Studies on the immunosuppressive properties of 7,25 dihydroxycholesterol--II. Effects on early steps of T-cell activation

Cholest-4,6-Dien-3-One Promote Epithelial-To-Mesenchymal Transition (EMT) in Biliary Tree Stem/Progenitor Cell Cultures In Vitro

Human biliary tree stem/progenitor cells (hBTSCs), reside in peribiliary glands, are mainly stimulated by primary sclerosing cholangitis (PSC) and cholangiocarcinoma. In these pathologies, hBTSCs displayed epithelial-to-mesenchymal transition (EMT), senescence characteristics, and impaired differentiation. Here, we investigated the effects of cholest-4,6-dien-3-one, an oxysterol involved in cholangiopathies, on hBTSCs biology. hBTSCs were isolated from donor organs, cultured in self-renewal control conditions, differentiated in mature cholangiocytes by specifically tailored medium, or exposed for 10 days to concentration of cholest-4,6-dien-3-one (0.14 mM). Viability, proliferation, senescence, EMT genes expression, telomerase activity, interleukin 6 (IL6) secretion, differentiation capacity, and HDAC6 gene expression were analyzed. Although the effect of cholest-4,6-dien-3-one was not detected on hBTSCs viability, we found a significant increase in cell proliferation, senescence, and IL6 secretion. Interestingly, cholest-4.6-dien-3-one impaired differentiation in mature cholangiocytes and, simultaneously, induced the EMT markers, significantly reduced the telomerase activity, and induced HDAC6 gene expression. Moreover, cholest-4,6-dien-3-one enhanced bone morphogenic protein 4 (Bmp-4) and sonic hedgehog (Shh) pathways in hBTSCs. The same pathways activated by human recombinant proteins induced the expression of EMT markers in hBTSCs. In conclusion, we demonstrated that chronic exposition of cholest-4,6-dien-3-one induced cell proliferation, EMT markers, and senescence in hBTSC, and also impaired the differentiation in mature cholangiocytes.

An oxysterol biomarker for 7-dehydrocholesterol oxidation in cell/mouse models for Smith-Lemli-Opitz syndrome

The level of 7-dehydrocholesterol (7-DHC) is elevated in tissues and fluids of Smith-Lemli-Opitz syndrome (SLOS) patients due to defective 7-DHC reductase. Although over a dozen oxysterols have been identified from 7-DHC free radical oxidation in solution, oxysterol profiles in SLOS cells and tissues have never been studied. We report here the identification and complete characterization of a novel oxysterol, 3β,5α-dihydroxycholest-7-en-6-one (DHCEO), as a biomarker for 7-DHC oxidation in fibroblasts from SLOS patients and brain tissue from a SLOS mouse model. Deuterated (d₇)-standards of 7-DHC and DHCEO were synthesized from d₇-cholesterol. The presence of DHCEO in SLOS samples was supported by chemical derivatization in the presence of d₇-DHCEO standard followed by HPLC-MS or GC-MS analysis. Quantification of cholesterol, 7-DHC, and DHCEO was carried out by isotope dilution MS with the d₇-standards. The level of DHCEO was high and correlated well with the level of 7-DHC in all samples examined (R = 0.9851). Based on our in vitro studies in two different cell lines, the mechanism of formation of DHCEO that involves 5α,6α-epoxycholest-7-en-3β-ol, a primary free radical oxidation product of 7-DHC, and 7-cholesten-3β,5α,6β-triol is proposed. In a preliminary test, a pyrimidinol antioxidant was found to effectively suppress the formation of DHCEO in SLOS fibroblasts.

Oxysterols from free radical chain oxidation of 7-dehydrocholesterol: product and mechanistic studies

Free radical chain oxidation of highly oxidizable 7-dehydrocholesterol (7-DHC), initiated by 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), was carried out at 37 degrees C in benzene for 24 h. Fifteen oxysterols derived from 7-DHC were isolated and characterized with 1D and 2D NMR spectroscopy and mass spectrometry. A mechanism that involves abstraction of hydrogen atoms at C-9 and/or C-14 is proposed to account for the formation of all of the oxysterols and the reaction progress profile. In either the H-9 or H-14 mechanism, a pentadienyl radical intermediate is formed after abstraction of H-9 or H-14 by a peroxyl radical. This step is followed by the well-precedented transformations observed in peroxidation reactions of polyunsaturated fatty acids such as oxygen addition, peroxyl radical 5-exo cyclization, and S(H)i carbon radical attack on the peroxide bond. The mechanism for peroxidation of 7-DHC also accounts for the formation of numerous oxysterol natural products isolated from fungal species, marine sponges, and cactaceous species. In a cell viability test, the oxysterol mixture from 7-DHC peroxidation was found to be cytotoxic to Neuro2a neuroblastoma cells in the micromolar concentration range. We propose that the high reactivity of 7-DHC and the oxysterols generated from its peroxidation may play important roles in the pathogenesis of Smith-Lemli-Opitz syndrome, X-linked dominant chondrodysplasia punctata, and cerebrotendinous xanthomatosis, all of these being metabolic disorders characterized by an elevated level of 7-DHC.

Oxysterols from human bile induce apoptosis of canine gallbladder epithelial cells in monolayer culture

Oxysterols have been detected in various mammalian organs and blood. Biliary epithelium is exposed to high concentrations of cholesterol, and we have identified three keto-oxysterols (cholest-4-en-3-one, cholesta-4,6-dien-3-one, cholesta-3,5-dien-7-one) in human bile and gallstones. Because the effects of oxysterols on biliary physiology are not well defined, we investigated their biological effects on dog gallbladder epithelial cells. Enriched medium (culture medium containing taurocholate and lecithin and cholesterol +/- various oxysterols) was applied to confluent monolayers of dog gallbladder epithelial cells in culture. Cytotoxicity and apoptosis were studied by morphological analysis and flow cytometry. Oxysterols in the mitochondrial fraction were identified by gas chromatography/mass spectrometry, whereas release of cytochrome c from mitochondria was assayed by spectrophotometry and Western blot analysis. Compared with cells treated with culture medium or with enriched medium containing cholesterol, oxysterol-treated cells showed significantly increased apoptosis (P < 0.05). Exogenously applied oxysterols were recovered from the mitochondrial fraction. Cytochrome c release from mitochondria was increased significantly by cholest-4-en-3-one, cholesta-4,6-dien-3-one, and 5beta-cholestan-3-one (all P < 0.05). Thus oxysterols recovered from human bile and gallstones induce apoptosis of biliary epithelium via a mitochondrial-dependent pathway and may play a role in the pathogenesis of chronic inflammation and carcinogenesis in the gallbladder.

7-ketocholesterol is an endogenous modulator for the arylhydrocarbon receptor

We have identified 7-ketocholesterol (7-KC) as an endogenous modulator that inhibits transactivation by the arylhydrocarbon receptor (AhR) through competitive binding against xenobiotic ligands. 7-KC binds AhR and displaces labeled dioxin (2,3,7,8-tetrachlorodibenzo(p)dioxin (TCDD)). IC(50) is 5 x 10(-7) m in vivo and 7 x 10(-6) m in vitro. These figures are consistent with its concentration in human blood plasma and tissues. Association with 7-KC prevents AhR binding to DNA. 7-KC blocks the TCDD-mediated transactivation of stably expressed reporter gene constructs in T47-D cells as well as the expression of the endogenous CYP 1A1 gene in HepG2 cells and in primary porcine aortic endothelial cells. Injection of 7-KC to rats blocks the induction of CYP 1A1 messenger RNA and protein in endothelial cells from myocardial blood vessels. The differential sensitivity of mammalian species to toxic effects of AhR ligands, especially dioxin (TCDD), correlates with the expression of 7-hydroxycholesterol dehydrogenase, which synthesizes 7-KC from 7-hydroxycholesterol. The documented involvement of AhR ligands in cardiovascular diseases through lipid peroxidation and endothelium dysfunction can now be examined in the context of displacement of this protective modulator.

Cholestan-3beta,5alpha,6beta-triol, but not 7-ketocholesterol, suppresses taurocholate-induced mucin secretion by cultured dog gallbladder epithelial cells

In order to investigate oxysterol-mediated effects on the biliary system, we studied the effects of cholestan-3beta,5alpha,6beta-triol (TriolC) and 7-ketocholesterol (7KC) on gallbladder epithelial cells. We compared their cell proliferation effects in cultured dog gallbladder epithelial cells (DGBE) to their effects in cultured human pulmonary artery endothelial cells (HPAE). Oxysterols inhibited cell proliferation in a dose-dependent fashion. Oxysterols inhibited cell growth to 50% of control at a higher dose for DGBE cells than for HPAE cells. TriolC was more cytotoxic than 7KC. We also investigated the effect of oxysterols on bile salt-induced mucin secretion by DGBE cells. TriolC suppressed mucin secretion by DGBE cells, whereas 7KC did not. These findings support the hypothesis that biliary oxysterols affect gallbladder mucosal function.

Oxysterols, but not cholesterol, inhibit human immunodeficiency virus replication in vitro

Oxysterols, oxygenated derivatives of cholesterol selected for their cytostatic activity and their inhibitory effect on cholesterol synthesis, have been investigated for their anti-human immunodeficiency virus (HIV) activity in vitro. The three oxysterols tested, 7 beta-hydroxycholesterol (7 beta-OHC), 25-hydroxycholesterol (25-OHC) and 7 beta, 25-dihydroxycholesterol (7,25-OHC), inhibit viral replication at micromolar concentrations. The selectivity indexes for 7 beta-OHC and 25-OHC are quite modest (2 to 8) but reproducible; the dihydroxycholesterol 7,25-OHC exhibited antiviral properties at concentrations 13- to 25-fold lower than the highest concentration tested at which no toxicity was measurable. Oxysterols are naturally occurring compounds, and we speculate on their physiological relevance in HIV-infected individuals.

Investigation of the potential genotoxicity of cholesterol oxidation products in two mammalian fibroblast cell lines

Cholesterol oxidation products (oxysterols) are generated during the cooking and processing of foods and may be produced endogenously in tissues and in the plasma membrane. A diverse range of biological functions have been ascribed to oxysterols, including atherogenicity, carcinogenicity, and mutagenicity, and in recent years concern has been expressed over the presence of oxysterols in food products. However, it is unclear whether oxysterols are capable of inducing genotoxic damage in cell culture systems. The aim of this study was to examine seven commonly occurring oxysterols (purity > 95%) for their cytotoxicity and ability to increase the frequency of DNA strand breaks and sister chromatid exchanges (SCE) in cells in culture. Two cell lines were employed in the study: Chinese hamster ovary (CHO) and Indian Muntjac (IM) fibroblasts. The 3-(4,5-dimethylthiozol-2-yl)-2,5-diphenyltetrazolium bromide assay, which is a measure of intracellular reductive metabolism based on the activity of mitochondrial dehydrogenases, was used as an index of cytotoxicity. The most cytotoxic oxysterols in constantly challenged CHO or IM cells (24-h exposure) proved to be 5 alpha-cholestane-3 beta,5,6 beta-triol and 25-hydroxy-cholesterol. The genotoxic potential of the oxysterols was assessed in CHO cells using the comet assay and IM cells using the SCE assay. The comet assay measures breaks in the DNA strand, whereas the exact mechanism of SCE formation is unclear but is believed to require DNA repair where genetic material becomes exchanged between the two sister chromatids. None of the oxysterols examined in this study affected baseline levels of DNA strand breaks or SCE relative to the negative control samples. This study indicates that, under the conditions used, the oxysterols investigated were not genotoxic.

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.

Cholesterol and non-cardiovascular disease: basic science

Cholesterol metabolism is of fundamental biological importance. This review examines the role of cholesterol metabolism in relation to non-cardiovascular disease (non-CVD). Particular attention is paid to the question of whether or not low levels of cholesterol may have harmful effects on cell function or lead to pathological processes. Many in vitro phenomena have been demonstrated at levels of cholesterol which are very low in comparison to physiological conditions. Nevertheless, low cholesterol is more favourable than high cholesterol for most aspects of cell function. There is no evidence that any catastrophic cellular response or pathological process occurs due to exposure of organisms to low cholesterol. On the other hand, the inflammatory process is a powerful and consistent cause of decreased cholesterol levels. This, together with other confounding factors, appears to explain a major component of the association between low cholesterol levels and non-CVD.

Proline dithiocarbamate inhibits N-nitrosodiethylamine induced liver carcinogenesis

A study was conducted to determine the toxicity of different dithiocarbamates and of disulfiram. In an experiment showing the cytotoxicity against murine spleen lymphocytes, proline dithiocarbamate (PDTC) and thioproline dithiocarbamate showed the lowest toxicity. Therefore one of them was selected and different doses of the hydrophilic PDTC were checked for their ability to affect the development of liver and oesophagus tumours induced in BD-6 rats by N-nitrosodiethylamine (NDEA). Rats were injected i.p. with 80 mg/kg NDEA once weekly for 10 weeks. Administration of PDTC, 1 h before and 24 h after the carcinogen, markedly decreased the number of rats developing NDEA-induced hepatocellular carcinoma and liver haemangioendothelioma. A 59%-77% reduction in the incidence of liver tumours was found in the different groups when the carcinogen was administered in combination with the inhibitor. For least 40 weeks after the start of the experiment PDTC protected the liver from NDEA carcinogenesis and did not shift the tumour development to any other organ. PDTC did not significantly affect the weight gain of the experimental animals. We conclude that parenteral administration of PDTC seems to represent a promising approach in chemoprevention of liver carcinogenesis.

Oxysterols: biological activities and physicochemical studies

To improve the understanding of the various biological activities of oxysterols (oxygenated derivatives of cholesterol), studies of their physicochemical properties have been undertaken. Oxysterols modify membrane dynamic properties which consequently trigger several biological effects. Despite the presence of at least one oxygenated group in addition to the C3 beta-hydroxyl, oxysterols insert perfectly into the lipidic bilayer of the membrane inducing a condensing effect similar to, but less potent than, that of cholesterol. In biological membranes oxysterols probably interact with membrane components as they are not easily exchanged after their incorporation into the cell membrane. These lipid-protein interactions are probably crucial for the expression of the biological activities of the oxysterols.

Membrane-related oxysterol function: preliminary results on the modification of protein kinase C activity and substrate phosphorylation by 7 beta,25-dihydroxycholesterol

Oxysterols exhibit a wide variety of biological activities, including potent immunosuppressive effects. 7 beta,25-Dihydroxycholesterol (7,25-OHC), a synthetic oxysterol, has been shown to strongly inhibit the lymphocyte response to different stimuli. This compound has been chosen as a model compound to investigate the mechanisms underlying the immunosuppressive effects of oxysterols. As protein kinase C (PKC) constitutes a key enzyme in the pathways leading to cell activation, we have studied the effect of 7,25-OHC on PKC activity in the cytosolic and particulate fractions of spleen cells. Lymphocytes treated with 7,25-OHC showed a decrease of the relative PKC activity in the particulate fractions compared to control cells. These results are confirmed by the observation that 7,25-OHC also reduces the phosphorylation of the endogenous PKC substrates. Thus oxysterols interfere with two membrane related phenomena, ie the modification of membrane PKC activity and the inhibition of the phosphorylation of the substrates of PKC located in the membrane. Previous results obtained by fluorescence polarisation revealed a modification of the membrane fluidity after oxysterol treatment. Furthermore, it has been demonstrated that oxysterols are incorporated into cell membranes. The alteration of the cell membrane could impair the signal transduction and may explain the immunosuppressive activity of oxysterol. Thus, along with other biological effects previously reported, oxysterols decrease membrane associated PKC activity in immune cells.

Polyoxygenated sterols and triterpenes: chemical structures and biological activities

Cholesterol and other sterols are precursors of many hormones of vertebrates (pregnane, androstane, estrane derivatives), invertebrates (ecdysteroids) and even of plants (brassinoids). The normal biosynthetic processes which begin by a series of oxidations lead to a family of compounds, some of which exhibit a wide variety of biological activities. Among the latter, those well-established are their inhibitory effect on the biosynthesis of cholesterol in mammalian cells, their toxic effect on tumor cells and their ability to modify some immunological responses. None of the members of this family has all of the activities just mentioned. The intensity of the effect depends markedly upon the specific structure of each compound.