Cellular control of ornithine decarboxylase activity by its antizyme

The Deficiency of Hypusinated eIF5A Decreases the Putrescine/Spermidine Ratio and Inhibits +1 Programmed Ribosomal Frameshifting during the Translation of Ty1 Retrotransposon in Saccharomyces cerevisiae

Programmed ribosomal frameshifting (PRF) exists in all branches of life that regulate gene expression at the translational level. The eukaryotic translation initiation factor 5A (eIF5A) is a highly conserved protein essential in all eukaryotes. It is identified initially as an initiation factor and functions broadly in translation elongation and termination. The hypusination of eIF5A is specifically required for +1 PRF at the shifty site derived from the ornithine decarboxylase antizyme 1 (OAZ1) in Saccharomyces cerevisiae. However, whether the regulation of +1 PRF by yeast eIF5A is universal remains unknown. Here, we found that Sc-eIF5A depletion decreased the putrescine/spermidine ratio. The re-introduction of Sc-eIF5A in yeast eIF5A mutants recovered the putrescine/spermidine ratio. In addition, the Sc-eIF5A depletion decreases +1 PRF during the decoding of Ty1 retrotransposon mRNA, but has no effect on -1 PRF during the decoding of L-A virus mRNA. The re-introduction of Sc-eIF5A in yeast eIF5A mutants restored the +1 PRF rate of Ty1. The inhibition of the hypusine modification of yeast eIF5A by GC7 treatment or by mutating the hypusination site Lys to Arg caused decreases of +1 PRF rates in the Ty1 retrotransposon. Furthermore, mutational studies of the Ty1 frameshifting element support a model where the efficient removal of ribosomal subunits at the first Ty1 frame 0 stop codon is required for the frameshifting of trailing ribosomes. This dependency is likely due to the unique position of the frame 0 stop codon distance from the slippery sequence of Ty1. The results showed that eIF5A is a trans-regulator of +1 PRF for Ty1 retrotransposon and could function universally in yeast.

Translational autoregulation of the S. cerevisiae high-affinity polyamine transporter Hol1

Polyamines, small organic polycations, are essential for cell viability, and their physiological levels are homeostatically maintained by post-transcriptional regulation of key biosynthetic enzymes. In addition to de novo synthesis, cells can also take up polyamines; however, identifying cellular polyamine transporters has been challenging. Here we show that the S. cerevisiae HOL1 mRNA is under translational control by polyamines, and we reveal that the encoded membrane transporter Hol1 is a high-affinity polyamine transporter and is required for yeast growth under limiting polyamine conditions. Moreover, we show that polyamine inhibition of the translation factor eIF5A impairs translation termination at a Pro-Ser-stop motif in a conserved upstream open reading frame on the HOL1 mRNA to repress Hol1 synthesis under conditions of elevated polyamines. Our findings reveal that polyamine transport, like polyamine biosynthesis, is under translational autoregulation by polyamines in yeast, highlighting the extensive control cells impose on polyamine levels.

Emerging Role of ODC1 in Neurodevelopmental Disorders and Brain Development

Ornithine decarboxylase 1 (ODC1 gene) has been linked through gain-of-function variants to a rare disease featuring developmental delay, alopecia, macrocephaly, and structural brain anomalies. ODC1 has been linked to additional diseases like cancer, with growing evidence for neurological contributions to schizophrenia, mood disorders, anxiety, epilepsy, learning, and suicidal behavior. The evidence of ODC1 connection to neural disorders highlights the need for a systematic analysis of ODC1 genotype-to-phenotype associations. An analysis of variants from ClinVar, Geno2MP, TOPMed, gnomAD, and COSMIC revealed an intellectual disability and seizure connected loss-of-function variant, ODC G84R (rs138359527, NC_000002.12:g.10444500C > T). The missense variant is found in ~1% of South Asian individuals and results in 2.5-fold decrease in enzyme function. Expression quantitative trait loci (eQTLs) reveal multiple functionally annotated, non-coding variants regulating ODC1 that associate with psychiatric/neurological phenotypes. Further dissection of RNA-Seq during fetal brain development and within cerebral organoids showed an association of ODC1 expression with cell proliferation of neural progenitor cells, suggesting gain-of-function variants with neural over-proliferation and loss-of-function variants with neural depletion. The linkage from the expression data of ODC1 in early neural progenitor proliferation to phenotypes of neurodevelopmental delay and to the connection of polyamine metabolites in brain function establish ODC1 as a bona fide neurodevelopmental disorder gene.

Polyamine Control of Translation Elongation Regulates Start Site Selection on Antizyme Inhibitor mRNA via Ribosome Queuing

Translation initiation is typically restricted to AUG codons, and scanning eukaryotic ribosomes inefficiently recognize near-cognate codons. We show that queuing of scanning ribosomes behind a paused elongating ribosome promotes initiation at upstream weak start sites. Ribosomal profiling reveals polyamine-dependent pausing of elongating ribosomes on a conserved Pro-Pro-Trp (PPW) motif in an inhibitory non-AUG-initiated upstream conserved coding region (uCC) of the antizyme inhibitor 1 (AZIN1) mRNA, encoding a regulator of cellular polyamine synthesis. Mutation of the PPW motif impairs initiation at the uCC's upstream near-cognate AUU start site and derepresses AZIN1 synthesis, whereas substitution of alternate elongation pause sequences restores uCC translation. Impairing ribosome loading reduces uCC translation and paradoxically derepresses AZIN1 synthesis. Finally, we identify the translation factor eIF5A as a sensor and effector for polyamine control of uCC translation. We propose that stalling of elongating ribosomes triggers queuing of scanning ribosomes and promotes initiation by positioning a ribosome near the start codon.

Control of gene expression by translational recoding

Like all rules, even the genetic code has exceptions: these are generically classified as “translational recoding.” Almost every conceivable mode of recoding has been documented, including signals that redefine translational reading frame and codon assignation. While first described in viruses, it is becoming clear that sequences that program elongating ribosomes to shift translational reading frame are widely used by organisms in all domains of life, thus expanding both the coding capacity of genomes and the modes through which gene expression can be regulated at the posttranscriptional level. Instances of programmed ribosomal frameshifting and stop codon reassignment are opening up new avenues for treatment of numerous inborn errors of metabolism. The implications of these findings on human health are only beginning to emerge.

Site-specific impacts on gene expression and behavior in fathead minnows (Pimephales promelas) exposed in situ to streams adjacent to sewage treatment plants

BACKGROUND

Environmental monitoring for pharmaceuticals and endocrine disruptors in the aquatic environment traditionally employs a variety of methods including analytical chemistry, as well as a variety of histological and biochemical endpoints that correlate with the fish fitness. It is now clear that analytical chemistry alone is insufficient to identify aquatic environments that are compromised because these measurements do not identify the biologically available dose. The biological endpoints that are measured are important because they relate to known impairments; however, they are not specific to the contaminants and often focus on only a few known endpoints. These studies can be enhanced by looking more broadly at changes in gene expression, especially if the analysis focuses on biochemical pathways. The present study was designed to obtain additional information for well-characterized sites adjacent to sewage treatment plants in MN that are thought to be impacted by endocrine disruptors.

RESULTS

Here we examine five sites that have been previously characterized and examine changes in gene expression in fathead minnows (Pimephales promelas) that have been caged for 48 h in each of the aquatic environments. We find that the gene expression changes are characteristic and unique at each of the five sites. Also, fish exposed to two of the sites, 7 and 12, present a more aggressive behavior compared to control fish.

CONCLUSION

Our results show that a short-term exposure to sewage treatment plant effluents was able to induce a site-specific gene expression pattern in the fathead minnow gonad and liver. The short-term exposure was also enough to affect fish sexual behavior. Our results also show that microarray analysis can be very useful at determining potential exposure to chemicals, and could be used routinely as a tool for environmental monitoring.

Androgen responsiveness and intrarenal localization of transcripts coding for the enzymes of polyamine metabolism in the mouse

Polyamines, spermidine (SPD), and spermine (SPM) are intracellular polycations required for cell growth and differentiation. Their biosynthetic precursor, the diamine putrescine (PUT), is produced by regulatory ornithine decarboxylase (ODC). Spermidine/spermine N1-acetyltransferase (SSAT) is the ODC counterpart in the degradation pathway which retroconverts SPM and SPD into PUT. Castration of male mice for 7 days resulted in a 40% decrease of the renal levels of both SSAT and ODC transcripts. Administration of 5-alpha-dihydrotestosterone (DHT) to castrated mice for the last 3 days before sacrifice caused the levels of ODC and SSAT mRNAs to increase by 250% and 180%, respectively. Thus activation of the retroconversion pathway of polyamine metabolism appears to contribute towards the increase in PUT production known to be caused by androgens in the mouse kidney. In situ hybridization histochemistry experiments showed that the SSAT transcript is expressed only by the epithelial cells of the straight and convoluted distal tubules of the nephron, while the expression of the ODC transcript is confined to the epithelium of the convoluted and straight portion of the proximal tubules. The separation of the biosynthetic from the degradation pathway along the nephron suggests that PUT is mostly produced in the distal tubule, where it may play a physiological role, independent of androgen action, in protecting tubular cells from the very low osmolarity to which they are exposed in this nephron segment.

Polyamine transport in mammalian cells. An update

The uptake and release of the natural polyamines putrescine, spermidine and spermine by mammalian cells are integral parts of the systems that regulate the intracellular concentrations of these biogenic amines according to needs. Although a general feature of all tissues, polyamine uptake into intestinal mucosa cells is perhaps the most obvious polyamine transport pathway of physiological and pathophysiological importance. Mutant cell lines lacking the ability to take up polyamines from the environment are capable of releasing polyamines. This indicates that uptake and release are functions of two different transport systems. The isolation of a transporter gene from a mammalian cell line is still lacking. Overaccumulation of polyamines is controlled by release and by a feedback regulation system that involves de novo synthesis of antizyme, a well known protein that also regulates the activity of ornithine decarboxylase. Recent work has demonstrated that Ca(2+)-signalling pathways are also involved. Although there is consensus about the importance of polyamine uptake inhibitors in the treatment of neoplastic disorders, a practically useful uptake inhibitor is still missing. However, the attempts to target tumours, and to increase the selectivity of cytotoxic agents by combining them with the polyamine structure, are promising. New, less toxic and more selective anticancer drugs can be expected from this approach.

Feedback regulation of mammalian ornithine decarboxylase. Studies using a transient expression system

Ornithine decarboxylase catalyzes the first step in the biosynthesis of polyamines in mammalian cells. The enzyme is subject to various control mechanisms to maintain adequate intracellular levels of polyamines. Polyamines exert a strong feedback control on ornithine decarboxylase. In a recent study [van Daalen Wetters, T., Macrae, M., Brabant, M., Sittler, A. & Coffino, P. (1989) Mol. Cell. Biol. 9, 5484-5490], it was concluded that feedback control of ornithine decarboxylase is mainly, if not exclusively, a posttranslational phenomenon. The existence of a fast-acting polyamine-stimulated component of ornithine decarboxylase degradation that acts on newly synthesized monomeric forms of the enzyme was postulated. In the present study we have used a transient expression system to test this hypothesis. The expression of ornithine decarboxylase in mock-transfected COS cells varied depending on the cellular supply of polyamines as has been found in other mammalian cells. Thus, supplementing the cells with exogenous spermidine resulted in a marked decrease in ornithine decarboxylase activity, whereas depletion of intracellular polyamines, using an ornithine decarboxylase inhibitor, gave a large increase in the cellular content of the enzyme. COS cells expressing an ornithine decarboxylase mRNA devoid of its 5' non-translated region did not exhibit any feedback control of the enzyme, neither in the presence of exogenous spermidine nor when the intracellular polyamine levels were depleted to the same extent as in the mock-transfected COS cells. The results strongly suggest that the feedback control of ornithine decarboxylase is not merely a posttranslational phenomenon.

Polyamine transport and ornithine decarboxylase activity in hypoxic pulmonary artery smooth muscle cells

Hypoxia causes remodeling of the pulmonary circulation that is dependent on increases in lungs polyamine contents. Mechanisms by which polyamines are regulated in hypoxic lung cells are unknown, but ornithine decarboxylase (ODC) activity, the initial enzyme in de novo biosynthesis, is depressed and polyamine transport is augmented in lungs from hypoxic rats (R.-T. Shiao et al. 1990. Am. J. Physiol. 259:L351-L358). To determine if hypoxia directly influences polyamine regulatory mechanisms in pulmonary vascular cells, we examined [14C]spermidine (SPD) transport and ODC activity in bovine main pulmonary artery smooth muscle cells (PASMCs) cultured under standard (culture medium Po2: greater than 100 mm Hg), "normoxic" (culture medium Po2: 50 to 70 mm Hg), or "hypoxic" (culture medium Po2: 18 to 30 mm Hg) conditions. Uptake of [14C]SPD in cells cultured under standard conditions was temperature- and concentration-dependent, exhibited saturation kinetics, and was abolished by metabolic inhibition. Modeling of transport according to Michaelis-Menten kinetics revealed that [14C]SPD uptake in cells cultured under standard conditions was characterized by Km and Vmax values of 0.78 microM and 4.5 pmol/min/10(6) cells, respectively. In comparison to cells cultured under standard conditions, Km was unaffected by culture under normoxic or hypoxic conditions while Vmax was increased to 18 pmol/min/10(6) cells in normoxic cells and to 33 pmol/min/10(6) cells in preparations cultured under hypoxic conditions. Inhibition of ODC with alpha-difluoromethylornithine (DFMO) also induced SPD transport, as evidenced by an increase in the Vmax to 65 pmol/min/10(6) cells. Both hypoxia- and DFMO-induced increases in [14C]SPD transport were suppressed by cycloheximide and actinomycin D, thus highlighting the importance of protein and RNA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of ornithine and S-adenosylmethionine decarboxylases in mouse brain

The activities of the two key enzymes in mammalian polyamine synthesis, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC) in mouse brain show distinct, but inverse, changes during ontogeny. The level of ODC activity is about 70 fold higher at the time of birth than in the adult mouse, whereas AdoMetDC activity is very low after birth and increases as the brain matures. The correlation between the changes in enzyme activities and in the levels of the corresponding mRNAs diminishes dramatically during development. The increase in AdoMetDC mRNA level exceeds the increase in enzyme activity by 100%. Whereas ODC mRNA level falls initially, in concert with decreasing enzyme activity, but then shows an abrupt rise to a very high level during the late period of brain maturation while the enzyme activity continues to decrease to an almost undetectable level. These data suggest the development-dependent appearance of post-transcriptional regulation mechanisms.

Stable ornithine decarboxylase in a rat hepatoma cell line selected for resistance to alpha-difluoromethylornithine

Ornithine decarboxylase (ODC) is extremely unstable in mammalian cells. This unusual characteristic facilitates rapid fluctuations in the activity of this enzyme in response to variations in its biosynthesis. Unfortunately, very little is known about the mechanism or regulation of this ODC-specific proteolytic pathway. This study describes the production and characterization of a variant of the rat hepatoma HTC cell line that is strikingly deficient in this pathway. This cell variant was induced by selection for growth in stepwise increasing concentrations (up to 10 mM) of the irreversible ODC inhibitor, alpha-difluoromethylornithine (DFMO). Resistance to this inhibitor appears to result from a combination of elevated (10X) ODC biosynthesis and inhibited degradation, producing greater than a 2000-fold increase in the level of ODC protein. In these variant cells (DH23b) inhibition of protein synthesis by cycloheximide did not result in rapid loss of enzyme activity or ODC protein determined by radioimmunoassay. Pulse-chase studies with [35S]methionine confirmed that this enzyme was not preferentially degraded, even when spermidine was added to the media. ODC purified from the variant cells was found to be identical to the control cell enzyme in size, isoelectric point, substrate binding kinetics, and sensitivity to the inhibitor DFMO. Also, as in the control cells, a major fraction of the ODC molecules extracted from DH23b cells was shown to be phosphorylated on a serine residue. The inability to detect physical or kinetic differences between the parent and the variant cell ODC suggests that the unusual stability of ODC in this cell is associated with a defect in a cellular mechanism for ODC-specific degradation.

Maturation of the rat cumulus-oocyte complex: structure and function

The cumulus cells that surround the mammalian oocyte become dispersed following the preovulatory surge of the pituitary gonadotropin, luteinizing hormone (LH). We have examined cumulus-oocyte complexes of PMSG-primed immature rats before and at 1, 2, 3, 4, 6, and 8 hr after injection of human chorionic gonadotropin (hCG), which acts on the rat ovary like the pituitary gonadotropin. Associations between projections of the cumulus cells and the oocyte were analyzed in thin sections. We observed that some cumulus projections were greatly enlarged where they associate with the oocyte. These enlarged regions were filled with numerous small vesicles. Gap junctions between cumulus cell projections and the oocytes were small. We quantitated the number and size of gap junctions between cumulus cells. The number of small gap junctions (less than 1 microM) between cumulus cells did not change significantly over the 8-hr period after hCG administration. Larger gap junctions, however, showed a general downward trend beginning after the third hour post hCG. Light microscopic observations of plastic sections revealed that dispersion of the cumulus oophorus is not observed until after 4 hr post-hCG, but between 4 and 8 hr after gonadotropin administration the cumulus becomes markedly dispersed. In the majority of the oocytes in these complexes the germinal vesicle (GV) displayed some irregularity in shape at 2 hr post-hCG, although absence of the GV was not observed until later. Our observations suggest a new means of communication in the cumulus-oocyte complex by the vesicle-filled enlargements of the cumulus cell projections at the oocyte surface. They further indicate that the decrease in metabolic coupling observed in rat cumulus-oocyte complexes soon after exposure to LH is not associated with a change in number and size of the gap junctions between the cumulus cells. We suggest that it is either the disruption of the gap junctions at the region of contact of the cumulus cell projections with the oocyte surface or the operation of a gating mechanism that blocks the junctional channels without affecting their morphological appearance that is responsible for uncoupling of the oocyte from the cumulus cells.

Conformational changes in ornithine decarboxylase enable recognition by antizyme

Rapid, polyamine-induced degradation of mammalian ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) (ODC) is though to be controlled by the availability of a small, ODC-binding protein termed antizyme. In this study we have investigated the ability of antizyme to bind ODC protein in various altered physiological states. In particular, cold, NaCl, spermidine and deprivation of coenzyme and substrate enhance enzyme-antizyme complex formation and are all found to promote ODC homodimer dissociation. Conversely, conditions that maintain the active ODC homodimer state prevent antizyme binding and inactivation of ODC. Further, covalent modification of ODC near its active site by difluoromethylornithine or phosphate also increases its sensitivity to antizyme. These results suggest that the initial signal in ODC degradation may actually be a subtle conformational change in the enzyme that enables antizyme to bind to the enzyme and may subsequently facilitate its degradation.

Epidermal growth factor: modulator of murine embryonic palate mesenchymal cell proliferation, polyamine biosynthesis, and polyamine transport

Polyamines (putrescine, spermidine, and spermine) are normal cellular constituents able to modulate cellular proliferation and differentiation in a number of tissues and cell types. This investigation explores the response of murine embryonic palate mesenchymal (MEPM) cells to epidermal growth factor (EGF) in terms of biosynthesis of putrescine and its transport across the plasma membrane and tests the hypothesis that polyamine transport can serve as an alternative mechanism (other than biosynthesis) for elevating intracellular polyamines during stimulation of MEPM cellular proliferation. MEPM cells treated with EGF were stimulated to proliferate and showed a dose- and time-dependent stimulation of ornithine decarboxylase (ODC) which was maximal at 4-6 hours. EGF also stimulated the initial rate of putrescine transport in a dose- and time-dependent manner. This stimulation was found to be maximal 3 hours after treatment and specific for the putrescine transport system. The kinetic parameters of putrescine transport shifted from 2.52 microM (Km) and 23.6 nmol/mg protein/15 minutes (Vmax) in nonstimulated cells to 4.48 microM (Km) and 39.8 nmol/mg protein/15 minutes (Vmax) in EGF-treated cells. This kinetic shift did not require de novo protein or RNA synthesis, as cycloheximide (10 micrograms/ml) and actinomycin D (50 micrograms/ml) had little effect on the ability of EGF to stimulate the initial rate of putrescine uptake. The rate of transport, however, was found to be inversely related to cell density. The addition of exogenous putrescine concomitantly with EGF blocked the induction of ODC, while in the presence of difluoromethylornithine (DFMO) (irreversible inhibitor of ODC) the initial rate of putrescine transport remained elevated throughout the time course studied. This stimulation of putrescine uptake caused by polyamine deprivation was reversed by exogenous putrescine and Ca++ while alpha-aminoisobutyric acid (AIB) further stimulated the rate of uptake. EGF's ability to stimulate cellular DNA synthesis was inhibited by DFMO. If DFMO-treated cells were stimulated with EGF in the presence of exogenous putrescine, this stimulatory effect was preserved. These studies indicate that the rate of polyamine transportation is highly responsive to a signal which initiates biosynthesis of polyamines. Further, this transportation system provides a compensatory mechanism allowing the cell to increase intracellular levels of polyamines when environmental conditions inhibit biosynthesis or when polyamines are abundant.

Induction of ornithine decarboxylase in adult rat hippocampal slices

Several factors involved in the regulation of ornithine decarboxylase (ODC) activity in adult rat brain tissue have been identified by using the in vitro hippocampal slice preparation. The same amino acids that have previously been reported to induce ODC in tissue culture, i.e., asparagine and glutamine, were found to produce a concentration- and time-dependent increase in ODC activity that reached a 100 fold the control value after 6 h of incubation. The effect of asparagine was totally blocked by inhibition of either protein or RNA synthesis, suggesting that the inducing amino acids increase ODC activity by stimulating the transcription of genes directly or indirectly regulating ODC activity. The effect of the inducing amino acids was potentiated by a variety of factors which by themselves did not modify ODC activity. In particular, opioid peptides markedly potentiated the effect of asparagine. Although the opiate antagonists naloxone and naltrexone totally blocked the effects of the opioid peptides on ODC induction, they also produced an inhibition of the asparagine-mediated increase in ODC activity. Other factors like dibutyryl cyclic AMP and insulin also potentiated the effects of asparagine on ODC activity. These results provide the first description of ODC induction in an in vitro preparation of adult brain tissue and indicate that the hippocampal slice preparation could be used to study the molecular mechanisms which regulate the expression and activity of ODC in the adult central nervous system. Moreover the data suggest possible mechanisms which may be involved in the induction of ODC in hippocampus by seizure activity.

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)

Ornithine decarboxylase lability in 2 transplantable highly deviated rat hepatomas

A strong ornithine decarboxylase (ODC)-inactivating capacity has been previously shown (M.F. Zuretti and E. Gravela (1983) Biochim. Biophys. Acta, 742, 269-277) to be bound to rat liver microsomes. Present results show that in 2 fast-growing transplantable tumors, the 3924A Morris hepatoma and the AH 130 Yoshida ascites hepatoma, microsomes are endowed with a greatly enhanced ODC-inactivating capacity, and, concurrently, ODC displays an extreme in vitro liability and an unusual thiol-dependency (most of the activity requires dithiothreitol supply to be determined). These data are at variance with those previously obtained in hepatomas induced by N-2-fluorenylacetamide (E. Gravela et al., (1983) Cancer Res., 42, 2298-2300). The possibility that ODC liability in the 2 hepatomas here studied may result from in vivo exposure to a strong microsomal activity is considered.

Regulation of ornithine decarboxylase activity by spermidine and the spermidine analogue N1N8-bis(ethyl)spermidine

Polyamine biosynthesis in intact cells can be exquisitely controlled with exogenous polyamines through the regulation of rate-limiting biosynthetic enzymes, particularly ornithine decarboxylase (ODC). In an attempt to exploit this phenomenon as an antiproliferative strategy, certain polyamine analogues have been identified [Porter, Cavanaugh, Stolowich, Ganis, Kelly & Bergeron (1985) Cancer Res. 45, 2050-2057] which lower ODC activity in intact cells, have no direct inhibitory effects on ODC, are incapable of substituting for spermidine (SPD) in supporting cell growth, and are growth-inhibitory at micromolar concentrations. In the present study, the most effective of these analogues, N1N8-bis(ethyl)SPD (BES), is compared with SPD in its ability to regulate ODC activity in intact L1210 cells and in the mechanism(s) by which this is accomplished. With respect to time and dose-dependence of ODC suppression, both polyamines closely paralleled one another in their response curves, although BES was slightly less effective than SPD. Conditions of minimal treatment leading to near-maximal ODC suppression (70-80%) were determined and found to be 3 microM for 2 h with either SPD or BES. After such treatment, ODC activity was fully recovered within 2-4 h when cells were re-seeded in drug-free media. By assessing BES or [3H]SPD concentrations in treated and recovered cells, it was possible to deduce that an intracellular accumulation of BES or SPD equivalent to less than 6.5% of the combined cellular polyamine pool was sufficient to invoke ODC regulatory mechanisms. Decreases in ODC activity after BES or SPD treatment were closely paralleled by concomitant decreases in ODC protein. Since cellular ODC mRNA was not similarly decreased by either BES or SPD, it was concluded that translational and/or post-translational mechanisms, such as increased degradation of ODC protein or decreased translation of ODC mRNA, were probably responsible for regulation of enzyme activity. Experimental evidence indicated that neither of these mechanisms seemed to be mediated by cyclic AMP or ODC-antizyme induction. On the basis of the consistent similarities between BES and SPD in all parameters studied, it is concluded that the analogue most probably acts by the same mechanisms as SPD in regulating polyamine biosynthesis.

Spermidine mediates degradation of ornithine decarboxylase by a non-lysosomal, ubiquitin-independent mechanism

The mechanism of spermidine-induced ornithine decarboxylase (ODC, E.C. 4.1.1.17) inactivation was investigated using Chinese hamster ovary (CHO) cells, maintained in serum-free medium, which display a stabilization of ODC owing to the lack of accumulation of putrescine and spermidine (Glass and Gerner: Biochem. J., 236:351-357, 1986; Sertich et al.: J. Cell Physiol., 127:114-120, 1986). Treatment of cells with 10 microM exogenous spermidine leads to rapid decay of ODC activity accompanied by a parallel decrease in enzyme protein. Analysis of the decay of [35S]methionine-labeled ODC and separation by two-dimensional electrophoresis revealed no detectable modification in ODC structure during enhanced degradation. Spermidine-mediated inactivation of ODC occurred in a temperature-dependent manner exhibiting pseudo-first-order kinetics over a temperature range of 22-37 degrees C. In cultures treated continuously, an initial lag was observed after treatment with spermidine, followed by a rapid decline in activity as an apparent critical concentration of intracellular spermidine was achieved. Treating cells at 22 degrees C for 3 hours with 10 microM spermidine, followed by removal of exogenous polyamine, and then shifting to varying temperatures, resulted in rates of ODC inactivation identical with that determined with a continuous treatment. Arrhenius analysis showed that polyamine mediated inactivation of ODC occurred with an activation energy of approximately 16 kcal/mol. Treatment of cells with lysosomotrophic agents (NH4Cl, chloroquine, antipain, leupeptin, chymostatin) had no effect on ODC degradation. ODC turnover was not dependent on ubiquitin-dependent proteolysis. Shift of ts85 cells, a temperature-sensitive mutant for ubiquitin conjugation, to 39 degrees C (nonpermissive for ubiquitin-dependent proteolysis) followed by addition of spermidine led to a rapid decline in ODC activity, with a rate similar to that seen at 32 degrees C (the permissive temperature). In contrast, spermidine-mediated ODC degradation was substantially decreased by inhibitors of protein synthesis (cycloheximide, emetine, and puromycin). These data support the hypothesis that spermidine regulates ODC degradation via a mechanism requiring new protein synthesis, and that this occurs via a non-lysosomal, ubiquitin-independent pathway.

Modulation of ornithine decarboxylase activity and ornithine decarboxylase-antizyme complex in rat heart by hormone and putrescine treatment

Ornithine decarboxylase was present in a cryptic, complexed form in an amount approximately equivalent to that of free ornithine decarboxylase activity in adult rat heart. Addition of isoproterenol (10 mg/kg) caused a notable rise in ornithine decarboxylase activity and a simultaneous decrease in the amount of the complexed enzyme. During the period of ornithine decarboxylase decay, when cardiac putrescine content had reached high values, the level of the complex increased above that of the control. Administration of putrescine (1.5 mmol/kg, twice) or dexamethasone (4 mg/kg) produced a decrease of heart ornithine decarboxylase activity, while it did not remarkably affect the level of complexed ornithine decarboxylase, therefore raising significantly the ratio of bound to total ornithine decarboxylase. Putrescine also elicited the appearance of free antizyme, concomitantly with the disappearance of free ornithine decarboxylase activity after 3-4 h of treatment. These results indicate that a significant amount of ornithine decarboxylase occurs in an inactive form in the heart under physiological conditions and that its absolute and relative levels may vary following stimuli which affect heart ornithine decarboxylase activity.

A novel synthetic vitamin-A-like compound (a polyprenoic acid derivative: E-5166) inhibits UVB-stimulated epidermal ornithine decarboxylase activity

Irradiation of skin with ultraviolet B (UVB) stimulates epidermal activity of ornithine decarboxylase (ODC), a rate limiting polyamine biosynthesing enzyme. The maximum stimulation of ODC activity by UVB was found at 24 after irradiation. Systemic administration of a polyprenoic acid derivative (E-5166) significantly inhibited the UVB-stimulated ODC activity in a dose-dependent manner. The inhibitory effect of E-5166 on the UVB-stimulated ODC activity was found to begin at 5 mg/kg body weight. These results indicate that because it inhibits ODC activity E-5166 can be utilized for the treatment of hyperproliferative skin diseases.

Purification of mouse brain ornithine decarboxylase reveals its presence as an inactive complex with antizyme

Mouse brain ornithine decarboxylase (ODC) was purified to near-homogeneity by using (NH4)2SO4 precipitation and chromatography on heparin-Sepharose, pyridoxamine phosphate-agarose and DEAE-cellulose. On SDS/polyacrylamide-gel electrophoresis, the final preparation gave one protein band similar to that obtained for purified mouse kidney enzyme, corresponding to an Mr of 53.000. The overall yield of the purification exceeded about 50-fold the total activity of the enzyme in the starting material. By affinity chromatography on ODC-bound Sepharose, the extra enzyme activity was shown to originate, at least partly, from the enzyme-antizyme complex. These results demonstrate that ODC in mouse brain occurs mainly in an inactive form and is activated during purification.

Posttranscriptional regulation of ornithine decarboxylase activity

We have used a Chinese hamster ovary cell line (DF3) that overproduces ornithine decarboxylase (ODC) to examine various parameters in the cell cycle-dependent regulation of this enzyme. Under a variety of conditions, alterations in the activity of ODC were accompanied by parallel changes in the levels of the protein, as measured by immunologically cross-reactive material (CRM). While putrescine has been known to suppress the induction of ODC, we have found that in DF3 cells 10(-4)M ornithine completely suppresses ODC activity. We also show that the levels of ODC mRNA are not modulated when the levels of ODC activity and CRM change drastically. The data can be interpreted in terms of models involving either an effect of putrescine on the translation of ODC mRNA, or on the activity of a relatively specific protease with ODC as its target.

Effects of putrescine on synthesis and degradation of ornithine decarboxylase in primary cultured hepatocytes

Changes in both synthesis rate and degradation rate of ornithine decarboxylase (ODC) were pursued in primary cultures of adult rat hepatocytes during the process of ODC induction caused by asparagine and glucagon and also during the process of rapid ODC decay caused by putrescine. The synthesis rate of ODC was determined by [35S]methionine incorporation into the enzyme, which was separated afterwards by immunoprecipitation and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The degradation rate of ODC was determined by following the decay of prelabeled ODC. The enzyme induction caused by asparagine (10 mM) and glucagon (1 microM) was due both to an increase in the synthesis rate and to a decrease in the degradation rate. Addition of 10 mM putrescine caused a rapid decay of ODC activity, which was faster than ODC decay in the presence of cycloheximide. This rapid decay in ODC activity was accompanied by slightly slower decay in ODC protein, which was due both to partial suppression of ODC synthesis and to several fold acceleration of ODC degradation.

Effect of adrenergic stimulation on ornithine decarboxylase activity in the rat spleen

The effect of a single administration of catecholamines on ornithine decarboxylase activity and polyamine biosynthesis in the rat spleen was investigated. Isoproterenol elicited a dose-dependent increase in spleen ODC activity which reached a maximum 4 hr after the administration of the drug. Putrescine content was also found to increase within a few hours, whereas S-adenosylmethionine decarboxylase activity and spermidine and spermine levels did not change significantly. Adrenaline and noradrenaline proved to be even more effective in increasing splenic ODC activity than isoproterenol. alpha- and beta-adrenergic antagonists prevented the ODC increase by catecholamines to a different extent.

Involvement of an "antizyme" in the inactivation of ornithine decarboxylase

DL-Allylglycine causes a marked increase in mouse brain ornithine decarboxylase (ODC) activity. The amount of immunoreactive enzyme protein increases concomitantly with the activity, but the enzyme protein decreases more slowly than that of the activity. The amount of immunoreactive ODC in brain is many hundred times that of the catalytically active enzyme. The fact that mouse brain cytosol contains high amounts of dissociable antizyme (an inactivating protein) indicates the existence of an inactive, immunoreactive ODC-antizyme pool. The total antizyme content does not change markedly, but instead there are significant changes in different antizyme pools. Putrescine concentrations start to increase 8 h after treatment with allylglycine and concomitantly with this increase, antizyme is released to inhibit enzyme activity. These results indicate the involvement of antizyme in the inactivation process of ODC.

Inhibition of rat heart ornithine decarboxylase by basic polypeptides

Purified and partially purified ornithine decarboxylase (ODC) from rat heart was inhibited by basic polypeptides in vitro. Poly-L-arginine, the most effective, was inhibitory at a concentration as low as 0.1 microgram/ml; protamine and histone clearly inhibited ODC at concentrations higher than 2 micrograms/ml, but poly-L-lysine was less effective. The ability to inhibit ODC appeared to correlate with the arginine-residue content of basic polypeptides. The inhibition effect could be decreased by increasing substrate concentration and ionic strength.

Ornithine decarboxylase activity in brain regulated by a specific macromolecule, the antizyme

Mouse brain ornithine decarboxylase activity is about 70-fold higher at the time of birth compared with that of adult mice. Enzyme activity declines rapidly after birth and reaches the adult level by 3 weeks. Immunoreactive enzyme concentration parallels very closely the decrease of enzyme activity during the first postnatal week, remaining constant thereafter. The content of brain antizyme, the macromolecular inhibitor to ornithine decarboxylase, in turn is very low during the first 7 days and starts then to increase and at the age of 3 weeks it is about six times the level of that in newborn mice. This may explain the decrease in enzyme activity during brain maturation, and suggests the regulation of polyamine biosynthesis by an antizyme-mediated mechanism in adult brain.

Ornithine decarboxylase modification and polyamine-stimulated enzyme inactivation in HTC cells

Ornithine decarboxylase isolated from HTC cells was separated into two distinct charged states by salt-gradient elution from DEAE-Sepharose columns. This charge difference between the enzyme forms was maintained in partially purified preparations, but enzyme form II was observed to change to form I in a time-dependent polyamine-stimulated fashion in crude cell homogenates. The enzyme modification that produces this charge diversity between the alternative enzyme states was further investigated for its role in enzyme activity induction, protein stability and rapid turnover. Inhibition of new protein synthesis by cycloheximide resulted in a much more rapid loss of form I enzyme than of form II, suggesting that during normal enzyme turnover the latter enzyme state may be derived from the former. Culture conditions that favour the stabilization of this usually labile enzyme generally induced an increased proportion of the enzyme in the form II charge state. In particular, inhibitors of synthesis of spermidine and spermine induced the stabilization of cellular ornithine decarboxylase and promoted a marked accumulation in form II. Conversely, polyamines added to the cells in culture induced a very rapid loss in both forms of the enzyme, an effect that could not be attributed merely to an inhibition of new enzyme synthesis. It appears that the polyamines, but not putrescine, may be an essential part of the rapid ornithine decarboxylase inactivation process and that they may function in part by stimulating the conversion of the more stable enzyme form II into the less stable enzyme state, form I.

Regulation of polyamine biosynthesis by antizyme and some recent developments relating the induction of polyamine biosynthesis to cell growth. Review

This review considers the role of antizyme, of amino acids and of protein synthesis in the regulation of polyamine biosynthesis. The ornithine decarboxylase of eukaryotic cells and of Escherichia coli can be non-competitively inhibited by proteins, termed antizymes, which are induced by di- and poly- amines. Some antizymes have been purified to homogeneity and have been shown to be structurally unique to the cell of origin. Yet, the E. coli antizyme and the rat liver antizyme cross react and inhibit each other's biosynthetic decarboxylases. These results indicate that aspects of the control of polyamine biosynthesis have been highly conserved throughout evolution. Evidence for the physiological role of the antizyme in mammalian cells rests upon its identification in normal uninduced cells, upon the inverse relationship that exists between antizyme and ornithine decarboxylase as well as upon the existence of the complex of ornithine decarboxylase and antizyme in vivo. Furthermore, the antizyme has been shown to be highly specific; its Keq for ornithine decarboxylase is 1.4 X 10(11) M-1. In addition, mammalian cells contain an anti-antizyme, a protein that specifically binds to the antizyme of an ornithine decarboxylase-antizyme complex and liberates free ornithine decarboxylase from the complex. In E. coli, in which polyamine biosynthesis is mediated both by ornithine decarboxylase and by arginine decarboxylase, three proteins (one acidic and two basic) have been purified, each of which inhibits both these enzymes. They do not inhibit the biodegradative ornithine and arginine decarboxylases nor lysine decarboxylase. The two basic inhibitors have been shown to correspond to the ribosomal proteins S20/L26 and L34, respectively. The relationship of the acidic antizyme to other known E. coli proteins remains to be determined. In mammalian cells, ornithine decarboxylase can be induced by a broad spectrum of compounds. These range from hormones and growth factors to natural amino acids such as asparagine and to non-metabolizable amino acid analogues such as alpha-amino-isobutyric acid. The amino acids that induce ornithine decarboxylase as well as those that promote polyamine uptake utilize the sodium dependent A and N transport systems. Consequently, they act in concert and increase intracellular polyamine levels by both mechanisms. The induction of ornithine decarboxylase by growth factors, such as NGF, EGF, and PDGF as well as by insulin requires the presence of these same amino acids and does not occur in their absence. However, the inducing amino acid need not be incorporated into protein nor covalently modified.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium dependence for increased antizyme inhibitory activity of ornithine decarboxylase in rat glioma cells

Ornithine decarboxylase activity was inhibited by the antizyme inhibitor protein in extracts from C6-2B rat glioma cells. Antizyme activity in C6-2B cells was increased 3- to 10-fold by micromolar concentrations of putrescine, spermidine and spermine. The calcium chelator EGTA (pCa 6.4) inhibited basal and polyamine-stimulated antizyme activity, and this inhibition was prevented by concurrent incubation with calcium, but not with magnesium. EGTA appeared to block antizyme synthesis, because the half-life values of antizyme activity in the presence of EGTA or cycloheximide were similar (121-143 min). Also, calcium readdition rapidly reversed EGTA inhibition of antizyme activity by a mechanism which could be blocked by cycloheximide. The ability of EGTA to inhibit spermidine-stimulated antizyme activity was not due to reduced spermidine uptake, because EGTA actually stimulated [3H]spermidine accumulation in the trichloroacetic acid-soluble fraction of C6-2B cells after 3 h.

Phosphorylation of rat heart ornithine decarboxylase by type-2 casein kinase

Highly purified preparations of rat heart ornithine decarboxylase are readily phosphorylated by rat liver type-2 casein kinase-TS at the same 54 KDa protein band which is also radiolabeled by 3H-DFMO. The reaction, which is stimulated by polylysine leads to the incorporation of up to 0.8 mol P/mol ornithine decarboxylase at seryl residue(s) included in a single 8.6 KDa CNBr fragment. Partially purified preparations of ornithine decarboxylase contain a type-2 casein kinase which promotes the phosphorylation of ornithine decarboxylase at the same CNBr fragment affected by rat liver casein kinase-TS.

Antizyme to ornithine decarboxylase is present in the liver of starved rats

Antizyme to ornithine decarboxylase (ODC) and ODC-antizyme complex were both present in liver cytosols of starved rats. The antizyme was identified by its molecular weight, kinetic properties, formation of a complex with ODC, and reversal of its inhibition by antizyme inhibitor. The average amount of antizyme in liver cytosols of starved rats was 0.1 unit/mg of protein, roughly corresponding to basal hepatic ODC activity in rats fed ad libitum. The presence of ODC-antizyme complex was detected by using antizyme inhibitor. These results indicate that antizyme participates in the regulation of ODC activity in vivo under physiological conditions.

Androgen induction of ornithine decarboxylase mRNA in mouse kidney as studied by complementary DNA

To investigate the mechanisms by which androgens regulate ornithine decarboxylase (OrnDCase; L-ornithine carboxy-lyase, EC 4.1.1.17) in mouse kidney, a cDNA clone encoding OrnDCase mRNA was prepared. Purification of OrnDCase mRNA from kidneys of androgen-treated mice was accomplished by immunoadsorption of renal polysomes to a protein A-Sepharose column and enrichment for poly(A)-containing RNA by oligo(dT)-cellulose. Double-stranded cDNA synthesized from this mRNA was inserted into the Pst I site of plasmid pBR322 by using oligo(dG . dC)-tailing and was propagated in Escherichia coli. Plasmids containing cDNA sequences coding for OrnDCase were identified by differential colony hybridization, by radioimmunological detection of OrnDCase-like antigens in bacterial cultures, and by cell-free translation of hybrid-selected mRNA followed by immunoprecipitation with monospecific OrnDCase antiserum. A restriction endonuclease fragment of the selected plasmid DNA (pODC54) was labeled by nick-translation and used to study changes in OrnDCase mRNA concentration. After a single dose of testosterone, renal OrnDCase mRNA concentration increased as soon as 6 hr and peaked 24 hr after steroid injection, as measured by RNA blot hybridization. Continuous androgen treatment for 4 days resulted in a 10- to 20-fold increase in OrnDCase mRNA concentration in normal animals, but no induction of this mRNA was detected in mice that have an inherent defect of the androgen receptor (testicular feminization). These results indicate that androgens regulate OrnD-Case synthesis in mouse kidney, at least in part, by increasing OrnDCase mRNA accumulation.

Epidermal growth factor stimulates ornithine decarboxylase activity in cultured mammalian keratinocytes

The influence of epidermal growth factor (EGF) on ornithine decarboxylase has been examined in cultured bovine keratinocytes. Keratinocyte ornithine decarboxylase activity was maximal at pH 6.3 in MES buffer in the presence of dithiothreitol and EDTA. When cultured cells, deprived of serum, were exposed to EGF, the activity of ornithine decarboxylase was stimulated severalfold. Enzyme activity increased in a dose-dependent manner with EGF. The time course of this stimulation is unlike any previously reported in cultured cells. The increase in activity was maximal by 8 h. A small dip in activity was seen between 8 and 12 h. Increased activity was sustained for as long as 24 h after exposure to EGF. The prolonged increased in enzyme activity was reduced by actinomycin D. When cycloheximide was added 1 h before EGF, ornithine decarboxylase activity was obliterated. This is the first demonstration of ornithine decarboxylase stimulation following exposure to EGF in cultured keratinocytes. The prolonged duration of ornithine decarboxylase stimulation is unexplained but may be related to processing of EGF by the keratinocytes.

Polyamines regulate calcium fluxes in a rapid plasma membrane response

Activation of cell-surface receptors often evokes changes in Ca2+ fluxes leading to an increase in cytosolic Ca2+, a generally accepted mediator of many cell responses. The molecular mechanisms by which surface agonists elicit these changes in Ca2+ flux have remained elusive. An increase in the polyamines putrescine, spermidine and spermine, and their rate-regulating, synthetic enzyme ornithine decarboxylase (ODC), is one of the earliest events that occur during cell growth, replication and differentiation. However, the precise physiological roles of the polyamines remain enigmatic. Recently, we found that testosterone induces an early (less than 60s), Ca2+- and receptor-dependent stimulation of endocytosis, hexose transport and amino acid transport in mouse kidney cortex involving the proximal tubules. This response is associated with increased Ca2+ fluxes and a mobilization of intracellular calcium, and is thought to represent a direct, receptor-mediated action of testosterone on the surface membrane. Polyamine synthesis was previously found to be essential for the long-term effects of testosterone on mouse kidney. We now report that testosterone evokes a rapid (less than 30 s), transient increase in ODC activity and a sustained increase in polyamines in kidney cortex. This polyamine synthesis is obligatory for stimulation of membrane transport functions and Ca2+ fluxes. These findings form the basis for a new theory of information flow in stimulus-response coupling in which the polyamines serve as messengers to generate a Ca2+ signal by increasing Ca2+ influx and mobilizing intracellular calcium via a cation-exchange reaction.

Regulation of polyamine biosynthesis in Escherichia coli by basic proteins

In Escherichia coli, the biosynthetic ornithine and arginine decarboxylases (EC 4.1.1.17 and 4.1.1.19, respectively) are responsible for the biosynthesis of polyamines from ornithine and arginine, respectively. When E. coli cells are grown in the presence of increasing amounts of polyamines, a progressive increase in the amount of antizyme 1 and antizyme 2 occurs. The amino acid compositions of antizymes 1 and 2 show them to be basic proteins; antizyme 1 has an amino acid composition similar to that of the E. coli histone-like protein HU and of the eukaryotic histone H2B; antizyme 2 is characterized by an unusually high arginine content. We find these proteins to be specific inhibitors of both the biosynthetic ornithine decarboxylase and the biosynthetic arginine decarboxylase. They do not inhibit the corresponding biodegradative ornithine and arginine decarboxylases, nor do they inhibit lysine decarboxylase or S-adenosylmethionine decarboxylase. These properties of the antizymes favor their function in the regulation of polyamine biosynthesis in E. coli. The ability of the purified antizymes to inhibit the ornithine and arginine decarboxylases is stabilized in acidic buffers and is lost upon prolonged exposure to solutions at neutral or basic pH.

Polyamine-stimulated alteration of the ornithine decarboxylase molecule in Physarum polycephalum

The molecular mechanism for polyamine-stimulated feedback modification of ornithine decarboxylase isolated from Physarum polycephalum was investigated by using two-dimensional polyacrylamide-gel electrophoresis. Partially purified A-form enzyme was converted into the B-form enzyme by isolated fractions of the Physarum A-B-converting protein, and the substrates and products were subsequently labelled by covalent addition of alpha-difluoro[14C]methylornithine, an enzyme-activated irreversible inhibitor. The active (A-form) and inactive (B-form) states of this enzyme were found to have the same Mr value, 52 000, yet they differed noticeably in their pI values, 5.45 and 5.65 respectively. In further experiments, the use of high-specific-radioactivity [3H]spermidine to stimulate this enzyme modification was shown not to result in the covalent attachment of this polyamine to ornithine decarboxylase. These results demonstrate that the polyamine-induced modification of ornithine decarboxylase in Physarum is not due to any of the mechanisms previously suggested for ornithine decarboxylase inactivation in this and other eukaryotes, namely phosphorylation, covalent polyamine addition or the non-covalent association of a specific low-Mr protein.

Polyamine-mediated protein phosphorylations: a possible target for intracellular polyamine action

Polyamines are well-known ubiquitous components of living cells. Although these polycations have been implicated in the regulation of major cellular functions such as DNA, RNA and protein synthesis occurring during cellular proliferation and/or differentiation processes, their mechanism of action at the molecular level has remained obscure. On the other hand, protein phosphorylation has emerged as a regulatory process of prime importance in cellular regulation. Data have recently been presented suggesting that polyamines may express at least part of their biological action through an effect upon selective protein phosphorylation systems. Two types of polyamine-sensitive protein kinases have been characterized in the last few years. The best known in molecular terms is the widespread casein kinase G (also termed casein kinase II), which represents a multifunctional protein kinase, at present classified as a messenger-independent activity. The other is a polyamine-dependent nuclear ornithine decarboxylase kinase characterized in Physarum polycephalum and several mammalian tissues. Both protein kinases are activated by polyamines in vitro at concentrations compatible with a physiological role, by a mechanism which most likely also involves an effect through the protein substrate conformation. Preliminary evidence suggests that both kinases may be implicated in the regulation of DNA-dependent RNA polymerase activities, although several other potential substrates have been suggested for casein kinase G. Another suggestion is that these kinases may also participate in the post-translational regulation of ornithine decarboxylase, the rate-limiting step in the polyamine biosynthetic pathway. A novel class of protein kinase activities may thus be defined as polyamine-mediated phosphorylation systems for which polyamines may function as intracellular messenger. Although their biological significance remains to be fully established, especially with regard to the definition of their specific intracellular target(s) and subsequent biological functions, these systems will be interesting to consider in future studies aimed at understanding the role of polyamines in cell regulation.

Effect of starvation and refeeding on polyamine concentrations and ornithine decarboxylase antizyme in mammary gland of lactating rats

Starvation caused a marked increase in putrescine content in mammary gland of lactating rats, together with a marked decrease in activity of ornithine decarboxylase and appearance of measurable ornithine decarboxylase antizyme. 2. Refeeding for 5 h caused disappearance of free antizyme and ornithine decarboxylase activity returned to the value in fed animals. Putrescine concentration remained elevated. 3. There was no significant change in nucleic acid content of mammary gland from starved rats, but spermidine and spermine contents increased significantly. 4. Refeeding for 5 h returned the spermidine content of mammary glands to 'fed' values, and significantly decreased the content of spermine, although it did not reach control values. Thus changes in polyamine content of mammary gland in starved rats are clearly dissociated from changes in either RNA content or activities of polyamine-synthetic decarboxylases. 5. Starvation caused a fall in the content of spermidine in liver, with no change in spermine content. Refeeding for 5 h returned the spermidine content to 'fed' values.