Signaling domains of cancer-associated glycolipids

Regulation of Glycosylation in Bone Metabolism

Glycosylation plays a crucial role in the maintenance of homeostasis in the body and at the onset of diseases such as inflammation, neurodegeneration, infection, diabetes, and cancer. It is also involved in bone metabolism. N- and O-glycans have been shown to regulate osteoblast and osteoclast differentiation. We recently demonstrated that ganglio-series and globo-series glycosphingolipids were essential for regulating the proliferation and differentiation of osteoblasts and osteoclasts in glycosyltransferase-knockout mice. Herein, we reviewed the importance of the regulation of bone metabolism by glycoconjugates, such as glycolipids and glycoproteins, including our recent results.

Globo-series Gb4 activates ERK and promotes the proliferation of osteoblasts

OBJECTIVES

Globo-series Gb4 (globoside) is involved in the immune system and disease pathogenesis. We recently reported that systemic Gb4 deficiency in mice led to decreased bone formation due to a reduction in osteoblast number. However, it remains unclear whether Gb4 expressed in osteoblasts promotes their proliferation. Therefore, we investigated the role of Gb4 in osteoblast proliferation in vitro.

METHODS

We examined osteoblast proliferation in Gb3 synthase knockout mice lacking Gb4. We investigated the effects of Gb4 synthase knockdown in the mouse osteoblast cell line MC3T3-E1 on its proliferation. Furthermore, we administered Gb4 to MC3T3-E1 cells in which Gb4 was suppressed by a glucosylceramide synthase (GCS) inhibitor and evaluated its effects on their proliferation. To elucidate the mechanisms by which Gb4 promotes osteoblast proliferation, the phosphorylated extracellular signal-regulated kinases 1 and 2 (ERK1/2) levels were measured in MC3T3-E1 cells.

RESULTS

Osteoblast proliferation was lower in Gb3 synthase knockout mice lacking Gb4 than in wild-type mice. Proliferation was inhibited by Gb4 synthase knockdown in MC3T3-E1 cells. Furthermore, the administration of Gb4 to MC3T3-E1 cells, in which a GCS inhibitor suppressed Gb4, promoted their proliferation. Moreover, it increased the phosphorylated ERK1/2 levels in MC3T3-E1 cells.

CONCLUSIONS

Our results suggest that Gb4 expressed in osteoblasts promotes their proliferation through ERK1/2 activation.

Multifunctional Proximity Labeling Strategy for Lipid Raft-Specific Sialic Acid Tracking and Engineering

Lipid raft-specific glycosylation has been implicated in many biological processes, including intracellular trafficking, cell adhesion, signal transduction, and host-pathogen interactions. The major predicament in lipid raft-specific glycosylation research is the unavailability of tools for tracking and manipulating glycans on lipid rafts at the microstructural level. To overcome this challenge, we developed a multifunctional proximity labeling (MPL) platform that relies on cholera toxin B subunit to localize horseradish peroxidase on lipid rafts. In addition to the prevailing electron-rich amino acids, modified sialic acid was included in the horseradish peroxidase-mediated proximity labeling substrate via purposefully designed chemical transformation reactions. In combination with sialic acid editing, the self-renewal of lipid raft-specific sialic acid was visualized. The MPL method enabled tracking of lipid raft dynamics under methyl-β-cyclodextrin and mevinolin treatments; in particular, the alteration of lipid rafts markedly affected cell migration. Furthermore, we embedded functional molecules into the method and implemented raft-specific sialic acid gradient engineering. Our novel strategy presents opportunities for tailoring lipid raft-specific sialic acids, thereby regulating interactions associated with lipid raft regions (such as cell-virus and cell-microenvironment interactions), and can aid in the development of lipid raft-based therapeutic regimens for tumors.

The role of lipid rafts in vesicle formation

The formation of membrane vesicles is a common feature in all eukaryotes. Lipid rafts are the best-studied example of membrane domains for both eukaryotes and prokaryotes, and their existence also is suggested in Archaea membranes. Lipid rafts are involved in the formation of transport vesicles, endocytic vesicles, exocytic vesicles, synaptic vesicles and extracellular vesicles, as well as enveloped viruses. Two mechanisms of how rafts are involved in vesicle formation have been proposed: first, that raft proteins and/or lipids located in lipid rafts associate with coat proteins that form a budding vesicle, and second, vesicle budding is triggered by enzymatic generation of cone-shaped ceramides and inverted cone-shaped lyso-phospholipids. In both cases, induction of curvature is also facilitated by the relaxation of tension in the raft domain. In this Review, we discuss the role of raft-derived vesicles in several intracellular trafficking pathways. We also highlight their role in different pathways of endocytosis, and in the formation of intraluminal vesicles (ILVs) through budding inwards from the multivesicular body (MVB) membrane, because rafts inside MVB membranes are likely to be involved in loading RNA into ILVs. Finally, we discuss the association of glycoproteins with rafts via the glycocalyx.

Extracellular vesicles released from ganglioside GD2-expressing melanoma cells enhance the malignant properties of GD2-negative melanomas

Exosomes (small extracellular vesicles: EVs) have attracted increasing attention from basic scientists and clinicians since they play important roles in cell-to-cell communication in various biological processes. Various features of EVs have been elucidated regarding their contents, generation and secretion mechanisms, and functions in inflammation, regeneration, and cancers. These vesicles are reported to contain proteins, RNAs, microRNAs, DNAs, and lipids. Although the roles of individual components have been rigorously studied, the presence and roles of glycans in EVs have rarely been reported. In particular, glycosphingolipids in EVs have not been investigated to date. In this study, the expression and function of a representative cancer-associated ganglioside, GD2, in malignant melanomas was investigated. Generally, cancer-associated gangliosides have been shown to enhance malignant properties and signals in cancers. Notably, EVs derived from GD2-expressing melanomas enhanced the malignant phenotypes of GD2-negative melanomas, such as cell growth, invasion, and cell adhesion, in a dose-dependent manner. The EVs also induced increased phosphorylation of signaling molecules such as EGF receptor and focal adhesion kinase. These results suggest that EVs released from cancer-associated ganglioside-expressing cells exert many functions that have been reported as a function of these gangliosides and regulate microenvironments, including total aggravation of heterogeneous cancer tissues, leading to more malignant and advanced cancer types.

Ganglioside GM3 prevents high fat diet-induced hepatosteatosis via attenuated insulin signaling pathway

Gangliosides, sialic acid-containing glycosphingolipids, are widely involved in regulations of signal transductions to control cellular functions. It has been suggested that GM3, the simplest structure among gangliosides, is involved in insulin resistance, whereas it remains unclear whether insulin signaling diminished by GM3 actually aggravates the pathological conditions in metabolic disorders. Moreover, the functional roles of gangliosides in the regulation of insulin signaling have not yet been fully elucidated in liver or hepatocytes despite that it is one of the major insulin-sensitive organs. To understand physiological roles of GM3 in metabolic homeostasis in liver, we conducted a high fat diet (HFD) loading experiment using double knockout (DKO) mice of GM2/GD2 synthase and GD3 synthase, which lack all gangliosides except GM3, as well as wild-type (WT) mice. DKO mice were strikingly resistant to HFD-induced hepatosteatosis, and hepatic lipogenesis-related molecules including insulin signaling components were down-regulated in HFD-fed DKO. Furthermore, we established primary hepatocyte cultures from DKO and WT mice, and examined their responses to insulin in vitro. Following insulin stimulation, DKO hepatocytes expressing GM3 showed attenuated expression and/or activations in the downstream components compared with WT hepatocytes expressing GM2. While insulin stimulation induced lipogenic proteins in hepatocytes from both genotypes, their expression levels were lower in DKO than in WT hepatocytes after insulin treatment. All our findings suggest that the modified gangliosides, i.e., a shift to GM3 from GM2, might exert a suppressive effect on lipogenesis by attenuating insulin signaling at least in mouse hepatocytes, which might result in protection of HFD-induced hepatosteatosis.

Simple separation of glycosphingolipids in the lower phase of a Folch's partition from crude lipid fractions using zirconium dioxide

A simple method was developed for the separation of glycosphingolipids (GSLs) from lipid mixtures, including phospholipids and cholesterol, using zirconium dioxide (zirconia, ZrO₂). Although this procedure does not incorporate a mild alkali treatment, which is commonly used for eliminating glycerophospholipids, it can be used to remove both alkali-resistant sphingomyelin and glycerophospholipids possessing ether bonds. Importantly, when GSLs were dissolved in organic solvent together with cholesterol (Chol) and phospholipids, and loaded onto ZrO₂, Chol did not bind to the ZrO₂ but both the GSLs and phospholipids did. When eluted with 5 mg/mL of 2,5-dihydroxybenzoic acid in methanol, GSLs but not phospholipids were recovered, leaving the phospholipids bound to the ZrO₂ particles. This method is particularly applicable for GSLs such as triglycosylceramides, tetraglycosylceramides and some pentaglycosylceramides, sulfatide and GM3 located in the lower phase of a Folch's partition, where significant amounts of phospholipids, Chol and neutral lipids reside along with GSLs. This method was successfully used to easily isolate GSLs from biological materials for their subsequent analysis by matrix-assisted laser desorption ionization time-of-flight mass spectrometry with high resolution.

Attenuation of Bone Formation through a Decrease in Osteoblasts in Mutant Mice Lacking the GM2/GD2 Synthase Gene

The ganglioside GD1a has been reported to promote the differentiation of mesenchymal stem cells to osteoblasts in cell culture systems. However, the involvement of gangliosides, including GD1a, in bone formation in vivo remains unknown; therefore, we herein investigated their roles in GM2/GD2 synthase-knockout (GM2/GD2S KO) mice without GD1a. The femoral cancellous bone mass was analyzed using three-dimensional micro-computed tomography. A histomorphometric analysis of bone using hematoxylin and eosin (HE) and tartrate-resistant acid phosphatase was performed to examine bone formation and resorption, respectively. Calcein double labeling was also conducted to evaluate bone formation. Although no significant differences were observed in bone mass or resorption between GM2/GD2S KO mice and wild-type (WT) mice, analyses of the parameters of bone formation using HE staining and calcein double labeling revealed less bone formation in GM2/GD2S KO mice than in WT mice. These results suggest that gangliosides play roles in bone formation.