Effect of spermine synthase on the sensitivity of cells to anti-tumour agents

Self-Assembled Alkylated Polyamine Analogs as Supramolecular Anticancer Agents

Conformationally restrained polyamine analog PG11047 is a well-known drug candidate that modulates polyamine metabolism and inhibits cancer cell growth in a broad spectrum of cancers. Here, we report a structure–activity relationship study of the PG11047 analogs (HPGs) containing alkyl chains of varying length, while keeping the unsaturated spermine backbone unchanged. Synthesis of higher symmetrical homologues was achieved through a synthetic route with fewer steps than the previous route to PG11047. The amphiphilic HPG analogs underwent self-assembly and formed spherically shaped nanoparticles whose size increased with the hydrophobic alkyl group’s increasing chain length. Assessment of the in vitro anticancer activity showed more than an eight-fold increase in the cancer cell inhibition activity of the analogs with longer alkyl chains compared to PG11047 in human colon cancer cell line HCT116, and a more than ten-fold increase in human lung cancer cell line A549. Evaluation of the inhibition of spermine oxidase (SMOX) showed no activity for PG11047, but activity was observed for its higher symmetrical homologues. Comparison with a reference SMOX inhibitor MDL72527 showed nine-fold better activity for the best performing HPG analog.

Unforeseen Possibilities To Investigate the Regulation of Polyamine Metabolism Revealed by Novel C-Methylated Spermine Derivatives

The biogenic polyamines, spermine (Spm) and spermidine, are organic polycations present in millimolar concentrations in all eukaryotic cells participating in the regulation of vital cellular functions including proliferation and differentiation. The design and biochemical evaluation of polyamine analogues are cornerstones of polyamine research. Here we synthesized and studied novel C-methylated Spm analogues: 2,11-dimethylspermine (2,11-Me2Spm), 3,10-dimethylspermine (3,10-Me2Spm), 2-methylspermine, and 2,2-dimethylspermine. The tested analogues overcame growth arrest induced by a 72 h treatment with α-difluoromethylornithine, an ornithine decarboxylase (ODC) inhibitor, and entered into DU145 cells via the polyamine transporter. 3,10-Me2Spm was a poor substrate of spermine oxidase and spermidine/spermine-N1-acetyltransferase (SSAT) when compared with 2,11-Me2Spm, thus resembling 1,12-dimethylspermine, which lacks the substrate properties required for the SSAT reaction. The antizyme (OAZ1)-mediated downregulation of ODC and inhibition of polyamine transport are crucial in the maintenance of polyamine homeostasis. Interestingly, 3,10-Me2Spm was found to be the first Spm analogue that did not induce OAZ1 and, consequently, was a weak downregulator of ODC activity in DU145 cells.

Rational design of small-molecule stabilizers of spermine synthase dimer by virtual screening and free energy-based approach

Snyder-Robinson Syndrome (SRS) is a rare mental retardation disorder which is caused by the malfunctioning of an enzyme, the spermine synthase (SMS), which functions as a homo-dimer. The malfunctioning of SMS in SRS patients is associated with several identified missense mutations that occur away from the active site. This investigation deals with a particular SRS-causing mutation, the G56S mutation, which was shown computationally and experimentally to destabilize the SMS homo-dimer and thus to abolish SMS enzymatic activity. As a proof-of-concept, we explore the possibility to restore the enzymatic activity of the malfunctioning SMS mutant G56S by stabilizing the dimer through small molecule binding at the mutant homo-dimer interface. For this purpose, we designed an in silico protocol that couples virtual screening and a free binding energy-based approach to identify potential small-molecule binders on the destabilized G56S dimer, with the goal to stabilize it and thus to increase SMS G56S mutant activity. The protocol resulted in extensive list of plausible stabilizers, among which we selected and tested 51 compounds experimentally for their capability to increase SMS G56S mutant enzymatic activity. In silico analysis of the experimentally identified stabilizers suggested five distinctive chemical scaffolds. This investigation suggests that druggable pockets exist in the vicinity of the mutation sites at protein-protein interfaces which can be used to alter the disease-causing effects by small molecule binding. The identified chemical scaffolds are drug-like and can serve as original starting points for development of lead molecules to further rescue the disease-causing effects of the Snyder-Robinson syndrome for which no efficient treatment exists up to now.

The function of spermine

Polyamines play important roles in cell physiology including effects on the structure of cellular macromolecules, gene expression, protein function, nucleic acid and protein synthesis, regulation of ion channels, and providing protection from oxidative damage. Vertebrates contain two polyamines, spermidine and spermine, as well as their precursor, the diamine putrescine. Although spermidine has an essential and unique role as the precursor of hypusine a post-translational modification of the elongation factor eIF5A, which is necessary for this protein to function in protein synthesis, no unique role for spermine has been identified unequivocally. The existence of a discrete spermine synthase enzyme that converts spermidine to spermine suggest that spermine must be needed and this is confirmed by studies with Gy mice and human patients with Snyder-Robinson syndrome in which spermine synthase is absent or greatly reduced. In both cases, this leads to a severe phenotype with multiple effects among which are intellectual disability, other neurological changes, hypotonia, and reduced growth of muscle and bone. This review describes these alterations and focuses on the roles of spermine which may contribute to these phenotypes including reducing damage due to reactive oxygen species, protection from stress, permitting correct current flow through inwardly rectifying K(+) channels, controlling activity of brain glutamate receptors involved in learning and memory, and affecting growth responses. Additional possibilities include acting as storage reservoir for maintaining appropriate levels of free spermidine and a possible non-catalytic role for spermine synthase protein.

Enhancing human spermine synthase activity by engineered mutations

Spermine synthase (SMS) is an enzyme which function is to convert spermidine into spermine. It was shown that gene defects resulting in amino acid changes of the wild type SMS cause Snyder-Robinson syndrome, which is a mild-to-moderate mental disability associated with osteoporosis, facial asymmetry, thin habitus, hypotonia, and a nonspecific movement disorder. These disease-causing missense mutations were demonstrated, both in silico and in vitro, to affect the wild type function of SMS by either destabilizing the SMS dimer/monomer or directly affecting the hydrogen bond network of the active site of SMS. In contrast to these studies, here we report an artificial engineering of a more efficient SMS variant by transferring sequence information from another organism. It is confirmed experimentally that the variant, bearing four amino acid substitutions, is catalytically more active than the wild type. The increased functionality is attributed to enhanced monomer stability, lowering the pKa of proton donor catalytic residue, optimized spatial distribution of the electrostatic potential around the SMS with respect to substrates, and increase of the frequency of mechanical vibration of the clefts presumed to be the gates toward the active sites. The study demonstrates that wild type SMS is not particularly evolutionarily optimized with respect to the reaction spermidine → spermine. Having in mind that currently there are no variations (non-synonymous single nucleotide polymorphism, nsSNP) detected in healthy individuals, it can be speculated that the human SMS function is precisely tuned toward its wild type and any deviation is unwanted and disease-causing.

Proteins are constantly subjected to evolutionary pressure to assure the organism's survival and reproduction. At the same time, the proteins' amino acid sequence undergoes mutations, some of which may cause diseases while others may be reflecting natural differences within the population (non-synonymous single nucleotide polymorphism, nsSNP). In this study we examine the human spermine synthase (HsSMS), for which currently there are no nsSNPs, while rare disease mutations are known to cause Snyder-Robinson syndrome. What is so special with this protein? Maybe the HsSMS is so well optimized for its function that any change of the wild type sequence should be degrading its performance. To check such a possibility, we engineered a mutant of HsSMS with enhanced stability, electrostatic and mechanical properties. The mutant was confirmed experimentally to be a better enzyme than the wild type. Thus, the HsSMS is not evolutionally optimized with respect to its enzymatic reaction, its amino acid sequence differs only in sick individuals and so far its sequence was found to be identical in all healthy individuals. Therefore, it can be speculated that the HsSMS function is precisely tuned toward the wild type characteristics such so any deviation is unwanted and is disease-causing.

Spermine synthase overexpression in vivo does not increase susceptibility to DMBA/TPA skin carcinogenesis or Min-Apc intestinal tumorigenesis

Numerous studies have demonstrated a link between elevated polyamine biosynthesis and neoplastic growth, but the specific contribution of spermine synthase to epithelial tumor development has never been explored in vivo. Mice with widespread overexpression of spermine synthase (CAG-SpmS) exhibit decreased spermidine levels, increased spermine and a significant rise in tissue spermine:spermidine ratio. We characterized the response of CAG-SpmS mice to two-stage skin chemical carcinogenesis as well as spontaneous intestinal carcinogenesis induced by loss of the Apc tumor suppressor in Apc (Min) (/+) (Min) mice. CAG-SpmS mice maintained the canonical increases in ornithine decarboxylase (ODC) activity, polyamine content and epidermal thickness in response to tumor promoter treatment of the skin. The induction of S-adenosylmethionine decarboxylase (AdoMetDC) activity and its product decarboxylated AdoMet were impaired in CAG-SpmS mice, and the spermine:spermidine ratio was increased 3-fold in both untreated and 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated skin. The susceptibility to 7,12-dimethylbenz[a]anthracene (DMBA)/TPA skin carcinogenesis was not altered in CAG-SpmS mice, and SpmS overexpression did not modify the previously described tumor resistance of mice with targeted antizyme expression or the enhanced tumor response in mice with targeted spermidine/spermine-N ( 1) -acetyltransferase expression. CAG-SpmS/Min mice also exhibited elevated spermine:spermidine ratios in the small intestine and colon, yet their tumor multiplicity and size was similar to Min mice. Therefore, studies in two of the most widely used tumorigenesis models demonstrate that increased spermine synthase activity and the resulting elevation of the spermine:spermidine ratio does not alter susceptibility to tumor development initiated by c-Ha-Ras mutation or Apc loss.

Use of (Gyro) Gy and spermine synthase transgenic mice to study functions of spermine

The polyamines putrescine, spermidine, and spermine are essential for mammalian cell growth, -differentiation, and cell death and have important physiological roles in all tissues. Many of the properties of polyamines that can be demonstrated in vitro are common to all three molecules with differences only in potency. Loss of any of the enzymes needed to make either putrescine or spermidine (which also -prevent the production of spermine) is lethal, but male mice lacking spermine synthase (SpmS) due to a deletion of part of the X chromosome are viable on the B6C3H background. These mice are termed Gyro (Gy) due to their circling behavior. They have a variety of abnormalities including deafness, neurological problems, small size, and a tendency to early death. They can therefore be used to evaluate the physiological function(s) uniquely provided by spermine. They also provide a potential animal model for Snyder-Robinson syndrome (SRS), a rare human inherited disease due to a loss of SpmS activity. An essential control in experiments using Gy mice is to demonstrate that the abnormal phenotypes exhibited by these mice are abolished by providing replacement spermine and this can be accomplished by breeding with CAG-SMS mice that express SpmS from a ubiquitous promoter. Techniques for identifying, characterizing, and using these mouse strains and limitations of this approach are described in this chapter.

A missense mutation, p.V132G, in the X-linked spermine synthase gene (SMS) causes Snyder-Robinson syndrome

Snyder-Robinson syndrome (SRS, OMIM 309583) is a rare X-linked syndrome characterized by mental retardation, marfanoid habitus, skeletal defects, osteoporosis, and facial asymmetry. Linkage analysis localized the related gene to Xp21.3-p22.12, and a G-to-A transition at point +5 of intron 4 of the spermine synthase gene, which caused truncation of the SMS protein and loss of enzyme activity, was identified in the original family. Here we describe another family with Snyder-Robinson syndrome in two Mexican brothers and a novel mutation (c.496T>G) in the exon 5 of the SMS gene confirming its involvement in this rare X-linked mental retardation syndrome.

Crystal structure of human spermine synthase: implications of substrate binding and catalytic mechanism

The crystal structures of two ternary complexes of human spermine synthase (EC 2.5.1.22), one with 5'-methylthioadenosine and spermidine and the other with 5'-methylthioadenosine and spermine, have been solved. They show that the enzyme is a dimer of two identical subunits. Each monomer has three domains: a C-terminal domain, which contains the active site and is similar in structure to spermidine synthase; a central domain made up of four beta-strands; and an N-terminal domain with remarkable structural similarity to S-adenosylmethionine decarboxylase, the enzyme that forms the aminopropyl donor substrate. Dimerization occurs mainly through interactions between the N-terminal domains. Deletion of the N-terminal domain led to a complete loss of spermine synthase activity, suggesting that dimerization may be required for activity. The structures provide an outline of the active site and a plausible model for catalysis. The active site is similar to those of spermidine synthases but has a larger substrate-binding pocket able to accommodate longer substrates. Two residues (Asp(201) and Asp(276)) that are conserved in aminopropyltransferases appear to play a key part in the catalytic mechanism, and this role was supported by the results of site-directed mutagenesis. The spermine synthase.5'-methylthioadenosine structure provides a plausible explanation for the potent inhibition of the reaction by this product and the stronger inhibition of spermine synthase compared with spermidine synthase. An analysis to trace possible evolutionary origins of spermine synthase is also described.

Targeting polyamine metabolism and function in cancer and other hyperproliferative diseases

The polyamines spermidine and spermine and their diamine precursor putrescine are naturally occurring, polycationic alkylamines that are essential for eukaryotic cell growth. The requirement for and the metabolism of polyamines are frequently dysregulated in cancer and other hyperproliferative diseases, thus making polyamine function and metabolism attractive targets for therapeutic intervention. Recent advances in our understanding of polyamine function, metabolic regulation, and differences between normal cells and tumour cells with respect to polyamine biology, have reinforced the interest in this target-rich pathway for drug development.

Characterization of transgenic mice with widespread overexpression of spermine synthase

A widespread increase in SpmS (spermine synthase) activity has been produced in transgenic mice using a construct in which the human SpmS cDNA was placed under the control of a composite CMV-IE (cytomegalovirus immediate early gene) enhancer-chicken beta-actin promoter. Four separate founder CAG/SpmS mice were studied. Transgenic expression of SpmS was found in all of the tissues examined, but the relative SpmS activities varied widely according to the founder animal and the tissue studied. Very large increases in SpmS activity were seen in many tissues. SpdS (spermidine synthase) activity was not affected. Although there was a statistically significant decline in spermidine content and increase in spermine, the alterations were small compared with the increase in SpmS activity. These results provide strong support for the concept that the levels of the higher polyamines spermidine and spermine are not determined only by the relative activities of the two aminopropyltransferases. Other factors such as availability of the aminopropyl donor substrate decarboxylated S-adenosylmethionine and possibly degradation or excretion must also influence the spermidine/spermine ratio. No deleterious effects of SpmS overexpression were seen. The mice had normal growth, fertility and behaviour up to the age of 12 months. However, breeding the CAG/SpmS mice with MHC (alpha-myosin heavy chain)/AdoMetDC (S-adenosylmethionine decarboxylase) mice, which have a large increase in S-adenosylmethionine decarboxylase expression in heart, was lethal. In contrast, breeding the CAG/SpmS mice with MHC/ODC (L-ornithine decarboxylase) mice, which have a large increase in cardiac ornithine decarboxylase expression, had a protective effect in preventing the small decrease in viability of the MHC/ODC mice.

Spermine Synthesis Is Required for Normal Viability, Growth, and Fertility in the Mouse

Spermidine is essential for viability in eukaryotes but the importance of the longer polyamine spermine has not been established. Spermine is formed from spermidine by the action of spermine synthase, an aminopropyltransferase, whose gene (SpmS) is located on the X chromosome. Deletion of part of the X chromosome that include SpmS in Gy mice leads to a striking phenotype in affected males that includes altered phosphate metabolism and symptoms of hypophosphatemic rickets, circling behavior, hyperactivity, head shaking, inner ear abnormalities, deafness, sterility, a profound postnatal growth retardation, and a propensity to sudden death. It was not clear to what extent these alterations were due to the loss of spermine synthase activity, since this chromosomal deletion extends well beyond the SpmS gene and includes at least one other gene termed Phex. We have bred the Gy carrier female mice with transgenic mice (CAG/SpmS mice) that express spermine synthase from the ubiquitous CAG promoter. The resulting Gy-CAG/SpmS mice had extremely high levels of spermine synthase and contained spermine in all tissues examined. These mice had a normal life span and fertility and a normal growth rate except for a reduction in body weight due to a loss of bone mass that was consistent with the observation that the derangement in phosphate metabolism is due to the loss of the Phex gene and was not restored. These results show that spermine synthesis is needed for normal growth, viability, and fertility in male mice and that regulation of spermine synthase content is not required.

Identification of novel Myc target genes with a potential role in lymphomagenesis

The c-Myc transcription factor regulates a wide set of genes involved in processes such as proliferation, differentiation and apoptosis. Therefore, altered expression of Myc leads to deregulation of a large number of target genes and, as a consequence, to tumorigenesis. For understanding Myc-induced transformation, identification of these target genes is essential. In this study, we searched for Myc target genes involved in lymphomagenesis using different mouse T and B cell lymphoma cell lines transformed by a conditional Myc-allele. Target genes obtained by microarray experiments were further subjected to a kinetic analysis of mRNA expression upon Myc inactivation/reactivation, bioinformatic examination of Myc binding sites and chromatin immunoprecipitation. This approach allowed us to define targets whose activation is a direct consequence of Myc binding. Among the 38 novel Myc targets, we identified several genes implicated in the tumor development. These genes are not only relevant for mouse lymphomas because we observed their upregulation in human lymphomas as well. Our findings further the understanding of Myc-induced lymphomagenesis and help toward developing more efficient antitumor strategies.

Genetic approaches to the cellular functions of polyamines in mammals

The polyamines putrescine, spermidine and spermine are organic cations shown to participate in a bewildering number of cellular reactions, yet their exact functions in intermediary metabolism and specific interactions with cellular components remain largely elusive. Pharmacological interventions have demonstrated convincingly that a steady supply of these compounds is a prerequisite for cell proliferation to occur. The last decade has witnessed the appearance of a substantial number of studies, in which genetic engineering of polyamine metabolism in transgenic rodents has been employed to unravel their cellular functions. Transgenic activation of polyamine biosynthesis through an overexpression of their biosynthetic enzymes has assigned specific roles for these compounds in spermatogenesis, skin physiology, promotion of tumorigenesis and organ hypertrophy as well as neuronal protection. Transgenic activation of polyamine catabolism not only profoundly disturbs polyamine homeostasis in most tissues, but also creates a complex phenotype affecting skin, female fertility, fat depots, pancreatic integrity and regenerative growth. Transgenic expression of ornithine decarboxylase antizyme has suggested that this unique protein may act as a general tumor suppressor. Homozygous deficiency of the key biosynthetic enzymes of the polyamines, ornithine and S-adenosylmethionine decarboxylase, as achieved through targeted disruption of their genes, is not compatible with murine embryogenesis. Finally, the first reports of human diseases apparently caused by mutations or rearrangements of the genes involved in polyamine metabolism have appeared.