Coactivator PRIP, the peroxisome proliferator-activated receptor-interacting protein, is a modulator of placental, cardiac, hepatic, and embryonic development

Diagnostic yield and novel candidate genes by next generation sequencing in 166 children with intrahepatic cholestasis

BACKGROUND AND AIMS

Cholestatic liver disease is a leading referral to pediatric liver transplant centers. Inherited disorders are the second most frequent cause of cholestasis in the first month of life.

METHODS

We retrospectively characterized the genotype and phenotype of 166 participants with intrahepatic cholestasis, and re-analyzed phenotype and whole-exome sequencing (WES) data from patients with previously undetermined genetic etiology for newly published genes and novel candidates. Functional validations of selected variants were conducted in cultured cells.

RESULTS

Overall, we identified disease-causing variants in 31% (52/166) of our study participants. Of the 52 individuals, 18 (35%) had metabolic liver diseases, 9 (17%) had syndromic cholestasis, 9 (17%) had progressive familial intrahepatic cholestasis, 3 (6%) had bile acid synthesis defects, 3(6%) had infantile liver failure and 10 (19%) had a phenocopy of intrahepatic cholestasis. By reverse phenotyping, we identified a de novo variant c.1883G > A in FAM111B of a case with high glutamyl transpeptidase (GGT) cholestasis. By re-analyzing WES data, two patients were newly solved, who had compound heterozygous variants in recently published genes KIF12 and USP53, respectively. Our additional search for novel candidates in unsolved WES families revealed four potential novel candidate genes (NCOA6, CCDC88B, USP24 and ATP11C), among which the patients with variants in NCOA6 and ATP11C recapitulate the cholestasis phenotype in mice models.

CONCLUSIONS

In a single-center pediatric cohort, we identified monogenic variants in 22 known human intrahepatic cholestasis or phenocopy genes, explaining up to 31% of the intrahepatic cholestasis patients. Our findings suggest that re-evaluating existing WES data from well-phenotyped patients on a regular basis can increase the diagnostic yield for cholestatic liver disease in children.

Regulation of Hepatic Gluconeogenesis by Nuclear Receptor Coactivator 6

Nuclear receptor coactivator 6 (NCOA6) is a transcriptional coactivator of nuclear receptors and other transcription factors. A general Ncoa6 knockout mouse was previously shown to be embryonic lethal, but we here generated liver-specific Ncoa6 knockout (Ncoa6 LKO) mice to investigate the metabolic function of NCOA6 in the liver. These Ncoa6 LKO mice exhibited similar blood glucose and insulin levels to wild type but showed improvements in glucose tolerance, insulin sensitivity, and pyruvate tolerance. The decrease in glucose production from pyruvate in these LKO mice was consistent with the abrogation of the fasting-stimulated induction of gluconeogenic genes, phosphoenolpyruvate carboxykinase 1 (Pck1) and glucose-6-phosphatase (G6pc). The forskolin-stimulated inductions of Pck1 and G6pc were also dramatically reduced in primary hepatocytes isolated from Ncoa6 LKO mice, whereas the expression levels of other gluconeogenic gene regulators, including cAMP response element binding protein (Creb), forkhead box protein O1 and peroxisome proliferator-activated receptor γ coactivator 1α, were unaltered in the LKO mouse livers. CREB phosphorylation via fasting or forskolin stimulation was normal in the livers and primary hepatocytes of the LKO mice. Notably, it was observed that CREB interacts with NCOA6. The transcriptional activity of CREB was found to be enhanced by NCOA6 in the context of Pck1 and G6pc promoters. NCOA6-dependent augmentation was abolished in cAMP response element (CRE) mutant promoters of the Pck1 and G6pc genes. Our present results suggest that NCOA6 regulates hepatic gluconeogenesis by modulating glucagon/cAMP-dependent gluconeogenic gene transcription through an interaction with CREB.

The pleiotropic peroxisome proliferator activated receptors: Regulation and therapeutics

The Peroxisome proliferator-activated receptors (PPARs) are key regulators of metabolic events in our body. Owing to their implication in maintenance of homeostasis, both PPAR agonists and antagonists assume therapeutic significance. Understanding the molecular mechanisms of each of the PPAR isotypes in the healthy body and during disease is crucial to exploiting their full therapeutic potential. This article is an attempt to present a rational analysis of the multifaceted therapeutic effects and underlying mechanisms of isotype-specific PPAR agonists, dual PPAR agonists, pan PPAR agonists as well as PPAR antagonists. A holistic understanding of the mechanistic dimensions of these key metabolic regulators will guide future efforts to identify novel molecules in the realm of metabolic, inflammatory and immunotherapeutic diseases.

Application of WES Towards Molecular Investigation of Congenital Cataracts: Identification of Novel Alleles and Genes in a Hospital-Based Cohort of South India

Congenital cataracts are the prime cause for irreversible blindness in children. The global incidence of congenital cataract is 2.2–13.6 per 10,000 births, with the highest prevalence in Asia. Nearly half of the congenital cataracts are of familial nature, with a predominant autosomal dominant pattern of inheritance. Over 38 of the 45 mapped loci for isolated congenital or infantile cataracts have been associated with a mutation in a specific gene. The clinical and genetic heterogeneity of congenital cataracts makes the molecular diagnosis a bit of a complicated task. Hence, whole exome sequencing (WES) was utilized to concurrently screen all known cataract genes and to examine novel candidate factors for a disease-causing mutation in probands from 11 pedigrees affected with familial congenital cataracts. Analysis of the WES data for known cataract genes identified causative mutations in six pedigrees (55%) in PAX6, FYCO1 (two variants), EPHA2, P3H2,TDRD7 and an additional likely causative mutation in a novel gene NCOA6, which represents the first dominant mutation in this gene. This study identifies a novel cataract gene not yet linked to human disease. NCOA6 is a transcriptional coactivator that interacts with nuclear hormone receptors to enhance their transcriptional activator function.

Nuclear receptor coactivator 6 promotes HTR-8/SVneo cell invasion and migration by activating NF-κB-mediated MMP9 transcription

Objectives

NCOA6 is a transcription coactivator; its deletion in mice results in growth retardation and lethality between 8.5 and 12.5 dpc with defects in the placenta. However, the transcription factor(s) and the mechanism(s) involved in the function of NCOA6 in placentation have not been elucidated. Here, the roles of NCOA6 in human cytotrophoblast invasion and migration were studied.

Materials and Methods

Human placenta tissues were collected from normal pregnancies and pregnancies complicated by early‐onset severe preeclampsia (sPE). Immunofluorescence, RT‐qPCR and Western blotting were used to determine NCOA6 expression. Transwell invasion/migration assays were performed to explore whether NCOA6 knockdown affected human placenta‐derived HTR‐8/SVneo cell invasion/migration. Gelatin zymography was performed to examine the change in the gelatinolytic activities of secreted MMP2 and MMP9. Luciferase reporter assays were used to explore whether NCOA6 coactivated NF‐κB‐mediated MMP9 transcription.

Results

NCOA6 is mainly expressed in the human placental trophoblast column, as well as in the EVTs. HTR‐8/SVneo cell invasion and migration were significantly attenuated after NCOA6 knockdown, and the secretion of MMP9 was decreased due to transcriptional suppression. NCOA6 was further found to coactivate NF‐κB‐mediated MMP9 transcription. Moreover, expression of NCOA6 was impaired in placentas of patients complicated by early‐onset sPE.

Conclusions

Thus, we demonstrated that NCOA6 is important for cytotrophoblast invasion/migration, at least partially, by activating NF‐κB‐mediated MMP9 transcription; the downregulation of NCOA6 may contribute to the pathogenesis of early‐onset sPE.

Genome-wide association mapping for milk fat composition and fine mapping of a QTL for de novo synthesis of milk fatty acids on bovine chromosome 13

Background

Bovine milk is widely regarded as a nutritious food source for humans, although the effects of individual fatty acids on human health is a subject of debate. Based on the assumption that genomic selection offers potential to improve milk fat composition, there is strong interest to understand more about the genetic factors that influence the biosynthesis of bovine milk and the molecular mechanisms that regulate milk fat synthesis and secretion. For this reason, the work reported here aimed at identifying genetic variants that affect milk fatty acid composition in Norwegian Red cattle. Milk fatty acid composition was predicted from the nation-wide recording scheme using Fourier transform infrared spectroscopy data and applied to estimate heritabilities for 36 individual and combined fatty acid traits. The recordings were used to generate daughter yield deviations that were first applied in a genome-wide association (GWAS) study with 17,343 markers to identify quantitative trait loci (QTL) affecting fatty acid composition, and next on high-density and sequence-level datasets to fine-map the most significant QTL on BTA13 (BTA for Bos taurus chromosome).

Results

The initial GWAS revealed 200 significant associations, with the strongest signals on BTA1, 13 and 15. The BTA13 QTL highlighted a strong functional candidate gene for de novo synthesis of short- and medium-chained saturated fatty acids; acyl-CoA synthetase short-chain family member 2. However, subsequent fine-mapping using single nucleotide polymorphisms (SNPs) from a high-density chip and variants detected by resequencing showed that the effect was more likely caused by a second nearby gene; nuclear receptor coactivator 6 (NCOA6). These findings were confirmed with results from haplotype studies. NCOA6 is a nuclear receptor that interacts with transcription factors such as PPARγ, which is a major regulator of bovine milk fat synthesis.

Conclusions

An initial GWAS revealed a highly significant QTL for de novo-synthesized fatty acids on BTA13 and was followed by fine-mapping of the QTL within NCOA6. The most significant SNPs were either synonymous or situated in introns; more research is needed to uncover the underlying causal DNA variation(s).

Electronic supplementary material

The online version of this article (doi:10.1186/s12711-017-0294-5) contains supplementary material, which is available to authorized users.

Prognostic significance and therapeutic implications of peroxisome proliferator-activated receptor γ overexpression in human pancreatic carcinoma

Peroxisome proliferator-activated receptor γ (PPARγ) is a ligand-activated nuclear receptor which has been implicated in carcinogenesis and angiogenesis in a wide range of cancers, including pancreatic carcinoma (PC). We aimed to characterize the prognosis and potential therapeutic implications of PPARγ in PC. Real-time RT-PCR and western blotting were used to quantify PPARγ expression in immortalized pancreatic epithelial cells, PC cell lines and freshly isolated matched tumor and non-tumor tissues. PPARγ protein expression was analyzed by immunohistochemistry (IHC) in archived tumor tissues from 101 PC patients. Furthermore, the effect of PPARγ on the cytotoxic action of gemcitabine (Gem) and 5-fluorouracil (5-FU) in PC cell lines was investigated in vitro using RNA interference techniques. Both PPARγ protein and mRNA were expressed at markedly higher levels in all of the PC cell lines and freshly isolated PC tissues, compared to normal immortalized pancreatic epithelial cells and the matched adjacent non-tumor tissues. High levels of PPARγ expression correlated significantly with tumor-node-metastasis (TNM) staging (P<0.001) and poor overall survival (P<0.001), especially in patients with advanced disease who received postoperative chemotherapy. While silencing of PPARγ significantly inhibit the cytotoxic effects of both gemcitabine and 5-fluorouracil in PC cells in vitro. This study suggests that high levels of PPARγ expression are associated with poor overall survival in PC. Additionally, PPARγ promotes chemoresistance in PC cells, indicating that PPARγ may represent a novel therapeutic target for PC.

Perturbation of NCOA6 leads to dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a progressive heart disease characterized by left ventricular dilation and contractile dysfunction. Although many candidate genes have been identified with mouse models, few of them have been shown to be associated with DCM in humans. Germline depletion of Ncoa6, a nuclear hormone receptor coactivator, leads to embryonic lethality and heart defects. However, it is unclear whether Ncoa6 mutations cause heart diseases in adults. Here, we report that two independent mouse models of NCOA6 dysfunction develop severe DCM with impaired mitochondrial function and reduced activity of peroxisome proliferator-activated receptor δ (PPARδ), an NCOA6 target critical for normal heart function. Sequencing of NCOA6-coding regions revealed three independent nonsynonymous mutations present in 5 of 50 (10%) patients with idiopathic DCM (iDCM). These data suggest that malfunction of NCOA6 can cause DCM in humans.

The Hippo effector Yorkie activates transcription by interacting with a histone methyltransferase complex through Ncoa6

The Hippo signaling pathway regulates tissue growth in Drosophila through the transcriptional coactivator Yorkie (Yki). How Yki activates target gene transcription is poorly understood. Here, we identify Nuclear receptor coactivator 6 (Ncoa6), a subunit of the Trithorax-related (Trr) histone H3 lysine 4 (H3K4) methyltransferase complex, as a Yki-binding protein. Like Yki, Ncoa6 and Trr are functionally required for Hippo-mediated growth control and target gene expression. Strikingly, artificial tethering of Ncoa6 to Sd is sufficient to promote tissue growth and Yki target expression even in the absence of Yki, underscoring the importance of Yki-mediated recruitment of Ncoa6 in transcriptional activation. Consistent with the established role for the Trr complex in histone methylation, we show that Yki, Ncoa6, and Trr are required for normal H3K4 methylation at Hippo target genes. These findings shed light on Yki-mediated transcriptional regulation and uncover a potential link between chromatin modification and tissue growth.

Yorkie promotes transcription by recruiting a histone methyltransferase complex

Hippo signaling limits organ growth by inhibiting the transcriptional coactivator Yorkie. Despite the key role of Yorkie in both normal and oncogenic growth, the mechanism by which it activates transcription has not been defined. We report that Yorkie binding to chromatin correlates with histone H3K4 methylation and is sufficient to locally increase it. We show that Yorkie can recruit a histone methyltransferase complex through binding between WW domains of Yorkie and PPxY sequence motifs of NcoA6, a subunit of the Trithorax-related (Trr) methyltransferase complex. Cell culture and in vivo assays establish that this recruitment of NcoA6 contributes to Yorkie's ability to activate transcription. Mammalian NcoA6, a subunit of Trr-homologous methyltransferase complexes, can similarly interact with Yorkie's mammalian homolog YAP. Our results implicate direct recruitment of a histone methyltransferase complex as central to transcriptional activation by Yorkie, linking the control of cell proliferation by Hippo signaling to chromatin modification.

PPAR-γ -- a possible drug target for complicated pregnancies

Peroxisome proliferator activated receptors (PPARs) are ligand-activated transcription factors expressed in trophoblasts, which regulate both cell differentiation and proliferation. In recent years, evidence has linked PPARs to playing an integral role in pregnancy; specifically, PPAR-β and PPAR-γ have been shown to play an integral role in placentation, with PPAR-γ additionally serving to regulate trophoblast differentiation. Recent evidence has shown that PPAR-γ expression is altered in many complications of pregnancy such as intrauterine growth restriction (IUGR), preterm birth, pre-clampsia and gestational diabetes. Thus, at present, accumulating evidence from the literature suggests both a pivotal role for PPAR-γ in the progression of a healthy pregnancy and the possibility that PPAR-γ may act as a therapeutic target in complicated pregnancies. This review aims to provide a succinct and comprehensive assessment of the role of PPAR-γ in normal pregnancy and pregnancy complications, and finally its potential as a therapeutic target in the treatment and/or prevention of adverse pregnancy outcomes.

Mediator complex dependent regulation of cardiac development and disease

Cardiovascular disease (CVD) is a leading cause of morbidity and mortality. The risk factors for CVD include environmental and genetic components. Human mutations in genes involved in most aspects of cardiovascular function have been identified, many of which are involved in transcriptional regulation. The Mediator complex serves as a pivotal transcriptional regulator that functions to integrate diverse cellular signals by multiple mechanisms including recruiting RNA polymerase II, chromatin modifying proteins and non-coding RNAs to promoters in a context dependent manner. This review discusses components of the Mediator complex and the contribution of the Mediator complex to normal and pathological cardiac development and function. Enhanced understanding of the role of this core transcriptional regulatory complex in the heart will help us gain further insights into CVD.

Modulation of the transcriptional activity of peroxisome proliferator-activated receptor gamma by protein-protein interactions and post-translational modifications

Peroxisome proliferator-activated receptor gamma (PPARγ) belongs to a nuclear receptor superfamily; members of which play key roles in the control of body metabolism principally by acting on adipose tissue. Ligands of PPARγ, such as thiazolidinediones, are widely used in the treatment of metabolic syndromes and type 2 diabetes mellitus (T2DM). Although these drugs have potential benefits in the treatment of T2DM, they also cause unwanted side effects. Thus, understanding the molecular mechanisms governing the transcriptional activity of PPARγ is of prime importance in the development of new selective drugs or drugs with fewer side effects. Recent advancements in molecular biology have made it possible to obtain a deeper understanding of the role of PPARγ in body homeostasis. The transcriptional activity of PPARγ is subject to regulation either by interacting proteins or by modification of the protein itself. New interacting partners of PPARγ with new functions are being unveiled. In addition, post-translational modification by various cellular signals contributes to fine-tuning of the transcriptional activities of PPARγ. In this review, we will summarize recent advancements in our understanding of the post-translational modifications of, and proteins interacting with, PPARγ, both of which affect its transcriptional activities in relation to adipogenesis.

Early embryonic lethality of mice with disrupted transcription cofactor PIMT/NCOA6IP/Tgs1 gene

PIMT (also known as PIPMT/NCOA6IP/Tgs1), first isolated as a transcription coactivator PRIP (NCOA6)-interacting 96-kDa protein with RNA-binding property, possesses RNA methyltransferase activity. As a transcription coactivator binding protein, PIMT enhances the nuclear receptor transcriptional activity and its methyltransferase property is involved in the formation of the 2,2,7-trimethylguanosine cap of non-coding small RNAs, but the in vivo functions of this gene have not been fully explored. To elucidate the biological functions, we used gene targeting to generate mice with a disrupted PIMT/Tgs1 gene. Disruption of PIMT gene results in early embryonic lethality due to impairment of development around the blastocyst and uterine implantation stages. We show that PIMT is expressed in all cells of the E3.5day blastocyst in the mouse. PIMT null mutation abolished PIMT expression in all cells of the blastocyst and caused a reduction in the expression of Oct4 and Nanog transcription factor proteins in the E3.5 blastocyst resulting in the near failure to form inner cell mass (ICM). With conditional deletion of PIMT gene, mouse embryonic fibroblasts (MEFs) exhibit defective wound healing in the scratch assay and a reduction in cell proliferation due to decreased G₀/G₁ transition and G₂/M phase cell cycle arrest. We conclude that PIMT/NCOA6IP, which is expressed in all cells of the 3.5 day stage blastocyst, is indispensable for early embryonic development.

PPAR Signaling in Placental Development and Function

With the major attention to the pivotal roles of PPARs in diverse aspects of energy metabolism, the essential functions of PPARγ and PPARβ/δ in placental development came as a surprise and were often considered a nuisance en route to their genetic analysis. However, these findings provided an opportune entrée into placental biology. Genetic and pharmacological studies, primarily of knockout animal models and cell culture, uncovered networks of PPARγ and PPARδ, their heterodimeric RXR partners, associated transcriptional coactivators, and target genes, that regulate various aspects of placental development and function. These studies furnish both specific information about trophoblasts and the placenta and potential hints about the functions of PPARs in other tissues and cell types. They reveal that the remarkable versatility of PPARs extends beyond the orchestration of metabolism to the regulation of cellular differentiation, tissue development, and trophoblast-specific functions. This information and its implications are the subject of this review.

Nuclear Receptor Cofactors in PPARgamma-Mediated Adipogenesis and Adipocyte Energy Metabolism

Transcriptional cofactors are integral to the proper function and regulation of nuclear receptors. Members of the peroxisome proliferator-activated receptor (PPAR) family of nuclear receptors are involved in the regulation of lipid and carbohydrate metabolism. They modulate gene transcription in response to a wide variety of ligands, a process that is mediated by transcriptional coactivators and corepressors. The mechanisms by which these cofactors mediate transcriptional regulation of nuclear receptor function are still being elucidated. The rapidly increasing array of cofactors has brought into focus the need for a clear understanding of how these cofactors interact in ligand- and cell-specific manners. This review highlights the differential effects of the assorted cofactors regulating the transcriptional action of PPARγ and summarizes the recent advances in understanding the physiological functions of corepressors and coactivators.

PPAR Action in Human Placental Development and Pregnancy and Its Complications

During pregnancy crucial anatomic, physiologic, and metabolic changes challenge the mother and the fetus. The placenta is a remarkable organ that allows the mother and the fetus to adapt to the new metabolic, immunologic, and angiogenic environment imposed by gestation. One of the physiologic systems that appears to have evolved to sustain this metabolic regulation is mediated by peroxisome proliferator-activated receptors (PPARs). In clinical pregnancy-specific disorders, including preeclampsia, gestational diabetes, and intrauterine growth restriction, aberrant regulation of components of the PPAR system parallels dysregulation of metabolism, inflammation and angiogenesis. This review summarizes current knowledge on the role of PPARs in regulating human trophoblast invasion, early placental development, and also in the physiology of clinical pregnancy and its complications. As increasingly indicated in the literature, pregnancy disorders, such as preeclampsia and gestational diabetes, represent potential targets for treatment with PPAR ligands. With the advent of more specific PPAR agonists that exhibit efficacy in ameliorating metabolic, inflammatory, and angiogenic disturbances, further studies of their application in pregnancy-related diseases are warranted.

Placental implications of peroxisome proliferator-activated receptors in gestation and parturition

The placenta is a transitory structure indispensable for the proper development of the embryo and fetus during mammalian gestation. Like other members of the nuclear receptor family, the peroxisome proliferator-activated receptors (PPARs) are known to be involved in the physiological and pathological events occurring during the placentation. This placental involvement has been recently reviewed focusing on the early stages of placental development (implantation and invasion, etc.), mouse PPARs knockout phenotypes, and cytotrophoblast physiology. In this review, we describe the placental involvement of PPARs (e.g., fat transport and metabolism, etc.) during the late stages of gestation and in the amniotic membranes, highlighting their roles in the inflammation process (e.g., chorioamnionitis), metabolic disorders (e.g., diabetes), and parturition.

Identification and characterization of the alternatively spliced nuclear receptor coactivator-6 isoforms

The nuclear receptor coactivator-6 (NCOA6, AIB3, PRIP, ASC-2, TRBP, RAP250 or NRC) is a co-activator for nuclear hormone receptors and certain other transcription factors. NCOA6 plays an important role in embryonic development, adipocyte differentiation, metabolism and breast carcinogenesis. The human and mouse NCOA6 genes had 15 and 14 previously identified exons, respectively. This study further identified an alternatively spliced exon 11b (E11b) in human or E10b in mouse, which codes a short polypeptide and a Stop codon, resulting in splicing variants lacking the last four exon-coded polypeptide. Analyses of mouse testis NCOA6 mRNAs identified four alternatively spliced variants, NCOA6-α (without E10b), -β (without E10a and E10b), -γ (with E10a and E10b) and -δ (without E10a but with E10b). These isoforms were detected in multiple mouse tissues and in MDA-MB-435 human cells. NCOA6-α and -β are mainly located in the nucleus; NCOA6-γ is located in both cytoplasm and nucleus; and NCOA6-δ is mainly located in mitochondria. The C-terminus coded by the last four exons was responsible for locating NCOA6-α and -β into the nucleus. The human E11a or mouse E10a-coded region is responsible for distributing NCOA6-γ in both cytoplasm and nucleus, while the region coded by E8-E9 in human or E7-E8 in mouse is responsible for directing NCOA6-δ to mitochondria. Our assays also demonstrated that NCOA6-α and -β could significantly enhance estrogen receptor α-mediated transcription, but NCOA6-γ and -δ were unable to do so. These results suggest that the diverse physiological function of NCOA6 may be mediated by multiple isoforms expressed in different tissues and localized in different subcellular compartments.

PRIC295, a Nuclear Receptor Coactivator, Identified from PPARα-Interacting Cofactor Complex

The peroxisome proliferator-activated receptor-α (PPARα) plays a key role in lipid metabolism and energy combustion. Chronic activation of PPARα in rodents leads to the development of hepatocellular carcinomas. The ability of PPARα to induce expression of its target genes depends on Mediator, an evolutionarily conserved complex of cofactors and, in particular, the subunit 1 (Med1) of this complex. Here, we report the identification and characterization of PPARα-interacting cofactor (PRIC)-295 (PRIC295), a novel coactivator protein, and show that it interacts with the Med1 and Med24 subunits of the Mediator complex. PRIC295 contains 10 LXXLL signature motifs that facilitate nuclear receptor binding and interacts with PPARα and five other members of the nuclear receptor superfamily in a ligand-dependent manner. PRIC295 enhances the transactivation function of PPARα, PPARγ, and ERα. These data demonstrate that PRIC295 interacts with nuclear receptors such as PPARα and functions as a transcription coactivator under in vitro conditions and may play an important role in mediating the effects in vivo as a member of the PRIC complex with Med1 and Med24.

Coactivators in PPAR-Regulated Gene Expression

Peroxisome proliferator-activated receptor (PPAR)alpha, beta (also known as delta), and gamma function as sensors for fatty acids and fatty acid derivatives and control important metabolic pathways involved in the maintenance of energy balance. PPARs also regulate other diverse biological processes such as development, differentiation, inflammation, and neoplasia. In the nucleus, PPARs exist as heterodimers with retinoid X receptor-alpha bound to DNA with corepressor molecules. Upon ligand activation, PPARs undergo conformational changes that facilitate the dissociation of corepressor molecules and invoke a spatiotemporally orchestrated recruitment of transcription cofactors including coactivators and coactivator-associated proteins. While a given nuclear receptor regulates the expression of a prescribed set of target genes, coactivators are likely to influence the functioning of many regulators and thus affect the transcription of many genes. Evidence suggests that some of the coactivators such as PPAR-binding protein (PBP/PPARBP), thyroid hormone receptor-associated protein 220 (TRAP220), and mediator complex subunit 1 (MED1) may exert a broader influence on the functions of several nuclear receptors and their target genes. Investigations into the role of coactivators in the function of PPARs should strengthen our understanding of the complexities of metabolic diseases associated with energy metabolism.

Lens fiber cell differentiation and denucleation are disrupted through expression of the N-terminal nuclear receptor box of NCOA6 and result in p53-dependent and p53-independent apoptosis

Nuclear receptor coactivator 6 (NCOA6) is a multifunctional protein implicated in embryonic development, cell survival, and homeostasis. An 81-amino acid fragment, dnNCOA6, containing the N-terminal nuclear receptor box (LXXLL motif) of NCOA6, acts as a dominant-negative (dn) inhibitor of NCOA6. Here, we expressed dnNCOA6 in postmitotic transgenic mouse lens fiber cells. The transgenic lenses showed reduced growth; a wide spectrum of lens fiber cell differentiation defects, including reduced expression of gamma-crystallins; and cataract formation. Those lens fiber cells entered an alternate proapoptotic pathway, and the denucleation (karyolysis) process was stalled. Activation of caspase-3 at embryonic day (E)13.5 was followed by double-strand breaks (DSBs) formation monitored via a biomarker, gamma-H2AX. Intense terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) signals were found at E16.5. Thus, a window of approximately 72 h between these events suggested prolonged though incomplete apoptosis in the lens fiber cell compartment that preserved nuclei in its cells. Genetic experiments showed that the apoptotic-like processes in the transgenic lens were both p53-dependent and p53-independent. Lens-specific deletion of Ncoa6 also resulted in disrupted lens fiber cell differentiation. Our data demonstrate a cell-autonomous role of Ncoa6 in lens fiber cell differentiation and suggest novel insights into the process of lens fiber cell denucleation and apoptosis.

Transcription coactivator PBP/MED1-deficient hepatocytes are not susceptible to diethylnitrosamine-induced hepatocarcinogenesis in the mouse

Nuclear receptor coactivator [peroxisome proliferator-activated receptor-binding protein (PBP)/mediator subunit 1 (MED1)] is a critical component of the mediator transcription complex. Disruption of this gene in the mouse results in embryonic lethality. Using the PBP/MED1 liver conditional null (PBP/MED1^ΔLiv) mice, we reported that PBP/MED1 is essential for liver regeneration and the peroxisome proliferator-activated receptor α ligand Wy-14,643-induced receptor-mediated hepatocarcinogenesis. We now examined the role of PBP/MED1 in genotoxic chemical carcinogen diethylnitrosamine (DEN)-induced and phenobarbital-promoted hepatocarcinogenesis. The carcinogenic process was initiated by a single intraperitoneal injection of DEN at 14 days of age and initiated cells were promoted with phenobarbital (PB) (0.05%) in drinking water. PBP/MED1^ΔLiv mice, killed at 1, 4 and 12 weeks, revealed a striking proliferative response of few residual PBP/MED1-positive hepatocytes that escaped Cre-mediated deletion of PBP/MED1 gene. No proliferative expansion of PBP/MED1 null hepatocytes was noted in the PBP/MED1^ΔLiv mouse livers. Multiple hepatocellular carcinomas (HCCs) developed in the DEN-initiated PBP/MED1^fl/fl and PBP/MED1^ΔLiv mice, 1 year after the PB promotion. Of interest is that all HCC developing in PBP/MED1^ΔLiv mice were PBP/MED1 positive. None of the tumors was PBP/MED1 negative implying that hepatocytes deficient in PBP/MED1 are not susceptible to neoplastic conversion. HCC that developed in PBP/MED1^ΔLiv mouse livers were transplantable in athymic nude mice and these maintained PBP/MED1^fl/fl genotype. PBP/MED1^fl/fl HCC cell line derived from these tumors expressed PBP/MED1 and deletion of PBP/MED1^fl/fl allele by adeno-Cre injection into tumors caused necrosis of tumor cells. These results indicate that PBP/MED1 is essential for the development of HCC in the mouse.

Roles of histone H3-lysine 4 methyltransferase complexes in NR-mediated gene transcription

Transcriptional regulation by nuclear hormone receptors (NRs) requires multiple coregulators that modulate chromatin structures by catalyzing a diverse array of posttranslational modifications of histones. Different combinations of these modifications yield dynamic functional outcomes, constituting an epigenetic histone code. This code is inscribed by histone-modifying enzymes and decoded by effector proteins that recognize specific covalent marks. One important modification associated with active chromatin structures is methylation of histone H3-lysine 4 (H3K4). Crucial roles for this modification in NR transactivation have been recently highlighted through our purification and subsequent characterization of a steady-state complex associated with ASC-2, a coactivator of NRs and other transcription factors. This complex, designated ASCOM for ASC-2 complex, contains H3K4-methyltransferase MLL3/HALR or its paralogue MLL4/ALR and represents the first Set1-like H3K4-methyltransferase complex to be reported in vertebrates. This review focuses on recent progress in our understanding of how ASCOM-MLL3 and ASCOM-MLL4 influence NR-mediated gene transcription and of their physiological function.

A tumor suppressive coactivator complex of p53 containing ASC-2 and histone H3-lysine-4 methyltransferase MLL3 or its paralogue MLL4

ASC-2, a multifunctional coactivator, forms a steady-state complex, named ASCOM (for ASC-2 COMplex), that contains the histone H3-lysine-4 (H3K4)-methyltransferase MLL3 or its paralogue MLL4. Somewhat surprisingly, given prior indications of redundancy between MLL3 and MLL4, targeted inactivation of the MLL3 H3K4-methylation activity in mice is found to result in ureter epithelial tumors. Interestingly, this phenotype is exacerbated in a p53(+/-) background and the tumorigenic cells are heavily immunostained for gammaH2AX, indicating a contribution of MLL3 to the DNA damage response pathway through p53. Consistent with the in vivo observations, and the demonstration of a direct interaction between p53 and ASCOM, cell-based assays have revealed that ASCOM, through ASC-2 and MLL3/4, acts as a p53 coactivator and is required for H3K4-trimethyation and expression of endogenous p53-target genes in response to the DNA damaging agent doxorubicin. In support of redundant functions for MLL3 and MLL4 for some events, siRNA-mediated down-regulation of both MLL3 and MLL4 is required to suppress doxorubicin-inducible expression of several p53-target genes. Importantly, this study identifies a specific H3K4 methytransferase complex, ASCOM, as a physiologically relevant coactivator for p53 and implicates ASCOM in the p53 tumor suppression pathway in vivo.

PRIP promotes tumor formation through enhancing serum-responsive factor-mediated FOS expression

PRIP (peroxisome proliferator-activator receptor interacting protein) is a nuclear receptor coactivator required for mammary gland development. To understand the function of PRIP in breast tumorigenesis, we established a mammary tumor cell line with the PRIP(Loxp/Loxp) genotype. By knocking out the PRIP gene in the tumor cell line, we demonstrated that PRIP deficiency led to inhibited tumor formation without affecting tumor cell proliferation. The PRIP deficiency was associated with decreased cell invasion and migration capabilities. We found that PRIP deficiency substantially reduced FOS gene expression. A chromatin immunoprecipitation assay revealed that PRIP was recruited to the FOS promoter. In addition, we demonstrated that PRIP also directly up-regulated the FOS gene expression in human breast cancer cells. Promoter analysis showed that PRIP acted through serum-responsive factor to regulate FOS gene expression. Finally, by re-expressing the FOS gene, we confirmed that the inhibited tumor formation of PRIP-deficient tumor cells was due to reduced expression of the FOS gene.

Targeted inactivation of MLL3 histone H3-Lys-4 methyltransferase activity in the mouse reveals vital roles for MLL3 in adipogenesis

Activating signal cointegrator-2 (ASC-2), a transcriptional coactivator of multiple transcription factors that include the adipogenic factors peroxisome proliferator-activated receptor gamma (PPARgamma) and C/EBPalpha, is associated with histone H3-Lys-4-methyltransferase (H3K4MT) MLL3 or its paralogue MLL4 in a complex named ASCOM (ASC-2 complex). Indeed, ASC-2-null mouse embryonic fibroblasts (MEFs) have been demonstrated to be refractory to PPARgamma-stimulated adipogenesis and fail to express the PPARgamma-responsive adipogenic marker gene aP2. However, the specific roles for MLL3 and MLL4 in adipogenesis remain undefined. Here, we provide evidence that MLL3 plays crucial roles in adipogenesis. First, MLL3(Delta/Delta) mice expressing a H3K4MT-inactivated mutant of MLL3 have significantly less white fat. Second, MLL3(Delta/Delta) MEFs are mildly but consistently less responsive to inducers of adipogenesis than WT MEFs. Third, ASC-2, MLL3, and MLL4 are recruited to the PPARgamma-activated aP2 gene during adipogenesis, and PPARgamma is shown to interact directly with the purified ASCOM. Moreover, although H3K4 methylation of aP2 is readily induced in WT MEFs, it is not induced in ASC-2(-/-) MEFs and only partially induced in MLL3(Delta/Delta) MEFs. These results suggest that ASCOM-MLL3 and ASCOM-MLL4 likely function as crucial but redundant H3K4MT complexes for PPARgamma-dependent adipogenesis.

PPAR-gamma signaling pathway in placental development and function: a potential therapeutic target in the treatment of gestational diseases

BACKGROUND

PPAR-gamma is a target for the treatment of metabolic disorders, as Pioglitazone and Rosiglitazone are already used against type 2 diabetes. Pleiotropic functions, such as antiproliferative and anti-inflammatory effects against several pathological states, including cardiovascular disease and cancer, are currently being explored in clinical studies.

OBJECTIVE

Evidence indicates that PPAR-gamma is expressed in the placenta, playing a crucial role in placental development and function, while PPAR-gamma ligands appear to modulate fetal membrane signals. Thus, in the last few years, the pivotal role of PPAR-gamma in placental biology has been the focus of extensive research, as diabetes appears to be the most common metabolic dysfunction in pregnancy.

METHODS

We aim to present data concerning the expression of PPAR-gamma in animal and human placenta, underlining its significance in normal placental development and several gestational diseases. The effects of PPAR-gamma ligands as modulators of placental biology in normal and certain pathological conditions are also discussed.

RESULTS/CONCLUSION

Current research provides substantial evidence that PPAR-gamma plays a pivotal role in placental biology and may reveal new perspectives in the treatment of gestational diseases.

Nuclear receptor coactivator/coregulator NCoA6(NRC) is a pleiotropic coregulator involved in transcription, cell survival, growth and development

NCoA6 (also referred to as NRC, ASC-2, TRBP, PRIP and RAP250) was originally isolated as a ligand-dependent nuclear receptor interacting protein. However, NCoA6 is a multifunctional coregulator or coactivator necessary for transcriptional activation of a wide spectrum of target genes. The NCoA6 gene is amplified and overexpressed in breast, colon and lung cancers. NCoA6 is a 250 kDa protein which harbors a potent N-terminal activation domain, AD1; and a second, centrally-located activation domain, AD2, which is necessary for nuclear receptor signaling. The intrinsic activation potential of NCoA6 is regulated by its C-terminal STL regulatory domain. Near AD2 is an LxxLL-1 motif which interacts with a wide spectrum of ligand-bound NRs with high-affinity. A second LxxLL motif (LxxLL-2) located towards the C-terminal region is more restricted in its NR specificity. The potential role of NCoA6 as a co-integrator is suggested by its ability to enhance transcriptional activation of a wide variety of transcription factors and from its in vivo association with a number of known cofactors including CBP/p300. NCoA6 has been shown to associate with at least three distinct coactivator complexes containing Set methyltransferases as core polypeptides. The composition of these complexes suggests that NCoA6 may play a fundamental role in transcriptional activation by modulating chromatin structure through histone methylation. Knockout studies in mice suggest that NCoA6 is an essential coactivator. NCoA6-/- embryos die between 8.5-12.5 dpc from general growth retardation coupled with developmental defects in the heart, liver, brain and placenta. NCoA6-/- MEFs grow at a reduced rate compared to WT MEFs and spontaneously undergo apoptosis, indicating the importance of NCoA6 as a prosurvival and anti-apoptotic gene. Studies with NCoA6+/- and conditional knockout mice suggest that NCoA6 is a pleiotropic coregulator involved in growth, development, wound healing and maintenance of energy homeostasis.

Tissue- and nuclear receptor-specific function of the C-terminal LXXLL motif of coactivator NCoA6/AIB3 in mice

Although the LXXLL motif of nuclear receptor (NR) coactivators is essential for interaction with NRs, its role has not been assessed in unbiased animal models. The nuclear receptor coactivator 6 (NCoA6; also AIB3, PRIP, ASC-2, TRBP, RAP250, or NRC) is a coactivator containing an N-terminal LXXLL-1 (L1) and a C-terminal L2. L1 interacts with many NRs, while L2 interacts with the liver X receptor alpha (LXRalpha) and the estrogen receptor alpha (ERalpha). We generated mice in which L2 was mutated into AXXAL (L2m) to disrupt its interaction with LXRalpha and ERalpha. NCoA6(L2m/L2m) mice exhibited normal reproduction, mammary gland morphogenesis, and ERalpha target gene expression. In contrast, when treated with an LXRalpha agonist, lipogenesis and the LXRalpha target gene expression were significantly reduced in NCoA6(L2m/L2m) mice. The induction of Cyp7A1 expression by a high-cholesterol diet was impaired in NCoA6(L2m/L2m) mice, which reduced bile acid synthesis in the liver and excretion in the feces and resulted in cholesterol accumulation in the liver and blood. These results demonstrate that L2 plays a tissue- and NR-specific role: it is required for NCoA6 to mediate LXRalpha-regulated lipogenesis and cholesterol/bile acid homeostasis in the liver but not required for ERalpha function in the mammary gland.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

PPARs and the Placenta

The discovery of the peroxisome proliferator-activated receptors (PPARs) in 1990s provided new insights in understanding the mechanisms involved in the control of energy homeostasis and in cell differentiation, proliferation, apoptosis and the inflammatory process. The PPARs became thus an exciting therapeutic target for diabetes, metabolic syndrome, atherosclerosis, and cancer. Unexpectedly, genetic studies performed in mice established that PPARgamma are essential for placental development. After a brief description of structural and functional features of PPARs, we will summarize in this review the most recent results concerning expression and the role of PPARs in placenta and of PPARgamma in human trophoblastic cells in particular.

Peroxisome proliferator-activated receptor gamma is highly expressed in pancreatic cancer and is associated with shorter overall survival times

PURPOSE

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor that has been implicated in carcinogenesis and progression of various solid tumors, including pancreatic carcinoma. We aimed to clarify the expression patterns of PPARgamma in pancreatic ductal carcinomas and to correlate these to clinicopathologic variables, including patient survival.

EXPERIMENTAL DESIGN

Array-based expression profiling of 19 microdissected carcinomas and 14 normal ductal epithelia was conducted. Additionally, Western blots of pancreatic cancer cell lines and paraffinized tissue of 129 pancreatic carcinomas were immunostained for PPARgamma. For statistical analysis, Fisher's exact test, chi2 test for trends, correlation analysis, Kaplan-Meier analysis, and Cox's regression were applied.

RESULTS

Expression profiles showed a strong overexpression of PPARgamma mRNA (change fold, 6.9; P=0.04). Immunohistochemically, PPARgamma expression was seen in 71.3% of pancreatic cancer cases. PPARgamma expression correlated positively to higher pT stages and higher tumor grade. Survival analysis showed a significant prognostic value for PPARgamma, which was found to be independent in the clinically important subgroup of node-negative tumors.

CONCLUSIONS

PPARgamma is commonly up-regulated in pancreatic ductal adenocarcinoma and might be a prognostic marker in this disease. Both findings corroborate the importance of PPARgamma in tumor progression of pancreatic cancer.

Transcription coactivator PRIP, the peroxisome proliferator-activated receptor (PPAR)-interacting protein, is redundant for the function of nuclear receptors PParalpha and CAR, the constitutive androstane receptor, in mouse liver

Disruption of the genes encoding for the transcription coactivators, peroxisome proliferator-activated receptor (PPAR)-interacting protein (PRIP/ASC-2/RAP250/TRBP/NRC) and PPAR-binding protein (PBP/TRAP220/DRIP205/MED1), results in embryonic lethality by affecting placental and multiorgan development. Targeted deletion of coactivator PBP gene in liver parenchymal cells (PBP(LIV-/-)) results in the near abrogation of the induction of PPARalpha and CAR (constitutive androstane receptor)-regulated genes in liver. Here, we show that targeted deletion of coactivator PRIP gene in liver (PRIP(LIV-/-)) does not affect the induction of PPARalpha-regulated pleiotropic responses, including hepatomegaly, hepatic peroxisome proliferation, and induction of mRNAs of genes involved in fatty acid oxidation system, indicating that PRIP is not essential for PPARalpha-mediated transcriptional activity. We also provide additional data to show that liver-specific deletion of PRIP gene does not interfere with the induction of genes regulated by nuclear receptor CAR. Furthermore, disruption of PRIP gene in liver did not alter zoxazolamine-induced paralysis, and acetaminophen-induced hepatotoxicity. Studies with adenovirally driven EGFP-CAR expression in liver demonstrated that, unlike PBP, the absence of PRIP does not prevent phenobarbital-mediated nuclear translocation/retention of the receptor CAR in liver in vivo and cultured hepatocytes in vitro. These results show that PRIP deficiency in liver does not interfere with the function of nuclear receptors PPARalpha and CAR. The dependence of PPARalpha- and CAR-regulated gene transcription on coactivator PBP but not on PRIP attests to the existence of coactivator selectivity in nuclear receptor function.

Regulation of insulin secretion and beta-cell mass by activating signal cointegrator 2

Activating signal cointegrator 2 (ASC-2) is a transcriptional coactivator of many nuclear receptors (NRs) and other transcription factors and contains two NR-interacting LXXLL motifs (NR boxes). In the pancreas, ASC-2 is expressed only in the endocrine cells of the islets of Langerhans, but not in the exocrine cells. Thus, we examined the potential role of ASC-2 in insulin secretion from pancreatic beta-cells. Overexpressed ASC-2 increased glucose-elicited insulin secretion, whereas insulin secretion was decreased in islets from ASC-2+/- mice. DN1 and DN2 are two dominant-negative fragments of ASC-2 that contain NR boxes 1 and 2, respectively, and block the interactions of cognate NRs with the endogenous ASC-2. Primary rat islets ectopically expressing DN1 or DN2 exhibited decreased insulin secretion. Furthermore, relative to the wild type, ASC-2+/- mice showed reduced islet mass and number, which correlated with increased apoptosis and decreased proliferation of ASC-2+/- islets. These results suggest that ASC-2 regulates insulin secretion and beta-cell survival and that the regulatory role of ASC-2 in insulin secretion appears to involve, at least in part, its interaction with NRs via its two NR boxes.

Epigenetic regulation of metabolism in children born small for gestational age

PURPOSE OF REVIEW

Epigenetic alterations are responsible for modulation of tissue-specific gene expression and genomic imprinting. Mechanisms include posttranslational modifications of core histones and DNA methylation. The review focuses on emerging data highlighting the potential for epigenetic modulation of gene expression in mediating early-life programming of increased risk of adult-onset disease. To illustrate these concepts, we focus on epigenetic programming of insulin resistance, obesity and type 2 diabetes, with emphasis on the potential role of the adipocyte and three of its products, fatty acids, leptin and tumour necrosis factor alpha.

RECENT FINDINGS

Recent studies have highlighted potential mechanisms underlying epigenetic modification of tissue function that may predispose to later development of insulin resistance. These include altered regulation of adipocyte clonal expansion and terminal differentiation via epigenetic modification of peroxisome proliferator-activated receptor gamma, Foxo1 or cyclin D1 expression and signalling that, via altered adipocyte lipid sequestration, alters lipid delivery to nonadipose tissue and, therefore, insulin action or modification of adipokine or cytokine expression and signalling. Direct epigenetic modification of insulin action in muscle may be achieved through altered stearoyl-CoA desaturase 1 expression, which correlates with low fatty acid oxidation.

SUMMARY

While poor early growth and an increased risk of type 2 diabetes in adulthood are undisputedly linked, the relative impact of environment or genotype remains unclear. Altered DNA methylation patterns could, potentially, serve as biomarkers for assessment of prognosis and could help in the development of prophylactic strategies.

Peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP) but not PPAR-interacting protein (PRIP) is required for nuclear translocation of constitutive androstane receptor in mouse liver

The constitutive androstane receptor (CAR) regulates transcription of phenobarbital-inducible genes that encode xenobiotic-metabolizing enzymes in liver. CAR is localized to the hepatocyte cytoplasm but to be functional, it translocates into the nucleus in the presence of phenobarbital-like CAR ligands. We now demonstrate that adenovirally driven EGFP-CAR, as expected, translocates into the nucleus of normal wild-type hepatocytes following phenobarbital treatment under both in vivo and in vitro conditions. Using this approach we investigated the role of transcription coactivators PBP and PRIP in the translocation of EGFP-CAR into the nucleus of PBP and PRIP liver conditional null mouse hepatocytes. We show that coactivator PBP is essential for nuclear translocation of CAR but not PRIP. Adenoviral expression of both PBP and EGFP-CAR restored phenobarbital-mediated nuclear translocation of exogenously expressed CAR in PBP null livers in vivo and in PBP null primary hepatocytes in vitro. CAR translocation into the nucleus of PRIP null livers resulted in the induction of CAR target genes such as CYP2B10, necessary for the conversion of acetaminophen to its hepatotoxic intermediate metabolite, N-acetyl-p-benzoquinone imine. As a consequence, PRIP-deficiency in liver did not protect from acetaminophen-induced hepatic necrosis, unlike that exerted by PBP deficiency. These results establish that transcription coactivator PBP plays a pivotal role in nuclear localization of CAR, that it is likely that PBP either enhances nuclear import or nuclear retention of CAR in hepatocytes, and that PRIP is redundant for CAR function.

The pleiotropic function of PPAR gamma in the placenta

At different stages of placental development the cytotrophoblasts differentiate into specialized cells that are vital for specific placental tasks. These types include the invasive trophoblasts, which are responsible for invasion of the placenta into the uterine wall, and syncytiotrophoblasts, which form a barrier between the maternal and fetal circulations, govern trans-placental transport of gas, nutrient and waste, and produce placental hormones. Recent research illuminated the role of the nuclear receptor peroxisome proliferator-activated receptor-gamma (PPAR gamma) in the areas of adipocyte and macrophage biology, insulin action, bioenergetics and inflammation. It was somewhat surprising that PPAR gamma was also found to play a pivotal role in placental biology. In this review we summarize recent data, which show that PPAR gamma is expressed in the placenta, particularly in trophoblasts, and is essential for placental development, trophoblast invasion, differentiation of cytotrophoblasts into syncytium, and regulation of fat accumulation in trophoblasts. PPAR gamma may also play a role in modulating fetal membrane signals toward parturition. The data presented here underscore the need for a focused investigation of the unique aspects of PPAR gamma function in trophoblasts, which may have direct implications for the use of PPAR gamma ligands during pregnancy.

Steroid receptor coregulator diversity: What can it mean for the stressed brain?

Glucocorticoid hormones modulate brain function and as such are crucial for responding and adjusting to physical and psychological stressors. Their effects are mediated via mineralo- and glucocorticoid receptors, which in large measure act as transcription factors to modulate transcription of target genes, in a receptor-, cell-, and state-specific manner. The nature and magnitude of these transcriptional effects depend on the presence and activity of downstream proteins, such as steroid receptor coactivators and corepressors (together: coregulators), many of which are expressed in the brain. We address the role of coregulators for mineralo- and glucocorticoid receptor-mediated modulation of gene transcription. We first address evidence from cell lines for the importance of coregulator stoichiometry for steroid signaling. The in vivo importance of coregulators-when possible specifically for glucocorticoid signaling in the brain-is discussed based on knockout mice, transient knockdown of steroid receptor coactivators, and distribution and regulation of coactivator expression in the brain. We conclude that for a better understanding of modulation of brain function by glucocorticoids, it is necessary to take into account the role of coregulators, and to assess their importance relative to changes in hormone levels and receptor expression.

From molecular action to physiological outputs: peroxisome proliferator-activated receptors are nuclear receptors at the crossroads of key cellular functions

Peroxisome proliferator-activated receptors (PPARs) compose a family of three nuclear receptors which act as lipid sensors to modulate gene expression. As such, PPARs are implicated in major metabolic and inflammatory regulations with far-reaching medical consequences, as well as in important processes controlling cellular fate. Throughout this review, we focus on the cellular functions of these receptors. The molecular mechanisms through which PPARs regulate transcription are thoroughly addressed with particular emphasis on the latest results on corepressor and coactivator action. Their implication in cellular metabolism and in the control of the balance between cell proliferation, differentiation and survival is then reviewed. Finally, we discuss how the integration of various intra-cellular signaling pathways allows PPARs to participate to whole-body homeostasis by mediating regulatory crosstalks between organs.

Transcription coactivator peroxisome proliferator-activated receptor-binding protein/mediator 1 deficiency abrogates acetaminophen hepatotoxicity

Peroxisome proliferator-activated receptor-binding protein (PBP), also known as thyroid hormone receptor-associated protein 220/vitamin D receptor-interacting protein 205/mediator 1, an anchor for multisubunit mediator transcription complex, functions as a transcription coactivator for nuclear receptors. Disruption of the PBP gene results in embryonic lethality around embryonic day 11.5 by affecting placental and multiorgan development. Here, we report that targeted deletion of PBP in liver parenchymal cells (PBP(Liv-/-)) results in the abrogation of hypertrophic and hyperplastic influences in liver mediated by constitutive androstane receptor (CAR) ligands phenobarbital (PB) and 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene, and of acetaminophen-induced hepatotoxicity. CAR interacts with the two nuclear receptor-interacting LXXLL (L, leucine; X, any amino acid) motifs in PBP in a ligand-dependent manner. We also show that PBP interacts with the C-terminal portion of CAR, suggesting that PBP is involved in the regulation of CAR function. Although the full-length PBP only minimally increased CAR transcriptional activity, a truncated form of PBP (amino acids 487-735) functioned as a dominant negative repressor, establishing that PBP functions as a coactivator for CAR. A reduction in CAR mRNA and protein level observed in PBP(Liv-/-) mouse liver suggests that PBP may regulate hepatic CAR expression. PBP-deficient hepatocytes in liver failed to reveal PB-dependent translocation of CAR to the nucleus. Adenoviral reconstitution of PBP in PBP(Liv-/-) mouse livers restored PB-mediated nuclear translocation of CAR as well as inducibility of CYP1A2, CYP2B10, CYP3A11, and CYP7A1 expression. We conclude that transcription coactivator PBP/TRAP220/MED1 is involved in the regulation of hepatic CAR function and that PBP deficiency in liver abrogates acetaminophen hepatotoxicity.

Development of structures and transport functions in the mouse placenta

The placenta is essential for sustaining the growth of the fetus during gestation, and defects in its function result in fetal growth restriction or, if more severe, fetal death. Several molecular pathways have been identified that are essential for development of the placenta, and mouse mutants offer new insights into the cell biology of placental development and physiology of nutrient transport.

Nuclear hormone receptor coregulator: role in hormone action, metabolism, growth, and development

Nuclear hormone receptor coregulator (NRC) (also referred to as activating signal cointegrator-2, thyroid hormone receptor-binding protein, peroxisome proliferator activating receptor-interacting protein, and 250-kDa receptor associated protein) belongs to a growing class of nuclear cofactors widely known as coregulators or coactivators that are necessary for transcriptional activation of target genes. The NRC gene is also amplified and overexpressed in breast, colon, and lung cancers. NRC is a 2063-amino acid protein that harbors a potent N-terminal activation domain (AD1) and a second more centrally located activation domain (AD2) that is rich in Glu and Pro. Near AD2 is a receptor-interacting domain containing an LxxLL motif (LxxLL-1), which interacts with a wide variety of ligand-bound nuclear hormone receptors with high affinity. A second LxxLL motif (LxxLL-2) located in the C-terminal region of NRC is more restricted in its nuclear hormone receptor specificity. The intrinsic activation potential of NRC is regulated by a C-terminal serine, threonine, leucine-regulatory domain. The potential role of NRC as a cointegrator is suggested by its ability to enhance transcriptional activation of a wide variety of transcription factors and from its in vivo association with a number of known transcriptional regulators including CBP/p300. Recent studies in mice indicate that deletion of both NRC alleles leads to embryonic lethality resulting from general growth retardation coupled with developmental defects in the heart, liver, brain, and placenta. NRC(-/-) mouse embryo fibroblasts spontaneously undergo apoptosis, indicating the importance of NRC as a prosurvival and antiapoptotic gene. Studies with 129S6 NRC(+/-) mice indicate that NRC is a pleiotropic regulator that is involved in growth, development, reproduction, metabolism, and wound healing.

Characterization of activating signal cointegrator-2 as a novel transcriptional coactivator of the xenobiotic nuclear receptor constitutive androstane receptor

Activating signal cointegrator-2 (ASC-2) is a recently isolated transcriptional coactivator protein for a variety of different transcription factors, including many members of the nuclear receptor superfamily. In this report, we demonstrate that ASC-2 also serves as a coactivator of the xenobiotic nuclear receptor constitutive androstane receptor (CAR). First, transcriptional activation by CAR was enhanced by cotransfected ASC-2 in CV-1 and HeLa cells. In contrast, CAR transactivation was significantly impaired in HepG2 cells stably expressing specific small interfering RNA directed against ASC-2. Consistent with these results, chromatin immunoprecipitation experiments revealed that ASC-2 is recruited to the known CAR target genes in a ligand-dependent manner. Secondly, CAR specifically interacted with the first LXXLL motif of ASC-2, and these interactions were stimulated by CAR agonist 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene and repressed by CAR inverse agonist androstanol, suggesting that this motif may mediate the interactions of ASC-2 and CAR in vivo. In support of this idea, DN1, a fragment of ASC-2 encompassing the first LXXLL motif, suppressed CAR transactivation, and coexpressed ASC-2 but not other LXXLL-type coactivators such as thyroid hormone receptor-associated protein 220 reversed this repression. Finally, CAR was recently found to play a pivotal role in effecting the severe acetaminophen-induced liver damage. Interestingly, transgenic mice expressing DN1 were resistant to the acetaminophen-induced hepatotoxicity and expression of a series of the known CAR target genes was specifically repressed in these transgenic mice. Taken together, these results strongly suggest that ASC-2 is a bona fide coactivator of the xenobiotic nuclear receptor CAR and mediate the specific xenobiotic response by CAR in vivo.

Haploid inactivation of the amplified-in-breast cancer 3 coactivator reduces the inhibitory effect of peroxisome proliferator-activated receptor gamma and retinoid X receptor on cell proliferation and accelerates polyoma middle-T antigen-induced mammary tumorigenesis in mice

The amplified-in-breast cancer 3 (AIB3) is a nuclear receptor coactivator amplified and overexpressed in human breast cancers. AIB3(-/-) mice die during gestation, whereas AIB3(+/-) mice exhibit normal development. Here, we demonstrate that AIB3 protein is mainly located in the nuclei of mammary epithelial cells and tumor cells and its levels are elevated in mammary epithelial cells at middle pregnant stage and in mammary tumor cells. To examine whether AIB3 reduction affects mammary tumorigenesis, we generated wild-type mouse mammary tumor virus/polyoma middle-T (WT/PyMT) and AIB3(+/-)/PyMT mice. Mammary tumor development in AIB3(+/-)/PyMT female and male mice was substantially accelerated compared with that in WT/PyMT mice, because of increased cell proliferation in early tumorigenic lesions, including ductal hyperplasia and mammary intraepithelial neoplasia. Tumor formation in nude mice that received premalignant AIB3(+/-)/PyMT mammary tissue was much faster than in nude mice that received transplants of premalignant WT/PyMT mammary tissue, which indicated that the accelerated tumorigenesis in AIB3(+/-)/PyMT mammary glands is due to a mammary epithelial autonomous defect. Expression of PyMT, estrogen receptor alpha and estrogen receptor alpha-regulated genes was unaffected in AIB3(+/-)/PyMT mammary glands, which suggests that the acceleration of mammary tumor formation in AIB3(+/-)/PyMT mice was not a consequence of changes in PyMT expression or in estrogen receptor function. Importantly, the inhibitory effects of peroxisome proliferator-activated receptor gamma (PPARgamma) and retinoid-X receptor (RXR) ligands on AIB3(+/-)/PyMT cell proliferation and the transcriptional function of PPARgamma in AIB3(+/-)/PyMT cells were reduced. Thus, AIB3 haplodeficiency may facilitate PyMT-induced tumorigenesis through a partial impairment of PPARgamma and RXR function. These results suggest that AIB3 may be a tumor suppressor that is required for the inhibition of cell proliferation by PPARgamma and RXR.

Biochemical and morphological effects of K-111, a peroxisome proliferator-activated receptor (PPAR)alpha activator, in non-human primates

K-111 has been characterized as a potent peroxisome proliferator-activated receptor (PPAR)alpha activator. Antidiabetic potency and amelioration of disturbed lipid metabolism were demonstrated in rodents, which were accompanied by elevations of peroxisomal enzymes and liver weight. To examine the possible therapeutic application of K-111 we have now assessed its efficacy in non-human primates with high transferability to humans. For this purpose obese, hypertriglyceridaemic, hyperinsulinaemic prediabetic rhesus monkeys were dosed sequentially with 0, 1, 3 and 10mg/kg per day orally over a period of 4 weeks each. In addition, the effect of K-111 on the peroxisome compartment was analyzed in cynomolgus monkeys using liver samples obtained following a 13-week oral toxicity study. In prediabetic monkeys, the reduction of hyperinsulinaemia and improvement of insulin-stimulated glucose uptake rate indicated amelioration of insulin resistance. These effects were nearly maximal at a dose of 3mg/kg per day, while triglycerides and body weight were lowered significantly in a dose-dependent manner. This reduction of body weight contrasts sharply with the adipogenic response observed with thiazolidinediones, another family of insulin-sensitizing agents. In young cynomolgus monkeys at a dosage of 5mg/kg per day and more, K-111 induced an up to three-fold increase in lipid beta-oxidation enzymes with an 1.5- to 2-fold increase in peroxisome volume density. This moderate increase in peroxisomal activity by K-111 in monkeys is consistent with its role as an PPARalpha activator and corresponds to the observations with fibrates in other low responder mammalian species. The increase in beta-oxidation may explain, at least in part, the lipid modulating effect as well as the antidiabetic potency of K-111. This pharmacological profile makes K-111 a highly promising drug candidate for clinical applications in the treatment of type 2 diabetes, dyslipidaemia, obesity and the metabolic syndrome.