(5S)-5-Benzylswainsonines as potent and selective inhibitors of Golgi α-mannosidase II: synthesis, enzyme evaluation and molecular modelling.

Development of novel anti-cancer therapeutics based on Golgi α-mannosidase II (GMII) inhibition is considerably impeded by an undesired co-inhibition of lysosomal α-mannosidase leading to severe side-effects. In this contribution, we describe a fully stereoselective synthesis of (5S)-5-[4-(halo)benzyl]swainsonines as highly potent and selective inhibitors of GMII. The synthesis starts from a previously reported aldehyde readily available from l-ribose, and the key features include an intramolecular reductive amination with substrate-controlled stereoselectivity and a late-stage derivatisation of the benzyl group via ipso-substitution. These novel swainsonine analogues were found to be nanomolar inhibitors of the Golgi-type α-mannosidase AMAN-2 (Ki = 23-75 nM) with excellent selectivity (selectivity index = 205-870) over the lysosomal-type Jack bean α-mannosidase. Finally, molecular docking and pKa calculations were performed to provide more insight into the structure of the inhibitor:enzyme complexes, and a pair interaction energy analysis (FMO-PIEDA) was carried out to rationalise the observed potency and selectivity of the inhibitors.

Golgi a-mannosidase II mediates the formation of vascular smooth muscle foam cells under inflammatory stress.

INTRODUCTION: Vascular smooth muscle cells (VSMCs)-based foam cell formation is a crucial factor in the atherosclerosis process. We aimed to explore the mechanism of Golgi a-mannosidase II (GMII) effects on the VSMCs-based foam cell formation. MATERIAL AND METHODS: VSMCs were exposed to different concentrations of low-density lipoproteins (LDLs), lipopolysaccharide (LPS), and/or GMII inhibitor (swainsonine). The qRT-PCR and western blot were used for expression analysis. Oil Red O staining was used to verify changes of lipid droplets in VSMCs. The translocation of the SCAP from the endoplasmic reticulum (ER) to Golgi was detected by immunofluorescence (IF). RESULTS: LPS disrupted the LDLs-mediated regulation of LDL receptor (LDLr) and increased intracellular cholesterol ester, which was inversely inhibited by swainsonine. The activity of a-mannosidase II and GMII expression were decreased by LDLs but increased by the addition of LPS. Conversely, LPS-induced enhancement was reversed by swainsonine. Additionally, swainsonine reversed the LPS-induced increase of intracellular lipid droplets in the presence of LDLs. Expression analysis demonstrated that LDLr, SCAP, and SREBP2 were up-regulated by LPS, but reversed by swainsonine in LDLs-treated cells. IF staining revealed that swainsonine inhibited the translocation of SCAP to Golgi under inflammatory stress. CONCLUSIONS: Collectively, swainsonine restrained LDLr expression to suppress the formation of VSMCs-based foam cells by reducing SREBP2 and SCAP under inflammatory stress conditions, suggesting that GMII contributes to the formation of VSMCs-based foam cells under inflammatory stress.

Swainsonine promotes apoptosis by impairing lysosomal function and inhibiting autophagic degradation in rat primary renal tubular epithelial cells.

Swainsonine (SW), an indolizidine alkaloid, is the primary toxin in locoweeds that causes toxicity syndrome in livestock. Current research shows that SW can induce both apoptosis and autophagy. However, the relationship between, and regulatory mechanism of, autophagy and apoptosis in SW-mediated cytotoxicity remain unclear. In this study, we investigated the role of autophagy and apoptosis in SW-induced cytotoxicity in rat primary renal tubular epithelial cells (RTECs). We examined the effect of SW on lysosomal function using western blotting, transmission electron microscopy, fluorescent microscopy, and flow cytometry. The results showed that SW induced both autophagy and apoptosis, and autophagy protected RTECs from cellular damage. Activating autophagy using rapamycin (Rapa) inhibited apoptosis, while suppressing autophagy using bafilomycin A1 (Baf A1) greatly enhanced SW-induced apoptosis. SW treatment suppressed the expression of lysosomal-related proteins, and co-incubation with SW and aloxistatin (E64d) further promoted apoptosis and LC3-II accumulation in RTECs. These results suggest that SW causes toxicity by disrupting lysosomal dysfunction, inhibiting autophagic degradation, and promoting apoptosis.

Curcumin Derivative GT863 Inhibits Amyloid-Beta Production via Inhibition of Protein N-Glycosylation.

Amyloid-ß (Aß) peptides play a crucial role in the pathogenesis of Alzheimer's disease (AD). Aß production, aggregation, and clearance are thought to be important therapeutic targets for AD. Curcumin has been known to have an anti-amyloidogenic effect on AD. In the present study, we performed screening analysis using a curcumin derivative library with the aim of finding derivatives effective in suppressing Aß production with improved bioavailability of curcumin using CHO cells that stably express human amyloid-ß precursor protein and using human neuroblastoma SH-SY5Y cells. We found that the curcumin derivative GT863/PE859, which has been shown to have an inhibitory effect on Aß and tau aggregation in vivo, was more effective than curcumin itself in reducing Aß secretion. We further found that GT863 inhibited neither ß- nor γ-secretase activity, but did suppress γ-secretase-mediated cleavage in a substrate-dependent manner. We further found that GT863 suppressed N-linked glycosylation, including that of the γ-secretase subunit nicastrin. We also found that mannosidase inhibitors that block the mannose trimming step of N-glycosylation suppressed Aß production in a similar fashion, as was observed as a result of treatment with GT863. Collectively, these results suggest that GT863 downregulates N-glycosylation, resulting in suppression of Aß production without affecting secretase activity.

Swainsonine protects H9c2 cells against lipopolysaccharide-induced apoptosis and inflammatory injury via down-regulating miR-429.

Pediatric myocarditis (PM) is usually related to myocardial dysfunction. Generally, 30% of PM patients will die or undergo heart transplantation. Swainsonine (SW) is a natural alkaloid and an anti-cancer substance. Our goal was to determine the roles of SW in PM in current study. H9c2 cells were pre-treated by lipopolysaccharide (LPS). Viability and apoptosis were evaluated utilizing CCK-8 assay and flow cytometry. Inflammatory cytokines' mRNA expression and production were assessed by western blot and ELISA. Western blot was utilized to distinguish apoptosis and immune-related factors expression. Sequentially, the abovementioned parameters were reassessed when miR-429 was overexpressed. LPS declined viability as well as raised apoptosis and inflammatory injury in H9c2 cells. SW alleviated apoptosis and inflammatory injury induced by LPS. MiR-429 expression was elevated by LPS and suppressed by SW. SW-induced the increasing of viability and the reduction of inflammatory injury were reversed by overexpression of miR-429. Eventually, SW inhibited p38MAPK/NF-κB pathway which activated by LPS via overexpressing miR-429. SW exerted its anti-apoptosis and anti-inflammatory function in LPS-treated H9c2 cells through p38MAPK/NF-κB pathway and down-regulation of miR-429.

Sesquiterpenoids and mycotoxin swainsonine from the locoweed endophytic fungus Alternaria oxytropis.

Oxytropiols A-J, ten undescribed guaiane-type sesquiterpenoids, and the mycotoxin swainsonine (SW) were isolated from the locoweed endophytic fungus Alternaria oxytropis. The chemical structures of these sesquiterpenoids were elucidated on the basis of HR-ESI-MS and NMR data including 1H, 13C, HSQC, 1H-1H COSY, HMBC, and NOESY spectra, and the absolute configurations of these compounds were determined using a modified Mosher's method and X-ray diffraction spectroscopy. A possible biosynthetic pathway of these guaiane-type sesquiterpenoids is discussed, and proposed that post-modification oxidative enzymes might form these highly polyhydroxylated structures. Compound 1 displayed biological effects on the root growth of Arabidopsis thaliana, and SW displayed cytotoxicity against A549 and HeLa cancer cell lines.

Swainsonine inhibits proliferation and collagen synthesis of NIH-3T3 cells by declining miR-21.

Swainsonine (SW) is an indolizidine alkaloid first discovered in Swainsona canescens. This study explored the effects of SW on mouse embryo fibroblast NIH-3T3 cell proliferation and collagen synthesis, as well as potential molecule mechanisms. We discovered that SW exposure lowered the viability and proliferation of NIH-3T3 cells. The collagen synthesis was reduced after SW exposure, as evidenced by declines of the mRNA and protein levels of collagen I (CoI I), collagen III (CoI III) and α-smooth muscle actin (α-SMA) in NIH-3T3 cells, as well as reduction of collagen concentration in the culture supernatant of NIH-3T3 cells. Mechanically, transforming growth factor ß1 (TGF-ß1) stimulation elevated the microRNA-21 (miR-21) expression in NIH-3T3 cells. SW reversed the TGF-ß1-caused elevation of miR-21. Up-regulation of miR-21 attenuated the inhibitory influences of SW on NIH-3T3 cell viability, proliferation and collagen synthesis. Silence of miR-21 had converse influence. Besides, SW inactivated PI3K/AKT and NF-κB pathways via declining miR-21. Altogether, SW inhibited the proliferation and collagen synthesis of fibroblast NIH-3T3 might be through declining miR-21 and then suppressing PI3K/AKT and NF-κB pathways. SW may be an effective therapeutic medicine for scar hyperplasia.

Complex N-glycan promotes CD133 mono-ubiquitination and secretion.

CD133 is a widely used cell surface marker of cancer stem cells that plays an important role in tumor initiation and metastasis. Increasing evidence shows that CD133 is secreted to the extracellular space. However, the underlying mechanisms of CD133 secretion remain largely unknown. In this study, we report that secreted CD133 has a complex-type N-glycosylation and is modified by beta1,6GlcNAc N-glycan. We found that inhibition of CD133 complex-type N-glycosylation by swainsonine does not affect the membrane localization of CD133, but significantly reduces CD133 secretion and promotes its accumulation in early endosomes. Moreover, swainsonine reduces CD133 secretion by reducing its mono-ubiquitination and inhibiting the interaction between CD133 and Tsg101. These findings reveal a new mechanism of glycosylation-dependent secretion of CD133.

Swainsonine, an alpha-mannosidase inhibitor, may worsen cervical cancer progression through the increase in myeloid derived suppressor cells population.

Cervical cancer, caused by high oncogenic risk Human Papillomavirus (HPV) infection, continues to be a public health problem, mainly in developing countries. Using peptide phage display as a tool to identify potential molecular targets in HPV associated tumors, we identified α-mannosidase, among other enriched sequences. This enzyme is expressed in both tumor and inflammatory compartment of the tumor microenvironment. Several studies in experimental models have shown that its inhibition by swainsonine (SW) led to inhibition of tumor growth and metastasis directly and indirectly, through activation of macrophages and NK cells, promoting anti-tumor activity. Therefore, the aim of this work was to test if swainsonine treatment could modulate anti-tumor immune responses and therefore interfere in HPV associated tumor growth. Validation of our biopanning results showed that cervical tumors, both tumor cells and leukocytes, expressed α-mannosidase. Ex vivo experiments with tumor associated macrophages showed that SW could partially modulate macrophage phenotype, decreasing CCL2 secretion and impairing IL-10 and IL-6 upregulation, which prompted us to proceed to in vivo tests. However, in vivo, SW treatment increased tumor growth. Investigation of the mechanisms leading to this result showed that SW treatment significantly induced the accumulation of myeloid derived suppressor cells in the spleen of tumor bearing mice, which inhibited T cell activation. Our results suggested that SW contributes to cervical cancer progression by favoring proliferation and accumulation of myeloid cells in the spleen, thus exacerbating these tumors systemic effects on the immune system, therefore facilitating tumor growth.

Swainsonine represses glioma cell proliferation, migration and invasion by reduction of miR-92a expression.

BACKGROUND: Swainsonine is a natural indolizidine alkaloid, its anti-tumor activity has been widely reported in varied cancers. This study aimed to investigate whether Swainsonine exerted anti-tumor impact on glioma cells, likewise uncovered the relative molecular mechanisms. METHODS: After administration with diverse concentrations of Swainsonine, cell growth, migration and invasion in U251 and LN444 cells were appraised by the common-used CCK-8, BrdU, flow cytometry and Transwell assays. MiR-92a mimic, inhibitor and the correlative NC were transfected into U251 and LN444 cells, and assessment of miR-92a expression was by utilizing qRT-PCR. Functions of miR-92a in above-mentioned cell biological processes were analyzed again in Swainsonine-treated cells. The momentous proteins of cell cycle, apoptosis and PI3K/AKT/mTOR pathway were ultimately examined by western blot. RESULTS: Swainsonine significantly hindered cell proliferation through decreasing cell viability, declining the percentage of BrdU cells, down-regulating CyclinD1 and up-regulating p16 expression. Enhancement of percentage of apoptotic cells was presented in Swainsonine-treated cells via activating cleaved-Caspase-3 and cleaved-Caspase-9. Additionally, Swainsonine impeded the abilities of migration and invasion by decreasing MMP-2, MMP-9, Vimentin and E-cadherin. Repression of miR-92a was observed in Swainsonine-treated cells, and miR-92a overexpression overturned the anti-tumor activity of Swainsonine in glioma cells. Finally, western blot assay displayed that Swainsonine hindered PI3K/AKT/mTOR pathway via regulating miR-92a. CONCLUSIONS: These discoveries corroborated that Swainsonine exerted anti-tumor impacts on glioma cells via repression of miR-92a, and inactivation of PI3K/AKT/mTOR signaling pathway.

N-glycan Remodeling Using Mannosidase Inhibitors to Increase High-mannose Glycans on Acid α-Glucosidase in Transgenic Rice Cell Cultures.

Glycoengineering of plant expression systems is a prerequisite for the production of biopharmaceuticals that are compatible with animal-derived glycoproteins. Large amounts of high-mannose glycans such as Man7GlcNAc2, Man8GlcNAc2, and Man9GlcNAc2 (Man7/8/9), which can be favorably modified by chemical conjugation of mannose-6-phosphate, are desirable for lysosomal enzyme targeting. This study proposed a rice cell-based glycoengineering strategy using two different mannosidase inhibitors, kifunensine (KIF) and swainsonine (SWA), to increase Man7/8/9 glycoforms of recombinant human acid α-glucosidase (rhGAA), which is a therapeutic enzyme for Pompe disease. Response surface methodology was used to investigate the effects of the mannosidase inhibitors and to evaluate the synergistic effect of glycoengineering on rhGAA. Both inhibitors suppressed formation of plant-specific complex and paucimannose type N-glycans. SWA increased hybrid type glycans while KIF significantly increased Man7/8/9. Interestingly, the combination of KIF and SWA more effectively enhanced synthesis of Man7/8/9, especially Man9, than KIF alone. These changes show that SWA in combination with KIF more efficiently inhibited ER α-mannosidase II, resulting in a synergistic effect on synthesis of Man7/8/9. In conclusion, combined KIF and SWA treatment in rice cell culture media can be an effective method for the production of rhGAA displaying dominantly Man7/8/9 glycoforms without genetic manipulation of glycosylation.

Swainsonine induces apoptosis of rat cardiomyocytes via mitochondria­mediated pathway.

Swainsonine is an Astragalus membranaceus extract. It is indole, alkaloid, and soluble in water. Its effect on rat cardiomyocytes apoptosis, and the mechanisms underlying that effect, were investigated by inducing apoptosis in H9c2 cells. This was detected by MTT assay, Annexin V-FITC/propidium iodide double staining and western blotting. Flow cytometry and fluorescence microscopy were used to confirm swainsonine's effect on mitochondrial membrane potential and levels of reactive oxygen species, while an ATP-dependent bioluminescence assay kit served to find the ATP contents. Assessment was also carried out for peroxisome proliferator activated receptor γ co-activator 1α (PGC-1α) expression levels as well as those of such apoptosis-associated proteins as Cytochrome c, Caspase-3, B-cell lymphoma 2 (Bcl-2) and Bcl-2-associated X protein (Bax). Overall, indications were that swainsonine may have the potential to inhibit viability of cells, decrease expression of PGC-1α, induce mitochondrial dysfunction, upregulate Cytochrome c, Bax and Caspase-3, and downregulate Bcl-2. The suggestion would be that apoptosis may be induced through signalling pathways in H9c2 cells mediated by mitochondria.

Integrin expression and glycosylation patterns regulate cell-matrix adhesion and alter with breast cancer progression.

Integrins are the major cell adhesion glycoproteins involved in cell-extracellular matrix (ECM) interaction and metastasis. Further, glycosylation on integrin is necessary for its proper folding and functionality. Herein, differential expression of integrins viz., αvß3 and αvß6 was examined in MDA-MB-231, MDA-MB-468 and MCF-10A cells, which signify three different stages of breast cancer development from highly metastatic to non-tumorigenic stage. The expression of αvß3 and αvß6 integrins at mRNA and protein levels was observed in all three cell lines and the results displayed a distinct pattern of expression. Highly metastatic cells showed enhanced expression of αvß3 than moderate metastatic and non-tumorigenic cells. The scenario was reversed in case of αvß6 integrin, which was strongly expressed in moderate metastatic and non-tumorigenic cells. N-glycosylation of αvß3 and αvß6 integrins is required for the attachment of cells to ECM proteins like fibronectin. The cell adhesion properties were found to be different in these cancer cells with respect to the type of integrins expressed. The results testify that αvß3 integrin in highly metastatic cells, αvß6 integrin in both moderate metastatic and non-tumorigenic cells play an important role in cell adhesion. The investigation typify that N-glycosylation on integrins is also necessary for cell-ECM interaction. Further, glycosylation inhibition by Swainsonine is found to be more detrimental to invasive property of moderate metastatic cells. Conclusively, types of integrins expressed as well as their N-glycosylation pattern alter during the course of breast cancer progression.

Swainsonine Inhibits Invasion and the EMT Process in Esophageal Carcinoma Cells by Targeting Twist1.

Esophageal cancer is a common gastrointestinal cancer, with a very high mortality rate in patients with metastasis. Swainsonine, a cytotoxic fungal alkaloid, has been shown to inhibit cell growth in esophageal cancer. In the present study, we explored the effects of swainsonine on cell invasion and metastasis in esophageal cancer cells. Human esophageal carcinoma cells were treated with different doses of swainsonine, and then cell viability, invasion, and apoptosis were measured. The mRNA and protein expressions of Twist1, apoptosis- and EMT-related factors, and PI3K/AKT pathway factors were detected by qRT-PCR and Western blot. Swainsonine had no effect on esophageal cancer cell viability and apoptosis, but it significantly decreased cell invasion in a dose-dependent manner. Swainsonine increased the expression of E-cadherin but decreased the expression of N-cadherin, vimentin, ZEB1, and snail in a dose-dependent manner, thereby inhibiting EMT. Last, we found that swainsonine inhibits cell invasion and EMT in the esophageal carcinoma cells by downregulation of Twist1 and deactivation of the PI3K/AKT signaling pathway.

Alteration of the PI3K/Akt signaling pathway by swainsonine affects 17ß-Estradiol secretion in ovary cells.

BACKGROUND: The ingestion of locoweed that contains the toxic indolizidine alkaloid swainsonine (SW) disrupts ovarian function, accompanied by delayed estrus, increased estrous cycle length, delayed conception, and abortion. GOALS: The direct effects of SW on ovary cell steroidogenesis remain unclear. MATERIALS AND METHODS: In this study, Chinese hamster ovary (CHO) cells were used to investigate the effects of SW on estradiol (E2) secretion and cell viability and the mechanisms involved in these processes. RESULTS: CHO cells were treated with SW. 17 ß-Estradiol mRNA expression was decreased in the SW group compared to that in the control group. Various concentrations of E2 and SW were added to cultured cells for 12 h and 36 h. Compared to the control group cells, CYP19A1 expression was decreased in the SW and SW + E2 treatment groups at 12 h and 36 h (P < 0.05). This showed that SW mainly inhibits the last step of estrogen synthesis. When CHO cells were treated with SW, the p-Akt protein levels were significantly decreased compared to that in the control group cells at 12 h and 36 h (P < 0.05). However, the p-Akt expression in the SW + E2 group was not significantly different compared to that in the control group cells (P > 0.05). When CHO cells were treated with SW and SW + E2, the PI3K protein levels were significantly down-regulated compared to that in the control group cells at 12 h and 36 h. CONCLUSION: Taken together, these studies demonstrate that SW is an inhibitor of PI3K/Akt signaling pathway. However, SW blocked PI3K activation in estrogen induction without blocking p-Akt activation in CHO cells. Therefore, SW + E2 blocked upstream but did not affect the downstream of the PI3K/Akt signaling pathway.

N-glycan content modulates kainate receptor functional properties.

KEY POINTS: Ionotropic glutamate receptor (iGluR) subunits are N-glycosylated at 4-12 sites, and Golgi processing produces mature receptors that contain high-mannose, hybrid and complex oligosaccharides. N-glycosylation is crucial for receptor biogenesis, influences receptor trafficking and provides a binding site for carbohydrate binding proteins. Glycan moieties are large, polar and occasionally charged, and they are attached at sites along iGluRs that position them for involvement in the structural changes underlying gating. Altering glycan content on kainate receptors (KARs), a subfamily of iGluRs, changes functional properties of the receptor, such as desensitization, recovery from desensitization and deactivation. We report the first observation that the charged trisaccharide HNK-1 is conjugated to native KARs, and we find that it substantially alters recombinant KAR functional properties. Our results show that the molecular composition of N-glycans can influence KAR biophysical properties, revealing a potential mechanism for fine-tuning the function of these receptors. ABSTRACT: Ionotropic glutamate receptors (iGluRs) are tetrameric proteins with between four and 12 consensus sites for N-glycosylation on each subunit, which potentially allows for a high degree of structural diversity conferred by this post-translational modification. N-glycosylation is required for proper folding of iGluRs in mammalian cells, although the impact of oligosaccharides on the function of successfully folded receptors is less clear. Glycan moieties are large, polar, occasionally charged and mediate many protein-protein interactions throughout the nervous system. Additionally, they are attached at sites along iGluR subunits that position them for involvement in the structural changes underlying gating. In the present study, we show that altering glycan content on kainate receptors (KARs) changes the functional properties of the receptors in a manner dependent on the identity of both the modified sugars and the subunit composition of the receptor to which they are attached. We also report that native KARs carry the complex capping oligosaccharide human natural killer-1. Glycosylation patterns probably differ between cell types, across development or with pathologies, and thus our findings reveal a potential mechanism for context-specific fine-tuning of KAR function through diversity in glycan structure.

iTRAQ-based quantitative proteomics discovering potential serum biomarkers in locoweed poisoned rabbits.

Locoism threatens the sustainable development of animal husbandry in areas around the world with intensified desertification, especially in the western United States, western China, Canada, and Mexico, among other countries. This study was conducted to discover potential serum biomarkers in locoweed-poisoned rabbits and lay a foundation for early diagnosis of locoism. We performed iTRAQ labeling coupled with two-dimensional liquid chromatography-tandem mass spectrometry (2D LC-MS/MS), comparing locoweed-poisoned rabbits and healthy controls. A total of 78 differentially-expressed proteins (fold change > 1.5 or < 0.67) were identified in the locoweed-poisoned rabbits compared to healthy controls. We found that 57.70% of differentially-expressed proteins were functionally related, and through bioinformatics analysis, we were able to construct a network mainly in complement and coagulation cascades. Significant differences in thrombospondin 4 (THBS4), kininogen 1 (KNG1), hemoglobin (HBB), and complement factor I (CFI) between locoweed poisoned animals and controls were found (P < 0.05) and validated by western blotting. These results suggested that locoweed could damage neurocytes, lower immunity, and form thrombi in rabbits. Our study proposes potential biomarkers for locoism diagnosis and also provides a new experimental basis to understand the pathogenesis of locoism.

N-glycosylation of human sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is essential for stability, secretion and activity.

Sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is a recently identified phosphodiesterase, which is a secreted N-linked glycoprotein. SMPDL3A is highly homologous to acid sphingomyelinase (aSMase), but unlike aSMase cannot cleave sphingomyelin. Rather, SMPDL3A hydrolyzes nucleotide tri- and diphosphates and their derivatives. While recent structural studies have shed light on these unexpected substrate preferences, many other aspects of SMPDL3A biology, which may give insight into its function in vivo, remain obscure. Here, we investigate the roles of N-glycosylation in the expression, secretion and activity of human SMPDL3A, using inhibitors of N-glycosylation and site-directed mutagenesis, with either THP-1 macrophages or CHO cells expressing human SMPDL3A. Tunicamycin (TM) treatment resulted in expression of non-glycosylated SMPDL3A that was not secreted, and was largely degraded by the proteasome. Proteasomal inhibition restored levels of SMPDL3A in TM-treated cells, although this non-glycosylated protein lacked phosphodiesterase activity. Enzymatic deglycosylation of purified recombinant SMPDL3A also resulted in significant loss of phosphodiesterase activity. Site-directed mutagenesis of individual N-glycosylation sites in SMPDL3A identified glycosylation of Asn69 and Asn222 as affecting maturation of its N-glycans and secretion. Glycosylation of Asn356 in SMPDL3A, an N-linked site conserved throughout the aSMase-like family, was critical for protection against proteasomal degradation and preservation of enzymatic activity. We provide the first experimental evidence for a predicted 22 residue N-terminal signal peptide in SMPDL3A, which is essential for facilitating glycosylation and is removed from the mature protein secreted from CHO cells. In conclusion, site-specific N-glycosylation is essential for the intracellular stability, secretion and activity of human SMPDL3A.

Sialylated ß1, 6 branched N-glycans modulate the adhesion, invasion and metastasis of hepatocarcinoma cells.

The mouse hepatocarcinoma cell lines Hca-F and Hca-P have been derived from hepatocarcinoma in mice and metastasize only to the lymph node. Hca-F cells displayed greater lymphatic metastasis ability than Hca-P cells. When the two cell lines were compared for cell surface sialylated ß1,6 branched N-glycans by flow cytometry using L-PHA and SNA, Hca-F cells were found to express significantly higher levels. To explore the effect of increased sialylated ß1,6 branched N-glycans on hepatocarcinoma progression, we inhibit their expression in Hca-F cells by using swainsonine treatment and RNA interference. We found that swainsonine treatment or GnT-V-shRNA transfection significantly inhibited the formation of ß1,6 branched N-glycans, and partially inhibited the expression of α2,6 sialic acids. Knockdown of sialylated ß1,6 branched N-glycans significantly attenuated the invasive and metastatic capability both in vitro and in vivo. Blockade of α2,6 sialic acid expression on Hca-F cell surface by the treatment with neuraminidase caused reduction in cellular adherence to lymph node. In addition, knockdown of sialylated ß1,6 branched N-glycans could decrease the expression of Notch1, NICD1, NICD2 and HES1 in Hca-F cells. Collectively, these findings suggest that increased sialylated ß1,6 branched N-glycans may contribute to hepatocarcinoma progression by altering the adhesive, invasive and metastatic ability to lymph node via Notch signaling pathway.

Betulinic acid and oleanolic acid, natural pentacyclic triterpenoids, interfere with N-linked glycan modifications to intercellular adhesion molecule-1, but not its intracellular transport to the cell surface.

Betulinic acid (3ß-hydroxy-20(29)-lupen-28-oic acid), oleanolic acid (3ß-hydroxy-olean-12-en-28-oic acid), and ursolic acid (3ß-hydroxy-urs-12-en-28-oic acid) are close structural isomers of natural pentacyclic triterpenoid carboxylic acids. We recently identified a unique biological effect of ursolic acid, its inhibition of the intracellular trafficking of glycoproteins. In the present study, we demonstrated that betulinic acid and oleanolic acid did not inhibit the interleukin-1α-induced expression of cell-surface intercellular adhesion molecule-1 (ICAM-1) in human lung carcinoma A549 cells. Nevertheless, betulinic acid and, to a lesser extent, oleanolic acid interfered with N-linked glycan modifications to ICAM-1 in a similar manner to castanospermine (an inhibitor of endoplasmic reticulum α-glucosidases I and II), but not swainsonine (an inhibitor of Golgi α-mannosidase II). Consistent with these results, betulinic acid and oleanolic acid inhibited yeast α-glucosidase activity, but not Jack bean α-mannosidase activity. Thus, to the best of our knowledge, this is the first study to show that betulinic acid and oleanolic acid interfere with N-linked glycan modifications to ICAM-1, but not its intracellular transport to the cell surface.