Silvère Baron

Androgen receptor expression is regulated by the phosphoinositide 3-kinase/Akt pathway in normal and tumoral epithelial cells

The androgen receptor (AR) is a ligand-responsive transcription factor known to play a central role in the pathogenesis of prostate cancer. However, the regulation of AR gene expression in the normal and pathological prostate remains poorly understood. This study focuses on the effect of the phosphoinositide 3-kinase (PI 3-kinase)/Akt axis on AR expression in vas deferens epithelial cells (VDEC), a suitable model to study androgen regulation of gene expression, and LNCaP cells (derived from a metastasis at the left supraclavicular lymph node from a 50-year-old patient with a confirmed diagnosis of metastatic prostate carcinoma). Taken together, our data show for the first time that the PI 3-kinase/Akt pathway is required for basal and dihydrotestosterone-induced AR protein expression in both VDEC and LNCaP. Inhibition of the PI 3-kinase/Akt pathway reduced AR expression and the decline in AR protein level correlated with a decrease in AR mRNA in VDEC but not in LNCaP. Since PI 3-kinase/Akt axis is active in prostate cancer, cross-talk between PI 3-kinase/Akt and AR signalling pathways may have implications for endocrine therapy.

Bile acids: from digestion to cancers.

Bile acids (BAs) are cholesterol metabolites that have been extensively studied these last decades. BAs have been classified in two groups. Primary BAs are synthesized in liver, when secondary BAs are produced by intestinal bacteria. Recently, next to their ancestral roles in digestion and fat solubilization, BAs have been described as signaling molecules involved in many physiological functions, such as glucose and energy metabolisms. These signaling pathways involve the activation of the nuclear receptor FXRα or of the G-protein-coupled receptor TGR5. These two receptors have selective affinity to different types of BAs and show different expression patterns, leading to different described roles of BAs. It has been suggested for long that BAs could be molecules linked to tumor processes. Indeed, as many other molecules, regarding analyzed tissues, BAs could have either protective or pro-carcinogen activities. However, the molecular mechanisms responsible for these effects have not been characterized yet. It involves either chemical properties or their capacities to activate their specific receptors FXRα or TGR5. This review highlights and discusses the potential links between BAs and cancer diseases and the perspectives of using BAs as potential therapeutic targets in several pathologies.

Targeting liver X receptors in human health: deadlock or promising trail?

Introduction: Liver X receptors (LXR) are transcription factors that belong to the nuclear receptor superfamily. Natural derivatives of cholesterol, known as oxysterols, have been identified as agonistic ligands of LXR. They are thus mainly considered to be intracellular cholesterol ‘sensors’ whose activation leads to decreased plasma cholesterol. Their implication in other physiologic processes currently prevents their use as therapeutic targets, because of potentially deleterious side effects. Areas covered: The various LXR agonists and antagonists, along with the physiological functions of LXR. Putative clinical targets including atherosclerosis, diabetes, Alzheimers disease, skin disorders, reproductive disorders and cancer. Expert opinion: LXR are promising pharmacological targets because of the high potential to develop ligands owing to the variety of natural or synthetic agonists. Three aspects should be developed to select a LXR-ligand for treatment of human disease: bio-availability; isoform specificity; tissue specificity. This will allow the development of selective liver X modulators (SLiMs). The challenge is to overcome deleterious side effects to establish LXR as new pharmacological targets.

Crosstalk between androgen receptor and epidermal growth factor receptor-signalling pathways: a molecular switch for epithelial cell differentiation.

In the male, androgens promote growth and differentiation of sex reproductive organs through ligand activation of the androgen receptor (AR). Here, we show that androgens are not major actors of the cell cycle arrest associated with the differentiation process, and that the epidermal growth factor (EGF)-mediated signalling interferes with AR activities to regulate androgen response when epithelial cells are differentiated. Higher AR expression and enhanced androgen responsiveness correlate with reduction of phosphorylated ERK1/2 over differentiation. These modifications are associated with recruitment of cells in phase G(0)/G(1), up-regulation of p27(kip1), down-regulation of p21(Cip1) and p53 proteins, and accumulation of hypo-phosphorylated Rb. Exposure to EGF reduces AR expression levels and blocks androgen-dependent transcription in differentiated cells. It also restores p53 and p21(Cip1) levels, Rb hyper-phosphorylation, ERK1/2 activation and promotes cell cycle re-entry as p27(kip1) protein levels are decreased. Treatment with a MEK inhibitor reverses the EGF-mediated AR down-regulation in differentiated cells, thus suggesting the existence of an inverse correlation between EGF and androgen signalling in non-tumoural epithelia. Interestingly, when androgen signalling is set in differentiated cells, dihydrotestosterone exerts an inhibitory effect on ERK activity but paradoxically does not modify EGFR (ErbB1) phosphorylation, indicating that androgens are able to disrupt the EGFR-ERK cascade. Overall, our data demonstrate the existence of a balance between AR and mitogen-activated protein kinase activities that favours either the maintenance of differentiated conditions or the enhancement of cell proliferation capacities.

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.

Liver X Receptors Protect from Development of Prostatic Intra-Epithelial Neoplasia in Mice

LXR (Liver X Receptors) act as “sensor” proteins that regulate cholesterol uptake, storage, and efflux. LXR signaling is known to influence proliferation of different cell types including human prostatic carcinoma (PCa) cell lines. This study shows that deletion of LXR in mouse fed a high-cholesterol diet recapitulates initial steps of PCa development. Elevation of circulating cholesterol in Lxrαβ-/- double knockout mice results in aberrant cholesterol ester accumulation and prostatic intra-epithelial neoplasia. This phenotype is linked to increased expression of the histone methyl transferase EZH2 (Enhancer of Zeste Homolog 2), which results in the down-regulation of the tumor suppressors Msmb and Nkx3.1 through increased methylation of lysine 27 of histone H3 (H3K27) on their promoter regions. Altogether, our data provide a novel link between LXR, cholesterol homeostasis, and epigenetic control of tumor suppressor gene expression.

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.

Oxysterol receptors, AKT and prostate cancer

Oxysterols derive from cholesterol oxidation. They display various biological activities such as regulating cholesterol, fatty acid and glucose homeostasis as well as cell survival/apoptosis balance. Oxysterols display these metabolic and transcriptional activities mainly through their nuclear receptors known as Liver X Receptors (LXRs) α and β. There is accumulating evidence that LXRs are key modulators of prostate cancer cell survival. Hence, LXR activation increases cholesterol efflux and induces a disruption of lipid rafts. The decrease of membrane cholesterol causes a down regulation of AKT survival pathway and consequently apoptosis. Moreover cholesterol is associated with an increased risk of developing aggressive forms of prostate cancer. These data highlight the interest of targeting the LXR-AKT axis in prostate carcinogenesis.

Oxysterol receptors and their therapeutic applications in cancer conditions

Introduction: Oxysterols are implicated in various cellular processes. Among their target proteins, liver X receptors (LXRs) α and β modulate the cell cycle in a large range of cancer cell lines. Besides their role as cholesterol sensors, LXRs are also involved in the proliferation/apoptosis balance regulation in various types of cancers. Areas covered: This review covers oxysterols and derivatives of cholesterol as well as synthetic or natural ligands (agonist/antagonist) of LXRs. Most tumor cell lines are sensitive to LXR activation. Indeed various cancers are concerned such as prostate, breast, glioblastoma, colorectal, and ovary tumors, and leukemia. Expert opinion: Developing the use of LXR ligands in human health, especially in the field of cancer, represents a novel and promising strategy. Despite a wide spectrum of applications, numerous adverse effects of LXR activation need to be solved before genuine clinical trials in humans. Future directions will be based on the engineering of selective LXRs modulators (SLiMs) as already done for nuclear steroid receptors.

Bile acids alter male fertility through G‐protein‐coupled bile acid receptor 1 signaling pathways in mice

Bile acids (BAs) are signaling molecules that are involved in many physiological functions, such as glucose and energy metabolism. These effects are mediated through activation of the nuclear and membrane receptors, farnesoid X receptor (FXR‐α) and TGR5 (G‐protein‐coupled bile acid receptor 1; GPBAR1). Although both receptors are expressed within the testes, the potential effect of BAs on testis physiology and male fertility has not been explored thus far. Here, we demonstrate that mice fed a diet supplemented with cholic acid have reduced fertility subsequent to testicular defects. Initially, germ cell sloughing and rupture of the blood‐testis barrier occur and are correlated with decreased protein accumulation of connexin‐43 (Cx43) and N‐cadherin, whereas at later stages, apoptosis of spermatids is observed. These abnormalities are associated with increased intratesticular BA levels in general and deoxycholic acid, a TGR5 agonist, in particular. We demonstrate here that Tgr5 is expressed within the germ cell lineage, where it represses Cx43 expression through regulation of the transcriptional repressor, T‐box transcription factor 2 gene. Consistent with this finding, mice deficient for Tgr5 are protected against the deleterious testicular effects of BA exposure. Conclusions: These data identify the testis as a new target of BAs and emphasize TGR5 as a critical element in testicular pathophysiology. This work may open new perspectives on the potential effect of BAs on testis physiology during liver dysfunction. (Hepatology 2014;60:1054‐1065)