An apoA-I mimetic peptide facilitates off-loading cholesterol from HDL to liver cells through scavenger receptor BI

Recombinant high-density lipoprotein targeted delivery of celastrol to promote foam cells lipophagy against early atherosclerosis

INTRODUCTION

Atherosclerosis serving as the main underlying factor of cardiovascular disease (CVD) remains the primary cause of mortality and morbidity globally, while the deposition of massive cholesterol in macrophage-derived foam cells exerts pivotal roles in the occurrence and progression of atherosclerosis. Celastrol (CEL) is a bioactive ingredient owning potent capability to modulate lipid metabolism, whereas the poor bioavailability and potential toxicity limit its clinical application.

OBJECTIVES

This study aims to design a CEL-loaded recombinant high-density lipoprotein (rHDL) delivery platform for active targeting, which may effectively promote lipid degradation in foam cells and reversely transport excessive cholesterol to the liver for metabolism in time.

METHODS

The rHDL loaded with CEL (CEL-rHDL) was prepared by the thin film dispersion method. Then the anti-atherosclerotic efficacy and targeted delivery to foam cells of atherosclerotic lesions were verified both in vitro and in vivo. RNA-sequence was applied to reveal the potential mechanism against early atherosclerosis, which was further validated through several molecular biology experiments.

RESULTS

The prepared CEL-rHDL increased the targeting efficiency to foam cells of atherosclerotic lesions, mitigated its off-target toxicity, and improved anti-atherosclerotic efficacy. Importantly, CEL-rHDL decreased lipid storage in foam cells by triggering lipophagy via the activation of Ca2+/CaMKKβ/AMPK/mTOR signaling pathway and reverse cholesterol transport (RCT).

CONCLUSION

A combination of hypolipidemic chemo-intervention with rHDL participated specific and reverse delivery may offer a promising strategy for biocompatible treatment of early atherosclerosis.

Apolipoprotein A1 Modulates Teff/Treg Balance Through Scavenger Receptor Class B Type I-Dependent Mechanisms in Experimental Autoimmune Uveitis

Purpose

Experimental autoimmune uveitis (EAU) is a representative animal model of human uveitis. In this study, we investigated whether apolipoprotein A1 (APOA1) can alleviate EAU and explored its underlying mechanism.

Methods

Mice were immunized with interphotoreceptor retinoid-binding protein 1-20 and treated with APOA1 or vehicle. The retinas, draining lymph nodes (DLNs), and spleens were analyzed. Isolated T cells were used for proliferation, differentiation, and function assays in vitro. Selective inhibitors and pathway agonists were used to study signaling pathways. The effect of APOA1 on peripheral blood mononuclear cells (PBMCs) from uveitis patients was also examined.

Results

Administration of APOA1 ameliorated EAU. APOA1 suppressed pathogenic CD4+ T cell expansion in DLNs and spleen, and decreased the infiltration of effector T (Teff) cells into retina. APOA1 also inhibited T cell proliferation and T helper 1 cell differentiation in vitro and promoted regulatory T (Treg) cell differentiation. APOA1 restricted inflammatory cytokine production from lipopolysaccharide-stimulated PBMCs. Mechanistic studies revealed that the effect of APOA1 was mediated by scavenger receptor class B type I (SR-BI) and downstream signals including phosphatidylinositol 3-kinase/Protein kinase B (PKB, or Akt), p38 mitogen-activated protein kinase, and nuclear factor–κB.

Conclusions

APOA1 ameliorates EAU by regulating the Teff/Treg partially through SR-BI. Our results suggest that APOA1 can be a therapeutic alternative for autoimmune uveitis.

Post-Stroke Administration of L-4F Promotes Neurovascular and White Matter Remodeling in Type-2 Diabetic Stroke Mice

Patients with type 2 diabetes mellitus (T2DM) exhibit a distinct and high risk of ischemic stroke with worse post-stroke neurovascular and white matter (WM) prognosis than the non-diabetic population. In the central nervous system, the ATP-binding cassette transporter member A 1 (ABCA1), a reverse cholesterol transporter that efflux cellular cholesterol, plays an important role in high-density lipoprotein (HDL) biogenesis and in maintaining neurovascular stability and WM integrity. Our previous study shows that L-4F, an economical apolipoprotein A member I (ApoA-I) mimetic peptide, has neuroprotective effects via alleviating neurovascular and WM impairments in the brain of db/db-T2DM stroke mice. To further investigate whether L-4F has neurorestorative benefits in the ischemic brain after stroke in T2DM and elucidate the underlying molecular mechanisms, we subjected middle-aged, brain-ABCA1 deficient (ABCA1-B/-B), and ABCA1-floxed (ABCA1fl/fl) T2DM control mice to distal middle cerebral artery occlusion. L-4F (16 mg/kg, subcutaneous) treatment was initiated 24 h after stroke and administered once daily for 21 days. Treatment of T2DM-stroke with L-4F improved neurological functional outcome, and decreased hemorrhage, mortality, and BBB leakage identified by decreased albumin infiltration and increased tight-junction and astrocyte end-feet densities, increased cerebral arteriole diameter and smooth muscle cell number, and increased WM density and oligodendrogenesis in the ischemic brain in both ABCA1-B/-B and ABCA1fl/fl T2DM-stroke mice compared with vehicle-control mice, respectively (p < 0.05, n = 9 or 21/group). The L-4F treatment reduced macrophage infiltration and neuroinflammation identified by decreases in ED-1, monocyte chemoattractant protein-1 (MCP-1), and toll-like receptor 4 (TLR4) expression, and increases in anti-inflammatory factor Insulin-like growth factor 1 (IGF-1) and its receptor IGF-1 receptor β (IGF-1Rβ) in the ischemic brain (p < 0.05, n = 6/group). These results suggest that post-stroke administration of L-4F may provide a restorative strategy for T2DM-stroke by promoting neurovascular and WM remodeling. Reducing neuroinflammation in the injured brain may contribute at least partially to the restorative effects of L-4F independent of the ABCA1 signaling pathway.

Serum apolipoprotein A-I depletion is causative to silica nanoparticles-induced cardiovascular damage

The rapid development of nanotechnology has greatly benefited modern science and engineering and also led to an increased environmental exposure to nanoparticles (NPs). While recent research has established a correlation between the exposure of NPs and cardiovascular diseases, the intrinsic mechanisms of such a connection remain unclear. Inhaled NPs can penetrate the air-blood barrier from the lung to systemic circulation, thereby intruding the cardiovascular system and generating cardiotoxic effects. In this study, on-site cardiovascular damage was observed in mice upon respiratory exposure of silica nanoparticles (SiNPs), and the corresponding mechanism was investigated by focusing on the interaction of SiNPs and their encountered biomacromolecules en route. SiNPs were found to collect a significant amount of apolipoprotein A-I (Apo A-I) from the blood, in particular when the SiNPs were preadsorbed with pulmonary surfactants. While the adsorbed Apo A-I ameliorated the cytotoxic and proinflammatory effects of SiNPs, the protein was eliminated from the blood upon clearance of the NPs. However, supplementation of Apo A-I mimic peptide mitigated the atherosclerotic lesion induced by SiNPs. In addition, we found a further declined plasma Apo A-I level in clinical silicosis patients than coronary heart disease patients, suggesting clearance of SiNPs sequestered Apo A-I to compromise the coronal protein's regular biological functions. Together, this study has provided evidence that the protein corona of SiNPs acquired in the blood depletes Apo A-I, a biomarker for prediction of cardiovascular diseases, which gives rise to unexpected toxic effects of the nanoparticles.

Role of macrophage autophagy in atherosclerosis: modulation by bioactive compounds

Atherosclerosis is a chronic inflammatory disease associated with lipid metabolism disorder. Autophagy is a catabolic process and contributes to maintaining cellular homeostasis. Substantial evidence suggests that defective autophagy is implicated in several diseases, including atherosclerosis, while increased autophagy mitigates atherosclerosis development. Thus, understanding the mechanisms of autophagy regulation and its association with atherosclerosis is vital to develop new therapies against atherosclerosis. Dietary bioactive compounds are non-nutrient natural compounds that include phenolics, flavonoids, and carotenoids. Importantly, these bioactive compounds possess anti-inflammatory, antioxidant, and antibacterial properties that may alleviate various chronic diseases. Recently, examining the effects of bioactive compounds on autophagy activity in atherogenesis has drawn considerable attention. The current review discusses the role of macrophage autophagy in the development and progression of atherosclerosis. We also summarize our current knowledge of the therapeutic potential of bioactive compounds on atherosclerosis and autophagy.

D-4F, an ApoA-I mimetic peptide ameliorating TRPA1-mediated nocifensive behaviour in a model of neurogenic inflammation

Background

High doses of capsaicin are recommended for the treatment of neuropathic pain. However, low doses evoke mechanical hypersensitivity. Activation of the capsaicin chemosensor transient receptor potential vanilloid 1 (TRPV1) induces neurogenic inflammation. In addition to the release of pro-inflammatory mediators, reactive oxygen species are produced. These highly reactive molecules generate oxidised phospholipids and 4-hydroxynonenal (4-HNE) which then directly activate TRP ankyrin 1 (TRPA1). The apolipoprotein A-I mimetic peptide D-4F neutralises oxidised phospholipids. Here, we asked whether D-4F ameliorates neurogenic hypersensitivity in rodents by targeting reactive oxygen species and 4-HNE in the capsaicin-evoked pain model.

Results

Co-application of D-4F ameliorated capsaicin-induced mechanical hypersensitivity and allodynia as well as persistent heat hypersensitivity measured by Randell–Selitto, von Frey and Hargreaves test, respectively. In addition, mechanical hypersensitivity was blocked after co-injection of D-4F with the reactive oxygen species analogue H₂O₂ or 4-HNE. In vitro studies on dorsal root ganglion neurons and stably transfected cell lines revealed a TRPA1-dependent inhibition of the calcium influx when agonists were pre-incubated with D-4F. The capsaicin-induced calcium influx in TRPV1-expressing cell lines and dorsal root ganglion neurons sustained in the presence of D-4F.

Conclusions

D-4F is a promising compound to ameliorate TRPA1-dependent hypersensitivity during neurogenic inflammation.

Current Therapies Focused on High-Density Lipoproteins Associated with Cardiovascular Disease

High-density lipoproteins (HDL) comprise a heterogeneous family of lipoprotein particles divided into subclasses that are determined by density, size and surface charge as well as protein composition. Epidemiological studies have suggested an inverse correlation between High-density lipoprotein-cholesterol (HDL-C) levels and the risk of cardiovascular diseases and atherosclerosis. HDLs promote reverse cholesterol transport (RCT) and have several atheroprotective functions such as anti-inflammation, anti-thrombosis, and anti-oxidation. HDLs are considered to be atheroprotective because they are associated in serum with paraoxonases (PONs) which protect HDL from oxidation. Polyphenol consumption reduces the risk of chronic diseases in humans. Polyphenols increase the binding of HDL to PON1, increasing the catalytic activity of PON1. This review summarizes the evidence currently available regarding pharmacological and alternative treatments aimed at improving the functionality of HDL-C. Information on the effectiveness of the treatments has contributed to the understanding of the molecular mechanisms that regulate plasma levels of HDL-C, thereby promoting the development of more effective treatment of cardiovascular diseases. For that purpose, Scopus and Medline databases were searched to identify the publications investigating the impact of current therapies focused on high-density lipoproteins.

Apolipoprotein A-1 mimetic peptide 4F promotes endothelial repairing and compromises reendothelialization impaired by oxidized HDL through SR-B1

Disruption of endothelial monolayer integrity is the primary instigating factor for many cardiovascular diseases. High density lipoprotein (HDL) oxidized by heme enzyme myeloperoxidase (MPO) is dysfunctional in promoting endothelial repair. Apolipoprotein A-1 mimetic 4F with its pleiotropic benefits has been proven effective in many in vivo models. In this study we investigated whether 4F promotes endothelial repair and restores the impaired function of oxidized HDL (Cl/NO₂-HDL) in promoting re-endothelialization. We demonstrate that 4F and Cl/NO₂-HDL act on scavenger receptor type I (SR-B1) using human aorta endothelial cells (HAEC) and SR-B1 ^(-/-) mouse aortic endothelial cells. Wound healing, transwell migration, lamellipodia formation and single cell migration assay experiments show that 4F treatment is associated with a recovery of endothelial cell migration and associated with significantly increased endothelial nitric oxide synthase (eNOS) activity, Akt phosphorylation and SR-B1 expression. 4F increases NO generation and diminishes oxidative stress. In vivo, 4F can stimulate cell proliferation and re-endothelialization in the carotid artery after treatment with Cl/NO₂-HDL in a carotid artery electric injury model but fails to do so in SR-B1^(-/-) mice. These findings demonstrate that 4F promotes endothelial cell migration and has a potential therapeutic benefit against early endothelial injury in cardiovascular diseases.

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4F restores the decreased ability of Cl/NO₂-HDL in promoting endothelial repair.

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4F increases NO generation and diminishes oxidative stress.

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4F increases eNOS activity, Akt phosphorylation and SR-B1 expression.

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4F can stimulate re-endothelialization in a carotid artery electric injury model.

HDL protects against doxorubicin-induced cardiotoxicity in a scavenger receptor class B type 1-, PI3K-, and Akt-dependent manner

Doxorubicin is a widely used chemotherapeutic with deleterious cardiotoxic side effects. HDL has been shown to protect cardiomyocytes in vitro against doxorubicin-induced apoptosis. Scavenger receptor class B type 1 (SR-B1), a high-affinity HDL receptor, mediates cytoprotective signaling by HDL through Akt. Here, we assessed whether increased HDL levels protect against doxorubicin-induced cardiotoxicity in vivo and in cardiomyocytes in culture and explored the intracellular signaling mechanisms involved, particularly the role of SR-B1. Transgenic mice with increased HDL levels through overexpression of human apolipoprotein A1 (apoA1^Tg/Tg) and wild-type mice (apoA1^+/+) with normal HDL levels were treated repeatedly with doxorubicin. After treatment, apoA1^+/+ mice displayed cardiac dysfunction, as evidenced by reduced left ventricular end-systolic pressure and +dP/d t, and histological analysis revealed cardiomyocyte atrophy and increased cardiomyocyte apoptosis after doxorubicin treatment. In contrast, apoA1^Tg/Tg mice were protected against doxorubicin-induced cardiac dysfunction and cardiomyocyte atrophy and apoptosis. When SR-B1 was knocked out, however, overexpression of apoA1 did not protect against doxorubicin-induced cardiotoxicity. Using primary neonatal mouse cardiomyocytes and human immortalized ventricular cardiomyocytes in combination with genetic knockout, inhibitors, or siRNA-mediated knockdown, we demonstrated that SR-B1 is required for HDL-mediated protection of cardiomyocytes against doxorubicin-induced apoptosis in vitro via a pathway involving phosphatidylinositol 3-kinase and Akt1/2. Our findings provide proof of concept that raising apoA1 to supraphysiological levels can dramatically protect against doxorubicin-induced cardiotoxicity via a pathway that is mediated by SR-B1 and involves Akt1/2 activation in cardiomyocytes. NEW & NOTEWORTHY We have identified an important role for the scavenger receptor class B type 1 in facilitating high-density lipoprotein-mediated protection of cardiomyocytes against stress-induced apoptosis and shown that increasing plasma high-density lipoprotein protects against the deleterious side effects of the chemotherapeutic and cardiotoxic drug doxorubicin.

Transient Receptor Potential Ankyrin 1 Channel Involved in Atherosclerosis and Macrophage-Foam Cell Formation

Transient receptor potential ankyrin 1 channel (TRPA1) plays an important role in the pathogenesis of inflammatory diseases, yet its role and the underlying mechanism in atherosclerosis remain unclear. We aimed to investigate the role of TRPA1 in atherosclerosis and foam-cell formation in vivo in mice and in vitro in mouse macrophages. Histopathology was examined by hematoxylin and eosin staining, levels of cytokines and lipid profile were evaluated by assay kits, and protein expression was determined by western blot analysis. TRPA1 expression was increased in macrophage foam cells in atherosclerotic aortas of apolipoprotein E-deficient (apoE^-/-) mice. Atherosclerotic lesions, hyperlipidemia and systemic inflammation were worsened with chronic administration of the TRPA1 channel antagonist HC030031 or genetic ablation of TRPA1 (TRPA1^-/-) in apoE^-/- mice. Treatment with allyl isothiocyanate (AITC, a TRPA1 agonist) retarded the progression of atherosclerosis in apoE^-/- mice but not apoE^-/-TRPA1^-/- mice. Mouse macrophages showed oxidized low-density lipoprotein (oxLDL) activated TRPA1 channels. OxLDL-induced lipid accumulation of macrophages was exacerbated by HC030031 or loss of function of TRPA1. Inhibition of TRPA1 activity did not alter oxLDL internalization but impaired cholesterol efflux by downregulating the ATP-binding cassette transporters. Furthermore, tumor necrosis factor-α-induced inflammatory response was attenuated in AITC-activated macrophages. TRPA1 may be a pivotal regulator in the pathogenesis of atherosclerosis and cholesterol metabolism of macrophage foam cells.

Novel Apo E-Derived ABCA1 Agonist Peptide (CS-6253) Promotes Reverse Cholesterol Transport and Induces Formation of preβ-1 HDL In Vitro

Apolipoprotein (apo) mimetic peptides replicate some aspects of HDL function. We have previously reported the effects of compound ATI-5261 on its ability to replicate many functions of native apo A-I in the process of HDL biogenesis. ATI-5261 induced muscle toxicity in wild type C57Bl/6 mice, increased CPK, ALT and AST and increase in triglyceride (Tg) levels. Aromatic phenylalanine residues on the non-polar face of ATI-5261, together with positively charged arginine residues at the lipid-water interface were responsible for these effects. This information was used to create a novel analog (CS-6253) that was non-toxic. We evaluated this peptide designed from the carboxyl terminus of apo E, in its ability to mimic apo A-I functionality. Our data shows that the lipidated particles generated by incubating cells overexpressing ABCA1 with lipid free CS-6253 enhances the rate of ABCA1 lipid efflux with high affinity interactions with native ABCA1 oligomeric forms and plasma membrane micro-domains. Interaction between ABCA1 and lipid free CS-6253 resulted in formation of nascent HDL-CS-6253 particles that are actively remodeled in plasma. Mature HDL-CS-6253 particles deliver cholesterol to liver cells via SR-BI in-vitro. CS-6253 significantly increases cholesterol efflux in murine macrophages and in human THP-1 macrophage-derived foam cells expressing ABCA1. Addition of CS-6253 to plasma dose-dependently displaced apo A-I from α-HDL particles and led to de novo formation of preβ-1 HDL that stimulates ABCA1 dependent cholesterol efflux efficiently. When incubated with human plasma CS-6253 was also found to bind with HDL and LDL and promoted the transfer of cholesterol from HDL to LDL predominantly. Our data shows that CS-6253 mimics apo A-I in its ability to promote ABCA1-mediated formation of nascent HDL particles, and enhances formation of preβ-1 HDL with increase in the cycling of apo A-I between the preβ and α-HDL particles in-vitro. These mechanisms are potentially anti-atherogenic.

Apolipoprotein E derived HDL mimetic peptide ATI-5261 promotes nascent HDL formation and reverse cholesterol transport in vitro

Modulation of the reverse cholesterol transport (RCT) pathway may provide a therapeutic target for the prevention and treatment of atherosclerotic cardiovascular disease (CVD). In the present study, we evaluated a novel 26-amino acid apolipoprotein mimetic peptide (ATI-5261) designed from the carboxyl terminal of apoE, in its ability to mimic apoA-I functionality in RCT in vitro. Our data shows that nascent HDL-like (nHDL) particles generated by incubating cells over-expressing ABCA1 with ATI-5261 increase the rate of specific ABCA1 dependent lipid efflux, with high affinity interactions with ABCA1. We also show that these nHDL particles interact with membrane micro-domains in a manner similar to nHDL apoA-I. These nHDL particles then interact with the ABCG1 transporter and are remodeled by plasma HDL-modulating enzymes. Finally, we show that these mature HDL-like particles are taken up by SR-BI for cholesterol delivery to liver cells. This ATI-5621-mediated process mimics apoA-I and may provide a means to prevent cholesterol accumulation in the artery wall. In this study, we propose an integrative physiology approach of HDL biogenesis with the synthetic peptide ATI-5261. These experiments provide new insights for potential therapeutic use of apolipoprotein mimetic peptides.

Apolipoprotein A-I and adenosine triphosphate-binding cassette transporter A1 expression alleviates lipid accumulation in hepatocytes

BACKGROUND AND AIM

Abnormal lipid metabolism may contribute to the pathogenesis of non-alcoholic steatohepatitis. ATP-binding cassette transporter A1 (ABCA1) mediates the transport of cholesterol and phospholipids from cells to high density lipoprotein apolipoproteins. The lipidation of apolipoprotein A-I (apoA-I) by ABCA1 is the rate-limiting step in reverse cholesterol transport and the generation of plasma high density lipoprotein. Here, we examined the effect of apoA-I or ABCA1 overexpression on hepatic lipid levels in BEL-7402 cells.

METHODS

Human ABCA1 or apoA-I was overexpressed in BEL-7402 hepatocytes by transfection and human apoA-I was overexpressed via adenoviral vector in C57BL/6J mice with MCD diet.

RESULTS

Overexpression of either apoA-I or ABCA1 resulted in an increase in cholesterol efflux and a decrease in cellular fatty acids and triglycerides. However, after repression of ABCA1 by its siRNA, overexpression of apoA-I failed to decrease both cellular fatty acids and triglycerides. ApoA-I or ABCA1 overexpression also resulted in a decrease in the expression of the endoplasmic reticulum stress-related proteins GRP78 and SREBP-1. Overexpression of apoA-I in mice also reduced hepatic lipid levels.

CONCLUSIONS

Expression of apoA-I or ABCA1 can reduce steatosis by decreasing lipid storage in hepatocytes through lipid transport and may also reduce endoplasmic reticulum stress, further lessening hepatic steatosis.

Molecules that mimic apolipoprotein A-I: potential agents for treating atherosclerosis

Certain amphipathic α-helical peptides can functionally mimic many of the properties of full-length apolipoproteins, thereby offering an approach to modulate high-density lipoprotein (HDL) for combating atherosclerosis. In this Perspective, we summarize the key findings and advances over the past 25 years in the development of peptides that mimic apolipoproteins, especially apolipoprotein A-I (apoA-I). This assemblage of information provides a reasonably clear picture of the state of the art in the apolipoprotein mimetic field, an appreciation of the potential for such agents in pharmacotherapy, and a sense of the opportunities for optimizing the functional properties of HDL.

Newer therapeutic strategies to alter high-density lipoprotein level and function

Measurements of low levels of high-density lipoprotein (HDL) cholesterol have been identified as a risk factor for premature coronary artery disease, however, to date, current pharmacologic approaches for raising HDL have provided little benefit, if at all, in reducing cardiovascular outcomes. It has been shown that HDL can modify many aspects of plaque pathogenesis. Its most established role is in reverse cholesterol transportation, but HDL can also affect oxidation, inflammation, cellular adhesion, and vasodilatation. Considering these potential benefits of HDL, newer treatments have been developed to modify HDL activity, which include the use of oral cholesteryl ester transfer protein inhibitors, apolipoprotein (apo)A-I infusions, apoA-I mimetics, drugs to increase apoA-I synthesis, and agonists of the liver X receptor. These new therapies are reviewed in this article.

Control of hypercholesterolemia and atherosclerosis using the cholesterol recognition/interaction amino acid sequence of the translocator protein TSPO

The translocator protein (18-kDa) TSPO is an ubiquitous high affinity cholesterol-binding protein reported to be present in the endothelial and smooth muscle cells of the blood vessels; its expression dramatically increased in macrophages found in atherosclerotic plaques. A domain in the carboxy-terminus of TSPO was identified and characterized as the cholesterol recognition/interaction amino acid consensus (CRAC). The ability of the CRAC domain to bind to cholesterol led us to hypothesize that this peptide could be used as an hypocholesterolemic, with potential anti-atherogenic properties, agent. We report herein the therapeutic benefit that resulted for the administration of the VLNYYVWR human CRAC sequence to guinea pigs fed with a high cholesterol diet and ApoE knock-out B6.129P2-Apoetm1Unc/J mice. CRAC treatment (3 and 30mg/kg once daily for 6 weeks) resulted in reduced circulating cholesterol levels in guinea pigs fed with 2% high cholesterol diet and ApoE knock-out B6.129P2-Apoetm1Unc/J mice. In high cholesterol fed guinea pigs, CRAC treatment administered once daily induced an increase in circulating HDL, decreased total, free and LDL cholesterol, and removed atheroma deposits in the aorta in a dose-dependent manner. The treatment also prevented the high cholesterol diet-induced increase in serum creatine kinase, total and isoforms, markers of neurological, cardiac and muscular damage. No toxicity was observed. Taken together these results support a role of TSPO in lipid homeostasis and atherosclerosis and indicate that CRAC may constitute a novel and safe treatment of hypercholesterolemia and atherosclerosis.

Intracellular cholesterol-binding proteins enhance HDL-mediated cholesterol uptake in cultured primary mouse hepatocytes

A major gap in our knowledge of rapid hepatic HDL cholesterol clearance is the role of key intracellular factors that influence this process. Although the reverse cholesterol transport pathway targets HDL to the liver for net elimination of free cholesterol from the body, molecular details governing cholesterol uptake into hepatocytes are not completely understood. Therefore, the effects of sterol carrier protein (SCP)-2 and liver fatty acid-binding protein (L-FABP), high-affinity cholesterol-binding proteins present in hepatocyte cytosol, on HDL-mediated free cholesterol uptake were examined using gene-targeted mouse models, cultured primary hepatocytes, and 22-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-amino]-23,24-bisnor-5-cholen-3β-ol (NBD-cholesterol). While SCP-2 overexpression enhanced NBD-cholesterol uptake, counterintuitively, SCP-2/SCP-x gene ablation also 1) enhanced the rapid molecular phase of free sterol uptake detectable in <1 min and initial rate and maximal uptake of HDL free cholesterol and 2) differentially enhanced free cholesterol uptake mediated by the HDL3, rather than the HDL2, subfraction. The increased HDL free cholesterol uptake was not due to increased expression or distribution of the HDL receptor [scavenger receptor B1 (SRB1)], proteins regulating SRB1 [postsynaptic density protein (PSD-95)/Drosophila disk large tumor suppressor (dlg)/tight junction protein (ZO1) and 17-kDa membrane-associated protein], or other intracellular cholesterol trafficking proteins (steroidogenic acute response protein D, Niemann Pick C, and oxysterol-binding protein-related proteins). However, expression of L-FABP, the single most prevalent hepatic cytosolic protein that binds cholesterol, was upregulated twofold in SCP-2/SCP-x null hepatocytes. Double-immunogold electron microscopy detected L-FABP sufficiently close to SRB1 for direct interaction, similar to SCP-2. These data suggest a role for L-FABP in HDL cholesterol uptake, a finding confirmed with SCP-2/SCP-x/L-FABP null mice and hepatocytes. Taken together, these results suggest that L-FABP, particularly in the absence of SCP-2, plays a significant role in HDL-mediated cholesterol uptake in cultured primary hepatocytes.

Novel HDL-directed pharmacotherapeutic strategies

The burden of atherothrombotic cardiovascular disease remains high despite currently available optimum medical therapy. To address this substantial residual risk, the development of novel therapies that attempt to harness the atheroprotective functions of HDL is a major goal. These functions include the critical role of HDL in reverse cholesterol transport, and its anti-inflammatory, antithrombotic, and antioxidant activities. Discoveries in the past decade have shed light on the complex metabolic and antiatherosclerotic pathways of HDL. These insights have fueled the development of HDL-targeted drugs, which can be classified among four different therapeutic approaches: directly augmenting apolipoprotein A-I (apo A-I) levels, such as with apo A-I infusions and upregulators of endogenous apo A-I production; indirectly augmenting apo A-I and HDL-cholesterol levels, such as through inhibition of cholesteryl ester transfer protein or endothelial lipase, or through activation of the high-affinity niacin receptor GPR109A; mimicking the functionality of apo A-I with apo A-I mimetic peptides; and enhancing steps in the reverse cholesterol transport pathway, such as via activation of the liver X receptor or of lecithin-cholesterol acyltransferase.