Retinoid modulation following the differentiation of the NB4 cell line using either ATRA or RAMBA. (A) ATRA and RAMBA chemical molecules. (B) Efficiency of differentiation (•) and cell growth inhibition (○) after treatment (black arrows) of NB4 cells with 1 μM of either ATRA or RAMBA. (C) Retinoid modulation in NB4 culture medium performed with gradient reversed-phase HPLC. Chromatogram of retinoids extracted from the culture medium after 24 hours of treatment with either ATRA or RAMBA (upper graphs). Retinoid modulation in the culture medium following NB4 cell differentiation kinetics by treatment with either ATRA or RAMBA (lower graphs). As the azole group makes RAMBA a specific and potent inhibitor of CYP26 p450-cytochromes, metabolism was only observed following ATRA treatment. Hydroxylated products were more rapidly eluted than parental isoforms. 13-cis RA (a), 13-cis RAMBA (a′), their stereoisomers ATRA (b) and RAMBA (b′). The identified metabolites were 4-oxo-ATRA (c), 4-hydroxy-ATRA (d), 18-hydroxy-ATRA (e), and 5,6-epoxy-ATRA (f). (D) Retinoid modulation in cytoplasm and nucleoplasm following NB4 cell differentiation.