Impairment of trophoblast survival and differentiation by LXR ligands is prevented by cholesterol but not ABCA1 silencing.

The Liver X Receptors (LXRs) drive the transcriptional response to excess intracellular cholesterol. Oxysterols, the products of cholesterol oxidation, are activating ligands for LXR that can accumulate under conditions of oxidative stress and disrupt cholesterol homeostasis. While activation of LXR inhibits trophoblast differentiation, the impact of LXR on trophoblast physiology or cholesterol homeostasis is incompletely understood. We sought to determine if the effects of LXR activation can be ameliorated through modification of cholesterol bioavailability or inhibition of LXR-driven cholesterol efflux in trophoblasts. We measured the effect of oxysterol exposure on BeWo cells and primary human trophoblasts (PHT cells) cultured in lipoprotein-deficient medium. We also measured the effect of the synthetic, LXR-specific ligand T0901317 on PHT cell differentiation and survival. Finally, we silenced the ATP-binding cassette transporter A1 (ABCA1), a transcriptional target of LXR that drives cholesterol efflux, to determine if inhibition of cholesterol efflux could block the effects of T0901317. Oxysterols inhibited BeWo survival and PHT cell differentiation, and these effects were blocked by cholesterol supplementation. T0901317 also inhibited PHT cell differentiation, and this effect was similarly blocked by cholesterol. Unlike cholesterol however, ABCA1 silencing did not modify the effect of T0901317 on PHT cell differentiation. Oxysterols and LXR inhibit trophoblast survival and differentiation exclusively in conditions of cholesterol scarcity. These findings underscore the importance of cholesterol homeostasis in the maintenance of placental function and suggest that pathways regulating cholesterol homeostasis may represent therapeutic targets to mitigate harmful sequelae of placental injury.

Harmon CM ，McGonigal S ，Larkin JC 《-》

The influence of ligand-activated LXR on primary human trophoblasts.

The Liver X Receptors (LXRs) are critical transcriptional regulators of cellular metabolism that promote cholesterol efflux and lipogenesis in response to excess intracellular cholesterol. In contrast, the Sterol Response Element Binding Protein-2 (SREBP2) promotes the synthesis and uptake of cholesterol. Oxysterols are products of cholesterol oxidation that accumulate in conditions associated with increased cellular levels of reactive oxygen species, such as hypoxia and oxidative stress, activating LXR and inhibiting SREBP2. While hypoxia and oxidative stress are commonly implicated in placental injury, the impact of the transcriptional regulation of cholesterol homeostasis on placental function is not well characterized. We measured the effects of the synthetic LXR ligand T0901317 and the endogenous oxysterol 25-hydroxycholesterol (25OHC) on differentiation, cytotoxicity, progesterone synthesis, lipid droplet formation, and gene expression in primary human trophoblasts. Exposure to T0901317 promoted lipid droplet formation and inhibited differentiation, while 25OHC induced trophoblast toxicity, promoted hCG and progesterone release at lower concentrations with inhibition at higher concentrations, and had no effect on lipid droplet formation. The discrepant effect of these ligands was associated with distinct changes in expression of LXR and SREBP2 target genes, with upregulation of ABCA1 following 25OHC and T090317 exposure, exclusive activation of the lipogenic LXR targets SREBP1c, ACC1 and FAS by T0901317, and exclusive inhibition of the SREBP2 targets LDLR and HMGCR by 25OHC. These findings implicate cholesterol oxidation as a determinant of trophoblast function and activity, and suggest that placental gene targets and functional pathways are selectively regulated by specific LXR ligands.

Larkin JC ，Sears SB ，Sadovsky Y 《-》

Oxysterols inhibit differentiation and fusion of term primary trophoblasts by activating liver X receptors.

Oxygenated cholesterol metabolites known as oxysterols display potent biological activities ranging from regulation of lipid homeostasis to cytotoxicity. Oxysterols have previously been shown to inhibit the invasion of first trimester trophoblasts, an effect which involves activation of the nuclear liver X receptors (LXRs). In the present study, we investigated the effects of several oxysterols on syncytialisation (differentiation and fusion) in term placental trophoblasts. Treatment of cultured term primary trophoblast cells with oxysterols [25-hydroxycholesterol, 7-ketocholesterol, 22(R)-hydroxycholesterol] and the synthetic LXR agonist T0901317 at non-toxic doses decreased expression of GCM-1 and HERV-W mRNA and reduced hCG secretion and placental alkaline phosphatase activity, indicative of diminished trophoblast differentiation. Furthermore, treatment with these compounds also decreased cell fusion measured by E-cadherin immunostaining and quantification of syncytialised nuclei. Treatment with an LXR antagonist (geranylgeranyl diphosphate) abrogated the inhibitory effects of oxysterols and T0901317 on trophoblast syncytialisation indicating that these effects are mediated by LXR. These findings suggest that oxysterols impair differentiation and fusion of term trophoblast cells via an LXR-dependent mechanism.

Aye IL ，Waddell BJ ，Mark PJ ，Keelan JA ... -  《-》

The liver X receptor (LXR) and its target gene ABCA1 are regulated upon low oxygen in human trophoblast cells: a reason for alterations in preeclampsia?

The Liver X receptors (LXR) alpha and beta and their target genes such as the ATP-binding cassette (ABC) transporters have been shown to be crucially involved in the regulation of cellular cholesterol homeostasis. The aim of this study was to characterize the role of LXR alpha/beta in the human placenta under normal physiological circumstances and in preeclampsia. We investigated the expression pattern of the LXRs and their target genes in the human placenta during normal pregnancy and in preeclampsia. Placental explants and cell lines were studied under different oxygen levels and pharmacological LXR agonists. Gene expressions (Taqman PCR) and protein levels (Western Blot) were combined with immunohistochemistry to analyze the expression of LXR and its target genes. In the human placenta, LXRA and LXRB expression increased during normal pregnancy. This was paralleled by the expression of their prototypical target genes, e.g., the cholesterol transporter ABCA1. Interestingly, early-onset preeclamptic placentae revealed a significant upregulation of ABCA1. Culture of JAr trophoblast cells and human first trimester placental explants under low oxygen lead to increased expression of LXRA and ABCA1 which was further enhanced by the LXR agonist T0901317. LXRA together with ABCA1 are specifically expressed in the human placenta and can be regulated by hypoxia. Deregulation of this system in early preeclampsia might be the result of placental hypoxia and hence might have consequences for maternal-fetal cholesterol transport.

Plösch T ，Gellhaus A ，van Straten EM ，Wolf N ，Huijkman NC ，Schmidt M ，Dunk CE ，Kuipers F ，Winterhager E ... -  《-》

Ozone-derived Oxysterols Affect Liver X Receptor (LXR) Signaling: A POTENTIAL ROLE FOR LIPID-PROTEIN ADDUCTS.

When inhaled, ozone (O3) interacts with cholesterols of airway epithelial cell membranes or the lung-lining fluid, generating chemically reactive oxysterols. The mechanism by which O3-derived oxysterols affect molecular function is unknown. Our data show that in vitro exposure of human bronchial epithelial cells to O3 results in the formation of oxysterols, epoxycholesterol-α and -β and secosterol A and B (Seco A and Seco B), in cell lysates and apical washes. Similarly, bronchoalveolar lavage fluid obtained from human volunteers exposed to O3 contained elevated levels of these oxysterol species. As expected, O3-derived oxysterols have a pro-inflammatory effect and increase NF-κB activity. Interestingly, expression of the cholesterol efflux pump ATP-binding cassette transporter 1 (ABCA1), which is regulated by activation of the liver X receptor (LXR), was suppressed in epithelial cells exposed to O3 Additionally, exposure of LXR knock-out mice to O3 enhanced pro-inflammatory cytokine production in the lung, suggesting LXR inhibits O3-induced inflammation. Using alkynyl surrogates of O3-derived oxysterols, our data demonstrate adduction of LXR with Seco A. Similarly, supplementation of epithelial cells with alkynyl-tagged cholesterol followed by O3 exposure causes observable lipid-LXR adduct formation. Experiments using Seco A and the LXR agonist T0901317 (T09) showed reduced expression of ABCA1 as compared with stimulation with T0901317 alone, indicating that Seco A-LXR protein adduct formation inhibits LXR activation by traditional agonists. Overall, these data demonstrate that O3-derived oxysterols have pro-inflammatory functions and form lipid-protein adducts with LXR, thus leading to suppressed cholesterol regulatory gene expression and providing a biochemical mechanism mediating O3-derived formation of oxidized lipids in the airways and subsequent adverse health effects.

Speen AM ，Kim HH ，Bauer RN ，Meyer M ，Gowdy KM ，Fessler MB ，Duncan KE ，Liu W ，Porter NA ，Jaspers I ... -  《-》