Synthesis and biological activity of high-affinity retinoic acid receptor antagonists

Molecular Interactions of Selective Agonists and Antagonists with the Retinoic Acid Receptor γ

All-trans retinoic acid (ATRA), the major active metabolite of all-trans retinol (vitamin A), is a key hormonal signaling molecule. In the adult organism, ATRA has a widespread influence on processes that are crucial to the growth and differentiation of cells and, in turn, the acquisition of mature cell functions. Therefore, there is considerable potential in the use of retinoids to treat diseases. ATRA binds to the retinoic acid receptors (RAR) which, as activated by ATRA, selectively regulate gene expression. There are three main RAR isoforms, RARα, RARβ, and RARγ. They each have a distinct role, for example, RARα and RARγ regulate myeloid progenitor cell differentiation and hematopoietic stem cell maintenance, respectively. Hence, targeting an isoform is crucial to developing retinoid-based therapeutics. In principle, this is exemplified when ATRA is used to treat acute promyelocytic leukemia (PML) and target RARα within PML-RARα oncogenic fusion protein. ATRA with arsenic trioxide has provided a cure for the once highly fatal leukemia. Recent in vitro and in vivo studies of RARγ have revealed the potential use of agonists and antagonists to treat diseases as diverse as cancer, heterotopic ossification, psoriasis, and acne. During the final drug development there may be a need to design newer compounds with added modifications to improve solubility, pharmacokinetics, or potency. At the same time, it is important to retain isotype specificity and activity. Examination of the molecular interactions between RARγ agonists and the ligand binding domain of RARγ has revealed aspects to ligand binding that are crucial to RARγ selectivity and compound activity and key to designing newer compounds.

Antagonizing RARγ Drives Necroptosis of Cancer Stem Cells

There is a need for agents that eliminate cancer stem cells, which sustain cancer and are also largely responsible for disease relapse and metastasis. Conventional chemotherapeutics and radiotherapy are often highly effective against the bulk of cancer cells, which are proliferating, but spare cancer stem cells. Therapeutics that target cancer stem cells may also provide a bona fide cure for cancer. There are two rationales for targeting the retinoic acid receptor (RAR)γ. First, RARγ is expressed selectively within primitive cells. Second, RARγ is a putative oncogene for a number of human cancers, including cases of acute myeloid leukemia, cholangiocarcinoma, and colorectal, renal and hepatocellular carcinomas. Prostate cancer cells depend on active RARγ for their survival. Antagonizing all RARs caused necroptosis of prostate and breast cancer stem cell-like cells, and the cancer stem cells that gave rise to neurospheres from pediatric patients’ primitive neuroectodermal tumors and an astrocytoma. As tested for prostate cancer, antagonizing RARγ was sufficient to drive necroptosis. Achieving cancer-selectively is a longstanding paradigm for developing new treatments. The normal prostate epithelium was less sensitive to the RARγ antagonist and pan-RAR antagonist than prostate cancer cells, and fibroblasts and blood mononuclear cells were insensitive. The RARγ antagonist and pan-RAR antagonist are promising new cancer therapeutics.

Targeting S100A9-ALDH1A1-retinoic acid signaling to suppress brain relapse in EGFR-mutant lung cancer

The epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) osimertinib has significantly prolonged progression-free survival (PFS) in EGFR-mutant lung cancer patients, including those with brain metastases. However, despite striking initial responses, osimertinib-treated patients eventually develop lethal metastatic relapse, often to the brain. Although osimertinib-refractory brain relapse is a major clinical challenge, its underlying mechanisms remain poorly understood. Using metastatic models of EGFR-mutant lung cancer, we show that cancer cells expressing high intracellular S100A9 escape osimertinib and initiate brain relapses. Mechanistically, S100A9 upregulates ALDH1A1 expression and activates the retinoic acid (RA) signaling pathway in osimertinib-refractory cancer cells. We demonstrate that the genetic repression of S100A9, ALDH1A1, or RA receptors (RAR) in cancer cells, or treatment with a pan-RAR antagonist, dramatically reduces brain metastasis. Importantly, S100A9 expression in cancer cells correlates with poor PFS in osimertinib-treated patients. Our study therefore identifies a novel, therapeutically targetable S100A9-ALDH1A1-RA axis that drives brain relapse.

From Propargylic Alcohols to Substituted Thiochromenes: gem-Disubstituent Effect in Intramolecular Alkyne Iodo/hydroarylation

This work describes the 6-endo-dig cyclization of S-aryl propargyl sulfides to afford 2H-thiochromenes. The substitution at the propargylic position plays a crucial role in allowing intramolecular silver-catalyzed alkyne hydroarylation and N-iodosuccinimide-promoted iodoarylation. Additionally, a PTSA-catalyzed thiolation reaction of propargylic alcohols was developed to synthesize the required tertiary S-aryl propargyl ethers. The applicability of merging these two methods is demonstrated by synthesizing the retinoic acid receptor antagonist AGN194310.

Retinoic acid receptor antagonists for male contraception: current status†

Retinoic acid receptor alpha (RARA), a nuclear receptor protein, has been validated as a target for male contraception by gene knockout studies and also pharmacologically using a pan-retinoic acid receptor antagonist. Retinoic acid receptor alpha activity is indispensable for the spermatogenic process, and therefore its antagonists have potential as male contraceptive agents. This review discusses the effects of systematic dosing regimen modifications of the orally bioavailable and reversible pan-antagonist BMS-189453 as well as studies with the alpha-selective antagonists BMS-189532 and BMS-189614 in a murine model. We also provide an overview of structure-activity studies of retinoic acid receptor alpha antagonists that provide insight for the design of novel alpha-selective ligands.

Design of synthetic retinoids

This chapter has been conceived as an introductory text to aid in the understanding of the key design strategies for the development of synthetic analogs of endogenous retinoids as ligands for the retinoic acid receptors (RARs) and retinoid X receptors (RXRs). The structure and binding characteristics of the endogenous retinoids are first explained to put the main chemical design challenges in context. Existing biochemical and structural data is then used to describe the guiding principles used to develop agonists and antagonists of the RARs and RXRs. In light of the increasing proliferation of biophysical methods that employ fluorescence measurements or molecular tags, we also examine the application of retinoids as probes and the chemical principles required to develop these tools.

Synthesis and Evaluation of Antileishmanial and Cytotoxic Activity of Benzothiopyrane Derivatives

In continuation of our efforts to identify promising antileishmanial agents based on the chroman scaffold, we synthesized several substituted 2H-thiochroman derivatives, including thiochromenes, thichromanones and hydrazones substituted in C-2 or C-3 with carbonyl or carboxyl groups. Thirty-two compounds were thus obtained, characterized, and evaluated against intracellular amastigotes of Leishmania (V)panamensis. Twelve compounds were active, with EC₅₀ values lower than 40 µM, but only four compounds displayed the highest antileishmanial activity, with EC₅₀ values below 10 µM; these all compounds possess a good Selectivity Index > 2.6. Although two active compounds were thiochromenes, a clear structure-activity relationship was not detected since each active compound has a different substitution pattern.

A Convenient Synthesis of 2-Amino-3-Cyano-4-Aryl-9,10-Dihydrobenzo[f] Chromene Derivatives Catalysed by KF/Al2O3

A series of 2-amino-3-cyano-4-aryl-9,10-dihydrobenzo[ f]chromene derivatives were synthesised from arylaldehyde, malononitrile with 7-methoxyl-1,2,3,4-tetrahydronaphthalene-2-one in ethyl alcohol at refluxing temperature catalysed by KF-Al2O3. The structure of the product was confirmed by X-ray analysis.

Unexpected Ring-Opening of a 2-Pyrone Ring in the Synthesis of 3-[(Z)-1-Hydroxy-3-Oxobut-1-Enyl]-2H-chromen-2-One Derivatives Catalysed by Kf-Alumina

A series of 3-[( Z)-1-hydroxy-3-oxobutenyl]-2 H-chromen-2-one derivatives were synthesised by an unexpected ring-opening of a 2-pyrone ring reaction of substituted salicylaldehydes, and 4-hydroxy-6-methyl-pyran-2-one in ethyl alcohol at room temperature catalysed by KF-Al2O3. The enol structure of the product not ketone was characterised by 1H NMR, IR and elemental analysis, and enol and ( Z)-structure further confirmed by X-ray diffraction analysis.

Probing biological activity through structural modelling of ligand-receptor interactions of 2,4-disubstituted thiazole retinoids

Retinoids, such as all-trans-retinoic acid (ATRA), regulate cellular differentiation and signalling pathways in chordates by binding to nuclear retinoic acid receptors (RARα/β/γ). Polar interactions between receptor and ligand are important for binding and facilitating the non-polar interactions and conformational changes necessary for RAR-mediated transcriptional regulation. The constraints on activity and RAR-type specificity with respect to the structural link between the polar and non-polar functions of synthetic retinoids are poorly understood. To address this, predictions from in silico ligand-RAR docking calculations and molecular dynamics simulations for a small library of stable, synthetic retinoids (designated GZ series) containing a central thiazole linker structure and different hydrophobic region substituents, were tested using a ligand binding assay and a range of cellular biological assays. The docking analysis showed that these thiazole-containing retinoids were well suited to the binding pocket of RARα, particularly via a favorable hydrogen bonding interaction between the thiazole and Ser232 of RARα. A bulky hydrophobic region (i.e., present in compounds GZ23 and GZ25) was important for interaction with the RAR binding pockets. Ligand binding assays generally reflected the findings from in silico docking, and showed that GZ25 was a particularly strongly binding ligand for RARα/β. GZ25 also exhibited higher activity as an inducer of neuronal differentiation than ATRA and other GZ derivatives. These data demonstrate that GZ25 is a stable synthetic retinoid with improved activity which efficiently regulates neuronal differentiation and help to define the key structural requirements for retinoid activity enabling the design and development of the next generation of more active, selective synthetic retinoids as potential therapeutic regulators of neurogenesis.

Regulation of human hematopoietic stem cell self-renewal by the microenvironment's control of retinoic acid signaling

The high expression of aldehyde dehydrogenase 1, also known as retinaldehyde dehydrogenase, by hematopoietic stem cells (HSCs) suggests an important role for retinoic acid (RA) signaling in determining the fate of these cells. We found that primitive human bone marrow-derived CD34(+)CD38(-) cells not only highly express aldehyde dehydrogenase 1, but also the RA receptor α. Despite the up-regulation of early components of RA signaling, the downstream pathway remained inactive in the primitive CD34(+)CD38(-) cells. Primitive hematopoietic cells rapidly undergo terminal differentiation when cultured away from their microenvironment; however, we found that inhibition of RA signaling maintained their primitive phenotype and function, and promoted their self-renewal. HSCs reside in a complex microenvironment that enforces the balance between self-renewal and differentiation. The exact physiologic mechanisms by which the niche controls HSC fate remain elusive. The embryonic gonadal microenvironment has recently been shown to determine germ-cell fate by degrading RA through expression of the P450 retinoid-inactivating enzyme CYP26B1. We found that the bone marrow microenvironment similarly can control primitive hematopoietic cell fate via modulation of retinoid bioavailability. Accordingly, we found that bone marrow stromal cell CYP26 was also able to inactivate retinoids in serum, preventing RA signaling. Thus, primitive hematopoietic cells appear to be intrinsically programmed to undergo RA-mediated differentiation unless prevented from doing so by bone marrow niche CYP26. Modulation of RA signaling also holds promise for clinical HSC expansion, a prerequisite for the wide-scale use of these cells in regenerative medicine and gene therapy.

C-Alkynylation of Chromones by Sonogashira Reaction

Sonogashira reaction of bromochromones and -flavones with a bromine atom on their benzene or heterocyclic ring with various terminal alkynes gave the desired products with nearly the same efficiency as the previously used iodine derivatives. The coupling reactions were performed in the presence of [tetrakis(triphenylphosphine)palladium(0)], copper(i) co-catalyst, and triethylamine, resulting in the formation of numerous hitherto unknown alkynylated oxygen heterocycles, and provide further proof for the applicability of this reaction for these O-heterocycles. Chromones with ethynyl functionality were prepared by removal of the trimethylsilyl protecting group and used as terminal alkynes in a second cross-coupling reaction.

Genetic deletion of Cyp26b1 negatively impacts limb skeletogenesis by inhibiting chondrogenesis

Cyp26b1, a retinoic acid (RA)-metabolising enzyme, is expressed in the developing limb bud, and Cyp26b1(-/-) mice present with severe limb defects. These malformations might be attributable to an RA-induced patterning defect; however, recent reports suggest that RA is dispensable for limb patterning. In this study, we examined the role of endogenous retinoid signalling in skeletogenesis using Cyp26b1(-/-) mice and transgenic mice in which Cyp26b1 is conditionally deleted under control of the Prrx1 promoter beginning at ~E9.5 (Prrx1Cre(+)/Cyp26b1(fl/fl)). We found that the limb phenotype in Prrx1Cre(+)/Cyp26b1(fl/fl) mice was less severe than that observed in Cyp26b1(-/-) animals and that a change in retinoid signalling contributed to the difference in phenotypes. We systematically examined the role of endogenous RA signalling in chondrogenesis and found that Cyp26b1(-/-) cells and limb mesenchymal cells treated with a CYP inhibitor, are maintained in a pre-chondrogenic state, exhibit reduced chondroblast differentiation and have modestly accelerated chondrocyte hypertrophy. Furthermore, Cyp26b1(-/-) mesenchyme exhibited an increase in expression of genes in a closely related tendogenic lineage, indicating that retinoid signals in the limb interfere with differentiation and maintain progenitor status. Together, these findings support an important function for RA in regulating the behaviour of mesenchymal progenitors, and their subsequent differentiation and maturation.

Lymphocytic choriomeningitis virus (LCMV) infection of macaques: a model for Lassa fever

Arenaviruses such as Lassa fever virus (LASV) and lymphocytic choriomeningitis virus (LCMV) are benign in their natural reservoir hosts, and can occasionally cause severe viral hemorrhagic fever (VHF) in non-human primates and in human beings. LCMV is considerably more benign for human beings than Lassa virus, however certain strains, like the LCMV-WE strain, can cause severe disease when the virus is delivered as a high-dose inoculum. Here we describe a rhesus macaque model for Lassa fever that employs a virulent strain of LCMV. Since LASV must be studied within Biosafety Level-4 (BSL-4) facilities, the LCMV-infected macaque model has the advantage that it can be used at BSL-3. LCMV-induced disease is rarely as severe as other VHF, but it is similar in cases where vascular leakage leads to lethal systemic failure. The LCMV-infected macaque has been valuable for describing the course of disease with differing viral strains, doses and routes of infection. By monitoring system-wide changes in physiology and gene expression in a controlled experimental setting, it is possible to identify events that are pathognomonic for developing VHF and potential treatment targets.

Retinoic Acid Receptor Antagonist Inhibits CD38 Antigen Expression on Human Hematopoietic CellsIn Vitro

The CD34+ CD38- subset of human hematopoietic stem cells are crucial for long-term ex-vivo expansion; conditions that decreased this specific sub-population reduced the self-renewal capacity and shortened the duration of the proliferative phase of the culture. Retinoids, such as all-trans retinoic acid (ATRA), have been shown to induce CD38 expression. ATRA present in serum may be responsible for the high CD38 of cells grown in serum-containing medium. In the present study we analyzed the effects of AGN 194310, a retinoic acid receptor pan-antagonist, on CD38 expression of human hematopoietic cells. Normal cells (cord blood derived CD34+ cells) and abnormal cells (myeloid leukemic lines) were studied when grown in either serum-containing or serum-free media. The results showed that both serum and ATRA enhanced differentiation and, thereby, reduced the proportion of CD34+ CD38- cells and total CD34+ cell expansion. AGN reversed these effects of serum and ATRA: it delayed differentiation and increased CD34+ CD38- cells. These results suggest that physiological ATRA levels in serum may prevent efficient cell expansion. AGN, by neutralizing ATRA, improves cell expansion in serum-containing cultures, thus making AGN a useful agent for ex vivo expansion of stem cells and other specific sub-populations for research and clinical use.

N-(4-hydroxyphenyl)retinamide induces apoptosis in human retinal pigment epithelial cells: retinoic acid receptors regulate apoptosis, reactive oxygen species generation, and the expression of heme oxygenase-1 and Gadd153

N-(4-hydroxyphenyl)retinamide (4HPR, fenretinide), a retinoic acid (RA) derivative and a potential cancer preventive agent, is known to exert its chemotherapeutic effects in cancer cells through induction of apoptosis. Earlier work from our laboratory has shown that relatively low concentrations of 4HPR induce neuronal differentiation of cultured human retinal pigment epithelial (ARPE-19) cells (Chen et al., 2003, J Neurochem 84:972-981). However, at higher concentrations of 4HPR, these cells showed morphological changes including cell shrinkage and cell death. Here we demonstrate that ARPE-19 cells treated with 4HPR exhibit a dose- and time-dependent induction of apoptosis as evidenced by morphological changes, mono- and oligonucleosome generation, and increased activity of caspases 2 and 3. The 4HPR-induced apoptosis as well as the activation of caspases 2 and 3 were blocked by both retinoic acid receptors (RAR) pan-antagonists, AGN193109 and AGN194310, and by an RARalpha-specific antagonist AGN194301. 4HPR treatment also increased reactive oxygen species (ROS) generation in ARPE-19 cells in a time-dependent manner as determined from the oxidation of 2',7'-dichlorofluorescin. In addition, the increase in the expression of heme oxygenase-1 (HO-1), a stress response protein, and the growth arrest and DNA damage-inducible transcription factor 153 (Gadd153) in response to the ROS generation were also blocked by these receptor antagonists. Pyrrolidine dithiocarbamate (PDTC), a free-radical scavenger, inhibited 4HPR-induced ROS generation, the expression of its downstream mediator, Gadd153, and apoptosis in the pretreated cells. Therefore, our results, clearly demonstrate that 4HPR induces apoptosis in ARPE-19 cells and that RARs mediate this process by regulating ROS generation as well as the expression of Gadd153 and HO-1.

Regulation of hematopoiesis by retinoid signaling

The discovery that retinoic acid efficiently stimulates the terminal differentiation of granulocytic leukemia cells had a major impact on clinical hematology, but has also inspired research into the normal function of the retinoid signaling pathway during hematopoiesis. New animal models and loss-of-function approaches have successfully revealed requirements for the pathway at defined embryonic stages that are relevant for distinct hematopoietic cell populations. For example, novel insight has been gained regarding the function of retinoids in yolk sac hematovascular development, fetal erythropoiesis, T-cell homing, and hematopoietic stem and progenitor cell biology. The lessons learned so far indicate that future development of sophisticated animal models will be needed to fully understand the intricacy and specificity of this complex signaling pathway, but that this effort will be productive and continue to inform both basic and clinical research on many fronts.

Independent roles for retinoic acid in segmentation and neuronal differentiation in the zebrafish hindbrain

Segmentation of the vertebrate hindbrain into rhombomeres is essential for the anterior-posterior patterning of cranial motor nuclei and their associated nerves. The vitamin A derivative, retinoic acid (RA), is an early embryonic signal that specifies rhombomeres, but its roles in neuronal differentiation within the hindbrain remain unclear. Here we have analyzed the formation of primary and secondary hindbrain neurons in the zebrafish mutant neckless (nls), which disrupts retinaldehyde dehydrogenase 2 (raldh2), and in embryos treated with retinoid receptor (RAR) antagonists. Mutation of nls disrupts secondary, branchiomotor neurons of the facial and vagal nerves, but not the segmental pattern of primary, reticulospinal neurons, suggesting that RA acts on branchiomotor neurons independent of its role in hindbrain segmentation. Very few vagal motor neurons form in nls mutants and many facial motor neurons do not migrate out of rhombomere 4 into more posterior segments. When embryos are treated with RAR antagonists during gastrulation, we observe more severe patterning defects than seen in nls. These include duplicated reticulospinal neurons and posterior expansions of rhombomere 4, as well as defects in branchiomotor neurons. However, later antagonist treatments after rhombomeres are established still disrupt branchiomotor development, suggesting that requirements for RARs in these neurons occur later and independent of segmental patterning. We also show that RA produced by the paraxial mesoderm controls branchiomotor differentiation, since we can rescue the entire motor innervation pattern by transplanting wild-type cells into the somites of nls mutants. Thus, in addition to its role in determining rhombomere identities, RA plays a more direct role in the differentiation of subsets of branchiomotor neurons within the hindbrain.

Identification of the molecular requirements for an RAR alpha-mediated cell cycle arrest during granulocytic differentiation

Retinoids are potent inducers of cell cycle arrest and differentiation of numerous cell types, notably granulocytes. However the mechanisms by which retinoids mediate cell cycle arrest during differentiation remain unclear. We have used myeloid differentiation to characterize the molecular pathways that couple cell cycle withdrawal to terminal differentiation. Using primary cells from mice deficient for either the cyclin-dependent kinase inhibitor (CDKi) p27(Kip1), the Myc antagonist Mad1, or both Mad1 and p27(Kip1), we observed that signals mediated through retinoic acid receptor alpha (RAR alpha), but not RAR beta or gamma, required both Mad1 and p27(Kip1) to induce cell cycle arrest and to accelerate terminal differentiation of granulocytes. Although RAR alpha did not directly regulate Mad1 or p27(Kip1), the RAR alpha target gene C/EBP epsilon directly regulated transcription of Mad1. Induction of C/EBP epsilon activity in granulocytic cells led to rapid induction of Mad1 protein and transcript, with direct binding of C/EBP epsilon to the Mad1 promoter demonstrated through chromatin immunoprecipitation assay. These data demonstrate that cell cycle arrest in response to RAR alpha specifically requires Mad1 and p27(Kip1) and that Mad1 is transcriptionally activated by CCAAT/enhancer-binding protein epsilon (C/EBP epsilon). Moreover, these data demonstrate selectivity among the RARs for cell cycle arrest pathways and provide a direct mechanism to link differentiation induction and regulation of the Myc antagonist Mad1.

Adenomatous polyposis coli (APC)-independent regulation of beta-catenin degradation via a retinoid X receptor-mediated pathway

Beta-catenin is a component of stable cell adherent complexes whereas its free form functions as a transcription factor that regulate genes involved in oncogenesis and metastasis. Free beta-catenin is eliminated by two adenomatous polyposis coli (APC)-dependent proteasomal degradation pathways regulated by glycogen synthase kinase 3beta (GSK3 beta) or p53-inducible Siah-1. Dysregulation of beta-catenin turnover consequent to mutations in critical genes of the APC-dependent pathways is implicated in cancers such as colorectal cancer. We have identified a novel retinoid X receptor (RXR)-mediated APC-independent pathway in the regulation of beta-catenin. In this proteasomal pathway, RXR agonists induce degradation of beta-catenin and RXR alpha and repress beta-catenin-mediated transcription. In vivo, beta-catenin interacts with RXR alpha in the absence of ligand, but RXR agonists enhanced the interaction. RXR agonist action was not impaired by GSK3 beta inhibitors or deletion of the GSK3 beta-targeted sequence from beta-catenin. In APC- and p53-mutated colorectal cancer cells, RXR agonists still inactivated endogenous beta-catenin via RXR alpha. Interestingly, deletion of the RXR alpha A/B region abolished ligand-induced beta-catenin degradation but not RXR alpha-mediated transactivation. RXR alpha-mediated inactivation of oncogenic beta-catenin paralleled a reduction in cell proliferation. These results suggest a potential role for RXR and its agonists in the regulation of beta-catenin turnover and related biological events.

BMP-2 mediates retinoid-induced apoptosis in medulloblastoma cells through a paracrine effect

The mechanisms of retinoid activity in tumors remain largely unknown. Here we establish that retinoids cause extensive apoptosis of medulloblastoma cells. In a xenograft model, retinoids largely abrogated tumor growth. Using receptor-specific retinoid agonists, we defined a subset of mRNAs that were induced by all active retinoids in retinoid-sensitive cell lines. We also identified bone morphogenetic protein-2 (BMP-2) as a candidate mediator of retinoid activity. BMP-2 protein induced medulloblastoma cell apoptosis, whereas the BMP-2 antagonist noggin blocked both retinoid and BMP-2-induced apoptosis. BMP-2 also induced p38 mitogen-activated protein kinase (MAPK), which is necessary for BMP-2- and retinoid-induced apoptosis. Retinoid-resistant medulloblastoma cells underwent apoptosis when treated with BMP-2 or when cultured with retinoid-sensitive medulloblastoma cells. Retinoid-induced expression of BMP-2 is thus necessary and sufficient for apoptosis of retinoid-responsive cells, and expression of BMP-2 by retinoid-sensitive cells is sufficient to induce apoptosis in surrounding retinoid-resistant cells.

Retinoic acid receptor antagonism in vivo expands the numbers of precursor cells during granulopoiesis

The role/s of retinoids in granulopoiesis has been recognised for many years, being powerful differentiation inducers. The physiological role/s of retinoic acid receptor (RAR)-mediated signalling during adult haemopoiesis has by contrast proved more elusive. The recent generation of highly specific pan-RAR antagonists has now made possible an assessment of the specific physiological role/s of RAR signalling, allowing the separation for the first time of the RAR and RXR pathways. Mice were treated with AGN194310, a synthetic retinoid that antagonises the physiological function of the three RAR isotypes (alpha, beta, gamma) but does not interact with RXRs. Analyses of the granulocytic lineage using Gr-1, c-Kit and CD11b antibodies, demonstrated that granulocyte numbers were strikingly increased across haemopoietic compartments in all AGN194310-treated mice. A significant increase in the frequency of progenitor cells containing granulocytes was observed in the bone marrow of mice following treatment with AGN194310. In contrast we were not able to detect any differences in cell death of either mature granulocytes or granulocytic progenitors from AGN194310-treated mice compared with control animals. These data demonstrate an essential role for RAR signalling in regulating the numbers of granulocytic precursors in vivo.

The prospects of retinoids in the treatment of prostate cancer

Prostate cancer is the most prevalent cancer amongst males and accounts for 13% of cancer deaths in this population in the US. Aggressive, androgen-independent, metastatic prostate cancer is incurable, and the search for new therapies has been directed towards identifying agents that block proliferation and induce differentiation and/or apoptosis of prostate cancer cells. Retinoid receptor agonists, such as all- retinoic acid, can induce apoptosis of prostate cancer cells, but clinical studies have demonstrated only mild to moderate efficacy. Retinoic acid receptor antagonists are a new class of retinoids, and pre-clinical studies have shown that they potently inhibit the growth of prostate cancer cells and induce apoptosis. Here, we review whether retinoids have a role in the fight against prostate cancer.

Requirement for RAR-mediated gene repression in skeletal progenitor differentiation

Chondrogenesis is a multistep process culminating in the establishment of a precisely patterned template for bone formation. Previously, we identified a loss in retinoid receptor-mediated signaling as being necessary and sufficient for expression of the chondroblast phenotype (Weston et al., 2000. J. Cell Biol. 148:679-690). Here we demonstrate a close association between retinoic acid receptor (RAR) activity and the transcriptional activity of Sox9, a transcription factor required for cartilage formation. Specifically, inhibition of RAR-mediated signaling in primary cultures of mouse limb mesenchyme results in increased Sox9 expression and activity. This induction is attenuated by the histone deacetylase inhibitor, trichostatin A, and by coexpression of a dominant negative nuclear receptor corepressor-1, indicating an unexpected requirement for RAR-mediated repression in skeletal progenitor differentiation. Inhibition of RAR activity results in activation of the p38 mitogen-activated protein kinase (MAPK) and protein kinase A (PKA) pathways, indicating their potential role in the regulation of chondrogenesis by RAR repression. Accordingly, activation of RAR signaling, which attenuates differentiation, can be rescued by activation of p38 MAPK or PKA. In summary, these findings demonstrate a novel role for active RAR-mediated gene repression in chondrogenesis and establish a hierarchical network whereby RAR-mediated signaling functions upstream of the p38 MAPK and PKA signaling pathways to regulate emergence of the chondroblast phenotype.

Antagonists of retinoic acid receptors (RARs) are potent growth inhibitors of prostate carcinoma cells

Novel synthetic antagonists of retinoic acid receptors (RARs) have been developed. To avoid interference by serum retinoids when testing these compounds, we established serum-free grown sub-lines (>3 years) of the prostate carcinoma lines LNCaP, PC3 and DU145. A high affinity pan-RAR antagonist (AGN194310, K(d) for binding to RARs = 2-5 nM) inhibited colony formation (by 50%) by all three lines at 16-34 nM, and led to a transient accumulation of flask-cultured cells in G1 followed by apoptosis. AGN194310 is 12-22 fold more potent than all-trans retinoic acid (ATRA) against cell lines and also more potent in inhibiting the growth of primary prostate carcinoma cells. PC3 and DU145 cells do not express RARbeta, and an antagonist with predominant activity at RARbeta and RARgamma (AGN194431) inhibited colony formation at concentrations (approximately 100 nM) commensurate with a K(d)value of 70 nM at RARgamma. An RARalpha antagonist (AGN194301) was less potent (IC(50) approximately 200 nM), but was more active than specific agonists of RARalpha and of betagamma. A component(s) of serum and of LNCaP-conditioned medium diminishes the activity of antagonists: this factor is not the most likely candidates IGF-1 and EGF. In vitro studies of RAR antagonists together with data from RAR-null mice lead to the hypothesis that RARgamma-regulated gene transcription is necessary for the survival and maintenance of prostate epithelium. The increased potencies of RAR antagonists, as compared with agonists, suggest that antagonists may be useful in the treatment of prostate carcinoma.

Phenylcyclohexene and phenylcyclohexadiene substituted compounds having retinoid antagonist activity

Retinoids are natural and synthetic analogues of the hormone retinoic acid. Systemic retinoid agonist therapy is usually associated with toxic side effects, such as mucocutaneous toxicity, which may be alleviated by the use of topical retinoid antagonists. We report the synthesis and biological activity of a new series of potent, RAR-specific antagonists substituted with phenylcyclohexene and phenylcyclohexadiene groups.