Retinoic acid induction of mouse cellular retinoic acid-binding protein-I gene expression is enhanced by sphinganine

Sonic Hedgehog-Gli1 Signaling and Cellular Retinoic Acid Binding Protein 1 Gene Regulation in Motor Neuron Differentiation and Diseases

Cellular retinoic acid-binding protein 1 (CRABP1) is highly expressed in motor neurons. Degenerated motor neuron-like MN1 cells are engineered by introducing SOD^G93A or AR-65Q to model degenerated amyotrophic lateral sclerosis (ALS) or spinal bulbar muscular atrophy neurons. Retinoic acid (RA)/sonic hedgehog (Shh)-induced embryonic stem cells differentiation into motor neurons are employed to study up-regulation of Crabp1 by Shh. In SOD^G93A or AR-65Q MN1 neurons, CRABP1 level is reduced, revealing a correlation of motor neuron degeneration with Crabp1 down-regulation. Up-regulation of Crabp1 by Shh is mediated by glioma-associated oncogene homolog 1 (Gli1) that binds the Gli target sequence in Crabp1′s neuron-specific regulatory region upstream of minimal promoter. Gli1 binding triggers chromatin juxtaposition with minimal promoter, activating transcription. Motor neuron differentiation and Crabp1 up-regulation are both inhibited by blunting Shh with Gli inhibitor GANT61. Expression data mining of ALS and spinal muscular atrophy (SMA) motor neurons shows reduced CRABP1, coincided with reduction in Shh-Gli1 signaling components. This study reports motor neuron degeneration correlated with down-regulation in Crabp1 and Shh-Gli signaling. Shh-Gli up-regulation of Crabp1 involves specific chromatin remodeling. The physiological and pathological implication of this regulatory pathway in motor neuron degeneration is supported by gene expression data of ALS and SMA patients.

Cellular Retinoic Acid Binding Proteins: Genomic and Non-genomic Functions and their Regulation

Cellular retinoic acid binding proteins (CRABPs) are high-affinity retinoic acid (RA) binding proteins that mainly reside in the cytoplasm. In mammals, this family has two members, CRABPI and II, both highly conserved during evolution. The two proteins share a very similar structure that is characteristic of a "β-clam" motif built up from10-strands. The proteins are encoded by two different genes that share a very similar genomic structure. CRABPI is widely distributed and CRABPII has restricted expression in only certain tissues. The CrabpI gene is driven by a housekeeping promoter, but can be regulated by numerous factors, including thyroid hormones and RA, which engage a specific chromatin-remodeling complex containing either TRAP220 or RIP140 as coactivator and corepressor, respectively. The chromatin-remodeling complex binds the DR4 element in the CrabpI gene promoter to activate or repress this gene in different cellular backgrounds. The CrabpII gene promoter contains a TATA-box and is rapidly activated by RA through an RA response element. Biochemical and cell culture studies carried out in vitro show the two proteins have distinct biological functions. CRABPII mainly functions to deliver RA to the nuclear RA receptors for gene regulation, although recent studies suggest that CRABPII may also be involved in other cellular events, such as RNA stability. In contrast, biochemical and cell culture studies suggest that CRABPI functions mainly in the cytoplasm to modulate intracellular RA availability/concentration and to engage other signaling components such as ERK activity. However, these functional studies remain inconclusive because knocking out one or both genes in mice does not produce definitive phenotypes. Further studies are needed to unambiguously decipher the exact physiological activities of these two proteins.

AP-1-mediated expression of brain-specific class IVa β-tubulin in P19 embryonal carcinoma cells

Expression of brain-specific phenotypes increased in all trans retinoic acid (ATRA)-induced neural differentiation of mouse P19 embryonal carcinoma cells. Among these phenotypes, expression of class IVa β-tubulin isotype (TUBB4a) was particularly enhanced in neural differentiation. Transient transfection assays employing a reporter construct found that ATRA-mediated regulatory region of the TUBB4a gene lay in the region from −83 nt to +137 nt relative to the +1 transcription start site. Site-directed mutagenesis in the AP-1 binding site at −29/−17 suggested that the AP-1 binding site was a critical region for ATRA-mediated TUBB4a expression. Chromatin immunoprecipitation experiments suggested participation of JunD and activating transcription factor-2 (ATF2) in TUBB4a expression. Additionally, exogenous induction of the dominant-negative (dn) type of JunD canceled ATRA-induced upregulation of TUBB4a, and the dn type of ATF2 suppressed even the basal activity. Further immunoblot study revealed an ATRA-mediated increase in JunD protein, while a significant amount of ATF2 protein was constantly produced. These results suggest that differentiation-mediated activation of JunD results in enhanced TUBB4a expression.

Chromatin remodeling and epigenetic regulation of the CrabpI gene in adipocyte differentiation

Retinoic acid (RA) acts by binding to nuclear RA receptors (RARs) to regulate a broad spectrum of downstream target genes in most cell types examined. In cytoplasm, RA binds specifically to cellular retinoic acid binding proteins I (CRABPI), and II. Although the function of CRABPI in animals remains the subject of debate, it is believed that CRABPI binding facilitates RA metabolism, thereby modulating the concentration of RA and the type of RA metabolites in cells. The basal promoter of the CrabpI gene is a housekeeping promoter that can be regulated by thyroid hormones (T3), DNA methylation, sphinganine, and ethanol acting on its upstream regulatory region. T3 regulation of CrabpI is mediated by the binding of thyroid hormone receptor (TR) to a TR response element (TRE) approximately 1 kb upstream of the basal promoter. Specifically, in the adipocyte differentiation process, T3 regulation is bimodal and closely associated with the cellular differentiation status: T3 activates CrabpI in predifferentiated cells (e.g., mesenchymal precursors or fibroblasts), but suppresses this gene once cells are committed to adipocyte differentiation. These disparate effects are functions of T3-triggered differential recruitment of coregulatory complexes in conjunction with chromatin looping/folding that alters the configuration of this genomic locus along adipocyte differentiation. Subsequent sliding, disassembly and reassembly of nucleosomes occur, resulting in specific changes in the conformation of the basal promoter chromatin at different stages of differentiation. This chapter summarizes studies illustrating the epigenetic regulation of CrabpI expression during adipocyte differentiation. Understanding the pathways regulating CrabpI in this specific context might help to illuminate the physiological role of CRABPI in vivo. This article is part of a special issue entitled: Retinoid and Lipid Metabolism.

RIP140 in thyroid hormone-repression and chromatin remodeling of Crabp1 gene during adipocyte differentiation

Cellular retinoic acid binding protein 1 (Crabp1) gene is biphasically (proliferation versus differentiation) regulated by thyroid hormone (T3) in 3T3-L1 cells. This study examines T3-repression of Crabp1 gene during adipocyte differentiation. T3 repression of Crabp1 requires receptor interacting protein 140 (RIP140). During differentiation, the juxtaposed chromatin configuration of Crabp1 promoter with its upstream region is maintained, but the 6-nucleosomes spanning thyroid hormone response element to transcription initiation site slide bi-directionally, with the third nucleosome remaining at the same position throughout differentiation. On the basal promoter, RIP140 replaces coactivators GRIP1 and PCAF and forms a repressive complex with CtBP1, HDAC3 and G9a. Initially active chromatin marks on this promoter, histone modifications H3-Ac and H3K4-me3, are weakened whereas repressive chromatin marks, H3K9-me3 and H3K27-me3 modification and recruitment of G9a, HP1α, HP1γ and H1, are intensified. This is the first study to examine chromatin remodeling, during the phase of hormone repression, of a bi-directionally regulated hormone target gene, and provides evidence for a functional role of RIP140 in chromatin remodeling to repress hormone target gene expression.

Expression of cellular retinoic acid-binding protein I is specific to neurons in adult transgenic mouse brain

Cellular retinoic acid binding protein I (CRABP-I) plays a role in retinoic acid (RA) metabolism or transport. This report shows specific neuronal expression of CRABP-I in adult transgenic mouse brain using CRABP-I promotor-driven lac-Z and neuron- and astrocyte-markers. Double staining indicates that CRABP-I is expressed in neurons and large cells (>12 microm) but to much lesser degree the astrocytes. CRABP-I-lac-Z(+) neurons were distributed throughout the brain, but in a very discreet pattern in each brain region. CRABP-I expression in specific populations of brain neurons suggests that RA is extensively metabolized in mature brains, mostly in neurons. Additionally, the genetic basis of its specific expression in these brain areas is located in the 5' regulatory region of this gene.

Transcriptional repression of the CTP:phosphocholine cytidylyltransferase gene by sphingosine

We examined the effects of the bioactive lipid, sphingosine, on the expression of the rate-limiting enzyme involved in surfactant phosphatidylcholine synthesis, CCTalpha (CTP:phosphocholine cytidylyltransferase alpha). Sphingosine decreased phosphatidylcholine synthesis by inhibiting CCT activity in primary alveolar type II epithelia. Sphingosine decreased CCTalpha protein and mRNA levels by approx. 50% compared with control. The bioactive lipid did not alter CCTalpha mRNA stability, but significantly inhibited its transcriptional rate. In murine lung epithelia, sphingosine selectively reduced CCTalpha promoter-reporter activity when transfected with a 2 kb CCTalpha promoter/luciferase gene construct. Sphingosine also decreased transgene expression in murine type II epithelia isolated from CCTalpha promoter-reporter transgenic mice harbouring this 2 kb proximal 5'-flanking sequence. Deletional analysis revealed that sphingosine responsiveness was mapped to a negative regulatory element contained within 814 bp upstream of the coding region. The results indicate that bioactive sphingolipid metabolites suppress surfactant lipid synthesis by inhibiting gene transcription of a key surfactant biosynthetic enzyme.

Signaling pathways in retinoid chemoprevention and treatment of cancer

The Vitamin A metabolite, retinoic acid, has been shown to have chemopreventive and therapeutic activity for certain cancers such as head and neck, cervical, neuroblastoma and promyelocytic leukemia. Retinoic acid achieves these activities by inducing differentiation and/or growth arrest. A large number of studies have investigated the mechanism(s) by which retinoic acid alters the behavior of premalignant and tumor cells. Although much important data has been obtained, the exact signaling pathways required for retinoic acid to exert its biological effects remains elusive. In this review, we outline the role and function of retinoid nuclear receptors, followed by a discussion of how major signaling pathways are affected in different tumor types by retinoids. We conclude by examining the effect of retinoic acid on G1 cell cycle regulatory proteins in various tumors.

Receptor interacting protein 140 as a thyroid hormone-dependent, negative co-regulator for the induction of cellular retinoic acid binding protein I gene

Over-expression of receptor interacting protein 140 (RIP140) suppressed thyroid hormone (T3) induction of cellular retinoic acid binding I protein (CRABPI) gene in P19 embryonal carcinoma cells. CRABPI induction by T3 is mediated by a direct-repeat four-element bound by T3 receptor (T3R) and retinoid receptor X (RXR). Three receptor-interacting domains (RIDs) in RIP140 mediate its interaction with T3R: one constitutive RID within the amino terminus, and two T3-dependent RIDs in the central portion and the carboxyl terminus. In co-immunoprecipitation and chromatin immunoprecipitation assays, RIP140 formed complexes with T3R/RXR in solution and on the endogenous target, the CRABPI promoter. T3 treatment resulted in elevated histone acetylation of the endogenous CRABPI gene promoter, but simultaneous expression of RIP140 resulted in significantly reduced histone acetylation of this promoter, primarily through the recruitment of HDAC4. This study presents the first evidence that over-expressed RIP140 acts as a T3-dependent negative co-regulator for T3 induction of the endogenous CRABPI gene in P19 cells.

Upregulation of cellular retinoic acid-binding protein I expression by ethanol

Acute and chronic ethanol ingestion cause embryopathy similar to that of hyper- or hypovitaminosis A. Experimental data have suggested interaction between vitamin A and alcohol signaling pathways at the level of metabolic interference, which ultimately affects the concentration of retinoic acid (RA) in animals. The present study was set up to examine the possible effects of alcohol on cellular RA binding protein I (CRABP-I) expression during embryonic development by using transgenic mouse embryos and P19 embryonal carcinoma cells as experimental models. It was found that expression of the mouse CRABP-I gene was elevated in developing embryos at mid-gestation stages as a result of ethanol consumption by the mothers. Specific elevation of this gene was detected in the limb bud and the gut. In the P19 model, the CRABP-I gene was directly upregulated by ethanol, which was not blocked by a protein synthesis inhibitor. Furthermore, the regulation of the CRABP-I gene by ethanol was mediated by the 5' upstream regulatory region of the CRABP-I gene promoter. A potential interaction of vitamin A and ethanol at the level of CRABP-I gene expression is discussed.

Characterization of a negative response DNA element in the upstream region of the cellular retinoic acid-binding protein-I gene of the mouse

A negative, regulatory DNA element from the mouse cellular retinoic acid-binding protein I gene promoter was identified. This DNA element, located approximately 1 kilobase upstream from the transcription initiation site of this gene, contained a pair of direct repeats (DRs) separated by 4 base pairs (DR4, TGACCTTTGGGGACCT). By examining a series of reporters deleted or mutated within this DR4 region, it was concluded that the core sequence of this DR4, including both repeats and the spacer, was required for suppressive activity in the mouse embryonal carcinoma cell line P19. From gel retardation experiments, it was concluded that both repeated sequences were essential for specific protein binding, but the spacer sequence was not as critical. Specific residues required for protein binding to this DR4 were identified. In P19 cells, retinoic acid induced the binding of nuclear factors to DR4 and suppressed the activities of the reporters containing this DR4. Co-expression of retinoic acid receptor beta or thyroid hormone receptor beta1 (T3Rbeta1) significantly inhibited the expression of this reporter in P19 cells. Gel retardation with in vitro-synthesized nuclear receptors demonstrated specific binding of this DR4 by T3Rbeta1 monomers, homodimers, or heterodimers of T3Rbeta1/retinoid receptor X beta. A biological function of DR4 in crabp-I gene regulation in P19 cells was suggested.

Molecular cloning and characterization of a mouse nuclear orphan receptor expressed in embryos and testes

A mouse cDNA encoding a putative DNA-binding protein of the zinc-finger type was isolated from an E8.5 mouse embryonic cDNA library. Sequence comparison revealed a high degree of homology between this mouse cDNA and the human and rat orphan receptor Tr2-11 isolated from prostate cDNA libraries. This transcript was detected in early-to-midgestation embryos and was seen to level off during later stages of development. In adult animals, a high level of expression was detected only in the testis, starting at postnatal day 18, a stage when active meiosis begins to occur. A specific antibody was raised, and immunoreactive signal was specifically located in the adlumenal compartment of the seminiferous tubule, where advanced germ cells reside. In mice fed a vitamin A-depleted diet, where the testes were depleted of advanced germ cells, expression of this protein could not be detected, suggesting a biological relation of this orphan receptor and male germ-cell differentiation. Using a retinoic acid response element (RARE)-containing reporter system, it was demonstrated that expression of this protein dramatically repressed both the basal and the retinoic acid (RA)-regulated promoter activities of this reporter. Thus, this orphan receptor could play a role in modulating both the basic transcription machinery and the RA signalling pathway during embryogenesis and male germ-cell differentiation.

Promoter and upstream regulatory activities of the mouse cellular retinoic acid-binding protein-I gene

The promoter and its upstream regulatory region of the mouse cellular retinoic acid-binding protein I (crabp-I) gene were examined in transgenic mouse embryos, a mouse embryonal carcinoma cell line P19, and a mouse embryonic fibroblast cell line 3T6. In transgenic mouse embryos, a beta-galactosidase reporter gene under the control of crabp-I promoter and its upstream regulatory region displayed a very specific pattern of expression characteristic of crabp-I gene expression during developmental stages. In tissue culture systems, the minimal promoter of this gene was identified, and regions containing positive and negative regulatory activities were dissected from the upstream 3-kilobase sequence using assays for transient reporter activity. It is concluded that the minimal promoter of the mouse crabp-I gene is located between 120 and 150 base pairs upstream from the transcription initiation site. Several cell type-specific positive and negative regulatory regions for this promoter have been identified. A region encoding a common negative regulatory activity in both P19 and 3T6 cells is also inhibitory to two heterologous promoters, and specific protein-DNA interactions between this DNA fragment and nuclear extracts of P19 and 3T6 are demonstrated by gel retardation experiments.