Cholesterol epoxide hydrolase and cancer

A stable epoxide as a potential endogenous estrogen metabolite: Possible significance in breast cancer?

Epoxides as reactive intermediates of estrogen metabolism have been considered to be potential precursors of the 2- and 4-hydroxy, catechol estrogens and even to be mutagenic/carcinogenic agents themselves. The labile nature of the intermediates has made proof of their existence difficult in natural biological conditions. In our studies on estrogen metabolism in vitro, in various tissues from several laboratory and domestic species, there was chromatographic evidence of formation of a stable estrogen metabolite that could be seen after incubation with radiolabeled estrone, but not with unlabeled substrate. Investigation with acid treatment of the metabolite yielded material detected as 6-hydroxy-estrone-suggesting the presence of an additional oxygen atom in the molecule. An identification of the "unknown compound" has not yet been made but, with this evidence, the properties revealed so far can best be met by assuming the presence of 5,6-epoxy-estrone. The recent favorable reports on the role of 5α,6α-epoxy-cholesterol in breast cancer has led to the hypothesis that the formation of a similar, stable epoxide of an estrogen could potentially be a compound of interest. If a metabolic pathway from estrone to 6-hydroxy-estrone through a stable epoxide has indeed been observed, it would suggest that identifying and screening for the enzymes responsible for its production, as opposed to those generating the catecholestrogens, could provide valuable information in relation to breast cancer. The balance in production of estrogen epoxides could be a key factor in determining normal health or risk of tumor development.

When cholesterol meets histamine, it gives rise to dendrogenin A: a tumour suppressor metabolite1

Dendrogenin A (DDA) is the first steroidal alkaloid (SA) to be identified in human tissues to date and arises from the stereoselective enzymatic conjugation of 5,6α-epoxycholesterol (5,6α-EC) with histamine (HA). DDA induces the re-differentiation of cancer cells in vitro and in vivo and prevents breast cancer (BC) and melanoma development in mice, evidencing its protective role against oncogenesis. In addition, DDA production is lower in BCs compared with normal tissues, suggesting a deregulation of its biosynthesis during carcinogenesis. The discovery of DDA reveals the existence of a new metabolic pathway in mammals which lies at the crossroads of cholesterol and HA metabolism and which leads to the production of this metabolic tumour suppressor.

Molecular and Biochemical Analysis of the Estrogenic and Proliferative Properties of Vitamin E Compounds

Tocols are vitamin E compounds that include tocopherols (TPs) and tocotrienols (TTs). These lipophilic compounds are phenolic antioxidants and are reportedly able to modulate estrogen receptor β (ERβ). We investigated the molecular determinants that control their estrogenicity and effects on the proliferation of breast cancer cells. Docking experiments highlighted the importance of the tocol phenolic groups for their interaction with the ERs. Binding experiments confirmed that they directly interact with both ERα and ERβ with their isoforms showing potencies in the following order: δ-tocols > γ-tocols > α-tocols. We also found that tocols activated the transcription of an estrogen-responsive reporter gene that had been stably transfected into cells expressing either ERα or ERβ. The role of the phenolic group in tocol-ER interaction was further established using δ-tocopherylquinone, the oxidized form of δ-TP, which had no ER affinity and did not induce ER-dependent transcriptional modulation. Tocol activity also required the AF1 transactivation domain of ER. We found that both δ-TP and δ-TT stimulated the expression of endogenous ER-dependent genes. However, whereas δ-TP induced the proliferation of ER-positive breast cancer cells but not ER-negative breast cancer cells, δ-TT inhibited the proliferation of both ER-positive and ER-negative breast cancer cells. These effects of δ-TT were found to act through the down regulation of HMG-CoA reductase (HMGR) activity, establishing that ERs are not involved in this effect. Altogether, these data show that the reduced form of δ-TP has estrogenic properties which are lost when it is oxidized, highlighting the importance of the redox status in its estrogenicity. Moreover, we have shown that δ-TT has antiproliferative effects on breast cancer cells independently of their ER status through the inhibition of HMGR. These data clearly show that TPs can be discriminated from TTs according to their structure.

Tamoxifen induces cellular stress in the nervous system by inhibiting cholesterol synthesis

Background

Tamoxifen (TAM) is an important cancer therapeutic and an experimental tool for effecting genetic recombination using the inducible Cre-Lox technique. Despite its widespread use in the clinic and laboratory, we know little about its effects on the nervous system. This is of significant concern because TAM, via unknown mechanisms, induces cognitive impairment in humans. A hallmark of cellular stress is induction of Activating Transcription Factor 3 (Atf3), and so to determine whether TAM induces cellular stress in the adult nervous system, we generated a knock-in mouse in which Atf3 promoter activity drives transcription of TAM-dependent Cre recombinase (Cre-ERT2); when crossed with tdtomato reporter mice, Atf3 induction results in robust and permanent genetic labeling of cells in which it is up-regulated even transiently.

Results

We found that granular neurons of the olfactory bulb and dentate gyrus, vascular cells and ependymal cells throughout the brain, and peripheral sensory neurons expressed tdtomato in response to TAM treatment. We also show that TAM induced Atf3 up-regulation through inhibition of cholesterol epoxide hydrolase (ChEH): reporter expression was mitigated by delivery in vitamin E-rich wheat germ oil (vitamin E depletes ChEH substrates), and was partially mimicked by a ChEH-specific inhibitor.

Conclusions

This work demonstrates that TAM stresses cells of the adult central and peripheral nervous systems and highlights concerns about clinical and experimental use of TAM. We propose TAM administration in vitamin E-rich vehicles such as wheat germ oil as a simple remedy.

Cholestane-3β,5α,6β-triol: high levels in Niemann-Pick type C, cerebrotendinous xanthomatosis, and lysosomal acid lipase deficiency

Niemann-Pick type C (NPC) is a progressive neurodegenerative disease characterized by lysosomal/endosomal accumulation of unesterified cholesterol and glycolipids. Recent studies have shown that plasma cholestane-3β,5α,6β-triol (CT) and 7-ketocholesterol (7-KC) could be potential biomarkers for the diagnosis of NPC patients. We aimed to know the sensitivity and specificity of these biomarkers for the diagnosis of NPC compared with other diseases that can potentially lead to oxysterol alterations. We studied 107 controls and 122 patients including 16 with NPC, 3 with lysosomal acid lipase (LAL) deficiency, 8 with other lysosomal diseases, 5 with galactosemia, 11 with cerebrotendinous xanthomatosis (CTX), 3 with Smith-Lemli-Opitz, 14 with peroxisomal biogenesis disorders, 19 with unspecific hepatic diseases, 13 with familial hypercholesterolemia, and 30 with neurological involvement and no evidence of an inherited metabolic disease. CT and 7-KC were analyzed by HPLC-ESI-MS/MS as mono-dimethylglycine derivatives. Levels of 7-KC were high in most of the studied diseases, whereas those of CT were only high in NPC, LAL, and CTX patients. Consequently, although CT is a sensitive biomarker of NPC disease, including those cases with doubtful filipin staining, it is not specific. 7-KC is a very unspecific biomarker.

The 2014 Bernard B. Brodie award lecture-epoxide hydrolases: drug metabolism to therapeutics for chronic pain

Dr. Bernard Brodie's legacy is built on fundamental discoveries in pharmacology and drug metabolism that were then translated to the clinic to improve patient care. Similarly, the development of a novel class of therapeutics termed the soluble epoxide hydrolase (sEH) inhibitors was originally spurred by fundamental research exploring the biochemistry and physiology of the sEH. Here, we present an overview of the history and current state of research on epoxide hydrolases, specifically focusing on sEHs. In doing so, we start with the translational project studying the metabolism of the insect juvenile hormone mimic R-20458 [(E)-6,7-epoxy-1-(4-ethylphenoxy)-3,7-dimethyl-2-octene], which led to the identification of the mammalian sEH. Further investigation of this enzyme and its substrates, including the epoxyeicosatrienoic acids, led to insight into mechanisms of inflammation, chronic and neuropathic pain, angiogenesis, and other physiologic processes. This basic knowledge in turn led to the development of potent inhibitors of the sEH that are promising therapeutics for pain, hypertension, chronic obstructive pulmonary disorder, arthritis, and other disorders.

TM6SF2 and MAC30, new enzyme homologs in sterol metabolism and common metabolic disease

Carriers of the Glu167Lys coding variant in the TM6SF2 gene have recently been identified as being more susceptible to non-alcoholic fatty liver disease (NAFLD), yet exhibit lower levels of circulating lipids and hence are protected against cardiovascular disease. Despite the physiological importance of these observations, the molecular function of TM6SF2 remains unknown, and no sequence similarity with functionally characterized proteins has been identified. In order to trace its evolutionary history and to identify functional domains, we embarked on a computational protein sequence analysis of TM6SF2. We identified a new domain, the EXPERA domain, which is conserved among TM6SF, MAC30/TMEM97 and EBP (D8, D7 sterol isomerase) protein families. EBP mutations are the cause of chondrodysplasia punctata 2 X-linked dominant (CDPX2), also known as Conradi-Hünermann-Happle syndrome, a defective cholesterol biosynthesis disorder. Our analysis of evolutionary conservation among EXPERA domain-containing families and the previously suggested catalytic mechanism for the EBP enzyme, indicate that TM6SF and MAC30/TMEM97 families are both highly likely to possess, as for the EBP family, catalytic activity as sterol isomerases. This unexpected prediction of enzymatic functions for TM6SF and MAC30/TMEM97 is important because it now permits detailed experiments to investigate the function of these key proteins in various human pathologies, from cardiovascular disease to cancer.

Antiestrogen-binding site ligands induce autophagy in myeloma cells that proceeds through alteration of cholesterol metabolism

Multiple myeloma (MM) is a malignancy characterized by the accumulation of clonal plasma cells in the bone marrow. Despite extensive efforts to design drugs targeting tumoral cells and their microenvironment, MM remains an incurable disease for which new therapeutic strategies are needed. We demonstrated here that antiestrogens (AEs) belonging to selective estrogen receptor modulators family induce a caspase-dependent apoptosis and trigger a protective autophagy. Autophagy was recognized by monodansylcadaverin staining, detection of autophagosomes by electronic microscopy, and detection of the cleaved form of the microtubule-associated protein light chain 3. Moreover, autophagy was inhibited by drugs such as bafilomycin A1 and 3-methyladenosine. Autophagy was mediated by the binding of AEs to a class of receptors called the antiestrogen binding site (AEBS) different from the classical estrogen nuclear receptors. The binding of specific ligands to the AEBS was accompanied by alteration of cholesterol metabolism and in particular accumulation of sterols: zymostenol or desmosterol depending on the ligand. This was due to the inhibition of the cholesterol-5,6-epoxide hydrolase activity borne by the AEBS. We further showed that the phosphoinositide 3-kinase/AKT/mammalian target of rapamycin pathway mediated autophagy signaling. Moreover, AEBS ligands restored sensitivity to dexamethasone in resistant MM cells. Since we showed previously that AEs arrest MM tumor growth in xenografted mice, we propose that AEBS ligands may have a potent antimyeloma activity alone or in combination with drugs used in clinic.

One step synthesis of 6-oxo-cholestan-3β,5α-diol

Cholesterol metabolism has been recently linked to cancer, highlighting the importance of the characterization of new metabolic pathways in the sterol series. One of these pathways is centered on cholesterol-5,6-epoxides (5,6-ECs). 5,6-ECs can either generate dendrogenin A, a tumor suppressor present in healthy mammalian tissues, or the carcinogenic cholestane-3β,5α,6β-triol (CT) and its putative metabolite 6-oxo-cholestan-3β,5α-diol (OCDO) in tumor cells. We are currently investigating the identification of the enzyme involved in OCDO biosynthesis, which would be highly facilitated by the use of commercially unavailable [(14)C]-cholestane-3β,5α,6β-triol and [(14)C]-6-oxo-cholestan-3β,5α-diol. In the present study we report the one-step synthesis of [(14)C]-cholestane-3β,5α,6β-triol and [(14)C]-6-oxo-cholestan-3β,5α-diol by oxidation of [(14)C]-cholesterol with iodide metaperiodate (HIO4).

Dendrogenin A arises from cholesterol and histamine metabolism and shows cell differentiation and anti-tumour properties

We previously synthesized dendrogenin A and hypothesized that it could be a natural metabolite occurring in mammals. Here we explore this hypothesis and report the discovery of dendrogenin A in mammalian tissues and normal cells as an enzymatic product of the conjugation of 5,6α-epoxy-cholesterol and histamine. Dendrogenin A was not detected in cancer cell lines and was fivefold lower in human breast tumours compared with normal tissues, suggesting a deregulation of dendrogenin A metabolism during carcinogenesis. We established that dendrogenin A is a selective inhibitor of cholesterol epoxide hydrolase and it triggered tumour re-differentiation and growth control in mice and improved animal survival. The properties of dendrogenin A and its decreased level in tumours suggest a physiological function in maintaining cell integrity and differentiation. The discovery of dendrogenin A reveals a new metabolic pathway at the crossroads of cholesterol and histamine metabolism and the existence of steroidal alkaloids in mammals.

It has been hypothesized that the steroidal alkaloid dendrogenin A (DDA) is a natural metabolite. de Medina et al. show that DDA is produced in mammalian tissues from 5,6α-epoxy-cholesterol and histamine metabolism, and that the compound displays cell differentiation and anti-tumour activities.

Exosomes as new vesicular lipid transporters involved in cell-cell communication and various pathophysiologies

Exosomes are nanovesicles that have emerged as a new intercellular communication system between an intracellular compartment of a donor cell towards the periphery or an internal compartment of a recipient cell. The bioactivity of exosomes resides not only in their protein and RNA contents but also in their lipidic molecules. Exosomes display original lipids organized in a bilayer membrane and along with the lipid carriers such as fatty acid binding proteins that they contain, exosomes transport bioactive lipids. Exosomes can vectorize lipids such as eicosanoids, fatty acids, and cholesterol, and their lipid composition can be modified by in-vitro manipulation. They also contain lipid related enzymes so that they can constitute an autonomous unit of production of various bioactive lipids. Exosomes can circulate between proximal or distal cells and their fate can be regulated in part by lipidic molecules. Compared to their parental cells, exosomes are enriched in cholesterol and sphingomyelin and their accumulation in cells might modulate recipient cell homeostasis. Exosome release from cells appears to be a general biological process. They have been reported in all biological fluids from which they can be recovered and can be monitors of specific pathophysiological situations. Thus, the lipid content of circulating exosomes could be useful biomarkers of lipid related diseases. Since the first lipid analysis of exosomes ten years ago detailed knowledge of exosomal lipids has accumulated. The role of lipids in exosome fate and bioactivity and how they constitute an additional lipid transport system are considered in this review.

The role of cholesterol metabolism and cholesterol transport in carcinogenesis: a review of scientific findings, relevant to future cancer therapeutics

While the unique metabolic activities of malignant tissues as potential targets for cancer therapeutics has been the subject of several recent reviews, the role of cholesterol metabolism in this context is yet to be fully explored. Cholesterol is an essential component of mammalian cell membranes as well as a precursor of bile acids and steroid hormones. The hypothesis that cancer cells need excess cholesterol and intermediates of the cholesterol biosynthesis pathway to maintain a high level of proliferation is well accepted, however the mechanisms by which malignant cells and tissues reprogram cholesterol synthesis, uptake and efflux are yet to be fully elucidated as potential therapeutic targets. High and low density plasma lipoproteins are the likely major suppliers of cholesterol to cancer cells and tumors, potentially via receptor mediated mechanisms. This review is primarily focused on the role(s) of lipoproteins in carcinogenesis, and their future roles as drug delivery vehicles for targeted cancer chemotherapy.

Emerging concepts on the role of exosomes in lipid metabolic diseases

Dysregulation of lipid metabolism involves cellular communication mediated by cell contacts or exchange of bioactive lipids bound to soluble carriers or to lipoproteins. An increasing field is that of cellular communication mediated by nanovesicles called exosomes. Those vesicles are released from an internal compartment of viable cells, circulate in all biological fluids and can transfer material from cell-to-cells. Involvement of exosome trafficking in the transcellular metabolism of eicosanoids and cholesterol-related diseases including cancer is developed hereafter.

5,6-Epoxy-cholesterols contribute to the anticancer pharmacology of tamoxifen in breast cancer cells

Tamoxifen (Tam) is a selective estrogen receptor modulator (SERM) that remains one of the major drugs used in the hormonotherapy of breast cancer (BC). In addition to its SERM activity, we recently showed that the oxidative metabolism of cholesterol plays a role in its anticancer pharmacology. We established that these effects were not regulated by the ER but by the microsomal antiestrogen binding site/cholesterol-5,6-epoxide hydrolase complex (AEBS/ChEH). The present study aimed to identify the oxysterols that are produced under Tam treatment and to define their mechanisms of action. Tam and PBPE (a selective AEBS/ChEH ligand) stimulated the production and the accumulation of 5,6α-epoxy-cholesterol (5,6α-EC), 5,6α-epoxy-cholesterol-3β-sulfate (5,6-ECS), 5,6β-epoxy-cholesterol (5,6β-EC) in MCF-7 cells through a ROS-dependent mechanism, by inhibiting ChEH and inducing sulfation of 5,6α-EC by SULT2B1b. We showed that only 5,6α-EC was responsible for the induction of triacylglycerol (TAG) biosynthesis by Tam and PBPE, through the modulation of the oxysterol receptor LXRβ. The cytotoxicity mediated by Tam and PBPE was triggered by 5,6β-EC through an LXRβ-independent route and by 5,6-ECS through an LXRβ-dependent mechanism. The importance of SULT2B1b was confirmed by its ectopic expression in the SULT2B1b(-) MDA-MB-231 cells, which became sensitive to 5,6α-EC, Tam or PBPE at a comparable level to MCF-7 cells. This study established that 5,6-EC metabolites contribute to the anticancer pharmacology of Tam and highlights a novel signaling pathway that points to a rationale for re-sensitizing BC cells to Tam and AEBS/ChEH ligands.