Detection of ornithine decarboxylase antizyme in mouse brain

Two zebrafish (Danio rerio) antizymes with different expression and activities

Cellular polyamines are regulated by a unique feedback mechanism involving ornithine decarboxylase (ODC) antizyme. The synthesis of mammalian antizyme requires a programmed translational frameshift event induced by polyamines. Antizyme represses ODC, a key enzyme for polyamine synthesis, through accelerating enzyme degradation by the 26 S proteasome. Antizyme also inhibits the cellular uptake of polyamines. In the present study we isolated two distinct zebrafish (Danio rerio) antizyme cDNA clones (AZS and AZL) from an embryonic library. Their sequences revealed that both clones required translational frameshifting for expression. Taking account of +1 frameshifting, AZS and AZL products were 214 and 218 residues long respectively and shared 51.8% amino acid identity. In rabbit reticulocyte lysates, both mRNA species were translated through spermidine-induced frameshifting. The presence of the two antizyme mRNA species in embryos, adult fish and a cultured cell line was confirmed by Northern blot analysis. The ratio of AZS mRNA to AZL mRNA in the adult fish was 1.8-fold higher than in the embryos. Whole-mount hybridization in situ demonstrated that both mRNA species are expressed in every tissue in embryo, but predominantly in the central nervous system and the eyes. Bacterial expression products of both cDNA species inhibited ODC activity, but only the AZS product accelerated ODC degradation in vitro. These results show that both zebrafish antizymes are induced by polyamines but their mRNA species are expressed differently during development. The difference in activities on ODC degradation suggests their functional divergence.

Restricted Zn2+ availability affects the antizyme-dependent ornithine decarboxylase degradation pathway in isolated primary cultured rat hepatocytes

We previously reported that lack of Zn2+ decreased ornithine decarboxylase (ODC) activity without any change in ODC messenger RNA levels and the half-life of ODC activity being about 2-fold more rapid in primary cultured adult rat hepatocytes, suggesting that lack of Zn2+ decreased ODC activity mainly by degrading the enzyme. The present investigations showed that the chelator, diethylenetriamine penta-acetic acid (DTPA), increased the ratio of ODC-antizyme complex to total ODC (about 2-fold) and caused a decrease in antizyme inhibitor, a protein inhibitor of ODC antizyme (about 50%). These results indicate that a restricted Zn2+ availability affects the antizyme-dependent ODC degradation pathway and consequently decreases ODC activity in primary cultured rat hepatocytes.

Activation of rat brain ornithine decarboxylase by GTP

The activity of ornithine decarboxylase (ODC) measured in different regions of rat brain was highest in the hippocampus and lowest in the cerebellum. The ODC activity of a crude extract of the cerebellum was increased by the addition of GTP to the enzyme assay. Following dissociation of the ODC-antizyme complex by Sephadex G-75 chromatography in buffer containing 0.25 M NaCl, the GTP-activatable ODC was found in every brain region analysed. This GTP-activatable brain ODC has greater affinity for antizyme than the non-GTP-activatable brain ODC or the kidney ODC. The irreversible inhibitor of ODC, alpha-difluoromethylornithine (DFMO), inhibited approx. 60% of the ODC activity of all brain regions, whereas kidney ODC was inhibited totally by DFMO. When extracts of brain and kidney were incubated at 55 degrees C, kidney ODC was rapidly inactivated, but brain ODC was more heat-stable. Brain ODC, but not kidney ODC, was activated by GTP and ATP, and also by their deoxy forms. The K1/2 for activation of the enzyme was 2 microM for GTP and 40 microM for ATP. Using partially purified brain ODC, the activation by GTP was irreversible. These results demonstrate for the first time that the GTP-activatable ODC exists in the brain and is associated with the antizyme. The possible mechanisms of activation by GTP, the significance of this finding for the regulation of brain ODC, and the similarities to and differences from the GTP-activatable ODC found in certain rodent and human tumours are all discussed.

Endogenous ornithine in search for CNS functions and therapeutic applications

The vertebrate brain has the machinery to transport arginine and ornithine, and to form within nerve endings from these amino acids glutamate and GABA, the major excitatory and inhibitory neurotransmitters. Ornithine aminotransferase is a key enzyme of the Arg-->Orn-->Glu-->GABA pathway; the physiological significance of this pathway is still unclear. With 5-fluoromethylornithine, a selective inactivator of ornithine aminotransferase, a tool is in our hands that allows us to study biochemical and behavioral consequences of elevated tissue ornithine concentrations. Increase of the rate of hepatic urea formation, and of ornithine decarboxylation are the most important changes in vertebrates following inactivation of ornithine aminotransferase. Administration of 5-fluoromethylornithine prevented the accumulation of lethal concentrations of ammonia in brain, and ameliorated pathological consequences of thioacetamide intoxication. Inhibition of ornithine catabolism has, therefore, potentials in the therapy of those hyperammonemic states which are characterized by a conditional deficiency of ornithine. The enhancement of polyamine formation due to elevated ornithine concentrations may allow us to favorably affect tissue regeneration following injury.

Changes in antizyme-ornithine decarboxylase complexes in tissues of hormone-treated rats

The presence of antizyme-ornithine decarboxylase complex in thymus and kidney of rats was demonstrated using the method of Y Murakami et al. [(1985) Biochem. J. 225, 689-697]. A very small amount of complex was found in kidney of control rats, accounting for only 1-3% of total enzyme in the tissue, while in thymus, approximately one-third of the total ornithine decarboxylase in thymus occurred as an antizyme-enzyme complex. After treatment with dexamethasone, both free ornithine decarboxylase and antizyme-ornithine decarboxylase decreased in thymus, the free enzyme activity decreasing more rapidly. In kidney, the concentration of the antizyme-ornithine decarboxylase complex increased after dexamethasone treatment, but only after the induction of free enzyme activity had reached its peak and begun to decrease. The pattern of the changes in amount of antizyme-ornithine decarboxylase complex after prolactin treatment differed from those observed in the dexamethasone-treated animals. In both kidney and thymus, the concentration of antizyme-ornithine decarboxylase complex increased concurrently with the induction of free enzyme activity. Both free and complexed ornithine decarboxylase had increased at 2.5 h after prolactin treatment and continued to increase to maximum specific activities at similar rates. In thymus, the amount of ornithine decarboxylase present as a complex reached 70% of the total in the tissue. In both thymus and kidney, the concentration of antizyme-ornithine decarboxylase complex decreased more slowly than did free enzyme activity. Free antizyme was observed only in thymus of dexamethasone-treated animals. The amount of measurable inhibitor was decreased if cycloheximide was given with dexamethasone.

Ornithine decarboxylase antizyme in kidneys of male and female mice

Antizyme, a protein inhibitor of ornithine decarboxylase (ODC), was shown to be induced in mouse kidney by repeated injection of putrescine. Antizyme was also present as a complex with ODC in the kidney of untreated mouse. The amount of the renal ODC-antizyme complex was 3-fold higher in male mice than in female mice. On the contrary, the proportion of ODC present as a complex with antizyme was 24-fold higher in females than in males, and the decay of renal ODC activity after cycloheximide treatment was about 5-fold more rapid in females than in males. Administration of testosterone to female mice, a procedure known to prolong the half-life of renal ODC, increased both ODC activity and the content of ODC-antizyme complex, but decreased the antizyme/ODC ratio in the kidney. These results are consistent with the previous observation in HTC cells that the decay rate of ODC activity in the presence of cycloheximide correlated well with the proportion of ODC present as a complex with antizyme, suggesting the ubiquitous role of antizyme in ODC degradation.

Changes in ornithine decarboxylase and antizyme activities in developing mouse brain

A macromolecular inhibitor to ornithine decarboxylase (ODC) present in mouse brain was identified as ODC antizyme [Fong, Heller & Canellakis (1976) Biochim. Biophys. Acta 428, 456-465; Heller, Fong & Canellakis (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 1858-1862] on the basis of kinetic properties, Mr and reversal of its inhibition by antizyme inhibitor. The brain antizyme, however, did not cross-react immunochemically with any of seven monoclonal antibodies to rat liver antizyme. ODC activity in mouse brain rapidly decreased after birth, in parallel with putrescine content, and almost disappeared by 3 weeks of age. Free antizyme activity appeared shortly after birth and increased gradually, whereas ODC-antizyme complex already existed at birth and then gradually decreased. Thus total amount of antizyme remained about the same throughout the developmental period in mouse brain. In addition to ODC-antizyme complex, inactive ODC protein was detected by radioimmunoassay in about the same level as the complex at 3 weeks of age. Upon cycloheximide treatment, both free ODC activity and ODC-antizyme complex rapidly disappeared, although free antizyme and the inactive ODC protein were both quite stable.

Properties of ornithine decarboxylase-antizyme from mammary gland of lactating rats

Ornithine decarboxylase-antizyme was induced in mammary gland of fasted lactating rats by administration of 1,3-diaminopropan-2-ol. Antizyme from mammary gland showed similar chemical and kinetic behavior to that previously reported by Canellakis and co-workers for antizyme from liver [J. S. Heller, W. F. Fong, and E. S. Canellakis (1976) Proc. Natl. Acad. Sci. USA 72, 1858-1862]; specifically the inhibitor was nondialyzable, heat labile, and ribonuclease insensitive, and the inhibition was time independent, proportional to the concentration of antizyme present, and noncompetitive with respect to the substrate, ornithine. However, ornithine decarboxylase-antizyme from mammary gland eluted from Sephadex G-75 with an apparent molecular mass of 55 kDa, compared with 27 kDa, for antizyme from liver under identical conditions. The elution pattern was unaffected by the presence of high salt concentrations, indicating that the larger size was not due to macromolecular complexes. The presence of antizyme-ornithine decarboxylase complex was detected in mammary gland of untreated lactating rats fasted for 6 or 24 h, thus indicating that antizyme plays a role in the regulation of ornithine decarboxylase in mammary gland under physiological conditions.

Transient elevation of intrasynaptosomal free calcium by putrescine

The role of polyamines in the regulation of free intrasynaptosomal Ca2+, [Ca2+]i was studied. After preincubation of rat brain synaptosomes with 5 mM difluormethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase, the K+-induced increase of [Ca2+]i was 33% less than that in non-treated synaptosomes. Putrescine (1 mM) added together with DFMO negated the effect of DFMO suggesting that abnormally low polyamine concentrations were the reason for the diminished K+-response. Putrescine alone did not alter the K+-response to [Ca2+]i. Instead putrescine (0.5 mM) caused a rapid (less than 10 s) transient increase in [Ca2+]i but did not simultaneously increase 45Ca2+ uptake into polarized synaptosomes. Neither spermidine nor spermine (0.5 mM) significantly altered [Ca2+]i. The results suggest that polyamines play a role in the regulation of free intrasynaptosomal Ca2+.

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.

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.

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.

Ornithine decarboxylase in motoneurons during regeneration

The activity of ornithine decarboxylase, the rate-limiting enzyme in polyamine synthesis, was assayed in the isolated facial nucleus of the rat at various times after axotomy of the facial nerve. In addition, it was measured 24 h after the second of a series of two lesions (conditioning lesion design) with various times between the first and second operations. Ornithine decarboxylase activity was found to increase 8 h after nerve transection and was maximum after 24 h (300% of control). Thereafter the activity declined to subnormal levels where it remained for several weeks. Ornithine decarboxylase activity did not increase again when a second axotomy was made 2 weeks after the first lesion. However, ornithine decarboxylase did respond to the second axotomy if it was carried out 3 weeks after the first lesion. Histochemical localization of ornithine decarboxylase demonstrated that the increase in enzyme activity was mainly confined to the perikarya of the motoneurons. These data suggest that this enzyme is somehow involved in triggering the "regeneration program" and clearly indicate that at least some aspects of the neuronal response to axotomy are not further stimulated by a conditioning lesion.

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.

Temporal profiles of proteins responsive to transient ischemia

The responses of long and short half-lived proteins to ischemia were measured in rat brain during 6 days of recovery from 30 min of transient forebrain ischemia produced by four-vessel occlusion. At the end of the ischemic interval, the neocortical activities of four vulnerable enzymes [ornithine (ODC) and S-adenosylmethionine (SAMDC) decarboxylases, and RNA polymerases I and II] were unchanged, but within 30 min of reperfusion, their activities dropped by 25-50%. The loss of substance P in the striatum and substantia nigra was slower, reaching about 50% by 12 h. On the other hand, the activities of 5 long half-lived enzymes did not change in the neocortex at 5 and 15 h of reperfusion and regional protein concentrations were essentially unaffected over 6 days survival. The rate and extent of normalization of the amounts or activities of the vulnerable proteins varied. RNA polymerase II and ODC activities were restored within 4 h, and ODC showed a biphasic increase in activity, with peaks at 10 h and 2-3 days. RNA polymerase I and SAMDC activities were restored by 18 h and 5 days, respectively, whereas substance P concentrations did not completely recover, even at 6-15 days. The greater the regional reduction of blood flow during ischemia, the larger the net change (gain or loss) of SAMDC or ODC activity and the longer the time required to normalize the activities of these enzymes. The average rate of proteolysis, assessed by measuring the rate of clearance of 14C from protein prelabeled with [14C]bicarbonate, was abnormal during the first 2 days of reperfusion. Postischemic changes in both protein synthesis and degradation could affect the amounts of some of the proteins responsive to transient ischemia.

Induction of brain ornithine decarboxylase during recovery from metabolic, mechanical, thermal, or chemical injury

Metabolic, mechanical, thermal, and chemical injury induced ornithine decarboxylase (ODC) activity in rat brain. A two- to sixfold increase in ODC activity was measured at 5-9 h after different modes of injury to the brain. During the early phase of recovery from transient ischemia, when average protein synthesis was less than 50% of control, ODC activity was increased nearly fivefold. The rise in activity could be blocked by anisomycin, or reduced by intracerebral injections of actinomycin D. Drilling burr holes into the skull, injection of the vehicle for actinomycin D, hyperthermia, and freezing lesions all caused increased ODC activity. Neurotoxic chemicals (ammonia, methionine sulfoximine, acrylamide, carbon tetrachloride, and anisomycin) also increased brain ODC activity, whereas other chemicals (mannitol and valine) did not. Treatments known to stimulate the synthesis of heat shock proteins (carotid occlusion, hyperthermia, Cd2+, canavanine, and ethanol) induced ODC activity in the liver, whereas only hyperthermia and ethanol caused significant increases in spleen ODC activity. All increases in ODC activity were blocked by difluoromethylornithine, an irreversible inhibitor of ODC. The cellular response to noxious or stressful stimuli includes the synthesis of a small number of proteins of unknown functions; ODC may be one of these "heat shock" or "trauma" proteins.

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.