Recent advances in the development of sialyltransferase inhibitors to control cancer metastasis: A comprehensive review

Metastasis accounts for the majority of cancer-associated mortalities, representing a huge health and economic burden. One of the mechanisms that enables metastasis is hypersialylation, characterized by an overabundance of sialylated glycans on the tumor surface, which leads to repulsion and detachment of cells from the original tumor. Once the tumor cells are mobilized, sialylated glycans hijack the natural killer T-cells through self-molecular mimicry and activatea downstream cascade of molecular events that result in inhibition of cytotoxicity and inflammatory responses against cancer cells, ultimately leading to immune evasion. Sialylation is mediated by a family of enzymes known as sialyltransferases (STs), which catalyse the transfer of sialic acid residue from the donor, CMP-sialic acid, onto the terminal end of an acceptor such as N-acetylgalactosamine on the cell-surface. Upregulation of STs increases tumor hypersialylation by up to 60% which is considered a distinctive hallmark of several types of cancers such as pancreatic, breast, and ovarian cancer. Therefore, inhibiting STs has emerged as a potential strategy to prevent metastasis. In this comprehensive review, we discuss the recent advances in designing novel sialyltransferase inhibitors using ligand-based drug design and high-throughput screening of natural and synthetic entities, emphasizing the most successful approaches. We analyse the limitations and challenges of designing selective, potent, and cell-permeable ST inhibitors that hindered further development of ST inhibitors into clinical trials. We conclude by analysing emerging opportunities, including advanced delivery methods which further increase the potential of these inhibitors to enrich the clinics with novel therapeutics to combat metastasis.

Functional characterization of a sialyltransferase-deficient mutant of Neisseria gonorrhoeae

Previous studies indicate that sialylation of lipopolysaccharide (LPS) by host CMP-N-acetylneuraminic acid (CMP-NANA) catalyzed by bacterial sialyltransferase rendered gonococci resistant to killing by phagocytes, to entry into epithelial cell lines, to killing by immune serum and complement, and to absorption of complement component C3. These results have been confirmed by comparing a sialyltransferase-deficient mutant (strain JB1) with its parent (strain F62) in appropriate tests. In contrast to F62, JB1 was very susceptible to killing by human polymorphonuclear phagocytes in opsonophagocytosis tests and incubation with CMP-NANA did not decrease the level of killing. The inherent resistance of F62 in these tests was probably due to LPS sialylation by CMP-NANA and lactate present in the phagocytes. A JB1 variant expressing the invasion-associated Opa protein was as able to enter Chang human conjunctiva epithelial cells as an Opa-positive variant of F62, suggesting that the sialyltransferase is not required for Opa-mediated entry. After incubation with CMP-NANA, the number of F62 variant gonococci entering cells but not that of JB1 variant gonococci was drastically reduced. Both JB1 and F62 were killed by incubation with rabbit antibody to gonococcal major outer membrane protein, protein I, and human complement, but only F62 was rendered resistant to the killing by incubation with CMP-NANA. Finally, both JB1 and F62 absorbed similar amounts of complement component C3 and the binding was decreased by incubation with CMP-NANA only for the wild type, F62.

Sialylation lessens the infectivity of Neisseria gonorrhoeae MS11mkC

In a human challenge experiment, the infectivity of gonococci with sialylated lipooligosaccharide (LOS) was compared with the infectivity of gonococci with unsialylated LOS. Volunteers were intraurethrally inoculated with approximately 5000 sialylated or unsialylated piliated, non-opaque (P+Opa-, transparent) colony type gonococci, strain MS11mkC. Five (83%) of 6 volunteers inoculated with unsialylated gonococci became infected; however, only 1 of 5 volunteers became infected with sialylated gonococci. The unsialylated gonococcal infections, with a median incubation time of 62 h (range, 32-98), were similar to previously described experimental infections. Gonococci shed by infected volunteers showed a transition from the P+Opa- phenotype of the inoculation strain to the P+Opa+ (piliated, opaque) phenotype 12-60 h before onset of disease. The subject with sialylated gonococcus infection had an extended incubation period, showing a progressive increase in the number of organisms shed until he became symptomatic on day 6 after inoculation. These results show that gonococci with sialylated LOS are less infective than gonococci with unsialylated LOS.

Identification and characterization of genes required for post-translational modification of Campylobacter coli VC167 flagellin

Two genes have been identified in Campylobacter coli VC167 which are required for the biosynthesis of post-translational modifications on flagellin proteins. The ptmA gene encodes a protein of predicted M(r) 28,486 which shows significant homology to a family of alcohol dehydrogenases from a variety of bacteria. The ptmB gene encodes a protein of predicted M(r) 26,598 with significant homology to CMP-N-acetylneuraminic acid synthetase enzymes involved in sialic acid capsular biosynthesis in Neisseria meninigitidis and Escherichia coli K1. Site-specific mutation of either ptmA or ptmB caused loss of reactivity with antisera specific to the post-translational modifications and a change in the isoelectric focusing fingerprints relative to the parent strains. Mutation of ptmB, but not of ptmA, caused a change in apparent M(r) of the flagellin subunit in SDS-PAGE gels. The ptmA and ptmB genes are present in other strains of Campylobacter. In a rabbit model the ptmA mutant showed a reduced ability to elicit protection against subsequent challenge with heterologous strains of the same Lior serotype compared to the parental wild-type strain. This suggests that the surface-exposed post-translational modifications may play a significant role in the protective immune response.

Site-specific labelling of the oligosaccharide chains of antibodies

This paper presents a new method for site-specific labelling of antibodies employing enzymatic reactions without oxidizing or reducing agents. IgG was first treated with immobilized sialidase from Clostridium perfringens to cleave bound NeuAc. CMP-9-deoxy-9-salizoyl-NeuAc, an activated sialic acid analogue, was labelled with 131I via the iodogen-method in high yields (> 95%). Then the oligosaccharide chains of antibodies were labelled yield with the radioactive NeuAc analogue by transfer using alpha-2,6-sialyltransferase from rat liver in 50%.

Biosynthesis of oligosaccharides in intact Golgi preparations from rat liver. Analysis of N-linked glycans labeled by UDP-[6-3H]galactose, CMP-[9-3H]N-acetylneuraminic acid, and [acetyl-3H]acetyl-coenzyme A

When a rat liver Golgi apparatus-enriched subcellular fraction is incubated with UDP-[3H]Gal, CMP-[3H] Neu5Ac, or [acetyl-3H]acetyl (Ac)-CoA, label is efficiently transferred to endogenous acceptors, which are resistant to added proteases, unless detergent is added at a sufficiently high concentration. Thus, the acceptors are within the lumen of intact compartments of correct topological orientation, which are likely to be similar to those of the Golgi apparatus in the intact cell. In each case, approximately 90% of the macromolecular radioactivity is specifically released by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase digestion, as labeled N-linked oligosaccharides. Label from UDP-[3H]Gal is transferred to several distinct N-linked oligosaccharides, and many of these carry sialic acid (Sia) residues. This amount increases if the transfer reaction is chased with CMP-Neu5Ac. A major fraction of the [3H]Gal is directly "covered" with Sia residues, indicating that at least a portion of the beta-galactosyltransferase(s) are co-localized with one or more sialyltransferases. The majority of the [3H]Gal is found in a beta 1,3-linkage, rather than the more common beta 1,4-linkage. The N-linked oligosaccharides labeled by CMP-[3H] Neu5Ac carry labeled Sia residues in either alpha 2,3 or alpha 2,6 linkage, and showed a range of charge distribution. The transferred [3H]Neu5Ac is not O-acetylated even when Ac-CoA is added at saturating concentrations, implying that the sialyltransferases and the O-acetyltransferase(s) are not functionally co-localized. However, approximately 20% of label released from N-linked oligosaccharides by sialidase does not co-migrate with authentic Neu5Ac in high performance liquid chromatography analysis, indicating that transferred [3H] Neu5Ac is modified by unknown enzymes in the Golgi. Most of the [3H]acetate transferred from [acetyl-3H] Ac-CoA to N-linked oligosaccharides is on Sia residues that are exclusively alpha 2,6-linked, and is enriched on tri- and tetra-antennary chains that do not appear to carry any 2,3-linked Sia residues. These data indicate a restricted substrate preference of the O-acetyltransferase(s). About one-quarter of the [3H]acetate transferred is sialidase-resistant, indicating either transfer to monosaccharides other than sialic acid, or to sialidase-resistant sialic acids. While most of these sialidase-resistant oligosaccharides remain negatively charged, about 10% are neutralized by sialidase, confirming transfer of [3H]acetate to monosaccharides other than sialic acid.

Lipo-oligosaccharides (LOS) of mucosal pathogens: molecular mimicry and host-modification of LOS

Immunochemical studies of the lipo-oligosaccharides (LOS) of the Gram-negative bacteria Neisseria gonorrhoeae and Neisseria meningitidis have revealed some interesting structural characteristics of these LOS that might relate to their roles during pathogenesis. The carbohydrate moieties of the LOS of pathogenic Neisseria mimic carbohydrates present in glycosphingolipids of human cells. Firstly, an LOS component present among a number of Neisseria species is antigenically and/or chemically identical to lactoneoseries glycosphingolipids present in human cells. The lactoneoseries LOS becomes sialylated on Neisseria gonorrhoeae when they are grown in the presence of cytidine 5'-monophospho-N-acetyl-neuraminic acid (CMP-NANA), the nucleotide sugar for sialic acid. Examination of gonococci present in exudates from males with natural infection indicates that sialylation also occurs in vivo. The mechanism for this process apparently involves a bacterial sialyltransferase scavenging available host CMP-NANA ("host-modification" of LOS) and transferring the sialic acid to the lactoneoserieslike LOS. Strains of N. meningitidis and Haemophilus influenzae also express similarly sialylated LOS suggesting that this is a common mechanism of pathogenesis among these bacteria. Additional examples of LOS that mimic other glycosphingolipid series have been identified also and the fact that multiple series can be expressed in a single population of gonococci suggests that a diverse set of LOS can be presented to the host during infection. It is possible that this diverse set of LOS serve different functions for the bacteria in various hosts and/or environments during infection.

Antibodies to N-terminal peptides of gonococcal porin are bactericidal when gonococcal lipopolysaccharide is not sialylated

Six synthetic 25-mer peptides corresponding to certain presumed surface-exposed regions of gonococcal porin protein I (PI) were made from strains FA19 (PIA) and MS11 (PIB). Four peptides were immunogenic in rabbits. Affinity-purified antisera against both PIA and PIB N-terminal peptides were bactericidal for homologous gonococci and many heterologous PI serovars. However, sialylation of gonococcal lipopolysaccharide (LPS) by growth of gonococci in the presence of cytidine monophosphate-neuraminic acid (CMP-NANA) abrogated the bactericidal activity of these antisera. Binding of anti-PI monoclonal antibodies to whole gonococci was reduced two- to fourfold by sialylation of LPS, suggesting that sialylation may inhibit bactericidal activity by masking porin epitopes. However, binding of anti-PII (Opa) monoclonal antibodies was not inhibited, yet complement-mediated killing was inhibited by sialylated LPS. Binding of complement components C3 and C9 was inhibited in the presence of either anti-PI or anti-PII monoclonals when gonococci were grown in the presence of CMP-NANA. Thus sialylation inhibited both anti-PI antibody binding and complement deposition, with a resultant decrease in bactericidal activity.

A highly sensitive fluorometric assay for sialyltransferase activity using CMP-9-fluoresceinyl-NeuAc as donor

This paper presents a very sensitive fluorometric assay for sialyltransferase activity based on the transfer of 5-acetamido-9-deoxy-9-fluoresceinylthioure-idoneuraminic acid onto distinct glycoproteins, thus allowing determination of acceptor specificities. Acceptor protein-bound fluorescence was quantified after gel filtration which separated fluorescent sialoglycoprotein from the fluorescence-labeled CMP-glycoside donor. Kinetic constants obtained for five different purified sialyltransferases indicated that CMP-9-fluoresceinyl-NeuAc was a suitable donor substrate for each enzyme, affording low Km values and Vmax values comparable in magnitude (15-100%) to that obtained with the parent CMP-NeuAc. Sensitivity was enhanced 200- to 1000-fold compared to the radiometric sialyltransferase assay as it is used routinely. The method was applied to determination of the kinetic properties of purified rat liver alpha 2,6-sialyltransferase with four separate glycoprotein acceptors differing in glycan structure, employing very small amounts of donor, acceptor, and enzyme, and to the study of sialyltransferase activity of the human promyelocytic cell line HL-60 toward three different acceptors.

Chemical behaviour of cytidine 5'-monophospho-N-acetyl-beta-D-neuraminic acid under neutral and alkaline conditions

The chemical behaviour of CMP-N-acetylneuraminic acid under neutral and different alkaline conditions has been investigated. The products formed were isolated by ion-exchange chromatography and gel filtration and analysed by colorimetric methods, thin-layer chromatography, combined gas-liquid chromatography/mass spectrometry and/or 360-MHz 1H-NMR spectroscopy. A maximum stability of CMP-N-acetylneuraminic acid was observed at pH8-11. In the tested pH range of 6-13, CMP and N-acetylneuraminic acid were formed in variable amounts as decomposition products. 2-Deoxy-2,3-dehydro-N-acetylneuraminic acid was produced at pH greater than 7; the amount of this substance increased with increasing pH. In anhydrous triethylamine its yield was 50%. A new neuraminic acid derivative, N-acetyl-beta-D-neuraminic acid 2-phosphate, could be isolated from the mixture of alkaline decomposition products of CMP-N-acetylneuraminic acid. The yield of this compound was maximum 22% in anhydrous triethylamine. Because 2-deoxy-2,3-dehydro-N-acetylneuraminic acid was formed under simulated physiological conditions, it is assumed that this compound, which occurs in tissues and fluids of man and animals, is derived from CMP-N-acetylneuraminic acid non-enzymically also under conditions in vivo.

Ganglioside biosynthesis in rat liver: alteration of sialyltransferase activities by nucleotides

CMP-NAcNeu:GM3 ganglioside sialyltransferase (GD3 synthase) was characterized with respect to regulation of activity by nucleotides and compared in this regard with other sialyltransferases of ganglioside biosynthesis. Nucleotides preferentially inhibited the activity of GD3 synthase. Di- and trinucleotides inhibited most strongly and cytidine nucleotides were the most inhibitory class. The mode of inhibition by CMP (competitive or noncompetitive) varied with storage conditions of Golgi apparatus membranes; CMP inhibition was decreased during a series of consecutive freeze-thawings of membranes. Also, GD3 synthase inhibition by CDP was only partially relieved by excess Mg2+. With lactosylceramide as the in vitro precursor, synthesis of GM3 was always less inhibited by cytidine nucleotides than was that of GD3 and GT3. An 8-fold reduction in the ratio GD3/GM3 in the reaction products was obtained at 1.5 mM CTP. Separate incubations for the sialylation of GM3 or GM1 showed cytidine nucleotides increased synthesis of GD1a relative to GD3 by 3.5-fold.

Ganglioside biosynthesis in rat liver: characterization of cytidine-5'-monophospho-n-acetylneuraminic acid:hematoside (GM3) sialyltransferase

CMP-NAcNeu:GM3 ganglioside sialytransferase (GD3 synthase) was concentrated 80-100-fold, relative to total homogenates, in Golgi apparatus fractions from rat liver. Ultrasound treatment of Golgi apparatus in a low salt medium extracted 40-60% of the original protein but did not dissociate the transferase from membranes. The acivity was greatest in the presence of certain detergents, had a pH optimum of 6.2, was stimulated by mg2+ and diacylphospholipids and was inhibited by lysophospholipids. Apparent Km values for CMP-NAcNeu and GM3 were about 0.8 and 0.2 mM, respectively. On chromatographic separation, virtually all the reaction product migrated as GD3. GD3 synthase appeared to be a glycoprotein since the activity bound to concanavalin A-Sepharose and was eluted, with increased specific activity, by alpha-methyl mannoside.

Sialyltransferases in young rat brain

1. There are more glycolipid acceptor sites for NeuNAc than for glycoproteins in 11--15 day old rat cerebra. 2. The glycolipid acceptors appear to be almost exclusively Cer-Glc-Gal and GM1 ganglioside and each is a substrate for a different sialyltransferase. 3. The sialyltransferase(s) that acted on glycoprotein could be differentiated from the ones that acted on the glycolipids. 4. The apparent Km for CMP-NeuNAc was the same for all four of the sialyltransferase reactions studied. 5. Electron microscopic examination and marker enzyme studies on continuous sucrose gradient fractions found that most of the sialyltransferase activities appeared to be localized in smooth microsomal membrane and the Golgi complex derivatives and not associated with the synaptosomes.

Ganglioside biosynthesis. Characterization of CMP-N-acetylneuraminic acid : lactosylceramide sialyltransferase in Golgi apparatus from rat liver

An enzyme that transfers sialic acid from GMP-sialic acid to lactosylceramide was concentrated 40-50 times in Golgi apparatus from rat liver relative to total homogenates. This enzyme required detergents as dispersing agents. Of the numerous detergents tested, the combination Tween 80-Triton CF-54 (1 : 2, w/w) was the most effective in stimulating the reaction. Two apparent pH optima, at 6.35 and 5.5, were observed. The enzyme showed no requirement for a divalent cation. The Km values calculated for CMP-N-acetylneuraminic acid and lactosylceramide were 2.7 - 10(-3) and 1.3 - 10(-4) M, respectively. The enzyme could not be dissociated from Golgi apparatus fractions by treatment with ultrasound, indicating that it is tightly associated with the membrane. The newly synthesized GM3, the product of the reaction, was incorporated into or became tightly associated with the membranes of the Golgi apparatus.

CMP-N-acetylneuraminic acid hydrolase, an ectoenzyme distributed unevenly over the hepatocyte surface

The regional localization of CMP-N-acetylneuramic acid hydrolase at the hepatocyte surface was studied by using plasma membranes and hepatocytes isolated from rat liver. 1. By homogenization of the rat liver plasma membrane preparations and subsequent discontinuous sucrose gradient centrifugation, one light and two heavy membrane fractions were obtained. The origin of these three subfractions is discussed based on the specific activities in the three fractions of 5'-nucleotidase, alakaline phosphatase and Mg2+-ATPase and on electron microscopic examination of the fractions. Evidence is given suggesting that the light fraction is derived from the bile canalicular surface of the plasma membrane, and that the heavy fractions are derived predominantly from the sinusoidal and lateral surfaces of the liver cell membrane. CMP-AcNeu hydrolase was present at highest specific activity in one of the heavy subfractions. Therefore it is concluded that CMP-AcNeu hdyrolase is located preferentially in the sinusoidal and/or lateral plasma membrane parts of the liver cell. 2. Experiments with intact and disintegrated hepatocytes isolated from rat liver indicated that CMP-AcNeu hydrolase is located at the surface of the cell membrane, with its functional group directed to the outside.

Properties and subcellular localization of CMP-N-acetylneuraminic acid hydrolase of calf kidney

The properties and subcellular distribution of CMP-N-acetylneuraminic acid (CMP-NAcNeu) hydrolase were studied in the cortex of calf kidney. The pH optimum was 9.0 in both Tris - HCl and glycine/NaOH buffer. The apparent Km was 0.47 mM and the apparent V 15.3 mumol/h/g wet wt of calf kidney cortex. A stimulation by divalent metal ions (Ca2+ and Mg2+) was demonstrated for the hydrolase. In the presence of Triton X-100 an increase in enzyme activity was observed. CMP-NAcNeu hydrolase was inhibited by EDTA, beta-mercaptoethanol, nucleoside phosphates and nucleotide-sugars. The inhibition was more pronounced when a sub-optimal CMP-NAcNeu concentration was used. The enzyme appeared to be localized in the plasma membranes. In the plasma membrane preparation of calf kidney cortex, which was derived mainly from the proximal tubule cells, the yield of CMP-NAcNeu hydrolase (13%) and its increase in specific activity (9-fold) was as high as for the plasma membrane marker enzymes. From subcellular distribution studies it appeared that the enzyme was localized mainly at the bursh border side of the plasma membrane of the proximal tubule cell.