Deficiency of scavenger receptor BI leads to impaired lymphocyte homeostasis and autoimmune disorders in mice

Scavenger receptor class B type I and immune dysfunctions

PURPOSE OF REVIEW

To summarize the recent findings about the roles of scavenger receptor class B type I (SR-BI) in immunity and discuss the underlying mechanisms by which SR-BI prevents immune dysfunctions.

RECENT FINDINGS

SR-BI is well known as a high-density lipoprotein (HDL) receptor playing key roles in HDL metabolism and in protection against atherosclerosis. Recent studies have indicated that SR-BI is also an essential modulator in immunity. SR-BI deficiency in mice causes immune dysfunctions, including increased atherosclerosis, elevated susceptibility to sepsis, impaired lymphocyte homeostasis, and autoimmune disorders. SR-BI exerts its protective roles through a variety of HDL-dependent and HDL-independent mechanisms. SR-BI is also involved in hepatitis C virus cell entry. A deficiency of SR-BI in humanized mice has been shown to decrease hepatitis C virus infectivity.

SUMMARY

SR-BI regulates immunity via multiple mechanisms and its deficiency causes numerous diseases. A comprehensive understanding of the roles of SR-BI in protection against immune dysfunctions may provide a therapeutic target for intervention against its associated diseases.

Scavenger receptor BI and high-density lipoprotein regulate thymocyte apoptosis in sepsis

OBJECTIVE

Thymocyte apoptosis is a major event in sepsis; however, how this process is regulated remains poorly understood.

APPROACH AND RESULTS

Septic stress induces glucocorticoids production which triggers thymocyte apoptosis. Here, we used scavenger receptor BI (SR-BI)-null mice, which are completely deficient in inducible glucocorticoids in sepsis, to investigate the regulation of thymocyte apoptosis in sepsis. Cecal ligation and puncture induced profound thymocyte apoptosis in SR-BI(+/+) mice, but no thymocyte apoptosis in SR-BI(-/-) mice because of lack of inducible glucocorticoids. Unexpectedly, supplementation of glucocorticoids only partly restored thymocyte apoptosis in SR-BI(-/-) mice. We demonstrated that high-density lipoprotein (HDL) is a critical modulator for thymocyte apoptosis. SR-BI(+/+) HDL significantly enhanced glucocorticoid-induced thymocyte apoptosis, but SR-BI(-/-) HDL had no such activity. Further study revealed that SR-BI(+/+) HDL modulates glucocorticoid-induced thymocyte apoptosis via promoting glucocorticoid receptor translocation, but SR-BI(-/-) HDL loses such regulatory activity. To understand why SR-BI(-/-) HDL loses its regulatory activity, we analyzed HDL cholesterol contents. There was 3-fold enrichment of unesterified cholesterol in SR-BI(-/-) HDL compared with SR-BI(+/+) HDL. Normalization of unesterified cholesterol in SR-BI(-/-) HDL by probucol administration or lecithin cholesteryl acyltransferase expression restored glucocorticoid-induced thymocyte apoptosis, and incorporating unesterified cholesterol into SR-BI(+/+) HDL rendered SR-BI(+/+) HDL dysfunctional. Using lckCre-GR(fl/fl) mice in which thymocytes lack cecal ligation and puncture-induced thymocyte apoptosis, we showed that lckCre-GR(fl/fl) mice were significantly more susceptible to cecal ligation and puncture-induced septic death than GR(fl/fl) control mice, suggesting that glucocorticoid-induced thymocyte apoptosis is required for protection against sepsis.

CONCLUSIONS

The findings in this study reveal a novel regulatory mechanism of thymocyte apoptosis in sepsis by SR-BI and HDL.

The atherogenic Scarb1 null mouse model shows a high bone mass phenotype

Scavenger receptor class B, type I (SR-BI), the Scarb1 gene product, is a receptor associated with cholesteryl ester uptake from high-density lipoproteins (HDL), which drives cholesterol movement from peripheral tissues toward the liver for excretion, and, consequently, Scarb1 null mice are prone to atherosclerosis. Because studies have linked atherosclerosis incidence with osteoporosis, we characterized the bone metabolism in these mice. Bone morphometry was assessed through microcomputed tomography and histology. Marrow stromal cells (MSCs) were used to characterize influence of endogenous SR-BI in cell functions. Total and HDL-associated cholesterol in null mice were increased by 32-60%, correlating with its role in lipoprotein metabolism. Distal metaphyses from 2- and 4-mo-old null mice showed correspondingly 46 and 37% higher bone volume fraction associated with a higher number of trabeculae. Histomorphometric analyses in 2-mo-old null male mice revealed 1.42-fold greater osteoblast surface, 1.37-fold higher percent mineralizing surface, and 1.69-fold enhanced bone formation rate. In vitro assays for MSCs from null mice revealed 37% higher proliferation rate, 48% more alkaline phosphatase activity, 70% greater mineralization potential and a 2-fold osterix (Sp7) expression, yet a 0.5-fold decrease in caveolin-1 (Cav1) expression. Selective uptake levels of HDL-associated cholesteryl oleate and estradiol were similar between MSC from wild-type and Scarb1 null mice, suggesting that its contribution to this process is not its main role in these cells. However, Scarb1 knockout stunted the HDL-dependent regulation of Cav1 genic expression. Scarb1 null mice are not prone to osteoporosis but show higher bone mass associated with enhanced bone formation.

SR-BI in bone marrow derived cells protects mice from diet induced coronary artery atherosclerosis and myocardial infarction

SR-BI deficient mice that are also hypomorphic for apolipoprotein E expression develop diet induced occlusive coronary artery atherosclerosis, myocardial infarction and early death. To test the role of SR-BI in bone marrow derived cells, we used bone marrow transplantation to generate SR-BI-null; apoE-hypomorphic mice in which SR-BI expression was restored solely in bone marrow derived cells. SR-BI-null; apoE-hypomorphic mice were transplanted with SR-BI^+/+apoE-hypomorphic, or control, autologous SR-BI-null; apoE-hypomorphic bone marrow. Four weeks later, mice were fed a high-fat, high-cholesterol, cholate-containing diet to induce coronary artery atherosclerosis. Mice transplanted with autologous bone marrow developed extensive aortic atherosclerosis and severe occlusive coronary artery atherosclerosis after 4 weeks of feeding. This was accompanied by myocardial fibrosis and increased heart weights. In contrast, restoration of SR-BI expression in bone marrow derived-cells reduced diet induced aortic and coronary artery atherosclerosis, myocardial fibrosis and the increase in heart weights in SR-BI-null; apoE-hypomorphic mice. Restoration of SR-BI in bone marrow derived cells did not, however, affect steady state lipoprotein cholesterol levels, but did reduce plasma levels of IL-6. Monocytes from SR-BI-null mice exhibited a greater capacity to bind to VCAM-1 and ICAM-1 than those from SR-BI^+/+ mice. Furthermore, restoration of SR-BI expression in bone marrow derived cells attenuated monocyte recruitment into atherosclerotic plaques in mice fed high fat, high cholesterol cholate containing diet. These data demonstrate directly that SR-BI in bone marrow-derived cells protects against both aortic and CA atherosclerosis.

Scavenger receptors in homeostasis and immunity

Scavenger receptors were originally identified by their ability to recognize and to remove modified lipoproteins; however, it is now appreciated that they carry out a striking range of functions, including pathogen clearance, lipid transport, the transport of cargo within the cell and even functioning as taste receptors. The large repertoire of ligands recognized by scavenger receptors and their broad range of functions are not only due to the wide range of receptors that constitute this family but also to their ability to partner with various co-receptors. The ability of individual scavenger receptors to associate with different co-receptors makes their responsiveness extremely versatile. This Review highlights recent insights into the structural features that determine the function of scavenger receptors and the emerging role that these receptors have in immune responses, notably in macrophage polarization and in the pathogenesis of diseases such as atherosclerosis and Alzheimer's disease.

A coding variant in SR-BI (I179N) significantly increases atherosclerosis in mice

Human coding variants in scavenger receptor class B member 1 (SR-BI; gene name SCARB1) have recently been identified as being associated with plasma levels of HDL cholesterol. However, a link between coding variants and atherosclerosis has not yet been established. In this study we set out to examine the impact of a SR-BI coding variant in vivo. A mouse model with a coding variant in SR-BI (I179N), identified through a mutagenesis screen, was crossed with Ldlr (-/-) mice, and these mice were maintained on a Western-type diet to promote atherosclerosis. Mice showed 56 and 125 % increased atherosclerosis in female and male Ldlr (-/-) Scarb1 (I179N) mice, respectively, when compared to gender-matched Ldlr (-/-) control mice. As expected, HDL cholesteryl ester uptake was impaired in Ldlr (-/-) Scarb1 (I179N) mice compared to Ldlr (-/-) control mice, with a net effect of increased small and very small LDL cholesterol in Ldlr (-/-) Scarb1 (I179N) mice being the most probable cause of the observed increased atherosclerosis. Our data show that non-null coding variants in SR-BI can have a large significant impact on atherosclerosis, even if plasma lipid levels are not dramatically affected, and that human mutations in other candidate lipid genes could significantly impact atherosclerosis.

Molecular mechanisms responsible for the reduced expression of cholesterol transporters from macrophages by low-dose endotoxin

OBJECTIVE

Atherosclerosis is characterized as a chronic inflammatory condition that involves cholesterol deposition in arteries. Together with scavenger receptor B1 (SR-B1), the ATP-binding cassette transporters ABCA1 and ABCG1 are the major components of macrophage cholesterol efflux. Recent studies have shown that low-grade inflammation plays a distinct regulatory role in the expression of SR-B1 and ABCA1/ABCG1. However, the mechanisms linking low-grade inflammation and cholesterol accumulation are poorly understood.

METHODS AND RESULTS

Using primary bone-marrow-derived macrophages, we demonstrate that subclinical low-dose lipopolysaccharide potently reduces the expression of SR-B1 and ABCA1/ABCG1, as well as cholesterol efflux from macrophages through interleukin-1 receptor-associated kinase 1 and Toll-interacting-protein. Low-dose lipopolysaccharide downregulates the nuclear levels of retinoic acid receptor-α, leading to their reduced binding to the promoters of SR-B1 and ABCA1/ABCG1. We observe that glycogen synthase kinase 3β activation by low-dose lipopolysaccharide through interleukin-1 receptor-associated kinase 1 and Toll-interacting-protein is responsible for reduced levels of retinoic acid receptor-α, and reduced expression of SR-B1 and ABCA1/ABCG1. Interleukin-1 receptor-associated kinase M, however, counteracts the function of interleukin-1 receptor associated kinase 1.

CONCLUSIONS

Collectively, our data reveal a novel intracellular network regulated by low-dose endotoxemia that disrupts cholesterol efflux from macrophages and leads to the pathogenesis of atherosclerosis.

Functions of scavenger receptor class B, type I in atherosclerosis

PURPOSE OF REVIEW

This review highlights the diverse roles of the high-affinity HDL receptor scavenger receptor class B, type I (SR-BI) in the modulation of global cholesterol homeostasis and vascular cell function, and the potential implications of these processes in atherosclerosis.

RECENT FINDINGS

SR-BI in the liver plays a critical role in reverse cholesterol transport and it dramatically impacts the characteristics of the HDL particle, and through reverse cholesterol transport it promotes an antiatherogenic lipid environment in the vascular wall. SR-BI in macrophages may influence their inflammatory phenotype. In endothelial cells, SR-BI mediates HDL-induced endothelial nitric oxide synthase activation and proliferation and migration, and in platelets SR-BI may be prothrombotic in the setting of dyslipidemia. Several polymorphisms of SR-BI have been reported in humans that influence receptor expression or function.

SUMMARY

In addition to regulating global lipid metabolism, SR-BI influences the functions of a variety of vascular cells relevant to atherosclerosis. Studies of SR-BI genetics in humans partially support the conclusions drawn from experimental models. However, because of the multiple functions of SR-BI, the diversity of cell types in which it is expressed, and the influence of the receptor on the characteristics of its own ligand, our understanding of the biology of the receptor is just emerging.

Signaling in innate immunity and inflammation

Inflammation is triggered when innate immune cells detect infection or tissue injury. Surveillance mechanisms involve pattern recognition receptors (PRRs) on the cell surface and in the cytoplasm. Most PRRs respond to pathogen-associated molecular patterns (PAMPs) or host-derived damage-associated molecular patterns (DAMPs) by triggering activation of NF-κB, AP1, CREB, c/EBP, and IRF transcription factors. Induction of genes encoding enzymes, chemokines, cytokines, adhesion molecules, and regulators of the extracellular matrix promotes the recruitment and activation of leukocytes, which are critical for eliminating foreign particles and host debris. A subset of PRRs activates the protease caspase-1, which causes maturation of the cytokines IL1β and IL18. Cell adhesion molecules and chemokines facilitate leukocyte extravasation from the circulation to the affected site, the chemokines stimulating G-protein-coupled receptors (GPCRs). Binding initiates signals that regulate leukocyte motility and effector functions. Other triggers of inflammation include allergens, which form antibody complexes that stimulate Fc receptors on mast cells. Although the role of inflammation is to resolve infection and injury, increasing evidence indicates that chronic inflammation is a risk factor for cancer.