Molecular characterization of rabbit scavenger receptor class B types I and II: portal to central vein gradient of expression in the liver

Receptors and Host Factors for Enterovirus Infection: Implications for Cancer Therapy

Enteroviruses, with their diverse clinical manifestations ranging from mild or asymptomatic infections to severe diseases such as poliomyelitis and viral myocarditis, present a public health threat. However, they can also be used as oncolytic agents. This review shows the intricate relationship between enteroviruses and host cell factors. Enteroviruses utilize specific receptors and coreceptors for cell entry that are critical for infection and subsequent viral replication. These receptors, many of which are glycoproteins, facilitate virus binding, capsid destabilization, and internalization into cells, and their expression defines virus tropism towards various types of cells. Since enteroviruses can exploit different receptors, they have high oncolytic potential for personalized cancer therapy, as exemplified by the antitumor activity of certain enterovirus strains including the bioselected non-pathogenic Echovirus type 7/Rigvir, approved for melanoma treatment. Dissecting the roles of individual receptors in the entry of enteroviruses can provide valuable insights into their potential in cancer therapy. This review discusses the application of gene-targeting techniques such as CRISPR/Cas9 technology to investigate the impact of the loss of a particular receptor on the attachment of the virus and its subsequent internalization. It also summarizes the data on their expression in various types of cancer. By understanding how enteroviruses interact with specific cellular receptors, researchers can develop more effective regimens of treatment, offering hope for more targeted and efficient therapeutic strategies.

Matcha Green Tea Powder does not Prevent Diet-Induced Arteriosclerosis in New Zealand White Rabbits Due to Impaired Reverse Cholesterol Transport

INTRODUCTION

Green tea is associated with decreased risk for cardiovascular disease and stroke. Matcha is a special kind of powdered green tea known for its use in the Japanese tea ceremony. Due to its influence on lipoprotein parameters, it has been postulated to exert antiatherogenic effects. This study investigates whether it modulates the high-density lipoprotein (HDL) function and thereby influences the atherogenic process in an animal model with a strong influence on humans' situation.

METHODS AND RESULTS

After a pretreatment phase based on a standard diet, 10 female New Zealand White (NZW) rabbits are fed a high-fat diet for 20 weeks. The treatment group is additionally administered 1% matcha during the whole experiment. Long-term matcha treatment leads to lowered HDL cholesterol, impaired cholesterol transport manifested by reduced in vitro cholesterol efflux capacity, reduced cholesteryl ester transfer protein (CETP)-mediated cholesterol ester (CE) transfer between HDL and triglyceride-rich particles, and reduced macrophage-specific in vivo transfer, where ian increased absorption of cholesterol in the liver but a decreased secretion into bile is observed. Pulse wave velocity, assessed by nuclear magnetic resonance, is increased in matcha-treated animals, and a similar trend is observed for atherosclerotic lesion formation.

CONCLUSION

Long-term matcha green tea treatment of hypercholesterolemic rabbits cause impaired reverse cholesterol transport and increased vascular stiffness, and susceptibility for atherosclerotic lesion development.

A Polysaccharide Purified From Ganoderma lucidum Acts as a Potent Mucosal Adjuvant That Promotes Protective Immunity Against the Lethal Challenge With Enterovirus A71

Enterovirus A71 (EV-A71), the pathogen responsible for the seasonal hand-foot-and-mouth epidemics, can cause significant mortality in infants and young children. The vaccine against EV-A71 could potentially prevent virus-induced neurological complications and mortalities occurring due to the high risk of poliomyelitis-like paralysis and fatal encephalitis. It is known that polysaccharide purified from Ganoderma lucidum (PS-G) can effectively modulate immune function. Here, we used PS-G as an adjuvant with the EV-A71 mucosal vaccine and studied its effects. Our data showed that PS-G-adjuvanted EV-A71 generated significantly better IgA and IgG in the serum, saliva, nasal wash, bronchoalveolar lavage fluid (BALF), and feces. More importantly, these antibodies could neutralize the infectivity of EV-A71 (C2 genotype) and cross-neutralize the B4, B5, and C4 genotypes of EV-A71. In addition, more EV-A71-specific IgA- and IgG- secreting cells were observed with the used of a combination of EV-A71 and PS-G. Furthermore, T-cell proliferative responses and IFN-γ and IL-17 secretions levels were notably increased in splenocytes when the EV-A71 vaccine contained PS-G. We also found that levels of IFN-γ and IL-17 released in Peyer's patch cells were significantly increased in EV-A71, after it was combined with PS-G. We further demonstrated that both CD4⁺ and CD8⁺ T cells were able to generate IFN-γ and IL-17 in the spleen. An easy-accessed model of hybrid hSCARB2⁺/⁺/stat-1^-/^- mice was used for EV-A71 infection and pathogenesis. We infected the mouse model with EV-A71, which was premixed with mouse sera immunized with the EV-A71 vaccine with or without PS-G. Indeed, in the EV-A71 + PS-G group, the levels of VP1-specific RNA sequences in the brain, spinal cord, and muscle decreased significantly. Finally, hSCARB2-Tg mice immunized via the intranasal route with the PS-G-adjuvanted EV-A71 vaccine resisted a subsequent lethal oral EV-A71 challenge. Taken together, these results demonstrated that PS-G could potentially be used as an adjuvant for the EV-A71 mucosal vaccine.

Identification and initial functional characterization of lysosomal integral membrane protein type 2 (LIMP-2) in turbot (Scophthalmus maximus L.)

The immune system protects organism from external pathogens, this progress starts with the pathogen recognition by pattern recognition receptors (PRRs). As a group of PRRs, the class B scavenger receptors showed important roles in phagocytosis. Among three class B scavenger receptors, lysosomal integral membrane protein type 2 (LIMP-2) was reported to present in the limiting membranes of lysosomes and late endosomes, but its immune roles in teleost species are still limited in handful species. Here, we characterized LIMP-2 gene in turbot, and its expression patterns in mucosal barriers following different bacterial infection, as well as ligand binding activities to different microbial ligands and agglutination assay with different bacteria. In our results, one SmLIMP2 gene was identified with a 1,593 bp open reading frame (ORF). The multiple species comparison and phylogenetic analysis showed the closest relationship to Paralichthys olivaceus, the genomic structure analysis and syntenic analysis revealed the conservation of LIMP-2 during evolution. In tissue distribution analysis, SmLIMP-2 was expressed in all the examined turbot tissues, with the highest expression level in brain, and the lowest expression level in liver. In addition, SmLIMP-2 was significantly up-regulated in all the mucosal tissues (skin, gill and intestine) following Gram-negative bacteria Vibrio anguillarum infection, and was only up-regulated in gill following Gram-positive bacteria Streptococcus iniae challenge. Finally, the rSmLIMP-2 showed strong binding ability to all the examined microbial ligands, and strong agglutination with Escherichia coli, Staphylococcus aureus and V. anguillarum. Taken together, our results suggested SmLIMP-2 played important roles in fish immune response to bacterial infection. However, further functional studies should be carried out to better characterize its detailed roles in teleost immunity.

Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.

Transgenic mouse model for the study of enterovirus 71 neuropathogenesis

Enterovirus 71 (EV71) typically causes mild hand-foot-and-mouth disease in children, but it can also cause severe neurological disease. Recently, epidemic outbreaks of EV71 with significant mortality have been reported in the Asia-Pacific region, and EV71 infection has become a serious public health concern worldwide. However, there is little information available concerning EV71 neuropathogenesis, and no vaccines or anti-EV71 drugs have been developed. Previous studies of this disease have used monkeys and neonatal mice that are susceptible to some EV71 strains as models. The monkey model is problematic for ethical and economical reasons, and mice that are more than a few weeks old lose their susceptibility to EV71. Thus, the development of an appropriate small animal model would greatly contribute to the study of this disease. Mice lack EV71 susceptibility due to the absence of a receptor for this virus. Previously, we identified the human scavenger receptor class B, member 2 (hSCARB2) as a cellular receptor for EV71. In the current study, we generated a transgenic (Tg) mouse expressing hSCARB2 with an expression profile similar to that in humans. Tg mice infected with EV71 exhibited ataxia, paralysis, and death. The most severely affected cells were neurons in the spinal cord, brainstem, cerebellum, hypothalamus, thalamus, and cerebrum. The pathological features in these Tg mice were generally similar to those of EV71 encephalomyelitis in humans and experimentally infected monkeys. These results suggest that this Tg mouse could represent a useful animal model for the study of EV71 infection.

Hypolipidemic effect of SR‑BI gene delivery by combining cationic liposomal microbubbles and ultrasound in hypercholesterolemic rats

High-density lipoprotein (HDL) is a key mediator in reverse cholesterol transport and is involved in a mechanism known as 'selective lipid uptake', a process mediated by scavenger receptor B type I (SR‑BI), which is a HDL receptor. The aim of the present study was to investigate the therapeutic effect of the SR‑BI gene when delivered by combining cationic liposomal microbubbles (CLMs) and ultrasound (US) in hypercholesterolemic rats. Hypercholesterolemia was induced by administration of excessive doses of vitamin D3 and cholesterol in rats. The CLMs consisted of perfluoropropane gas encapsulated in a phospholipid shell using the sonication‑lyophilization method. The SR‑BI gene, mixed with the self‑made microbubbles, was transfected into hypercholesterolemic rat arteries using therapeutic US. SR‑BI protein expression was determined by western blot analysis 2 days post-transfection. Two weeks after transfection, total cholesterol (TC), triglyceride (TG), low-density lipoprotein (LDL) and HDL serum concentrations were measured. Transfection efficiency of the SR‑BI gene in the SR‑BI + US/CLM group increased 6‑7‑fold compared with the SR‑BI group. Two weeks after transfection, plasma lipid levels in treated hypercholesterolemic rats were observed to be significantly reduced compared with rats that did not receive treatment. However, no significant change was observed in the SR‑BI group compared with that in the SR‑BI + US/CLM group. Results of the present study indicate that the combination of US and CLMs loaded with the SR‑BI gene may exert a protective role in hypercholesterolemia.

Contrast imaging and gene delivery through the combined use of novel cationic liposomal microbubbles and ultrasound in rat carotid arteries

INTRODUCTION

Lipid-coated cationic microbubbles represent a new class of agents with both diagnostic and therapeutic applications. The main goal of this study was to evaluate the efficiency of gene transfer through the combined use of microbubbles and ultrasound in rat carotid arteries. Furthermore, we assessed whether the cationic liposomal microbubbles could allow long-term enhanced imaging, comparing with SonoVue(®).

MATERIAL AND METHODS

Normal rat carotid arteries were imaged after intravenous bolus injections of 0.5 ml/kg of two contrast agents (SonoVue(®) and the cationic liposomal microbubbles). Forty Sprague-Dawley rats were divided into 5 groups according to ultrasound parameters and were treated with or without microbubbles. All rats were sacrificed after being transfected for 2 days. The level of protein expression was determined by western blot analysis.

RESULTS

The enhancing time of self-made microbubbles was much longer than that of SonoVue(®) in rat carotid arteries (p < 0.05). The results of the western blot analysis revealed that the expression of SR-BI DNA in the carotid artery was highest in the SR-BI + US/CLM group (p < 0.05).

CONCLUSIONS

These results suggest that the novel cationic liposomal microbubbles enhance image quality over a longer period than does SonoVue(®). Additionally, the combination of ultrasound and this new type of microbubble can act synergistically to increase SR-BI DNA transfection.

Inhibition of hepatic scavenger receptor-class B type I by RNA interference decreases atherosclerosis in rabbits

Objective

Scavenger receptor-class B type I (SR-BI), the receptor for HDL-cholesterol, plays a key role in HDL metabolism, whole body cholesterol homeostasis, and reverse cholesterol transport. We investigated the in vivo impact of hepatic SR-BI inhibition on lipoprotein metabolism and the development of atherosclerosis employing RNA interference.

Methods

Small hairpin RNA plasmid specific for rabbit SR-BI was complexed with galactosylated poly-l-lysine, allowing an organ-selective, receptor-mediated gene transfer. Rabbits were fed a cholesterol-rich diet, and were injected with plasmid-complexes once a week.

Results

After 2 weeks of treatment hepatic SR-BI mRNA levels were reduced by 80% accompanied by reduced SR-BI protein levels and a modulation of the lipoprotein profile. Rabbits treated with SR-BI-specific plasmid-complexes displayed higher cholesteryl ester transfer from HDL to apoB-containing lipoproteins, lower HDL-cholesterol, and higher VLDL-cholesterol levels, when compared to controls. In a long-term study, this gene therapeutic intervention led to a similar modulation of the lipoprotein profile, to lower total cholesterol levels, and most importantly to a 50% reduction of the relative atherosclerotic lesion area.

Conclusion

Our results are another indication that the role of SR-BI in lipoprotein metabolism and atherogenesis in rabbits – a CETP-expressing animal model displaying a manlike lipoprotein profile may be different from the one found in rodents.

Scavenger receptor class B member 1 protein: hepatic regulation and its effects on lipids, reverse cholesterol transport, and atherosclerosis

Scavenger receptor class B member 1 (SR-BI, also known as SCARB1) is the primary receptor for the selective uptake of cholesterol from high-density lipoprotein (HDL). SR-BI is present in several key tissues; however, its presence and function in the liver is deemed the most relevant for protection against atherosclerosis. Cholesterol is transferred from HDL via SR-BI to the liver, which ultimately results in the excretion of cholesterol via bile and feces in what is known as the reverse cholesterol transport pathway. Much of our knowledge of SR-BI hepatic function and regulation is derived from mouse models and in vitro characterization. Multiple independent regulatory mechanisms of SR-BI have been discovered that operate at the transcriptional and post-transcriptional levels. In this review we summarize the critical discoveries relating to hepatic SR-BI cholesterol metabolism, atherosclerosis, and regulation of SR-BI, as well as alternative functions that may indirectly affect atherosclerosis.

Aldosterone production in human adrenocortical cells is stimulated by high-density lipoprotein 2 (HDL2) through increased expression of aldosterone synthase (CYP11B2)

Adrenal aldosterone production is regulated by physiological agonists at the level of early and late rate-limiting steps. Numerous studies have focused on the role of lipoproteins including high-density lipoprotein (HDL) as cholesterol providers in this process; however, recent research suggests that HDL can also act as a signaling molecule. Herein, we used the human H295R adrenocortical cell model to study the effects of HDL on adrenal aldosterone production and CYP11B2 expression. HDL, especially HDL2, stimulated aldosterone synthesis by increasing expression of CYP11B2. HDL treatment increased CYP11B2 mRNA in both a concentration- and time-dependent manner, with a maximal 19-fold increase (24 h, 250 μg/ml of HDL). Effects of HDL on CYP11B2 were not additive with natural agonists including angiotensin II or K(+). HDL effects were likely mediated by a calcium signaling cascade, because a calcium channel blocker and a calmodulin kinase inhibitor abolished the CYP11B2-stimulating effects. Of the two subfractions of HDL, HDL2 was more potent than HDL3 in stimulating aldosterone and CYP11B2. Further studies are needed to identify the active components of HDL, which regulate aldosterone production.

The thyromimetic T-0681 protects from atherosclerosis

This report describes studies in hyperlipidemic New Zealand White (NZW) rabbits investigating the impact of the liver-selective thyromimetic T-0681 on lipoprotein metabolism and the development of atherosclerosis. Prolonged treatment with T-0681 increased the hepatic expression of both LDL receptor and scavenger receptor class B, type I without affecting cholesteryl ester transfer protein activity. Upregulation of hepatic lipoprotein receptors was accompanied by a marked decrease of apolipoprotein B-containing lipoproteins, reflected by a 60% reduction of plasma cholesterol and a >70% reduction of plasma triglyceride levels. Most importantly, T-0681 reduced the development of atherosclerosis by 80% in NZW rabbits on high-cholesterol chow. Our data suggest that liver-selective thyromimetics, such as T-0681, may prove to be useful therapeutic agents against the development of atherosclerosis in humans.

Influence of aspirin on SR-BI expression in human carotid plaques

BACKGROUND

We recently showed that aspirin promotes scavenger receptor class-B type I (SR-BI) protein expression in vitro in primary human macrophages and in vivo in resident peritoneal macrophages of mice.

METHODS

We compared SR-BI and CD68 expression in carotid atherosclerotic specimens from endarterectomized patients with (n=38) or without (n=19) low-dose aspirin medication (100 mg/day) prior to endarterectomy.

RESULTS

We found no differences concerning expression of CD68, indicating that aspirin did not influence macrophage content within atherosclerotic plaques. However, aspirin increased the expression of SR-BI protein in the analyzed specimens. In human THP-1-derived macrophages, induction of SR-BI protein by aspirin was abrogated by concomitant pharmacological inhibition of nuclear factor-kappa B (NF-kappaB). In in vitro experiments employing cultured primary macrophages from NF-kappaB/p50 KO mice, aspirin was not able to influence SR-BI expression. Additionally, no considerable effects on SR-BI expression were observed in vivo in resident macrophages of NF-kappaB/p50 KO mice orally treated with low or high doses of aspirin, respectively.

CONCLUSIONS

We suggest that aspirin treatment might lead to enhanced expression of SR-BI in human plaque macrophages and that this effect is dependent on the presence of NF-kappaB.

The inhibition of endocytosis affects HDL-lipid uptake mediated by the human scavenger receptor class B type I

The scavenger receptor SR-BI plays an important role in the hepatic clearance of HDL cholesterol and other lipids, driving reverse cholesterol transport and contributing to protection against atherosclerosis in mouse models. We characterized the role of endocytosis in lipid uptake from HDL, mediated by the human SR-BI, using a variety of approaches to inhibit endocytosis, including hypertonic shock, potassium or energy depletion and disassembly of the actin cytoskeleton. Our studies revealed that unlike mouse SR-BI, human SR-BI-mediated HDL-lipid uptake was reduced by inhibition of endocytosis. This was not dependent on the cytoplasmic C-terminus of SR-BI. Monitoring the uptake of both the protein and lipid components of HDL revealed that although overall lipid uptake was decreased, the degree of selective lipid uptake was increased. These data suggest that that endocytosis is a dynamic regulator of SR-BI's selective lipid uptake activity.

Effects of amino acid substitutions at glycine 420 on SR-BI cholesterol transport function

Scavenger receptor class B type I (SR-BI) facilitates the uptake of HDL cholesteryl esters (CEs) in a two-step process involving binding of HDL to its extracellular domain and transfer of HDL core CEs to a metabolically active membrane pool, where they are subsequently hydrolyzed by a neutral CE hydrolase. Recently, we characterized a mutant, G420H, which replaced glycine 420 in the extracellular domain of SR-BI with a histidine residue and had a profound effect on SR-BI function. The G420H mutant receptor exhibited a reduced ability to mediate selective HDL CE uptake and was unable to deliver HDL CE for hydrolysis, despite the fact that it retained the ability to bind HDL. This did not hold true if glycine 420 was replaced with an alanine residue; G420A maintained wild-type HDL binding and cholesterol transport activity. To further understand the role that glycine 420 plays in SR-BI function and why there was a disparity between replacing glycine 420 with a histidine versus an alanine, we generated a battery of point mutants by substituting glycine 420 with amino acids possessing side chains that were charged, hydrophobic, polar, or bulky and tested the resulting mutants for their ability to support HDL binding, HDL cholesterol transport, and delivery for hydrolysis. The results indicated that substitution with a negatively charged residue or a proline impaired cell surface expression of SR-BI or its interaction with HDL, respectively. Furthermore, substitution of glycine 420 with a positively charged residue reduced HDL CE uptake as well as its subsequent hydrolysis.

Hepatic ENPP1 expression is induced in diabetic rabbits

The ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) is an inhibitor of the insulin receptor. Variants of ENPP1 are associated with infantile arterial calcification, obesity, and insulin resistance. To study the functional relevance of this protein in vivo, we cloned rabbit ENPP1 and studied its regulation in experimentally induced diabetes mellitus. We amplified and sequenced the complete coding sequence of rabbit ENPP1 gene out of a liver cDNA library using redundant primers deduced from other species. Next, we performed quantitative PCR of ENPP1 to study the tissue distribution of ENPP1 expression and its regulation in an alloxan-dependent diabetes model. The putative rabbit ENPP1 protein contains 873 amino acids and is highly conserved when compared with human ENPP1 (90% amino acid identity). Particularly high levels of ENPP1 mRNA expression were found in adipose tissue. Quantitative PCR analysis revealed a significant upregulation of ENPP1 transcription in liver (p = 0.025) and brain (p = 0.034) of diabetic rabbits compared with controls. Hepatic ENPP1 expression is induced in diabetic rabbits when compared with controls. This approximately twofold upregulation of ENPP1 mRNA in rabbit liver parallels previous findings in patients with type 2 diabetes mellitus. We provide further molecular information about ENPP1 as a potential pharmacologic target and characterize its regulation in an insulin-dependent diabetes mellitus animal model.

Aspirin regulates expression and function of scavenger receptor‐BI in macrophages: studies in primary human macrophages and in mice

Scavenger receptor class B type I (SR-BI) has been shown to be expressed in human atherosclerotic plaque macrophages, where it is believed to reduce atherosclerosis by promoting cholesterol efflux. In this study we investigated the influence of aspirin and other NSAIDs on SR-BI expression and function in cultured human macrophages as well as in different mouse strains. Incubation of human macrophages with 0.5 mmol/l aspirin resulted in increased SR-BI protein expression and increased uptake of HDL-associated [3H]cholesteryl oleate without changes of SR-BI mRNA levels. In contrast, using 5 mmol/l of aspirin, SR-BI expression and function were significantly decreased. Sodium salicylate exerted similar effects on SR-BI expression, whereas no effects were observed using known COX1/2 inhibitors ibuprofen and naproxen, respectively. In in vivo studies low-dose aspirin treatment (6 mg/kg.day) induced SR-BI expression in wild-type and PPAR-alpha knockout mice, respectively, whereas the opposite effect was observed upon high-dose aspirin treatment (60 mg/kg.day) in these animals. We could show that COX-independent effects of aspirin were able to enhance expression of SR-BI in macrophages in a post-transcriptional, PPAR-alpha independent way, suggesting a novel pharmacologic effect of aspirin.

High density lipoprotein endocytosis by scavenger receptor SR-BII is clathrin-dependent and requires a carboxyl-terminal dileucine motif

The high density lipoprotein (HDL) receptor Scavenger Receptor BII (SR-BII) is encoded by an alternatively spliced mRNA from the SR-BI gene and is expressed in various tissues. SR-BII protein differs from SR-BI only in the carboxyl-terminal cytoplasmic tail, which, as we showed previously, must contain a signal that confers predominant intracellular expression and rapid endocytosis of HDL. We have shown that SR-BII mediates HDL endocytosis through aclathrin-dependent, caveolae-independent pathway. Two candidate amino acid motifs were identified in the tail that could mediate association with clathrin-containing endocytic vesicles: a putative dileucine motif at position 492-493 and an overlapping tyrosine-based YXXZ motif starting at position 489. Although substitution of tyrosine at position 489 with alanine or histidine did not affect endocytosis, substitution L492A resulted in increased surface binding of HDL and reduced HDL particle endocytosis. Substitution L493A had a less dramatic effect. No other regions in the carboxyl-terminal tail appeared to contain motifs required for HDL endocytosis. Substitutions of leucine at position 492 with the hydrophobic amino acids valine or phenylalanine also reduced HDL endocytosis, stressing the importance of leucine at this position. Introducing the SR-BII YTPLL motif into the carboxyl-terminal cytoplasmic tail of SR-BI converted SR-BI into an endocytic receptor resembling SR-BII. These results demonstrated that SR-BII differs from SR-BI in subcellular localization and trafficking and suggest that the two isoforms differ in the manner in which they target ligands intracellularly.

Probucol enhances the expression of human hepatic scavenger receptor class B type I, possibly through a species-specific mechanism

OBJECTIVE

Scavenger receptor class B type I (SR-BI) is a major receptor for high-density lipoproteins (HDL) in the liver, which is the terminus of reverse cholesterol transport. Overexpression of SR-BI attenuated experimental atherosclerosis in murine models, concomitant with a reduction in plasma HDL-cholesterol levels. Probucol is known to be a potent hypolipidemic drug to regress xanthoma formation and carotid atherosclerosis in conjunction with a marked reduction in HDL-cholesterol levels. The aim of the present study was to know the effect of probucol on the expression of SR-BI and the underlying mechanism.

METHODS AND RESULTS

We found that probucol increased the expression of SR-BI proteins in in vitro human liver cells and an in vivo rabbit model, but not in wild-type C57Bl6 mice. The decay curve of SR-BI protein was markedly retarded in probucol-treated HepG2 cells in the presence of cycloheximide, indicating that probucol may stabilize human SR-BI protein. To determine the underlying mechanism for the observed species-specific effect, we conducted the following host-swap experiments, in which SR-BI was transfected or expressed in heterologous cells or hosts. Probucol did not increase human SR-BI protein in the liver of transgenic mice carrying the entire human SR-BI genome. Although probucol could stabilize even murine SR-BI, when transfected into a human cell line, HepG2, human SR-BI was not stabilized in a mouse hepatoma cell line, Hepa 1-6, treated with probucol.

CONCLUSIONS

Probucol enhances hepatic SR-BI protein expression, possibly through species-specific stabilization of the protein.

Increased plasma levels of LDL cholesterol in rabbits after adenoviral overexpression of human scavenger receptor class B type I

Scavenger receptor class B type I (SR-BI), a CD36 family member, plays a key role in high-density lipoprotein (HDL) metabolism, reverse cholesterol transport, and whole body cholesterol homeostasis, and is shown to be involved in the development of atherosclerosis in mice. In this report, we describe the effects of the adenoviral overexpression of human SR-BI (hSR-BI) in New Zealand White (NZW) rabbits, a wild-type animal model that expresses cholesteryl ester transfer protein (CETP) in plasma, displays a manlike lipoprotein profile, and is susceptible to atherosclerosis. A total of 1x10(12) adenoviral particles containing either hSR-BI or lacZ complementary deoxyribonucleic acid (control) were infused into the ear vein of NZW rabbits. Transgene expression was ascertained by TaqMan Real Time polymerase chain reaction measurements. Rabbits infected with Ad/hSR-BI (adenoviral plasmids containing hSR-BI) showed a faster clearance of administered [3H]HDL cholesterol and significantly decreased apolipoprotein (apo) A-I levels when compared to control rabbits, respectively. Interestingly, we found markedly increased levels of low-density lipoprotein (LDL) cholesterol exclusively in SR-BI-overexpressing rabbits. These changes were not accompanied by alterations in LDL receptor expression but by increased levels of CE transfer in these animals. By lowering HDL cholesterol and increasing plasma apoB-containing lipoprotein levels, the overexpression of SR-BI leads to a lipoprotein pattern, which is believed to enhance the development of atherosclerosis. The role of SR-BI in lipoprotein metabolism and atherogenesis in rabbits--a CETP-expressing animal model displaying a manlike lipoprotein profile--may therefore be different from the one found in rodents.

Pioglitazone increases the fractional catabolic and production rates of high-density lipoproteins apo AI in the New Zealand White Rabbit

Pioglitazone is an agonist of the peroxisome proliferator-activated receptor gamma (PPARgamma) that raises HDL-cholesterol plasma in humans. Whether pioglitazone-mediated modifications in HDL-apolipoprotein AI (apo AI) turnover in vivo contribute to this effect has not been completely elucidated. Therefore, we performed kinetic studies of HDL-apo AI radiolabeled with 125I in male New Zealand White rabbits after 6 weeks of 0.6 (n = 8), 1.75 (n = 8), and 2.6 mg/kg/day (n = 7) pioglitazone and vehicle (n = 12) treatment. Fractional catabolic rate (FCR) of HDL-apo AI was significantly higher in 1.75 and 2.6 mg/kg pioglitazone-treated animals, as compared with control rabbits (0.057+/-0.014 and 0.049+/-0.01 versus 0.025+/-0.005 pools/h, respectively); these changes were associated to a similar increase in apo AI production rates (PR) (1.24+/-0.62 and 1.14+/-0.40 versus 0.53+/-0.17 mg/kg/h, p < 0.01). Consequently, apo AI plasma levels in pioglitazone-treated animals were similar to those of controls. The apo AI-FRC and -PR correlated with the relative proportion of the HDL3c subclass, as determined by polyacrylamide gradient electrophoresis. Our data demonstrate that pioglitazone markedly modifies apo AI kinetics and enhances the proportion of small HDL3c particles, despite the unchanged apo AI concentration. Whether or not the pioglitazone-induced structural changes of HDL contribute to the anti-atherosclerotic effects of the drug remains to be determined.

Glycine 420 near the C-terminal transmembrane domain of SR-BI is critical for proper delivery and metabolism of high density lipoprotein cholesteryl ester

Scavenger receptor BI, SR-BI, is a physiologically relevant receptor for high density lipoprotein (HDL) that mediates the uptake of cholesteryl esters and delivers them to a metabolically active membrane pool where they are subsequently hydrolyzed. A previously characterized SR-BI mutant, A-VI, with an epitope tag inserted into the extracellular domain near the C-terminal transmembrane segment, revealed a separation-of-function between SR-BI-mediated HDL cholesteryl ester uptake and cholesterol efflux to HDL, on one hand, and cholesterol release to small unilamellar phospholipid vesicle acceptors and an increased cholesterol oxidase-sensitive pool of membrane free cholesterol on the other. To further elucidate amino acid residues responsible for this separation-of-function phenotype, we engineered alanine substitutions and point mutations in and around the site of epitope tag insertion, and tested these for various cholesterol transport functions. We found that changing amino acid 420 from glycine to histidine had a profound effect on SR-BI function. Despite the ability to mediate selective HDL cholesteryl ester uptake, the G420H receptor had a greatly reduced ability to: 1) enlarge the cholesterol oxidase-sensitive pool of membrane free cholesterol, 2) mediate cholesterol efflux to HDL, even at low concentrations of HDL acceptor where binding-dependent cholesterol efflux predominates, and 3) accumulate cholesterol mass within the cell. Most importantly, the G420H mutant was unable to deliver the HDL cholesteryl ester to a metabolically active membrane compartment for efficient hydrolysis. These observations have important implications regarding SR-BI function as related to its structure near the C-terminal transmembrane domain.