C3 halogen and c8'' substituents on stilbene arotinoids modulate retinoic Acid receptor subtype function

Solid-Phase Synthesis of Cereblon-Recruiting Selective Histone Deacetylase 6 Degraders (HDAC6 PROTACs) with Antileukemic Activity

In this work, we utilized the proteolysis targeting chimera (PROTAC) technology to achieve the chemical knock-down of histone deacetylase 6 (HDAC6). Two series of cereblon-recruiting PROTACs were synthesized via a solid-phase parallel synthesis approach, which allowed the rapid preparation of two HDAC6 degrader mini libraries. The PROTACs were either based on an unselective vorinostat-like HDAC ligand or derived from a selective HDAC6 inhibitor. Notably, both PROTAC series demonstrated selective degradation of HDAC6 in leukemia cell lines. The best degraders from each series (denoted A6 and B4) were capable of degrading HDAC6 via ternary complex formation and the ubiquitin-proteasome pathway, with DC₅₀ values of 3.5 and 19.4 nM, respectively. PROTAC A6 demonstrated promising antiproliferative activity via inducing apoptosis in myeloid leukemia cell lines. These findings highlight the potential of this series of degraders as effective pharmacological tools for the targeted degradation of HDAC6.

Overview of all-trans-retinoic acid (ATRA) and its analogues: Structures, activities, and mechanisms in acute promyelocytic leukaemia

All-trans-retinoic acid (ATRA) is effective for preventing cancer and treating skin diseases and acute promyelocytic leukaemia (APL). These pharmacological effects of ATRA are mainly mediated by retinoid X receptors (RXRs) and retinoic acid receptors (RARs). This article provides a comprehensive overview of the clinical progress on and the molecular mechanisms of ATRA in the treatment of APL. ATRA can promote the transcriptional activation of differentiation-related genes and regulate autophagy by inhibiting mTOR, which results in anti-APL effects. In detail, the structures, pharmacological effects, and clinical studies of 68 types of ATRA analogues are described. These compounds have excellent antitumour therapeutic potential and could be used as lead compounds for further development and research.

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.

Distinct retinoic acid receptor (RAR) isotypes control differentiation of embryonal carcinoma cells to dopaminergic or striatopallidal medium spiny neurons

Embryonal carcinoma (EC) cells are pluripotent stem cells extensively used for studies of cell differentiation. Although retinoic acid (RA) is a powerful inducer of neurogenesis in EC cells, it is not clear what specific neuronal subtypes are generated and whether different RAR isotypes may contribute to such neuronal diversification. Here we show that RA treatment during EC embryoid body formation is a highly robust protocol for generation of striatal-like GABAergic neurons which display molecular characteristics of striatopallidal medium spiny neurons (MSNs), including expression of functional dopamine D2 receptor. By using RARα, β and γ selective agonists we show that RARγ is the functionally dominant RAR in mediating RA control of early molecular determinants of MSNs leading to formation of striatopallidal-like neurons. In contrast, activation of RARα is less efficient in generation of this class of neurons, but is essential for differentiation of functional dopaminergic neurons, which may correspond to a subpopulation of inhibitory dopaminergic neurons expressing glutamic acid decarboxylase in vivo.

Isolation and Asymmetric Total Synthesis of Perforanoid A

A novel limonoid, perforanoid A, was isolated, and an asymmetric total synthesis was achieved in 10 steps. The key steps are chiral tertiary aminonaphthol mediated enantioselective alkenylation of an aldehyde to an allylic alcohol, Pd-catalyzed coupling of the allylic alcohol with vinyl ether to form the γ-lactone ring, and cyclopentenone ring formation through a Rh-catalyzed Pauson-Khand reaction. Preliminary studies show that perforanoid A is cytotoxic towards HEL, K562, and CB3 tumor cell lines.

Studies on Antibiotics Active against Resistant Bacteria. Total Synthesis of MRSA-Active Tetarimycin A and Its Analogues

Making use of the Hauser-Kraus annulation as a key step, the first total synthesis of tetarimycin A has been accomplished in a highly convergent and operationally simple manner. Preliminary SAR not only validated that tetarimycin A exhibited potent activity against MRSA and VRE at a low MIC value but also identified that the hydroxyl group at C-10 was essential for antibacterial activities.

Modulation of Retinoic Acid Receptor Subtypes by 5- and 8-Substituted (Naphthalen-2-yl)-based Arotinoids

Retinoid receptors (RARs and RXRs) transduce the signals of their natural and synthetic ligands (retinoids and rexinoids) to cellular transcriptional machinery to induce gene programs that control diverse biological and physiological effects on organisms. All-trans-retinoic acid, the natural ligand for RARs, is used therapeutically for the treatment of acute promyelocytic leukemia (APL), whereas the synthetic rexinoid bexarotene (a representative member of the aromatic retinoids or arotinoids) is approved for the treatment of cutaneous T-cell lymphoma (CTCL). Other retinoids have found applications in the topical treatment of skin disorders. In continuation of previous work on the naphthalene-based arotinoid scaffold, we synthesized a new series of (3-halo)benzoic acids connected to C5- or C8-substituted naphthyl rings via (E)-ethenyl and amide and, for the C5 series, (E)-chalcone linkers. These compounds were evaluated as RAR modulators in comparison with previously described dihydronaphthalene arotinoids with the same substitution pattern. Transactivation studies in this series revealed an absence of synergy between small halogen atoms (F, Cl) at C3 and the groups at C5 or C8, as had been observed on some of the dihydronaphthalene analogues. Instead, non-halogenated 4-(2-naphthamido)benzoic acid derivatives transactivated toward the RARβ subtype in preference to the paralogues. The derivatives with bulkier substituents at C8 were characterized as dual RARβ/RARα antagonists, and (E)-4-[(8-(phenylethynyl)naphthalene-2-yl)ethenyl]benzoic acid (11 c), with an ethenyl connector, was shown to be a potent antagonist of RARα.

An Unexpected Mode Of Binding Defines BMS948 as A Full Retinoic Acid Receptor β (RARβ, NR1B2) Selective Agonist

Retinoic acid is an important regulator of cell differentiation which plays major roles in embryonic development and tissue remodeling. The biological action of retinoic acid is mediated by three nuclear receptors denoted RARα, β and γ. Multiple studies support that RARβ possesses functional characteristics of a tumor suppressor and indeed, its expression is frequently lost in neoplastic tissues. However, it has been recently reported that RARβ could also play a role in mammary gland tumorigenesis, thus demonstrating the important but yet incompletely understood function of this receptor in cancer development. As a consequence, there is a great need for RARβ-selective agonists and antagonists as tools to facilitate the pharmacological analysis of this protein in vitro and in vivo as well as for potential therapeutic interventions. Here we provide experimental evidences that the novel synthetic retinoid BMS948 is an RARβ-selective ligand exhibiting a full transcriptional agonistic activity and activating RARβ as efficiently as the reference agonist TTNPB. In addition, we solved the crystal structures of the RARβ ligand-binding domain in complex with BMS948 and two related compounds, BMS641 and BMS411. These structures provided a rationale to explain how a single retinoid can be at the same time an RARα antagonist and an RARβ full agonist, and revealed the structural basis of partial agonism. Finally, in addition to revealing that a flip by 180° of the amide linker, that usually confers RARα selectivity, accounts for the RARβ selectivity of BMS948, the structural analysis uncovers guidelines for the rational design of RARβ-selective antagonists.

Dual RXR Agonists and RAR Antagonists Based on the Stilbene Retinoid Scaffold

Arotinoids containing a C5,C8-diphenylnaphthalene-2-yl ring linked to a (C3-halogenated) benzoic acid via an ethenyl connector (but not the corresponding naphthamides), which are prepared by Horner-Wadsworth-Emmons reaction of naphthaldehydes and benzylphosphonates, display the rather unusual property of being RXR agonists (15-fold induction of the RXR reporter cell line was achieved at 3- to 10-fold lower concentration than 9-cis-retinoic acid) and RAR antagonists as shown by transient transactivation studies. The binding of such bulky ligands suggests that the RXR ligand-binding domain is endowed with some degree of structural elasticity.

The class I-specific HDAC inhibitor MS-275 modulates the differentiation potential of mouse embryonic stem cells

Exploitation of embryonic stem cells (ESC) for therapeutic use and biomedical applications is severely hampered by the risk of teratocarcinoma formation. Here, we performed a screen of selected epi-modulating compounds and demonstrate that a transient exposure of mouse ESC to MS-275 (Entinostat), a class I histone deacetylase inhibitor (HDAC), modulates differentiation and prevents teratocarcinoma formation. Morphological and molecular data indicate that MS-275-primed ESCs are committed towards neural differentiation, which is supported by transcriptome analyses. Interestingly, in vitro withdrawal of MS-275 reverses the primed cells to the pluripotent state. In vivo, MS275-primed ES cells injected into recipient mice give only rise to benign teratomas but not teratocarcinomas with prevalence of neural-derived structures. In agreement, MS-275-primed ESC are unable to colonize blastocysts. These findings provide evidence that a transient alteration of acetylation alters the ESC fate.

Targeting the heat shock protein 90 dimer with dimeric inhibitors

The design, synthesis, and biological evaluation of conformationally constrained coumermycin A1 analogues are reported. Compounds were evaluated against both breast cancer (SKBr3 and MCF7) and prostate cancer (PC3 mm2, A549, and HT29) cell lines. Non-noviosylated coumermycin A1 analogues that manifest potent antiproliferative activity resulting from Hsp90 inhibition are provided, wherein replacement of the stereochemically complex noviose sugar with readily available piperidine rings resulted in ∼100 fold increase in antiproliferative activities as compared to coumermycin A1, producing small molecule Hsp90 inhibitors that exhibit nanomolar activities.

Retinoic acid receptor modulators: a perspective on recent advances and promises

IMPORTANCE OF THE FIELD

Retinoids are currently used in the clinic for the treatment of skin diseases and acute promielocytic leukemia and are known to contribute to early development and organogenesis in embryo and throughout life. Most of these activities are primarily due to the binding of the retinoid to the retinoic acid receptors (RARs, subtypes α, β and γ). Ligand modulates, via allosteric conformational changes, the ability of RARs to interact with different sets of co-regulators. Structure-based insights on the ligand-binding domain of the ligand-bound RARs have clearly linked retinoid function to co-activator (CoA) recruitment for agonists, CoA dissociation for antagonists and corepressor stabilization for inverse agonists.

AREAS COVERED IN THIS REVIEW

To help understand ligand-modulated RAR action as a consequence of its interaction with different sets of co-regulators, we present the chemical engineering of subsets of retinoid chemotypes (rexinoids, i.e., the ligands of the retinoid X receptors, α, β and γ, with impact in the treatment of cancer and metabolic diseases, are not covered) that display the whole range of ligand functions, including subtype- and isotype-selectivities.

WHAT THE READER WILL GAIN

An understanding of the correlation of retinoid ligand structure and function. Structural insights into ligand action and retinoid chemotypes. Potential for clinical application of retinoid receptor modulators.

TAKE HOME MESSAGE

Potential pharmacological/therapeutic applications of these chemical tools extend beyond cancer prevention and therapy to the treatment of autoimmune and neurodegenerative diseases.

Retinoid-independent motor neurogenesis from human embryonic stem cells reveals a medial columnar ground state

A major challenge in neurobiology is to understand mechanisms underlying human neuronal diversification. Motor neurons (MNs) represent a diverse collection of neuronal subtypes, displaying differential vulnerability in different human neurodegenerative diseases. The ability to manipulate cell subtype diversification is critical to establish accurate, clinically relevant in vitro disease models. Retinoid signalling contributes to caudal precursor specification and subsequent MN subtype diversification. Here we investigate the necessity for retinoic acid in motor neurogenesis from human embryonic stem cells. We show that activin/nodal signalling inhibition, followed by sonic hedgehog agonist treatment, is sufficient for MN precursor specification, which occurs even in the presence of retinoid pathway antagonists. Importantly, precursors mature into HB9/ChAT-expressing functional MNs. Furthermore, retinoid-independent motor neurogenesis results in a ground state biased to caudal, medial motor columnar identities from which a greater retinoid-dependent diversity of MNs, including those of lateral motor columns, can be selectively derived in vitro.

Nolz1 promotes striatal neurogenesis through the regulation of retinoic acid signaling

Background

Nolz1 is a zinc finger transcription factor whose expression is enriched in the lateral ganglionic eminence (LGE), although its function is still unknown.

Results

Here we analyze the role of Nolz1 during LGE development. We show that Nolz1 expression is high in proliferating neural progenitor cells (NPCs) of the LGE subventricular zone. In addition, low levels of Nolz1 are detected in the mantle zone, as well as in the adult striatum. Similarly, Nolz1 is highly expressed in proliferating LGE-derived NPC cultures, but its levels rapidly decrease upon cell differentiation, pointing to a role of Nolz1 in the control of NPC proliferation and/or differentiation. In agreement with this hypothesis, we find that Nolz1 over-expression promotes cell cycle exit of NPCs in neurosphere cultures and negatively regulates proliferation in telencephalic organotypic cultures. Within LGE primary cultures, Nolz1 over-expression promotes the acquisition of a neuronal phenotype, since it increases the number of β-III tubulin (Tuj1)- and microtubule-associated protein (MAP)2-positive neurons, and inhibits astrocyte generation and/or differentiation. Retinoic acid (RA) is one of the most important morphogens involved in striatal neurogenesis, and regulates Nolz1 expression in different systems. Here we show that Nolz1 also responds to this morphogen in E12.5 LGE-derived cell cultures. However, Nolz1 expression is not regulated by RA in E14.5 LGE-derived cell cultures, nor is it affected during LGE development in mouse models that present decreased RA levels. Interestingly, we find that Gsx2, which is necessary for normal RA signaling during LGE development, is also required for Nolz1 expression, which is lost in Gsx2 knockout mice. These findings suggest that Nolz1 might act downstream of Gsx2 to regulate RA-induced neurogenesis. Keeping with this hypothesis, we show that Nolz1 induces the selective expression of the RA receptor (RAR)β without altering RARα or RARγ. In addition, Nozl1 over-expression increases RA signaling since it stimulates the RA response element. This RA signaling is essential for Nolz1-induced neurogenesis, which is impaired in a RA-free environment or in the presence of a RAR inverse agonist. It has been proposed that Drosophila Gsx2 and Nolz1 homologues could cooperate with the transcriptional co-repressors Groucho-TLE to regulate cell proliferation. In agreement with this view, we show that Nolz1 could act in collaboration with TLE-4, as they are expressed at the same time in NPC cultures and during mouse development.

Conclusions

Nolz1 promotes RA signaling in the LGE, contributing to the striatal neurogenesis during development.

Directional Weak Intermolecular Interactions: σ-Hole Bonding

The prototypical directional weak interactions, hydrogen bonding and σ-hole bonding (including the special case of halogen bonding) are reviewed in a united picture that depends on the anisotropic nature of the molecular electrostatic potential around the donor atom. Qualitative descriptions of the effects that lead to these anisotropic distributions are given and examples of the importance of σ-hole bonding in crystal engineering and biological systems are discussed.

Inverse agonists and antagonists of retinoid receptors

Nuclear receptors (NRs) are ligand-inducible transcription factors that regulate a plethora of cell biological phenomena, thus orchestrating complex events like development, organ homeostasis, immune function, and reproduction. Due to their regulatory potential, NRs are major drug targets for a variety of diseases, including cancer and metabolic diseases, and had a major societal impact following the development of contraceptives and abortifacients. Not surprisingly in view of this medical and societal importance, a large amount of diverse NR ligands have been generated and the corresponding structural and functional analyses have provided a deep insight into the molecular basis of ligand action. What we have learnt is that ligands regulate, via allosteric conformational changes, the ability of NRs to interact with different sets of coregulators which in turn recruit enzymatically active complexes, the workhorses of the ligand-induced epigenetic and transcription-regulatory events. Thus, ligands essentially direct the communication of a given NR with its intracellular environment at the chromatin and extragenomic level to modulate gene programs directly at the chromatin level or via less well-understood extranuclear actions. Here we will review our current structural and mechanistic insight into the functionalities of subsets of retinoid and rexinoid ligands that act generically as antagonists but follow different mechanistic principles, resulting in "classical" or neutral antagonism, or inverse agonism. In addition, we describe the chemical features and guidelines for the synthesis of retinoids/rexinoids that exert specific functions and we provide protocols for a number of experimental approaches that are useful for studies of the agonistic and antagonistic features of NR ligands.