Polyamine metabolism in enucleated mouse L-cells

Evidence for Polyamine, Biogenic Amine, and Amino Acid Adduction Resulting from Metabolic Activation of Diosbulbin B

Dioscorea bulbifera L. (DBL), a traditional Chinese medicine, is a well-known herb with hepatotoxicity, and the biochemical mechanisms of the toxic action remain unknown. Diosbulbin B (DSB), a major component of DBL, can induce severer liver injury which requires cytochrome P450-catalyzed oxidation of the furan ring. It is reported that a cis-enedial reactive intermediate resulting from metabolic activation of DSB can react with thiols and amines to form pyrrole or pyrroline derivatives. In this study, we investigated the interaction of the reactive intermediate with polyamines, biogenic amines, and amino acids involved in the polyamine metabolic pathway, including putrescine, spermidine, spermine, histamine, arginine, ornithine, lysine, glutamine, and asparagine. Seven DSB-derived amine adducts were detected in microsomal incubations supplemented with DSB and individual amines. Six adducts were observed in cultured rat primary hepatocytes after exposure to DSB. DSB was found to induce apoptosis and cell death in time- and concentration-dependent manners. Apparently, the observed apoptosis was associated with the detected amine adduction. The findings facilitate the understanding of the mechanisms of toxic action of DSB.

Endogenous polyamine function--the RNA perspective

Recent progress with techniques for monitoring RNA structure in cells such as ‘DMS-Seq’ and ‘Structure-Seq’ suggests that a new era of RNA structure-function exploration is on the horizon. This will also include systematic investigation of the factors required for the structural integrity of RNA. In this context, much evidence accumulated over 50 years suggests that polyamines play important roles as modulators of RNA structure. Here, we summarize and discuss recent literature relating to the roles of these small endogenous molecules in RNA function. We have included studies directed at understanding the binding interactions of polyamines with polynucleotides, tRNA, rRNA, mRNA and ribozymes using chemical, biochemical and spectroscopic tools. In brief, polyamines bind RNA in a sequence-selective fashion and induce changes in RNA structure in context-dependent manners. In some cases the functional consequences of these interactions have been observed in cells. Most notably, polyamine-mediated effects on RNA are frequently distinct from those of divalent cations (i.e. Mg²⁺) confirming their roles as independent molecular entities which help drive RNA-mediated processes.

Subcellular localization of antizyme inhibitor 2 in mammalian cells: Influence of intrinsic sequences and interaction with antizymes

Ornithine decarboxylase (ODC) and the antizyme inhibitors (AZIN1 and AZIN2), regulatory proteins of polyamine levels, are antizyme-binding proteins. Although it is widely recognized that ODC is mainly a cytosolic enzyme, less is known about the subcellular distribution of AZIN1 and AZIN2. We found that these proteins, which share a high degree of homology in their amino acid sequences, presented differences in their subcellular location in transfected mammalian cells. Whereas ODC was mainly present in the cytosol, and AZIN1 was found predominantly in the nucleus, interestingly, AZIN2 was located in the ER-Golgi intermediate compartment (ERGIC) and in the cis-Golgi network, apparently not related to any known cell-sorting sequence. Our results rather suggest that the N-terminal region may be responsible for this particular location, since its deletion abrogated the incorporation of the mutated AZIN2 to the ERGIC complex and, on the other hand, the substitution of this sequence for the corresponding sequence in ODC, translocated ODC from cytosol to the ERGIC compartment. Furthermore, the coexpression of AZIN2 with any members of the antizyme family induced a shift of AZIN2 from the ERGIC to the cytosol. These findings underline the complexity of the AZs/AZINs regulatory system, supporting early evidence that relates these proteins with additional functions other than regulating polyamine homeostasis.

Distribution patterns of ornithine decarboxylase in cells and tissues: facts, problems, and postulates

Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis. Increased polyamine levels are required for growth, differentiation, and transformation of cells. In situ detection of ODC in cells and tissues has been performed with biochemical, enzyme cytochemical, immunocytochemical, and in situ hybridization techniques. Different localization patterns at the cellular level have been described, depending on the type of cells or tissues studied. These patterns varied from exclusively cytoplasmic to both cytoplasmic and nuclear. These discrepancies can be partially explained by the (lack of) sensitivity and/or specificity of the methods used, but it is more likely that (sub)cellular localization of ODC is cell type-specific and/or depends on the physiological status (growth, differentiation, malignant transformation, apoptosis) of cells. Intracellular translocation of ODC may be a prerequisite for its regulation and function.

Spontaneous cleavage of bleomycin-induced abasic sites in chromatin and their mutagenicity in mammalian shuttle vectors

The stability of oxidized abasic sites induced by bleomycin and neocarzinostatin was examined in chromatin reconstituted from a supercoiled plasmid and core histones. Most of the drug-induced abasic sites were found to undergo spontaneous cleavage in chromatin, probably by reaction with histone amine groups. However, there was considerable heterogeneity in the rate of spontaneous cleavage, with some sites being cleaved almost immediately and some remaining intact even after 7 h. Bleomycin-induced abasic sites with closely opposed strand breaks were more unstable than lone abasic sites. Neocarzinostatin-induced abasic sites, which have a different chemical structure, were cleaved somewhat more slowly than those induced by bleomycin. To assess the mutagenic potential of bleomycin-induced abasic sites, bleomycin-treated shuttle vectors were transfected into mammalian cells, and mutations in progeny plasmids were sequenced. Bleomycin treatment resulted primarily in deletions of various sizes in the shuttle vectors, including a number of one-base deletions occurring at potential bleomycin damage sites. However, under certain conditions, substitutions occurring at expected sites of bleomycin attack were also observed. The results suggest that bleomycin-induced abasic sites have only a slight potential to produce base substitutions in mammalian cells and that a substantial fraction of the double-strand breaks induced by bleomycin and most of the double-strand breaks induced by neocarzinostatin are the result of spontaneous cleavage of abasic sites with closely opposed strand breaks. Inaccurate repair of these double-strand breaks may account for the large deletions, and perhaps the one-base deletions, induced by bleomycin.

Spermidine nuclear acetylation in rat hepatocytes and in logarithmically growing rat hepatoma cells: comparison with histone acetylation

Spermidine acetylation has been studied in nuclear homogenates and in entire nuclei from rat hepatocytes and rat hepatoma tissue culture (HTC) cells, isolated at different stages of logarithmic growth, and compared to histone acetylation. Under all experimental conditions, N8-acetylspermidine was the predominant product of the reaction (90%). Unlike histone, spermidine acetylation in HTC cell and hepatocyte entire nuclei was almost absent or strikingly reduced relative to acetylation using nuclear homogenates as the enzyme sources. This was due to the lack of a free minor pool of spermidine, most likely lost during the purification of entire nuclei. Thus, preincubation of intact nuclei in the presence of spermidine restored activities to values observed using nuclear sonicates. Spermidine acetylation in HTC cell nuclei fluctuated moderately during cell growth, being stimulated immediately after initiation of proliferation and decreasing progressively as cultures reached high cell density. This pattern corroborated that of N8-acetylspermidine intracellular accumulation induced by culturing cells in the presence of 1 mM 7-amino-2-heptanone, a competitive inhibitor of N8-acetylspermidine deacetylase. Histone acetylation during HTC cell growth was not markedly different qualitatively from that of spermidine. Moreover, spermidine and histone acetylations in hepatocyte nuclei were of the same order of magnitude as those seen in rat hepatoma cell nuclei. Finally, inhibition of deacetylation of N8-acetylspermidine had no apparent deleterious effects on cell and growth. It remains to be determined whether the acetylation step is of higher physiological importance, in particular, and as discussed in nuclear spermidine turnover.

Biochemistry of the polyamines

The naturally-occurring polyamines exist in the free form, as N-acetyl derivatives and bound to protein. Their biosynthesis is subject to sensitive control, particularly of ornithine decarboxylase. This enzyme may be multifunctional and a key regulatory protein. Studies, principally with selective inhibitors, have elucidated the roles of polyamines in cell proliferation. Oxidized polyamines, in contrast, can be potent mitotic inhibitors. These effects are reviewed in terms of their chemistry and biochemistry. Their principal distinctions are that they can be made or degraded intracellularly, they can associate electrostatically with macromolecules by means of their spaced cationic groups, and these can be readily converted to covalent bonds.

Glucocorticoid-mediated inhibition of ornithine decarboxylyase activity in S49 lymphoma cells

Incubation of wild-type S49 lymphoma cells with glucocorticoids, such as dexamethasone and hydrocortisone, inhibits the activity of ornithine decarboxylase (L-ornithine carboxylyase, EC 4.1.1.17), the rate-limiting enzyme in the pathway of polyamine biosynthesis. The kinetics of this inhibition are more rapid than the glucocorticoid-mediated growth arrest in the G1 phase of the cell cycle or in glucocorticoid-mediated cytolysis of these cells. The inhibition of ornithine decarboxylase activity by corticosteroids is specific for steroids of the glucocorticoid class. Results obtained with variant S49 cells having lesions in the pathways of glucocorticoid or cyclic AMP action indicate that cytoplasmic glucocorticoid receptors, as well as nuclear transfer of steroid--receptor complexes, are required for the inhibition of ornithine decarboxylase activity but that this inhibition does not require hormonal activation of adenylate cyclase or cyclic AMP-dependent protein kinase. Because glucocorticoid-mediated inhibition of ornithine decarboxylase occurs when cellular protein synthesis has decreased less than 20%, this inhibition may represent a specific glucocorticoid-mediated deinduction of ornithine decarboxylase in S49 cells. Inhibition of ornithine decarboxylase activity may offer a useful marker for suppression of growth and cell cycle progression in these and other lymphoma cells.

A mutant of Chinese hamster ovary cells resistant to alpha-methyl- and alpha-difluoromethylornithine

We describe a mutant of Chinese hamster ovary cells which is resistant to elevated levels of alpha-methylornithine and alpha-difluoromethylornithine, reversible and enzyme-activated irreversible inhibitors, respectively, of the enzyme ornithine decarboxylase (ODC). The mutant cells have significantly elevated levels of enzyme activity compared to wild-type cells, but several of the physical parameters of the enzyme are completely normal: Michaelis-Menten parameter, Km, affinity for the analog, and half-life. The temporal regulation of this activity in synchronized cells is not perturbed, and the suppression of ODC activity by the addition of putrescine is still observed. Indirect experiments suggest increased concentrations of ODC mRNA in the mutant cells.

Increased nuclear conjugated polyamines and transglutaminase during liver regeneration

The nuclear content of conjugated polyamines increased during rat liver regeneration. Conjugated polyamines isolated from the acid-precipitable fraction of nuclei required peptide bond hydrolysis for release of the parent compounds. The most striking change occurred in conjugated putrescine which fluctuated in a biphasic manner; maximal nuclear levels 12-fold and 25-fold above those of sham-operated controls were achieved at 4 and 42 hr after hepatectomy, respectively. Conjugated spermidine and spermine increased 3- and 2-fold respectively within 4 hr and remained high throughout the 48 hr studied. When expressed on the basis of mg of nuclear protein, the maximal conjugated putrescine increased 19-fold, conjugated spermidine increased 2-fold, and conjugated spermine decreased by 50%. Therefore, the spermidine and spermine conjugates may be of a more constitutive nature whereas the large changes in the nuclear conjugation of putrescine associated with the onset of growth may play a regulatory role. The nucleus also contained transglutaminase (R-glutaminyl-peptide:amine gamma-glutamyl-yltransferase, EC 2.3.2.13), an enzyme shown in vitro to conjugate polyamines covalently to proteins. The specific activity of the nuclear enzyme increased rapidly after partial hepatectomy to a level 3-fold above control at 4 hr and 7-fold above control at 42 hr. The increased conjugating activity resulted from an increase in detectable maximal velocity and not a change in affinity of the enzyme for putrescine (Km congruent to 0.4 mM). There was also a 3-fold increase at 42 hr in the number of nuclear amine acceptor sites present to which radiolabeled putrescine could be conjugated by endogenous enzyme.

Enzyme induction in a temperature-sensitive cell cycle mutant of Chinese hamster fibroblasts

A temperature-sensitive (ts) cell cycle mutant of Chinese hamster fibroblasts with a block in G1 was investigated. Attention was on the expression of the activity of three enzymes: ornithine decarboxylase (ODC) S-adenosylmethionine decarboxylase (SAMDC), and thymidine kinase (TK). ODC and SAMDC activities are normally induced in the middle of, or late in, the G1 phase, while TK activity starts to appear at the G1/S boundary. In the ts mutant released from serum starvation at the nonpermissive temperature (40.8 degrees C), we find no effect on the expression of SAMDC activity, a significantly reduced level of ODC activity compared to the control at the permissive temperature (34 degrees C), and no induction of TK activity. Results presented here and in a previous publication (Landy-Otsuka and Scheffler, '78) suggest that the decrease in ODC activity is due to an effect of the nonpermissive temperature on a post-transcriptional step, possibly a very rapid inactivation of the enzyme. The absence of TK activity, on the other hand, appears to be due to a block in transcription at the nonpermissive temperature.

Changes in the nuclear polyamine content of chick erythrocytes during embryonic development

The polyamine content of the circulating erythrocyte population in the embryonic chick was studied during its development. Total cellular polyamine content fell dramatically between 5 and 7 days of development, paralleling the decrease in metabolic activity exhibited by these cells. Nuclei were isolated from the erythrocytes by a non-aqueous technique, which not only eliminated the polyamine loss that occurred with aqueous isolation, but also prevented redistribution of the polyamines from the cytoplasm. Nuclear spermidine and spermine contents decreased markedly between 5 and 6 days of development from 31 to 10 pmol/microgram of DNA and from 33 to 18 pmol/microgram of DNA respectively. Thereafter the spermine content remained constant, but the spermidine content continued to decline. Good correlations between spermidine and RNA contents were observed in both cells and nuclei, and similarly between spermine and RNA contents in cells, but no such correlation was observed between spermine and RNA in nuclei.

Inhibition of lymphocyte proliferation by polyamines requires ruminant-plasma polyamine oxidase

Spermine and spermidine in vitro are potent inhibitors of proliferation of phytohaemagglutinin-stimulated rat thymic lymphocytes, lymphoma cells and human lymphoblastic leukaemia cells, but only in media supplemented by foetal calf serum. This inhibition is shown to be due to a bovine plasma polyamine oxidase, with a high specificity for these polyamines. Spontaneously dividing lymphocytes are not subject to this inhibition. This, plus direct evidence from synchronous cultures of EB2 cells demonstrates that the inhibition is expressed in the late G1 or G1/S interface of the cell cycle. Putrescine was not an inhibitor in the presence of foetal calf serum but became so in the presence of human pregnancy serum, possibly due to the action of diamine oxidase.

Cytoplasmic regulation of two G1-specific temperature-sensitive functions

tsAF8 and ts13 cells are temperature-sensitive (ts) mutants of BHK cells that specifically arrest, at nonpermissive temperature, in the G1 phase of the cell cycle. These two mutants can complement each other. Both cell lines can be made quiescent by serum deprivation (G0). When subsequently stimulated by serum, they can enter S phase at 34 degrees C but not at 39.5 degrees-40.6 degrees C. We have used these mutants to determine whether the nucleus is needed during the G0 leads to S transition for the expression of the G1 ts functions. For this purpose, we fused cytoplasts of G0-tsAF8 with whole ts13 cells in G0, and cytoplasts of G0-ts13 with whole tsAF8 cells in G0. Serum stimulation at the nonpermissive temperature induced DNA synthesis in both types of such fusion products. No DNA synthesis was induced by serum stimulation at the nonpermissive temperature in fusion products constructed between either G0-tsAF8 cytoplasts and whole G0-tsAF8 cells or G0-ts13 cytoplasts and whole G0-ts13 cells. These results demonstrate that the information for these two ts functions, which are required for entry of serum-stimulated cells into the S phase, are already present in the cytoplasm of G0 cells--that is, before serum stimulation commits them to the transition from the nonproliferating to the proliferating state.

Kinetics of polyamine synthesis and turnover in mouse fibroblasts

Kinetics of polyamine synthesis and degradation were studied in mouse fibroblasts growing in suspension culture. The approach was to prelabel cells with radioactive polyamines and to observe the rate of loss of radioactivity and the rate of decrease in specific activity of these compounds in cells. Radioactive putrescine declined with a half-life of 1.5-2h, whether derived directly from exogenous putrescine or indirectly from ornithine. Much of this turnover was due to excretion, the kinetics of which suggested that a steady-state was being established between putrescine inside and outside the cells. Within 5h of medium change, cells growing at a density of 5x10(5)cells/ml had supplied putrescine to the medium to a concentration of about 1mum. When cells were prelabelled with either putrescine or spermidine, radioactivity in cell spermidine declined with a half-life of 60h. This rate of turnover is sufficient to provide all the spermine required by the cell. Spermine synthesis was the only observed reaction of spermidine, although some excretion into the growth medium was detected. Spermine was not degraded at a detectable rate as long as cells were growing exponentially; in stationary phase, degradation to spermidine, which was excreted, became significant. The half-lives of the specific activities of spermine, spermidine and putrescine were 24, 15 and 1.5h respectively. From these values, the rate of synthesis of each was calculated. Spermidine was synthesized at 6.8 times the rate of spermine, and putrescine was synthesized at 0.46nmol/10(6)cells per h, twice the rate of spermidine. The significance of these kinetic parameters is discussed.