First synthesis of ent-desmosterol and its conversion to ent-deuterocholesterol

Asymmetric Total Syntheses of Euphol and Tirucallol

Asymmetric de novo syntheses of euphol and tirucallol have been accomplished by way of a concise sequence of chemical steps featuring several modern stereoselective transformations. The preparative solution described for these complex problems in natural product synthesis departs significantly from biomimetic polyene cyclization chemistry, which has been leveraged to address related tetracyclic triterpenoid targets. In particular, a diastereoselective Friedel-Crafts-type cyclization was employed to establish a tetracycle bearing a stereodefined quaternary center at C9 (steroid numbering) that provided access to intermediates of relevance for introducing the C10 and C14 quaternary centers by sequential stereospecific 1,2-alkyl shifts (C9 → C10 and C15 → C14). Finally, the stereodefined C17 side chain was introduced in a single step by late-stage stereoselective conjugate addition to an intermediate possessing a D-ring enone. Notably, these de novo asymmetric syntheses are the first of their kind, providing completely synthetic access to enantiodefined euphane and tirucallane systems. Overall, each synthesis has been accomplished in fewer than 20 linear chemical steps from a simple Hajos-Parrish-derived ketone through a sequence that features just 15 chromatographic operations.

Chirality Effect on Cholesterol Modulation of Protein Function

Cholesterol is a key steroidal, lipid biomolecule found abundantly in plasma membranes of eukaryotic cells. It is an important structural component of cellular membranes and regulates membrane fluidity and permeability. Cholesterol is also essential for normal functioning of key proteins including ion-channels, G protein-coupled receptors (GPCRs), membrane bound steroid receptors, and receptor kinases. It is thought that cholesterol exerts its actions via specific binding to chiral proteins and lipids as well as through non-specific physiochemical interactions. Distinguishing between the specific and the non-specific interactions can be difficult. Although much remains unclear, progress has been made in recent years by utilizing ent-cholesterol, the enantiomer of natural cholesterol (nat-cholesterol) as a probe. Ent-Cholesterol is the non-superimposable mirror image of nat-cholesterol and exhibits identical physiochemical properties as nat-cholesterol. Hence, if the biological effects of cholesterol result solely due to membrane effects, it is expected that there will be no difference between ent-cholesterol and nat-cholesterol. However, when direct binding with chiral proteins and lipids is involved, the enantiomer is expected to potentially elicit significantly different, measurable effects due to formation of diastereomeric complexes. In this chapter, we have reviewed the literature related to ent-cholesterol and its use as a probe for various biophysical and biological interactions of cholesterol.

A new triterpene ester and other chemical constituents from the aerial parts of Anodendron paniculatum and their cytotoxic activity

The aim of the research was to study the active constituents of Anodendron paniculatum Roxb. (Apocynaceae). A new triterpene ester, named anopaniester (1), and cycloartenol (2), ursolic acid (3), esculenic acid (4), bis-(2-ethylhexyl) phthalate (5), desmosterol (6), stigmasterol (7), vaniline (8), and (E)-phytol (9), were isolated from the aerial parts of A. paniculatum. Compounds 3 and 6 showed the significant inhibitory effect (IC50 values ranging from 30.89 ± 3.60 to 44.37 ± 5.40 μg/ml) against tested human cancer cell lines LU-1 and MKN-7. The compounds 1-4, and 6 were isolated from this genus Anodendron for the first time.

First synthesis of 22-oxa-chenodeoxycholic acid analogue

In this study, we report the first synthesis of 22-oxa-chenodeoxycholic acid analogue via androstenedione and progesterone, in 11 and 8 steps with overall yields of 6.4% and 12.7%, respectively. We anticipate this will help to facilitate the development of new drugs.

Identification of a cholesterol-binding pocket in inward rectifier K(+) (Kir) channels

Cholesterol is the major sterol component of all mammalian plasma membranes. Recent studies have shown that cholesterol inhibits both bacterial (KirBac1.1 and KirBac3.1) and eukaryotic (Kir2.1) inward rectifier K(+) (Kir) channels. Lipid-sterol interactions are not enantioselective, and the enantiomer of cholesterol (ent-cholesterol) does not inhibit Kir channel activity, suggesting that inhibition results from direct enantiospecific binding to the channel, and not indirect effects of changes to the bilayer. Furthermore, conservation of the effect of cholesterol among prokaryotic and eukaryotic Kir channels suggests an evolutionary conserved cholesterol-binding pocket, which we aimed to identify. Computational experiments were performed by docking cholesterol to the atomic structures of Kir2.2 (PDB: 3SPI) and KirBac1.1 (PDB: 2WLL) using Autodock 4.2. Poses were assessed to ensure biologically relevant orientation and then clustered according to location and orientation. The stability of cholesterol in each of these poses was then confirmed by molecular dynamics simulations. Finally, mutation of key residues (S95H and I171L) in this putative binding pocket found within the transmembrane domain of Kir2.1 channels were shown to lead to a loss of inhibition by cholesterol. Together, these data provide support for this location as a biologically relevant pocket.

Synthesis of a smoothened cholesterol: 18,19-di-nor-cholesterol

Herein, we report the first synthesis of a demethylated form of cholesterol (18,19-di-nor-cholesterol), in which the C18 and C19 methyl groups of the β-face were eliminated. Recent molecular simulations modeling 18,19-di-nor-cholesterol have suggested that cholesterol's opposing rough β-face and smooth α-face play necessary roles in cholesterol's membrane condensing abilities and, additionally, that specific facial preferences are displayed as cholesterol interacts with different neighboring lipids and transmembrane proteins. Inspired by these poorly characterized biochemical interactions, an extensive 18-step synthesis was completed as part of a collaborative effort, wherein synthesizing a "smoothened" cholesterol analogue would provide a direct way to experimentally measure the significance of the β-face methyl groups. Starting from known perhydrochrysenone A, the synthesis of 18,19-di-nor-cholesterol was accomplished with an excellent overall yield of 3.5%. The use of the highly stereoselective Dieckmann condensation and the employment of Evans' chiral auxiliary were both key to ensuring the success of this synthesis.

Lipid raft: A floating island of death or survival

Lipid rafts are microdomains of the plasma membrane enriched in cholesterol and sphingolipids, and play an important role in the initiation of many pharmacological agent-induced signaling pathways and toxicological effects. The structure of lipid rafts is dynamic, resulting in an ever-changing content of both lipids and proteins. Cholesterol, as a major component of lipid rafts, is critical for the formation and configuration of lipid raft microdomains, which provide signaling platforms capable of activating both pro-apoptotic and anti-apoptotic signaling pathways. A change of cholesterol level can result in lipid raft disruption and activate or deactivate raft-associated proteins, such as death receptor proteins, protein kinases, and calcium channels. Several anti-cancer drugs are able to suppress growth and induce apoptosis of tumor cells through alteration of lipid raft contents via disrupting lipid raft integrity.

Cholesterol tuning of BK ethanol response is enantioselective, and is a function of accompanying lipids

In the search to uncover ethanol's molecular mechanisms, the calcium and voltage activated, large conductance potassium channel (BK) has emerged as an important molecule. We examine how cholesterol content in bilayers of 1,2-dioleoyl-3-phosphatidylethanolamine (DOPE)/sphingomyelin (SPM) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylserine (POPS) affect the function and ethanol sensitivity of BK. In addition, we examine how manipulation of cholesterol in biological membranes modulates ethanol's actions on BK. We report that cholesterol levels regulate the change in BK channel open probability elicited by 50 mM ethanol. Low levels of cholesterol (<20%, molar ratio) supports ethanol activation, while high levels of cholesterol leads to ethanol inhibition of BK. To determine if cholesterol affects BK and its sensitivity to ethanol through a direct cholesterol-protein interaction or via an indirect action on the lipid bilayer, we used the synthetic enantiomer of cholesterol (ent-CHS). We found that 20% and 40% ent-CHS had little effect on the ethanol sensitivity of BK, when compared with the same concentration of nat-CHS. We accessed the effects of ent-CHS and nat-CHS on the molecular organization of DOPE/SPM monolayers at the air/water interface. The isotherm data showed that ent-CHS condensed DOPE/SPM monolayer equivalently to nat-CHS at a 20% concentration, but slightly less at a 40% concentration. Atomic force microscopy (AFM) images of DOPE/SPM membranes in the presence of ent-CHS or nat-CHS prepared with LB technique or vesicle deposition showed no significant difference in topographies, supporting the interpretation that the differences in actions of nat-CHS and ent-CHS on BK channel are not likely from a generalized action on bilayers. We conclude that membrane cholesterol influences ethanol's modulation of BK in a complex manner, including an interaction with the channel protein. Finally, our results suggest that an understanding of membrane protein function and modulation is impossible unless protein and surrounding lipid are considered as a functional unit.

Enantioselective protein-sterol interactions mediate regulation of both prokaryotic and eukaryotic inward rectifier K+ channels by cholesterol

Cholesterol is the major sterol component of all mammalian cell plasma membranes and plays a critical role in cell function and growth. Previous studies have shown that cholesterol inhibits inward rectifier K⁺ (Kir) channels, but have not distinguished whether this is due directly to protein-sterol interactions or indirectly to changes in the physical properties of the lipid bilayer. Using purified bacterial and eukaryotic Kir channels reconstituted into liposomes of controlled lipid composition, we demonstrate by ⁸⁶Rb⁺ influx assays that bacterial Kir channels (KirBac1.1 and KirBac3.1) and human Kir2.1 are all inhibited by cholesterol, most likely by locking the channels into prolonged closed states, whereas the enantiomer, ent-cholesterol, does not inhibit these channels. These data indicate that cholesterol regulates Kir channels through direct protein-sterol interactions likely taking advantage of an evolutionarily conserved binding pocket.

Relationship between phosphatidylinositol 4-phosphate synthesis, membrane organization, and lateral diffusion of PI4KIIalpha at the trans-Golgi network

Type II phosphatidylinositol 4-kinase IIalpha (PI4KIIalpha) is the dominant phosphatidylinositol kinase activity measured in mammalian cells and has important functions in intracellular vesicular trafficking. Recently PI4KIIalpha has been shown to have important roles in neuronal survival and tumorigenesis. This study focuses on the relationship between membrane cholesterol levels, phosphatidylinositol 4-phosphate (PI4P) synthesis, and PI4KIIalpha mobility. Enzyme kinetic measurements, sterol substitution studies, and membrane fragmentation analyses all revealed that cholesterol regulates PI4KIIalpha activity indirectly through effects on membrane structure. In particular, we found that cholesterol levels determined the distribution of PI4KIIalpha to biophysically distinct membrane domains. Imaging studies on cells expressing enhanced green fluorescent protein (eGFP)-tagged PI4KIIalpha demonstrated that cholesterol depletion resulted in morphological changes to the juxtanuclear membrane pool of the enzyme. Lateral membrane diffusion of eGFP-PI4KIIalpha was assessed by fluorescence recovery after photobleaching (FRAP) experiments, which revealed the existence of both mobile and immobile pools of the enzyme. Sterol depletion decreased the size of the mobile pool of PI4KIIalpha. Further measurements revealed that the reduction in the mobile fraction of PI4KIIalpha correlated with a loss of trans-Golgi network (TGN) membrane connectivity. We conclude that cholesterol modulates PI4P synthesis through effects on membrane organization and enzyme diffusion.

Synthesis of 19-Trideuterated ent-Testosterone and the GABA(A) Receptor Potentiators ent-Androsterone and ent-Etiocholanolone

19-Trideuteromethyl enantiomers of androgens namely ent-testosterone, ent-androsterone and ent-etiocholanolone were prepared by total synthesis. The isotope labeling at the carbon-19 angular methyl group was achieved by using deuterated methyl iodide (99.5% d(3)) for introduction of C-19 before closure of the steroid A-ring. This method yields 19,19,19-trideuterated steroids without increasing the number of steps involved in the total synthesis of ent-androgens. Analysis by mass spectrometry showed no loss of deuterium during incorporation of C-19 into ent-testosterone. The availability of the compounds will enable these ent-androgens to be distinguished by mass spectrometry from their natural enantiomers in future pharmacokinetic and metabolic studies.

Synthesis, characterization, and receptor interaction profiles of enantiomeric bile acids

Bile acids are endogenous steroid detergents with receptor-mediated physiologic actions including activation of the G-protein coupled receptor TGR5 and gene regulation mediated by nuclear receptors. In this study, we report the first synthesis of enantiomeric lithocholic acid (ent-LCA, ent-1) and chenodeoxycholic acid (ent-CDCA, ent-2) via ent-testosterone (3). ent-1 was synthesized in 21 total steps in 4.2% yield, whereas ent-2 was obtained in 23 total steps in 0.8% yield. Critical micelle concentrations of the enantiomeric bile acids were found to be identical to their natural counterparts. Furthermore, enantiomeric bile acids were also tested for their ability to modulate bile acid activated proteins: farnesoid X receptor, vitamin D receptor, pregnane X receptor, and TGR5. Interestingly, ent-1 and ent-2 showed differential interactions with these proteins as compared to their corresponding natural bile acids. These data highlight the potential for using enantioselectivity as a way to distinguish between receptor and nonreceptor-mediated functions of natural bile acids.

Rapid transient absorption and biliary secretion of enantiomeric cholesterol in hamsters

To probe the pathway and specificity of cholesterol absorption, the synthetic enantiomer of cholesterol (ent-cholesterol) and cholesterol were labeled with deuterium, gavaged into hamsters, and measured by negative ion mass spectrometry. Initial uptake of both tracers into the intestinal mucosa at 30 min was similar but cholesterol was temporarily retained there, whereas mucosal ent-cholesterol declined rapidly with concomitantly increased enrichment in both the systemic circulation and the gut lumen. In a 3 day fecal recovery study, ent-cholesterol was quantitatively recovered in the stool, whereas cholesterol absorption was 53.2%. ent-Cholesterol given by intracardiac injection was selectively secreted into bile, and the ratio of ent-cholesterol to cholesterol tracers in the gut lumen increased down the length of the small bowel, with the largest value being found in stool. ent-Cholesterol is efficiently taken up by the intestinal mucosa and undergoes transient enterohepatic recirculation, but it is quantitatively eliminated over 3 days as a result of selective secretion into bile and selective enrichment within the lumen of the intestine. These findings suggest that cholesterol absorption is structurally specific and likely to be mediated by enantiospecific cellular proteins.

Sterol intermediates from cholesterol biosynthetic pathway as liver X receptor ligands

The liver X receptors (LXRs) are ligand-activated transcription factors that regulate the expression of genes controlling lipid metabolism. Oxysterols bind LXRs with high affinity in vitro and are implicated as ligands for the receptor. We showed previously that accumulation of selected dietary sterols, in particular stigmasterol, is associated with activation of LXR in vivo. In the course of the defining of structural features of stigmasterol that confer LXR agonist activity, we determined that the presence of an unsaturated bond in the side chain of the sterol was necessary and sufficient for activity, with the C-24 unsaturated cholesterol precursor sterols desmosterol and zymosterol exerting the largest effects. Desmosterol failed to increase expression of the LXR target gene, ABCA1, in LXRalpha/beta-deficient mouse fibroblasts, but was fully active in cells lacking cholesterol 24-, 25-, and 27-hydroxylase; thus, the effect of desmosterol was LXR-dependent and did not require conversion to a side chain oxysterol. Desmosterol bound to purified LXRalpha and LXRbeta in vitro and supported the recruitment of steroid receptor coactivator 1. Desmosterol also inhibited processing of the sterol response element-binding protein-2 and reduced expression of hydroxymethylglutaryl-CoA reductase. These observations are consistent with specific intermediates in the cholesterol biosynthetic pathway regulating lipid homeostasis through both the LXR and sterol response element-binding protein pathways.

Synthesis of ent-25-hydroxycholesterol

25-Hydroxycholesterol (25-HC) appears to play a role in several important biological processes, including regulating cellular cholesterol levels and promoting apoptosis. However, in most cases the mechanisms by which 25-HC elicits its biological effects are not known. Insights into mechanisms of 25-HC action can be gained by studying the activity of its enantiomer (ent-25-HC). ent-25-HC is physically and chemically identical to 25-HC; however, 25-HC and ent-25-HC can be distinguished in chiral environments, like a protein binding site. In order to probe the mechanisms of 25-HC action, we have synthesized the enantiomer of 25-HC (ent-25-HC).

Comparison of effects of U18666A and enantiomeric U18666A on sterol synthesis and induction of apoptosis

Treatment of animals or cells with the amphipathic tertiary amine U18666A {3beta-[2-(diethylamino) ethoxy]androst-5-en-17-one} provides models for several human diseases (e.g., cataracts, Niemann-Pick disease, and epilepsy). Although U18666A can inhibit several enzymes in the cholesterol synthesis pathway, we hypothesized that induction of these varied conditions was due to physical effects of the amine rather than to inhibition of specific proteins. To test this possibility we compared the capacity of U18666A and its enantiomer, ent-U18666A, to inhibit net sterol synthesis and induce apoptosis in cultured bovine lens epithelial cells. Nonenantiospecific actions dependent on the physical properties of these mirror image molecules would be identical, but effects dependent upon enantiospecific interactions would be different for the enantiomers. At the same concentrations, both forms of the compound equally inhibited sterol synthesis and induced apoptosis. These observations supported a generalized mechanism of enzyme inhibition such as perturbation of the microenvironment of endoplasmic enzymes and alteration of membrane order, perhaps of the mitochondrial membrane, to explain induction of apoptosis.

Investigating the allosterism of acyl-CoA:cholesterol acyltransferase (ACAT) by using various sterols: in vitro and intact cell studies

ACAT1 (acyl-CoA:cholesterol acyltransferase 1) is thought to have two distinct sterol-binding sites: a substrate-binding site and an allosteric-activator site. In the present work, we investigated the structural features of various sterols as substrates and/or activators in vitro. The results show that without cholesterol, the plant sterol sitosterol is a poor substrate for ACAT. In the presence of cholesterol, ACAT1-mediated esterification of sitosterol is highly activated while ACAT2-mediated esterification of sitosterol is only moderately activated. For ACAT1, we show that the stereochemistry of the 3-hydroxy group at steroid ring A is a critical structural feature for a sterol to serve as a substrate, but less critical for activation. Additionally, enantiomeric cholesterol, which has the same biophysical properties as cholesterol in membranes, fails to activate ACAT1. Thus ACAT1 activation by cholesterol is the result of stereo-specific interactions between cholesterol and ACAT1, and is not related to the biophysical properties of phospholipid membranes. To demonstrate the relevance of the ACAT1 allosteric model in intact cells, we showed that sitosterol esterification in human macrophages is activated upon cholesterol loading. We further show that the activation is not due to an increase in ACAT1 protein content, but is partly due to an increase in the cholesterol content in the endoplasmic reticulum where ACAT1 resides. Together, our results support the existence of a distinct sterol-activator site in addition to the sterol-substrate site of ACAT1 and demonstrate the applicability of the ACAT1 allosteric model in intact cells.

Steroids: reactions and partial synthesis

This article reviews the progress in the chemistry of the steroids that was published between January and December 2003. The reactions and partial synthesis of estrogens, androgens, pregnanes, cholic acid derivatives, cholestanes and vitamin D analogues are covered. There are 152 references.

The Enantiomer of Cholesterol

Cholesterol plays a variety of significant roles in biological systems. However, the mechanisms by which cholesterol functions remain largely unclear. The enantiomer of cholesterol (ent-cholesterol)--which has identical physical properties, but opposite three-dimensional configuration compared to cholesterol--is a unique tool that can be used to better understand the mechanisms of cholesterol function. We review the literature pertaining to ent-cholesterol, focusing in particular on its use in biological studies.

Lipid rafts: heterogeneity on the high seas

Lipid rafts are membrane microdomains that are enriched in cholesterol and glycosphingolipids. They have been implicated in processes as diverse as signal transduction, endocytosis and cholesterol trafficking. Recent evidence suggests that this diversity of function is accompanied by a diversity in the composition of lipid rafts. The rafts in cells appear to be heterogeneous both in terms of their protein and their lipid content, and can be localized to different regions of the cell. This review summarizes the data supporting the concept of heterogeneity among lipid rafts and outlines the evidence for cross-talk between raft components. Based on differences in the ways in which proteins interact with rafts, the Induced-Fit Model of Raft Heterogeneity is proposed to explain the establishment and maintenance of heterogeneity within raft populations.

Cholesterol depletion results in site-specific increases in epidermal growth factor receptor phosphorylation due to membrane level effects. Studies with cholesterol enantiomers

In A431 cells, depletion of cholesterol with methyl-beta-cyclodextrin induced an increase in both basal and epidermal growth factor (EGF)-stimulated EGF receptor phosphorylation. This increase in phosphorylation was site-specific, with significant increases occurring at Tyr845, Tyr992, and Tyr1173, but only minor changes at Tyr1045 and Tyr1068. The elevated level of receptor phosphorylation was associated with an increase in the intrinsic kinase activity of the EGF receptor kinase, possibly as a result of the cyclodextrin-induced enhancement of the phosphorylation of Tyr845, a site in the kinase activation loop known to be phosphorylated by pp60src. Cholesterol and its enantiomer (ent-cholesterol) were used to investigate the molecular basis for the modulation of EGF receptor function by cholesterol. Natural cholesterol (nat-cholesterol) was oxidized substantially more rapidly than ent-cholesterol by cholesterol oxidase, a protein that contains a specific binding site for the sterol. By contrast, the ability of nat- and ent-cholesterol to interact with sphingomyelins and phosphatidylcholine and to induce lipid condensation in a monolayer system was the same. These data suggest that, whereas cholesterol-protein interactions may be sensitive to the absolute configuration of the sterol, sterol-lipid interactions are not. nat- and ent-cholesterol were tested for their ability to physically reconstitute lipid rafts following depletion of cholesterol. nat- and ent-cholesterol reversed to the same extent the enhanced phosphorylation of the EGF receptor that occurred following removal of cholesterol. Furthermore, the enantiomers showed similar abilities to reconstitute lipid rafts in cyclodextrin-treated cells. These data suggest that cholesterol most likely affects EGF receptor function because of its physical effects on membrane properties, not through direct enantioselective interactions with the receptor.

Differential interaction of the two cholesterol-dependent, membrane-damaging toxins, streptolysin O and Vibrio cholerae cytolysin, with enantiomeric cholesterol

Membrane cholesterol is essential to the activity of at least two structurally unrelated families of bacterial pore-forming toxins, represented by streptolysin O (SLO) and Vibrio cholerae cytolysin (VCC), respectively. Here, we report that SLO and VCC differ sharply in their interaction with liposome membranes containing enantiomeric cholesterol (ent-cholesterol). VCC had very low activity with ent-cholesterol, which is in line with a stereospecific mode of interaction of this toxin with cholesterol. In contrast, SLO was only slightly less active with ent-cholesterol than with cholesterol, suggesting a rather limited degree of structural specificity in the toxin-cholesterol interaction.