Convenient and Sensitive Measurement of Lactosylceramide Synthase Activity Using Deuterated Glucosylceramide and Mass Spectrometry

Lactosylceramide is necessary for the biosynthesis of almost all classes of glycosphingolipids and plays a relevant role in pathways involved in neuroinflammation. It is synthesized by the action of galactosyltransferases B4GALT5 and B4GALT6, which transfer galactose from UDP-galactose to glucosylceramide. Lactosylceramide synthase activity was classically determined in vitro by a method based on the incorporation of radiolabeled galactose followed by the chromatographic separation and quantitation of the product by liquid scintillation counting. Here, we used deuterated glucosylceramide as the acceptor substrate and quantitated the deuterated lactosylceramide product by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). We compared this method with the classical radiochemical method and found that the reactions have similar requirements and provide comparable results in the presence of high synthase activity. Conversely, when the biological source lacked lactosylceramide synthase activity, as in the case of a crude homogenate of human dermal fibroblasts, the radiochemical method failed, while the other provided a reliable measurement. In addition to being very accurate and sensitive, the proposed use of deuterated glucosylceramide and LC-MS/MS for the detection of lactosylceramide synthase in vitro has the relevant advantage of avoiding the costs and discomforts of managing radiochemicals.

A sensitive method for determining UDP-glucose: ceramide glucosyltransferase (UGCG) activity in biological samples using deuterated glucosylceramide as acceptor substrate

Glucosylceramide synthase (UGCG) is a key enzyme in the biosynthesis of glycosphingolipids and its activity is related to the resistance to anticancer drugs and is involved in the derangement of metabolism in various diseases. Moreover, UGCG acts as a major controller of the balanced levels of individual brain sphingolipids that may trigger neurodegeneration in Gaucher disease and in Parkinson disease associated to pathogenic variants in the glucocerebrosidase-encoding gene GBA. We have developed an effective method for determining UGCG activity in vitro using deuterated ceramide as an acceptor, and quantitation of the formed deuterated glucosylceramide by liquid chromatography coupled with tandem mass spectrometry. The method enabled us to determine the kinetic parameters of UGGC and the effect of the inhibitor GZ667161 on the enzyme activity expressed in model cells, as well as to measure UGCG specific activity in human fibroblasts using a simple crude cell homogenate. This novel approach may be useful in determining the actual UGCG activity levels in patient cells and tissues of animal models of diseases, and to study novel drugs targeting glycosphingolipid metabolism.

Epigenetic Regulation of Glycosylation

Expression of glycosylation-related genes (or glycogenes) is strictly regulated by transcription factors and epigenetic processes, both in normal and in pathological conditions. In fact, glycosylation is an essential mechanism through which proteins and lipids are modified to perform a variety of biological events, to adapt to environment, and to interact with microorganisms.Many glycogenes with a role in normal development are epigenetically regulated. Essential studies were performed in the brain, where expression of glycogenes like MGAT5B, B4GALNT1, and ST8Sia1 are under the control of histone modifications, and in the immune system, where expression of FUT7 is regulated by both DNA methylation and histone modifications. At present, epigenetic regulation of glycosylation is still poorly described under physiological conditions, since the majority of the studies were focused on cancer. In fact, virtually all types of cancers display aberrant glycosylation, because of both genetic and epigenetic modifications on glycogenes. This is also true for many other diseases, such as inflammatory bowel disease, diabetes, systemic lupus erythematosus, IgA nephropathy, multiple sclerosis, and cardiovascular diseases.A deeper knowledge in epigenetic regulation of glycogenes is essential, since research in this field could be helpful in finding novel and personalized therapeutics.

Epigenetic Regulation of Glycosylation in Cancer and Other Diseases

In the last few decades, the newly emerging field of epigenetic regulation of glycosylation acquired more importance because it is unraveling physiological and pathological mechanisms related to glycan functions. Glycosylation is a complex process in which proteins and lipids are modified by the attachment of monosaccharides. The main actors in this kind of modification are the glycoenzymes, which are translated from glycosylation-related genes (or glycogenes). The expression of glycogenes is regulated by transcription factors and epigenetic mechanisms (mainly DNA methylation, histone acetylation and noncoding RNAs). This review focuses only on these last ones, in relation to cancer and other diseases, such as inflammatory bowel disease and IgA1 nephropathy. In fact, it is clear that a deeper knowledge in the fine-tuning of glycogenes is essential for acquiring new insights in the glycan field, especially if this could be useful for finding novel and personalized therapeutics.

A novel nonsense and inactivating variant of ST3GAL3 in two infant siblings suffering severe epilepsy and expressing circulating CA19.9

Three missense variants of ST3GAL3 are known to be responsible for a congenital disorder of glycosylation determining a neurodevelopmental disorder (intellectual disability/epileptic encephalopathy). Here we report a novel nonsense variant, p.Y220*, in two dichorionic infant twins presenting a picture of epileptic encephalopathy with impaired neuromotor development. Upon expression in HEK-293T cells, the variant appears totally devoid of enzymatic activity in vitro, apparently accumulated with respect to the wild-type or the missense variants, as detected by western blot, and in large part properly localized in the Golgi apparatus, as assessed by confocal microscopy. Both patients were found to efficiently express the CA19.9 antigen in the serum despite the total loss of ST3GAL3 activity, which thus appears replaceable from other ST3GALs in the synthesis of the sialyl-Lewis a epitope. Kinetic studies of ST3GAL3 revealed a strong preference for lactotetraosylceramide as acceptor and gangliotetraosylceramide was also efficiently utilized in vitro. Moreover, the p.A13D missense variant, the one maintaining residual sialyltransferase activity, was found to have much lower affinity for all suitable substrates than the wild-type enzyme with an overall catalytic efficiency almost negligible. Altogether the present data suggest that the apparent redundancy of ST3GALs deduced from knock-out mouse models only partially exists in humans. In fact, our patients lacking ST3GAL3 activity synthesize the CA19.9 epitope sialyl-Lewis a, but not all glycans necessary for fine brain functions, where the role of minor gangliosides deserves further attention.

Complementary Use of Carbohydrate Antigens Lewis a, Lewis b, and Sialyl-Lewis a (CA19.9 Epitope) in Gastrointestinal Cancers: Biological Rationale Towards A Personalized Clinical Application

Carbohydrate antigen 19.9 (CA19.9) is used as a tumor marker for clinical and research purposes assuming that it is abundantly produced by gastrointestinal cancer cells due to a cancer-associated aberrant glycosylation favoring its synthesis. Recent data has instead suggested a different picture, where immunodetection on tissue sections matches biochemical and molecular data. In addition to CA19.9, structurally related carbohydrate antigens Lewis a and Lewis b are, in fact, undetectable in colon cancer, due to the down-regulation of a galactosyltransferase necessary for their synthesis. In the pancreas, no differential expression of CA19.9 or cognate glycosyltransferases occurs in cancer. Ductal cells only express such Lewis antigens in a pattern affected by the relative levels of each glycosyltransferase, which are genetically and epigenetically determined. The elevation of circulating antigens seems to depend on the obstruction of neoplastic ducts and loss of polarity occurring in malignant ductal cells. Circulating Lewis a and Lewis b are indeed promising candidates for monitoring pancreatic cancer patients that are negative for CA19.9, but not for improving the low diagnostic performance of such an antigen. Insufficient biological data are available for gastric and bile duct cancer. Studying each patient in a personalized manner determining all Lewis antigens in the surgical specimens and in the blood, together with the status of the tissue-specific glycosylation machinery, promises fruitful advances in translational research and clinical practice.

Simple and Complex Sugars in Parkinson's Disease: a Bittersweet Taste

Neuronal homeostasis depends on both simple and complex sugars (the glycoconjugates), and derangement of their metabolism is liable to impair neural function and lead to neurodegeneration. Glucose levels boost glycation phenomena, a wide series of non-enzymatic reactions that give rise to various intermediates and end-products that are potentially dangerous in neurons. Glycoconjugates, including glycoproteins, glycolipids, and glycosaminoglycans, contribute to the constitution of the unique features of neuron membranes and extracellular matrix in the nervous system. Glycosylation defects are indeed frequently associated with nervous system disturbances and neurodegeneration. Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms associated with the loss of dopaminergic neurons in the pars compacta of the substantia nigra. Neurons present intracytoplasmic inclusions of α-synuclein aggregates involved in the disease pathogenesis together with the impairment of the autophagy-lysosome function, oxidative stress, and defective traffic and turnover of membrane components. In the present review, we selected relevant recent contributions concerning the direct involvement of glycation and glycosylation in α-synuclein stability, impaired autophagy and lysosomal function in PD, focusing on potential models of PD pathogenesis provided by genetic variants of glycosphingolipid processing enzymes, especially glucocerebrosidase (GBA). Moreover, we collected data aimed at defining the glycomic profile of PD patients as a tool to help in diagnosis and patient subtyping, as well as those pointing to sugar-related compounds with potential therapeutic applications in PD.

SSEA3 and Sialyl Lewis a Glycan Expression Is Controlled by B3GALT5 LTR through Lamin A-NFYA and SIRT1-STAT3 Signaling in Human ES Cells

B3GALT5 is involved in the synthesis of embryonic stem (ES) cell marker glycan, stage-specific embryonic antigen-3 (SSEA3). This gene has three native promoters and an integrated retroviral long terminal repeat (LTR) promoter. We found that B3GALT5-LTR is expressed at high levels in human ES cells. B3GALT5-LTR is also involved in the synthesis of the cancer-associated glycan, sialyl Lewis a. Sialyl Lewis a is expressed in ES cells and its expression decreases upon differentiation. Retinoic acid induced differentiation of ES cells, decreased the short form of NFYA (NFYAs), increased phosphorylation of STAT3, and decreased B3GALT5-LTR expression. NFYAs activated, and constitutively-active STAT3 (STAT3C) repressed B3GALT5-LTR promoter. The NFYAs and STAT3C effects were eliminated when their binding sites were deleted. Retinoic acid decreased the binding of NFYA to B3GALT5-LTR promoter and increased phospho-STAT3 binding. Lamin A repressed NFYAs and SSEA3 expression. SSEA3 repression mediated by a SIRT1 inhibitor was reversed by a STAT3 inhibitor. Repression of SSEA3 and sialyl Lewis a synthesis mediated by retinoic acid was partially reversed by lamin A short interfering RNA (siRNA) and a STAT3 inhibitor. In conclusion, B3GALT5-LTR is regulated by lamin A-NFYA and SIRT1-STAT3 signaling that regulates SSEA3 and sialyl Lewis a synthesis in ES cells, and sialyl Lewis a is also a ES cell marker.

The Link between Gaucher Disease and Parkinson's Disease Sheds Light on Old and Novel Disorders of Sphingolipid Metabolism

Sphingolipid metabolism starts with the biosynthesis of ceramide, a bioactive lipid and the backbone for the biosynthesis of complex sphingolipids such as sphingomyelin and glycosphingolipids. These are degraded back to ceramide and then to sphingosine, which enters the ceramide–sphingosine-1-phosphate signaling pathway or is further degraded. Several enzymes with multiple catalytic properties and subcellular localizations are thus involved in such metabolism. Hereditary defects of lysosomal hydrolases have been known for several years to be the cause of lysosomal storage diseases such as gangliosidoses, Gaucher disease, Niemann–Pick disease, Krabbe disease, Fabry disease, and Farber disease. More recently, many other inborn errors of sphingolipid metabolism have been recognized, involving enzymes responsible for the biosynthesis of ceramide, sphingomyelin, and glycosphingolipids. Concurrently, epidemiologic and biochemical evidence has established a link between Gaucher disease and Parkinson’s disease, showing that glucocerebrosidase variants predispose individuals to α-synuclein accumulation and neurodegeneration even in the heterozygous status. This appears to be due not only to lysosomal overload of non-degraded glucosylceramide, but to the derangement of vesicle traffic and autophagy, including mitochondrial autophagy, triggered by both sphingolipid intermediates and misfolded proteins. In this review, old and novel disorders of sphingolipid metabolism, in particular those of ganglioside biosynthesis, are evaluated in light of recent investigations of the link between Gaucher disease and Parkinson’s disease, with the aim of better understanding their pathogenic mechanisms and addressing new potential therapeutic strategies.

Gangliosides and Tumors

Tumor-associated gangliosides play important roles in regulation of signal transduction induced by growth-factor receptors including EGFR, FGFR, HGF and PDGFR in a specific microdomain called glycosynapse in the cancer cell membranes, and in interaction with glycan recognition molecules involved in cell adhesion and immune regulation including selectins and siglecs. As the genes involved in the synthesis and degradation of tumor-associated gangliosides were identified, biological functions became clearer from the experimental results employing forced overexpression and/or knockdown/knockout of the genes. Studies on the regulatory mechanisms for their expression also achieved great advancements. Epigenetic silencing of glycan-related genes is a dominant mechanism in glycan alteration at early stages of carcinogenesis. Development of hypoxia resistance involving activation of a transcription factor HIF, and acquisition of cancer stem cell-like characteristics through epithelial-mesenchymal transition are important mechanisms for glycan modulations in the later stages of cancer progression. In the initial stages of studies, the gangliosides which specifically appear in cancers attracted attention under the name of tumor-associated gangliosides. However, it became apparent that not only the cancer-associated gangliosides but also the normal gangliosides present in nonmalignant cells and tissues perform important biological functions, and some of them tend to disappear in cancer cells resulting in the loss of the physiological functions, and this sometimes facilitates progression of cancers.

Epigenetic Bases of Aberrant Glycosylation in Cancer

In this review, the sugar portions of glycoproteins, glycolipids, and glycosaminoglycans constitute the glycome, and the genes involved in their biosynthesis, degradation, transport and recognition are referred to as “glycogenes“. The extreme complexity of the glycome requires the regulatory layer to be provided by the epigenetic mechanisms. Almost all types of cancers present glycosylation aberrations, giving rise to phenotypic changes and to the expression of tumor markers. In this review, we discuss how cancer-associated alterations of promoter methylation, histone methylation/acetylation, and miRNAs determine glycomic changes associated with the malignant phenotype. Usually, increased promoter methylation and miRNA expression induce glycogene silencing. However, treatment with demethylating agents sometimes results in silencing, rather than in a reactivation of glycogenes, suggesting the involvement of distant methylation-dependent regulatory elements. From a therapeutic perspective aimed at the normalization of the malignant glycome, it appears that miRNA targeting of cancer-deranged glycogenes can be a more specific and promising approach than the use of drugs, which broad target methylation/acetylation. A very specific type of glycosylation, the addition of GlcNAc to serine or threonine (O-GlcNAc), is not only regulated by epigenetic mechanisms, but is an epigenetic modifier of histones and transcription factors. Thus, glycosylation is both under the control of epigenetic mechanisms and is an integral part of the epigenetic code.

Selectin Ligands Sialyl-Lewis a and Sialyl-Lewis x in Gastrointestinal Cancers

The tetrasaccharide structures Siaα2,3Galβ1,3(Fucα1,4)GlcNAc and Siaα2,3Galβ1,4(Fucα1,3)GlcNAc constitute the epitopes of the carbohydrate antigens sialyl-Lewis a (sLe^a) and sialyl-Lewis x (sLe^x), respectively, and are the minimal requirement for selectin binding to their counter-receptors. Interaction of sLe^x expressed on the cell surface of leucocytes with E-selectin on endothelial cells allows their arrest and promotes their extravasation. Similarly, the rolling of cancer cells ectopically expressing the selectin ligands on endothelial cells is potentially a crucial step favoring the metastatic process. In this review, we focus on the biosynthetic steps giving rise to selectin ligand expression in cell lines and native tissues of gastrointestinal origin, trying to understand whether and how they are deregulated in cancer. We also discuss the use of such molecules in the diagnosis of gastrointestinal cancers, particularly in light of recent data questioning the ability of colon cancers to express sLe^a and the possible use of circulating sLe^x in the early detection of pancreatic cancer. Finally, we reviewed the data dealing with the mechanisms that link selectin ligand expression in gastrointestinal cells to cancer malignancy. This promising research field seems to require additional data on native patient tissues to reach more definitive conclusions.

TNF up-regulates ST3GAL4 and sialyl-Lewisx expression in lung epithelial cells through an intronic ATF2-responsive element

We have previously shown that tumor necrosis factor (TNF) induced the up-regulation of the sialyltransferase gene ST3GAL4 (α2,3-sialyltransferase gene) BX transcript through mitogen- and stress-activated kinase 1/2 (MSK1/2), extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) signaling pathways. This up-regulation resulted in sialyl-Lewis^x (sLe^x) overexpression on high-molecular-weight glycoproteins in inflamed airway epithelium and increased the adhesion of Pseudomonas aeruginosa PAO1 and PAK strains to lung epithelial cells. In the present study, we describe a TNF-responsive element in an intronic region of the ST3GAL4 gene, whose TNF-dependent activity is repressed by ERK/p38 and MSK1/2 inhibitors. This TNF-responsive element contains potential binding sites for ETS1 and ATF2 transcription factors related to TNF signaling. We also show that ATF2 is involved in TNF responsiveness, as well as in TNF-induced ST3GAL4 BX transcript and sLe^x overexpression in A549 lung epithelial cells. Moreover, we show that TNF induces the binding of ATF2 to the TNF-responsive element. Altogether, these data suggest that ATF2 could be a potential target to prevent inflammation-induced P. aeruginosa binding in the lung of patients suffering from lung diseases such as chronic bronchitis or cystic fibrosis.

Unexpected distribution of CA19.9 and other type 1 chain Lewis antigens in normal and cancer tissues of colon and pancreas: Importance of the detection method and role of glycosyltransferase regulation

BACKGROUND

CA19.9 antigen has been assumed as an abundant product of cancer cells, due to the reactivity found by immunohistochemical staining of cancer tissues with anti-CA19.9 antibody.

METHODS

Expression and biosynthesis of type 1 chain Lewis antigens in the colon and the pancreas were studied by immunodetection in tissue sections and lysates, quantification of glycosyltransferase transcripts, bisulfite sequencing, and chromatin immunoprecipitation assays.

RESULTS

CA19.9 was poorly detectable in normal colon mucosa and almost undetectable in colon cancer, while it was easily detected in the pancreatic ducts, together with Lewis b antigen, under both normal and cancer conditions. B3GALT5 transcripts were down-regulated in colon cancer, while they remained expressed in pancreatic cancer. Even ST3GAL3 transcript appeared well expressed in the pancreas but poorly in the colon, irrespective of normal or cancer conditions. CpG islands flanking B3GALT5 native promoter presented an extremely low degree of methylation in pancreatic cancer with respect to colon cancer. In a DNA region about 1kb away from the B3GALT5 retroviral promoter, a stretch of CG dinucleotides presented a methylation pattern potentially associated with transcription. Such a DNA region and the transcription factor binding site provided overlapping results by chromatin immunoprecipitation assays, corroborating the hypothesis.

CONCLUSIONS

CA19.9 appears as a physiological product whose synthesis strongly depends on the tissue specific and epigenetically-regulated expression of B3GALT5 and ST3GAL3.

GENERAL SIGNIFICANCE

CA19.9 and other Lewis antigens acquire tumor marker properties in the pancreas due to mechanisms giving rise to reabsorption into vessels and elevation in circulating levels.

Control of Glycosylation-Related Genes by DNA Methylation: the Intriguing Case of the B3GALT5 Gene and Its Distinct Promoters

Glycosylation is a metabolic pathway consisting of the enzymatic modification of proteins and lipids through the stepwise addition of sugars that gives rise to glycoconjugates. To determine the full complement of glycoconjugates that cells produce (the glycome), a variety of genes are involved, many of which are regulated by DNA methylation. The aim of the present review is to briefly describe some relevant examples of glycosylation-related genes whose DNA methylation has been implicated in their regulation and to focus on the intriguing case of a glycosyltransferase gene (B3GALT5). Aberrant promoter methylation is frequently at the basis of their modulation in cancer, but in the case of B3GALT5, at least two promoters are involved in regulation, and a complex interplay is reported to occur between transcription factors, chromatin remodelling and DNA methylation of typical CpG islands or even of other CpG dinucleotides. Transcription of the B3GALT5 gene underwent a particular evolutionary fate, so that promoter hypermethylation, acting on one transcript, and hypomethylation of other sequences, acting on the other, cooperate on one gene to obtain full cancer-associated silencing. The findings may also help in unravelling the complex origin of serum CA19.9 antigen circulating in some patients.