Some β‐apocarotenoids function as antagonists of retinoic acid receptors by directly competing at the ligand binding site

Eccentric cleavage of β‐carotene at double bonds other than the central double bond yields β‐apocarotenals and β‐apocarotenones. The aldehydes can presumably be oxidized in vivo to the carboxylic acids. We synthesized all possible β‐apocarotenoids (BACs) and screened for their activity on retinoic acid receptors (RARs) in transactivation assays using cells transfected with RAR constructs and RARE‐luciferase reporters. Some BACs antagonized retinoic acid (RA)‐induced transcription. We then examined whether these BACs serve as high‐affinity ligands for RARs by using competitive binding assays in which potential antagonists compete with radiolabeled RA for binding to recombinant RARs. β‐apo‐10′‐Carotenoic acid, β‐apo‐12′‐carotenoic acid, β‐apo‐14′‐carotenal, β‐apo‐14′‐carotenoic acid, and β‐apo‐13‐carotenone were tested for their ability to compete with labeled RA on RARα, RARβ, and RARγ. β‐apo‐13‐Carotenone has a two carbon shorter polyene chain than RA and results from the cleavage of the double bond next to the central one. Its affinity for all receptor subtypes was nearly identical with that of RA and less than 10nM. β‐apo‐14′‐Carotenal and β‐apo‐14′‐carotenoic acid that are two carbons longer than RA (and result from the same cleavage as β‐apo‐13‐carotenone) had observed K i s on all RARs in the range of 20–50 nM. Finally, β‐apo‐12′‐carotenoic acid and β‐apo‐10′‐carotenoic acid competed for labeled RA binding to the recombinant RARs with K i s of 250–700nM. Thus, eccentric cleavage products of BC may be biologically active compounds in nuclear receptor signaling. Grant Funding Source : National Institutes of Health