Insights into estrogen receptor alpha modulation by cholestenoic acids

Oxysterols are a family of over 25 cholesterol metabolites naturally produced by enzymatic or radical oxidation. They are involved in many physiological and pathological pathways. Although their activity has been mainly attributed to the modulation of the Liver X Receptors (LXR), it is currently accepted that oxysterols are quite promiscuous compounds, acting at several targets at the same time. The promiscuity of the oxysterols with the Estrogen Receptor α (ERα) is crucial in several pathologies such as ER⁺ breast cancer, inflammation and atherosclerosis. Regarding this matter, we have previously reported the synthesis, LXR activity and binding mode of a family of cholestenoic acid analogs with a modified side chain. Here we report the transcriptional activity on the ERα triggered by these compounds and details on the molecular determinants involved in their activities in order to establish structure-activity relationships to shed light over the molecular basis of the promiscuity of these compounds on ER/LXR responses. Our results show that 3β-hydroxy-5-cholestenoic acid can interact with the ERα receptor in a way similar to 26-hydroxycholesterol and is an agonist of the receptor. Using molecular dynamics simulations, we were able to predict the ERα activity of a set of cholestenoic acid analogs with changes in the flexibility and/or steric requirements of the side chain, some of which exhibited selective activation of ERα or LXR.

Multiple Targets for Oxysterols in Their Regulation of the Immune System

Oxysterols, or cholesterol oxidation products, are naturally occurring lipids which regulate the physiology of cells, including those of the immune system. In contrast to effects that are mediated through nuclear receptors or by epigenetic mechanism, which take tens of minutes to occur, changes in the activities of cell-surface receptors caused by oxysterols can be extremely rapid, often taking place within subsecond timescales. Such cell-surface receptor effects of oxysterols allow for the regulation of fast cellular processes, such as motility, secretion and endocytosis. These cellular processes play critical roles in both the innate and adaptive immune systems. This review will survey the two broad classes of cell-surface receptors for oxysterols (G-protein coupled receptors (GPCRs) and ion channels), the mechanisms by which cholesterol oxidation products act on them, and their presence and functions in the different cell types of the immune system. Overall, this review will highlight the potential of oxysterols, synthetic derivatives and their receptors for physiological and therapeutic modulation of the immune system.

The Liver under the Spotlight: Bile Acids and Oxysterols as Pivotal Actors Controlling Metabolism

Among the myriad of molecules produced by the liver, both bile acids and their precursors, the oxysterols are becoming pivotal bioactive lipids which have been underestimated for a long time. Their actions are ranging from regulation of energy homeostasis (i.e., glucose and lipid metabolism) to inflammation and immunity, thereby opening the avenue to new treatments to tackle metabolic disorders associated with obesity (e.g., type 2 diabetes and hepatic steatosis) and inflammatory diseases. Here, we review the biosynthesis of these endocrine factors including their interconnection with the gut microbiota and their impact on host homeostasis as well as their attractive potential for the development of therapeutic strategies for metabolic disorders.

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.

Cholestenoic acid analogues as inverse agonists of the liver X receptors

Liver X Receptors (LXRs) are ligand dependent transcription factors activated by oxidized cholesterol metabolites (oxysterols) that play fundamental roles in the transcriptional control of lipid metabolism, cholesterol transport and modulation of inflammatory responses. In the last decade, LXRs have become attractive pharmacological targets for intervention in human metabolic diseases and thus, several efforts have concentrated on the development of synthetic analogues able to modulate LXR transcriptional response. In this sense, we have previously found that cholestenoic acid analogues with a modified side chain behave as LXR inverse agonists. To further investigate the structure-activity relationships and to explore how cholestenoic acid derivatives interact with the LXRs, we evaluated the LXR biological activity of new analogues containing a C24-C25 double bond. Furthermore, a microarray assay was performed to evaluate the recruitment of coregulators to recombinant LXR LBD upon ligand binding. Also, conventional and accelerated molecular dynamics simulations were applied to gain insight on the molecular determinants involved in the inverse agonism. As LXR inverse agonists emerge as very promising candidates to control LXR activity, the cholestenoic acid analogues here depicted constitute a new relevant steroidal scaffold to inhibit LXR action.

Omics analysis of oxysterols to better understand their pathophysiological role

High amounts of cholesterol have been definitely associated with the pathogenesis of several diseases, including metabolic and neurodegenerative disorders, cardiovascular diseases, and cancer. In all these pathologies the exacerbation of pro-oxidant and inflammatory responses is a consistent feature. In this scenario, species derived from enzymatic and non-enzymatic cholesterol oxidation, namely oxysterols, are strongly suspected to play a primary role. The consideration of these bioactive lipids is therefore helpful in investigating pathological mechanisms and may also acquire clinical value for the diagnosis and treatment of diseases. For this purpose and considering that a great number of oxysterols may be present together in the body, the employment of lipidomics technology certainly represents a powerful strategy for the simultaneous detection and characterization of these compounds in biological specimens. In this review, we will discuss the applicability of the lipidomics approach in the study of the association between oxysterols and diseases.

Fast liquid chromatography-mass spectrometry reveals side chain oxysterol heterogeneity in breast cancer tumour samples

Oxysterols can contribute to proliferation of breast cancer through activation of the Estrogen Receptors, and to metastasis through activation of the Liver X Receptors. Endogenous levels of both esterified and free sidechain-hydroxylated oxysterols were examined in breast cancer tumours from Estrogen Receptor positive and negative breast tumours, using a novel fast liquid chromatography tandem mass spectrometry method. Multiple aliquots of five milligram samples of 22 tumours were analysed for oxysterol content to assess intra- and inter-tumour variation. Derivatization was performed with Girard T reagent (with and without alkaline hydrolysis) and sample clean-up was performed using a robust automatic on-line column switching system ("AFFL"). Oxysterols were separated isocratically on a 2.1 mm inner diameter column packed with ACE SuperPhenylHexyl core shell particles using a mobile phase consisting of 0.1% formic acid in H₂O/methanol/acetonitrile (57/10/33, v/v/v) followed by a wash out step (0.1% formic acid in methanol/acetonitrile, 50/50, v/v). The total analysis time, including sample clean-up and column reconditioning, was 8 min (80% time reduction compared to other on-line systems). Analysis revealed large intra-tumour variations of sidechain oxysterols, resulting in no significant differences in endogenous oxysterols levels between Estrogen Receptor positive and Estrogen Receptor negative breast cancers. However, a correlation between esterified and free 27-hydroxycholesterol was observed. The same correlation was not observed for 24S-hydroxycholesterol or 25-hydroxycholesterol. The oxysterol heterogeneity of tumour tissue is a critical factor when assessing the role of these lipids in cancer.

Inhibition of Hedgehog Signaling in Fibroblasts, Pancreatic, and Lung Tumor Cells by Oxy186, an Oxysterol Analogue with Drug-Like Properties

The widespread involvement of the Hedgehog (Hh) signaling pathway in human malignancies has motivated the clinical development of Smoothened (Smo) antagonists, such as vismodegib and sonidegib. However, Smo antagonists have failed to benefit patients suffering from Hh pathway-dependent solid tumors, such as pancreatic, colorectal, or ovarian cancer. Hh-dependent cancers are often driven by activating mutations that occur downstream of Smo and directly activate the transcription factors known as glioma-associated oncogenes (Gli1-3). Hence, the direct targeting of Gli could be a more effective strategy for achieving disease modification compared to Smo antagonism. In this study, we report on the biological and pharmacological evaluation of Oxy186, a semisynthetic oxysterol analogue, as a novel inhibitor of Hh signaling acting downstream of Smo, with encouraging drug-like properties. Oxy186 exhibits strong inhibition of ligand-induced Hh signaling in NIH3T3-E1 fibroblasts, as well as in constitutively activated Hh signaling in Suppressor of Fused (Sufu) null mouse embryonic fibroblast (MEF) cells. Oxy186 also inhibits Gli1 transcriptional activity in NIH3T3-E1 cells expressing exogenous Gli1 and Gli-dependent reporter constructs. Furthermore, Oxy186 suppresses Hh signaling in PANC-1 cells, a human pancreatic ductal adenocarcinoma (PDAC) tumor cell line, as well as PANC-1 cell proliferation in vitro, and in human lung cancer cell lines, A549 and H2039.

Oxysterols from a Marine Sponge Inflatella sp. and Their Action in 6-Hydroxydopamine-Induced Cell Model of Parkinson's Disease

Four new oxysterols 1⁻4 along with previously known oxygenated sterols 5⁻14 were isolated from the sponge Inflatella sp., collected from the Sea of Okhotsk. Structures of 1⁻4 were elucidated by the detailed NMR spectroscopic and mass-spectrometric analyses as well as by comparison of the corresponding experimental data with those reported in literature. The influence of compounds 1⁻14 on the viability of neuronal Neuro2a cells treated by 6-hydroxydopamine and reactive oxygen species (ROS) formation in these cells was investigated.

Bobbing of Oxysterols: Molecular Mechanism for Translocation of Tail-Oxidized Sterols through Biological Membranes

Translocation of sterols between cellular membrane leaflets is of key importance in membrane organization, dynamics, and signaling. We present a novel translocation mechanism that differs in a unique manner from the established ones. The bobbing mechanism identified here is demonstrated for tail-oxidized sterols, but is expected to be viable for any molecule containing two polar centers at the opposite sides of the molecule. The mechanism renders translocation across a lipid membrane possible without a change in molecular orientation. For tail-oxidized sterols, the bobbing mechanism provides an exceptionally facile means to translocate these signaling molecules across membrane structures and may thus represent an important pathway in the course of their biological action.

Formation of oxysterols during thermal processing and frozen storage of cooked minced meat

BACKGROUND

Cholesterol is susceptible to oxidation and the formation of oxysterols, which could have a negative health effect. The formation and distribution of oxysterols was investigated in meatloaves prepared under different baking regimes with an increased temperature or prolonged time. The effect of frozen storage and marjoram addition on the level of oxysterols was also investigated.

RESULTS

The effect of baking regime on the content and distribution of oxysterols was determined. Temperature was the most important factor affecting 7-ketocholesterol formation in baked meatloaf. Its content was significantly higher after baking at 250 °C compared to at 180 °C. The content of 7-ketocholesterol increased from the centre (87 µg kg^-1 ) to the surface (122 µg kg^-1 ) of baked meatloaves prepared under standard conditions. The level of α-tocopherol and its distribution was also affected by the baking regime. A higher level of 7-ketocholesterol was found in baked meatloaves after their frozen storage. The addition of marjoram did not change this level.

CONCLUSION

Inadequate culinary conditions used for preparation of baked meat can contribute to an increased oxysterol intake in the diet. Frozen storage did not stop oxysterol formation. The inhibition effect of marjoram on sterols oxidation was not confirmed. © 2017 Society of Chemical Industry.

Design and Characterization of a Therapeutic Non-phospholipid Liposomal Nanocarrier with Osteoinductive Characteristics To Promote Bone Formation

Sterosomes are recently developed types of non-phospholipid liposomes formed from single-chain amphiphiles and high content of sterols. Although sterosomes presented significantly increased stability compared to conventional phospholipid liposomes, current sterosome biomaterials are not truly bioactive and have no intrinsic therapeutic effects. The purpose of this study was to develop a sterosome formulation with osteoinductive properties by an effective selection of sterol, one of the sterosome components. Oxysterols are oxidized derivatives of cholesterol and are known to stimulate osteogenesis and bone formation. Thus, 20S-hydroxycholesterol (Oxy), one of the most potent oxysterols for bone regeneration, was examined as a promising candidate molecule to form fluid lamellar phases with a single-chain amphiphile, namely, stearylamine (SA). First, the optimal composition was identified by investigating the phase behavior of SA/Oxy mixtures. Next, the capacity of the optimized SA/Oxy sterosomes to promote osteogenic differentiation of bone marrow stromal cells was assessed in vitro in a hydrogel environment. Furthermore, we explored the effects of osteogenic oxysterol sterosomes in vivo with the mouse critical-sized calvarial defect model. Our results showed that SA/Oxy sterosomes induced osteogenic differentiation in vitro and enhanced calvarial healing without delivery of additional therapeutic agents, indicating their intrinsic bone-forming potential. This study suggests a promising non-phospholipid liposomal platform with osteoinductive properties for delivery of small molecular drugs and/or other therapeutic genes for enhanced bone formation.

Plant sterols, cholesterol precursors and oxysterols: Minute concentrations-Major physiological effects

Non-cholesterol sterols are present in our body at very low concentrations as compared to cholesterol. Small changes in the structure of sterol molecules confer them highly distinct biological activities. The best-known example are steroid hormones derived from cholesterol. During the past decade, our knowledge of also other biomolecules related to or derived from cholesterol, particularly plant sterols, biosynthetic precursors of cholesterol, and oxysterols, has expanded rapidly. In this review article we recapitulate the latest insights into the properties and physiological activities of these non-cholesterol sterols, as well as their importance in disease processes and potential as diagnostic biomarkers.

Synthesis of side-chain oxysterols and their enantiomers through cross-metathesis reactions of Δ22 steroids

A synthetic route that utilizes a cross-metathesis reaction with Δ²² steroids has been developed to prepare sterols with varying C-27 side-chains. Natural sterols containing hydroxyl groups at the 25 and (25R)-26 positions were prepared. Enantiomers of cholesterol and (3β,25R)-26-hydroxycholesterol (27-hydroxycholesterol) trideuterated at C-19 were prepared for future biological studies.

Oxysterols formation: A review of a multifactorial process

Dietary sterols are nutritionally interesting compounds which can suffer oxidation reactions. In the case of plant sterols, they are being widely used for food enrichment due to their hypocholesterolemic properties. Besides, cholesterol and plant sterols oxidation products are associated with the development of cardiovascular and neurodegenerative diseases, among others. Therefore, the evaluation of the particular factors affecting sterol degradation and oxysterols formation in foods is of major importance. The present work summarizes the main results obtained in experiments which aimed to study four aspects in this context: the effect of the heating treatment, the unsaturation degree of the surrounding lipids, the presence of antioxidants on sterols degradation, and at last, oxides formation. The use of model systems allowed the isolation of some of these effects resulting in more accurate data. Thus, these results could be applied in real conditions.

Bile acid synthesis precursors in subjects with genetic hypercholesterolemia negative for LDLR/APOB/PCSK9/APOE mutations. Association with lipids and carotid atherosclerosis

Some oxysterols are precursors of bile acid synthesis and play an important role in cholesterol homeostasis. However, if they are involved in the pathogeny of genetic hypercholesterolemia has not been previously explored. We have studied non-cholesterol sterol markers of cholesterol synthesis (lanosterol and desmosterol) and oxysterols (7α-hydroxy-4-cholesten-3-one, 24S-hydroxycholesterol and 27-hydroxycholesterol) in 200 affected subjects with primary hypercholesterolemia of genetic origin, negative for mutations in LDLR, APOB, PCSK9 and APOE genes (non-FH GH) and 100 normolipemic controls. All studied oxysterols and cholesterol synthesis markers were significantly higher in affected subjects than controls (P<0.001). Ratios of oxysterols to total cholesterol were higher in non-FH GH than in controls, although only 24S-hydroxycholesterol showed statistical significance (P<0.001). Cholesterol synthesis markers had a positive correlation with BMI, triglycerides, cholesterol and apoB in control population. However, these correlations disappeared in non-FH GH with the exception of a weak positive correlation for non-HDL cholesterol and apoB. The same pattern was observed for oxysterols with high positive correlation in controls and absence of correlation for non-FH GH, except non-HDL cholesterol for 24S-hydroxycholesterol and 27-hydroxycholesterol and apoB for 27-hydroxycholesterol. All non-cholesterol sterols had positive correlation among them in patients and in controls. A total of 65 (32.5%) and 35 (17.5%) affected subjects presented values of oxysterols ratios to total cholesterol above the 95th percentile of the normal distribution (24S-hydroxycholesterol and 27-hydroxycholesterol, respectively). Those patients with the highest levels of 24S-hydroxycholesterol associated an increase in the carotid intima media thickness. These results suggest that bile acid metabolism is affected in some patients with primary hypercholesterolemia of genetic origin, negative for mutations in the candidate genes, and may confer a higher cardiovascular risk. Our results confirm that cholesterol synthesis overproduction is a primary defect in non-HF GH and suggest that subjects with non-FH GH show high levels of oxysterols in response to hepatic overproduction of cholesterol.

Mitochondrial dysfunctions in 7-ketocholesterol-treated 158N oligodendrocytes without or with α-tocopherol: Impacts on the cellular profil of tricarboxylic cycle-associated organic acids, long chain saturated and unsaturated fatty acids, oxysterols, cholesterol and cholesterol precursors

In multiple sclerosis (MS) a process of white matter degradation leading to demyelination is observed. Oxidative stress, inflammation, apoptosis, necrosis and/or autophagy result together into a progressive loss of oligodendrocytes. 7-ketocholesterol (7KC), found increased in the cerebrospinal fluid of MS patients, triggers a rupture of RedOx homeostasis associated with mitochondrial dysfunctions, aptoptosis and autophagy (oxiapoptophagy) in cultured murine oligodendrocytes (158N). α-tocopherol is able to mild the alterations induced by 7KC partially restoring the cellular homeostasis. In presence of 7KC, the amount of adherent 158N cells was decreased and oxidative stress was enhanced. An increase of caspase-3 and PARP degradation (evidences of apoptosis), and an increased LC3-II/LC3-I ratio (criterion of autophagy), were detected. These events were associated with a decrease of the mitochondrial membrane potential (ΔΨ_m) and by a decrease of oxidative phosphorylation revealed by reduced NAD⁺ and ATP. The cellular lactate was higher while pyruvate, citrate, fumarate, succinate (tricarboxylic acid (TCA) cycle intermediates) were significantly reduced in exposed cells, suggesting that an impairment of mitochondrial respiratory functions could lead to an increase of lactate production and to a reduced amount of ATP and acetyl-CoA available for the anabolic pathways. The concentration of sterol precursors lathosterol, lanosterol and desmosterol were significantly reduced together with satured and unsatured long chain fatty acids (C16:0 - C18:0, structural elements of membrane phospholipids). Such reductions were milder with α-tocopherol. It is likely that the cell death induced by 7KC is associated with mitochondrial dysfunctions, including alterations of oxidative phosphorylation, which could result from lipid anabolism dysfunctions, especially on TCA cycle intermediates. A better knowledge of mitochondrial associated dysfunctions triggered by 7KC will contribute to bring new information on the demyelination processes which are linked with oxidative stress and lipid peroxidation, especially in MS.

Lipid deregulation in UV irradiated skin cells: Role of 25-hydroxycholesterol in keratinocyte differentiation during photoaging

Skin photoaging due to UV irradiation is a degenerative process that appears more and more as a growing concern. Lipids, including oxysterols, are involved in degenerative processes; as skin cells contain various lipids, the aim of our study was to evaluate first, changes in keratinocyte lipid levels induced by UV exposure and second, cellular effects of oxysterols in cell morphology and several hallmarks of keratinocyte differentiation. Our mass spectrometry results demonstrated that UV irradiation induces changes in lipid profile of cultured keratinocytes; in particular, ceramides and oxysterols, specifically 25-hydroxycholesterol (25-OH), were increased. Using holography and confocal microscopy analyses, we highlighted cell thickening and cytoskeletal disruption after incubation of keratinocytes with 25-OH. These alterations were associated with keratinocyte differentiation patterns: autophagy stimulation and intracellular calcium increase as measured by cytofluorometry, and increased involucrin level detected by immunocytochemistry. To conclude, oxysterol deregulation could be considered as a common marker of degenerative disorders. During photoaging, 25-OH seems to play a key role inducing morphological changes and keratinocyte differentiation.

Synthesis, in vitro and in vivo biological evaluation of new oxysterols as modulators of the liver X receptors

Liver X Receptor (LXR) modulators have shown potential as drugs since they target genes affecting metabolism and fatty acid synthesis. LXR antagonists are of particular interest since they are able to reduce the synthesis of complex fatty acids and glucose uptake. Based on molecular modeling, five new cholesterol mimics were synthesized, where four contained a hydroxyl group in the 22-S-position. The new compounds were screened in vitro against several genes affecting lipid metabolism. The compound that performed best in vitro was a dimethylamide derivative of 22(S)-hydroxycholesterol and it was chosen for in vivo testing. However, the blood plasma analysis from the in vivo tests revealed a concentration lower than needed to give any response, indicating either rapid metabolism or low bioavailability.

Fluorinated oxysterol analogues: Synthesis, molecular modelling and LXRβ activity

Liver X receptors (LXRs) are nuclear receptors that play central roles in the transcriptional control of lipid metabolism. The ability of LXRs to integrate metabolic and inflammation signalling makes them attractive targets for intervention in human metabolic diseases. Several oxidized metabolites of cholesterol (oxysterols) are endogenous LXR ligands, that modulate their transcriptional responses. While 25R-cholestenoic acid is an agonist of the LXRs, the synthetic analogue 27-norcholestenoic acid that lacks the 25-methyl is an inverse agonist. This change in the activity profile is triggered by a disruption of a key interaction between residues His435 and Trp457 that destabilizes the H11-H12 region of the receptor and favors the binding of corepressors. The introduction of fluorine atoms on the oxysterol side chain can favor both hydrophobic interactions as well as hydrogen bonds with the fluorine atoms and may thus induce changes in the receptor that may lead to changes in the activity profile. To evaluate these effects we have synthesized two fluorinated 27-nor-steroids, analogues of 27-norcholestenoic acid, the 25,25-difluoroacid and the corresponding 26-alcohol. The key step was a Reformatsky reaction on the C-24 cholenaldehyde, with ethyl bromodifluoroacetate under high intensity ultrasound (HIU) irradiation, followed by a Barton-McCombie type deoxygenation. Activity was evaluated in a luciferase reporter assay in the human HEK293T cells co-transfected with full length human LXRβ expression vector. The 25,25-difluoro-27-norcholestenoic acid was an inverse agonist and antagonist similar to its non-fluorinated analogue while its reduced derivative 25,25-difluoro-27-norcholest-5-ene-3β,26-diol was an agonist. Molecular dynamics simulation of the ligand-receptor complexes showed that the difluoroacid disrupted the His435-Trp457 interaction although the resulting conformational changes were different from those induced by the non-fluorinated analogue. In the case of the difluoroalcohol, the fluorine atoms actively participated in the interaction with several residues in the ligand binding pocket leading to a stabilization of the active receptor conformation.

Cholesterol photo-oxidation: A chemical reaction network for kinetic modeling

In this work we studied the effect of polyunsaturated fatty acids (PUFAs) methyl esters on cholesterol photo-induced oxidation. The oxidative routes were modeled with a chemical reaction network (CRN), which represents the first application of CRN to the oxidative degradation of a food-related lipid matrix. Docosahexaenoic acid (DHA, T-I), eicosapentaenoic acid (EPA, T-II) and a mixture of both (T-III) were added to cholesterol using hematoporphyrin as sensitizer, and were exposed to a fluorescent lamp for 48h. High amounts of Type I cholesterol oxidation products (COPs) were recovered (epimers 7α- and 7β-OH, 7-keto and 25-OH), as well as 5β,6β-epoxy. Fitting the experimental data with the CRN allowed characterizing the associated kinetics. DHA and EPA exerted different effects on the oxidative process. DHA showed a protective effect to 7-hydroxy derivatives, whereas EPA enhanced side-chain oxidation and 7β-OH kinetic rates. The mixture of PUFAs increased the kinetic rates several fold, particularly for 25-OH. With respect to the control, the formation of β-epoxy was reduced, suggesting potential inhibition in the presence of PUFAs.

C(16)-C(22) oxygen-bridged analogues of ceDAF-12 and LXR ligands

The DAF-12 receptor in nematodes and the Liver X Receptor (LXR) in mammals are structurally related transcription factors that play key roles in determining the life span of the organism. Both types of receptors are activated by oxysterols, cholesterol metabolites with oxidized side chains. Restricting the movement of the oxysterol side chain to certain orientations may have profound effects in the activity profile, however this has not been explored so far. In a first attempt to obtain analogues of natural ligands of DAF-12 and LXR with restricted side chain mobility we introduced a 16,22-oxygen bridge in 26-hydroxycholesterol, a cholestenoic acid and a dafachronic acid (5-7). Diosgenin was used as starting material, the key step to obtain the 16,22 epoxy functionality was the one pot formation and reduction of a cyclic hemiketal via the oxocarbenium ion using sodium cyanoborohydride. All new compounds were characterized by NMR and mass spectrometry and assayed as ceDAF-12 or LXR ligands in transactivation cell-based assays. The dafachronic acid analogue 7 behaved as a ceDAF-12 agonist.

A highly stereoselective synthesis of C-24 and C-25 oxysterols from desmosterol

A new highly stereoselective construction of the side chain of the C-24 and C-25 oxysterols has been achieved by using desmosterol acetate as the starting material and an improved Sharpless catalytic asymmetric dihydroxylation with 100% d.e. (diastereomeric excess) as the key step. The result is much better than the usual asymmetric dihydroxylation procedure. t-Butyl nitrite/2,2,6,6-tetramethylpiperidine N-oxyl radical/FeCl3 catalyst system was developed to activate molecular oxygen for the aerobic oxidation of 24-hydroxycholesterol and the 24-ketocholesterol was obtained in 86.2% yield. The oxidation system has never been reported before. The mechanism for the catalytic aerobic oxidation was also proposed.

Oxysterols and mechanisms of survival signaling

Oxysterols, a family of oxidation products of cholesterol, are increasingly drawing attention of scientists to their multifaceted biochemical properties, several of them of clear relevance to human pathophysiology. Taken up by cells through both vesicular and non-vesicular ways or often generated intracellularly, oxysterols contribute to modulate not only the inflammatory and immunological response but also cell viability, metabolism and function by modulating several signaling pathways. Moreover, they have been recognized as elective ligands for the most important nuclear receptors. The outcome of such a complex network of intracellular reactions promoted by these cholesterol oxidation products appears to be largely dependent not only on the type of cells, the dynamic conditions of the cellular and tissue environment but also on the concentration of the oxysterols. Here focus has been given to the cascade of molecular events exerted by relatively low concentrations of certain oxysterols that elicit survival and functional signals in the cells, with the aim to contribute to further expand the knowledge about the biological and physiological potential of the biochemical reactions triggered and modulated by oxysterols.