N-(4-HYDROXYPHENYL)RETINAMIDE - INTERACTIONS WITH RETINOID-BINDING PROTEINS RECEPTORS/

The cellular transport, metabolism and biological activity of retinoid s are mediated by their specific binding proteins and nuclear receptor s, For an understanding of the mode of action of retinoids with potent ial cancer chemopreventive or other biological activity, it is importa nt to study their interactions with these binding proteins and recepto rs, In our attempts to understand the action of N-(4-hydroxyphenyl)ret inamide (4HPR) and other retinamides in the prevention of cancer, we o bserved that 4HPR binds to a serum protein with a molecular size of si milar to 20 000, The retinoid, however, did not show any binding affin ity for cellular retinol-binding protein (CRBP) or for cellular retino ic acid-binding protein (CRABP). However, it showed binding affinity f or the nuclear receptors of retinoic acid (RARs) equivalent to 15% of that of retinoic acid, The physicochemical properties of the 4HPR bind ing protein in the serum were identical to those of serum retinol bind ing protein (RBP), Antibodies against RBP quantitatively immunoprecipi tated the protein-4HPR complex, confirming that the retinoid specifica lly binds to RBP. Although retinol and 4HPR cross-competed for RBP bin ding, N-phenylretinamide, in which the 4-hydroxyl group is absent, and N-(4-methoxyphenyl)retinamide, a major cellular metabolite of 4HPR, i n which the hydroxyl group is blocked, did not show affinity for the b inding protein, The results indicate that the hydroxyl group of 4HPR i s essential for binding of this type of retinoid to RBP. Thus, our stu dies suggest that serum transport of 4HPR may be facilitated by RBP. T o bind more efficiently to CRBP, CRABP, or RARs/RXRs, the retinoid may require further metabolic change.