Polyamine acetylation modulates polyamine metabolic flux, a prelude to broader metabolic consequences

S-adenosylmethionine decarboxylase overexpression inhibits mouse skin tumor promotion

Neoplastic growth is associated with increased polyamine biosynthetic activity and content. Tumor promoter treatment induces the rate-limiting enzymes in polyamine biosynthesis, ornithine decarboxylase (ODC), and S-adenosylmethionine decarboxylase (AdoMetDC), and targeted ODC overexpression is sufficient for tumor promotion in initiated mouse skin. We generated a mouse model with doxycycline (Dox)-regulated AdoMetDC expression to determine the impact of this second rate-limiting enzyme on epithelial carcinogenesis. TetO-AdoMetDC (TAMD) transgenic founders were crossed with transgenic mice (K5-tTA) that express the tetracycline-regulated transcriptional activator within basal keratinocytes of the skin. Transgene expression in TAMD/K5-tTA mice was restricted to keratin 5 (K5) target tissues and silenced upon Dox treatment. AdoMetDC activity and its product, decarboxylated AdoMet, both increased approximately 8-fold in the skin. This enabled a redistribution of the polyamines that led to reduced putrescine, increased spermine, and an elevated spermine:spermidine ratio. Given the positive association between polyamine biosynthetic capacity and neoplastic growth, it was somewhat surprising to find that TAMD/K5-tTA mice developed significantly fewer tumors than controls in response to 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13-acetate chemical carcinogenesis. Importantly, tumor counts in TAMD/K5-tTA mice rebounded to nearly equal the levels in the control group upon Dox-mediated transgene silencing at a late stage of tumor promotion, which indicates that latent viable initiated cells remain in AdoMetDC-expressing skin. These results underscore the complexity of polyamine modulation of tumor development and emphasize the critical role of putrescine in tumor promotion. AdoMetDC-expressing mice will enable more refined spatial and temporal manipulation of polyamine biosynthesis during tumorigenesis and in other models of human disease.

Complex N-acetylation of triethylenetetramine

Triethylenetetramine (TETA) is an efficient copper chelator that has versatile clinical potential. We have recently shown that spermidine/spermine-N(1)-acetyltransferase (SSAT1), the key polyamine catabolic enzyme, acetylates TETA in vitro. Here, we studied the metabolism of TETA in three different mouse lines: syngenic, SSAT1-overexpressing, and SSAT1-deficient (SSAT1-KO) mice. The mice were sacrificed at 1, 2, or 4 h after TETA injection (300 mg/kg i.p.). We found only N(1)-acetyltriethylenetetramine (N(1)AcTETA) and/or TETA in the liver, kidney, and plasma samples. As expected, SSAT1-overexpressing mice acetylated TETA at an accelerated rate compared with syngenic and SSAT1-KO mice. It is noteworthy that SSAT1-KO mice metabolized TETA as syngenic mice did, probably by thialysine acetyltransferase, which had a K(m) value of 2.5 ± 0.3 mM and a k(cat) value of 1.3 s(-1) for TETA when tested in vitro with the human recombinant enzyme. Thus, the present results suggest that there are at least two N-acetylases potentially metabolizing TETA. However, their physiological significance for TETA acetylation requires further studies. Furthermore, we detected chemical intramolecular N-acetyl migration from the N(1) to N(3) position of N(1)AcTETA and N(1),N(8)-diacetyltriethylenetetramine in an acidified high-performance liquid chromatography sample matrix. The complex metabolism of TETA together with the intramolecular N-acetyl migration may explain the huge individual variations in the acetylation rate of TETA reported earlier.

A combined model of hepatic polyamine and sulfur amino acid metabolism to analyze S-adenosyl methionine availability

Many molecular details remain to be uncovered concerning the regulation of polyamine metabolism. A previous model of mammalian polyamine metabolism showed that S-adenosyl methionine availability could play a key role in polyamine homeostasis. To get a deeper insight in this prediction, we have built a combined model by integration of the previously published polyamine model and one-carbon and glutathione metabolism model, published by different research groups. The combined model is robust and it is able to achieve physiological steady-state values, as well as to reproduce the predictions of the individual models. Furthermore, a transition between two versions of our model with new regulatory factors added properly simulates the switch in methionine adenosyl transferase isozymes occurring when the liver enters in proliferative conditions. The combined model is useful to support the previous prediction on the role of S-adenosyl methionine availability in polyamine homeostasis. Furthermore, it could be easily adapted to get deeper insights on the connections of polyamines with energy metabolism.

Inactivation of eukaryotic initiation factor 5A (eIF5A) by specific acetylation of its hypusine residue by spermidine/spermine acetyltransferase 1 (SSAT1)

eIF5A (eukaryotic translation initiation factor 5A) is the only cellular protein containing hypusine [Nϵ-(4-amino-2-hydroxybutyl)lysine]. eIF5A is activated by the post-translational synthesis of hypusine and the hypusine modification is essential for cell proliferation. In the present study, we report selective acetylation of the hypusine and/or deoxyhypusine residue of eIF5A by a key polyamine catabolic enzyme SSAT1 (spermidine/spermine-N1-acetyltransferase 1). This enzyme normally catalyses the N1-acetylation of spermine and spermidine to form acetyl-derivatives, which in turn are degraded to lower polyamines. Although SSAT1 has been reported to exert other effects in cells by its interaction with other cellular proteins, eIF5A is the first target protein specifically acetylated by SSAT1. Hypusine or deoxyhypusine, as the free amino acid, does not act as a substrate for SSAT1, suggesting a macromolecular interaction between eIF5A and SSAT1. Indeed, the binding of eIF5A and SSAT1 was confirmed by pull-down assays. The effect of the acetylation of hypusine on eIF5A activity was assessed by comparison of acetylated with non-acetylated bovine testis eIF5A in the methionyl-puromycin synthesis assay. The loss of eIF5A activity by this SSAT1-mediated acetylation confirms the strict structural requirement for the hypusine side chain and suggests a possible regulation of eIF5A by hypusine acetylation/deacetylation.

Does polyamine oxidase activity influence the oxidative metabolism of children who suffer of diabetes mellitus?

Diabetes mellitus is a metabolic disease characterized by inadequate secretion of insulin. Polyamine oxidase (PAO), a FAD-containing enzyme is involved in the biodegradation of Sp and Spd, catalyzing the oxidative deamination of Sp and Spd, resulting in production of ammonia (NH(3)), corresponding amino aldehydes and H(2)O(2). Malondialdehyde (MDA) and acrolein (CH2=CHCHO), potentially toxic agents, which induce oxidative stress in mammalian cells, are then spontaneously formed from aminoaldehydes. The main signs of oxidative stress in diabetic children were the values of HbA1c and MDA levels. Polyamines have an insulin-like action. Antiglycation property of spermine and spermidine has been recently confirmed. There are no data in the literature about plasma polyamine oxidase (PAO) activities in children with type 1 diabetes. The idea of this study was to evaluate the polyamine metabolism through the estimation of polyamine oxidase activity. We have study children with newly diagnosed type 1 diabetes mellitus (n = 35, age group of 5-16 years, as well as age-matched healthy control subjects (n = 25). The biochemical investigations were done on diabetic children who have the pathological values of glucose (9.11-17.33 mmol/l) and glycosylated Hb (7.57-14.49% HbA(1c)). The children in the control group have referent values of glucose and glycated hemoglobin (4.11-5.84 mmol/L and HbA(1c) 4.22-6.81% of the total Hb. Glucose levels in blood plasma and glycosylated hemoglobin in erythrocythes hemolysates (HbA1c) were measured by using standard laboratory methods. PAO activity in venous blood plasma and the amount of malondialdehyde (MDA) were measured by the spectrophotometric methods. PAO activity, glycemia, HbA1c and MDA were significantly increased in diabetic children compared to the control subjects. PAO activity in children with type 1 diabetes mellitus was very high. The findings of higher blood HbA(1C) and MDA levels confirm the presence of oxidant stress in children with type 1 diabetes mellitus and demonstrate that PAO activity may participate in these circumstances.

The role of spermidine/spermine N1-acetyltransferase in endotoxin-induced acute kidney injury

The expression of catabolic enzymes spermidine/spermine N(1)-acetyltransferase (SSAT) and spermine oxidase (SMO) increases after ischemic reperfusion injury. We hypothesized that polyamine catabolism is upregulated and that this increase in catabolic response contributes to tissue damage in endotoxin-induced acute kidney injury (AKI). SSAT mRNA expression peaked at threefold 24 h following LPS injection and returned to background levels by 48 h. The activity of SSAT correlated with its mRNA levels. The expression of SMO also increased in the kidney after LPS administration. Serum creatinine levels increased significantly at approximately 15 h, peaking by 24 h, and returned to background levels by 72 h. To test the role of SSAT in endotoxin-induced AKI, we injected wild-type (SSAT-wt) and SSAT-deficient (SSAT-ko) mice with LPS. Compared with SSAT-wt mice, the SSAT-ko mice subjected to endotoxic-AKI had less severe kidney damage as indicated by better preservation of kidney function. The role of polyamine oxidation in the mediation of kidney injury was examined by comparing the severity of renal damage in SSAT-wt mice treated with MDL72527, an inhibitor of both polyamine oxidase and SMO. Animals treated with MDL72527 showed significant protection against endotoxin-induced AKI. We conclude that increased polyamine catabolism through generation of by-products of polyamine oxidation contributes to kidney damage and that modulation of polyamine catabolism may be a viable approach for the treatment of endotoxin-induced AKI.

Suppression of exogenous gene expression by spermidine/spermine N1-acetyltransferase 1 (SSAT1) cotransfection

Spermidine/spermine N(1)-acetyltransferase 1 (SSAT1), which catalyzes the N(1)-acetylation of spermidine and spermine to form acetyl derivatives, is a rate-limiting enzyme in polyamine catabolism. We now report a novel activity of transiently transfected SSAT1 in suppressing the exogenous expression of other proteins, i.e. green fluorescent protein (GFP) or GFP-eIF5A. Spermidine/spermine N(1)-acetyltransferase 2 (SSAT2) or inactive SSAT1 mutant enzymes (R101A or R101K) were without effect. The loss of exogenous gene expression is not due to accelerated protein degradation, because various inhibitors of proteases, lysosome, or autophagy did not mitigate the effects. This SSAT1 effect cannot be attributed to the depletion of overall cellular polyamines or accumulation of N(1)-acetylspermidine (N(1)-AcSpd) because of the following: (i) addition of putrescine, spermidine, spermine, or N(1)-AcSpd did not restore the expression of GFP or GFP-eIF5A; (ii) depletion of cellular polyamines with alpha-difluoromethylornithine, an inhibitor of ornithine decarboxylase, did not inhibit exogenous gene expression; and (iii) N(1),N(11)-bis(ethyl)norspermine caused a drastic depletion of cellular polyamines through induction of endogenous SSAT1 but did not block exogenous gene expression. SSAT1 transient transfection did not affect stable expression of GFP, and stably expressed SSAT1 did not affect exogenous expression of GFP, suggesting that only transiently (episomally) expressed SSAT1 blocks exogenous (episomal) expression of other proteins. SSAT1 may regulate exogenous gene expression by blocking steps involved in transcription/translation from an episomal vector by targeting non-polyamine substrate(s) critical for this pathway.

Polyamine catabolism is enhanced after traumatic brain injury

Polyamines spermine and spermidine are highly regulated, ubiquitous aliphatic cations that maintain DNA structure and function as immunomodulators and as antioxidants. Polyamine homeostasis is disrupted after brain injuries, with concomitant generation of toxic metabolites that may contribute to secondary injuries. To test the hypothesis of increased brain polyamine catabolism after traumatic brain injury (TBI), we determined changes in catabolic enzymes and polyamine levels in the rat brain after lateral controlled cortical impact TBI. Spermine oxidase (SMO) catalyzes the degradation of spermine to spermidine, generating H2O2 and aminoaldehydes. Spermidine/spermine-N(1)-acetyltransferase (SSAT) catalyzes acetylation of these polyamines, and both are further oxidized in a reaction that generates putrescine, H2O2, and aminoaldehydes. In a rat cortical impact model of TBI, SSAT mRNA increased subacutely (6-24 h) after TBI in ipsilateral cortex and hippocampus. SMO mRNA levels were elevated late, from 3 to 7 days post-injury. Polyamine catabolism increased as well. Spermine levels were normal at 6 h and decreased slightly at 24 h, but were normal again by 72 h post-injury. Spermidine levels also decreased slightly (6-24 h), then increased by approximately 50% at 72 h post-injury. By contrast, normally low putrescine levels increased up to sixfold (6-72 h) after TBI. Moreover, N-acetylspermidine (but not N-acetylspermine) was detectable (24-72 h) near the site of injury, consistent with increased SSAT activity. None of these changes were seen in the contralateral hemisphere. Immunohistochemical confirmation indicated that SSAT and SMO were expressed throughout the brain. SSAT-immunoreactivity (SSAT-ir) increased in both neuronal and nonneuronal (likely glial) populations ipsilateral to injury. Interestingly, bilateral increases in cortical SSAT-ir neurons occurred at 72 h post-injury, whereas hippocampal changes occurred only ipsilaterally. Prolonged increases in brain polyamine catabolism are the likely cause of loss of homeostasis in this pathway. The potential for simple therapeutic interventions (e.g., polyamine supplementation or inhibition of polyamine oxidation) is an exciting implication of these studies.

Disrupting polyamine homeostasis as a therapeutic strategy for neuroblastoma

MYC genes are deregulated in a plurality of human cancers. Through direct and indirect mechanisms, the MYC network regulates the expression of > 15% of the human genome, including both protein-coding and noncoding RNAs. This complexity has complicated efforts to define the principal pathways mediating MYC's oncogenic activity. MYC plays a central role in providing for the bioenergetic and biomass needs of proliferating cells, and polyamines are essential cell constituents supporting many of these functions. The rate-limiting enzyme in polyamine biosynthesis, ODC, is a bona fide MYC target, as are other regulatory enzymes in this pathway. A wealth of data link enhanced polyamine biosynthesis to cancer progression, and polyamine depletion may limit the malignant transformation of preneoplastic lesions. Studies with transgenic cancer models also support the finding that the effect of MYC on tumor initiation and progression can be attenuated through the repression of polyamine production. High-risk neuroblastomas (an often lethal embryonal tumor in which MYC activation is paramount) deregulate numerous polyamine enzymes to promote the expansion of intracellular polyamine pools. Selective inhibition of key enzymes in this pathway, e.g., using DFMO and/or SAM486, reduces tumorigenesis and synergizes with chemotherapy to regress tumors in preclinical models. Here, we review the potential clinical application of these and additional polyamine depletion agents to neuroblastoma and other advanced cancers in which MYC is operative.

Polyamine catabolism and disease

In addition to polyamine homoeostasis, it has become increasingly clear that polyamine catabolism can play a dominant role in drug response, apoptosis and the response to stressful stimuli, and contribute to the aetiology of several pathological states, including cancer. The highly inducible enzymes SSAT (spermidine/spermine N1-acetyltransferase) and SMO (spermine oxidase) and the generally constitutively expressed APAO (N1-acetylpolyamine oxidase) appear to play critical roles in many normal and disease processes. The dysregulation of polyamine catabolism frequently accompanies several disease states and suggests that such dysregulation may both provide useful insight into disease mechanism and provide unique druggable targets that can be exploited for therapeutic benefit. Each of these enzymes has the potential to alter polyamine homoeostasis in response to multiple cell signals and the two oxidases produce the reactive oxygen species H2O2 and aldehydes, each with the potential to produce pathological states. The activity of SSAT provides substrates for APAO or substrates for the polyamine exporter, thus reducing the intracellular polyamine concentration, the net effect of which depends on the magnitude and rate of any increase in SSAT. SSAT may also influence cellular metabolism via interaction with other proteins and by perturbing the content of acetyl-CoA and ATP. The goal of the present review is to cover those aspects of polyamine catabolism that have an impact on disease aetiology or treatment and to provide a solid background in this ever more exciting aspect of polyamine biology.

Polyamine biosynthesis impacts cellular folate requirements necessary to maintain S-adenosylmethionine and nucleotide pools

Folate (vitamin B9) is utilized for synthesis of both S-adenosylmethionine (AdoMet) and deoxythymidine monophosphate (dTMP), which are required for methylation reactions and DNA synthesis, respectively. Folate depletion leads to an imbalance in both AdoMet and nucleotide pools, causing epigenetic and genetic damage capable of initiating tumorigenesis. Polyamine biosynthesis also utilizes AdoMet, but polyamine pools are not reduced under a regimen of folate depletion. We hypothesized that high polyamine biosynthesis, due to the high demand on AdoMet pools, might be a factor in determining sensitivity to folate depletion. We found a significant correlation (P<0.001) between polyamine biosynthesis and the amount of folate required to sustain cell line proliferation. We manipulated polyamine biosynthesis by genetic and pharmacological intervention and mechanistically demonstrated that we could thereby alter AdoMet pools and increase or decrease demand on folate availability needed to sustain cellular proliferation. Furthermore, growing a panel of cell lines with 100 nM folate led to imbalanced nucleotide and AdoMet pools only in cells with endogenously high polyamine biosynthesis. These data demonstrate that polyamine biosynthesis is a critical factor in determining sensitivity to folate depletion and may be particularly important in the prostate, where biosynthesis of polyamines is characteristically high due to its secretory function.

Spermidine/spermine-N1-acetyltransferase ablation protects against liver and kidney ischemia-reperfusion injury in mice

Expression of spermine/spermidine-N1-acetyltransferase (SSAT), the rate-limiting enzyme of polyamine backconversion cascade, increases after ischemia-reperfusion injuries (IRI). We hypothesized that SSAT plays an important role in the mediation of IRI. To test our hypothesis, wild-type (SSAT-wt) and SSAT-deficient (SSAT-ko) mice were subjected to liver or kidney IRI by ligation of hepatic or renal arteries. The liver and kidney content of putrescine (Put), a downstream by-product of SSAT activity, increased in SSAT-wt animals but not in SSAT-ko animals after IRI, indicating that polyamine backconversion is not functional in SSAT-deficient mice. When subjected to hepatic IRI, SSAT-ko mice were significantly protected against liver damage compared with SSAT-wt mice. Similarly, SSAT-ko animals subjected to renal IRI showed significantly greater protection against damage to kidney tubules than SSAT-wt mice. These studies indicate that SSAT-deficient animals are protected against IRI and suggest that SSAT is an important mediator of the tissue damage in IRI.

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.