates of rats depleted of putrescine by treatment with the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine. The translation of the S-adenosylmethionine decarboxylase mRNA in the reticulocyte lysates was strongly inhibited by the addition of spermidine or spermine demonstrating that polyamines directly inhibit the synthesis of S-adenosylmethionine decarboxylase. cDNA clones corresponding to S-adenosylmethionine decarboxylase were isolated using prostatic mRNA from polysomes enriched in S-adenosylmethionine decarboxylase by immunopurification. The use of these probes showed that rat ventral prostate contains two S-adenosylmethionine decarboxylase mRNA species of approximately 3.4 and 2.1 kb which differ in the 3' non-translated sequence. The sequence of these cDNAs will enable the amino acid sequence of the precursor to be obtained. This will provide evidence on the origin of the pyruvate prosthetic group of S-adenosylmethionine decarboxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Enzyme regulation as an approach to interference with polyamine biosynthesis--an alternative to enzyme inhibition

The progress reviewed here would seem to validate the regulatory approach to interference with polyamine biosynthesis as an antiproliferative strategy. To our knowledge, this is the first example, among anticancer drugs, of pharmacological intervention of a biochemical pathway based strictly on regulatory control. Several features of polyamine biology naturally favor this approach and may account for its relative success. These include (a) the nature of the regulatory mechanisms themselves, (b) the exquisite sensitivity of the pathway to regulatory control, (c) the rapid turnover of ODC and AdoMetDC, (d) the different structural specificity of ODC and AdoMetDC regulation versus growth-dependent functions, and (e) the direct dependence of growth on sustained polyamine biosynthesis. As such, the regulatory approach to interference with polyamine biosynthesis offers several advantages over the use of specific enzyme inhibitors (Table 10). Of these, perhaps, the more significant are the facts that more than one enzyme can be simultaneously and specifically suppressed and that compensatory mechanisms, which otherwise counter the effects of enzyme inhibitors (11), are not invoked. We are encouraged by the concurrence of in vitro mechanistic findings with the predictions of the hypothesis for the regulatory approach and by the in vitro and in vivo growth inhibitory effects of the analogs against murine leukemia. One disadvantage of the regulatory analogs, such as BESm, has been that, as with specific polyamine inhibitors such as DFMO, analog-induced polyamine depletion results in cytostatic growth inhibition. While this response may help to minimize host toxicities, it clearly compromises antitumor activity. An intriguing exception to this generality has recently been found among human lung carcinoma cell lines. Previously, Luk et al. (93, 94) and others (95) reported that, among a spectrum of human lung carcinoma lines, small cell carcinoma was exquisitely sensitive to the ODC inhibitor, DFMO. Not only did these cells display a cessation of growth but also an inability to survive during DFMO-induced polyamine depletion. Studies extending these findings to long term maintenance therapy in human small cell lung carcinoma implants in athymic mice revealed sustained growth inhibition of the tumor for longer than one year (96). Casero et al. (97) now find that human large cell carcinoma, which is otherwise refractory to chemotherapeutic intervention, displays a cytotoxic response in vitro to polyamine depletion induced by BES or BESm but not by DFMO.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of polyamine depletion on serum stimulation of quiescent 3T3 murine fibroblast cells

Numerous reports have shown that polyamines are required for cell proliferation. A current model for regulating commitment to DNA replication in cultured fibroblasts stimulated from quiescence by serum addition postulates sequential action by specific growth factors. To temporally localize polyamine-dependent steps within this defined sequence, mouse Balb/c-3T3 fibroblasts were partially depleted of polyamines by treatment with DL-alpha-difluoromethylornithine (DFMO), next rendered quiescent by serum deprivation, then stimulated by 10% serum with or without exogenous putrescine (Pu). Depletion of polyamines was verified by HPLC, and entry of cells into S phase was monitored by autoradiography. After 24 h of incubation with [3H]-thymidine, polyamine-depleted cells had labeling indices similar to quiescent cells if they were serum-stimulated without Pu, but progressed to S phase to the same degree as control cultures if polyamines were restored by adding Pu at the time of serum stimulation. These observations suggested that commitment of quiescent cells to DNA replication may require polyamines. To determine if polyamine-dependent steps occur during the pre-commitment period (up to 12 h after serum stimulation) or only in traverse of G1 (12 h to 24 h, post-commitment), polyamine-depleted quiescent cells were serum-stimulated for 12 h without Pu, then returned to low serum with Pu. Labeling indices of these cultures remained nearly as low as those of unstimulated cells. Reducing serum concentration from 10% to 0.5% at 12 h after stimulation did not effect labeling indices of control cells not depleted of polyamines by DFMO. These results supported the postulated requirement for polyamines during pre-commitment events. However, polyamine-deficient quiescent cells serum-stimulated without Pu for periods longer than 24 h had labeling indices at 36 and 48 h significantly greater than at 24 h. This suggested that polyamine depletion may decrease the rate at which quiescent cells commit to DNA replication, rather than producing an absolute blockade during the pre-commitment period.

Red blood cell polyamine levels and host toxicity during continuous alpha-difluoromethylornithine infusion

The dose effects of continuous alpha-difluoromethylornithine (DFMO) infusion on red blood cell (RBC) polyamine levels, host toxicity and tumor growth were determined. Male rats with and without a transplantable methylcholanthrene-induced sarcoma received intravenously either 0.45% NaCl or DFMO at 500 mg, 1,000 mg, or 2,000 mg/kg body wt/day for 6 or 12 days. Dose-related inhibition of tumor growth was noted after the 12-day treatment. There were no changes in host carcass weight, food intake, plasma albumin, hematocrit or white blood cell counts. Platelet suppression was associated with the 1,000- and 2,000-mg doses with the 12-day treatment. Morphometry of the small intestine revealed mild but significant shortening of villi in the duodenum and jejunum at the 2,000-mg dose, but none of the animals developed diarrhea. The 500-mg DFMO dose reduced the rate of tumor growth without inducing platelet suppression or altering intestinal morphology. A decrease in RBC putrescine levels was noted at all doses. RBC spermidine levels increased with the 500-mg dose. RBC spermine levels were higher at all doses compared with controls. These results suggest that thrombocytopenia is the major dose-limiting side-effect of continuous DFMO infusion but does not occur at a dose of 500 mg/kg body wt/day.