Retinoid X receptors (RXR␣, -␤, and -␥) occupy a central position in the nuclear receptor superfamily, because they form heterodimers with many other family members and hence are involved in the control of a variety of (patho)physiologic processes. Selective RXR ligands, referred to as rexinoids, are already used or are being developed for cancer therapy and have promise for the treatment of metabolic diseases. However, important side effects remain associated with existing rexinoids. Here we describe the rational design and functional characterization of a spectrum of RXR modulators ranging from partial to pure antagonists and demonstrate their utility as tools to probe the implication of RXRs in cell biological phenomena. One of these ligands renders RXR activity particularly sensitive to coactivator levels and has the potential to act as a cell-specific RXR modulator. A combination of crystallographic and fluorescence anisotropy studies reveals the molecular details accounting for the agonist-to-antagonist transition and provides direct experimental evidence for a correlation between the pharmacological activity of a ligand and its impact on the structural dynamics of the activation helix H12. Using RXR and its cognate ligands as a model system, our correlative analysis of 3D structures and dynamic data provides an original view on ligand actions and enables the establishment of mechanistic concepts, which will aid in the development of selective nuclear receptor modulators.crystal structure ͉ ligand design ͉ nuclear receptor ͉ agonist ͉ antagonist N uclear Receptor (NR)-controlled gene expression relies on a mechanism in which NRs recruit coregulators that are part of multiprotein complexes. These complexes correspond to chromatin-modifying and transcription-initiating machineries that act at target gene promoters in a precisely timed and sequential fashion (1). The binding of a ligand to the ligandbinding domain (LBD) of NRs constitutes the initial step of this regulatory process. In this context, the C-terminal helix H12 of LBDs plays a key role, because its position, which depends on the bound ligand, determines the type of coregulator recruited by the receptor (2). Structural studies have shown that in agonistbound NR LBDs, H12 adopts the so-called ''active'' or ''holo'' conformation and provides a binding surface for short NR interaction motifs of coactivators (3). In contrast, antagonists prevent H12 from adopting the holo position (4).Therapeutically, retinoid X receptor (RXR)-selective ligands, referred to as rexinoids, are used in cancer therapy, and previously uncharacterized rexinoid-based therapeutic paradigms are currently being explored. In addition, rexinoids have promise for use in the therapy of metabolic diseases (5, 6), but important side effects associated with existing compounds limit their use. Improved understanding of the biological role and the structural biology of RXR (7, 8) will allow the synthesis of selective modulators that might overcome the limitations of current drugs. Here,...

Apart from PML-retinoic acid receptor-A (RARA) acute promyelocytic leukemia all other acute myeloid leukemias (AML) are unresponsive to retinoid differentiation therapy. However, elevating the levels of cyclic AMP (cAMP) confers onto retinoid X receptor (RXR)-selective agonists (''rexinoids'') the ability to induce terminal granulocyte differentiation and apoptosis of all-trans retinoic acid-resistant and insensitive AML cells and patients' blasts. Protein kinase A activation leads to corepressor release from the RAR subunit of the RAR-RXR heterodimer, resulting in ''desubordination'' of otherwise silent RXR, which acquires transcriptional competence in response to cognate ligands. Rexinoid-cAMP induction of endogenous RARB is blunted in mouse embryo fibroblasts lacking RARs, but reintroduction of exogenous RARA reestablishes responsiveness, thus confirming that the RARA-RXR heterodimer is the rexinoid mediator. The apoptogenic effect of this treatment involves enhanced expression of the death receptor DR5 and its cognate ligand, tumor necrosis factorrelated apoptosis inducing ligand, both of which are known to induce apoptosis in a tumor cell-selective manner and lead to the activation of initiator caspases. Immunohistochemistry confirmed induction of tumor necrosis factor-related apoptosis inducing ligand and DR5 in AML patient blasts cultured ex vivo. AML patients' blasts responded to rexinoid-cAMP combination treatment with induction of maturation and apoptosis, independent of karyotype, immunophenotype, and French-American-British classification status. Clonogenic assays revealed complete inhibition of blast clonogenicity in four out of five tested samples. Our results suggest that despite the genetic, morphologic, and clinical variability of this disease, the combination of rexinoids and cAMP-elevating drugs, such as phosphodiesterase inhibitors, might lead to a novel therapeutic option for AML patients by inducing a tumorselective death pathway. (Cancer Res 2005; 65(19): 8754-65)

Mineralocorticoid (MR) and glucocorticoid (GR) receptors are two closely-related members of the steroid nuclear receptor family of transcription factors that bind common ligands in the brain (corticosterone and cortisol) and supposedly have identical hormone response elements. This raises the important question of how they can elicit differential biological actions in neurons in which they are often colocalized. One plausible explanation is that they differentially recruit proteins (coregulators or other receptor-interacting factors) through cell-specific interactions with regions that diverge between MR and GR to modulate target gene transcription in a receptor-specific manner. We therefore performed a yeast-two-hybrid screening of a human brain cDNA library with an AF1-containing region of the human MR as bait. This screening revealed several potential MR-interacting partners; among them were several clones bearing homology to DAXX, FLASH, and FAF-1, all previously implicated in apoptosis. Coexpression of candidate clones in a mouse hippocampal cell line confirmed these interactions in a mammalian neural cell environment as well. In transient transactivation assays, DAXX and FLASH influenced MR-and GRdriven transcription of the MMTV-Luc reporter similarly; in contrast, although FAF-1 did not transactivate GR, it did selectively stimulate MR-mediated transcription. Thus, the present findings, that 1) DAXX, FLASH, and FAF-1 modulate the transcriptional activities of MR and GR and that 2) FAF-1 selectively coactivates only MR, provide possible clues for how these closely related receptors might differentially influence neuronal function.

Apart from PML-RARα acute promyelocytic leukemia all other acute myeloid leukemias (AML) are unresponsive to retinoid differentiation therapy. However, in our study we show that elevating the levels of cyclic AMP (cAMP) confers onto retinoid X receptor (RXR)-selective agonists (“rexinoids”) the ability to induce terminal granulocyte differentiation and apoptosis of all-trans retinoic acid-resistant and insensitive AML cell lines and patients’ AML blasts. Protein kinase A activation leads to co-repressor release from the RAR subunit of the RAR-RXR heterodimer, resulting in “de-subordination” of otherwise silent RXR, which acquires transcriptional competence in response to cognate ligands. Rexinoid-cAMP induction of endogenous RARβ is blunted in mouse embryo fibroblasts lacking RARs, but re-introduction of exogenous RARα re-establishes responsiveness, thus confirming that the RARα-RXR heterodimer is the rexinoid mediator. The apoptogenic effect of this treatment involves enhanced expression of the death receptor DR5 and its cognate ligand, the tumor necrosis factor-related apoptosis inducing ligand (TRAIL), both of which are known to induce apoptosis in a tumor cell-selective manner and lead to the activation of initiator caspases. Immunohistochemistry confirmed induction of TRAIL and DR5 in AML patient blasts cultured “ex vivo”. AML patients’ blasts responded to rexinoid-cAMP combination treatment with induction of maturation and apoptosis, independent of karyotype, immunophenotype, and FAB classification. Clonogenic assays revealed complete inhibition of blast clonogenicity in four out of five tested samples. Indeed, it is known that cAMP levels can be elevated also by treating cells with 3′, 5′-cAMP phosphodiesterase inhibitors (PDEi’s). These observation and the clinical availability of the corresponding drugs provide a rationale for initiating clinical studies addressing the efficacy of combinatorial PDEi-rexinoid therapy in AML patients. Together with our recent finding that a very promising class of epigenetic anti-tumor drugs operates through activation of TRAIL expression (Nat Med2005, 11(1):7784), the possibility to target both the ligand (TRAIL) by HDAC inhibitors and the cognate receptors (DR4, DR5) by the above described rexinoid crosstalk may represent a promising therapeutic option which might lead, despite the genetic, morphologic, and clinical variability of AML, to a novel therapeutic option for AML patients by inducing a tumor-selective death pathway.