Synthesis and biological evaluation of a new series of sterols as potential hypocholesterolemic agents

Evolution of a Short and Stereocontrolled Synthesis of (+)-7,20-Diisocyanoadociane

A full account of the development of a concise and highly stereoselective synthesis of (+)-7,20-diisocyanoadociane (DICA)─a structurally complex isocyanoditerpene with potent antiplasmodial activity─is described. The strategy that evolved relies on the rapid construction of unsaturated tricyclic precursors designed to undergo stereocontrolled Birch reductions and a subsequent "bay ring" formation to generate the isocycloamphilectane core. This report is divided into three sections: (1) a description of the initial strategy and the results that focused our efforts on a single route to the DICA core, (2) a discussion of the precise choreography needed to enable a first-generation formal synthesis of (±)-DICA, and (3) the execution of a 13-step second-generation synthesis of (+)-DICA that builds on important lessons learned from the first-generation effort.

Side-chain oxysterols modulate cholesterol accessibility through membrane remodeling

Side-chain oxysterols, such as 25-hydroxycholesterol (25-HC), are key regulators of cholesterol homeostasis. New evidence suggests that the alteration of membrane structure by 25-HC contributes to its regulatory effects. We have examined the role of oxysterol membrane effects on cholesterol accessibility within the membrane using perfringolysin O (PFO), a cholesterol-dependent cytolysin that selectively binds accessible cholesterol, as a sensor of membrane cholesterol accessibility. We show that 25-HC increases cholesterol accessibility in a manner dependent on the membrane lipid composition. Structural analysis of molecular dynamics simulations reveals that increased cholesterol accessibility is associated with membrane thinning, and that the effects of 25-HC on cholesterol accessibility are driven by these changes in membrane thickness. Further, we find that the 25-HC antagonist LY295427 (agisterol) abrogates the membrane effects of 25-HC in a nonenantioselective manner, suggesting that agisterol antagonizes the cholesterol-homeostatic effects of 25-HC indirectly through its membrane interactions. These studies demonstrate that oxysterols regulate cholesterol accessibility, and thus the availability of cholesterol to be sensed and transported throughout the cell, by modulating the membrane environment. This work provides new insights into how alterations in membrane structure can be used to relay cholesterol regulatory signals.

Synthesis of the enantiomer of the oxysterol-antagonist LY295427

Cellular cholesterol homeostasis is regulated by oxygenated cholesterol metabolites called oxysterols. While the importance of oxysterols in the acute regulation of cholesterol homeostasis is known, the precise molecular mechanisms through which oxysterols exert their effects remain to be elucidated. LY295427 (1) is a known antagonist of the cholesterol-homeostatic effects of 25-hydroxycholesterol (25-HC), a biologically active oxysterol. In order to examine the mechanism of action of this antagonism, and to further explore recent evidence suggesting that the membrane effects of 25-HC contribute to acute cholesterol regulation, we synthesized the enantiomer of LY295427 (ent-LY295427). ent-LY295427 (2) will serve as a unique probe to provide insight into the role of transcription-independent mechanisms in regulation of cholesterol homeostasis.

Characterization of NVX-207, a novel betulinic acid-derived anti-cancer compound

BACKGROUND

Development of betulinic acid derivatives for clinical use has been hampered by adverse pharmacological and physico-chemical characteristics of this class of compounds. We here present a novel semi-synthetic betulinic acid-derived drug candidate well suited for further clinical development.

MATERIALS AND METHODS

In vitro activity and mode of action of NVX-207 were determined using normal as well as cancer cell lines. Gene expression profiling was performed with Affymetrix U133 microarrays. NVX-207 binding partners were identified using a heterobifunctional chemical crosslinker system. Potential binding proteins were identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) analysis. Clinical studies were conducted in canine cancer patients suffering from spontaneously arising pre-treated tumours.

RESULTS

NVX-207 showed anti-tumour activity (mean IC(50) = 3.5 microM) against various human and canine cell lines. NVX-207-induced apoptosis was associated with activation of the intrinsic apoptotic pathway via cleavage of caspases -9, -3, -7 and of poly (ADP-ribose) polymerase (PARP). Global gene expression profiling demonstrated regulation of genes associated with lipid metabolism, most notably an upregulation of genes coding for insulin-induced gene 1 (Insig-1), low-density lipoprotein receptor (LDL-R) and of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA). NVX-207 bound to apolipoprotein A-I, a major regulator of lipid metabolism and cholesterol transport. A phase I/II study in dogs suffering from naturally occurring cancer receiving local treatment of NVX-207 (10 mg mL(-1)) showed excellent clinical responses including a complete remission in so far 5/5 treated animals.

CONCLUSIONS

NVX-207 is well tolerated and has significant anti-cancer activity in vitro and in vivo in dogs with treatment-resistant malignancies.

Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: oxysterols block transport by binding to Insig

Cholesterol synthesis in animals is controlled by the regulated transport of sterol regulatory element-binding proteins (SREBPs) from the endoplasmic reticulum to the Golgi, where the transcription factors are processed proteolytically to release active fragments. Transport is inhibited by either cholesterol or oxysterols, blocking cholesterol synthesis. Cholesterol acts by binding to the SREBP-escort protein Scap, thereby causing Scap to bind to anchor proteins called Insigs. Here, we show that oxysterols act by binding to Insigs, causing Insigs to bind to Scap. Mutational analysis of the six transmembrane helices of Insigs reveals that the third and fourth are important for Insig's binding to oxysterols and to Scap. These studies define Insigs as oxysterol-binding proteins, explaining the long-known ability of oxysterols to inhibit cholesterol synthesis in animal cells.

Effects of 25-Hydroxycholesterol on Cholesterol Esterification and Sterol Regulatory Element-binding Protein Processing Are Dissociable

The regulatory pool of cholesterol is located in the endoplasmic reticulum (ER) and is key to how mammalian cells sense and respond to changes in cellular cholesterol levels. The extent of cholesterol esterification by the ER-resident protein, acyl-coenzyme A:cholesterol acyl-transferase (ACAT), has become the standard method for monitoring cholesterol transport to the ER and is assumed to reflect the regulatory pool of ER cholesterol. The oxysterol, 25-hydroxycholesterol (25HC), is thought to trigger intracellular cholesterol transport to the ER. In support of this contention, we confirmed previous reports that 25HC activates cholesterol esterification and is a potent suppressor of the sterol regulatory element-binding protein (SREBP) pathway. Processing of the ER membrane-bound SREBP into a soluble transcription factor is controlled by cholesterol levels in the ER. In this study, we addressed whether or not cholesterol esterification necessarily reflects cholesterol movement to the cholesterol homeostatic machinery in the ER as determined by SREBP processing. We found that three agents that inhibited the ability of 25HC to induce cholesterol esterification (progesterone, nigericin, and monensin) did not have a corresponding effect on 25HC suppression of SREBP processing. Moreover, ACAT inhibition did not alter the sensitivity of SREBP processing to 25HC. Therefore, cholesterol esterification by the ER-resident protein ACAT is dissociable from cholesterol transport to the cholesterol homeostatic machinery in the ER. In light of our results, we question the security of previous work that has inferred cholesterol transport to the ER regulatory pool based solely on cholesterol esterification.

The hypocholesterolemic agent LY295427 up-regulates INSIG-1, identifying the INSIG-1 protein as a mediator of cholesterol homeostasis through SREBP

Oxysterols regulate cholesterol homeostasis through liver X receptor (LXR; cholesterol-lowering)- and sterol regulatory element-binding protein (SREBP; cholesterol-raising)-mediated signaling pathways. Previously we reported that the hypocholesterolemic agent LY295427 (4alpha-allylcholestan-3alpha-ol) reverses oxysterol-mediated suppression of SREBP processing. We now report that LY295427 increases expression of insulin-induced gene-1 (INSIG-1) and restores SREBP processing in cells treated with oxysterols. In cells overexpressing the INSIG-1 gene, by contrast, SREBP processing is suppressed and oxysterol regulation is disrupted. SREBP processing is not restored by addition of LY295427, but is restored by increasing the levels of SREBP cleavage-activating protein (SCAP). These findings suggest that the INSIG-1 protein alters sterol balance by modulating SREBP processing jointly with SCAP. To test whether the action of oxysterols on SREBP processing is mediated through endogenous INSIG-1 protein, we used RNAi to lower the expression of the INSIG-1 gene, and found that reduced INSIG-1 protein levels caused the loss of SREBP regulation by oxysterols. We conclude that: (i) INSIG-1 gene expression is suppressed by oxysterols; (ii) LY295427 treatment counters the suppressive effects of oxysterols on SREBP processing, resulting in the expression of the INSIG-1 gene; and (iii) INSIG-1 gene expression affects SREBP processing. Taken together, these data suggest that INSIG-1 plays a critical role in regulating cholesterol concentrations in the cell.

The hypocholesterolemic agent LY295427 reverses suppression of sterol regulatory element-binding protein processing mediated by oxysterols

The sterol LY295427 reduces plasma cholesterol levels in animals by increasing the expression of hepatic low density lipoprotein (LDL) receptors. Here we trace the hypocholesterolemic activity of LY295427 to an ability to reverse oxysterol-mediated suppression of sterol regulatory element-binding protein (SREBP) processing. Micromolar concentrations of LY295427 induced the metabolism of LDL in oxysterol-treated cultured cells and inhibited the stimulation of cholesteryl ester synthesis mediated by oxysterols. cDNA microarray and RNA blotting experiments revealed that LY295427 increased levels of the LDL receptor mRNA and those of other SREBP target genes. The compound stimulated the accumulation of SREBPs in the nuclei of cells grown in the presence of oxysterols within 4-6 h of addition to the medium. Induction required components of the normal SREBP-processing pathway, including the SREBP cleavage-activating protein and the Site 1 protease. LY295427 overcame the suppression of SREBP processing mediated by several oxysterols but not by LDL-derived cholesterol. We conclude that LY295427 achieves a therapeutically desirable end point by an unique mechanism of action.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Up-regulation of low density lipoprotein receptor by a novel isobenzofranone derivative, MD-700

Stimulatory effects of a novel isobenzofranone, MD-700, on low density lipoprotein (LDL) receptor activity were investigated in vitro and in vivo. MD-700 at 0.03 microg/ml elevated the expression of LDL receptor in HepG2 cells within 4 h. Corresponding to this, uptake of fluorescent labeled-LDL (3,3'-dioctadecylindocarbocyanine-LDL) by the cells increased linearly in time- and dose-dependent manner by MD-700 for up to 12 h. In the experiment using HepG2 cells transiently transfected with promoter-luciferase gene constructs, MD-700 increased luciferase activity in a dose-dependent manner from 0.03 to 0.1 microg/ml. In contrast, luciferase activity was not stimulated by MD-700 in construct with a deleted sterol regulatory element (SRE)-1, suggesting importance of SRE-1 in stimulation of the LDL receptor gene promoter by MD-700. Binding experiments on liver membranes from MD-700-treated hamsters showed about a 60% increase in 125I-labeled LDL binding. A Scatchard plot revealed that MD-700 increased the maximal binding without affecting binding affinity. In contrast to findings with an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, pravastatin, MD-700 had no effect on the sterol synthesis in hamster liver homogenates. These results suggest that MD-700 stimulates the expression of LDL receptor, presumably in a manner independent of change in sterol metabolism, and thereby promotes LDL clearance. Hypocholesterolemic actions of MD-700 in hamsters were then examined. MD-700 lowered serum cholesterol levels in hamsters fed normal chow or a high-fat diet. Fractionation of serum lipoproteins demonstrated that MD-700 selectively decreased LDL and very low density lipoprotein cholesterol. Dose-dependent decrease in serum cholesterol was also seen in hypercholesterolemic rats. Thus, the hypocholesterolemic action of MD-700 may be attributed to up-regulation of the LDL receptor, based on stimulation of the transcription of the LDL receptor gene. Although pravastatin stimulates LDL uptake and lowers serum cholesterol in a manner similar to that seen with MD-700, the mechanism responsible for hypocholesterolemic action appears to differ.

Distribution of exogenous 25-hydroxycholesterol in Hep G2 cells between two different pools

Binding of [26,27-(3)H]25-hydroxycholesterol (25HC) to human hepatoma Hep G2 cells was saturated within 120 min. Two intracellular pools of 25HC were identified in a pulse-chase experiment: (i) an exchangeable pool which was in dynamic equilibrium with 25HC in the medium (t(1/2) of reversible exchange 15 min) and (ii) an unexchangeable pool which remained in cells during incubation in medium containing LPDS. 25HC from the exchangeable pool inhibits cholesterol biosynthesis, decreases the HMG CoA reductase mRNA level and stimulates cholesterol acylation. 25HC from the unexchangeable pool was partially bound to cytosolic proteins and apparently utilized for metabolic transformation. Incubation of Hep G2 cells with [26,27-(3)H]25HC in the presence of a 30-fold molar excess of 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one was found to cause (i) 2-fold decrease in the binding of [26,27-(3)H]25HC to cytosolic proteins (sedimentation constant of radioactive complex was 4-5 S) and (ii) the 35% inhibition of 25HC transformation to polar metabolites.

Synthesis of 4 alpha-(2-propenyl)-5,6-secocholestan-3 alpha-ol, a novel B-ring seco analog of the hypocholesterolemic agent 4 alpha-(2-propenyl)-5 alpha-cholestan-3 alpha-ol

4 alpha-(2-Propenyl)-5 alpha-cholestan-3 alpha-ol (LY295427) was previously identified from a CHO cell-based assay to be a potent LDL receptor up-regulator and had demonstrated to be an effective agent in lowering plasma cholesterol levels in hypercholesterolemic hamsters. In order to investigate the effect of flexibility of the 3 alpha-hydroxy-bearing A-ring on the activity, 4 alpha-(2-propenyl)-5,6-secocholestan-3 alpha-ol (11), a B-ring seco analog of LY295427, is thus synthesized from cholest-4-en-3-one. Test results indicate that 11 is not active in the CHO cell-based LDL receptor/luciferase assay at concentrations up to 20 micrograms/mL. The result underlines the importance of maintaining the A-B-C-D ring rigidity of the 3 alpha-sterols in terms of binding to the putative oxysterol receptor.

The synthesis and in vitro activity of some delta 7,9(11)-lanostadienes

The synthesis of delta 7,9(11)-lanostadiene derivatives functionalized at C(32) starting from 3 beta-acetoxy-7 alpha,32-epoxylanostan-11-one has been presented. The delta 7,9(11) moiety was efficiently introduced in three steps in 71% yield by the regioselective abstraction of allylic 8 beta hydrogen. The formyl group of the key intermediate, 3 beta-benzoyloxylanosta-7,9(11)-dien-32-al, has been stereoselectively alkylated into (32S) derivative, whereas its oxidation unexpectedly afforded 3 beta-benzoyloxy-7-oxolanost-8-ene-32,11 alpha-lactone and not the corresponding acid. delta 7,9(11)-lanostadienes possessing HC(32)=O, C(32) [symbol: see text] N, HC(32S)CH3OH, H2C(32)OH, as well as some 11-keto lanostenes, were tested in vitro against several purified cholesterogenic enzymes showing moderate activity, with most the active aldehyde 16 having IC50 = 86 microM.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Substrate-based inhibitors of lanosterol 14 alpha-methyl demethylase: I. Assessment of inhibitor structure-activity relationship and cholesterol biosynthesis inhibition properties

A series of 15-, 32-, and 15,32-substituted lanost-8-en-3 beta-ols is described which function as inhibitors of cholesterol biosynthesis. These agents inhibit lanosterol 14 alpha-methyl demethylase activity as well as suppress HMG-CoA reduction activity in cultured cells. Several of these agents are extremely potent as both demethylase inhibitors and reductase suppressors, while others are more selective in their activities. Selected regio double bond isomers show preference for demethylase inhibition with the following order: delta 8 > delta 7 > delta 6 = unsaturated sterols. Comparisons also show that 4,4-dimethyl sterols are always more potent demethylase inhibitors and reductase suppressors than their 4,4-bisnomethyl counterparts. However, evaluation of an extensive oxylanosterol series leads us to conclude that demethylase inhibition and reductase suppression are not parallel in the same molecule. In addition, the oxylanosterols, but not the oxycholesterols, are able to disrupt coordinate regulation of HMG-CoA reductase from the LDL receptor. Thus, oxylanosterol treatment at levels which suppress reductase activity enhances LDL receptor activity. These results demonstrate that compounds can be made which (1) are selective reductase suppressors enabling dissection of the dual inhibitor nature of these compounds and (2) maximize reductase suppression and LDL receptor induction without demethylase inhibition which could lead to novel agents for serum cholesterol lowering.