The carboxy-terminal hydrophobic domain of TIG3, a class II tumor suppressor protein, is required for appropriate cellular localization and optimal biological activity

TIG3 is a recently discovered class II tumor suppressor protein, originally isolated from retinoid-treated cultured epidermal keratinocytes, that suppresses the proliferation of a variety of epithelial cell types. In the present study, we examine the ability of this protein to reduce CHO, T47D and HaCaT cell proliferation, and the role of the carboxy-terminal hydrophobic domain in this regulation. Vector-mediated expression of the full length TIG3 protein, TIG31-164, results in a 50-70% reduction colony formation efficiency. Expression of a truncated mutant, TIG31-134, that lacks the putative carboxy-terminal membrane-anchoring domain, results in a partial loss of ability to suppress colony formation. The fact that the truncated protein remains partially active suggests that both the amino- and carboxy-terminal regions of TIG3 are required for optimal growth suppression. The full-length protein is distributed in a perinuclear location, and is not present in the nucleus. TIG31-134, in contrast, is distributed in the cytoplasm. Thus, a change in location is associated with the partial loss of activity. We also monitored the distribution of green fluorescent protein (GFP)-TIG3 fusion proteins. GFP-TIG31-164 was localized in a pattern similar to that observed for TIG31-164, while GFP-TIG31-134 displayed a distribution pattern similar to GFP. This suggests that the c-terminal hydrophobic domain has an important role in determining the intracellular localization of TIG3. In addition, GFP-TIG31-164 retains the ability to inhibit cell function, while GFP-TIG31-134 is inactive.

Retinoid-related orphan receptor gamma (RORgamma) is essential for lymphoid organogenesis and controls apoptosis during thymopoiesis

To identify the physiological functions of the retinoid-related orphan receptor gamma (RORgamma), a member of the nuclear receptor superfamily, mice deficient in RORgamma function were generated by targeted disruption. RORgamma(-/-) mice lack peripheral and mesenteric lymph nodes and Peyer's patches, indicating that RORgamma expression is indispensable for lymph node organogenesis. Although the spleen is enlarged, its architecture is normal. The number of peripheral blood CD3(+) and CD4(+) lymphocytes is reduced 6- and 10-fold, respectively, whereas the number of circulating B cells is normal. The thymus of RORgamma(-/-) mice contains 74.4% +/- 8.9% fewer thymocytes than that of wild-type mice. Flow cytometric analysis showed a decrease in the CD4(+)CD8(+) subpopulation. Terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL) staining demonstrated a 4-fold increase in apoptotic cells in the cortex of the thymus of RORgamma(-/-) mice. The latter was supported by the observed increase in annexin V-positive cells. RORgamma(-/-) thymocytes placed in culture exhibit a dramatic increase in the rate of "spontaneous" apoptosis. This increase is largely associated with CD4(+)CD8(+) thymocytes and may, at least in part, be related to the greatly reduced level of expression of the anti-apoptotic gene Bcl-X(L). Flow cytometric analysis demonstrated a 6-fold rise in the percentage of cells in the S phase of the cell cycle among thymocytes from RORgamma(-/-) mice. Our observations indicate that RORgamma is essential for lymphoid organogenesis and plays an important regulatory role in thymopoiesis. Our findings support a model in which RORgamma negatively controls apoptosis in thymocytes.

Expression of a retinoid-inducible tumor suppressor, Tazarotene-inducible gene-3, is decreased in psoriasis and skin cancer

Tazarotene-induced gene-3 (TIG-3), isolated from human keratinocytes treated with the retinoic acid receptor-selective retinoid Tazarotene, is homologous to H-rev, a class II tumor suppressor. TIG-3 gene localized to chromosome 11q23, a site of loss of heterozygosity in several malignancies. Retinoids influence epidermal differentiation and are used to treat and prevent skin cancer. Therefore, we studied TIG-3 mRNA expression in psoriasis and in basal and SCCs by in situ hybridization and a quantitative QT-RT-PCR assay. Psoriasis lesions had significantly lower staining (median, 3) than paired normal control skin (median, 4; P = 0.012). TIG-3 mRNA was significantly higher in normal control skin (P = 0.001), in paired adjacent skin (median, 3; P = 0.007), and in overlying epidermis (median, 3.0; P = 0.0001) than in 21 SCC specimens as a group (median, 1.5).

Requirement for RORgamma in thymocyte survival and lymphoid organ development

Most developing thymocytes undergo apoptosis because they cannot interact productively with molecules encoded by the major histocompatibility complex. Here, we show that mice lacking the orphan nuclear hormone receptor RORgamma lose thymic expression of the anti-apoptotic factor Bcl-xL. RORgamma thus regulates the survival of CD4+8+ thymocytes and may control the temporal window during which thymocytes can undergo positive selection. RORgamma was also required for development of lymph nodes and Peyer's patches, but not splenic follicles. In its absence, there was loss of a population of CD3-CD4+CD45+ cells that normally express RORgamma and that are likely early progenitors of lymphoid organs. Hence, RORgamma has critical functions in T cell repertoire selection and lymphoid organogenesis.

Down-regulation of the orphan nuclear receptor ROR gamma t is essential for T lymphocyte maturation

Thymocyte development is a tightly regulated process. CD4+CD8+ double-positive (DP) immature thymocytes exhibit distinct phenotypic features from mature T cells; they express only 10% of surface TCR that are found on mature T cells and do not proliferate and produce IL-2 in response to stimulation. In this report we show that transgenic expression of the orphan nuclear receptor ROR gamma t in mature T cells down-regulates their surface TCR expression. The ROR gamma t transgene inhibits IL-2 production by mature T cells, and this inhibition may be partially due to the inhibitory effect of ROR gamma t on c-Rel transcription. Furthermore, ectopic expression of ROR gamma t inhibits the proliferation of mature and immature T cells. These results, together with its predominant expression in DP thymocytes, suggest that ROR gamma t controls these distinct phenotypic features of DP thymocytes. Our data suggest that down-regulation of ROR gamma t expression in thymocytes is essential for their maturation.

Evidence for the presence of peroxisome proliferator-activated receptor (PPAR) alpha and gamma and retinoid Z receptor in cartilage. PPARgamma activation modulates the effects of interleukin-1beta on rat chondrocytes

Peroxisome proliferator-activated receptor (PPAR) alpha, PPARgamma, and retinoid acid receptor-related orphan receptor (ROR) alpha are members of the nuclear receptor superfamily of ligand-activated transcription factors. Although they play a key role in adipocyte differentiation, lipid metabolism, or glucose homeostasis regulation, recent studies suggested that they might be involved in the inflammation control and especially in the modulation of the cytokine production. This strongly suggests that these transcriptional factors could modulate the deleterious effects of interleukin-1 (IL-1) on cartilage. However, to date, their presence in cartilage has never been investigated. By quantitative reverse transcription-polymerase chain reaction, Western blot, and immunocytochemistry analysis, we demonstrated, for the first time, the presence of PPARalpha, PPARgamma, and RORalpha in rat cartilage, at both mRNA and protein levels. Comparatively, the PPARalpha mRNA content in cartilage was much lower than in the liver but not significantly different to that of the adipose tissue. PPARgamma mRNA expression in cartilage was weak, when compared with adipose tissue, but similar to that found in the liver. RORalpha mRNA levels were similar in the three tissues. mRNA expression of the three nuclear receptors was very differently modulated by IL-1 or mono-iodoacetate treatments. This indicates that they should be unequally involved in the effects of IL-1 on chondrocyte, which is in accordance with results obtained in other cell types. Indeed, we showed that 15d-PGJ2 mainly, but also the drug troglitazone, that are ligands of PPARgamma could significantly counteract the decrease in proteoglycan synthesis and NO production induced by IL-1. By contrast, PPARalpha ligands such as Wy-14,643 or clofibrate had no effect on this process. Therefore, the presence of PPARgamma in chondrocytes opens up new perspectives to modulate the effects of cytokines on cartilage by the use of specific ligands. The function of the two other transcription factors, PPARalpha and RORalpha identified in chondrocytes remains to be explored.

Transcription therapy for acute promyelocytic leukaemia

Transcription factors are proteins that regulate gene transcription and expression. In many cases of acute leukaemia chromosomal aberrations are translocations of transcription factors which change their expression and induce the leukaemic phenotype. These abnormal transcription factors are tumour-specific and can be targets for novel treatments approaches. Acute promyelocytic leukaemia (APL) is a distinct and unique subtype of acute myeloid leukaemia (AML) characterised by a reciprocal translocation between chromosomes 15 and 17 t(15q22;17q21). The breakpoints of chromosome 15 and 17 are in the PML and RARalpha genes, respectively, forming the fusion PML-RARalpha gene expressed exclusively and universally in APL. The normal RARalpha is an all-trans retinoic acid- (ATRA-)dependent transcription factor involved in the normal differentiation of myeloid cells. The aberrant fusion PML-RARalpha protein remains sensitive to ATRA and underlies the pathogenesis of the APL. ATRA modulation of gene transcription mediated by PML-RARalpha results in a major clinical response. Almost all newly diagnosed APL cases can be induced into complete remission with ATRA with or without chemotherapy by in vivo differentiation of the APL cells. Randomised clinical trials have shown that the most significant effect of ATRA is an additive or synergistic activity with chemotherapy to improve the long-term outcome of the disease. On the other hand, ATRA with or without induction chemotherapy did not increase the complete remission rate compared to chemotherapy alone. In addition, the relapse rate was significantly lower for patients randomised to induction with concurrent ATRA/chemotherapy than with ATRA followed by chemotherapy. Chemotherapy and/or ATRA maintenance may further improve the long-term outcome compared to no maintenance. PML-RARalpha fusion transcripts can be assayed by RT-PCR to identify PCR positive cells during remission, which are highly predictable of a subsequent haematological relapse. The goal of therapy has been modified to induce a molecular remission with a negative PCR to the PML-RARalpha transcript. This is the first example of an effective response to treatment with a ligand binding to a mutated form of its natural transcription factor. The transcription factor mutation, caused by translocation to another gene, underlies the pathogenesis of the disease.

Cloning and characterization of a novel retinoid-inducible gene 1(RIG1) deriving from human gastric cancer cells

Retinoids exert wide-spectrum anti-tumor activities, which are mediated via the induction of growth arrest, differentiation or apoptosis. To determine whether the effects of retinoids are mediated by specific gene activation or repression, SC-M1 CL23 gastric cancer cells, pretreated with either vehicle alone or all-trans retinoic acid (10 microM) for 1 day, were analyzed using the technique of differential display. A novel retinoid-inducible gene 1 (RIG1) was isolated. The full-length RIG1 cDNA contained 768 base pairs and encoded a protein of 164 amino acids with a molecular weight of 18 kDa. The RIG1 gene was ubiquitously expressed in normal tissue, and its expression was positively associated with cellular density. Nucleotide sequence analysis demonstrated that the RIG1 gene was similar to a recently-isolated TIG3 gene, and displayed 54% nucleotide sequence homology with a type II tumor suppressor gene H-REV-107-1. RIG1 cDNA, however, contained an extra 32 base pairs located at its 5' end and revealed three base pair differences for the remaining sequences leading to two amino acids substitution between the two encoded proteins. All-trans retinoic acid increased the level of RIG1 mRNA in a time- and concentration-dependent manner in SC-M1 CL23 gastric cancer cells. This was not observed for the H-REV-107-1 gene. The RIG1 regulation was related to cellular retinoid sensitivity. Both retinoic acid receptor alpha- and retinoic acid receptor gamma-selective agonists increased RIG1 mRNA level, and the retinoid x receptor-selective agonist potentiated this regulation. In conclusion, the cDNA of a novel retinoid-inducible gene RIG1 has been cloned. This gene is regulated by retinoic acid through the heterodimer of retinoic acid receptor and retinoid x receptor.

RORgammaT, a thymus-specific isoform of the orphan nuclear receptor RORgamma / TOR, is up-regulated by signaling through the pre-T cell receptor and binds to the TEA promoter

TEA (T early alpha) is a genetic element located upstream of the TCR-Jalpha cluster. Thymocytes from mice carrying a targeted deletion of TEA do not rearrange their TCRalpha locus on a window spanning the first nine Jalpha segments. This led us to the hypothesis of TEA having a "rearrangement focusing" activity on the 5' side of the TCR-Jalpha region. We analyzed DNAseI and "phylogenetic" footprints within the TEA promoter in an attempt to identify trans-acting factors that could account for its regulatory function on DNA accessibility. One of these footprints corresponded to a putative DNA-binding site for an orphan nuclear receptor of the ROR / RZR family. The RORgammaT cDNA clone was isolated from a thymus library using a probe corresponding to the DNA-binding domain of RORgamma / TOR. RORgammaT is a thymus-specific isoform of RORgamma, expressed almost exclusively in immature double-positive thymocytes. RORgammaT binds, to the TEA promoter in vitro. Lastly, the expression of RORgammaT is stimulated in two situations that mimic activation through the pre-TCR and in which the thymocytes have their TCR-alpha locus in an "open", yet unrearranged DNA configuration. We propose that the expression of RORgammaT may be part of the pre-TCR activation cascade leading to the maturation of alpha / beta T cells and may participate in the regulation of DNA accessibility in the TCR-Jalpha locus.

Melatonin induces gamma-glutamylcysteine synthetase mediated by activator protein-1 in human vascular endothelial cells

In the present study, we show that melatonin induces the expression of gamma-glutamylcysteine synthetase (gamma-GCS), the rate-limiting enzyme of glutathione (GSH) synthesis, in ECV304 human vascular endothelial cells. One micromolar melatonin induced the expression of gamma-GCS mRNA followed by an increase in the concentration of GSH with a peak at 24 h. An electrophoretic mobility shift assay showed that melatonin stimulates the DNA-binding activity of activator protein-1 (AP-1) as well as retinoid Z receptor/retinoid receptor-related orphan receptor alpha (RZR/RORalpha). ECV304 cells transiently transfected with a plasmid containing the gamma-GCS promoter-luciferase construct showed increased luciferase activity when treated with melatonin. The melatonin-dependent luciferase activity was found in the gamma-GCS promoter containing AP-1 site. The luciferase activity mediated by AP-1 was repressed in the promoter containing RZR/RORalpha site. In addition, cell cycle analysis showed that melatonin increases the number of cells in the G0/G1 phase; however, treatment of the cells with buthionine sulfoximine, a specific inhibitor of gamma-GCS, abolished the effect of melatonin on the cell cycle, suggesting induction of cell arrest by melatonin requires GSH. As conclusion, induction of GSH synthesis by melatonin protects cells against oxidative stress and regulates cell proliferation.

Identification and characterization of a retinoid-induced class II tumor suppressor/growth regulatory gene

Retinoids, synthetic and natural analogs of retinoic acid, exhibit potent growth inhibitory and cell differentiation activities that account for their beneficial effects in treating hyperproliferative diseases such as psoriasis, actinic keratosis, and certain neoplasias. Tazarotene is a synthetic retinoid that is used in the clinic for the treatment of psoriasis. To better understand the mechanism of retinoid action in the treatment of hyperproliferative diseases, we used a long-range differential display-PCR to isolate retinoid-responsive genes from primary human keratinocytes. We have identified a cDNA, tazarotene-induced gene 3 (TIG3; Retinoic Acid Receptor Responder 3) showing significant homology to the class II tumor suppressor gene, H-rev 107. Tazarotene treatment increases TIG3 expression in primary human keratinocytes and in vivo in psoriatic lesions. Increased TIG3 expression is correlated with decreased proliferation. TIG3 is expressed in a number of tissues, and expression is reduced in cancer cell lines and some primary tumors. In breast cancer cell lines, retinoid-dependent TIG3 induction is observed in lines that are growth suppressed by retinoids but not in nonresponsive lines. Transient over-expression of TIG3 in T47D or Chinese hamster ovary cells inhibits colony expansion. Finally, studies in 293 cells expressing TIG3 linked to an inducible promoter demonstrated decreased proliferation with increased TIG3 levels. These studies suggest that TIG3 may be a growth regulator that mediates some of the growth suppressive effects of retinoids.

RORgamma t, a novel isoform of an orphan receptor, negatively regulates Fas ligand expression and IL-2 production in T cells

We have identified RORgamma t, a novel, thymus-specific isoform of the orphan nuclear receptor RORgamma that is expressed predominantly in CD4+ CD8+ double-positive thymocytes. Ectopic expression of RORgamma t protects T cell hybridomas from activation-induced cell death by inhibiting the upregulation of Fas ligand. Following hybridoma stimulation, RORgamma t also inhibits IL-2 production but does not affect the induction of Nur-77 and Egr-3 nor the upregulation of CD69. Both the ligand-binding and DNA-binding domains of RORgamma t are required for this effect. We propose that the role of RORgamma t expression in immature thymocytes is to inhibit Fas ligand expression and cytokine secretion following engagement of their TCR during positive or negative selection.

Motor neuron-derived retinoid signaling specifies the subtype identity of spinal motor neurons

The diversification of neuronal cell types in the vertebrate central nervous system depends on inductive signals provided by local organizing cell groups of both neural and nonneural origin. The influence of signals provided by postmitotic neurons on the fate of neurons born at subsequent development stages, however, remains unclear. We provide evidence that a retinoid-mediated signal provided by one subset of early-born spinal motor neurons imposes a local variation in the number of motor neurons generated at different axial levels and also specifies the identity of a later-born subset of motor neurons. Thus, in the vertebrate central nervous system the distinct fates of late-born neurons may be acquired in response to signals provided by early-born neurons.

Induction of the nuclear orphan receptor RORgamma during adipocyte differentiation of D1 and 3T3-L1 cells

Here, we analyzed the expression of the three members of the retinoid-like orphan receptor (ROR) nuclear receptor subfamily during adipocyte differentiation. RORalpha and RORgamma mRNA were upregulated during adipocyte differentiation in preadipocyte D1 and 3T3-L1 cells, whereas RORbeta mRNA could not be detected. The induction of RORalpha and RORgamma mRNA succeeded the induction of peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer binding protein alpha and occurred at a similar time interval as did the increase in aP2 and lipoprotein lipase mRNA. Like the expression of PPARgamma and aP2, the induction of RORgamma mRNA was repressed by tumor necrosis factor alpha and transforming growth factor beta. The induction of adipogenesis by prostaglandin D2 and two thiazolidinediones in the multipotent stem cells C3H10T1/2 was also accompanied by an induction in RORgamma mRNA. In contrast to parental cells, clofibrate induces adipogenesis and RORalpha and RORgamma mRNA in BALB/c3T3 cells that ectopically express PPARgamma. RORgamma mediates its effect on transcription through specific response elements. Cotransfection of RORalpha or RORgamma and (RORgamma response element)4-chloramphenicol acetyltransferase into preadipocyte D1 cells induced transactivation of chloramphenicol acetyltransferase about 100-fold, suggesting that ROR plays a role in the regulation of gene expression in adipocytes. The nuclear orphan receptor Rev-ErbAalpha, which did not exhibit transactivation function, was able to inhibit transactivation by RORgamma at two different levels. Our results show that RORgamma is induced during adipocyte differentiation in D1 and 3T3-L1 cells and functions as an active transcription factor, suggesting a role for RORgamma in the regulation of gene expression during this differentiation process.

Effects of conformationally restricted synthetic retinoids on ovarian tumor cell growth

We have used conformationally restricted retinoids to investigate the role of individual RAR subtypes and RXR in mediating the growth response of ovarian tumor cells to retinoids. Our results show that treatment of all-trans-RA-sensitive CAOV-3 cells with retinoids that bind and activate a single RAR or RXR led to a partial inhibition of growth. Treatment of all-trans-RA- resistant SKOV-3 cells did not alter growth. Maximum inhibition of growth, comparable to that observed following treatment with natural retinoids such as all-trans-RA and 9-cis-RA, was obtained only following treatment with a combination of an RAR-selective compound and an RXR-selective one. These results suggest that activation of both RAR and RXR classes is required in order to obtain maximum inhibition of ovarian tumor cell growth by retinoids. In addition, one compound, AHPN, was found to inhibit both RA-sensitive CAOV-3 and RA-resistant SKOV-3 cells. Further study of the effects of this retinoid showed that AHPN acts through an apoptotic pathway. Taken together, our results suggest that retinoids may serve as effective anti-proliferative agents in the treatment of ovarian cancer.

Genomic structure and chromosomal mapping of the nuclear orphan receptor ROR gamma (RORC) gene

The nuclear orphan receptor subfamily ROR/RZR is part of the steroid and thyroid hormone/retinoid receptor superfamily and consists of three different genes, alpha, beta, and gamma. In this study, we determined the genomic structure of mouse ROR gamma and the chromosomal localization of both mouse ROR gamma and human ROR gamma (HGMW-approved symbol RORC). The genomic structure of the mouse ROR gamma gene was derived from the analysis of P1 vector clones containing large genomic fragments encoding ROR gamma. These results revealed that the mROR gamma gene has a complex structure consisting of 11 exons separated by 10 introns spanning more than 21 kb of genomic DNA. The DNA-binding domain is contained in two exons, 3 and 4, each encoding one zinc-finger. The splice site between exon 3 and exon 4 is identical to that found in RAR and TR3 receptors. ROR gamma is expressed as two mRNAs, 2.3 and 3.0 kb in size, that are derived by the use of alternative polyadenylation signals. We show by fluorescence in situ hybridization that the mouse ROR gamma gene is located on chromosome 3, in a region that corresponds to band 3F2.1-2.2. The human ROR gamma was mapped to chromosome region 1q21. The results demonstrate that the ROR gamma genes are located in chromosomal regions that are syntenic between mouse and human.

A role for ultraspiracle, the Drosophila RXR, in morphogenetic furrow movement and photoreceptor cluster formation

Many of the same genes needed for proper eye and limb development in vertebrates, such as hairy, hedgehog, patched and cyclic AMP-dependent protein kinase A, are responsible for patterning Drosophila imaginal discs, the tissues that will give rise to the adult cuticle structures. This is well demonstrated in the control of morphogenetic furrow movement and differentiation in the eye imaginal disc. We report that ultraspiracle, the gene encoding the Drosophila cognate of the Retinoid X Receptor, is required for normal morphogenetic furrow movement and ommatidial cluster formation. Examination of the expression of genes involved in regulating the furrow suggests that ultraspiracle defines a novel regulatory pathway in eye differentiation.

Structural and functional evidence for activation of a chick retinoid X receptor by eicosanoids

The retinoid X receptors (RXR-alpha, RXR-beta and RXR-gamma) are members of the steroid-thyroid hormone receptor superfamily of ligand-dependent transcription factors. They appear to function as auxiliary proteins that regulate high-affinity DNA binding and enhance transcriptional activity through heterodimer formation with other members of the superfamily. The RXR-alpha, RXR-beta and RXR-gamma proteins bind and are activated by the naturally occurring retinoid, 9-cis-retinoic acid. Structural similarities are apparent between retinoic acid and various eicosanoids, raising the possibility that eicosanoids may also activate retinoid receptors in vivo. We present evidence that lipoxygenase metabolites of arachidonic acid at submicromolar concentrations are capable of activating RXR-gamma activity in transient transfection assays. In addition, molecular modelling predicts conformational similarities between some lipoxygenase products and retinoic acid. Consistent with this, hydroxyeicosatetraenoic acids are known to mimic some actions of retinoids in cell-based assays. These observations raise the possibility that eicosanoids, already known to act both as local hormones and as intracellular second messengers, may also have a direct role in transcriptional activation via nuclear receptors.

Cloning of a cDNA encoding the murine orphan receptor RZR/ROR gamma and characterization of its response element

In this study, we describe the cloning of the mouse homologue of the orphan receptor, RZR/ROR gamma, a member of the nuclear receptor superfamily, from a mouse muscle cDNA library. The amino acid sequence of mouse ROR gamma (mROR gamma) is highly homologous to that of human ROR gamma, with an overall identity of 88%. Northern blot analysis using RNA from different tissues showed that mROR gamma was found to be highly expressed in skeletal muscle, liver and kidney. Analysis of the ROR gamma-response element using in vitro synthesized ROR gamma revealed that it binds as a monomer to response elements composed of a single core motif GGTCA preceded by a 6 bp AT-rich sequence. The ROR gamma-binding specificity was further defined by mutational analysis of the consensus RORE. ROR gamma was able to activate RORE-dependent transcription of the CAT reporter gene in mouse fibroblast D1 cells. ROR alpha 1 and ROR gamma inhibit the transactivation induced by GAL4(DBD)-ROR gamma in fibroblast D1 cells suggesting that these receptors compete for binding to the same coactivators.

Functional analysis of retinoid Z receptor beta, a brain-specific nuclear orphan receptor

The retinoid Z receptor beta (RZR beta), an orphan receptor, is a member of the retinoic acid receptor (RAR)/thyroid hormone receptor (TR) subfamily of nuclear receptors. RZR beta exhibits a highly restricted brain-specific expression pattern. So far, no natural RZR beta target gene has been identified and the physiological role of the receptor in transcriptional regulation remains to be elucidated. Electrophoretic mobility shift assays reveal binding of RZR beta to monomeric response elements containing the sequence AnnTAGGTCA, but RZR beta-mediated transactivation of reporter genes is only achieved with two property spaced binding sites. We present evidence that RZR beta can function as a cell-type-specific transactivator. In neuronal cells, GaI-RZR beta fusion proteins function as potent transcriptional activators, whereas no transactivation can be observed in nonneuronal cells. Mutational analyses demonstrate that the activation domain (AF-2) of RZR beta and RAR alpha are functionally interchangeable. However, in contrast to RAR and TR, the RZR beta AF-2 cannot function autonomously as a transactivation domain. Furthermore, our data define a novel repressor function for the C-terminal part of the putative ligand binding domain. We propose that the transcriptional activity of RZR beta is regulated by an interplay of different receptor domains with coactivators and corepressors.

Thiazolidine diones, specific ligands of the nuclear receptor retinoid Z receptor/retinoid acid receptor-related orphan receptor alpha with potent antiarthritic activity

Rat adjuvant arthritis is a chronic T cell-dependent autoimmune disease with many similarities to rheumatoid arthritis. We have identified a class of thiazolidine diones with high potency in suppressing chronic inflammation and joint destruction in this experimental model. The lead compound CGP 52608 (1-(3-allyl-4-oxothiazolidine-2-ylidene)-4-methylthiosemicarbazone) exhibits antiarthritic activity at daily oral doses between 0.01 and 1 mg/kg and was shown to specifically activate the retinoid Z receptor/retinoid acid receptor-related orphan receptor alpha (RZR/RORalpha) in low nanomolar concentrations. This receptor is a novel member of the superfamily of ligand-inducible transcription factors, and we have recently identified the pineal gland hormone melatonin as a natural ligand. Structure-activity relationship studies with 13 closely related analogues of CGP 52608 revealed a striking correlation between RZR/RORalpha activation and antiarthritic activity. We therefore suggest that nuclear signaling via RZR/RORalpha is a key mechanism in mediating the antiarthritic effects of these thiazolidine diones and may open a novel therapeutic approach for the treatment of rheumatoid arthritis and other autoimmune diseases. The existence of a nuclear melatonin receptor may lead to a better understanding of the immunomodulatory actions of melatonin.

The nuclear receptor for melatonin represses 5-lipoxygenase gene expression in human B lymphocytes

The two subtypes of retinoid Z receptor (RZR alpha and beta) and the three splicing variants of retinoid orphan receptor (ROR alpha 1, alpha 2, and alpha 3) form a subfamily within the superfamily of nuclear hormone receptors. Very recently we found that the pineal gland hormone melatonin is a natural ligand of RZR alpha and RZR beta. Ligand-induced transcriptional control is therefore proposed to mediate physiological functions of melatonin in the brain where RZR beta is expressed, but also in peripheral tissues, where RZR alpha was found. However, no natural RZR responding genes have been identified yet. Here, we report that a response element in the promoter of 5-lipoxygenase binds specifically RZR alpha and ROR alpha 1, but not ROR alpha 2 and alpha 3. 5-Lipoxygenase is a key enzyme in the biosynthesis of leukotrienes, which are known to be allergic and inflammatory mediators. We could show that the activity of the whole 5-lipoxygenase promoter as well as of the RZR response element fused to the heterologous thymidine kinase promoter could be repressed by melatonin. The hormone down-regulated the expression of 5-lipoxygenase about 5-fold in B lymphocytes, which express RZR alpha. In contrast, 5-lipoxygenase mRNA levels were not affected in differentiated monocytic and granulocytic cell lines, which do not express RZR alpha. This indicates that 5-lipoxygenase is the first natural RZR alpha responding gene. Furthermore, our results open up a new perspective in understanding the involvement of melatonin in inflammatory and immunological reactions.

ROR gamma: the third member of ROR/RZR orphan receptor subfamily that is highly expressed in skeletal muscle

In this study, we describe the identification and cloning of a novel member of the nuclear receptor superfamily. This orphan receptor, referred to as ROR gamma, belongs to the ROR/RZR subfamily. The open reading frame of ROR gamma encodes a protein of 560 amino acid residues with a predicted molecular mass of 63 kD. The amino acid sequence of ROR gamma exhibits a 50 and 51% identity with those of ROR alpha/RZR alpha and RZR beta, respectively, whereas the DNA-binding domains were 89% identical. ROR gamma was localized on human chromosome 1. Northern blot analysis using RNA from multiple tissues indicated that ROR gamma is expressed in several tissues but is most highly expressed in skeletal muscle.