INDUCTION OF THE OXIDATIVE CATABOLISM OF RETINOIC ACID IN MCF-7 CELLS

Cytochrome P450-dependent oxidation is a pathway for all-trans-retinoi c acid (all-trans-RA) catabolism. Induction of this catabolic pathway was studied in MCF-7 breast cancer cells. MCF-7 cells showed low const itutive all-trans-RA catabolism. Concentration-dependent induction was obtained by preincubation of the cells with all-trans-RA (10(-9) to 1 0(-6) M). Onset of induction was fast, being detectable within 60 min, with maximal induction (45-fold) obtained after 16 h. Enzymatic chara cterization of induced all-trans-RA catabolism showed an estimated K-m value (Michaelis-Menten constant) of 0.33 mu M and a V-max value (max imal velocity of an enzyme-catalysed reaction) of 54.5 fmol polar all- trans-RA metabolites 10(6) cells(-1) h(-1). These kinetic parameters r epresent the overall formation of polar metabolites from all-trans-RA. Induction of all-trans-RA catabolism was also obtained with other ret inoids, CH55 >> 19-cis-RA = all-trans-RA > 9-cis-RA > 4-keto-all-trans -RA > 4-keto-13-cis-RA > retinol. The potency of the retinoids to indu ce all-trans-RA catabolism was correlated to their retinoic acid recep tor affinity (Crettaz et al, 1990; Repa et al, 1990; Sani et al, 1990) . Induction of all-trans-RA catabolism was inhibited by actinomycin D. Furthermore, all-trans-RA did not increase cytosolic retinoic acid-bi nding protein (CRABP) mRNA levels. These data suggest that induction o f all-trans-RA catabolism in MCF-7 cells is a retinoic acid receptor-m ediated gene transcriptional event. Induced all-trans-RA catabolism wa s inhibited by various retinoids with decreasing potency in the order: all-trans-RA > 4-keto-all-trans-RA > 13-cis-RA > 9-cis-RA > 4-keto-19 -cis-RA > retinol > CH55. The antitumoral compound liarozole-fumarate inhibited all-trans-RA catabolism with a potency similar to that of al l-trans-RA.