Α-mannosidase Inhibitor Research Articles

Inhibition of N-linked glycosylation by tunicamycin induces E-cadherin-mediated cell–cell adhesion and inhibits cell proliferation in undifferentiated human colon cancer cells

Aberrant protein glycosylation and disassembly of E-cadherin-mediated cell-cell adhesion are characteristics of epithelial cancer. However, the relationship between these two events in colorectal cancer remains to be defined. In this study, we analyzed whether N-glycan expression is crucial for the loss of E-cadherin-mediated cell-cell adhesion in human colorectal cancer cells. Differentiated Caco-2 and undifferentiated HCT-116 colon cancer cells were used as models of stable and unstable adherens junctions (AJs), respectively. Complex-type N-glycans were detected using the lectins E-PHA (Phaseolus vulgaris E.) and L-PHA (Phaseolus vulgaris L.). To study E-cadherin-mediated AJ assembly, we examined the effects of swainsonine, an inhibitor of α-mannosidase II, and tunicamycin, a drug that inhibits the biosynthesis of N-glycans, via western blot, immunofluorescence, differential extraction in Triton X-100, and electron microscopy. Cell proliferation and apoptosis were examined by crystal violet staining and flow cytometry, respectively. We observed positive labeling for E-PHA and L-PHA lectins in both cell lines; however, HCT-116 cells had increased E-cadherin-linked complex-type N-glycans. Interestingly, tunicamycin, but not swainsonine, was able to induce functional E-cadherin-mediated cell-cell adhesion in undifferentiated HCT-116 cells, as shown by the increased association of E-cadherin with the actin cytoskeleton. Moreover, in HCT-116 cells, tunicamycin also induced the formation of tight cell-cell contacts, and it inhibited cell proliferation without triggering apoptosis. Collectively, our results demonstrate for the first time that altered N-glycan expression plays an important role in the loss of AJ stability in undifferentiated colorectal cancer cells and that this loss may be associated with the progression of colorectal cancer.

Selective Yolk Deposition and Mannose Phosphorylation of Lysosomal Glycosidases in Zebrafish

The regulation and function of lysosomal hydrolases during yolk consumption and embryogenesis in zebrafish are poorly understood. In an effort to better define the lysosomal biochemistry of this organism, we analyzed the developmental expression, biochemical properties, and function of several glycosidases in zebrafish eggs, embryos, and adult tissues. Our results demonstrated that the specific activity of most enzymes increases during embryogenesis, likely reflecting a greater need for turnover within the embryo as yolk-derived nutrients are depleted. Analysis of glycosidase activity in zebrafish and medaka eggs revealed selective deposition of enzymes required for the degradation of N-linked glycans, including an abundance of acidic mannosidases. Treatment of zebrafish embryos with the α-mannosidase inhibitor swainsonine resulted in the accumulation of glycosylated vitellogenin fragments and demonstrated a function for maternally deposited acid α-mannosidase in yolk consumption. Surprisingly, we also found that, unlike mammals, acid α-glucosidase from zebrafish and medaka does not appear to be modified with mannose 6-phosphate residues. We further showed these residues were not acquired on human acid α-glucosidase when expressed in zebrafish embryos, suggesting unique differences in the ability of the human and zebrafish N-acetylglucosamine-1-phosphotransferase to recognize and modify certain lysosomal glycosidases. Together, these results provide novel insight into the role of acidic glycosidases during yolk utilization and the evolution of the mannose 6-phosphate targeting system in vertebrates.

Concise and practical route to tri- and tetra-hydroxy seven-membered iminocyclitols as glycosidase inhibitors from d-(+)-glucurono-γ-lactone

An efficient and short total synthesis of tetrahydroxy-1c and trihydroxy-azepane 1d is reported in 72% and 57% overall yields, respectively, from d-(+)-glucurono-γ-lactone. Thus, d-glucuronolactone 2 on acetonide protection, DIBAL-H reduction and one-pot intermolecular reductive amination followed by –NCbz protection afforded 6-(N-benzyl-N-benzyloxycarbonyl) amino-6-deoxy-1,2-O-isopropylidene-α-d-gluco-1,4-furanose 5a. 1,2-Acetonide hydrolysis in 5a and Pd-mediated intramolecular reductive aminocyclization afforded tetrahydroxyazepane 1c. An analogous pathway with 5-deoxy-1,2-O-isopropylidene-α-d-glucurono-6,3-lactone 3b gave trihydroxy-azepane 1d. Glycosidase inhibitory activity of 1c/1d was studied and 1d was found to be potent inhibitor of α-mannosidase and β-galactosidase.

Novel 2-[(benzylamino)methyl]pyrrolidine-3,4-diol derivatives as α-mannosidase inhibitors and with antitumor activities against hematological and solid malignancies

Novel α-mannosidase inhibitors of the type (2R,3R,4S)-2-({[(1R)-2-hydroxy-1-arylethyl]amino}methyl)pyrrolidine-3,4-diol have been prepared and assayed for their anticancer activities. Compound 30 with the aryl group=4-trifluoromethylbiphenyl inhibits the proliferation of primary cells and cell lines of different origins, irrespective of Bcl-2 expression levels, inducing a G2/Mcell cycle arrest and by modification of genes involved in cell cycle progression and survival.

Development of a transformation system in the swainsonine producing, slow growing endophytic fungus, Undifilum oxytropis

Undifilum oxytropis (Phylum: Ascomycota; Family: Pleosporaceae) is a slow growing endophytic fungus that produces a toxic alkaloid, swainsonine. This endophyte resides in locoweeds, which are perennial flowering legumes. Consumption of this fungus by grazing animals induces a neurological disorder called locoism. The alkaloid swainsonine, an α-mannosidase inhibitor, is responsible for the field toxicity related to locoism. Little is known about the biosynthetic pathway of swainsonine in endophytic fungi. Genetic manipulation of endophytic fungi is important to better understand biochemical pathways involved in alkaloid synthesis, but no transformation system has been available for studying such enzymes in Undifilum. In this study we report the development of protoplast and transformation system for U. oxytropis. Fungal mycelia required for generating protoplasts were grown in liquid culture, then harvested and processed with various enzymes. Protoplasts were transformed with a fungal specific vector driving the expression of Enhanced Green Florescent Protein (EGFP). The quality of transformed protoplasts and transformation efficiency were monitored during the process. In all cases, resistance to antibiotic hygromycin B was maintained. Such manipulation will open avenues for future research to decipher fungal metabolic pathways.

A long-wavelength fluorescent substrate for continuous fluorometric determination of α-mannosidase activity: Resorufin α- d-mannopyranoside

A simple and reliable continuous assay for measurement of α-mannosidase activity is described and demonstrated for analysis with two recombinant human enzymes using the new substrate resorufin α- d-mannopyranoside (Res-Man). The product of enzyme reaction, resorufin, exhibits fluorescence emission at 585 nm with excitation at 571 nm and has a p K a of 5.8, allowing continuous measurement of fluorescence turnover at or near physiological pH values for human lysosomal and Drosophila Golgi α-mannosidases. The assay performed using recombinant Drosophila Golgi α-mannosidase (dGMII) has been shown to give the kinetic parameters K m of 200 μM and V max of 11 nmol/min per nmol dGMII. Methods for performing the assay using several concentrations of the known α-mannosidase inhibitor swainsonine are also presented, demonstrating a potential for use of the assay as a simple method for high-throughput screening of inhibitors potentially useful in cancer treatment.

Crystal Structure of the LG1-3 Region of the Laminin α2 Chain

Laminins are large heterotrimeric glycoproteins with many essential functions in basement membrane assembly and function. Cell adhesion to laminins is mediated by a tandem of five laminin G-like (LG) domains at the C terminus of the α chain. Integrin binding requires an intact LG1-3 region, as well as contributions from the coiled coil formed by the α, β, and γ chains. We have determined the crystal structure at 2.8-Å resolution of the LG1-3 region of the laminin α2 chain (α2LG1-3). The three LG domains adopt typical β-sandwich folds, with canonical calcium binding sites in LG1 and LG2. LG2 and LG3 interact through a substantial interface, but LG1 is completely dissociated from the LG2-3 pair. We suggest that the missing γ chain tail may be required to stabilize the interaction between LG1 and LG2-3 in the biologically active conformation. A global analysis of N-linked glycosylation sites shows that the β-sandwich faces of LG1 are free of carbohydrate modifications in all five laminin α chains, suggesting that these surfaces may harbor the integrin binding site. The α2LG1-3 structure provides the first atomic view of the integrin binding region of laminins.

Synthesis and X-ray Structure of (2R, 3R, 4R, 5R)-3,4,5-Tris-Benzyloxy-2-Benzyloxymethyl-Piperidin-1-ol, the N-Hydroxy-Analogue of 2,3,4,6-Tetra-O-Benzyl-1-Deoxymannojirimycin

The title compound 8 was obtained for the first time by hydride-mediated reduction of the d-fructose-derived nitrone 9. The N-hydroxy piperidine 8 is a precursor of the N-hydroxy analogue of DMJ, a potent inhibitor of α-mannosidases. It was isolated as colorless crystals (triclinic, P1 space group) exhibiting the following cell parameters: a = 9.947(2) A; b = 12.155(2) A; c = 13.864(5) A; α = 100.98(3)°; β = 97.94(2)°; γ = 109.50(1)°. The X-ray analysis of a monocrystal of 8 allowed confirmation of its relative configurations and showed the anti orientation of its N-hydroxy group. This structural feature should be useful for considering the interaction of N-hydroxy iminosugars with the recognition site of carbohydrate processing enzymes. The title compound 8 was prepared for the first time by hydride-mediated reduction of the d-fructose-derived nitrone 9. Compound 8 is a precursor of the N-hydroxy analogue of DMJ, a potent inhibitor of α-mannosidases. The X-ray analysis allowed confirmation of the relative configurations of 8 and showed the anti orientation of its N-hydroxy group. This structural feature will be useful for considering the interaction of N-hydroxy-iminosugars with specific biological targets.

Inhibition of α-mannosidase attenuates endoplasmic reticulum stress-induced neuronal cell death

N-glycosylation is crucial for proper folding of most of the proteins in the endoplasmic reticulum (ER). The N-glycans in the ER are mainly constructed of mannose. In this study, we examined whether inhibition of mannose trimming in the ER affects the susceptibility of PC-12 cells to ER stress. Pretreatment with 100 μM α-mannosidase inhibitor 1-deoxymannojirimycin (DMJ) in PC-12 cells significantly attenuated the cytotoxicity by ER stressors tunicamycin (TM), thapsigargin (TG), and amyloid β1-42 (Aβ1-42), and reduced caspase-3 activation by TM and TG. Pretreatment with DMJ also protected primary cultured mouse cortical neurons from Aβ1-42 toxicity. With regard to the effect of DMJ pretreatment on ER stress signaling in PC-12 cells, DMJ attenuated TM- and TG-induced CHOP expression and TG stimulated JNK phosphorylation, which is associated with ER stress dependent cell death. Next, we examined the effect of mannose oligosaccharides, which have similar structures to N-glycans in the ER, on amyloidogenesis of Aβ1-42 that causes ER stress dependent neuronal cell death. Mannopentaose (M5) and Man 9GlcNAc 2 (M9) oligosaccharides significantly inhibited the amyloidogenesis of Aβ1-42. Our data suggests that inhibition of N-glycan processing in the ER attenuates ER stress-induced cell death by increasing high-mannose type oligosaccharides that reduce protein aggregation, such as amyloidogenesis.

Functionalized pyrrolidine inhibitors of human type II α-mannosidases as anti-cancer agents: Optimizing the fit to the active site

Refining the chemical structure of functionalized pyrrolidine-based inhibitors of Golgi α-mannosidase II (GMII) to optimize binding affinity provided a lead molecule that demonstrated nanomolar competitive inhibition of α-mannosidases II and an optimal fit in the active site of Drosophila GMII by X-ray crystallography. Esters of this lead compound also inhibited the growth of human glioblastoma and brain-derived endothelial cells more than the growth of non-tumoral human fibroblasts, suggesting their potential for anti-cancer therapy.

Synthetic studies towards a new scaffold, spirobicycloimidazoline

A new scaffold consisting of a carbocycle and a substituted imidazoline in an orthogonal arrangement was synthesized as a potential specific inhibitor of glycosidases. The spirobicycloimidazoline, (5 R,6 R,7 R,8 R)-8-(hydroxymethyl)-2-phenyl-1,3-diazaspiro[4.4]non-1-ene-6,7-diol, was synthesized from methyl 2- O- p-methoxybenzyl-3,4-di- O-benzyl-α/β- d- gluco-6-enopyranoside via (1 R,2 S,3 S,4 R,5 S)-3,4-bis(benzyloxy)-2-(4-methoxybenzyloxy)-5-vinyl-cyclopentanol. The ring contraction of the 6-enopyranoside in the presence of zirconocene equivalent (‘Cp 2Zr’) reagent gave exclusively the corresponding cyclopentanol without cleavage of the PMB protecting group. In the course of the study, a new α-mannosidase inhibitor, (1 R,2 R,3 R,5 R)-5-amino-3-hydroxymethyl-cyclopentane-1,2-diol, was also discovered.

Glycoside Esters of 5-Aminolevulinic Acid for Photodynamic Therapy of Cancer

Aliphatic and ethylene glycol esters of 5-aminolevulinic acid (ALA) are very efficient precursors of the photosensitizer protoporphyrin IX (PpIX) for photodynamic therapy; however, they diffuse passively across the cell membrane and thus lack cell selectivity. We evaluated whether alpha-glucose, alpha-mannose, or beta-galactose esters of ALA would present improved properties as precursors of PpIX. Esterification was performed either at the position O-1 or O-6 of the sugars with or without an ethylene glycol linker, and these glycoside esters of ALA were evaluated in human cells. The results demonstrated that glycoside esters of ALA are efficient precursors of PpIX in human cancer and angiogenic endothelial cells, comparable to free ALA, but not in normal human fibroblasts. PpIX production was confirmed by fluorescence microscopy and photodynamic treatment of cells. The O-1 or O-6 positions of functionalization and the nature of the sugar moiety did not influence PpIX production. The presence of the ethylene glycol linker generally resulted in decreased PpIX production. The uptake of these glycoside esters of ALA by cells was not decreased in the presence of high concentrations of the related sugars. Inhibitors of alpha-glucosidases or alpha-mannosidases did not decrease PpIX production. These results suggest the involvement of active non-glycoside-specific membrane transporter(s) for uptake and of esterases rather than glycosidases in the release of ALA from the glycoside esters of ALA.

Cell surface N-glycosylation and sialylation regulate galectin-3-induced apoptosis in human diffuse large B cell lymphoma

Galectin-3 is a soluble endogenous lectin in vertebrates and is implicated in a variety of biological functions, including tumor cell adhesion, proliferation, differentiation, apoptosis, cancer progression and metastasis. In the present study, we analyzed the role of galectin-3 in apoptosis in human malignant lymphoma. Galectin-3 induced cell death in the HBL-2 human diffuse large B cell lymphoma cell line. A morphological examination and annexin V assays revealed that galectin-3-induced cell death is consistent with apoptosis and swainsonine, a potent inhibitor of alpha-mannosidase II, which catalyzes the synthesis of complex type N-linked oligosaccharides, inhibited galectin-3-induced apoptosis in HBL-2 cells. These results suggest that galectin-3 induces apoptosis in HBL-2 cells by interacting with cell surface N-linked oligosaccharides. Furthermore, treatment of cells with Vibrio Cholerae neuraminidase enhanced galectin-3-induced apoptosis, suggesting that cell surface sialylation regulates galectin-3-induced apoptosis in human B cell lymphoma. In conclusion, our results indicate that galectin-3-induced apoptosis is regulated by cell surface expression of N-glycans and sialic acid in human diffuse large B cell lymphoma. This mechanism may be involved in the malignant behavior of human lymphoma cells.

D-Fructose-6-Phosphate Aldolase-Catalyzed One-Pot Synthesis of Iminocyclitols

A one-pot chemoenzymatic method for the synthesis of a variety of new iminocyclitols from readily available, non-phosphorylated donor substrates has been developed. The method utilizes the recently discovered fructose-6-phosphate aldolase (FSA), which is functionally distinct from known aldolases in its tolerance of different donor substrates as well as acceptor substrates. Kinetic studies were performed with dihydroxyacetone (DHA), the presumed endogenous substrate for FSA, as well as hydroxy acetone (HA) and 1-hydroxy-2-butanone (HB) as donor substrates, in each case using glyceraldehyde-3-phosphate as acceptor substrate. Remarkably, FSA used the three donor substrates with equal efficiency, with kcat/KMvalues of 33, 75, and 20 M-1 s-1, respectively. This level of donor substrate tolerance is unprecedented for an aldolase. Furthermore, DHA, HA, and HB were accepted as donors in FSA-catalyzed aldol reactions with a variety of azido- and Cbz-amino aldehyde acceptors. The broad substrate tolerance of FSA and the ability to circumvent the need for phosphorylated substrates allowed for one-pot synthesis of a number of known and novel iminocyclitols in good yields, and in a very concise fashion. New iminocyclitols were assayed as inhibitors against a panel of glycosidases. Compounds 15 and 16 were specific alpha-mannosidase inhibitors, and 24 and 26 were potent and selective inhibitors of beta-N-acetylglucosaminidases in the submicromolar range. Facile access to these compounds makes them attractive core structures for further inhibitor optimization.

Synthesis of aza- and thia-spiroheterocycles and attempted synthesis of spiro sulfonium compounds related to salacinol

The synthesis of aza- and thia-spiroheterocycles and the attempted synthesis of spiro sulfonium compounds related to salacinol are described. The binding of the nanomolar inhibitor swainsonine to Drosophila Golgi α-mannosidase II (dGMII) involves a large contribution of interactions between the six-membered ring of the inhibitor and the hydrophobic pocket within the enzyme active site. Salacinol, a naturally occurring sulfonium ion, is one of the active principles in the aqueous extracts of Salacia reticulata that are traditionally used in Sri Lanka and India for the treatment of diabetes. Spiro aza- and thia-heterocycles and a spiro analogue of salacinol were designed with the expectation that the hydrocarbon portions would make hydrophobic contributions to binding. The former sets of compounds were synthesized successfully but the salacinol analogue proved to be elusive. The stereochemistry of the final compounds was determined by means of 1D-NOESY experiments. The aza- and thia-heterocycles were not effective inhibitors of Golgi α-mannosidase II or human maltase glucoamylase.

Conformationally locked thiosugars as potent α-mannosidase inhibitors: Synthesis, biochemical and docking studies

A series of thiosugar derivatives (thiolevomannosans) derived from mannose were synthesized and their inhibitory activity was tested against α-mannosidase (jack bean). These inhibitors were found to be more potent than the well-known inhibitors like kifunensine and deoxymannojirimycin based on docking and biochemical studies. The sulfone derivative 10 was shown to be the best inhibitor of α-mannosidase with the K i value of 350 nM.

Aminocyclopentanols as sugar mimics. Synthesis from unsaturated bicyclic lactones by Overman rearrangement

Bicyclic cyclopentane lactones, prepared from bromodeoxyaldonolactones, were transformed into aminocyclopentanols with an Overman rearrangement as the key step. Two of the compounds prepared, 7 and 19, were found to be good inhibitors of jack bean alpha-mannosidase and beta-D-N-acetylglucosaminidase, respectively.

Beneficial effect of intracellular free high-mannose oligosaccharides on cryopreservation of mammalian cells and proteins

The cryoprotective effect of intracellular free high-mannose oligosaccharides (HMOS) on mammalian cells and proteins was examined by monitoring PC-12 cell viability and assaying protein kinase C (PKC)-ε activity. 1-Deoxymannojirimycin, an inhibitor of α-mannosidase, to cause an increase in intracellular free HMOS, significantly rescued PC-12 cells with 2-h freezing insult at −15 °C in a concentration (1–50 mM)- and pretreatment time (48–72 h)-dependent manner, as compared with unpretreated cells; full rescue from freezing injury was obtained with 1-deoxymannojirimycin at more than 25 mM for 48-h pretreatment and more than 3 mM for 72- and 96-h pretreatment. For PC-12 cells pretreated with 1-deoxymannojirimycin at 1 mM for 72 h, thawed cell viability after more than 8-w cryopreservation at −80 °C in 10% (v/v) dimethyl sulfoxide was much higher than that for cells without pretreatment. PKC-ε activity was well preserved after 16-h cryopreservation at −20 °C in the presence of mannose 9- N-acetylglucosamine 2 (Man9-GlcNAc2) (1 mM), an HMOS, while the activity was reduced to 15% without Man9-GlcNAc2. Collectively, the results of the present study suggest that intracellular free HMOS is a key molecule to protect mammalian cells and proteins from freezing injury; in other words, HMOS could be a new target for cryopreservation of mammalian cells and proteins.

Antiviral effects of glycosylation and glucose trimming inhibitors on human parainfluenza virus type 3

Endoplasmic reticulum (ER) α-glucosidase inhibitors block the trimming of N-linked glycosylation and thus prevent the production of several viruses. The present study investigates the antiviral effects of the α-glucosidase and α-mannosidase inhibitors (castanospermine, 1-deoxynojirimycin, bromoconduritol, deoxymannojirimycin and swainsonine) on human parainfluenza virus type 3 (HPIV3). The α-glucosidase inhibitors (castanospermine, 1-deoxynojirimycin) in recombinant expression systems reduced the surface and intracellular expression of both HPIV3 F and HN proteins. On the other hand, α-mannosidase inhibitors prevented processing of the oligosaccharides on HPIV3 glycoproteins into the complex form. Consequently, α-glycosidase inhibitors (castanospermine and 1-deoxynojirimycin) significantly inhibited viral fusion activity. We demonstrated that the α-glucosidase inhibitors (castanospermine and 1-deoxynojirimycin) reduced the infectivity of newly released viral particles. We postulate that α-glucosidase inhibitors can prevent the first steps of HPIV3 envelope glycoprotein processing and that the inhibition of glucose trimming has antiviral effects.

Evidence that maturation of the N-linked glycans of the respiratory syncytial virus (RSV) glycoproteins is required for virus-mediated cell fusion: The effect of α-mannosidase inhibitors on RSV infectivity

Glycan heterogeneity of the respiratory syncytial virus (RSV) fusion (F) protein was demonstrated by proteomics. The effect of maturation of the virus glycoproteins-associated glycans on virus infectivity was therefore examined using the α-mannosidase inhibitors deoxymannojirimycin (DMJ) and swainsonine (SW). In the presence of SW the N-linked glycans on the F protein appeared in a partially mature form, whereas in the presence of DMJ no maturation of the glycans was observed. Neither inhibitor had a significant effect on G protein processing or on the formation of progeny virus. Although the level of infectious virus and syncytia formation was not significantly affected by SW-treatment, DMJ-treatment correlated with a one hundred-fold reduction in virus infectivity. Our data suggest that glycan maturation of the RSV glycoproteins, in particular those on the F protein, is an important step in virus maturation and is required for virus infectivity.