all-trans-Retinoic acid-induced expression and regulation of retinoic acid 4-hydroxylase (CYP26) in human promyelocytic leukemia

all-trans-Retinoic acid (ATRA) induces complete remission in majority of patients with acute promyelocytic leukemia (APL). However, accelerated metabolism of ATRA that is induced by chronic daily administration of oral ATRA has been implicated as one of the mechanisms leading to a reduced sensitivity or resistance to ATRA therapy. We investigated the expression and regulation of CYP26, a novel p450 enzyme, which is highly specific for ATRA, in promyelocytic leukemia cells (NB4 and HL-60). We found that treatment of NB4 cells with a pharmacological concentration of ATRA (1 microM) induced rapid and dose-dependent expression of CYP26 mRNA. The CYP26 expression returned to pretreatment levels in both cells after ATRA was removed from the media. Retinoic acid receptor-alpha (RARalpha) specific antagonist (CD2503) totally abolished the ATRA-induced expression of CYP26 mRNA in HL-60 and NB4 cells. Furthermore, HL-60R, a HL-60 subclone expressing nonfunctional RAR because of a point mutation in the ligand-binding domain of RARalpha, failed to show CYP26 mRNA expression in response to ATRA. ATRA-induced expression of CYP26 was restored in HL-60R cells retrovirally transduced with RARalpha, but not in those cells transduced with the other retinoid receptors. In conclusion, ATRA induces expression of CYP26 in myeloid and promyelocytic leukemia cells and this expression is modulated by RARalpha. The induction of CYP26 expression by ATRA treatment might be related to a substrate-driven feedback mechanism to regulate intracellular concentrations of ATRA and its over expression in some clones may be partly responsible for reduced sensitivity or resistance to ATRA therapy.