Identification of a second human retinoic acid receptor

We have previously described a human complementary DNA that encodes a novel protein which is homologous to members of the steroid/thyroid nuclear receptor multigene family. This novel protein (hap for hepatoma) exhibits strong homology with the human retinoic acid receptor (RAR) which has been recently characterized. To test the possibility that the hap protein might also be a retinoid receptor, a chimaeric receptor was created by replacing the putative DNA binding domain of hap with that of the human oestrogen receptor (ER). The resulting hap-ER chimaera was then tested for its ability to trans-activate an oestrogen-responsive reporter gene (vit-tk-CAT) in the presence of possible receptor ligands. Here we show that retinoic acid (RA) at physiological concentrations is effective in inducing the expression of this reporter gene by the hap-ER chimaeric receptor. This demonstrates the existence of two human retinoic acid receptors designated RAR-alpha and RAR-beta.     

9-cis retinoic acid stereoisomer binds and activates the nuclear receptor RXR alpha.

Vitamin A (retinol) and its natural derivatives are required for many physiological processes. The activity of retinoids is thought to be mediated by interactions with two subfamilies of nuclear retinoic acid receptors, RAR and RXR. The RARs bind all-trans retinoic acid (t-RA) with high affinity and alter gene expression as a consequence of this direct ligand interaction. RXR alpha is activated by t-RA, yet has little binding affinity for this ligand. t-RA may be converted to a more proximate ligand that directly binds and activates RXR alpha, and we have developed a method of nuclear receptor-dependent ligand trapping to test this hypothesis. Here we report the identification of a stereoisomer of retinoic acid, 9-cis retinoic acid, which directly binds and activates RXR alpha. These results suggest a new role for isomerization in the physiology of natural retinoids.     

A new retinoic acid receptor identified from a hepatocellular carcinoma

Processes as diverse as growth, vision and reproduction depend on the presence of vitamin A and its metabolites (retinoids), but the molecular mechanisms which govern these diverse actions remain unclear (for reviews see refs 1,2). A crucial advance recently was the isolation of a specific nuclear receptor for retinoic acid, one of the physiologically active vitamin A derivatives. This nuclear receptor is a member of the steroid/thyroid hormone receptor family. Our analysis of an uncharacterized member of this class of intracellular receptors, encoded by a complementary DNA clone from a human placental library, has led us to discover a second retinoic acid receptor. This new receptor is expressed at high levels in a number of epithelial-type tissues. The gene for the receptor was first identified in a hepatocellular carcinoma where it surrounds a site of integration of hepatitis B virus. Activation by this virus may play a role in tumour development in liver cells, where it is normally not expressed.     

四株肺癌细胞系的维甲酸受体表达的研究

目的：观察来源于不同组织学类型的人肺癌细胞系（肺巨细胞癌细胞系PLA-801,肺鳞状上皮细胞癌细胞系C-57,个旧肺腺癌细胞系GLC-82和肺腺癌细胞系PC-84045）的维甲酸受体的含量和分布特征。方法：作ELISA和免疫组化显色计算机图象分析方法,对4株具有不同侵袭潜能,不同组织学类型的肺癌细胞系的维甲酸受体（RARs、RAR和RXRs）进行检测。结果：4株细胞系的RARβ的表达极少,甚至缺如;RARs主要分布在癌细胞胞浆中,但RXRs主要位于细胞核内,表达水平较高;RARs和RXRs在PLA-801细胞中表达最低。结论：肺癌中普遍存在着RAR的表达分布异常,这些改变可能与肺癌的异常分化有关,亦可能是肺癌细胞对维甲酸诱导分化不敏感的重要原因。RXRs较高水平的表达且主要位于核内,提示用9-顺式维甲酸来诱导肺癌的分化效果可能会较好。     

A family of candidate taste receptors in human and mouse

The gustatory system of mammals can sense four basic taste qualities, bitter, sweet, salty and sour, as well as umami, the taste of glutamate. Previous studies suggested that the detection of bitter and sweet tastants by taste receptor cells in the mouth is likely to involve G-protein-coupled receptors. Although two putative G-protein-coupled bitter/sweet taste receptors have been identified, the chemical diversity of bitter and sweet compounds leads one to expect that there is a larger number of different receptors. Here we report the identification of a family of candidate taste receptors (the TRBs) that are members of the G-protein-coupled receptor superfamily and that are specifically expressed by taste receptor cells. A cluster of genes encoding human TRBs is located adjacent to a Prp gene locus, which in mouse is tightly linked to the SOA genetic locus that is involved in detecting the bitter compound sucrose octaacetate. Another TRB gene is found on a human contig assigned to chromosome 5p15, the location of a genetic locus (PROP) that controls the detection of the bitter compound 6-n-propyl-2-thiouracil in humans.     

Identification of a novel retinoic acid receptor in regenerative tissues of the newt

In urodele amphibians, the progenitor cells that regenerate amputated limbs (known as the blastema) normally replace only the missing structures. After systemic delivery of retinoic acid (RA), more proximal structures are also formed, indicating that RA can control position specification in the proximal-distal axis of the regenerating limb. According to dose and experimental context, retinoids can also re-specify the anteroposterior axis of the limb, induce deletions of skeletal elements, or block re-growth completely. To study the molecular basis of these morphogenetic effects, we screened complementary DNA libraries of newt regenerative tissues (limbs and tails) for hormone nuclear receptors activated by RA. Two functional retinoic acid receptors (RARs) were identified, one of which is the newt homologue of the human alpha-receptor (RAR alpha). The second receptor, called RAR delta, is novel. Sequence analysis suggests that the composite newt RAR previously reported is chimaeric, consisting of 5'RAR-beta-like and 3' RAR delta clones. We conclude that multiple RARs are expressed during limb regeneration in amphibians and suggest that receptor heterogeneity may underlie the different effects of retinoids on limb morphogenesis.     

Spatial and temporal expression of the retinoic acid receptor in the regenerating amphibian limb

Retinoic acid is known to have dramatic effects on vertebrate limb pattern in development and regeneration, supporting a model in which a gradient of retinoic acid serves as a morphogen to differentially supply positional information to a developing limb. The discovery of a retinoic acid receptor (RAR) and its homology to the steroid and thyroid hormone receptors provided a potential molecular mechanism for limb morphogenesis. One prediction of this model is that the receptor must be expressed in the developing and regenerating limb anlage. We investigated the expression of the RAR in the adult newt, Notophthalmus viridescens, whose amputated limbs are capable of regenerating and upon which retinoic acid can act to alter pattern. We report the cloning of cDNAs encoding a functional newt RAR and the localization of high and uniform levels of RAR mRNA specifically in the regenerating cells that control limb pattern. These results indicate that the morphogenic field is established through differential activation of pre-existing retinoic acid receptors rather than differential expression of the RAR gene.     

Excitatory glycine receptors containing the NR3 family of NMDA receptor subunits

The N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) serves critical functions in physiological and pathological processes in the central nervous system, including neuronal development, plasticity and neurodegeneration. Conventional heteromeric NMDARs composed of NR1 and NR2A-D subunits require dual agonists, glutamate and glycine, for activation. They are also highly permeable to Ca2+, and exhibit voltage-dependent inhibition by Mg2+. Coexpression of NR3A with NR1 and NR2 subunits modulates NMDAR activity. Here we report the cloning and characterization of the final member of the NMDAR family, NR3B, which shares high sequence homology with NR3A. From in situ and immunocytochemical analyses, NR3B is expressed predominantly in motor neurons, whereas NR3A is more widely distributed. Remarkably, when co-expressed in Xenopus oocytes, NR3A or NR3B co-assembles with NR1 to form excitatory glycine receptors that are unaffected by glutamate or NMDA, and inhibited by D-serine, a co-activator of conventional NMDARs. Moreover, NR1/NR3A or -3B receptors form relatively Ca2+-impermeable cation channels that are resistant to Mg2+, MK-801, memantine and competitive antagonists. In cerebrocortical neurons containing NR3 family members, glycine triggers a burst of firing, and membrane patches manifest glycine-responsive single channels that are suppressible by D-serine. By itself, glycine is normally thought of as an inhibitory neurotransmitter. In contrast, these NR1/NR3A or -3B 'NMDARs' constitute a type of excitatory glycine receptor.     

Homodimer formation of retinoid X receptor induced by 9-cis retinoic acid.

Retinoid response pathways are mediated by two classes of receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). A central question is whether distinct response pathways are regulated by these two classes of receptors. The observation that the stereoisomer 9-cis-retinoic acid binds with high affinity to RXRs suggested that this retinoid has a distinct role in controlling RXR activity, but it was almost simultaneously discovered that RXRs function as auxiliary receptors for RARs and related receptors, and are essential for DNA binding and function of those receptors. Hence, although RARs seem to operate effectively only as heterodimeric RAR/RXR complexes, RXRs themselves apparently function predominantly, if not exclusively, as auxiliary receptors. Here we report that 9-cis-retinoic acid induces RXR homodimer formation. Our results demonstrate a new mechanism for retinoid action by which a ligand-induced homodimer mediates a distinct retinoid response pathway.     

A novel population of T-cell receptor alpha beta-bearing thymocytes which predominantly expresses a single V beta gene family

Recent studies have demonstrated that CD3 is expressed on a subset of thymocytes with a CD4-CD8- (double negative) phenotype. At least some of these cells bear the CD3-associated gamma delta T-cell receptor (TCR gamma delta). Here we describe a second subset of double negative thymocytes which expresses CD3-associated alpha beta receptors (TCR alpha beta). Surprisingly, these cells express predominantly the products of a single V beta gene family (V beta 8). These CD4-CD8-, TCR alpha beta+ cells appear relatively late in ontogeny (between birth and day 5 of life) and thus are unlikely to be the precursors to the TCR alpha beta-bearing cells (CD4+CD8- and CD4-CD8+) already present at birth. They can be selectively expanded in vitro by stimulation with a monoclonal antibody to V beta 8 (F23.1) in the presence of interleukin I (IL-1). We propose that this cell type is a unique T-cell population distinguishable from typical TCR alpha beta+ T cells by its CD4-CD8- phenotype and a restricted TCR V beta repertoire. Analysis of the unique phenotype of these cells suggests that they may represent the normal counterpart of the defective CD4-CD8- T cells found in the lpr autoimmune mouse.     

对"实位移是虚位移中的一个"传统提法的质疑

国内几乎所有理论力学教材和相关教材，在对虚位移的论述中，基本上认为“实位移是虚位移中的一个”．对此提出了疑问，认为“实位移不是虚位移中的一个”，并且在此前提下，对虚位移原理的充分性给予了证明。     

Convergence of 9-cis retinoic acid and peroxisome proliferator signalling pathways through heterodimer formation of their receptors.

Peroxisomes are cytoplasmic organelles which are important in mammals in modulation of lipid homeostasis, including the metabolism of long-chain fatty acids and conversion of cholesterol to bile salts (reviewed in refs 1 and 2). Amphipathic carboxylates such as clofibric acid have been used in man as hypolipidaemic agents and in rodents they stimulate the proliferation of peroxisomes. These agents, termed peroxisome proliferators, and all-trans retinoic acid activate genes involved in peroxisomal-mediated beta-oxidation of fatty acids. Here we show that the receptor activated by peroxisome proliferators and the retinoid X receptor-alpha (ref. 6) form a heterodimer that activates acyl-CoA oxidase gene expression in response to either clofibric acid or the retinoid X receptor-alpha ligand, 9-cis retinoic acid, an all-trans retinoic acid metabolite; simultaneous exposure to both activators results in a synergistic induction of gene expression. These data demonstrate the coupling of the peroxisome proliferator and retinoid signalling pathways and provide evidence for a physiological role for 9-cis retinoic acid in modulating lipid metabolism.