S-adenosylmethionine decarboxylase degradation by the 26 S proteasome is accelerated by substrate-mediated transamination

Polyamine Metabolism in Leishmania Parasites: A Promising Therapeutic Target

Parasites of the genus Leishmania cause a variety of devastating and often fatal diseases in humans and domestic animals worldwide. The need for new therapeutic strategies is urgent because no vaccine is available, and treatment options are limited due to a lack of specificity and the emergence of drug resistance. Polyamines are metabolites that play a central role in rapidly proliferating cells, and recent studies have highlighted their critical nature in Leishmania. Numerous studies using a variety of inhibitors as well as gene deletion mutants have elucidated the pathway and routes of transport, revealing unique aspects of polyamine metabolism in Leishmania parasites. These studies have also shed light on the significance of polyamines for parasite proliferation, infectivity, and host-parasite interactions. This comprehensive review article focuses on the main polyamine biosynthetic enzymes: ornithine decarboxylase, S-adenosylmethionine decarboxylase, and spermidine synthase, and it emphasizes recent discoveries that advance these enzymes as potential therapeutic targets against Leishmania parasites.

Increased polyamines as protective disease modifiers in congenital muscular dystrophy

Most Mendelian disorders, including neuromuscular disorders, display extensive clinical heterogeneity that cannot be solely explained by primary genetic mutations. This phenotypic variability is largely attributed to the presence of disease modifiers, which can exacerbate or lessen the severity and progression of the disease. LAMA2-deficient congenital muscular dystrophy (LAMA2-CMD) is a fatal degenerative muscle disease resulting from mutations in the LAMA2 gene encoding Laminin-α2. Progressive muscle weakness is predominantly observed in the lower limbs in LAMA2-CMD patients, whereas upper limbs muscles are significantly less affected. However, very little is known about the molecular mechanism underlying differential pathophysiology between specific muscle groups. Here, we demonstrate that the triceps muscles of the dy2j/dy2j mouse model of LAMA2-CMD demonstrate very mild myopathic findings compared with the tibialis anterior (TA) muscles that undergo severe atrophy and fibrosis, suggesting a protective mechanism in the upper limbs of these mice. Comparative gene expression analysis reveals that S-Adenosylmethionine decarboxylase (Amd1) and Spermine oxidase (Smox), two components of polyamine pathway metabolism, are downregulated in the TA but not in the triceps of dy2j/dy2j mice. As a consequence, the level of polyamine metabolites is significantly lower in the TA than triceps. Normalization of either Amd1 or Smox expression in dy2j/dy2j TA ameliorates muscle fibrosis, reduces overactive profibrotic TGF-β pathway and leads to improved locomotion. In summary, we demonstrate that a deregulated polyamine metabolism is a characteristic feature of severely affected lower limb muscles in LAMA2-CMD. Targeted modulation of this pathway represents a novel therapeutic avenue for this devastating disease.

Chemoproteomic profiling and discovery of protein electrophiles in human cells

Activity-based protein profiling (ABPP) serves as a chemical proteomic platform to discover and characterize functional amino acids in proteins on the basis of their enhanced reactivity towards small-molecule probes. This approach, to date, has mainly targeted nucleophilic functional groups, such as the side chains of serine and cysteine, using electrophilic probes. We show here that "reverse-polarity" (RP)-ABPP using clickable, nucleophilic hydrazine probes can capture and identify protein-bound electrophiles in cells, including the pyruvoyl cofactor of S-adenosyl-l-methionine decarboxylase (AMD1), which we find is dynamically controlled by intracellular methionine concentrations, and a heretofore unknown modification – an N-terminally bound glyoxylyl group – in the poorly characterized protein secernin-3. RP-ABPP thus provides a versatile method to monitor the metabolic regulation of electrophilic cofactors and discover novel types of electrophilic modifications on proteins in human cells.

A chemical proteomic strategy is described for the discovery of protein-bound electrophilic groups in human cells and used to characterize dynamic regulation of the pyruvoyl catalytic cofactor in S-adenosyl-l-methionine decarboxylase and to discover an N-terminal glyoxylyl modification on Secernin proteins.

A proteomic analysis of p53-independent induction of apoptosis by bortezomib in 4T1 breast cancer cell line

The 26S proteasome is a proteolytic enzyme found in both cytoplasm and nucleus. In this study, we examined the differential expression of proteasome inhibitor bortezomib-induced proteins in p53-deficient 4T1 cells. It was found that GRP78 and TCEB2 were over-expressed in response to treatment with bortezomib for 24h. Next, we analyzed the expression of intracellular proteins in response to treatment with 100nM bortezomib for 24h by label-free LC-MS/MS. These analyses showed that Hsp70, the 26S proteasome non-ATPase regulatory subunit 14 and sequestosome 1 were increased at least 2 fold in p53-deficient 4T1 cells. The proteins identified by label-free LC-MS/MS were then analyzed by Ingenuity Pathway Analysis (IPA) Tool to determine biological networks affected by inhibition of the 26S proteasome. The analysis results showed that post-translational modifications, protein folding, DNA replication, energy production and nucleic acid metabolism were found to be among the top functions affected by the 26S proteasome inhibition. The biological network analysis indicated that ubiquitin may be the central regulator of the pathways modulated after bortezomib-treatment. Further investigation of the mechanism of the proteins modulated in response to the proteasomal inhibition may lead to the design of more effective and novel therapeutic strategies for cancer.

BIOLOGICAL SIGNIFICANCE

Although the proteasome inhibitor bortezomib is approved and used for the treatment of human cancer (multiple myeloma), the mechanism of action is not entirely understood. A number of studies showed that proteasome inhibitors induced apoptosis through upregulation of tumor suppressor protein p53. However, the role of tumor suppressor protein p53 in bortezomib-induced apoptosis is controversial and not well-understood. The tumor suppressor p53 is mutated in at least 50% of human cancers and is strongly induced by proteasomal inhibition. Some also reported that the proteasome inhibitor can induce apoptosis in a p53-independent manner. Also, it is reported that Noxa, a target of p53, is induced in response to proteasomal inhibition in a p53-independent manner. However, we have also previously reported that neither Puma nor Noxa are induced by proteasomal inhibition in p53-null 4T1 breast cancer cells, which is commonly used for in vivo breast cancer tumor models. The current results provided additional targets of proteasome inhibitor bortezomib and may therefore help in understanding the p53-independent mechanism of apoptosis induction by proteasome inhibitors. In addition, the results presented in this current study report for the first time that proteasomal subunit Psmd14, anti-apoptotic GRP78, anti apoptotic protein Card10, Dffb, Traf3 and Trp53bp2 are regulated and overexpressed in response to proteasome inhibitor bortezomib in p53-deficient 4T1 cells. Therefore, novel therapeutic strategies targeting these anti-apoptotic or pro-apoptotic proteins as well as inhibiting the proteasome simultaneously may be more effective against cancer cells. The proteins identified here present new avenues for the development of anti-cancer drugs.

Leishmania donovani: proteasome-mediated down-regulation of methionine adenosyltransferase

Methionine adenosyltransferase (MAT) is an important enzyme for metabolic processes, to the extent that its product, S-adenosylmethionine (AdoMet), plays a key role intrans-methylation,trans-sulphuration and polyamine synthesis. Previous studies have shown that a MAT-overexpressing strain ofLeishmania donovanicontrols AdoMet production, keeping the intracellular AdoMet concentration at levels that are compatible with cell survival. This unexpected result, together with the fact that MAT activity and abundance changed with time in culture, suggests that different regulatory mechanisms acting beyond the post-transcriptional level are controlling this protein. In order to gain an insight into these mechanisms, several experiments were carried out to explain the MAT abundance during promastigote cell growth. Determination of MAT turnover in cycloheximide (CHX)-treated cultures resulted in a surprising 5-fold increase in MAT turnover compared to CHX-untreated cultures. This increase agrees with a stabilization of the MAT protein, whose integrity was maintained during culture. The presence of proteasome inhibitors, namely MG-132, MG-115, epoxomycin and lactacystin in the culture medium prevented MAT degradation in both MAT-overexpressing and ‘mock-transfected’ leishmanial strains. The role of the ubiquitin (Ub) pathway in MAT down-regulation was supported using immunoprecipitation experiments. Immunoprecipitated MAT cross-reacted with anti-Ub antibodies, which provides evidence of a proteasome-mediated down-regulation of the leishmanial MAT abundance.

Polyamines, androgens, and skeletal muscle hypertrophy

The naturally occurring polyamines, spermidine, spermine, and their precursor putrescine, play indispensible roles in both prokaryotic and eukaryotic cells, from basic DNA synthesis to regulation of cell proliferation and differentiation. The rate-limiting polyamine biosynthetic enzymes, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase, are essential for mammalian development, with knockout of the genes encoding these enzymes, Odc1 and Amd1, causing early embryonic lethality in mice. In muscle, the involvement of polyamines in muscle hypertrophy is suggested by the concomitant increase in cardiac and skeletal muscle mass and polyamine levels in response to anabolic agents including β-agonists. In addition to β-agonists, androgens, which increase skeletal mass and strength, have also been shown to stimulate polyamine accumulation in a number of tissues. In muscle, androgens act via the androgen receptor to regulate expression of polyamine biosynthetic enzyme genes, including Odc1 and Amd1, which may be one mechanism via which androgens promote muscle growth. This review outlines the role of polyamines in proliferation and hypertrophy, and explores their possible actions in mediating the anabolic actions of androgens in muscle.

Defining the role of polyamines in colon carcinogenesis using mouse models

Genetics and diet are both considered important risk determinants for colorectal cancer, a leading cause of death in the US and worldwide. Genetically engineered mouse (GEM) models have made a significant contribution to the characterization of colorectal cancer risk factors. Reliable, reproducible, and clinically relevant animal models help in the identification of the molecular events associated with disease progression and in the development of effictive treatment strategies. This review is focused on the use of mouse models for studying the role of polyamines in colon carcinogenesis. We describe how the available mouse models of colon cancer such as the multiple intestinal neoplasia (Min) mice and knockout genetic models facilitate understanding of the role of polyamines in colon carcinogenesis and help in the development of a rational strategy for colon cancer chemoprevention.

BSCTV C2 attenuates the degradation of SAMDC1 to suppress DNA methylation-mediated gene silencing in Arabidopsis

Plant viruses are excellent tools for studying microbial-plant interactions as well as the complexities of host activities. Our study focuses on the role of C2 encoded by Beet severe curly top virus (BSCTV) in the virus-plant interaction. Using BSCTV C2 as bait in a yeast two-hybrid screen, a C2-interacting protein, S-adenosyl-methionine decarboxylase 1 (SAMDC1), was identified from an Arabidopsis thaliana cDNA library. The interaction was confirmed by an in vitro pull-down assay and a firefly luciferase complemention imaging assay in planta. Biochemical analysis further showed that the degradation of the SAMDC1 protein was inhibited by MG132, a 26S proteasome inhibitor, and that C2 could attenuate the degradation of the SAMDC1 protein. Genetic analysis showed that loss of function of SAMDC1 resulted in reduced susceptibility to BSCTV infection and reduced viral DNA accumulation, similar to the effect of BSCTV C2 deficiency. Bisulfite sequencing analysis further showed that C2 deficiency caused enhanced DNA methylation of the viral genome in infected plants. We also showed that C2 can suppress de novo methylation in the FWA transgenic assay in the C2 transgene background. Overexpression of SAMDC1 can mimic the suppressive activity of C2 against green fluorescent protein-directed silencing. These results suggest that C2 interferes with the host defense mechanism of DNA methylation-mediated gene silencing by attenuating the 26S proteasome-mediated degradation of SAMDC1.

Polyamines in cancer

Polyamines are organic cations shown to control gene expression at the transcriptional, posttranscriptional, and translational levels. Multiple cellular oncogenic pathways are involved in regulation of transcription and translation of polyamine-metabolizing enzymes. As a consequence of genetic alterations, expression levels and activities of polyamine-metabolizing enzymes change rapidly during tumorigenesis resulting in high levels of polyamines in many human epithelial tumors. This review summarizes the mechanisms of polyamine regulation by canonical tumor suppressor genes and oncogenes, as well as the role of eukaryotic initiation factor 5A (EIF5A) in cancer. The importance of research utilizing pharmaceutical inhibitors and cancer chemopreventive strategies targeting the polyamine pathway is also discussed.

Spermine synthase activity affects the content of decarboxylated S-adenosylmethionine

dcAdoMet (decarboxylated S-adenosylmethionine) is an essential intermediate in the synthesis of polyamines. Its content is normally very low, amounting to less than 5% of that of S-adenosylmethionine itself. It was found that in mice lacking spermine synthase there was a large increase in dcAdoMet and that overexpression of spermine synthase reduced the amount of this nucleoside. There was also an increase in dcAdoMet in cells derived from patients with Snyder–Robinson syndrome, a rare X-linked recessive human disease caused by SMS gene mutations that greatly reduce the content of spermine synthase. These results suggest that there is an inverse relationship between the amount of spermine synthase protein and the content of dcAdoMet and raise the possibility that some of the abnormalities seen in mammals deficient in spermine synthase might be due to changes in dcAdoMet pools.

Mammalian polyamine metabolism and function

Polyamines are ubiquitous small basic molecules that play multiple essential roles in mammalian physiology. Their cellular content is highly regulated and there is convincing evidence that altered metabolism is involvement in many disease states. Drugs altering polyamine levels may therefore have a variety of important targets. This review will summarize the current state of understanding of polyamine metabolism and function, the regulation of polyamine content, and heritable pathological conditions that may be derived from altered polyamine metabolism.

Polyamine homoeostasis

The polyamines are essential for a variety of functions in the mammalian cell. Although their specific effects have not been fully elucidated, it is clear that the cellular polyamines have to be kept within certain levels for normal cell function. Polyamine homoeostasis in mammalian cells is achieved by a complex network of regulatory mechanisms affecting synthesis and degradation, as well as membrane transport of polyamines. The two key enzymes in the polyamine biosynthetic pathway, ODC (ornithine decarboxylase) and AdoMetDC (S-adenosylmethionine decarboxylase), are strongly regulated by feedback mechanisms at several levels, including transcriptional, translational and post-translational. Some of these mechanisms have been shown to be truly unique and include upstream reading frames and ribosomal frameshifting, as well as ubiquitin-independent proteasomal degradation. SSAT (spermidine/spermine N1-acetyltransferase), which is a crucial enzyme for degradation and efflux of polyamines, is also highly regulated by polyamines. A cellular excess of polyamines rapidly induces SSAT, resulting in increased degradation/efflux of the polyamines. The polyamines appear to induce both transcription and translation of the SSAT mRNA. However, the major part of the polyamine-induced increase in SSAT is caused by a marked stabilization of the enzyme against degradation by the 26S proteasome. In addition, active transport of extracellular polyamines into the cell contributes to cellular polyamine homoeostasis. Depletion of cellular polyamines rapidly induces an increased uptake of exogenous polyamines, whereas an excess of polyamines down-regulates the polyamine transporter(s). However, the protein(s) involved in polyamine transport and the exact mechanisms by which the polyamines regulate the transporter(s) are not yet known.

S-Adenosylmethionine decarboxylase

S-Adenosylmethionine decarboxylase is a key enzyme for the synthesis of polyamines in mammals, plants and many other species that use aminopropyltransferases for this pathway. It catalyses the formation of S-adenosyl-1-(methylthio)-3-propylamine (decarboxylated S-adenosylmethionine), which is used as the aminopropyl donor. This is the sole function of decarboxylated S-adenosylmethionine. Its content is therefore kept very low and is regulated by variation in the activity of S-adenosylmethionine decarboxylase according to the need for polyamine synthesis. All S-adenosylmethionine decarboxylases have a covalently bound pyruvate prosthetic group, which is essential for the decarboxylation reaction, and have similar structures, although they differ with respect to activation by cations, primary sequence and subunit composition. The present chapter describes these features, the mechanisms for autocatalytic generation of the pyruvate from a proenzyme precursor and for the decarboxylation reaction, and the available inhibitors of this enzyme, which have uses as anticancer and anti-trypanosomal agents. The intricate mechanisms for regulation of mammalian S-adenosylmethionine decarboxylase activity and content are also described.

Phosphorylation of eIF2alpha in response to 26S proteasome inhibition is mediated by the haem-regulated inhibitor (HRI) kinase

The present study demonstrates that even brief inhibition of degradation by the 26S proteasome inhibits global protein synthesis, mediated through increased phosphorylation of eIF2alpha (eukaryotic translational initiation factor 2alpha) by the HRI (haem-regulated inhibitor) kinase. Exposure of COS-7 cells to the proteasome inhibitor MG-132 (the proteasome inhibitor carbobenzoxy-L-leucyl-L-leucyl-leucinal) for 4 h resulted in a 55-60% decrease in protein synthesis rate compared with control cells. This repression of protein synthesis after treatment with MG-132 is not due to induction of apoptosis, which is known to occur after longer periods of 26S inhibition. Instead, we observed a significantly increased phosphorylation of eIF2alpha, which is known to repress global protein synthesis. In three MEF (mouse embryonic fibroblast) knockout cell lines lacking one of the four kinases known to phosphorylate eIF2alpha, increased phosphorylation of eIF2alpha still occurred after inhibition of the 26S proteasome. These three cell lines included a deletion of the PKR (double-stranded-RNA-dependent protein kinase); a deletion of the PERK (PKR-like endoplasmic reticulum resident kinase); or a deletion of the GCN2 (positive general control of transcription-2) kinase, indicating that none of these kinases was primarily responsible for the observed phosphorylation of eIF2alpha. In contrast, in a fourth MEF knockout cell line, HRI(-/-) cells lacking the HRI kinase failed to increase eIF2alpha phosphorylation upon proteasome inhibitor treatment (MG-132 or various doses of Bortezomib), indicating that the HRI kinase is the primary kinase activated by brief treatment of MEFs with 26S proteasome inhibitors.

Ubiquitin dependent and independent protein degradation in the regulation of cellular polyamines

Protein degradation mediated by the ubiquitin/proteasome system is the major route for the degradation of cellular proteins. In this pathway the ubiquitination of the target proteins is manifested via the concerted action of several enzymes. The ubiquinated proteins are then recognized and degraded by the 26S proteasome. There are few reports of proteins degraded by the 26S protesome without ubiquitination, with ornithine decarboxylase being the most notable representative of this group. Interestingly, while the degradation of ODC is independent of ubiquitination, the degradation of other enzymes of the polyamine biosynthesis pathway is ubiquitin dependent. The present review describes the degradation of enzymes and regulators of the polyamine biosynthesis pathway.

Ornithine decarboxylase and S-adenosylmethionine decarboxylase in trypanosomatids

The production of polyamines has been shown to be an effective target for a drug against the West African form of sleeping sickness caused by Trypanosoma brucei gambiense. T. brucei belongs to the group of protozoan parasites classed as trypanosomatids. Parasitic species of this group are the causative agents of various tropical diseases besides African sleeping sickness, e.g. Chagas' disease (Trypanosoma cruzi), cutaneous (Lesihmania spp.) and visceral (Leishmania donovani) leishmaniasis. The metabolism of polyamines in the parasites is a potential target for the development of new drugs for treatment of these diseases. The key steps in polyamine synthesis are catalysed by ODC (ornithine decarboxylase) and AdoMetDC (S-adenosylmethionine decarboxylase). In the present paper, some of the available information on ODC and AdoMetDC in trypanosomatids will be described and discussed.

Overproduction of cardiac S-adenosylmethionine decarboxylase in transgenic mice

The present study was designed to provide a better understanding of the role played by AdoMetDC (S-adenosylmethionine decarboxylase), the key rate-controlling enzyme in the synthesis of spermidine and spermine, in controlling polyamine levels and the importance of polyamines in cardiac physiology. The alphaMHC (alpha-myosin heavy chain) promoter was used to generate transgenic mice with cardiac-specific expression of AdoMetDC. A founder line (alphaMHC/AdoMetDC) was established with a >100-fold increase in AdoMetDC activity in the heart. Transgene expression was maximal by 1 week of age and remained constant into adulthood. However, the changes in polyamine levels were most pronounced during the first week of age, with a 2-fold decrease in putrescine and spermidine and a 2-fold increase in spermine. At later times, spermine returned to near control levels, whereas putrescine and spermidine levels remained lower, suggesting that compensatory mechanisms exist to limit spermine accumulation. The alphaMHC/AdoMetDC mice did not display an overt cardiac phenotype, but there was an increased cardiac hypertrophy after beta-adrenergic stimulation with isoprenaline ('isoproterenol'), as well as a small increase in spermine content. Crosses of the alphaMHC/AdoMetDC with alphaMHC/ornithine decarboxylase mice that have a >1000-fold increase in cardiac ornithine decarboxylase were lethal in utero, presumably due to increase in spermine to toxic levels. These findings suggest that cardiac spermine levels are highly regulated to avoid polyamine-induced toxicity and that homoeostatic mechanisms can maintain non-toxic levels even when one enzyme of the biosynthetic pathway is greatly elevated but are unable to do so when two biosynthetic enzymes are increased.