Retinoic acid receptors and retinoid X receptors (RAR/RXR)

Retinoid receptors, comprising the retinoic acid receptor (RAR) and retinoid X receptor (RXR) families, are essential ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily [1, 4]. Each family includes three subtypes—alpha, beta, and gamma—which primarily function as RAR/RXR heterodimers to control the expression of genes critical for embryonic development, cell differentiation, and tissue homeostasis [2, 8]. In their unliganded state, these receptors bind to retinoic acid response elements (RAREs) and recruit corepressors to silence gene transcription; however, the binding of ligands such as all-trans retinoic acid (ATRA) triggers a conformational change that recruits coactivators to initiate gene expression [3, 11]. Furthermore, RXRs serve as obligate heterodimeric partners for several other nuclear receptors, including the vitamin D receptor and peroxisome proliferator-activated receptors, thereby acting as central nodes in multiple metabolic and signaling pathways [11, 15]. These receptors are major therapeutic targets in oncology and dermatology, with drugs like tretinoin and bexarotene used to treat acute promyelocytic leukemia (APL) and cutaneous T-cell lymphoma, respectively [7, 9]. Despite their clinical utility, targeting retinoid receptors is associated with significant safety challenges, most notably severe teratogenicity and the potential for differentiation syndrome in leukemia patients [12, 15].

Other names

Retinoid receptorsRARRXRNR1BNR2BRetinoic acid receptor alphaRetinoic acid receptor betaRetinoic acid receptor gammaRetinoid X receptor alphaRetinoid X receptor betaRetinoid X receptor gammaNR1B1NR1B2NR1B3NR2B1NR2B2NR2B3

Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) function as ligand-activated transcription factors that primarily form RAR/RXR heterodimers [1, 4]. In the absence of a ligand, these heterodimers bind to retinoic acid response elements (RAREs) in the DNA and recruit corepressor complexes, such as NCoR and SMRT, along with histone deacetylases (HDACs) to maintain a repressed chromatin state [3, 11]. Upon binding of an agonist ligand, such as all-trans retinoic acid to RAR or 9-cis retinoic acid to RXR, the receptors undergo a conformational change that leads to the dissociation of corepressors and the recruitment of coactivator complexes with histone acetyltransferase (HAT) activity [4, 16]. This remodeling of chromatin facilitates the assembly of the transcriptional machinery and the activation of target gene expression, which regulates processes like cell differentiation, growth arrest, and apoptosis [2, 8].