Scavenger Receptor Class B Type I Promoter Research Articles

Recombinant Humanized IgG1 Antibody Promotes Reverse Cholesterol Transport through FcRn-ERK1/2-PPARα Pathway in Hepatocytes

Hyperlipidemia-associated lipid disorders are considered the cause of atherosclerotic cardiovascular disease. Reverse cholesterol transport (RCT) is a mechanism by which excess peripheral cholesterol is transported to the liver and further converted into bile acid for excretion from the body in feces, which contributes to reducing hyperlipidemia as well as cardiovascular disease. We previously found that the recombinant humanized IgG1 antibody promotes macrophages to engulf lipids and increases cholesterol efflux to high-density lipoprotein (HDL) through ATP-binding cassette sub-family A1 (ABCA1), one of the key proteins related to RCT. In the present study, we explored other RCT related proteins expression on hepatocytes, including scavenger receptor class B type I (SR-BI), apolipoprotein A-I (ApoA-I), and apolipoprotein A-II (ApoA-II), and its modulation mechanism involved. We confirmed that the recombinant humanized IgG1 antibody selectively activated ERK1/2 to upregulate SR-BI, ApoA-I, and ApoA-II expression in mice liver and human hepatocellular carcinoma cell lines HepG2 cells. The rate-limiting enzymes of bile acid synthesis, including cholesterol 7α-hydroxylase (CYP7A1) and sterol 27-hydroxylase (CYP27A1), exhibited a significant increase when treated with the recombinant humanized IgG1 antibody, as well as increased excretion of bile acids in feces. Besides, abolishment or mutation of peroxisome proliferator-activated receptor α (PPARα)/RXR binding site on SR-BI promoter eliminated SR-BI reporter gene luciferase activity even in the presence of the recombinant humanized IgG1 antibody. Knock down the neonatal Fc receptor (FcRn) on hepatocytes impaired the effect of recombinant humanized IgG1 antibody on activation of ERK1/2, as well as upregulation of SR-BI, ApoA-I, and ApoA-II expression. In conclusion, one of the mechanisms on the recombinant humanized IgG1 antibody attenuates hyperlipidemia in ApoE-/- mice model fed with high-fat-diet might be through reinforcement of liver RCT function in an FcRn-ERK1/2-PPARα dependent manner.

The Distinct Role of the HDL Receptor SR-BI in Cholesterol Homeostasis of Human Placental Arterial and Venous Endothelial Cells.

As opposed to adults, high-density lipoprotein (HDL) is the main cholesterol carrying lipoprotein in fetal circulation. The major HDL receptor, scavenger receptor class B type I (SR-BI), contributes to local cholesterol homeostasis. Arterial endothelial cells (ECA) from human placenta are enriched with cholesterol compared to venous endothelial cells (ECV). Moreover, umbilical venous and arterial plasma cholesterol levels differ markedly. We tested the hypothesis that the uptake of HDL-cholesteryl esters differs between ECA and ECV because of the differential expression of SR-BI. We aimed to identify the key regulators underlying these differences and the functional consequences. Immunohistochemistry was used for visualization of SR-BI in situ. ECA and ECV were isolated from the chorionic plate of human placenta and used for RT-qPCR, Western Blot, and HDL uptake assays with 3H- and 125I-labeled HDL. DNA was extracted for the methylation profiling of the SR-BI promoter. SR-BI regulation was studied by exposing ECA and ECV to differential oxygen concentrations or shear stress. Our results show elevated SR-BI expression and protein abundance in ECA compared to ECV in situ and in vitro. Immunohistochemistry demonstrated that SR-BI is mainly expressed on the apical side of placental endothelial cells in situ, allowing interaction with mature HDL circulating in the fetal blood. This was functionally linked to a higher increase of selective cholesterol ester uptake from fetal HDL in ECA than in ECV, and resulted in increased cholesterol availability in ECA. SR-BI expression on ECV tended to decrease with shear stress, which, together with heterogeneous immunostaining, suggests that SR-BI expression is locally regulated in the placental vasculature. In addition, hypomethylation of several CpG sites within the SR-BI promoter region might contribute to differential expression of SR-BI between chorionic arteries and veins. Therefore, SR-BI contributes to a local cholesterol homeostasis in ECA and ECV of the human feto-placental vasculature.

Low-density lipoprotein upregulate SR-BI through Sp1 Ser702 phosphorylation in hepatic cells

Scavenger receptor class B type I (SR-BI) is one of the key proteins in the process of reverse cholesterol transport (RCT), and its major function is to uptake high density lipoprotein (HDL) cholesterol from plasma into liver cells. The regulation of SR-BI expression is important for controlling serum lipid content and reducing the risks of cardiovascular diseases. Here we found that SR-BI expression was significantly increased by LDL in vivo and in vitro, and the transcription factor specific protein 1 (Sp1) plays a critical role in this process. Results from co-immunoprecipitation experiments indicate that the activation of SR-BI was associated with Sp1-recruited protein complexes in the promoter region of SR-BI, where histone acetyltransferase p300 was recruited and histone deacetylase HDAC1 was dismissed. As a result, histone acetylation increased, leading to activation of SR-BI transcription. With further investigation, we found that LDL phosphorylated Sp1 through ERK1/2 pathway, which affected Sp1 protein complexes formation in SR-BI promoter. Using mass spectrometry and site directed mutagenesis, a new Sp1 phosphorylation site Ser702 was defined to be associated with Sp1-HDAC1 interaction and may be important in SR-BI activation, shedding light on the knowledge of delicate mechanism of hepatic HDL receptor SR-BI gene modulation by LDL.

Cell-Specific Polymorphism and Hormonal Regulation of DNA Methylation in Scavenger Receptor Class B, Type I

The scavenger receptor class B, type I (SR-BI), is a cell-surface glycoprotein that mediates selective uptake of high density lipoprotein (HDL)-derived cholesteryl ester. SR-BI plays an important role in cellular delivery of cholesterol. Both human and rodent SR-BI are expressed most abundantly in the liver parenchymal cells and steroidogenic cells of the adrenal gland and gonads, where the selective pathway exhibits its highest activity. In steroidogenic cells, the expression of SR-BI is regulated by trophic hormones (adrenocorticotropic hormone or gonadotropins luteinizing hormone or follicle-stimulating hormone) in concert with the regulation of steroid hormone production. DNA methylation has been implicated in a large number of biological processes mainly by regulating gene expression. The SR-BI promoter contains one CpG island (CGI) in its promoter and seven CGIs in its intronic regions. Here, we studied the DNA methylation status of SR-BI gene and provide evidence that the DNA methylation is cell specific in this gene promoter as well as in intronic regions. The DNA methylation in the SR-BI promoter is subject to N(6), 2'-O-dibutyryladenosine3':5'-cyclic monophosphate regulation in mouse adrenal Y1 cells and mouse Leydig tumor cells (MLTCs). The seven intron CGIs are methylated differentially in Y1 cells, MLTCs, ovarian granulosa cells, and mouse liver hepa 1-6 cells. Our experiments raised the possibility that DNA methylation participates in hormonal regulation of SR-BI expression in a tissue-specific manner. We further suggest that the cell-specific DNA methylation in SR-BI intronic regions may be associated with specific biological function(s) of these regions, including regulation of gene expression.

Effects of Extracts from Tiaozhi Granule and Its Components on Expression of Scavenger Receptor Class B Type I.

Sera from the rats with different drug treatments (atorvastatin, Tiaozhi granule, or its extracts) were collected. LO-2 cells or HepG2 cells were pretreated with different sera as the following groups randomly: (1) blank control group, (2) positive control group (atorvastatin group), (3) Tiaozhi granule water extract groups, (4) Tiaozhi granule alcohol extract groups, and (5) alcohol extracts for each component: Pollen Typhae Angustifoliae, Curcuma longa L., and Rhizoma Alismatis. LO-2 cells were cotransfected with plasmid carrying SR-BI and pRL-TK promoter genes. Promoter activity was measured by the luciferase reporter gene assay. The mRNA and protein expressions of SR-BI were examined using real-time PCR and western blot analyses. Our results show that promoter activity and mRNA and protein expression levels of the SR-BI were significantly upregulated by Tiaozhi granules alcohol or water extracts in a dose-dependent manner. Pollen Typhae Angustifoliae alcohol extract with a high dosage could also increase SR-BI activity and expression, but not the extracts from Curcuma longa L. and Rhizoma Alismatis. Both Tiaozhi granule alcohol and water extracts can upregulate SR-BI gene expression. Among the components, Pollen Typhae Angustifoliae are important for the regulatory effect coordinating with Curcuma longa L. and Rhizoma Alismatis.

Increased DNA methylation of scavenger receptor class B type I contributes to inhibitory effects of prenatal caffeine ingestion on cholesterol uptake and steroidogenesis in fetal adrenals

Steroid hormones synthesized from cholesterol in the fetal adrenal are crucial for fetal development. We have observed the inhibited fetal adrenal corticosterone synthesis and increased intrauterine growth retardation (IUGR) rate in rats under prenatal caffeine ingestion. The aim of this study is to evaluate the effects of prenatal caffeine ingestion on cholesterol supply in fetal adrenal steroidogenesis in rats and explore the underlying epigenetic mechanisms. Pregnant Wistar rats were treated with 60mg/kg·d caffeine from gestational day (GD) 7 to GD17. Histological changes of fetal adrenals and increased IUGR rates were observed in the caffeine group. There were significantly decreased steroid hormone contents and cholesterol supply in caffeine-treated fetal adrenals. Data from the gene expression array suggested that prenatal caffeine ingestion caused increased expression of genes related to DNA methylation and decreased expression of genes related to cholesterol uptake. The following conjoint analysis of DNA methylation array with these differentially expressed genes suggested that scavenger receptor class B type I (SR-BI) may play an important role in caffeine-induced cholesterol supply deficiency. Moreover, real-time RT-PCR and immunohistochemical detection certified the inhibitory effects of caffeine on both mRNA expression and protein expression of SR-BI in the fetal adrenal. And the increased DNA methylation frequency in the proximal promoter of SR-BI was confirmed by bisulfite-sequencing PCR. In conclusion, prenatal caffeine ingestion can induce DNA hypermethylation of the SR-BI promoter in the rat fetal adrenal. These effects may lead to decreased SR-BI expression and cholesterol uptake, which inhibits steroidogenesis in the fetal adrenal.

Induction of macrophage scavenger receptor type BI expression by tamoxifen and 4-hydroxytamoxifen

ObjectiveScavenger receptor type BI (SR-BI) is an HDL receptor that is expressed by macrophages. SR-BI expression is tightly linked to the development of atherosclerosis. Tamoxifen has been shown to be atheroprotective. However, the involved mechanisms have not been fully elucidated. Methods and resultsIn this study, we investigated the effect of tamoxifen and 4-hydroxytamoxifen on macrophage SR-BI expression. Macrophage cell lines and peritoneal macrophages isolated from wild-type mice were used to determine changes in SR-BI mRNA and protein expression in response to tamoxifen and 4-hydroxytamoxifen. We observed that tamoxifen and 4-hydroxytamoxifen increased SR-BI protein expression in a macrophage cell line derived from female mice (J774 cells) but not in a line derived from male mice (RAW cells). Similar observations were obtained in primary macrophages isolated from wild-type male and female mice. Thus, the induction of macrophage SR-BI expression by tamoxifen and 4-hydroxytamoxifen is sex-dependent. Furthermore, we observed that SR-BI expression was induced by activating the oestrogen receptor (ER, specifically ERα) but was inhibited by inactivating the ER. However, the increased macrophage SR-BI protein expression was independent of transcription because SR-BI mRNA expression and promoter activity were not influenced by tamoxifen and 4-hydroxytamoxifen. Instead, tamoxifen increased the stability of macrophage SR-BI protein. Tamoxifen administration to mice had no effect on hepatic SR-BI protein expression but improved the serum lipid profile. ConclusionOur study demonstrates that tamoxifen and 4-hydroxytamoxifen induce macrophage SR-BI protein expression via a post-transcriptional mechanism.

Upregulation of scavenger receptor class B type I expression by activation of FXR in hepatocyte

Objective The farnesoid X receptor (FXR), a member of the nuclear receptor superfamily, has been proposed to play an important role in the pathogenesis of cardiovascular diseases by regulating the metabolism and transport of cholesterol and triglyceride. Scavenger receptor class B type I (SR-BI), a high-density lipoprotein receptor, plays an important role in decreasing lipid metabolism-associated cardiovascular diseases by regulating reverse cholesterol transport. Recent studies have shown that SR-BI expression is upregulated by several nuclear receptors. However, the role of FXR in the regulation of SR-BI expression is not well known. In the present study, we investigate the regulation of SR-BI by FXR in hepatocyte and the corresponding mechanism. Methods and results Treatment of human hepatoma cell line HepG2 with FXR ligands resulted in upregulation of SR-BI at the levels of both mRNA and protein. Reporter assays showed that activation of FXR significantly enhanced the SR-BI promoter activity. Electrophoretic mobility shift and chromatin immunoprecipitation assays indicated that FXR induced SR-BI expression by binding to a novel FXR element (FXRE), a directed repeat DNA motif, DR8 (−703 AGGCCAcgttctagAGCTCA −684). The in vivo experiment demonstrated that gavaging mice with a natural ligand of FXR increased SR-BI expression in liver tissues. Conclusions FXR can directly upregulate SR-BI expression in hepatocyte, and DR8 is a likely novel FXRE that is involved in SR-BI regulation. FXR may serve as a novel molecular target for manipulating SR-BI expression in hepatocyte.

Induction of Krüppel-like factor 4 by high-density lipoproteins promotes the expression of scavenger receptor class B type I

Krüppel-like factor 4 (KLF4) is an evolutionarily conserved zinc finger-containing transcription factor. In the present study, peripheral blood mononuclear cells and phorbol 12-myristate 13-acetate-differentiated THP-1 cells were treated with oxidized low-density lipoproteins and high-density lipoproteins to determine the expression of KLF4 and scavenger receptor class B type I (SR-BI). A full-length cDNA of KLF4 or short interference RNA against KLF4 was transfected into THP-1 cells, and the subsequent expressions of SR-BI were analysed by real-time PCR and western blot. The binding and transcriptional activities of KLF4 to the SR-BI promoter were detected by electrophoretic mobility shift assay, chromatin immunoprecipitation assay and luciferase reporter assay. The results showed that induction of KLF4 by high-density lipoproteins could promote the expression of SR-BI, resulting from the binding to putative KLF4 binding element on the promoter of SR-BI. All results indicate a potential function of KLF4 in the pathogenesis of atherosclerosis through the regulation effect on atherosclerotic-related genes.

Feedback Inhibition of Human Scavenger Receptor Class B Type I Gene Expression by Glucocorticoid in Adrenal and Ovarian Cells

Scavenger receptor class B type I (SR-BI) facilitates the reverse transport of excess cholesterol from peripheral tissues to the liver via high-density lipoproteins. In steroidogenic tissues, SR-BI supplies cholesterol for steroid hormone production. We show here that the transcription of the human SR-BI gene is subject to feedback inhibition by glucocorticoid in adrenal and ovarian cells. SR-BI mRNA levels were increased in adrenals from corticosterone-insufficient Crh(-/-) mice, whereas corticosterone replacement by oral administration inhibited SR-BI gene expression in these mice. SR-BI mRNA levels were increased in adrenals from wild-type mice treated with metyrapone, a drug that blocks corticosterone synthesis. Experiments in adrenocortical H295R and ovarian SKOV-3 cells using cycloheximide and siRNA-mediated gene silencing revealed that glucocorticoid-mediated inhibition of SR-BI gene transcription requires de novo protein synthesis and the glucocorticoid receptor (GR). No direct binding of GR to the SR-BI promoter could be demonstrated in vitro and in vivo, suggesting an indirect mechanism of repression of SR-BI gene transcription by GR in adrenal cells. Deletion analysis established that the region of the human SR-BI promoter between nucleotides -201 and -62 is sufficient to mediate repression by glucocorticoid. This region contains putative binding sites for transcriptional repressors that could play a role in SR-BI gene regulation in response to glucocorticoid. In summary, this is the first report showing that glucocorticoid suppress SR-BI expression suggesting that steroidogenic tissues maintain steroid hormone homeostasis by prohibiting SR-BI-mediated high-density lipoprotein cholesterol uptake when the endogenous levels of glucocorticoid are elevated.

The Transcriptional Factor Prolactin Regulatory Element-Binding Protein Mediates the Gene Transcription of Adrenal Scavenger Receptor Class B Type I via 3′,5′-Cyclic Adenosine 5′-Monophosphate

Prolactin regulatory element-binding (PREB) protein is a transcription factor that regulates prolactin promoter activity in the rat anterior pituitary. The PREB protein is not only expressed in the anterior pituitary but also in the adrenal gland. However, the role of PREB in the adrenal gland is not clearly understood. Scavenger receptor class B type I (SR-BI) is a receptor for high-density lipoprotein that mediates the cellular uptake of high-density lipoprotein-cholesteryl ester and is a major route for cholesterol delivery to the steroidogenic pathway in the adrenal gland. In the present study, we have examined the role of PREB in regulating SR-BI. SR-BI expression was found to be regulated by cAMP, which stimulated the expression of PREB in a dose-dependent manner. Conversely, overexpression of PREB using a PREB-expressing adenovirus increased the expression of the SR-BI protein in the adrenocortical cell line Y-1. In addition, PREB induced the expression of the luciferase reporter protein that was under the control of the SR-BI promoter. EMSA showed that PREB mediates its transcriptional effect by binding to the PREB-responsive cis-element of the SR-BI promoter. Finally, we used small interfering RNA to inhibit PREB expression in the Y-1 cells and demonstrated that the knockdown of PREB expression attenuated the effects of cAMP on SR-BI expression. In summary, our data showed that in the adrenal gland, PREB regulates the transcription of the SR-BI gene via cAMP.

Hyperglycemia suppresses hepatic scavenger receptor class B type I expression

Hyperglycemia is a major risk factor for atherosclerotic disease. Hepatic scavenger receptor class B type I (SR-BI) binds HDL particles that mediate reverse cholesterol transport and thus lowers the risk of atherosclerosis. Here we examined glucose regulation of SR-BI gene expression in both HepG2 cells and whole animals. Results showed that hepatic SR-BI mRNA, protein, and uptake of cholesterol from HDL were halved following 48 h of exposure to 22.4 vs. 5.6 mM glucose. As in the case of the cell culture model, hepatic expression of SR-BI was lower in diabetic rats than in euglycemic rats. Transcriptional activity of the human SR-BI promoter paralleled endogenous expression of the gene, and this activity was dependent upon the dose of glucose. Next, we used inhibitors of select signal transduction pathways to demonstrate that glucose suppression of SR-BI was sensitive to the p38 MAPK inhibitor. Expression of a constitutively active p38 MAPK inhibited SR-BI promoter activity in the presence or absence of glucose. A dominant-negative p38 MAPK abolished the inhibitory effect of glucose on promoter activity. Deletional analysis located a 50-bp fragment of the promoter that mediated the effects of glucose. Within this DNA fragment there were several specificity protein-1 (Sp1) binding sites, and cellular knockdown of Sp1 abrogated its suppression by glucose. Together, these results indicate that the glucose suppression of SR-B1 expression is partially mediated by the activation of the p38 MAPK-Sp1 pathway and raise the possibility that the inhibition of hepatic SR-BI expression under high-glucose conditions provides a mechanism for accelerated atherosclerosis in diabetics.

Activation of the rat scavenger receptor class B type I gene by PPARα

Peroxisomal proliferator activated receptor α (PPARα) is activated by fibrate drugs which are known to protect against atherosclerosis. The present study examines the effects of PPARα on SR-BI expression. For this study, a rat SR-BI promoter-luciferase reporter gene construct was co-transfected into different cell lines with expression vectors that encode for PPARα ± retinoic X receptor α (RXRα). PPARα/RXR increased the activity of the SR-BI promoter, an effect that was enhanced by clofibrate. Sequence analysis of the rat SR-BI promoter revealed the presence of a putative peroxisomal proliferator response element (PPRE) at bp −1622. Electrophoretic mobility shift assays demonstrated that PPARα and RXRα are able to bind to the SR-BI PPRE motif. In addition, mutational analysis studies confirmed that this PPRE motif is responsible for the PPARα/RXRα-dependent activation of the rat SR-BI promoter in the cell lines examined.

The role of cyclic AMP response element binding protein in transactivation of scavenger receptor class B type I promoter in transfected cells and in primary cultures of rat theca-interstitial cells

In the ovary, lutropin (LH) stimulates the selective uptake and transport of cholesterol for steroid biosynthesis from HDL particles via the scavenger receptor class B type I (SR-BI). Furthermore the expression of SR-BI mRNA in the ovary is stimulated by LH and cyclic AMP (cAMP). Since the promoter of the rat SR-BI gene is devoid of consensus cyclic AMP response element (CRE) sequences, this study examined if cAMP response element binding protein (CREB) plays a role in the transactivation of SR-BI promoter (SR-BIpr). The transactivation of SR-BIpr was examined in transfected 293T cells and human granulosa SVOG-4o cells, and in primary cultures of rat theca-interstitial cells infected with adenoviral constructs containing the SR-BIpr and a luciferase reporter gene. Dose-related increases in SR-BRpr activity ranging from 2- to 4-fold was induces by 293T cells co-transfected with the catalytic subunit of protein kinase A (cPKA). Co-transfections with CREB and cPKA produced a concentration-dependent increase ranging from 6- to 32-fold. The cAMP-mediated transactivation was significantly attenuated by co-transfection with CREB M1, a non-phosphorylatable, dominant-negative form of CREB. An increase in transactivation of SR-BIpr activity was also seen in SVOG-4o cells co-transfected with CREB. In primary cultures of rat theca-interstitial (T-I) cells infected with an adenoviral construct of SR-BIpr, forskolin produced a marked increase in promoter activity. These data indicate that stimulation of the cAMP-PKA–CREB pathway enhances rat SR-BIpr activity and substantiate the role of CREB as an intermediary in this process. The absence of canonical CRE sequences in the rat SR-BIpr suggests that the activation of SR-BI by CREB may occur either through non-canonical CRE sequences or through additional transcription factors that cooperate with CREB in the activation of SR-BI promoter activity.

Bile acids reduce SR-BI expression in hepatocytes by a pathway involving FXR/RXR, SHP, and LRH-1

Hepatic SR-BI mediates uptake of circulating cholesterol into liver hepatocytes where a part of the cholesterol is metabolised to bile acids. In the hepatocytes, bile acids reduce their own synthesis by a negative feedback loop to prevent toxic high levels of bile acids. Bile acid-activated FXR/RXR represses expression of CYP7A1, the rate-limiting enzyme during bile acid synthesis, by inducing the expression of SHP, which inhibits LXR/RXR and LRH-1-transactivation of CYP7A1. The present paper presents data indicating that CDCA suppresses SR-BI expression by the same pathway. As previously reported, LRH-1 induces SR-BI promoter activity. Here we show that CDCA or over-expression of SHP inhibit this transactivation. No FXR-response element was identified in the bile acid-responsive region of the SR-BI promoter (−1200 bp/−937 bp). However, a binding site for LRH-1 was characterised and shown to specifically bind LRH-1. The present study shows that also the SR-BI-mediated supply of cholesterol, the substrate for bile acid synthesis, is feedback regulated by bile acids.

Insulin-like growth factor-i regulation of hepatic scavenger receptor class BI.

High-density lipoprotein mediates a normal physiological process called reverse cholesterol transport. This process enables the transfer of cholesterol from peripheral tissues to the liver for further metabolism and eventual secretion in the form of bile. The scavenger receptor of the B class (SR-BI), human homolog of SR-BI, and CD36 and LIMPII analogous-1 (CLA-1) are different names for the same receptor that facilitates hepatocellular uptake of cholesterol from high-density lipoprotein. The pivotal role of this receptor in enterohepatic circulation of cholesterol and bile salts underlies our interest to study the regulation of hepatic SR-BI gene in response to the actions of IGF-I. The results of our studies showed that endogenous expression of SR-BI/CLA-1 was suppressed by exposure to GH or IGF-I in cultured HepG2 cells. This observation extended to a whole animal model of rats continuously infused with IGF-I. IGF-I decreased transcriptional activity of the SR-BI promoter. However, the inhibitory effect of IGF-I on SR-BI/CLA-1 promoter activity was abrogated by wortmannin, a specific inhibitor of phosphoinositide 3-kinase (PI3-K). Exposure of HepG2 cells to IGF-I elicited a rapid phosphorylation of Akt. We also demonstrated that the constitutively active form of both p110, a subunit of PI3-K, and Akt inhibited activity of the human SR-BI/CLA-1 promoter. Furthermore, the dominant-negative mutant of Akt abolished the ability of IGF-I to suppress activity of the SR-BI/CLA-1 promoter. In conclusion, PI3-K/Akt pathways participate in IGF-I-suppression of SR-BI/CLA-1 expression, which suggests that the activation of Akt plays an important role in cholesterol metabolism in liver.

Association between a novel 11-base pair deletion mutation in the promoter region of the scavenger receptor class B type I gene and plasma HDL cholesterol levels in Taiwanese Chinese.

Scavenger receptor class B type I (SR-BI) is a multiligand cell-surface receptor that mediates the selective uptake of lipid from HDL cholesterol (HDL-C) into cells. This study hypothesized an association between functional variants in the promoter region of SR-BI gene and HDL-C levels. We identified 2 novel mutations in the SR-BI gene promoter region by using single-strand conformation polymorphism. One mutation was an 11-bp CCCCGCCCCGT deletion mutation from positions -140 to -150 relative to the transcription start site, corresponding to an Sp1 binding site; the other was a C-->T substitution at position -142. Twenty-six of 690 unrelated subjects were heterozygous for the -140 to -150 deletion mutation, and the allele frequency in this population was 0.02. This study showed that the deletion variant prevented binding of Sp1 to this region of the SR-BI promoter and effectively reduced transcriptional activities in HepG2 cells. Notably, the -140 to -150 deletion mutation was significantly associated with increased HDL-C levels and explained approximately 0.5% of the variation in HDL-C levels in this population. A genetic variant at the SR-BI gene promoter region might explain a significant proportion of individual differences in HDL-C levels among Taiwanese Chinese. Our results require further replication in an independent population.

Hepatic scavenger receptor class B, type I is stimulated by peroxisome proliferator-activated receptor γ and hepatocyte nuclear factor 4α

Excessive cellular cholesterol is transported to the liver by a pathway called ‘reverse cholesterol transport.’ Scavenger receptor class B, type I (SR-BI) mediates cholesterol uptake in the liver. Polyunsaturated fatty acids, known to activate peroxisome proliferator-activated receptor (PPAR), have been reported to increase hepatic cholesterol uptake. We found in the present study that PPARγ induces expression of SR-BI in rat hepatocytes, liver endothelial cells, and Kupffer cells. In contrast, PPARα increased SR-BI levels only in hepatocytes and liver endothelial cells. PPARγ/RXR binds to a response element between −459 and −472 bp in the human SR-BI promoter. Furthermore, hepatocyte nuclear factor 4α (HNF4α) was found to enhance PPARγ-mediated SR-BI transcription. Thiazolidinedione (TZD)-activated PPARγ/RXR increased hepatic SR-BI levels, which may lead to increased hepatic cholesterol uptake and less accumulation of lipids in peripheral tissues. The present results are in agreement with previous reports, indicating that specific PPARγ-agonists (such as TZDs) protect against atherosclerosis.

Regulation of the scavenger receptor BI and the LDL receptor by activators of aldosterone production, angiotensin II and PMA, in the human NCI-H295R adrenocortical cell line

In human adrenal cells, cholesterol for steroidogenesis is derived from both high-density lipoproteins (HDL) via the Scavenger Receptor Class B Type I (SR-BI) and low-density lipoproteins (LDL) via the LDL receptor pathway. We have previously shown that, in the human adrenocortical carcinoma cell line, NCI-H295R, SR-BI and LDL receptor expression and steroidogenesis are coordinately regulated by activators of protein kinase A (PKA) leading to glucocorticoid synthesis. In the present study, we studied whether SR-BI and LDL receptor expression are regulated by activators of the protein kinase C (PKC) signaling pathway, such as angiotensin II, which stimulate mineralocorticoid synthesis. First, it is shown that, in NCI-H295R cells, aldosterone synthesis is stimulated by a phorbol ester (phorbol-12-myristate-13 acetate, PMA), a potent PKC activator. Northern blot analysis indicated that both angiotensin II and PMA stimulated SR-BI expression in a time-dependent manner. LDL receptor expression is slightly stimulated by PMA. The induction of SR-BI gene expression occurs at the transcriptional level, via an activation of the human SR-BI promoter, as shown by transient transfection experiments. Finally, SR-BI protein level was increased in angiotensin II- and PMA-stimulated cells, resulting in higher lipoprotein binding and specific cholesteryl ester (CE) uptake from HDL, as well from LDL after angiotensin II and PMA stimulation.

Liver receptor homolog 1 controls the expression of the scavenger receptor class B type I.

The scavenger receptor class B type I (SR-BI), which mediates selective cellular cholesterol uptake from high-density lipoproteins (HDLs), plays a key role in reverse cholesterol transport. The orphan nuclear receptor liver receptor homolog 1 (LRH-1) and SR-BI are co-expressed in liver and ovary, suggesting that LRH-1 might control the expression of SR-BI in these tissues. LRH-1 induces human and mouse SR-BI promoter activity by binding to an LRH-1 response element in the promoter. Retroviral expression of LRH-1 robustly induces SR-BI, an effect associated with histone H3 acetylation on the SR-BI promoter. The decrease in SR-BI mRNA levels in livers of LRH-1(+/-) animals provides in vivo evidence that LRH-1 regulates SR-BI expression. Our data demonstrate that SR-BI is an LRH-1 target gene and underscore the pivotal role of LRH-1 in reverse cholesterol transport.