The recently identified retinoic acid (RA)-metabolizing cytochrome P450RAI-1 (CYP26A1) has been implicated in accelerated metabolism and rapid clearance of all-trans-retinoic acid (ATRA) during prolonged oral administration in patients with acute promyelocytic leukemia (APL), leading to a progressive decline in plasma drug levels. We studied induction and regulation of CYP26A1 expression and ATRA metabolism in human intestinal (Caco-2), liver (HepG2), endothelial (HUVEC), and APL (NB4) cell lines. ATRA rapidly induced upregulation of CYP26A1 mRNA expression in a dose-dependent manner. Other retinoids (retinol, 9-cis-RA, and 13-cis-RA) also induced significant CYP26A1 expression in HepG2 and NB4 cells. CYP26A1 mRNA expression in HepG2 cells returned to baseline in 48 h upon removal of ATRA from the culture medium, suggesting that the expression is reversible and requires the presence of ATRA. In endothelial cells, however, a higher concentration of ATRA (10 microM) was required to induce expression of CYP26A1. A specific RA receptor-alpha antagonist totally inhibited ATRA-induced expression of CYP26A1, indicating that RA receptor-alpha plays a major role in CYP26A1 expression in HepG2 cells. Liposomal incorporation of ATRA has been shown to alter its metabolism. Therefore, we also tested CYP26A1 expression after administration of free ATRA and liposomal ATRA (L-ATRA). L-ATRA induced lower CYP26A1 expression and metabolic activity in HepG2 and NB4 cells when compared with free ATRA. Pretreatment of cells with free ATRA resulted in higher metabolic activity as indicated by conversion of radiolabeled [3H]-ATRA into its metabolites (4-oxo-RA and 4-hydroxy-RA), which was associated with lower nuclear localization of [3H]-ATRA when compared with pretreatment with L-ATRA. Our data suggest that upregulation of CYP26A1 expression in intestinal, endothelial, liver, and APL cells and metabolism of ATRA may play a role in rapid clearance of ATRA after continuous oral administration. Therapeutic strategies such as liposomal encapsulation and intermittent administration of ATRA may circumvent accelerated ATRA metabolism and improve the treatment of APL.