Oxysterol nuclear receptor LXRbeta regulates cholesterol homeostasis and contractile function in mouse uterus

A pilot study of the association between maternal mid-pregnancy cholesterol and oxysterol concentrations and labor duration

BACKGROUND

Previous animal model studies have highlighted a role for cholesterol and its oxidized derivatives (oxysterols) in uterine contractile activity, however, a lipotoxic state associated with hypercholesterolemia may contribute to labor dystocia. Therefore, we investigated if maternal mid-pregnancy cholesterol and oxysterol concentrations were associated with labor duration in a human pregnancy cohort.

METHODS

We conducted a secondary analysis of serum samples and birth outcome data from healthy pregnant women (N = 25) with mid-pregnancy fasting serum samples collected at 22-28 weeks of gestation. Serum was analyzed for total-C, HDL-C, and LDL-C by direct automated enzymatic assay and oxysterol profile including 7α-hydroxycholesterol (7αOHC), 7β-hydroxycholesterol (7βOHC), 24-hydroxycholesterol (24OHC), 25-hydroxycholesterol (25OHC), 27-hydroxycholesterol (27OHC), and 7-ketocholesterol (7KC) by liquid chromatography-selected ion monitoring-stable isotope dilution-atmospheric pressure chemical ionization-mass spectroscopy. Associations between maternal second trimester lipids and labor duration (minutes) were assessed using multivariable linear regression adjusting for maternal nulliparity and age.

RESULTS

An increase in labor duration was observed for every 1-unit increment in serum 24OHC (0.96 min [0.36,1.56], p < 0.01), 25OHC (7.02 min [1.92,12.24], p = 0.01), 27OHC (0.54 min [0.06, 1.08], p < 0.05), 7KC (8.04 min [2.7,13.5], p < 0.01), and total oxysterols (0.42 min [0.18,0.06], p < 0.01]. No significant associations between labor duration and serum total-C, LDL-C, or HDL-C were observed.

CONCLUSIONS

In this cohort, mid-pregnancy concentrations of maternal oxysterols (24OHC, 25OHC, 27OHC, and 7KC) were positively associated with labor duration. Given the small population and use of self-reported labor duration, subsequent studies are required for confirmation.

Role of cholesterol homeostasis and its efflux pathways in cancer progression

Tumor cells show high avidity for cholesterol in order to support their inherent nature to divide and proliferate. This results in the rewiring of cholesterol homeostatic pathways by influencing not only de novo synthesis but also uptake or efflux pathways of cholesterol. Recent findings have pointed towards the importance of cholesterol efflux in tumor pathogenesis. Cholesterol efflux is the first and foremost step in reverse cholesterol transport and any perturbation in this pathway may lead to the accumulation of intracellular cholesterol, thereby altering the cellular equilibrium. This review addresses the different mechanisms of cholesterol efflux from the cell and highlights their role and regulation in context to tumor development. There are four different routes by which cholesterol can be effluxed from the cell namely, 1) passive diffusion of cholesterol to mature HDL particles, 2) SR-B1 mediated facilitated diffusion, 3) Active efflux to apo A1 via ABCA1 and 4) ABCG1 mediated efflux to mature HDL. These molecular players facilitating cholesterol efflux are engaged in a complex interplay with different signaling pathways. Thus, an understanding of the efflux pathways, their regulation and cross-talk with signaling molecules may provide novel prognostic markers and therapeutic targets to combat the onset of carcinogenesis.

Molecular characterization and homology modeling of liver X receptor in Asian seabass, Lates calcarifer: predicted functions in reproduction and lipid metabolism

Liver X receptor (LXR) is a ligand-activated transcription factor that plays vital roles in maintaining cholesterol and lipid homeostasis. Much work has been done on mammalian LXRs, but the role of LXR in fish remains unclear. In the present study, LXR gene was identified from adult Asian seabass, Lates calcarifer, and its predicted protein structure was docked with several cholesterol derivatives at the binding site. The LXR cDNA consisted of 1495 bp encoding a putative LXR protein of 494 amino acids. The Asian seabass LXR retained many important structural features found in LXRs of other fishes and mammals, such as putative signal peptide, activation function-1 (AF-1) domain, DNA-binding domain (DBD), ligand-binding domain (LBD), activation function-2 (AF-2) domain, and eight conserved cysteine residues. The deduced amino acid sequence of LXR shared significant identity with those of other species ranging from 65.7 to 95.8%. The homology modeling and in silico molecular docking demonstrated that Asian seabass LXR could interact with cholesterol derivatives at amino acid residues Phe274 and Ile312. Real-time PCR further revealed that LXR transcripts are ubiquitously expressed in all tissues examined, with the highest levels detected in the gonad followed by the liver. Given the well-known importance of cholesterol-mediated signaling in these tissues, Asian seabass LXR may reasonably be involved in reproduction and lipid metabolism.

Liver X Receptors: A Possible Link between Lipid Disorders and Female Infertility

A close relationship exists between cholesterol and female reproductive physiology. Indeed, cholesterol is crucial for steroid synthesis by ovary and placenta, and primordial for cell structure during folliculogenesis. Furthermore, oxysterols, cholesterol-derived ligands, play a potential role in oocyte maturation. Anomalies of cholesterol metabolism are frequently linked to infertility. However, little is known about the molecular mechanisms. In parallel, increasing evidence describing the biological roles of liver X receptors (LXRs) in the regulation of steroid synthesis and inflammation, two processes necessary for follicle maturation and ovulation. Both of the isoforms of LXRs and their bona fide ligands are present in the ovary. LXR-deficient mice develop late sterility due to abnormal oocyte maturation and increased oocyte atresia. These mice also have an ovarian hyper stimulation syndrome in response to gonadotropin stimulation. Hence, further studies are necessary to explore their specific roles in oocyte, granulosa, and theca cells. LXRs also modulate estrogen signaling and this could explain the putative protective role of the LXRs in breast cancer growth. Altogether, clinical studies would be important for determining the physiological relevance of LXRs in reproductive disorders in women.

How do Spondias mombin L (Anacardiaceae) leaves extract increase uterine smooth muscle contractions to facilitate child birth in parturient women?

Background

Spondias mombin L. (Anacardiaceae) leaves were used in Togolese folk to treat dystocia, expel placenta and manage post-partum hemorrhage during child birth.

Objectives

This study aimed to establish how the extract of S. mombin leaves increase uterine smooth muscle contractions relevant to its traditional use to facilitate child birth.

Methods

Tests were performed on uterus muscle strips from Sprague-Dawley rats. Central portion of uterine horns were dissected, cleaned of surrounding fat and loose connective tissue, and cut longitudinally into strips which were placed in the organ bath for isometric tension record in presence of different substances.

Results

S. mombin leaves extract increased uterine spontaneous contractions. This effect was reduced by indomethacin (2 × 10-6 M), yohimbine (2 × 10-6 M) and 2-aminoethoxydiphenyl borate (2-APB) (5 × 10-5 M), but not by atropine (3.45 × 10-8 M) and cholesterol (2.5 mg/ml).

Conclusion

The pharmacological justification for the traditional use of S. mombin leaves to treat dystocia and expel placenta was that its hydro-ethanolic extract induced prostaglandins release, α2-adrenoceptors stimulation, calcium release from internal stores and lifted inhibitory effect of cholesterol on uterine contractions in order to increase uterine smooth muscle contractions.

The impact of 27-hydroxycholesterol on endometrial cancer proliferation

Endometrial cancer (EC) is the most common gynaecological malignancy. Obesity is a major risk factor for EC and is associated with elevated cholesterol. 27-hydroxycholesterol (27HC) is a cholesterol metabolite that functions as an endogenous agonist for Liver X receptor (LXR) and a selective oestrogen receptor modulator (SERM). Exposure to oestrogenic ligands increases risk of developing EC; however, the impact of 27HC on EC is unknown. Samples of stage 1 EC (n = 126) were collected from postmenopausal women undergoing hysterectomy. Expression of LXRs (NR1H3, LXRα; NR1H2, LXRβ) and enzymes required for the synthesis (CYP27A1) or breakdown (CYP7B1) of 27HC were detected in all grades of EC. Cell lines originating from well-, moderate- and poorly-differentiated ECs (Ishikawa, RL95, MFE 280 respectively) were used to assess the impact of 27HC or the LXR agonist GW3965 on proliferation or expression of a luciferase reporter gene under the control of LXR- or ER-dependent promoters (LXRE, ERE). Incubation with 27HC or GW3965 increased transcription via LXRE in Ishikawa, RL95 and MFE 280 cells (P < 0.01). 27HC selectively activated ER-dependent transcription (P < 0.001) in Ishikawa cells and promoted proliferation of both Ishikawa and RL95 cells (P < 0.001). In MFE 280 cells, 27HC did not alter proliferation but selective targeting of LXR with GW3965 significantly reduced cell proliferation (P < 0.0001). These novel results suggest that 27HC can contribute to risk of EC by promoting proliferation of endometrial cancer epithelial cells and highlight LXR as a potential therapeutic target in the treatment of advanced disease.

Omega-3 fatty acids modulate the lipid profile, membrane architecture, and gene expression of leiomyoma cells

Uterine leiomyomas (fibroids or myomas) are the most common benign tumors of premenopausal women and new medical treatments are needed. This study aimed to determine the effects of omega-3 fatty acids on the lipid profile, membrane architecture and gene expression patterns of extracellular matrix components (collagen1A1, fibronectin, versican, or activin A), mechanical signaling (integrin β1, FAK, and AKAP13), sterol regulatory molecules (ABCG1, ABCA1, CAV1, and SREBF2), and mitochondrial enzyme (CYP11A1) in myometrial and leiomyoma cells. Myometrial tissues had a higher amount of arachidonic acid than leiomyoma tissues while leiomyoma tissues had a higher level of linoleic acid than myometrial tissues. Treatment of primary myometrial and leiomyoma cells with eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) reduced the monounsaturated fatty acid (MUFA) content and increased the polyunsaturated fatty acid (PUFA) content in both cell types. Myometrial and leiomyoma cell membranes were in the liquid-crystalline phase, but EPA- and DHA-treated cells had decreased membrane fluidity. While we found no changes in the mRNA expression of ECM components, EPA and DHA treatment reduced levels of ABCG1, ABCA1, and AKAP13 in both cell types. EPA and DHA also reduced FAK and CYP11A1 expression in myometrial cells. The ability of omega-3 fatty acids to remodel membrane architecture and downregulate the expression of genes involved in mechanical signaling and lipid accumulation in leiomyoma cells offers to further investigate this compound as preventive and/or therapeutic option.

Hypercholesterolemia impairs oxytocin-induced uterine contractility in late pregnant mouse

High cholesterol is known to negatively affect uterine contractility inex vivoconditions. The aim of the present study was to reveal the effect ofin vivohypercholesterolemia on spontaneous and oxytocin-induced uterine contractility in late pregnant mouse uterus. Female Swiss albino mice were fed with high cholesterol (HC) diet (0.5% sodium cholate, 1.25% cholesterol and 15% fat) for 6 weeks and then throughout the gestation period after mating. On day 19 of gestation, serum cholesterol level was increased more than 3-fold while triglycerides level was reduced in HC diet-fed animals as compared to control animals fed with a standard diet. In tension experiments, neither the mean integral tension of spontaneous contractility nor the response to CaCl2in high K+-depolarized tissues was altered, but the oxytocin-induced concentration-dependent contractile response in uterine strips was attenuated in hypercholesterolemic mice as compared to control. Similarly, hypercholesterolemia dampened concentration-dependent uterine contractions elicited by a GNAQ protein activator,Pasteurella multocidatoxin. However, it had no effect on endogenous oxytocin level either in plasma or in uterine tissue. It also did not affect the prostaglandin release in oxytocin-stimulated tissues. Western blot data showed a significant increase in caveolin-1 and GRK6 proteins but decline in oxytocin receptor, GNAQ and RHOA protein expressions in hypercholesterolemic mouse uterus. The results of the present study suggest that hypercholesterolemia may attenuate the uterotonic action of oxytocin in late pregnancy by causing downregulation of oxytocin receptors and suppressing the signaling efficacy through GNAQ and RHOA proteins.

Parturition dysfunction in obesity: time to target the pathobiology

Over a third of women of childbearing age in the United States are obese, and during pregnancy they are at increased risk for delayed labor onset and slow labor progress that often results in unplanned cesarean delivery. The biology behind this dysfunctional parturition is not well understood. Studies of obesity-induced changes in parturition physiology may facilitate approaches to optimize labor in obese women. In this review, we summarize known and proposed biologic effects of obesity on labor preparation, contraction/synchronization, and endurance, drawing on both clinical observation and experimental data. We present evidence from human and animal studies of interactions between obesity and parturition signaling in all elements of the birth process, including: delayed cervical ripening, prostaglandin insensitivity, amniotic membrane strengthening, decreased myometrial oxytocin receptor expression, decreased myocyte action potential initiation and contractility, decreased myocyte gap junction formation, and impaired myocyte neutralization of reactive oxygen species. We found convincing clinical data on the effect of obesity on labor initiation and successful delivery, but few studies on the underlying pathobiology. We suggest research opportunities and therapeutic interventions based on plausible biologic mechanisms.

Identification of the Functions of Liver X Receptor-β in Sertoli Cells Using a Targeted Expression-Rescue Model

Liver X receptors (LXRs) are key regulators of lipid homeostasis and are involved in multiple testicular functions. The Lxrα(-/-);Lxrβ(-/-) mice have illuminated the roles of both isoforms in maintenance of the epithelium in the seminiferous tubules, spermatogenesis, and T production. The requirement for LXRβ in Sertoli cells have been emphasized by early abnormal cholesteryl ester accumulation in the Lxrβ(-/-) and Lxrα(-/-);Lxrβ(-/-) mice. Other phenotypes, such as germ cell loss and hypogonadism, occur later in life in the Lxrα(-/-);Lxrβ(-/-) mice. Thus, LXRβ expression in Sertoli cells seems to be essential for normal testicular physiology. To decipher the roles of LXRβ within the Sertoli cells, we generated Lxrα(-/-);Lxrβ(-/-):AMH-Lxrβ transgenic mice, which reexpress Lxrβ in Sertoli cells in the context of Lxrα(-/-);Lxrβ(-/-) mice. In addition to lipid homeostasis, LXRβ is necessary for maintaining the blood-testis barrier and the integrity of the germ cell epithelium. LXRβ is also implicated in the paracrine action of Sertoli cells on Leydig cells to modulate T synthesis. The Lxrα(-/-);Lxrβ(-/-) and Lxrα(-/-);Lxrβ(-/-):AMH-Lxrβ mice exhibit lipid accumulation in germ cells after the Abcg8 down-regulation, suggesting an intricate LXRβ-dependent cooperation between the Sertoli cells and germ cells to ensure spermiogenesis. Further analysis revealed also peritubular smooth muscle defects (abnormal lipid accumulation and disorganized smooth muscle actin) and spermatozoa stagnation in the seminiferous tubules. Together the present work elucidates specific roles of LXRβ in Sertoli cell physiology in vivo beyond lipid homeostasis.

ATP-binding cassette transporters in reproduction: a new frontier

BACKGROUND

The transmembrane ATP-binding cassette (ABC) transporters actively efflux an array of clinically relevant compounds across biological barriers, and modulate biodistribution of many physiological and pharmacological factors. To date, over 48 ABC transporters have been identified and shown to be directly and indirectly involved in peri-implantation events and fetal/placental development. They efflux cholesterol, steroid hormones, vitamins, cytokines, chemokines, prostaglandins, diverse xenobiotics and environmental toxins, playing a critical role in regulating drug disposition, immunological responses and lipid trafficking, as well as preventing fetal accumulation of drugs and environmental toxins.

METHODS

This review examines ABC transporters as important mediators of placental barrier functions and key reproductive processes. Expression, localization and function of all identified ABC transporters were systematically reviewed using PubMed and Google Scholar websites to identify relevant studies examining ABC transporters in reproductive tissues in physiological and pathophysiological states. Only reports written in English were incorporated with no restriction on year of publication. While a major focus has been placed on the human, extensive evidence from animal studies is utilized to describe current understanding of the regulation and function of ABC transporters relevant to human reproduction.

RESULTS

ABC transporters are modulators of steroidogenesis, fertilization, implantation, nutrient transport and immunological responses, and function as 'gatekeepers' at various barrier sites (i.e. blood-testes barrier and placenta) against potentially harmful xenobiotic factors, including drugs and environmental toxins. These roles appear to be species dependent and change as a function of gestation and development. The best-described ABC transporters in reproductive tissues (primarily in the placenta) are the multidrug transporters p-glycoprotein and breast cancer-related protein, the multidrug resistance proteins 1 through 5 and the cholesterol transporters ABCA1 and ABCG1.

CONCLUSIONS

The ABC transporters have various roles across multiple reproductive tissues. Knowledge of efflux direction, tissue distribution, substrate specificity and regulation of the ABC transporters in the placenta and other reproductive tissues is rapidly expanding. This will allow better understanding of the disposition of specific substrates within reproductive tissues, and facilitate development of novel treatments for reproductive disorders as well as improved approaches to protecting the developing fetus.

Hepatotoxicity induced by neonatal exposure to diethylstilbestrol is maintained throughout adulthood via the nuclear receptor SHP

BACKGROUND

Liver physiology is sensitive to estrogens, which suggests that the liver might be a target of estrogenic endocrine disrupters (EED). However, the long-term consequences of neonatal exposure to EED on liver physiology have rarely been studied. The nuclear receptor small heterodimer partner (SHP) mediates the deleterious effects of neonatal exposure to diethylstilbestrol (DES) on male fertility.

OBJECTIVES

As SHP is involved in liver homeostasis, we aimed to determine whether neonatal estrogenic exposure also affected adult liver physiology through SHP. Male mouse pups were exposed to DES in the first 5 days of life.

RESULTS

DES exposure leads to alterations in the postnatal bile acid (BA) synthesis pathway. Neonatal DES-exposure affected adult liver BA metabolism and subsequently triglyceride (TG) homeostasis. The wild-type males neonatally exposed to DES exhibited increased liver weight and altered liver histology in the adult age. The use of deficient male mice revealed that SHP mediates the deleterious effects of DES treatment. These long-term effects of DES were associated with differently timed alterations in the expression of epigenetic factors.

CONCLUSIONS

However, the molecular mechanisms by which neonatal exposure persist to affect the adult liver physiology remain to be defined. In conclusion, we demonstrate that neonatal DES exposure alters adult hepatic physiology in an SHP-dependent manner.

Changes in mouse uterine transcriptome in estrus and proestrus

Changes in the CD-1 mouse uterine transcriptome during proestrus and estrus were investigated to help elucidate mechanisms of uterine tissue remodeling during the estrus cycle and their regulation by estrogen and progesterone in preparation of the uterus for pregnancy. Mice were staged beginning at 6 weeks of age, and uterine horns were harvested after monitoring two estrus cycles. Microarray analysis of whole uterine horn RNA identified 2428 genes differentially expressed in estrus compared to proestrus, indicating there is extensive remodeling of mouse uterus during the estrus cycle, affecting ~10% of all protein-encoding genes. Many (~50%) of these genes showed the same differential expression in independent analyses of isolated uterine lumenal epithelial cells. Changes in gene expression associated with structural alterations of the uterus included remodeling of the extracellular matrix, changes in cell keratins and adhesion molecules, activation of mitosis and changes in major histocompatibility complex class II (MHCII) presentation, complement and coagulation cascades, and cytochrome P450 expression. Signaling pathways regulated during the estrus cycle, involving ligand-gated channels, Wnt and hedgehog signaling, and transcription factors with poorly understood roles in reproductive tissues, included several genes and gene networks that have been implicated in pathological states. Many of the molecular pathways and biological functions represented by the genes differentially expressed from proestrus to estrus are also altered during the human menstrual cycle, although not necessarily at the corresponding phases of the cycle. These findings establish a baseline for further studies in the mouse model to dissect mechanisms involved in uterine tissue response to endocrine disruptors and the development of reproductive tract diseases.

Emerging roles for LXRs and LRH-1 in female reproduction

Nutritional status is known to control female reproductive physiology. Many reproductive pathologies such as anorexia nervosa, dystocia and preeclampsia, have been linked to body mass index and to metabolic syndrome. Lipid metabolism has also been associated with ovarian, uterine and placental functions. Among the regulators of lipid homeostasis, the Liver X Receptors (LXRs) and the Liver Receptor Homolog-1 (LRH-1), two members of the nuclear receptor superfamily, play a central role. LXRs are sensitive to intracellular cholesterol concentration and decrease plasma cholesterol, allowing to considering them as "cholesterol sensors". LRH-1 shares many target-genes with LXRs and has been considered for a long time as a real orphan nuclear receptor, but recent findings showed that phospholipids are ligands for this nuclear receptor. Acting in concert, LXRs and LRH-1 could thus be sensitive to slight modifications in cellular lipid balance, tightly maintaining their cellular concentrations. These last years, the use of transgenic mice clarified the roles of these nuclear receptors in many physiological functions. This review will be focused on the roles of LXRs and LRH-1 on female reproduction. Their contribution to ovarian endocrine and exocrine functions, as well as uterine and placental physiology will be discussed. The future challenge will thus be to target these nuclear receptors to prevent lipid-associated reproductive diseases in women.

Dietary cholesterol-induced post-testicular infertility

This work shows that an overload of dietary cholesterol causes complete infertility in dyslipidemic male mice (the Liver X Receptor-deficient mouse model). Infertility resulted from post-testicular defects affecting the fertilizing potential of spermatozoa. Spermatozoa of cholesterol-fed lxr−/− animals were found to be dramatically less viable and motile, and highly susceptible to undergo a premature acrosome reaction. We also provide evidence, that this lipid-induced infertility is associated with the accelerated appearance of a highly regionalized epididymal phenotype in segments 1 and 2 of the caput epididymidis that was otherwise only observed in aged LXR-deficient males. The epididymal epithelial phenotype is characterized by peritubular accumulation of cholesteryl ester lipid droplets in smooth muscle cells lining the epididymal duct, leading to their transdifferentiation into foam cells that eventually migrate through the duct wall, a situation that resembles the inflammatory atherosclerotic process. These findings establish the high level of susceptibility of epididymal sperm maturation to dietary cholesterol overload and could partly explain reproductive failures encountered by young dyslipidemic men as well as ageing males wishing to reproduce.

PPARs and Female Reproduction: Evidence from Genetically Manipulated Mice

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear receptors controlling many important physiological processes, including lipid and glucose metabolism, energy homeostasis, inflammation, as well as cell proliferation and differentiation. In the past decade, intensive study of PPARs has shed novel insight into prevention and treatment of dyslipidemia, insulin resistance, and type 2 diabetes. Recently, a large body of research revealed that PPARs are also functionally expressed in reproductive organs and various parts of placenta during pregnancy, which strongly suggests that PPARs might play a critical role in reproduction and development, in addition to their central actions in energy homeostasis. In this review, we summarize recent findings elucidating the role of PPARs in female reproduction, with particular focus on evidence from gene knockout and transgenic animal model study.

Evolution of promiscuous nuclear hormone receptors: LXR, FXR, VDR, PXR, and CAR

Nuclear hormone receptors (NHRs) are transcription factors that work in concert with co-activators and co-repressors to regulate gene expression. Some examples of ligands for NHRs include endogenous compounds such as bile acids, retinoids, steroid hormones, thyroid hormone, and vitamin D. This review describes the evolution of liver X receptors α and β (NR1H3 and 1H2, respectively), farnesoid X receptor (NR1H4), vitamin D receptor (NR1I1), pregnane X receptor (NR1I2), and constitutive androstane receptor (NR1I3). These NHRs participate in complex, overlapping transcriptional regulation networks involving cholesterol homeostasis and energy metabolism. Some of these receptors, particularly PXR and CAR, are promiscuous with respect to the structurally wide range of ligands that act as agonists. A combination of functional and computational analyses has shed light on the evolutionary changes of NR1H and NR1I receptors across vertebrates, and how these receptors may have diverged from ancestral receptors that first appeared in invertebrates.

Liver X receptors, lipids and their reproductive secrets in the male

Liver X receptor (LXR) α and LXRβ belong to the nuclear receptor superfamily. For many years, they have been called orphan receptors, as no natural ligand was identified. In the last decade, the LXR natural ligands have been shown to be oxysterols, molecules derived from cholesterol. While these nuclear receptors have been abundantly studied for their roles in the regulation of lipid metabolism, it appears that they also present crucial activities in reproductive organs such as testis and epididymis, as well as prostate. Phenotypic analyses of mice lacking LXRs (lxr-/-) pointed out their physiological activities in the various cells and organs regulating reproductive functions. This review summarizes the impact of LXR-deficiency in male reproduction, highlighting the novel information coming from the phenotypic analyses of lxrα-/-, lxrβ-/- and lxrα;β-/- mice. This article is part of a Special Issue entitled: Translating nuclear receptor from health to disease.

Nuclear receptor-driven alterations in bile acid and lipid metabolic pathways during gestation

Nuclear receptor signalling is essential for physiological processes such as metabolism, development, and reproduction. Alterations in the endocrine state that naturally occur during pregnancy result in maternal adaptations to support the feto-placental unit. A series of studies have shown that nuclear receptor signalling is involved in maternal adaptations of bile acid, cholesterol, and lipid homeostasis pathways to ensure maintenance of the nutritional demands of the fetus. We discuss regulation of hepatic nuclear receptors and their target genes in pregnancy and their impact on the development of disorders such as intrahepatic cholestasis of pregnancy and oestrogen-induced hepatotoxicity. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Liver X receptor (LXR) and the reproductive system--a potential novel target for therapeutic intervention

Liver X receptor (LXR) alpha and beta are ligand-activated transcription factors that regulate the expression of genes involved in the removal of cholesterol from cells by high-density lipoproteins, the transport of cholesterol to the liver and the biliary excretion of cholesterol. LXRs are activated by oxygenated cholesterol derivatives such as 24(S),25-epoxycholesterol or 24(S)-, 25- and 27-hydroxycholesterol. In this review, we will discuss the role of LXR in the reproductive system and perspectives on the application of LXR agonists in the treatment of reproductive pathologies. Interestingly, progressive age-related impairment of fertility is observed in both female and male LXR knockout mice. Reduced fertility in female LXR knockout mice is associated with resistance to follicular fluid meiosis-activating sterol (FF-MAS), the intermediate of cholesterol synthesis generated in the ovaries that is responsible for stimulating oocyte meiosis partially in a LXR-dependent manner. Female mice lacking both LXR isoforms exhibit symptoms of ovarian hyperstimulation syndrome when treated with pharmacological doses of gonadotropins. LXR agonists have mainly been considered as potential anti-atherosclerotic medications. However, experimental studies suggest that natural or synthetic LXR agonists may also effectively treat some reproductive pathologies, such as infertility, impaired uterine contractility, hormone-dependent cancers and insulin resistance in patients with polycystic ovarian syndrome. However, the specific adverse effects of LXR agonists on the reproductive system must also be considered. Adverse effects of LXR agonists include impaired trophoblast invasion, excessive transplacental cholesterol transport from the mother to the fetus leading to fetal hypercholesterolemia, and augmented estrogen deficiency after menopause.

Regulation of cholesterol homeostasis by liver X receptors

Cellular cholesterol levels reflect a balance between uptake, efflux, and endogenous synthesis. The sterol-responsive transcription factors liver X receptors (LXRalpha and LXRbeta) help maintain cholesterol homeostasis, not only through promotion of cholesterol efflux from peripheral tissues but also through suppression of de novo synthesis and exogenous cholesterol uptake. In this review, we summarize the important role of LXRs in regulating expression of key members that keep cholesterol levels in balance.

Absence of nuclear receptors for oxysterols liver X receptor induces ovarian hyperstimulation syndrome in mice

Ovarian hyperstimulation syndrome is a frequent complication occurring during in vitro fertilization cycles. It is characterized by a massive ovarian enlargement associated with an accumulation of extra vascular fluid. Here we show that liver X receptor (LXR)-alpha and LXR-beta deficient mice present many clinical and biological signs of ovarian hyperstimulation syndrome: ovarian enlargement, hemorrhagic corpora lutea, increased ovarian vascular permeability, and elevated estradiol. Ovulation stimulation resulted in excessive ovarian response to exogenous gonadotropins because follicle number and estradiol production were higher in transgenic mice. LXR deficiency also leads to perturbations in general inflammatory status, associated with ovarian il-6 deregulation. Upon treatment with the synthetic LXR agonist T09101317, serum estradiol and expression of star and cyp11a1 genes were markedly increased in wild-type mice, showing that LXRs are key regulators of ovarian steroidogenesis. These results suggest that LXRs control the ovulation by regulating endocrine and vascular processes.

Minireview: liver X receptor beta: emerging roles in physiology and diseases

Liver X receptors, LXRalpha and LXRbeta, are nuclear receptors belonging to the large family of transcription factors. After activation by oxysterols, LXRs play a central role in the control of lipid and carbohydrate metabolism as well as inflammation. The role of LXRalpha has been extensively studied, particularly in the liver and macrophages. In the liver it prevents cholesterol accumulation by increasing bile acid synthesis and secretion into the bile through ATP-binding cassette G5/G8 transporters, whereas in macrophages it increases cholesterol reverse transport. The function of LXRbeta is still under investigation with most of the current knowledge coming from the study of phenotypes of LXRbeta-/- mice. With these mice new emerging roles for LXRbeta have been demonstrated in the pathogenesis of diseases such as amyotrophic lateral sclerosis and chronic pancreatitis. The present review will focus on the abnormalities described so far in LXRbeta-/- mice and the insight gained into the possible roles of LXRbeta in human diseases.

Liver X receptor-alpha regulates proopiomelanocortin (POMC) gene transcription in the pituitary

The liver X receptors (LXR-alpha and -beta) are nuclear oxysterol receptors that play pivotal roles in regulating the expression of genes involved in cholesterol transport and metabolism. Recently, several groups have reported that the LXRs also regulate adrenal steroidogenesis. However, the roles of LXRs in the hypothalami-pituitary-adrenal axis, especially whether they regulate proopiomelanocortin (POMC) gene expression in the pituitary, remain to be elucidated. In this report, we demonstrate that LXR mRNA is expressed in the pituitary and that at the protein level, LXR-alpha is dominantly expressed. Next, we show that the LXR agonist TO901317 (TO) increased POMC mRNA levels and the number of cells immunostained with anti-ACTH antibody in the mouse pituitary. We also confirmed that TO elevated plasma ACTH and serum corticosterone levels in vivo and increased the total tissue content of immunoreactive ACTH in the pituitary. TO activated the rat POMC gene promoter (-706/+64 bp) in GH3 and AtT-20 cells. Silencing of LXR-alpha mRNA expression in GH3 cells with small interfering RNA specific to LXR-alpha caused a loss of promoter activity induced by the LXR ligand, suggesting that LXR-alpha directly regulates the POMC gene promoter. EMSAs also demonstrated that the retinoid X receptor-alpha/LXR-alpha heterodimer bound to the region between -73 and -52 bp in the rat POMC gene promoter, and this site was responsible for the induction by TO, as confirmed by chromatin immunoprecipitation assays using AtT-20 cells. Our findings provide the first evidence that LXR-alpha positively regulates the POMC gene promoter at the transcriptional level and suggest LXR-alpha to be a coordinator for cross talk between lipid metabolism and neuroendocrinology.

Ligand specificity and evolution of liver X receptors

Liver X receptors (LXRs) are key regulators of lipid and cholesterol metabolism in mammals. Little is known, however, about the function and evolution of LXRs in non-mammalian species. The present study reports the cloning of LXRs from African clawed frog (Xenopus laevis), Western clawed frog (Xenopus tropicalis), and zebrafish (Danio rerio), and their functional characterization and comparison with human and mouse LXRs. Additionally, an ortholog of LXR in the chordate invertebrate Ciona intestinalis was cloned and functionally characterized. Ligand specificities of the frog and zebrafish LXRs were very similar to LXRalpha and LXRbeta from human and mouse. All vertebrate LXRs studied were activated robustly by the synthetic ligands T-0901317 and GW3965 and by a variety of oxysterols. In contrast, Ciona LXR was not activated by T-0901317 or GW3965 but was activated by a limited number of oxysterols, as well as some androstane and pregnane steroids. Pharmacophore analysis, homology modeling, and docking studies of Ciona LXR predict a receptor with a more restricted ligand-binding pocket and less intrinsic disorder in the ligand-binding domain compared to vertebrate LXRs. The results suggest that LXRs have a long evolutionary history, with vertebrate LXRs diverging from invertebrate LXRs in ligand specificity.

Endoglin (CD105) expression is regulated by the liver X receptor alpha (NR1H3) in human trophoblast cell line JAR

Human implantation involves invasion of the uterine wall and remodeling of uterine arteries by extravillous cytotrophoblasts. Defects in these early steps of placental development lead to poor placentation and are often associated with preeclampsia, a frequent complication of human pregnancy. One of the complex mechanisms controlling trophoblast invasion involves the activation of the liver X receptor beta (or NR1H2, more commonly known as LXRbeta) by oxysterols known as potent LXR activators. This activation of LXRbeta leads to a decrease of trophoblast invasion. The identification of new target genes of LXR in the placenta could aid in the understanding of their physiological roles in trophoblast invasion. In the present study, we show that the endoglin (ENG) gene is a direct target of the liver X receptor alpha (NR1H3, also known as LXRalpha). ENG, whose gene is highly expressed in syncytiotrophoblasts, is part of the transforming growth factor (TGF) receptor complex that binds several members of the TGFbeta superfamily. In the human placenta, ENG has been shown to be involved in the inhibition of trophoblast invasion. Treatment of human choriocarcinoma JAR cells with T0901317, a synthetic LXR-selective agonist, leads to a significant increase in ENG mRNA and protein levels. Using transfection and electrophoretic mobility shift assays, we demonstrate that LXR (as a heterodimer with the retinoid X receptor) is able to bind the ENG promoter on an LXR response element and mediates the activation of ENG gene expression by LXRalpha in JAR cells. This study suggests a novel mechanism by which LXR may regulate trophoblast invasion in pathological pregnancy such as preeclampsia.

Liver X receptors and epididymal epithelium physiology

AIM

To investigate the roles of liver X receptors (LXR) in the lipid composition and gene expression regulation in the murine caput epididymidis. LXR are nuclear receptors for oxysterols, molecules derived from cholesterol metabolism that are present in mammals as two isoforms: LXRalpha, which is more specifically expressed in lipid-metabolising tissues, such as liver, adipose and steroidogenic tissues, and macrophages, whereas LXRbeta is ubiquitous. Their importance in reproductive physiology has been sustained by the fact that male mice in which the function of both LXR has been disrupted have fertility disturbances starting at the age of 5 months, leading to complete sterility by the age of 9 months. These defects are associated with epididymal epithelial degeneration in caput segments one and two, and with a sperm midpiece fragility, leading to the presence of isolated sperm heads and flagella when luminal contents are recovered from the cauda epididymidis.

METHODS

The lipid composition of the caput epididymidis of wild-type and LXR-deficient mice was assessed using oil red O staining on tissue cryosections and lipid extraction followed by high performance liquid chromatography or gas chromatography. Gene expression was checked by quantitative real time polymerase chain reaction.

RESULTS

Using LXR-deficient mice, we showed an alteration of the lipid composition of the caput epididymidis as well as a significantly decreased expression of the genes encoding SREBP1c, SCD1 and SCD2, involved in fatty acid metabolism.

CONCLUSION

Altogether, these results show that LXR are important regulators of epididymal function, and play a critical role in the lipid maturation processes occurring during sperm epididymal maturation.

Multiple roles of the nuclear receptors for oxysterols liver X receptor to maintain male fertility

Oxysterol nuclear receptors liver X receptor (LXR)alpha and LXRbeta are known to regulate lipid homeostasis in cells exposed to high amounts of cholesterol and/or fatty acids. In order to elucidate the specific and redundant roles of the LXRs in the testis, we explored the reproductive phenotypes of mice deficient of LXRalpha, LXRbeta, and both, of which only the lxralpha;beta-/- mice are infertile by 5 months of age. We demonstrate that LXRalpha-deficient mice had lower levels of testicular testosterone that correlated with a higher apoptotic rate of the germ cells. LXRbeta-deficient mice showed increased lipid accumulation in the Sertoli cells and a lower proliferation rate of the germ cells. In lxralpha;beta-/- mice, fatty acid metabolism was affected through a decrease of srebp1c and increase in scd1 mRNA expression. The retinoid acid signaling pathway was also altered in lxralpha;beta-/- mice, with a higher accumulation of all-trans retinoid receptor alpha, all-trans retinoid receptor beta, and retinoic aldehyde dehydrogenase-2 mRNA. Combination of these alterations might explain the deleterious phenotype of infertility observed only in lxralpha;beta-/- mice, even though lipid homeostasis seemed to be first altered. Wild-type mice treated with a specific LXR agonist showed an increase of testosterone production involving both LXR isoforms. Altogether, these data identify new roles of each LXR, collaborating to maintain both integrity and functions of the testis.