Association of retinoic acid receptor genes with meningomyelocele.

BACKGROUND: Neural tube defects (NTDs) occur in as many as 0.5-2 per 1000 live births in the United States. One of the most common and severe neural tube defects is meningomyelocele (MM) resulting from failed closure of the caudal end of the neural tube. MM has been induced by retinoic acid teratogenicity in rodent models. We hypothesized that genetic variants influencing retinoic acid (RA) induction via retinoic acid receptors (RARs) may be associated with risk for MM. METHODS: We analyzed 47 single nucleotide polymorphisms (SNPs) that span across the three retinoic acid receptor genes using the SNPlex genotyping platform. Our cohort consisted of 610 MM families. RESULTS: One variant in the RARA gene (rs12051734), three variants in the RARB gene (rs6799734, rs12630816, rs17016462), and a single variant in the RARG gene (rs3741434) were found to be statistically significant at p < 0.05. CONCLUSION: RAR genes were associated with risk for MM. For all associated SNPs, the rare allele conferred a protective effect for MM susceptibility.

Analysis of endogenous retinoic acid receptor expression and function in murine melanoma cells

There have been numerous reports over the past several years on the ability of vitamin A analogs (retinoids) to modulate cell proliferation, malignant transformation, morphogenesis, and differentiation in a wide variety of cell types and organisms. Two families of nuclear retinoid-inducible, trans-acting, transcription-enhancing receptors that bear strong DNA sequence homology to thyroid and steroid hormone receptors have recently been discovered. The retinoic acid receptors (RARs) and retinoid X receptors (RXRs) each have at least three types designated $\alpha,$ $\beta,$ and $\gamma,$ which are encoded by separate genes and expressed in a tissue and cell type-specific manner. We have been interested in the mechanism by which retinoids inhibit tumor cell proliferation and induce differentiation. As a model system we have employed several murine melanoma cell lines (S91-C2, K1735P, and B16-F1), which are sensitive to the growth-inhibitory and differentiation-inducing effects of RA, as well as a RA-resistant subclone of one of the cell lines (S91-C154), in order to study the role of the nuclear RARs in these effects. The initial phase of this project consisted of the characterization of the expression pattern of the three known RAR and RXR types in the murine melanoma cell lines in order to determine whether any differences exist which may elucidate a role for any of the receptors in RA-induced growth inhibition and differentiation. The novel finding was made that the RAR-$\beta$ gene is rapidly induced from undetectable levels by RA treatment at the mRNA and protein level, and that the induction of RAR-$\beta$ by other biologically active retinoids correlated with their ability to inhibit the growth of the highly RA-sensitive S91-C2 cell line. This suggests a role for RAR-$\beta$ in the growth inhibiting effect of retinoids. The second phase of this project involves the stable expression of RAR-$\beta$ in the S91-C2 cells and the RAR-$\beta$ receptor-null cell line, K1735P. These studies have indicated an inverse correlation between RAR-$\beta$ expression and proliferation rate. ^

Retinoic acid receptor beta and angiopoietin-like protein 1 are involved in the regulation of human androgen biosynthesis

Androgens are essential for sexual development and reproduction. However, androgen regulation in health and disease is poorly understood. We showed that human adrenocortical H295R cells grown under starvation conditions acquire a hyperandrogenic steroid profile with changes in steroid metabolizing enzymes HSD3B2 and CYP17A1 essential for androgen production. Here we studied the regulatory mechanisms underlying androgen production in starved H295R cells. Microarray expression profiling of normal versus starved H295R cells revealed fourteen differentially expressed genes; HSD3B2, HSD3B1, CYP21A2, RARB, ASS1, CFI, ASCL1 and ENC1 play a role in steroid and energy metabolism and ANGPTL1, PLK2, DUSP6, DUSP10 and FREM2 are involved in signal transduction. We discovered two new gene networks around RARB and ANGPTL1, and show how they regulate androgen biosynthesis. Transcription factor RARB stimulated the promoters of genes involved in androgen production (StAR, CYP17A1 and HSD3B2) and enhanced androstenedione production. For HSD3B2 regulation RARB worked in cooperation with Nur77. Secretory protein ANGPTL1 modulated CYP17A1 and DUSP6 expression by inducing ERK1/2 phosphorylation. By contrast, our studies revealed no evidence for hormones or cell cycle involvement in regulating androgen biosynthesis. In summary, these studies establish a firm role for RARB and ANGPTL1 in the regulation of androgen production in H295R cells.

Retardation of skeletal development and cervical abnormalities in transgenic mice expressing a dominant-negative retinoic acid receptor in chondrogenic cells

Skeletal formation is a fundamental element of body patterning and is strictly regulated both temporally and spatially by a variety of molecules. Among these, retinoic acid (RA) has been shown to be involved in normal skeletal development. However, its pleiotropic effects have caused difficulty in identifying its crucial target cells and molecular mechanisms for each effect. Development of cartilage primordia is an important process in defining the skeletal structures. To address the role of RA in skeletal formation, we have generated mice expressing a dominant-negative retinoic acid receptor (RAR) in chondrogenic cells by using the type II collagen α1 promoter, and we have analyzed their phenotypes. These mice exhibited small cartilage primordia during development and retarded skeletal formation in both embryonic and postnatal periods. They also showed selective degeneration in their cervical vertebrae combined with homeotic transformations, but not in their extremities. The cervical phenotypes are reminiscent of phenotypes involving homeobox genes. We found that the expression of Hoxa-4 was indeed reduced in the cartilage primordia of cervical vertebrae of embryonic day 12.5 embryos. These observations demonstrate that endogenous RA acts directly on chondrogenic cells to promote skeletal growth in both embryonic and growing periods, and it regulates the proper formation of cervical vertebrae. Furthermore, RA apparently specifies the identities of the cervical vertebrae through the regulation of homeobox genes in the chondrogenic cells. Great similarities of the phenotypes between our mice and reported RAR knockout mice revealed that chondrogenic cells are a principal RA target during complex cascades of skeletal development.

SMRT corepressor interacts with PLZF and with the PML-retinoic acid receptor α (RARα) and PLZF-RARα oncoproteins associated with acute promyelocytic leukemia

Retinoic acid receptors (RARs) are hormone-regulated transcription factors that control key aspects of normal differentiation. Aberrant RAR activity may be a causal factor in neoplasia. Human acute promyelocytic leukemia, for example, is tightly linked to chromosomal translocations that fuse novel amino acid sequences (denoted PML, PLZF, and NPM) to the DNA-binding and hormone-binding domains of RARα. The resulting chimeric receptors have unique transcriptional properties that may contribute to leukemogenesis. Normal RARs repress gene transcription by associating with ancillary factors denoted corepressors (also referred to as SMRT, N-CoR, TRAC, or RIP13). We report here that the PML-RARα and PLZF-RARα oncoproteins retain the ability of RARα to associate with corepressors, and that this corepressor association correlates with certain aspects of the leukemic phenotype. Unexpectedly, the PLZF moiety itself can interact with SMRT corepressor. This interaction with corepressor is mediated, in part, by a POZ motif within PLZF. Given the presence of POZ motifs in a number of known transcriptional repressors, similar interactions with SMRT may play a role in transcriptional silencing by a variety of both receptor and nonreceptor transcription factors.

A single dose of kainic acid elevates the levels of enkephalins and activator protein-1 transcription factors in the hippocampus for up to 1 year

Neuronal plasticity plays a very important role in brain adaptations to environmental stimuli, disease, and aging processes. The kainic acid model of temporal lobe epilepsy was used to study the long-term anatomical and biochemical changes in the hippocampus after seizures. Using Northern blot analysis, immunocytochemistry, and Western blot analysis, we have found a long-term elevation of the proconvulsive opioid peptide, enkephalin, in the rat hippocampus. We have also demonstrated that an activator protein-1 transcription factor, the 35-kDa fos-related antigen, can be induced and elevated for at least 1 year after kainate treatment. This study demonstrated that a single systemic injection of kainate produces almost permanent increases in the enkephalin and an activator protein-1 transcription factor, the 35-kDa fos-related antigen, in the rat hippocampus, and it is likely that these two events are closely associated with the molecular mechanisms of induction of long-lasting enhanced seizure susceptibility in the kainate-induced seizure model. The long-term expression of the proenkephalin mRNA and its peptides in the kainate-treated rat hippocampus also suggests an important role in the recurrent seizures of temporal lobe epilepsy.

Leukemia-associated retinoic acid receptor α fusion partners, PML and PLZF, heterodimerize and colocalize to nuclear bodies

In acute promyelocytic leukemia (APL), the typical t(15;17) and the rare t(11;17) translocations express, respectively, the PML/RARα and PLZF/RARα fusion proteins (where RARα is retinoic acid receptor α). Herein, we demonstrate that the PLZF and PML proteins interact with each other and colocalize onto nuclear bodies (NBs). Furthermore, induction of PML expression by interferons leads to a recruitment of PLZF onto NBs without increase in the levels of the PLZF protein. PML/RARα and PLZF/RARα localize to the same microspeckled nuclear domains that appear to be common targets for the two fusion proteins in APL. Although PLZF/RARα does not affect the localization of PML, PML/RARα delocalizes the endogenous PLZF protein in t(15;17)-positive NB4 cells, pointing to a hierarchy in the nuclear targeting of these proteins. Thus, our results unify the molecular pathogenesis of APL with at least two different RARα gene translocations and stress the importance of alterations of PLZF and RARα nuclear localizations in this disease.

Retinoic acid induces proteasome-dependent degradation of retinoic acid receptor α (RARα) and oncogenic RARα fusion proteins

Analyzing the pathways by which retinoic acid (RA) induces promyelocytic leukemia/retinoic acid receptor α (PML/RARα) catabolism in acute promyelocytic leukemia (APL), we found that, in addition to caspase-mediated PML/RARα cleavage, RA triggers degradation of both PML/RARα and RARα. Similarly, in non-APL cells, RA directly targeted RARα and RARα fusions to the proteasome degradation pathway. Activation of either RARα or RXRα by specific agonists induced degradation of both proteins. Conversely, a mutation in RARα that abolishes heterodimer formation and DNA binding, blocked both RARα and RXRα degradation. Mutations in the RARα DNA-binding domain or AF-2 transcriptional activation region also impaired RARα catabolism. Hence, our results link transcriptional activation to receptor catabolism and suggest that transcriptional up-regulation of nuclear receptors by their ligands may be a feedback mechanism allowing sustained target-gene activation.

Ligand-dependent activation of transcription in vitro by retinoic acid receptor α/retinoid X receptor α heterodimers that mimics transactivation by retinoids in vivo

All-trans and 9-cis retinoic acids (RA) signals are transduced by retinoic acid receptor/retinoid X receptor (RAR/RXR) heterodimers that act as functional units controlling the transcription of RA-responsive genes. With the aim of elucidating the underlying molecular mechanisms, we have developed an in vitro transcription system using a chromatin template made up of a minimal promoter and a direct repeat with 5-spacing-based RA response element. RARα and RXRα were expressed in and purified from baculovirus-infected Sf9 cells, and transcription was carried out by using naked DNA or chromatin templates. Transcription from naked templates was not affected by the presence of RA and/or RAR/RXR heterodimers. In contrast, very little transcription occurred from chromatin templates in the absence of RA or RAR/RXR heterodimers whereas their addition resulted in a dosage-dependent stimulation of transcription that never exceeded that occurring on naked DNA templates. Most importantly, the addition of synthetic agonistic or antagonistic retinoids to the chromatin transcription system mimicked their stimulatory or inhibitory action in vivo, and activation by a RXR-specific retinoid was subordinated to the binding of an agonist ligand to the RAR partner. Moreover, the addition of the p300 coactivator generated a synergistic enhancement of transcription. Thus, the dissection of this transcription system ultimately should lead to the elucidation of the molecular mechanisms by which RAR/RXR heterodimers control transcription in a ligand-dependent manner.

An initiator element mediates autologous downregulation of the human type A γ-aminobutyric acid receptor β1 subunit gene

The regulated expression of type A γ-aminobutyric acid receptor (GABAAR) subunit genes is postulated to play a role in neuronal maturation, synaptogenesis, and predisposition to neurological disease. Increases in GABA levels and changes in GABAAR subunit gene expression, including decreased β1 mRNA levels, have been observed in animal models of epilepsy. Persistent exposure to GABA down-regulates GABAAR number in primary cultures of neocortical neurons, but the regulatory mechanisms remain unknown. Here, we report the identification of a TATA-less minimal promoter of 296 bp for the human GABAAR β1 subunit gene that is neuron specific and autologously down-regulated by GABA. β1 promoter activity, mRNA levels, and subunit protein are decreased by persistent GABAAR activation. The core promoter, 270 bp, contains an initiator element (Inr) at the major transcriptional start site. Three concatenated copies of the 10-bp Inr and its immediate 3′ flanking sequence produce full neural specific activity that is down-regulated by GABA in transiently transfected neocortical neurons. Taking these results together with those of DNase I footprinting, electrophoretic mobility shift analysis, and 2-bp mutagenesis, we conclude that GABA-induced down-regulation of β1 subunit mRNAs involves the differential binding of a sequence-specific basal transcription factor(s) to the Inr. The results support a transcriptional mechanism for the down-regulation of β1 subunit GABAAR gene expression and raises the possibility that altered levels of sequence-specific basal transcription factors may contribute to neurological disorders such as epilepsy.

Remodeling of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit composition in hippocampal neurons after global ischemia

Transient global ischemia induces selective delayed cell death, primarily of principal neurons in the hippocampal CA1. However, the molecular mechanisms underlying ischemia-induced cell death are as yet unclear. The present study shows that global ischemia triggers a pronounced and cell-specific reduction in GluR2 [the subunit that limits Ca2+ permeability of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors] in vulnerable CA1 neurons, as evidenced by immunofluorescence of brain sections and Western blot analysis of microdissected hippocampal subfields. At 72 h after ischemia (a time before cell death), virtually all CA1 pyramidal neurons exhibited greatly reduced GluR2 immunolabeling throughout their somata and dendritic processes. GluR2 immunolabeling was unchanged in pyramidal cells of the CA3 and granule cells of the dentate gyrus, regions resistant to ischemia-induced damage. Immunolabeling of the AMPA receptor subunit GluR1 was unchanged in CA1, CA3, and dentate gyrus. Western analysis indicated that GluR2 subunit abundance was markedly reduced in CA1 at 60 and 72 h after the ischemic insult; GluR1 abundance was unchanged in all subfields at all times examined. These findings, together with the previous observation of enhanced AMPA-elicited Ca2+ influx in postischemic CA1 neurons, show that functional GluR2-lacking, Ca2+-permeable AMPA receptors are expressed in vulnerable neurons before cell death. Thus, the present study provides an important link in the postulated causal chain between global ischemia and delayed death of CA1 pyramidal neurons.

Requirement for the lpA1 lysophosphatidic acid receptor gene in normal suckling behavior

Although extracellular application of lysophosphatidic acid (LPA) has been extensively documented to produce a variety of cellular responses through a family of specific G protein-coupled receptors, the in vivo organismal role of LPA signaling remains largely unknown. The first identified LPA receptor gene, lpA1/vzg-1/edg-2, was previously shown to have remarkably enriched embryonic expression in the cerebral cortex and dorsal olfactory bulb and postnatal expression in myelinating glia including Schwann cells. Here, we show that targeted deletion of lpA1 results in approximately 50% neonatal lethality, impaired suckling in neonatal pups, and loss of LPA responsivity in embryonic cerebral cortical neuroblasts with survivors showing reduced size, craniofacial dysmorphism, and increased apoptosis in sciatic nerve Schwann cells. The suckling defect was responsible for the death among lpA1(−/−) neonates and the stunted growth of survivors. Impaired suckling behavior was attributable to defective olfaction, which is likely related to developmental abnormalities in olfactory bulb and/or cerebral cortex. Our results provide evidence that endogenous lysophospholipid signaling requires an lp receptor gene and indicate that LPA signaling through the LPA1 receptor is required for normal development of an inborn, neonatal behavior.

Constitutive overexpression of Cu/Zn superoxide dismutase exacerbates kainic acid-induced apoptosis of transgenic-Cu/Zn superoxide dismutase neurons.

Cu/Zn superoxide dismutase (Cu/Zn SOD) is a key enzyme in the metabolism of oxygen free radicals. The gene resides on chromosome 21 and is overexpressed in patients with Down syndrome. Cultured neurons of transgenic Cu/Zn SOD (Tg-Cu/Zn SOD) mice with elevated activity of Cu/Zn SOD were used to determine whether constitutive overexpression of Cu/Zn SOD creates an indigenous oxidative stress that predisposes the Tg-Cu/Zn SOD neurons to added insults. Neurons from three independently derived Tg-Cu/Zn SOD strains showed higher susceptibility than nontransgenic neurons to kainic acid (KA)-mediated excitotoxicity, reflected by an earlier onset and enhanced apoptotic cell death. This higher susceptibility of transgenic neurons to KA-mediated apoptosis was associated with a chronic prooxidant state that was manifested by reduced levels of cellular glutathione and altered [Ca2+]i homeostasis. The data are compatible with the thesis that overexpression of Cu/Zn SOD creates chronic oxidative stress in the transgenic neurons, which exacerbates their susceptibility to additional insults such as KA-mediated excitotoxicity.

Cell-type and promoter-context dependent retinoic acid receptor (RAR) redundancies for RAR beta 2 and Hoxa-1 activation in F9 and P19 cells can be artefactually generated by gene knockouts.

By using RAR type (alpha, beta, or gamma)-specific synthetic retinoids and a pan-retinoic X receptor (RXR)-specific ligand, we have investigated the contribution of RARs and RXRs in the activation of RA target genes and the differentiation of embryonal carcinoma cells. We demonstrate cell-type- and promoter context-dependent functional redundancies that differ between the three RAR types for mediating the induction of RARbeta2 and Hoxa-1 in wild-type, RARgamma-/- and RARalpha-/- F9 cells and in P19 cells. The extent of redundancy between RARs is further modulated by the synergistic activation of RXRs with a pan-RXR agonist. We also demonstrate that the expression of RARbeta2 is auto-inducible in RARgamma-/- but not in wild-type F9 cells, indicating that the functional redundancies observed between RARs in gene disruption studies can be artefactually generated. Thus, even though all three RARs can functionally substitute each other for inducing the expression of RA target genes and cell differentiation, one RAR can cell-specifically override the activity of the other RARs. Interestingly, only RARgamma can mediate the retinoic acid-induced differentiation of wild-type F9 cells, whereas the differentiation of P19 cells can be mediated by either RARalpha or RARgamma.