Recent advances in the mechanisms of action and physiological functions of the retinoid-related orphan receptors (RORs)

Activation of the Blue Opsin Gene in Cone Photoreceptor Development by Retinoid-Related Orphan Receptor β

Color vision requires the expression of opsin photopigments with different wavelength sensitivities in retinal cone photoreceptors. The basic color visual system of mammals is dichromatic, involving differential expression in the cone population of two opsins with sensitivity to short (S, blue) or medium (M, green) wavelengths. However, little is known of the factors that directly activate these opsin genes and thereby contribute to the S or M opsin identity of the cone. We report that the orphan nuclear receptor RORβ (retinoid-related orphan receptor β) activates the S opsin gene (Opn1sw) through binding sites upstream of the gene. RORβ lacks a known physiological ligand and activates the Opn1sw promoter modestly alone but strongly in synergy with the retinal cone-rod homeobox factor (CRX), suggesting a cooperative means of enhancing RORβ activity. Comparison of wild-type and mutant lacZ reporter transgenes showed that the RORβ-binding sites in Opn1sw are required for expression in mouse retina. RORβ-deficient mice fail to induce S opsin appropriately during postnatal cone development. Photoreceptors in these mice also lack outer segments, indicating additional functions for RORβ in photoreceptor morphological maturation. The results identify Opn1sw as a target gene for RORβ and suggest a key role for RORβ in regulating opsin expression in the color visual system.

NABP1, a novel RORgamma-regulated gene encoding a single-stranded nucleic-acid-binding protein

RORgamma2 (retinoid-related orphan receptor gamma2) plays a critical role in the regulation of thymopoiesis. Microarray analysis was performed in order to uncover differences in gene expression between thymocytes of wild-type and RORgamma-/- mice. This analysis identified a novel gene encoding a 22 kDa protein, referred to as NABP1 (nucleic-acid-binding protein 1). This subsequently led to the identification of an additional protein, closely related to NABP1, designated NABP2. Both proteins contain an OB (oligonucleotide/oligosaccharide binding) motif at their N-terminus. This motif is highly conserved between the two proteins. NABP1 is highly expressed in the thymus of wild-type mice and is greatly suppressed in RORgamma-/- mice. During thymopoiesis, NABP1 mRNA expression is restricted to CD4+CD8+ thymocytes, an expression pattern similar to that observed for RORgamma2. These observations appear to suggest that NABP1 expression is regulated either directly or indirectly by RORgamma2. Confocal microscopic analysis showed that the NABP1 protein localizes to the nucleus. Analysis of nuclear proteins by size-exclusion chromatography indicated that NABP1 is part of a high molecular-mass protein complex. Since the OB-fold is frequently involved in the recognition of nucleic acids, the interaction of NABP1 with various nucleic acids was examined. Our results demonstrate that NABP1 binds single-stranded nucleic acids, but not double-stranded DNA, suggesting that it functions as a single-stranded nucleic acid binding protein.

Plasticity in the expression of the steroid receptor coactivator 1 in the Japanese quail brain: effect of sex, testosterone, stress and time of the day

Analysis of nuclear receptor action on the eukaryotic genome highlights the importance of coactivators on gene transcription. The steroid receptor coactivator-1 in particular is the focus of an intense research and physiological or behavioral studies have confirmed that it plays a major role in the modulation of steroid and thyroid receptors activity. However, little is known about the regulation of steroid receptor coactivator-1 expression the brain. The goal of this study was to determine the potential factors modulating steroid receptor coactivator-1 synthesis in Japanese quail by quantification of its mRNA with real time quantitative polymerase chain reaction and of the corresponding protein via Western blotting. Contrary to previously published results from our laboratory [Charlier TD, Lakaye B, Ball GF, Balthazart J (2002) The steroid receptor coactivator SRC-1 exhibits high expression in steroid-sensitive brain areas regulating reproductive behaviors in the quail brain. Neuroendocrinology 76:297-315], we found here that sexually mature females had a higher concentration of steroid receptor coactivator-1 in the preoptic area/hypothalamus compared with males. Steroid receptor coactivator-1 expression in the male preoptic area/hypothalamus was up-regulated by testosterone and tended to be decreased by stress. We also identified a significant correlation between the time of the day and the expression of the coactivator in the optic lobes, hippocampus, telencephalon and hindbrain but the pattern of changes in expression as a function of the time of the day varied from one brain area to another. Together, these data support the idea that steroid receptor coactivator-1 is not constitutively expressed but rather is finely regulated by steroids, stress and possibly other unidentified factors.

Lactoferrin interaction with retinoid signaling: cell growth and apoptosis in mammary cells

Lactoferrin (Lf) is a multifunctional iron-binding protein that was first identified in mammary secretions, but is synthesized by most mammalian tissues. The protein has a signal sequence that dictates secretion; it also has a nuclear localization sequence that facilitates entry into the cell nucleus. The mechanism of the latter action is currently unknown, but is thought to occur via a Lf receptor. Lactoferrin content of mammary tissue and secretions varies with developmental state; it is synthesized in mammary tissue at high levels during both pregnancy and involution, and during mammary infections. Using fluorescent (FITC)-labeled holo-bLf, we show that bovine primary epithelial cells and MCF-7 breast cancer cells do not translocate the exogenously added Lf to the nucleus after culture in serum free media (SFM). However, the supplementation of SFM with 1microM all-trans retinoic acid (atRA) caused breast cancer cells to gain the capacity to take up labeled bLf into the cell nucleus. Primary bovine mammary cells (MeBo) exhibited similar capacity in culture. This suggests that in addition to Lf, one or more components modulated by atRA, are necessary for nuclear translocation to occur. Transfection experiments with atRA treated MCF-7 cells containing retinoic acid response element reporter constructs showed that the extracellular application of lactoferrin alters reporter gene expression. Lactoferrin increased a DR5 luciferase response element in a dose-dependent manner only when atRA was applied. Immunocytochemical markers for the cell cycle (Ki67) and apoptotic events (Caspase-3 and PARP-85) showed that lactoferrin alters the atRA-induced phenotype, blocking apoptosis and maintaining cell cycle activity in both MCF-7 and MeBo cells in the presence of 1muM atRA. We propose that nuclear lactoferrin interacts with retinoic acid signaling pathways in cells and alters/blocks the signals so that cells remain in the cell cycle and/or do not enter the apoptotic pathway.

RORA, a large common fragile site gene, is involved in cellular stress response

Common fragile sites (CFSs) are large genomic regions present in all individuals that are highly unstable and prone to breakage and rearrangement, especially in cancer cells with genomic instability. Eight of the 90 known CFSs have been precisely defined and five of these span genes that extend from 700 kb to over 1.5 Mb of genomic sequence. Although these genes reside within some of the most unstable chromosomal regions in the human genome, they are highly conserved evolutionarily. These genes are targets for large chromosomal deletions and rearrangements in cancer and are frequently inactivated in multiple tumor types. There is also an association between these genes and cellular responses to stress. Based upon the association between large genes and CFSs, we began to systematically test other large genes derived from chromosomal regions that were known to contain a CFS. In this study, we demonstrate that the 730 kb retinoic acid receptor-related orphan receptor alpha (RORA) gene is derived from the middle of the FRA15A (15q22.2) CFS. Although this gene is expressed in normal breast, prostate and ovarian epithelium, it is frequently inactivated in cancers that arise from these organs. RORA was previously shown to be involved in the cellular response to hypoxia and here we demonstrate changes in the amount of RORA message produced in cells exposed to a variety of different cellular stresses. Our results demonstrate that RORA is another very large CFS gene that is inactivated in multiple tumors. In addition, RORA appears to play a critical role in responses to cellular stress, lending further support to the idea that the large CFS genes function as part of a highly conserved stress response network that is uniquely susceptible to genomic instability in cancer cells.

Retinoid-related Orphan Receptors (RORs): Roles in Cellular Differentiation and Development

Retinoid-related orphan receptors RORalpha, -beta, and -gamma are transcription factors belonging to the steroid hormone receptor superfamily. During embryonic development RORs are expressed in a spatial and temporal manner and are critical in the regulation of cellular differentiation and the development of several tissues. RORalpha plays a key role in the development of the cerebellum particularly in the regulation of the maturation and survival of Purkinje cells. In RORalpha-deficient mice, the reduced production of sonic hedgehog by these cells appears to be the major cause of the decreased proliferation of granule cell precursors and the observed cerebellar atrophy. RORalpha has been implicated in the regulation of a number of other physiological processes, including bone formation. RORbeta expression is largely restricted to several regions of the brain, the retina, and pineal gland. Mice deficient in RORbeta develop retinal degeneration that results in blindness. RORgamma is essential for lymph node organogenesis. In the intestine RORgamma is required for the formation of several other lymphoid tissues: Peyer's patches, cryptopatches, and isolated lymphoid follicles. RORgamma plays a key role in the generation of lymphoid tissue inducer (LTi) cells that are essential for the development of these lymphoid tissues. In addition, RORgamma is a critical regulator of thymopoiesis. It controls the differentiation of immature single-positive thymocytes into double-positive thymocytes and promotes the survival of double-positive thymocytes by inducing the expression of the anti-apoptotic gene Bcl-X(L). Interestingly, all three ROR receptors appear to play a role in the control of circadian rhythms. RORalpha positively regulates the expression of Bmal1, a transcription factor that is critical in the control of the circadian clock. This review intends to provide an overview of the current status of the functions RORs have in these biological processes.

Enhanced susceptibility of staggerer (RORαsg/sg) mice to lipopolysaccharide-induced lung inflammation

The retinoid-related orphan receptor α (RORα), a member of the ROR subfamily of nuclear receptors, has been implicated in the control of a number of physiological processes, including the regulation of several immune functions. To study the potential role of RORα in the regulation of innate immune responses in vivo, we analyzed the induction of airway inflammation in response to lipopolysaccharide (LPS) challenge in wild-type and staggerer (RORαsg/sg) mice, a natural mutant strain lacking RORα expression. Examination of hematoxylin and eosin-stained lung sections showed that RORαsg/sg mice displayed a higher degree of LPS-induced inflammation than wild-type mice. Bronchoalveolar lavage (BAL) was performed at 3, 16, and 24 h after LPS exposure to monitor the increase in inflammatory cells and the level of several cytokines/chemokines. The increased susceptibility of RORαsg/sg mice to LPS-induced airway inflammation correlated with a higher number of total cells and neutrophils in BAL fluids from LPS-treated RORαsg/sg mice compared with those from LPS-treated wild-type mice. In addition, IL-1β, IL-6, and macrophage inflammatory protein-2 were appreciably more elevated in BAL fluids from LPS-treated RORαsg/sg mice compared with those from LPS-treated wild-type mice. The enhanced susceptibility of RORαsg/sg mice appeared not to be due to a repression of IκBα expression. Our observations indicate that RORαsg/sg mice are more susceptible to LPS-induced airway inflammation and are in agreement with the hypothesis that RORα functions as a negative regulator of LPS-induced inflammatory responses.