Deletion of the cancer-amplified coactivator AIB3 results in defective placentation and embryonic lethality

Nuclear receptor coactivator 6 is a critical regulator of NLRP3 inflammasome activation and gouty arthritis

Transcriptional coactivators regulate the rate of gene expression in the nucleus. Nuclear receptor coactivator 6 (NCOA6), a coactivator, has been implicated in embryonic development, metabolism, and cancer pathogenesis, but its role in innate immunity and inflammatory diseases remains unclear. Here, we demonstrated that NCOA6 was expressed in monocytes and macrophages and that its level was increased under proinflammatory conditions. Unexpectedly, nuclear NCOA6 was found to translocate to the cytoplasm in activated monocytes and then become incorporated into the inflammasome with NLRP3 and ASC, forming cytoplasmic specks. Mechanistically, NCOA6 associated with the ATP hydrolysis motifs in the NACHT domain of NLRP3, promoting the oligomerization of NLRP3 and ASC and thereby instigating the production of IL-1β and active caspase-1. Of note, Ncoa6 deficiency markedly inhibited NLRP3 hyperactivation caused by the Nlrp3R258W gain-of-function mutation in macrophages. Genetic ablation of Ncoa6 substantially attenuated the severity of two NLRP3-dependent diseases, folic-induced acute tubular necrosis and crystal-induced arthritis, in mice. Consistent with these findings, NCOA6 was highly expressed in macrophages derived from gout patients, and NCOA6-positive macrophages were significantly enriched in gout macrophages according to the transcriptome profiling results. Conclusively, NCOA6 is a critical regulator of NLRP3 inflammasome activation and is therefore a promising target for NLRP3-dependent diseases, including gout.

Nuclear receptor coactivator 6 (NCoA6) promotes cell proliferation, migration, and invasion in pancreatic cancer

BACKGROUND

Nuclear receptor coactivator 6 (NCoA6) is overexpressed in various cancers and considered a multifunctional coactivator of various transcription factors and nuclear receptors. However, the role of NCoA6 in pancreatic ductal adenocarcinoma (PDAC) remains unclear.

METHODS

NCoA6 expression data in PDAC were extracted from TCGA and GTEx databases, and their correlation with survival outcomes were analyzed using the Kaplan-Meier plotter database. NCoA6 protein expression in PDAC tissues was evaluated using immunohistochemistry. RNA-sequencing technology was used to sequence the transcriptome of NCoA6-silenced PANC-1 cells, followed by differential expression, GO/KEGG and GSEA analyses. The effects of NCoA6 on cell proliferation, migration, invasion, cell cycle, and apoptosis were determined in two representative cell lines (PANC-1 and SW1990). Western blotting, qPCR, and co-immunoprecipitation were performed to explore the mechanism of action of NCoA6 in PDAC cells.

RESULTS

NCoA6 expression was markedly increased in PDAC tissues, and high NCoA6 expression was associated with poor survival prognosis. However, there was no significant relationship between NCoA6 expression and metastasis in PDAC patients. Our RNA-sequencing data analysis found 1194 significant differentially expressed genes between the control and NCoA6-silenced PANC-1 cells. GO/KEGG analysis results mainly focused on cytokine production, cytokine activity, and cytokine-cytokine receptor interactions. GSEA results showed that the knockdown of NCoA6 affected the expression of histone deacetylase 1 (HDAC1) targeted genes. NCoA6 knockdown suppressed proliferation, migration, and invasion of PDAC cells. Finally, western blotting, qPCR, and co-immunoprecipitation results showed that NCoA6 interacted with HDAC1 and that NCoA6 expression was negatively correlated with F-box and WD repeat domain-containing 7 (FBW7) and caudal-related homeobox transcription factor 2 (CDX2) expression in pancreatic cancer.

CONCLUSIONS

NCoA6 has a profound effect on cell proliferation, migration, invasion, and prognosis of PDAC and is potentially related to the expression of HDAC1, FBW7, and CDX2. Our results may provide novel therapeutic strategies for PDAC patients.

The Landscape of AhR Regulators and Coregulators to Fine-Tune AhR Functions

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates numerous cellular responses. Originally investigated in toxicology because of its ability to bind environmental contaminants, AhR has attracted enormous attention in the field of immunology in the last 20 years. In addition, the discovery of endogenous and plant-derived ligands points to AhR also having a crucial role in normal cell physiology. Thus, AhR is emerging as a promiscuous receptor that can mediate either toxic or physiologic effects upon sensing multiple exogenous and endogenous molecules. Within this scenario, several factors appear to contribute to the outcome of gene transcriptional regulation by AhR, including the nature of the ligand as such and its further metabolism by AhR-induced enzymes, the local tissue microenvironment, and the presence of coregulators or specific transcription factors in the cell. Here, we review the current knowledge on the array of transcription factors and coregulators that, by interacting with AhR, tune its transcriptional activity in response to endogenous and exogenous ligands.

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.

COMPASS and SWI/SNF complexes in development and disease

The Trithorax group (TrxG) of proteins is a large family of epigenetic regulators that form multiprotein complexes to counteract repressive developmental gene expression programmes established by the Polycomb group of proteins and to promote and maintain an active state of gene expression. Recent studies are providing new insights into how two crucial families of the TrxG - the COMPASS family of histone H3 lysine 4 methyltransferases and the SWI/SNF family of chromatin remodelling complexes - regulate gene expression and developmental programmes, and how misregulation of their activities through genetic abnormalities leads to pathologies such as developmental disorders and malignancies.

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.

The transcriptional co-activator NCOA6 promotes estrogen-induced GREB1 transcription by recruiting ERα and enhancing enhancer-promoter interactions

Estrogen and its cognate receptor, ERα, regulate cell proliferation, differentiation, and carcinogenesis in the endometrium by controlling gene transcription. ERα requires co-activators to mediate transcription via mechanisms that are largely uncharacterized. Herein, using growth-regulating estrogen receptor binding 1 (GREB1) as an ERα target gene in Ishikawa cells, we demonstrate that nuclear receptor co-activator 6 (NCOA6) is essential for estradiol (E2)/ERα-activated GREB1 transcription. We found that NCOA6 associates with the GREB1 promoter and enhancer in an E2-independent manner and that NCOA6 knockout reduces chromatin looping, enhancer-promoter interactions, and basal GREB1 expression in the absence of E2. In the presence of E2, ERα bound the GREB1 enhancer and also associated with its promoter, and p300, myeloid/lymphoid or mixed-lineage leukemia protein 4 (MLL4), and RNA polymerase II were recruited to the GREB1 enhancer and promoter. Consequently, the levels of the histone modifications H3K4me1/3, H3K9ac, and H3K27ac were significantly increased; enhancer and promoter regions were transcribed; and GREB1 mRNA was robustly transcribed. NCOA6 knockout reduced ERα recruitment and abolished all of the aforementioned E2-induced events, making GREB1 completely insensitive to E2 induction. We also found that GREB1-deficient Ishikawa cells are much more resistant to chemotherapy and that human endometrial cancers with low GREB1 expression predict poor overall survival. These results indicate that NCOA6 has an essential role in ERα-mediated transcription by increasing enhancer-promoter interactions through chromatin looping and by recruiting RNA polymerase II and the histone-code modifiers p300 and MLL4. Moreover, GREB1 loss may predict chemoresistance of endometrial cancer.

Steroid Hormone Receptor Coregulators in Endocrine Cancers

Coregulators span a broad and extensive domain in modulating cellular transcriptional activity. Studies have established a dynamic role for such coregulators in various endocrine cancers. Steroid hormone receptors (SHRs) play a pivotal role in such endocrine cancers, and interact abundantly with transcriptional coregulators in altering gene expression. Several families of coregulators have implications in propagating the development, progression and invasion of breast, prostate, and other hormone-responsive cancers. This mini-review aims to discuss different classes of coregulators involved in endocrine cancers and highlight unique information regarding each family with relevance to mechanism, intervention, and novel directions being investigated. © 2016 IUBMB Life, 68(7):504-515, 2016.

Mechanisms of uterine estrogen signaling during early pregnancy in mice: an update

Adherence of an embryo to the uterus represents the most critical step of the reproductive process. Implantation is a synchronized event between the blastocyst and the uterine luminal epithelium, leading to structural and functional changes for further embryonic growth and development. The milieu comprising the complex process of implantation is mediated by estrogen through diverse but interdependent signaling pathways. Mouse models have demonstrated the relevance of the expression of estrogen-modulated paracrine factors to uterine receptivity and implantation window. More importantly, some factors seem to serve as molecular links between different estrogen pathways, promoting cell growth, acting as molecular chaperones, or amplifying estrogenic effects. Abnormal expression of these factors can lead to implantation failure and infertility. This review provides an overview of several well-characterized signaling pathways that elucidates the molecular cross talk involved in the uterus during early pregnancy.

The cancer COMPASS: navigating the functions of MLL complexes in cancer. Cancer Genet. 2015;208:178-191. [PMID: 25794446 DOI: 10.1016/j.cancergen.2015.01.005]

The mixed-lineage leukemia family of histone methyltransferases (MLL1-4, or KMT2A-D) were previously linked to cancer through the founding member, MLL1/KMT2A, which is often involved in translocation-associated gene fusion events in childhood leukemias. However, in recent years, a multitude of tumor exome sequencing studies have revealed that orthologues MLL3/KMT2C and MLL2/KMT2D are mutated in a significant percentage of a large variety of malignancies, particularly solid tumors. These unexpected findings necessitate a deeper inspection into the activities and functional differences between the MLL/KMT2 family members. This review provides an overview of this protein family and its relation to cancers, focusing on the recent links between MLL3/KMT2C and MLL2/4/KMT2D and their potential roles as tumor suppressors in an assortment of cell types.

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.

Nuclear receptor coactivator-6 attenuates uterine estrogen sensitivity to permit embryo implantation

Uterine receptivity to embryo implantation is coordinately regulated by 17β-estradiol (E(2)) and progesterone (P(4)). Although increased E(2) sensitivity causes infertility, the mechanisms underlying the modulation of E(2) sensitivity are unknown. We show that nuclear receptor coactivator-6 (NCOA6), a reported coactivator for estrogen receptor α (ERα), actually attenuates E(2) sensitivity to determine uterine receptivity to embryo implantation under normal physiological conditions. Specifically, conditional knockout of Ncoa6 in uterine epithelial and stromal cells does not decrease, but rather markedly increases E(2) sensitivity, which disrupts embryo implantation and inhibits P(4)-regulated genes and decidual response. NCOA6 enhances ERα ubiquitination and accelerates its degradation, while loss of NCOA6 causes ERα accumulation in stromal cells during the preimplantation period. During the same period, NCOA6 deficiency also caused a failure in downregulation of steroid receptor coactivator-3 (SRC-3), a potent ERα coactivator. Therefore, NCOA6 controls E(2) sensitivity and uterine receptivity by regulating multiple E(2)-signaling components.

Regulation of hepatic insulin sensitivity by activating signal co-integrator-2

ASC-2 (activating signal co-integrator-2, also known as AIB3 and NCoA6) is a transcriptional co-activator and regulates insulin secretion and β-cell survival. The present study was performed to elucidate the role of ASC-2 in the regulation of insulin sensitivity. Although islet cells from 10-week-old ASC-2+/- mice secreted less insulin than wild-type islets, there was no significant difference in glucose tolerance between ASC-2+/- and wild-type mice. However, ASC-2+/- mice did show increased insulin sensitivity compared with wild-type mice in insulin tolerance tests. Consistently, the levels of phosphorylated Akt were higher in ASC-2+/- hepatocytes than in wild-type hepatocytes after insulin treatment. Moreover, decreases in phosphoenol pyruvate carboxykinase mRNA in refed mice were more prominent in ASC-2+/- livers than in wild-type livers. Interestingly, the expression levels of SOCS1 (suppressor of cytokine signalling 1) and SOCS3, well-known insulin signalling inhibitors, were decreased in ASC-2+/- hepatocytes and increased in ASC-2-overexpressing hepatocytes. Furthermore, ASC-2 was recruited to the promoter region of SOCS1 and potentiated the transcription by SREBP-1c (sterol-regulatory-element-binding protein-1c). This transcription-activating function of ASC-2 was diminished by mutations of SREBP-1c-binding sites in the SOCS1 promoter. Taken together, these results suggest that ASC-2 negatively affects hepatic insulin sensitivity, at least in part, through induction of the insulin signalling inhibitors SOCS1 and SOCS3.

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.

Placental PPARγ regulates spatiotemporally diverse genes and a unique metabolic network

The nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) is essential for placental development. For insights into its functions in the placenta, we screened for PPARγ-regulated genes by integrating expression profiles of Pparg-null and Rxra-null placentas with those of WT and Pparg-null trophoblast stem cells differentiated in the presence or absence of a PPARγ agonist. Intersection of these paradigms identified high-probability PPARγ target genes. A few of these genes were previously reported as PPARγ targets in other tissues, but most are new in the context of either PPARγ or placental biology. Transcriptional profiling demonstrated a widespread role for the coactivator NCOA6/AIB3, but not MED1/PBP, in PPARγ-dependent placental gene expression. Spatial and temporal expression analyses revealed that PPARγ impacts genes in diverse trophoblast lineages and during different stages of differentiation. We further validated the Ldhb gene, which encodes the H isoform of lactate dehydrogenase, as a robust PPARγ target in trophoblasts, and propose a hypothetical model that integrates it with a network of PPARγ-regulated genes into a novel pathway of placental fuel metabolism. These findings offer insights not only into the placental functions of PPARγ, but also into unique, previously unsuspected biosynthetic functions of trophoblasts.

Variants at chromosome 20 (ASIP locus) and melanoma risk

Agouti signaling protein (ASIP) locus on chromosome 20q11 is implicated, as shown by genome-wide association studies, in phenotype variation and melanoma risk. We genotyped 837 melanoma cases and 1,154 controls for 21 single nucleotide polymorphisms (SNPs) informative for 495 polymorphisms at the locus. Our data showed an increased risk of melanoma (odds ratio [OR] 1.27, 95% confidence interval [95% CI] 1.03-1.57) in carriers of the rs4911414 variant, located 120 kb upstream of ASIP. The main effect of rs4911414, as reported previously, was in tandem with a 10 kb adjacent polymorphism rs1015362; two constituted risk-associated haplotype/diplotype. Except for rs1015363, none of the 12 tagging SNPs, genotyped to cover 239.9 kb region with polymorphisms linked to rs4911414 and rs1015362, were associated with melanoma. Our data confirmed a previous association of melanoma risk (OR 1.82, 95% CI 1.37-2.41) with rs4911442, located in intron 5 of the nuclear receptor coactivator 6 (NCOA6) gene. The rs910871, one of the six variants, genotyped to cover NCOA6, showed an association with melanoma risk (OR 1.33, 95% CI 1.04-1.70). Both, rs4911442 and rs910871 were in moderate linkage with a, previously reported, risk-associated rs910873 polymorphism. A haplotype from the variants within NCOA6 showed an association with risk of melanoma (OR 1.49, 95% CI 1.17-1.88). Interaction between risk-associated polymorphisms and previously genotyped melanocortin receptor 1 (MC1R) variants, in our study, was not statistically significant. Nevertheless, the carriers of the variant alleles over the background of MC1R variants were at a higher risk than the carriers not enriched for MC1R variants. Our data confirmed the association of different variants at chromosome 20q11 with melanoma risk.

Role of histone methylation and demethylation in adipogenesis and obesity

Adipocyte differentiation is a complex developmental process that involves the coordinated interplay of numerous transcription factors. PPARγ has emerged as a master regulator of adipogenesis and recent global target gene analysis demonstrated that PPARγ targets many genes encoding chromatin modification enzymes as well as genes of lipid metabolism and storage. Among such modification enzymes are histone lysine methyltransferases, which play important roles in transcriptional regulation. Histone methyltransferases are involved in PPARγ gene expression and subsequent adipogenesis. In addition, recent studies revealed that demethylation of histone H3 at lys9 is associated with resistance to obesity. We here review the role of histone methylation and demethylation in adipogenesis, metabolism and obesity.

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.

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.

Coactivators in PPAR-Regulated Gene Expression. PPAR Res. 2010;2010. [PMID: 20814439 PMCID: PMC2929611 DOI: 10.1155/2010/250126]

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.

The cooperative function of nuclear receptor coactivator 1 (NCOA1) and NCOA3 in placental development and embryo survival

Nuclear receptor coactivator 1 [NCOA1/steroid receptor coactivator (SRC)-1] and NCOA3 (SRC-3/AIB1/ACTR) constitute two thirds of the SRC (steroid receptor coactivator) family. Although in vitro experiments have suggested overlapping functions between NCOA1 and NCOA3, their in vivo functional relationship is poorly understood. In this study, NCOA1 and NCOA3 double knockout mice were generated to determine the compensatory roles of NCOA1 and NCOA3 in development. NCOA1(-/-) mice survived normally, whereas most NCOA3(-/-) embryos were viable at embryonic d 13.5 (E13.5). In contrast, the majority of double-knockout (DKO) embryos died by E13.5. NCOA1 and NCOA3 are expressed in the labyrinth, and labyrinths of NCOA1(+/-);NCOA3(-/-) and DKO placentas were small compared with wild-type and single-knockout labyrinths. DKO labyrinths exhibited low densities of maternal blood sinuses and fetal capillaries and displayed fetomaternal blood transfusion. At the interface between maternal and fetal circulations, layer I sinusoidal trophoblast giant cells showed a reduced density of microvilli. Layer III syncytiotrophoblasts appeared to accumulate large lipid droplets and have reduced density and deepened invaginations of the intrasyncytial bays. The endothelial layer in DKO labyrinth showed abnormal morphologies and had large lipid droplets. Furthermore, disruption of NCOA1 and NCOA3 increased labyrinth trophoblast proliferation and their progenitor gene expression but decreased their differentiation gene expression. NCOA1 and NCOA3 deficiencies also affected the expression of several genes for placental morphogenesis including TGFβ-, peroxisome proliferator-activated receptor-β-, and peroxisome proliferator-activated receptor-γ-regulated genes and for glucose transportation including GLUT1 and Cx26. These findings demonstrate that NCOA1 and NCOA3 cooperatively regulate placental morphogenesis and embryo survival.

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.

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.

Generation and validation of a mouse line with a floxed SRC-3/AIB1 allele for conditional knockout

The steroid receptor coactivator-3 (SRC-3), also known as AIB1, ACTR, p/CIP and NCOA3, is a transcriptional coactivator for nuclear receptors and certain other transcription factors. SRC-3 is widely expressed and plays important physiological functions and pathogenic roles in breast and prostate cancers. SRC-3 knockout (SRC-3(-/-)) mice display genetic background-dependent embryonic lethality and multiple local and systemic abnormalities. Since both the partial lethality and the systemic effects caused by global SRC-3 knockout interfere with downstream investigation of tissue-specific function of SRC-3, we have generated floxed SRC-3 (SRC-3(f/f)) mice with conditional alleles carrying loxP sites in introns 10 and 12 by a gene-targeting strategy. The two SRC-3(f/f) mouse lines (A and B) are indistinguishable from wild type mice. To test if deletion of the floxed exons 11 and 12 for SRC-3 nuclear receptor interaction domains and disruption of its downstream sequence for transcriptional activation domains would inactivate SRC-3 function, SRC-3(f/f) mice were crossbred with EIIa-Cre mice to generate SRC-3(d/d) mice with germ line deletion of the floxed SRC-3 gene. Both lines of SRC-3(d/d) mice exhibited growth retardation and low IGF-I levels, which was similar to that observed in SRC-3(-/-) mice. The line A SRC-3(d/d) mice showed normal viability, while line B SRC-3(d/d) mice showed partial lethality similar to SRC-3-/- mice, probably due to variable distributions of genetic background during breeding. These results demonstrate that the floxed SRC-3 mouse lines have been successfully established. These mice will be useful for investigating the cell type- and developmental stage-specific functions of SRC-3.

The AIB1 oncogene promotes breast cancer metastasis by activation of PEA3-mediated matrix metalloproteinase 2 (MMP2) and MMP9 expression. Mol Cell Biol. 2008;28:5937-5950. [PMID: 18644862 DOI: 10.1128/MCB.00579-08]

Amplified-in-breast cancer 1 (AIB1) is an overexpressed transcriptional coactivator in breast cancer. Although overproduced AIB1 is oncogenic, its role and underlying mechanisms in metastasis remain unclear. Here, mammary tumorigenesis and lung metastasis were investigated in wild-type (WT) and AIB1(-/-) mice harboring the mouse mammary tumor virus-polyomavirus middle T (PyMT) transgene. All WT/PyMT mice developed massive lung metastasis, but AIB1(-/-)/PyMT mice with comparable mammary tumors had significantly less lung metastasis. The recipient mice with transplanted AIB1(-/-)/PyMT tumors also had much less lung metastasis than the recipient mice with transplanted WT/PyMT tumors. WT/PyMT tumor cells expressed mesenchymal markers such as vimentin and N-cadherin, migrated and invaded rapidly, and formed disorganized cellular masses in three-dimensional cultures. In contrast, AIB1(-/-)/PyMT tumor cells maintained epithelial markers such as E-cadherin and ZO-1, migrated and invaded slowly, and still formed polarized acinar structures in three-dimensional cultures. Molecular analyses revealed that AIB1 served as a PEA3 coactivator and formed complexes with PEA3 on matrix metalloproteinase 2 (MMP2) and MMP9 promoters to enhance their expression in both mouse and human breast cancer cells. In 560 human breast tumors, AIB1 expression was found to be positively associated with PEA3, MMP2, and MMP9. These findings suggest a new alternative strategy for controlling the deleterious roles of these MMPs in breast cancer by inhibiting their upstream coregulator AIB1.

Steroid receptor coactivator 3 maintains circulating insulin-like growth factor I (IGF-I) by controlling IGF-binding protein 3 expression

Steroid receptor coactivator 3 (SRC-3/AIB1/ACTR/NCoA-3) is a transcriptional coactivator for nuclear receptors including vitamin D receptor (VDR). Growth hormone (GH) regulates insulin-like growth factor I (IGF-I) expression, and IGF-I forms complexes with acid-labile subunit (ALS) and IGF-binding protein 3 (IGFBP-3) to maintain its circulating concentration and endocrine function. This study demonstrated that the circulating IGF-I was significantly reduced in SRC-3(-/-) mice with the C57BL/6J background. However, SRC-3 deficiency affected neither GH nor ALS expression. The low IGF-I level in SRC-3(-/-) mice was not due to the failure of IGF-I mRNA and protein synthesis but was a consequence of rapid degradation. The rapid IGF-I degradation was associated with drastically reduced IGFBP-3 levels. Because IGF-I and IGFBP-3 stabilize each other, SRC-3(-/-) mice were crossbred with the liver-specific transthyretin (TTR)-IGF-I transgenic mice to assess the relationship between reduced IGF-I and IGFBP-3. In SRC-3(-/-)/TTR-IGF-I mice, the IGF-I level was significantly increased over that in SRC-3(-/-) mice, but the IGFBP-3 level failed to increase proportionally, indicating that the low IGFBP-3 level is a responsible factor that limits the IGF-I level in SRC-3(-/-) mice. Furthermore, IGFBP-3 mRNA was reduced in SRC-3(-/-) mice. The IGFBP-3 promoter activity induced by vitamin D, through VDR, was diminished in SRC-3(-/-) cells, suggesting an important role of SRC-3 in VDR-mediated transactivation of the IGFBP-3 gene. In agreement with the role of SRC-3 in VDR function, the expression of several VDR target genes was also reduced in SRC-3(-/-) mice. Therefore, SRC-3 maintains IGF-I in the circulation through enhancing VDR-regulated IGFBP-3 expression.

Activating signal cointegrator-2 is an essential adaptor to recruit histone H3 lysine 4 methyltransferases MLL3 and MLL4 to the liver X receptors

Activating signal cointegrator-2 (ASC-2), a coactivator of multiple nuclear receptors and transcription factors, including the liver X receptors (LXRs), is associated with histone H3 lysine 4 (H3K4) methyltransferase (H3K4MT) MLL3 or its paralogue MLL4 in a steady-state complex named ASCOM (ASC-2 complex). ASCOM belongs to Set1-like complexes, a conserved family of related H3K4MT complexes. ASC-2 binds to many nuclear receptors in a ligand-dependent manner through its two LXXLL motifs. In particular, the second motif has been shown to specifically recognize LXRs. However, the exact role for neither ASC-2 nor MLL3/4 in LXR transactivation is clearly defined. Here, we show that the key function of ASC-2 in transactivation by LXRs is to present MLL3 and MLL4 to LXRs. Thus, ASC-2 is required for ligand-induced recruitment of MLL3 and MLL4 to LXRs, and LXR ligand T1317 induces not only expression of LXR-target genes but also their H3K4-trimethylation. Strikingly, both of these ligand effects are ablated in ASC-2-null cells but only partially suppressed in cells expressing an enzymatically inactivated mutant MLL3. Our results also reveal that transactivation by LXRs does not appear to require other Set1-like complexes. Taken together, these results suggest that ASCOM-MLL3 and ASCOM-MLL4 play redundant but essential roles in ligand-dependent H3K4 trimethylation and expression of LXR-target genes, and that ASC-2 is likely a key determinant for LXRs to function through ASCOM but not other Set1-like complexes.

Preparation, culture, and immortalization of mouse embryonic fibroblasts

Genetically manipulated transgenic and gene-targeted mouse models are indispensible tools used in defining the role of genes in development and organ function. Similarly, cells isolated from these mice bear the same genetic alterations and therefore can be extremely useful for studying the molecular and cellular mechanisms and regulatory gene networks involving the mutated gene under well defined culture conditions. In this unit, detailed protocols regarding isolation of mouse embryonic fibroblasts (MEFs) from mouse embryos, culture of MEFs, and immortalization of MEFs are described. These protocols should be particularly useful to those investigators who intend to establish primarily cultured MEFs and to immortalize primary MEFs into stable cell cultures with a permanent growth feature from wild-type and mutant mouse embryos.

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.

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.

Differential tumor suppressor properties and transforming growth factor-beta responsiveness of p57KIP2 in leukemia cells with aberrant p57KIP2 promoter DNA methylation

To investigate if the tumor suppressor properties of p57KIP2 are dependent on its DNA methylation status, we studied the impact of several stress stimuli in leukemic cell lines with different p57KIP2 promoter DNA methylation levels. p57KIP2 reactivation was observed after stimulation with transforming growth factor-beta, other cytokines, high-density culture or serum withdrawal in p57KIP2 promoter unmethylated cells but not in methylated cells. In these cells, p57KIP2 reactivation required the use of a hypomethylating agent or a histone deacetylase inhibitor. Overexpression of p57KIP2 in p57KIP2 promoter methylated leukemic cell lines resulted in cell growth arrest and the induction of apoptosis. In contrast, overexpression of p57KIP2 in partially methylated cells only resulted in a moderate inhibition of cell growth and had no impact on apoptosis. Transduction of unmethylated cells expressing high levels of p57KIP2 with p57KIP2 short hairpin RNA resulted in increased cell proliferation. These results suggest that the tumor suppressive properties of p57KIP2 in leukemia may depend on the intrinsic promoter DNA methylation status of the gene.

Molecular mechanisms controlling the coupled development of myocardium and coronary vasculature

Cardiac failure affects 1.5% of the adult population and is predominantly caused by myocardial dysfunction secondary to coronary vascular insufficiency. Current therapeutic strategies improve prognosis only modestly, as the primary cause -- loss of normally functioning cardiac myocytes -- is not being corrected. Adult cardiac myocytes are unable to divide and regenerate to any significant extent following injury. New cardiac myocytes are, however, created during embryogenesis from progenitor cells and then by cell division from existing cardiac myocytes. This process is intimately linked to the development of coronary vasculature from progenitors originating in the endothelium, the proepicardial organ and neural crest. In this review, we systematically evaluate approx. 90 mouse mutations that impair heart muscle growth during development. These studies provide genetic evidence for interactions between myocytes, endothelium and cells derived from the proepicardial organ and the neural crest that co-ordinate myocardial and coronary vascular development. Conditional knockout and transgenic rescue experiments indicate that Vegfa, Bmpr1a (ALK3), Fgfr1/2, Mapk14 (p38), Hand1, Hand2, Gata4, Zfpm2 (FOG2), Srf and Txnrd2 in cardiac myocytes, Rxra and Wt1 in the proepicardial organ, EfnB2, Tek, Mapk7, Pten, Nf1 and Casp8 in the endothelium, and Bmpr1a and Pax3 in neural crest cells are key molecules controlling myocardial development. Coupling of myocardial and coronary development is mediated by BMP (bone morphogenetic protein), FGF (fibroblast growth factor) and VEGFA (vascular endothelial growth factor A) signalling, and also probably involves hypoxia. Pharmacological targeting of these molecules and pathways could, in principle, be used to recreate the embryonic state and achieve coupled myocardial and coronary vascular regeneration in failing hearts.

Gene amplification and associated loss of 5' regulatory sequences of CoAA in human cancers

CoAA is an RRM-containing transcriptional coactivator that stimulates transcriptional activation and regulates alternative splicing. We show that the CoAA gene is amplified at the chromosome 11q13 locus in a subset of primary human cancers including non-small cell lung carcinoma, squamous cell skin carcinoma and lymphoma. Analysis of 42 primary tumors suggests that CoAA amplifies independently from the CCND1 locus. Detailed mapping of three CoAA amplicons reveals that the amplified CoAA gene is consistently located at the 5' boundaries of the amplicons. The CoAA coding and basal promoter sequences are retained within the amplicons but upstream silencing sequences are lost. CoAA protein is overexpressed in tumors containing the amplified CoAA gene. RNA dot blot analysis of 100 cases of primary tumors suggests elevated CoAA mRNA expression. CoAA positively regulates its own basal promoter in transfection assays. Thus, gene amplification, loss of silencing sequence and positive feedback regulation may lead to drastic upregulation of CoAA protein. CoAA has transforming activities when tested in soft agar assays, and CoAA is homologous to oncoproteins EWS and TLS, which regulate alternative splicing. These data imply that CoAA may share a similar oncogenic mechanism with oncogene EWS and that CoAA deregulation may alter the alternative splicing of target genes.

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.

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.

Mice lacking the amplified in breast cancer 1/steroid receptor coactivator-3 are resistant to chemical carcinogen-induced mammary tumorigenesis

Amplified in breast cancer 1 (AIB1; steroid receptor coactivator-3, p/CIP, RAC3, ACTR, TRAM-1, or NCoA-3) is a transcriptional coactivator for nuclear receptors and certain other transcription factors and is a newly defined oncogene overexpressed in human breast cancer. Although the role and molecular mechanism of AIB1 in normal physiology and in breast cancer are currently under intensive investigation, the role of AIB1 in determination of the susceptibility of mammary gland to chemical carcinogens remains uncharacterized. In this study, we used back-crossed FVB wild-type (WT) and AIB1 mutant mice to assess the role of AIB1 in mammary gland development and in carcinogen-induced tumorigenesis. We show that mammary ductal growth was delayed in AIB1-/- mice with FVB strain background, and mammary ductal outgrowths emanating from the AIB1-/- mammary epithelial transplants in WT mice also were attenuated, indicating that the role of AIB1 in mammary ductal growth is a mammary epithelial autonomous function. In mice treated with the chemical carcinogen 7,12-dimethylbenz[a]anthracene (DMBA), AIB1 deficiency protected the mammary gland, but not the skin, from tumorigenesis. AIB1 deficiency suppressed the up-regulation of the insulin receptor substrate (IRS)-1 and IRS-2 and thereby inhibited the activation of Akt, expression of cyclin D1, and cell proliferation. The suppression of these components for insulin-like growth factor-I signaling might be partially responsible for the decreased DMBA-induced mammary tumor initiation and progression in AIB1-/- mice. Our results suggest that AIB1 may serve as a potential target for prevention of carcinogen-induced breast cancer initiation and for treatment of breast cancer progression.

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.

The Ligand-Dependent Interaction of Mineralocorticoid Receptor with Coactivator and Corepressor Peptides Suggests Multiple Activation Mechanisms

We investigated the coregulator (coactivator and corepressor) interactions with the mineralocorticoid receptor (MR) that lead to activation and inhibition of the receptor in the presence of agonist and/or antagonist. Our results indicate that MR ligand binding domain (LBD) interacts strongly with only a few specific coactivator peptides in the presence of the agonist aldosterone and that these interactions are blocked by the antagonist eplerenone. We also discovered that cortisol, the preferred physiological ligand for the glucocorticoid receptor in humans, is a partial MR agonist/antagonist, providing a possible molecular explanation of the tissue-selective effects of glucocorticoids on MR. However, when we examined the coactivator and corepressor peptide interactions in the presence of cortisol, we found that MR bound with cortisol or aldosterone interacted with the same set of peptides. Thus, the partial agonism shown by cortisol is unlikely to be the result of differential interaction with known coactivators and corepressors. On the other hand, we have identified coactivator binding groove mutations that are critical for cortisol activation but not for aldosterone activation, suggesting that the two steroids induce different MR LBD conformations. In addition, we also show that cortisol becomes full agonist when S810L mutation is introduced in the LBD of MR. Interestingly, MR antagonists, such as eplerenone and progesterone, become partial agonist/antagonist of S810L but are still able to recruit LXXLL peptides to the mutant receptor. Together, these findings suggest a model to explain the MR activation by various ligands.