Effects of hepatic expression of the high-density lipoprotein receptor SR-BI on lipoprotein metabolism and female fertility

Follicular Fluid Proteomic Analysis of Women Undergoing Assisted Reproduction Suggests That Apolipoprotein A1 Is a Potential Fertility Marker

Infertility affects millions worldwide, posing a significant global health challenge. The proteomic analysis of follicular fluid provides a comprehensive view of the complex molecular landscape within ovarian follicles, offering valuable information on the factors influencing oocyte development and on the overall reproductive health. The follicular fluid is derived from the plasma and contains various proteins that can have different roles in oocyte health and infertility, and this fluid is a critical microenvironment for the developing oocytes as well. Using the high-performance liquid chromatography-mass spectrometry method, we investigated the protein composition of the follicular fluid, and after classification, we carried out relative quantification of the identified proteins in the pregnant (P) and non-pregnant (NP) groups. Based on the protein-protein interaction analysis, albumin and apolipoprotein A1 (ApoA1) were found to be hub proteins, and the quantitative comparison of the P and NP groups resulted in a significantly lower concentration of ApoA1 and high-density lipoprotein cholesterol in the P group. As both molecules are involved in the cholesterol transport, we also investigated their role in the development of oocytes and in the prediction of fertility.

Effect of hyperlipidemia on the outcome of in vitro fertilization in non-obese patients with polycystic ovary syndrome

Introduction

It is little known whether hyperlipidemia alone has adverse effects on the outcome of in vitro fertilization (IVF) in patients with polycystic ovarian syndrome (PCOS).

Methods

The PCOS patients with body mass index (BMI) < 30 kg/m2 were performed IVF or intracytoplasmic sperm injection treatment, including 208 fresh cycles and 127 frozen embryo transfer (FET) cycles. All the patients were divided into hyperlipidemia and control groups, and embryo quality and pregnancy outcomes between the two groups were compared.

Results

In the fresh cycles, total gonadotropin dosage in the control group was significantly lower than that in the hyperlipidemia group, and serum estradiol levels on trigger day were reversed (P < 0.05). The embryo fragment score was positively correlated with serum low-density lipoprotein level (r = 0.06, P < 0.05) and negatively with serum high-density lipoprotein (HDL) and lipoprotein A levels (r = -0.489 and -0.085, P < 0.01). Logistic regression analysis found that HDL was beneficial for clinical pregnancy (OR = 0.355, 95% CI: 0.135-0.938, P < 0.05). In the FET cycles, there were no differences in pulse index, systolic/diastolic ratio and serum estradiol and progesterone levels between the two groups, but resistance index in the hyperlipidemia group was significantly higher than that in the control group (P < 0.05).

Conclusion

Hyperlipidemia may increase the dosage of gonadotropin and have adverse effect on the embryo quality, endometrial receptivity, and clinical outcomes of lean PCOS patients. It is recommended that the non-obese patients with hyperlipidemia and PCOS perform lipid-lowering treatment before undergoing embryo transfer.

Serum opacity factor normalizes erythrocyte morphology in Scarb1-/- mice in an HDL-free cholesterol-dependent way

Compared with WT mice, HDL receptor-deficient (Scarb1-/-) mice have higher plasma levels of free cholesterol (FC)-rich HDL and exhibit multiple pathologies associated with a high mol% FC in ovaries, platelets, and erythrocytes, which are reversed by lowering HDL. Bacterial serum opacity factor (SOF) catalyzes the opacification of plasma by targeting and quantitatively converting HDL to neo HDL (HDL remnant), a cholesterol ester-rich microemulsion, and lipid-free APOA1. SOF delivery with an adeno-associated virus (AAVSOF) constitutively lowers plasma HDL-FC and reverses female infertility in Scarb1-/- mice in an HDL-dependent way. We tested whether AAVSOF delivery to Scarb1-/- mice will normalize erythrocyte morphology in an HDL-FC-dependent way. We determined erythrocyte morphology and FC content (mol%) in three groups-WT, untreated Scarb1-/- (control), and Scarb1-/- mice receiving AAVSOF-and correlated these with their respective HDL-mol% FC. Plasma-, HDL-, and tissue-lipid compositions were also determined. Plasma- and HDL-mol% FC positively correlated across all groups. Among Scarb1-/- mice, AAVSOF treatment normalized reticulocyte number, erythrocyte morphology, and erythrocyte-mol% FC. Erythrocyte-mol% FC positively correlated with HDL-mol% FC and with both the number of reticulocytes and abnormal erythrocytes. AAVSOF treatment also reduced FC of extravascular tissues to a lesser extent. HDL-FC spontaneously transfers from plasma HDL to cell membranes. AAVSOF treatment lowers erythrocyte-FC and normalizes erythrocyte morphology and lipid composition by reducing HDL-mol% FC.

The Association between Circulating Lipids and Female Infertility Risk: A Univariable and Multivariable Mendelian Randomization Analysis

BACKGROUND

Although observational studies have demonstrated that blood lipids are associated with female infertility, the causality of this association remains unclear. We performed a univariable and multivariable Mendelian randomization (MR) analysis to evaluate the causal relationship between blood lipids and female infertility.

METHODS

Single-nucleotide polymorphisms associated with lipid traits in univariate analysis were obtained from the Million Veteran Program (MVP) and Global Lipids Genetics Consortium (GLGC), involving up to 215,551 and 188,577 European individuals, respectively. Blood lipids in multivariate analysis were obtained from the latest genome-wide association study meta-analysis with lipid levels in 73 studies encompassing >300,000 participants. Data on female infertility were obtained from the FinnGen Consortium R6 release, which included 6481 samples and 75,450 controls. Subsequently, MR analysis was performed using inverse variance-weighted (IVW), weighted median, weighted-mode, simple-mode and MR-Egger regression to demonstrate the causal relationship between lipids and female infertility.

RESULTS

After controlling confounding factors including body mass index and age at menarche, two-sample MR demonstrated that genetically predicted LDL-C and TC were causally associated with the risk of female infertility (When the genetic instruments come from the MVP database, LDL-C and female infertility, IVW OR: 1.13, 95% CI: 1.001-1.269, p = 0.047; TC and female infertility, IVW OR: 1.16, 95% CI: 1.018-1.317, p = 0.025, and when the genetic instruments came from the GLGC database, LDL-C and female infertility, IVW OR: 1.10, 95% CI: 1.008-1.210, p = 0.033; TC and female infertility, IVW OR: 1.14, 95% CI: 1.024-1.258, p = 0.015). However, the IVW estimate showed that HDL-C was not significantly associated with the risk of female infertility (when the genetic instruments came from the MVP database, IVW OR: 1.00, 95% CI: 0.887-1.128, p = 0.999; when the genetic instruments came from the GLGC database, IVW OR: 1.00, 95% CI: 0.896-1.111, p = 0.968). The multivariable MR analysis also provided evidence that LDL-C (OR: 1.12, 95% CI: 1.006-1.243, p = 0.042) was significantly associated with the risk of female infertility after considering the correlation of all lipid-related traits.

CONCLUSION

These findings support a causal relationship between increased LDL-cholesterol and increased female infertility risk. Furthermore, the association between lipid-related traits and female infertility risk merits more studies.

RNA sequencing and expression analysis reveal a role for Lhx9 in the haploinsufficient adult mouse ovary

Understanding the molecular pathways that underpin ovarian development and function is vital for improving the research approaches to investigating fertility. Despite a significant improvement in our knowledge of molecular activity in the ovary, many questions remain unanswered in the quest to understand factors influencing fertility and ovarian pathologies such as cancer. Here, we present an investigation into the expression and function of the developmental transcription factor LIM Homeobox 9 (LHX9) in the adult mouse ovary. We have characterized Lhx9 expression in several cell types of the mature ovary across follicle stages. To evaluate possible LHX9 function in the adult ovary, we investigated ovarian anatomy and transcription in an Lhx9^+/- knockout mouse model displaying subfertility. Despite a lack of gross anatomical differences between genotypes, RNA-sequencing found that 90 differentially expressed genes between Lhx9^{+/ -} and Lhx9^+/+ mice. Gene ontology analyses revealed a reduced expression of genes with major roles in ovarian steroidogenesis and an increased expression of genes associated with ovarian cancer. Analysis of the ovarian epithelium revealed Lhx9^{+/ -} mice have a disorganized epithelial phenotype, corresponding to a significant increase in epithelial marker gene expression. These results provide an analysis of Lhx9 in the adult mouse ovary, suggesting a role in fertility and ovarian epithelial cancer.

Implications of High-Density Cholesterol Metabolism for Oocyte Biology and Female Fertility

Cholesterol is an essential component of animal cells. Different regulatory mechanisms converge to maintain adequate levels of this lipid because both its deficiency and excess are unfavorable. Low cell cholesterol content promotes its synthesis and uptake from circulating lipoproteins. In contrast, its excess induces the efflux to high-density lipoproteins (HDL) and their transport to the liver for excretion, a process known as reverse cholesterol transport. Different studies suggest that an abnormal HDL metabolism hinders female fertility. HDL are the only lipoproteins detected in substantial amounts in follicular fluid (FF), and their size and composition correlate with embryo quality. Oocytes obtain cholesterol from cumulus cells via gap junctions because they cannot synthesize cholesterol de novo and lack HDL receptors. Recent evidence has supported the possibility that FF HDL play a major role in taking up excess unesterified cholesterol (UC) from the oocyte. Indeed, genetically modified mouse models with disruptions in reverse cholesterol transport, some of which show excessive circulating UC levels, exhibit female infertility. Cholesterol accumulation can affect the egg´s viability, as reported in other cell types, and activate the plasma membrane structure and activity of membrane proteins. Indeed, in mice deficient for the HDL receptor Scavenger Class B Type I (SR-B1), excess circulating HDL cholesterol and UC accumulation in oocytes impairs meiosis arrest and hinders the developmental capacity of the egg. In other cells, the addition of cholesterol activates calcium channels and dysregulates cell death/survival signaling pathways, suggesting that these mechanisms may link altered HDL cholesterol metabolism and infertility. Although cholesterol, and lipids in general, are usually not evaluated in infertile patients, one study reported high circulating UC levels in women showing longer time to pregnancy as an outcome of fertility. Based on the evidence described above, we propose the existence of a well-regulated and largely unexplored system of cholesterol homeostasis controlling traffic between FF HDL and oocytes, with significant implications for female fertility.

High Free Cholesterol Bioavailability Drives the Tissue Pathologies in Scarb1-/- Mice

Objective: Overall and atherosclerosis-associated mortality is elevated in humans with very high HDL (high-density lipoprotein) cholesterol concentrations. Mice with a deficiency of the HDL receptor, Scarb1 (scavenger receptor class B type 1), are a robust model of this phenotype and exhibit several additional pathologies. We hypothesized that the previously reported high plasma concentration of free cholesterol (FC)-rich HDL in Scarb1-/- mice produces a state of high HDL-FC bioavailability that increases whole-body FC and dysfunction in multiple tissue sites. Approach and Results: The higher mol% FC in Scarb1-/- versus WT (wild type) HDL (41.1 versus 16.0 mol%) affords greater FC bioavailability for transfer to multiple sites. Plasma clearance of autologous HDL-FC mass was faster in WT versus Scarb1-/- mice. FC influx from Scarb1-/- HDL to LDL (low-density lipoprotein) and J774 macrophages was greater ([almost equal to]4x) than that from WT HDL, whereas FC efflux capacity was similar. The higher mol% FC of ovaries, erythrocytes, heart, and macrophages of Scarb1-/- versus WT mice is associated with previously reported female infertility, impaired cell maturation, cardiac dysfunction, and atherosclerosis. The FC contents of other tissues were similar in the two genotypes, and these tissues were not associated with any overt pathology. In addition to the differences between WT versus Scarb1-/- mice, there were many sex-dependent differences in tissue-lipid composition and plasma FC clearance rates. Conclusions: Higher HDL-FC bioavailability among Scarb1-/- versus WT mice drives increased FC content of multiple cell sites and is a potential biomarker that is mechanistically linked to multiple pathologies.

Class B Scavenger Receptors BI and BII Protect against LPS-Induced Acute Lung Injury in Mice by Mediating LPS

Recent studies suggest an anti-inflammatory protective role for class B scavenger receptor BI (SR-BI) in endotoxin-induced inflammation and sepsis. Other data, including ours, provide evidence for an alternative role of SR-BI, facilitating bacterial and endotoxin uptake and contributing to inflammation and bacterial infection. Enhanced endotoxin susceptibility of SR-BI-deficient mice due to their anti-inflammatory glucocorticoid deficiency complicates the understanding of SR-BI's role in endotoxemia/sepsis, calling for the use of alternative models. In this study, using human SR-BI (hSR-BI) and hSR-BII transgenic mice, we found that SR-BI and, to a lesser extent, its splicing variant SR-BII protect against LPS-induced lung damage. At 20 h after intratracheal LPS instillation, the extent of pulmonary inflammation and vascular leakage was significantly lower in hSR-BI and hSR-BII transgenic mice than in wild-type mice. Higher bronchoalveolar lavage fluid (BALF) inflammatory cell count and protein content and lung tissue neutrophil infiltration found in wild-type mice were associated with markedly (2 to 3 times) increased proinflammatory cytokine production compared to these parameters in transgenic mice following LPS administration. The markedly lower endotoxin levels detected in BALF of transgenic versus wild-type mice and the significantly increased BODIPY-LPS uptake observed in lungs of hSR-BI and hSR-BII mice 20 h after the i.t. LPS injection suggest that hSR-BI- and hSR-BII-mediated enhanced LPS clearance in the airways could represent the mechanism of their protective role against LPS-induced acute lung injury.

Ovarian cholesterol efflux: ATP-binding cassette transporters and follicular fluid HDL regulate cholesterol content in mouse oocytes†

High density lipoproteins (HDL) take up cholesterol from peripheral tissues via ABC transporters and deliver it to the liver via scavenger receptor class B type I (SR-B1). HDL are the main lipoproteins present in follicular fluid (FF). They are thought to derive from plasma, but their origin is still controversial. SR-B1 knock-out (KO) mice have provided important evidence linking HDL metabolism and female fertility. These mice have cholesterol-rich circulating HDL and female infertility that can be restored by treating mice with the cholesterol-lowering drug probucol. Ovulated oocytes from SR-B1 KO females are dysfunctional and show excess cholesterol. The mechanisms explaining the contribution of FF HDL to oocyte cholesterol homeostasis are unknown. Here, using quantitation of filipin fluorescence we show that in SR-B1 KO ovaries, cholesterol excess is first observed in immature oocytes in antral follicles. By performing cross-transplant experiments between WT and apolipoprotein A-I deficient (ApoA-I KO) mice, which lack the main protein component of HDL, we provide evidence supporting the plasmatic origin of FF HDL. Also, we demonstrate that probucol treatment in SR-B1 KO females results in lowering of cholesterol content in their oocytes. Incubation of oocytes from SR-B1 KO mice with purified WT HDL reduces their cholesterol content, suggesting that HDL promote efflux of excess cholesterol from oocytes. In agreement with this hypothesis, we identified ABC transporters in oocytes and observed that ABCA1 KO oocytes have excess cholesterol and lower viability than WT oocytes.

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.

SR-B1: A Unique Multifunctional Receptor for Cholesterol Influx and Efflux

The scavenger receptor, class B type 1 (SR-B1), is a multiligand membrane receptor protein that functions as a physiologically relevant high-density lipoprotein (HDL) receptor whose primary role is to mediate selective uptake or influx of HDL-derived cholesteryl esters into cells and tissues. SR-B1 also facilitates the efflux of cholesterol from peripheral tissues, including macrophages, back to liver. As a regulator of plasma membrane cholesterol content, SR-B1 promotes the uptake of lipid soluble vitamins as well as viral entry into host cells. These collective functions of SR-B1 ultimately affect programmed cell death, female fertility, platelet function, vasculature inflammation, and diet-induced atherosclerosis and myocardial infarction. SR-B1 has also been identified as a potential marker for cancer diagnosis and prognosis. Finally, the SR-B1-linked selective HDL-cholesteryl ester uptake pathway is now being evaluated as a gateway for the delivery of therapeutic and diagnostic agents. In this review, we focus on the regulation and functional significance of SR-B1 in mediating cholesterol movement into and out of cells.

Gugulipid causes hypercholesterolemia leading to endothelial dysfunction, increased atherosclerosis, and premature death by ischemic heart disease in male mice

For proper cholesterol metabolism, normal expression and function of scavenger receptor class B type I (SR-BI), a high-density lipoprotein (HDL) receptor, is required. Among the factors that regulate overall cholesterol homeostasis and HDL metabolism, the nuclear farnesoid X receptor plays an important role. Guggulsterone, a bioactive compound present in the natural product gugulipid, is an antagonist of this receptor. This natural product is widely used globally as a natural lipid-lowering agent, although its anti-atherogenic cardiovascular benefit in animal models or humans is unknown. The aim of this study was to determine the effects of gugulipid on cholesterol homeostasis and development of mild and severe atherosclerosis in male mice. For this purpose, we evaluated the impact of gugulipid treatment on liver histology, plasma lipoprotein cholesterol, endothelial function, and development of atherosclerosis and/or ischemic heart disease in wild-type mice; apolipoprotein E knockout mice, a model of atherosclerosis without ischemic complications; and SR-B1 knockout and atherogenic–diet-fed apolipoprotein E hypomorphic (SR-BI KO/ApoER61^h/h) mice, a model of lethal ischemic heart disease due to severe atherosclerosis. Gugulipid administration was associated with histological abnormalities in liver, increased alanine aminotransferase levels, lower hepatic SR-BI content, hypercholesterolemia due to increased HDL cholesterol levels, endothelial dysfunction, enhanced atherosclerosis, and accelerated death in animals with severe ischemic heart disease. In conclusion, our data show important adverse effects of gugulipid intake on HDL metabolism and atherosclerosis in male mice, suggesting potential and unknown deleterious effects on cardiovascular health in humans. In addition, these findings reemphasize the need for rigorous preclinical and clinical studies to provide guidance on the consumption of natural products and regulation of their use in the general population.

Carboxy-terminal deletion of the HDL receptor reduces receptor levels in liver and steroidogenic tissues, induces hypercholesterolemia, and causes fatal heart disease

The HDL receptor SR-BI mediates the transfer of cholesteryl esters from HDL to cells and controls HDL abundance and structure. Depending on the genetic background, loss of SR-BI causes hypercholesterolemia, anemia, reticulocytosis, splenomegaly, thrombocytopenia, female infertility, and fatal coronary heart disease (CHD). The carboxy terminus of SR-BI (⁵⁰⁵QEAKL⁵⁰⁹) must bind to the cytoplasmic adaptor PDZK1 for normal hepatic-but not steroidogenic cell-expression of SR-BI protein. To determine whether SR-BI's carboxy terminus is also required for normal protein levels in steroidogenic cells, we introduced into SR-BI's gene a ⁵⁰⁷Ala/STOP mutation that produces a truncated receptor (SR-BIΔCT). As expected, the dramatic reduction of hepatic receptor protein in SR-BIΔCT mice was similar to that in PDZK1 knockout (KO) mice. Unlike SR-BI KO females, SR-BIΔCT females were fertile. The severity of SR-BIΔCT mice's hypercholesterolemia was intermediate between those of SR-BI KO and PDZK1 KO mice. Substantially reduced levels of the receptor in adrenal cortical cells, ovarian cells, and testicular Leydig cells in SR-BIΔCT mice suggested that steroidogenic cells have an adaptor(s) functionally analogous to hepatic PDZK1. When SR-BIΔCT mice were crossed with apolipoprotein E KO mice (SR-BIΔCT/apoE KO), pathologies including hypercholesterolemia, macrocytic anemia, hepatic and splenic extramedullary hematopoiesis, massive splenomegaly, reticulocytosis, thrombocytopenia, and rapid-onset and fatal occlusive coronary arterial atherosclerosis and CHD (median age of death: 9 wk) were observed. These results provide new insights into the control of SR-BI in steroidogenic cells and establish SR-BIΔCT/apoE KO mice as a new animal model for the study of CHD.

Human SR-BI and SR-BII Potentiate Lipopolysaccharide-Induced Inflammation and Acute Liver and Kidney Injury in Mice

The class B scavenger receptors BI (SR-BI) and BII (SR-BII) are high-density lipoprotein receptors that recognize various pathogens, including bacteria and their products. It has been reported that SR-BI/II null mice are more sensitive than normal mice to endotoxin-induced inflammation and sepsis. Because the SR-BI/II knockout model demonstrates multiple immune and metabolic disorders, we investigated the role of each receptor in the LPS-induced inflammatory response and tissue damage using transgenic mice with pLiv-11-directed expression of human SR-BI (hSR-BI) or human SR-BII (hSR-BII). At 6 h after i.p. LPS injection, transgenic hSR-BI and hSR-BII mice demonstrated markedly higher serum levels of proinflammatory cytokines and 2- to 3-fold increased expression levels of inflammatory mediators in the liver and kidney, compared with wild-type (WT) mice. LPS-stimulated inducible NO synthase expression was 3- to 6-fold higher in the liver and kidney of both transgenic strains, although serum NO levels were similar in all mice. Despite the lower high-density lipoprotein plasma levels, both transgenic strains responded to LPS by a 5-fold increase of plasma corticosterone levels, which were only moderately lower than in WT animals. LPS treatment resulted in MAPK activation in tissues of all mice; however, the strongest response was detected for hepatic extracellular signal-regulated protein kinase 1 and 2 and kidney JNK of both transgenic mice. Histological examination of hepatic and renal tissue from LPS-challenged mice revealed more injury in hSR-BII, but not hSR-BI, transgenic mice versus WT controls. Our findings demonstrate that hSR-BII, and to a lesser extent hSR-BI, significantly increase LPS-induced inflammation and contribute to LPS-induced tissue injury in the liver and kidney, two major organs susceptible to LPS toxicity.

Endothelial Expression of Scavenger Receptor Class B, Type I Protects against Development of Atherosclerosis in Mice

The role of scavenger receptor class B, type I (SR-BI) in endothelial cells (EC) was examined in several novel transgenic mouse models expressing SR-BI in endothelium of mice with normal C57Bl6/N, apoE-KO, or Scarb1-KO backgrounds. Mice were also created expressing SR-BI exclusively in endothelium and liver. Endothelial expression of the Tie2-Scarb1 transgene had no significant effect on plasma lipoprotein levels in mice on a normal chow diet but on an atherogenic diet, significantly decreased plasma cholesterol levels, increased plasma HDL cholesterol (HDL-C) levels, and protected mice against atherosclerosis. In 8-month-old apoE-KO mice fed a normal chow diet, the Tie2-Scarb1 transgene decreased aortic lesions by 24%. Mice expressing SR-BI only in EC and liver had a 1.5 ± 0.1-fold increase in plasma cholesterol compared to mice synthesizing SR-BI only in liver. This elevation was due mostly to increased HDL-C. In EC culture studies, SR-BI was found to be present in both basolateral and apical membranes but greater cellular uptake of cholesterol from HDL was found in the basolateral compartment. In summary, enhanced expression of SR-BI in EC resulted in a less atherogenic lipoprotein profile and decreased atherosclerosis, suggesting a possible role for endothelial SR-BI in the flux of cholesterol across EC.

Increased plasma cholesterol esterification by LCAT reduces diet-induced atherosclerosis in SR-BI knockout mice

LCAT, a plasma enzyme that esterifies cholesterol, has been proposed to play an antiatherogenic role, but animal and epidemiologic studies have yielded conflicting results. To gain insight into LCAT and the role of free cholesterol (FC) in atherosclerosis, we examined the effect of LCAT over- and underexpression in diet-induced atherosclerosis in scavenger receptor class B member I-deficient [Scarab(-/-)] mice, which have a secondary defect in cholesterol esterification. Scarab(-/-)×LCAT-null [Lcat(-/-)] mice had a decrease in HDL-cholesterol and a high plasma ratio of FC/total cholesterol (TC) (0.88 ± 0.033) and a marked increase in VLDL-cholesterol (VLDL-C) on a high-fat diet. Scarab(-/-)×LCAT-transgenic (Tg) mice had lower levels of VLDL-C and a normal plasma FC/TC ratio (0.28 ± 0.005). Plasma from Scarab(-/-)×LCAT-Tg mice also showed an increase in cholesterol esterification during in vitro cholesterol efflux, but increased esterification did not appear to affect the overall rate of cholesterol efflux or hepatic uptake of cholesterol. Scarab(-/-)×LCAT-Tg mice also displayed a 51% decrease in aortic sinus atherosclerosis compared with Scarab(-/-) mice (P < 0.05). In summary, we demonstrate that increased cholesterol esterification by LCAT is atheroprotective, most likely through its ability to increase HDL levels and decrease pro-atherogenic apoB-containing lipoprotein particles.

Expression and regulation of scavenger receptor class B type 1 in the rat ovary and uterus during the estrous cycle

Scavenger receptor class B type 1 (SR-B1) preferentially mediates the selective uptake of high density lipoprotein-cholesterol ester and the delivery of cholesterol for steroidogenesis. Although multiple analyses have investigated the function of SR-B1 in the liver, adrenal and ovary, its expression in rat ovary and uterus during the estrous cycle is lacking. In the present study, real-time PCR, western blot and immunohistochemistry (IHC) were used to investigate SR-B1 expression in the rat ovary and uterus during the estrous cycle. The results demonstrated that ovarian SR-B1 expression was in a stage-dependent manner, continuously increased from proestrus and kept elevated during metoestrus, while uterine SR-B1 expression decreased from proestrus to diestrus. To determine whether ovarian and uterine SR-B1 expression were affected by sex steroid hormones, immature rats were treated with 17 β-estradiol (E2), progesterone (P4), or their antagonists from postnatal days 24-26. Results showed that the levels of SR-B1 mRNA and protein were significantly up-regulated by E2 in both the ovary and uterus. IHC results showed that SR-B1 was primarily localized in the oocytes, theca internal cells (T-I) of follicles, interstitial cells (IC) as well as corpus luteum (CL), but not granulosa cells (GC) in the ovary during the estrous cycle. Uterine SR-B1 was highly expressed in the endometrial luminal epithelial cells (LEC) and glandular epithelial cells (GEC) as well as in the circular muscle (CM) cells, and weak staining in stromal cells (SC) through estrous cycle. Taken together, SR-B1 expression in the ovary and uterus across the estrous cycle demonstrate that SR-B1 may be involved in uterine function, follicular development as well as luteal function.

HDL biogenesis, remodeling, and catabolism

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research.

Low-density lipoprotein receptor affects the fertility of female mice

Low-density lipoprotein receptor (LDLR) has been demonstrated to play a central role in lipoprotein metabolism, with Ldlr-deficient (Ldlr–/–) mice developing severe dyslipidemia. In the present study we investigated whether Ldlr knockout could harm female reproduction and explored the mechanisms involved. The results indicate that although the number of litters born to Ldlr–/– mice did not differ significantly from that born to controls, the number of pups per litter was significantly lower in the former group. Interestingly, although Ldlr–/– mice were obese, the weight of their ovaries was lower than that in control mice. Serum cholesterol levels was significantly higher in Ldlr–/– mice than in their wild-type counterparts. In contrast, there were significant decreases in cholesterol, triglyceride and total lipid levels in ovaries of Ldlr–/– mice. Both ovarian lipid deposition, as detected by Oil red O staining, and lipid droplets, as evaluated by transmission electron microscopy, supported decreased lipid levels in ovaries from Ldlr–/– mice. In addition, Ldlr–/– mice had fewer ovarian follicles, more atretic follicles, lower oestrogen levels and spent significantly less time in oestrus than did the controls. Superovulation assays indicated immature Ldlr–/– mice ovulated fewer ova than controls. These results indicate that lack of Ldlr results in dyslipidaemia and poor fertility.

Excess cholesterol induces mouse egg activation and may cause female infertility

The HDL receptor scavenger receptor, class B type I (SR-BI) controls the structure and fate of plasma HDL. Female SR-BI KO mice are infertile, apparently because of their abnormal cholesterol-enriched HDL particles. We examined the growth and meiotic progression of SR-BI KO oocytes and found that they underwent normal germinal vesicle breakdown; however, SR-BI KO eggs, which had accumulated excess cholesterol in vivo, spontaneously activated, and they escaped metaphase II (MII) arrest and progressed to pronuclear, MIII, and anaphase/telophase III stages. Eggs from fertile WT mice were activated when loaded in vitro with excess cholesterol by a cholesterol/methyl-β-cyclodextrin complex, phenocopying SR-BI KO oocytes. In vitro cholesterol loading of eggs induced reduction in maturation promoting factor and MAPK activities, elevation of intracellular calcium, extrusion of a second polar body, and progression to meiotic stages beyond MII. These results suggest that the infertility of SR-BI KO females is caused, at least in part, by excess cholesterol in eggs inducing premature activation and that cholesterol can activate WT mouse eggs to escape from MII arrest. Analysis of SR-BI KO female infertility raises the possibility that abnormalities in cholesterol metabolism might underlie some cases of human female infertility of unknown etiology.

Genetic alterations affecting cholesterol metabolism and human fertility

Single nucleotide polymorphisms (SNPs) represent genetic variations among individuals in a population. In medicine, these small variations in the DNA sequence may significantly impact an individual's response to certain drugs or influence the risk of developing certain diseases. In the field of reproductive medicine, a significant amount of research has been devoted to identifying polymorphisms which may impact steroidogenesis and fertility. This review discusses current understanding of the effects of genetic variations in cholesterol metabolic pathways on human fertility that bridge novel linkages between cholesterol metabolism and reproductive health. For example, the role of the low-density lipoprotein receptor (LDLR) in cellular metabolism and human reproduction has been well studied, whereas there is now an emerging body of research on the role of the high-density lipoprotein (HDL) receptor scavenger receptor class B type I (SR-BI) in human lipid metabolism and female reproduction. Identifying and understanding how polymorphisms in the SCARB1 gene or other genes related to lipid metabolism impact human physiology is essential and will play a major role in the development of personalized medicine for improved diagnosis and treatment of infertility.

Lipids and oocyte developmental competence: the role of fatty acids and β-oxidation

Metabolism and ATP levels within the oocyte and adjacent cumulus cells are associated with quality of oocyte and optimal development of a healthy embryo. Lipid metabolism provides a potent source of energy and its importance during oocyte maturation is being increasingly recognised. The triglyceride and fatty acid composition of ovarian follicular fluid has been characterised for many species and is influenced by nutritional status (i.e. dietary fat, fasting, obesity and season) as well as lactation in cows. Lipid in oocytes is a primarily triglyceride of specific fatty acids which differ by species, stored in distinct droplet organelles that re-localise during oocyte maturation. The presence of lipids, particularly saturated vs unsaturated fatty acids, in in vitro maturation systems affects oocyte lipid content as well as developmental competence. Triglycerides are metabolised by lipases that have been localised to cumulus cells as well as oocytes. Fatty acids generated by lipolysis are further metabolised by β-oxidation in mitochondria for the production of ATP. β-oxidation is induced in cumulus–oocyte complexes (COCs) by the LH surge, and pharmacological inhibition of β-oxidation impairs oocyte maturation and embryo development. Promoting β-oxidation with l-carnitine improves embryo development in many species. Thus, fatty acid metabolism in the mammalian COC is regulated by maternal physiological and in vitro environmental conditions; and is important for oocyte developmental competence.

LCAT deficiency does not impair amyloid metabolism in APP/PS1 mice

A key step in plasma HDL maturation from discoidal to spherical particles is the esterification of cholesterol to cholesteryl ester, which is catalyzed by LCAT. HDL-like lipoproteins in cerebrospinal fluid (CSF) are also spherical, whereas nascent lipoprotein particles secreted from astrocytes are discoidal, suggesting that LCAT may play a similar role in the CNS. In plasma, apoA-I is the main LCAT activator, while in the CNS, it is believed to be apoE. apoE is directly involved in the pathological progression of Alzheimer's disease, including facilitating β-amyloid (Aβ) clearance from the brain, a function that requires its lipidation by ABCA1. However, whether apoE particle maturation by LCAT is also required for Aβ clearance is unknown. Here we characterized the impact of LCAT deficiency on CNS lipoprotein metabolism and amyloid pathology. Deletion of LCAT from APP/PS1 mice resulted in a pronounced decrease of apoA-I in plasma that was paralleled by decreased apoA-I levels in CSF and brain tissue, whereas apoE levels were unaffected. Furthermore, LCAT deficiency did not increase Aβ or amyloid in APP/PS1 LCAT(-/-) mice. Finally, LCAT expression and plasma activity were unaffected by age or the onset of Alzheimer's-like pathology in APP/PS1 mice. Taken together, these results suggest that apoE-containing discoidal HDLs do not require LCAT-dependent maturation to mediate efficient Aβ clearance.

Nonesterified fatty acids and spontaneous preterm birth: a factor analysis for identification of risk patterns

We considered that accumulation of nonesterified (free) fatty acids (NEFAs) in the first trimester of pregnancy would mark women at excess risk of spontaneous preterm birth (sPTB) and examined the interplay between NEFAs, lipids, and other markers to explore pathways to sPTB. In a case-control study nested in the Pregnancy Exposures and Preeclampsia Prevention Study (Pittsburgh, Pennsylvania, 1997-2001), we assayed NEFA levels in nonfasting serum collected at a mean gestational week of 9.4 (range, 4-20 weeks) in 115 women with sPTB (<37 weeks) and 222 women with births occurring at ≥37 weeks. C-reactive protein, total cholesterol, low-density lipoprotein and high-density lipoprotein (HDL) cholesterol, triglycerides, and uric acid were also measured. Polytomous logistic regression models were used to evaluate tertiles of NEFA levels and sPTB at <34 weeks and 34-36 weeks; factor analysis was used to characterize patterns of biomarkers. Women with NEFA levels in the highest tertile versus the lowest were 2.02 (95% confidence interval: 1.13, 3.48) times more likely to have sPTB, after adjustment for covariates. Risk of sPTB before 34 weeks was particularly high among women with high NEFA levels (odds ratio = 3.73, 95% confidence interval: 1.33, 10.44). Six biomarker patterns were identified, and 2 were associated with sPTB: 1) increasing NEFA and HDL cholesterol levels and 2) family history of gestational hypertension. NEFA levels early in pregnancy were independently associated with sPTB, particularly before 34 weeks. We also detected a novel risk pattern suggesting that NEFAs together with HDL cholesterol may be related to sPTB.

Hepatic scavenger receptor BI protects against polymicrobial-induced sepsis through promoting LPS clearance in mice

Recent studies revealed that scavenger receptor BI (SR-BI or Scarb1) plays a critical protective role in sepsis. However, the mechanisms underlying this protection remain largely unknown. In this study, using Scarb1(I179N) mice, a mouse model specifically deficient in hepatic SR-BI, we report that hepatic SR-BI protects against cecal ligation and puncture (CLP)-induced sepsis as shown by 75% fatality in Scarb1(I179N) mice, but only 21% fatality in C57BL/6J control mice. The increase in fatality in Scarb1(I179N) mice was associated with an exacerbated inflammatory cytokine production. Further study demonstrated that hepatic SR-BI exerts its protection against sepsis through its role in promoting LPS clearance without affecting the inflammatory response in macrophages, the glucocorticoid production in adrenal glands, the leukocyte recruitment to peritoneum or the bacterial clearance in liver. Our findings reveal hepatic SR-BI as a critical protective factor in sepsis and point out that promoting hepatic SR-BI-mediated LPS clearance may provide a therapeutic approach for sepsis.

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.

Functional genomics of the human high-density lipoprotein receptor scavenger receptor BI: an old dog with new tricks

PURPOSE OF REVIEW

The athero-protective role of scavenger receptor BI (SR-BI) is primarily attributed to its ability to selectively transfer cholesteryl esters from high-density lipoproteins (HDLs) to the liver during reverse cholesterol transport (RCT). In this review, we highlight recent findings that reveal the impact of SR-BI on lipid levels and cardiovascular disease in humans. Moreover, additional responsibilities of SR-BI in modulating adrenal and platelet function, as well as female fertility in humans, are discussed.

RECENT FINDINGS

Heterozygote carriers of P297S, S112F and T175A-mutant SR-BI receptors were identified in patients with high HDL-cholesterol levels. HDL from P297S-SR-BI carriers was unable to mediate macrophage cholesterol efflux, whereas hepatocytes expressing P297S-SR-BI were unable to mediate the selective uptake of HDL-cholesteryl esters. S112F and T175A-mutant receptors exhibited similar impaired cholesterol transport functions in vitro. Reduced SR-BI function in P297S carriers was also associated with decreased steroidogenesis and altered platelet function. Further, human population studies identified SCARB1 variants associated with female infertility.

SUMMARY

Identification of SR-BI variants confirms the key role of this receptor in influencing lipid levels and RCT in humans. A deeper understanding of the contributions of SR-BI to steroidogenesis, platelet function and fertility is required in light of exploration of HDL-raising therapies aimed at reducing cardiovascular risk.

Dietary manipulation and social isolation alter disease progression in a murine model of coronary heart disease

BACKGROUND

Mice with a deficiency in the HDL receptor SR-BI and low expression of a modified apolipoprotein E gene (SR-BI KO/ApoeR61(h/h)) called 'HypoE' when fed an atherogenic, 'Paigen' diet develop occlusive, atherosclerotic coronary arterial disease (CHD), myocardial infarctions (MI), and heart dysfunction and die prematurely (50% mortality ~40 days after initiation of this diet). Because few murine models share with HypoE mice these cardinal, human-like, features of CHD, HypoE mice represent a novel, small animal, diet-inducible and genetically tractable model for CHD. To better describe the properties of this model, we have explored the effects of varying the composition and timing of administration of atherogenic diets, as well as social isolation vs. group housing, on these animals.

METHODOLOGY/PRINCIPAL FINDINGS

HypoE mice were maintained on a standard lab chow diet (control) until two months of age. Subsequently they received one of three atherogenic diets (Paigen, Paigen without cholate, Western) or control diet for varying times and were housed in groups or singly, and we determined the plasma cholesterol levels, extent of cardiomegaly and/or survival. The rate of disease progression could be reduced by lowering the severity of the atherogenic diet and accelerated by social isolation. Disease could be induced by Paigen diets either containing or free of cholate. We also established conditions under which CHD could be initiated by an atherogenic diet and then subsequently, by replacing this diet with standard lab chow, hypercholesterolemia could be reduced and progression to early death prevented.

CONCLUSIONS/SIGNIFICANCE

HypoE mice provide a powerful, surgery-free, diet-'titratable' small animal model that can be used to study the onset of recovery from occlusive, atherosclerotic CHD and heart failure due to MI. HypoE mice can be used for the analysis of the effects of environment (diet, social isolation) on a variety of features of cardiovascular disease.

Extrahepatic high-density lipoprotein receptor SR-BI and apoA-I protect against deep vein thrombosis in mice

OBJECTIVE

Deep vein thrombosis (DVT) and pulmonary embolism are frequent causes of morbidity and mortality. The goal of our study was to determine whether plasma high-density lipoprotein (HDL), which inversely correlates with the risk of cardiovascular events, affects DVT.

METHODS AND RESULTS

Using a murine DVT model of inferior vena cava stenosis, we demonstrated that deficiency of the HDL receptor, scavenger receptor class B type I (SR-BI), promotes venous thrombosis. As SR-BI(-/-) mice have increased plasma cholesterol levels and abnormal HDL particles, we tested SR-BI(-/-) mice with an SR-BI liver transgene that normalizes both parameters. These mice also exhibited increased susceptibility to DVT, indicating a protective role of extrahepatic SR-BI. Mice lacking the major HDL apolipoprotein apoA-I or endothelial nitric oxide synthase (eNOS) (a downstream target of endothelial SR-BI signaling) also had a prothrombotic phenotype. Intravenous infusion of human apoA-I, an HDL component and SR-BI ligend, prevented DVT in wild-type but not SR-BI(-/-) or eNOS(-/-) mice, suggesting that its effect is mediated by SR-BI and eNOS. Intravenous apoA-I infusion abolished histamine-induced platelet-endothelial interactions, which are important for DVT initiation.

CONCLUSIONS

An apoA-I (HDL)-SR-BI-eNOS axis is highly protective in DVT and may provide new targets for prophylaxis and treatment of venous thrombosis.

Scavenger receptor class B type 1 gene polymorphisms and female fertility

PURPOSE OF REVIEW

Multiple studies have demonstrated a role for scavenger receptor class B type 1 (SR-B1) in female fertility. Recent studies have implicated specific SR-B1 gene polymorphisms in decreased progesterone production and suboptimal fertility outcomes.

RECENT FINDINGS

The lipoprotein receptor SR-B1 has been known to mediate selective uptake of lipids into steroidogenic tissues such as the ovaries. SR-B1 plays a major role in the ability of the corpus luteum to produce progesterone, which is known to play a key role in sustaining early pregnancy. Animal studies have demonstrated that deficiency in SR-B1 results in subfertility that can be restored with addition of SR-B1 function. Single-nucleotide polymorphisms in SCARB1, the gene encoding SR-B1, have been associated with human lipid levels. Women undergoing infertility treatment with low SR-B1 expression in granulosa cells were noted to have plasma estradiol levels half the normal levels and a significantly lower number of retrieved oocytes. In vitro, deficiency of SR-B1 is associated with lower progesterone secretion in human granulosa cells. Certain SR-B1 polymorphisms have been associated with lower follicular progesterone levels and a significantly lower clinical pregnancy rate.

SUMMARY

Deficiency of SR-B1, particularly due to single-nucleotide polymorphisms, could explain some features of female human infertility.

Clinical impact of scavenger receptor class B type I gene polymorphisms on human female fertility

BACKGROUND

The goal of this study was to evaluate the association of SCARB1 single nucleotide polymorphisms (SNPs) and fertility outcomes in women undergoing IVF.

METHODS

Between November 2007 and March 2010, granulosa cells and follicular fluid were collected from women undergoing IVF. Five SCARB1 SNPs were sequenced and progesterone levels were measured in the follicular fluid. Fertility measurements were defined as the presence of gestational sac(s) and fetal heartbeat(s).

RESULTS

The study group consisted of 274 women (mean age of 36.4 ± 4.6 years). The racial/ethnic composition was 55% Caucasian (n = 152), 25% African-American (n = 68), 12% Asian (n = 34), 5% Hispanic, (n = 14) and 2% other (n = 6). There was a significant difference in the genotype frequencies of the SCARB1 SNPs across the groups. Subjects who were homozygous for the minor allele in the rs5888 SNP had higher follicular progesterone levels than those who were homozygous for the major allele (P = 0.03). In the Caucasian group, carriers of the minor A allele of the rs4238001 SNP had lower follicular progesterone levels compared with homozygous carriers of the major G allele (P = 0.04). In this group, follicular progesterone levels were highly predictive of the rs4238001 SNP (P = 0.03). In the entire cohort, minor allele carriers of rs4238001 did not have any viable fetuses at Day 42 following embryo transfers (P = 0.04). In the African-American group in particular, there was also an association between rs10846744 and gestational sac(s) (P = 0.006), and fetal heartbeat(s) (P = 0.005).

CONCLUSIONS

In part, SCARB1 rs4238001 and rs10846744 SNPs may contribute to human female infertility.

High density lipoprotein: it's not just about lipid transport anymore

Plasma levels of high density lipoprotein cholesterol (HDL-C) have long been associated with protection against cardiovascular disease (CVD) in large populations. However, HDL-C has been significantly less useful for predicting CVD risk in individual patients. This has ignited a new debate on the merits of measuring HDL quantity versus quality in terms of protective potential. In addition, numerous recent studies have begun to uncover HDL functions that vary surprisingly from traditional lipid transport roles. In this paper, we review recent findings that point to important functions for HDL that go well beyond lipid transport. These discoveries suggest that HDL might be a platform that mediates protection from a host of disease states ranging from CVD to diabetes to infectious disease.

Deficiency of scavenger receptor class B type I negatively affects progesterone secretion in human granulosa cells

Our goal was to examine the effect of deficiency of the lipoprotein receptor, scavenger receptor class B type I (SR-BI), on progesterone secretion in human granulosa cells (HGL5). Scrambled or SR-BI small interfering RNA [knockdown (KD)] cells were exposed to dimethylsulfoxide [DMSO, vehicle for forskolin (Fo)], Fo, serum, high-density lipoprotein, low-density lipoprotein (LDL), or Fo plus lipoproteins or serum for 24 h. Progesterone secretion was lower in all of the SR-BI KD cells regardless of treatment. We examined progesterone secretion in SR-BI KD, LDL receptor KD, and double KD cells incubated with DMSO, Fo, LDL, or Fo + LDL for 6-24 h. As compared with scrambled cells, progesterone secretion was lower in SR-BI and double KD cells regardless of treatment; whereas progesterone secretion was only lower in LDL receptor KD cells incubated with LDL and Fo + LDL. We measured phosphorylation of hormone-sensitive lipase (pHSL) expression, intracellular total cholesterol (TC) mass, and progesterone secretion in scrambled and SR-BI KD cells incubated with DMSO or Fo for 2-24 h. The expression of pHSL was similar between the cells and conditions. The mean change in TC mass and progesterone secretion was lower in SR-BI KD cells exposed to DMSO and Fo. Incubating SR-BI KD cells with 22-hydroxy cholesterol did not overcome the reduction in progesterone secretion. At different time points, RNA expression of steroidogenic acute regulatory protein, side-chain cleavage, and 3β-hydroxysteroid dehydrogenase was significantly lower in SR-BI KD cells incubated with Fo. In conclusion, SR-BI protein deficiency, in part, might explain progesterone deficiency in some infertile women.

In vitro and in vivo analysis of the binding of the C terminus of the HDL receptor scavenger receptor class B, type I (SR-BI), to the PDZ1 domain of its adaptor protein PDZK1

The PDZ1 domain of the four PDZ domain-containing protein PDZK1 has been reported to bind the C terminus of the HDL receptor scavenger receptor class B, type I (SR-BI), and to control hepatic SR-BI expression and function. We generated wild-type (WT) and mutant murine PDZ1 domains, the mutants bearing single amino acid substitutions in their carboxylate binding loop (Lys(14)-Xaa(4)-Asn(19)-Tyr-Gly-Phe-Phe-Leu(24)), and measured their binding affinity for a 7-residue peptide corresponding to the C terminus of SR-BI ((503)VLQEAKL(509)). The Y20A and G21Y substitutions abrogated all binding activity. Surprisingly, binding affinities (K(d)) of the K14A and F22A mutants were 3.2 and 4.0 μM, respectively, similar to 2.6 μM measured for the WT PDZ1. To understand these findings, we determined the high resolution structure of WT PDZ1 bound to a 5-residue sequence from the C-terminal SR-BI ((505)QEAKL(509)) using x-ray crystallography. In addition, we incorporated the K14A and Y20A substitutions into full-length PDZK1 liver-specific transgenes and expressed them in WT and PDZK1 knock-out mice. In WT mice, the transgenes did not alter endogenous hepatic SR-BI protein expression (intracellular distribution or amount) or lipoprotein metabolism (total plasma cholesterol, lipoprotein size distribution). In PDZK1 knock-out mice, as expected, the K14A mutant behaved like wild-type PDZK1 and completely corrected their hepatic SR-BI and plasma lipoprotein abnormalities. Unexpectedly, the 10-20-fold overexpressed Y20A mutant also substantially, but not completely, corrected these abnormalities. The results suggest that there may be an additional site(s) within PDZK1 that bind(s) SR-BI and mediate(s) productive SR-BI-PDZK1 interaction previously attributed exclusively to the canonical binding of the C-terminal SR-BI to PDZ1.

Restoration of high-density lipoprotein levels by cholesteryl ester transfer protein expression in scavenger receptor class B type I (SR-BI) knockout mice does not normalize pathologies associated with SR-BI deficiency

OBJECTIVE

Disruption of scavenger receptor class B type I (SR-BI) in mice impairs high-density lipoprotein (HDL)-cholesterol (HDL-C) delivery to the liver and induces susceptibility to atherosclerosis. In this study, it was investigated whether introduction of cholesteryl ester transfer protein (CETP) can normalize HDL-C transport to the liver and reduce atherosclerosis in SR-BI knockout (KO) mice.

METHODS AND RESULTS

Expression of human CETP in SR-BI(KO) mice resulted in decreased plasma HDL-C levels, both on chow diet (1.8-fold, P<0.001) and on challenge with Western-type diet (1.6-fold, P<0.01). Furthermore, the presence of CETP partially normalized the abnormally large HDL particles observed in SR-BI(KO) mice. Unexpectedly, expression of CETP in SR-BI(KO) mice did not reduce atherosclerotic lesion development, probably because of consequences of SR-BI deficiency, including the persistence of higher VLDL-cholesterol (VLDL-C) levels, unchanged elevated free cholesterol/total cholesterol ratio, and the increased oxidative status of the animals. In addition, CETP expression did not normalize other characteristics of SR-BI deficiency, including female infertility, reticulocytosis, thrombocytopenia, and impaired platelet aggregation.

CONCLUSIONS

CETP restores HDL-C levels in SR-BI(KO) mice, but it does not change the susceptibility to atherosclerosis and other typical characteristics that are associated with SR-BI disruption. This may indicate that the pathophysiology of SR-BI deficiency is not a direct consequence of changes in the HDL pool.

Scavenger receptor-B1 and luteal function in mice

During luteinization, circulating high-density lipoproteins supply cholesterol to ovarian cells via the scavenger receptor-B1 (SCARB1). In the mouse, SCARB1 is expressed in cytoplasm and periphery of theca, granulosa, and cumulus cells of developing follicles and increases dramatically during formation of corpora lutea. Blockade of ovulation in mice with meloxicam, a prostaglandin synthase-2 inhibitor, resulted in follicles with oocytes entrapped in unexpanded cumulus complexes and with granulosa cells with luteinized morphology and expressing SCARB1 characteristic of luteinization. Mice bearing null mutation of the Scarb1 gene (SCARB1(-/-)) had ovaries with small corpora lutea, large follicles with hypertrophied theca cells, and follicular cysts with blood-filled cavities. Plasma progesterone concentrations were decreased 50% in mice with Scarb1 gene disruption. When SCARB1(-/-) mice were treated with a combination of mevinolin [an inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase (HMGR)] and chloroquine (an inhibitor of lysosomal processing of low-density lipoproteins), serum progesterone was further reduced. HMGR protein expression increased in SCARB1(-/-) mice, independent of treatment. It was concluded that theca, granulosa, and cumulus cells express SCARB1 during follicle development, but maximum expression depends on luteinization. Knockout of SCARB1(-/-) leads to ovarian pathology and suboptimal luteal steroidogenesis. Therefore, SCARB1 expression is essential for maintaining normal ovarian cholesterol homeostasis and luteal steroid synthesis.

High-density lipoprotein metabolism and the human embryo

BACKGROUND

High-density lipoprotein (HDL) appears to be the dominant lipoprotein particle in human follicular fluid (FF). The reported anti-atherogenic properties of HDL have been attributed in part to reverse cholesterol transport. The discoveries of the scavenger receptor class B type I (SR-BI) and the ATP-binding cassette A1 lipid (ABCA1) transporter have generated studies aimed at unraveling the pathways of HDL biogenesis, remodeling and catabolism. The production of SR-BI and ABCA1 knockout mice as well as other lipoprotein metabolism-associated mutants has resulted in reduced or absent fertility, leading us to postulate the existence of a human hepatic-ovarian HDL-associated axis of fertility. Here, we review an evolving literature on the role of HDL metabolism on mammalian fertility and oocyte development.

METHODS

An extensive online search was conducted of published articles relevant to the section topics discussed. All relevant English language articles contained in Pubmed/Medline, with no specific time frame for publication, were considered for this narrative review. Cardiovascular literature was highly cited due to the wealth of relevant knowledge on HDL metabolism, and the dearth thereof in the reproductive field.

RESULTS

Various vertebrate models demonstrate a role for HDL in embryo development and fertility. In our clinical studies, FF levels of HDL cholesterol and apolipoprotein AI levels were negatively associated with embryo fragmentation, but not with embryo cell cleavage rate. However, the HDL component, paraoxonase 1 arylesterase activity, was positively associated with embryo cell cleavage rate.

CONCLUSIONS

HDL contributes to intra-follicular cholesterol homeostasis which appears to be important for successful oocyte and embryo development.

Loss of PDZK1 causes coronary artery occlusion and myocardial infarction in Paigen diet-fed apolipoprotein E deficient mice

BACKGROUND

PDZK1 is a four PDZ-domain containing protein that binds to the carboxy terminus of the HDL receptor, scavenger receptor class B type I (SR-BI), and regulates its expression, localization and function in a tissue-specific manner. PDZK1 knockout (KO) mice are characterized by a marked reduction of SR-BI protein expression ( approximately 95%) in the liver (lesser or no reduction in other organs) with a concomitant 1.7 fold increase in plasma cholesterol. PDZK1 has been shown to be atheroprotective using the high fat/high cholesterol ('Western') diet-fed murine apolipoprotein E (apoE) KO model of atherosclerosis, presumably because of its role in promoting reverse cholesterol transport via SR-BI.

PRINCIPAL FINDINGS

Here, we have examined the effects of PDZK1 deficiency in apoE KO mice fed with the atherogenic 'Paigen' diet for three months. Relative to apoE KO, PDZK1/apoE double KO (dKO) mice showed increased plasma lipids (33% increase in total cholesterol; 49 % increase in unesterified cholesterol; and 36% increase in phospholipids) and a 26% increase in aortic root lesions. Compared to apoE KO, dKO mice exhibited substantial occlusive coronary artery disease: 375% increase in severe occlusions. Myocardial infarctions, not observed in apoE KO mice (although occasional minimal fibrosis was noted), were seen in 7 of 8 dKO mice, resulting in 12 times greater area of fibrosis in dKO cardiac muscle.

CONCLUSIONS

These results show that Paigen-diet fed PDZK1/apoE dKO mice represent a new animal model useful for studying coronary heart disease and suggest that PDZK1 may represent a valuable target for therapeutic intervention.

Follicular fluid high density lipoprotein-associated micronutrient levels are associated with embryo fragmentation during IVF

Purpose

To investigate whether follicular fluid lipid-soluble micronutrients are associated with embryo morphology parameters during IVF.

Methods

Follicle fluid and oocytes were obtained prospectively from 81 women. Embryo morphology parameters were used as surrogate markers of oocyte health. HDL lipids and lipid-soluble micronutrients were analyzed by high-pressure liquid chromatography. Non-parametric bi-variate analysis and multivariable ordinal logistic regression models were employed to examine associations between biochemical and embryo morphology parameters.

Results

Follicular fluid HDL cholesterol (r = −0.47, p < 0.01), α-tocopherol (r = −0.41, p < 0.01), δ-tocopherol (r = −0.38, p < 0.05) and β-cryptoxanthine (r = −0.42, p < 0.01) are negatively correlated with embryo fragmentation. Ordinal logistic regression models indicate that a 0.1 μmol/L increase in β-cryptoxanthine, adjusted for γ-tocopherol, is associated with a 75% decrease in the cumulative odds of higher embryo fragmentation (p = 0.010).

Conclusion

Follicular fluid HDL micronutrients may play an important role in the development of the human oocyte as evident by embryo fragmentation during IVF.

Scavenger receptor BI facilitates hepatic very low density lipoprotein production in mice

Scavenger receptor BI (SR-BI) is a selective uptake receptor for HDL cholesterol but is also involved in the catabolism of apolipoprotein (apo)B-containing lipoproteins. However, plasma levels of apoB-containing lipoproteins increase following hepatic SR-BI overexpression, suggesting that SR-BI not solely mediates their catabolism. We therefore tested the hypothesis that hepatic SR-BI impacts on VLDL production. On day 7 following adenovirus (Ad)-mediated overexpression of SR-BI, VLDL-triglyceride and VLDL-apoB production rates were significantly increased (P < 0.001), whereas VLDL production was significantly lower in SR-BI knockout mice compared with controls (P < 0.05). In mice injected with AdSR-BI, hepatic cholesterol content increased (P < 0.001), microsomal triglyceride transfer protein activity was higher (P < 0.01) and expression of sterol-regulatory element binding protein (SREBP)2 and its target genes was decreased (P < 0.01). Conversely, in SR-BI knockout mice, microsomal triglyceride transfer protein activity was lower and expression of SREBP2 target genes was increased (P < 0.01). Finally, we demonstrate in vitro in isolated primary hepatocytes as well as in vivo that cholesterol derived from HDL and taken up via SR-BI into the liver can be resecreted within VLDL. These data indicate that hepatic SR-BI expression is linked to VLDL production, and within liver, a metabolic shunt might exist that delivers HDL cholesterol, at least in part, to a pool from which cholesterol is mobilized for VLDL production. These results might have implications for HDL-based therapies against atherosclerotic cardiovascular disease, especially with SR-BI as target.

Age-related variations in follicular apolipoproteins may influence human oocyte maturation and fertility potential

OBJECTIVE

To investigate involvement of specific apolipoproteins in the process of human oocyte maturation and age-related infertility as molecular constituents of follicular fluid.

DESIGN

Laboratory-based observational study.

SETTING

Basic science laboratory at a large academic institution.

PATIENT(S)

Follicular fluid obtained from healthy women aged 18 to 45 years undergoing in vitro fertilization for unexplained infertility, ovulatory dysfunction, tubal disease, male factor infertility, or oocyte donation.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Specific concentration of apolipoproteins and content of lipoprotein particles in follicular fluid and blood plasma as related to reproductive aging.

RESULT(S)

We registered a decline of follicular apolipoprotein A1 (Apo A1) and apolipoprotein CII (Apo CII) and an increase of the apolipoprotein E (Apo E) with age, which parallels a lower number of retrieved mature oocytes in older women. Follicular apolipoprotein A1, apolipoprotein B (Apo B), apolipoprotein E, and apolipoprotein C II are present in diverse heterogeneous complexes including very-low-density lipoproteins (VLDL), intermediate-low-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) that vary with patient age and differ from the blood plasma lipoprotein complexes.

CONCLUSION(S)

Age-related variation in follicular apolipoprotein content and distribution in the cholesterol particles may be associated with the decrease in production of mature oocytes and the age-related decline in fertility potential.

Normal hepatic cell surface localization of the high density lipoprotein receptor, scavenger receptor class B, type I, depends on all four PDZ domains of PDZK1

PDZK1 is a four PDZ domain-containing scaffold protein that binds to scavenger receptor class B, type I (SR-BI), the high density lipoprotein receptor, by its first PDZ domain (PDZ1). PDZK1 knock-out mice exhibit a >95% decrease in hepatic SR-BI protein and consequently an approximately 70% increase in plasma cholesterol in abnormally large high density lipoprotein particles. These defects are corrected by hepatic overexpression of full-length PDZK1 but not the PDZ1 domain alone, which partially restores SR-BI protein abundance but not cell surface expression or function. We have generated PDZK1 knock-out mice with hepatic expression of four PDZK1 transgenes encoding proteins with nested C-terminal truncations: pTEM, which lacks the three C-terminal residues (putative PDZ-binding motif), and PDZ1.2, PDZ1.2.3, or PDZ1.2.3.4, which contain only the first two, three, or four N-terminal PDZ domains, respectively, but not the remaining C-terminal sequences. Hepatic overexpression of pTEM restored normal hepatic SR-BI abundance, localization, and function. Hepatic overexpression of PDZ1.2 or PDZ1.2.3 partially restored SR-BI abundance ( approximately 12 or approximately 30% of wild type, respectively) but did not (PDZ1.2) or only slightly (PDZ1.2.3) restored hepatic SR-BI cell surface localization and function. Hepatic overexpression of PDZ1.2.3.4 completely restored SR-BI protein abundance, cell surface expression, and function (normalization of plasma cholesterol levels). Thus, all four PDZ domains in PDZK1, but not PDZ1-3 alone, are sufficient for its normal control of the abundance, localization, and therefore function of hepatic SR-BI, whereas the residues C-terminal to the PDZ4 domain, including the C-terminal putative PDZ-binding domain, are not required.

Overexpression of the PDZ1 domain of PDZK1 blocks the activity of hepatic scavenger receptor, class B, type I by altering its abundance and cellular localization

PDZK1 is a four-PDZ domain-containing scaffold protein that, via its first PDZ domain (PDZ1), binds to the C terminus of the high density lipoprotein (HDL) receptor scavenger receptor, class B, type I (SR-BI). Abolishing PDZK1 expression in PDZK1 knock-out (KO) mice leads to a post-transcriptional, tissue-specific decrease in SR-BI protein level and an increase in total plasma cholesterol carried in abnormally large HDL particles. Here we show that, although hepatic overexpression of PDZK1 restored normal SR-BI protein abundance and function in PDZK1 KO mice, hepatic overexpression of only the PDZ1 domain was not sufficient to restore normal SR-BI function. In wild-type mice, overexpression of the PDZ1 domain overcame the activity of the endogenous hepatic PDZK1, resulting in a 75% reduction in hepatic SR-BI protein levels and intracellular mislocalization of the remaining SR-BI. As a consequence, the plasma lipoproteins in PDZ1 transgenic mice resembled those in PDZK1 KO mice (hypercholesterolemia due to large HDL). These results indicate that the PDZ1 domain can control the abundance and localization, and therefore the function, of hepatic SR-BI and that structural features of PDZK1 in addition to its SR-BI-binding PDZ1 domain are required for normal hepatic SR-BI regulation.

Thrombocytopenia and platelet abnormalities in high-density lipoprotein receptor-deficient mice

OBJECTIVE

High-density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediated cellular uptake of lipoprotein cholesterol controls HDL structure and plasma HDL and biliary cholesterol levels. In SR-BI knockout (KO) mice, an unusually high plasma unesterified-to-total cholesterol ratio (UC:TC) and abnormally large HDL particles apparently contribute to pathology, including female infertility, susceptibility to atherosclerosis and coronary heart disease, and anemia. Here we examined the influence of SR-BI deficiency on platelets.

METHODS AND RESULTS

The high plasma UC:TC ratio in SR-BI KO mice was correlated with platelet abnormalities, including high cholesterol content, abnormal morphologies, high clearance rates, and thrombocytopenia. One day after platelets from wild-type mice were infused into SR-BI KO mice, they exhibited abnormally high cholesterol content and clearance rates similar to those of endogenous platelets. Platelets from SR-BI KO mice exhibited in vitro a blunted aggregation response to the agonist ADP but a normal response to PAR4.

CONCLUSIONS

In SR-BI KO mice abnormal circulating lipoproteins, particularly their high UC:TC ratio-rather than the absence of SR-BI in platelets themselves-induce defects in platelet structure and clearance, together with a mild defect in function.

Influence of PDZK1 on lipoprotein metabolism and atherosclerosis

PDZK1 is a scaffold protein containing four PDZ protein interaction domains, which bind to the carboxy termini of a number of membrane transporter proteins, including ion channels (e.g., CFTR) and cell surface receptors. One of these, the HDL receptor, scavenger receptor class B type I (SR-BI), exhibits a striking, tissue-specific dependence on PDZK1 for its expression and activity. In PDZK1 knockout (KO) mice there is a marked reduction of SR-BI protein expression (approximately 95%) in the liver, but not in steroidogenic tissues or, as we show in this report, in bone marrow- or spleen-derived macrophages, or lung-derived endothelial cells. Because of hepatic SR-BI deficiency, PDZK1 KO mice exhibit dyslipidemia characterized by elevated plasma cholesterol carried in abnormally large HDL particles. Here, we show that inactivation of the PDZK1 gene promotes the development of aortic root atherosclerosis in apolipoprotein E (apoE) KO mice fed with a high fat/high cholesterol diet. However, unlike complete SR-BI-deficiency in SR-BI/apoE double KO mice, PDZK1 deficiency in PDZK1/apoE double knockout mice did not result in development of occlusive coronary artery disease or myocardial infarction, presumably because of their residual expression of SR-BI. These findings demonstrate that deficiency of an adaptor protein essential for normal expression of a lipoprotein receptor promotes atherosclerosis in a murine model. They also define PDZK1 as a member of the family of proteins that is instrumental in preventing cardiovascular disease by maintaining normal lipoprotein metabolism.

Identification of the molecular target of small molecule inhibitors of HDL receptor SR-BI activity

Scavenger receptor, class B, type I (SR-BI), controls high-density lipoprotein (HDL) metabolism by mediating cellular selective uptake of lipids from HDL without the concomitant degradation of the lipoprotein particle. We previously identified in a high-throughput chemical screen of intact cells five compounds (BLT-1-5) that inhibit SR-BI-dependent lipid transport from HDL, but do not block HDL binding to SR-BI on the cell surface. Although these BLTs are widely used to examine the diverse functions of SR-BI, their direct target(s), SR-BI itself or some other component of the SR-BI pathway, has not been identified. Here we show that SR-BI in the context of a membrane lipid environment is the target of BLT-1, -3, -4, and -5. The analysis using intact cells and an in vitro system of purified SR-BI reconstituted into liposomes was aided by information derived from structure-activity relationship (SAR) analysis of the most potent of these BLTs, the thiosemicarbazone BLT-1. We found that the sulfur atom of BLT-1 was crucially important for its inhibitory activity, because changing it to an oxygen atom resulted in the isostructural, but essentially inactive, semicarbazone derivative BLT-1sc. SAR analysis also established the importance of BLT-1's hydrophobic tail. BLTs and their corresponding inactive compounds can be used to explore the mechanism and function of SR-BI-mediated selective lipid uptake in diverse mammalian experimental models. Consequently, BLTs may help determine the therapeutic potential of SR-BI-targeted pharmaceutical drugs.

Lipoprotein receptor expression during luteinization of the ovarian follicle

Ovarian follicles luteinize after ovulation, requiring structural and molecular remodeling along with exponential increases in steroidogenesis. Cholesterol substrates for luteal steroidogenesis are imported via scavenger receptor-BI (SR-BI) and the low-density lipoprotein (LDL) receptor from circulating high-density lipoproteins and LDL. SR-BI mRNA is expressed in pig ovaries at all stages of folliculogenesis and in the corpus luteum (CL). An 82-kDa form of SR-BI predominates throughout, is weakly present in granulosa cells, and is robustly expressed in the CL, along with the less abundant 57-kDa form. Digestion of N-linked carbohydrates substantially reduced the SR-BI mass in luteal cells, indicating that differences between forms is attributable to glycosylation. Immunohistochemistry revealed SR-BI to be concentrated in the cytoplasm of follicular granulosa cells, although found mostly at the periphery of luteal cells. To examine receptor dynamics during gonadotropin-induced luteinization, pigs were treated with an ovulatory stimulus, and ovaries were collected at intervals to ovulation. SR-BI in granulosa cell cytoplasm increased through the periovulatory period, with migration to the cell periphery as the CL matured. In vitro culture of follicles with human chorionic gonadotropin induced time-dependent upregulation of 82-kDa SR-BI in granulosa cells. SR-BI and LDL receptor were reciprocally expressed, with the latter highest in follicular granulosa cells, declining precipitously with CL formation. We conclude that luteinization causes upregulation of SR-BI expression, its posttranslational maturation by glycosylation, and insertion into luteal cell membranes. Expression of the LDL receptor is extinguished during luteinization, indicating dynamic regulation of cholesterol importation to maintain elevated steroid output by the CL.

Functional LCAT deficiency in human apolipoprotein A-I transgenic, SR-BI knockout mice

Reduction of plasma LCAT activity has been observed in several conditions in which the size of HDL particles is increased; however, the mechanism of this reduction remains elusive. We investigated the plasma activity, mass, and in vivo catabolism of LCAT and its association with HDL particles in human apolipoprotein A-I transgenic, scavenger receptor class B type I knockout (hA-ITg SR-BI-/-) mice. Compared with hA-ITg mice, hA-ITg SR-BI-/- mice had a 4-fold higher total plasma cholesterol concentration, which occurred predominantly in 13-18 nm diameter HDL particles, a significant reduction in plasma esterified cholesterol-total cholesterol (EC/TC) ratio, and significantly lower plasma LCAT activity, suggesting a decrease in LCAT protein. However, LCAT protein in plasma, hepatic mRNA for LCAT, and in vivo turnover of 35S-radiolabeled LCAT were similar in both genotypes of mice. HDL from hA-ITg SR-BI-/- mice was enriched in sphingomyelin (SM), relative to phosphatidylcholine, and had less associated [35S]LCAT radiolabel and endogenous LCAT activity compared with HDL from hA-ITg mice. We conclude that the decreased EC/TC ratio in the plasma of hA-ITg SR-BI-/- mice is attributed to a reduction in LCAT reactivity with SM-enriched HDL particles.

PDZK1 Is Required for Maintaining Hepatic Scavenger Receptor, Class B, Type I (SR-BI) Steady State Levels but Not Its Surface Localization or Function

PDZK1 is a multi-PDZ domain-containing adaptor protein that binds to the C terminus of the high density lipoprotein receptor, scavenger receptor, class B, type I (SR-BI), and controls the posttranscriptional, tissue-specific expression of this lipoprotein receptor. In the absence of PDZK1 (PDZK1(-/-) mice), murine hepatic SR-BI protein levels are very low (<5% of control). As a consequence, abnormal plasma lipoprotein metabolism ( approximately 1.5-1.7-fold increased total plasma cholesterol carried in both normal size and abnormally large high density lipoprotein particles) resembles, but is not as severely defective as, that in SR-BI(-/-) mice. Here we show that the total plasma cholesterol levels and size distribution of lipoproteins are virtually identical in SR-BI(-/-) and SR-BI(-/-)/PDZK1(-/-) mice, indicating that most, if not all of the effects of PDZK1 on lipoprotein metabolism are likely because of the effects of PDZK1 on SR-BI. Hepatic overexpression of wild-type SR-BI in PDZK1(-/-) mice restored near or greater than normal levels of cell surface-expressed, functional SR-BI protein levels in the livers of SR-BI(-/-)/PDZK1(-/-) mice and consequently restored apparently normal lipoprotein metabolism in the absence of PDZK1. Thus, PDZK1 is important for maintaining adequate steady state levels of SR-BI in the liver but is not essential for cell surface expression or function of hepatic SR-BI.