Maintenance Of Cholesterol Homeostasis Research Articles

Exploring Phytochemical Mechanisms in the Prevention of Cholesterol Dysregulation: A Review.

Although cholesterol plays a key role in many physiological processes, its dysregulation can lead to several metabolic diseases. Statins are a group of drugs widely used to lower cholesterol levels and cardiovascular risk but may lead to several side effects in some patients. Therefore, the development of a plant-based therapeutic adjuvant with cholesterol-lowering activity is desirable. The maintenance of cholesterol homeostasis encompasses multiple steps, including biosynthesis and metabolism, uptake and transport, and bile acid metabolism; issues arising in any of these processes could contribute to the etiology of cholesterol-related diseases. An increasing body of evidence strongly indicates the benefits of phytochemicals for cholesterol regulation; traditional Chinese medicines prove beneficial in some disease models, although more scientific investigations are needed to confirm their effectiveness. One of the main functions of cholesterol is bile acid biosynthesis, where most bile acids are recycled back to the liver. The composition of bile acid is partly modulated by gut microbes and could be harmful to the liver. In this regard, the reshaping effect of phytochemicals on gut microbiota has been widely reported in the literature for its significance. Therefore, we reviewed studies conducted over the past 5 years elucidating the regulatory effects of phytochemicals or herbal medicines on cholesterol metabolism. In addition, their effects on the recomposition of gut microbiota and bile acid metabolism due to modulation are discussed. This review aims to provide novel insights into the treatment of cholesterol dysregulation and the anticipated development of natural-based compounds in the near and far future.

Direct binding to sterols accelerates endoplasmic reticulum-associated degradation of HMG CoA reductase

The maintenance of cholesterol homeostasis is crucial for normal function at both the cellular and organismal levels. Two integral membrane proteins, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) and Scap, are key targets of a complex feedback regulatory system that operates to ensure cholesterol homeostasis. HMGCR catalyzes the rate-limiting step in the transformation of the 2-carbon precursor acetate to 27-carbon cholesterol. Scap mediates proteolytic activation of sterol regulatory element-binding protein-2 (SREBP-2), a membrane-bound transcription factor that controls expression of genes involved in the synthesis and uptake of cholesterol. Sterol accumulation triggers binding of HMGCR to endoplasmic reticulum (ER)-localized Insig proteins, leading to the enzyme's ubiquitination and proteasome-mediated ER-associated degradation (ERAD). Sterols also induce binding of Insigs to Scap, which leads to sequestration of Scap and its bound SREBP-2 in the ER, thereby preventing proteolytic activation of SREBP-2 in the Golgi. The oxygenated cholesterol derivative 25-hydroxycholesterol (25HC) and the methylated cholesterol synthesis intermediate 24,25-dihydrolanosterol (DHL) differentially modulate HMGCR and Scap. While both sterols promote binding of HMGCR to Insigs for ubiquitination and subsequent ERAD, only 25HC inhibits the Scap-mediated proteolytic activation of SREBP-2. We showed previously that 1,1-bisphosphonate esters mimic DHL, accelerating ERAD of HMGCR while sparing SREBP-2 activation. Building on these results, our current studies reveal specific, Insig-independent photoaffinity labeling of HMGCR by photoactivatable derivatives of the 1,1-bisphosphonate ester SRP-3042 and 25HC. These findings disclose a direct sterol binding mechanism as the trigger that initiates the HMGCR ERAD pathway, providing valuable insights into the intricate mechanisms that govern cholesterol homeostasis.

The role of ATP-binding cassette transporter G1 (ABCG1) in Alzheimer's disease: A review of the mechanisms.

The maintenance of cholesterol homeostasis is essential for central nervous system function. Consequently, factors that affect cholesterol homeostasis are linked to neurological disorders and pathologies. Among them, ATP-binding cassette transporter G1 (ABCG1) plays a significant role in atherosclerosis. However, its role in Alzheimer's disease (AD) is unclear. There is inconsistent information regarding ABCG1's role in AD. It can increase or decrease amyloid β (Aβ) levels in animals' brains. Clinical studies show that ABCG1 is involved in AD patients' impairment of cholesterol efflux capacity (CEC) in the cerebrospinal fluid (CSF). Lower Aβ levels in the CSF are correlated with ABCG1-mediated CEC dysfunction. ABCG1 modulates α-, β-, and γ-secretase activities in the plasma membrane and may affect Aβ production in the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) cell compartment. Despite contradictory findings regarding ABCG1's role in AD, this review shows that ABCG1 has a role in Aβ generation via modulation of membrane secretases. It is, however, necessary to investigate the underlying mechanism(s). ABCG1 may also contribute to AD pathology through its role in apoptosis and oxidative stress. As a result, ABCG1 plays a role in AD and is a candidate for drug development.

Expression of components involved in cholesterol homeostasis maintenance during experimental autoimmune encephalomyelitis in rat spinal cord.

Dysregulations in cholesterol homeostasis contribute to the pathogenesis of multiple sclerosis (MS) and its best described animal model, experimental autoimmune encephalomyelitis (EAE). Cholesterol is an important component of myelin, which is necessary for signal transmission between neurons. Demyelination leads to the formation of oxysterols, degradation products of cholesterol that are ligands for nuclear liver X receptors (LXRs). Genes regulated by LXRs are involved in cholesterol efflux, absorption, transport, and excretion, which we investigated in this study. In this study, we detected changes in gene expression of Srebf1, Ldlr, Soat1, Abca1, Lrp1, and Npc1, all of which are important in the regulation of cholesterol homeostasis, during the course of EAE in male and female rats. In particular, differential expression of Srebf1, Ldlr, and Soat1 was observed in the spinal cord of male and female rats during EAE. Moreover, these genes are altered during EAE. In contrast, the expression of Abca1 and Lrp1 was significantly affected only by sex. In male animals, the expression of Npc1 is conspicuously reduced in EAE pathology. Thus, our study confirms the involvement of enzymes of cholesterol metabolism in the pathophysiology of EAE, with sex and disease progression affecting the expression of these genes. These findings may improve the understanding of neurodegenerative diseases associated with impaired lipid metabolism in the brain, such as MS/EAE.

Is microRNA-33 an Appropriate Target in the Treatment of Atherosclerosis?

The maintenance of cholesterol homeostasis is a complicated process involving regulation of cholesterol synthesis, dietary uptake and bile acid synthesis and excretion. Reverse cholesterol transport, described as the transfer of cholesterol from non-hepatic cells, including foam cells in atherosclerotic plaques, to the liver and then its excretion in the feces is important part of this regulation. High-density lipoproteins are the key mediators of reverse cholesterol transport. On the other hand, microRNA-33 was identified as a key regulator of cholesterol homeostasis. Recent studies indicate the impact of microRNA-33 not only on cellular cholesterol efflux and HDL production but also on bile metabolism in the liver. As proper coordination of cholesterol metabolism is essential to human health, discussion of recent findings in this field may open new perspectives in the microRNA-dependent treatment of a cholesterol imbalance.

Emerging role of HDL in brain cholesterol metabolism and neurodegenerative disorders

High-density lipoproteins (HDL) play a key role in cholesterol homeostasis maintenance in the central nervous system (CNS), by carrying newly synthesized cholesterol from astrocytes to neurons, to support their lipid-related physiological functions. As occurs for plasma HDL, brain lipoproteins are assembled through the activity of membrane cholesterol transporters, undergo remodeling mediated by specific enzymes and transport proteins, and finally deliver cholesterol to neurons by a receptor-mediated internalization process. A growing number of evidences indicates a strong association between alterations of CNS cholesterol homeostasis and neurodegenerative disorders, in particular Alzheimer's disease (AD), and a possible role in this relationship may be played by defects in brain HDL metabolism. In the present review, we summarize and critically examine the current state of knowledge on major modifications of HDL and HDL-mediated brain cholesterol transport in AD, by taking into consideration the individual steps of this process. We also describe potential and encouraging HDL-based therapies that could represent new therapeutic strategies for AD treatment. Finally, we revise the main plasma and brain HDL modifications in other neurodegenerative disorders including Parkinson's disease (PD), Huntington's disease (HD), and frontotemporal dementia (FTD).

Balancing cholesterol in the brain: from synthesis to disposal

The cholesterol is a vital component of cell membranes and myelin sheaths, and a precursor for essential molecules such as steroid hormones. In humans, cholesterol is partially obtained through the diet, while the majority is synthesized in the body, primarily in the liver. However, the limited exchange between the central nervous system and peripheral circulation, due to the presence of the blood-brain barrier, necessitates cholesterol in the brain to be exclusively acquired from local de novo synthesis. This cholesterol is reutilized efficiently, rendering a much slower overall turnover of the compound in the brain as compared with the periphery. Furthermore, brain cholesterol is regulated independently from peripheral cholesterol. Numerous enzymes, proteins, and other factors are involved in cholesterol synthesis and metabolism in the brain. Understanding the unique mechanisms and pathways involved in the maintenance of cholesterol homeostasis in the brain is critical, considering perturbations to these processes are implicated in numerous neurodegenerative diseases. This review focuses on the developing understanding of cholesterol metabolism in the brain, discussing the sites and processes involved in its synthesis and regulation, as well as the mechanisms involved in its distribution throughout, and elimination from, the brain.

Glycoursodeoxycholic Acid Ameliorates Atherosclerosis and Alters Gut Microbiota in Apolipoprotein E–Deficient Mice

BackgroundAlthough glycoursodeoxycholic acid (GUDCA) has been associated with the improvement of metabolic disorders, its effect on atherosclerosis remains elusive. This study aimed to investigate the role of GUDCA in the development of atherosclerosis and its potential mechanisms.Methods and ResultsHuman THP‐1 macrophages were used to investigate the effect of GUDCA on oxidized low‐density lipoprotein–induced foam cell formation in vitro. We found that GUDCA downregulated scavenger receptor A1 mRNA expression, reduced oxidized low‐density lipoprotein uptake, and inhibited macrophage foam cell formation. In an in vivo study, apolipoprotein E–deficient mice were fed a Western diet for 10 weeks to induce atherosclerosis, and then were gavaged once daily with or without GUDCA for 18 weeks. Parameters of systemic metabolism and atherosclerosis were detected. We found that GUDCA improved cholesterol homeostasis and protected against atherosclerosis progression as evidenced by reduced plaque area along with lipid deposition, ameliorated local chronic inflammation, and elevated plaque stability. In addition, 16S rDNA sequencing showed that GUDCA administration partially normalized the Western diet–associated gut microbiota dysbiosis. Interestingly, the changes of bacterial genera (Alloprevotella, Parabacteroides, Turicibacter, and Alistipes) modulated by GUDCA were correlated with the plaque area in mice aortas.ConclusionsOur study for the first time indicates that GUDCA attenuates the development of atherosclerosis, probably attributable to the inhibition of foam cell formation, maintenance of cholesterol homeostasis, and modulation of gut microbiota.

The Interplay of ABC Transporters in Aβ Translocation and Cholesterol Metabolism: Implicating Their Roles in Alzheimer's Disease.

The occurrence of Alzheimer's disease (AD) worldwide has been progressively accelerating at an alarming rate, without any successful therapeutic strategy for the disease mitigation. The complexity of AD pathogenesis needs to be targeted with an alternative approach, as provided by the superfamily of ATP-binding cassette (ABC) transporters, which constitutes an extensive range of proteins, capable of transporting molecular entities across biological membranes. These protein moieties have been implicated in AD, based upon their potential in lipid transportation, resulting in maintenance of cholesterol homeostasis. These transporters have been reported to target the primary hallmark of AD pathogenesis, namely, beta-amyloid hypothesis, which is associated with accumulation of beta-amyloid (Aβ) plaques in AD patients. The ABC transporters have been observed to be localized to the capillary endothelial cells of the blood-brain barrier and neural parenchymal cells, where they exhibit different roles, consequently influencing the neuronal expression of Aβ peptides. The review highlights different families of ABC transporters, ABCB1 (P-glycoprotein), ABCA (ABCA1, ABCA2, and ABCA7), ABCG2 (BCRP; breast cancer resistance protein), ABCG1 and ABCG4, as well as ABCC1 (MRP; multidrug resistance protein) in the CNS, and their interplay in regulating cholesterol metabolism and Aβ peptide load in the brain, simultaneously exerting protective effects against neurotoxic substrates and xenobiotics. The authors aim to establish the significance of this alternative approach as a novel therapeutic target in AD, to provide the researchers an opportunity to evaluate the potential aspects of ABC transporters in AD treatment.

Cholesterol Regulation in Age-Related Macular Degeneration: A Framework for Mathematical Modelling of Drusen Biogenesis

In age-related macular degeneration (AMD), there is, in common with many other age-related diseases, the need to distinguish between changes in the ageing eye that lead to disease and those changes that are considered part of a healthy, ageing eye. Various studies investigating the multitude of mechanisms involved in the aetiology of AMD exist within the field of ophthalmology and related medical fields, yet many aspects of it remain poorly understood and only a limited number of therapies are available. A recent study relates drusen's topographically cellular characteristics to the neural retina's metabolic needs and associated cholesterol involvement within the retina. In particular, there is a need to fully understand the maintenance of cholesterol homeostasis in the retina to prevent normal ageing processes from being perturbed towards maculopathy. Here, we present an extensive review of the clinical and physiological features of the ageing retina, as well as mechanisms implicated in pathology, synthesised from a vast body of the published literature. We use this novel synthesis to construct a comprehensive process schematic, encompassing all key species and physiological processes such as nutrients, waste and lipoprotein management. We are therefore able to express these processes in a mathematical language via a comprehensive modelling framework, comprising a set of twenty-three equations spanning three distinct biological compartments. This very general modelling framework may now be adapted to more focused studies on individual mechanisms, processes or components underlying of the many facets of AMD. As an example of such a focused application, we conclude this article with a one-compartment, four-species model of the retinal pigment epithelium, which considers the parametric conditions under which either cholesterol homeostasis or unregulated accumulation of cholesterol may obtain in the ageing eye.

Disturbances in Cholesterol Homeostasis and Non-alcoholic Fatty Liver Diseases.

Non-alcoholic fatty liver disease (NAFLD) is a major health problem associated with obesity and other features of the metabolic syndrome including insulin resistance and dyslipidemia. The accumulation of lipids in hepatocytes causes liver damage and triggers inflammation, fibrosis, and cirrhosis. Beside fatty acids and triglycerides, evidence showed an increased accumulation of free cholesterol in the liver with subsequent toxic effects contributing to liver damage. The maintenance of cholesterol homeostasis in the body requires a balance between several pathways responsible for cholesterol synthesis, transport and conversion into bile acids. Intestinal absorption is also one of the major determinants of cholesterol homeostasis. The nature of changes in cholesterol homeostasis associated with NAFLD has been a subject of extensive investigations. In this article, we will attempt to provide a brief overview of the current knowledge about the disturbances in cholesterol metabolism associated with NAFLD and discuss how certain molecular targets of these pathways could be exploited for the treatment of this multifactorial disease.

Hypolipidemic effect of N-acetylcysteine against dexamethasone-induced hyperlipidemia in rats

Objective: The liver is a key metabolic organ involved in lipid metabolism and maintenance of cholesterol homeostasis in the body. However, hypercholesterolemia and oxidative stress is associated with the fatty liver which is the major risk factor associated with cardiovascular diseases (CVDs). The objective of this study was to investigate the hypolipidemic property of N-acetylcysteine (NAC) in dexamethasone-induced hyperlipidemic rats. Materials and Methods: Dexamethasone (10 mg/kg) was administered on alternate days intraperitoneally for 28 days to induce hyperlipidemia. NAC (50 mg/kg and 100 mg/kg) was daily administered intraperitoneally for 28 days. After 24 h of the last treatment blood and liver samples were collected. Results: The relative body and liver weights, activities of serum alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, lipid profile, and antioxidant defense markers were measured. The result indicated that the treatment of hyperlipidemic rats with 50 and 100 mg/kg NAC significantly (P < 0.05) prevented dexamethasone-induced body weight loss and restored liver weight. In addition, NAC reduced the elevation of hepatic enzymes activities induced by dexamethasone. Moreover, NAC exhibits hypolipidemic effect as demonstrated by reversal of serum levels of total cholesterol, triacylglycerol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and atherogenic index. Conclusion: These findings indicated that NAC was able to restore dyslipidemia induced by dexamethasone through improving liver function parameters and augments antioxidant defense systems. Altogether the anti- hyperlipidemic effects exhibited by NAC might have been mediated partly through antioxidant actions and could be beneficial against CVDs.

ABC Transporters, Cholesterol Efflux, and Implications for Cardiovascular Diseases.

Most types of cells in the body have no or very limited capacity of catabolizing cholesterol, so cholesterol efflux is essential for cholesterol homeostasis. There are multiple mechanisms responsible for cellular cholesterol efflux. Among them, the active efflux pathways are mediated primarily by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1. ABCA1 is essential for cholesterol and phospholipid efflux to apolipoprotein A-I and high density lipoprotein (HDL) biogenesis. ABCG1 promotes cholesterol efflux primarily to HDL particles. Atherosclerotic cardiovascular disease is a chronic inflammatory disease characterized by marked macrophage foam cell accumulation in atherosclerotic plaques and associated pro-inflammatory responses in lesional cells. Findings from both animal and human studies indicate a critical role of disturbed cholesterol homeostasis in pro-inflammatory responses in these cells, particularly in lesional macrophages. ABCA1 and ABCG1 are highly expressed in macrophages, particularly in response to cholesterol accumulation, and are essential in maintenance of cholesterol homeostasis. Functional deficiency of ABCA1 and ABCG1 in macrophage markedly increases atherogenesis, with exacerbated inflammatory responses. ABCA1 and ABCG1 also play a critical role in control of hematopoietic stem and progenitor cell (HSPC) proliferation and extramedullary hematopoiesis. Hematopoietic ABCA1 and ABCG1 deficiencies cause marked HSPC expansion and extramedullary hematopoiesis, particularly in hypercholesterolemia, and lead to marked monocytosis and neutrophilia with exacerbated pro-inflammatory responses. All these contribute to atherosclerosis. In this chapter, we describe these findings and discuss the current understanding of the underlying mechanisms. We also discuss other ABC transporters such as ABCG4, which also promotes cholesterol efflux to HDL and controls megakaryocyte proliferation and platelet biogenesis. By this pathway, ABCG4 also modulates atherogenesis. Therapeutic approaches targeting the pathways and mechanisms described also are discussed.

PMP22 Regulates Cholesterol Trafficking and ABCA1-Mediated Cholesterol Efflux.

The absence of functional peripheral myelin protein 22 (PMP22) is associated with shortened lifespan in rodents and severe peripheral nerve myelin abnormalities in several species including humans. Schwann cells and nerves from PMP22 knock-out (KO) mice show deranged cholesterol distribution and aberrant lipid raft morphology, supporting an unrecognized role for PMP22 in cellular lipid metabolism. To examine the mechanisms underlying these abnormalities, we studied Schwann cells and nerves from male and female PMP22 KO mice. Whole-cell current-clamp recordings in cultured Schwann cells revealed increased membrane capacitance and decreased membrane resistance in the absence of PMP22, which was consistent with a reduction in membrane cholesterol. Nerves from PMP22-deficient mice contained abnormal lipid droplets, with both mRNA and protein levels of apolipoprotein E (apoE) and ATP-binding cassette transporter A1 (ABCA1) being highly upregulated. Despite the upregulation of ABCA1 and apoE, the absence of PMP22 resulted in reduced localization of the transporter to the cell membrane and diminished secretion of apoE. The absence of PMP22 also impaired ABCA1-mediated cholesterol efflux capacity. In nerves from ABCA1 KO mice, the expression of PMP22 was significantly elevated and the subcellular processing of the overproduced protein was aberrant. In wild-type samples, double immunolabeling identified overlapping distribution of PMP22 and ABCA1 at the Schwann cell plasma membrane and the two proteins were coimmunoprecipitated from Schwann cell and nerve lysates. Together, these results reveal a novel role for PMP22 in regulating lipid metabolism and cholesterol trafficking through functional interaction with the cholesterol efflux regulatory protein ABCA1.SIGNIFICANCE STATEMENT Understanding the subcellular events that underlie abnormal myelin formation in hereditary neuropathies is critical for advancing therapy development. Peripheral myelin protein 22 (PMP22) is an essential peripheral myelin protein because its genetic abnormalities account for ∼80% of hereditary neuropathies. Here, we demonstrate that in the absence of PMP22, the cellular and electrophysiological properties of the Schwann cells' plasma membrane are altered and cholesterol trafficking and lipid homeostasis are perturbed. The molecular mechanisms for these abnormalities involve a functional interplay among PMP22, cholesterol, apolipoprotein E, and the major cholesterol-efflux transporter protein ATP-binding cassette transporter A1 (ABCA1). These findings establish a critical role for PMP22 in the maintenance of cholesterol homeostasis in Schwann cells.

Abstract 2419: Lipid droplets promote glioblastoma cell growth through maintaining cholesterol homeostasis mediated by autophagy

Abstract Lipid droplets (LDs) have been studied as the major storage site of neutral lipids for decades, especially in the progression of metabolic diseases. Recently, the functions of LDs in cancers have begun to gain significant attention. Here, we provide evidence that LDs promote glioblastoma (GBM) cell growth through autophagy mediated maintenance of cholesterol homeostasis. Our data showed that LDs in tumor tissues from GBM patients are engulfed by autophagic vacuoles and lysosomes. Cellular cholesterol deprivation or removal of plasma membrane cholesterol by methyl-β-cyclodextrin (MβCD) activates autophagy to hydrolyze LDs to support GBM cell survival, whereas inhibition of autophagy blocks transportation of cholesterol to plasma membrane, leading to cell death. Fluorescence imaging reveals direct trafficking of LD-stored cholesterol to the plasma membrane through autophagic liberation. Our data demonstrate that LDs serve as cholesterol reservoir for GBM cell growth, suggesting that targeting LD-autophagy-cholesterol homeostasis metabolic pathway represents a potential means to treat GBM and other malignancies. Citation Format: Feng Geng, Xiang Cheng, Chunming Cheng, Arnab Chakravarti, Deliang Guo. Lipid droplets promote glioblastoma cell growth through maintaining cholesterol homeostasis mediated by autophagy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2419.

Lipidated apolipoprotein E4 structure and its receptor binding mechanism determined by a combined cross-linking coupled to mass spectrometry and molecular dynamics approach.

Apolipoprotein E (apoE) is a forefront actor in the transport of lipids and the maintenance of cholesterol homeostasis, and is also strongly implicated in Alzheimer’s disease. Upon lipid-binding apoE adopts a conformational state that mediates the receptor-induced internalization of lipoproteins. Due to its inherent structural dynamics and the presence of lipids, the structure of the biologically active apoE remains so far poorly described. To address this issue, we developed an innovative hybrid method combining experimental data with molecular modeling and dynamics to generate comprehensive models of the lipidated apoE4 isoform. Chemical cross-linking combined with mass spectrometry provided distance restraints, characterizing the three-dimensional organization of apoE4 molecules at the surface of lipidic nanoparticles. The ensemble of spatial restraints was then rationalized in an original molecular modeling approach to generate monomeric models of apoE4 that advocated the existence of two alternative conformations. These two models point towards an activation mechanism of apoE4 relying on a regulation of the accessibility of its receptor binding region. Further, molecular dynamics simulations of the dimerized and lipidated apoE4 monomeric conformations revealed an elongation of the apoE N-terminal domain, whereby helix 4 is rearranged, together with Arg172, into a proper orientation essential for lipoprotein receptor association. Overall, our results show how apoE4 adapts its conformation for the recognition of the low density lipoprotein receptor and we propose a novel mechanism of activation for apoE4 that is based on accessibility and remodeling of the receptor binding region.

Intestinal Farnesoid X Receptor Controls Transintestinal Cholesterol Excretion in Mice

The role of the intestine in the maintenance of cholesterol homeostasis increasingly is recognized. Fecal excretion of cholesterol is the last step in the atheroprotective reverse cholesterol transport pathway, to which biliary and transintestinal cholesterol excretion (TICE) contribute. The mechanisms controlling the flux of cholesterol through the TICE pathway, however, are poorly understood. We aimed to identify mechanisms that regulate and stimulate TICE. We performed studies with C57Bl/6J mice, as well as with mice with intestine-specific knockout of the farnesoid X receptor (FXR), mice that express an FXR transgene specifically in the intestine, and ABCG8-knockout mice. Mice were fed a control diet or a diet supplemented with the FXR agonist PX20606, with or without the cholesterol absorption inhibitor ezetimibe. Some mice with intestine-specific knockout of FXR were given daily injections of fibroblast growth factor (FGF)19. To determine fractional cholesterol absorption, mice were given intravenous injections of cholesterol D5 and oral cholesterol D7. Mice were given 13C-acetate in drinking water for measurement of cholesterol synthesis. Bile cannulations were performed and biliary cholesterol secretion rates were assessed. In a separate set of experiments, bile ducts of male Wistar rats were exteriorized, allowing replacement of endogenous bile by a model bile. In mice, we found TICE to be regulated by intestinal FXR via induction of its target gene Fgf15 (FGF19 in rats and human beings). Stimulation of this pathway caused mice to excrete up to 60% of their total cholesterol content each day. PX20606 and FGF19 each increased the ratio of muricholate:cholate in bile, inducing a more hydrophilic bile salt pool. The altered bile salt pool stimulated robust secretion of cholesterol into the intestinal lumen via the sterol-exporting heterodimer adenosine triphosphate binding cassette subfamily G member 5/8 (ABCG5/G8). Of note, the increase in TICE induced by PX20606 was independent of changes in cholesterol absorption. Hydrophilicity of the bile salt pool, controlled by FXR and FGF15/19, is an important determinant of cholesterol removal via TICE. Strategies that alter bile salt pool composition might be developed for the prevention of cardiovascular disease. Transcript profiling: http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?token=irsrayeohfcntqx&acc=GSE74101.

Biosynthesis and biotransformation of bile acids

Bile acids are steroidal compounds, which contain 24 carbon atoms. They can be classified into two major groups: primary and secondary. The most abundant bile acids: The primary bile acids include cholic acid and chenodeoxycholic acid, while the major secondary bile acids are deoxycholic acid and litocholic acid. Bile acids are important physiological agents for intestinal absorption of nutrients and are used for biliary lipid secretion, toxic metabolites and xenobiotics. The aim of this paper is to analyze biosynthesis and biotransformation of bile acids, as preparation for practical usage in laboratory and clinical conditions. Topic: Biosynthesis and biotransformation of bile acids: The biosynthesis of bile acids is the dominant metabolic pathway for catabolism of cholesterol in humans. The classical route of biosynthesis of bile acids is embarking on the conversion of cholesterol into 7α-hydroxycholesterol using enzyme 7α-cholesterol hydroxylase (CYP7A1). This enzyme is one of the microsomal cytochrome P450 enzyme is localized exclusively in the liver. Classical road is the main road in the biosynthesis of bile acids, and its total contribution amounts to 90% for people, and 75% in mice. CYP 7A1 enzyme is considered to be sensitive to the inhibition of carbon monoxide, and the condition for the effect of NADPH, the oxygen, lecithin, and the NADPH-cytochrome P450 reductase. Bile acids are important signaling molecules and metabolic controls which activate the nuclear receptor and the G protein-coupled receptors (GPCR), a signaling lipid regulation of the liver, glucose and energy homeostasis. Also, bile acids maintain metabolic homeostasis. Biotransformation of bile acids: The conversion of cholesterol into bile acids just important for maintenance of cholesterol homeostasis, but also to prevent the accumulation of cholesterol, triglycerides and toxic metabolites as well as violations of the liver and other organs. Enterohepatic circulation of bile acids from the liver to the intestine and back to the liver occupies the most important role in the processes of absorption and distribution, as well as in metabolic regulation and homeostasis. Conclusions: This physiological process is complicated and regulates the membrane transport system in the liver and intestine by means of nuclear receptors. It is very dangerous fact that toxic bile acids may be causes of inflammation, apoptosis and cell death. On the other hand activated GPCR signaling and nuclear bile acid protects against inflammation of the liver, intestine and macrophages. Bile acid metabolism disorders cause cholestatic liver disease, dyslipidemia, fatty liver disease, cardiovascular disease and diabetes.

Abstract 402: miRNA Targeting of Oxysterol Binding Protein-like 6 (OSBPL6) Regulates Cholesterol Trafficking and Efflux

Cholesterol homeostasis is fundamental to human health, and is thus, tightly regulated. MicroRNAs exert potent effects on biological pathways, including cholesterol metabolism, by repressing genes with related functions. We reasoned that this mode of pathway regulation could be exploited to identify novel genes involved in cholesterol homeostasis. Here, we identify oxysterol binding protein-like 6 ( OSBPL6 ) as a novel target of 2 miRNA hubs regulating cholesterol homeostasis: miR-33 and miR-27b. Characterization of OSBPL6 revealed that it is transcriptionally regulated in macrophages and hepatocytes by LXR and in response to cholesterol loading, and in mice and non-human primates by western diet feeding. OSBPL6 encodes the OSBPL-related protein 6 (ORP6), which contains a pleckstrin homology membrane-targeting domain and a FFAT domain predicted to interact with VAP proteins in the ER. Indeed, subcellular localization studies showed that ORP6 is associated with the plasma membrane, early endosomes, lysosomes and ER, suggesting a putative role for ORP6 in cholesterol trafficking between these compartments. Consistent with this, siRNA-mediated knockdown of OSBPL6 results in aberrant clustering of endosomes and promotes the accumulation of free cholesterol in these structures. As a consequence, esterification of cholesterol at the ER is reduced, triggering activation of SREBP2. Conversely, ORP6 overexpression enhances cholesterol trafficking and efflux to apoA1 in macrophages and hepatocytes. Moreover, we show that hepatic expression of OSBPL6 is positively correlated with plasma levels of HDL cholesterol in a cohort of 200 healthy individuals, whereas its expression is reduced in human atherosclerotic plaques. Collectively, these studies identify ORP6 as a novel regulator of cholesterol trafficking that is part of the miR-33 and miR-27b target gene networks that contribute to the maintenance of cholesterol homeostasis.

MiRNA Targeting of Oxysterol-Binding Protein-Like 6 Regulates Cholesterol Trafficking and Efflux.

Cholesterol homeostasis is fundamental to human health and is, thus, tightly regulated. MicroRNAs exert potent effects on biological pathways, including cholesterol metabolism, by repressing genes with related functions. We reasoned that this mode of pathway regulation could be exploited to identify novel genes involved in cholesterol homeostasis. Here, we identify oxysterol-binding protein-like 6 (OSBPL6) as a novel target of 2 miRNA hubs regulating cholesterol homeostasis: miR-33 and miR-27b. Characterization of OSBPL6 revealed that it is transcriptionally regulated in macrophages and hepatocytes by liver X receptor and in response to cholesterol loading and in mice and nonhuman primates by Western diet feeding. OSBPL6 encodes the OSBPL-related protein 6 (ORP6), which contains dual membrane- and endoplasmic reticulum-targeting motifs. Subcellular localization studies showed that ORP6 is associated with the endolysosomal network and endoplasmic reticulum, suggesting a role for ORP6 in cholesterol trafficking between these compartments. Accordingly, knockdown of OSBPL6 results in aberrant clustering of endosomes and promotes the accumulation of free cholesterol in these structures, resulting in reduced cholesterol esterification at the endoplasmic reticulum. Conversely, ORP6 overexpression enhances cholesterol trafficking and efflux in macrophages and hepatocytes. Moreover, we show that hepatic expression of OSBPL6 is positively correlated with plasma levels of high-density lipoprotein cholesterol in a cohort of 200 healthy individuals, whereas its expression is reduced in human atherosclerotic plaques. These studies identify ORP6 as a novel regulator of cholesterol trafficking that is part of the miR-33 and miR-27b target gene networks that contribute to the maintenance of cholesterol homeostasis.