Conformationally defined 6-s-trans-retinoic acid analogs. 3. Structure- activity relationships for nuclear receptor binding, transcriptional activity, and cancer chemopreventive activity

Academic Article

Abstract

  • We recently demonstrated that conformationally defined 6-s-trans- retinoic acid (RA) analogs were effective in the prevention of skin papillomas (Vaezi et al. J. Med. Chem. 1994, 37, 44994507) and selective agonists for nuclear receptor binding and activation (Alam et al. J. Med. Chem. 1995, 38, 2302-2310). In order to probe important structure-activity relationships, we evaluated a homologous series of four 6-s-trans-retinoids that are 8-(2'-cyclohexen-1'-ylidene)3,7-dimethyl-2,4,6-octatrienoic acids with different substituents at 2' (R2) and 3' (R1) positions on the cyclohexene ring. UAB1 (R1 = R2 = H), UAB4 (R1 = R2 = Me), UAB7 (R1 = Me, R2 = iPr), and UAB8 (R1 = Et, R2 = iPr) contain alkyl R groups that mimic, to different extents, portions of the trimethylcyclohexenyl ring of RA. Both 9Z- and all-E-isomers of these retinoids were evaluated in binding assays for cellular retinoic acid-binding proteins (CRABP-I and CRABP-II), a nuclear retinoic acid receptor (RARα), and a nuclear retinoid X receptor (RXRα). The all-E-isomers of UAB retinoids bound tightly to CRABPs and RARα, the binding affinity of the all-E-isomer increased systematically from UAB1 to UAB8, and binding for the latter was comparable to that of all-E-RA. In contrast to RA, the (9Z)-UAB retinoids were at least 200-fold less active than the all-E-isomers in binding to RARα. The (9Z)-UAB isomers exhibited increasingly stronger binding to RXRα, and (9Z)-UAB8 was nearly as effective as (9Z)-RA in binding affinity. The retinoids were also evaluated in gene expression assays mediated by RARα and RXRα homodimers or RARα/RXRα heterodimers. Consistent with the binding affinities, the (all-E)-UAB retinoids activated gene transcription mediated by RARer homodimers or RARα/RXRα heterodimers, while the (9Z)-UAB isomers activated only the RXRα homodimer-mediated transcription. The all-E- and 9Z-isomers of the UAB retinoids were further evaluated for their capacity to prevent the induction of mouse skin papillomas. When compared to RA, only the (all-E)-UAB retinoids containing bulky R1 and R2 groups were effective in this chemoprevention assay. (9Z)-RA displayed equal capacity as RA to prevent papillomas, while the 9Z-isomers of the UAB retinoids were much less effective. Taken together, these studies demonstrate that the cyclohexenyl ring substituents of 6-s- trans-UAB retinoids are important for their biological activities and that the chemopreventive effect of the all-E-isomers of these retinoids correlates well with their capacity to bind to RARs and activate RAR/RXR-mediated transcription.
  • Published In

    Digital Object Identifier (doi)

    Author List

  • Muccio DD; Brouillette WJ; Alam M; Vaezi MF; Sani BP; Venepally P; Reddy L; Li E; Norris AW; Simpson-Herren L
  • Start Page

  • 3625
  • End Page

  • 3635
  • Volume

  • 39
  • Issue

  • 19