Liver X receptors regulate adrenal steroidogenesis and hypothalamic-pituitary-adrenal feedback

NR5A1/SF-1 Collaborates with Inhibin α and the Androgen Receptor

Steroidogenic factor 1 (SF-1) is a nuclear receptor that regulates steroidogenesis and reproductive development. NR5A1/SF-1 variants are associated with a broad spectrum of phenotypes across individuals with disorders of sex development (DSDs). Oligogenic inheritance has been suggested as an explanation. SF-1 interacts with numerous partners. Here, we investigated a constellation of gene variants identified in a 46,XY severely undervirilized individual carrying an ACMG-categorized 'pathogenic' NR5A1/SF-1 variant in comparison to the healthy carrier father. Candidate genes were revealed by whole exome sequencing, and pathogenicity was predicted by different in silico tools. We found variants in NR1H2 and INHA associated with steroidogenesis, sex development, and reproduction. The identified variants were tested in cell models. Novel SF-1 and NR1H2 binding sites in the AR and INHA gene promoters were found. Transactivation studies showed that wild-type NR5A1/SF-1 regulates INHA and AR gene expression, while the NR5A1/SF-1 variant had decreased transcriptional activity. NR1H2 was found to regulate AR gene transcription; however, the NR1H2 variant showed normal activity. This study expands the NR5A1/SF-1 network of interacting partners, while not solving the exact interplay of different variants that might be involved in revealing the observed DSD phenotype. It also illustrates that understanding complex genetics in DSDs is challenging.

Identification of a novel class of cortisol biosynthesis inhibitors and its implications in a therapeutic strategy for hypercortisolism

AIMS

Dysregulation of adrenocortical steroid (corticosteroids) biosynthesis leads to pathological conditions such as Cushing's syndrome. Although several classes of steroid biosynthesis inhibitors have been developed to treat cortisol overproduction, limitations such as insufficient efficacy, adverse effects, and/or tolerability still remain. The present study aimed to develop a new class of small molecules that inhibit cortisol production, and investigated their putative modes of action.

MAIN METHODS

We screened an in-house chemical library with drug-like chemical scaffolds using human adrenocortical NCI-H295R cells. We then evaluated and validated the effects of the selected compounds at multiple regulatory steps of the adrenal steroidogenic pathway. Finally, genome-wide RNA expression analysis coupled with gene enrichment analysis was conducted to infer possible action mechanisms.

KEY FINDINGS

A subset of benzimidazolylurea derivatives, including a representative compound (designated as CJ28), inhibited both basal and stimulated production of cortisol and related intermediate steroids. CJ28 attenuated the mRNA expression of multiple genes involved in steroidogenesis and cholesterol biosynthesis. Furthermore, CJ28 significantly attenuated de novo cholesterol biosynthesis, which contributed to its suppression of cortisol production.

SIGNIFICANCE

We identified a novel chemical scaffold that exerts inhibitory effects on cortisol and cholesterol biosynthesis via coordinated transcriptional silencing of gene expression networks. Our findings also reveal an additional adrenal-directed pharmacological strategy for hypercortisolism involving a combination of inhibitors targeting steroidogenesis and de novo cholesterol biosynthesis.

Short communication: Photoperiod impacts ovarian extracellular matrix and metabolic gene expression in Siberian hamsters

Ovarian cyclicity is variable in adult Siberian hamsters (Phodopus sungorus), who respond to long breeding season photoperiods with follicle development and ovulation, while short photoperiods typical of the non-breeding season induce gonadal atrophy. Recent RNAseq results identified ovarian matrix components and regulators of metabolism as differentially regulated by photoperiod; however, the impact of photoperiod across a full cycle of ovarian regression and recrudescence had not been explored for additional regulators of ovarian metabolism and extracellular matrix components. We hypothesized that matrix and metabolism-related genes would be expressed differentially across photoperiods that mimic breeding and non-breeding season daylengths. Hamsters were housed in one of four photoperiod groups: long day (16 h of light per day: 8 h of dark; LD, controls), short day regressed (8 L:16D; SD, regressed), and females exposed to SD then transferred to LD to stimulate return of ovarian function for 2 (early recrudescence), or 8 (late recrudescence) weeks. Plasma leptin concentrations along with expression of ovarian versican and liver-receptor homolog-1/Nr582 mRNA decreased in SD compared to LD and late recrudescence, while vimentin mRNA expression peaked in early and late recrudescence. Ovarian expression of fibronectin and extracellular matrix protein-1 was low in LD ovaries and increased in regressed and recrudescing groups. Expression of hyaluronidase-2, nectin-2, liver-X receptors-α and-β, and adiponectin mRNA peaked in late recrudescence, with no changes noted for adiponectin receptor-1 and -2. The results offer a first look at the parallels between expression of these genes and the dynamic remodeling that occurs during ovarian regression and recrudescence.

Comparison of the qualitative and quantitative optical coherence tomographic features between sudden acquired retinal degeneration syndrome and normal eyes in dogs

OBJECTIVE

To quantitatively and qualitatively characterize the retinal optical coherence tomographic features of sudden acquired retinal degeneration syndrome (SARDS) and SARDS suspect dogs.

ANIMALS STUDIED

Fourteen SARDS affected dogs, 11 age-, breed-, and sex-matched control dogs, and two SARDS suspect dogs.

PROCEDURES

Spectral-domain optical coherence tomography (OCT) images were used to evaluate the quantitative features, including thickness, intereye asymmetry, and longitudinal changes in retinal layer thickness and the qualitative features, including retinal architecture and vitreous haze.

RESULTS

Mean outer retinal layer thickness (ORT), outer nuclear layer thickness (ONL), and photoreceptor layer thickness (PRL) were significantly lower in the SARDS group, whereas mean inner retinal layer thickness was significantly higher in the SARDS group than in the control group. While thickness values of all retinal layers did not differ significantly between paired eyes in each group, the absolute intereye asymmetries in the ORT (p < .0001), ONL (p = .008), and PRL (p < .0001) were significantly higher in the SARDS group than in the control group. Some SARDS patients and SARDS suspects had a greater PRL than the control group, and serial OCT evaluation showed an increase in PRL in one SARDS suspect. Vitreous haze severity was greater in the SARDS group than in the control group (vitreous relative intensity, p = .030).

CONCLUSIONS

We described the OCT features of SARDS patients and suspects. In particular, PRL thickening in the SARDS suspects might indicate an early change in SARDS. Although further studies are needed, this finding might provide new insights into the pathogenesis of SARDS.

Nuclear Receptors as Regulators of Pituitary Corticotroph Pro-Opiomelanocortin Transcription

The hypothalamic–pituitary–adrenal (HPA) axis plays a critical role in adaptive stress responses and maintaining organism homeostasis. The pituitary corticotroph is the central player in the HPA axis and is regulated by a plethora of hormonal and stress related factors that synergistically interact to activate and temper pro-opiomelanocortin (POMC) transcription, to either increase or decrease adrenocorticotropic hormone (ACTH) production and secretion as needed. Nuclear receptors are a family of highly conserved transcription factors that can also be induced by various physiologic signals, and they mediate their responses via multiple targets to regulate metabolism and homeostasis. In this review, we summarize the modulatory roles of nuclear receptors on pituitary corticotroph cell POMC transcription, describe the unique and complex role these factors play in hypothalamic–pituitary–adrenal axis (HPA) regulation and discuss potential therapeutic targets in disease states.

Metabolism of cholesterol and progesterone is differentially regulated in primary trophoblastic subtypes and might be disturbed in recurrent miscarriages

During pregnancy, extravillous trophoblasts (EVTs) invade the maternal decidua and remodel the local vasculature to establish blood supply for the growing fetus. Compromised EVT function has been linked to aberrant pregnancy associated with maternal and fetal morbidity and mortality. However, metabolic features of this invasive trophoblast subtype are largely unknown. Using primary human trophoblasts isolated from first trimester placental tissues, we show that cellular cholesterol homeostasis is differentially regulated in EVTs compared with villous cytotrophoblasts. Utilizing RNA-sequencing, gene set-enrichment analysis, and functional validation, we provide evidence that EVTs display increased levels of free and esterified cholesterol. Accordingly, EVTs are characterized by increased expression of the HDL-receptor, scavenger receptor class B type I, and reduced expression of the LXR and its target genes. We further reveal that EVTs express elevated levels of hydroxy-delta-5-steroid dehydrogenase 3 beta- and steroid delta-isomerase 1 (HSD3B1) (a rate-limiting enzyme in progesterone synthesis) and are capable of secreting progesterone. Increasing cholesterol export by LXR activation reduced progesterone secretion in an ABCA1-dependent manner. Importantly, HSD3B1 expression was decreased in EVTs of idiopathic recurrent spontaneous abortions, pointing toward compromised progesterone metabolism in EVTs of early miscarriages. Here, we provide insights into the regulation of cholesterol and progesterone metabolism in trophoblastic subtypes and its putative relevance in human miscarriage.

The Pituitary Transcriptional Response Related to Feed Conversion in Pigs

Over the decades, pig breeding objectives have focused on improving the meat content in the carcass without taking into consideration the more effective fattening indicators that affect feed conversion. At present, pig growth traits associated particularly with animal feeding have become crucial due to their economic significance. This is especially evident in countries where pigs are maintained on large farms. The present study indicates that pituitary differentially expressed genes (DEGs) are activated in response to variable feed conversion (FC) in pigs. The experiment included two native Polish breeds: Puławska and Złotnicka White (ZW). The whole pituitary transcriptome was sequenced using next-generation sequencing (NGS) technology. The RNA-seq method identified over 500 and 300 DEGs in the pituitaries of the ZW and the Puławska pig populations, respectively, that were associated with hormonal regulation, notch signaling, and Wnt pathways. Lower FC in the ZW pigs favoured increased fat content in the body and significantly higher prolactin expression. The obtained results indicate that low FC values in pigs are related to slower growth or increased fat content, which suggests various pituitary responses. Therefore, the identified candidate genes were not directly associated with feed conversion values but with other factors. However, the present study delivers new insights into pituitary regulation in pigs.

SIRT2 plays a novel role on progesterone, estradiol and testosterone synthesis via PPARs/LXRα pathways in bovine ovarian granular cells

SIRT2 has been shown to possess NAD⁺-dependent deacetylase and desuccinylase enzymatic activities, it also regulates metabolism homeostasis in mammals. Previous data has suggested that resveratrol, a potential activator of Sirtuins, played a stimulation role in steroidogenesis. Unfortunately, to date, the physiological roles of SIRT2 in ovarian granular cells (GCs) are largely unknown. Here, we studied the function and molecular mechanisms of SIRT2 on steroid hormone synthesis in GCs from Qinchuan cattle. Immunohistochemistry and western blotting showed that SIRT2 was expressed not only in GCs and cumulus cells, but also in oocytes and theca cells. We found that the secretion of progesterone was induced, whereas that of estrogen and testosterone secretion was suppressed by treatment with the SIRT2 inhibitor (Thiomyristoyl or SirReal2) or siRNA. Additionally, the PPARs/LXRα signaling pathways were suppressed by SIRT2 siRNA or inhibitors. The mRNA expression of CYP17, aromatase and StAR was suppressed, but the abundance of CYP11A1 mRNA was induced by SIRT2 inhibition. Furthermore, the PPARα agonist or PPARγ antagonist could mimic the effects of SIRT2 inhibition on hormones levels and gene expression associated with steroid hormone biosynthesis. In turn, those effects were abolished by the LXRα agonist (LXR-623). Together, these data support the hypothesis that SIRT2 regulates steroid hormone synthesis via the PPARs/LXRα pathways in GCs.

Oxysterol-related-binding-protein related Protein-2 (ORP2) regulates cortisol biosynthesis and cholesterol homeostasis

Oxysterol binding protein-related protein 2 (ORP2) is a lipid binding protein that has been implicated in various cellular processes, including lipid sensing, cholesterol efflux, and endocytosis. We recently identified ORP2 as a member of a protein complex that regulates glucocorticoid biosynthesis. Herein, we examine the effect of silencing ORP2 on adrenocortical function and show that the ORP2 knockdown cells exhibit reduced amounts of multiple steroid metabolites, including progesterone, 11-deoxycortisol, and cortisol, but have increased concentrations of androgens, and estrogens. Moreover, silencing ORP2 suppresses the expression of most proteins required for cortisol production and reduces the expression of steroidogenic factor 1 (SF1). ORP2 silencing also increases cellular cholesterol, concomitant with decreased amounts of 22-hydroxycholesterol and 7-ketocholesterol, two molecules that have been shown to bind to ORP2. Further, we show that ORP2 binds to liver X receptor (LXR) and is required for nuclear LXR expression. LXR and ORP2 are recruited to the CYP11B1 promoter in response to cAMP signaling. Additionally, ORP2 is required for the expression of other LXR target genes, including ABCA1 and the LDL receptor (LDLR). In summary, we establish a novel role for ORP2 in regulating steroidogenic capacity and cholesterol homeostasis in the adrenal cortex.

Effect of retinoic acid on human adrenal corticosteroid synthesis

AIMS

Retinoic acid has recently yielded promising results in the treatment of Cushing's disease, i.e., excess cortisol secretion due to a pituitary corticotropin (ACTH)-secreting adenoma. In addition to its effect on the tumoral corticotrope cell, clinical results suggest an additional adrenal site of action. Aim of this study was to evaluate whether retinoic acid modulates cortisol synthesis and secretion by human adrenals in vitro.

MAIN METHODS

Primary cultures from 10 human adrenals specimens were incubated with 10nM, 100nM and 1μM retinoic acid with and without 10nM ACTH for 24h. Cortisol levels were measured by radioimmunoassay and CYP11A1, STAR and MC2R gene expression analyzed by real-time PCR.

KEY FINDINGS

Retinoic acid increased cortisol secretion (149.5±33.01%, 151.3±49.45% and 129.3±8.32% control secretion for 10nM, 100nM and 1μM respectively, p<0.05) and potentiated STAR expression (1.51±0.22, 1.56±0.15 and 1.59±0.14 fold change over baseline, for 10nM, 100nM and 1μM respectively, p<0.05). Concurrently, retinoic acid markedly blunted constitutional and ACTH-induced MC2R expression (0.66±0.11, 0.62±0.08 and 0.53±0.07 fold change over baseline, for 10nM, 100nM and 1μM respectively, p<0.05; 0.71±0.10, 0.51±0.07 and 0.51±0.08 fold change over ACTH alone, for 10nM, 100nM and 1μM respectively, p<0.05). No effect on CYP11A1 was observed.

SIGNIFICANCE

Retinoic acid stimulates cortisol synthesis and secretion in human adrenals and at the same time markedly blunts ACTH receptor transcription. These results reveal a novel, adrenal effect of retinoic acid which may contribute to its efficacy in patients with Cushing's disease.

Decreased cholesterol uptake and increased liver x receptor-mediated cholesterol efflux pathways during prostaglandin F2 alpha-induced and spontaneous luteolysis in sheep

In nonprimate species, it has been well established that prostaglandin F2 alpha (PGF2alpha) initiates luteolysis. Changes in intracellular cholesterol concentrations caused by modulation of cholesterol uptake and efflux may mediate PGF2alpha-induced luteolysis. These changes in cholesterol efflux and uptake are controlled, in part, by the liver x receptors (LXR) alpha (NR1H3) and beta (NR1H2), nuclear receptors that increase expression of genes necessary for cholesterol efflux or limiting cholesterol uptake. Therefore, we hypothesized that PGF2alpha reduces expression of cholesterol uptake and increases expression of cholesterol efflux genes, mediated in part by enhanced LXR activity. To test this hypothesis, an induced luteolysis model was used whereby ewes were treated during their midluteal phase with saline or PGF2alpha and corpora lutea (CL) collected 12, 24, or 48 h later for determination of mRNA and protein concentrations by quantitative real-time PCR and Western blot analysis, respectively. As a complementary approach, CL undergoing spontaneous luteolysis were compared to midluteal phase CL. The lipoprotein receptors responsible for cholesterol uptake were significantly decreased in both luteolysis models. Expression of the LXR target gene ATP binding cassette subfamily A1 (ABCA1), an important mediator of cholesterol efflux, was significantly increased in both experimental models. Chromatin immunoprecipitation confirmed that PGF2alpha treatment resulted in enhanced NR1H3 and NR1H2 binding to the ABCA1 promoter. Qualitative changes in lipid droplet distribution were also observed following PGF2alpha treatment. These data support the hypothesis that reduced cholesterol uptake and increased efflux mediate luteolysis in sheep, which is partially controlled by PGF2alpha stimulation of LXR activity.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Production of corticosteroid hormones in vitro by adrenals in rats with streptozotocin-induced diabetes

We studied baseline and ACTH-stimulated in vitro production of corticosteroids by rat adrenals. Production of the basic corticosteroids pregnenolone (early precursor in corticosteroid synthesis), progesterone (intermediate precursor in synthesis of gluco- and mineralocorticoid hormones), and corticosterone (major glucocorticoid hormone in rodents) in animals with streptozotocin-induced diabetes was enhanced by 1.8-2.0 times in comparison with the control animals. Addition of ACTH to the incubation medium stimulated pregnenolone production by the adrenals equally in the control and experimental (diabetic) groups, while the increase in corticosterone production was less pronounced in the experimental group. Stimulation of corticosterone production in response to ACTH after saturation of the incubation medium with pregnenolone was also less pronounced in diabetic rats.

Alterations of LXRα and LXRβ expression in the hypothalamus of glucose-intolerant rats

Liver X receptor (LXR) α and β are nuclear receptors that are crucial for the regulation of carbohydrate and lipid metabolism. Activation of LXRs in the brain facilitates cholesterol clearance and improves cognitive deficits, thus they are considered as promising drug targets to treat diseases such as atherosclerosis and Alzheimer's disease. Nevertheless, little is known about the function and localization of LXRs in the brain. Here, we studied the expression of LXR in the brains of rats that received free access to 10% (w/v) fructose group (FG) in their beverages or water control drinks (control group (CG)). After 6 weeks rats in the FG presented with hypertriglyceridemia, hyperinsulinemia, and became glucose intolerant, suggesting a progression toward type 2 diabetes. We found that hypothalamic LXR expression was altered in fructose-fed rats. Rats in the FG presented with a decrease in LXRβ levels while showing an increase in LXRα expression in the hypothalamus but not in the hippocampus, cerebellum, or neocortex. Moreover, both LXRα and β expression correlated negatively with insulin and triglyceride levels. Interestingly, LXRβ showed a negative correlation with the area under the curve during the glucose tolerance test in the CG and a positive correlation in the FG. Immunocytochemistry revealed that the paraventricular and ventromedial nuclei express mainly LXRα whereas the arcuate nucleus expresses LXRβ. Both LXR immunosignals were found in the median preoptic area. This is the first study showing a relationship between glucose and lipid homeostasis and the expression of LXRs in the hypothalamus, suggesting that LXRs may trigger neurochemical and neurophysiological responses for the control of food intake and energy expenditure through these receptors.

A genome-wide expression profile of adrenocortical cells in knockout mice lacking steroidogenic acute regulatory protein

Steroidogenic acute regulatory protein (StAR) facilitates cholesterol transfer into the inner mitochondrial membrane in the acute phase of steroidogenesis. Mice lacking StAR (Star(-/-)) share phenotypes with human individuals having congenital lipoid adrenal hyperplasia including compromised production of steroid hormones and florid accumulation of cholesterol esters in adrenal glands and gonads. To define a specific pattern of molecular changes with StAR deficiency, we performed transcriptome analysis of adrenal cells selectively isolated by fluorescent-activated cell sorting at embryonic d 17.5 or 18.5 in seven wild-type (Star(+/+)) or four Star(-/-) mice having the transgene targeting the enhanced green fluorescent protein to cell lineages that express StAR. A gene expression profile was obtained by whole-mouse genome microarray and confirmed by quantitative real-time PCR, identifying 1206 and 767 significantly up-regulated and down-regulated genes, respectively, in Star(-/-) mice compared with Star(+/+) mice (fold difference ≥ 2 and P value < 0.05 with false discovery rate < 0.2). In Star(-/-) mice, expression levels of genes involved in cholesterol efflux and the inflammatory response were significantly up-regulated, whereas those related to steroid hormone biosynthesis or cholesterol biosynthesis and influx were not significantly changed. Immunoreactive Iba1 or F4/80 (macrophage marker) in adrenal glands of Star(-/-) mice was detected not only in an increased number of resident macrophages but also in most adrenocortical cells. These findings expand our understanding of the pathophysiology of adrenal glands with the disruption of StAR and propose a reciprocal interaction between adrenocortical cells and resident macrophages inside adrenal glands of Star(-/-) mice.

Liver x receptor modulation of gene expression leading to proluteolytic effects in primate luteal cells

The expressions of genes involved in cholesterol efflux increase, whereas those involved in extracellular cholesterol uptake decrease, during spontaneous functional regression of the primate corpus luteum (CL). This may result from liver x receptor (LXR) alpha (official symbol NR1H3) and/or beta (official symbol NR1H2) control of luteal gene transcription, because these nuclear receptor superfamily members are key regulators of cellular cholesterol homeostasis. Therefore, studies were conducted to assess endogenous LXR ligands in the primate CL through the luteal phase, and to determine the effect of synthetic or natural LXR ligands on cholesterol efflux and uptake in functional primate luteal cells. Using high-performance liquid chromatography tandem mass spectrometry, three LXR ligands were identified and quantified in the rhesus macaque CL, including 22R-hydroxycholesterol (22ROH), 27-hydroxycholesterol (27OH), and desmosterol. Levels of 22ROH paralleled serum progesterone concentrations, whereas mean levels of 27OH tended to be higher following the loss of progesterone synthesis. Desmosterol was present throughout the luteal phase. Functional macaque luteal cells treated with the synthetic LXR agonist T0901317 or physiologically relevant concentrations of the endogenous luteal ligands 22ROH, 27OH, and desmosterol had increased expression of various known LXR target genes and greater cholesterol efflux. Additionally, T0901317 reduced low-density lipoprotein receptor protein and extracellular low-density lipoprotein uptake, whereas 27OH decreased low-density lipoprotein receptor protein, most likely via a posttranslational mechanism. Collectively, these data support the hypothesis that LXR activation causes increased cholesterol efflux and decreased extracellular cholesterol uptake. In theory, these effects could deplete the primate CL of cholesterol needed for steroidogenesis, ultimately contributing to functional regression.

Sterol regulation of metabolism, homeostasis, and development

Sterol metabolites are critical signaling molecules that regulate metabolism, development, and homeostasis. Oxysterols, bile acids (BAs), and steroids work primarily through cognate sterol-responsive nuclear hormone receptors to control these processes through feed-forward and feedback mechanisms. These signaling pathways are conserved from simple invertebrates to mammals. Indeed, results from various model organisms have yielded fundamental insights into cholesterol and BA homeostasis, lipid and glucose metabolism, protective mechanisms, tissue differentiation, development, reproduction, and even aging. Here, we review how sterols act through evolutionarily ancient mechanisms to control these processes.

Liver X receptor-α/β expression ratio is increased in ACTH-secreting pituitary adenomas

The liver X receptors (LXR-α and -β) 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. In the previous report, we demonstrated that LXR-α is dominantly expressed in the pituitary and that LXR-α positively regulates the proopiomelanocortin (POMC) gene promoter at the transcriptional level. In this report, we evaluated the expression levels of LXR-α and -β gene in the human pituitary tumor. Even though LXR-α mRNA levels are not significantly increased in ACTH-secreting adenomas, LXR-α/β expression ratio is significantly higher than other pituitary tumors including normal pituitaries. Furthermore, in At-T20 cells, which express POMC gene, overexpression of LXR-β decreased POMC gene promoter activities. Thus, we concluded that LXR-α/β gene expression ratio is a critical factor to activate POMC gene expression in ACTH-secreting pituitary adenomas.

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.

Bile duct ligation: step-by-step to cholangiocyte inflammatory tumorigenesis

Chronic liver inflammation after murine bile duct ligation could evolve according to three interrelated phenotypes, which would have different metabolic, functional and histologic characteristics. Liver injury secondary to extrahepatic cholestasis would induce an early ischemic-reperfusion phenotype with cholangiocyte depolarization, abnormal ion transport, hypometabolism with anaerobic glycolysis and hepatocytic apoptosis. This phenotype, in turn, could trigger the switch to a leukocytic phenotype by the cholangiocytes, with an intense anaplerotic activity, hypermetabolism, extracellular matrix degradation and moderated proliferation to create a pseudotissue with metabolic autonomy and paracrine functions. In the long-term cholestasis-drive tumorigenesis, the tumorous tissue would principally consist of cholangiocyte parenchyma, with an impressive biosynthetic activity through the tricarboxylic cell cycle. In terms of the tumorous stroma, made up by fibroplasia and angiogenesis, it would favor the tumor trophism. In conclusion, the great intensity and persistence in the expression of these phenotypes by the cholestatic cholangiocyte would favor chronic inflammatory tumorigenesis.

Group X secretory phospholipase A2 regulates the expression of steroidogenic acute regulatory protein (StAR) in mouse adrenal glands

We developed C57BL/6 mice with targeted deletion of group X secretory phospholipase A(2) (GX KO). These mice have approximately 80% higher plasma corticosterone concentrations compared with wild-type (WT) mice under both basal and adrenocorticotropic hormone (ACTH)-induced stress conditions. This increased corticosterone level was not associated with increased circulating ACTH or a defect in the hypothalamic-pituitary axis as evidenced by a normal response to dexamethasone challenge. Primary cultures of adrenal cells from GX KO mice exhibited significantly increased corticosteroid secretion compared with WT cells. Conversely, overexpression of GX secretory phospholipase A(2) (sPLA(2)), but not a catalytically inactive mutant form of GX sPLA(2), significantly reduced steroid production 30-40% in Y1 mouse adrenal cell line. This effect was reversed by the sPLA(2) inhibitor, indoxam. Silencing of endogenous M-type receptor expression did not restore steroid production in GX sPLA(2)-overexpressing Y1 cells, ruling out a role for this sPLA(2) receptor in this regulatory process. Expression of steroidogenic acute regulatory protein (StAR), the rate-limiting protein in corticosteroid production, was approximately 2-fold higher in adrenal glands of GX KO mice compared with WT mice, whereas StAR expression was suppressed in Y1 cells overexpressing GX sPLA(2). Results from StAR-promoter luciferase reporter gene assays indicated that GX sPLA(2) antagonizes StAR promoter activity and liver X receptor-mediated StAR promoter activation. In summary, GX sPLA(2) is expressed in mouse adrenal glands and functions to negatively regulate corticosteroid synthesis, most likely by negatively regulating StAR expression.

The liver X receptor: control of cellular lipid homeostasis and beyond Implications for drug design

Liver X receptor (LXR) α and β are nuclear receptors that control cellular metabolism. LXRs modulate the expression of genes involved in cholesterol and lipid metabolism in response to changes in cellular cholesterol status. Because of their involvement in cholesterol homeostasis, LXRs have emerged as promising drug targets for anti-atherosclerotic therapies. In rodents, synthetic LXR agonists promote cellular cholesterol efflux, transport and excretion. As a result, the progression of atherosclerosis is halted. However, pharmacological LXR activation also induces hepatic steatosis and promotes the secretion of atherogenic triacylglycerol-rich VLDL particles by the liver, complicating the clinical application of LXR agonists. The more recently emerged roles of LXRs in fat tissue, pituitary and brain may have implications for treatment of obesity and Alzheimer disease. In addition to the improvements in atherosclerosis, LXR activation exerts beneficial effects on glucose control in mouse models of type 2 diabetes. Future therapeutic strategies aiming to exert beneficial effects on cholesterol and glucose homeostasis, while circumventing the undesired effects on hepatic lipid metabolism, should target specific LXR-mediated processes. Therefore, tissue and/or isotype-specific effects of LXR action need to be established. The consequences of combinatorial drug approaches and the identification of the co-regulatory networks involved in the LXR-mediated control of particular genes may contribute to development of novel LXR agonists. Finally, pathway analyses of LXR actions provide tools to evaluate and optimize the effectiveness of novel therapeutic strategies to prevent and/or treat metabolic diseases.

Genomic and neural analysis of the estradiol-synthetic pathway in the zebra finch

Background

Steroids are small molecule hormones derived from cholesterol. Steroids affect many tissues, including the brain. In the zebra finch, estrogenic steroids are particularly interesting because they masculinize the neural circuit that controls singing and their synthesis in the brain is modulated by experience. Here, we analyzed the zebra finch genome assembly to assess the content, conservation, and organization of genes that code for components of the estrogen-synthetic pathway and steroid nuclear receptors. Based on these analyses, we also investigated neural expression of a cholesterol transport protein gene in the context of song neurobiology.

Results

We present sequence-based analysis of twenty steroid-related genes using the genome assembly and other resources. Generally, zebra finch genes showed high homology to genes in other species. The diversity of steroidogenic enzymes and receptors may be lower in songbirds than in mammals; we were unable to identify all known mammalian isoforms of the 3β-hydroxysteroid dehydrogenase and 17β-hydroxysteroid dehydrogenase families in the zebra finch genome assembly, and not all splice sites described in mammals were identified in the corresponding zebra finch genes. We did identify two factors, Nobox and NR1H2-RXR, that may be important for coordinated transcription of multiple steroid-related genes. We found very little qualitative overlap in predicted transcription factor binding sites in the genes for two cholesterol transport proteins, the 18 kDa cholesterol transport protein (TSPO) and steroidogenic acute regulatory protein (StAR). We therefore performed in situ hybridization for TSPO and found that its mRNA was not always detected in brain regions where StAR and steroidogenic enzymes were previously shown to be expressed. Also, transcription of TSPO, but not StAR, may be regulated by the experience of hearing song.

Conclusions

The genes required for estradiol synthesis and action are represented in the zebra finch genome assembly, though the complement of steroidogenic genes may be smaller in birds than in mammals. Coordinated transcription of multiple steroidogenic genes is possible, but results were inconsistent with the hypothesis that StAR and TSPO mRNAs are co-regulated. Integration of genomic and neuroanatomical analyses will continue to provide insights into the evolution and function of steroidogenesis in the songbird brain.

The reverse cholesterol transport system as a potential mediator of luteolysis in the primate corpus luteum

The cessation of progesterone (P(4)) production (i.e. functional regression), arguably the key event in luteolysis of the primate corpus luteum (CL), is poorly understood. Previously, we found that genes encoding proteins involved in cholesterol uptake decreased, while those involved in cholesterol efflux (reverse cholesterol transport, RCT) increased in expression during spontaneous functional regression of the rhesus macaque CL, thereby potentially depleting the cholesterol reserves needed for steroidogenesis. Therefore, a comprehensive analysis of the components necessary for RCT was performed. RCT components were expressed (mRNA and/or protein) in the macaque CL including cholesterol sensors (liver X receptors alpha or NR1H3; and beta or NR1H2), efflux proteins (ATP-binding cassette subfamilies A1 (ABCA1) and G1), acceptors (apolipoproteins A1 or APOA1; and E or APOE), and plasma proteins facilitating high-density lipoprotein formation (lecithin:cholesterol acyltransferase or LCAT; phospholipid transfer protein or PLTP). ABCA1, APOE, PLTP, and NR1H3 increased, while lipoprotein receptors decreased, in expression (mRNA and/or protein) through the period of functional regression. The expression of APOA1 and APOE, as well as NR1H3, was greatest in the CL and tissues involved in regulating cholesterol homeostasis. Immunolocalization studies revealed that RCT proteins and lipoprotein receptors were expressed in large luteal cells, which possess intracellular cholesterol reserves during periods of P(4) synthesis. Lipid staining revealed changes in luteal cholesterol ester/lipid distribution that occurred following functional regression. These results indicate that decreased cholesterol uptake and increased RCT may be critical for the initiation of primate luteolysis by limiting intracellular cholesterol pools required for steroidogenesis.

Minireview: Nuclear receptor-controlled steroid hormone synthesis and metabolism

Steroid hormones are essential in normal physiology whereas disruptions in hormonal homeostasis represent an important etiological factor for many human diseases. Steroid hormones exert most of their functions through the binding and activation of nuclear hormone receptors (NRs or NHRs), a superfamily of DNA-binding and often ligand-dependent transcription factors. In recent years, accumulating evidence has suggested that NRs can also regulate the biosynthesis and metabolism of steroid hormones. This review will focus on the recent progress in our understanding of the regulatory role of NRs in hormonal homeostasis and the implications of this regulation in physiology and diseases.

Transcriptional regulation of steroidogenic genes: STARD1, CYP11A1 and HSD3B

Expression of the genes that mediate the first steps in steroidogenesis, the steroidogenic acute regulatory protein (STARD1), the cholesterol side-chain cleavage enzyme, cytochrome P450scc (CYP11A1) and 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase (HSD3B), is tightly controlled by a battery of transcription factors in the adrenal cortex, the gonads and the placenta. These genes generally respond to the same hormones that stimulate steroid production through common pathways such as cAMP signaling and common actions on their promoters by proteins such as NR5A and GATA family members. However, there are distinct temporal, tissue and species-specific differences in expression between the genes that are defined by combinatorial regulation and unique promoter elements. This review will provide an overview of the hormonal and transcriptional regulation of the STARD1, CYP11A1 and specific steroidogenic HSD3B genes in the adrenal, testis, ovary and placenta and discuss the current knowledge regarding the key transcriptional factors involved.

Liver X Receptors as Therapeutic Targets for Managing Cholesterol: Implications for Atherosclerosis and Other Inflammatory Conditions

Atherosclerosis is a disease characterized by excess cholesterol and inflammation in the blood vessels. The liver X receptors (alpha and beta) are members of the nuclear hormone receptor family that are activated by endogenous cholesterol metabolites. These receptors are widely expressed with a tissue distribution that includes the liver, intestine and macrophage. Upon activation, these receptors have been shown to increase reverse cholesterol transport from the macrophage back to the liver to aid in the removal of excess cholesterol. More recently, they have also been shown to inhibit the inflammatory response in macrophages. These functions are accomplished through direct regulation of gene transcription. Herein, we will describe the key benefits and potential risks of targeting the LXRs for the treatment of atherosclerosis.

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.

Liver X receptor agonist downregulates hepatic apoM expression in vivo and in vitro

It has been demonstrated that apolipoprotein M (apoM), a recently discovered HDL apolipoprotein, has antiatherosclerotic properties, which may be mediated by the enhancement of reversed cholesterol transportation and/or hepatic cholesterol catabolism. The detailed mechanisms are unknown yet. Liver X receptor (LXR) belongs to the nuclear receptor superfamily and is a ligand-activated transcription factor involved in the regulation of lipid metabolism and inflammation. Activation of LXR in the cell cultures results in an enhancement of cholesterol efflux to apoAI. In the present study, we investigated effects of the LXR agonist, T0901317 on hepatic apoM expression in vivo and in vitro. Serum apoM levels in mice given T0901317 at 10 mg or 100 mg/kg for 7 days were reduced by 12-17% (P<0.05). In HepG2 cell cultures, apoM mRNA levels were significantly lower in presence of 25 microM T0901317 (37.1%) than in control cells (P<0.001). A similar reduction was found by the addition of 9-cis retinoic acid (RA). Twenty-five micromolar T0901317 together with 100 nM RA decreased apoM mRNA expression by 65% (P<0.001). Thus, the LXR agonist T0901317 significantly downregulates apoM mRNA expression in vivo and in vitro, which indicates that apoM is another novel target gene regulated by the LXR. The combination of RA and T0901317 showed additive effects, which suggests that apoM expression can be modulated by LXR/RXR pathway.

Effects of the synthetic liver X receptor agonist T0901317 on the growth hormone and thyroid hormone axes in male rats

Liver X receptors (LXRs), activated by oxysterols, play an important role in the regulation of lipid and glucose metabolism, which is also markedly dependent on thyroid hormone and growth hormone (GH) status. Here, we investigated how a 1-week exposure to the synthetic LXR agonist T0901317 affected GH secretion and thyroid hormone status in male rats. While the pulse frequency of GH secretion was marginally affected there was a highly significant decrease in the triiodo-L-thyronine/thyroxine (T3/T4) ratio in plasma. This effect was associated with decreased expression of deiodinase 1 (DIO1) and 2 (DIO2) mRNA in the liver and thyroid gland, respectively. Expression of sterol regulatory element binding protein-1c (SREBP-1c), the hallmark of stimulated lipogenesis, was markedly increased in both thyroid and pituitary implying that protracted pharmacological LXR activation may promote lipid accumulation in these endocrine tissues. These findings suggest that attention must be given to pituitary hormone dependent axes when developing therapeutic strategies based on agonism of the LXRs, e.g. for treatment of atherosclerosis.

Adrenocortical dysregulation as a major player in insulin resistance and onset of obesity

The aim of this review is to explore the dysregulation of adrenocortical secretions as a major contributor in the development of obesity and insulin resistance. Disturbance of adipose tissue physiology is one of the primary events in the development of pathologies associated with the metabolic syndrome, such as obesity and type 2 diabetes. Several studies indicate that alterations in metabolism of glucocorticoids (GC) and androgens, as well as aldosterone in excess, are involved in the emergence of metabolic syndrome. Cross talk among adipose tissue, the hypothalamo-pituitary complex, and adrenal gland activity plays a major role in the control of food intake, glucose metabolism, lipid storage, and energy balance. Perturbation of this cross talk induces alterations in the regulatory mechanisms of adrenocortical steroid synthesis, secretion, degradation, and/or recycling, at the level of the zonae glomerulosa (aldosterone), fasciculata (GC and GC metabolites), and reticularis (androgens and androgen precursors DHEA and DHEAS). As a whole, these adrenocortical perturbations contribute to the development of metabolic syndrome at both the paracrine and systemic level by favoring the physiological dysregulation of organs responsive to aldosterone, GC, and/or androgens, including adipose tissue.

Transcriptional regulation of adrenocortical steroidogenic gene expression

By serving as ligands for nuclear and plasma membrane receptors, steroid hormones are key regulators of a diverse array of physiological processes. These hormones are synthesized from cholesterol in tissues such as the adrenal cortex, ovaries, testes, and placenta. Because steroid hormones control the expression of numerous genes, steroidogenic cells utilize multiple mechanisms that ensure tight control of the synthesis of these molecules. This review will give an overview of the molecular mechanisms by which the expression of steroidogenic genes is regulated in the human adrenal cortex.