Role of Retinoic Acid Receptor Gamma in the Rhino Mouse and Rabbit Irritation Models of Retinoid Activity

Abstract

The three retinoic acid receptors (RARΑ, RARΒ and RARΓ) are known to modulate the transcription of target genes through interaction of the individual receptors with their naturally occurring ligand, retinoic acid (RA). Since RA has multiple effects in vivo, considerable effort has recently been devoted to finding selective compounds to elucidate the functions of individual receptors and to relate these functions to specific in vivo effects. The racemic synthetic retinoid 6-[(5,5,8,8-tetramethyl- 5,6,7,8-tetrahydro-2 – naphthyl)hydroxy-methyl]-2-naphthalene carboxylic acid has recently been identified as an RARΓ-selective agonist. A synthetic method involving lipase-mediated transformation has been developed to prepare the individual enantiomers. Discrimination between the two enantiomers is seen in both transcriptional activity and binding to recombinant receptors with the (S)-enantiomer being the more active. Differences between the two compounds are also seen in the Rhino mouse utriculi reduction assay and the rabbit irritation model. In both animal models, the (S)-enantiomer consistently gave a greater response. Taken together, these results suggest that the activity and irritation seen with RA and related compounds is receptor mediated. Further, the strong selectivity of the compounds reported here for RARΓ suggests that this receptor plays an important role in these in vivo biological activities. The discrimination between these enantiomers may be useful in the design of novel retinoids with uniquely defined biological properties.

Enantiomer discrimination illustrated by high-resolution crystal structures of the human nuclear receptor hRARγ

“…The ligand flexibility depends strongly on the length of the bridge between the TTN and the benzoic acid moieties. Retinoids where the TTN moiety and a naphthoic acid are connected by a single atom (8,9) have only two rotatable dihedral angles, whereas BMS270395 has three. This flexibility allows the latter to adapt to the pocket and to maintain the hydrogen bond to Met-272.…”

“…Numerous synthetic agonist and antagonist retinoids differentiate RXR from RAR or are selective for the RAR␣, ␤, or ␥ isotypes. The different activity of RAR␥-specific enantiomers has been described for two retinoids (8,9). However, the structural reasons for enantiomer discrimination has remained obscure.…”

The N-terminal portion of domain E of retinoic acid receptors alpha and beta is essential for the recognition of retinoic acid and various analogs.

“…as described (21,25 Retinoid Binding Assays. Chimeric receptors were ligated into the Asp718 and Xma I sites of pETl5b for bacterial expression using the T7 system (Novagen).…”

A Novel Molecular Target for Breast Cancer Prevention and Treatment

“…Transactivation by liganded RARy is reported to correlate with retinoid toxicity (53,54). The lack of retinoid receptor activation activity by MM 11453 suggests that toxic side effects characteristic of retinoid receptor activation (reviewed in Ref.…”