Overexpression of LOXIN Protects Endothelial Progenitor Cells From Apoptosis Induced by Oxidized Low Density Lipoprotein

LOX-1 variants modulate the severity of cardiovascular disease: state of the art and future directions

Atherosclerosis is one of the major causes of cerebral infarction and many other ischemic cardio-cerebrovascular diseases. Although large randomized clinical trials have highlighted the impressive benefits of lipid-lowering therapies, the 50-70% of patients who have achieved their lipid-lowering goal remain at high cardiovascular disease risk. For this reason, there is a need to investigate other markers of atherosclerosis progression. LOX-1 is a scavenger receptor that accepts oxidized low-density lipoproteins as major ligand and internalizes it by endocytosis favoring its retention in subendothelial layer and triggering a wide variety of proatherogenic events. However, other factors such as cytokines, shear stress, and advanced glycation end-products can upregulate LOX-1. LOX-1 is encoded by the OLR1 gene, located in the p12.3-p13 region of chromosome 12. OLR1 gene has different isoforms induced by splicing, or single-nucleotide polymorphisms (SNPs). According to some authors, the expression of these isoforms induces a different effect on atherosclerosis and cardiovascular disease. In particular, LOXIN, an isoform lacking part of the functional domain, exerts an important role in atherosclerosis protection. In other cases, studies on SNPs showed an association with more severe forms, like in the case of 3'UTR polymorphisms. The knowledge of these variants can give rise to the development of new preventive therapies and can lead to the identification of subjects at greater risk of cardiovascular event. In this review, we reported the state of the art regarding SNPs with known effects on OLR1 splicing and how LOX-1 variants modulate the severity of cardiovascular disease.

Loxin Reduced the Inflammatory Response in the Liver and the Aortic Fatty Streak Formation in Mice Fed with a High-Fat Diet

Oxidized low-density lipoprotein (ox-LDL) is the most harmful form of cholesterol associated with vascular atherosclerosis and hepatic injury, mainly due to inflammatory cell infiltration and subsequent severe tissue injury. Lox-1 is the central ox-LDL receptor expressed in endothelial and immune cells, its activation regulating inflammatory cytokines and chemotactic factor secretion. Recently, a Lox-1 truncated protein isoform lacking the ox-LDL binding domain named LOXIN has been described. We have previously shown that LOXIN overexpression blocked Lox-1-mediated ox-LDL internalization in human endothelial progenitor cells in vitro. However, the functional role of LOXIN in targeting inflammation or tissue injury in vivo remains unknown. In this study, we investigate whether LOXIN modulated the expression of Lox-1 and reduced the inflammatory response in a high-fat-diet mice model. Results indicate that human LOXIN blocks Lox-1 mediated uptake of ox-LDL in H4-II-E-C3 cells. Furthermore, in vivo experiments showed that overexpression of LOXIN reduced both fatty streak lesions in the aorta and inflammation and fibrosis in the liver. These findings were associated with the down-regulation of Lox-1 in endothelial cells. Then, LOXIN prevents hepatic and aortic tissue damage in vivo associated with reduced Lox-1 expression in endothelial cells. We encourage future research to understand better the underlying molecular mechanisms and potential therapeutic use of LOXIN.

Accumulation of LDL/ox-LDL in the necrotic region participates in osteonecrosis of the femoral head: a pathological and in vitro study

Background

Osteonecrosis of the femoral head (ONFH) is a common but intractable disease that appears to involve lipid metabolic disorders. Although numerous studies have demonstrated that high blood levels of low-density lipoprotein (LDL) are closely associated with ONFH, there is limited evidence to explain the pathological role of LDL. Pathological and in vitro studies were performed to investigate the role of disordered metabolism of LDL and oxidized LDL (ox-LDL) in the femoral head in the pathology of ONFH.

Methods

Nineteen femoral head specimens from patients with ONFH were obtained for immunohistochemistry analysis. Murine long-bone osteocyte Y4 cells were used to study the effects of LDL/ox-LDL on cell viability, apoptosis, and metabolism process of LDL/ox-LDL in osteocytes in normoxic and hypoxic environments.

Results

In the pathological specimens, marked accumulation of LDL/ox-LDL was observed in osteocytes/lacunae of necrotic regions compared with healthy regions. In vitro studies showed that ox-LDL, rather than LDL, reduced the viability and enhanced apoptosis of osteocytes. Pathological sections indicated that the accumulation of ox-LDL was significantly associated with impaired blood supply. Exposure to a hypoxic environment appeared to be a key factor leading to LDL/ox-LDL accumulation by enhancing internalisation and oxidation of LDL in osteocytes.

Conclusions

The accumulation of LDL/ox-LDL in the necrotic region may contribute to the pathology of ONFH. These findings could provide new insights into the prevention and treatment of ONFH.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-021-01601-x.

Alternative Splicing in Cardiovascular Disease-A Survey of Recent Findings

Alternative splicing, a driver of posttranscriptional variance, differs from canonical splicing by arranging the introns and exons of an immature pre-mRNA transcript in a multitude of different ways. Although alternative splicing was discovered almost half a century ago, estimates of the proportion of genes that undergo alternative splicing have risen drastically over the last two decades. Deep sequencing methods and novel bioinformatic algorithms have led to new insights into the prevalence of spliced variants, tissue-specific splicing patterns and the significance of alternative splicing in development and disease. Thus far, the role of alternative splicing has been uncovered in areas ranging from heart development, the response to myocardial infarction to cardiac structural disease. Circular RNAs, a product of alternative back-splicing, were initially discovered in 1976, but landmark publications have only recently identified their regulatory role, tissue-specific expression, and transcriptomic abundance, spurring a renewed interest in the topic. The aim of this review is to provide a brief insight into some of the available findings on the role of alternative splicing in cardiovascular disease, with a focus on atherosclerosis, myocardial infarction, heart failure, dilated cardiomyopathy and circular RNAs in myocardial infarction.

Association of OLR1 gene polymorphisms with the risk of coronary artery disease: A systematic review and meta-analysis

BACKGROUND

Oxidized LDL receptor 1 (OLR1) encodes LOX-1, LOXIN, and OLR1D4 transcript variants. Up-regulation of LOX-1 and down-regulation of LOXIN have an essential role in causing coronary artery disease (CAD). Discovery of risk single nucleotide polymorphisms (SNPs) in OLR1 gene is clinically important as these polymorphisms could be candidate biomarkers of CAD.

OBJECTIVES

The purpose of this study is quantitative evidence synthesis on how OLR1 polymorphisms in the haplotype block impact the risk of CAD.

METHODS

First, a systematic keyword-based search in PubMed, Web of Science, and Scopus was conducted. After data extraction, pooled odds ratios (ORs) and 95% confidence intervals (CIs) were calculated for OLR1 polymorphisms and CAD. Twelve case-control studies, including 6,238 cases and 15,773 controls, were concluded in the meta-analysis.

RESULTS

Our findings demonstrate significant association of OLR1 polymorphisms in the haplotype block with CAD risk in all genetic models (allelic model: OR = 1.19, 95%CI = 1.06-1.34; additive model: OR = 1.54, 95%CI = 1.16-2.05; recessive model: OR = 1.26, 95%CI = 1.04-1.53; dominant model: OR = 1.28, 95%CI = 1.09-1.51). Subgroup analysis based on the type of polymorphism revealed that rs1050283 (3'UTR*188 C > T) and rs3736235 (IVS4-14 A > G) are more significantly associated with the risk of CAD compared to other polymorphisms in the haplotype block.

CONCLUSIONS

We found a significant association between OLR1 polymorphisms in the haplotype block, especially rs1050283 and rs3736235, with CAD. We also suggest that precise determination of disease association with polymorphisms in a haplotype requires investigation of all SNPs rather than a single SNP in that specific haplotype.

Proteolytic Regulation of the Lectin-Like Oxidized Lipoprotein Receptor LOX-1

The lectin-like oxidized-LDL (oxLDL) receptor LOX-1, which is broadly expressed in vascular cells, represents a key mediator of endothelial activation and dysfunction in atherosclerotic plaque development. Being a member of the C-type lectin receptor family, LOX-1 can bind different ligands, with oxLDL being the best characterized. LOX-1 mediates oxLDL uptake into vascular cells and by this means can promote foam cell formation. In addition, LOX-1 triggers multiple signaling pathways, which ultimately induce a pro-atherogenic and pro-fibrotic transcriptional program. However, the molecular mechanisms underlying this signal transduction remain incompletely understood. In this regard, proteolysis has recently emerged as a regulatory mechanism of LOX-1 function. Different proteolytic cleavages within the LOX-1 protein can initiate its turnover and control the cellular levels of this receptor. Thereby, cleavage products with individual biological functions and/or medical significance are produced. Ectodomain shedding leads to the release of a soluble form of the receptor (sLOX1) which has been suggested to have diagnostic potential as a biomarker. Removal of the ectodomain leaves behind a membrane-bound N-terminal fragment (NTF), which despite being devoid of the ligand-binding domain is actively involved in signal transduction. Degradation of this LOX-1 NTF, which represents an athero-protective mechanism, critically depends on the aspartyl intramembrane proteases Signal peptide peptidase-like 2a and b (SPPL2a/b). Here, we present an overview of the biology of LOX-1 focusing on how proteolytic cleavages directly modulate the function of this receptor and, what kind of pathophysiological implications this has in cardiovascular disease.

Role of LOX-1 (Lectin-Like Oxidized Low-Density Lipoprotein Receptor 1) as a Cardiovascular Risk Predictor: Mechanistic Insight and Potential Clinical Use

Atherosclerosis, the underlying cause of cardiovascular disease (CVD), is a worldwide cause of morbidity and mortality. Reducing ApoB-containing lipoproteins-chiefly, LDL (low-density lipoprotein)-has been the main strategy for reducing CVD risk. Although supported by large randomized clinical trials, the persistence of residual cardiovascular risk after effective LDL reduction has sparked an intense search for other novel CVD biomarkers and therapeutic targets. Recently, Lox-1 (lectin-type oxidized LDL receptor 1), an innate immune scavenger receptor, has emerged as a promising target for early diagnosis and cardiovascular risk prediction and is also being considered as a treatment target. Lox-1 was first described as a 50 kDa transmembrane protein in endothelial cells responsible for oxLDL (oxidized LDL) recognition, triggering downstream pathways that intensify atherosclerosis via endothelial dysfunction, oxLDL uptake, and apoptosis. Lox-1 is also expressed in platelets, where it enhances platelet activation, adhesion to endothelial cells, and ADP-mediated aggregation, thereby favoring thrombus formation. Lox-1 was also identified in cardiomyocytes, where it was implicated in the development of cardiac fibrosis and myocyte apoptosis, the main determinants of cardiac recovery following an ischemic insult. Together, these findings have revealed that Lox-1 is implicated in all the main steps of atherosclerosis and has encouraged the development of immunoassays for measurement of sLox-1 (serum levels of soluble Lox-1) to be used as a potential CVD biomarker. Finally, the recent development of synthetic Lox-1 inhibitors and neutralizing antibodies with promising results in animal models has made Lox-1 a target for drug development. In this review, we discuss the main findings regarding the role of Lox-1 in the development, diagnosis, and therapeutic strategies for CVD prevention and treatment.

LOX-1 and atherosclerotic-related diseases

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), a scavenger receptor of oxidized low-density lipoprotein (ox-LDL) found in various cells, plays a crucial role in the formation and progression of atherosclerotic plaques. Animal studies have suggested that LOX-1 mediates the balance between internalization and degeneration of endothelial cells, thereby contributing to various steps in the atherosclerotic process, from initiation to plaque rupture. Under pathological conditions, the extracellular domain of membrane bound LOX-1 can be largely proteolytically cleaved into a soluble form (sLOX-1), which is proportional and linked to the LOX-1 expression level. Circulating levels of sLOX-1 are regarded as a risk biomarker for plaque rupture and acute coronary syndrome (ACS). Recently, studies have shown that sLOX-1 is also elevated in patients with acute stroke and can be a predictive biomarker for acute stroke. With the discovery of the vital role of LOX-1 in atherosclerosis, there is growing focus on the influence of LOX-1 in atherosclerotic-related diseases, including coronary arterial disease(CAD), stroke, and other cardiovascular events. Genetic polymorphisms of LOX-1 have been investigated and have been found to modulate the risk of these diseases. Most polymorphisms have been found to be risk factors, except for the splicing isoform LOXIN. This review concludes with a discussion of the potential future applications of LOX-1 for atherosclerotic-related diseases.

Galectin-3 exacerbates ox-LDL-mediated endothelial injury by inducing inflammation via integrin β1-RhoA-JNK signaling activation

Oxidized low‐density lipoprotein (Ox‐LDL)‐induced endothelial cell injury plays a crucial role in the pathogenesis of atherosclerosis (AS). Plasma galectin‐3 (Gal‐3) is elevated inside and drives diverse systemic inflammatory disorders, including cardiovascular diseases. However, the exact role of Gal‐3 in ox‐LDL‐mediated endothelial injury remains unclear. This study explores the effects of Gal‐3 on ox‐LDL‐induced endothelial dysfunction and the underlying molecular mechanisms. In this study, Gal‐3, integrin β1, and GTP‐RhoA in the blood and plaques of AS patients were examined by ELISA and western blot respectively. Their levels were found to be obviously upregulated compared with non‐AS control group. CCK8 assay and flow cytometry analysis showed that Gal‐3 significantly decreased cell viability and promoted apoptosis in ox‐LDL‐treated human umbilical vascular endothelial cells (HUVECs). The upregulation of integrinβ1, GTP‐RhoA, p‐JNK, p‐p65, p‐IKKα, and p‐IKKβ induced by ox‐LDL was further enhanced by treatment with Gal‐3. Pretreatment with Gal‐3 increased expression of inflammatory factors (interleukin [IL]‐6, IL‐8, and IL‐1β), chemokines(CXCL‐1 and CCL‐2) and adhesion molecules (VCAM‐1 and ICAM‐1). Furthermore, the promotional effects of Gal‐3 on NF‐κB activation and inflammatory factors in ox‐LDL‐treated HUVECs were reversed by the treatments with integrinβ1‐siRNA or the JNK inhibitor. We also found that integrinβ1‐siRNA decreased the protein expression of GTP‐RhoA and p‐JNK, while RhoA inhibitor partially reduced the upregulated expression of p‐JNK induced by Gal‐3. In conclusion, our finding suggests that Gal‐3 exacerbates ox‐LDL‐mediated endothelial injury by inducing inflammation via integrin β1‐RhoA‐JNK signaling activation.

Targeting LOX-1 in atherosclerosis and vasculopathy: current knowledge and future perspectives

LOX-1 (lectin-like oxidized low-density lipoprotein receptor-1; also known as OLR1) is the dominant receptor that recognizes and internalizes oxidized low-density lipoproteins (ox-LDLs) in endothelial cells. Several genetic variants of LOX-1 are associated with the risk and severity of coronary artery disease. The LOX-1-ox-LDL interaction induces endothelial dysfunction, leukocyte adhesion, macrophage-derived foam cell formation, smooth muscle cell proliferation and migration, and platelet activation. LOX-1 activation eventually leads to the rupture of atherosclerotic plaques and acute cardiovascular events. In addition, LOX-1 can be cleaved to generate soluble LOX-1 (sLOX-1), which is a useful diagnostic and prognostic marker for atherosclerosis-related diseases in human patients. Of therapeutic relevance, several natural products and clinically used drugs have emerged as LOX-1 inhibitors that have antiatherosclerotic actions. We hereby provide an updated overview of role of LOX-1 in atherosclerosis and associated vascular diseases, with an aim to highlighting the potential of LOX-1 as a novel theranostic tool for cardiovascular disease prevention and treatment.

Platelet-rich Plasma Protects HUVECs against oX-LDL-induced Injury

Platelet-rich plasma (PRP) contains a variety of cytokines, some of which ameliorate oX-LDL (oxidized low-density lipoprotein)-induced endothelial cell (EC) injury. Therefore, we hypothesized that PRP might alleviate oX-LDL-induced injury.

METHODOLOGY

Human umbilical vein endothelial cells (HUVECs) were divided into four groups: a PPP (platelet-poor plasma) group, an oX-LDL group, an oX-LDL+PRP group and a PRP group. CCK-8 (Cell Counting Kit) assay, Annexin V-FITC/7-AAD and Hochest 33342 staining were performed to assess cell proliferation and apoptosis. Tube formation and cell migration assays were performed to evaluate HUVEC-mediated vasculogenesis and migration. Expression levels of Bcl-2, Bax, caspase-3, cleaved caspase-3, PI3K, Akt, eNOS p-Akt, p-eNOS, IL-6 and IL-1 were detected by western blotting or immunofluorescence.

PRINCIPAL FINDINGS

PRP promoted HUVEC proliferation in a non-linear pattern, protected HUVECs against oX-LDL-induced apoptosis and attenuated oX-LDL-mediated inhibition of HUVEC migration and vasculogenesis. Additionally, compared to the PPP group, PRP downregulated pro-apoptotic proteins (ratio of Bax/Bcl-2, caspase-3 and cleaved caspase-3) as well as IL-6 and IL-1. Moreover, the PI3K/Akt/eNOS pathway was activated by PRP and inactivated by oX-LDL.

CONCLUSIONS

It was demonstrated that PRP protected HUVECs against oX-LDL-induced injury and that the PI3K/Akt/eNOS pathway was activated in this process.

The anti-proliferative effects of oleanolic acid on A7r5 cells-Role of UCP2 and downstream FGF-2/p53/TSP-1

Vascular smooth muscle cell (VSMC) proliferation is a major contributor to atherosclerosis. This study investigated the inhibitory effects of oleanolic acid (OA) against oxidized low-density lipoprotein (ox-LDL)-induced VSMC proliferation in A7r5 cells and explored underlying molecular mechanism. The cell proliferation was quantified with cell counting kit-8 (CCK-8), in which ox-LDL significantly increased A7r5 cells proliferation, while OA pretreatment effectively alleviated such changes without inducing overt cytotoxicity, as indicated by lactate dehydrogenase (LDH) assay. Quantitative real-time RT-PCR (qRT-PCR) and Western blotting revealed increased UCP2 and FGF-2 expression levels as well as decreased p53 and TSP-1 expression levels in A7r5 cells following ox-LDL exposure, while OA pretreatment reversed such changes. Furthermore, inhibiting UCP2 with genipin remarkably reversed the changes in the expression levels of FGF-2, p53, and TSP-1 induced by ox-LDL exposure; silencing FGF-2 with siRNA did not significantly change the expression levels of UCP2 but effectively reversed the changes in the expression levels of p53 and TSP-1, and activation of p53 with PRIMA-1 only significantly affected the changes in the expression levels of TSP-1, but not in UCP2 or FGF-2, suggesting a UCP-2/FGF-2/p53/TSP-1 signaling in A7r5 cells response to ox-LDL exposure. Additionally, co-treatment of OA and genipin exhibited similar effects to the expression levels of UCP2, FGF-2, p53, and TSP-1 as OA or genipin solo treatment in ox-LDL-exposed A7r5 cells, suggesting the involvement of UCP-2/FGF-2/p53/TSP-1 in the mechanism of OA. In conclusion, OA inhibits ox-LDL-induced VSMC proliferation in A7r5 cells, the mechanism involves the changes in UCP-2/FGF-2/p53/TSP-1.

LOX-1 and Its Splice Variants: A New Challenge for Atherosclerosis and Cancer-Targeted Therapies

Alternative splicing (AS) is a process in which precursor messenger RNA (pre-mRNA) splicing sites are differentially selected to diversify the protein isoform population. Changes in AS patterns have an essential role in normal development, differentiation and response to physiological stimuli. It is documented that AS can generate both “risk” and “protective” splice variants that can contribute to the pathogenesis of several diseases including atherosclerosis. The main endothelial receptor for oxidized low-density lipoprotein (ox-LDLs) is LOX-1 receptor protein encoded by the OLR1 gene. When OLR1 undergoes AS events, it generates three variants: OLR1, OLR1D4 and LOXIN. The latter lacks exon 5 and two-thirds of the functional domain. Literature data demonstrate a protective role of LOXIN in pathologies correlated with LOX-1 overexpression such as atherosclerosis and tumors. In this review, we summarize recent developments in understanding of OLR1 AS while also highlighting data warranting further investigation of this process as a novel therapeutic target.

Hypercholesterolemia Tunes Hematopoietic Stem/Progenitor Cells for Inflammation and Atherosclerosis

As the pathological basis of cardiovascular disease (CVD), atherosclerosis is featured as a chronic inflammation. Hypercholesterolemia is an independent risk factor for CVD. Accumulated studies have shown that hypercholesterolemia is associated with myeloid cell expansion, which stimulates innate and adaptive immune responses, strengthens inflammation, and accelerates atherosclerosis progression. Hematopoietic stem/progenitor cells (HSPC) in bone marrow (BM) expresses a panel of lipoprotein receptors to control cholesterol homeostasis. Deficiency of these receptors abrogates cellular cholesterol efflux, resulting in HSPC proliferation and differentiation in hypercholesterolemic mice. Reduction of the cholesterol level in the lipid rafts by infusion of reconstituted high-density lipoprotein (HDL) or its major apolipoprotein, apoA-I, reverses hypercholesterolemia-induced HSPC expansion. Apart from impaired cholesterol metabolism, inhibition of reactive oxygen species production suppresses HSPC activation and leukocytosis. These data indicate that the mechanisms underlying the effects of hypercholesterolemia on HSPC proliferation and differentiation could be multifaceted. Furthermore, dyslipidemia also regulates HSPC-neighboring cells, resulting in HSPC mobilization. In the article, we review how hypercholesterolemia evokes HSPC activation and mobilization directly or via its modification of BM microenvironment. We hope this review will bring light to finding key molecules to control HSPC expansion, inflammation, and atherosclerosis for the treatment of CVD.

Roles of LOX-1 in microvascular dysfunction

Studies from human and animal models with metabolic disease and hypertension highlight atrophic remodeling, reduced lumen size and thinner vascular walls of microvessels with profound density reduction. This impaired vascular response limits the perfusion of peripheral tissues inducing organ damage. These conditions are strongly associated with oxidative stress and in particular with the up-regulation of lectin-like oxidized low density lipoprotein receptor-1 (LOX-1). Several factors such as cytokines, shear stress, and advanced glycation end-products, especially oxLDL, can up-regulate LOX-1. The activation of this receptor induces the production of adhesion molecules, cytokines and the release of reactive oxygen species via NADPH oxidase. LOX-1 is considered a potent mediator of endothelial dysfunction and it is significantly associated with reduced microvascular endothelium NO-dependent vasodilation in hypercholesterolemia and hypertension. Microvascular endothelial cells increased the expression of IL-6 in association with the increased concentration of LDL and its degree of oxidation. Moreover, increased IL-6 levels are associated with up-regulation of LOX-1 in a dose-dependent manner. Another consequence of microvascular inflammation is the generation of small amounts of ROS, similar to those induced by low concentration of oxLDL (<5 μg/mL) which induces capillary tube formation of endothelial cells, through LOX-1 up-regulation. In light of its central role, LOX-1 represents an attractive therapeutic target for the treatment of human atherosclerotic diseases and microvascular disorders.