Endocytosis of apolipoprotein A-V by members of the low density lipoprotein receptor and the VPS10p domain receptor families

SORL1 is a receptor for tau that promotes tau seeding

Sortilin-related receptor 1 (SORL1) is an intracellular sorting receptor genetically implicated in Alzheimer's disease (AD) that impacts amyloid precursor protein trafficking. The objective of these studies was to test the hypothesis that SORL1 binds tau, modulates its cellular trafficking and impacts the aggregation of cytoplasmic tau induced by pathological forms of tau. Using surface plasmon resonance measurements, we observed high-affinity binding of tau to SORL1 and the vacuolar protein sorting 10 domain of SORL1. Interestingly, unlike LDL receptor-related protein 1, SORL1 binds tau at both pH 7.4 and pH 5.5, revealing its ability to bind tau at endosomal pH. Immunofluorescence studies confirmed that exogenously added tau colocalized with SORL1 in H4 neuroglioma cells, while overexpression of SORL1 in LDL receptor-related protein 1-deficient Chinese hamster ovary (CHO) cells resulted in a marked increase in the internalization of tau, indicating that SORL1 can bind and mediate the internalization of monomeric forms of tau. We further demonstrated that SORL1 mediates tau seeding when tau RD P301S FRET biosensor cells expressing SORL1 were incubated with high molecular weight forms of tau isolated from the brains of patients with AD. Seeding in H4 neuroglioma cells is significantly reduced when SORL1 is knocked down with siRNA. Finally, we demonstrate that the N1358S mutant of SORL1 significantly increases tau seeding when compared to WT SORL1, identifying for the first time a potential mechanism that connects this specific SORL1 mutation to Alzheimer's disease. Together, these studies identify SORL1 as a receptor that contributes to trafficking and seeding of pathogenic tau.

Diagnostic potential of SORT1 gene in coronary artery disease

BACKGROUND

Genome-wide association studies identify SORT1 gene associated with risk of coronary artery disease (CAD). Sortilin protein enhances LDL absorption, form cell development, and atherosclerosis in macrophages.

AIM

We therefore explored SORT1 expression in CAD patients and its gene expression's predictive usefulness for the severity of the disease.

METHODOLOGY

This is a case control study and Quantitative real-time PCR; Sandwich ELISA and western blotting were used to determine the expression of SORT1 gene at the mRNA and protein level in two hundred healthy controls and two hundred patients with various CAD syndromes.

RESULTS

CAD patients exhibit higher SORT1 gene expression in CAD patients, a higher concentration of sortilin in their plasma, and distinct expression patterns in various CAD syndromes. The study reveals a positive correlation between gene expression and the severity of coronary artery stenosis, the number of diseased vessels, and the presence of diabetes. ROC curve analysis of SORT1 gene expression both at mRNA and protein level showed strong discrimination between significant CAD and control subjects.

CONCLUSION

Therefore, elevated SORT1 gene expression in various CAD syndromes may be a potential biomarker for the disease.

Targeting the vivid facets of apolipoproteins as a cardiovascular risk factor in rheumatoid arthritis

Mostly, cardiovascular diseases are blamed for casualties in rheumatoid arthritis (RA) patients. Customarily, dyslipidemia is probably the most prevalent underlying cause of untimely demise in people suffering from RA as it hastens the expansion of atherosclerosis. The engagement of inflammatory cytokines like tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1), interleukin-6 (IL-6), etc., is crucial in the progression and proliferation of both RA and abnormal lipid parameters. Thus, lipid abnormalities should be monitored frequently in patients with both primary and advanced RA stages. An advanced lipid profile examination, i.e., direct role of apolipoproteins associated with various lipid molecules is a more dependable approach for better understanding of the disease and selecting suitable therapeutic targets. Therefore, studying their apolipoproteins is more relevant than assessing RA patients' altered lipid profile levels. Among the various apolipoprotein classes, Apo A1 and Apo B are primarily being focused. In addition, it also addresses how calculating Apo B:Apo A1 ratio can aid in analyzing the disease's risk. The marketed therapies available to control lipid abnormalities are associated with many other risk factors. Hence, directly targeting Apo A1 and Apo B would provide a better and safer option.

Interactions between red and processed meat consumption and APOA5 gene variants associated with the incidence of metabolic syndrome in Korean adults

Background

Metabolic syndrome (MetS) is characterized by the coexistence of disorders such as diabetes, hypertension, hyperlipidemia, and obesity and is affected by genetic factors. Previous genome-wide association studies (GWAS) suggested that APOA5 gene variants were significantly associated with MetS and its components. Dietary factors such as red and processed meat consumption can cause chronic diseases, including hypertension, diabetes, and vascular depression. The aim of this study was to investigate the modulation of the incidence of MetS by the interaction between APOA5 rs662799 polymorphism and red and processed meat consumption.

Methods

In this prospective cohort study, 3266 participants were collected from the Korea Association REsource (KARE) cohort of the Korean Genome and Epidemiology Study (KoGES) from 2001 to 2016. APOA5 rs662799 polymorphism was extracted by GWAS using the Korean Chip. Red and processed meat consumption data were assessed using a semi-quantitative food frequency questionnaire.

Results

The incidence of MetS in carriers of the minor G allele of rs662799 (AG + GG) and the third tertile of red and processed meat consumption (serving/day) was higher than those with the major allele of rs662799 (AA) and the first tertile of red and processed meat consumption (HR 1.70, 95% CI 1.30–2.22, p interaction = 0.002).

Conclusions

An association between the presence of the minor alleles of rs662799 and high red and processed meat consumption and the incidence of MetS was observed in Korean adults.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12263-022-00707-w.

AP-4 regulates neuronal lysosome composition, function, and transport via regulating export of critical lysosome receptor proteins at the trans-Golgi network

The adaptor protein complex-4 or AP-4 is known to mediate autophagosome maturation through regulating sorting of transmembrane cargo such as ATG9A at the Golgi. There is a need to understand AP-4 function in neurons, as mutations in any of its four subunits cause a complex form of hereditary spastic paraplegia (HSP) with intellectual disability. While AP-4 has been implicated in regulating trafficking and distribution of cargo such as ATG9A and APP, little is known about its effect on neuronal lysosomal protein traffic, lysosome biogenesis and function. In this study, we demonstrate that in human iPSC-derived neurons AP-4 regulates lysosome composition, function and transport via regulating export of critical lysosomal receptors, including Sortilin 1, from the trans-Golgi network to endo-lysosomes. Additionally, loss of AP-4 causes endo-lysosomes to stall and build up in axonal swellings potentially through reduced recruitment of retrograde transport machinery to the organelle. These findings of axonal lysosome build-up are highly reminiscent of those observed in Alzheimer's disease as well as in neurons modelling the most common form of HSP, caused by spastin mutations. Our findings implicate AP-4 as a critical regulator of neuronal lysosome biogenesis and altered lysosome function and axonal endo-lysosome transport as an underlying defect in AP-4 deficient HSP. Additionally, our results also demonstrate the utility of the human i³Neuronal model system in investigating neuronal phenotypes observed in AP-4 deficient mice and/or the human AP-4 deficiency syndrome. [Media: see text] [Media: see text].

Sorting through the extensive and confusing roles of sortilin in metabolic disease

Sortilin is a post-Golgi trafficking receptor homologous to the yeast vacuolar protein sorting receptor 10 (VPS10). The VPS10 motif on sortilin is a 10-bladed β-propeller structure capable of binding more than 50 proteins, covering a wide range of biological functions including lipid and lipoprotein metabolism, neuronal growth and death, inflammation, and lysosomal degradation. Sortilin has a complex cellular trafficking itinerary, where it functions as a receptor in the trans-Golgi network, endosomes, secretory vesicles, multivesicular bodies, and at the cell surface. In addition, sortilin is associated with hypercholesterolemia, Alzheimer's disease, prion diseases, Parkinson's disease, and inflammation syndromes. The 1p13.3 locus containing SORT1, the gene encoding sortilin, carries the strongest association with LDL-C of all loci in human genome-wide association studies. However, the mechanism by which sortilin influences LDL-C is unclear. Here, we review the role sortilin plays in cardiovascular and metabolic diseases and describe in detail the large and often contradictory literature on the role of sortilin in the regulation of LDL-C levels.

Mechanisms regulating the sorting of soluble lysosomal proteins

Lysosomes are key regulators of many fundamental cellular processes such as metabolism, autophagy, immune response, cell signalling and plasma membrane repair. These highly dynamic organelles are composed of various membrane and soluble proteins, which are essential for their proper functioning. The soluble proteins include numerous proteases, glycosidases and other hydrolases, along with activators, required for catabolism. The correct sorting of soluble lysosomal proteins is crucial to ensure the proper functioning of lysosomes and is achieved through the coordinated effort of many sorting receptors, resident ER and Golgi proteins, and several cytosolic components. Mutations in a number of proteins involved in sorting soluble proteins to lysosomes result in human disease. These can range from rare diseases such as lysosome storage disorders, to more prevalent ones, such as Alzheimer’s disease, Parkinson’s disease and others, including rare neurodegenerative diseases that affect children. In this review, we discuss the mechanisms that regulate the sorting of soluble proteins to lysosomes and highlight the effects of mutations in this pathway that cause human disease. More precisely, we will review the route taken by soluble lysosomal proteins from their translation into the ER, their maturation along the Golgi apparatus, and sorting at the trans-Golgi network. We will also highlight the effects of mutations in this pathway that cause human disease.

Coronary Heart Disease in Type 2 Diabetes Mellitus: Genetic Factors and Their Mechanisms, Gene-Gene, and Gene-Environment Interactions in the Asian Populations

Asians are more susceptible to type 2 diabetes mellitus (T2D) and its coronary heart disease (CHD) complications than the Western populations, possibly due to genetic factors, higher degrees of obesity, insulin resistance, and endothelial dysfunction that could occur even in healthy individuals. The genetic factors and their mechanisms, along with gene-gene and gene-environment interactions associated with CHD in T2D Asians, are yet to be explored. Therefore, the objectives of this paper were to review the current evidence of genetic factors for CHD, summarize the proposed mechanisms of these genes and how they may associate with CHD risk, and review the gene-gene and gene-environment interactions in T2D Asians with CHD. The genetic factors can be grouped according to their involvement in the energy and lipoprotein metabolism, vascular and endothelial pathology, antioxidation, cell cycle regulation, DNA damage repair, hormonal regulation of glucose metabolism, as well as cytoskeletal function and intracellular transport. Meanwhile, interactions between single nucleotide polymorphisms (SNPs) from different genes, SNPs within a single gene, and genetic interaction with environmental factors including obesity, smoking habit, and hyperlipidemia could modify the gene's effect on the disease risk. Collectively, these factors illustrate the complexities of CHD in T2D, specifically among Asians.

Serum Sortilin Is Associated with Coronary Artery Calcification and Cardiovascular and Cerebrovascular Events in Maintenance Hemodialysis Patients

OBJECTIVE

To analyze the role of serum sortilin in coronary artery calcification (CAC) and cardiovascular and cerebrovascular events (CCE) in maintenance hemodialysis (MHD) patients.

METHODS

One hundred eleven patients with MHD ≥3 months were included in this study. The general data, clinical features, hematological data, and medication history of the patients were recorded. Eighty-five cases were examined by vascular color Doppler ultrasound, cardiac color Doppler ultrasound, lateral lumbar radiography, and coronary artery calcification score. The patients were followed up for a median time of 45 months. The primary endpoint was CCE or death from a vascular event, and the role of sortilin in this process was analyzed.

RESULTS

Among 85 MHD patients, 51 cases (60.00%) had different degrees of CAC. There were significant differences in diabetes, dialysis time, serum phosphorus, calcium-phosphorus product, medical history of phosphate binders, sortilin, and carotid artery plaque between 4 different degrees of calcification groups (p < 0.05). Logistic regression analysis showed that diabetes (OR = 5.475; 95% CI: 1.794-16.71, p = 0.003), calcium-phosphorus product (OR = 2.953; 95% CI: 1.198-7.279, p = 0.019), and sortilin (OR = 1.475 per 100 pg/mL; 95% CI: 1.170-1.858, p = 0.001) were independent risk factors for CAC. During the follow-up, 28 cases of 111 patients (25.23%) suffered from CCE. There were significant differences in CCE between mild, moderate, and severe CAC groups and noncalcification groups (p < 0.05). Cox regression analysis showed that diabetes mellitus (HR 3.424; 95% CI: 1.348-8.701, p = 0.010), CAC (HR 5.210; 95% CI: 1.093-24.83, p = 0.038), and serum sortilin (HR = 8.588; 95% CI: 1.919-38.43, p = 0.005) were independent risk factors for CCE. Besides, we proposed a cutoff value of 418 pg/mL for serum sortilin level, which was able to predict the occurrence of CCE with 75.0% sensitivity and 71.9% specificity. The area under the curve was 0.778 (95% CI: 0.673-0.883).

CONCLUSION

Sortilin is newly found to be independently associated with CAC and CCE in MHD patients.

Genetics and regulation of HDL metabolism

The inverse association between plasma HDL cholesterol (HDL-C) levels and risk for cardiovascular disease (CVD) has been demonstrated by numerous epidemiological studies. However, efforts to reduce CVD risk by pharmaceutically manipulating HDL-C levels failed and refused the HDL hypothesis. HDL-C levels in the general population are highly heterogeneous and are determined by a combination of genetic and environmental factors. Insights into the causes of HDL-C heterogeneity came from the study of monogenic HDL deficiency syndromes but also from genome wide association and Μendelian randomization studies which revealed the contribution of a large number of loci to low or high HDL-C cases in the general or in restricted ethnic populations. Furthermore, HDL-C levels in the plasma are under the control of transcription factor families acting primarily in the liver including members of the hormone nuclear receptors (PPARs, LXRs, HNF-4) and forkhead box proteins (FOXO1-4) and activating transcription factors (ATFs). The effects of certain lipid lowering drugs used today are based on the modulation of the activity of specific members of these transcription factors. During the past decade, the roles of small or long non-coding RNAs acting post-transcriptionally on the expression of HDL genes have emerged and provided novel insights into HDL regulation and new opportunities for therapeutic interventions. In the present review we summarize recent progress made in the genetics and the regulation (transcriptional and post-transcriptional) of HDL metabolism.

Olanzapine leads to nonalcoholic fatty liver disease through the apolipoprotein A5 pathway

The antipsychotic drug olanzapine was reported to induce nonalcoholic fatty liver disease (NAFLD), whereas the underlying mechanism remains incompletely understood. This study investigated whether apolipoprotein A5 (apoA5) and sortilin, two interactive factors involved in NAFLD pathogenesis, are implicated in olanzapine-induced NAFLD. In our study, at week 8, olanzapine treatment successfully induced hepatic steatosis in female C57 BL/6 J mice, which was independent of body weight gain. Likewise, olanzapine effectively mediated hepatocyte steatosis in HepG2 cells characterized by substantially elevated intracellular lipid droplets. Increased plasma triglyceride concentration and decreased plasma apoA5 levels were observed in mice treated with 8-week olanzapine. Surprisingly, olanzapine markedly enhanced hepatic apoA5 protein levels in mice, without a significant effect on rodent hepatic ApoA5 mRNA. Our in vitro study showed that olanzapine reduced apoA5 protein levels in the medium and enhanced apoA5 protein levels in hepatocytes, whereas this drug exerted no effect on hepatocyte APOA5 mRNA. By transfecting APOA5 siRNA into HepG2 cells, it was demonstrated that APOA5 knockdown effectively reversed olanzapine-induced hepatocyte steatosis in vitro. In addition, olanzapine drastically increased sortilin mRNA and protein levels in vivo and in vitro. Interestingly, SORT1 knockdown reduced intracellular apoA5 protein levels and increased medium apoA5 protein levels in vitro, without affecting intracellular APOA5 mRNA levels. Furthermore, SORT1 knockdown greatly ameliorated hepatocyte steatosis in vitro. This study provides the first evidence that sortilin inhibits the hepatic apoA5 secretion that is attributable to olanzapine-induced NAFLD, which provides new insight into effective strategies against NAFLD for patients with schizophrenia administered olanzapine.

Kernel machine SNP set analysis finds the association of BUD13, ZPR1, and APOA5 variants with metabolic syndrome in Tehran Cardio-metabolic Genetics Study

Metabolic syndrome (MetS) is one of the most important risk factors for cardiovascular disease. The 11p23.3 chromosomal region plays a potential role in the pathogenesis of MetS. The present study aimed to assess the association between 18 single nucleotide polymorphisms (SNPs) located at the BUD13, ZPR1, and APOA5 genes with MetS in the Tehran Cardio-metabolic Genetics Study (TCGS). In 5421 MetS affected and non-affected participants, we analyzed the data using two models. The first model (MetS model) examined SNPs' association with MetS. The second model (HTg-MetS Model) examined the association of SNPs with MetS affection participants who had a high plasma triglyceride (TG). The four-gamete rules were used to make SNP sets from correlated nearby SNPs. The kernel machine regression models and single SNP regression evaluated the association between SNP sets and MetS. The kernel machine results showed two sets over three sets of correlated SNPs have a significant joint effect on both models (p < 0.0001). Also, single SNP regression results showed that the odds ratios (ORs) for both models are almost similar; however, the p-values had slightly higher significance levels in the HTg-MetS model. The strongest ORs in the HTg-MetS model belonged to the G allele in rs2266788 (MetS: OR = 1.3, p = 3.6 × 10^–7; HTg-MetS: OR = 1.4, p = 2.3 × 10^–11) and the T allele in rs651821 (MetS: OR = 1.3, p = 2.8 × 10^–7; HTg-MetS: OR = 1.4, p = 3.6 × 10^–11). In the present study, the kernel machine regression models could help assess the association between the BUD13, ZPR1, and APOA5 gene variants (11p23.3 region) with lipid-related traits in MetS and MetS affected with high TG.

Circulating sortilin levels are associated with inflammation in patients with moyamoya disease

BACKGROUND

Systemic inflammation has been implicated in the pathogenesis of moyamoya disease (MMD). Sortilin is a critical regulator of proinflammatory cytokine secretion in several cell types. The present study investigated the association between circulating sortilin and proinflammatory cytokine levels and the occurrence of MMD.

METHODS

Forty-two MMD cases and 76 age- and sex-matched controls were enrolled in this study between January 2018 and June 2019 at the Affiliated Hospital of Jining Medical University. The demographic and clinical characteristics were evaluated, and the circulating serum and cerebrospinal fluid (CSF) levels of sortilin, sortilin-related receptor with A-type repeats (SorLA), and proinflammatory cytokines including C-reactive protein (CRP), interleukin (IL)-6, interferon (IFN)-γ were measured by enzyme-linked immunosorbent assay. Linear regression and correlation analyses were used to estimate the associations between sortilin, SorLA, and proinflammatory cytokine levels.

RESULTS

MMD patients had higher serum levels of sortilin (P = 0.012), CRP (P = 0.013), IL-6 (P = 0.004), and IFN-γ (P = 0.033) than healthy controls. In MMD patients, serum sortilin was positively correlated with serum proinflammatory cytokines (CRP: r = 0.459, P = 0.0022; IL-6: r = 0.445, P = 0.0032; and IFN-γ: r = 0.448, P = 0.0029) and CSF sortilin (r = 0.440, P = 0.0035); the latter was positively correlated with CSF levels of CRP (r = 0.542, P = 0.0002), IL-6 (r = 0.440, P = 0.0036), and IFN-γ (r = 0.443, P = 0.0033).

CONCLUSIONS

Elevated sortilin level is associated MMD onset and may be a clinically useful biomarker along with proinflammatory cytokine levels.

The Association Between Sortilin and Inflammation in Patients with Coronary Heart Disease

Purpose

Inflammation is a key contributor to coronary heart disease (CHD). Sortilin is a sorting receptor and has been identified as a critical regulator of inflammatory response. Therefore, our study aimed to determine the link between circulating sortilin levels, proinflammatory cytokine levels, and the occurrence of CHD.

Patients and Methods

Our study included 227 CHD patients and 101 matched healthy individuals. Circulating serum levels of sortilin and proinflammatory cytokines, including IL-1β, IL-6 and TNF-α, were assessed by a double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA). Linear regression and correlation analyses were used to estimate the associations between sortilin and proinflammatory cytokines. Moreover, six single-nucleotide polymorphisms (SNPs) spanning the sortilin and SORL1 genes were genotyped.

Results

Elevated levels of sortilin (P=0.027) and proinflammatory cytokines IL-1β (P=0.013), IL-6 (P=0.000) and TNF-α (P=0.010) were observed in CHD patients compared to those in healthy controls. Furthermore, sortilin levels were significantly positively correlated with IL-1β (r=0.252, P=0.0001), IL-6 (r=0.250, P=0.0001) and TNF-α (r=0.180, P=0.0064) levels. Notably, sortilin polymorphisms were revealed to be associated with the occurrence of CHD and varying sortilin levels. Subjects with the rs599839 AA risk genotype for CHD had significantly higher sortilin levels than those with the GG and GA genotypes (P=0.000); the same tendency was also observed in the levels of the proinflammatory cytokines IL-1β (P=0.003) and TNF-α (P=0.000). Similarly, GG carriers of rs464218 with increased sortilin levels were found to be at increased risk for CHD (P=0.014). The levels of IL-1β (P=0.025) and IL-6 (P=0.015) were also increased in these patients.

Conclusion

Our findings reveal that high sortilin levels may interact with inflammatory response to contribute to the occurrence of CHD. Considering that our clinical evidence suggests for the first time that sortilin involves in inflammatory response in CHD, the mechanistic role of sortilin in the progression of CHD deserves detailed investigation.

Characterization of Human Induced Pluripotent Stem Cell-Derived Hepatocytes with Mature Features and Potential for Modeling Metabolic Diseases

There is a strong anticipated future for human induced pluripotent stem cell-derived hepatocytes (hiPS-HEP), but so far, their use has been limited due to insufficient functionality. We investigated the potential of hiPS-HEP as an in vitro model for metabolic diseases by combining transcriptomics with multiple functional assays. The transcriptomics analysis revealed that 86% of the genes were expressed at similar levels in hiPS-HEP as in human primary hepatocytes (hphep). Adult characteristics of the hiPS-HEP were confirmed by the presence of important hepatocyte features, e.g., Albumin secretion and expression of major drug metabolizing genes. Normal energy metabolism is crucial for modeling metabolic diseases, and both transcriptomics data and functional assays showed that hiPS-HEP were similar to hphep regarding uptake of glucose, low-density lipoproteins (LDL), and fatty acids. Importantly, the inflammatory state of the hiPS-HEP was low under standard conditions, but in response to lipid accumulation and ER stress the inflammation marker tumor necrosis factor α (TNFα) was upregulated. Furthermore, hiPS-HEP could be co-cultured with primary hepatic stellate cells both in 2D and in 3D spheroids, paving the way for using these co-cultures for modeling non-alcoholic steatohepatitis (NASH). Taken together, hiPS-HEP have the potential to serve as an in vitro model for metabolic diseases. Furthermore, differently expressed genes identified in this study can serve as targets for future improvements of the hiPS-HEP.

Regulatory Roles of Sortilin and SorLA in Immune-Related Processes

Sortilin, also known as Neurotensin Receptor-3, and the sorting-related receptor with type-A repeats (SorLA) are both members of the Vps10p domain receptor family. Initially identified in CNS cells, they are expressed in various other cell types where they exert multiple functions. Although mostly studied for its involvement in Alzheimer’s disease, SorLA has recently been shown to be implicated in immune response by regulating IL-6-mediated signaling, as well as driving monocyte migration. Sortilin has been shown to act as a receptor, as a co-receptor and as an intra- and extracellular trafficking regulator. In the last two decades, deregulation of sortilin has been demonstrated to be involved in many human pathophysiologies, including neurodegenerative disorders (Alzheimer and Parkinson diseases), type 2 diabetes and obesity, cancer, and cardiovascular pathologies such as atherosclerosis. Several studies highlighted different functions of sortilin in the immune system, notably in microglia, pro-inflammatory cytokine regulation, phagosome fusion and pathogen clearance. In this review, we will analyze the multiple roles of sortilin and SorLA in the human immune system and how their deregulation may be involved in disease development.

Analysis of causal effect of APOA5 variants on premature coronary artery disease

Apolipoprotein A5 (APOA5) regulates the metabolisms of triglyceride and HDL. APOA5 variants have been linked to coronary artery disease (CAD), but their causal roles are not well studied yet. This study aims to identify the causal effects of APOA5 variants on premature CAD. Sequencing analysis of APOA5 in 128 premature, familiar CAD patients from GeneQuest identified 11 genomic variants, including p.S19W (rs3135506). SKAT analysis showed that all sequenced variants, in aggregate, significantly increased the risk of premature CAD (P-skat = 0.037). Individually, the p.S19W variant was significantly associated with risk of premature CAD (OR = 2.30, P = 0.008) in an independent set of 342 premature CAD patients and 537 controls after adjusting for covariates of sex, age, hypertension, body mass index, triglycerides (TGs), and total, LDL-, and HDL-cholesterol levels. Meanwhile, p.S19W significantly correlated with HDL-C levels (P = 0.048) and TG levels (P = 0.025). Mediation analysis yielded a mediation effect of p.S19W on risk of premature CAD through HDL-C (OR = 0.98, P = 0.040) and TG (OR = 0.98, P = 0.042), suggesting a causal relationship between p.S19W and premature CAD partially through its effects on HDL-C and TG levels. These results suggest that APOA5 variation regulates TG and HDL levels, thus displaying a causal role in the development of CAD.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

Trafficking in Alzheimer's Disease: Modulation of APP Transport and Processing by the Transmembrane Proteins LRP1, SorLA, SorCS1c, Sortilin, and Calsyntenin

The amyloid precursor protein (APP), one key player in Alzheimer's disease (AD), is extensively processed by different proteases. This leads to the generation of diverging fragments including the amyloid β (Aβ) peptide, which accumulates in brains of AD patients. Subcellular trafficking of APP is an important aspect for its proteolytic conversion, since the various secretases which cleave APP are located in different cellular compartments. As a consequence, altered subcellular targeting of APP is thought to directly affect the degree to which Aβ is generated. The mechanisms underlying intracellular APP transport are critical to understand AD pathogenesis and can serve as a target for future pharmacological interventions. In the recent years, a number of APP interacting proteins were identified which are implicated in sorting of APP, thereby influencing APP processing at different angles of the secretory or endocytic pathway. This review provides an update on the proteolytic processing of APP and the interplay of the transmembrane proteins low-density lipoprotein receptor-related protein 1, sortilin-receptor with A-type repeats, SorCS1c, sortilin, and calsyntenin. We discuss the specific interactions with APP, the capacity to modulate the intracellular itinerary and the proteolytic conversion of APP, a possible involvement in the clearance of Aβ, and the implications of these transmembrane proteins in AD and other neurodegenerative diseases.

Apolipoprotein A5 regulates intracellular triglyceride metabolism in adipocytes

It has previously been demonstrated that apolipoprotein A5 (apoA5) can be internalized by human adipocytes and significantly decreases intracellular triglyceride content. In the present study, endocytosis of apoA5 by adipocytes under different conditions, and the underlying mechanism by which apoA5 regulates cellular triglyceride storage, was investigated. The results revealed that the apoA5 protein was detected in human subcutaneous abdominal adipose tissues. In addition, the uptake of apoA5 was attenuated in human obese adipose tissues and in cultured adipocytes with hypertrophy or insulin resistance. Low-density lipoprotein receptor protein 1 (LRP1) knockdown in adipocytes resulted in a decrease in internalized apoA5 content, suggesting that LRP1 serves a role in apoA5 uptake. Treatment of adipocytes with apoA5 decreased the expression of the lipid droplet-associated proteins such as cidec and perilipin. ApoA5-treated adipocytes demonstrated an increase in lipolysis activity and expression of uncoupling protein 1, which is the molecular effector of thermogenesis in brown adipocytes. These results suggested that decreased triglyceride accumulation in adipocytes induced by apoA5 may be associated with enhanced lipolysis and energy expenditure, which may result from reduced expression of cidec and perilipin. In conclusion, the present study demonstrated a novel role of apoA5 in regulating the intracellular triglyceride metabolism of adipocytes. The results of the present study suggested that apoA5 may serve as a potential therapeutic target for the treatment of obesity and its related disorders.

Soluble LR11 associates with aortic root calcification in asymptomatic treated male patients with familial hypercholesterolemia

BACKGROUND AND AIMS

Despite statin treatment, a high prevalence of severe vascular calcification is found in patients with familial hypercholesterolemia (FH). We assessed the relation between the circulating soluble form of low-density lipoprotein receptor relative with 11 ligand-binding repeats (sLR11), a risk factor for cardiovascular disease, and vascular calcification in asymptomatic statin-treated heterozygous FH patients.

METHODS

In 123 asymptomatic heterozygous FH patients (age 40-69 years), aortic root (ARC), aortic valve (AVC) and coronary artery calcification (CAC) were determined with CT-based calcium scoring expressed in Agatston units. Plasma sLR11 levels were measured by sandwich ELISA.

RESULTS

Seventy-three patients displayed ARC, 48 had AVC and 96 CAC. Plasma sLR11 levels were positively correlated with the presence of ARC (r = 0.2, p = 0.03), but not with AVC or CAC. The correlation between sLR11 levels and ARC was restricted to male FH patients (r = 0.31, p = 0.006). Multivariate logistic analyses showed that the association of plasma sLR11 with the presence of ARC was independent of other determinants (Adjusted Odds Ratio, 2.01 (95% CI = 1.28-3.16) p = 0.002).

CONCLUSIONS

Plasma sLR11 is associated with ARC in male FH patients and may be mechanistically involved in the differential distribution of atherosclerotic lesions in the vasculature.

Sorting receptor SORLA: cellular mechanisms and implications for disease

Sorting-related receptor with A-type repeats (SORLA) is an intracellular sorting receptor that directs cargo proteins, such as kinases, phosphatases, and signaling receptors, to their correct location within the cell. The activity of SORLA assures proper function of cells and tissues, and receptor dysfunction is the underlying cause of common human malignancies, including Alzheimer’s disease, atherosclerosis, and obesity. Here, we discuss the molecular mechanisms that govern sorting of SORLA and its cargo in multiple cell types, and why genetic defects in this receptor results in devastating diseases.

Genetic predisposition of human plasma triglyceride concentrations

The issue of plasma triglyceride levels relative to the risk of development of cardiovascular disease, as well as overall mortality, has been actively discussed for many years. Like other cardiovascular disease risk factors, final plasma TG values have environmental influences (primarily dietary habits, physical activity, and smoking), and a genetic predisposition. Rare mutations (mainly in the lipoprotein lipase and apolipoprotein C2) along with common polymorphisms (within apolipoprotein A5, glucokinase regulatory protein, apolipoprotein B, apolipo-protein E, cAMP responsive element binding protein 3-like 3, glycosylphosphatidylinositol-anchored HDL-binding protein 1) play an important role in determining plasma TG levels.

Genetics of non-alcoholic fatty liver disease: From susceptibility and nutrient interactions to management

Genetics plays an important role in determining the susceptibility of an individual to develop a disease. Complex, multi factorial diseases of modern day (diabetes, cardiovascular disease, hypertension and obesity) are a result of disparity between the type of food consumed and genes, suggesting that food which does not match the host genes is probably one of the major reasons for developing life style diseases. Non-alcoholic fatty liver is becoming a global epidemic leading to substantial morbidity. While various genotyping approaches such as whole exome sequencing using next generation sequencers and genome wide association studies have identified susceptibility loci for non-alcoholic fatty liver disease (NAFLD) including variants in patatin-like phospholipase domain containing 3 and transmembrane 6 superfamily member 2 genes apart from others; nutrient based studies emphasized on a combination of vitamin D, E and omega-3 fatty acids to manage fatty liver disease. However majority of the studies were conducted independent of each other and very few studies explored the interactions between the genetic susceptibility and nutrient interactions. Identifying such interactions will aid in optimizing the nutrition tailor made to an individual’s genetic makeup, thereby aiding in delaying the onset of the disease and its progression. The present topic focuses on studies that identified the genetic susceptibility for NAFLD, nutritional recommendations, and their interactions for better management of NAFLD.

Sortilin: A novel regulator in lipid metabolism and atherogenesis

Several lines of evidence have shown that SORT1 gene within 1p13.3 locus is an important modulator of the low-density lipoprotein-cholesterol (LDL-C) level and atherosclerosis risk. Here, we summarize the effects of SORT1, which codes for sortilin, on lipid metabolism and development of atherosclerosis and explore the mechanisms underlying sortilin effects on lipid metabolism especially in hepatocytes and macrophages. Recent epidemiological evidence demonstrated that sortilin has been implicated as the causative factor and regulates lipid metabolism in vivo. Hepatic sortilin overexpression leads to both increased and decreased LDL-C levels by several different mechanisms, suggesting the complex roles of sortilin in hepatic lipid metabolism. Macrophage sortilin causes internalization of LDL and probably a reduction in cholesterol efflux, resulting in the intracellular accumulation of excessive lipids. In addition, sortilin deficiency in an atherosclerotic mouse model results in decreased aortic atherosclerotic lesion. Sortilin involves in lipid metabolism, promotes the development of atherosclerosis, and possibly becomes a potential therapeutic target for atherosclerosis treatment.

Neuroendocrinological and Epigenetic Mechanisms Subserving Autonomic Imbalance and HPA Dysfunction in the Metabolic Syndrome

Impact of environmental stress upon pathophysiology of the metabolic syndrome (MetS) has been substantiated by epidemiological, psychophysiological, and endocrinological studies. This review discusses recent advances in the understanding of causative roles of nutritional factors, sympathomedullo-adrenal (SMA) and hypothalamic-pituitary adrenocortical (HPA) axes, and adipose tissue chronic low-grade inflammation processes in MetS. Disturbances in the neuroendocrine systems for leptin, melanocortin, and neuropeptide Y (NPY)/agouti-related protein systems have been found resulting directly in MetS-like conditions. The review identifies candidate risk genes from factors shown critical for the functioning of each of these neuroendocrine signaling cascades. In its meta-analytic part, recent studies in epigenetic modification (histone methylation, acetylation, phosphorylation, ubiquitination) and posttranscriptional gene regulation by microRNAs are evaluated. Several studies suggest modification mechanisms of early life stress (ELS) and diet-induced obesity (DIO) programming in the hypothalamic regions with populations of POMC-expressing neurons. Epigenetic modifications were found in cortisol (here HSD11B1 expression), melanocortin, leptin, NPY, and adiponectin genes. With respect to adiposity genes, epigenetic modifications were documented for fat mass gene cluster APOA1/C3/A4/A5, and the lipolysis gene LIPE. With regard to inflammatory, immune and subcellular metabolism, PPARG, NKBF1, TNFA, TCF7C2, and those genes expressing cytochrome P450 family enzymes involved in steroidogenesis and in hepatic lipoproteins were documented for epigenetic modifications.

Soluble sortilin is released by activated platelets and its circulating levels are associated with cardiovascular risk factors

OBJECTIVE

Sortilin is involved multilaterally in the development of atherosclerosis. Here, we examine the release of soluble sortilin (sSortilin) from platelets and assess the association between circulating levels of sSoritlin and atherothrombosis such as coronary artery disease (CAD).

METHODS AND RESULTS

sSortilin levels measured in healthy subjects were higher in serum than in plasma (38.4 ± 8.7 vs. 15.8 ± 2.9 ng/mL; p < 0.0001). Platelets were shown to contain both membrane-bound sortilin and its soluble form lacking the cytoplasmic tail. Stimulation of platelet-rich plasma with collagen induced sSortilin release concomitantly with platelet aggregation, and the release was suppressed by aspirin. In clinical evaluation, plasma sSortilin was detected at significantly higher levels in cardiovascular risk patients with hypertension, dyslipidemia, and/or diabetes without CAD (non-CAD, 18.7 ± 3.3 ng/mL) than in patients with CAD under aspirin therapy (17.1 ± 3.6 ng/mL; p < 0.01) or in healthy controls (16.8 ± 2.9 ng/mL; p < 0.01). In these patients, plasma sSortilin levels were significantly correlated with platelet counts (rs = 0.33; p = 0.0085) and showed significant positive associations with cardiovascular risk factors: low-density lipoprotein cholesterol (rs = 0.37; p = 0.0023), triglycerides (rs = 0.28; p = 0.023), and serum uric acid (rs = 0.30; p = 0.017) in non-CAD, and γ-glutamyltransferase (rs = 0.43; p = 0.020) and high-sensitivity C-reactive protein (rs = 0.33, p = 0.0022) in CAD.

CONCLUSION

Elevated plasma sSortilin levels may be associated with in vivo platelet activation and could be a risk factor for atherothrombosis.

Chylomicrons: Advances in biology, pathology, laboratory testing, and therapeutics

The adequate absorption of lipids is essential for all mammalian species due to their inability to synthesize some essential fatty acids and fat-soluble vitamins. Chylomicrons (CMs) are large, triglyceride-rich lipoproteins that are produced in intestinal enterocytes in response to fat ingestion, which function to transport the ingested lipids to different tissues. In addition to the contribution of CMs to postprandial lipemia, their remnants, the degradation products following lipolysis by lipoprotein lipase, are linked to cardiovascular disease. In this review, we will focus on the structure-function and metabolism of CMs. Second, we will analyze the impact of gene defects reported to affect CM metabolism and, also, the role of CMs in other pathologies, such as atherothrombotic cardiovascular disease and diabetes mellitus. Third, we will provide an overview of the laboratory tests currently used to study CM disorders, and, finally, we will highlight current treatments in diseases affecting CMs.

Effects of Statin Therapy on Plasma Proprotein Convertase Subtilisin/kexin Type 9 and Sortilin Levels in Statin-Naive Patients with Coronary Artery Disease

AIM

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a key regulator of serum low-density lipoprotein (LDL) cholesterol levels, and sortilin is linked to lipoprotein metabolism. Although statin therapy increases PCSK9 levels, effects of this therapy on plasma sortilin levels have not been evaluated. The purpose of the present study was to examine the effects of statins on plasma PCSK9 and sortilin levels, and association of statin-induced increase in PCSK9 levels with sortilin.

METHODS

Serum lipid levels and plasma PCSK9 and sortilin levels were measured at baseline and 8 months after statin therapy in 90 statin-naive patients with coronary artery disease (CAD). Pitavastatin 4 mg/day was used to treat 44 patients and pravastatin 20 mg/day to treat the remaining 46 patients.

RESULTS

For both statin groups, significant increases in hetero-dimer PCSK9 levels (pitavastatin: 31%, p＜0.0001; pravastatin: 34%, p=0.03) and decreases in sortilin levels (pitavastatin: －8%, p=0.02; pravastatin: －16%, p=0.002) were observed. Although a reduction in LDL cholesterol was greater in the pitavastatin group than in the pravastatin group, no significant differences were observed in percentage changes in hetero-dimer PCSK9 and sortilin levels. A significant positive correlation was observed between percentage changes in hetero-dimer PCSK9 levels and those in sortilin levels (pitavastatin: r=0.359, p=0.02; pravastatin: r=0.276, p=0.06).

CONCLUSIONS

Use of pitavastatin and pravastatin increased plasma PCSK9 and decreased sortilin levels. Statin-induced increases in PCSK9 were associated with changes in sortilin in statin-naive patients with CAD.

A new role under sortilin's belt in cancer

The neurotensin receptor-3 also known as sortilin was the first member of the small family of vacuolar protein sorting 10 protein domain (Vps10p) discovered two decades ago in the human brain. The expression of sortilin is not confined to the nervous system but sortilin is ubiquitously expressed in many tissues. Sortilin has multiple roles in the cell as a receptor or a co-receptor, in protein transport of many interacting partners to the plasma membrane, to the endocytic pathway and to the lysosomes for protein degradation. Sortilin could be considered as the cells own shuttle system. In many human diseases including neurological diseases and cancer, sortilin expression has been shown to be deregulated. In addition, some studies have highlighted that the extracellular domain of sortilin is shedded into the culture media by an unknown mechanism. Sortilin can be released in exosomes and appears to control some mechanisms of exosome biogenesis. In lung cancer cells, sortilin can associate with two receptor tyrosine kinase receptors called the TES complex found in exosomes. Exosomes carrying the TES complex can convey a microenvironment control through the activation of ErbB signaling pathways and the release of angiogenic factors. Deregulation of sortilin function is now emerging to be implicated in four major human diseases- cardiovascular disease, Type 2 diabetes mellitus, Alzheimer disease and cancer.

Coordinated Regulation of Vasopressin Inactivation and Glucose Uptake by Action of TUG Protein in Muscle

In adipose and muscle cells, insulin stimulates the exocytic translocation of vesicles containing GLUT4, a glucose transporter, and insulin-regulated aminopeptidase (IRAP), a transmembrane aminopeptidase. A substrate of IRAP is vasopressin, which controls water homeostasis. The physiological importance of IRAP translocation to inactivate vasopressin remains uncertain. We previously showed that in skeletal muscle, insulin stimulates proteolytic processing of the GLUT4 retention protein, TUG, to promote GLUT4 translocation and glucose uptake. Here we show that TUG proteolysis also controls IRAP targeting and regulates vasopressin action in vivo. Transgenic mice with constitutive TUG proteolysis in muscle consumed much more water than wild-type control mice. The transgenic mice lost more body weight during water restriction, and the abundance of renal AQP2 water channels was reduced, implying that vasopressin activity is decreased. To compensate for accelerated vasopressin degradation, vasopressin secretion was increased, as assessed by the cosecreted protein copeptin. IRAP abundance was increased in T-tubule fractions of fasting transgenic mice, when compared with controls. Recombinant IRAP bound to TUG, and this interaction was mapped to a short peptide in IRAP that was previously shown to be critical for GLUT4 intracellular retention. In cultured 3T3-L1 adipocytes, IRAP was present in TUG-bound membranes and was released by insulin stimulation. Together with previous results, these data support a model in which TUG controls vesicle translocation by interacting with IRAP as well as GLUT4. Furthermore, the effect of IRAP to reduce vasopressin activity is a physiologically important consequence of vesicle translocation, which is coordinated with the stimulation of glucose uptake.

Apolipoprotein A-V deficiency enhances chylomicron production in lymph fistula mice

Apolipoprotein A-V (apoA-V), a liver-synthesized apolipoprotein discovered in 2001, strongly modulates fasting plasma triglycerides (TG). Little is reported on the effect of apoA-V on postprandial plasma TG, an independent predictor for atherosclerosis. Overexpressing apoA-V in mice suppresses postprandial TG, but mechanisms focus on increased lipolysis or clearance of remnant particles. Unknown is whether apoA-V suppresses the absorption of dietary lipids by the gut. This study examines how apoA-V deficiency affects the steady-state absorption and lymphatic transport of dietary lipids in chow-fed mice. Using apoA-V knockout (KO, n = 8) and wild-type (WT, n = 8) lymph fistula mice, we analyzed the uptake and lymphatic transport of lipids during a continuous infusion of an emulsion containing [(3)H]triolein and [(14)C]cholesterol. ApoA-V KO mice showed a twofold increase in (3)H (P < 0.001) and a threefold increase in (14)C (P < 0.001) transport into the lymph compared with WT. The increased lymphatic transport was accompanied by a twofold reduction (P < 0.05) in mucosal (3)H, suggesting that apoA-V KO mice more rapidly secreted [(3)H]TG out of the mucosa into the lymph. ApoA-V KO mice also produced chylomicrons more rapidly than WT (P < 0.05), as measured by the transit time of [(14)C]oleic acid from the intestinal lumen to lymph. Interestingly, apoA-V KO mice produced a steadily increasing number of chylomicron particles over time, as measured by lymphatic apoB output. The data suggest that apoA-V suppresses the production of chylomicrons, playing a previously unknown role in lipid metabolism that may contribute to the postprandial hypertriglyceridemia associated with apoA-V deficiency.

Association of six genetic variants with myocardial infarction

Although various genes that confer susceptibility to myocardial infarction (MI) have been identified for Caucasian populations in genome-wide association studies (GWAS), genetic variants related to this condition in Japanese individuals have not been identified definitively. The aim of the present study was to examine an association of MI in Japanese individuals with 29 polymorphisms identified as susceptibility loci for MI or coronary artery disease in Caucasian populations by meta-analyses of GWAS. The study subjects comprised 1,824 subjects with MI and 2,329 controls. Genotypes of the polymorphisms were determined by Luminex bead-based multiplex assay. To compensate for multiple comparisons, we adopted the criterion of a false discovery rate (FDR) of <0.05 for statistical significance for association. Comparisons of allele frequencies by the χ(2) test revealed that rs9369640 of the phosphatase and actin regulator 1 gene (PHACTR1, FDR=0.0007), rs4977574 of the CDKN2B antisense RNA 1 gene (CDKN2B-AS1, FDR=0.0038), rs264 of the lipoprotein lipase gene (LPL, FDR=0.0061), rs599839 of the proline/serine-rich coiled-coil 1 gene (PSRC1, FDR=0.0118), rs9319428 of the fms-related tyrosine kinase 1 gene (FLT1, FDR=0.0118) and rs12413409 of the cyclin and CBS domain divalent metal cation transport mediator 2 gene (CNNM2, FDR=0.0300) were significantly associated with MI. Multivariate logistic regression analysis with adjustment for covariates revealed that rs9369640 (P=0.0005; odds ratio, 0.89), rs4977574 (P=0.0001; odds ratio, 1.50), rs264 (P=0.0405; odds ratio, 0.85), rs599839 (P=0.0003; odds ratio, 0.68), rs9319428 (P=0.0155; odds ratio, 1.20) and rs12413409 (P=0.0076; odds ratio, 0.66) were significantly (P<0.05) associated with MI. PHACTR1, CDKN2B-AS1, LPL, PSRC1, FLT1 and CNNM2 may thus be susceptibility loci for MI in Japanese individuals.

Amyloid-Forming Properties of Human Apolipoproteins: Sequence Analyses and Structural Insights

Apolipoproteins are protein constituents of lipoproteins that transport cholesterol and fat in circulation and are central to cardiovascular health and disease. Soluble apolipoproteins can transiently dissociate from the lipoprotein surface in a labile free form that can misfold, potentially leading to amyloid disease. Misfolding of apoA-I, apoA-II, and serum amyloid A (SAA) causes systemic amyloidoses, apoE4 is a critical risk factor in Alzheimer's disease, and apolipoprotein misfolding is also implicated in cardiovascular disease. To explain why apolipoproteins are over-represented in amyloidoses, it was proposed that the amphipathic α-helices, which form the lipid surface-binding motif in this protein family, have high amyloid-forming propensity. Here, we use 12 sequence-based bioinformatics approaches to assess amyloid-forming potential of human apolipoproteins and to identify segments that are likely to initiate β-aggregation. Mapping such segments on the available atomic structures of apolipoproteins helps explain why some of them readily form amyloid while others do not. Our analysis shows that nearly all amyloidogenic segments: (i) are largely hydrophobic, (ii) are located in the lipid-binding amphipathic α-helices in the native structures of soluble apolipoproteins, (iii) are predicted in both native α-helices and β-sheets in the insoluble apoB, and (iv) are predicted to form parallel in-register β-sheet in amyloid. Most of these predictions have been verified experimentally for apoC-II, apoA-I, apoA-II and SAA. Surprisingly, the rank order of the amino acid sequence propensity to form amyloid (apoB>apoA-II>apoC-II≥apoA-I, apoC-III, SAA, apoC-I>apoA-IV, apoA-V, apoE) does not correlate with the proteins' involvement in amyloidosis. Rather, it correlates directly with the strength of the protein-lipid association, which increases with increasing protein hydrophobicity. Therefore, the lipid surface-binding function and the amyloid-forming propensity are both rooted in apolipoproteins' hydrophobicity, suggesting that functional constraints make it difficult to completely eliminate pathogenic apolipoprotein misfolding. We propose that apolipoproteins have evolved protective mechanisms against misfolding, such as the sequestration of the amyloidogenic segments via the native protein-lipid and protein-protein interactions involving amphipathic α-helices and, in case of apoB, β-sheets.

Sortilin: the mechanistic link between genes, cholesterol, and coronary artery disease

Hyperlipidemia is an important risk factor for cardiovascular disease globally, but there is still much mystery surrounding the topic of lipid regulation. Many studies have attempted to assess the underlying genetic basis of low-density lipoprotein (LDL) metabolism. Recently, multiple genome-wide association studies identified genes that strongly associate with plasma lipid concentration and cardiovascular disease. Compelling evidence linking the SORT1 gene to both LDL cholesterol (LDL-C) levels and the risk of coronary artery disease emerged from the data, prompting the search for the molecules and mechanisms responsible for this association. Three recent studies explored this relation through sortilin, the gene product of SORT1, and an intracellular trafficking molecule. Careful, hypothesis-driven experimental designs elucidated the potential mechanisms of sortilin's role in LDL-C metabolism. However, each study's conclusions differed in the details of SORT1's association to LDL-C and the subcellular mechanisms at work. Nevertheless, these 3 studies demonstrate how a complex disease such as hyperlipidemia can be evaluated from the scope of the genome down through the level of cellular regulation. Their findings serve as a platform for further study of LDL-C metabolism and hyperlipidemia while also providing lessons on how to better study other complex diseases.

Revisiting the structure of the Vps10 domain of human sortilin and its interaction with neurotensin

Sortilin is a multifunctional receptor involved in sorting and apoptosis. We have previously reported a 2.0-Å structure of the Vps10 ectodomain in complex with one of its ligands, the tridecapeptide neurotensin. Here we set out to further characterize the structural properties of sortilin and its interaction with neurotensin. To this end, we have determined a new 2.7 Å structure using a crystal grown with a 10-fold increased concentration of neurotensin. Here a second peptide fragment was observed within the Vps10 β-propeller, which may in principle either represent a second molecule of neurotensin or the N-terminal part of the molecule bound at the previously identified binding site. However, in vitro binding experiments strongly favor the latter hypothesis. Neurotensin thus appears to bind with a 1:1 stoichiometry, and whereas the N-terminus does not bind on its own, it enhances the affinity in context of full-length neurotensin. We conclude that the N-terminus of neurotensin probably functions as an affinity enhancer for binding to sortilin by engaging the second binding site. Crystal packing differs partly from the previous structure, which may be due to variations in the degree and pattern of glycosylations. Consequently, a notable hydrophobic loop, not modeled previously, could now be traced. A computational analysis suggests that this and a neighboring loop may insert into the membrane and thus restrain movement of the Vps10 domain. We have, furthermore, mapped all N-linked glycosylations of CHO-expressed human sortilin by mass spectrometry and find that their locations are compatible with membrane insertion of the hydrophobic loops.

Fish oil and fenofibrate prevented phosphorylation-dependent hepatic sortilin 1 degradation in Western diet-fed mice

Obesity and diabetes are associated with hepatic triglyceride overproduction and hypertriglyceridemia. Recent studies have found that the cellular trafficking receptor sortilin 1 (Sort1) inhibits hepatic apolipoprotein B secretion and reduces plasma lipid levels in mice, and its hepatic expression was negatively associated with plasma lipids in humans. This study investigated the regulation of hepatic Sort1 under diabetic conditions and by lipid-lowering fish oil and fenofibrate. Results showed that hepatic Sort1 protein, but not mRNA, was markedly lower in Western diet-fed mice. Knockdown of hepatic Sort1 increased plasma triglyceride in mice. Feeding mice a fish oil-enriched diet completely restored hepatic Sort1 levels in Western diet-fed mice. Fenofibrate also restored hepatic Sort1 protein levels in Western diet-fed wild type mice, but not in peroxisome proliferator-activated receptor α (PPARα) knock-out mice. PPARα ligands did not induce Sort1 in hepatocytes in vitro. Instead, fish oil and fenofibrate reduced circulating and hepatic fatty acids in mice, and n-3 polyunsaturated fatty acids prevented palmitate inhibition of Sort1 protein in HepG2 cells. LC/MS/MS analysis revealed that Sort1 phosphorylation at serine 793 was increased in obese mice and in palmitate-treated HepG2 cells. Mutations that abolished phosphorylation at Ser-793 increased Sort1 stability and prevented palmitate inhibition of Sort1 ubiquitination and degradation in HepG2 cells. In summary, therapeutic strategies that prevent posttranslational hepatic Sort1 down-regulation in obesity and diabetes may be beneficial in improving dyslipidemia.

Low-density lipoprotein receptors in liver: old acquaintances and a newcomer

The lipoprotein receptors low-density lipoprotein receptor (LDLR), the low-density lipoprotein receptor-related protein 1 (LRP1) and megalin/LRP2 share characteristic structural elements. In addition to their well-known roles in endocytosis of lipoproteins and systemic lipid homeostasis, it has been established that LRP1 mediates the endocytotic clearance of a multitude of extracellular ligands and regulates diverse signaling processes such as growth factor signaling, inflammatory signaling pathways, apoptosis, and phagocytosis in liver. Here, possible functions of LRP1 expression in hepatocytes and non-parenchymal cells in healthy and injured liver are discussed. Recent studies indicate the expression of megalin (LRP2) by hepatic stellate cells, myofibroblasts and Kupffer cells and hypothesize that LRP2 might represent another potential regulator of hepatic inflammatory processes. These observations provide the experimental framework for the systematic and dynamic analysis of the LDLR family during chronic liver injury and fibrogenesis.

Sortilin and the risk of cardiovascular disease

Plasma low-density lipoprotein cholesterol (LDL-C) levels are a key determinant of the risk of cardiovascular disease, which is why many studies have attempted to elucidate the pathways that regulate its metabolism. Novel latest-generation sequencing techniques have identified a strong association between the 1p13 locus and the risk of cardiovascular disease caused by changes in plasma LDL-C levels. As expected for a complex phenotype, the effects of variation in this locus are only moderate. Even so, knowledge of the association is of major importance, since it has unveiled a new metabolic pathway regulating plasma cholesterol levels. Crucial to this discovery was the work of three independent teams seeking to clarify the biological basis of this association, who succeeded in proving that SORT1, encoding sortilin, was the gene in the 1p13 locus involved in LDL metabolism. SORT1 was the first gene identified as determining plasma LDL levels to be mechanistically evaluated and, although the three teams used different, though appropriate, experimental methods, their results were in some ways contradictory. Here we review all the experiments that led to the identification of the new pathway connecting sortilin with plasma LDL levels and risk of myocardial infarction. The regulatory mechanism underlying this association remains unclear, but its discovery has paved the way for considering previously unsuspected therapeutic targets and approaches.