Sialyltransferase activity and atherosclerosis

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

Desialylation of ATG5 by sialidase (NEU1) promotes macrophages autophagy and exacerbates inflammation under hypoxia

During the process of atherosclerosis (AS), hypoxia induces plaque macrophage inflammation, promoting lipid accumulation. Autophagy is a cell homeostasis process that increases tolerance to stressors like oxidative stress and hypoxia. However, the specific mechanism by which hypoxia initiates autophagy and the inflammation of macrophages remains to be elucidated. Here, we found that hypoxia-induced macrophage inflammation was mediated by autophagy. Then, the effect of hypoxia on autophagy was investigated in terms of post-translational modifications of proteins. The results showed that desialylation of the autophagy protein ATG5 under hypoxic conditions enhanced protein stability by affecting its charge effect and promoted the formation of the ATG5-ATG12-ATG16L complex, further increasing autophagosome formation. And NEU1, a key enzyme in sialic acid metabolism, was significantly up-regulated under hypoxic conditions and was identified as an interacting protein of ATG5, affecting the sialylation of ATG5. In addition, the knockdown or inhibition of NEU1 reversed hypoxia-induced autophagy and inflammatory responses. In conclusion, our data reveal a key mechanism of autophagy regulation under hypoxia involving ATG5 sialylation and NEU1, suggesting that NEU1 may be a potential target for the prevention and treatment of atherosclerosis.

Sialic acids promote macrophage M1 polarization and atherosclerosis by upregulating ROS and autophagy blockage

Accumulating evidence suggests that sialic acids is closely related to atherosclerosis. However, the effects and underlying mechanisms of sialic acids in atherosclerosis have been not defined. Macrophages are one of the most important cells during plaque progression. In this study, we investigated the role of sialic acids in the M1 macrophage polarization and pathogenesis of atherosclerosis. Here we found that sialic acids can promote the polarization of RAW264.7 cells to the M1 phenotype, thereby promoting the expression of proinflammatory cytokines in vitro. The proinflammatory effect of sialic acids may result from the inhibition of LKB1-AMPK-Sirt3 signaling pathway to upregulate intracellular ROS and impairing autophagy-lysosome system to block autophagic flux. In the APOE^-/- mice, sialic acids in plasma increased during the development of atherosclerosis. Moreover, exogenous supplement of sialic acids can promote plaque progression in aortic arch and aortic sinus being accompanied by the differentiation of macrophages into M1 type in peripheral tissues. These studies demonstrated that sialic acids can promote macrophage polarization toward the M1 phenotype to accentuate atherosclerosis via inducing mitochondrial ROS and blocking autophagy, thus providing clue to a novel therapeutic strategy for atherosclerosis.

Lipoprotein sialylation in atherosclerosis: Lessons from mice

Sialylation is a dynamically regulated modification, which commonly occurs at the terminal of glycan chains in glycoproteins and glycolipids in eukaryotic cells. Sialylation plays a key role in a wide array of biological processes through the regulation of protein-protein interactions, intracellular localization, vesicular trafficking, and signal transduction. A majority of the proteins involved in lipoprotein metabolism and atherogenesis, such as apolipoproteins and lipoprotein receptors, are sialylated in their glycan structures. Earlier studies in humans and in preclinical models found a positive correlation between low sialylation of lipoproteins and atherosclerosis. More recent works using loss- and gain-of-function approaches in mice have revealed molecular and cellular mechanisms by which protein sialylation modulates causally the process of atherosclerosis. The purpose of this concise review is to summarize these findings in mouse models and to provide mechanistic insights into lipoprotein sialylation and atherosclerosis.

Sialic acid metabolism as a potential therapeutic target of atherosclerosis

Sialic acid (Sia), the acylated derivative of the nine-carbon sugar neuraminic acid, is a terminal component of the oligosaccharide chains of many glycoproteins and glycolipids. In light of its important biological and pathological functions, the relationship between Sia and coronary artery disease (CAD) has been drawing great attentions recently. Large-scale epidemiological surveys have uncovered a positive correlation between plasma total Sia and CAD risk. Further research demonstrated that N-Acetyl-Neuraminic Acid, acting as a signaling molecule, triggered myocardial injury via activation of Rho/ROCK-JNK/ERK signaling pathway both in vitro and in vivo. Moreover, there were some evidences showing that the aberrant sialylation of low-density lipoprotein, low-density lipoprotein receptor and blood cells was involved in the pathological process of atherosclerosis. Significantly, the Sia regulates immune response by binding to sialic acid-binding immunoglobulin-like lectin (Siglecs). The Sia-Siglecs axis is involved in the immune inflammation of atherosclerosis. The generation of Sia and sialylation of glycoconjugate both depend on many enzymes, such as sialidase, sialyltransferase and trans-sialidase. Abnormal activation or level of these enzymes associated with atherosclerosis, and inhibitors of them might be new CAD treatments. In this review, we focus on summarizing current understanding of Sia metabolism and of its relevance to atherosclerosis.

Recombinant Sialyltransferase Infusion Mitigates Infection-Driven Acute Lung Inflammation

Inappropriate inflammation exacerbates a vast array of chronic and acute conditions with severe health risks. In certain situations, such as acute sepsis, traditional therapies may be inadequate in preventing severe organ damage or death. We have previously shown cell surface glycan modification by the circulating sialyltransferase ST6Gal-1 regulates de novo inflammatory cell production via a novel extrinsic glycosylation pathway. Here, we show that therapeutic administration of recombinant, bioactive ST6Gal-1 (rST6G) mitigates acute inflammation in a murine model mimicking acute exacerbations experienced by patients with chronic obstructive pulmonary disease (COPD). In addition to suppressing proximal neutrophil recruitment at onset of infection-mediated inflammation, rST6G also muted local cytokine production. Histologically, exposure with NTHI, a bacterium associated with COPD exacerbations, in rST6G-treated animals revealed consistent and pronounced reduction of pulmonary inflammation, characterized by smaller inflammatory cuffs around bronchovascular bundles, and fewer inflammatory cells within alveolar walls, alveolar spaces, and on pleural surfaces. Taken together, the data advance the idea that manipulating circulatory ST6Gal-1 levels has potential in managing inflammatory conditions by leveraging the combined approaches of controlling new inflammatory cell production and dampening the inflammation mediator cascade.

The blood-borne sialyltransferase ST6Gal-1 is a negative systemic regulator of granulopoiesis

Responding to systemic demands in producing and replenishing end-effector blood cells is predicated on the appropriate delivery and interpretation of extrinsic signals to the HSPCs. The data presented herein implicate the systemic, extracellular form of the glycosyltransferase ST6Gal-1 in the regulation of late-stage neutrophil development. ST6Gal-1 is typically a membrane-bound enzyme sequestered within the intracellular secretory apparatus, but an extracellular form is released into the blood from the liver. Both human and murine HSPCs, upon exposure to extracellular ST6Gal-1 ex vivo, exhibited decreased proliferation, diminished expression of the neutrophilic primary granule protein MPO, and decreased appearance of CD11b+ cells. HSPC suppression was preceded by decreased STAT-3 phosphorylation and diminished C/EBPα expression, without increased apoptosis, indicating attenuated G-CSF receptor signaling. A murine model to raise systemic ST6Gal-1 level was developed to examine the role of the circulatory enzyme in vivo. Our results show that systemic ST6Gal-1 modified the cell surface of the GMP subset of HSPCs and decreased marrow neutrophil reserves. Acute airway neutrophilic inflammation by LPS challenge was used to drive demand for new neutrophil production. Reduced neutrophil infiltration into the airway was observed in mice with elevated circulatory ST6Gal-1 levels. The blunted transition of GMPs into GPs in vitro is consistent with ST6Gal-1-attenuated granulopoiesis. The data confirm that circulatory ST6Gal-1 is a negative systemic regulator of granulopoiesis and moreover suggest a clinical potential to limit the number of inflammatory cells by manipulating blood ST6Gal-1 levels.

Birth Cohort, Age, and Sex Strongly Modulate Effects of Lipid Risk Alleles Identified in Genome-Wide Association Studies

Insights into genetic origin of diseases and related traits could substantially impact strategies for improving human health. The results of genome-wide association studies (GWAS) are often positioned as discoveries of unconditional risk alleles of complex health traits. We re-analyzed the associations of single nucleotide polymorphisms (SNPs) associated with total cholesterol (TC) in a large-scale GWAS meta-analysis. We focused on three generations of genotyped participants of the Framingham Heart Study (FHS). We show that the effects of all ten directly-genotyped SNPs were clustered in different FHS generations and/or birth cohorts in a sex-specific or sex-unspecific manner. The sample size and procedure-therapeutic issues play, at most, a minor role in this clustering. An important result was clustering of significant associations with the strongest effects in the youngest, or 3rd Generation, cohort. These results imply that an assumption of unconditional connections of these SNPs with TC is generally implausible and that a demographic perspective can substantially improve GWAS efficiency. The analyses of genetic effects in age-matched samples suggest a role of environmental and age-related mechanisms in the associations of different SNPs with TC. Analysis of the literature supports systemic roles for genes for these SNPs beyond those related to lipid metabolism. Our analyses reveal strong antagonistic effects of rs2479409 (the PCSK9 gene) that cautions strategies aimed at targeting this gene in the next generation of lipid drugs. Our results suggest that standard GWAS strategies need to be advanced in order to appropriately address the problem of genetic susceptibility to complex traits that is imperative for translation to health care.