The nuclear hormone receptor coactivator NRC is a pleiotropic modulator affecting growth, development, apoptosis, reproduction, and wound repair

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.

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.

The many faces of the zinc finger protein 335 in brain development and immune system

Zinc finger protein 335 (ZNF335) plays a crucial role in the methylation and, consequently, regulates the expression of a specific set of genes. Variants of the ZNF335 gene have been identified as risk factors for microcephaly in a variety of populations worldwide. Meanwhile, ZNF335 has also been identified as an essential regulator of T-cell development. However, an in-depth understanding of the role of ZNF335 in brain development and T cell maturation is still lacking. In this review, we summarize current knowledge of the molecular mechanisms underlying the involvement of ZNF335 in neuronal and T cell development across a wide range of pre-clinical, post-mortem, ex vivo, in vivo, and clinical studies. We also review the current limitations regarding the study of the pathophysiological functions of ZNF335. Finally, we hypothesize a potential role for ZNF335 in brain disorders and discuss the rationale of targeting ZNF335 as a therapeutic strategy for preventing brain disorders.

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.

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.

Network analysis reveals dysregulated functional patterns in type II diabetic skin

Skin disorders are one of the most common complications of type II diabetes (T2DM). Long-term effects of high blood glucose leave individuals with T2DM more susceptible to cutaneous diseases, but its underlying molecular mechanisms are unclear. Network-based methods consider the complex interactions between genes which can complement the analysis of single genes in previous research. Here, we use network analysis and topological properties to systematically investigate dysregulated gene co-expression patterns in type II diabetic skin with skin samples from the Genotype-Tissue Expression database. Our final network consisted of 8812 genes from 73 subjects with T2DM and 147 non-T2DM subjects matched for age, sex, and race. Two gene modules significantly related to T2DM were functionally enriched in the pathway lipid metabolism, activated by PPARA and SREBF (SREBP). Transcription factors KLF10, KLF4, SP1, and microRNA-21 were predicted to be important regulators of gene expression in these modules. Intramodular analysis and betweenness centrality identified NCOA6 as the hub gene while KHSRP and SIN3B are key coordinators that influence molecular activities differently between T2DM and non-T2DM populations. We built a TF-miRNA-mRNA regulatory network to reveal the novel mechanism (miR-21-PPARA-NCOA6) of dysregulated keratinocyte proliferation, differentiation, and migration in diabetic skin, which may provide new insights into the susceptibility of skin disorders in T2DM patients. Hub genes and key coordinators may serve as therapeutic targets to improve diabetic skincare.

Collagen/Chitosan Gels Cross-Linked with Genipin for Wound Healing in Mice with Induced Diabetes

Diabetes mellitus continues to be one of the most common diseases often associated with diabetic ulcers. Chitosan is an attractive biopolymer for wound healing due to its biodegradability, biocompatibility, mucoadhesiveness, low toxicity, and hemostatic effect. A panel of hydrogels based on chitosan, collagen, and silver nanoparticels were produced to treat diabetic wounds. The antibacterial activity, cytotoxicity, swelling, rheological properties, and longitudinal sections of hydrogels were studied. The ability of the gels for wound healing was studied in CD1 mice with alloxan-induced diabetes. Application of the gels resulted in an increase in VEGF, TGF-b1, IL-1b, and TIMP1 gene expression and earlier wound closure in a comparison with control untreated wounds. All gels increased collagen deposition, hair follicle repair, and sebaceous glands formation. The results of these tests show that the obtained hydrogels have good mechanical properties and biological activity and have potential applications in the field of wound healing. However, clinical studies are required to compare the efficacy of the gels as animal models do not reproduce full diabetes pathology.

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.

Topical L-thyroxine: The Cinderella among hormones waiting to dance on the floor of dermatological therapy?

Topical hormone therapy with natural or synthetic ligands of nuclear hormone receptors such as glucocorticoids, vitamin D analogues and retinoids has a long and highly successful tradition in dermatology. Yet the dermatological potential of thyroid hormone receptor (TR) agonists has been widely ignored, despite abundant clinical, cell and molecular biology, mouse in vivo, and human skin and hair follicle organ culture data documenting a role of TR-mediated signalling in skin physiology and pathology. Here, we review this evidence, with emphasis on wound healing and hair growth, and specifically highlight the therapeutic potential of repurposing topical L-thyroxine (T4) for selected applications in future dermatological therapy. We underscore the known systemic safety and efficacy profile of T4 in clinical medicine, and the well-documented impact of thyroid hormones on, for example, human epidermal and hair follicle physiology, hair follicle epithelial stem cells and pigmentation, keratin expression, mitochondrial energy metabolism and wound healing. On this background, we argue that short-term topical T4 treatment deserves careful further preclinical and clinical exploration for repurposing as a low-cost, effective and widely available dermatotherapeutic, namely in the management of skin ulcers and telogen effluvium, and that its predictable adverse effects are well-manageable.

Phenotypic variation reveals sites of evolutionary constraint in the androgenic signaling pathway

Steroid hormone systems play an important role in shaping the evolution of vertebrate sexual traits, but several aspects of this relationship remain unclear. For example, we currently know little about how steroid signaling complexes are adapted to accommodate the emergence of behavior in response to sexual selection. We use downy woodpeckers (Dryobates pubescens) to evaluate how the machinery underlying androgen action can evolve to accommodate this bird's main territorial signal, the drum. We focus specifically on modifications to androgenic mechanisms in the primary neck muscle that actuates the hammering movements underlying this signal. Of the signaling components we examine, we find that levels of circulating testosterone (T) and androgen receptor (AR) expression are consistently increased in a way that likely enhances androgenic regulation of drumming. By contrast, the expression of nuclear receptor co-factors-the 'molecular rheostats' of steroid action-show no such relationship in our analyses. If anything, co-factors are expressed in directions that would presumably hinder androgenic regulation of the drum. These findings therefore collectively point to T levels and AR as the more evolutionarily labile components of the androgenic system, in that they are likely more apt to change over time to support sexual selection for territorial signaling in woodpeckers. Yet the signaling elements that fine-tune AR's functional effects on the genome-namely the receptor's transcriptional co-factors-do not change in such a manner, and thus may be under tighter evolutionary constraint.

Genetically modified laboratory mice with sebaceous glands abnormalities

Sebaceous glands (SG) are exocrine glands that release their product by holocrine secretion, meaning that the whole cell becomes a secretion following disruption of the membrane. SG may be found in association with a hair follicle, forming the pilosebaceous unit, or as modified SG at different body sites such as the eyelids (Meibomian glands) or the preputial glands. Depending on their location, SG fulfill a number of functions, including protection of the skin and fur, thermoregulation, formation of the tear lipid film, and pheromone-based communication. Accordingly, SG abnormalities are associated with several diseases such as acne, cicatricial alopecia, and dry eye disease. An increasing number of genetically modified laboratory mouse lines develop SG abnormalities, and their study may provide important clues regarding the molecular pathways regulating SG development, physiology, and pathology. Here, we summarize in tabulated form the available mouse lines with SG abnormalities and, focusing on selected examples, discuss the insights they provide into SG biology and pathology. We hope this survey will become a helpful information source for researchers with a primary interest in SG but also as for researchers from unrelated fields that are unexpectedly confronted with a SG phenotype in newly generated mouse lines.

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.

Transition from inflammation to proliferation: a critical step during wound healing

The ability to rapidly restore the integrity of a broken skin barrier is critical and is the ultimate goal of therapies for hard-to-heal-ulcers. Unfortunately effective treatments to enhance healing and reduce scarring are still lacking. A deeper understanding of the physiology of normal repair and of the pathology of delayed healing is a prerequisite for the development of more effective therapeutic interventions. Transition from the inflammatory to the proliferative phase is a key step during healing and accumulating evidence associates a compromised transition with wound healing disorders. Thus, targeting factors that impact this phase transition may offer a rationale for therapeutic development. This review summarizes mechanisms regulating the inflammation-proliferation transition at cellular and molecular levels. We propose that identification of such mechanisms will reveal promising targets for development of more effective therapies.

Finding the most appropriate mouse model of juvenile CLN3 (Batten) disease for therapeutic studies: the importance of genetic background and gender

Mutations in the CLN3 gene cause a fatal neurodegenerative disorder: juvenile CLN3 disease, also known as juvenile Batten disease. The two most commonly utilized mouse models of juvenile CLN3 disease are Cln3-knockout (Cln3^−/−) and Cln3^Δex7/8-knock-in mice, the latter mimicking the most frequent disease-causing human mutation. To determine which mouse model has the most pronounced neurological phenotypes that can be used as outcome measures for therapeutic studies, we compared the exploratory activity, motor function and depressive-like behavior of 1-, 3- and 6-month-old Cln3^−/− and Cln3^Δex7/8-knock-in mice on two different genetic backgrounds (129S6/SvEv and C57BL/6J). Although, in many cases, the behavior of Cln3^−/− and Cln3^Δex7/8 mice was similar, we found genetic-background-, gender- and age-dependent differences between the two mouse models. We also observed large differences in the behavior of the 129S6/SvEv and C57BL/6J wild-type strains, which highlights the strong influence that genetic background can have on phenotype. Based on our results, Cln3^−/− male mice on the 129S6/SvEv genetic background are the most appropriate candidates for therapeutic studies. They exhibit motor deficits at 1 and 6 months of age in the vertical pole test, and they were the only mice to show impaired motor coordination in the rotarod test at both 3 and 6 months. Cln3^−/− males on the C57BL/6J background and Cln3^Δex7/8 males on the 129S6/SvEv background also provide good outcome measures for therapeutic interventions. Cln3^−/− (C57BL/6J) males had serious difficulties in climbing down (at 1 and 6 months) and turning downward on (at 1, 3 and 6 months) the vertical pole, whereas Cln3^Δex7/8 (129S6/SvEv) males climbed down the vertical pole drastically slower than wild-type males at 3 and 6 months of age. Our study demonstrates the importance of testing mouse models on different genetic backgrounds and comparing males and females in order to find the most appropriate disease model for therapeutic studies.

Detrimental role of prolonged sleep deprivation on adult neurogenesis

Adult mammalian brains continuously generate new neurons, a phenomenon called adult neurogenesis. Both environmental stimuli and endogenous factors are important regulators of adult neurogenesis. Sleep has an important role in normal brain physiology and its disturbance causes very stressful conditions, which disrupt normal brain physiology. Recently, an influence of sleep in adult neurogenesis has been established, mainly based on sleep deprivation studies. This review provides an overview on how rhythms and sleep cycles regulate hippocampal and subventricular zone neurogenesis, discussing some potential underlying mechanisms. In addition, our review highlights some interacting points between sleep and adult neurogenesis in brain function, such as learning, memory, and mood states, and provides some insights on the effects of antidepressants and hypnotic drugs on adult neurogenesis.

NRC-interacting factor directs neurite outgrowth in an activity-dependent manner

Nuclear hormone receptor coregulator-interacting factor 1 (NIF-1) is a zinc finger nuclear protein that was initially identified to enhance nuclear hormone receptor transcription via its interaction with nuclear hormone receptor coregulator (NRC). NIF-1 may regulate gene transcription either by modulating general transcriptional machinery or remodeling chromatin structure through interactions with specific protein partners. We previously reported that the cytoplasmic/nuclear localization of NIF-1 is regulated by the neuronal Cdk5 activator p35, suggesting potential neuronal functions for NIF-1. The present study reveals that NIF-1 plays critical roles in regulating neuronal morphogenesis at early stages. NIF-1 was prominently expressed in the nuclei of developing rat cortical neurons. Knockdown of NIF-1 expression attenuated both neurite outgrowth in cultured cortical neurons and retinoic acid (RA)-treated Neuro-2a neuroblastoma cells. Furthermore, activity-induced Ca(2+) influx, which is critical for neuronal morphogenesis, stimulated the nuclear localization of NIF-1 in cortical neurons. Suppression of NIF-1 expression reduced the up-regulation of neuronal activity-dependent gene transcription. These findings collectively suggest that NIF-1 directs neuronal morphogenesis during early developmental stages through modulating activity-dependent gene transcription.

The cancer COMPASS: navigating the functions of MLL complexes in cancer

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.

Impaired hair growth and wound healing in mice lacking thyroid hormone receptors

Both clinical and experimental observations show that the skin is affected by the thyroidal status. In hypothyroid patients the epidermis is thin and alopecia is common, indicating that thyroidal status might influence not only skin proliferation but also hair growth. We demonstrate here that the thyroid hormone receptors (TRs) mediate these effects of the thyroid hormones on the skin. Mice lacking TRα1 and TRβ (the main thyroid hormone binding isoforms) display impaired hair cycling associated to a decrease in follicular hair cell proliferation. This was also observed in hypothyroid mice, indicating the important role of the hormone-bound receptors in hair growth. In contrast, the individual deletion of either TRα1 or TRβ did not impair hair cycling, revealing an overlapping or compensatory role of the receptors in follicular cell proliferation. In support of the role of the receptors in hair growth, TRα1/TRβ-deficient mice developed alopecia after serial depilation. These mice also presented a wound-healing defect, with retarded re-epithelialization and wound gaping, associated to impaired keratinocyte proliferation. These results reinforce the idea that the thyroid hormone nuclear receptors play an important role on skin homeostasis and suggest that they could be targets for the treatment of cutaneous pathologies.

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.

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.

Altered sensitivity of cerebellar granule cells to glutamate receptor overactivation in the Cln3(Δex7/8)-knock-in mouse model of juvenile neuronal ceroid lipofuscinosis

The juvenile onset form of neuronal ceroid lipofuscinoses (JNCL) is a recessively inherited lysosomal storage disorder characterized by progressive neurodegeneration. JNCL results from mutations in the CLN3 gene that encodes a lysosomal membrane protein with unknown function. Utilizing a Cln3-knock-out mouse model of JNCL that was created on the 129S6/SvEv genetic background, we have previously demonstrated that CLN3-deficient cerebellar granule cells (CGCs) have a selectively increased sensitivity to AMPA-type glutamate receptor-mediated toxicity. Our recent findings that CGCs from 129S6/SvEv and C57BL/6J wild type (WT) mice have significant differences in glutamate receptor expression and in excitotoxic vulnerability indicated that the genetic background possibly have a strong influence on how glutamate receptor function is dysregulated in CLN3-deficient neurons. Indeed, here we show that in the Cln3(Δex7/8)-knock-in mouse model, that is on the C57BL/6J genetic background, mimics the most frequent mutation observed in JNCL patients and considered a null mutant, the sensitivity of CGCs to both AMPA- and NMDA-type glutamate receptor overactivations is altered. Cultured wild type and Cln3(Δex7/8) CGCs were equally sensitive to AMPA toxicity after 2 or 3 weeks in vitro, whereas the subunit-selective AMPA receptor agonist, CPW-399, induced significantly more cell death in mature, 3-week-old Cln3(Δex7/8) cultures. NMDA receptor-mediated toxicity changed during in vitro development: Cln3(Δex7/8) CGCs were less sensitive to high concentration of NMDA after 2 weeks in culture but became more vulnerable than their WT counterparts after 3 weeks in vitro. Abnormally altered glutamate receptor function in the cerebellum may result in motor deficits, and we confirmed that 7-week-old Cln3(Δex7/8) mice, similarly to Cln3-knock-out mice, have a motor coordination deficit as measured by an accelerating rotarod. Our results demonstrate altered glutamate receptor function in Cln3(Δex7/8) neurons and suggest that both AMPA and NMDA receptors are potential therapeutic targets in JNCL.

Altered sensitivity to excitotoxic cell death and glutamate receptor expression between two commonly studied mouse strains

Alterations in glutamatergic synapse function have been implicated in the pathogenesis of many different neurological disorders, including ischemia, epilepsy, Parkinson's disease, Alzheimer's disease, and Huntington's disease. While studying glutamate receptor function in juvenile Batten disease on the C57BL/6J and 129S6/S(v)E(v) mouse backgrounds, we noticed differences unlikely to be due to mutation difference alone. We report here that primary cerebellar granule cell cultures from C57BL/6J mice are more sensitive to N-methyl-D-aspartate (NMDA)-mediated cell death. Moreover, sensitivity to AMPA-mediated excitotoxicity is more variable and is dependent on the treatment conditions and age of the cultures. Glutamate receptor surface expression levels examined in vitro by in situ ELISA and in vivo by Western blot in surface cross-linked cerebellar samples indicated that these differences in sensitivity likely are due to strain-dependent differences in cell surface receptor expression levels. We propose that differences in glutamate receptor expression and in excitotoxic vulnerability should be taken into consideration in the context of characterizing disease models on the C57BL/6J and 129S6/S(v)E(v) mouse backgrounds.

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.

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.

ASCOM controls farnesoid X receptor transactivation through its associated histone H3 lysine 4 methyltransferase activity

Activating signal cointegrator-2 (ASC-2), a coactivator of multiple nuclear receptors and transcription factors, belongs to a steady-state complex named ASCOM (for ASC-2 complex), which contains histone H3 lysine 4 (H3K4) methyltransferase MLL3 or its paralog MLL4. ASC-2 binds to many nuclear receptors in a ligand-dependent manner through its two LxxLL motifs. Here we show that the first LxxLL motif of ASC-2 shows relatively weak but specific interaction with the nuclear receptor farnesoid X receptor (FXR) and that ASCOM plays crucial roles in FXR transactivation. Our results reveal that ASC-2, MLL3, and MLL4 are recruited to FXR target genes in a ligand-dependent manner. We further show that the recruitment of MLL3 requires ASC-2 and that FXR ligand induces not only expression of FXR-target genes but also their H3K4 trimethylation in a manner dependent on the presence of ASC-2, MLL3, and MLL4. In addition, MLL3 and MLL4 function redundantly with FXR transactivation. Correspondingly, expression of FXR target genes is partially impaired in mice expressing an enzymatically inactivated mutant form of MLL3, and these mice show disrupted bile acid homeostasis. Overall, these results suggest that ASCOM-MLL3 and ASCOM-MLL4 play redundant but essential roles in FXR transactivation via their H3K4 trimethylation activity.

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.

Identification and characterization of a novel nuclear protein complex involved in nuclear hormone receptor-mediated gene regulation

NRC/NCoA6 plays an important role in mediating the effects of ligand-bound nuclear hormone receptors as well as other transcription factors. NRC interacting factor 1 (NIF-1) was cloned as a novel factor that interacts in vivo with NRC. Although NIF-1 does not directly interact with nuclear hormone receptors, it enhances activation by nuclear hormone receptors presumably through its interaction with NRC. To further understand the cellular and biological function of NIF-1, we identified NIF-1-associated proteins by in-solution proteolysis followed by mass spectrometry. The identified components revealed factors involved in histone methylation and cell cycle control and include Ash2L, RbBP5, WDR5, HCF-1, DBC-1, and EMSY. Although the NIF-1 complex contains Ash2L, RbBP5, and WDR5, suggesting that the complex might methylate histone H3-Lys-4, we found that the complex contains a H3 methyltransferase activity that modifies a residue other than H3-Lys-4. The identified components form at least two distinctly sized NIF-1 complexes. DBC-1 and EMSY were identified as integral components of an NIF-1 complex of approximately 1.5 MDa and were found to play an important role in the regulation of nuclear receptor-mediated transcription. Stimulation of the Sox9 and HoxA1 genes by retinoic acid receptor-alpha was found to require both DBC-1 and EMSY in addition to NIF-1 for maximal transcriptional activation. Interestingly, NRC was not identified as a component of the NIF-1 complex, suggesting that NIF-1 and NRC do not exist as stable in vitro purified complexes, although the separate NIF-1 and NRC complexes appear to functionally interact in the cell.

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.

The nuclear receptor coactivator AIB3 is a modulator of HOMA beta-cell function in nondiabetic children

OBJECTIVE

The amplified in breast cancer-3 protein (AIB3) is a nuclear coactivator involved in proliferation, apoptosis and development. AIB3 loss of function causes deficient insulin secretion in mice, indicating that AIB3 participates in beta-cell regulation. Our objective was to evaluate genetic variants located on AIB3 associated with beta-cell function in children and to analyse the effect of AIB3 overexpression on gene expression in insulin 1 (INS-1) beta-pancreatic cells.

DESIGN

Polymorphisms from AIB3 were genotyped in 148 children with normal or low birthweights for gestational age. The effect of AIB3 overexpression on gene expression was analysed by real-time polymerase chain reaction (PCR) in INS-1 cells.

RESULTS

AIB3 variants were associated with homeostasis model assessment of beta-cell function (HOMA-beta-cell) in children with normal or low birthweights for gestational age, but not with HOMA of insulin resistance (HOMA-IR), or with birthweight. AIB3 overexpression increased the expression of genes involved in signalling, such as IRS-1, IRS-2, IGF-II receptor or Foxo1, or of genes that control insulin secretion, such as Cplx2, Glut2 or Kv3.1 in INS-1 cells.

CONCLUSIONS

Our results suggest that AIB3 contributes to the maintenance of beta-cell function in nondiabetic children and regulates gene expression in INS-1 cells.

ADAM12: a potential target for the treatment of chronic wounds

Wound healing is a complex process involving multiple cellular events, including cell proliferation, migration, and tissue remodeling. A disintegrin and metalloprotease 12 (ADAM12) is a membrane-anchored metalloprotease, which has been implicated in activation-inactivation of growth factors that play an important role in wound healing, including heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) and insulin growth factor (IGF) binding proteins. Here, we report that expression of ADAM12 is fivefold upregulated in the nonhealing edge of chronic ulcers compared to healthy skin, based on microarrays of biopsies taken from five patients and from healthy controls (p = 0.013). The increase in ADAM12 expression in chronic ulcers was confirmed by quantitative real-time polymerase chain reaction (RT-PCR). Moreover, immunohistochemical analysis demonstrated a pronounced increase in the membranous and intracellular signal for ADAM12 in the epidermis of chronic wounds compared to healthy skin. These findings, coupled with our previous observations that lack of keratinocyte migration contributes to the pathogenesis of chronic ulcers, prompted us to evaluate how the absence of ADAM12 affects the migration of mouse keratinocytes. Skin explants from newborn ADAM12-/- or wild-type (WT) mice were used to quantify keratinocyte migration out of the explants over a period of 7 days. We found a statistically significant increase in the migration of ADAM12-/- keratinocytes compared to WT control (p = 0.0014) samples. Taken together, the upregulation of ADAM12 in chronic wounds and the increased migration of keratinocytes in the absence of ADAM12 suggest that ADAM12 is an important mediator of wound healing. We hypothesize that increased expression of ADAM12 in chronic wounds impairs wound healing through the inhibition of keratinocyte migration and that topical ADAM12 inhibitors may therefore prove useful for the treatment of chronic wounds.

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.

Transcriptional control of wound repair

Injury to the skin initiates a complex process of events involving inflammation as well as the formation and remodeling of new tissue. These processes result in at least partial reconstitution of the injured skin. However, wounds in adult mammals heal with a scar, which is accompanied by functional and aesthetic impairments. In addition to this problem, a large number of patients, in particular in the aged population, suffer from chronic, nonhealing ulcers. Therefore, there is a strong need to improve the wound healing process. This requires a thorough understanding of the underlying molecular and cellular mechanisms. During the past several years, important regulators of the wound healing process have been identified. In particular, the growth factors and matrix proteins, which orchestrate skin repair, have been characterized in detail. By contrast, much less is known about the transcription factors, which regulate gene expression at the wound site. This review summarizes recent data on the expression of transcription factors in skin wounds and their functions in the repair process.

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.

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.

Coactivator as a target gene specificity determinant for histone H3 lysine 4 methyltransferases

Activating signal cointegrator-2 (ASC-2), a coactivator of multiple transcription factors that include retinoic acid receptor (RAR), associates with histone H3-K4 methyltranferases (H3K4MTs) MLL3 and MLL4 in mixed-lineage leukemia. Here, we show that mice expressing a SET domain mutant of MLL3 share phenotypes with isogenic ASC2+/- mice and that expression and H3-K4 trimethylation of RAR target gene RAR-beta2 are impaired in ASC-2-null mouse embryo fibroblasts (MEFs) or in MEFs expressing siRNAs against both MLL3 and MLL4. We also show that MLL3 and MLL4 are found in distinct ASC-2-containing complexes rather than in a common ASC-2 complex, and they are recruited to RAR-beta2 by ASC-2. In contrast, RAR-beta2 expression is intact in MEFs devoid of menin, a component of MLL1 and MLL2 H3K4MT complexes. These results suggest that ASC-2 confers target gene specificity to MLL3 and MLL4 H3K4MT complexes and that recruitment of H3K4MTs to their target genes generally involves interactions between integral components of H3K4MT complexes and transcription factors.

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.

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 Book of Opposites: The Role of the Nuclear Receptor Co-regulators in the Suppression of Epidermal Genes by Retinoic Acid and Thyroid Hormone Receptors

Transcriptional regulation by nuclear receptors occurs through complex interactions that involve DNA response elements, co-activators/co-repressors, and histone modifying enzymes. Very little is known about how molecular interplay of these components may determine tissue specificity of hormone action. We have shown previously that retinoic acid (RA) and thyroid hormone (T3) repress transcription of a specific group of epidermal keratin genes through a novel mechanism that utilizes receptors homodimers. In this paper, we have analyzed the epidermal specificity of RA/T3 action by testing the role of co-repressors and co-activators in regulation of epidermal genes. Using transient co-transfections, northern blots, antisense oligonucleotides, and a histone deacetylase (HDAC) inhibitor, trichostatin A, we found that in the context of specific keratin RE (KRE), co-activators and histone acetylase become co-repressors of the RA/T3 receptors in the presence of their respective ligands. Conversely, co-repressors and HDAC become co-activators of unliganded T3Ralpha. The receptor-co-activator interaction is intact and occurs through the NR-box. Therefore, the role of co-activator is to associate with liganded receptors whereas the KRE-receptor interaction determines specific transcriptional signal, in this case repression. This novel molecular mechanism of transcriptional repression conveys how RA and T3 target specific groups of epidermal genes, thus exerting intrinsic tissue specificity.

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.

From an Enhanceosome to a Repressosome: Molecular Antagonism between Glucocorticoids and EGF Leads to Inhibition of Wound Healing

Wound healing in its complexity depends on the concerted activity of many signaling pathways. Here, we analyzed how the simultaneous presence of glucocorticoids (GC), retinoic acid (RA) and epidermal growth factor (EGF) affect wound healing at the molecular, cellular and tissue levels. We found that GC inhibit wound healing by inhibiting keratinocyte migration, whereas RA does not. Furthermore, GC block EGF-mediated migration, whereas RA does not. On the molecular level, these compounds target expression of one of the earliest markers of wound healing, cytoskeletal components, keratins K6 and K16. Both GC and RA repress their transcription, whereas EGF induces it. Interestingly, the GC inhibition is mediated by a repressosome complex consisting of four monomers of the GC receptor, beta-catenin and coactivator-associated-arginine-methyltransferase-1. GC are dominant, EGF cannot rescue GC-mediated inhibition. Pre-treatment of keratinocytes with GC shifts the balance towards the repressosome, allowing for dominant inhibition of K6 even in the presence of EGF or c-fos/c-jun. Although RA receptor gamma and glucocorticoid receptor bind to the same response element repressing transcription of keratins K6/K16, RA receptor interacts with the components of the EGF-enhanceosome (co-activators: glucocorticoid-receptor-interactive protein-1(GRIP-1)/steroid-receptors coactivator-1 (SRC-1)) without breaking it. Consequently, RA has a co-dominant effect with EGF: when present simultaneously, their effects balance each other. When keratinocytes are pre-treated with mitogen-activated protein kinase (MAPK) inhibitor, thus blocking EGF, the balance is shifted towards the RA repression. Similar to clinical findings, pre-treatment of keratinocytes with RA blocks GC-mediated inhibition. In summary, our results identify complex molecular mechanisms through which RA alleviates GC-mediated inhibition of wound healing.