Loss of transcriptional repression of three sterol-regulated genes in mutant hamster cells

A cholesterol-binding bacterial toxin provides a strategy for identifying a specific Scap inhibitor that blocks lipid synthesis in animal cells

Lipid synthesis is regulated by the actions of Scap, a polytopic membrane protein that binds cholesterol in membranes of the endoplasmic reticulum (ER). When ER cholesterol levels are low, Scap activates SREBPs, transcription factors that upregulate genes for synthesis of cholesterol, fatty acids, and triglycerides. When ER cholesterol levels rise, the sterol binds to Scap, triggering conformational changes that prevent activation of SREBPs and halting synthesis of lipids. To achieve a molecular understanding of how cholesterol regulates the Scap/SREBP machine and to identify therapeutics for dysregulated lipid metabolism, cholesterol-mimetic compounds that specifically bind and inhibit Scap are needed. To accomplish this goal, we focused on Anthrolysin O (ALO), a pore-forming bacterial toxin that binds cholesterol with a specificity and sensitivity that is uncannily similar to Scap. We reasoned that a small molecule that would bind and inhibit ALO might also inhibit Scap. High-throughput screening of a ~300,000-compound library for ALO-binding unearthed one molecule, termed UT-59, which binds to Scap's cholesterol-binding site. Upon binding, UT-59 triggers the same conformation changes in Scap as those induced by cholesterol and blocks activation of SREBPs and lipogenesis in cultured cells. UT-59 also inhibits SREBP activation in the mouse liver. Unlike five previously reported inhibitors of SREBP activation, UT-59 is the only one that acts specifically by binding to Scap's cholesterol-binding site. Our approach to identify specific Scap inhibitors such as UT-59 holds great promise in developing therapeutic leads for human diseases stemming from elevated SREBP activation, such as fatty liver and certain cancers.

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.

Sterols in an intramolecular channel of Smoothened mediate Hedgehog signaling

Smoothened (SMO), a class Frizzled G protein-coupled receptor (class F GPCR), transduces the Hedgehog signal across the cell membrane. Sterols can bind to its extracellular cysteine-rich domain (CRD) and to several sites in the seven transmembrane helices (7-TMs) of SMO. However, the mechanism by which sterols regulate SMO via multiple sites is unknown. Here we determined the structures of SMO-Gi complexes bound to the synthetic SMO agonist (SAG) and to 24(S),25-epoxycholesterol (24(S),25-EC). A novel sterol-binding site in the extracellular extension of TM6 was revealed to connect other sites in 7-TMs and CRD, forming an intramolecular sterol channel from the middle side of 7-TMs to CRD. Additional structures of two gain-of-function variants, SMOD384R and SMOG111C/I496C, showed that blocking the channel at its midpoints allows sterols to occupy the binding sites in 7-TMs, thereby activating SMO. These data indicate that sterol transport through the core of SMO is a major regulator of SMO-mediated signaling.

Dipyridamole Inhibits Lipogenic Gene Expression by Retaining SCAP-SREBP in the Endoplasmic Reticulum

Sterol regulatory element-binding proteins (SREBPs) are master transcriptional regulators of the mevalonate pathway and lipid metabolism and represent an attractive therapeutic target for lipid metabolic disorders. SREBPs are maintained in the endoplasmic reticulum (ER) in a tripartite complex with SREBP cleavage-activating protein (SCAP) and insulin-induced gene protein (INSIG). When new lipid synthesis is required, the SCAP-SREBP complex dissociates from INSIG and undergoes ER-to-Golgi transport where the N-terminal transcription factor domain is released by proteolysis. The mature transcription factor translocates to the nucleus and stimulates expression of the SREBP gene program. Previous studies showed that dipyridamole, a clinically prescribed phosphodiesterase (PDE) inhibitor, potentiated statin-induced tumor growth inhibition. Dipyridamole limited nuclear accumulation of SREBP, but the mechanism was not well resolved. In this study, we show that dipyridamole selectively blocks ER-to-Golgi movement of the SCAP-SREBP complex and that this is independent of its PDE inhibitory activity.

Oxysterols provide innate immunity to bacterial infection by mobilizing cell surface accessible cholesterol

Cholesterol 25-hydroxylase (CH25H) is an interferon-stimulated gene that converts cholesterol to the oxysterol 25-hydroxycholesterol (25HC). Circulating 25HC modulates essential immunological processes including antiviral immunity, inflammasome activation and antibody class switching; and dysregulation of CH25H may contribute to chronic inflammatory disease and cancer. Although 25HC is a potent regulator of cholesterol storage, uptake, efflux and biosynthesis, how these metabolic activities reprogram the immunological state of target cells remains poorly understood. Here, we used recently designed toxin-based biosensors that discriminate between distinct pools of plasma membrane cholesterol to elucidate how 25HC prevents Listeria monocytogenes from traversing the plasma membrane of infected host cells. The 25HC-mediated activation of acyl-CoA:cholesterol acyltransferase (ACAT) triggered rapid internalization of a biochemically defined fraction of cholesterol, termed 'accessible' cholesterol, from the plasma membrane while having little effect on cholesterol in complexes with sphingomyelin. We show that evolutionarily distinct bacterial species, L. monocytogenes and Shigella flexneri, exploit the accessible pool of cholesterol for infection and that acute mobilization of this pool by oxysterols confers immunity to these pathogens. The significance of this signal-mediated membrane remodelling pathway probably extends beyond host defence systems, as several other biologically active oxysterols also mobilize accessible cholesterol through an ACAT-dependent mechanism.

Twin enzymes, divergent control: The cholesterogenic enzymes DHCR14 and LBR are differentially regulated transcriptionally and post-translationally

Cholesterol synthesis is a tightly regulated process, both transcriptionally and post-translationally. Transcriptional control of cholesterol synthesis is relatively well-understood. However, of the ∼20 enzymes in cholesterol biosynthesis, post-translational regulation has only been examined for a small number. Three of the four sterol reductases in cholesterol production, 7-dehydrocholesterol reductase (DHCR7), 14-dehydrocholesterol reductase (DHCR14), and lamin-B receptor (LBR), share evolutionary ties with a high level of sequence homology and predicted structural homology. DHCR14 and LBR uniquely share the same Δ-14 reductase activity in cholesterol biosynthesis, yet little is known about their post-translational regulation. We have previously identified specific modes of post-translational control of DHCR7, but it is unknown whether these regulatory mechanisms are shared by DHCR14 and LBR. Using CHO-7 cells stably expressing epitope-tagged DHCR14 or LBR, we investigated the post-translational regulation of these enzymes. We found that DHCR14 and LBR undergo differential post-translational regulation, with DHCR14 being rapidly turned over, triggered by cholesterol and other sterol intermediates, whereas LBR remained stable. DHCR14 is degraded via the ubiquitin-proteasome system, and we identified several DHCR14 and DHCR7 putative interaction partners, including a number of E3 ligases that modulate DHCR14 levels. Interestingly, we found that gene expression across an array of human tissues showed a negative relationship between the C14-sterol reductases; one enzyme or the other tends to be predominantly expressed in each tissue. Overall, our findings indicate that whereas LBR tends to be the constitutively active C14-sterol reductase, DHCR14 levels are tunable, responding to the local cellular demands for cholesterol.

Structural basis for itraconazole-mediated NPC1 inhibition

Niemann-Pick C1 (NPC1), a lysosomal protein of 13 transmembrane helices (TMs) and three lumenal domains, exports low-density-lipoprotein (LDL)-derived cholesterol from lysosomes. TMs 3–7 of NPC1 comprise the Sterol-Sensing Domain (SSD). Previous studies suggest that mutation of the NPC1-SSD or the addition of the anti-fungal drug itraconazole abolishes NPC1 activity in cells. However, the itraconazole binding site and the mechanism of NPC1-mediated cholesterol transport remain unknown. Here, we report a cryo-EM structure of human NPC1 bound to itraconazole, which reveals how this binding site in the center of NPC1 blocks a putative lumenal tunnel linked to the SSD. Functional assays confirm that blocking this tunnel abolishes NPC1-mediated cholesterol egress. Intriguingly, the palmitate anchor of Hedgehog occupies a similar site in the homologous tunnel of Patched, suggesting a conserved mechanism for sterol transport in this family of proteins and establishing a central function of their SSDs.

Niemann-Pick C1 (NPC1) exports low-density-lipoprotein (LDL)-derived cholesterol from lysosomes and comporses a Sterol-Sensing Domain (SSD). Here authors report a cryo-EM structure of human NPC1 bound to itraconazole which reveals how this binding site in the center of NPC1 blocks a putative lumenal tunnel linked to the SSD.

Valosin-containing protein mediates the ERAD of squalene monooxygenase and its cholesterol-responsive degron

Squalene monooxygenase (SM) is an essential rate-limiting enzyme in cholesterol synthesis. SM degradation is accelerated by excess cholesterol, and this requires the first 100 amino acids of SM (SM N100). This process is part of a protein quality control pathway called endoplasmic reticulum-associated degradation (ERAD). In ERAD, SM is ubiquitinated by MARCH6, an E3 ubiquitin ligase located in the endoplasmic reticulum (ER). However, several details of the ERAD process for SM remain elusive, such as the extraction mechanism from the ER membrane. Here, we used SM N100 fused to GFP (SM N100-GFP) as a model degron to investigate the extraction process of SM in ERAD. We showed that valosin-containing protein (VCP) is important for the cholesterol-accelerated degradation of SM N100-GFP and SM. In addition, we revealed that VCP acts following ubiquitination of SM N100-GFP by MARCH6. We demonstrated that the amphipathic helix (Gln62–Leu73) of SM N100-GFP is critical for regulation by VCP and MARCH6. Replacing this amphipathic helix with hydrophobic re-entrant loops promoted degradation in a VCP-dependent manner. Finally, we showed that inhibiting VCP increases cellular squalene and cholesterol levels, indicating a functional consequence for VCP in regulating the cholesterol synthesis pathway. Collectively, we established VCP plays a key role in ERAD that contributes to the cholesterol-mediated regulation of SM.

Lysosomal cholesterol export reconstituted from fragments of Niemann-Pick C1

Niemann-Pick C1 (NPC1) is a polytopic membrane protein with 13 transmembrane helices that exports LDL-derived cholesterol from lysosomes by carrying it through the 80 Å glycocalyx and the 40 Å lipid bilayer. Transport begins when cholesterol binds to the N-terminal domain (NTD) of NPC1, which projects to the surface of the glycocalyx. Here, we reconstitute cholesterol transport by expressing the NTD as a fragment separate from the remaining portion of NPC1. When co-expressed, the two NPC1 fragments reconstitute cholesterol transport, indicating that the NTD has the flexibility to interact with the remaining parts of NPC1 even when not covalently linked. We also show that cholesterol can be transferred from the NTD of one full-length NPC1 to another NPC1 molecule that lacks the NTD. These data support the hypothesis that cholesterol is transported through interactions between two or more NPC1 molecules.

Manipulating Cholesterol Status Within Cells

Cellular cholesterol levels are intricately controlled to maintain homeostasis. Here, we describe ways in which cellular cholesterol status can be manipulated for the study of cholesterol homeostasis, including sterol starvation (by culturing cells in lipoprotein-deficient serum and pretreating/treating with the cholesterol-lowering drug, statin) and sterol enrichment (using cholesterol complexed to cyclodextrin, and low-density lipoprotein). We also describe how to prepare lipoprotein-deficient serum and complex cholesterol to cyclodextrin.

Phosphorylation regulates activity of 7-dehydrocholesterol reductase (DHCR7), a terminal enzyme of cholesterol synthesis

Cholesterol is essential for survival, but too much or too little can cause disease. Thus, cholesterol levels must be kept within close margins. 7-dehydrocholesterol reductase (DHCR7) is a terminal enzyme of cholesterol synthesis, and is essential for embryonic development. Largely, DHCR7 research is associated with the developmental disease Smith-Lemli-Opitz syndrome, which is caused by mutations in the DHCR7 gene. However, little is known about what regulates DHCR7 activity. Here we provide evidence that phosphorylation plays a role in controlling DHCR7 activity, which may provide a means to divert flux from cholesterol synthesis to vitamin D production. DHCR7 activity was significantly decreased when we used pharmacological inhibitors against two important kinases, AMP-activated protein kinase and protein kinase A. Moreover, mutating a known phosphorylated residue, S14, also decreased DHCR7 activity. Thus, we demonstrate that phosphorylation modulates DHCR7 activity in cells, and contributes to the overall synthesis of cholesterol, and probably vitamin D.

Triazoles inhibit cholesterol export from lysosomes by binding to NPC1

Niemann-Pick C1 (NPC1), a membrane protein of lysosomes, is required for the export of cholesterol derived from receptor-mediated endocytosis of LDL. Lysosomal cholesterol export is reportedly inhibited by itraconazole, a triazole that is used as an antifungal drug [Xu et al. (2010) Proc Natl Acad Sci USA 107:4764-4769]. Here we show that posaconazole, another triazole, also blocks cholesterol export from lysosomes. We prepared P-X, a photoactivatable cross-linking derivative of posaconazole. P-X cross-linked to NPC1 when added to intact cells. Cross-linking was inhibited by itraconazole but not by ketoconazole, an imidazole that does not block cholesterol export. Cross-linking of P-X was also blocked by U18666A, a compound that has been shown to bind to NPC1 and inhibit cholesterol export. P-X also cross-linked to purified NPC1 that was incorporated into lipid bilayer nanodiscs. In this in vitro system, cross-linking of P-X was inhibited by itraconazole, but not by U18666A. P-X cross-linking was not prevented by deletion of the N-terminal domain of NPC1, which contains the initial binding site for cholesterol. In contrast, P-X cross-linking was reduced when NPC1 contained a point mutation (P691S) in its putative sterol-sensing domain. We hypothesize that the sterol-sensing domain has a binding site that can accommodate structurally different ligands.

Fatostatin blocks ER exit of SCAP but inhibits cell growth in a SCAP-independent manner

Sterol regulatory element-binding protein (SREBP) transcription factors are central regulators of cellular lipid homeostasis and activate expression of genes required for fatty acid, triglyceride, and cholesterol synthesis and uptake. SREBP cleavage activating protein (SCAP) plays an essential role in SREBP activation by mediating endoplasmic reticulum (ER)-to-Golgi transport of SREBP. In the Golgi, membrane-bound SREBPs are cleaved sequentially by the site-1 and site-2 proteases. Recent studies have shown a requirement for the SREBP pathway in the development of fatty liver disease and tumor growth, making SCAP a target for drug development. Fatostatin is a chemical inhibitor of the SREBP pathway that directly binds SCAP and blocks its ER-to-Golgi transport. In this study, we determined that fatostatin blocks ER exit of SCAP and showed that inhibition is independent of insulin-induced gene proteins, which function to retain the SCAP-SREBP complex in the ER. Fatostatin potently inhibited cell growth, but unexpectedly exogenous lipids failed to rescue proliferation of fatostatin-treated cells. Furthermore, fatostatin inhibited growth of cells lacking SCAP Using a vesicular stomatitis virus glycoprotein (VSVG) trafficking assay, we demonstrated that fatostatin delays ER-to-Golgi transport of VSVG. In summary, fatostatin inhibited SREBP activation, but fatostatin additionally inhibited cell proliferation through both lipid-independent and SCAP-independent mechanisms, possibly by general inhibition of ER-to-Golgi transport.

Geranylgeranyl-regulated transport of the prenyltransferase UBIAD1 between membranes of the ER and Golgi

UbiA prenyltransferase domain-containing protein-1 (UBIAD1) utilizes geranylgeranyl pyrophosphate (GGpp) to synthesize the vitamin K2 subtype menaquinone-4. Previously, we found that sterols trigger binding of UBIAD1 to endoplasmic reticulum (ER)-localized HMG-CoA reductase, the rate-limiting enzyme in synthesis of cholesterol and nonsterol isoprenoids, including GGpp. This binding inhibits sterol-accelerated degradation of reductase, which contributes to feedback regulation of the enzyme. The addition to cells of geranylgeraniol (GGOH), which can become converted to GGpp, triggers release of UBIAD1 from reductase, allowing for its maximal degradation and permitting ER-to-Golgi transport of UBIAD1. Here, we further characterize geranylgeranyl-regulated transport of UBIAD1. Results of this characterization support a model in which UBIAD1 continuously cycles between the ER and medial-trans Golgi of isoprenoid-replete cells. Upon sensing a decline of GGpp in ER membranes, UBIAD1 becomes trapped in the organelle where it inhibits reductase degradation. Mutant forms of UBIAD1 associated with Schnyder corneal dystrophy (SCD), a human eye disease characterized by corneal accumulation of cholesterol, are sequestered in the ER and block reductase degradation. Collectively, these findings disclose a novel sensing mechanism that allows for stringent metabolic control of intracellular trafficking of UBIAD1, which directly modulates reductase degradation and becomes disrupted in SCD.

Identification of NPC1 as the target of U18666A, an inhibitor of lysosomal cholesterol export and Ebola infection

Niemann-Pick C1 (NPC1) is a lysosomal membrane protein that exports cholesterol derived from receptor-mediated uptake of LDL, and it also mediates cellular entry of Ebola virus. Cholesterol export is inhibited by nanomolar concentrations of U18666A, a cationic sterol. To identify the target of U18666A, we synthesized U-X, a U18666A derivative with a benzophenone that permits ultraviolet-induced crosslinking. When added to CHO cells, U-X crosslinked to NPC1. Crosslinking was blocked by U18666A derivatives that block cholesterol export, but not derivatives lacking blocking activity. Crosslinking was prevented by point mutation in the sterol-sensing domain (SSD) of NPC1, but not by point mutation in the N-terminal domain (NTD). These data suggest that the SSD contains a U18666A-inhibitable site required for cholesterol export distinct from the cholesterol-binding site in the NTD. Inasmuch as inhibition of Ebola requires 100-fold higher concentrations of U18666A, the high affinity U16888A-binding site is likely not required for virus entry.

DOI:http://dx.doi.org/10.7554/eLife.12177.001

Cholesterol is a type of fat molecule and is a vital component of animal cell membranes. It is taken up into cells within particles called low density lipoproteins (LDLs) that are then digested in cell compartments known as lysosomes to release the cholesterol. Then, the cholesterol leaves the lysosome with the help of a transport protein called NPC1. Mutations in the gene that encodes NPC1 lead to the accumulation of cholesterol in lysosomes; this can cause a devastating illness that affects the brain, liver and other organs. The NPC1 protein also plays a crucial role in allowing Ebola viruses to infect animal cells and multiply.

U18666A is a drug that blocks the movement of cholesterol out of lysosomes and also inhibits Ebola virus infections, but it was not known what components it targets in cells. Lu et al. used a technique called ultraviolet-induced crosslinking to identify the proteins that U18666A can bind to. The experiments show that U18666A can directly bind to a site that is within a section of the NPC1 protein called the sterol-sensing domain. The binding of U18666A to this site blocks the movement of cholesterol out of lysosomes.

Lu et al.’s findings indicate that the sterol-sensing domain of NPC1 plays a crucial role in cholesterol’s export from lysosomes. A future challenge is to use structural biology techniques (such as X-ray crystallography or cryo-electron microscope tomography) to understand the three-dimensional structure of NPC1.

DOI:http://dx.doi.org/10.7554/eLife.12177.002

The terminal enzymes of cholesterol synthesis, DHCR24 and DHCR7, interact physically and functionally

Cholesterol is essential to human health, and its levels are tightly regulated by a balance of synthesis, uptake, and efflux. Cholesterol synthesis requires the actions of more than twenty enzymes to reach the final product, through two alternate pathways. Here we describe a physical and functional interaction between the two terminal enzymes. 24-Dehydrocholesterol reductase (DHCR24) and 7-dehydrocholesterol reductase (DHCR7) coimmunoprecipitate, and when the DHCR24 gene is knocked down by siRNA, DHCR7 activity is also ablated. Conversely, overexpression of DHCR24 enhances DHCR7 activity, but only when a functional form of DHCR24 is used. DHCR7 is important for both cholesterol and vitamin D synthesis, and we have identified a novel layer of regulation, whereby its activity is controlled by DHCR24. This suggests the existence of a cholesterol "metabolon", where enzymes from the same metabolic pathway interact with each other to provide a substrate channeling benefit. We predict that other enzymes in cholesterol synthesis may similarly interact, and this should be explored in future studies.

Forward genetic screening for regulators involved in cholesterol synthesis using validation-based insertional mutagenesis

Somatic cell genetics is a powerful approach for unraveling the regulatory mechanism of cholesterol metabolism. However, it is difficult to identify the mutant gene(s) due to cells are usually mutagenized chemically or physically. To identify important genes controlling cholesterol biosynthesis, an unbiased forward genetics approach named validation-based insertional mutagenesis (VBIM) system was used to isolate and characterize the 25-hydroxycholesterol (25-HC)-resistant and SR-12813-resisitant mutants. Here we report that five mutant cell lines were isolated. Among which, four sterol-resistant mutants either contain a truncated NH₂-terminal domain of sterol regulatory element-binding protein (SREBP)-2 terminating at amino acids (aa) 400, or harbor an overexpressed SREBP cleavage-activating protein (SCAP). Besides, one SR-12813 resistant mutant was identified to contain a truncated COOH-terminal catalytic domain of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase). This study demonstrates that the VBIM system can be a powerful tool to screen novel regulatory genes in cholesterol biosynthesis.

The sterol-based transcriptional control of human 7-dehydrocholesterol reductase (DHCR7): Evidence of a cooperative regulatory program in cholesterol synthesis

The enzyme 7-dehydrocholesterol reductase (DHCR7) catalyzes the final step of cholesterol synthesis via the Kandutsch-Russell pathway, and is crucial in maintaining cellular cholesterol levels. Its absence leads to the devastating fetal developmental disorder Smith-Lemli-Opitz Syndrome (SLOS). How this enzyme is regulated has implications in controlling not only cholesterol synthesis, but also the synthesis of Vitamin D - another product of 7-dehydrocholesterol. In this study, we look specifically at how DHCR7 is regulated by the sterol regulatory element-binding protein-2 (SREBP-2) transcription factor. Sterol regulation has previously been studied in the rat DHCR7 promoter, but we have found that its regulatory elements are not all conserved in humans. Rather, the human promoter contains two binding sites for SREBP-2 (at -155 and -55) and a binding site for the nuclear factor-Y (NF-Y) cofactor (at -136). The -155 site is a particularly responsive sterol regulatory element (SRE) which is well conserved in mammals, and was possibly overlooked in the rat promoter study. The exact location of the weaker -55 site (close to the known rat SRE) may have shifted during evolution. Furthermore, we established that the two SREs that bind SREBP-2 work in cooperation to synergistically activate DHCR7. We have previously characterized the SREs in DHCR24, the final enzyme in the alternate Bloch pathway of cholesterol synthesis. Here, comparison of the sterol regulation of these terminal enzymes demonstrates the unique cooperative system that helps to control cholesterol synthesis.

Three pools of plasma membrane cholesterol and their relation to cholesterol homeostasis

When human fibroblasts take up plasma low density lipoprotein (LDL), its cholesterol is liberated in lysosomes and eventually reaches the endoplasmic reticulum (ER) where it inhibits cholesterol synthesis by blocking activation of SREBPs. This feedback protects against cholesterol overaccumulation in the plasma membrane (PM). But how does ER know whether PM is saturated with cholesterol? In this study, we define three pools of PM cholesterol: (1) a pool accessible to bind 125I-PFO*, a mutant form of bacterial Perfringolysin O, which binds cholesterol in membranes; (2) a sphingomyelin(SM)-sequestered pool that binds 125I-PFO* only after SM is destroyed by sphingomyelinase; and (3) a residual pool that does not bind 125I-PFO* even after sphingomyelinase treatment. When LDL-derived cholesterol leaves lysosomes, it expands PM's PFO-accessible pool and, after a short lag, it also increases the ER's PFO-accessible regulatory pool. This regulatory mechanism allows cells to ensure optimal cholesterol levels in PM while avoiding cholesterol overaccumulation.

Sterol regulatory element-binding protein (SREBP) cleavage regulates Golgi-to-endoplasmic reticulum recycling of SREBP cleavage-activating protein (SCAP)

Sterol regulatory element-binding protein (SREBP) transcription factors are central regulators of cellular lipogenesis. Release of membrane-bound SREBP requires SREBP cleavage-activating protein (SCAP) to escort SREBP from the endoplasmic reticulum (ER) to the Golgi for cleavage by site-1 and site-2 proteases. SCAP then recycles to the ER for additional rounds of SREBP binding and transport. Mechanisms regulating ER-to-Golgi transport of SCAP-SREBP are understood in molecular detail, but little is known about SCAP recycling. Here, we have demonstrated that SCAP Golgi-to-ER transport requires cleavage of SREBP at site-1. Reductions in SREBP cleavage lead to SCAP degradation in lysosomes, providing additional negative feedback control to the SREBP pathway. Current models suggest that SREBP plays a passive role prior to cleavage. However, we show that SREBP actively prevents premature recycling of SCAP-SREBP until initiation of SREBP cleavage. SREBP regulates SCAP in human cells and yeast, indicating that this is an ancient regulatory mechanism.

Signaling regulates activity of DHCR24, the final enzyme in cholesterol synthesis

The role of signaling in regulating cholesterol homeostasis is gradually becoming more widely recognized. Here, we explored how kinases and phosphorylation sites regulate the activity of the enzyme involved in the final step of cholesterol synthesis, 3β-hydroxysterol Δ24-reductase (DHCR24). Many factors are known to regulate DHCR24 transcriptionally, but little is known about its posttranslational regulation. We developed a system to specifically test human ectopic DHCR24 activity in a model cell-line (Chinese hamster ovary-7) using siRNA targeted only to hamster DHCR24, thus ensuring that all activity could be attributed to the human enzyme. We determined the effect of known phosphorylation sites and found that mutating certain residues (T110, Y299, and Y507) inhibited DHCR24 activity. In addition, inhibitors of protein kinase C ablated DHCR24 activity, although not through a known phosphorylation site. Our data indicate a novel mechanism whereby DHCR24 activity is regulated by signaling.

Doxorubicin blocks proliferation of cancer cells through proteolytic activation of CREB3L1

Doxorubicin is used extensively for chemotherapy of diverse types of cancer, yet the mechanism through which it inhibits proliferation of cancer cells remains unclear. Here we report that doxorubicin stimulates de novo synthesis of ceramide, which in turn activates CREB3L1, a transcription factor synthesized as a membrane-bound precursor. Doxorubicin stimulates proteolytic cleavage of CREB3L1 by Site-1 Protease and Site-2 Protease, allowing the NH₂-terminal domain of CREB3L1 to enter the nucleus where it activates transcription of genes encoding inhibitors of the cell cycle, including p21. Knockdown of CREB3L1 mRNA in human hepatoma Huh7 cells and immortalized human fibroblast SV589 cells conferred increased resistance to doxorubicin, whereas overexpression of CREB3L1 in human breast cancer MCF-7 cells markedly enhanced the sensitivity of these cells to doxorubicin. These results suggest that measurement of CREB3L1 expression may be a useful biomarker in identifying cancer cells sensitive to doxorubicin.

DOI:http://dx.doi.org/10.7554/eLife.00090.001

Cancer is a broad term to describe over 200 diseases that are caused by cells proliferating in an out-of-control manner. Cell replication and division are normally very tightly regulated, and as cells become old, damaged or mutated, they are either repaired or undergo programmed cell death (apoptosis). However, if defective cells continue to replicate, the resulting clusters of abnormal cells can become cancerous.

With so many different types of cancer, there is no ‘magic bullet’ to cure all of them. Many cancer therapies are targeted, relying on drugs that block the spread of cancer by interfering with specific molecules involved in the growth and progression of certain tumors. However, the fact that diseased cells replicate faster than normal cells in many forms of cancer makes it possible to use non-specific drugs, such as doxorubicin, to treat tumors when targeted therapies are not available.

Doxorubicin can induce DNA breaks in a variety of different cancers by inhibiting the activity of topoisomerase II but a consistent relationship between the inhibition of this enzyme and the blocking of cell proliferation has not been established. This lack of understanding of the mechanism through which doxorubicin inhibits cell proliferation makes it difficult to identify cancer patients who are most likely to benefit from doxorubicin treatment.

Denard et al. have now shown that doxorubicin blocks cell replication by cleaving a transcription factor called CREB3L1. This latest work builds on previous work in which they showed that cleavage of this transcription factor can inhibit the replication of cells infected with hepatitis C virus. It has been known since 2000 that CREB3L1 is a membrane protein with one end inside the lumen of the endoplasmic reticulum, and the other end (which is terminated with an NH₂ group) in the cytosol of the cell. When CREB3L1 is cleaved, the NH₂-terminal domain travels into the nucleus of the cell, where it drives the transcription of genes that suppress the cell cycle. Denard et al. clearly show that doxorubicin triggers the cleavage of CREB3L1 by stimulating the production of ceramide molecules. Thus, It might be possible, with further research, to use CREB3L1 as a biomarker to identify tumors that are suitable for treatment by doxorubicin.

DOI:http://dx.doi.org/10.7554/eLife.00090.002

Ancient ubiquitous protein-1 mediates sterol-induced ubiquitination of 3-hydroxy-3-methylglutaryl CoA reductase in lipid droplet-associated endoplasmic reticulum membranes

Sterol-induced binding to Insigs in endoplasmic reticulum (ER) membranes triggers ubiquitination of the cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase. This ubiquitination, which is mediated by Insig-associated ubiquitin ligases gp78 and Trc8, is obligatory for extraction of reductase from lipid droplet-associated ER membranes into the cytosol for proteasome-mediated, ER-associated degradation (ERAD). In this study, we identify lipid droplet-associated, ancient, ubiquitous protein-1 (Aup1) as one of several proteins that copurify with gp78. RNA interference (RNAi) studies show that Aup1 recruits the ubiquitin-conjugating enzyme Ubc7 to lipid droplets and facilitates its binding to both gp78 and Trc8. The functional significance of these interactions is revealed by the observation that RNAi-mediated knockdown of Aup1 blunts sterol-accelerated ubiquitination of reductase, which appears to occur in lipid droplet-associated membranes and subsequent ERAD of the enzyme. In addition, Aup1 knockdown inhibits ERAD of Insig-1, another substrate for gp78, as well as that of membrane-bound precursor forms of sterol-regulatory, element-binding protein-1 and -2, transcription factors that modulate expression of genes encoding enzymes required for cholesterol synthesis. Considered together, these findings not only implicate a role for Aup1 in maintenance of intracellular cholesterol homeostasis, but they also highlight the close connections among ERAD, lipid droplets, and lipid droplet-associated proteins.

A key regulator of cholesterol homoeostasis, SREBP-2, can be targeted in prostate cancer cells with natural products

There is growing evidence showing that prostate cancer cells have perturbed cholesterol homoeostasis, accumulating cholesterol to promote cell growth. Consequently, cholesterol-lowering drugs such as statins are being evaluated in prostate cancer treatment. Furthermore, natural products such as betulin (from birch tree bark) and tocotrienol (a minor form of vitamin E) have been shown to lower cholesterol levels. Using these drugs and oxysterols, we have determined which aspects of cholesterol homoeostasis should be targeted in prostate cancer, e.g. cellular cholesterol levels are increased by the transcription factor SREBP-2 (sterol-regulatory-element-binding protein isoform 2), whereas LXR (liver X receptor) promotes cholesterol efflux. Whereas betulin exerted non-specific effects on cell viability, tocotrienols produced a strong direct correlation between SREBP-2 activity and cell viability. Mechanistically, tocotrienols lowered SREBP-2 activity by degrading mature SREBP-2 independently of the proteasome. In contrast, no correlation was seen between LXR activity and cell viability, implying that SREBP-2 is a better target than LXR for prostate cancer treatment. Lastly, androgen-dependent and -independent LNCaP cells were both sensitive to tocotrienols. Overall, this suggests that tocotrienols and other drugs targeting the SREBP-2 pathway are a potential therapeutic option for prostate cancer.

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.

Sterols regulate 3β-hydroxysterol Δ24-reductase (DHCR24) via dual sterol regulatory elements: cooperative induction of key enzymes in lipid synthesis by Sterol Regulatory Element Binding Proteins

3β-Hydroxysterol Δ24-reductase (DHCR24) catalyzes a final step in cholesterol synthesis, and has been ascribed diverse functions, such as being anti-apoptotic and anti-inflammatory. How this enzyme is regulated transcriptionally by sterols is currently unclear. Some studies have suggested that its expression is regulated by Sterol Regulatory Element Binding Proteins (SREBPs) while another suggests it is through the Liver X Receptor (LXR). However, these transcription factors have opposing effects on cellular sterol levels, so it is likely that one predominates. Here we establish that sterol regulation of DHCR24 occurs predominantly through SREBP-2, and identify the particular region of the DHCR24 promoter to which SREBP-2 binds. We demonstrate that sterol regulation is mediated by two sterol regulatory elements (SREs) in the promoter of the gene, assisted by two nearby NF-Y binding sites. Moreover, we present evidence that the dual SREs work cooperatively to regulate DHCR24 expression by comparison to two known SREBP target genes, the LDL receptor with one SRE, and farnesyl-diphosphate farnesyltransferase 1, with two SREs.

The endogenous regulator 24(S),25-epoxycholesterol inhibits cholesterol synthesis at DHCR24 (Seladin-1)

The oxysterol 24(S),25-epoxycholesterol (24,25EC) can affect cholesterol metabolism at multiple points. Previously, we proposed that 24,25EC has an especially significant role in fine-tuning cholesterol synthesis, since it parallels cholesterol production, and without it, acute cholesterol synthesis is exaggerated. 24,25EC is structurally similar to desmosterol, a substrate for the enzyme 3β-hydroxysterol ∆(24)-reductase (DHCR24, also called Seladin-1) which catalyzes a final step in cholesterol synthesis. In this study, we reveal a novel mode by which 24,25EC can regulate cholesterol synthesis, by interfering with DHCR24, resulting in the rapid accumulation of the substrate desmosterol, at the expense of cholesterol. This effect was independent of DHCR24 protein levels, and was observed in multiple mammalian cell-lines, including those of hepatic and neuronal origin. Conversely, overexpression of DHCR24 blunted the inhibition by 24,25EC. We also determined that the specificity of this effect was restricted to certain side-chain oxysterols, notably those oxygenated at C-25. Importantly, endogenous levels of 24,25EC, manipulated by genetic and pharmacological methods, were sufficient to reduce DHCR24 activity. Together, our work introduces a novel role for 24,25EC in cholesterol homeostasis, through its rapid inhibition of cholesterol synthesis at DHCR24. Also, our work provides new insights into a little studied area, the post-transcriptional regulation of DHCR24, an important enzyme in human health and disease.

Akt acutely activates the cholesterogenic transcription factor SREBP-2

Akt is an essential protein kinase for cell growth, proliferation, and survival. Perturbed Akt regulation is associated with a number of human diseases, such as cancer and diabetes. Recently, evidence has emerged that Akt is involved in the regulation of the sterol-regulatory element binding proteins, which are master transcriptional regulators of lipid metabolism. This offers a means by which synthesis of new membrane can be coordinated with cell growth and proliferation. However, the link between Akt and sterol-regulatory element binding protein-2, the major isoform participating in cholesterol regulation, is relatively unexplored. In the present study, we employed insulin-like growth factor-1 as an inducer of Akt signalling, and showed that it increased sterol-regulatory element binding protein-2 activation acutely (within 1h). This insulin-like growth factor-1-induced sterol-regulatory element binding protein-2 activation was blunted when Akt was inhibited pharmacologically or molecularly with small interfering RNA. Furthermore, we employed a rapalog heterodimerisation system to specifically and rapidly activate Akt, and found that sterol-regulatory element binding protein-2 activation was increased in response to Akt activation. Together, this study provides compelling evidence that Akt contributes to the acute regulation of cholesterol metabolism through activating sterol-regulatory element binding protein-2.

Natural products reveal cancer cell dependence on oxysterol-binding proteins

Cephalostatin 1, OSW-1, ritterazine B and schweinfurthin A are natural products that potently, and in some cases selectively, inhibit the growth of cultured human cancer cell lines. The cellular targets of these small molecules have yet to be identified. We have discovered that these molecules target oxysterol binding protein (OSBP) and its closest paralog, OSBP-related protein 4L (ORP4L)--proteins not known to be involved in cancer cell survival. OSBP and the ORPs constitute an evolutionarily conserved protein superfamily, members of which have been implicated in signal transduction, lipid transport and lipid metabolism. The functions of OSBP and the ORPs, however, remain largely enigmatic. Based on our findings, we have named the aforementioned natural products ORPphilins. Here we used ORPphilins to reveal new cellular activities of OSBP. The ORPphilins are powerful probes of OSBP and ORP4L that will be useful in uncovering their cellular functions and their roles in human diseases.

Cholesterol-dependent degradation of squalene monooxygenase, a control point in cholesterol synthesis beyond HMG-CoA reductase

Exquisite control of cholesterol synthesis is crucial for maintaining homeostasis of this vital yet potentially toxic lipid. Squalene monooxygenase (SM) catalyzes the first oxygenation step in cholesterol synthesis, acting on squalene before cyclization into the basic steroid structure. Using model cell systems, we found that cholesterol caused the accumulation of the substrate squalene, suggesting that SM may serve as a flux-controlling enzyme beyond 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR, considered as rate limiting). Cholesterol accelerated the proteasomal degradation of SM which required the N-terminal domain, partially conserved in vertebrates but not in lower organisms. Unlike HMGR, SM degradation is not mediated by Insig, 24,25-dihydrolanosterol, or side-chain oxysterols, but rather by cholesterol itself. Importantly, SM's N-terminal domain conferred cholesterol-regulated turnover on heterologous fusion proteins. Furthermore, proteasomal inhibition almost totally eliminated squalene accumulation, highlighting the importance of this degradation mechanism for the control of SM and suggesting this as a possible control point in cholesterol synthesis.

Intramembrane glycine mediates multimerization of Insig-2, a requirement for sterol regulation in Chinese hamster ovary cells

Sterol-induced binding of endoplasmic reticulum (ER) membrane proteins Insig-1 and Insig-2 to SREBP cleavage-activating protein (Scap) and HMG-CoA reductase triggers regulatory events that limit cholesterol synthesis in animal cells. Binding of Insigs to Scap prevents proteolytic activation of sterol-regulatory element binding proteins (SREBPs), membrane-bound transcription factors that enhance cholesterol synthesis, by trapping Scap-SREBP complexes in the ER. Insig binding to reductase causes ubiquitination and subsequent proteasome-mediated degradation of the enzyme from ER membranes, slowing a rate-limiting step in cholesterol synthesis. Here, we report the characterization of mutant Chinese hamster ovary cells, designated SRD-20, that are resistant to 25-hydroxycholesterol, which potently inhibits SREBP activation and stimulates degradation of reductase. SRD-20 cells were produced by mutagenesis of Insig-1-deficient SRD-14 cells, followed by selection in 25-hydroxycholesterol. DNA sequencing reveals that SRD-20 cells harbor a point mutation in one Insig-2 allele that results in production of a truncated, nonfunctional protein, whereas the other allele contains a point mutation that results in substitution of glutamic acid for glycine-39. This glycine residue localizes to the first membrane-spanning segment of Insig-2 and is also present in the corresponding region of Insig-1. Mutant forms of Insig-1 and Insig-2 containing the Glu-to-Gly substitution fail to confer sterol regulation upon overexpressed Scap and reductase. These studies identify the intramembrane glycine as a key residue for normal sterol regulation in animal cells.

Cholesterol feedback: from Schoenheimer's bottle to Scap's MELADL

Cholesterol biosynthesis is among the most intensely regulated processes in biology. Synthetic rates vary over hundreds of fold depending on the availability of an external source of cholesterol. Studies of this feedback regulatory process have a rich history. The field began 75 years ago when Rudolf Schoenheimer measured cholesterol balance in mice in a bottle. He found that cholesterol feeding led to decreased cholesterol synthesis, thereby introducing the general phenomenon by which end products of biosynthetic pathways inhibit their own synthesis. Recently, cholesterol feedback has been explained at a molecular level with the discovery of membrane-bound transcription factors called sterol regulatory element-binding proteins (SREBPs), and an appreciation of the sterol-sensing role of their partner, an escort protein called Scap. The key element in Scap is a hexapeptide sequence designated MELADL (rhymes with bottle). Thus, over 75 years, Schoenheimer's bottle led to Scap's MELADL. In addition to their basic importance in membrane biology, these studies have implications for the regulation of plasma cholesterol levels and consequently for the development of atherosclerotic plaques, myocardial infarctions, and strokes. In this article we review the major milestones in the cholesterol feedback story.

Statins and stroke

Statins play an important role in brain ischemia. These drugs reduce cholesterol levels, which have been related to a reduction in vascular event risk, but they also have other functions besides cholesterol metabolism, called pleiotropic effects. Statins play an important role during the acute phase of ischemia, and might have neuroprotective effects, as they act in several mechanisms during the acute phase of stroke, such as in nitric oxide (NO) and glutamate metabolism, inflammation, platelet aggregation, immune responses and apoptosis. They also have other functions that can be related, with better long-term outcome, to neurorepair mechanisms. Statins promote angiogenesis, endogenous cell proliferation, neurogenesis and new synapse formation.

Switch-like control of SREBP-2 transport triggered by small changes in ER cholesterol: a delicate balance

Animal cells control their membrane lipid composition within narrow limits, but the sensing mechanisms underlying this control are largely unknown. Recent studies disclosed a protein network that controls the level of one lipid-cholesterol. This network resides in the endoplasmic reticulum (ER). A key component is Scap, a tetrameric ER membrane protein that binds cholesterol. Cholesterol binding prevents Scap from transporting SREBPs to the Golgi for activation. Using a new method to purify ER membranes from cultured cells, we show that Scap responds cooperatively to ER cholesterol levels. When ER cholesterol exceeds 5% of total ER lipids (molar basis), SREBP-2 transport is abruptly blocked. Transport resumes when ER cholesterol falls below the 5% threshold. The 5% threshold is lowered to 3% when cells overexpress Insig-1, a Scap-binding protein. Cooperative interactions between cholesterol, Scap, and Insig create a sensitive switch that controls the cholesterol composition of cell membranes with remarkable precision.

Unsaturated fatty acids inhibit proteasomal degradation of Insig-1 at a postubiquitination step

Proteasomes mediate the regulated degradation of Insig-1, a membrane protein of the endoplasmic reticulum (ER) that plays a crucial role in lipid metabolism. We showed previously that sterols inhibit this degradation by blocking ubiquitination of Insig-1. Here we show that unsaturated fatty acids stabilize Insig-1 without affecting its ubiquitination. Instead unsaturated fatty acids inhibit extraction of ubiquitinated Insig-1 from membranes, a process known to be mediated by valosin-containing protein and necessary for ER-associated degradation. Valosin-containing protein is recruited to Insig-1 through the action of another protein, Ubxd8. Unsaturated fatty acids block the binding between Ubxd8 and Insig-1, thereby abrogating the membrane extraction of Insig-1. Unsaturated fatty acid-mediated stabilization of Insig-1 enhances the ability of sterols to inhibit proteolytic activation of SREBP-1, which activates transcription of genes involved in fatty acid synthesis. The current study provides a molecular mechanism for regulation of proteasome-mediated ER protein degradation at a postubiquitination step.

Hypoxia Stimulates Degradation of 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase through Accumulation of Lanosterol and Hypoxia-Inducible Factor-mediated Induction of Insigs

Endoplasmic reticulum-associated degradation of the enzyme 3-hydroxy-3-methylglutaryl-CoA reductase represents one mechanism by which cholesterol synthesis is controlled in mammalian cells. The key reaction in this degradation is binding of reductase to Insig proteins in the endoplasmic reticulum, which is stimulated by the cholesterol precursor lanosterol. Conversion of lanosterol to cholesterol requires removal of three methyl groups, which consumes nine molecules of dioxygen. Here, we report that oxygen deprivation (hypoxia) slows demethylation of lanosterol and its metabolite 24,25-dihydrolanosterol, causing both sterols to accumulate in cells. In addition, hypoxia increases the amount of Insig-1 and Insig-2 in a response mediated by hypoxia-inducible factor (HIF)-1alpha. Accumulation of lanosterol together with increased Insigs accelerates degradation of reductase, which ultimately slows a rate-determining step in cholesterol synthesis. These results define a novel oxygen-sensing mechanism mediated by the combined actions of methylated intermediates in cholesterol synthesis and the hypoxia-activated transcription factor HIF-1alpha.

Amplification of the gene for SCAP, coupled with Insig-1 deficiency, confers sterol resistance in mutant Chinese hamster ovary cells

The endoplasmic reticulum membrane proteins Insig-1 and Insig-2 limit cholesterol synthesis, in part through their sterol-dependent binding to sterol-regulatory element binding protein (SREBP) cleavage-activating protein (SCAP). This binding prevents proteolytic processing of SREBPs, membrane-bound transcription factors that enhance cholesterol synthesis. We report here the characterization of mutant Chinese hamster ovary (CHO) cells, designated SRD-19, that are resistant to 25-hydroxycholesterol, a potent inhibitor of SREBP processing. SRD-19 cells were produced by mutagenesis of Insig-1-deficient SRD-14 cells, followed by selection in high levels of 25-hydroxycholesterol. 25-Hydroxycholesterol fails to suppress SREBP processing in SRD-19, even though they express normal levels of Insig-2. The number of copies of the gene encoding SCAP was found to be increased by 4-fold in SRD-19 cells compared with wild-type CHO cells, leading to the overproduction of SCAP mRNA and protein. Our data indicate that overproduced SCAP saturates the remaining Insig-2 in SRD-19 cells, thus explaining their resistance to 25-hydroxycholesterol. Consistent with this conclusion, regulated SREBP processing is restored in SRD-19 cells upon transfection of plasmids encoding either Insig-1 or Insig-2. These results highlight the importance of SCAP/Insig ratios in normal sterol-regulated processing of SREBPs in cultured cells.

SREBP-2 positively regulates transcription of the cholesterol efflux gene, ABCA1, by generating oxysterol ligands for LXR

Cholesterol accumulation and removal are regulated by two different transcription factors. SREBP-2 (sterol-regulatory-element-binding protein-2) is best known to up-regulate genes involved in cholesterol biosynthesis and uptake, whereas LXR (liver X receptor) is best known for up-regulating cholesterol efflux genes. An important cholesterol efflux gene that is regulated by LXR is the ATP-binding cassette transporter, ABCA1 (ATP-binding cassette transporter-A1). We have previously shown that statin treatment down-regulated ABCA1 expression in human macrophages, probably by inhibiting synthesis of the LXR ligand 24(S),25-epoxycholesterol. However, it was subsequently reported that ABCA1 expression is down-regulated by SREBP-2 through binding of SREBP-2 to an E-box element in ABCA1's proximal promoter. As statin treatment induces SREBP-2 activation, this may provide an alternative explanation for the statin-mediated down-regulation of ABCA1. In the present study, we employed a set of CHO (Chinese-hamster ovary) mutant cell lines to investigate the role of SREBP-2 in the regulation of ABCA1. We observed increased ABCA1 mRNA levels in SREBP-2-overexpressing cells and decreased levels in cells lacking a functional SREBP-2 pathway, which were restored when the SREBP-2 pathway was reinstated. Moreover, ABCA1 gene expression was positively associated with synthesis of 24(S),25-epoxycholesterol in these cell lines. In studies using a human ABCA1 promoter reporter assay, mutation of the E-box motif had a similar response as the wild-type construct to either statin treatment or addition of 24(S),25-epoxycholesterol. By contrast, these responses were completely ablated when the DR4 element to which LXR binds was mutated. These results support the idea that 24(S),25-epoxycholesterol and statin treatment influence ABCA1 transcription via supply of an LXR ligand and not through an SREBP-2/E-box-related mechanism. In addition, our results indicate a critical role of SREBP-2 as a positive regulator of ABCA1 gene expression by enabling the generation of oxysterol ligands for LXR.

A Chinese hamster ovarian cell line imports cholesterol by high density lipoprotein degradation

Plasma high density lipoprotein (HDL) is inversely associated with the development of atherosclerosis. HDL exerts its atheroprotective role through involvement in reverse cholesterol transport in which HDL is loaded with cholesterol at the periphery and transports its lipid load back to the liver for disposal. In this pathway, HDL is not completely dismantled but only transfers its lipids to the cell. Here we present evidence that a Chinese hamster ovarian cell line (CHO7) adapted to grow in lipoprotein-deficient media degrades HDL and concomitantly internalizes HDL-derived cholesterol. Delivery of HDL cholesterol to the cell was demonstrated by a down-regulation of cholesterol biosynthesis, an increase in total cellular cholesterol content and by stimulation of cholesterol esterification after HDL treatment. This HDL degradation pathway is distinct from the low density lipoprotein (LDL) receptor pathway but also degrades LDL. 25-Hydroxycholesterol, a potent inhibitor of the LDL receptor pathway, down-regulated LDL degradation in CHO7 cells only in part and did not down-regulate HDL degradation. Dextran sulfate released HDL bound to the cell surface of CHO7 cells, and heparin treatment released protein(s) contributing to HDL degradation. The involvement of heparan sulfate proteoglycans and lipases in this HDL degradation was further tested by two inhibitors genistein and tetrahydrolipstatin. Both blocked HDL degradation significantly. Thus, we demonstrate that CHO7 cells degrade HDL and LDL to supply themselves with cholesterol via a novel degradation pathway. Interestingly, HDL degradation with similar properties was also observed in a human placental cell line.

Localization and role of NPC1L1 in cholesterol absorption in human intestine

Recent studies have documented the presence of Niemann-Pick C1-Like 1 (NPC1L1) in the small intestine and its capacity to transport cholesterol in mice and rats. The current investigation was undertaken to explore the localization and function of NPC1L1 in human enterocytes. Cell fractionation experiments revealed an NPC1L1 association with apical membrane of the enterocyte in human jejunum. Signal was also detected in lysosomes, endosomes, and mitochondria. Confirmation of cellular NPC1L1 distribution was obtained by immunocytochemistry. Knockdown of NPC1L1 caused a decline in the ability of Caco-2 cells to capture micellar [(14)C]free cholesterol. Furthermore, this NPC1L1 suppression resulted in increased and decreased mRNA levels and activity of HMG-CoA reductase, the rate-limiting step in cholesterol synthesis, and of ACAT, the key enzyme in cholesterol esterification, respectively. An increase was also noted in the transcriptional factor sterol-regulatory element binding protein that modulates cholesterol homeostasis. Efforts were devoted to define the impact of NPC1L1 knockdown on other mediators of cholesterol uptake. RT-PCR evidence is presented to show the significant decrease in the levels of scavenger receptor class B type I (SR-BI) with no changes in ABCA1, ABCG5, and cluster determinant 36 in NPC1L1-deficient Caco-2 cells. Together, our data suggest that NPC1L1 contributes to intestinal cholesterol homeostasis and possibly cooperates with SR-BI to mediate cholesterol absorption in humans.

Regulation of cholesterol 25-hydroxylase expression by vitamin D3 metabolites in human prostate stromal cells

Vitamin D3 plays an important role in the control of cell proliferation and differentiation. Cholesterol 25-hydroxylase (CH25H) is an enzyme converting cholesterol into 25-hydroxycholesterol. Vitamin D3 as well as 25-hydroxycholesterol has been shown to inhibit cell growth and induce cell apoptosis. Here we show that 10 nM 1alpha,25(OH)2D3 and 500 nM 25OHD3 upregulate CH25H mRNA expression in human primary prostate stromal cells (P29SN). Protein synthesis inhibitor cycloheximide does not block 1alpha,25(OH)2D3 mediated upregulation of CH25H mRNA. Transcription inhibitor actinomycin D blocks basal level as well as 1alpha,25(OH)2D3 induced CH25H mRNA expression. 1alpha,25(OH)2D3 has no effect on CH25H mRNA stability. 25-Hydroxycholesterol significantly decreased the P29SN cell number. A CH25H enzyme inhibitor, desmosterol, increases basal cell number but has no significant effect on vitamin D3 treated cells. Our data suggest that ch25h could be a vitamin D3 target gene and may partly mediate anti-proliferative action of vitamin D3 in human primary prostate stromal cells.

Juxtamembranous aspartic acid in Insig-1 and Insig-2 is required for cholesterol homeostasis

Insig-1 and Insig-2 are closely related proteins of the endoplasmic reticulum (ER) that mediate feedback control of cholesterol synthesis by sterol-dependent binding to the following two membrane proteins: the escort protein Scap, thus preventing proteolytic processing of sterol regulatory element-binding proteins; and the cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase, thus inducing the ubiquitination and ER-associated degradation of the enzyme. Here, we report that the conserved Asp-205 in Insig-1, which abuts the fourth transmembrane helix at the cytosolic side of the ER membrane, is essential for its dual function. When Asp-205 was mutated to alanine, the mutant Insig-1 lost the ability to bind to Scap and, thus, was unable to suppress the cleavage of sterol regulatory element-binding proteins. The mutant Insig-1 was ineffective also in accelerating sterol-stimulated degradation of 3-hydroxy-3-methylglutaryl CoA reductase. Alanine substitution of the corresponding aspartic acid in Insig-2 produced the same dual defects. These studies identify a single amino acid residue that is crucial for the function of Insig proteins in regulating cholesterol homeostasis in mammalian cells.

Involvement of Akt in ER-to-Golgi transport of SCAP/SREBP: a link between a key cell proliferative pathway and membrane synthesis

Akt is a critical regulator of cell growth, proliferation, and survival that is activated by phosphatidylinositol 3-kinase (PI3K). We investigated the effect of PI3K inhibition on activation of sterol regulatory element binding protein-2 (SREBP-2), a master regulator of cholesterol homeostasis. SREBP-2 processing increased in response to various cholesterol depletion approaches (including statin treatment) and this increase was blunted by treatment with a potent and specific inhibitor of PI3K, LY294002, or when a plasmid encoding a dominant-negative form of Akt (DN-Akt) was expressed. LY294002 also suppressed SREBP-2 processing induced by insulin-like growth factor-1. Furthermore, LY294002 treatment down-regulated SREBP-2 or -1c gene targets and decreased cholesterol and fatty acid synthesis. Fluorescence microscopy studies indicated that LY294002 disrupts transport of the SREBP escort protein, SCAP, from the endoplasmic reticulum to the Golgi. This disruption was also shown by immunofluorescence staining when DN-Akt was expressed. Taken together, our studies indicate that the PI3K/Akt pathway is involved in SREBP-2 transport to the Golgi, contributing to the control of SREBP-2 activation. Our results provide a crucial mechanistic link between the SREBP and PI3K/Akt pathways that may be reconciled teleologically because synthesis of new membrane is an absolute requirement for cell growth and proliferation.

Sterol-regulated ubiquitination and degradation of Insig-1 creates a convergent mechanism for feedback control of cholesterol synthesis and uptake

This paper describes a convergent mechanism for the feedback control of cholesterol synthesis and uptake mediated by SREBPs, membrane bound transcription factors. Endoplasmic reticulum (ER) bound SREBPs form complexes with Scap, a polytopic ER protein. In sterol-overloaded cells, Scap/SREBP binds to Insig-1, which retains the complex in the ER. Upon sterol deprivation, the Scap/SREBP complex dissociates from Insig-1, which is then ubiquitinated on lysines 156 and 158 and degraded in proteasomes. Scap/SREBP moves to the Golgi, where SREBP is processed to liberate a nuclear fragment that activates genes for cholesterol synthesis and uptake and the gene for Insig-1. Ubiquitination is not necessary for release of Scap/SREBP from Insig-1, but it establishes a requirement for synthesis of new Insig-1 for feedback inhibition. When the new Insig-1 and cholesterol converge on Scap, Scap/SREBP binds to Insig-1, preventing ubiquitination. The Insig-1/Scap/SREBP complex accumulates in the ER, ready for liberation when the cell is again sterol deprived.

Dual roles for cholesterol in mammalian cells

The structural features of sterols required to support mammalian cell growth have not been fully defined. Here, we use mutant CHO cells that synthesize only small amounts of cholesterol to test the capacity of various sterols to support growth. Sterols with minor modifications of the side chain (e.g., campesterol, beta-sitosterol, and desmosterol) supported long-term growth of mutant cells, but sterols with more complex modifications of the side chain, the sterol nucleus, or the 3-hydroxy group did not. After 60 days in culture, the exogenous sterol comprised >90% of cellular sterols. Inactivation of residual endogenous synthesis with the squalene epoxidase inhibitor NB-598 prevented growth in beta-sitosterol and greatly reduced growth in campesterol. Growth of cells cultured in beta-sitosterol and NB-598 was restored by adding small amounts of cholesterol to the medium. Surprisingly, enantiomeric cholesterol also supported cell growth, even in the presence of NB-598. Thus, sterols fulfill two roles in mammalian cells: (i) a bulk membrane requirement in which phytosterols can substitute for cholesterol and (ii) other processes that specifically require small amounts of cholesterol but are not enantioselective.

Identification of FBL2 as a geranylgeranylated cellular protein required for hepatitis C virus RNA replication

We recently reported that Hepatitis C virus (HCV) RNA replication requires one or more geranylgeranylated host proteins. Using a combination of [(3)H]mevalonate labeling, coimmunoprecipitation, and bioinformatic search, we identified a geranylgeranylated host protein required for HCV RNA replication. This protein, FBL2, contains an F box domain and a CAAX motif (CVIL). It forms a stable immunoprecipitable complex with the HCV nonstructural protein 5A (NS5A). The association of FBL2 with NS5A requires the CAAX motif of FBL2, but not the F box. Deletion of the F box created a dominant-negative protein that inhibited replication of HCV RNA when overexpressed in Huh7-K2040 cells; this inhibition was overcome by coexpression of NS5A. siRNA-mediated knockdown of FBL2 mRNA by 70% in Huh7-HP cells reduced HCV RNA by 65%; this reduction was overcome by expression of a cDNA encoding a wobble mutant of FBL2. The current data indicate that geranylgeranylated FBL2 binds to NS5A in a reaction crucial for HCV RNA replication.

Isolation of Sterol-resistant Chinese Hamster Ovary Cells with Genetic Deficiencies in Both Insig-1 and Insig-2

Insig-1 and Insig-2, a pair of endoplasmic reticulum (ER) membrane proteins, mediate feedback control of cholesterol synthesis through their sterol-dependent binding to the following two polytopic ER membrane proteins: sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) and 3-hydroxy-3-methylglutaryl-coenzyme A reductase. Sterol-induced binding of Insigs to SCAP prevents the proteolytic processing of SREBPs, membrane-bound transcription factors that enhance the synthesis of cholesterol, by retaining complexes between SCAP and SREBP in the ER. Sterol-induced binding of Insigs to reductase leads to the ubiquitination and ER-associated degradation of the enzyme, thereby slowing a rate-controlling step in cholesterol synthesis. Here we report the isolation of a new line of mutant Chinese hamster ovary cells, designated SRD-15, deficient in both Insig-1 and Insig-2. The SRD-15 cells were produced by gamma-irradiation of Insig-1-deficient SRD-14 cells, followed by selection in high levels of the oxysterol, 25-hydroxycholesterol. Sterols neither inhibit SREBP processing nor promote reductase ubiquitination/degradation in SRD-15 cells. Sterol regulation of SREBP processing and reductase ubiquitination/degradation is fully restored in SRD-15 cells when they are transfected with expression plasmids encoding either Insig-1 or Insig-2. These results demonstrate an absolute requirement for Insig proteins in the regulatory system that mediates lipid homeostasis in animal cells.