Aurélie Vega

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.

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)

Bile Acid Alters Male Mouse Fertility in Metabolic Syndrome Context.

Bile acids have recently been demonstrated as molecules with endocrine activities controlling several physiological functions such as immunity and glucose homeostases. They act mainly through two receptors, the nuclear receptor Farnesol-X-Receptor alpha (FXRα) and the G-protein coupled receptor (TGR5). These recent studies have led to the idea that molecules derived from bile acids (BAs) and targeting their receptors must be good targets for treatment of metabolic diseases such as obesity or diabetes. Thus it might be important to decipher the potential long term impact of such treatment on different physiological functions. Indeed, BAs have recently been demonstrated to alter male fertility. Here we demonstrate that in mice with overweight induced by high fat diet, BA exposure leads to increased rate of male infertility. This is associated with the altered germ cell proliferation, default of testicular endocrine function and abnormalities in cell-cell interaction within the seminiferous epithelium. Even if the identification of the exact molecular mechanisms will need more studies, the present results suggest that both FXRα and TGR5 might be involved. We believed that this work is of particular interest regarding the potential consequences on future approaches for the treatment of metabolic diseases.

Bile acid-FXRα pathways regulate male sexual maturation in mice.

The bile acid receptor Farnesol-X-Receptor alpha (FRXα) is a member of the nuclear receptor superfamily. FRXα is expressed in the interstitial compartment of the adult testes, which contain the Leydig cells. In adult, short term treatment (12 hours) with FRXα agonist inhibits the expression of steroidogenic genes via the induction of the Small heterodimer partner (SHP). However the consequences of FRXα activation on testicular pathophysiology have never been evaluated. We demonstrate here that mice fed a diet supplemented with bile acid during pubertal age show increased incidence of infertility. This is associated with altered differentiation and increase apoptosis of germ cells due to lower testosterone levels. At the molecular level, next to the repression of basal steroidogenesis via the induction expression of Shp and Dax-1, two repressors of steroidogenesis, the main action of the BA-FRXα signaling is through lowering the Leydig cell sensitivity to the hypothalamo-pituitary axis, the main regulator of testicular endocrine function. In conclusion, BA-FRXα signaling is a critical actor during sexual maturation.

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.

Bile acid homeostasis controls CAR signaling pathways in mouse testis through FXRalpha

Bile acids (BAs) are molecules with endocrine activities controlling several physiological functions such as immunity, glucose homeostasis, testicular physiology and male fertility. The role of the nuclear BA receptor FXRα in the control of BA homeostasis has been well characterized. The present study shows that testis synthetize BAs. We demonstrate that mice invalidated for the gene encoding FXRα have altered BA homeostasis in both liver and testis. In the absence of FXRα, BA exposure differently alters hepatic and testicular expression of genes involved in BA synthesis. Interestingly, Fxrα-/- males fed a diet supplemented with BAs show alterations of testicular physiology and sperm production. This phenotype was correlated with the altered testicular BA homeostasis and the production of intermediate metabolites of BAs which led to the modulation of CAR signaling pathways within the testis. The role of the CAR signaling pathways within testis was validated using specific CAR agonist (TCPOBOP) and inverse agonist (androstanol) that respectively inhibited or reproduced the phenotype observed in Fxrα-/- males fed BA-diet. These data open interesting perspectives to better define how BA homeostasis contributes to physiological or pathophysiological conditions via the modulation of CAR activity.

Identification of the Link Between the Hypothalamo-Pituitary Axis and the Testicular Orphan Nuclear Receptor NR0B2 in Adult Male Mice

The small heterodimer partner (SHP, nuclear receptor subfamily 0, group B, member 2; NR0B2) is an atypical nuclear receptor known mainly for its role in bile acid homeostasis in the enterohepatic tract. We previously showed that NR0B2 controls testicular functions such as testosterone synthesis. Moreover, NR0B2 mediates the deleterious testicular effects of estrogenic endocrine disruptors leading to infertility. The endocrine homeostasis is essential for health, because it controls many physiological functions. This is supported by a large number of studies demonstrating that alterations of steroid activity lead to several kinds of diseases such as obesity and infertility. Within the testis, the functions of the Leydig cells are mainly controlled by the hypothalamo-pituitary axis via LH/chorionic gonadotropin (CG). Here, we show that LH/CG represses Nr0b2 expression through the protein kinase A-AMP protein kinase pathway. Moreover, using a transgenic mouse model invalidated for Nr0b2, we point out that NR0B2 mediates the repression of testosterone synthesis and subsequent germ cell apoptosis induced by exposure to anti-GnRH compound. Together, our data demonstrate a new link between hypothalamo-pituitary axis and NR0B2 in testicular androgen metabolism, making NR0B2 a major actor of testicular physiology in case of alteration of LH/CG levels.

Male fertility: Is spermiogenesis the critical step for answering biomedical issues?

Regarding male fertility, biomedical issues have opposite goals to treat infertility or develop contraceptive drugs. Recently, the identification of the molecular mechanisms involved in germ cell differentiation suggest that spermiogenesis has to be put at the crossroad to reach these goals. Concerning fertility issues, citizens in our modern world are schizophrenic. On one side, couples have the possibility to control conception; and on the other side, more and more couples suffer from the misfortune of being infertile. These two societal problems lead to intensive research and conflicting government policies. However, these opposing goals rely on a better understanding of germ cell differentiation.