The Relationship Between Lipid Peroxidation and LDL Desialylation in Experimental Atherosclerosis

The emerging role of glycans and the importance of sialylation in cardiovascular disease

Glycosylation is the process by which glycans (i.e. 'sugars') are enzymatically attached to proteins or lipids to form glycoconjugates. Growing evidence points to glycosylation playing a central role in atherosclerosis. Glycosylation occurs in all human cells and post-translationally modifies many signalling molecules that regulate cardiovascular disease, affecting their binding and function. Glycoconjugates are present in abundance on the vascular endothelium and on circulating lipoproteins, both of which have well-established roles in atherosclerotic plaque development. Sialic acid is a major regulator of glycan function and therefore the process of sialylation, in which sialic acid is added to glycans, is likely to be entwined in any regulation of atherosclerosis. Glycans and sialylation regulators have the potential to present as new biomarkers that predict atherosclerotic disease or as targets for pharmacological intervention, as well as providing insights into novel cardiovascular mechanisms. Moreover, the asialoglycoprotein receptor 1 (ASGR1), a glycan receptor, is emerging as an exciting new regulator of lipid metabolism and coronary artery disease. This review summarises the latest advances in the growing body of evidence that supports an important role for glycosylation and sialylation in the regulation of atherosclerosis.

Prospects for the Use of Sialidase Inhibitors in Anti-atherosclerotic Therapy

The most typical feature of atherogenesis in humans at its early stage is the formation of foam cells in subendothelial arterial intima, which occurs as the consequence of intracellular cholesterol deposition. The main source of lipids accumulating in the arterial wall is circulating low-density lipoprotein (LDL). However, LDL particles should undergo proatherogenic modification to acquire atherogenic properties. One of the known types of atherogenic modification of LDL is enzymatic deglycosilation, namely, desialylation, which is the earliest change in the cascade of following multiple LDL modifications. The accumulating data make sialidases an intriguing and plausible therapeutic target, since pharmacological modulation of activity of these enzymes may have beneficial effects in several pathologies, including atherosclerosis. The hypothesis exists that decreasing LDL enzymatic desialylation may result in the prevention of lipid accumulation in arterial wall, thus breaking down one of the key players in atherogenesis at the cellular level. Several drugs acting as glycomimetics and inhibiting sialidase enzymatic activity already exist, but the concept of sialidase inhibition as an anti-atherosclerosis strategy remains unexplored to date. This review is focused on the potential possibilities of the repurposing of sialidase inhibitors for pathogenetic anti-atherosclerotic therapy.

Siglec-E retards atherosclerosis by inhibiting CD36-mediated foam cell formation

BACKGROUND

The accumulation of lipid-laden macrophages, foam cells, within sub-endothelial intima is a key feature of early atherosclerosis. Siglec-E, a mouse orthologue of human Siglec-9, is a sialic acid binding lectin predominantly expressed on the surface of myeloid cells to transduce inhibitory signal via recruitment of SH2-domain containing protein tyrosine phosphatase SHP-1/2 upon binding to its sialoglycan ligands. Whether Siglec-E expression on macrophages impacts foam cell formation and atherosclerosis remains to be established.

METHODS

ApoE-deficient (apoE^-/-) and apoE/Siglec-E-double deficient (apoE^-/-/Siglec-E^-/-) mice were placed on high fat diet for 3 months and their lipid profiles and severities of atherosclerosis were assessed. Modified low-density lipoprotein (LDL) uptake and foam cell formation in wild type (WT) and Siglec-E^-/-- peritoneal macrophages were examined in vitro. Potential Siglec-E-interacting proteins were identified by proximity labeling in conjunction with proteomic analysis and confirmed by coimmunoprecipitation experiment. Impacts of Siglec-E expression and cell surface sialic acid status on oxidized LDL uptake and signaling involved were examined by biochemical assays.

RESULTS

Here we show that genetic deletion of Siglec-E accelerated atherosclerosis without affecting lipid profile in apoE^-/- mice. Siglec-E deficiency promotes foam cell formation by enhancing acetylated and oxidized LDL uptake without affecting cholesterol efflux in macrophages in vitro. By performing proximity labeling and proteomic analysis, we identified scavenger receptor CD36 as a cell surface protein interacting with Siglec-E. Further experiments performed in HEK293T cells transiently overexpressing Siglec-E and CD36 and peritoneal macrophages demonstrated that depletion of cell surface sialic acids by treatment with sialyltransferase inhibitor or sialidase did not affect interaction between Siglec-E and CD36 but retarded Siglec-E-mediated inhibition on oxidized LDL uptake. Subsequent experiments revealed that oxidized LDL induced transient Siglec-E tyrosine phosphorylation and recruitment of SHP-1 phosphatase in macrophages. VAV, a downstream effector implicated in CD36-mediated oxidized LDL uptake, was shown to interact with SHP-1 following oxidized LDL treatment. Moreover, oxidized LDL-induced VAV phosphorylation was substantially lower in WT macrophages comparing to Siglec-E^-/- counterparts.

CONCLUSIONS

These data support the protective role of Siglec-E in atherosclerosis. Mechanistically, Siglec-E interacts with CD36 to suppress downstream VAV signaling involved in modified LDL uptake.

Sesamol: a powerful functional food ingredient from sesame oil for cardioprotection

Phytophenols are important bioactive food based chemical entities, largely present in several natural sources. Among them, sesamol is one of the key natural phenols found in sesame seeds, Piper cubeba etc. Several studies have reported that sesame oil is a potent cardioprotective functional food. Papers on the utility of sesamol in sesame oil (the chemical name of sesamol is methylenedioxyphenol, MDP) have appeared in the literature, though there is no single concise review on the usefulness of sesamol in sesame oil in CVD in the literature. Cardiovascular disease (CVD) is the most challenging health problem encountered by the global population. There has been increasing interest in the growth of effective cardiovascular therapeutics, specifically of natural origin. Among various natural sources of chemicals, phytochemicals are micronutrients and bio-compatible scaffolds having an extraordinary efficacy at multiple disease targets with minimal or no adverse effect. This review offers a perspective on the existing literature on functional ingredients in sesame oil with particular focus on sesamol and its derivatives having nutritional and cardioprotective properties. This is demonstrated to have shown a specifically modulating oxidative enzyme myeloperoxidase (MPO) and other proteins which are detrimental to human well-being. The molecular mechanism of cardioprotection by this food ingredient is primarily attributed to the methylenedioxy group present in the sesamol component.

The role of sialic acids in the initiation of atherosclerosis

Atherosclerosis is a major cause of disease-related mortality around the globe. The main characteristic of the disease is an accumulation of plaque on the arterial wall and subsequent erosion or rupture of some plaques. Atherosclerosis often leads to cardiovascular disease and such acute complications as myocardial infarction or ischemic stroke due to thrombus formation. Most recent advances in atherosclerotic research state that the modifications of low-density lipoprotein (LDL) are one of the most significant stages in the disease initiation, and among these modifications desialylation is of particular interest. Sialic acids are widely expressed on all types of cells of many organisms and participate in numerous biological processes. Regarding atherosclerosis, sialidases that are responsible for the regulation of the sialic component of different molecules, are probably one of the most crucial enzymatic families. Sufficient sialylation of vascular endothelium defines its susceptibility to an atherogenic plaque formation. Moreover, the desialylation of LDL provokes an accumulation of cholesterol and lipids in the arterial walls. According to the multiple involvements of sialic acids and related enzymes, sialidases, in the initiation and development of atherosclerosis, the deeper understanding of their exact role, as well as cellular and molecular mechanisms, will allow creating more targeted and effective therapeutic and diagnostic approaches.

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.

Electronegative LDL: An Active Player in Atherogenesis or a By- Product of Atherosclerosis?

Low-density lipoproteins (LDLs) are the major plasma carriers of cholesterol. However, LDL particles must undergo various molecular modifications to promote the development of atherosclerotic lesions. Modified LDL can be generated by different mechanisms, but as a common trait, show an increased electronegative charge of the LDL particle. A subfraction of LDL with increased electronegative charge (LDL(-)), which can be isolated from blood, exhibits several pro-atherogenic characteristics. LDL(-) is heterogeneous, due to its multiple origins but is strongly related to the development of atherosclerosis. Nevertheless, the implication of LDL(-) in a broad array of pathologic conditions is complex and in some cases anti-atherogenic LDL(-) properties have been reported. In fact, several molecular modifications generating LDL(-) have been widely studied, but it remains unknown as to whether these different mechanisms are specific or common to different pathological disorders. In this review, we attempt to address these issues examining the most recent findings on the biology of LDL(-) and discussing the relationship between this LDL subfraction and the development of different diseases with increased cardiovascular risk. Finally, the review highlights the importance of minor apolipoproteins associated with LDL(-) which would play a crucial role in the different properties displayed by these modified LDL particles.

N-Acetylneuraminic acid attenuates hypercoagulation on high fat diet-induced hyperlipidemic rats

Background and objective

N-Acetylneuraminic acid (Neu5Ac), a type of sialic acid, has close links with cholesterol metabolism and is often used as a biomarker in evaluating the risk of cardiovascular diseases. However, most studies on the health implications of Neu5Ac have focused on its effects on the nervous system, while its effects on cardiovascular risk factors have largely been unreported. Thus, the effects of Neu5Ac on coagulation status in high fat diet (HFD)-induced hyperlipidemic rats were evaluated in this study.

Methods

Sprague Dawley male rats were divided into five different groups and fed with HFD alone, HFD low-dose Neu5Ac, HFD high-dose Neu5Ac, HFD simvastatin (10 mg/kg day), and normal pellet alone. Food was given ad libitum while body weight of rats was measured weekly. After 12 weeks of intervention, rats were sacrificed and serum and tissue samples were collected for biochemistry and gene expression analysis, respectively.

Results

The results showed that Neu5Ac could improve lipid metabolism and hyperlipidemia-associated coagulation. Neu5Ac exerted comparable or sometimes better physiological effects than simvastatin, at biochemical and gene expression levels.

Conclusions

The data indicated that Neu5Ac prevented HFD-induced hyperlipidemia and associated hypercoagulation in rats through regulation of lipid-related and coagulation-related genes and, by extension, induced metabolite and protein changes. The implications of the present findings are that Neu5Ac may be used to prevent coagulation-related cardiovascular events in hyperlipidemic conditions. These findings are worth studying further.