An emerging role for retinoid X receptor α in malignant hematopoiesis

Retinoid X Receptor agonists as selective modulators of the immune system for the treatment of cancer

Upon heterodimerizing with other nuclear receptors, retinoid X receptors (RXR) act as ligand-dependent transcription factors, regulating transcription of critical signaling pathways that impact numerous hallmarks of cancer. By controlling both inflammation and immune responses, ligands that activate RXR can modulate the tumor microenvironment. Several small molecule agonists of these essential receptors have been synthesized. Historically, RXR agonists were tested for inhibition of growth in cancer cells, but more recent drug discovery programs screen new molecules for inhibition of inflammation or activation of immune cells. Bexarotene is the first successful example of an effective therapeutic that molecularly targets RXR; this drug was approved to treat cutaneous T cell lymphoma and is still used as a standard of care treatment for this disease. No additional RXR agonists have yet achieved FDA approval, but several promising novel compounds are being developed. In this review, we provide an overview of the multiple mechanisms by which RXR signaling regulates inflammation and tumor immunity. We also discuss the potential of RXR-dependent immune cell modulation for the treatment or prevention of cancer and concomitant challenges and opportunities.

Potential therapeutic uses of rexinoids

The discovery of nuclear receptors, particularly retinoid X receptors (RXR), and their involvement in numerous pathways related to development sparked interest in their immunomodulatory properties. Genetic models using deletion or overexpression of RXR and the subsequent development of several small molecules that are agonists or antagonists of this receptor support a promising therapeutic role for these receptors in immunology. Bexarotene was approved in 1999 for the treatment of cutaneous T cell lymphoma. Several other small molecule RXR agonists have since been synthesized with limited preclinical development, but none have yet achieved FDA approval. Cancer treatment has recently been revolutionized with the introduction of immune checkpoint inhibitors, but their success has been restricted to a minority of patients. This review showcases the emerging immunomodulatory effects of RXR and the potential of small molecules that target this receptor as therapies for cancer and other diseases. Here we describe the essential roles that RXR and partner receptors play in T cells, dendritic cells, macrophages and epithelial cells, especially within the tumor microenvironment. Most of these effects are site and cancer type dependent but skew immune cells toward an anti-inflammatory and anti-tumor effect. This beneficial effect on immune cells supports the promise of combining rexinoids with approved checkpoint blockade therapies in order to enhance efficacy of the latter and to delay or potentially eliminate drug resistance. The data compiled in this review strongly suggest that targeting RXR nuclear receptors is a promising new avenue in immunomodulation for cancer and other chronic inflammatory diseases.

Molecular docking, dynamics, and pharmacology studies on bexarotene as an agonist of ligand-activated transcription factors, retinoid X receptors

Retinoid X receptors (RXRs) belong to the nuclear receptor superfamily, and upon ligand activation, these receptors control gene transcription via either homodimerization with themselves or heterodimerization with the partner-nuclear receptor. The protective effects of RXRs and RXR agonists have been reported in several neurodegenerative diseases, including in the retina. This study was aimed to prioritize compounds from natural and synthetic origin retinoids as potential RXR agonists by molecular docking and molecular dynamic simulation strategies. The docking studies indicated bexarotene as a lead compound that can activate various RXR receptor isoforms (α, β, and γ) and has a strong binding affinity to the receptor protein than retinoic acid, which is known as a natural endogenous RXR agonist. Dynamic simulation studies confirmed that the hydrogen bonding and hydrophobic interactions were highly stable in all the three isoforms of the RXR-bexarotene complex. To further validate the significance of the RXR receptor in neurons, in vitro pharmacological treatment of neuronal SH-SY5Y cells with bexarotene was performed. In vitro data from SH-SY5Y cells confirmed that bexarotene activated RXR-simulated neurite outgrowth significantly. We conclude that bexarotene could be potentially used as an exogenous activator of RXRs and emerge as a good drug target for several neurodegenerative disorders.

miR-27a-3p targeting RXRα promotes colorectal cancer progression by activating Wnt/β-catenin pathway

This study aimed to elucidate how miR-27a-3p modulates the Wnt/β-catenin signaling pathway to promote colorectal cancer (CRC) progression. Our results showed that the expression of miR-27a-3p was up-regulated in CRC and closely associated with histological differentiation, clinical stage, distant metastasis and CRC patients' survival. miR-27a-3p mimic suppressed apoptosis and promoted proliferation, migration, invasion of CRC cells in vitro and in vivo. Whereas miR-27a-3p inhibitor promoted apoptosis and suppressed proliferation, migration, invasion of CRC cells in vitro and in vivo. Furthermore, RXRα was the target gene of miR-27a-3p in CRC. miR-27a-3p expression negatively correlated with RXRα expression in CRC tissues. The underlining mechanism study showed that miR-27a-3p/RXRα/Wnt/β-catenin signaling pathway is involved in CRC progression. In conclusion, our findings first demonstrate that miR-27a-3p is a prognostic and/or potential therapeutic biomarker for CRC patients and RXRα as miR-27a-3p targeting gene plays an important role in activation of the Wnt/β-catenin pathway during CRC progression.

The multi-faceted role of retinoid X receptor in bone remodeling

Retinoid X receptors (RXRs) form a unique subclass within the nuclear receptor (NR) superfamily of ligand-dependent transcription factors. RXRs are obligatory partners for a number of other NRs, placing RXRs in a coordinating role at the crossroads of multiple signaling pathways. In addition, RXRs can function as self-sufficient homodimers. Recent advances have revealed RXRs as novel regulators of osteoclastogenesis and bone remodeling. This review outlines the versatility of RXR action in the control of transcription of bone-forming osteoblasts and bone-resorbing osteoclasts, both through heterodimerization with other NRs and through RXR homodimerization. RXR signaling is currently a major therapeutic target and, therefore, knowledge of how RXR signaling affects bone remodeling creates enormous potential for the translation of basic research findings into successful clinical therapies to increase bone mass and improve bone quality.

Targeting truncated RXRα for cancer therapy

Retinoid X receptor-alpha (RXRα), a unique member of the nuclear receptor superfamily, is a well-established drug target, representing one of the most important targets for pharmacologic interventions and therapeutic applications for cancer. However, how RXRα regulates cancer cell growth and how RXRα modulators suppress tumorigenesis are poorly understood. Altered expression and aberrant function of RXRα are implicated in the development of cancer. Previously, several studies had demonstrated the presence of N-terminally truncated RXRα (tRXRα) proteins resulted from limited proteolysis of RXRα in tumor cells. Recently, we discovered that overexpression of tRXRα can promote tumor growth by interacting with tumor necrosis factor-alpha-induced phosphoinositide 3-kinase and NF-κB signal transduction pathways. We also identified nonsteroidal anti-inflammatory drug Sulindac and analogs as effective inhibitors of tRXRα activities via a unique binding mechanism. This review discusses the emerging roles of tRXRα and modulators in the regulation of cancer cell survival and death as well as inflammation and our recent understanding of tRXRα regulation by targeting the alternate binding sites on its surface.

NSC-640358 acts as RXRα ligand to promote TNFα-mediated apoptosis of cancer cell

Retinoid X receptor α (RXRα) and its N-terminally truncated version tRXRα play important roles in tumorigenesis, while some RXRα ligands possess potent anti-cancer activities by targeting and modulating the tumorigenic effects of RXRα and tRXRα. Here we describe NSC-640358 (N-6), a thiazolyl-pyrazole derived compound, acts as a selective RXRα ligand to promote TNFα-mediated apoptosis of cancer cell. N-6 binds to RXRα and inhibits the transactivation of RXRα homodimer and RXRα/TR3 heterodimer. Using mutational analysis and computational study, we determine that Arg316 in RXRα, essential for 9-cis-retinoic acid binding and activating RXRα transactivation, is not required for antagonist effects of N-6, whereas Trp305 and Phe313 are crucial for N-6 binding to RXRα by forming extra π–π stacking interactions with N-6, indicating a distinct RXRα binding mode of N-6. N-6 inhibits TR3-stimulated transactivation of Gal4-DBD-RXRα-LBD by binding to the ligand binding pocket of RXRα-LBD, suggesting a strategy to regulate TR3 activity indirectly by using small molecules to target its interacting partner RXRα. For its physiological activities, we show that N-6 strongly inhibits tumor necrosis factor α (TNFα)-induced AKT activation and stimulates TNFα-mediated apoptosis in cancer cells in an RXRα/tRXRα dependent manner. The inhibition of TNFα-induced tRXRα/p85α complex formation by N-6 implies that N-6 targets tRXRα to inhibit TNFα-induced AKT activation and to induce cancer cell apoptosis. Together, our data illustrate a new RXRα ligand with a unique RXRα binding mode and the abilities to regulate TR3 activity indirectly and to induce TNFα-mediated cancer cell apoptosis by targeting RXRα/tRXRα.

Clearance of PML/RARA-bound promoters suffice to initiate APL differentiation

PML/RARA loss or detachment from target promoters suffices to differentiate APL cells. PML/RARA degradation by arsenic thus explains arsenic-induced differentiation.

Nuclear receptors and pathogenesis of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a devastating disease with a median overall survival time of 5 mo and the five years survival less than 5%, a rate essentially unchanged over the course of the years. A well defined progression model of accumulation of genetic alterations ranging from single point mutations to gross chromosomal abnormalities has been introduced to describe the origin of this disease. However, due to the its subtle nature and concurring events PDAC cure remains elusive. Nuclear receptors (NR) are members of a large superfamily of evolutionarily conserved ligand-regulated DNA-binding transcription factors functionally involved in important cellular functions ranging from regulation of metabolism, to growth and development. Given the nature of their ligands, NR are very tempting drug targets and their pharmacological modulation has been widely exploited for the treatment of metabolic and inflammatory diseases. There are now clear evidences that both classical ligand-activated and orphan NR are involved in the pathogenesis of PDAC from its very early stages; nonetheless many aspects of their role are not fully understood. The purpose of this review is to highlight the striking connections that link peroxisome proliferator activated receptors, retinoic acid receptors, retinoid X receptor, androgen receptor, estrogen receptors and the orphan NR Nur, chicken ovalbumin upstream promoter transcription factor II and the liver receptor homologue-1 receptor to PDAC development, connections that could lead to the identification of novel therapies for this disease.

Retinoid X receptors in macrophage biology

Retinoid X receptors (RXRs) form a distinct and unique subclass within the nuclear receptor (NR) superfamily of ligand-dependent transcription factors. RXRs regulate a plethora of genetic programs, including cell differentiation, the immune response, and lipid and glucose metabolism. Recent advances reveal that RXRs are important regulators of macrophages, key players in inflammatory and metabolic disorders. This review outlines the versatility of RXR action in the control of macrophage gene transcription through its heterodimerization with other NRs or through RXR homodimerization. We also highlight the potential of RXR-controlled transcriptional programs as targets for the treatment of pathologies associated with altered macrophage function, such as atherosclerosis, insulin resistance, autoimmunity, and neurodegeneration.