High Density Lipoprotein Protects against Polymicrobe-induced Sepsis in Mice

HDL has been considered to be a protective factor in sepsis; however, most contributing studies were conducted using the endotoxic animal model, and evidence from clinically relevant septic animal models remains limited and controversial. Furthermore, little is known about the roles of HDL in sepsis other than LPS neutralization. In this study, we employed cecal ligation and puncture (CLP), a clinically relevant septic animal model, and utilized apoA-I knock-out (KO) and transgenic mice to elucidate the roles of HDL in sepsis. ApoA-I-KO mice were more susceptible to CLP-induced septic death as shown by the 47.1% survival of apoA-I-KO mice versus the 76.7% survival of C57BL/6J (B6) mice (p = 0.038). ApoA-I-KO mice had exacerbated inflammatory cytokine production during sepsis compared with B6 mice. Further study indicated that serum from apoA-I-KO mice displayed less capacity for LPS neutralization compared with serum from B6 mice. In addition, apoA-I-KO mice had less LPS clearance, reduced corticosterone generation, and impaired leukocyte recruitment in sepsis. In contrast to apoA-I-KO mice, apoA-I transgenic mice were moderately resistant to CLP-induced septic death compared with B6 mice. In conclusion, our findings reveal multiple protective roles of HDL in CLP-induced sepsis. In addition to its well established role in neutralization of LPS, HDL exerts its protection against sepsis through promoting LPS clearance and modulating corticosterone production and leukocyte recruitment. Our study supports efforts to raise HDL levels as a therapeutic approach for sepsis.     

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

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

Scavenger Receptor BI Protects against Septic Death through Its Role in Modulating Inflammatory Response

Sepsis is a leading cause of death that is characterized by uncontrolled inflammatory response. In this study, we report that scavenger receptor BI (SR-BI), a high density lipoprotein receptor, is a critical survival factor of sepsis. We induced sepsis using an established septic animal model, cecal ligation and puncture (CLP). CLP induced 100% fatality in SR-BI-null mice but only 21% fatality in wild type littermates. SR-BI-null mice exhibited aberrant inflammatory responses with delayed inflammatory cytokine generation at the early stage of sepsis and highly elevated inflammatory cytokine production 20 h after CLP treatment. To understand the mechanisms underlying SR-BI protection, we elucidated the effect of mac ro phage SR-BI on inflammatory cytokine generation. Macrophages from SR-BI-null mice produced significantly higher levels of inflammatory cytokines than those of wild type controls in response to LPS. Importantly, transgenic mice overexpressing SR-BI were more resistant to CLP-induced septic death. Using an HEK-Blue^TM cell system, we demonstrated that expression of SR-BI suppressed TLR4-mediated NF-κB activation. To understand why SR-BI-null mice had a delayed inflammatory response, we elucidated the effect of SR-BI on LPS clearance during sepsis. Compared with wild type controls, SR-BI-null mice had lower plasma LPS levels in the early stage of sepsis and elevated plasma LPS levels 20 h following CLP treatment. In conclusion, our findings demonstrate that SR-BI is a critical protective modulator of sepsis in mice. SR-BI exerts its protective function through its role in modulating inflammatory response in mac ro phages and facilitating LPS recruitment and clearance.Sepsis is one of the major causes of death that claims over 215,000 lives and costs $16.7 billion per year in America alone (1–4). The death rate from sepsis is high, exceeding 50%, due to poor understanding of the disease (5). Identifying molecules involved in sepsis, especially endogenous protective modulators, is of great importance not only in understanding the mechanisms but also in providing new insights for efficient therapies.Scavenger receptor BI (SR-BI² or Scarb1) is a 75-kDa membrane protein expressed in the liver, endothelial cells, macrophages, and steroidogenic tissues (6, 7). It is a well established high density lipoprotein (HDL) receptor. It mediates intracellular uptake of cholesterol ester from HDL, which plays a key role in regulating plasma cholesterol levels and steroidogenesis (8–11). Mice deficient in SR-BI have a 2-fold increase in plasma cholesterol levels and develop cardiovascular diseases (10, 12–16). Recent studies reveal that SR-BI is a multifunctional protein. It activates endothelial nitric-oxide synthase in endothelial cells in the presence of HDL (17–20), induces apoptosis in the absence of HDL/endothelial nitric-oxide synthase (21), and protects against nitric oxide (NO)-induced oxidative damage (22). Emerging evidence indicates that specific expression of SR-BI in macrophages provides protection against the development of atherosclerosis, and importantly, it seems that SR-BI exerts this protection independently of its role in regulating lipoprotein metabolism or cholesterol efflux (23–25). The mechanisms underlying the protection of macrophage SR-BI against atherosclerosis are unclear.We recently reported that SR-BI protects against endotoxin-induced animal death through suppression of NO-induced cytotoxicity (22). A recent study by Cai et al. (26) confirmed the protective role of SR-BI in LPS-induced animal death, and the authors demonstrated that SR-BI-mediated glucocorticoid synthesis contributes to its protective function. These studies suggest that SR-BI might play a role in sepsis. Interestingly, using in vitro cell culture, Vishnyakova et al. (27) reported recently that SR-BI enhances the uptake of Gram-negative bacteria, which raises a possibility that this SR-BI-mediated bacterial uptake may facilitate bacterial infection and therefore play a deleterious role in sepsis. Given the limitations of endotoxemia animal model (28), it is of importance to determine whether and how SR-BI plays a protective role in sepsis. In this study, we assessed the role of SR-BI in sepsis using an established septic animal model, cecal ligation and puncture (CLP). We demonstrated that SR-BI is a critical survival factor of sepsis in mice. In contrast to significant protection against LPS-induced animal death by SR-BI-mediated corticosterone generation, corticosterone did not provide protection against CLP-induced septic animal death, indicating that SR-BI has other functions than regulating corticosterone production in sepsis. Using a transgenic animal model, primary macrophages, and HEK-Blue cell system, we demonstrated that SR-BI modulates inflammatory response in macrophages via TLR4 signaling, which contributes to protection against septic death.     

Caveolin-1 Protects against Sepsis by Modulating Inflammatory Response,Alleviating Bacterial Burden,and Suppressing Thymocyte Apoptosis

Sepsis is a leading cause of death, which is characterized by uncontrolled inflammatory response. In this study, we report that caveolin-1, a major component of caveolae, is a critical survival factor of sepsis. We induced sepsis using a well established sepsis animal model, cecal ligation and puncture (CLP). CLP induced 67% fatality in caveolin-1 null mice, but only 27% fatality in wild type littermates (p = 0.015). Further studies revealed that mice deficient in caveolin-1 exhibited marked increase in tumor necrosis factor-α and interleukin-6 production 20 h following CLP treatment, indicating uncontrolled inflammatory responses in the absence of caveolin-1. Caveolin-1 null mice also had a significant increase in bacteria number recovered from liver and spleen, indicating elevated bacterial burdens. In addition, caveolin-1 null mice had a 2-fold increase in thymocyte apoptosis compared with wild type littermates, indicating caveolin-1 as a critical modulator of thymocyte apoptosis during sepsis. In conclusion, our findings demonstrate that caveolin-1 is a critical protective modulator of sepsis in mice. Caveolin-1 exerts its protective function likely through its roles in modulating inflammatory response, alleviating bacterial burdens, and suppressing thymocyte apoptosis.     

Scavenger receptor-BI is a receptor for lipoprotein(a)

Scavenger receptor class B type I (SR-BI) is a multi-ligand receptor that binds a variety of lipoproteins, including high density lipoprotein (HDL) and low density lipoprotein (LDL), but lipoprotein(a) [Lp(a)] has not been investigated as a possible ligand. Stable cell lines (HEK293 and HeLa) expressing human SR-BI were incubated with protein- or lipid-labeled Lp(a) to investigate SR-BI-dependent Lp(a) cell association. SR-BI expression enhanced the association of both ¹²⁵I- and Alexa Fluor-labeled protein from Lp(a). By confocal microscopy, SR-BI was also found to promote the internalization of fluorescent lipids (BODIPY-cholesteryl ester (CE)- and DiI-labeled) from Lp(a), and by immunocytochemistry the cellular internalization of apolipoprotein(a) and apolipoprotein B. When dual-labeled (³H-cholesteryl ether,¹²⁵I-protein) Lp(a) was added to cells expressing SR-BI, there was a greater relative increase in lipid uptake over protein, indicating that SR-BI mediates selective lipid uptake from Lp(a). Compared with C57BL/6 control mice, transgenic mice overexpressing human SR-BI in liver were found to have increased plasma clearance of ³H-CE-Lp(a), whereas mouse scavenger receptor class B type I knockout (Sr-b1-KO) mice had decreased plasma clearance (fractional catabolic rate: 0.63 ± 0.08/day, 1.64 ± 0.62/day, and 4.64 ± 0.40/day for Sr-b1-KO, C57BL/6, and human scavenger receptor class B type I transgenic mice, respectively). We conclude that Lp(a) is a novel ligand for SR-BI and that SR-BI mediates selective uptake of Lp(a)-associated lipids.     

Macrophage SR-BI regulates LPS-induced pro-inflammatory signaling in mice and isolated macrophages

Scavenger receptor BI (SR-BI), an HDL receptor, plays a key role in reverse cholesterol transport. In mice, disruption of SR-BI results in hypersensitivity to lipopolysaccharide (LPS) and bacteria-induced septic shock due to adrenal insufficiency and abnormal hepatic pathogen clearance. In this study, we identify an anti-inflammatory role of macrophage SR-BI. Using bone marrow transplantation, we report an enhanced pro-inflammatory response to LPS in wild-type (WT) mice receiving SR-BI-null compared with WT bone marrow cells and a reduced response in SR-BI-null mice receiving WT compared with SR-BI-null cells. Although significant, SR-BI deficiency limited to bone marrow-derived cells promoted a relatively modest enhancement of the inflammatory response to LPS in mice compared with the effect of whole-body SR-BI deletion. Consistent with earlier findings, SR-BI-null primary macrophages exhibited a greater inflammatory cytokine response to LPS than control macro phages. In addition, we showed that overexpression of SR-BI in J774 macrophages attenuated the inflammatory response to LPS. The LPS-induced cytokine expression in both WT and SR-BI-null macrophages was dependent not only on NFκB as previously reported but also on JNK and P38 cell signaling pathways. The increased inflammatory signaling in SR-BI-null cells was not related to alterations in cellular cholesterol content. We conclude that SR-BI plays an important function in regulating the macrophage inflammatory response to LPS.     

Noncanonical Role of the PDZ4 Domain of the Adaptor Protein PDZK1 in the Regulation of the Hepatic High Density Lipoprotein Receptor Scavenger Receptor Class B,Type I (SR-BI)

The four PDZ (PDZ1 to PDZ4) domain-containing adaptor protein PDZK1 controls the expression, localization, and function of the HDL receptor scavenger receptor class B, type I (SR-BI), in hepatocytes in vivo. This control depends on both the PDZ4 domain and the binding of SR-BI''s cytoplasmic C terminus to the canonical peptide-binding sites of either the PDZ1 or PDZ3 domain (no binding to PDZ2 or PDZ4). Using transgenic mice expressing in the liver domain deletion (ΔPDZ2 or ΔPDZ3), domain replacement (PDZ2→1), or target peptide binding-negative (PDZ4(G389P)) mutants of PDZK1, we found that neither PDZ2 nor PDZ3 nor the canonical target peptide binding activity of PDZ4 were necessary for hepatic SR-BI regulatory activity. Immunohistochemical studies established that the localization of PDZK1 on hepatocyte cell surface membranes in vivo is dependent on its PDZ4 domain and the presence of SR-BI. Analytical ultracentrifugation and hydrogen deuterium exchange mass spectrometry suggested that the requirement of PDZ4 for localization and SR-BI regulation is not due to PDZ4-mediated oligomerization or induction of conformational changes in the PDZ123 portion of PDZK1. However, surface plasmon resonance analysis showed that PDZ4, but not the other PDZ domains, can bind vesicles that mimic the plasma membrane. Thus, PDZ4 may potentiate PDZK1''s regulation of SR-BI by promoting its lipid-mediated attachment to the cytoplasmic membrane. Our results show that not all of the PDZ domains of a multi-PDZ domain-containing adaptor protein are required for its biological activities and that both canonical target peptide binding and noncanonical (peptide binding-independent) capacities of PDZ domains may be employed by a single such adaptor for optimal in vivo activity.     

Cholesteryl ester transfer protein expression partially attenuates the adverse effects of SR-BI receptor deficiency on cholesterol metabolism and atherosclerosis

Scavenger receptor SR-BI significantly contributes to HDL cholesterol metabolism and atherogenesis in mice. However, the role of SR-BI may not be as pronounced in humans due to cholesteryl ester transfer protein (CETP) activity. To address the impact of CETP expression on the adverse effects associated with SR-BI deficiency, we cross-bred our SR-BI conditional knock-out mouse model with CETP transgenic mice. CETP almost completely restored the abnormal HDL-C distribution in SR-BI-deficient mice. However, it did not normalize the elevated plasma free to total cholesterol ratio characteristic of hepatic SR-BI deficiency. Red blood cell and platelet count abnormalities observed in mice liver deficient for SR-BI were partially restored by CETP, but the elevated erythrocyte cholesterol to phospholipid ratio remained unchanged. Complete deletion of SR-BI was associated with diminished adrenal cholesterol stores, whereas hepatic SR-BI deficiency resulted in a significant increase in adrenal gland cholesterol content. In both mouse models, CETP had no impact on adrenal cholesterol metabolism. In diet-induced atherosclerosis studies, hepatic SR-BI deficiency accelerated aortic lipid lesion formation in both CETP-expressing (4-fold) and non-CETP-expressing (8-fold) mice when compared with controls. Impaired macrophage to feces reverse cholesterol transport in mice deficient for SR-BI in liver, which was not corrected by CETP, most likely contributed by such an increase in atherosclerosis susceptibility. Finally, comparison of the atherosclerosis burden in SR-BI liver-deficient and fully deficient mice demonstrated that SR-BI exerted an atheroprotective activity in extra-hepatic tissues whether CETP was present or not. These findings support the contention that the SR-BI pathway contributes in unique ways to cholesterol metabolism and atherosclerosis susceptibility even in the presence of CETP.     

Montelukast abrogates prednisolone‐induced hepatic injury in rats: Modulation of mitochondrial dysfunction,oxidative/nitrosative stress,and apoptosis

The aim of this study was to investigate the protective effect of montelukast (MTK) against prednisolone‐induced hepatic injury in rats. Twenty‐eight male albino rats were categorized into four equal groups. Group I served as the control group; group II: rats orally received prednisolone (5 mg·kg^?1·d^?1) for 30 days; groups III and IV: rats orally received MTK at 10 and 20 mg·kg^?1·d^?1, respectively, simultaneously with prednisolone for 30 days. Serum liver enzymes, hepatic mitochondrial function, oxidative/nitrosative stress, and inflammatory and apoptotic markers were evaluated, and the results were confirmed by histopathological examination. MTK showed significant hepatic protection evidenced by alleviated histological lesion and improvement of mitochondrial function, oxidative/nitrosative stress, and inflammatory and apoptotic changes induced by prednisolone, with more profound protection in higher MTK dose (20 mg·kg^?1). In view of these findings, we can conclude that MTK may have hepatoprotective potential, beyond its therapeutic value for asthmatic patients during their course of corticosteroid therapy.     

Hormone-sensitive lipase deficiency affects the expression of SR-BI,LDLr, and ABCA1 receptors/transporters involved in cellular cholesterol uptake and efflux and disturbs fertility in mouse testis

Hormone-sensitive lipase (HSL) hydrolyse acylglycerols, cholesteryl and retinyl esters. HSL is a key lipase in mice testis, as HSL deficiency results in male sterility. The present work study the effects of the deficiency and lack of HSL on the localization and expression of SR-BI, LDLr, and ABCA1 receptors/transporters involved in uptake and efflux of cholesterol in mice testis, to determine the impact of HSL gene dosage on testis morphology, lipid homeostasis and fertility. The results of this work show that the lack of HSL in mice alters testis morphology and spermatogenesis, decreasing sperm counts, sperm motility and increasing the amount of Leydig cells and lipid droplets. They also show that there are differences in the localization of HSL, SR-BI, LDLr and ABCA1 in HSL^+/+, HSL^+/− and HSL^−/− mice. The deficiency or lack of HSL has effects on protein and mRNA expression of genes involved in lipid metabolisms in mouse testis. HSL^−/− testis have augmented expression of SR-BI, LDLr, ABCA1 and LXRβ, a critical sterol sensor that regulate multiple genes involved in lipid metabolism; whereas LDLr expression decreased in HSL^+/− mice. Plin2, Abca1 and Ldlr mRNA levels increased; and LXRα (Nr1h3) and LXRβ (Nr1h2) decreased in testis from HSL^−/− compared with HSL^+/+; with no differences in Scarb1. Together these data suggest that HSL deficiency or lack in mice testis induces lipid homeostasis alterations that affect the cellular localization and expression of key receptors/transporter involved in cellular cholesterol uptake and efflux (SR-BI, LDRr, ABCA1); alters normal cellular function and impact fertility.     

Novel ENU-Induced Point Mutation in Scavenger Receptor Class B,Member 1, Results in Liver Specific Loss of SCARB1 Protein

Cardiovascular disease (CVD) is the largest cause of premature death in human populations throughout the world. Circulating plasma lipid levels, specifically high levels of LDL or low levels of HDL, are predictive of susceptibility to CVD. The scavenger receptor class B member 1 (SCARB1) is the primary receptor for the selective uptake of HDL cholesterol by liver and steroidogenic tissues. Hepatic SCARB1 influences plasma HDL-cholesterol levels and is vital for reverse cholesterol transport. Here we describe the mapping of a novel N-ethyl-N-nitrosourea (ENU) induced point mutation in the Scarb1 gene identified in a C57BL/6J background. The mutation is located in a highly conserved amino acid in the extracellular loop and leads to the conversion of an isoleucine to an asparagine (I179N). Homozygous mutant mice express normal Scarb1 mRNA levels and are fertile. SCARB1 protein levels are markedly reduced in liver (∼90%), but not in steroidogenic tissues. This leads to ∼70% increased plasma HDL levels due to reduced HDL cholesteryl ester selective uptake. Pdzk1 knockout mice have liver-specific reduction of SCARB1 protein as does this mutant; however, in vitro analysis of the mutation indicates that the regulation of SCARB1 protein in this mutant is independent of PDZK1. This new Scarb1 model may help further our understanding of post-translational and tissue-specific regulation of SCARB1 that may aid the important clinical goal of raising functional HDL.     

Identification of Novel Pathways That Control Farnesoid X Receptor-mediated Hypocholesterolemia

Farnesoid X receptor (FXR) plays important regulatory roles in bile acid, lipoprotein, and glucose homeostasis. Here, we have utilized Fxr^−/− mice and mice deficient in scavenger receptor class B type I (SR-BI), together with an FXR-specific agonist and adenovirus expressing hepatocyte nuclear factor 4α or constitutively active FXR, to identify the mechanisms by which activation of FXR results in hypocholesterolemia. We identify a novel pathway linking FXR to changes in hepatic p-JNK, hepatocyte nuclear factor 4α, and finally SR-BI. Importantly, we demonstrate that the FXR-dependent increase in SR-BI results in both hypocholesterolemia and an increase in reverse cholesterol transport, a process involving the transport of cholesterol from peripheral macrophages to the liver for excretion into the feces. In addition, we demonstrate that FXR activation also induces an SR-BI-independent increase in reverse cholesterol transport and reduces intestinal cholesterol absorption. Together, these data indicate that FXR is a promising therapeutic target for treatment of hypercholesterolemia and coronary heart disease.     

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 (Lys¹⁴-Xaa₄-Asn¹⁹-Tyr-Gly-Phe-Phe-Leu²⁴), and measured their binding affinity for a 7-residue peptide corresponding to the C terminus of SR-BI (⁵⁰³VLQEAKL⁵⁰⁹). 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 (⁵⁰⁵QEAKL⁵⁰⁹) 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.     

Procollagen C-endopeptidase Enhancer Protein 2 (PCPE2) Reduces Atherosclerosis in Mice by Enhancing Scavenger Receptor Class B1 (SR-BI)-mediated High-density Lipoprotein (HDL)-Cholesteryl Ester Uptake

Studies in human populations have shown a significant correlation between procollagen C-endopeptidase enhancer protein 2 (PCPE2) single nucleotide polymorphisms and plasma HDL cholesterol concentrations. PCPE2, a 52-kDa glycoprotein located in the extracellular matrix, enhances the cleavage of C-terminal procollagen by bone morphogenetic protein 1 (BMP1). Our studies here focused on investigating the basis for the elevated concentration of enlarged plasma HDL in PCPE2-deficient mice to determine whether they protected against diet-induced atherosclerosis. PCPE2-deficient mice were crossed with LDL receptor-deficient mice to obtain LDLr^−/−, PCPE2^−/− mice, which had elevated HDL levels compared with LDLr^−/− mice with similar LDL concentrations. We found that LDLr^−/−, PCPE2^−/− mice had significantly more neutral lipid and CD68+ infiltration in the aortic root than LDLr^−/− mice. Surprisingly, in light of their elevated HDL levels, the extent of aortic lipid deposition in LDLr^−/−, PCPE2^−/− mice was similar to that reported for LDLr^−/−, apoA-I^−/− mice, which lack any apoA-I/HDL. Furthermore, LDLr^−/−, PCPE2^−/− mice had reduced HDL apoA-I fractional clearance and macrophage to fecal reverse cholesterol transport rates compared with LDLr^−/− mice, despite a 2-fold increase in liver SR-BI expression. PCPE2 was shown to enhance SR-BI function by increasing the rate of HDL-associated cholesteryl ester uptake, possibly by optimizing SR-BI localization and/or conformation. We conclude that PCPE2 is atheroprotective and an important component of the reverse cholesterol transport HDL system.     

Genetic deficiency of NADPH oxidase does not diminish,but rather enhances,LPS-induced acute inflammatory responses in vivo

Reactive oxygen species (ROS) and oxidative stress are thought to play a central role in the etiology of cell dysfunction and tissue damage in sepsis. However, there is limited and controversial evidence from in vivo studies that ROS mediate cell signaling processes that elicit acute inflammatory responses during sepsis. Because NADPH oxidase is one of the main cellular sources of ROS, we investigated the role of this enzyme in lipopolysaccharide (LPS)-induced acute inflammation in vivo, utilizing mice deficient in the gp91^phox or p47^phox subunits of NADPH oxidase. Age-and body weight-matched C57BL/6J wild-type (WT) and gp91^phox?/? and p47^phox?/? mice were injected ip with 50 μg LPS or saline vehicle and sacrificed at various time points up to 24 h. We found that LPS-induced acute inflammatory responses in serum and tissues were not significantly diminished in gp91^phox?/? and p47^phox?/? mice compared to WT mice. Rather, genetic deficiency of NADPH oxidase was associated with enhanced gene expression of inflammatory mediators and increased neutrophil recruitment to lung and heart. Furthermore, no protection from LPS-induced septic death was observed in either knockout strain. Our findings suggest that NADPH oxidase-mediated ROS production and cellular redox signaling do not promote, but instead limit, LPS-induced acute inflammatory responses in vivo.     

High density lipoprotein metabolism in low density lipoprotein receptor-deficient mice

The LDL receptor (LDLR) and scavenger receptor class B type I (SR-BI) play physiological roles in LDL and HDL metabolism in vivo. In this study, we explored HDL metabolism in LDLR-deficient mice in comparison with WT littermates. Murine HDL was radiolabeled in the protein (¹²⁵I) and in the cholesteryl ester (CE) moiety ([³H]). The metabolism of ¹²⁵I-/[³H]HDL was investigated in plasma and in tissues of mice and in murine hepatocytes. In WT mice, liver and adrenals selectively take up HDL-associated CE ([³H]). In contrast, in LDLR^−/− mice, selective HDL CE uptake is significantly reduced in liver and adrenals. In hepatocytes isolated from LDLR^−/− mice, selective HDL CE uptake is substantially diminished compared with WT liver cells. Hepatic and adrenal protein expression of lipoprotein receptors SR-BI, cluster of differentiation 36 (CD36), and LDL receptor-related protein 1 (LRP1) was analyzed by immunoblots. The respective protein levels were identical both in hepatic and adrenal membranes prepared from WT or from LDLR^−/− mice. In summary, an LDLR deficiency substantially decreases selective HDL CE uptake by liver and adrenals. This decrease is independent from regulation of receptor proteins like SR-BI, CD36, and LRP1. Thus, LDLR expression has a substantial impact on both HDL and LDL metabolism in mice.     

Rutaecarpine suppresses atherosclerosis in ApoE−/− mice through upregulating ABCA1 and SR-BI within RCT

ABCA1 and scavenger receptor class B type I (SR-BI)/CD36 and lysosomal integral membrane protein II analogous 1 (CLA-1) are the key transporter and receptor in reverse cholesterol transport (RCT). Increasing the expression level of ABCA1 and SR-BI/CLA-1 is antiatherogenic. The aim of the study was to find novel antiatherosclerotic agents upregulating expression of ABCA1 and SR-BI/CLA-1 from natural compounds. Using the ABCA1p-LUC and CLA-1p-LUC HepG2 cell lines, we found that rutaecarpine (RUT) triggered promoters of ABCA1 and CLA-1 genes. RUT increased ABCA1 and SR-BI/CLA-1 expression in vitro related to liver X receptor alpha and liver X receptor beta. RUT induced cholesterol efflux in RAW264.7 cells. ApoE-deficient (ApoE^−/−) mice treated with RUT for 8 weeks showed ∼68.43, 70.23, and 85.56% less en face lesions for RUT (L), RUT (M), and RUT (H) groups, respectively, compared with the model group. Mouse macrophage-specific antibody and filipin staining indicated that RUT attenuated macrophages and cholesterol accumulations in atherosclerotic lesions, respectively. Additionally, ABCA1 and SR-BI expression was highly induced by RUT in livers of ApoE^−/− mice. Meanwhile, RUT treatment significantly increased the fecal ³H-cholesterol excretion, which demonstrated that RUT could promote RCT in vivo. RUT was identified to be a candidate that protected ApoE^−/− mice from developing atherosclerosis through preferentially promoting activities of ABCA1 and SR-BI within RCT.     

Lipopolysaccharide Is Cleared from the Circulation by Hepatocytes via the Low Density Lipoprotein Receptor

Sepsis is the leading cause of death in critically ill patients. While decreased Proprotein Convertase Subtilisin/Kexin type 9 (PCSK9) function improves clinical outcomes in murine and human sepsis, the mechanisms involved have not been fully elucidated. We tested the hypothesis that lipopolysaccharide (LPS), the major Gram-negative bacteria endotoxin, is cleared from the circulation by hepatocyte Low Density Lipoprotein Receptors (LDLR)—receptors downregulated by PCSK9. We directly visualized LPS uptake and found that LPS is rapidly taken up by hepatocytes into the cell periphery. Over the course of 4 hours LPS is transported towards the cell center. We next found that clearance of injected LPS from the blood was reduced substantially in Ldlr knockout (Ldlr-/-) mice compared to wild type controls and, simultaneously, hepatic uptake of LPS was also reduced in Ldlr-/- mice. Specifically examining the role of hepatocytes, we further found that primary hepatocytes isolated from Ldlr-/- mice had greatly decreased LPS uptake. In the HepG2 immortalized human hepatocyte cell line, LDLR silencing similarly resulted in decreased LPS uptake. PCSK9 treatment reduces LDLR density on hepatocytes and, therefore, was another independent strategy to test our hypothesis. Incubation with PCSK9 reduced LPS uptake by hepatocytes. Taken together, these findings demonstrate that hepatocytes clear LPS from the circulation via the LDLR and PCSK9 regulates LPS clearance from the circulation during sepsis by downregulation of hepatic LDLR.