High-density lipoprotein binding to scavenger receptor-BI activates endothelial nitric oxide synthase

High-density lipoproteins and atherosclerosis

High-density lipoproteins (HDLs) are strongly related to risk of atherosclerotic cardiovascular disease. Low levels of HDL cholesterol are a major cardiovascular risk factor, and overexpression of the major HDL protein, apolipoprotein (apo) A-I, markedly inhibits progression and even induces regression of atherosclerosis in animal models. Clinical data regarding the effect of increasing HDL cholesterol on vascular events are limited. HDL remains an important potential target for therapeutic intervention. A variety of gene products are involved in the regulation of HDL metabolism. Yet, the mechanisms by which HDL inhibits atherosclerosis are not yet fully understood. There remains much to be learned about HDL metabolism and its relation to atherosclerosis and other cardiovascular risk factors.

High-density lipoprotein regulates calcification of vascular cells

Accumulating evidence has suggested the protective role of HDL in cardiovascular disease processes. Calcification is a common feature of atherosclerotic lesions and contributes to cardiovascular complications due to the loss of aortic resilience and function. Recent studies have suggested that vascular calcification shares several features with skeletal bone formation at the cellular and molecular levels. These include the presence of osteoblast-like calcifying vascular cells in the artery wall that undergo osteoblastic differentiation and calcification in vitro. We hypothesized that HDL may also protect against vascular calcification by regulating the osteogenic activity of these calcifying vascular cells. When treated with HDL, alkaline phosphatase activity, a marker of osteogenic differentiation of osteoblastic cells, was significantly reduced in those cells. Prolonged treatment with HDL also inhibited calcification of these cells, further supporting the antiosteogenic differentiation property of HDL when applied to vascular cells. Furthermore, HDL inhibited the osteogenic activity that was induced by inflammatory cytokines interleukin (IL)-1beta and IL-6 as well as by minimally oxidized LDL. HDL also partially inhibited the IL-6-induced activation of signal transducer and activator of transcription 3 in calcifying vascular cells, suggesting that HDL may inhibit cytokine-induced signal transduction pathways. The inhibitory effects of HDL were mimicked by lipids extracted from HDL but not by HDL-associated apolipoproteins or reconstituted HDL. Furthermore, oxidation of HDL rendered it pro-osteogenic. Taken together, these results suggest that HDL regulates the osteoblastic differentiation and calcification of vascular cells and that vascular calcification may be another target of HDL action in the artery wall.

Sphingosine 1-phosphate signaling in atherosclerosis and vascular biology

PURPOSE OF REVIEW

Sphingosine 1-phosphate is a novel lipid mediator which exerts various actions on endothelial cells and vascular smooth muscle cells. In this review, we discuss the latest findings about the molecule in vascular biology.

RECENT FINDINGS

It has been demonstrated that most sphingosine 1-phosphate-induced actions are mediated by the Edg-family of its receptors. Sphingosine 1-phosphate stimulates the migration and proliferation of endothelial cells and is cytoprotective towards them. The involvement of phosphoinositide 3-kinase and nitric oxide in sphingosine 1-phosphate downstream signaling in endothelial cells was recently reported, as was the enhancement of endothelial barrier integrity induced by the molecule. Sphingosine 1-phosphate inhibits migration of vascular smooth muscle cells and this inhibition was reported to be mediated by inhibition of Rac. Sphingosine 1-phosphate is concentrated in the lipoprotein fraction in plasma, and high-density lipoprotein exerted endothelial cytoprotection through its component of this molecule.

SUMMARY

Sphingosine 1-phosphate might play a critical role in the development of atherosclerosis.

A carboxyl-terminal PDZ-interacting domain of scavenger receptor B, type I is essential for cell surface expression in liver

Scavenger receptor B, type I (SR-BI) was recently shown to interact with a PDZ domain-containing protein, PDZK1 (CLAMP/Diphor-1/CAP70/NaPi-Cap1), but the importance of this interaction in vivo in terms of SR-BI function has not been determined. In an effort to elucidate the role of this interaction in vivo, the PDZK1-interacting domain of SR-BI was identified and mutated and expressed liver-specifically in mice. The PDZKI-interacting domain on SR-BI was identified as the last three carboxyl-terminal amino acids, Arg-Lys-Leu. A mutant SR-BI (SR-BIdel509) that lacked only the leucine in the PDZ-interacting domain failed to interact with PDZK1 in vitro, while showing normal selective uptake function in nonpolarized cells. Transgenic mice with liver overexpression of SR-BIdel509 showed marked accumulation of SR-BI mRNA with only a moderate increase in SR-BI protein in liver, with no reduction in plasma cholesterol levels. Measurement of cell surface SR-BI levels and HDL cholesteryl ester-selective uptake in primary hepatocytes from transgenic mice revealed that SR-BIdel509 was not expressed at the plasma membrane correlating with normal levels of selective uptake compared with hepatocytes from nontransgenic littermates. This study indicates that the PDZK1-interacting domain of SR-BI is essential for cell surface expression of SR-BI in liver and suggests that PDZK1 or other PDZ domain proteins may play an important role in regulating SR-BI cell surface expression and hence reverse cholesterol transport.

FEEL-1, a novel scavenger receptor with in vitro bacteria-binding and angiogenesis-modulating activities

Employing the expression cloning technique, we cloned a novel scavenger receptor that is structurally unrelated to other scavenger receptors. The cloned receptor contained fasciclin (Fas-1), epidermal growth factor (EGF)-like, laminin-type EGF-like, and link domains. Based on the domain structures, we temporarily named it FEEL-1 (fasciclin, EGF-like, laminin-type EGF-like, and link domain-containing scavenger receptor-1). A data base search suggested the presence of a paralogous gene of FEEL-1, the full-length cDNA of this gene was also cloned, and its nucleotide sequence was determined. The deduced amino acid sequence of the clone indicated that its domain organization is similar to FEEL-1, and we named this clone FEEL-2. The effect of monoclonal antibodies against FEEL-1 indicated that FEEL-1 is the major receptor for 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbo-cyanine perchlorate (DiI)-labeled acetylated low density lipoprotein (DiI-Ac-LDL) in human umbilical vein endothelial cells. Reverse transcription and PCR analysis revealed that both FEEL-1 and FEEL-2 were expressed in several tissues and expressed highly in the spleen and lymph node. On the other hand, only FEEL-1 was expressed in mononuclear cells, particularly resting CD14(+) cells. The transient expression of FEEL-1 and FEEL-2 in Chinese hamster ovary cells demonstrated that both FEELs could bind to DiI-Ac-LDL. Both receptors were also found to bind to Gram-negative and Gram-positive bacteria. These results suggest that FEELs play important roles in the defense mechanisms against bacterial infection. Finally, the phenotypic effect of the inhibition of FEEL-1 on vascular remodeling was tested in vitro using the Matrigel tube formation assay, and we found a marked reduction in the degree of cell-cell interaction in anti-FEEL-1 monoclonal antibody-treated cells, suggesting the role of this receptor in angiogenesis.

A lipoprotein lipase activator, NO-1886 prevents impaired endothelium-dependent relaxation of aorta caused by exercise in aged rats

Exercise decreases plasma total cholesterol and triglycerides, and simultaneously, increases high density lipoprotein (HDL) cholesterol. As a result, exercise is believed to aid in preventing atherosclerosis. However, we do not know whether exercise protects against the development of atherosclerosis in the elderly. The aim of this study was to ascertain whether the lipoprotein lipase activator NO-1886 had an effect on the prevention of atherosclerosis in aged rats which undergo exercise. Exercise for 3 months did not affect plasma lipids but decreased the accumulation of visceral fat in 2-year-old rats (aged rat). Exercise also resulted in an elevation of plasma lipid peroxide (LPO) levels and impaired the endothelium-dependent relaxation of the thoracic aorta caused by acetylcholine in aged rats. On the other hand, NO-1886 decreased plasma triglycerides and increased HDL cholesterol and suppressed the elevation of plasma LPO levels caused by exercise. Furthermore, NO-1886 prevented impaired endothelium-dependent relaxation caused by exercise. In summary, the results of our study indicate that exercise may cause impaired endothelium-dependent relaxation by elevation of LPO in aged rats, and that NO-1886 prevents this impaired endothelium-dependent relaxation of aorta by reducing plasma triglycerides, elevating HDL cholesterol, and suppressing the elevation of plasma LPO caused by exercise.

Regulation of peroxisome proliferator-activated receptor-gamma-mediated gene expression. A new mechanism of action for high density lipoprotein

Cellular cholesterol content reflects a balance of lipid influx by lipoprotein receptors and endogenous synthesis and efflux to cholesterol acceptor particles. The beneficial effect of high density lipoprotein (HDL) in protecting against the development of cardiovascular disease is thought to be mediated predominately through its induction of cellular cholesterol efflux and "reverse cholesterol transport" from peripheral tissues to the liver. We tested the hypothesis that HDL could inhibit cellular lipid accumulation by modulating expression of peroxisome proliferator-activated receptor-gamma (PPARgamma)-responsive genes. To this end, we evaluated expression of two PPARgamma-responsive genes, CD36, a receptor for oxidized low density lipoprotein, and aP2, a fatty acid-binding protein. HDL decreased expression of macrophage CD36 and aP2 in a dose-dependent manner. HDL also decreased aP2 expression in fibroblasts, reduced accumulation of lipid, and slowed differentiation of fibroblasts into adipocytes. HDL stimulated mitogen-activated protein (MAP) kinase activity, and inhibition of CD36 expression was blocked by co-incubation with a MAP kinase inhibitor. HDL increased expression of PPARgamma mRNA and protein, induced translocation of PPARgamma from the cytoplasm to the nucleus, and increased PPARgamma phosphorylation. Our data demonstrate that despite induction and translocation of PPARgamma in response to HDL, MAP kinase-mediated phosphorylation of PPARgamma inhibited expression of PPARgamma-responsive genes and suggest mechanisms by which HDL may inhibit cellular lipid accumulation.

Plasma lipoproteins behave as carriers of extracellular sphingosine 1-phosphate: is this an atherogenic mediator or an anti-atherogenic mediator?

Sphingosine 1-phosphate (S1P) concentration in plasma and serum has been estimated to be within 200-900 nM. Among plasma and serum components, S1P is concentrated in lipoprotein fractions with a rank order of high-density lipoprotein (HDL)>low-density lipoprotein (LDL)>very low-density lipoprotein (VLDL)>lipoprotein-deficient plasma (LPDP) when expressed as the per unit amount of protein. It is well known that LDL, especially oxidized LDL, is closely correlated and HDL is inversely correlated, with the risk of cardiovascular disease, such as atherosclerosis. Evidence was presented that a part of HDL-induced actions previously reported are mediated by the lipoprotein-associated S1P. Furthermore, S1P content in LDL was markedly decreased during its oxidation. This paper will discuss whether S1P is an atherogenic mediator or an anti-atherogenic mediator.

Regulation of endothelial nitric oxide synthase: location, location, location

Endothelial nitric oxide synthase (eNOS) is expressed in vascular endothelium, airway epithelium, and certain other cell types where it generates the key signaling molecule nitric oxide (NO). Diminished NO availability contributes to systemic and pulmonary hypertension, atherosclerosis, and airway dysfunction. Complex mechanisms underly the cell specificity of eNOS expression, and co- and post-translational processing leads to trafficking of the enzyme to plasma membrane caveolae. Within caveolae, eNOS is the downstream target member of a signaling complex in which it is functionally linked to both typical G protein-coupled receptors and less typical receptors such as estrogen receptor (ER) alpha and the high-density lipoprotein receptor SR-BI displaying novel actions. This compartmentalization facilitates dynamic protein-protein interactions and calcium- and phosphorylation-dependent signal transduction events that modify eNOS activity. Further understanding of these mechanisms will enable us to take preventive and therapeutic advantage of the powerful actions of NO in multiple cell types.

Lipases and HDL metabolism

Plasma levels of high-density lipoprotein (HDL) cholesterol are strongly inversely associated with atherosclerotic cardiovascular disease, and overexpression of HDL proteins, such as apolipoprotein A-I in animals, reduces progression and even induces regression of atherosclerosis. Therefore, HDL metabolism is recognized as a potential target for therapeutic intervention of atherosclerotic vascular diseases. The antiatherogenic properties of HDL include promotion of cellular cholesterol efflux and reverse cholesterol transport, as well as antioxidant, anti-inflammatory and anticoagulant properties. The molecular regulation of HDL metabolism is not fully understood, but it is influenced by several extracellular lipases. Here, we focus on new developments and insights into the role of secreted lipases on HDL metabolism and their relationship to atherosclerosis.

High density lipoprotein binding to scavenger receptor, Class B, type I activates endothelial nitric-oxide synthase in a ceramide-dependent manner

Recently it has been demonstrated that high density lipoprotein (HDL) binding to scavenger receptors, class B, type I (SR-BI) stimulates endothelial nitric-oxide synthase (eNOS) activity. In the present studies we used a Chinese hamster ovary cell system and a human microvascular endothelial cell line to confirm that HDL stimulates eNOS activity in a SR-BI-dependent manner. Importantly, we have extended these studies to examine the mechanism whereby HDL binding to SR-BI stimulates eNOS. eNOS can be stimulated by an increase in intracellular calcium, by phosphorylation by Akt kinase, or by an increase in intracellular ceramide. Calcium imagining studies and experiments with the calcium chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester demonstrated that HDL binding to SR-BI does not induce an increase in intracellular calcium. Antibodies specific for activated Akt kinase demonstrated that HDL binding to SR-BI does not induce Akt kinase activation. However, HDL binding to SR-BI caused a reversible increase in intracellular ceramide levels from 97 +/- 14 pmol/mg of protein to 501 +/- 21 pmol/mg of protein. In addition, C(2)-ceramide stimulated eNOS to the same extent as HDL, whereas C(2)-dihydroceramide did not stimulate eNOS. We conclude that HDL binding to SR-BI stimulates eNOS by increasing intracellular ceramide levels and is independent of an increase in intracellular calcium or Akt kinase phosphorylation.

Loss of SR-BI expression leads to the early onset of occlusive atherosclerotic coronary artery disease, spontaneous myocardial infarctions, severe cardiac dysfunction, and premature death in apolipoprotein E-deficient mice

Murine models of atherosclerosis, such as the apolipoprotein E (apoE) or the LDL receptor knockout mice, usually do not exhibit many of the cardinal features of human coronary heart disease (CHD), eg, spontaneous myocardial infarction, severe cardiac dysfunction, and premature death. Here we show that mice with homozygous null mutations in the genes for both the high density lipoprotein receptor SR-BI and apoE (SR-BI/apoE double knockout [dKO] mice) exhibit morphological and functional defects with similarities to those seen in human CHD. When fed a standard chow diet, these hypercholesterolemic animals developed significant atherosclerotic lesions in the aortic sinus as early as 4 to 5 weeks after birth. We now show that they also exhibited extensive lipid-rich coronary artery occlusions and spontaneously developed multiple myocardial infarctions and cardiac dysfunction (eg, enlarged hearts, reduced ejection fraction and contractility, and ECG abnormalities). Their coronary arterial lesions, which were strikingly similar to human atherosclerotic plaques, exhibited evidence of cholesterol clefts and extensive fibrin deposition, indicating hemorrhage and clotting. All of the dKO mice died by 8 weeks of age (50% mortality at 6 weeks). Thus, SR-BI/apoE dKO mice provide a new murine model for CHD and may help better define the role of lipoprotein metabolism and atherosclerosis in the pathogenesis of myocardial infarction and cardiac dysfunction. Furthermore, these animals may be useful for preclinical testing of potential genetic and/or pharmacological therapies for CHD.