Spermidine/spermine N1-acetyltransferase mRNA levels show marked and region-specific changes in the early phase after transient forebrain ischemia

Acrolein is involved in ischemic stroke-induced neurotoxicity through spermidine/spermine-N1-acetyltransferase activation

BACKGROUND AND PURPOSE

Ischemic stroke is the most common type of cerebrovascular event and is responsible for approximately 85% of all strokes in Taiwan. Neurons contain high concentrations of polyamines, which are prone to various pathological states in the brain and are perturbed after cerebral ischemia. Acrolein, an α,β-unsaturated aldehyde, has been suggested as the primary culprit of neuronal damage in stroke patients. However, the mechanism by which acrolein induces neuronal damage during ischemic stroke is not clear.

METHODS

Urinary 3-hydroxypropyl mercapturic acid (3-HPMA), an acrolein-glutathione (GSH) metabolite, plasma acrolein-protein conjugates (Acr-PC) and plasma GSH levels were analyzed to correlate disease severity and prognosis of stroke patients compared with control subjects. In vivo middle cerebral artery occlusion (MCAO) animal models and an in vitro oxygen glucose deprivation (OGD) stroke model were used to investigate the mechanisms of acrolein-induced neuronal damage.

RESULTS

A deregulated acrolein metabolism, including significantly increased plasma Acr-PC levels, decreased urinary 3-HPMA levels and decreased plasma GSH levels, was found in stroke patients compared to control subjects. We further observed that acrolein was produced during ischemia resulting in brain damage in in vivo MCAO animal model. The induction of acrolein in neuronal cells during OGD occurred due to the increased expression of spermidine/spermine N1-acetyltransferase (SSAT) by NF-kB pathway activation. In addition, acrolein elicited a vicious cycling of oxidative stress resulting in neurotoxicity. Finally, N-acetylcysteine effectively prevented OGD-induced neurotoxicity by scavenging acrolein.

CONCLUSION

Overall, our current results demonstrate that acrolein is a culprit of neuronal damage through GSH depletion in stroke patients. The mechanism underlying the role of acrolein in stroke-related neuronal damage occurs through SSAT-induced polyamine oxidation by NF-kB pathway activation. These results provide a novel mechanism of neurotoxicity in stroke patients, aid in the development of neutralizing or preventive measures, and further our understanding of neural protection.

Polyamine Catabolism in Acute Kidney Injury

Acute kidney injury (AKI) refers to an abrupt decrease in kidney function. It affects approximately 7% of all hospitalized patients and almost 35% of intensive care patients. Mortality from acute kidney injury remains high, particularly in critically ill patients, where it can be more than 50%. The primary causes of AKI include ischemia/reperfusion (I/R), sepsis, or nephrotoxicity; however, AKI patients may present with a complicated etiology where many of the aforementioned conditions co-exist. Multiple bio-markers associated with renal damage, as well as metabolic and signal transduction pathways that are involved in the mediation of renal dysfunction have been identified as a result of the examination of models, patient samples, and clinical data of AKI of disparate etiologies. These discoveries have enhanced our ability to diagnose AKIs and to begin to elucidate the mechanisms involved in their pathogenesis. Studies in our laboratory revealed that the expression and activity of spermine/spermidine N1-acetyltransferase (SAT1), the rate-limiting enzyme in polyamine back conversion, were enhanced in kidneys of rats after I/R injury. Additional studies revealed that the expression of spermine oxidase (SMOX), another critical enzyme in polyamine catabolism, is also elevated in the kidney and other organs subjected to I/R, septic, toxic, and traumatic injuries. The maladaptive role of polyamine catabolism in the mediation of AKI and other injuries has been clearly demonstrated. This review will examine the biochemical and mechanistic basis of tissue damage brought about by enhanced polyamine degradation and discuss the potential of therapeutic interventions that target polyamine catabolic enzymes or their byproducts for the treatment of AKI.

Spermidine/spermine-N(1)-acetyltransferase: a key metabolic regulator

Spermidine/spermine-N(1)-acetyltransferase (SSAT) regulates cellular polyamine content. Its acetylated products are either excreted from the cell or oxidized by acetylpolyamine oxidase. Since polyamines play critical roles in normal and neoplastic growth and in ion channel regulation, SSAT is a key enzyme in these processes. SSAT is very highly regulated. Its content is adjusted in response to alterations in polyamine content to maintain polyamine homeostasis. Certain polyamine analogs can mimic the induction of SSAT and cause a loss of normal polyamines. This may have utility in cancer chemotherapy. SSAT activity is also induced via a variety of other stimuli, including toxins, hormones, cytokines, nonsteroidal anti-inflammatory agents, natural products, and stress pathways, and by ischemia-reperfusion injury. These increases are initiated by alterations in Sat1 gene transcription reinforced by alterations at the other regulatory steps, including protein turnover, mRNA processing, and translation. Transgenic manipulation of SSAT activity has revealed that SSAT activity links polyamine metabolism to lipid and carbohydrate metabolism by means of alterations in the content of acetyl-CoA and ATP. A high level of SSAT stimulates flux through the polyamine biosynthetic pathway, since biosynthetic enzymes are induced in response to the fall in polyamines. This sets up a futile cycle in which ATP is used to generate S-adenosylmethionine for polyamine biosynthesis and acetyl-CoA is consumed in the acetylation reaction. A variety of other effects of increased SSAT activity include death of pancreatic cells, blockage of regenerative tissue growth, behavioral changes, keratosis follicularis spinulosa decalvans, and hair loss. These are very likely due to changes in polyamine and putrescine levels, although increased oxidative stress via the oxidation of acetylated polyamines may also contribute. Recently, it was found that the SSAT protein and/or a related protein, thialysine acetyltransferase, interacts with a number of other important proteins, including the hypoxia-inducible factor-1 alpha-subunit, the p65 subunit of NF-kappaB, and alpha9beta1-integrin, altering the function of these proteins. It is not yet clear whether this functional alteration involves protein acetylation, local polyamine concentration changes, or other effects. It has been suggested that SSAT may also be a useful target in diseases other than cancer, but the wide-ranging physiological and pathophysiological effects of altered SSAT expression will require very careful limitation of such strategies to the relevant cells to avoid toxic effects.

Myocardial SSAT induction via AMPK signaling and its implication for ischemic injury

Spermidine/spermine N-1-acetyl-transferase (SSAT) is a catabolic enzyme that participates in polyamine metabolism. SSAT has been reported to be induced in some organs subjected to ischemia-reperfusion, but its induction mechanism has not been clarified, and little is known about SSAT regulation by ischemia per se. We induced regional ischemia of rat heart by coronary ligation and found that SSAT expression increased in ischemic myocardium. In neonatal rat cardiomyocytes and HEK293 cells, SSAT was up-regulated at the transcriptional step primarily by ATP depletion rather than oxygen deprivation. Moreover, an AMPK inhibitor compound C and AMPKalpha1-silencing RNAs attenuated the SSAT induction by ATP depletion, and an AMPK activator AICAR induced SSAT expression even without ATP depletion. When SSAT was suppressed using siRNA, the caspase activities and Bax/Bcl-2 ratios further increased in ATP depletion. These results suggest that myocardial SSAT is induced by AMPK signaling and function as a cardioprotectant under ATP-depleted conditions.

The neuroprotective effects of N1-dansyl-spermine in the gerbil model of cerebral ischaemia

The effects of N1-dansyl-spermine, a polyamine antagonist, and ifenprodil, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, were investigated in the gerbil model of global cerebral ischaemia. Transient forebrain ischaemia was induced by 5-min bilateral occlusion of the common carotid arteries. N1-dansyl-spermine (2, 5 and 10 mg/kg) and ifenprodil (30 mg/kg) were administered intraperitoneally 30 min after bilateral carotid artery occlusion. On histological examination, 4 days (96 h) after ischaemia, there was a significant decrease in neuronal density of the hippocampal CA1 subfield. This reduction in neuronal density was attenuated in those animals treated with the 5 or 10 mg/kg dose of N1-dansyl-spermine and those treated with 30 mg/kg ifenprodil. However, unlike ifenprodil, N1-dansyl-spermine failed to attenuate the ischaemia-induced increase in locomotor activity. This demonstrates that polyamines play a significant role in the neuronal damage produced after cerebral ischaemia, while casting doubt on the suggestion that increased locomotor activity correlates with CA1 pyramidal cell damage.

Effects of delayed administration of (-)-epigallocatechin gallate, a green tea polyphenol on the changes in polyamine levels and neuronal damage after transient forebrain ischemia in gerbils

(-)-Epigallocatechin gallate has a potent antioxidant property and can reduce free radical-induced lipid peroxidation as a green tea polyphenol. In previous study, systemic administration of (-)-epigallocatechin gallate immediately after ischemia has been shown to inhibit the hippocampal neuronal damage in the gerbil model of global ischemia. Polyamines are thought to be important in the generation of brain edema and neuronal cell damage associated with various types of excitatory neurotoxicity. We examined the effects of delayed administration of (-)-epigallocatechin gallate on the changes in polyamine levels and neuronal damage after transient global ischemia in gerbils. To produce transient global ischemia, both common carotid arteries were occluded for 3 min with micro-clips. The gerbils were treated with (-)-epigallocatechin gallate (50 mg/kg, i.p.) at 1 or 3 h after ischemia. The polyamines; putrescine, spermidine, and spermine levels were examined using high performance liquid chromatography in the cerebral cortex and hippocampus 24 h after ischemia. Putrescine levels in the cerebral cortex and hippocampus were increased significantly after ischemia and the delayed administrations of (-)-epigallocatechin gallate (1 or 3 h after ischemia) attenuated the increases. Only minor changes were noted in the spermidine and spermine levels after ischemia. In histology, neuronal injuries in the hippocampal CA1 regions were evaluated quantitatively 5 days after ischemia. (-)-Epigallocatechin gallate administered 1 h or 3 after ischemia significantly reduced hippocampal neuronal damage. The present results show that the delayed administrations of (-)-epigallocatechin gallate inhibit the transient global ischemia-induced increase of putrescine levels in the cerebral cortex and hippocampus. (-)-Epigallocatechin gallate is neuroprotective against neuronal damage even when administered up to 3 h after global ischemia. These findings suggest that (-)-epigallocatechin gallate may be promising in the acute treatment of stroke.

Cloning, sequencing, and heterologous expression of the murine peroxisomal flavoprotein, N1-acetylated polyamine oxidase

The aminoacyl sequences of three regions of pure bovine N1-acetylated polyamine oxidase (PAO) were obtained and used to search GenBankTM. This led to the cloning and sequencing of a complete coding cDNA for murine PAO (mPAO) and the 5'-truncated coding region of the bovine pao (bpao) gene. A search of GenBankTM indicated that mpao maps to murine chromosome 7 as seven exons. The translated amino acid sequences of mpao and bpao have a -Pro-Arg-Leu peroxisomal targeting signal at the extreme C termini. A beta-alpha-beta FAD-binding motif is present in the N-terminal portion of mPAO. This and several other regions of mPAO and bPAO are highly similar to corresponding sections of other flavoprotein amine oxidases, although the overall identity of aligned sequences indicates that PAO represents a new subfamily of flavoproteins. A fragment of mpao was used as a probe to establish the relative transcription levels of this gene in various mature murine tissues and murine embryonic and breast tissues at different developmental stages. An Escherichia coli expression system has been developed for manufacturing mPAO at a reasonable level. The mPAO so produced was purified to homogeneity and characterized. It was demonstrated definitively that PAO oxidizes N1-acetylspermine to spermidine and 3-acetamidopropanal and that it also oxidizes N1-acetylspermidine to putrescine and 3-acetamidopropanal. Thus, this is the classical polyamine oxidase (EC 1.5.3.11) that is defined as the enzyme that oxidizes these N1-acetylated polyamines on the exo-side of their N4-amino groups. This enzyme is distinguishable from the plant polyamine oxidase that oxidizes spermine on the endo-side of the N4-nitrogen. It differs also from mammalian spermine oxidase that oxidizes spermine (but not N1-acetylspermine or N1-acetylspermidine) at the exo-carbon of its N4-amino group. This report provides details of the biochemical, spectral, oxidation-reduction, and steady-state kinetic properties of pure mPAO.

Expression of SSAT, a novel biomarker of tubular cell damage, increases in kidney ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) is the major cause of acute renal failure in native and allograft kidneys. Identifying the molecules and pathways involved in the pathophysiology of renal IRI will yield valuable new diagnostic and therapeutic information. To identify differentially regulated genes in renal IRI, RNA from rat kidneys subjected to an established renal IRI protocol (bilateral occlusion of renal pedicles for 30 min followed by reperfusion) and time-matched kidneys from sham-operated animals was subjected to suppression subtractive hybridization. The level of spermidine/spermine N(1)-acetyltransferase (SSAT) mRNA, an essential enzyme for the catabolism of polyamines, increased in renal IRI. SSAT expression was found throughout normal kidney tubules, as detected by nephron segment RT-PCR. Northern blots demonstrated that the mRNA levels of SSAT are increased by greater than threefold in the renal cortex and by fivefold in the renal medulla at 12 h and returned to baseline at 48 h after ischemia. The increase in SSAT mRNA was paralleled by an increase in SSAT protein levels as determined by Western blot analysis. The concentration of putrescine in the kidney increased by approximately 4- and approximately 7.5-fold at 12 and 24 h of reperfusion, respectively, consistent with increased functional activity of SSAT. To assess the specificity of SSAT for tubular injury, a model of acute renal failure from Na(+) depletion (without tubular injury) was studied; SSAT mRNA levels remained unchanged in rats subjected to Na(+) depletion. To distinguish SSAT increases from the effects of tubular injury vs. uremic toxins, SSAT was increased in cis-platinum-treated animals before the onset of renal failure. The expression of SSAT mRNA and protein increased by approximately 3.5- and >10-fold, respectively, in renal tubule epithelial cells subjected to ATP depletion and metabolic poisoning (an in vitro model of kidney IRI). Our results suggest that SSAT is likely a new marker of tubular cell injury that distinguishes acute prerenal from intrarenal failure.

Ornithine decarboxylase knockdown exacerbates transient focal cerebral ischemia-induced neuronal damage in rat brain

Transient cerebral ischemia leads to increased expression of ornithine decarboxylase (ODC). Contradicting studies attributed neuroprotective and neurotoxic roles to ODC after ischemia. Using antisense oligonucleotides (ODNs), the current study evaluated the functional role of ODC in the process of neuronal damage after transient focal cerebral ischemia induced by middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats. Transient MCAO significantly increased the ODC immunoreactive protein levels and catalytic activity in the ipsilateral cortex, which were completely prevented by the infusion of antisense ODN specific for ODC. Transient MCAO in rats infused with ODC antisense ODN increased the infarct volume, motor deficits, and mortality compared with the sense or random ODN-infused controls. Results of the current study support a neuroprotective or recovery role, or both, for ODC after transient focal ischemia.

Regulation of SSAT expression by synaptic activity

Neuronal activity is a requirement for the plasticity and normal development of the central nervous system. We have used differential cloning techniques to identify an immediate-early gene (IEG) that is rapidly induced in neurons by activity in both adult and developmental models of plasticity. Here we describe the key regulatory enzyme of polyamine catabolism, spermidine/spermine N1-acetyltransferase (SSAT), as a neuronal IEG. In the rat brain, kainate-induced seizures result in a 5.5-fold increase in the amount of SSAT mRNA above basal levels and the enzymatic activity is increased twofold. Expression of SSAT mRNA is rapidly and transiently upregulated in the cerebral cortex and hippocampus by seizure-induced neuronal activation. In hippocampal neurons, SSAT expression is dynamically responsive to synaptic activity in the long-term potentiation (LTP) paradigm. In developing brain, region-specific expression of SSAT mRNA is first detected at postnatal day 9 (P9) and subsequently increases through days P15, P20, before reaching maximal level in adult animals. This dynamic transcriptional and translational control suggests that SSAT may play a role in activity-dependent neuronal plasticity and development.

Spermidine/spermine N1-acetyl transferase activity in rat brain following transient focal cerebral ischemia and reperfusion

The polyamine system is very sensitive to different pathological states of brain and is perturbed after central nervous system (CNS) injury. Spermidine/Spermine N(1)-acetyl transferase (SSAT) is the key enzyme responsible for interconversion of spermine and spermidine to spermidine and putrescine respectively. In the present study, SSAT activity was evaluated in the rat CNS, following transient focal cerebral ischemia and reperfusion. The middle cerebral artery (MCA) was occluded for 2 h in male spontaneously hypertensive rats by an intraluminal suture technique. Animals were sacrificed at 3-24 h reperfusion following the MCA occlusion and SSAT activity was assayed in cortex and striatum. Results showed that SSAT activity was significantly increased at 12 h reperfusion in cortex and at 9, 12 and 18 h reperfusion in striatum following ischemia compared to sham or contralateral controls. These results demonstrate that polyamine catabolism in the rat CNS is altered following MCA occlusion. In the in vitro ischemia study, SSAT activity was evaluated in primary cortical neuronal cultures at 6-24 h re-oxygenation intervals following oxygen-glucose deprivation for 1 h, and the results from this group show that the enzyme activity increased by about 62% (P<0.05) at 24 h re-oxygenation. This study suggests that the increased SSAT activity may contribute to the increase in putrescine during the post-ischemic period.

The cellular localization of the L-ornithine decarboxylase/polyamine system in normal and diseased central nervous systems

Natural polyamines, spermidine and spermine, and their precursor putrescine, are of considerable importance for the developing and mature nervous system. They exhibit a number of neurophysiological and metabolic effects in the nervous system, including control of nucleic acid and protein synthesis, modulation of ionic channels and calcium-dependent transmitter release. The polyamine system is also known to be involved in various brain pathologic events (seizures, stroke, Alzheimer's disease and others). While cerebral polyamine concentrations and the activities of polyamine-metabolizing enzymes have been studied in great detail, much less is known about the cells that are responsible for cerebral polyamine synthesis and interconversion. With the present review the attempt is made to show how exact knowledge about the regional distribution and cellular localization of polyamines and the polyamine-synthesizing enzymatic machinery (and especially of L-ornithine decarboxylase) may help to better understand the functional interplay between polyamines and other endogenous agents (transmitters, receptors, growth factors neuroactive drugs etc.). Polyamines have been localized both in neurones and glial cells. However, the main cellular locus of the ODC is the neuron--both in the immature and adult central nervous system. Each period of normal brain development and ageing seems to have its own, characteristic temporo-spatial pattern of neuronal ODC expression. During strong functional activation (kindling, epileptic seizures, neural transplantation) astrocytes and other non-neuronal cells do also express ODC and other polyamine-metabolizing enzymes. Astroglial expression of ODC is accompanied by an increase in glial fibrillary acidic protein in these cells. This shift in the cellular mechanisms of polyamine metabolism is currently far from being understood. In human brain diseases (Alzheimer's disease, schizophrenia) certain neurones show an increased expression of ODC, the first and rate-limiting enzyme of polyamine metabolism. Since polyamines are structurally related to psychoactive drugs (neuroleptics, antidepressants) the polyamine system might be of importance as a putative target for drug intervention in psychiatry.

Simultaneous assay of ornithine decarboxylase and polyamines after central nervous system injury in gerbil and rat

Ornithine decarboxylase (ODC) is considered the rate-limiting enzyme in polyamine biosynthesis. An increase in putrescine (a natural polyamine) synthesis after central nervous system (CNS) injury appears to be involved in blood-brain barrier dysfunction, development of vasogenic edema and neuronal death. An improved method is described to determine the ODC activity as well as polyamine levels from the same brain tissue. The polyamine results showed no significant differences from data obtained with the conventional assay. The advantages of this method are to: (1) minimize the number of animals needed for the study, and (2) eliminate any internal inconsistencies resulting from use of two independent groups of animals for ODC and polyamine measurements. Using this method, ODC activities and polyamine levels were measured in cortices and hippocampi from global transient ischemia of gerbils and traumatic brain injury (TBI) of rats.

Putrescine-stimulated intracellular Ca2+ release for invasiveness of rat ascites hepatoma cells

Our previous study showed that treatment of highly invasive rat ascites hepatoma (LC-AH) cells with alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, decreased both their intracellular level of putrescine and their in vitro invasion of a monolayer of calf pulmonary arterial endothelial (CPAE) cells, and that both these decreases were completely reversed by exogenous putrescine, but not spermidine or spermine. Here we show that all adhering control (DFMO-untreated) cells migrated beneath CPAE monolayer with morphological change from round to cauliflower-shaped cells (migratory cells). DFMO treatment increased the number of cells that remained round without migration (nonmigratory cells). Exogenous putrescine, but not spermidine or spermine, induced transformation of all nonmigratory cells to migratory cells with a concomitant increase in their intracellular Ca2+ level, [Ca2+]i. The putrescine-induced increase in their [Ca2+]i preceded their transformation and these effects of putrescine were not affected by antagonists of the voltage-gated Ca2+ channel, but were completely suppressed by ryanodine, which also suppressed the invasiveness of the control cells. The DFMO-induced decreases in both [Ca2+]i and the invasiveness of the cells were restored by thapsigargin, which elevated [Ca2+]i by inhibiting endoplasmic Ca2+-ATPase, indicating that thapsigargin mimics the effects of putrescine. These results support the idea that putrescine is a cofactor for Ca2+ release through the Ca2+ channel in the endoplasmic reticulum that is inhibited by ryanodine, this release being initiated by cell adhesion and being a prerequisite for tumor cell invasion.

Regional and cellular distribution of spermidine/spermine N1-acetyltransferase (SSAT) mRNA in the rat central nervous system

Spermidine/spermine N1-acetyltransferase (SSAT) is the key enzyme responsible for polyamine interconversion. SSAT mRNA (visualized by in situ hybridization histochemistry) was shown to have a wide but heterogeneous distribution in the central nervous system (CNS) at both regional and cellular levels. The highest labelling was observed in hippocampus (pyramidal and polymorph neurons) and olfactory bulb. Present data suggest that polyamine metabolism in the CNS is not homogeneous but rather that the preferential production of a polyamine species is region- and cell type-specific.