Effectors of rapid homeostatic responses of endoplasmic reticulum cholesterol and 3-hydroxy-3-methylglutaryl-CoA reductase

How active cholesterol coordinates cell cholesterol homeostasis: Test of a hypothesis

How do cells coordinate the diverse elements that regulate their cholesterol homeostasis? Our model postulates that membrane cholesterol forms simple complexes with bilayer phospholipids. The phospholipids in the plasma membrane are of high affinity; consequently, they are fully complexed with the sterol. This sets the resting level of plasma membrane cholesterol. Cholesterol in excess of the stoichiometric equivalence point of these complexes has high chemical activity; we refer to it as active cholesterol. It equilibrates with the low affinity phospholipids in the intracellular membranes where it serves as a negative feedback signal to a manifold of regulatory proteins that rein in ongoing cholesterol accretion. We tested the model with a review of the literature regarding fourteen homeostatic proteins in enterocytes. It provided strong albeit indirect support for the following hypothesis. Active cholesterol inhibits cholesterol uptake and biosynthesis by suppressing both the expression and the activity of the gene products activated by SREBP-2; namely, HMGCR, LDLR and NPC1L1. It also reduces free cell cholesterol by serving as the substrate for its esterification by ACAT and for the synthesis of side-chain oxysterols, 27-hydroxycholesterol in particular. The oxysterols drive cholesterol depletion by promoting the destruction of HMGCR and stimulating sterol esterification as well as the activation of LXR. The latter fosters the expression of multiple homeostatic proteins, including four transporters for which active cholesterol is the likely substrate. By nulling active cholesterol, the manifold maintains the cellular sterol at its physiologic set point.

27-Hydroxycholesterol acts on estrogen receptor α expressed by POMC neurons in the arcuate nucleus to modulate feeding behavior

Oxysterols are metabolites of cholesterol that regulate cholesterol homeostasis. Among these, the most abundant oxysterol is 27-hydroxycholesterol (27HC), which can cross the blood-brain barrier. Because 27HC functions as an endogenous selective estrogen receptor modulator, we hypothesize that 27HC binds to the estrogen receptor α (ERα) in the brain to regulate energy balance. Supporting this view, we found that delivering 27HC to the brain reduced food intake and activated proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (POMCARH) in an ERα-dependent manner. In addition, we observed that inhibiting brain ERα, deleting ERα in POMC neurons, or chemogenetic inhibition of POMCARH neurons blocked the anorexigenic effects of 27HC. Mechanistically, we further revealed that 27HC stimulates POMCARH neurons by inhibiting the small conductance of the calcium-activated potassium (SK) channel. Together, our findings suggest that 27HC, through its interaction with ERα and modulation of the SK channel, inhibits food intake as a negative feedback mechanism against a surge in circulating cholesterol.

Is reverse cholesterol transport regulated by active cholesterol?

This review considers the hypothesis that a small portion of plasma membrane cholesterol regulates reverse cholesterol transport in coordination with overall cellular homeostasis. It appears that almost all of the plasma membrane cholesterol is held in stoichiometric complexes with bilayer phospholipids. The minor fraction of cholesterol that exceeds the complexation capacity of the phospholipids is called active cholesterol. It has an elevated chemical activity and circulates among the organelles. It also moves down its chemical activity gradient to plasma HDL, facilitated by the activity of ABCA1, ABCG1, and SR-BI. ABCA1 initiates this process by perturbing the organization of the plasma membrane bilayer, thereby priming its phospholipids for translocation to apoA-I to form nascent HDL. The active excess sterol and that activated by ABCA1 itself follow the phospholipids to the nascent HDL. ABCG1 similarly rearranges the bilayer and sends additional active cholesterol to nascent HDL, while SR-BI simply facilitates the equilibration of the active sterol between plasma membranes and plasma proteins. Active cholesterol also flows downhill to cytoplasmic membranes where it serves both as a feedback signal to homeostatic ER proteins and as the substrate for the synthesis of mitochondrial 27-hydroxycholesterol (27HC). 27HC binds the LXR and promotes the expression of the aforementioned transport proteins. 27HC-LXR also activates ABCA1 by competitively displacing its inhibitor, unliganded LXR. § Considerable indirect evidence suggests that active cholesterol serves as both a substrate and a feedback signal for reverse cholesterol transport. Direct tests of this novel hypothesis are proposed.

Oxysterols Protect Epithelial Cells Against Pore-Forming Toxins

Many species of bacteria produce toxins such as cholesterol-dependent cytolysins that form pores in cell membranes. Membrane pores facilitate infection by releasing nutrients, delivering virulence factors, and causing lytic cell damage - cytolysis. Oxysterols are oxidized forms of cholesterol that regulate cellular cholesterol and alter immune responses to bacteria. Whether oxysterols also influence the protection of cells against pore-forming toxins is unresolved. Here we tested the hypothesis that oxysterols stimulate the intrinsic protection of epithelial cells against damage caused by cholesterol-dependent cytolysins. We treated epithelial cells with oxysterols and then challenged them with the cholesterol-dependent cytolysin, pyolysin. Treating HeLa cells with 27-hydroxycholesterol, 25-hydroxycholesterol, 7α-hydroxycholesterol, or 7β-hydroxycholesterol reduced pyolysin-induced leakage of lactate dehydrogenase and reduced pyolysin-induced cytolysis. Specifically, treatment with 10 ng/ml 27-hydroxycholesterol for 24 h reduced pyolysin-induced lactate dehydrogenase leakage by 88%, and reduced cytolysis from 74% to 1%. Treating HeLa cells with 27-hydroxycholesterol also reduced pyolysin-induced leakage of potassium ions, prevented mitogen-activated protein kinase cell stress responses, and limited alterations in the cytoskeleton. Furthermore, 27-hydroxycholesterol reduced pyolysin-induced damage in lung and liver epithelial cells, and protected against the cytolysins streptolysin O and Staphylococcus aureus α-hemolysin. Although oxysterols regulate cellular cholesterol by activating liver X receptors, cytoprotection did not depend on liver X receptors or changes in total cellular cholesterol. However, oxysterol cytoprotection was partially dependent on acyl-CoA:cholesterol acyltransferase (ACAT) reducing accessible cholesterol in cell membranes. Collectively, these findings imply that oxysterols stimulate the intrinsic protection of epithelial cells against pore-forming toxins and may help protect tissues against pathogenic bacteria.

A basic model for cell cholesterol homeostasis

Cells manage their cholesterol by negative feedback using a battery of sterol-responsive proteins. How these activities are coordinated so as to specify the abundance and distribution of the sterol is unclear. We present a simple mathematical model that addresses this question. It assumes that almost all of the cholesterol is associated with phospholipids in stoichiometric complexes. A small fraction of the sterol is uncomplexed and thermodynamically active. It equilibrates among the organelles, setting their sterol level according to the affinity of their phospholipids. The activity of the homeostatic proteins in the cytoplasmic membranes is then set by their fractional saturation with uncomplexed cholesterol in competition with the phospholipids. The high-affinity phospholipids in the plasma membrane (PM) are filled to near stoichiometric equivalence, giving it most of the cell sterol. Notably, the affinity of the phospholipids in the endomembranes (EMs) is lower by orders of magnitude than that of the phospholipids in the PM. Thus, the small amount of sterol in the EMs rests far below stoichiometric capacity. Simulations match a variety of experimental data. The model captures the essence of cell cholesterol homeostasis, makes coherent a diverse set of experimental findings, provides a surprising prediction and suggests new experiments.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

A Comprehensive Review on Oxysterols and Related Diseases

The present review aims to provide a complete and comprehensive summary of current literature relevant to oxysterols and related diseases. Oxidation of cholesterol leads to the formation of a large number of oxidized products, generally known as oxysterols. They are intermediates in the biosynthesis of bile acids, steroid hormones, and 1,25- dihydroxyvitamin D3. Although oxysterols are considered as metabolic intermediates, there is a growing body of evidence that many of them are bioactive, and their absence or excess may be part of the cause of a disease phenotype. These compounds derive from either enzymatic or non-enzymatic oxidation of cholesterol. This study provides comprehensive information about the structures, formation, and types of oxysterols even when involved in certain disease states, focusing on their effects on metabolism and linkages with these diseases. The role of specific oxysterols as mediators in various disorders, such as degenerative (age-related) and cancer-related disorders, has now become clearer. Oxysterol levels may be employed as suitable markers for the diagnosis of specific diseases or in predicting the incidence rate of diseases, such as diabetes mellitus, Alzheimer's disease, multiple sclerosis, osteoporosis, lung cancer, breast cancer, and infertility. However, further investigations may be required to confirm these mentioned possibilities.

Active cholesterol 20 years on

This review considers the following hypotheses, some well-supported and some speculative. Almost all of the sterol molecules in plasma membranes are associated with bilayer phospholipids in complexes of varied strength and stoichiometry. These complexes underlie many of the material properties of the bilayer. The small fraction of cholesterol molecules exceeding the binding capacity of the phospholipids is thermodynamically active and serves diverse functions. It circulates briskly among the cell membranes, particularly through contact sites linking the organelles. Active cholesterol provides the upstream feedback signal to multiple mechanisms governing plasma membrane homeostasis, pegging the sterol level to a threshold set by its phospholipids. Active cholesterol could also be the cargo for various inter-organelle transporters and the form excreted from cells by reverse transport. Furthermore, it is integral to the function of caveolae; a mediator of Hedgehog regulation; and a ligand for the binding of cytolytic toxins to membranes. Active cholesterol modulates a variety of plasma membrane proteins-receptors, channels and transporters-at least in vitro.

Endogenous sterol intermediates of the mevalonate pathway regulate HMGCR degradation and SREBP-2 processing

Sterol-regulated HMG-CoA reductase (HMGCR) degradation and SREBP-2 cleavage are two major feedback regulatory mechanisms governing cholesterol biosynthesis. Reportedly, lanosterol selectively stimulates HMGCR degradation, and cholesterol is a specific regulator of SREBP-2 cleavage. However, it is unclear whether other endogenously generated sterols regulate these events. Here, we investigated the sterol intermediates from the mevalonate pathway of cholesterol biosynthesis using a CRISPR/Cas9-mediated genetic engineering approach. With a constructed HeLa cell line expressing the mevalonate transporter, we individually deleted genes encoding major enzymes in the mevalonate pathway, used lipidomics to measure sterol intermediates, and examined HMGCR and SREBP-2 statuses. We found that the C4-dimethylated sterol intermediates, including lanosterol, 24,25-dihydrolanosterol, follicular fluid meiosis activating sterol, testis meiosis activating sterol, and dihydro-testis meiosis activating sterol, were significantly upregulated upon mevalonate loading. These intermediates augmented both degradation of HMGCR and inhibition of SREBP-2 cleavage. The accumulated lanosterol induced rapid degradation of HMGCR, but did not inhibit SREBP-2 cleavage. The newly synthesized cholesterol from the mevalonate pathway is dispensable for inhibiting SREBP-2 cleavage. Together, these results suggest that lanosterol is a bona fide endogenous regulator that specifically promotes HMGCR degradation, and that other C4-dimethylated sterol intermediates may regulate both HMGCR degradation and SREBP-2 cleavage.

Steady state analysis of influx and transbilayer distribution of ergosterol in the yeast plasma membrane

Background

The transbilayer sterol distribution between both plasma membrane (PM) leaflets has long been debated. Recent studies in mammalian cells and in yeast show that the majority of sterol resides in the inner PM leaflet. Since sterol flip-flop in model membranes is rapid and energy-independent, a mechanistic understanding for net enrichment of sterol in one leaflet is lacking. Import of ergosterol in yeast can take place via the ABC transporters Aus1/Pdr11 under anaerobic growth conditions, eventually followed by rapid non-vesicular sterol transport to the endoplasmic reticulum (ER). Little is known about how these transport steps are dynamically coordinated.

Methods

Here, a kinetic steady state model is presented which considers sterol import via Aus1/Pdr11, sterol flip-flop across the PM, bi-molecular complex formation and intracellular sterol release followed by eventual transport to and esterification of sterol in the ER. The steady state flux is calculated, and a thermodynamic analysis of feasibility is presented.

Results

It is shown that the steady state sterol flux across the PM can be entirely controlled by irreversible sterol import via Aus1/Pdr11. The transbilayer sterol flux at steady state is a non-linear function of the chemical potential difference of sterol between both leaflets. Non-vesicular release of sterol on the cytoplasmic side of the PM lowers the attainable sterol enrichment in the inner leaflet. Including complex formation of sterol with phospholipids or proteins can explain several puzzling experimental observations; 1) rapid sterol flip-flop across the PM despite net sterol enrichment in one leaflet, 2) a pronounced steady state sterol gradient between PM and ER despite fast non-vesicular sterol exchange between both compartments and 3) a non-linear dependence of ER sterol on ergosterol abundance in the PM.

Conclusions

A steady state model is presented that can account for the observed sterol asymmetry in the yeast PM, the strong sterol gradient between PM and ER and threshold-like expansion of ER sterol for increasing sterol influx into the PM. The model also provides new insight into selective uptake of cholesterol and its homeostasis in mammalian cells, and it provides testable predictions for future experiments.

Lipids and their (un)known effects on ER-associated protein degradation (ERAD)

Endoplasmic reticulum (ER)-associated protein degradation (ERAD) is a conserved cellular process that apart from protein quality control and maintenance of ER membrane identity has pivotal functions in regulating the lipid composition of the ER membrane. A general trigger for ERAD activation is the exposure of normally buried protein domains due to protein misfolding, absence of binding partners or conformational changes. Several feedback loops for ER lipid homeostasis exploit the induction of conformational changes in key enzymes of lipid biosynthesis or in ER membrane-embedded transcription factors upon shortage or abundance of specific lipids, leading to enzyme degradation or mobilization of transcription factors. Similarly, an insufficient amount of lipids triggers ERAD of apolipoproteins during lipoprotein formation. Lipids might even have a role in ER protein quality control: when proteins destined for ER export are covalently modified with lipids their ER residence time and their susceptibility to ERAD is reduced. Here we summarize and compare the various interconnections of lipids with ER membrane proteins and ERAD. This article is part of a Special Issue entitled Endoplasmic reticulum platforms for lipid dynamics edited by Shamshad Cockcroft and Christopher Stefan.

The Chemical Potential of Plasma Membrane Cholesterol: Implications for Cell Biology

Cholesterol is abundant in plasma membranes and exhibits a variety of interactions throughout the membrane. Chemical potential accounts for thermodynamic consequences of molecular interactions, and quantifies the effective concentration (i.e., activity) of any substance participating in a process. We have developed, to our knowledge, the first method to measure cholesterol chemical potential in plasma membranes. This was accomplished by complexing methyl-β-cyclodextrin with cholesterol in an aqueous solution and equilibrating it with an organic solvent containing dissolved cholesterol. The chemical potential of cholesterol was thereby equalized in the two phases. Because cholesterol is dilute in the organic phase, here activity and concentration were equivalent. This equivalence allowed the amount of cholesterol bound to methyl-β-cyclodextrin to be converted to cholesterol chemical potential. Our method was used to determine the chemical potential of cholesterol in erythrocytes and in plasma membranes of nucleated cells in culture. For erythrocytes, the chemical potential did not vary when the concentration was below a critical value. Above this value, the chemical potential progressively increased with concentration. We used standard cancer lines to characterize cholesterol chemical potential in plasma membranes of nucleated cells. This chemical potential was significantly greater for highly metastatic breast cancer cells than for nonmetastatic breast cancer cells. Chemical potential depended on density of the cancer cells. A method to alter and fix the cholesterol chemical potential to any value (i.e., a cholesterol chemical potential clamp) was also developed. Cholesterol content did not change when cells were clamped for 24-48 h. It was found that the level of activation of the transcription factor STAT3 increased with increasing cholesterol chemical potential. The cholesterol chemical potential may regulate signaling pathways.

The Potential of Isoprenoids in Adjuvant Cancer Therapy to Reduce Adverse Effects of Statins

The mevalonate pathway provides sterols for membrane structure and nonsterol intermediates for the post-translational modification and membrane anchorage of growth-related proteins, including the Ras, Rac, and Rho GTPase family. Mevalonate-derived products are also essential for the Hedgehog pathway, steroid hormone signaling, and the nuclear localization of Yes-associated protein and transcriptional co-activator with PDZ-binding motif, all of which playing roles in tumorigenesis and cancer stem cell function. The phosphatidylinositol-4,5-bisphosphate 3-kinase-AKT-mammalian target of rapamycin complex 1 pathway, p53 with gain-of-function mutation, and oncoprotein MYC upregulate the mevalonate pathway, whereas adenosine monophosphate-activated protein kinase and tumor suppressor protein RB are the downregulators. The rate-limiting enzyme, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), is under a multivalent regulation. Sterol regulatory element binding protein 2 mediates the sterol-controlled transcriptional downregulation of HMGCR. UbiA prenyltransferase domain-containing protein-1 regulates the ubiquitination and proteasome-mediated degradation of HMGCR, which is accelerated by 24, 25-dihydrolanosterol and the diterpene geranylgeraniol. Statins, competitive inhibitors of HMGCR, deplete cells of mevalonate-derived intermediates and consequently inhibit cell proliferation and induce apoptosis. Clinical application of statins is marred by dose-limiting toxicities and mixed outcomes on cancer risk, survival and mortality, partially resulting from the statin-mediated compensatory upregulation of HMGCR and indiscriminate inhibition of HMGCR in normal and tumor cells. Tumor HMGCR is resistant to the sterol-mediated transcriptional control; consequently, HMGCR is upregulated in cancers derived from adrenal gland, blood and lymph, brain, breast, colon, connective tissue, embryo, esophagus, liver, lung, ovary, pancreas, prostate, skin, and stomach. Nevertheless, tumor HMGCR remains sensitive to isoprenoid-mediated degradation. Isoprenoids including monoterpenes (carvacrol, L-carvone, geraniol, perillyl alcohol), sesquiterpenes (cacalol, farnesol, β-ionone), diterpene (geranylgeranyl acetone), “mixed” isoprenoids (tocotrienols), and their derivatives suppress the growth of tumor cells with little impact on non-malignant cells. In cancer cells derived from breast, colon, liver, mesothelium, prostate, pancreas, and skin, statins and isoprenoids, including tocotrienols, geraniol, limonene, β-ionone and perillyl alcohol, synergistically suppress cell proliferation and associated signaling pathways. A blend of dietary lovastatin and δ-tocotrienol, each at no-effect doses, suppress the growth of implanted murine B16 melanomas in C57BL6 mice. Isoprenoids have potential as adjuvant agents to reduce the toxicities of statins in cancer prevention or therapy.

Discovery of a potent HMG-CoA reductase degrader that eliminates statin-induced reductase accumulation and lowers cholesterol

Statins are inhibitors of HMG-CoA reductase, the rate-limiting enzyme of cholesterol biosynthesis, and have been clinically used to treat cardiovascular disease. However, a paradoxical increase of reductase protein following statin treatment may attenuate the effect and increase the side effects. Here we present a previously unexplored strategy to alleviate statin-induced reductase accumulation by inducing its degradation. Inspired by the observations that cholesterol intermediates trigger reductase degradation, we identify a potent degrader, namely Cmpd 81, through structure-activity relationship analysis of sterol analogs. Cmpd 81 stimulates ubiquitination and degradation of reductase in an Insig-dependent manner, thus dramatically reducing protein accumulation induced by various statins. Cmpd 81 can act alone or synergistically with statin to lower cholesterol and reduce atherosclerotic plaques in mice. Collectively, our work suggests that inducing reductase degradation by Cmpd 81 or similar chemicals alone or in combination with statin therapy can be a promising strategy for treating cardiovascular disease.

Molecular Pathways Underlying Cholesterol Homeostasis

Cholesterol is an essential molecule that exerts pleiotropic actions. Although its presence is vital to the cell, its excess can be harmful and, therefore, sustaining cholesterol homeostasis is crucial to maintaining proper cellular functioning. It is well documented that high plasma cholesterol concentration increases the risk of atherosclerotic heart disease. In the last decades, several studies have investigated the association of plasma cholesterol concentrations and the risk of cardiovascular diseases as well as the signaling pathways involved in cholesterol homeostasis. Here, we present an overview of several mechanisms involved in intestinal cholesterol absorption, the regulation of cholesterol synthesis and uptake. We also discuss the importance of reverse cholesterol transport and transintestinal cholesterol transport to maintain cholesterol homeostasis and prevent atherosclerosis development. Additionally, we discuss the influence of dietary cholesterol on plasma cholesterol concentration and the new recommendations for cholesterol intake in a context of a healthy dietary pattern.

Proteostatic Tactics in the Strategy of Sterol Regulation

In eukaryotes, the synthesis and uptake of sterols undergo stringent multivalent regulation. Both individual enzymes and transcriptional networks are controlled to meet changing needs of the many sterol pathway products. Regulation is tailored by evolution to match regulatory constraints, which can be very different in distinct species. Nevertheless, a broadly conserved feature of many aspects of sterol regulation is employment of proteostasis mechanisms to bring about control of individual proteins. Proteostasis is the set of processes that maintain homeostasis of a dynamic proteome. Proteostasis includes protein quality control pathways for the detection, and then the correction or destruction, of the many misfolded proteins that arise as an unavoidable feature of protein-based life. Protein quality control displays not only the remarkable breadth needed to manage the wide variety of client molecules, but also extreme specificity toward the misfolded variants of a given protein. These features are amenable to evolutionary usurpation as a means to regulate proteins, and this approach has been used in sterol regulation. We describe both well-trod and less familiar versions of the interface between proteostasis and sterol regulation and suggest some underlying ideas with broad biological and clinical applicability.

Ring finger protein 145 (RNF145) is a ubiquitin ligase for sterol-induced degradation of HMG-CoA reductase

Cholesterol biosynthesis is tightly regulated in the cell. For example, high sterol concentrations can stimulate degradation of the rate-limiting cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase, HMGCR). HMGCR is broken down by the endoplasmic reticulum membrane-associated protein complexes consisting of insulin-induced genes (Insigs) and the E3 ubiquitin ligase gp78. Here we found that HMGCR degradation is partially blunted in Chinese hamster ovary (CHO) cells lacking gp78 (gp78-KO). To identify other ubiquitin ligase(s) that may function together with gp78 in triggering HMGCR degradation, we performed a small-scale short hairpin RNA-based screening targeting endoplasmic reticulum-localized E3s. We found that knockdown of both ring finger protein 145 (Rnf145) and gp78 genes abrogates sterol-induced degradation of HMGCR in CHO cells. We also observed that RNF145 interacts with Insig-1 and -2 proteins and ubiquitinates HMGCR. Moreover, the tetrapeptide sequence YLYF in the sterol-sensing domain and the Cys-537 residue in the RING finger domain were essential for RNF145 binding to Insigs and RNF145 E3 activity, respectively. Of note, amino acid substitutions in the YLYF or of Cys-537 completely abolished RNF145-mediated HMGCR degradation. In summary, our study reveals that RNF145, along with gp78, promotes HMGCR degradation in response to elevated sterol levels and identifies residues essential for RNF145 function.

Sterol Metabolism and Transport in Atherosclerosis and Cancer

Cholesterol is a vital lipid molecule for mammalian cells, regulating fluidity of biological membranes, and serving as an essential constituent of lipid rafts. Mammalian cells acquire cholesterol from extracellular lipoproteins and from de novo synthesis. Cholesterol biosynthesis generates various precursor sterols. Cholesterol undergoes metabolic conversion into oxygenated sterols (oxysterols), bile acids, and steroid hormones. Cholesterol intermediates and metabolites have diverse and important cellular functions. A network of molecular machineries including transcription factors, protein modifiers, sterol transporters/carriers, and sterol sensors regulate sterol homeostasis in mammalian cells and tissues. Dysfunction in metabolism and transport of cholesterol, sterol intermediates, and oxysterols occurs in various pathophysiological settings such as atherosclerosis, cancers, and neurodegenerative diseases. Here we review the cholesterol, intermediate sterol, and oxysterol regulatory mechanisms and intracellular transport machineries, and discuss the roles of sterols and sterol metabolism in human diseases.

Hypoxia-inducible factor 1α activates insulin-induced gene 2 (Insig-2) transcription for degradation of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase in the liver

Cholesterol synthesis is a highly oxygen-consuming process. As such, oxygen deprivation (hypoxia) limits cholesterol synthesis through incompletely understood mechanisms mediated by the oxygen-sensitive transcription factor hypoxia-inducible factor 1α (HIF-1α). We show here that HIF-1α links pathways for oxygen sensing and feedback control of cholesterol synthesis in human fibroblasts by directly activating transcription of the INSIG-2 gene. Insig-2 is one of two endoplasmic reticulum membrane proteins that inhibit cholesterol synthesis by mediating sterol-induced ubiquitination and subsequent endoplasmic reticulum-associated degradation of the rate-limiting enzyme in the pathway, HMG-CoA reductase (HMGCR). Consistent with the results in cultured cells, hepatic levels of Insig-2 mRNA were enhanced in mouse models of hypoxia. Moreover, pharmacologic stabilization of HIF-1α in the liver stimulated HMGCR degradation via a reaction that requires the protein's prior ubiquitination and the presence of the Insig-2 protein. In summary, our results show that HIF-1α activates INSIG-2 transcription, leading to accumulation of Insig-2 protein, which binds to HMGCR and triggers its accelerated ubiquitination and degradation. These results indicate that HIF-mediated induction of Insig-2 and degradation of HMGCR are physiologically relevant events that guard against wasteful oxygen consumption and inappropriate cell growth during hypoxia.

27-Hydroxycholesterol regulates cholesterol synthesis and transport in C6 glioma cells

The oxysterol 27-Hydroxycholesterol (27-OHC) is a major cholesterol metabolite that can cross the blood brain barrier (BBB) from peripheral circulation to the brain. Currently, the role of 27-OHC on cholesterol homeostasis in astrocytes and the underlying mechanisms are not defined. Since all brain cholesterol is essentially synthesized in brain itself and astrocytes as net producers of cholesterol are essential for normal brain function, here we investigated the effects of 27-OHC on cholesterol synthesis and transport in C6 glioma cells. C6 cells were treated with 5, 10 and 20μM 27-OHC for 24h and the cell viability and apoptosis, the cholesterol levels and metabolism-related mediators, genes and proteins were subsequently assessed using cell-counting kit (CCK)-8, Amplex red, ELISA, real-time PCR and Western blot, respectively. We found that 27-OHC decreased cholesterol levels by down-regulating the expression of sterol-regulated element binding protein-1 (SREBP-1a), 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CR) and low density lipoprotein receptor (LDLR) and promoted cholesterol transport by up-regulating the expression of peroxisome proliferator-activated receptors-γ (PPAR-γ), liver X receptor-α (LXR-α), ATP-binding cassette transporter protein family member A1 (ABCA1) and apolipoprotein E (ApoE)genes. Our results suggested that 27-OHC may represent a sensitive modulator of cholesterol metabolism disorder by suppressing cholesterol synthesis and stimulating cholesterol transport in astrocytes.

Endoplasmic Reticulum-Associated Degradation and Lipid Homeostasis

The endoplasmic reticulum is the port of entry for proteins into the secretory pathway and the site of synthesis for several important lipids, including cholesterol, triacylglycerol, and phospholipids. Protein production within the endoplasmic reticulum is tightly regulated by a cohort of resident machinery that coordinates the folding, modification, and deployment of secreted and integral membrane proteins. Proteins failing to attain their native conformation are degraded through the endoplasmic reticulum-associated degradation (ERAD) pathway via a series of tightly coupled steps: substrate recognition, dislocation, and ubiquitin-dependent proteasomal destruction. The same ERAD machinery also controls the flux through various metabolic pathways by coupling the turnover of metabolic enzymes to the levels of key metabolites. We review the current understanding and biological significance of ERAD-mediated regulation of lipid metabolism in mammalian cells.

Imaging approaches for analysis of cholesterol distribution and dynamics in the plasma membrane

Cholesterol is an important lipid component of the plasma membrane (PM) of mammalian cells, where it is involved in control of many physiological processes, such as endocytosis, cell migration, cell signalling and surface ruffling. In an attempt to explain these functions of cholesterol, several models have been put forward about cholesterol's lateral and transbilayer organization in the PM. In this article, we review imaging techniques developed over the last two decades for assessing the distribution and dynamics of cholesterol in the PM of mammalian cells. Particular focus is on fluorescence techniques to study the lateral and inter-leaflet distribution of suitable cholesterol analogues in the PM of living cells. We describe also several methods for determining lateral cholesterol dynamics in the PM including fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), single particle tracking (SPT) and spot variation FCS coupled to stimulated emission depletion (STED) microscopy. For proper interpretation of such measurements, we provide some background in probe photophysics and diffusion phenomena occurring in cell membranes. In particular, we show the equivalence of the reaction-diffusion approach, as used in FRAP and FCS, and continuous time random walk (CTRW) models, as often invoked in SPT studies. We also discuss mass spectrometry (MS) based imaging of cholesterol in the PM of fixed cells and compare this method with fluorescence imaging of sterols. We conclude that evidence from many experimental techniques converges towards a model of a homogeneous distribution of cholesterol with largely free and unhindered diffusion in both leaflets of the PM.

Cholesterol homeostasis: How do cells sense sterol excess?

Cholesterol is vital in mammals, but toxic in excess. Consequently, elaborate molecular mechanisms have evolved to maintain this sterol within narrow limits. How cells sense excess cholesterol is an intriguing area of research. Cells sense cholesterol, and other related sterols such as oxysterols or cholesterol synthesis intermediates, and respond to changing levels through several elegant mechanisms of feedback regulation. Cholesterol sensing involves both direct binding of sterols to the homeostatic machinery located in the endoplasmic reticulum (ER), and indirect effects elicited by sterol-dependent alteration of the physical properties of membranes. Here, we examine the mechanisms employed by cells to maintain cholesterol homeostasis.

Cholesterol-mediated Degradation of 7-Dehydrocholesterol Reductase Switches the Balance from Cholesterol to Vitamin D Synthesis

Cholesterol is detrimental to human health in excess but is also essential for normal embryogenesis. Hence, enzymes involved in its synthesis possess many layers of regulation to achieve balanced cholesterol levels. 7-Dehydrocholesterol reductase (DHCR7) is the terminal enzyme of cholesterol synthesis in the Kandutsch-Russell pathway, converting 7-dehydrocholesterol (7DHC) to cholesterol. In the absence of functional DHCR7, accumulation of 7DHC and a lack of cholesterol production leads to the devastating developmental disorder, Smith-Lemli-Opitz syndrome. This study identifies that statin treatment can ameliorate the low DHCR7 expression seen with common Smith-Lemli-Opitz syndrome mutations. Furthermore, we show that wild-type DHCR7 is also relatively labile. In an example of end-product inhibition, cholesterol accelerates the proteasomal degradation of DHCR7, resulting in decreased protein levels and activity. The loss of enzymatic activity results in the accumulation of the substrate 7DHC, which leads to an increased production of vitamin D. Thus, these findings highlight DHCR7 as an important regulatory switch between cholesterol and vitamin D synthesis.

Active membrane cholesterol as a physiological effector

Sterols associate preferentially with plasma membrane sphingolipids and saturated phospholipids to form stoichiometric complexes. Cholesterol in molar excess of the capacity of these polar bilayer lipids has a high accessibility and fugacity; we call this fraction active cholesterol. This review first considers how active cholesterol serves as an upstream regulator of cellular sterol homeostasis. The mechanism appears to utilize the redistribution of active cholesterol down its diffusional gradient to the endoplasmic reticulum and mitochondria, where it binds multiple effectors and directs their feedback activity. We have also reviewed a broad literature in search of a role for active cholesterol (as opposed to bulk cholesterol or lipid domains such as rafts) in the activity of diverse membrane proteins. Several systems provide such evidence, implicating, in particular, caveolin-1, various kinds of ABC-type cholesterol transporters, solute transporters, receptors and ion channels. We suggest that this larger role for active cholesterol warrants close attention and can be tested easily.

ABCA1-dependent sterol release: sterol molecule specificity and potential membrane domain for HDL biogenesis

Mammalian cells synthesize various sterol molecules, including the C30 sterol, lanosterol, as cholesterol precursors in the endoplasmic reticulum. The build-up of precursor sterols, including lanosterol, displays cellular toxicity. Precursor sterols are found in plasma HDL. How these structurally different sterols are released from cells is poorly understood. Here, we show that newly synthesized precursor sterols arriving at the plasma membrane (PM) are removed by extracellular apoA-I in a manner dependent on ABCA1, a key macromolecule for HDL biogenesis. Analysis of sterol molecules by GC-MS and tracing the fate of radiolabeled acetate-derived sterols in normal and mutant Niemann-Pick type C cells reveal that ABCA1 prefers newly synthesized sterols, especially lanosterol, as the substrates before they are internalized from the PM. We also show that ABCA1 resides in a cholesterol-rich membrane domain resistant to the mild detergent, Brij 98. Blocking ACAT activity increases the cholesterol contents of this domain. Newly synthesized C29/C30 sterols are transiently enriched within this domain, but rapidly disappear from this domain with a half-life of less than 1 h. Our work shows that substantial amounts of precursor sterols are transported to a certain PM domain and are removed by the ABCA1-dependent pathway.

Deficiency in the Lipid Exporter ABCA1 Impairs Retrograde Sterol Movement and Disrupts Sterol Sensing at the Endoplasmic Reticulum

Cellular cholesterol homeostasis involves sterol sensing at the endoplasmic reticulum (ER) and sterol export from the plasma membrane (PM). Sterol sensing at the ER requires efficient sterol delivery from the PM; however, the macromolecules that facilitate retrograde sterol transport at the PM have not been identified. ATP-binding cassette transporter A1 (ABCA1) mediates cholesterol and phospholipid export to apolipoprotein A-I for the assembly of high density lipoprotein (HDL). Mutations in ABCA1 cause Tangier disease, a familial HDL deficiency. Several lines of clinical and experimental evidence suggest a second function of ABCA1 in cellular cholesterol homeostasis in addition to mediating cholesterol efflux. Here, we report the unexpected finding that ABCA1 also plays a key role in facilitating retrograde sterol transport from the PM to the ER for sterol sensing. Deficiency in ABCA1 delays sterol esterification at the ER and activates the SREBP-2 cleavage pathway. The intrinsic ATPase activity in ABCA1 is required to facilitate retrograde sterol transport. ABCA1 deficiency causes alternation of PM composition and hampers a clathrin-independent endocytic activity that is required for ER sterol sensing. Our finding identifies ABCA1 as a key macromolecule facilitating bidirectional sterol movement at the PM and shows that ABCA1 controls retrograde sterol transport by modulating a certain clathrin-independent endocytic process.

Flux analysis of cholesterol biosynthesis in vivo reveals multiple tissue and cell-type specific pathways

Two parallel pathways produce cholesterol: the Bloch and Kandutsch-Russell pathways. Here we used stable isotope labeling and isotopomer analysis to trace sterol flux through the two pathways in mice. Surprisingly, no tissue used the canonical K–R pathway. Rather, a hybrid pathway was identified that we call the modified K–R (MK–R) pathway. Proportional flux through the Bloch pathway varied from 8% in preputial gland to 97% in testes, and the tissue-specificity observed in vivo was retained in cultured cells. The distribution of sterol isotopomers in plasma mirrored that of liver. Sterol depletion in cultured cells increased flux through the Bloch pathway, whereas overexpression of 24-dehydrocholesterol reductase (DHCR24) enhanced usage of the MK–R pathway. Thus, relative use of the Bloch and MK–R pathways is highly variable, tissue-specific, flux dependent, and epigenetically fixed. Maintenance of two interdigitated pathways permits production of diverse bioactive sterols that can be regulated independently of cholesterol.

DOI:http://dx.doi.org/10.7554/eLife.07999.001

Cholesterol is important for animals, both as an essential component of the membrane that surrounds cells and as a building block to make hormones and other biologically important molecules. However, cells limit how much cholesterol they make because an excess of this fatty molecule can cause serious health problems, including heart disease and stroke.

Cholesterol is made via a complex process that involves more than 30 different steps, which can be organized into two biochemical pathways (named the Bloch pathway and the Kandutsch–Russell pathway). The enzymes that carry out the steps in these pathways have been characterized in detail. Less is known about which of the two pathways is actually used in different cells and tissues, or how much cholesterol each pathway produces. This is partly because it is difficult to distinguish between the closely related intermediate molecules that are formed in each pathway.

Mitsche et al. have now used mass spectrometry and isotope labeling techniques to analyze the relative contributions of the two cholesterol-making pathways in both cells grown in the laboratory and in mice. The experiments show that many cells use the Bloch pathway. However, no cells were found to use the Kandutsch–Russell pathway as it was originally described. Rather, some of the cells used a hybrid pathway where the production of cholesterol was started using the Bloch pathway and then after a certain number of steps, the process switched to using part of the Kandutsch–Russell pathway. Mitsche et al. referred to this mixed system as the ‘modified Kandutsch–Russell pathway’.

Mitsche et al. next examined the flow of molecules through these two pathways in different tissues and observed that the Bloch pathway is exclusively used in the testes and adrenal glands, which produce high levels of cholesterol. In contrast, the skin and brain use the modified Kandutsch–Russell pathway. In some tissues, a fraction of the building blocks that can be used to make cholesterol were instead diverted to make other products. This suggests that animals have maintained the two pathways over the course of evolution to enable them to generate a variety of products, which can be used to carry out different biological processes. One challenge following this work will be to use the newly developed methods to analyze other complex biochemical pathways.

DOI:http://dx.doi.org/10.7554/eLife.07999.002

snoRNA U17 regulates cellular cholesterol trafficking

Cholesterol is required for the growth and viability of mammalian cells and is an obligate precursor for steroid hormone synthesis. Using a loss-of-function screen for mutants with defects in intracellular cholesterol trafficking, a Chinese hamster ovary cell mutant with haploinsufficiency of the U17 snoRNA was isolated. U17 is an H/ACA orphan snoRNA, for which a function other than ribosomal processing has not previously been identified. Through expression profiling, we identified hypoxia-upregulated mitochondrial movement regulator (HUMMR) mRNA as a target that is negatively regulated by U17 snoRNA. Upregulation of HUMMR in U17 snoRNA-deficient cells promoted the formation of ER-mitochondrial contacts, decreasing esterification of cholesterol and facilitating cholesterol trafficking to mitochondria. U17 snoRNA and HUMMR regulate mitochondrial synthesis of steroids in vivo and are developmentally regulated in steroidogenic tissues, suggesting that the U17 snoRNA-HUMMR pathway may serve a previously unrecognized, physiological role in gonadal tissue maturation.

27-Hydroxycholesterol contributes to disruptive effects on learning and memory by modulating cholesterol metabolism in the rat brain

Cholesterol metabolism is important for neuronal function in the central nervous system (CNS). The oxysterol 27-hydroxycholesterol (27-OHC) is a cholesterol metabolite that crosses the blood-brain barrier (BBB) and may be a useful substitutive marker for neurodegenerative diseases. However, the effects of 27-OHC on learning and memory and the underlying mechanisms are unclear. To determine this mechanism, we investigated learning and memory and cholesterol metabolism in rat brain following the injection of various doses of 27-OHC into the caudal vein. We found that 27-OHC increased cholesterol levels and upregulated the expression of liver X receptor-α (LXR-α) and adenosine triphosphate (ATP)-binding cassette transporter protein family member A1 (ABCA1). In addition, 27-OHC decreased the expression of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CR) and low-density lipoprotein receptor (LDLR) in rat brain tissues. These findings suggest that 27-OHC may negatively modulate cognitive effects and cholesterol metabolism in the brain.

Obesity, cholesterol metabolism, and breast cancer pathogenesis

Obesity and altered lipid metabolism are risk factors for breast cancer in pre- and post-menopausal women. These pathologic relationships have been attributed in part to the impact of cholesterol on the biophysical properties of cell membranes and to the influence of these changes on signaling events initiated at the membrane. However, more recent studies have indicated that the oxysterol 27-hydroxycholesterol (27HC), and not cholesterol per se, may be the primary biochemical link between lipid metabolism and cancer. The enzyme responsible for production of 27HC from cholesterol, CYP27A1, is expressed primarily in the liver and in macrophages. In addition, significantly elevated expression of this enzyme within breast tumors has also been observed. It is believed that 27HC, acting through the liver X receptor in macrophages and possibly other cells, is involved in maintaining organismal cholesterol homeostasis. It has also been shown recently that 27HC is an estrogen receptor agonist in breast cancer cells and that it stimulates the growth and metastasis of tumors in several models of breast cancer. These findings provide the rationale for the clinical evaluation of pharmaceutical approaches that interfere with cholesterol/27HC synthesis as a means to mitigate the impact of cholesterol on breast cancer pathogenesis. Cancer Res; 74(18); 4976-82. ©2014 AACR.

Genetic alterations affecting cholesterol metabolism and human fertility

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

Characterization of cholesterol homeostasis in telomerase-immortalized Tangier disease fibroblasts reveals marked phenotype variability

We compared the consequences of an ABCA1 mutation that produced an apparent lack of atherosclerosis (Tangier family 1, N935S) with an ABCA1 mutation with functional ABCA1 knockout that was associated with severe atherosclerosis (Tangier family 2, Leu(548):Leu(575)-End), using primary and telomerase-immortalized fibroblasts. Telomerase-immortalized Tangier fibroblasts of family 1 (TT1) showed 30% residual cholesterol efflux capacity in response to apolipoprotein A-I, whereas telomerase-immortalized Tangier fibroblasts of family 2 (TT2) showed only 20%. However, there were a number of secondary differences that were often stronger and may help to explain the more rapid development of atherosclerosis in family 2. First, the total cellular cholesterol content increase was 2-3-fold and 3-5-fold in TT1 and TT2 cells, respectively. The corresponding increase in esterified cholesterol concentration was 10- and 40-fold, respectively. Second, 24-, 25-, and 27-hydroxycholesterol concentrations were moderately increased in TT1 cells, but were increased as much as 200-fold in TT2 cells. Third, cholesterol biosynthesis was moderately decreased in TT1 cells, but was markedly decreased in TT2 cells. Fourth, potentially atheroprotective LXR-dependent SREBP1c signaling was normal in TT1, but was rather suppressed in TT2 cells. Cultivated primary Tangier fibroblasts were characterized by premature aging in culture and were associated with less obvious biochemical differences. In summary, these results may help to understand the differential atherosclerotic susceptibility in Tangier disease and further demonstrate the usefulness of telomerase-immortalized cells in studying this cellular phenotype. The data support the contention that side chain-oxidized oxysterols are strong suppressors of cholesterol biosynthesis under specific pathological conditions in humans.

Box C/D small nucleolar RNA (snoRNA) U60 regulates intracellular cholesterol trafficking

Mobilization of plasma membrane (PM) cholesterol to the endoplasmic reticulum is essential for cellular cholesterol homeostasis. The mechanisms regulating this retrograde, intermembrane cholesterol transfer are not well understood. Because mutant cells with defects in PM to endoplasmic reticulum cholesterol trafficking can be isolated on the basis of resistance to amphotericin B, we conducted an amphotericin B loss-of-function screen in Chinese hamster ovary (CHO) cells using insertional mutagenesis to identify genes that regulate this trafficking mechanism. Mutant line A1 displayed reduced cholesteryl ester formation from PM-derived cholesterol and increased de novo cholesterol synthesis, indicating a deficiency in retrograde cholesterol transport. Genotypic analysis revealed that the A1 cell line contained one disrupted allele of the U60 small nucleolar RNA (snoRNA) host gene, resulting in haploinsufficiency of the box C/D snoRNA U60. Complementation and mutational studies revealed the U60 snoRNA to be the essential feature from this locus that affects cholesterol trafficking. Lack of alteration in predicted U60-mediated site-directed methylation of 28 S rRNA in the A1 mutant suggests that the U60 snoRNA modulates cholesterol trafficking by a mechanism that is independent of this canonical function. Our study adds to a growing body of evidence for participation of small noncoding RNAs in cholesterol homeostasis and is the first to implicate a snoRNA in this cellular function.

Hepatic cholesterol homeostasis: is the low-density lipoprotein pathway a regulatory or a shunt pathway?

OBJECTIVE

The hypothesis that cholesterol that enters the cell within low-density lipoprotein (LDL) particles rapidly equilibrates with the regulatory pool of intracellular cholesterol and maintains cholesterol homeostasis by reducing cholesterol and LDL receptor synthesis was validated in the fibroblast but not in the hepatocyte. Accordingly, the present studies were designed to compare the effects of cholesterol that enters the hepatocyte within an LDL particle with those of cholesterol that enters via other lipoprotein particles.

APPROACH AND RESULTS

We measured cholesterol synthesis and esterification in hamster hepatocytes treated with LDL and other lipoprotein particles, including chylomicron remnants and VLDL. Endogenous cholesterol synthesis was not significantly reduced by uptake of LDL, but cholesterol esterification (280%) and acyl CoA:cholesterol acyltransferase 2 expression (870%) were increased. In contrast, cholesterol synthesis was significantly reduced (70% decrease) with other lipoprotein particles. Furthermore, more cholesterol that entered the hepatocyte within LDL particles was secreted within VLDL particles (480%) compared with cholesterol from other sources.

CONCLUSIONS

Much of the cholesterol that enters the hepatocyte within LDL particles is shunted through the cell and resecreted within VLDL particles without reaching equilibrium with the regulatory pool.

Sterol homeostasis requires regulated degradation of squalene monooxygenase by the ubiquitin ligase Doa10/Teb4

Sterol homeostasis is essential for the function of cellular membranes and requires feedback inhibition of HMGR, a rate-limiting enzyme of the mevalonate pathway. As HMGR acts at the beginning of the pathway, its regulation affects the synthesis of sterols and of other essential mevalonate-derived metabolites, such as ubiquinone or dolichol. Here, we describe a novel, evolutionarily conserved feedback system operating at a sterol-specific step of the mevalonate pathway. This involves the sterol-dependent degradation of squalene monooxygenase mediated by the yeast Doa10 or mammalian Teb4, a ubiquitin ligase implicated in a branch of the endoplasmic reticulum (ER)-associated protein degradation (ERAD) pathway. Since the other branch of ERAD is required for HMGR regulation, our results reveal a fundamental role for ERAD in sterol homeostasis, with the two branches of this pathway acting together to control sterol biosynthesis at different levels and thereby allowing independent regulation of multiple products of the mevalonate pathway. DOI:http://dx.doi.org/10.7554/eLife.00953.001.

Controlling cholesterol synthesis beyond 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR)

3-Hydroxy-3-methylglutaryl-CoA reductase (HMGCR) is the target of the statins, important drugs that lower blood cholesterol levels and treat cardiovascular disease. Consequently, the regulation of HMGCR has been investigated in detail. However, this enzyme acts very early in the cholesterol synthesis pathway, with ∼20 subsequent enzymes needed to produce cholesterol. How they are regulated is largely unexplored territory, but there is growing evidence that enzymes beyond HMGCR serve as flux-controlling points. Here, we introduce some of the known regulatory mechanisms affecting enzymes beyond HMGCR and highlight the need to further investigate their control.

Oxysterol generation and liver X receptor-dependent reverse cholesterol transport: not all roads lead to Rome

Cell cholesterol metabolism is a tightly regulated process, dependent in part on activation of nuclear liver X receptors (LXRs) to increase expression of genes mediating removal of excess cholesterol from cells in the reverse cholesterol transport pathway. LXRs are thought to be activated predominantly by oxysterols generated enzymatically from cholesterol in different cell organelles. Defects resulting in slowed release of cholesterol from late endosomes and lysosomes or reduction in sterol-27-hydroxylase activity lead to specific blocks in oxysterol production and impaired LXR-dependent gene activation. This block does not appear to be compensated by oxysterol production in other cell compartments. The purpose of this review is to summarize current knowledge about oxysterol-dependent activation by LXR of genes involved in reverse cholesterol transport, and what these defects of cell cholesterol homeostasis can teach us about the critical pathways of oxysterol generation for expression of LXR-dependent genes.

Oxysterols in cancer cell proliferation and death

Oxysterols have been shown to interfere with proliferation and cause the death of many cancer cell types, such as leukaemia, glioblastoma, colon, breast and prostate cancer cells, while they have little or no effect on senescent cells. The mechanisms by which oxysterols may influence proliferation are manifold: they control the transcription and the turnover of the key enzyme in cholesterol synthesis, 3-hydroxy-3-methylglutaryl CoA reductase, by binding to Insig-1, Insig-2 and liver X receptors. Oxysterols are thought to be generated in proportion to the rate of cholesterol synthesis. Although there is no consensus about the mechanism by which these oxysterols are generated in vivo, it clearly has to be ubiquitous. The 25- and the 27-cholesterol hydroxylases, present in almost all tissues, are possible candidates. Cholesterol uptake from lipoproteins, intracellular vesicle transport and lipid transfer are also modified by oxysterols. Oxysterols interfere with ERK, hedgehog and wnt pathways of proliferation and differentiation. When administered in vitro to cancer cell lines, oxysterols invariably both slow down proliferation and provoke cell death. Perhaps is it sufficient to stop proliferation of a cancer to provoke its eradication. Therefore, the two facets of oxysterol action that seem important for cancer treatment, cytostaticity and cytotoxicity, will be discussed.

The cholesterol derivative 27-hydroxycholesterol reduces steatohepatitis in mice

BACKGROUND & AIMS

Non-alcoholic steatohepatitis is characterized by hepatic steatosis with inflammation. Although steatosis is benign and reversible, inflammation can increase liver damage. Hepatic inflammation has been associated with accumulation of cholesterol in lysosomes of Kupffer cells. 27-Hydroxycholesterol (27HC), a derivative of cholesterol formed by CYP27A1, can mobilize cholesterol from the lysosomes to the cytoplasm. We investigated whether 27HC can change the intracellular distribution cholesterol and reduce hepatic inflammation in mice.

METHODS

We transplanted bone marrow from irradiated wild-type or Cyp27a1(-/-) mice to mice that do not express the low density lipoprotein receptor (Ldlr(-/-)), which are hyperlipidemic; 9 weeks later, mice were fed either regular chow or a high-fat, high-cholesterol (HFC) diet for 3 months. In a separate experiment, Ldlr(-/-) mice were given subcutaneous injections of 27HC and placed on regular chow or HFC diets for 3 weeks. Blood and liver tissues samples were collected and analyzed for intracellular cholesterol distribution and inflammation.

RESULTS

In Ldlr(-/-) mice that received bone marrow transplants from Cyp27a1(-/-) mice, lysosomes of Kupfer cells had a greater accumulation of cholesterol than those of mice that received bone marrow from wild-type mice, after the HFC diet. Liver histology and gene expression analyses showed increased inflammation and liver damage in mice given bone marrow transplants from Cyp27a1(-/-) mice and placed on the HFC diet. Administration of 27HC to Ldlr(-/-) mice, following the HFC diet, reduced the accumulation of lysosomal cholesterol and hepatic inflammation, compared with mice that were not given 27HC.

CONCLUSIONS

Accumulation of cholesterol in lysosomes of Kupfer cells promotes hepatic inflammation in mice. The cholesterol derivative 27HC reduces accumulation of cholesterol in lysosomes and might be used to treat non-alcoholic steatohepatitis.

Mechanisms of oxysterol-induced disease: insights from the biliary system

Oxysterols are oxidized species of cholesterol that are derived from exogenous (e.g. dietary) and endogenous (in vivo) sources. Oxysterols play critical roles in normal physiologic functions as well as in pathophysiologic processes in a variety of organ systems. This review provides an overview of oxysterol biology from the vantage point of the biliary system. Several oxysterols have been identified in human bile in the context of biliary tract infection and inflammation. This finding has led to investigations regarding the potential pathophysiologic significance of biliary oxysterols in diseases affecting the biliary system, with an emphasis on cholangiocarcinoma. Emerging evidence implicates specific oxysterols in the development and progression of this malignancy. This review will summarize the literature on oxysterols in the biliary system and discuss how the accumulated evidence contributes to a hypothesis describing the molecular basis of cholangiocarcinogenesis.

Cholesterol through the looking glass: ability of its enantiomer also to elicit homeostatic responses

How cholesterol is sensed to maintain homeostasis has been explained by direct binding to a specific protein, Scap, or through altering the physical properties of the membrane. The enantiomer of cholesterol (ent-cholesterol) is a valuable tool in distinguishing between these two models because it shares nonspecific membrane effects with native cholesterol (nat-cholesterol), but not specific binding interactions. This is the first study to compare ent- and nat-cholesterol directly on major molecular parameters of cholesterol homeostasis. We found that ent-cholesterol suppressed activation of the master transcriptional regulator of cholesterol metabolism, SREBP-2, almost as effectively as nat-cholesterol. Importantly, ent-cholesterol induced a conformational change in the cholesterol-sensing protein Scap in isolated membranes in vitro, even when steps were taken to eliminate potential confounding effects from endogenous cholesterol. Ent-cholesterol also accelerated proteasomal degradation of the key cholesterol biosynthetic enzyme, squalene monooxygenase. Together, these findings provide compelling evidence that cholesterol maintains its own homeostasis not only via direct protein interactions, but also by altering membrane properties.