The orphan nuclear receptor DAX-1 functions as a potent corepressor of the constitutive androstane receptor (NR1I3)

Modulation of nuclear receptor function: Targeting the protein-DNA interface

Nuclear receptors (NRs) are a superfamily of ligand-dependent transcription factors that modulate several biological processes. Traditionally, modulation of NRs has been focused on the development of ligands that recognize and bind to the ligand binding domain (LBD), resulting in activation or repression of transcription through the recruitment of coregulators. However, for more severe diseases, such as breast and prostate cancer, the conventional treatment addressing LBD modulation is not always successful, due to tumor resistance. To overcome these challenges and aiming to modulate NR activity by inhibiting the NR-DNA interaction, new studies focus on the development of molecules targeting alternative sites and domains on NRs. Here, we discuss two different approaches for this alternative NR modulation: one targeting the NR DNA binding domain (DBD); and the other targeting the DNA sites recognized by NRs. Our aim is to present the challenges and perspectives for developing specific inhibitors for each purpose, alongside with already reported examples.

Role of Nuclear Constitutive Androstane Receptor in Regulation of Hepatocyte Proliferation and Hepatocarcinogenesis

Activation of the constitutive androstane receptor (CAR) in hepatocytes occurs as a body adaptation in response to a number of external influences, and its functional activity is primarily related to induction of enzymes detoxifying xenobiotics. However, special attention was recently given to CAR due to the fact that its key role becomes unveiled in various physiological and pathophysiological processes occurring in the liver: gluconeogenesis, metabolism of fatty acids and bilirubin, hormonal regulation, proliferation of hepatocytes, and hepatocarcinogenesis. Here we review the main pathways and mechanisms that elevate hepatocyte proliferative activity related to CAR and whose disturbance may be a pivotal factor in hepatocarcinogenesis.

Regulation of PXR and CAR by protein-protein interaction and signaling crosstalk

INTRODUCTION

Protein-protein interaction and signaling crosstalk contribute to the regulation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR) and broaden their cellular function.

AREA COVERED

This review covers key historic discoveries and recent advances in our understanding of the broad function of PXR and CAR and their regulation by protein-protein interaction and signaling crosstalk.

EXPERT OPINION

PXR and CAR were first discovered as xenobiotic receptors; however, it is clear that PXR and CAR perform a much broader range of cellular functions through protein-protein interaction and signaling crosstalk, which typically mutually affect the function of all the partners involved. Future research on PXR and CAR should, therefore, look beyond their xenobiotic function.

Structures and regulation of non-X orphan nuclear receptors: A retinoid hypothesis

Nuclear receptors are defined as a family of ligand regulated transcription factors [1-6]. While this definition reflects that ligand binding is a key property of nuclear receptors, it is still a heated subject of debate if all the nuclear receptors (48 human members) can bind ligands (ligands referred here to both physiological and synthetic ligands). Recent studies in nuclear receptor structure biology and pharmacology have undoubtedly increased our knowledge of nuclear receptor functions and their regulation. As a result, they point to new avenues for the discovery and development of nuclear receptor regulators, including nuclear receptor ligands. Here we review the recent literature on orphan nuclear receptor structural analysis and ligand identification, particularly on the orphan nuclear receptors that do not heterodimerize with retinoid X receptors, which we term as non-X orphan receptors. We also propose a speculative "retinoid hypothesis" for a subset of non-X orphan nuclear receptors, which we hope to help shed light on orphan nuclear receptor biology and drug discovery. This article is part of a Special Issue entitled 'Orphan Nuclear Receptors'.

Regulation of gene expression by CAR: an update

The constitutive androstane receptor (CAR), a member of the nuclear receptor superfamily, is a well-known xenosensor that regulates hepatic drug metabolism and detoxification. CAR activation can be elicited by a large variety of xenobiotics, including phenobarbital (PB) which is not a directly binding CAR ligand. The mechanism of CAR activation is complex and involves translocation from the cytoplasm into the nucleus, followed by further activation steps in the nucleus. Recently, epidermal growth factor receptor (EGFR) has been identified as a PB-responsive receptor, and PB activates CAR by inhibiting the EGFR signaling. In addition to regulation of drug metabolism, activation of CAR has multiple biological end points such as modulation of xenobiotic-elicited liver injury, and the role of CAR in endobiotic functions such as glucose metabolism and cholesterol homeostasis is increasingly recognized. Thus, investigations on the molecular mechanism of CAR activation are critical for the real understanding of CAR-mediated processes. Here, we summarize the current understanding of mechanisms by which CAR activators regulate gene expression through cellular signaling pathways and the roles of CAR on xenobiotic-elicited hepatocellular carcinoma, liver injury, glucose metabolism and cholesterol homeostasis.

Deciphering the roles of the constitutive androstane receptor in energy metabolism

The constitutive androstane receptor (CAR) is initially defined as a xenobiotic nuclear receptor that protects the liver from injury. Detoxification of damaging chemicals is achieved by CAR-mediated induction of drug-metabolizing enzymes and transporters. More recent research has implicated CAR in energy metabolism, suggesting a therapeutic potential for CAR in metabolic diseases, such as type 2 diabetes and obesity. A better understanding of the mechanisms by which CAR regulates energy metabolism will allow us to take advantage of its effectiveness while avoiding its side effects. This review summarizes the current progress on the regulation of CAR nuclear translocation, upstream modulators of CAR activity, and the crosstalk between CAR and other transcriptional factors, with the aim of elucidating how CAR regulates glucose and lipid metabolism.

Role of Orphan Nuclear Receptor DAX-1/NR0B1 in Development, Physiology, and Disease

DAX-1/NR0B1 is an unusual orphan receptor that has a pivotal role in the development and function of steroidogenic tissues and of the reproductive axis. Recent studies have also indicated that this transcription factor has an important function in stem cell biology and in several types of cancer. Here I critically review the most important findings on the role of DAX-1 in development, physiology, and disease of endocrine tissues since the cloning of its gene twenty years ago.

Post-transcriptional regulation by miR-137 underlies the low abundance of CAR and low rate of bilirubin clearance in neonatal mice

AIM

Jaundice, potentially fatal encephalopathy, is common in approximately two-thirds of all well term infants. It is largely due to low expression of constitutive androstane receptor (CAR) in newborns; however, the mechanisms for this low expression were poorly understood.

MATERIALS AND METHODS

Expression of miR-137 and CAR was compared between neonatal and adult mice and between healthy and a mouse model of obstructive jaundice (OJ) using real-time RT-PCR and Western blot methods. Rate of bilirubin clearance was measured. DNA methylation of miR-137 was analyzed.

KEY FINDINGS

Inverse expressions of miR-137 and CAR were consistently observed between newborn and adult mice, with a significantly higher miR-137 level and lower CAR protein and mRNA levels in neonatal liver than in adult liver. Similar reciprocal relationship was found existing between adult OJ mice and healthy control animals with a higher miR-137 level and lower CAR protein and mRNA levels in OJ than in healthy mice. Forced expression of miR-137 in primary hepatocytes repressed CAR protein levels, which was prevented by the inhibitor of miR-137. Knockdown of endogenous miR-137 by its inhibitor increased the rate of bilirubin clearance in OJ mice. Finally, we found that miR-137 was epigenetically over-activated due to hypomethylation in neonatal mice and in adult OJ mice, relative to adult healthy animals.

SIGNIFICANCE

Our findings indicate that miR-137 is a repressor of CAR and thus a critical determinant of bilirubin clearance and may be considered a molecular target for the treatment of neonatal hyperbilirubinemia.

Gene expression profile analysis of Manila clam (Ruditapes philippinarum) hemocytes after a Vibrio alginolyticus challenge using an immune-enriched oligo-microarray

Background

The Manila clam (Ruditapes philippinarum) is a cultured bivalve with worldwide commercial importance, and diseases cause high economic losses. For this reason, interest in the immune genes in this species has recently increased. The present work describes the construction of the first R. philippinarum microarray containing immune-related hemocyte sequences and its application to study the gene transcription profiles of hemocytes from clams infected with V. alginolyticus through a time course.

Results

The complete set of sequences from R. philippinarum available in the public databases and the hemocyte sequences enriched in immune transcripts were assembled successfully. A total of 12,156 annotated sequences were used to construct the 8 ×15 k oligo-microarray. The microarray experiments yielded a total of 579 differentially expressed transcripts. Using the gene expression results, the associated Gene Ontology terms and the enrichment analysis, we found different response mechanisms throughout the experiment. Genes related to signaling, transcription and apoptosis, such as IL-17D, NF-κB or calmodulin, were typically expressed as early as 3 hours post-challenge (hpc), while characteristic immune genes, such as PGRPs, FREPs and defense proteins appeared later at 8 hpc. This immune-triggering response could have affected a high number of processes that seemed to be activated 24 hpc to overcome the Vibrio challenge, including the expression of many cytoskeleton molecules, which is indicative of the active movement of hemocytes. In fact functional studies showed an increment in apoptosis, necrosis or cell migration after the infection. Finally, 72 hpc, activity returned to normal levels, and more than 50% of the genes were downregulated in a negative feedback of all of the previously active processes.

Conclusions

Using a new version of the R. philippinarum oligo-microarray, a putative timing for the response against a Vibrio infection was established. The key point to overcome the challenge seemed to be 8 hours after the challenge, when we detected immune functions that could lead to the destruction of the pathogen and the activation of a wide variety of processes related to homeostasis and defense. These results highlight the importance of a fast response in bivalves and the effectiveness of their innate immune system.

Intronic DNA elements regulate Nrf2 chemical responsiveness of the human microsomal epoxide hydrolase gene (EPHX1) through a far upstream alternative promoter

In humans, microsomal epoxide hydrolase (mEH) contributes important biological functions that underlie both detoxification and bioactivation fates arising from exposures to foreign chemicals. Previously, we discovered that human mEH gene transcription is initiated from alternative promoters. The respective transcripts are programmed with tissue specificity and the upstream E1b promoter contributes predominantly to mEH expression. The results presented demonstrate that exposures to the Nrf2 activators, sulforaphane (SFN) and tert-butylhydroquinone (tBHQ), markedly activate E1b transcription in human lung and liver cells. Genomic analyses identified two major DNase I hypersensitive regions (HS-1 and HS-2) within the ~15 kb intervening sequence separating E1b from the downstream E1 promoter. In BEAS-2B cells, the Nrf2 effectors, SFN and tBHQ, selectively activated the more distal HS-2 through an antioxidant response element (ARE). An activator protein 1/12-O-tetradecanoylphorbol-13-acetate interaction was further identified within the HS-2 enhancer that functioned to additionally contribute to ARE-mediated induction responsiveness of the E1b promoter. The results demonstrate that ARE modulation, integrated with additional transcriptional complexes, regulates the tissue-specific expression of mEH and that these processes likely coordinate both the protective and bioactivation functions contributed by mEH activities in human tissues.

MicroRNA-561 promotes acetaminophen-induced hepatotoxicity in HepG2 cells and primary human hepatocytes through downregulation of the nuclear receptor corepressor dosage-sensitive sex-reversal adrenal hypoplasia congenital critical region on the X chromosome, gene 1 (DAX-1)

One of the major mechanisms involved in acetaminophen (APAP)-induced hepatotoxicity is hepatocyte nuclear factor 4α (HNF4α)-mediated activation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR). In the present study, we investigated the role of miR-561 and its target gene DAX-1 encoding a corepressor of HNF4α in the process of APAP-induced hepatotoxicity. We used both human hepatocellular liver carcinoma cell line (HepG2) cells and primary human hepatocytes in this study and monitored the levels of reactive oxygen species, lactate dehydrogenase, and glutathione. Our bioinformatics study suggests an association between miR-561 and DAX-1, but not HNF4α. Treatment of HepG2 cells with APAP significantly reduced the expression of DAX-1 in a concentration-dependent manner. miR-561 was induced by APAP treatment in HepG2 cells. Transfection of HepG2 cells with an miR-561 mimic exacerbated APAP-induced hepatotoxicity. HNF4α is physically associated with DAX-1 in HepG2 cells. A decreased protein level of DAX-1 by APAP treatment was also enhanced by miR-561 mimic transfection in HepG2 cells and primary human hepatocytes. The basal and APAP-induced expression of PXR and CAR was enhanced by miR-561 mimic transfection; however, transfection of HepG2 cells or primary human hepatocytes with a miR-561 inhibitor or DAX-1 small interfering RNA reversed these effects. Additionally, the chromatin immunoprecipitation assay revealed that recruitment of DAX-1 onto the PXR promoter was inversely correlated with the recruitment of peroxisome proliferator-activated receptor-α coactivator-1α and HNF4α on APAP treatment. These results indicate that miR-561 worsens APAP-induced hepatotoxicity via inhibition of DAX-1 and consequent transactivation of nuclear receptors.