Regulation of retinoidal actions by diazepinylbenzoic acids. Retinoid synergists which activate the RXR-RAR heterodimers

Retinoic acid receptor-α up-regulates proopiomelanocortin gene expression in AtT20 corticotroph cells

Cushing's disease is a disorder caused by excessive ACTH secretion from a corticotroph tumor of the pituitary gland. Although its standard therapy is a transsphenoidal surgery, innovation of novel medical treatments for the disease is urgently necessary. Retinoic acid (RA) has been reported to suppress adrenocorticotropic hormone (ACTH) secretion in Cushing's disease. However, the role of RA receptor (RAR) in proopiomelanocortin (Pomc) gene expression remains uncertain. We here examined the involvement of RARα in Pomc regulation using AtT20 corticotroph cells. Surprisingly, a synthetic RARα agonist Am80 increased Pomc mRNA expression, CRH-induced ACTH secretion, and Pomc promoter activity. Small interfering RNA-mediated RARα-knockdown suppressed both basal and Am80-induced Pomc promoter activity. RARα-overexpression dose-dependently increased Pomc promoter activity. Pomc promoter mutation analysis revealed that both Tpit and NeuroD1 binding elements were responsible for the Am80-mediated effect. Am80 increased Tpit expression while RAR antagonist LE540 suppressed the increase. Tpit-overexpression increased Pomc promoter activity. Mammalian two-hybrid assay revealed that Am80 induced NeuroD1-RARα interaction. NeuroD1-overexpression enhanced the Am80-induced Pomc promoter activity, which was suppressed by NeuroD1 truncated mutant-overexpression. RARα thus positively regulates ACTH secretion/Pomc gene expression through interaction with NeuroD1 and Tpit expression increase. The present observation will be useful for the future development of the RA/retinoid-derived therapeutics of the disease.

Downregulation of STRA6 expression in epidermal keratinocytes leads to hyperproliferation-associated differentiation in both in vitro and in vivo skin models

Retinoids are known to affect skin cell proliferation and differentiation and are key molecules that target retinoid and retinoic acid receptors (RXRs and RARs), leading to physiological and pharmacologic effects. Our aim was to elucidate the role of the retinol-binding protein receptor STRA6, mediating cellular uptake of retinol, on skin structure and function. Our results indicate that STRA6 is constitutively expressed in human epidermal keratinocytes and dermal fibroblasts and is regulated via RAR/RXR-mediated pathways. HaCaT (Human adult low Calcium high Temperature) cells with stable STRA6 knockdown (STRA6KD) showed increased proliferation. Consistently, human organotypic 3D skin models using stable STRA6KD HaCaT cells showed a significantly thicker epidermis and enhanced expression of activation, differentiation, and proliferation markers. The effects were reversible after treatment with free retinol. Human skin reconstitution employing STRA6KD HaCaT cells leads to massive epithelial thickening under in vivo conditions in SCID mice. We propose that STRA6KD could lead to cellular vitamin A deficiency in keratinocytes. Consequently, STRA6 has a role for regulating retinoid homeostasis and in helping to program signaling that drives proliferation and differentiation of human skin cells. By its influence on hyperproliferation-associated differentiation, STRA6 could also have a role in skin regeneration and could be a target for pharmacological approaches to improve wound healing.

Retinoid X receptor agonists modulate Foxp3⁺ regulatory T cell and Th17 cell differentiation with differential dependence on retinoic acid receptor activation

Retinoic acid (RA) enhances TGF-β-dependent differentiation of Foxp3(+) inducible regulatory T cells (iTregs) and inhibits Th17 differentiation by binding to the RA receptor (RAR)/retinoid X receptor (RXR) heterodimer. The major physiologic RA, all-trans-RA, binds to RAR but not to RXR at physiological concentrations. It remained unclear whether RXR-mediated stimulation affected the iTregs and Th17 differentiation. We found in this study that the RXR agonists, PA024 and tributyltin, augmented the ability of all-trans-RA or the RAR agonist Am80 to enhance CD4(+)CD25(-) T cells to acquire Foxp3 expression and suppressive function. However, they failed to enhance Foxp3 expression in the presence of the RAR antagonist LE540, suggesting that the effect depends on RAR-mediated signals. They exerted the effect largely by augmenting the ability of all-trans-RA to suppress the production of IL-4, IL-21, and IFN-γ that inhibited Foxp3 expression. Agonists of peroxisome proliferator-activated receptors and liver X receptors (LXRs), permissive partners of RXR, failed to enhance Foxp3 expression. In contrast, RXR agonists and LXR agonists suppressed IL-17 expression. The RXR-mediated suppression was not canceled by blocking RAR stimulation but was likely to involve permissive activation of LXRs. All-trans-RA and an agonist of RXR or LXR additively suppressed IL-17 expression when the all-trans-RA concentration was low. RXR agonists also suppressed Ccr6 expression that is essential for Th17 cells to enter the CNS. Accordingly, tributyltin treatment of mice ameliorated experimental autoimmune encephalomyelitis through regulating Th17 cell activities. These results suggest that RXR stimulation modulates Foxp3(+) iTreg and Th17 differentiation with differential dependence on RAR-mediated stimulation.

Retinoid X receptor activation is essential for docosahexaenoic acid protection of retina photoreceptors

We have established that docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, promotes survival of rat retina photoreceptors during early development in vitro and upon oxidative stress by activating the ERK/MAPK signaling pathway. Here we have investigated whether DHA turns on this pathway through activation of retinoid X receptors (RXRs) or by inducing tyrosine kinase (Trk) receptor activation. We also evaluated whether DHA release from phospholipids was required for its protective effect. Addition of RXR antagonists (HX531, PA452) to rat retinal neuronal cultures inhibited DHA protection during early development in vitro and upon oxidative stress induced with Paraquat or H2O2. In contrast, the Trk inhibitor K252a did not affect DHA prevention of photoreceptor apoptosis. These results imply that activation of RXRs was required for DHA protection whereas Trk receptors were not involved in this protection. Pretreatment with 4-bromoenol lactone, a phospholipase A2 inhibitor, blocked DHA prevention of oxidative stress-induced apoptosis of photoreceptors. It is noteworthy that RXR agonists (HX630, PA024) also rescued photoreceptors from H2O2-induced apoptosis. These results provide the first evidence that activation of RXRs prevents photoreceptor apoptosis and suggest that DHA is first released from phospholipids and then activates RXRs to promote the survival of photoreceptors.

An open-label phase I/II study of tamibarotene in patients with advanced hepatocellular carcinoma

AIM

Tamibarotene is a synthetic retinoid expected to inhibit tumor-cell proliferation and to induce apoptosis by selective interaction with retinoic acid receptor α/β. We conducted an open-label phase I/II study to determine the maximum tolerated dose (MTD) and recommended dose (RD), and to evaluate the pharmacokinetics, efficacy, and safety profiles for advanced hepatocellular carcinoma (HCC).

METHODS

Patients with histologically confirmed, measurable, unresectable HCC of Child-Pugh classification A or B and with no effective systemic or local therapies were eligible. In phase I, patients were assigned based on the 3 + 3 dose escalation criteria to receive tamibarotene at 8, 12, and 16 mg/day. The RD determined in phase I was employed for phase II. The planned sample size in phase II was 25, including the RD-treated patients in phase I.

RESULTS

Thirty-six patients were enrolled. No patients experienced dose-limiting toxicity (DLT) at 8 mg/day. However, two out of six patients experienced the DLTs at 12 mg/day: one experienced thrombosis in a limb vein and pulmonary artery, and the other experienced an increase of γ-GTP. The MTD and RD were determined as 12 and 8 mg/day, respectively. In phase II, one patient achieved partial response, and seven achieved stable disease. The disease control rate was 32 % (95 % CI: 15.0-53.5). The following drug-related serious adverse events were reported: thrombosis in a limb vein, pulmonary artery, and portal vein; interstitial lung disease; and vomiting.

CONCLUSIONS

Tamibarotene demonstrated the inhibition of tumor cell growth in advanced HCC with acceptable tolerance.

Structure-activity relationship study on benzoic acid part of diphenylamine-based retinoids

Based on structure-activity relationship studies of the benzoic acid part of diphenylamine-based retinoids, the potent RXR agonist 4 was derivatized to obtain retinoid agonists, synergists, and an antagonist. Cinnamic acid derivatives 5 and phenylpropionic acid derivatives 6 showed retinoid agonistic and synergistic activities, respectively. The difference of the activities is considered to be due to differences in the flexibility of the carboxylic acid-containing substituent on the diphenylamine skeleton. Compound 7, bearing a methyl group at the meta position to the carboxyl group, was an antagonist, dose-dependently inhibiting HL-60 cell differentiation induced by 3.3 × 10(-10)M Am80.

Increased dietary intake of vitamin A promotes aortic valve calcification in vivo

OBJECTIVE

Calcific aortic valve disease (CAVD) is a major public health problem with no effective treatment available other than surgery. We previously showed that mature heart valves calcify in response to retinoic acid (RA) treatment through downregulation of the SRY transcription factor Sox9. In this study, we investigated the effects of excess vitamin A and its metabolite RA on heart valve structure and function in vivo and examined the molecular mechanisms of RA signaling during the calcification process in vitro.

METHODS AND RESULTS

Using a combination of approaches, we defined calcific aortic valve disease pathogenesis in mice fed 200 IU/g and 20 IU/g of retinyl palmitate for 12 months at molecular, cellular, and functional levels. We show that mice fed excess vitamin A develop aortic valve stenosis and leaflet calcification associated with increased expression of osteogenic genes and decreased expression of cartilaginous markers. Using a pharmacological approach, we show that RA-mediated Sox9 repression and calcification is regulated by classical RA signaling and requires both RA and retinoid X receptors.

CONCLUSIONS

Our studies demonstrate that excess vitamin A dietary intake promotes heart valve calcification in vivo. Therefore suggesting that hypervitaminosis A could serve as a new risk factor of calcific aortic valve disease in the human population.

Advances in drug design with RXR modulators

INTRODUCTION

Retinoid X receptors (subtypes RXRα or NR2B1, RXRβ or NR2B2 and RXRγ or NR2B3, which originate from three distinct genes) are promiscuous partners with heterodimeric associations to other members of the Nuclear Receptor (NR) superfamily. Some of the heterodimers are "permissive" and transcriptionally active in the presence of either an RXR ligand ("rexinoid") or a NR partner ligand, whereas others are "non-permissive" and unresponsive to rexinoids alone. In rodent models, rexinoids and partner agonists (mainly PPARγ, LXR, FXR) produce beneficial effects on insulin sensitization, diabetes and obesity, but secondary effects have also been noted, such as a raise in tryglyceride levels, supression of the thyroid hormone axis and induction of hepatomegaly.

AREAS COVERED

The authors review recent advances in rexinoid design, including further optimization of known scaffolds, and the discovery of novel RXR modulators by virtual ligand screening or from bioactive natural products. The understanding of rexinoid functions in permissive and non-permissive heterodimers is firmly based on structural knowledge. By strenghtening or disrupting the interaction surface with coregulators rexinoids exert agonist or (partial) antagonist activities. The activity state of the heterodimer can also be fine-tuned by the cellular context and the nature of coregulators.

EXPERT OPINION

The synthetic chemistry toolbox has provided a panel of agonists, partial (ant)agonists and/or heterodimer-selective rexinoids starting from existing, naturally occurring or serendipitously discovered scaffolds. These compounds have an unexplored therapeutic potential that might overcome some of the current limitations of rexinoids in therapy, such as hypertriglyceridemia.

Decalpenic acid induces early osteoblastic markers in pluripotent mesenchymal cells via activation of retinoic acid receptor γ

Decalpenic acid is a natural small molecule previously isolated from the fermentation broth of fungi that induces early osteoblastic markers in pluripotent mesenchymal cells. Treatment of mouse pluripotent mesenchymal C3H10T1/2 cells with decalpenic acid gave rise to a morphological change similar to that induced by the treatment with retinoic acid, i.e. the cells adopted a more elongated spindle shape. Using a retinoic acid response element reporter and receptor activity assays, we show that decalpenic acid is a new retinoid with selectivity towards retinoic acid receptors γ and α. The induction of early osteoblastic markers by decalpenic acid was significantly inhibited by treatment with the retinoid antagonist, LE540, or with small interfering RNA-mediated knockdown of retinoic acid receptor γ. These results demonstrated that decalpenic acid induces early osteoblastic markers in pluripotent mesenchymal cells through activation of retinoic acid receptor γ.

Mesenchymal cells regulate retinoic acid receptor-dependent cortical thymic epithelial cell homeostasis

The vitamin A metabolite and transcriptional modulator retinoic acid (RA) is recognized as an important regulator of epithelial cell homeostasis in several tissues. Despite the known importance of the epithelial compartment of the thymus in T cell development and selection, the potential role of RA in the regulation of thymic cortical and medullary epithelial cell homeostasis has yet to be addressed. In this study, using fetal thymus organ cultures, we demonstrate that endogenous RA signaling promotes thymic epithelial cell (TEC) cell-cycle exit and restricts TEC cellularity preferentially in the cortical TEC compartment. Combined gene expression, biochemical, and functional analyses identified mesenchymal cells as the major source of RA in the embryonic thymus. In reaggregate culture experiments, thymic mesenchyme was required for RA-dependent regulation of TEC expansion, highlighting the importance of mesenchyme-derived RA in modulating TEC turnover. The RA-generating potential of mesenchymal cells was selectively maintained within a discrete Ly51(int)gp38(+) subset of Ly51(+) mesenchyme in the adult thymus, suggesting a continual role for mesenchymal cell-derived RA in postnatal TEC homeostasis. These findings identify RA signaling as a novel mechanism by which thymic mesenchyme influences TEC development.

Retinoic acid receptor agonists regulate expression of ATP-binding cassette transporter G1 in macrophages

ABC transporter G1 (ABCG1) plays a pivotal role in HDL-mediated cholesterol efflux and atherogenesis. We investigated whether, and how, retinoic acid receptors (RARs) regulate ABCG1 expression in macrophages. All-trans retinoic acid (ATRA), an RAR ligand, increased ABCG1 protein levels and apoA-I/HDL-mediated cholesterol efflux from the macrophages. Both ATRA and other RAR agonists, TTNPB and Am580, increased major transcripts driven by promoter B upstream of exon 5, though minor transcripts driven by promoter A upstream of exon 1 were only increased by ATRA. The stimulatory effects of ATRA on ABCG1 expression were completely abolished in the presence of RAR/RXR antagonists but were only partially canceled in the presence of an LXR antagonist. Adenovirus with overexpressed oxysterol sulfotransferase abolished the LXR pathway, as previously reported, and ATRA-responsiveness in ABCA1/ABCG1 expressions were respectively attenuated by 38 and 22% compared to the control virus. Promoter assays revealed that ABCG1 levels were regulated more by promoter B than promoter A, and ATRA activated promoter B in a liver X receptor-responsive element (LXRE)-dependent manner. Further, LXRE-B in intron 7, but not LXRE-A in intron 5, enhanced ATRA responsiveness under overexpression of all RAR isoforms-RARα/β/γ. In contrast, the activation of promoter B by TTNPB depended on LXRE-B and RARα, but not on RARβ/γ. Finally, chromatin immunoprecipitation and gel-shift assays revealed a specific and direct repeat 4-dependent binding of RARα to LXRE-B. In conclusion, RAR ligands increase ABCA1/G1 expression and apoA-I/HDL-mediated cholesterol efflux from macrophages, and modulate ABCG1 promoter activity via LXRE-dependent mechanisms.

Imposex development in Nucella lapillus--evidence for the involvement of retinoid X receptor and androgen signalling pathways in vivo

Imposex in female gastropods is a widely documented masculinisation phenomenon in response to tributyltin (TBT) exposure. Although it is generally accepted that imposex is a case of endocrine disruption the underlying mechanisms are controversially discussed with aromatase inhibition and retinoid X receptor (RXR) signalling pathways as two conflicting hypotheses. Hence, we performed injection experiments with the marine dogwhelk Nucella lapillus. As expected TBT induced imposex in all test specimens while the natural RXR ligand 9 cis-retinoic acid did not cause significant effects. Additionally, TBT effects were suppressed if the organotin compound was simultaneously injected with an androgen receptor inhibitor (cyproterone acetate) but not if co-administered with the synthetic RXR antagonist HX531. In contrast, the injection of the RXR agonist HX630 resulted in imposex development in nearly 100% females. Therefore, the results provide evidence for the involvement of the RXR and the androgen signalling pathway. Further investigations are necessary to resolve the biochemical mechanism of imposex development.

Discovery of a Potent Retinoid X Receptor Antagonist Structurally Closely Related to RXR Agonist NEt-3IB

We discovered a potent retinoid X receptor (RXR) antagonist, 6-[N-ethyl-N-(5-isobutoxy-4-isopropyl-2-(E)-styrylphenyl)amino]nicotinic acid (13e), that is structurally closely related to the RXR full agonist 6-[N-ethyl-N-(3-isobutoxy-4-isopropylphenyl)amino]nicotinic acid (NEt-3IB) (4). Compound 13e was synthesized via a simple route from 11, a methyl ester precursor of 4. Because 11 possesses high electrophilic reactivity because of the amino and alkoxy groups, it was readily transformed to 12 by iodization, and the iodine atom of 12 was converted to a C-C or C-N bond by means of palladium-catalyzed reaction to afford 13. Transcriptional activation assay revealed that 13g (in which the iodine atom was replaced with an amino group) is a weak RXR agonist, while 13d (a phenyl group), 13e (a styryl group), and 13f (an anilino group) are RXR antagonists. Among them, 13e was found to be more potent than the known RXR antagonist PA452 (9).

Neuritogenic activity of a genipin derivative in retinal ganglion cells is mediated by retinoic acid receptor β expression through nitric oxide/S-nitrosylation signaling

Genipin, a herbal iridoid, is known to have both neuroprotective and neuritogenic activity in neuronal cell lines. As it is structurally similar to tetrahydrobiopterin, its activity is believed to be nitric oxide (NO)-dependent. We previously proposed a novel neuroprotective activity of a genipin derivative, (1R)-isoPropyloxygenipin (IPRG001), whereby it reduces oxidative stress in RGC-5, a neuronal precursor cell line of retinal origin through protein S-nitrosylation. In the present study, we investigated another neuritogenic property of IPRG001 in RGC-5 cells and retinal explant culture where in we focused on the NO-cGMP-dependent and protein S-nitrosylation pathways. IPRG001 stimulated neurite outgrowth in RGC-5 cells and retinal explant culture through NO-dependent signaling, but not NO-dependent cGMP signaling. Neurite outgrowth with IPRG001 requires retinoic acid receptor β (RARβ) expression, which is suppressed by an RAR blocking agent and siRNA inhibition. Thereby, we hypothesized that RARβ expression is mediated by protein S-nitrosylation. S-nitrosylation of histone deacetylase 2 is a key mechanism in chromatin remodeling leading to transcriptional gene activation. We found a parallelism between S-nitrosylation of histone diacetylase 2 and the induction of RARβ expression with IPRG001 treatment. The both neuroprotective and neuritogenic activities of genipin could be a new target for the regeneration of retinal ganglion cells after glaucomatous conditions.

Retinoic acid inhibits NFATc1 expression and osteoclast differentiation

Ingestion of excess vitamin A appears to correlate with an increased fracture risk, an outcome that is likely mediated by retinoic acids (RAs); these are vitamin A metabolites that have dramatic effects on skeletal development. We studied the impacts of RA and isoform-specific RA receptor (RAR) agonists (α, β, and γ) on osteoclast formation (osteoclastogenesis) in two model systems: RAW264.7 cells and murine bone marrow-derived monocytes. The pan-RAR agonists, all-trans and 9-cis RA, inhibited receptor activator of nuclear factor kappa B ligand (RANKL)-mediated osteoclast differentiation in a concentration-dependent manner. Isoform-specific RAR agonists (α, β, and γ) also inhibited osteoclastogenesis, with the RARα agonist producing the most consistent reductions in both osteoclast number and size and total area covered. Inhibition of osteoclastogenesis correlated with reductions in expression, DNA binding, and nuclear abundance of nuclear factor of activated T cells c1 (NFATc1), a transcription factor critical for osteoclastogenesis. The upregulation of three NFATc1-responsive genes, cathepsin K, dendritic cell-specific transmembrane protein and osteoclast-associated receptor were similarly reduced following RA or RAR agonist exposure. These results suggest that RA blocks in vitro RANKL-mediated osteoclastogenesis by decreasing NFATc1 function.

6-arylcoumarins as novel nonsteroidal type progesterone antagonists: an example with receptor-binding-dependent fluorescence

Various 6-arylcoumarin derivatives were designed and synthesized as candidate nonsteroidal type progesterone antagonists. 6-Bromocoumarin derivatives were prepared from the corresponding 4-substituted 2-acetoxy-5-bromobenzaldehyde by employing the Still-Gennari modification of the Horner-Wadsworth-Emmons olefination reaction and were converted to 6-arylcoumarins by means of Suzuki-Miyaura cross-coupling reactions. The biological activities of these coumarin derivatives were evaluated by means of alkaline phosphatase assay in the T47D human breast carcinoma cell line. Among the synthesized compounds, 36 (IC(50) = 0.12 μM) and 38 (IC(50) = 0.065 μM), bearing a five-membered heterocycle, showed potent PR antagonist activity. Competitive binding assay showed that compounds 8 and 34 have potent PR binding affinity. The fluorescence of compound 8 was dependent on the solvent properties and was increased in the presence of PR ligand binding domain. This property might be applicable to the development of fluorescence probes for studies on PR.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Retinoic acid receptor antagonist LE540 attenuates wakefulness via the dopamine D1 receptor in mice

Vitamin A is a common lipophilic vitamin, and its function is mainly mediated by the binding of its metabolite retinoic acid to retinoic acid receptors (RARs) and retinoid X receptors. Recently, it was reported that the expression of the RARb (an RAR subtype) gene determines the contribution of the delta oscillation in the sleep electroencephalogram (EEG) patterns in mice. We also reported that 4-week dietary deficiency of vitamin A (VAD) causes the attenuation of delta power in sleep and spontaneous activity in mice. However, our previous study could not clarify whether the attenuation of delta power by VAD is attributed to the suppression of RARs. To address this problem, we investigated whether the chronic administration of LE540 (30mg/kg/day), an antagonist of RARs, for 1 or 4weeks attenuated EEG delta power during sleep in mice. Consequently, 4-week LE540 administration induced a significant attenuation of wakefulness and delta power in non-rapid eye movement sleep. Western blot analysis revealed a significant decrease in the expression of dopamine D1 receptor (D1DR) in the striatum and tyrosine hydroxylase in the midbrain of mice that were administered LE540 for 4weeks. High-performance liquid chromatography analysis of striatal tissue revealed a significant decrease in the homovanillic acid/dopamine ratio. Meanwhile, dopamine levels did not change in these mice. Our results suggest that the 4-week antagonism of RARs induces the attenuation of delta power. However, the attenuation of delta power may be elicited indirectly by the decrease of wakefulness followed by the hypo-expression of dopamine receptors especially D1DR.

Concise palladium-catalyzed synthesis of dibenzodiazepines and structural analogues

A general and highly efficient protocol for the synthesis of dibenzodiazepines and their structural analogues is reported. In the presence of catalytic quantities of palladium, readily accessible precursors are cross-coupled with ammonia and then spontaneously undergo an intramolecular condensation to form the corresponding dibenzodiazepines in one step. This new strategy is applicable to the construction of a wide variety of dibenzooxazepines and other structurally related heterocycles.

Establishment of a rescue program for anorectal malformations induced by retinoic acid in mice

AIMS OF STUDY

Retinoid-mediated signal transduction plays a crucial role in the embryogenesis of various organs. We previously reported the successful induction of anorectal malformations in mice using retinoic acid (RA). Retinoic acid controls the expression of essential target genes for cell differentiation, morphogenesis, and apoptosis through a complicated interaction in which RA receptors form heterodimers with retinoid X receptors. In the present study, we investigated whether the retinoid antagonist, LE135, could prevent the induction of anorectal malformations (ARMs) in mice.

METHODS

Retinoic acid was intraperitoneally administered as 100 mg/kg of all-trans RA on E9; and then the retinoid antagonist, LE135, was intraperitoneally administered to pregnant ICR strain mice on the eighth gestational day (E8), 1 day before administration of RA (group B) or on E9, simultaneously (group C) with RA administration. All of the embryos were obtained from the uteri on E18. Frozen sections were evaluated for concentric layers around the endodermal epithelium by hematoxylin and eosin staining.

RESULTS

In group A, all of the embryos demonstrated ARM with rectoprostatic urethral fistula, or rectocloacal fistula, and all of the embryos showed the absence of a tail. In group B, 36% of the embryos could be rescued from ARM. However, all of the rescued embryos had a short tail that was shorter than their hind limb. The ARM rescue rates in group B were significantly improved compared to those in group A (P < .01). In group C, 45% of the embryos were rescued from ARM, but all of the rescued embryos had short tail. The ARM rescue rate in group C was significantly improved compared to that in group A (P < .01). However, there was no significant difference in the ARM rescue rate between group B and Group C.

CONCLUSION

The present study provides evidence that in the hindgut region, RAR selective retinoid antagonist, LE135, could rescue embryos from ARM. However, the disturbance of all-trans RA acid was limited to the caudal region. Further study to establish an appropriate rescue program for ARM in a mouse model might suggest a step toward protection against human ARM in the future.

Modulation of RXR function through ligand design

As the promiscuous partner of heterodimeric associations, retinoid X receptors (RXRs) play a key role within the Nuclear Receptor (NR) superfamily. Some of the heterodimers (PPAR/RXR, LXR/RXR, FXR/RXR) are "permissive" as they become transcriptionally active in the sole presence of either an RXR-selective ligand ("rexinoid") or a NR partner ligand. In contrast, "non-permissive" heterodimers (including RAR/RXR, VDR/RXR and TR/RXR) are unresponsive to rexinoids alone but these agonists superactivate transcription by synergizing with partner agonists. Despite their promiscuity in heterodimer formation and activation of multiple pathways, RXR is a target for drug discovery. Indeed, a rexinoid is used in the clinic for the treatment of cutaneous T-cell lymphoma. In addition to cancer RXR modulators hold therapeutical potential for the treatment of metabolic diseases. The modulation potential of the rexinoid (as agonist or antagonist ligand) is dictated by the precise conformation of the ligand-receptor complexes and the nature and extent of their interaction with co-regulators, which determine the specific physiological responses through transcription modulation of cognate gene networks. Notwithstanding the advances in this field, it is not yet possible to predict the correlation between ligand structure and physiological response. We will focus on this review on the modulation of PPARγ/RXR and LXR/RXR heterodimer activities by rexinoids. The genetic and pharmacological data from animal models of insulin resistance, diabetes and obesity demonstrate that RXR agonists and antagonists have promise as anti-obesity agents. However, the treatment with rexinoids raises triglycerides levels, suppresses the thyroid hormone axis, and induces hepatomegaly, which has complicated the development of these compounds as therapeutic agents for the treatment of type 2 diabetes and insulin resistance. The discovery of PPARγ/RXR and LXR/RXR heterodimer-selective rexinoids, which act differently than PPARγ or LXR agonists, might overcome some of these limitations.

Midbrain dopaminergic neurons utilize nitric oxide/cyclic GMP signaling to recruit ERK that links retinoic acid receptor stimulation to up-regulation of BDNF

Stimulation of retinoic acid receptors (RARs) protects midbrain dopaminergic neurons, presumably via up-regulation of brain-derived neurotrophic factor (BDNF) expression. The present study was focused on unexplored signaling mechanisms linking RAR stimulation to BDNF expression. Rat midbrain slice cultures treated with an RAR agonist Am80 showed increased tissue levels of BDNF mRNA and protein as compared to cultures without treatment. Am80-induced increase in BDNF expression was observed in dopaminergic neurons, which was blocked by inhibition of extracellular signal-regulated kinase (ERK) activation. We also found that Am80 increased neuronal nitric oxide synthase expression in dopaminergic neurons even during ERK inhibition, and this increase was accompanied by 8-nitro-cyclic GMP formation. Notably, the effect of Am80 on BDNF expression was attenuated by inhibitors of nitric oxide synthase, soluble guanylyl cyclase and cyclic GMP-dependent protein kinase (PKG). Am80-induced ERK phosphorylation in dopaminergic neurons was also attenuated by inhibition of soluble guanylyl cyclase and PKG. Moreover, 8-Br-cyclic GMP induced ERK phosphorylation and BDNF expression in dopaminergic neurons. These results suggest that, by recruiting cyclic GMP and PKG, neuronal nitric oxide synthase-derived nitric oxide plays a novel and essential role in RAR signaling leading to ERK-dependent BDNF up-regulation in midbrain dopaminergic neurons.

Retinoic acid production by intestinal dendritic cells

Subpopulations of dendritic cells (DCs) in the small intestine and its related lymphoid organs can produce retinoic acid (RA) from vitamin A (retinol). Through the RA production, these DCs play a pivotal role in imprinting lymphocytes with gut-homing specificity, and contribute to the development of immune tolerance by enhancing the differentiation of Foxp3(+) regulatory T cells and inhibiting that of inflammatory Th17 cells. The RA-producing capacity in these DCs mostly depends on the expression of retinal dehydrogenase 2 (RALDH2, ALDH1A2). It is likely that the RALDH2 expression is induced in DCs by the microenvironmental factors in the small intestine and its related lymphoid organs. The major factor responsible for the RALDH2 expression appears to be GM-CSF. RA itself is essential for the GM-CSF-induced RALDH2 expression. IL-4 and IL-13 also enhance RALDH2 expression, but are dispensable. Toll-like receptor-mediated signals can also enhance the GM-CSF-induced RALDH2 expression in immature DCs.

A retinoic acid receptor agonist Am80 rescues neurons, attenuates inflammatory reactions, and improves behavioral recovery after intracerebral hemorrhage in mice

Am80 (tamibarotene) is a retinoic acid receptor (RAR) agonist clinically available for treatment of acute promyelocytic leukemia. As intracerebral hemorrhage (ICH) accompanies inflammatory reactions in the brain and also because retinoids may suppress activation of microglia, we investigated the effect of Am80 on collagenase-induced experimental model of ICH in adult mice. Daily oral administration of Am80 (5 mg/kg) starting from 1 day before or from up to 6 hours after intrastriatal injection of collagenase significantly inhibited the decrease in the number of striatal neurons at 3 days after the insult. Am80 showed no significant effect on the hematoma size and the extent of edema associated with hemorrhage. Prominent expression of RARα was observed in activated microglia/macrophages, and the number of activated microglia/macrophages in the perihematoma region was lower in Am80-treated mice than in vehicle-treated mice. Am80 treatment also reduced areas affected by hemorrhage-associated oxidative stress as indicated by nitrotyrosine immunoreactivity, and attenuated heme oxygenase-1 expression in activated microglia/macrophages. Moreover, Am80-treated mice exhibited better recovery from hemorrhage-induced neurologic deficits than vehicle-treated mice. These results suggest that RAR is a promising target of neuroprotective therapy for ICH.

Efficient induction of CCR9 on T cells requires coactivation of retinoic acid receptors and retinoid X receptors (RXRs): exaggerated T Cell homing to the intestine by RXR activation with organotins

The active vitamin A metabolite retinoic acid (RA) imprints gut-homing specificity on lymphocytes upon activation by inducing the expression of α4β7 integrin and CCR9. RA receptor (RAR) activation is essential for their expression, whereas retinoid X receptor (RXR) activation is not essential for α4β7 expression. However, it remains unclear whether RXR activation affects the RA-dependent CCR9 expression on T cells and their gut homing. The major physiological RA, all-trans-RA, binds to RAR but not to RXR at physiological concentrations. Cell-surface CCR9 expression was often induced on a limited population of murine naive CD4(+) T cells by all-trans-RA or the RAR agonist Am80 alone upon CD3/CD28-mediated activation in vitro, but it was markedly enhanced by adding the RXR agonist PA024 or the RXR-binding environmental chemicals tributyltin and triphenyltin. Accordingly, CD4(+) T cells treated with the combination of all-trans-RA and tributyltin migrated into the small intestine upon adoptive transfer much more efficiently than did those treated with all-trans-RA alone. Furthermore, naive TCR transgenic CD4(+) T cells transferred into wild-type recipients migrated into the small intestinal lamina propria following i.p. injection of Ag, and the migration was enhanced by i.p. injection of PA024. We also show that PA024 markedly enhanced the all-trans-RA-induced CCR9 expression on naturally occurring naive-like regulatory T cells upon activation, resulting in the expression of high levels of α4β7, CCR9, and Foxp3. These results suggest that RXR activation enhances the RAR-dependent expression of CCR9 on T cells and their homing capacity to the small intestine.

Encapsulation of a hydrophobic drug into a polymer-micelle core explored with synchrotron SAXS

Synchrotron small-angle X-ray scattering (SAXS) at the SPring-8 40B2 and 45XU beamlines was carried out on aqueous solutions of (PEG-P(Asp(Bzl))): partially benzyl-esterified poly(ethylene glycol)-block-poly(aspartic acid) with LE540 loaded up to 8.3 wt %, where LE540 is a very hydrophobic retinoid antagonist drug. The scattering profiles showed characteristic features for core-shell spherical micelles, confirming that P(Asp(Bzl)) forms a hydrophobic core and PEG forms a hydrophilic shell. Before the addition of LE540, a diffraction peak was observed around q = 4 nm(-1), where q is the magnitude of the scattering vector. This peak can be attributed to ordering between alpha-helices made of P(Asp(Bzl)), the so-called nonspecific hexatic arrangement. The P(Asp(Bzl)) helices disappeared as LE540 was added. This result can be interpreted by assuming a uniform distribution of LE540 in the core. By use of a core-shell spherical micelle model, the SAXS data could be well fitted for all of the samples. The analysis indicated that the core radius increases sigmoidally from 5.9 to 6.9 nm upon addition of LE540 whereas the shell radius stayed at 12.5-12.8 nm. The aggregation number that is the average number of PEG-P(Asp(Bzl))'s consisting of one micelle slightly increased from 145 to 182.

Inhibition of mammary carcinoma cell growth by RXR is mediated by the receptor's oligomeric switch

Ligands that activate the nuclear receptor retinoid X receptor (RXR) display potent anticarcinogenic activities, but the mechanisms by which these compounds inhibit carcinoma cell growth are poorly understood. While RXR can regulate gene expression due to its intrinsic ligand-activated transcription function, this receptor can also regulate transcription by functioning as a ligand-controlled DNA architectural factor. It was thus reported that apo-RXR self-associates into tetramers and that each dimer within these tetramers can separately bind to an RXR response element. Hence, DNA binding by RXR tetramers may bring distant genomic regions into close physical proximity. As ligand binding induces the dissociation of RXR tetramers into dimers, it can alter gene expression by modulating the DNA architecture. Here, we show that inhibition of mammary carcinoma cell growth by RXR ligands stems from the ability of these compounds to regulate the oligomeric state of RXR and is independent of the direct intrinsic transcriptional activity of the receptor. The data suggest that compounds that trigger dissociation of RXR tetramers may comprise a novel class of anticarcinogenic agents.

Synthetic retinoids: structure-activity relationships

Retinoid signalling pathways are involved in numerous processes in cells, particularly those mediating differentiation and apoptosis. The endogenous ligands that bind to the retinoid receptors, namely all-trans-retinoic acid (ATRA) and 9-cis-retinoic acid, are prone to double-bond isomerisation and to oxidation by metabolic enzymes, which can have significant and deleterious effects on their activities and selectivities. Many of these problems can be overcome through the use of synthetic retinoids, which are often much more stable, as well as being more active. Modification of their molecular structures can result in retinoids that act as antagonists, rather than agonists, or exhibit a large degree of selectivity for particular retinoid-receptor isotypes. Several such selective retinoids are likely to be of value as pharmaceutical agents with reduced toxicities, particularly in cancer therapy, as reagents for controlling cell differentiation, and as tools for elucidating the precise roles that specific retinoid signalling pathways play within cells.

Retinoid receptor subtype-selective modulators through synthetic modifications of RARgamma agonists

A series of retinoids designed to interfere with the repositioning of H12 have been synthesized to identify novel RARgamma antagonists based on the structure of known RARgamma agonists. The transcriptional activities of the novel ligands were revealed by cell-based reporting assays, using engineered cells containg RAR subtype-selective fusions of the RAR ligand-binding domains with the yeast GAL4 activator DNA-binding domain and the cognate luciferase reporter gene. Whereas none of the ligands exhibited features of a selective RARgamma antagonist, some of them are endowed with interesting activities. In particular 24a acts as a pan-RAR agonist that induces at high concentration a higher transactivation potential on RARalpha than TTNPB and synergizes at low concentration with TTNPB-bound RARalpha but not RARbeta or RARgamma. Similarly, 24c synergizes with TTNPB-bound RARgamma and exhibits RARalpha,beta antagonist activity. Compounds 24b and 25b are strong RARalpha,beta-selective antagonists without agonist or antagonist activities for RARgamma. Compounds 24b and 24c display weak RXR antagonist activity. In addition several pan-antagonists and partial agonist/antagonists have been defined.

The ATRA-dependent overexpression of the glutamate transporter EAAC1 requires RARbeta induction

The mechanisms underlying trafficking and membrane targeting of EAAC1, the rodent counterpart of the human EAAT3 carrier for anionic amino acids, are well characterized. In contrast, much less is known on the regulation of Slc1a1, the gene that encodes for the transporter. We have recently found that all-trans retinoic acid (ATRA) stimulates EAAC1 expression and anionic amino acid transport in C6 rat glioma cells. We report here that the ATRA effect on EAAC1 activity was inhibited by the specific RAR antagonist LE540 and mimicked by Am80, a RAR agonist, but not by the RXR agonist HX630. Moreover, the ATRA-dependent induction of Slc1a1 mRNA required the synthesis of a protein intermediate and was not associated with changes in the messenger half-life. ATRA treatment induced the expression of both Rarb mRNA and RARbeta protein several hours before the induction of Slc1a1, while the mRNA for RFX1, a transcription factor recently involved in Slc1a1 transcription, was unchanged. In addition, Rarb silencing markedly inhibited the ATRA-dependent increase of both Rarb and Slc1a1 mRNAs. We conclude that in C6 glioma cells the induction of Slc1a1 by ATRA requires the synthesis of RARbeta, suggesting that the receptor is involved in the regulation of the transporter gene.

Retinoic acid receptor stimulation protects midbrain dopaminergic neurons from inflammatory degeneration via BDNF-mediated signaling

Functions of retinoic acid receptors (RARs) in adult CNS have been poorly characterized. Here we investigated potential neuroprotective action of tamibarotene (Am80), an RARalpha/beta agonist available for the treatment of acute promyelocytic leukemia, on midbrain dopaminergic neurons. Am80 protected dopaminergic neurons in rat midbrain slice culture from injury mediated by lipopolysaccharide-activated microglia, without affecting production of nitric oxide, a key mediator of cell injury. The effect of Am80 was mimicked by another RAR agonist, TAC-101, but not by a retinoid X receptor agonist, HX630, and HX630 did not synergize with Am80. We observed neuronal expression of RARalpha and RARbeta in midbrain slice culture and also found that Am80 increased tissue level of brain-derived neurotrophic factor (BDNF) mRNA. Exogenous BDNF prevented dopaminergic neurodegeneration, and the neuroprotective effect of Am80 was suppressed by a TrkB inhibitor, K252a, or by anti-BDNF neutralizing antibody. These results reveal a novel action of RARs mediated by enhancement of BDNF expression. Finally, oral administration of Am80 prevented dopaminergic cell loss in the substantia nigra induced by local injection of lipopolysaccharide in mice, indicating that RARs are a promising target of therapeutics for neurodegenerative disorders.