Localization and regulation of SR-BI in membrane rafts of HepG2 cells

HDLR-SR-BI Expression and Cholesterol Uptake are Regulated via Indoleamine-2,3-dioxygenase 1 in Macrophages under Inflammation

Macrophages play crucial roles in inflammation, and their dysfunction is a contributing factor to various human diseases. Maintaining the balance of cholesterol and lipid metabolism is central to macrophage function, and any disruption in this balance increases the risk of conditions such as cardiovascular disease, atherosclerosis, and others. HDLR-SR-BI (SR-BI) is pivotal for reverse cholesterol transport and cholesterol homeostasis. Our studies demonstrate that the expression of SR-BI is reduced along with a decrease in cholesterol uptake in macrophages, both of which are regulated by the activation of NF-κB. Furthermore, we have discovered that indoleamine-2,3-dioxygenase 1 (IDO1), which is a critical player in tryptophan (Trp) catabolism, is crucial to the regulation of SR-BI expression. Inflammation leads to elevated levels of IDO1 and the associated Trp catabolite kynurenine (KYN) in macrophages. Interestingly, knockdown or inhibition of IDO1 results in the downregulation of LPS-induced inflammation, decreased KYN levels, and the restoration of SR-BI expression as well as cholesterol uptake in macrophages. Beyond LPS, stimulation with pro-inflammatory cytokine IFNγ exhibits similar trends in inflammatory response, IDO1 regulation, and cholesterol uptake in macrophages. These observations suggest that IDO1 plays a critical role in SR-BI expression and cholesterol uptake in macrophages under inflammation.

The two major splice variants of scavenger receptor BI differ by their interactions with lipoproteins and cellular localization in endothelial cells

The scavenger receptor BI (SR-BI) facilitates the transport of both HDL and LDL through endothelial cells. Its two splice variants, SR-BIvar1 and SR-BIvar2, differ in their carboxy terminal domains. Only SR-BIvar1 contains the putative binding sites for the adapter proteins PDZ domain containing protein 1 (PDZK1) and dedicator of cytokinesis 4 (DOCK4), which limit the cell surface abundance and internalization of the receptor. To investigate the cellular localization of the SR-BI variants and their interaction with lipoproteins in endothelial cells, EA.hy926 cells were stably transfected with vectors encoding untagged, GFP- or mCherry-tagged constructs of the two SR-BI variants. Additionally, the cells were transfected with shRNAs against PDZK1 or DOCK4. Microscopy investigation showed that SR-BIvar1 was predominantly localized on the cell surface together with clathrin whereas SR-BIvar2 was absent from the cell surface but retrieved in endosomes and lysosomes. Accordingly, only SR-BIvar1 increased lipoprotein binding to endothelial while HDL and LDL uptake were enhanced by both variants. Silencing of PDZK1 or DOCK4 only reduced HDL association in SR-BIvar2 overexpressing cells while LDL association was reduced both in WT and SR-BIvar2 overexpressing cells. In conclusion, either SR-BI variant facilitates the uptake of HDL and LDL into endothelial cells, however by different mechanisms and trafficking routes. This dual role may explain why the loss of DOCK4 or PDZK1 differently affects the uptake of HDL and LDL in different endothelial cells.

Mitigating Vascular Inflammation by Mimicking AIBP Mechanisms: A New Therapeutic End for Atherosclerotic Cardiovascular Disease

Atherosclerosis, characterized by the accumulation of lipoproteins and lipids within the vascular wall, underlies a heart attack, stroke, and peripheral artery disease. Endothelial inflammation is the primary component driving atherosclerosis, promoting leukocyte adhesion molecule expression (e.g., E-selectin), inducing chemokine secretion, reducing the production of nitric oxide (NO), and enhancing the thrombogenic potential. While current therapies, such as statins, colchicine, anti-IL1β, and sodium-glucose cotransporter 2 (SGLT2) inhibitors, target systemic inflammation, none of them addresses endothelial cell (EC) inflammation, a critical contributor to disease progression. Targeting endothelial inflammation is clinically significant because it can mitigate the root cause of atherosclerosis, potentially preventing disease progression, while reducing the side effects associated with broader anti-inflammatory treatments. Recent studies highlight the potential of the APOA1 binding protein (AIBP) to reduce systemic inflammation in mice. Furthermore, its mechanism of action also guides the design of a potential targeted therapy against a particular inflammatory signaling pathway. This review discusses the unique advantages of repressing vascular inflammation or enhancing vascular quiescence and the associated benefits of reducing thrombosis. This approach offers a promising avenue for more effective and targeted interventions to improve patient outcomes.

LDL Transcytosis by the Arterial Endothelium-Atherosclerosis by a Thousand Cuts?

PURPOSE OF REVIEW

The accumulation of LDL in the arterial intima is an initiating event in atherosclerosis. After decades of controversy, it is now clear that transcytosis of LDL across an intact endothelial monolayer contributes to its intimal deposition. We review recent observations in this field and address the question of whether LDL transcytosis can be manipulated therapeutically.

RECENT FINDINGS

The development of a live-cell imaging method for studying transcytosis using total internal reflection fluorescence (TIRF) microscopy has catalyzed recent discoveries. LDL transcytosis is mediated by SR-BI and ALK1. Estrogen down-regulates SR-BI and inhibits LDL transcytosis, while the nuclear structural protein HMGB1 promotes LDL transcytosis. LDL transcytosis by ALK1 is independent of the receptor's kinase activity and is antagonized by BMP9, ALK1's canonical ligand. Inflammation stimulates LDL transcytosis. Identifying the function and mechanisms of LDL transcytosis may ultimately permit its therapeutic manipulation.

High-density lipoprotein cholesterol and carcinogenesis

High-density lipoprotein cholesterol (HDLC) has been recognized to be associated with atherosclerosis. In the past few years many studies have found that HDLC is also related to tumor development and progression. Despite some opposing views, a large number of studies support a negative association between HDLC and tumor incidence. Measuring serum HDLC concentrations may facilitate assessment of the prognosis of cancer patients and provide a biomarker for tumors. However, there is a lack of molecular mechanism studies on the link between HDLC and tumors. In this review we discuss the impact of HDLC on the incidence and prognosis of cancer in different systems, as well as prospects for the prediction and treatment of cancer in the future.

The role of cholesterol and mitochondrial bioenergetics in activation of the inflammasome in IBD

Inflammatory Bowel Disease (IBD) is characterized by a loss of intestinal barrier function caused by an aberrant interaction between the immune response and the gut microbiota. In IBD, imbalance in cholesterol homeostasis and mitochondrial bioenergetics have been identified as essential events for activating the inflammasome-mediated response. Mitochondrial alterations, such as reduced respiratory complex activities and reduced production of tricarboxylic acid (TCA) cycle intermediates (e.g., citric acid, fumarate, isocitric acid, malate, pyruvate, and succinate) have been described in in vitro and clinical studies. Under inflammatory conditions, mitochondrial architecture in intestinal epithelial cells is dysmorphic, with cristae destruction and high dynamin-related protein 1 (DRP1)-dependent fission. Likewise, these alterations in mitochondrial morphology and bioenergetics promote metabolic shifts towards glycolysis and down-regulation of antioxidant Nuclear erythroid 2-related factor 2 (Nrf2)/Peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) signaling. Although the mechanisms underlying the mitochondrial dysfunction during mucosal inflammation are not fully understood at present, metabolic intermediates and cholesterol may act as signals activating the NLRP3 inflammasome in IBD. Notably, dietary phytochemicals exhibit protective effects against cholesterol imbalance and mitochondrial function alterations to maintain gastrointestinal mucosal renewal in vitro and in vivo conditions. Here, we discuss the role of cholesterol and mitochondrial metabolism in IBD, highlighting the therapeutic potential of dietary phytochemicals, restoring intestinal metabolism and function.

Neurological aspects of SARS-CoV-2 infection: lipoproteins and exosomes as Trojan horses

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) primarily targets lipid-producing cells for viral tropism. In this review, we connect systemic lipid couriers, particularly high-density lipoproteins (HDLs) and exosomes, with the neurological facets of SARS-CoV-2 infection. We discuss how SARS-CoV-2 preferentially targets lipid-secreting cells and usurps host cell lipid metabolism for efficient replication and systemic spreading. Besides providing natural veils for viral materials against host immunity, the inherent properties of some of these endogenous lipid particles to traverse the blood-brain barrier (BBB) also offer alternative routes for SARS-CoV-2 neurotropism. Importantly, virus-driven neurological aberrations mediated by HDLs and exosomes are fueled by lipid rafts, which are implicated in the production and transmigration of these lipid particles across the BBB. Finally, we discuss how repurposing existing drugs targeting lipid rafts and cholesterol homeostasis may be beneficial toward alleviating the global coronavirus disease 2019 (COVID-19) disease burden.

Viral Hepatitis, Cholesterol Metabolism, and Cholesterol-Lowering Natural Compounds

Hepatitis is defined as inflammation of the liver; it can be acute or chronic. In chronic cases, the prolonged inflammation gradually damages the liver, resulting in liver fibrosis, cirrhosis, and sometimes liver failure or cancer. Hepatitis is often caused by viral infections. The most common causes of viral hepatitis are the five hepatitis viruses—hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV), and hepatitis E virus (HEV). While HAV and HEV rarely (or do not) cause chronic hepatitis, a considerable proportion of acute hepatitis cases caused by HBV (sometimes co-infected with HDV) and HCV infections become chronic. Thus, many medical researchers have focused on the treatment of HBV and HCV. It has been documented that host lipid metabolism, particularly cholesterol metabolism, is required for the hepatitis viral infection and life cycle. Thus, manipulating host cholesterol metabolism-related genes and proteins is a strategy used in fighting the viral infections. Efforts have been made to evaluate the efficacy of cholesterol-lowering drugs, particularly 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, in the treatment of hepatitis viral infections; promising results have been obtained. This review provides information on the relationships between hepatitis viruses and host cholesterol metabolism/homeostasis, as well as the discovery/development of cholesterol-lowering natural phytochemicals that could potentially be applied in the treatment of viral hepatitis.

Plasma Membrane Lipids: An Important Binding Site for All Lipoprotein Classes

Cholesterol is one of the main constituents of plasma membranes; thus, its supply is of utmost importance. This review covers the known mechanisms of cholesterol transfer from circulating lipoprotein particles to the plasma membrane, and vice versa. To achieve homeostasis, the human body utilizes cellular de novo synthesis and extracellular transport particles for supply of cholesterol and other lipids via the blood stream. These lipoprotein particles can be classified according to their density: chylomicrons, very low, low, and high-density lipoprotein (VLDL, LDL, and HDL, respectively). They deliver and receive their lipid loads, most importantly cholesterol, to and from cells by several redundant routes. Defects in one of these pathways (e.g., due to mutations in receptors) usually are not immediately fatal. Several redundant pathways, at least temporarily, compensate for the loss of one or more of them, but the defects trigger systemic diseases, such as atherosclerosis later on. Recently, intracellular membrane–membrane contact sites were shown to be involved in intracellular cholesterol transfer and the plasma membrane itself has been proposed to act as a binding site for lipoprotein-mediated cargo unloading.

Expanding the toolbox of exosome-based modulators of cell functions

Exosomes are cell-derived extracellular vesicles and play important roles in mediating intercellular communications. Due to their unique advantages in transporting a variety of biomolecules, exosomes have been emerging as a new class of nanocarriers with great potential for therapeutic applications. Despite advancements in loading chemotherapeutics and interfering RNAs into exosomes, active incorporation of protein molecules into exosomes remains challenging owing to their distinctive physicochemical properties and/or a lack of knowledge of cargo sorting during exosome biogenesis. Here we report the generation of a novel type of engineered exosomes with actively incorporated membrane proteins or soluble protein cargos, named genetically infused functionally tailored exosomes (GIFTed-Exos). Through genetic fusion with exosome-associated tetraspanin CD9, transmembrane protein CD70 and glucocorticoid-induced tumor necrosis factor receptor family-related ligand (GITRL) could be displayed on exosome surface, resulting in GIFTed-Exos with excellent T-cell co-stimulatory activities. By genetically linking to a CD9-photocleavable protein fusion, fluorescent protein mCherry, apoptosis-inducing protein apoptin, and antioxidant enzyme catalase could be effectively packed into exosomes for light-controlled release. The generated GIFTed-Exos display notable in vitro and in vivo activities for delivering distinct types of protein cargos to target cells. As a possibly general approach, GIFTed-Exos provide new opportunities to create exosomes with new functions and properties for biomedical research.

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

HDL nanoparticles have wound healing and anti-inflammatory properties and can topically deliver miRNAs

microRNAs regulate numerous biological processes, making them potential therapeutic agents. Problems with delivery and stability of these molecules have limited their usefulness as treatments. We demonstrate that synthetic high-density lipoprotein nanoparticles (HDL NPs) topically applied to the intact ocular surface are taken up by epithelial and stromal cells. microRNAs complexed to HDL NPs (miR-HDL NPs) are similarly taken up by cells and tissues and retain biological activity. Topical treatment of diabetic mice with either HDL NPs or miR-HDL NPs significantly improved corneal re-epithelialization following wounding compared with controls. Mouse corneas with alkali burn-induced inflammation, topically treated with HDL NPs, displayed clinical, morphological and immunological improvement. These results should yield a novel HDL NP-based eye drop for patients with compromised wound healing ability (diabetics) and/or corneal inflammatory diseases (e.g. dry eye).

The uptake mechanism and intracellular fate of Paraoxonase-1 in endothelial cells

Paraoxonase-1 (PON1) is a high-density lipoprotein (HDL)-associated lactonase that plays a significant role in the anti-atherosclerotic activity of HDL. However, several studies have shown that PON1 localizes in cells, where it operates independently of HDL. Previously, we showed that PON1 localizes in endothelial cells (ECs), and impairs vasodilation mediated by the endothelium-derived hyperpolarizing factor (EDHF) 5,6-δ-DHTL. However, the internalization pathway of PON1 into ECs, and the intracellular fate of PON1 are unknown. Therefore, the present study aimed to elucidate the uptake mechanism, intracellular trafficking and the function of PON1 in ECs. ‏We conducted a series of inhibition experiments of fluorescently labeled recombinant PON1 (rePON1) in ECs, followed by FACS analyses. We found that rePON1 binds the EC membrane via specific binding sites located in lipid-rafts/caveolae microdomains that are shared with HDL, and internalized through dynamin-dependent endocytosis. Qualitative assessments of the intracellular trafficking of rePON1, using confocal z-stack images, showed colocalization of the labeled rePON1 with early and late endosome/lysosome markers. Accordingly, a "pulse-chase" incubation of rePON1, followed by lactonase activity measurement in EC lysate, revealed that rePON1 retains its lactonase activity after binding to the cells. However, this activity decreases over time. Finally, induction of endothelial dysfunction with high glucose, angiotensin II, or palmitic acid increased rePON1 uptake by ECs. In conclusion, these results indicate that free PON1 interacts with ECs via binding sites located in lipid-rafts/caveolae, where it is enzymatically active and regulates endothelial functions. However, once internalized, PON1 is degraded. Additionally, alteration in endothelial function affects PON1 uptake by ECs.

Scavenger Receptor Class B Member 1 Independent Uptake of Transthyretin by Cultured Hepatocytes Is Regulated by High Density Lipoprotein

Thyroid hormone (thyroxine, T4) is essential for the normal function of all cell types and is carried in serum bound to several proteins including transthyretin. Recently, evidence has emerged of alternate pathways for hormone entry into cells that are dependent on hormone binding proteins. Transthyretin and transthyretin bound T4 are endocytosed by placental trophoblasts through the high-density lipoprotein receptor, Scavenger Receptor Class B Type 1 (SR-B1). High density lipoprotein (HDL) affects the expression and function of SR-B1 in trophoblast cells. SR-B1 is also expressed in hepatocytes and we sought to determine if hepatocyte SR-B1 was involved in transthyretin or transthyretin-T4 uptake and whether uptake was affected by HDL. Transthyretin and transthyretin-T4 uptake by hepatocytes is not dependent on SR-B1. HDL treatment reduced SR-B1 expression. However, pretreatment of hepatocytes with HDL increased uptake of transthyretin-T4. Knockdown of SR-B1 expression using siRNA also increased transthyretin-T4 uptake. Coaddition of HDL to transthyretin uptake experiments blocked both transthyretin and transthyretin-T4 uptake. Hepatocyte uptake of transthyretin-T4 uptake is influenced by, but is not dependent on, SR-B1 expression. HDL also decreases transthyretin-T4 uptake and therefore diet or drugs may interfere with this process. This suggests that multiple lipoprotein receptors may be involved in the regulation of uptake of transthyretin-T4 in a cell-type specific manner. Further study is required to understand this important process.

Citrus Flavanones Enhance β-Carotene Uptake in Vitro Experiment Using Caco-2 Cell: Structure-Activity Relationship and Molecular Mechanisms

Flavonoids can interfere with the absorption of carotenoids. In this study, the inherent mechanisms of 12 citrus flavanones for β-carotene (Bc) cellular uptake and the structure-activity relationship were investigated. The results showed that multiple hydroxyl groups had the lowest promoting effect. O-Glycosylation at C7 of the A ring led to the greatest promoting effect on Bc absorption. O-Glycosylation at C7 exhibited a strong affinity with the cell membrane and subsequently fluidized the cell membrane. Aglycon molecules significantly induced transient increases of paracellular permeability by decreasing tight junction proteins (ZO-1, claudin-1) expression. In addition, citrus flavanones might enhance scavenger receptor class B type I (SR-BI) expression via their actions as agonists of peroxisome proliferator-activated receptor-gamma (PPARγ). Catechol structure in the B-ring attenuated the activate action of SR-BI expression. The structure-dependent membrane permeability and activation of specific membrane proteins are mechanistically associated with the promoting effect on Bc cellular uptake by citrus flavanones.

Reconstituted HDL: Drug Delivery Platform for Overcoming Biological Barriers to Cancer Therapy

Drug delivery to malignant tumors is limited by several factors, including off-target toxicities and suboptimal benefits to cancer patient. Major research efforts have been directed toward developing novel technologies involving nanoparticles (NPs) to overcome these challenges. Major obstacles, however, including, opsonization, transport across cancer cell membranes, multidrug-resistant proteins, and endosomal sequestration of the therapeutic agent continue to limit the efficiency of cancer chemotherapy. Lipoprotein-based drug delivery technology, "nature's drug delivery system," while exhibits highly desirable characteristics, it still needs substantial investment from private/government stakeholders to promote its eventual advance to the bedside. Consequently, this review focuses specifically on the synthetic (reconstituted) high-density lipoprotein rHDL NPs, evaluating their potential to overcome specific biological barriers and the challenges of translation toward clinical utilization and commercialization. This highly robust drug transport system provides site-specific, tumor-selective delivery of anti-cancer agents while reducing harmful off-target effects. Utilizing rHDL NPs for anti-cancer therapeutics and tumor imaging revolutionizes the future strategy for the management of a broad range of cancers and other diseases.

Correlation of CD81 and SCARB1 polymorphisms on virological responses in Iranian patients with chronic hepatitis C virus genotype 1

The cluster of differentiation 81 (CD81) and scavenger receptor class B member 1 (SCARB1) plays an important role in the entry of hepatitis C virus (HCV). We assessed the correlation of five single nucleotide polymorphisms (SNPs) of CD81 (rs800136, rs2651842, rs2522012, rs800146, and rs708564) and SCARB1 rs10846744 polymorphisms with treatment responses in 395 treatment-naïve patients with chronic HCV (CHC) genotype 1 treated with pegylated interferon-α and ribavirin (pegIFN-α/RBV). The frequency of rapid virologic response (RVR), complete early virologic response (cEVR) and sustained virologic response (SVR) were 57.2%, 55.2%, and 58.2%, respectively. RVR, cEVR, and SVR were significantly associated with CD81 rs800136 (CC), CD81 rs2651842 (AA), CD81 rs708564 (TT), and SCARB1 rs10846744 (CC). High rates of RVR, cEVR, and SVR were reported for the CD81 rs800136 (CC), CD81 rs2651842 (AA), and CD81 rs708564 (TT) genotypes when correlated with higher levels of low-density lipoprotein (LDL) and lower levels of high-density lipoprotein (HDL) as well as lower levels of HDL and LDL in the SCARB1 rs10846744 (CC) genotype. In addition, patients with GG genotype had higher fasting blood glucose (FBS) level than those with CC genotype. In conclusion, CD81 and SCARB1 SNPs may serve as powerful predictor factors for treatment responses in CHC patients, and this effect is correlated with serum lipoprotein and FBS levels.

High scavenger receptor class B type I expression is related to tumor aggressiveness and poor prognosis in lung adenocarcinoma: A STROBE compliant article

Scavenger receptor class B type I (SR-B1) is highly expressed in a variety of cancers, including prostate, breast and ovarian. However, the relationship between SR-B1 and lung adenocarcinoma is unknown. We analyzed the expression of SR-B1 in a well-characterized lung adenocarcinoma tissue microarray by immunohistochemistry, in 90 cancerous and 90 adjacent normal lung tissues. Results showed that the positive expression rate of SR-B1 in cancer tissues (86/90, 96%) was significantly higher than that of adjacent tissues (50/90, 56%) (P < .001). And SR-B1 overexpression in lung adenocarcinoma tissue was significantly higher than that of adjacent normal tissue (P < .001), accounting for 67% of cases. This elevated SR-B1 expression was associated with AJCC stage (P < .001), T stage (P = .012), N stage (P = .002), and lymph node positivity (P < .001). The Kaplan-Meier survival analysis indicated that patients with high SR-B1 expression had a shorter overall survival (P < .001). On the multivariate analysis, SR-B1 was an independent prognostic factor for outcomes after adjustment for other prognostic factors (P = .038). In conclusion, high SR-B1 expression is associated with conventional pathologic parameters that represent tumor aggressiveness and may purport a poor clinical prognosis in lung adenocarcinoma.

High density lipoproteins selectively promote the survival of human regulatory T cells

HDLs appear to affect regulatory T cell (Treg) homeostasis, as suggested by the increased Treg counts in HDL-treated mice and by the positive correlation between Treg frequency and HDL-cholesterol levels in statin-treated healthy adults. However, the underlying mechanisms remain unclear. Herein, we show that HDLs, not LDLs, significantly decreased the apoptosis of human Tregs in vitro, whereas they did not alter naïve or memory CD4⁺ T cell survival. Similarly, oleic acid bound to serum albumin increased Treg survival. Tregs bound and internalized high amounts of HDL compared with other subsets, which might arise from the higher expression of the scavenger receptor class B type I by Tregs; accordingly, blocking this receptor hindered HDL-mediated Treg survival. Mechanistically, we showed that HDL increased Treg ATP concentration and mitochondrial activity, enhancing basal respiration, maximal respiration, and spare respiratory capacity. Blockade of FA oxidation by etoxomir abolished the HDL-mediated enhanced survival and mitochondrial activity. Our findings thus suggest that Tregs can specifically internalize HDLs from their microenvironment and use them as an energy source. Furthermore, a novel implication of our data is that enhanced Treg survival may contribute to HDLs' anti-inflammatory properties.

SR-BI: Linking Cholesterol and Lipoprotein Metabolism with Breast and Prostate Cancer

Studies have demonstrated the significant role of cholesterol and lipoprotein metabolism in the progression of cancer. The SCARB1 gene encodes the scavenger receptor class B type I (SR-BI), which is an 82-kDa glycoprotein with two transmembrane domains separated by a large extracellular loop. SR-BI plays an important role in the regulation of cholesterol exchange between cells and high-density lipoproteins. Accordingly, hepatic SR-BI has been shown to play an essential role in the regulation of the reverse cholesterol transport pathway, which promotes the removal and excretion of excess body cholesterol. In the context of atherosclerosis, SR-BI has been implicated in the regulation of intracellular signaling, lipid accumulation, foam cell formation, and cellular apoptosis. Furthermore, since lipid metabolism is a relevant target for cancer treatment, recent studies have focused on examining the role of SR-BI in this pathology. While signaling pathways have initially been explored in non-tumoral cells, studies with cancer cells have now demonstrated SR-BI's function in tumor progression. In this review, we will discuss the role of SR-BI during tumor development and malignant progression. In addition, we will provide insights into the transcriptional and post-transcriptional regulation of the SCARB1 gene. Overall, studying the role of SR-BI in tumor development and progression should allow us to gain useful information for the development of new therapeutic strategies.

AIBP: A Novel Molecule at the Interface of Cholesterol Transport, Angiogenesis, and Atherosclerosis

Cardiovascular disease, which is often driven by hypercholesterolemia and subsequent coronary atherosclerosis, is the number-one cause of morbidity and mortality in the United States. Based on long-term epidemiological studies, high-density lipoprotein cholesterol (HDL-C) levels are inversely correlated with risk for coronary artery disease (CAD). HDL-mediated reverse cholesterol transport (RCT) is responsible for cholesterol removal from the peripheral tissues and return to the liver for final elimination.1 In atherosclerosis, intraplaque angiogenesis promotes plaque growth and increases plaque vulnerability. Conceivably, the acceleration of RCT and disruption of intraplaque angiogenesis would inhibit atherosclerosis and reduce CAD. We have identified a protein called apoA-I binding protein (AIBP) that augments HDL functionality by accelerating cholesterol efflux. Furthermore, AIBP inhibits vascular endothelial growth factor receptor 2 activation in endothelial cells and limits angiogenesis.2 The following discusses the prospect of using AIBP as a novel therapeutic approach for the treatment of CAD.

CD81 and CLDN1 polymorphisms and hepatitis C virus infection susceptibility: a case control study

CD81 and CLDN1 interact to form a CD81-CLDN1 co-receptor complex that is crucial in hepatitis C virus (HCV) entry. Variations in the two genes were shown to influence immunological functions; therefore, we hypothesized that polymorphisms in these genes may contribute to HCV susceptibility. A case-control study consisting of 461 patients and 461 controls was conducted to explore the associations between CD81 rs708564 and CLDN1 rs893051 and HCV susceptibility in a Chinese population. We found a decreased HCV risk associated with the CD81 rs708564 TT (odds ratio (OR) = 0.66, 95% CI = 0.44-0.98) genotype. The gene-gene interaction between CD81 and CLDN1 polymorphisms also decreased HCV risk in a joint multiplicative manner (OR for the presence of both CD81 rs708564 TT and CLDN1 rs893051 GG genotypes = 0.59, 95% CI = 0.36-0.97). Furthermore, the CD81 rs708564 TT genotype conferred a more pronounced decrease in HCV susceptibility in combination with lower levels of high-density lipoprotein cholesterol (HDL-C; OR = 0.71, 95% CI = 0.52-0.96), and higher levels of low-density lipoprotein cholesterol (OR = 0.24, 95% CI = 0.09-0.65). We also observed a decreased HCV susceptibility in individuals with higher HDL-C levels who carried the CLDN1 rs893051 G/C genotype. These findings suggest that homozygous CD81 rs708564 TT may be a genetic modifier for avoiding HCV infection whether as a sole single nucleotide polymorphism or combined with the CLDN1 rs893051 GG genotype, and this effect is associated with serum levels of lipoprotein.

Liver growth factor induces testicular regeneration in EDS-treated rats and increases protein levels of class B scavenger receptors

The aim of the present work was to determine the effects of liver growth factor (LGF) on the regeneration process of rat testes after chemical castration induced by ethane dimethanesulfonate (EDS) by analyzing some of the most relevant proteins involved in cholesterol metabolism, such as hormone sensitive lipase (HSL), 3β-hydroxysteroid dehydrogenase (3β-HSD), scavenger receptor SR-BI, and other components of the SR family that could contribute to the recovery of steroidogenesis and spermatogenesis in the testis. Sixty male rats were randomized to nontreated (controls) and LGF-treated, EDS-treated, and EDS + LGF-treated groups. Testes were obtained on days 10 (T1), 21 (T2), and 35 (T3) after EDS treatment, embedded in paraffin, and analyzed by immunohistochemistry and Western blot. LGF improved the recovery of the seminiferous epithelia, the appearance of the mature pattern of Leydig cell interstitial distribution, and the expression of mature SR-BI. Moreover, LGF treatment resulted in partial recovery of HSL expression in Leydig cells and spermatogonia. No changes in serum testosterone were observed in control or LGF-treated rats, but in EDS-castrated animals LGF treatment induced a progressive increase in serum testosterone levels and 3β-HSD expression. Based on the pivotal role of SR-BI in the uptake of cholesteryl esters from HDL, it is suggested that the observed effects of LGF would facilitate the provision of cholesterol for sperm cell growth and Leydig cell recovery.

Impact of lipids and lipoproteins on hepatitis C virus infection and virus neutralization

Hepatitis C virus (HCV) infections represent a major global health problem. End-stage liver disease caused by chronic HCV infection is a major indication for liver transplantation. However, after transplantation the engrafted liver inevitably becomes infected by the circulating virus. Direct acting antivirals are not yet approved for use in liver transplant patients, and limited efficacy and severe side effects hamper the use of pegylated interferon combined with ribavirin in a post-transplant setting. Therefore, alternative therapeutic options need to be explored. Viral entry represents an attractive target for such therapeutic intervention. Understanding the mechanisms of viral entry is essential to define the viral and cellular factors involved. The HCV life cycle is dependent of and associated with lipoprotein physiology and the presence of lipoproteins has been correlated with altered antiviral efficacy of entry inhibitors. In this review, we summarise the current knowledge on how lipoprotein physiology influences the HCV life cycle. We focus especially on the influence of lipoproteins on antibodies that target HCV envelope proteins or antibodies that target the cellular receptors of the virus. This information can be particularly relevant for the prevention of HCV re-infection after liver transplantation.

Enzymatic oxidation of cholesterol: properties and functional effects of cholestenone in cell membranes

Bacterial cholesterol oxidase is commonly used as an experimental tool to reduce cellular cholesterol content. That the treatment also generates the poorly degradable metabolite 4-cholesten-3-one (cholestenone) has received less attention. Here, we investigated the membrane partitioning of cholestenone using simulations and cell biological experiments and assessed the functional effects of cholestenone in human cells. Atomistic simulations predicted that cholestenone reduces membrane order, undergoes faster flip-flop and desorbs more readily from membranes than cholesterol. In primary human fibroblasts, cholestenone was released from membranes to physiological extracellular acceptors more avidly than cholesterol, but without acceptors it remained in cells over a day. To address the functional effects of cholestenone, we studied fibroblast migration during wound healing. When cells were either cholesterol oxidase treated or part of cellular cholesterol was exchanged for cholestenone with cyclodextrin, cell migration during 22 h was markedly inhibited. Instead, when a similar fraction of cholesterol was removed using cyclodextrin, cells replenished their cholesterol content in 3 h and migrated similarly to control cells. Thus, cholesterol oxidation produces long-term functional effects in cells and these are in part due to the generated membrane active cholestenone.

HDL endocytosis and resecretion

HDL removes excess cholesterol from peripheral tissues and delivers it to the liver and steroidogenic tissues via selective lipid uptake without catabolism of the HDL particle itself. In addition, endocytosis of HDL holo-particles has been debated for nearly 40years. However, neither the connection between HDL endocytosis and selective lipid uptake, nor the physiological relevance of HDL uptake has been delineated clearly. This review will focus on HDL endocytosis and resecretion and its relation to cholesterol transfer. We will discuss the role of HDL endocytosis in maintaining cholesterol homeostasis in tissues and cell types involved in atherosclerosis, focusing on liver, macrophages and endothelium. We will critically summarize the current knowledge on the receptors mediating HDL endocytosis including SR-BI, F1-ATPase and CD36 and on intracellular HDL transport routes. Dependent on the tissue, HDL is either resecreted (retro-endocytosis) or degraded after endocytosis. Finally, findings on HDL transcytosis across the endothelial barrier will be summarized. We suggest that HDL endocytosis and resecretion is a rather redundant pathway under physiologic conditions. In case of disturbed lipid metabolism, however, HDL retro-endocytosis represents an alternative pathway that enables tissues to maintain cellular cholesterol homeostasis.

Scavenger receptors in homeostasis and immunity

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

Exosome-mediated transmission of hepatitis C virus between human hepatoma Huh7.5 cells

Recent evidence indicates there is a role for small membrane vesicles, including exosomes, as vehicles for intercellular communication. Exosomes secreted by most cell types can mediate transfer of proteins, mRNAs, and microRNAs, but their role in the transmission of infectious agents is less established. Recent studies have shown that hepatocyte-derived exosomes containing hepatitis C virus (HCV) RNA can activate innate immune cells, but the role of exosomes in the transmission of HCV between hepatocytes remains unknown. In this study, we investigated whether exosomes transfer HCV in the presence of neutralizing antibodies. Purified exosomes isolated from HCV-infected human hepatoma Huh7.5.1 cells were shown to contain full-length viral RNA, viral protein, and particles, as determined by RT-PCR, mass spectrometry, and transmission electron microscopy. Exosomes from HCV-infected cells were capable of transmitting infection to naive human hepatoma Huh7.5.1 cells and establishing a productive infection. Even with subgenomic replicons, lacking structural viral proteins, exosome-mediated transmission of HCV RNA was observed. Treatment with patient-derived IgGs showed a variable degree of neutralization of exosome-mediated infection compared with free virus. In conclusion, this study showed that hepatic exosomes can transmit productive HCV infection in vitro and are partially resistant to antibody neutralization. This discovery sheds light on neutralizing antibodies resistant to HCV transmission by exosomes as a potential immune evasion mechanism.

Imaging the cytosolic drug delivery mechanism of HDL-like nanoparticles

PURPOSE

Molecular therapeutics often require an effective nanoparticle-based delivery strategy to transport them to cytosolic organelles to be functional. Recently, a cytosolic delivery strategy based on the scavenger receptor class B type I (SR-BI) mediated pathway has shown great potential for the effective delivery of theranostics agents into the cytoplasm of cells without detrimental endosomal entrapment. This study elucidates this unique delivery mechanism for improving cytosolic drug delivery.

METHODS

Multifluorophore-labeled HDL-mimicking peptide phospholipid scaffold (HPPS) nanoparticles were developed. Fluorescence imaging was utilized to examine HPPS transporting payloads into cells step by step through sequential inhibition studies.

RESULTS

HPPS specifically recognizes and binds to SR-BI, then interacts with SR-BI, which results in direct transport of payload molecules into the cell cytoplasm without entire particles internalization. The cytosolic transport of payloads occurred through a temperature- and energy-independent pathway, and was also different from actin- and clathrin-mediated endocytosis. Furthermore, this transport was significantly inhibited by disruption of lipid rafts using filipin or methyl-β-cyclodextrin.

CONCLUSIONS

The cytosolic delivery of payloads by HPPS via SR-BI targeting is predominately mediated through a lipid rafts/caveolae-like pathway. This cytosolic delivery strategy can be utilized for transporting molecular therapeutics that require their action sites to be within cytosolic organelles to enhance therapeutic effect.

Sphingomyelin in high-density lipoproteins: structural role and biological function

High-density lipoprotein (HDL) levels are an inverse risk factor for cardiovascular diseases, and sphingomyelin (SM) is the second most abundant phospholipid component and the major sphingolipid in HDL. Considering the marked presence of SM, the present review has focused on the current knowledge about this phospholipid by addressing its variable distribution among HDL lipoparticles, how they acquire this phospholipid, and the important role that SM plays in regulating their fluidity and cholesterol efflux from different cells. In addition, plasma enzymes involved in HDL metabolism such as lecithin-cholesterol acyltransferase or phospholipid transfer protein are inhibited by HDL SM content. Likewise, HDL SM levels are influenced by dietary maneuvers (source of protein or fat), drugs (statins or diuretics) and modified in diseases such as diabetes, renal failure or Niemann-Pick disease. Furthermore, increased levels of HDL SM have been shown to be an inverse risk factor for coronary heart disease. The complexity of SM species, described using new lipidomic methodologies, and their distribution in different HDL particles under many experimental conditions are promising avenues for further research in the future.

Myeloid cell-specific serine palmitoyltransferase subunit 2 haploinsufficiency reduces murine atherosclerosis

Serine palmitoyltransferase (SPT) is the first and rate-limiting enzyme of the de novo biosynthetic pathway of sphingomyelin (SM). Both SPT and SM have been implicated in the pathogenesis of atherosclerosis, the development of which is driven by macrophages; however, the role of SPT in macrophage-mediated atherogenesis is unknown. To address this issue, we have analyzed macrophage inflammatory responses and reverse cholesterol transport, 2 key mediators of atherogenesis, in SPT subunit 2-haploinsufficient (Sptlc2(+/-)) macrophages. We found that Sptlc2(+/-) macrophages have significantly lower SM levels in plasma membrane and lipid rafts. This reduction not only impaired inflammatory responses triggered by TLR4 and its downstream NF-κB and MAPK pathways, but also enhanced reverse cholesterol transport mediated by ABC transporters. LDL receptor-deficient (Ldlr(-/-)) mice transplanted with Sptlc2(+/-) bone marrow cells exhibited significantly fewer atherosclerotic lesions after high-fat and high-cholesterol diet feeding. Additionally, Ldlr(-/-) mice with myeloid cell-specific Sptlc2 haploinsufficiency exhibited significantly less atherosclerosis than controls. These findings suggest that SPT could be a novel therapeutic target in atherosclerosis.

Biomimetic, synthetic HDL nanostructures for lymphoma

New therapies that challenge existing paradigms are needed for the treatment of cancer. We report a nanoparticle-enabled therapeutic approach to B-cell lymphoma using synthetic high density lipoprotein nanoparticles (HDL-NPs). HDL-NPs are synthesized using a gold nanoparticle template to control conjugate size and ensure a spherical shape. Like natural HDLs, biomimetic HDL-NPs target scavenger receptor type B-1, a high-affinity HDL receptor expressed by lymphoma cells. Functionally, compared with natural HDL, the gold NP template enables differential manipulation of cellular cholesterol flux in lymphoma cells, promoting cellular cholesterol efflux and limiting cholesterol delivery. This combination of scavenger receptor type B-1 binding and relative cholesterol starvation selectively induces apoptosis. HDL-NP treatment of mice bearing B-cell lymphoma xenografts selectively inhibits B-cell lymphoma growth. As such, HDL-NPs are biofunctional therapeutic agents, whose mechanism of action is enabled by the presence of a synthetic nanotemplate. HDL-NPs are active in B-cell lymphomas and potentially, other malignancies or diseases of pathologic cholesterol accumulation.

Sphingolipid metabolism and atherosclerosis

Atherosclerosis is the major cause of mortality in the developed countries. Although presently known risk factors have some predictive value for the disease, a major part of the variability in this process remains unexplained. It is extremely important to find new approaches for better understanding of the disease and for treating it. Exploration of the sphingolipid metabolism is one of these approaches. Sphingolipids are a large class of lipids with structural and signaling functions. Recent researches indicated that these lipids play important roles in the development of atherosclerosis. In this chapter, we summarized the major findings in the field.

Soluble FLT1 binds lipid microdomains in podocytes to control cell morphology and glomerular barrier function

Vascular endothelial growth factor and its receptors, FLK1/KDR and FLT1, are key regulators of angiogenesis. Unlike FLK1/KDR, the role of FLT1 has remained elusive. FLT1 is produced as soluble (sFLT1) and full-length isoforms. Here, we show that pericytes from multiple tissues produce sFLT1. To define the biologic role of sFLT1, we chose the glomerular microvasculature as a model system. Deletion of Flt1 from specialized glomerular pericytes, known as podocytes, causes reorganization of their cytoskeleton with massive proteinuria and kidney failure, characteristic features of nephrotic syndrome in humans. The kinase-deficient allele of Flt1 rescues this phenotype, demonstrating dispensability of the full-length isoform. Using cell imaging, proteomics, and lipidomics, we show that sFLT1 binds to the glycosphingolipid GM3 in lipid rafts on the surface of podocytes, promoting adhesion and rapid actin reorganization. sFLT1 also regulates pericyte function in vessels outside of the kidney. Our findings demonstrate an autocrine function for sFLT1 to control pericyte behavior.

Efficient systemic delivery of siRNA by using high-density lipoprotein-mimicking peptide lipid nanoparticles

The main challenge for RNAi therapeutics lies in systemic delivery of siRNA to the correct tissues and transporting them into the cytoplasm of targeted cells, at safe, therapeutic levels. Recently, we developed a high-density lipoprotein-mimicking peptide-phospholipid scaffold (HPPS) and demonstrated its direct cytosolic delivery of siRNA in vitro, thereby bypassing endosomal trapping.

AIM

We investigate the in vivo implementation of HPPS for siRNA delivery.

METHOD & RESULTS

After systemic administration in KB tumor-bearing mice, HPPS prolonged the blood circulation time of cholesterol-modified siRNA (chol-siRNA) by a factor of four, improved its biodistribution and facilitated its uptake in scavenger receptor class B type I overexpressed tumors. For therapeutic targeting to the bcl-2 gene, the HPPS-chol-si-bcl-2 nanoparticles downregulated Bcl-2 protein, induced enhanced apoptosis (2.5-fold) in tumors when compared with controls (saline, HPPS, HPPS-chol-si-scramble and chol-si-bcl-2) and significantly inhibited tumor growth with no adverse effect.

CONCLUSION

HPPS is a safe, efficient nanocarrier for RNAi therapeutics in vivo.

Recombinant high-density lipoprotein nanoparticles containing gadolinium-labeled cholesterol for morphologic and functional magnetic resonance imaging of the liver

Background

Natural high-density lipoproteins (HDL) possess important physiological functions to the transport of cholesterol from the peripheral tissues to the liver for metabolic degradation and excretion in the bile.

Methods and results

In this work, we took advantage of this pathway and prepared two different gadolinium (Gd)-DTPA-labeled cholesterol-containing recombinant HDL nanoparticles (Gd-chol-HDL) and Gd-(chol)₂-HDL as liver-specific magnetic resonance imaging (MRI) contrast agents. The reconstituted HDL nanoparticles had structural similarity to native HDL, and could be taken up by HepG2 cells via interaction with HDL receptors in vitro. In vivo MRI studies in rats after intravenous injections of 10 μmol gadolinium per kg of recombinant HDL nanoparticles indicated that both nanoparticles could provide signal enhancement in the liver and related organs. However, different T₁-weighted image details suggested that they participated in different cholesterol metabolism and excretion pathways in the liver.

Conclusion

Such information could be highly useful to differentiate functional changes as well as anatomic differences in the liver. These cholesterol-derived contrast agents and their recombinant HDL preparations may warrant further development as a new class of contrast agents for MRI of the liver and related organs.

Knockdown of scavenger receptor class B type I reduces prostate specific antigen secretion and viability of prostate cancer cells

BACKGROUND

Scavenger Receptor Class B Type I (SR-BI) facilitates influx of cholesterol to the cell from lipoproteins in the circulation. This influx of cholesterol may be important for many cellular functions, including synthesis of androgens. Castration-resistant prostate cancer tumors are able to synthesize androgens de novo in order to supplement the loss of exogenous sources often induced by androgen deprivation therapy. Silencing of SR-BI may impact the ability of prostate cancer cells, particularly those of castration-resistant state, to maintain the intracellular supply of androgens by removing a supply of cholesterol.

METHODS

SR-BI expression was knocked down using small interfering RNA in LNCaP and C4-2 cells. The effect of down-regulation of SR-BI on PSA production, cell toxicity, and cell viability was measured in both cell types. In addition, compensatory cholesterol synthesis activity was measured using the radiolabeled precursor, (14) C-acetate.

RESULTS

SR-BI protein expression is higher basally in C4-2 cells than LNCaP cells. Silencing of SR-BI expression to greater than 85% reduced PSA production in LNCaP and C4-2 SRBI-KD cells by 55% and 58% compared to negative control cells, respectively. SR-BI-KD C4-2 cells demonstrated significantly reduced cell viability (>25%) compared the NC cells.

CONCLUSIONS

The down-regulation of SR-BI significantly impacts PSA production of prostate cancer cells, as well as the viability of C4-2 cells in the presence and absence of HDL. This may indicate a deficiency in cholesterol availability to the androgen synthesis pathway or may implicate a role for SR-BI in prostate cancer signal transduction pathways.

Chlamydia pneumoniae entry into epithelial cells by clathrin-independent endocytosis

A gram-negative obligate intracellular bacterium, Chlamydia pneumoniae, is a common respiratory pathogen. Here, we examined the invasion and attachment of C. pneumoniae K6 into nonphagocytic HL epithelial cell line by manipulating host plasma membranes by using cholesterol-depleting methyl-beta-cyclodextrin (MβCD) and cholesterol-loading MβCD complexed cholesterol (chol-MβCD). The invasion was attenuated by MβCD-treatment while chol-MβCD augmented the attachment and invasion. In addition, the invasion was inhibited by cholesterol sequestering reagents, nystatin and filipin. Furthermore, exposure of host cells to sphingomyelinase inhibited the invasion. RNA interference was used to assay the role of clathrin and human scavenger receptor B, type I (SR-BI) in the entry of C. pneumoniae into A549 lung epithelial adenocarcinoma cells. In contrast to Chlamydia trachomatis L2, the entry of C. pneumoniae was found to be independent of clathrin. In addition, the entry was found to be SR-BI-independent, but interestingly, the chlamydial growth was attenuated in the SR-BI-silenced cells. These findings suggest that the attachment and invasion of C. pneumoniae into nonphagocytic epithelial cells is dependent on the formation of cholesterol- and sphingomyelin-rich plasma membrane microdomains, and the entry is a clathrin-independent process. In addition, our data indicate that SR-BI supports the growth of C. pneumoniae in epithelial cells.

Scavenger receptor class B type I and the hypervariable region-1 of hepatitis C virus in cell entry and neutralisation

Hepatitis C virus (HCV) infection is a leading cause of chronic liver disease worldwide and represents a major public health problem. Viral attachment and entry - the first encounter of the virus with the host cell - are major targets of neutralising immune responses. Thus, a detailed understanding of the HCV entry process offers interesting opportunities for the development of novel therapeutic strategies. Different cellular or soluble host factors mediate HCV entry, and considerable progress has been made in recent years to decipher how they induce HCV attachment, internalisation and membrane fusion. Among these factors, the scavenger receptor class B type I (SR-BI/SCARB1) is essential for HCV replication in vitro, through its interaction with the HCV E1E2 surface glycoproteins and, more particularly, the HVR1 segment located in the E2 protein. SR-BI is an interesting receptor because HCV, whose replication cycle intersects with lipoprotein metabolism, seems to exploit some aspects of its physiological functions, such as cholesterol transfer from high-density lipoprotein (HDL), during cell entry. SR-BI is also involved in neutralisation attenuation and therefore could be an important target for therapeutic intervention. Recent results suggest that it should be possible to identify inhibitors of the interaction of HCV with SR-BI that do not impair its important physiological properties, as discussed in this review.

Adenovirus-mediated sphingomyelin synthase 2 increases atherosclerotic lesions in ApoE KO mice

Background

Sphingomyelin synthase 2 (SMS2) contributes to de novo sphingomyelin (SM) biosynthesis. Its activity is related to SM levels in the plasma and the cell membrane. In this study, we investigated the possibility of a direct relationship between SMS and atherosclerosis.

Methods

The Adenovirus containing SMS2 gene was given into 10-week ApoE KO C57BL/6J mice by femoral intravenous injection. In the control group, the Adenovirus containing GFP was given. To confirm this model, we took both mRNA level examination (RT-PCR) and protein level examination (SMS activity assay).

Result

We generated recombinant adenovirus vectors containing either human SMS2 cDNA (AdV-SMS2) or GFP cDNA (AdV-GFP). On day six after intravenous infusion of 2 × 10¹¹ particle numbers into ten-week-old apoE KO mice, AdV-SMS2 treatment significantly increased liver SMS2 mRNA levels and SMS activity (by 2.7-fold, 2.3-fold, p < 0.001, respectively), compared to AdV-GFP treated mice. Moreover, plasma total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triglyceride (TG), and sphingomyelin (SM) levels were significantly increased by 39% (p < 0.05), 42% (p < 0.05), 68% (p < 0.001), and 45% (p < 0.05), respectively. Plasma high-density lipoprotein cholesterol (HDL-C), phosphatidylcholine (PC), and PC/SM ratio were decreased by 42% (p < 0.05), 18% (p < 0.05), and 45% (p < 0.05), respectively. On day 30, the atherosclerotic lesions on the aortic arch of AdV-SMS2 treated mice were increased, and the lesion areas on the whole aorta and in the aortic root were significantly increased (p < 0.001). Furthermore, the collagen content in the aorta root was significantly decreased (p < 0.01).

Conclusions

Our results present direct morphological evidence for the pro-atherogenic capabilities of SMS2. SMS2 could be a potential target for treating atherosclerosis.

Cellular cholesterol delivery, intracellular processing and utilization for biosynthesis of steroid hormones

Steroid hormones regulate diverse physiological functions such as reproduction, blood salt balance, maintenance of secondary sexual characteristics, response to stress, neuronal function and various metabolic processes. They are synthesized from cholesterol mainly in the adrenal gland and gonads in response to tissue-specific tropic hormones. These steroidogenic tissues are unique in that they require cholesterol not only for membrane biogenesis, maintenance of membrane fluidity and cell signaling, but also as the starting material for the biosynthesis of steroid hormones. It is not surprising, then, that cells of steroidogenic tissues have evolved with multiple pathways to assure the constant supply of cholesterol needed to maintain optimum steroid synthesis. The cholesterol utilized for steroidogenesis is derived from a combination of sources: 1) de novo synthesis in the endoplasmic reticulum (ER); 2) the mobilization of cholesteryl esters (CEs) stored in lipid droplets through cholesteryl ester hydrolase; 3) plasma lipoprotein-derived CEs obtained by either LDL receptor-mediated endocytic and/or SR-BI-mediated selective uptake; and 4) in some cultured cell systems from plasma membrane-associated free cholesterol. Here, we focus on recent insights into the molecules and cellular processes that mediate the uptake of plasma lipoprotein-derived cholesterol, events connected with the intracellular cholesterol processing and the role of crucial proteins that mediate cholesterol transport to mitochondria for its utilization for steroid hormone production. In particular, we discuss the structure and function of SR-BI, the importance of the selective cholesterol transport pathway in providing cholesterol substrate for steroid biosynthesis and the role of two key proteins, StAR and PBR/TSO in facilitating cholesterol delivery to inner mitochondrial membrane sites, where P450scc (CYP11A) is localized and where the conversion of cholesterol to pregnenolone (the common steroid precursor) takes place.

High density lipoprotein inhibits the activation of sterol regulatory element-binding protein-1 in cultured cells

A link between cellular uptake of high density lipoprotein (HDL) and regulation of sterol regulatory element-binding protein-1 (SREBP-1) was investigated in vitro. HDL decreased nuclear SREBP-1 levels as well as SREBP-1 target gene expression in HepG2 and HEK293 cells. However, HDL did not repress an exogenously expressed, constitutively active form of SREBP-1. HDL increased cellular cholesterol levels, and cellular cholesterol depletion by methyl-beta-cyclodextrin abolished the effects of HDL. These results suggest that HDL inhibits the activation of SREBP-1 through a cholesterol-dependent mechanism, which may play an important role in regulating lipid synthetic pathways mediated by SREBP-1.