Glutamate-mediated excitotoxicity and neurodegeneration in Alzheimer?s disease

Alzheimer's disease (AD) is the most common form of dementia, accounting for 60-70% of cases in subjects over 65 years of age. Several postulates have been put forward that relate AD neuropathology to intellectual and functional impairment. These range from free-radical-induced damage, through cholinergic dysfunction, to beta-amyloid-induced toxicity. However, therapeutic strategies aimed at improving the cognitive symptoms of patients via choline supplementation, cholinergic stimulation or beta-amyloid vaccination, have largely failed. A growing body of evidence suggests that perturbations in systems using the excitatory amino acid L-glutamate (L-Glu) may underlie the pathogenic mechanisms of (e.g.) hypoxia-ischemia, epilepsy, and chronic neurodegenerative disorders such as Huntington's disease and AD. Almost all neurons in the CNS carry the N-methyl-D-aspartate (NMDA) subtype of ionotropic L-glutamate receptors, which can mediate post-synaptic Ca2+ influx. Excitotoxicity resulting from excessive activation of NMDA receptors may enhance the localized vulnerability of neurons in a manner consistent with AD neuropathology, as a consequence of an altered regional distribution of NMDA receptor subtypes. This review discusses mechanisms for the involvement of the NMDA receptor complex and its interaction with polyamines in the pathogenesis of AD. NMDA receptor antagonists have potential for the therapeutic amelioration of AD.

Mammalian polyamine metabolism and function

Polyamines are ubiquitous small basic molecules that play multiple essential roles in mammalian physiology. Their cellular content is highly regulated and there is convincing evidence that altered metabolism is involvement in many disease states. Drugs altering polyamine levels may therefore have a variety of important targets. This review will summarize the current state of understanding of polyamine metabolism and function, the regulation of polyamine content, and heritable pathological conditions that may be derived from altered polyamine metabolism.

Polyamines: metabolism and implications in human diseases

Classically, polyamines is a family of molecules (i.e. putrescine, spermine, spermidine) derived from ornithine according to a decarboxylation/condensative process. More recently, it has been demonstrated that arginine can be metabolised according to the same pathway leading to agmatine formation. Polyamines are essential for the growth, the maintenance and the function of normal cells. The complexity of their metabolism and the fact that polyamines homeostasis is tightly regulated support the idea that polyamines are essential to cell survival. Multiple abnormalities in the control of polyamines metabolism might be implicated in several pathological processes. This paper summarises the knowledge about polyamines metabolism and point out the importance of these molecules as a target for the development of therapeutic tools and nutritional strategy.

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.

Biological significance of dietary polyamines

Polyamines are classically known by their names of putrescine, spermine, and spermidine. They are synthesized endogenously from ornithine and are interconvertible. In addition, an exogenous supply of polyamines is provided by dietary intake and by intestinal absorption from the products of bacterial metabolism. Polyamine uptake occurs almost entirely in the gut, and afterward the various forms are metabolized in different tissues under the strict regulation of ornithine decarboxylase, which is the first enzyme involved in their synthesis. Polyamines are eliminated from the organism by means of oxidation reactions, appearing in urine in all their metabolic forms. Polyamines play an important role in regulating cell growth and proliferation, the stabilization of negative charges of DNA, RNA transcription, protein synthesis, apoptosis, and the regulation of the immune response. They are components of breast milk and might be important in neonatal gut maturation, for which reason the possible supplementation of infant formulas with these compounds is under study.

Polyamine Modulon in Escherichia coli: Genes Involved in the Stimulation of Cell Growth by Polyamines

We have recently proposed an idea to explain how polyamines enhance cell growth in Escherichia coli. Since most polyamines exist as polyamine-RNA complexes, our idea is that polyamines stimulate several kinds of protein synthesis which are important for cell growth at the level of translation. We found that synthesis of oligopeptide binding protein (OppA), which is important for nutrient supply, adenylate cyclase (Cya), RNA polymerase sigma(38) subunit (RpoS), transcription factor of iron transport operon (FecI), and transcription factor of growth-related genes including rRNA and some kinds of tRNA synthesis (Fis) was enhanced by polyamines at the level of translation. We proposed that a group of genes whose expression is enhanced by polyamines at the level of translation be referred to as a "polyamine modulon." By DNA microarray, we found that 309 of 2,742 mRNA species were up-regulated by polyamines. Among the 309 up-regulated genes, transcriptional enhancement of at least 58 genes might be attributable to increased levels of the transcription factors Cya, RpoS, FecI, and Fis. This unifying molecular mechanism is proposed to underlie the physiological role of polyamines in controlling the growth of Escherichia coli.

Polyamine dependence of normal cell-cycle progression

The driving force of the cell cycle is the activities of cyclin-dependent kinases (CDKs). Key steps in the regulation of the cell cycle therefore must impinge upon the activities of the CDKs. CDKs exert their functions when bound to cyclins that are expressed cyclically during the cell cycle. Polyamine biosynthesis varies bicyclically during the cell cycle with peaks in enzyme activities at the G(1)/S and S/G(2) transitions. The enzyme activities are regulated at transcriptional, translational and post-translational levels. When cells are seeded in the presence of drugs that interfere with polyamine biosynthesis, cell cycle progression is affected within one cell cycle after seeding. The cell cycle phase that is most sensitive to polyamine biosynthesis inhibition is the S phase, while effects on the G(1) and G(2)/M phases occur at later time points. The elongation step of DNA replication is negatively affected when polyamine pools are not allowed to increase normally during cell proliferation. Cyclin A is expressed during the S phase and cyclin A/CDK2 is important for a normal rate of DNA elongation. Cyclin A expression is lowered in cells treated with polyamine biosynthesis inhibitors. Thus, polyamines may affect S phase progression by participating in the regulation of cyclin A expression.

Modulation of channel function by polyamines

Polyamines are found in every cell of the body and the intricate enzymatic reactions responsible for their metabolism and transport in mammalian cells are now well understood. Despite intense efforts, elucidation of the role of polyamines has suffered in that little information of physiological relevance has surfaced. Recently, recombinant receptor techniques and increased availability of polyamine analogues have revealed, as discussed here by David Johnson, modulation of NMDA receptors by polyamines by reversal of tonic proton inhibition, and a function for these compounds as 'intrinsic rectifier factors' for K+ channels.

Polyamine regulation of ribosome pausing at the upstream open reading frame of S-adenosylmethionine decarboxylase

Synthesis of S-adenosylmethionine decarboxylase (AdoMetDC), a key regulated enzyme in the pathway of polyamine biosynthesis, is feedback-controlled at the level of translation by spermidine and spermine. The peptide product of an upstream open reading frame (uORF) in the mRNA is solely responsible for polyamine regulation of AdoMetDC translation. Using a primer extension inhibition assay and in vitro protein synthesis reactions, we found ribosomes paused at or close to the termination codon of the uORF. This pause was greatly diminished with the altered uORFs' sequences that abolish uORF regulation in vivo. The half-life of the ribosome pause was related to the concentration of polyamines present but was unaffected by magnesium concentration. Furthermore, inhibition of translation initiation at a reporter gene placed downstream of the AdoMetDC uORF directly correlated with the stability of the ribosome pause at the uORF. These observations are consistent with a model in which regulation of ribosome pausing at the uORF by polyamines controls ribosome access to the downstream AdoMetDC reading frame.

Ozone impact on the photosynthetic apparatus and the protective role of polyamines

One of the primary plant mechanisms protecting leaf cells against enhanced atmospheric ozone is the accumulation of polyamines, generally observed as an increase in putrescine level, and in particular its bound form to thylakoid membranes. Ozone-sensitive plants of tobacco (cultivar Bel W3) in contrast to ozone-tolerant Bel B, are not able to increase their endogenous thylakoid membrane-bound putrescine when they are exposed to an atmosphere with enhanced ozone concentration, resulting in reduction of their photosynthetic rates and consequently reduction in plant biomass formation. In comparison to the tolerant cultivar Bel B, a prolongation of ozone exposure thus can lead to typical visible symptoms (necrotic spots) in leaves of the sensitive plant. Exogenously manipulated increase of the cellular putrescine levels of the ozone-sensitive Bel W3 is sufficient to revert these effects, whereas a reduction in endogenous putrescine levels of the tolerant cultivar Bel B renders them sensitive to ozone treatment. The results of this work reveal a regulator role for polyamines in adaptation of the photosynthetic apparatus and consequently to its protection in an environment polluted by ozone.

Facilitation of currents through rat Ca2+-permeable AMPA receptor channels by activity-dependent relief from polyamine block

1. In outside-out patches excised from human embryonic kidney (HEK) 293 cells expressing Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptor (AMPAR) channels, currents activated by 1 ms glutamate pulses at negative membrane potentials facilitated during and following a repetitive (2 to 100 Hz) agonist application. The degree of facilitation depended on subunit type, membrane potential and stimulation frequency being antagonized by a slow recovery from desensitization. 2. Activity-dependent current facilitation occurred in Ca2+-permeable but not in Ca2+-impermeable AMPAR channels. Current facilitation, however, does not depend on Ca2+ flux. Rather it reflects a relief from the block of Ca2+-permeable AMPARs by intracellular polyamines since facilitation occurred only in the presence of polyamines and since facilitated currents had a nearly linear current-voltage relation (I-V). 3. Relief from polyamine block was use dependent and occurred mainly in open channels. The relief mechanism was determined primarily by membrane potential rather than by current flow. 4. In closed channels the degree of polyamine block was independent of membrane potential. The voltage dependence of the rate of relief from the block in open channels rather than the voltage dependence of the block underlies the inwardly rectifying shape of the I-V at negative potentials. 5. Currents through native Ca2+-permeable AMPAR channels in outside-out or nucleated patches from either hippocampal basket cells or a subtype of neocortical layer II nonpyramidal cells also showed facilitation. 6. It is concluded that a use-dependent relief from polyamine block during consecutive AMPAR channel openings underlies current facilitation. This polyamine-AMPAR interaction may represent a new activity-dependent postsynaptic mechanism for control of synaptic signalling.

Mast cells in neuroimmune function: neurotoxicological and neuropharmacological perspectives

Mast cells are located in close proximity to neurons in the peripheral and central nervous systems, suggesting a functional role in normal and aberrant neurodegenerative states. They also possess many of the features of neurons, in terms of monoaminergic systems, responsiveness to neurotrophins and neuropeptides and the ability to synthesise and release bioactive neurotrophic factors. Mast cells are able to secrete an array of potent mediators which may orchestrate neuroinflammation and affect the integrity of the blood-brain barrier. The 'cross-talk' between mast cells, lymphocytes, neurons and glia constitutes a neuroimmune axis which is implicated in a range of neurodegenerative diseases with an inflammatory and/or autoimmune component, such as multiple sclerosis and Alzheimer's disease. Mast cells appear to make an important contribution to developing, mature and degenerating nervous systems and this should now be recognised when assessing the neurotoxic potential of xenobiotics.

Increased polyamines may downregulate interleukin 2 production in rheumatoid arthritis

Polyamines downregulate immune reactivity. RA is associated with decreased IL 2 production. In this study, we present evidence to suggest that excessive polyamines can contribute to the IL 2 deficiency in RA. Blocking polyamine production with inhibitors of ornithine decarboxylase results in increased IL 2 production by RA PBMC. Moreover, polyamine oxidase (PAO) inhibitors and catalase also increase IL 2 production by RA PBMC. This effect of PAO inhibition is monocyte mediated. After 3 d in culture, RA PBMC produce three times more IL 2 than do normal PBMC. This rise is prevented by exogenous spermidine but only in the presence of monocytes. The concentration of polyamines in RA PBMC and synovial fluid MNC is 2-20-fold higher than in normal cells. Thus, polyamines and their oxidation products downregulate IL 2 production by RA PBMC and may account for the decreased T cell effector function seen in this disease.

Depolarization-dependent effect of flavonoids in rat uterine smooth muscle contraction elicited by CaCl2

1. The effects of the flavonoids genistein (3-60 microM), kaempferol (3-60 microM) and quercetin (1-100 microM) on KCl (60 mM)-induced tonic contraction in rat uterus and their modifications with the inhibitor of cAMP-dependent protein kinases (TPCK, 3 microM), the inhibitor of ornithine decarboxylase [alpha-difluoromethyl ornithine (DFMO), 10 mM] and the polyamine spermine (1 mM) have been assayed. The effects of the three flavonoids were also studied on the contraction elicited by CaCl2 (30 microM to 10 mM) on rat uterus incubated in medium lacking calcium and supplemented with 33, 60 or 90 mM of KCl. For comparison, the effects of the calcium channel blockers nifedipine and verapamil and the activator of adenylyl cyclase forskolin were assayed on contractions induced by KCl and CaCl2. 2. Genistein (IC50: 20.2 +/- 1.0 microM, n = 11), kaempferol (IC50: 10.1 +/- 0.8 microM, n = 8) and quercetin (IC50: 13.2 +/- 0.5 microM, n = 8) relaxed the tonic contraction induced by KCl (60 mM) in a concentration-dependent way. Verapamil (IC50: 70.1 +/- 5.8 nM, n = 7), nifedipine (IC50: 8.4 +/- 0.7 nM, n = 6) and forskolin (IC50: 0.62 +/- 0.08 microM, n = 14) also relaxed the KCl-induced contraction. TPCK (3 microM) significantly antagonized the effect of quercetin, kaempferol and forskolin (P < 0.01) but did not modify the effect of genistein. 3. Spermine (1 mM) increased the effects of genistein and verapamil and antagonized the effect of quercetin but did not modify those of kaempferol and forskolin. DFMO (10 mM) did not modify the effect of quercetin but increased that of genistein and antagonized those of kaempferol and forskolin. The addition of spermine (1 mM) plus DFMO (10 mM) antagonized the effect of quercetin. Spermine counteracted the effect of DFMO on forskolin but not on genistein. 4. KCl (33, 60 or 90 mM) did not produce contraction in calcium-free solution, but CaCl2 (30 microM to 10 mM) induced concentration-dependent contraction after depolarizing with KCl. The EC50 values for CaCl2 were: 0.74 +/- 0.08 (n = 12), 0.34 +/- 0.03 (n = 14) and 0.48 +/- 0.02 (n = 12) mM in a medium with 33, 60 or 90 mM of KCl, respectively. 5. Genistein (20 microM), kaempferol (10 microM), quercetin (15 microM), verapamil (70 nM), nifedipine (10 nM) and forskolin (0.5 microM) inhibited the concentration-response curve to CaCl2 in medium supplemented with 33, 60 or 90 mM of KCl. The effect of kaempferol was independent of the concentration of KCl in the incubation medium. However, the inhibitory effect of genistein on CaCl2-induced contraction was inversely related to the concentration of KCl in the medium. On the contrary, the effect of quercetin was directly related to the concentration of KCl in the medium. 6. The antagonism of verapamil, nifedipine and forskolin on CaCl2-induced contraction seems to be related to the degree of depolarization because increasing the KCl in the medium counteracted their effects. 7. Our results suggest that (1) cAMP contributes to the relaxant effects of quercetin and kaempferol on KCl (60 mM)-induced tonic contraction; (2) polyamines are involved in the relaxant effects of forskolin and kaempferol on KCl-induced tonic contraction but not on CaCl2-induced contraction in the depolarized uterus, and (3) the flavonoids assayed also possess a calcium antagonist action but show a different behavior toward the calcium channel blockers and the cAMP enhancer forskolin.

Estrogens and colorectal cancer

In recent years, several lines of epidemiologic, clinical and experimental evidences have been reported showing that estrogen hormones may be involved in malignant colorectal tumors. The sex differences in site-specific incidence, the increased incidence of colonic cancer in women with breast cancer, the protective effect of increasing parity and the reduced risk among women taking postmenopausal hormones, are all elements suggesting that sex hormones may play a role. Male rats experimentally exposed to the carcinogen dimethylhydrazine, have twice the risk of developing colon cancer and significantly shorter survival times than their female counterparts. Along with the clinical, experimental and epidemiologic findings there are also biologic reasons why estrogen may be protective. Most estrogen action appears to be exerted via the estrogen receptors (ERs) on target cells. ERs have been reported in several solid tumors including gastrointestinal neoplasms such as esophageal, gallbladder, gastric and colorectal cancer. At the end of 1995, a second ER (ER-beta) was cloned from the rat prostate cDNA library and subsequently, the human and mouse homologs. Its demonstration in normal and neoplastic human colorectal tissues and "in vitro" in colonic epithelial cells, has renewed interest in investigating the existence of two ER subtypes. The presence of two ERs could explain the selective actions of estrogens on different target tissues and, particularly, on the gastrointestinal tract. Finally, our studies suggest that estrogens and their receptors play an important role in the growth and progression of colorectal tumors, by interacting with other molecules required for cell proliferation like growth factors and polyamines.

Polyamines and prostatic cancer

The importance of polyamines in prostatic growth and differentiation has prompted studies to evaluate the clinical relevance of the ornithine decarboxylase/polyamine system in prostatic cancer. These studies show that differences in biological behaviour of prostatic (cancer) cells are associated with changes in polyamine levels and/or the activity of their metabolic enzymes. Faulty antizyme regulation of polyamine homoeostasis may play an important role in the growth and progression of prostatic carcinoma. Treatment of human prostate carcinoma cells with inhibitors of polyamine metabolic enzymes or polyamine analogues induces cell growth arrest or (apoptotic) cell death. Our recent in vitro studies using conformationally restricted polyamine analogues show that these compounds inhibit cell growth, probably by inducing antizyme-mediated degradation of ornithine decarboxylase. Sensitivity of human prostate cancer cells for these compounds was increased in the absence of androgens. These results suggest that these analogues might have chemotherapeutic potential in case prostatic cancer has become androgen-independent. Pilot data in an in vivo model show that these analogues have effects on tumour cell proliferation, vascularity, blood perfusion and tissue hypoxia. Overall, these studies show that polyamines may serve as important biomarkers of prostatic malignancy and provide a promising target for chemotherapy of prostatic cancer.

Transgenic mouse models for studies of the role of polyamines in normal, hypertrophic and neoplastic growth

Transgenic mice expressing proteins altering polyamine levels in a tissue-specific manner have considerable promise for evaluation of the roles of polyamines in normal, hypertrophic and neoplastic growth. This short review summarizes the available transgenic models. Mice with large increases in ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase or antizyme, a protein regulating polyamine synthesis by reducing polyamine transport and ODC in the heart, have been produced using constructs in which the protein is expressed from the alpha -myosin heavy-chain promoter. These mice are useful in studies of the role of polyamines in hypertrophic growth. Expression from keratin promoters has been used to target increased synthesis of ODC, spermidine/spermine-N(1)-acetyltransferase (SSAT) and antizyme in the skin. Such expression of ODC leads to an increased sensitivity to chemical and UV carcinogenesis. Expression of antizyme inhibits carcinogenesis in skin and forestomach. Expression of SSAT increases the incidence of skin papillomas and their progression to carcinomas in response to a two-stage carcinogenesis protocol. These results establish the importance of polyamines in carcinogenesis and neoplastic growth and these transgenic mice will be valuable experimental tools to evaluate the importance of polyamines in mediating responses to oncogenes and studies of cancer chemoprevention.

Polyamines and ethylene interact in rice grains in response to soil drying during grain filling

This study tested the hypothesis that the interaction between polyamines and ethylene may mediate the effects of soil drying on grain filling of rice (Oryza sativa L.). Two rice cultivars were pot grown. Three treatments, well-watered, moderate soil drying (MD), and severe soil drying (SD), were imposed from 8 d post-anthesis until maturity. The endosperm cell division rate, grain-filling rate, and grain weight of earlier flowering superior spikelets showed no significant differences among the three treatments. However, those of the later flowering inferior spikelets were significantly increased under MD and significantly reduced under SD when compared with those which were well watered. The two cultivars showed the same tendencies. MD increased the contents of free spermidine (Spd) and free spermine (Spm), the activities of S-adenosyl-L-methionine decarboxylase and Spd synthase, and expression levels of polyamine synthesis genes, and decreased the ethylene evolution rate, the contents of 1-aminocylopropane-1-carboxylic acid (ACC) and hydrogen peroxide, the activities of ACC synthase, ACC oxidase, and polyamine oxidase, and the expression levels of ethylene synthesis genes in inferior spikelets. SD exhibited the opposite effects. Application of Spd, Spm, or an inhibitor of ethylene synthesis to rice panicles significantly reduced ethylene and ACC levels, but significantly increased Spd and Spm contents, grain-filling rate, and grain weight of inferior spikelets. The results were reversed when ACC or an inhibitor of Spd and Spm synthesis was applied. The results suggest that a potential metabolic interaction between polyamines and ethylene biosynthesis responds to soil drying and mediates the grain filling of inferior spikelets in rice.

Polyamines and mRNA stability in regulation of intestinal mucosal growth

The mammalian intestinal epithelium is a rapidly self-renewing tissue in the body, and its homeostasis is preserved through strict regulation of epithelial cell proliferation, growth arrest, and apoptosis. Polyamines are necessary for normal intestinal mucosal growth and decreasing cellular polyamines inhibits cell proliferation and disrupts epithelial integrity. An increasing body of evidence indicates that polyamines regulate intestinal epithelial cell renewal by virtue of their ability to modulate expression of various genes and that growth inhibition following polyamine depletion results primarily from the activation of growth-inhibiting genes rather than a simple decrease in expression of growth-promoting genes. In this review article, we will focus on changes in expression of growth-inhibiting genes following polyamine depletion and further analyze in some detail the mechanisms through which mRNA stability is regulated by RNA-binding proteins.

Polyamine depletion induces rapid NF-kappa B activation in IEC-6 cells

The proliferation of the rat intestinal mucosal IEC-6 cell line requires polyamines, whose synthesis is catalyzed by the enzyme ornithine decarboxylase (ODC). ODC inhibition leads to polyamine depletion, as well as inhibition of both cell proliferation and apoptosis by regulating gene expression. The NF-kappa B transcription factor regulates genes involved in apoptotic, immune, and inflammatory responses. In the present study we tested the hypothesis that NF-kappa B is activated following ODC inhibition. We found that the inhibition of ODC by alpha-difluoromethylornithine (DFMO) resulted in a approximately 50% decrease in intracellular putrescine levels within 1 h. NF-kappa B is activated by DFMO through the degradation of the inhibitory protein I kappa B alpha that sequesters NF-kappa B in the cytoplasm. The DFMO-induced NF-kappa B complexes contain the p65 and p50 members of the Rel protein family. DFMO-induced NF-kappa B activation was accompanied by the translocation of p65 from the cytoplasm into the nucleus. DFMO selectively inhibited a gene reporter construct dependent on the kappa B site present in the HLA-B7 gene. In contrast, DFMO had no effect on a gene reporter construct dependent on the kappa B site present in the interleukin-8 gene. Thus, we report that ODC inhibition activates the NF-kappa B transcription factor, which may mediate the altered physiological state of intestinal cells that occurs following polyamine depletion.

Role of polyamines in mediating malignant transformation and oncogene expression

Previous studies have demonstrated that polyamines accumulate in cancer cells and that overproduction of ornithine decarboxylase (ODC), which catalyzes polyamine synthesis, elicits the acquisition of the transformed phenotype. However, it was not clear whether the expression of ODC and the accumulation of polyamines are only innocent by-products of the transformation process. In this study we confirm previous findings what polyamines can trigger the transformation of immortalized cultured cells. In addition to NIH 3T3 fibroblasts, studied previously, rat kidney epithelial cells or fibroblasts also grew in soft agar in the presence of polyamines. It has also been demonstrated that spermidine, preferentially stimulated the transcription and the expression of c-myc while those of c-fos were preferentially stimulated by putrescine. These findings suggest that the effect of polyamines on cellular transformation, could be explained, at least partially, by stimulation of proto-oncogene expression.

The non-enzymatic hydrolysis of oligoribonucleotides VI. The role of biogenic polyamines

Single-stranded oligoribonucleotides containing UA and CA phosphodiester bonds can be hydrolyzed specifically under non-enzymatic conditions in the presence of spermidine, a biogenic amine found in a wide variety of organisms. In the present study, the rate of oligonucleotide and tRNA(i)(Met)hydrolysis was measured in the presence of spermidine and other biogenic amines. It was found that spermine [H(3)N(+)(CH(2))(3)(+)NH(2)(CH(2))(4)(+)NH(2)(CH(2))(3)(+)NH(3)] and putrescine [H(3)N(+)(CH(2))(4)(+)NH(3)] can replace spermidine [H(3)N(+)-(CH(2))(4)(+)NH(2)(CH(2))(3)(+)NH(3)] to induce the hydrolysis. For all three polyamines, a bell-shaped cleavage rate versus concentration relationship was observed. The maximum rate of hydrolysis was achieved at 0.1, 1.0 and 10 mM spermine, spermidine and putrescine, respectively. Moreover, we found that the hydrolysis requires at least two linked amino groups since two aminoalcohols, 2-aminoethanol and 3-aminopropanol, were not able to induce the cleavage of the phospho-diester bond. The optimal cleavage rate of the oligo-ribonucleotides was observed when amino groups were separated by tri- or tetramethylene linkers. The methylation of the amino groups reduced the ability of diamines to induce oligoribonucleotide hydrolysis. Non-enzymatic cleavage of tRNA(i)(Met)from Lupinus luteus and tRNA(i)(Met)from Escherichia coli demonstrate that both RNAs hydrolyze as expected from principles derived from oligoribonucleotide models.

Polyamine regulation of plasma membrane phospholipid flip-flop during apoptosis

During apoptosis, phosphatidylserine (PS) is moved from the plasma membrane inner leaflet to the outer leaflet where it triggers recognition and phagocytosis of the apoptotic cell. Although the mechanisms of PS appearance during apoptosis are not well understood, it is thought that declining activity of the aminophospholipid translocase and calcium-mediated, nonspecific flip-flop of phospholipids play a role. As previous studies in the erythrocyte ghost have shown that polyamines can alter flip-flop of phospholipids, we asked whether alterations in cellular polyamines in intact cells undergoing apoptosis would affect PS appearance, either by altering aminophospholipid translocase activity or phospholipid flip-flop. Cells of the human leukemic cell line, HL-60, were incubated with or without the ornithine decarboxylase inhibitor, difluoromethylornithine (DFMO), and induced to undergo apoptosis by ultraviolet irradiation. Whereas DFMO treatment resulted in profound depletion of putrescine and spermidine (but not spermine), it had no effect on caspase activity, DNA fragmentation, or plasma membrane vesiculation, typical characteristics of apoptosis. Notably, DFMO treatment prior to ultraviolet irradiation did not alter the decline in PS inward movement by the aminophospholipid translocase as measured by the uptake of 6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aminocaproyl] (NBD)-labeled PS detected in the flow cytometer. Conversely, the appearance of endogenous PS in the plasma membrane outer leaflet detected with fluorescein isothiocyanate-labeled annexin V and enhanced phospholipid flip-flop detected by the uptake of 1-palmitoyl-1-[6-[(7-nitro-2-1, 3-benzoxadiazol-4-yl)aminocaproyl]-sn-glycero-3-phosphocholine (NBD-PC) seen during apoptosis were significantly inhibited by prior DFMO treatment. Importantly, replenishment of spermidine, by treatment with exogenous putrescine to bypass the metabolic blockade by DFMO, restored both enhanced phospholipid flip-flop and appearance of PS during apoptosis. Such restoration was seen even in the presence of cycloheximide but was not seen when polyamines were added externally just prior to assay. Taken together, these data show that intracellular polyamines can modulate PS appearance resulting from nonspecific flip-flop of phospholipids across the plasma membrane during apoptosis.