Formation and interconversion of putrescine and spermidine in mammalian cells

The Trypanosoma cruzi Diamine Transporter Is Essential for Robust Infection of Mammalian Cells

Trypanosoma cruzi is incapable of synthesizing putrescine or cadaverine de novo, and, therefore, salvage of polyamines from the host milieu is an obligatory nutritional function for the parasite. A high-affinity diamine transporter (TcPOT1) from T. cruzi has been identified previously that recognizes both putrescine and cadaverine as ligands. In order to assess the functional role of TcPOT1 in intact parasites, a Δtcpot1 null mutant was constructed by targeted gene replacement and characterized. The Δtcpot1 mutant lacked high-affinity putrescine-cadaverine transport capability but retained the capacity to transport diamines via a non-saturable, low-affinity mechanism. Transport of spermidine and arginine was not impacted by the Δtcpot1 lesion. The Δtcpot1 cell line exhibited a significant but not total defect in its ability to subsist in Vero cells, although initial infection rates were not affected by the lesion. These findings reveal that TcPOT1 is the sole high-affinity diamine permease in T. cruzi, that genetic obliteration of TcPOT1 impairs the ability of the parasite to maintain a robust infection in mammalian cells, and that a secondary low-affinity uptake mechanism for this key parasite nutrient is operative but insufficient for optimal infection.

Effects of Saccharomyces boulardii on intestinal mucosa

Saccharomyces boulardii (S. boulardii) is a non-pathogenic biotherapeutic agent, widely prescribed in a lyophilized form in many countries over the world. S. boulardii acts as a shuttle liberating effective enzymes, proteins and trophic factors during its intestinal transit that improve host immune defenses, digestion, and absorption of nutrients. In addition, S. boulardii secretes during its intestinal transit polyamines, mainly spermine and spermidine that regulate gene expression and protein synthesis. In this review, we will focus on the interactions of the yeast with the host intestinal mucosa.

Regulation of Polyamine and Cancer

This review describes my work in the field of polyamine research for the last 35 years. My research started with developing the improved synthesis of decarboxylated S-adenosylmethionine and then moved to the purification of spermidine synthase from rat prostate. I also took considerable efforts to find the synthetic procedure for various polyamines with high yield in order to prepare (15)N-labeled polyamines. On the basis of these methodological work, I searched for the inhibitor of spermidine synthase and found trans-4-methylcyclohexylamine (MCHA), the most effective one at the present time. I also developed a new analytical method for polyamines using stable isotope and ionspray ionization mass spectrometry (IS-MS). Based on these studies I examined the role of polyamines in liver regeneration and found that oral administration of MCHA effectively changed the concentration of polyamines and inhibited the hepatic growth. I also found the close relationship between the concentration ratio of spermidine to spermine and the extent of liver regeneration. These results may shed new light on the control of cell growth by polyamine in vivo.

Meniscal replacement in dogs. Tissue regeneration in two different materials with similar properties

In earlier studies, meniscal replacement with a porous polymer implant led to regeneration of neo-meniscal tissue. To evaluate the influence of the chemical properties on the tissue regeneration in the implant, in the present study, the meniscus in the dog's knee was replaced with either an aromatic 4,4-diphenylmethanediisocyanate based polyesterurethane implant (Estane) (n = 6) or with an aliphatic 1,4-butanediisocyanate based polyesterurethane implant (PCLPU) (n = 6). After 6 months, the knee joints were resected and the tissue behavior in the two different prostheses was evaluated microscopically. In both prostheses, a meniscus-like distribution of the tissue phenotype was found with collagen type I in the peripheral fibrous zones and collagen type II in the central, more cartilaginous zones. The compression-stress behavior of the implant-tissue construct remained in between the stiffness of the polymer material and that of the native meniscus. The PCLPU implant seemed to provoke less synovial tissue reaction. After meniscectomy solely, in 5 out of 6 cases, a meniscus-like regenerate was formed. Furthermore, the articular cartilage degeneration after placing a PCLPU implant did also not exceed the degeneration after the Estane implant or after meniscectomy. The differences between these two implants did not seem to influence the tissue regeneration in the implant. However, PCLPU seemed to evoke less tissue reaction and, therefore, is thought to be less or even nontoxic as compared with the Estane implant. Therefore, for studies in the future, the authors prefer the PCLPU prostheses for replacement of the meniscus.

Effect of ME-3451-106, an aqueous extract of Stichaster striatus with inhibitory activity of voluntary alcohol intake, in genetically drinker rats: Isolation and identification of the active fraction

The aqueous extract obtained from Stichaster striatus Müller & Troschel (Asteroidea, Stichasteridae) has been shown to possess activity as an alcohol appetite inhibitor after oral administration in a rat model with a genetically established excessive appetite for alcohol (Wistar rats, lineage UChB). A significant decrease in the consumption of ethanol was observed (unrelated to a possible disulfiram effect) without a change in the normal food or water intake during the experimentation period. A bio-guided fractionation of the extract was carried out in order to identify the most active fraction, in which the presence of a group of natural endogenous polyamines in undetermined proportions is suspected. Our hypothesis was to relate the activity obtained for the original ME-3451-106 extract with the presence of these polyamines in the extract in question. The activity shown by a series of commercially available polyamines (putrescine (Pu), spermidine (SPD) and spermine (SP)) in inhibiting voluntary ethanol intake lends support to our hypothesis. The extract was selected on the basis of oral tradition, which claimed that the consumption of a "soup" obtained by boiling starfish, later identified as Stichaster striatus, prevented the appearance of alcoholism in laborers on properties entrusted to the Jesuit order during the middle period of the Spanish conquest of America (17-18th century).

-Bioactive factors in milk-

Human milk as well as the milk of several mammalian species contains, beside major nutrients and anti-infectious and immunocompetent substances, a group of biologically active substances called "milk-borne trophic factors" or "growth modulators". Milk-borne trophic can be classified into three groups: hormones and trophic peptides; nucleotides, nucleosides and derived substances; and polyamines, especially spermine and spermidine. Certain hormones and peptides such as growth hormone, insulin, insulin like-growth factor I (IGF-I), epidermal growth factor (EGF), prolactin and growth hormone releasing factor (GHRF) can influence directly newborn's metabolism after intestinal absorption and promote growth and differentiation of several organs and target tissues. They could exert a cytoprotective effect against toxins and toxic substances and reduce the potential risk of necrotizing enterocolitis. Nucleotides are present in human milk at high levels, and are precursors of nucleic acids, which implies that they can enhance growth and differentiation of several organs and tissues, especially the liver. Nucleotides from milk enhance lipid metabolism, lipoprotein synthesis and liver cell function and regeneration. In addition, they have a determinant action on the development of the gut associated lymphoid tissue (GALT). Lastly, polyamines, mainly spermine and spermidine, are polycationic substances virtually present in all cells, whose concentration in human milk is about ten times higher than in infant formulae. In addition, spermine and spermidine levels increase markedly during the first 3 days of lactation reaching, after 1 week, plateau levels which are respectively 12 and eight times higher than the levels measured at day 0. Although several experimental studies have shown that polyamines from the milk of lactating mammals determine important mitogenic, metabolic and immunological effects promoting growth and differentiation of the immature gastrointestinal tract of the offspring, their beneficial effects on growth and differentiation of the gastrointestinal tract in humans remain hypothetical. As a consequence, enrichment of milk formulae in one or in several trophic factors is an important but complex goal. Its practical realization is not realistic today because of a too great number of incertitudes. The most important is related to potential beneficial or adverse effects emerging at short or at long term and to the individual interactions of these substances which could be agonist and antagonist because they are naturally present in milk as a "complex cocktail" whose composition changes during the lactation period.

Transgenic mice overexpressing ornithine and S-adenosylmethionine decarboxylases maintain a physiological polyamine homoeostasis in their tissues

Recent work has shown that transgenic mice overexpressing human ornithine decarboxylase display no marked changes in the tissue concentrations of spermidine or spermine in spite of a dramatic increase in putrescine levels. In the tissues of transgenic mice carrying the human spermidine synthase gene and in those of hybrid mice overexpressing both ornithine decarboxylase and spermidine synthase, spermidine and spermine levels remain within normal limits. To test whether the amount of the propylamine group donor, decarboxylated S-adenosylmethionine, limits the conversion of putrescine into the higher polyamines, we have produced transgenic mouse lines harbouring the rat S-adenosylmethionine decarboxylase gene in their genome. However, neither these mice nor the hybrid mice overexpressing both ornithine decarboxylase and S-adenosylmethionine decarboxylase displayed significant changes in their spermidine and spermine tissue levels. To study the mechanism by which cells maintain the constancy of the polyamine concentrations, we have determined the metabolic flux of polyamines in transgenic primary fibroblasts using pulse labelling. The results indicate that the polyamine flow is faster in transgenic primary fibroblasts than in non-transgenic fibroblasts and that the intracellular homoeostasis of higher polyamines is maintained at least partly by the acetylation of spermidine and spermine and their secretion into the medium.

Metabolism and action of amino acid analog anti-cancer agents

The preclinical pharmacology, antitumor activity and toxicity of seven of the more important amino acid analogs, with antineoplastic activity, is discussed in this review. Three of these compounds are antagonists of L-glutamine: acivicin, DON and azaserine; and two are analogs of L-aspartic acid: PALA and L-alanosine. All five of these antimetabolites interrupt cellular nucleotide synthesis and thereby halt the formation of DNA and/or RNA in the tumor cell. The remaining two compounds, buthionine sulfoximine and difluoromethylornithine, are inhibitors of glutathione and polyamine synthesis, respectively, with limited intrinsic antitumor activity; however, because of their powerful biochemical actions and their low systemic toxicities, they are being evaluated as chemotherapeutic adjuncts to or modulators of other more toxic antineoplastic agents.

Feedback regulation of polyamine synthesis in Ehrlich ascites tumor cells. Analysis using nonmetabolizable derivatives of putrescine and spermine

Ornithine decarboxylase (ODC) is subject to feedback regulation by the polyamines. Thus, addition of putrescine, spermidine or spermine to cells causes inhibition of ODC mRNA translation. Putrescine and spermine are readily converted into spermidine. Therefore, it is conceivable that the inhibition of ODC synthesis observed in putrescine- and spermine-supplemented cells is instead an effect of spermidine. To examine this possibility we have used two analogs of putrescine and spermine, namely 1,4-dimethylputrescine and 5,8-dimethylspermine, which cannot be converted into spermidine. Both analogs were found to inhibit the incorporation of [35S]methionine into ODC protein to approximately the same extent, suggesting that putrescine as well as spermine exert a negative feedback control of ODC mRNA translation in the cell. In addition to suppressing ODC synthesis, both analogs were found to increase the turnover rate of the enzyme. 5,8-Dimethylspermine caused a marked decrease in the activity of S-adenosylmethionine decarboxylase (AdoMetDC). This effect was not obtained with 1,4-dimethylputrescine, indicating that spermine, but not putrescine, exerts a negative control of AdoMetDC. Treatment with 1,4-dimethylputrescine caused extensive depletion of the cellular putrescine and spermidine content, but accumulation of spermine. 5,8-Dimethylspermine treatment, on the other hand, effectively depleted the spermine content and had less effect on the putrescine and spermidine content, at least initially. Nevertheless, the total polyamine content was more extensively reduced by treatment with 5,8-dimethylspermine than with 1,4-dimethylputrescine. Accordingly, only 5,8-dimethylspermine treatment exerted a significant inhibitory effect on Ehrlich ascites tumor cell growth.

Reduced amounts of S-adenosylmethionine decarboxylase in the adrenal glands of rats following administration of piribedil or 2-deoxyglucose

The activity of S-adenosylmethionine decarboxylase (SAM-DC) is decreased in the adrenal gland of the rat following physical stress, metabolic stress or administration of dopamine agonists [M. Ekker and T. L. Sourkes, Endocrinology 120, 1299 (1987)]. Immunotitration studies with a serum directed against purified rat liver SAM-DC show that the reduction in activity of the enzyme following administration of 2-deoxyglucose or piribedil was paralleled by a decrease in the amount of immuno-reactive protein. There was no difference in the half-life of SAM-DC activity between piribedil-treated rats and controls. The properties of an extensively purified preparation of the adrenal enzyme resembled those of SAM-DC obtained from rat liver. It is suggested that the reduction in adrenal SAM-DC activity and protein content caused by stress is due to a reduction in the rate of synthesis of the enzyme.

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)

Polyamines in normal and cancer cells

Based on available evidence, it appears that polyamines are critical for proliferation of both normal and transformed cells. Although the requirement of polyamines for DNA replication and cell proliferation is established, the molecular events in which the polyamines are essential are yet unknown. Furthermore, transformed and cancer cells, possibly because of their higher proliferative rate, appear to be more dependent on polyamine metabolism than their normal counterparts. This has been shown by the in vivo response of tumor models and human tumor xenografts in nude mice to polyamine depletion by DFMO. Although there has been associated toxicity to the host, the inhibition of cell proliferation has been higher in the implanted tumors than in the host. DFMO, a specific irreversible inhibitor of ODC, has been used extensively in studies which have shed light on the role of polyamines in cell proliferation and differentiation. DFMO has shown interesting anti-tumor effect in a number of experimental tumor models. Currently, DFMO clinical trials are being completed, and it will be of interest to see whether this polyamine inhibitor, or other newer polyamine analogs and inhibitors, will find a place in the treatment of neoplastic disorders.

Increased activity of spermidine N1-acetyltransferase in the adrenal cortex of rats following administration of exogenous spermidine, carbamylcholine, 2-deoxyglucose and dopamine agonists

The activity of N1-acetyltransferase was increased in the dissected adrenal cortex of the rat following a single administration of spermidine. The activity was maximal 6-8 h after the onset of treatment. The increase in enzyme activity was abolished when the rats were given cycloheximide 2 h after spermidine; this suggests that increased activity results from an augmentation in the synthesis of the enzyme. Adrenocortical spermidine N1-acetyltransferase was also induced by carbamylcholine, 2-deoxyglucose, apomorphine and piribedil, drugs that are known to cause induction of ornithine decarboxylase in that organ. Hypophysectomized rats showed reduced activity of spermidine N1-acetyltransferase when compared to sham-operated controls, and carbamylcholine no longer elicited an increase in enzyme activity in such animals. Adrenocortical spermidine N1-acetyltransferase activity of hypophysectomized rats is induced by corticotropin (ACTH). These results suggest a hormonal control over the activity of the enzyme in the adrenal cortex with ACTH acting as a mediator.

Effect of 1,25-dihydroxyvitamin D on S-adenosylmethionine decarboxylase in chick intestine

The effect of 1,25-dihydroxyvitamin D on the activity of S-adenosylmethionine decarboxylase in the duodenal mucosa of vitamin D-deficient chicks was investigated. Enzyme activity increased dose-dependently in a biphasic manner with maximal responses at 1 and 6 h, due to an increase in Vmax in both cases. A second dose of 1,25-dihydroxyvitamin D, administered 6 h after the first, resulted in a significant increase in activity 1 h later, confirming the rapidity of the response. This early response was not seen with ornithine decarboxylase. The increase in S-adenosylmethionine decarboxylase activity particularly at 6 h may be due to a rise in cytosolic calcium, since hydrocortisone, an inhibitor of 1,25-dihydroxyvitamin D-stimulated calcium absorption, attenuates this enzyme's activity. Inhibitors of polyamine biosynthesis such as DL-alpha-difluoromethyl ornithine and methylglyoxal bis(guanylhydrazone) had no effect on calcium absorption, but the significance of this in evaluating the importance of 1,25-dihydroxyvitamin D stimulation of S-adenosylmethionine decarboxylase activity needs further investigation.

Polyamines in mammalian ageing: an oncological problem, too? A review

This review surveys the literature about changes in polyamine contents and levels of activity of the enzymes involved in the polyamine biosynthetic pathway in organs of ageing mammals. The literature about changes in the polyamine levels in physiological fluids in healthy ageing humans is also reviewed. Generally speaking, decreases in the levels of the main polyamines (noticeably putrescine and spermidine) are observed in different mammalian organs with ageing. The polyamine levels in serum and in urine of healthy human beings are also age-related, declining progressively with increasing age. Some major enzymes (i.e., ornithine decarboxylase (EC 4.1.1.17) and S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50) involved in the polyamine biosynthetic pathway show similar trends. Hormonal induction of ornithine decarboxylase activity is strongly reduced in organs of aged animals, as it is in neoplastic organs. There is also some evidence for an age-related decrease in the level of ornithine decarboxylase and its inducibility in mammalian cells cultured in vitro. Some in vitro effects of spermidine and spermine on aged structures or systems are briefly summarized. There is no evidence yet that this generally reduced capacity of mammalian aged organs for polyamine biosynthesis is one of the factors responsible for the well known high incidence of some neoplasias in elderly humans. In view of the typical stimulatory effects of the tumour promoters on polyamine biosynthesis in target tissues and the effects of senescence on the same metabolic pathway, it can be excluded that the ageing process has a tumour promoting effect by itself. However, although the exact mechanism responsible for the increased occurrence of some tumors during mammalian senescence is still obscure, there are enough experimental data (both in humans and in animals) to indicate that the reduced polyamine biosynthetic capacity of aged mammals can account for the slower course of some tumors in elderly patients.

Role of polyamines in differentiation of 3T3-L1 fibroblasts into adipocytes

The role of polyamines in the differentiation of 3T3-L1 fibroblasts into adipose cells was studied. This conversion was blocked by the addition of alpha-difluoromethylornithine, an enzyme-activated irreversible inhibitor of ornithine decarboxylase, which prevented a rise in spermidine content in the differentiating cells. The inhibition of differentiation could be overcome completely by the provision of exogenous putrescine, spermidine, or spermine. Partial reversal could be produced by exposure to nonphysiological homologues of the natural polyamines such as 1,3-diaminopropane, 1,5-diaminopentane, and sym-norspermine. Reversal of the inhibition of differentiation by exogenous polyamines required a period of exposure to the amines, indicating that the lack of differentiation is not due simply to an obligatory role for polyamines in the biosynthesis of lipids. These results indicate that spermidine is required for the differentiation, but spermidine alone was not able to replace insulin and 1-methyl-3-isobutylxanthine in stimulating conversion to adipocytes. Therefore spermidine appears to be necessary but not sufficient for differentiation to occur. Finally, the elevation of spermidine content that occurs during the conversion of fibroblasts to adipocytes did not correlate with an increased activity of the polyamine biosynthetic enzymes. This implies that the increase must be regulated by changes in the rate of degradation or excretion of the polyamines.

Determination of free and total polyamines in human serum and urine by ion-pairing high-performance liquid chromatography using a radial compression module. Application to blood polyamine determination in cancer patients treated or not treated with an ornithine decarboxylase inhibitor

A sensitive and rapid determination of free and total polyamines (putrescine, cadaverine, spermidine and spermine) in urine and serum is described. The procedure is based on reversed-phase high-performance liquid chromatographic separation using radial compression module (Radialpak C 8). The samples are purified with a silica gel Sep-Pak cartridge. The polyamines are converted to dansyl chloride derivatives and separated using a linear gradient of triethylammonium phosphate-methanol within 15 min. The lower limits of detection are 10 pmoles for spermine and 5 pmoles for other polyamines. This method is applied to cancer patients treated by cytotoxic chemotherapy with or without difluoromethylornithine (DFMO). All four polyamines are significantly increased in these patients before treatment. On day 8, after onset of treatment, the levels of polyamines in patients not treated with DFMO are more elevated than on day 1, while in patients treated with DFMO the levels are decreased. However, DFMO does not seem to modify the treatment result. The patients which have a low level of putrescine before and during treatment, do not respond to treatment. Perhaps this low level is the consequence of conjugation of this polyamine?

Some regulatory and integrative aspects of purine nucleotide biosynthesis and its control: an overview

The regulation and integration of purine nucleotide biosynthesis is considered from the viewpoint of the main groups of reaction sequences involved and with respect to some specific organs and tissues. Inhibiting either IMP dehydrogenase or adenylosuccinate synthetase in rat liver in vitro reduced the rate of purine do novo synthesis with respect to the purine remaining in the tissue and did not materially affect the rate with respect to the purines extruded into the incubation medium. These results are considered in contrast to the results of previous studies in cultured lymphoblasts. The relative activities of purine de novo synthesis and of purine salvage have been assessed in different tissues by the activities of amidophosphoribosyltransferase and hypoxanthine phosphoribosyltransferase (HPRT), respectively. Changes in purine de novo synthesis as measured by [14C]formate incorporation into cellular purines were reflected in the amidophosphoribosyltransferase activities. The capacity of different tissues to synthesize purines de novo is widespread and the role of the liver as the main site of purine de novo synthesis in vivo and exporting purines to other tissues appears questionable. Regulatory mechanisms may well be tissue specific. The age-related changes in the activity of the purine de novo synthesis and purine salvage pathways, respectively, in the brain suggest that it is physiological or neuropharmacological functions of the developed brain rather than cell division and organogenesis which require a high level of purine salvage relative to purine de novo synthesis. This is compatible with the observation that purine de novo synthesis alone can meet the needs for additional purine nucleotides which lectin induced lymphocyte transformation involves. The mechanism whereby purine de novo synthesis is initiated during lectin induced lymphoblast transformation remains obscure.

Conversion of exogenous spermidine into putrescine after administration to rats

Administration of large, but non-toxic doses of spermidine (0.4-1.25 mmol/kg) led to a substantial increase in putrescine in liver, kidney and a number of other tissues including muscle. The increase in putrescine peaked at 6 h after treatment and was completely prevented by administration of cycloheximide 3 h after the spermidine suggesting that the induction of a new protein was required. This protein is likely to be spermidine N1-acetyltransferase which was induced by the treatment with spermidine and increased 3-4-fold in liver and kidney within 6 h. N1-Acetylspermidine was detected in tissues at this time after spermidine treatment and experiments in which labeled spermidine was given indicated that a substantial fraction of the administered spermidine was converted into N1-acetylspermidine and into putrescine. These results suggest that the rise in putrescine after spermidine treatment is brought about by the production of N1-acetylspermidine which is converted into putrescine by the action of polyamine oxidase. The limiting step in this conversion is the activity of the acetylase which is induced in response to the rise in spermidine content. The acetylase/oxidase pathway, therefore, provides a means by which polyamine levels can be regulated and excess polyamine disposed of.

Polyamine metabolism and function

Polyamines are ubiquitous organic cations of low molecular weight. The content of these amines is closely regulated by the cell according to the state of growth. The reactions responsible for the biosynthesis and interconversion of the polyamines and their precursor putrescine are described and the means by which polyamine content can be varied in response to exogenous stimuli are discussed. The role of polyamines in the cell cycle, cell division, tissue growth, and differentiation is considered. Recent studies using highly specific inhibitors of polyamine biosynthesis such as alpha-difluoromethylornithine to prevent accumulation of polyamines have indicated that the synthesis of polyamines is intimately associated with these processes. Such inhibitors have great potential for investigation of the cellular role of polyamines.

Regulation of polyamine content in cultured fibroblasts

The content of putrescine and of the polyamines (spermidine and spermine) and the activities of their biosynthetic enzymes were measured in 3T3 mouse fibroblasts and SV40-transformed mouse fibroblasts over the entire period from subculturing in fresh medium until confluence. The transformed cells had a substantially higher content of putrescine and spermidine than the 3T3 cells and higher activities of all of the biosynthetic enzymes. However, the ratio of spermine synthase to spermidine synthase was higher in the 3T3 cells, which correlated with their higher spermine-to-spermidine ratio. All of the biosynthetic enzymes increased in activity during cell growth. Ornithine decarboxylase increased 20-fold with a maximum at 24-36 h after culturing whereas S-adenosylmethionine decarboxylase increased 3-fold at the same time. Spermidine synthase increased 10- to 16-fold during the growth period whereas spermine synthase increased 2- to 3-fold. The relative enzyme activities and the changes in total polyamine content suggested that 1) the activity of S-adenosylmethionine decarboxylase limited the production of the polyamines and 2) the relative amounts of spermidine and spermine synthase determined the predominant polyamine that the available decarboxylated S-adenosylmethionine is used to synthesize. When 3T3 cells become quiescent at confluence, there was a substantial fall in the intracellular spermidine level because of a greatly increased excretion of spermidine into the medium. Spermine content also fell because there was an increased conversion of spermine into spermidine, which was then excreted. The specific excretion of spermidine did not occur with the transformed SV-3T3 cells.

Comparison of S-adenosylmethionine decarboxylases from rat liver and muscle

S-Adenosylmethionine decarboxylase was purified to homogeneity from rat liver and from rat psoas. The major step involved affinity chromatography on methylglyoxal bis-(guanylhydrazone) linked to Sepharose. The muscle enzyme was more tightly retained to this absorbent, and the enzymes from the two sources could readily be separated by chromatography on this material. The psoas and liver enzymes could not be distinguished by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, both giving a single band corresponding to an Mr of 32 500, but were separated by electrophoresis under nondenaturing conditions and by isoelectric focusing (the isoelectric points were 5.3 for psoas and 5.7 for liver enzyme). The liver and psoas enzymes also differed in respect to Km for S-adenosylmethionine, the degree to which they were activated by putrescine, and their sensitivity to inhibition by methylglyoxal bis(guanylhydrazone) and related compounds. These results indicate that there are two forms of S-adenosylmethionine decarboxylase. The presence of a particular form could, therefore, be important both in the regulation of polyamine levels and also in the pharmacology involving inhibitors of polyamine synthesis.

Effects of inhibitors of ornithine and S-adenosylmethionine decarboxylases on L6 myoblast proliferation

The role of polyamines in myoblast proliferation was studied by treating cells of Yaffe's L6 line of rat myoblasts with inhibitors of polyamine synthesis. Both an irreversible inhibitor of ornithine decarboxylase--difluoromethyl-ornithine (DFMO)--and a competitive inhibitor of S-adenosyl-methionine decarboxylase--methylglyoxal-bis(guanylhydrazone) (MGBG)--depressed spermidine levels and inhibited myoblast proliferation. Spermine levels were not significantly depressed by either inhibitor and putrescine levels were decreased only by DFMO. Putrescine and spermidine, but not magnesium, prevented inhibition of myoblast proliferation by DFMO and MGBG; determination of 14C-DFMO uptake in the presence and absence of these compounds demonstrated that they did not reduce the rate or extent of inhibitor uptake and thus prevent its inhibition of ornithine decarboxylase. Thus it seems likely that these inhibitors reduce cell proliferation by inhibiting polyamine formation. Addition of spermidine to the cells led to a substantial reduction in the activity of S-adenosyl-methionine-decarboxylase, suggesting that the enzyme is subject to negative regulation by the products of the polyamine biosynthetic pathway. Unexpectedly, addition of spermidine also increased intracellular putrescine levels; this apparently resulted from conversion of spermidine to putrescine. Addition of putrescine or spermidine in the absence of serum did not increase the rate of myoblast proliferation although it did elevate intracellular polyamine levels as expected. We conclude that some threshold level of one or more polyamines (probably spermidine) is necessary but not sufficient for initiation and maintenance of myoblast proliferation in culture.

Differences between tissues in response of S-adenosylmethionine decarboxylase to administration of polyamines

1. Administration of spermidine or sym-norspermidine decreased the activity of AdoMet (S-adenosylmethionine) decarboxylase in extracts prepared from rat liver, Kidney, psoas, diaphragm, soleus and small intestine, but not heart. The decline in psoas, diaphragm and soleus was much greater than that in liver and kidney. The difference in sensitivity to spermidine could not be explained by changes in the uptake and accumulation of the polyamine, because much higher contents were found in liver and kidney that in diaphragm and psoas. 2. Spermidine administration also led to a substantial increase in putrescine in all tissues examined. However, the rise in putrescine was not responsible for the decline in AdoMet decarboxylase activity, since norspermidine, which cannot form putrescine, also produced the decline. Also, administration of putrescine or 1,3-diaminopropane did not decrease AdoMet decarboxylase. 3. The decline in skeletal-muscle AdoMet decarboxylase activity in response to spermidine may be due to an increased rate of degradation of the enzyme protein. The t1/2 (half-time) for the decline in activity after inhibition of protein synthesis by cycloheximide was almost halved in the psoas of spermidine-treated rats. Spermidine treatment did not change the t1/2 in liver. 4. These results raise the possibility that there are at least two different forms of AdoMet decarboxylase and that the enzyme from psoas or diaphragm differs from that in liver. Additional support for this hypothesis was obtained by comparing the activation by putrescine of AdoMet decarboxylase from these tissues. The liver enzyme was stimulated 10-fold, but the muscle enzyme was stimulated 30-fold.