Novel approaches to treating cardiovascular disease: lessons from Tangier disease

MicroRNA-328-5p Alleviates Macrophage Lipid Accumulation through the Histone Deacetylase 3/ATP-binding cassette transporter A1 pathway

MicroRNA-328 (miR-328) was reported to protect against atherosclerosis, but its role in foam cell formation remains unknown. The aim of this study was to investigate the effect of miR-328-5p on macrophage lipid accumulation and the underlying mechanisms. The results showed that miR-328-5p expression was robustly decreased in oxidized low-density lipoprotein (ox-LDL)-treated macrophages. Treatment of human acute monocytic leukemia cel (THP-1) macrophage-derived foam cells with a miR-328-5p mimic markedly increased [³ H]-cholesterol efflux, inhibited lipid droplet accumulation, and decreased intracellular total cholesterol (TC), free cholesterol (FC) and cholesteryl ester (CE) contents. Upregulation of miR-328-5p also reduced the expression of histone deacetylase 3 (HDAC3) but increased the levels of ATP-binding cassette transporter A1 (ABCA1) in THP-1 macrophage-derived foam cells. Mechanistically, miR-328-5p inhibited HDAC3 expression by directly targeting its 3'UTR, thereby promoting ABCA1 expression and the subsequent cholesterol efflux. Furthermore, miR-328-5p mimic treatment did not affect the uptake of Dil-ox-LDL or the expression of scavenger receptor-A (SR-A), thrombospondin receptor (CD36) and ABCG1. Taken together, these findings suggest that miR-328-5p alleviates macrophage lipid accumulation through the HDAC3/ABCA1 pathway.

A novel splicing mutation in the ABCA1 gene, causing Tangier disease and familial HDL deficiency in a large family

Tangier disease is a rare disorder of lipoprotein metabolism that presents with extremely low levels of HDL cholesterol and apoprotein A-I. It is caused by mutations in the ATP-binding cassette transporter A1 (ABCA1) gene. Clinical heterogeneity and mutational pattern of Tangier disease are poorly characterized. Moreover, also familial HDL deficiency may be caused by mutations in ABCA1 gene. ATP-binding cassette transporter A1 (ABCA1) gene mutations in a patient with Tangier disease, who presented an uncommon clinical history, and in his family were found and characterized. He was found to be compound heterozygous for two intronic mutations of ABCA1 gene, causing abnormal pre-mRNAs splicing. The novel c.1510-1G > A mutation was located in intron 12 and caused the activation of a cryptic splice site in exon 13, which determined the loss of 22 amino acids of exon 13 with the introduction of a premature stop codon. Five heterozygous carriers of this mutation were also found in proband's family, all presenting reduced HDL cholesterol and ApoAI (0.86 ± 0.16 mmol/L and 92.2 ± 10.9 mg/dL respectively), but not the typical features of Tangier disease, a phenotype compatible with the diagnosis of familial HDL deficiency. The other known mutation c.1195-27G > A was confirmed to cause aberrant retention of 25 nucleotides of intron 10 leading to the insertion of a stop codon after 20 amino acids of exon 11. Heterozygous carriers of this mutation also showed the clinical phenotype of familial HDL deficiency. Our study extends the catalog of pathogenic intronic mutations affecting ABCA1 pre-mRNA splicing. In a large family, a clear demonstration that the same mutations may cause Tangier disease (if in compound heterozygosis) or familial HDL deficiency (if in heterozygosis) is provided.

Monocyte and macrophage dynamics during atherogenesis

Vascular inflammation is associated with and in large part driven by changes in the leukocyte compartment of the vessel wall. Here, we focus on monocyte influx during atherosclerosis, the most common form of vascular inflammation. Although the arterial wall contains a large number of resident macrophages and some resident dendritic cells, atherosclerosis drives a rapid influx of inflammatory monocytes (Ly-6C(+) in mice) and other monocytes (Ly-6C(-) in mice, also known as patrolling monocytes). Once in the vessel wall, Ly-6C(+) monocytes differentiate to a phenotype consistent with inflammatory macrophages and inflammatory dendritic cells. The phenotype of these cells is modulated by lipid uptake, Toll-like receptor ligands, hematopoietic growth factors, cytokines, and chemokines. In addition to newly recruited macrophages, it is likely that resident macrophages also change their phenotype. Monocyte-derived inflammatory macrophages have a short half-life. After undergoing apoptosis, they may be taken up by surrounding macrophages or, if the phagocytic capacity is overwhelmed, can undergo secondary necrosis, a key event in forming the necrotic core of atherosclerotic lesions. In this review, we discuss these and other processes associated with monocytic cell dynamics in the vascular wall and their role in the initiation and progression of atherosclerosis.

Phospholipid transfer protein is regulated by liver X receptors in vivo

Liver X receptors (LXR) belong to the nuclear receptor superfamily that can regulate important lipid metabolic pathways. The plasma phospholipid transfer protein (PLTP) is known to mediate transfer of phospholipids from triglyceride-rich lipoproteins to high density lipoprotein (HDL) and plays a critical role in HDL metabolism. We report here that a specific LXR agonist, T0901317, elevated HDL cholesterol and phospholipid in C57/BL6 mice and generated enlarged HDL particles that were enriched in cholesterol, ApoAI, ApoE, and phospholipid. The appearance of these HDL particles upon oral dosing of T0901317 in C57/BL6 mice was closely correlated with the increased plasma PLTP activity and liver PLTP mRNA levels. Nuclear run-on assay indicated that the effect of LXR agonist on PLTP expression was at the transcriptional level. In mouse peritoneal macrophage cells, PLTP expression was also up-regulated by the LXR/RXR (retinoid X receptor) heterodimer. However, cholesterol efflux in mouse peritoneal macrophage cells from PLTP-deficient mice (PLTP0) was not significantly different from wild type animals. Although in PLTP-deficient mice, the induction of HDL cholesterol as well as HDL particle size increase persisted, the extent of the induction was greatly attenuated. We conclude that PLTP is a direct target gene of LXRs in vivo and plays an important role in LXR agonist-mediated HDL cholesterol and size increase in mice.

Increased atherosclerosis in hyperlipidemic mice with inactivation of ABCA1 in macrophages

The ATP-binding cassette transporter A1 (ABCA1) encodes a membrane protein that promotes cholesterol and phospholipid efflux from cells. Mutations in ABCA1 lead to HDL deficiency and tissue accumulation of macrophages in patients with homozygous Tangier disease. In this study, we examined whether the complete absence of ABCA1 or selected inactivation in macrophages is accompanied by an increase in atherosclerotic lesion progression in hypercholesterolemic apolipoprotein E-deficient (apoE(-/-)) mice and LDLR receptor-deficient (LDLr(-/-)) mice. The absence of ABCA1 led to reduced plasma cholesterol levels in both the apoE(-/-) and LDLr(-/-) mice, along with severe skin xanthomatosis characterized by marked foamy macrophages and cholesterol ester accumulation. However, the complete absence of ABCA1 did not affect the development, progression, or composition of atherosclerotic lesions in either the LDLr(-/-) or the apoE(-/-) mice fed a chow or atherogenic diet. In contrast, bone marrow transplantation studies demonstrated that the selective inactivation of ABCA1 in macrophages markedly increased atherosclerosis and foam cell accumulation in apoE(-/-). Taken together, these findings demonstrate that the complete absence of ABCA1 has a major impact on plasma lipoprotein homeostasis, and the proposed antiatherogenic effect resulting from ABCA1 deficiency is compensated by a less atherogenic profile. ABCA1 deficiency in macrophages, however, demonstrates the antiatherogenic properties of ABCA1 independent of plasma lipids and HDL levels.

Novel agents for managing dyslipidaemia

An elevated low-density lipoprotein (LDL) cholesterol level is a strong predictor of coronary heart disease (CHD) risk. Over the past seven years, equally strong evidence has accumulated that lowering LDL cholesterol with HMG-CoA reductase inhibitors or statins reduces CHD risk and there is now widespread use of these agents for the primary and secondary prevention of CHD. Treatment issues remain regarding the appropriate degree of LDL cholesterol reduction and whether, in people with very high levels, it would be preferable to achieve the LDL cholesterol goal with a powerful statin alone or combined with an agent that lowers LDL cholesterol by a different mechanism. The main focus in the development of novel agents is the patient with low high-density lipoprotein (HDL) cholesterol, usually associated with hypertriglyceridaemia. Already prevalent as a risk factor for CHD, this abnormality has been linked with insulin resistance, which is likely to increase greatly over the next decade, along with increasing obesity and diabetes. Agents that have potent HDL cholesterol raising capacity include cholesteryl ester transfer protein (CETP) inhibitors, retinoid X receptor (RXR) selective agonists, specific peroxisome proliferator-activated receptor (PPAR) agonists and oestrogen-like compounds. Another area of development involves agents that will lower both cholesterol and triglyceride levels, such as partial inhibitors of microsomal triglyceride transfer protein (MTP) and perhaps squalene synthase inhibitors and agonists of AMP kinase. Future emphasis will be on correcting all lipid abnormalities for the prevention of CHD, not just lowering LDL cholesterol.